Hot-rolled steel sheet

Abstract

Provided is a hot-rolled steel sheet which has a prescribed chemical composition, and which has a metallographic structure that contains, in area%, 80-98% in total of at least one of ferrite and bainite and 2-10% of martensite, and which has a boundary having a difference in orientation of 15 degrees or more as a grain boundary. When a region surrounded by the grain boundary and having an equivalent circle diameter of 0.3 [mu] m or more is defined as a crystal grain, the proportion of the crystal grain having a difference in orientation within the crystal grain of 5-14 DEG is 10-60% in area%; the surface roughness Ra is less than 1.50 [mu] m, and the difference between the maximum value and the minimum value of the surface roughness Ra is 0.50 [mu] m or less.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet. Background Art

[0002] In recent years, in the automotive industry, from the viewpoint of improving fuel efficiency, weight reduction of vehicle bodies has been required. In order to balance weight reduction of vehicle bodies and collision safety, one effective method is to increase the strength of the steel sheets used. Against this background, development of high-strength steel sheets has been underway. On the other hand, while increasing the strength, the workability of the steel sheets generally decreases. Therefore, in the development of high-strength steel sheets, it is important to ensure workability at a certain level or higher and to pursue high strength.

[0003] In connection with this, in Patent Document 1, there is described a hot-rolled steel sheet characterized in that it has a prescribed chemical composition, the structure contains ferrite and bainite in a total area ratio of 80 to 98% and martensite in an area ratio of 2 to 10%, and in the structure, when boundaries with an orientation difference of 15° or more are defined as grain boundaries and regions surrounded by the grain boundaries and having an equivalent circle diameter of 0.3 μm or more are defined as grains, the ratio of the grains having an orientation difference of 5 to 14° in the grains is 10 to 60% in terms of area ratio. Further, in Patent Document 1, it is taught that by setting the ratio of the above grains having an orientation difference of 5 to 14° in the grains to 10 to 60% in terms of area ratio, tensile flange formability and ductility can be improved while maintaining high strength, and further, by controlling the total area ratio of ferrite and bainite and the area ratio of martensite in the structure within a prescribed range, notch fatigue characteristics can be improved.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: International Publication No. 2016 / 133222 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] For example, in order to increase the strength of the steel sheet, Si may be contained in the steel sheet in a relatively large amount. However, if Si is contained in the steel sheet in a relatively large amount, tiger-striped scale patterns called Si scale may sometimes be formed on the surface of the steel sheet. If such scale patterns are formed, chemical conversion treatability may sometimes deteriorate, resulting in a decrease in corrosion resistance after painting. Therefore, in Patent Document 1, it is taught that by limiting the Si content in the hot-rolled steel sheet to 0.100% or less, a decrease in corrosion resistance after painting caused by the formation of such scale patterns is suppressed.

[0009] On the other hand, in the automotive industry and the like, further weight reduction of steel materials is required. In order to achieve such weight reduction, it is necessary to make the steel materials have higher strength than before. Therefore, even when Si is contained in a relatively large amount for further high strength and the like, there is also a high demand for high-strength steel sheets in which the stretch flangeability, ductility, notch fatigue characteristics, and corrosion resistance after coating are improved as described in Patent Document 1.

[0010] Accordingly, an object of the present invention is to provide a hot-rolled steel sheet which, despite its high strength, has improved stretch flangeability, ductility, and notch fatigue characteristics and excellent corrosion resistance after coating.

[0011] Means for Solving the Problem

[0012] In order to achieve the above object, the present inventors have conducted research focusing particularly on the metallographic structure and surface properties of hot-rolled steel sheets. As a result, the present inventors have found that by configuring the metallographic structure of a hot-rolled steel sheet having a specified chemical composition to contain at least one of ferrite and bainite and martensite in a specific ratio and further controlling the ratio of crystal grains within a specified range, stretch flangeability, ductility, and notch fatigue characteristics can be improved, and on this basis, by controlling the surface roughness Ra and its deviation of the steel sheet within a specified range, the corrosion resistance after coating can be enhanced, thereby completing the present invention.

[0013] The present invention capable of achieving the above object is as follows.

[0014] (1) A hot-rolled steel sheet, characterized in that the chemical composition thereof contains, by mass%, C: 0.020 to 0.070%,

[0015] Si: more than 0.100% and 2.000% or less,

[0016] Mn: 0.60 to 2.00%,

[0017] Ti: 0.015 to 0.200%,

[0018] sol.Al: 0.010 to 1.000%,

[0019] P: 0.100% or less,

[0020] S: 0.030% or less,

[0021] N: 0.0060% or less,

[0022] O: 0.0100% or less,

[0023] Nb: 0 to 0.050%,

[0024] V: 0 to 0.300%,

[0025] Cr: 0 to 2.00%,

[0026] Ni: 0 to 2.00%,

[0027] Cu: 0 to 2.00%,

[0028] Mo: 0 to 1.000%,

[0029] B: 0 to 0.0100%,

[0030] Sb: 0 to 1.00%,

[0031] Ca: 0 to 0.0100%,

[0032] Mg: 0 to 0.0100%,

[0033] Hf: 0 to 0.0100%,

[0034] REM: 0 to 0.1000%,

[0035] Bi: 0 to 0.0100%,

[0036] As: 0 to 0.0100%,

[0037] Zr: 0 to 1.00%,

[0038] Co: 0 to 1.00%,

[0039] Zn: 0 to 1.00%,

[0040] W: 0 to 1.00%,

[0041] Sn: 0 to 1.00%, and

[0042] The balance: composed of Fe and impurities, and

[0043] Satisfies 0.110 < [Si] + [sol.Al] ≤ 2.500, where [Si] and [sol.Al] are the contents (mass %) of the respective elements;

[0044] The metal structure it has, by area %, contains:

[0045] At least one of ferrite and bainite: totaling 80 to 98%, and

[0046] Martensite: 2 to 10%,

[0047] When boundaries with an orientation difference of 15° or more are defined as grain boundaries and regions surrounded by the grain boundaries with an equivalent circle diameter of 0.3 μm or more are defined as grains, the proportion of grains with an orientation difference of 5 to 14° within the grains is 10 to 60% by area;

[0048] The surface roughness Ra is less than 1.50 μm, and the difference between the maximum value and the minimum value of the surface roughness Ra is 0.50 μm or less.

[0049] (2) The hot-rolled steel sheet according to (1) above, characterized in that the chemical composition contains, by mass%,

[0050] Nb: 0.001 to 0.050%,

[0051] V: 0.001 to 0.300%,

[0052] Cr: 0.01 to 2.00%,

[0053] Ni: 0.01 to 2.00%,

[0054] Cu: 0.01 to 2.00%,

[0055] Mo: 0.001 to 1.000%,

[0056] B: 0.0001 to 0.0100%,

[0057] Sb: 0.01 to 1.00%,

[0058] Ca: 0.0001 to 0.0100%,

[0059] Mg: 0.0001 to 0.0100%,

[0060] Hf: 0.0001 to 0.0100%,

[0061] REM: 0.0001 to 0.1000%,

[0062] Bi: 0.0001 to 0.0100%,

[0063] As: 0.0001 to 0.0100%,

[0064] Zr: 0.01 to 1.00%,

[0065] Co: 0.01 to 1.00%,

[0066] Zn: 0.01 to 1.00%,

[0067] W: 0.01 to 1.00%, and

[0068] At least one of Sn: 0.01 to 1.00%.

[0069] (3) The hot-rolled steel sheet according to (1) or (2) above, characterized in that the hot-rolled steel sheet is a coated steel sheet having a coating layer on at least one surface.

[0070] (4) A component, characterized in that the component comprises the hot-rolled steel sheet according to any one of (1) to (3) above.

[0071] Advantages of the Invention

[0072] According to the present invention, it is possible to provide a hot-rolled steel sheet which, despite being of high strength, has improved stretch flangeability, ductility and notch fatigue characteristics and excellent corrosion resistance after coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 It is a view showing the shape of a formed product having a saddle shape used in the saddle-type stretch flange test method.

[0074] Figure 2 It is a view showing the shape of a fatigue test piece for evaluating notch fatigue characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0075] <Hot-rolled Steel Sheet>

[0076] The chemical composition of the hot-rolled steel sheet according to an embodiment of the present invention is by mass%

[0077] C: 0.020 to 0.070%,

[0078] Si: More than 0.100% and 2.000% or less,

[0079] Mn: 0.60 to 2.00%,

[0080] Ti: 0.015 to 0.200%,

[0081] sol.Al: 0.010 to 1.000%,

[0082] P: 0.100% or less,

[0083] S: 0.030% or less,

[0084] N: 0.0060% or less,

[0085] O: 0.0100% or less,

[0086] Nb: 0 to 0.050%,

[0087] V: 0 to 0.300%,

[0088] Cr: 0 to 2.00%,

[0089] Ni: 0 to 2.00%,

[0090] Cu: 0 to 2.00%,

[0091] Mo: 0 to 1.000%,

[0092] B: 0 to 0.0100%,

[0093] Sb: 0 to 1.00%,

[0094] Ca: 0 to 0.0100%,

[0095] Mg: 0 to 0.0100%,

[0096] Hf: 0 to 0.0100%,

[0097] REM: 0 to 0.1000%,

[0098] Bi: 0 to 0.0100%,

[0099] As: 0 to 0.0100%,

[0100] Zr: 0 to 1.00%,

[0101] Co: 0 to 1.00%,

[0102] Zn: 0 to 1.00%,

[0103] W: 0 to 1.00%,

[0104] Sn: 0 to 1.00%, and

[0105] The balance: composed of Fe and impurities, and

[0106] Satisfies 0.110 < [Si] + [sol.Al] ≤ 2.500, where [Si] and [sol.Al] are the contents (mass %) of the respective elements;

[0107] The metallographic structure it has, in area %, contains:

[0108] At least one of ferrite and bainite: totaling 80 to 98%, and

[0109] Martensite: 2 to 10%,

[0110] When the boundary with an orientation difference of 15° or more is defined as the grain boundary, and the region surrounded by the grain boundary and having an equivalent circle diameter of 0.3 μm or more is defined as the grain, the proportion of the grains with an orientation difference of 5 to 14° within the grains is 10 to 60% in area %;

[0111] The surface roughness Ra is less than 1.50 μm, and the difference between the maximum value and the minimum value of the surface roughness Ra is 0.50 μm or less.

