CN120917169A

CN120917169A - Steel plates, components and their manufacturing methods

Info

Publication number: CN120917169A
Application number: CN202480021123.5A
Authority: CN
Inventors: 知场三周; 木村英之; 松井洋一郎; 野口琴未
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-03-31
Filing date: 2024-03-21
Publication date: 2025-11-07
Also published as: EP4667610A1; KR20250150146A; JPWO2024203779A1; MX2025011405A; JP7704309B2; WO2024203779A1

Abstract

This invention provides steel plates, components, and methods for manufacturing them, all possessing excellent ductility, porosity, and chemical conversion properties, and a tensile strength of 780 MPa or higher. A steel plate has a composition containing C, Si, Mn, P, S, sol.Al, and N within a specified range by mass%, satisfying formula (1), and a steel microstructure such that the area ratio of polygonal ferrite is within a specified range. The maximum concentration of P [Pm] within 1 μm of the plate thickness from the surface of the steel plate is 0.025% by mass or higher, and it satisfies formula (2). [Si]/[Mn]≤0.35 …Formula (1) [Pm]/[Pi]≥1.5 …Formula (2)

Description

Steel sheet, member, and method for producing same

Technical Field

The present invention relates to a steel sheet and a member having excellent chemical conversion properties, which are suitable for press-formed articles having a complicated shape used after a press-forming process in automobiles, home appliances, and the like, and a method for producing the same.

Background

In view of the worldwide increase in CO ₂ emissions, there is a further demand for weight reduction of the vehicle body due to the increase in strength of the steel sheet for automobiles, and applications from the conventional 440 MPa-grade cold-rolled steel sheet to 590 MPa-grade or higher strength steel sheets are also being advanced for vehicle bodies and seat parts. In general, when a steel sheet is made stronger, the press formability such as ductility and stretch flangeability is reduced, cracks are likely to occur during press forming, and the degree of freedom in shape is reduced, so that the steel sheet is limited to being applied to a member of a simple shape. Therefore, in order to apply a high-strength steel sheet to a complex-shaped member, it is important to increase the strength of the steel sheet while maintaining or improving the formability.

Against such a background, TRIP steel in which retained austenite (retained γ) is dispersed in the microstructure of a steel sheet has been developed as a technique for improving the ductility of the steel sheet. In the production of TRIP steel, a heat treatment process is used in which soaking and holding are followed by austempering treatment in which isothermal holding in a bainite transformation temperature range is performed, and in which after cooling, the steel is once cooled to a temperature range between a martensite transformation start temperature (Ms point) and a martensite transformation end temperature (Mf point), and then reheating and holding are performed to stabilize retained austenite, so-called Q & P: quenching & Partitioning (quenching and distribution of C from martensite to austenite). In any heat treatment process, since residual γ is formed in the microstructure, a large amount of Si capable of suppressing carbide precipitation is added. In addition, Q & P is a heat treatment process in which a part of the microstructure undergoes martensitic transformation during cooling and the martensitic structure is tempered by subsequent reheating and retained, whereby the difference in hardness between different phases in the microstructure is reduced and not only ductility but also hole expansibility is improved.

For example, patent document 1 discloses a steel sheet and a method for producing the same, in which a cold rolled steel sheet containing 0.6 to 2.5% of Si is held at a first soaking temperature of 750 ℃ or higher, then cooled to a cooling stop temperature in a temperature range of 150 to 350 ℃ and then heated to a temperature range of 350 to 500 ℃ to thereby ensure retained austenite of 5 to 15% in volume fraction, and achieve both TS of 980MPa or higher and ductility of 17% in elongation, and also have excellent hole expansibility of 50% in hole expansibility.

On the other hand, as the Si content increases, si is known to be enriched on the annealed steel sheet surface to form Si-based oxides, which deteriorate the chemical conversion treatability. To solve this problem, for example, patent document 2 discloses a method of improving chemical conversion treatability by adding Ni so as not to enrich Si on the surface of a steel sheet.

Patent document 3 discloses a method of improving chemical conversion treatability by forming an mn—si composite oxide on a surface by appropriately controlling the content of Mn enriched on the surface together with Si so that the Si/Mn ratio is 0.40 or less.

Patent document 4 discloses a method of directly removing Si-based oxide by acid washing or brush polishing after annealing to improve chemical conversion treatability.

Prior art literature

Patent literature

Patent document 1 Japanese patent No. 5821911

Patent document 2 Japanese patent No. 2951480

Patent document 3 Japanese patent No. 3889768

Patent document 4 Japanese patent laid-open publication No. 2003-201518

Disclosure of Invention

Problems to be solved by the invention

As described above, although it is effective to add Si in order to improve ductility of a high-strength steel sheet, si content is in a trade-off relationship with chemical conversion treatability of the steel sheet in the case of positively utilizing Si to secure high workability. The methods disclosed in patent documents 2 and 4 are effective as a method for improving chemical conversion treatability in steel having a high Si content, but it is also desirable to establish other techniques for adjusting the alloy elements contained therein, annealing conditions, and the like.

Further, as a result of the study by the present inventors, it was found that the method disclosed in patent document 3 does not always ensure good chemical conversion treatability.

As described above, as a technique for a high-strength steel sheet having excellent ductility and chemical conversion treatability and hole expansibility, a new technique is required to be established.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel sheet and a member having excellent ductility, hole expansibility, chemical conversion treatability, and tensile strength of 780MPa or more, and a method for producing the same.

Here, the tensile strength means a Tensile Strength (TS) obtained in accordance with JIS Z2241 (2011).

Further, 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) When TS is 780MPa or more and less than 980MPa, EL is 16.0% or more;

(B) When TS is 980MPa or more and 1180MPa or less, EL is 14.0% or more;

(C) When TS is 1180MPa or more, EL is 12.0% or more.

Further, excellent hole expansibility means that, in order to secure hole expansibility necessary for practical use, the hole expansibility λ (%) (= { (d-d ₀)/d₀ } ×100) obtained by a predetermined hole expansion test according to JFST1001 is 45% or more at any TS level.

Further, excellent chemical conversion treatability means that degreasing (treatment temperature: 40 ℃ C., treatment time: 120 seconds, spray degreasing, degreasing agent: FC-E2011 manufactured by Nippon Kagaku Co., ltd.), surface adjustment (pH 9.5, treatment temperature: room temperature, treatment time: 20 seconds, surface conditioner: PL-X manufactured by Nippon Kagaku Co., ltd.) and chemical conversion treatment (treatment temperature: 35 ℃ C., treatment time: 120 seconds, chemical conversion treatment liquid: PALBOND PB-L3065 manufactured by Nippon Kagaku Co., ltd.) were performed, and then the exposed area of the steel base was less than 10% with respect to the whole area.

Means for solving the problems

In order to solve the above problems, the present inventors have conducted intensive studies on steel components, heat treatment conditions and microstructure affecting ductility and chemical conversion treatability with respect to various steel sheets having a tensile strength of 780MPa or more. As a result, it has been found that a steel structure comprising, by mass%, 0.05 to 0.25% of C, 0.30 to 1.50% of Si, 1.5 to 4.5% of Mn, 0.005 to 0.050% of P, 0.01% or less of S, less than 1.0% of sol.Al, less than 0.015% of N, and the balance of the composition comprising iron and unavoidable impurities is formed, and that a steel structure having a polygonal ferrite area ratio of 10% or more and 70% or less, a total area ratio of 20% or more and 80% or less of upper bainite, tempered martensite and lower bainite, a volume ratio of 5% or more and 20% or less of residual austenite (residual γ), a quenched martensite area ratio of 13% or less (including 0%), and further comprising a residual structure is formed, and that a steel structure having a maximum P concentration of 1 μm or less in the steel surface direction from the steel sheet surface as measured by a plate thickness analysis method is formed, and that the steel structure has a cold rolling property of 0.025% or less is obtained for the purpose of having a high heat-expansibility and a local heat-seal strength.

