JP6515281B2 - Cold rolled steel sheet and method of manufacturing the same - Google Patents

Description

  The present invention relates to a cold rolled steel sheet and a method of manufacturing the same, and more particularly to a cold rolled steel sheet having high strength and excellent in workability and a method of manufacturing the same.

  In recent years, from the viewpoint of global environmental protection, development of steel materials that contribute to energy saving is required. In the fields of automotive steel plates and construction structural steel plates, the demand for particularly lightweight high-strength steel plates is increasing. Since steel plates used in these fields are often processed into products by press forming or the like, not only strength characteristics but also excellent formability are required.

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

  In Patent Document 2, the ferrite formed upon cooling is refined by accumulating the orientation of ferrite to an orientation advantageous to the Young's modulus and refining the austenite grains, and the TS is 590 MPa or more, and the YR is 0. 0. It is disclosed that a high-strength thin steel sheet excellent in rigidity and having a Young's modulus of at least 65 and a Young's modulus of at least 225 GPa can be obtained.

  Patent Document 3 has a composite structure in which one or two types of low-temperature transformation phases, martensite and bainite, are used as a main phase, and while suppressing development of a specific texture, while having high strength, It has been shown that a cold rolled steel sheet excellent in ductility and stretch flangeability can be obtained.

  Further, 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 controlling the average grain size of the cementite phase and having high elongation and stretch flangeability.

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 heat-rolled steel plate which is the material, the structure can be refined without containing a large amount of precipitation elements, and excellent ductility can be achieved. It is possible to manufacture the cold rolled steel sheet which it has. In the cold-rolled steel sheet obtained, since the material of the hot-rolled steel sheet as the material has a fine structure, the structure after cold rolling and recrystallization is also fine, and the austenite generated therefrom is also fine. , Has a fine structure.

  However, due to the growth of recrystallized recrystallized grains, most of the preferential nucleation sites for austenite transformation such as grain boundaries, fine carbide particles and low temperature transformation phase present in the hot rolled steel sheet have disappeared during heating during annealing. After that, austenite transformation occurs with the grain boundaries of the structure after recrystallization as nucleation sites.

  Therefore, although the cold-rolled steel sheet obtained by the method disclosed in Patent Document 1 has a fine structure, the refinement of austenite grains in the annealing process is restricted in that the structure after recrystallization is assumed, It can not be said that the fine structure of the heat-rolled steel sheet can be sufficiently utilized for refining 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 refining the structure after cold rolling and 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 non-recrystallized ferrite, but the state of the carbide of the hot rolled steel sheet which greatly affects nucleation of austenite is sufficiently considered. There is still room for consideration.

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

  As mentioned above, in patent documents 1-4, either the control method of the state of the carbide in a hot rolled sheet steel or the control method of recrystallization in the heating process of annealing is not enough, and improves ductility of a steel plate Effects are not obtained enough. From this, there is room to improve formability by selecting a combination of initial structure control and a heating method of annealing suitable for refining the structure of the steel sheet.

  An object of the present invention is to provide a cold rolled steel sheet having high strength and excellent processability, and a method of manufacturing the same.

  The inventors of the present invention conducted intensive studies on a method of obtaining a cold rolled steel sheet which is excellent in ductility and stretch flangeability while having high strength. As a result, the following findings were obtained.

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

  (B) Moreover, it becomes possible to improve ductility by containing the second phase containing retained austenite. Then, by reducing the second phase, the decrease in stretch flangeability is suppressed as much as possible.

  (C) Further, by making the metal structure of the cold-rolled steel sheet present at a close distance between retained austenites different from each other in crystal orientation, it is possible to significantly improve the workability of the steel sheet.

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

(1) Chemical composition is in 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%,
Remainder: Fe and impurities,
Satisfy the following equation (i),
The area ratio contains at least 50% in total of bainite and tempered martensite, and 3.0% or more of retained austenite,
A cold-rolled steel sheet having a metallographic structure in which the proportion of the residual austenite grains having another residual austenite grain different in crystal orientation by 10 ° or more in the range of the closest contact distance of 1 μm or less among the retained austenite 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 the above (1), wherein the metal structure contains 5.0% or more of ferrite at an area ratio, and an average crystal grain size of the ferrite is 4.0 μm or less.

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

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

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

(6) The chemical composition is in mass%,
B: 0.0003 to 0.005%
The cold rolled steel sheet according to any one of the above (1) to (5), which contains

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

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

(9) A steel material having a chemical composition according to any one of the above (1) or (3) to (7),
Hot rolling is performed after the rolling completion temperature is _{Ar 3} or more and the rolling reduction in final rolling is 20% or more, and
Then apply cold rolling,
Subsequently, heat treatment the average heating rate between 500 ° C. of _{A c1} point + 10 ° C. was heated so that the 15 ° C. / s or _more, soaking or 10s in a temperature range of _{A c3} point + 100 ° C. from _{A c3} point After applying
The manufacturing method of the cold-rolled steel plate which cools so that the average cooling rate between 650 degreeC and 500 degreeC may become 10 degrees C / s or more, and hold | maintains 10s or more in the temperature range of 500 degrees C to 300 degrees C.

  (10) After 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 Ms point or less and Ms point over 100 ° C. The method for producing a cold-rolled steel sheet according to the above (9), wherein the temperature range is maintained for 1 s or more, and then the temperature range is 300 ° C. to 500 ° C. .

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

  (12) In the cooling after the hot rolling, the cold according to any one of (9) to (11) above, wherein the time required for cooling from the hot completion temperature to 750 ° C. is 0.4 s or less Method of manufacturing rolled steel sheet.

  (13) The manufacturing method of the cold rolled steel sheet in any one of said (9) to (12) whose soaking holding time in the said heat processing is less than 30 s.

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

  According to the present invention, the structure after cold rolling and annealing can be effectively refined without containing a large amount of precipitation elements such as Ti and Nb, and ductility and stretch flangeability are excellent. It becomes possible to obtain a high strength cold rolled steel sheet. 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 construction structure, and the like.