[0112] As described above, it is known that while the strength of a steel sheet is increased, properties such as stretch flange formability are reduced, and in association with the formation of scale patterns (hereinafter also referred to as Si scale patterns) due to Si scale, the corrosion resistance after painting sometimes decreases. First, in an embodiment of the present invention, by configuring the metal structure of a hot-rolled steel sheet having a prescribed chemical composition to contain at least one of ferrite and bainite and martensite in a specific ratio, more specifically, to contain at least one of ferrite and bainite: 80 to 98% in total by area% and martensite: 2 to 10%, it is possible to improve strength, stretch flange formability, ductility, and notch fatigue characteristics in good balance. In addition, when a region surrounded by a boundary with an orientation difference of 15° or more and having an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain, crystal grains having an orientation difference of 5 to 14° within the crystal grains are effective for improving strength, stretch flange formability, and ductility. Therefore, by appropriately controlling the ratio of such crystal grains, more specifically, controlling it within a range of 10 to 60% by area%, it is possible to further improve the balance between strength, stretch flange formability, and ductility.

[0113] On the other hand, if the corrosion resistance after painting is described in more detail, the steel sheet is generally painted after performing a chemical conversion treatment such as a zinc phosphate treatment. In the chemical conversion treatment, by causing a chemical reaction between the treatment liquid and Fe eluted from the steel sheet, a chemical conversion treatment film based on chemical conversion treatment crystals is densely formed on the surface of the steel sheet, whereby the corrosion resistance after painting can be improved. However, if oxides generated due to the formation of Si scale patterns remain on the surface of the steel sheet, the elution of Fe is hindered, and a portion where the chemical conversion treatment film is not formed, called "lack of hiding", appears, or a chemical conversion treatment film that does not contain Fe and that would not normally be formed is generated because Fe does not elute. As a result, the corrosion resistance after painting sometimes decreases. Thus, the present inventors conducted research focusing particularly on the surface state of the hot-rolled steel sheet in order to improve the corrosion resistance after painting by suppressing the formation of Si scale patterns while improving stretch flange formability, ductility, and notch fatigue characteristics. As a result, the present inventors found that by controlling the surface roughness Ra of the hot-rolled steel sheet to be less than 1.50 μm and at the same time controlling the difference between the maximum value and the minimum value of the surface roughness Ra to be 0.50 μm or less, the formation of Si scale patterns can be suppressed, and thereby the corrosion resistance after painting of the hot-rolled steel sheet can be significantly improved.

[0114] More specifically, in a steel sheet containing relatively more Si, Si oxide is formed at the interface between Fe oxide (also called scale) generated on the surface of the steel sheet and the steel during the hot rolling process. Since this Si oxide firmly adheres the Fe oxide to the steel, sometimes the scale cannot be sufficiently removed even if scale removal using high-pressure water or the like is employed thereafter. In such a case, the insufficiently removed scale is pressed into the surface of the steel sheet due to subsequent rolling such as finish rolling, and as a result, irregularities are generated on the surface of the steel sheet, deteriorating the surface properties. Due to such poor scale removal and the subsequent deterioration of the surface properties caused by rolling, Si scale patterns are generated in the hot-rolled steel sheet after pickling.

[0115] Then, the present inventors found that, as will be described in detail later in relation to the manufacturing method of the hot-rolled steel sheet, by appropriately controlling the temperature conditions between rough rolling and finish rolling, scale can be sufficiently or completely removed by scale removal before finish rolling, and in association therewith, scale pressing into the surface of the steel sheet and generating irregularities on the surface of the steel sheet can be significantly suppressed during subsequent finish rolling. The present inventors further studied the relationship between the surface properties of the steel sheet and the generation of Si scale patterns. As a result, the present inventors found that by sufficiently or completely removing the scale by scale removal before finish rolling, the surface roughness Ra of the finally obtained hot-rolled steel sheet is less than 1.50 μm and the difference between the maximum value and the minimum value of the surface roughness Ra is 0.50 μm or less, the generation of Si scale patterns can be significantly suppressed, and in association therewith, a hot-rolled steel sheet having excellent corrosion resistance after coating can be obtained. That is to say, according to the embodiment of the present invention, even though it is a high strength, for example, a high strength of 540 MPa or more in tensile strength, the stretch flange property, ductility, and notch fatigue property can be improved, and excellent corrosion resistance after coating can be achieved. Therefore, the hot-rolled steel sheet of the embodiment of the present invention is particularly useful in the automotive field where these properties are required because it can surely balance the opposite properties of high strength and excellent workability, and further has excellent corrosion resistance after coating.

[0116] Hereinafter, the hot-rolled steel sheet of the embodiment of the present invention will be described in more detail. In the following description, unless otherwise specified, "%" as the unit of the content of each element means "mass%". In addition, in this specification, "~" indicating a numerical range is used to mean including the values described before and after it as the lower limit value and the upper limit value without special specification.

[0117] [C: 0.020 to 0.070%]

[0118] C is an element effective in increasing the strength of the steel plate. In addition, C forms carbides and / or carbonitrides with Ti and Nb in the steel, which also contributes to precipitation strengthening based on the formed precipitates and refinement of the microstructure due to the pinning effect of these precipitates. To fully obtain these effects, the C content is set to 0.020% or more. The C content can also be 0.022% or more, 0.025% or more, 0.028% or more, or 0.030% or more. On the other hand, if C is contained excessively, the stretch flange formability and weldability may sometimes decrease. Therefore, the C content is set to 0.070% or less. The C content can also be 0.065% or less, 0.060% or less, 0.055% or less, or 0.050% or less.

[0119] [Si: exceeding 0.100% and 2.000% or less]

[0120] Si is an element effective in increasing the strength as a solid solution strengthening element. To fully obtain such an effect, the Si content is set to exceed 0.100%. The Si content can also be 0.102% or more, 0.105% or more, 0.108% or more, 0.110% or more, 0.120% or more, 0.150% or more, 0.200% or more, 0.300% or more, 0.500% or more, 0.600% or more, 0.700% or more, 0.800% or more, or 1.000% or more. On the other hand, if Si is contained excessively, sometimes the formation of Si oxide scale patterns cannot be fully suppressed even when appropriately controlling the surface roughness Ra of the steel plate and the difference between its maximum and minimum values. Therefore, the Si content is set to 2.000% or less. The Si content can also be 1.800% or less, 1.600% or less, 1.400% or less, or 1.200% or less.

[0121] [Mn: 0.60 - 2.00%]

[0122] Mn is an element effective in increasing the strength as an element for hardenability and solid solution strengthening. To fully obtain these effects, the Mn content is set to 0.60% or more. The Mn content can also be 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if Mn is contained excessively, sometimes the stretch flange formability decreases. Therefore, the Mn content is set to 2.00% or less. The Mn content can also be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.

[0123] [Ti: 0.015 - 0.200%]

[0124] Ti precipitates finely as carbide (TiC) in steel and is an element that increases the strength of steel through precipitation strengthening. In addition, Ti is also an element that fixes C by forming carbide and inhibits the formation of cementite, which is harmful to stretch flangeability. In order to fully obtain these effects, the Ti content is set to 0.015% or more. The Ti content can also be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if Ti is contained excessively, the carbide becomes coarse and sometimes the ductility is reduced. Therefore, the Ti content is set to 0.200% or less. The Ti content can also be 0.180% or less, 0.170% or less, 0.150% or less, or 0.120% or less.

[0125] [sol.Al: 0.010 - 1.000%]

[0126] sol.Al is an element that acts as a deoxidizer for molten steel. In order to fully obtain such an effect, the sol.Al content is set to 0.010% or more. The sol.Al content can also be 0.012% or more, 0.015% or more, or 0.020% or more. On the other hand, if sol.Al is contained excessively, coarse oxides are formed and sometimes the toughness and ductility are reduced, resulting in fracture during rolling. Therefore, the sol.Al content is set to 1.000% or less. The sol.Al content can also be 0.800% or less, 0.600% or less, or 0.400% or less. In addition, sol.Al means acid-soluble Al, indicating the dissolved Al present in steel in a solid solution state.

[0127] [P: 0.100% or less]

[0128] If P is contained excessively, it sometimes has an adverse effect on weldability, etc. Therefore, the P content is set to 0.100% or less. The P content can also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and can also be 0%, but excessive reduction will lead to an increase in cost. Therefore, the P content can also be 0.001% or more, 0.003% or more, or 0.005% or more.

[0129] [S: 0.030% or less]

[0130] If the content is excessive, a large amount of MnS is generated, sometimes reducing the toughness. Therefore, the Si content is set to 0.030% or less. The S content can also be 0.020% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly limited and can also be 0%, but excessive reduction will lead to an increase in cost. Therefore, the S content can also be 0.001% or more, 0.002% or more, or 0.003% or more.

[0131] [N: 0.0060% or less]

[0132] N preferentially forms precipitates with Ti over C, sometimes reducing the Ti that is effective in fixing C. Therefore, the N content is set to 0.0060% or less. The N content can also be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and can also be 0%, but excessive reduction will lead to an increase in cost. Therefore, the N content can also be 0.0001% or more or 0.0005% or more.

