CN117751204A - Cold rolled steel plate and manufacturing method thereof - Google Patents

Abstract

The cold-rolled steel sheet has a predetermined chemical composition, and when a range of 1/8 to 3/8 of the sheet thickness from the surface in the sheet thickness direction is set as a t/4 portion and a range of 20 [ mu ] m from the surface in the sheet thickness direction is set as a surface layer portion, the microstructure in the t/4 portion includes, in terms of volume ratio: 0% -10.0% of residual austenite; and 1 or 2 of 90.0 to 100% of martensite and tempered martensite, wherein the ratio of the dislocation density of the surface layer portion to the dislocation density of the t/4 portion is 0.80 or more, the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is 0.90 or more, and the cold-rolled steel sheet has a tensile strength of 1310MPa or more.

Description

Cold rolled steel plate and manufacturing method thereof

Technical field

The present invention relates to cold-rolled steel plates and their manufacturing methods.

This application claims priority based on Japanese Patent Application No. 2021-120895 filed in Japan on July 21, 2021, the contents of which are incorporated herein by reference.

Background technique

In today's highly differentiated industrial technology field, materials used in each technical field are required to have special and high-level properties. For example, steel plates for automobiles are required to have high strength from considerations of the global environment and in order to improve fuel efficiency by reducing the weight of the vehicle body. When a high-strength steel plate is applied to an automobile body, the thickness of the steel plate can be reduced to reduce the weight of the vehicle body, while also imparting desired strength to the vehicle body.

In recent years, the requirements for automobile steel sheets have become more advanced. Among automobile steel sheets, especially cold-rolled steel sheets used in vehicle body frame components, high strength is required, and steel sheets with a tensile strength of 1310 MPa or more are required.

In response to the above requirements, for example, Patent Document 1 discloses, as a high-strength steel plate used for automobile parts and the like, a high-strength steel plate with excellent delayed fracture resistance and a tensile strength of 1470 MPa or more, which has a specified Composition: It has a specified steel plate structure mainly composed of martensite and bainite, and the average number of inclusions with an average particle size of 5 μm or more in a cross section perpendicular to the rolling direction is 5.0 inclusions/ ^mm2 or less. .

In addition, Patent Document 2 discloses a thin steel plate having a steel structure having a ferrite area ratio of 30% or less (including 0%) and a bainite area ratio of 5% or less (including 0%). , the area ratio of martensite and tempered martensite is more than 70% (including 100%), the area ratio of retained austenite is less than 2.0% (including 0%), and the position is within the range of 0 to 20 μm from the surface of the steel plate. The ratio of dislocation density to the dislocation density in the center of the plate thickness is 90% to 110%, and the average cementite particle size within the upper 10% from the surface of the steel plate to a depth of 100 μm is 300 nm or less, where along the length direction of the steel plate The maximum warpage of the steel plate when cut to a length of 1m is 15mm or less. Patent Document 2 discloses that this thin steel plate has a tensile strength of 980 MPa or more and can also obtain a tensile strength of 2000 MPa or more.

In addition, Patent Document 3 discloses a high-strength steel plate with excellent delayed fracture resistance, whose chemical composition (C, Si, Mn, Al, P, S) satisfies the prescribed range, and the remainder contains iron and unavoidable The impurities account for more than 95 area% of martensite in the entire structure, and the structure from a position at a depth of 10 μm in the thickness direction from the surface of the steel plate to a position at a depth of 1/4 of the plate thickness meets the requirements. The relational formula, and the tensile strength of the high-strength steel plate is above 1180MPa.

existing technical documents

patent documents

Patent Document 1: Japanese Patent No. 6729835

Patent Document 2: International Publication No. 2020/026838

Patent Document 3: Japanese Patent Application Publication No. 2013-104081

Contents of the invention

Invent the problem to be solved

As mentioned above, high-strength steel plates having a tensile strength of 1310 MPa or more have been proposed in the past. Generally speaking, the main structure of such high-strength steel sheets is martensite and/or tempered martensite.

As a result of research by the inventors of the present invention, they found that when a high-strength steel plate whose main structure is martensite or tempered martensite is applied with a load that causes deformation, the load is removed, the load is left for a certain period of time, and then the steel plate is again When a load is applied, the flow stress when the load is applied again becomes lower than the flow stress when the load is first applied (hereinafter sometimes simply referred to as flow stress reduction). However, in Patent Documents 1 to 3, there is no study on the reduction of flow stress when a load is applied again, and there is room for improvement.

The present invention was completed in view of the above. An object of the present invention is to provide a cold-rolled steel sheet having a structure mainly composed of martensite and tempered martensite, and a case where a load is applied, the load is removed, the load is left to stand for a certain period, and then the load is applied again. , the flow stress when the load is applied again can be suppressed from becoming lower than the flow stress when the load is first applied (the decrease in the flow stress is suppressed).

Means used to solve problems

The inventors of the present invention studied the reasons why the above-mentioned reduction in flow stress occurs. As a result, it was found that even if the structure is martensite and/or tempered martensite throughout the plate thickness direction, the flow stress is reduced when the dislocation density in the structure is different depending on the position in the plate thickness direction.

In addition, the inventors of the present invention conducted further studies and found that even when the difference in dislocation density at each position in the plate thickness direction is small, the flow stress may be reduced. The inventors of the present invention conducted further research on this reason. As a result, it was found that even if the difference in dislocation density in the plate thickness direction is small, when the dislocations are mainly movable dislocations, the flow stress is reduced.

The present invention was completed in view of the above knowledge. The gist of the present invention is as follows.

