JP7741417B2 - Cold-rolled steel sheet, its manufacturing method, and welded joint - Google Patents

Description

The present invention relates to a cold-rolled steel sheet, a manufacturing method thereof, and a welded joint.
This application claims priority based on Japanese Patent Application No. 2021-168157, filed on October 13, 2021, the contents of which are incorporated herein by reference.

In today's world, where industrial technology fields have become highly specialized, the materials used in each field are required to have specialized and advanced performance. For example, with regard to steel sheets for automobiles, high strength is required to improve fuel efficiency by reducing the weight of the vehicle body, out of consideration for the global environment. When high-strength steel sheets are used in automobile bodies, the desired strength can be imparted to the body while reducing the thickness of the steel sheets and reducing the body weight.

In recent years, the demands for automotive steel sheets have become even more sophisticated, and high strength is now required of automotive steel sheets, particularly cold-rolled steel sheets used for body frame parts, with steel sheets having a tensile strength of 1310 MPa or more being required.

In response to such demands, for example, Patent Document 1 discloses a high-strength steel sheet to be used for automobile parts and the like, which has a predetermined chemical composition, a predetermined steel sheet structure mainly composed of martensite and bainite, an average number of inclusions having an average grain size of 5 μm or more in a cross section perpendicular to the rolling direction of 5.0 pieces/ ^mm2 or less, excellent delayed fracture resistance, and a tensile strength of 1470 MPa or more.

Patent Document 2 also discloses a thin steel sheet having a steel structure in which the ferrite area ratio is 30% or less (including 0%), the bainite area ratio is 5% or less (including 0%), the martensite and tempered martensite area ratio is 70% or more (including 100%), the retained austenite area ratio is 2.0% or less (including 0%), the ratio of the dislocation density within 0-20 μm from the steel sheet surface to the dislocation density at the center of the sheet thickness is 90% to 110%, and the average cementite particle size within the top 10% of the diameters up to a depth of 100 μm from the steel sheet surface is 300 nm or less, and the maximum warpage of the steel sheet when sheared at a length of 1 m in the longitudinal direction is 15 mm or less. Patent Document 2 also indicates that this thin steel sheet has a tensile strength of 980 MPa or more, and can also achieve a tensile strength of 2000 MPa or more.

Patent document 3 also discloses a high-strength steel plate with excellent resistance to delayed fracture, in which the chemical composition (C, Si, Mn, Al, P, S) satisfies specified ranges, with the remainder consisting of iron and unavoidable impurities, martensite accounts for 95% or more by area of the entire structure, the structure from a position 10 μm deep from the steel plate surface in the plate thickness direction to a position 1/4 of the plate thickness depth satisfies a specified relationship, and the tensile strength is 1180 MPa or more.

Japanese Patent No. 6729835 International Publication No. 2020/026838 Japanese Patent Application Publication No. 2013-104081

As mentioned above, high-strength steel sheets with a tensile strength of 1310 MPa or more have been proposed. Such high-strength steel sheets generally contain a large amount of alloying elements, such as Mn, and segregation of alloying elements such as Mn is observed within the steel sheet. It is also known that P, which is contained as an impurity along with Mn, also segregates within the steel sheet. The segregation of Mn and P occurs due to the partitioning of elements between the solid and liquid phases during dendrite growth during solidification from molten steel. Because these elements diffuse slowly within steel, the segregation during solidification cannot be eliminated by the degree of heating during hot rolling, annealing, or the like after solidification.
As a result of investigations by the present inventors, it was found that when steel plates having such segregation are welded, the joint strength may decrease in the heat-affected zone of the weld due to the segregation in the steel plate. However, Patent Documents 1 to 3 do not take into consideration the joint strength after welding.
Therefore, an object of the present invention is to provide a steel plate that can provide sufficiently high joint strength after welding, assuming that the steel plate is an ultra-high strength steel plate having a tensile strength of 1310 MPa or more, and to provide a welded joint using this steel plate.

The present inventors investigated the reason why the segregation of Mn and P reduces the joint strength. As a result, they found that the difference in the Mn content (concentration difference) causes a difference in the hardness of martensite in the weld heat-affected zone, and that the co-segregation of Mn and P makes cracks more likely to occur. They also found that Mn and P are likely to segregate at prior γ (austenite) grain boundaries.
Therefore, the present inventors have investigated means for suppressing the segregation of Mn and P to the prior γ grain boundaries.
As a result, they found that the segregation of Mn and P to the prior γ grain boundaries can be suppressed by subjecting the cast slab to breakdown (BD) and high-temperature heat treatment (SP treatment) prior to hot rolling, and further by applying a large reduction in the hot rolling.
Furthermore, it was found that when a steel plate in which such segregation is suppressed is used, the deterioration of joint properties after welding can be suppressed.

