CN109477182A

CN109477182A - High-strength steel sheet and its manufacturing method

Info

Publication number: CN109477182A
Application number: CN201780046036.5A
Authority: CN
Inventors: 柿内艾丽佳; 村上俊夫; 大谷茂生; 二村裕; 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: 2019-03-15
Anticipated expiration: 2037-07-21
Also published as: CN109477182B; JP6762798B2; JP2018021233A; EP3495523A4; WO2018025675A1; EP3495523A1; US20190185958A1; EP3495523B1

Abstract

A kind of high-strength steel sheet, wherein, contain the C:0.15 mass mass of %~0.35 %, Si's and Al is total: 0.5 mass of mass %~3.0 %, the Mn:1.0 mass mass of %~4.0 %, P:0.05 mass % or less, S:0.01 mass % or less, surplus is made of Fe and inevitable impurity, in structure of steel, ferrite divides rate to be 5% or less, tempered martensite and the total of tempering bainite divide rate to be 60% or more, remained austenite content is 10% or more, the average-size of MA is at 1.0 μm or less, half breadth with the concentration distribution of the Mn in the carbon thickening region of remained austenite content equivalent is 0.3 mass % or more, the q value of small angle X ray scattering is 1nm^{- 1}Scattering strength in 1.0cm^{- 1}Below.

Description

High-strength steel sheet and its manufacturing method

Technical field

This disclosure relates to can the high-strength steel sheet used in the various uses headed by auto parts.

Background technique

For the steel plate of automobile part etc., in order to realize lightweight and crashworthiness together, the raising of intensity is taken into account With the raising of impact property.

Such as patent document 1 discloses a kind of high-strength steel sheet, by heating of plate blank to 1210 DEG C or more, controls hot rolling item Part, so that 0.5 μm of fine TiN particle below be made to generate, inhibiting the partial size of the starting point as low temperature damage is 1 μm or more Thus the generation of AlN particle attempts to improve impact property.

A kind of high-strength steel sheet is disclosed in patent document 2, and C amount is made to be higher than 0.45% and 0.77% hereinafter, making Mn amount be 0.1% or more, 0.5% hereinafter, make Si amount be 0.5% hereinafter, and regulation Cr, Al, N, O additive amount, while making iron 50% or more of ferritic partial size becomes the mesh-shape tissue engaged with hard, to try hard to improve impact resistant characteristic.

A kind of high-strength steel sheet is disclosed in patent document 3, by the Mn of addition 3.5~10%, makes residual austenite The amount of body is 10% or more, makes 1.5 μm of the equispaced of retained austenite hereinafter, to seek the improvement of impact resistant characteristic.

Patent document 4 discloses a kind of tensile strength with 980~1180MPa, and shows good deep drawability High-strength steel sheet.

[existing technical literature]

[patent document]

No. 5240421 specifications of [patent document 1] Japanese Patent No.

[patent document 2] Japanese Unexamined Patent Publication 2015-105384 bulletin

[patent document 3] Japanese Unexamined Patent Publication 2012-251239 bulletin

[patent document 4] Japanese Unexamined Patent Publication 2009-203548 bulletin

In order to realize the further lightweight of steel plate used in automobile part, need it thinner, while being also equipped with and filling The intensity and impact property divided.In short, requiring the steel plate of higher tensile strength and excellent impact characteristics.

In addition, not only requiring high-tensile in the various uses using automobile part as representative and impact being special Property, but also there is excellent strength-ductility balanced, high-yield-ratio, the formability that swells and excellent hole expansibility.

It is specifically wanted respectively about tensile strength, strength-ductility balanced, yield tensile ratio, deep draw characteristics and hole expansibility Ask as follows.

About tensile strength, it is desirable that in 980MPa or more.In order to improve the stress for capableing of load in use, in addition to high tensile Other than intensity (TS), it is also necessary to have high-yield strength (YS).In addition, from the viewpoint of ensuring crashworthiness etc., it is also necessary to The yield strength of steel plate is improved, in addition to showing strength characteristics stabilization in collision, it is also desirable to fracture when thering is inhibition to deform Characteristic.Therefore, specifically, it is desirable that have 0.75 or more yield tensile ratio (YR=YS/TS), and as replacement destruction characteristic Evaluation index, it is desirable that the plate thickness reduction of fracture when tension test improves.In addition, the basic nature as automotive sheet Can, also require the strength of joint of spot-welding part.Specifically, it is desirable that the cross tensile strength of spot-welding part is 6kN or more.

About strength-ductility balanced, it is desirable that the product (TS × EL) of TS and breaking elongation (EL) be 20000MPa% with On.In addition, formability when in order to ensure part forming, also requiring the hole expansibility λ for indicating hole expandability is 20% or more, Yi Jibiao The limit bulging height (bulging height) for showing the formability that swells is 16mm or more.

But all meets these requirements in high-strength steel sheet and be difficult, so wanting disclosed in the Patent Documents 1 to 4 Seek a kind of high-strength steel sheet that can all meet these requirements.

Summary of the invention

Embodiments of the present invention as reply precisely in order to requiring and making, it is intended that providing a kind of anti- It is disconnected when tensile strength (TS), yield tensile ratio (YR), product (TS × EL), the hole expansibility (λ), tension test of (TS) and breaking elongation (EL) The cross tensile strength (stretching of SW cross) for splitting the plate thickness reduction (RA) in portion, limit bulging height and spot-welding part is in Gao Shui Flat high-strength steel sheet and its manufacturing method.

Mode 1 of the invention is a kind of high-strength steel sheet, wherein contain

The C:0.15 mass mass of %~0.35 %,

Si's and Al is total: 0.5 mass of mass %~3.0 %,

The Mn:1.0 mass mass of %~4.0 %,

P:0.05 mass % or less,

S:0.01 mass % hereinafter,

Surplus is made of Fe and inevitable impurity,

In structure of steel,

Ferrite divide rate be 5% hereinafter,

Tempered martensite and the total of tempering bainite divide rate to be 60% or more,

Remained austenite content is 10% or more,

The average-size of MA be 1.0 μm hereinafter,

Half breadth with the concentration distribution of the Mn in carbon that remained austenite content is equivalent thickening region is 0.3 mass % More than,

The q value of small angle X ray scattering is 1nm^{- 1}Scattering strength in 1.0cm^{- 1}Below.

Mode 2 of the invention, according to high-strength steel sheet described in mode 1, wherein C amount is 0.30 mass % or less.

Mode 3 of the invention, the high-strength steel sheet according to mode 1 or 2, wherein Al amount is lower than 0.10 mass %.