It is the figure which showed the relationship between the nearest distance between the retained austenite grains in which misorientation differs mutually, and the number fraction of retained austenite grains in an example.

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

1. Chemical composition The reasons for limitation of each element are as follows. In the following description, “%” of the content means “mass%”.

C: 0.05 to 0.30%
C is an element having an effect of enhancing the strength of steel. Further, it has the function of stabilizing austenite by concentrating it in austenite, increasing the volume fraction of retained austenite in a cold rolled steel sheet, and improving ductility. Furthermore, C has the effect of lowering the transformation point, and as a result, in the hot rolling process, the hot rolling is completed at a lower temperature range, and the microstructure of the hot rolled steel sheet can be refined. In the annealing step, in combination with the recrystallization suppressing action of ferrite in the temperature rising process by C, the temperature range of the _Ac1 point + 10 ° C or higher is reached while maintaining the high state of the non-recrystallization rate of ferrite by rapid heating. The austenite grains can be refined during the annealing of the cold rolled steel sheet.

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

Si: 0.5 to 2.5%
Si is an element having a function of strengthening the steel by promoting the formation of bainite, martensite and tempered martensite which form the main phase of the cold rolled steel sheet according to the present invention. Furthermore, in order to promote the formation of retained austenite and to have the effect of improving the ductility of the steel, in the present invention, it is necessary to contain a certain amount or more. If the Si content is less than 0.5%, the above effects can not be obtained. On the other hand, when the Si content exceeds 2.5%, not only the ductility is significantly reduced, 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, 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 enhancing the strength of steel. In addition, since the austenitizing temperature is lowered, the soaking temperature can be lowered in the annealing step, and the grain growth can be suppressed to keep the structure fine. This makes it 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 an effect which ensures the area ratio of bainite, martensite, and tempered martensite required in the cooling after annealing.

  If the Mn content is less than 0.5%, the above effects can not be obtained. On the other hand, when 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 which is contained as an impurity and segregates at grain boundaries to embrittle the material. When the P content exceeds 0.1%, the embrittlement due to the above action becomes remarkable. Therefore, the P content is 0.1% or less. The P content is preferably 0.06% or less. The lower limit of the P content is preferably as low as possible. The lower limit of the P content 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 that is contained as an impurity and forms sulfide-based inclusions in the steel to reduce the ductility of the steel. When the S content exceeds 0.05%, the ductility is significantly reduced due to the above-mentioned action. Therefore, the S content is 0.05% or less. The S content is preferably 0.008% or less, more preferably 0.003% or less. The lower the S content, the more preferable, so the lower limit need not be limited.

sol. Al: 0 to 1.0%
Al is an element having an action of enhancing ductility. Therefore, Al may be contained if necessary. However, since Al has the effect of raising the transformation point, sol. If the Al content exceeds 1.0%, the hot rolling has to be completed at a higher temperature range. As a result, it is difficult to miniaturize the structure of the heat-rolled steel plate, and therefore, it is also difficult to miniaturize the structure of the cold-rolled steel plate. Therefore, sol. The Al content is 1.0% or less. sol. The Al content is preferably 0.7% or less. If you want to get the above effect, sol. The Al content is preferably 0.1% or more, 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 which precipitate in the steel as carbides and / or nitrides and have the function of promoting the refinement of the steel structure by suppressing the grain growth of austenite in the annealing step. Therefore, one or two of Ti and Nb may be contained.

  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 effects, 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 having the effect of enhancing the strength of steel. In addition, Mo suppresses the grain growth of crystal grains, and also has an action of refining the structure. Therefore, one or more selected from Cr, Mo and V may be contained, as necessary.

  However, if the Cr content exceeds 1.0%, ferrite transformation is excessively suppressed, and a target structure can not be secured. In addition, when the content of Mo and V exceeds 0.3%, a large amount of precipitates are formed in the heating stage of the hot rolling process, and the ductility is significantly reduced. Therefore, the content of each element is as described above. The Mo content is preferably 0.25% or less. In order to obtain the above effects, 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-described elements are contained in combination, the total content is preferably 1.2% or less.

B: 0 to 0.005%
B is an element having the effect of enhancing the strength of the steel by enhancing the hardenability of the steel and promoting the formation of a low temperature transformation phase. Therefore, B may be contained as needed. However, when the B content exceeds 0.005%, the steel is excessively hardened and the ductility is significantly reduced. Therefore, the B content is made 0.005% or less. When it is desired to obtain the above effects, the B content is preferably made 0.0003% or more.

Ca: 0 to 0.003%
REM: 0 to 0.003%
Ca and REM are elements having the function of refining the oxides and / or nitrides precipitated in the solidification process of the molten steel to improve the integrity of the slab. Therefore, if necessary, one or two of these elements may be contained. However, since each element is expensive, the content of each element is made 0.003% or less. When it is desired to obtain the above effects, 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 combinedly included, it is preferable to make the total content into 0.005% or less.

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

  The steel plate of the present invention has a chemical composition composed of the elements C to REM described above, and the balance Fe and impurities.

  Here, the term "impurity" is a component which is mixed due to various factors such as ore, scrap, etc. and various factors of the manufacturing process when industrially producing a steel plate, and is allowed within a range which does not adversely affect the present invention Means one.

2. Metallographic Structure The steel plate of the present invention contains, in terms of area ratio, 50% or more of bainite and tempered martensite in total and 3.0% or more of retained austenite, and the closest contact distance in the retained austenite is 1 μm or less Have a metallographic structure in which the proportion of other retained austenite grains differing in crystal orientation by 10 ° or more is 50% or more.

Total area ratio of bainite and tempered martensite: 50% or more The metallographic structure of the cold rolled steel sheet according to the present invention has bainite or a mixed structure of bainite and tempered martensite as a main phase. By increasing the area ratio of bainite and tempered martensite to make the microstructure homogeneous, it is possible to suppress the formation of minute voids when the steel plate is processed. In addition, bainite and tempered martensite also have an effect of increasing the strength of the steel sheet because they have a relatively hard structure. Therefore, the total area ratio of bainite and tempered martensite needs to be 50% or more, preferably 60% or more.

