JP6696208B2 - High strength steel sheet manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a high-strength steel sheet, and particularly to a method for manufacturing a high-strength steel sheet having excellent ductility and stretch flangeability.

  In recent years, from the perspective of protecting the global environment, we are working on reducing carbon dioxide emissions in many fields. Automobile manufacturers are also actively developing technologies for reducing the weight of vehicles to reduce fuel consumption. However, it is not easy to reduce the weight of the vehicle body because the emphasis is also placed on the improvement of collision resistance in order to ensure the safety of passengers.

  Therefore, in order to achieve both the weight reduction of the vehicle body and the collision resistance, it is considered to reduce the thickness of the member by using the high strength steel plate. Therefore, a steel sheet having both high strength and excellent formability has been strongly desired, and several techniques have been proposed so far in order to meet these requirements.

  For example, in Patent Document 1, high strength for automobiles having excellent collision resistance and formability, in which residual austenite having an average crystal grain size of 5 μm or less is dispersed in ferrite having an average crystal grain size of 10 μm or less, is excellent. A steel plate is disclosed.

  Patent Document 2 discloses a high-strength steel sheet excellent in elongation and stretch flangeability, in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains.

  Patent Document 3 has an area ratio of 75% or more of ferrite phase, 1% or more of bainitic ferrite phase and 1% or more and 10% or less of pearlite phase. Further, the area ratio of martensite phase is 10%. % Or less, and the martensite area ratio / (bainitic ferrite area ratio + perlite area ratio) ≦ 0.6 is satisfied, and the ratio of the Mn concentration in the ferrite phase and the Mn concentration in the second phase is 0. A high-strength hot-dip galvanized steel sheet having excellent workability and impact resistance of 0.70 or more is disclosed.

Patent Document 4 has a two-phase structure in which an area ratio of tempered martensite is 5% or more and 95% or less and the balance is ferrite, and an average Mn concentration C _{Mn · α} in ferrite and tempered martensite Disclosed is a high-strength cold-rolled steel sheet having an excellent balance between elongation and stretch-flangeability, which has a ratio C _{Mn · α} / C _{Mn · M} of 0.95 or more with respect to the average Mn concentration C _{Mn · M.}

  In Patent Document 5, after completion of hot rolling, after cooling to a temperature range of 720 ° C. or less within 1 second and allowing the material to stay in a temperature range of more than 500 ° C. and 720 ° C. or less for a stay time of 1 second or more and 20 seconds or less, It discloses a method for producing a high-strength hot-rolled steel sheet which has good ductility and stretch-flangeability and is wound in a temperature range of 350 ° C. or higher and 500 ° C. or lower.

In Patent Document 6, after completion of hot rolling, after cooling to a temperature range of 780 ° C. or less within 0.4 seconds, winding and cold rolling (Ac ₃ points −40 ° C.) or more temperature range. Discloses a method for producing a cold-rolled steel sheet having good ductility and stretch-flangeability, which is obtained by subjecting the steel to a soaking treatment at 500 ° C., cooling to a temperature range of 500 ° C. or lower and 300 ° C. or higher, and holding the temperature for 30 seconds or longer. ..

JP, 11-61326, A JP 2005-179703 A International Publication No. 2011/090179 JP, 2010-156032, A JP2012-251200 International Publication No. 2013/005714

Japan Iron and Steel Institute Basic Research Group, Bainite Research Group, Recent Research on Bainite Structure and Transformation Behavior of Low Carbon Steel: Final Report of Bainite Research Group, Japan Iron and Steel Institute, July 1994

  In general, a steel sheet containing a retained austenite in the metal structure shows a large elongation due to the effect of transformation-induced plasticity (TRIP) generated by the transformation of the retained austenite during processing, but due to the formation of hard martensite Spreadability is impaired. The steel sheet described in Patent Document 1 is said to have improved ductility and hole expandability due to the refinement of ferrite and retained austenite, but the hole expansion ratio is at most 1.5, and it is said that it has sufficient press formability. It's hard to say. Further, in order to increase the work hardening index and improve the collision resistance, it is necessary to make the main phase a soft ferrite phase, and it is difficult to obtain high tensile strength.

  Further, in the technique described in Patent Document 2, in order to refine the second phase to a nano size and disperse it in the crystal grains, a large amount of expensive elements such as Cu and Ni are contained, and high temperature and long time It is necessary to carry out solution treatment, resulting in significant increase in manufacturing cost and decrease in productivity.

  Further, according to Patent Document 3, the distribution of Mn in the steel is made uniform, and the ratio of the Mn concentration in the ferrite phase and the Mn concentration in the second phase is set to 0.70 or more, whereby the strain due to press working is suppressed. Even if it is not introduced, the absorbed energy up to a low strain region of about 5% is large, and it is possible to improve the collision resistance. However, it is necessary to make the main phase a soft ferrite phase, and it is difficult to obtain high tensile strength.

  Then, according to Patent Document 4, by setting the ratio of the Mn concentration in the ferrite and the Mn concentration in the martensite to be a certain value or more, the difference in hardness between the ferrite and the martensite becomes small, and at the interface between the ferrite and the martensite. It is said that by reducing the stress concentration, good stretch-flangeability can be obtained with a tensile strength of more than 1000 MPa. However, the strength-ductility balance (TS × EL) is less than 16000 MPa ·%, and it is difficult to apply it to members requiring ductility.

  By the way, since there are various processing modes for automobile parts, the required formability differs depending on the member to which it is applied, but ductility and stretch flangeability are positioned as important indicators of formability, It is desired to combine these at a high level. Further, in recent years, it has been desired to have higher strength than conventional ones.

  The techniques disclosed in Patent Document 5 and Patent Document 6 described above are excellent techniques that meet such requirements, but a facility for performing rapid cooling immediately after hot rolling is essential. Furthermore, there is a problem that it is difficult to control the temperature of the steel sheet because rapid cooling of several hundreds of degrees Celsius / second or more is continued up to a temperature near 700 ° C.

  The present invention has been made in order to solve the above problems, without high-speed cooling immediately after hot rolling, high strength, and a method for manufacturing a steel sheet having excellent ductility and stretch flangeability. The purpose is to provide.

  The present inventors have earnestly studied the relationship between the chemical composition of high-strength steel sheets and the steel structure and mechanical properties, and as a result, have obtained the following findings.

  (A) The steel structure is preferably hard to obtain high strength, and the steel structure is preferably homogeneous to obtain excellent stretch flangeability. Therefore, in order to have both high strength and excellent stretch flangeability, bainitic ferrite having a hard and homogeneous structure is suitable, and it is important to make a steel structure mainly composed of bainitic ferrite. .