Mode 4 of the invention is a kind of manufacturing method of high-strength steel sheet, wherein comprise the following steps:

The process for preparing rolled stock, it is total containing the C:0.15 mass mass of %~0.35 %, Si and Al: 0.5 matter Mass %, the Mn:1.0 mass of %~3.0 mass %, the P:0.05 mass of %~4.0 % or less, S:0.01 mass % are measured hereinafter, remaining Amount is made of Fe and inevitable impurity；

By the rolled stock in Ac₁Point and 0.2 × Ac₁+ 0.8 × Ac of point₃After being kept for 5 seconds or more at a temperature of between point, add Heat is to Ac₃Or more temperature and kept for 5~600 seconds and the process that carries out austenitizing；

After the austenitizing, with 10 DEG C/sec or more of average cooling rate, it is cooled to from 650 DEG C or more of temperature 100 DEG C more than or lower than 300 DEG C between the cooling process stopped until temperature；

With 30 DEG C/sec or more of average heating rate, it is heated to from the cooling stopping temperature in 300 DEG C~500 Process until the relation reheating temperature of DEG C range；

Under the relation reheating temperature, when being 10000~14500 and keep to meet the tempering parameter P as defined in formula (1) Between the process that is kept for 1~300 second mode；With

After the holding, being cooled to 200 DEG C from the relation reheating temperature with 10 DEG C/sec or more of average cooling rate is Process only.

P=T × (20+log (t/3600)) ... (1)

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

Mode 5 of the invention, according to manufacturing method described in mode 4, wherein cold until the cooling stopping temperature But in, comprising: be cooled to the temperature as 650 DEG C or more using 0.1 DEG C/sec of average cooling rate more than or lower than 10 DEG C/sec Chilling start temperature until process；It is cooled to from the chilling start temperature with 10 DEG C/sec of average cooling rate or more Process until the cooling stopping temperature.

Mode 6 of the invention, the manufacturing method according to mode 4 or 5, wherein the tempering parameter be 11000~ 14000, the retention time is 1~150 second.

Embodiment according to the present invention is capable of providing tensile strength (TS), yield tensile ratio (YR), (TS) and breaking elongation (EL) the plate thickness reduction (RA) (impact property) of fracture when product (TS × EL), hole expansibility (λ), tension test, pole Limit bulging height and the cross tensile strength (stretching of SW cross) of spot-welding part are in high-caliber high-strength steel sheet and its manufacture Method.

Detailed description of the invention

Fig. 1 is the manufacturing method for illustrating the high-strength steel sheet of embodiments of the present invention, the diagram being especially heat-treated.

Specific embodiment

The present inventors' sharp study as a result, it has been found that, in the steel with defined ingredient, by making structure of steel (metal Tissue) in, reach ferrite point rate: 5% hereinafter, tempered martensite and tempering bainite total point of rate: 60% or more, it is remaining γ amount: the average-size of 10% or more, MA: 1.0 μm hereinafter, be equivalent in the part of retained austenite, i.e. carbon thickening region The half breadth of the concentration distribution of Mn: 0.3 mass % or more, furthermore the q value of small angle X ray scattering is 1nm^{- 1}Scattering strength: 1.0cm^{- 1}Hereinafter, can then obtain tensile strength (TS), yield tensile ratio (YR), the product (TS × EL) of (TS) and breaking elongation (EL), Plate thickness reduction (RA) (impact property), limit bulging height and the spot-welding part of fracture when hole expansibility (λ), tension test Cross tensile strength (stretching of SW cross) be in high-caliber high-strength steel sheet.

Although details are aftermentioned, there is Mn to be thickened region, be to manufacture for the high-strength steel sheet of embodiments of the present invention When heat treatment austenitizing process in, by Ac₁Point and Ac₃The two-phase coexisting region of the centre of point, in more detail, It is exactly in Ac₁Point~0.2 × Ac₁+ 0.8 × Ac of point₃After being kept for the stipulated time at a temperature of between point, then with Ac₃Or more temperature Degree is kept for the stipulated time and is formed.In addition, being also formed in heat treatment corresponding with retained austenite (with retained austenite Amount is same amount of) carbon thickening region.Moreover, the carbon thickening region in, formed Mn thickening region and Mn be not thickened region this The two.That is, existing among carbon thickening region (retained austenite) containing more Mn and really not so region.Therefore, if It is thickened the distribution of region (that is, corresponding to whole retained austenites) measurement Mn concentration in entire carbon, then Mn concentration has certain journey Degree or more deviation.Specifically, the half breadth of the concentration distribution of Mn is 0.3 mass % or more.

In this way, the Mn amount for containing retained austenite is fluctuated, it is meant that can have has the residual of various stability Remaining austenite.Just cause to process the low retained austenite of stability of induced transformation and with large strain with lesser dependent variable The retained austenite mixing that the stability for processing induced transformation is high can just be caused, can cause to add in various strain regions Work induced transformation.As a result, high n-value can be proposed in wide strain region, it can be improved strain dispersibility and realize high swollen Shape processability.

The high-strength steel sheet of embodiments of the present invention presented below and its details of manufacturing method.

1. structure of steel

Illustrate the details of the structure of steel of the high-strength steel sheet of embodiments of the present invention below.

In the explanation of structure of steel below, have for can be improved with such tissue the mechanism of various characteristics into The case where row illustrates.These are the mechanism that the present inventors are expected based on the cognition obtained at present, it is noted that, It is not to define the range of technology of the invention.

(1) ferrite divides rate: 5% or less

Ferrite is typically of problem, although excellent in workability, intensity is low.As a result, if ferrite content is more, Then yield tensile ratio reduces.Therefore, make 5% or less ferrite point rate (5 volume % or less).

Ferrite point rate is preferably 3% hereinafter, more preferably 1% or less.

Ferrite point rate can be acquired by being observed with optical microscopy with the region of counting method measurement white.That is, By such method, ferrite point rate can be acquired with area ratio (area %).It then, can be straight with the value that area ratio acquires It connects and is used as the value of volume ratio (volume %).

(2) total rate of dividing of tempered martensite and tempering bainite: 60% or more

60% or more the total point of rate (60 volume % or more) for making tempered martensite and tempering bainite, can make high-strength Degree is taken into account with high hole expandability.Total point of rate of tempered martensite and tempering bainite is preferably 70% or more.

Tempered martensite and tempering bainite amount (total to divide rate), can acquire, that is, in the following way for carrying out The section progress SEM observation of nital corrosion, measurement MA (that is, the martensite of retained austenite and as-quenched It is total) divide rate, above-mentioned ferrite point rate and MA points of rate are subtracted from whole structure of steel and is acquired.

(3) remained austenite content: 10% or more

Retained austenite is in the processing of punch process etc., it may occur that mutually becomes martensite due to processing induced transformation TRIP phenomenon can obtain big elongation percentage.In addition, being formed by martensite with high rigidity.Therefore, it can obtain excellent It is strength-ductility balanced.Make 10% or more remained austenite content (10 volume % or more), can be realized TS × EL is Excellent in this way strength-ductility balanced of 20000MPa% or more.

