JP2016125116A - Cold rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet having both of high strength and excellent processability and a manufacturing method therefor.SOLUTION: There is provided a cold rolled steel sheet having a chemical composition containing, by mass%, C:0.05 to 0.30%, Si:0.5 to 2.5%, Mn:0.5 to 3.5%, P:0.100% or less, S:0.050% or less, sol.Al:0 to 1.00%, Cr:0 to 1.00%, Mo:0 to 0.30%, V:0 to 0.30%, B:0 to 0.0050%, Ca:0 to 0.003%, REM:0 to 0.003%, one or more selected from Ti:0.080% or less and Nb:0.050% or less and the balance of Fe with impurities and satisfying [0.003≤Ti+Nb≤0.100] and a metallic structure containing, by area%, bainite:50% or more, retained austenite:3.0% or more and ferrite:5.0% or more, where an average crystal particle diameter of the ferrite is 4.0 μm or less and a percentage of crystal particles existing neighboring to crystal particles of retained austenite having a crystal particle diameter of 0.2 to 1.0 μm is 50% or more.SELECTED DRAWING: None

Description

  The present invention relates to a cold-rolled steel sheet and a method for producing the same, and particularly to a cold-rolled steel sheet having both high strength and excellent workability and a method for producing the same.

  In recent years, development of steel materials contributing to energy saving has been demanded from the viewpoint of protecting the global environment. In fields such as automotive steel sheets and building structural steel sheets, there is a growing demand for particularly high-strength steel sheets that are lightweight. Steel sheets used in these fields are often processed into products by press forming or the like, so that not only strength characteristics but also excellent workability is required.

  As a steel sheet having excellent strength characteristics, for example, in Patent Document 1, ferrite has a main phase, and the average crystal grain size of ferrite at a depth position of ¼ of the plate thickness and the average crystal grain size at 700 ° C. It is disclosed that a hot-rolled steel plate and a cold-rolled steel plate excellent in thermal stability and mechanical properties capable of withstanding the heat of the welding or hot dipping process can be obtained by adjusting the increasing speed of the steel sheet.

  Patent Document 2 discloses that the ferrite orientation is accumulated in an orientation advantageous for Young's modulus and the austenite grains are refined to refine the ferrite produced during cooling, and the tensile strength (TS) is 590 MPa. As described above, it is disclosed that a high-strength thin steel sheet having a yield ratio (YR) of 0.65 or more and a Young's modulus of 225 GPa or more and excellent in rigidity can be obtained.

  Furthermore, Patent Document 3 discloses a composite structure having one or two of martensite and bainite, which are low-temperature transformation phases, as a main phase, and has high strength by suppressing the development of a specific texture. However, it is disclosed that a cold-rolled steel sheet having excellent ductility and stretch flangeability can be obtained.

International Publication No. 2007/015541 JP 2007-92131 A International Publication No. 2013/125399

  According to the method disclosed in Patent Document 1, by refining the structure of the hot-rolled steel sheet, which is a raw material, the structure can be refined without containing a large amount of precipitated elements, and excellent ductility can be achieved. It is possible to manufacture a cold-rolled steel sheet having the same. The obtained cold-rolled steel sheet has a fine structure because the hot-rolled steel sheet, which is the raw material, has a fine structure after cold rolling and after annealing, and austenite resulting therefrom is also fine. And having a fine structure.

  However, in the method disclosed in Patent Document 1, most of the preferential nucleation sites of austenite transformation such as grain boundaries, fine carbide particles, and low-temperature transformation phase existing in the hot-rolled steel sheet are caused by the growth of recrystallized grains of ferrite. After disappearing during heating during annealing, austenite transformation occurs with the grain boundaries of the recrystallized structure as nucleation sites.

  Therefore, although the cold-rolled steel sheet obtained by the method disclosed in Patent Document 1 has a fine structure, the structure is refined using austenite nucleated from the ferrite structure after recrystallization in the annealing process. In addition, it is difficult to say that the very fine structure of the hot-rolled steel sheet is fully utilized for the refinement of the structure. In particular, when annealing is performed in an austenite single phase region, it is difficult to utilize the fine structure of the hot-rolled steel sheet for the refinement of the structure after cold rolling and after annealing.

  In the method disclosed in Patent Document 2, since the winding after hot rolling is performed at a high temperature, it is considered that most of Ti and Nb are precipitated as carbides before annealing. . Furthermore, the method disclosed in Patent Document 3 is a method of refining the structure of a cold-rolled steel sheet by transforming a large number of fine austenite from unrecrystallized ferrite, but greatly affects the austenite grain size during annealing. The control of Ti and Nb precipitates is not fully considered, leaving room for study.

  As described above, in the methods disclosed in Patent Documents 1 to 3, the structure control method in the hot-rolled steel sheet or the recrystallization control method in the annealing heating process is not sufficient, and the ductility of the steel sheet is improved. The effect is not sufficiently obtained. From this, there remains room for improving workability by selecting a combination of initial structure control and annealing heating method suitable for refining the structure of the steel sheet.

  This invention is made | formed in view of the said present condition, and it aims at providing the cold-rolled steel plate which has high intensity | strength and the outstanding workability, and its manufacturing method.

  As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.

  (A) Stretch flangeability can be improved by setting the microstructure of the cold rolled steel sheet as a main phase of bainite that is easy to obtain a uniform hardness distribution.

  (B) Moreover, it becomes possible to improve ductility by containing the 2nd phase containing a ferrite and a retained austenite. And the fall of stretch flangeability can be suppressed as much as possible by refining the 2nd phase.

  (C) Furthermore, the workability of the steel sheet is greatly improved by including fine ferrite and retained austenite as the second phase in the metal structure of the cold-rolled steel sheet, and making them adjacent to each other. Is possible.

  The present invention has been made on the basis of the above-described knowledge, and the gist thereof is in the cold-rolled steel sheet and the manufacturing method thereof shown below.

(1) The chemical composition is mass%,
C: 0.05 to 0.30%
Si: 0.5 to 2.5%
Mn: 0.5 to 3.5%
P: 0.100% or less,
S: 0.050% or less,
sol. Al: 0 to 1.00%,
Cr: 0 to 1.00%,
Mo: 0 to 0.30%,
V: 0 to 0.30%,
B: 0 to 0.0050%,
Ca: 0 to 0.003%, and
REM: 0 to 0.003%,
One or more selected from Ti: 0.080% or less and Nb: 0.050% or less;
Balance: Fe and impurities,
Satisfying the following formula (i)
Metal structure is area%,
Bainite: 50% or more
Retained austenite: 3.0% or more, and
Ferrite: 5.0% or more,
The average crystal grain size of the ferrite is 4.0 μm or less, and
A cold-rolled steel sheet in which the proportion of ferrite crystal grains present adjacent to the residual austenite crystal grains having a crystal grain size of 0.2 to 1.0 µm is 50% or more.
0.003 ≦ Ti + Nb ≦ 0.100 (i)
However, each element symbol in the formula (i) represents the content (% by mass) of each element.