[ Si ]/[ Mn ]. Ltoreq.0.35..formula (1)

[ Pm ]/[ P ] is not less than 1.5..formula (2)

In the formula (1), si is Si content (mass%), [ Mn ] is Mn content (mass%),

In the formula (2), P is the P content (mass%).

The present invention has been completed based on the above-described findings, and the gist thereof is as follows.

[1] A steel sheet, comprising:

Comprises, in mass%, 0.05 to 0.25% of C, 0.30 to 1.50% of Si, 1.5 to 4.5% of Mn, 0.005 to 0.050% of P, less than 0.01% of S, less than 1.0% of sol.Al, less than 0.015% of N, satisfying the following formula (1), and the balance being iron and unavoidable impurities, and

A steel structure having a polygonal ferrite area ratio of 10% to 70%, a total area ratio of upper bainite, tempered martensite and lower bainite of 20% to 80%, a volume ratio of retained austenite of 5% to 20%, and a quenched martensite area ratio of 13% or less (including 0%),

The maximum concentration [ Pm ] of P within 1 μm in the plate thickness direction from the steel plate surface is 0.025 mass% or more, and satisfies the following formula (2).

[ Si ]/[ Mn ]. Ltoreq.0.35..formula (1)

[ Pm ]/[ P ] is not less than 1.5..formula (2)

In the formula (1), si is Si content (mass%), [ Mn ] is Mn content (mass%),

In the formula (2), P is the P content (mass%).

[2] The steel sheet according to the above item [1], wherein the composition further comprises one or more selected from the group consisting of 0.1% or less of Ti, 0.001% or less of B, 1% or less of Cu, 1% or less of Ni, 1% or less of Cr, 0.5% or less of Mo, 0.5% or less of V, 0.1% or less of Nb, 0.0050% or less of Mg, 0.0050% or less of Ca, 0.1% or less of Sn, 0.1% or less of Sb, and 0.0050% or less of REM.

[3] A member comprising the steel sheet of [1] or [2 ].

[4] A method for producing a steel sheet by hot-rolling, pickling and cold-rolling a steel slab having the composition of the above item [1] or [2] and annealing the obtained cold-rolled steel sheet,

The annealing includes:

A soaking maintaining step of heating the cold-rolled steel sheet in a furnace atmosphere having a dew point of-40 ℃ or lower to a soaking temperature of a _c1 point +20 ℃ or higher and a _c3 point or lower and Tc or higher calculated by the formula (3), and maintaining the soaking temperature for 30 to 500 seconds;

a first cooling step of cooling the heat-soaked material to a first cooling stop temperature of 350-550 ℃ at a first average cooling rate of 2-50 ℃ per second within a temperature range from the heat-soaked material to the first cooling stop temperature;

A second cooling step of stopping cooling at the first cooling stop temperature, staying at the first cooling stop temperature for 10 to 60 seconds in a temperature range of 350 to 550 ℃ and then cooling at a second average cooling rate of 2 to 50 ℃ per second to a second cooling stop temperature of 100 to 300 ℃, and

And a reheating-holding step of heating from the second cooling stop temperature to a reheating temperature of from the second cooling stop temperature +50 ℃ to 450 ℃ at an average heating rate of 2.0 ℃ per second or more, and holding the reheating temperature for 60s to 3000 s.

Tc(°C)=663-1.2×exp(20/t)×Tdp ...(3)

Here, t represents the holding time(s) at the soaking temperature, and Tdp represents the dew point (°c).

[5] A method for producing a steel sheet by hot-rolling, pickling and cold-rolling a steel slab having the composition described in the above [1] or [2], and annealing the obtained cold-rolled steel sheet,

The annealing includes:

a cooling step of cooling the heat-soaked material to a cooling stop temperature of 100-300 ℃ at an average cooling rate of 2-50 ℃ per second to the cooling stop temperature, and

And a reheating-holding step of holding the reheating temperature at a temperature of at least 50 ℃ and at most 450 ℃ from the cooling-stopping temperature at an average heating rate of at least 2.0 ℃ per second for at least 60s and at most 3000 s.

Tc(°C)=663-1.2×exp(20/t)×Tdp ...(3)

[6] A method for producing a member, comprising the step of forming or joining the steel sheet of [1] or [2] to produce a member.

Effects of the invention

According to the present invention, a steel sheet and a member having high strength with a tensile strength TS of 780MPa or more and excellent ductility, hole expansibility and chemical conversion treatability can be obtained.

When the steel sheet of the present invention is applied to a skeleton member of an automobile body, a difficult-to-form member having a complicated shape can be produced by cold press working, and thus the steel sheet can contribute significantly to the weight saving of the automobile body. The chemical conversion treatability is improved without using expensive alloy elements and post-treatment after annealing, and the material cost can be reduced.

Drawings

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

Detailed Description

The present invention will be specifically described below. The present invention is not limited to the following embodiments.

(Steel plate)

The steel sheet has a steel structure which has excellent tensile strength TS of 780MPa or more and is excellent in ductility, hole expansibility and chemical conversion treatment, and contains, by mass, 0.05-0.25% C, 0.30-1.50% Si, 1.5-4.5% Mn, 0.005-0.050% P, 0.01% S, less than 1.0% sol.Al, less than 0.015% N, and a composition comprising iron and unavoidable impurities in balance, wherein the total area ratio of upper bainite and tempered martensite is 10% or more and 70% or less, the total area ratio of upper bainite and tempered martensite is 20% or more and 80% or less, the volume ratio of retained austenite is 5% or more and 20% or less, the area ratio of quenched martensite is 13% or less (including 0%) and the maximum plate thickness concentration in the direction P measured by a glow discharge analysis method from the surface [ plate thickness ] is 1 [ mu ] m or less and 2% or more by mass from the surface to the maximum plate thickness [ Pm ] in the thickness direction of 1 [ Pm or less.

[ Si ]/[ Mn ]. Ltoreq.0.35..formula (1)

[ Pm ]/[ P ] is not less than 1.5..formula (2)

In the formula (1), si is Si content (mass%), [ Mn ] is Mn content (mass%),

In the formula (2), P is the P content (mass%).

The steel sheet of the present invention will be described below in the order of the composition and the steel structure. First, the reason why the composition of the components of the present invention is limited will be described. In the following description, unless otherwise specified, the term "% of the steel component" means% by mass.

<C:0.05~0.25%>

C is contained from the viewpoint of improving ductility by securing a predetermined strength by transformation strengthening and securing a predetermined amount of retained austenite (hereinafter also referred to as retained γ). When the C content is less than 0.05%, these effects cannot be sufficiently ensured.

On the other hand, the upper limit of the C content is set to 0.25% from the viewpoints of hole expansibility important in press formability, spot welding at the time of assembling into a vehicle body after forming into an automobile member, and weldability important at the time of laser welding.

Therefore, the C content is set to 0.05 to 0.25%. The C content is preferably 0.08% or more, more preferably 0.10% or more. The C content is preferably 0.22% or less, 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 carbide formation in martensite and bainite, and securing a predetermined amount of residual γ to increase ductility. When the Si content is less than 0.30%, these effects cannot be sufficiently ensured.

On the other hand, when the Si content exceeds 1.50%, good chemical conversion treatability cannot be ensured even in the production method defined in the present invention.

Therefore, the Si content is set to 0.30 to 1.50%. The Si content is preferably 0.35% or more, more preferably 0.40% or more. The Si content is preferably 1.20% or less, more preferably 1.00% or less.

<Mn:1.5~4.5%>

Mn is contained from the viewpoint of improving hardenability of a steel sheet, promoting high strength by transformation strengthening, and similarly to Si, suppressing the formation of carbides in bainite, promoting the formation of retained austenite contributing to ductility, and improving ductility. In order to obtain these effects, the Mn content needs to be 1.5% or more.