[0133] [O: 0.0100% or less]

[0134] O is an element mixed in the manufacturing process. If O is contained excessively, sometimes coarse inclusions are formed, reducing the toughness of the steel sheet. Therefore, the O content is set to 0.0100% or less. The O content can also be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and can also be 0%, but in order to reduce it to less than 0.0001%, refining takes time, resulting in a reduction in productivity. Therefore, the O content can also be 0.0001% or more or 0.0005% or more.

[0135] The basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet can also contain at least one of the following optional elements as needed to replace a part of the remaining Fe.

[0136] [Nb: 0 to 0.050%]

[0137] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel and contributes to the micro-refinement of the structure through the pinning effect, thereby contributing to the high strength of the steel sheet. In addition, Nb is also an element that fixes C by forming carbides and / or carbonitrides and inhibits the formation of cementite, which is harmful to stretch flangeability. The Nb content can also be 0%, but in order to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content can also be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if Nb is contained excessively, sometimes coarse carbides, etc. are generated in the steel, reducing the ductility of the steel sheet. Therefore, the Nb content is set to 0.050% or less. The Nb content can also be 0.040% or less, 0.030% or less, or 0.020% or less.

[0138] [V: 0 to 0.300%]

[0139] V is an element that contributes to strength improvement through precipitation strengthening, etc. The V content can also be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more. The V content can also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if V is contained excessively, the effect reaches saturation, and it may lead to an increase in manufacturing cost. Therefore, the V content is preferably 0.300% or less. The V content can also be 0.200% or less, 0.100% or less, or 0.080% or less.

[0140] [Cr: 0 to 2.00%]

[0141] Cr is an element that improves the hardenability of steel and contributes to strength improvement. The Cr content can also be 0%, but in order to obtain such an effect, the Cr content is preferably 0.01% or more. The Cr content can also be 0.03% or more or 0.05% or more. On the other hand, even if Cr is contained excessively, the effect reaches saturation, and it may lead to an increase in manufacturing cost. Therefore, the Cr content is preferably 2.00% or less. The Cr content can also be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0142] [Ni: 0 to 2.00%]

[0143] [Cu: 0 to 2.00%]

[0144] Ni and Cu are elements that contribute to the increase in strength through precipitation strengthening or solid solution strengthening. The Ni and Cu contents can also be 0%, but in order to achieve such an effect, the contents of these elements are preferably 0.01% or more, and can also be 0.03% or more or 0.05% or more. On the other hand, even if these elements are contained excessively, the effect reaches saturation, and it may lead to an increase in manufacturing cost. Therefore, the Ni and Cu contents are preferably 2.00% or less, and can also be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0145] [Mo: 0 to 1.000%]

[0146] Mo is an element that improves the hardenability of steel and contributes to the increase in strength. The Mo content can also be 0%, but in order to achieve such an effect, the Mo content is preferably 0.001% or more. The Mo content can also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if Mo is contained excessively, the deformation resistance during hot working increases, and sometimes the equipment load increases. Therefore, the Mo content is preferably 1.000% or less. The Mo content can also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0147] [B: 0 to 0.0100%]

[0148] B segregates at grain boundaries to increase the grain boundary strength, thereby improving the low-temperature toughness. The B content can also be 0%, but in order to achieve such an effect, the B content is preferably 0.0001% or more. The B content can also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, even if B is contained excessively, the effect reaches saturation, and it may lead to an increase in manufacturing cost. Therefore, the B content is preferably 0.0100% or less. The B content can also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[0149] [Sb: 0 to 1.00%]

[0150] Sb is an element effective in improving corrosion resistance. The Sb content can also be 0%, but in order to achieve such an effect, the Sb content is preferably 0.01% or more. The Sb content can also be 0.02% or more or 0.05% or more. On the other hand, if Sb is contained excessively, sometimes the toughness decreases. Therefore, the Sb content is preferably 1.00% or less. The Sb content can also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[0151] [Ca: 0 to 0.0100%]

[0152] [Mg: 0 to 0.0100%]

[0153] [Hf: 0 to 0.0100%]

[0154] Ca, Mg, and Hf are elements that can control the morphology of non-metallic inclusions. The contents of Ca, Mg, and Hf can also be 0%, but in order to achieve such an effect, the contents of these elements are preferably above 0.0001% respectively, and can also be above 0.0005%, 0.0010%, or 0.0015%. On the other hand, even if these elements are excessively contained, the effect reaches saturation, and containing more than necessary elements in the steel plate will lead to an increase in manufacturing cost. Therefore, the contents of Ca, Mg, and Hf are preferably below 0.0100% respectively, and can also be below 0.0050%, 0.0030%, or 0.0020%.

[0155] [REM: 0 to 0.1000%]

[0156] REM is an element that can control the morphology of non-metallic inclusions. The content of REM can also be 0%, but in order to achieve such an effect, the content of REM is preferably above 0.0001%. The content of REM can also be above 0.0005%, 0.0010%, or 0.0015%. On the other hand, even if REM is excessively contained, the effect reaches saturation, and containing more than necessary REM in the steel plate will lead to an increase in manufacturing cost. Therefore, the content of REM is preferably below 0.1000%. The content of REM can also be below 0.0500%, 0.0100%, 0.0050%, 0.0030%, or 0.0020%. In this specification, REM refers to the general term of 17 elements including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 as lanthanide elements, and the content of REM is the total content of these elements.

[0157] [Bi: 0 to 0.0100%]

[0158] [As: 0 to 0.0100%]

[0159] Bi and As are elements effective in improving corrosion resistance. The contents of Bi and As may also be 0%, but in order to obtain such an effect, the contents of these elements are preferably 0.0001% or more, and may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if these elements are contained excessively, the effect reaches saturation, and containing these elements more than necessary in the steel sheet will lead to an increase in manufacturing cost. Therefore, the contents of Bi and As are preferably 0.0100% or less, and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0160] [Zr: 0 to 1.00%]

[0161] Zr is an element that can control the morphology of non-metallic inclusions. The content of Zr may also be 0%, but in order to obtain such an effect, the content of Zr is preferably 0.01% or more. The content of Zr may also be 0.05% or more or 0.10% or more. On the other hand, even if Zr is contained excessively, the effect reaches saturation, and containing Zr more than necessary in the steel sheet will lead to an increase in manufacturing cost. Therefore, the content of Zr is preferably 1.00% or less. The content of Zr may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0162] [Co: 0 to 1.00%]

[0163] Co is an element that contributes to the improvement of hardenability and / or heat resistance. The content of Co may also be 0%, but in order to obtain these effects, the content of Co is preferably 0.01% or more. The content of Co may also be 0.05% or more or 0.10% or more. On the other hand, if Co is contained excessively, sometimes the hot workability decreases, and it also leads to an increase in raw material cost. Therefore, the content of Co is preferably 1.00% or less. The content of Co may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0164] [Zn: 0 to 1.00%]

[0165] Zn is an element effective in controlling the shape of inclusions. In order to obtain such an effect, the content of Zn is preferably 0.01% or more. The content of Zn may also be 0.05% or more or 0.10% or more. On the other hand, even if Zn is contained excessively, the effect reaches saturation, resulting in an increase in manufacturing cost. Therefore, the content of Zn is preferably 1.00% or less. The content of Zn may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0166] [W: 0 to 1.00%]

[0167] W is an element that improves the hardenability of steel and contributes to the increase in strength. The content of W may also be 0%, but in order to obtain such an effect, the content of W is preferably 0.01% or more. The content of W may also be 0.05% or more or 0.10% or more. On the other hand, if W is contained excessively, the weldability sometimes decreases. Therefore, the content of W is preferably 1.00% or less. The content of W may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0168] [Sn: 0 to 1.00%]

[0169] Sn is an element effective for improving the corrosion resistance. The content of Sn may also be 0%, but in order to obtain such an effect, the content of Sn is preferably 0.01% or more. The content of Sn may also be 0.02% or more or 0.05% or more. On the other hand, if Sn is contained excessively, the toughness sometimes decreases. Therefore, the content of Sn is preferably 1.00% or less. The content of Sn may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[0170] In the hot-rolled steel sheet according to the embodiment of the present invention, the remaining portion other than the above elements is composed of Fe and impurities. Impurities refer to components such as those mixed in due to various reasons in the manufacturing process, represented by raw materials such as ores and scraps, when manufacturing the hot-rolled steel sheet industrially.

[0171] [0.110 < [Si] + [sol.Al] ≤ 2.500]

[0172] The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention needs to satisfy the following formula.

[0173] 0.110 < [Si] + [sol.Al] ≤ 2.500

[0174] In the formula, [Si] and [sol.Al] are the contents (mass %) of respective elements. As described above, when a region surrounded by boundaries with an orientation difference of more than 15° and having an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain, crystal grains with an orientation difference of 5 to 14° within the crystal grains are effective for improving strength and stretch flange formability. Therefore, in the hot-rolled steel sheet according to an embodiment of the present invention, as will be described in detail later, by controlling the proportion of such crystal grains within a range of 10 to 60% by area%, the balance between strength and stretch flange formability is improved. In addition to the effects described for each element, Si and sol.Al are also elements effective for controlling the proportion of crystal grains with an orientation difference of 5 to 14° within the crystal grains within a range of 10 to 60%. It is considered that this is due to the increase in the temperature of the Ar3 point and the reduction in the transformation strain introduced into the crystal grains by containing Si and sol.Al. In order to sufficiently obtain these effects, the chemical composition of the hot-rolled steel sheet according to an embodiment of the present invention is controlled such that the total content of Si and sol.Al exceeds 0.110%, that is, [Si] + [sol.Al] > 0.110. From the viewpoint of further improving these effects, the total content of Si and sol.Al is preferably set to 0.120% or more, and may be 0.150% or more, 0.200% or more, or 0.300% or more. On the other hand, if the total content of Si and sol.Al is too high, ferrite formation may be promoted and the strength may be reduced. Therefore, the total content of Si and sol.Al is set to 2.500% or less, that is, [Si] + [sol.Al] ≤ 2.500. The total content of Si and sol.Al may also be 2.000% or less, 1.500% or less, 1.000% or less, or 0.800% or less.

