CN109477182B

CN109477182B - High-strength steel sheet and method for producing same

Info

Publication number: CN109477182B
Application number: CN201780046036.5A
Authority: CN
Inventors: 柿内艾丽佳; 村上俊夫; 大谷茂生; 二村裕一; 村田忠夫
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2016-08-03
Filing date: 2017-07-21
Publication date: 2021-06-18
Anticipated expiration: 2037-07-21
Also published as: JP6762798B2; JP2018021233A; EP3495523A4; WO2018025675A1; EP3495523A1; CN109477182A; US20190185958A1; EP3495523B1

Abstract

A high-strength steel sheet, which contains C: 0.15 to 0.35 mass%, total of Si and Al: 0.5 to 3.0 mass%, Mn: 1.0 to 4.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, the balance being Fe and unavoidable impurities, the ferrite fraction being 5% or less, the total fraction of tempered martensite and tempered bainite being 60% or more, the retained austenite amount being 10% or more, the average size of MA being 1.0 [ mu ] m or less, the half-value width of the concentration distribution of Mn in a carbon-thickened region equivalent to the retained austenite amount being 0.3 mass% or more, and the q-value of small-angle X-ray scattering being 1nm^－1Has a scattering intensity of 1.0cm^－1The following.

Description

High-strength steel sheet and method for producing same

Technical Field

The present disclosure relates to a high-strength steel sheet that can be used in various applications including automobile parts.

Background

Steel sheets for use in automobile parts and the like are improved in strength and impact resistance in order to achieve both weight reduction and collision safety.

For example, patent document 1 discloses a high-strength steel sheet in which a slab is heated to 1210 ℃ or higher, hot rolling conditions are controlled to generate fine TiN particles of 0.5 μm or less, and generation of AlN particles having a particle size of 1 μm or more, which are the starting point of low-temperature fracture, is suppressed, thereby attempting to improve impact resistance.

Patent document 2 discloses a high-strength steel sheet in which the C content is higher than 0.45% and 0.77% or less, the Mn content is 0.1% or more and 0.5% or less, the Si content is 0.5% or less, the addition amounts of Cr, Al, N, and O are defined, and 50% or more of the ferrite grain size is a mesh structure to be bonded to a hard material, thereby improving the collision resistance.

Patent document 3 discloses a high-strength steel sheet in which 3.5 to 10% of Mn is added to make the amount of retained austenite 10% or more and the average spacing of retained austenite 1.5 μm or less, thereby improving collision resistance.

Patent document 4 discloses a high-strength steel sheet having a tensile strength of 980 to 1180MPa and exhibiting good deep drawability.

[ Prior art documents ]

[ patent document ]

[ patent document 1 ] specification of Japanese patent No. 5240421

[ patent document 2] Japanese patent laid-open No. 2015-105384

[ patent document 3 ] Japanese patent laid-open No. 2012 and 251239

[ patent document 4 ] Japanese patent laid-open No. 2009-203548

In order to achieve further weight reduction of steel sheets used for automobile parts, it is necessary to make them thinner and to have sufficient strength and impact resistance. In short, a steel sheet having a higher tensile strength and excellent impact characteristics is required.

In addition, in various applications represented by automobile parts, not only high tensile strength and impact properties, but also excellent strength-ductility balance, high yield ratio, swelling formability, and excellent hole expansibility are required.

The tensile strength, strength-ductility balance, yield ratio, deep drawing characteristics, and hole expansibility are specifically required as follows.

The tensile strength is required to be 980MPa or more. In order to increase the stresses that can be loaded during use, a high Yield Strength (YS) is required in addition to a high Tensile Strength (TS). In addition, from the viewpoint of ensuring collision safety and the like, it is necessary to increase the yield strength of the steel sheet, and also to have a property of suppressing fracture at the time of deformation in order to stably exhibit the strength characteristics at the time of collision. Therefore, specifically, a Yield Ratio (YR) of 0.75 or more is required, and as an evaluation index in place of fracture characteristics, it is required to improve the sheet thickness reduction rate at the fracture portion in the tensile test. Further, as basic performance of the steel sheet for automobiles, joint strength of spot welded portions is also required. Specifically, the cross tensile strength of the spot-welded portion is required to be 6kN or more.

Regarding the strength-ductility balance, the product (TS × EL) of TS and total Elongation (EL) is required to be 20000 MPa% or more. In order to ensure formability during part forming, it is also required that the hole expansibility λ indicating hole expandability be 20% or more and the limit bulging height (bulging height) indicating bulging formability be 16mm or more.

However, it is difficult for the high-strength steel sheets disclosed in patent documents 1 to 4 to satisfy all of these requirements, and therefore a high-strength steel sheet capable of satisfying all of these requirements is required.

Disclosure of Invention

An embodiment of the present invention has been made to meet such a demand, and an object thereof is to provide a high-strength steel sheet having high levels of all of Tensile Strength (TS), product (TS × EL) of Yield Ratio (YR) and (TS) and total Elongation (EL), hole expansion ratio (λ), sheet thickness reduction Ratio (RA) of a fracture portion in a tensile test, ultimate bulging height, and cross tensile strength (SW cross stretch) of spot-welded portions, and a method for manufacturing the same.

Mode 1 of the present invention is a high-strength steel sheet comprising

C: 0.15 to 0.35 mass percent,

Total of Si and Al: 0.5 to 3.0 mass percent,

Mn: 1.0-4.0 mass%,

P: 0.05 mass% or less,

S: 0.01 mass% or less of a surfactant,

the balance being Fe and unavoidable impurities,

in the steel structure, the steel is provided with a plurality of steel bars,

the ferrite fraction is 5% or less,

the total fraction of tempered martensite and tempered bainite is 60% or more,

the retained austenite amount is 10% or more,

the average size of MA is 1.0 μm or less,

the half-value width of the Mn concentration distribution in the carbon-thickened region equivalent to the retained austenite amount is 0.3 mass% or more,

q value of 1nm for small angle X-ray scattering^－1Has a scattering intensity of 1.0cm^－1The following.

Embodiment 2 of the present invention is the high-strength steel sheet according to embodiment 1, wherein the C content is 0.30 mass% or less.

Embodiment 3 of the present invention is the high-strength steel sheet according to

embodiment

1 or 2, wherein the Al content is less than 0.10 mass%.

