JP2016028172A - Cold rolled steel sheet and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet combining high strength with excellent workability, and a production method therefor.SOLUTION: The cold rolled steel sheet is provided which has a chemical composition that contains, by mass%, C:0.05 to 0.30%, Si:0.5 to 2.5%, Mn:0.5 to 3.5%, P:0.1% or less, S:0.05% or less, sol.Al:0 to 1.0%, Ti:0 to 0.050%, Nb:0 to 0.030%, Cr:0 to 1.0%, Mo:0 to 0.3%, V:0 to 0.3%, B:0 to 0.005%, Ca:0 to 0.003%, REM:0 to 0.003% and the balance Fe with impurities, and that satisfies [0≤Ti+Nb≤0.070]. The cold rolled steel sheet having the above chemical composition has metallographic structure that contains, by area ratio, 50% or more of bainite and tempered martensite in total and 3.0% or more of retained austenite and that has a ratio of the retained austenite in which other retained austenite having crystal orientation with difference of 10° or more is present in a range of most proximity distance of 1 μm or less, to the retained austenite of 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 more particularly to a cold-rolled steel sheet having 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.

  In Patent Document 1, welding is performed by adjusting the average crystal grain size of ferrite at a depth of 1/4 of the plate thickness and the rate of increase of the average crystal grain size at 700 ° C. while using ferrite as a main phase. In addition, it is disclosed that a hot-rolled steel sheet and a cold-rolled steel sheet excellent in thermal stability and mechanical properties that can withstand the heat of the hot-dip plating process are obtained.

  In Patent Document 2, 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, so that TS is 590 MPa or more and YR is 0.00. It is disclosed that a high-strength thin steel sheet having a rigidity of 65 or higher and a Young's modulus of 225 GPa or higher can be obtained.

  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 by suppressing the development of a specific texture, while having high strength, It has been shown that a cold-rolled steel sheet having excellent ductility and stretch flangeability can be obtained.

  Patent Document 4 discloses a high-strength, high-ductility cold-rolled steel sheet having a structure composed of a ferrite and a cementite phase and having high elongation and stretch flangeability in which the average particle diameter of the cementite phase is controlled.

International Publication No. 2007/015541 JP 2007-92131 A International Publication No. 2013/125399 JP 2003-183775 A

  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 resulting cold-rolled steel sheet has a fine structure because the hot-rolled steel sheet, which is the material, has a fine structure after cold rolling and recrystallization, and the austenite produced therefrom is also fine. And having a fine structure.

  However, due to the growth of recrystallized grains of ferrite, most of the preferential nucleation sites of austenite transformation such as grain boundaries, fine carbide particles and low-temperature transformation phase existing in hot-rolled steel sheets disappear during heating during annealing. After that, the 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 austenite grain refinement in the annealing process is restricted in that it assumes the structure after recrystallization, It cannot be said that the fine structure of the hot-rolled steel sheet can be fully utilized for the refinement of the structure after cold rolling and annealing. In particular, when annealing is performed in the austenite single phase region, it is difficult to utilize the fine structure of the hot-rolled steel sheet for cold rolling and refinement of the structure 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 the particle diameter of cementite is not fine. Further, the method disclosed in Patent Document 3 is a method of transforming fine austenite from unrecrystallized ferrite. However, sufficient consideration is given to the state of carbide in the hot-rolled steel sheet that greatly affects nucleation of austenite. There is still room for consideration.

  Furthermore, the manufacturing method disclosed in Patent Document 4 is characterized in that the hot-rolled steel sheet is annealed before cold rolling, but when such annealing is performed, the concentration of alloy elements such as Mn into cementite is reduced. Therefore, it takes time to dissolve cementite in the annealing process. Therefore, the grain size of austenite during annealing becomes coarse.

  As described above, in Patent Documents 1 to 4, either the control method of the state of carbide in the hot-rolled steel sheet or the control method of recrystallization in the heating process of annealing is not sufficient, and the ductility of the steel sheet is improved. The effect is not obtained sufficiently. 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.

  An object of this invention is to provide the cold-rolled steel plate which has high intensity | strength and the outstanding workability, and its manufacturing method.

  The inventors of the present invention have made extensive studies on a method for obtaining a cold-rolled steel sheet having high strength and excellent ductility and stretch flangeability. As a result, the following knowledge was obtained.

  (A) Stretch flangeability can be improved by using bainite and tempered martensite, which are easy to obtain a uniform hardness distribution, as the main phase as the metal structure of the cold-rolled steel sheet.

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

  (C) Furthermore, it is possible to greatly improve the workability of the steel sheet by allowing the retained austenite having different crystal orientations to exist in a close distance in the metal structure of the cold-rolled steel sheet.

  The present invention has been made on the basis of the above findings, and the gist of the present invention is the following cold-rolled steel sheet and its manufacturing method.

(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: 0.1% or less,
S: 0.05% or less,
sol. Al: 0 to 1.0%,
Ti: 0 to 0.050%,
Nb: 0 to 0.030%,
Cr: 0 to 1.0%,
Mo: 0 to 0.3%,
V: 0 to 0.3%
B: 0 to 0.005%,
Ca: 0 to 0.003%,
REM: 0 to 0.003%,
Balance: Fe and impurities,
Satisfying the following formula (i)
In area ratio, bainite and tempered martensite in total 50% or more, residual austenite 3.0% or more,
A cold-rolled steel sheet having a metal structure in which the proportion of the remaining austenite having another retained austenite grain having a crystal orientation different by 10 ° or more in a range where the closest distance is 1 μm or less is 50% or more.
0 ≦ Ti + Nb ≦ 0.070 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate.

  (2) The cold-rolled steel sheet according to (1), wherein the metal structure has an area ratio of 5.0% or more of ferrite, and an average crystal grain size of the ferrite is 4.0 μm or less.

