KR20240046196A - Cold rolled steel sheet and its manufacturing method, and welded joints - Google Patents

Abstract

This cold-rolled steel sheet has a predetermined chemical composition, and the metal structure at a position of 1/4 to 3/4 of the sheet thickness in the direction from the surface to the sheet thickness includes retained austenite of 0% or more and 10.0% or less by volume, It contains one or both types of martensite and tempered martensite of 90.0% or more and 100% or less, and in the metal structure at the above position, the P content at the sphere γ grain boundary is 10.0 mass% or less, and the sphere γ The Mn content at the grain boundary is 10.0 mass% or less, and the tensile strength is 1310 MPa or more.

Description

Cold rolled steel sheet and its manufacturing method, and welded joints

The present invention relates to cold rolled steel sheets, methods for manufacturing the same, and welded joints.

This application claims priority based on Japanese Patent Application No. 2021-168157, filed in Japan on October 13, 2021, and uses the content here.

Today, the industrial technology field is highly specialized, and materials used in each technology field are required to have special yet high performance. For example, regarding steel sheets for automobiles, high strength is required in consideration of the global environment and to improve fuel efficiency by reducing the weight of the vehicle body. When a high-strength steel plate is applied to the body of a car, the thickness of the steel plate can be thinned to lighten the car body and provide the desired strength to the car body.

In recent years, requirements for automotive steel sheets have become more sophisticated, and among automotive steel sheets, especially cold rolled steel sheets used for car body frame parts, high strength has been required, and steel sheets with a tensile strength of 1310 MPa or more are required.

In response to such a request, for example, in Patent Document 1, a high-strength steel sheet used in automobile parts, etc. has a predetermined composition, has a predetermined steel sheet structure mainly composed of martensite and bainite, and has a cross section perpendicular to the rolling direction. A high-strength steel sheet with an average number of inclusions with an average particle diameter of 5 μm or more of 5.0 pieces/mm 2 or less, excellent delayed fracture resistance, and a tensile strength of 1470 MPa or more is disclosed.

Additionally, in Patent Document 2, the ferrite area ratio is 30% or less (including 0%), the bainite area ratio is 5% or less (including 0%), and the martensite and tempered martensite area ratios are 70%. or more (inclusive of 100%), the retained austenite area ratio is 2.0% or less (inclusive of 0%), and the ratio of the dislocation density within the range of 0 to 20 ㎛ from the surface of the steel sheet to the dislocation density at the center of the sheet thickness is 90%. A foil having a steel structure in which the average of the top 10% of cementite particle diameters from the surface of the steel sheet to a depth of 100 ㎛ is 300㎚ or less, and the maximum deflection of the steel sheet when sheared to a length of 1 m in the longitudinal direction of the steel sheet is 15 mm or less. A steel plate is disclosed. Patent Document 2 discloses that this steel sheet has a tensile strength of 980 MPa or more, and that a tensile strength of 2000 MPa or more can be obtained.

In addition, Patent Document 3 states that the chemical composition (C, Si, Mn, Al, P, S) satisfies the specified range, the balance is made up of iron and inevitable impurities, and martensite occupies the entire structure is 95%. % or more, and the structure from a position of 10 ㎛ depth in the direction of the sheet thickness from the surface of the steel sheet to a position of a depth of 1/4 of the sheet thickness satisfies a predetermined relational expression, and the tensile strength is 1180 MPa or more. An excellent high-strength steel plate is disclosed.

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

As described above, conventionally, high-strength steel sheets with a tensile strength of 1310 MPa or more have been proposed. Such high-strength steel sheets generally have a high content of alloying elements such as Mn, and segregation of alloying elements such as Mn is observed within the steel sheets. Additionally, it is known that P, which is contained as an impurity along with Mn, is also segregated within the steel sheet. Segregation of Mn and P occurs when elements are distributed between the solid phase and the liquid phase during dendrite growth during solidification from molten steel. Since diffusion of these elements in steel is delayed, segregation during solidification cannot be eliminated by the degree of heating during hot rolling or annealing after solidification.

As a result of examination by the present inventors, it was found that when steel sheets having such segregation are welded, the joint strength sometimes decreases in the heat-affected zone of the weld zone due to the segregation of the steel sheets. However, in Patent Documents 1 to 3, the joint strength after welding is not considered.

Therefore, the subject of the present invention is to provide a steel plate that can obtain a sufficiently high joint strength after welding, assuming an ultra-high strength steel plate with a tensile strength of 1310 MPa or more. Furthermore, the problem is to provide a welded joint using this steel plate.

The present inventors investigated the reason why joint strength decreases due to segregation of Mn and P. As a result, it was found that differences in martensite hardness occur in the weld heat-affected zone due to differences in Mn content (concentration differences), and that this is because both Mn and P are segregated, making cracks more likely to occur. Additionally, it was found that Mn and P are likely to segregate at the old γ (austenite) grain boundaries.

Therefore, the present inventors studied means for suppressing segregation of Mn and P at the old γ grain boundaries.

As a result, the cast slab was subjected to breakdown (BD) and high-temperature heat treatment (SP treatment) prior to hot rolling, and further subjected to counter pressure during hot rolling, resulting in Mn and P in the old γ grain boundaries. It was discovered that segregation can be suppressed.

Additionally, it was found that when a steel plate in which such segregation was suppressed was used, the deterioration of the joint properties after welding could be suppressed.