  (2) The cold-rolled steel sheet according to (1) above, wherein an average crystal grain size of the retained austenite is 1.0 μm or less.

(3) The chemical composition is mass%,
sol. Al: 0.10 to 1.00%,
The cold-rolled steel sheet according to (1) or (2) above.

(4) The chemical composition is mass%,
Cr: 0.03-1.00%,
Mo: 0.01 to 0.30%, and
V: 0.01-0.30%,
The cold-rolled steel sheet according to any one of (1) to (3) above, which contains one or more selected from:

(5) The chemical composition is mass%,
B: 0.0003 to 0.0050%,
The cold-rolled steel sheet according to any one of (1) to (4) above, which contains

(6) The chemical composition is mass%,
Ca: 0.0005 to 0.003%, and
REM: 0.0005 to 0.003%,
The cold-rolled steel sheet according to any one of (1) to (5) above, which contains one or more selected from:

(7) The method for producing a cold-rolled steel sheet according to any one of (1) to (6) above,
A hot rolling step in which the steel material having the chemical composition according to any one of (1) or (3) to (6) is hot-rolled at a rolling completion temperature of Ar ₃ or more;
After the hot rolling step, cooling to 600 ° C. within 5 s, further cooling to 550 ° C. or lower and winding, producing a hot rolled steel sheet,
A cold rolling step of cold rolling the hot-rolled steel sheet at a total rolling reduction of 20% or more;
After the cold rolling step, a first heating step of heating to Ac ₁ point + 10 ° C. so that the average heating rate is 15 ° C./s or more in a temperature range from 500 ° C. to Ac ₁ point + 10 ° C., and then A second heating step of heating to a temperature range from Ac ₃ points to Ac ₃ points + 100 ° C. at an average heating rate of 0.2 to 2.0 ° C./s and holding for 30 s or more, and the first heating step And an annealing step in which the second heating step is allowed to stay at 700 to 800 ° C. for 50 s or more, and further cooled so that the average cooling rate is 10 ° C./s or more in the temperature range from 650 ° C. to 500 ° C.,
The manufacturing method of a cold-rolled steel sheet including the overaging process process hold | maintained at 500-300 degreeC for 10 s or more after the said annealing process.

  (8) In the hot rolling step, the rolling reduction in the final rolling is set to 30% or more, and in the cooling step, after the hot rolling is finished, the temperature is cooled to 600 ° C. within 1 s. A method for producing a cold-rolled steel sheet.

  According to the present invention, it becomes possible to effectively refine the structure after cold rolling and after annealing without containing a large amount of precipitation elements such as Ti and Nb, and has high strength, and A cold-rolled steel sheet having excellent ductility and stretch flangeability of the steel sheet and a method for producing the cold-rolled steel sheet can be provided. Therefore, the cold-rolled steel sheet according to the present invention can be suitably used as a steel sheet for automobiles and a steel sheet for building structures.

  The present invention will be described in detail below.

(A) Chemical composition In the cold-rolled steel sheet of the present invention, the reason for limiting the chemical composition is as follows. In the following description, “%” for the content means “% by mass”.

C: 0.05-0.30%
C is an element having an effect of increasing the strength of steel. Moreover, it has the effect | action which stabilizes austenite by concentrating in austenite, raises the volume ratio of the retained austenite in a cold-rolled steel plate, and improves ductility. Further, C has an action of lowering the transformation point, and as a result, in the hot rolling process, hot rolling is completed in a lower temperature range, and the microstructure of the hot rolled steel sheet can be refined. . In the annealing process, coupled with the effect of suppressing the recrystallization of ferrite in the temperature rising process by C, the temperature range of not less than the unrecrystallized ratio of ferrite by rapid heating (Ac ₁ point + 10 ° C.) or higher This makes it possible to refine the austenite grains during the annealing of the cold-rolled steel sheet.

  If the C content is less than 0.05%, the above effect cannot be obtained. On the other hand, when the C content exceeds 0.30%, the workability and weldability of the cold-rolled steel sheet are significantly deteriorated. Therefore, the C content is 0.05 to 0.30%. The C content is preferably 0.08% or more, and more preferably 0.10% or more. Further, the C content is preferably 0.25% or less.

Si: 0.5 to 2.5%
Si is an element having an action of increasing the strength of steel by promoting the generation of bainite that forms the main phase of the cold-rolled steel sheet according to the present invention. Furthermore, since it has the effect | action which accelerates | stimulates the production | generation of a retained austenite and improves the ductility of steel, in this invention, it is necessary to contain more than fixed amount. If the Si content is less than 0.5%, the above effect cannot be obtained. On the other hand, when the Si content exceeds 2.5%, not only the ductility is significantly lowered, but also the plating property is impaired. Therefore, the Si content is 0.5 to 2.5%. The Si content is preferably 0.8% or more, and more preferably 1.0% or more. Moreover, it is preferable that Si content is 2.0% or less.

Mn: 0.5 to 3.5%
Mn is an element having an effect of increasing the strength of steel. Moreover, since it has the effect | action which lowers austenitizing temperature, soaking temperature can be made low in an annealing process. Therefore, the structure can be kept fine by suppressing grain growth. Thereby, it becomes possible to refine the microstructure of the cold-rolled steel sheet. Moreover, since there exists an effect | action which improves the hardenability of steel, it also has the effect of ensuring the area ratio of bainite required in the cooling after annealing.

  If the Mn content is less than 0.5%, the above effect cannot be obtained. On the other hand, if the Mn content exceeds 3.5%, the steel is excessively strengthened and the ductility is significantly impaired. Therefore, the Mn content is 0.5 to 3.5%. The Mn content is preferably 1.0% or more, and preferably 3.0% or less.

P: 0.100% or less P is an element that is contained as an impurity and segregates at grain boundaries to embrittle the material. When the P content exceeds 0.100%, embrittlement due to the above action becomes significant. Therefore, the P content is 0.100% or less. The P content is preferably 0.060% or less. The lower the P content, the better. Therefore, it is not necessary to specifically limit the lower limit, but it is preferably 0.001% or more from the viewpoint of cost.