[1] The cold-rolled steel sheet according to one aspect of the present invention has the following chemical composition, including in mass % C: 0.150 to 0.500%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0200% or less, O: 0.020% or less, Ni: 0 to 1.000%, Mo: 0 to 1.000%, Cr: 0 to 2.000%, B: 0 to 0.010 %, As: 0～0.050%, Co: 0～0.500%, Ti: 0～0.500%, Nb: 0～0.500%, V: 0～0.500%, Cu: 0～0.500%, W: 0～0.100% , Ta: 0～0.100%, Ca: 0～0.050%, Mg: 0～0.050%, La: 0～0.050%, Ce: 0～0.050%, Y: 0～0.050%, Zr: 0～0.050%, Sb: 0 to 0.050%, Sn: 0 to 0.050%, and the remainder: Fe and impurities, set the distance from the surface to the range of 1/8 to 3/8 of the plate thickness in the thickness direction as t/4 part , when the range of 20 μm from the surface in the plate thickness direction is set as the surface layer portion, the microstructure in the t/4 portion includes: 0% to 10.0% retained austenite in terms of volume ratio; and 90.0% to 100% of one or two types of martensite and tempered martensite, the ratio of the dislocation density of the above-mentioned surface layer part to the dislocation density of the above-mentioned t/4 part is 0.80 or more, and the ratio of the dislocation density of the above-mentioned surface layer part The ratio of the hardness to the hardness of the t/4 portion is 0.90 or more, and the tensile strength of the cold-rolled steel sheet is 1310 MPa or more.

[2] The cold-rolled steel sheet according to [1], which may have a coating layer formed of zinc, aluminum, magnesium, or one or more alloys thereof on the surface.

[3] A method for manufacturing a cold-rolled steel sheet according to another aspect of the present invention has the following steps: a hot-rolling step of hot-rolling a slab having the chemical composition described in [1] to produce a hot-rolled steel sheet, The above-mentioned hot-rolled steel sheet is coiled in a state where the temperature of the central portion in the width direction is more than 600°C and 700°C or less, and the temperature of the edge portion at a position 20 mm away from the width direction end is 600°C or less; cold rolling process , which pickles the above-mentioned hot-rolled steel plate after the above-mentioned hot-rolling process, and cold-rolls it at a reduction rate of 30 to 90% to obtain a cold-rolled steel plate; and an annealing step, which heats the above-mentioned cold-rolled steel plate to a temperature exceeding Ac3°C. The annealing temperature is maintained at the above-mentioned annealing temperature until the average cooling rate of the above-mentioned cold-rolled steel sheet after the above-mentioned maintenance becomes 10°C/second or more until 400°C, and the cooling stop temperature is from 400°C to 100°C or less. cooling to the above-mentioned cooling stop temperature so that the average cooling rate becomes 15°C/second or more; a heat treatment step in which the above-mentioned cold-rolled steel sheet after the above-mentioned annealing step is heated to a temperature range of 200 to 350°C and maintained within the above-mentioned temperature range; and a skin pass rolling process, in which the cold-rolled steel sheet after the heat treatment process is skin-pass rolled at a reduction rate of 0.1% or more, and the thickness of the central portion in the width direction of the cold-rolled steel sheet after the cold rolling process is equal to The difference in plate thickness at the edge is 10 μm or more.

[4] The method of manufacturing a cold-rolled steel sheet according to [3], wherein in the annealing step, zinc, aluminum, magnesium, or one or more alloys thereof may be formed on the front and back surfaces of the cold-rolled steel sheet. The film layer formed.

Effect of the invention

According to the above aspect of the present invention, it is possible to provide a cold-rolled steel plate having a structure mainly composed of martensite and tempered martensite, and a method for manufacturing the same when a load is applied and the load is removed. When the load is applied again after being left for a certain period of time, the flow stress when the load is applied again can be suppressed from becoming lower than the flow stress when the load is first applied.

Detailed ways

A cold-rolled steel plate according to one embodiment of the present invention (cold-rolled steel plate according to this embodiment) and a manufacturing method for obtaining the cold-rolled steel plate will be described.

The cold-rolled steel sheet of this embodiment has a predetermined chemical composition, and the distance from the surface in the direction of the thickness of the sheet is in the range of 1/8 to 3/8 of the sheet thickness as t/4, and the distance from the surface to the above-mentioned sheet is When the range of 20 μm in the thickness direction is set as the surface layer portion, the microstructure (metallic structure) in the t/4 portion includes, in terms of volume ratio: 0% to 10.0% of retained austenite; and 90.0% to 100% % of one or both of martensite and tempered martensite, the ratio of the dislocation density of the above-mentioned surface layer part to the above-mentioned t/4 part is 0.80 or more, the hardness of the above-mentioned surface layer part and the above-mentioned t The hardness ratio of /4 parts is more than 0.90. In addition, the tensile strength of cold-rolled steel plates is 1310MPa or more.

They are explained separately below.

In the description, regarding the range represented by “～”, in principle, the values at both ends are included in the range as the lower limit value and the upper limit value. However, values expressed as "over" and "under" are not included in the range.

[chemical components]

First, the chemical composition is explained.

In this embodiment, "%" of the content of each element means "mass %".

C: 0.150～0.500%

C is an element related to the hardness of martensite and tempered martensite, and is required to increase the strength of the steel plate. In order to obtain a tensile strength of 1310 MPa or more, a C content of at least 0.150% or more is required. Therefore, the C content is set to 0.150% or more. The C content is preferably 0.180% or more, more preferably 0.200% or more.

On the other hand, if the C content exceeds 0.500%, weldability deteriorates and formability deteriorates. Therefore, the C content is set to 0.500% or less. The C content is preferably 0.350% or less, more preferably 0.300% or less.

Si: 0.01～2.00%

Si is a solid solution strengthening element and is an element effective in strengthening the steel plate. In order to obtain this effect, the Si content is set to 0.01% or more. The Si content is preferably set to 0.10% or more, more preferably 0.20% or more.

On the other hand, if the Si content becomes excessive, the formability decreases and the wettability with the plating layer decreases. Therefore, the Si content is set to 2.00% or less. The Si content is preferably set to 1.80% or less, more preferably 1.70% or less.

Mn: 0.50～3.00%

Mn is an element that improves hardenability and promotes the formation of martensite. If the Mn content is less than 0.50%, it becomes difficult to obtain the target microstructure. Therefore, the Mn content is set to 0.50% or more.

On the other hand, if the Mn content becomes excessive, the effect of improving the hardenability due to segregation of Mn is reduced, and the raw material cost increases. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.80% or less.

P: 0.0200% or less

P is an element contained in steel as an impurity, and is an element that segregates at grain boundaries and embrittles steel. Therefore, it is preferable that the P content is as low as possible, and it may be 0%. However, the P content is set to 0.0200% or less in consideration of the P removal time and cost. The P content is preferably 0.0150% or less, more preferably 0.0100% or less.

