[0001]The present invention relates to a high-strength hot-rolled steel sheet having excellent bending workability.
　The present application claims priority based on Japanese Patent Application No. 2019-43961 filed in Japan on March 11, 2019, the contents of which are incorporated herein by reference.
Background technology
[0002]
　So-called hot-rolled steel sheets manufactured by hot rolling are widely used as relatively inexpensive structural materials and as materials for structural members of automobiles and industrial equipment. In particular, hot-rolled steel sheets used for undercarriage parts, bumper parts, shock absorbing members, etc. of automobiles are being strengthened from the viewpoints of weight reduction, durability, shock absorbing capacity, etc., and at the same time, they are complicated. It is also required to have excellent moldability that can withstand molding into a flexible shape.
[0003]
　However, since the formability of the hot-rolled steel sheet tends to decrease as the strength of the material increases, it is a difficult problem to achieve both high strength and good formability.
　In particular, in recent years, there has been an increasing demand for weight reduction of undercarriage parts of automobiles, and it has become an important issue to realize high tensile strength of 780 MPa or more and excellent bending workability.
[0004]
　Non-Patent Document 1 reports that bending workability is improved by controlling a single structure such as ferrite, bainite, and martensite by structure control.
　Further, in Patent Document 1, bending is performed by combining ferrite having excellent bending workability as the main phase, suppressing crack generation by reducing the surface roughness of the steel sheet, and suppressing crack propagation by fine precipitates. It has been reported that workability is improved. Patent Document 2 reports a technique for improving bending workability by making the hardness of the surface layer of a steel sheet lower than that of the central portion of the steel sheet.
[0005]
　Further, Patent Document 3 has a predetermined chemical composition, a plate thickness of 3 to 15 mm, an area ratio of a surface defect caused by a scale of 20% or less, a yield stress YP (L) in the rolling direction, and a width direction. Yield stress YP (C) is 400 MPa or more, and YP (L) and YP (C) satisfy | YP (C) -YP (L) | / YP (C) ≤ 0.04. Has been reported.
Prior art literature
Patent documents
[0006]
Patent Document 1: Japanese Patent No. 6179584
Patent Document 2: Japanese Patent Application Laid-Open No. 2015-98629
Patent Document 3: Japanese Patent Application Laid-Open No. 2014-118592
Non-patent literature
[0007]
Non-Patent Document 1: Yamazaki et al., Journal of the Japan Society for Technology of Plasticity), vol. 36 (1995), No. 416, p. 973
Outline of the invention
Problems to be solved by the invention
[0008]
　However, in the techniques reported in Patent Documents 1 and 2 and Non-Patent Document 1, it may be difficult to achieve both tensile strength and bending workability at the same time. Further, in Patent Document 3, the area ratio of the surface flaws caused by the scale is set to 20% or less, but the conditions in the finish rolling process are not satisfied and the steel sheet is leveled. In some cases, the area ratio of a portion 10 μm or more lower than the average height of the steel sheet surface is more than 20%, and the bending workability is inferior.
　Therefore, an object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent tensile strength and bending workability.
Means to solve problems
[0009]
(1) The hot-rolled steel sheet according to one aspect of the present invention has C: 0.030 to 0.250%, Si: 0.05 to 2.50%, Mn: 1.00 in mass% as a chemical component. ~ 4.00%, sol. Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0200% or less, N: 0.01000% or less, Ti: 0 to 0.20%, Nb: 0 to 0. 20%, B: 0 to 0.010%, V: 0 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 1.0%, Cu: 0 to 1.0%, Co: 0 to 1.0%, W: 0 to 1.0%, Ni: 0 to 1.0%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, REM: 0 to 0.01 %, Zr: 0 to 0.01%, and the balance: Fe and impurities, the area ratio of the scale scratched part, which is 10 μm or more lower than the average height of the steel plate surface, is 20% or less, and the tensile strength is high. It is 780 MPa or more.
(2) The hot-rolled steel sheet according to (1) may have an average aspect ratio of 5 or less on the scale scratched portion.
(3) The hot-rolled steel sheet according to (1) or (2) has Ti: 0.001 to 0.20%, Nb: 0.001 to 0.20%, B in mass% as the chemical component. : 0.001 to 0.010%, V: 0.005 to 1.0%, Cr: 0.005 to 1.0%, Mo: 0.005 to 1.0%, Cu: 0.005 to 1 .0%, Co: 0.005 to 1.0%, W: 0.005 to 1.0%, Ni: 0.005 to 1.0%, Ca: 0.0003 to 0.01%, Mg: It may contain at least one species composed of the group consisting of 0.0003 to 0.01%, REM: 0.0003 to 0.01%, and Zr: 0.0003 to 0.01%.
The invention's effect
[0010]
　According to one embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet having an excellent tensile strength of 780 MPa or more and 1470 MPa or less and having excellent bending workability capable of suppressing the occurrence of internal bending cracks.
