Hot rolled steel sheet

Abstract

Provided is a hot rolled steel sheet having a predetermined chemical composition and a microstructure comprising, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, wherein when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area %, 10 to 60%, and a surface roughness Ra is less than 1.50 μm and a difference of maximum value and minimum value in the surface roughness Ra is 0.50 μm or less.

Description

FIELD

[0001] The present invention relates to a hot rolled steel sheet.BACKGROUND

[0002] In recent years, in the automotive industry, lighter weight of car bodies has been sought from the viewpoint of improvement of fuel efficiency. To achieve both lighter weight of car bodies and collision safety, increasing the strength of the steel sheet used is one effective method. A high strength steel sheet is being developed from this background. On the other hand, along with higher strength, the workability of a steel sheet generally falls. For this reason, in development of a high strength steel sheet, it is important to secure a certain level or more of workability while raising the strength.

[0003] In relation to this, PTL 1 describes a hot rolled steel sheet having a predetermined chemical composition and having a structure containing, by area ratio, a total of 80 to 98% of ferrite, and bainite and 2 to 10% of martensite and, when deeming a boundary with an orientation difference of 15° or more in that structure as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, having a ratio of crystal grains with an orientation difference in the grains of 5 to 14° of an area ratio of 10 to 60%. Further, PTL 1 teaches that by making the ratio of crystal grains with an orientation difference in the grains of 5 to 14° an area ratio of 10 to 60%, it is possible to improve the stretch flangeability and ductility while being high in strength and further that by controlling the total area ratio of ferrite and bainite and the area ratio of martensite in the structure to within predetermined ranges, it is possible to improve the notch fatigue property.CITATIONS LISTPatent Literature

[0004] [PTL 1] WO20161133222SUMMARYTechnical Problem

[0005] For example, to make a steel sheet high in strength, sometimes Si is included in the steel sheet in a relatively large amount. However, if including Si in steel in a relatively large amount, sometimes a tiger stripe shaped scale pattern called “Si scale” is formed at the steel sheet surface. If such a scale pattern is formed, the chemical convertibility deteriorates and sometimes the post-painting corrosion resistance is lowered. For this reason, PTL 1 teaches that by limiting the Si content in the hot rolled steel sheet to 0.100% or less, a drop in post-painting corrosion resistance due to formation of the scale pattern is suppressed.

[0006] On the other hand, in the automobile industry, etc., further lighter weight of steel materials is being sought. To achieve such lighter weight, a need arises for further raising the strength of steel materials even more than up to now. Therefore, there is a high need for a high strength steel sheet which, even if containing a relatively large amount of Si for further raising the strength, is improved in stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance as described in PTL 1.

[0007] Therefore, the present invention has as its object the provision of a hot rolled steel sheet which, despite being high strength, is improved in stretch flangeability, ductility, and notch fatigue property and is excellent in post-painting corrosion resistance.Solution to Problem

[0008] The inventors engaged in studies to achieve the above object focusing in particular on the microstructure and surface properties of a hot rolled steel sheet. As a result, the inventors discovered that by configuring the microstructure of a hot rolled steel sheet having a predetermined chemical composition by at least one of ferrite and bainite and martensite in specific ratios and further controlling the ratio of crystal grains to within a predetermined range, it is possible to improve the stretch flangeability, ductility, and notch fatigue property and that by additionally controlling the surface roughness Ra of the steel sheet and its variation to within predetermined ranges, it is possible to improve the post-painting corrosion resistance and thereby completed the present invention.

[0009] The present invention able to achieve this object is as follows:

[0010] (1) A hot rolled steel sheet having a chemical composition comprising, by mass %,

[0011] C: 0.020 to 0.070%,

[0012] Si: more than 0.100 to 2.000%,

[0013] Mn: 0.60 to 2.00%,

[0014] Ti: 0.015 to 0.200%,

[0015] sol. Al: 0.010 to 1.000%,

[0016] P: 0.100% or less,

[0017] S: 0.030% or less,

[0018] N: 0.0060% or less,

[0019] O: 0.0100% or less,

[0020] Nb: 0 to 0.050%,

[0021] V: 0 to 0.300%,

[0022] Cr: 0 to 2.00%,

[0023] Ni: 0 to 2.00%,

[0024] Cu: 0 to 2.00%,

[0025] Mo: 0 to 1.000%,

[0026] B: 0 to 0.0100%,

[0027] Sb: 0 to 1.00%,

[0028] Ca: 0 to 0.0100%,

[0029] Mg: 0 to 0.0100%,

[0030] Hf: 0 to 0.0100%,

[0031] REM: 0 to 0.1000%,

[0032] Bi: 0 to 0.0100%,

[0033] As: 0 to 0.0100%,

[0034] Zr: 0 to 1.00%,

[0035] Co: 0 to 1.00%,

[0036] Zn: 0 to 1.00%,

[0037] W: 0 to 1.00%,

[0038] Sn: 0 to 1.00%, and

[0039] balance: Fe, and impurities, and

[0040] satisfying 0.110<[Si]+[sol. Al]≤2.500, wherein [Si] and [sol. Al] are the contents (mass %) of the elements, and

[0041] a microstructure comprising, by area %,

[0042] at least one of ferrite and bainite: 80 to 98% in total, and

[0043] martensite: 2 to 10%, wherein

[0044] when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area %, 10 to 60%, and

[0045] a surface roughness Ra is less than 1.50 μm and a difference of a maximum value and minimum value in the surface roughness Ra is 0.50 μm or less.

[0046] (2) The hot rolled steel sheet according to the above (1), wherein the chemical composition contains, by mass %, at least one of

[0047] Nb: 0.001 to 0.050%,

[0048] V: 0.001 to 0.300%,

[0049] Cr: 0.01 to 2.00%,

[0050] Ni: 0.01 to 2.00%,

[0051] Cu: 0.01 to 2.00%,

[0052] Mo: 0.001 to 1.000%,

[0053] B: 0.0001 to 0.0100%,

[0054] Sb: 0.01 to 1.00%,

[0055] Ca: 0.0001 to 0.0100%,

[0056] Mg: 0.0001 to 0.0100%,

[0057] Hf: 0.0001 to 0.0100%,

[0058] REM: 0.0001 to 0.1000%,

[0059] Bi: 0.0001 to 0.0100%,

[0060] As: 0.0001 to 0.0100%,

[0061] Zr: 0.01 to 1.00%,

[0062] Co: 0.01 to 1.00%,

[0063] Zn: 0.01 to 1.00%,

[0064] W: 0.01 to 1.00%, and

[0065] Sn: 0.01 to 1.00%.

