JP6835046B2 - Thin steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a thin steel sheet and a method for producing the same. The thin steel sheet of the present invention has a tensile strength (TS) of 780 MPa or more, and has excellent strength homogeneity, surface texture, and plate shape. Therefore, the thin steel plate of the present invention is suitable as a material for a skeleton member for automobiles.

In recent years, from the viewpoint of global environmental conservation, improvement of automobile fuel efficiency has been aimed at in the entire automobile industry for the purpose of regulating _{CO 2 emissions.} In recent years, the amount of high-strength steel sheets used as materials for automobile parts has been increasing because it is most effective to reduce the weight of automobiles by thinning the thickness of the parts used to improve the fuel efficiency of automobiles.

Many steel sheets utilize martensite, which is a hard phase, in order to obtain steel sheet strength. On the other hand, when martensite is generated, the plate shape deteriorates due to transformation strain. Since deterioration of the plate shape adversely affects the dimensional accuracy at the time of molding, the plate has been corrected by leveler processing or skin pass rolling (tempering rolling) so as to obtain the desired dimensional accuracy. On the other hand, since plastic deformation is locally applied by these leveler processing and skin pass rolling, non-uniformity of work hardening is caused, and fluctuation of yield strength is unavoidable in the width direction. Since the yield strength affects the dimensional accuracy at the time of molding, a high-strength steel sheet having excellent homogeneity is desired. In order not to deteriorate this homogeneity, it is necessary to suppress the deterioration of the plate shape during martensitic transformation, whereas various techniques have been proposed so far.

For example, Patent Document 1, after heating the _{A c1} to 900 ° C., subjected to average water cooling or air-water cooling to a temperature range at a cooling rate of 30~500 ℃ / s (Ms + 10 ℃) ~ (Ms + 100 ℃), followed by ( Gas cooling is performed in the temperature range of Ms-30 ° C) to (Ms-100 ° C), followed by water cooling or air-water cooling at an average cooling rate of 30 to 1000 ° C / s below 400 ° C, and gas cooling. It is said that an ultra-high-strength cold-rolled steel plate that eliminates shape defects can be obtained by contacting the steel plate inside with a pair or more of rolls held at a temperature of (Ms + 10 ° C.) to (Ms + 100 ° C.).

In Patent Document _2, after annealing at transformation point _{A c1} temperature above rapidly cooled at an average cooling rate of more than 400 ° C. / sec from 650 to 750 ° C., then, at a temperature of 100~450 ℃, 100~1200sec holding It is said that a steel sheet having a good shape can be obtained by performing temper rolling so that the average roughness Ra of the surface of the steel sheet is 1.4 μm or more after the annealing treatment.

Japanese Unexamined Patent Publication No. 2000-160254 JP-A-2009-79255

In the technique proposed in Patent Document 1, it is necessary to contact the heating roll for the purpose of eliminating temperature unevenness during gas cooling, but the cooling rate is significantly lower than that of water cooling, and bainite is inevitably generated. To do. When bainite is formed, not only the desired steel sheet strength cannot be obtained, but also it causes strength variation.

In the technique proposed in Patent Document 2, a rolling roll having a surface roughness Ra of 5.0 to 10.0 μm is transferred to the plate surface to obtain a desired surface roughness. However, even with this method, the influence of local work hardening cannot be avoided, and a steel sheet having both strength homogeneity and a steel sheet shape cannot be obtained.

Neither of the techniques described in the patent documents can combine strength homogeneity with steel plate shape and surface texture. An object of the present invention is to provide a thin steel sheet having a tensile strength of 780 MPa or more and having good strength homogeneity, a steel sheet shape and surface properties, and a method for producing the same.