Remained austenite content is preferably 15% or more.

In the high-strength steel sheet of embodiments of the present invention, the most of of retained austenite exists in the form of MA.It is so-called MA is the abbreviation of martensite-austenite constituent, is the complex (complex group of martensite and austenite It knits).

Remained austenite content can utilize X-ray diffraction, acquire ferrite (containing tempered martensite and not in X-ray diffraction The martensite of tempering) with the diffracted intensity ratio of austenite, then by calculating acquirement.As x-ray source, it is able to use Co-K α Ray.

(4) average-size of MA: 1.0 μm or less

MA is hard phase, and when deformation forms point as hole in parent phase/hard phase interface neighborhood and works.MA size is thicker Greatly, it can more cause strain to be concentrated to parent phase/hard phase interface, be easy to happen to be formed in parent phase/hard phase interface neighborhood hole For the destruction of starting point.

Therefore, making MA size, especially MA average-size is 1.0 μm or less fine in this way, inhibits to destroy, then can make to expand Porosity λ is improved.

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

The average-size of MA can acquire in the following way, that is, for passing through the section of nital corrosion, With SEM with 3000 times or more observations, 3 visuals field more than, total 200 μm or more of straight line is drawn in the arbitrary position in photo, survey The slice length that the straight line intersects with MA is measured, the average value of the slice length is calculated.

(5) half breadth with the Mn concentration distribution in the carbon of remained austenite content equivalent thickening region is 0.3 mass % More than

As above-mentioned, the most of of retained austenite exists in the form of MA, only identifies remnants using optical microscopy or SEM Austenite is had any problem.Retained austenite is compared with ferrite etc., since the solid solution limit of carbon is big, so by carrying out at aftermentioned heat Reason, and it is thickened carbon in retained austenite.Therefore, it using EPMA (Electron Probe Micro Analyzer), carries out The element image of carbon, by acquired from the high measurement point of concentration of carbon sequentially through above-mentioned X-ray diffraction and retained austenite The measurement point for measuring equivalent is thickened region as carbon, and it is retained austenite which can be thickened to region decision.That is, for example, remaining When Ovshinsky scale of construction is 15 volume %, in the measurement point for measuring carbon amounts by element image, the side selection high from concentration of carbon 15%, and the measurement point (carbon thickening region) that these concentration of carbons are high may determine that as retained austenite.

Therefore " region is thickened with the carbon of remained austenite content equivalent ", it is meant that be equivalent to the (right therewith of retained austenite Answer) region.

Then, about the concentration distribution with the Mn in the carbon of remained austenite content equivalent thickening region, especially Mn is dense The half breadth for spending distribution is also able to use EPMA measurement.Make be considered as carbon thickening region measurement point Mn amount distribution Graphically, hereby it is possible to obtaining half breadth.

The half breadth of the Mn concentration distribution is bigger, and the deviation of the Mn concentration in expression retained austenite is bigger, and (Mn's is dense The range for spending distribution is wide).In the high-strength steel sheet of embodiments of the present invention, the half breadth of the concentration distribution of Mn is 0.3 Quality % or more, preferably 0.5 mass % or more, more preferably 0.6 mass % or more, further preferably 0.75 mass % with On.

In this way, the Mn amount fluctuation for containing retained austenite (carbon thickening region), is capable of forming the remnants low from stability The austenite retained austenite wide to the such stability range of the high retained austenite of stability.The low residual austenite of stability Processing induced transformation just occurs under small dependent variable, becomes martensite for body.The high retained austenite of stability, will not be by small Dependent variable cause to process induced transformation, only apply big dependent variable just and can cause and process induced transformation and become martensite. Therefore, if the retained austenite with the wide stability of range exists, the processing by just and when small dependent variable, cross over To when processing progress and big dependent variable, processing induced transformation can constantly occur.As a result, can be answered across roomy Become range and propose high n-value, can be improved strain dispersibility and realize high bulging processability.

(6) the q value of small angle X ray scattering is 1nm^{- 1}Scattering strength in 1.0cm^{- 1}Below

So-called small angle X ray scattering is for steel plate X-ray irradiation, and measurement penetrates the scattering of the X-ray of steel plate, thus The size distribution of minuteness particle contained in steel plate (for example, being dispersed in the cementite particle in steel plate) can be acquired.In this hair In the steel plate of bright embodiment, by small angle X ray scattering, the conduct microfine being dispersed in tempered martensite can be acquired The size distribution of the cementite particle of son.Specifically, it in small angle X ray scattering, with q value and scattering strength, can solve It analyses the size of the particle of cementite and its divides rate.

Q value is the index of the size of the particle (such as cementite particle) in steel plate.It is so-called that " q value is 1nm^{- 1}", correspond to The cementite particle of partial size about 1nm.Scattering strength is the index of the volume fraction of the particle (such as cementite particle) in steel plate. Scattering strength is stronger, indicates that the volume fraction of cementite is bigger.

About the scattering strength of a certain q value, the volume of the cementite particle of size corresponding to the q value is semi-quantitatively indicated Divide rate.For example, q value is 1nm^{- 1}Scattering strength, semi-quantitatively indicate the volume fraction of the fine cementite particle of about 1nm.

That is, q value is 1nm^{- 1}Scattering strength it is big, indicate about 1nm fine cementite particle volume fraction it is big.? " q value is 1nm^{- 1}Scattering strength in 1.0cm^{- 1}In steel plate below ", it is meant that about 1nm's is fine present in the steel plate The volume fraction of cementite particle (is equivalent to scattering strength in 1.0cm in defined value^{- 1}Value) below.As described below, " q Value is 1nm^{- 1}Scattering strength in 1.0cm^{- 1}Steel plate below ", because the volume fraction of the cementite of about 1nm is suppressed low, It is believed that impact resistant excellent.

In the high ductile steel containing remaining γ, the preferably carbon state that concentrates on retained austenite, it is generally desirable to not deposit In cementite.The partial size dispersed in steel is the fine cementite of 1nm or so, can interfere moving for dislocation and make the changes of steel Shape ability reduces.Therefore, in the steel more than the volume fraction of the cementite of partial size about 1nm, destruction when deformation is promoted, resistance to Collision characteristic can reduce.

The steel plate of embodiments of the present invention, by inhibiting low the volume fraction of fine cementite, more specifically It says, is to make q value 1nm^{- 1}Scattering strength be in 1cm^{- 1}Hereinafter, fine to make to be formed by the lath of tempered martensite Carbide reduce, to improve the deformability in martensite.Inhibit steel plate to destroy in collision as a result, makes steel plate Impact resistant characteristic improves.