  (B) Bainitic ferrite has a structure with poor ductility. For this reason, it is difficult to secure ductility if the steel structure is mainly composed of bainitic ferrite.

  (C) By including retained austenite in the steel structure, ductility can be improved by the TRIP effect. However, when the retained austenite is coarse, coarse voids are formed by martensite generated by the work-induced transformation, so that stretch flangeability is deteriorated. In order to maintain stretch flangeability and improve ductility, it is effective to contain fine retained austenite to make martensite generated by the work-induced transformation fine and suppress the formation of coarse voids. ..

  (D) Since fine retained austenite has high stability against deformation due to three-dimensional restraint from surrounding crystal grains, local deformation occurs in the high strength steel sheet before the TRIP effect is sufficiently exhibited, and ductility is improved. May not be achieved.

  (E) In order to improve the ductility of the high-strength steel sheet, it is effective to add soft polygonal ferrite to increase the work hardening index in the initial stage of deformation, but in addition to the content of polygonal ferrite, By controlling the average grain size of and the ratio of the Mn concentrations in the bainitic ferrite and the polygonal ferrite in an appropriate range, it is possible to improve the ductility while maintaining the stretch flangeability. The average grain size of the polygonal ferrite contributes to the strengthening of the polygonal ferrite by the grain boundary strengthening and the Mn concentration contributes to the strengthening of the polygonal ferrite by the solid solution strengthening. By controlling the hardness difference, it becomes possible to combine high strength and ductility and stretch flangeability at a high level.

(F) By subjecting the steel sheet to soaking for a predetermined time in the temperature range of (Ac ₃ points −50 ° C.) to (Ac ₃ points −2 ° C.), Mn is distributed between ferrite and austenite. By subsequently performing the second annealing, the area ratio, grain size, and Mn concentration of bainitic ferrite, polygonal ferrite, and retained austenite can be appropriately controlled. As a result, a steel sheet having high strength and excellent ductility and stretch flangeability can be obtained without performing rapid cooling immediately after hot rolling.

  The present invention was made on the basis of the above findings, and has as its gist the following method for producing a high-strength steel sheet.

(1) A method for producing a high-strength steel sheet in which a main phase is bainitic ferrite and a second phase has a metal structure containing polygonal ferrite and retained austenite,
A method for producing a high-strength steel sheet, which comprises the following steps (A) to (C).
(A)% by mass,
C: 0.04% or more and less than 0.50%,
Si: 0.10% or more and less than 3.0%,
Mn: 1.5-8.0%,
P: 0.10% or less,
S: 0.030% or less,
sol. Al: 0.01 to 2.0%,
N: 0.010% or less,
Ti: 0 to 0.20%,
Nb: 0 to 0.10%,
V: 0 to 0.50%,
Cr: 0% or more and less than 1.0%,
Mo: 0 to 0.50%,
Ni: 0 to 1.0%,
B: 0 to 0.0050%,
Ca: 0 to 0.020%,
Mg: 0 to 0.020%,
REM: 0 to 0.020%,
Cu: 0 to 1.0%,
Bi: 0 to 0.020%,
The balance: Fe and impurities,
For a slab having a chemical composition that satisfies the following formula (i),
A hot rolling process in which hot rolling is performed to complete rolling in a temperature range of Ar ₃ points to 1100 ° C. to obtain a hot rolled steel sheet, and the hot rolling steel sheet is wound in a temperature range of 650 ° C. or less.
0.5 ≦ Si + sol. Al ≦ 3.0 (i)
However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel sheet.
(B) Primary holding the hot-rolled steel sheet obtained in the step (A) in the temperature range of (Ac ₃ points −50 ° C.) to (Ac ₃ points −2 ° C.) for a time satisfying the following expression (ii). Annealing process.
1.4 × 10 ⁻⁸ × exp {26500 / (T + 273)} ≦ t ≦ 4.0 × 10 ⁵ (ii)
However, the meaning of each symbol in the above formula (ii) is as follows.
t: Primary annealing holding time (seconds)
T: Primary annealing temperature (° C)
(C) After holding the annealed steel sheet obtained in the step (B) in a temperature range of (Ac ₃ points-40 ° C) or higher and lower than (Ac ₃ points + 100 ° C), it is cooled to a temperature range of 500 ° C to Ms point. Then, the secondary annealing step of maintaining the temperature range for 30 seconds or more.

  (2) The method for producing a high-strength steel sheet according to (1), wherein after the step (B), the annealed steel sheet is cold-rolled with a total reduction of 30% or more and less than 80%.

  (3) The high-strength steel sheet according to (1) or (2), wherein after the step (A), the hot-rolled steel sheet is subjected to cold rolling with a total reduction of 30% or more and less than 80%. Manufacturing method.

(4) The chemical composition is mass%,
Ti: 0.005 to 0.20%,
Nb: 0.002-0.10%, and V: 0.005-0.50%,
The method for producing a high-strength steel sheet according to any one of (1) to (3) above, which comprises one or more selected from

(5) The chemical composition is mass%,
Cr: 0.05% or more and less than 1.0%,
Mo: 0.02-0.50%,
Ni: 0.05 to 1.0%, and B: 0.0002 to 0.0050%,
The method for producing a high-strength steel sheet according to any one of (1) to (4) above, which comprises one or more selected from

(6) The chemical composition is mass%,
Ca: 0.0005 to 0.020%,
Mg: 0.0005 to 0.020%, and REM: 0.0005 to 0.020%,
The method for producing a high-strength steel sheet according to any one of (1) to (5) above, which comprises one or more selected from

(7) The chemical composition is% by mass,
Cu: 0.05-1.0%
The method for producing a high-strength steel sheet according to any one of (1) to (6) above, which comprises:

(8) The chemical composition is mass%,
Bi: 0.0005 to 0.020%
The method for producing a high-strength steel sheet according to any one of (1) to (7) above, which comprises:

  According to the present invention, it is possible to obtain a steel sheet having high strength and excellent ductility and stretch flangeability. Therefore, the high-strength steel sheet produced by the method according to the present invention is suitable for use as a material for automobile members, mechanical structural members, building members, and the like.

  The manufacturing method of the high strength steel plate according to the present invention includes the following steps (A) to (C). Hereinafter, each requirement of the present invention will be described in detail.

(A) Hot Rolling Step In the step (A), a slab having a predetermined chemical composition is subjected to hot rolling to complete rolling in a temperature range of Ar ₃ points to 1100 ° C. to obtain a hot rolled steel sheet, and 650 Wind in the temperature range below ℃. Each configuration will be described in more detail.

(A-1) Slab chemical composition The reasons for limiting each element are as follows. In addition, in the following description, "%" regarding the content means "mass%".