[0066] (3) The hot rolled steel sheet according to the above (1) or (2), wherein the hot rolled steel sheet is a painted steel sheet provided with a paint layer on at least one surface.

[0067] (4) A part including the hot rolled steel sheet according to any one of the above (1) to (3).ADVANTAGEOUS EFFECTS OF INVENTION

[0068] According to the present invention, it is possible to provide a hot rolled steel sheet which, despite being high strength, is improved in stretch flangeability, ductility, and notch fatigue property and is excellent in post-painting corrosion resistance.BRIEF DESCRIPTION OF DRAWINGS

[0069] FIG. 1 is a view showing a shape of a saddle shaped article used in a saddle type stretch flanging test method.

[0070] FIG. 2 is a view showing a shape of a fatigue test piece used for evaluating a notch fatigue property.DESCRIPTION OF EMBODIMENTS<Hot Rolled Steel Sheet>

[0071] The hot rolled steel sheet according to an embodiment of the present invention is characterized by having a chemical composition comprising, by mass %,

[0072] C: 0.020 to 0.070%,

[0073] Si: more than 0.100 to 2.000%,

[0074] Mn: 0.60 to 2.00%,

[0075] Ti: 0.015 to 0.200%,

[0076] sol. Al: 0.010 to 1.000%,

[0077] P: 0.100% or less,

[0078] S: 0.030% or less,

[0079] N: 0.0060% or less,

[0080] O: 0.0100% or less,

[0081] Nb: 0 to 0.050%,

[0082] V: 0 to 0.300%,

[0083] Cr: 0 to 2.00%,

[0084] Ni: 0 to 2.00%,

[0085] Cu: 0 to 2.00%,

[0086] Mo: 0 to 1.000%,

[0087] B: 0 to 0.0100%,

[0088] Sb: 0 to 1.00%,

[0089] Ca: 0 to 0.0100%,

[0090] Mg: 0 to 0.0100%,

[0091] Hf: 0 to 0.0100%,

[0092] REM: 0 to 0.1000%,

[0093] Bi: 0 to 0.0100%,

[0094] As: 0 to 0.0100%,

[0095] Zr: 0 to 1.00%,

[0096] Co: 0 to 1.00%,

[0097] Zn: 0 to 1.00%,

[0098] W: 0 to 1.00%,

[0099] Sn: 0 to 1.00%, and

[0100] balance: Fe, and impurities, and

[0101] satisfying 0.110<[Si]+[sol. Al]≤2.500, wherein [Si] and [sol. Al] are the contents (mass %) of the elements, and

[0102] a microstructure comprising, by area %,

[0103] at least one of ferrite and bainite: 80 to 98% in total, and

[0104] martensite: 2 to 10%, wherein

[0105] when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area %, 10 to 60%, and

[0106] a surface roughness Ra is less than 1.50 μm and a difference of a maximum value and minimum value in the surface roughness Ra is 0.50 μm or less.

[0107] As explained above, it is known that the stretch flangeability and other properties fall along with higher strength of a steel sheet and that due to the formation of a scale pattern due to Si scale (below, also referred to as an “Si scale pattern”), sometimes the post-painting corrosion resistance falls. First, in an embodiment of the present invention, by making the microstructure of the hot rolled steel sheet having a predetermined chemical composition include at least one of ferrite and bainite and martensite in specific ratios, more specifically by making it include, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, it is possible to improve the strength and the stretch flangeability, ductility, and notch fatigue property with a good balance. In addition, if defining a region surrounded by grain boundaries with an orientation difference of 15° or more and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, crystal grains with an orientation difference in grains of 5 to 14° are effective for improving the strength and the stretch flangeability and ductility. For this reason, by suitably controlling the ratios of the crystal grains, more specifically by controlling it to within a range of, by area %, 10 to 60%, it becomes possible to further improve the balance of the strength and the stretch flangeability and ductility.

[0108] On the other hand, if explaining post-painting corrosion resistance in more detail, a steel sheet is generally treated by zinc phosphate or other chemical conversion, then painted. In chemical conversion, a chemical reaction is caused between the treatment solution and the Fe eluted from the steel sheet to cause the formation of a dense chemical conversion coating comprised of chemically converted crystals on the steel sheet surface and thereby improve the post-painting corrosion resistance. However, if the steel sheet surface has oxides derived from the formation of the Si scale pattern remaining on it, elution of Fe is obstructed and portions called “bald spots” where the chemical conversion coating is not formed appear or Fe is not eluted and therefore a chemical conversion coating not including Fe which originally should not be formed is formed and as a result, sometimes the post-painting corrosion resistance falls. Therefore, the inventors engaged in studies to realize improvement of the post-painting corrosion resistance by suppressing the formation of the Si scale pattern in addition to improvement of the stretch flangeability, ductility, and notch fatigue property, in particular focusing on the surface properties of a hot rolled steel sheet. As a result, the inventors discovered that by controlling the surface roughness Ra of the hot rolled steel sheet to less than 1.50 μm and controlling the difference of the maximum value and minimum value in the surface roughness Ra to 0.50 μm or less, it is possible to suppress the formation of the Si scale pattern and thereby possible to remarkably improve the post-painting corrosion resistance of the hot rolled steel sheet.

[0109] Explained more specifically, in a steel sheet containing a relatively large amount of Si, Si oxides are formed at the interface of the Fe oxides (also called “scale”) formed at the steel sheet surface at the time of the hot rolling step and the steel. The Si oxides cause the Fe oxides to be strongly attached to the steel, therefore sometimes the scale cannot be sufficiently removed even by subsequent descaling using high pressure water, etc. In such a case, the scale not sufficiently removed is pressed into the steel sheet surface due to the subsequent finish rolling or other rolling. As a result, asperities are formed at the steel sheet surface and the surface properties deteriorate. Due to such poor descaling and deterioration of the surface properties due to the subsequent rolling, the Si scale pattern is formed at the hot rolled steel sheet after pickling.

[0110] Therefore, the inventors discovered that, as explained later in detail relating to the method of production of a hot rolled steel sheet, by suitably controlling the temperature conditions between the rough rolling and the finish rolling, it is possible to sufficiently or completely remove scale by descaling before finish rolling and that, along with this, it is possible to remarkably keep scale from being pressed into the steel sheet surface and forming asperities at the steel sheet surface at the time of the subsequent finish rolling. The inventors further studied the relationship between the surface properties of a steel sheet and formation of the Si scale pattern. As a result, the inventors discovered that by sufficiently or completely removing the scale by descaling before the finish rolling so that the surface roughness Ra of the finally obtained hot rolled steel sheet becomes less than 1.50 μm and the difference of the maximum value and minimum value in the surface roughness Ra becomes 0.50 μm or less, it is possible to remarkably suppress the formation of the Si scale pattern and along with that obtain a hot rolled steel sheet having excellent post-painting corrosion resistance. That is, according to an embodiment of the present invention, despite being high strength, for example, a high strength of a tensile strength of 540 MPa or more, it is possible to achieve an improvement of the stretch flangeability, ductility, and notch fatigue property and realize excellent post-painting corrosion resistance. Therefore, since the hot rolled steel sheet according to an embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability and is also excellent in post-painting corrosion resistance, it is particularly useful in use in the automobile field where achievement of both of these properties is sought.

