JP7541653B1 - Steel plates and members, and their manufacturing methods - Google Patents

Abstract

To provide a steel sheet that combines high strength, high YS, excellent ductility, and excellent bendability. To provide a steel sheet having a specified composition and steel structure. In particular, to appropriately control the particle number density of retained austenite, the ratio of particles with an aspect ratio of 3 or less, the average C concentration, the average interparticle distance, the maximum interparticle distance, and [Mn]C/[Mn] in the steel structure.

Description

The present invention relates to a steel plate, a component made from the steel plate, and a method for manufacturing the same.

In recent years, from the viewpoint of protecting the global environment, the automobile industry has been attempting to reduce exhaust gases such as _CO2 . Specifically, the steel sheets used as the raw material for automobile parts are made stronger and thinner to reduce the weight of the car body and improve fuel efficiency. This is an attempt to reduce the amount of exhaust gas.

As an example of a steel sheet that can be used as a material for such an automobile component, Patent Document 1 describes:
"In mass percent,
C: 0.05 or more, 0.35% or less,
Si: 0.05% or more, 2.5% or less,
Mn: 0.6% or more, 3.0% or less,
P: 0.001% or more, 0.1% or less,
S: 0.0002% or more, 0.05% or less,
N: 0.0010% or more, 0.020% or less,
Al: 0.001% or more, 2.0% or less,
and a steel composition consisting of the remainder iron and unavoidable impurities, the metal structure of the steel sheet containing one or more of ferrite, bainite and tempered martensite, and containing 3% or more retained austenite, the average grain size of the austenite being 1 μm or more and 8 μm or less, and at interfaces where the austenite grains contact the ferrite, bainite and tempered martensite, 50% or more austenite grains have a central concentration Cgc of the austenite grains and a grain boundary concentration Cgb of the austenite grains in the range that satisfies (Equation 1).
Cgb/Cgc≧1.1 (Equation 1)
has been disclosed.

In Patent Document 2,
"In mass percent,
C: 0.10-0.50%,
Mn: 1.0-3.0%
Si: 0.005-2.5%,
Al: 0.005-2.5%,
Contains
P: 0.05% or less,
S: 0.02% or less,
A galvannealed steel sheet having excellent ductility and corrosion resistance, characterized in that N is limited to 0.006% or less, the sum of Si and Al is Si+Al≧0.8%, the microstructure contains, by area ratio, 10-75% ferrite and 2-30% retained austenite, and the amount of C in the retained austenite is 0.8-1.0%.
has been disclosed.

In Patent Document 3,
"In mass percent,
C: 0.05% or more, 0.35% or less,
Si: 0.05% or more, 2.0% or less,
Mn: 0.8% or more, 3.0% or less,
P: 0.0010% or more, 0.1% or less,
S: 0.0005% or more, 0.05% or less,
1. A high-strength steel sheet excellent in elongation and press forming stability, comprising: N: 0.0010% or more and 0.010% or less; Al: 0.01% or more and 2.0% or less; the balance being iron and unavoidable impurities; and a microstructure having, in terms of area ratio, a ferrite phase and a bainite phase of 10% or more and 93% or less in total, an area ratio of a retained austenite phase of 5% or more and 30% or less, and an area ratio of a martensite phase of 5% or more and 20% or less, the retained austenite phase being in a lath-like and island-like form, and an area ratio γi of the island-like retained austenite phase and an area ratio γ of the total retained austenite phase satisfying the following formula (1):
0.7≧γi/γ≧0.3...Formula (1)''
has been disclosed.

JP 2012-031505 A JP 2011-168816 A JP 2012-041573 A

Generally, increasing the strength of steel plate reduces its ductility. However, steel plate used as the raw material for automotive components is required to have both high strength and excellent ductility, specifically, improved total elongation (hereinafter simply referred to as El) in tensile tests.

Furthermore, among automotive components, steel sheets used in automotive frame structural components, in particular, are required to have high component strength when press-formed. For example, increasing the yield stress (hereinafter simply referred to as YS) of the steel sheet is effective in improving the strength of automotive components.

Furthermore, steel sheets used for automobile structural components are formed into complex shapes, so they require excellent formability, especially excellent bendability.

However, the steel sheets disclosed in Patent Documents 1 to 3 do not satisfy all of the required characteristics. In addition, the technology in Patent Document 2 requires that the steel be held for a long time after annealing in order to stabilize the retained austenite. This requires larger annealing equipment, which raises concerns about increased equipment costs.

The present invention has been developed in order to meet the above-mentioned requirements, and has an object to provide a steel plate having high strength, high YS, excellent ductility, and excellent bendability, together with an advantageous manufacturing method thereof.
Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.

Here, high strength and high YS mean that the tensile strength (hereinafter also referred to as TS) and YS measured in a tensile test in accordance with JIS Z 2241 respectively satisfy the following formulas.
・TS
780MPa≦TS
・Y.S.
When 780MPa≦TS<980MPa, 420MPa≦YS
When 980MPa≦TS, 550MPa≦YS

Excellent ductility means that the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula:
When 780MPa≦TS<980MPa, 19%≦El
When 980MPa≦TS, 10%≦El

The excellent bendability means that R/t measured by a V-bend test in accordance with JIS Z 2248 satisfies the following formula:
In the case of 780 MPa ≦ TS < 980 MPa, 2.0 ≧ R / t
In the case of 980 MPa ≦ TS, 4.0 ≧ R / t
Where:
R: Limit bending radius (mm)
t: thickness of steel plate (mm)
It is.

In order to achieve the above object, the inventors have conducted extensive research and have obtained the following findings.
(1) After adjusting the composition of the components to fall within a predetermined range, the total area ratio of ferrite and bainite and the area ratio of martensite are controlled to 10% or more and 87% or less, respectively, which makes it possible to achieve both high strength and excellent ductility.
(2) The area ratio of the retained austenite is controlled to 3% or more, and the number density of the particles constituting the retained austenite is controlled to 0.05 particles/ ^μm2 or more. This improves both ductility and bendability.
(3) Among the particles constituting the retained austenite, the particles having an aspect ratio of 3 or less are controlled to satisfy the following (A), (B), (C), and (D). This improves the stability of the retained austenite. As a result, the processing-induced transformation from the retained austenite to hard fresh martensite is suppressed. Here, the processing-induced transformation occurs during primary processing such as press forming. In addition, by controlling the distribution form of the retained austenite, it is possible to suppress stress concentration even if hard fresh martensite is generated. This further improves both ductility and bendability.
(A) The ratio of particles with an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio (B) The average C concentration of particles with an aspect ratio of 3 or less: 0.3 mass% or more (C) The average value of the shortest distance between particles with an aspect ratio of 3 or less: 5 μm or less (D) The maximum value of the shortest distance between particles with an aspect ratio of 3 or less: 15 μm or less (4) The Mn concentration distribution in the steel is appropriately controlled, that is, [Mn] _C /[Mn] is controlled to be 1.05 to 2.00. This further improves both ductility and bendability. Here, [Mn] _C is the average Mn concentration (mass%) of the Mn-enriched region in the steel, and [Mn] is the average Mn concentration (mass%) in the steel.
The present invention was completed based on the above findings and through further investigation.

That is, the gist and configuration of the present invention are as follows.
1. In mass percent,
C: 0.05% or more and 0.20% or less,
Si: 0.1% or more and 1.8% or less,
Mn: 1.5% or more and 3.0% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0500% or less,
Al: 0.010% or more and 1.000% or less,
N: 0.0100% or less, and one or two of Nb and Ti: 0.005% or more and 0.200% or less in total;
The balance being Fe and unavoidable impurities;
Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total,
Area ratio of martensite: 10% or more and 87% or less; and area ratio of retained austenite: 3% or more;
The number density of particles constituting the retained austenite is 0.05 particles/ ^μm2 or more,
Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the following (A), (B), (C), and (D),
a steel structure in which [ _Mn ]/[Mn] is 1.05 or more and 2.00 or less, [ _Mn ] is an average Mn concentration (mass%) in Mn-enriched regions in the steel, and [Mn] is an average Mn concentration (mass%) in the steel;
A steel plate having a tensile strength of 780 MPa or more.
(A) The ratio of particles having an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio; (B) The average C concentration of particles having an aspect ratio of 3 or less: 0.3 mass% or more; (C) The average value of the shortest distance between particles having an aspect ratio of 3 or less: 5 μm or less; (D) The maximum value of the shortest distance between particles having an aspect ratio of 3 or less: 15 μm or less.

2. The composition further comprises, in mass %,
V: 0.45% or less,
B: 0.010% or less,
Cr: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Sb: 0.1% or less,
Sn: 0.1% or less,
Cu: 1.0% or less,
Ta: 0.1% or less,
W: 0.2% or less,
Mg: 0.01% or less,
Zn: 0.02% or less,
Co: 0.02% or less,
Zr: 0.2% or less,
Ca: 0.02% or less,
Se: 0.02% or less,
Te: 0.02% or less,
Ge: 0.02% or less,
As: 0.05% or less,
Sr: 0.02% or less,
Cs: 0.02% or less,
Hf: 0.02% or less,
Pb: 0.02% or less,
2. The steel plate according to 1 above, containing at least one selected from Bi: 0.02% or less and REM: 0.02% or less.

3. The steel sheet according to 1 or 2 above, having a soft layer having a thickness of 1 μm or more and 50 μm or less.
Here, the soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the plate thickness of the steel plate.

4. A steel sheet as described in any one of 1 to 3 above, having a zinc-plated layer on the surface.

5. The steel sheet according to claim 4, wherein the zinc-plated layer is a hot-dip zinc-plated layer or an alloyed hot-dip zinc-plated layer.

