Patent Application
20240035105
This disclosure relates to a hot-dip galvanized steel sheet that is suitably used for automobile members and the like, and a method of manufacturing the hot-dip galvanized steel sheet.
In recent years, enhancement of fuel efficiency of automobiles has become an important issue from the viewpoint of global environment protection. Therefore, there is a growing trend to increase the strength and reduce the thickness of steel sheets used as materials for automobile members to reduce the weight of automotive bodies. Further, the steel sheets used for automobile members are formed into complicated shapes, so that they are required to have good workability.
In response to such a request, for example, JP 2012-172159 A (PTL 1) describes “a high-strength cold-rolled steel sheet with excellent uniform deformability and local deformability, which contains, in mass %,
Further, J P 2009-249733A (PTL 2) describes “a high-strength steel sheet having excellent hardenability with very little aging deterioration, which contains, in mass %,
However, from the viewpoint of rust resistance of automobile bodies, steel sheets used as materials for automobile members are sometimes subjected to zinc or zinc alloy coating or plating, such as hot-dip galvanizing.
However, when the steel sheets described in PTLs 1 and 2 are subjected to hot-dip galvanizing, the coating or plating quality such as coating or plating appearance and coating or plating adhesion may be insufficient. Therefore, it is desired to make improvement in this regard.
It could thus be helpful to provide a hot-dip galvanized steel sheet that has both high strength and good workability, as well as excellent coating quality.
It is also helpful to provide a method of manufacturing the hot-dip galvanized steel sheet.
As a result of intensive studies, we discovered the following.
(a) To obtain good workability, it is necessary to improve the hole expansion formability and the elongation of a steel sheet. From the viewpoint of preventing cracking during forming, it is effective to increase the yield ratio YR (=yield stress (YS)/tensile strength (TS)) of a steel sheet.
(b) To obtain high strength, it is effective to use martensite. On the other hand, the use of ferrite is effective in obtaining excellent elongation. Further, to obtain excellent hole expansion formability, it is necessary to reduce the hardness difference between ferrite, which is a soft phase, and martensite, which is a hard phase. This can be effectively achieved by using bainite, which is an intermediate phase. Further, the use of bainite increases the yield ratio.
(c) That is, when the steel microstructure is a complex structure in which ferrite, martensite and bainite are controlled to predetermined area ratios (hereinafter referred to simply as “complex structure”), it is possible to achieve both high strength and good workability.
(d) Furthermore, to obtain good coating or plating quality, it is effective to
That is, it is effective to use Si and Mn in terms of increasing the strength of a steel sheet. However, elements such as Si and Mn are oxidizable elements, which combine with oxygen to form oxides on the steel sheet surface. The presence of such Si and Mn oxides on the surface of the base steel sheet during the coating or plating treatment reduces the wettability of the base steel sheet by a coating or plating bath (hot dip zinc), causing poor coating or plating appearance such as non-coating or non-plating and deterioration of coating or plating adhesion.
In this regard, if internal oxidation is caused in the surface layer of the base steel sheet to form oxides of Si and Mn before the coating or plating treatment, these oxides in the surface layer of the base steel sheet serve as a barrier, and the formation of oxides on the surface of the base steel sheet (hereinafter referred to as external oxidation) is suppressed. As a result, the coating or plating quality such as coating or plating appearance and coating or plating adhesion is improved.
Further, the coating or plating quality, especially coating or plating adhesion, is improved by containing an appropriate amount of Fe in a hot-dip galvanized layer.
(e) In addition, it is important to properly control the annealing conditions prior to the coating or plating treatment and the coating or plating treatment conditions to create a complex structure as described above, to form oxides of Si and Mn in the surface layer of the base steel sheet by causing internal oxidation in the surface layer of the base steel sheet, and also to contain an appropriate amount of Fe in the hot-dip galvanized layer. It is particularly important to control the atmosphere during the holding of annealing and to control the temperature of the cold-rolled steel sheet when it enters the coating or plating bath in the coating or plating treatment.
Specifically, when the dew point is set in a range of −20° C. or higher and 5° C. or lower and a certain amount of oxygen is ensured in the holding atmosphere of annealing, the internal oxidation in the surface layer of the base steel sheet is promoted. On the other hand, when the hydrogen concentration is set to 3 mass % or more and 20 mass % or less, oxides that have been formed on the surface of the base steel sheet (and oxides that have been formed during the holding of annealing) are reduced. Therefore, it is important to suppress the external oxidation while introducing sufficient oxygen from the atmosphere into the interior (surface layer) of the base steel sheet. It is also important to promote the diffusion of Fe from the base steel sheet to the coated or plated layer by setting the temperature of the cold-rolled steel sheet when it enters the coating or plating bath to at least 10° C. higher than the coating or plating bath temperature.
The present disclosure is based on these discoveries and further studies.
We thus provide the following.
[1] A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, wherein
[2] The hot-dip galvanized steel sheet according to aspect [1], wherein the other metallic phases have an area ratio of 5% or less.
[3] The hot-dip galvanized steel sheet according to aspect [1] or [2], wherein the hot-dip galvanized layer contains Fe in an amount of 8.0 mass % or less.
[4] The hot-dip galvanized steel sheet according to any one of aspects [1] to [3], wherein the hot-dip galvanized layer has a coating weight of 20 g/m2 or more per surface.
[5] The hot-dip galvanized steel sheet according to any one of aspects [1] to [4], wherein the chemical composition of the base steel sheet further contains, in mass %, at least one selected from the group consisting of
[6] The hot-dip galvanized steel sheet according to any one of aspects [1] to [5], wherein the chemical composition of the base steel sheet further contains, in mass %,
[7] A method of manufacturing a hot-dip galvanized steel sheet, comprising:
According to the present disclosure, it is possible to obtain a hot-dip galvanized steel sheet having both high strength and good workability, as well as excellent coating quality.