[0111] Below, the hot rolled steel sheet according to an embodiment of the present invention will be explained in more detail. In the following explanation, the “%” of the units of contents of the elements, unless otherwise indicated, means “mass %”. Further, in this Description, the “to” showing a numerical range, unless otherwise indicated, is used in the sense of the numerical values described before and after the same being included as the lower limit value and the upper limit value.[C: 0.020 to 0.070%]

[0112] C is an element effective for raising the strength of a steel sheet. Further, C forms carbides and / or carbonitrides with Ti and Nb in the steel and also contributes to the precipitation strengthening based on the precipitates formed and the refinement of the structure by the pinning effect of the precipitates. To sufficiently obtain these effects, the C content is 0.020% or more. The C content may also be 0.022% or more, 0.025% or more, 0.028% or more, or 0.030% or more. On the other hand, if excessively containing C, sometimes the stretch flangeability and weldability fall. Therefore, the C content is 0.070% or less. The C content may also be 0.065% or less, 0.060% or less, 0.055% or less, or 0.050% or less.[Si: More than 0.100 to 2.000%]

[0113] Si is an element effective for raising strength as a solid solution strengthening element. To sufficiently obtain such an effect, the Si content is more than 0.100%. The Si content may 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 excessively containing Si, even if suitably controlling the surface roughness Ra of the steel sheet and the difference between the maximum value and minimum value, sometimes formation of the Si scale pattern cannot be sufficiently suppressed. Therefore, the Si content is 2.000% or less. The Si content may also be 1.800% or less, 1.600% or less, 1.400% or less, or 1.200% or less.[Mn: 0.60 to 2.00%]

[0114] Mn is an element effective for hardenability and for raising strength as a solid solution strengthening element. To sufficiently obtain these effects, the Mn content is 0.60% or more. The Mn content may also be 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if excessively containing Mn, sometimes the stretch flangeability falls. Therefore, the Mn content is 2.00% or less. The Mn content may also be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.[Ti: 0.015 to 0.200%]

[0115] Ti is an element finely precipitating in steel as a carbide (TiC) and improving the strength of the steel by precipitation strengthening. Further, Ti is an element forming carbides to fix C and suppress the formation of the cementite harmful to the stretch flangeability. To sufficiently obtain these effects, the Ti content is 0.015% or more. The Ti content may also be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if excessively containing Ti, the carbides become coarser and sometimes the ductility falls. Therefore, the Ti content is 0.200% or less. The Ti content may also be 0.180% or less, 0.170% or less, 0.150% or less, or 0.120% or less.[sol. Al: 0.010 to 1.000%]

[0116] sol. Al is an element acting as a deoxidizer of molten steel. To sufficiently obtain such an effect, the sol. Al content is 0.010% or more. The sol. Al content may also be 0.012% or more, 0.015% or more, or 0.020% or more. On the other hand, if excessively containing sol. Al, coarse oxides are formed and the toughness and ductility fall sometimes leading to fracture during rolling. Therefore, the sol. Al content is 1.000% or less. The sol. Al content may also be 0.800% or less, 0.600% or less, or 0.400% or less. Note that “sol. Al” means acid soluble Al and indicates the dissolved Al present in the steel in a dissolved state.[P: 0.100% or Less]

[0117] P, if excessively contained, sometimes disadvantageously affects the weldability, etc. Therefore, the P content is 0.100% or less. The P content may 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 prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the P content may also be 0.001% or more, 0.003% or more, or 0.005% or more.[S: 0.030% or Less]

[0118] S, if excessively contained, sometimes forms a large amount of MnS and lowers the toughness. Therefore, the Si content is 0.030% or less. The S content may 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 prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the S content may also be 0.001% or more, 0.002% or more, or 0.003% or more.[N: 0.0060% or Less]

[0119] N forms precipitates with Ti more preferentially than C and sometimes causes a reduction in the Ti effective for fixing the C. Therefore, the N content is 0.0060% or less. The N content may 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 prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the N content may also be 0.0001% or more or 0.0005% or more.[O: 0.0100% or Less]

[0120] O is an element entering in the production process. If excessively containing 0, coarse inclusions are formed and sometimes the toughness of the steel sheet is lowered. Therefore, the O content may also be 0.0100% or less. The O content may 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 prescribed and may also be 0%, but for reduction to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the O content may also be 0.0001% or more or 0.0005% or more.

[0121] The basic chemical composition of the hot rolled steel sheet according to an embodiment of the present invention is as explained above. Further, the hot rolled steel sheet may, in accordance with need, contain at least one of the following optional elements in place of part of the balance of Fe.[Nb: 0 to 0.050%]

[0122] Nb is an element forming carbides, nitrides, and / or carbonitrides in steel to contribute to refinement of the structure due to the pinning effect and in turn higher strength of the steel sheet. Further, Nb is an element forming carbides and / or carbonitrides to thereby fix C and suppress the formation of the cementite harmful to stretch flangeability. The Nb content may also be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may also be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes the ductility of the steel sheet falls. Therefore, the Nb content is preferably 0.050% or less. The Nb content may also be 0.040% or less, 0.030% or less, or 0.020% or less.[V: 0 to 0.300%]

[0123] V is an element contributing to improvement of strength by precipitation strengthening, etc. The V content may also be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if excessively including V, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the V content is preferably 0.300% or less. The V content may also be 0.200% or less, 0.100% or less, or 0.080% or less.[Cr: 0 to 2.00%]

[0124] Cr is an element raising the hardenability of steel and contributing to improvement of strength. The Cr content may also be 0%, but to obtain such an effect, the Cr content is preferably 0.01% or more. The Cr content may also be 0.03% or more or 0.05% or more. On the other hand, even if excessively including Cr, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the Cr content is preferably 2.00% or less. The Cr content may 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.[Ni: 0 to 2.00%][Cu: 0 to 2.00%]