The present invention has been made in light of the above findings.
[1] A cold-rolled steel sheet according to one embodiment of the present invention has, in mass%, C: 0.200% or more and 0.450% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.6% or more and 3.5% or less, Al: 0.001% or more and 0.100% or less, Ti: 0.001% or more and 0.100% or less, N: 0.0100% or less, P: 0.0400% or less, S: 0.0100% or less, O: 0.0060% or less, B: 0% or more and 0.0 100% or less, Mo: 0% or more, 0.500% or less, Nb: 0% or more, 0.200% or less, Cr: 0% or more, 2.00% or less, V: 0% or more, 0.500% or less, Co: 0% or more, 0.500% or less, Ni: 0% or more, 1.000% or less, Cu: 0% or more, 1.000% or less, W: 0% or more, 0.100% or less, Ta: 0% or more, 0.100% or less, Sn: 0% or more, 0.050% or less, Sb: 0% or more and 0.050% or less, As: 0% or more, 0.050% or less, Mg: 0% or more, 0.050% or less, Ca: 0% or more, 0.040% or less, Y: 0% or more, 0.050% or less, Zr: 0% or more, 0.050% or less, La: 0% or more, 0.050% or less, Ce: 0% or more, 0.050% or less, and the balance: Fe and impurities, and the metal is located at a position of 1/4 to 3/4 of the plate thickness from the surface in the plate thickness direction. The structure contains, by volume, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or both of martensite and tempered martensite, and in the metal structure at the position, the P content at the prior γ grain boundary is 3.6% by mass or more and 10.0% by mass or less, and the Mn content at the prior γ grain boundary is 3.6% by mass or more and 10.0% by mass or less, and the tensile strength is 1310 MPa or more.
[2] A method for producing a cold-rolled steel sheet according to another aspect of the present invention is a method for producing a cold-rolled steel sheet according to [1], comprising: a continuous casting step of obtaining a slab having the chemical composition according to [1] by continuous casting; a breakdown step of reducing the thickness of the slab by rolling it down at a temperature range of 850 to 1000°C at a rolling reduction rate of 30 to 60%; a high-temperature heat treatment step of heating the slab after the breakdown step to 1000°C to 1300°C, holding the temperature for 5 to 20 hours, and cooling it; a hot-rolling step of hot-rolling the slab after the high-temperature heat treatment step to obtain a hot-rolled steel sheet; a coiling step of coiling the hot-rolled steel sheet in a temperature range of 400 to 650°C; and a step of pickling the hot-rolled steel sheet after the coiling step and cold-rolling it at a rolling reduction rate of 20 to 80% to obtain a cold-rolled steel sheet. the cold-rolled steel sheet is heated to an annealing temperature of more than Ac3°C at an average heating rate of 2°C/sec or more, held at the annealing temperature for 60 to 300 seconds, and cooled to 250°C or less at an average cooling rate of 10°C/sec or more; and a holding step of holding the cold-rolled steel sheet after the annealing step at 150 to 400°C for 500 seconds or less, wherein in the hot-rolling step, finish rolling is performed using a rolling mill having four or more stands, and when the first stand is the first stand and the final stand is the n-th stand, the sheet thickness reduction rate at each of the n-3rd stand to the n-th stand is 30% or more, and the rolling temperature at the n-th stand is 900°C or less.
[3] In the method for producing a cold-rolled steel sheet according to [2], a coating layer containing zinc, aluminum, magnesium or an alloy thereof may be formed on the front and back surfaces of the steel sheet in the annealing step.
[4] A method for manufacturing a welded joint according to another aspect of the present invention is a welded joint in which a plurality of steel plates are joined, and at least one of the steel plates is the cold-rolled steel plate described in [1].

According to the above aspect of the present invention, it is possible to provide an ultra-high strength steel plate having a tensile strength of 1310 MPa or more, which has a sufficiently high joint strength after welding, as well as a welded joint.

FIG. 1 is a diagram showing the shape of a test piece for an Auger test.

This article describes a cold-rolled steel sheet according to one embodiment of the present invention (the cold-rolled steel sheet according to this embodiment), its manufacturing method, and a welded joint obtained using the cold-rolled steel sheet according to this embodiment.

[Cold rolled steel sheet]
The cold-rolled steel sheet according to this embodiment has a predetermined chemical composition, and the metal structure at a position from the surface to ¼ to ¾ of the thickness in the sheet thickness direction contains, in volume fractions, 0% to 10.0% retained austenite and 90.0% to 100% martensite and one or both of tempered martensite, and in the metal structure at the above-mentioned position, the P content at the prior γ grain boundaries is 10.0 mass% or less and the Mn content at the prior γ grain boundaries is 10.0 mass% or less, and the tensile strength of the steel sheet is 1310 MPa or more.

<Chemical composition>
First, the chemical composition will be described. In this embodiment, the % of the content of each element means % by mass.

C: 0.200% or more, 0.450% or less C is an element that is related to the hardness of martensite and tempered martensite and is necessary for increasing the strength of the steel sheet and the strength of the joint after welding. To obtain a tensile strength of 1310 MPa or more, the C content is set to 0.200% or more. The C content is preferably 0.210% or more, and more preferably 0.220% or more.
On the other hand, if the C content exceeds 0.450%, the weldability and formability deteriorate. Therefore, the C content is set to 0.450% or less. The C content is preferably 0.350% or less, and more preferably 0.300% or less.

Si: 0.01% or more, 2.50% or less Si is a solid solution strengthening element and is an element that is effective in increasing the strength of steel sheets. To achieve this effect, the Si content is set to 0.01% or more. The Si content is preferably set to 0.10% or more, and more preferably set to 0.20% or more.
On the other hand, if the Si content is excessive, the formability and wettability with the plating decrease. Therefore, the Si content is set to 2.50% or less. The Si content is preferably 2.00% or less, and more preferably 1.80% or less.

Mn: 0.6% or more, 3.5% or less Mn is an element that segregates at prior γ grain boundaries to improve the hardenability of steel and promotes the formation of martensite. If the Mn content is less than 0.6%, it becomes difficult to obtain the target microstructure. Therefore, the Mn content is set to 0.6% or more. The Mn content is preferably 1.0% or more.
On the other hand, excessive Mn content may result in deterioration of platability, workability, and weldability. In particular, deterioration of weldability is caused by Mn segregation at prior γ grain boundaries. Therefore, the Mn content is set to 3.5% or less. The Mn content is preferably 3.0% or less.

Al: 0.001% or more, 0.100% or less Al is an element that has the effect of deoxidizing molten steel. For deoxidation, the Al content is set to 0.001% or more. Furthermore, like Si, Al has the effect of increasing the stability of austenite, so it may be contained to obtain retained austenite.
On the other hand, if the Al content is too high, not only will surface defects due to alumina be more likely to occur, but the transformation point will also rise significantly, increasing the volume fraction of ferrite. In this case, it will be difficult to obtain the desired metal structure, and sufficient tensile strength will not 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, and even more preferably 0.030% or less.

Ti: 0.001% or more, 0.100% or less Ti is an element that combines with N to form TiN and contributes to the refinement of γ grains. The refinement of γ grains can suppress the P content at the γ grain boundaries. To achieve this effect, the Ti content is set to 0.001% or more. The Ti content is preferably 0.005% or more.
On the other hand, if the Ti content is excessive, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes non-uniform, and formability is impaired. Therefore, the Ti content is set to 0.100% or less.

