阅读说明：本技术 钢板及其制造方法 (Steel sheet and method for producing same ) 是由 小野义彦 木谷靖 秋元浩幸 松井洋一郎 于 2020-03-26 设计创作，主要内容包括：本发明提供一种钢板及其制造方法,该钢板具有高延展性且焊接部显示优异的耐HAZ软化特性。一种钢板,具有特定的成分组成和钢组织,在钢组织中,以面积率计包含6～90％的铁素体,由上贝氏体、新鲜马氏体、回火马氏体、下贝氏体、残留γ的1种或2种以上构成的组织合计为10～94％,以体积率计包含3～20％的残留γ；与粒子宽度为0.17～0.80μm、长径比为4～25的残留γ-(UB)相接而存在的宽度0.8～7μm、长度2～15μm、长径比为2.2以上的上贝氏体的面积率S-(UB)相对于上述由上贝氏体等1种或2种以上构成的组织的面积率S-(2nd)的比率：(S-(UB)/S-(2nd))×100(％)为2.0～15％；存在于上述由上贝氏体等1种或2种以上构成的组织内部的、长径比为3.5以下、圆等效粒径为0.02～0.25μm的粒子的分布密度N-(θ)为7个/μm～(2)以下；圆等效粒径为1.3～20μm、长径比为3以下的粒子的合计面积率S-(γBlock)为5％以下。(The invention provides a steel sheet having high ductility and exhibiting excellent HAZ softening resistance in a welded portion, and a method for manufacturing the same. A steel sheet having a specific composition and a steel structure, wherein the steel structure contains 6 to 90% by area of ferrite, 10 to 94% by total of 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ, and 3 to 20% by volume of residual γ; and a particle width of 0.17 to 0.80 μm and a length-diameter ratio of 4 to 25 UB An area ratio S of upper bainite having a width of 0.8 to 7 μm, a length of 2 to 15 μm and an aspect ratio of 2.2 or more UB An area ratio S to the structure composed of 1 or 2 or more kinds of upper bainite 2nd The ratio of (A) to (B): (S) UB /S 2nd ) X 100 (%) is 2.0-15%; a distribution density N of particles having an aspect ratio of 3.5 or less and a circle-equivalent particle diameter of 0.02 to 0.25 μm, which are present in the interior of the structure composed of 1 or 2 or more species such as upper bainite θ Is 7 pieces/mum 2 The following; a total area ratio S of particles having a circle-equivalent particle diameter of 1.3 to 20 μm and an aspect ratio of 3 or less γBlock Is 5% or less.)

1. A steel sheet comprising, as a component composition, in mass%

C:0.06～0.25％、

Si:0.1～2.5％、

Mn：2.0～3.2％、

P: less than 0.02 percent,

S: less than 0.01 percent,

Al: less than 1.0% and including 0%,

N: less than 0.015 percent,

and the total content of Si and sol.Al is 0.7-2.5%,

the remainder being made up of Fe and unavoidable impurities;

the steel structure comprises 6-90% ferrite by area percentage, 10-94% in total of a structure consisting of 1 or more than 2 of upper bainite, fresh martensite, tempered martensite, lower bainite and residual gamma, and 3-20% residual gamma by volume percentage;

and residual gamma of 0.17 to 0.80 μm in particle width and 4 to 25 in aspect ratio_UBAn area ratio S of upper bainite having a width of 0.8 to 7 μm, a length of 2 to 15 μm, and an aspect ratio of 2.2 or more_UBAn area ratio S of the structure composed of 1 or 2 or more kinds of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_2ndRatio (S) of_UB/S_2nd) X 100 (%) is 2.0-15%;

a distribution density N of particles having an aspect ratio of 3.5 or less and a circle equivalent particle diameter of 0.02 to 0.25 [ mu ] m, which are present in the interior of the structure comprising 1 or 2 or more of the upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_θIs 7 pieces/mum²Below and including 0 pieces/. mu.m²；

A total area ratio S of particles having a circle-equivalent particle diameter of 1.3 to 20 μm and an aspect ratio of 3 or less_γBlockIs 5% or less and includes 0%.

2. The steel sheet according to claim 1, wherein the distribution density N of particles having an aspect ratio of 3.6 to 15 and a particle width of 0.14 to 0.30 μm, which are present inside the structure comprising 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ, is within a range of 0.6 to 0.30 μm_Fine0.03 to 0.4 pieces/mum²。

3. The steel sheet according to claim 1 or 2, wherein the composition further contains, in mass%, a metal element selected from the group consisting of

Ti：0.002～0.1％、

B：0.0002～0.01％

1 or 2 of them.

4. Steel sheet according to any one of claims 1 to 3, wherein the composition further comprises, in mass%, (iii) a metal selected from the group consisting of

Cu：0.005～1％、

Ni：0.01～1％、

Cr：0.01～1.0％、

Mo：0.01～0.5％、

V：0.003～0.5％、

Nb：0.002～0.1％、

Zr: 0.005 to 0.2% and

W：0.005～0.2％

1 or 2 or more.

5. Steel sheet according to any one of claims 1 to 4, wherein the composition further comprises, in mass%, (iii) a metal selected from the group consisting of

Ca：0.0002～0.0040％、

Ce：0.0002～0.0040％、

La：0.0002～0.0040％、

Mg：0.0002～0.0030％、

Sb: 0.002 to 0.1% and

Sn：0.002～0.1％

1 or 2 or more.

6. Steel sheet according to any one of claims 1 to 5, wherein the tensile strength is between 590 and 1600 MPa.

7. The steel sheet according to any one of claims 1 to 6, wherein the steel sheet has a zinc plating layer on a surface thereof.

8. A method for manufacturing a steel sheet, comprising hot-rolling and cold-rolling a steel slab having the composition according to any one of claims 1 to 5, and then heating the resulting cold-rolled steel sheet on a continuous annealing line: heating at the temperature of 660-740 ℃ and 1-6 ℃/s, heating at the temperature of 740-770 ℃ and 1-6 ℃/s, and further annealing at the annealing temperature of 770-850 ℃; then, in a temperature range of 770-700 ℃, at an average cooling rate: cooling at 1-2000 ℃/s, and further cooling at an average cooling rate within a temperature range of 700-500 ℃: cooling at 8-2000 ℃/s, maintaining the temperature range of 500-405 ℃ for 13-200 sec, and further cooling from 405 ℃ to a cooling stop temperature Tsq of 170-270 ℃, wherein the average cooling speed is as follows: cooling at 1-50 ℃/s, and then cooling from the cooling stop temperature Tsq to 350 ℃ at an average heating rate: heating at 2 ℃/s or more, holding at 350-500 ℃ for 20-3000 sec, cooling to room temperature, and further cooling from the post-annealing to the average heating rate: in the step of heating at a temperature of 2 ℃/s or more, the residence time is 50s or less in the temperature range of 170 to 250 ℃.

9. The method for producing a steel sheet according to claim 8, wherein in the step of cooling from a cooling stop temperature Tsq in the range of 405 ℃ to 170 ℃ to 270 ℃, the cooling rate in the range of 320 ℃ to 270 ℃ is 0.3 ℃/s or more and less than 20 ℃/s.

10. The method for producing a steel sheet according to claim 8 or 9, wherein a dew point in the step of annealing at an annealing temperature of 770 to 850 ℃ is-45 ℃ or higher.

11. The method for producing a steel sheet according to any one of claims 8 to 10, wherein the average cooling rate is set in the range of 700 to 500 ℃ in terms of the average cooling rate: performing a zinc plating treatment or an alloying zinc plating treatment between the step of cooling at 8-2000 ℃/s and the step of maintaining at a temperature of 500-405 ℃ for 13-200 sec.

12. The method for producing a steel sheet according to any one of claims 8 to 10, wherein the step of maintaining the steel sheet at 350 to 500 ℃ for 20 to 3000sec is followed by a zinc plating treatment or an alloyed zinc plating treatment.

Technical Field

The present invention relates to a steel sheet and a method for manufacturing the same, which can be suitably used for roll forming applications in which the steel sheet is used in a roll forming process in automobiles, home appliances, and the like.

Background

In recent years, due to further increasing demands for weight reduction of automobile bodies, 980 to 1470MPa grade high strength steel sheets have also been applied to automobile frame parts, bumpers, seat parts, and the like. Further, the use of laser welding has also been advancing to reduce the weight and increase the rigidity of parts. For example, the use of tailor welded blanks, which are produced by joining steel sheets having different thicknesses and strengths before rolling, has been developed, including the laser welding of the end portions and flange portions of rolled parts to form a closed cross-section structure, the shortening of flanges, and the improvement of strength of welded portions in conventional spot welding.

However, when 590 to 1470MPa grade high-strength steel sheet is applied to automobile parts, rolling fracture due to reduction in ductility is likely to occur. In addition, since hard martensite is often used in the microstructure of these steel sheets in order to improve strength and ductility, if laser welding is applied to these steel sheets, large softening due to softening of martensite occurs in a heat affected zone (hereinafter referred to as HAZ). As a result, the following problems occur: the HAZ softened portion is broken during roll forming, and the HAZ softened portion is broken preferentially when the part is deformed, which reduces the strength of the part. Therefore, it is desired that these high-strength steel sheets have excellent formability and a low HAZ softening.

