Patent Application
20090280350
The present invention relates to steel sheet excellent in deep drawability, press formability, punchability, and other workability and a method of production of that steel sheet.
For sheet steel for automobiles or home electrical appliances, in addition to the needs for higher strength and lighter weight, excellent workability enabling working in press forming and other work processes without causing cracks or wrinkles is required.
The workability of steel sheet depends on the texture of the αFe phase or the γFe phase. In particular, by increasing {222} plane integration of the crystals at the steel sheet surface, it is possible to improve the workability. For this reason, several methods have been proposed for controlling the texture to raise the workability of the steel.
Japanese Patent Publication (A) No. 6-2069 discloses high strength cold rolled steel sheet and hot dip galvanized steel sheet wherein the amounts of Si, Mn, and P are controlled based on a fixed relationship with the X-ray diffraction intensities of the {222} planes and {200} planes parallel to the steel sheet surface so as to secure deep drawability.
Japanese Patent Publication (A) No. 8-13081 discloses an enameling use high strength cold rolled steel sheet and a method of production of the same wherein the amount of Nb is defined by the amount of C and, furthermore, the hot rolling and cold rolling conditions are defined so as to control the (111) texture
Japanese Patent Publication (A) No. 10-18011 discloses a hot dip galvannealed steel sheet and method of production of the same wherein, when, among the X-ray diffraction intensities, the ratio of the {200} plane intensity and the {222} plane intensity, that is, I(200)/I(222), becomes less than 0.17, there are no longer streak like defects at the plating surface and wherein when the final rolling temperature of the hot rolling is made Ar3+30° C. or more, the X-ray diffraction intensity ratio I(200)/I(222) becomes less than 0.17.
Japanese Patent Publication (A) No. 11-350072 discloses very low carbon cold rolled steel sheet with a content of C in the steel of 0.01% or less which, when the particle size of the ferrite at the surface layer part accounting for 1/10 of the total thickness from the surface of the steel sheet is a and the particle size of the ferrite at the inner layer part accounting for ½ of the total thickness centered at the center of thickness is b, satisfies a−b≧0.5, a≧7.0, and b≦7.5 and which, if controlling the ratio I(222)/I(200) of X-ray diffraction intensities from the {222} plane and the {200} plane to be 5.0 or more at the part of 1/15 the total sheet thickness from the surface of the steel sheet and to be 12 or more at the center part of sheet thickness of the steel sheet, it is possible to reduce the orange skin peel state of the steel sheet at the time of press formation.
In this way, in the past, to improve the workability of a steel sheet, the technique has been devised of increasing the {222} plane integration of the αFe phase or γFe phase. This has been used to optimize the steel sheet ingredients, rolling conditions, temperature conditions, etc.
Furthermore, Japanese Patent Publication (A) No. 2006-144116 discloses high Al content steel sheet having an Al content of 6.5 mass % to 10 mass % wherein the {222} plane integration of the αFe crystals is made 60% to 95% or the {200} plane integration is made 0.01% to 15% so as to improve the workability.
Furthermore, the above publication discloses a method of raising the plane integration of specific planes in high Al content steel sheet comprising treating the surface of matrix steel sheet having an Al content of 3.5 mass % to less than 6.5 mass % by hot dip Al coating to deposit Al alloy, cold rolling, then performing diffusion heat treatment.
Further, when punching steel sheet, a small size of the burrs formed at the cross-section is sought as one aspect of the workability, so in the past various methods have been proposed for suppressing the formation of burrs.
Japanese Patent Publication (A) No. 3-277739 discloses steel sheet hardened at its surface so as to make the burrs formed at the time of shearing extremely small and give a soft hardness distribution inside the steel sheet so as to prevent reduction of the press formability. Specifically, steel sheet having an r value (Rankford value) of 1.7 to 2 and having a burr height at the time of punching of 12 to 40 μm is disclosed.
Japanese Patent Publication (A) No. 8-188850 discloses cold rolled steel sheet comprised of very low carbon steel to which S is added in an amount of 0.003 to 0.03% so as to satisfy a fixed formula and raised in deep drawability and punchability. Specifically, steel sheet having an r value of 2.2 to 2.6 and a burr height at the time of punching of 30 to 80 μm is disclosed.
As explained above, in the past, techniques have been devised for optimizing the steel sheet ingredients, rolling conditions, temperature conditions, etc. so as to raise the {222} plane integration of the αFe phase or γFe phase. These have met the needs for improvement of the workability of steel sheet.
However, meeting more sophisticated requirements is difficult with the prior art. A new perspective is required.
That is, in steel sheet with a {222} plane integration of the conventional extent, the punchability becomes poor in the working process. Further, the plastic flowability required in complicated press forming is insufficient. It has not been possible to meet the needs for more sophisticated working or higher efficiency of the working process.
Specifically, the above steel sheet had the problem of formation of burrs at the cross-section at the time of punching and the need for a chamfering process to remove the formed burrs.
Further, the above steel sheet had the problem of insufficient slip of the steel sheet with the die surface at the time of press formation by a complicated die and therefore the inability to form shapes more complicated than in the past.
The steel sheet disclosed in Japanese Patent Publication (A) No. 2006-144116 has a {222} plane integration for raising the workability higher than the past and has workability enough for forming foil for forming a honeycomb structure, but has a large Al content, so cannot be used as usual processing use steel sheet for sophisticated working or for higher efficiency of the working process.
Further, the methods disclosed in Japanese Patent Publication (A) No. 6-2069, Japanese Patent Publication (A) No. 8-13081, Japanese Patent Publication (A) No. 10-18011, and Japanese Patent Publication (A) No. 11-350072 enable integration of the {222} planes up to a certain ratio, but there are limits to the improvement of the plane integration with just setting the ingredient conditions and conditions in the annealing and other conventional processes.
In the method disclosed in Japanese Patent Publication (A) No. 2006-144116, the conventional process is augmented by a step of deposition of an Al alloy on the matrix surface by hot dip Al coating so as to raise the {222} plane integration.
However, the above method is a method improving the {222} plane integration only when using a matrix having an Al content of 3.5 mass % to less than 6.5 mass %. If just applying this method to steel sheet with a low Al content, it is difficult to raise or lower the integration of specific planes.
Furthermore, the methods disclosed in Japanese Patent Publication (A) No. 3-277739 and Japanese Patent Publication (A) No. 8-188850 succeed in reducing the formation of burrs accompanying punching to a certain extent, but have not reached the point of enabling elimination of the chamfering step for removing the burrs.
Therefore, the inventors studied art for plating or otherwise treating the surface of steel sheet to control the texture further. The present invention has as its object the provision of “less than 6.5 mass % Al content steel sheet” excellent in workability having an unprecedentedly high level of {222} plane integration and free from formation of burrs at the cross-section at the time of punching.
Further, the present invention has as its object the provision of a method of production for producing a “less than 6.5 mass % Al content steel sheet” having an unprecedentedly high {222} plane integration.
The inventors discovered that in steel sheet with an Al content of less than 6.5 mass %, if (x1) making the {222} plane integration of the Fe crystals a high specific range and/or (x2) making the {200} plane integration of the Fe crystals a low specific range, no burrs form at the cross-section at the time of punching and unprecedentedly excellent workability is obtained.
Furthermore, the inventors discovered that, as techniques for effectively integrating specific crystal planes by a high ratio in steel sheet having an Al content of less than 6.5 mass %, (y1) depositing a second layer on the surface of a matrix steel sheet having an Al content of less than 3.5 mass % (the matrix steel sheet being referred to as the “first layer” and the layer provided on its surface being referred to as the “second layer”), then heat treating this to integrate specific crystal planes to a high level, by making the content of Cr in the matrix steel sheet 12 mass % or less and, further, (y2) depositing a second layer on a matrix steel sheet having an Al content of less than 6.5 mass %, then cold rolling, then removing the second layer and performing heat treatment were effective.
Below, the gist of the present invention will be described.
(1) Steel sheet having a high {222} plane integration comprised of steel sheet having an Al content of less than 6.5 mass %, characterized by one or both of:
(3) Steel sheet having a high {222} plane integration comprising steel sheet having an Al content of less than 6.5 mass % on at least one surface of which a second layer is formed and having the second layer and steel sheet partially alloyed, characterized by one or both of:
(4) Steel sheet having a high {222} plane integration comprising steel sheet having an Al content of less than 6.5 mass % on at least one surface of which a second layer is deposited and alloyed with the steel sheet, characterized by one or both of:
(5) Steel sheet having a high {222} plane integration as set forth in any of (1) to (4) characterized in that said {222} plane integration is 60% to 95%.
(6) Steel sheet having a high {222} plane integration as set forth in any of (2) to (5) characterized in that said second layer contains at least one element from among Fe, Al, Co, Cu, Cr, Ga, Hf, Hg, In, Mn, Mo, Nb, Ni, Pb, Pd, Pt, Sb, Si, Sn, Ta, Ti, V, W, Zn, and Zr.
(7) Steel sheet having a high {222} plane integration as set forth in any of (1) to (6) characterized in that the thickness of the steel sheet is 5 μm to 5 mm.
(8) Steel sheet having a high {222} plane integration as set forth in any of (2) to (7) characterized in that the thickness of the second layer is 0.01 μm to 500 μm.
(9) A method of production of steel sheet having a high {222} plane integration having
(10) A method of production of steel sheet having a high {222} plane integration having
(11) A method of production of steel sheet having a high {222} plane integration having:
(12) A method of production of steel sheet having a high {222} plane integration having:
(13) A method of production of steel sheet having a high {222} plane integration as set forth in any of (9) to (12), said method of production of steel sheet having a high {222} plane integration characterized by control to obtain one or both of:
(14) A method of production of steel sheet having a high {222} plane integration as set forth in any of (9) to (12), said method of production of steel sheet having a high {222} plane integration characterized by control to obtain one or both of:
(15) A method of production of steel sheet having a high {222} plane integration as set forth in any of (9) to (12), said method of production of steel sheet having a high {222} plane integration characterized in that the second layer contains at least one element among Fe, Al, Co, Cu, Cr, Ga, Hf, Hg, In, Mn, Mo, Nb, Ni, Pb, Pd, Pt, Sb, Si, Sn, Ta, Ti, V, W, Zn, and Zr.
(16) A method of production of steel sheet having a high {222} plane integration, said method of production of steel sheet having a high {222} plane integration characterized by having
(17) A method of production of steel sheet having a high {222} plane integration, said method of production of steel sheet having a high {222} plane integration characterized by having
(18) A method of production of steel sheet having a high {222} plane integration, said method of production of steel sheet having a high {222} plane integration characterized by having
(19) A method of production of steel sheet having a high {222} plane integration, said method of production of steel sheet having a high {222} plane integration characterized by having
(20) A method of production of steel sheet having a high {222} plane integration as set forth in any one of (9) to (19) characterized in that the thickness of the steel sheet serving as said matrix is 10 μm to 10 mm.
(21) A method of production of steel sheet having a high {222} plane integration as set forth in any one of (9) to (19) characterized in that the thickness of the second layer is 0.05 μm to 1000 μm.