The present invention was made in consideration of the above knowledge. The main point of the present invention is as follows.

[1] The cold rolled steel sheet according to one aspect of the present invention contains, 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% Hereinafter, 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% Hereinafter, 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% Hereinafter, 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% Hereinafter, 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: having a chemical composition consisting of Fe and impurities, from the surface to the sheet thickness direction. The metal structure at the position of 1/4 to 3/4 of the plate thickness is one of retained austenite with a volume ratio of 0% or more and 10.0% or less, martensite and tempered martensite with a volume ratio of 90.0% or more and 100% or less, or Two types are included, and in the metal structure at the above position, the P content at the old γ grain boundary is 10.0 mass% or less, the Mn content at the old γ grain boundary is 10.0 mass% or less, and the tensile strength is 1310 MPa. That's it.

[2] A method for producing a cold-rolled steel sheet according to another aspect of the present invention includes a continuous casting process of obtaining a slab having the chemical composition described in [1] by continuous casting, and placing the slab in a temperature range of 850 to 1000°C. A breakdown process of reducing the thickness by performing reduction at a reduction ratio of 30 to 60%, and a high temperature heat treatment of heating the slab after the breakdown process to 1000°C to 1300°C, holding for 5 to 20 hours, and cooling. A hot rolling process of hot rolling the slab after the high temperature heat treatment process to obtain a hot rolled steel sheet, a coiling process of winding the hot rolled steel sheet in a temperature range of 400 to 650° C., and the hot rolling after the coiling process. A cold rolling process of obtaining a cold rolled steel sheet by pickling the steel sheet and cold rolling it at a reduction ratio of 20 to 80%, heating the cold rolled steel sheet to an annealing temperature of more than Ac3°C at an average temperature increase rate of 2°C/sec or more, and An annealing process of maintaining the annealing temperature for 60 to 300 seconds and cooling to 250°C or lower at an average cooling rate of 10°C/sec or more, and maintaining the cold rolled steel sheet after the annealing process at 150 to 400°C for 500 seconds or less. A holding process is provided, and in the hot rolling process, when finish rolling is performed using a rolling mill having four or more stands, and the first stand is the first stand and the final stand is the nth stand, n- The sheet thickness reduction rate at each stand from stand 3 to the nth stand is set to 30% or more, and the rolling temperature at the nth stand is set to 900° C. or lower.

[3] In the method for manufacturing a cold rolled steel sheet described in [2], in the annealing process, a film layer containing zinc, aluminum, magnesium or an alloy thereof may be formed on the front and back surfaces of the steel sheet.

[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 steel plate 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 sheet with a tensile strength of 1310 MPa or more, a steel sheet in which a sufficiently high joint strength is obtained after welding, and a welded joint.

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

A cold-rolled steel sheet according to one embodiment of the present invention (cold-rolled steel sheet according to this embodiment), a manufacturing method thereof, and a weld joint obtained using the cold-rolled steel sheet according to this embodiment will be described.

[Cold rolled steel plate]

The cold-rolled steel sheet according to the present embodiment has a predetermined chemical composition, and the metal structure at a position of 1/4 to 3/4 of the sheet thickness in the sheet thickness direction from the surface has a residual volume ratio of 0% or more and 10.0% or less. It contains austenite and one or two types of martensite and tempered martensite of 90.0% or more and 100% or less, and in the metal structure at the above position, the P content at the old γ grain boundary is 10.0 mass% or less, Additionally, the Mn content at the sphere γ grain boundary is 10.0% by mass or less, and the tensile strength of the steel sheet is 1310 MPa or more.

<Chemical composition>

First, the chemical composition is explained. In this embodiment, % content of each element means mass %.

C: 0.200% or more, 0.450% or less

C is related to the hardness of martensite and tempered martensite, and is an element necessary to increase the strength of the steel sheet and the joint strength after welding. In order 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, more preferably 0.220% or more.

On the other hand, if the C content exceeds 0.450%, weldability deteriorates and formability deteriorates. Therefore, the C content is set to 0.450% or less. The C content is preferably 0.350% or less, 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 effective in increasing the strength of steel sheets. To obtain this effect, the Si content is set to 0.01% or more. The Si content is preferably 0.10% or more, and more preferably 0.20% or more.

On the other hand, when the Si content is excessive, the formability decreases and the wettability with plating decreases. Therefore, the Si content is set to 2.50% or less. The Si content is preferably 2.00% or less, more preferably 1.80% or less.

Mn: 0.6% or more, 3.5% or less

Mn is an element that segregates at the sphere γ grain boundaries to improve the quenching properties of steel, and is an element that 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, if the Mn content is excessive, there is a risk that plating properties, processability, and weldability may decrease. In particular, the decline in weldability is due to segregation of Mn at the sphere 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. Additionally, since Al, like Si, has the effect of increasing the stability of austenite, it may be included to obtain retained austenite.

On the other hand, if the Al content is too high, not only are surface scratches caused by alumina likely to occur, but the transformation point also increases significantly, and the volume fraction of ferrite increases. In this case, it becomes difficult to obtain the desired metal structure, and sufficient tensile strength is not 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. By refining γ, the P content at the γ grain boundary can be suppressed. To obtain 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, when 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, 0.0100% or less

N is an element that combines 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 high, coarse precipitates are formed and the formability deteriorates. 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 is an element that segregates at grain boundaries and embrittles the steel. For this reason, the lower the P content, the more preferable it is, and may be 0%. However, taking P removal time and cost into consideration, the P content is set to 0.0400% or less. The P content is preferably 0.0200% or less, more preferably 0.0150% or less.