[0125] Ni, and Cu are elements contributing to improvement of strength by precipitation strengthening or solid solution strengthening. The Ni and Cu contents may also be 0%, but to obtain such an effect, the contents of these elements are preferably 0.01% or more and may also be 0.03% or more or 0.05% or more. On the other hand, even if excessively including these elements, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the Ni and Cu contents are preferably respectively 2.00% or less and may 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.[Mo: 0 to 1.000%]

[0126] Mo is an element raising the hardenability of steel and contributing to improvement of the strength. The Mo content may also be 0%, but to obtain such an effect, the Mo content is preferably 0.001% or more. The Mo content may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if excessively containing Mo, the deformation resistance at the time of hot working increases and sometimes the load on the facilities becomes greater. Therefore, the Mo content is preferably 1.000% or less. The Mo content may also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.[B: 0 to 0.0100%]

[0127] B segregates at the grain boundaries and raises the intergranular strength to raise the low temperature toughness. The B content may also be 0%, but to obtain such an effect, the B content is preferably 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, even if excessively including B, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.[Sb: 0 to 1.00%]

[0128] Sb is an element effective for improvement of the corrosion resistance. The Sb content may also be 0%, but to obtain such an effect, the Sb contents is preferably 0.01% or more. The Sb content may also be 0.02% or more or 0.05% or more. On the other hand, if excessively Sb, sometimes a drop in toughness is invited. Therefore, Sb content is preferably 1.00% or less. The Sb content may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.[Ca: 0 to 0.0100%][Mg: 0 to 0.0100%][Hf: 0 to 0.0100%]

[0129] Ca, Mg, and Hf are elements enabling control of the form of the nonmetallic inclusions. The Ca, Mg, and Hf contents may also be 0%, but to obtain such an effect, the contents of these elements are preferably respectively 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 excessively containing these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Ca, Mg, and Hf contents are preferably respectively 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.[REM: 0 to 0.1000%]

[0130] An REM is an element enabling control of the form of nonmetallic inclusion. The REM content may be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing a REM, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the REM content is preferably 0.1000% or less. The REM content may also be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The “REM” in this Description is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.[Bi: 0 to 0.0100%][As: 0 to 0.0100%]

[0131] Bi and As are elements effective for improvement of the corrosion resistance. The Bi and As contents may also be 0%, but to obtain such an effect, the contents of these elements are preferably respectively 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 excessively including these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Bi and As contents are preferably respectively 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.[Zr: 0 to 1.00%]

[0132] Zr is an element enabling control of the form of nonmetallic inclusions. The Zr content may also be 0%, but to obtain such an effect, the Zr content is preferably 0.01% or more. The Zr content may also be 0.05% or more or 0.10% or more. On the other hand, even if excessively containing Zr, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Zr content is preferably 1.00% or less. The Zr content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.[Co: 0 to 1.00%]

[0133] Co is an element contributing to improvement of the hardenability and / or heat resistance. The Co content may also be 0%, but to obtain these effects, the Co content is preferably 0.01% or more. The Co content may also be 0.05% or more or 0.10% or more. On the other hand, if excessively containing Co, the hot workability sometimes falls. This also leads to an increase in material costs. Therefore, the Co content is preferably 1.00% or less. The Co content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.[Zn: 0 to 1.00%]

[0134] Zn is an element effective for control of the shape of the inclusions. To obtain such an effect, the Zn content is preferably 0.01% or more. The Zn content may also be 0.05% or more or 0.10% or more. On the other hand, even if excessively including Zn, the effect becomes saturated and a rise in production costs is invited. Therefore, the Zn content is preferably 1.00% or less. The Zn content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.[W: 0 to 1.00%]

[0135] W is an element raising the hardenability of steel and contributes to improvement of strength. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.01% or more. The W content may also be 0.05% or more or 0.10% or more. On the other hand, if excessively containing W, sometimes the weldability falls. Therefore, the W content is preferably 1.00% or less. The W content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.[Sn: 0 to 1.00%]

[0136] Sn is an element effective for improvement of the corrosion resistance. The Sn content may also be 0%, but to obtain such an effect, the Sn content is preferably 0.01% or more. The Sn content may also be 0.02% or more or 0.05% or more. On the other hand, if excessively Sn, sometimes a drop in toughness is invited. Therefore, Sn content is preferably 1.00% or less. The Sn content may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[0137] In the hot rolled steel sheet according to an embodiment of the present invention, the balance aside from the above elements is comprised of Fe and impurities. The “impurities” are constituents, etc., entering due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing the hot rolled steel sheet.[0.110<[Si]+[sol. Al]≤2.500]

[0138] The chemical composition of the hot rolled steel sheet according to an embodiment of the present invention has to satisfy the following formula:0.11<[Si]+[sol. Al]≤2.5

[0139] wherein [Si] and [sol. Al] are the contents (mass %) of the elements. As explained previously, if defining a region surrounded by boundaries with orientation differences of 15° or more and with a circle equivalent diameter of 0.3 μm or more as a crystal grain, crystal grains with orientation differences in the grains of 5 to 14° are effective for improvement of the strength and stretch flangeability. For this reason, in the hot rolled steel sheet according to an embodiment of the present invention, as explained in detail later, by controlling the ratio of these crystal grains to within a range of, by area %, 10 to 60%, the balance of the strength and stretch flangeability is improved. Si and sol. Al are elements effective for controlling the ratio of crystal grains with an orientation difference in the grains of 5 to 14° to within a range of 10 to 60% in addition to the effects explained regarding the individual elements. This is believed to be because by inclusion of Si and sol. Al, the temperature of the Ar3 point rises and the transformation strain introduced into the grains becomes smaller. To sufficiently obtain these effects, the chemical composition of the hot rolled steel sheet according to an embodiment of the present invention is controlled so that the total contents of Si and sol. Al becomes more than 0.110%, i.e., satisfies [Si]+[sol. Al]>0.110. From the viewpoint of further enhancing these effects, the total content of Si and sol. Al is preferably 0.120% or more and may also 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, formation of ferrite is promoted and sometimes the strength falls. Therefore, the total content of Si and sol. Al is 2.500% or less, i.e., [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.