A brief description of the drawing
[0011]
FIG. 1 is a schematic view of a hot-rolled steel plate according to the present embodiment when the plate surface is viewed in a plan view.
FIG. 2 is a cross-sectional view of a hot-rolled steel sheet according to the present embodiment when cut along the PP'line of FIG. 1 in the plate thickness direction.
Mode for carrying out the invention
[0012]
　Hereinafter, the hot-rolled steel sheet according to the embodiment of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention. In addition, the following numerical range includes the lower limit value and the upper limit value. Numerical values ​​that indicate "greater than" or "less than" are not included in the numerical range. "%" Regarding the content of each element means "mass%".
[0013]
　First, the findings of the present inventors who came up with the present invention will be described.
　Conventionally, cracks in the bending process of a steel sheet have generally occurred from the surface of the steel sheet on the outside of the bending or near the end face, but as the strength of the steel sheet increases, minute cracks may occur on the inside of the bending. .. Conventional knowledge has not shown a method for suppressing minute cracks generated inside such bending.
[0014]
　The present inventors conducted an diligent investigation on the bending workability of a high-strength steel sheet, and found that the higher the steel sheet strength, the more likely it is that cracks will occur from the inside of the bending during bending (hereinafter referred to as internal bending cracks). ).
　The mechanism of internal bending cracking is presumed as follows. During bending, compressive stress is generated inside the bend. At first, the entire inside of the bend is deformed uniformly while processing proceeds, but when the amount of processing increases, the deformation cannot be carried out only by uniform deformation, and the deformation progresses due to the concentration of strain locally (generation of shear deformation zone).
[0015]
　As this shear band grows further, cracks along the shear band are generated from the inner surface of the bend and grow. The reason why in-bending cracks are more likely to occur as the strength increases is that uniform deformation is less likely to proceed due to the decrease in work hardening ability due to the increase in strength, and biased deformation is likely to occur at an early stage of processing ( It is presumed that a shear band is generated (or under loose processing conditions).
　According to the research by the present inventors, internal bending cracks are likely to occur in steel sheets having a tensile strength of 780 MPa class or higher, become more prominent in steel sheets of 980 MPa class or higher, and become a more prominent problem in steel sheets of 1180 MPa class or higher. I understood.
[0016]
　From the above estimation process of bending internal cracks, the present inventors considered that suppressing the local concentration of strain at the initial stage would lead to the occurrence of cracks, and eagerly searched for a method for suppressing the cracks. As a result, it was clarified that the surface texture of the steel sheet is related to the initial local strain concentration, and it was discovered that the internal bending crack can be suppressed by controlling the surface texture.
　The present inventors have also found an effective hot rolling method for obtaining the above-mentioned surface texture. It has been clarified that the phenomenon that the surface scale is pressed against the steel sheet by the roll during hot rolling has a great influence on the final surface texture, and in order to control the surface texture, the scale growth during hot rolling is controlled. It became clear that this can be achieved by spraying water on the surface of the steel sheet during rolling under certain conditions.
[0017]
1. 1. Chemical
　composition The composition of the hot-rolled steel sheet according to the present embodiment will be described in detail below. The hot-rolled steel sheet according to the present embodiment contains a basic element as a chemical component, and if necessary, a selective element, and the balance is composed of Fe and impurities.
[0018]
(C: 0.030% or more and 0.250% or less)
　C is an important element for ensuring the strength of the steel sheet. If the C content is less than 0.030%, the tensile strength of 780 MPa or more cannot be secured. Therefore, the C content is 0.030% or more, preferably 0.050% or more.
　On the other hand, if the C content exceeds 0.250%, the weldability deteriorates, so the upper limit is set to 0.250%. The C content is preferably 0.200% or less, more preferably 0.150% or less.
[0019]
(Si: 0.05% or more and 2.50% or less)
　Si is an important element whose material strength can be increased by solid solution strengthening. If the Si content is less than 0.05%, the yield strength is lowered, so the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more, more preferably 0.30% or more.
　On the other hand, if the Si content exceeds 2.50%, the surface texture deteriorates, so the Si content is set to 2.50% or less. The Si content is preferably 2.00% or less, more preferably 1.50% or less.
[0020]
(Mn: 1.00% or more and 4.00% or less)
　Mn is an element effective for increasing the mechanical strength of the steel sheet. If the Mn content is less than 1.00%, it is not possible to secure a tensile strength of 780 MPa or more. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more, more preferably 2.00% or more.
　On the other hand, when Mn is added excessively, the structure becomes non-uniform due to Mn segregation, and the bending workability is lowered. Therefore, the Mn content is set to 4.00% or less, preferably 3.00% or less, and more preferably 2.60% or less.
[0021]
(Sol.
　Al : 0.001% or more and 2.000% or less) Al is an element having an action of deoxidizing steel to make a steel sheet sound. sol. If the Al content is less than 0.001%, it cannot be sufficiently deoxidized. The Al content is 0.001% or more. However, when sufficient deoxidation is required, it is more desirable to add 0.010% or more. More preferably, sol. The Al content is 0.020% or more.