(3) The chemical composition is mass%,
sol. Al: 0.1 to 1.0%
The cold-rolled steel sheet according to (1) or (2), which contains

(4) The chemical composition is mass%,
Ti: 0.005-0.050% and Nb: 0.003-0.030%
The cold-rolled steel sheet according to any one of (1) to (3), wherein the cold-rolled steel sheet contains one or two selected from:

(5) The chemical composition is mass%,
Cr: 0.03-1.0%,
Mo: 0.01-0.3% and V: 0.01-0.3%
The cold-rolled steel sheet according to any one of (1) to (4), which contains one or more selected from

(6) The chemical composition is mass%,
B: 0.0003 to 0.005%
The cold-rolled steel sheet according to any one of (1) to (5), containing

(7) The chemical composition is mass%,
Ca: 0.0005-0.003% and REM: 0.0005-0.003%
The cold-rolled steel sheet according to any one of (1) to (6), wherein the cold-rolled steel sheet contains one or two selected from:

  (8) The cold-rolled steel sheet according to any one of (1) to (7), wherein the steel sheet surface has a plating layer.

(9) A steel material having the chemical composition according to any one of (1) or (3) to (7) above,
The rolling completion temperature is _Ar3 point or higher, and after performing hot rolling with a rolling reduction of 20% or higher in the final rolling, cooling is performed,
Then cold-rolled,
Subsequently, heat treatment is performed so that the average heating rate between 500 ° C. and A _c1 point + 10 ° C. is 15 ° C./s or more, and the temperature is maintained at 10 s or more in the temperature range from _{Ac 3} point to _{Ac 3} point + 100 ° C. After applying
The manufacturing method of the cold-rolled steel sheet which cools so that the average cooling rate between 650 degreeC and 500 degreeC may be 10 degrees C / s or more, and hold | maintains for 10 seconds or more in the temperature range of 500 degreeC to 300 degreeC.

  (10) After performing the heat treatment, cooling is performed so that the average cooling rate between 650 ° C. and 500 ° C. is 10 ° C./s or more, and then the Ms point or less and exceeds the Ms point−100 ° C. The method for producing a cold-rolled steel sheet according to the above (9), which is held for 1 s or more in a temperature range, then heated to a temperature range of 300 ° C. to 500 ° C. and held for 10 s or more in the temperature range. .

  (11) In the cooling after the hot rolling, the residence time in the temperature range from 750 ° C. to 550 ° C. is set to less than 15 s, and then winding is performed at a temperature of 550 ° C. or less. The manufacturing method of the cold-rolled steel sheet of description.

  (12) The cooling according to any one of (9) to (11) above, wherein in the cooling after the hot rolling, the time required for cooling from the hot completion temperature to 750 ° C. is 0.4 s or less. A method for producing rolled steel sheets.

  (13) The method for producing a cold-rolled steel sheet according to any one of (9) to (12), wherein a soaking time in the heat treatment is less than 30 s.

  (14) The method for producing a cold-rolled steel sheet according to any one of (9) to (13), wherein the steel sheet surface is plated in the course of cooling after the heat treatment.

  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 it has excellent ductility and stretch flangeability. A high-strength cold-rolled steel sheet can be obtained. Therefore, the cold-rolled steel sheet according to the present invention is suitable for use as a steel sheet for automobiles, a steel sheet for building structures, and the like.

It is the figure which showed the relationship between the closest distance between the residual austenite grains from which an orientation difference mutually differs, and the number fraction of a retained austenite grain in an Example.

  Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition The reasons for limiting each element are 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. Furthermore, C has an action of lowering the transformation point. As a result, in the hot rolling process, the 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, combined with the effect of suppressing the recrystallization of ferrite in the temperature rising process by C, the temperature reaches a temperature range of _Ac1 point + 10 ° C. or higher while maintaining a high unrecrystallized ratio of ferrite by rapid heating. 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. 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 formation of bainite, martensite, and tempered martensite that form 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 an austenitizing temperature, a soaking temperature can be made low in an annealing process, and a structure | tissue can be kept fine by suppressing grain growth. Thereby, it becomes possible to refine the microstructure of the cold-rolled steel sheet. Moreover, since it has the effect | action which improves the hardenability of steel, it also has the effect of ensuring the area ratio of a bainite, a martensite, and a tempered martensite 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: 0.1% 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 0.1%, embrittlement due to the above action becomes significant. Therefore, the P content is 0.1% or less. The P content is preferably 0.06% or less. The lower the P content, the better. Therefore, the lower limit is not particularly limited, but is preferably 0.001% or more from the viewpoint of cost.

S: 0.05% 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. If the S content exceeds 0.05%, the ductility is significantly reduced by the above action. Therefore, the S content is 0.05% 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 1.0%
Al is an element having an effect of increasing ductility. Therefore, Al may be included as necessary. However, since Al has an action of raising the transformation point, sol. If the Al content exceeds 1.0%, 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. Al content shall be 1.0% or less. sol. The Al content is preferably 0.7% or less. If the above effect is desired, sol. The Al content is preferably 0.1% or more, and more preferably 0.2% or more.

Ti: 0 to 0.050%
Nb: 0 to 0.030%
0 ≦ Ti + Nb ≦ 0.070 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate.
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, you may contain 1 type or 2 types of Ti and Nb.

  However, when the content of each element exceeds the above upper limit value or when the above formula (i) is not satisfied, the ductility is significantly reduced. Therefore, the content and total content of each element are as described above. The Ti content is preferably 0.030% or less, and the Nb content is preferably 0.020% or less. The total content of Ti and Nb is preferably 0.030% or less. In order to obtain the above effect, it is preferable to contain one or two selected from Ti: 0.005% or more and Nb: 0.003% or more.

Cr: 0 to 1.0%,
Mo: 0 to 0.3%,
V: 0 to 0.3%
Cr, Mo, and V are all elements that have an effect of increasing the strength of steel. Mo also has an effect of suppressing grain growth of the crystal grains and making the structure finer. Therefore, you may contain 1 or more types selected from Cr, Mo, and V as needed.

  However, if the Cr content exceeds 1.0%, ferrite transformation is excessively suppressed, and the target structure cannot be ensured. On the other hand, if the contents of Mo and V exceed 0.3%, a large amount of precipitates are generated in the heating stage of the hot rolling process, and 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 obtain the above effect, it is preferable to contain one or more selected from Cr: 0.03% or more, Mo: 0.01% or more, and V: 0.01% or more. When two or more of the above elements are contained in a composite manner, the total content is preferably 1.2% or less.