N: 0.0001% or more and 0.0100% or less N is an element that bonds with Ti to form TiN. To form TiN, the N content is set to 0.0001% or more.
On the other hand, if the N content is too high, coarse precipitates are formed, deteriorating formability. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less.

P: 0.0400% or less P is an element contained in steel as an impurity, and segregates at grain boundaries to embrittle the steel. Therefore, the lower the P content, the better, and 0% is acceptable. However, taking into consideration the time and cost required for removing P, the P content is set to 0.0400% or less. The P content is preferably 0.0200% or less, and more preferably 0.0150% or less.
From the viewpoint of the cost of refining and the like, the P content may be set to 0.0001% or more.

S: 0.0100% or less S is an element contained in steel as an impurity, and forms sulfide-based inclusions that deteriorate the formability of steel sheet. Therefore, the lower the S content, the better, and 0% is acceptable. However, taking into consideration the time and cost required for removing S, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.
From the viewpoint of the cost of refining and the like, the S content may be set to 0.0001% or more.

O: 0.0060% or less O is an element that can be contained as an impurity. If the O content exceeds 0.0060%, coarse oxides are formed in the steel, resulting in reduced formability. Therefore, the O content is set to 0.0060% or less. The O content is preferably 0.0050% or less, and more preferably 0.0030% or less. The O content may be 0%, but from the viewpoint of costs such as refining, the O content may be set to 0.0005% or more, or 0.0010% or more.

In the chemical composition of the cold-rolled steel sheet according to this embodiment, the balance excluding the above elements is basically Fe and impurities. Impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process and are permissible to be contained within a range that does not clearly deteriorate the properties of the cold-rolled steel sheet according to this embodiment.
On the other hand, the chemical composition of the cold-rolled steel sheet according to this embodiment may contain one or more elements selected from the group consisting of B, Mo, Nb, Cr, V, Co, Ni, Cu, W, Ta, Sn, Sb, As, Mg, Ca, Y, Zr, La, and Ce in the ranges described below for the purpose of improving various properties. These elements do not necessarily need to be contained, so the lower limit is 0%. Furthermore, as long as the content is within the ranges described below, the effects of the cold-rolled steel sheet according to this embodiment are not impaired even if these elements are contained as impurities.

B: 0% or more, 0.0100% or less; Mo: 0% or more, 0.500% or less; Cr: 0% or more, 2.000% or less; Ni: 0% or more, 1.000% or less; As: 0% or more, 0.050% or less. B, Mo, Cr, Ni, and As are elements that improve hardenability and contribute to increasing the strength of the steel sheet. Therefore, these elements may be contained. To fully achieve the above effects, it is preferable that the B content be 0.0001% or more, the Mo content, the Cr content, and the Ni content be 0.010% or more, and the As content be 0.001% or more. More preferably, the B content is 0.0010% or more, the Mo content and the Cr content are 0.100% or more, and the As content is 0.005% or more. It is not essential to achieve the above effects. Therefore, there is no need to particularly set lower limits for the B content, Mo content, Cr content, Ni content, and As content, and the lower limits are 0%.
On the other hand, if B, Mo, Cr, Ni, or As is contained in excess, the effects of the above actions saturate and it becomes uneconomical. Therefore, if these elements are contained, the B content should be 0.0100% or less, the Mo content should be 0.500% or less, the Cr content should be 2.000% or less, the Ni content should be 1.000% or less, and the As content should be 0.050% or less. The B content is preferably 0.0030% or less, the Mo content is preferably 0.300% or less, the Cr content is preferably 1.000% or less, the Ni content is 0.500% or less, and the As content is preferably 0.030% or less.

Nb: 0% or more, 0.200% or less V: 0% or more, 0.500% or less Cu: 0% or more, 1.000% or less W: 0% or more, 0.100% or less Ta: 0% or more, 0.100% or less Nb, V, Cu, W, and Ta are elements that have the effect of improving the strength of the steel sheet by precipitation hardening. Therefore, they may be contained. To fully obtain the above effects, the Nb content, V content, Cu content, W content, and/or Ta content are each preferably 0.001% or more.
On the other hand, excessive content of these elements increases the recrystallization temperature, makes the metal structure of the cold-rolled steel sheet non-uniform, and impairs formability. Therefore, the Nb content is set to 0.200% or less, the V content is set to 0.500% or less, the Cu content is set to 1.000% or less, and the W content and the Ta content are each set to 0.100% or less.

Co: 0% or more, 0.500% or less Co is an element effective in improving the strength of steel sheet. The Co content may be 0%, but in order to obtain the above effect, 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, resulting in a risk of deteriorating formability. For this reason, the Co content is set to 0.500% or less.

Ca: 0% or more, 0.040% or less Mg: 0% or more, 0.050% or less La: 0% or more, 0.050% or less Ce: 0% or more, 0.050% or less Y: 0% or more, 0.050% or less Zr: 0% or more, 0.050% or less Sb: 0% or more, 0.050% or less Ca, Mg, La, Ce, Y, Zr, and Sb are elements that contribute to finely dispersing inclusions in steel, and this finely dispersing contributes to improving the formability of the steel sheet. Therefore, these elements may be contained. To achieve the above effects, it is preferable to contain one or more of Ca, Mg, La, Ce, Y, Zr, and Sb, and to set the content of each to 0.001% or more.
On the other hand, excessive inclusion of these elements deteriorates ductility, and therefore the Ca content is set to 0.040% or less, and the Mg content, La content, Ce content, Y content, Zr content, and Sb content are each set to 0.050% or less.

Sn: 0% or more and 0.050% or less Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet. 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 the embrittlement of ferrite. If the Sn content exceeds 0.050%, the 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 according to this embodiment can be determined by the following method.
For example, measurement can be performed on chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G1201 (2014). In this case, the chemical composition is the average content across the entire plate thickness. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.
When the steel sheet has a coating layer on its surface, the coating layer can be removed by mechanical grinding or the like before analyzing the chemical composition. When the coating layer is a plating layer, the plating layer can be removed by dissolving it in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet has been added.