C: 0.04% or more and less than 0.50% C has the function of increasing the strength of the steel sheet by solid solution strengthening and the function of stabilizing retained austenite. When the C content is less than 0.04%, it becomes difficult to secure the desired steel plate strength and the retained austenite area ratio. On the other hand, when the C content is 0.50% or more, pearlite is preferentially generated, and it becomes difficult to obtain a target retained austenite area ratio. Therefore, the C content is 0.04% or more and less than 0.50%. The C content is preferably 0.06% or more, more preferably 0.10% or more. Further, the C content is preferably 0.40% or less.

Si: 0.10% or more and less than 3.0% Si has the function of increasing the strength of the steel sheet by solid solution strengthening and the function of soundening the steel by deoxidation. Further, the action of delaying the precipitation of cementite and increasing the area ratio of retained austenite contributes to the improvement of ductility. If the Si content is less than 0.10%, it is difficult to obtain the effect of the above action. On the other hand, when the Si content is 3.0% or more, the surface properties and chemical conversion treatability of the steel sheet, and the ductility and weldability deteriorate significantly. Further, the A ₃ transformation point is remarkably increased, which makes stable hot rolling difficult. Therefore, the Si content is set to 0.10% or more and less than 3.0%.

  As will be described later, in the present invention, Si and sol. Although the total content of Al (Si + sol.Al) is important, Si is sol. It has a higher solid solution strengthening ability than Al. Therefore, when higher strength is required, the Si content is preferably 0.50% or more, more preferably 0.80% or more, and further preferably 1.0% or more. Further, the Si content is preferably 2.5% or less.

Mn: 1.5-8.0%
Mn has the effect of enhancing the hardenability of steel and promoting the formation of bainitic ferrite. If the Mn content is less than 1.5%, it is difficult to secure the desired amount of bainitic ferrite. On the other hand, when the Mn content exceeds 8.0%, ferrite transformation is excessively suppressed, and it becomes difficult to secure the target amount of polygonal ferrite. Therefore, the Mn content is set to 1.5 to 8.0%. The Mn content is preferably 2.0% or more, more preferably 2.3% or more. Further, the Mn content is preferably 6.0% or less.

P: 0.10% or less Although P is an element generally contained as an impurity, it is also an element having an action of enhancing strength by solid solution strengthening. Therefore, P may be positively contained. However, P is an element that easily segregates, and if its content exceeds 0.10%, the formability and toughness significantly decrease due to the segregation of grain boundaries. Therefore, the P content is 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less, and further preferably 0.020% or less. The lower limit of the P content does not have to be specified in particular, but it is preferably 0.001% or more from the viewpoint of refining cost.

S: 0.030% or less S is an element contained as an impurity and forms a sulfide-based inclusion in the steel to reduce the formability of the steel sheet. When the S content exceeds 0.030%, the moldability is significantly reduced. Therefore, the S content is 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.005% or less, and further preferably 0.001% or less. The lower limit of the S content need not be specified in particular, but it is preferably 0.0001% or more from the viewpoint of suppressing an increase in refining cost.

sol. Al: 0.01 to 2.0%
Similar to Si, Al has a function of deoxidizing steel and soundening the steel sheet. Further, the action of delaying the precipitation of cementite and increasing the area ratio of retained austenite contributes to the improvement of ductility. sol. If the Al content is less than 0.01%, it is difficult to obtain the effect due to the above action. On the other hand, sol. When the Al content exceeds 2.0%, the A ₃ transformation point is remarkably increased, which makes stable hot rolling difficult. Therefore, sol. The Al content is 0.01 to 2.0%. sol. The Al content is preferably 0.03% or more. In addition, sol. The Al content is preferably 1.5% or less, more preferably 1.0% or less.

N: 0.010% or less N is an element contained as an impurity and has an effect of reducing the formability of the steel sheet. When the N content is more than 0.010%, the moldability is significantly reduced. Therefore, the N content is 0.010% or less. The N content is preferably 0.0080% or less, and more preferably 0.0070% or less. The lower limit of the N content does not have to be specified in particular, but in consideration of the case where one or more kinds of Ti, Nb, and V are contained to refine the steel structure as described later, precipitation of carbonitrides is promoted. Therefore, the N content is preferably 0.0010% or more, more preferably 0.0020% or more.

  In the steel sheet of the present invention, in addition to the above elements, one or more selected from the following amounts of Ti, Nb, V, Cr, Mo, Ni, B, Ca, Mg, REM, Cu and Bi. May be included.

Ti: 0 to 0.20%
Nb: 0 to 0.10%
V: 0 to 0.50%
Ti, Nb and V precipitate in the steel as carbides or nitrides, and have the effect of refining the steel structure by the pinning effect. Therefore, one or more selected from these elements may be contained. However, even if it is contained excessively, the effect due to the above-mentioned action is saturated and it becomes uneconomical. Therefore, the Ti content is 0.20% or less, the Nb content is 0.10% or less, and the V content is 0.50% or less. In order to more reliably obtain the effects of these elements, it is preferable to satisfy at least one of Ti: 0.005% or more, Nb: 0.002% or more, and V: 0.005% or more. ..

Cr: 0% or more and less than 1.0% Mo: 0 to 0.50%
Ni: 0 to 1.0%
B: 0 to 0.0050%
Cr, Mo, Ni and B have a function of enhancing hardenability. Further, Mo has the function of precipitating carbides in the steel to increase the strength. Further, Ni has an effect of effectively suppressing grain boundary cracking of the slab due to Cu when Cu is contained as described later. Therefore, one or more selected from these elements may be contained.

  However, when the Cr content is 1.0% or more, the chemical conversion treatability is significantly deteriorated. Therefore, the Cr content is less than 1.0%. In order to obtain the effect of the above action more reliably, the Cr content is preferably 0.05% or more. Further, when the Mo content exceeds 0.50%, the effect due to the above-mentioned action is saturated, which is disadvantageous in cost. Therefore, the Mo content is 0.50% or less. It is preferably 0.20% or less. The Mo content is preferably 0.02% or more in order to more reliably obtain the effects of the above action.

  Since Ni is an expensive element, the inclusion of a large amount is disadvantageous in cost. Therefore, the Ni content is 1.0% or less. In order to obtain the effect of the above action more reliably, the Ni content is preferably 0.05% or more. Further, when the B content exceeds 0.0050%, the moldability is remarkably deteriorated. Therefore, the B content is 0.0050% or less. In order to obtain the effect of the above action more reliably, the B content is preferably 0.0002% or more.