[0140] The chemical composition of the hot rolled steel sheet according to an embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the hot rolled steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-nondispersive type infrared absorption method.[Microstructure][At Least One of Ferrite and Bainite: 80 to 98% in Total and Martensite: 2 to 10%]

[0141] The microstructure of the hot rolled steel sheet according to an embodiment of the present invention includes, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%. By the microstructure of the hot rolled steel sheet being comprised of these structure, it is possible to improve the strength, stretch flangeability, ductility, and notch fatigue property with a good balance. If the total area ratio of the at least one of ferrite and bainite is low or the area ratio of the martensite is high, in particular the balance of the strength and stretch flangeability falls and the desired properties cannot be obtained. Therefore, the total area ratio of the at least one of ferrite, and bainite is 80% or more and, for example, may also be 82% or more, 85% or more, 88% or more, or 90% or more. Similarly, the area ratio of martensite is 10% or less and, for example, may 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 the at least one of ferrite and bainite is high or the area ratio of martensite is low, sometimes in particular the balance of strength and notch fatigue property fall and the desired properties cannot be obtained. Therefore, the total area ratio of the at least one of ferrite and bainite is 98% or less and, for example, may also be 96% or less, 94% or less, or 92% or less. Similarly, the area ratio of martensite is 2% or more and may also be, for example, 3% or more, 4% or more, or 5% or more.

[0142] The microstructure of the hot rolled steel sheet may contain either of ferrite and bainite, preferably includes both of ferrite and bainite. Therefore, either of the area ratios of ferrite and bainite may be 0%, for example, they may also respectively be 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more. Similarly, the area ratios of ferrite and bainite may also be, for example, respectively 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less. From the viewpoint of improvement of the ductility of the hot rolled steel sheet, the area ratio of bainite is preferably 80% or less, more preferably is 70% or less.[Balance Structures]

[0143] The balance structures other than the ferrite, bainite, and martensite may also be an area % of 0%, but if there are balance structures present, the balance structures may be at least one of retained austenite and pearlite. The area ratio of the balance structures is not particularly limited, but, for example, may be 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flangeability, the area ratio of the balance structures is preferably, for example, 10% or less and may also be 8% or less, 6% or less, or 5% or less.[Identification of Microstructure and Calculation of Area Ratios]

[0144] The microstructure at the hot rolled steel sheet is identified and the area ratios are calculated by observation under an optical microscope after corrosion using a Nital reagent or LePera solution and by X-ray diffraction. The structure is observed under an optical microscope at a sheet thickness cross-section in a direction parallel to the rolling direction and vertical to the sheet surface. Specifically, first, a sample is taken from the hot rolled steel sheet and the observed surface of the sample is etched by Nital. Next, an optical microscope is used to obtain a structural photograph of a 300 μm×300 μm field at a ¼ depth position in sheet thickness. The image is analyzed to calculate the area ratios of the ferrite and pearlite and the total area ratio of bainite and martensite. Crystal grains not including sub structures equiaxially can be identified as ferrite while those including sub structures can be identified as bainite and martensite. Next, the observed surface of the sample is corroded by a LePera solution. In the same way, an optical microscope is used to obtain a structural photograph of a 300 μm×300 μm field at a ¼ depth position in sheet thickness. The image is analyzed to calculate the total area ratio of the retained austenite and martensite. The image analysis is performed using the “Analyze” function of the image analysis software “ImageJ”. Due to this, it is possible to calculate the individual area ratios and total area ratio. Here, the “Imager” is an open source public domain image processing software which is widely utilized among persons skilled in the art. Next, the sample is face cut from the rolling surface normal direction down to a ¼ depth in the sheet thickness. This is then measured by X-ray diffraction to calculate the volume ratio of the retained austenite. The volume ratio of retained austenite is equivalent to the area ratio, so this is made the area ratio of the retained austenite. The area ratio of the retained austenite obtained is subtracted from the total area ratio of the residual austenite and martensite calculated previously to calculate the area ratio of the martensite. Finally, the area ratio of the obtained martensite is subtracted from the total area ratio of the bainite and martensite calculated previously in the same way to calculate the area ratio of the bainite.[Ratio of Crystal Grains With Orientation Difference in Grains of 5 to 14°: Area % of 10 to 60%]

[0145] In the microstructure of the hot rolled steel sheet according to an embodiment of the present invention, if deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is controlled to within a range of, by area %, 10 to 60%. Crystal grains having such an orientation difference in grains are effective for raising the strength and the stretch flangeability. While not intending to be constrained to any specific theory, it is believed that the crystal orientation difference in the grains is correlated with the dislocation density contained in the crystal grains. In general, an increase in the dislocation density in grains causes improvement of strength, but lowers the workability. However, in crystal grains with an orientation difference in grains controlled to 5 to 14°, it is believed possible to raise the strength without lowering the workability. As opposed to this, crystal grains with an orientation difference in grains of less than 5° are excellent in workability, but difficult to make high in strength. On the other hand, crystal grains with an orientation difference in grains of more than 14° differ in deformation ability in the crystal grains, therefore do not necessarily contribute to improvement of the stretch flangeability. Therefore, in the hot rolled steel sheet according to an embodiment of the present invention, it becomes possible to suitably control the ratio of crystal grains with an orientation difference in grains of 5 to 14°, more specifically to control it to within a range of, by area %, 10 to 60%, to achieve the desired steel sheet strength while improving the stretch flangeability and to further improve the balance of strength and the stretch flangeability. If the ratio of crystal grains with an orientation difference in grains of 5 to 14° is small, sometimes the stretch flangeability falls. Therefore, from the viewpoint of improvement of the stretch flangeability, the ratio of crystal grains with an orientation difference in grains of 5 to 14° may also be 15% or more, 18% or more, or 20% or more. On the other hand, if the ratio of crystal grains with an orientation difference in grains of 5 to 14° is large, sometimes the ductility falls. Therefore, from the viewpoint of improvement of ductility, the ratio of crystal grains with an orientation difference in grains of 5 to 14° may also be 55% or less, 50% or less, 45% or less, or 40% or less.[Measurement of Ratio of Crystal Grains with Orientation Difference in Grains of 5 to 14°]

[0146] The ratio of crystal grains with an orientation difference in grains of 5 to 14° is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from a steel sheet so that a sheet thickness cross-section in a direction parallel to the rolling direction and vertical to the sheet surface becomes the examined surface. Next, a region of 200 μm in the rolling direction of the steel sheet and 100 μm in the rolling surface normal direction at a ¼ depth position in sheet thickness from the steel sheet surface is analyzed by EBSD at 0.2 μm measurement intervals so as to acquire crystal orientation information. Here, the EBSD analysis is performed using an apparatus comprised of a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and an EBSD detector (HIKARI detector made by TSL) by a 50 to 300 points / s analysis speed. Next, for the obtained crystal orientation information, a region with an orientation difference of 15° or more and a circle equivalent diameter of 0.3 μm or more is defined as a crystal grain, the average orientation difference in the grain of each crystal grain is calculated, and the ratio of crystal grains with an orientation difference in grains of 5 to 14° is found. Such a defined crystal grain or average orientation difference in grains can be calculated using the software “OIM Analysis™” attached to the EBSD analysis apparatus. In the present invention, the “orientation difference in grains” expresses the grain orientation spread (GOS). The value of the orientation difference in grains, as described in “Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-Ray Diffraction Method”, Hidehiko Kimura et al., Transactions of the JSME (A Edition), vol. 71, no. 712, 2005, p. 1722-1728, is found as the average value of the misorientation among all measurement points from the crystal orientation becoming the reference in the same crystal grain. In the embodiment of the present invention, the reference crystal orientation is the orientation of the average of all measurement points in the same crystal grain. The value of GOS can be calculated using the software “OIM Analysis™ Version 7.0.1” attached to the EBSD analysis apparatus.[Surface Roughness Ra: Less Than 1.50 μm and Difference of Maximum Value and Minimum Value in Surface Roughness Ra: 0.50 μm or Less]

