JP4910898B2 - High strength steel plate and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel sheet having a tensile strength of 780 MPa or more and a manufacturing method thereof. The present invention is a high-strength steel sheet having a tensile strength of 780 MPa or more, which is suitable as a material for structural members formed into various shapes including press molding such as drawing and overhanging, particularly in industrial fields such as automobiles and machinery, and the production thereof. Regarding the method.

  In recent years, various machines and devices have been strongly promoted to achieve high performance and light weight, and many techniques for increasing the strength of steel sheets have been developed. However, in general, increasing the strength of a steel sheet is accompanied by deterioration of ductility, so that it has been considered very difficult to manufacture a steel sheet having both good workability and high strength. However, after annealing in a two-phase region to a low-carbon steel sheet with a combined addition of Si and Mn, it is rapidly cooled to 350 to 550 ° C. and kept in that temperature range for a short time or by stepped cooling. -When the structure of stenite partially transformed into bainite and finally composed of ferrite + bainite + retained austenite, the retained austenite undergoes strain-induced transformation during deformation during processing and exhibits a large elongation. '' Since this phenomenon was found, attempts have been made to produce high ductility and high strength steel sheets using this phenomenon.

  For example, Patent Document 1 includes 0.4 to 1.8% Si (hereinafter, “%” representing the component ratio is “mass%”) and 0.2 to 2.5% Mn, If necessary, a steel plate containing one or more of P, Ni, Cu, Cr, Ti, Nb, V, and Mo is heated to a ferrite + austenite two-phase region, and then cooled to 500 to 350 ° C. A method is disclosed in which the mixed structure is realized by holding in a temperature range of 30 seconds to 30 minutes to obtain a high-strength steel sheet exhibiting high ductility.

In Patent Document 2, as a method for producing a high-strength steel sheet exhibiting high ductility, a steel sheet containing 0.7 to 2.0% Si and 0.5 to 2.0% Mn is annealed in the annealing process. + After heating to austenite two-phase region, hold constant temperature for 10 to 50 seconds between 650 and 450 ° C. in the cooling process. -A method for producing a mixed structure steel plate containing stenite is disclosed.
JP-A 61-157625 Japanese Patent Laid-Open No. 60-43430

  In practice, however, steel sheets having the above mixed structure generally do not necessarily have good formability during press working even if they exhibit good ductility in a tensile test, and are sufficiently satisfactory as a working steel sheet. It was not a thing. For example, when the mixed-structure steel sheet is processed, most of the retained austenite is strain-induced and transformed into high carbon martensite in the later stage of deformation, so that the local ductility becomes extremely poor. This phenomenon appears remarkably in the case of stretch flange processing such as hole expansion, so that the hole expansion property of the mixed structure steel plate is inferior to that of the conventional low carbon steel plate. This is because when punching is performed by punching, the high carbon martensite generated by strain-induced transformation is very hard and cracks are generated, and this crack expands and propagates during subsequent hole expansion. It is considered.

  This invention is made | formed in view of the said present condition, The subject is providing the high strength steel plate excellent in the workability in which ductility and hole expansibility were compatible, and its manufacturing method.

Then, when this inventor repeated earnest research in order to solve the said subject, the following knowledge was able to be acquired.
(1) In order to ensure a tensile strength of 780 MPa or more and ensure ductility, the ferrite volume ratio needs to be within a specific range.

  (2) In local deformation such as hole expansion, in addition to excessively hard martensite being the starting point of cracks, crack starting points may exist inside low-temperature transformation phases other than martensite. Coarse iron carbide was observed.

  (3) The content of other alloy elements such as Si, Al and Mn and the production conditions are controlled to adjust the amount of C in the austenite and the transformation process of the austenite in the annealing process before transformation, and the origin of cracks By suppressing the formation of excessively hard martensite or coarse iron carbide, it is possible to produce a high-strength steel sheet excellent in workability that achieves both ductility and hole expansibility.