On the other hand, when the Mn content exceeds 4.5%, the bainite transformation is significantly delayed, a predetermined amount of retained austenite cannot be ensured, and the ductility is lowered. When the Mn content exceeds 4.5%, it is difficult to suppress the formation of coarse quenched martensite due to the low temperature of the martensite transformation start temperature, and the stretch flangeability (hole expansibility) is deteriorated.

Therefore, the Mn content is set to 1.5% or more and 4.5% or less. The Mn content is preferably 1.8% or more, more preferably 2.0% or more. The Mn content is preferably 3.5% or less, more preferably 3.0% or less.

<P:0.005~0.050%>

P is an element for strengthening steel. In addition, since a surface-enriched portion of P is generated on the surface of the annealed steel sheet by appropriately controlling the P content, P is an element capable of improving chemical conversion treatability, and from this point of view, the P content is set to 0.005% or more.

On the other hand, when the content of P is large, the spot weldability is deteriorated. From this point of view, 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, more preferably 0.009% or more. The P content is preferably 0.040% or less, more preferably 0.030% or less.

< S > 0.01% or less

S is an element having an effect of improving the scale peelability during hot rolling and an effect of suppressing nitriding during annealing, but adversely affecting the spot weldability, bendability, and hole expansibility. In order to reduce these adverse effects, at least the S content is set to 0.01% or less, preferably 0.0050% or less.

In addition, S may not be contained, but is a large cost for reducing to less than 0.0001%, so that the S content is preferably set to 0.0001% or more from the viewpoint of manufacturing cost. The S content is more preferably 0.0005% or more, still more preferably 0.0010% or more.

< Sol.Al less than 1.0% >

Al is contained for deoxidization or for the purpose of obtaining residual gamma. The lower limit of sol.al is not particularly limited, and in order to stably perform deoxidation, the content of sol.al is preferably set to 0.005% or more.

On the other hand, when the sol.al content is 1.0% or more, the Al-based coarse inclusions increase greatly, and the stretch flangeability (hole expansibility) decreases. Further, 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 in the present invention. Therefore, the sol.al content is set to 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 a nitride such as BN, alN, tiN in steel, and since the formability (hole expansibility) of the extended flange is lowered, the content thereof needs to be limited. Therefore, the N content is set to less than 0.015%. The N content is preferably 0.010% or less, more preferably 0.006% or less.

Although N may not be contained, it is necessary to use a large cost for reducing N to less than 0.0001%, and therefore, the N content is preferably 0.0001% or more from the viewpoint of manufacturing cost. The N content is more preferably 0.0005% or more, still more preferably 0.001% or more.

Si/Mn is less than or equal to 0.35

In the formula (1), the [ Si ] is Si content (mass%) and the [ Mn ] is Mn content (mass%).

The [ Si ]/[ Mn ] (Si/Mn ratio) determines the Si to Mn composition ratio of the surface oxide formed at the time of annealing. When [ Si ]/[ Mn ] exceeds 0.35 in the range of the production conditions specified in the present invention, good chemical conversion treatability cannot be ensured. Therefore, [ Si ]/[ Mn ] is set to 0.35 or less. The [ Si ]/[ Mn ] is preferably 0.32 or less, more preferably 0.30 or less. The lower limit is not particularly limited, and [ Si ]/[ Mn ] is preferably 0.10 or more, more preferably 0.15 or more.

The composition of the steel sheet of the present invention contains the above-mentioned constituent elements as essential components, and the balance contains iron (Fe) and unavoidable impurities. The steel sheet of the present invention preferably has a composition including Fe and unavoidable impurities in balance.

The composition of the steel sheet of the present invention may further contain one or two or more selected from the following as optional elements (optional elements) in addition to the above-mentioned components.

Less than 0.1% of B, less than 1% of Cu, less than 1% of Ni, less than 1% of Cr, less than 0.5% of Mo, less than 0.1% of Nb, less than 0.0050% of Ca, less than 0.0050% of Sn, less than 0.1% of Sb, and less than 0.0050% of REM

< Ti 0.1% or less >

Ti has an effect of fixing N in steel in the form of TiN, and an effect of improving hot ductility and an effect of improving hardenability of B. In addition, the effect of refining the structure by precipitation of TiC is obtained. In order to obtain these effects, the Ti content is preferably set to 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably set to 0.008% or more. The Ti content is more preferably 0.010% or more.

On the other hand, when the Ti content exceeds 0.1%, the ductility decreases due to an increase in rolling load and an increase in precipitation strengthening amount, and therefore, when Ti is contained, the Ti content is set to 0.1% or less. The Ti content is preferably 0.05% or less, more preferably 0.03% or less.

< B > 0.001% or less

B is an element that improves hardenability of steel, and has an advantage that tempered martensite and/or bainite having a predetermined area ratio are easily formed. Therefore, the B content is preferably set to 0.0005% or more.

On the other hand, when the B content exceeds 0.001%, enrichment to the oxide occurs during annealing, which promotes coarsening of the oxide and deteriorates the chemical conversion treatability. Therefore, in the case of containing B, the B content is set to 0.001% or less.

< Cu 1% or less >

Cu improves corrosion resistance in the environment of use of automobiles. In addition, the corrosion product of Cu has an effect of coating the surface of the steel sheet to inhibit hydrogen from penetrating into the steel sheet. Cu is an element mixed when the scrap is used as a raw material, and by allowing Cu to be mixed, a reuse material can be used as a raw material, and the manufacturing cost can be reduced. From such a viewpoint, cu is preferably contained in an amount of 0.005% or more, and further from the viewpoint of improving the delayed fracture resistance, cu is more preferably contained in an amount of 0.05% or more. The Cu content is more preferably 0.10% or more. More preferably, the Cu content is 0.25% or more, still more preferably 0.50% or more.

However, if the Cu content is too high, surface defects are generated, and therefore, when Cu is contained, the Cu content is set to 1% or less.

< Ni 1% or less >

Ni is also an element having an effect of improving corrosion resistance, similar to Cu. In addition, ni has an effect of suppressing the generation of surface defects that are easily generated when Cu is contained. Therefore, ni is preferably contained in an amount of 0.01% or more. The Ni content is more preferably 0.04% or more, still more preferably 0.06% or more.

However, when the Ni content is too high, the formation of scale in the heating furnace becomes uneven, which may cause surface defects. In addition, this also results in an increase in cost. Therefore, when Ni is contained, the Ni content is set to 1% or less. The Ni content is preferably 0.5% or less, more preferably 0.3% or less.

< Cr 1% or less >

Cr may be contained in consideration of the effect of improving hardenability of steel and the effect of suppressing carbide formation in martensite, upper bainite, and lower bainite. In order to obtain such an effect, the Cr content is preferably set to 0.01% or more. The Cr content is more preferably 0.03% or more, still more preferably 0.06% or more.

However, when Cr is excessively contained, pitting corrosion resistance is deteriorated, and therefore, when Cr is contained, the Cr content is set to 1% or less. The Cr content is preferably 0.3% or less, more preferably 0.1% or less.

< Mo 0.5% or less >

Mo may be contained in consideration of the effect of improving hardenability of steel and the effect of suppressing carbide formation in martensite, upper bainite, and lower bainite. In order to obtain such an effect, the Mo content is preferably set to 0.01% or more. The Mo content is more preferably 0.03% or more, still more preferably 0.06% or more. More preferably, the Mo content is 0.1% or more, still more preferably 0.2% or more.

However, mo significantly deteriorates the chemical conversion treatability of the cold-rolled steel sheet, and therefore, when Mo is contained, the Mo content is set to 0.5% or less.

< V0.5% or less >

V may be contained in consideration of the effect of improving hardenability of steel, the effect of suppressing carbide formation in martensite, upper bainite, and lower bainite, the effect of refining the structure, and the effect of improving delayed fracture resistance by precipitation of carbide. In order to obtain these effects, the V content is preferably set to 0.003% or more. The V content is more preferably 0.005% or more, still more preferably 0.010% or more. The V content is more preferably 0.020% or more, still more preferably 0.050% or more.