  The bainite contained in the cold rolled steel sheet according to the present invention is generated during overageing treatment after cooling from annealing soaking, or during overageing treatment after reheating after cooling to a temperature lower than the Ms point. Although this bainite has a lower hardness than tempered martensite, a certain elongation is secured. By including this structure, high strength can be obtained while securing good processability. In order to obtain the above effects, the area ratio of bainite contained in the microstructure as the main phase is preferably 10% or more, and more preferably 20% or more. In addition, when the bainite transformation proceeds, carbon can be concentrated to untransformed austenite, and 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 effects can be stably obtained by incorporating steel in the above-described amount.

  Further, the tempered martensite contained in the cold rolled steel sheet according to the present invention is martensite formed 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 tissue tempered by being reheated and maintained to a temperature range of 300 to 500 ° C., which exceeds the Ms point. Since carbon is precipitated as cementite, tempered martensite is softer than martensite and harder than bainite, so that the strength of the steel plate can be enhanced while obtaining a homogeneous microstructure. Furthermore, once martensite is generated during heat treatment, an effect of promoting bainite transformation that occurs after reheating can be obtained. As a result, since the concentration of C to retained austenite formed with bainite transformation is promoted, the stability of retained austenite can be increased. As a result, the workability of the steel sheet, particularly the hole expansibility, 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 metallographic structure of the cold rolled steel sheet according to the present invention contains retained austenite as a second phase. Since retained austenite has the effect of improving the elongation of the steel sheet, it is possible to secure an even more excellent 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 retained austenite grains having different crystal orientations in the range of the closest contact distance of 1 μm or less: 50% or more As described above, locally retained austenite having different crystal orientations in the metal structure of a cold rolled steel sheet When many are arranged in the region, when a steel plate is processed, retained austenite grains of various orientations undergo processing induced transformation. Then, along with that, since bainite and ferrite are also deformed under compression in various directions, deformation of the microstructure proceeds more uniformly and isotropically, local deformation is suppressed, and voids in the microstructure and Micro cracks are less likely to occur. As a result, good processability can be obtained.

  On the other hand, when retained austenite contained in a cold-rolled steel sheet has a wide range and the same crystal orientation, a large amount of retained austenite of the same orientation locally undergoes processing-induced transformation, so the strain anisotropy accompanying the processing-induced transformation The larger, the deformation of the microstructure increases the non-uniformity. As a result, strain and stress concentrate locally, leading to premature fracture, so that good ductility can not be obtained. Therefore, in the present invention, it is necessary to set the percentage of residual austenite particles having another residual austenite grain having a crystal orientation different by 10 ° or more in the range of the closest contact distance of 1 μm or less to 50% or more. The proportion of the retained austenite grains is preferably 55% or more.

The 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 a second phase. Ferrite has the effect of significantly improving the elongation of the steel sheet, and by making the structure finer, the effect of suppressing the development of fine cracks at the time of hole expansion processing can also be obtained. As a result, while the elongation is significantly improved, the decrease in the hole expansion ratio can be kept small, and a balance between the excellent elongation and the hole expansion ratio can be imparted to the steel plate. Therefore, it is preferable to contain 5.0% or more of fine ferrite having an average crystal grain size of 4.0 μm or less in area ratio.

  In addition, although pearlite and / or cementite may be mixed in the metallographic structure, a total area ratio of 10% or less is acceptable.

  The area ratio of bainite, tempered martensite and ferrite can be measured by structure analysis using SEM-EBSD. The ratio of tempered martensite and bainite contained in the main phase is the heat treatment of the corresponding steel plate under the same annealing conditions, and the thermal expansion during cooling of the annealing is measured, and the amount of expansion below the Ms point and the Ms point And the expansion amount at 500 ° C. or less, and can be obtained by multiplying the total area ratio described above by the respective ratio. Further, as the area ratio of retained austenite, the volume fraction obtained by the X-ray diffraction method is used as the area ratio as it is. The distance between the retained austenites and the misorientation can be determined by obtaining the information on the orientation and coordinates of the crystal grains from the above-described SEM-EBSD structure analysis results.

  Furthermore, the average grain size of the ferrite contained as the second phase can be obtained by using SEM-EBSD to obtain the grain size of the ferrite surrounded by large angle grain boundaries having a tilt angle of 15 ° or more. The SEM-EBSD is a method of measuring the orientation of a micro area by electron backscattering diffraction (EBSD) in a scanning electron microscope (SEM). The crystal grain size can be calculated by analyzing the obtained orientation map. The average grain size of the ferrite in the present invention means the average value of the equivalent circle diameters obtained by the following formula. However, Ai in the following formula represents the area of the ith ferrite grain, and di represents the equivalent circle diameter of the ith ferrite grain.

  In the present invention, measurement values at a thickness of 1⁄4 depth of the steel plate are adopted as the values of the area ratio and the average grain size.

3. Plated 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., as a surface treated steel sheet. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanization, Zn-Ni alloy plating and the like. 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. Ru.

  The plating adhesion amount is not particularly limited, and may be the same as in the prior art. In addition, it is also possible to further enhance corrosion resistance by forming a suitable chemical conversion coating on the plating surface (for example, by applying and drying a silicate-based chromium-free chemical conversion solution). Furthermore, it can also be coated with an organic resin film.

4. Manufacturing Method The method for manufacturing the cold rolled steel sheet according to the present invention is not particularly limited. For example, the steel material having the above-described chemical composition is subjected to hot rolling as described below and then cooled to obtain a hot rolled steel sheet. After producing it, it can be manufactured by applying cold rolling and further heat treatment such as annealing. The heat treatment is described below as annealing.