<Metal structure (microstructure)>
In the cold-rolled steel sheet according to this embodiment, the metal structure at a position from the surface to ¼ to ¾ of the sheet thickness in the sheet thickness direction (in the range of t/4 to 3t/4, where t is the sheet thickness) contains, in volume fractions, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or both of martensite and tempered martensite.
The retained austenite contributes to improving the formability of the steel sheet by improving the uniform elongation of the steel sheet due to the TRIP effect. Therefore, retained austenite (retained γ) may be contained. To obtain the above effect, the volume fraction of the retained austenite is preferably 1.0% or more. The volume fraction of the retained austenite is more preferably 2.0% or more, and even more preferably 3.0% or more.
On the other hand, if the volume fraction of retained austenite is excessive, the grain size of the retained austenite becomes large. Such retained austenite with a large grain size becomes coarse and hard martensite after deformation. In this case, crack initiation points are more likely to occur, and the formability of the cold-rolled steel sheet decreases. For this reason, 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, and more preferably 7.0% or less.
The structure other than the retained austenite includes one or both of martensite and tempered martensite.
Martensite (so-called fresh martensite) and tempered martensite are aggregates of lath-shaped crystal grains and contribute greatly to improving strength. Therefore, the cold-rolled steel sheet according to this embodiment contains martensite and tempered martensite at a total volume ratio of 90.0 to 100%.
Unlike martensite, tempered martensite is a hard structure containing fine iron-based carbides inside due to tempering. Although tempered martensite contributes less to improving strength than martensite, it is not brittle and has ductility. Therefore, when it is desired to further improve formability, it is preferable to increase the volume fraction of tempered martensite. 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 contain ferrite and bainite in addition to retained austenite, martensite, and tempered martensite.
Since ferrite is a soft structure that improves the uniform elongation of the cold-rolled steel sheet and consequently contributes to improving workability, when ferrite is contained, the total of retained austenite and ferrite may be 5% or more or more than 5%. On the other hand, if the volume fraction of ferrite exceeds 3%, the tensile strength of the steel sheet may decrease, so the volume fraction of ferrite is preferably 3% or less.
Pearlite is a structure that has strength intermediate between martensite and ferrite, but is poor in deformability and deteriorates workability, so it is preferable that pearlite is substantially not contained.
The reason why the metal structure at a position from the surface to 1/4 to 3/4 of the plate thickness, centered at a position 1/2 of the plate thickness from the surface in the plate thickness direction, is specified is that in the cold-rolled steel plate according to this embodiment, the metal structure at this position is a representative structure of the steel plate and has a strong correlation with the properties.

The volume fraction of each structure in the metal structure (microstructure) at a position of 1/4 to 3/4 of the sheet thickness from the surface of the cold-rolled steel sheet according to this embodiment in the sheet thickness direction is measured as follows.
That is, the volume fractions of ferrite, bainite, martensite, tempered martensite, and pearlite were determined by taking a test piece from any position in the rolling direction and width direction of the steel sheet, polishing a longitudinal cross section parallel to the rolling direction, and observing the structure revealed by nital etching in a range of 1/4 to 3/4 of the sheet thickness from the surface in the sheet thickness direction using an SEM. In the SEM observation, five 30 μm × 50 μm fields of view were observed at a magnification of 3000 times, and the area fractions of each structure were measured from the observed images and their average values were calculated. Since there was no change in the structure in the direction perpendicular to the rolling direction (the steel sheet width direction), the area fractions of the longitudinal cross section parallel to the rolling direction were considered to be equal to the volume fractions, and the area fractions obtained by the structure observation were used as the respective volume fractions.

When measuring the area ratio of each structure, areas where the substructure is not visible and has low brightness are considered to be ferrite. Areas where the substructure is not visible and has high brightness are considered to be martensite or retained austenite. Areas where the substructure is visible are considered to be tempered martensite or bainite.

Bainite and tempered martensite can be further distinguished by careful observation of intragranular carbides.
Specifically, tempered martensite is composed of martensite laths and cementite formed within the laths. Since there are two or more types of crystal orientation relationships between martensite laths and cementite, the cementite that constitutes tempered martensite has multiple variants. On the other hand, bainite is classified into upper bainite and lower bainite. Upper bainite is easily distinguishable from tempered martensite because it is composed of lath-shaped bainitic ferrite and cementite formed at the lath interfaces. Lower bainite is composed of lath-shaped bainitic ferrite and cementite formed within the laths. Unlike tempered martensite, the crystal orientation relationship between bainitic ferrite and cementite is one type, and the cementite that constitutes lower bainite has the same variant. Therefore, lower bainite and tempered martensite can be distinguished based on the cementite variant.
On the other hand, martensite and retained austenite cannot be clearly distinguished by SEM observation, and therefore the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite, calculated by the method described below, from the volume fraction of the structure determined to be martensite or retained austenite.

The volume fraction of retained austenite is determined by taking a test piece from any position on the steel plate, chemically polishing the rolled surface from the surface of the steel plate to a position 1/4 of the plate thickness, and quantifying the (200), (210) area integral intensity of ferrite and the (200), (220), and (311) area integral intensity of austenite using MoKα radiation.