[0147] In the hot rolled steel sheet according to an embodiment of the present invention, the surface roughness Ra is controlled to less than 1.50 μm and the difference of the maximum value and minimum value in the surface roughness Ra is controlled to 0.50 μm or less. As explained later in detail relating to the method of production of the hot rolled steel sheet, by scale being sufficiently or completely removed by descaling before finish rolling, it is possible to keep asperities from being formed at the steel sheet surface due to scale being pressed into the steel sheet surface at the time of the later finish rolling. In an embodiment of the present invention, by realizing surface properties giving a surface roughness Ra of the finally obtained hot rolled steel sheet of less than 1.50 μm and a difference of the maximum value and minimum value in the surface roughness Ra of 0.50 μm or less, the formation of the Si scale pattern due to the scale being pressed into the steel sheet surface can be remarkably suppressed and, along with this, excellent post-painting corrosion resistance can be achieved. If it is not possible to sufficiently suppress the formation of the Si scale pattern and the surface roughness Ra of the hot rolled steel sheet becomes 1.50 μm or more and / or the difference of the maximum value and minimum value in the surface roughness Ra becomes more than 0.50 μm, as a result, the post-painting corrosion resistance of the hot rolled steel sheet falls. For example, even if the surface roughness Ra is less than 1.50 μm, if the difference of the maximum value and minimum value in the surface roughness Ra is more than 0.50 μm, the variation in the surface roughness is great, so it cannot be said that formation of an Si scale pattern was sufficiently suppressed. In this case, for example, it becomes no longer possible to realize uniform chemical conversion after that and as a result the post-painting corrosion resistance falls. Therefore, in an embodiment of the present invention, it is not sufficient to just control the surface roughness Ra to less than 1.50 μm. In addition to this, controlling the difference of the maximum value and minimum value in the surface roughness Ra to 0.50 μm or less is extremely important.

[0148] From the viewpoint of suppressing the formation of the Si scale pattern to improve the post-painting corrosion resistance, the lower the surface roughness Ra and the difference of the 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, most preferably 0.80 μm or less. Similarly, the difference of the maximum value and minimum value in the surface roughness Ra is preferably 0.40 μm or less, more preferably is 0.30 μm or less.

[0149] The lower limits of the surface roughness Ra and the difference of the maximum value and minimum value in the same are not particularly prescribed, but, for example, the surface roughness Ra may 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 of the maximum value and minimum value in 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.[Measurement of Surface Roughness Ra and Difference of Maximum Value and Minimum Value in Same]

[0150] The surface roughness Rain the hot rolled steel sheet and the difference of the maximum value and minimum value in the same are measured in the following way. More specifically, first, if the hot rolled steel sheet has scale on the surface, the sample is pickled, then sent on for measurement of roughness. The pickling is performed under conditions of dipping in hydrochloric acid with a hydrochloric acid concentration of 3 to 10 vol % at a 85 to 98° C. temperature for 20 to 300 seconds. Further, the pickling may be performed at one time or may be performed divided into several times in accordance with need. The above pickling time (20 to 300 seconds) means the duration of the pickling in the case of performing the pickling just one time and means the total duration of the pickling if performing the pickling several times. By making the pickling temperature 85° C. or more, it is possible to sufficiently remove the oxides at the surface layer. Next, the sample surface of the pickled hot rolled steel sheet is measured for roughness of the surface in the rolling direction at different measurement locations using locations at 50 mm intervals along the width direction as the measurement locations. Ten or more points are preferably measured, but if the sheet width is insufficient, positions 50 mm apart in the rolling direction are similar measured and 10 or more points measured. The measurement length at each measurement location is 5 mm. Contour curve filters with cutoff values λc and λs are successively applied to the measurement cross-section curve obtained by measurement to obtain a roughness curve. Specifically, the component with a wavelength λc of 0.8 mm or less and component with a wavelength λs of 2.5 μm or more are removed from the obtained measurement results to obtain a roughness curve. Based on the obtained roughness curve, the arithmetic average roughnesses of the individual measurement locations are calculated based on JIS B 0601: 2013. The average value of all of the obtained arithmetic average roughnesses is determined as the surface roughness Ra. Similarly, the difference of the maximum value and minimum value in the surface roughness Ra is determined from all of the obtained arithmetic average roughnesses.

[0151] If the hot rolled steel sheet is provided with a plating layer and paint or other surface coating on its surface, the base iron surface obtained after removing the surface coating is measured. The method of removing the surface coating can be suitably selected in accordance with the type of the surface coating within a range not affecting the surface roughness of the base iron. For example, if the surface coating is formed by electrogalvanization, electric Zn—Ni alloy plating, hot dip galvanization, hot dip galvannealing, hot dip Zn—Al alloy plating, hot dip Zn—Al—Mg alloy plating, hot dip Zn—Al—Mg—Si alloy plating, or other zinc plating layer, dilute hydrochloric acid to which an inhibitor is added may be used to dissolve the galvanized layer. Due to this, it is possible to peel off only the galvanized layer from the steel sheet. An “inhibitor” is an additive used for suppressing changes in roughness by prevention of over dissolution of the base iron. For example, it is possible to use hydrochloric acid diluted to 5 vol % to which is added an “ibit No. 700BK” corrosion inhibitor for hydrochloric acid pickling made by Asahi Chemical Co., Ltd. to a 0.6 g / L concentration. Further, if the surface coating is hot dip aluminum plating or other aluminum plating layer, based on the description in JIS G 3314: 2019, the steel sheet is successively dipped in a sodium hydroxide aqueous solution and dilute hydrochloric acid aqueous solution to which hexamethylene tetramine is added. The Al plating is made to dissolve by dipping the steel sheet until the foaming due to the dissolution of the plating dies down. Further, if the surface coating was formed by electroplating, a release agent (Neo-Rever SP-751: made by Sansaikako) is used to peel off the electrodeposited paint.[Sheet Thickness]

[0152] The hot rolled steel sheet according to an embodiment of the present invention is not particularly limited, but in general has a sheet thickness of 1.0 to 6.0 mm. For example, the sheet thickness may also be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and / or may also be 5.0 mm or less or 4.0 mm or less.

