WO2020004561A1

WO2020004561A1 - Hot-rolled steel sheet, high-strength cold-rolled steel sheet, and manufacturing methods therefor

Info

Publication number: WO2020004561A1
Application number: PCT/JP2019/025644
Authority: WO
Inventors: 啓志桂; 暢宏岩元; 伸一竹松; 冬樹吉田; 隆志山下; 和彦安樂
Original assignee: 東洋鋼鈑株式会社; 株式会社中山製鋼所
Priority date: 2018-06-29
Filing date: 2019-06-27
Publication date: 2020-01-02
Also published as: TW202012642A; CN112313352B; TWI809136B; JPWO2020004561A1; JP7217274B2; KR20210028610A; CN112313352A

Abstract

Provided are: a high-strength cold-rolled steel sheet for which little load is required during cold working and which has superior formability and ductility such that certain conditions are satisfied with regard to the evaluation of formability and retained austenite; a hot-rolled steel sheet which serves as the material of the high-strength cold-rolled steel sheet; and manufacturing methods for the high-strength cold-rolled steel sheet and the hot-rolled steel sheet, each of the manufacturing methods including a first step for subjecting a rolling material to rough rolling with a total rolling reduction of 30% or greater, a second step for subjecting the rolling material to finish rolling with a total rolling reduction of 40% or greater using a plurality of different diameter rolls having diameters that differ from one another in a temperature environment of 800ºC or greater, and a third step for winding the rolled product in a temperature environment of 700ºC or greater.

Description

Hot-rolled steel sheet, high-strength cold-rolled steel sheet, and methods for producing them

The present invention relates to a high-strength cold-rolled steel sheet and a hot-rolled steel sheet having excellent formability, and more particularly, to draw out a high-strength cold-rolled steel sheet having ductility that can withstand severe processing such as deep drawing and excellent formability. Hot-rolled steel sheet and a method for producing the same.

For example, automobiles that are indispensable as modern means of transportation use parts manufactured by pressing a high-strength steel plate. In manufacturing such a high-strength steel sheet, a manufacturing method such as a so-called hot press method may be used.
The hot press method has the advantage that the amount of springback is extremely small and the shape is freezingly good because the steel sheet is softened in a high temperature environment and hot pressed. Another advantage is that parts having very high strength can be provided with high precision by the quenching effect at the time of hot pressing.

However, in the above-described hot press method, it is essential to heat the steel sheet to a high temperature before press working, and it is necessary to drop the scale after hot pressing. Therefore, although the hot press method has the above-mentioned advantages, it also has a drawback that the work efficiency is generally extremely low and the cost is high. Further, there is another drawback that the press molding die comes into contact with the heated steel plate, so that the life of the die is relatively short, which also causes an increase in manufacturing cost.

On the other hand, in addition to the above-mentioned automotive parts, for example, display frame parts used for mobile phones, notebook computers, and the like are often formed as cold-rolled steel sheets using a so-called cold press. Such cold working is generally a method of working in a temperature environment of 720 ° C. or lower, and also has a feature that the metal structure of the steel sheet becomes dense.

In recent years, there is a severe demand for weight reduction and miniaturization of information equipment and automobile parts in recent years, and it is necessary to make cold-rolled steel sheets thin in order to reduce the weight and size of these parts at low cost. Since the strength of a pressed steel sheet cannot be ensured with the same strength in a thinned steel sheet, it is necessary to provide a thin steel sheet having high strength. On the other hand, it is also necessary to consider that if only the strength is pursued, the ductility is reduced and cracks occur during press molding.

In order to respond to such demands, for example, high-strength and high-ductility materials (hereinafter, also referred to as “TRIP steel”) as exemplified in Patent Documents 1 to 4 have been proposed.
For example, in Patent Document 1, a hot-rolled steel sheet having a tensile strength of 1000 MPa or less is cold-rolled at a rolling rate of 60% or more in total to form a cold-rolled steel sheet, and further, an annealing treatment is performed at a soaking temperature of 750 ° C. or more. It is disclosed that a high-strength cold-rolled steel sheet having a tensile strength of 1280 MPa or more and a breaking elongation of 3% or more can be obtained by cooling at a temperature of 3 ° C./s to 100 ° C./s.

Japanese Patent No. 5717631 JP 2013-76162 A JP 2012-41573 A JP 2012-214868 A

However, the conventional technologies including Patent Documents 1 to 4 cannot satisfy the needs of the market, and have the following problems.
First, the high-strength cold-rolled steel sheet disclosed in Patent Literature 1 certainly has excellent properties in which both strength and ductility are compatible. However, in order to further reduce the weight and size, a high-strength cold-rolled steel sheet having higher ductility is required. It is expected that a high strength cold rolled steel sheet will be desired.

Although Patent Documents 2 to 4 which disclose TRIP steel are referred to as having excellent formability, practical evaluations such as deep drawing formability have hardly been performed, and the contents of the examples are also limited. The limit draw ratio is described in some documents, and the evaluation of formability is clearly insufficient. In addition, in these documents, regarding the evaluation of retained austenite which is a key element in TRIP steel, the amount of retained austenite is only described to the extent described, and such a TRIP steel may cause local cracking and the like. There is still room for improvement.

The present invention has been made in view of solving such problems as an example, and has a small load at the time of cold working, and furthermore, formability and ductility satisfying certain conditions with respect to evaluation of formability and retained austenite. It is an object of the present invention to provide a high-strength cold-rolled steel sheet excellent in heat resistance, a hot-rolled steel sheet as a material thereof, and a method for producing these.

In order to solve the above-mentioned problems, a method for producing a hot-rolled steel sheet as a material for a high-strength cold-rolled steel sheet according to an embodiment of the present invention includes the following steps: (1) C: 0.1 to 0 0.3%, Si: 1.0 to 2.0%, Mn: 1.0 to 2.5%, Cr: 0.5% or less, Ni: 1.0% or less, P: 0.01% or less, S: 0.006% or less, N: 0.015% or less, Cu: 0.5% or less, the remainder being a rolling material having a composition of Fe and unavoidable impurities at a rolling reduction of 30% or more in total. A first step of roughly rolling the material, and after the first step, a plurality of different diameters different from each other in a temperature environment of 800 ° C. or more to suppress agglomeration and coarsening of retained austenite after cold rolling and annealing. A second step of finish rolling the rolled material at a total reduction of 40% or more using a diameter roll; After the second step, characterized in that it comprises a third step it is tensile strength for retracting the rolling stock to produce a hot rolled steel sheet in the following 900MPa at a temperature environment of more than 700 ° C..

In the method for manufacturing a hot-rolled steel sheet according to the above (1), (2) in the first step, it is preferable to perform rough rolling on the rolled material in a temperature environment of 1100 ° C. or more.

Further, in the method for producing a hot-rolled steel sheet according to the above (1) or (2), (3) in the second step, the average draft per rolling mill in the pre-finishing stage is 40% or more; In addition, it is preferable to perform finish rolling so that the cumulative strain under rolling by a rolling mill in the latter stage of finishing is 0.5 or more.

