JP2020019992A - Thin steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: 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. SOLUTION: A specific component composition, a ferrite area ratio of 0% or more 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. It has a steel structure (including 100%) and a retained austenite area ratio of 2.0% or less (including 0%), and has a standard deviation of yield strength in the width direction of 30 MPa or less and a length of 1 m. A thin steel sheet having a maximum warp amount of 10 mm or less when sheared is used. [Selection diagram] Fig. 1

Description

  The present invention relates to a thin steel sheet and a method for manufacturing the same. The thin steel sheet of the present invention has a tensile strength (TS) of 780 MPa or more and has both excellent strength homogeneity, surface properties, and sheet shape. For this reason, the thin steel sheet of the present invention is suitable for a raw material of an automobile skeleton member.

2. Description of the Related Art In recent years, from the viewpoint of global environmental protection, the automobile industry as a whole has been pursuing improvement in fuel efficiency for the purpose of regulating CO ₂ emissions. In order to improve the fuel efficiency of automobiles, it is most effective to reduce the weight of automobiles by reducing the thickness of parts used. Therefore, in recent years, the use of high-strength steel sheets as materials for automobile parts has been increasing.

  Many steel sheets utilize martensite, which is a hard phase, to obtain steel sheet strength. On the other hand, when forming martensite, the plate shape is deteriorated by transformation strain. Deterioration of the shape of the sheet adversely affects the dimensional accuracy at the time of molding. Therefore, the sheet has been corrected by leveler processing or skin pass rolling (temper rolling) to obtain a desired dimensional accuracy. On the other hand, since plastic deformation is locally applied by the leveler processing and the skin pass rolling, the work hardening becomes non-uniform, and the fluctuation of the yield strength in the width direction is inevitable. Since the yield strength affects the dimensional accuracy at the time of forming, a high-strength steel sheet excellent in homogeneity is desired. In order not to deteriorate the homogeneity, it is necessary to suppress the deterioration of the plate shape during the martensitic transformation, but 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 to a temperature range of (Ms-30 ° C.) to (Ms-100 ° C.), and then water or gas-water is cooled to 400 ° C. or less at an average cooling rate of 30 to 1000 ° C./s. It is stated that an ultra-high-strength cold-rolled steel sheet that eliminates shape defects can be obtained by bringing the steel sheet inside into contact with one or more pairs of rolls maintained 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 stated that a temper-rolling process is performed so that the average roughness Ra of the steel sheet surface becomes 1.4 μm or more after the tempering treatment is performed, thereby obtaining a steel sheet having a good steel sheet shape.

JP-A-2000-160254 JP 2009-79255 A

  In the technique proposed in Patent Document 1, it is necessary to contact a heating roll for the purpose of eliminating temperature unevenness during gas cooling, but the cooling rate is remarkably lower than water cooling, and bainite is inevitably generated. I do. The formation of bainite not only makes it impossible to obtain a desired steel sheet strength, but also causes a variation in strength.

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

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

  In order to solve the above-mentioned problems, the present inventors have intensively studied requirements for a thin steel sheet having a tensile strength of 780 MPa or more and good strength uniformity and shape of the steel sheet. The thickness of the thin steel sheet targeted in this case is 0.4 mm or more and 2.6 mm or less. Generally, as the strength of a steel sheet increases, the alloy element concentration increases, which adversely affects spot weldability. Therefore, attention was paid to martensite, which can efficiently obtain strength in consideration of spot weldability. On the other hand, it is effective to water-cool the steel sheet in order to obtain martensite efficiently, but the martensitic transformation during water cooling occurs rapidly and non-uniformly, so that the shape of the steel sheet is deteriorated by transformation strain. As a result of investigating the reduction of the adverse effect due to the transformation strain, it was found that the difference in cooling rate between the front and back surfaces of the steel sheet induces non-uniform martensitic transformation and deteriorates the steel sheet shape. As a result of further investigating the difference between the cooling rates of the front and back surfaces of the steel sheet, it was found that the difference occurs in the case where the fluctuation in the normal direction to the plate surface when the steel sheet enters the water tank is large. If the plate shape is good, excessive straightening is not required, and a desired shape can be obtained by ordinary skin pass rolling (temper rolling), and the work hardens uniformly, so that the yield strength varies in the width direction. Can be suppressed. The present invention has been completed based on the above findings, and the gist is as follows.

