Patent Grant
8815025
This disclosure relates to a high strength steel sheet having a strength not lower than 780 MPa and excellent in the balance between the strength (TS) and the uniform elongation (U·EL) and suitable for use as a raw material of the member to which is applied some working such as a press forming, a bending process or a stretch flanging process.
With enhancement of the attentions paid to the environmental problem, efforts are being made in an attempt to decrease the weight of the part by increasing the strength of the part and by decreasing the thickness of the part. Further, with expansion of the field to which a high strength steel sheet is applied, the press forming tends to be employed widely for performing a complex process even in the case of handling a high strength steel sheet, with the result that required is a material having a high strength and, at the same, excellent in the workability.
Particularly, in the field of the automobile, the high strength steel sheet is required to exhibit various properties in addition to the balance between the strength and the stretch flange-ability. To be more specific, required are (1) a high yield ratio (YS/TS>0.7) in view of the safety in the event of a car crash, (2) an excellent balance between the strength and the uniform elongation (TS×U·EL>12,000) in view of the bulging properties, and (3) a good plating capability in view of the durability of the part (in general, Si<0.5% is one of the absolutely required conditions). Particularly, concerning the uniform elongation, i.e., requirement (2) given above, an improvement in the uniform elongation is a very important factor nowadays because the ductility until the starting of the necking after the yield point has come to be required in accordance with the complex shaping of the part and the shortening of the press forming time, which are required nowadays. However, it is very difficult for the conventional technology to satisfy simultaneously all the requirements (1) to (3) given above.
It was customary in the past to use a high strength steel sheet for the manufacture of a structural part and, thus, the stretch flangeability has been evaluated as more important than the bulging properties. Therefore, many methods have been proposed to date for satisfying the requirements for both the high strength and the high stretch flangeability. For example, proposed in each of JP-A-7-11382 and JP-A-6-200351 identified hereinafter is a steel sheet exhibiting an excellent hole expanding ratio in spite of a high strength not lower than 700 MPa. Specifically, it is proposed in patent document 1 that TiC or NbC is precipitated in the acicular ferrite structure so as to obtain a steel sheet excellent in the hole expanding ratio. On the other hand, it is proposed in JP-A-6-200351 that, in order to increase the hole expanding ratio of the steel sheet, at least 85% of the structure of the steel sheet is formed of a polygonal ferrite, that TiC is precipitated, and that Mo is dissolved. JP-A-7-11382 and JP-A-6-200351 also propose the methods of manufacturing the particular steel sheets. However, where TiC or NbC is utilized for precipitation strengthening as in the patent documents quoted above, it is unavoidable for the precipitate to be enlarged and coarsened, leading to a lowered strength. It is also difficult to secure a sufficient stretch flangeability because the enlarged and coarsened precipitates provide the starting points and the propagating route of the cracking.
In order to overcome the problems pointed out above, proposed in JP-A-2004-143518 referred to hereinafter is a steel sheet containing ferrite as a main phase and having V carbonitride, which has an average carbide diameter not larger than 50 nm, precipitated within the ferrite grains. It is taught that the steel of the particular structure permits improving the total elongation, the hole expanding ratio and the fatigue resistance. However, the structure obtained by this method consists mainly of ferrite and pearlite and is not intended to utilize the retained austenite and martensite (It is taught that it is highly desirable for the amount of the second phase to be 0%). It is not reasonable to state that the steel sheet proposed in patent document 3 is satisfactory in the balance between the strength and the uniform elongation. On the other hand, a steel sheet having a high YS/TS ratio, a good stretch flanging property, and a satisfactory plating property and a method of manufacturing the particular steel are disclosed in each of JP-A-2002-322539, JP-A-2002-322540, JP-A-2002-322541, JP-A-2002-322543, JP-A-2003-89848, JP-A-2003-138343 and JP-A-2003-138344 referred to hereinafter. It is taught that the steel sheet exhibiting the excellent properties can be obtained by the construction that the structure is formed of ferrite and the ferrite structure is reinforced by superfine precipitates containing Ti and Mo and having an average precipitate diameter not larger than 10 nm. The method proposed in these patent documents is highly effective in respect of requirement (1) referred to previously. However, the particular method is incapable of obtaining not only a ferrite single phase structure but also a good balance between the strength and the uniform elongation.