Embodiment 4 of the present invention is a method for manufacturing a high-strength steel sheet, including the steps of:

a step of preparing a rolled material, which comprises C: 0.15 to 0.35 mass%, total of Si and Al: 0.5 to 3.0 mass%, Mn: 1.0 to 4.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, the balance being Fe and unavoidable impurities;

subjecting the rolled material to Ac₁Point and 0.2 × Ac₁Point +0.8 × Ac₃Maintaining at the temperature between the points for 5 seconds or more, and heating to Ac₃Maintaining the temperature at the above temperature for 5 to 600 seconds to austenitize;

a step of cooling the austenite from a temperature of 650 ℃ or higher to a cooling stop temperature of 100 ℃ or higher and lower than 300 ℃ at an average cooling rate of 10 ℃/sec or higher;

heating the sheet from the cooling stop temperature to a reheating temperature in the range of 300 to 500 ℃ at an average heating rate of 30 ℃/sec or more;

a step of holding the steel sheet at the reheating temperature so as to satisfy a tempering parameter P defined by the formula (1) of 10000 to 14500 and a holding time of 1 to 300 seconds; and

and cooling the steel sheet from the reheating temperature to 200 ℃ at an average cooling rate of 10 ℃/sec or more after the holding.

P＝T×(20+log(t/3600))…(1)

Here, T: temperature (K), t: time (seconds).

An aspect 5 of the present invention is the manufacturing method according to the aspect 4, wherein the cooling to the cooling stop temperature includes: a step of cooling the steel sheet to a quenching start temperature which is a temperature of 650 ℃ or higher at an average cooling rate of 0.1 ℃/sec or higher and less than 10 ℃/sec; and cooling the steel sheet from the rapid cooling start temperature to the cooling stop temperature at an average cooling rate of 10 ℃/sec or more.

Mode 6 of the present invention is the production method according to

mode

4 or 5, wherein the tempering parameter is 11000 to 14000, and the holding time is 1 to 150 seconds.

According to the embodiments of the present invention, it is possible to provide a high-strength steel sheet and a manufacturing method thereof, in which the Tensile Strength (TS), the product (TS × EL) of the Yield Ratio (YR) and (TS) and the total Elongation (EL), the hole expansion ratio (λ), the sheet thickness reduction Ratio (RA) of a fracture portion in a tensile test (impact resistance), the ultimate bulging height, and the cross tensile strength (SW cross tensile) of a spot-welded portion are all at high levels.

Drawings

Fig. 1 is a diagram illustrating a method for manufacturing a high-strength steel sheet according to an embodiment of the present invention, particularly, a heat treatment.

Detailed Description

As a result of intensive studies, the inventors have found that in steel having a predetermined composition, by making the steel structure (metal structure) have a ferrite fraction: the content of the active ingredients is less than 5%,total fraction of tempered martensite and tempered bainite: more than 60%, residual γ amount: above 10%, average size of MA: 1.0 μm or less, which corresponds to the half-value width of the concentration distribution of Mn in the retained austenite region, i.e., the carbon-densified region: 0.3 mass% or more, and a q value of small-angle X-ray scattering of 1nm^－1Scattering intensity of (2): 1.0cm^－1High-strength steel sheets having high levels of Tensile Strength (TS), product (TS × EL) of the Yield Ratio (YR) and (TS) and total Elongation (EL), hole expansion ratio (λ), sheet thickness reduction Ratio (RA) of a fracture portion in a tensile test (impact resistance), ultimate bulging height, and cross tensile strength (SW cross tensile) of spot-welded portions can be obtained as follows.

Although described in detail later, the high-strength steel sheet according to the embodiment of the present invention has an Mn-enriched region in which Ac passes through an austenitizing step of heat treatment during production₁And Ac₃The two-phase coexistence region in the middle of the point, more specifically, Ac₁Point 0.2 × Ac₁Point +0.8 × Ac₃After a predetermined time at a temperature between the points, it is further treated with Ac₃The temperature above the predetermined temperature is maintained for a predetermined time. In addition, a carbon-thickened region corresponding to the retained austenite (the same amount as the retained austenite amount) is formed during the heat treatment. In this carbon densified region, both an Mn densified region and an Mn undensified region are formed. That is, in the carbon-densified region (retained austenite), there are regions containing more Mn and not so. Therefore, if the distribution of the Mn concentration is measured over the entire carbon-densified region (i.e., corresponding to the entire retained austenite), the Mn concentration has a deviation of some degree or more. Specifically, the half-value width of the Mn concentration distribution is 0.3 mass% or more.

Thus, the variation in the amount of Mn contained in the retained austenite means that retained austenite having various degrees of stability can be provided. The retained austenite having a low degree of stability, which causes the work-induced transformation with a small strain amount, and the retained austenite having a high degree of stability, which causes the work-induced transformation with a large strain amount, are mixed, and the work-induced transformation can be induced in various strain regions. As a result, the n value can be increased in a wide strain region, and the strain dispersion can be improved to realize high bulging workability.

The following shows details of the high-strength steel sheet according to the embodiment of the present invention and the manufacturing method thereof.

1. Steel structure

The steel structure of the high-strength steel sheet according to the embodiment of the present invention will be described in detail below.

In the following description of the steel structure, a mechanism that has such a structure and can improve various properties is sometimes described. These are mechanisms that the present inventors have thought based on the knowledge obtained at present, but it should be noted that these are not limitative to the scope of the technique of the present invention.

(1) Ferrite fraction: less than 5%

Ferrite generally has a problem of low strength although it is excellent in workability. As a result, when the amount of ferrite is large, the yield ratio decreases. Therefore, the ferrite fraction is 5% or less (5% by volume or less).

The ferrite fraction is preferably 3% or less, more preferably 1% or less.

The ferrite fraction can be determined by observing with an optical microscope and measuring a white region by a dot counting method. That is, by such a method, the ferrite fraction can be obtained as an area ratio (area%). Then, the value obtained by the area ratio can be used as it is as a value of the volume ratio (vol%).

(2) Total fraction of tempered martensite and tempered bainite: over 60 percent

The total fraction of tempered martensite and tempered bainite is 60% or more (60% by volume or more), and high strength and high hole expansibility can be achieved at the same time. The total fraction of tempered martensite and tempered bainite is preferably 70% or more.

The amounts (total fractions) of tempered martensite and tempered bainite can be determined by SEM observation of a cross section subjected to nital corrosion, measuring the fraction MA (i.e., the total of retained austenite and martensite in a quenched state), and subtracting the ferrite fraction and MA fraction from the total steel structure.

(3) Retained austenite amount: over 10 percent

In the retained austenite, a TRIP phenomenon occurs in which the retained austenite is transformed into martensite by work-induced transformation in a working such as a press working, and a large elongation can be obtained. In addition, the formed martensite has high hardness. Therefore, an excellent strength-ductility balance can be obtained. When the retained austenite content is 10% or more (10% by volume or more), an excellent strength-ductility balance with TS × EL of 20000 MPa% or more can be achieved.

The retained austenite amount is preferably 15% or more.