B: 0 to 0.005%
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 0.005%, the steel is excessively hardened and the ductility is significantly reduced. Therefore, the B content is 0.005% or less. In order to obtain the above effect, the B content is preferably set to 0.0003% or more.

Ca: 0 to 0.003%
REM: 0 to 0.003%
Ca and REM are elements having an action of increasing the soundness of a slab by refining oxides and / or nitrides precipitated in the solidification process of molten steel. Therefore, you may contain 1 type or 2 types of these elements as needed. However, since any element is expensive, the content of each element is set to 0.003% or less. In order to obtain the above effect, it is preferable to contain one or two selected from Ca: 0.0005% or more and REM: 0.0005% or more. In addition, when 2 types of said elements are contained in combination, it is preferable that the total content shall be 0.005% or less.

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

  The steel sheet of the present invention has a chemical composition comprising the above-described elements from C to REM, the remainder Fe and impurities.

  Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel sheets, and are permitted within a range that does not adversely affect the present invention. Means something.

2. Metal structure The steel sheet according to the present invention contains an area ratio of bainite and tempered martensite in a total of 50% or more and residual austenite of 3.0% or more, and the closest distance of the residual austenite is 1 μm or less. In addition, a metal structure in which another residual austenite grain having a crystal orientation different by 10 ° or more exists is 50% or more.

Total area ratio of bainite and tempered martensite: 50% or more The metal structure of the cold-rolled steel sheet according to the present invention has a main phase of bainite or a mixed structure composed of bainite and tempered martensite. By increasing the area ratio of bainite and tempered martensite and making the microstructure homogeneous, the generation of minute voids when the steel sheet is processed can be suppressed. Moreover, since bainite and tempered martensite are relatively hard structures, they also have an effect of increasing the strength of the steel sheet. Therefore, the total area ratio of bainite and tempered martensite needs to be 50% or more, and preferably 60% or more.

  The bainite contained in the cold-rolled steel sheet according to the present invention is generated during the overaging treatment after cooling from annealing soaking, or during the overaging treatment when reheated after cooling to a temperature below the Ms point. This bainite has a lower hardness than tempered martensite, but a certain elongation is ensured. By including this structure, high strength can be obtained while ensuring good workability. In order to obtain the above effect, the area ratio of bainite contained in the microstructure as the main phase is preferably 10% or more, and more preferably 20% 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 adding the above-mentioned amount of Si to the steel.

  Moreover, the tempered martensite contained in the cold-rolled steel sheet according to the present invention is a martensite produced when cooled to a temperature range below the Ms point and exceeding the Ms point−100 ° C. in the cooling process after annealing. Then, it is a structure that has been tempered by being reheated and held to a temperature range of 300 to 500 ° C. beyond the Ms point. Since tempered martensite is precipitated as cementite and is softer than martensite and harder than bainite, the strength of the steel sheet can be increased while obtaining a homogeneous microstructure. Furthermore, once martensite is generated during the heat treatment, the effect of promoting the bainite transformation that occurs after reheating can be obtained. As a result, since the concentration of C into the retained austenite generated with the bainite transformation is promoted, the stability of the retained austenite can be increased. As a result, the workability of the steel sheet, particularly the hole expandability is improved. The area ratio of tempered martensite contained in the microstructure as the main phase is preferably 10% or more, and more preferably 20% or more.

Area ratio of retained 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 retained austenite area ratio. Therefore, the retained austenite area ratio needs to be 3.0% or more. The area ratio of retained austenite is preferably 5.0% or more.

Proportion of residual austenite grains with different crystal orientations in the range where the closest distance is 1 μm or less: 50% or more As described above, residual austenites with different crystal orientations are locally localized in the metal structure of the cold-rolled steel sheet. When a large number are arranged in the region, residual austenite grains having various orientations undergo a processing-induced transformation when the steel sheet is processed. Along with this, bainite and ferrite are also deformed under compression in various directions, so that the deformation of the microstructure proceeds more uniformly and isotropically, local deformation is suppressed, voids in the microstructure and Micro cracks are less likely to occur. As a result, good workability can be obtained.

  On the other hand, when the retained austenite contained in the cold-rolled steel sheet has the same crystal orientation in a wide range, a large amount of retained austenite with the same orientation locally undergoes work-induced transformation, so the strain anisotropy associated with the work-induced transformation is Larger and microstructure deformation increases non-uniformity. As a result, strain and stress are concentrated locally, leading to early breakage, so that good ductility cannot be obtained. Therefore, in the present invention, it is necessary to set the ratio of the remaining austenite having another retained austenite grain having a crystal orientation different by 10 ° or more in the range where the closest distance is 1 μm or less to 50% or more. The ratio of the residual austenite grains is preferably 55% or more.

Area ratio of ferrite having an average crystal grain size of 4.0 μm or less: 5.0% or more The metal structure of the cold-rolled steel sheet according to the present invention may further contain fine ferrite as the second phase. Ferrite has the effect of remarkably improving the elongation of the steel sheet. Further, by making the structure finer, the effect of suppressing the development of fine cracks during the hole expanding process can 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, it is preferable to contain fine ferrite having an average crystal grain size of 4.0 μm or less in an area ratio of 5.0% 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, tempered martensite, and ferrite can be measured by a structural analysis using SEM-EBSD. The ratio of tempered martensite and bainite contained in the main phase is determined by heat-treating the corresponding steel sheet under the same annealing conditions, measuring the thermal expansion during cooling of the annealing, the amount of expansion below the Ms point, and the Ms point. It can obtain | require by calculating | requiring the ratio with the amount of expansion | swelling in 500 degrees C or less exceeding each, and multiplying each ratio to the above-mentioned total area rate. Furthermore, the area ratio of the retained austenite is the volume ratio obtained by the X-ray diffraction method as it is. The distance and orientation difference between the retained austenites can be obtained by obtaining information on the orientation and coordinates of the crystal grains from the structure analysis result of the SEM-EBSD.

  Furthermore, the average crystal grain size of the ferrite contained as the second phase can be obtained by using SEM-EBSD to determine the crystal grain size of ferrite surrounded by a large-angle grain boundary having an inclination angle of 15 ° or more. 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). The crystal grain size can be calculated by analyzing the obtained orientation map. The average crystal grain size of ferrite in the present invention means an average value of equivalent circle diameters obtained by the following formula. However, Ai in the following formula 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 plate | board thickness 1/4 depth of a steel plate is employ | adopted about the value of an area rate and an average crystal grain size.