　On the other hand, sol. When the Al content exceeds 2.000%, the weldability is remarkably lowered, oxide-based inclusions are increased, and the surface texture is remarkably deteriorated. Therefore, sol. The Al content is 2.000% or less, preferably 1.500% or less, more preferably 1.000% or less, and most preferably 0.080% or less. In addition, sol. Al means an acid-soluble Al that is not an oxide such as Al 2 O 3 and is soluble in an acid.
[0022]
　The hot-rolled steel sheet according to this embodiment contains impurities as a chemical component. The term "impurity" refers to substances mixed from ore or scrap as a raw material, or from the manufacturing environment, etc., when steel is industrially manufactured. For example, it means an element such as P, S, N. These impurities are preferably limited as follows in order to fully exert the effects of the present embodiment. Further, since the content of impurities is preferably small, it is not necessary to limit the lower limit value, and the lower limit value of impurities may be 0%.
[0023]
(P: 0.100% or less)
　P is an impurity generally contained in steel, but P may be positively contained because it has an effect of increasing tensile strength. However, when the P content exceeds 0.100%, the deterioration of weldability becomes remarkable. Therefore, the P content is limited to 0.100% or less. The P content is preferably limited to 0.050% or less. In order to obtain the effect of the above action more reliably, the P content may be 0.001% or more.
[0024]
(S: 0.0200% or less)
　S is an impurity contained in steel, and the smaller the amount, the more preferable it is from the viewpoint of weldability. When the S content exceeds 0.0200%, the weldability is significantly lowered, the MnS precipitation amount is increased, and the low temperature toughness is lowered. Therefore, the S content is limited to 0.0200% or less. The S content is preferably limited to 0.0100% or less, more preferably 0.0050% or less. From the viewpoint of desulfurization cost, the S content may be 0.0010% or more.
[0025]
(N: 0.01000% or less)
　N is an impurity contained in steel, and the smaller the amount, the more preferable it is from the viewpoint of weldability. If the N content exceeds 0.01000%, the weldability is significantly reduced. Therefore, the N content may be limited to 0.01000% or less, preferably 0.00500% or less.
[0026]
　The hot-rolled steel sheet according to the present embodiment may contain a selective element in addition to the basic elements and impurities described above. For example, instead of a part of Fe which is the balance described above, Ti, Nb, B, V, Cr, Mo, Cu, Co, W, Ni, Ca, Mg, REM, and Zr are contained as selective elements. May be good. These selective elements may be contained according to the purpose. Therefore, it is not necessary to limit the lower limit values ​​of these selective elements, and the lower limit value may be 0%. Further, even if these selective elements are contained as impurities, the above effects are not impaired.
[0027]
(Ti: 0% or more and 0.20% or less)
　Ti is an element as TiC that precipitates on ferrite or bainite of the steel sheet structure during cooling or winding of the steel sheet and contributes to the improvement of strength. Further, when Ti exceeds 0.20%, the above effect is saturated and the economic efficiency is lowered. Therefore, the Ti content is set to 0.20% or less. The Ti content is preferably 0.18% or less, more preferably 0.15% or less. In order to obtain the above effect preferably, the Ti content may be 0.001% or more. It is preferably 0.02% or more.
[0028]
(Nb: 0% or more and 0.20% or less)
　Like Ti, Nb is an element that precipitates as NbC, improves the strength, remarkably suppresses recrystallization of austenite, and refines the grain size of ferrite. Is. When Nb exceeds 0.20%, the above effects are saturated and economic efficiency is reduced. Therefore, the Nb content is set to 0.20% or less. It is preferably 0.15% or less, more preferably 0.10% or less. In order to obtain the above effect preferably, the Nb content may be 0.001% or more. It is preferably 0.005% or more.
[0029]
　In the hot-rolled steel sheet according to the present embodiment, at least of Ti: 0.001% or more and 0.20% or less and Nb: 0.001% or more and 0.20% or less in mass% as chemical components. It is preferable to contain one kind.
[0030]
(B: 0% or more and 0.010% or less)
　B segregates at the grain boundaries to improve the grain boundary strength, so that the roughness of the punched cross section at the time of punching can be suppressed. Therefore, B may be contained. Even if the B content exceeds 0.010%, the above effect is saturated and economically disadvantageous. Therefore, the upper limit of the B content is set to 0.010% or less. The B content is preferably 0.005% or less, more preferably 0.003% or less. In order to obtain the above effect preferably, the B content may be 0.001% or more.