(22) A method of production of steel sheet having a high {222} plane integration as set forth in any one of (9) to (19) characterized by, before depositing said second layer, preheat treating the steel sheet.
(23) A method of production of steel sheet having a high {222} plane integration as set forth in (22) characterized in that the temperature of said preheat treatment is 700 to 1100° C.
(24) A method of production of steel sheet having a high {222} plane integration as set forth in (22) or (23) characterized in that an atmosphere of said preheat treatment is at least one of a vacuum, an insert gas atmosphere, and a hydrogen atmosphere.
(25) A method of production of steel sheet having a high {222} plane integration as set forth in any of (9) to (19) characterized in that said step of depositing the second layer on the steel sheet is by plating.
(26) A method of production of steel sheet having a high {222} plane integration as set forth in any of (9) to (19) characterized in that said step of depositing the second layer on the steel sheet is by roll cladding.
(27) A method of production of steel sheet having a high {222} plane integration as set forth in any of (9) to (19) characterized in that a reduction rate in said step of cold rolling is 30% to 95%.
(28) A method of production of steel sheet having a high {222} plane integration as set forth in any of (9) to (19) characterized in that a heat treatment temperature in said step of heat treatment is 600° C. to 1000° C. and a heat treatment time is 30 seconds or more.
(29) A method of production of steel sheet having a high {222} plane integration as set forth in any of (9) to (19) characterized in that a heat treatment temperature in said step of heat treatment is over 1000° C.
The steel having a high {222} plane integration of the present invention (the present invention steel sheet) sheet is an unprecedented steel sheet excellent in workability which has an Al content of less than 6.5 mass % and a high {222} plane integration and has a low {200} plane integration, so not being formed with burrs at the cross-section at the time of punching.
For this reason, the present invention steel sheet can easily be worked to various shapes including conventional shapes to special shapes and for example are useful for outer panels for auto parts, home electrical appliance parts, etc. requiring complicatedly shaped press formation and other various structural materials, functional materials, etc.
In the method of production of the present invention, in steel sheet having an Al content of less than 6.5 mass %, it is possible to increase the {222} plane integration or to lower the {200} plane integration easily and effectively. Further, the method of production of the present invention enables the production of the present invention steel sheet having a high {222} plane integration without production of new facilities by just switching processes of existing facilities easily and at low cost.
Below, the present invention will be explained in detail.
The inventors discovered that by making the Al content of the steel sheet less than 6.5 mass % and (x1) raising the {222} plane integration of the Fe crystal phase to 60% to 99% and/or (x2) lowering the {200} plane integration to 0.01% to 15%, it is possible to provide unprecedented steel sheet excellent in workability free from the occurrence of burrs at the cross-section at the time of punching.
The inventors disclosed “high Al content steel sheet having an Al content of 6.5 mass % to 10 mass %” having a {222} plane integration of an αFe phase of 60% to 95% and/or a {200} plane integration of an αFe phase of 0.01% to 15% in Japanese Patent Publication (A) No. 2006-144116.
The above method of production of steel sheet is characterized by depositing an Al alloy on at least one surface of steel sheet containing Al in 3.5 mass % to 6.5 mass %, applying working strain by cold working, then applying heat treatment for making the Al diffuse.
The inventors, after this, tackled the development of technology for further raising the {222} plane integration in steel sheet having an Al content of less than 6.5 mass % and ran various experiments.
As a result, regarding the method for integrating specific crystal planes, the inventors found that by using a matrix steel sheet having an Al content of less than 3.5 mass %, making the content of Cr of the matrix steel sheet 12 mass % or less, depositing a second layer comprised of not only Al, but also another metal on the steel sheet, then heat treating this to make the steel sheet recrystallize, it is possible to raise the {222} plane integration.
This is based on the discovery disclosed in Japanese Patent Publication (A) No. 2006-144116 that “at the time of cold rolling, the special dislocation structures to be formed in the steel sheet are effectively formed and that due to the heat treatment, recrystallization nuclei are efficiently formed for making the dislocation structures grow to a {222} plane texture.
That is, according to the present invention, even if the ingredients of the steel sheet are ingredients where the Al content after recrystallization becomes less than 6.5 mass %, the frequency of occurrence of the above recrystallization nuclei tends to become higher and as a result steel sheet having a higher {222} plane integration can be obtained.
Note that, in the present invention, the content of Cr in the matrix steel sheet is preferably less than 10 mass %. With such a Cr content, it is possible to more easily raise the {222} plane integration.
When using matrix steel sheet having an Al content of less than 6.5 mass %, it is possible to deposit a second layer on the steel sheet surface, cold roll the sheet, then remove the second layer to obtain, by subsequent heat treatment, a high {222} plane integration.
This phenomenon is basically also considered to arise based on the mechanism of formation of recrystallization nuclei.
Below, details of the present invention will further described.
The present invention steel sheet, at ordinary temperature, is comprised or one or both of an αFe phase and γFe phase. The Al content is less than 6.5 mass %.
If the Al content becomes 6.5 mass % or more, it is not possible to easily obtain a high {222} plane texture. Not only this, the tensile elongation at break falls. Even if having a high {222} plane integration, sufficient workability cannot be obtained.
That is, in steel sheet having an Al content of 6.5 mass % or more, no matter how one raises the {222} plane integration and, further, no matter how one lowers the {200} plane integration, burrs end up forming at the cross-section at the time of punching. Therefore, in the present invention steel sheet, the Al content was made less than 6.5 mass %.
The Al content of the present invention steel sheet is preferably 0.001 mass % or more. If the Al is 0.001 mass % or more, the yield at the time of production will rise. More preferably, it is 0.11 mass % or more. If Al becomes 0.11 mass % or more, the {222} plane integration becomes higher and as a result a higher workability can be obtained.
The inventors discovered that by depositing a second layer on at least one side of a matrix steel sheet having an Al content of less than 3.5 mass % and then heat treating this to make the steel sheet recrystallize, it is possible to raise the {222} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface very high.
The steel sheet having a high {222} plane integration of the present invention (the present invention steel sheet) is excellent in deep drawability, punchability, and other workability.
Since the Al content of the matrix steel sheet is less than 3.5 mass %, even if the second layer contains Al, in the production process, the steel sheet is resistant to shrinkage and other deformation. The Al content of the matrix steel sheet is preferably 0.001 mass % or more. If the Al is 0.001 mass % or more, the production yield of the matrix steel sheet is improved.
The present invention steel sheet is comprised of one or both of an αFe phase and γFe phase.
The αFe phase is an Fe crystal phase of a structure of a body centered orientation, while the γFe phase is an Fe crystal phase of a structure of a face centered orientation. The Fe crystal phase includes phases where other atoms replace part of the Fe or enter between the Fe atoms.
The present invention steel sheet has an Al content of less than 6.5 mass % and is characterized in that a {222} plane integration of one or both of the αFe phase and γFe phase is 60% to 99% and a {200} plane integration of one or both of the αFe phase and γFe phase is 0.01% to 15%.
If the above plane integration is in the range of the present invention, the value for evaluation of the drawability, that is, the average r value (Rankford value), becomes 2.5 or more. Furthermore, at the time of punching, excellent workability free of formation of burrs at the cross-section can be obtained.
The plane integration was measured by X-ray diffraction using MoKα rays. The {222} plane integration of the αFe phase and the {200} plane integration of the αFe phase were found as follows.
The integrated intensities of the 11 α crystal planes of Fe parallel to a sample surface, that is, {110}, {200}, {211}, {310}, {222}, {321}, {411}, {420}, {332}, {521}, and {442}, were measured. The measurement values were respectively divided by the theoretical integrated intensities of a sample of random orientation, then the ratios with the {200} intensity or {222} intensity were found by percentages.
For example, the ratio with the {222} intensity is expressed by the following formula (1).
{222} plane integration=[{i(222)/I(222)}/{Σi(hkl)/I(hkl)}]×100 (1)
where the symbols are as follows:
In the same way, the {222} plane integration of the Fe phase and the {200} plane integration of the γFe phase were found as follows:
The integrated intensities of the 6 γ crystal planes of Fe parallel to the sample surface, that is, {111}, {200}, {220}, {311}, {331}, and {420}, were measured. The measurement values were respectively divided by the theoretical integrated intensities of a sample of a random orientation, then the ratios with the {200} intensity or {222} intensity were found by percentages.
For example, the ratio with the {222} intensity is expressed by the following formula (2).
{222} plane integration=[{i(111)/I(111)}/{Σi(hkl)/I(hkl)}]×100 (2)
where the symbols are as follows:
For αFe crystal grains, separately, the EBSP (Electron Backscattering Diffraction Pattern) method may also be used to find the {222} plane integration.
The area rate of the {222} planes with respect to the total area of the crystal planes measured by the EPSP method becomes the {222} integration. Therefore, even by the EBSP method, in the present invention steel sheet, the {222} plane integration becomes 60% to 99%.
In the present invention, it is not necessary that the values obtained by all analysis methods satisfy the range prescribed by the present invention. The effect of the present invention is obtained if the value obtained by one analysis method satisfies the range of the present invention.
Further, in the EPSP method, the {222} plane deviates from the steel sheet surface. This deviation is preferably within 30°.
The deviation of the {222} plane is observed by the L cross-section. The area ratio of the crystal grains with deviation of the {222} plane of 30° or less is preferably 80 to 99.9%.
Furthermore, the area ratio of the crystal grains with deviation of the {222} plane in the L cross-section of 0 to 10° is more preferably 40 to 98%.
The “average r value” means the average plastic strain ratio found by JIS Z 2254 and is a value calculated by the following formula:
Average r value=(r0+2r45+r90)/4 (3)
Here, r0, r45, and r90 are the plastic strain ratios measured when taking test samples in directions of 0°, 45°, and 90° with respect to the rolling direction of the sheet surface.
Note that the integrated intensity of the sample having a random orientation may also be found by measurement using a sample prepared in advance.
In the present invention steel sheet, (i) a {222} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface is 60% to 99% and/or (ii) a {200} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface is 0.01% to 15%.
If the {222} plane integration is less than 60% and the {200} plane integration is over 15%, cracks and breakage easily occur at the time of drawing, bending, and rolling. Further, burrs occur at the cross-section at the time of punching.
If the {222} plane integration is over 99% and the {200} plane integration is less than 0.01%, the effect of the present invention becomes saturated and production also becomes difficult.
Therefore, the texture of the present invention steel sheet was defined as in the above.
Note that the {222} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface is preferably 60% to 95%. If the {222} plane integration is in the above range, production becomes easier and the yield is improved.
The {200} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface is preferably 0.01% to 10%. If the {200} plane integration is in the above range, burrs will not occur at the cross-section at the time of punching.
One method for producing the present invention steel sheet is comprised of a step of depositing a second layer on at least one surface of a matrix steel sheet having an Al content of less than 6.5%, a step of cold rolling the steel sheet on which the second layer is deposited, a step of removing the second layer from the cold rolled steel sheet, and a step of heat treating the steel sheet from which the second layer has been removed to make the steel sheet recrystallize.
To obtain a high {222} plane integration, it is essential to cold roll the matrix steel sheet in the state with the second layer deposited on it.