6. A component made using a steel plate described in any one of 1 to 5 above.

7. A steel slab having the component composition described in 1 or 2 above,
A slab heating step in which the slab is heated under the conditions of a slab heating temperature of 1220° C. or more and a slab heating time of 1.0 hour or more;
Next, the steel slab is
Finish rolling end temperature: 840°C or more and 1000°C or less,
Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C.: 10 ° C. / sec or more, and
A hot rolling process in which hot rolling is performed under the condition of a coiling temperature of 620°C or less to obtain a hot-rolled steel sheet;
Next, the hot-rolled steel sheet is
A cold rolling process in which cold rolling is performed under a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet;
Next, the cold rolled steel sheet is
A temperature increasing step in which the temperature is increased at an average temperature increasing rate in a temperature range from 600° C. to 750° C. of 1° C./sec or more and 15° C./sec or less;
Next, the cold rolled steel sheet is
Annealing temperature: 750°C or higher and 920°C or lower; and
Annealing time: annealing under conditions of 1 second to 30 seconds;
Next, the cold rolled steel sheet is
A cooling process in which the average cooling rate in the annealing temperature range from -30 ° C. to 600 ° C. is 5 ° C./sec or more and 100 ° C./sec or less, and the cooling stop temperature is 400 ° C. or more and 600 ° C. or less;
Next, the cold rolled steel sheet is
A residence time in a temperature range of 400° C. to 600° C.: a residence step of 1 second to 90 seconds;
The method for producing a steel sheet comprising the steps of:

8. The dew points of the atmospheres in the temperature increasing step and the annealing step are both −35° C. or higher;
The method for producing a steel sheet according to claim 7, wherein the dew point of the atmosphere in the cooling step is −35° C. or lower.

9. A method for producing a steel sheet as described in 7 or 8, further comprising a zinc plating process in which the cold-rolled steel sheet is subjected to a zinc plating process after the retention process.

10. A method for manufacturing a steel sheet as described in 9, wherein the zinc plating process is a hot-dip zinc plating process or an alloyed hot-dip zinc plating process.

11. A method for manufacturing a component, comprising the step of subjecting a steel plate described in any one of 1 to 5 to at least one of forming and joining processes to form a component.

According to the present invention, a steel sheet having high strength, high YS, excellent ductility, and excellent bendability can be obtained. In addition, since the steel sheet of the present invention has high strength, high YS, excellent ductility, and excellent bendability, it can be used extremely advantageously as a material for automobile frame structural members having complex shapes.

1 is an example of an observation image used for measuring the particle number density, etc., of retained austenite.

The present invention will be described based on the following embodiments.
[1] Steel Plate First, the composition of the steel plate according to one embodiment of the present invention will be described. Note that the unit of the composition is "mass%", but hereinafter, unless otherwise specified, it will be simply shown as "%".

C: 0.05% or more and 0.20% or less C has the effect of increasing the strength of martensite and bainite and generating appropriate amounts of these phases. This effect makes it possible to ensure a predetermined strength. Here, if the C content is less than 0.05%, the strength of martensite decreases. In addition, the area ratio of ferrite increases excessively, making it difficult to obtain a predetermined strength. On the other hand, if the C content exceeds 0.20%, TS becomes excessively high and El decreases. In addition, the strength of martensite increases excessively, decreasing bendability. Therefore, the C content is set to 0.05% or more and 0.20% or less. The C content is preferably 0.07% or more, more preferably 0.09% or more. In addition, the C content is preferably 0.18% or less, more preferably 0.17% or less.

Si: 0.1% or more and 1.8% or less Si is an element that improves the strength of the steel sheet by solid solution strengthening. In addition, Si is an element that improves ductility while suppressing strength reduction by increasing the strength of ferrite. Furthermore, Si is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Si is an element that affects the area ratio of ferrite. Here, if the Si content is less than 0.1%, the area ratio of ferrite decreases and ductility decreases. On the other hand, if the Si content is excessive, particularly if it exceeds 1.8%, it will lead to a significant increase in the rolling load during hot rolling and cold rolling. It will also lead to a decrease in toughness. Therefore, the Si content is set to 0.1% or more and 1.8% or less. The Si content is preferably 0.3% or more, more preferably 0.5% or more. The Si content is preferably 1.5% or less, more preferably 1.0% or less.

Mn: 1.5% or more and 3.0% or less Mn is contained to improve the hardenability of the steel and to ensure a predetermined area ratio of martensite and bainite. Here, if the Mn content is less than 1.5%, the hardenability is insufficient and ferrite is excessively generated. This makes it difficult to achieve a TS of 780 MPa or more. On the other hand, if Mn is excessively contained, the bainite transformation is delayed and it becomes difficult to obtain a predetermined amount of retained austenite. This leads to a decrease in ductility. Therefore, the Mn content is set to 1.5% or more and 3.0% or less. The Mn content is preferably 1.65% or more, more preferably 1.8% or more. The Mn content is preferably 2.85% or less, more preferably 2.7% or less.

P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS of the steel sheet. In order to obtain such an effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates to the prior austenite grain boundary and embrittles the grain boundary. Therefore, when bending stress is applied to the steel sheet, voids are generated and cracks grow along the prior austenite grain boundary, and the desired bendability cannot be obtained. Therefore, the P content is set to 0.100% or less. Due to the constraints of production technology, the P content is preferably 0.002% or more. In addition, the P content is preferably 0.050% or less, more preferably 0.030% or less.

S: 0.0500% or less S forms MnS, etc., and reduces ductility. In addition, when Ti is contained together with S, TiS, Ti(C,S), etc. are formed, which may reduce bendability. Therefore, the S content is set to 0.0500% or less. The S content is preferably 0.0100% or less, more preferably 0.0080% or less, and further preferably 0.0050% or less. The lower limit of the S content is not particularly limited. The S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more.

Al: 0.010% or more and 1.000% or less Al is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Al is an element that affects the area ratio of ferrite. Here, when the Al content is less than 0.010%, the area ratio of ferrite decreases and ductility decreases. On the other hand, when the Al content exceeds 1.000%, the area ratio of ferrite increases excessively, making it difficult to make TS 780 MPa or more. Therefore, the Al content is 0.010% or more and 1.000% or less. The Al content is preferably 0.015% or more, more preferably 0.030% or more. The Al content is preferably 0.500% or less, more preferably 0.100% or less.

N: 0.0100% or less N is an element that generates nitride-based precipitates such as AlN that pin grain boundaries, and can be contained to improve elongation. However, if the N content exceeds 0.0100%, the nitride-based precipitates such as AlN become coarse, and the elongation decreases. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0070% or less, more preferably 0.0050% or less. The lower limit of the N content is not particularly limited. Due to constraints on production technology, the N content is preferably 0.0006% or more.

One or both of Nb and Ti: 0.005% or more and 0.200% or less in total Nb and Ti are elements that contribute to improving TS, ductility, and further bendability through the refinement of prior austenite grains. That is, Nb and Ti contribute to increasing TS through the refinement of the internal structure of martensite and bainite due to the refinement of prior austenite grains. Furthermore, Nb and Ti also contribute to increasing TS through the formation of fine precipitates, such as carbides, nitrides, and carbonitrides, in the hot rolling process and annealing process. In addition, Nb and Ti increase the nucleation sites of ferrite and bainite in the cooling process and retention process by the refinement of prior austenite grains and the formation of fine precipitates, and promote ferrite transformation and bainite transformation. Furthermore, with the increase in the nucleation sites of ferrite and bainite, it becomes possible to appropriately control the distribution state of particles constituting retained austenite. In particular, for particles with an aspect ratio of 3 or less, the above-mentioned (C) and (D) can be appropriately controlled. As a result, Nb and Ti contribute to improving ductility and further bending property. In order to obtain these effects, the total content of Nb and Ti is set to 0.005% or more. On the other hand, if the content of Nb and Ti becomes excessive, a large amount of coarse precipitates and inclusions are generated, which leads to a decrease in ductility and bending property. In particular, when Nb and Ti are added in combination, the precipitates are stabilized and tend to remain as coarse inclusions. Therefore, the total content of Nb and Ti is set to 0.200% or less. The total content of Nb and Ti is preferably 0.008% or more, more preferably 0.010% or more, even more preferably 0.011% or more, and even more preferably 0.015% or more. The total content of Nb and Ti is preferably 0.150% or less, more preferably 0.080% or less.

As long as the total content of Nb and Ti is 0.005% or more and 0.200% or less, the respective contents of Nb and Ti are not particularly limited.
For example, the Nb content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Nb content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.080% or less.
The Ti content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Ti content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.080% or less.

The basic composition of the steel sheet according to one embodiment of the present invention has been described above, but the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities. Here, the steel sheet according to one embodiment of the present invention preferably has a composition containing the above basic components, with the balance consisting of Fe and unavoidable impurities. The steel sheet according to one embodiment of the present invention may contain at least one selected from the following as an optional added element in addition to the above basic components.
V: 0.45% or less,
B: 0.010% or less,
Cr: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Sb: 0.1% or less,
Sn: 0.1% or less,
Cu: 1.0% or less,
Ta: 0.1% or less,
W: 0.2% or less,
Mg: 0.01% or less,
Zn: 0.02% or less,
Co: 0.02% or less,
Zr: 0.2% or less,
Ca: 0.02% or less,
Se: 0.02% or less,
Te: 0.02% or less,
Ge: 0.02% or less,
As: 0.05% or less,
Sr: 0.02% or less,
Cs: 0.02% or less,
Hf: 0.02% or less,
Pb: 0.02% or less,
Bi: 0.02% or less and REM: 0.02% or less. Since the effects of the present invention can be obtained as long as the optional elements are contained in amounts not exceeding the upper limits, no lower limit is set. When the optional elements are contained in amounts less than the preferred lower limits described below, the elements are considered to be contained as unavoidable impurities.

V: 0.45% or less Like Nb and Ti, V increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, in the hot rolling process and annealing process. In order to obtain such an effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, if the V content exceeds 0.45%, a large amount of coarse precipitates and inclusions may be generated, and ductility may decrease. Therefore, when V is contained, the V content is preferably 0.45% or less. The V content is more preferably 0.060% or less.

B: 0.010% or less B is an element that enhances hardenability by segregating at the austenite grain boundary. In addition, B is an element that controls the generation and grain growth of ferrite in the cooling process after the annealing process. In order to obtain such an effect, it is preferable that the B content is 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.010%, the amount of nitride-based precipitates such as BN becomes excessive, and ductility may decrease. Therefore, when B is contained, the B content is preferably 0.010% or less. The B content is more preferably 0.0050% or less, and further preferably 0.0030% or less.

Cr: 1.0% or less Cr is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Cr content to 0.0005% or more. Moreover, the Cr content is more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.0%, the area ratio of martensite increases and ductility may decrease. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less. Moreover, the Cr content is more preferably 0.60% or less, and further preferably 0.30% or less.

Ni: 1.0% or less Ni is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Ni content to 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.0%, the area ratio of martensite increases, and ductility may decrease. Therefore, when Ni is contained, the Ni content is preferably 1.0% or less. The Ni content is more preferably 0.5% or less.

Mo: 1.0% or less Mo is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Mo content to 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.0%, the area ratio of martensite increases, and the desired ductility may not be obtained. Therefore, when Mo is contained, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.5% or less, and further preferably 0.3% or less.

Sb: 0.1% or less Sb is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to make the TS 780 MPa or more. Therefore, it is preferable to set the Sb content to 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.1%, the castability decreases. Therefore, when Sb is contained, the Sb content is preferably 0.1% or less. The Sb content is more preferably 0.06% or less, and further preferably 0.04% or less.

Sn: 0.1% or less Sn suppresses oxidation and nitridation near the steel sheet surface, thereby suppressing the decrease in the content of C and B near the steel sheet surface. This suppresses excessive generation of ferrite near the steel sheet surface, contributing to making the TS 780 MPa or more. From this perspective, the Sn content is preferably 0.002% or more. However, if the Sn content exceeds 0.1%, the castability decreases. Therefore, when Sn is contained, the Sn content is preferably 0.1% or less. The Sn content is more preferably 0.04% or less, and even more preferably 0.02% or less.

Cu: 1.0% or less Cu is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, it is preferable to set the Cu content to 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.0%, the area ratio of martensite increases excessively, which may reduce ductility. In addition, a large amount of coarse precipitates and inclusions may be generated, and such coarse precipitates and inclusions may reduce ductility. Therefore, when Cu is contained, the Cu content is preferably 1.0% or less. The Cu content is more preferably 0.2% or less.

Ta: 0.1% or less Ta, like Ti, Nb and V, increases TS by forming fine precipitates, such as carbides, nitrides and carbonitrides, in the hot rolling process and annealing process. In addition, Ta is partially dissolved in Nb carbides and Nb carbonitrides to generate composite precipitates such as (Nb, Ta) (C, N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS. In order to obtain such an effect, it is preferable that the Ta content is 0.001% or more. On the other hand, if the Ta content exceeds 0.1%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may reduce ductility and even bendability. Therefore, when Ta is contained, the Ta content is preferably 0.1% or less. The Ta content is more preferably 0.05% or less.

W: 0.2% or less Like Ti, Nb and V, W increases TS by forming fine precipitates, such as carbides, nitrides and carbonitrides, in the hot rolling process and annealing process. In order to obtain such an effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more. On the other hand, if the W content exceeds 0.2%, a large amount of coarse precipitates and inclusions are generated, which leads to a decrease in ductility. Therefore, when W is contained, the W content is preferably 0.2% or less. The W content is more preferably 0.060% or less.

Mg: 0.01% or less Mg is an effective element for spheroidizing the shape of inclusions such as sulfides and oxides to improve the hole expandability and bendability of steel sheets. In order to obtain such an effect, it is preferable to set the Mg content to 0.0001% or more. However, if the Mg content exceeds 0.01%, the surface quality will deteriorate. In addition, it may even lead to a deterioration in bendability. Therefore, when Mg is contained, the Mg content is preferably 0.01% or less. The Mg content is more preferably 0.005% or less, and even more preferably 0.001% or less.

Zn: 0.02% or less Zn is an element that is effective in making the shape of inclusions spherical and improving the bendability of the steel sheet. In order to obtain such an effect, the Zn content is preferably 0.001% or more. On the other hand, if the Zn content exceeds 0.02%, a large amount of coarse precipitates and inclusions are generated, which may lead to a decrease in bendability. Therefore, when Zn is contained, the Zn content is preferably 0.02% or less.

Co: 0.02% or less Like Zn, Co is an effective element for making the shape of inclusions spherical and improving the bendability of the steel sheet. In order to obtain such an effect, it is preferable that the Co content is 0.001% or more. On the other hand, if the Co content exceeds 0.02%, a large amount of coarse precipitates and inclusions are generated, which may lead to a decrease in bendability. Therefore, when Co is contained, the Co content is preferably 0.02% or less.

Zr: 0.2% or less Zr contributes to high strength through refinement of prior austenite grains. Zr also contributes to high strength through reduction of block size and vein grain size, which are internal structural units of martensite and bainite, by refinement of prior austenite grains. Furthermore, Zr improves castability. In order to obtain such an effect, it is preferable that the Zr content is 0.001% or more. However, if a large amount of Zr is added, the amount of coarse precipitates of ZrN and ZrS that remain in an undissolved state in the slab heating process increases, and ductility decreases. Therefore, when Zr is contained, the Zr content is preferably 0.2% or less. The Zr content is more preferably 0.05% or less, and further preferably 0.01% or less.

Ca: 0.02% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.02%, a large amount of coarse inclusions may be generated, which may reduce ductility and bendability. Surface quality may also deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.02% or less. The lower limit of the Ca content is not particularly limited. For example, the Ca content is preferably 0.0005% or more. In addition, due to constraints on production technology, the Ca content is more preferably 0.0010% or more.

Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.02% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective for improving the bendability of steel sheet. In order to obtain such an effect, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM exceed 0.02% each, or if the content of As exceeds 0.05%, a large amount of coarse precipitates and inclusions are generated, and the bendability may be deteriorated. Therefore, when these elements are contained, it is preferable that the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM are each 0.02% or less, and the content of As is 0.05% or less. Note that Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM may be contained alone or in combination.

Elements other than those mentioned above are Fe and unavoidable impurities.

Next, the steel structure of the steel plate according to one embodiment of the present invention will be described.
The steel structure of the steel plate according to one embodiment of the present invention is
Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total,
Area ratio of martensite: 10% or more and 87% or less, and
Area ratio of retained austenite: 3% or more,
The number density of particles constituting the retained austenite is 0.05 particles/ ^μm2 or more,
Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the above (A), (B), (C), and (D),
The steel structure has a ratio of [Mn] _C /[Mn] of 1.05 to 2.00, [Mn] _C being the average Mn concentration (mass %) of Mn-enriched regions in the steel, and [Mn] being the average Mn concentration (mass %) in the steel.
The reasons for each of the limitations will be explained below. The area ratio of each phase is the ratio of the area that each phase occupies to the area of the entire steel structure.

Total area ratio of one or both of ferrite and bainite (hereinafter also referred to as total area ratio of ferrite and bainite): 10% or more and 87% or less Ferrite and bainite are soft, and therefore are effective in obtaining excellent ductility. In order to obtain the desired ductility, the total area ratio of ferrite and bainite is set to 10% or more. On the other hand, if the area ratio of ferrite and bainite becomes excessive, it becomes difficult to achieve a TS of 780 MPa or more. Therefore, the total area ratio of ferrite and bainite is set to 87% or less. The total area ratio of ferrite and bainite is preferably 20% or more, more preferably 30% or more. In addition, the total area ratio of ferrite and bainite is preferably 75% or less, more preferably 65% or less. In addition, ferrite and bainite may be contained alone, or both of them may be contained.

Area ratio of martensite: 10% or more and 87% or less Martensite is hard and is a structure necessary for increasing the strength of steel plate. If the area ratio of martensite is less than 10%, the desired TS cannot be obtained. On the other hand, an excessive increase in the area ratio of martensite causes a decrease in ductility. Therefore, the area ratio of martensite is 10% or more and 87% or less. The area ratio of martensite is preferably 20% or more, more preferably 30% or more. In addition, the area ratio of martensite is preferably 75% or less, more preferably 65% or less.
Martensite is a hard structure that is generated by transformation from austenite at a martensite transformation point (also simply referred to as Ms point) or lower. Martensite includes both so-called fresh martensite, which is as quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.

Area ratio of retained austenite: 3% or more Retained austenite is a structure necessary for achieving both strength and ductility. Here, if the area ratio of retained austenite is less than 3%, it is not possible to achieve both strength and ductility. Therefore, the area ratio of retained austenite is set to 3% or more. The area ratio of retained austenite is preferably 5% or more, more preferably 7% or more. There is no particular upper limit for the area ratio of retained austenite. However, if the amount of retained austenite becomes excessive, for example, when the steel plate is formed into a part, the retained austenite is transformed into martensite, and the number of starting points of bending cracks increases. Therefore, the area ratio of retained austenite is preferably 20% or less, more preferably 15% or less.
The term "retained austenite" refers to austenite that does not transform into ferrite, martensite, bainite, or other metallic phases and remains. The retained austenite is generated, for example, when elements such as C are concentrated in austenite, causing the martensite transformation point to be lower than room temperature (austenite remains without transforming).

The area ratio of the remaining structure other than the above is preferably 15% or less. The area ratio of the remaining structure is more preferably 10% or less, and further preferably 5% or less. The area ratio of the remaining structure may be 0%.
The remaining structure is not particularly limited, and examples thereof include carbides such as pearlite and cementite. The type of the remaining structure can be confirmed, for example, by observation using a scanning electron microscope (SEM). Pearlite is formed from austenite at a relatively high temperature and is a structure consisting of lamellar ferrite and cementite.

Here, the total area ratio of ferrite and bainite and the area ratio of martensite are measured at a 1/4 position of the sheet thickness of the steel sheet as follows.
That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste, and then the observation surface of the sample is finish-polished using alumina. Next, the observation surface of the sample is etched with nital to reveal the structure. Then, the observation surface of the sample is observed in five fields of view at a magnification of 1500 times using a SEM (Scanning Electron Microscope). Next, the following regions are color-coded (defined) from the obtained structure image using Adobe Photoshop from Adobe Systems. Then, the total area ratio of ferrite and bainite, and the area ratio of martensite are calculated by the point counting method. Specifically, 16×15 lattice points are set at intervals of 4.8 μm in an area of 82 μm×57 μm in actual length of each SEM image. Then, the number of lattice points on ferrite, bainite, and martensite is counted. Then, the number of lattice points on ferrite, bainite, and martensite is divided by the number of all lattice points, and multiplied by 100 to calculate the total area ratio of ferrite and bainite, and the area ratio of martensite.
Ferrite: A black region with a blocky shape. Ferrite is a structure consisting of crystal grains with a BCC lattice. Ferrite is formed by transformation from austenite at relatively high temperatures.
Bainite: A region that is black to dark gray in color and has a blocky or amorphous shape. As described above, bainite is a hard structure in which fine carbides are dispersed in needle-like or plate-like ferrite. Bainite is formed from austenite at a relatively low temperature (Ms point or higher). Bainite contains a relatively small number of carbides.
Martensite: A region that is white to light gray in color. As described above, martensite is a hard structure that is generated by transformation from austenite at or below the Ms point. Martensite includes both so-called fresh martensite, which is as quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.

The area ratio of retained austenite is measured at a 1/4 position of the sheet thickness of the steel sheet as follows.
That is, the steel plate is mechanically ground in the plate thickness direction (depth direction) to a position of 1/4 of the plate thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. CoKα rays are used as the incident X-rays, and the ratio of the diffraction intensity of each of the (200), (220) and (311) faces of fcc iron (austenite) to the diffraction intensity of each of the (200), (211) and (220) faces of bcc iron is obtained. Next, the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensity of each face. Then, the retained austenite is considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.

Furthermore, the area ratio of the remaining structure is determined by subtracting the total area ratio of ferrite and bainite, the area ratio of martensite, and the area ratio of retained austenite determined as described above from 100%.
[Area ratio of remaining structure (%)] = 100 - [Total area ratio of ferrite and bainite (%)] - [Area ratio of martensite (%)] - [Area ratio of retained austenite (%)]

Number density of particles constituting the retained austenite (hereinafter also referred to as the particle number density of the retained austenite): 0.05 pieces/μm ² or more In a steel sheet according to one embodiment of the present invention, it is important to set the particle number density of the retained austenite to 0.05 pieces/μm ² or more. The hard martensite (hereinafter also referred to as the processing-induced martensite) generated by processing-induced transformation from the retained austenite during processing (hereinafter also referred to simply as processing) to form the steel sheet into a part is a structure that promotes the generation of voids and the growth of cracks during processing. However, by setting the particle number density of the retained austenite to 0.05 pieces/μm ² , the stress on the processing-induced martensite is dispersed to suppress the stress concentration, and the generation of voids and the growth of cracks can be suppressed. Therefore, the particle number density of the retained austenite is set to 0.05 pieces/μm ² or more. The particle number density of the retained austenite is preferably 0.15 pieces/μm ² or more, more preferably 0.25 pieces/μm ² or more. The upper limit of the particle number density of the retained austenite is not particularly limited. However, if the particle number density of the retained austenite becomes excessive, the number of bending crack initiation points increases. Therefore, the particle number density of the retained austenite is preferably 100 particles/ ^μm2 or less, more preferably 10 particles/ ^μm2 or less.

(A) The ratio of particles with an aspect ratio of 3 or less to all particles constituting the retained austenite (hereinafter also referred to as the ratio of particles with an aspect ratio of 3 or less): 60% or more in area ratio. When the aspect ratio of the retained austenite is large, stress concentration is likely to occur around the processing-induced martensite generated by processing. This promotes the generation of voids and the propagation of cracks, and reduces ductility and bendability. Therefore, the ratio of particles with an aspect ratio of 3 or less is set to 60% or more in area ratio. The ratio of particles with an aspect ratio of 3 or less is preferably 65% or more, more preferably 70% or more in area ratio. The upper limit of the ratio of particles with an aspect ratio of 3 or less is not particularly limited, and may be 100% in area ratio.

(B) Average C concentration of particles with aspect ratio of 3 or less: 0.3 mass% or more In the steel sheet according to one embodiment of the present invention, it is important to set the average C concentration of particles with aspect ratio of 3 or less to 0.3 mass% or more. That is, the higher the average C concentration of particles with aspect ratio of 3 or less, the higher the stability of the retained austenite, and an excellent balance between strength and ductility can be obtained. When the average C concentration of particles with aspect ratio of 3 or less is less than 0.3 mass%, a good balance between strength and ductility cannot be obtained. Furthermore, since the stability of the retained austenite is low, for example, the amount of retained austenite that becomes processing-induced martensite by processing increases, and bendability decreases. Therefore, the average C concentration of particles with aspect ratio of 3 or less is set to 0.3 mass% or more. The average C concentration of particles with aspect ratio of 3 or less is preferably 0.5 mass% or more, more preferably 0.7 mass% or more. The upper limit of the average C concentration of particles with aspect ratio of 3 or less is not particularly limited. However, if the C concentration of particles having an aspect ratio of 3 or less is excessively high, the progress of the work-induced transformation from retained austenite to martensite may be excessively suppressed, and sufficient work hardening ability may not be obtained. Therefore, the average C concentration of particles having an aspect ratio of 3 or less is preferably 2.0 mass% or less.

(C) Average value of the shortest distance between particles with an aspect ratio of 3 or less (hereinafter also referred to as the average interparticle distance value): 5 μm or less In a steel sheet according to one embodiment of the present invention, it is important to uniformly disperse the particles of retained austenite. By uniformly dispersing the particles of retained austenite, particularly the particles with an aspect ratio of 3 or less, stress concentration on the processing-induced transformation martensite caused by processing is suppressed, and ductility and bendability are improved. For this purpose, it is important to set the average interparticle distance value to 5 μm or less. The average interparticle distance value is preferably 4 μm or less, more preferably 3 μm or less. The lower limit of the average interparticle distance value is not particularly limited. However, from the viewpoint of cost and productivity, the average interparticle distance value is preferably 0.8 μm or more.

(D) Maximum value of the shortest distance between particles with an aspect ratio of 3 or less (hereinafter also referred to as the maximum interparticle distance value): 15 μm or less In a steel sheet according to one embodiment of the present invention, it is important to uniformly disperse the retained austenite. By uniformly dispersing the particles of the retained austenite, particularly the particles with an aspect ratio of 3 or less, stress concentration on the processing-induced transformation martensite caused by processing is suppressed, and ductility and bendability are improved. Here, if the maximum interparticle distance value is large, particularly exceeding 15 μm, the particles of the retained austenite will be present locally, resulting in a decrease in ductility and bendability. Therefore, the maximum interparticle distance value is set to 15 μm or less. The maximum interparticle distance value is preferably 10 μm or less, more preferably 7 μm or less. The lower limit of the maximum interparticle distance value is not particularly limited. For example, the maximum interparticle distance value is preferably 1 μm or more.

Here, the particle number density of the retained austenite, the ratio of particles having an aspect ratio of 3 or less, the average C concentration of particles having an aspect ratio of 3 or less, the average interparticle distance, and the maximum interparticle distance are each determined as follows using EBSD (electron backscatter diffraction) attached to an FE-SEM.
That is, a sample is cut out from the steel plate so that the plate thickness section (L section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste. Next, the observation surface of the sample is finish-polished using alumina. Next, the observation position is set to 1/4 of the plate thickness position of the steel plate, and a region of 50 μm x 50 μm is observed by EBSD. Then, using phase map, image analysis is performed by ImageJ, and the aspect ratio and center of gravity position of each particle of the observed retained austenite are obtained by the particle analysis function. For reference, an example of the observation image is shown in FIG. 1. In the observation image, the white area is the particle of the retained austenite. Note that the particles constituting the retained austenite are those having a circle equivalent diameter of 0.8 μm or more. Then, the number of particles constituting the retained austenite is counted, and the number of particles is divided by the area of the observation region to obtain the particle number density of the retained austenite. The area occupied by particles having an aspect ratio of 3 or less among all the particles constituting the retained austenite counted above is divided by the area occupied by all the particles constituting the retained austenite, and the result is multiplied by 100 to determine the ratio of particles with an aspect ratio of 3 or less. Furthermore, for each particle having an aspect ratio of 3 or less, the distance to the nearest particle having an aspect ratio of 3 or less (the distance between the centers of gravity of the particles) is determined. The average and maximum of the calculated distances are then taken as the average interparticle distance and the maximum interparticle distance, respectively.

The average C concentration of particles having an aspect ratio of 3 or less is determined as follows.
That is, for the sample used in the above EBSD observation, the observation position is set to ¼ of the sheet thickness position of the steel sheet in the same manner as in EBSD, and the C concentration is measured in a grid pattern in a 23 μm square area with measurement intervals of 0.1 μm. Next, an area of particles with an aspect ratio of 3 or less is extracted from the EBSD phase map, and the average value of the C concentration at each measurement point in that area is taken as the average C concentration of particles with an aspect ratio of 3 or less.

[Mn] _C /[Mn]: 1.05 or more and 2.00 or less The fact that [Mn] _C /[Mn], which is the ratio of the average Mn concentration of the Mn-enriched region in the steel to the average Mn concentration in the steel, is large means that the enrichment of Mn in the austenite has progressed in the annealing process. The Mn concentration of the austenite in the steel sheet immediately after the annealing process is one of the factors that determine whether the phase transformed from the austenite in the cooling process and the retention process after the annealing process is ferrite and bainite or martensite. Here, if Mn is excessively enriched in the austenite in the annealing process, it leads to a delay in the ferrite transformation and bainite transformation in the cooling process. As a result, the desired area ratio of ferrite and bainite cannot be obtained, and ductility and bendability may be reduced. In addition, the delay in the ferrite transformation and the bainite transformation suppresses the enrichment of C in the untransformed austenite. Therefore, the retained austenite that contributes to improving ductility is not obtained sufficiently. Therefore, [Mn] _C /[Mn] is set to 2.00 or less. [Mn] _C /[Mn] is preferably set to 1.80 or less, more preferably set to 1.60 or less. On the other hand, when [Mn] _C /[Mn] is set to 1.05 or more, that is, when the distribution of Mn concentration is moderately non-uniform, ferrite transformation and bainite transformation are promoted in austenite with a low Mn concentration. Accordingly, C concentration in untransformed austenite progresses. This results in good ductility and bendability. Therefore, [Mn] _C /[Mn] is set to 1.05 or more. [Mn] _C /[Mn] is preferably set to 1.10 or more, more preferably set to 1.15 or more.

Here, [Mn] _C /[Mn] is calculated as follows.
That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished using diamond paste. Next, the observation surface of the sample is finish-polished using alumina. Next, the observation position is set to 1/4 of the plate thickness of the steel plate, and the Mn concentration is measured in a grid pattern with a measurement interval of 0.1 μm in a 23 μm square area by EPMA. Then, the average value of the Mn concentration of all the measurement points is the average Mn concentration [Mn] (mass%) in the steel. In addition, the top 10% of the areas with the highest Mn concentration among all the measurement points are set as the Mn-enriched areas. Then, the average value of the Mn concentration measured in the Mn-enriched areas is set as the average Mn concentration [Mn] _C (mass%) of the Mn-enriched areas in the steel. Then, [Mn] _C is divided by [Mn] to obtain [Mn] _C / [Mn].

In addition, in the steel sheet according to one embodiment of the present invention, it is preferable to have a soft layer having a thickness of 1 μm or more and 50 μm or less. In particular, by having a soft layer having a thickness of 1 μm or more and 50 μm or less in the thickness direction from the steel sheet surface, better bendability can be obtained. Therefore, it is preferable to have a soft layer in the thickness direction from the steel sheet surface, and it is preferable that the thickness is 1 μm or more. However, if an excessive soft layer is formed, it becomes difficult to obtain the desired TS. Therefore, when a soft layer is present, it is preferable that the thickness is 50 μm or less. The thickness of the soft layer is more preferably 40 μm or less.

The soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the sheet thickness of the steel sheet. The thickness of the soft layer is measured as follows.
That is, the surface of the thickness cross section (L cross section) parallel to the rolling direction of the steel plate is smoothed by wet polishing. Next, using a Vickers hardness tester, hardness measurements are performed at 1 μm intervals in the thickness (depth) direction from a position at a depth of 1 μm from the surface of the steel plate to a position at a depth of 100 μm under a load of 10 gf. Also, under the same conditions, hardness measurements are performed at 20 μm intervals in the thickness (depth) direction from a position at a depth of 100 μm from the surface of the steel plate to the center position of the thickness. Then, the hardness obtained at the 1/4 position of the thickness of the steel plate is set as the reference hardness, and the depth position at which the hardness is 65% or less of the reference hardness on the surface side of the 1/4 position of the thickness of the steel plate is specified. Then, the distance from the surface of the steel plate to the deepest depth position at which the hardness is 65% or less of the reference hardness (hereinafter also referred to as the depth of the region at which the hardness is 65% or less of the reference hardness) is measured. This measurement is carried out at five points spaced at intervals of 3 mm or more in the rolling direction, and the average value of the depth of the region where the hardness is 65% or less of the measured reference hardness is regarded as the thickness of the soft layer.
In addition, since the steel structure of a steel plate is generally approximately symmetrical up and down in the plate thickness direction, any one of the surfaces (front and back) of the steel plate is used as a representative in measuring the thickness of the soft layer. For example, any one of the surfaces (front and back) of the steel plate may be used as the starting point of the plate thickness position (plate thickness 0 position) such as the 1/4 plate thickness position. In addition, when the soft layer is present on only one side of the steel plate, the surface on which the soft layer is present is used as the starting point of the plate thickness position (plate thickness 0 position). The thickness of the soft layer is the thickness per side.

Moreover, in the longitudinal direction (rolling direction) of the steel sheet, the smaller the fluctuation range of the soft phase thickness, the better. In particular, if the fluctuation range of the soft phase thickness exceeds 20 μm, the bendability may vary in the longitudinal direction of the steel sheet, and the bendability may be locally reduced. Therefore, it is preferable that the fluctuation range of the soft phase thickness is 20 μm or less. The fluctuation range of the soft phase thickness is preferably 15 μm or less, more preferably 10 μm or less. The lower limit of the fluctuation range of the soft phase thickness is not particularly limited. The fluctuation range of the soft phase thickness may be 0 μm.
Here, the fluctuation range of the soft phase thickness is the difference between the maximum and minimum values of the depth of the region having a hardness of 65% or less of the reference hardness measured in the above-mentioned measurement of the soft phase thickness (maximum value-minimum value).

Next, the mechanical properties of the steel plate according to one embodiment of the present invention will be described.

Tensile strength (TS): 780 MPa or more The tensile strength of the steel plate according to one embodiment of the present invention is 780 MPa or more. The upper limit of the tensile strength of the steel plate according to one embodiment of the present invention is not particularly limited. However, for example, the tensile strength of the steel plate according to one embodiment of the present invention is preferably less than 1180 MPa.

The yield stress (YS), total elongation (El) and R/t of the steel plate according to one embodiment of the present invention are as described above. The tensile strength (TS), yield stress (YS), total elongation (El) and R/t are measured as described later in the examples.

The steel sheet according to one embodiment of the present invention may have a zinc-plated layer on the surface. The zinc-plated layer may be provided on only one surface of the steel sheet, or on both surfaces. The zinc-plated layer refers to a plating layer containing Zn as the main component (Zn content of 50.0 mass% or more). Examples of the zinc-plated layer include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer. The steel sheet having a zinc-plated layer can also be called a zinc-plated steel sheet. The above-mentioned steel sheets having a hot-dip galvanized layer and alloyed hot-dip galvanized coating can also be called hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA), respectively.

Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% or more and 1.0 mass% or less of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% or more and 3.5 mass% or less. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.

The galvannealed layer is preferably composed of, for example, Zn, 20% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more and 3.5% by mass or less. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

In addition, the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g/ ^m2 or more and 80 g/ ^m2 or less.

The plating weight of the zinc plating layer is measured as follows.
That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% aqueous hydrochloric acid solution. Next, a steel sheet to be used as a test material is immersed in the treatment solution to dissolve the zinc plating layer. The mass loss of the test material before and after dissolution is then measured, and the value is divided by the surface area of the steel sheet (the surface area of the part that was covered with plating) to calculate the plating coverage (g/ ^m2 ).

The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more and 3.5 mm or less.

[2] Members Next, members according to one embodiment of the present invention will be described.
A member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material). For example, the raw material steel plate is subjected to at least one of forming and joining to form a member.
Here, the above steel plate has a TS of 780 MPa or more, a high YS, excellent ductility, and excellent bendability. Therefore, the member according to one embodiment of the present invention has high strength and is particularly suitable for application to complex-shaped members used in the automotive field.

[3] Manufacturing Method of Steel Sheet Next, a manufacturing method of a steel sheet according to one embodiment of the present invention will be described.

A method for producing a steel sheet according to one embodiment of the present invention includes the steps of:
A steel slab having the above-mentioned composition is
A slab heating step in which the slab is heated under the conditions of a slab heating temperature of 1220° C. or more and a slab heating time of 1.0 hour or more;
Next, the steel slab is
Finish rolling end temperature: 840°C or more and 1000°C or less,
Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C.: 10 ° C. / sec or more, and
A hot rolling process in which hot rolling is performed under the condition of a coiling temperature of 620°C or less to obtain a hot-rolled steel sheet;
Next, the hot-rolled steel sheet is
A cold rolling process in which cold rolling is performed under a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet;
Next, the cold rolled steel sheet is
A temperature increasing step in which the temperature is increased at an average temperature increasing rate in a temperature range from 600° C. to 750° C. of 1° C./sec or more and 15° C./sec or less;
Next, the cold rolled steel sheet is
Annealing temperature: 750°C or higher and 920°C or lower; and
Annealing time: annealing under conditions of 1 second to 30 seconds;
Next, the cold rolled steel sheet is
A cooling process in which the average cooling rate in the annealing temperature range from -30 ° C. to 600 ° C. is 5 ° C./sec or more and 100 ° C./sec or less, and the cooling stop temperature is 400 ° C. or more and 600 ° C. or less;
Next, the cold rolled steel sheet is
and a residence step of maintaining the mixture in a temperature range of 400° C. to 600° C. for a residence time of 1 second to 90 seconds.
Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and the steel plate.

First, a steel slab having the above-mentioned composition is prepared. For example, a steel material is melted to obtain molten steel having the above-mentioned composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. Then, the obtained molten steel is solidified to obtain a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited. For example, a continuous casting method, an ingot casting method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, a continuous casting method is preferable. In addition, a conventional method in which the steel slab is cooled to room temperature after being produced and then heated again can be applied. Furthermore, energy-saving processes such as direct rolling (a method in which the steel slab is not cooled to room temperature, but is charged into a heating furnace as a hot piece and hot rolled) and direct rolling (a method in which the steel slab is immediately rolled after being slightly kept at heat) can also be applied without any problems.

[Slab heating process]
Next, the steel slab is heated. At this time, in the method for producing a steel plate according to an embodiment of the present invention, it is important to satisfy the following conditions.

Slab heating temperature: 1220°C or higher When the slab heating temperature is 1220°C or higher, the coarse precipitates of Nb and Ti generated during casting are sufficiently dissolved. This makes it possible to reduce the coarse precipitates. In addition, it is possible to uniformly disperse the particles of the retained austenite, in particular to control the average interparticle distance and the maximum interparticle distance within a predetermined range. This improves the ductility and bendability of the steel sheet that is the final product. Therefore, the slab heating temperature is 1220°C or higher. The slab heating temperature is preferably 1230°C or higher, more preferably 1240°C or higher. The upper limit of the slab heating temperature is not particularly limited. For example, the slab heating temperature is preferably 1400°C or lower. The slab heating temperature is the maximum temperature reached by the steel slab in the slab heating process.

Slab heating time: 1.0 hour or more When the slab heating temperature is 1220°C or more and the slab heating time is 1.0 hour or more, the coarse precipitates of Nb and Ti generated during casting are sufficiently dissolved. This makes it possible to reduce the coarse precipitates. In addition, it is possible to uniformly disperse the particles of the retained austenite, in particular to control the average interparticle distance and the maximum interparticle distance within a predetermined range. This improves the ductility and bendability of the steel sheet that is the final product. Therefore, the slab heating time is 1.0 hour or more. The slab heating time is preferably 1.1 hours or more, more preferably 1.2 hours or more. There is no particular limit to the upper limit of the slab heating time. For example, the slab heating time is preferably 3.0 hours or less. The slab heating time is a holding time in a temperature range of 1220°C or more.

[Hot rolling process]
Next, the steel slab is subjected to hot rolling to obtain a hot-rolled steel sheet. In this hot rolling process, it is important to satisfy the following conditions:

Finish rolling end temperature: 840°C or more and 1000°C or less From the viewpoint of suppressing the concentration of Mn in the steel structure of the steel sheet to be the final product, it is important to suppress the concentration of Mn (variation in Mn concentration) in the structure of the steel sheet before the annealing process. Here, if the finish rolling end temperature is less than 840°C, the generation of ferrite is promoted, and ferrite is generated excessively before the coiling of the hot-rolled steel sheet. As a result, C is concentrated in the untransformed austenite. Excessive concentration of C in the untransformed austenite promotes pearlite transformation. That is, pearlite is generated excessively in the steel structure of the hot-rolled steel sheet obtained after hot rolling. Pearlite is a lamellar structure of ferrite and cementite, and Mn is concentrated in the cementite. As a result, variation in Mn concentration occurs. Therefore, the finish rolling end temperature is 840°C or more. The finish rolling end temperature is preferably 850°C or more. On the other hand, if the finish rolling end temperature is excessively high, it may be difficult to cool the steel sheet to the coiling temperature described below. Therefore, the finish rolling end temperature is set to 1000° C. or less. The finish rolling end temperature is preferably 950° C. or less, and more preferably 920° C. or less.

Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C. (hereinafter, also referred to as the first cooling rate): 10 ° C./sec or more As described above, from the viewpoint of suppressing the concentration of Mn in the steel structure of the final product, it is important to suppress Mn concentration (variation in Mn concentration) in the structure of the steel sheet before the annealing process. Here, if the first cooling rate is slow, the amount of ferrite generated during cooling becomes excessive, leading to the concentration of C in untransformed austenite. Excessive concentration of C in untransformed austenite promotes pearlite transformation. That is, in the steel structure of the hot-rolled steel sheet obtained after hot rolling, pearlite is excessively generated. As described above, pearlite is a lamellar structure of ferrite and cementite, and Mn is concentrated in cementite. As a result, variation in Mn concentration occurs. Furthermore, if the first cooling rate is slow, precipitates such as carbides and nitrides of Nb and Ti become coarse. Therefore, the effect of increasing TS and improving ductility due to refinement of prior austenite grains and fine precipitates cannot be obtained. Therefore, the first cooling rate is set to 10°C/sec or more. The first cooling rate is preferably 15°C/sec or more. The upper limit of the first cooling rate is not particularly limited. However, from the viewpoint of energy saving of cooling equipment, the first cooling rate is preferably set to 1000°C/sec or less.

Coiling temperature: 620°C or less If the coiling temperature exceeds 620°C, the amount of pearlite produced during coiling is excessively large, and Mn concentration is promoted. The lower the coiling temperature, the less pearlite is produced, so the lower the coiling temperature is, the more preferable it is. Furthermore, from the viewpoint of finely precipitating carbides and nitrides of Nb and Ti, the lower the coiling temperature is, the more preferable it is. Therefore, the coiling temperature is set to 620°C or less. The coiling temperature is preferably 600°C or less, more preferably 580°C or less. On the other hand, if the coiling temperature is less than 400°C, the steel sheet may be excessively hardened and may cause breakage during cold rolling. Therefore, the coiling temperature is preferably 400°C or more. The coiling temperature is more preferably 450°C or more.

In addition, descaling may be performed as appropriate to remove the primary and secondary scale formed on the surface of the hot-rolled steel sheet. Before cold rolling the hot-rolled steel sheet, it is advisable to thoroughly pickle the sheet to reduce the amount of remaining scale. In addition, from the viewpoint of reducing the load during cold rolling, the hot-rolled steel sheet may be optionally subjected to hot-rolled sheet annealing.

[Cold rolling process]
Next, the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet.

Reduction ratio: 20% or more and 80% or less The reduction ratio in cold rolling is 20% or more. That is, if the reduction ratio is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, and the TS and bendability of the final product are reduced. Therefore, the reduction ratio is 20% or more. On the other hand, if the reduction ratio exceeds 80%, the shape of the steel sheet is likely to be defective. In addition, there is a risk of non-uniformity in the steel structure and non-uniformity in the amount of zinc coating due to temperature unevenness in the annealing process. Therefore, the reduction ratio is 80% or less. The reduction ratio is preferably 30% or more. In addition, the reduction ratio is preferably 70% or less.

[Temperature increasing process]
The cold-rolled steel sheet is then heated to an annealing temperature. At this time, it is important to appropriately control the average heating rate in the temperature range from 600°C to 750°C.

Average heating rate in the temperature range from 600°C to 750°C (hereinafter also referred to as heating rate): 1°C/sec or more and 15°C/sec or less When the residence time of the cold-rolled steel sheet in the temperature range from 600°C to 750°C (hereinafter also referred to as the heating temperature range) in the heating process is reduced, Mn diffuses and the concentration of Mn in austenite is suppressed. That is, the longer the residence time in the above heating temperature range, the more the concentration of Mn in austenite is promoted. Therefore, it is effective to shorten the residence time in the heating temperature range, in other words, to increase the heating rate. In addition, by shortening the residence time in the heating temperature range, the coarsening of prior austenite grains is suppressed. As a result, it is possible to secure a predetermined amount of particle ratios with aspect ratios of 3 or less constituting the retained austenite. Furthermore, from the viewpoint of promoting ferrite transformation and bainite transformation and uniformly dispersing the retained austenite, it is better to have a fast heating rate. Therefore, the heating rate is set to 1°C/sec or more. The heating rate is preferably 2° C./sec or more, more preferably 3° C./sec or more. On the other hand, if the heating rate exceeds 15° C./sec, Mn concentration in austenite during the heating step is excessively suppressed. Therefore, the heating rate is set to 15° C./sec or less. The heating rate is preferably 12° C./sec or less, more preferably 9° C./sec or less.

Atmospheric dew point: -35°C or higher From the viewpoint of forming a soft layer of a desired thickness from the steel sheet surface in the sheet thickness direction and obtaining excellent bendability, it is preferable that the atmospheric dew point in the temperature-raising step is -35°C or higher. If the atmospheric dew point is less than -35°C, it is difficult to form a soft phase of a desired thickness. Therefore, it is preferable that the atmospheric dew point in the temperature-raising step is -35°C or higher. The atmospheric dew point in the temperature-raising step is more preferably -20°C or higher, and further preferably -10°C or higher. The upper limit of the atmospheric dew point in the temperature-raising step is not particularly limited. In order to set TS within a suitable range, the atmospheric dew point in the temperature-raising step is preferably 15°C or lower, more preferably 5°C or lower.

[Annealing process]
Next, the cold-rolled steel sheet is annealed under the conditions of an annealing temperature of 750° C. to 920° C. and an annealing time of 1 second to 30 seconds.

Annealing temperature: 750°C or more and 920°C or less When the annealing temperature is less than 750°C, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and the desired TS cannot be obtained. On the other hand, when the annealing temperature exceeds 920°C, the desired area ratio of ferrite and bainite cannot be obtained, and ductility decreases. Therefore, the annealing temperature is 750°C or more and 920°C or less. The annealing temperature is preferably 880°C or less. The annealing temperature is the maximum temperature reached in the annealing process.

Annealing time: 1 second or more and 30 seconds or less In the method for producing a steel sheet according to one embodiment of the present invention, the annealing time is important for controlling the aspect ratio of the grains constituting the retained austenite. That is, the shorter the annealing time, the better from the following viewpoints.
- It suppresses grain growth during annealing and suppresses the concentration of Mn in austenite.
- Promotes ferrite transformation and bainite transformation, and reduces the aspect ratio of the particles that make up retained austenite.
- Promotes C concentration in particles with an aspect ratio of 3 or less.
・It suppresses the coarsening of austenite (particles) during annealing and disperses the retained austenite uniformly.
Therefore, the annealing time is set to 30 seconds or less, preferably 25 seconds or less, and more preferably 20 seconds or less.
On the other hand, if the annealing time is less than 1 second, the coarse Fe-based precipitates do not dissolve, and the elongation decreases. Therefore, the annealing time is set to 1 second or more. The annealing time is preferably 3 seconds or more, and more preferably 5 seconds or more. The annealing time is the holding time at the annealing temperature.

Dew point of atmosphere: -35°C or higher From the viewpoint of forming a soft layer of a desired thickness from the steel sheet surface in the sheet thickness direction and obtaining excellent bendability, it is preferable to set the dew point of the atmosphere to -35°C or higher in the annealing process following the above-mentioned heating process. If the dew point of the atmosphere is less than -35°C, it becomes difficult to form a soft phase of a desired thickness. Therefore, it is preferable to set the dew point of the atmosphere in the annealing process to -35°C or higher. The dew point of the atmosphere in the annealing process is more preferably -20°C or higher, and further preferably -10°C or higher. The upper limit of the dew point of the atmosphere in the annealing process is not particularly limited. In order to set the TS within a suitable range, the dew point of the atmosphere in the annealing process is preferably 15°C or lower, more preferably 5°C or lower.

[Cooling process]
Next, the cold-rolled steel sheet annealed as described above is cooled under the following conditions.

Average cooling rate in the temperature range from annealing temperature -30 ° C. to 600 ° C.: 5 ° C./sec or more and 100 ° C./sec or less In this cooling step, in order to generate ferrite and bainite, it is necessary to appropriately control the cooling rate, particularly the average cooling rate in the temperature range from annealing temperature -30 ° C. to 600 ° C. (hereinafter also referred to as the second cooling rate). If the second cooling rate is slow, ferrite is generated excessively. In addition, pearlite is also generated excessively, and TS is reduced. Furthermore, an appropriate amount of retained austenite is not obtained. Therefore, the second cooling rate is set to 5 ° C./sec or more. The second cooling rate is preferably 9 ° C./sec or more, more preferably 12 ° C./sec or more. On the other hand, if the second cooling rate exceeds 100 ° C./sec, ferrite transformation and bainite transformation are excessively suppressed, and ductility may be reduced. Therefore, the second cooling rate is set to 100 ° C./sec or less. The second cooling rate is preferably 75 ° C./sec or less, more preferably 50 ° C./sec or less.

Cooling stop temperature: 400°C or more and 600°C or less When the cooling stop temperature is less than 400°C, the aspect ratio of bainite increases, and accordingly, the number of particles with an aspect ratio of more than 3 increases among all particles constituting the retained austenite. Therefore, the cooling stop temperature is set to 400°C or more. The cooling stop temperature is preferably 430°C or more, more preferably 460°C or more. On the other hand, when the cooling stop temperature exceeds 600°C, there is a risk that pearlite is excessively generated and the desired TS cannot be obtained. Therefore, the cooling stop temperature is set to 600°C or less. The cooling stop temperature is preferably 570°C or less, more preferably 540°C or less.

Dew point of atmosphere: -35°C or less From the viewpoint of homogenizing the soft phase formed from the steel sheet surface in the sheet thickness direction, it is preferable to set the dew point of the atmosphere in the cooling step to -35°C or less. That is, if the dew point of the atmosphere during cooling exceeds -35°C, the soft phase may not be homogenized and unevenness may occur. Therefore, it is preferable to set the dew point of the atmosphere in the cooling step to -35°C or less. The dew point of the atmosphere in the cooling step is more preferably -40°C or less. Note that the lower limit of the dew point of the atmosphere in the cooling step is not limited. From the viewpoint of controllability, the dew point of the atmosphere in the cooling step is preferably -60°C or more, more preferably -55°C or more.

[Retention process]
Next, the cold-rolled steel sheet cooled as described above is retained in a temperature range of 400° C. or more and 600° C. or less for 1 second or more and 90 seconds or less.

Residence temperature range: 400°C or more and 600°C or less The residence temperature range is set to 400°C or more and 600°C or less from the viewpoint of securing an appropriate amount of bainite and residual austenite. If the residence temperature range is less than 400°C, the number of particles with an aspect ratio of more than 3 increases among all particles constituting the residual austenite. On the other hand, if the residence temperature range exceeds 600°C, ferrite and bainite may be excessively generated, and the desired TS may not be obtained. Therefore, the residence temperature range is set to 400°C or more and 600°C or less. The residence temperature range is preferably 420°C or more, more preferably 440°C or more. In addition, the residence temperature range is preferably 560°C or less, more preferably 520°C or less. In addition, when performing a galvanizing process described later, particularly a hot-dip galvanizing process or an alloyed hot-dip galvanizing process, it is preferable to reheat the cold-rolled steel sheet immediately before the galvanizing process so that the temperature of the sheet entering the galvanizing bath is higher than the temperature of the galvanizing bath.

Residence time: 1 second or more and 90 seconds or less In order to ensure an appropriate amount of retained austenite, it is necessary to appropriately control the residence time in the above residence temperature range (hereinafter, simply referred to as residence time). Here, the longer the residence time, the more the retained austenite increases. Therefore, the residence time is set to 1 second or more. The residence time is preferably 7 seconds or more, more preferably 15 seconds or more. On the other hand, if the residence time is too long, the amount of bainite becomes excessive, and martensite required to ensure strength cannot be obtained. Therefore, the residence time is set to 90 seconds or less. The residence time is preferably 80 seconds or less, more preferably 70 seconds or less. Note that the residence time here does not include the residence time in the temperature range of 400°C or more and 600°C or less (before cooling is stopped) in the above cooling step.

Furthermore, after the above-mentioned retention process, the cold-rolled steel sheet may be subjected to further surface treatments such as chemical conversion treatment or organic coating treatment.

[Zinc plating process]
Next, the cold rolled steel sheet may be optionally subjected to a galvanizing treatment. Examples of the galvanizing treatment include a hot-dip galvanizing treatment and a hot-dip galvannealing treatment. The treatment conditions may be in accordance with conventional methods.

For example, in the case of hot-dip galvanizing, it is preferable to immerse the cold-rolled steel sheet in a galvanizing bath at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like. The galvanizing bath is not particularly limited as long as it has the composition of the galvanized layer described above, but it is preferable to use a plating bath with an Al content of 0.10% by mass to 0.23% by mass, with the balance consisting of Zn and unavoidable impurities. In addition, when hot-dip galvanizing and alloyed hot-dip galvanizing, which will be described later, are performed, it is preferable to reheat the cold-rolled steel sheet immediately before the process so that the temperature of the sheet entering the plating bath is higher than the plating bath temperature.

In addition, in the case of alloying hot-dip galvanizing treatment, it is preferable to perform alloying treatment at a temperature range of 450°C to 600°C after performing hot-dip galvanizing treatment as described above. If the alloying temperature is less than 450°C, the Zn-Fe alloying rate may be excessively slow, making alloying difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite may transform into pearlite, resulting in a decrease in TS and ductility. Therefore, the alloying temperature in the alloying treatment is preferably 450°C to 600°C. The alloying temperature in the alloying treatment is more preferably 460°C or higher, and even more preferably 470°C or higher. The alloying temperature in the alloying treatment is more preferably 580°C or lower, and even more preferably 560°C or lower.

The plating weight is preferably 20 g/m ² or more and 80 g/m ² or less per side. The plating weight can be adjusted by gas wiping or the like.

The steel sheet obtained as described above may be further subjected to temper rolling. If the elongation rate of temper rolling exceeds 2.00%, the yield stress increases, and the dimensional accuracy when forming the steel sheet into a member may decrease. Therefore, the elongation rate of temper rolling is preferably 2.00% or less. The lower limit of the elongation rate of temper rolling is not particularly limited. From the viewpoint of productivity, the elongation rate of temper rolling is preferably 0.05% or more. In addition, temper rolling may be performed on a device continuous with the annealing device for performing each of the above-mentioned steps (online), or may be performed on a device discontinuous with the annealing device for performing each of the steps (offline). In addition, the number of rolling times of temper rolling may be one or more than two. In addition, rolling by a leveler or the like may be performed as long as it can impart an elongation rate equivalent to that of temper rolling.

From the viewpoint of productivity, it is preferable to carry out a series of processes such as the above-mentioned annealing process and galvanizing process in a continuous annealing line (CAL) or a hot-dip galvanizing line (CGL). After the hot-dip galvanizing process, wiping is possible to adjust the coating weight.

The conditions other than those described above are not particularly limited and may be the same as in the ordinary case. According to the method for producing a steel sheet according to one embodiment of the present invention described above, a steel sheet having high strength, high YS, excellent ductility, and excellent bendability can be obtained, and the steel sheet can be suitably used, for example, for automobile parts.

[4] Manufacturing Method of Member Next, a manufacturing method of a member according to one embodiment of the present invention will be described.
A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a component.
Here, the molding method is not particularly limited, and for example, a general processing method such as press molding can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, riveting, crimping, etc. can be used. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.

A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The steel slab was then heated under the conditions shown in Table 2, and hot rolling consisting of rough rolling and finish rolling was performed on the steel slab to obtain a hot-rolled steel sheet. The slab heating time of No. 13 is the holding time at the slab heating temperature. The obtained hot-rolled steel sheet was then pickled and cold-rolled under the conditions shown in Table 2 to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheet was then subjected to a heating process, an annealing process, a cooling process, and a galvanizing process for some of the steel sheets under the conditions shown in Table 2 to obtain the final steel sheet. Conditions not specified were as per usual.

Here, in the galvanizing process, hot-dip galvanizing or alloyed hot-dip galvanizing was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA). In Table 2, the type of galvanizing process is also indicated as "GI" or "GA". In addition, the type of galvanizing process in Table 2 indicated as "CR" means that the galvanizing process was not performed and the steel remains cold-rolled.

Here, in the hot dip galvanizing treatment, the plating bath contained 0.20 mass% Al, with the balance consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The plating coating weight was about 45 to 72 g/ ^m2 per side (double-sided plating). The composition of the zinc plating layer of the finally obtained GI was 0.1 to 1.0 mass% Fe, 0.2 to 1.0 mass% Al, with the balance consisting of Zn and unavoidable impurities.

In the galvannealing treatment, the plating bath contained 0.14 mass% Al, with the balance consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The plating coating weight was about 45 g/ ^m2 per side (double-sided plating). The alloying temperature was 520°C. The composition of the finally obtained zinc plating layer of GA was 7-15 mass% Fe, 0.1-1.0 mass% Al, with the balance consisting of Zn and unavoidable impurities.

Using the steel plate thus obtained, the steel structure of the steel plate was identified, the particle number density of the retained austenite, (A) the particle ratio (area%) of aspect ratio: 3 or less, (B) the average C concentration (mass%) of aspect ratio: 3 or less, (C) the average interparticle distance (μm), (D) the maximum interparticle distance (μm), [Mn] _C /[Mn], and the thickness of the soft layer were measured in the above-mentioned manner. The measurement results are shown in Table 3. In the steel plate having a soft layer, the soft layer was formed on both sides of the steel plate and had the same thickness on both sides. In addition, in No. 25, the soft layer was not confirmed (the thickness of the soft layer was less than 1 μm), so the column of the thickness of the soft layer in Table 2 is marked with "0".

Further, a tensile test and a V-bend test were carried out according to the following procedures, and the tensile strength (TS), yield stress (YS), total elongation (El) and R/t were evaluated according to the following criteria.
・TS
Pass: 780MPa≦TS
Fail: TS<780MPa
・Y.S.
Passed:
When 780MPa≦TS<980MPa, 420MPa≦YS
When 980MPa≦TS, 550MPa≦YS
Failed:
When 780MPa≦TS<980MPa, YS<420MPa
When 980 MPa ≦ TS, YS < 550 MPa
・El
Passed:
When 780MPa≦TS<980MPa, 19%≦El
When 980MPa≦TS, 10%≦El
Failed:
When 780 MPa ≦ TS < 980 MPa, El < 19%
When 980MPa≦TS, El<10%
R/t
Passed:
In the case of 780 MPa ≦ TS < 980 MPa, 2.0 ≧ R / t
In the case of 980 MPa ≦ TS, 4.0 ≧ R / t
Failed:
In the case of 780 MPa ≦ TS < 980 MPa, R / t > 2.0
In the case of 980 MPa ≦ TS, R / t > 4.0

The tensile test was performed in accordance with JIS Z 2241. That is, JIS No. 5 test pieces were taken from the obtained steel plate so that the longitudinal direction was perpendicular to the rolling direction of the steel plate. Using the taken test pieces, a tensile test was performed at a crosshead speed of 10 mm/min, and TS, YS, and El were measured. The results are shown in Table 3.

The V (90°) bending test was performed in accordance with JIS Z 2248. That is, a test piece of 100 mm x 35 mm was taken from the steel plate by shearing and end face grinding. Here, the 100 mm side was taken so as to be parallel to the rolling perpendicular (C) direction (width direction). Next, using the taken test piece, a V (90°) bending test was performed under the following conditions.
Bending radius R: Varies in 0.5 mm increments Test method: Die support, punch press Forming load: 10 tons
Test speed: 30 mm/min
Holding time: 5 seconds Bending direction: perpendicular to rolling (C) direction The test was performed three times, and the minimum bending radius at which no cracks occurred in any of the three tests was taken as R. R/t was calculated by dividing R by the plate thickness t. The test pieces were observed at a magnification of 25 times using a Leica stereo microscope, and it was determined that cracks had occurred if a crack with a length of 200 μm or more was confirmed. The results are shown in Table 3.

As shown in Table 3, all of the inventive examples passed the TS, YS, El and R/t standards. In addition, all of the members obtained by forming or joining using the steel plates of the inventive examples had the desired shapes without cracking, and met the pass criteria for TS, YS, El and R/t.
On the other hand, in the comparative example, at least one of TS, YS, El and R/t was unsatisfactory.

According to the present invention, a steel sheet is obtained that has high strength, high YS, excellent ductility, and excellent bendability. Moreover, the steel sheet can be used extremely advantageously as a material for automobile frame structural members that have complex shapes. This allows for improved fuel efficiency by reducing the weight of the vehicle body, making the steel sheet extremely valuable in industry.

Claims (18)

In mass percent,
C: 0.05% or more and 0.20% or less,
Si: 0.1% or more and 1.8% or less,
Mn: 1.5% or more and 3.0% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0500% or less,
Al: 0.010% or more and 1.000% or less,
N: 0.0100% or less, and one or two of Nb and Ti: 0.005% or more and 0.200% or less in total;
The balance being Fe and unavoidable impurities;
Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total,
Area ratio of martensite: 10% or more and 87% or less; and area ratio of retained austenite: 3% or more;
The number density of particles constituting the retained austenite is 0.05 particles/ ^μm2 or more,
Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the following (A), (B), (C), and (D),
a steel structure in which [ _Mn ]/[Mn] is 1.05 or more and 2.00 or less, [ _Mn ] is an average Mn concentration (mass%) in Mn-enriched regions in the steel, and [Mn] is an average Mn concentration (mass%) in the steel;
A steel plate having a tensile strength of 780 MPa or more.
(A) The ratio of particles having an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio; (B) The average C concentration of particles having an aspect ratio of 3 or less: 0.3 mass% or more; (C) The average value of the shortest distance between particles having an aspect ratio of 3 or less: 5 μm or less; (D) The maximum value of the shortest distance between particles having an aspect ratio of 3 or less: 15 μm or less. The composition further comprises, in mass%,
V: 0.45% or less,
B: 0.010% or less,
Cr: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Sb: 0.1% or less,
Sn: 0.1% or less,
Cu: 1.0% or less,
Ta: 0.1% or less,
W: 0.2% or less,
Mg: 0.01% or less,
Zn: 0.02% or less,
Co: 0.02% or less,
Zr: 0.2% or less,
Ca: 0.02% or less,
Se: 0.02% or less,
Te: 0.02% or less,
Ge: 0.02% or less,
As: 0.05% or less,
Sr: 0.02% or less,
Cs: 0.02% or less,
Hf: 0.02% or less,
Pb: 0.02% or less,
The steel plate according to claim 1, containing at least one selected from the group consisting of Bi: 0.02% or less and REM: 0.02% or less. The steel sheet according to claim 1, having a soft layer having a thickness of 1 μm or more and 50 μm or less.
Here, the soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the plate thickness of the steel plate. The steel sheet according to claim 2, having a soft layer having a thickness of 1 μm or more and 50 μm or less.
Here, the soft layer is a region where the hardness is 65% or less of the hardness at the 1/4 position of the plate thickness of the steel plate. The steel sheet according to any one of claims 1 to 4, having a zinc-plated layer on its surface. The steel sheet according to claim 5, wherein the zinc-coated layer is a hot-dip galvanized layer or a hot-dip galvannealed layer. A member made using the steel plate according to any one of claims 1 to 4. A member made using the steel plate according to claim 5. A member made using the steel plate according to claim 6. Area ratio of one or both of ferrite and bainite: 10% or more and 87% or less in total,
Area ratio of martensite: 10% or more and 87% or less, and
Area ratio of retained austenite: 3% or more
and
The number density of particles constituting the retained austenite is 0.05 particles/μm2 ^or more,
Among the particles constituting the retained austenite, particles having an aspect ratio of 3 or less satisfy the following (A), (B), (C), and (D),
_{The steel} has a steel structure in which [Mn] _/ [Mn] is 1.05 or more and 2.00 or less, [Mn] is an average Mn concentration (mass%) in Mn-enriched regions in the steel, and [Mn] is an average Mn concentration (mass%) in the steel,
A method for producing a steel plate having a tensile strength of 780 MPa or more, comprising the steps of:
The method comprises:
A steel slab having the composition according to claim 1 or 2,
A slab heating step in which the slab is heated under the conditions of a slab heating temperature of 1220° C. or more and a slab heating time of 1.0 hour or more;
Next, the steel slab is
Finish rolling end temperature: 840°C or more and 1000°C or less,
Average cooling rate in the temperature range from the finish rolling end temperature to 700 ° C.: 10 ° C. / sec or more, and
A hot rolling process in which hot rolling is performed under the condition of a coiling temperature of 620°C or less to obtain a hot-rolled steel sheet;
Next, the hot-rolled steel sheet is
A cold rolling process in which cold rolling is performed under a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet;
Next, the cold rolled steel sheet is
A temperature increasing step in which the temperature is increased at an average temperature increasing rate in a temperature range from 600° C. to 750° C. of 1° C./sec or more and 15° C./sec or less;
Next, the cold rolled steel sheet is
Annealing temperature: 750°C or higher and 920°C or lower; and
Annealing time: annealing under conditions of 1 second to 30 seconds;
Next, the cold rolled steel sheet is
A cooling process in which the average cooling rate in the annealing temperature range from -30 ° C. to 600 ° C. is 5 ° C./sec or more and 100 ° C./sec or less, and the cooling stop temperature is 400 ° C. or more and 600 ° C. or less;
Next, the cold rolled steel sheet is
A residence time in a temperature range of 400° C. to 600° C.: a residence step of 1 second to 90 seconds;
The method for producing a steel sheet comprising the steps of:
(A) The ratio of particles having an aspect ratio of 3 or less to all particles constituting the retained austenite: 60% or more in terms of area ratio
(B) Aspect ratio: Average C concentration of particles of 3 or less: 0.3% by mass or more
(C) Aspect ratio: Average value of shortest distance between particles of 3 or less: 5 μm or less
(D) Aspect ratio: Maximum shortest distance between particles of 3 or less: 15 μm or less The dew points of the atmosphere in the temperature increasing step and the annealing step are both −35° C. or higher;
The method for producing a steel sheet according to claim 10, wherein a dew point of the atmosphere in the cooling step is −35° C. or lower. The method for manufacturing a steel sheet according to claim 10 further includes a galvanizing process in which the cold-rolled steel sheet is galvanized after the retention process. The method for manufacturing a steel sheet according to claim 11 further includes a galvanizing process in which the cold-rolled steel sheet is galvanized after the retention process. The method for manufacturing a steel sheet according to claim 12, wherein the galvanizing treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment. The method for manufacturing a steel sheet according to claim 13, wherein the galvanizing treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment. A method for manufacturing a component, comprising the steps of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining processes to produce a component. A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 5 to at least one of forming and joining processes to produce a component. A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 6 to at least one of forming and joining processes to produce a component.

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