[0031]
(V: 0% or more and 1.0% or less) (Cr: 0% or more and 1.0% or less) (Mo: 0% or more and 1.0% or less) (Cu: 0% or more and 1.0% or less) (Co : 0% or more and 1.0% or less) (W: 0% or more and 1.0% or less) (Ni: 0% or more and 1.0% or less)
　V, Cr, Mo, Cu, Co, W, Ni Is an element that is effective in ensuring stable strength. Therefore, these elements may be contained. However, even if each of the elements is contained in an amount of more than 1.0%, the effect of the above action is likely to be saturated, which may be economically disadvantageous. Therefore, the V content, Cr content, Mo content, Cu content, Co content, W content, and Ni content are each preferably 1.0% or less. In order to obtain the effect of the above action more reliably, V: 0.005% or more, Cr: 0.005% or more, Mo: 0.005% or more, Cu: 0.005% or more, Co: 0. It is preferable that at least one of 005% or more, W: 0.005% or more and Ni: 0.005% or more is contained.
[0032]
(Ca: 0% or more and 0.01% or less) (Mg: 0% or more and 0.01% or less) (REM: 0% or more and 0.01% or less) (Zr: 0% or more and 0.01% or less)
　Ca, Mg, REM, and Zr are all elements that contribute to inclusion control, particularly fine dispersion of inclusions, and have an action of increasing toughness. Therefore, one or more of these elements may be contained. However, if each of the elements is contained in an amount of more than 0.01%, deterioration of the surface texture may become apparent. Therefore, the content of each element is preferably 0.01% or less. In order to obtain the effect of the above action more reliably, the content of at least one of these elements is preferably 0.0003% or more.
　Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and is at least one of them. The content of REM means the total content of at least one of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.
[0033]
　In the hot-rolled steel sheet according to the present embodiment, Ca: 0.0003% or more and 0.01% or less, Mg: 0.0003% or more and 0.01% or less, REM: 0. It is preferable to contain at least one of 0003% or more and 0.01% or less and Zr: 0.0003% or more and 0.01% or less.
[0034]
　The above-mentioned steel composition may be measured by a general method for analyzing steel. For example, the steel component may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured by using the combustion-infrared absorption method, N may be measured by using the inert gas melting-heat conductivity method, and O may be measured by using the inert gas melting-non-dispersion infrared absorption method.
[0035]
　2. Surface properties
　The surface properties of the hot-rolled steel sheet according to this embodiment need to have an area ratio of 20% or less of scale scratches having a depth of 10 μm or more. If the area ratio of the scale scratched portion exceeds 20%, strain is concentrated locally on the scale scratched portion at the initial stage of bending, which causes cracks in the bending inside. From this point of view, the area ratio is preferably 10% or less, and more preferably 5% or less.
　The detailed definition of the scale scratch is as follows. Using a device such as a digital microscope (for example, RH-2000 (manufactured by Hirox Co., Ltd.)) that acquires the target 3D image data by analyzing the depth of focus, 3D image data in the range of 3000 μm × 3000 μm on the surface of the hot-rolled steel sheet can be obtained. get. FIG. 1 is a schematic view when the plate surface of the hot-rolled steel sheet according to the present embodiment is viewed in a plan view, and FIG. 2 is a hot-rolled view according to the present embodiment when cut along the PP'line of FIG. It is sectional drawing in the plate thickness direction of a steel plate. The average height position of the highest height position and the lowest height position in the acquired 3D image data is defined as the average height position I, and the area where the height position is 10 μm or more lower than the average height position I. Is defined as the scale wound portion 10. Next, as shown in FIG. 1, a plan view of 3D image data in a range of 3000 μm × 3000 μm on the surface of the hot-rolled steel sheet 100 is created from the upper part in the rolling direction, and all the scale scratches 10 included in the range are created. The area ratio of the scale scratched portion 10 is calculated by dividing the projected area by the total projected area of ​​the range.
　That is, if there is no region whose height position is 10 μm or more lower than the average height position within the range of 3000 μm × 3000 μm, there is no scale scratched portion within that range.
[0036]
　Next, the scale scratches as described above are often generated due to the scale existing on the surface layer being pressed by the roll during finish rolling, and extend in the direction (C direction) perpendicular to the rolling direction (L direction). I have something to do. If the scale scratched portion is extended in one direction, strain concentration is likely to occur particularly on the scale scratched portion in the bending process with the bending axis as the bending axis, which causes cracks in the bending.
　From this point of view, it is desirable that the average aspect ratio of the scale scratches is 5 or less. The average aspect ratio is preferably 3 or less, more preferably 2 or less.
[0037]
　The detailed definition of the average aspect ratio of the scale scratches is as follows. A method of obtaining the aspect ratio of the scale scratched portion will be described with reference to FIG. As shown in FIG. 1, the length when a line segment parallel to the rolling direction X is drawn to be the longest inside one scale scratched portion 10 is the length of the scale scratched portion 10 in the L direction. Let A be the length B of the scale scratched portion 10 in the C direction when the line segment running in the direction (C direction) Y perpendicular to the rolling direction (L direction) is drawn so as to be the longest. The value A / B or B / A obtained by dividing the larger value of the length A in the L direction and the length B in the C direction by a smaller value is defined as the aspect ratio of the scale scratched portion 10.