At this time, if the second layer is not deposited on at least one surface of the matrix steel sheet, a high {222} plane integration cannot be obtained. If making the second layer deposit on both surfaces of the steel sheet and then cold rolling, the effect of the present invention can be improved more.
At the time of heat treatment to make the steel sheet recrystallize, the second layer does not necessarily have to be deposited. The second layer deposited on the steel sheet may therefore be removed before heat treatment.
For example, when the elements forming the second layer would diffuse into the steel sheet at the time of heat treatment and have a detrimental effect on the mechanical properties etc., if removing the second layer before heat treatment, it would be possible to obtain only the effect of improvement of the {222} plane integration.
A steel sheet on at least one surface of which a second layer is deposited and having one or both of a {222} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface of 60% to 99% and a {200} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface of 0.01% to 15% is included in the present invention steel sheet.
If the {222} plane integration is less than 60% and the {200} plane integration is over 15%, cracks and breakage will easily occur at the time of drawing, bending, and rolling and, further, burrs will form at the cross-section at the time of punching.
If the {222} plane integration is over 99% and the {200} plane integration is less than 0.01%, the effect of the present invention becomes saturated and production further becomes difficult.
Here, if the second layer is deposited on the steel sheet, it is possible to prevent internal oxidation, corrosion, etc. of the steel sheet and possible to make the steel sheet more sophisticated in functions.
The method of production of this steel sheet includes a step of depositing the second layer on at least one surface of a matrix steel sheet having an Al content of less than 3.5 mass %, a step of cold rolling the sheet in the state with the second layer deposited, and a step of heat treating the steel sheet to make the steel sheet recrystallize.
To obtain a higher {222} plane integration, it is preferable to cold roll the matrix steel sheet in a state with the second layer deposited.
When heat treating the steel sheet to make it recrystallize in the subsequent steps, even if the second layer is deposited on at least one surface, the effects of the present invention can be obtained. If the second layer is deposited on both surfaces of the matrix steel sheet, the effect of the present invention is further improved.
Steel sheet wherein the second layer and the steel sheet are partially alloyed and having one or both of a {222} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface of 60% to 99% and a {200} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface of 0.01% to 15% is also included in the present invention steel sheet.
If the {222} plane integration is less than 60% and the {200} plane integration is over 15%, cracks and breakage will easily occur at the time of drawing, bending, and rolling and, further, burrs will form at the cross-section at the time of punching.
If the {222} plane integration is over 99% and the {200} plane integration is less than 0.01%, the effect of the present invention becomes saturated and production further becomes difficult.
If the second layer is deposited on the steel sheet surface and part of the second layer is alloyed with the steel sheet, internal oxidation, corrosion, etc. of the steel sheet can be prevented, peeling of the second layer can be prevented, and the steel sheet can be made more sophisticated in function.
To obtain a higher {222} plane integration, it is preferable to cold roll the matrix steel sheet in a state with the second layer deposited on at least one surface. If the second layer is deposited on both surfaces of the matrix steel sheet, the effect of the present invention is further improved.
In the steps after this, the steel sheet has to be heat treated to make it recrystallize. At this time, if part of the second layer deposited on one or both surfaces is alloyed with the matrix steel sheet, a higher {222} plane integration can be obtained.
Here, the second layer and the steel sheet partially alloying means, for example, the second layer and the steel sheet partially alloying near their boundary by mutual diffusion.
Steel sheet where the second layer and steel sheet are alloyed and having one or both of a {222} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface of 60% to 99% and a {200} plane integration of one or both of an αFe phase and γFe phase with respect to the steel sheet surface of 0.01% to 15% is also included in the present invention steel sheet.
If the {222} plane integration is less than 60 and the {200} plane integration is over 15%, cracks and breakage will easily occur at the time of drawing, bending, and rolling and, further, burrs will form at the cross-section at the time of punching.
If the {222} plane integration is over 99% and the {200} plane integration is less than 0.01%, the effect of the present invention becomes saturated and production further becomes difficult.
If the second layer is deposited on the steel sheet surface and the second layer alloys with the steel sheet, the mechanical properties or functionality of the steel sheet will be improved in accordance with the elements making up the second layer. For example, when the element forming the second layer is Al, the high temperature oxidation resistance and corrosion resistance of the steel sheet will be improved.
To obtain a higher {222} plane integration, it is preferable to cold roll the matrix steel sheet in a state with the second layer deposited, then heat treat the steel sheet to make it recrystallize.
At the time of cold rolling, the second layer has to be deposited on at least one surface of the matrix steel sheet, preferably both surfaces. After this, after the heat treatment step, the second layer completely alloys with the steel sheet whereby a higher {222} plane integration can be obtained.
In the present invention steel sheet having the second layer, the second layer is preferably a metal.
The preferable elements forming the second layer are at least one element among Fe, Al, Co, Cu, Cr, Ga, Hf, Hg, In, Mn, Mo, Nb, Ni, Pb, Pd, Pt, Sb, Si, Sn, Ta, Ti, V, W, Zn, and Zr.
The above elements have the common feature of being alloying elements with Fe. Particularly preferably, the elements are at least one element among Al, Cr, Ga, Mo, Nb, P, Sb, Si, Sn, Ti, V, W, and Zn which become solid solute in αFe and tend to stabilize the a phase.
Further, more preferably, the elements are at least one element among Al, Cr, Mo, Si, Sn, Ti, V, W, and Zn which become solid solute in αFe and tend to stabilize the a phase more.
For example, as the second layer, it is possible to select an Al alloy, Zn alloy, Sn alloy, etc.
Further, in the method of production of the present invention steel sheet, the second layer applied to the surface of the matrix steel sheet is, in the same way as the above, preferably a metal.
The preferable elements forming the second layer are at least one element among Fe, Al, Co, Cu, Cr, Ga, Hf, Hg, In, Mn, Mo, Nb, Ni, Pb, Pd, Pt, Sb, Si, Sn, Ta, Ti, V, W, Zn, and Zr.
The above elements have the common feature of being alloying elements with Fe. Particularly preferably, the elements are at least one element among Al, Cr, Ga, Mo, Nb, P, Sb, Si, Sn, Ti, V, W, and Zn which become solid solute in αFe and tend to stabilize the α phase.
Further, more preferably, the elements are at least one element among Al, Cr, Mo, Si, Sn, Ti, V, W, and Zn which become solid solute in αFe and tend to stabilize the α phase more.
For example, as the second layer, it is possible to select an Al alloy, Zn alloy, Sn alloy, etc.
Here, when the second layer includes Al, the preferable Al content of the matrix steel sheet is less than 3.5 mass %. If the Al concentration of the matrix steel sheet is 3.5 mass % or more, if heat treating the sheet with the Al alloy deposited as the second layer, shrinkage will occur during the heat treatment and the dimensional precision will remarkably drop.
Therefore, in the present invention steel sheet, when the second layer contains Al, the Al content of the matrix steel sheet is made less than 3.5 mass %.
When the second layer does not contain Al, the Al content of the matrix steel sheet is made less than 6.5 mass %.
When the production process includes a step of depositing on at least one surface a second layer of at least one element among Fe, Co, Cu, Cr, Ga, Hf, Hg, In, Mn, Mo, Nb, Ni, Pb, Pd, Pt, Sb, Si, Sn, Ta, Ti, V, W, Zn, and Zr, if the Al content of the matrix steel sheet is 6.5 mass % or more, the tensile elongation at break of the obtained steel sheet falls and even if having a high {222} plane integration, sufficient workability will no longer be obtained and burrs will form at the cross-section at the time of punching.
Therefore, the Al content of the steel sheet when the second layer does not contain Al is made less than 6.5 mass %.
Note that even if the second layer contains Al, if removing the second layer before the heat treatment, no shrinkage will occur. Therefore, when removing the second layer before heat treatment, the Al content of the matrix steel sheet is preferably less than 6.5 mass %.
In this method of production, the method of omitting the step of removing the second layer so as to raise the work efficiency is also included in the present invention.
Further, the method of heat treating the sheet to alloy part or all of the second layer and produce steel sheet having a high {222} plane integration is also included in the present invention.
In the present invention, the alloyed region of the steel sheet and second layer is defined as follows.
When the element of the greatest content in the second layer is “A”, the region where the Fe content is 0.5 mass % higher than the Fe content of the second layer before alloying and the content of A is 0.1 mass % higher than the content of A of the matrix steel sheet before alloying is defined as an “alloyed region”.
Further, the ratio of alloying is the ratio of the alloyed region in the overall region. In the present invention steel sheet, by forming the alloyed region in accordance with the above definition, a more superior workability can be obtained.
Furthermore, if the Fe content and/or A content become large and intermetallic compounds etc. are formed, a higher effect of the present invention can be obtained.
Note that the alloying ratio for example can be found by using EPMA etc., analyzing the distribution of contents of the Fe and element A at the L cross-section, identifying the alloyed region, finding that area, and finding the ratio of the area of the identified region to the overall area.
The thickness of the steel sheet of the present invention is preferably 5 μm to 5 mm. This is the thickness including the second layer. If the thickness of the steel sheet is less than 5 μm, the production yield falls so this is not suitable for practical application.
If the thickness of the steel sheet exceeds 5 mm, the {222} plane integration will sometimes not fall in the range of the present invention. Therefore, the thickness of the steel sheet is preferably 5 μm to 5 mm.
The thickness of the steel sheet is more preferably 100 μm to 3 mm. If the thickness of the steel sheet is 3 mm or less, the effect of suppression of the formation of burrs at the cross-section at the time of punching becomes more remarkable.
If the thickness of the steel sheet is 100 μm or more, the {222} plane integration becomes higher and more easily controlled. Similarly, the effect of suppression of formation of burrs becomes more remarkable.
In the thickness of the steel sheet in the present invention, the thickness of the second layer is preferably 0.01 μm to 500 μm. When the steel sheet and the second layer are partially alloyed, the thickness of the alloyed part is included in the thickness of the second layer. When the second layer is deposited at both surfaces, this is the thicknesses of the two surfaces in total.
The second layer has the function of improving the {222} plane integration at the time of production and can be left after production and used as a rust-preventive and protective coating of the steel sheet.
If the thickness of the second layer is over 500 μm, the possibility of peeling rises, so 500 μm or less is preferable. If the thickness of the second layer is less than 0.01 μm, the coating will easily tear and the rust-preventive and protective effect will be reduced.
Therefore, the thickness of the second layer is preferably 0.01 μm or more. The case where the entire thickness of the steel sheet is alloyed is also preferable. In this case, the second layer may be considered to have disappeared.
In the method of production of the present invention steel sheet, the thickness of the matrix steel sheet is 10 μm to 10 mm. If the thickness of the matrix steel sheet is less than 10 μm, the production yield will drop in the steps from cold rolling on so this is not suitable for practical application in some cases.
If the thickness of the matrix steel sheet is over 10 mm, the {222} plane integration may not fall in the range of the present invention.
Therefore, the thickness of the matrix steel sheet is preferably 10 μm to 10 mm.
A thickness of the matrix steel sheet of over 130 μm to 7 mm is more preferable. In this range of thickness, an efficient and sufficient increase in the {222} plane integration can be expected and production of steel sheet able to suppress the formation of burrs at the time of punching becomes easy.