From the viewpoint of costs such as refining, the P content may be 0.0001% or more.

S: 0.0200% or less

S is an element contained in steel as an impurity and forms sulfide-based inclusions to deteriorate the formability of the steel sheet. Therefore, it is preferable that the S content is as low as possible, and it may be 0%. However, the S content is set to 0.0200% or less in consideration of the removal time and cost of S. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less.

From the viewpoint of costs such as refining, the S content may be 0.0001% or more.

Al: 0.100% or less

Al is an element that has the effect of deoxidizing molten steel. The cold-rolled steel sheet of this embodiment does not necessarily need to contain Al, and the Al content may be 0%. However, Al may be contained for deoxidation. In this case, the Al content is preferably 0.001% or more. In addition, like Si, Al has the effect of improving the stability of austenite, so in order to obtain retained austenite, Al may be contained.

On the other hand, if the Al content is too high, not only surface defects due to alumina will easily occur, but also the transformation point will rise significantly, and the volume fraction of ferrite will increase. In this case, it becomes difficult to obtain the desired metal structure, and sufficient tensile strength cannot be obtained. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.050% or less, more preferably 0.040% or less, still more preferably 0.030% or less.

N: 0.0200% or less

N is an element that may be contained in steel as an impurity, and is an element that generates coarse precipitates and deteriorates formability. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0100% or less, more preferably 0.0060% or less. The lower the N content, the more preferred it is, and it may be 0%. However, from the viewpoint of costs such as refining, it may be 0.0001% or more.

O: 0.020% or less

O is an element contained as an impurity. If the O content exceeds 0.020%, coarse oxides are formed in the steel and the formability is reduced. Therefore, the O content is set to 0.020% or less. The O content is preferably set to 0.010% or less, more preferably 0.005% or less. The O content may be 0%, but from the viewpoint of costs such as refining, the O content may be 0.0001% or more, or 0.001% or more.

The chemical composition of the cold-rolled steel sheet of this embodiment is basically based on the fact that the remainder other than the above-mentioned elements is Fe and impurities. Impurities refer to elements that are mixed from steel raw materials and/or during the steelmaking process and are allowed to exist within a range that does not significantly deteriorate the characteristics of the cold-rolled steel sheet of the present embodiment.

On the other hand, the chemical composition of the cold-rolled steel sheet of this embodiment may contain Ni, Mo, Cr, B, As, Co, Ti, Nb, V, One or more of Cu, W, Ta, Ca, Mg, La, Ce, Y, Zr, Sb, and Sn replace part of Fe. These elements do not need to be contained, so the lower limit is 0%. In addition, if the content is within the range described below, even if these elements are contained as impurities, the effects of the cold-rolled steel sheet of this embodiment will not be inhibited.

Ni: 0～1.000%

Mo: 0～1.000%

Cr: 0～2.000%

B: 0～0.010%

As: 0～0.050%

Ni, Mo, Cr, B, and As are elements that improve hardenability and contribute to high strength of steel plates. Therefore, these elements may also be contained. In order to fully obtain the above effects, it is preferable that the Ni content, Mo content, and Cr content be 0.010% or more, the B content be 0.0001% or more, and/or the As content be 0.001% or more. More preferably, the Ni content, Mo content, and Cr content are 0.050% or more, the B content is 0.001% or more, and the As content is 0.005% or more. It is not necessary to obtain the effects described above. Therefore, there is no need to specifically limit the lower limits of Ni content, Mo content, Cr content, B content, and As content, and their lower limits are 0%.

On the other hand, even if these elements are contained excessively, the effects produced by the above-mentioned actions are saturated and become uneconomical. Therefore, when these elements are contained, the Ni content and the Mo content are 1.000% or less, the Cr content is 2.000% or less, the B content is 0.010% or less, and the As content is 0.050% or less. The Ni content and Mo content are each preferably 0.500% or less, the Cr content is preferably 1.000% or less, the B content is preferably 0.0060% or less, and the As content is preferably 0.030% or less.

Co: 0～0.500%

Co is an element effective in improving the strength of steel plates. The Co content may be 0%, but in order to obtain the above effects, the Co content is preferably 0.010% or more, and more preferably 0.100% or more.

On the other hand, if the Co content is too high, the elongation of the steel sheet may decrease and the formability may decrease. Therefore, the Co content is set to 0.500% or less.

Ti: 0～0.500％

Nb: 0～0.500%

V: 0～0.500%

Cu: 0～0.500%

W: 0～0.100%

Ta: 0～0.100%

Ti, Nb, V, Cu, W, and Ta are elements that have the effect of improving the strength of the steel plate through precipitation hardening. Therefore, these elements may also be contained. In order to fully obtain the above-mentioned effects, it is preferable to contain one or more types of Ti, Nb, V, Cu, W, and Ta, and the content of each is 0.001% or more.

On the other hand, if these elements are contained excessively, the recrystallization temperature will rise, the metal structure of the cold-rolled steel sheet will become non-uniform, and the formability will be impaired. Therefore, the Ti content, Nb content, V content, and Cu content are each set to 0.500% or less. In addition, the W content and the Ta content are each set to 0.100% or less.

Ca: 0～0.050%

Mg: 0～0.050%

La: 0～0.050%

Ce: 0～0.050%

Y: 0～0.050%

Zr: 0～0.050%

Sb: 0～0.050%

Ca, Mg, La, Ce, Y, Zr, and Sb are elements that contribute to the fine dispersion of inclusions in steel, and the fine dispersion contributes to the improvement of the formability of the steel sheet. Therefore, these elements may also be contained. In order to obtain the above effects, it is preferable to contain one or more types of Ca, Mg, La, Ce, Y, Zr, and Sb so that the content of each is 0.001% or more.

On the other hand, if these elements are contained excessively, the ductility deteriorates. Therefore, the contents of Ca, Mg, La, Ce, Y, Zr, and Sb are each set to 0.050% or less.

Sn: 0～0.050%

Sn is an element that suppresses the coarsening of crystal grains and contributes to improvement in the strength of the steel plate. Therefore, Sn may be contained.

On the other hand, Sn is an element that may cause a decrease in the cold formability of the steel sheet due to embrittlement of ferrite. If the Sn content exceeds 0.050%, adverse effects become significant, so the Sn content is set to 0.050% or less. The Sn content is preferably 0.040% or less.