　Aspect ratios are measured for all scale scratches 10 in a field of view of 3000 μm × 3000 μm (three scale scratches 10 in the example shown in FIG. 1), and the average value thereof is defined as an average aspect ratio.
[0038]
3. 3.
　Steel sheet structure The hot-rolled steel sheet according to the present embodiment may have any phase of ferrite, pearlite, bainite, fresh martensite and tempered martensite, pearlite, retained austenite, etc. as a constituent phase of the steel structure. , A compound such as bainite may be contained in the structure.
　For example, in% area, 80% or less of ferrite, 0 to 100% of bainite or martensite, and other retained austenite: 25% or less and pearlite: 5% or less can be contained.
[0039]
4. Mechanical Properties
　The hot-rolled steel sheet according to this embodiment has a tensile strength (TS) of 780 MPa or more as a sufficient strength that contributes to weight reduction of automobiles. On the other hand, since it is difficult to make it more than 1470 MPa in the configuration of this embodiment, the practical upper limit of the tensile strength is 1470 MPa or less. Therefore, it is not necessary to set the upper limit of the tensile strength in particular, but in the present embodiment, the upper limit of the substantial tensile strength can be set to 1470 MPa.
　The tensile test may be performed in accordance with JIS Z2241 (2011).
　In the hot-rolled steel sheet according to the present embodiment, the limit bending R / t value, which is an index value of the bending internal crackability, is preferably 2.0 or less. The R / t values ​​are, for example, bending (L-axis bending) in which a strip-shaped test piece is cut out from the width direction 1/2 position of the hot-rolled steel sheet and the bending ridge line is parallel to the rolling direction (L direction). Bending is performed in accordance with JIS Z2248 (V block 90 ° bending test) for both bending (C-axis bending) in which the bending ridge is parallel to the direction perpendicular to the rolling direction (C direction), and occurs inside the bending. The cracks can be investigated and found. The minimum bending radius that does not cause cracks with a length of 30 μm or more can be obtained, and the value obtained by dividing the average value of the minimum bending radii of the L axis and the C axis by the plate thickness can be used as the limit bending R / t as an index value of bendability. can.
[0040]
5. Manufacturing Method
　Next, a preferable manufacturing method of the hot-rolled steel sheet according to the present embodiment will be described.
[0041]
　The manufacturing process prior to hot rolling is not particularly limited. That is, after melting in a blast furnace, an electric furnace, or the like, various secondary smelting may be performed, and then casting may be performed by a method such as ordinary continuous casting, casting by the ingot method, or thin slab casting. In the case of continuous casting, the cast slab may be cooled to a low temperature and then heated again and then hot-rolled, or the cast slab may be hot-rolled as it is after casting without being cooled to a low temperature. .. Scrap may be used as the raw material.
[0042]
　The cast slab is heated. In this heating step, the slab is heated to a temperature of 1100 ° C. or higher and 1300 ° C. or lower, and then held for 30 minutes or longer. When Ti or Nb is added, it is heated to a temperature of 1200 ° C. or higher and 1300 ° C. or lower, and then held for 30 minutes or longer. If the heating temperature is less than 1200 ° C., the precipitate elements Ti and Nb are not sufficiently dissolved, so that sufficient precipitation strengthening cannot be obtained during subsequent hot rolling, and the precipitate remains as coarse carbides, resulting in formability. It is not preferable because it deteriorates. Therefore, when Ti and Nb are contained, the heating temperature of the slab is set to 1200 ° C. or higher. On the other hand, if the heating temperature exceeds 1300 ° C., the amount of scale generated increases and the yield decreases, so the heating temperature is set to 1300 ° C. or lower. The heating holding time is preferably 30 minutes or more in order to sufficiently dissolve Ti and Nb. Further, in order to suppress excessive scale loss, the heating holding time is preferably 10 hours or less, and more preferably 5 hours or less.
[0043]
　Next, a rough rolling step is performed in which the heated slab is roughly rolled to obtain a rough rolled plate.
　In rough rolling, the slab may have a desired size and shape, and the conditions thereof are not particularly limited. The thickness of the rough-rolled plate affects the amount of temperature decrease from the tip to the tail of the hot-rolled plate from the start of rolling to the completion of rolling in the finish rolling process, so it should be determined in consideration of this. Is preferable.
[0044]
　The rough-rolled plate is subjected to finish rolling. In this finish rolling process, multi-step finish rolling is performed. In this embodiment, finish rolling is performed in a temperature range of 850 ° C to 1200 ° C under the condition of satisfying the following formula (2).
　K'/ Si * ≧ 2.5 ... (2)
　Here, when Si ≧ 0.35, Si * = 140√Si, and when Si <0.35, Si * = 80. In addition, Si represents the Si content (mass%) of the steel sheet.
[0045]
　Further, K'in the above formula (2) is represented by the following formula (3).