3. Plating layer A plated layer may be provided on the surface of the cold-rolled steel sheet for the purpose of improving the corrosion resistance, etc., to obtain 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 plating adhesion amount is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by forming a suitable chemical conversion treatment film on the plating surface (for example, by applying and drying a silicate-based chromium-free chemical conversion treatment solution). Furthermore, it can be coated with an organic resin film.

4). 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, with respect to the steel raw material which has said chemical composition, after giving the hot rolling shown below, it cools and a hot-rolled steel plate is made. After production, it can be produced by cold rolling and further heat treatment such as annealing. Hereinafter, the heat treatment will be described as annealing.

4-1 Hot rolling and cooling after rolling In the present invention, the structure of a hot-rolled steel sheet that is a base material before cold rolling is refined. In particular, it is preferable to have a fine structure including a large amount of austenite grain boundaries, block boundaries of bainite and martensite, and ferrite grain boundaries. Moreover, cementite exists on the grain boundaries and boundaries, so that it functions more effectively as a nucleation site. Therefore, the structure of the hot-rolled steel sheet is preferably a structure in which cementite is finely dispersed.

  Such a hot-rolled steel sheet having a fine structure can be produced by hot rolling shown below and subsequent cooling. 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 may be used while maintaining the high temperature during continuous casting, or may be used after being cooled to room temperature and then reheated.

  The temperature of the slab used for hot rolling is preferably 1000 ° C. or higher. When 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 the structure is rolled with the transformed ferrite included. There is a fear. For this reason, the heating temperature is preferably sufficiently high so that 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.

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. Since it is necessary to maintain the steel sheet in the austenite temperature range during rolling, the rolling completion temperature is preferably set to _Ar3 or higher.

  Moreover, the austenite structure becomes finer by increasing the reduction amount of the final rolling. Therefore, in the hot rolling, the rolling reduction in the final rolling is preferably 20% or more, and more preferably 25% or more. This austenite grain boundary remains as a prior austenite grain boundary in the structure of bainite and martensite after the hot-rolled steel sheet is cooled, so that the austenite grain boundary is more densely distributed in the microstructure of the hot-rolled sheet. become. Since this former austenite grain boundary becomes a nucleation site during annealing, the austenite nucleation number in the annealing heating process can be remarkably increased.

The total reduction amount of the hot-rolled steel sheet is also preferably 40% or more, and the thickness reduction rate when the temperature of the material to be rolled is in the temperature range of _{Ar 3} point to _{Ar 3} point + 150 ° C. is 40% or more. More preferably. Rolling does not have to be performed in one pass, and may be a continuous multiple-pass rolling. More strain energy is introduced into the 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 amount of reduction per pass is preferably 60%.

  Cooling after the end of rolling is preferably performed by the following method. In the cooling after hot rolling, the time required for cooling from the hot completion temperature to 750 ° C. after rolling is preferably 0.4 s or less, and more preferably 0.2 s or less. By this cooling, it is preferable to cool to a temperature of 750 ° C. or lower, and more preferable to cool to a temperature of 700 ° C. or lower. The cooling method is preferably water cooling.

  The cooling rate at the start of cooling is preferably 400 ° C./s or more, more preferably 600 ° C./s or more, and still more preferably 800 ° C./s or more. The reason for defining the cooling method after hot rolling as described above is to make the structure of the hot-rolled steel sheet fine and increase the density of nucleation sites in annealing. By rapidly cooling after hot rolling, the recovery of strain introduced in austenite and the consumption due to recrystallization are suppressed as much as possible, and the strain energy accumulated in the steel is transformed from austenite to ferrite or bainite. It can be used as much as possible. The reason for setting the cooling rate at the start of cooling from the rolling completion temperature to 400 ° C./s or more is 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.

  After completion of rolling, the residence time in the temperature range from 750 ° C. to 550 ° C. is preferably less than 15 s, and more preferably less than 10 s. This is for suppressing the formation of coarse pearlite in the microstructure of the hot-rolled steel sheet. When a large amount of pearlite is generated, the number of nucleation during annealing decreases. The area ratio of pearlite is preferably less than 10%, more preferably less than 5%. When it stays for a long time in the above temperature range, steel alloy elements diffuse into pearlite or cementite, and the concentration of Mn and the like in the cementite increases. As a result, the dissolution of cementite during annealing is suppressed, and it is necessary to maintain the annealing for a long time, resulting in an adverse effect on the refinement of the structure.

  As a temperature and residence time suitable for tissue control, for example, cooling can be performed for about 6 s from 680 ° C. to 630 ° C. Thereby, fine ferrite can be introduced into the structure of the hot rolled sheet.

  Then, it cools to the coiling temperature of a steel plate. 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 the crystal grains and cementite fine in the microstructure of the hot-rolled steel sheet. In addition, when winding temperature becomes 300 degrees C or less, the intensity | strength of a hot-rolled steel plate will increase and cold rolling may become difficult. In this case, you may anneal at the temperature of 650 degrees C or less before cold rolling.

  The hot-rolled steel sheet produced by the above hot-rolling process has a structure in which the second phase such as martensite or cementite is finely dispersed. Thus, it is preferable to cold-roll and anneal the hot-rolled steel sheet in which the second phase such as cementite is finely dispersed. Because the cementite existing on these large-angle grain boundaries is the preferential nucleation site for austenite transformation, rapid austenite annealing described later generates a large number of austenite and recrystallized ferrite from these positions to refine the structure. This is because it is possible to plan.

  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 grain size defined by the large-angle grain boundaries with an inclination angle of 15 ° or more is set to 6 μm or less. Can be further increased.

  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, a structure obtained by tempering them, and a mixed structure thereof.

4-2 Cold Rolling The hot rolled steel sheet produced by the above hot rolling is pickled and then cold rolled. Cold rolling may be performed using a normal method, and the rolling reduction is usually 20% or more. When the cold rolling rate is increased, the density of grain boundaries in the structure is increased, so that the number of nucleation during annealing heating can be increased.