S: 0.050% or less S is an element which is contained as an impurity, and forms sulfide-based inclusions in the steel to lower the ductility of the steel. When S content exceeds 0.050%, the ductility fall by the said effect | action will become remarkable. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.008% or less, and more preferably 0.003% or less. Since the S content is preferably as low as possible, there is no need to limit the lower limit.

sol. Al: 0 to 1.00%
Al is an element having an effect of increasing ductility. Therefore, you may contain Al as needed. However, since Al has an action of raising the transformation point, sol. If the Al content exceeds 1.00%, hot rolling must be completed in a higher temperature range. As a result, it is difficult to refine the structure of the hot-rolled steel sheet, and therefore, it is also difficult to refine the structure of the cold-rolled steel sheet. Therefore, sol. The Al content is 1.00% or less. sol. The Al content is preferably 0.70% or less. In order to sufficiently obtain the above effects, sol. The Al content is preferably 0.10% or more, and more preferably 0.20% or more.

Cr: 0 to 1.00%
Mo: 0 to 0.30%
V: 0 to 0.30%
Cr, Mo, and V are all elements that have an effect of increasing the strength of steel. Moreover, Mo has the effect | action which suppresses the grain growth of a crystal grain and refines | miniaturizes a structure | tissue, V has the effect | action which promotes the transformation to a ferrite and improves the ductility of a steel plate. Therefore, you may contain 1 or more types selected from Cr, Mo, and V as needed.

  However, if the Cr content exceeds 1.00%, ferrite transformation is excessively suppressed, and the target structure cannot be ensured. On the other hand, if the contents of Mo and V exceed 0.30%, a large amount of precipitates are generated in the heating stage of the hot rolling process, and the ductility is significantly reduced. Therefore, the content of each element is as described above. The Mo content is preferably 0.25% or less. In order to sufficiently obtain the above effects, the Cr content is preferably 0.03% or more, the Mo content is preferably 0.01% or more, and the V content is 0.01% or more. It is preferable. In addition, when 2 or more types of said elements are contained complexly, it is preferable that the total content shall be 1.20% or less.

B: 0 to 0.0050%
B is an element having an effect of increasing the strength of the steel by enhancing the hardenability of the steel and promoting the generation of the low temperature transformation phase. Therefore, you may contain B as needed. However, if the B content exceeds 0.0050%, the steel is excessively hardened and the ductility is significantly reduced. Therefore, the B content is 0.0050% or less. In order to sufficiently obtain the above effect, the B content is preferably 0.0003% or more.

Ca: 0 to 0.003%
REM: 0 to 0.003%
Both Ca and REM have the effect of increasing the soundness of the slab by refining oxides and nitrides that precipitate during the solidification process of the molten steel. Therefore, you may contain 1 or more types selected from these elements as needed. However, since any element is expensive, the content of each element is set to 0.003% or less. The total content of these elements is preferably 0.005% or less. In order to obtain the effect of the above action more reliably, the contents of Ca and REM are each preferably 0.0005% or more.

Ti: 0.080% or less Nb: 0.050% or less 0.003 ≦ Ti + Nb ≦ 0.100 (i)
However, each element symbol in the formula (i) represents the content (% by mass) of each element.
Ti and Nb are elements having the effect of promoting refinement of the steel structure by precipitating in the steel as carbides and / or nitrides and suppressing the grain growth of austenite in the annealing process. Therefore, in this invention, it is necessary to contain 1 or more types selected from Ti: 0.080% or less and Nb: 0.050% or less, and to satisfy the said (i) Formula.

  However, when the Ti content exceeds 0.080%, or when the Nb content exceeds 0.050%, the ductility is significantly reduced. Therefore, when Ti is contained, the content is 0.080% or less, and when Nb is contained, the content is 0.050% or less. The Ti content is preferably 0.050% or less, and the Nb content is preferably 0.030% or less. In addition, the total content of one or more selected from Ti and Nb is preferably 0.040% or less. In order to sufficiently obtain the above effect, when Ti is contained, the content is preferably 0.005% or more, and when Nb is contained, the content is 0.003% or more. It is preferable to do.

The balance: Fe and impurities The cold-rolled steel sheet of the present invention contains the above-described elements, and the balance has a chemical composition that is Fe and impurities. "Impurity" is a component that is mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and is allowed within a range that does not adversely affect the present invention. Means.

(B) Metal structure The cold-rolled steel sheet of the present invention has an area%, bainite: 50% or more, retained austenite: 3.0% or more, and ferrite: 5.0% or more. The average crystal grain size of the ferrite is 4.0 μm or less, and among the ferrites, the ratio of the ferrite crystal grains present adjacent to the residual austenite crystal grains having a crystal grain size of 0.2 to 1.0 μm is 50% or more.

Bainite: 50% or more The metal structure of the cold-rolled steel sheet according to the present invention has bainite as the main phase. By increasing the area ratio of bainite and making the microstructure homogeneous, it is possible to suppress the generation of minute voids when the steel sheet is processed. Moreover, since bainite is a relatively hard structure, it also has the effect of increasing the strength of the steel sheet. Therefore, the area ratio of bainite needs to be 50% or more. The area ratio of bainite is preferably 60% or more.

Residual austenite: 3.0% or more The metal structure of the cold-rolled steel sheet according to the present invention contains retained austenite as the second phase. Since retained austenite has the effect of improving the elongation of the steel sheet, it is possible to ensure even better elongation by increasing the area ratio of retained austenite. Therefore, the area ratio of retained austenite needs to be 3.0% or more. The area ratio of retained austenite is preferably 5.0% or more. The average crystal grain size of retained austenite is preferably 1.0 μm or less. This is because by increasing the crystal grain size, the stability of retained austenite to processing deformation increases, so stress-induced transformation is likely to occur later in processing, and the work hardening rate of the material, and thus uniform elongation, is improved. It is to do.

Ferrite: 5.0% or more The metal structure of the cold-rolled steel sheet according to the present invention further contains fine ferrite as the second phase. Ferrite has the effect of remarkably improving the elongation of the steel sheet. Further, by making the structure fine, the effect of suppressing the development of fine cracks during hole expansion processing can also be obtained. As a result, while significantly improving the elongation, the decrease in the hole expansion rate can be kept small, and a balance between the elongation and the hole expansion rate excellent in the steel sheet can be imparted. Therefore, the area ratio of ferrite is set to 5.0% or more. Moreover, in order to acquire the said effect, the average crystal grain diameter of a ferrite shall be 4.0 micrometers or less.

Among the ferrites described above, the ratio of the ferrite crystal grains present adjacent to the residual austenite crystal grains having a crystal grain size of 0.2 to 1.0 μm: 50% or more. The presence of a sufficient volume of retained austenite of 2 to 1.0 μm makes the improvement of elongation, particularly uniform elongation, more remarkable. This is presumably because the stress applied to the retained austenite when the steel sheet is processed is relaxed by the deformation of the adjacent ferrite, so that the stress-induced transformation of the retained austenite is delayed until a high strain is applied. As a result, stress-induced transformation is likely to occur in the later stage of work deformation (high strain region), and the work hardening coefficient in the strain region can be increased.