In the high-strength steel sheet according to the embodiment of the present invention, most of the retained austenite exists in the form of MA. MA is an abbreviation of martentite-austenit constancy, and is a complex structure of martensite and austenite (complex structure).

The retained austenite amount can be obtained by calculating the diffraction intensity ratio of ferrite (including tempered martensite and untempered martensite in X-ray diffraction) to austenite by X-ray diffraction. As the X-ray source, Co-K alpha rays can be used.

(4) Average size of MA: 1.0 μm or less

MA is a hard phase which, when deformed, acts as a pore forming point in the vicinity of the matrix/hard phase interface. The larger MA size causes strain concentration toward the matrix/hard phase interface, and thus tends to cause failure starting from pores formed in the neighborhood of the matrix/hard phase interface.

Therefore, the pore expansion ratio λ can be improved by making the MA size, particularly the average MA size, as fine as 1.0 μm or less to suppress the fracture.

The average size of MA is preferably 0.8 μm or less.

The average size of MA can be determined by observing a cross section etched by the nital etching solution with an SEM at 3000 times or more of 3 visual fields or more, drawing a total of 200 μm or more straight lines at arbitrary positions in the photograph, measuring the length of a section where the straight lines intersect MA, and calculating the average value of the length of the section.

(5) The half-value width of the Mn concentration distribution in the carbon-thickened region equivalent to the retained austenite amount is 0.3 mass% or more

As described above, most of the retained austenite exists in the form of MA, and it is difficult to identify only the retained austenite by an optical microscope or SEM. The retained austenite has a larger solid solution limit of carbon than ferrite, and therefore, by performing heat treatment described later, carbon is thickened in the retained austenite. Therefore, the carbon thickening region can be determined as retained austenite by mapping carbon elements using epma (electron Probe Micro analyzer) and determining the measuring points equivalent to the retained austenite amount obtained from the measuring points having a high carbon concentration by the X-ray diffraction as the carbon thickening region. That is, for example, when the retained austenite amount is 15 vol%, 15% is selected from the measurement points for measuring the carbon amount by elemental mapping, and these measurement points (carbon thickened regions) having a high carbon concentration can be determined as retained austenite.

Therefore, the "carbon-thickened region equal to the retained austenite amount" means a region corresponding to (corresponding to) the retained austenite.

Then, the concentration distribution of Mn in the carbon-thickened region equivalent to the retained austenite amount, particularly the half width of the Mn concentration distribution, can also be measured using EPMA. The half-value width can be obtained by patterning the distribution of the Mn amount at the measurement point considered as the carbon densified region.

The larger the half width of the Mn concentration distribution, the larger the variation of the Mn concentration in the retained austenite (the wider the range of the Mn concentration distribution). In the high-strength steel sheet according to the embodiment of the present invention, the half-value width of the concentration distribution of Mn is 0.3 mass% or more, preferably 0.5 mass% or more, more preferably 0.6 mass% or more, and still more preferably 0.75 mass% or more.

In this way, the amount of Mn contained in the retained austenite (carbon-thickened region) is fluctuated, and retained austenite having a wide stability range from retained austenite having a low stability to retained austenite having a high stability can be formed. The retained austenite having a low stability undergoes work-induced transformation to form martensite with a small strain amount. The retained austenite having a high degree of stability does not cause work-induced transformation by a small amount of strain, and only by applying a large amount of strain, work-induced transformation is caused to become martensite. Therefore, if retained austenite having a wide range of stability exists, the work-induced transformation continues to occur when the amount of strain is small from the start of working and when the amount of strain is large as the working progresses. As a result, the n value can be increased over a wide strain range, and the strain dispersibility can be improved to realize high bulging workability.

(6) Q value of 1nm for small angle X-ray scattering^－1Has a scattering intensity of 1.0cm^－1The following

The small-angle X-ray scattering is a method in which a steel sheet is irradiated with X-rays, and the size distribution of fine particles (for example, cementite particles dispersed in the steel sheet) contained in the steel sheet can be obtained by measuring the scattering of the X-rays transmitted through the steel sheet. In the steel sheet according to the embodiment of the present invention, the size distribution of cementite particles as fine particles dispersed in tempered martensite can be determined by small-angle X-ray scattering. Specifically, in the small-angle X-ray scattering, the size and fraction of the cementite particles can be analyzed by using the q value and the scattering intensity.

The q value is an index of the size of particles (e.g., cementite particles) in the steel sheet. The so-called "q value is 1nm^－1", corresponds to cementite particles having a particle size of about 1 nm. The scattering intensity is an index of the volume fraction of particles (e.g., cementite particles) in the steel sheet. The stronger the scattering intensity, the larger the volume fraction of cementite.

The scattering intensity at a certain q-value is semi-quantitatively represented by the volume fraction of cementite particles having a size corresponding to the q-value. For example, q is 1nm^－1The scattering intensity of (2) represents a semi-quantitative volume fraction of fine cementite particles of about 1 nm.

I.e. q is 1nm^－1The scattering intensity of (2) is large, and indicates that the volume fraction of fine cementite particles having a particle size of about 1nm is large. At "q value of 1nm^－1Has a scattering intensity of 1.0cm^－1The steel sheet "means fine carburization of about 1nm existing in the steel sheetThe volume fraction of the bulk particles was set to a predetermined value (corresponding to a scattering intensity of 1.0 cm)^－1Value of (d) below. As explained below, the "q value is 1nm^－1Has a scattering intensity of 1.0cm^－1The steel sheet "below is considered to have excellent collision resistance properties because the volume fraction of cementite of about 1nm is suppressed to be low.

In the high ductility steel containing residual γ, a state in which carbon is concentrated in the residual austenite is preferable, and ideally no cementite is present. Fine cementite having a particle size of about 1nm dispersed in the steel hinders the movement of dislocations and lowers the deformability of the steel. Therefore, in a steel material having a large volume fraction of cementite with a grain size of about 1nm, breakage during deformation is promoted, and collision resistance is lowered.

The steel sheet according to the embodiment of the present invention is obtained by suppressing the volume fraction of fine cementite, more specifically, by setting the q value to 1nm^－1Has a scattering intensity of 1cm^－1In this way, the formation of fine carbides in the laths of tempered martensite is reduced, and the deformability of the martensite is improved. This suppresses the steel sheet from being broken at the time of collision, and improves the collision resistance of the steel sheet.

The measurement of small-angle X-ray scattering was carried out using a Nano-viewer manufactured by RIGAKU, Mo tube ball. A disk-shaped sample having a diameter of 3mm was cut out from a steel sheet, and a sample having a thickness of 20 μm was cut out from the vicinity of the sheet thickness of 1/4. The q value is 0.1-10 nm^－1The data of (1). Wherein the value for q is 1nm^－1The absolute intensity is obtained.