Furthermore, in the cold-rolled steel sheet according to this embodiment, in the metal structure at a position of ¼ to ¾ of the sheet thickness from the surface in the sheet thickness direction, the P content at the prior γ grain boundaries is 10.0 mass% or less, and the Mn content at the prior γ grain boundaries is 10.0 mass% or less.
Segregation of Mn and P typically occurs due to elemental partitioning between the solid and liquid phases during dendrite growth during solidification in the continuous casting process. When welding is performed on a steel sheet with segregation, differences in the Mn content (concentration difference) cause differences in the hardness of martensite in the heat-affected zone of the weld (partial hardening), resulting in differences in joint strength after welding. This is presumably because the Ms point changes depending on the Mn content. Additionally, co-segregation of Mn and P makes cracking more likely. Therefore, to improve joint strength after welding, it is necessary to reduce the segregation of Mn and P. Therefore, in the cold-rolled steel sheet according to this embodiment, the segregation of Mn and P is suppressed. More specifically, the P content at the prior γ grain boundaries is set to 10.0 mass% or less, and the Mn content at the prior γ grain boundaries is set to 10.0 mass% or less.
If the P content exceeds 10.0 mass % or the Mn content exceeds 10.0 mass % at the prior γ grain boundaries, the strength of the welded joint obtained by welding decreases due to differences in hardness and cracks.
The P content and Mn content in the prior γ grain boundary are each preferably 8.0 mass % or less, and more preferably 6.0 mass % or less.
Furthermore, although there are no lower limits for the P content and the Mn content at the prior γ grain boundaries, since both P and Mn are elements that segregate at grain boundaries, it is not easy to keep the P content at 80 times or less the P content of the base material, and it is difficult in principle to keep the Mn content at 1.01 times or less the Mn content of the base material. Therefore, taking these into consideration, the P content and the Mn content at the prior γ grain boundaries may each be 3.6% or more.
The reason why the degree of segregation is specified in the metal structure at a position of 1/4 to 3/4 of the plate thickness in the plate thickness direction from the surface, with the position of 1/2 of the plate thickness in the plate thickness direction from the surface as the center, is that the degree of segregation at this position is larger than at other positions, and the effect of segregation mitigation in a region including this position can be evaluated.
Although the effects of macro and semi-macro segregation of both P and Mn have been evaluated in the past, the effects of these elements on segregation at prior γ grain boundaries as described above have not been clarified. The present inventors have newly discovered that joints with high joint strength can be obtained by controlling the P content and Mn content at the prior γ grain boundaries.

The P content and Mn content at the prior γ grain boundary are measured by the following method.
A test piece for Auger testing of the size shown in Figure 1 is cut from a steel plate at a position 1/4 to 3/4 of the plate thickness from the surface, with the center at a position 1/2 of the plate thickness from the surface. This test piece is immersed in an aqueous solution of ammonium thiocyanate with a concentration of 20 mass% for 48 hours. After immersion, an impact test is performed on the test piece to obtain a fracture surface. In the impact test, the test piece is cooled with liquid nitrogen and then broken by hitting it with a hammer in a vacuum. This results in a grain boundary (prior γ grain boundary) fracture surface, and Auger electron spectroscopy is performed on this fracture surface to measure the P content and Mn content. This determines the P content and Mn content at the prior γ grain boundary.
The measuring device is not particularly limited, but for example, a JAMP-9500F manufactured by JEOL Ltd. is used. Furthermore, during the measurement, a portion of the grain boundary fracture surface where no precipitates are present is measured at least three times, and the AES peaks of P and Mn are measured. With reference to the non-patent document (Analytical Chemistry, Vol. 35 (1986)), the AES peak intensities are subjected to sensitivity correction using the respective relative sensitivity factors (RSFs) to determine the grain boundary segregation concentrations.

To reduce the P and Mn content at the prior γ grain boundaries (reduce segregation at prior γ grain boundaries), it is also effective to refine the crystal grains by performing a large reduction in hot rolling, as described below. Therefore, in the cold-rolled steel sheet according to this embodiment, it is preferable that the prior γ grain size (average grain size) be 15 μm or less.

The prior γ grain size can be measured by the following method.
A test specimen is taken from any position in the rolling direction and width direction of the steel plate, and a longitudinal cross section parallel to the rolling direction is polished. The structure revealed using a picric acid saturated aqueous solution is observed using an optical microscope within a range of 1/4 to 3/4 of the plate thickness from the surface. Among the structures revealed using the picric acid saturated aqueous solution, black, mesh-like lines are determined to be prior-γ grain boundaries. If no mesh-like black lines appear in the picric acid saturated aqueous solution, they can be revealed by adding a surfactant or changing the immersion temperature in the picric acid saturated aqueous solution to approximately 20 to 80°C. For observation using an optical microscope, a magnification of 200 to 1000 times is selected to obtain images of the structure. Three images containing at least 200 or more crystal grains are taken, and the average prior-γ (austenite) grain size is measured for the images using the point counting method.
The method for measuring the prior γ grain size is not limited to the above, and measurement is also possible using other techniques such as inverse analysis of prior austenite using SEM-EBSD.

The cold-rolled steel sheet according to the present embodiment described above may have a coating layer containing zinc, aluminum, magnesium, or an alloy thereof on its surface. The coating layer may consist essentially of zinc, aluminum, magnesium, or an alloy thereof. The presence of the coating layer on the steel sheet surface improves corrosion resistance. The coating layer may be a known coating layer.
For example, when steel sheets are used in a corrosive environment, there are concerns about holes and other problems, so even if the strength is increased, it may not be possible to reduce the thickness below a certain level. One of the purposes of increasing the strength of steel sheets is to reduce the weight by reducing the thickness, so even if a high-strength steel sheet is developed, its application areas will be limited if its corrosion resistance is low. A coating layer containing zinc, aluminum, magnesium, or an alloy thereof on the surface is preferable because it improves corrosion resistance and expands the range of application.
When the steel sheet has a coating layer (for example, a plating layer) on its surface, the "surface" in the "position 1/4 to 3/4 of the thickness from the surface of the steel sheet" means the surface of the base steel excluding the coating layer.

The thickness of the cold-rolled steel sheet in this embodiment is not limited to a specific range, but considering strength, versatility, and manufacturability, a thickness of 1.0 to 2.0 mm is preferable.

<Tensile strength>
The cold-rolled steel sheet according to this embodiment has a tensile strength (TS) of 1310 MPa or more, which is a strength that contributes to reducing the weight of an automobile body. From the viewpoint of impact absorption, the tensile strength is preferably 1400 MPa or more, and more preferably 1470 MPa or more.
There is no need to set an upper limit, but since an increase in tensile strength may result in a decrease in formability, the tensile strength may be set to 2000 MPa or less.

[Welded joints]
The welded joint according to the present embodiment is obtained by joining the cold-rolled steel sheet according to the present embodiment and another steel sheet (which may be the cold-rolled steel sheet according to the present embodiment) by welding. Therefore, the welded joint according to the present embodiment is a welded joint in which a plurality of steel sheets are joined, and at least one of the steel sheets is the cold-rolled steel sheet according to the present embodiment described above.
In the welded joint according to this embodiment, steel plates are joined via a welded portion, and if the welding is spot welding, the steel plates are joined via the spot welded portion.