4-3 Annealing The steel sheet obtained by the above cold rolling is subjected to the following annealing. First, the steel sheet is heated so that the average heating rate between 500 ° C. and _{Ac 1} point + 10 ° C. is 15 ° C./s or more. By rapidly heating, recrystallization during heating is suppressed, and heating can be performed up to _Ac1 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, packet block boundaries, and ferrite grain boundaries of hot-rolled steel sheets as the main nucleation sites. If the average heating rate between 500 ° C. and _{Ac 1} point + 10 ° C. is less than 15 ° C./s, recrystallization occurs and the grain boundary of the hot-rolled steel sheet disappears, so the above effect cannot be obtained.

  By increasing the nucleation number of austenite, the austenite grains during annealing can be remarkably refined, and as a result, in the microstructure of the cold-rolled steel sheet, residual austenite having a crystal orientation difference of 10 ° or more The closest distance can be shortened. It is also possible to refine the ferrite and residual austenite in the microstructure.

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 between 500 ° C. and _{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 considering that temperature control becomes difficult.

When the _{unrecrystallized} ratio in the region not subjected to austenite transformation at the point of reaching _Ac1 point + 10 ° C. is less than 30%, 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 during annealing become coarse and the final structure becomes coarse. Therefore, it is preferable that the unrecrystallized ratio in the region not transformed to austenite at the point of reaching _Ac1 point + 10 ° C. is 30% or more.

  If the temperature at which the above rapid heating is started is before the start of recrystallization, a sufficient effect is exhibited. 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. In addition, since the austenite transformation can be caused while keeping the cementite fine by starting rapid heating from room temperature, it is preferable to refine the structure.

  In order to obtain a sufficiently rapid heating rate, it is preferable to use energization heating, induction heating, or direct flame heating, but heating by 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.

After heating to A _c1 point + 10 ° C., heating is further performed from A _c3 point to an annealing temperature (soaking temperature) of A _c3 point + 100 ° C. The heating rate at this time can be set to an arbitrary rate. By reducing the heating rate, sufficient time can be taken to promote recrystallization of ferrite. Also, the heating rate can be changed such that only the first part is the same rapid heating as the rapid heating and the subsequent heating rate is lower.

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, the annealing temperature needs to be _Ac3 point or higher. When annealed at a temperature lower than this, an austenite single phase state is not obtained during annealing, and recrystallization of ferrite does not occur, so that a processed ferrite structure remains. Then, the orientation of the {100} <011> to {211} <011> orientation groups in the texture of the cold-rolled steel sheet increases, and the workability of the steel sheet decreases. On the other hand, when annealing is performed at a temperature exceeding the _Ac3 point + 100 ° C., austenite grains grow rapidly and the final structure becomes coarse. Therefore, it is preferable that the annealing temperature is in a temperature range from A _c3 point to A _c3 point + 100 ° C. In order to refine the structure, the annealing temperature is more preferably set to _Ac3 point + 50 ° C. or lower.

  The soaking time for annealing is preferably 10 s or longer. If the soaking time is less than 10 s, the dissolution of pearlite or cementite and the transformation to austenite do not proceed sufficiently, so that the workability of the cold-rolled steel sheet is lowered. In addition, temperature unevenness during annealing is liable to occur, causing a problem in manufacturing stability. Therefore, it is preferable that the annealing holding time is 10 s or longer and the transformation to austenite is sufficiently advanced. On the other hand, if the holding is performed for an excessively long time, the condition of the dispersed state of retained austenite defined by the present invention may not be satisfied due to the growth of austenite grains. For this reason, the soaking time is preferably less than 10 minutes. In addition, the retained austenite having different crystal orientations can be made closer to each other as the soaking time is shorter. Therefore, the soaking time is more preferably less than 30 s.

  After the soaking, the cooling is preferably performed so that the average cooling rate between 650 ° C. and 500 ° C. is 10 ° C./s or more. 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, a large amount of ferrite is generated during cooling, and the stretch flangeability is deteriorated.

  In the above cooling process, cooling can be stopped in a temperature range of 500 ° C. to 300 ° C., and overaging treatment can be performed in that temperature range. In the overaging treatment, by controlling the soaking temperature and the holding time, bainite having an appropriate area ratio is generated in the cold-rolled steel sheet, and the concentration of residual austenite is promoted by promoting the concentration of C into untransformed austenite. Generate. For this reason, it is preferable to hold | maintain 10 s or more in the temperature range from 500 degreeC to 300 degreeC. More preferable heat treatment conditions are a temperature in the range of 350 ° C. to 450 ° C. and a holding time in the range of 100 s to 600 s.

  After the above cooling, after stopping the cooling in the temperature range below the Ms point and exceeding the Ms point−100 ° C., it is reheated to exceed the Ms point and rise from 300 ° C. to 500 ° C. It is more preferable to warm and perform an overaging treatment in that temperature range.

  Before the overaging treatment, the cooling stop temperature is set to a temperature lower than or equal to the Ms point and higher than the Ms point to −100 ° C., a part of the microstructure is transformed into martensite, the Ms point is exceeded, and 300 By reheating from ℃ to 500 ℃, the structure of the cold-rolled steel sheet can be made a structure containing tempered martensite.

  When the cooling stop temperature is Ms point −100 ° C. or lower, the area ratio of bainite and retained austenite in the final structure is excessively reduced, and good workability cannot be obtained. The cooling stop temperature is more preferably set to a temperature not higher than the Ms point and higher than the Ms point −80 ° C. On the other hand, when the reheating temperature exceeds 500 ° C., untransformed austenite is decomposed into carbides, so that there is a possibility that good workability cannot be obtained.

  In addition, after performing the above-described overaging treatment or overaging treatment after reheating, it is cooled as it is to room temperature by a method such as standing cooling or water cooling, and then further, for example, hot dip galvanizing or alloying hot dip zinc A hot dipping process such as plating can be performed. In addition, the above hot dipping treatment can be performed continuously with the above reheating and holding.

  If the cooling rate is excessively low, or if soaking is performed for a long time at a high temperature, not only the desired structural fraction cannot be obtained, but also the workability of the steel sheet deteriorates due to transformation of residual austenite to carbide, etc. Cause it. For this reason, it is desirable that the holding time (including plating and / or overaging) in the temperature range of 500 ° C. to 300 ° C. during cooling is less than 2000 s. Although the cooling method can be performed by any method, for example, cooling by gas, mist, or water is possible.