　K '= D × (DT-930) × 1.5 + sigma ((FT n -930) × S n ) · · · (3)
　Here, D is sprayed per unit time of the water pressure descaling prior to the start of finish rolling amount (M 3 / min), DT is the steel plate temperature (° C.) at the time of hydraulic descaling before the start of finish rolling, FT n is the steel plate temperature (° C.) at the nth stage of finish rolling , Sn is n of finish rolling. -The amount of spraying per hour (m 3 / min) when water is sprayed onto the steel sheet between the 1st and nth stages .
[0046]
　Si * is a parameter related to the steel sheet component that indicates the ease of forming scale scratches. When the amount of Si in the steel sheet component is large, the scale generated on the surface layer during hot rolling grows from wustite (FeO), which is relatively easy to descale and difficult to form scale scratches on the steel sheet, so as to take root in the steel sheet. It changes to fire light (Fe 2 SiO 4 ), which makes it easy to make scale scratches . Therefore, the larger the amount of Si, that is, the larger the Si * , the easier it is for scale scratches on the surface layer to be formed. Here, the easiness of forming scale scratches on the surface layer by adding Si becomes particularly effective when 0.35% by mass or more of Si is added. Therefore, when 0.35% by mass or more is added, Si * becomes a function of Si, but when it is less than 0.35% by mass, it becomes a constant.
[0047]
　K'is a parameter of manufacturing conditions indicating the difficulty of forming scale scratches. The first item of the above formula (3) is that when hydraulic descaling is performed before the start of finish rolling in order to suppress the formation of scale scratches, the larger the amount of hydraulic descaling sprayed per hour, the higher the steel sheet temperature. The higher the value, the more effective it is from the viewpoint of descaling. When performing a plurality of descaling before the start of finish rolling, the descaling value closest to the finish rolling and before the first spray spraying step S 1 is used.
　The second item of the above formula (3) is a section showing the effect of descaling the scale that could not be completely peeled off by descaling before finishing and the scale that was reformed during finish rolling during finish rolling. Yes, it is shown that spraying a large amount of water onto the steel sheet in the form of a spray at a high temperature makes it easier to descale.
　Considering the mechanism of descaling control, the original parameter of the manufacturing conditions indicating the difficulty of forming scale scratches is the product of "parameters related to temperature" and "parameters related to the amount of water sprayed", and finish rolling is performed. It is considered that it will be integrated in the temperature range to be performed. This is due to the idea of ​​promoting descaling by spraying more water at a higher temperature.
　In order to make the parameters simpler in controlling the manufacturing conditions, the present inventors use a parameter K'(Equation 3) corresponding to summing up the above-mentioned original parameters divided between each roll. By doing so, it has been found that the surface roughness can be controlled.
　Here, it is conceivable that the parameter K'is different from the above-mentioned original parameter depending on the number of stands of the finishing rolling mill, the distance between rolls, and the plate passing speed. However, the present inventors can set the above parameter K'if the number of finishing rolling stands is 5 to 8, the distance between rolls is 4500 mm to 7000 mm, and the plate passing speed (speed after passing the final stage) is in the range of 400 to 900 mmp. It has been confirmed that the surface roughness can be controlled by using it.
[0048]
If the ratio 　of the parameter K'in the manufacturing conditions indicating the difficulty of forming the scale scratches and the parameter Si * regarding the steel plate component indicating the ease of forming the scale scratches is 2.5 or more, the area ratio of the scale scratches Can be 20% or less, and the occurrence of cracks inside the bend can be suppressed. It is preferably 3.0 or more, and more preferably 3.5 or more.
　When K'/ Si * is set to 3.0 or more, the average aspect ratio of the scale scratched portion can be set to 5 or less, and more preferable surface texture can be obtained. The reason for this is not completely clear, but it can be estimated as follows. That is, when the roll pushes the scale into the steel sheet to form a scale scratch, the scale is divided at a certain interval in the direction of the roll axis, that is, in the direction perpendicular to the rolling direction (L direction) (C direction). While being pushed into the steel plate. This is the reason why the scale scratched portion tends to have a shape extending in the C direction. At this time, it is estimated that the thicker the scale thickness, the longer the interval at which the scale is divided, and the aspect ratio of the scale scratched portion may also increase. Therefore, it is considered that the aspect ratio of the scale scratched portion can be reduced by increasing K'/ Si * so that the scale is less likely to exist during the finish rolling.
[0049]
　Further, in finish rolling, it is desirable to satisfy the condition represented by the following formula (4).
　F ≧ 0.5 ... (4)
　F is the total time (xy) of the time (x seconds) from the start to the completion of finish rolling, excluding the time (y seconds) in which the steel sheet is in contact with the roll. The ratio of the time (z seconds) that the surface of the steel sheet is covered with the water film is shown. That is, it is represented by F = z / (xy).
[0050]
　The scale that grows during finish rolling as described above can also cause scale scratches to form on the steel sheet, but the growth can be suppressed by covering the surface of the steel sheet with a water film, so the surface of the steel sheet is covered with a water film. The longer the time covered with, the more desirable. Examples of the method of covering the surface of the steel sheet with a water film include spraying water between the rolls.