  [1] In mass%, C: 0.05% to 0.35%, Si: 0.01% to 2.0%, Mn: 0.8% to 3.2%, 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, the balance of the component composition of Fe and unavoidable impurities and the ferrite area ratio 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%), and retained austenite area ratio is 2. 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 warpage of the sheet steel when sheared at a length of 1 m is 10 mm or less. Some steel sheets.

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

  [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 the steel sheet after the hot rolling step and cold rolling. After heating the steel sheet after the cold rolling step at 760 ° C. or more, water quenching is performed at 600 ° C. or more, 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 reduced. An annealing step of water-cooling to 100 ° C. or less so as to be 30 mm or less and heating again at 100 ° C. or more and 350 ° C. or less.

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

  According to the present invention, the thin steel sheet of the present invention has both high strength of tensile strength (TS): 780 MPa or more, excellent strength homogeneity, good surface properties 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 showing an example of bainite. It is a figure which shows the amount of fluctuation typically.

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

  First, the component composition of the steel sheet of the present invention will be described. In the following description, “%” representing the content of a component means “% by mass”.

C: 0.05% or more and 0.35% or less C is an element that relates to the hardness of martensite and tempered martensite, and contributes to increasing the strength of a steel sheet. In order to obtain a tensile strength of 780 MPa or more, it is necessary to contain at least 0.05% or more of the C content. Preferably it is 0.06% or more. On the other hand, when the C content exceeds 0.35%, practical use is extremely difficult due to spot weldability and the like. Therefore, the C content is set to 0.35% or less. Preferably it is 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 elongation, the Si content is set to 0.01% or more. Preferably it is 0.1% or more. On the other hand, when the Si content exceeds 2.0%, the chemical conversion property deteriorates remarkably, and the steel sheet becomes unsuitable as a steel sheet for automobiles. Preferably it is 1.7% or less.

Mn: 0.8% or more and 3.2% or less Mn is an element effective for increasing the hardenability of a steel sheet and obtaining martensite. When the amount of Mn is insufficient, the strength in the width direction changes due to insufficient hardenability. To suppress this, Mn must be contained at 0.8% or more. It is preferably at least 1.1%. On the other hand, even when Mn is excessively contained, the effect of hardenability is not only saturated, but also adversely affects productivity such as castability. From the above, the Mn content was set to 3.2% or less. Preferably it is 2.9% or less. In order to suppress the change in the bendability, the Mn content is more preferably set to 2.3% or less.

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

S: 0.005% or less S forms coarse sulfides in the steel, which expands during hot rolling to become wedge-shaped inclusions, which adversely affects weldability. Therefore, since S is also a harmful element, it is preferable to reduce S as much as possible. In the present invention, the content of S is set to 0.005% or less because it can be allowed up to 0.005%. Preferably, the content is 0.003% or less, but for use under more severe welding conditions, the content is more preferably suppressed to 0.001% or less. In production, 0.0002% may be inevitably mixed.

Al: 0.005% or more and 0.10% or less When Al is added as a deoxidizing agent at the steelmaking stage, the Al content needs to be 0.005% or more. On the other hand, Al forms a coarse oxide that deteriorates weldability and the like. Therefore, the upper limit of the Al content is set to 0.10%. Preferably it is 0.07% or less.

N: 0.0060% or less N is a harmful element that has an adverse effect on surface properties because it deteriorates normal-temperature aging and causes unexpected cracking. Therefore, it is desirable to reduce N as much as possible. In the present invention, up to 0.0060% is acceptable. Preferably it is 0.0050% or less. Although it is desirable to reduce the N content as much as possible, 0.0005% may be unavoidably mixed in production.

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

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

  Next, the steel structure of the thin steel sheet of the present invention will be described. The steel structure of the 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%), and a martensite and tempered martensite area ratio of 10% or more. (Including 100%), and the retained austenite area ratio is 2.0% or less (including 0%).

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

The bainite area ratio is 5% or less (including 0%)
In order to form bainite, it is necessary to maintain isothermally 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 leads to softening of the steel sheet, and inevitable cooling unevenness in the width direction, so that uniform steel sheet strength cannot be obtained. In the present invention, since it is intended to form martensite by water cooling, it is not possible to maintain the isothermal temperature from 200 ° C. to 400 ° C. On the other hand, bainite may be slightly generated during the water cooling process, and the upper limit (including 0%) is set to 5% in the present invention. Preferably it is 3% or less.

  The bainite to be treated in the present invention has a structure containing bainitic ferrite containing dislocations more than polygonal ferrite, and does not cover tempered martensite and lower bainite which cannot be distinguished by a scanning electron microscope. Bainitic ferrite is ferrite that shows corrosion by scanning electron microscope after corrosion appears by 1% nital etching. A representative example is shown in FIG.

Martensite and tempered martensite area ratio is 10% or more (including 100%)
In the present invention, a 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 structure needs to be 10% or more (including 100%) in total. Preferably, it is at least 20%.

Retained austenite area ratio is 2.0% or less (including 0%)
In order to generate the residual austenite in excess of 2.0%, it is essential for the steel composition of the present invention to use a method other than bainite generation and water cooling. In the present invention, since the production method other than the production of bainite and water cooling is not intended, 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%.

  In addition, as other structures other than the ferrite, bainite, martensite, tempered martensite, and retained austenite, pearlite, cementite, and the like can be given. The structure indicates 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 or tempered martensite, and these are not included in the area ratio of cementite. If they remain isolated within the ferrite grains, they are not counted in the area ratio. Since it is difficult to discriminate it from martensite from a scanning electron microscope, it is necessary to confirm it by an EBSD method or a TEM diffraction image. The area ratio of the cementite which remains isolated in the ferrite grains is preferably 2% or less, 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 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, Rmax-Rave in the bendability evaluation described in the examples is 0.8 mm or less. Preferably it is 0.7 mm or less, more preferably 0.6 mm or less.

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

  Next, a method for manufacturing a thin steel sheet according to the present invention will be described. The method for producing a thin steel sheet according to 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 component composition.

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

  The hot rolling conditions in the hot rolling may be set as appropriate.

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

  The annealing step is to heat the steel sheet after the cold-rolling step at 760 ° C. or more, then water-quench at 600 ° C. or more, and in the water quenching, the normal direction of the steel sheet surface when the steel sheet enters the water tank. This is a step of cooling with water to 100 ° C. or less so that the fluctuation amount becomes 30 mm or less, and heating again at 100 ° C. or more and 350 ° C. or less. The annealing step is preferably performed in a continuous annealing line.

Heating to 760 ° C. or higher Heating is intended to generate austenite which contributes to the formation of martensite, and it is necessary to heat to 760 ° C. or higher to obtain a desired martensite area ratio. Preferably it is 780 ° C or higher. The upper limit of the heating temperature is preferably 900 ° C. or less, more preferably 875 ° C. or less, because coarsening of austenite grains causes rough skin and deteriorates surface properties.

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

The present invention is characterized in that the amount of fluctuation in the normal direction of the steel sheet surface when the steel sheet enters the water tank is controlled to 30 mm or less. By controlling the state of the steel sheet entering the water tank, the fluctuation of the cooling rate during water cooling is suppressed. is there. If the position of the steel sheet is largely fluctuated with respect to the normal direction of the sheet surface when the steel sheet enters the water tank, the water retention state on the steel sheet surface becomes non-uniform and the cooling rate fluctuates. In order to suppress this fluctuation, it is possible to suppress the fluctuation by increasing the tension, for example, as compared with the related art, but there is a risk of breakage during passing. By setting the variation of the position of the steel sheet relative to the normal direction of the sheet surface to 30 mm or less, the fluctuation of the cooling rate is suppressed, and a desired steel sheet shape is obtained. The preferred variation is 20 mm or less. The smaller the fluctuation amount is, the more preferable. Note that the amount of change is the amount of deviation from the case where the steel sheet goes straight, and FIG. 2 schematically shows the amount of change.