  Therefore, in the present invention, among the ferrites present in the metal structure, the ratio of the ferrite crystal grains present adjacent to the residual austenite crystal grains having a crystal grain size of 0.2 to 1.0 μm is 50% or more. To do. The ratio of the ferrite crystal grains is preferably 55% or more.

  In addition, although pearlite and / or cementite may mix in a metal structure, if these total area ratios are 10% or less, it is accept | permitted.

  The area ratio of bainite and ferrite can be measured by structural analysis using SEM-EBSD. Further, the area ratio of retained austenite is the volume ratio obtained by the X-ray diffraction method as it is. Judgment of the adjacent state of ferrite and retained austenite can be obtained by obtaining information on the coordinates of crystal grains from the structure analysis result of the SEM-EBSD. Here, SEM-EBSD is a method of measuring the orientation of a minute region by electron beam backscatter diffraction (EBSD) in a scanning electron microscope (SEM).

  Furthermore, the average grain size of the ferrite contained as the second phase is targeted for ferrite surrounded by large-angle grain boundaries with an inclination angle of 15 ° or more by analyzing the obtained orientation map using SEM-EBSD. It is obtained by determining the crystal grain size. In addition, the average crystal grain diameter of the ferrite in the present invention means an average value of equivalent circle diameters obtained by the following formula (ii). In the following formula (ii), Ai represents the area of the i th ferrite grain, and di represents the equivalent circle diameter of the i th ferrite grain.

  In addition, in this invention, the measured value in the depth of the board thickness 1/4 of a steel plate is employ | adopted about the value of an area rate and an average crystal grain size.

(C) Manufacturing method Although there is no restriction | limiting in particular about the manufacturing method of the cold-rolled steel plate which concerns on this invention, For example, the hot rolling shown below with respect to the steel raw material (it is also called "slab") which has said chemical composition. After producing a hot-rolled steel sheet by performing a process and a cooling process, it can be manufactured by performing a cold rolling process, further performing a heat treatment process such as annealing, and finally performing an overaging treatment process. Below, a heat treatment process is demonstrated as an annealing process.

In this specification, the Ar ₃ point is the temperature at which ferrite transformation starts at the time of cooling, the Ac ₁ point is the temperature at which austenite begins to be produced at the time of heating, and the Ac ₃ point is the transformation of ferrite to austenite at the time of heating. Means the temperature to complete.

(C-1) Hot rolling process In this invention, in the hot-rolled steel plate used as the raw material before cold rolling, most of Ti and Nb are made to exist in the solid solution state. By keeping these elements in a solid solution state, Ti and Nb carbides can be precipitated in the heating process of the subsequent heat treatment step. Since these precipitates are precipitated from the grain boundaries, block boundaries, and dislocations contained in the unrecrystallized ferrite, a large number of precipitation sites are obtained, so that very fine precipitates are obtained. be able to. As a result, since the growth of austenite grains during the soaking in the annealing process can be effectively suppressed, a sufficient fine graining effect can be obtained by containing a small amount of Ti and Nb.

  The total amount of Ti and Nb in the solid solution state contained in the hot-rolled steel sheet is the total content of Ti and Nb contained in the hot-rolled steel sheet, and Ti and Nb carbides contained in the electrolytic residue of the hot-rolled steel sheet and It can be determined as a difference in the total amount of nitride.

  Moreover, it is preferable that the structure of a hot-rolled steel plate is fine. Specifically, the structure of the hot-rolled steel sheet is preferably a structure in which cementite particles are finely dispersed while containing a large amount of boundaries such as prior austenite grain boundaries, block boundaries of bainite and martensite, and ferrite grain boundaries. . When the cementite particles are present on the boundary having a crystal orientation difference of 15 ° or more, the boundary functions particularly effectively as an austenite nucleation site in the subsequent heat treatment. From the above, refining the structure of the hot-rolled steel sheet is effective for refining the final structure.

  Such a hot-rolled steel sheet having a fine structure can be produced by the following hot rolling step and subsequent cooling step. A slab having the above-described chemical composition is produced by continuous casting, and this is subjected to a hot rolling process. At this time, the slab may be used while maintaining the high temperature during continuous casting, or may be used after being reheated after being cooled to room temperature.

  The temperature of the slab used for hot rolling is preferably 1000 ° C. or higher. If the temperature of the slab is lower than 1000 ° C., in addition to giving an excessive load to the rolling mill, the temperature may be lowered to the ferrite transformation temperature during rolling, and the steel may be rolled with the transformed ferrite contained in the structure. There is. From this, it is preferable that the heating temperature be sufficiently high, that is, 1000 ° C. or higher so that the hot rolling can be completed in the austenite temperature range. If a temperature drop occurs during rolling, the steel sheet may be heated again between rough rolling and finish rolling.

The hot rolling is preferably performed using a lever mill or a tandem mill. From the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the final few passes (for example, 3 to 10 passes). During rolling, since it is necessary to maintain the steel sheet in the austenite temperature range, it is preferable that the rolling completion temperature be Ar ₃ or higher.

  Further, by increasing the reduction amount of the final rolling, the austenite structure immediately after hot rolling becomes finer, and the structure generated during cooling after hot rolling can be made fine. Therefore, in hot rolling, the rolling reduction in final rolling is preferably 20% or more, and more preferably 30% or more. Since the austenite grain boundaries after hot rolling remain as old austenite grain boundaries in the bainite and martensite structures after cooling the hot-rolled steel sheets, the former austenite grain boundaries are more closely packed in the microstructure of the hot-rolled steel sheets. Will be distributed. Since this former austenite grain boundary becomes a nucleation site in the annealing process, the number of austenite nucleation in the heating process of the annealing process can be remarkably increased.

The total rolling reduction of the hot-rolled steel sheet in the temperature range from Ar ₃ point to Ar ₃ point + 150 ° C. is preferably 40% or more, and more preferably 60% or more. Rolling does not have to be performed in one pass, and may be continuous multi-pass rolling. By rolling in a plurality of continuous passes, more strain energy can be introduced into austenite, the transformation driving force to ferrite or bainite can be increased, and the hot-rolled steel sheet can be further refined. However, since this also increases the load on the rolling equipment, the upper limit of the rolling reduction per pass is preferably 60%.

(C-2) Cooling process It is preferable to perform a cooling process with the following method. In the cooling step after the hot rolling step, the time required for cooling from the rolling completion temperature to 600 ° C. after the end of rolling is preferably 5 s or less, and more preferably 2 s or less. More preferably, the time required for starting cooling immediately after finishing rolling and cooling to 600 ° C. is 1 s or less. It is preferable to cool to a temperature of 600 ° C. or lower by this cooling step, and it is more preferable to cool to a temperature of 550 ° C. or lower. The cooling method is preferably water cooling. The cooling rate is preferably 100 ° C./s or more, and more preferably 400 ° C./s or more.