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

S: 0.0100% or less

S is an element contained in steel as an impurity, and is an element that forms sulfide-based inclusions and deteriorates the formability of the steel sheet. For this reason, the lower the S content, the more preferable it is, and may be 0%. However, taking S removal time and cost into consideration, 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 costs such as refining, the S content may be 0.0001% or more.

O: 0.0060% or less

O is an element that may be contained as an impurity. If the O content exceeds 0.0060%, coarse oxides are formed in the steel and formability deteriorates. Therefore, the O content is set to 0.0060% or less. The O content is preferably 0.0050% or less, 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 0.0005% or more, or 0.0010% or more.

In the chemical composition of the cold rolled steel sheet according to the present embodiment, the balance excluding the above elements is basically Fe and impurities. Impurities are elements that are mixed from steel raw materials and/or during the steelmaking process and are allowed to be contained within a range that does not clearly deteriorate the characteristics of the cold rolled steel sheet according to the present embodiment.

Meanwhile, the chemical composition of the cold rolled steel sheet according to the present embodiment is B, Mo, Nb, Cr, V, Co, Ni, Cu, W, Ta, Sn, Sb, As, Mg for the purpose of improving various properties. , Ca, Y, Zr, La, and Ce may be contained in the range described later. Since these elements do not need to be contained, the lower limit is 0%. In addition, if the content is within the range described later, the effect of the cold rolled steel sheet according to this embodiment is 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 steel sheets. Therefore, these elements may be contained. In order to sufficiently obtain the above effect, it is preferable that the B content is 0.0001% or more, the Mo content, Cr content, and Ni content are each 0.010% or more, and the As content is 0.001% or more. More preferably, the B content is 0.0010% or more, the Mo content and Cr content are each 0.100% or more, and the As content is 0.005% or more. It is not essential to obtain the above effects. For this reason, there is no need to specifically limit the lower limits of the B content, Mo content, Cr content, Ni content, and As content, and the lower limit is 0%.

On the other hand, even if B, Mo, Cr, Ni, and As are contained excessively, the effects due to the above actions are saturated and it becomes uneconomical. Therefore, when included, the B content is 0.0100% or less, the Mo content is 0.500% or less, the Cr content is 2.000% or less, the Ni content is 1.000% or less, and the As content is 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 preferably 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 steel sheets through precipitation hardening. Therefore, it may be contained. In order to sufficiently obtain the above effects, it is preferable that the Nb content, V content, Cu content, W content, and/or Ta content are each 0.001% or more.

On the other hand, if these elements are contained excessively, the recrystallization temperature increases, the metal structure of the cold rolled steel sheet becomes non-uniform, and formability is impaired. 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 and Ta contents 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 sheets. 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, there is a risk that the elongation of the steel sheet will decrease and the formability will decrease. 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 the fine dispersion of inclusions in steel, and are elements that contribute to improving the formability of steel sheets through this fine dispersion. Therefore, these elements may be contained. In order to obtain the above effect, it is preferable to contain one or more of Ca, Mg, La, Ce, Y, Zr, and Sb, and to set each content to 0.001% or more.

On the other hand, if these elements are contained excessively, ductility deteriorates. 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, 0.050% or less

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

On the other hand, Sn is an element that may cause a decrease in the cold formability of steel sheets 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. Sn content is preferably 0.040% or less.

The chemical composition of the cold rolled steel sheet according to this embodiment can be obtained by the following method.

For example, in accordance with JIS G 1201 (2014), cuttings can 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, 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. do.

When the steel sheet has a coating layer on the surface, the chemical composition can be analyzed after removing the coating layer by mechanical grinding or the like. When the film layer is a plating layer, it may be removed by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet.

<Metal structure (micro structure)>

In the cold rolled steel sheet according to the present embodiment, the metal structure is at a position of 1/4 to 3/4 of the sheet thickness in the direction from the surface to the sheet thickness (if the sheet thickness is t, it is in the range of t/4 to 3t/4), It includes one or two types of retained austenite, which is 0% or more and 10.0% or less by volume, and martensite and tempered martensite, which is 90.0% or more and 100% or less.

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. In order to obtain the above effect, it is preferable that the volume ratio of retained austenite is 1.0% or more. The volume ratio of retained austenite is more preferably 2.0% or more, and even more preferably 3.0% or more.

On the other hand, when the volume fraction of retained austenite becomes excessive, the grain size of retained austenite increases. Such retained austenite with a large grain size becomes coarse yet hard martensite after deformation. In this case, the origin of cracks is likely to occur, and the formability of the cold rolled steel sheet deteriorates. For this reason, the volume fraction of retained austenite is set to 10.0% or less. The volume ratio of retained austenite is preferably 8.0% or less, and more preferably 7.0% or less.

As a structure other than retained austenite, it includes one or two types of martensite and tempered martensite.

Martensite (so-called fresh martensite) and tempered martensite are aggregates of lath-phase crystal grains and greatly contribute to improving strength. Therefore, the cold rolled steel sheet according to the present embodiment contains martensite and tempered martensite in a total volume ratio of 90.0 to 100%.