However, when V is contained in a large amount, the castability is remarkably deteriorated, and therefore, when V is contained, the V content is set to 0.5% or less. The V content is preferably 0.3% or less, more preferably 0.2% 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 contained from the viewpoints of the effect of refining the steel structure and increasing the strength, the effect of promoting bainitic transformation by grain refining, the effect of improving bendability, and the effect of improving the delayed fracture resistance. In order to obtain these effects, the Nb content is preferably set to 0.010% or more. The Nb content is preferably 0.015% or more, more preferably 0.020% or more.

However, when a large amount of Nb is contained, precipitation strengthening becomes too strong and ductility decreases. Further, an increase in rolling load and a deterioration in castability are caused. Therefore, when Nb is contained, the Nb content is set to 0.1% or less. The Nb content is preferably 0.08% or less, more preferably 0.05% or less.

< Mg 0.0050% or less >

Mg fixes O as MgO, contributing to improvement of formability such as bendability. Therefore, the Mg content is preferably set to 0.0002% or more. The Mg content is preferably 0.0010% or more, more preferably 0.0015% or more.

On the other hand, when a large amount of Mg is added, the surface quality and bendability are deteriorated, and therefore, when Mg is contained, the Mg content is set to 0.0050% or less. Preferably, the Mg content is 0.0040% or less.

< Ca 0.0050% or less >

Ca fixes S in the form of CaS, contributing to improvement of bendability and improvement of delayed fracture resistance. Therefore, the Ca content is preferably set to 0.0002% or more. The Ca content is more preferably 0.0005% or more, still more preferably 0.0010% or more.

On the other hand, when a large amount of Ca is added, the surface quality and bendability are deteriorated, and therefore, when Ca is contained, the Ca content is set to 0.0050% or less. The Ca content is preferably 0.0040% or less.

< Sn 0.1% or less >

Sn suppresses oxidation and nitridation of the surface layer portion of the steel sheet, and suppresses the decrease in the content of C, B in the surface layer caused by this. By this effect, ferrite formation in the surface layer portion of the steel sheet is suppressed, and the strength is increased, and the fatigue resistance is improved. From such a viewpoint, the Sn content is preferably set to 0.003% or more. The Sn content is more preferably 0.010% or more, and still more preferably 0.015% or more. The Sn content is preferably 0.020% or more, more preferably 0.030% or more.

On the other hand, when the Sn content exceeds 0.1%, the castability deteriorates. Further, sn segregates in the original γ grain boundary, and the delayed fracture resistance is deteriorated. Therefore, when Sn is contained, the Sn content is set to 0.1% or less.

< Sb 0.1% or less >

Sb suppresses oxidation and nitridation of the surface layer portion of the steel sheet, and suppresses the decrease in the content of C, B in the surface layer caused by this. By this effect, ferrite formation in the surface layer portion of the steel sheet is suppressed, and the strength is increased, and the fatigue resistance is improved. From such a viewpoint, the Sb content is preferably set to 0.002% or more. The Sb content is more preferably 0.004% or more, still more preferably 0.006% or more. More preferably, the Sb content is 0.008% or more, still more preferably 0.010% or more. The Sb content is preferably 0.015% or more, more preferably 0.030% or more.

On the other hand, if the Sb content exceeds 0.1%, the castability is deteriorated, and the original γ grain boundary segregates, and the delayed fracture resistance is deteriorated. Therefore, when Sb is contained, the Sb content is set to 0.1% or less.

< REM 0.0050% or less >

REM is an element that suppresses adverse effects of sulfide on stretch-flange formability by spheroidizing the shape of sulfide, and improves stretch-flange formability. In order to obtain these effects, the REM content is preferably set to 0.0005% or more. The REM content is more preferably 0.0010% or more, and still more preferably 0.0020% or more.

On the other hand, when the REM content exceeds 0.0050%, the effect of improving the stretch flangeability is saturated, and therefore, when REM is contained, the REM content is set to 0.0050% or less.

In the present invention, REM refers to lanthanoids of scandium (Sc) having an atomic number 21, yttrium (Y) having an atomic number 39, and lutetium (Lu) having an atomic number 57 from lanthanum (La) to lutetium (71). The REM concentration in the present invention means the total content of one or more elements selected from the above REM.

In the case where the above-mentioned optional components are contained below the lower limit value, the optional elements contained below the lower limit value do not impair the effects of the present invention. Therefore, in the case where the above optional element is contained below the lower limit value, the above optional element is contained as an unavoidable impurity.

Next, the mechanical properties of the steel sheet (cold-rolled steel sheet excellent in material stability) to which the present invention is directed will be described.

The Tensile Strength (TS) of the steel sheet of the present invention is set to 780MPa or more. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of achieving other properties, the tensile strength is preferably 1300MPa or less.

The steel sheet of the present invention has excellent ductility, and exhibits an elongation EL of 16.0% or more when TS is 780MPa or more and less than 980MPa, 14.0% or more when TS is 980MPa or more and less than 1180MPa, and 12.0% or more when TS is 1180MPa or more. Further, as the hole expansibility, a hole expansibility λ of 45% or more is ensured. This significantly improves the press forming stability.

In the evaluation of tensile characteristics, a JIS5 tensile test piece was cut from the center position of the board width, and a tensile test was performed at n=3 (according to JIS Z2241 (2011)). For each evaluation, an average value of 3 points was used. A steel sheet having a tensile strength of 780MPa or more was used as a high-strength steel sheet. A steel sheet having excellent ductility, wherein the total elongation EL is 16.0% or more when TS is 780MPa or more and less than 980MPa, 14.0% or more when TS is 980MPa or more and less than 1180MPa, and 12.0% or more when TS is 1180MPa or more. Regarding hole expansibility, a hole expansibility λ (%) (= { (d-d ₀)/d₀ } ×100) obtained by a predetermined hole expansion test according to JFST1001 was set to 45% or more as an essential condition of the present invention.

Next, the steel structure of the steel sheet of the present invention will be described.

< Area ratio of polygonal ferrite: 10% or more and 70% or less >

From the viewpoint of securing high ductility, the polygonal ferrite is set to 10% or more in terms of area ratio, and is preferably set to 20% or more in order to obtain higher ductility.

On the other hand, when the polygonal ferrite exceeds 70%, the desired strength may not be obtained, and therefore the polygonal ferrite is set to 70% or less, preferably 65% or less, and more preferably 60% in terms of area ratio.

< Total area ratio of upper bainite and tempered martensite and lower bainite: 20% to 80%

The total area ratio of the upper bainite, tempered martensite, and lower bainite is set to 20% or more in order to obtain a desired strength, and is preferably set to 25% or more in order to obtain a higher strength.

On the other hand, when the total area ratio of the upper bainite, tempered martensite, and lower bainite exceeds 80%, the ductility is lowered due to an excessive increase in strength, and therefore the area ratio is set to 80% or less. More preferably, the content is 75% or less, and still more preferably, 70% or less.

< Volume fraction of retained austenite (retained. Gamma.) > 5% or more and 20% or less

When the volume fraction of retained austenite is less than 5%, the desired ductility may not be ensured. In addition, when the volume fraction of retained austenite is less than 5%, the desired strength may not be ensured. In addition, when the volume fraction of retained austenite is less than 5%, desired hole expansibility may not be ensured.

From the viewpoint of ductility, the volume ratio of the retained austenite is set to 5% or more, preferably 7% or more.

On the other hand, when the retained austenite exceeds 20%, the stretch flangeability (hole expansibility) is reduced, and thus the retained austenite is set to 20% or less. Preferably 15% or less, more preferably 13% or less.

< Area ratio of quenched martensite: 13% or less (including 0%) >

The hard quenched martensitic structure reduces λ, and therefore the area ratio needs to be suppressed. In order to obtain 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.