Various methods utilizing the retained austenite (retained γ) are proposed as a measure for improving the balance between the strength and the uniform elongation or between the strength and the entire elongation (EL). For example, a steel sheet excellent in the balance between the strength and the entire elongation and a method of manufacturing the particular steel sheet are disclosed in JP-A-2000-336455 referred to herein later. It is taught that the steel sheet has a composition containing 0.5 to 20 wt % of Si and 0.005 to 0.3 wt % of Ti, that the steel sheet contains ferrite having an average grain diameter smaller than 2.5 μm as a main component, and that the steel sheet has a structure containing bainite having an average grain diameter not larger than 5 μm and at least 5% of the retained γ. However, since the steel sheet is strengthened mainly in this prior art by grain refinement, it is difficult to obtain the requirement of YS/TS>0.7. It is also difficult to obtain the strength not lower than 780 MPa.
Disclosed in each of JP-A-4-228538 and JP-A-2003-321738 referred to hereinafter are a steel sheet having a strength not lower than 780 MPa and an excellent balance between the strength and the entire elongation and a method of manufacturing the particular steel sheet. It is disclosed in JP-A-4-228538 that the ratio of the polygonal ferrite space factor rate to the average grain diameter of the polygonal ferrite is set at 7 or more, and that Si is added in a large amount so as to obtain the steel sheet noted above. On the other hand, JP-A-2003-321738 teaches that the ferrite in the retained γ steel having Si added thereto in an amount of 0.5 wt % or more is reinforced by fine precipitates containing Ti and Mo so as to obtain the steel sheet noted above. In each of these methods, however, required is Si in an amount of 0.5 wt % or more so as to deteriorate the surface properties and to lower the plating capability of the steel sheet.
As a measure for obtaining a retained γ steel without adding a large amount of Si, disclosed in, for example, JP-A-6-264183 referred to hereinafter is a steel sheet excellent in the balance between the strength and the entire elongation. It is taught that the steel sheet contains 0.8 to 2.5 wt % of Sol. Al and that a fine polygonal ferrite containing at least 5% by volume of retained γ constitutes the main phase of the steel sheet. JP-A-6-264183 also discloses a method of manufacturing the particular steel sheet. In this prior art, a fine polygonal ferrite is used as the main phase of the steel sheet in order to improve the hole expanding ratio. It should be noted in this connection that the fine polygonal ferrite is solid-solution-strengthened by Si alone, or is precipitation-strengthened by TiC or NbC, with the result that the precipitates are enlarged and coarsened in the re-heating stage for applying a molten zinc plating to the surface of the steel sheet so as to give rise to the difficulty that the crystal grains are enlarged and coarsened so as to lower the strength and the hole expanding ratio. In addition, in order to obtain a fine polygonal ferrite, it is necessary to heat the steel sheet between rolls of at least two rear stage stands of a finish rolling mill in a temperature region of Ar3−50° C. to Ar3+100° C. with the total rolling reduction in this temperature region set at 30% or more. It is possible to supply current directly to the roll for heating the roll in order to heat the steel sheet between rolls of the finish rolling mill. In this method, however, special facilities are required. In addition, such a large power as 1,500 kVA is required, leaving room for further improvement in view of the energy saving.
We provide a high strength steel sheet having a high strength not lower than 780 MPa, a good balance between the strength and a stretch flangeability, a high yield ratio (YS/TS>0.7), an excellent balance between the strength and the uniform elongation (TS×U·EL>12,000), and a good plating property (in general, the condition of Si<0.5% is one of the absolutely required conditions).
We conducted an extensive research on a high tensile steel sheet having a strength not lower than 780 MPa in an attempt to optimize the components and the structure of the steel sheet in a method of improving the balance between the strength and the uniform elongation while retaining a high yield ratio and a good plating property, arriving at findings (i) to (iii) given below:
We provide aspects (1) to (8) given below:
We will now describe our disclosure more in detail in respect of the metal structure, the chemical components and the manufacturing conditions.
(Metal Structure)
The metal structure will now be described first.
The high strength hot rolled steel sheet has a complex structure including three phases of the ferrite phase, the bainite phase and the retained austenite phase. The complex structure may possibly include the martensite phase. In the steel sheet, the ferrite phase is strengthened by the composite carbide containing Ti and Mo, or the composite carbide Ti, V and Mo. The particular construction of the complex structure will now be described.