In order to solve the above problems, the present inventors have diligently studied the requirements for a thin steel sheet having a tensile strength of 780 MPa or more and good strength homogeneity and shape of the steel sheet. The thickness of the thin steel plate targeted in this case is 0.4 mm or more and 2.6 mm or less. In general, the concentration of alloying elements increases as the strength of the steel sheet increases, which adversely affects the spot weldability. Therefore, we focused on martensite, which can efficiently obtain strength in consideration of spot weldability. On the other hand, in order to efficiently obtain martensite, it is effective to cool the steel sheet with water, but since the martensitic transformation during water cooling occurs rapidly and non-uniformly, the shape of the steel sheet is deteriorated by the transformation strain. As a result of investigating the reduction of adverse effects due to transformation strain, it was found that the difference in cooling rate between the front and back surfaces of the steel sheet induces non-uniformity of martensitic transformation and worsens the shape of the steel sheet. As a result of further investigating the difference in the cooling rate between the front and back surfaces of the steel sheet, it was found that it occurs when the fluctuation in the normal direction with respect to the plate surface when the steel sheet enters the water tank is large. When the plate shape is good, excessive straightening is not required, a desired shape can be obtained by ordinary skin pass rolling (tempering rolling), and work hardening is performed uniformly, so that the yield strength fluctuates in the width direction. It was found that it can suppress. The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] In terms of mass%, C: 0.05% or more and 0.35% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.8% or more and 3.2% or less, P: 0. 05% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, N: 0.0060% or less, component composition with the balance consisting of Fe and unavoidable impurities, and ferrite area ratio 0% or more and 90% or less, bainite area ratio of 5% or less (including 0%), martensite and tempered martensite area ratio of 10% or more (including 100%), retained austenite area ratio of 2. It has a steel structure of 0% or less (including 0%), the standard deviation of the yield strength in the width direction is 30 MPa or less, and the maximum warp amount of the sheet steel when sheared at a length of 1 m is 10 mm or less. A thin steel plate.

[2] Further, the component composition is, in mass%, V: 0.001% or more and 1% or less, Ti: 0.001% or more and 0.3% or less, Nb: 0.001% or more and 0.3% or less. , Cr: 0.001% or more and 1.0% or less, Mo: 0.001% or more and 1.0% or less, Ni: 0.01% or more and 1.0% or less, Cu: 0.01% or more and 1.0 % Or less, B: 0.0002% or more and 0.0050% or less, Sb: 0.001% or more and 0.050% or less, REM: 0.0002% or more and 0.050% or less, Mg: 0.0002% or more 0 The thin steel sheet according to [1], which contains any one or more of .050% or less and Ca: 0.0002% or more and 0.050% or less.

[3] A hot-rolling step of hot-rolling a steel material having the component composition according to [1] or [2], and a cold-rolling step of pickling and cold-rolling a steel sheet after the hot-rolling step. After heating the steel sheet after the cold rolling process at 760 ° C. or higher, water-annealing is performed at 600 ° C. or higher, and the amount of fluctuation in the normal direction of the steel sheet surface when the steel sheet enters the water tank in the water-annealing A method for producing a thin steel sheet, comprising an annealing step of water-cooling to 100 ° C. or lower so as to be 30 mm or less and reheating at 100 ° C. or higher and 350 ° C. or lower.

[4] The method for producing a thin steel plate according to [3], wherein in the annealing step, one or more pairs of steel plate restraining rolls are installed within 1 m above and below the water surface of the water tank to adjust the fluctuation amount.

According to the present invention, the thin steel sheet of the present invention has a high tensile strength (TS) of 780 MPa or more, excellent strength homogeneity, good surface texture and steel sheet shape. If the thin steel sheet of the present invention is applied to an automobile part, the weight of the automobile part can be further reduced.

It is a figure which shows an example of bainite. It is a figure which shows the fluctuation amount schematically.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.

First, the composition of the thin steel sheet of the present invention will be described. In the following description, "%" representing the content of the component means "mass%".

C: 0.05% or more and 0.35% or less C is an element that is related to the hardness of martensite and tempered martensite and contributes to increasing the strength of the steel sheet. Tensile strength: In order to obtain 780 MPa or more, it is necessary to contain at least 0.05% or more of C content. It is preferably 0.06% or more. On the other hand, if the C content exceeds 0.35%, it is extremely difficult to put it into practical use due to spot weldability and the like. Therefore, the C content was set to 0.35% or less. It is preferably 0.25% or less.