< Residual tissue >

The steel structure is composed of the remaining structure other than the above. The area ratio of the remaining tissue is preferably set to 5% or less. The remaining structure may be unrecrystallized ferrite, carbide, pearlite. These tissues may be determined by SEM observation as described later.

< Maximum concentration of P [ Pm ] of 1 μm or less in the plate thickness direction from the surface of the steel plate is 0.025% by mass or more and satisfies formula (2) >

[ Pm ]/[ P ] is not less than 1.5..formula (2)

In the formula (2), P (also referred to as [ Pi ]) is the P content (mass%).

As a result of intensive studies on various elements affecting the chemical conversion treatability, the surface enrichment thereof, and the oxide species formed during annealing, it was revealed that the chemical conversion treatability could not be sufficiently ensured even under the manufacturing conditions in which oxide formation was not observed. As a result of quantitatively evaluating the maximum concentration of P near the surface layer of a steel sheet having ensured chemical conversion treatability by a method described later, it was found that when the luminous intensity of P measured by GDS (glow discharge analysis) in the sheet thickness direction from the surface was analyzed, the maximum concentration [ Pm ] of P within 1 μm in the sheet thickness direction from the steel sheet surface was 0.025 mass% or more and satisfied the steel structure of formula (2), and good chemical conversion treatability was ensured.

The detailed mechanism is not clear, but it is considered that it is important that the maximum concentration of P in the surface layer is locally high with respect to the steel component, and if the maximum concentration of P is insufficient, the shape of the chemical conversion crystal after the chemical conversion treatment is scaly, so that local surface enrichment of P has an effect of suppressing the formation of Si-based oxide or si—mn-based oxide on the surface that adversely affects the chemical conversion treatability.

[ Pm ] is preferably 0.030% by mass or more, more preferably 0.035% by mass or more. The upper limit is not particularly limited, but [ 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. The upper limit is not particularly limited, but [ Pm ]/[ Pi ] is preferably 10.0 or less, more preferably 9.0 or less.

Next, a method for measuring a 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 a plate thickness cross section parallel to the rolling direction, mirror polishing, etching with a1 vol% nitric acid ethanol solution, observing a range of 25 μm×20 μm of 10 views at a 1/4 thickness position by SEM at 5000 times, photographing, and quantifying the obtained structure photograph by image analysis.

Polygonal ferrite is a ferrite which is hardly accompanied with carbide inside and is relatively equiaxed. Under SEM is the region that appears to be the darkest.

Upper bainite is a ferrite structure internally accompanied by the formation of carbides or retained austenite that appear white under SEM. When it is difficult to distinguish upper bainite from polygonal ferrite, the area ratio is calculated by classifying the region of ferrite having an aspect ratio of 2.0 or less as polygonal ferrite, and classifying the region having an aspect ratio of >2.0 as upper bainite. Here, regarding the aspect ratio, the long axis length a, which is the longest particle length, is obtained, and the particle length when the particles traverse longest in the direction perpendicular thereto is defined as the short axis length b, and a/b is defined as the aspect ratio.

Tempered martensite and lower bainite are regions that are internally accompanied by precipitation of lath-like substructures and carbides under SEM.

Quenched martensite (fresh martensite) is a block-like region that appears whitish without a sub-structure being seen inside under SEM.

The remaining structure is a structure containing at least one of unrecrystallized ferrite, carbide and pearlite, and the unrecrystallized ferrite is a structure that can be confirmed by SEM as ferrite having a black contrast, which is a deformed structure introduced by rolling, and the carbide and pearlite are structures that can be confirmed by white contrast. The carbide has a structure with a particle diameter of 1 μm or less, and the pearlite has a lamellar (lamellar) structure, so that it can be distinguished.

The volume fraction of the retained austenite was obtained by X-ray diffraction from the surface layer by chemical polishing to a 1/4 thickness position. The volume fraction of retained austenite was calculated from the intensity ratio of the ferrite (200), (211), (220) surface to the austenite (200), (220), (311) surface using a Co-K.alpha.radiation source. Here, since the retained austenite is randomly distributed, the volume fraction of the retained austenite obtained by X-ray diffraction can be used as the area fraction of the retained austenite.

The surface enrichment of the surface enrichment portion of P on the surface of the steel sheet was analyzed by sputtering under conditions of an Ar gas pressure of 600Pa, a high-frequency output of 35W, a measurement time interval of 0.1s, and a measurement time of 150s using GDS (manufactured by Shimadzu corporation), the surface enrichment of P was measured, and the maximum concentration [ Pm ] of P within 1 μm in the thickness direction from the surface of the steel sheet was obtained from a standard curve obtained in advance. Under the measurement conditions, the measurement position d (μm) from the surface was obtained by using the formula d=ts/1.7 (μm) using the sputtering time ts.

In the present invention, as shown in fig. 1, the maximum concentration ([ Pm ]) is a value obtained by converting the highest intensity value of P into mass% using a standard curve in the measurement time of 150 s.

As a method for converting to the mass%, the correlation between the Intensity (concentration) of the P element obtained by GDS and the amount of P was determined from data obtained by measurement under the same conditions using a standard material having a known amount of P, and the Intensity of P of examples thus measured was converted to a concentration.

In fig. 1, pi is the P content (mass%) in the steel sheet.

(Method for producing Steel sheet)

Next, a method for manufacturing the steel sheet of the present invention will be described.

< First embodiment >

The method for producing a steel sheet according to the first embodiment of the present invention is a method for producing a steel sheet by subjecting a steel sheet having the above-described composition to hot rolling, pickling and cold rolling, and then annealing the obtained cold-rolled steel sheet, wherein the annealing includes a soaking and holding step of heating the cold-rolled steel sheet to a soaking temperature of at least a point A _c1 ℃ and at most a point A _c3 and at most Tc calculated by the formula (3) in a furnace atmosphere having a dew point of at most-40 ℃, holding the soaking temperature for 30-500 seconds, a first cooling step of cooling the cold-rolled steel sheet to the first cooling and stopping temperature at a first average cooling rate of 2-50 ℃ in a temperature range from the soaking temperature to a first cooling and stopping temperature of 350-550 ℃, and a second cooling step of stopping cooling the cold-rolled steel sheet to a soaking temperature of at most 60 ℃ and at most 100-300 ℃ in a temperature range of at most 2-50 ℃ after stopping cooling the first cooling and holding the cold-rolled steel sheet to a second average cooling and holding the soaking temperature to a second average cooling and at most 300 ℃ from the soaking temperature of at most 2-450 ℃ to a second average cooling and at most 0 ℃ after stopping the heating.

Tc (° C) =663-1.2×exp (20/t) ×tdp..formula (3)

Here, t represents a holding time (soaking holding time)(s) at the soaking temperature, and Tdp represents a dew point (°c).

< Hot Rolling >

Examples of the method for hot rolling a billet include a method for heating a billet and then rolling the billet, a method for directly rolling a billet after continuous casting without heating the billet, and a method for heating a billet after continuous casting for a short period of time and then rolling the billet. The hot rolling may be performed by a conventional method, and for example, the billet heating temperature may be set to 1100 ℃ or higher. The billet heating temperature may be set to 1300 ℃ or lower. The soaking temperature may be set to 20 minutes or more. The soaking temperature may be set to 300 minutes or less. The finish rolling temperature may be set to be equal to or higher than the a _r3 transformation point. The finish rolling temperature may be set to a _r3 transformation point +200℃. The winding temperature may be set to 400 ℃ or higher. The winding temperature may be set to 720 ℃ or less. The winding temperature is preferably controlled from the viewpoint of suppressing plate thickness fluctuation and stably securing high strength. Specifically, the winding temperature is preferably set to 430 ℃ or higher. The winding temperature is preferably set to 530 ℃ or less.

The transformation point a _r3 can be calculated from the composition of the steel sheet and the following empirical formula (a).

Point a _r3 (°c) =910-310× [ C ] -80× [ Mn ] -20× [ Cu ] -15× [ Cr ] -55× [ Ni ] -80× [ Mo ])

(In the above formula, [ M ] is the content (mass%) of the element M in the steel slab, and the value of the element not contained is zero (0))

< Pickling >

The acid washing is carried out according to a conventional method.

< Cold Rolling >

The cold rolling may be performed according to a conventional method, and the rolling reduction (cumulative rolling reduction) may be set to 30% or more. The rolling reduction (cumulative rolling reduction) may be 85% or less. The rolling reduction is preferably controlled from the viewpoint of stably securing high strength and reducing anisotropy. Specifically, the rolling reduction is preferably set to 35% or more. When the rolling load is high, the annealing treatment for softening may be performed by CAL (continuous annealing line) or BAF (box annealing furnace) at 450 to 730 ℃.

< Annealing >

For a cold-rolled steel sheet (cold-rolled steel sheet) manufactured according to a conventional method, annealing was performed under the following conditions. The annealing equipment is not particularly limited, and is preferably implemented by a Continuous Annealing Line (CAL) from the viewpoint of ensuring productivity and desired heating and cooling rates.

[ Soaking step of heating to a soaking temperature of at least A _c1 +20 ℃ and at most A _c3 ℃ and at least Tc in a furnace atmosphere having a dew point of at most-40 ℃ and maintaining the soaking temperature for 30-500 s ]

The dew point affects oxide formation on the surface of the annealed steel sheet, and when the dew point exceeds-40 ℃, the amount of oxide formed on the surface of the steel sheet excessively increases, and thus chemical conversion treatability is deteriorated. Therefore, the dew point is set to-40 ℃ or lower.

The lower limit is not particularly limited, and the dew point is preferably set to-70 ℃ or higher, more preferably set to-60 ℃ or higher.

The steel sheet obtained in the present invention contains a soft ferrite structure, and thus ductility is improved. Therefore, the soaking temperature is set to be a _c1 point +20 ℃ or higher and a _c3 point or lower, at which ferrite is formed.

Further, by setting the soaking temperature to Tc (°c) or higher, the surface enrichment amount of P in the surface enrichment portion of P formed on the surface of the steel sheet can be ensured to be the amount specified in the present invention. Tc is calculated from the dew point and the soaking hold time in equation (3).

Tc (° C) =663-1.2×exp (20/t) ×tdp..formula (3)

Here, t represents a holding time(s) at the soaking temperature, tdp represents a dew point (°c).

When the soaking temperature is lower than Tc, the surface enrichment of the predetermined P cannot be ensured, and the chemical conversion treatability deteriorates. Therefore, in the furnace atmosphere having a dew point of-40 ℃ or lower, the soaking temperature is set to a _c1 point +20 ℃ or higher and a _c3 point or lower and Tc (°c) or higher.

If the holding time at the soaking temperature (soaking holding time) is less than 30 seconds, the austenite formation at the soaking temperature may not proceed sufficiently, the polygonal ferrite may become large, the desired total area ratio of upper bainite, tempered martensite, and lower bainite may not be obtained, the desired strength may not be obtained, and the residual austenite may not be obtained sufficiently, and the desired ductility may not be ensured.

On the other hand, if the holding time at the soaking temperature (soaking holding time) exceeds 500 seconds, coarsening of the structure is significantly generated, and therefore the desired strength cannot be ensured.

Therefore, the time (soaking time) for holding at the annealing temperature is set to 30 to 500 seconds.

The holding time (soaking time) at the soaking temperature is preferably 60 seconds or longer, more preferably 100 seconds or longer. The holding time (soaking time) at the soaking temperature is preferably 400 seconds or less, more preferably 300 seconds or less.

Note that a _c1 and a _c3 obtained by empirical formulas of the following formulas (4) and (5) can be used for the above-mentioned formulas a _c1 and a _c3.

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] … (5)

Here, [ M ] is mass% of each element.

[ First cooling step of cooling to a first cooling stop temperature at a first average cooling rate of 2 to 50 ℃ per second within a temperature range from the soaking temperature to the first cooling stop temperature of 350 to 550 ]

After maintaining the soaking temperature of a _c1 to a _c3 and a Tc (after the soaking maintaining step), cooling is performed at a first average cooling rate of 2 to 50 ℃ per second in a temperature range from the soaking temperature to a first cooling stop temperature of 350 to 550 ℃. When the temperature is less than 2 ℃ per second, ferrite transformation during cooling proceeds excessively, since the desired polygonal ferrite amount is not obtained, the first average cooling rate is set to 2 ℃ per second or more. The first average cooling rate is preferably 5 ℃ or more per second.

On the other hand, when the first average cooling rate becomes too large, the plate shape deteriorates, and thus is set to 50 ℃ per second or less. The first average cooling rate is preferably below 40 ℃ per second, more preferably less than 30 ℃ per second.

Here, the first average cooling rate means "(soaking temperature (°c) -first cooling stop temperature (°c))/cooling time (seconds) from the soaking temperature to the first cooling stop temperature)".

The second cooling step (1) is to stop cooling at the first cooling stop temperature and then to stay at a stay temperature of 350-550 ℃ for 10-60 s

In the temperature range (retention temperature) of 350 ℃ to 550 ℃ below the first cooling stop temperature, upper bainite is formed, and a large amount of residual γ can be obtained, and ductility can be improved, as compared with a production method in which retention at the temperature is not performed for 10s or more and 60s or less. In the present disclosure, the second cooling step (1) is performed at a residence temperature of 350 to 550 ℃ for 10 to 60 seconds, and the presence or absence of the residence can be determined in consideration of desired characteristics.

The bainite phase is a latent phase and must remain at that temperature for a certain period of time in order to obtain the desired amount of bainite. When the residence temperature range including the residence start temperature (=first cooling stop temperature) and the residence end temperature deviates from the range of 350 to 550 ℃ and/or when the residence time (hereinafter also referred to as residence time) is less than 10s, a desired amount of bainite may not be obtained, and the formation of retained austenite may be suppressed, thereby failing to obtain desired ductility.

On the other hand, if the residence time exceeds 60 seconds, the enrichment of C from bainite to bulk non-phase-change γ proceeds, which leads to an increase in the amount of residual bulk structure, and there is a concern that λ is reduced. Therefore, the residence time is set to 10s or more and 60s or less. The residence time is preferably 20s or more. The residence time is preferably 50 seconds or less.

A second cooling step (2) of cooling to a second cooling stop temperature of 100-300 ℃ at a second average cooling rate of 2-50 ℃ per second

After the above-mentioned residence, the bainite transformation is not excessively performed, and it is necessary to rapidly cool the steel. If the average cooling rate (second average cooling rate) in the temperature range from the above-mentioned dwell-end temperature to the second cooling stop temperature of 100 ℃ or more and 300 ℃ or less is less than 2 ℃/s, bainite transformation may excessively proceed, so that retained austenite excessively increases, and the amount of desired quenched martensite cannot be ensured, resulting in a decrease in strength. At a second average cooling rate of less than 2C/s, the desired ductility and hole expansibility may not be obtained.

Therefore, the second average cooling rate in the temperature range from the stop temperature to the second cooling stop temperature of 100 ℃ to 300 ℃ is set to 2 ℃ or more. The second average cooling rate is preferably set to 5 ℃ per second or more, more preferably 8 ℃ per second or more.

When the cooling rate in this temperature range becomes excessively large, the plate shape deteriorates, and therefore the cooling rate (second average cooling rate) in this temperature range is set to 50 ℃ per second or less. Preferably 40℃/s or less.

When the second cooling stop temperature exceeds 300 ℃, the tempered martensite or lower bainite does not reach a predetermined area ratio, and the area ratio of the annealed quenched martensite increases, whereby hole expansibility is deteriorated.

Therefore, the second cooling stop temperature is set to 300 ℃ or less. The second cooling stop temperature is preferably 280 ℃ or less.