The thickness of the second layer deposited on the matrix steel sheet before cold rolling is preferably 0.05 μm to 1000 μm. When the steel sheet and the second layer are alloyed, the thickness of the alloyed part is included in the thickness of the second layer. When the second layer is deposited at both surfaces, this becomes the thicknesses of the two surfaces in total.
If the thickness of the second layer is less than 0.05 μm, the {222} plane integration becomes lower and may not fall in the range of the present invention, so 0.05 μm or more is preferable.
Even when the thickness of the second layer exceeds 1000 μm, the {222} plane integration becomes lower and may not fall in the range of the present invention, so 1000 μm or less is preferable.
To express more superior effects of the present invention, the matrix steel sheet before deposition of the second layer is preferably given preheat treatment.
This preheat treatment causes rearrangement of the dislocations accumulated in the process of production of the matrix steel sheet. Therefore, causing recrystallization is preferable, but there is not necessarily a need to cause recrystallization.
The preheat treatment temperature is preferably 700° C. to 1100° C. If the preheat treatment temperature is less than 700° C., changes in the dislocation structure for obtaining more superior effects of the present invention are hard to occur, so the preheat treatment temperature is made 700° C. or more.
If the preheat treatment temperature exceeds 1100° C., the steel sheet surface is formed with an unpreferable oxide film. This has a detrimental effect on the later deposition of the second layer and the cold rolling, so the preheat treatment temperature is made 1100° C. or less.
The atmosphere of the preheat treatment may be a vacuum, inert gas atmosphere, hydrogen atmosphere, or weak acidic atmosphere. In any atmosphere, the effect of the present invention can be obtained, but an atmosphere is sought of conditions not forming on the steel sheet surface an oxide film having a detrimental effect on the deposition of the second layer after the preheat treatment or on the cold rolling.
The preheat treatment time does not particularly have to be limited, but if considering the production of the steel sheet etc., several seconds to several hours are suitable.
The second layer may be deposited on the steel sheet by the hot dip method, electroplating method, dry process, cladding, etc. No matter which method is used, the effect of the present invention can be obtained. Further, it is also possible to add desired alloy elements to the second layer deposited and simultaneously alloy it.
The cold rolling is performed with the second layer deposited on the steel sheet. The reduction rate is 30% to 95%.
If the reduction rate is less than 30%, the {222} plane integration of the steel sheet obtained after heat treatment is low and sometimes will not reach the range of the present invention. If the reduction rate is over 95%, the increase in plane integration becomes saturated and the production cost increases. Therefore, the reduction rate is made 30% to 95%.
When removing the second layer before the heat treatment, as the method of removal, mechanical removal by polishing etc. or chemical removal by dissolution by a strong acid or strong alkali aqueous solution may be applied.
For example, in the case of an Al plated steel sheet, the steel sheet is dipped in an aqueous solution of caustic soda to remove the plating ingredient. As a result, in the heat treatment process, the effect of the Al ingredient can be eliminated.
The heat treatment for causing the steel sheet to recrystallize can be performed in a vacuum atmosphere, Ar atmosphere, H2 atmosphere, or other nonoxidizing atmosphere. At this time, preferably the heat treatment temperature is 600° C. to 1000° C. and the heat treatment time is 30 seconds or more.
If the heat treatment temperature is 600° C. or more, the {222} plane integration becomes higher and more easily reaches the range of the present invention. At a heat treatment temperature of 1000° C. or less and a heat treatment time of less than 30 seconds, in the same way, the {222} plane integration becomes higher and more easily reaches the range of the present invention.
Therefore, preferably the heat treatment temperature is 600° C. to 1000° C. and the heat treatment time is 30 seconds or more.
If the heat treatment temperature is over 1000° C., a high {222} plane integration can be obtained without restriction by the heat treatment time. In particular, if over 1000° C., even with less than 30 seconds heat treatment time, the {222} plane integration can be easily increased.
Note that the heat treatment temperature is more preferably 1300° C. or less. If the heat treatment temperature is 1300° C. or less, the flatness of the steel sheet and other sheet properties become more superior.
The temperature rise rate at the time of the heat treatment is preferably 1° C./min to 1000° C./min. If the temperature rise rate is 1000° C./min or less, a higher {222} plane integration can be easily obtained. If the temperature rise rate is 1° C./min or more, the productivity is remarkably improved.
Therefore, a temperature rise rate of 1° C./min to 1000° C./min is preferable.
The heat treatment performed in the state with the second layer deposited is designed to make the steel sheet recrystallize and also to make the elements included in the second layer diffuse into the steel.
If the elements contained in the second layer diffuse into the steel, the {222} plane integration is improved more and the high temperature oxidation resistance and mechanical properties are improved, so in the method of production of the present invention steel sheet, the diffusion of elements included in the second layer is positively utilized.
The matrix steel sheet preferably has a content of Cr of 12 mass % or less under the above Al content. A Cr content of less than 10 mass % is more preferable.
Further, the matrix steel sheet is a steel sheet with a C content of 2.0 mass % or less and includes as impurities slight amounts of Mn, P, S, etc. For example, carbon steel is included in the matrix steel sheet of the present invention. Furthermore, alloyed steel containing alloy elements such as Ni and Cr in addition to C is also included in the matrix steel sheet of the present invention.
The alloy elements which the matrix steel sheet may contain are Si, Al, Mo, W, V, Ti, Nb, B, Cu, Co, Zr, Y, Hf, La, Ce, N, O, etc.
Below, examples will be used to explain the present invention in more detail.
The Al content of the matrix steel sheet was changed to investigate the manufacturability and {222} plane integration.
Matrix steel sheets of ingredients of five different types of Al content were produced. The Al contents were, by mass %, 3.0% (ingredients A), 3.4% (ingredients E), 4.0% (ingredients B), 6.0% (ingredients C), and 7.5% (ingredients D). In addition, the ingredients included C: 0.008%, Si: 0.2%, Mn: 0.4%, Cr: 20.0%, Zr: 0.08%, La: 0.08%, and a balance of iron and unavoidable impurities.
By each of these ingredients, ingots were produced by vacuum melting and hot rolled to try to reduce them to 3.0 mm thickness.
In the case of the ingredients A, B, C, and E, the ingots could be easily hot rolled to 3.0 mm thick steel sheets, but in the case of the ingredients D, the steel sheet frequently broke during the hot rolling, so hot rolling could not be continued.
In this way, if the Al content of the matrix steel sheet is over the range of the present invention at 6.5% or more, production becomes difficult. Therefore, production of steel sheet of the ingredients D was foregone and the steel sheets of the ingredients A, B, C, and E were cold rolled to 0.4 mm thickness.
The main phases of the steel sheets of the ingredients A, B, C, and E at ordinary temperature were αFe phases. X-ray diffraction was used to measure the texture of the αFe phase of each matrix steel sheet and, in the same way as above, the plane integration was calculated.
It was confirmed that the {222} plane integration was, in the ingredients A, 32%, ingredients B, 31%, ingredients C, 31%, and ingredients E, 30%, while the {200} plane integration was, in the ingredients A, 16%, ingredients B, 15%, ingredients C, 16%, and ingredients E, 16%.
Each steel sheet was heat treated at 800° C.×10 sec in a hydrogen atmosphere before forming the second layer. After this, the hot dip method was used to deposit Al alloy on the surface of the matrix steel sheet.
The composition of the plating bath was, by mass %, 90% Al-10% Si. The Al alloy was deposited on both surfaces of each steel sheet.
The amount of deposition, for each steel sheet as a whole, was controlled to give an Al content by mass % of 3.5% (ingredients A), 4.5% (ingredients B), 6.4% (ingredients C), and 6.4% (ingredients E).
With the Al alloy deposited as the second layer, each steel sheet was cold rolled by a reduction rate of 70%. Next, it was heat treated in a vacuum under conditions of 1000° C.×120 min to cause the steel sheet to recrystallize.
At this time, the steel sheets of the ingredients B and C shrank during the heat treatment and remarkably dropped in dimensional precision.
When the second layer does not include Al, if the Al content in the matrix steel sheet is outside the range of the present invention at 3.5% or more, it was confirmed that shrinkage occurs during heat treatment and use for practical applications is difficult.
On the other hand, if the Al content of the matrix steel sheet is in the range of the present invention at less than 3.5%, no shrinkage occurs and use is possible for practical applications.
A second layer not containing Al was deposited on a matrix steel sheet having an Al content of 3.5% or more and similar heat treatment was performed. In this case, no shrinkage occurred during the heat treatment.
When using steel sheets of the ingredients A and E as matrix steel sheets, the {222} plane integrations of the obtained steel sheets were respectively 82% and 83% and the {200} plane integrations were respectively 0.5% and 0.8%. Both integrations were in the range of the present invention.
Furthermore, these steel sheets were measured for the average r value. It was confirmed that the average r value was a high level of 2.5 or more. These steel sheets had excellent drawability.
In this way, it was confirmed that steel sheets produced by the method of production of the present invention were in the range of the present invention with a {222} plane integration of the αFe phase parallel to the steel sheet surface of 60% or more or with a {200} plane integration parallel to the steel sheet surface of 15% or less.
The results of production of steel sheet having a high {222} plane integration using an Al alloy as the second layer are shown.
The ingredients of the matrix steel sheet were, by mass %, Al: 1.5%, C: 0.008%, Si: 0.1%, Mn: 0.2%, Cr: 18%, Ti: 0.1%, and a balance of iron and unavoidable impurities.
The matrix steel sheet was a steel sheet obtained by producing an ingot by the vacuum melting method, hot rolling the ingot to obtain steel sheet of 3.8 mm thickness, then cold rolling it to obtain steel sheet of 0.8 mm thickness.
The main phase of the matrix steel sheet at ordinary temperature was the αFe phase. X-ray diffraction was used to measure the texture of the αFe phase of the matrix steel sheet whereupon it was confirmed that the {222} plane integration was 36% and the {200} plane integration was 20%.
Part of the matrix steel sheet was heat treated at 800° C.×10 sec in a hydrogen atmosphere before plating. Al alloy was deposited on the surface of the matrix steel sheet using the hot dip method.
The composition of the plating bath was, by mass %, 90% Al-10% Si. The Al alloy was deposited on both surfaces of the steel sheet. The thickness of the deposited Al alloy was controlled to be uniform in the steel sheet surface.
The steel sheet with the Al alloy deposited was cold rolled. After this, it was heat treated in a nonoxidizing atmosphere. Before the heat treatment, if necessary, the Al alloy deposited on the surface was removed.
The Al alloy was removed by dipping the steel sheet in heated caustic soda 10% aqueous solution to dissolve the Al alloy in the solution.
As comparative examples, cases where the Al alloy was deposited, then the sheets were not cold rolled were also studied.
Table 1 shows the alloying ratio, {222} plane integration of the αFe phase, {200} plane integration of the αFe phase, and Al content for steel sheets produced under various conditions. The plane integration was obtained by measurement using X-ray diffraction and calculation by the above-mentioned calculation processing method.