Tempered martensite, unlike martensite, is a hard structure that contains fine iron-based carbides inside by tempering. Tempered martensite has a small contribution to strength improvement compared to martensite, but is not brittle and is a ductile structure, so when it is desired to further improve formability, it is desirable to increase the volume fraction of tempered martensite. For example, the volume ratio of tempered martensite is 85.0% or more.

On the other hand, when it is desired to obtain high strength, it is desirable to increase the volume ratio of martensite.

The microstructure may contain ferrite and bainite in addition to retained austenite, martensite, and tempered martensite.

Ferrite is a soft structure and improves the uniform elongation of cold rolled steel sheets, and as a result, it is a structure that contributes to the improvement of workability. Therefore, when ferrite is included, the total of retained austenite and ferrite is 5% or more. You may include it if possible. 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 a strength intermediate between martensite and ferrite, but has insufficient deformability and deteriorates workability, so it is preferable not to include it substantially.

In the cold-rolled steel sheet according to the present embodiment, defining the metal structure at a position of 1/4 to 3/4 of the sheet thickness from the surface centered on the position of 1/2 of the sheet thickness in the direction from the surface to the sheet thickness is as follows. This is because the metal structure at the location is a representative structure of a steel plate and has a strong correlation with its properties.

The volume ratio of each structure in the metal structure (microstructure) at a position of 1/4 to 3/4 of the sheet thickness in the sheet thickness direction from the surface of the cold rolled steel sheet according to the present embodiment is measured as follows.

In other words, the volume ratio of ferrite, bainite, martensite, tempered martensite, and pearlite is determined by collecting a test piece from an arbitrary position in the rolling direction and width direction of the steel sheet, grinding the longitudinal section parallel to the rolling direction, and measuring the volume ratio from the surface. The structure revealed by nital etching in the range of 1/4 to 3/4 of the sheet thickness in the sheet thickness direction is observed using an SEM. In SEM observation, 5 fields of view of 30 ㎛ × 50 ㎛ are observed at a magnification of 3000 times, the area ratio of each tissue is measured from the observed images, and the average value is calculated. There is no change in structure in the direction perpendicular to the rolling direction (width direction of steel sheet), the area ratio of the longitudinal section parallel to the rolling direction is assumed to be equivalent to the volume ratio, and the area ratio obtained from tissue observation is taken as the respective volume ratio. .

When measuring the area ratio of each structure, the region in which the lower structure does not protrude and the luminance is low is taken as ferrite. Additionally, the region where the underlying structure does not protrude and the brightness is high is considered martensite or retained austenite. Additionally, the area where the lower structure protrudes is referred to as tempered martensite or bainite.

Bainite and tempered martensite can also be distinguished by carefully observing the carbides within the grains.

Specifically, tempered martensite is composed of martensite lath and cementite generated inside the lath. At this time, since there are two or more types of crystal orientation relationships between martensite lath and cementite, cementite constituting tempered martensite has a plurality of variants. Meanwhile, bainite is classified into upper bainite and lower bainite. Since the upper bainite is composed of bainitic ferrite on the lath phase and cementite generated at the lath interface, it can be easily distinguished from tempered martensite. The lower bainite is composed of bainitic ferrite on the lath and cementite generated inside the lath. At this time, unlike tempered martensite, the crystal orientation relationship between bainitic ferrite and cementite is one type, and the cementite constituting the lower bainite has the same variant. Therefore, lower bainite and tempered martensite can be distinguished based on the variant of cementite.

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

The volume fraction of retained austenite was determined by collecting a test piece from an arbitrary position of the steel sheet, chemically polishing the rolled surface from the surface of the steel sheet to a position of 1/4 of the sheet thickness, and measuring the ferrite by MoKα rays (200), (210). ) It is quantified from the area strength and the (200), (220), and (311) area strength of austenite.

In addition, in the cold rolled steel sheet according to the present embodiment, in the metal structure at a position of 1/4 to 3/4 of the sheet thickness in the direction from the surface to the sheet thickness, the P content at the old γ grain boundary is 10.0% by mass or less, and The Mn content at the old γ grain boundary is 10.0 mass% or less.

Segregation of Mn and P occurs due to element distribution between the solid phase and the liquid phase during dendrite growth during solidification in a normal continuous casting process. When welding is performed on a steel sheet with segregation, differences in martensite hardness (partially hardening) occur in the heat-affected zone of the weld zone due to differences in Mn content (concentration differences), resulting in differences in joint strength after welding. . This is presumed to be because the Ms point changes depending on the difference in Mn content. In addition, cracks are more likely to occur due to co-segregation of Mn and P. Therefore, in order to increase the 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, segregation of Mn and P is suppressed. More specifically, the P content at the old γ grain boundaries is 10.0 mass% or less, and the Mn content at the old γ grain boundaries is 10.0 mass% or less.

At the old γ grain boundary, if the P content exceeds 10.0 mass% or the Mn content exceeds 10.0 mass%, the strength of the welded joint obtained by welding decreases due to differences in hardness and cracks.

The P content and Mn content of the sphere γ grain boundary are each preferably 8.0 mass% or less, and more preferably 6.0 mass% or less.

In addition, the lower limits of the P content and Mn content at the old γ grain boundaries are not limited, but since both P and Mn are elements that segregate at the grain boundaries, it is not easy to set the P content to 80 times or less than the P content of the base material if it is P. However, if it is Mn, it is difficult in principle to suppress it to less than 1.01 times the Mn content of the base material. Therefore, taking these into consideration, the P content and Mn content of the old γ grain boundary may each be 3.6% or more.