The chemical composition of the cold-rolled steel sheet of this embodiment can be determined by the following method.

For example, in accordance with JISG1201 (2014), the cut powder may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In this case, the chemical composition is the average content over the entire plate thickness. C and S that cannot be measured by ICP-AES can be measured using the combustion-infrared absorption method, N can be measured using the inactive gas fusion-thermal conductivity method, and O can be measured using the inactive gas fusion-non-dispersive infrared absorption method. It can be measured by method.

When the steel plate has a coating layer on the surface, the coating layer can be removed by mechanical grinding, etc., and then the chemical composition can be analyzed. When the coating layer is a plating layer, the plating layer can also be removed by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel plate.

[Microstructure (metallic structure)]

In this embodiment, the range from a position that is 1/8 of the plate thickness to 3/8 of the plate thickness, centered at a position that is 1/4 of the plate thickness in the plate thickness direction from the surface, is set as t /4 part ((1/4)t part) will be explained by assuming that the range from the surface to 20 μm in the plate thickness direction is a surface layer part.

[Microstructure of part t/4: Contains by volume: 0% to 10.0% of retained austenite; and 90.0% to 100% of one or two of martensite and tempered martensite ]

Retained austenite increases the uniform elongation of the steel plate by utilizing the TRIP effect, which helps improve the formability of the steel plate. Therefore, retained austenite (retained γ) may be contained. In order to obtain the above effects, the volume fraction of retained austenite is preferably set to 1.0% or more. The volume fraction of retained austenite is more preferably 2.0% or more, further preferably 3.0% or more.

On the other hand, if the volume ratio of the retained austenite becomes excessive, the particle size of the retained austenite becomes larger. Such retained austenite with a large particle size becomes coarse and hard martensite after deformation. In this case, the origin of cracks becomes likely to occur, and the formability decreases. Therefore, the volume fraction of retained austenite is set to 10.0% or less. The volume fraction of retained austenite is preferably 8.0% or less, more preferably 7.0% or less.

The structure other than retained austenite includes one or both of martensite and tempered martensite.

Martensite (so-called primary martensite) and tempered martensite are a collection of lath-shaped crystal grains and contribute significantly to strength improvement. Therefore, the cold-rolled steel sheet of this embodiment contains 90.0 to 100% of martensite and tempered martensite based on the total volume ratio.

Tempered martensite is different from martensite in that it is a hard structure containing fine iron-based carbides inside due to tempering. Tempered martensite contributes less to strength improvement than martensite, but since it is a non-brittle and ductile structure, when it is desired to further improve formability, it is preferable to increase tempered martensite. volume ratio. For example, the volume fraction of tempered martensite is 85.0% or more.

On the other hand, when high strength is desired, it is preferable to increase the volume fraction of martensite.

The microstructure may also contain bainite in addition to retained austenite, martensite and tempered martensite. Preferably, ferrite and pearlite are not included.

The volume fraction of each structure in the microstructure of the t/4 portion of the cold-rolled steel sheet according to this embodiment is measured as follows.

That is, regarding the volume fraction of ferrite, bainite, martensite, tempered martensite, and pearlite, a test piece was taken from any position in the rolling direction and width direction of the steel plate, and the test piece was taken parallel to the rolling direction. The longitudinal section (section parallel to the plate thickness direction) was ground, and the structure revealed by nitric ethanol etching in the range of 1/8 to 3/8 of the plate thickness (t/4 part) from the surface was observed using SEM. . In SEM observation, five fields of view of 30 μm × 50 μm were observed at a magnification of 3000 times. From the observed images, the area ratio of each tissue was measured and the average value was calculated. There is no structural change in the direction perpendicular to the rolling direction (steel plate width direction), and the area ratio and volume ratio of the longitudinal section parallel to the rolling direction are equal, so the area ratio obtained from the structure observation is regarded as the respective volume ratio.

When measuring the area ratio of each structure, a region in which the lower structure is not shown and has low brightness is defined as ferrite. In addition, a region with high brightness that does not exhibit an underlying structure is defined as martensite or retained austenite. In addition, the region in which the lower structure appears is defined as tempered martensite or bainite.

Bainite and tempered martensite can be distinguished by closer inspection of the carbides within the grains.

Specifically, tempered martensite is composed of martensite laths and cementite generated inside the laths. At this time, there are two or more crystal orientation relationships between martensite laths and cementite, so the cementite constituting tempered martensite has multiple variations. Bainite, on the other hand, is divided into upper bainite and lower bainite. Upper bainite is composed of lath-shaped bainite ferrite and cementite generated at the lath interface, so it can be easily distinguished from tempered martensite. Lower bainite is composed of lath-shaped bainite ferrite and cementite generated inside the laths. At this time, the crystal orientation relationship between bainite ferrite and cementite is different from that of tempered martensite and is one type, and the cementite constituting lower bainite has the same modification. Therefore, lower bainite can be distinguished from tempered martensite based on the modification of cementite.

On the other hand, martensite and retained austenite cannot be clearly distinguished by SEM observation. Therefore, the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite calculated by a method described below from the volume fraction of the structure judged to be martensite or retained austenite.

Regarding the volume fraction of retained austenite, a test piece was taken from any position on the steel plate, and the rolled surface was chemically ground from the surface of the steel plate to a position of 1/4 of the plate thickness. ), (210) area integral strength and the (200), (220) and (311) area integral strength of austenite were quantified.

[The ratio of the dislocation density in the surface layer to the dislocation density in the t/4 part: 0.80 or more]

The microstructure of the cold-rolled steel sheet according to this embodiment is mainly composed of martensite and/or tempered martensite in which martensite is tempered.

Martensite is obtained by holding a steel plate in the austenite single-phase region and then rapidly cooling it. However, when the steel plate is cooled using a general method, martensite changes in the structure depending on the position of the steel plate in the thickness direction. differences in characteristics (such as the density of dislocations contained). This difference is caused by the difference in the time point of the phase transition. That is, during cooling, the temperature of the area close to the surface of the steel plate first decreases, and then the temperature inside the steel plate decreases. Therefore, the phase transformation from austenite to martensite occurs first on the surface layer side of the steel plate. Since the martensite transformation is an exothermic reaction, the martensite generated on the surface layer side is maintained at a higher temperature for a longer period of time than the internal martensite, thereby causing tempering to progress. If tempered, the dislocation density in martensite decreases.