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 then cooling it at 2 ° C./s. It is obtained from the thermal expansion measurement result during that time. Further, the points A _c1 and A _c3 are obtained from a thermal expansion curve measured when the cold-rolled steel sheet is heated to 1100 ° C. at a heating rate of 2 ° C./s. Furthermore, non-recrystallization ratio occupied in the region not austenitic transformation in the ferrite at the time it reaches the A _c1 point + 10 ° C., after the steel plate was to cold rolling and the temperature was raised to A _c1 point + 10 ° C., immediately It can be obtained by water cooling, photographing the structure with SEM, and measuring the fraction of the recrystallized structure and the processed structure on the structure photograph.

  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 hot rolled at the hot rolling completion temperature and final rolling reduction shown in Table 2 using a small test mill. After the completion of rolling, hot-rolled steel sheets having a thickness of 2.0 to 2.6 mm were produced under the conditions shown in Table 2. Table 3 also shows _{Ar 3} points of steel types A to H.

  Immediately after the completion of rolling or after cooling for a predetermined time at the rolling completion temperature (FT), cooling by water cooling was performed. Cooling by water cooling is carried out in two steps. After cooling to 750 ° C. or less by the first cooling, it is allowed to cool for 3 to 15 s, and as the second cooling, at a cooling rate of 30 to 100 ° C./s. Water cooling was performed to cool to the coiling temperature. Table 2 shows the cooling rate of the first cooling, the time required for cooling to 750 ° C., the residence time in the temperature range from 750 ° C. to 550 ° C., and the coiling temperature. Winding was performed by putting the steel plate into a furnace and performing slow cooling simulating winding.

  Table 2 shows the average crystal grain size of the hot-rolled steel sheet obtained by the above hot rolling. The crystal grain size of the hot-rolled steel sheet 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 ¼ depth position of the steel sheet. About the BCC phase which consists of a site and a bainite, it analyzed and calculated | required the particle size which consists of a large angle grain boundary with an inclination angle of 15 degrees or more.

The hot-rolled steel sheet thus obtained was pickled with hydrochloric acid and then cold-rolled at the rolling reduction shown in Table 3 to make the steel sheet thickness 1.0 to 1.3 mm. . Then, using a laboratory-scale annealing facility, a temperature range from room temperature to 750 ° C. was heated at an average heating rate shown in Table 3, and then the annealing temperature (soaking temperature) and soaking time shown in Table 3 were used. Annealing was performed, and a temperature range of 650 ° C. to 500 ° C. was cooled at an average cooling rate shown in Table 3 to obtain a cold-rolled steel sheet. Cooling after soaking was performed with nitrogen gas. Table 3 also shows points A _c1 and A _{c3 of} steel types A to H.

  Further, the steel sheet without plating treatment in Table 3 was over-aged under the conditions shown in Table 3 in the cooling process. Further, the steel plate with plating treatment was held at 400 ° C. for 60 seconds, and then heated to 460 ° C. to simulate immersion in a hot dip galvanizing bath, and further heated to 500 ° C. to perform an alloying treatment. After these overaging treatments or plating treatments, it was cooled to room temperature to obtain cold-rolled steel plates or hot-dip galvanized steel plates.

  The metal structure and mechanical properties of the cold-rolled steel sheet thus manufactured were examined as follows.

  The area ratios of bainite, tempered martensite, and ferrite were determined by structural analysis of the cross-sectional structure in the rolling direction at a thickness of ¼ depth of the steel sheet using the SEM-EBSD apparatus described above. In this example, since the cooling stop temperature after annealing is high, tempered martensite is not included in the structure in all steel sheets. 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). As in the case of the hot-rolled steel sheet described above, the ferrite grain size is obtained by using a SEM-EBSD device (JSM-7001F, JSM-7001F) as a cross-sectional structure from the width direction of the cold-rolled steel sheet. It was determined as an average crystal grain size defined by a large-angle grain boundary of ° or more.

  Further, the closest distance of residual austenite having a crystal orientation different by 10 ° or more was obtained by obtaining the Euler angles and the position coordinates of the austenite crystal grains from the analysis result of the SEM-EBSD and calculating it. . In this measurement, about 0.13 μm or more of retained austenite was used as an object, and 2000 or more of retained austenite was used as an object of measurement when analyzing one microstructure.

  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 mechanical properties of the cold-rolled steel sheet after annealing were investigated by a tensile test and a hole expansion test. The tensile test was performed using a JIS No. 5 tensile test piece, and 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 4 respectively. In the present invention, a material satisfying the following formula (ii) having a balance of TS × El and TS × λ was determined to have good strength and workability. The value on the right side of the formula (ii) is more preferably 196000.
6.2 × TS × El + TS × λ> 190000 (ii)

Among test numbers 1 to 11 manufactured using steel type A, test number 3 as a comparative example has a low heating rate during annealing, test number 4 has a long residence time at 750 to 550 ° C. after hot rolling, and the test In No. 6, the final rolling reduction ratio of hot rolling does not reach 20%, and in Test No. 10, the residence time of 750 to 550 ° C. is excessively long and the coiling temperature exceeds 600 ° C. Was inappropriate. As a result, the structure of the hot-rolled steel sheet becomes coarse, and as a result, the proportion of the retained austenite grains having different crystal orientations with a shortest closest distance decreases. Furthermore, in Test No. 5, the soaking temperature of annealing was lower than the _Ac3 point, and in Test No. 9, the cooling rate after annealing was low. Therefore, not only the ratio of the retained austenite grains having different crystal orientations but the closest distance is small, the bainite area ratio is lowered, and the ferrite grain size is coarsened. In Test No. 5, the processed ferrite remained after annealing. As a result, the mechanical properties became insufficient.

  On the other hand, Test Nos. 1, 2, 7, 8 and 11 which are examples of the present invention had appropriate manufacturing conditions, so that the metal structure satisfied the provisions of the present invention and resulted in excellent strength and workability. Among them, Test No. 2 sets the soaking time in annealing to 30 s or less, and Test Nos. 7 and 8 set the reduction ratio in hot rolling to 33%, rapidly cooling and soaking in annealing. The holding time was 30 s or less, and test number 11 was 40% hot rolling reduction and was rapidly cooled, so the closest distance of residual austenite grains with different crystal orientations compared to test number 1 The ratio of small is high and exhibited better mechanical properties.

  In addition, as an example of the result of calculating the closest distance between retained austenites having different crystal orientations by 10 ° or more, the results of Test No. 2 as an example of the present invention and Test No. 3 as a comparative example are shown in FIG. The vertical axis in FIG. 1 indicates the number fraction of retained austenite. From FIG. 1, in test number 2, most of the retained austenite grains have a closest distance of 1 μm or less, and the ratio exceeds 50%, while in test number 3, the ratio of the residual austenite whose closest distance exceeds 1 μm. You can see that there are many. As described above, in the metal structure of the steel sheet according to the present invention, residual austenite having an orientation difference of 10 ° or more exists in the vicinity, which makes the deformation in the microstructure uniform and improves the workability of the steel sheet. Improve.

  Similarly, in the results of other steel types, test number 16 which is a comparative example has a high annealing temperature, test numbers 19, 33, 36 and 38 have a low heating rate during annealing, and test number 22 is hot rolled. The subsequent residence time of 750 to 550 ° C. is excessively long, the coiling temperature exceeds 600 ° C., and Test Nos. 23 and 27 do not reach a final rolling reduction ratio of 20%. Nos. 26 and 29 had a long residence time at 750 to 550 ° C. after hot rolling, and the production conditions were inappropriate. As a result, the proportion of residual austenite grains having different crystal orientations with a shortest closest distance decreased, and mechanical properties became insufficient.

  Test numbers 39 and 40 have a low Si content, and test number 41 has a low C and Si content. Therefore, the area ratio of bainite is low, and residual austenite is hardly generated. It became.

  On the other hand, test numbers 12-15, 17, 18, 20, 21, 24, 25, 28, 30-32, 34, 35 and 37, which are examples of the present invention, have a chemical composition and a metal structure of the present invention. Since the requirements were satisfied, the strength and workability were excellent.

Ingots of steel types K to S having chemical compositions shown in Table 5 were melted in a vacuum induction furnace, hot forged, and then cut into slab-shaped steel pieces for use in hot rolling. Each of the obtained slabs was hot-rolled and cooled under the conditions shown in Table 6 in the same manner as in the method in Example 1 to produce hot-rolled steel sheets having a thickness of 2.0 to 2.6 mm. _{A r3} point grades K~S also shown in Table 6.

  Table 6 shows the average crystal grain size of the hot-rolled steel sheet obtained by the above hot rolling. The crystal grain size of the hot-rolled steel sheet was measured in the same manner as described in Example 1.

The hot-rolled steel sheet thus obtained was pickled with hydrochloric acid, and then subjected to cold rolling at the rolling reduction shown in Table 7, so that the thickness of the steel sheet was 1.0 to 1.3 mm. . Then, after annealing at an average heating rate, a soaking temperature (annealing temperature), and a soaking time (holding time) shown in Table 7 using a laboratory-scale annealing facility, the “average cooling rate” shown in Table 7 was used. The product was cooled to the temperature described as “Cooling stop temperature” in the same table. Further, the steel sheet without plating treatment in Table 7 was reheated from the cooling stop temperature and heated to 400 ° C., and at that temperature, an overaging treatment was performed to keep the soaking for 330 s. The steel sheet with plating treatment is reheated from the cooling stop temperature, raised to 400 ° C., held at that temperature for 60 s, heated to 460 ° C. to simulate immersion in a hot dip galvanizing bath, and further heated to 500 ° C. And heat treatment was performed to simulate the alloying treatment. Then, the steel plate without a plating process and the steel plate with a plating process were cooled to normal temperature with the average cooling rate of 2 degrees C / s, and the cold-rolled steel plate was obtained. Cooling after soaking was performed with nitrogen gas. _{A c1} point steels _{K~S, A c3} point and Ms point are also shown in Table 7.

  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 7, 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 the metal structure of the cold-rolled steel sheet thus manufactured, the area ratio of bainite, tempered martensite and ferrite was determined using the above-described SEM-EBSD apparatus in the cross-sectional structure in the rolling direction at a sheet thickness of ¼ depth. Obtained by analysis. Furthermore, the ratio of bainite and tempered martensite, which are the main phases, is determined by measuring the thermal expansion during cooling of the annealing, heat-treating the corresponding steel plate under the same annealing conditions, and calculating the expansion amount below the Ms point and the Ms point It calculated | required by calculating | requiring the ratio with the expansion amount in 500 degrees C or less exceeding, and multiplying each ratio to the above-mentioned total area rate. Further, the area ratio of the retained austenite phase, and the closest distance of retained austenite having a grain size of 10 ° or more with different crystal orientations were determined by the same method as in Example 1 above.

  Also, the mechanical properties of the cold-rolled steel sheet were determined by the same method as in Example 1.

Table 8 shows the results of the mechanical property investigation. In the present invention, in the same manner as in Example 1, when the balance of TS × El and TS × λ satisfies the following formula (ii), it was determined that the strength and workability were good. The value on the right side of the formula (ii) is more preferably 196000.
6.2 × TS × El + TS × λ> 190000 (ii)

  Among test numbers 42 to 49 manufactured using steel type K, test number 44 as a comparative example has a low heating rate during annealing, and test number 45 has an excessively long residence time of 750 to 550 ° C. after hot rolling. At the same time, the coiling temperature exceeded 600 ° C., and the production conditions were inappropriate. As a result, the structure of the cold-rolled steel sheet becomes coarse, and as a result, the proportion of the austenite grains having different crystallographic orientations with a shortest closest distance decreases. In Test No. 48, since the cooling stop temperature after soaking of the annealing was lower than the Ms point of −100 ° C., the area ratio of the tempered martensite was increased, so that the retained austenite area ratio was lowered. In Test No. 49, the cooling rate between 500 and 650 ° C. after soaking of the annealing was low. For this reason, not only the ratio of the residual austenite grains having different crystal orientations but the closest distance is small, the total area ratio of bainite and tempered martensite is lowered, and the grain size of ferrite is coarsened. As a result, the mechanical properties became insufficient.