The present invention has been completed on the basis of the above knowledge and the like, and the gist thereof is as follows.
(1) By mass%, C: 0.08 to 0.3%, Si: 0.25 to 1.1%, Mn: more than 2%, 3.5% or less, P: 0.028% or less, S: 0.01% or less, sol. Al: 0.03 to 1% and N: 0.01% or less, with the balance being Fe and impurities and having a steel composition satisfying the following formula (1), the steel structure is ferrite, low-temperature transformation phase And the volume fraction VF of the ferrite is 0.1 to 0.80, satisfies the following formula (2), the volume fraction VA of the residual austenite is 0.30 or less, and the low temperature transformation A high-strength steel sheet having a tensile strength of 780 MPa or more, wherein the particle size of iron carbide contained in the phase is 500 nm or less.

0.2 ≦ (Si + Al) /Mn≦0.80 (1)
0.15 ≦ C / (1-VF) ≦ 0.50 (2)
Here, Si, Al, Mn, and C in the formula indicate the content (unit: mass%) of each element.
(2) By mass%, C: 0.08 to 0.3%, Si: 0.25 to 1.1%, Mn: more than 2%, 3.5% or less, P: 0.028% or less, S: 0.01% or less, sol. Al: 0.03 to 1% and N: 0.01% or less, further Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less and B: 0 The steel composition contains one or more selected from the group consisting of 0.005% or less , the balance is Fe and impurities, and satisfies the following formula (3). The steel structure is ferrite, low temperature transformation A volume fraction VF of the ferrite is 0.1 to 0.80, satisfies the following formula (2), the volume fraction of residual austenite VA is 0.30 or less, and the low temperature transformation phase A high-strength steel sheet having a tensile strength of 780 MPa or more, wherein the iron carbide contained in the steel has a particle size of 500 nm or less.

0.2 ≦ (Si + Al) / (Mn + Ni) ≦ 0.80 (3)
0.15 ≦ C / (1-VF) ≦ 0.50 (2)
Here, Si, Al, Mn, Ni, and C indicate the content (unit: mass%) of each element.
(3) The high-strength steel plate according to (1) or (2) above, wherein a plating layer is provided on the steel plate surface.
(4) By mass%, C: 0.08 to 0.3%, Si: 0.25 to 1.1%, Mn: more than 2%, 3.5% or less, P: 0.028% or less, S: 0.01% or less, sol. Hot rolled into a steel ingot or steel slab containing Al: 0.03 to 1% and N: 0.01% or less, the balance being Fe and impurities and having a steel composition satisfying the following formula (1) In the method for producing a high-strength steel sheet, after the descaling, the obtained hot-rolled steel sheet is subjected to cold rolling, and then the obtained cold-rolled steel sheet is subjected to continuous annealing or further plating. The coiling temperature after rolling is set to 450 to 700 ° C., and the cold-rolled steel sheet is heated to a temperature range equal to or higher than the Ac1 transformation point in the continuous annealing, and a temperature range of less than 350 ° C. at an average cooling rate of 3 ° C./second or more. The method for producing a high-strength steel sheet as described in (1) above, wherein
0.2 ≦ (Si + Al) /Mn≦0.8 (1)
Here, Si, Al, and Mn in the formula indicate the content (unit: mass%) of each element.
(5) By mass%, C: 0.08 to 0.3%, Si: 0.25 to 1.1%, Mn: more than 2%, 3.5% or less, P: 0.028% or less, S: 0.01% or less, sol. Al: 0.03 to 1% and N: 0.01% or less, further Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less and B: A steel ingot or steel slab containing one or two or more selected from the group consisting of 0.005% or less and having the steel composition satisfying the following formula (3) with the balance consisting of Fe and impurities , In the method for producing a high-strength steel sheet, after rolling and descaling, the obtained hot-rolled steel sheet is cold-rolled, and then the obtained cold-rolled steel sheet is subjected to continuous annealing or further plating. The coiling temperature after hot rolling is set to 450 to 700 ° C., and the cold-rolled steel sheet is heated to a temperature range equal to or higher than the Ac1 transformation point in the continuous annealing, and the temperature is less than 350 ° C. at an average cooling rate of 3 ° C./second or more. As described in (2) above, which is cooled to an area Method of manufacturing a high-strength steel sheet.
0.2 ≦ (Si + Al) / (Mn + Ni) ≦ 0.80 (3)
Here, Si, Al, Mn, and Ni in the formula indicate the content (unit: mass%) of each element.
(6) In the continuous annealing, after the cold-rolled steel sheet is cooled to a temperature range of less than 350 ° C., the cold-rolled steel sheet is allowed to stay in the temperature range of 200 to 350 ° C. for 500 seconds or less and then cooled to room temperature. The method for producing a high-strength steel sheet according to the above (4) or (5).