If there is such a difference in dislocation density, when a load that causes deformation is applied, the load is removed, the load is left to stand for a certain period, and then the load is applied again, the flow stress when the load is applied again becomes larger than the flow stress when the load is first applied. Low stress. Therefore, in the cold-rolled steel sheet of this embodiment, the ratio (ρ _{s /ρ t/4 ) of the dislocation density (ρ s )} of the surface layer part to the dislocation density (ρ _{t/4 )} of the t/4 part is: 0.80 or above. (ρ _s /ρ _t/4 ) is preferably 0.85 or more, and more preferably 0.90 or more.

The dislocation density of the t/4 portion is preferably 5.2×10 ¹⁵ m ^-2 or more. Therefore, considering the above-mentioned preferable range of ρ _s /ρ _t/4 , the dislocation density of the surface layer portion is preferably 4.2×10 ¹⁵ m ⁻² or more.

The dislocation density at each position is determined by the following method.

The position 20 μm from the surface of the steel plate is considered as the representative structure of the surface layer, and the position 1/4 of the plate thickness from the surface is considered the representative structure of the t/4 part. A sample ground 20 μm from the surface and a sample ground from the surface are produced. A sample with 1/4 of the plate thickness ground from the surface was subjected to X-ray diffraction after strain was removed by chemical polishing on each ground surface. Using the modified Williamson-Hall method and the modified Warren-Averbach method from the X-ray diffraction curve obtained by X-ray diffraction, the position at a position 20 μm from the surface is found. Dislocation density and dislocation density at a distance from the surface that is 1/4 of the plate thickness. Specifically, the dislocation density was determined according to the method described in ISIJ Int. vol. 50 (2010) pages 875-882. The dislocation density at a position 20 μm from the surface was defined as the dislocation density in the surface layer portion, and the dislocation density at a position 1/4 of the plate thickness from the surface was defined as the dislocation density in the t/4 portion.

[The ratio of the hardness of the surface part to the hardness of the t/4 part: 0.90 or more]

As described above, even if the difference in dislocation density in the plate thickness direction is reduced, when the dislocations are mainly movable dislocations, a load that causes deformation is applied, the load is removed, the load is left for a certain period, and then the load is applied again. In the case of , the flow stress when the load is applied again becomes lower than the flow stress when the load is first applied.

Therefore, in the cold-rolled steel sheet of this embodiment, dislocations in the surface layer portion are particularly immobilized. In this embodiment, the ratio of the hardness of the surface layer part to the hardness of the t/4 part is used as an index of whether the dislocation is immobilized.

When (ρ _s /ρ _t/4 ) is 0.80 or more, if the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is 0.90 or more, dislocations are immobilized and a decrease in flow stress can be prevented. The hardness ratio is preferably 0.92 or more, more preferably 0.94 or more, and still more preferably 0.95 or more.

As will be described later, dislocations can be immobilized by skin pass rolling on a steel plate in which a plate thickness difference is intentionally provided.

There is a correlation between tensile strength and hardness, so the hardness of the t/4 portion is preferably 360 Hv or more. Therefore, considering the preferred range of the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion described above, the hardness of the surface layer portion is preferably 324 Hv or more.

Hardness is determined by the following method.

A cutting surface perpendicular to the rolling direction of the steel plate and parallel to the plate thickness direction is formed, and mirror polished. Next, on the cut surface, the Vickers hardness was measured at four points each based on JISZ2244-1 (2020) at a position 20 μm from the surface of the steel plate and a position 1/4 of the plate thickness from the surface. The load in Vickers hardness measurement is set to 2kgf. The average value of the hardness measurement values at a position 20 μm from the surface of the steel plate is set as the hardness of the surface layer, and the average value of the hardness measurement values at a position 1/4 of the plate thickness from the surface is set as t/ 4 parts hardness.

The cold-rolled steel sheet of the present embodiment may have a coating layer containing zinc, aluminum, magnesium, or one or more alloys thereof on the surface (one side or both sides), or may be made of zinc, aluminum, magnesium, or an alloy thereof. A coating layer formed of more than one alloy (which is allowed to contain impurities, etc.).

By providing a coating layer on the surface, corrosion resistance is improved. If steel sheets for automobiles are concerned about perforations due to corrosion, they may not be able to be thinned below a certain plate thickness even if the steel sheets are strengthened. One of the purposes of increasing the strength of steel plates is to reduce their weight by thinning them. Therefore, even if high-strength steel plates are developed, application areas will be limited if the corrosion resistance is low. As a method to solve these problems, it is considered to form a coating layer on the surface in order to improve the corrosion resistance.

The coating layer is, for example, a hot-dip galvanizing layer, an alloyed hot-dip galvanizing layer, an electrolytic zinc plating layer, an aluminum plating layer, a Zn-Al alloy plating layer, an Al-Mg alloy plating layer, or a Zn-Al-Mg alloy plating layer.

When there is a coating layer on the surface, the surface used as a reference for the above-mentioned t/4 portion and the like is the surface of the base metal excluding the coating layer.

[tensile strength]

In the cold-rolled steel sheet of the present embodiment, the tensile strength (TS) is targeted to be 1310 MPa or more as a strength that contributes to lightweighting of the automobile body. From the viewpoint of impact absorption, the tensile strength is preferably 1,400 MPa or more, and more preferably 1,470 MPa or more.

There is no need to limit the upper limit, but if the tensile strength increases, the formability may decrease, so the tensile strength may be 2000 MPa or less.

＜Manufacturing method＞

The cold-rolled steel sheet of this embodiment does not depend on the manufacturing method. As long as it has the above-mentioned characteristics, its effects can be obtained. However, the cold-rolled steel sheet can be stably manufactured by the following manufacturing method.

Specifically, the cold-rolled steel sheet of this embodiment can be produced using a manufacturing method including the following steps (I) to (VI).