Ca: 0 to 0.020%
Mg: 0 to 0.020%
REM: 0 to 0.020%
Ca, Mg and REM have the effect of enhancing the formability by adjusting the shape of the inclusions. Therefore, one or more selected from these elements may be contained. However, if the contents of these elements exceed the above upper limits, the inclusions in the steel become excessive, which may rather reduce the formability. Therefore, the Ca content is 0.020% or less, the Mg content is 0.020% or less, and the REM content is 0.020% or less. The content of each element is preferably 0.010% or less, more preferably 0.005% or less. In order to obtain the effect of the above action more reliably, it is preferable to contain at least one of the above elements in an amount of 0.0005% or more.

  Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements. In the case of lanthanoid, it is industrially added in the form of misch metal.

Cu: 0 to 1.0%
Cu has the action of precipitating at a low temperature and increasing the strength, so it may be contained in the steel. However, if the Cu content exceeds 1.0%, intergranular cracking of the slab may occur. Therefore, the Cu content is 1.0% or less. The Cu content is preferably less than 0.5%, more preferably less than 0.3%. The Cu content is preferably 0.05% or more in order to more reliably obtain the effect of the above action.

Bi: 0 to 0.020%
Bi has a function of enhancing the formability by refining the solidified structure, and thus may be contained in steel. However, if the Bi content exceeds 0.020%, the effect due to the above-mentioned action is saturated, which is disadvantageous in terms of cost. Therefore, the Bi content is 0.020% or less. The Bi content is preferably 0.010% or less. In order to obtain the effect of the above action more reliably, the Bi content is preferably 0.0005% or more.

0.5 ≦ Si + sol. Al ≦ 3.0 (i)
However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel sheet.
As described above, Si and Al both have the effect of suppressing the precipitation of cementite and increasing the area ratio of retained austenite, and improve the ductility. Therefore, in the present invention, Si and sol. The total content with Al (Si + sol.Al) is specified.

Si + sol. When the value of Al is less than 0.5, the desired retained austenite area ratio cannot be obtained because the above action is insufficient, and ductility deteriorates. On the other hand, Si + sol. When the value of Al exceeds 3.0, the A ₃ transformation point is remarkably increased, and stable hot rolling becomes difficult. Therefore, it is necessary to satisfy the above equation (i). Si + sol. The value of Al is preferably 1.0 or more, more preferably 1.2 or more. In addition, Si + sol. The Al value is preferably 2.5 or less, and more preferably 2.2 or less.

  In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities.

  The "impurity" is a component that is mixed by ores, raw materials such as scrap, and various factors in the manufacturing process when industrially manufacturing steel, and is allowed within a range that does not adversely affect the present invention. Means something.

(A-2) Hot rolling conditions Slab heating temperature: 1350 ° C or lower The temperature of the slab to be subjected to hot rolling is preferably 1350 ° C or lower, more preferably 1280 ° C or lower, from the viewpoint of suppressing scale loss. preferable. The lower limit of the temperature of the slab subjected to hot rolling does not have to be particularly limited, and may be a temperature at which hot rolling can be completed at Ar ₃ points or more as described later.

Rolling completion temperature: Ar ₃ points to 1100 ° C
The hot rolling is completed in a temperature range of Ar ₃ or higher in order to transform the austenite after the completion of rolling to refine the metal structure of the hot rolled steel sheet. When the rolling completion temperature is lower than Ar ₃ point, ferrite transformation occurs during hot rolling, and a coarse low-temperature transformation phase expanded in the rolling direction is generated in the metallographic structure of the hot-rolled steel sheet. As a result, the metal structure after annealing becomes coarse, and the ductility and stretch flangeability tend to deteriorate. Therefore, the completion temperature of hot rolling needs to be Ar ₃ point or higher.

  On the other hand, when the rolling completion temperature exceeds 1100 ° C., the metallographic structure of the hot-rolled steel sheet becomes coarse, and the metallographic structure after annealing becomes coarse, and ductility and stretch-flangeability easily deteriorate. Therefore, the completion temperature of hot rolling needs to be 1100 ° C or lower. The completion temperature is preferably 1050 ° C. or lower.

  When the hot rolling includes rough rolling and finish rolling, the rough rolled material may be heated between the rough rolling and the finish rolling in order to complete the finish rolling at the above temperature. At this time, it is desirable to suppress the temperature fluctuation over the entire length of the rough rolled material at 140 ° C. or lower at the start of finish rolling by heating so that the rear end of the rough rolled material becomes higher in temperature than the front end. This improves the uniformity of product properties within the coil.

  The heating method of the rough rolled material may be performed by using a known means. For example, a solenoid type induction heating device is provided between the rough rolling mill and the finish rolling mill, and the heating temperature rise amount is controlled based on the temperature distribution in the longitudinal direction of the rough rolling material on the upstream side of the induction heating device. May be.

  In the hot rolling, it is preferable to use a revers mill or a tandem mill as the multi-pass rolling. Particularly, from the viewpoint of industrial productivity, it is more preferable to perform rolling using a tandem mill at least in the final several stages.

Primary cooling after completion of rolling: Cooling is started within 10 seconds after completion of rolling, and cooling is performed at a cooling rate of 5 ° C./second or more. After completion of rolling, when the time until cooling starts exceeds 10 seconds, or cooling rate Is less than 5 ° C., the metallographic structure of the hot-rolled steel sheet becomes coarse, and the metallographic structure after annealing becomes coarse, and ductility and stretch-flangeability are likely to deteriorate. Therefore, the primary cooling after the completion of rolling is preferably started within 10 seconds and cooled at a cooling rate of 5 ° C./second or more.

Winding temperature: 650 ° C. or less Annealing (primary annealing) at a two-phase region temperature of ferrite and austenite after hot rolling promotes distribution of Mn between the ferrite and austenite, and then performs annealing (second annealing). The subsequent annealing) makes it possible to control the relationship between the area ratio of polygonal ferrite and the Mn concentration in the bainitic ferrite and the Mn concentration in the polygonal ferrite within a desired range. To obtain the effect, the coiling temperature after hot rolling needs to be 650 ° C. or lower. When the coiling temperature exceeds 650 ° C., pearlite is easily generated, Mn distribution proceeds between ferrite and pearlite, and a structure suitable for ductility and stretch flangeability cannot be obtained. The winding temperature is preferably lower than 400 ° C, more preferably lower than 300 ° C.

(B) Primary annealing step The hot rolled steel sheet obtained in the step (A) is annealed at a two-phase region temperature of ferrite and austenite. This annealing is called "primary annealing" in the present invention. By promoting the distribution of Mn between ferrite and austenite by the primary annealing, it becomes easy to obtain a metal structure suitable for ductility and stretch flangeability. The primary annealing conditions will be described in detail below.