The total volume of the ferrite phase and the bainite phase is not smaller than 80% and the volume of the bainite phase is 5% to 60%:
The volume of the retained γ phase is 3 to 20%:
Composite carbides containing Ti and Mo, and composite carbides containing Ti, Mo and V:
The average carbide diameter of the composite carbides is not larger than 30 nm:
The chemical components will now be described. Incidentally, the expression “%” used in the following description denotes “mass %”.
C: 0.05 to 0.25%:
Si: less than 0.5%:
Mn: 0.5 to 3.0%:
P: not larger than 0.06%:
S: not larger than 0.01%:
Sol. Al: 0.50 to 3.0%:
N: not larger than 0.02%:
Mo: 0.1 to 0.8%:
Ti: 0.02 to 0.40%:
V: 0.05 to 0.50%:
The manufacturing conditions (hot rolling conditions) employed will now be described.
The steel sheet can be manufactured by hot rolling a slab having the chemical compositions described above. All the steel making methods generally known to the art can be employed for manufacturing the steel sheet and, thus, the steel making method need not be limited. For example, it is appropriate to use a converter or an electric furnace in the melting stage, followed by performing a secondary refining by using a vacuum degassing furnace. Concerning the casting method, it is desirable to employ a continuous casting method in view of the productivity and the product quality.
It is possible to employ the ordinary process comprising the steps of casting a molten steel, cooling once the cast steel to room temperature, and re-heating the steel so as to subject the steel to a hot rolling. It is also possible to employ a direct rolling process in which the steel immediately after the casting, or the steel further heated after the casting for imparting an additional heat, is hot rolled. In any of these cases, the effect on the steels is not affected. Further, in the hot rolling, it is possible to perform the heating after the rough rolling and before the finish rolling, to perform a continuous hot rolling by joining a rolling material after the rough rolling stage, or to perform the heating and the continuous rolling of the rolling material. In any of these cases, the effect of the present invention is not impaired. Incidentally, it is desirable for the heating temperature of the slab in the range of 1,200 to 1,300° C. in order to dissolve the carbide. Also, it is desirable for the temperature of finish rolling in the hot rolling process to be not lower than 800° C. in order to lower the load of the rolling and to secure the surface properties. Further, it is desirable for the finish rolling temperature to be not higher than 1,050° C. for grain refining.
In the steel sheet, the bainite transformation is utilized for promoting the generation of the retained γ, and the bainite phase is utilized for improving the strength of the steel sheet. It is appropriate to set the coiling temperature after the hot rolling process in a manner to fall within a range of 350° C. to 580° C. in order to generate the bainite phase. If the coiling temperature exceeds 580° C., cementite is precipitated after the coiling process. By contraries, the martensite phase is generated if the coiling temperature is lower than 350° C. to deteriorate the uniform elongation. It follows that it is appropriate to coil the hot rolled steel sheet in the temperature range of 350° C. to 580° C., preferably within a range of 400° C. to 530° C. Incidentally, in order to obtain abovementioned microstructure, it is desirable for the steel sheet after the hot rolling stage to be cooled at an average cooling rate of 30° C./s to 150° C. If the average cooling rate after the hot rolling step is lower than 30° C./s, the ferrite grains and the composite carbide grains contained in the ferrite phase are enlarged and coarsened so as to lower the strength of the steel sheet. Therefore it is preferable that the average cooling rate is not lower than 30° C./s. If the average cooling rate after the hot rolling step is higher than 150° C./s, it is difficult to generate the ferrite grains and the carbide. Therefore it is preferable that the average cooling rate is not higher than 150° C./s.