Under such a background, for example, TRIP steel in which residual γ is dispersed in a microstructure of a steel sheet has been developed as a technique for improving ductility of the steel sheet. For example, patent document 1 discloses: by mixing a mixture containing C: 0.10-0.45%, S: 0.5 to 1.8%, Mn: keeping 0.5-3.0% of steel at 350-500 ℃ for 1-30 min after annealing to generate residual gamma, thereby obtaining TS: 80kgf/mm²Above and having TS × El ≥ 2500kgf/mm²High ductility steel sheet.

Patent document 2 discloses: by mixing a mixture containing C: 0.10 to 0.25%, Si: 1.0-2.0%, Mn: 1.5 to 3.0% of steel is cooled to 450 to 300 ℃ at a temperature of 10 ℃/s or more after annealing, and is maintained for 180 to 600 seconds, and the ductility is obtained by controlling the ratio of retained austenite to be 5% or more, bainitic ferrite to be 60% or more, and polygonal ferrite to be 20% or less, in terms of area ratio: el and stretch flange formability: a steel sheet excellent in lambda.

Patent document 3 discloses: the catalyst contains C: 0.10 to 0.28%, Si: 1.0-2.0%, Mn: 1.0-3.0% of steel in A₃Annealing in a temperature region above the transformation point, and then cooling at a cooling rate of 1-10 ℃: slowly cooling to A at 1-10 DEG C₃-250～A₃-20 ℃ to produce ferrite, and then cooling the steel plate to a bainite transformation temperature region (320-450 ℃) while avoiding ferrite transformation at a cooling rate of 11 ℃/sec or more, thereby obtaining bainitic ferrite in terms of area ratio: 30-65%, polygonal ferrite: 30-50%, retained austenite: 5-20% of TS xEl is more than or equal to 23000 MPa.

As a technique for improving the HAZ softening resistance, for example, patent document 4 discloses a technique of: in the case of a composition containing martensite: 5-40% of a steel sheet having a microstructure of tensile strength of 780MPa or more, which steel sheet contains C: 0.05 to 0.20%, Si: 0.005-1.3%, Mn: 1.0-3.2%, Mo: 0.05 to 0.5%, further comprising Nb: 0.005-0.05%, Ti: 0.001 to 0.05% of 1 or 2 or more species, thereby forming fine carbides in the heat-affected zone and suppressing temper softening of martensite.

Further, in patent document 5, a composition containing C: 0.01 to 0.3%, Si: 0.005-2.5%, Mn: 0.01-3%, Mo: 0.01 to 0.3%, Nb: 0.001 to 0.1% of a steel sheet is annealed in a high temperature region where a structure close to a γ single phase is formed, and then cooled and held at a temperature of 200 to 450 ℃, thereby containing 50 to 97% of bainite or bainitic ferrite as a main phase and 3 to 50% of austenite as a 2 nd phase, whereby a steel sheet excellent in ductility, stretch flange formability, and weldability can be obtained.

Documents of the prior art

Patent document

Patent document 1 Japanese examined patent publication No. 6-35619

Patent document 2 Japanese patent No. 4411221

Patent document 3 Japanese patent No. 4716359

Patent document 4 Japanese patent No. 3881559

Patent document 5 Japanese patent No. 3854506

Disclosure of Invention

However, the conventional TRIP steel described in patent document 1 has the following problems: although El is excellent, the HAZ softening resistance of the weld is significantly deteriorated. That is, if plastic deformation is applied to the HAZ softened portion of the laser-welded steel sheet, breakage occurs in the HAZ that is softer than the base material, and therefore, it is difficult to apply laser welding, or it takes time and effort to provide a weld line at a portion where deformation is not applied.

In the technique described in patent document 2, bainitic ferrite is mainly used as a microstructure, and ferrite is suppressed to a small amount, so that the stretch flange formability is excellent, but the ductility is not necessarily high.

In the technique described in patent document 3, the ductility is improved by using ferrite as a microstructure, but there is a problem that it is difficult to achieve high strength of 980MPa or more in particular by using ferrite. In addition, the HAZ softening resistance is significantly deteriorated.

In the technique described in patent document 4, although the tempering softening resistance of martensite is improved, the improvement effect of the HAZ softening resistance is insufficient. Softening of martensite in the HAZ cannot be sufficiently suppressed, and a sufficient improvement effect cannot be obtained particularly in a strength steel sheet of 980MPa or higher.

The technique described in patent document 5 is effective in improving a material having higher strength when the laser welding speed is high and the heat input is small, but in the case of laser output: 4-6 kW, welding speed: the HAZ softening becomes large under normal or low-speed welding conditions of 3-5 mpm, and the HAZ has a problem of breakage when a tensile test is performed on an interface including a laser welding line in a direction perpendicular to a tensile axis. Further, ductility is not necessarily high, and further improvement in ductility is desired. In addition, a large amount of expensive Mo and Nb needs to be added, and cost reduction is also required.

As described above, in the prior art, particularly, in the high strength steel sheet of 980MPa class or more, no steel sheet having high ductility and excellent HAZ softening resistance has been found.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a steel sheet having high ductility and exhibiting excellent HAZ softening resistance in a welded portion, and a method for manufacturing the same.

The present inventors have made intensive studies on means for ensuring both high ductility and excellent HAZ softening resistance, and have reached the following findings.

(i) In the microstructure of the steel, the residual gamma (residual gamma) having a particle width of 0.17 to 0.80 μm and an aspect ratio of 4 to 25 is added_UB) Adjacent, coarse upper bainite is formed in a predetermined amount. Coarse upper bainite is soft and is formed by leaving gamma residues_UBThe C in the upper bainite can be retained in the γ direction adjacent to the coarse upper bainite_UBMoving to form a residual gamma region not easily affected by heat and a residual gamma region not easily affected by heatA region composed of soft bainite having a small C content. In addition, such a region also has an effect of improving ductility.

(ii) In the interior of a structure comprising 1 or 2 or more types of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ, the distribution density N of particles having an aspect ratio of 3.5 or less and a circle-equivalent particle diameter of 0.02 to 0.25 μm is set to be_θIs 7 pieces/mum²The following. The region having a large distribution density of the particles means hard tempered martensite or lower bainite. In this region, further tempering is performed during the welding process, and softening is likely to occur. Therefore, by reducing the region having a high particle density, the region having a low particle density in which softening due to tempering is less likely to occur is regarded as the main structure, and the HAZ softening resistance is improved.

(iii) The total area ratio S of particles having a circle equivalent particle diameter of 1.3 to 20 μm and an aspect ratio of 3 or less_γBlockThe content is controlled to be less than 5%. The coarse particles are mostly fresh martensite, which significantly increases the strength of the base material, but on the other hand, softening by heat input is easily generated, which significantly deteriorates the HAZ softening resistance.

(iv) Such a tissue can be obtained as follows: the heating rate in the annealing step is controlled, and the annealing is performed by holding the alloy at around 450 ℃ for a predetermined time in the cooling step after annealing, cooling the alloy to around 200 ℃, and then reheating the alloy at around 400 ℃ for holding the alloy. As a result, both ductility and HAZ softening resistance can be improved.

The present invention has been made in view of the above, and specifically provides the following.

[1] A steel sheet having a specific composition and a steel structure, wherein the steel sheet contains, as the composition, a steel sheet having a specific composition and a specific steel structure

C:0.06～0.25％、

Si:0.1～2.5％、

Mn：2.0～3.2％、

P: less than 0.02 percent,

S: less than 0.01 percent,

Al: less than 1.0% (including 0%),

N: less than 0.015 percent,

total of Si and sol. al content: 0.7 to 2.5 percent of Si + sol-Al,

the remainder being made up of Fe and unavoidable impurities;

the steel structure contains 6-90% ferrite by area percentage, 10-94% in total of the structure composed of 1 or more than 2 of upper bainite, fresh martensite, tempered martensite, lower bainite and residual gamma, and 3-20% residual gamma by volume percentage;

and residual gamma of 0.17 to 0.80 μm in particle width and 4 to 25 in aspect ratio_UBAn area ratio S of upper bainite having a width of 0.8 to 7 μm, a length of 2 to 15 μm and an aspect ratio of 2.2 or more_UBAn area ratio S to the structure composed of 1 or 2 or more of the upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_2ndThe ratio of (A) to (B): (S)_UB/S_2nd) X 100 (%) is 2.0-15%;

a distribution density N of particles having an aspect ratio of 3.5 or less and a circle equivalent particle diameter of 0.02 to 0.25 μm, which are present in the interior of a structure comprising 1 or 2 or more species of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_θIs 7 pieces/mum²The following (including 0/. mu.m)²)；

A total area ratio S of particles having a circle-equivalent particle diameter of 1.3 to 20 μm and an aspect ratio of 3 or less_γBlock5% or less (including 0%).