The alloying ratio of the steel sheet was found as follows: At the L cross-section, in a field of the L direction 1 mm×entire thickness, the EPMA (Electron Probe Micro-Analysis) method was used to measure the plane distribution of the Fe content and the plane distribution of the Al content.
Further, a region of Fe≧0.5 mass % and Al≧1.6 mass % was deemed an alloyed region and its area was found as the alloyed area. The alloying ratio was calculated by dividing the alloyed area by the L direction 1 mm×total thickness area.
In No. 1 of Comparative Example 1, the amount of deposition of the Al alloy was controlled by adjusting the plating thickness so that the Al content of the steel sheet as a whole became 3.2%. The Al alloy was removed without cold rolling after plating. Furthermore, the steel sheet was heated treated under conditions of 950° C.×10 min to make the steel sheet recrystallize.
As a result, the {222} plane integration and the {200} plane integration were outside the range of the present invention. The Al content of the obtained steel sheet was the same as the matrix steel sheet, that is, 1.5%, since the Al alloy was removed.
In No. 2 of Comparative Example 2, the step of depositing an Al alloy as the second layer was omitted. The matrix steel sheet was cold rolled by a reduction rate of 50%, then the steel sheet was heat treated under conditions of 950° C.×10 min to make the steel sheet recrystallize.
In this case as well, the {222} plane integration and the {200} plane integration were outside the range of the present invention.
In No. 3 of an invention example, the amount of deposition of the Al alloy was controlled by adjusting the plating thickness to become 3.2% of the steel sheet as whole. After plating, the steel sheet was cold rolled by a reduction rate of 50%, then the Al alloy was removed and the steel sheet was heat treated under conditions of 950° C.×0.1 min to make the steel sheet recrystallize.
As a result, the {222} plane integration was outside the range of the present invention, but the {200} plane integration was in the range of the present invention. The Al content in the obtained steel sheet was the same as the matrix, that is, 1.5%, since the Al alloy was removed.
In Nos. 4 and 5 of Invention Examples 1 and 2, each steel sheet was heat treated at 800° C., then Al alloy was deposited on the steel sheet surface so that the Al content became 3.2% at the steel sheet as a whole. After this, the steel sheet was cold rolled at a reduction rate of 50% to make it thinner.
After the Al alloy was removed, in No. 4, the steel sheet was heat treated under conditions of 950° C.×1 min, while in No. 5, the steel sheet was heat treated under conditions of 950° C.×10 min, to make the steel sheets recrystallize.
As a result, in both Nos. 4 and 5 of Invention Examples 1 and 2, it was confirmed that the {222} plane integration and the {200} plane integration were controlled to within the range of the present invention and the Al content was also in the range of the present invention. The Al content in the obtained steel sheet was the same as the matrix, that is, 1.5%, since the Al alloy was removed.
In No. 6 of Invention Example 3, the heat treatment before deposition of the Al alloy was omitted from No. 5 of the invention example, but it was confirmed that the {222} plane integration and the {200} plane integration were both controlled to within the range of the present invention and that the Al content was also in the range of the present invention.
The Al content in the obtained steel sheet was the same as the matrix, that is, 1.5%, since the Al alloy was removed.
In Nos. 7 and 8 of Invention Examples 4 and 5, before depositing the Al alloy, the steel sheet was heat treated at 800° C. then the Al alloy was deposited.
The amount of deposition of the Al alloy in No. 7 was controlled to give an Al content of 3.2% in the steel sheet as a whole. The amount of deposition of the Al alloy in No. 8 was similarly controlled to give an Al content of 6.0% in the steel sheet as a whole. After this, the two steel sheets were cold rolled at a reduction rate of 50% to make them thinner.
The removal of the Al alloy was omitted, the rolling oil was removed from the steel sheet surface, then the steel sheet was heat treated under conditions of 1000° C.×120 min to make the steel sheet recrystallize. Due to this heat treatment, the Al alloy deposited on the steel sheet surface was completely alloyed with the steel sheet.
It was confirmed that the obtained {222} plane integration and the {200} plane integration were both controlled to within the range of the present invention and that the Al content was also in the range of the present invention.
In No. 9 of Comparative Example 4, compared with Nos. 7 and 8 of the invention examples, the amount of deposition of the second layer was increased. The amount of deposition of the Al alloy was controlled to give an Al content of 7.5% in the steel sheet as a whole.
The other steps were the same as in Nos. 7 and 8 of the invention examples. Due to the heat treatment, the Al alloy deposited on the steel sheet surface was completely alloyed with the steel sheet.
As a result, the Al content of the steel sheet became 7.5% or ended up exceeding the range of the present invention. The {222} plane integration of this steel sheet was considerably improved, but failed to reach the range of the present invention.
Tensile tests were run. As a result, it was learned that the elongation at break was 10% or less and the toughness was low. From this, it was learned that the No. 9 steel sheet was not suited for practical application.
In No. 10 of Comparative Example 5, the Al alloy was deposited on the steel sheet surface so that the Al content became 3.2% in the steel sheet as a whole. The cold rolling after deposition of the Al alloy was omitted. After depositing the Al alloy, the steel sheet was heat treated under conditions of 1050° C.×0.17 min to make the steel sheet recrystallize.
As a result, the {222} plane integration and the {200} plane integration were both outside the range of the present invention.
In Nos. 11 and 12 of Invention Examples 6 and 7, before depositing the Al alloy, the steel sheet was heat treated at 800° C. and Al alloy was deposited on the steel sheet surface so that the Al content became 3.2% at the steel sheet as a whole.
After this, in No. 11 of Invention Example 6, the steel sheet was cold rolled by a reduction rate of 50% to make it thinner. In No. 12 of Invention Example 7, the steel sheet was cold rolled by a reduction rate of 75% to make it thinner.
The removal of the Al alloy was omitted and the steel sheet was heat treated under conditions of 1050° C.×0.17 min to make the steel sheet recrystallize.
As a result, in each steel sheet, it was confirmed that the {222} plane integration and the {200} plane integration were both controlled to within the range of the present invention and the Al content was also in the range of the present invention.
Each of the above steel sheets was tested for burr resistance. A 10.0 mmφ punch and a 10.3 mmφ die were used for punching and the burr height around the punched hole was measured by a point micrometer.
As a result, it was confirmed that the burr height was a high level of 23 to 65 μm in the comparative examples, but was an extremely low level of 4 to 9 μm in the invention examples.
The steel sheets of the above examples were measured for the average r value, whereupon it was confirmed that in the steel sheets of the invention examples, the average r value was at a high level of 2.5 or more, but in the steel sheets of the comparative examples, the average r value was less than 2.5 or measurement was not possible.
Therefore, the steel sheets of the invention examples have excellent drawability. Further, the steel sheets of the invention examples were subjected to Erichsen tests and the extruded surfaces were observed whereupon excellent press workability was also confirmed.
The steel sheet produced by the method of production of the present invention in this way was confirmed to have a {222} plane integration of αFe phase parallel to the steel sheet surface of 60% or more and a {200} plane integration of the αFe phase parallel to the steel sheet surface of 15% or less or both in the range of the present invention.
As a result, it was confirmed that the steel sheet produced by the method of production of the present invention achieved both excellent burr resistance and drawability.
The results of using a Zn alloy as the deposit (second layer) to produce steel sheet having a high {222} plane integration are shown.
The matrix steel sheet was a steel sheet obtained by using the vacuum melting method to obtain an ingot of ingredients, by mass %, of an Al content of 0.01% and also C: 0.005%, Si: 0.2%, Mn: 0.5%, Ti: 0.05%, and a balance of iron and unavoidable impurities, hot rolling to a thickness of 3.2 mm, then cold rolling to a thickness of 1.8 mm.
The main phase of the matrix steel sheet at ordinary temperature was an αFe phase. X-ray diffraction was used to measure the texture of the αFe phase of the matrix steel sheet whereupon it was confirmed that the {222} plane integration was 28% and the {200} plane integration was 19%.
Part of the matrix steel sheet was heat treated by 770° C.×5 sec in a hydrogen atmosphere before plating.
On the surface of the matrix steel sheet, the electroplating method was used to deposit an Zn alloy. For the plating bath, a sulfuric acid type acidic solution was used. The deposited plating was, by mass %, a 94% Zn-6% Ni alloy. The thickness of the deposited Zn alloy was controlled to become uniform in the steel sheet surface.
The steel sheet on which the Zn alloy was deposited was cold rolled, then heat treated in a nonoxidizing atmosphere. Before the heat treatment, if necessary, the Zn alloy deposited on the steel sheet surface was removed. The Zn alloy was removed by dipping the steel sheet into a heated hydrochloric acid 10% aqueous solution to make the Zn alloy dissolve in the solution.
As comparative examples, the case of deposition of an Al alloy, then not performing cold rolling was also studied.
Table 2 shows the alloying ratio, the {222} plane integration of the αFe phase, the {200} plane integration of the αFe phase, and the Al content of steel sheet produced under various conditions. Note that the plane integration was found by measurement using X-ray diffraction and calculation by the above-mentioned calculation processing method.
The alloying ratio of the steel sheet was found as follows: At the L cross-section, in a field of the L direction 1 mm×entire thickness, the EPMA method was used to measure the plane distribution of the Fe content and the plane distribution of the Zn content.
Further, a region of Fe≧0.5 mass % and Zn≧0.1 mass % was deemed an alloyed region and its area was found as the alloyed area. The alloying ratio was calculated by dividing the alloyed area by the L direction 1 mm×total thickness area.
Note that, the area ratios obtained by using the EBSP method to separately observe by the L cross-section the crystal grains with a deviation of the {222} plane with respect to the steel sheet surface of 0 to 30° and the crystal grains with a deviation of the {222} plane with respect to the steel sheet surface of 0 to 10° are described.
Further, the above steel sheet was tested for burr resistance. A 30.0 mmφ punch and a 30.6 mmφ die were used for punching and the burr height around the punched hole was measured by a point micrometer.
In No. 13 of Comparative Example 6, Zn alloy of a thickness of 0.8 μm was deposited on the steel sheet surface. The cold rolling was omitted and the Zn alloy was removed, then the steel sheet was heat treated under conditions of 1050° C.×0.1 min to make the steel sheet recrystallize.
As a result, the {222} plane integration and the {200} plane integration of this steel sheet were both outside the range of the present invention.
In No. 14 of Comparative Example 7, the deposition of the Zn alloy was omitted and the steel sheet was cold rolled by a reduction rate of 70%. After this, the steel sheet was heat treated under conditions of 1050° C.×0.1 min to make the steel sheet recrystallize. In this case as well, the {222} plane integration and the {200} plane integration were both outside the range of the present invention.
In No. 15 of Invention Example 8, after heat treatment at 770° C., Zn alloy of a thickness of 0.8 μm was deposited on the steel sheet surface. After this, the steel sheet was cold rolled by a reduction rate of 70% to make it thinner. Furthermore, the Zn alloy was removed, then the steel sheet was heat treated under conditions of 1050° C.×0.1 min to make the steel sheet recrystallize.
As a result, it was confirmed that the {222} plane integration and the {200} plane integration were in the range of the present invention and the Al content was also in the range of the present invention.