(7) Other steel structures:

in the present specification, the steel structure other than ferrite, tempered martensite, tempered bainite, retained austenite and cementite is not particularly specified. However, in addition to the steel structure such as ferrite, pearlite, untempered bainite, untempered martensite, and the like exist. If the steel structure of ferrite or the like satisfies the above-described structure conditions, the effects of the present invention can be exhibited even if pearlite or the like is present in the steel.

2. Composition of

The composition of the high-strength steel sheet according to the embodiment of the present invention will be described below. The description will be made mainly for the basic elements C, Si, Al, Mn, P and S.

The unit% in the component composition means all of mass%.

(1)C：0.15～0.35％

C increases the amount of the desired structure, particularly the amount of the residual γ, and is an element necessary for ensuring the properties such as the high strength-ductility balance (TS × EL balance), and in order to effectively exhibit such effects, it is necessary to add 0.15% or more. However, above 0.35% is not suitable for welding. Preferably 0.18% or more, more preferably 0.20% or more. Further, it is preferably 0.30% or less. If the C content is 0.25% or less, welding can be performed more easily.

(2) Total of Si and Al: 0.5 to 3.0 percent

Si and Al inhibit the precipitation of cementite, respectively, and have the effect of retaining residual austenite. In order to effectively exhibit such an effect, it is necessary to add 0.5% or more of Si and Al in total. However, if the total of Si and Al is more than 3.0%, the deformability of the steel is reduced, and TS × EL and the bulging height are reduced. Preferably 0.7% or more, more preferably 1.0% or more. Further, it is preferably 2.5% or less.

Further, Al may be added in an amount of less than 0.10 mass% to the extent that it functions as a deoxidizing element, or may be added in an amount of more than 0.7 mass% for the purpose of, for example, suppressing the formation of cementite and increasing the retained austenite amount.

(3)Mn：1.0～4.0％

Mn suppresses the formation of ferrite. Further, Mn forms Mn-thickened regions and forms retained austenite having different degrees of stability, and is an essential element for improving the bulging workability. In order to effectively exhibit such an effect, it is necessary to add 1.0% or more. However, if it exceeds 4.0%, the temperature range for heating the two-phase region is difficult to control to be narrow, and the temperature becomes too low, so that Ac is used even when it is used₁Point 0.2 × Ac₁Point +0.8 × Ac₃The temperature between the points is maintained for a specified time, and the phase change is not causedIn some cases, the Mn-thickened region cannot be formed. Preferably 1.5% or more, more preferably 2.0% or more. Further, it is preferably 3.5% or less.

(4) P: less than 0.05%

P is inevitably present as an impurity element. If more than 0.05% of P is present, EL and λ deteriorate. Therefore, the content of P is 0.05% or less (including 0%). Preferably 0.03% or less (including 0%).

(5) S: less than 0.01%

S is inevitably present as an impurity element. If more than 0.01% of S is present, sulfide-based inclusions such as MnS are formed, which become starting points of cracks and decrease λ. Therefore, the content of S is 0.01% or less (including 0%). Preferably 0.005% or less (including 0%).

(6) Balance of

In a preferred embodiment, the balance is iron and unavoidable impurities. As inevitable impurities, trace elements (for example, As, Sb, Sn, etc.) introduced by the conditions of raw materials, manufacturing facilities, etc. are allowed to be mixed. In addition, for example, as P and S, the smaller the content is, the more preferable the content is, and therefore, the impurities are inevitable, but in this composition range, there are elements separately specified as described above. Therefore, in the present specification, the term "inevitable impurities" constituting the balance is a concept excluding elements whose composition ranges are separately defined.

However, the present invention is not limited to this embodiment. Any other element may be further contained as long as the characteristics of the high-strength steel sheet according to the embodiment of the present invention can be maintained.

3. Characteristics of

As described above, in the high-strength steel sheet according to the embodiment of the present invention, TS, YR, TS × EL, λ, collision resistance, ultimate bulging height, and SW cross stretch are all at high levels. Hereinafter, these characteristics of the high-strength steel sheet according to the embodiment of the present invention will be described in detail.

(1) Tensile Strength (TS)

Has a TS of 980MPa or more. Preferably, the TS is 1180MPa or more. This is because, if TS is less than 980MPa, excellent fracture characteristics can be obtained more reliably, but the load received at the time of collision is low, which is not preferable.

(2) Yield Ratio (YR)

Has a yield ratio of 0.75 or more. This can achieve high yield strength in combination with the high tensile strength described above, and a final product obtained by processing such as deep drawing can be used under high stress. Preferably having a yield ratio of 0.80 or more.

(3) Product of TS and Total Elongation (EL) (TS × EL)

TS × EL is 20000 MPa% or more. TS × EL of 20000 MPa% or more, and a high level of strength-ductility balance can be obtained, which has both high strength and high ductility. Preferably, TS × EL is 23000 MPa% or more.

(4) Bulging formability (ultimate bulging height)

The ultimate bulging height is an index for evaluating bulging formability. The ultimate bulging height, in the load-stroke diagram, is the punching stroke at which a fracture occurs with a sharply decreasing load.

More specifically, a test piece having a diameter of 120mm, a die having a diameter of 53.6mm and a shoulder radius of 8mm, and a ball head punch having a diameter of 50mm were used to sandwich a polyethylene sheet for lubrication between the punch and a steel plate, and the bulging was performed with a blank holder pressure of 1000kgf, and the height at break (punching stroke) was measured to determine the limit bulging height.

The high-strength steel sheet according to the embodiment of the present invention has a limit bulging height of 20mm or more, preferably 21mm or more.

(5) Hole expansion ratio (lambda)

The hole expansion ratio λ was determined in accordance with the japan iron and steel union specification JFS T1001. Digging diameter d on test piece₀(d₀10mm) was pressed into the hole by a punch having a tip angle of 60 °, and the diameter d of the hole was measured at the point when the generated crack penetrated through the thickness of the test piece, and the hole expansion ratio was determined according to the following equation.

λ(％)＝{(d－d₀)/d₀}×100

The high-strength steel sheet according to the embodiment of the present invention has a hole expansion ratio λ of 20% or more, preferably 30% or more. This can provide excellent workability such as press formability.

(6) Sheet thickness reduction ratio in tensile test (R5 tensile sheet thickness reduction ratio)

A test piece having a circular-arc-shaped notch with a radius of 5mm on test piece No. 5 was used, and the test was conducted with the deformation rate of the tensile test set to 10mm/min, and the sample was fractured. Thereafter, a cross section is observed by the thickness t in the plate thickness direction of the cross section₁Divided by the original sheet thickness t₀Value of (t)₁/t₀) The thickness reduction rate is defined as the sheet thickness reduction rate.

