JP5824283B2 - High strength steel plate with excellent formability at room temperature and warm temperature - Google Patents

Description

The present invention relates to a high strength steel plate excellent in formability between room temperature and temperature. The high-strength steel sheet of the present invention includes a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet.

  Thin steel plates used for automobile frame parts are required to have high strength in order to realize collision safety and fuel efficiency improvement. Therefore, it is required to ensure press formability while increasing the strength of the steel sheet to 980 MPa class or higher. It is known that in a high-strength steel sheet of 980 MPa class or higher, it is effective to use steel utilizing the TRIP effect to achieve both high strength and formability (for example, see Patent Document 1).

Patent Document 1 discloses a high-strength steel plate containing bainite or bainitic ferrite as a main phase and containing retained austenite (γ _R ) in an area ratio of 3% or more. However, this high-strength steel sheet has a tensile strength at room temperature of 980 MPa or more and the total elongation does not reach 20%, and further improvement in mechanical properties (hereinafter also simply referred to as “characteristics”) is required.

  In addition, although the TRIP steel sheet is excellent in formability, the load at the time of press forming is increased by the amount of strength, so it is difficult to apply the TRIP steel sheet depending on the size of the part.

  As a technique for reducing the press forming load, a steel plate is formed by controlling the subsequent cooling while reducing the load by press forming in a high temperature range of about 900 ° C. called hot press (or hot stamping). There has been proposed a technique for realizing a high strength by converting the structure into martensite (see, for example, Patent Document 2). However, this technology has manufacturing problems such as significant oxidation of the steel sheet during heating, long heating time, and necessity of cooling control, so it is possible to achieve both load reduction and high strength at lower temperatures. Technology development was sought.

JP 2003-193193 A JP 2011-31254 A

The present invention has been made by paying attention to the above circumstances, and its purpose is to have not only the formability at room temperature but also the effect of reducing the molding load at warm while ensuring room temperature strength of 980 MPa class or higher. and to provide a high strength steel plate.

The invention described in claim 1
% By mass (hereinafter the same for chemical components)
C: 0.02-0.3%
Si: 1.0-3.0%,
Mn: 1.8-3.0%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
Al: 0.001 to 0.1%,
N: 0.002 to 0.008%
And the balance has a component composition consisting of iron and impurities,
The area ratio for all tissues (hereinafter the same for tissues)
Bainitic ferrite: 50-75 %,
Residual austenite: 3% or more,
Martensite + said retained austenite: 10 to 35 %,
Ferrite: 15-40%
Having a structure containing each phase of
C concentration (Cγ _R ) in the residual austenite is 0.3 to 1.2% by mass,
Part of N in the component composition is solute N, and the amount of solute N is 12 ppm or less (including 0 ppm),
High strength steel sheet excellent in formability between room temperature and temperature and a tensile strength at room temperature is not less than 980 MPa.

The invention described in claim 2
The high-strength steel sheet having excellent formability at room temperature and warm according to claim 1, wherein the dislocation density in the whole structure is 5 x 10 ¹⁵ m ^-2 or less.

The invention according to claim 3
Ingredient composition further
Cr: 0.01 to 3.0%
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
B: The high-strength steel sheet excellent in formability at room temperature and warm according to claim 1 or 2 containing one or more of 0.00001 to 0.01%.

The invention according to claim 4
Ingredient composition further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
The high-strength steel sheet having excellent formability at room temperature and warm according to any one of claims 1 to 3, wherein the REM contains one or more of 0.0001 to 0.01%.

According to the present invention, bainitic ferrite: 50-75 %, retained austenite: 3% or more, martensite + residual austenite: 10-35 %, ferrite: 15-40% A C concentration (Cγ _R ) in the retained austenite is 0.3 to 1.2% by mass, a part of N in the component composition is solute N, and the amount of solute N Is not higher than 12ppm (including 0ppm), ensuring high room temperature strength of 980MPa class or higher, and not only formability at room temperature but also warm forming load reduction effect. Can now be provided.

As described above, the present inventors pay attention to a TRIP steel sheet containing bainitic ferrite having a substructure (matrix) with a high dislocation density and residual austenite (γ _R ), similar to the above-described conventional technology, Further studies have been made to further improve not only the formability at room temperature but also the effect of reducing the molding load at warm temperatures while ensuring room temperature strength.