Si: 0.01% or more and 2.0 or less Si is an element effective for increasing the elongation of the steel sheet. From the viewpoint of increase in elongation, the Si content was set to 0.01% or more. It is preferably 0.1% or more. On the other hand, if the Si content exceeds 2.0%, the chemical conversion processability is remarkably deteriorated and it becomes unsuitable as a steel sheet for automobiles. Therefore, the Si content is set to 2.0% or less. It is preferably 1.7% or less.

Mn: 0.8% or more and 3.2% or less Mn is an element effective for increasing the hardenability of steel sheets and obtaining martensite. When Mn is insufficient, the strength fluctuates in the width direction due to insufficient hardenability. To suppress this, Mn needs to be contained in an amount of 0.8% or more. It is preferably 1.1% or more. On the other hand, even if Mn is excessively contained, not only the effect of hardenability is saturated, but also the castability and the like adversely affect the productivity. From the above, the Mn content was set to 3.2% or less. It is preferably 2.9% or less. In order to suppress fluctuations in bendability, it is more preferable that the Mn content is 2.3% or less.

P: 0.05% or less P is a harmful element that causes low temperature brittleness and lowers weldability, so it is preferable to reduce it as much as possible. In the present invention, the P content can be up to 0.05%. The preferred P content is 0.02% or less, but it is more preferably suppressed to 0.01% or less for use under more severe welding conditions. On the other hand, 0.002% may be unavoidably mixed in manufacturing.

S: 0.005% or less S forms coarse sulfide in steel, which stretches during hot rolling and becomes wedge-shaped inclusions, which adversely affects weldability. Therefore, since S is also a harmful element, it is preferable to reduce it as much as possible. In the present invention, since 0.005% can be tolerated, the S content is set to 0.005% or less. It is preferably 0.003% or less, but it is more preferably suppressed to 0.001% or less for use under more severe welding conditions. In manufacturing, 0.0002% may be unavoidably mixed.

Al: 0.005% or more and 0.10% or less When Al is added as a deoxidizer at the stage of steelmaking, it is necessary to contain 0.005% or more of Al content. On the other hand, Al forms a coarse oxide that deteriorates weldability and the like. Therefore, the upper limit of Al content was set to 0.10%. It is preferably 0.07% or less.

N: 0.0060% or less N is a harmful element that adversely affects the surface properties because it deteriorates the aging at room temperature and causes unexpected cracks. Therefore, it is desirable to reduce N as much as possible. In the present invention, up to 0.0060% is acceptable. It is preferably 0.0050% or less. It is desirable to reduce the N content as much as possible, but 0.0005% may be unavoidably mixed in during manufacturing.

The above are the essential components of the thin steel sheet of the present invention, but further, in terms of mass%, V: 0.001% or more and 1% or less, Ti: 0.001% or more and 0.3% or less, Nb: 0.001%. More than 0.3% or less, Cr: 0.001% or more and 1.0% or less, Mo: 0.001% or more and 1.0% or less, Ni: 0.01% or more and 1.0% or less, Cu: 0. 01% or more and 1.0% or less, B: 0.0002% or more and 0.0050% or less, Sb: 0.001% or more and 0.050% or less, REM: 0.0002% or more and 0.050% or less, Mg: One or more of 0.0002% or more and 0.050% or less and Ca: 0.0002% or more and 0.050% or less may be contained as an optional element. These optional elements are elements used for ensuring hardenability, adjusting strength, controlling inclusions, etc. However, even if these optional elements are contained in the above range, the effect of the present invention is not impaired.

Components other than the above components are Fe and unavoidable impurities. When an arbitrary element is contained below the above lower limit value, the arbitrary element contained below the lower limit value shall be included as an unavoidable impurity.

Subsequently, the steel structure of the thin steel sheet of the present invention will be described. The steel structure of the thin steel sheet of the present invention has a ferrite area ratio of 0% or less and 90% or less, a bainite area ratio of 5% or less (including 0%), martensite and a tempered martensite area ratio of 10% or more. (Including 100%), the residual austenite area ratio is 2.0% or less (including 0%).

Ferrite area ratio is 0% or more and 90% or less Since ferrite is soft, if the area ratio exceeds 90%, the desired steel sheet strength cannot be obtained. Therefore, the upper limit of the ferrite area ratio was set to 90%. It is preferably 80% or less. Further, even if the ferrite area ratio is 0%, the effect of the present invention is not lost.