[0153] The hot rolled steel sheet according to an embodiment of the present invention may also be a painted steel sheet provided with at paint layer on at least one surface. If the hot rolled steel sheet is provided with a plating layer and / or a chemical conversion coating, the paint layer can be formed on the plating layer and / or chemical conversion coating. The paint layer is not particularly limited and many be any suitable paint layer known to persons skilled in the art. The thickness of the paint layer is not particularly limited. The paint layer can have any suitable thickness. A paint layer generally includes an electrodeposited paint layer and may also include an intermediate paint layer, base coat layer, and clear coat layer on top of that.

[0154] The hot rolled steel sheet according to an embodiment of the present invention, as explained above, despite being high in strength, is improved in stretch flangeability, ductility, and notch fatigue property and achieves excellent post-painting corrosion resistance. For this reason, the hot rolled steel sheet according to an embodiment of the present invention can realize both the contradictory properties of high strength and excellent workability and, further, is excellent in post-painting corrosion resistance, therefore is particularly useful for use in parts, etc., of technical fields in which these features are sought. In a preferable embodiment, an auto part containing the hot rolled steel sheet according to an embodiment of the present invention, in particular a part selected from the suspension, chassis, bumpers, etc., of an automobile, is provided. Such an auto part need only include the hot rolled steel sheet according to an embodiment of the present invention in a portion of the same. Therefore, it satisfies the features of the chemical compositions, microstructure, and surface properties explained earlier in at least a portions of the part. At portions of the hot rolled steel sheet which do not directly contact the die in press-forming or other shaping and which are low in degree of working, the features of the microstructure and surface properties do not particularly change before and after shaping.[Mechanical Properties][Tensile Strength: TS]

[0155] According to the hot rolled steel sheet having the above chemical composition and microstructure, it is possible to achieve a high tensile strength, specifically a 540 MPa or more tensile strength. The tensile strength is preferably 600 MPa or more, 700 MPa or more, 780 MPa or more, or 850 MPa or more. According to the hot rolled steel sheet according to an embodiment of the present invention, despite having such an extremely high tensile strength, by the specific combination of the chemical composition, microstructure, and surface toughness Ra explained above, it is possible to remarkably improve the stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance. The upper limit of the tensile strength is not particularly prescribed, but for example the tensile strength of the hot rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, or 1180 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.[Total Elongation: El]

[0156] According to the hot rolled steel sheet having the above-mentioned chemical composition and microstructure, in addition to the high tensile strength, it is possible to improve the total elongation, more specifically possible to achieve a 15.0% or more total elongation. The total elongation is preferably 18.0% or more, more preferably 20.0% or more, most preferably 22.0% or more. The upper limit is not particularly prescribed, but, for example, the total elongation is 40.0% or less or 35.0% or less. The total elongation is measured by taking a JIS No. 5 test piece from an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.<Method of Production of Hot Rolled Steel Sheet>

[0157] Next, a preferable method of production of the hot rolled steel sheet according to an embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for production of the hot rolled steel sheet according to an embodiment of the present invention and is not intended to limit the hot rolled steel sheet to one produced by the method of production explained below.

[0158] The method of production of the hot rolled steel sheet according to an embodiment of the present invention comprises:

[0159] (A) hot rolling including heating a slab having a chemical composition explained above in relation to the hot rolled steel sheet, then rough rolling, descaling by high pressure water, and finish rolling it, and satisfying the following (A1) to (A4) conditions:

[0160] (A1) a heating temperature of the slab is a solubilization temperature (SRTmin) ° C. expressed by the following formula 1 or more and 1170° C. or more,

[0161] (A2) a highest heating temperature from completion of rough rolling to the start of high pressure water descaling before finish rolling is T° C. expressed by the following formula 2 or more,

[0162] (A3) a cumulative strain (εeff.) of the later three stages of finish rolling expressed by the following formula 3 is 0.50 to 0.60,SRT⁢min=7000 / {2.75-log⁡([Ti]×[C])}-273formula⁢ 1

[0163] where [Ti] and [C] are the contents (mass %) of the elements in the steel

[0164] (A4) an end temperature of finish rolling is Ar3+30° C. or more,T=1081+63×[Si]+27×[sol. Al]+10×[sol. Al]⁢ / [Si]-8⁢ / [Si]-1⁢ / [sol. Al]formula⁢ 2

[0165] where [Si] and [sol. Al] are the contents (mass %) of the elements in the steelε⁢eff. =∑ε⁢i⁡(t,T)formula⁢ 3whereε⁢i⁡(t,T)=ε⁢i⁢0 / exp⁢{(t / τ⁢R)2 / 3}τ⁢R=τ0·exp⁡(Q / RT)τ0=8.46×10-6Q=183200⁢JR=8.314J / K·mol

[0166] εi0 indicates the logarithmic strain at the time of rolling reduction, “t” indicates the cumulative time (s) up right before the cooling of the pass, and T indicates the rolling temperature (° C.) at the pass, and

[0167] (B) cooling including primary cooling the finish rolled steel sheet by an average cooling speed of 10° C. / s or more down to a temperature region of 650 to 750° C., holding in the temperature region for 3.0 to 10.0 seconds, then secondary cooling down to 100° C. or less by an average cooling speed of 30° C. / s or more. Below, the steps will be explained in detail.[(A) Hot Rolling Step][(A1) Heating Temperature of Slab]

[0168] First, a slab having the chemical composition explained above in relation to the hot rolled steel sheet is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be produced by an ingot making method or a thin slab casting method. The heating temperature of the slab has to be the solubilization temperature (SRTmin) ° C. expressed by the following formula 1 or more.SRT⁢min=7000 / {2.75-log⁡([Ti]×[C])}-273formula⁢ 1

[0169] where [Ti] and [C] are the contents (mass %) of the elements in the steel.