By applying the hot-dip galvanized steel sheet of the present disclosure to automobile members, the performance of automobile bodies can be significantly improved.
The present disclosure will be described based on the following embodiments.
First, the chemical composition of a base steel sheet of a hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described. The “%” representations below indicating the chemical composition are in “mass %” unless stated otherwise.
C: 0.09% or More and 0.17% or Less
C is an element that improves the hardenability. C also plays a role in increasing the strength of ferrite. Therefore, it is required to contain C to ensure a desired tensile strength (TS) of 750 MPa or more. When the C content is less than 0.09%, the desired tensile strength cannot be obtained. Therefore, the C content is set to 0.09% or more. The C content is preferably 0.10% or more and more preferably 0.11% or more. On the other hand, if the C content exceeds 0.17%, the stability of austenite increases, and it is difficult to form bainite. In addition, the strength of martensite increases excessively, and the yield ratio decreases. Therefore, the C content is set to 0.17% or less. The C content is preferably 0.16% or less and more preferably 0.15% or less.
Si: 0.3% or More and 1.1% or Less
Si is a solid-solution-strengthening element. Si also plays a role in increasing the yield ratio by increasing the strength of ferrite. To obtain this effect, the Si content is set to 0.3% or more. The Si content is preferably 0.4% or more and more preferably 0.5% or more. On the other hand, if the Si content is too high, Si concentrates on the surface of the base steel sheet, causing external oxidation and deteriorating the coating quality such as coating appearance. Therefore, the Si content is set to 1.1% or less. The Si content is preferably 1.0% or less and more preferably 0.9% or less.
Mn: 1.9% or More and 2.7% or Less
Mn is an element that improves the hardenability of steel. Therefore, it is required to contain Mn to ensure the desired tensile strength. When the Mn content is less than 1.9%, the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.9% or more. The Mn content is preferably 2.0% or more and more preferably 2.1% or more. On the other hand, if the Mn content is too high, Mn concentrates on the surface of the base steel sheet, causing external oxidation and deteriorating the coating quality such as coating appearance. In addition, Mn tends to concentrate into austenite during, for example, the holding of annealing, and the strength of martensite that transforms from austenite excessively increases. Therefore, the Mn content is set to 2.7% or less. The Mn content is preferably 2.6% or less and more preferably 2.5% or less.
P: 0.10% or Less
P is an element that strengthens steel. However, if the P content is too high, P segregates to grain boundaries and deteriorates the hole expansion formability. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.05% or less and more preferably 0.03% or less. Although the lower limit of the P content is not particularly limited, it is preferably 0.001% or more from the viewpoint of cost, for example. The P content is more preferably 0.003% or more and even more preferably 0.005% or more.
S: 0.050% or Less
S is an element that deteriorates the elongation through the formation of MnS and the like. If Ti is contained together with S, the hole expansion formability may be deteriorated due to the formation of, for example, TiS and Ti(C,S). Therefore, the S content is set to 0.050% or less. The S content is preferably 0.030% or less, more preferably 0.020% or less, and even more preferably 0.015% or less. Although the lower limit of the S content is not particularly limited, it is preferably 0.0002% or more from the viewpoint of cost, for example. The S content is more preferably 0.0005% or more.
Al: 0.01% or More and 0.20% or Less
Al is an element added as a deoxidizing material. Al also plays a role in reducing coarse inclusions in the steel and improving the hole expansion formability. When the Al content is less than 0.01%, the above effect is insufficient. Therefore, the Al content is set to 0.01% or more. The Al content is preferably 0.02% or more. On the other hand, if the Al content exceeds 0.20%, nitride-based precipitates such as AlN are coarsened, and the hole expansion formability is deteriorated. Therefore, the Al content is set to 0.20% or less. The Al content is preferably 0.17% or less and more preferably 0.15% or less.
N: 0.10% or Less
N is an element that contributes to the improvement of hole expansion formability by forming nitride-based precipitates such as AlN that pin crystal grain boundaries. However, if the N content exceeds 0.10%, nitride-based precipitates such as AlN are coarsened, and the hole expansion formability is deteriorated. Therefore, the N content is set to 0.10% or less. The N content is preferably 0.05% or less and more preferably 0.010% or less. Although the lower limit of the N content is not particularly limited, it is preferably 0.0006% or more from the viewpoint of cost, for example. The N content is more preferably 0.0010% or more.
The base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a chemical composition containing the above elements and the balance of Fe (iron) and inevitable impurities. It is particularly preferable that the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a chemical composition containing the above elements, with the balance consisting of Fe and inevitable impurities.
The above describes the basic chemical composition of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure. The base steel sheet may contain, as optional elements, at least one selected from the group consisting of
Further, it may contain, as optional elements, at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr, where the selected elements are contained in a total amount of 0.1% or less.
If any of the above optional elements is contained in an amount less than the suitable lower limit described below, this element is regarded as an inevitable impurity.
Nb: 0.040% or Less
Nb contributes to increasing the strength through the refinement of prior γ grains and the formation of fine precipitates. In addition, the fine precipitates increase the strength of ferrite and contribute to increasing the yield ratio. To obtain this effect, the Nb content is preferably 0.0010% or more. The Nb content is more preferably 0.0015% or more and even more preferably 0.0020% or more. On the other hand, an excessively high Nb content results in an excessive amount of carbonitride-based precipitate, which deteriorates the hole expansion formability. Therefore, when Nb is contained, the Nb content is preferably 0.040% or less. The Nb content is more preferably 0.035% or less and even more preferably 0.030% or less.