Primary annealing temperature: (Ac ₃ points-50 ° C) to (Ac ₃ points-2 ° C)
The primary annealing temperature is (Ac ₃ points −50 ° C.) to (Ac ₃ points −2 ° C.). By annealing at a temperature in this range, it is possible to control the area ratio and average particle size of polygonal ferrite, and the relationship between the Mn concentration in bainitic ferrite and the Mn concentration in polygonal ferrite to a desired range. Becomes If the primary annealing temperature is less than (Ac ₃ points −50 ° C.), coarse polygonal ferrite is likely to be generated, and stretch flangeability is deteriorated. On the other hand, when the primary annealing temperature exceeds (Ac ₃ points-2 ° C), it becomes difficult to secure a desired amount of polygonal ferrite, and ductility deteriorates.

Primary annealing holding time: Satisfies the following expression (ii) The primary annealing holding time needs to satisfy the following expression (ii) in relation to the above primary annealing temperature.
1.4 × 10 ⁻⁸ × exp {26500 / (T + 273)} ≦ t ≦ 4.0 × 10 ⁵ (ii)
However, the meaning of each symbol in the above formula (ii) is as follows.
t: Primary annealing holding time (seconds)
T: Primary annealing temperature (° C)

Since the diffusion rate of Mn is very slow, it is maintained at the above-mentioned primary annealing temperature for a predetermined time to promote the distribution of Mn from ferrite to austenite. By subjecting this annealed steel sheet to secondary annealing as described below, it becomes possible to control the relationship between the Mn concentration in the bainitic ferrite and the Mn concentration in the polygonal ferrite within a desired range. When the retention time is less than the left side of the equation (ii) [1.4 × 10 ⁻⁸ × exp {6500 / (T + 273)}], the distribution of Mn is insufficient, so the Mn concentration in the bainitic ferrite and the polygonal The ratio to the Mn concentration in ferrite does not reach a desired value, and ductility deteriorates. On the other hand, when it is held for a long time, the ratio of Mn concentration approaches an equilibrium state, so even if annealing is performed for more than 4.0 × 10 ⁵ seconds, the heat treatment cost only increases.

(C) Secondary Annealing Step The annealed steel sheet obtained in the step (B) is annealed after being subjected to treatments such as descaling and degreasing according to known methods, if necessary. This annealing is called "secondary annealing" in the present invention. By performing secondary annealing, bainitic ferrite and retained austenite are generated. Thereby, it becomes easy to obtain a metal structure suitable for ductility and stretch flangeability. The secondary annealing conditions will be described in detail below.

Secondary annealing temperature: (Ac ₃ points −40 ° C.) or higher and less than (Ac ₃ points + 100 ° C.) Secondary annealing temperature must be (Ac ₃ points −40 ° C.) or higher. This is because a main phase is bainitic ferrite and a metal structure containing retained austenite is obtained in the second phase. In order to increase the area ratio of the bainitic ferrite and improve the stretch flangeability, the secondary annealing temperature is preferably a temperature higher than (Ac ₃ point −20 ° C.), and a temperature higher than Ac ₃ point. More preferably. However, if the secondary annealing temperature becomes too high, the austenite becomes excessively coarse, and the Mn distribution promoted by the primary annealing decreases due to diffusion, and the ductility and stretch flangeability deteriorate. Therefore, the secondary annealing temperature needs to be lower than (Ac ₃ point + 100 ° C.). The secondary annealing temperature is preferably lower than (Ac ₃ point + 50 ° C.), and more preferably lower than (Ac ₃ point + 20 ° C.).

Secondary annealing retention time: less than 150 seconds The lower limit of the retention time at the secondary annealing temperature does not need to be particularly limited, but in order to obtain stable mechanical properties, it is preferably a time of more than 15 seconds, 60 seconds. It is more preferable that the time exceeds. On the other hand, if the holding time is too long, the Mn distributed in the primary annealing is diffused, and the ductility and stretch-flangeability are likely to deteriorate. Therefore, the holding time is preferably less than 150 seconds, more preferably less than 120 seconds.

  In the heating process in the secondary annealing, in order to suppress the diffusion of Mn distributed in the primary annealing, the average heating rate is preferably 5 ° C./sec or more, more preferably 20 ° C./sec or more, and 100 ° C. / Sec or more is more preferable.

  In the cooling process after soaking of the secondary annealing, in order to promote the formation of fine polygonal ferrite and improve the ductility, cooling from the secondary annealing temperature to 50 ° C or more is performed at a cooling rate of less than 10 ° C / sec. It is preferable to carry out. The cooling rate after soaking is preferably less than 5 ° C./second, more preferably less than 3 ° C./second, and even more preferably less than 2 ° C./second. Since Mn is distributed in the steel sheet in the primary annealing, polygonal ferrite is preferentially generated from a site having a low Mn concentration. Thereby, the relationship between the area ratio of polygonal ferrite suitable for ductility and stretch-flangeability and the Mn concentration in bainitic ferrite and the Mn concentration in polygonal ferrite can be easily obtained.

  Further, in order to obtain a metal structure having bainitic ferrite as a main phase, it is preferable that the average cooling rate in the temperature range of 650 to 500 ° C. is 15 ° C./sec or more. It is more preferable that the average cooling rate in the temperature range of 650 to 450 ° C. is 15 ° C./sec or more. Since the area ratio of bainitic ferrite increases as the cooling rate becomes faster, the average cooling rate is more preferably more than 30 ° C./sec, and even more preferably more than 50 ° C./sec. On the other hand, if the cooling rate is too fast, the shape of the steel sheet is impaired, so the cooling rate in the temperature range of 650 to 500 ° C. is preferably 200 ° C./sec or less, and more preferably less than 150 ° C./sec. Preferably, it is less than 130 ° C./second, more preferably less than 130 ° C./second.

  Further, in order to obtain the retained austenite, it is necessary to cool to a temperature range of 500 ° C. to Ms point and hold the temperature range for 30 seconds or more. In order to improve the stability of the retained austenite and improve the ductility and stretch flangeability, the holding temperature range is preferably 475 ° C to (Ms point + 20 ° C), and 450 ° C to (Ms point + 40 ° C). ) Is more preferable, and it is further preferable that it is 430 ° C. to (Ms point + 60 ° C.). Further, since the stability of the retained austenite increases as the holding time is increased, the holding time is preferably 60 seconds or longer, more preferably 120 seconds or longer, and more than 300 seconds. More preferable.

The Ms point is calculated from the element content using the following formula (iii).
Ms point (° C.) = 561-407 × C-7.3 × Si-37.8 × Mn-20.5 × Cu-19.5 × Ni-19.8 × Cr-4.5 × Mo ... (Iii)
However, each element symbol in the above formula (iii) represents the content (mass%) of each element contained in the steel sheet.