[0175] The chemical composition of the hot-rolled steel sheet according to an embodiment of the present invention can be measured by a general analysis method. For example, the chemical composition of the hot-rolled steel sheet can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S can be measured by combustion-infrared absorption method, N can be measured by inert gas fusion-thermal conductivity method, and O can be measured by inert gas fusion-non-dispersive infrared absorption method.

[0176] [Metallographic structure]

[0177] [At least one of ferrite and bainite: total 80 to 98%, and martensite: 2 to 10%]

[0178] In the metal structure of the hot-rolled steel sheet according to an embodiment of the present invention, in terms of area%, it contains at least one of ferrite and bainite: a total of 80 to 98%, and martensite: 2 to 10%. By forming the metal structure of the hot-rolled steel sheet from these structures, it is possible to improve strength, stretch flange formability, ductility, and notch fatigue characteristics in a good balance. If the total area ratio of at least one of ferrite and bainite is low or the area ratio of martensite is high, the balance between strength and stretch flange formability is particularly reduced, and sometimes the desired characteristics cannot be obtained. Therefore, the total area ratio of at least one of ferrite and bainite is set to 80% or more, for example, it can also be 82% or more, 85% or more, 88% or more, or 90% or more. Similarly, the area ratio of martensite is set to 10% or less, for example, it can also be 9% or less, 8% or less, 7% or less, or 6% or less. On the other hand, if the total area ratio of at least one of ferrite and bainite is high or the area ratio of martensite is low, the balance between strength and notch fatigue characteristics is particularly reduced, and sometimes the desired characteristics cannot be obtained. Therefore, the total area ratio of at least one of ferrite and bainite is set to 98% or less, for example, it can also be 96% or less, 94% or less, or 92% or less. Similarly, the area ratio of martensite is set to 2% or more, for example, it can also be 3% or more, 4% or more, or 5% or more.

[0179] The metal structure of the hot-rolled steel sheet only needs to contain any one of ferrite and bainite, and preferably contains both ferrite and bainite. Therefore, either one of the area ratios of ferrite and bainite can be 0%, for example, they can also be 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more respectively. Similarly, the area ratios of ferrite and bainite can also be 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less respectively. From the viewpoint of improving the ductility of the hot-rolled steel sheet, the area ratio of bainite is preferably 80% or less, and more preferably 70% or less.

[0180] [Remaining part of the structure]

[0181] The remaining part of the structure other than ferrite, bainite, and martensite can also be 0% in terms of area%, but in the case where there is a remaining part of the structure, the remaining part of the structure can be at least one of retained austenite and pearlite. The area ratio of the remaining part of the structure is not particularly limited, for example, it can also be 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flange formability, the area ratio of the remaining part of the structure is preferably 10% or less, and can also be 8% or less, 6% or less, or 5% or less.

[0182] [Identification of the metal structure and calculation of the area ratio]

[0183] The identification of the metallographic structure and the calculation of the area ratio in the hot-rolled steel sheet are carried out by optical microscope observation after etching with nitric acid ethanol reagent or LePera solution and X-ray diffraction method. The microstructural observation using the optical microscope is performed on the plate thickness cross-section in the direction parallel to the rolling direction and perpendicular to the plate surface. Specifically, first, a specimen is collected from the hot-rolled steel sheet, and the observation surface of the specimen is etched with nitric acid ethanol. Then, image analysis is performed on the microstructural photograph obtained at a field of view of 300 μm × 300 μm at a depth of 1 / 4 of the plate thickness using an optical microscope, whereby the area ratios of ferrite and pearlite, and the total area ratio of bainite and martensite are calculated. Grains with equiaxed grains and no lower organizer can be identified as ferrite, and grains containing a lower organizer are identified as bainite and martensite. Next, using the specimen etched with LePera on the observation surface, similarly, image analysis is performed on the microstructural photograph obtained at a field of view of 300 μm × 300 μm at a depth of 1 / 4 of the plate thickness using an optical microscope, whereby the total area ratio of retained austenite and martensite is calculated. The image analysis is performed using the "Analyze" function of the image analysis software "Image J", whereby the above-mentioned area ratios and total area ratios can be calculated. Here, "Image J" is an open-source public domain image processing software that has been widely used among those skilled in the art. Next, using the specimen milled from the normal direction of the rolling surface to a depth of 1 / 4 of the plate thickness, the volume ratio of retained austenite is calculated by X-ray diffraction measurement. The volume ratio of retained austenite is the same as the area ratio, so it is used as the area ratio of retained austenite. The area ratio of martensite is calculated by subtracting the obtained area ratio of retained austenite from the total area ratio of retained austenite and martensite calculated previously. Finally, similarly, the area ratio of bainite is calculated by subtracting the obtained area ratio of martensite from the total area ratio of bainite and martensite calculated previously.

[0184] [Ratio of grains with an orientation difference of 5 to 14° within the grain: 10 to 60% by area%]

[0185] In the metallographic structure of the hot-rolled steel sheet according to the embodiment of the present invention, when boundaries with an orientation difference of 15° or more are defined as grain boundaries and regions surrounded by such grain boundaries and having an equivalent circle diameter of 0.3 μm or more are defined as grains, the proportion of grains with an orientation difference of 5 to 14° within the grains is controlled within a range of 10 to 60% by area. Grains having such an orientation difference within the grains are effective for improving strength and stretch flangeability. Without being bound by any specific theory, it is considered that the crystal orientation difference within the grains is related to the dislocation density contained in the grains. Generally speaking, an increase in the dislocation density within the grains brings about an increase in strength, but on the other hand, it reduces workability. However, in grains with an orientation difference within the grains controlled to 5 to 14°, it is considered that strength can be increased without reducing workability. In contrast, grains with an orientation difference within the grains of less than 5° have excellent workability but are difficult to be strengthened to a high strength. On the other hand, grains with an orientation difference within the grains exceeding 14° have different deformation abilities within the grains, and thus do not necessarily contribute to the improvement of stretch flangeability. Therefore, in the hot-rolled steel sheet according to the embodiment of the present invention, by appropriately controlling the proportion of grains with an orientation difference within the grains of 5 to 14°, more specifically, controlling it within a range of 10 to 60% by area, it is possible to improve stretch flangeability while achieving the desired steel sheet strength, and further improve the balance between strength and stretch flangeability. If the proportion of grains with an orientation difference within the grains of 5 to 14° is small, stretch flangeability may sometimes decrease. Therefore, from the viewpoint of improving stretch flangeability, the proportion of grains with an orientation difference within the grains of 5 to 14° may also be 15% or more, 18% or more, or 20% or more. On the other hand, if the proportion of grains with an orientation difference within the grains of 5 to 14° is large, ductility may sometimes decrease. Therefore, from the viewpoint of improving ductility, the proportion of grains with an orientation difference within the grains of 5 to 14° may also be 55% or less, 50% or less, 45% or less, or 40% or less.

[0186] [Measurement of the proportion of grains with an orientation difference within the grains of 5 to 14°]

[0187] The proportion of grains with an orientation difference within the grain of 5 to 14° is measured by the Electron BackScattered Diffraction (EBSD) method. More specifically, first, a specimen is collected from the steel plate in such a way that the cross-section of the plate thickness in the direction parallel to the rolling direction and perpendicular to the plate surface becomes the observation surface. Next, at a depth position of 1 / 4 of the plate thickness from the surface of the steel plate, EBSD analysis is performed on a region of 200 μm in the rolling direction of the steel plate and 100 μm in the direction normal to the rolling surface at a measurement interval of 0.2 μm, thereby obtaining crystal orientation information. Here, the EBSD analysis is carried out using a device composed of a thermal field emission type scanning electron microscope (e.g., JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) at an analysis speed of 50 to 300 points / second. Then, for the obtained crystal orientation information, a region with an orientation difference of 15° or more and an equivalent circle diameter of 0.3 μm or more is defined as a grain, the average orientation difference within the grain of the grain is calculated, and the proportion of grains with an orientation difference within the grain of 5 to 14° is obtained. The grains and the average orientation difference within the grain defined as above can be calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. In the present invention, the so-called "orientation difference within the grain" represents the orientation dispersion within the grain, that is, "Grain Orientation Spread (GOS)". The value of the orientation difference within the grain is obtained as the average value of the orientation differences between the crystal orientation serving as a reference and all measurement points within the same grain, as described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel Based on EBSD Method and X-ray Diffraction Method", Hidehiko Kimura et al., Transactions of the Japan Society of Mechanical Engineers, Series A, Vol. 71, No. 712, 2005, p. 1722-1728. In the embodiment of the present invention, the crystal orientation serving as a reference is the orientation obtained by averaging all measurement points within the same grain. The value of GOS can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" attached to the EBSD analysis device.