[Manufacturing method]
The cold-rolled steel sheet according to this embodiment can obtain the effects as long as it has the above-described characteristics regardless of the manufacturing method, but can be stably manufactured by the manufacturing method described below.
Specifically, the cold-rolled steel sheet according to this embodiment can be produced by a production method including the following steps (I) to (VIII).
(I) a continuous casting step for obtaining a slab having a predetermined chemical composition by continuous casting;
(II) a breakdown process in which the slab is reduced in thickness at a reduction rate of 30 to 60% in a temperature range of 850 to 1000 ° C.;
(III) a high-temperature heat treatment step of heating the slab after the breakdown step to 1000 to 1300 ° C, holding it for 5 to 20 hours, and cooling it;
(IV) a hot rolling step of hot rolling the slab after the high-temperature heat treatment step to obtain a hot-rolled steel sheet;
(V) a winding step of winding the hot-rolled steel sheet in a temperature range of 400 to 650°C;
(VI) a cold rolling step of pickling the hot-rolled steel sheet after the coiling step and cold-rolling it at a reduction ratio of 20 to 80% to obtain a cold-rolled steel sheet;
(VII) an annealing step in which the cold-rolled steel sheet is heated to an annealing temperature exceeding Ac3°C at an average heating rate of 2°C/sec or more, held at the annealing temperature for 60 to 300 seconds, and cooled to 250°C or less at an average cooling rate of 10°C/sec or more;
(VIII) A holding step of holding the cold-rolled steel sheet after the annealing step at 150 to 400°C for 500 seconds or less.
Furthermore, the welded joint according to this embodiment can be obtained by a manufacturing method that further includes the following steps.
(IX) A welding step of welding the cold-rolled steel sheet after the holding step to another steel sheet.
Preferred conditions for each step will be explained below.

<Continuous casting process>
In the continuous casting process, a slab having a predetermined chemical composition (which does not substantially change in the subsequent processes, and therefore has the same chemical composition as the cold-rolled steel sheet according to this embodiment) is obtained by continuous casting.

<Breakdown (BD) process>
<High-temperature heat treatment (SP treatment) step>
In the breakdown process, the slab obtained in the continuous casting process is reduced in thickness by reduction (BD) at a temperature range of 850 to 1000°C with a reduction ratio of 30 to 60%. If the slab obtained in the continuous casting process is at a temperature lower than 850°C, it is heated before reduction. If the temperature of the slab is in the range of 850 to 1000°C, heating is not necessary.
Thereafter, in the high-temperature heat treatment step, the slab after the breakdown step is heated to 1000 to 1300° C., held at that temperature for 5 to 20 hours (SP treatment), and then cooled.
The SP treatment alleviates the segregation of Mn and P. However, if the segregation of Mn and P is to be alleviated by the SP treatment alone, a treatment at a significantly high temperature or for a long time is required. Therefore, in the manufacturing method of the cold-rolled steel sheet according to the present embodiment, BD is performed before the SP treatment to sufficiently alleviate the segregation.
By performing BD, the diffusion constant increases and the segregation zone thickness decreases. Therefore, by performing SP treatment after BD, the segregation of Mn and P can be alleviated within a practically feasible range of temperature and time. If either of these conditions is not met, sufficient effects cannot be obtained.
In the past, the BD process or the SP process was sometimes performed alone to reduce macrosegregation and semimacrosegregation. However, the effect of the BD process or the SP process in reducing the P content or the Mn content at the prior γ grain boundaries was not clear. Furthermore, it was not known that by combining the BD process and the SP process and then performing a large reduction in hot rolling as described below, the P content and the Mn content at the prior γ grain boundaries could be reduced to a predetermined range more than when the BD process or the SP process was performed alone. For this reason, combining these processes was not commonly performed.

<Hot rolling process>
In the hot rolling step, the slab after the BD and SP treatments is heated and hot rolled to obtain a hot-rolled steel sheet.
The heating temperature prior to hot rolling is not limited, but if it is less than 1100°C, there is a concern that carbides and sulfides generated during the period from casting to the SP treatment step will not dissolve and will become coarse, resulting in coarse grain sizes during annealing, so the heating temperature is preferably 1100°C or higher. The upper limit of the heating temperature is not particularly specified, but is generally 1300°C or lower.
In the hot rolling process, recrystallization is utilized to refine the γ grains and suppress P segregation to the grain boundaries.
For this reason, in the hot rolling process, rough rolling and finish rolling are usually performed, and in this finish rolling, a rolling mill having four or more stands is used, with the first stand being the first stand and the final stand being the nth stand. In this case, the thickness reduction rate at each of the (n-3)th stand to the nth stand is set to 30% or more, and the rolling temperature at the final stand (nth stand) is set to 900°C or less. That is, for example, in a rolling mill having seven stands, the thickness reduction rate at each of the fourth stand, fifth stand, sixth stand, and seventh stand is set to 30% or more, and the rolling temperature at the seventh stand is set to 900°C or less. In this finish rolling, the austenite grain size is refined by recrystallization during rolling, and the refined grain boundaries are used as diffusion paths to promote the diffusion of Mn, P, etc., and mitigate segregation.
If even one of the thickness reduction rates in each stand is less than 30%, or if the rolling temperature in the nth stand exceeds 900°C, the hot-rolled structure becomes coarse and mixed-grained, and the structure after the annealing process described below also becomes coarse. If the hot-rolling completion temperature is less than 830°C, the rolling reaction force increases, making it difficult to stably obtain the target thickness. For this reason, the rolling temperature in the final stand is preferably 830°C or higher. Furthermore, even if the reduction rate is greater than 50%, the effect of grain refinement saturates, and the equipment load increases excessively due to the increase in rolling load. Therefore, it is preferable that the thickness reduction rates in the n-3th stand to the nth stand are each 50% or less.
Furthermore, the finish rolling is performed using a rolling mill having four or more stands in order to perform continuous rolling with short inter-pass times for the final four passes of rolling, because if the inter-pass times are long, strain will recover between passes even if reduction is performed with a large thickness reduction rate, and strain will not accumulate sufficiently.