(D) Other Steps (D-1) Casting Step There is no particular limitation on the method for producing the slab used as the material for the hot rolling in the step (A). The steel having the above-mentioned chemical composition is melted by a known means and then made into a steel ingot by a continuous casting method, or a steel obtained by a slab rolling after making a steel ingot by any casting method. To be cut off. In the continuous casting step, in order to suppress the occurrence of surface defects caused by inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold.

  The steel ingot or billet may be subjected to hot rolling after being reheated once cooled, and the ingot in the high temperature state after continuous casting or the billet in the high temperature state after slabbing as it is Alternatively, it may be subjected to hot rolling by keeping it warm or by performing auxiliary heating. In addition, in this invention, such a steel ingot and a steel piece are generically called a slab.

(D-2) Cold rolling step Cold rolling ratio: 30% or more and less than 80% Cold annealing according to a conventional method for the annealed steel sheet after the step (B) and / or the hot rolled steel sheet after the step (A). It may be rolled. Before cold rolling, the annealed steel sheet and / or hot rolled steel sheet may be descaled by pickling or the like. In cold rolling, in order to promote recrystallization, uniformize the metallographic structure after cold rolling and annealing, and further improve stretch flangeability, the cold rolling ratio (total rolling reduction in cold rolling) is 30% or more. It is preferable that More preferably, the cold pressing rate is 40% or more. This further refines the grain structure of the annealed metal structure, improves the texture, and further improves ductility and stretch flangeability. From this point of view, the cold pressing rate is more preferably 50% or more, and particularly preferably 60% or more. On the other hand, if the cold pressing rate is too high, the rolling load increases and rolling becomes difficult. Therefore, the cold pressing rate is preferably less than 80%, more preferably less than 70%.

(D-3) Plating Step When an electroplated steel sheet is produced, the annealed steel sheet produced by the above-mentioned method is subjected to a known pretreatment for cleaning and adjusting the surface, if necessary, It suffices to carry out electroplating according to a conventional method, and the chemical composition and the amount of adhesion of the plating film are not limited. Examples of the type of electroplating include electrogalvanizing and electroplating Zn-Ni alloy.

  In the case of producing a hot dip plated steel sheet, in the cooling process of the step (C), the steel sheet is held in a temperature range of 500 ° C. to Ms for 30 seconds or more, and then the steel sheet is heated if necessary, and then a plating bath is used. Immerse and apply hot dip. In order to increase the stability of retained austenite and improve the ductility and stretch flangeability, the holding temperature range is preferably 475 ° C to (Ms point + 20 ° C), and 450 ° C to (Ms point + 40 ° C). More preferably, it is more preferably 430 ° C. to (Ms point + 60 ° C.).

  In addition, the stability of the retained austenite increases as the holding time in the temperature range of 500 ° C. to Ms increases, so that the holding time is preferably 60 seconds or longer, more preferably 120 seconds or longer, and 300 More preferably, the time exceeds seconds. The alloying treatment may be performed by reheating after hot dipping. The chemical composition and the adhesion amount of the plating film are not limited. Examples of the type of hot-dip galvanizing, hot-dip galvanizing, hot-dip aluminum coating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plating, etc. To be done.

  The plated steel sheet may be subjected to an appropriate chemical conversion treatment after plating in order to further improve its corrosion resistance. The chemical conversion treatment is preferably carried out by using a non-chromium type chemical conversion treatment liquid (eg, silicate type, phosphate type, etc.) instead of the conventional chromate treatment.

  The annealed steel plate and the plated steel plate thus obtained may be subjected to temper rolling according to a conventional method. However, if the elongation percentage of the temper rolling is high, the ductility deteriorates. Therefore, the elongation percentage of the temper rolling is preferably 1.0% or less, more preferably 0.5% or less.

  The high-strength steel sheet produced by the method according to the present invention has a bainitic ferrite as a main phase, and a metal structure containing polygonal ferrite and retained austenite in the second phase. The metal structure of the steel sheet according to the present invention will be described below.

Area ratio of bainitic ferrite: 60.0-95.0%
Bainitic ferrite has a hard and homogeneous structure, and is a structure suitable for having both high strength and excellent stretch flangeability. Therefore, in the steel sheet according to the present invention, the main phase needs to be bainitic ferrite. In the present invention, bainitic ferrite being the main phase means that the area ratio of bainitic ferrite is 60.0% or more.

  The area ratio of the bainitic ferrite is not particularly limited, but if it is less than 60.0%, it becomes difficult to obtain desired strength and stretch flangeability. On the other hand, if the area ratio of bainitic ferrite exceeds 95.0%, the ductility may be significantly reduced. Therefore, the area ratio of bainitic ferrite is preferably 60.0 to 95.0%. The area ratio of bainitic ferrite is more preferably 70.0% or more.

  Bainitic ferrite is distinguished from polygonal ferrite in that it exhibits a lath-like or plate-like morphology and high dislocation density, and is distinguished from bainite in the absence of iron carbide inside and at the interface. Here, bainitic ferrite is, as described in Non-Patent Document 1, an intermediate between a microstructure containing polygonal ferrite or pearlite formed by a diffusive mechanism and martensite formed by a non-diffusive shearing mechanism. A transformation structure in a stage, which is mainly a lath- or plate-shaped bainitic ferrite and a granular bainitic as described in Non-Patent Document 1 pages 125 to 127 as an optical microscope observation structure. It is composed of ferrite and pseudopolygonal ferrite. In the present invention, all of these are collectively referred to as bainitic ferrite.

  Further, in the steel sheet according to the present invention, the second phase contains polygonal ferrite and retained austenite. The area ratio and particle size of polygonal ferrite and retained austenite are not particularly limited, but are preferably in the ranges shown below.

Area ratio of polygonal ferrite: 2.0 to 25.0%
The area ratio of the polygonal ferrite is preferably 2.0 to 25.0% in order to improve the ductility by containing the soft polygonal ferrite. If the area ratio of polygonal ferrite is less than 2.0%, it is difficult to obtain the effect of improving ductility. On the other hand, if it exceeds 25.0%, not only the stretch flangeability is deteriorated but also it becomes difficult to secure a desired strength. The area ratio of polygonal ferrite is more preferably 5.0% or more, further preferably 6.0% or more. The area ratio is more preferably 20.0% or less, further preferably 15.0% or less.

Area ratio of retained austenite: 3.0 to 35.0%
Retained austenite has a function of enhancing ductility by transformation-induced plasticity (TRIP). If the retained austenite area ratio is less than 3.0%, it is difficult to obtain the effect of the above action. On the other hand, if the area ratio of the retained austenite exceeds 35.0%, the hard martensite generated by the work-induced transformation may deteriorate the stretch flangeability. Therefore, the retained austenite area ratio is preferably 3.0 to 35.0%. The retained austenite area ratio is more preferably 4.0% or more, and further preferably 6.0% or more.