(I) A hot rolling process in which a slab having a predetermined chemical composition is hot-rolled to produce a hot-rolled steel sheet, and the temperature of the hot-rolled steel sheet in the center portion in the width direction is between more than 600°C and less than 700°C at a distance of Coiling is performed in a state where the temperature of the edge portion at the 20 mm width direction end is 600°C or lower;

(III) a cold rolling process, which involves pickling the hot-rolled steel sheet after the hot-rolling process and cold-rolling it at a reduction rate of 30 to 90% to obtain a cold-rolled steel sheet;

(IV) An annealing step in which the cold-rolled steel sheet is heated to an annealing temperature exceeding Ac3°C, maintained at the annealing temperature, and the cold-rolled steel sheet after the holding is cooled to an average cooling rate of 10°C up to 400°C. / second or more, and the average cooling rate from 400°C to the cooling stop temperature of 100°C or less becomes 15°C/second or more to the above-mentioned cooling stop temperature;

(V) A heat treatment process in which the above-mentioned cold-rolled steel sheet after the above-mentioned annealing process is heated to a temperature range of 200 to 350°C and maintained within the above-mentioned temperature range;

(VI) A skin pass rolling step in which the cold-rolled steel sheet after the above heat treatment step is skin pass rolled at a reduction rate of 0.1% or more.

In addition, in the manufacturing method of the cold-rolled steel sheet according to this embodiment, the difference between the thickness of the central portion in the width direction of the cold-rolled steel sheet and the thickness of the edge portion of the cold-rolled steel sheet after the cold rolling process is 10 μm or more.

Each process is explained below.

[Hot rolling process]

In the hot rolling process, a slab having the same chemical composition as the cold-rolled steel sheet of this embodiment is hot-rolled to obtain a hot-rolled steel sheet. In order to satisfy the temperature during coiling described later, hot rolling is preferably performed under conditions such that the completion temperature of finish rolling becomes Ac3°C or higher. The upper limit of the finishing temperature of finish rolling is not particularly limited, but is generally 950°C or less.

This hot-rolled steel sheet was coiled in a state where the temperature of the center portion in the width direction was more than 600°C and 700°C or less, and the temperature of the edge portion at a position 20 mm from the width direction end was 600°C or less.

In order to make the winding temperature of the edge part lower than that of the center part in the width direction, the edge part is cooled so that the cooling rate may become faster than that of the center part. For example, only the edge portion of the hot-rolled steel plate may be water-cooled, or when the entire steel plate is water-cooled, the amount of cooling water at the edge portion may be set to be greater than that at the center portion in the width direction.

When the edge portion is water-cooled and then rolled up, it is tempered by heat transfer from the width-direction center portion where the temperature is higher, and therefore is softened more than the width-direction center portion. As a result, in a state cooled to near room temperature, the strength of the edge portion becomes lower than the strength of the center portion in the width direction.

By subjecting such a steel plate having a strength difference in the width direction to cold rolling described below, a plate thickness difference occurs between the center portion and the edge portion of the steel plate in the width direction.

If the winding temperature in the width direction center part exceeds 700°C, the width direction center part becomes soft. In addition, if the winding temperature at the center portion in the width direction is 600° C. or less, the temperature difference with the edge portion becomes small, or the edge portion cannot be sufficiently tempered. The coiling temperature in the center part is preferably 620°C or higher.

In addition, if the winding temperature of the edge portion exceeds 600° C., the softening effect due to tempering cannot be sufficiently obtained. In addition, if the coiling temperature of the edge portion is 400° C. or lower, although the steel is tempered by heat transfer from the center portion in the width direction, the cold rolling load increases because the strength becomes higher, and cracks may occur. Therefore, the winding temperature of the edge portion is preferably more than 400°C, more preferably 450°C or more.

When the plate thickness difference between the width direction center part and the edge part is to be made larger, the difference in coiling temperature between the width direction center part and the edge part is preferably 50°C or more, more preferably 75°C or more, and even more preferably Above 100℃.

The manufacturing method of the slab to be used for hot rolling is not particularly limited. In the preferred manufacturing method of the illustrated slab, steel having the above-mentioned chemical composition is melted by a known method and then made into a steel ingot by a continuous casting method, or an arbitrary casting method is used to make a steel ingot and then subjected to billet rolling. methods to make steel billets. In the continuous casting process, in order to suppress the occurrence of surface defects caused by inclusions, it is preferable to generate an external flow such as electromagnetic stirring in the molten steel in the mold. As for the steel ingot or billet, the temporarily cooled product can be reheated and supplied to hot rolling, or the steel ingot in a high-temperature state after continuous casting or the steel billet in a high-temperature state after billet rolling can be directly supplied to the steel ingot or billet. Hot rolling, heat preservation for hot rolling, or auxiliary heating for hot rolling. In this embodiment, such steel ingots and steel slabs are collectively referred to as "slabs" as raw materials for hot rolling.

[Cold rolling process]

In the cold rolling process, the hot rolled steel sheet after the hot rolling process is pickled and cold rolled at a reduction rate of 30 to 90% to obtain a cold rolled steel sheet.

The pickling conditions are not particularly limited, and known conditions may be sufficient.

In the cold rolling process, a steel plate having a difference in strength in the width direction is cold rolled, thereby obtaining a steel plate having a difference in plate thickness in the width direction (cold-rolled steel plate).

If the cold rolling reduction ratio (cumulative reduction ratio) is less than 30%, a sufficient plate thickness difference cannot be provided. In addition, if the reduction ratio of cold rolling exceeds 90%, the cold rolling load becomes excessively large and cold rolling becomes difficult.

In the method for manufacturing a cold-rolled steel sheet according to this embodiment, the thickness of the central portion in the width direction and the thickness of the edge portion of the cold-rolled steel sheet after the cold rolling process are obtained by passing through the hot rolling process and the cold rolling process. Cold-rolled steel plates with a thickness difference of 10 μm or more. The plate thickness difference is preferably 15 μm or more.

There is no upper limit to the difference in plate thickness. However, if the difference in plate thickness is large, cracks may occur in the thin portion of the plate and the hole expandability may decrease. Therefore, from the viewpoint of formability, the plate thickness difference may be 55 μm or less.

The plate thickness at the center portion in the width direction and the plate thickness at the edge portion can be measured by installing a scanning plate thickness gauge on the exit side of the cold rolling mill.