The measurement of small angle X ray scattering is measured using RIGAKU society Nano-viewer, Mo pipe ball.From steel plate The disc shaped test piece for cutting Φ 3mm is cut the sample of 20 μ m-thicks near plate thickness 1/4, is used as sample.Q value takes 0.1~ 10nm^{- 1}Data.It wherein, is 1nm for q value^{- 1}And acquire absolute intensity.

(7) other structure of steel:

In the present specification, other than the ferrite, tempered martensite, tempering bainite, retained austenite and cementite Structure of steel there is no special provision.But other than the structure of steel of these ferrites etc., there is also pearlite, it is not tempered Bainite and the martensite not being tempered etc..The structure of steel of ferrite etc., if meeting organization condition above-mentioned, even if in steel There are pearlite etc., can still play effect of the invention.

2. composition

It is illustrated below for the composition of the high-strength steel sheet of embodiments of the present invention.Mainly for basic element C, Si, Al, Mn, P and S are illustrated.

In addition, being indicated at the % for being grouped as middle unit, the entirely meaning of quality %.

(1) C:0.15~0.35%

C increases the amount of expected tissue, especially remnants γ, be for ensuring that it is high-intensitive-ductility balanced (TS × EL balance) etc. characteristic necessary to element need to add 0.15% or more to effectively play such effect.But It is unsuitable for welding higher than 0.35%.Preferably 0.18% or more, more preferably 0.20% or more.Additionally, it is preferred that being 0.30% Below.If C amount 0.25% hereinafter, if can more easily be welded.

(2) Si's and Al is total: 0.5~3.0%

Si and Al inhibits the precipitation of cementite respectively, has the function of keeping retained austenite remaining.In order to effectively play Such effect needs to add Si and Al total 0.5% or more.But if the total of Si and Al is higher than 3.0%, the change of steel Shape ability reduces, and TS × EL and bulging height reduce.Preferably 0.7% or more, more preferably 1.0% or more.Additionally, it is preferred that being 2.5% or less.

In addition, can be the additive amount of this degree to work as deoxidant element about Al, 0.10 matter can be lower than % is measured, in addition, making increased purpose of remained austenite content etc. for the formation for for example inhibiting cementite, 0.7 matter can also be added Measure % or more amounts more in this way.

(3) Mn:1.0~4.0%

Mn inhibits ferritic formation.In addition, Mn, which forms Mn, is thickened region, the different retained austenite of stability is formed, It is that bulging processability is made to improve the indispensable element of institute.In order to effectively play such effect, need to add 1.0% with On.But if being higher than 4.0%, the temperature range of two phase regions heating is uncontrollable that narrow and temperature becomes too low, because This, even if with Ac₁Point~0.2 × Ac₁+ 0.8 × Ac of point₃Temperature between point is kept for the stipulated time, and phase transformation will not be carried out, be deposited The case where cannot form the thickening region Mn.Preferably 1.5% or more, more preferably 2.0% or more.Additionally, it is preferred that being 3.5% Below.

(4) P:0.05% or less

P is inevitably present as impurity element.If being deteriorated with the presence of P, EL and the λ higher than 0.05%.Therefore, P Content be 0.05% or less (contain 0%).Preferably 0.03% or less (containing 0%).

(5) S:0.01% or less

S is inevitably present as impurity element.If the S higher than 0.01% exists, the sulfide-based of MnS etc. is formed Field trash becomes the starting point of crackle and reduces λ.Therefore, the content of S was 0.01% or less (containing 0%).Preferably 0.005% (contain 0%) below.

(6) surplus

In a preferred embodiment, surplus is iron and inevitable impurity.As inevitable impurity, permit Perhaps microelement (for example, As, Sb, the Sn's etc.) brought by the situation of raw material, goods and materials, manufacturing equipment etc. is mixed into.In addition, example Such as, as P and S in this way, in general, the content the few the more preferred, therefore it is inevitable impurity, but in this compositing range, had The element made separate stipulations as so above-mentioned.Therefore, in the present specification, " inevitable impurity " this feelings of surplus are constituted Condition is the concept removed other than the element that its compositing range is made separate stipulations.

But it is not limited by this embodiment.As long as being able to maintain that the spy of the high-strength steel sheet of embodiments of the present invention Property, it can further contain arbitrary other elements.

3. characteristic

Such as the high-strength steel sheet of above-mentioned embodiments of the present invention, TS, YR, TS × EL, λ, impact resistant characteristic, limit bulging Height and the stretching of SW cross are in high level.Hereinafter, these characteristics of the high-strength steel sheet for embodiments of the present invention It is described in detail.

(1) tensile strength (TS)

TS with 980MPa or more.It is preferred that TS is 1180MPa or more.This is because, if TS is lower than 980MPa, although Receiving load when can more reliably obtain excellent fracture characteristics, but collide is lower, therefore is not preferred.

(2) yield tensile ratio (YR)

With 0.75 or more yield tensile ratio.Realize that high yield is strong thus, it is possible to combine with above-mentioned high-tensile Degree, the final product as obtained from the processing of deep-draw processing etc. can use under high stress.It is preferred that with 0.80 or more Yield tensile ratio.

(3) product (TS × EL) of TS and breaking elongation (EL)

TS × EL is 20000MPa% or more.TS × EL with 20000MPa% or more can be had height simultaneously Intensity and high ductibility it is high-caliber strength-ductility balanced.It is preferred that TS × EL is 23000MPa% or more.

(4) formability that swells (limit bulging height)

Limit bulging height is the index for the evaluation for the formability that swells.Limit bulging height, in load-stroke line In figure for load sharply reduction fracture occur when punching stroke.

In more detail, with the test film of Φ 120mm, with Φ 53.6mm, the bulb of the punch die of shoulder radius 8mm and Φ 50mm Formed punch, presss from both sides the polythene strip every lubrication between formed punch and steel plate, and pressure-pad-force carries out bulging, measurement fracture as 1000kgf When height (punching stroke), acquire limit bulging height.

The high-strength steel sheet of embodiments of the present invention, limit bulging height are 20mm or more, preferably 21mm or more.

(5) hole expansibility (λ)

Hole expansibility λ follows Nippon Steel alliance specification JFS T1001 and acquires.Diameter d is dug on test film₀(d₀=10mm) Perforation, be that 60 ° of formed punch is pressed into the perforation by toe angle, measure generation cracking penetrated through test film plate thickness this The diameter d of the perforation at moment, acquires hole expansibility according to the following formula.

λ (%)={ (d-d₀)/d₀}×100

The high-strength steel sheet of embodiments of the present invention, hole expansibility λ are 20% or more, preferably 30% or more.Thus, it is possible to Access the excellent processability such as press formability.