In order to solve the above problems, a method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention is obtained by the method for manufacturing a hot-rolled steel sheet according to any one of the above (1) to (3). A fourth step of cold rolling the hot rolled steel sheet to produce a cold rolled steel sheet.

In the method for manufacturing a high-strength cold-rolled steel sheet according to (4), (5) in the fourth step, the hot-rolled steel sheet may be cold-rolled at a rolling reduction of 60% or more in total. preferable.

In the method of manufacturing a high-strength cold-rolled steel sheet according to the above (5), (6) after the fourth step, the cold-rolled steel sheet is annealed at a soaking temperature equal to or higher than the Ac point and then cooled and held. Preferably, the method further includes a fifth step.

In the method for producing a high-strength cold-rolled steel sheet according to any one of the above (4) to (6), (7) the hot-rolled steel sheet has a thickness of 1.2 to 3.0 mm, It is preferable that the thickness of the rolled steel sheet is 0.01 to 0.6 mm.

In order to further solve the above-described problems, the hot-rolled steel sheet according to one embodiment of the present invention has a content of (8) mass% of C: 0.1 to 0.3% and Si: 1.0 to 1.0%. 2.0%, Mn: 1.0 to 2.5%, Cr: 0.5% or less, Ni: 1.0% or less, P: 0.01% or less, S: 0.006% or less, N: 0.015% or less, Cu: 0.5% or less, the balance has a composition of Fe and inevitable impurities, the thickness is 1.2 to 3.0 mm, and the tensile strength is 900 MPa or less. And

In order to further solve the above-mentioned problems, the high-strength cold-rolled steel sheet according to one embodiment of the present invention has a content of (9) mass% of C: 0.1 to 0.3%, Si: 1. 0 to 2.0%, Mn: 1.0 to 2.5%, Cr: 0.5% or less, Ni: 1.0% or less, P: 0.01% or less, S: 0.006% or less, N: 0.015% or less, Cu: 0.5% or less, the balance has a composition of Fe and inevitable impurities, the main phase is a bainite structure, and in addition to the bainite structure, a ferrite structure, a martensite structure, and a residual One or more residual austenite grains having an austenite structure and having a particle size of less than 10 μm per unit area of 10 μm square are dispersed, and a tensile strength TS is 700 MPa or more and 1400 MPa or less, and when the elongation at break is EL%, TS ≧ 1400 -(30 × EL).
In order to obtain the high-strength cold-rolled steel sheet according to (9), the content is the same as described above in terms of mass%, the thickness is 1.2 to 3.0 mm, and the tensile strength is It is preferable to use a hot-rolled steel sheet having a strength of 900 MPa or less as a material.

In the high-strength cold-rolled steel sheet according to the above (9), it is preferable that (10) the n value indicating the work hardening property of the high-strength cold-rolled steel sheet is 0.20 or more.

In the high-strength cold-rolled steel sheet according to the above (9) or (10), it is preferable that (11) the volume ratio of the retained austenite structure is 8% or more.

において In the high-strength cold-rolled steel sheet according to any one of the above (9) to (11), (12) the high-strength cold-rolled steel sheet preferably has a thickness of 0.01 to 0.6 mm.

Further, in the high-strength cold-rolled steel sheet according to any one of the above (9) to (12), (13) it is preferable that a critical overhang height of the high-strength cold-rolled steel sheet is 6.5 mm or more. .

In the high-strength cold-rolled steel sheet according to any one of the above (9) to (13), it is preferable that (14) the critical drawing ratio of the high-strength cold-rolled steel sheet is 2.0 or more.

In the high-strength cold-rolled steel sheet according to any one of (9) to (14), (15) it is preferable that Δr in the high-strength cold-rolled steel sheet is in a range of ± 0.7.

Further, in the high-strength cold-rolled steel sheet according to any one of the above (9) to (15), (16) the ear ratio of the high-strength cold-rolled steel sheet is preferably 10% or less.

According to the present invention, it is possible to realize an excellent high-strength cold-rolled steel sheet which has a small load at the time of cold working and can achieve both high formability and high strength in a high dimension. Alternatively, according to the present invention, it is possible to provide a hot-rolled steel sheet as a material for realizing such an excellent high-strength cold-rolled steel sheet.

It is a figure showing typically finish rolling mill 1 in this embodiment. It is a cross-sectional structure | tissue photograph by the EBSD method of the cold rolled steel sheet in this embodiment. It is a cross-sectional structure | tissue photograph by the EBSD method of the cold rolled steel plate manufactured out of the range of this invention.

As a result of diligent research on steel sheets with high strength and excellent ductility, the inventors found that ideal ductility was achieved by adopting appropriate component composition, hot rolling conditions, cold rolling conditions, annealing conditions, etc. It has been found that a preferable high-strength steel sheet can be obtained. That is, a slab having an appropriate component range was subjected to high-pressure rolling by rough rolling in hot rolling, further high-strain rolling in finish rolling was completed at a high temperature, and air-cooled for a predetermined time (for example, several seconds). It has been found that by starting cooling afterwards, by winding the steel sheet cooled in an appropriate temperature environment, it is possible to obtain a hot-rolled steel sheet that can be easily cold-worked and has excellent structure uniformity. Further, it has been found that a high-strength cold-rolled steel sheet excellent in formability can be manufactured by performing appropriate cold rolling on the hot-rolled steel sheet and performing final annealing under appropriate conditions.
Hereinafter, details of the steel plate and the like in the present embodiment that embody the above knowledge will be described.

<Rolled material>
A slab piece having a specific composition can be used as the rolled material used in the method for manufacturing a high-strength cold-rolled steel sheet and the hot-rolled steel sheet as the material of the present embodiment. As for the composition, as content in mass%, C: 0.1 to 0.3%, Si: 1.0 to 2.0%, Mn: 1.0 to 2.5%, Cr: 0. 5% or less, Ni: 1.0% or less, P: 0.01% or less, S: 0.006% or less, N: 0.015% or less, Cu: 0.5% or less, the balance being Fe and inevitable Impurities.

C (carbon) is an important element for stabilizing the retained austenite structure, which is a feature of the present embodiment. As described above, the content of C is required to be 0.1 to 0.3%. If the C content is less than 0.1%, the required stability of the retained austenite structure cannot be obtained. On the other hand, if the C content exceeds 0.3%, for example, when a steel plate is welded, there is a problem that the welded portion is excessively hardened and is easily broken from the welded portion.

Si (silicon) is also an important element for stabilizing the retained austenite structure. As for the amount of Si, a content of 1.0 to 2.0% is required as described above. Further, Si is an element that also contributes to improvement of the strength of the steel sheet by solid solution strengthening. As the Si content increases, the stability of the retained austenite structure of the steel sheet and the volume ratio thereof increase. The reason for defining the Si content as described above in the present embodiment is as follows. That is, when the Si content is less than 1.0%, the composite structure and material properties of the steel sheet required for the present embodiment cannot be obtained. On the other hand, if the amount of Si exceeds 2.0%, a favorable balance between the strength of the steel sheet and the ductility required for the present embodiment cannot be obtained. Further, from the viewpoint of cost reduction, in the present embodiment, the upper limit of the amount of Si is set to 2.0%.