[0188] [Surface roughness Ra: less than 1.50 μm, and the difference between the maximum value and the minimum value of surface roughness Ra: 0.50 μm or less]

[0189] In the hot-rolled steel sheet according to an embodiment of the present invention, the surface roughness Ra is controlled to be less than 1.50 μm, and the difference between the maximum value and the minimum value of the surface roughness Ra is controlled to be 0.50 μm or less. As will be described in detail later in connection with the manufacturing method of the hot-rolled steel sheet, by removing the scale sufficiently or completely by descaling before finish rolling, it is possible to suppress the scale from being pressed into the surface of the steel sheet during subsequent finish rolling and to generate unevenness on the surface of the steel sheet. In the embodiment of the present invention, by achieving a surface property such that the surface roughness Ra of the finally obtained hot-rolled steel sheet is less than 1.50 μm and the difference between the maximum value and the minimum value of the surface roughness Ra is 0.50 μm or less, it is possible to significantly suppress the generation of Si scale patterns caused by the pressing of the scale into the surface of the steel sheet. In association therewith, it is possible to achieve excellent corrosion resistance after coating. If the generation of Si scale patterns cannot be sufficiently suppressed and the surface roughness Ra of the hot-rolled steel sheet is 1.50 μm or more and / or the difference between the maximum value and the minimum value of the surface roughness Ra exceeds 0.50 μm, as a result, the corrosion resistance after coating of the hot-rolled steel sheet is reduced. For example, even if the surface roughness Ra is less than 1.50 μm, but if the difference between the maximum value and the minimum value of the surface roughness Ra exceeds 0.50 μm, since the deviation of the surface roughness is large, it cannot be said that the generation of Si scale patterns is sufficiently suppressed. In this case, for example, uniform chemical conversion treatment thereafter cannot be achieved, and as a result, the corrosion resistance after coating is reduced. Therefore, in the embodiment of the present invention, it is not sufficient to merely control the surface roughness Ra to be less than 1.50 μm. In addition, it is extremely important to control the difference between the maximum value and the minimum value of the surface roughness Ra to be 0.50 μm or less.

[0190] From the viewpoint of suppressing the generation of Si scale patterns and improving the corrosion resistance after coating, the lower the surface roughness Ra and the difference between its maximum value and minimum value, the more preferable. For example, the surface roughness Ra is preferably 1.40 μm or less, more preferably 1.20 μm or less or 1.00 μm or less, and most preferably 0.80 μm or less. Similarly, the difference between the maximum value and the minimum value of the surface roughness Ra is preferably 0.40 μm or less, more preferably 0.30 μm or less. The lower limit of the surface roughness Ra and the difference between its maximum value and minimum value is not particularly limited. For example, the surface roughness Ra may also be 0.20 μm or more, 0.30 μm or more, 0.40 μm or more, or 0.50 μm or more. Similarly, the difference between the maximum value and the minimum value of the surface roughness Ra may also be 0 μm or more, 0.02 μm or more, 0.03 μm or more, 0.05 μm or more, or 0.07 μm or more.

[0191] [Measurement of surface roughness Ra and the difference between its maximum value and minimum value]

[0192] The surface roughness Ra of the hot-rolled steel sheet and the difference between its maximum value and minimum value are measured as follows. More specifically, first, when the hot-rolled steel sheet has a scale on its surface, after the pickling treatment of the specimen, it is subjected to roughness measurement. The pickling treatment is carried out under the conditions of immersing in hydrochloric acid with a hydrochloric acid concentration of 3 to 10% by volume at a temperature of 85 to 98 °C for 20 to 300 seconds. In addition, the pickling can be carried out once or, if necessary, in multiple times. The above pickling time (20 seconds to 300 seconds) refers to the time of this pickling when pickling is carried out only once, and refers to the total time of these picklings when pickling is carried out multiple times. By setting the pickling temperature to 85 °C or higher, the oxides on the surface layer can be sufficiently removed. Next, for the surface of the specimen of the pickled hot-rolled steel sheet, the positions spaced 50 mm apart in the width direction are taken as the measurement positions, and the surface roughness is measured along the rolling direction at each measurement position. It is preferably measured at 10 or more points, but in the case of insufficient plate width, the same measurement is carried out at positions 50 mm apart in the rolling direction, and it is specified to measure at 10 or more points. The measurement length of each measurement position is set to 5 mm. By successively applying the profile curve filters of the cut-off values λc and λs to the measured cross-sectional curve obtained by the measurement, a roughness curve is obtained. Specifically, the components with a wavelength λc of 0.8 mm or less and the components with a wavelength λs of 2.5 μm or more are removed from the obtained measurement results, and a roughness curve is obtained. Based on the obtained roughness curve, in accordance with JIS B 0601:2013, the arithmetic mean roughness of each measurement position is calculated. The average value of all the obtained arithmetic mean roughnesses is determined as the surface roughness Ra. Similarly, the difference between the maximum value and the minimum value of the surface roughness Ra is determined from the maximum value and the minimum value among all the obtained arithmetic mean roughnesses.

[0193] When the hot-rolled steel sheet has a surface treatment film such as a coating and painting on its surface, the base surface obtained after removing the surface treatment film is measured. The method for removing the surface treatment film can be appropriately selected according to the type of the surface treatment film within the range that does not affect the surface roughness of the base. For example, when the surface treatment film is a zinc coating such as electro-galvanized, electroplated Zn-Ni alloy, hot-dip galvanized, alloyed hot-dip galvanized, hot-dip Zn-Al alloy, hot-dip Zn-Al-Mg alloy, hot-dip Zn-Al-Mg-Si alloy, etc., it is possible to dissolve the galvanized layer by using dilute hydrochloric acid added with a corrosion inhibitor. Thus, it is possible to peel only the galvanized layer from the steel sheet. The so-called corrosion inhibitor is an additive used to suppress the change in roughness caused by preventing the excessive dissolution of the base. For example, it is possible to use the corrosion inhibitor for hydrochloric acid pickling produced by Asahi Chemical Industry Co., Ltd. added in dilute hydrochloric acid at a concentration of 0.6 g / L when diluted to 5% by volume. A substance formed by "No.700BK". Additionally, in the case where the surface treatment film is an aluminum coating such as hot-dip aluminizing, according to the description in JIS G3314:2019, it is successively immersed in a dilute hydrochloric acid aqueous solution added with sodium hydroxide aqueous solution and hexamethylenetetramine until the foaming caused by the dissolution of the coating layer subsides, thereby dissolving the Al coating layer. Additionally, in the case where the surface treatment film is electrophoretic coating, the electrophoretic coating film is peeled off using a peeling agent (NEOREVER SP-751: manufactured by Sankai Chemical Co., Ltd.).

[0194] [Plate thickness]

[0195] The hot-rolled steel sheet of the embodiment of the present invention is not particularly limited. Generally speaking, it has a plate thickness of 1.0 to 6.0 mm. For example, the plate thickness can also be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or can also be 5.0 mm or less or 4.0 mm or less.

[0196] The hot-rolled steel sheet of the embodiment of the present invention can also be a coated steel sheet having a coating layer on at least one surface. In the case where the hot-rolled steel sheet has a coating layer and / or a chemical conversion treatment film, the coating layer can be formed on the coating layer and / or the chemical conversion treatment film. As the coating layer, there is no particular limitation, and it can be any suitable coating layer known to those skilled in the art. The film thickness of the coating layer is also not particularly limited and can have any suitable film thickness. The coating layer usually includes an electrophoretic coating layer, and further can also include an intermediate coating layer, a primer layer, and a clear coat layer thereon.

[0197] As described above, the hot-rolled steel sheet of the embodiment of the present invention, although having high strength, can also improve the stretch flangeability, ductility, and notch fatigue characteristics, and can achieve excellent corrosion resistance after coating. Therefore, the hot-rolled steel sheet of the embodiment of the present invention is particularly useful in the use of components and the like in technical fields that require these characteristics because it can surely balance the opposite characteristics of high strength and excellent workability, and furthermore, the corrosion resistance after coating is also excellent. In a preferred embodiment, automotive components, particularly components selected from the running parts, chassis, and bumpers of automobiles, etc., containing the hot-rolled steel sheet of the embodiment of the present invention can be provided. These automotive components can include the hot-rolled steel sheet of the embodiment of the present invention in at least a part, and thus, in at least a part of these components, the characteristics of the chemical composition, metal structure, and surface properties described above are satisfied. For the parts of the hot-rolled steel sheet that do not come into direct contact with the mold during forming such as press forming and have a relatively low degree of processing, the characteristics of the metal structure and surface properties will not change particularly before and after forming.

[0198] [Mechanical properties]

[0199] [Tensile strength: TS]

[0200] Based on the hot-rolled steel sheet having the above chemical composition and metallographic structure, a high tensile strength can be achieved, specifically, a tensile strength of 540 MPa or more. The tensile strength is preferably 600 MPa or more, 700 MPa or more, 780 MPa or more, or 850 MPa or more. For the hot-rolled steel sheet according to the embodiment of the present invention, although having such a very high tensile strength, through the specific combination of the chemical composition, metallographic structure, and surface roughness Ra described above, the stretch flangeability, ductility, notch fatigue characteristics, and corrosion resistance after painting can be significantly improved. The upper limit of the tensile strength is not particularly limited. For example, the tensile strength of the hot-rolled steel sheet can also be 1470 MPa or less, 1250 MPa or less, or 1180 MPa or less. The tensile strength is measured by collecting a JIS No. 5 test piece from the direction (C direction) parallel to the direction perpendicular to the rolling direction of the hot-rolled steel sheet in the length direction of the test piece and performing a tensile test according to JIS Z 2241:2011.

[0201] [Total elongation: El]

[0202] Based on the hot-rolled steel sheet having the above chemical composition and metallographic structure, in addition to the high tensile strength, the total elongation can also be improved. More specifically, a total elongation of 15.0% or more can be achieved. The total elongation is preferably 18.0% or more, more preferably 20.0% or more, and most preferably 22.0% or more. The upper limit of the total elongation is not particularly limited. For example, it can also be 40.0% or less or 35.0% or less. The total elongation is measured by collecting a JIS No. 5 test piece from the direction (C direction) parallel to the direction perpendicular to the rolling direction of the hot-rolled steel sheet in the length direction of the test piece and performing a tensile test according to JIS Z 2241:2011.

[0203] <Manufacturing method of hot-rolled steel sheet>

[0204] Next, a preferred manufacturing method of the hot-rolled steel sheet according to the embodiment of the present invention will be described. The following description aims to illustrate the characteristic method for manufacturing the hot-rolled steel sheet according to the embodiment of the present invention and does not aim to limit the hot-rolled steel sheet to the hot-rolled steel sheet manufactured by the manufacturing method described below.