The sheet thickness reduction rate in this test is 50% or more, preferably 52% or more, and more preferably 55% or more. Thus, the steel sheet is hard to break even if it is deformed seriously at the time of collision, and therefore, a steel sheet having excellent impact resistance characteristics can be obtained.

(7) Cross tensile strength of spot welding

The cross tensile strength of spot welding was evaluated in accordance with JIS Z3137. The spot welding conditions used were two 1.4mm steel plates stacked. With a dome radius (ドームラジアス) type electrode, spot welding was performed by increasing the current from 6kA to 12kA by 0.5kA under a pressurizing force of 4kN, and the current value (lowest current value) generated by spatter (ちり) during welding was examined. The cross joint welded by spot welding was used as a cross tensile strength measurement sample at a current lower than the minimum current value by 0.5 kA. The cross tensile strength was "good" at 6kN or more. The cross tensile strength is preferably 8kN or more, and more preferably 10kN or more.

When the cross tensile strength is 6kN or more, a part having high joining strength at the time of welding can be obtained when an automobile part or the like is produced from a steel sheet.

4. Manufacturing method

Next, a method for manufacturing a high-strength steel sheet according to an embodiment of the present invention will be described.

The inventors have found that a rolled material having a predetermined composition has the desired steel structure by performing heat treatment (multi-stage austempering treatment) described in detail below, and as a result, a high-strength steel sheet having the desired properties is obtained.

The details thereof will be described below.

The rolled material subjected to the heat treatment is usually produced by hot rolling and then cold rolling. However, the present invention is not limited thereto, and can be produced by performing either hot rolling or cold rolling. In addition, conditions for hot rolling and cold rolling are not particularly limited.

(1) Austenitizing

In the austenitizing step, [2] in FIG. 1]A rolled material was treated with a reducing agent of Ac₁And Ac₃The two-phase coexistence region in the middle of the point, more specifically, Ac₁Point and 0.2 × Ac₁Point +0.8 × Ac₃Temperature T between points₁(Ac₁≤T₁≤0.2×Ac₁Point +0.8 × Ac₃) After holding for more than 5 seconds, as shown in [3 ] of FIG. 1]、[4]Shown, cut off Ac₃Temperature T above the point₂(Ac₃≤T₂) To heat up the temperature T₂Keeping the temperature for 5 to 600 seconds to make the steel into austenite.

Heating to a temperature T₁And keeping for more than 5 seconds. The holding time is preferably 900 seconds or less. Also, the holding temperature T₁E.g. [2] of FIG. 1]May also be present in Ac₁Point and 0.2 × Ac₁Point +0.8 × Ac₃Held at a fixed temperature between the points, e.g. also at Ac₁Point and 0.2 × Ac₁Point +0.8 × Ac₃Heating slowly between points, etc., to make it at Ac₁Point and 0.2 × Ac₁Point +0.8 × Ac₃The points vary from point to point. In this way, by holding the ferrite and austenite in a relatively low temperature region in the two-phase coexisting region, more Mn is distributed to the austenite side among the coexisting ferrite and austenite, and thus a Mn-thickened region can be obtained. Then, the Mn concentration of the austenite formed in the Mn-thickened region and remaining as retained austenite even after the heat treatment is high, so that variation in the Mn concentration in the carbon-thickened region can be increased, and high swelling formability can be achieved.

If the temperature T is₁Below Ac of₁In this case, the amount of Mn-thickened austenite is small, and variation in Mn concentration in the retained austenite (carbon-thickened region) is small, so that sufficient swelling formability cannot be obtained.

If the temperature T is₁Higher than 0.2 × Ac₁Point +0.8 × Ac₃In this case, the Mn concentration of austenite becomes low, and the variation in Mn concentration in the retained austenite (carbon-thickened region) becomes small, so that sufficient swelling formability cannot be obtained.

If the temperature T is₁If the retention time is shorter than 5 seconds, the diffusion time of Mn becomes short, the Mn thickening into austenite becomes insufficient, the variation of Mn in the retained austenite (carbon thickened region) becomes small, and sufficient swelling formability cannot be obtained.

Preferred temperature T₁The holding time is preferably 900 seconds or less from the viewpoint of productivity.

Preferred temperature T₁At 0.9 × Ac₁Point +0.1 × Ac₃Point and 0.3 × Ac₁Point +0.7 × Ac₃Between points, temperature T₁The following holding time is 10 seconds to 800 seconds. More preferably the temperature T₁At 0.8 × Ac₁Point +0.2 × Ac₃Point and 0.4 × Ac₁Point +0.6 × Ac₃Between points, temperature T₁The holding time is 30 seconds to 600 seconds.

In FIG. 1, the number is [1 ]]Shown to temperature T₁The heating rate of (2) is preferably 5 to 20 ℃/sec.

Secondly, as shown in [3 ] of FIG. 1]Of [4 ]]Shown that the temperature is raised to Ac₃Temperature T above the point₂(Ac₃≤T₂) And at a temperature T₂Retained to be austenitized. Temperature T₂The holding time is 5-600 seconds.

By heating to Ac₃Temperature T above the point₂After heating to a temperature T₁In this case, the ferrite portion is also austenite. The portion of Mn that re-transforms to austenite does not thicken. Therefore, the austenite contains the above-described Mn-thickened region and the Mn-uncritized region, and the retained austenite can be enlarged in the high-strength steel sheet after heat treatmentThe variation in the Mn concentration (carbon-thickened region) enables high swelling formability to be achieved.

If the temperature T is₂Below Ac of₃Point, or temperature T₂When the holding time is shorter than 5 seconds, the ferrite fraction of the obtained high-strength steel sheet is higher than 5%, and YR decreases.

If the temperature T is₂If the Mn concentration is too high, Mn in the Mn-enriched region formed first diffuses, and the variation in Mn concentration may become too small. Thus, the temperature T₂Preferably Ac₃Point +50 ℃ or lower.

If the temperature T is₂When the retention time is longer than 600 seconds, the Mn concentration in the Mn-thickened region is reduced by diffusion, the variation in Mn concentration in the retained austenite is reduced, and the swelling formability is reduced.

Preferred temperature T₂Is Ac₃Point +10 ℃ or higher, temperature T₂The retention time is 10-450 seconds. More preferably the temperature T₂Is Ac₃Point +20 ℃ or higher, temperature T₂The retention time is 20-300 seconds.

[3 ] of FIG. 1]Shown slave temperature T₁To temperature T₂The heating of (3) is preferably performed at a heating rate of 0.1 ℃/sec or more and less than 10 ℃/sec.

Also, with respect to Ac₁Point and Ac₃The point can be obtained by measurement, but it is preferable to calculate the point by a commonly known formula using the composition.