(6) the plate thickness reduction in tension test (R5 stretches plate thickness reduction)

Using the test film for the arc-shaped notch for being equipped with radius 5mm on No. 5 test films, make the deformation speed of tension test Degree is tested for 10mm/min, is broken sample.Thereafter, cross-section observation is carried out, with the thickness t in the plate thickness direction in section₁ Divided by the plate thickness t of script₀Value (t₁/t₀), as plate thickness reduction.

Plate thickness reduction in the test is 50% or more, preferably 52% or more, more preferably 55% or more.As a result, It is difficult to be broken severely deformed when collision, therefore the steel plate for the impact property for having excellent can be obtained.

(7) the cross tensile strength of spot welding

The cross tensile strength of spot welding is evaluated in accordance with JIS Z 3137.The condition of spot welding is used two The steel plate of 1.4mm is overlapped.With dome radius (ドー system ラジアス) electrode of type makes electric current from 6kA at plus-pressure 4kN Range to 12kA increases 0.5kA every time and carries out spot welding, the current value (minimum current for (Chi り) generation of splashing when investigation is welded Value).Spot welding is carried out with electric current 0.5kA lower than the minimum current value, is drawn the cruciform joint being thus welded as cross Stretch the measuring sample of intensity.The above are " good " in 6kN for cross tensile strength.In addition, cross tensile strength is preferably 8kN More than, more preferably 10kN or more.

If cross tensile strength is 6kN or more, when manufacturing automobile part etc. by steel plate, when can obtain welding The high part of bond strength.

4. manufacturing method

Next, being illustrated for the manufacturing method of the high-strength steel sheet of embodiments of the present invention.

Present inventors found that (more by carrying out the aftermentioned heat treatment of details for the rolled stock with defined composition The means of isothermal quenching of grade), to have above-mentioned desired structure of steel, as a result, can obtain that there are above-mentioned expected characteristics High-strength steel sheet.

Illustrate its details below.

Implement the rolled stock of heat treatment, usually after hot rolling, carries out cold rolling and manufacture.But it's not limited to that, carries out Any one of hot rolling and cold rolling can manufacture.In addition, the condition of hot rolling and cold rolling is not particularly limited.

(1) austenitizing

In austenitizing process, as shown in [2] of Fig. 1, by rolled stock in Ac₁Point and Ac₃The two-phase coexistence of the centre of point Domain is exactly in more detail in Ac₁Point and 0.2 × Ac₁+ 0.8 × Ac of point₃Temperature T between point₁(Ac₁≤T₁≤0.2×Ac₁ + 0.8 × Ac of point₃) under kept for 5 seconds or more after, as shown in [3] of Fig. 1, [4], then end Ac₃Or more temperature T₂(Ac₃≤ T₂), with heating temperature T₂It keeps being allowed to austenitizing in 5~600 seconds.

It is heated to temperature T₁Kept for 5 seconds or more.Preferably remaining the time is 900 seconds or less.In addition, keeping temperature T₁, such as Fig. 1 [2], can also be in Ac₁Point and 0.2 × Ac₁+ 0.8 × Ac of point₃It is kept at a temperature of fixation between point, for example, it is also possible to Ac₁Point and 0.2 × Ac₁+ 0.8 × Ac of point₃Be slowly heated between point etc., it is allowed in Ac₁Point and 0.2 × Ac₁+ 0.8 × Ac of point₃Point Between change.In this way, being kept by the relatively low temperature field in the two-phase coexisting region of ferrite and austenite, the iron coexisted Among ferritic and austenite, Ovshinsky side is assigned to more Mn, and thus, it is possible to obtain Mn thickening region.Then, it is formed in this Even if Mn thickening region, the Mn concentration after heat treatment as the remaining austenite of retained austenite are still high, it is possible thereby to plus The deviation of the concentration of the Mn in big carbon thickening region can be realized height and swell formability.

If temperature T₁Lower than Ac₁The amount of point, the then austenite for having Mn to be thickened is few, in retained austenite (carbon thickening region) The deviation of Mn concentration becomes smaller, and cannot obtain the formability that adequately swells.

If temperature T₁Higher than 0.2 × Ac₁+ 0.8 × Ac of point₃Point, then the Mn concentration of austenite is lower, and (carbon is thick for retained austenite Change region) in the deviation of Mn concentration become smaller, the formability that adequately swells cannot be obtained.

If temperature T₁Under time of the retention time than 5 seconds it is short, then the time of Mn diffusion is insufficient, thickening of the Mn to austenite Insufficient, the deviation of the Mn in retained austenite (carbon thickening region) becomes smaller, and cannot obtain the formability that adequately swells.

Preferable temperature T₁Under retention time long aspect, but from the viewpoint of productivity, preferably 900 seconds or less.

Preferable temperature T₁In 0.9 × Ac₁+ 0.1 × Ac of point₃Point and 0.3 × Ac₁+ 0.7 × Ac of point₃Between point, temperature T₁Under Retention time is 10 seconds or more and 800 seconds or less.More preferable temperature T₁In 0.8 × Ac₁+ 0.2 × Ac of point₃Point and 0.4 × Ac₁Point+ 0.6×Ac₃Between point, temperature T₁Under retention time be 30 seconds or more and 600 seconds or less.

In addition, being used as in Fig. 1 shown in [1] to temperature T₁Heating speed, preferably 5~20 DEG C/sec.

Secondly as shown in [4] of [3] of Fig. 1, it is warming up to Ac₃Or more temperature T₂(Ac₃≤T₂) and with temperature T₂It keeps And carry out austenitizing.Temperature T₂Under retention time be 5~600 seconds.

By being heated to Ac₃Or more temperature T₂, it is being heated to temperature T₁The ferritic part Shi Zuowei also becomes Ovshinsky Body.Again the part Mn for mutually becoming austenite is not thickened.Therefore, there are the thickening regions of above-mentioned Mn in austenite, together When there are the regions that Mn is not thickened, in high-strength steel sheet after heat treatment, retained austenite (carbon thickening region) can be increased Mn concentration deviation, can be realized height and swell formability.

If temperature T₂Lower than Ac₃Point or temperature T₂Under retention time it is shorter than 5 seconds, then the iron of obtained high-strength steel sheet Ferritic divides rate to reduce higher than 5%, YR.

If temperature T₂Excessively high, the Mn in the Mn thickening region being initially formed is spread, and the deviation of Mn concentration is possible to become It is small.Therefore, temperature T₂Preferably Ac₃+ 50 DEG C or less of point.

If temperature T₂Under retention time it is longer than 600 seconds, then due to spread cause Mn thickening region Mn concentration be lower, it is residual The deviation of Mn concentration in remaining austenite becomes smaller, and the formability that swells reduces.

Preferable temperature T₂For Ac₃+ 10 DEG C or more of point, temperature T₂Under retention time be 10~450 seconds.More preferable temperature T₂ For Ac₃+ 20 DEG C or more of point, temperature T₂Under retention time be 20~300 seconds.