  According to the present invention, a high-strength steel sheet having excellent ductility and good workability such as hole expansibility, in particular, a high-strength thin steel sheet for structural members such as automobiles can be stably obtained. The effect is brought about.

In the present invention, the reason why the steel composition and production conditions are defined as described above will be described.
First, the reason for defining the steel composition is as follows. In the present specification, “%” indicating the steel composition is “mass%”.

C: 0.08 to 0.3%
C has the effect | action which raises the tensile strength of a steel plate. If the C content is less than 0.08%, it may be difficult to ensure the intended tensile strength of the present invention, so the lower limit of the C content is 0.08%. On the other hand, if the C content exceeds 0.3%, the steel plate becomes excessively hard, and it becomes difficult to process the steel plate in a normal plate making process. Therefore, the C content is 0.08 to 0.3%. Preferably it is 0.08 to 0.2%. Furthermore, if considering weldability, 0.08 to 0.15% is most preferable.

Si: 0.25 to 1.1%
Si is a ferrite stabilizing element and has an effect of increasing the C concentration of the austenite phase which is balanced by increasing the ferrite volume fraction, and is an important element in the present invention. Si also has the effect of suppressing tempering of the low temperature transformation phase. Due to these two effects, the formation of coarse iron carbide harmful to hole expansion by tempering at high temperature is suppressed, and 0.25% or more is contained in order to ensure the hole expansion property. However, if Si exceeds 1.1%, not only the surface quality deteriorates due to the Si scale unique to the Si-added steel sheet, but the C concentration of the austenite phase is excessively increased, resulting in excessively hard martensite. Since the hole-expanding property is rather deteriorated by the generation of the site, the Si content is set to 1.1% or less. Preferably, it is 0.8% or less.

Mn: more than 2% and 3.5% or less Mn is an element necessary for ensuring the strength of a tensile strength of 780 MPa or more and is contained in excess of 2%. However, if it exceeds 3.5%, the formation of ferrite is suppressed and ductility deteriorates, so 3.5% should be made the upper limit. More preferably, it is 2.8% or less.

P: 0.028% or less P is an element unavoidably contained in steel as an impurity, and is preferably as low as possible. In particular, if the content exceeds 0.028%, the deterioration of the toughness of the welded portion is significant, so the P content is determined to be 0.028% or less.

S: 0.01% or less S is an element inevitably contained in steel as an impurity, and the lower one is preferable. In particular, when the content exceeds 0.01%, the deterioration of the toughness of the welded portion is remarkable, so the S content is determined to be 0.01% or less.

sol. Al: 0.03 to 1%
Al is a ferrite stabilizing element and has an effect of increasing the C concentration of the austenite phase which is balanced by increasing the ferrite volume fraction, and is an important element in the present invention. Moreover, it is necessary for deoxidizing the molten steel together with this, and it is necessary to contain at least 0.03% or more. However, since weldability becomes a problem when the Al content exceeds 1%, Al is determined to be 1% or less. Preferably, it is 0.7% or less. Al has a stronger effect of forming ferrite than Si, and is preferably contained in an amount of 0.3% or more.