[2]According to [1]The steel sheet is characterized in that the distribution density N of particles having an aspect ratio of 3.6 to 15 and a particle width of 0.14 to 0.30 [ mu ] m, which are present in the structure comprising 1 or 2 or more species of upper bainite, fresh martensite, tempered martensite, lower bainite and residual gamma-phase_Fine0.03 to 0.4 pieces/mum²。

[3] The steel sheet according to the above [1] or [2], wherein the composition further comprises, in mass%

Ti：0.002～0.1％、

B: 0.0002-0.01% of 1 or 2 kinds.

[4] The steel sheet according to any one of the above [1] to [3], wherein the composition further comprises, in mass%, [

Cu：0.005～1％、

Ni：0.01～1％、

Cr：0.01～1.0％、

Mo：0.01～0.5％、

V：0.003～0.5％、

Nb：0.002～0.1％、

Zr: 0.005-0.2% and W: 0.005-0.2% of 1 or more than 2.

[5] The steel sheet according to any one of the above [1] to [4], wherein the composition further comprises, in mass%, [

Ca：0.0002～0.0040％、

Ce：0.0002～0.0040％、

La：0.0002～0.0040％、

Mg：0.0002～0.0030％、

Sb: 0.002-0.1% and Sn: 0.002-0.1% of 1 or more than 2.

[6] The steel sheet according to any one of [1] to [5], wherein the tensile strength is 590 to 1600 MPa.

[7] The steel sheet according to any one of claims 1 to 6, wherein a zinc plating layer is provided on a surface of the steel sheet.

[8] A method for manufacturing a steel sheet, wherein a steel blank having the composition of any one of [1] to [5] is hot-rolled and cold-rolled, and then the obtained cold-rolled steel sheet is heated in a continuous annealing line at 1-6 ℃/s at a temperature range of 660-740 ℃, 1-6 ℃/s at a temperature range of 740-770 ℃, and further annealed at an annealing temperature of 770-850 ℃; then, in a temperature range of 770-700 ℃, at an average cooling rate: cooling at 1-2000 ℃/s, and further cooling at an average cooling rate within a temperature range of 700-500 ℃: cooling at 8-2000 ℃/s, maintaining the temperature range of 500-405 ℃ for 13-200 sec, and further cooling from 405 ℃ to a cooling stop temperature Tsq of 170-270 ℃, wherein the average cooling speed is as follows: cooling at 1-50 ℃/s, and then cooling from the cooling stop temperature Tsq to 350 ℃ at an average heating rate: heating at 2 ℃/s or more, holding at 350 to 500 ℃ for 20 to 3000sec, cooling to room temperature, and further cooling from the post-annealing cooling to the average heating rate: in the step of heating at a temperature of 2 ℃/s or more, the residence time in the temperature range of 170 to 250 ℃ is 50s or less.

[9] The method for producing a steel sheet according to item [8], wherein in the step of cooling from a cooling stop temperature Tsq in the range of 405 ℃ to 170 ℃ to 270 ℃, the cooling rate in the range of 320 ℃ to 270 ℃ is 0.3 ℃/s or more and less than 20 ℃/s.

[10] The method for producing a steel sheet according to claim 8 or 9, wherein a dew point in the step of annealing at an annealing temperature of 770 to 850 ℃ is-45 ℃ or higher.

[11] The method for producing a steel sheet according to any one of [8] to [10], wherein the average cooling rate in a temperature range of 700 to 500 ℃ is: performing a zinc plating treatment or an alloying zinc plating treatment between the step of cooling at 8-2000 ℃/s and the step of maintaining at a temperature of 500-405 ℃ for 13-200 sec.

[12] The method for producing a steel sheet according to any one of [8] to [10], wherein the step of holding at 350 to 500 ℃ for 20 to 3000sec is followed by zinc plating or alloyed zinc plating.

According to the present invention, a steel sheet having both high ductility and excellent HAZ softening resistance in a welded portion can be obtained. Further, according to the present invention, high strength can be achieved.

Drawings

Fig. 1 is a diagram showing an example of an SEM image.

Fig. 2 is a diagram for explaining aspect ratio, particle width, and particle length.

Detailed Description

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

The steel sheet of the present invention has a specific composition and a specific steel structure. Therefore, the steel sheet of the present invention will be described in the order of the composition and the steel structure.

The steel sheet of the present invention contains the following components. In the following description, "%" as a unit of component content means "% by mass".

C：0.06～0.25％

C has the following functions: by increasing the strength of the molten portion and the quenched portion from the γ region in the welded joint, deformation in the HAZ is suppressed, and the HAZ softening resistance is improved. In addition, from the viewpoint of securing a predetermined strength by securing the area ratio of tempered martensite, from the viewpoint of securing the volume ratio of residual γ to improve ductility, from the viewpoint of thickening in residual γ to stabilize residual γ to improve ductility, C needs to be contained. If the content of C is less than 0.06%, these effects cannot be sufficiently ensured, so the lower limit thereof is 0.06%. Preferably 0.09% or more, more preferably 0.11% or more. If the C content is more than 0.25%, transformation of upper bainite at the time of intermediate holding during cooling is delayed, and it is difficult to sufficiently secure the retained γ_UBThe amount of upper bainite formed contiguously. As a result, the HAZ softening resistance and ductility are reduced. Therefore, the upper limit of the C content is 0.25%. From the viewpoint of improving the HAZ softening resistance and ductility, the C content is desirably 0.22% or less. From the viewpoint of improving the HAZ softening resistance and ductility, the C content is further desirably 0.20% or less.

Si:0.1～2.5％

Si has the following functions: by suppressing the formation of carbides in martensite and bainite and increasing the amount of solid solution strengthening which is less susceptible to thermal influence, the HAZ softening resistance in the welded portion is improved, the stability of the retained γ is improved, and the ductility is improved. From these viewpoints, the Si content is 0.1% or more. From these viewpoints, the Si content is preferably 0.6% or more. More preferably 0.8% or more, and still more preferably 1.1% or more. If the Si content is more than 2.5%, the rolling load becomes extremely high, and the production of a thin plate becomes difficult. In addition, chemical conversion treatability and toughness of the weld zone are deteriorated. Therefore, the content of Si is 2.5% or less. From the viewpoint of ensuring chemical treatability and toughness of the material and the welded portion, the content of Si is preferably less than 2.0%. From the viewpoint of securing toughness of the welded portion, the content of Si is 1.8% or less, and more preferably 1.5% or less.

Mn:2.0～3.2％

Mn is an important element from the viewpoint of securing strength by securing tempered martensite and/or bainite of a predetermined area ratio, from the viewpoint of stabilizing the residual γ by lowering the Ms point of the residual γ and improving ductility, from the viewpoint of enhancing ductility by suppressing the generation of carbide in bainite similarly to Si, and from the viewpoint of enhancing ductility by increasing the volume ratio of the residual γ. In order to obtain these effects, the Mn content is set to 2.0% or more. The Mn content is preferably 2.3% or more from the viewpoint of stabilizing the residual γ to improve ductility. More preferably 2.5% or more, and still more preferably 2.7% or more. If the Mn content is more than 3.2%, upper bainite transformation in the course of cooling is delayed, making it difficult to sufficiently secure the residual γ_UBThe amount of upper bainite formed contiguously. As a result, the HAZ softening resistance and ductility are reduced. Therefore, the Mn content is 3.2% or less. From the viewpoint of promoting bainite transformation to ensure high ductility, the Mn content is preferably 3.0% or less. More preferably 2.9% or less.

P: less than 0.02%

P is an element for reinforcing steel, but if the content thereof is too large, the spot weldability deteriorates. Therefore, P is set to 0.02% or less. From the viewpoint of improving the spot weldability, P is preferably 0.01% or less. P may not be included, but the content of P is preferably 0.001% or more from the viewpoint of production cost.

S is less than 0.01%

S is the following element: the steel sheet has an effect of improving scale removability during hot rolling and an effect of suppressing nitriding during annealing, but has a great adverse effect on spot weldability, bendability, and hole expansibility. In order to reduce these adverse effects, S is set to at least 0.01% or less. In the present invention, the content of C, Si, and Mn is high, so that the spot weldability is easily deteriorated, and from the viewpoint of improving the spot weldability, S is preferably 0.0020% or less, and more preferably less than 0.0010%. S may not be included, but the S content is preferably 0.0001% or more from the viewpoint of production cost.

Al: less than 1.0% (including 0%)

Al has the following effects: the HAZ softening resistance is improved by suppressing the formation of carbides and promoting the transformation of upper bainite. The lower limit of al is not specified, but is preferably 0.01% or more for stable deoxidation. On the other hand, if sol.al is 1.0% or more, the strength of the material is extremely lowered, and the chemical conversion treatability is also adversely affected, so sol.al is less than 1.0%. The amount of sol.al may be 0%, but in order to obtain high strength, sol.al is preferably less than 0.50%, and more preferably less than 0.10% or less.

N: less than 0.015 percent

N is an element that forms a nitride such as BN, AlN, or TiN in steel, and is an element that reduces hot rolling ductility and surface quality of steel. In addition, in the steel containing B, there is a disadvantage that the effect of B is lost by the formation of BN. If the N content is 0.015% or more, the surface quality is significantly deteriorated. Therefore, the N content is set to less than 0.015%. N may not be included, but the content of N is preferably 0.0001% or more from the viewpoint of production cost.