From temperature T shown in [3] of Fig. 1₁To temperature T₂Heating, preferably with 0.1 DEG C/sec more than or lower than 10 DEG C/sec Heating speed carries out.

In addition, about Ac₁Point and Ac₃Point is formed, by generally known calculation although can also be acquired by measurement with it Formula, which calculate, to be advisable.

For example, Ac can be calculated with following (1) formulas and 2 (formulas)₁Point and Ac₃Point is (for example, referring to " Lesley's steel material Material " ball is kind, (1985)).

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

Ac₃Point (DEG C)=910-203 × [C]^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, [] indicate the wherein described element with content shown in quality %.

(2) be cooled to 100 DEG C more than or lower than 300 DEG C between it is cooling stop temperature until

After above-mentioned austenitizing, as shown in [6] of Fig. 1, with 10 DEG C/sec or more of average cooling rate, it is cooled to 100 Cooling between DEG C more than or lower than 300 DEG C stops temperature T₃Until.At 100 DEG C or more, lower than 300 DEG C within the temperature range of control Refrigeration but stops temperature, so as to adjust the amount for not becoming martensite and remaining austenite mutually, controls final retained austenite Amount.

In cooling, at least between 650 DEG C~300 DEG C, cooled down with 10 DEG C/sec of average cooling rate or more.Make to put down Equal cooling velocity is 10 DEG C/sec or more, is to become fine geneva phosphor bodies to inhibit the ferritic formation in cooling Tissue.

It as the preference of such cooling, can enumerate, as shown in [5] of Fig. 1, by 650 DEG C or more chillings Start temperature T₄, cooling (slow cooling) is carried out with 0.1 DEG C/sec of relatively low average cooling rate more than or lower than 10 DEG C/sec, such as Shown in [6] of Fig. 1, from chilling start temperature T₄To 300 DEG C or less cooling stopping temperature T₃, with 10 DEG C/sec of average cooling rate with On carry out cooling (chilling).In addition, by making chilling start temperature T₄It is 650 DEG C or more, can inhibits in cooling (slow cooling) Ferritic formation.

If cooling velocity is slower than 10 DEG C/sec, ferrite is formed, and YR is reduced.In addition, MA becomes thick, hole expansibility thus It reduces.

If cooling stop temperature T₃Lower than 100 DEG C, then remained austenite content is insufficient.As a result, although TS is got higher, EL is reduced, TS × EL underbalance.

If cooling stop temperature T₃It is 300 DEG C or more, then coarse non-transformed austenite increases, even if through cooling thereafter Still remaining, final MA size becomes thick, and hole expansibility λ is reduced.

In addition, preferred cooling velocity is 15 DEG C/DEG C or more, more preferably 20 DEG C/s or more.Preferred cooling stopping temperature Spend T₃For 120 DEG C or more and 280 DEG C hereinafter, more preferably 140 DEG C or more and 260 DEG C or less.

As shown in [7] of Fig. 1, temperature T can also be stopped cooling₃Lower holding.Preferred retention time when as holding, It can enumerate 1~150 second.Even if the retention time is longer than 150 seconds, how the characteristic of obtained steel plate is not improved yet, steel plate Productivity but reduces, therefore preferably at 150 seconds or less.

(3) until being again heated to 300 DEG C~500 DEG C of temperature range

As shown in [8] of Fig. 1, from above-mentioned cooling stopping temperature T₃It rises with 30 DEG C/sec or more of reheating speed, heating To the relation reheating temperature T for being in 300 DEG C~500 DEG C ranges₅Until.By instant heating, it can shorten and can promote carbide Precipitation and growth temperature field residence time, be able to suppress the formation of fine carbide.Preferably reheating speed is 60 DEG C/s or more, more preferably 70 DEG C/s.

Such instant heating, such as can reach in the method for high-frequency heating, electrified regulation etc..

Reach relation reheating temperature T₅Afterwards, as shown in [9] of Fig. 1, with this temperature T₅It keeps.At this moment, preferably make formula below (1) the tempering parameter P represented by is 10000 or more and 14500 hereinafter, also, the retention time is 1~150 second.Present embodiment Steel plate tempering parameter P by formula below (1) indicate.

P=T (K) × (20+log (t/3600) ... (1)

Here, T is tempering temperature (K), t is to maintain time (second).

When reheating, the carbon of super saturated solid solution is reallocated in martensite.It specifically, is exactly to occur following two A phenomenon, that is, diffusion of the carbon from martensite to austenite；It is precipitated with the carbide (cementite) in the lath of martensite.This two Among a phenomenon, if being kept for a long time under low temperature, it is easy to happen the precipitation of carbide.In addition, even if with high temperature dwell When holding, if heating speed is slow or the retention time is too long, carbide can be also precipitated.On the other hand, carbon is from martensite to austenite Diffusion, depend on diffusion velocity strongly, therefore the processing through the short time just can be carried out sufficiently at high temperature.

The particle for the cementite being present in martensite easily becomes the starting point of impact wreckage, becomes to drop impact resistant characteristic Low reason.Therefore, when reheating, it is desirable to inhibit on one side the analysis of the carbide (cementite) in the lath of martensite Out, promote reheating processing as diffusion of the carbon from martensite to austenite on one side.Therefore, effectively implement rapidly to add The heat treatment of heat and high temperature and short time.

But it in order to make sufficient carbon spread generation obtain expected tensile strength, needs that temperature and time will be used as Connector tempering parameter P control in a certain range.

If tempering parameter P, less than 10000, diffusion of the carbon from martensite to austenite does not occur sufficiently, and austenite becomes It is unstable, it cannot ensure remained austenite content, therefore TS × EL underbalance.In addition, if tempering parameter P is greater than 14500, i.e., Make short time processing that can not prevent the formation of carbide, cannot ensure remained austenite content, TS × EL balance deterioration.In addition, Even if tempering parameter is appropriate, if heating speed is too low, overlong time then still will form carbide in martensite lath, collision becomes Cracking progress when shape is easy to happen, impact resistant deterioration in characteristics.The amount of carbide in martensite lath, can be according to small angle X The scattering strength of ray scattering acquires.

If relation reheating temperature T₅Lower than 300 DEG C, then Carbon diffusion is insufficient, cannot get enough remained austenite contents, and TS × EL is reduced.If relation reheating temperature T₅Higher than 500 DEG C, then retained austenite resolves into cementite and ferrite, and retained austenite is not Foot, characteristic cannot ensure.

If shorter than 1 second without holding or retention time, it is also possible to deficiencies for Carbon diffusion.It is therefore preferable that with again Heating temperature T₅Holding in progress 1 second or more.If the retention time is longer than 150 seconds, it is also possible to be precipitated as cementite for carbon. Therefore, the retention time is preferably 150 seconds or less.