Bainite area ratio is 5% or less (including 0%)
In order to generate bainite, it is necessary to maintain the isothermal temperature in the range of about 200 ° C. to 400 ° C. without slow cooling or cooling to room temperature after heating in the annealing step. Slow cooling causes softening of the steel sheet, and uneven cooling in the width direction is unavoidable, so that uniform steel sheet strength cannot be obtained. Since the present invention is intended to generate martensite by water cooling, it is not possible to maintain an isothermal temperature from 200 ° C. to 400 ° C. On the other hand, bainite may be slightly formed in the water cooling process, and in the present invention, the upper limit is 5% (including 0%). It is preferably 3% or less.

The bainite targeted in the present invention is a structure containing bainitic ferrite containing dislocations rather than polygonal ferrite, and does not include lower bainite that cannot be distinguished by tempered martensite and a scanning electron microscope. Bainitic ferrite is a ferrite in which corrosion marks are observed on a scanning electron microscope after corrosion appears by 1% nightal etching. A typical example is shown in FIG.

Martensite and refurbished martensite area ratio is 10% or more (including 100%)
In the present invention, desired strength is obtained with martensite and tempered martensite (tempered martensite). In order to obtain a tensile strength of 780 MPa or more, the above-mentioned structure needs to be 10% or more (including 100%) in total. Preferably, it is 20% or more.

Residual austenite area ratio is 2.0% or less (including 0%)
In order to produce more than 2.0% of retained austenite, the steel composition of the present invention requires production by a method other than bainite formation or water cooling. Since the present invention does not intend a production method other than bainite formation or water cooling, the upper limit of the area ratio of retained austenite is set to 2.0%. The present invention is not impaired even if the retained austenite is 0%.

Examples of structures other than the above-mentioned ferrite, bainite, martensite, tempered martensite, and retained austenite include pearlite and cementite. The structure represents insufficient annealing or insufficient cooling capacity in the present invention, and the area ratio is preferably 1% or less, more preferably 0%. Cementite is often contained in bainite and tempered martensite, and these are not counted as the area ratio of cementite. If it remains isolated in the ferrite grains, it is not counted in the area ratio. Since it is difficult to distinguish martensite from a scanning electron microscope, it is necessary to confirm it by the EBSD method or a diffraction image of TEM. The area ratio of cementite remaining isolated in the ferrite grains is preferably 2% or less, and more preferably 0%.

Next, the characteristics of the thin steel sheet of the present invention will be described.

The thin steel sheet of the present invention has high strength. Specifically, the tensile strength (TS) measured by the method described in the examples is 780 MPa or more. The upper limit of TS is not particularly limited, but is preferably 2500 MPa or less from the viewpoint of balance with other characteristics.

The thin steel sheet of the present invention has strength homogeneity. Specifically, the standard deviation of the yield strength in the width direction measured by the method described in the examples is 30 MPa or less.

The thin steel sheet of the present invention has excellent surface properties. Specifically, the Rmax-Rave in the bendability evaluation described in the examples is 0.8 mm or less. It is preferably 0.7 mm or less, more preferably 0.6 mm or less.

The thin steel plate of the present invention has an excellent plate shape. Specifically, the maximum amount of warpage measured by the method described in the examples is 10 mm or less.

Next, the method for manufacturing the thin steel sheet of the present invention will be described. The method for producing a thin steel sheet of the present invention includes a hot rolling step, a cold rolling step, and an annealing step.

The hot rolling step is a step of hot rolling a steel material having the above composition.

The melting method for producing the above steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be adopted. Further, the secondary refining may be performed in a vacuum degassing furnace. After that, it is preferable to use a slab (steel material) by a continuous casting method from the viewpoint of productivity and quality. Further, a slab may be formed by a known casting method such as an ingot-block rolling method or a thin slab continuous casting method.

Further, the hot rolling conditions in hot rolling may be appropriately set.

The cold rolling step is a step of pickling and cold rolling the steel sheet after the hot rolling step. The conditions for pickling and cold rolling are not particularly limited and may be set as appropriate.