  In order to further improve the molding load reduction effect in the warm, the present inventors have used the TRIP phenomenon (used to obtain strength at room temperature when processed in a temperature range of 100 to 250 ° C. ( It is effective to reduce the strength in the warm (the temperature range of 100 to 250 ° C.) by suppressing the transformation behavior from retained austenite to martensite by increasing the amount of dissolved N. Thought.

Specifically, in order to achieve both high strength at room temperature and improvement in the effect of reducing the molding load at warm temperature, by introducing ferrite with an area ratio of 5 to 40%, the matrix (matrix) strength was low, the area ratio of residual austenite (gamma _R) 3% or more, C concentration in the gamma _R a (C gamma _R) with 0.3-1.2 wt%, TRIP phenomenon (strain-induced Room temperature strength by accelerating transformation and promoting work hardening to improve strength, and further reducing the amount of solute N to suppress strain aging near 200 ° C. It has been found that the effect of reducing the molding load in the warm state can coexist.

  And further examination was advanced based on the said knowledge, and it came to complete this invention.

  Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention is based on the structure of TRIP steel as in the above-described prior art, and particularly contains a predetermined amount of ferrite and a predetermined amount of γ _R having a predetermined carbon concentration, Furthermore, it is different from the above prior art in that it contains a predetermined amount of solute N.

<Bainitic ferrite: 50-85%>
“Bainitic ferrite” in the present invention has a substructure having a lath-like structure with a high dislocation density in the bainite structure and is free of carbides in the structure. It is clearly different, and is also different from the polygonal ferrite structure having a substructure with little or no dislocation density, or a quasi-polygonal ferrite structure having a substructure such as fine subgrains (Japan Iron and Steel Institute Fundamental Study Group) Issued “Steel Bainite Photobook-1”). This structure exhibits an acicular shape when observed with an optical microscope or SEM, and is difficult to distinguish. Therefore, in order to determine a clear difference from a bainite structure or a polygonal / ferrite structure, the structure of the lower structure by TEM observation is determined. Identification is necessary.

  Thus, the balance of strength and formability can be improved by using bainitic ferrite having a uniform and fine structure, high ductility, high dislocation density and high strength as the parent phase.

In the steel sheet of the present invention, the amount of the bainitic ferrite structure needs to be 50 to 85% (preferably 60 to 85%, more preferably 70 to 85%) in terms of area ratio with respect to the entire structure. is there. This is because the effect of the bainitic ferrite structure is effectively exhibited. Note that the amount of the bainitic ferrite structure is determined by the balance with γ _R, and it is recommended that the amount be controlled appropriately so that desired characteristics can be exhibited.

<Contains 3% or more of retained austenite (γ _R ) in area ratio with respect to the entire structure>
γ _R is useful for improving the total elongation, and in order to effectively exhibit such action, the area ratio is 3% or more (preferably 5% or more, more preferably 10% or more) with respect to the entire structure. It is necessary to exist.

<Martensite + Retained austenite (γ _R ): 10 to 45%>
For securing strength, but introduces some martensite in the tissue, since the moldability amount of martensite is too large can not be secured, 10% total area fraction of martensite + gamma _R for all tissues It is limited to 45% or less (preferably 12% or more, more preferably 16% or more).

<Ferrite: 5-40%>
Since ferrite is a soft phase, it does not contribute to high strength, but it is effective in increasing ductility. Therefore, in order to increase the balance between strength and elongation, the area ratio that can guarantee strength is 5% or more (preferably 10% or more, more preferably 15% or more) and 40% or less (preferably 35% or less, more preferably 30% or less).

<C concentration in the retained austenite _{(γ R) (Cγ R)} : 0.3~1.2 wt%>
Cγ _R is, γ _R at the time of processing is an indicator that affects the stability of the transformation to martensite. When C gamma _R is too low, gamma for _R is unstable, after stressing, since the deformation-induced martensitic transformation occurs prior to plastic deformation, not bulging property can be obtained. On the other hand, when the C gamma _R is too high, gamma _R becomes too stable, since the addition of machining work-induced martensitic transformation does not occur, not too bulging property can be obtained. To obtain a sufficient stretch forming property, C gamma _R is required to be 0.3-1.2 mass%. Preferably it is 0.4-0.9 mass%.