Mn (manganese) is an element required to increase the strength of a steel sheet. The amount of Mn needs to be 1.0 to 2.5% as described above. When the amount of Mn is less than 1.0, the amount of ferrite increases and high steel sheet strength cannot be obtained. On the other hand, when the amount of Mn exceeds 2.5%, martensite is easily generated, and a composite structure required in the present embodiment cannot be obtained. Therefore, in the present embodiment, the above-described Mn amount is defined.

The amount of Cr (chromium) needs to be 0.5% or less as described above. This is because when the Cr content exceeds 0.5%, the Ac1 transformation point rises, and there is a problem that the cost increases when annealing is performed at the Ac1 transformation point or higher. Therefore, in the present embodiment, the above-described amount of Cr is defined.

The amount of Ni (nickel) needs to be 1.0% or less as described above. The strength of the steel sheet can be improved by adding Ni. If the Ni content exceeds 1.0%, martensite is likely to be generated, and a composite structure required in the present embodiment cannot be obtained. In the present embodiment, Ni is defined as described above from the viewpoint of cost.

P (phosphorus) needs to be reduced as much as possible to improve the weldability of the steel sheet. Therefore, in this embodiment, the P content is set to 0.01% or less.

S (sulfur) also needs to be reduced as much as possible to improve the weldability of the steel sheet. Therefore, in the present embodiment, the S amount is set to 0.006% or less.

N (nitrogen) is an element necessary for stabilizing the austenite structure like carbon. On the other hand, the reason for setting the N content to 0.015% or less in the present embodiment is that if the N content exceeds 0.015%, the weldability of the steel sheet is reduced.

Cu (copper) is an element necessary for improving the strength by solid solution strengthening or precipitation strengthening, so that a certain amount can be added. On the other hand, the reason for setting the Cu content to 0.5% or less in the present embodiment is that there is a possibility that embrittlement may occur during hot rolling.

組成 As for the composition of the rolled material of the present embodiment, the balance is Fe and inevitable impurities. The unavoidable impurities refer to components that are included even without intentional addition. Specific examples of such inevitable impurities include Zn: 0.03% or less, and Sn: 0.3% or less.

<Production method of high-strength cold-rolled steel sheet and hot-rolled steel sheet as its material>
The method for manufacturing a high-strength cold-rolled steel sheet according to the present embodiment includes a hot rolling step described below and a cold rolling step. Further, in particular, in the method for producing a high-strength cold-rolled steel sheet and a hot-rolled steel sheet as a material of the present embodiment, in the hot rolling step, the rolling material having the above composition is rolled at a rolling reduction of 30% or more in total. A first step of roughly rolling the material, and after the first step, a plurality of different diameters different from each other in a temperature environment of 800 ° C. or more to suppress agglomeration and coarsening of retained austenite after cold rolling and annealing. A second step of finishing rolling the rolled material at a total reduction of 40% or more using a diameter roll, and after the second step, winding the rolled material in a temperature environment of 700 ° C or more. And producing a hot-rolled steel sheet having a tensile strength of 900 MPa or less.
Hereinafter, a method for producing the high-strength cold-rolled steel sheet and a hot-rolled steel sheet as a material thereof will be described in detail.

<Steel making>
First, a slab (rolled material) adjusted to the above component range by a known method is prepared. Known equipment such as a converter and an electric furnace can be used for preparing the slab.

<Hot rolling>
The hot rolling process in the present embodiment includes a rough rolling process, a finish rolling process, and a winding process, as described later.

(4) The obtained rolled material is first heated to 1100 ° C. or more, and then roughly rolled at a rolling reduction of 30% or more in total (first step). If the heating temperature of the rolled material is lower than 1100 ° C., it is not preferable because the active decomposition and solid solution of N is insufficient and the hot rolling load is increased.

After the first step, the rolled material is finish-rolled under a temperature environment of 800 ° C. or more using a plurality of rolls having different diameters with a total reduction of 40% or more (second step). . Specifically, it is preferable to perform finish rolling using, for example, a six rolling mill or a seven rolling mill.

At this time, the finishing pre-rolling mills 1 to 3 (in the case of six rolling mills) or the finishing pre-rolling mills 1 to 4 (in the case of seven rolling mills) are used. Rolling can be performed at an average draft of 40% or more. In this case, it is preferable that the cumulative strain under the rolling of the finishing third rolling mill is 0.5 or more. If the cumulative strain is less than 0.5, the agglomeration and grains of the retained austenite become large, and the shape of the retained austenite grains becomes rolled flat, which is not preferable because it causes a large anisotropy. .

The “cumulative distortion” is obtained by weighting and integrating the distortion in each stage (each pass) of the subsequent three stands in consideration of the strength of the influence on the metal structure. The final stage (final pass) and the preceding stage When the distortions at the (previous pass) and the pre-previous stage (pre-previous pass) are ε _n , ε _n−1 , and ε _n−2 respectively,
ε _C = ε _n + ε _n−1 / 2 + ε _n−2 / 4
Ε _C represented by
The strain ε is the difference between the thickness h ₀ of the steel sheet at the entrance side of each stand (each step or each pass at the time of rough rolling) and the thickness h ₁ at the exit side as an average thickness of both. Ε = (h ₀ −h ₁ ) / {(h ₀ + h ₁ ) / 2}
Can be represented by

In the finish rolling, it is necessary to suppress poor biting of the steel sheet into the rolling rolls within 5 m of the biting of the top of the rolled material into the rolling mill. Therefore, the topmost portion of the above-mentioned rolled material is subjected to the first-stage rolling mills 1 to 5 (in the case of six-stage finishing rolling mill) or the first-stage rolling mills 1 to 6 (in the case of seven-stage finishing rolling mill) as necessary. Preferably, the rolling reduction is performed by adding a rolling reduction of 10% or less of the planned rolling reduction (original rolling reduction for predetermined rolling) of the rolling mill.

Furthermore, in order to prevent the occurrence of slip between the rolling material during rolling and the rolling roll, it is preferable to use a special high grip roll as a work roll of the first to third rolling mills from the final rolling mill. As the special high grip roll, a roll or the like disclosed in Japanese Patent No. 5214905 can be appropriately used.

Next, the above-mentioned different diameter roll will be described. As the different-diameter roll used in the present embodiment, for example, a known different-diameter roll as disclosed in JP-A-2007-33017 can be used. That is, the different-diameter roll refers to a pair of upper and lower work rolls in which the diameter is not equal, and the average roll diameter of each pair of work rolls is less than 600 mm in diameter. Since such a different-diameter roll has a small work roll diameter, high-pressure rolling can be performed with a low rolling load.

FIG. 1 schematically illustrates an example of a suitable finishing mill in the present embodiment.
The finishing mill 1 is a finishing rolling mill having six stages (six stands). As shown in the figure, the three stands in the first stage are provided with so-called CVC mills F1, F2 and F3. The mill F1 is a quadruple rolling mill composed of work rolls 1a and 1b and backup rolls 1c and 1d as shown in FIG. 1, and the work rolls 1a and 1b are relatively moved (shifted) in the axial direction. Appropriate crowns (CVCs or continuous changes in diameter) are applied to the roll surface to allow control of the shape of the steel sheet. The above configuration can be similarly applied to the other two-stage CVC mills F2 and F3. By using such mills F1, F2, F3, the shape accuracy of the steel sheet obtained through the subsequent mills F4, F5, F6 can be increased.

いわゆる As subsequent three stands, so-called different diameter roll mills F4, F5 and F6 are arranged. The different-diameter roll mill F4, which is the fourth stand counted from the mill 1, is a quadruple rolling mill composed of work rolls 4a and 4b and backup rolls 4c and 4d as shown in FIG. As shown in the figure, different diameters are used. Only the lower large-diameter roll 4b of the work rolls 4a and 4b is rotationally driven by a motor or the like (not shown), and the upper small-diameter roll 4a is rotatable so that no driving force is applied. And In addition, such a configuration can be the same for the other two-stage roll mills F5 and F6 provided at the rear. As for the rear mill, a CVC mill may be used similarly to the front mill. The stand intervals of all six stands may be equal or different.

The roll rolls F4, F5, and F6 with three rear stands have a relatively low rolling load because the roll diameter is small and a shear force acts on the steel plate to drive only one of the work rolls (4b, etc.). However, high rolling reduction can be performed. Specifically, it is possible to realize, for example, rolling close to a draft of 50%. As a result, there is an advantage that problems such as roll flattening and edge drop do not occur because the rolling load is small.

Curtain wall type water cooling means 11, 12, and 13 may be arranged on the respective outlet sides of the three stand different-diameter roll mills F4, F5, and F6 arranged at the subsequent stage. Further, also in the run-out table 20 arranged on the downstream side of the finishing mill 1, water cooling means 20a and 20b may be arranged so that the steel sheet can be effectively cooled.
In addition, it is preferable that the temperature of the steel sheet on the outlet side of the finishing mill be 800 ° C. or higher.

(4) After the steel sheet that has been finish-rolled as described above is air-cooled for about several seconds (for example, 2 to 6 seconds), it is cooled with water and wound up. The main feature of the present embodiment is that the winding temperature at this time is set to 700 ° C. or higher. If the winding temperature is lower than 700 ° C., the strength of the steel sheet is increased, so that the cold rolling performed after the hot rolling disadvantageously occurs. From the above viewpoint, in the present embodiment, it is important that the winding temperature after hot rolling is set to 700 ° C. or higher. On the other hand, the upper limit of the winding temperature in the present embodiment is preferably 900 ° C. or less. The reason for setting the upper limit of the winding temperature is that if the temperature is too high, scale formation is promoted, and it takes time to remove the scale in the subsequent pickling.

By the hot rolling process described above, a hot-rolled steel sheet having a thickness of 1.2 mm to 3.0 mm can be obtained as an example. Although the thickness of the hot-rolled steel sheet may be less than 1.2 mm, it is necessary to keep in mind that if the thickness is less than 1.2 mm, the load applied to the rolling roll during hot rolling may be excessively increased. . The thickness of the hot-rolled steel sheet may exceed 3.0 mm. However, when the thickness exceeds 3.0 mm, it is noted that the load applied to the rolling rolls in the subsequent cold rolling step also increases too much. There is a need.

引張 Further, the tensile strength of the hot-rolled steel sheet obtained as described above is preferably 900 MPa or less. If the tensile strength exceeds 900 MPa, the load applied to the rolling rolls during the cold rolling process performed after the hot rolling is undesirably increased.

<Pickling>
The obtained hot-rolled steel sheet is pickled by a known method in order to remove the scale on the surface generated in the hot rolling step.

<Cold rolling>
Next, the hot-rolled steel sheet obtained as described above is subjected to cold rolling. In the process of the cold-rolled steel sheet according to the present embodiment, it is preferable that the cold rolling is performed at a rolling reduction (reduction rate) of 60% or more in total once or a plurality of times. Further, in the present embodiment, the method of cold rolling and the number of times of cold rolling are not particularly limited, and can be appropriately selected according to the target plate thickness.

厚み The thickness of the finally obtained cold-rolled steel sheet is not particularly limited, but is preferably, for example, in the range of 0.01 mm to 0.6 mm. It should be noted that when the thickness of the final cold-rolled steel sheet is less than 0.01 mm, the rigidity of the obtained cold-rolled steel sheet decreases. Therefore, it should be noted that the shape is easily deformed when used for products such as gaskets of gasoline engines of automobiles. On the other hand, when the thickness exceeds 0.6 mm, it is necessary to pay attention to the fact that the product may be heavier than the designed value or a required miniaturization may not be realized.

<Annealing>
By performing annealing after the above-described cold rolled steel sheet process, the work-hardened steel sheet can be softened, or the distortion of the steel sheet during the cold rolled steel sheet can be removed. The annealing step in this embodiment may be continuous annealing or batch annealing. In the case of performing the cold rolling a plurality of times in the above-described cold rolling step, annealing can be performed each time.

温度 The temperature at the time of annealing is preferably 500 ° C or more. If the temperature is lower than 500 ° C., recrystallization does not occur in the steel sheet, and if the steel sheet is not softened, the rolling load in the next step increases, which is not preferable.

In the present embodiment, it is preferable that the last annealing includes a soaking step and a cooling step. By the soaking step and the cooling step, (1) the main phase exceeding 50% of the structure of the steel sheet is made bainite, and further, as phases other than bainite, for example, a ferrite phase, a martensite phase, a retained austenite phase, and the like are provided. At the same time, (2) it is possible to provide a state in which the retained austenite grains of less than 10 μm are present in an arbitrary region of the steel sheet.

Furthermore, by the above-mentioned soaking step and cooling step, the steel sheet structure can be in a state where the retained austenite grains are uniformly dispersed. In the present embodiment, a high-strength cold-rolled steel sheet having high strength and good formability can be obtained by controlling the retained austenite grains in the above-described dispersed state.

In the present embodiment, "retained austenite grains are uniformly dispersed in the structure of the steel sheet" means that a given number of retained austenite grains of less than 10 μm are contained in an arbitrary region of the steel sheet. Define what you say. More specifically, when an arbitrary 10 μm × 10 μm region of a steel sheet is defined as a unit area, a case where one or more retained austenite grains are included per arbitrary unit area is uniformly dispersed. .
At this time, it is further preferable that the ratio of the retained austenite structure to the steel sheet is not less than a certain value. More specifically, in the uniformly dispersed state, it can be said that the volume ratio of the retained austenite structure to the steel sheet is more preferably 8% or more.