  On the other hand, test numbers 42, 43, 46, and 47, which are examples of the present invention, had appropriate manufacturing conditions, so that the metal structure satisfied the provisions of the present invention, and the microstructure contained an appropriate amount of tempered martensite. As a result, mechanical properties excellent in strength and workability, in particular, hole expandability were obtained. Among them, test number 43 in which cooling after hot rolling was performed rapidly, and test number 46 in which the rolling reduction in hot rolling was quenched at 33% or more and the holding temperature in annealing was 30 s or less, Compared with test number 42, the ratio of the austenite grains with different crystallographic orientations with the shortest closest distance was high and exhibited better mechanical properties.

Similarly, in the results of other steel types, test numbers 52 and 60, which are comparative examples, have a low heating rate during annealing, test number 62 has a high soaking temperature, and test number 66 is 750 to 750 after hot rolling. The residence time at 550 ° C. was excessively long, the coiling temperature exceeded 600 ° C., and the production conditions were inappropriate. As a result, the proportion of residual austenite grains having different crystal orientations with a shortest closest distance decreased, and mechanical properties became insufficient. In Test No. 53, the soaking temperature of annealing was lower than the _Ac3 point, and the processed ferrite structure remained in the microstructure even after annealing, resulting in poor mechanical properties.

  In addition, since test numbers 67 and 68 have a low Si content and test number 69 has a low C content, the total area ratio of bainite and tempered martensite is low, and residual austenite is hardly generated. As a result, the balance between ductility and the ductility was poor.

  On the other hand, test numbers 50, 51, 54 to 59, 61 and 63 to 65, which are examples of the present invention, are excellent in strength and workability because the chemical composition and the metal structure satisfy the provisions of the present invention. As a result.

  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 it has excellent ductility and stretch flangeability. A high-strength cold-rolled steel sheet can be obtained. Therefore, the cold-rolled steel sheet according to the present invention is suitable for use as a steel sheet for automobiles, a steel sheet for building structures, and the like.

Claims (14)

Chemical composition is mass%,
C: 0.05 to 0.30%
Si: 0.5 to 2.5%
Mn: 0.5 to 3.5%
P: 0.1% or less,
S: 0.05% or less,
sol. Al: 0 to 1.0%,
Ti: 0 to 0.050%,
Nb: 0 to 0.030%,
Cr: 0 to 1.0%,
Mo: 0 to 0.3%,
V: 0 to 0.3%
B: 0 to 0.005%,
Ca: 0 to 0.003%,
REM: 0 to 0.003%,
Balance: Fe and impurities,
Satisfying the following formula (i)
In area ratio, bainite and tempered martensite in total 50% or more, residual austenite 3.0% or more,
A cold-rolled steel sheet having a metal structure in which the proportion of the remaining austenite having another retained austenite grain having a crystal orientation different by 10 ° or more in a range where the closest distance is 1 μm or less is 50% or more.
0 ≦ Ti + Nb ≦ 0.070 (i)
However, each element symbol in a formula represents content (mass%) of each element contained in a steel plate.   The cold-rolled steel sheet according to claim 1, wherein the metal structure has an area ratio of 5.0% or more of ferrite, and an average crystal grain size of the ferrite is 4.0 µm or less. The chemical composition is mass%,
sol. Al: 0.1 to 1.0%
The cold-rolled steel sheet according to claim 1 or 2, which contains The chemical composition is mass%,
Ti: 0.005-0.050% and Nb: 0.003-0.030%
The cold-rolled steel sheet according to any one of claims 1 to 3, comprising one or two selected from the above. The chemical composition is mass%,
Cr: 0.03-1.0%,
Mo: 0.01-0.3% and V: 0.01-0.3%
The cold-rolled steel sheet according to any one of claims 1 to 4, comprising one or more selected from the above. The chemical composition is mass%,
B: 0.0003 to 0.005%
The cold-rolled steel sheet according to any one of claims 1 to 5, comprising: The chemical composition is mass%,
Ca: 0.0005-0.003% and REM: 0.0005-0.003%
The cold-rolled steel sheet according to any one of claims 1 to 6, comprising one or two selected from:   The cold-rolled steel sheet according to any one of claims 1 to 7, wherein the steel sheet has a plating layer. A steel material having the chemical composition according to claim 1 or claim 3 to claim 7,
The rolling completion temperature is _Ar3 point or higher, and after performing hot rolling with a rolling reduction of 20% or higher in the final rolling, cooling is performed,
Then cold-rolled,
Subsequently, heat treatment is performed so that the average heating rate between 500 ° C. and A _c1 point + 10 ° C. is 15 ° C./s or more, and the temperature is maintained at 10 s or more in the temperature range from _{Ac 3} point to _{Ac 3} point + 100 ° C. After applying
The manufacturing method of the cold-rolled steel sheet which cools so that the average cooling rate between 650 degreeC and 500 degreeC may be 10 degrees C / s or more, and hold | maintains for 10 seconds or more in the temperature range of 500 degreeC to 300 degreeC.   After performing the heat treatment, cooling is performed so that the average cooling rate between 650 ° C. and 500 ° C. is 10 ° C./s or more, and then at a temperature range below the Ms point and above the Ms point−100 ° C. The method for producing a cold-rolled steel sheet according to claim 9, wherein after holding for 1 s or longer, the Ms point is exceeded and heating is performed from 300 ° C to 500 ° C, and the temperature is held for 10 s or longer.   The cold rolling according to claim 9 or 10, wherein in the cooling after the hot rolling, a residence time in a temperature range of 750 ° C to 550 ° C is set to less than 15 s, and then winding is performed at a temperature of 550 ° C or less. A method of manufacturing a steel sheet.   The manufacturing of the cold-rolled steel sheet according to any one of claims 9 to 11, wherein in the cooling after the hot rolling, the time required for cooling from the hot completion temperature to 750 ° C is 0.4 s or less. Method.   The method for producing a cold-rolled steel sheet according to any one of claims 9 to 12, wherein a soaking time in the heat treatment is less than 30 s.   The method for producing a cold-rolled steel sheet according to any one of claims 9 to 13, wherein the steel sheet surface is plated during cooling after the heat treatment.

Images