For example, Ac can be calculated by using the following formulae (1) and (2)₁Point and Ac₃(see, for example, Lesley iron and Steel materials science, Maruzen (1985)).

Ac₁Point (. degree. C.) 723+29.1 × [ Si ]]－10.7×[Mn]+16.9×[Cr]－16.9×[Ni] (1)

Ac₃Point (. degree. C.) 910-]^1/2+44.7×[Si]－30×[Mn]+700×[P]+400×[Al]+400×[Ti]+104×[V]－11×[Cr]+31.5×[Mo]－20×[Cu]－15.2×[Ni] (2)

Here, [ ] denotes the content shown in mass% of the elements described therein.

(2) Cooling to a cooling stop temperature of 100-300 deg.C

After the above-mentioned austenitization, as shown in [6 ] of FIG. 1]The cooling is carried out at an average cooling rate of 10 ℃/sec or more to a cooling stop temperature T of 100 ℃ or higher and lower than 300 DEG C₃Until now. The cooling stop temperature is controlled within a temperature range of 100 ℃ or more and less than 300 ℃, thereby adjusting the amount of austenite remaining without being transformed into martensite and controlling the final retained austenite amount.

In the cooling, the cooling is carried out at an average cooling rate of 10 ℃/sec or more at least between 650 ℃ and 300 ℃. The reason why the average cooling rate is set to 10 ℃/sec or more is to suppress formation of ferrite during cooling and to form a microstructure mainly composed of fine martensite.

As a preferable example of such cooling, there can be mentioned [5 ] in FIG. 1]Shown as a rapid cooling onset temperature T of 650 ℃ or higher₄Cooling (slow cooling) is performed at a relatively low average cooling rate of 0.1 ℃/sec or more and less than 10 ℃/sec as shown in [6 ] of FIG. 1]Shown, starting from the quenching temperature T₄Cooling stop temperature T below 300 DEG C₃The cooling (rapid cooling) is performed at an average cooling rate of 10 ℃/sec or more. Also, by making the quenching start temperature T₄Is 650 ℃ or higher, and can suppress ferrite formation during cooling (slow cooling).

When the cooling rate is slower than 10 ℃/sec, ferrite is formed and YR decreases. Further, MA becomes coarse, and thus the hole expansion ratio is decreased.

If the cooling stop temperature T₃Below 100 c, the retained austenite amount is insufficient. As a result, although TS becomes high, EL decreases, and TS × EL balance is insufficient.

If the cooling stop temperature T₃At 300 ℃ or higher, coarse austenite increases, remains even after subsequent cooling, and finally the MA size becomes coarse, and the hole expansion ratio λ decreases.

The cooling rate is preferably 15 ℃ C/C or higher, more preferably 20 ℃ C/s or higher. Preferred cooling stop temperature T₃Is 120 ℃ or higher and 280 ℃ or lower, more preferably 140 ℃ or higher and 260 ℃ or lower.

As in fig. 1[7]As shown, the temperature may be set at the cooling stop temperature T₃And (4) keeping. The preferable holding time for holding is 1 to 150 seconds. Even if the holding time is longer than 150 seconds, the properties of the obtained steel sheet are not improved so much, and the productivity of the steel sheet is lowered, and therefore, it is preferably 150 seconds or less.

(3) Then heating to 300-500 deg.C

Such as [8] of FIG. 1]Shown from the above-mentioned cooling stop temperature T₃Heating the mixture to a reheating temperature T in the range of 300-500 ℃ at a reheating speed of more than 30 ℃/s₅Until now. By rapid heating, the residence time in the temperature range in which precipitation and growth of carbide are promoted can be shortened, and formation of fine carbide can be suppressed. The reheating rate is preferably 60 ℃/s or more, more preferably 70 ℃/s.

Such rapid heating can be achieved by, for example, high-frequency heating or electric heating.

Reaches the reheating temperature T₅Then, as in [9 ] of FIG. 1]Shown at this temperature T₅And (4) maintaining. In this case, the tempering parameter P represented by the following formula (1) is preferably 10000 to 14500 inclusive, and the holding time is preferably 1 to 150 seconds. The tempering parameter P of the steel sheet of the present embodiment is represented by the following formula (1).

P＝T(K)×(20+log(t/3600)…·(1)

Here, T is the tempering temperature (K) and T is the holding time (sec).

During reheating, carbon supersaturated and solid-dissolved in martensite is redistributed. Specifically, two phenomena occur, namely, diffusion of carbon from martensite to austenite; and carbide (cementite) precipitation in the martensitic lath. Among these two phenomena, when the holding is performed at a low temperature for a long time, carbide precipitation is likely to occur. In addition, even when the steel is held at a high temperature, if the heating rate is slow or the holding time is too long, carbide precipitates. On the other hand, since the diffusion of carbon from martensite to austenite strongly depends on the diffusion rate, the carbon can be sufficiently diffused by a short treatment at a high temperature.

The particles of cementite present in martensite are likely to become starting points of collision fracture, and cause a reduction in collision resistance. Therefore, in reheating, it is desired to perform reheating treatment for suppressing precipitation of carbide (cementite) in the martensite lath and promoting diffusion of carbon from martensite to austenite. Therefore, it is effective to perform rapid heating and heat treatment at a high temperature for a short time.

However, in order to obtain a desired tensile strength by causing sufficient carbon diffusion, it is necessary to control the tempering parameter P, which is a combination factor of temperature and time, within a certain range.

If the tempering parameter P is less than 10000, the diffusion of carbon from martensite to austenite does not occur sufficiently, austenite becomes unstable, and the retained austenite amount cannot be secured, so the TS × EL balance is insufficient. If the tempering parameter P is more than 14500, the formation of carbide cannot be prevented even by a short treatment, the retained austenite amount cannot be secured, and the TS × EL balance is deteriorated. Even if the tempering parameters are appropriate, if the heating rate is too low and the heating time is too long, carbides are formed in the martensite lath, and the crack growth during the impact deformation is likely to occur, thereby deteriorating the impact resistance. The amount of carbides in the martensite laths can be determined from the scattering intensity of the small-angle X-ray scattering.

If reheating temperature T₅Below 300 ℃, the diffusion of carbon is insufficient, a sufficient retained austenite amount cannot be obtained, and TS × EL decreases. If reheating temperature T₅If the temperature is higher than 500 ℃, the retained austenite is decomposed into cementite and ferrite, and the retained austenite is insufficient and the properties cannot be ensured.

If the holding is not performed or the holding time is shorter than 1 second, the diffusion of carbon may be insufficient as well. Therefore, the reheating temperature T is preferably set to₅The holding is performed for 1 second or more. If the holding time is longer than 150 seconds, carbon may similarly be precipitated as cementite. Therefore, the holding time is preferably 150 seconds or less.

