WO2024070890A1

WO2024070890A1 - Steel sheet, member, and production methods therefor

Info

Publication number: WO2024070890A1
Application number: PCT/JP2023/034300
Authority: WO
Inventors: 大洋浅川; 英之木村; 真平吉岡
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-09-30
Filing date: 2023-09-21
Publication date: 2024-04-04
Also published as: JPWO2024070890A1

Abstract

Provided are: a steel sheet having high strength, excellent in terms of ductility and hole expansibility, and having highly stable mechanical properties along the longitudinal direction of the coil; and a method for producing the steel sheet.　The steel sheet has a composition containing, in terms of mass%, 0.08-0.35% C, 0.4-3.0% Si, 1.5-3.5% Mn, up to 0.02% P, up to 0.01% S, up to 1.0% sol. Al, and up to 0.015% N, the remainder comprising Fe and unavoidable impurities, and has a steel structure in which the areal content of ferrite is 5% or less (including 0%), the total areal content of tempered martensite and lower bainite is 70% or higher, the volume content of retained austenite is 5-15%, and the areal content of fresh martensite is 10% or less (including 0%). The steel sheet has a standard deviation of coil-longitudinal-direction tensile strength (TS) of 30 MPa or less.

Description

Steel plates, components and their manufacturing methods

The present invention relates to steel sheets and components used in various applications such as automobiles and home appliances, and to methods for manufacturing them.

In recent years, automobile components have become stronger in order to reduce the weight of automobile bodies, and high-strength steel sheets with a tensile strength (TS) of 1180 MPa or more are being used in automobile frame parts and seat parts. In general, the ductility and stretch flange formability of steel sheets decrease as the strength increases, so steel sheets with a TS of 1180 MPa or more are more likely to crack during press forming.

In order to obtain excellent press formability in high-strength steel plates with a TS of 1180 MPa or more, it is important to make the steel plate structure uniformly tempered martensite, improve hole expansion properties, and improve ductility by finely dispersing retained austenite.

Patent Document 1 discloses a high-strength cold-rolled steel sheet with excellent workability and impact resistance, containing, by mass%, C: 0.05-0.3%, Si: 0.3-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.02% or less, Al: 0.010-0.5%, ferrite: 20% or more, tempered martensite: 10-60%, martensite: 0-10%, retained austenite: 3-15%, and having a steel structure in which the average crystal grain size of the low-temperature transformation phase consisting of martensite, tempered martensite, and retained austenite is 3 μm or less. The technology described in Patent Document 1 utilizes a process known as Q&P (Quenching & Partitioning) in which the material is cooled to a temperature range between the martensite transformation start temperature (Ms) and the martensite transformation completion temperature (Mf) during the cooling process, and then reheated and held to stabilize the residual γ. In recent years, progress has been made in the development of high-strength steel with excellent ductility and stretch flange formability and its manufacturing method using this process.

Patent Document 2 discloses a high-strength steel plate with excellent workability, containing, by mass%, C: 0.05-0.5%, Si: 0.01-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.02% or less, and Al: 0.010-0.5%, with a steel structure containing, by area ratio, 0-10% ferrite, 0-10% martensite, 60-95% tempered martensite, and 5-20% retained austenite as determined by X-ray diffraction, with a tensile strength of 1200 MPa or more and a hole expansion ratio of 50% or more.

In Patent Document 3, a steel plate containing, by mass%, C: 0.10% to 0.73%, Si: 3.0% or less, Mn: 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less is heated to the austenite single phase region or the (austenite + ferrite) two-phase region, and then, using the martensitic transformation start temperature Ms as an index, the target cooling stop temperature is set to a temperature range below Ms and above (Ms - 150°C). This method of manufacturing high-strength steel plate has excellent workability and tensile strength (TS) and excellent stability of mechanical properties, and in which the coldest part of the steel plate in the width direction is held in a temperature range from the target cooling stop temperature to (cooling stop temperature + 15°C) for 15 seconds to 100 seconds, is disclosed.

Japanese Patent No. 5463685 Japanese Patent No. 5402007 Patent No. 5333298

Each of the conventional technologies mentioned above has the following issues:

In the technology described in Patent Document 1, controlling the cooling stop temperature during annealing is extremely important in order to obtain excellent strength, ductility, and hole expansion properties. However, if the cooling rate is fast, the cooling stop temperature is likely to vary, which can lead to variations in the mechanical properties along the coil's length and reduced press formability.

In addition, in the technology described in Patent Document 2, control of the cooling stop temperature during annealing is extremely important in order to obtain excellent strength, ductility, and hole expandability. However, because the cooling rate is fast at 20°C/s or more, the cooling stop temperature is prone to variation, which can cause variation in the mechanical properties in the longitudinal direction of the coil and lead to reduced press formability.

The technology described in Patent Document 3 provides a high-strength steel plate with excellent stability of mechanical properties in the width direction by heating to the austenite single-phase region or the (austenite + ferrite) two-phase region, followed by cooling with a target cooling stop temperature in a temperature range below Ms and above (Ms-150°C), and by holding the coldest part of the steel plate in the width direction in a temperature range from the target cooling stop temperature to (cooling stop temperature + 15°C) for a time of 15 to 100 seconds. However, there is no mention of the stability of mechanical properties in the longitudinal direction of the coil, and there remains a possibility that variations in the mechanical properties in the longitudinal direction of the coil may occur due to variations in the hot-rolled structure and reheating holding temperature, leading to a decrease in press formability.

The present invention was made to solve these problems, and aims to provide steel plates and components that are high in strength, have excellent ductility and hole expansion properties, and have excellent stability of mechanical properties in the longitudinal direction of the coil, as well as methods for manufacturing the same.

In the present invention, high strength refers to a tensile strength TS evaluated in accordance with JIS Z2241 (2011) of 1180 MPa or more.
Excellent ductility refers to a total elongation (EL) evaluated in accordance with JIS Z2241 (2011) of 11.0% or more.
Excellent hole expandability refers to a state in which a hole of 10 mm in diameter is punched into a 100 mm x 100 mm steel plate with a clearance of 12% of the plate thickness, and a 60° conical punch is pressed into the hole while pressing with a wrinkle pressing force of 88.2 kN using a die with an inner diameter of 75 mm, and the hole diameter at the crack initiation limit is measured, where Df is the hole diameter at the time of crack initiation (mm), and D0 is the initial hole diameter (mm), and the limit hole expansion ratio λ (%) = {(Df - D0) / D0} x 100 is 40% or more.
"Excellent stability of mechanical properties in the longitudinal direction of the coil" refers to a standard deviation of TS evaluated in accordance with JIS Z2241 (2011) of 30 MPa or less using a total of 20 JIS No. 5 tensile test pieces taken at equal intervals in the longitudinal direction of the coil, including test pieces taken at positions 10 m from the leading and trailing ends of the coil, as JIS No. 5 tensile test pieces parallel to the rolling direction.