[0051]
　K '/ Si * ≧ 2.5 at the same time satisfies a ≧ 0.5 K F' / Si * than when satisfying only ≧ 2.5, it is possible to reduce the area ratio of the scale flaw, bending inner crack Occurrence can be suppressed more. Further, when F ≧ 0.5 is satisfied, the average aspect ratio of the recessed portion can be set to 5 or less even in the range of 3.0> K'/ Si * ≧ 2.5, and the occurrence of cracks inside the bending can be further suppressed. From this viewpoint, F is preferably 0.6 or more, and more preferably 0.7 or more.
[0052]
　The plate thickness after finish rolling shall be less than 5.0 mm. This is because hot rolling has the effect of reducing unevenness by crushing the surface of the steel sheet after the scale has been removed with a roll, and this effect is obtained when the plate thickness after finish rolling is less than 5.0 mm. This is because it becomes easier. The plate thickness after finish rolling is preferably 4.5 mm or less. On the other hand, when the plate thickness after finish rolling is 5.0 mm or more, the specified surface condition cannot be obtained and the bendability deteriorates.
[0053]
　Following the finish rolling, a cooling step and a winding step are performed.
　In the hot-rolled steel sheet of the present embodiment, excellent bending workability is achieved by controlling the surface texture rather than controlling the base structure, so that the conditions of the cooling process and the winding process are not particularly limited. Therefore, the cooling step and the winding step after the multi-step finish rolling may be performed by a conventional method.
[0054]
　The hot-rolled steel sheet may be pickled if necessary after cooling. The pickling treatment may be carried out, for example, in hydrochloric acid having a concentration of 3 to 10% at a temperature of 85 ° C. to 98 ° C. for 20 seconds to 100 seconds.
[0055]
　The hot-rolled steel sheet may be subjected to skin pass rolling after cooling, if necessary. Skin pass rolling has the effects of preventing stretcher strains that occur during processing and shaping, and of shape correction.
Example
[0056]
　The hot-rolled steel sheet according to the present invention will be described in more detail below with reference to an example. However, the following examples are examples of the hot-rolled steel sheet of the present invention, and the hot-rolled steel sheet of the present invention is not limited to the following aspects. The conditions in the examples described below are one-condition examples adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to these one-condition examples. In the present invention, various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0057]
　The steels with the chemical components shown in Table 1 are cast, and after casting, they are heated as they are or after being cooled to room temperature and then reheated to a temperature range of 1200 ° C to 1300 ° C, and then the slab is rolled at a temperature of 1100 ° C or higher. A rough-rolled plate was produced by rough-rolling.
　The rough-rolled plate was finish-rolled to the finish-rolled plate thickness shown in Tables 2 and 3 using the following three types of finish-rolling machines.
　Roller A: 7 stands, distance between rolls 5500 mm, plate speed 700 mpm Roller
　B: Number of stands 6, distance between rolls 5500 mm, plate speed 600 mpm Roller
　C: Number of stands 7, distance between rolls 6000 mm, sheet passing speed 700mpm
　then subjected to hot rolling under the conditions described in Table 2 and Table 3. The finish rolling mills used are also shown in Tables 2 and 3. Since the steel plate temperature DT at the time of descaling is almost the same as the finish rolling start temperature here, K'was calculated using the steel plate temperature FT 1 of the first stage of finish rolling . After the finish rolling was completed, cooling and winding were performed in each of the cooling patterns shown below with the aim of forming the hot-rolled sheet structure into bainite, ferrite-bainite, and martensite.
[0058]
(Bainite pattern: Cooling pattern B)
　The hot-rolled steel sheet produced by this pattern is finished-rolled, cooled at a cooling rate of 20 ° C./sec or higher to a winding temperature of 450 ° C. to 550 ° C., and then wound into a coil. A cooling step and a winding step were performed.
[0059]
(Ferrite-Bainite pattern: Cooling pattern F + B)
　After finish rolling, the hot-rolled steel sheet produced by this pattern is cooled to a cooling stop temperature range of 600 to 750 ° C at an average cooling rate of 20 ° C / sec or more, and the cooling stop temperature. It was obtained by holding for 2 to 4 seconds within the range, and then performing a cooling step and a winding step of winding in a coil at a winding temperature of 500 to 600 ° C at an average cooling rate of 20 ° C./sec or more. .. When it is necessary to clearly determine the temperature, holding time, etc. in this step, the temperature and time are set using the Ar3 temperature of the following formula. In the following formula, C, Si, Mn, Ni, Cr, Cu, and Mo represent the content of each element in the unit: mass%.
　Ar3 (° C.) = 870-390C + 24Si-70Mn-50Ni-5Cr-20Cu + 80Mo
[0060]
(Martensite pattern: Cooling pattern Ms)
　After the finish rolling is completed, the hot-rolled steel sheet produced by this pattern is cooled to a winding temperature of 100 ° C or less at an average cooling rate of 20 ° C / sec or more, and then wound into a coil. Manufactured by taking, cooling and winding steps.