Preferred reheating temperature T₅320 to 480 ℃, and more preferably a reheating temperature T₅Is 340 to 460 ℃.

The preferable tempering parameter P is 10500-14500, and the preferable holding time is 1-150 seconds. The tempering parameter P is more preferably 11000-14000, and the holding time is preferably 1-100 seconds, more preferably 1-60 seconds.

After reheating, as shown in [10] of FIG. 1, the steel sheet may be cooled to a temperature of 200 ℃ or lower, for example, room temperature. The preferred average cooling rate for cooling to 200 ℃ or lower is 10 ℃/sec.

The high-strength steel sheet according to the embodiment of the present invention can be obtained by the heat treatment described above.

In the case of those skilled in the art who have come into contact with the above-described method for producing a high-strength steel sheet according to the embodiment of the present invention, there is a possibility that the high-strength steel sheet according to the embodiment of the present invention can be obtained by a trial and error method using a production method different from the above-described production method.

[ examples ] A method for producing a compound

1. Sample preparation

After producing a cast material having a chemical composition shown in table 1 by vacuum melting, the cast material was hot forged into a steel sheet having a thickness of 30mm, and then hot rolled. In addition, Ac calculated from the composition is also described in Table 1₃And (4) point.

The hot rolling conditions do not substantially affect the final structure and properties of the steel sheet, and the steel sheet is heated to 1200 ℃ and then rolled in multiple stages to a thickness of 2.5 mm. At this time, the finishing temperature of hot rolling was 880 ℃. Thereafter, the steel sheet was cooled to 600 ℃ at 30 ℃/sec, the cooling was stopped, the steel sheet was inserted into a furnace heated to 600 ℃, and then the steel sheet was held for 30 minutes, and thereafter furnace-cooled to obtain a hot-rolled steel sheet.

The hot-rolled steel sheet was pickled to remove scale on the surface, and then cold-rolled to 1.4 mm. The cold-rolled sheet was subjected to heat treatment to obtain a sample. The heat treatment conditions are shown in table 2. In table 2, for example, the numbers shown in [ ] such as [2] correspond to the same procedures as those shown in fig. 1.

In Table 2, in sample No.1, austenitizing was not divided into temperatures T₁And temperature T₂Two stages, only at equivalent temperatureT₂Ac of (a)₃The temperature above the point is maintained.

Sample No.9 is a sample held at the reheating temperature after cooling to the reheating temperature (a sample obtained by skipping the steps corresponding to [7] to [8] in fig. 1), instead of cooling to a cooling stop temperature between 100 ℃ and less than 300 ℃.

Samples 15 and 31 to 36 were heated to a temperature T₂And a rapid cooling onset temperature T₄The same sample. That is, after austenitizing, the steel sheet is cooled to a cooling stop temperature T in one stage₃The sample thus obtained.

Reheating corresponding to [8] was performed by an electric heating method.

In tables 1 to 4, the numerical values with asterisks (h) are outside the range of the embodiments of the present invention.

[ TABLE 1 ]

[ TABLE 2]

2. Steel structure

For each sample, the ferrite fraction, the total fraction of tempered martensite and tempered bainite (described as "tempered M/B" in table 3), the retained austenite amount (retained γ amount), the average size of MA, the half width of the Mn concentration distribution in the carbon densified region, and the q value of small-angle X-ray scattering of 1nm were obtained by the above-described method^－1The scattering intensity of (2). For the measurement of the retained austenite amount, a two-dimensional micro-domain X-ray diffraction apparatus (RINT-RAPIDII) manufactured by リガク was used. The results obtained are shown in table 3.

In the present example, the steel structure (residual structure) other than the steel structure shown in table 3, except for the sample No.9, was martensite which was not tempered, and the sample No.9 was bainite which was not tempered.

[ TABLE 3 ]

3. Mechanical characteristics

The obtained samples were measured for YS, TS, and EL using a tensile tester, and YR and TS × EL were calculated. Further, the hole expansion ratio λ, the limit bulging height (bulging height), the cross tensile strength of the spot welded portion (SW cross tensile), and the R5 tensile sheet thickness reduction ratio were obtained by the above-described methods. The results obtained are shown in table 4.

[ TABLE 4 ]

The results of Table 4 were examined. Sample Nos. 12 to 15, 18, 21 and 29 to 36 are examples satisfying all the requirements (composition, production conditions and steel structure) specified in the embodiment of the present invention. All of these samples achieved a Tensile Strength (TS) of 980MPa or more, a Yield Ratio (YR) of 0.75 or more, TS × EL of 20000 MPa% or more, a hole expansion ratio (λ) of 20% or more, a SW cross stretch of 16mm or more and 6kN or more in terms of ultimate bulge height, and a R5 tensile sheet thickness reduction Ratio (RA) of 50% or more.

In contrast, in sample No.1, austenitizing was not divided into temperatures T₁And temperature T₂Two stages, only at a temperature T₂Ac of (a)₃Since the temperature is maintained at the point or higher, the value of the half width of the Mn concentration distribution in the carbon densified region is small, and the limit swell height is low. Furthermore, because [7]]The retention time was as long as 300 seconds, so that carbide (cementite) was precipitated. In addition, since the scattering intensity of small-angle X-ray scattering is large, the volume fraction of cementite of about 1nm can be said to be large. As a result, the collision resistance (sheet thickness reduction rate) is lowered.

In sample No.2, since the temperature T is maintained₁Since the Mn concentration distribution in the carbon-thickened region has a small value of half-value width, the ultimate bulge height becomes low.

In sample No.3, since the temperature T was maintained₁Since the Mn concentration distribution in the carbon densified region has a high value, the half-value width of the Mn concentration distribution is small, and the limit swell height is low. Furthermore [7]The retention time was as long as 300 seconds, and therefore carbide (cementite) was precipitated. In addition, since the scattering intensity of small-angle X-ray scattering is large, the volume fraction of cementite of about 1nm can be said to be large. As a result, the collision resistance (sheet thickness reduction rate) is lowered.

In samples No.4 and 5, since the heating was carried out to the heating temperature T₁And after holding, select and T₁The same temperature is taken as the heating temperature T₂Therefore, austenitization cannot be performed at a sufficiently high temperature. Therefore, the amount of ferrite is large, the total fraction of tempered martensite and tempered bainite is low, and the value of the half width of the Mn concentration distribution in the carbon-densified region is small. As a result, the tensile strength, yield ratio and ultimate swell height become low.

Sample No.6, heating temperature T₂The amount of ferrite increases, and as a result, the yield ratio decreases.

In sample No.7, the cooling stop temperature T was set₃Since the total fraction of tempered martensite and tempered bainite is high, the average size of MA is large. As a result, the hole expansion ratio becomes low.