As described above, in the present embodiment, in order to achieve both high strength and excellent ductility in a high dimension, for example, several μm order exceeding 0.1 μm (a size exceeding 0.1 μm and less than 10 μm, It is important that the retained austenite grains (having a size of more than 1 μm to less than 10 μm, which are particularly significant) are dispersed in the structure of the steel sheet in the above-mentioned state.
The “size of retained austenite grains” described above means a particle size in the present embodiment. Specifically, when one residual austenite grain is included per arbitrary unit area, the longest part of the grain is defined as the grain size. Further, when a plurality of retained austenite grains are contained per arbitrary unit area, the grain size is measured for each of them in the same manner as in the case of one piece, and the average value thereof is adopted.

First, the above-mentioned soaking step will be described in detail. In the soaking step in the present embodiment, the soaking temperature of the steel sheet is preferably set to the Ac1 transformation point or higher and 1000 ° C or lower, and the soaking is preferably maintained for 30 seconds or longer. If the soaking temperature is lower than the Ac1 transformation point, the steel sheet has a microstructure having ferrite as a matrix, and thus the steel sheet strength required in the present embodiment cannot be obtained. On the other hand, when the soaking temperature exceeds 1000 ° C., there is no particular advantage and there is a disadvantage in cost. Therefore, the soaking temperature is set to 1000 ° C. in the present embodiment.

Next, the cooling step will be described. The cooling step in this embodiment is a step subsequent to the above-mentioned soaking step. After cooling the steel sheet at a cooling rate of 10 ° C./s to 100 ° C./s to a holding temperature of 350 to 500 ° C., the cooling step is performed for 60 seconds or more to 720 °. Preferably, the step is to hold for not more than seconds. When the cooling rate is less than 10 ° C./s, the steel sheet has a microstructure mainly composed of ferrite, so that the steel sheet strength required in the present embodiment cannot be obtained. On the other hand, if the above-mentioned cooling rate exceeds 100 ° C./s, cooling equipment such as water cooling is required instead of gas cooling, which increases costs, which is not preferable. Further, if the holding time is less than 60 seconds or exceeds 720 seconds, the amount of retained austenite (γ _R amount) required for the TRIP effect decreases.

If the holding temperature is lower than 350 ° C., the ratio of the martensite structure increases, and the steel sheet elongation required in the present embodiment cannot be obtained. On the other hand, when the holding temperature exceeds 500 ° C., the ferrite phase of the steel sheet increases, so that the strength of the steel sheet required in the present embodiment cannot be obtained.

<Temperature rolling etc.>
The cold-rolled steel sheet obtained as described above may be subjected to temper rolling for surface roughness adjustment, electroplating of Zn, Ni, Sn or the like for rust prevention and chemical conversion treatment as necessary. Can be.

<Laminate>
The tempered rolled sheet obtained as described above, the cold-rolled steel sheet obtained by performing the electroplating and the chemical conversion treatment, if necessary, a thermoplastic resin film or a thermosetting resin on at least one side of the steel sheet. The film can be coated.

Examples of the thermoplastic resin used for such a film include (1) olefin-based resin films such as polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, ethylene-acrylic ester copolymer, and ionomer; ) Polyesters such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, ethylene terephthalate / isophthalate copolymer, (3) polyamides such as nylon 6, nylon 6.6, nylon 11, nylon 12, and (4) polyvinyl chloride And polyvinylidene chloride.
Examples of the thermosetting resin include an epoxy resin and a vinyl ester resin.

These thermoplastic resins or thermosetting resins include, for the purpose of improving properties such as strength, inorganic fibers such as glass fiber, carbon fiber, boron fiber, silicon carbide fiber, and alumina fiber, aramid fiber, and polyparaphenylene benzobis. A fiber reinforcing agent such as an organic fiber such as an oxazole fiber, an aluminum fiber, an alumina fiber, a SUS fiber, a metal fiber such as a copper fiber may be mixed. Examples of the form of the reinforcing fiber include a nonwoven fabric, a chopped fiber, a combination of a nonwoven fabric and a woven or knitted fabric, and the like. In addition to the fiber reinforcing agent, known additives such as a dye, a flame retardant, an antibacterial agent, an antioxidant, a plasticizer, and a lubricant may be mixed.
These thermoplastic resin films or thermosetting resin films have different characteristics in terms of heat resistance, corrosion resistance, impact resistance, and adhesion to a steel plate, but can be used properly depending on the application.

When the steel sheet is coated with these thermoplastic resin films or thermosetting resin films, an adhesive can be used as necessary, such as an epoxy-based adhesive, a phenol-based adhesive, and an amide-based adhesive. , A urethane-based adhesive, an acid-modified olefin resin-based adhesive, a copolyamide-based adhesive, a copolyester-based adhesive, a blend thereof, and the like.

<Press molding>
The cold rolled steel sheet obtained as described above can be applied as a material for press forming.

<Cold rolled steel sheet>
Next, the cold rolled steel sheet in the present embodiment will be described in detail. The cold-rolled steel sheet of the present embodiment is obtained by the above-described manufacturing method.
The composition of the cold rolled steel sheet in the present embodiment is as follows: C: 0.1 to 0.3%, Si: 1.0 to 2.0%, Mn: 1.0 to 2. 5%, Cr: 0.5% or less, Ni: 1.0% or less, P: 0.01% or less, S: 0.006% or less, N: 0.015% or less, Cu: 0.5% or less The balance has the composition of Fe and inevitable impurities. The content of each element is the same as the description of the above-mentioned rolled material, and thus the description is omitted here.

冷 The cold-rolled steel sheet according to the present embodiment is characterized in that, in its structure, the main phase has a bainite structure, and further, as phases other than the bainite, for example, a ferrite phase, a martensite phase, and a retained austenite phase. Further, the cold-rolled steel sheet according to the present embodiment is characterized in that retained austenite grains having a size of less than 10 μm are present in an arbitrary region of the steel sheet.

Furthermore, the cold-rolled steel sheet according to the present embodiment is characterized in that the retained austenite grains are uniformly dispersed. That is, the cold-rolled steel sheet according to the present embodiment is characterized in that an arbitrary region contains a predetermined number or more of retained austenite grains having a grain size of less than 10 μm. Specifically, when an arbitrary area of 10 μm × 10 μm of the cold-rolled steel sheet of the present embodiment is defined as a unit area, one or more residual austenite grains having a particle size of less than 10 μm are included per arbitrary unit area. It is characterized by the following.

In the present embodiment, more preferably, the unit area contains one or more residual austenite grains having a particle size of 0.1 μm to less than 10 μm (more preferably, more than 1 μm to less than 10 μm, which is particularly significant). preferable. More preferably, the number is eight or more. As described above, by distributing the retained austenite grains having a particle size in the above-described range to the cold-rolled steel sheet, a cold-rolled steel sheet having higher strength and higher ductility can be obtained. As a result, they have found that both the formability and the strength can be achieved even when the thickness of the cold-rolled steel sheet of the present embodiment is reduced or when the cold-rolled steel sheet is formed into a small part.