<Solution N amount: 15 ppm or less (including 0 ppm)>
Since solute N lowers the ductility of ferrite due to strain aging, limiting the amount of solute N ensures ductility particularly in the temperature range of 100 to 250 ° C. where dynamic strain aging occurs. Preferably it is 10 ppm or less.

<Others: Bainite (including 0%)>
The steel sheet of the present invention may be composed of only the above structure (a mixed structure of bainitic ferrite, martensite, retained austenite, and ferrite). You may have bainite as a structure | tissue. Although this structure can inevitably remain in the manufacturing process of the steel sheet of the present invention, the smaller the number, the better. It is recommended to control the area ratio to 5% or less, more preferably 3% or less with respect to the entire structure. Is done.

<Dislocation density: 5 × 10 ¹⁵ m ⁻² or less>
In the temperature range of about 300 ° C. or lower, the dislocation strengthening mechanism has a small temperature dependency. Therefore, when the TRIP effect is reduced at 100 to 250 ° C., the dislocation density is lowered to some extent to reduce the strength more reliably. It is desirable to keep it 5 × 10 ¹⁵ m ⁻² or less. More preferably, it is 4 × 10 ¹⁵ m ⁻² or less, and particularly preferably 3 × 10 ¹⁵ m ⁻² or less.

[Each phase area ratio, gamma C concentration (C gamma _R) in _R, solute N amount, and each method of measuring the dislocation density]
Here, each phase area ratio, C concentration in the γ _R (Cγ _R), solute N amount, and will be described the method of measuring the dislocation density.

Regarding the area ratio of each phase of the structure in the steel sheet, the white area is defined as “martensite + residual austenite (γ _R )” by repeller corrosion of the steel sheet and observation with a transmission electron microscope (TEM; magnification: 1500 times). After identifying the tissue, the area ratio of each phase was measured by observation with an optical microscope (magnification 1000 times).

Note that the gamma _R area ratio and gamma C concentration in _R of (C gamma _R), was ground to a thickness of 1/4 of each sample steel plates was measured by X-ray diffraction method from the chemical polishing (ISIJ Int.Vol.33, (1933), No.7, p.776).

  As for the area ratio of ferrite, each test steel sheet was subjected to nital corrosion, and the black area was identified as ferrite by observation with a scanning electron microscope (SEM; magnification 2000 times) to obtain the area ratio.

  The amount of dissolved N is calculated by measuring the amount of precipitated N by extraction residue analysis (mesh diameter 0.1 μm) and subtracting the total amount of precipitated N from the total amount of N in steel. did.

  The dislocation density was measured by a method of measuring with an X-ray half width (see paragraphs [0021] to [0032] of JP-A-2008-144233).

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.02-0.3%
C is an essential element for obtaining a desired main structure (bainitic ferrite + martensite + γ _R ) while ensuring high strength, and 0. It is necessary to add 02% or more (preferably 0.05% or more, more preferably 0.10% or more). However, if it exceeds 0.3%, it is not suitable for welding.

Si: 1.0-3.0%
Si is an element that effectively suppresses the generation of carbides by decomposition of γ _R. In particular, Si is useful as a solid solution strengthening element. In order to exhibit such an action effectively, it is necessary to add 1.0% or more of Si. Preferably it is 1.1% or more, More preferably, it is 1.2% or more. However, if Si is added in excess of 3.0%, the formation of bainitic ferrite + martensite structure is hindered, and the hot deformation resistance is increased and the weld is easily embrittled. Adversely affects the surface properties of the steel sheet, so the upper limit is made 3.0%. Preferably it is 2.5% or less, More preferably, it is 2.0% or less.

Mn: 1.8-3.0%
In addition to effectively acting as a solid solution strengthening element, Mn also exerts an effect of promoting transformation and promoting the formation of bainitic ferrite + martensite structure. Furthermore, it is an element necessary for stabilizing γ and obtaining a desired γ _R. It also contributes to improving hardenability. In order to exhibit such an action effectively, it is necessary to add 1.8% or more. Preferably it is 1.9% or more, more preferably 2.0% or more. However, if added over 3.0%, adverse effects such as slab cracking are observed. Preferably it is 2.8% or less, more preferably 2.5% or less.

P: 0.1% or less (including 0%)
P is inevitably present as an impurity element, but is an element that may be added to ensure desired γ _R. However, when it exceeds 0.1%, secondary workability deteriorates. More preferably, it is 0.03% or less.

S: 0.01% or less (including 0%)
S is also an element unavoidably present as an impurity element, forms sulfide inclusions such as MnS, and becomes a starting point of cracking and deteriorates workability. Preferably it is 0.01% or less, More preferably, it is 0.005% or less.

Al: 0.001 to 0.1%
Al is added as a deoxidizer and, in combination with Si, effectively suppresses the formation of carbides by decomposition of γ _R , and further forms nitrides (AlN) to form solid solution N. It is an element effective for suppressing the amount. In order to exhibit such an action effectively, it is necessary to add 0.001% or more of Al. However, even if added excessively, the effect is saturated and is economically wasteful, so the upper limit is made 0.1%.

N: 0.002 to 0.008%
N dissolves in steel and lowers the ductility of ferrite due to strain aging, so the upper limit of N content is limited to 0.008%. On the other hand, the lower limit of the N content is set to 0.002% in consideration of the manufacturing cost of the steel material.

  The steel of the present invention basically contains the above components, and the balance is substantially iron and unavoidable impurities, but the following allowable components can be added as long as the effects of the present invention are not impaired. .

Cr: 0.01 to 3.0%
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
B: One or more elements of 0.00001 to 0.01% These elements are useful elements for strengthening steel and are effective in stabilizing γ _R and securing a predetermined amount. In order to effectively exhibit such an action, Mo: 0.01% or more (more preferably 0.02% or more), Cu: 0.01% or more (more preferably 0.1% or more), Ni : 0.01% or more (more preferably 0.1% or more) and B: 0.00001% or more (more preferably 0.0002% or more) are recommended. However, Cr is 3.0%, Mo is 1.0%, Cu and Ni are each 2.0%, and even if B is added over 0.01%, the above effect is saturated, economically. It is useless. More preferably, Cr is 2.0% or less, Mo is 0.8% or less, Cu is 1.0% or less, Ni is 1.0% or less, and B is 0.0030% or less.

Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
REM: One or more of 0.0001 to 0.01% These elements are effective elements for controlling the form of sulfide in steel and improving workability. Here, examples of the REM (rare earth element) used in the present invention include Sc, Y, and lanthanoid. In order to effectively exhibit the above-mentioned action, Ca and Mg are each added to 0.0005% or more (more preferably 0.0001% or more), and REM is added to 0.0001% or more (more preferably 0.0002% or more). It is recommended to do. However, even if Ca and Mg are added in an amount of 0.01% and REM is added in excess of 0.01%, the above effects are saturated, which is economically wasteful. More preferably, Ca and Mg are 0.003% or less, and REM is 0.006% or less.

[Warm processing method]
It is particularly recommended that the steel sheet of the present invention be processed within 3600 s (more preferably within 1200 s) after heating to an appropriate temperature between 100 and 250 ° C.

Formability can be maximized by processing before the decomposition of γ _R occurs under the temperature condition where the stability of γ _R is optimal.

  Parts processed by this warm processing method have a uniform strength after cooling within the cross section, and there are fewer low-strength parts than parts with a large strength distribution in the same cross section, thus increasing the part strength. be able to.

That is, a steel sheet containing γ _R generally has a low yield ratio and a high work hardening rate in a low strain region. Therefore, in the region where the applied strain amount is small, the strength after applying the strain, in particular, the strain amount dependency of the yield stress becomes very large. When a part is formed by press working, the amount of strain applied varies depending on the part, and there is a region where strain is hardly applied partially. For this reason, a large strength difference may occur between a region where machining is performed and a region where machining is not performed in the component, and a strength distribution may be formed in the component. When such an intensity distribution exists, deformation and buckling occur due to the yielding of the low-strength region, so that the part having the lowest strength is rate-determined.

The reason why the yield stress is low in the steel containing γ _R is thought to be that when γ _R is introduced, martensite formed at the same time introduces mobile dislocations in the surrounding matrix during transformation. Therefore, if this dislocation movement is prevented even in a region where the amount of processing is small, the yield stress can be improved and the component strength can be increased. In order to suppress the movement of movable dislocations, it is effective to heat the material to eliminate the movable dislocations or to stop it by strain aging such as solute carbon, which can increase the yield stress.

Therefore, when a steel sheet containing γ _R is heated to an appropriate temperature between 100 ° C. and 250 ° C. and press-formed (warm processing), the yield strength increases even in a portion with a small strain, and the strength distribution in the part decreases. As a result, the component strength can be improved.

  Next, the preferable manufacturing method for obtaining the said steel plate of this invention is demonstrated below.

[Preferred production method of the steel sheet of the present invention]
The steel sheet according to the present invention is manufactured by hot rolling a steel material satisfying the above component composition, performing a nitride precipitation treatment, cold rolling, and further performing a heat treatment.

[Hot rolling conditions]
The hot rolling conditions are not particularly limited. For example, the hot rolling finishing temperature (rolling end temperature, FDT) may be 800 to 900 ° C., and the winding temperature may be 400 to 600 ° C.

[Nitride precipitation treatment conditions]
By heating to 500-700 ° C. and holding for 60 s or more before cold rolling, solid solution N in the steel is precipitated as AlN which is a nitride by Al and fixed, thereby reducing the amount of solid solution N To do.

[Cold rolling conditions]
Moreover, it heat-processes on the following heat processing conditions, setting the cold rolling rate in the case of cold rolling as 30 to 70%.

[Heat treatment conditions]
Regarding the heat treatment conditions, the temperature is rapidly raised at a predetermined heating rate, soaked in the high temperature side temperature range of the ferrite + austenite (α + γ) two-phase region, and the majority of the structure is austenitized, and then rapidly cooled at a predetermined cooling rate. After supercooling, a desired structure can be obtained by holding at the supercooling temperature for a predetermined time and performing austempering. It should be noted that plating or further alloying treatment may be performed without significantly degrading the desired structure and within the range not impairing the action of the present invention.

  Specifically, the cold-rolled material after the cold rolling is rapidly heated at a heating rate of 10 ° C./s or more, and is 10 to 10 in a temperature range of (0.4Ac1 + 0.6Ac3) to (0.1Ac1 + 0.9Ac3). After holding for 60 s, it is cooled rapidly to a temperature range of 350 to 500 ° C. at an average cooling rate of 10 ° C./s or more, and held for 10 to 1800 s at this quenching stop temperature (supercooling temperature). After austempering, cool to room temperature.

<Rapid heating at a heating rate of 10 ° C./s or higher>
This is because by shortening the temperature raising time, it is possible to suppress the fixation of N by a nitride-forming element such as Al and secure a solid solution N amount.

<Holding for 10 to 60 seconds in a temperature range of (0.4Ac1 + 0.6Ac3) to (0.1Ac1 + 0.9Ac3)>
This is because a majority of the structure is austenitized by holding it in the temperature region on the high temperature side of the two-phase region, and the fraction of bainitic ferrite produced by reverse transformation from austenite during cooling is ensured.

<At an average cooling rate of 10 ° C./s or more, it is rapidly cooled to a temperature range of 350 to 500 ° C. and supercooled, and this rapid cooling stop temperature (supercooling temperature) is maintained for 10 to 1800 s>
This is because the desired structure is obtained by the austempering process.

  In order to confirm the effect of the present invention, the influence of the mechanical properties of the high-strength steel sheet at room temperature and warm when the component composition and heat treatment conditions were changed was investigated. Test steels having the respective component compositions shown in Table 1 below were melted in vacuum to form a slab having a thickness of 30 mm, and then the slab was heated to 1200 ° C., rolling end temperature (FDT) 900 ° C., and winding temperature 500 The steel sheet was hot-rolled at 2.4 ° C. to a thickness of 2.4 mm, the hot-rolled material was heated again to the precipitation treatment temperature T0 and held for t0 seconds, and then cold-rolled at a cold rolling rate of 50% to obtain a thickness of 1.2 mm The cold-rolled material was subjected to the heat treatment shown in Table 2 below. Specifically, the cold-rolled material is heated to a soaking temperature T1 ° C., held at that temperature for a soaking time t1 second, and then cooled to a cooling stop temperature (supercooling temperature) T2 at a cooling rate of CR1 ° C./s. After cooling and holding at that temperature for t2 seconds, it was air-cooled, or after holding at the cooling stop temperature (supercooling temperature) T2 ° C. for t2 seconds, and further holding at the holding temperature T3 ° C. for t3 seconds, then it was air-cooled.

The steel sheet thus obtained, by a measuring method described in the section of [Description of the Invention, each phase area ratio, C concentration (C gamma _R) in gamma _R, solute N amount The dislocation density was measured.

  Moreover, in order to evaluate the mechanical properties at room temperature and warm, the tensile strength (TS) and elongation (EL) at room temperature and the tensile strength (TS) at 150 ° C. were measured for the steel sheet. . Then, ΔTS = TS at warm (150 ° C.) − TS at room temperature was calculated as an index for evaluating the load reduction effect by warm forming.

  These results are shown in Table 3.

As shown in these tables, steel No. which is the steel sheet of the present invention. 1 to 3 and 10 to 17 all use steel types satisfying the range of the component composition of the present invention, and as a result of heat treatment under the recommended heat treatment conditions, satisfy the requirements of the structure provision of the present invention, A high-strength steel sheet excellent in room temperature and warm elongation (EL) and warm forming load reduction effect (ΔTS) was obtained while securing a strength (TS) of 980 MPa or more.

  In contrast, steel No., which is a comparative steel. 4 to 9 all use steel grades that do not satisfy the requirements of the component composition defined in the present invention, so that the heat treatment is performed under the recommended heat treatment conditions, but the requirements of the structure provision of the present invention are not satisfied, room temperature strength At least one of the properties of (TS), room temperature and warm elongation (EL), and warm load reduction effect (ΔTS) is inferior.

  Moreover, steel No. which is another comparative steel. Although 18-21, 24, and 28 all used the steel grade which satisfies the range of the component composition of this invention, as a result of performing heat processing on the conditions outside the recommended heat processing conditions, the requirements of the structure | tissue of this invention are satisfied. Moreover, at least one of the properties of room temperature strength (TS), room temperature and warm elongation (EL), and warm load reduction effect (ΔTS) is inferior.

Claims (4)

% By mass (hereinafter the same for chemical components)
C: 0.02-0.3%
Si: 1.0-3.0%,
Mn: 1.8-3.0%,
P: 0.1% or less (including 0%),
S: 0.01% or less (including 0%),
Al: 0.001 to 0.1%,
N: 0.002 to 0.008%
And the balance has a component composition consisting of iron and impurities,
The area ratio for all tissues (hereinafter the same for tissues)
Bainitic ferrite: 50-75 %,
Residual austenite: 3% or more,
Martensite + said retained austenite: 10 to 35 %,
Ferrite: 15-40%
Having a structure containing each phase of
C concentration (Cγ _R ) in the residual austenite is 0.3 to 1.2% by mass,
Part of N in the component composition is solute N, and the amount of solute N is 12 ppm or less (including 0 ppm),
A high-strength steel sheet having excellent formability at room temperature and warm, wherein the tensile strength at room temperature is 980 MPa or more. The high-strength steel sheet excellent in formability at room temperature and warm according to claim 1, wherein the dislocation density in the entire structure is 5 × 10 ¹⁵ m ⁻² or less. Ingredient composition further
Cr: 0.01 to 3.0%
Mo: 0.01 to 1.0%,
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
The high-strength steel sheet excellent in formability at room temperature and warm according to claim 1 or 2, wherein B: one or more of 0.00001 to 0.01%. Ingredient composition further
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
The high-strength steel sheet excellent in formability at room temperature and warm according to any one of claims 1 to 3, wherein the REM contains one or more of 0.0001 to 0.01%.