In sample No.8, the heating temperature T was set₁Since the retention time of the Mn layer is short, the value of the half width of the Mn concentration distribution in the carbon densified region is small, and as a result, the limit swell height becomes low.

Sample No.9, heating temperature T₂Long holding time, and cooling stop temperature T₃High. Therefore, the total fraction of tempered martensite and tempered bainite is 0%, the average size of MA is large, and the value of the half-value width of the concentration distribution of Mn is small. As a result, the tensile strength, hole expansibility, and ultimate swell height are low. Furthermore, because the temperature is maintained for 300 seconds ([9 ]]Retention time) so that the formation of carbides is also low. These results are a reduction in the hole expansion ratio λ.

In sample No.10, the cooling stop temperature T₃Low retained austenite content, and TS × EL value and ultimate swell as a resultThe height of the figure becomes low.

In sample No.11, the reheating rate of [8] was as slow as 30 ℃/sec, and therefore carbide (cementite) was precipitated. In addition, since the scattering intensity of small-angle X-ray scattering is large, the volume fraction of cementite of about 1nm can be said to be large. As a result, the collision resistance (sheet thickness reduction rate) is lowered.

In sample No.16, the quenching initiation temperature T was set₄Since the amount of ferrite is low, the total fraction of tempered martensite and tempered bainite is low. As a result, the tensile strength and yield ratio are low. Furthermore, because [9 ]]The retention time was as long as 300 seconds, so that carbide (cementite) was precipitated. In addition, since the scattering intensity of small-angle X-ray scattering is large, the volume fraction of cementite of about 1nm can be said to be large. As a result, the collision resistance (sheet thickness reduction rate) is lowered.

In sample No.17, the reheating rate of [8] was as slow as 15 ℃/sec, and carbide (cementite) was precipitated. In addition, since the scattering intensity of small-angle X-ray scattering is large, the volume fraction of cementite of about 1nm can be said to be large. As a result, the collision resistance (sheet thickness reduction rate) is lowered.

Sample No.19, reheating temperature T₅High, so the parameter is up to 14604, and the retained austenite amount is small. As a result, the value of TS × EL and the limit bulging height become low. In addition, since the scattering intensity of small-angle X-ray scattering is large, the volume fraction of cementite of about 1nm can be said to be large. As a result, the collision resistance (sheet thickness reduction rate) is lowered.

Sample No.20, reheating temperature T₅Low, so the parameter is as low as 9280 and the retained austenite amount becomes small. As a result, the value of TS × EL and the limit bulging height become low.

Sample No.22 had a low C content and a small retained austenite content, and as a result, the value of TS × EL and the ultimate bulging height became low.

Sample No.23 had a large Mn content and a small retained austenite content, and as a result, the value of TS × EL and the ultimate bulging height were low.

Sample No.24 had a small Mn content and a large ferrite content, and the total amount of tempered martensite and tempered bainite was insufficient. As a result, the tensile strength and yield ratio become low.

In sample No.25, the Si + Al content was low, the total fraction of tempered martensite and tempered bainite and the retained austenite content were low, and the MA average size was large. As a result, the value of TS × EL, the hole expansion ratio and the limit bulging height become low.

Sample No.26 had a large amount of C, resulting in a low SW cross tensile strength.

In sample No.27, the Si + Al content was large, and as a result, the TS × EL value and the limit bulging height were low.

In sample No.28, since the temperature T was maintained₁Since the Mn concentration distribution in the carbon densified region has a high half width, the limit swell height becomes low.

4. Summary of the invention

As described above, it was confirmed that the steel sheet satisfying the composition and steel structure defined in the embodiment of the present invention has high Tensile Strength (TS), product (TS × EL) of the Yield Ratio (YR) and (TS) and total Elongation (EL), hole expansion ratio (λ), plate thickness reduction Ratio (RA) of the fracture portion in the tensile test, ultimate bulging height, and cross tensile strength of the spot-welded portion.

Further, it was confirmed that according to the manufacturing method of the embodiment of the present invention, a steel sheet satisfying the composition and steel structure defined in the embodiment of the present invention can be manufactured.

This application is accompanied with Japanese patent application No. 2016, 8, 3, 2016, and priority claim of application No. 2016-. Japanese application No. 2016-153110 is hereby incorporated by reference.

Claims

1. A high-strength steel sheet comprising

C: 0.15 to 0.35 mass percent,

Total of Si and Al: 0.5 to 3.0 mass percent,

Mn: 1.0-4.0 mass%,

P: 0.05 mass% or less,

S: 0.01 mass% or less of a surfactant,

the balance being Fe and unavoidable impurities,

in the steel structure, the steel is provided with a plurality of steel bars,

the ferrite fraction is 5 vol% or less,

the total fraction of tempered martensite and tempered bainite is 60 vol% or more,

the retained austenite amount is 10 vol% or more,

the average size of MA is 1.0 μm or less,

2. The high-strength steel sheet according to claim 1, wherein the amount of C is 0.30% by mass or less.

3. The high-strength steel sheet according to claim 1 or 2, wherein the amount of Al is less than 0.10 mass%.

4. A method for manufacturing a high-strength steel sheet, comprising the steps of:

a step of preparing a rolled material containing C: 0.15 to 0.35 mass%, total of Si and Al: 0.5 to 3.0 mass%, Mn: 1.0 to 4.0 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, the balance being Fe and unavoidable impurities;

a step of cooling the austenite to a cooling stop temperature of 100 ℃ or higher and lower than 300 ℃ at an average cooling rate of 10 ℃/sec or higher from a temperature of 650 ℃ or higher;

heating the steel sheet from the cooling stop temperature to a reheating temperature in the range of 300 to 500 ℃ at an average heating rate of 30 ℃/sec or more;

a step of maintaining the tempering temperature T at 10000 to 14500 for 1 to 150 seconds while satisfying a tempering parameter P defined by formula (1); and

a step of cooling the steel sheet to 200 ℃ at an average cooling rate of 10 ℃/sec or more from the reheating temperature after the holding,

wherein the cooling up to the cooling stop temperature includes: a step of cooling the steel sheet to a quenching start temperature of 650 ℃ or higher at an average cooling rate of 0.1 ℃/sec or higher and less than 10 ℃/sec; and a step of cooling the steel sheet from the rapid cooling start temperature to the cooling stop temperature at an average cooling rate of 10 ℃/sec or more,

P＝T×(20+log(t/3600))…(1)

here, T: reheating temperature in K; t: hold time in seconds.

5. The method for manufacturing a high-strength steel sheet according to claim 4, wherein the tempering parameter is 11000 to 14000, and the holding time is 1 to 150 seconds.