Si+sol.Al：0.7～2.5％

Si and sol.al both have an effect of improving the HAZ softening resistance, and in order to sufficiently exert these effects, the total content thereof needs to be 0.7% or more. More preferably 0.8% or more. More preferably 0.9% or more, and particularly preferably 1.1% or more. If these contents are too large, the strength is significantly reduced by excessively increasing ferrite and upper bainite, and therefore, these contents need to be 2.5% or less in total. More preferably 2.0% or less.

The steel sheet of the present invention may contain the following optional elements in addition to the above components.

Ti：0.002～0.1％

Ti has an effect of fixing N in the steel to TiN to improve hot rolling ductility and an effect of improving hardenability of B. In addition, precipitation of TiC and refinement of the structure due to this precipitation also have an effect of improving the HAZ softening resistance. In order to obtain such an effect, it is desirable to set the Ti content to 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. More preferably 0.010% or more. On the other hand, if the Ti content is more than 0.1%, an increase in rolling load and a decrease in ductility due to an increase in precipitation strengthening amount are caused, and therefore, it is desirable that the Ti content is 0.1% or less. More preferably 0.05% or less. In order to ensure high ductility, Ti is preferably 0.03% or less.

B：0.0002～0.01％

B is an element for improving the hardenability of steel, and has an advantage that tempered martensite and/or bainite having a predetermined area ratio can be easily formed. In addition, by increasing the hardenability in the vicinity of the welded portion, a hard structure is formed in the vicinity of the welded portion, and thereby the HAZ softening resistance is improved. In addition, the delayed fracture resistance is improved. In order to obtain such an effect of B, the content of B is preferably 0.0002% or more. Further, the B content is more preferably 0.0005% or more. More preferably 0.0010% or more. On the other hand, if the B content is more than 0.01%, not only the effect is saturated but also a significant reduction in hot rolling ductility is brought about, and surface defects are generated. Therefore, the B content is preferably 0.01% or less. More preferably 0.0050% or less. A more preferable range is 0.0030% or less.

Cu：0.005～1％

Cu improves corrosion resistance in the use environment of automobiles. Further, corrosion products of Cu coat the surface of the steel sheet, and have an effect of suppressing hydrogen intrusion into the steel sheet. Cu is an element mixed when scrap iron is used as a raw material, and by allowing Cu to be mixed, a recycled material can be used as a raw material, and the production cost can be reduced. From such a viewpoint, Cu is preferably contained in an amount of 0.005% or more, and from the viewpoint of the delayed fracture resistance property, it is desirable to contain Cu in an amount of 0.05% or more. More preferably 0.10% or more. However, if the Cu content is too large, surface defects may be generated, and therefore, the Cu content is desirably 1% or less. More preferably 0.4% or less, and still more preferably 0.2% or less.

Ni：0.01～1％

Like Cu, Ni is an element having an effect of improving corrosion resistance. In addition, Ni has an effect of suppressing surface defects that are likely to occur when Cu is contained. Therefore, it is desirable to contain 0.01% or more of Ni. More preferably 0.04% or more, and still more preferably 0.06% or more. However, if the Ni content is too large, scale formation in the heating furnace becomes uneven, and this may cause surface defects. In addition, it also causes an increase in cost. Therefore, the Ni content is desirably 1% or less. More preferably 0.4% or less, and still more preferably 0.2% or less.

Cr：0.01～1.0％

Cr has an effect of improving the hardenability of steel and an effect of suppressing the formation of carbides in martensite and upper and lower bainite, and therefore Cr may be contained. Further, by increasing the hardenability in the vicinity of the welded portion, a hard phase is formed in the vicinity of the welded portion, and thereby the HAZ softening resistance is improved. In order to obtain such effects, the Cr content is desirably 0.01% or more. More preferably 0.03% or more, and still more preferably 0.06% or more. However, if Cr is contained excessively, pitting corrosion resistance deteriorates, so the Cr content is desirably 1.0% or less. More preferably 0.8% or less, and still more preferably 0.4% or less.

Mo：0.01～0.5％

Mo has an effect of improving the hardenability of steel and an effect of suppressing the formation of carbides in martensite and upper and lower bainite, and therefore Mo may be contained. Further, by increasing the hardenability in the vicinity of the welded portion, a hard phase is formed in the vicinity of the welded portion, and thereby the HAZ softening resistance is improved. In order to obtain such an effect, the Mo content is desirably 0.01% or more. More preferably 0.03% or more, and still more preferably 0.06% or more. However, Mo significantly deteriorates the chemical conversion treatability of the cold-rolled steel sheet, and therefore the content thereof is preferably 0.5% or less. From the viewpoint of improving the chemical conversion treatability, Mo is more preferably 0.15% or less, and still more preferably less than 0.10%.

V：0.003～0.5％

V is contained because it has an effect of improving the hardenability of steel, an effect of suppressing the formation of carbides in martensite and upper/lower bainite, an effect of refining the structure, and an effect of precipitating carbides and improving the delayed fracture resistance. Further, by increasing the hardenability in the vicinity of the welded portion, a hard phase is formed in the vicinity of the welded portion, and thereby the HAZ softening resistance is improved. In order to obtain the effect, the V content is desirably 0.003% or more. More preferably 0.005% or more, and still more preferably 0.010% or more. However, if V is contained in a large amount, castability is significantly deteriorated, and therefore, the V content is desirably 0.5% or less. More preferably 0.3% or less, and still more preferably 0.1% or less.

Nb：0.002～0.1％

Nb has an effect of refining the steel structure and increasing the strength, an effect of promoting bainite transformation by grain refining, an effect of improving the bendability, and an effect of improving the delayed fracture resistance, and therefore, Nb may be contained. Further, by increasing the hardenability in the vicinity of the welded portion, a hard phase is formed in the vicinity of the welded portion, and thereby the HAZ softening resistance is improved. In order to obtain the effect, the Nb content is desirably 0.002% or more. More preferably 0.004% or more, and still more preferably 0.010% or more. However, if Nb is contained in a large amount, precipitation strengthening becomes excessively strong, and ductility deteriorates. In addition, the rolling load is increased, and the castability is deteriorated. Therefore, the Nb content is desirably 0.1% or less. More preferably 0.05% or less, and still more preferably 0.03% or less.

Zr：0.005～0.2％

Zr is contained because it has an effect of improving the hardenability of steel, an effect of suppressing the formation of carbides in bainite, an effect of refining the structure, and an effect of precipitating carbides and improving the delayed fracture resistance. In order to obtain such an effect, the Zr content is desirably 0.005% or more. More preferably 0.008% or more, and still more preferably 0.010% or more. However, if Zr is contained in a large amount, coarse precipitates of ZrN and ZrS remaining without solid solution increase during heating of the billet before hot rolling, and delayed fracture resistance deteriorates. Therefore, the Zr content is desirably 0.2% or less. More preferably 0.15% or less, and still more preferably 0.08% or less.

W：0.005～0.2％

W may be contained because it has an effect of improving the hardenability of steel, an effect of suppressing the formation of carbides in bainite, an effect of refining the structure, an effect of precipitating carbides and an effect of improving the delayed fracture resistance. In order to obtain such an effect, the W content is desirably 0.005% or more. More preferably 0.008% or more, and still more preferably 0.010% or more. However, if W is contained in a large amount, coarse precipitates such as WN and WS remaining undissolved during heating of the material before hot rolling increase, and delayed fracture resistance deteriorates. Therefore, the W content is desirably 0.2% or less. More preferably 0.15% or less, and still more preferably 0.08% or less.

Ca：0.0002～0.0040％

Ca fixes S to CaS, contributing to improvement of bendability and improvement of delayed fracture resistance. Therefore, the Ca content is preferably 0.0002% or more. More preferably 0.0005% or more, and still more preferably 0.0010% or more. However, since addition of a large amount of Ca deteriorates the surface quality and bendability, the Ca content is desirably 0.0040% or less. More preferably 0.0035% or less, and still more preferably 0.0020% or less.

Ce：0.0002～0.0040％

Ce fixes S similarly to Ca, and contributes to improvement of bendability and improvement of delayed fracture resistance. Therefore, the Ce content is preferably 0.0002% or more. More preferably 0.0004% or more, and still more preferably 0.0006% or more. However, since the surface quality and bendability deteriorate if a large amount of Ce is added, the Ce content is desirably 0.0040% or less. More preferably 0.0035% or less, and still more preferably 0.0020% or less.

La：0.0002～0.0040％

La fixes S similarly to Ca, and contributes to improvement of bendability and improvement of delayed fracture resistance. Therefore, the La content is preferably 0.0002% or more. More preferably 0.0004% or more, and still more preferably 0.0006% or more. However, if a large amount of La is added, the surface quality and bendability deteriorate, and therefore, it is desirable that the La content be 0.0040% or less. More preferably 0.0035% or less, and still more preferably 0.0020% or less.