[Width trimming process]

After cutting, if the difference between the plate thickness at the center portion in the width direction and the plate thickness at the edge portion becomes 10 μm or more, width trimming may be performed by cutting an arbitrary width from the end portion in the width direction of the steel plate.

By performing width trimming, even if cracks or defects occur at the end of the cold-rolled steel plate, the steel plate can be supplied to the next process by cutting off this part, which is preferable in terms of cost and yield.

[Annealing process]

In the annealing process, the cold-rolled steel sheet after the cold rolling process or, if necessary, further subjected to the width trimming process is heated to an annealing temperature exceeding Ac3° C. and maintained at the annealing temperature.

If the annealing temperature is Ac3° C. or less, the structure will not undergo sufficient austenite transformation, and the desired microstructure mainly composed of martensite cannot be obtained after the annealing process.

On the other hand, excessive high-temperature heating such that the annealing temperature exceeds 900° C. causes an increase in manufacturing costs. Therefore, the annealing temperature is preferably set to 900°C or lower.

The temperature (°C) at Ac3 point can be obtained by the following method.

Ac3＝910-(203×C ^1/2 )+44.7×Si-30×Mn+700×P-20×Cu-15.2×Ni-

11×Cr+31.5×Mo+400×Ti+104×V+120×Al

Among them, the element symbols contained in the formula refer to the content in mass % of the elements contained in the steel plate.

The holding time at the annealing temperature is preferably 40 to 135 seconds.

If the holding time is less than 40 seconds, austenitization may not proceed sufficiently. In addition, if the holding time exceeds 135 seconds, productivity decreases.

The cold-rolled steel sheet after the above-mentioned maintenance is cooled so that the average cooling rate to 400°C becomes 10°C/second or more, and the average cooling rate from 400°C to the cooling stop temperature of 100°C or lower becomes 15°C/second or more. Cooling stop temperature.

If the average cooling rate up to 400°C is less than 10°C/sec, ferrite may be generated in the microstructure. In addition, when the average cooling rate from 400°C to the cooling stop temperature (100°C or lower) is less than 15°C/sec, or the cooling stop temperature exceeds 100°C, bainite may be generated in the microstructure. In these cases, the desired microstructure cannot be obtained.

In the annealing process, a coating layer made of zinc, aluminum, magnesium, or one or more alloys thereof is formed on the surface (one surface or both surfaces) of the cold-rolled steel sheet.

When forming a coating layer, for example, in the case of hot dip plating, as long as the average cooling rate to 400°C becomes 10°C/second or more and the average cooling from 400°C to the cooling stop temperature of 100°C or less When the speed is in the range of 15°C/second or more, the steel plate is immersed in the plating bath during cooling to form a hot-dip coating on the surface, and the steel plate is kept in the temperature range of about 450 to 470°C for 10 to 40 seconds.

When alloying the hot-dip coating, it is preferable to immerse the steel plate in a plating bath and heat the steel plate with the hot-dip galvanized layer to a temperature range of 470 to 550° C. (alloying temperature), Maintain within this temperature range for 10 to 40 seconds. If the alloying temperature is lower than 470°C, there is a possibility that alloying will not proceed sufficiently. On the other hand, if the alloying temperature exceeds 550° C., the alloying proceeds excessively, and the Fe concentration in the plating layer may exceed 15% due to the formation of Γ phase, resulting in deterioration of corrosion resistance. More preferably, the alloying temperature is 480°C or higher. In addition, the alloying temperature is more preferably 540°C or lower.

[Heat treatment process]

By cooling in the annealing step (cooling after holding), untransformed austenite transforms into martensite in the microstructure of the cold-rolled steel sheet. However, it is possible that part of the austenite will not transform and become retained austenite.

Such a cold-rolled steel sheet is heated to a temperature range of 200 to 350° C. and maintained within this temperature range.

Through this heat treatment, part or all of the martensite becomes tempered martensite. When the microstructure is to have a main component of tempered martensite, it is preferable to set the holding time to 1 second or more.

If the heating temperature is lower than 200°C, there is a possibility that the martensite will not be tempered sufficiently to bring about satisfactory changes in the microstructure and mechanical properties. When the heating temperature exceeds 350° C., the dislocation density in the tempered martensite may decrease, resulting in a decrease in tensile strength.

[Skin rolling process]

In the skin pass rolling process, the cold rolled steel sheet after the heat treatment process is skin pass rolled at a reduction rate of 0.1% or more.

As described above, the cold-rolled steel sheet after the heat treatment process has a thickness difference of 10 μm or more between the central portion and the edge portion in the width direction.

When such a cold-rolled steel plate is skin-pass rolled, when it is bitten by the roll, due to the difference in plate thickness, the length direction of the steel plate is not perpendicular to the axial direction of the roll but is bitten at a predetermined angle. enter. The reduction rate can be selected arbitrarily in the setting of the skin pass rolling mill. However, when there is a difference in plate thickness, the amount of strain introduced by the reduction rate is assumed to be set when the plate thickness is uniform. , the amount of strain introduced into the surface layer can be further increased.

In this embodiment, by subjecting a cold-rolled steel sheet having a predetermined plate thickness difference to skin pass rolling to introduce strain into the surface layer portion, the dislocation density in the surface layer portion can be increased and the dislocations can be immobilized.

However, if the reduction ratio is less than 0.1%, sufficient effects cannot be obtained, so the reduction ratio is set to 0.1% or more. The upper limit of the reduction rate is not limited, but if it exceeds 1.5%, the productivity significantly decreases, so it is preferably set to less than 1.5%.

Generally speaking, skin pass rolling with a reduction ratio of 0.1% or more is not performed on steel plates with a tensile strength of 1310 MPa or more. However, in this embodiment, based on the above-mentioned new knowledge discovered by the inventors of the present invention, , perform skin pass rolling with a reduction rate of 0.1% or more.

Example

Slabs (steel types A to W) having the chemical compositions shown in Tables 1-1 to 1-2 (unit is mass %, the balance is Fe and impurities) were produced by continuous casting.

Using these slabs, hot rolling was performed so that the finishing temperature of the finish rolling would be Ac3°C or higher. By changing the cooling conditions of the central portion and the edge portion, coiling was performed under the conditions in Table 2-1, and hot rolling was obtained. steel plate.