  The reason for prescribing the cooling method after the hot rolling process as described above is to pass through the precipitation nose of carbides of Ti and Nb in a short time and to suppress the precipitation as much as possible in the cooling after the hot rolling process. . Moreover, it is for making the structure of a hot-rolled steel sheet fine and increasing the density of nucleation sites in the annealing process. By quickly performing the cooling process after the hot rolling process, the strain recovered in the austenite and the consumption due to recrystallization are suppressed as much as possible, and the strain energy accumulated in the steel is changed from austenite to ferrite or bainite. It can be used to the maximum extent as a transformation driving force. The reason why the cooling rate from the rolling completion temperature is more preferably 400 ° C./s or more is also to increase the transformation driving force as described above. Thereby, the number of transformation nucleation to ferrite etc. can be increased and the structure of a hot-rolled steel sheet can be refined. By using a hot-rolled steel sheet having a microstructure produced in this way as a raw material, the structure of the cold-rolled steel sheet can be further refined.

  You may cool after cooling to 600 degreeC from rolling completion temperature. The cooling time is preferably 60 s or less, and more preferably 15 s or less. When the cooling time exceeds 60 s, the structure of the hot-rolled steel sheet may become coarse due to the growth of ferrite. The cooling time is preferably 3 seconds or longer.

  As a cooling temperature and a cooling time suitable for tissue control, for example, the cooling can be performed for about 6 seconds in a temperature range of 500 to 600 ° C. Thereby, fine ferrite can be introduced into the structure of the hot-rolled steel sheet.

  Thereafter, the steel sheet is cooled to the coiling temperature. The cooling method at this time can be performed at an arbitrary cooling rate by a method selected from water cooling, mist cooling, and gas cooling (including air cooling). The coiling temperature of the steel sheet is preferably 550 ° C. or lower in order to make Ti and Nb in the hot-rolled steel sheet as solid as possible. Note that when the coiling temperature is less than 250 ° C., the strength of the hot-rolled steel sheet increases and cold rolling may be difficult. For this reason, it is preferable that winding temperature shall be 250 degreeC or more.

  The hot-rolled steel sheet produced by the hot rolling process described above can contain 0.010% or more of Ti and Nb in a solid solution state. As a result, Ti and Nb carbides can be precipitated in the heating process of the subsequent annealing step. Since this precipitate is precipitated starting from the dislocation structure contained in the unrecrystallized ferrite, a large number of precipitation sites can be obtained, so that a very fine precipitate can be obtained. As a result, since the growth of austenite grains during the soaking in the annealing process can be effectively suppressed, a sufficient grain refining effect can be obtained even if the contents of Ti and Nb are very small.

  Furthermore, the metal structure of the hot-rolled steel sheet is a structure in which phases such as martensite or cementite are finely dispersed. Thus, it is preferable to perform a cold rolling process and an annealing process on the hot rolled steel sheet in which phases such as cementite are finely dispersed. Because the cementite existing on these large-angle grain boundaries is the preferential nucleation site for the austenite transformation, a large number of austenite and recrystallized ferrite are generated from these positions by rapid heating in the annealing process described later, and the structure This is because it is possible to reduce the size.

  In addition, a sufficiently large amount of large-angle grain boundaries are introduced into the microstructure of the hot-rolled steel sheet, and the average crystal grain size defined by the large-angle grain boundaries with an inclination angle of 15 ° or more becomes 6 μm or less. The site can be increased further. For this miniaturization, it is effective to increase the reduction ratio in the hot rolling process, shorten the subsequent cooling time, and increase the cooling rate.

  The metal structure of the hot-rolled steel sheet obtained by the hot rolling step can be a ferrite structure containing pearlite as the second phase, a structure composed of bainite and martensite, and a structure obtained by mixing them.

(C-3) Cold rolling process It is preferable to perform cold rolling after pickling the hot-rolled steel sheet produced by said hot rolling process and cooling process as needed. Cold rolling may be performed using a normal method, and the total rolling reduction is usually 20% or more. When the cold rolling rate is increased, the density of grain boundaries in the structure increases, so that the number of nucleation can be increased during heating in the annealing process.

(C-4) Annealing Step In the annealing step following the cold rolling step, it is preferable to perform a first heating step of heating the steel sheet from room temperature to Ac ₁ point + 10 ° C. In the first heating step, the average heating rate in the temperature range from 500 ° C. to Ac ₁ point + 10 ° C. is preferably 15 ° C./s or more. By rapidly heating in this temperature range, recrystallization during heating can be suppressed, and as a result, it can be heated to Ac ₁ point + 10 ° C. while leaving an unrecrystallized structure. As a result, a large number of austenite can be nucleated with cementite existing on large-angle grain boundaries such as prior austenite grain boundaries, block boundaries, and ferrite grain boundaries of the hot-rolled steel sheet as the main nucleation sites. When the average heating rate in the temperature range from 500 ° C. to Ac ₁ point + 10 ° C. is less than 15 ° C./s, recrystallization occurs and the grain boundary of the hot-rolled steel sheet disappears, and the above effect is obtained. There may not be.

Increasing the heating rate not only increases the unrecrystallized rate, but also increases the driving force for reverse transformation, which is preferable for increasing nucleation of austenite. Therefore, the average heating rate in the temperature range from 500 ° C. to Ac ₁ point + 10 ° C. is more preferably 30 ° C./s or more, and further preferably 100 ° C./s or more. The upper limit of the average heating rate is not particularly set, but is preferably set to 1000 ° C./s or less in consideration of difficulty in temperature control.

When the temperature reaches Ac ₁ point + 10 ° C., the non-recrystallization rate in the region not transformed to austenite is preferably 30% or more. When the non-recrystallization ratio in the region not subjected to austenite transformation is less than 30% when Ac reaches ₁ point + 10 ° C., the region in which austenite transformation has proceeded after completion of the recrystallization occupies most. As a result, since the austenite transformation proceeds from the grain boundaries of the recrystallized grains, the austenite grains in the annealing process become coarse and the final structure also becomes coarse.

  The temperature at which the above rapid heating is started is not particularly limited because it exhibits a sufficient effect as long as it is before the start of recrystallization. The start temperature of rapid heating is preferably Ts-30 ° C. or lower with respect to the softening start temperature (recrystallization start temperature) Ts measured at a heating rate of 10 ° C./s. For example, even if rapid heating is started from about 500 ° C., a sufficient fine graining effect can be obtained. Further, by starting rapid heating from room temperature, the austenite transformation can be caused while maintaining the cementite fine, so that the structure can be refined.

  The heating rate in the temperature range from room temperature to 500 ° C. in the heating process of the annealing step is preferably 15 ° C./s or more. This is because a large number of cementite particles can be obtained by suppressing the coarsening of cementite generated during heating, and the number of austenite nucleation sites can be increased.

After heating to Ac ₁ point + 10 ° C., it is preferable to perform a second heating step of heating to an annealing temperature (soaking temperature) from Ac ₃ point to Ac ₃ point + 100 ° C. The heating rate at this time can be an arbitrary rate, but by reducing the heating rate, sufficient time can be taken to promote recrystallization of ferrite, so the average heating rate is 0.2 to It is preferably 2.0 ° C./s. Also, the heating rate can be changed so that only the first part is rapid heating as in the first heating step and the subsequent heating rate is lower.

In the annealing step, the transformation to austenite is sufficiently advanced to eliminate the processed ferrite structure and dissolve carbides in the steel sheet. Therefore, it is preferable annealing temperature is Ac ₃ points or more. When annealing is performed at a temperature lower than this, an austenite single-phase state is not obtained during the annealing process, or ferrite recrystallization does not occur, and thus a processed ferrite structure may remain. If it becomes so, the integration degree of the orientation group of {100} <011> to {211} <011> will become strong in the texture of a cold-rolled steel plate, and the workability of a steel plate will fall. This decrease in workability becomes prominent when the degree of integration of orientation groups from {100} <011> to {211} <011> is 6 or more. On the other hand, when annealing is performed at a temperature exceeding Ac ₃ point + 100 ° C., abrupt grain growth of austenite grains occurs, and the final structure may become coarse. Therefore, the annealing temperature is preferably Ac ₃ point + 100 ° C. or less. In order to refine the structure, the annealing temperature is more preferably Ac ₃ points + 50 ° C. or lower.

  The soaking time at the soaking temperature is preferably 30 s or more. If the soaking time is less than 30 s, the dissolution of pearlite or cementite and the transformation to austenite do not proceed sufficiently, and the workability of the cold-rolled steel sheet may be reduced. In addition, temperature unevenness during soaking is likely to occur, which may cause problems in manufacturing stability. Therefore, it is preferable that the soaking time is set to 30 seconds or longer and the transformation to austenite is sufficiently advanced. On the other hand, if the holding is carried out for an excessively long time, the austenite grain growth may fail to satisfy the dispersed austenite condition defined by the present invention. For this reason, the soaking time is preferably less than 10 minutes.

  In order to obtain a sufficiently rapid heating rate, it is preferable to use energization heating, induction heating, or direct flame heating, but heating with a radiant tube is also possible as long as the requirements of the present invention are satisfied. Furthermore, by applying these heating devices, the heating time of the steel sheet can be greatly shortened, the annealing equipment can be made more compact, and the effects of improving productivity and reducing capital investment costs can be expected. Moreover, it is also possible to add the rapid heating apparatus to the existing continuous annealing line and hot dip plating line to carry out the above heating.

  In the first heating step and the second heating step, the residence time at 700 to 800 ° C. is preferably set to 50 s or longer. As a result, precipitation of Ti and Nb, which was in a solid solution state before the heat treatment, occurs in the unrecrystallized ferrite structure. Since TiC and NbC are generated by using dislocations included in the unrecrystallized structure as precipitation sites, a very large number of nucleation sites are obtained. As a result, the particle size of the precipitate can be made fine. The average particle size of the precipitated particles is approximately 5 nm to 20 nm.

  If the temperature range in which the steel sheet is retained is less than 700 ° C., recrystallization of the steel occurs, dislocations at grain boundaries and boundaries, which are precipitation sites, decrease, and the effect of refining Ti and Nb carbides is sufficient. It may not be obtained. Further, when the temperature range exceeds 800 ° C., most of the structure is transformed into austenite, so that dislocations at grain boundaries and boundaries may be reduced.

  By finely precipitating Ti and Nb carbides in this way, it is possible to remarkably suppress austenite grain growth during soaking in the annealing process. As a result, austenite can be made finer even under the condition of soaking at 30 so as to keep at soaking temperature. By making the austenite finer in this way, the ferrite transformation in cooling after soaking is effectively promoted, and the diffusion of carbon atoms to the austenite side associated therewith causes the carbon of the carbon at the interface between ferrite and austenite. A local thickening zone can be created. As a result, ferrite and retained austenite can be made adjacent to each other in the metal structure after the annealing step.

  After the soaking is maintained, cooling is preferably performed so that the average cooling rate is 10 ° C./s or more in the temperature range from 650 ° C. to 500 ° C. By setting the average cooling rate in the temperature range to 10 ° C./s or more, the area ratio of the low temperature transformation phase in the cold rolled steel sheet structure can be increased. On the other hand, when the average cooling rate in the temperature range is less than 10 ° C./s, ferrite is coarsely generated during cooling, and the stretch flangeability may be deteriorated.

(C-5) Overaging treatment step After the annealing step, it is preferable to carry out an overaging treatment for 10 seconds or more at 500 to 300 ° C. By this control, a low-temperature transformation phase having an appropriate area ratio is generated in the cold-rolled steel sheet, and residual austenite is generated by promoting the diffusion of carbon atoms into untransformed austenite. For this reason, it is preferable to hold | maintain at 500-300 degreeC for 10 s or more. More preferably, the temperature range is 450 to 350 ° C. and the holding time is 100 to 600 s.

  If the cooling rate is excessively low or kept at a high temperature for a long time, not only the desired structure fraction cannot be obtained, but also the workability of the steel sheet deteriorates due to transformation of residual austenite to carbide, etc. Cause. For this reason, even if it hold | maintains at 500-300 degreeC in the middle of cooling for a long time, it is desirable to set it as less than 2000 s. Although the cooling method can be performed by any method, for example, cooling by gas, mist, or water is possible.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Ingots of steel types A to H having the chemical compositions shown in Table 1 were melted in a vacuum induction furnace, hot forged, and then cut into slab-shaped steel pieces for use in hot rolling. Each obtained slab was heated for about 1 h at a temperature of 1000 ° C. or higher, and then subjected to a hot rolling process under the conditions shown in Table 2 using a small test mill to produce a hot rolled steel sheet having a thickness of 2.8 mm. did.

  Immediately after the completion of rolling or after cooling for a predetermined time at the rolling completion temperature, cooling by water cooling was performed. Cooling by water cooling was performed in two steps, and in the first cooling, the water was cooled to the cooling temperature shown in Table 2. Then, about the hot-rolled steel plate water-cooled to 600 degrees C or less, after allowing to cool, the 2nd cooling was performed. In the second cooling, water cooling was performed up to the coiling temperature. In addition, steel plate No. In 10, 13, 25, and 27, the water was cooled to the coiling temperature (650 ° C.) in the first cooling, and the subsequent cooling and the second cooling were not performed. Table 2 shows the cooling conditions. After cooling to the winding temperature, it was cooled to room temperature at 20 ° C./h as a heat treatment (slow cooling) for simulating winding. The heat treatment for simulating winding was performed by putting a steel plate in a furnace.

  Table 2 shows the time taken for cooling from the completion of rolling to 600 ° C. in the cooling step. In addition, the steel plate No. 1 wound up at 650 ° C. For 10, 13, 25 and 27, the cooling time was not described.

  Table 3 shows the average crystal grain size of the hot-rolled steel sheet obtained by the above hot rolling process and cooling process. The crystal grain size is measured by using a SEM-EBSD device (JSM-7001F, JSM-7001F) for the structure of the cross section in the rolling direction at the depth position of the steel sheet thickness ¼, ferrite, martensite, bainite. For the BCC phase consisting of

  Moreover, the total amount of solid solution Ti and Nb contained in the hot-rolled steel sheet was determined by the following method. First, the total amount of Ti and Nb contained in the produced hot-rolled steel sheet was determined by ICP-AES analysis. Furthermore, the same steel plate was electrolyzed, the residue of Ti and Nb precipitated as nitride or carbide was extracted, and the amount was determined by ICP-AES analysis. The total amount of solute Ti and Nb contained in the hot-rolled steel sheet was determined as the difference between the total amount of Ti and Nb contained above and the total amount of precipitated Ti and Nb.

The hot-rolled steel sheet obtained by the above method was pickled with hydrochloric acid and then cold-rolled at a rolling reduction shown in Table 2 to make the steel sheet thickness 1.4 mm. Then, using laboratory-scale annealing equipment, it is heated from room temperature to Ac ₁ point + 10 ° C. (first heating step), and then heated to the annealing temperature (soaking temperature) (second heating step), Retained. Table 2 shows the heating conditions. Then, in the temperature range of 650 degreeC to 500 degreeC, it cooled so that an average cooling rate might become the value shown in Table 2, and the cold-rolled steel plate was obtained. Cooling was performed with nitrogen gas.

  Furthermore, the obtained cold-rolled steel sheet was over-aged under the conditions shown in Table 2. Then, it cooled to room temperature and steel plate No. 1-33 were obtained.

In the present invention, the Ar ₃ point is obtained by processing a steel ingot melted in a vacuum induction furnace into a cylindrical sample having a diameter of 8 mm and a height of 12 mm, heating it to 900 ° C., and cooling it at 2 ° C./s. , And obtained from the result of thermal expansion measurement during that time. Moreover, Ac ₁ point and Ac ₃ point were calculated | required from the thermal expansion curve measured when the steel plate which cold-rolled was heated up to 1100 degreeC with the heating rate of 2 degree-C / s. Table 1 shows the values of Ar ₃ point, Ac ₁ point and Ac ₃ point.

  Steel plate No. manufactured in this way The metal structures and mechanical properties of 1-33 were examined as follows.

  The area ratios of bainite and ferrite were determined by structural analysis of the cross-sectional structure in the rolling direction at a depth of ¼ of the steel sheet using the SEM-EBSD apparatus described above. At this time, ferrite was defined as having an average misorientation value of 0.5 ° or less in the crystal grains. Further, the volume ratio of the retained austenite phase was determined by an X-ray diffraction method, and this was defined as the area ratio of retained austenite (residual γ). The average crystal grain size of ferrite and retained austenite is obtained by using a SEM-EBSD device (JSM-7001F, manufactured by JEOL Ltd.) as the cross-sectional structure from the width direction of the cold-rolled steel plate, as in the case of the hot-rolled steel plate. The average crystal grain size defined by a large-angle grain boundary having an inclination angle of 15 ° or more was used.

  Further, the adjacent state of ferrite and retained austenite was obtained by obtaining the position coordinates of the crystal grains of ferrite and austenite from the analysis result of the SEM-EBSD and calculating it. In this measurement, residual austenite of 0.2 to 1.0 μm was targeted. Further, ferrite was defined as crystal grains surrounded by a 15 ° grain boundary and having an average misorientation within 0.5 ° within the grains.

  In the EBSD analysis of the structure containing the retained austenite phase, it is easily affected by the surface state of the sample. Therefore, in this example, the area ratio (γEBSD) of retained austenite obtained by EBSD analysis is used as an index of analysis accuracy. It was assumed that the evaluation satisfied (γEBSD / γXRD)> 0.7 with respect to the volume fraction of retained austenite (γXRD) obtained by X-ray diffraction described later.

  The degree of accumulation of orientation groups {100} <011> to {211} <011> in the texture of the cold-rolled steel sheet is determined by the average X-ray intensity in the depth plane of the steel sheet thickness 1/2 by the X-ray diffraction method. It was measured by obtaining. The average X-ray intensity of the {100} <011> to {211} <011> orientation groups is the ODF (azimuth distribution) analyzed from the measurement results of the {200}, {110}, and {211} positive pole plots of ferrite. Function). In addition, the X-ray intensity of the random structure | tissue which does not have the texture required in X-ray intensity measurement was calculated | required by carrying out X-ray diffraction of powdery steel. As the apparatus, RINT-2500HL / PC manufactured by Rigaku Electronics Co., Ltd. was used.

Steel plate No. The mechanical properties 1 to 33 were investigated by a tensile test and a hole expansion test. The tensile test was performed using a No. 5 test piece according to JIS Z 2241 (2011), and the tensile strength (TS) and elongation at break (total elongation, El) were determined. The hole expansion test was performed according to JIS Z 2256 (2010), and the hole expansion ratio (λ) was obtained. The value of TS × El was used as an index of balance between strength and ductility, and the value of TS × λ was used as an index of balance between strength and stretch flangeability. These results are shown in Table 3. In the present invention, those satisfying the following formulas (iii) and (iv) where the values of TS × El and TS × λ satisfy the following formulas (iii) and (iv): The workability was judged to be poor. The value on the right side of the formula (iii) is preferably 26500.
TS × El> 25500 (iii)
TS × λ> 40000 (iv)

  Steel plate No. manufactured using steel type A 1-10, steel plate No. which is an example of the present invention. Nos. 1, 3, 5 and 7 were excellent in strength and workability because the chemical composition and metal structure satisfied the provisions of the present invention. Among them, the steel plate No. 5 and 7 are states of the initial structure for making the austenite structure fine in the annealing process by setting the rolling reduction of the final rolling in the hot rolling process to 30% or more and performing the cooling rapidly in the cooling process. Therefore, the ratio of the residual austenite crystal grains having a crystal grain size of 0.2 to 1.0 μm and the adjacent ferrite crystal grains was high, and better mechanical properties were exhibited.