Preferred relation reheating temperature T₅It is 320~480 DEG C, preferred relation reheating temperature T₅It is 340~460 DEG C.

It is preferred that tempering parameter P is 10500~14500, at this moment the preferred retention time is 1~150 second.Preferred tempering Parameter P is 11000~14000, and at this moment the preferred retention time is 1~100 second, more preferably 1~60 second.

After reheating, as shown in [10] of Fig. 1, such as such 200 DEG C of temperature below can be cooled to room temperature.As 200 DEG C of preferred average cooling rates below are cooled to, 10 DEG C/sec can be enumerated.

By above heat treatment, the high-strength steel sheet of embodiments of the present invention can be obtained.

If it is this field of the manufacturing method of the high-strength steel sheet for the embodiment for contacting present invention mentioned above Then there is following possibility in technical staff, that is, can by trial and error method, with the manufacturing method different from above-mentioned manufacturing method, Obtain the high-strength steel sheet of embodiments of the present invention.

[embodiment]

1. sample makes

The casting material is hot-forged after manufacturing the casting material with chemical composition described in table 1 by vacuum melting And after becoming the steel plate of plate thickness 30mm, implement hot rolling.In addition, also describing the with good grounds calculated Ac of composition in table 1₃Point.

The condition of hot rolling will not apply influence substantially to the final tissue and characteristic of this patent, be heated to 1200 DEG C Afterwards, reach plate thickness 2.5mm through multistage rolling.At this moment, the end temperature of hot rolling is 880 DEG C.Thereafter, 600 are cooled to 30 DEG C/sec DEG C, stop cooling, after insertion is heated in 600 DEG C of furnace, is kept for 30 minutes, thereafter, progress furnace is cold, as hot rolled steel plate.

After the oxide skin for removing surface to hot rolled steel plate implementation pickling, implementation is cold-rolled to 1.4mm.To the cold-reduced sheet into Row heat treatment, obtains sample.Heat treatment condition is shown in table 2.In addition, such as [2] are shown in [] in this way in table 2 Number, it is corresponding with the process being identically numbered being shown in Fig. 1 in [].

In table 2, in sample No.1, austenitizing temperature T is not divided into₁With temperature T₂Two stages carry out, only with suitable In temperature T₂Ac₃Or more temperature keep.

Sample No.9 is (to be to skip in Fig. 1 with the sample that this temperature is kept after being cooled to relation reheating temperature and be equivalent to [7] The sample of the process of~[8]), with substitution be cooled to 100 DEG C more than or lower than 300 DEG C between cooling stop temperature.

Sample 15 and 31~36 is to make heating temperature T₂With chilling start temperature T₄Identical sample.That is, being in austenite After change, a stage is cooled to cooling stopping temperature T₃Until sample.

The reheating for being equivalent to [8] is carried out by electrified regulation method.

In addition, in 1~table of table 4, the numerical value with asterisk (*), indicate to be detached from embodiments of the present invention range it Outside.

[table 1]

[table 2]

2. structure of steel

The total of ferrite point rate, tempered martensite and tempering bainite is acquired according to the above method for each sample Rate is divided (to describe the Mn for being thickened region for " tempering M/B "), remained austenite content (remaining γ amount), the average-size of MA, carbon in table 3 The half breadth of concentration distribution and the q value of small angle X ray scattering are 1nm^{- 1}Scattering strength.The measurement of remained austenite content In, use Co., Ltd.'s リガ Network two dimension zone X ray detecting diffraction device (RINT-RAPIDII).Obtain as the result is shown in table In 3.

In addition, in the present embodiment, the structure of steel (residue tissue) other than structure of steel described in table 3 removes sample No.9 Sample in addition is the martensite not being tempered, and sample No.9 is the bainite not being tempered.

[table 3]

3. mechanical property

YS, TS, EL are measured using cupping machine for obtained sample, calculate YR and TS × EL.In addition, according to upper The method stated acquire hole expansibility λ, limit bulging height (bulging height), the cross tensile strength of spot-welding part (stretching of SW cross) and R5 stretches plate thickness reduction.Obtain as the result is shown in table 4.

[table 4]

The result of investigation table 4.Sample No.12~15,18,21 and 29~36 are to meet in embodiments of the present invention to advise The embodiment of fixed whole important documents (composition, manufacturing condition and structure of steel).The tension that these samples reach 980MPa or more is strong Spend (TS), 0.75 or more yield tensile ratio (YR), TS × EL of 20000MPa% or more, 20% or more hole expansibility (λ), the limit is swollen Shape height is 16mm or more, the SW cross of 6kN or more stretches and 50% or more R5 stretches plate thickness reduction (RA).

In contrast, austenitizing is not divided into temperature T in sample No.1₁With temperature T₂Two stages carry out, only with It is equivalent to temperature T₂Ac₃Or more temperature keep, therefore carbon thickening region Mn concentration distribution half breadth value Small, limit bulging height is low.In addition, carbide (cementite) is precipitated because [7] retention time is up to 300 seconds.In addition, Because the scattering strength of small angle X ray scattering is big, the volume fraction of the cementite of about 1nm is big.As a result, resistance to touch Hit characteristic (plate thickness reduction) reduction.

In sample No.2, because keeping temperature T₁It is low, so the half breadth of the concentration distribution of the Mn in carbon thickening region It is worth small, limit bulging height is lower.

In sample No.3, because keeping temperature T₁Height, so the half breadth of the concentration distribution of the Mn in carbon thickening region Value it is small, limit bulging height is low.Furthermore [7] retention time is up to 300 seconds, therefore carbide (cementite) is precipitated.In addition, by It is big in the scattering strength of small angle X ray scattering, so the volume fraction of the cementite of about 1nm is big.As a result, impact resistant Characteristic (plate thickness reduction) reduces.

In sample No.4 and 5, because being heated to heating temperature T₁And after keeping, selection and T₁Identical temperature is as heating Temperature T₂, so austenitizing cannot be carried out with sufficiently high temperature.Therefore, ferrite content is more, tempered martensite and tempering shellfish The total of family name's body divides rate low, and the value of the half breadth of the concentration distribution of the Mn in carbon thickening region becomes smaller.As a result, tension is strong Degree, yield tensile ratio and limit bulging height are lower.

In sample No.6, heating temperature T₂Low, ferrite quantitative change is more, as a result, yield tensile ratio is lower.

In sample No.7, because cooling stop temperature T₃Height, so tempered martensite and the total of tempering bainite divide rate It is low, and the average-size of MA is big.As a result, hole expansibility is lower.

In sample No.8, because of heating temperature T₁Under retention time it is short, so the concentration point of the Mn in carbon thickening region The value of the half breadth of cloth is small, as a result, limit bulging height is lower.