  The roll may be installed within 1 m from the water surface, for example, to restrain the steel plate when the plate enters. This distance includes both underwater and on the water surface (FIG. 2 shows the case on the water surface). It is preferably within 500 mm. The width and material of the roll are not particularly limited, and may be, for example, smaller than the width of a steel plate that restricts only the center portion in the width direction. When restraining, it is necessary to select a material, a roll position, and a roll peripheral speed so as not to scratch the plate surface. Therefore, the material of the roll is not limited to a metal material, and may be a material containing a heat-resistant rubber or resin. The roll position may be a pair of rolls constrained from the front and back surfaces of the steel sheet, or a plurality of roll positions. Further, a pair of rolls having a large distance between the rolls may be used. In the case of a pair of rolls separated by a roll distance, if the roll is pushed excessively against the steel sheet, that is, if the roll is located at a position obstructing the course of the steel sheet, the shape and surface properties are deteriorated. Preferably, it is not more than 2 mm.

Water cooling to 100 ° C. or less If the temperature after water cooling exceeds 100 ° C., the martensitic transformation proceeds after water cooling so as to adversely affect the shape. When the martensitic transformation does not occur, the bainite transformation proceeds unevenly, so that a variation in strength is induced in the width direction. Therefore, the temperature of the steel sheet after leaving the water tank needs to be 100 ° C. or less. Preferably it is 80 ° C. or lower.

Reheating at 100 ° C. or more and 350 ° C. or less It is necessary to reheat after water cooling and to temper the martensite generated during water cooling to increase the ductility to enable molding for automobiles. If the reheating temperature is lower than 100 ° C., the required ductility cannot be obtained. On the other hand, if the tempering is performed at a temperature exceeding 350 ° C., the martensite is excessively tempered, so that a desired steel sheet strength cannot be obtained. From the above, the reheating temperature was set to 100 ° C. or more and 350 ° C. or less. Preferably, it is 130 ° C. or more and 260 ° C. or less.

  A 250-mm-thick steel material having the composition shown in Table 1 was hot-rolled under hot-rolling conditions shown in Table 2 to obtain a hot-rolled sheet having a sheet thickness of 2.6 to 4.4 mm. A cold-rolled sheet having a thickness of 1.4 mm was formed by cold rolling, and subjected to annealing under the conditions shown in Table 2 in a continuous annealing line to produce a steel sheet for evaluation. After annealing, only normal skin pass rolling at an elongation of 0.2% was performed, and evaluation was performed by the following method without performing skin pass rolling or leveler correction twice or more. The fluctuation of the steel sheet with respect to the normal direction of the sheet surface at the time of entering the water surface was determined by installing a camera directly above the water surface, investigating the amount of deviation from an ideal steel sheet entry position, and calculating the maximum amount of deviation per meter. The amount of change in the steel sheet relative to the normal direction of the sheet surface when entering the water tank was controlled by installing one roll for each of the front and back surfaces of the steel sheet at positions 300 mm above and below the water surface and changing the amount of pushing.

(I) Microstructure observation (area ratio of steel structure)
The steel sheet was cut out from the steel sheet so that the cross section of the sheet thickness parallel to the rolling direction became the observation surface. The center of the sheet thickness was exposed to corrosion with 1% nital, and it was enlarged by 2000 times with a scanning electron microscope to obtain a sheet thickness of 1/4 t portion (t). Was taken for 10 fields of view. Ferrite is a structure in which corrosion marks are not observed in grains, and tempered martensite is a structure in which a large number of fine cementite having a size of 500 nm or less and corrosion marks in grains are observed. Martensite is a structure observed with a scanning electron microscope with a contrast higher than that of ferrite, and is a structure in which precipitation of cementite is not observed in 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 determined by a scanning electron microscope was recorded as the martensite area ratio. The bainite structure was for bainitic ferrite having corrosion marks.

  The area ratio of the tissue was determined by drawing 20 horizontal lines and 20 vertical lines each having an actual length of 30 μm on the obtained photograph so as to form a grid, identifying the tissue at the intersection, and determining the number of intersections of each tissue with respect to all the intersections. Was determined by a cutting method, in which the ratio of was used as the area ratio of each tissue.