<Winding process>
In the coiling step, the hot-rolled steel sheet after the hot rolling step is coiled at a coiling temperature of 400°C or higher and 650°C or lower.
If the coiling temperature exceeds 650°C, an internal oxide layer is formed, and pickling properties deteriorate.
On the other hand, if the coiling temperature is less than 400°C, the strength of the steel sheet becomes excessive, the cold rolling load becomes excessive, and productivity deteriorates.

<Cold rolling process>
In the cold rolling process, the hot-rolled steel sheet after the coiling process is pickled under known conditions, and then cold-rolled at a reduction rate (thickness reduction rate) of 20 to 80% to obtain a cold-rolled steel sheet.
If the sheet thickness reduction rate is less than 20%, the accumulation of strain in the steel sheet becomes insufficient, the nucleation sites of austenite become non-uniform, and the degree of segregation of Mn and P at the prior γ grain boundaries increases.
On the other hand, if the sheet thickness reduction rate exceeds 80%, the cold rolling load becomes excessive, resulting in a deterioration in productivity.
Therefore, the thickness reduction rate is set to 20% or more and 80% or less. The thickness reduction rate is preferably 30% or more and 80% or less. There are no restrictions on the cold rolling method, and the number of rolling passes and the reduction rate per pass may be set appropriately.

<Annealing process>
In the annealing step, the cold-rolled steel sheet obtained in the cold rolling step is heated to an annealing temperature of more than Ac3°C at an average heating rate of 2°C/second or more, held at this annealing temperature for 60 to 300 seconds, and then cooled to 250°C or less at an average cooling rate of 10°C/second or more.
If the average temperature rise rate is less than 2°C/sec, productivity will decrease, the grain size will become coarse, and the degree of segregation of Mn and P at the prior γ grain boundaries will increase, which is undesirable.
If the annealing temperature is Ac3°C or lower or the holding time is less than 60 seconds, the gamma transformation may be insufficient and the target structure may not be obtained after the annealing step. On the other hand, if the annealing time exceeds 300 seconds, productivity decreases.
If the average cooling rate is less than 10°C/sec or the cooling stop temperature exceeds 250°C, ferrite or bainite may be generated, and the target metal structure may not be obtained. On the other hand, setting the cooling stop temperature to less than 150°C not only requires a significant capital investment, but also saturates its effect at temperatures below 150°C. Therefore, it is preferable to set the cooling stop temperature to 150°C or higher.
The temperature (°C) of the Ac3 point can be determined 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

In the annealing process, a coating layer containing zinc, aluminum, magnesium, or an alloy thereof may be formed on the surface of the steel sheet to enhance its corrosion resistance. For example, during cooling after holding, the steel sheet may be immersed in a coating bath to form a hot-dip coating within a range that satisfies the above-mentioned average cooling rate. This hot-dip coating may also be heated to a predetermined temperature to alloy it, resulting in an alloyed hot-dip coating. The coating layer may also contain Fe, Al, Mg, Mn, Si, Cr, Ni, Cu, etc. Any of the above methods may be used to form a coating layer intended to enhance corrosion resistance. Known plating and alloying conditions may be applied depending on the coating composition.

<Holding process>
In the holding step, the cold-rolled steel sheet after the annealing step is held at 150 to 400° C. for 500 seconds or less.
The holding step tempers part or all of the martensite to form tempered martensite. If the holding temperature is less than 150°C, the martensite is not sufficiently tempered, and the effect of the holding step cannot be fully obtained.
If the holding temperature exceeds 400°C, the dislocation density in the tempered martensite decreases, which may result in a decrease in tensile strength. If the holding time exceeds 500 seconds, the tensile strength decreases and productivity also decreases.
There is no lower limit to the holding time, but when the metal structure is to be mainly tempered martensite, the holding time is preferably 100 seconds or more.
If the temperature of the cold-rolled steel sheet has dropped to less than 150°C before the holding step, heating may be carried out as necessary.

<Welding process>
In the welding step, the cold-rolled steel sheet after the holding step is welded to another steel sheet. The other steel sheet is not limited and may be the cold-rolled steel sheet according to the present embodiment or may be a different steel sheet. Furthermore, welding may be performed multiple times to join three or more steel sheets.
There are no limitations on the welding method, but when considering application to automobile parts, spot welding is preferred.

Slabs (steel types A to X) having the chemical compositions shown in Tables 1-1 and 1-2 (units are mass %, the remainder is Fe and impurities) were produced by continuous casting.
These slabs were heated to the temperature in Table 2-1, and reduced at the reduction rates in Table 2-1 to reduce the thickness and perform breakdown. Thereafter, they were heated to the temperature in Table 2-1 and held there while undergoing SP treatment.
The slab after the SP treatment was heated to 1100 to 1300°C, hot rolled, and coiled at the coiling temperature shown in Table 2-2 to obtain a hot-rolled steel sheet. In the hot rolling, a hot rolling mill having seven stands was used for finish rolling, and the reduction ratios from the third stand to the final stand and the rolling temperature at the final stand were as shown in Table 2-2.
The hot-rolled steel sheets were pickled under known conditions and then cold-rolled at the reduction ratios shown in Table 2-2 to obtain cold-rolled steel sheets with thicknesses of 1.0 to 2.0 mm. However, some of the hot-rolled steel sheets had high strength and could not be cold-rolled.
The obtained cold-rolled steel sheets were annealed under the conditions shown in Table 2-3, and then held under the conditions shown in Table 2-3.
Furthermore, some cold-rolled steel sheets were heated or cooled to (galvanized bath temperature -40) °C to (galvanized bath temperature +50) °C during annealing (cooling stage), and then immersed in a galvanized bath to perform galvanization (in the table, examples with or without plating are indicated as "prepared"). Furthermore, some of the galvanized cold-rolled steel sheets were further heated to a temperature range of 470 to 550 °C to perform alloying (in the table, examples with or without alloying are indicated as "prepared").