Mg：0.0002～0.0030％

Mg fixes O to MgO, contributing to improvement of delayed fracture resistance. Therefore, the Mg content is preferably 0.0002% or more. More preferably 0.0004% or more, and still more preferably 0.0006% or more. However, if Mg is added in a large amount, the surface quality and bendability deteriorate, and therefore, the Mg content is desirably 0.0030% or less. More preferably 0.0025% or less, and still more preferably 0.0010% or less.

Sb:0.002～0.1％

Sb suppresses oxidation and nitridation of the surface layer portion of the steel sheet, thereby suppressing a decrease in the content of C, B in the surface layer. Further, by suppressing the above reduction in the content of C, B, the delayed fracture resistance is improved while suppressing the generation of ferrite at the surface layer portion of the steel sheet, thereby achieving high strength. From such a viewpoint, the Sb content is desirably 0.002% or more. More preferably 0.004% or more, and still more preferably 0.006% or more. However, if the Sb content is more than 0.1%, castability deteriorates, and segregation to the old γ -grain boundary occurs, and the delayed fracture resistance of the sheared edge face deteriorates. Therefore, the Sb content is desirably 0.1% or less. More preferably 0.04% or less, and still more preferably 0.03% or less.

Sn:0.002～0.1％

Sn suppresses oxidation and nitridation of the surface layer portion of the steel sheet, thereby suppressing a decrease in the content of C, B in the surface layer. Further, by suppressing the above reduction in the content of C, B, the delayed fracture resistance is improved while suppressing the generation of ferrite at the surface layer portion of the steel sheet, thereby achieving high strength. From such a viewpoint, the Sn content is desirably 0.002% or more. More preferably 0.004% or more, and still more preferably 0.006% or more. However, if the Sn content is more than 0.1%, the castability is deteriorated. In addition, Sn segregates to the old γ grain boundaries, and the delayed fracture resistance of the sheared edge faces deteriorates. Therefore, the Sn content is desirably 0.1% or less. More preferably 0.04% or less, and still more preferably 0.03% or less.

The balance other than the above is Fe and inevitable impurities. In addition, when any of the above components is contained in an amount less than the lower limit value, any element contained in an amount less than the lower limit value does not impair the effects of the present invention. Therefore, when the above-mentioned arbitrary element is contained in an amount smaller than the lower limit, the above-mentioned arbitrary element is regarded as an inevitable impurity.

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

Ferrite: 6 to 90 percent

In order to ensure high ductility, the ferrite is 6% or more in area ratio. From the viewpoint of ensuring high ductility, ferrite is preferably 8% or more in terms of area percentage, and more preferably more than 10%. However, since an excessive increase in ferrite causes a decrease in strength and suppresses the formation of upper bainite, ferrite is 90% or less in area ratio. From the viewpoint of securing strength, ferrite is preferably 85% or less, and more preferably 70% or less in area ratio. Here, ferrite refers to polygonal ferrite.

A structure composed of 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ: 10 to 94 percent

In order to ensure predetermined strength and ductility, the total area ratio of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ, which are the remaining portions other than polygonal ferrite, is 10 to 94%. The lower limit is preferably 15% or more, and more preferably 30% or more. It is considered that the area ratios of the respective structures of the upper bainite, the fresh martensite, the tempered martensite, and the lower bainite are mostly in the following ranges. The upper bainite is 1 to 30% in terms of area ratio. The fresh martensite is 0 to 10% in terms of area ratio. The tempered martensite is 2 to 60% in area ratio. The lower bainite accounts for 2-70% of the area ratio.

Residual gamma: 3 to 20 percent

In order to ensure high ductility, the residual γ is 3% or more by volume relative to the entire steel structure. More preferably 6% or more, and still more preferably 8% or more. The residual γ content includes both residual γ and martensite, which are generated adjacent to the upper bainite, and residual γ and martensite, which are generated adjacent to the lower bainite. If the amount of retained γ is excessively increased, the strength is lowered, the stretch flange formability is lowered, and the delayed fracture resistance is deteriorated. Therefore, the volume fraction of the residual γ is set to 20% or less. More preferably 15% or less. The "volume fraction" may be regarded as "area fraction".

And residual gamma of 0.17 to 0.80 μm in particle width and 4 to 25 in aspect ratio_UBAn area ratio S of upper bainite having a width of 0.8 to 7 μm, a length of 2 to 15 μm and an aspect ratio of 2.2 or more_UBAn area ratio S to a structure consisting of 1 or 2 or more kinds of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_2ndThe ratio of (A) to (B): (S)_UB/S_2nd) X 100 (%) is 2.0-15%

In the latter manufacturing method, by holding the steel in an intermediate temperature region of 500 to 405 ℃ in the cooling process, a predetermined amount of residual γ can be secured adjacent to upper bainite (bainitic ferrite) containing almost no carbide_UB. The residual gamma_UBThe particle has a particle width of 0.17 to 0.80 μm and an aspect ratio of 4 to 25. By making the residue gamma_UBAdjacent to the coarse upper bainite to leave the C in the upper bainite in the residual gamma direction_UBBy moving, thickening can be locally performed. As a result, the upper bainite does not substantially contain solid solution C, and is therefore less susceptible to thermal influence. In addition, the residual gamma is formed by making the upper bainite adjacent_UBCarbon is discharged, and thus residual γ that is not easily affected by heat can be generated. In addition, upper bainite and residual γ, which are effective for improving the resistance to HAZ softening_UBThe side-by-side structure has a width of 0.8 to 4 μm, a length of 2 to 15 μm, and an aspect ratio of 2.2 or more. The area ratio S of the upper bainite needs to be controlled_UBThe total area ratio S of the microstructure composed of 1 or 2 or more kinds of fresh martensite, tempered martensite, lower bainite and residual gamma_2ndIn the present invention, by making the ratio: (S)_UB/S_2nd) X 100 (%) is 2.0% or more, therebyThe HAZ softening resistance is improved. In addition, ductility is improved. From the viewpoint of improving the HAZ softening resistance, (S)_UB/S_2nd) X 100 (%) is preferably 3.0% or more. On the other hand, the area ratio S of the upper bainite_UBIf too much, the strength of the base material is greatly reduced. In addition, the ductility, stretch flange formability, and delayed fracture resistance are deteriorated. Thus, will (S)_UB/S_2nd) X 100 (%) is 15% or less. Preferably 12% or less, more preferably 10% or less. The area ratio S is defined as_UB、S_2ndRefers to the area ratio of the entire steel structure.

A distribution density N of particles having an aspect ratio of 3.5 or less and a circle equivalent particle diameter of 0.02 to 0.25 μm, which are present in the interior of a structure comprising 1 or 2 or more species of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_θIs 7 pieces/mum²The following (including 0/. mu.m)²)

Particles having an aspect ratio of 3.5 or less and a circle equivalent particle diameter of 0.02 to 0.25 μm, which are present in the interior of a structure comprising 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ, are mainly carbides. The region with a dense carbide distribution is hard tempered martensite and/or lower bainite, and is easily softened by the influence of heat. Therefore, from the viewpoint of improving the HAZ softening resistance, the distribution density N of particles having an aspect ratio of 3.5 or less and a circle-equivalent particle diameter of 0.02 to 0.25 μm existing in the interior of a structure composed of 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ is 3.5 or less_θIs 7 pieces/mum²The following. Preferably 6 pieces/. mu.m²Hereinafter, more preferably, 4 pieces/. mu.m²The following. The lower limit of the distribution density of the particles is not particularly limited, but is preferably 0 particles/. mu.m². The carbide is remarkably generated when the cooling rate is too high in a temperature range of 320 ℃ or less, or when the holding time is too long at a low temperature of 250 ℃ or less.

Exists in upper bainite, fresh martensite, tempered martensite, lower bainite and residual martensiteA distribution density N of particles having an aspect ratio of 3.6 to 15 and a particle width of 0.14 to 0.30 μm within a structure of 1 or 2 or more species of gamma_Fine0.03 to 0.4 pieces/mum²(preferred conditions).

Particles having an aspect ratio of 3.6 to 15 and a particle width of 0.14 to 0.30 μm existing in the interior of a structure composed of 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ are mainly residual γ generated in the lower bainite (including the outer peripheral portion). The particles have both an effect of improving ductility and an effect of improving the HAZ softening resistance. In order to obtain the effect, the distribution density N of particles having an aspect ratio of 3.6 to 15 and a particle width of 0.14 to 0.30 μm existing in the inside of a structure comprising 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_FinePreferably 0.03 pieces/. mu.m²The above. More preferably 0.04 pieces/. mu.m²The above. More preferably 0.05/. mu.m²The above. An excessive increase of the particles leads to a decrease of λ, and thus, the distribution density N of the particles_FinePreferably 0.4 pieces/. mu.m²The following. More preferably 0.3 pieces/. mu.m²The following.

A total area ratio of particles having a circle-equivalent particle diameter of 1.3 to 20 μm and an aspect ratio of 3 or less: s_γBlockIs 5% or less (including 0%)

The structure is mainly fresh martensite. TRIP steel obtained by conventional austempering contains many such particles and has poor HAZ softening resistance. In the cooling step after annealing, the bulk structure can be reduced by rapidly cooling the steel sheet to 405 ℃ or lower, and further, by moderately cooling the steel sheet to 270 ℃ or lower at a slow cooling rate in a low-temperature region of 320 ℃ or lower. In order to obtain excellent HAZ softening resistance, the total area ratio of particles having a circle-equivalent particle diameter of 1.3 to 20 μm and an aspect ratio of 3 or less is required to be as follows: s_γBlockIs 5% or less. More preferably 4% or less, and still more preferably 3% or less.

Next, a method of measuring the steel structure will be described.

Ferrite, residual gamma_UBThe shape and area ratio of the upper bainite are measured according to the following method: a plate thickness cross section parallel to the rolling direction was cut out, mirror polished, etched with 3 vol.% nitric acid alcohol solution (nital), and 8 fields were observed at a thickness position of 1/4 by SEM at 5000 × magnification. The ferrite has almost no carbide inside, and is mainly equiaxed polygonal ferrite with an aspect ratio of less than 2.2. In SEM, the areas that appear most dark are the most intense. Residual gamma_UBWhite particles in the SEM. The upper bainite has almost no carbide inside, and is a black-appearing region in sem (scanning electron microscope), similarly to ferrite. Classifying the region with an aspect ratio of 2.2 or more as upper bainite (bainitic ferrite), and calculating the area ratio S_UB. Here, as shown in fig. 2, the aspect ratio is obtained by obtaining the major axis length a where the particle length is longest, and the minor axis length b is the particle length where the particle is longest when it crosses the perpendicular direction, and the aspect ratio is a/b. When a plurality of particles are in contact with each other, the particles are divided at the positions of the broken lines shown in fig. 2 so that the regions where the particles are in contact with each other are substantially equally divided, and the sizes of the particles are measured.

An area ratio S of a structure comprising 1 or 2 or more species of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_2ndThe measurement was performed by the same method as ferrite. The area ratio is an area ratio of a region other than the ferrite. Here, the area ratio of carbide is extremely small, and therefore, the area ratio is included in the above.

The volume fraction of the residual γ was determined by X-ray diffraction by chemically polishing the position separated from the surface layer 1/4 by thickness. On the incident X-ray, the area fraction of retained austenite was calculated from the intensity ratios of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite using a Co — K α source. Here, since the residual γ is randomly distributed, the volume fraction of the residual γ obtained by X-ray diffraction is equal to the area fraction of the residual γ in the steel structure.

An aspect ratio of 3.5 or less in a structure composed of 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γAnd the distribution density N of particles having a circle equivalent particle diameter of 0.02 to 0.25 μm_θThe measurement was similarly carried out from the SEM photograph. The region to be measured for the distribution density is the above-described structure, and ferrite is not contained. That is, the number of particles having the size present inside (including the outer periphery) of the structure is determined, and the distribution density N is determined by dividing the number by the area of the structure_θ。

Further, the circle-equivalent particle diameter (circle-equivalent particle diameter) was calculated as a circle-equivalent particle diameter by observing each particle with SEM and calculating the area ratio thereof.

A distribution density N of particles having an aspect ratio of 3.6 to 40 and a particle width of 0.14 to 0.30 [ mu ] m, which are present in the interior of a structure comprising 1 or 2 or more species of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ_FineThe measurement was similarly carried out from the SEM photograph. The region to be measured for the distribution density is the above-described structure, and ferrite is not contained. That is, the number of particles having the size present inside (including the outer periphery) of the structure is determined, and the distribution density N is determined by dividing the number by the area of the structure_Fine。

A total area ratio S of particles having a circle-equivalent particle diameter of 1.3 to 20 μm and an aspect ratio of 3 or less_γBlockThe measurement was similarly carried out from the SEM photograph.

Fig. 1 shows an example of the SEM photograph.

For the steel sheet used for the observation of fig. 1, 0.18% C-1.5% Si-2.8% Mn steel was heated to 630 ℃ at 20 ℃/s, and then heated to 630 to 800 ℃ at a constant heating rate of 3 ℃/s. The steel sheet is obtained by annealing at 800 ℃ で, cooling the steel sheet to 450 ℃ at a constant cooling rate of 20 ℃/s, maintaining the steel sheet at 450 ℃ for 30sec, cooling the steel sheet to 320 ℃ at a constant cooling rate of 15 ℃/s, cooling the steel sheet to 6 ℃/s from 320 ℃ to 270 ℃, cooling the steel sheet to 200 ℃ at 5 ℃/s, immediately heating the steel sheet to 400 ℃ at 15 ℃/s after 200 ℃, maintaining the steel sheet at 400 ℃ for 10min, and cooling the steel sheet to 100 ℃ or lower at 10 ℃/s. The 1/4-thick portion of the vertical section in the rolling direction was polished, etched with a 3% nital solution, and observed by SEM.

According to the SEM photograph, upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ were separated and evaluated. The upper bainite (a) contains almost no carbide, hardly has any streak-like distortion (LAS interface) inside, is substantially the same as ferrite, and has a black structure having a width of 0.8 to 7 μm, a length of 2 to 15 μm, and an aspect ratio of 2.2 or more. Particles having a particle width of 0.17 to 0.80 μm and an aspect ratio of 4 to 25 are residual γ_UB(b) In that respect Tempered martensite (c) is 1 μm per inside of the structure²Contains 2.0 to 20 fine carbide regions having an aspect ratio of 3.5 or less and an equivalent circle diameter of 0.02 to 0.25 μm. Lower bainite (d) is in the interior of the structure at 1 μm per unit²Comprises 0 to 4 film-like particles having a particle width of 0.14 to 0.30 μm and an aspect ratio of 3.6 to 15, each 1 μm²Contains 0 to 1.9 fine carbide regions having an aspect ratio of 3.5 or less and an equivalent circle diameter of 0.02 to 0.25 μm. Tempered martensite and lower bainite can have a streak-like strain (LAS interface) inside them, and are slightly gray in color compared with ferrite and upper bainite. The fresh martensite (e) is a block-like white particle having an aspect ratio of about 3 or less and a circle-equivalent particle diameter of 0.26 μm or more. e also mixed with a portion of the lumpy γ. The black region in which carbide is hardly formed and the aspect ratio is 2.1 or less is polygonal ferrite (f).

The tensile strength of the steel sheet of the present invention is preferably 590MPa or more. More preferably 980 or more, and still more preferably 1180MPa or more. The upper limit of the tensile strength is preferably 1600MPa or less, and more preferably 1450MPa or less, from the viewpoint of achieving other properties at the same time.

In addition, the steel sheet of the present invention may have a zinc plating layer on the surface of the steel sheet. The zinc plating layer may include an alloyed zinc plating layer obtained by alloying.

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

Hot rolling

As the hot rolling of the steel slab, there are a method of heating and then rolling the slab, and a method of not heating the slab after the continuous castingAnd a method of direct rolling, a method of rolling a continuously cast billet by applying a short-time heat treatment, and the like. The hot rolling may be carried out by a conventional method, and for example, the billet may be heated at 1100 to 1300 ℃ at a soaking temperature of 20 to 300min and at a finish rolling temperature of Ar₃Transformation point-Ar₃The transformation point is +200 ℃ and the winding temperature is 400-720 ℃. The winding temperature is preferably 430 to 530 ℃ from the viewpoint of suppressing the variation in sheet thickness and ensuring stability at high strength.

Cold rolling

In the cold rolling, the rolling rate is 30-85%. From the viewpoint of ensuring stability at high strength and reducing anisotropy, the rolling reduction is preferably 45 to 85%. When the rolling load is high, the annealing treatment for softening may be performed at 450 to 730 ℃ in a CAL (continuous annealing line) or BAF (box annealing furnace).

Annealing on a continuous annealing line

After hot rolling and cold rolling a steel slab having a predetermined composition, annealing is performed on a continuous annealing line under the following predetermined conditions. The annealing facility is not particularly limited, and is preferably performed in a Continuous Annealing Line (CAL) or a continuous hot dip galvanizing line (CGL) from the viewpoint of ensuring productivity and desired heating and cooling rates.

Heating rate in the temperature range of 660-740 ℃: 1-6 ℃/s

The heat treatment is carried out slowly at a temperature of 1 to 6 ℃/s in the temperature range, thereby improving the HAZ softening resistance. If the heating rate in this temperature range is too high, austenite is generated by reverse transformation from the state before recrystallization, and the austenite is excessively refined. In finely dispersed austenite, even if it is held at around 450 ℃ in the subsequent cooling process, coarse upper bainite cannot be sufficiently formed, and the improvement effect of the HAZ softening resistance by the holding cannot be expressed. In order to obtain such an effect, the heating rate in the temperature range of 660 to 740 ℃ needs to be 6 ℃/s or less. Preferably 5 ℃/s or less. If the heating rate is too slow, productivity is impaired, so the heating rate is 1 ℃/s or more. Preferably 2 ℃/s or more.

Heating rate in a temperature range of 740 to 770 ℃: 1-6 ℃/s

The heat treatment is carried out slowly at a temperature of 1 to 6 ℃/s in the temperature range, thereby improving the HAZ softening resistance. If the heating rate in this temperature range is too high, the nucleation size of austenite increases excessively, and austenite is excessively refined. In finely dispersed austenite, even if it is held at around 450 ℃ in the subsequent cooling process, coarse upper bainite cannot be sufficiently formed, and the improvement effect of the HAZ softening resistance by the holding cannot be expressed. Preferably 5 ℃/s or less.

Annealing temperature: 770-850 DEG C

The annealing temperature is set to 770 to 850 ℃ in order to ensure tempered martensite and/or bainite in a predetermined area ratio and a residual γ in a predetermined volume ratio. If the annealing temperature is below 770 deg.C, the amount S of upper bainite_UBThe HAZ softening resistance is lowered. If the annealing temperature is higher than 850 ℃, the amount of ferrite generated is reduced and ductility is reduced.

A dew point of-45 ℃ or higher (preferable condition) at an annealing temperature of 770 to 850 DEG C

By controlling the dew point in annealing at an annealing temperature of 770 to 850 ℃ to-45 ℃ or higher, the formation of a decarburized layer in the surface layer can be promoted, and the distribution density N of particles (mainly carbides) having an aspect ratio of 3.5 or less and a circle equivalent particle diameter of 0.02 to 0.25 μm, which are present in the inside of a structure comprising 1 or 2 or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual γ, is 3.5 or less_θWill be reduced. As a result, excessive temper softening in the surface layer is suppressed, and the HAZ softening resistance is improved. In order to obtain such an effect, the dew point in annealing at an annealing temperature of 770 to 850 ℃ is preferably-45 ℃ or higher, more preferably-40 ℃ or higher, and still more preferably-35 ℃ or higher. Note that, in the dew point of more than 10 ℃, there is a possibility that the roller is deteriorated by pickup or the like, and therefore, the dew point is preferably 10 ℃ or less.

Average cooling rate in the temperature range of 770 to 700 ℃: 1-2000 ℃/s

After annealing, in a temperature range of 770-700 ℃, at an average cooling rate: 1 to 2Cooling was carried out at 000 ℃/s. If the average cooling rate is slower than 1 deg.C/S, ferrite is generated in a large amount and the amount of upper bainite S is caused_UBDecrease in strength, decrease in HAZ softening resistance, and decrease in λ. More preferably 3 ℃/s or more. On the other hand, if the average cooling rate is too high, the plate shape deteriorates, and therefore, the average cooling rate is 2000 ℃/s or less. Preferably 100 ℃/s or less, more preferably less than 30 ℃/s.

Average cooling rate in a temperature range of 700 to 500 ℃: 8-2000 ℃/s

Cooling at a temperature of 700-500 ℃ at a temperature of 8 ℃/s or more. If the average cooling rate is slower than 8 deg.C/S, ferrite is generated in a large amount and results in an amount S of upper bainite_UBDecrease in strength, decrease in HAZ softening resistance, and decrease in λ. More preferably 10.0 ℃/s or more. On the other hand, if the average cooling rate is too high, the plate shape deteriorates, and therefore, the average cooling rate is 2000 ℃/s or less. Preferably 100 ℃/s or less, more preferably less than 30 ℃/s.

Zinc plating treatment or alloyed zinc plating treatment (preferred conditions)

Average cooling rate in a temperature range of 700 to 500 ℃: the zinc plating treatment or the alloying zinc plating treatment can be performed between cooling at 8 to 2000 ℃/s and holding at a temperature of 500 to 405 ℃ for 13 to 200 sec. The zinc plating treatment is preferably performed by immersing the steel sheet in a molten zinc plating bath at 440 to 500 ℃, and then adjusting the plating deposition amount by gas wiping or the like. Further, when alloying zinc plating, it is preferable to perform alloying by staying in a temperature range of 460 to 580 ℃ for 1 to 120 seconds. The average cooling rate in the temperature range of 700 to 500 ℃ is: after cooling at 8-2000 ℃/s, the mixture can be heated to a temperature of 500 ℃ or higher as required. The zinc plating is preferably formed using a zinc plating bath containing 0.08 to 0.25 mass% of Al. The zinc-plated steel sheet may be subjected to various coating treatments such as resin coating and grease coating.

Holding time at a temperature range of 500-405 ℃: 13 to 200sec

By passing in this temperature regionThe steel is retained for a predetermined period of time, whereby upper bainite can be formed with little carbide precipitation. In addition, a high-density residual gamma of C can be formed adjacent to the above_UB. By the maintenance in this temperature region, (S) can be ensured_UB/S_2nd) X 100 (%) is 2.0% or more, improving the HAZ softening resistance. From this viewpoint, the holding time in the temperature range of 500 to 405 ℃ is set to 13sec or more. Preferably 15sec or more. On the other hand, even if the holding time is longer than 200sec, the generation of bainite is still stagnant, and if it is longer than 200sec, carbon thickening to an unconverted γ in a lump state proceeds, resulting in an increase in the residual amount of the lump structure. As a result, the HAZ softening resistance is deteriorated. Therefore, the holding time in the temperature range of 500 to 405 ℃ is set to 13 to 200 sec. From the viewpoint of improving stretch flange formability, the holding time in the temperature range of 500 to 405 ℃ is preferably 100sec or less. The temperature region is maintained in such a manner that the average cooling rate in the temperature range is reduced to 7.3 ℃/s or less. The temperature range to be maintained is preferably 410 ℃ or higher, and more preferably 430 ℃ or higher, from the viewpoint of improving ductility. Further, it is preferably 490 ℃ or lower, and more preferably 480 ℃ or lower.

Average cooling rate from 405 ℃ to a cooling stop temperature Tsq of 170 to 270 ℃: 1 to 50 ℃/s

The cooling is moderately and slowly performed in a temperature range from 405 ℃ to a cooling stop temperature Tsq of 170-270 ℃. Thus, the formation of martensite and lower bainite and the thickening of γ adjacent to carbon can suppress the formation of carbides having an aspect ratio of 3.5 or less and a circle-equivalent grain diameter of 0.02 to 0.25 μm, and the formation of fresh martensite in the form of blocks having a circle-equivalent grain diameter of 1.3 to 20 μm and an aspect ratio of 3 or less, thereby softening martensite and lower bainite and obtaining an effect of improving the resistance to HAZ softening. In addition, ductility is also increased. From these viewpoints, the average cooling rate in the temperature range is set to 1 to 50 ℃/s. From the viewpoint of suppressing the formation of carbide, the average cooling rate in this temperature range is desirably less than 15 ℃/s, and more preferably less than 10 ℃/s.

Average cooling rate in the range of 320 to 270 ℃ in the step of cooling from a cooling stop temperature Tsq in the range of 405 to 170 to 270 ℃: 0.3 ℃/s or more and less than 20 ℃/s (preferable conditions)

Carbon is distributed from martensite and/or lower bainite to γ, and in order to suppress carbides having an aspect ratio of 3.5 or less and a circle-equivalent particle diameter of 0.02 to 0.25 μm, the distribution density N of particles having an aspect ratio of 3.6 to 15 and a particle width of 0.14 to 0.30 μm is set to be_Fine0.03 to 0.4 pieces/mum²Preferably, the cooling is carried out slowly in the range of 320 to 270 ℃. From such a viewpoint, the cooling rate in this temperature region is preferably 0.3 ℃/s or more and less than 20 ℃/s, and more preferably 0.3 ℃/s or more and less than 10 ℃/s.

Cooling stop temperature Tsq: 170-270 DEG C

So that the total area ratio S of particles having a circle equivalent particle diameter of 1.3 to 20 μm and an aspect ratio of 3 or less_γBlock5% or less, and the cooling stop temperature Tsq is required to be in the range of 170 to 270 ℃ in order to secure the residual γ amount.

Average heating rate in the temperature range from the cooling stop temperature Tsq to 350 ℃: 2 ℃/s or more

Further, by heating at a temperature ranging from the cooling stop temperature Tsq to 350 ℃ for a short time, precipitation of carbide is suppressed, and high ductility can be ensured. Therefore, the average heating rate in the temperature range from the cooling stop temperature Tsq to 350 ℃ is set to 2 ℃/s or more. From such a viewpoint, the average heating rate is preferably 5 ℃/s or more, and more preferably 10 ℃/s or more. The upper limit of the average heating rate is not particularly limited, but is preferably 50 ℃/s or less, and more preferably 30 ℃/s or less.

Cooling after annealing to average heating rate: a residence time in a temperature range of 170 to 250 ℃ in a heating step of 2 ℃/s or more: less than 50s

Diffusion of C into γ in martensite and/or lower bainite held in a temperature region of 250 ℃ or lower is delayed, and carbide precipitation proceeds. As a result, these tissues are hardened, and the HAZ softening resistance is deteriorated. Therefore, from cooling after annealing to the average heating rate described below: the residence time in the temperature range of 170 to 250 ℃ in the heating step of 2 ℃/s or more is required to be 50s or less. From such a viewpoint, the residence time is preferably 30 seconds or less.

The retention time at 350-500 ℃ is as follows: 20 to 3000sec

For residual gamma generated by intermediate holding (holding for 13-200 sec at 500-405 deg.C)_UBAnd martensite and lower bainite are adjacent to each other, and the remaining γ is distributed with C to stabilize them, and in view of improving ductility and HAZ softening resistance, the region in which the martensite and lower bainite are distributed in bulk is subdivided by bainite transformation as untransformed γ, and in view of improving λ, the martensite and lower bainite are held at a temperature of 350 to 500 ℃ for 20 to 3000 sec. The holding time in this temperature range is preferably 240sec or more from the viewpoint of further improving ductility, and more preferably 300sec or more from the viewpoint of improving the HAZ softening resistance.

After that, the steel sheet is cooled to room temperature, and temper rolling may be performed on the steel sheet from the viewpoint of stabilizing the roll formability such as adjustment of surface roughness and flattening of the sheet shape, and from the viewpoint of increasing the Yield Strength (YS). The elongation of temper rolling is preferably 0.1-0.5%. In addition, the plate shape may be flattened by a leveler.

In addition, the average cooling rate in the temperature range of 700 to 500 ℃ is: the step of cooling at 8 to 2000 ℃/s, the step of holding at 500 to 405 ℃ for 13 to 200sec, and the step of holding at 350 to 500 ℃ for 20 to 3000sec without performing the zinc plating treatment or the alloyed zinc plating treatment, and then the step of performing the zinc plating treatment or the alloyed zinc plating treatment (preferable conditions) may be performed. When the zinc plating treatment is performed, it is preferable to perform the zinc plating treatment by immersing the steel sheet in a molten zinc plating bath at 440 to 500 ℃, and then adjusting the plating adhesion amount by gas wiping or the like. Further, when alloying zinc plating, it is preferable to perform alloying by staying in a temperature range of 460 to 580 ℃ for 1 to 120 seconds. From the viewpoint of suppressing the decomposition of the residual γ, it is more preferably 550 ℃. The zinc plating is preferably formed using a zinc plating bath having an Al content of 0.08 to 0.25 mass%. The zinc-plated steel sheet may be subjected to various coating treatments such as resin coating and grease coating.

From the viewpoint of improving stretch flange formability, the heat treatment may be performed at a low temperature of 100 to 300 ℃ for 30sec to 10 days after the heat treatment or after the temper rolling. By this processing, there are generated: tempering of martensite generated at the time of final cooling or temper rolling, and hydrogen intruded into the steel sheet at the time of annealing are released from the steel sheet. Hydrogen can be reduced to less than 0.1ppm in a low temperature heat treatment. In addition, electroplating may be performed. After the plating, the low-temperature heat treatment is preferably performed from the viewpoint of reducing hydrogen in the steel.

According to the present invention, TS × El ≧ 17000MPa · which is important as an index of formability of a complex-shaped part in which stretch forming and stretch flange forming are mixed can be secured, and breakage of the HAZ portion from the welded portion can be suppressed.

Example 1

The steel sheets of the present invention and the steel sheets of the comparative examples were produced by treating cold rolled steel sheets having a thickness of 1.4mm and a composition of components shown in Table 1 under annealing conditions shown in Table 2.

After a step of holding a part of the steel sheets (cold-rolled steel sheet: CR) at a temperature in the range of 350 to 500 ℃, the steel sheets are subjected to hot-dip zinc plating treatment to obtain hot-dip zinc-plated steel sheets (GI). Here, the steel sheet is immersed in a zinc plating bath at 440 to 500 ℃ to perform a molten zinc plating treatment, and then the plating deposition amount is adjusted by gas wiping or the like. The molten zinc plating uses a zinc plating bath having an Al content of 0.10 to 0.22%. Further, a part of the hot-dip galvanized steel sheet was subjected to alloying treatment after the hot-dip galvanizing treatment to obtain an alloyed hot-dip galvanized steel sheet (GA). Here, the alloying treatment is performed in a temperature range of 460 to 580 ℃. Further, a part of the steel sheet (cold rolled steel sheet: CR) was subjected to electroplating to obtain an electro-galvanized steel sheet (EG).

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

Tensile test pieces of JIS5 were collected from the obtained steel sheets, and a tensile test (according to JIS Z2241) was carried out. TS and El are shown in Table 3.

From the obtained steel sheet, 2 sheets of steel sheet of 150mm in the vertical rolling direction and 125mm in the rolling direction (end face grinding treatment) were collected, and 2 sheets were aligned in the rolling direction and aligned in butt joint, and laser welding was performed at the butt joint position. Condition a is a gap between abutting surfaces: 0mm, condition B is the gap between the abutting surfaces: 0.15 mm. In condition B, the cross-sectional area of the molten portion becomes smaller, and therefore, the evaluation is more strict. In laser welding, Nd-YAG laser was used, and the spot diameter at the focal point position: 0.6mm, focal position: 4mm at the upper part of the steel plate, Ar as protective gas, and laser output: 4.2kW, welding speed: 3.7 m/min. A tensile test piece No. JIS5 (according to JIS Z2241) was sampled from the welded member so that the weld line was perpendicular to the tensile axis and located at the center of the length of the test piece. The base material was judged to be broken when the breaking position was 2.0mm or more from the weld line (when a part was more than 2.0 mm), and the welded portion was judged to be broken when the crack was less than 2.0mm and progressed along the weld line (when the crack progressed in the HAZ portion or the molten portion). The fracture morphology in the laser welding condition a was determined to be the case of base material fracture (∘), and was determined to be excellent in the HAZ softening resistance.

The embodiment of the invention meets the condition that TS multiplied by El is more than or equal to 17000MPa, and the laser welding part can also break the base material.

In the invention examples, nos. 1, 11, 12, 18, 26, 29, 32, 33 and 37 were (S)_UB/S_2nd) X 100 (%) is 3.0 or more, N_θIs5 pieces/. mu.m²Below, N_FineIs 0.03 pieces/. mu.m²Above, S_γBlockThe ratio is 5% or less, and the condition B having a gap during butt welding causes base material breakage, and the ductility is particularly excellent as TS × El ≥ 19000 MPa%.

[ Table 2]

The same underlines are intended to be outside the scope of the present invention.

*1: heating rate in the temperature range of 660-740 DEG C

*2: heating speed within the temperature range of 740-770 DEG C

*3: average cooling rate in the temperature range of 770-700 DEG C

*4: average cooling rate in temperature range of 700-500 DEG C

*5: a holding time in a temperature range of 500 to 405 DEG C

*6: average cooling rate in temperature range from 405 to cooling stop temperature Tsq

*7: average cooling rate in temperature range of 320-270 DEG C

*8: the retention time of the temperature range of 170-250 ℃ in the cooling-heating process after annealing

*9: average heating rate in temperature range from cooling stop temperature Tsq to 350 DEG C

*10: a holding time in a temperature range of 350 to 500 DEG C

*11: CR: cold-rolled steel sheet (non-plating treatment), GA: alloyed molten zinc-plated steel sheet, GI: molten zinc plated steel sheet (alloying treatment for non-zinc plating), EG: electrogalvanized steel sheet

[ Table 3]

The same underlines are intended to be outside the scope of the present invention. O: breaking of base metal NG: fracture of welded part

*12: a structure comprising 1 or more than 2 kinds of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual gamma-phase

Example 2

The steel sheets of the present invention and the steel sheets of the comparative examples were produced by treating cold rolled steel sheets having a thickness of 1.4mm and a composition of components shown in Table 1 under annealing conditions shown in Table 4.

The average cooling rate in the temperature range of 700 to 500 ℃ is: performing a zinc plating treatment or an alloying zinc plating treatment between the step of cooling at 8-2000 ℃/s and the step of maintaining at a temperature of 500-405 ℃ for 13-200 sec. Here, the steel sheet is immersed in a zinc plating bath at 440 to 500 ℃ to perform a molten zinc plating treatment, and then the plating deposition amount is adjusted by gas wiping or the like. The molten zinc plating is performed using a zinc plating bath containing A1 in an amount of 0.10 to 0.22%. Further, a part of the hot-dip galvanized steel sheet was subjected to alloying treatment after the hot-dip galvanizing treatment to obtain an alloyed hot-dip galvanized steel sheet (GA). Here, the alloying treatment of zinc plating is performed in a temperature range of 460 to 580 ℃.

The steel structure, tensile test, and HAZ softening resistance of the obtained steel were evaluated in the same manner as in example 1, and the measurement results are shown in table 5.

The embodiment of the invention meets the condition that TS multiplied by El is more than or equal to 17000MPa, and the laser welding part can also break the base material.

In the invention example, No.3 (S)_UB/S_2nd) X 100 (%) is 3.0 or more, N_θIs5 pieces/. mu.m²Below, N_FineIs 0.03 pieces/. mu.m²Above, S_γBlockThe ratio is 5% or less, and the condition B having a gap during butt welding causes base material breakage, and the ductility is particularly excellent as TS × El ≥ 19000 MPa%.

Industrial applicability

The present invention has high ductility and excellent HAZ softening resistance, and can be preferably used for roll forming applications in automobiles, home appliances, and the like, which are subjected to a roll forming step.