  Note that there are methods such as X-ray diffraction, electron beam backscattering diffraction (EBSP) analysis, and magnetic measurement as quantitative methods for residual austenite, and quantitative values may differ depending on the methods. The area ratio of retained austenite specified in the present invention is a value measured by X-ray diffraction.

  The steel sheet according to the present invention may contain cementite, pearlite, martensite, and the like as the balance in addition to the above-described structure. If the total area ratio of the remaining structure excluding bainitic ferrite, polygonal ferrite, and retained austenite exceeds 15.0%, stretch flangeability may deteriorate. From the viewpoint of moldability, the area ratio of the balance is preferably 15.0% or less. The area ratio of the remaining portion is more preferably 10.0% or less, further preferably 6.0% or less.

Average particle size of polygonal ferrite: 0.3 to 10.0 μm
If the average particle size of polygonal ferrite is less than 0.3 μm, the ductility may deteriorate, so it is preferable to set it to 0.3 μm or more. On the other hand, if the average particle size exceeds 10.0 μm, coarse voids may be generated at the interface between polygonal ferrite and bainitic ferrite, and stretch flangeability may deteriorate. From the viewpoint of reducing the hardness difference from bainitic ferrite and improving stretch-flangeability by grain refinement, the average grain size of polygonal ferrite is preferably 10.0 μm or less.

  The average particle size of polygonal ferrite is more preferably 0.8 μm or more, further preferably 1.2 μm or more. The average particle size is more preferably 7.0 μm or less, further preferably 5.0 μm or less. The average particle size of polygonal ferrite in the present invention is the equivalent circle diameter determined by image analysis using a scanning electron microscope (SEM) observation image and EBSP analysis results.

  The average aspect ratio of polygonal ferrite is preferably 3.0 or less, more preferably 2.5 or less from the viewpoint of reducing anisotropy and improving stretch flangeability. The average aspect ratio of polygonal ferrite is a value represented by (major axis length) / (minor axis length) obtained by elliptic approximation from image analysis using SEM observation images and EBSP analysis results. Is.

Average particle size of retained austenite: 1.0 μm or less If the average particle size of retained austenite exceeds 1.0 μm, stretch flangeability may be deteriorated due to martensite generated by the work-induced transformation. Therefore, the average particle size of the retained austenite is preferably 1.0 μm or less. The average particle size of the retained austenite is more preferably 0.8 μm or less, further preferably 0.6 μm or less.

0.25 <[Mn] _PF / [Mn] _BF <0.70 (iv)
However, the meaning of each symbol in the above formula (iv) is as follows.
[Mn] _PF : average Mn concentration (mass%) in polygonal ferrite
[Mn] _BF : Average Mn concentration (mass%) in bainitic ferrite
In order to control the difference in hardness between bainitic ferrite and polygonal ferrite so as to combine ductility and stretch-flangeability at a high level, the Mn concentration in the bainitic ferrite and the Mn concentration in the polygonal ferrite are as described above ( It is preferable to satisfy the formula iv).

  When the average Mn concentration in the polygonal ferrite is 0.70 times or more the average Mn concentration in the bainitic ferrite, the solid solution strengthening amount of the polygonal ferrite due to Mn is large, and it is difficult to obtain the desired ductility. On the other hand, when the average Mn concentration in the polygonal ferrite is 0.25 times or less of the average Mn concentration in the bainitic ferrite, the hardness difference between the bainitic ferrite and the polygonal ferrite is large, so that it is coarse during deformation. Voids are likely to occur, and stretch flangeability may be reduced.

  The metal structure is identified and the area ratio is calculated by the following method. First, a vertical cross section in the rolling direction of a steel sheet is mirror-polished, then subjected to Nital corrosion, and the metal structure is identified by structure observation using an optical microscope or SEM. Next, the total area ratio of bainitic ferrite and martensite and the area ratio of polygonal ferrite at a position of 1/4 depth of the plate thickness from the steel plate surface are calculated from EBSP analysis, respectively. Further, the retained austenite area ratio is obtained by X-ray diffraction measurement using a sample which is chamfered from the rolling surface normal direction to a depth of 1/4 of the plate thickness.

  Furthermore, using a repeller-corroded sample, an area of 200 μm in the rolling direction × 50 μm in the rolling surface normal direction was photographed with an optical microscope for a vertical section in the rolling direction at a depth of 1/4 of the thickness from the surface of the steel sheet, and commercially available image processing software The total area ratio of retained austenite and martensite is calculated by the binarization process using "Image-Pro". Value obtained by subtracting the total area ratio of retained austenite and martensite obtained by repeller corrosion from the sum of the area ratio of bainitic ferrite and martensite calculated by EBSP analysis and the residual area ratio of retained austenite obtained by X-ray diffraction measurement. Is the area ratio of bainitic ferrite.

  The value obtained by subtracting the total area ratio of the bainitic ferrite, polygonal ferrite, and retained austenite obtained above from 100% is taken as the area ratio of the residual structure. The average grain size of polygonal ferrite and retained austenite is calculated from EBSP analysis and transmission electron microscope (TEM) observation of a thin film test piece taken from the surface of the steel sheet at a depth of 1/4 of the sheet thickness.

  The Mn concentrations in polygonal ferrite and bainitic ferrite are determined by the following method using an electron beam microanalyzer (FE-EPMA) equipped with a field emission electron gun. First, a mapping analysis of the Mn concentration is performed by FE-EPMA at an interval of 0.1 μm for a 50 μm × 50 μm region at a depth position of 1/4 of the plate thickness from the steel plate surface. Next, the metal structure in the same field of view is identified from the EBSP analysis by the method described above. With respect to the identified bainitic ferrite and polygonal ferrite, an average value of Mn concentrations at arbitrary 10 points is calculated to obtain Mn concentrations of the bainitic ferrite and the polygonal ferrite.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to these Examples.

  A 180 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and hot forged into a steel piece having a thickness of 30 mm. This steel slab was hot-rolled in a test small tandem mill under the conditions shown in Table 2 to obtain a hot-rolled steel plate having a plate thickness of 2 to 4 mm. This hot rolled steel sheet was heat-treated at various primary annealing temperatures shown in Table 2 for a predetermined time and then cooled to room temperature.

  Then, using the continuous annealing simulator, the obtained primary annealed steel sheets were heated to various secondary annealing temperatures shown in Table 2 and held for a predetermined time. Then, it cooled under various conditions shown in Table 2, hold | maintained at the cooling stop temperature shown in Table 2 for 330 seconds, Then, it cooled to room temperature and obtained the annealed steel plate.