In No. 16 of Invention Example 9, the heat treatment before deposition of the Zn alloy was omitted from No. 15 of the invention examples, but it was confirmed that the {222} plane integration and the {200} plane integration were both controlled to be within the range of the present invention and the Al content was also in the range of the present invention.
In Nos. 17 and 18 of Invention Examples 10 and 11, before deposition of the Zn alloy, the steel sheet was heat treated at 770° C. then the Zn alloy was deposited.
In No. 17, Zn alloy of a thickness of 0.8 μm was deposited on the steel sheet surface. In No. 18, Zn alloy of a thickness of 0.4 μm was deposited on the steel sheet surface. After this, the two steel sheets were cold rolled by a reduction rate of 70% to make them thinner.
The removal of the Zn alloy was omitted, the rolling oil on the steel sheet surface was removed, then the steel sheet was heat treated under conditions of 1050° C.×0.1 min to make the steel sheet recrystallize. Due to this heat treatment, part of the Zn alloy deposited on the steel sheet surface alloyed with the steel sheet.
The alloying ratio was 30% in No. 17 and 60 in No. 18. It was confirmed that the obtained {222} plane integration and {200} plane integration were both controlled to within the range of the present invention and the Al content was also in the range of the present invention.
In No. 19 of Comparative Example 8, Zn alloy of a thickness of 0.8 μm was deposited on the steel sheet surface. The cold rolling after deposition of the Zn alloy was omitted. After the deposition of the Zn alloy, the steel sheet was heat treated under conditions of 750° C.×10 min to make the steel sheet recrystallize.
As a result, the {222} plane integration and the {200} plane integration were both outside the range of the present invention.
In Nos. 20 and 21 of Invention Examples 12 and 13, before the deposition of the Zn alloy, the steel sheet was heat treated at 770° C., then Zn alloy of a thickness of 0.8 μm was deposited on the steel sheet surface.
After this, in No. 20, the steel sheet was cold rolled by a reduction rate of 30% to make it thinner. In No. 21, the steel sheet was cold rolled by a reduction rate of 87% to make it thinner.
The removal of the Al alloy was omitted and the steel sheet was heat treated under conditions of 750° C.×10 min to make the steel sheet recrystallize.
As a result, in each steel sheet, it was confirmed that the {222} plane integration and the {200} plane integration were both in the range of the present invention and that the Al content was also in the range of the present invention.
It was confirmed that in the steel sheets of the comparative examples, the burr height was a high level of 82 to 92 μm, but in the steel sheets of the invention examples, it was an extremely low level of 7 to 9 μm.
The steel sheets of the above examples were measured for the average r value. It was confirmed that in the steel sheets of the invention examples, the average r value was a high level of 2.5 or more, but in the steel sheets of the comparative examples, it was less than 2.5.
From these results, it was confirmed that in the steel sheets produced by the method of production of the present invention, both an excellent burr resistance and drawability are achieved.
Therefore, the steel sheets produced by the method of production of the present invention were examined at their extruded surfaces in Erichsen tests and confirmed to be excellent in press workability as well.
In this way it was confirmed that the steel sheet produced by the method of production of the present invention had a {222} plane integration of the αFe phase parallel with respect to the steel sheet surface of 60% or more and a {200} plane integration of the αFe phase parallel to the steel sheet surface of 15% or less or in the range of the present invention.
The results of using Cu as the deposit (second layer) to produce steel sheet having a high {222} plane integration are shown.
The ingredients of the matrix steel sheet were, by mass %, ingredients including Al: 0.015%, C: 0.15%, Si: 0.1%, Mn: 1.5%, Mo: 0.5%, and a balance of iron and unavoidable impurities.
As the matrix steel sheet, steel sheets obtained by using the vacuum melting method to produce an ingot and hot rolling the ingot to thicknesses of 15 mm, 10 mm, and 3.8 mm were used.
Further, cold rolled sheets obtained by cold rolling 3.8 mm steel sheet to thicknesses of 2.0 mm, 1.0 mm, 0.1 mm, 0.01 mm, and 0.005 mm were used as the matrix steel sheet.
The main phase of the matrix steel sheet at ordinary temperature was the αFe phase. X-ray diffraction was used to measure the texture of the αFe phase of the matrix steel sheet whereupon it was confirmed that the {222} plane integration was 36 to 40% and the {200} plane integration was 17 to 22%.
Before deposition of Cu, the matrix steel sheet was heat treated at 850° C.×10 sec in a hydrogen atmosphere. After this, different thicknesses of Cu were deposited on the two surfaces of matrix steel sheets. The Cu was deposited by the cladding method, the electroplating method, or the sputtering method.
The thickness of the Cu was changed, in the cladding method, by changing the thickness of the Cu sheet clad, in the plating method, by changing the conducted current and dipping time, and, further, in the sputtering method, by changing the sputtering time. For the plating bath, a sulfuric acid solution was used.
The steel sheet on which the Cu was deposited was cold rolled, then the steel sheet was heat treated in a nonoxidizing atmosphere.
Table 3 shows the {222} plane integration of the αFe phase and the {200} plane integration of the αFe phase of steel sheets produced under various conditions. Note that the plane integration was obtained by measurement by X-ray diffraction and calculation by the above-mentioned calculation processing method.
In Nos. 22 to 27 of Invention Examples 14 to 19, Cu was deposited on matrix steel sheet having a thickness of 2.0 mm by the cladding method, electroplating method, or sputtering method to a thickness in the range of the present invention as shown in Table 3.
With the Cu as deposited, the steel sheet was cold rolled by a reduction rate of 60%. Next, the removal of the second layer was omitted and the steel sheet was heat treated under conditions of 1020° C.×0.3 min to make the steel sheet recrystallize.
In each steel sheet, the {222} plane integration was in the range of the present invention. In No. 22 where the thickness of the second layer when depositing the second layer was over 1000 μm and in No. 27 where the thickness of the second layer less than 0.05 μm, the {222} plane integration fell somewhat and the {222} plane integration was over 15%.
In No. 22 of Invention Example 14, the thickness of the second layer after production was over 500 μm and peeling occurred somewhat easily. In No. 27 of Invention Example 19, the thickness of the second layer after production was less than 0.01 μm, the coating tore easily, and there was some problem in terms of rust prevention.
In Nos. 28 to 33 of Invention Examples 20 to 25, 2 μm of Cu was deposited on matrix steel sheet of a thickness of 0.005 to 15 mm by the electroplating method. Next, with the Cu as deposited, the steel sheet was cold rolled by a reduction rate of 50%. The removal of the second layer was omitted and the steel sheet was heat treated under conditions of 900° C.×60 min to make the steel sheet recrystallize.
In each steel sheet, the {222} plane integration was in the range of the present invention, but in No. 28 where the thickness of the matrix steel sheet at the time of deposition was over 10 mm and in No. 33 where the thickness of the matrix steel sheet was less than 10 μm, the {222} plane integration fell somewhat and, furthermore, the {222} plane integration exceeded 15%.
The steel sheets of the above invention examples were measured for the average r value. It was confirmed that in the steel sheets of the invention examples, the average r value was a high level of 2.5 or more. Therefore, the steel sheets of the invention examples had excellent drawability.
In this way it was confirmed that the steel sheet produced by the method of production of the present invention had a {222} plane integration of the αFe phase parallel with respect to the steel sheet surface of 60% or more and a {200} plane integration of the αFe phase parallel to the steel sheet surface of 15% or less or in the range of the present invention.
The results of using Cr as the deposit (second layer) to produce steel sheet having a high {222} plane integration are shown.
The ingredients of the matrix steel sheet were, by mass %, ingredients including Al: 0.02%, C: 0.06%, Si: 0.2%, Mn: 0.4%, Cr: 13.1%, Ni: 11.2%, and a balance of iron and unavoidable impurities.
As the matrix steel sheet, steel sheet obtained by using the vacuum melting method to produce an ingot and hot rolling the ingot to a thickness of 3.0 mm and, furthermore, cold rolling to a thickness of 0.8 mm was used.
The main phase of the matrix steel sheet at ordinary temperature was the γFe phase. X-ray diffraction was used to measure the texture of the γFe phase of the matrix steel sheet and the plane integration was calculated in the same way as above. It was confirmed that the {222} plane integration was 24% and that the {200} plane integration was 21%.
Part of the matrix steel sheet was heat treated at 950° C.×10 sec in a hydrogen atmosphere before Cr plating.
Cr was deposited on the surface of the matrix steel sheet using the electroplating method. For the plating bath, a chrome sulfate solution was used. The thickness of the deposited Cr was 0.6 μm. This was controlled to become uniform in the steel sheet surface.
The steel sheet on which the Cr was deposited was cold rolled, then the steel sheet was heated treated in a nonoxidizing atmosphere. Before the heat treatment, if necessary, the Cr deposited on the steel sheet surface was removed. The Cr was removed by mechanical polishing.
Table 4 shows the alloying ratio, the {222} plane integration of the γFe phase, the {200} plane integration of the γFe phase, and the Al content of steel sheet produced under various conditions. Note that the plane integration was found by measurement using X-ray diffraction and calculation by the above-mentioned calculation processing method.
The alloying ratio of the steel sheet was found as follows: At the L cross-section, in a field of the L direction 1 mm×entire thickness, the EPMA method was used to measure the plane distribution of the Fe content and the plane distribution of the Cr content.
Further, a region of Fe≧0.5 mass % and Cr≧13.2 mass % was deemed an alloyed region and its area was found as the alloyed area. The alloying ratio was calculated by dividing the alloyed area by the L direction 1 mm×total thickness area.
In No. 34 of Comparative Example 9, Cr of a thickness of 0.6 μm was deposited on the steel sheet surface. The cold rolling was omitted and the Cr was removed, then the steel sheet was heat treated under conditions of 1050° C.×0.2 min to make the steel sheet recrystallize.
As a result, the {222} plane integration and the {200} plane integration of this steel sheet were both outside the range of the present invention.
In No. 35 of Comparative Example 10, the deposition of Cr was omitted and the steel sheet was cold rolled by a reduction rate of 75% without any deposit. After this, the steel sheet was heat treated under conditions of 1050° C.×0.2 min to make the steel sheet recrystallize.
In this case as well, the {222} plane integration and the {200} plane integration were both outside the range of the present invention.
In No. 36 of Invention Example 26, the sheet was heat treated at 950° C., then Cr of a thickness of 0.6 μm was deposited on the steel sheet surface. After this, the steel sheet was cold rolled by a reduction rate of 75% to make it thinner.
Furthermore, the Cr was removed, then the steel sheet was heat treated under conditions of 1050° C.×0.2 min to make the steel sheet recrystallize.
As a result, it was confirmed that the {222} plane integration and the {200} plane integration were both controlled to within the range of the present invention and that the Al content was also in the range of the present invention.
Further, using a tensile test, it was confirmed that the steel sheet of Invention Example 26 has a high toughness.
In No. 37 of Invention Example 27, the heat treatment before deposition of Cr was omitted from No. 36 of the invention example, but it was confirmed that the {222} plane integration and the {200} plane integration were both controlled to within the range of the present invention and the Al content was also in the range of the present invention.
In No. 38 of Comparative Example 11, before deposition of Cr, the steel sheet was heat treated at 950° C., then Cr was deposited and the sheet was cold rolled by a reduction rate of 75% to make it thinner.
The removal of the Cr was omitted and the rolling oil on the steel sheet surface was removed, then the steel sheet was heat treated under conditions of 400° C.×0.2 min. At this time, the steel sheet was not made to recrystallize.
As a result, neither the obtained {222} plane integration and the {200} plane integration were in the range of the present invention.
In Nos. 39 to 41 of Invention Examples 28 to 30, before depositing the Cr, the steel sheet was heat treated at 950° C., then Cr was deposited. In each case, the steel sheet was cold rolled by a reduction rate of 75% to make it thinner.
Removal of the Cr was omitted, then rolling oil on the steel sheet surface was removed, then, in No. 39, the steel sheet was heat treated under conditions of 1050° C.×0.2 min, in No. 40, it was heat treated under conditions of 1100° C.×0.2 min, and, further, in No. 41, it was heat treated under conditions of 1150° C.×0.2 min to make the steel sheet recrystallize.
Part of the deposited Cr alloyed with the steel sheet. The ratio of alloying was, in No. 39, 10%, in No. 40, 30%, and in No. 41, 60%.
It was confirmed that the obtained {222} plane integration and the {200} plane integration were both controlled to within the range of the present invention and that the Al content was also in the range of the present invention.
The steel sheets of the above examples were measured for the average r value. It was confirmed that in the steel sheets of the invention examples, the average r value was a high level of 2.5 or more, but in the steel sheets of the comparative examples, it was less than 2.5.
From these results, it was learned that the steel sheets produced by the method of production of the present invention had excellent drawability.
In this way it was confirmed that the steel sheet produced by the method of production of the present invention had a {222} plane integration of the αFe phase parallel with respect to the steel sheet surface of 60% or more and a {200} plane integration of the αFe phase parallel to the steel sheet surface of 15% or less or in the range of the present invention.
The results of using Al alloy as the second layer and changing the thickness of the second layer to produce steel sheet having a high {222} plane integration are shown.
The ingredients of the matrix steel sheet were, by mass %, ingredients including Al: 0.039%, C: 0.0019%, Si: 0.011%, Mn: 0.13%, N: 0.002%, Ti: 0.061%, Cr: 0.002% or less and a balance of iron and unavoidable impurities.
The matrix steel sheet was steel sheet obtained by using the vacuum melting method to produce an ingot and hot rolling the ingot to a thickness of 3.0 mm. Note that pickling was used to remove the scale from the steel sheet surface.
The main phase of the matrix steel sheet at ordinary temperature was the αFe phase. X-ray diffraction was used to measure the texture of the αFe phase of the matrix steel sheet and the plane integration was calculated in the same way as above. It was confirmed that the {222} plane integration was 19% and that the {200} plane integration was 17%.
This matrix steel sheet was heat treated at 780° C.×10 sec in a hydrogen atmosphere before plating. On the surface of the matrix steel sheet, Al alloy was deposited by the hot dip method. The composition of the plating bath was, by mass %, 90% Al-10% Si. The alloy was deposited on the two surfaces of the steel sheet.
The amount of plating deposition was controlled by, before the plating solidified, using a wiping gas to blow nitrogen over the steel sheet surface to blow off unnecessary plating.
The steel sheet on which the Al alloy was deposited was cold rolled to reduce it to a thickness of 0.8 mm. After this, this steel sheet was heat treated in a nonoxidizing atmosphere to make the steel sheet recrystallize and promote diffusion of Al.
Table 5 shows the alloying ratio of the produced steel sheet, the {222} plane integration of the αFe phase, the {200} plane integration of the αFe phase, and the Al content of steel sheet produced under various conditions. Note that the plane integration was found by measurement using X-ray diffraction and calculation by the above-mentioned calculation processing method.
The alloying ratio of the steel sheet was found as follows: At the L cross-section, in a field of the L direction 1 mm×entire thickness, the EPMA method was used to measure the plane distribution of the Fe content and the plane distribution of the Al content.
Further, a region of Fe≧0.5 mass % and Al≧0.139 mass % was deemed an alloyed region and its area was found as the alloyed area. The alloying ratio was calculated by dividing the alloyed area by the L direction 1 mm×total thickness area.
Note that, the area ratios obtained by using the EBSP method to separately observe by the L cross-section the crystal grains with a deviation of the {222} plane with respect to the steel sheet surface of 0 to 30° and the crystal grains with a deviation of the {222} plane with respect to the steel sheet surface of 0 to 10° are described.
Further, the above steel sheet was tested for burr resistance. A 10.0 mmφ punch and a 10.3 mmφ die were used for punching and the burr height around the punched hole was measured by a point micrometer.
In Nos. 42 to 44 of Comparative Examples 12 to 14, the step of deposition of the Al alloy was omitted and the steel sheet was cold rolled by a reduction rate of 73% without any deposits. After this, the steel sheet was heat treated under conditions of 700 to 1010° C. to make the steel sheet recrystallize.
In this case, the {222} plane integration and the {200} plane integration were both outside the range of the present invention. The burr height was a large value of 51 to 57 μm.
In No. 45 to 47 of Invention Examples 31 to 33, Al alloy of 5 μm thickness in total of the front and back was deposited. Further, the steel sheet was cold rolled to a thickness of 0.8 mm, then the steel sheet was heat treated under conditions of 700 to 1010° C. to make the steel sheet recrystallize.
In this case, the {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 12 to 14 μm or remarkably lower than the comparative examples.
In Nos. 48 to 57 of Invention Examples 34 to 40, Al alloy of 10 to 40 μm thickness in total of the front and back was deposited. Further, the steel sheet was cold rolled to a thickness of 0.8 mm, then the steel sheet was heat treated under conditions of 700 to 1010° C. to make the steel sheet recrystallize. At this time, the temperature rise rate was changed.
In each case, the {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 5 to 8 μm or a remarkably small value.
The steel sheets of the above examples were measured for the average r value. It was confirmed that in the steel sheets of the invention examples, the average r value was a high level of 2.5 or more, but in the steel sheets of the comparative examples, it was less than 2.5.
From these results, it was learned that the steel sheets of the invention examples have excellent drawability.
Further, an Erichsen test was performed and the extruded surfaces were examined whereupon it was confirmed that the steel sheets of the invention examples are also excellent in press formability.
In this way it was confirmed that the steel sheet produced by the method of production of the present invention had a {222} plane integration of the αFe phase parallel with respect to the steel sheet surface of 60% or more and a {200} plane integration of the αFe phase parallel to the steel sheet surface of 15% or less or in the range of the present invention and that both excellent burr resistance and drawability were achieved.
The results of changing the Cr content of the matrix steel sheet to examine the manufacturability and the {222} plane integration are shown.
The matrix steel sheet was produced by four types of ingredients with different Cr content. The Cr content was, by mass %, 13.0% (ingredients F), 11.9% (ingredients G), 6.0% (ingredients H), and 0.002% or less (detection limit or less) (ingredients I). In addition, C: 0.083%, Si: 0.11%, Mn: 0.23%, Al: 0.002%, N: 0.003, and a balance of iron and unavoidable impurities were included in the ingredients.
By each of these ingredients, vacuum melting was used to produce an ingot and the ingot was hot rolled to reduce it to a thickness of 3.5 mm. Next, the four types of steel sheets were cold rolled to a thickness of 1.3 mm.
The main phases of the steel sheets of the ingredients F, G, H, and I at ordinary temperature were the αFe phases. X-ray diffraction was used to measure the texture of the αFe phase of the matrix steel sheet and the plane integration was calculated in the same way as above.
It was confirmed that the {222} plane integration was, with the ingredients F, 8%, the ingredients G, 9%, the ingredients H, 9%, and the ingredients I, 8%, while the {200} plane integration was, with the ingredients F, 28%, the ingredients G, 30%, the ingredients H, 31%, and the ingredients I, 29%.
The electroplating method was used to deposit Sn on the surface of the matrix steel sheet as the second layer. The plating bath was a sulfuric acid acidic solution. The process was controlled to give a basis weight per side of 1 g/m2. Both surfaces were plated. Before the electroplating, no preheat treatment was applied.
With the Sn deposited as the second layer in this way, each steel sheet was cold rolled by a reduction rate of 40% to obtain steel sheet of a thickness of 0.78 mm. For comparison, steel sheets of the ingredients F, G, H, and I with no Sn deposited were also cold rolled by a reduction rate of 40%.
Next, each steel sheet was heat treated in vacuum at a temperature rise rate of 100° C./min under conditions of 1100° C.×60 min to make the steel sheet recrystallize. At this time, at each steel sheet, the Sn of the steel sheet surface diffused in the steel and was completely alloyed.
For comparison, steel sheet without Sn deposited was similarly heat treated.
The obtained eight types of steel sheets were measured for the {222} plane integration and the {200} plane integration. The {222} area integration of the steel sheets on which Sn was deposited was, for the ingredients F, 65%, the ingredients G, 75%, the ingredients H, 79%, and the ingredients I, 85%, while the {200} plane integration was, for the ingredients F, 12%, the ingredients G, 4%, the ingredients H, 2.5%, and the ingredients I, 1.4.
In each case, the plane integration was within the range of the present invention, but it was learned that if the Cr contained is, by mass %, less than 12.0%, a particularly high {222} plane integration can be obtained.
On the other hand, the plane integration of the steel sheets on which Sn was not deposited was, for the ingredients F, 21%, the ingredients G, 12%, the ingredients H, 11%, and the ingredients I, 12 and the {200} plane integration was, for the ingredients F, 16%, the ingredients G, 17%, the ingredients H, 16%, and the ingredients I, 16%.
The burr resistance was evaluated by using 10.0 mmφ punch and a 10.3 mmφ die for punching and measuring the burr height around the punched hole by a point micrometer.
The burr height of the steel sheets on which Sn was deposited was, for the ingredients F, 9 μm, the ingredients G, 7 μm, the ingredients H, 6 μm, and the ingredients I, 5 μm. It was confirmed that each steel sheet had excellent properties.
The burr height of the steel sheets on which Sn was not deposited was, for the ingredients F, 46 μm, the ingredients G, 52 μm, the ingredients H, 63 μm, and the ingredients I, 68 μm. It was confirmed that each steel sheet suffered from large burrs.
Furthermore, each steel sheet was measured for the average r value, whereupon it was confirmed that the average r value of a steel sheet on which Sn was deposited was a high level of 2.5 or more. The average r value for a steel sheet on which Sn was not deposited was about 1.1.
From this, it was learned that steel sheet on which Sn was deposited has excellent drawability. Further, an Erichsen test was performed and the extruded surface was examined. As a result, it was confirmed that steel sheet on which Sn was deposited is excellent in press formability as well.
In this way it was confirmed that the steel sheet produced by the method of production of the present invention had a {222} plane integration of the αFe phase parallel with respect to the steel sheet surface of 60% or more and a {200} plane integration of the αFe phase parallel to the steel sheet surface of 15% or less or in the range of the present invention.