The annealing step is a process in which a steel sheet after a cold-rolling process is heated at 760 ° C. or higher and then water-quenched at 600 ° C. or higher. This is a step of water cooling to 100 ° C. or lower so that the fluctuation amount is 30 mm or less, and reheating at 100 ° C. or higher and 350 ° C. or lower. The annealing step is preferably performed on a continuous annealing line.

Heating to 760 ° C or higher The purpose of heating is to produce austenite that contributes to the formation of martensite, and it is necessary to heat to 760 ° C or higher to obtain the desired martensite area ratio. It is preferably 780 ° C. or higher. The upper limit of the heating temperature is preferably 900 ° C. or lower, more preferably 875 ° C. or lower, because the austenite grains become coarse and cause rough skin and deteriorate the surface texture.

Water quenching at 600 ° C. or higher If the temperature is excessively lowered before the start of water quenching, the desired steel sheet characteristics cannot be obtained due to the strength fluctuation in the width direction due to the steel sheet temperature distribution and the formation of ferrite and bainite. From the above viewpoint, the water quenching start temperature was set to 600 ° C. or higher. It is preferably 640 ° C. or higher. Further, the upper limit of the water quenching start temperature is not particularly limited, and a higher temperature is preferable when ferrite formation is not intended. In actual production, quenching is often performed when the temperature drops by about 10 ° C from the annealing temperature.

Controlling the amount of fluctuation of the steel sheet surface in the normal direction when the steel plate enters the water tank to 30 mm or less The present invention is characterized in that the fluctuation of the cooling rate during water cooling is suppressed by controlling the state of steel plate entering the water tank. is there. If the position of the steel sheet fluctuates greatly with respect to the normal direction of the plate surface when the steel sheet enters the water tank, the retention state of water on the surface of the steel sheet becomes non-uniform and the cooling rate fluctuates. In order to suppress this fluctuation, for example, it is possible to suppress the fluctuation by increasing the tension more than before, but there is a risk of breakage at the time of passing the plate. By setting the fluctuation amount of the steel plate position with respect to the plate surface normal direction to 30 mm or less, this fluctuation of the cooling rate can be suppressed and a desired steel plate shape can be obtained. The preferred amount of variation is 20 mm or less. Further, the smaller the fluctuation amount, the more preferable. The amount of fluctuation is the amount of deviation when the steel sheet goes straight, and the amount of fluctuation is schematically shown in FIG.

To restrain the steel plate when entering the plate, for example, the roll may be installed within 1 m from the water surface. This distance includes both underwater and on the surface of the water (FIG. 2 shows the case on the surface of the water). It is preferably within 500 mm. The width and material of the roll are not particularly limited, and may be narrower than the width of the steel plate that restrains only the center portion in the width direction, for example. When restraining, it is necessary to select the material, roll position, and roll peripheral speed so that the plate surface is not scratched. Therefore, the material of the roll is not limited to the one made of metal, and may be a material containing rubber and resin having heat resistance. The roll position may be a pair of rolls restrained from the front and back surfaces of the steel sheet, or may be a plurality of roll positions. Further, a pair of rolls having a distance between the rolls may be used. In the case of a pair of rolls with a long roll distance, if the roll is pushed excessively against the steel plate, that is, if the roll is in a position that obstructs the course of the steel plate, the shape and surface properties will deteriorate, so the pushing amount is 5 mm or less. It is preferably 2 mm or less, more preferably 2 mm or less.

Water cooling to 100 ° C. or lower When the temperature after water cooling exceeds 100 ° C., martensitic transformation proceeds after water cooling to the extent that the shape is adversely affected. When the martensitic transformation does not occur, the bainite transformation proceeds non-uniformly, which induces intensity fluctuation in the width direction. Therefore, the temperature of the steel sheet after being discharged from the water tank needs to be 100 ° C. or lower. It is preferably 80 ° C. or lower.

Reheating at 100 ° C. or higher and 350 ° C. or lower After water cooling, it is necessary to reheat and reheat the martensite generated during water cooling to achieve high ductility that enables molding for automobiles. If the reheating temperature is less than 100 ° C, the required ductility cannot be obtained. On the other hand, when the temperature is higher than 350 ° C., the martensite is excessively baked, so that the desired steel sheet strength cannot be obtained. From the above, the reheating temperature was set to 100 ° C. or higher and 350 ° C. or lower. Preferably, it is 130 ° C. or higher and 260 ° C. or lower.