  Note that some of the above hot-rolled steel sheets and / or primary annealed steel sheets were subjected to pickling and cold rolling and then annealed.

  The obtained annealed steel sheet was mirror-polished at a cross section perpendicular to the rolling direction of the steel sheet, corroded with a nital corrosive liquid or a repeller corrosive liquid, and then microstructure-observed using an optical microscope or SEM. Further, the crystal orientation was measured and analyzed by the EBSP method using a sample prepared by electrolytic polishing after mirror polishing. Further, a thin piece for TEM observation was sampled from the surface of the steel plate at a depth of 1/4 of the plate thickness, and the vicinity of the hole opened by the twin jet electrolysis method was observed by TEM. Further, the structures in the same visual field were analyzed by the FE-EPMA and EBSP methods, and the Mn concentrations in the bainitic ferrite and the polygonal ferrite were determined.

  Tensile properties and stretch flangeability were evaluated as mechanical properties. As for the tensile properties, a tensile test was carried out according to JIS Z 2241 (2011), and the tensile strength (TS) and the total elongation (El) were measured. For stretch flangeability, a hole expansion test was carried out in accordance with JIS Z 2256 (2010), and a hole expansion ratio (λ) was obtained.

  Table 3 shows the metallographic structure and mechanical properties of the obtained steel sheets. In the present invention, a steel sheet having a tensile strength of 780 MPa or more, a TS × EL value of 18000 MPa ·% or more, and a TS × λ value of 50000 MPa ·% or more has high tensile strength and strength. -Ductility balance and strength-Strong flangeability balance is judged to be excellent.

  With reference to Tables 1 to 3, Test Nos. 1 to 20, which are examples of the present invention, have high tensile strength (TS), and have an excellent strength-ductility balance (TS × El) and an excellent strength-stretch flange balance. (TS × λ). On the other hand, in test numbers 21 to 31 which are comparative examples manufactured by a method that does not satisfy the requirements of the present invention, the characteristics of TS × El or TS × λ or both are inferior.

  According to the present invention, it is possible to obtain a steel sheet having high strength and excellent ductility and stretch flangeability. Therefore, the high-strength steel sheet produced by the method according to the present invention is suitable for use as a material for automobile members, mechanical structural members, building members, and the like.

Claims (8)

The metal structure is the area%
Bainitic ferrite: 60.0-95.0%,
Polygonal ferrite: 2.0-25.0%,
Retained austenite: 3.0 to 35.0%,
Remainder: 15.0% or less, and
Average particle size of polygonal ferrite: 0.3 to 10.0 μm,
Average particle size of retained austenite: 1.0 μm or less,
A method for producing a high-strength steel sheet that satisfies the following equation (iv) ,
A method for producing a high-strength steel sheet, which comprises the following steps (A) to (C).
0.25 <[Mn] _PF / [Mn] _BF <0.70 (iv)
However, the meaning of each symbol in the above formula (iv) is as follows.
[Mn] _PF : average Mn concentration (mass%) in polygonal ferrite
[Mn] _BF : Average Mn concentration (mass%) in bainitic ferrite
(A)% by mass,
C: 0.04% or more and less than 0.50%,
Si: 0.10% or more and less than 3.0%,
Mn: 1.5-8.0%,
P: 0.10% or less,
S: 0.030% or less,
sol. Al: 0.01 to 2.0%,
N: 0.010% or less,
Ti: 0 to 0.20%,
Nb: 0 to 0.10%,
V: 0 to 0.50%,
Cr: 0% or more and less than 1.0%,
Mo: 0 to 0.50%,
Ni: 0 to 1.0%,
B: 0 to 0.0050%,
Ca: 0 to 0.020%,
Mg: 0 to 0.020%,
REM: 0 to 0.020%,
Cu: 0 to 1.0%,
Bi: 0 to 0.020%,
The balance: Fe and impurities,
For a slab having a chemical composition that satisfies the following formula (i),
A hot rolling process in which hot rolling is performed to complete rolling in a temperature range of Ar ₃ points to 1100 ° C. to obtain a hot rolled steel sheet, and the hot rolling steel sheet is wound in a temperature range of 650 ° C. or less.
0.5 ≦ Si + sol. Al ≦ 3.0 (i)
However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel sheet.
(B) Primary holding the hot-rolled steel sheet obtained in the step (A) in the temperature range of (Ac ₃ points −50 ° C.) to (Ac ₃ points −2 ° C.) for a time satisfying the following expression (ii). Annealing process.
1.4 × 10 ⁻⁸ × exp {26500 / (T + 273)} ≦ t ≦ 4.0 × 10 ⁵ (ii)
However, the meaning of each symbol in the above formula (ii) is as follows.
t: Primary annealing holding time (seconds)
T: Primary annealing temperature (° C)
(C) After holding the annealed steel sheet obtained in the step (B) in a temperature range of (Ac ₃ points-40 ° C) or higher and lower than (Ac ₃ points + 100 ° C), it is cooled to a temperature range of 500 ° C to Ms point. Then, the secondary annealing step of maintaining the temperature range for 30 seconds or more.   The method for producing a high-strength steel sheet according to claim 1, wherein after the step (B), the annealed steel sheet is cold-rolled with a total reduction of 30% or more and less than 80%.   The method for producing a high-strength steel sheet according to claim 1 or 2, wherein after the step (A), the hot-rolled steel sheet is subjected to cold rolling with a total reduction of 30% or more and less than 80%. The chemical composition is% by mass,
Ti: 0.005 to 0.20%,
Nb: 0.002-0.10%, and V: 0.005-0.50%,
The method for producing a high-strength steel sheet according to any one of claims 1 to 3, containing at least one selected from the group consisting of: The chemical composition is% by mass,
Cr: 0.05% or more and less than 1.0%,
Mo: 0.02-0.50%,
Ni: 0.05 to 1.0%, and B: 0.0002 to 0.0050%,
The method for producing a high-strength steel sheet according to any one of claims 1 to 4, containing at least one selected from the group consisting of: The chemical composition is% by mass,
Ca: 0.0005 to 0.020%,
Mg: 0.0005 to 0.020%, and REM: 0.0005 to 0.020%,
The method for producing a high-strength steel sheet according to any one of claims 1 to 5, containing at least one selected from the group consisting of: The chemical composition is% by mass,
Cu: 0.05-1.0%
The method for producing a high-strength steel sheet according to any one of claims 1 to 6, further comprising: The chemical composition is% by mass,
Bi: 0.0005 to 0.020%
The method for producing a high-strength steel sheet according to any one of claims 1 to 7, further comprising: