JP6379716B2 - Cold-rolled steel sheet and manufacturing method thereof - Google Patents

Description

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

  Regarding the technique for improving the mechanical properties of the cold-rolled steel sheet, Patent Document 1 discloses that the microstructure of the longitudinal section is made of steel having a specific chemical composition, and has a lath-like residue with bainitic ferrite as the main phase. A high-strength steel sheet excellent in elongation, stretch flangeability and weldability is shown in which the austenite is 3% or more and the block-like retained austenite is 1% to 1/2 of the lath-like retained austenite space factor. However, the block-like retained austenite in this steel sheet has an average particle size of 10 μm or less, specifically, as shown in the examples, it is as coarse as 2.2 to 8.9 μm, so it adversely affects the formability of the steel sheet. It is thought to affect.

Patent Document 2 has a specific chemical composition, and the temperature range from 500 ° C. to Ac ₁ transformation point is increased at an average temperature increase rate of 10 ° C./s or more, and from Ac ₁ transformation point to (Ac ₃ transformation point). + 30 ° C.) and kept for 10 s or more, then cooled to a temperature range of (Ms point−100 ° C.) to (Ms point−250 ° C.) at an average cooling rate of 10 ° C./s or more, 350 to A high-strength hot-dip galvanized steel sheet that has high strength and high workability and is excellent in impact resistance is obtained by reheating to 600 ° C. and holding for 1 to 600 seconds. The steel sheet obtained by this method is mainly composed of ferrite and tempered martensite, and the second phase is limited to a small amount of untempered martensite (hereinafter simply referred to as “martensite”), bainite and residual austenite. Organization. Thus, it is difficult to secure stretch flangeability (hole expansibility) in a steel plate composed of a large amount of ferrite and a hard second phase.

  Patent Document 3 discloses a method of cold rolling using a hot-rolled steel sheet produced by starting cooling in a short time after hot rolling. For example, after cooling to 720 ° C. or lower at a cooling rate of 400 ° C./second or higher within 0.4 seconds after hot rolling, by holding for 2 seconds or longer in a temperature range of 600 to 720 ° C., It is disclosed that a hot-rolled steel sheet having a fine structure with a small ferrite as the main phase is produced and subjected to ordinary cold rolling and annealing.

  Patent Document 4 has a specific chemical composition, contains at least 40 area% of martensite and bainite as the main phase, and has an average particle size as the second phase as necessary. A cold-rolled steel sheet having a microstructure containing 3% by area or more of ferrite of 4.0 μm or less and having high strength and good workability is shown.

JP 2007-32236 A International Publication No. 2009/054539 International Publication No. 2007/015541 International Publication No. 2013/125399

  The cold-rolled steel sheet according to the method disclosed in Patent Document 3 has a fine recrystallized structure in annealing after cold rolling by making the structure of the hot-rolled steel sheet, which is the material, fine. That is, if the fine hot-rolled steel sheet is used, austenite generated from the grain boundaries and the like can obtain a large number of nucleation numbers, so that a cold-rolled steel sheet having a fine structure can be obtained. However, since the annealing method after cold rolling is a normal one, recrystallization occurs in the heating process during annealing. That is, austenite transformation occurs after most of the preferential nucleation sites of austenite transformation such as large-angle grain boundaries and fine carbide particles existing in the hot-rolled steel sheet have disappeared during heating during annealing. Therefore, there is room for improvement in that the nucleation site existing in the fine structure of the hot-rolled steel sheet is fully utilized.

  In the case of the technique disclosed in Patent Document 4, in order to obtain a high-strength steel sheet, it is inevitable that the low-temperature transformation phase, which is the main phase, contains hard martensite, and the hardness distribution in the microstructure is relatively It was big. For this reason, there is room for further improvement in terms of reducing the hardness distribution in the microstructure and suppressing the occurrence of fine cracks when processing the steel sheet.

  On the other hand, the present invention effectively refines the structure after cold rolling and annealing without containing a large amount of precipitation elements such as Ti and Nb, and further, retained austenite (hereinafter referred to as “residual γ”). It is an object of the present invention to provide a cold-rolled steel sheet having high strength and excellent elongation and stretch flangeability, and a method for producing the same, by increasing and stabilizing the C concentration.

  As a structure for obtaining excellent elongation and stretch flangeability while having high strength, the present inventors have prepared a composite containing a proper amount of ferrite in a homogeneous microstructure mainly composed of tempered martensite and bainite. We focused on suppressing the development of specific texture that adversely affects elongation and stretch flangeability as well as the structure.

  Furthermore, a soft phase such as ferrite improves elongation, but there is a concern that the stretch flangeability (hole expandability) may be reduced. The study proceeded based on the organization design.

  As a method for obtaining the above-described structure, the present inventors have made various trials based on the following [1] and [2].

[1] In the annealing process after hot rolling, the austenite transformation is advanced before the recrystallization is completed, rather than the conventional annealing method in which the austenite transformation is advanced after the completion of the recrystallization. , Ac The strength of a specific texture is lowered by performing soaking of annealing at _three or more points. Further, the austenite is refined to refine the ferrite generated during cooling after annealing.

  [2] After annealing, cool to a temperature range below the Ms point and above the [Ms point-100 ° C] to generate an appropriate amount of martensite, and then reheat to a temperature range above the Ms point to burn the martensite. While returning, the bainite transformation is advanced. In this way, once martensite is generated and then reheated and transformed into bainite, the martensite generated by the cooling is tempered and softened, so that the hardness difference in the microstructure can be reduced. .

  As a result, the following new findings were obtained.

  1) In the conventional annealing method in which austenite transformation proceeds after completion of recrystallization, austenite transformation occurs mainly at the grain boundary of the structure after recrystallization, so the number of nucleation of austenite is small, and it is caused by reverse transformation during annealing. There is a limit to the refinement of austenite grains, that is, prior austenite grains after annealing (hereinafter also referred to as “old austenite grains”).

  On the other hand, according to the annealing method in which the austenite transformation is advanced before the completion of recrystallization by rapid heating to a temperature range where austenite is generated, large-angle grain boundaries and fine grain boundaries that are preferential nucleation sites for austenite transformation in hot rolled steel Since austenite transformation occurs from carbide particles, island martensite, and bainite, austenite grains generated in the annealing process are dramatically refined. As a result, the structure of the cold-rolled steel sheet after annealing is effectively refined.

  2) On the other hand, in the annealing method in which the austenite transformation is advanced before completion of recrystallization by rapid heating to a temperature range in which such austenite is generated, the processed ferrite structure tends to remain, so that a specific texture is developed. The tendency which the workability of a steel plate falls is shown.

On the other hand, when annealing is performed at a moderately high temperature range of Ac ₃ points or more, the processed ferrite structure can be eliminated while maintaining the fine austenite structure. Is suppressed, and excellent elongation and stretch flangeability can be secured.

  3) Although it becomes possible to improve the elongation of the cold-rolled steel sheet by containing ferrite, the structure containing a soft phase such as ferrite is formed from the interface between the soft phase and the hard phase when the steel sheet is processed. Since cracks tend to occur, the stretch flangeability generally tends to decrease.

  However, as described above, since the structure of the cold-rolled steel sheet after annealing is effectively refined, the ferrite is also refined, so that the generation and progress of fine cracks are effective when the steel sheet is processed. Therefore, a decrease in stretch flangeability is suppressed. For this reason, inclusion of fine ferrite makes it possible to improve elongation and ensure excellent stretch flangeability.

  4) It becomes possible to further improve the elongation of the cold-rolled steel sheet by including the residual γ that exhibits the effect of improving the elongation by the processing-induced transformation.

  In particular, in an annealing process after cold rolling, a steel sheet obtained by an annealing method in which austenite transformation proceeds before completion of recrystallization has a residual γ (hereinafter referred to as “lumpy residual” with an aspect ratio of less than 5 in the total residual γ. The fraction of “γ” is sometimes increased. This is presumably because residual γ existing on the prior austenite grain boundaries, packet boundaries, or block boundaries increases due to refinement of the prior austenite grains. Such a lump-like residual γ is more stable to processing strain than the residual γ generated between laths of bainite and martensite, and increases the work hardening coefficient in a high strain region. For this reason, the elongation of the steel sheet is effectively increased.

  However, in general, the residual γ is transformed into hard martensite by processing-induced transformation, which causes cracks when the steel sheet is processed. For this reason, a structure containing residual γ is generally concerned with a decrease in stretch flangeability. On the other hand, as described above, due to the refinement of the residual γ due to the effective refinement of the structure of the cold-rolled steel sheet after annealing and the increase in the fraction of the residual residual γ, the stretch flangeability is reduced. It is suppressed. For this reason, inclusion of fine residual γ having a small aspect ratio makes it possible to further improve the elongation and ensure excellent stretch flangeability.

  5) After holding the soaking in annealing, cooling to a temperature below the Ms point and above the [Ms point−100 ° C.] transforms part of the austenite to martensite, and then the temperature above the Ms point. Reheat to area. This transforms the remaining austenite into bainite. Furthermore, the martensite produced by the cooling is tempered by reheating and softens, whereby a high-strength and homogeneous microstructure can be obtained. As a result, the strength and stretch flangeability of the steel sheet can be improved.

  6) Furthermore, in the above-described bainite transformation, the concentration of C into untransformed austenite proceeds, the untransformed austenite is stabilized, and residual γ can be included in the cold-rolled steel sheet. Thereby, the elongation of the steel sheet can be remarkably improved.

  7) As described above, the annealing method in which the austenite transformation is advanced before the completion of recrystallization in the annealing step after cold rolling is performed by using large-angle grain boundaries and fine carbide particles that are preferential nucleation sites for austenite transformation in hot-rolled steel sheets. Further, nucleation of austenite transformation occurs from island-like martensite or bainite, and the prior austenite grains are effectively refined. Therefore, as a method for producing a hot-rolled steel sheet, a hot-rolled steel sheet that contains these austenite transformation preferential nucleation sites at a high density is suitable. As a manufacturing method of such a hot-rolled steel sheet, for example, a hot-rolled steel sheet obtained by a manufacturing method described in Patent Document 3 is cited. By applying the annealing method to this hot-rolled steel sheet, the austenite grains in the annealing process are further refined, and the structure of the cold-rolled steel sheet after annealing is further refined.

  The present inventors have found that, as a result of the above-described structure refinement, the high strength of the cold-rolled steel sheet can be secured and the balance between elongation and stretch flangeability can be significantly improved.

In the present invention, <1> chemical composition is mass%,
C: 0.06 to 0.3%, Si: 0.6 to 2.5%, Mn: 0.6 to 3.5%, P: 0.1% or less, S: 0.05% or less, Ti : 0 to 0.08%, Nb: 0 to 0.04%, total content of Ti and Nb: 0 to 0.10%, sol. Al: 0 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0 0.003%, REM: 0-0.003%, balance: Fe and impurities,
At the 1/4 depth position of the plate thickness,
The microstructure is tempered martensite and bainite as the main phase in a total of 50 area% or more, bainite is 10 area% or more, and ferrite satisfying the following formula (1) as the second phase is 5 area% or more, ,
The ratio of the nano hardness of the other structure to the ferrite is 2.5 or less,
At the half depth position of the plate thickness,
The average texture X-ray intensity of the orientation group of {100} <011> to {211} <011> is less than 6 in a ratio with respect to the average X-ray intensity of a random structure having no texture.
It is a cold-rolled steel sheet.
dF ≦ 4.0 (1)
However, dF in the above formula (1) is an average particle diameter (μm) of ferrite defined by a large-angle grain boundary having an inclination angle of 15 ° or more.

  The main phase in the microstructure means a phase having the largest area ratio, and the second phase means that all other phases are included.

  The cold-rolled steel sheet according to the present invention further has one or more features described in the following <2> to <9> as a more preferable state.

  <2> The microstructure contains 3% by area or more of retained austenite as the second phase.

<3> Among the retained austenite as the second phase, the retained austenite having an aspect ratio of less than 5 satisfies the following formulas (2) and (3).
dAs ≦ 1.5 (2)
rAs ≧ 50 (3)
However, dAs in the above formula (2) is an average particle size (μm) of retained austenite having an aspect ratio of less than 5, and rAs in formula (3) is based on the total retained austenite of retained austenite having an aspect ratio of less than 5. Area ratio (%).

  <4> The chemical composition contains, in mass%, at least one selected from Ti: 0.005 to 0.08% and Nb: 0.003 to 0.04%, and the total content of Ti and Nb The amount is 0.10% or less.

  <5> The chemical composition is mass%, and sol. Al: 0.1 to 1.0% is contained.

  <6> One or more types selected from the chemical composition is mass%, Cr: 0.03 to 1%, Mo: 0.01 to 0.3%, and V: 0.01 to 0.3%. contains.

  <7> The chemical composition is% by mass and contains B: 0.0003 to 0.005%.

  <8> The chemical composition contains one or more selected from Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.003% by mass%.

  <9> A plating layer is provided on the steel plate surface.

  Moreover, this invention is also a manufacturing method of the cold-rolled steel plate which has the following processes.

<10> After cold rolling the hot rolled steel sheet having the chemical composition according to any one of <1> and <4> to <8>, a temperature range up to [Ac ₁ point + 10 ° C.] After heating at an average heating rate of 15 ° C./second or higher, and then holding at a temperature in the temperature range of ₃ points or higher Ac [ ₃ points + 100 ° C.] for 10 seconds or higher, an average cooling rate of 10 ° C./second or higher And then cooled to a temperature in the temperature range below the Ms point and above the [Ms point−100 ° C.], and reheated to a temperature in the temperature range of 350 to 500 ° C. above the Ms point. At a position of 1/4 depth of the plate thickness where holding is performed for 10 seconds or more , the microstructure is tempered martensite and bainite as a main phase in total of 50 area% or more, and bainite is 10 area% or more, The ferrite satisfying the following formula (1) as the phase is 5 And the ratio of the nano hardness of the other structure to ferrite is 2.5 or less, and the texture is {100} <011> from {100} <011> at the 1/2 depth position of the plate thickness. 211} The method for producing a cold-rolled steel sheet, wherein the average X-ray intensity of the <011> orientation group is less than 6 as a ratio to the average X-ray intensity of a random structure having no texture .
dF ≦ 4.0 (1)
However, dF in the above formula (1) is an average particle diameter (μm) of ferrite defined by a large-angle grain boundary having an inclination angle of 15 ° or more.

  As a more preferable manufacturing method, the manufacturing method of the cold-rolled steel sheet according to the present invention further includes one or more features of <11> to <13> below.

  <11> The hot-rolled steel sheet is obtained by winding at 300 ° C. or less after completion of hot rolling, and then performing heat treatment in a temperature range of 500 to 700 ° C.

<12> After completion of hot rolling in which the hot-rolled steel sheet completes rolling at ₃ or more points of Ar, a cooling rate Crate (T) satisfying the following formula (4) is applied to the rolling completion temperature from [rolling completion temperature−100. The average grain size of the BCC phase defined by the large-angle grain boundaries having an inclination angle of 15 ° or more obtained by a hot rolling process for cooling the temperature range up to [° C.] is 6 μm or less.

In the above formula (4),
IC (T) = 0.1-3 × 10 ⁻³ × T + 4 × 10 ⁻⁵ × T ² −5 × 10 ⁻⁷ × T ³ + 5 × 10 ⁻⁹ × T ⁴ −7 × 10 ⁻¹¹ × T ⁵
Crate (T) is a cooling rate (° C./second) and is a positive value.
T is a relative temperature at which the rolling completion temperature is zero (° C.), that is, T = [temperature of the steel sheet during cooling (° C.) − Rolling completion temperature (° C.)], and is a negative value,
When there is a temperature at which Crate (T) is zero, a value obtained by dividing the residence time (Δt) at that temperature by IC (T) is added as the integral of that interval.

  <13> Exceeding the Ms point of the above <10> and reheating to a temperature in the temperature range of 350 to 500 ° C. After holding at that temperature for 10 seconds or more, the cold-rolled steel sheet is plated It further has a process.

  According to the present invention, it is possible to effectively refine the structure after cold rolling and annealing without containing a large amount of precipitation elements such as Ti and Nb, and high strength excellent in elongation and stretch flangeability. A cold-rolled steel sheet and a manufacturing method thereof can be realized.

  Hereinafter, the cold-rolled steel sheet and the manufacturing method thereof according to the present invention will be described. In the following description, “%” relating to chemical composition is all “mass%”. Further, in the present invention, the average particle diameter means an average value of equivalent circle diameters obtained by the equation (5) using SEM-EBSD.

1. Chemical composition of steel sheet C: 0.06 to 0.3%
C has the effect | action which raises the intensity | strength of steel. C also has an effect of stabilizing austenite by concentrating in austenite, increasing the area ratio of residual γ in the cold-rolled steel sheet, and improving elongation. Furthermore, in a hot rolling process and an annealing process, it has the effect | action which refines | miniaturizes a microstructure. That is, C has an effect of lowering the transformation point. As a result, in the hot rolling process, the hot rolling can be completed at a lower temperature range, and the microstructure of the hot rolled steel sheet can be refined. In the annealing process, combined with the effect of suppressing the recrystallization of ferrite in the temperature rising process by C, the temperature range of [Ac ₁ point + 10 ° C.] or higher while maintaining a high unrecrystallized ratio of ferrite by rapid heating Therefore, it becomes possible to refine the microstructure of the cold-rolled steel sheet. However, if the C content is less than 0.06%, it is difficult to obtain the effect of the above action. On the other hand, when the C content exceeds 0.3%, the workability and weldability of the cold-rolled steel sheet are significantly deteriorated. Therefore, the C content is 0.06 to 0.3%. The C content is preferably 0.10% or more, and more preferably 0.16% or more. The C content is preferably 0.25% or less.

Si: 0.6 to 2.5%
In order to improve the hardenability of steel, Si suppresses the formation of pearlite during cooling after annealing in the present invention, thereby enabling austenite to be kept at a low temperature and promoting the formation of martensite and bainite. effective. Therefore, it has the effect of increasing the strength of the steel by promoting the formation of tempered martensite and bainite that form the main phase of the cold-rolled steel sheet according to the present invention. Furthermore, it has the effect | action which promotes the production | generation of residual (gamma) and improves elongation of steel. However, if the Si content is less than 0.6%, an excessive amount of pearlite and ferrite are generated during cooling, so that tempered martensite and bainite cannot be obtained in a total area of 50 area% or more. On the other hand, if the Si content exceeds 2.5%, the stretch flangeability may be significantly deteriorated or the plating property may be impaired. Therefore, the Si content is set to 0.6 to 2.5%. The Si content is preferably 0.8% or more, and more preferably 1.0% or more. Moreover, it is preferable that content of Si is 2.0% or less.

Mn: 0.6 to 3.5%
Mn has the effect | action which raises the intensity | strength of steel. Moreover, since Mn has the effect | action which lowers | transforms a transformation temperature, it is set as the temperature range more than [Ac ₁ point +10 degreeC] in the annealing process, maintaining the state in which the unrecrystallization rate of a ferrite is high by rapid heating. It becomes easy, and this makes it possible to refine the microstructure of the cold-rolled steel sheet. However, if the Mn content is less than 0.6%, it is difficult to obtain the effect by the above action. On the other hand, if the Mn content exceeds 3.5%, the steel is excessively strengthened and the elongation may be significantly impaired. Therefore, the Mn content is set to 0.6 to 3.5%. The Mn content is preferably 0.8% or more, and more preferably 1.0% or more. The Mn content is preferably 3.0% or less.

P: 0.1% or less P is contained as an impurity and segregates at grain boundaries to embrittle the material. When the amount exceeds 0.1%, embrittlement becomes significant. Therefore, an upper limit is set for the P content to 0.1% or less. The P content is preferably 0.06% or less. In addition, since it is preferable that the P content is as low as possible, it is not necessary to provide a lower limit, but an extreme reduction leads to an increase in manufacturing cost. Therefore, the desirable lower limit of the P content is 0.001%.

S: 0.05% or less S is contained as an impurity, and forms sulphide inclusions in the steel to reduce workability. If the amount exceeds 0.05%, the workability is significantly reduced. There is a case. Therefore, an upper limit is set for the S content to 0.05% or less. The S content is preferably 0.008% or less, more preferably 0.003% or less. Note that the lower the S content, the better. Therefore, it is not necessary to provide a lower limit, but extreme reduction leads to an increase in manufacturing cost. Therefore, the desirable lower limit of the S content is 0.001%.

Ti: 0 to 0.08%, Nb: 0 to 0.04%, total content of Ti and Nb: 0 to 0.10%
Ti and Nb precipitate in the steel as carbides and nitrides, and have the effect of promoting refinement of the steel structure by suppressing the austenite grain growth in the annealing process. Therefore, you may contain 1 or more types of Ti and Nb as needed. However, when the Ti and Nb contents exceed 0.08% and 0.04%, respectively, or the total contents thereof exceed 0.10%, the workability may be significantly deteriorated. Therefore, the upper limits of Ti, Nb and the total amount thereof when contained are 0.08%, 0.04% and 0.10%, respectively. Here, the Ti content is preferably 0.05% or less, and more preferably 0.03% or less. Moreover, it is preferable that Nb content shall be 0.02% or less.

  In order to stably obtain the above-described effects of Ti and Nb, the Ti content is preferably 0.005% or more. The Nb content is preferably 0.003% or more.

  Said Ti and Nb can be contained only in any 1 type or 2 types of composites. In addition, it is preferable that the total amount in the case of containing two types in combination is 0.007% or more. The total amount is preferably 0.05% or less, and more preferably 0.03% or less.

sol. Al: 0 to 1.0%
Al has an effect of increasing elongation. Therefore, Al may be contained as necessary. However, since Al raises the transformation point, the content of Al becomes high, especially sol. When the content of Al (“acid-soluble Al”) exceeds 1.0%, it is necessary to complete hot rolling in a higher temperature range. As a result, it is difficult to refine the structure of the hot-rolled steel sheet, and it is also difficult to refine the structure of the cold-rolled steel sheet. Therefore, sol. The Al content is 1.0% or less. Note that sol. The Al content is preferably 0.7% or less, and more preferably 0.5% or less. On the other hand, in order to stably obtain the above-described effects of Al, sol. The Al content is preferably 0.1% or more.

Cr: 0 to 1.0%, Mo: 0 to 0.3%, V: 0 to 0.3%
Cr, Mo and V all have the effect of increasing the strength of the steel. Further, Mo suppresses the growth of crystal grains and refines the structure, and V promotes the transformation to ferrite and improves the workability of the steel sheet. Therefore, you may contain 1 or more types of Cr, Mo, and V as needed. However, if the Cr content exceeds 1.0%, the ferrite transformation is excessively suppressed, and the target structure may not be ensured. Further, if the Mo content exceeds 0.3% or the V content exceeds 0.3%, a large amount of carbide precipitates, and the workability of the steel may deteriorate. Therefore, the upper limits of the Cr, Mo and V amounts when contained are 1.0%, 0.3% and 0.3%, respectively. In addition, it is preferable that content of Cr, Mo, and V shall be 0.8% or less, 0.25% or less, and 0.25% or less, respectively.

  In order to stably obtain the above-described effects of Cr, Mo, and V, the Cr content is preferably 0.03% or more. The Mo content is preferably 0.01% or more. Furthermore, the V content is preferably 0.01% or more.

  Said Cr, Mo, and V can be contained only in any 1 type or 2 or more types of composites. In addition, it is preferable that the total amount in the case of containing 2 or more types in combination is 1.0% or less.

B: 0 to 0.005%
B has the effect | action which raises the intensity | strength of steel by improving the hardenability of steel and promoting the production | generation of a martensite and a bainite. Therefore, you may contain B as needed. However, if the B content exceeds 0.005%, the steel may be excessively hardened, and the elongation may be significantly reduced. Therefore, the upper limit of the B amount in the case of inclusion is 0.005%. The B content is preferably 0.003% or less. On the other hand, in order to stably obtain the effect of B described above, the B content is preferably 0.0003% or more.

Ca: 0 to 0.003%, REM: 0 to 0.003%
Ca and REM have the effect | action which refines | miniaturizes the oxide and nitride which precipitate in the solidification process of molten steel, and improves the soundness of a slab. Therefore, you may contain 1 or more types of Ca and REM as needed. However, since both are expensive elements, the upper limit of the amount of Ca and REM in the case of inclusion is 0.003%. In addition, it is preferable that content of Ca and REM shall be 0.002% or less and 0.002% or less, respectively.

  On the other hand, in order to stably obtain the effect of Ca described above, the Ca content is preferably set to 0.0005% or more. Similarly, in order to stably obtain the effect of REM, the REM content is preferably set to 0.0005% or more.

  Said Ca and REM can be contained only in any 1 type, or 2 types of composites, and it is preferable to make 0.005% or less the total amount when combining and containing 2 types.

  “REM” is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of one or more elements of REM. Further, REM is generally contained in misch metal. For this reason, for example, it may be added in the form of misch metal and contained so that the amount of REM falls within the above range.

  The chemical composition of the steel sheet according to the present invention is the above-described elements, with the balance being Fe and impurities.

  The “impurity” refers to a material that is mixed due to various factors in the manufacturing process, including raw materials such as ore or scrap, when industrially manufacturing steel materials.

2. Cold rolled steel sheet 2-1: Microstructure In the cold rolled steel sheet according to the present invention, tempered martensite and bainite as a main phase are 50 area% or more in total and 10 areas of bainite at the 1/4 depth position of the sheet thickness. % Or more, ferrite that satisfies the following formula (1) as the second phase exhibits a microstructure of 5 area% or more.
dF ≦ 4.0 (1)
However, dF in the above formula (1) is an average particle diameter (μm) of ferrite defined by a large-angle grain boundary having an inclination angle of 15 ° or more.

2-1-1: Main phase:
By including tempered martensite and bainite at a high fraction in the microstructure, a homogeneous microstructure can be obtained while increasing the strength of the cold-rolled steel sheet. Can increase the sex.

  “Tempered martensite” according to the present invention means that martensite produced when cooled to a temperature in the temperature range below the Ms point and above [Ms point−100 ° C.] in the cooling process after annealing, , A structure that has been tempered by being reheated and held at a temperature in the temperature range of 350 to 500 ° C. beyond the Ms point. Tempered martensite is a softer and more toughened structure than martensite, because the solid solution of carbon precipitates as cementite. By including this tempered martensite structure, the hardness difference in the microstructure is reduced as compared with martensite. Specifically, the ratio of the nano hardness of the other structure to the ferrite is 2.5 or less at the 1/4 depth position of the plate thickness, and the hardness difference in the microstructure is reduced. For this reason, when a cold-rolled steel plate is processed, a fine crack is hard to generate | occur | produce and the intensity | strength of a cold-rolled steel plate can be raised, without having a big bad influence on stretch flangeability. The tempered martensite contained in the microstructure as the main phase is preferably present in an amount of 10 area% or more, and more preferably 20 area% or more.

  The “bainite” according to the present invention is reheated during cooling after annealing, or after cooling to a temperature in the temperature range below the Ms point and exceeding [Ms point−100 ° C.], exceeding the Ms point, and Although it is a structure that is generated when held at a temperature in the temperature range of 350 to 500 ° C. and has a lower hardness than the above-described tempered martensite according to the present invention, a certain elongation is ensured. By including this structure, high strength can be obtained while ensuring good processability. In order to obtain the above effect, bainite contained in the microstructure as the main phase needs to be present in an area of 10% by area or more. Preferably it is 20 area% or more. Further, when the bainite transformation proceeds, carbon is concentrated in the untransformed austenite, and the retained austenite can be included in the structure of the cold-rolled steel sheet, whereby the elongation of the cold-rolled steel sheet can be improved. The above effect can be stably obtained by including the above-mentioned amount of Si in the steel.

  If the total area ratio of tempered martensite and bainite is less than 50%, the hardness distribution in the microstructure becomes large, and fine cracks are likely to occur during processing deformation, making it difficult to obtain excellent stretch flangeability. . Therefore, the microstructure needs to be 50 area% or more in total of tempered martensite and bainite as main phases. In addition, it is preferable that a tempered martensite and a bainite are 60 area% or more in total as a main phase. The total mentioned above may be 95 area%, but is preferably 92 area% or less in order to include ferrite and residual γ in the second phase. Note that bainite contains bainitic ferrite.

2-1-2: Second Phase Next, the microstructure is a ferrite satisfying the above formula (1) as the second phase, specifically, an average particle diameter defined by a large-angle grain boundary having an inclination angle of 15 ° or more. Ferrite having a dF of 4.0 μm or less must be present in an area of 5 area% or more. By including the above ferrite as the second phase in the microstructure, the elongation of the cold-rolled steel sheet can be increased.

  That is, as the second phase in the microstructure, the average particle diameter of ferrite defined by a large-angle grain boundary with an inclination angle of 15 ° or more (hereinafter simply referred to as “average diameter of ferrite”) dF is as fine as 4.0 μm or less. Then, since the generation | occurrence | production and progress of a fine crack are suppressed when a cold-rolled steel plate is processed, favorable stretch flangeability is obtained. Refinement of ferrite is achieved by rapidly heating in annealing to refine austenite during annealing. In addition, it is preferable that the average particle diameter dF of a ferrite is 3.5 micrometers or less, and it is so preferable that it is small, but when it becomes excessively fine, it becomes difficult to include 5 area% or more of ferrite. In the case of industrial production, about 0.1 μm is considered to be the lower limit.

  However, even if the average particle diameter dF of the ferrite is as small as 4.0 μm or less, if the area ratio of the ferrite is less than 5% as the second phase, good elongation cannot be imparted to the cold-rolled steel sheet. There is a case. Therefore, the ferrite described above is 5 area% or more as the second phase. In addition, even if the area ratio of the ferrite is too large, the stretch flangeability may be deteriorated. Therefore, the ferrite as the second phase is desirably less than 40 area%, and less than 30 area%. More desirable.

  The microstructure preferably contains 3% by area or more of residual γ as the second phase. By including the residual γ in the above ratio in the microstructure, it is possible to ensure a further excellent elongation in the cold-rolled steel sheet. A more preferable area ratio of residual γ is 5% or more. The area ratio of residual γ is preferably 25% or less.

  Among the residual γ, the massive residual γ satisfying the above formula (2) (residual γ having an aspect ratio of less than 5) has the effect of greatly improving the elongation of the cold-rolled steel sheet, and because it is fine, it has a stretch flange. Does not adversely affect sex. For this reason, by including the above-mentioned massive residual γ in the microstructure so as to satisfy the formula (3), it is possible to impart more excellent elongation to the cold-rolled steel sheet and ensure excellent stretch flangeability.

  Since the massive residual γ occupying most of the residual γ is fine, the generated martensite becomes fine after the residual γ is transformed into martensite during the processing of the cold-rolled steel sheet. For this reason, a decrease in stretch flangeability accompanying martensitic transformation is suppressed. In addition, the massive residual γ tends to be formed adjacent to the ferrite, and in order to make work hardening due to work-induced transformation more remarkable, the aspect ratio generated between laths such as martensite is an elongated shape exceeding 5 Compared to the residual γ, it is considered that the effect of improving the elongation, particularly the uniform elongation and the n value, is high. Since the bulk residual γ having such characteristics occupies most of the residual γ, the workability of the cold-rolled steel sheet can be improved.

  In the microstructure, as the second phase, the average particle diameter dAs of the massive residual γ is more preferably 1.0 μm or less, and the smaller is more preferable, but in the case of industrial production, about 0.03 μm is the lower limit. It is thought that it becomes.

  In the microstructure, as the second phase, the area ratio rAs of the massive residual γ to the total residual γ is more preferably 60% or more, and the more the more, the more preferable, but in the case of industrial production, 95 % Is considered the upper limit.

  In the present invention, the bainite transformation is promoted by forming a small amount of martensite when cooled to a temperature in the cooling range after annealing to a temperature below the Ms point and above the [Ms point−100 ° C.]. In addition, since the concentration of C to the residual γ further proceeds, the C concentration is high and the stability is enhanced, and the effect of remarkably improving the stretch flangeability is also obtained.

  As the second phase contained in the microstructure, there is, for example, martensite in addition to the above structure. Martensite tends to form when cooled to room temperature when austenite grains are not sufficiently stabilized during soaking after annealing reheating. The martensite fraction can be lowered by once transforming an appropriate amount of martensite before reheating and promoting the stabilization of residual γ. As a result, the hardness difference of the microstructure is reduced, and the hole expansion rate can be improved.

  As already described, in the present invention, the average particle diameter refers to the average value of equivalent circle diameters determined by the following formula (5) using SEM-EBSD. Note that 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). By analyzing the obtained orientation map, it is possible to calculate an average particle diameter as an average value of equivalent circle diameter.

  For example, the average particle diameter dF of ferrite can be determined from the average value of equivalent circle diameters obtained by the equation (5) for ferrite surrounded by large-angle grain boundaries with an inclination angle of 15 ° or more using SEM-EBSD. it can.

  The average particle diameter dAs of the aggregated residual γ having an aspect ratio of less than 5 can also be obtained by the same method.

  The area ratio of the main phase and the second phase can be measured by a structure analysis using SEM-EBSD. Further, the area ratio of the residual γ is obtained by directly using the volume fraction obtained by the X-ray diffraction method as the area ratio.

  In this invention, the measured value in the plate | board thickness 1/4 depth of a steel plate is employ | adopted about any above average particle diameter and area ratio.

2-2: Ratio of nano-hardness of other structure to ferrite The cold-rolled steel sheet according to the present invention has a ratio of nano-hardness of other structure to ferrite of ¼ depth position at a thickness of 2. 5 or less. When the above conditions are satisfied, the hardness difference in the microstructure is reduced, so that it is difficult for fine cracks to occur when the cold-rolled steel sheet is processed. The strength of the rolled steel sheet can be increased.

2-3: Texture In the cold-rolled steel sheet according to the present invention, {100} <in the 45 ° section of the orientation distribution function (hereinafter referred to as “ODF”) with respect to the texture at a half depth position of the sheet thickness. The average X-ray intensity of the orientation groups from 011> to {211} <011> is less than 6 in terms of the ratio to the average X-ray intensity of a random structure having no texture.

  When the texture of the orientation group from {100} <011> to {211} <011> develops, the workability of steel deteriorates. Therefore, the workability of steel is improved by reducing the X-ray intensity ratio of the orientation group. When the average X-ray intensity of the orientation group is 6 or more with respect to the average X-ray intensity of a random structure having no texture, it is difficult to ensure good elongation and stretch flangeability.

  Therefore, the average of the X-ray intensities of the above azimuth group is set to be less than 6 as a ratio to the average of the X-ray intensities of random tissues having no texture. This ratio is preferably less than 5, more preferably less than 4, and a value closer to 1 is preferable for improving stretch flangeability, but is actually influenced (inherited) by the texture of the structure before annealing. It is difficult to obtain a completely random structure, and about 2 is the lower limit. The texture {hkl} <uvw> represents a crystal orientation in which the vertical direction of the plate and the normal line of {hkl} are parallel and the rolling direction and <uvw> are parallel.

  The X-ray intensity in this specific direction is obtained by positively polishing the {200}, {110}, and {211} planes of the ferrite phase on the plate surface after chemically polishing the steel plate to 1/2 plate thickness with hydrofluoric acid. It is obtained by analyzing ODF by the series expansion method using the measured value.

  The X-ray intensity of a random structure not having a texture is obtained by performing the same measurement as described above using powdered steel.

2-4: Plating layer A plating layer may be provided on the surface of the above-described cold-rolled steel sheet for the purpose of improving the corrosion resistance and the like, and may be a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electro-Zn—Ni alloy plating. Examples of the hot dip plating layer include hot dip galvanizing, alloying hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. The

  The amount of plating adhesion is not particularly limited, and may be the same as the conventional one. In addition, it is possible to further enhance the corrosion resistance by forming an appropriate chemical conversion film on the plating surface, for example, by applying and drying a silicate-based chromium-free chemical conversion treatment liquid. Furthermore, it can be coated with an organic resin film.

3. Manufacturing Method 3-1: Hot Rolling for Hot Rolled Steel Sheet Production and Cooling after Rolling In the present invention, the structure of the cold rolled steel sheet is refined by annealing, which will be described later. You may use what was manufactured. However, in order to further refine the structure of the cold-rolled steel sheet, it is preferable to refine the structure of the hot-rolled steel sheet used for cold rolling. Specifically, it is preferable to refine the grains surrounded by the large-angle grain boundaries having an inclination angle of 15 ° or more and to finely disperse the second phase such as cementite and martensite. This is because the number of nucleation sites for the austenite transformation in annealing of the cold-rolled steel sheet can be increased.

  When hot-rolled steel sheets with a microstructure are cold-rolled and then rapidly heated and annealed, rapid heating can suppress the disappearance of nucleation sites due to recrystallization during the heating process, so there are many austenites and recrystallized ferrites. It becomes easier to nucleate and refine the final structure.

  In the present invention, the preferred hot-rolled steel sheet used for cold rolling specifically means that the average grain size of the BCC phase defined by the large-angle grain boundaries having an inclination angle of 15 ° or more satisfies 6 μm or less. The average particle size is preferably 5 μm or less, and this average particle size can also be determined by SEM-EBSD.

  When the average particle size of the BCC phase of the hot-rolled steel sheet is 6 μm or less, the cold-rolled steel sheet can be further refined, and the mechanical properties can be further improved. In addition, since the average particle diameter of the said BCC phase of a hot-rolled steel plate is so preferable that it is small, the minimum is not provided, but about 1.0 micrometer is considered to become a minimum in the case of normal industrial manufacture. The BCC phase used here includes ferrite, bainite and martensite, and consists of one or more of them. Although martensite is not exactly a BCC phase, it is treated as a BCC phase for convenience because the average particle size is determined by SEM-EBSD analysis.

  Such a hot-rolled steel sheet having a fine structure can be produced, for example, by the following hot rolling and cooling methods.

  A slab having the above-described chemical composition is produced by continuous casting, and this is subjected to hot rolling. At this time, the slab can be used while maintaining the high temperature during continuous casting, or it can be cooled to room temperature and then reheated.

  The heating temperature of the slab used for hot rolling is preferably 1000 ° C. or higher. That is, if the heating temperature of the slab is lower than 1000 ° C., in addition to giving an excessive load to the rolling mill, the temperature is lowered to the ferrite transformation temperature during rolling, and rolling is performed with the transformed ferrite contained in the structure. There is a risk that. From this, it is preferable that the heating temperature of the slab is set to 1000 ° C. or higher so that the hot rolling can be completed in the austenite temperature range, particularly in terms of product characteristics. However, heating to an excessively high temperature causes adverse effects such as an increase in scale loss and a decrease in yield, so the upper limit is preferably set to 1350 ° C.

Hot rolling is 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 last several stages. In order to maintain the steel sheet in the austenite temperature range during rolling, the rolling completion temperature needs to be Ar ₃ or higher.

The total rolling reduction of the hot rolling is preferably 40% or more in terms of sheet thickness reduction rate when the temperature of the material to be rolled is in the temperature range from Ar ₃ point to [Ar ₃ point + 150 ° C.]. This total reduction amount is more preferably 60% or more. Rolling does not have to be performed in one pass, and may be continuous multi-pass rolling. By increasing the reduction amount, more strain energy is introduced into the austenite, the transformation driving force to the BCC phase can be increased, and the BCC phase of 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 amount of reduction per pass is preferably 60%.

  The cooling after the end of rolling is preferably performed by the method described in detail below.

  First, the cooling from the rolling completion temperature is performed by cooling the temperature range from the rolling completion temperature to [rolling completion temperature−100 ° C.] at a cooling rate Crate (T) that satisfies the following formula (4).

As already stated, in the above formula (4),
IC (T) = 0.1-3 × 10 ⁻³ × T + 4 × 10 ⁻⁵ × T ² −5 × 10 ⁻⁷ × T ³ + 5 × 10 ⁻⁹ × T ⁴ −7 × 10 ⁻¹¹ × T ⁵
Crate (T) is a cooling rate (° C./second) and is a positive value.
T is a relative temperature at which the rolling completion temperature is zero (° C.), that is, T = [temperature of the steel sheet during cooling (° C.) − Rolling completion temperature (° C.)], and is a negative value,
When there is a temperature at which Crate (T) is zero, a value obtained by dividing the residence time (Δt) at that temperature by IC (T) is added as the integral of that interval.

  The above formula (4) is austenite non-recrystallization temperature range before the strain energy accumulated in the steel sheet by hot rolling is consumed by recovery / recrystallization after completion of hot rolling [rolling completion temperature− It represents the conditions for cooling to [100 ° C.]. Specifically, IC (T) is a value obtained from calculation related to body diffusion of Fe atoms, and represents the time from the completion of hot rolling to the start of austenite recovery. Further, [1 / (Crate (T) · IC (T))] is a value obtained by standardizing the time required for cooling at 1 ° C. at the cooling rate Crate (T) by IC (T), that is, recovery / recrystallization. Represents the fraction of the cooling time with respect to the time until the strain energy disappears. Therefore, the value obtained by integrating [1 / (Crate (T) · IC (T))] between T = 0 and −100 ° C. is an index representing the loss of strain energy during cooling. By limiting this value, the cooling conditions (cooling rate and residence time) necessary for cooling at 100 ° C. before the strain energy disappears by a certain amount are specified. The value on the right side of the above formula (4) is preferably 3.0, more preferably 2.0, and still more preferably 1.0. Note that the lower limit of the value on the left side of Equation (4) is about 0.11.

  In a preferred cooling method satisfying the above formula (4), the primary cooling from the rolling completion temperature is started at a cooling rate of 400 ° C./second or more, and a temperature section of 30 ° C. or more is cooled at this cooling rate. Is preferably performed. This temperature interval is preferably 60 ° C. or higher, more preferably 100 ° C. or higher. The cooling rate of the primary cooling is more preferably 600 ° C./second or more, and particularly preferably 800 ° C./second or more. This primary cooling can also be started after holding the rolling completion temperature for a short time of 5 seconds or less. However, since it will not satisfy | fill Formula (4) if it cools for a long time at rolling completion temperature, it is preferable that the time hold | maintained at rolling completion temperature shall be less than 0.4 second.

  In addition, cooling is started by water cooling at a cooling rate of 400 ° C./second or more (preferably 600 ° C./second or more, more preferably 800 ° C./second or more), and a temperature interval of 30 ° C. or more and 80 ° C. or less is set at this cooling rate. After cooling, a water cooling stop period of 0.2 second or more and 1.5 seconds or less (preferably 1 second or less) is provided, during which the plate shape such as the plate thickness and the plate width is measured, and then 50 ° C./second It is also preferable to perform cooling (secondary cooling) at the above speed. By measuring the plate shape in this manner, it is possible to perform feedback control of the plate shape, and productivity is improved. During the water cooling stop period, it may be cooled or air cooled.

  Both the primary cooling and the secondary cooling are industrially performed by water cooling.

  When the cooling condition from the rolling completion temperature to the temperature of [rolling completion temperature−100 ° C.] satisfies the above formula (4), the recovery of strain introduced into the austenite by hot rolling and the consumption due to recrystallization are suppressed as much as possible. Thus, the strain energy accumulated in the steel can be utilized to the maximum as the transformation driving force from the austenite to the BCC phase. If the cooling rate of the primary cooling from the rolling completion temperature is 400 ° C./second or more, it is effective to increase the transformation driving force in the same manner as described above. Thereby, the number of transformation nucleation from an austenite to a BCC phase 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.

  After performing primary cooling or primary cooling and secondary cooling as described above, and before cooling to the coiling temperature, the steel sheet is held for an arbitrary period of time, thereby comprising ferrite transformation, Nb, and Ti. Structure control such as precipitation of fine particles may be performed. Conditions of temperature and holding time suitable for tissue control are, for example, to cool for about 3 to 15 seconds in a temperature range of 600 to 680 ° C. Under this condition, fine ferrite can be introduced into the hot rolled sheet structure.

  Then, it cools to the coiling temperature of a steel plate. The cooling 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 set to 600 ° C. or less from the viewpoint of more surely refining the structure.

  The hot-rolled steel sheet produced by the above hot rolling and the cooling process after rolling has a sufficiently large number of large-angle grain boundaries introduced, and the average grain size defined by the large-angle grain boundaries with an inclination angle of 15 ° or more is 6 μm or less. It becomes a structure in which the second phase such as martensite and cementite is finely dispersed. As described above, it is preferable to subject the hot-rolled steel sheet having a large amount of large-angle grain boundaries and finely dispersed the second phase to cold rolling and annealing. Since these large-angle grain boundaries and the fine second phase are the preferential nucleation sites for the austenite transformation, it is possible to generate a large number of austenite and recrystallized ferrite from these positions by rapid thermal annealing, and to refine the structure. This is because it becomes possible.

  The structure of the hot-rolled steel sheet can be a ferrite structure containing pearlite as the second phase, a structure composed of bainite and martensite, or a mixed structure thereof.

3-2: Heat treatment of hot-rolled steel sheet The above-mentioned hot-rolled steel sheet may be heat-treated at a temperature of 500 to 700 ° C. This heat treatment is particularly suitable for hot-rolled steel sheets wound up at 300 ° C. or lower.

  This heat treatment can be performed by a method in which a hot-rolled coil is passed through a continuous annealing line or a method in which a coil is used in a batch annealing furnace. In heating the hot-rolled steel sheet, the heating rate up to the annealing temperature of 500 ° C. can be performed at any rate from the gradual heating of about 10 ° C./hour to the rapid heating of 30 ° C./second.

  The soaking temperature for heat treatment is in the range of 500 to 700 ° C. The holding time in this temperature range is not particularly limited, but is preferably 3 hours or more. The holding time is preferably 15 hours or shorter, more preferably 10 hours or shorter from the viewpoint of suppressing coarsening of the carbide.

  By performing such a heat treatment of the hot-rolled steel sheet, fine carbides can be dispersed at the grain boundaries, packet boundaries, and block boundaries in the hot-rolled steel sheet. By combining with, carbide can be dispersed more finely. As a result, the austenite nucleation sites can be increased during the heat treatment, and the final structure can be refined. The heat treatment of the hot-rolled steel sheet also has the effect of softening the hot-rolled steel sheet and reducing the load on the cold rolling equipment.

3-3: Pickling and cold rolling of hot-rolled steel sheet The hot-rolled steel sheet produced by the above method is pickled and then cold-rolled. These may be carried out by conventional methods. Cold rolling can be performed using a lubricating oil. The cold rolling rate need not be specified, but is usually 20% or more. If the cold rolling rate exceeds 85%, the burden on the cold rolling equipment increases, so the cold rolling rate is preferably 85% or less.

3-4: Annealing of Cold Rolled Steel Sheet In the annealing of the steel sheet obtained by the cold rolling described above, the temperature range up to [Ac ₁ point + 10 ° C.] is heated at an average heating rate of 15 ° C./second or more. By this treatment, the unrecrystallized ratio in the region not transformed to austenite when reaching [Ac ₁ point + 10 ° C.] becomes 30% or more. By heating to [Ac ₁ point + 10 ° C.] while leaving the non-recrystallized structure in the above proportion, a large number of fine austenite can be nucleated using the large angle grain boundaries and the second phase of the hot rolled steel sheet as nucleation sites. it can. At this time, it is preferable that the structure of the hot-rolled steel sheet is fine because more nucleation can be obtained. By increasing the number of nucleation of austenite, the austenite grains during annealing can be remarkably refined, and the ferrite, martensite and bainite substructures produced thereafter, and the residual γ can be refined. Can do. In addition, by reducing the austenite grains, the number of ferrite nucleation generated during cooling after annealing increases, so the area ratio of ferrite increases. Thereby, the elongation of the steel sheet is significantly improved.

On the other hand, when the non-recrystallization rate in the region not austenite transformed at the time of reaching [Ac ₁ point + 10 ° C.] is less than 30%, the region in which the austenite transformation has progressed after completion of recrystallization will occupy the majority. . As a result, since the austenite transformation proceeds from the grain boundaries of the recrystallized grains in such a region, the austenite grains being annealed become coarse and the final structure becomes coarse. Further, since the number of nucleation sites during cooling decreases, the area ratio of ferrite becomes very small or very coarse ferrite is generated. As a result, either elongation or stretch flangeability is deteriorated.

Accordingly, the average heating in the temperature range up to [Ac ₁ point + 10 ° C.] so that the non-recrystallization ratio in the region not austenite transformed at the time of reaching [Ac ₁ point + 10 ° C.] becomes 30% by area or more. The speed is 15 ° C./second or more. The average heating rate is preferably 30 ° C./second or more, more preferably 80 ° C./second or more, and particularly preferably 100 ° C./second or more. The upper limit of the average heating rate is not particularly set, but is preferably set to 1000 ° C./second or less in consideration of difficulty in temperature control. On the other hand, when the average heating rate is less than 15 ° C./second, the recrystallization of ferrite proceeds before reaching [Ac ₁ point + 10 ° C.], and the unrecrystallized ratio in the region not transformed to austenite is 30 area%. Will be less than. As a result, the number of nucleation of austenite decreases and austenite grains during annealing become coarse, so that the final structure becomes coarse or the area ratio of ferrite decreases.

  The temperature at which the rapid heating at 15 ° C./second or more is started is arbitrary as long as it is before the start of recrystallization. For example, even if the rapid heating is started from about 600 ° C., a sufficient fine graining effect can be obtained. Moreover, even if rapid heating is started from room temperature, the present invention is not adversely affected. Moreover, the heating rate in the temperature range before the rapid heating is started is also arbitrary.

  In order to obtain a sufficiently rapid heating rate, it is preferable to use energization heating, induction heating, or direct fire 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 the hot dipping line to carry out the heating.

After heating to [Ac ₁ point + 10 ° C.], it is further heated to a temperature in the temperature range of Ac ₃ point or more and [Ac ₃ point + 100 ° C.] or less as the annealing temperature and held for 10 seconds or more. By this treatment, the structure of the cold rolled steel sheet is made into an austenite single phase state. In the present invention, this treatment is called “annealing”. In addition, said heating rate can be made into arbitrary speeds. For example, the heating rate is changed so that only the first partial temperature range is the same rapid heating as the average heating rate of 15 ° C./second or more and the subsequent temperature range is a lower heating rate. The temperature can also be raised to the annealing temperature.

In the annealing process, the transformation to austenite is sufficiently advanced to eliminate the processed ferrite structure and dissolve carbides in the steel sheet. For this reason, annealing temperature shall be Ac ₃ points or more. The reason why an austenite single phase state is obtained is that the ferrite contained in the metal structure of the cold rolled steel sheet according to the present invention is a ferrite transformed from austenite during cooling after annealing. By forming ferrite, carbon is swept out to austenite during cooling, and a high concentration region of carbon is formed locally (near the boundary between ferrite and austenite). In the high concentration region of carbon, massive retained austenite is easily formed, and the proportion of massive retained austenite can be increased in the structure of the cold-rolled steel sheet. At the same time, through the austenite single phase state, the X-ray intensity of the orientation group of {100} <011> to {211} <011> in the texture of the cold-rolled steel sheet can be reduced. With these, excellent workability can be obtained. On the other hand, when annealing is performed at a temperature exceeding [Ac ₃ points + 100 ° C.], abrupt grain growth of austenite grains occurs, and the final structure becomes coarse. For this reason, after heating at an average heating rate of 15 ° C./second or higher up to [Ac ₁ point + 10 ° C.], it is further heated to a temperature within a range of Ac ₃ points or higher and [Ac ₃ points + 100 ° C.] or lower. . In order to refine the structure, the heating temperature of Ac ₃ point or higher as the annealing temperature is preferably [Ac ₃ point + 70 ° C.] or lower, and more preferably [Ac ₃ point + 50 ° C.] or lower.

The holding time at a temperature in the temperature range of Ac ₃ points or higher and [Ac ₃ points + 100 ° C.] or lower, which is the annealing temperature, is 10 seconds or longer. If Ac is ₃ points or more, by holding for 10 seconds or more, dissolution of carbide and transformation to austenite proceed sufficiently, and excellent workability can be obtained. On the other hand, if the time is less than 10 seconds, temperature unevenness during annealing is likely to occur, and soaking in the steel sheet becomes insufficient, causing a problem in the stability of product characteristics. Accordingly, the annealing holding time is 10 seconds or more.

  The upper limit of the annealing holding time is not particularly defined. However, when holding for an excessively long time, it becomes difficult to obtain the final structure defined by the present invention due to the growth of austenite grains. The time is preferably less than 10 minutes.

  After holding for 10 seconds or more at the above annealing temperature, cooling is performed at an average cooling rate of 10 ° C./second or more to a temperature in the temperature range below the Ms point and above [Ms point−100 ° C.]. By setting the average cooling rate in the temperature range to 10 ° C./second or more, the area ratio of tempered martensite and bainite in the final cold-rolled steel sheet structure can be increased. In addition, an appropriate amount of ferrite can be generated during cooling. On the other hand, when the cooling rate is less than 10 ° C./second, a large amount of ferrite is generated during cooling, and the stretch flangeability is deteriorated. Therefore, the cooling rate in the said temperature range after annealing holding shall be 10 degrees C / sec or more. Cooling can be performed by an arbitrary method, for example, cooling with gas, mist, or water is possible. In order to produce 5% by area or more of ferrite, the average cooling rate is preferably 500 ° C./second or less.

  By performing such cooling to a temperature in the temperature range below the Ms point and exceeding [Ms point−100 ° C.], after the martensitic transformation, the temperature range beyond the Ms point and 350 to 500 ° C. By reheating to a temperature of tempered, tempered martensite can be included in the structure of the cold-rolled steel sheet. Since this tempered martensite is softer than martensite and harder than bainite, the strength of the steel sheet can be increased while obtaining a homogeneous microstructure. Moreover, the bainite transformation which occurs after reheating can be promoted by causing the martensitic transformation. For this reason, the concentration of C into untransformed austenite accompanying the bainite transformation further proceeds, and the C concentration of residual γ in the cold-rolled steel sheet increases and can be stabilized. As a result, the elongation can be remarkably improved.

  In the present invention, the cooling stop temperature needs to be a temperature not higher than the Ms point and exceeding [Ms point−100 ° C.]. By stopping the cooling in the above range, the tempered martensite fraction of the cold-rolled steel sheet is controlled to an appropriate amount, and the balance between strength and workability can be stably improved. Furthermore, once martensite is generated, the bainite transformation is promoted and the concentration of C to residual γ further proceeds, so that the C concentration is high and the stability is enhanced. As a result, unstable austenite is prevented from transforming into martensite, the martensite fraction of the final structure is lowered, and the effect of significantly improving stretch flangeability is also obtained. When the cooling stop temperature is [Ms point −100 ° C.] or less, the fraction of tempered martensite in the final structure increases, and the fraction of bainite and residual γ is excessively reduced. A balance between strength and workability cannot be obtained. The cooling stop temperature is preferably set to a temperature not higher than the Ms point and higher than [Ms point−80 ° C.].

  In addition, in order to reduce the amount of martensite transformation due to temperature unevenness in the steel plate, the amount of martensite transformation can be made constant by maintaining a constant time at the cooling stop temperature and ensuring soaking of the steel plate. It is preferable for stabilizing the product characteristics.

  Following the above cooling, the austenite is transformed into bainite by reheating to a temperature in the temperature range of 350 to 500 ° C. beyond the Ms point and holding at that temperature for 10 seconds or more.

  When the reheating temperature exceeds 500 ° C., untransformed austenite is decomposed into carbides, so that excellent workability cannot be obtained. The retention time at 350 to 500 ° C. exceeding the Ms point after reheating is 10 seconds or more, and the bainite transformation is sufficiently advanced. Even if the holding time at a temperature in the above temperature range exceeds 2000 seconds, the untransformed austenite cannot be decomposed into carbides to obtain excellent workability, so the upper limit of the holding time is preferably 2000 seconds. In addition, after being reheated and held as described above, it is cooled as it is to room temperature by an appropriate method such as standing cooling or water cooling, and then further hot-dip plating such as hot-dip galvanization or alloyed hot-dip galvanization, for example. Processing can be performed. In addition, the above hot dipping treatment can be performed in succession to the above reheating and holding.

  Both the temperature and the cooling rate described in the above manufacturing method refer to the temperature and the cooling rate at the surface portion.

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

Steels A to J having the chemical compositions shown in Table 1 were melted in a vacuum induction furnace and then ingoted to produce ingots. Table 1 also shows Ar ₃ points, Ac ₁ point, and Ac ₃ points of steels A to J. Steels A to I in Table 1 are steels whose chemical compositions are within the range defined by the present invention, while Steel J is a steel whose chemical composition is outside the conditions defined by the present invention.

  Each of the ingots was hot forged and then cut into slab-shaped steel pieces for use in hot rolling. Each obtained slab was heated for 1 hour at a temperature of 1000 to 1300 ° C. according to its chemical composition, and then hot-rolled at a rolling completion temperature shown in Table 2 using a small test mill. In addition, after completion of hot rolling, a hot rolled steel sheet having a thickness of 2.0 to 2.6 mm was produced under various conditions shown in the same table.

Cooling after the completion of rolling was water cooling, and was carried out by one of the following methods.
1) Immediately after completion of rolling, only primary cooling is performed with a temperature drop exceeding 100 ° C .;
2) After cooling at rolling completion temperature (FT) for a predetermined time, only primary cooling is performed with a temperature drop exceeding 100 ° C .; or 3) Primary cooling is performed immediately after completion of rolling, and rolling completion temperature (FT) Then, the primary cooling is stopped at the stage of cooling at 80 ° C., and the mixture is allowed to cool at that temperature for a predetermined time, and then the secondary cooling is performed.

  After stopping the primary cooling in the above conditions 1) and 2), or after stopping the secondary cooling in 3), the mixture is allowed to cool for 3 to 15 seconds, and further cooled with water at a cooling rate of 30 to 100 ° C./second, Cooled to coiling temperature. Thereafter, the steel sheet was placed in a furnace and subjected to slow cooling simulating winding. The value of the left side of Formula (4) and the average particle diameter of the BCC phase of the hot-rolled steel sheet are also shown in Table 2.

  The average grain size of the BCC phase of the hot-rolled steel sheet is measured by using a SEM-EBSD device (JSM-7001F, manufactured by JEOL Ltd.) as a cross-sectional structure in the width direction of the steel sheet. It was calculated | required by analyzing the particle size of the BCC phase prescribed | regulated by. The particle diameter d was determined using the following formula (5).

  In the above formula (5), Ai represents the area of the i-th grain, and di represents the equivalent circle diameter of the i-th grain.

  Some hot-rolled steel sheets were subjected to hot-rolled sheet heat treatment under the conditions shown in Table 2 using a heating furnace.

  The hot-rolled steel sheet thus obtained was pickled with hydrochloric acid and cold-rolled at the rolling reduction shown in Table 2 to make the steel sheet thickness 1.0 to 1.3 mm.

Subsequently, the cold-rolled steel sheet having a thickness of 1.0 to 1.3 mm obtained as described above was subjected to an average heating rate, a soaking temperature (annealing temperature), and a soaking temperature shown in Table 2 using an experimental annealing apparatus. After annealing at the time (holding time), it was cooled to the temperature described as “cooling stop temperature” in the same table at “average cooling rate” described in Table 2. Further, reheating from the cooling stop temperature to raise the temperature to 400 ° C. or 425 ° C., and at that temperature, over-aging treatment for holding soaking for 330 seconds, or alloying hot dip galvanizing treatment after holding soaking for 60 seconds After performing heat treatment simulating the above, it was cooled to room temperature at 2 ° C./second to obtain a cold-rolled steel sheet. Specific conditions for the heat treatment applied to each steel plate are the following A to D.
A: Over-aging treatment of holding at 400 ° C. for 330 seconds.
B: Over-aging treatment of holding at 425 ° C. for 330 seconds.
C: Heat treatment simulating an galvanizing bath immersion by heating to 460 ° C., followed by an alloying treatment heated to 500 ° C., after an overaging treatment at 400 ° C. for 60 seconds.
D: A heat treatment in which, after an over-aging treatment of holding at 425 ° C. for 60 seconds, heating to 460 ° C. simulates hot dip galvanizing bath immersion, and further heats to 500 ° C. to simulate alloying treatment.

  The Ms point takes a different value depending on the annealing temperature, the amount of ferrite generated during cooling, and the like. For this reason, in Table 2, Ms point calculated | required from the thermal expansion curve measured when the steel plate which cold-rolled was cooled from predetermined | prescribed annealing temperature was described. In Table 2, the above A and B are described as “No” in the “Plating treatment” column, and C and D are described as “Yes” in the “Plating treatment” column.

Table 2 shows the unrecrystallized ratio in the region not transformed to austenite in the ferrite when [Ac ₁ point + 10 ° C.] is reached. This value was determined by the following method. That is, using a steel sheet that had been cold-rolled according to the production conditions of the present invention, the temperature was raised to [Ac ₁ point + 10 ° C.] at the heating rate shown in each example, and then immediately cooled with water. The structure was photographed by SEM, and the region excluding martensite on the structure photograph, that is, the region excluding the region that was transformed to austenite when [Ac ₁ point + 10 ° C.] was reached, By measuring the fraction, the unrecrystallized rate was determined.

  The microstructure, texture and mechanical properties of the cold-rolled steel sheet obtained as described above were examined as follows.

  The average grain size of ferrite and the average grain size of residual γ in the cold-rolled steel sheet are the same as those described for the hot-rolled steel sheet in the cross-sectional structure in the width direction at the ¼ depth position of the steel sheet. -Determined using EBSD device. In the EBSD analysis of a tissue containing a residual γ phase, there is a concern that the residual γ cannot be measured accurately due to an error caused by a sample preparation method or the like. For this reason, in this embodiment, the residual γ area fraction (γEBSD) obtained by EBSD analysis as an index of analysis accuracy is expressed as follows with respect to the volume fraction (γXRD) of residual γ obtained by X-ray diffraction described later ( The evaluation was premised on satisfying γEBSD / γXRD)> 0.7.

  The total area ratio of tempered martensite and bainite, which are main phases, and the area ratio of ferrite were determined by structural analysis using SEM-EBSD. Furthermore, the ratio of the tempered martensite and bainite contained in the main phase is determined by measuring the thermal expansion during cooling of the annealing of the corresponding steel material under the same annealing conditions, and the amount of expansion below the Ms point and the Ms point. The ratio of the amount of expansion above 500 ° C. was obtained, and obtained by multiplying the above total area ratio by the respective ratio. Further, the volume ratio of the austenite phase was determined by the X-ray diffraction method described later, and this was defined as the area ratio of residual γ. In the microstructure, the portion excluding the tempered martensite, bainite, ferrite and residual γ is mainly composed of martensite.

  The nano hardness in the microstructure was measured using TRIBOSCOPE manufactured by HYSTRON at the position of the steel sheet thickness ¼ depth. In other words, the unevenness of the microstructure on the sample surface was investigated in advance with a TRIBOSCOPE cantilever, and after identifying the ferrite and other structures in the microstructure, the hardness of each structure was measured at 15 points, and the average value was calculated. The tissue was nano-hard.

  In addition, the ratio of the nano hardness of other tissues to the nano hardness of the ferrite was determined. The “other structure” includes tempered martensite and bainite which are main phases, martensite, and residual γ. In addition, the nano hardness of the ferrite in this measurement was 2.0 to 3.5 GPa, and the nano hardness of other tissues was 4.5 to 16.0 GPa. The test force (indentation force) for measurement was 1000 μN.

  In Table 3, “−” in “Ferrite” in the “Average particle size (μm)” column indicates that the fraction is lower than the conditions defined in the present invention, and thus is excluded from the evaluation. “-” In the “Nano hardness ratio” column indicates that the area ratio of ferrite is small, and when measuring the hardness, it interferes with the surrounding bainite, etc., and the exact hardness cannot be measured. It was outside.

  The texture of the cold-rolled steel sheet was measured by X-ray diffraction in a plane at a half depth position of the sheet thickness. The strength of the orientation group from {100} <011> to {211} <011> is an ODF analyzed from the measurement results of the {200}, {110}, and {211} positive pole plots of ferrite, and {100} < 011>, {411} <011>, and {211} <011> as the average intensity. In the X-ray measurement, the X-ray intensity of a random structure having no required texture was obtained by X-ray diffraction of powdered steel. The apparatus used was RINT-2500HL / PC manufactured by Rigaku Electronics Co., Ltd.

The mechanical properties of the cold rolled steel sheet were investigated by a tensile test and a hole expansion test. For the tensile test, No. 5 test piece described in JIS Z 2241: 2011 was used, and tensile strength (hereinafter also referred to as “TS”. The unit is “MPa”) and elongation at break (total elongation, hereinafter “ Also referred to as “El.” The unit is “%”. The hole expansion test was performed according to JIS Z 2256: 2010, and the hole expansion ratio (hereinafter also referred to as “λ”. The unit is “%”) was obtained. The value of [TS × El] was calculated as an index of balance between strength and elongation, and the value of [TS × λ] was calculated as an index of balance between strength and stretch flangeability. The mechanical properties that the cold-rolled steel sheet of the present invention should satisfy are that the balance of strength, elongation, and stretch flangeability is at a high level, and therefore all the following formulas (6) to (8) are satisfied. Aimed at.
TS × EL> 19000 (6),
TS × λ> 40000 (7),
5.45 × (TS × EL) + (TS × λ)> 172000 (8).

  Table 3 summarizes the above survey results.

  From Table 3, the “invention example” steel plate No. 1 satisfying the requirements of the present invention. In this case, it is clear that all are the high-strength cold-rolled steel sheets that satisfy all of the above formulas (6) to (8) and have an excellent balance between elongation and stretch flangeability.

  On the other hand, the “Comparative Example” steel plate No. 1 deviating from the requirements of the present invention. As for the case of using steel J whose chemical composition deviates from the conditions specified in the present invention, as well as in the case of using steels A to J whose chemical composition is within the range specified in the present invention, at least the above formula (6 ) To (8) and does not satisfy any of the above steel plate Nos. It is inferior to the balance of elongation and stretch flangeability compared with the case of.

According to the present invention, it is possible to effectively refine the structure after cold rolling and annealing without containing a large amount of precipitation elements such as Ti and Nb, and high strength excellent in elongation and stretch flangeability. A cold-rolled steel sheet and a manufacturing method thereof can be realized.

Claims (13)

Chemical composition is mass%,
C: 0.06 to 0.3%, Si: 0.6 to 2.5%, Mn: 0.6 to 3.5%, P: 0.1% or less, S: 0.05% or less, Ti : 0 to 0.08%, Nb: 0 to 0.04%, total content of Ti and Nb: 0 to 0.10%, sol. Al: 0 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0 0.003%, REM: 0-0.003%, balance: Fe and impurities,
At the 1/4 depth position of the plate thickness,
The microstructure is tempered martensite and bainite as the main phase in a total of 50 area% or more, bainite is 10 area% or more, and ferrite satisfying the following formula (1) as the second phase is 5 area% or more, ,
The ratio of the nano hardness of the other structure to the ferrite is 2.5 or less,
At the half depth position of the plate thickness,
The average texture X-ray intensity of the orientation group of {100} <011> to {211} <011> is less than 6 in a ratio with respect to the average X-ray intensity of a random structure having no texture.
Cold rolled steel sheet.
dF ≦ 4.0 (1)
However, dF in the above formula (1) is an average particle diameter (μm) of ferrite defined by a large-angle grain boundary having an inclination angle of 15 ° or more.   The cold-rolled steel sheet according to claim 1, wherein the microstructure contains 3 area% or more of retained austenite as the second phase. The cold-rolled steel sheet according to claim 2, wherein the retained austenite having an aspect ratio of less than 5 satisfies the following formulas (2) and (3) in the retained austenite as the second phase.
dAs ≦ 1.5 (2)
rAs ≧ 50 (3)
However, dAs in the above formula (2) is an average particle size (μm) of retained austenite having an aspect ratio of less than 5, and rAs in formula (3) is based on the total retained austenite of retained austenite having an aspect ratio of less than 5. Area ratio (%).   The chemical composition contains one or more selected from Ti: 0.005 to 0.08% and Nb: 0.003 to 0.04% by mass%, and the total content of Ti and Nb is 0. The cold-rolled steel sheet according to any one of claims 1 to 3, which is 10% or less.   The chemical composition is mass% and sol. The cold-rolled steel sheet according to any one of claims 1 to 4, comprising Al: 0.1 to 1.0%.   The chemical composition contains, in mass%, at least one selected from Cr: 0.03 to 1%, Mo: 0.01 to 0.3% and V: 0.01 to 0.3%. The cold-rolled steel sheet according to any one of claims 1 to 5.   The cold-rolled steel sheet according to any one of claims 1 to 6, wherein the chemical composition contains B: 0.0003 to 0.005% in mass%.   The chemical composition according to any one of claims 1 to 7, wherein the chemical composition contains one or more selected from Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.003% in mass%. Cold-rolled steel sheet as described in 1.   The cold-rolled steel sheet according to any one of claims 1 to 8, which has a plating layer on the steel sheet surface. After cold rolling the hot-rolled steel sheet having the chemical composition according to any one of claims 1 and 4 to 8, the temperature range up to [Ac ₁ point + 10 ° C] is 15 ° C / second or more. After heating at an average heating rate, and then holding at a temperature in the temperature range of Ac ₃ points or more and [Ac ₃ points + 100 ° C.] or less for 10 seconds or more, an average cooling rate of 10 ° C./second or more and an Ms point or less, And it is cooled to a temperature in the temperature range exceeding [Ms point−100 ° C.], and is further reheated to a temperature in the temperature range of 350 to 500 ° C. exceeding the Ms point, and kept at that temperature for 10 seconds or more. At a ¼ depth position of the plate thickness, the microstructure is tempered martensite and bainite as the main phase in total of 50 area% or more, and bainite is 10 area% or more, and the second phase is represented by the following formula (1 ) Ferrite that satisfies 5) Further, the ratio of the nanohardness of the other structure to the ferrite is 2.5 or less, and the texture is {100} <011> to {211} <011> at the 1/2 depth position of the plate thickness. A method for producing a cold-rolled steel sheet, wherein the average X-ray intensity of the orientation group is less than 6 in a ratio to the average X-ray intensity of a random structure having no texture .
dF ≦ 4.0 (1)
However, dF in the above formula (1) is an average particle diameter (μm) of ferrite defined by a large-angle grain boundary having an inclination angle of 15 ° or more.
  The cold-rolled steel sheet according to claim 10, wherein the hot-rolled steel sheet is obtained by winding at a temperature of 300 ° C or less after completion of hot rolling, and then performing a heat treatment in a temperature range of 500 to 700 ° C. Production method. After completion of hot rolling, the hot-rolled steel sheet completes rolling at ₃ or more points of Ar, at a cooling rate Crate (T) satisfying the following formula (4), from the rolling completion temperature to [rolling completion temperature −100 ° C.]. The cooling according to claim 10 or 11, wherein the average particle diameter of the BCC phase obtained by a hot rolling step for cooling the temperature range of BCC phase defined by a large angle grain boundary having an inclination angle of 15 ° or more is 6 µm or less. A method for producing rolled steel sheets.

In the above formula (4),
IC (T) = 0.1-3 × 10 ⁻³ × T + 4 × 10 ⁻⁵ × T ² −5 × 10 ⁻⁷ × T ³ + 5 × 10 ⁻⁹ × T ⁴ −7 × 10 ⁻¹¹ × T ⁵
Crate (T) is a cooling rate (° C./second) and is a positive value.
T is a relative temperature (° C.) at which the rolling completion temperature is zero, and is a negative value.
When there is a temperature at which Crate (T) is zero, a value obtained by dividing the residence time (Δt) at that temperature by IC (T) is added as the integral of that interval. The method further includes a step of plating the cold-rolled steel sheet after reheating to a temperature in the temperature range of 350 to 500 ° C. exceeding the Ms point of claim 10 and holding at that temperature for 10 seconds or more. The manufacturing method of the cold-rolled steel plate in any one of Claim 10-12.