A steel material having a wall thickness of 250 mm having the composition shown in Table 1 is hot-rolled under the hot-rolling conditions shown in Table 2 to obtain a hot-rolled plate having a thickness of 2.6 to 4.4 mm, pickled and then cooled. A cold-rolled sheet having a plate thickness of 1.4 mm was obtained by inter-rolling, and annealing under the conditions shown in Table 2 was performed on a continuous annealing line to produce a steel sheet to be evaluated. After annealing, only normal skin pass rolling with an elongation rate of 0.2% was performed, and evaluation was performed by the following method without performing skin pass rolling or leveler correction more than once. As for the fluctuation of the steel plate with respect to the plate surface normal direction at the time of approaching the water surface, a camera was installed directly above the water surface, the amount of deviation from the ideal steel plate approach position was investigated, and the maximum amount of deviation per 1 m was obtained. The amount of fluctuation of the steel plate with respect to the normal direction of the plate surface at the time of entering the water tank was controlled by installing one roll on each of the front and back surfaces of the steel plate at a position 300 mm above and below the water surface and changing the pushing amount.

(I) Structure observation (area ratio of steel structure)
Cut out from the steel plate so that the cross section of the plate thickness parallel to the rolling direction becomes the observation surface, corrode the central part of the plate thickness with 1% nital, and magnify it 2000 times with a scanning electron microscope to make the plate thickness 1/4 t part (t). Was photographed for 10 fields of view. Ferrite is a structure in which no corrosion marks are observed in the grains, and tempered martensite is a structure in which a large number of fine cementites having an orientation of 500 nm or less and corrosion marks are observed in the grains. Martensite is a structure observed with a whiter contrast than ferrite with a scanning electron microscope, and is a structure in which no cementite precipitation is observed in the grains. Since retained austenite is also observed in the same form as martensite, the value obtained by subtracting the retained austenite fraction by XRD described later from the martensite area obtained by the scanning electron microscope was recorded as the martensite area ratio. The bainite structure was targeted for bainitic ferrite with corrosion marks.

For the area ratio of the above tissues, draw 20 horizontal lines and 20 vertical lines each having an actual length of 30 μm in a grid pattern with respect to the obtained photograph, identify the tissues at the intersections, and the number of intersections of each tissue with respect to all the intersections. Was determined by the cutting method, where the ratio of was taken as the area ratio of each tissue.

(Ii) Measurement of retained austenite fraction by XRD After polishing the steel plate to a plate thickness of 1/4 position, the surface further polished by 0.1 mm by chemical polishing was subjected to fcc iron (austenite) using Mo Kα rays with an X-ray diffractometer. ), The (200) plane, the (311) plane, and the (200) plane, the (211) plane, and the (220) plane of the bcc iron (ferrite) are measured, and the bcc iron (ferrite) plane is measured. ) Fcc iron (austenite) to the integrated reflection intensity from each surface The ratio of austenite obtained from the intensity ratio of the integrated reflection intensity from each surface was defined as the retained austenite fraction.

(Iii) Tensile test A JIS No. 5 tensile test piece was prepared from the obtained steel sheet in a direction perpendicular to the rolling direction, and a tensile test in accordance with the provisions of JIS Z 2241 (2011) was performed five times to obtain an average yield strength (iii). YS), tensile strength (TS), and total elongation (El) were determined. The crosshead speed of the tensile test was 10 mm / min. In Table 3, the tensile strength: 780 MPa or more was set as the mechanical property of the steel sheet obtained by the steel of the present invention.

Further, JIS No. 5 tensile test pieces in the direction parallel to the rolling direction were continuously collected without gaps in the plate width direction. At this time, when the length was insufficient in the plate width direction and a position where the JIS No. 5 tensile test piece could not be collected occurred, the plate width of more than 0 mm and less than 35 mm at the center of the plate width was discarded. The yield strength was investigated and the standard deviation was obtained, and the value was set to the range of 30 MPa or less obtained in the present invention.