Further, it is desirable for the cooling process to include the steps of cooling the hot rolled steel sheet to a temperature region falling within the range of 600° C. to 750° C. at an average cooling rate not lower than 30° C./s, air-cooling the steel sheet within the temperature range of 600° C. to 750° C. for 1 to 10 seconds, further cooling the steel sheet to the coiling temperature at an average cooling rate not lower than 10° C./s and, then, coiling the steel sheet in the temperature range of 350° C. to 580° C. The particular cooling process makes it possible to obtain easily the micro structure described above. It should be noted that, if the average cooling rate after the hot rolling step is lower than 30° C./s, the ferrite grains and the composite carbide grains contained in the ferrite phase are enlarged and coarsened so as to lower the strength of the steel sheet. Further, if the air-cooling is performed for 1 to 10 second in the temperature range of 600° C. to 750° C., it is possible to promote the ferrite transformation, to promote the C diffusion in the untransformed γ, and to promote the fine precipitation of composite carbides containing Ti—Mo or Ti—V—Mo in the formed ferrite. If the air-cooling temperature exceeds 750° C., the precipitates are rendered large and coarse to lower the strength of the steel sheet. On the other hand, if the air-cooling temperature is lower than 600° C., the composite carbides are not precipitated sufficiently to lower the strength of the steel sheet. Further, if the air-cooling time is shorter than 1 second, the composite carbides are not precipitated sufficiently. On the other hand, if the air-cooling time is longer than 10 seconds, the ferrite transformation proceeds excessively, resulting in failure to obtain the bainite phase in an amount not smaller than 5%. Also, if the average cooling rate after the air-cooling stage is lower than 10° C./s, pearlite is formed and the stretch flanging ratio is lowered.
Incidentally, the upper limits in respect of the cooling rate after the hot rolling stage and the cooling rate after the air-cooling stage are not particularly specified in the present invention. However, it is desirable for the cooling rate after the hot rolling stage to be not higher than 700° C./s and for the cooling rate after the air-cooling stage to be not higher than 200° C./s.
Incidentally, it is possible to apply plating such as a hot dipping or an electric galvanising to the steel sheet to form a zinc-based plated coating on the surface of the steel sheet. Naturally, the high strength steel sheet of the present invention includes a galvanized steel sheet obtained by forming a zinc-based plated coating on the surface of the steel sheet by the plating treatment described above. It is also possible to apply a chemical treatment to the surface of the steel sheet.
Since the high strength steel sheet exhibits a good workability, the steel sheet retains a good workability even if a plated coating of galvanizing system is formed on the surface. Incidentally, the zinc-based plating noted above denotes the zinc plating and the plating based on zinc. It is possible for the plating to include alloying elements such as Al and Cr in addition to zinc. Incidentally, in the case of the steel sheet having a galvanized plated coating formed on the surface, it is possible to apply the alloying treatment to the plated surface of the steel sheet. When it comes to the annealing temperature before the plating stage in the case of applying the plating by a hot dipping in molten zinc, zinc is not plated on the surface of the steel sheet if the heating temperature is lower than 450° C. On the other hand, the uniform elongation of the steel sheet tends to be lowered, if the annealing temperature exceeds Ac3. Such being the situation, it is desirable for the heating temperature to fall within the range of 450° C. to Ac3.
In the steel sheet, there is no difference in properties between the steel sheet having a black skin surface and the steel sheet after cleaning with an acid. The temper rolling is not particularly limited as far as the temper rolling employed in general is applied. Further, it is desirable to apply the galvanising after the pickling. However, it is possible to apply the zinc-based plating by a hot dipping in a molten metal even after the pickling with an acid or to apply the plating to the steel sheet having a black skin surface.
Slabs having the chemical compositions shown in Table 1 were heated to various temperatures, followed by hot rolling the heated slabs to obtain hot rolled steel sheets each having a thickness of 2.0 mm. In preparing the hot rolled steel sheets, the heating temperature, the finish rolling temperature, the cooling rate, and the coiling temperature were changed. The hot rolled steel sheets were pickled thereby preparing samples. For obtaining the hole expanding ratio λ providing a criterion of the stretch flangeability, a steel sample sized 130 mm square was cut out from the steel sheet, followed by making a cutting hole, 10 mmΦ, in the sample by drilling. Then, a conical punch of 60° was pushed up from below and the hole diameter d was measured when the crack penetrated through the steel sheet. The hole expanding ratio λ(%) was calculated by the formula given below:
λ(%)=100·(d−10)/10.
The mechanical properties were obtained by taking out a JIS 5 tensile strength test piece in a direction of 90° from the rolling direction and by applying a tensile strength test to the test piece. For determining the composition of the composite carbides such as the amounts of Ti, Mo and V contained in the composite carbides, a thin film sample was prepared from the steel sheet, and the composition was determined by the energy dispersion type X-ray spectroscopic apparatus (EDX) of a transmission electron microscope (TEM). Also, for determining the average particle size of the composite carbides, not less than 100 ferrite grains were observed with an observation magnification of 200,000, and the diameters were converted into the diameters of the corresponding circles by an image processing based on the areas of the individual composite carbides. Further, the diameters obtained by the conversion were averaged to obtain the particle size of the composite carbides. The micro structure was identified by using an optical microscope and a scanning electron microscope (SEM) to obtain the area percentage of ferrite and the area percentage of bainite. The area percentage of ferrite and the area percentage of bainite were used as the volume percentage of ferrite and the volume percentage of bainite. Also, the amount of the retained γ (volume percentage) was obtained by the X-ray diffraction.