N: 0.01% or less N is an element inevitably contained in steel as an impurity, and its content is preferably low. In particular, if the N content exceeds 0.01%, the amount of Al consumed as AlN is large and the effect of Al addition becomes small, and the deterioration of ductility due to AlN becomes conspicuous, so the upper limit of the N content Was determined to be 0.01%.

Cu: 1%, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, B: 0.005% or less Cu, Ni, Cr, Mo, B improves strength. At least one kind can be contained for the purpose. When the content exceeds each of the above upper limits, the ductility is remarkably deteriorated.

0.2 ≦ (Si + Al) /Mn≦0.80 (1)
0.2 ≦ (Si + Al) / (Mn + Ni) ≦ 0.80 (3)
In the present invention, it is important to increase the ferrite volume fraction to ensure ductility and to optimize the hardness of the low temperature transformation phase by appropriate concentration of C in the austenite during the annealing process. The above formulas (1) and (3) are related to the ease of ferrite formation in the cooling process from the soaking of the continuous annealing. When the value of the formula is less than 0.2, there are few soft ferrites and the ductility is insufficient. When 0.80 is exceeded, too much ferrite is formed, an excessively hard low-temperature transformation phase is formed, and the hole expandability is poor. Preferably it is 0.3-0.6.

  Formula (3) is applied instead of Formula (1) in the case of a steel composition containing Ni. Therefore, in the present invention, Ni acts not only as a strength improving element like Cu and Mo but also as Mn as a ferrite suppressing element.

  The high-strength steel sheet according to the present invention is composed of a ferrite phase, a low-temperature transformation phase, and a retained austenite phase, and their quantitative relationship is as follows. In addition, the volume ratio of each structure | tissue represents the volume ratio of the structure | tissue when the whole volume is set to 1.

Ferrite volume fraction VF: 0.1 to 0.80
In the present invention, the ferrite volume fraction is limited to 0.1 or more in order to improve the ductility while ensuring a tensile strength of 780 MPa or more. On the other hand, if it exceeds 0.8, it will be difficult to ensure the tensile strength, so it is limited to 0.80 or less. Preferably it is 0.4-0.7.

  Such adjustment of the ferrite volume fraction may change the blending amount of ferrite stabilizing elements such as Si and Al and austenite stabilizing elements such as Mn and Ni, or change the soaking temperature and cooling rate of annealing conditions. Those skilled in the art can easily understand from the above description.

In addition, the ferrite which comprises the low temperature transformation phase is not included in the ferrite volume fraction here.
0.15 ≦ C / (1-VF) ≦ 0.50 (2)
In order to control the amount of C contained in the second phase other than ferrite (low-temperature transformation phase and retained austenite), the value of C / (1-VF) is limited to 0.15 to 0.50. If it is less than 0.15, the hardness of the low temperature transformation phase necessary for securing the strength when VF ≧ 0.1 is not obtained, and if it exceeds 0.50, the volume fraction of very hard martensite and residual austenite is low. Increases and causes deterioration of hole expansibility.

Residual austenite volume fraction VA: 0.30 or less In the present invention, retained austenite is a phase inevitably mixed. While it is a convenient structure for increasing the elongation of a steel sheet, martensite produced by strain-induced transformation by processing is very hard and therefore causes deterioration of hole expansibility. Therefore, the retained austenite is limited to 0.30 or less in volume ratio.

The volume fraction of retained austenite can be adjusted by appropriately controlling the amount of C in the second phase other than ferrite and the conditions such as the soaking temperature and cooling rate in the annealing process according to the amount of C.
In the present invention, the steel structure is determined by measuring the retained austenite volume fraction by the X-ray reflection intensity, and the ferrite volume fraction and the size of the iron carbide by the cross-sectional electron micrograph.

Particle size of iron carbide contained in low-temperature transformation phase: 500 nm or less The low-temperature transformation phase of the present invention refers to martensite, bainite, and those tempered, and the high-strength steel sheet of the present invention has a steel structure. Any one or more of these must be included. In order to avoid that the low-temperature transformation phase of the present invention becomes excessively hard and causes deterioration of hole expansibility, a part of C is precipitated as fine iron carbide inside the phase. That is, the low temperature transformation phase of the present invention preferably contains iron carbide having a particle size of 1 to 100 nm.
However, although the mechanism is not necessarily clear, coarse iron carbide may occur in the low-temperature transformation phase, and when the particle size of the iron carbide exceeds 500 nm, the hole expandability is caused by the generation of cracks starting from the particle size. In order to bring about deterioration, it must be limited to 500 nm or less.

  Such coarse iron carbide is produced by (1) Si content of less than 0.5%, (2) Hot rolling coiling temperature of 600 ° C. or higher, and (3) 600 to 350 during cooling in the annealing process. This is remarkable when conditions such as a long holding time between ° C. overlap. In order to make the size of the iron carbide 500 nm or less, these conditions are avoided under normal conditions, and Cr or Mo is preferably added.

The amount of the low-temperature transformation phase, that is, the volume ratio is not particularly limited, but preferably it is about 0.25 in volume ratio.
The method for producing a high-strength steel sheet according to the present invention can realize the steel structure as described above by performing hot rolling, cold rolling and heat treatment on a steel ingot or steel slab having the steel composition as described above. There is no particular limitation, but an example will be described.

  When the steel having the composition according to the present invention is hot-rolled and then wound at a low temperature, it becomes hardened and hardened so that subsequent cold-rolling becomes difficult. On the other hand, when it is wound at a high temperature, since it contains a large amount of easily oxidizable elements such as Si, Mn, and Al, oxidation of the surface of the hot-rolled steel sheet becomes remarkable, and the remaining scale after pickling becomes a problem. Therefore, it was decided that the winding after hot rolling should be performed at 450 to 700 ° C., which can avoid the above disadvantages. However, since it is desired that the hot-rolled steel sheet be easily pickled and cold-rolled as much as possible, the coiling temperature is preferably 500 to 650 ° C. As a descaling method, mechanical descaling, shot blasting, grinding, etc. under skin pass pressure can be selected in addition to the above pickling.

  In continuous annealing of a cold-rolled steel sheet, soaking is performed in a temperature range equal to or higher than the Ac1 transformation point in order to finally form a structure including a low-temperature transformation phase. However, if the heating temperature is too low, it takes too much time for the cementite to re-dissolve, and if it is too high, the heating cost increases and the load on the equipment is large, so it is desirable to soak at 800 to 860 ° C. .

  Subsequent cooling is performed at an average cooling rate of 3 ° C./second or more to less than 350 ° C. in order to obtain an appropriate C concentration in the austenite during the annealing process and to suppress the formation of coarse iron carbide. When the average cooling rate is less than 3 ° C./second or the cooling end temperature is 350 ° C. or more, a structure containing very hard martensite or coarse carbide is generated, and the strength is increased. It may decrease or the hole expandability may deteriorate.

  From the viewpoint of further improving the hole expanding property by appropriately softening the low-temperature transformation phase, it is preferable to stay in the temperature range of 200 to 350 ° C. for 500 seconds or less after the cooling. Here, if the temperature exceeds 350 ° C., the iron carbide in the low-temperature transformation phase becomes coarse and the intended strength may not be obtained. Preferably it is 330 degrees C or less. On the other hand, when the temperature is lower than 200 ° C., it is difficult to sufficiently obtain the effect of further improving the hole expanding property by appropriately softening the low temperature transformation phase. Further, if the staying time exceeds 500 seconds, the transformation phase may become excessively soft and the intended strength may not be obtained. Preferably it is within 400 seconds. In order to surely obtain the above effect, the staying time is preferably 50 seconds or longer, and more preferably 100 seconds or longer.