(Ii) Measurement of residual austenite fraction by XRD After polishing a steel sheet to a position having a thickness of 1/4 and further polishing it by 0.1 mm by chemical polishing, fcc iron (austenite) ), (200), (220), and (311) planes, and the (200), (211), and (220) planes of bcc iron (ferrite). ) The ratio of austenite obtained from the intensity ratio of the integrated reflection intensity from each surface to fcc iron (austenite) with respect to 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 was performed five times in accordance with the provisions of JIS Z 2241 (2011) to obtain an average yield strength ( YS), tensile strength (TS), and total elongation (El) were determined. The crosshead speed in the tensile test was 10 mm / min. In Table 3, tensile strength: 780 MPa or more was defined as the mechanical properties of the steel sheet required for the steel of the present invention.

  In addition, JIS No. 5 tensile test pieces in the direction parallel to the rolling direction were continuously sampled without any gap in the sheet width direction. At this time, when the length was insufficient in the plate width direction and a position where a JIS No. 5 tensile test piece could not be collected occurred, the plate width at the central portion of the plate width exceeding 0 mm and less than 35 mm was discarded. The yield strength was investigated to determine the standard deviation, and a value of 30 MPa or less was defined as the range required in the present invention.

(Iv) Evaluation of bendability In some cases, a molded member may be rejected due to a crack in a bent portion. This is due to local deterioration of bendability and is often caused by cracks on the surface of the steel sheet, and the cracks on the surface of the steel sheet can be caused by applying two or more skin pass rollings or leveler processing on a steel sheet of inferior shape. appear. In the present invention, since the shape correction is not required, which locally deteriorates the bending property, the local deterioration of the bending property 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 were cut out from the center portion in the width direction, and the sheared end faces were ground. After that, 90 ° V by the V block method in accordance with JISZ2248 (1996). A sample for bending evaluation was prepared by a bending test (the bending ridge line is in the rolling direction). The vicinity of the bending apex was observed with a 20-fold optical microscope or 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 crack and the maximum value (Rmax) of the minimum bending radii of the 50 evaluations. The level of Rmax-Rave = 0.8 mm or less was set as a preferable range in the present invention.

(V) Evaluation of steel sheet shape A sheet obtained by shearing a cold-rolled steel sheet that does not shear in the width direction to a length of 1 m in the length direction is placed on a horizontal table, and the maximum height of the steel sheet with respect to the installed table is referred to as “maximum warpage”. And the results are shown in Table 3. A steel sheet shape having a maximum warpage of 10 mm or less was determined according to the present invention.

  In all of the examples of the present invention, the tensile strength TS is 780 MPa or more, and it can be seen that good steel sheet strength uniformity, surface properties and steel sheet shape were obtained. On the other hand, in the comparative examples outside the range of the present invention, the tensile strength did not reach 780 MPa, or the steel sheet strength uniformity, surface properties or steel sheet shape required in the present invention could not be obtained.

Claims (4)

In 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,
The balance being a component composition comprising 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%), residual austenite area A steel structure having a rate of 2.0% or less (including 0%).
A thin steel sheet having a standard deviation of the yield strength in the width direction of 30 MPa or less and a maximum warpage of the sheet steel of 10 mm or less when sheared at a length of 1 m. The component composition further includes, 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% to 0.050%,
REM: 0.0002% to 0.050%,
The thin steel sheet according to claim 1, comprising one or more of Mg: 0.0002% to 0.050% and Ca: 0.0002% to 0.050%. A hot rolling step of hot rolling a steel material having the component composition according to claim 1 or 2,
Pickling the steel sheet after the hot rolling step, a cold rolling step of cold rolling,
After the steel sheet after the cold rolling step is heated at 760 ° C. or more, water quenching is performed at 600 ° C. or more, 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 of producing a thin steel sheet, comprising: an annealing step of water-cooling to 100 ° C or lower and heating again at 100 ° C to 350 ° C so that   4. The method for producing a thin steel sheet according to claim 3, wherein in the annealing step, one or more pairs of steel sheet restraining rolls are installed within 1 m above and below the water surface of the water tank as a base point, and the variation is adjusted. 5.