[0205] The manufacturing method of the hot-rolled steel sheet according to the embodiment of the present invention is characterized in that it includes:

[0206] (A) A hot-rolling process, which includes heating a slab having the above-described chemical composition associated with the hot-rolled steel sheet, then performing rough rolling, descaling with high-pressure water, and finish rolling, and satisfying the following conditions (A1) to (A4),

[0207] (A1) The heating temperature of the slab is at least the solution temperature (SRTmin) °C represented by the following formula 1 and at least 1170 °C,

[0208] (A2) The maximum heating temperature from the end of rough rolling to the start of descaling with high-pressure water before finish rolling is at least T °C represented by the following formula 2,

[0209] (A3) The cumulative strain (εeff.) of the last three passes of finish rolling represented by the following formula 3 is 0.50 to 0.60,

[0210] SRTmin = 7000 / {2.75 - log([Ti]×[C])} - 273 Formula 1

[0211] Here, [Ti] and [C] are the contents (mass %) of the respective elements in the steel,

[0212] (A4) The finish rolling temperature is at least Ar3 + 30 °C,

[0213] T = 1081 + 63×[Si] + 27×[sol.Al] + 10×[sol.Al] / [Si] - 8 / [Si] - 1 / [sol.Al] Formula 2

[0214] Here, [Si] and [sol.Al] are the contents (mass %) of the respective elements in the steel,

[0215] εeff. = Σεi(t, T) Formula 3

[0216] Here,

[0217] εi(t, T) = εi0 / exp{(t / τR) 2 / 3}

[0218] τR = τ0·exp(Q / RT)

[0219] τ0 = 8.46×10 -6

[0220] Q = 183200J

[0221] R = 8.314J / K·mol

[0222] εi0 represents the logarithmic strain during rolling, t represents the cumulative time (seconds) until just before cooling in that pass, T represents the rolling temperature (°C) in that pass; and

[0223] (B) Cooling process, which includes: cooling the hot-rolled steel plate at an average cooling rate of 10 °C / s or more to a temperature range of 650 - 750 °C once, holding for 3.0 - 10.0 seconds in the said temperature range, and then cooling to below 100 °C at an average cooling rate of 30 °C / s or more.

[0224] Hereinafter, each process will be described in detail.

[0225] [(A) Hot rolling process]

[0226] [(A1) Heating temperature of slab]

[0227] First, heat the slab having the above-described chemical composition associated with the hot-rolled steel plate. From the viewpoint of productivity, the slab used is preferably cast by the continuous casting method, but can also be manufactured by the ingot casting method or the thin slab casting method. The heating temperature of the slab needs to be set at or above the solution temperature (SRTmin) °C represented by the following formula 1.

[0228] SRTmin = 7000 / {2.75 - log([Ti]×[C])} - 273 Formula 1

[0229] Here, [Ti] and [C] are the contents (mass %) of the respective elements in the steel.

[0230] The slab used contains relatively many alloy elements, especially Ti. Therefore, it is necessary to dissolve the alloy elements in the slab, especially to dissolve Ti sufficiently. If the heating temperature of the slab is lower than the solution temperature (SRTmin) °C, Ti will not be dissolved sufficiently. If Ti is not dissolved sufficiently during slab heating, in the cooling process after the hot rolling process, etc., Ti will precipitate finely in the steel in the form of carbide (TiC), making it difficult to improve the strength of the steel by precipitation strengthening. In addition to this, by forming carbide (TiC) to fix C, it is also difficult to suppress the formation of cementite harmful to the stretch flange property. In addition, the heating temperature of the slab needs to be set at 1170 °C or more. It is necessary to remove the scale generated on the surface of the steel plate during heating in the scale removal after heating, but when it is lower than 1170 °C, the removal of the scale for scale removal becomes uneven, and sometimes the formation of Si scale patterns cannot be sufficiently suppressed.

[0231] [(A2) Maximum heating temperature from the end of rough rolling to the start of high-pressure water scale removal before finish rolling: T °C or more]

[0232] In this manufacturing method, for the heated slab, in order to adjust the plate thickness, etc., rough rolling is carried out before finish rolling, and then the maximum heating temperature from the end of rough rolling to the start of high-pressure water scale removal before finish rolling is controlled to be T °C or more represented by the following formula 2.

[0233] T = 1081 + 63×[Si] + 27×[sol.Al] + 10×[sol.Al] / [Si] - 8 / [Si] - 1 / [sol.Al] Formula 2

[0234] Here, [Si] and [sol.Al] are the contents (mass %) of the respective elements in the steel.

[0235] In a steel sheet containing relatively more Si, more specifically, a steel sheet containing Si in an amount exceeding 0.100%, Si oxide is formed at the interface between the Fe oxide (also called scale) generated on the steel sheet surface during the hot rolling process and the steel. Since this Si oxide firmly adheres the Fe oxide to the steel, sometimes the scale cannot be sufficiently removed even by using high-pressure water or the like for scale removal. In such a case, the insufficiently removed scale is pressed into the steel sheet surface due to subsequent finish rolling, and as a result, unevenness is generated on the steel sheet surface, deteriorating the surface properties. Therefore, it is important to control the maximum heating temperature from the end of rough rolling to the start of high-pressure water scale removal before finish rolling to T °C or higher represented by the above Formula 2. It is considered that by such temperature control, the scale properties that are easily removed during high-pressure water scale removal can be changed, and the scale can be easily removed or completely removed by subsequent high-pressure water scale removal. In this case, the generation of scale removal defects and the Si scale pattern caused by subsequent finish rolling and the pressing of scale into the steel sheet surface can be significantly suppressed. In connection with this, in the finally obtained hot-rolled steel sheet, a surface property with a surface roughness Ra lower than 1.50 μm and a difference between the maximum value and the minimum value of the surface roughness Ra of 0.50 μm or less can be achieved, and excellent corrosion resistance after coating can be realized.

[0236] Rough rolling not only needs to ensure the desired thin slab size, but also sometimes requires appropriate temperature control in relation to high-pressure water scale removal before finish rolling. For example, in the case where reheating using equipment such as a bar heater is not performed between rough rolling and finish rolling, the maximum heating temperature from the end of rough rolling to the start of high-pressure water scale removal before finish rolling should correspond to the outlet side temperature of rough rolling. Therefore, in such a case, it is necessary to control the outlet side temperature of rough rolling to T °C or higher represented by the above Formula 2. On the other hand, in the case where reheating using equipment such as a bar heater is performed between rough rolling and finish rolling, it is only necessary to control the maximum heating temperature at any time point from the end of rough rolling to high-pressure water scale removal before finish rolling to T °C or higher. Therefore, the outlet side temperature of rough rolling is not necessarily limited, and an appropriate temperature can be suitably selected.

[0237] In this manufacturing method, after rough rolling, high-pressure water is used to remove the scale from the rolled material before finish rolling. The scale removal with high-pressure water is carried out using high-pressure water with a sprayed water pressure of 10 to 50 MPa. By using such high-pressure water, the scale can be sufficiently or completely removed from the rolled material after rough rolling. Even in the hot-rolled steel sheet after finish rolling, scale growth occurs. However, by sufficiently or completely removing the scale before finish rolling, as described above, the pressing of the scale into the steel sheet surface during finish rolling can be suppressed. Therefore, even if the scale grows thereafter, it only forms uniformly on the entire steel sheet surface, and the formation of such scale does not have any influence on the generation of Si scale patterns. In addition, such scale can be relatively easily removed by performing an appropriate pickling treatment after hot rolling.

[0238] [(A3) Cumulative strain (εeff.) in the last three passes of finish rolling: 0.50 to 0.60]

[0239] The slab that undergoes scale removal with high-pressure water after rough rolling is then subjected to finish rolling. In this manufacturing method, for example, it is preferably carried out using a tandem mill composed of four or more rolling stands for finish rolling. In this manufacturing method, in order to control the proportion of grains with an intragranular orientation difference of 5 to 14° within the range of 10 to 60% by area, during the finish rolling of the heated slab, it is necessary to carry out the subsequent cooling process on the basis of making the cumulative strain (εeff.) in the last three passes (the final three passes) 0.50 to 0.60. This is for the following reasons. Grains with an intragranular orientation difference of 5 to 14° are generated by undergoing a phase transformation in a quasi-equilibrium state at a relatively low temperature. Therefore, by limiting the dislocation density of austenite before the phase transformation within a certain range during the hot rolling process and limiting the cooling rate within a certain range during the subsequent cooling process, the generation of grains with an intragranular orientation difference of 5 to 14° can be controlled. That is to say, by controlling the cumulative strain in the last three passes of finish rolling and the subsequent cooling, the nucleation frequency of grains with an intragranular orientation difference of 5 to 14° and their subsequent growth rate can be controlled. As a result, the area ratio of grains with an intragranular orientation difference of 5 to 14° in the hot-rolled steel sheet obtained after cooling can be controlled. More specifically, the dislocation density of austenite introduced by finish rolling is mainly related to the nucleation frequency, and the cooling rate after finish rolling is mainly related to the growth rate.

[0240] When the cumulative strain in the last three stands of finish rolling is less than 0.50, the dislocation density of the introduced austenite is not sufficient, and the proportion of grains with an orientation difference of 5 to 14° within the grains is less than 10%. On the other hand, if the cumulative strain in the last three stands of finish rolling exceeds 0.60, recrystallization of austenite occurs during hot rolling, and the accumulated dislocation density during phase transformation decreases. As a result, the proportion of grains with an orientation difference of 5 to 14° within the grains is also less than 10%. In this manufacturing method, the cumulative strain (εeff.) in the last three stands of finish rolling is calculated by the following formula 3.