The degree of segregation is defined as the metal structure at a position of 1/4 to 3/4 of the sheet thickness in the direction from the surface to the sheet thickness, centered on the position of 1/2 of the sheet thickness from the surface to the sheet thickness at this position. Since the degree of segregation is greater than other locations, this is to evaluate the effect of alleviating segregation in the area including that location.

Conventionally, the influence of macro- and semi-macro segregation of both P and Mn has been evaluated, but the influence of segregation on the sphere γ grain boundaries as described above has not been clearly identified. It is a new finding obtained by the present inventors that a joint with high joint strength can be obtained by controlling the P content and Mn content at the old γ grain boundary.

The P content and Mn content at the old γ grain boundary are measured by the following method.

A test piece for an auger test of the size shown in FIG. 1 is cut out from a position of 1/4 to 3/4 of the sheet thickness from the surface, centered on a position of 1/2 of the sheet thickness from the surface of the steel sheet. This test piece is immersed in an ammonium thiocyanate aqueous solution with a concentration of 20% by mass for 48 hours. An impact test is performed on the test piece after immersion 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. As a result, the fracture surface becomes a grain boundary (old γ grain boundary) fracture surface, so Auger electron spectroscopic analysis is performed on this fracture surface to measure the P content and Mn content. Thereby, the P content and Mn content at the old γ grain boundary are obtained.

The measuring device is not particularly limited, but is performed using, for example, JAMP-9500F manufactured by Nippon Electronics. Additionally, during measurement, the portion on the grain boundary fracture where no precipitates exist is measured at least three times, and the AES peaks of P and Mn are measured. Referring to a non-patent document (Analytical Chemistry, vol. 35 (1986)), sensitivity correction is performed on this AES peak intensity using each relative sensitivity factor (RSF) to determine the grain boundary segregation concentration.

It is also effective to reduce the P content and Mn content at the old γ grain boundaries (reduce segregation at the old γ grain boundaries) by refining the crystal grains by performing counter-pressure reduction by hot rolling as described later. Therefore, in the cold rolled steel sheet according to this embodiment, it is preferable that the sphere γ grain size (average grain size) is 15 μm or less.

The above-mentioned sphere γ particle size can be measured by the following method.

A test piece is taken from an arbitrary position in the rolling direction and width direction of the steel sheet, the longitudinal surface parallel to the rolling direction is polished, and the test piece is tested in the range of 1/4 to 3/4 of the sheet thickness from the surface to the sheet thickness direction. , the tissue extruded using a saturated aqueous solution of picric acid is observed using an optical microscope. Among the structures extruded in a saturated aqueous solution of picric acid, the black, network-shaped lines are judged to be spherical γ grain boundaries. In cases where a black network-like line does not appear in the saturated aqueous solution of picric acid, the appearance becomes possible by adding a surfactant or changing the temperature of immersion in the saturated aqueous solution of picric acid to about 20 to 80°C. In observation using an optical microscope, an arbitrary magnification of 200 to 1000 times is selected to acquire an image of the tissue. Three images containing at least 200 or more crystal grains are taken, and the average particle size of the old γ (austenite) grain size is measured for the captured images using a point calculation method.

The method of measuring the old γ grain size is not limited to the above, and as another method, measurement is also possible by using reverse analysis of old austenite using SEM-EBSD.

The cold-rolled steel sheet according to the present embodiment described above may have a film layer containing zinc, aluminum, magnesium, or alloys thereof on the surface. The film layer may be substantially made of zinc, aluminum, magnesium, or alloys thereof. The presence of a film layer on the surface of the steel sheet improves corrosion resistance. The coating layer may be a known coating layer.

For example, when steel sheets are used in a corrosive environment, there is a risk of perforation, etc., so even if the strength is increased, there are cases where the sheet thickness cannot be reduced below a certain certain thickness. One of the goals of increasing the strength of steel sheets is to reduce their weight by reducing their thickness, so even if high-strength steel sheets are developed, their application areas are limited if their corrosion resistance is low. It is preferable to have a film layer containing zinc, aluminum, magnesium or alloys thereof on the surface because corrosion resistance is improved and the applicable range is expanded.

When a steel sheet has a coating layer (for example, a plating layer) on the surface, “surface” in “a position 1/4 to 3/4 thick from the surface of the steel sheet” means the surface of the base iron excluding the coating layer.

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

<Tensile strength>

In the cold rolled steel sheet according to the present embodiment, the tensile strength (TS) is set to 1310 MPa or more, which is the strength that contributes to reducing the weight of the automobile body. From the viewpoint of shock absorption, the tensile strength is preferably 1400 MPa or more, and more preferably 1470 MPa or more.

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

[Welded joint]

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 (the cold-rolled steel sheet according to the present embodiment may be sufficient) by welding. Therefore, the welded joint according to the present embodiment is a welded joint in which a plurality of steel plates are joined, and at least one steel plate is the cold rolled steel plate according to the present embodiment described above.

In the welded joint according to this embodiment, steel plates are joined through a weld, and if the welding is spot welding, they are joined through a spot weld.

[Manufacturing method]

The cold-rolled steel sheet according to the present embodiment is not limited to the manufacturing method, and the effect can be obtained as long as it has the above-mentioned characteristics. However, it can be stably manufactured according to the following manufacturing method.

Specifically, the cold rolled steel sheet according to this embodiment can be manufactured by a manufacturing method including the following steps (I) to (VIII).

(I) a continuous casting process to obtain a slab having a predetermined chemical composition by continuous casting,

(II) a breakdown process of reducing the thickness of the slab by reducing the slab at a reduction rate of 30 to 60% in a temperature range of 850 to 1000°C;

(III) a high-temperature heat treatment process in which the slab after the breakdown process is heated to 1000 to 1300° C., maintained for 5 to 20 hours, and cooled;

(IV) a hot rolling process to obtain a hot rolled steel sheet by hot rolling the slab after the high temperature heat treatment process;

(V) a coiling process of winding the hot-rolled steel sheet in a temperature range of 400 to 650°C;

(VI) a cold rolling process of pickling the hot rolled steel sheet after the coiling process and cold rolling it at a reduction ratio of 20 to 80% to obtain a cold rolled steel sheet;

(VII) Heating the cold-rolled steel sheet to an annealing temperature above Ac3°C at an average temperature increase rate of 2°C/sec or more, maintaining the annealing temperature for 60 to 300 seconds, and cooling to 250°C or lower at an average cooling rate of 10°C/sec or more. An annealing process, cooling until

(VIII) A holding process of maintaining the cold rolled steel sheet after the annealing process at 150 to 400°C for 500 seconds or less.

In addition, the welded joint according to this embodiment can be obtained by a manufacturing method further including the following steps.

(IX) A welding process for welding the cold rolled steel sheet and other steel sheets after the holding process.

Hereinafter, preferred conditions for each process will be described.

<Continuous casting process>

In the continuous casting process, a slab having a predetermined chemical composition (the chemical composition is substantially the same as that of the cold rolled steel sheet according to the present embodiment because the chemical composition does not substantially change in the subsequent process) is obtained through continuous casting.

<Breakdown (BD) process>

<High temperature heat treatment (SP treatment) process>

In the breakdown process, the slab obtained in the continuous casting process is subjected to reduction (BD) at a reduction ratio of 30 to 60% in a temperature range of 850 to 1000° C. to reduce the thickness. If the slab obtained in the continuous casting process has a temperature lower than 850°C, it is heated and then reduced. If the temperature of the slab is in the range of 850 to 1000°C, heating does not need to be performed.

Thereafter, in the high-temperature heat treatment process, the slab after the breakdown process is heated to 1000 to 1300°C, maintained at that temperature for 5 to 20 hours (SP treatment), and then cooled.

SP treatment alleviates segregation of Mn and P. However, even if the segregation of Mn and P is attempted to be alleviated with SP treatment alone, treatment at a significantly high temperature or for a long time is required. Therefore, in the method for manufacturing a cold rolled steel sheet according to the present embodiment, sufficient mitigation of segregation is attempted by performing BD before SP treatment.

By performing BD, the effects of increasing the diffusion constant and decreasing the thickness of the segregation zone are obtained. Therefore, by performing SP treatment after performing BD, segregation of Mn and P can be alleviated at temperature and time within a practically possible range. If either of the conditions is outside the above conditions, sufficient effect will not be obtained.

Conventionally, in order to reduce macro-segregation or semi-macro segregation, there were cases where the BD process or the SP process was performed alone. However, the effect of reducing the P content or Mn content of the sphere γ grain boundaries by the BD process or the SP process was not clear. In addition, by combining the BD process and the SP process and performing large pressure reduction by hot rolling as described later, the P content and Mn content of the old γ grain boundaries can be reduced to a predetermined level compared to the case where the BD process or the SP process is performed alone. It was not known to what extent it could be reduced. Therefore, combining these processes has not been commonly performed.

<Hot rolling process>

In the hot rolling process, 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 lower than 1100°C, there is concern that carbides and sulfides generated during the casting to SP treatment process will not be dissolved and will become coarse, causing the grain size to become coarse during annealing. Therefore, 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 γ and suppress P segregation at grain boundaries.

For this reason, in the hot rolling process, rough rolling and finish rolling are usually performed, and in this finish rolling, it is performed using a rolling mill having four or more stands, with the first stand being the first stand and the final stand being the nth stand. In the case of a stand, the sheet thickness reduction rate at each stand from the n-3rd 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 lower. That is, for example, if it is a rolling mill with seven stands, the sheet thickness reduction rates at the fourth stand, fifth stand, sixth stand, and seventh stand are each set to 30% or more, and the rolling temperature at the seventh stand is set to 30% or more. Keep the temperature below 900℃. In this finish rolling, the austenite grain size is made fine by recrystallization during rolling, and by using this refined grain boundary as a diffusion path, diffusion of Mn, P, etc. is promoted and segregation is alleviated.

If the rate of reduction in sheet thickness at each stand is less than 30%, or the rolling temperature at the nth stand is greater than 900°C, the hot rolling structure becomes coarse and mixed, and the structure after the annealing process described later also becomes coarse. If the completion temperature of hot rolling is less than 830°C, the rolling reaction force increases, making it difficult to stably obtain the target plate thickness. For this reason, it is preferable that the rolling temperature in the final stand is 830°C or higher. Furthermore, even if the reduction ratio is greater than 50%, the effect of grain refinement is saturated and the equipment load becomes excessively high due to an increase in the rolling load. Therefore, it is preferable that the plate thickness reduction rate in the n-3th stand to the nth stand is respectively 50% or less.