Ti: 0.030% or Less
Ti, like Nb, contributes to increasing the strength through the refinement of prior γ grains and the formation of fine precipitates. In addition, the fine precipitates increase the strength of ferrite and contribute to increasing the yield ratio. To obtain this effect, the Ti content is preferably 0.0010% or more. The Ti content is more preferably 0.0015% or more and even more preferably 0.0020% or more. On the other hand, an excessively high Ti content results in an excessive amount of carbonitride-based precipitate, which deteriorates the hole expansion formability. Therefore, when Ti is contained, the Ti content is preferably 0.030% or less. The Ti content is more preferably 0.025% or less and even more preferably 0.020% or less.
B: 0.0030% or Less
B is an element that improves the hardenability of steel. The inclusion of B renders it possible to achieve the desired tensile strength even when the Mn content is low. To obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, a B content of 0.0030% or more results in an excessive amount of nitride-based precipitate such as BN, which deteriorates the hole expansion formability. Therefore, when B is contained, the B content is preferably 0.0030% or less. The B content is more preferably 0.0025% or less and even more preferably 0.0020% or less.
Cr: 0.3% or Less
Cr is an element that improves the hardenability of steel. To obtain this effect, the Cr content is preferably 0.005% or more. However, an excessively high Cr content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when Cr is contained, the Cr content is preferably 0.3% or less. The Cr content is more preferably 0.2% or less and even more preferably 0.1% or less.
Mo: 0.2% or Less
Mo, like Cr, is an element that improves the hardenability of steel. To obtain this effect, the Mo content is preferably 0.005% or more. However, an excessively high Mo content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when Mo is contained, the Mo content is preferably 0.2% or less. The Mo content is more preferably 0.1% or less and even more preferably 0.04% or less.
V: 0.065% or Less.
V, like Cr, is an element that improves the hardenability of steel. To obtain this effect, the V content is preferably 0.005% or more. However, an excessively high V content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when V is contained, the V content is preferably 0.065% or less. The V content is more preferably 0.050% or less and even more preferably 0.035% or less.
At least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of 0.1% or less.
Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr are elements that increase the strength without deteriorating the coating quality. To obtain this effect, the content of these elements is preferably 0.0010% or more, either singly or in total. However, when the total content of these elements exceeds 0.1%, the above effect is saturated. Therefore, when at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr are contained, the total content of these elements is preferably 0.1% or less.
The balance other than the aforementioned elements is Fe and inevitable impurities.
Next, the steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.
The steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure is a complex structure where
Area Ratio of Ferrite: 30% or More and 85% or Less
Ferrite is a necessary phase from the viewpoint of obtaining desired elongation. Therefore, the area ratio of ferrite is set to 30% or more. The area ratio of ferrite is preferably 35% or more and more preferably 40% or more. On the other hand, an excess of ferrite reduces the area ratio of martensite required to ensure the strength, rendering it difficult to ensure the strength. It also suppresses the formation of bainite and reduces the hole expansion formability and the yield ratio. Therefore, the area ratio of ferrite is set to 85% or less. The area ratio of ferrite is preferably 80% or less.
As used herein, the ferrite is a microstructure containing crystal grains of BCC lattice, which is formed by transformation from austenite at relatively high temperatures.
Area Ratio of Martensite: 5% or More and 30% or Less
Martensite contributes to the improvement of strength and is a phase necessary for ensuring the desired tensile strength. Therefore, the area ratio of martensite is set to 5% or more. The area ratio of martensite is preferably 8% or more and more preferably 10% or more. On the other hand, an excess of martensite deteriorates the elongation. Therefore, the area ratio of martensite is set to 30% or less. The area ratio of martensite is preferably 28% or less and more preferably 25% or less.
As used herein, the martensite refers to a hard microstructure formed from austenite at or below the martensite transformation temperature (also referred to simply as “Ms point”), which includes both so-called fresh martensite as quenched and so-called tempered martensite where fresh martensite is reheated and tempered.
Area Ratio of Bainite: 10% or More and 60% or Less
Bainite is a phase necessary for improving the hole expansion formability and increasing the yield ratio. Therefore, the area ratio of bainite is set to 10% or more. The area ratio of bainite is preferably 15% or more and more preferably 20% or more. On the other hand, an excess of bainite deteriorates the elongation. Therefore, the area ratio of bainite is set to 60% or less. The area ratio of bainite is preferably 55% or less and more preferably 50% or less.
As used herein, the bainite is a hard microstructure in which fine carbides are dispersed in needle-like or plate-like ferrite, and it is formed from austenite at relatively low temperatures (at or above the martensitic transformation temperature).
Area Ratio of Other Metallic Phases: 15% or Less
The steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure may contain metallic phases other than martensite, ferrite, and bainite. It is acceptable if the total area ratio of other metallic phases is 15% or less. Therefore, the area ratio of other metallic phases is set to 15% or less. The area ratio of other metallic phases is preferably 10% or less and more preferably 5% or less. The area ratio of other metallic phases may be 0%.
Examples of the other metallic phases include pearlite, retained austenite, and non-recrystallized ferrite. Among these phases, pearlite and non-recrystallized ferrite deteriorate the workability (El and λ), so that the total area ratio of pearlite and non-recrystallized ferrite is set to 5% or less. The area ratios of pearlite and non-recrystallized ferrite may each be 0%. Because retained austenite does not deteriorate the workability (El and λ), there is no problem if the area ratio of retained austenite is 15% or less. The area ratio of retained austenite is preferably 10% or less and more preferably 5% or less. The area ratio of retained austenite may be 0% or less.
As used herein, the pearlite is a microstructure containing ferrite and needle-like cementite. The retained austenite is austenite remaining without being transformed into martensite. The non-recrystallized ferrite is ferrite that is not recrystallized, in which crystal grains include sub-boundaries.
As used herein, the area ratio of each phase is measured as follows.
A test piece is collected from the base steel sheet of the hot-dip galvanized steel sheet so that an L-section parallel to the rolling direction serves as a test surface. Next, the test surface of the test piece is subjected to mirror polishing, and the microstructure is revealed with a nital solution. The test surface of the test piece with the revealed microstructure is observed with a SEM at a magnification of 1500×, and the area ratio of martensite, the area ratio of ferrite, and the area ratio of bainite at the ¼ thickness position of the base steel sheet are measured with a point counting method.
In the SEM image, martensite is a white microstructure. Further, fine carbides are precipitated inside tempered martensite among the martensite. Ferrite is a black microstructure. Bainite has white carbides precipitated in a black microstructure. Each phase in the SEM image is identified based on the above description. However, depending on the plane orientation of block grains and the degree of etching, it may be difficult to reveal the internal carbides. In that case, etching is thoroughly performed for confirmation.
The total area ratio of the other metallic phases is calculated by subtracting the area ratio of martensite, the area ratio of ferrite, and the area ratio of bainite from 100%.
Among the other metallic phases, pearlite is a microstructure containing ferrite and needle-like cementite as described above. Based on this, pearlite is identified in the SEM image, and the area ratio of pearlite is measured. Non-recrystallized ferrite has sub-boundaries inside crystal grains as described above. Based on this, non-recrystallized ferrite is identified in the SEM image, and the area ratio of non-recrystallized ferrite is measured.
The Area Ratio of Retained Austenite is Measured as Follows.
The base steel sheet of the hot-dip galvanized steel sheet is polished in the thickness direction (depth direction) to the ¼ thickness position and then chemically polished by 0.1 mm to obtain an observation plane. Next, the observation plane is observed with the X-ray diffraction method. Using a Mo Kα source as an incident X-ray, ratios of the diffraction intensity of each of (200), (220) and (311) planes of fcc iron (austenite) to the diffraction intensity of each of (200), (211), and (220) planes of bcc iron are determined, and the volume fraction of retained austenite is calculated based on the ratio of diffraction intensity of each plane. Next, assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area ratio of retained austenite.
Amount of oxygen present as oxide in the surface layer of the base steel sheet (hereinafter also referred to as “amount of oxygen in oxide form in the surface layer of the base steel sheet”): 0.05 g/m2 or more and 0.50 g/m2 or less per surface
As described above, it is effective to use Si and Mn in terms of increasing the strength of a steel sheet. However, elements such as Si and Mn are oxidizable elements, which combine with oxygen to form oxides on the steel sheet surface. The presence of such Si and Mn oxides on the surface of the base steel sheet during coating treatment reduces the wettability of the base steel sheet by a coating bath (hot-dip zinc), causing poor coating appearance such as non-coating and deterioration of coating adhesion.
In this regard, if internal oxidation is caused in the surface layer of the base steel sheet to form oxides of Si and Mn before the coating treatment, these oxides present in the surface layer of the base steel sheet serve as a barrier, and the formation of oxides on the surface of the base steel sheet (hereinafter referred to as “external oxidation”) is suppressed. As a result, the coating quality such as coating appearance and coating adhesion is improved. Therefore, the amount of oxygen in oxide form in the surface layer of the base steel sheet is set to 0.05 g/m2 or more per surface (note that all the amount of oxygen described below is the amount for one surface). The amount of oxygen in oxide form in the surface layer of the base steel sheet is preferably 0.06 g/m2 or more. On the other hand, if the amount of oxygen in oxide form in the surface layer of the base steel sheet exceeds 0.50 g/m2, the oxides promote fracture and deteriorate the elongation and the hole expansion formability. Therefore, the amount of oxygen in oxide form in the surface layer of the base steel sheet is set to 0.50 g/m2 or less. The amount of oxygen in oxide form in the surface layer of the base steel sheet is preferably 0.45 g/m2 or less.
As used herein, the surface layer is an area from the surface of the base steel sheet to a position at a depth of 100 μm.
Oxides are compounds of oxygen and elements such as Si, Mn, Fe, P, Al, Nb, Ti, B, Cr, Mo, and V contained in the base steel sheet, and the oxides are mainly Si oxides and Mn oxides.
The amount of internal oxidation is inversely related to the amount of external oxidation. Therefore, if external oxidation occurs in the base steel sheet, the amount of oxygen in oxide form in the surface layer of the base steel sheet is less than 0.05 g/m2.
The amount of oxygen in oxide form in the surface layer of the base steel sheet is measured with an “impulse furnace-infrared absorption method”.
First, the hot-dip galvanized layer is removed from the hot-dip galvanized steel sheet. The method of removing the hot-dip galvanized layer is not limited if the hot-dip galvanized layer can be totally removed. Examples thereof include pickling, alkali dissolution, and mechanical polishing.
Next, the amount of oxygen in the steel of the base steel sheet is measured. The measured value is taken as the total amount of oxygen OI (g) contained in the base steel sheet.
Next, at least the surface layers (an area from the surface of the base steel sheet to a position at a depth of 100 μm) on both sides of the base steel sheet is removed by polishing, and the amount of oxygen in the steel of the base steel sheet is measured after the surface layers have been removed. The measured value is taken as OH (g).
The amount of oxygen in oxide form in the surface layer of the base steel sheet is calculated based on the following formula.
[Amount of oxygen in oxide form in the surface layer of the base steel sheet]{OI(g)−OH(g)×([thickness of the base steel sheet before polishing(mm)]/[thickness of the base steel sheet after polishing(mm)])}÷([surface area of the base steel sheet(per surface)(m2)]÷2
In the above formula, the amount of oxygen in oxide form in the surface layer of the base steel sheet is calculated by
The thickness of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure is preferably 0.2 mm or more. The thickness is preferably 3.2 mm or less.
Next, the hot-dip galvanized layer of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.
Fe Content in Hot-Dip Galvanized Layer: 0.40 Mass % or More
It is preferable to contain a large amount of Fe in the hot-dip galvanized layer to improve the coating adhesion. Therefore, the Fe content in the hot-dip galvanized layer is set to 0.40 mass % or more. The Fe content in the hot-dip galvanized layer is preferably 0.50 mass % or more. On the other hand, an excess of Fe in the hot-dip galvanized layer results in the formation of a hard Fe—Zn alloy phase in the hot-dip galvanized layer. As a result, the coating itself is likely to be broken, resulting in deterioration of coating adhesion. Therefore, the Fe content in the hot-dip galvanized layer is preferably 8.0 mass % or less. The Fe content in the hot-dip galvanized layer is more preferably 7.5 mass % or less and even more preferably 7.0 mass % or less.
Coating Weight in Hot-Dip Galvanized Layer: 20 g/m2 or More Per Surface
A large coating weight is desirable to improve the corrosion resistance. Therefore, the coating weight is preferably 20 g/m2 or more per surface (note that all the coating weight described below is the amount for one surface). The coating weight is more preferably 25 g/m2 or more and even more preferably 30 g/m2 or more. The upper limit of the coating weight is not particularly limited. However, if the coating weight exceeds 120 g/m2, the above effect is saturated. Therefore, the coating weight is preferably 120 g/m2 or less.
The Fe content and the coating weight in the hot-dip galvanized layer are measured as follows.
After degreasing the surface of the hot-dip galvanized steel sheet as a test piece, the mass of the test piece is weighed for the first time. Next, two or three drops of inhibitor, which is a corrosion inhibitor for Fe, are added to 30 cc of 1:3 HCl solution (HCl solution with a concentration of 25 vol. %), and then the test piece is immersed in the solution to dissolve the hot-dip galvanized layer of the test piece. After dissolving the hot-dip galvanized layer (when there is no more H2 gas formed on the surface of the test piece), the solution is collected. After the test piece is collected and dried, the mass of the test piece is weighed for the second time.
The coating weight is calculated by the following formula.
[Coating weight(g/m2)]([mass of the test piece weighed for the first time(g)]−[mass of the test piece weighed for the second time(g)])÷[coated area of the test piece(area covered by the hot-dip galvanized layer in the test piece before dissolving the hot-dip galvanized layer)(m2)]
The masses of Fe, Zn, and Al dissolved in the collected solution (hereinafter referred to as dissolved amount of Fe, dissolved amount of Zn, and dissolved amount of Al) are measured with the inductively coupled plasma (ICP) method, and the Fe content in the hot-dip galvanized layer is determined by the following formula.
[Fe content in the hot-dip galvanized layer(mass %)[dissolved amount of Fe(g)]/([dissolved amount of Fe(g)]+[dissolved amount of Zn(g)]+[dissolved amount of Al(g)])×100
The hot-dip galvanized layer is mainly composed of Zn and is basically composed of Zn and the aforementioned Fe. Depending on the composition of the coating bath, the hot-dip galvanized layer may contain 0.30 mass % or less, specifically 0.15 mass % to 0.30 mass %, of Al. The balance other than Zn, Fe and Al is inevitable impurities. The hot-dip galvanized layer may be provided on only one side or on both sides of the base steel sheet.
Next, the mechanical properties of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.
The hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a tensile strength (TS) of 750 MPa or more. The tensile strength (TS) is preferably 780 MPa or more. Although the upper limit of the tensile strength is not particularly limited, a tensile strength of less than 980 MPa is preferred considering the balance with other properties.
Further, from the viewpoint of workability,
As used herein, the tensile strength (TS), the yield stress (YS), and the elongation (El) are measured as follows.
A JIS No. 5 test piece with a gauge length of 50 mm and a gauge width of 25 mm is collected from the center of the width of the hot-dip galvanized steel sheet, with the rolling direction being the longitudinal direction. Next, the collected JIS No. 5 test piece is subjected to a tensile test in accordance with the provisions of JIS Z 2241 (2011) to measure the tensile strength (TS), the yield stress (YS), and the elongation (El). The tensile speed is 10 mm/min.
Further, λ is the maximum hole expansion ratio (%), which is measured as follows.
A 100 mm square test piece is collected from the center of the width of the hot-dip galvanized steel sheet. Next, the collected test piece is subjected to a hole expanding test according to the Japan Iron and Steel Federation standard JFST1001 to measure k. Specifically, after punching a hole with a diameter of 10 mm in the test piece, a 60-degree conical punch is pressed into the hole while the surrounding area is being restrained, and the diameter of the hole at the crack initiation limit is measured. The maximum hole expansion ratio λ (%) is determined by the following formula.
Maximum hole expansion ratioλ(%)={(Df−D0)/D0}×100
“Excellent coating quality” means that there is no peeling of the hot-dip galvanized layer in a ball impact test under the following conditions, and that there is no non-coating defect in the hot-dip galvanized layer (preferably, there is no uneven coating appearance) found by appearance observation. The non-coating defect refers to an area of several micrometers to several millimeters in size where the base steel sheet is exposed without the hot-dip galvanized layer.
Conditions of Ball Impact Test
Next, a method of manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.
The method of manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present disclosure comprises
In the following description, “temperature” is the surface temperature of the steel sheet or slab unless otherwise specified. The surface temperature of the steel sheet or slab is measured, for example, using a radiation thermometer.
Hot Rolling Process
In this process, a steel material (steel slab) having the chemical composition described above is subjected to hot rolling to obtain a hot-rolled steel sheet.
The steel material used is preferably obtained by continuous casting to prevent macro-segregation of components. The steel material can also be obtained by ingot casting or thin slab casting.
The following describes the optimum manufacturing conditions of the hot rolling process.
Slab Heating Temperature: 1200° C. or Higher
If the heating temperature of the slab is lower than 1200° C., precipitates such as AlN are not sufficiently dissolved. As a result, precipitates such as AlN may be coarsened during the hot rolling, which deteriorates the hole expansion formability. Therefore, the heating temperature of the slab is preferably 1200° C. or higher. The heating temperature of the slab is more preferably 1230° C. or higher and even more preferably 1250° C. or higher. The upper limit of the heating temperature of the slab is not particularly limited, but 1400° C. or lower is preferred. The heating temperature of the slab is more preferably 1350° C. or lower.
Rolling Finish Temperature: 840° C. or Higher and 900° C. or Lower
If the rolling finish temperature is lower than 840° C., inclusions and coarse carbides may be formed, which deteriorates the hole expansion formability. The quality of the interior of the base steel sheet may also be deteriorated. Therefore, the rolling finish temperature is preferably 840° C. or higher. The rolling finish temperature is more preferably 860° C. or higher. On the other hand, if the holding time at high temperatures is increased, coarse inclusions may be formed, which deteriorates the hole expansion formability. Therefore, the rolling finish temperature is preferably 900° C. or lower. The rolling finish temperature is more preferably 880° C. or lower.
Coiling Temperature: 450° C. or Higher and 650° C. or Lower
The steel material is subjected to hot rolling as described above to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is coiled. When the coiling temperature is higher than 650° C., the surface of the steel substrate may be decarburized. This may cause a difference in microstructure between the interior and the surface of the base steel sheet, resulting in uneven alloy concentration. Further, coarse carbides and nitrides may be formed, which deteriorates the hole expansion formability. Therefore, the coiling temperature is preferably 650° C. or lower. The coiling temperature is more preferably 630° C. or lower. On the other hand, the coiling temperature is preferably 450° C. or higher to prevent deterioration of cold rolling manufacturability. The coiling temperature is more preferably 470° C. or higher.
The hot-rolled steel sheet may be subjected to pickling after coiling. The conditions of the pickling are not particularly limited, and conventional methods may be followed. Further, the hot-rolled steel sheet may be subjected to heat treatment after coiling to soften the microstructure.
Cold Rolling Process
In this process, the hot-rolled steel sheet obtained in the hot rolling process is subjected to cold rolling to obtain a cold-rolled steel sheet. There is no limit on the cold rolling ratio if the sheet thickness is controlled within a desired range. However, if the cold rolling ratio is too small, it is difficult to cause recrystallization in the subsequent annealing process. That is, non-recrystallized ferrite may be formed, which deteriorates the elongation. Therefore, the cold rolling ratio is preferably 20% or more. The cold rolling ratio is more preferably 30% or more. On the other hand, if the cold rolling ratio is too high, it is also difficult to cause recrystallization in the subsequent annealing process due to excessive strain. That is, non-recrystallized ferrite may be formed, which deteriorates the elongation. Therefore, the cold rolling ratio is preferably 90% or less. The cold rolling ratio is more preferably 80% or less.
Annealing Process
In this process, the cold-rolled steel sheet obtained in the cold rolling process is heated to an annealing temperature, held at the annealing temperature, and then cooled.
Further, from the viewpoint of creating a complex structure as described above, forming oxides of Si and Mn in the surface layer of the base steel sheet by causing internal oxidation in the surface layer of the base steel sheet, and containing an appropriate amount of Fe in the hot-dip galvanized layer, it is important in this process to set
Average heating rate: 1° C./s or higher and 7° C./s or lower
The average heating rate is preferably a low rate so that ferrite is recrystallized and the desired area ratio of ferrite is ensured. Therefore, the average heating rate is set to 7° C./s or lower. The average heating rate is preferably 6° C./s or lower and more preferably 5° C./s or lower. On the other hand, as the average heating rate decreases, Mn, which diffuses at a low rate, also concentrates into austenite and stabilizes the austenite. As a result, it is difficult to cause bainite transformation, and the desired complex structure cannot be obtained. Therefore, the average heating rate is set to 1° C./s or higher. The average heating rate is preferably 2° C./s or higher and more preferably 3° C./s or higher.
Annealing temperature: (AC1 point+50° C.) or higher and (AC3 point+20° C.) or lower
If the annealing temperature is lower than (AC1 point+50° C.), coarse Fe-based precipitates are formed, which deteriorates the strength and the hole expansion formability. Therefore, the annealing temperature is set to (AC1 point+50° C.) or higher. The annealing temperature is preferably (AC1 point+60° C.) or higher. On the other hand, if the annealing temperature exceeds (AC3 point+20° C.), the area ratio of ferrite decreases, and the elongation deteriorates. Therefore, the annealing temperature is set to (AC3 point+20° C.) or lower. The annealing temperature is preferably (AC3 point+10° C.) or lower.
As used herein, the AC1 point and the AC3 point are calculated by the following formulas, respectively. Note that in the following formulas, (% element symbol) refers to the content (mass %) of each element in the chemical composition of the base steel sheet. If the element is not contained (including cases where it is inevitably contained), it is calculated as 0.
AC1=723+22(% Si)−18(% Mn)+17(% Cr)+4.5(% Mo)+16(% V)
AC3=910−203√(% C)+45(% Si)−30(% Mn)−20(% Cu)−15(% Ni)+11(% Cr)+32(% Mo)+104(% V)+400(% Ti)+460(% Al)
The annealing temperature may be constant during the holding. The annealing temperature may not be constant during the holding, if it is within the above temperature range and the temperature fluctuation range is within ±10° C. of the set temperature.
Annealing time: 1 second or longer and 40 seconds or shorter
The annealing time is an important condition to transform austenite to bainite. From the viewpoint of avoiding concentration of Mn in austenite, i.e., avoiding excessive stabilization of austenite and obtaining an appropriate amount of bainite, the annealing time is preferably short. Therefore, the annealing time is set to 40 seconds or shorter. The annealing time is preferably 30 seconds or shorter and more preferably 25 seconds or shorter. On the other hand, if the annealing time is shorter than 1 second, recrystallization of ferrite is not promoted, resulting in deteriorated hole expansion formability. Therefore, the annealing time is set to 1 second or longer. The annealing time is preferably 5 seconds or longer. The annealing time is the holding time at the annealing temperature.
Dew Point of Holding Atmosphere: −20° C. or Higher and 5° C. or Lower
As described above, it is necessary to ensure a certain amount of oxygen in the holding atmosphere to cause internal oxidation in the surface layer of the base steel sheet and to form appropriate amounts of Si and Mn oxides in the surface layer of the base steel sheet. Further, it is necessary to raise the dew point to some extent from the viewpoint of ensuring an appropriate amount of Fe in the hot-dip galvanized layer. Therefore, the dew point of the holding atmosphere is set to −20° C. or higher. The dew point of the holding atmosphere is preferably −18° C. or higher and more preferably −15° C. or higher. On the other hand, if the dew point is too high, excessive internal oxidation is caused in the surface layer of the base steel sheet, which deteriorates the elongation and the hole expansion formability. If the dew point is too high, iron diffusion is excessively promoted during the coating treatment, resulting in excessive diffusion of iron in the coated layer. Therefore, the dew point of the holding atmosphere is set to 5° C. or lower. The dew point of the holding atmosphere is preferably 0° C. or lower.
Hydrogen Concentration in Holding Atmosphere: 3 Mass % or More and 20 Mass % or Less.
To promote internal oxidation in the surface layer of the base steel sheet and to ensure the coating weight of the hot-dip galvanized layer, the oxides formed on the surface of the base steel sheet (and formed during the holding of the annealing process) need to be reduced. Therefore, the hydrogen concentration in the holding atmosphere is set to 3 mass % or more. The hydrogen concentration in the holding atmosphere is preferably 5 mass % or more. On the other hand, if the hydrogen concentration in the holding atmosphere is too high, hydrogen penetrates into the steel, and the elongation and the hole expansion formability are deteriorated. Therefore, the hydrogen concentration in the holding atmosphere is set to 20 mass % or less. The hydrogen concentration in the holding atmosphere is preferably 17 mass % or less.
Primary Cooling Rate: 10° C./s or Higher
During the cooling process in a temperature range from the annealing temperature to the primary cooling stop temperature, it is necessary to properly control the cooling rate to form bainite. That is, if the primary cooling rate is low, pearlite is formed in addition to ferrite, and an appropriate amount of bainite cannot be obtained. Therefore, the primary cooling rate is set to 10° C./s or higher. The primary cooling rate is preferably 12° C./s or higher and more preferably 15° C./s or higher. The upper limit of the primary cooling rate is not limited, because a high primary cooling rate is preferred to suppress pearlite transformation. For example, there is no problem if the primary cooling rate reaches 2000° C./s or higher by water cooling or like.
Primary Cooling Stop Temperature: 450° C. or Higher and 600° C. or Lower
The primary cooling stop temperature is set to 450° C. or higher and 600° C. or lower to suppress pearlite transformation during the primary cooling and to ensure the specified amount of bainite during the secondary cooling. That is, if the primary cooling stop temperature exceeds 600° C., pearlite transformation is accelerated during the secondary cooling. Therefore, the primary cooling stop temperature is set to 600° C. or lower. The primary cooling stop temperature is preferably 580° C. or lower and more preferably 560° C. or lower. On the other hand, if the primary cooling stop temperature is lower than 450° C., bainite transformation is suppressed during the secondary cooling, rendering it difficult to ensure the specified fraction of bainite. Therefore, the primary cooling stop temperature is set to 450° C. or higher. The primary cooling stop temperature is preferably 460° C. or higher and more preferably 470° C. or higher.
Secondary Cooling Time: 20 Seconds or Longer and 100 Seconds or Shorter
In the secondary cooling process from the primary cooling stop temperature to the secondary cooling stop temperature following the primary cooling process, it is necessary properly control the secondary cooling time to form bainite. That is, a long secondary cooling time promotes bainite transformation. Therefore, the secondary cooling time is set to 20 seconds or longer. The secondary cooling time is preferably 25 seconds or longer and more preferably 30 seconds or longer. On the other hand, if the secondary cooling time is too long, bainite is excessively formed, and the area ratio of martensite necessary for ensuring strength cannot be obtained. Therefore, the secondary cooling time is set to 100 seconds or shorter. The secondary cooling time is preferably 90 seconds or shorter and more preferably 80 seconds or shorter.