In the present embodiment, the above-mentioned particle size is measured by the EBSD method (measuring device: as an example, TSL Solutions Co., Ltd., OIM analysis, measurement range: 50 × 50 μm, measurement STEP: 0.1 μm, CI value: 0.05) As described above, the measurement can be performed using the “CleanUP processing: measured by GrainDilation”. FIG. 2 is a photograph of a cross-sectional structure of a cold-rolled steel sheet (Example 10) manufactured in the scope of the present invention according to the EBSD method. In this example, the retained austenite is shown in white, and it is shown that one or more retained austenite grains having a particle size of less than 10 μm are contained per unit area of 10 μm × 10 μm (FIG. 2). See right). FIG. 3 is a photograph of a cross-sectional structure of a cold-rolled steel sheet (Comparative Example 11) manufactured outside the scope of the present invention, as measured by the EBSD method. From this figure, it is confirmed that one or more residual austenite grains having a particle size of less than 10 μm are not present per unit area of 10 μm × 10 μm.

Further, in the cold-rolled steel sheet of the present embodiment, the ratio of the retained austenite grains in the steel sheet structure is preferably equal to or more than a certain value. That is, when a large number of retained austenite grains are present, the TRIP phenomenon occurs, and good strength and formability can be obtained. In the present embodiment, the volume ratio of the retained austenite structure to the steel sheet is preferably 8% or more. By adopting such a composition, it is possible to obtain a cold-rolled steel sheet in which the desired strength and ductility of the steel sheet are both achieved at a high level.

The cold-rolled steel sheet according to the present embodiment is further characterized in that the tensile strength TS is 700 MPa or more and 1400 MPa or less. Furthermore, when the breaking elongation is EL%, the following formula is satisfied.
TS ≧ 1400- (30 × EL)
The above-described tensile strength and elongation at break can be measured according to JIS Z 2241.

冷 In addition, the cold-rolled steel sheet of the present embodiment preferably further has a work-hardening index n value, which is a value indicating work-hardening characteristics, of 0.20 or more. The work hardening index n value is a numerical value that is considered to be good as the bending workability increases as the value increases, and takes a value of 0 ≦ n ≦ 1 (Ichi Sudo: Material Testing Method, Uchida Ritsuruhosha, ( 1976), p. In order to realize excellent strength, ductility, formability, and the like, the cold-rolled steel sheet of the present embodiment preferably has a work hardening index (n value) of 0.20 or more in a direction parallel to the rolling direction.

冷 Since the cold-rolled steel sheet of the present embodiment has the above-described configuration, it has excellent formability when processed. Specifically, the cold rolled steel sheet of the present embodiment preferably has a critical overhang height of 6.5 mm or more. That is, when the overhang test is performed based on JIS Z 2247 and measured, the height at which a crack occurs during overhang is defined as the overhang height, and the overhang height is 6.5 mm or more. Is preferred.

冷 The cold-rolled steel sheet of the present embodiment preferably has a critical drawing ratio of 2.0 or more. That is, the ratio (D / d) of the maximum blank diameter D obtained in the deep drawing test and drawn without breaking and the punch diameter d is defined as the limit drawing ratio (LDR). In the present embodiment, when LDR ≧ 2.0, it is determined that the deep drawability is good.

In the cold-rolled steel sheet of the present embodiment, it is preferable that the value of Δr expressed below is in a range of ± 0.7. If the value of Δr is large, unnecessary ears are generated at the time of molding, so it is preferable to set the value as small as possible.
Δr = (r ₀ −r ₉₀ ) / 2−r ₄₅
Here, r ₀ is a value obtained by cutting out a No. 5 test piece from the cold-rolled annealed sheet from the L direction (rolling direction) and conforming to JIS Z2254. Similarly, with respect to r ₄₅ and r ₉₀ , No. 5 test pieces from the cold-rolled annealed sheet in the D direction (a direction at 45 ° to the rolling direction) and the C direction (a direction at 90 ° to the rolling direction), respectively. Is a value obtained in accordance with JIS Z2254.

Further, the cold rolled steel sheet of the present embodiment preferably has an ear ratio after deep drawing of 10% or less. That is, in the deep drawing test, a cylindrical drawing test is performed, the height of the ear after forming is measured, and the ear ratio represented by the following formula is measured.
Ear ratio = Δh (h _Max −h _Min ) / h _Ave × 100
h _Max : maximum ear height, h _Min : minimum ear height, h _Ave : average ear height The lower the ear ratio, the flatter the ear, and it can be determined that the moldability is good. In the present embodiment, the ear ratio is preferably 10% or less.

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

<Example 1>
Molten steel having the components shown in Table 1 was used as a slab (rolled material) by a continuous casting method. The thickness of the slab was 230 mm. Subsequently, the slab was heated to 1250 ° C., and then roughly rolled at a rolling reduction of 80%. Thereafter, finish rolling was performed at 1050 ° C. using a six-roll mill shown in FIG. The finishing pre-rolling mills 1 to 3 were used, and the average rolling reduction per finishing pre-rolling mill was 43%. The cumulative strain under the rolling of the finishing three-stage rolling mill was 0.5. The temperature of the steel sheet on the exit side of the finishing mill was 900 ° C.

(4) The steel sheet finish-rolled as described above was air-cooled for 3 seconds, cooled with water, and wound at 750 ° C. Thus, a hot-rolled steel sheet having a thickness of 1.8 mm was obtained. The tensile strength of the obtained hot-rolled steel sheet was 700 MPa.

酸 After the hot-rolled steel sheet obtained as described above was pickled, cold rolling was performed twice at a total reduction rate of 83%. In the annealing after cold rolling, after a soaking step at 800 ° C. for 60 seconds, a cooling step of cooling at a cooling rate of 60 ° C./s to a holding temperature of 400 ° C., and a cooling step of holding for 180 seconds, finally yielding a thickness A 0.3 mm cold-rolled steel sheet was obtained.

[Measurement of volume ratio of retained austenite grains]
The measurement of the volume ratio of retained austenite grains in the obtained cold-rolled steel sheet was performed by an X-ray diffraction method. As a measuring device, XRD: SmartLab manufactured by Rigaku was used.
The obtained cold-rolled steel sheet was wet-polished to a position of 1/4 thickness from the surface and then finish-polished by chemical polishing to obtain a measurement sample. The X-ray source used was a Cu tube, the measurement was 40 to 140 ° (2θ / θ), the entrance slit was 2 mm, the entrance, and the light receiving slit was 1/5 deg. And

Furthermore, after measuring the integrated intensities of the (200), (211) planes of the ferrite phase and the (200), (220), (311) planes of the austenite phase, the Rigaku RINT2000 / PC software Instructions for the residual austenite determination program According to the procedure described in the book, smoothing treatment, background removal, strength calculation, and quantitative calculation were respectively performed to determine the volume ratio of retained austenite.
Table 3 shows the obtained results. In the present invention, when the volume ratio of the retained austenite grains is 8% or more, it can be determined that the phase structure is good.

[Measurement of distribution state and particle size of retained austenite particles]
The distribution state of the retained austenite grains was measured by an EBSD (electron back scattering difference) method using a scanning electron microscope (SEM). As a measuring device, TSL Solutions OIM analysis was used. A scanning electron microscope (FE-SEM (SU8020) manufactured by Hitachi High-Technologies Corporation) was measured using the same sample as the measurement sample in the X-ray diffraction method.
In Example 1, as an example, ○ was given when eight or more retained austenite grains having a particle size of less than 10 μm per unit area of 10 μm × 10 μm were included, and × was given when none of them was included. Further, the average value of the particle diameters of all the retained austenite grains observed per unit area was calculated. Table 3 shows the results.

[Mechanical properties (tensile test)]
A sample was taken from the obtained cold-rolled steel sheet such that the tensile direction was parallel to the rolling direction of the steel sheet, and a JIS No. 13B test piece was prepared. Using the obtained test piece, a tensile test was performed in accordance with JIS Z 2241, and the tensile strength (TS) and the elongation at break (EL) were measured. From the obtained values of the tensile strength (TS) and the elongation at break (EL), when the formula of “tensile strength (TS) ≧ 1400-30 × elongation at break (EL)” was satisfied, the result was evaluated as ○, and when not satisfied, the result was evaluated as ×. . Table 3 shows the results.

[Mechanical properties (n value)]
The n value was calculated based on JIS Z 2253 using the results obtained by the tensile test. Table 3 shows the results of the obtained n values. In the present invention, when the n value is 0.20 or more, it can be determined that the moldability is good.

[Evaluation of formability (critical overhang height)]
Using the obtained cold-rolled steel sheet, an overhang test was performed based on JIS Z 2247, and a value of a critical overhang height was obtained. The overhang test was performed using a punch having a diameter of 10 mm. The height at which a crack occurred during overhang was defined as the limit overhang height. Table 3 shows the obtained numerical values.
According to the overhang test, it is possible to evaluate the combined effect due to both the total elongation characteristics and the local ductility of the steel sheet. In the present invention, when the overhang height is 6.5 mm or more, it can be determined that the moldability is good.

[Formability evaluation (critical drawing ratio)]
A deep drawing test was performed using the obtained cold-rolled steel sheet, and a value of a limit drawing ratio (LDR) was obtained. The deep drawing test was performed by a cylindrical drawing test. The test conditions were as follows: punch diameter: 30 mm, Rp: 3.0 mm, die diameter: 30.7 mm, Rd: 2.5 mm, wrinkle holding force: 10 kN, molding speed: 2.5 mm / s. Lubrication was performed under high lubrication conditions using a lubricating oil and a polyethylene sheet. The ratio (D / d) of the maximum blank diameter D that was drawn out without breaking and the punch diameter d was defined as the limit drawing ratio (LDR). Table 3 shows the obtained results. In the present invention, when LDR ≧ 2.0 or more, it can be determined that the deep drawability is good.

[Formability evaluation (Δr)]
The value of Δr was obtained as follows. Using the obtained cold-rolled steel sheet, each No. 5 test piece was examined from L direction (rolling direction), D direction (direction forming 45 ° with the rolling direction) and C direction (direction forming 90 ° with the rolling direction). I cut it out. The respective r values (r _L = r ₀ , r _D = r ₄₅ , r _C = r ₉₀ ) were determined in accordance with the provisions of JIS Z 2254, and the Δr value was calculated by the following equation.
Δr = (r ₀ −r ₉₀ ) / 2−r ₄₅
Table 3 shows the obtained results. In the present invention, when Δr is within the range of ± 0.7, it can be determined that the moldability is good.

[Evaluation of moldability (ear ratio)]
Ear ratio was calculated as follows. Using the obtained cold-rolled steel sheet, a deep drawing test was performed in the same manner as described above. The height of the ear after the deep drawing was measured, and the ear ratio was calculated using the following equation.
Ear ratio = Δh (h _Max −h _Min ) / h _Ave × 100
h _Max : maximum ear height, h _Min : minimum ear height, h _Ave : average ear height Table 3 shows the obtained results. In the present invention, when the ear ratio is 10% or less, it can be determined that the moldability is good.

<Examples 2 to 21, Comparative Examples 1 to 13>
In the same manner as in Example 1, Examples 2 to 21 and Comparative Examples 1 to 13 were performed. The cold-rolled steel sheets to be used had the components shown in Table 1, and were subjected to the same conditions as in Example 1 except that the rolling was performed under the conditions shown in Table 3 or Table 4. The obtained results are shown in Tables 3 and 4, respectively. Table 2 shows the mechanical properties of the hot-rolled steel sheet after hot rolling.

について Regarding the steel types of the slab pieces shown in Table 1, steel types 1 to 3 are slab pieces falling within the component range of the present invention, while steel types 4 to 6 are slab pieces outside the component range of the present invention. Using the slab pieces of these steel types 1 to 6, respective values of the examples and comparative examples were obtained.

に関して Regarding the mechanical properties of the hot-rolled steel sheet after hot rolling shown in Table 2, the properties were shown for steel type 2 and steel type 4. Since the steel type 2 has a winding temperature (CT) of 700 ° C. or higher, it is possible to obtain a property of a tensile strength (TS) of 900 MPa or less.

On the other hand, since the winding temperature (CT) of steel type 4 was as low as 480 ° C, the tensile strength (TS) was as high as 1034 MPa. As a result, in the subsequent cold rolling, the thickness could not be reduced to the target of 0.6 mm or less, and cracks occurred when the number of times of rolling and the rolling load were increased, so the cold rolling was stopped. In addition, regarding the steel types 5 to 7, the test in which the winding temperature was changed was not performed because the winding temperature was low and the steel might be hardened and cracked during cold rolling. In the present invention, in order to reduce the load at the time of cold rolling, it was determined that the case where the tensile strength after hot rolling was 900 MPa or less was good.

In Table 2, “FT (Finishing Temperature)” indicates the coil temperature on the exit side of the finishing mill, “YP (Yield Point)” indicates the yield point, and “EL (Elongation) indicates the elongation at break.

に関して Regarding the results shown in Table 4, in Comparative Examples 1, 3, 4, and 7, the main phase was martensite because the holding temperature during the soaking step was as low as 300 ° C. As a result, a certain amount or more of retained austenite grains could not be secured, and as a result, elongation was insufficient.

In Comparative Examples 2 and 9, in the cooling step following the soaking step, the holding for a certain time was not performed, so that a certain amount of retained austenite grains could not be secured, and the characteristics were not satisfied.

In Comparative Examples 5 and 8, since the holding temperature in the cooling step was too high, a certain amount of retained austenite grains could not be secured, and the characteristics were not satisfied.

In Comparative Example 6, since the soaking temperature in the soaking step was equal to or higher than the Ac3 transformation point, the phase configuration was different from that in the case where the soaking temperature was equal to or lower than the Ac3 transformation point, and as a result, the amount of retained austenite grains could not be satisfied. .

In Comparative Examples 10 and 11, the Cr transformation point was increased because Cr was added to the slab piece in a certain amount or more. As a result, at a soaking temperature of 800 ° C., a bainite phase and an austenite phase were not obtained, and a ferrite phase became a main phase. As a result, a cold rolled steel sheet satisfying the balance between strength and ductility could not be obtained.

In Comparative Example 12, since a slab piece lacked Si, which is an austenite-stable element, constant retained austenite grains could not be secured. As a result, a cold-rolled steel sheet satisfying the balance between strength and ductility could not be obtained.

In Comparative Example 13, since the amount of C and the amount of Si were small, constant retained austenite grains could not be obtained, and ferrite was the main phase. As a result, a cold-rolled steel sheet satisfying the strength could not be obtained.

As is clear from the above, in the examples of the present invention, a high-strength cold-rolled steel sheet excellent in formability was obtained by satisfying the phase constitution, mechanical property values, and formability standards shown in the table. On the other hand, in the comparative example, because it was not manufactured under appropriate conditions, it did not satisfy any of the phase structure, mechanical properties, and formability standards, and was not satisfied as a high-strength cold-rolled steel sheet excellent in formability. Was determined to be.

It goes without saying that the above-described embodiment and each example can be variously modified without departing from the spirit of the present invention.

As described above, according to the hot-rolled steel sheet and the cold-rolled steel sheet of the present invention and the method for producing them, it is possible to obtain a high-strength cold-rolled steel sheet excellent in formability and a hot-rolled steel sheet as a material thereof. Among these, the high-strength cold-rolled steel sheet of the present invention has excellent formability without cracking even when it is formed into a small part by press forming or the like as a thin plate. Further, the high-strength cold-rolled steel sheet of the present invention can realize a demand for miniaturization and weight reduction of a molded product, and has extremely high industrial applicability.
The high-strength cold-rolled steel sheet of the present invention can be used for gaskets of gasoline engines of automobiles, housings of notebook computers and smartphones, frame parts of electronic devices, and the like.

DESCRIPTION OF SYMBOLS 1 Finish rolling mill F4, F5, F6 Different

diameter roll mills

4a, 4b Work rolls 4c, 4d Backup rolls 11, 12, 13 Water cooling means 20a, 20b Water cooling means

Claims

As content in mass%, C: 0.1 to 0.3%, Si: 1.0 to 2.0%, Mn: 1.0 to 2.5%, Cr: 0.5% or less, Ni: : 1.0% or less, P: 0.01% or less, S: 0.006% or less, N: 0.015% or less, Cu: 0.5% or less, with the balance being Fe and inevitable impurities. Rolled material,
A first step of roughly rolling the rolled material at a rolling reduction of 30% or more in total;
After the first step, a second step of finish-rolling the rolled material at a total reduction of 40% or more using a plurality of different-diameter rolls having different diameters in a temperature environment of 800 ° C or more;
After the second step, a third step of producing a hot-rolled steel sheet having a tensile strength of 900 MPa or less by winding the rolled material under a temperature environment of 700 ° C. or more,
A method for producing a hot-rolled steel sheet, comprising:
(4) The method for producing a hot-rolled steel sheet according to (1), wherein in the first step, rough rolling is performed on the rolled material under a temperature environment of 1100 ° C. or higher.
In the second step, finish rolling is performed so that the average draft per rolling mill in the pre-finishing stage is 40% or more, and the cumulative reduction in rolling by the rolling mill in the post-finishing stage is 0.5 or more. The method for producing a hot-rolled steel sheet according to claim 1.
A fourth step of cold-rolling a hot-rolled steel sheet obtained by the method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 3 to produce a cold-rolled steel sheet. Manufacturing method of high strength cold rolled steel sheet.
5. The method of manufacturing a high-strength cold-rolled steel sheet according to claim 4, wherein in the fourth step, the hot-rolled steel sheet is cold-rolled at a total reduction of 60% or more.
6. The method for producing a high-strength cold-rolled steel sheet according to claim 5, further comprising a fifth step of, after the fourth step, annealing the cold-rolled steel sheet at a soaking temperature equal to or higher than the Ac1 point and cooling and holding the cold-rolled steel sheet.
The high-strength cold-rolled steel according to any one of claims 4 to 6, wherein the hot-rolled steel sheet has a thickness of 1.2 to 3.0 mm, and the cold-rolled steel sheet has a thickness of 0.01 to 0.6 mm. Steel plate manufacturing method.
As content in mass%, C: 0.1 to 0.3%, Si: 1.0 to 2.0%, Mn: 1.0 to 2.5%, Cr: 0.5% or less, Ni: : 1.0% or less, P: 0.01% or less, S: 0.006% or less, N: 0.015% or less, Cu: 0.5% or less, the balance has a composition of Fe and unavoidable impurities. And
A hot-rolled steel sheet having a thickness of 1.2 to 3.0 mm and a tensile strength of 900 MPa or less.
As content in mass%, C: 0.1 to 0.3%, Si: 1.0 to 2.0%, Mn: 1.0 to 2.5%, Cr: 0.5% or less, Ni: : 1.0% or less, P: 0.01% or less, S: 0.006% or less, N: 0.015% or less, Cu: 0.5% or less, the balance has a composition of Fe and unavoidable impurities. And
The main phase is a bainite structure, further including a ferrite structure, martensite structure and residual austenite structure in addition to the bainite structure,
One or more residual austenite grains of less than 10 μm are dispersed per unit area of 10 μm square,
Tensile strength TS is 700 MPa or more and 1400 MPa or less, and
When the elongation at break is EL%,
TS ≧ 1400- (30 × EL)
A high-strength cold-rolled steel sheet characterized by satisfying the following conditions.
高 The high-strength cold-rolled steel sheet according to claim 9, wherein the n value indicating the work hardening property of the high-strength cold-rolled steel sheet is 0.20 or more.
The high-strength cold-rolled steel sheet according to claim 9 or 10, wherein the volume ratio of the retained austenite structure is 8% or more.
The high-strength cold-rolled steel sheet according to any one of claims 9 to 11, wherein the high-strength cold-rolled steel sheet has a thickness of 0.01 to 0.6 mm.
The high-strength cold-rolled steel sheet according to any one of claims 9 to 12, wherein a critical overhang height of the high-strength cold-rolled steel sheet is 6.5 mm or more.
The high-strength cold-rolled steel sheet according to any one of claims 9 to 13, wherein the critical draw ratio of the high-strength cold-rolled steel sheet is 2.0 or more.
The high-strength cold-rolled steel sheet according to any one of claims 9 to 14, wherein Δr in the high-strength cold-rolled steel sheet is in a range of ± 0.7.
The high-strength cold-rolled steel sheet according to any one of claims 9 to 15, wherein the ear ratio of the high-strength cold-rolled steel sheet is 10% or less.