The results of changing the Al content of the matrix steel sheet to examine the manufacturability and the {222} plane integration are shown.
The matrix steel sheet was produced by four types of ingredients with different Al content. The Al content was, by mass %, 7.5% (ingredients J), 6.4% (ingredients K), 3.4% (ingredients L), and 0.002% or less (ICP detection limit or less) (ingredients M). In addition, C: 0.083%, Si: 0.11%, Mn: 0.23%, Cr: 0.002% or less (ICP analysis detection limit or less), N: 0.003, and a balance of iron and unavoidable impurities were included in the ingredients.
By each of these ingredients, vacuum melting was used to produce an ingot and the ingot was hot rolled to reduce it to a thickness of 2.8 mm.
The ingots of the ingredients K, L, and M could be easily hot rolled to steel sheets, but the ingot of the ingredients J frequently broke during hot rolling so hot rolling could not be continued.
In this way, if the Al content of the matrix steel sheet is over the range of the present invention at 6.5% or more, production is difficult, so production of steel sheet of the ingredients J was foregone. Next, the steel sheets of the ingredients K, L, and M were cold rolled to 1.6 mm thicknesses.
The main phases of the steel sheets of the ingredients K, L, and M at ordinary temperature were the αFe phases. X-ray diffraction was used to measure the texture of the αFe phase of the matrix and the plane integration was calculated in the same way as above. It was confirmed that the {222} plane integration was, for the ingredients K, 11%, the ingredients L, 12%, and the ingredients M, 12%, while the {200} plane integration was, for the ingredients K, 8%, the ingredients L, 7%, and the ingredients M, 8%.
Each matrix steel sheet, before formation of the second layer, was heat treated at 750° C.×10 sec in a hydrogen atmosphere. After this, the hot dip method was used to deposit Zn on the surface of the matrix steel sheet.
The composition of the plating bath was 95% Zn-5% Fe. The Zn alloy was deposited on both surfaces of the steel sheet. The amount of deposition, in total for the front and back, was made 80 g/m2. The amounts of deposition on the front and back were made as uniform as possible.
With the Zn alloy deposited as the second layer, each steel sheet was cold rolled by a reduction rate of 50% to obtain steel sheet of a thickness of 0.80 mm.
For comparison, steel sheets of the ingredients K, L, and M with no Zn alloy deposited were also cold rolled by a reduction rate of 50% to a thickness of 0.80 mm.
Next, each steel sheet was heat treated in a vacuum at a temperature rise rate of 10° C./min under conditions of 1100° C.×60 min to make the steel sheet recrystallize. At this time, in each steel sheet, the Zn alloy of the steel sheet surface diffused in the steel and was completely alloyed.
For comparison, steel sheet without Zn alloy deposited was similarly heat treated.
The obtained eight types of steel sheets were measured for the {222} plane integration and the {200} plane integration. The {222} area integration of the steel sheets on which Zn alloy was deposited was, for the ingredients K, 78%, the ingredients L, 85%, the ingredients M, 90%, and the ingredients I, 85%, while the {200} plane integration was, for the ingredients K, 1.4%, the ingredients L, 0.6%, and the ingredients M, 0.4%.
In each case, the plane integration was within the range of the present invention, but it was learned that if the Al contained is, by mass %, less than 3.5%, a particularly high {222} plane integration can be obtained.
On the other hand, the plane integration of the steel sheets on which Zn alloy was not deposited was, for the ingredients K, 36%, the ingredients L, 32%, and the ingredients M, 25%, and the {200} plane integration was, for the ingredients K, 17%, the ingredients L, 19%, and the ingredients M, 16%.
The burr resistance was evaluated by using 10.0 mmφ punch and a 10.3 mmφ die for punching and measuring the burr height around the punched hole by a point micrometer.
The burr height of the steel sheets on which Zn was deposited was, for the ingredients K, 7 μm, the ingredients L, 5 μm, and the ingredients M, 5 μm. It was confirmed that each steel sheet had excellent properties.
The burr height of the steel sheets on which Zn alloy was not deposited was, for the ingredients K, 52 μm, the ingredients L, 57 μm, and the ingredients M, 65 μm. It was confirmed that each steel sheet suffered from large burrs.
Furthermore, each steel sheet was measured for the average r value, whereupon it was confirmed that the average r value of a steel sheet on which Zn alloy was deposited was a high level of 2.5 or more. The average r value for a steel sheet on which Zn alloy was not deposited was about 1.1.
From this, it was learned that steel sheet on which Zn alloy was deposited has excellent drawability.
Further, an Erichsen test was performed on each steel sheet and the extruded surface was examined. As a result, it was confirmed that steel sheet on which Zn alloy was deposited is excellent in press formability as well.
In this way it was confirmed that the steel sheet produced by the method of production of the present invention had a {222} plane integration of the αFe phase parallel with respect to the steel sheet surface of 60% or more and a {200} plane integration of the αFe phase parallel to the steel sheet surface of 15% or less or in the range of the present invention.
The results of using Mo, Cr, Ge, Si, Ti, W, and V metal as the deposits of the second layer to produce steel sheet having a high {222} plane integration are shown.
The hot rolled sheets of the thicknesses of 2.8 mm of the ingredients K, L, and M used in Example 8 were used as the matrix steel sheets. Steel sheets of the ingredients K, L, and M were cold rolled to 0.4 mm thickness.
The main phases of the steel sheets of the ingredients K, L, and M at ordinary temperature were αFe phases. X-ray diffraction was used to measure the texture of the αFe phase of each matrix steel sheet and the plane integration was calculated in the same way as above.
It was confirmed that the {222} plane integration was, for the ingredients K, 15%, the ingredients L, 17%, and the ingredients M, 16%, while the {200} plane integration was, for the ingredients K, 7%, the ingredients L, 6%, and the ingredients M, 8%.
Before sputtering for depositing the second layer, each matrix steel sheet was heat treated at 620° C.×60 sec in an Ar atmosphere. The sputtering method was used to deposit on the surface of the matrix steel sheet a second layer of Mo, Cr, Ge, Si, Ti, W, and V metal.
Metal target materials of purities of 99.9% or more were prepared and the thicknesses per side were controlled to 1 μm to form coatings on the two surfaces.
With the second layer comprised of each metal as deposited, each steel sheet was cold rolled by a reduction rate of 62.5% to obtain steel sheet of a thickness of 0.15 mm.
For comparison, steel sheets of the ingredients K, L, and M on which no second layer comprised of a metal is deposited were also cold rolled by a reduction rate of 62.5% to a thickness of 0.15 mm.
Next, each steel sheet was heat treated in vacuum at a temperature rise rate of 500° C./min under conditions of 1150° C.×15 sec to make the steel sheet recrystallize.
At this time, in each steel sheet, the second layer metal at the steel sheet surface diffused in the steel and was completely alloyed. For comparison, steel sheets on which no second layer metal was deposited were similarly heat treated.
Table 6 shows the alloying ratio, the {222} plane integration of the αFe phase, the {200} plane integration of the αFe phase, and the Al content of steel sheet produced under various conditions. The plane integration was found by measurement using X-ray diffraction and calculation by the above-mentioned calculation processing method.
The alloying ratio of the steel sheet was found as follows: At the L cross-section, in a field of the L direction 1 mm×entire thickness, the EPMA method was used to measure the plane distribution of the Fe content and the plane distribution of the content of the deposited metal elements among Mo, Cr, Ge, Si, Ti, W, and V.
Further, a region of Fe≧0.5 mass % and a content of the deposited metal element among Mo, Cr, Ge, Si, Ti, W, and V≧0.1 mass % was deemed an alloyed region and its area was found as the alloyed area. The alloying ratio was calculated by dividing the alloyed area by the L direction 1 mm×total thickness area.
Note that, the area ratios obtained by using the EBSP method to separately observe by the L cross-section the crystal grains with a deviation of the {222} plane with respect to the steel sheet surface of 0 to 30° and the crystal grains with a deviation of the {222} plane with respect to the steel sheet surface of 0 to 10° are described.
Further, the above steel sheet was tested for burr resistance. A 10.0 mmφ punch and a 10.3 mmφ die were used for punching and the burr height around the punched hole was measured by a point micrometer.
In Nos. 60 to 62 of Comparative Examples 15 to 17, the deposition of the metal of the second layer was omitted. In this case, the {222} plane integration and the {200} plane integration were both outside the range of the present invention. The burr height was a large value of 42 to 63 μm.
In Nos. 63 to 65 of Invention Examples 43 to 45, Mo was deposited as the second layer. The {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 8 to 9 μm or much lower than the comparative examples.
In Nos. 66 to 68 of Invention Examples 46 to 48, Cr metal was deposited as the second layer. The {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 7 to 8 μm or much lower than the comparative examples.
In Nos. 69 to 71 of Invention Examples 49 to 51, Si metal was deposited as the second layer. The {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 7 to 8 μm or much lower than the comparative examples.
In Nos. 72 to 74 of Invention Examples 52 to 54, Ge metal was deposited as the second layer. The {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 8 to 9 μm or much lower than the comparative examples.
In Nos. 75 to 77 of Invention Examples 55 to 57, Ti metal was deposited as the second layer. The {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 7 to 8 μm or much lower than the comparative examples.
In Nos. 78 to 80 of Invention Examples 58 to 60, W metal was deposited as the second layer. The {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 7 to 9 μm or much lower than the comparative examples.
In Nos. 81 to 83 of Invention Examples 60 to 63, V metal was deposited as the second layer. The {222} plane integration and the {200} plane integration were both in the range of the present invention. The burr height was 6 to 8 μm or much lower than the comparative examples.
The steel sheets of the above examples were measured for the average r value. It was confirmed that in the steel sheets of the invention examples, the average r value was a high level of 2.5 or more, but in the steel sheets of the comparative examples, it was less than 2.5.
Therefore, it was learned that the steel sheets of the invention examples have excellent drawability.
In this way it was confirmed that the steel sheet produced by the method of production of the present invention had a {222} plane integration of the αFe phase parallel with respect to the steel sheet surface of 60% or more and a {200} plane integration of the αFe phase parallel to the steel sheet surface of 15% or less or in the range of the present invention and that excellent burr resistance and drawability were both achieved.
As explained above, the present invention steel sheet has the unprecedented superior workability of absence of formation of burrs at the cross-section at the time of punching, so can be easily worked into various shapes including everything from conventional shapes to special sheets.
Therefore, the present invention steel sheet is for example useful for outer panels of auto parts, home electrical appliances, etc. requiring press formation into complicated shapes and other various structural materials and functional materials.
Further, the method of production of the present invention enables the {222} plane integration to be raised and/or the {200} plane integration to be lowered easily and effectively even in steel sheet having an Al content of less than 6.5 mass %.
Therefore, according to the method of production of the present invention, it is possible to produce steel sheet having a high {222} plane integration (the present invention steel sheet) without production of new facilities by just switching processes of existing facilities easily and at low cost.
Therefore, the present invention is high in industrial applicability in the manufacturing industries utilizing the various structural materials and functional materials.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-314240 | Nov 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/072997 | 11/21/2007 | WO | 00 | 4/27/2009 |
