WO2016152675A1

WO2016152675A1 - High-strength steel sheet having excellent workability

Info

Publication number: WO2016152675A1
Application number: PCT/JP2016/058311
Authority: WO
Inventors: 隼矢中田; 俊夫村上; 裕一二村; 康二粕谷
Original assignee: 株式会社神戸製鋼所
Priority date: 2015-03-23
Filing date: 2016-03-16
Publication date: 2016-09-29
Also published as: JP6434348B2; JP2016180138A

Abstract

A high-strength steel sheet including specific amounts of each of C, Si, Mn, and Al, the remainder being iron and unavoidable impurities, polygonal ferrite, tempered martensite, bainitic ferrite, a mixed structure (MA) of fresh martensite and residual austenite, and the total of polygonal ferrite and bainitic ferrite being specific area ratios, and the Mn concentration in the MA being at least 1.2 times the Mn content of the steel sheet as a whole.

Description

High strength steel plate with excellent workability

The present invention relates to a high-strength steel sheet excellent in workability, and in particular, to a high-strength steel sheet with improved strength-elongation balance and stretch flangeability. The steel plate types of the high-strength steel plates according to the present invention include cold-rolled steel plates as well as various plated steel plates such as hot-dip galvanized steel plates and galvannealed steel plates.

High-strength steel plates are used for automobile frame parts. Steel sheets used as frame parts for automobiles are required to be cold-rolled steel sheets and alloyed hot-dip galvanized steel sheets with increased strength for the purpose of improving fuel efficiency by reducing the weight of the vehicle body. On the other hand, excellent workability is also required in order to form a complex shaped part.

For this reason, it has been desired to provide a high-strength steel sheet having high strength and enhanced elongation (total elongation; EL) and stretch flangeability (hole expansion rate: λ), for example, tensile strength (TS). For steel sheets of 980 MPa, TS × EL is 25000 MPa ·% or more, yield strength (YS) is 550 MPa or more, and λ is 20% or more.

In order to answer the above request, various materials have been proposed as steel plate materials.

For example, in Patent Document 1, a hot rolled coil having a component composition similar to that of the high-strength steel sheet according to the present invention and a structure mainly composed of bainite and martensite is held for 1 hour or more in the range of 550 ° C. to Ac1 point. Batch hot-rolled sheet annealing step, cold-rolled step, and cold-rolled plate are heated to Ac1-Ac3 temperature at an average heating rate of Ac1 point to Ac1 point + 50 ° C) at 5 ° C / s or higher and held for 5 seconds or longer A continuous recrystallization annealing, followed by an austempering process of cooling to 350 to 500 ° C. at a cooling rate of 5 ° C./s or more and maintaining the temperature for 10 to 600 s, It has a composite structure composed of ferrite having a volume ratio of 70% or more as a main phase and a second phase having an average particle diameter of 3 μm or less and containing at least 3% residual austenite by volume ratio, and an r value of 1.1. more than And strength - ductility balance TS × EL is disclosed that a high strength cold rolled steel sheet is 22000MPa ·% or more is obtained.

This high-strength cold-rolled steel sheet is common to the high-strength steel sheet according to the present invention in that a batch hot-rolled sheet annealing step (corresponding to the preliminary annealing treatment of the present invention) is performed in the manufacturing method.

However, the cooling stop temperature when quenching from the two-phase region temperature to the austempering temperature is 350 to 500 ° C., which is higher than 50 ° C. or more and less than 350 ° C. of the present invention.

For this reason, the ferrite fraction in the final structure is 70% or more, and contains more ferrite (corresponding to the polygonal ferrite of the present invention) than the maximum value of polygonal ferrite in the present invention: 65% or less. This is clearly different from the high-strength steel sheet according to the present invention.

Further, Patent Document 2 has a component composition that is similar to that of the present invention, and in volume fraction, ferrite phase: 40 to 70%, bainite phase: 15 to 35%, tempered martensite phase: 5 to 25% and retained austenite phase: 2 to 20%, and the ratio of the martensite phase with the major axis length ≧ 10 μm in the total volume fraction of the tempered martensite phase is 30% or less. A high-strength cold-rolled steel sheet having a tensile strength TS of 1180 MPa or more with improved workability such as stretch flangeability and bendability is disclosed.