4-1 Cooling after Hot Rolling and Rolling In the present invention, the structure of a hot-rolled steel plate to be 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 grain boundaries of ferrite. In addition, cementite is more effective as a nucleation site because it exists on grain boundaries and boundaries. Therefore, the structure of the heat-rolled steel plate is preferably a structure in which cementite is finely dispersed.

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

  The temperature of the slab to be subjected to hot rolling is preferably 1000 ° C. or higher. 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 drops to the ferrite transformation temperature during rolling, and rolling is performed with the transformed ferrite in the structure There is a fear. From this, it is preferable that the heating temperature be sufficiently high so that the hot rolling can be completed in the austenite temperature range. If a temperature drop occurs during rolling, the steel sheet may be reheated 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 at least for the final few stages. Since it is necessary to maintain the steel plate in the austenite temperature range during rolling, it is preferable to set the rolling completion temperature to _{Ar 3} or more.

  In addition, the austenite structure becomes finer by increasing the reduction of the final rolling. Therefore, in hot rolling, the rolling reduction in final rolling is preferably 20% or more, and more preferably 25% or more. Since the austenite grain boundaries remain as prior austenite grain boundaries in the bainite and martensite structure after cooling the hot rolled steel sheet, the prior austenite grain boundaries are more closely distributed in the hot rolled sheet microstructure. become. Since this prior-austenite grain boundary serves as a nucleation site during annealing, it is possible to significantly increase the number of austenite nucleation in the heating process of annealing.

With respect to the overall reduction of the heat-rolled steel sheet, it is preferable to set the sheet thickness reduction rate to 40% or more when the temperature of the material to be rolled is in the temperature range of _Ar3 point to _Ar3 point + 150 ° C. It is more preferable to do. The rolling does not have to be performed in one pass, but may be continuous multiple rolling. More strain energy can be introduced into austenite, the transformation driving force to ferrite or bainite can be increased, and the hot rolled steel sheet can be finer. However, in order to increase the load on the rolling mill, it is preferable to set the upper limit of the rolling reduction per pass to 60%.

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

  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 finer and to increase the density of nucleation sites in annealing. By rapidly cooling after hot rolling, strain energy accumulated in the steel is controlled from transformation from austenite to ferrite or bainite while minimizing recovery of strain introduced into austenite and consumption due to recrystallization. It can be fully utilized as a force. The reason for setting the cooling rate at the start of cooling from the rolling completion temperature to 400 ° C./s or more is also to increase the transformation driving force as described above. Thereby, the number of transformation nucleation to ferrite etc. can be increased, and the structure of the hot rolled steel sheet can be refined. By using the hot-rolled steel plate having a fine structure manufactured in this manner as a material, the structure of the cold-rolled steel plate can be further refined.

  After completion of rolling, the residence time in the temperature range of 750 ° C. to 550 ° C. is preferably less than 15 s, and more preferably less than 10 s. This is to suppress the formation of coarse pearlite in the microstructure of the heat-rolled steel sheet. The formation of a large amount of pearlite reduces the number of nucleation during annealing. The area ratio of pearlite is preferably less than 10%, more preferably less than 5%. When staying for a long time in the above temperature range, alloy elements of the steel diffuse into perlite or cementite, and the concentration of Mn and the like in cementite increases. As a result, the dissolution of cementite in annealing is suppressed, and the annealing retention for a long time is required, which causes the problem of the refinement of the structure.

  As temperature and residence time suitable for tissue control, for example, cooling for about 6 seconds from 680 ° C. to 630 ° C. can be performed. Fine ferrite can be introduced into the structure of the hot-rolled sheet by this.

  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 less in order to refine the grains and cementite in the microstructure of the hot rolled steel sheet. When the coiling temperature is 300 ° C. or less, the strength of the hot rolled steel sheet may increase, which may make cold rolling difficult. In this case, annealing may be performed at a temperature of 650 ° C. or less before cold rolling.

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

  In addition, a large amount of large angle grain boundaries is introduced into the microstructure of the heat-rolled steel sheet, and the above-mentioned preferential nucleation site is achieved by setting the average grain diameter specified by the large angle grain boundaries of 15 ° or more 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 consisting of bainite and martensite, or a structure obtained by tempering them, and a mixed structure thereof.

4-2 Cold rolling After hot-rolled steel plate produced by the above-mentioned hot rolling is pickled, it is cold-rolled. The cold rolling may be performed using a conventional method, and the rolling reduction is usually 20% or more. Increasing the cold-rolling rate increases the density of grain boundaries in the structure, which can increase the number of nucleation during annealing heating.

4-3 Annealing The following annealing is applied to the steel sheet obtained by the above-mentioned cold rolling. First, the steel plate is heated so that the average heating rate between 500 ° C. and A _c1 point + 10 ° C. is 15 ° C./s or more. By rapidly heating, recrystallization in the middle of heating is suppressed, and heating can be performed to the _Ac1 point + 10 ° C while leaving the _{unrecrystallized} structure. As a result, a large number of austenite can be nucleated with cementite existing on large angle grain boundaries such as former austenite grain boundaries, packet block boundaries, and ferrite grain boundaries of the heat-rolled steel sheet as main nucleation sites. If the average heating rate between 500 ° C. and the _Ac1 point + 10 ° C. is less than 15 ° C./s, recrystallization will occur and the grain boundaries of the hot rolled steel sheet will disappear, so the above effect can not be obtained.

  By increasing the number of nucleation of austenite, the austenite grains during annealing can be significantly reduced, and as a result, in the microstructure of the cold-rolled steel sheet, retained austenite having a crystal orientation difference of 10 ° or more The closest contact distance can be shortened. In addition, ferrite and retained austenite in the microstructure can also be refined.

Increasing the heating rate not only increases the unrecrystallization rate but also increases the driving force of reverse transformation, which is preferable in increasing nucleation of austenite. Therefore, the average heating rate between 500 ° C. and the _Ac1 point + 10 ° C. is more preferably 30 ° C./s or more, and still more preferably 100 ° C./s or more. Although the upper limit in particular of an average heating rate is not provided, it is preferable to set it as 1000 degrees C / s or less, considering that temperature control becomes difficult.

When the _Ac point + 10 ° C. is reached, if the unrecrystallizing ratio in the non-austenite-transformed region is less than 30%, the region in which austenite transformation has progressed after recrystallization is complete. As a result, since austenite transformation proceeds from grain boundaries of recrystallized grains, the austenite grains during annealing become coarse, and the final structure also becomes coarse. Therefore, it is preferable that the unrecrystallized rate occupied to the area | region which is not austenite transformation at the time of reaching _Ac1 point +10 _degreeC becomes 30% or more.

  The temperature at which the above-mentioned rapid heating is started exerts a sufficient effect before recrystallization starts. 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 granulation effect can be obtained. In addition, by starting rapid heating from room temperature, it is possible to cause austenite transformation while keeping cementite fine, so it is preferable for refining the structure.

  The heating method preferably uses electric heating, induction heating or direct flame heating to obtain a sufficiently rapid heating rate, but heating with radiant tubes is also possible as long as the requirements of the present invention are satisfied. Furthermore, the application of these heating devices significantly shortens the heating time of the steel sheet, making it possible to make the annealing equipment more compact, and the effects of improving productivity and reducing equipment investment costs can also be expected. Moreover, it is also possible to add the rapid-heating apparatus to the existing continuous annealing line and the hot dip plating line, and to implement the said heating.

After heating to A _c1 point + 10 ° C., it is further heated from the A _c3 point to an annealing temperature (soaking temperature) of A _c3 point + 100 ° C. The heating rate at this time can be any rate. By lowering 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 is a lower heating rate.

In the annealing step, transformation to austenite is sufficiently advanced to extinguish the processed ferrite structure and dissolve carbides in the steel sheet. For this reason, the annealing temperature needs to be set to an _Ac3 point or more. In the case of annealing at a temperature lower than this, a worked austenite structure remains because the austenite single phase state is not obtained during annealing or recrystallization of ferrite does not occur. Then, in the texture of the cold rolled steel sheet, the orientation group of {100} <011> to {211} <011> is strengthened, and the workability of the steel sheet is reduced. On the other hand, when annealing is performed at a temperature higher than the _Ac3 point + 100 ° C, rapid grain growth of austenite grains occurs to coarsen the final structure. Therefore, it is preferable to set the annealing temperature to a temperature range from _Ac3 point to _Ac3 point + 100 ° C. In order to _refine the structure, it is more preferable to set the annealing temperature to an _Ac3 point + 50 ° C or less.

  Moreover, it is preferable to make the soaking holding | maintenance time of annealing into 10 s or more. If the soaking holding time is less than 10 s, the meltability of pearlite or cementite and the transformation to austenite do not sufficiently advance, so the workability of a cold-rolled steel sheet decreases. In addition, temperature unevenness is likely to occur during annealing and holding, which causes a problem in manufacturing stability. Therefore, it is preferable to set the annealing holding time to 10 s or more and sufficiently advance the transformation to austenite. On the other hand, when holding for an excessively long time, there is a possibility that the growth condition of austenite grains can not satisfy the condition of the dispersed state of retained austenite specified by the present invention. Therefore, the soaking holding time is preferably less than 10 minutes. Further, as the soaking holding time is shorter, retained austenite different in crystal orientation can be present closer to each other. Therefore, the soaking holding time is more preferably less than 30 seconds.

  After soaking, 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 above 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 above temperature range is less than 10 ° C./s, a large amount of ferrite is formed during cooling, and the stretch flangeability is deteriorated.

  In the above-described cooling process, the cooling can be stopped in the temperature range of 500 ° C. to 300 ° C., and the overaging treatment can be performed in the temperature range. In the over-aging treatment, retained austenite can be formed by controlling bainite of an appropriate area ratio in cold-rolled steel sheet and controlling the concentration of holding C to untransformed austenite by controlling the soaking holding temperature, holding time, etc. 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 cooling described above, cooling is stopped at a temperature range not exceeding the Ms point and exceeding the Ms point -100 ° C, and then the temperature is raised to the temperature range of 300 ° C to 500 ° C by exceeding the Ms point by reheating. It is more preferable to warm and perform overaging treatment in the temperature range.

  Before the overaging treatment, the cooling stop temperature is set to a temperature below the Ms point and a temperature above the Ms point -100 ° C., and after transforming a part of the microstructure to martensite, the Ms point is exceeded and 300 By reheating to a temperature range of 0 C to 500 0 C, the structure of the cold rolled steel sheet can be made a structure containing tempered martensite.

  When the cooling stop temperature is less than Ms point -100 ° C., the area ratio of bainite and retained austenite in the final structure is excessively reduced, and good workability can not be obtained. In addition, as for the stop temperature of the said cooling, it is more preferable to set it as the temperature which is below Ms point and exceeds Ms point -80 degreeC. When the reheating temperature exceeds 500 ° C., untransformed austenite is decomposed into carbides, and there is a possibility that good workability can not be obtained.

  In addition, after performing said superaging treatment or the overaging treatment after reheating, it cools to normal temperature as it is by methods, such as free cooling or water cooling, and also, for example, hot dip galvanization or alloying molten zinc A hot dip plating process, such as plating, can be performed. Further, the above-described hot-dip plating treatment can also be performed continuously to the above-described reheating and holding.

  If the cooling rate is too low, or if soaking at high temperatures for long periods of time, not only the desired structural fraction can not be obtained, but the retained austenite is transformed to carbides, etc. to deteriorate the workability of the steel sheet. It causes you to Therefore, it is desirable that the holding time (including plating and / or overaging) in the temperature range of 500 ° C. to 300 ° C. during cooling be less than 2000 s. The cooling method can be performed by any method, but for example, cooling by gas, mist or water is possible.

In the present invention, at _{Ar 3} points, a steel ingot melted in a vacuum induction furnace is processed into a cylindrical sample having a diameter of 8 mm and a height of 12 mm, heated to 900 ° C., and cooled at 2 ° C./s. , It obtains from the thermal expansion measurement result in the meantime. Further, the _Ac1 point and the _Ac3 point are determined from the thermal expansion curve measured when the cold-rolled steel plate is heated to 1100 ° C. at a heating rate of 2 ° C./s. Furthermore, the _{unrecrystallized} ratio occupied in the austenite-non-transformed region of the ferrite at the time of reaching the _Ac1 point + 10 ° C immediately after _raising the steel sheet subjected to cold rolling to the _Ac1 point + 10 ° C It can be obtained by water cooling, photographing the structure by SEM, and measuring the fraction of the recrystallized structure and the processed structure on the structure photograph.

  EXAMPLES Hereinafter, the present invention will be more specifically described by way of examples, but the present invention is not limited to these examples.

Steel ingots of steel types A to H having chemical compositions shown in Table 1 were melted in a vacuum induction furnace, hot forged, and then cut into slab-like steel pieces in order to be subjected to hot rolling. After heating each obtained slab at a temperature of 1000 ° C. or more for about 1 h, hot rolling was performed using a small test mill at a completion temperature of hot rolling and a final rolling reduction shown in Table 2. After completion of rolling, a hot-rolled steel plate having a thickness of 2.0 to 2.6 mm was produced under the conditions shown in Table 2. _{A r3} point grades A~H also shown in Table 2.

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

  The average grain size of the hot rolled steel sheet obtained by the above hot rolling is shown together in Table 2. The grain size of the heat-rolled steel sheet is measured by using a SEM-EBSD apparatus (JSM-7001F manufactured by Nippon Denshi Co., Ltd.) to determine the structure of the cross section in the rolling direction at the 1⁄4 depth position of the steel sheet. Regarding the BCC phase consisting of the site and bainite, the grain size consisting of large angle grain boundaries with an inclination angle of 15 ° or more was analyzed and determined.

The hot-rolled steel sheet thus obtained was pickled with hydrochloric acid and then cold-rolled at a rolling reduction shown in Table 3 to make the thickness of the steel sheet 1.0 to 1.3 mm. . Thereafter, using a laboratory scale annealing facility, the temperature range from room temperature to 750 ° C. is heated at an average heating rate shown in Table 3, and then the annealing temperature (soaking temperature) and soaking holding time shown in Table 3 Annealing was performed, and the 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. _{A c1} point and _{A c3} point grades A~H also shown in Table 3.

  Furthermore, the steel sheets without the plating treatment of Table 3 were subjected to overaging treatment under the conditions shown in Table 3 in the cooling process. Furthermore, after holding for 60 s at 400 ° C., the steel sheet with a plating treatment was heated to 460 ° C. to simulate immersion in a hot dip galvanizing bath, and further heated to 500 ° C. to perform heat treatment to simulate alloying treatment. After these overageing treatment or plating treatment, it was cooled to normal temperature to obtain a cold rolled steel plate or a hot-dip galvanized steel plate.

  The metallographic structure and mechanical properties of the cold rolled steel sheet thus produced were examined as follows.

  The area ratio of bainite, tempered martensite and ferrite was determined by structural analysis in the cross-sectional structure in the rolling direction at a plate thickness of 1⁄4 depth using the above-mentioned SEM-EBSD apparatus. In the present embodiment, since the cooling stop temperature after annealing is high, tempered martensite is not included in the structure in all steel plates. Further, the volume fraction of the retained austenite phase is determined by X-ray diffraction, and this is taken as the area fraction of retained austenite (residual γ). The grain size of the ferrite is, as in the case of the above-described hot-rolled steel plate, the structure of the cross-section from the width direction of the cold-rolled steel plate using a SEM-EBSD apparatus (JSM-7001F manufactured by JEOL Ltd.), the inclination angle It was determined as an average grain size defined by large angle grain boundaries of at least °.

  Furthermore, the closest contact distance of retained austenite different in crystal orientation by 10 ° or more was obtained by obtaining the Euler angles and position coordinates of the austenite crystal grains from the analysis result of the above-mentioned SEM-EBSD, and calculating it. . In this measurement, residual austenite of about 0.13 μm or more was targeted, and in analyzing one microstructure, 2000 or more residual austenite were subjected to measurement.

  The EBSD analysis of the structure containing the retained austenite phase is susceptible to the surface state of the sample. Therefore, in this example, the area ratio of retained austenite (γEBSD) obtained by EBSD analysis is an index of analysis accuracy. The premise of the evaluation is that the ratio of (γEBSD / γXRD)> 0.7 is satisfied 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 the tensile strength (TS) and the breaking elongation (total elongation, El) were determined. The hole expansion test was performed according to JIS Z 2256 (2010) to determine the hole expansion ratio λ (%). The value of TS × El was used as an index of balance of strength and ductility, and the value of TS × λ was used as an index of balance of strength and stretch flangeability. The results are shown in Table 4 respectively. In the present invention, it was determined that the strength and the processability were good if the balance of TS × El and TS × λ satisfied the following equation (ii). The value of the right side of the equation (ii) is more preferably 196000.
6.2 × TS × El + TS × λ> 190000 ・ ・ ・ (ii)

Among the test numbers 1 to 11 manufactured using steel type A, the test No. 3 which is a comparative example has a low heating rate at the time of annealing, and the test No. 4 has a long residence time of 750 to 550 ° C. after hot rolling. No. 6 did not reach the final rolling reduction of 20% in hot rolling, and test No. 10 had an excessively long residence time of 750 to 550 ° C., and the winding temperature exceeded 600 ° C., respectively. Was inappropriate. Due to this, the structure of the heat-rolled steel plate becomes coarse, and as a result, the proportion of the small closest contact distance of residual austenite grains having different crystal orientations decreases. Furthermore, test No. 5 had a soaking temperature of annealing lower than the A _c3 point, and test No. 9 had a low cooling rate after annealing. As a result, not only the proportion of those with a small closest distance to residual austenite grains having different crystal orientations decreases, but the area fraction of bainite also decreases, and the grain size of ferrite is coarsened. In Test No. 5, the processed ferrite remained even after annealing. As a result, the mechanical properties became insufficient.

  On the other hand, in Test Nos. 1, 2, 7, 8 and 11 which are inventive examples, the manufacturing conditions were appropriate, and the metal structure satisfied the definition of the present invention, and the result was excellent in strength and processability. Among them, test No. 2 sets the soaking holding time in annealing to 30 s or less, and tests No. 7 and 8 set the rolling reduction in hot rolling to 33% and perform cooling rapidly and soaking in annealing The holding time is set to 30 s or less, and test No. 11 is 40% in hot rolling and the cooling is performed rapidly. Therefore, compared with test No. 1, the closest contact distance of retained austenite grains different in crystal orientation The proportion of small ones was high and exhibited better mechanical properties.

  In addition, FIG. 1 shows the results of Test No. 2 which is an example of the present invention and Test No. 3 which is a comparative example, as an example of the result of calculating the closest contact distance between retained austenites different in crystal orientation by 10 ° or more. The vertical axis in FIG. 1 indicates the number fraction of retained austenite. From FIG. 1, test No. 2 has many retained austenite grains having a closest contact distance of 1 μm or less, and the ratio exceeds 50%, while test No. 3 has a ratio of retained austenite having a closest touch distance of more than 1 μm It can be seen that there are many. As described above, in the metallographic structure of the steel sheet according to the present invention example, retained austenite having an orientation difference of 10 ° or more exists in close proximity, which makes deformation in the microstructure uniform and improves the workability of the steel sheet. Improve.

  Also in the results of other steel types, similarly, the test No. 16 as a comparative example has a high soaking temperature for annealing, the test Nos. 19, 33, 36 and 38 have a low heating rate at the time of annealing, and the test No. 22 has a hot rolling With a subsequent residence time of 750-550 ° C excessively, the coiling temperature exceeds 600 ° C, and the test numbers 23 and 27 do not reach the final rolling reduction of 20% of the hot rolling, and further, the test number No. 26 and 29 had a long residence time of 750 to 550 ° C. after hot rolling, and the production conditions were respectively inadequate. As a result, the proportion of retained austenite grains having different crystallographic orientations with a small closest distance is low, resulting in insufficient mechanical properties.

  In addition, Test Nos. 39 and 40 have low Si content, and Test No. 41 has low C and Si contents, so the area ratio of bainite is low, and almost no retained austenite is generated, so the ductility is significantly inferior. It became.

  On the other hand, Test Nos. 12 to 15, 17, 18, 20, 21, 24, 25, 28, 30 to 32, 34, 35 and 37, which are inventive examples, have chemical compositions and metal structures of the present invention. As the requirements were satisfied, the result was excellent in strength and processability.

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

  The average grain size of the hot rolled steel sheet obtained by the above hot rolling is also shown in Table 6. The measurement of the crystal grain size of the hot-rolled steel sheet was performed in the same manner as the method described in Example 1.

The hot-rolled steel sheet thus obtained was pickled with hydrochloric acid and then cold-rolled at a reduction ratio shown in Table 7 to make the thickness of the steel sheet 1.0 to 1.3 mm. . After that, after annealing with a laboratory scale annealing facility and using the average heating rate, soaking temperature (annealing temperature), and soaking time (holding time) shown in Table 7, the “average cooling rate described in Table 7 It cooled to the temperature described in the same table as "cooling stop temperature" by "." Furthermore, the steel plates without plating treatment of Table 7 were reheated from the cooling stop temperature to raise the temperature to 400 ° C., and were subjected to overaging treatment at the temperature to perform soaking for 330 s. The steel sheet with plating treatment is reheated from the cooling stop temperature to raise the temperature to 400 ° C. After holding for 60 seconds at that temperature, it is heated to 460 ° C. to simulate immersion in a hot dip galvanizing bath, and further 500 ° C. It heats up to heat treatment which simulates alloying treatment. Thereafter, the steel plate without plating treatment and the steel plate with plating treatment were cooled to normal temperature at an average cooling rate of 2 ° C./s to obtain a cold rolled steel plate. 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 different values depending on the annealing temperature, the amount of ferrite formed during cooling, and the like. For this reason, Table 7 describes Ms points obtained from the thermal expansion curve measured when the cold-rolled steel sheet was cooled from a predetermined annealing temperature.

  In the metal structure of the cold-rolled steel plate manufactured in this way, the area ratio of bainite, tempered martensite and ferrite is a structure in the cross-sectional structure in the rolling direction at a thickness of 1⁄4 of the steel plate using the above-mentioned SEM-EBSD apparatus. It asked by analysis. Furthermore, the ratio of bainite which is the main phase to tempered martensite heats the corresponding steel plate under the same annealing conditions, measures the thermal expansion during the cooling of the annealing, and the amount of expansion below the Ms point and the Ms point The ratio to the amount of expansion at 500 ° C. or less was determined, and the ratio was calculated by multiplying the total area ratio described above by each ratio. Further, the area ratio of the retained austenite phase, the grain diameter of ferrite, and the closest distance of retained austenite different in crystal orientation by 10 ° or more were determined by the same method as in Example 1 described above.

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

The investigation results of the mechanical properties are shown in Table 8. In the present invention, in the same manner as in Example 1, it was determined that the strength and the processability were good if the balance of TS × El and TS × λ satisfied the following equation (ii). The value of the right side of the equation (ii) is more preferably 196000.
6.2 × TS × El + TS × λ> 190000 ・ ・ ・ (ii)

  Of the test numbers 42 to 49 manufactured using steel type K, the test number 44 which is a comparative example has a low heating rate at the time of annealing, and the test number 45 has an excessively long residence time of 750 to 550 ° C. after hot rolling. At the same time, the winding temperature exceeded 600 ° C., and the production conditions were inadequate. Due to this, the structure of the cold-rolled steel plate becomes coarse, and as a result, the proportion of retained austenite grains having different crystallographic orientations with a small closest distance decreases. In Test No. 48, since the cooling stop temperature after soaking in annealing was lower than the Ms point −100 ° C., the area ratio of tempered martensite was increased, whereby the retained austenite area ratio was decreased. Moreover, the test number 49 had the low cooling rate between 500-650 degreeC after soaking of annealing. Therefore, not only the ratio of the small closest contact distance of retained austenite grains having different crystal orientations decreases, but also the area ratio of the total of bainite and tempered martensite decreases, and the grain size of ferrite is coarsened. As a result, the mechanical properties became insufficient.