[0061]
　Each hot-rolled steel sheet was pickled with hydrochloric acid having a concentration of 3 to 10% at a temperature of 85 ° C. to 98 ° C. for 20 seconds to 100 seconds to peel off the scale.
　The scale scratches were measured as follows. Using RH-2000 (manufactured by Hirox Co., Ltd.), 3D image data in the range of 3000 μm × 3000 μm on the surface of the hot-rolled steel sheet is acquired, and the area ratio (%) and aspect ratio of the scale scratches defined above are calculated. bottom.
[0062]
　The tensile strength is JIS using the JIS No. 5 test piece collected so that the direction (C direction) perpendicular to the rolling direction (L direction) is the longitudinal direction from the position of 1/4 of the width direction of the hot-rolled steel sheet. A tensile test was carried out in accordance with the provisions of Z 2241 (2011), and tensile strength TS (MPa), butt elongation (total elongation) EL (%), and limit bending (R / t) were determined. The respective measurement results are shown in Tables 4 and 5. As the tensile strength TS, those having a tensile strength of 780 MPa or more were accepted.
[0063]
　As the bending test piece, a strip-shaped test piece having a size of 100 mm × 30 mm was cut out from a position 1/2 in the width direction of the hot-rolled steel sheet and subjected to the following test.
　Z2248 (Z2248 (C-axis bending) for both bending where the bending ridge is parallel to the rolling direction (L direction) and bending where the bending ridge is parallel to the direction perpendicular to the rolling direction (C direction) (C-axis bending) Bending workability was investigated in accordance with the V block 90 ° bending test), the minimum bending radius without cracks was obtained, and the value obtained by dividing the average value of the minimum bending radii of the L and C axes by the plate thickness was the limit bending. R / t was used as an index value of bendability. Those having a limit bending R / t of 2.0 or less were regarded as acceptable.
　However, the presence or absence of cracks is determined by mirror-polishing the cross section of the test piece after the V block 90 ° bending test cut on a surface parallel to the bending direction and perpendicular to the plate surface, and then observing the cracks with an optical microscope. When the crack length observed inside the bend exceeds 30 μm, it is judged that there is a crack.
[0064]
[table 1]

[0065]
[Table 2]

[0066]
[Table 3]

[0067]
[Table 4]

[0068]
[Table 5]

[0069]
　As shown in Tables 1-5, all the properties were suitable in the examples satisfying the requirements of the present invention. On the other hand, in the comparative example in which at least one of the requirements of the present invention is not satisfied, at least one characteristic is not suitable.
Code description
[0070]
　10 Scale scratches
　100 Hot-rolled steel sheet
　X Rolling direction (L direction)
　Y Direction perpendicular to rolling direction (C direction)
　T Plate thickness direction
　I Average height position

WE CLAIMS

[Claim 1]As chemical components, in mass%,
　C: 0.030 to 0.250%,
　Si: 0.05 to 2.50%,
　Mn: 1.00 to 4.00%,
　sol. Al: 0.001 to 2.000%,
　P: 0.100% or less,
　S: 0.0200% or less,
　N: 0.01000% or less,
　Ti: 0 to 0.20%,
　Nb: 0 to 0. 20%,
　B: 0 to 0.010%,
　V: 0 to 1.0%,
　Cr: 0 to 1.0%,
　Mo: 0 to 1.0%,
　Cu: 0 to 1.0%,
　Co: 0 to 1.0%,
　W: 0 to 1.0%,
　Ni: 0 to 1.0%,
　Ca: 0 to 0.01%,
　Mg: 0 to 0.01%,
　REM: 0 to 0.01 %,
　Zr: 0 to 0.01%, and the
　balance: Fe and impurities, and
　the area ratio of the scale scratched portion, which is a portion lower than the average height of the steel plate surface by 10 μm or more, is 20% or less.
　A hot-rolled steel sheet having a tensile strength of 780 MPa or more.
[Claim 2]
　The hot-rolled steel sheet according to claim 1, wherein the average aspect ratio of the scale scratched portion is 5 or less.
[Claim 3]
　As the chemical components, in
　terms of mass%, Ti: 0.001 to 0.20%,
　Nb: 0.001 to 0.20%,
　B: 0.001 to 0.010%,
　V: 0.005 to 1. 0%,
　Cr: 0.005 to 1.0%,
　Mo: 0.005 to 1.0%,
　Cu: 0.005 to 1.0%,
　Co: 0.005 to 1.0%,
　W: 0 .005 to 1.0%,
　Ni: 0.005 to 1.0%,
　Ca: 0.0003 to 0.01%,
　Mg: 0.0003 to 0.01%,
　REM: 0.0003 to 0.01 The hot-rolled steel plate according to claim 1 or 2, wherein the hot-rolled steel plate contains at least one type composed of a group consisting of %,
　Zr: 0.0003 to 0.01%