The metal structure of the obtained cold-rolled steel sheet was observed at positions from t/4 to 3t/4 using the method described above, and the total volume fraction of martensite and tempered martensite, and the volume fractions of retained austenite, ferrite, bainite, and pearlite were determined.

In addition, using the method described above, the P content and Mn content at the prior γ grain boundaries were measured in the metal structure at positions t/4 to 3t/4.

In addition, JIS No. 5 test pieces were taken from the obtained cold-rolled steel sheets perpendicular to the rolling direction, and the tensile strength was measured in accordance with JIS Z2241:2011.

Furthermore, two of the obtained cold-rolled steel sheets were stacked together, and spot welding was performed on the sheet assemblies, and the joint properties were evaluated.
For welding, a servo motor pressure type single phase AC welding machine (power frequency 50 Hz) was used, and a DR type electrode made of Cr-Cu with a tip curvature radius of 40 mm and a tip diameter of 6 mm was used.
The welding conditions were a pressure of 440 kgf, a current application time of 0.28 seconds, and a hold time of 0.1 seconds. The welding current was set to a condition that would result in a nugget diameter of 5√t.
Then, a cross tension test was carried out on the produced joints in accordance with JIS Z3137 (1999) (performed under each condition n=2).
The joint properties were evaluated as △ (Fair) if they were improved by 5% or more compared to conventional steel sheets that had not been subjected to segregation relaxation (steel sheets having the same chemical composition and manufactured under the same conditions except for the breakdown process, high-temperature heat treatment process, and hot rolling process), ◯ (Good) if they were improved by 10% or more, ◎ (Excellent) if they were improved by 20% or more, and × (NG) if they showed no improvement.

As can be seen from Tables 1-1 to 3, in Nos. 1 to 30, which are examples of the present invention (invention examples), the chemical composition, metal structure, Mn content at the prior γ grain boundary, and P content (segregation) are within the ranges of the present invention, and as a result, they have a high strength of 1310 MPa or more and sufficient joint strength.
On the other hand, in Comparative Examples Nos. 31 to 47, which are chemical compositions or manufacturing methods outside the range of the present invention, at least one of the chemical composition, metal structure, Mn content at the prior γ grain boundary, and P content (segregation degree) is outside the range of the present invention, and either the tensile strength or the joint strength is insufficient.

The present invention provides ultra-high-strength steel plates with a tensile strength of 1,310 MPa or more, which provide sufficiently high joint strength after welding, as well as welded joints. These steel plates and welded joints contribute to reducing the weight of automobile bodies, and therefore have high industrial applicability.

Claims (4)

In mass%,
C: 0.200% or more, 0.450% or less,
Si: 0.01% or more, 2.50% or less,
Mn: 0.6% or more, 3.5% or less,
Al: 0.001% or more, 0.100% or less,
Ti: 0.001% or more, 0.100% or less,
N: 0.0100% or less,
P: 0.0400% or less,
S: 0.0100% or less,
O: 0.0060% or less,
B: 0% or more, 0.0100% or less,
Mo: 0% or more, 0.500% or less,
Nb: 0% or more, 0.200% or less,
Cr: 0% or more, 2.00% or less,
V: 0% or more, 0.500% or less,
Co: 0% or more, 0.500% or less,
Ni: 0% or more, 1.000% or less,
Cu: 0% or more, 1.000% or less,
W: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Sn: 0% or more, 0.050% or less,
Sb: 0% or more, 0.050% or less,
As: 0% or more, 0.050% or less,
Mg: 0% or more, 0.050% or less,
Ca: 0% or more, 0.040% or less,
Y: 0% or more, 0.050% or less,
Zr: 0% or more, 0.050% or less,
La: 0% or more, 0.050% or less,
Ce: 0% or more and 0.050% or less, and
The balance has a chemical composition consisting of Fe and impurities,
A metal structure at a position of 1/4 to 3/4 of the plate thickness from the surface in the plate thickness direction includes, by volume fraction, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or both of martensite and tempered martensite,
In the metal structure at the position, the P content at the prior γ grain boundary is 3.6 mass% or more and 10.0 mass% or less, and the Mn content at the prior γ grain boundary is 3.6 mass% or more and 10.0 mass% or less,
The tensile strength is 1310 MPa or more.
A cold-rolled steel sheet characterized by: The method for producing a cold-rolled steel sheet according to claim 1,
a continuous casting step for obtaining a slab having the chemical composition according to claim 1 by continuous casting;
a breakdown process in which the slab is reduced in thickness at a reduction rate of 30 to 60% in a temperature range of 850 to 1000°C;
a high-temperature heat treatment step of heating the slab after the breakdown step to 1000°C to 1300°C, holding it for 5 to 20 hours, and cooling it;
a hot rolling step of hot rolling the slab after the high-temperature heat treatment step to obtain a hot-rolled steel sheet;
a winding step of winding the hot-rolled steel sheet in a temperature range of 400 to 650°C;
a cold rolling step of pickling the hot-rolled steel sheet after the coiling step and cold-rolling it at a reduction ratio of 20 to 80% to obtain a cold-rolled steel sheet;
An annealing step of heating the cold-rolled steel sheet to an annealing temperature of more than Ac3°C at an average heating rate of 2°C/second or more, holding the annealing temperature for 60 to 300 seconds, and cooling the cold-rolled steel sheet to 250°C or less at an average cooling rate of 10°C/second or more;
A holding step of holding the cold-rolled steel sheet after the annealing step at 150 to 400 ° C. for 500 seconds or less;
Equipped with
In the hot rolling step,
When finish rolling is performed using a rolling mill having four or more stands, with the first stand being the first stand and the last stand being the nth stand, the thickness reduction rate at each of the (n-3)th stand to the nth stand is 30% or more, and the rolling temperature at the nth stand is 900°C or less.
A method for producing a cold-rolled steel sheet. In the annealing step, a coating layer containing zinc, aluminum, magnesium or an alloy thereof is formed on the front and back surfaces of the steel sheet.
The method for producing a cold-rolled steel sheet according to claim 2 . A welded joint in which a plurality of steel plates are joined, wherein at least one steel plate is the cold-rolled steel plate according to claim 1.
A welded joint characterized by: