WO2024203779A1 - Steel sheet, member and production methods for those - Google Patents

Abstract

Provided are: a steel sheet which exhibits excellent ductility, bore expanding properties and chemical conversion properties, while having a tensile strength of 780 MPa or more; a member; and a production method for the steel sheet and a production method for the member.　The steel sheet has a component composition which contains, in mass%, C, Si, Mn, P, S, sol. Al and N in predetermined ranges, and satisfies formula (1), and a steel structure in which the area ratio or the like of polygonal ferrite is within a predetermined range. With respect to the steel sheet, the maximum concentration [Pm] of P in a portion within 1 μm from the steel sheet surface in the sheet thickness direction is 0.025 mass% or more, and formula (2) is satisfied. Formula (1): [Si]/[Mn] ≤ 0.35 Formula (2): [Pm]/[Pi] ≥ 1.5

Description

Steel plates, components and their manufacturing methods

The present invention relates to steel sheets, components, and their manufacturing methods that are suitable for use in press-molded products with complex shapes that are used in automobiles, home appliances, etc. through press molding processes and that have excellent chemical conversion treatability.

In the background of the tightening of global _CO2 emission regulations, there is a growing demand for the weight reduction of automobiles by increasing the strength of steel sheets for automobiles, and the application of high-strength steel sheets of 590 MPa class or more to body and seat parts is being promoted from the existing 440 MPa class cold-rolled steel sheets. In general, when the strength of a steel sheet is increased, the press formability such as ductility and stretch flangeability decreases, cracks are easily generated during press forming, and the degree of freedom of shape decreases, so that the application is limited to parts with simple shapes. Therefore, in order to apply high-strength steel sheets to parts with complex shapes, it is important to increase the strength of the steel sheet while maintaining or improving the formability.

In light of this background, TRIP steel, which has retained austenite (residual γ) dispersed in the microstructure of the steel sheet, has been developed as a technology for improving the ductility of the steel sheet. TRIP steel is manufactured using austempering, which involves isothermal holding in the bainite transformation temperature range after soaking, and a heat treatment process called Q&P; Quenching & Partitioning (quenching and partitioning of carbon from martensite to austenite), in which the steel is cooled once in the cooling process to a temperature range between the martensite transformation start temperature (Ms point) and the martensite transformation completion temperature (Mf point), and then reheated and held to stabilize the retained austenite. In both heat treatment processes, a large amount of Si, which can suppress carbide precipitation, is added to form retained γ in the microstructure. Furthermore, Q&P is a heat treatment process in which a portion of the structure is transformed into martensite during the cooling process, and then the martensite structure is tempered by reheating and holding, thereby reducing the hardness difference between different phases in the structure and improving not only ductility but also hole expandability.
For example, Patent Document 1 discloses a steel sheet having excellent hole expansion properties with a hole expansion ratio of 50%, in which a cold-rolled steel sheet containing 0.6 to 2.5% Si is held at a first soaking temperature of 750°C or higher, cooled to a cooling stop temperature in the temperature range of 150 to 350°C, and then reheated to a temperature range of 350 to 500°C, thereby securing a volume fraction of 5 to 15% retained austenite, and achieving both a TS of 980 MPa or higher and ductility with an elongation of 17%.

On the other hand, it is known that as the Si content increases, Si concentrates on the steel sheet surface after annealing, and Si-based oxides are formed, which deteriorates the chemical conversion treatability. To address this issue, for example, Patent Document 2 discloses a method of improving the chemical conversion treatability by adding Ni to prevent Si from concentrating on the steel sheet surface.
Furthermore, Patent Document 3 discloses a method for improving chemical conversion treatability by appropriately controlling the content of Mn, which concentrates on the surface together with Si, so that the Si/Mn ratio is 0.40 or less, thereby forming a Mn-Si composite oxide on the surface.
Furthermore, Patent Document 4 discloses a method for improving chemical conversion treatability by directly removing Si-based oxides by pickling or brushing after annealing.

Patent No. 5821911 Patent No. 2951480 Patent No. 3889768 JP 2003-201538 A

As described above, the addition of Si is effective in improving the ductility of high-strength steel sheets, but when Si is actively utilized to ensure high workability, there is a trade-off between the Si content and the chemical conversion treatability of the steel sheet. The methods disclosed in Patent Documents 2 and 4 are effective as methods for improving the chemical conversion treatability of steels with a high Si content, but there has been a demand for the establishment of other techniques that adjust the alloy elements to be contained, annealing conditions, etc.
Furthermore, the inventors' investigations revealed that even in the method disclosed in Patent Document 3, good chemical conversion treatability is not necessarily ensured.
Thus, there has been a demand for the establishment of a new technology for producing high-strength steel sheet that has excellent ductility and chemical conversion treatability, as well as hole expandability.

The present invention was made in consideration of the above circumstances, and its purpose is to provide steel plates, components, and manufacturing methods thereof that have excellent ductility, hole expansion properties, and chemical conversion treatability, and have a tensile strength of 780 MPa or more.

Here, tensile strength refers to the tensile strength (TS) obtained in accordance with JIS Z2241 (2011).

In addition, excellent ductility means that the total elongation EL obtained in accordance with JIS Z2241 (2011) satisfies any one of the following (A) to (C).
(A) TS: 780 MPa or more and less than 980 MPa, EL: 16.0% or more,
(B) TS: 980 MPa or more and less than 1180 MPa, EL: 14.0% or more,
(C) TS: 1180 MPa or more, EL: 12.0% or more

In addition, excellent hole expandability means that the hole expanding ratio λ (%) (= {(dd−d0)/ _d0 } × ₁₀₀ ) obtained by a hole expanding test in accordance with the JFST1001 regulations is 45% or more at any TS level in order to ensure the hole expandability required for practical use.

Excellent chemical conversion treatability means that after degreasing (treatment temperature: 40°C, treatment time: 120 seconds, spray degreasing, degreaser: FC-E2011 manufactured by Nippon Parkerizing Co., Ltd.), surface conditioning (pH 9.5, treatment temperature: room temperature, treatment time: 20 seconds, surface conditioner: PL-X manufactured by Nippon Parkerizing Co., Ltd.), and then chemical conversion treatment using a zinc phosphate chemical conversion treatment solution (chemical conversion treatment solution temperature: 35°C, treatment time: 120 seconds, chemical conversion treatment solution: Palbond PB-L3065 manufactured by Nippon Parkerizing Co., Ltd.), the area where the base steel is exposed is less than 10% of the total area.

In order to solve the above problems, the present inventors have conducted extensive research into the steel components, heat treatment conditions, and microstructures that affect the ductility and chemical conversion treatability of various thin steel sheets having a tensile strength of 780 MPa or more. As a result, the following components, in mass %, were found to be C: 0.05-0.25%, Si: 0.30-1.50%, Mn: 1.5-4.5%, P: 0.005-0.050%, S: 0.01% or less, sol. The composition contains Al: less than 1.0% and N: less than 0.015%, satisfies the following formula (1), with the balance consisting of iron and unavoidable impurities, the area ratio of polygonal ferrite is 10% to 70%, the total area ratio of upper bainite, tempered martensite, and lower bainite is 20% to 80%, the volume ratio of retained austenite (residual γ) is 5% to 20%, the area ratio of quenched martensite is 13% or less (including 0%), and the steel structure consists of the remaining structure. When the emission intensity of P measured by glow discharge analysis from the steel sheet surface in the sheet thickness direction is analyzed, the maximum concentration of P within 1 μm in the sheet thickness direction from the steel sheet surface is 0.025 mass% or more, and the following formula (2) is satisfied. By making the steel structure locally concentrated with P, a high-strength cold-rolled steel sheet having excellent ductility, hole expandability, and chemical conversion treatability can be obtained.
[Si]/[Mn]≦0.35...Formula (1)
[Pm]/[P]≧1.5...Formula (2)
In the formula (1), [Si] is the Si content (% by mass), [Mn] is the Mn content (% by mass),
In formula (2), [P] is the P content (mass%).

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
[1] In mass%,
C: 0.05-0.25%,
Si: 0.30-1.50%,
Mn: 1.5-4.5%,
P: 0.005-0.050%,
S: 0.01% or less,
sol. Al: less than 1.0%;
N: less than 0.015%;
The following formula (1) is satisfied:
The balance being iron and unavoidable impurities;
Area ratio of polygonal ferrite: 10% or more and 70% or less,
Total area ratio of upper bainite, tempered martensite, and lower bainite: 20% or more and 80% or less,
Volume fraction of retained austenite: 5% or more and 20% or less,
A steel structure having an area ratio of quenched martensite of 13% or less (including 0%);
having
A steel sheet having a maximum P concentration [Pm] of 0.025 mass% or more within 1 μm from the steel sheet surface in the sheet thickness direction, and satisfying the following formula (2):
[Si]/[Mn]≦0.35...Formula (1)
[Pm]/[P]≧1.5...Formula (2)
In the formula (1), [Si] is the Si content (% by mass), [Mn] is the Mn content (% by mass),
In formula (2), [P] is the P content (mass%).
[2] The component composition further includes, in mass%,
Ti: 0.1% or less,
B: 0.001% or less,
Cu: 1% or less,
Ni: 1% or less,
Cr: 1% or less,
Mo: 0.5% or less,
V: 0.5% or less,
Nb: 0.1% or less,
Mg: 0.0050% or less,
Ca: 0.0050% or less,
Sn: 0.1% or less,
Sb: 0.1% or less,
REM: 0.0050% or less,
The steel sheet according to the above [1], containing one or more selected from the following:
[3] A member made using the steel plate according to [1] or [2] above.
[4] A method for producing a steel sheet, comprising the steps of: subjecting a steel slab having the composition according to [1] or [2] to hot rolling, pickling and cold rolling; and then annealing the resulting cold-rolled steel sheet;
The annealing is
A soaking temperature holding step of heating the cold-rolled steel sheet to a soaking temperature of A _c1 point +20 ° C. or more and A _c3 point or less and Tc or more calculated by formula (3) in a furnace atmosphere having a dew point of −40 ° C. or less, and holding the soaking temperature for 30 to 500 s;
A first cooling step of cooling the temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550 ° C. at a first average cooling rate of 2 to 50 ° C./s to the first cooling stop temperature;
After stopping the cooling at the first cooling stop temperature, the material is allowed to stay in a temperature range of 350 to 550 ° C. for 10 to 60 s,
A second cooling step of cooling to a second cooling stop temperature of 100 to 300 ° C. at a second average cooling rate of 2 to 50 ° C./s;
A reheating and holding step of heating from the second cooling stop temperature to a reheating temperature of 50° C. to 450° C. at an average heating rate of 2.0° C./s or more from the second cooling stop temperature and holding the temperature for 60 s to 3000 s;
A method for manufacturing a steel sheet, comprising:
Tc (℃) = 663-1.2 x exp (20/t) x Tdp (3)
Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.).
[5] A method for producing a steel sheet, comprising: subjecting a steel slab having the composition according to [1] or [2] to hot rolling, pickling and cold rolling; and then annealing the resulting cold-rolled steel sheet;
The annealing is
A soaking temperature holding step of heating the cold-rolled steel sheet to a soaking temperature of A _c1 point +20 ° C. or more and A _c3 point or less and Tc or more calculated by formula (3) in a furnace atmosphere having a dew point of −40 ° C. or less, and holding the soaking temperature for 30 to 500 s;
A cooling step of cooling from the soaking temperature to a cooling stop temperature of 100 to 300 ° C. at an average cooling rate of 2 to 50 ° C./s;
A reheating and holding step of heating from the cooling stop temperature to a reheating temperature of 50°C to 450°C at an average heating rate of 2.0°C/s or more from the cooling stop temperature and holding the temperature for 60 s to 3000 s;
A method for manufacturing a steel sheet, comprising:
Tc (℃) = 663-1.2 x exp (20/t) x Tdp (3)
Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.).
[6] A method for manufacturing a component, comprising the step of subjecting the steel plate according to [1] or [2] to at least one of forming and joining to form a component.

According to the present invention, it is possible to obtain a steel plate and a member which have a high strength, tensile strength TS of 780 MPa or more, and which also have excellent ductility, hole expandability and chemical conversion treatability.
When the steel sheet of the present invention is applied to the frame members of an automobile body, it can be used to manufacture difficult-to-form parts of complex shapes by cold pressing, which can greatly contribute to reducing the weight of the automobile body. It is possible to reduce material costs because there is no need to use expensive alloying elements or to improve chemical conversion treatability through post-treatment after annealing.

FIG. 1 is a graph for explaining the maximum P concentration [Pm] of the present invention.

The present invention will be described in detail below. Note that the present invention is not limited to the following embodiments.

(Steel plate)
The steel plate of the present invention contains, by mass, C: 0.05 to 0.25%, Si: 0.30 to 1.50%, Mn: 1.5 to 4.5%, P: 0.005 to 0.050%, S: 0.01% or less, sol. Al: less than 1.0%, and N: less than 0.015%, and satisfies the following formula (1), with the balance being iron and unavoidable impurities; and has a component composition in which the area ratio of polygonal ferrite is 10% or more and 70% or less, and a total area ratio of upper bainite, tempered martensite, and lower bainite is 20% or more and 80% or less. and a steel structure in which the volume fraction of retained austenite is 5% or more and 20% or less, and the area fraction of quenched martensite is 13% or less (including 0%). When the phosphorus emission intensity is analyzed from the surface in the sheet thickness direction by glow discharge analysis, the maximum concentration of phosphorus [Pm] within 1 μm in the sheet thickness direction from the steel sheet surface is 0.025 mass% or more, and the following formula (2) is satisfied. The steel sheet has a high strength of tensile strength TS of 780 MPa or more, and is excellent in ductility, hole expandability and chemical conversion treatability.
[Si]/[Mn]≦0.35...Formula (1)
[Pm]/[P]≧1.5...Formula (2)
In the formula (1), [Si] is the Si content (% by mass), [Mn] is the Mn content (% by mass),
In formula (2), [P] is the P content (mass%).

The steel sheet of the present invention will be explained below in the order of chemical composition and steel structure. First, the reasons for limiting the chemical composition of the present invention will be explained. In the following explanation, all percentages indicating the steel composition are mass percentages unless otherwise specified.

<C: 0.05-0.25%>
C is contained from the viewpoint of securing a predetermined strength by transformation strengthening and improving ductility by securing a predetermined amount of retained austenite (hereinafter also referred to as retained γ). If it is less than this, these effects cannot be sufficiently ensured.
On the other hand, the upper limit of the C content is set at 0.25% due to concerns about hole expandability, which is important in press formability, and weldability, which is important in spot welding or laser welding when assembling the material into a car body after forming into an automobile component. Let us assume that.
For this reason, the C content is set to 0.05 to 0.25%. The C content is preferably 0.08% or more, and more preferably 0.10% or more. , preferably 0.22% or less, and more preferably 0.20% or less.

<Si: 0.30-1.50%>
Si is contained from the viewpoint of strengthening ferrite to increase strength, and from the viewpoint of suppressing the formation of carbides in martensite and bainite, securing a predetermined amount of retained γ, and improving ductility. If it is less than 30%, these effects cannot be sufficiently ensured.
On the other hand, if the Si content exceeds 1.50%, good chemical conversion treatability cannot be ensured even with the production method specified in the present invention.
For this reason, the Si content is set to 0.30 to 1.50%. The Si content is preferably 0.35% or more, and more preferably 0.40% or more. is 1.20% or less, more preferably 1.00% or less.

<Mn: 1.5 to 4.5%>
Mn improves the hardenability of steel sheets and promotes high strength through transformation strengthening, and like Si, it suppresses the formation of carbides in bainite and promotes the formation of retained austenite that contributes to ductility, thereby improving ductility. In order to obtain these effects, the Mn content must be 1.5% or more.
On the other hand, if the Mn content exceeds 4.5%, the bainite transformation is significantly delayed, the required amount of retained austenite cannot be secured, and the ductility is reduced. Lowering the martensitic transformation start temperature makes it difficult to suppress the formation of coarse quenched martensite, and the stretch flangeability (hole expandability) deteriorates.
For this reason, the Mn content is set to 1.5% or more and 4.5% or less. The Mn content is preferably 1.8% or more, and more preferably 2.0% or more. is preferably 3.5% or less, and more preferably 3.0% or less.

<P: 0.005-0.050%>
P is an element that strengthens steel. By appropriately controlling the P content, a P-enriched surface portion is formed on the surface of the steel sheet after annealing, thereby improving the chemical conversion treatability. From this viewpoint, the P content is set to 0.005% or more.
On the other hand, a high P content deteriorates spot weldability, and from this viewpoint, the P content is set to 0.050% or less.
Therefore, the P content is set to 0.005 to 0.050%. The P content is preferably 0.007% or more, and more preferably 0.009% or more. The content is preferably 0.040% or less, and more preferably 0.030% or less.

<S: 0.01% or less>
S has the effect of improving the scale peeling property during hot rolling and the effect of suppressing nitriding during annealing, but it is an element that has a negative effect on spot weldability, bendability, and hole expandability. In order to reduce the adverse effects, the S content is at least 0.01% or less, and preferably 0.0050% or less.
Although it is not necessary to include S, it is very costly to reduce the S content to less than 0.0001%, so it is preferable to set the S content to 0.0001% or more from the viewpoint of manufacturing costs. The amount is more preferably 0.0005% or more, and further preferably 0.0010% or more.

<Sol. Al: Less than 1.0%>
Al is contained for the purpose of deoxidation or obtaining residual γ. Although there is no particular lower limit for sol. Al, in order to perform stable deoxidation, the sol. Al content is preferably 0.005% or more.
On the other hand, when the sol. Al content is 1.0% or more, the amount of coarse Al-based inclusions increases, and the stretch flange formability (hole expandability) decreases. In addition, Al is an element that deteriorates the chemical conversion treatability of the steel sheet, and when the sol. Al content is 1.0% or more, good chemical conversion treatability cannot be ensured even in the present invention. For this reason, the sol. Al content is less than 1.0%. The sol. Al content is preferably 0.80% or less, more preferably 0.06% or less.

<N: less than 0.015%>
N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces stretch flange formability (hole expandability), so its content must be limited. Therefore, the N content is less than 0.015%. The N content is preferably 0.010% or less, and more preferably 0.006% or less.
Although N may not be contained, it is very costly to reduce the N content to less than 0.0001%. Therefore, the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The N content is more preferably 0.0005% or more, and further preferably 0.001% or more.

<[Si]/[Mn]≦0.35...Formula (1)>
In formula (1), [Si] is the Si content (% by mass), and [Mn] is the Mn content (% by mass).
[Si]/[Mn] (Si/Mn ratio) determines the component ratio of Si and Mn in the surface oxide formed during annealing. If [Si]/[Mn] exceeds 0.35, good chemical conversion treatability cannot be ensured. Therefore, [Si]/[Mn] is set to 0.35 or less. [Si]/[Mn] is preferably 0. The ratio [Si]/[Mn] is preferably 0.10 or more, more preferably 0.15 or more, although there is no particular lower limit. be.

The composition of the steel plate in the present invention contains the above-mentioned elemental elements as the basic components, with the remainder being iron (Fe) and unavoidable impurities. It is preferable that the composition of the steel plate in the present invention has a composition in which the remainder is Fe and unavoidable impurities.

The composition of the steel sheet of the present invention may contain, in addition to the above-mentioned components, one or more elements selected from the following as optional elements (optional elements).
Ti: 0.1% or less, B: 0.001% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less.

<Ti: 0.1% or less>
Ti has the effect of fixing N in steel as TiN, improving hot ductility, and of improving the hardenability of B. It also has the effect of refining the structure by precipitating TiC. In order to obtain this effect, the Ti content is preferably 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. The content is preferably 0.010% or more.
On the other hand, if the Ti content exceeds 0.1%, it leads to an increase in the rolling load and a decrease in ductility due to an increase in the amount of precipitation strengthening, so when Ti is contained, the Ti content is set to 0.1% or less. The Ti content is preferably 0.05% or less, and more preferably 0.03% or less.

<B: 0.001% or less>
B is an element that improves the hardenability of steel and has the advantage of easily forming tempered martensite and/or bainite with a specified area ratio. Therefore, the B content is preferably 0.0005% or more. .
On the other hand, if the B content exceeds 0.001%, the oxides are concentrated during annealing, which promotes the coarsening of the oxides and deteriorates the chemical conversion treatability. The content shall be 0.001% or less.

<Cu: 1% or less>
Cu improves corrosion resistance in the environment in which the automobile is used. In addition, the corrosion product of Cu covers the surface of the steel sheet, and has the effect of suppressing hydrogen penetration into the steel sheet. Cu is an element that is mixed in when scrap is used as a raw material, and by allowing the inclusion of Cu, recycled materials can be used as raw materials, and manufacturing costs can be reduced. From this viewpoint, it is preferable to contain Cu at 0.005% or more, and furthermore, from the viewpoint of improving delayed fracture resistance, it is more preferable to contain Cu at 0.05% or more. The Cu content is more preferably 0.10% or more. More preferably, the Cu content is 0.25% or more, and even more preferably, 0.50% or more.
However, if the Cu content is too high, it will cause surface defects, so if Cu is contained, the Cu content is set to 1% or less.

<Ni: 1% or less>
Ni, like Cu, is an element that improves corrosion resistance. Ni also has the effect of suppressing the occurrence of surface defects that tend to occur when Cu is contained. For this reason, it is desirable to contain Ni at 0.01% or more. The Ni content is more preferably 0.04% or more, and even more preferably 0.06% or more.
However, if the Ni content is too high, the scale generation in the heating furnace becomes non-uniform, which may cause surface defects. It also leads to increased costs. For this reason, when Ni is contained, the Ni content is set to 1% or less. Preferably, the Ni content is 0.5% or less, and more preferably, 0.3% or less.

<Cr: 1% or less>
Cr can be added to improve the hardenability of the steel and to suppress the formation of carbides in martensite and upper/lower bainite. To obtain such effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more, and further preferably 0.06% or more.
However, an excessive Cr content deteriorates the pitting corrosion resistance, so when Cr is contained, the Cr content is set to 1% or less, preferably 0.3% or less, and more preferably 0.1% or less.

<Mo: 0.5% or less>
Mo can be added because of its effect of improving the hardenability of steel and its effect of suppressing the formation of carbides in martensite and upper/lower bainite. To obtain such effects, the Mo content should be 0.01 The Mo content is preferably 0.03% or more, and more preferably 0.06% or more. The Mo content is more preferably 0.1% or more. and even more preferably, equal to or greater than 0.2%.
However, Mo significantly deteriorates the chemical conversion treatability of the cold-rolled steel sheet, so if Mo is contained, the Mo content is set to 0.5% or less.

<V: 0.5% or less>
V is added because of its effects of improving the hardenability of steel, suppressing the formation of carbides in martensite and upper/lower bainite, refining the structure, and precipitating carbides to improve delayed fracture resistance. In order to obtain these effects, the V content is preferably 0.003% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. % or more. Even more preferably, the V content is 0.020% or more, and even more preferably, 0.050% or more.
However, if a large amount of V is contained, castability is significantly deteriorated, so if V is contained, the V content is set to 0.5% or less. Preferably, the V content is set to 0.3% or less. The V content is preferably 0.2% or less, more preferably 0.1% or less.

<Nb: 0.1% or less>
Nb can be added because of its effects of refining the steel structure to increase strength, promoting bainite transformation through grain refinement, improving bendability, and enhancing delayed fracture resistance. In order to obtain this, the Nb content is preferably 0.010% or more. The Nb content is preferably 0.015% or more, and more preferably 0.020% or more.
However, if a large amount of Nb is contained, precipitation strengthening becomes too strong, and ductility decreases. In addition, it causes an increase in rolling load and deterioration of castability. For this reason, when Nb is contained, the Nb content is set to 0.1%. Preferably, the Nb content is 0.08% or less, and more preferably 0.05% or less.

<Mg: 0.0050% or less>
Mg fixes O as MgO and contributes to improving formability such as bendability. Therefore, the Mg content is preferably 0.0002% or more. The Mg content is preferably 0.0010 % or more, and more preferably 0.0015% or more.
On the other hand, if a large amount of Mg is added, the surface quality and bendability are deteriorated, so when Mg is contained, the Mg content is set to 0.0050% or less. Preferably, the Mg content is set to 0.0040% or less. .

<Ca: 0.0050% or less>
Ca fixes S as CaS and contributes to improving bendability and delayed fracture resistance. Therefore, the Ca content is preferably 0.0002% or more. The Ca content is more preferably is 0.0005% or more, and more preferably 0.0010% or more.
On the other hand, when a large amount of Ca is added, it deteriorates the surface quality and bendability, so when Ca is contained, the Ca content is set to 0.0050% or less. Preferably, the Ca content is 0.0040% or less. .

<Sn: 0.1% or less>
Sn suppresses oxidation and nitridation in the surface layer of the steel sheet, and suppresses the resulting reduction in the content of C and B in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing the strength, and From this viewpoint, the Sn content is preferably 0.003% or more, more preferably 0.010% or more, and further preferably 0.015% or more. The Sn content is preferably 0.020% or more, and more preferably 0.030% or more.
On the other hand, if the Sn content exceeds 0.1%, castability is deteriorated. In addition, Sn segregates at the prior γ grain boundaries, and the delayed fracture resistance is deteriorated. Therefore, when Sn is contained, Sn The content shall be 0.1% or less.

<Sb: 0.1% or less>
Sb suppresses oxidation and nitridation in the surface layer of the steel sheet, and suppresses the resulting reduction in the content of C and B in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing the strength, and From this viewpoint, the Sb content is preferably 0.002% or more, more preferably 0.004% or more, and further preferably 0.006% or more. % or more. More preferably, the Sb content is 0.008% or more, and even more preferably, 0.010% or more. The Sb content is preferably 0.015% or more, More preferably, it is 0.030% or more.
On the other hand, if the Sb content exceeds 0.1%, castability is deteriorated, and the Sb segregates at the prior γ grain boundaries, deteriorating the delayed fracture resistance. shall be 0.1% or less.

<REM: 0.0050% or less>
REM is an element that suppresses the adverse effect of sulfides on stretch flangeability by making the shape of sulfides spheroidal, thereby improving stretch flangeability. The REM content is preferably 0.0005% or more. The REM content is more preferably 0.0010% or more, and further preferably 0.0020% or more.
On the other hand, if the REM content exceeds 0.0050%, the effect of improving the stretch flangeability is saturated, so when REM is contained, the REM content is set to 0.0050% or less.
In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs.

When the optional components are contained below the lower limit, the optional elements contained below the lower limit do not impair the effects of the present invention. Therefore, when the optional elements are contained below the lower limit, the optional elements are considered to be contained as unavoidable impurities.

Next, we will explain the mechanical properties of the steel sheet that is the subject of this invention (cold-rolled steel sheet with excellent material stability).

The steel plate of the present invention has a tensile strength (TS) of 780 MPa or more. There is no particular upper limit to the tensile strength, but from the viewpoint of compatibility with other properties, it is preferable that the tensile strength is 1300 MPa or less.

In the steel sheet of the present invention, excellent ductility is ensured by ensuring a total elongation EL of 16.0% or more when TS is 780 MPa or more and less than 980 MPa, EL of 14.0% or more when TS is 980 MPa or more and less than 1180 MPa, and EL of 12.0% or more when TS is 1180 MPa or more. In addition, hole expansion is ensured by ensuring a hole expansion ratio λ of 45% or more. This significantly improves the stability of press forming.

The tensile properties are evaluated by taking a JIS No. 5 tensile test piece from the center position of the sheet width, and performing a tensile test (in accordance with JIS Z2241 (2011)) with N=3. Each evaluation is based on the average value of three points. A high-strength steel sheet is a steel sheet having a tensile strength of 780 MPa or more. A steel sheet having a total elongation EL of 16.0% or more when TS is 780 MPa or more and less than 980 MPa, 14.0% or more when TS is 980 MPa or more and less than 1180 MPa, and 12.0% or more when TS is 1180 MPa or more is considered to have excellent ductility. In addition, the essential condition of the present invention is that the hole expansion ratio λ (%) (= {(d- _d0 )/ _d0 } x 100) obtained by a hole expansion test in accordance with the provisions of JFST1001 is 45% or more.

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

<Area ratio of polygonal ferrite: 10% or more and 70% or less>
From the viewpoint of ensuring high ductility, the area ratio of polygonal ferrite is set to 10% or more, and in order to obtain even higher ductility, it is preferably set to 20% or more.
On the other hand, if the polygonal ferrite exceeds 70%, the desired strength may not be obtained. Therefore, the area ratio of polygonal ferrite is set to 70% or less, preferably 65% or less, and more preferably 60%.

<Total area ratio of upper bainite, tempered martensite and lower bainite: 20% or more and 80% or less>
In order to obtain a desired strength, the total area ratio of upper bainite, tempered martensite and lower bainite is set to 20% or more, and in order to obtain even higher strength, it is preferably set to 25% or more.
On the other hand, if the total area ratio of upper bainite, tempered martensite and lower bainite exceeds 80%, ductility decreases due to excessively high strength, so the area ratio is set to 80% or less, more preferably 75% or less, and further preferably 70% or less.

<Volume fraction of retained austenite (retained γ): 5% or more and 20% or less>
If the volume fraction of the retained austenite is less than 5%, the desired ductility may not be ensured. Also, if the volume fraction of the retained austenite is less than 5%, the desired strength may not be ensured. Also, if the volume fraction of the retained austenite is less than 5%, the desired hole expandability may not be ensured.
From the viewpoint of ductility, the volume fraction of retained austenite is set to 5% or more, and preferably 7% or more.
On the other hand, if the residual austenite exceeds 20%, the stretch flangeability (hole expandability) decreases, so the residual austenite is set to 20% or less, preferably 15% or less, and more preferably 13% or less.

<Area ratio of quenched martensite: 13% or less (including 0%)>
Since the hard quenched martensite structure reduces λ, its area ratio needs to be suppressed. In order to obtain the λ required for practical use, the area ratio of the quenched martensite is set to 13% or less. In order to obtain λ more stably, the area ratio of the quenched martensite is preferably 11% or less, more preferably 9% or less. The area ratio of the quenched martensite may be 0% or 5% or more.

<Remainder structure>
The steel structure is made up of the remaining structure other than the above. The area ratio of the remaining structure is preferably 5% or less. The remaining structure may be unrecrystallized ferrite, carbide, or pearlite. These structures may be determined by SEM observation as described later.

<The maximum concentration of P within 1 μm from the steel sheet surface in the sheet thickness direction [Pm] is 0.025 mass% or more and satisfies formula (2)>
[Pm]/[P]≧1.5...Formula (2)
In formula (2), [P] (which can also be written as [Pi]) is the P content (mass%).
As a result of thorough investigation of various elements that affect chemical conversion treatability, their surface concentration, and the type of oxide formed during annealing, it has become clear that chemical conversion treatability cannot be sufficiently ensured even under manufacturing conditions in which no oxide formation is observed. As a result of quantitatively evaluating the maximum P concentration near the surface of steel sheets with ensured chemical conversion treatability by the method described below, it has been found that, when the emission intensity of P measured by GDS (glow discharge spectroscopy) from the surface in the sheet thickness direction is analyzed, good chemical conversion treatability can be ensured by making the maximum P concentration [Pm] within 1 μm in the sheet thickness direction from the steel sheet surface 0.025 mass% or more and making the steel structure satisfy the formula (2).
Although the detailed mechanism is unknown, it is important that the maximum concentration of P in the surface layer is locally high relative to the steel components. Furthermore, since the shape of the chemical crystals after chemical conversion treatment is scaly if this maximum concentration of P is insufficient, it is believed that the local surface concentration of P has the effect of suppressing the formation of surface Si-based oxides and Si-Mn-based oxides that have an adverse effect on chemical conversion treatability.
[Pm] is preferably 0.030 mass% or more, more preferably 0.035 mass% or more. Although there is no particular upper limit, [Pm] is preferably 0.100 mass% or less, more preferably 0.090 mass% or less.
[Pm]/[Pi] is preferably 1.7 or more, more preferably 1.9 or more. Although there is no particular upper limit, [Pm]/[Pi] is preferably 10.0 or less, more preferably 9.0 or less.

Next, a method for measuring the steel structure will be described.
The area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite) were measured by cutting out a cross section of the sheet thickness parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. At the 1/4 thickness position, an area of 25 μm × 20 μm was observed in 10 fields of view at 5,000 times magnification using an SEM, and the photographed structure was quantified by image analysis.
Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. It is the area that appears the blackest under SEM.
Upper bainite is a ferrite structure with the formation of carbides or retained austenite inside that appear white under SEM. When it is difficult to distinguish between upper bainite and polygonal ferrite, the region of ferrite with an aspect ratio of ≦2.0 is classified as polygonal ferrite, and the region with an aspect ratio of >2.0 is classified as upper bainite, and the area ratio is calculated. Here, the aspect ratio is calculated by determining the major axis length a at which the particle length is the longest, and the minor axis length b at the particle length when it crosses the particle the longest in the direction perpendicular to the major axis length a, and a/b is the aspect ratio.
Tempered martensite and lower bainite are regions that are accompanied by a lath-shaped substructure and carbide precipitation inside when viewed under an SEM.
Hardened martensite (fresh martensite) is a blocky region that appears white under an SEM with no internal substructure visible.
The remaining structure is a structure containing at least one of non-recrystallized ferrite, carbide, and pearlite, and can be confirmed by SEM as black contrast ferrite containing deformed structure introduced by rolling, carbide, and pearlite, respectively. Carbide is a structure with a particle size of 1 μm or less, and pearlite is a lamellar (layer) structure, so it can be distinguished.

The volume fraction of retained austenite is determined by chemically polishing the surface layer at 1/4 thickness and then performing X-ray diffraction. A Co-Kα source is used for the incident X-rays, and the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Here, since the retained austenite is randomly distributed, the volume fraction of retained austenite determined by X-ray diffraction can be taken as the area fraction of retained austenite.

The amount of surface enrichment of the P surface enriched part on the steel sheet surface is measured by performing sputtering analysis in the depth direction (sheet thickness direction) using a GDS (manufactured by Shimadzu Corporation) under conditions of Ar gas pressure: 600 Pa, high frequency output: 35 W, measurement time interval: 0.1 s, and measurement time: 150 s, and the amount of surface enrichment of P is measured, and the maximum P concentration [Pm] within 1 μm in the sheet thickness direction from the steel sheet surface is obtained using a calibration curve obtained in advance. Note that under these measurement conditions, the measurement position d (μm) from the surface is obtained by the formula d = ts/1.7 (μm) using the sputtering time ts.
In the present invention, as shown in FIG. 1, the highest P intensity value during the measurement time of 150 seconds is converted into mass % using a calibration curve and is defined as the maximum concentration ([Pm]).
The method of converting to mass% involves using a standard material having a known amount of P, measuring the data under the same conditions, determining the correlation between the intensity of the P element obtained by GDS and the amount of P, and converting the measured P intensity of the example into a concentration.
In FIG. 1, Pi is the P content (mass%) in the steel sheet.

(Method of manufacturing steel sheet)
Next, a method for producing a steel sheet according to the present invention will be described.
First Embodiment
The manufacturing method of the steel sheet according to the first embodiment of the present invention is a manufacturing method of a steel sheet in which a steel slab having the above-mentioned composition is subjected to hot rolling, pickling and cold rolling, and then the obtained cold-rolled steel sheet is annealed. The annealing is performed in a furnace atmosphere having a dew point of −40° C. or less, at a temperature of A _c1 point +20° C. or more. _a first cooling step of cooling to a first cooling stop temperature at a first average cooling rate of 2 to 50°C/s in a temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550°C; a second cooling step of stopping the cooling at the first cooling stop temperature, retaining the steel in a temperature range of 350 to 550°C for 10 to 60 s, and then cooling to a second cooling stop temperature of 100 to 300°C at a second average cooling rate of 2 to 50°C/s; and a reheating step of heating from the second cooling stop temperature to a reheating temperature of the second cooling stop temperature + 50°C or more and 450°C or less at an average heating rate of 2.0°C/s or more, and retaining the steel for 60 to 3000 s.
Tc (℃) = 663-1.2 x exp (20/t) x Tdp...Formula (3)
Here, t represents the holding time at the above-mentioned soaking temperature (soaking holding time) (s), and Tdp represents the dew point (° C.).

<Hot rolling>
The method of hot rolling a steel slab includes a method of rolling the slab after heating, a method of directly rolling the slab after continuous casting without heating it, and a method of rolling the slab after continuous casting by applying a short-term heat treatment. Hot rolling may be performed according to a conventional method, for example, the slab heating temperature may be 1100°C or higher. The slab heating temperature may be 1300°C or lower. The soaking temperature may be 20 min or higher. The soaking temperature may be 300 min or lower. The finish rolling temperature may be A _r3 transformation point or higher. The finish rolling temperature may be A _r3 transformation point + 200°C or lower. The coiling temperature may be 400°C or higher. The coiling temperature may be 720°C or lower. It is preferable to control the coiling temperature from the viewpoint of suppressing thickness fluctuation and stably ensuring high strength. Specifically, the coiling temperature is preferably 430°C or higher. The winding temperature is preferably 530° C. or less.
The _Ar3 transformation point can be calculated from the components of the steel sheet and the following empirical formula (A).
A _r3 point (°C) = 910-310×[C]-80×[Mn]-20×[Cu]-15×[Cr]-55×[Ni]-80×[Mo]...Formula (A)
(In the above formula, [M] is the content (mass%) of element M in the steel slab, and the value of an element that is not contained is zero (0).)

<Pickling>
The pickling may be carried out in a conventional manner.

<Cold rolling>
Cold rolling may be performed according to a conventional method, and the rolling ratio (cumulative rolling ratio) may be 30% or more. The rolling ratio (cumulative rolling ratio) may be 85% or less. The rolling ratio is preferably controlled from the viewpoint of stably securing high strength and reducing anisotropy. Specifically, the rolling ratio is preferably 35% or more. When the rolling load is high, softening annealing treatment can be performed at 450 to 730°C in a CAL (continuous annealing line) or BAF (box annealing furnace).

<Annealing>
A cold-rolled steel sheet manufactured according to a conventional method is annealed under the following conditions. The annealing equipment is not particularly limited, but it is preferable to perform the annealing in a continuous annealing line (CAL) from the viewpoints of productivity and ensuring the desired heating rate and cooling rate.

[Soaking process: in a furnace atmosphere with a dew point of -40°C or less, heat to a soaking temperature of A _c1 point +20°C or more and A _c3 point or less and Tc or more, and hold at the soaking temperature for 30 to 500 s]
The dew point affects the formation of oxides on the steel sheet surface during annealing, and if the dew point exceeds -40°C, the amount of oxides formed on the steel sheet surface increases excessively, deteriorating the chemical conversion treatability. For this reason, the dew point is set to -40°C or lower.
Although there is no particular lower limit, the dew point is preferably −70° C. or higher, and more preferably −60° C. or higher.

The steel sheet obtained in the present invention contains a soft ferrite structure, which improves ductility, and therefore the soaking temperature is set to be in the range of A _c1 point +20° C. to A _c3 point, at which ferrite is formed.

Furthermore, by setting the soaking temperature to Tc (°C) or higher, the amount of P concentrated at the surface-concentrated portion of the P formed on the steel sheet surface can be ensured to be within the range stipulated in the present invention. Tc is calculated from the dew point and the soaking holding time in formula (3): Tc (°C) = 663 - 1.2 x exp (20/t) x Tdp ... formula (3)
Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.).
If the soaking temperature is lower than Tc, the desired amount of P concentration on the surface cannot be ensured, and the chemical conversion treatability deteriorates. Therefore, in a furnace atmosphere with a dew point of -40°C or lower, the soaking temperature is set to be equal to or higher than A _c1 point +20°C and equal to or lower than A _c3 point, and equal to or higher than Tc (°C).

Furthermore, if the time for holding at the soaking temperature (soaking holding time) is less than 30 seconds, formation of austenite at the soaking temperature may not be sufficient, resulting in an increase in polygonal ferrite, and the desired total area ratio of upper bainite, tempered martensite, and lower bainite may not be obtained, making it difficult to obtain the desired strength. Also, there may be cases where a sufficient amount of retained austenite is not obtained, making it difficult to ensure the desired ductility.
On the other hand, if the time for holding at the above-mentioned soaking temperature (soaking holding time) exceeds 500 seconds, the structure will become significantly coarse, making it impossible to ensure the desired strength.
Therefore, the time for which the steel sheet is held at the annealing temperature (soaking time) is set to 30 to 500 seconds.
The time for which the material is held at the soaking temperature (soaking time) is preferably 60 seconds or more, more preferably 100 seconds or more. The time for which the material is held at the soaking temperature (soaking time) is preferably 400 seconds or less, more preferably 300 seconds or less.

Incidentally, for A _c1 and A _c3 , A _c1 and A _c3 obtained from the empirical formulas (4) and (5) below may be used.
A _c1 =723+22×[C]-18×[Si]+17×[Cr]+4.5×[Mo]+16×[V] ...Formula (4)
A _c3 =910-203×([C]) ^1/2 +44.7×[Si]-30×[Mn]+700×[P]+400×[sol. Al]-20×[Cu]+31.5×[Mo]+104×[V]+400×[Ti]...Formula (5)
Here, [M] is the mass % of each element.

[First cooling step: cooling to the first cooling stop temperature at a first average cooling rate of 2 to 50 ° C./s in the temperature range from the soaking temperature to the first cooling stop temperature of 350 to 550 ° C.]
After being held at a soaking temperature that is equal to or higher than A _c1 point + 20° C. and equal to or lower than A _c3 point and is equal to or higher than Tc (after the soaking temperature holding step), the material is cooled at a first average cooling rate of 2 to 50° C./s in a temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550° C.
If the first average cooling rate is less than 2° C./s, the ferrite transformation during cooling proceeds excessively and the desired amount of polygonal ferrite cannot be obtained, so the first average cooling rate is set to 2° C./s or more, and preferably 5° C./s or more.
On the other hand, if the first average cooling rate is too high, the sheet shape deteriorates, so the first average cooling rate is set to 50° C./s or less. The first average cooling rate is preferably 40° C./s or less, and more preferably less than 30° C./s.
Here, the first average cooling rate is "(soaking temperature (°C)-first cooling stop temperature (°C))/cooling time (seconds) from the soaking temperature to the first cooling stop temperature."

[Second cooling step (1): After stopping cooling at the first cooling stop temperature, the material is held at a holding temperature of 350 to 550° C. for 10 s to 60 s]
By forming upper bainite at or below the above-mentioned first cooling stop temperature and in the temperature range of 350°C to 550°C (residence temperature), a large amount of retained γ can be obtained, thereby improving ductility, compared to a manufacturing method that does not involve retention at that temperature for 10 s to 60 s. In the disclosure of the present invention, whether or not to carry out the second cooling step (1): retention at a retention temperature of 350 to 550°C for 10 s to 60 s may be determined in consideration of the desired properties.
Bainite transformation has an incubation period, and in order to obtain the desired amount of bainite, the material must be held at that temperature for a certain period of time. If the holding temperature range, including the holding start temperature (= first cooling stop temperature) and holding end temperature, is outside the range of 350 to 550°C and/or the holding time (hereinafter also referred to as holding time) is less than 10 seconds, the desired amount of bainite is not obtained, the formation of retained austenite is suppressed, and the desired ductility may not be obtained.
On the other hand, if the holding time exceeds 60 seconds, the concentration of C from bainite to the blocky untransformed γ progresses, leading to an increase in the amount of remaining blocky structure, and there is a concern of a decrease in λ. Therefore, the holding time is set to 10 seconds or more and 60 seconds or less. This holding time is preferably 20 seconds or more. Moreover, this holding time is preferably 50 seconds or less.

[Second cooling step (2): Cooling to a second cooling stop temperature of 100 to 300 ° C. at a second average cooling rate of 2 to 50 ° C./s]
After the above-mentioned retention, it is necessary to cool quickly so that the bainite transformation does not proceed excessively. If the average cooling rate (second average cooling rate) in the temperature range from the retention end temperature to the second cooling stop temperature of 100°C or more and 300°C or less is less than 2°C/s, the bainite transformation will proceed excessively, and the amount of retained austenite will increase excessively, and the desired amount of quenched martensite will not be secured, which may lead to a decrease in strength. If the second average cooling rate is less than 2°C/s, the desired ductility and hole expandability may not be obtained.
Therefore, the second average cooling rate in the temperature range from the retention end temperature to the second cooling stop temperature of 100° C. or more and 300° C. or less is set to 2° C./s or more. The second average cooling rate is preferably 5° C./s or more, and more preferably 8° C./s or more.
If the cooling rate in this temperature range is too high, the plate shape deteriorates, so the cooling rate in this temperature range (second average cooling rate) is set to 50° C./s or less, preferably 40° C./s or less.
If the second cooling stop temperature exceeds 300°C, the area ratio of tempered martensite or lower bainite will not be the specified ratio, and the area ratio of quenched martensite after annealing will increase, resulting in deterioration of hole expandability.
For this reason, the second cooling stop temperature is set to 300° C. or lower, and preferably, the second cooling stop temperature is 280° C. or lower.
On the other hand, if the second cooling stop temperature is less than 100°C, martensitic transformation occurs excessively, and it may not be possible to obtain a predetermined amount of retained γ, which deteriorates ductility. For this reason, the second cooling stop temperature is set to 100°C or higher. The second cooling stop temperature is preferably 220°C or higher.
Here, the second average cooling rate is "retention end temperature (°C)-second cooling stop temperature (°C)/cooling time (seconds) from the retention end temperature to the second cooling stop temperature".

[Reheating and holding step: heating from the second cooling stop temperature to a reheating temperature of 50°C to 450°C (second cooling stop temperature) at an average heating rate of 2.0°C/s or more, and holding for 60 s to 3000 s]
After the second cooling step, in order to promote C distribution from martensite to austenite, the steel plate is heated from the second cooling stop temperature to a reheating temperature of not less than 50° C. and not more than 450° C. above the second cooling stop temperature.
If the reheating temperature is less than the second cooling stop temperature + 50°C, the effect of C distribution from martensite to austenite cannot be obtained, and the desired volume fraction of retained austenite cannot be obtained. If the reheating temperature exceeds 450°C, excessive tempering of martensite occurs, and the desired TS may not be obtained. Furthermore, the decomposition reaction of austenite occurs, and the desired volume fraction of retained austenite cannot be obtained. For this reason, the reheating temperature is set to be equal to or higher than the second cooling stop temperature + 50°C and equal to or lower than 450°C.
Furthermore, if the average heating rate is less than 2.0° C./s, carbide precipitation is promoted rather than carbon partitioning, and the desired volume fraction of retained austenite cannot be obtained.
Therefore, the average heating rate is set to 2.0° C./s or more. The average heating rate is preferably 4.0° C./s or more, and more preferably 6.0° C./s or more. The average heating rate is preferably 50.0° C./s or less, and more preferably 35.0° C./s or less.

Holding at a reheating temperature of 50°C or more and 450°C or less than the second cooling stop temperature is performed from the viewpoint of adjusting the strength by tempering the formed martensite and promoting C enrichment in the residual γ. If the holding time at the reheating temperature is less than 60 s, tempering is insufficient and high-strength martensite is formed, and bainite transformation does not occur sufficiently, suppressing C enrichment in the residual γ. As a result, the residual γ decreases and the quenched martensite increases, and the desired ductility, hole expandability, or both may not be secured.
On the other hand, if the holding time at the reheating temperature exceeds 3000 seconds, a decomposition reaction of the retained austenite occurs, the desired volume fraction of the retained austenite cannot be obtained, and ductility cannot be ensured.
Therefore, the holding time at the reheating temperature is set to 60 seconds or more and 3000 seconds or less. The holding time at the reheating temperature is preferably 100 seconds or more, more preferably 150 seconds or more. The holding time at the reheating temperature is preferably 2500 seconds or less, more preferably 2000 seconds or less.

Second Embodiment
The manufacturing method of the steel sheet according to the second embodiment of the present invention is a manufacturing method of a steel sheet in which a steel slab having the above-mentioned composition is subjected to hot rolling, pickling and cold rolling, and then the obtained cold-rolled steel sheet is annealed. The annealing is a soaking holding process in which the cold-rolled steel sheet is heated to a soaking temperature that is A _c1 point +20 ° C. or more and A _c3 point or less, and is equal to or higher than Tc calculated by formula (3) in a furnace atmosphere having a dew point of -40 ° C. or less, and held at the soaking temperature for 30 to 500 s, and a cooling step in which the temperature range from the soaking temperature to a cooling stop temperature of 100 to 300 ° C. is cooled to the cooling stop temperature at an average cooling rate of 2 to 50 ° C./s, and a reheating holding step in which the cooling stop temperature is heated from the cooling stop temperature to a reheating temperature of 50 ° C. or more and 450 ° C. or less at an average heating rate of 2.0 ° C./s or more, and held for 60 s to 3000 s. This is a manufacturing method of a steel sheet.
Tc (℃) = 663-1.2 x exp (20/t) x Tdp...Formula (3)
Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.).

In the second embodiment, the treatments in the soaking steps of hot rolling, pickling, cold rolling, and annealing can be performed under the same conditions as in the first embodiment.
In the second embodiment, the first cooling step in the annealing of the first embodiment can be omitted.
In the second embodiment, the cooling step in the annealing corresponds to the second cooling step in the annealing in the first embodiment. However, in the cooling step in this embodiment, the retention treatment in the second cooling step in the first embodiment (retention for 10 to 60 s in a temperature range of 350 to 550° C.) can be omitted.
In addition, the reheating and holding step in the annealing of the second embodiment can be substantially the same as the reheating and holding step in the annealing of the first embodiment, except that the second cooling stop temperature is the cooling stop temperature.
In the present embodiment, the cooling step in the annealing will be mainly described below.

[Cooling process: cooling from the soaking temperature to a cooling stop temperature of 100 to 300 ° C. at an average cooling rate of 2 to 50 ° C./s]
After the soaking process, the steel must be cooled quickly so that the bainite transformation does not proceed excessively. If the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 100°C to 300°C is less than 2°C/s, the ferrite transformation proceeds excessively, the desired amount of ferrite is not secured, and strength may decrease. Also, if the average cooling rate is less than 2°C/s, the desired residual γ is not secured due to the excessive ferrite transformation, and ductility may not be obtained.
Therefore, the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 100° C. or more and 300° C. or less is set to 2° C./s or more. The average cooling rate is preferably 5° C./s or more, and more preferably 8° C./s or more.
If the cooling rate in this temperature range is too high, the plate shape will deteriorate, so the cooling rate (average cooling rate) in this temperature range is set to 50° C./s or less, preferably 40° C./s or less.
If the cooling stop temperature exceeds 300°C, the tempered martensite or lower bainite will not have a specified area ratio, and the area ratio of quenched martensite after annealing will increase, making it impossible to secure retained γ, which may result in deterioration of ductility.
Moreover, if the cooling stop temperature exceeds 300° C., the desired hole expandability may not be obtained. For this reason, the cooling stop temperature is set to 300° C. or less. The cooling stop temperature is preferably 280° C. or less.
On the other hand, if the cooling stop temperature is less than 100° C., martensitic transformation occurs excessively, so that a predetermined amount of retained austenite cannot be obtained, and ductility may deteriorate. For this reason, the cooling stop temperature is set to 100° C. or higher. The cooling stop temperature is preferably 120° C. or higher.
Here, the average cooling rate is "soaking temperature (° C.)−cooling stop temperature (° C.)/cooling time (seconds) from the soaking temperature to the cooling stop temperature."

[Thickness]
The steel sheet of the present invention obtained as described above preferably has a thickness of 0.5 mm or more, and more preferably has a thickness of 3.0 mm or less.

(Components and manufacturing methods for components)
Next, the member of the present invention and the method for producing the same will be described.

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

The steel plate of the present invention has a tensile strength of 780 MPa or more, and has excellent ductility, hole expandability, and chemical conversion treatability. Therefore, the member obtained using the steel plate of the present invention also has a tensile strength of 780 MPa or more, and has excellent ductility, hole expandability, and chemical conversion treatability. Furthermore, the use of the member of the present invention makes it possible to reduce the weight. Therefore, the member of the present invention can be suitably used, for example, in vehicle body frame parts.

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

Example 1
A slab having the chemical composition shown in Table 1 produced by continuous casting was heated to 1200°C, with a soaking time of 200 min, a finish rolling temperature of 860°C or higher, and a coiling temperature of 550°C. After the hot rolling process, the slab was cold rolled at a rolling ratio of 50% to produce a cold-rolled steel sheet having a thickness of 1.4 mm. The cold-rolled steel sheet was treated under the annealing conditions shown in Table 2 to produce the steel sheet of the present invention and the steel sheet of the comparative example.

The steel structure was measured by the following method. The measurement results are shown in Table 3.
The area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite) were measured by cutting out a cross section of the plate thickness parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. At the 1/4 thickness position, an area of 25 μm × 20 μm was observed in 10 fields of view at 5,000 times magnification using an SEM, and the photographed structure was quantified by image analysis.
Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. It is the area that appears the blackest under SEM.
Upper bainite is a ferrite structure with the formation of carbides or retained austenite inside that appear white under SEM. When it is difficult to distinguish between upper bainite and polygonal ferrite, the area of ferrite with an aspect ratio of ≦2.0 was classified as polygonal ferrite, and the area of ferrite with an aspect ratio of >2.0 was classified as upper bainite, and the area ratio was calculated. Here, the aspect ratio was calculated by determining the major axis length a at which the particle length is the longest, and the minor axis length b at the particle length when it crosses the particle the longest in the direction perpendicular to the major axis length a, and a/b was defined as the aspect ratio.
Tempered martensite and lower bainite are regions that are accompanied by a lath-shaped substructure and carbide precipitation inside when viewed under an SEM.
Hardened martensite (fresh martensite) is a blocky region that appears white under an SEM with no internal substructure visible.
The remaining structure is a carbide and/or pearlite structure, which can be confirmed by a white contrast under SEM. Carbide is a structure with a particle size of 1 μm or less, and pearlite is a lamellar (layer) structure, so it can be distinguished.

The volume fraction of retained austenite was determined by chemically polishing the surface layer at 1/4 thickness and then performing X-ray diffraction. A Co-Kα source was used for the incident X-rays, and the volume fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

From the obtained steel plate, JIS No. 5 tensile test pieces were taken, and a tensile test (based on JIS Z2241 (2011)) was performed with N=3. Each evaluation was based on the average value of three points. Steel plates with a tensile strength of 780 MPa or more were judged to have excellent strength. The total elongation EL was judged to be excellent when TS was 780 MPa or more and less than 980 MPa, EL: 16.0% or more, when TS was 980 MPa or more and less than 1180 MPa, EL: 14.0% or more, and when TS was 1180 MPa or more, EL: 12.0% or more.
In addition, a hole expanding test in accordance with the JFST1001 standard was performed with N=3, and the average hole expanding ratio λ(%) (={(dd ₀ )/d ₀ }×100) was calculated. A value of 45% or more was determined to be excellent in hole expanding property.
The measurement results are shown in Table 3.

For the steel sheets after annealing, a sputtering analysis was performed in the depth direction to measure the amount of P in the surface-enriched areas of the steel sheet surface using a GDS (manufactured by Shimadzu Corporation) under conditions of Ar gas pressure: 600 Pa, high frequency output: 35 W, measurement time interval: 0.1 s, and measurement time: 150 s, and the maximum P concentration near the surface layer (within 1 μm in the sheet thickness direction from the steel sheet surface) was measured. In this measurement, a calibration curve for P was obtained using standard materials with various P contents from 0.005 to 0.020 mass%.

The annealed steel sheet was degreased and surface-conditioned, and then chemically treated using a zinc phosphate chemical conversion treatment solution. Specifically, the chemical conversion treatment was performed as follows: degreasing step: treatment temperature; 40°C, treatment time; 120 seconds, spray degreasing, surface conditioning step: pH 9.5, treatment temperature; room temperature, treatment time; 20 seconds, chemical conversion treatment step: temperature of chemical conversion treatment solution; 35°C, treatment time; 120 seconds. Note that, as the treatment agents in the degreasing step, surface conditioning step, and chemical conversion treatment step, respectively, degreaser: FC-E2011, surface conditioning agent: PL-X, and chemical conversion treatment solution: Palbond PB-L3065, manufactured by Nihon Parkerizing Co., Ltd., were used. The surface chemical conversion structure was observed by SEM observation at a magnification of 1000 times in five fields (areas of 50,000 μm2 or ^more ), and the area where the base steel was exposed was evaluated as ◯ when it was less than 10% of the total area, and x when it was 10% or more. The results are shown in Table 3.

The examples of the present invention shown in Tables 2 and 3 were excellent in strength, ductility, hole expansion property and chemical conversion treatment property, whereas the comparative examples were inferior in all of these properties.

Example 2
A slab produced by continuous casting having the composition shown in Table 1 was heated to 1200°C, and subjected to a hot rolling process in which the soaking time was 200 min, the finish rolling temperature was 860°C or higher, and the coiling temperature was 550°C. The cold-rolled steel sheet having a thickness of 1.4 mm was produced by cold rolling at a rolling ratio of 50%, and was treated under the annealing conditions shown in Table 4 to produce the steel sheet of the present invention and the steel sheet of the comparative example. The same evaluation as in Example 1 was carried out. The results are shown in Table 5.

The examples of the present invention shown in Tables 4 and 5 were excellent in strength, ductility, hole expansion property and chemical conversion treatment property, whereas the comparative examples were inferior in all of these properties.

In addition, it was found that the components obtained by forming and joining the steel plate of the present invention have excellent strength, ductility, hole expandability and chemical treatability, just like the steel plate of the present invention, because the steel plate of the present invention has excellent strength, ductility, hole expandability and chemical treatability.

Claims (6)

In mass percent,
C: 0.05-0.25%,
Si: 0.30-1.50%,
Mn: 1.5-4.5%,
P: 0.005-0.050%,
S: 0.01% or less,
sol. Al: less than 1.0%;
N: less than 0.015%;
The following formula (1) is satisfied:
The balance being iron and unavoidable impurities;
Area ratio of polygonal ferrite: 10% or more and 70% or less,
Total area ratio of upper bainite, tempered martensite, and lower bainite: 20% or more and 80% or less,
Volume fraction of retained austenite: 5% or more and 20% or less,
A steel structure having an area ratio of quenched martensite of 13% or less (including 0%);
having
A steel sheet having a maximum P concentration [Pm] of 0.025 mass% or more within 1 μm from the steel sheet surface in the sheet thickness direction, and satisfying the following formula (2):
[Si]/[Mn]≦0.35...Formula (1)
[Pm]/[P]≧1.5...Formula (2)
In the formula (1), [Si] is the Si content (% by mass), [Mn] is the Mn content (% by mass),
In formula (2), [P] is the P content (mass%). The composition further includes, in mass%,
Ti: 0.1% or less,
B: 0.001% or less,
Cu: 1% or less,
Ni: 1% or less,
Cr: 1% or less,
Mo: 0.5% or less,
V: 0.5% or less,
Nb: 0.1% or less,
Mg: 0.0050% or less,
Ca: 0.0050% or less,
Sn: 0.1% or less,
Sb: 0.1% or less,
The steel plate according to claim 1, further comprising one or more selected from the following: REM: 0.0050% or less. A member made using the steel plate according to claim 1 or 2. A method for producing a steel sheet, comprising the steps of: subjecting a steel slab having the composition according to claim 1 or 2 to hot rolling, pickling and cold rolling; and then annealing the resulting cold-rolled steel sheet,
The annealing is
A soaking temperature holding step of heating the cold-rolled steel sheet to a soaking temperature of A _c1 point +20 ° C. or more and A _c3 point or less and Tc or more calculated by formula (3) in a furnace atmosphere having a dew point of −40 ° C. or less, and holding the soaking temperature for 30 to 500 s;
A first cooling step of cooling the temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550 ° C. at a first average cooling rate of 2 to 50 ° C./s to the first cooling stop temperature;
After stopping the cooling at the first cooling stop temperature, the material is allowed to stay in a temperature range of 350 to 550 ° C. for 10 to 60 s,
A second cooling step of cooling to a second cooling stop temperature of 100 to 300 ° C. at a second average cooling rate of 2 to 50 ° C./s;
A reheating and holding step of heating from the second cooling stop temperature to a reheating temperature of 50° C. to 450° C. at an average heating rate of 2.0° C./s or more from the second cooling stop temperature and holding the temperature for 60 s to 3000 s;
A method for manufacturing a steel sheet, comprising:
Tc (℃) = 663-1.2 x exp (20/t) x Tdp (3)
Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.). A method for producing a steel sheet, comprising the steps of: subjecting a steel slab having the composition according to claim 1 or 2 to hot rolling, pickling and cold rolling; and then annealing the resulting cold-rolled steel sheet,
The annealing is
A soaking temperature holding step of heating the cold-rolled steel sheet to a soaking temperature of A _c1 point +20 ° C. or more and A _c3 point or less and Tc or more calculated by formula (3) in a furnace atmosphere having a dew point of −40 ° C. or less, and holding the soaking temperature for 30 to 500 s;
A cooling step of cooling from the soaking temperature to a cooling stop temperature of 100 to 300 ° C. at an average cooling rate of 2 to 50 ° C./s;
A reheating and holding step of heating from the cooling stop temperature to a reheating temperature of 50°C to 450°C at an average heating rate of 2.0°C/s or more from the cooling stop temperature and holding the temperature for 60 s to 3000 s;
A method for manufacturing a steel sheet, comprising:
Tc (℃) = 663-1.2 x exp (20/t) x Tdp (3)
Here, t is the holding time (s) at the soaking temperature, and Tdp is the dew point (° C.). A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 1 or 2 to at least one of forming and joining processes to produce a component.

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