(Iv) Evaluation of bendability The molded member may be rejected due to cracks in the bent portion. This is due to the local deterioration of bendability, often due to cracks on the surface of the steel sheet, and the cracks on the surface of the steel sheet are when a steel sheet with an inferior shape is subjected to skin pass rolling or leveler processing more than once. appear. In the present invention, since shape correction that causes local deterioration of bendability is not required, this local deterioration of bendability can also be suppressed. In order to evaluate this, 50 strip-shaped samples having a width of 100 mm and a length of 40 mm are cut out from the center portion in the width direction, and after grinding the sheared end face, 90 ° V by the V block method according to the regulations of JISZ2248 (1996). A sample for bending evaluation was prepared by a bending test (the bending ridge line is the rolling direction). The vicinity of the bending apex was observed with a 20x optical microscope or a loupe to determine the presence or absence of cracks. Table 3 shows the average value (Rave) of the minimum bending radii of the pressing dies that did not cause cracks and the maximum value (Rmax) of the minimum bending radii among 50 sheets evaluated. A level of Rmax-Rave = 0.8 mm or less was set as a preferable range in the present invention.

(V) Steel plate shape evaluation A cold-rolled steel plate that does not shear in the width direction is placed on a horizontal table that has been sheared to a length of 1 m in the length direction, and the maximum height of the steel sheet with respect to the installed table is the "maximum warpage amount". The results are shown in Table 3. The steel plate shape obtained in the present invention has a maximum warp amount of 10 mm or less.

In each of the examples of the present invention, the tensile strength TS: 780 MPa or more, and it can be seen that good steel sheet strength homogeneity, surface texture and steel sheet shape were obtained. On the other hand, in the comparative example outside the scope of the present invention, the tensile strength did not reach 780 MPa, or the steel sheet strength homogeneity, surface texture, or steel sheet shape required in the present invention could not be obtained.

Claims (4)

By mass%
C: 0.05% or more and 0.35% or less,
Si: 0.01% or more and 2.0% or less,
Mn: 0.8% or more and 3.2% or less,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.005% or more and 0.10% or less,
N: 0.0060% or less,
Ingredient composition with the balance consisting of Fe and unavoidable impurities,
Ferrite area ratio is 0% or more and 90% or less, bainite area ratio is 5% or less (including 0%), martensite and tempered martensite area ratio is 10% or more (including 100%), retained austenite area. Has a steel structure with a rate of 2.0% or less (including 0%),
A thin steel sheet having a tensile strength of 780 MPa or more, a standard deviation of yield strength in the width direction of 30 MPa or less, and a maximum warp amount of a sheet steel sheet when sheared at a length of 1 m is 10 mm or less. The component composition is further increased by mass%.
V: 0.001% or more and 1% or less,
Ti: 0.001% or more and 0.3% or less,
Nb: 0.001% or more and 0.3% or less,
Cr: 0.001% or more and 1.0% or less,
Mo: 0.001% or more and 1.0% or less,
Ni: 0.01% or more and 1.0% or less,
Cu: 0.01% or more and 1.0% or less,
B: 0.0002% or more and 0.0050% or less,
Sb: 0.001% or more and 0.050% or less,
REM: 0.0002% or more and 0.050% or less,
The thin steel sheet according to claim 1, which contains any one or more of Mg: 0.0002% or more and 0.050% or less and Ca: 0.0002% or more and 0.050% or less. The method for manufacturing a thin steel sheet according to claim 1 or 2, wherein the steel material is hot-rolled and hot-rolled.
A cold rolling process in which the steel sheet after the hot rolling process is pickled and cold-rolled,
The steel sheet after the cold rolling step is heated at 760 ° C. or higher and then water-quenched at 600 ° C. or higher, and the amount of variation in the normal direction of the steel sheet surface when the steel sheet enters the water tank in the water quenching is 30 mm. A method for producing a thin steel sheet, which comprises an annealing step of water-cooling to 100 ° C. or lower and reheating at 100 ° C. or higher and 350 ° C. or lower so as to be as follows. The method for manufacturing a thin steel plate according to claim 3, wherein in the annealing step, one or more pairs of steel plate restraining rolls are installed within 1 m above and below the water surface of the water tank to adjust the fluctuation amount.