These hot-rolled steel sheets were cold-rolled under the conditions of Table 2-1, and cold-rolled steel sheets having the plate thickness differences shown in Table 2-1 were obtained.

For these cold-rolled steel sheets, annealing, heat treatment, and skin pass rolling were performed under the conditions shown in Table 2-2.

In addition, some cold-rolled steel sheets are hot-dip galvanized during annealing. By changing the holding temperature after immersion in the plating bath, a part of the plating layer is alloyed. The holding time is set to 10 to 40 seconds. In Table 3-1, GI indicates that a hot-dip galvanized layer is formed, and GA indicates that an alloyed hot-dip galvanized layer is formed.

From the obtained cold-rolled steel sheet, the microstructure of the t/4 portion was observed using the method described above, and the properties of martensite, tempered martensite, bainite, retained austenite, ferrite, and pearlite were measured. volume ratio.

The measurement results of the volume fractions of martensite, tempered martensite, bainite, and retained austenite are shown in Table 3-1. Although not shown in the table, Nos. 38 and 39 formed ferrite in addition to martensite, tempered martensite, bainite, and retained austenite.

In addition, from the obtained cold-rolled steel sheet, the dislocation density and hardness of the surface layer part and the t/4 part were measured using the above-mentioned method.

The results are shown in Table 3-2. However, regarding the expression of dislocation density, "5.0E+15" in the table means 5.0×10 ¹⁵ , and when it is described as YE+X, the others also mean Y×10 ^x in the same way.

In addition, the tensile strength of the obtained cold-rolled steel sheet was determined. The tensile strength is determined by the following method.

The tensile strength (TS) is measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate and performing a tensile test in accordance with JIS Z 2241 (2011).

The results are shown in Table 3-2.

In addition, in order to evaluate the reduction of flow stress, a post-prestrain tensile test was performed according to the following method.

The change in flow stress is evaluated by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate, and applying 0.1% preconditioning to the test piece in accordance with JIS Z 2241 (2011). After straining, the test piece was left to stand for 1 day after removing the load, and the stress when the test piece was re-stretched was compared with the stress when the 0.1% pre-strain load was applied. The case where the flow stress increased when strain was applied again, the same case, or the case where the amount of decrease was less than 40 MPa was set as A (excellent), and the case where the amount of decrease was 40 MPa or more and less than 80 MPa was set as B (good). ), and the case where the reduction amount is 80 MPa or more is set as C (the target is not met).

The results are shown in Table 3-2.

[Table 1-1]

[Table 1-2]

[table 2-1]

[Table 2-2]

[Table 3-1]

[Table 3-2]

As can be seen from Tables 1-1 to 3-2, Nos. 1 to 30 as invention examples have a tensile strength of 1310 MPa or more, and the reduction in flow stress is also suppressed to less than 80 MPa.

On the other hand, in Nos. 31 to 45 as comparative examples, a tensile strength of 1310 MPa or more could not be obtained, or a decrease in flow stress of 80 MPa or more was observed.

Claims (4)

1. A cold-rolled steel plate, characterized in that it has the following chemical composition, including in mass %: C: 0.150～0.500%, Si: 0.01～2.00%, Mn: 0.50～3.00%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0200% or less, O: 0.020% or less, Ni: 0～1.000%, Mo: 0～1.000%, Cr: 0～2.000%, B: 0～0.010%, As: 0～0.050%, Co: 0～0.500%, Ti: 0～0.500%, Nb: 0～0.500%, V: 0～0.500%, Cu: 0～0.500%, W: 0～0.100%, Ta: 0～0.100%, Ca: 0～0.050%, Mg: 0～0.050%, La: 0～0.050%, Ce: 0～0.050%, Y: 0～0.050%, Zr: 0～0.050%, Sb: 0～0.050%, Sn: 0～0.050%, and Remainder: Fe and impurities, The range of 1/8 to 3/8 of the plate thickness from the surface in the plate thickness direction is set as the t/4 part, and the range of 20 μm from the surface in the plate thickness direction is set as the surface layer. part, the microstructure in the t/4 part includes, in terms of volume ratio: 0% to 10.0% of retained austenite; and 90.0% to 100% of martensite and tempered martensite. species or 2 species, The ratio of the dislocation density of the surface layer portion to the dislocation density of the t/4 portion is 0.80 or more, The ratio of the hardness of the surface layer part to the hardness of the t/4 part is 0.90 or more, The tensile strength of the cold-rolled steel plate is above 1310MPa. 2. The cold-rolled steel sheet according to claim 1, characterized in that the surface has a coating layer formed of zinc, aluminum, magnesium or one or more alloys thereof. 3. A method for manufacturing cold-rolled steel plates, which is characterized by having the following steps: A hot rolling process in which a slab having the chemical composition according to claim 1 is hot-rolled to produce a hot-rolled steel sheet, and the temperature of the hot-rolled steel sheet in the center portion in the width direction is between 600°C and 700°C. ℃ or lower, and the temperature of the edge part at a position 20 mm away from the width direction end is 600 ℃ or lower. Winding is performed; A cold rolling process, which includes pickling the hot-rolled steel plate after the hot-rolling process and cold-rolling it at a reduction rate of 30 to 90% to obtain a cold-rolled steel plate; An annealing step in which the cold-rolled steel sheet is heated to an annealing temperature exceeding Ac3°C, held at the annealing temperature, and the cold-rolled steel sheet after the holding is cooled to an average cooling rate of 10 to 400°C. °C/second or more, and cool to the cooling stop temperature in such a manner that the average cooling rate from 400°C to the cooling stop temperature of 100°C or lower becomes 15°C/second or more; A heat treatment process, which heats the cold-rolled steel plate after the annealing process to a temperature range of 200 to 350°C and maintains it within the temperature range; and A skin pass rolling process, which performs skin pass rolling on the cold-rolled steel plate after the heat treatment process at a reduction rate of 0.1% or more, The difference between the thickness of the central portion in the width direction of the cold-rolled steel sheet and the thickness of the edge portion of the cold-rolled steel sheet after the cold-rolling process is 10 μm or more. 4. The manufacturing method of a cold-rolled steel plate according to claim 3, characterized in that, in the annealing process, a layer of zinc, aluminum or magnesium or one or more of them is formed on the front and back sides of the cold-rolled steel plate. The coating layer formed by the alloy.