Additionally, finish rolling is performed using a rolling mill with four or more stands in order to achieve continuous rolling with a short inter-pass time between the final four passes of rolling. This is because, if the time between passes is long, even if reduction is performed at a large sheet thickness reduction rate, the strain is recovered between passes and the strain is not sufficiently accumulated.

<Winding process>

In the coiling process, the hot rolled steel sheet after the hot rolling process is wound at a coiling temperature of 400°C or higher and 650°C or lower.

If the coiling temperature exceeds 650°C, an internal oxidation layer is formed and pickling properties deteriorate.

On the other hand, when the coiling temperature is lower 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 (plate thickness reduction rate) of 20 to 80% to obtain a cold rolled steel sheet.

If the sheet thickness reduction rate is less than 20%, strain accumulation in the steel sheet becomes insufficient, austenite nucleation sites become non-uniform, and the degree of segregation of Mn and P at the sphere γ grain boundaries increases.

On the other hand, when the sheet thickness reduction rate exceeds 80%, the cold rolling load becomes excessive and productivity deteriorates.

Therefore, the plate thickness reduction rate is set to be 20% or more and 80% or less. The plate thickness reduction rate is preferably 30% or more and 80% or less. There are no restrictions on the method of cold rolling, and the number of rolling passes and the reduction ratio for each pass can be set as appropriate.

<Annealing process>

In the annealing process, the cold rolled steel sheet obtained in the cold rolling process is heated to an annealing temperature exceeding Ac3°C at an average temperature increase rate of 2°C/sec or more, held at this annealing temperature for 60 to 300 seconds, and after holding at 10°C/sec. Cool to 250℃ or less at an average cooling rate of more than a second.

If the average temperature increase rate is less than 2°C/sec, productivity decreases, the grain size becomes coarse, and the degree of segregation of Mn and P at the old γ grain boundaries increases, which is not preferable.

If the annealing temperature is Ac3°C or lower or the holding time is less than 60 seconds, the γ transformation may not be sufficient and the target structure may not be obtained after the annealing process. On the other hand, if the annealing time is more than 300 seconds, productivity decreases.

If the average cooling rate is less than 10°C/sec or the cooling stop temperature is more than 250°C, there is concern that ferrite or bainite will be generated and the target metal structure will not be obtained. On the other hand, setting the cooling stop temperature to less than 150°C not only requires a large investment in equipment, but also the effect is saturated even if it is set to less than 150°C. Therefore, it is preferable that the cooling stop temperature is 150°C or higher.

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

In the annealing process, from the viewpoint of improving the corrosion resistance of the steel sheet, a film layer containing zinc, aluminum, magnesium, or an alloy thereof may be formed on the surface of the steel sheet. For example, during cooling after holding, the steel sheet may be immersed in a plating bath to form hot-dip plating within a range that can satisfy the above average cooling rate. Additionally, this hot dip plating may be heated to a predetermined temperature and alloyed to form alloyed hot dip plating. Additionally, the plating layer may further contain Fe, Al, Mg, Mn, Si, Cr, Ni, Cu, etc. As a plating layer for the purpose of increasing corrosion resistance, any of the above methods may be used. Plating conditions and alloying conditions may be known, depending on the composition of the plating.

<Maintenance process>

In the holding process, the cold rolled steel sheet after the annealing process is held at 150 to 400°C for 500 seconds or less.

Through the holding process, part or all of the martensite is tempered to become tempered martensite. If the holding temperature is less than 150°C, martensite is not sufficiently tempered, and the effect is not fully obtained.

If the holding temperature exceeds 400°C, the dislocation density in tempered martensite may decrease, resulting in a decrease in tensile strength. Additionally, if the holding time is more than 500 seconds, the tensile strength decreases and productivity decreases.

The lower limit of the holding time is not limited, but when the metal structure is mainly tempered martensite, the holding time is preferably 100 seconds or more.

When the temperature of the cold rolled steel sheet drops to less than 150°C before the holding process, heating may be performed as needed.

<Welding process>

In the welding process, the cold rolled steel sheet after the holding process and other steel sheets are welded. The other steel sheets are not limited, and may be the cold rolled steel sheets according to this embodiment or may be different. Additionally, welding may be performed multiple times to join three or more steel plates.

The welding method is not limited, but when considering application to automobile parts, spot welding is preferable.

Example

Slabs (steel grades A to

These slabs were heated to the temperature in Table 2-1, and reduced at the reduction ratio in Table 2-1 to reduce the thickness and perform breakdown. After that, SP treatment was performed by heating and maintaining the temperature in Table 2-1.

The slab after 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. During hot rolling, a hot rolling mill with 7 stands was used for the finish rolling, and the reduction ratios of the 3 stands before the final stand to the final stand and the rolling temperature at the final stand were as shown in Table 2-2.

This hot-rolled steel sheet was pickled under known conditions and then cold-rolled at the reduction ratio shown in Table 2-2 to obtain a cold-rolled steel sheet with a sheet thickness of 1.0 to 2.0 mm. However, some hot rolled steel sheets had high strengths and could not be cold rolled.

The obtained cold rolled steel sheet was annealed under the conditions shown in Table 2-3, and then maintained under the conditions shown in Table 2-3.