On the other hand, when the second cooling stop temperature is lower than 100 ℃, martensite transformation may excessively occur, and thus a predetermined amount of residual γ or the like cannot be obtained, and therefore ductility is deteriorated. Therefore, the second cooling stop temperature is set to 100 ℃ or higher. The second cooling stop temperature is preferably 220 ℃ or higher.

Here, the second average cooling rate means "residence end temperature (°c) -second cooling stop temperature (°c)/cooling time (seconds) from the residence end temperature to the second cooling stop temperature".

[ Reheating-maintaining step of maintaining a reheating temperature of at least 60s and at most 3000s by heating from a second cooling stop temperature to a second cooling stop temperature+50 ℃ and at most 450 ℃ at an average heating rate of at least 2.0 ℃ per second ]

After the second cooling step, the steel sheet is heated from the second cooling stop temperature to a reheating temperature of from +50 ℃ to 450 ℃ inclusive, in order to promote the distribution of C from martensite to austenite.

When the reheating temperature is lower than the second cooling stop temperature +50℃ C, the effect of partitioning C from martensite to austenite is not obtained, and the desired volume ratio of retained austenite is not obtained. If the reheating temperature exceeds 450 ℃, tempering of martensite may occur excessively, and the desired TS may not be obtained. In addition, the retained austenite of a desired volume ratio is not obtained due to the decomposition reaction of austenite. Therefore, the reheating temperature is set to be not less than +50 ℃ and not more than 450 ℃ of the second cooling stop temperature.

In addition, when the average heating rate is less than 2.0 ℃ per second, carbide precipitation is promoted as compared with carbon distribution, and as a result, a desired volume fraction of retained austenite is not obtained.

Therefore, the average heating rate is set to 2.0 ℃ per second or more. The average heating rate is preferably 4.0 ℃ per second or more, more preferably 6.0 ℃ per second or more. The average heating rate is preferably 50.0 ℃ per second or less, more preferably 35.0 ℃ per second or less.

From the standpoint of promoting strength adjustment and enrichment of C into residual gamma by tempering treatment of the formed martensite, the holding is performed at a reheating temperature of from +50 ℃ to 450 ℃ inclusive of the second cooling stop temperature. If the holding time at the reheating temperature is less than 60s, there are cases where tempering is insufficient to form martensite having high strength, bainite transformation is not sufficiently generated, and C is suppressed from being enriched into residual γ, so that residual γ is reduced, and quenched martensite is increased, whereby desired ductility, hole expansibility, or any of them cannot be ensured.

On the other hand, if the holding time at the reheating temperature exceeds 3000 seconds, the decomposition reaction of the retained austenite occurs, and the retained austenite of a desired volume ratio is not obtained, and ductility cannot be ensured.

Therefore, the holding time at the reheating temperature is set to 60s to 3000 s. The holding time at the reheating temperature is preferably 100s or more, more preferably 150s or more. The holding time at the reheating temperature is preferably 2500 seconds or less, more preferably 2000 seconds or less.

< Second embodiment >

The method for producing a steel sheet according to a second embodiment of the present invention is a method for producing a steel sheet by annealing a cold-rolled steel sheet obtained by hot rolling, pickling and cold rolling a steel sheet having the above-described composition, wherein the annealing includes a soaking maintaining step of heating the cold-rolled steel sheet to a soaking temperature of at least a _c1 point +20 ℃ and at most a _c3 point and at least Tc calculated by formula (3) in a furnace atmosphere having a dew point of at most minus 40 ℃, and maintaining the soaking temperature at the soaking temperature for 30 to 500 seconds, a cooling step of cooling the cold-rolled steel sheet to the cooling stop temperature at an average cooling rate of 2 to 50 ℃ per second in a temperature range from the soaking temperature to a cooling stop temperature of 100 to 300 ℃, and a reheating maintaining step of maintaining the cold-rolled steel sheet at an average heating rate of at least 2.0 ℃ from the cooling stop temperature to a reheating temperature of at most +50 ℃ and at most 450 ℃ for 60 to 3000 seconds.

Tc (° C) =663-1.2×exp (20/t) ×tdp..formula (3)

In the second embodiment, the treatment in the soaking step of hot rolling, pickling, cold rolling, and annealing may be performed under the same conditions as in the first embodiment.

In the second embodiment, the process in the first cooling step in the annealing of the first embodiment may be omitted.

In the second embodiment, the cooling step during annealing corresponds to the second cooling step during annealing in the first embodiment, but the stay treatment (stay in the temperature range of 350 to 550 ℃ for 10 to 60 seconds) in the second cooling step in the first embodiment may be omitted in the cooling step in the present embodiment.

In addition, the reheating maintenance step in the annealing of the second embodiment may be set to substantially the same conditions as the reheating maintenance step in the annealing of the first embodiment except that the second cooling stop temperature is set to be the cooling stop temperature.

In the following, a cooling step during annealing will be mainly described in the present embodiment.

[ Cooling step of cooling from the soaking temperature to a cooling stop temperature of 100 to 300 ℃ at an average cooling rate of 2 to 50 ℃ per second ]

After the treatment in the soaking and holding step, it is necessary to rapidly cool down the steel so as not to excessively undergo bainite transformation. If the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature of 100 ℃ to 300 ℃ inclusive is less than 2 ℃ per second, ferrite transformation proceeds excessively, and the desired amount of ferrite cannot be ensured, resulting in a decrease in strength. In addition, when the average cooling rate is less than 2 ℃ per second, the desired residual γ cannot be ensured due to 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 ℃ to 300 ℃ is set to 2 ℃ per second or more. The average cooling rate is preferably set to 5 ℃ per second or more, more preferably 8 ℃ per second or more.

When the cooling rate in this temperature range is too high, the plate shape is deteriorated, and therefore the cooling rate (average cooling rate) in this temperature range is set to 50 ℃ per second or less. Preferably 40℃/s or less.

When the cooling stop temperature exceeds 300 ℃, the tempered martensite or the lower bainite does not reach a predetermined area ratio, and the area ratio of the annealed quenched martensite increases, so that the residual γ cannot be ensured, and the delay property deteriorates.

In addition, when the cooling stop temperature exceeds 300 ℃, desired hole expansibility may not be obtained. Therefore, the cooling stop temperature is set to 300 ℃ or less. The cooling stop temperature is preferably 280 ℃ or less.

On the other hand, when the cooling stop temperature is lower than 100 ℃, martensite transformation excessively occurs, so that a predetermined amount of retained austenite cannot be obtained, and ductility may be deteriorated. Therefore, the cooling stop temperature is set to 100 ℃ or higher. The cooling stop temperature is preferably 120 ℃ or higher.

Here, the average cooling rate means "soaking temperature (°c) -cooling stop temperature (°c)/cooling time (seconds) from the soaking temperature to the cooling stop temperature".

[ Plate thickness ]

The thickness of the steel sheet of the present invention obtained as described above is preferably set to 0.5mm or more. The thickness of the sheet is preferably 3.0mm or less.

(Member and method for manufacturing Member)

Next, the member and the method of manufacturing the same of the present invention will be described.

The member of the present invention is a member obtained by subjecting the steel sheet of the present invention to at least one of forming and joining processes. The method for producing a member according to the present invention includes a step of forming the steel sheet according to the present invention or a step of joining the steel sheet to produce a member.

The steel plate has tensile strength of 780MPa or more and excellent ductility, hole expansibility and chemical conversion treatability. Therefore, the member obtained by using the steel sheet of the present invention has a tensile strength of 780MPa or more, and is excellent in ductility, hole expansibility and chemical conversion treatability. In addition, if the member of the present invention is used, weight reduction can be achieved. Therefore, the member of the present invention can be suitably used for a vehicle body frame member, for example.

The molding may be performed by a general method such as press molding, without limitation. The joining process may be general welding such as spot welding and arc welding, rivet joining, and rivet joining, without limitation.