[0241] εeff. = Σεi(t, T) Formula 3

[0242] Here,

[0243] εi(t, T) = εi0 / exp{(t / τR) 2 / 3}

[0244] τR = τ0·exp(Q / RT)

[0245] τ0 = 8.46×10 -6

[0246] Q = 183200J

[0247] R = 8.314J / K·mol

[0248] εi0 represents the logarithmic strain during rolling reduction, t represents the cumulative time (seconds) until just before cooling in this pass, T represents the rolling temperature (°C) in this pass,

[0249] [(A4) Finish rolling end temperature: Ar3 + 30°C or higher]

[0250] In this manufacturing method, the finish rolling end temperature needs to be set at Ar3 + 30°C or higher. If the finish rolling end temperature is lower than Ar3 + 30°C, when ferrite is generated in a part of the structure due to the deviation of the composition in the steel plate and the rolling temperature, it is possible to process the ferrite. The processed ferrite sometimes causes a reduction in ductility. In addition, if the finish rolling end temperature is lower than Ar3 + 30°C, sometimes the proportion of grains with an orientation difference of 5 to 14° within the grains exceeds 60% and becomes too high. In this manufacturing method, Ar3 (°C) is calculated by the following formula 4 based on the chemical composition of the hot-rolled steel plate.

[0251] Ar3 = 901 - 325×[C] + 33×[Si] + 287×[P] + 40×[sol.Al] - 92×([Mn] + [Mo] + [Cu]) - 46×([Cr] + [Ni]) Formula 4

[0252] Here, [C], [Si], [P], [sol.Al], [Mn], [Mo], [Cu], [Cr], and [Ni] are the contents (mass %) of respective elements in the steel, and are 0 when the element is not contained.

[0253] [(B) Cooling process]

[0254] In this manufacturing method, for the steel sheet after finish rolling, two-stage cooling is performed in the subsequent cooling process. Specifically, first, the steel sheet after finish rolling is cooled once to a temperature range of 650 to 750°C at an average cooling rate of 10°C / s or more, held in this temperature range for 3.0 to 10.0 seconds, and then cooled twice to 100°C or less at an average cooling rate of 30°C / s or more. By performing such two-stage cooling in combination with conditions such as (A3) and (A4) in the hot rolling process, a quasi-equilibrium phase transformation occurs in the desired lower temperature range, whereby the proportion of grains with an orientation difference of 5 to 14° within the grains can be surely controlled within the range of 10 to 60% by area. In contrast, if the average cooling rate of the first cooling is less than 10°C / s or the cooling stop temperature of the first cooling exceeds 750°C, a quasi-equilibrium phase transformation occurs at a relatively high temperature, and the proportion of grains with an orientation difference of 5 to 14° within the grains is less than 10%. In addition, if the cooling stop temperature of the first cooling is less than 650°C, a quasi-equilibrium phase transformation occurs at a temperature lower than the desired temperature range, and similarly, the proportion of grains with an orientation difference of 5 to 14° within the grains is less than 10%. Furthermore, even if the holding time at 650 to 750°C is less than 3.0 seconds, the proportion of grains with an orientation difference of 5 to 14° within the grains is similarly less than 10%. On the other hand, if the holding time at 650 to 750°C exceeds 10.0 seconds or the average cooling rate of the second cooling is less than 30°C / s, cementite harmful to the stretch flange property is likely to be generated. In addition, if the cooling stop temperature of the second cooling exceeds 100°C, the area ratio of martensite is less than 2%. The upper limit of the average cooling rate of the first and second coolings is not particularly limited. For example, considering the equipment capacity of the cooling equipment, the average cooling rate of the first and second coolings can also be set to 200°C / s or less.

[0255] The hot-rolled steel sheet produced by the above manufacturing method contains at least one of ferrite and bainite in an area percentage of 80 to 98% in total, and martensite in an area percentage of 2 to 10%. When the boundaries with an orientation difference of 15° or more are regarded as grain boundaries and the regions surrounded by these grain boundaries and having an equivalent circle diameter of 0.3 μm or more are defined as grains, a metallographic structure can be obtained in which the proportion of grains with an orientation difference of 5 to 14° in the grains is 10 to 60% in area percentage. As a result, although it has high strength, the stretch flange property, ductility, and notch fatigue property can be significantly improved. In addition, in the obtained hot-rolled steel sheet, since the surface roughness Ra is controlled to be less than 1.50 μm and the difference between the maximum value and the minimum value of the surface roughness Ra is controlled to be 0.50 μm or less, the generation of Si scale patterns can be significantly suppressed, and associated therewith, excellent corrosion resistance after painting can be achieved. Therefore, according to the hot-rolled steel sheet produced by the above manufacturing method, the contradictory properties of high strength and excellent workability can be surely balanced, and furthermore, the corrosion resistance after painting is also excellent, so it is particularly useful in the automotive field where these properties are required.

[0256] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited by any of these examples.

[0257] Examples

[0258] In the following examples, hot-rolled steel sheets of the embodiments of the present invention were produced under various conditions, and the tensile strength, stretch flange property, ductility, notch fatigue property, and corrosion resistance after painting of the obtained hot-rolled steel sheets were investigated.

[0259] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1 and Table 2, and these slabs were heated under the conditions shown in Table 3, and then hot-rolled. The hot rolling was carried out by rough rolling, descaling with high-pressure water, and finish rolling. The rough rolling was carried out under the same conditions in all the inventive examples and comparative examples. The descaling with high-pressure water was carried out using high-pressure water with a sprayed water pressure of 15 MPa, and the maximum heating temperature from the end of rough rolling to the start of descaling with high-pressure water before finish rolling is shown in Table 3. In addition, the finish rolling was carried out using a tandem mill composed of 5 rolling stands, and the cumulative strain (εeff.) of the last 3 stages of the finish rolling and the finish rolling end temperature are shown in Table 3. Then, the hot-rolled steel sheet after finish rolling was cooled once and cooled twice under the conditions shown in Table 3 to obtain a hot-rolled steel sheet having a plate thickness of 2.9 mm.

[0260]

[0261]

[0262]

[0263] The properties of the obtained hot-rolled steel sheet are measured and evaluated by the following methods.

[0264] [Tensile strength (TS) and total elongation (El)]

[0265] The tensile strength (TS) and total elongation (El) are measured as follows: A JIS No. 5 test piece is collected from the direction (C direction) parallel to the rolling right angle direction of the hot-rolled steel sheet in the length direction of the test piece, and a tensile test is carried out in accordance with JIS Z 2241:2011.

[0266] [Evaluation of stretch flangeability]

[0267] The stretch flangeability is evaluated by the saddle-type stretch flange test method using a saddle-shaped formed product. Specifically, pressure processing is performed on a formed product having a saddle shape that simulates the stretch flange shape composed of a straight portion and an arc portion shown in Figure 1 The stretch flangeability is evaluated by the limit forming height at this time. In the saddle-type stretch flange test method, a saddle-shaped formed product with the corner radius of curvature R set to 50 - 60 mm and the opening angle θ set to 120° is used, and the limit forming height H (mm) when the clearance at the time of blanking the corner is set to 11% is measured. Here, the clearance represents the ratio of the clearance between the blanking die and the punch to the thickness of the test piece. Since the clearance is actually determined by the combination of the blanking tool and the plate thickness, 11% means the range satisfying 10.5 - 11.5%. Regarding the determination of the limit forming height H, after forming, the presence or absence of cracks with a length of more than 1 / 3 of the plate thickness is visually observed, and the limit forming height where no cracks exist is set. The product of the tensile strength TS (MPa) and the limit forming height H (mm) (TS×H) is used as an index for evaluating the stretch flangeability. When TS×H≥19500 MPa·mm, it is evaluated that the stretch flangeability is improved.

[0268] [Evaluation of ductility]

[0269] When the product of TS (MPa) and El (%) (TS×El) satisfies TS×El≥13500 MPa·%, it is evaluated that the ductility is improved.

[0270] [Evaluation of notch fatigue characteristics]

[0271] The notch fatigue characteristics are evaluated as follows. Specifically, from the same position as the collection position of the tensile test piece, a Figure 2A fatigue test was conducted on a fatigue test piece having the shape shown. The fatigue test piece was ground to a depth of about 0.05 mm from the outermost surface. A stress-controlled axial fatigue test was carried out with a stress ratio R = 0.1 and a frequency of 5 Hz, and the stress at which no fracture occurred after 10 million cycles was defined as the notch fatigue limit (FL), and the notch fatigue characteristics were evaluated based on this. As a result of the test, when FL / TS≥0.25 was satisfied, the notch fatigue characteristics were evaluated as being improved.

[0272] [Evaluation of chemical conversion treatability]

[0273] The chemical conversion treatability was evaluated as follows. Specifically, first, the manufactured hot-rolled steel sheet was pickled, and then phosphating chemical conversion treatment was carried out to make a zinc phosphate film with an adhesion of 2.5 g / m 2 adhere. At this stage, as an evaluation of the chemical conversion treatability, the presence or absence of an uncovered part and the P ratio were measured. The uncovered part refers to the part where the chemical conversion treatment film is not adhered, and the P ratio refers to the value represented by P / (P + H), where P is the X-ray diffraction intensity of the (100) plane of lithiophorite (FeZn2(PO4)2·4H2O) and H is the X-ray diffraction intensity of the (020) plane of hopeite (Zn3(PO4)2·4H2O) measured using an X-ray diffractometer.