In sample No.9, heating temperature T₂Under retention time it is long, it is in addition cooling to stop temperature T₃It is high.Therefore, tempered martensite Body and the total of tempering bainite divide the average-size that rate is 0%, MA big, and the value of the half breadth of the concentration distribution of Mn becomes It is small.As a result, tensile strength, hole expansibility and limit bulging height are low.In addition, because ([9] are protected within 300 seconds with the holding of this temperature Hold the time), so the formation of carbide is also few.These are the result is that hole expansibility λ is reduced.

It is cooling to stop temperature T in sample No.10₃Low, remained austenite content is few, as a result, the value and the limit of TS × EL Bulging height is lower.

In sample No.11, because [8] reheat speed slowly up to 30 DEG C/sec, carbide (cementite) is precipitated.In addition, Since the scattering strength of small angle X ray scattering is big, so the volume fraction of the cementite of about 1nm is big.As a result, resistance to touch Hit characteristic (plate thickness reduction) reduction.

In sample No.16, because of chilling start temperature T₄It is low, so ferrite content is more, tempered martensite and tempering bayesian The total of body divides rate low.As a result, tensile strength and yield tensile ratio are low.In addition, be up to 300 seconds because of [9] retention time, Carbide (cementite) is precipitated.In addition, the scattering strength due to small angle X ray scattering is big, so the volume of the cementite of about 1nm Point rate is big.As a result, impact resistant characteristic (plate thickness reduction) reduces.

In sample No.17, because [8] reheat speed slowly up to 15 DEG C/sec, carbide (cementite) is precipitated.In addition, Since the scattering strength of small angle X ray scattering is big, so the volume fraction of the cementite of about 1nm is big.As a result, resistance to touch Hit characteristic (plate thickness reduction) reduction.

In sample No.19, relation reheating temperature T₅Height, therefore parameter is up to 14604, remained austenite content is few.As a result, The value and limit bulging height of TS × EL is lower.In addition, the scattering strength due to small angle X ray scattering is big, so the infiltration of about 1nm The volume fraction of carbon body is big.As a result, impact resistant characteristic (plate thickness reduction) reduces.

In sample No.20, relation reheating temperature T₅It is low, therefore parameter, down to 9280, remained austenite content tails off.Its result It is that the value and limit bulging height of TS × EL is lower.

In sample No.22, C amount is low, and remained austenite content is few, as a result, the value of TS × EL and limit bulging height become It is low.

In sample No.23, Mn amount is more, and remained austenite content is few, as a result, the value of TS × EL and limit bulging height become It is low.

In sample No.24, Mn amount is few, and ferrite content is more, and the total amount of tempered martensite and tempering bainite is insufficient.It is tied Fruit is that tensile strength and yield tensile ratio are lower.

In sample No.25, Si+Al amount is low, and tempered martensite and the total of tempering bainite divide rate and remained austenite content Low, MA is average to be become large-sized.As a result, the value of TS × EL, hole expansibility and limit bulging height are lower.

In sample No.26, C amount is more, as a result, SW cross tensile strength is lower.

In sample No.27, Si+Al amount is more, as a result, the value of TS × EL and limit bulging height are lower.

In sample No.28, because keeping temperature T₁Height, so the half breadth of the concentration distribution of the Mn in carbon thickening region Value it is small, limit bulging height is lower.

4. summarizing

In this way, meeting the steel plate of composition and structure of steel as defined in embodiments of the present invention, it is able to confirm that tensile strength (TS), fracture when yield tensile ratio (YR), the product (TS × EL) of (TS) and breaking elongation (EL), hole expansibility (λ), tension test The cross tensile strength of plate thickness reduction (RA), limit bulging height and spot-welding part is in high level.

It additionally is able to confirm, the manufacturing method of embodiment according to the present invention can manufacture and meet implementation of the invention The composition of mode defined and the steel plate of structure of steel.

The application with the applying date for the Japanese Patent Application on the 3rd of August in 2016, Patent 2016-153110 with The claim of priority of basis application.Patent 2016-153110 is incorporated into this specification due to reference.

Claims

1. a kind of high-strength steel sheet, contains

The C:0.15 mass mass of %~0.35 %,

Si's and Al is total: 0.5 mass of mass %~3.0 %,

The Mn:1.0 mass mass of %~4.0 %,

P:0.05 mass % or less,

S:0.01 mass % hereinafter,

Surplus is made of Fe and inevitable impurity,

In structure of steel,

Ferrite divide rate be 5% hereinafter,

Remained austenite content is 10% or more,

The average-size of MA 1.0 μm or less,

Half breadth with the concentration distribution of the Mn in the carbon thickening region of remained austenite content equivalent is 0.3 mass % or more,

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

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

4. a kind of manufacturing method of high-strength steel sheet, wherein comprise the following steps:

Prepare the process of rolled stock, the rolled stock contains the total of the C:0.15 mass mass of %~0.35 %, Si and Al: 0.5 Mass %, Mn:1.0 mass mass %, the P:0.05 mass of %~4.0 of quality %~3.0 % or less, S:0.01 mass % hereinafter, Surplus is made of Fe and inevitable impurity；

By the rolled stock in Ac₁Point and 0.2 × Ac₁+ 0.8 × Ac of point₃After being kept for 5 seconds or more at a temperature of between point, it is heated to Ac₃Or more temperature and kept for 5~600 seconds and the process that carries out austenitizing；

After the austenitizing, 100 DEG C are cooled to 10 DEG C/sec or more of average cooling rate from 650 DEG C or more of temperature The process until cooling stopping temperature between more than or lower than 300 DEG C；

It is heated to 30 DEG C/sec or more of average heating rate in 300 DEG C~500 DEG C ranges from the cooling stopping temperature Relation reheating temperature until process；

At the relation reheating temperature T, to meet the tempering parameter P of formula (1) defined for 10000~14500 and retention time t The process kept for 1~150 second mode；With

After the holding, from the relation reheating temperature until 10 DEG C/sec or more of average cooling rate is cooled to 200 DEG C Process,

P=T × (20+log (t/3600)) ... (1)

Here, T: relation reheating temperature, unit K；T: retention time, unit are the second.

5. the manufacturing method of high-strength steel sheet according to claim 4, wherein cold until the cooling stopping temperature But in, comprising: be cooled to the urgency of 650 DEG C or more of temperature with 0.1 DEG C/sec of average cooling rate more than or lower than 10 DEG C/sec Process until cold start temperature；Institute is cooled to from the chilling start temperature with 10 DEG C/sec of average cooling rate or more State the process until cooling stopping temperature.

6. the manufacturing method of high-strength steel sheet according to claim 4 or 5, wherein the tempering parameter be 11000~ 14000, the retention time is 1~150 second.