Examples

Example 1]

The steel slabs produced by continuous casting having the composition shown in table 1 were heated to 1200 ℃, subjected to a hot rolling step in which the soaking time was 200 minutes, the finish rolling temperature was 860 ℃ or higher, and the coiling temperature was 550 ℃ and then cold rolled at a rolling reduction of 50%, and cold rolled steel sheets having a thickness of 1.4mm produced by this were treated under annealing conditions shown in table 2, whereby steel sheets of the present invention and steel sheets of comparative examples were produced.

The steel structure was measured by the following method. The measurement results are shown in Table 3.

The remaining tissue is carbide and/or pearlite, and can be confirmed by SEM with white contrast. The carbide has a structure with a particle diameter of 1 μm or less, and the pearlite has a lamellar (lamellar) structure, so that it can be distinguished.

The volume fraction of the retained austenite was obtained by X-ray diffraction from the surface layer by chemical polishing to a 1/4 thickness position. The volume fraction of retained austenite was calculated from the intensity ratio of the ferrite (200), (211), (220) surface to the austenite (200), (220), (311) surface using a Co-K.alpha.radiation source.

From the obtained steel sheet, a JIS5 tensile test piece was cut, and a tensile test was performed at n=3 (according to JIS Z2241 (2011)). For each evaluation, an average value of 3 points was used. A steel sheet having a tensile strength of 780MPa or more was judged to be excellent in strength. The total elongation EL was found to be excellent in ductility, with 16.0% or more of EL when TS was 780MPa or more and less than 980MPa, 14.0% or more of EL when TS was 980MPa or more and less than 1180MPa, and 12.0% or more of EL when TS was 1180MPa or more.

Further, a predetermined hole expansion test according to JFST1001 was performed with n=3, and the average of hole expansion ratios λ (%) (= { (d-d ₀)/d₀ } ×100) was calculated, and 45% or more was determined to be excellent in hole expansion property.

The measurement results are shown in table 3.

In the annealed steel sheet, the surface enrichment of the surface enrichment portion of P on the surface of the steel sheet was analyzed by sputtering under conditions of an Ar gas pressure of 600Pa, a high-frequency output of 35W, a measurement time interval of 0.1s, and a measurement time of 150s using GDS (manufactured by Shimadzu corporation), and the maximum concentration of P in the vicinity of the surface layer (within 1 μm in the sheet thickness direction from the surface of the steel sheet) was measured. In the present measurement, a standard curve of P is obtained by using standard materials having various P contents of 0.005 to 0.020% by mass.

Degreasing and surface conditioning are carried out on the annealed steel plate, and then chemical conversion treatment is carried out by using zinc phosphate chemical conversion treatment liquid. Specifically, the chemical conversion treatment was performed under conditions of a treatment temperature of 40 ℃ in the degreasing step, a treatment time of 120 seconds, a pH of 9.5 in the spray degreasing and surface conditioning step, a treatment temperature of room temperature, a treatment time of 20 seconds, a temperature of the chemical conversion treatment liquid of 35 ℃ in the chemical conversion treatment step, and a treatment time of 120 seconds. As the treating agents in the degreasing step, the surface conditioning step, and the chemical conversion treatment step, a degreasing agent made by Nippon Kagaku Co., ltd., FC-E2011, a surface conditioner, PL-X, and a chemical conversion treatment liquid, PALBOND PB-L3065, were used in this order. SEM observation was performed in 5 fields of view (regions of 50000 μm ² or more) at a magnification, whereby the surface chemical conversion structure was observed, and the case where the exposed region of the steel base was less than 10% with respect to the entire region was evaluated as good, and the case where the exposed region of the steel base was 10% or more with respect to the entire region was evaluated as x. The results are shown in Table 3.

The inventive examples shown in tables 2 and 3 were excellent in strength, ductility, hole expansibility and chemical conversion treatability, whereas the comparative examples were poor.

Example 2]

The steel slabs produced by continuous casting having the composition shown in table 1 were heated to 1200 ℃, subjected to a hot rolling step in which the soaking time was 200 minutes, the finish rolling temperature was 860 ℃ or higher, and the coiling temperature was 550 ℃ and then cold rolled at a rolling reduction of 50%, and cold rolled steel sheets having a thickness of 1.4mm produced by this were treated under the annealing conditions shown in table 4, whereby steel sheets of the present invention and steel sheets of comparative examples were produced. The same evaluation as in example 1 was performed. The results are shown in Table 5.

The inventive examples shown in tables 4 and 5 were excellent in strength, ductility, hole expansibility and chemical conversion treatment properties, whereas the comparative examples were poor.

It was also found that the steel sheet of the present invention example was excellent in strength, ductility, hole expansibility and chemical conversion treatability, and therefore the member obtained by forming the steel sheet of the present invention example and the member obtained by joining the steel sheet were excellent in strength, ductility, hole expansibility and chemical conversion treatability, similarly to the steel sheet of the present invention example.

Claims

1. A steel sheet, comprising:

The maximum concentration [ Pm ] of P within 1 μm from the steel plate surface in the plate thickness direction is 0.025 mass% or more, and satisfies the following formula (2),

[ Si ]/[ Mn ]. Ltoreq.0.35..formula (1)

[ Pm ]/[ P ] is not less than 1.5..formula (2)

In the formula (1), si is Si content (mass%), [ Mn ] is Mn content (mass%),

In the formula (2), P is the P content (mass%).

2. The steel sheet according to claim 1, further comprising one or more selected from the group consisting of 0.1% or less of Ti, 0.001% or less of B, 1% or less of Cu, 1% or less of Ni, 1% or less of Cr, 0.5% or less of Mo, 0.5% or less of V, 0.1% or less of Nb, 0.0050% or less of Mg, 0.0050% or less of Ca, 0.1% or less of Sn, 0.1% or less of Sb, and 0.0050% or less of REM, as the component composition.

3. A member comprising the steel sheet according to claim 1 or 2.

4. A method for producing a steel sheet by hot-rolling, pickling and cold-rolling a steel slab having the composition described in claim 1 or 2 and then annealing the obtained cold-rolled steel sheet,

The annealing includes:

a soaking maintaining step of heating the cold-rolled steel sheet in a furnace atmosphere having a dew point of-40 ℃ or lower to a soaking temperature of a _c1 point +20 ℃ or higher and a _c3 point or lower and Tc or higher calculated by formula (3), and maintaining the soaking temperature for 30 to 500 seconds;

a first cooling step of cooling to a first cooling stop temperature at a first average cooling rate of 2-50 ℃ per second in a temperature range from the soaking temperature to the first cooling stop temperature of 350-550 ℃;

A second cooling step of stopping cooling at the first cooling stop temperature, staying for 10 to 60 seconds in a temperature range of 350 to 550 ℃ and then cooling to a second cooling stop temperature of 100 to 300 ℃ at a second average cooling rate of 2 to 50 ℃ per second, and

A reheating-holding step of heating from the second cooling stop temperature to a reheating temperature of from a second cooling stop temperature +50 ℃ to 450 ℃ at an average heating rate of 2.0 ℃ or more, holding for 60s to 3000s,

Tc(°C)=663-1.2×exp(20/t)×Tdp ...(3)

Here, t represents the holding time(s) at the soaking temperature, tdp represents the dew point (°c).

5. A method for producing a steel sheet by hot-rolling, pickling and cold-rolling a steel slab having the composition described in claim 1 or 2 and then annealing the obtained cold-rolled steel sheet,

The annealing includes:

A cooling step of cooling the heat soaked at an average cooling rate of 2-50 ℃ per second to a cooling stop temperature of 100-300 ℃ and

A reheating-holding step of heating from the cooling stop temperature to a reheating temperature of at least +50 ℃ and at most 450 ℃ at an average heating rate of at least 2.0 ℃ per second, holding for at least 60s and at most 3000s,

Tc(°C)=663-1.2×exp(20/t)×Tdp ...(3)

6. A method for producing a member, comprising the step of forming the steel sheet according to claim 1 or 2, or joining the steel sheet to produce a member.