The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that it is possible to homogenize the structure of the hot-rolled sheet in the longitudinal direction of the coil by controlling the temperature during coiling, and to reduce the variation in the cooling stop temperature in the longitudinal direction of the coil by slowly cooling from near the Ms point to the cooling stop temperature in the annealing process, and as a result, it is possible to significantly reduce the variation in mechanical properties.

More specifically, the present invention provides the following:
[1] In mass%,
C: 0.08 to 0.35%,
Si: 0.4 to 3.0%,
Mn: 1.5 to 3.5%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 1.0% or less,
N: 0.015% or less;
The balance has a composition consisting of Fe and unavoidable impurities,
Ferrite area ratio: 5% or less (including 0%),
Total area ratio of tempered martensite and lower bainite: 70% or more;
Volume fraction of retained austenite: 5 to 15%;
The area ratio of fresh martensite is 10% or less (including 0%).
A steel plate having a standard deviation of tensile strength TS in the longitudinal direction of the coil of 30 MPa or less.
[2] The component composition, in mass%,
B: 0.01% or less,
Ti: 0.1% or less,
Cu: 1% or less,
Ni: 1% or less,
Cr: 1.5% or less,
Mo: 1.0% or less,
V: 0.5% or less,
Nb: 0.1% or less,
The steel plate according to [1], containing one or more selected from Zr: 0.2% or less and W: 0.2% or less.
[3] The component composition, in mass%,
Ca: 0.0040% or less,
Ce: 0.0040% or less,
La: 0.0040% or less,
Mg: 0.0040% or less,
The steel sheet according to [1] or [2], containing one or more selected from Sb: 0.1% or less and Sn: 0.1% or less.
[4] The steel sheet according to any one of [1] to [3], having a plating layer on a surface of the steel sheet.
[5] A member made using the steel plate according to any one of [1] to [4].
[6] A steel slab having a component composition according to any one of [1] to [3],
After holding the slab at a heating temperature of 1100°C or higher for 1800s or more,
Finish hot rolling is performed at a finish rolling temperature of 850°C or higher,
Cooling is performed in a temperature range from the finish rolling temperature to 650° C. at an average cooling rate of 40° C./s or more,
a hot rolling process in which the hot-rolled steel sheet is obtained by coiling the steel sheet at a coiling temperature of 600° C. or less;
The hot-rolled steel sheet,
A cold rolling process in which the steel sheet is cold-rolled at a rolling ratio of 30% or more to obtain a cold-rolled steel sheet;
The cold-rolled steel sheet,
After heating in the temperature range from 700° C. to (Ac ₃ −10° C.) at an average heating rate HR1 of 0.5° C./s or more,
(Ac ₃ -10 ° C) or higher for 30 seconds or more,
Cooling is performed at an average cooling rate CR1 of 10 ° C./s or more in a temperature range from the annealing temperature to a slow cooling start temperature T1 which is equal to or higher than (Ms-30 ° C.) and equal to or lower than (Ms+30 ° C.),
The temperature range from the annealing start temperature T1 to the annealing stop temperature T2, which is equal to or higher than (Ms-220°C) and equal to or lower than (Ms-100°C), is cooled at an average cooling rate CR2 of 1 to 10°C/s,
The temperature range from the slow cooling stop temperature T2 to a reheating holding temperature T3 of 300° C. or more and 450° C. or less is heated at an average heating rate HR2 of 2° C./s or more,
The reheating holding temperature T3 is held for 20 s or more and 3000 s or less,
and an annealing step of cooling the steel sheet in a temperature range from the reheating holding temperature T3 to 50°C at an average cooling rate CR3 of 0.1°C/s or more.
[7] In the annealing step, during cooling from the annealing temperature to the slow cooling start temperature T1, or during reheating and holding at the reheating holding temperature T3, hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment is performed. The method for producing a steel sheet according to [6].
[8] The method for producing a steel sheet according to [6], further comprising the step of: performing an electroplating treatment after the annealing step.
[9] A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of [1] to [4] to at least one of forming and joining to form a component.

The present invention provides steel plates, components, and methods for manufacturing the same that are high strength, have excellent ductility and hole expansion properties, and have excellent stability of mechanical properties in the longitudinal direction of the coil.

The following describes an embodiment of the present invention. Note that the present invention is not limited to the following embodiment.

The steel plate of the present invention contains, by mass%, C: 0.08-0.35%, Si: 0.4-3.0%, Mn: 1.5-3.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: 1.0% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities. It has a steel structure with an area ratio of ferrite: 5% or less (including 0%), a combined area ratio of tempered martensite and lower bainite: 70% or more, a volume ratio of retained austenite: 5-15%, and an area ratio of fresh martensite: 10% or less (including 0%), and the standard deviation of the tensile strength (TS) in the longitudinal direction of the coil is 30 MPa or less.

First, the chemical composition of the steel sheet of the present invention will be described.
In the following description of the composition of the components, the unit of content "%" means "mass %." Furthermore, in the present invention, "high strength" refers to a tensile strength TS of 1180 MPa or more.

(C: 0.08 to 0.35%)
C is contained to increase the strength of tempered martensite or lower bainite and ensure a TS of 1180 MPa or more. If the C content is less than 0.08%, the strength of tempered martensite and lower bainite is low, and the desired TS cannot be stably obtained. If the C content is less than 0.08%, the desired ductility cannot be obtained. Therefore, the C content is set to 0.08% or more. The C content is preferably 0.10% or more, and more preferably 0.14% or more.
On the other hand, excessive addition of C leads to a decrease in hole expandability due to an increase in the number density of carbides, a decrease in ductility, and even a deterioration in the shape fixability of parts due to an excessive increase in YS. Therefore, the C content is set to 0.35% or less. The C content is preferably 0.30% or less, and more preferably 0.25% or less.

(Si: 0.4 to 3.0%)
Silicon improves the strength of the steel sheet by solid solution strengthening, and furthermore, by suppressing the coarsening of carbides, suppresses the decrease in strength due to tempering. If the silicon content is less than 0.4%, the desired TS cannot be obtained stably, so the silicon content is set to 0.4% or more. The silicon content is preferably 1.0% or more, and more preferably 1.4% or more.
On the other hand, excessive addition of Si leads to a significant decrease in chemical conversion treatability and plating property. Therefore, the Si content is set to 3.0% or less. The Si content is preferably 2.5% or less, and more preferably 2.0% or less.

(Mn: 1.5 to 3.5%)
Mn is an element effective in improving hardenability. If the Mn content is less than 1.5%, ferrite or pearlite is generated excessively. As a result, the desired TS cannot be obtained because the tempered martensite and lower bainite are not sufficiently obtained. Also, if the Mn content is less than 1.5%, the desired hole expandability cannot be obtained. Therefore, the Mn content is set to 1.5% or more. The Mn content is preferably 2.0% or more, and more preferably 2.4% or more.
On the other hand, excessive addition of Mn forms coarse MnS, which significantly reduces the hole expandability and bendability. Therefore, the Mn content is set to 3.5% or less, and preferably 3.0% or less.

(P: 0.02% or less)
P is an effective element for strengthening steel, but excessive addition of P significantly reduces spot weldability. Therefore, the P content is set to 0.02% or less, and preferably 0.01% or less.
There is no particular lower limit for the P content, but since a large amount of cost is required to make the P content less than 0.002%, the P content is preferably 0.002% or more.

(S: 0.01% or less)
S forms coarse sulfides with Mn, which reduces hole expandability and bendability. Therefore, the S content is set to 0.01% or less. The S content is preferably 0.002% or less, and more preferably 0.001% or less.
Although there is no particular restriction on the lower limit of the S content, a large amount of cost is required to make the S content less than 0.0002%, so the S content is preferably 0.0002% or more.

(sol. Al: 1.0% or less)
Al is an element added as a deoxidizer in the steelmaking process. If the sol. Al content exceeds 1.0%, inclusions such as Al ₂ O ₃ and AlN increase, which reduces hole expandability and bendability. Therefore, the sol. Al content is set to 1.0% or less. The sol. Al content is preferably 0.2% or less, and more preferably 0.05% or less.
Although there is no particular lower limit for the sol. Al content, in order to obtain a sufficient deoxidizing effect, the sol. Al content is preferably 0.001% or more, more preferably 0.010% or more, and further preferably 0.020% or more.

(N: 0.015% or less)
If N is added excessively, a large amount of inclusions such as AlN is generated, which reduces the hole expandability and bendability. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.008% or less, and more preferably 0.005% or less.
Although there is no particular restriction on the lower limit of the N content, a large amount of cost is required to make the N content less than 0.001%, so the N content is preferably 0.001% or more.

The composition of the steel sheet of the present invention contains the above-mentioned components as basic components, and the balance other than the composition of the components contains iron (Fe) and inevitable impurities. In the steel sheet of the present invention, it is preferable that the balance has a composition consisting of Fe and inevitable impurities.
Examples of unavoidable impurities include Zn, Co, etc., and in the present invention, even if these elements are contained within the range of a normal steel composition, the effect of the present invention is not impaired.

In addition to these basic components, in place of the iron (Fe) and part of the unavoidable impurities, one or more elements selected from B, Ti, Cu, Ni, Cr, Mo, V, Nb, Zr, and W may be added as necessary. Furthermore, one or more elements selected from Ca, Ce, La, Mg, Sb, and Sn may be added as necessary.
Specifically, the component composition of the steel sheet of the present invention may appropriately contain the following (A) and/or (B) as optional elements.
(A) In terms of mass%, one or more elements selected from B: 0.01% or less, Ti: 0.1% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.5% or less, Mo: 1.0% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, and W: 0.2% or less. (B) In terms of mass%, one or more elements selected from Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0040% or less, Sb: 0.1% or less, and Sn: 0.1% or less.

([Group A] B: 0.01% or less, Ti: 0.1% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.5% or less, Mo: 1.0% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, W: 0.2% or less)
These elements may be added for the purpose of stably obtaining the desired TS by grain refinement and precipitation strengthening. On the other hand, if added excessively, coarse precipitates are generated and hole expandability and bendability are deteriorated, so when B is contained, the B content is 0.01% or less, when Ti is contained, the Ti content is 0.1% or less, when Cu is contained, the Cu content is 1% or less, when Ni is contained, the Ni content is 1% or less, when Cr is contained, the Cr content is 1.5% or less, when Mo is contained, the Mo content is 1.0% or less, when V is contained, the V content is 0.5% or less, when Nb is contained, the Nb content is 0.1% or less, when Zr is contained, the Zr content is 0.2% or less, and when W is contained, the W content is 0.2% or less.

The B content is preferably 0.0050% or less, more preferably 0.0030% or less, and more preferably 0.0003% or more.
The Ti content is preferably 0.080% or less, more preferably 0.050% or less. The Ti content is preferably 0.001% or more, and more preferably 0.010% or more.
The Cu content is preferably 0.50% or less, more preferably 0.20% or less. The Cu content is preferably 0.001% or more, and more preferably 0.030% or more.
The Ni content is preferably 0.50% or less, more preferably 0.20% or less. The Ni content is preferably 0.001% or more, more preferably 0.030% or more.
The Cr content is preferably 1.2% or less, more preferably 1.0% or less. The Cr content is preferably 0.001% or more, and more preferably 0.100% or more.
The Mo content is preferably 0.50% or less, more preferably 0.20% or less. The Mo content is further preferably 0.10% or less. The Mo content is preferably 0.001% or more. The Mo content is more preferably 0.010% or more.
The V content is preferably 0.50% or less, more preferably 0.20% or less. The V content is further preferably 0.05% or less. The V content is preferably 0.001% or more. The V content is more preferably 0.005% or more.
The Nb content is preferably 0.08% or less, more preferably 0.05% or less. The Nb content is preferably 0.001% or more, and more preferably 0.010% or more.
The Zr content is preferably 0.1% or less, more preferably 0.05% or less. The Zr content is preferably 0.001% or more, and more preferably 0.010% or more.
The W content is preferably 0.1% or less, more preferably 0.05% or less, and further preferably 0.03% or less.
The W content is preferably 0.001% or more, and more preferably 0.005% or more.

([Group B] Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0040% or less, Sb: 0.1% or less, Sn: 0.1% or less)
These elements may be added for the purpose of improving hole expandability and bendability by controlling inclusions. When the amount of addition exceeds a certain amount, the effect is saturated, so when Ca is contained, the Ca content is 0.0040% or less, when Ce is contained, the Ce content is 0.0040% or less, when La is contained, the La content is 0.0040% or less, when Mg is contained, the Mg content is 0.0040% or less, when Sb is contained, the Sb content is 0.1% or less, and when Sn is contained, the Sn content is 0.1% or less.

The Ca content is preferably 0.0030% or less, more preferably 0.0010% or less, and preferably 0.0003% or more.
The Ce content is preferably 0.0030% or less, more preferably 0.0010% or less, and preferably 0.0003% or more.
The La content is preferably 0.0030% or less. The La content is more preferably 0.0010% or less. Also, the La content is preferably 0.0003% or more.
The Mg content is preferably 0.0030% or less. Also, the Mg content is preferably 0.0003% or more. The Mg content is further preferably 0.0010% or more.
The Sb content is preferably 0.05% or less, more preferably 0.02% or less, and more preferably 0.0003% or more. The Sb content is further preferably 0.0020% or more.
The Sn content is preferably 0.05% or less, more preferably 0.02% or less, and more preferably 0.0003% or more. The Sn content is further preferably 0.0020% or more.

If the optional components are contained in amounts less than the lower limit, the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. If the optional elements are contained in amounts less than the lower limit, the optional elements are considered to be contained as unavoidable impurities.

Next, we will explain the structure (microstructure) of the steel plate of the present invention.

(Ferrite area ratio: 5% or less (including 0%))
Although ferrite contributes to improving ductility, due to the difference in hardness between ferrite and hard phases such as tempered martensite, it becomes the origin of voids during punching and press molding, deteriorating hole expandability. If the area ratio of ferrite exceeds 5%, the desired hole expandability may not be obtained.
Therefore, the area ratio of ferrite is set to 5% or less, preferably 3% or less, and more preferably 0%.

(Total area ratio of tempered martensite and lower bainite: 70% or more)
In order to stably obtain a TS of 1180 MPa or more, the total area ratio of tempered martensite and lower bainite is set to 70% or more, preferably 80% or more, and more preferably 85% or more. Although tempered martensite and lower bainite have different transformation timings, they are low-temperature transformation products and have similar effects on mechanical properties, so they are evaluated based on the total area ratio.
Although there is no particular upper limit, the total area ratio of tempered martensite and lower bainite is preferably 95% or less, and more preferably 93% or less.

(Volume fraction of retained austenite: 5 to 15%)
The retained austenite contributes to improving uniform elongation due to the TRIP effect. In order to obtain the desired ductility, the volume fraction of the retained austenite is set to 5% or more. If the volume fraction of the retained austenite is less than 5%, the desired ductility may not be obtained, the desired hole expandability may not be obtained, and the desired stability of mechanical properties in the longitudinal direction of the coil may not be obtained. The volume fraction of the retained austenite is preferably 7% or more, more preferably 9% or more.
On the other hand, if the retained austenite is excessively generated, the hole expandability may decrease. Also, if the retained austenite exceeds 15% by volume, the desired ductility cannot be obtained. Therefore, the retained austenite is set to 15% or less.

(Area ratio of fresh martensite: 10% or less (including 0%))
Fresh martensite is very hard, and therefore becomes the starting point of voids during punching or press molding, thereby reducing hole expandability. If the fresh martensite content exceeds 10%, the hole expandability is significantly deteriorated, so the fresh martensite content is set to 10% or less, preferably 5% or less, and more preferably 3% or less. The fresh martensite content may be 0%.

In the present invention, the object of the present invention can be achieved if the above ferrite, tempered martensite, lower bainite, retained austenite, and fresh martensite are satisfied. Other remaining structures, such as pearlite and upper bainite, may be included as long as the total amount is 5% or less.

The steel sheet of the present invention may also have a plating layer on the surface of the steel sheet. The type of plating layer is not particularly limited, but may be a zinc plating layer, such as an electrolytic zinc plating layer, a hot-dip zinc plating layer, or an alloyed hot-dip zinc plating layer.

Next, we will explain how to measure the microstructure of steel sheets.

The area ratios of ferrite, tempered martensite, lower bainite, and fresh martensite are measured by cutting a cross section of the plate parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. Using an SEM, 10 fields of view are observed at 5,000x magnification at the 1/4 thickness position, and the area ratios are measured using the point count method (in accordance with ASTM E562-83 (1988)). In the above observations, ferrite is the area that appears the blackest under the SEM, and is an equiaxed area with almost no carbides inside. Tempered martensite and lower bainite are areas that appear gray under the SEM, and are areas where lath-shaped substructures and carbide precipitation are observed. Fresh martensite is the area that appears white and lumpy under the SEM, and is an area where no substructures are observed inside.

The volume fraction of retained austenite is determined by X-ray diffraction using steel plates that have been mechanically ground and polished with oxalic acid to a depth of 100 μm or more so that the measurement position is on 1/4 of the plate thickness. A Co-Kα source is used for the incident X-rays, and the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Here, since the retained austenite is randomly distributed, the volume fraction of retained austenite determined by X-ray diffraction is equal to the area fraction.

The steel plate of the present invention has a tensile strength TS evaluated in accordance with JIS Z2241 (2011) of 1180 MPa or more, and is therefore high in strength.
In addition, the steel sheet of the present invention has a total elongation (EL) evaluated in accordance with JIS Z2241 (2011) of 11.0% or more, and is excellent in ductility.
In addition, the steel plate of the present invention is punched with a diameter of 10 mm in a 100 mm x 100 mm steel plate with a clearance of 12% of the plate thickness, and a 60° conical punch is pressed into the hole with a wrinkle holding force of 88.2 kN using a die with an inner diameter of 75 mm to measure the hole diameter at the crack occurrence limit, where Df is the hole diameter (mm) at the time of crack occurrence and D0 is the initial hole diameter (mm), and the limit hole expansion ratio λ (%) = {(Df - D0) / D0} x 100 is 40% or more, and the steel plate has excellent hole expandability.
In addition, the steel sheet of the present invention has a JIS No. 5 tensile test piece in a direction parallel to the rolling direction, and a total of 20 pieces are taken at equal intervals in the longitudinal direction of the coil, including test pieces taken at positions 10 m from the head and tail ends of the coil. The standard deviation of TS evaluated in accordance with JIS Z2241 (2011) is 30 MPa or less, and the steel sheet has excellent stability of mechanical properties in the longitudinal direction of the coil.
Furthermore, the steel sheet of the present invention may have a standard deviation of EL in the longitudinal direction of the coil of 1.5% or less.

Next, the manufacturing method of the steel plate of the present invention will be described. Note that the temperatures when heating or cooling the steel slab (steel material), steel plate, etc. shown below refer to the surface temperatures of the steel slab (steel material), steel plate, etc., unless otherwise specified.
The method for producing a steel sheet of the present invention includes a hot rolling step in which a steel slab having the above-mentioned composition is held at a slab heating temperature of 1100°C or higher for 1800s or more, followed by finish hot rolling at a finish rolling temperature of 850°C or higher, cooling at an average cooling rate of 40°C/s or higher in the temperature range from the finish rolling temperature to 650°C, and coiling at a coiling temperature of 600°C or lower to obtain a hot-rolled steel sheet, a cold rolling step in which the hot-rolled steel sheet is cold-rolled at a rolling reduction rate of 30% or higher to obtain a cold-rolled steel sheet, and a cold rolling step in which the cold-rolled steel sheet is heated at an average heating rate of 0.5°C/s or higher in the temperature range from 700°C to (Ac ₃ -10 _° C), followed by coiling at a coiling temperature of 600°C or lower. and an annealing process of holding the annealing temperature at an annealing temperature of (Ms-10°C) or higher for 30 s or more, cooling a temperature range from the annealing temperature to a slow cooling start temperature T1 of (Ms-30°C) or higher and (Ms+30°C) or lower at an average cooling rate CR1 of 10°C/s or more, cooling a temperature range from the slow cooling start temperature T1 to a slow cooling stop temperature T2 of (Ms-220°C) or higher and (Ms-100°C) or lower at an average cooling rate CR2 of 1 to 10°C/s, heating a temperature range from the slow cooling stop temperature T2 to a reheating holding temperature T3 of 300°C or higher and 450°C or lower at an average heating rate HR2 of 2°C/s or more, holding the reheating holding temperature T3 for 20 s or more and 3000 s or less, and cooling a temperature range from the reheating holding temperature T3 to 50°C at an average cooling rate CR3 of 0.1°C/s or more.

In the present invention, the steelmaking process can be carried out according to a conventional method.
The hot rolling process, the pickling process, the cold rolling process, and the annealing process will be described below.

[Hot rolling process]
Methods for hot rolling a steel slab include a method of reheating a steel slab cooled to room temperature and then rolling it, a method of directly rolling a steel slab after continuous casting without heating it, and a method of rolling a steel slab after continuous casting by subjecting it to a short-term heat treatment. In the present invention, the steel slab is held at a slab heating temperature of 1100°C or higher for 1800s or more by any of the above methods, and then finish hot rolling is performed at a finish rolling temperature of 850°C or higher. Then, the steel slab is cooled at an average cooling rate of 40°C/s or higher in the temperature range from the finish rolling temperature to 650°C, and coiled at a coiling temperature of 600°C or lower to obtain a hot-rolled steel sheet.

(Slab heating temperature: 1100°C or higher)
(Slab heating holding time: 1800 s or more)
If the slab heating temperature is less than 1100° C., inclusions such as MnS remain, and the hole expandability decreases. Therefore, the slab heating temperature is set to 1100° C. or higher. The slab heating temperature is preferably 1180° C., and more preferably 1200° C. or higher.
Also, when the slab heating holding time is less than 1800 seconds, a large amount of inclusions such as MnS may remain, and the hole expandability may decrease. Therefore, the slab heating holding time is set to 1800 seconds or more.
Although there are no upper limits for the slab heating temperature and the slab heating holding time, from the viewpoint of production costs, the slab heating temperature is preferably 1,300° C. or less, and the slab heating holding time is preferably 3 hours or less.

(Finish rolling temperature: 850°C or higher)
If the finish rolling temperature is less than 850°C, ferrite will form during hot rolling, resulting in a non-uniform structure after rolling, which may cause variations in mechanical properties in the longitudinal direction of the coil after annealing. Therefore, the finish rolling temperature is set to 850°C or higher.
There is no particular upper limit, but it is preferably 950° C. or less.

(Average cooling rate from finish rolling temperature to 650°C: 40°C/s or more)
If the average cooling rate from the finish rolling temperature to 650°C is less than 40°C/s, ferrite and pearlite are generated during cooling, and the hot-rolled structure is likely to become non-uniform. In this case, the grain size and the amount of retained austenite in the coil longitudinal direction after annealing vary, causing variations in strength and ductility. Therefore, the average cooling rate from the finish rolling temperature to 650°C is set to 40°C/s or more. This average cooling rate is preferably 60°C/s or more.
The average cooling rate here is "(finish rolling temperature (°C)-650°C)/cooling time from the finish rolling temperature to 650°C (seconds)".

(Coiling temperature: 600°C or less)
If the coiling temperature exceeds 600°C, ferrite and pearlite are likely to be generated, and the hot-rolled structure becomes non-uniform due to the variation in the coiling temperature in the longitudinal direction of the coil. In this case, the grain size in the longitudinal direction of the coil after annealing varies, which causes variations in strength and ductility. Therefore, the coiling temperature is set to 600°C or less. The coiling temperature is preferably 550°C or less. Although the lower limit of the coiling temperature is not particularly specified, if the coiling temperature is less than 400°C, the hot-rolled structure may become hard due to the generation of martensite, and the cold rolling load may increase excessively. Therefore, the coiling temperature is preferably 400°C or more.

In addition, after the hot rolling process, the hot-rolled steel sheet can be subjected to a heat treatment as necessary in order to reduce the cold rolling load.

[Pickling process]
After the hot rolling step, pickling may be carried out to remove scale from the surface layer of the hot-rolled sheet. The pickling method is not particularly limited, and may be carried out according to a conventional method.

[Cold rolling process]
(Rolling ratio (cold rolling ratio): 30% or more)
The cold rolling reduction (cumulative cold rolling reduction) is set to 30% or more from the viewpoint of promoting recrystallization in the subsequent annealing heating and stabilizing the material. Although there is no particular upper limit for the cold rolling reduction, if it exceeds 95%, the cold rolling load may increase excessively. Therefore, the cold rolling reduction is preferably 95% or less.

[Annealing process]
(Average heating rate HR1 from 700°C to (Ac ₃ -10°C): 0.5°C/s or more)
If the average heating rate HR1 from 700°C to (Ac ₃ -10°C) is less than 0.5°C/s, C concentration from ferrite to austenite progresses during heating, and the C concentration distribution in the steel sheet becomes uneven, resulting in non-uniform material properties. In addition, if the C concentration distribution in the steel sheet becomes uneven, the variation in mechanical properties will become even greater due to fluctuations in the cooling stop temperature and reheating temperature in the coil longitudinal direction. Therefore, the average heating rate HR1 from 700°C to (Ac ₃ -10°C) is set to 0.5°C/s or more. The average heating rate HR1 from 700°C to (Ac ₃ -10°C) is preferably 1.0°C/s or more, more preferably 1.5°C/s or more.
The average heating rate HR1 is preferably 50° C./s or less, and more preferably 20° C./s or less.
The average heating rate HR1 is "(Ac ₃ -10° C.) -700° C.)/heating time (seconds) from 700° C. to (Ac ₃ -10° C.)".

(Annealing temperature: (Ac ₃ -10°C) or higher)
(Holding time (annealing time): 30 seconds or more)
In order to control the area ratio of ferrite within a desired range, the annealing temperature is set to be equal to or higher than (Ac ₃ - 10° C.). If the annealing temperature is lower than (Ac ₃ - 10° C.), the desired stability of mechanical properties in the longitudinal direction of the coil may not be obtained.
Although there is no upper limit for the annealing temperature, if it exceeds (Ac ₃ +50° C.), the austenite grain size becomes significantly coarsened, and the balance between strength and ductility may deteriorate.
Therefore, the annealing temperature is preferably (Ac ₃ +50° C.) or lower.

If the holding time (annealing time) is less than 30 seconds, the carbides will remain undissolved, resulting in reduced hole expansion and bendability. Therefore, the holding time should be 30 seconds or more. The holding time is preferably 60 seconds or more.

_Ac3 is calculated by the following formula. In the formula, the [element symbol] means the content (mass%) of each element. (Leslie Steel Materials Science, Maruzen Co., Ltd., published May 31, 1985, p. 273)
_Ac3 (°C) = 910 - 203 x [C] ^1/2 - 15.2 x [Ni] + 44.7 x [Si] + 104 x [V] + 31.5 x [Mo] + 13.1 x [W] - (30 x [Mn] + 11 x [Cr] + 20 x [Cu] - 700 x [P] - 400 x [sol. Al] - 120 x [As] - 400 x [Ti])

(Average cooling rate CR1 from annealing temperature to slow cooling start temperature T1: 10 ° C./s or more)
(Slow cooling start temperature T1: martensite transformation start temperature Ms ± 30°C ((Ms - 30°C) or more, (Ms + 30°C) or less))
If CR1 is less than 10°C/s, ferrite is excessively generated, and the desired tempered martensite and lower bainite cannot be obtained, and the desired strength may not be obtained. Therefore, CR1 is set to 10°C/s or more. CR1 is preferably 15°C/s or more.
Although there is no upper limit for CR1, an excessive increase in the average cooling rate may promote uneven cooling in the longitudinal direction of the coil, which may lead to a decrease in material uniformity in the longitudinal direction of the coil. Therefore, CR1 is preferably 100° C./s or less.
The average cooling rate CR1 is "(annealing temperature (°C) - slow cooling start temperature (T1) (°C)) / cooling time (seconds) from the annealing temperature to the slow cooling start temperature (T1)".

If T1 exceeds (Ms+30°C), ferrite and pearlite are generated excessively, and the desired tempered martensite and lower bainite are not obtained, and the desired strength may not be obtained. Also, if T1 exceeds (Ms+30°C), the desired hole expandability cannot be obtained. Therefore, T1 is set to (Ms+30°C) or less. T1 is preferably (Ms+20°C) or less, and more preferably (Ms+10°C) or less.
On the other hand, if T1 is less than (Ms-30°C), the desired amount of retained austenite may not be obtained, and the desired ductility may not be obtained. Furthermore, if T1 is less than (Ms-30°C), the desired stability of mechanical properties in the longitudinal direction of the coil may not be obtained. Therefore, T1 is set to be equal to or greater than (Ms-30°C). T1 is preferably equal to or greater than (Ms-20°C), and more preferably equal to or greater than (Ms-10°C).

The martensitic transformation start temperature Ms (°C) can be determined by using a Formaster testing machine, using a cylindrical test piece (diameter 3 mm × height 10 mm), holding the test piece at an annealing temperature of ( _Ac3 -10°C) or higher, and then quenching the test piece at a cooling rate of 30°C/s or higher using helium gas, and measuring the volume change.

(Average cooling rate CR2 from slow cooling start temperature T1 to slow cooling stop temperature T2: 1 to 10 ° C./s)
(Slow cooling stop temperature T2: (Ms-220°C) or higher and (Ms-100°C) or lower)
By setting the average cooling rate CR2 from T1 to T2 to 10°C/s or less, the variation in the cooling stop temperature is reduced, and the transformation amount of martensite and lower bainite becomes uniform in the coil longitudinal direction, so that the variation in the mechanical properties in the coil longitudinal direction can be suppressed. Furthermore, by setting CR2 to 10°C/s or less, carbon is distributed from martensite and lower bainite to austenite during cooling, and the austenite is stabilized. As a result, even if the reheating temperature varies, the decomposition of the residual austenite is suppressed, and the variation in the mechanical properties in the coil longitudinal direction is suppressed. Therefore, CR2 is set to 10°C/s or less. If CR2 is less than 1°C/s, the line length increases and the production efficiency decreases, so CR2 is set to 1°C/s or more.
The average cooling rate CR2 is "(slow cooling start temperature T1 (°C)-slow cooling stop temperature T2 (°C))/cooling time (seconds) from the slow cooling start temperature T1 to the slow cooling stop temperature T2."

If T2 is less than (Ms-220°C), martensitic transformation will proceed excessively, the desired amount of retained austenite will not be obtained, and ductility will decrease. Therefore, T2 is set to be (Ms-220°C) or more. T2 is preferably (Ms-200°C) or more, and more preferably (Ms-180°C) or more.
On the other hand, if T2 exceeds (Ms-100°C), C is not sufficiently distributed from martensite and lower bainite to austenite during slow cooling, so decomposition of austenite occurs during the reheating and holding process, which causes variation in mechanical properties in the longitudinal direction of the coil. Therefore, T2 is set to (Ms-100°C) or less.

(Average heating rate HR2 from slow cooling stop temperature T2 to reheating holding temperature T3: 2° C./s or more)
By heating from the slow cooling stop temperature T2 to the reheating holding temperature T3 in a short time, carbide precipitation can be suppressed and high ductility can be ensured. Therefore, the average heating rate HR2 is set to 2°C/s or more. HR2 is preferably 5°C/s or more, and more preferably 10°C/s or more. There is no particular upper limit to the average heating rate HR2, but as the average heating rate HR2 increases, it may become more difficult to maintain the uniformity of the steel sheet temperature. Therefore, HR2 is preferably 50°C/s or less, and more preferably 20°C/s or less.
The average heating rate HR2 is "reheating holding temperature T3 (°C) - slow cooling stop temperature T2 (°C) / heating time (seconds) from the slow cooling stop temperature T2 to the reheating holding temperature T3."

(Reheating holding temperature T3: 300°C or more, 450°C or less)
(Reheating holding time: 20 s or more, 3000 s or less)
The reheating is performed to stabilize austenite by C distribution. If the reheating temperature is less than 300°C, C distribution is not sufficient and the desired amount of retained austenite cannot be obtained, which may result in a decrease in ductility. Therefore, the reheating temperature T3 is set to 300°C or higher. T3 is preferably 330°C or higher, and more preferably 350°C or higher.
On the other hand, if the reheating temperature T3 exceeds 450°C, austenite transforms into pearlite, and the desired amount of retained austenite is not obtained, which may lead to a decrease in ductility. If the reheating temperature T3 exceeds 450°C, the desired tensile strength cannot be obtained. Therefore, the reheating temperature T3 is set to 450°C or less. T3 is preferably 420°C or less.

Furthermore, if the reheating holding time (holding time (residence time) at the reheating holding temperature T3) is less than 20 seconds, sufficient C distribution does not occur, and the desired amount of retained austenite cannot be obtained. Therefore, the reheating holding time is set to 20 seconds or more. The reheating holding time is preferably 50 seconds or more, and more preferably 100 seconds or more.
Since the effect of carbon distribution by reheating and holding is saturated at more than 3000 seconds, the reheating and holding time is set to 3000 seconds or less, preferably 1500 seconds or less, and more preferably 600 seconds or less.

(Average cooling rate CR3 from reheating holding temperature T3 to 50°C: 0.1°C/s or more)
If the average cooling rate CR3 from the reheating holding temperature T3 to 50°C is less than 0.1°C/s, there is a concern that the ductility will decrease due to softening caused by excessive tempering and carbide precipitation. Therefore, the average cooling rate CR3 from the reheating holding temperature T3 to 50°C is set to 0.1°C/s or more. CR3 is preferably 5°C/s or more, and more preferably 8°C/s or more.
CR3 is preferably 100° C./s or less, and more preferably 50° C./s or less.
The average cooling rate CR3 is "(reheating holding temperature T3) (°C) - 50°C) / cooling time from reheating holding temperature T3 to 50°C (seconds)".

[Hot-dip plating treatment]
In the present invention, in the annealing step, hot-dip galvanizing treatment can be performed during cooling from the annealing temperature to the slow cooling start temperature T1, or during reheating and holding at the reheating and holding temperature T3. The hot-dip galvanizing treatment may be hot-dip galvanizing treatment. When hot-dip galvanizing treatment is performed, it is preferable to immerse the steel sheet in a zinc plating bath at 440°C or more and 500°C or less, perform hot-dip galvanizing treatment, and then adjust the coating weight by gas wiping or the like. For hot-dip galvanizing, it is preferable to use a zinc plating bath having an Al content of 0.10% or more and 0.22% or less.
In addition, after the hot dip galvanizing treatment, a galvanizing alloying treatment can be performed. When performing a galvanizing alloying treatment, it is preferable to perform the galvanizing alloying treatment in a temperature range of 480° C. or more and 600° C. or less after immersion in the galvanizing bath.

[Temper rolling]
In the present invention, from the viewpoint of stabilizing press formability and increasing YS, the steel sheet after annealing can be subjected to temper rolling. The elongation rate is preferably 0.1% or more. The elongation rate is preferably 0.5% or less.

[Leveller correction]
In the present invention, in order to straighten the sheet shape, the steel sheet after annealing may be subjected to leveller straightening. The leveller straightening method is not particularly specified, and may be carried out according to a conventional method.

[Electroplating process]
In the present invention, after the annealing step, a surface treatment such as electroplating can be carried out.

The thickness of the steel plate of the present invention thus obtained is preferably 0.5 mm or more. Also, the thickness of the steel plate of the present invention is preferably 2.0 mm or less.
The plate width is preferably 600 mm or more, and 1700 mm or less.
Furthermore, the steel sheet of the present invention may have, but is not particularly limited to, a sheet length (length in the longitudinal direction of the coil) of 100 m or more, and a sheet thickness of 4000 m or less.

Next, we will explain the components of the present invention and their manufacturing method.

The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining processes. The manufacturing method of the member of the present invention also includes a step of subjecting the steel plate of the present invention to at least one of forming and joining processes to form the member.

The steel plate of the present invention has a tensile strength of 1180 MPa or more, excellent ductility and hole expandability, and excellent stability of mechanical properties in the longitudinal direction of the coil. Therefore, members obtained using the steel plate of the present invention also have high strength, excellent ductility and hole expandability, and excellent stability of mechanical properties in the longitudinal direction of the coil. Furthermore, the use of the members of the present invention makes it possible to reduce weight. Therefore, the members of the present invention can be suitably used, for example, for vehicle body frame parts. The members of the present invention also include welded joints.

For the forming process, general processing methods such as pressing can be used without restrictions. For the joining process, general welding methods such as spot welding and arc welding, riveting, crimping, etc. can be used without restrictions.

The present invention will be specifically described with reference to examples. However, the scope of the invention is not limited to the examples.

A slab having the chemical composition shown in Table 1 was held at a slab heating temperature of 1210°C for 3000 seconds, then hot rolled at a finishing rolling temperature of 880°C, cooled at an average cooling rate of 65°C/s in the temperature range from the finishing rolling temperature to 650°C, and coiled at the coiling temperature shown in Table 2 to produce a hot-rolled steel sheet with a thickness of 2.8 mm. The hot-rolled steel sheet was cold rolled at a reduction rate of 50%, producing a cold-rolled steel sheet with a thickness of 1.4 mm and a total length of 1500 m.

Thereafter, the cold-rolled steel sheet was annealed under the conditions shown in Table 2. Among the annealing conditions, the average heating rate HR1 in heating from 700° C. to (Ac ₃ −10° C.) was set to 2.0° C./s.
In addition, No. 11 had a steel sheet surface subjected to electrolytic galvanizing treatment (EG), and No. 12 had a steel sheet surface subjected to hot-dip galvanizing treatment. In addition, No. 12 had a steel sheet surface subjected to alloying treatment (GA) at 510° C. for 10 seconds in order to make the plated layer into an alloyed hot-dip galvanized layer.

The steel structure was measured using the method described above, and the results are shown in Table 3.

Tensile strength (TS) and total elongation (EL) were evaluated in accordance with JIS Z2241 (2011). JIS No. 5 tensile test pieces were prepared from the obtained steel plates and tensile tests were conducted. Steels with a TS of 1180 MPa or more were judged to have excellent strength, and steels with an EL of 11.0% or more were judged to have excellent ductility.

The hole expandability was evaluated in accordance with the Japan Iron and Steel Federation standard JFST1001. After cutting each of the obtained steel sheets into 100 mm x 100 mm, a hole with a diameter of 10 mm was punched with a clearance of 12% of the sheet thickness, and a 60° conical punch was pressed into the hole while pressing with a wrinkle pressing force of 88.2 kN using a die with an inner diameter of 75 mm to measure the hole diameter at the crack initiation limit, and the limit hole expansion ratio λ (%) was calculated from the formula (1), and it was determined that those with λ of 40% or more had excellent hole expandability.
Limit hole expansion ratio λ (%) = {(Df-D0)/D0} × 100 ... (1)
Here, Df is the hole diameter (mm) when a crack occurs, and D0 is the initial hole diameter (mm).

The stability of the mechanical properties in the longitudinal direction of the coil was evaluated as follows. First, a total of 20 JIS No. 5 tensile test pieces were taken parallel to the rolling direction, including test pieces taken 10 m from the leading and trailing ends of the coil, and test pieces located between these test pieces were taken at equal intervals in the longitudinal direction of the coil. The above-mentioned tensile test was then carried out on these 20 test pieces, and evaluation was carried out by finding the standard deviations of TS and EL. Coils with a standard deviation of TS in the longitudinal direction of the coil of 30 MPa or less were judged to have excellent stability of the mechanical properties in the longitudinal direction of the coil. There was no particular regulation on the standard deviation of EL in the longitudinal direction of the coil, but a standard deviation of 1.5% or less was judged to have excellent stability of the mechanical properties in the longitudinal direction of the coil.

The examples of the present invention shown in Tables 2 and 3 are excellent in strength, ductility, hole expansion property, and stability of mechanical properties, whereas the comparative examples are inferior in one or more of these. Furthermore, in the examples of the present invention, the standard deviation of TS in the longitudinal direction of the coil could be reduced to 30 MPa or less, and the standard deviation of EL in the longitudinal direction of the coil could be reduced to 1.5% or less.

In addition, using the steel plate of the present invention, it was found that the components obtained by molding, the components obtained by joining, and the components obtained by further molding and joining have high strength and excellent ductility, hole expandability, and stability of mechanical properties in the longitudinal direction of the coil, similar to the steel plate of the present invention, because the steel plate of the present invention has high strength and excellent ductility, hole expandability, and stability of mechanical properties in the longitudinal direction of the coil.

Claims

In mass percent,
C: 0.08 to 0.35%,
Si: 0.4 to 3.0%,
Mn: 1.5 to 3.5%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 1.0% or less,
N: 0.015% or less;
The balance has a composition consisting of Fe and unavoidable impurities,
Ferrite area ratio: 5% or less (including 0%),
Total area ratio of tempered martensite and lower bainite: 70% or more;
Volume fraction of retained austenite: 5 to 15%;
The area ratio of fresh martensite is 10% or less (including 0%).
A steel plate having a standard deviation of tensile strength in the longitudinal direction of the coil of 30 MPa or less.
The component composition is, in mass%,
B: 0.01% or less,
Ti: 0.1% or less,
Cu: 1% or less,
Ni: 1% or less,
Cr: 1.5% or less,
Mo: 1.0% or less,
V: 0.5% or less,
Nb: 0.1% or less,
The steel plate according to claim 1, further comprising one or more selected from the group consisting of Zr: 0.2% or less and W: 0.2% or less.
The component composition is, in mass%,
Ca: 0.0040% or less,
Ce: 0.0040% or less,
La: 0.0040% or less,
Mg: 0.0040% or less,
The steel plate according to claim 1 or 2, containing one or more selected from the group consisting of Sb: 0.1% or less and Sn: 0.1% or less.
The steel sheet according to any one of claims 1 to 3, having a plating layer on the surface of the steel sheet.
A member made using the steel plate according to any one of claims 1 to 4.
A steel slab having a component composition according to any one of claims 1 to 3,
After holding the slab at a heating temperature of 1100°C or higher for 1800s or more,
Finish hot rolling is performed at a finish rolling temperature of 850°C or higher,
Cooling is performed in a temperature range from the finish rolling temperature to 650° C. at an average cooling rate of 40° C./s or more,
a hot rolling process in which the hot-rolled steel sheet is obtained by coiling the steel sheet at a coiling temperature of 600° C. or less;
The hot-rolled steel sheet,
A cold rolling process in which the steel sheet is cold-rolled at a rolling ratio of 30% or more to obtain a cold-rolled steel sheet;
The cold-rolled steel sheet,
After heating in the temperature range from 700° C. to (Ac 3 −10° C.) at an average heating rate HR1 of 0.5° C./s or more,
(Ac 3 -10 ° C) or higher for 30 seconds or more,
Cooling is performed at an average cooling rate CR1 of 10 ° C./s or more in a temperature range from the annealing temperature to a slow cooling start temperature T1 which is equal to or higher than (Ms-30 ° C.) and equal to or lower than (Ms+30 ° C.),
The temperature range from the annealing start temperature T1 to the annealing stop temperature T2, which is equal to or higher than (Ms-220°C) and equal to or lower than (Ms-100°C), is cooled at an average cooling rate CR2 of 1 to 10°C/s,
The temperature range from the slow cooling stop temperature T2 to a reheating holding temperature T3 of 300° C. or more and 450° C. or less is heated at an average heating rate HR2 of 2° C./s or more,
The reheating holding temperature T3 is held for 20 s or more and 3000 s or less,
and an annealing step of cooling the steel sheet in a temperature range from the reheating holding temperature T3 to 50°C at an average cooling rate CR3 of 0.1°C/s or more.
The method for manufacturing a steel sheet according to claim 6, wherein in the annealing step, hot-dip galvanizing or alloying hot-dip galvanizing is performed during cooling from the annealing temperature to the slow-cooling start temperature T1 or during reheating and holding at the reheating and holding temperature T3.
The method for manufacturing steel sheet according to claim 6, further comprising the step of performing an electroplating process after the annealing process.
A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.