In addition, for some cold rolled steel sheets, in the middle of annealing (cooling stage), they are heated or cooled to (galvanizing bath temperature -40)°C to (galvanizing bath temperature +50)°C and immersed in a galvanizing bath. Zinc plating was performed (in the table, an example in which plating was performed or not was “yes”). In addition, some of the cold rolled steel sheets that had been galvanized were further alloyed by heating to a temperature range of 470 to 550°C (in the table, examples where the presence or absence of alloying is “yes”).

[Table 1-1]

[Table 1-2]

[Table 2-1]

[Table 2-2]

[Table 2-3]

For the obtained cold-rolled steel sheet, the metal structure was observed at the position of t/4 to 3t/4 in the manner described above, and the total volume fraction of martensite, tempered martensite, retained austenite, ferrite, bainite, and pearlite were measured. The volume ratio was obtained.

In addition, in the metal structure at the position from t/4 to 3t/4, the P content and Mn content at the sphere γ grain boundary were measured using the method described above.

Additionally, a JIS No. 5 test piece was taken from the obtained cold rolled steel sheet at a right angle to the rolling direction, and the tensile strength was measured according to JIS Z 2241:2011.

In addition, spot welding was performed on the obtained plate in which two cold rolled steel sheets were overlapped, and the joint characteristics were evaluated.

A servo motor pressurized single-phase AC welder (power frequency 50 Hz) was used for welding, 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 as the electrode.

The welding conditions were 440kgf of pressing force, 0.28sec of energization time, and 0.1sec of hold time. The welding current was set to obtain 5√t as the nugget diameter.

Then, a cross tensile test was performed on the manufactured joint in accordance with JIS Z 3137 (1999) (conducted under each condition n=2).

Joint properties are improved by more than 5% compared to conventional steel sheets that have not undergone segregation mitigation (steel sheets with the same chemical composition for each steel sheet and equivalent manufacturing conditions other than the breakdown process, high-temperature heat treatment process, and hot rolling process). Those that improved by 10% or more were evaluated as ○ (Good), those that improved by 20% or more were evaluated as ◎ (Excellent), and those that did not improve were evaluated as × (NG).

[Table 3]

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 sphere γ grain boundary, and P content (part stone strength) is within the scope of the present invention, and as a result, it has a high strength of 1310 MPa or more and also has sufficient joint strength.

On the other hand, in Nos. 31 to 47, which are comparative examples whose chemical composition or production method is outside the scope of the present invention, at least one of the chemical composition, metal structure, Mn content at the sphere γ grain boundary, and P content (segregation degree) is consistent with the present invention. is outside the range, and either the tensile strength or the joint strength is not sufficient.

According to the present invention, it is possible to provide an ultra-high-strength steel plate with a tensile strength of 1310 MPa or more, a steel plate with sufficiently high joint strength after welding, and a welded joint. These steel plates and welded joints contribute to reducing the weight of automobile bodies, and thus 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, 0.050% or less, and
Residue: Fe and impurities
It has a chemical composition consisting of,
The metal structure at the position of 1/4 to 3/4 of the sheet thickness in the direction from the surface to the sheet thickness is 0% or more and 10.0% or less of retained austenite, 90.0% or more and 100% or less of martensite and tempering. Contains one or two types of martensite,
In the metal structure at the above position, the P content at the old γ grain boundaries is 10.0 mass% or less, and the Mn content at the old γ grain boundaries is 10.0 mass% or less,
A tensile strength of 1310 MPa or more,
Cold rolled steel sheet characterized in that. A continuous casting process to obtain a slab having the chemical composition described in claim 1 by continuous casting,
A breakdown process of reducing the thickness of the slab by reducing the slab at a reduction rate of 30 to 60% in a temperature range of 850 to 1000 ° C.;
A high-temperature heat treatment process in which the slab after the breakdown process is heated to 1000°C to 1300°C, maintained for 5 to 20 hours, and then cooled;
A hot rolling process to obtain a hot rolled steel sheet by hot rolling the slab after the high temperature heat treatment process;
A winding process of winding the hot rolled steel sheet in a temperature range of 400 to 650°C,
A cold rolling process of pickling the hot rolled steel sheet after the coiling process and cold rolling it at a reduction ratio of 20 to 80% to obtain a cold rolled steel sheet;
The cold-rolled steel sheet is heated to an annealing temperature above Ac3°C at an average temperature increase rate of 2°C/sec or more, held at the annealing temperature for 60 to 300 seconds, and cooled to 250°C or lower at an average cooling rate of 10°C/sec or more. an annealing process,
A holding process of maintaining the cold rolled steel sheet after the annealing process at 150 to 400° C. for 500 seconds or less.
Equipped with
In the hot rolling process,
When finish rolling is performed using a rolling mill with four or more stands, and the first stand is the first stand and the final stand is the nth stand, the The sheet thickness reduction rate is each 30% or more, and the rolling temperature at the nth stand is 900°C or less.
A method of manufacturing a cold rolled steel sheet, characterized in that. According to paragraph 2,
In the annealing process, a film layer containing zinc, aluminum, magnesium or an alloy thereof is formed on the front and back surfaces of the steel sheet.
A method of manufacturing a cold rolled steel sheet, characterized in that. It is a welded joint in which a plurality of steel plates are joined, and at least one steel plate is the cold rolled steel plate according to claim 1,
A welded joint, characterized in that.