  Thus, according to the present invention, during processing, a large elongation due to strain-induced transformation of retained austenite is ensured as in the prior art, while a sufficient amount of soft ferrite is present, and the amount of carbon in the low-temperature transformation phase is also increased. Since there is little, local ductility is ensured, and excellent hole expansibility is realized. Further, in the steel composition of the present invention, the strength can be secured at 780 MPa or more due to the presence of such a low temperature transformation phase.

  The obtained steel plate may be electroplated with various metals. Alternatively, even when hot-dip plating is performed during the cooling of continuous annealing, the effects of the present invention are not lost as long as the cooling conditions and the residence time are satisfied. An example of the plating metal is zinc from a practical viewpoint.

  Steel of each component composition shown in Table 1 was melted and cast to obtain a slab. Next, after soaking at 1250 ° C. for 1 hour, it was rolled with a hot rolling mill so that the finishing temperature was 900 ° C., and a 5 mm thick hot rolled steel sheet was obtained. And as a winding simulation, the steel sheet is cooled to a temperature of 550 to 650 ° C. immediately after hot rolling by forced air cooling or water spray cooling, and then charged into an electric furnace maintained at the temperature. And then kept in the furnace at a cooling rate of 20 ° C./hour. Next, the obtained hot-rolled steel sheet was descaled by surface grinding to obtain a cold-rolled base material having a thickness of 2.8 mm, which was cold-rolled to a thickness of 1.4 mm.

  The obtained cold-rolled steel sheet was heated to 820 ° C. at 10 ° C./second in an infrared heating furnace as a continuous annealing simulation, held at that temperature for 40 seconds, and then an average cooling rate of 2-15 ° C./second. Was cooled to a temperature of 210 to 380 ° C. and held at that temperature for 80 to 700 seconds.

Then, steel numbers 20 to 28 were subjected to electrogalvanizing on both sides with an adhesion amount of 35 g / m 2 per side.
A JIS No. 5 tensile test piece was collected from each of the obtained steel plates and subjected to a tensile test, and a hole expansion test was performed. In the hole expansion test, a 60 mm conical punch was pushed in with a burr on the outer side of a test piece punched with a 10 mm diameter hole with a clearance of 12% on one side, and the hole expansion rate was determined by measuring the hole diameter at the crack initiation limit.

Here, the hole expansion rate is defined as follows.
Hole expansion rate (%) = (Hole diameter at crack limit (mm) −10) / 10 × 100
In the present invention, TS (MPa) × EL (%) ≧ 15000 and TS (MPa) × hole expansion ratio (%) ≧ 30000 are high strength excellent in workability in which ductility and hole expansion are compatible. A steel plate was used.

  For each annealed plate, the residual austenite volume fraction was measured by X-ray reflection intensity measurement, and the ferrite volume fraction was also measured using an electron micrograph of the cross-sectional structure. Furthermore, the size of the maximum iron carbide in the visual field corresponding to 10 μm square in the low-temperature transformation phase was measured by a high-magnification electron micrograph.

  These results are shown in Table 2. From the results shown in Table 2, it is shown that steel that is outside the range defined by the present invention cannot achieve both high elongation and hole expandability at the intended strength level.

Claims (6)

In mass%, C: 0.08 to 0.3%, Si: 0.25 to 1.1%, Mn: more than 2%, 3.5% or less, P: 0.028% or less, S: 0.01 % Or less, sol. Al: 0.03 to 1%, and N: 0.01% or less, the balance is made of Fe and impurities and has a steel composition satisfying the following formula (1), the steel structure is ferrite, low temperature transformation And the volume fraction VF of the ferrite is 0.1 to 0.80 and satisfies the following formula (2), the volume fraction VA of the residual austenite is 0.30 or less, and the low temperature A high-strength steel sheet having a tensile strength of 780 MPa or more, wherein the grain size of iron carbide contained in the transformation phase is 500 nm or less.
0.2 ≦ (Si + Al) /Mn≦0.8 (1)
0.15 ≦ C / (1-VF) ≦ 0.50 (2)
Here, Si, Al, Mn, and C in the formula indicate the content (unit: mass%) of each element. In mass%, C: 0.08 to 0.3%, Si: 0.25 to 1.1%, Mn: more than 2%, 3.5% or less, P: 0.028% or less, S: 0.01 % Or less, sol. Al: 0.03 to 1% and N: 0.01% or less, further Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less and B: The steel composition contains one or more selected from the group consisting of 0.005% or less , the balance is Fe and impurities, and satisfies the following formula (3). The steel structure is ferrite, low temperature It comprises a transformation phase and residual austenite, the ferrite volume fraction VF is 0.1 to 0.80, satisfies the following formula (2), the residual austenite volume fraction VA is 0.30 or less, and the low temperature transformation A high-strength steel sheet having a tensile strength of 780 MPa or more, wherein the particle size of iron carbide contained in the phase is 500 nm or less.
0.2 ≦ (Si + Al) / (Mn + Ni) ≦ 0.80 (3)
0.15 ≦ C / (1-VF) ≦ 0.50 (2)
Here, Si, Al, Mn, Ni, and C indicate the content (unit: mass%) of each element.   The high-strength steel plate according to claim 1, further comprising a plating layer on the steel plate surface. In mass%, C: 0.08 to 0.3%, Si: 0.25 to 1.1%, Mn: more than 2%, 3.5% or less, P: 0.028% or less, S: 0.01 % Or less, sol. Hot rolled into a steel ingot or steel slab containing Al: 0.03 to 1% and N: 0.01% or less, the balance being Fe and impurities and having a steel composition satisfying the following formula (1) In the method for producing a high-strength steel sheet, after the descaling, the obtained hot-rolled steel sheet is subjected to cold rolling, and then the obtained cold-rolled steel sheet is subjected to continuous annealing or further plating. The coiling temperature after rolling is set to 450 to 700 ° C., and the cold-rolled steel sheet is heated to a temperature range equal to or higher than the Ac1 transformation point in the continuous annealing, and a temperature range of less than 350 ° C. at an average cooling rate of 3 ° C./second or more. The method for producing a high-strength steel sheet according to claim 1, wherein the steel sheet is cooled to a low temperature.
0.2 ≦ (Si + Al) /Mn≦0.8 (1)
Here, Si, Al, and Mn in the formula indicate the content (unit: mass%) of each element. In mass%, C: 0.08 to 0.3%, Si: 0.25 to 1.1%, Mn: more than 2%, 3.5% or less, P: 0.028% or less, S: 0.01 % Or less, sol. Al: 0.03 to 1% and N: 0.01% or less, further Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less and B: A steel ingot or steel slab containing one or two or more selected from the group consisting of 0.005% or less and having the steel composition satisfying the following formula (3) with the balance consisting of Fe and impurities , In the method for producing a high-strength steel sheet, after rolling and descaling, the obtained hot-rolled steel sheet is cold-rolled, and then the obtained cold-rolled steel sheet is subjected to continuous annealing or further plating. The coiling temperature after hot rolling is set to 450 to 700 ° C., and the cold-rolled steel sheet is heated to a temperature range equal to or higher than the Ac1 transformation point in the continuous annealing, and the temperature is less than 350 ° C. at an average cooling rate of 3 ° C./second or more. 3. Cooling to a zone Method for producing a high strength steel sheet.
0.2 ≦ (Si + Al) / (Mn + Ni) ≦ 0.80 (3)
Here, Si, Al, Mn, and Ni in the formula indicate the content (unit: mass%) of each element.   In the continuous annealing, after cooling the cold-rolled steel sheet to a temperature range of less than 350 ° C., the cold-rolled steel sheet is allowed to stay in a temperature range of 200 to 350 ° C. for 500 seconds or less and then cooled to room temperature. The manufacturing method of the high strength steel plate of Claim 4 or Claim 5 to do.