[0274] The phosphating chemical conversion treatment is a treatment using a chemical solution mainly composed of phosphoric acid and Zn ions, and is a chemical reaction that forms crystals called lithiophorite between the Fe ions dissolved from the steel sheet. In the phosphating chemical conversion treatment, it is important (1) to dissolve Fe ions to promote the reaction and (2) to form lithiophorite crystals densely on the steel sheet surface. In particular, regarding (1), if an oxide resulting from the formation of Si scale remains on the steel sheet surface, the dissolution of Fe is hindered and an uncovered part appears, or since Fe does not dissolve, an abnormal chemical conversion treatment film that does not contain Fe, such as hopeite, which would not normally be formed, is generated. As a result, the corrosion resistance after painting sometimes decreases. The presence or absence of the uncovered part is judged by observation based on a scanning electron microscope. Specifically, about 20 fields of view are observed at a magnification of 1000 times, and the case where the entire surface is uniformly adhered and no uncovered part can be seen is evaluated as "no" for the uncovered part and rated as "A". In addition, when the number of fields of view where the uncovered part can be seen is 5% or less, it is rated as "B". When it exceeds 5%, it is evaluated as "C" as having an uncovered part.

[0275] On the other hand, the P ratio was measured using an X-ray diffractometer. The P ratio represents the ratio of hopeite to phosphophyllite in the film obtained by phosphoric acid chemical conversion treatment. This means that the higher the P ratio, the more phosphophyllite is contained, and the more densely the phosphophyllite crystals are formed on the surface of the steel sheet. Thus, when the P ratio is 0.80 or more and the evaluation of the uncovered portion is A or B, the chemical conversion processability is evaluated as excellent.

[0276] [Evaluation of corrosion resistance after painting]

[0277] The corrosion resistance after painting was evaluated as follows. Specifically, first, electrophoretic coating with a thickness of 25 μm was performed on the steel sheet after chemical conversion treatment, and then a painting baking treatment at 170 °C for 20 minutes was carried out. A cut with a length of 130 mm was made in the electrophoretic coating film with a pointed knife until reaching the substrate (base material). For this steel sheet, under the salt spray conditions specified in JIS Z 2371:2015, 5% brine spray was continuously carried out at a temperature of 35 °C for 700 hours. After the brine spray, a tape with a width of 24 mm and a length of 130 mm (Nichiban 405A-24 JIS Z 1522:2009) was attached parallel to the cut portion on the cut portion, and the maximum coating film peeling width when peeling it off was measured. When the maximum coating film peeling width is 4.0 mm or less, the corrosion resistance after painting is evaluated as excellent.

[0278] A hot-rolled steel sheet with a tensile strength TS of 540 MPa or more, TS×H≥19500 MPa·mm, TS×El≥13500 MPa·%, FL / TS≥0.25, and a maximum coating film peeling width ≤4.0 mm was evaluated as having improved stretch flangeability, ductility, and notch fatigue characteristics despite being a high-strength steel sheet, and excellent corrosion resistance after painting. The results are shown in Tables 4 and 5.

[0279] Table 4

[0280]

[0281] The underlines indicate outside the scope of the present invention or the characteristics are not preferred.

[0282] Table 5

[0283]

[0284] The underlines indicate outside the scope of the present invention or the characteristics are not preferred.

[0285] Referring to Tables 1 to 5, in Comparative Example 4, since the heating temperature of the slab was low, the removal of scale by high-pressure water became uneven, and thus the generation of Si scale patterns could not be sufficiently suppressed. As a result, the difference between the maximum and minimum values of the surface roughness Ra exceeded 0.50 μm, and the chemical conversion treatability and the corrosion resistance after painting decreased. In Comparative Example 5, since the maximum heating temperature from the end of rough rolling to the start of high-pressure water descaling before finish rolling was low, the scale could not be sufficiently removed even by subsequent high-pressure water descaling. As a result, the surface roughness Ra was 1.50 μm or more, the difference between the maximum and minimum values of the surface roughness Ra exceeded 0.50 μm, and the chemical conversion treatability and the corrosion resistance after painting decreased. It is considered that in Comparative Example 6, since the cumulative strain (εeff.) in the last three stages of finish rolling was high, recrystallization of austenite occurred during hot rolling, and the stored dislocation density at the time of phase transformation decreased. As a result, the proportion of grains with an orientation difference of 5 to 14° within the grains was less than 10%, and the stretch flange formability decreased. It is considered that in Comparative Example 7, since εeff. was low, the dislocation density of the introduced austenite was not sufficient. As a result, similarly, the proportion of grains with an orientation difference of 5 to 14° within the grains was less than 10%, and the stretch flange formability decreased. In Comparative Example 8, since the finish rolling temperature was low, the proportion of grains with an orientation difference of 5 to 14° within the grains exceeded 60%, and the ductility decreased. It is considered that in Comparative Example 9, since the average cooling rate of the first cooling in the cooling process was low, a quasi-equilibrium phase transformation occurred at a relatively high temperature. As a result, the proportion of grains with an orientation difference of 5 to 14° within the grains was less than 10%, and the stretch flange formability decreased. It is considered that in Comparative Example 10, since the cooling stop temperature of the first cooling was high, a quasi-equilibrium phase transformation also occurred at a relatively high temperature. As a result, the proportion of grains with an orientation difference of 5 to 14° within the grains was less than 10%, and the stretch flange formability decreased. It is considered that in Comparative Example 11, since the cooling stop temperature of the first cooling was low, a quasi-equilibrium phase transformation occurred at a temperature lower than the desired temperature range. As a result, similarly, the proportion of grains with an orientation difference of 5 to 14° within the grains was less than 10%, and the stretch flange formability decreased. In Comparative Example 12, since the holding time at 650 to 750°C in the first cooling was short, similarly, the proportion of grains with an orientation difference of 5 to 14° within the grains was less than 10%, and the stretch flange formability decreased. In Comparative Example 13, since the cooling stop temperature of the second cooling in the cooling process was high, the area ratio of martensite was less than 2%. As a result, the TS and the notch fatigue characteristics decreased.

[0286] In Comparative Example 30 and Comparative Example 32, the C and Mn contents are relatively high, resulting in a decrease in stretch flangeability. In Comparative Example 31 and Comparative Example 33, the C and Mn contents are relatively low, and thus sufficient strength cannot be obtained. In Comparative Example 34, due to the relatively high sol.Al content, cracks are generated during rolling, and subsequent tests cannot be carried out. In Comparative Example 35, since the total content of Si and sol.Al is relatively high, ferrite formation is promoted, and the area ratio of martensite is less than 2%. As a result, the TS decreases. In Comparative Example 36, due to the relatively high Ti content, the carbide (TiC) becomes coarse, and the ductility decreases. It is considered that in Comparative Example 37, due to the relatively low Ti content, the formation of cementite cannot be sufficiently suppressed, and as a result, the stretch flangeability decreases.

[0287] In contrast, in the hot-rolled steel sheets of all the inventive examples, by having a specified chemical composition and further appropriately controlling the respective conditions in the manufacturing method, a hot-rolled steel sheet having the following metallographic structure can be obtained: including at least one of ferrite and bainite: 80 to 98% in total by area%, and martensite: 2 to 10%, the proportion of grains having an orientation difference of 5 to 14° within the grains is 10 to 60% by area%, the surface roughness Ra is less than 1.50 μm, and the difference between the maximum value and the minimum value of the surface roughness Ra is 0.50 μm or less. In addition, as a result, although having a high strength with a tensile strength of 540 MPa or more, the stretch flangeability, ductility, notch fatigue characteristics, and corrosion resistance after coating can be significantly improved. In addition, in all the inventive examples, the area ratio of the remaining structure is 10% or less, and when there is a remaining structure, the remaining structure is at least one of retained austenite and pearlite.

Claims

1. A hot-rolled steel plate, characterized in that: The chemical composition thereof, by mass%, consists of C: 0.020 to 0.070%, Si: more than 0.100% and 2.000% or less, Mn: 0.60 to 2.00%, Ti: 0.015 to 0.200%, sol.Al: 0.010 to 1.000%, P: 0.100% or less, S: 0.030% or less, N: 0.0060% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V：0～0.300％、 Cr：0～2.00％、 Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.000%, B：0～0.0100％、 Sb: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, REM: 0 to 0.1000%, Bi: 0 to 0.0100%, As: 0 to 0.0100%, Zr：0～1.00％、 Co: 0 to 1.00%, Zn: 0 to 1.00%, W：0～1.00％、 Sn: 0 to 1.00%, and the balance: Fe and impurities, and satisfies 0.110 < [Si] + [sol.Al] ≤ 2.500, where [Si] and [sol.Al] are the contents of the respective elements by mass%; The metallographic structure thereof, by area%, contains: at least one of ferrite and bainite: a total of 80 to 98%, and martensite: 2 to 10%, When the boundary with an orientation difference of 15° or more is defined as a grain boundary and the region surrounded by the grain boundary and having an equivalent circle diameter of 0.3 μm or more is defined as a grain, the proportion of the grains with an orientation difference of 5 to 14° within the grains is 10 to 60% by area%; The surface roughness Ra is less than 1.50 μm, and the difference between the maximum value and the minimum value of the surface roughness Ra is 0.50 μm or less.

2. The hot-rolled steel sheet according to claim 1, characterized in that: The chemical composition contains, by mass%, Nb: 0.001 to 0.050%, V：0.001～0.300％、 Cr：0.01～2.00％、 Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.001 to 1.000%, B：0.0001～0.0100％、 Sb: 0.01 to 1.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, REM: 0.0001 to 0.1000%, Bi: 0.0001 to 0.0100%, As: 0.0001 to 0.0100%, Zr：0.01～1.00％、 Co: 0.01 to 1.00%, Zn: 0.01 to 1.00%, W: 0.01 to 1.00%, and at least one of Sn: 0.01 to 1.00%.

3. The hot-rolled steel sheet according to claim 1 or 2, characterized in that: The hot-rolled steel sheet is a coated steel sheet having a coating layer on at least one surface.

4. A component, characterized in that: The component includes the hot-rolled steel sheet according to claim 1 or 2.

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