Secondary Cooling Stop Temperature: 400° C. or Higher and 500° C. or Lower
The secondary cooling stop temperature is set to 400° C. or higher and 500° C. or lower from the viewpoint of ensuring the specified fraction of bainite and controlling the temperature of the cold-rolled steel sheet when it enters the coating bath in the coating treatment process, which will be described later, within the specified range. That is, if the secondary cooling stop temperature exceeds 500° C., bainite transformation is accelerated during the secondary cooling, and the fraction of bainite becomes too high. Therefore, the secondary cooling stop temperature is set to 500° C. or lower. The secondary cooling stop temperature is preferably 495° C. or lower and more preferably 490° C. or lower. On the other hand, if the secondary cooling stop temperature is lower than 400° C., it is difficult to control the temperature of the cold-rolled steel sheet when it enters the coating bath to a temperature at least 10° C. higher than the coating bath temperature even if heat treatment is applied immediately before the coating treatment, especially in a case of using a continuous annealing hot-dip galvanizing line (CGL). Therefore, the secondary cooling stop temperature is set to 400° C. or higher. The secondary cooling stop temperature is preferably 420° C. or higher and more preferably 440° C. or higher.
Coating Treatment Process
In this process, the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment after the annealing treatment.
Further, in this process, it is important that the temperature of the cold-rolled steel sheet when it enters the coating bath be at least 10° C. higher than the coating bath temperature.
Temperature of the cold-rolled steel sheet when it enters the coating bath: coating bath temperature+10° C. or higher
To ensure an appropriate amount of Fe in the hot-dip galvanized layer, it is necessary to control the temperature of the cold-rolled steel sheet when it enters the coating bath higher than the coating bath temperature, especially to a temperature at least 10° C. higher than the coating bath temperature. The temperature of the cold-rolled steel sheet when it enters the coating bath is preferably at least 15° C. higher than the coating bath temperature and more preferably at least 20° C. higher than the coating bath temperature. The upper limit of the temperature of the cold-rolled steel sheet when it enters the coating bath is not particularly limited, but it is preferably 500° C. or lower.
The coating bath is basically composed of Zn, and it may contain 0.15 mass % to 0.30 mass % of Al. The balance other than Zn and Al is inevitable impurities.
The Coating Bath Temperature is Preferably 440° C. to 500° C.
In addition, the annealing process and the coating treatment process may be performed on a continuous annealing line (CAL) or on a continuous annealing hot-dip galvanizing line (CGL). Each process may be performed by batch processing.
The conditions of each process other than the above are not limited, and conventional methods may be followed. After the annealing process, temper rolling may be performed for shape adjustment.
According to the above manufacturing method, it is possible to obtain a hot-dip galvanized steel sheet that has both high strength and good workability as well as excellent coating quality, and this hot-dip galvanized steel sheet can be suitably used for automotive members.
Steel materials having the chemical compositions listed in Table 1 (with the balance being Fe and inevitable impurities) were melted in a vacuum melting furnace and then subjected to blooming to obtain bloomed materials with a thickness of 27 mm. The obtained bloomed materials were subjected to hot rolling under the conditions listed in Table 2 to obtain hot-rolled steel sheets with a thickness of 4.0 mm. Next, the obtained hot-rolled steel sheets were ground to a thickness of 3.0 mm, and then they were subjected to cold rolling under the conditions listed in Table 2 to obtain cold-rolled steel sheets with a thickness 0.9 mm to 1.8 mm. Next, the obtained cold-rolled steel sheets were subjected to annealing and coating treatment under the conditions listed in Table 2 to obtain hot-dip galvanized steel sheets with hot-dip galvanized layers on both sides. Blank cells in Table 1 indicate that the element is not intentionally added (it is not necessarily 0 mass %, and it may be inevitably contained).
Next, each obtained hot-dip galvanized steel sheet was used to identify the microstructure in the base steel sheet, measure the amount of oxygen in oxide form in the surface layer of the base steel sheet, and measure the coating weight and the Fe content per surface in the hot-dip galvanized layer, according to the procedure described above.
The results are listed in Table 3.
For the identification of the microstructure in the base steel sheet (point counting method), 16×15 grids were evenly spaced over an area to be observed by a SEM (an area of 82 μm×57 μm). The number of grid points in each phase was counted, and a ratio of the number of grid points occupied by each phase to the total number of grid points was taken as the area ratio of each phase. Note that the area ratio of each phase was the average value of the area ratios of each phase obtained from three separate SEM images.
Further, the mechanical properties of each of the obtained hot-dip galvanized steel sheets were measured according to the procedure described above. The results are listed in Table 4.
The Desired Tensile Strength (TS) is 750 MPa or More.
From the viewpoint of workability, the desired TS×El is 18000 MPa·% 20 or more, TS×λ is 40000 MPa·% or more, and yield ratio YR (=YS/TS) is 0.55 or more.
Furthermore, the coating quality (coating adhesion and coating appearance) of each of the obtained hot-dip galvanized steel sheets was examined according to the procedure described above and evaluated according to the following criteria. The evaluation results are listed in Table 4.
Coating Adhesion
Coating Appearance
As listed in Table 4, all Examples had both high strength and good workability, as well as excellent coating quality.
On the other hand, at least one of strength, workability and coating quality was insufficient in Comparative Examples.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-215782 | Dec 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/046368 | 12/15/2021 | WO |