Further, an alloying galvanizing was applied to parts of steels A, J, L and AA under a heating temperature of 680° C. which is not higher than Ac3 and an alloying temperature of 560° C., which was maintained for 60 seconds, by using a continuous galvanizing line. In order to evaluate the outer appearance of the plated layer and the adhesivity of the plating, a 180° bending test was conducted based on JIS Z 2248, followed by attaching a tape (Dunplonpro No. 375 manufactured by Nitto Kako K.K.) to the bent portion and subsequently peeling off the tape to visually observe the surface state after the peeling off of the tape. The samples having the plating not peeled off at all were evaluated as “good”, and the samples having the plating peeled off such that the peeling was recognized by the naked eyes was evaluated as “poor.”
Table 2 shows the manufacturing conditions, Table 3 shows the properties of the steel sheet samples after the hot rolling and the pickling, and Table 4 shows the properties of the steel sheet samples after the galvanizing. As apparent from the experimental data, any of the Inventive Examples was found to exhibit a high yield ratio (YS/TS), compared with the Comparative Examples, and was also found to be excellent in the balance between the strength and the uniform elongation, in the stretch flangeability, and in the plating property. In contrast, the steel sheet samples for the Comparative Examples failing to fall within our range in at least one condition was found to fail to satisfy simultaneously all the properties including the high yield ratio, a good balance between the strength and the uniform elongation, a good stretch flangeability, and a good plating property.
We thus provide a high strength hot rolled steel sheet used in various fields including, for example, the use as a steel sheet for an automobile.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-340554 | Nov 2005 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5470529 | Nomura et al. | Nov 1995 | A |
| 20050081966 | Kashima et al. | Apr 2005 | A1 |
| 20050133124 | Kawano et al. | Jun 2005 | A1 |
| Number | Date | Country |
|---|---|---|
| 0 997 548 | May 2000 | EP |
| 1 350 859 | Oct 2003 | EP |
| 1 486 574 | Dec 2004 | EP |
| 4-228538 | Aug 1992 | JP |
| 6-200351 | Jul 1994 | JP |
| 06-264183 | Sep 1994 | JP |
| 6-264183 | Sep 1994 | JP |
| 7-11382 | Jan 1995 | JP |
| 2000-336455 | Dec 2000 | JP |
| 2002-322539 | Nov 2002 | JP |
| 2002-322540 | Nov 2002 | JP |
| 2002-322541 | Nov 2002 | JP |
| 2002-322543 | Nov 2002 | JP |
| 2003-89848 | Mar 2003 | JP |
| 2003-138343 | May 2003 | JP |
| 2003-138344 | May 2003 | JP |
| 2003-321738 | Nov 2003 | JP |
| 2004-027249 | Jan 2004 | JP |
| 2004-143518 | May 2004 | JP |
| 2004-143518 | May 2004 | JP |
| 03010351 | Feb 2003 | WO |
| Entry |
|---|
| JP 2004-143518. Machine Translation. |
| Higgins, Raymond A., Engineering Metallurgy, Part I: Applied Physical Metallurgy, 6th edition, Arnold Publishing, 1993. Ch. 13, pp. 285-332. (58 pages total, including front matter and table of contents). |
| Higgins, Raymond A., Engineering Metallurgy, Part I: Applied Physical Metallurgy, Sixth edition, Arnold, 1993, Ch. 8, pp. 160-178. |
| Computer-Generated Translation of JP 06-264183, originally published in the Japanese language on Sep. 20, 1994. |
| Computer-generated translation of JP 2004-027249, published in Japanese on Jan. 29, 2004. |
| “Glossary of Metallurgical and Metalworking Terms,” Metals Handbook, ASM International, 2002, term(s): galvanize. |
| Number | Date | Country | |
|---|---|---|---|
| 20070119521 A1 | May 2007 | US |