  On the other hand, in Test Nos. 42, 43, 46 and 47 which are inventive examples, since the manufacturing conditions were appropriate, the metal structure satisfied the definition of the present invention, and the microstructure contained an appropriate amount of tempered martensite. As a result, mechanical properties excellent in strength and processability, in particular, hole expandability, were obtained. Among them, Test No. 43 in which cooling after hot rolling was performed rapidly, and Test No. 46 in which quenching was performed with a rolling reduction of 33% or more in hot rolling and a holding temperature of 30 s or less in annealing, Compared to Test No. 42, the proportion of retained austenite grains having different crystallographic orientations with a smaller closest distance is higher, and exhibits better mechanical properties.

Also in the results of other steel types, similarly, the test numbers 52 and 60 as comparative examples have a low heating rate at the time of annealing, the test number 62 has a high soaking temperature for annealing, and the test number 66 has 750 to 5050 after hot rolling. The residence time of 550 ° C. was excessively long, the winding temperature exceeded 600 ° C., and the production conditions were respectively inadequate. As a result, the proportion of retained austenite grains having different crystallographic orientations with a small closest distance is low, resulting in insufficient mechanical properties. In Test No. 53, the soaking temperature of the annealing was lower than the A _c3 point, and the machined ferrite structure remained in the microstructure even after the annealing, resulting in inferior mechanical properties.

  In addition, Test Nos. 67 and 68 have low Si content and Test No. 69 has low C content, so the total area ratio of bainite and tempered martensite is low, and almost no retained austenite is generated, so the strength is The balance between this and ductility was poor.

  On the other hand, Test Nos. 50, 51, 54 to 59, 61 and 63 to 65, which are inventive examples, are excellent in strength and processability because the chemical composition and metal structure satisfy the definition of the present invention. It became a result.

  According to the present invention, the structure after cold rolling and annealing can be effectively refined without containing a large amount of precipitation elements such as Ti and Nb, and ductility and stretch flangeability are excellent. It becomes possible to obtain a high strength cold rolled steel sheet. 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 construction structure, and the like.

Claims (13)

The chemical composition is in 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%,
Remainder: Fe and impurities,
Satisfy the following equation (i),
The area ratio contains 20% or more of bainite and 10% or more of tempered martensite , and is 50% or more in total, contains 3.0% or more of retained austenite, and 5.0% or more of ferrite Perlite and / or cementite containing 24% or less and the balance 10% or less in total
The average grain size of ferrite is 4.0 μm or less,
A cold-rolled steel sheet having a metallographic structure in which the proportion of the residual austenite grains having another residual austenite grain different in crystal orientation by 10 ° or more in the range of the closest contact distance of 1 μm or less among the retained austenite 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 chemical composition is, in mass%,
sol. Al: 0.1 to 1.0%
The cold rolled steel sheet according to claim 1, which contains The chemical composition is, in mass%,
Ti: 0.005 to 0.050% and Nb: 0.003 to 0.030%
The cold rolled steel sheet of Claim 1 or 2 containing 1 type or 2 types selected from (1 ) . The chemical composition is, in mass%,
Cr: 0.03 to 1.0%,
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 3 , which contains one or more selected from the following. The chemical composition is, in mass%,
B: 0.0003 to 0.005%
The cold rolled steel sheet according to any one of claims 1 to 4 , which contains The chemical composition is, in mass%,
Ca: 0.0005 to 0.003% and REM: 0.0005 to 0.003%
The cold rolled steel sheet according to any one of claims 1 to 5 , which contains one or two selected from the following. The cold rolled steel sheet according to any one of claims 1 to 6 , having a plated layer on the surface of the steel sheet. It is a manufacturing method of the cold rolled steel sheet in any one of Claim 1- Claim 7, Comprising:
The steel material having the chemical composition according to any one of up to claim 1 or we claim 6,
Hot rolling is performed after the rolling completion temperature is _{Ar 3} or more and the rolling reduction in final rolling is 20% or more, and
Then apply cold rolling,
Subsequently, heat treatment the average heating rate between 500 ° C. of _{A c1} point + 10 ° C. was heated so that the 15 ° C. / s or _more, soaking or 10s in a temperature range of _{A c3} point + 100 ° C. from _{A c3} point After applying
The manufacturing method of the cold-rolled steel plate which cools so that the average cooling rate between 650 degreeC and 500 degreeC may become 10 degrees C / s or more, and hold | maintains 10s or more in the temperature range of 500 degrees C to 300 degrees C. After 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 8 , wherein after holding for 1 s or more, heating is performed to a temperature range of 300 ° C. to 500 ° C. above the Ms point and maintained for 10 s or more in the temperature range. The cold rolling according to claim 8 or 9 , wherein in the cooling after the hot rolling, the residence time in the temperature range of 750 ° C. to 550 ° C. is less than 15 s, and then winding is performed at a temperature of 550 ° C. or less. Method of manufacturing steel plate. The cold rolled steel sheet according to any one of claims 8 to 10 , wherein the time required for cooling from the hot completion temperature to 750 ° C in the cooling after the hot rolling is 0.4 s or less. Method. The manufacturing method of the cold rolled steel plate in any one of Claim 8 to 11 whose soaking holding time in the said heat processing is less than 30 s. The method for producing a cold rolled steel sheet according to any one of claims 8 to 12 , wherein the steel sheet surface is subjected to a plating treatment in the middle of cooling after the heat treatment.

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