On the other hand, steel plate No. which is a comparative example. No. 2 has a low heating rate in the annealing process. No. 4 has a short residence time at 700 to 800 ° C. in the annealing process. 6 and 10 had a long cooling time after rolling to 600 ° C. For this reason, the austenite grain growth cannot be effectively suppressed during the annealing process, the structure becomes coarse, and the ferrite adjacent to the residual austenite crystal grains having a crystal grain size of 0.2 to 1.0 μm. The proportion of crystal grains became lower. As a result, sufficient steel sheet elongation could not be obtained. Steel plate No. No. 8 has a slow cooling rate in the annealing process. In No. 9, since the soaking temperature in the annealing process was lower than Ac ₃ , the ferrite grains became coarse and sufficient mechanical properties could not be obtained. Steel plate No. In No. 9, since the degree of integration of the orientation groups {100} <011> to {211} <011> is high, the processed ferrite remained after the annealing step, and the mechanical properties were further deteriorated.

  In the case of steel plates manufactured using other steel types, the steel plate No. which is an example of the present invention is similarly applied. Nos. 11, 12, 14, 16, 20, 21, 23, 24, 26 and 28 were excellent in strength and workability because the chemical composition and metal structure satisfied the provisions of the present invention.

On the other hand, steel plate No. which is a comparative example. Nos. 13, 19, 25 and 27 have a long cooling time from rolling to 600 ° C. Nos. 15 and 29 have a low heating rate in the annealing process. No. 17 has a short residence time at 700 to 800 ° C. in the annealing process. In No. 22, the soaking temperature in the annealing process was higher than Ac ₃ points + 100 ° C. For this reason, the austenite grain growth cannot be effectively suppressed during the annealing process, the structure becomes coarse, and the ferrite adjacent to the residual austenite crystal grains having a crystal grain size of 0.2 to 1.0 μm. The proportion of crystal grains became lower. As a result, sufficient steel sheet elongation could not be obtained.

  Moreover, steel plate No. which is a comparative example. No. 18 had a low retention temperature in the overaging treatment step, resulting in a decrease in the area ratio of retained austenite and a marked deterioration in ductility.

  Furthermore, steel plate No. which is a comparative example. In Nos. 30 and 31, since the steel type G used does not contain Ti and Nb, the grain growth of austenite cannot be effectively suppressed during the annealing process, the structure becomes coarse, and the crystal grain size is 0.1. The ratio of the residual austenite crystal grains of 2 to 1.0 μm and the adjacent ferrite crystal grains was low. As a result, sufficient steel sheet elongation could not be obtained. Moreover, steel plate No. which is a comparative example. In Nos. 32 and 33, since the Si content of the steel type H used was smaller than the range specified in the present invention, the area ratio of retained austenite was low, and the ductility was extremely inferior.

According to the present invention, it becomes possible to effectively refine the structure after cold rolling and after annealing without containing a large amount of precipitation elements such as Ti and Nb, and has high strength, and A cold-rolled steel sheet having excellent ductility and stretch flangeability of the steel sheet and a method for producing the cold-rolled steel sheet can be provided. Therefore, the cold-rolled steel sheet according to the present invention can be suitably used as a steel sheet for automobiles and a steel sheet for building structures.

Claims (8)

Chemical composition is mass%,
C: 0.05 to 0.30%
Si: 0.5 to 2.5%
Mn: 0.5 to 3.5%
P: 0.100% or less,
S: 0.050% or less,
sol. Al: 0 to 1.00%,
Cr: 0 to 1.00%,
Mo: 0 to 0.30%,
V: 0 to 0.30%,
B: 0 to 0.0050%,
Ca: 0 to 0.003%, and
REM: 0 to 0.003%,
One or more selected from Ti: 0.080% or less and Nb: 0.050% or less;
Balance: Fe and impurities,
Satisfying the following formula (i)
Metal structure is area%,
Bainite: 50% or more
Retained austenite: 3.0% or more, and
Ferrite: 5.0% or more,
The average crystal grain size of the ferrite is 4.0 μm or less, and
A cold-rolled steel sheet having a ferrite grain ratio of 50% or more adjacent to the retained austenite crystal grains having a grain size of 0.2 to 1.0 μm among the ferrites.
0.003 ≦ Ti + Nb ≦ 0.100 (i)
However, each element symbol in the formula (i) represents the content (% by mass) of each element.   The cold-rolled steel sheet according to claim 1, wherein an average crystal grain size of the retained austenite is 1.0 μm or less. The chemical composition is mass%,
sol. Al: 0.10 to 1.00%,
The cold-rolled steel sheet according to claim 1 or 2, which contains The chemical composition is mass%,
Cr: 0.03-1.00%,
Mo: 0.01 to 0.30%, and
V: 0.01-0.30%,
The cold-rolled steel sheet according to any one of claims 1 to 3, comprising one or more selected from the above. The chemical composition is mass%,
B: 0.0003 to 0.0050%,
The cold-rolled steel sheet according to any one of claims 1 to 4, comprising: The chemical composition is mass%,
Ca: 0.0005 to 0.003%, and
REM: 0.0005 to 0.003%,
The cold-rolled steel sheet according to any one of claims 1 to 5, comprising one or more selected from the above. A method for producing a cold-rolled steel sheet according to any one of claims 1 to 6,
A hot rolling step of hot rolling the steel material having the chemical composition according to claim 1 or claim 3 to claim 6 at a rolling completion temperature of Ar ₃ or higher,
After the hot rolling step, it is cooled to 600 ° C. within 5 s, further cooled to 550 ° C. or lower and wound to produce a hot-rolled steel sheet,
A cold rolling step of cold rolling the hot rolled steel sheet at a total rolling reduction of 20% or more;
After the cold rolling step, a first heating step of heating to Ac ₁ point + 10 ° C. so that the average heating rate is 15 ° C./s or more in a temperature range from 500 ° C. to Ac ₁ point + 10 ° C., and then A second heating step of heating to a temperature range from Ac ₃ points to Ac ₃ points + 100 ° C. at an average heating rate of 0.2 to 2.0 ° C./s and holding for 30 s or more, the first heating step And an annealing step in which the second heating step is allowed to stay at 700 to 800 ° C. for 50 s or more, and further cooled so that the average cooling rate is 10 ° C./s or more in the temperature range from 650 ° C. to 500 ° C .;
The manufacturing method of a cold-rolled steel plate including the overaging treatment process hold | maintained at 500-300 degreeC for 10 s or more after the said annealing process.   The cold-rolled steel sheet according to claim 7, wherein in the hot rolling step, the rolling reduction in the final rolling is 30% or more, and in the cooling step, the hot rolling is cooled to 600 ° C within 1 s after the hot rolling is completed. Production method.