The high-strength cold-rolled steel sheet is a high-strength steel sheet according to the present invention in that the first annealing (corresponding to the pre-annealing treatment of the present invention) is performed in the temperature range of 400 to 800 ° C. after the hot rolling in the manufacturing method. And in common.

However, when rapidly cooling from the two-phase region temperature, which is the second annealing, to the cooling stop temperature, the cooling stop temperature is 300 to 500 ° C. (380 to 500 ° C. in the invention example of the embodiment), which is 50 ° C. or more of the present invention. It is higher than less than 350 ° C.

For this reason, the fresh martensite fraction is lowered, and the formed fresh martensite is all tempered by the third annealing (corresponding to the tempering treatment of the present invention) to become only tempered martensite, and the final structure is not yet. There is no tempered fresh martensite, and therefore no MA, so at least in this respect it is clearly different from the high strength steel sheet according to the invention.

Japanese Unexamined Patent Publication No. 2008-291304 Japanese Laid-Open Patent Publication No. 2012-237042

Therefore, the object of the present invention is that the tensile strength (TS) is 980 MPa or more, the tensile strength-elongation balance (TS × EL) is 25000 MPa ·% or more, the yield strength (YS) is 550 MPa or more, and the stretch flangeability (λ) is An object of the present invention is to provide a high-strength steel sheet excellent in workability and capable of securing 20% or more.

The high-strength steel sheet excellent in workability according to the first invention of the present invention is
% By mass
C: 0.05 to 0.50%,
Si: 1.0 to 3.0%,
Mn: 1.0 to 5.0%,
Al: 0.001 to 0.10%
Each
The balance consists of iron and inevitable impurities,
Among the inevitable impurities, P, S, and N are
P: 0.1% or less,
S: 0.01% or less,
N: 0.01% or less
Each having a component composition limited to
The area ratio for all tissues
Polygonal ferrite: 40% or more
Tempered martensite: 10% or more,
Bainitic ferrite: 5% or more
Mixed structure of fresh martensite and retained austenite (hereinafter, this mixed structure is referred to as “MA”): 5% or more,
Polygonal ferrite + bainitic ferrite: 70% or less in total
Has an organization consisting of
The Mn concentration in the MA is 1.2 times or more of the Mn content of the whole steel sheet.
It is characterized by this.

The high-strength steel sheet excellent in workability according to the second invention of the present invention is
In the first invention,
Ingredient composition is further mass%,
Cr: 0.05 to 1.0%,
Mo: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
B: 0.0001 to 0.002%
Any one or more of
Is.

The high-strength steel sheet excellent in workability according to the third invention of the present invention is
In the first or second invention,
Ingredient composition is further mass%,
Ti: 0.01 to 0.15%,
Nb: 0.01 to 0.15%,
V: 0.01 to 0.15%
Any 1 type or 2 types or more of these are included.

According to the present invention, a soft polygonal ferrite is used as a main phase, a predetermined amount of bainitic ferrite is introduced, and Mn is further concentrated in the MA to ensure elongation while maintaining the elongation in the MA. In addition to hard fresh martensite, a predetermined amount of tempered martensite is introduced to improve the strength (tensile strength, yield strength) and stretch flangeability due to the presence of the tempered martensite and bainitic ferrite. The tensile strength (TS) is 980 MPa or more, the tensile strength-elongation balance (TS × EL is 25000 MPa ·% or more, the yield strength (YS) is 550 MPa or more, and the stretch flangeability (λ) is 20%. It was possible to provide a high-strength steel sheet excellent in workability that can secure the above.

It is a drawing substitute photograph which shows an example of the steel structure of the high strength steel plate which concerns on this invention.

Hereinafter, the present invention will be described in more detail.

First, the structure that characterizes the high-strength steel sheet excellent in workability according to the present invention (hereinafter also referred to as “the steel sheet of the present invention”) will be described.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is characterized in that the parent phase is polygonal ferrite, and tempered martensite and bainitic ferrite are partially introduced into this, and further MA containing Mn is added. It is what.

<Polygonal ferrite: 40% or more>
Polygonal ferrite is a soft phase and is an effective structure for enhancing the ductility of a steel sheet. The content of polygonal ferrite with respect to the entire structure needs to be 40% or more, preferably 45% or more, and more preferably 50% or more in terms of area ratio in order to ensure the ductility of the steel sheet. In addition, the upper limit of polygonal ferrite is set in relation to the conditions of “Bainitic ferrite: 5% or more” and “Polygonal ferrite + Bainitic ferrite: 70% or less in total” as specified separately. 65%.
The “polygonal ferrite” in the present invention is a general term for the polygonal ferrite structure and the quasi-polygonal ferrite structure described in “Basic Steel Research Group published by the Japan Iron and Steel Institute”. Is.

<Tempered martensite: 10% or more>
By partially introducing tempered martensite, stretch flangeability can be improved while maintaining tensile strength. The tempered martensite content in the entire structure needs to be 10% or more, preferably 12% or more, and more preferably 14% or more in terms of area ratio in order to ensure stretch flangeability.

<Bainitic ferrite: 5% or more>
By partially introducing bainitic ferrite, the tensile strength-elongation balance can be improved. The content of bainitic ferrite with respect to the entire structure needs to be 5% or more, preferably 7% or more, more preferably 9% or more in terms of area ratio in order to ensure a tensile strength-elongation balance.
In the present invention, “bainitic ferrite” means that the bainite structure has a lower structure having a lath-like structure with a high dislocation density, and has no carbide in the structure. Is clearly different, and is also different from a polygonal ferrite structure with a substructure with little or no dislocation density, or a quasi-polygonal ferrite structure with a substructure such as fine subgrains. (See “Bay Night Photo Collection-1” published by the Society).

<MA: 5% or more>
By partially introducing MA, the yield strength and tensile strength-elongation balance can be improved. The MA content in the entire structure needs to be 5% or more, preferably 7% or more, and more preferably 9% or more in terms of area ratio in order to ensure the yield strength and tensile strength-elongation balance.
Note that “MA” in the present invention is a mixed structure of fresh martensite and retained austenite, and it is difficult to separate (discriminate) fresh martensite and retained austenite by microscopic observation. Fresh martensite refers to a state in which untransformed austenite is martensitic transformed in the process of cooling the steel sheet from the heating temperature to the MS point or less, and is distinguished from tempered martensite after tempering.

<Polygonal ferrite + Bainitic ferrite: 70% or less in total>
If the total amount of polygonal ferrite and bainitic ferrite is too large, the amounts of tempered martensite and MA are insufficient, and at least one of yield strength, tensile strength-elongation balance, and stretch flangeability cannot be secured. The total content of polygonal ferrite and bainitic ferrite with respect to the entire structure must be limited to 70% or less, preferably 69% or less, and more preferably 68% or less in terms of area ratio.

<Mn concentration in MA: 1.2 times or more of Mn content of whole steel plate>
By concentrating Mn in MA, the retained austenite constituting MA is stabilized, work-induced transformation can be performed in a higher strain region, and the work hardening degree in the high strain region is increased, so that ductility is improved. . In order to effectively exhibit such an action, the Mn concentration in the MA is 1.2 times or more, preferably 1.25 times or more, more preferably 1.3 times or more of the Mn content of the whole steel sheet. And

[Each measurement method of area ratio of each phase and Mn concentration in MA]
Here, each measuring method of the area ratio of each phase and the Mn concentration in MA will be described.

The area ratios of polygonal ferrite, MA, and tempered martensite were measured as follows. That is, the steel plate was mirror-polished and corroded with 3% nital solution to reveal the metal structure, and then a secondary electron image of a field emission scanning electron microscope (FE-SEM) for approximately 5 fields of 24 μm × 18 μm region. It was observed (magnification 5000 times). An example of the observation photograph is shown in FIG.
And by image analysis, the region that does not contain cementite and appears concave due to corrosion is polygonal ferrite, the region that does not contain cementite and appears to be convex on the polygonal ferrite is MA, and the region that contains cementite They were identified as tempered martensite and the respective area ratios were calculated.

By subtracting the total area ratio of polygonal ferrite, MA and tempered martensite from 100% with the remaining structure other than “polygonal ferrite”, “MA” and “tempered martensite” as bainitic ferrite, The area ratio of nittic ferrite was calculated.

Regarding the Mn concentration in MA, a Mn concentration distribution in MA was measured using a field emission electron beam microanalyzer (FE-EPMA), and the average value was taken as the Mn concentration in MA.

[Component composition of the steel sheet of the present invention]
Below, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%. In addition, “content” of each component may be simply referred to as “amount”.

C: 0.05 to 0.50%
C is an important element for improving the strength of the steel sheet. In order to exhibit the effect of improving the strength effectively, it is necessary to contain C 0.05% or more, preferably 0.08% or more, more preferably 0.12% or more. However, when the amount of C is excessive, coarse carbides are likely to precipitate during tempering, and the stretch flangeability is deteriorated and the weldability is also adversely affected. Therefore, the amount of C is preferably 0.50% or less, preferably Is 0.45% or less, more preferably 0.40% or less.

Si: 1.0 to 3.0%
Si is a useful element that has the effect of suppressing the coarsening of carbide particles during tempering, contributes to improvement in stretch flangeability, and also contributes to an increase in yield strength of the steel sheet as a solid solution strengthening element. In order to effectively exhibit such an action, it is necessary to contain Si by 1.0% or more, preferably 1.1% or more, and more preferably 1.2% or more. However, if the amount of Si becomes excessive, weldability is remarkably lowered, so the Si amount is 3.0% or less, preferably 2.9% or less, and more preferably 2.8% or less.

Mn: 1.0 to 5.0%
Mn, like Si, has an effect of suppressing the coarsening of cementite during tempering, contributes to the improvement of stretch flangeability, and is a useful element that also contributes to an increase in the yield strength of the steel sheet as a solid solution strengthening element. is there. Moreover, it has the effect of suppressing the ferrite transformation at the time of cooling by improving hardenability. In order to effectively exhibit such an action, it is necessary to contain Mn at 1.0% or more, preferably 1.1% or more, and more preferably 1.2% or more. However, if the amount of Mn becomes excessive, the amount of MA in the final structure becomes excessive, and conversely the stretch flangeability is lowered. Therefore, the amount of Mn is 5.0% or less, preferably 4.8% or less. Preferably it is 4.6% or less.

Al: 0.001 to 0.10%
Al is a useful element added as a deoxidizer. In order to effectively exhibit the action as a deoxidizer, it is necessary to contain Al 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more. However, if the amount of Al is excessive, the cleanliness of the steel is deteriorated, so the amount of Al is 0.10% or less, preferably 0.08% or less, and more preferably 0.06% or less.

The steel sheet of the present invention contains the above elements as essential components, and the balance is iron and unavoidable impurities (P, S, N, O, etc.). Among the unavoidable impurities, P, S, and N are as follows: It can be contained up to each allowable range.

P: 0.1% or less P is unavoidably present as an impurity element, and contributes to an increase in strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and makes the grain boundaries brittle, thereby improving the bendability. Since it deteriorates, the amount of P is limited to 0.1% or less, preferably 0.05% or less, and more preferably 0.03% or less.

S: 0.01% or less
S is also unavoidably present as an impurity element, and forms MnS inclusions and becomes a starting point of a crack at the time of bending deformation, thereby lowering the bendability. Therefore, the amount of S is 0.01% or less, preferably 0.8. It is limited to 005% or less, more preferably 0.003% or less.

N: 0.01% or less
N is also unavoidably present as an impurity element and lowers the workability of the steel sheet by strain aging, so the N content is 0.01% or less, preferably 0.005% or less, more preferably 0.003% or less. Restrict.

In addition, Cr, Mo, Ti, Nb, V, B, Ni, Cu, Zr, and the like can be included as allowable components within the range not impairing the action of the present invention. Of these allowable components, Cr, Mo Ni, B, Ti, Nb and V are recommended to be contained within the following permissible ranges.

Cr: 0.05 to 1.0%,
Mo: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
B: 0.0001 to 0.002%
Any one or two or more of these elements are useful for enhancing the hardenability and improving the strength of the steel sheet. In order to effectively exhibit hardenability, the Cr, Mo, and Ni contents are each 0.05% or more, more preferably 0.1% or more, and the B content is 0.0001% or more, more preferably Is recommended to be 0.0002% or more. However, if these elements are contained excessively, the workability deteriorates and the cost becomes high, so the contents of Cr, Mo and Ni are 1.0% or less, further 0.8% or less, respectively. The content is desirably limited to 0.002% or less, more preferably 0.001% or less.

Ti: 0.01 to 0.15%,
Nb: 0.01 to 0.15%,
V: 0.01 to 0.15%
Any one or more of
These elements are useful as precipitation strengthening elements for steel. In order to effectively exert the precipitation strengthening action, it is recommended that the content of these elements is 0.01% or more, more preferably 0.02% or more. However, if these elements are contained excessively, the workability deteriorates, so the content of these elements is preferably limited to 0.15% or less, and further to 0.10% or less.

Next, preferable production conditions for obtaining the steel sheet of the present invention will be described below.

[Preferred production method of the steel sheet of the present invention]
First, steel having the above component composition is melted and made into a slab (steel material) by ingot forming or continuous casting, and then hot-rolled (hot rolled) at a finishing temperature of 900 ° C. or less (preferably 880 ° C. or less). The coiling temperature after hot rolling is set to 600 to 700 ° C., and then cooled to room temperature to obtain a hot rolled sheet. In this way, the hot-rolled sheet has a bainite or pearlite single-phase structure or a two-phase structure containing ferrite.

Subsequently, a tempering parameter ξ calculated by the following equation (1) is set to 16 at a pre-annealing temperature Tpa (unit: ° C.) of 500 ° C. to Ac 1 (preferably 510 ° C. to [Ac 1-10 ° C.]). A pre-annealing process is performed using a batch furnace, a UAD (United Annealing Department) furnace, or the like under a condition of holding a pre-annealing holding time tpa (unit: h) of ˜20 (preferably 16.5 to 19.5).
ξ = (Tpa + 273) · {log (tpa) +20} / 1000 (1)
By this preliminary annealing treatment, the carbide is coarsened and Mn is concentrated in the carbide.

Next, after removing the scale from this pre-annealed material by pickling or the like, it is subjected to cold rolling (cold rolling) to obtain a cold rolled sheet. For this cold-rolled sheet, for example, using a continuous annealing line (CAL), Ac1 to Ac3 (preferably [Ac1 + 10 ° C.] to [Ac3-10 ° C.]) that are two-phase region temperatures in which polygonal ferrite and austenite are mixed. The carbide is austenitized by performing an annealing treatment under a condition that the annealing heating temperature is maintained for an annealing holding time of 50 s or more (preferably 55 s or more). In this carbide, austenite having a high Mn concentration can be formed because Mn is concentrated by the preliminary annealing treatment.

Rapid cooling from the two-phase region temperature (annealing heating temperature) of this polygonal ferrite and austenite to a supercooling stop temperature of 50 ° C. or more and less than 350 ° C. (preferably 100 to 300 ° C.) at a cooling rate of 10 to 50 ° C./s. Thus, an annealed material in which a mixed structure (MA) of fresh martensite and retained austenite in which Mn is concentrated is formed.

The annealed material is further subjected to an austempering treatment at austempering temperature of 400 to 500 ° C. (preferably 410 to 490 ° C.) for 30 to 1200 s (preferably 40 to 600 s) for austenating time. The steel sheet of the present invention (cold rolled steel sheet) is obtained. By this austempering treatment, bainite is formed and a part of the fresh martensite formed by the rapid cooling is tempered to form tempered martensite. Thereby, the balance of strength, ductility, and stretch flangeability can be improved. Furthermore, by improving the stability of retained austenite constituting the MA by concentrating Mn in the final structure so that the Mn concentration is 1.2 times or more of the Mn content of the entire steel sheet. The tensile strength-elongation balance can be improved.

In addition, the steel sheet of the present invention may be a plated steel sheet by performing a plating process on the annealed material after the annealing process and then performing the austempering process.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Test steels having the respective component compositions shown in Table 1 below were vacuum-melted to form a slab having a thickness of 30 mm, the slab was soaked to 1150 ° C., hot-rolled at a finishing temperature of 880 ° C., and then 600 ° C. Was rolled to prepare a hot-rolled sheet having a thickness of 2.5 mm. This hot-rolled sheet was pre-annealed under the conditions shown in Table 2 below using a batch furnace. After pickling this pre-annealed material, it was cold-rolled to a thickness of 1.5 mm, and further subjected to annealing treatment and austempering treatment under the conditions shown in Table 2 to produce a test steel sheet (cold rolled steel sheet) did.

In addition, although description of N content was abbreviate | omitted in following Table 1, N content was an impurity level of 0.01% or less in all the steel types.

In addition, Ac1 and Ac3 in Table 1 were obtained using the following formulas (1) and (2) (see translation by Kosei Shigeyasu, “Leslie Steel Materials Science”, Maruzen Co., Ltd., 1985, p. 273).

Ac1 (° C.) = 723 + 29.1 [Si] −10.7 [Mn] +16.9 [Cr] −16.9 [Ni] (1)
Ac3 (° C.) = 910−203√ [C] +44.7 [Si] −30 [Mn] +700 [P] +400 [Al] +400 [Ti] +104 [V] −11 [Cr] +31.5 [Mo] −20 [Cu] -15.2 [Ni] (2)
However, [] shows content (mass%) of each element.

About each said test steel plate, the area ratio of each phase and the Mn concentration in MA were measured by each measuring method demonstrated in the above-mentioned section of [Mode for carrying out the invention].

Further, in order to evaluate the strength and workability of each of the test steel plates, the yield strength YS, the tensile strength TS, and the elongation (total elongation) EL were measured by a tensile test, and the stretch flangeability λ was measured by a hole expansion test. Was measured. The tensile test was carried out in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis in a direction perpendicular to the rolling direction. Moreover, the hole expansion test was performed in accordance with the iron standard JFST001, the hole expansion rate was measured, and this was defined as stretch flangeability λ.

The measurement results are shown in Table 3 below. In the table, the mechanical properties (hereinafter, also simply referred to as “characteristics”) of the test steel sheet are tensile strength (TS) of 980 MPa or more, YS: 550 MPa or more, TS × EL: 25000 MPa ·% or more, λ: 20% Those satisfying all of the above were accepted (◯), and those not satisfying at least one were rejected (x).

As shown in Table 3 above, steel No. which is an invention steel (evaluation is ○). 1 to 3, 7, 11, 17, 21, 22, and 25 satisfy the requirements of the organization provision of the present invention as a result of being manufactured using the steel grade that satisfies the requirements of the composition provision of the present invention under the recommended production conditions. It is an invented steel and its characteristics satisfy the acceptance criteria.

In contrast, steel No., which is a comparative steel (evaluation of x). 4 to 6, 8 to 10, 12 to 16, 18 to 20, 23, 24, 26, and 27 do not satisfy at least one of the component provision and the structure provision of the present invention, and the characteristics satisfy the acceptance criteria Not.

That is, No. 4-6, 8-10, 12-16, 18, and 23 are manufactured using conditions that deviate from the recommended manufacturing conditions, although the steel types satisfying the requirements of the component provisions of the present invention are used. The organization regulations are not met and the characteristics are inferior.

On the other hand, steel No. Although Nos. 19 and 24 are manufactured under recommended manufacturing conditions, steel types that partially deviate from the requirements of the component provisions of the present invention are used, so the requirements of the structure provisions are not satisfied and the characteristics are inferior.

From the above results, the applicability of the present invention was confirmed.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on Mar. 23, 2015 (Japanese Patent Application No. 2015-060140), the contents of which are incorporated herein by reference.

The high strength steel sheet of the present invention has a tensile strength (TS) of 980 MPa or more, a tensile strength-elongation balance (TS × EL) of 25,000 MPa ·% or more, a yield strength (YS) of 550 MPa or more, and stretch flangeability (λ). However, it is excellent in workability by securing 20% or more, and is particularly useful for a framework part for automobiles.

Claims

% By mass
C: 0.05 to 0.50%,
Si: 1.0 to 3.0%,
Mn: 1.0 to 5.0%,
Al: 0.001 to 0.10%
Each
The balance consists of iron and inevitable impurities,
Among the inevitable impurities, P, S, and N are
P: 0.1% or less,
S: 0.01% or less,
N: 0.01% or less
Each having a component composition limited to
The area ratio for all tissues
Polygonal ferrite: 40% or more
Tempered martensite: 10% or more,
Bainitic ferrite: 5% or more
Mixed structure of fresh martensite and retained austenite (hereinafter, this mixed structure is referred to as “MA”): 5% or more,
Polygonal ferrite + bainitic ferrite: 70% or less in total
Has an organization consisting of
The Mn concentration in the MA is 1.2 times or more of the Mn content of the whole steel sheet.
A high-strength steel sheet with excellent workability.
The high-strength steel sheet excellent in workability according to claim 1, wherein the component composition further includes at least one of the following (a) and (b).
(A) By mass%, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%, Ni: 0.05 to 1.0%, B: 0.0001 to 0.002% Any one or two or more of (b) by mass: Ti: 0.01 to 0.15%, Nb: 0.01 to 0.15%, V: 0.01 to 0.15% Or one or more