[0170] The slab used contains relatively large amounts of alloy elements, in particular, contains Ti. For this reason, the alloy elements have to be made to dissolve in the slab. In particular, Ti has to be made to sufficiently dissolve. If the heating temperature of the slab is less than the solubilization temperature (SRTmin° C.), Ti will not sufficiently dissolve. If Ti does not sufficiently dissolve at the time of slab heating, it becomes difficult to make Ti finely precipitate as carbides (TiC) in the steel in the cooling step after the hot rolling step, etc., to improve the strength of the steel by precipitation strengthening. In addition, it also becomes difficult to form carbides (TiC) to fix C and suppress the formation of the cementite harmful to stretch flangeability. Further, the heating temperature of the slab has to be 1170° C. or more. The scale formed at the steel sheet surface at the time of heating has to be removed at the descaling after heating, but if less than 1170° C., the scale is unevenly removed by descaling and sometimes formation of the Si scale pattern cannot be sufficiently suppressed.[(A2) Highest Heating Temperature From Completion of Rough Rolling to Start of High Pressure Water Descaling Before Finish Rolling: T° C. or More]

[0171] In the present method of production, the heated slab is adjusted in sheet thickness, etc., by rough rolling before finish rolling, then the highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before the finish rolling is controlled to T° C. or more shown in the following formula 2.T=1081+63×[Si]+27×[sol. Al]+10×[sol. Al]⁢ / [Si]-8⁢ / [Si]-1⁢ / [sol. Al]formula⁢ 2

[0172] where [Si] and [sol. Al] are the contents (mass %) of the elements in the steel.

[0173] In a steel sheet contain a relatively large amount of Si, more specifically a steel sheet containing Si in a more than 0.100% amount, Si oxides are formed at the interface of the Fe oxides formed at the steel sheet surface (also called “scale”) at the time of the hot rolling step and the steel. The Si oxides strongly fix the Fe oxides at the steel, so sometimes the scale cannot be sufficiently removed even by subsequent descaling using high pressure water, etc. In such a case, the not sufficiently removed scale is pressed into the steel sheet surface by the subsequent finish rolling and as a result asperities are formed at the steel sheet surface and the surface properties deteriorate. Therefore, controlling the highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before finish rolling to the T° C. expressed by the above formula 2 becomes important. Due to such temperature control, it is believed possible to change to properties of scale facilitating removal by the subsequent high pressure water descaling. It is then possible to easily remove the scale by subsequent high pressure water descaling or possible to completely remove it. In this case, it becomes possible to remarkably suppress poor descaling and formation of the Si scale pattern due to scale being pressed into the steel sheet surface in the subsequent finish rolling. Along with this, in the finally obtained hot rolled steel sheet, it is possible to realize surface properties giving a surface roughness Ra of less than 1.50 μm and a difference of the maximum value and minimum value in the surface roughness Ra of 0.50 μm or less and becomes possible to achieve excellent post-painting corrosion resistance.

[0174] Rough rolling secures the desired sheet bar dimensions. Not only that, sometimes suitable temperature control is required in the relationship with the high pressure water descaling before finish rolling. For example, if reheating is not performed using a bar heater or other facility between the rough rolling and finish rolling, the highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before finish rolling corresponds to the exit side temperature of the rough rolling. Therefore, in such a case, it becomes necessary to control the exit side temperature of the rough rolling to the T° C. expressed by the above formula 2 or more. On the other hand, if reheating is performed using a bar heater or other facility between the rough rolling and finish rolling, the highest heating temperature at any point from completion of rough rolling to high pressure water descaling before finish rolling may be controlled to the T° C. expressed by the above formula 2 or more. Therefore, the exit side temperature of the rough rolling is not necessarily limited. A suitable temperature may be suitably selected.

[0175] In the present method of production, after the completion of the rough rolling and before the finish rolling, the rolled material is descaled by high pressure water. High pressure water descaling is performed using sprayed high pressure water with a water pressure of 10 to 50 MPa. By using such high pressure water, it becomes possible to sufficiently or completely remove scale from the rough rolled material. Scale grows in the hot rolled steel sheet after finish rolling. However, by sufficiently or completely removing the scale before the finish rolling, as explained above, it is possible to keep scale from being pressed into the steel sheet surface at the finish rolling. For this reason, even if the scale subsequently grows, scale cannot be uniformly formed over the steel sheet surface as a whole. Formation of such scale does not affect the formation of the Si scale pattern in any way. Further, such scale can be relatively easily removed by suitable pickling after the hot rolling.[(A3) Cumulative Strain (εeff) of Later Three Stages of Finish Rolling: 0.50 to 0.60]

[0176] The rough rolled then high pressure water descaled slab is next finish rolled. In the present method of production, for example, a tandem rolling machine comprised of four or more rolling stands is preferably used for finish rolling. In the present method of production, to control the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° to within a range of, by area %, 10 to 60%, in the finish rolling performed on a heated slab, the later explained cooling step has to be performed after making the cumulative strain (εeff.) of the later three stages (final three stages) 0.50 to 0.60. This is due to the following reason. The crystal grains with an orientation difference in the grains of 5 to 14° are formed by transformation in the paraequilibrium state by a relatively low temperature. For this reason, by limiting the dislocation density of austenite before transformation in the hot rolling step to a certain range and limiting the cooling speed in the subsequent cooling step to a certain range, it becomes possible to control the formation of the crystal grains with an orientation difference in the grains of 5 to 14°. That is, by controlling the cumulative strain of the later three stages of the finish rolling and the subsequent cooling, it is possible to control the frequency of formation of nuclei of the crystal grains with an orientation difference in the grains of 5 to 14° and the subsequent growth speed. As a result, it is possible to control the area ratio of crystal grains with an orientation difference in the grains of 5 to 14° in the hot rolled steel sheet obtained after cooling. More specifically, the dislocation density of austenite introduced by the finish rolling is mainly related to the formation of nuclei while the cooling speed after finish rolling is mainly related to the speed of growth.

[0177] If the cumulative strain of the later three stages of finish rolling is less than 0.50, the dislocation density of austenite introduced is not sufficient and the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° becomes less than 10%. On the other hand, if the cumulative strain of the later three stages of finish rolling is more than 0.60, during hot rolling, recrystallization of austenite occurs and the cumulative dislocation density at the time of transformation falls. As a result, similarly the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° becomes less than 10%. In the present method of production, the cumulative strain (εeef.) of the later three stages of finish rolling is found by the following formula 3:ε⁢eff. =∑ε⁢i⁡(t,T)formula⁢ 3whereε⁢i⁡(t,T)=ε⁢i⁢0 / exp⁢{(t / τ⁢R)2 / 3}τ⁢R=τ0·exp⁡(Q / RT)τ0=8.46×10-6Q=183200⁢JR=8.314J / K·mol

[0178] εi0 indicates the logarithmic strain at the time of rolling reduction, “t” indicates the cumulative time (s) up right before the cooling of the pass, and T indicates the rolling temperature (° C.) at the pass[(A4) End Temperature of Finish Rolling: Ar3+30° C. or More]

[0179] In the present method of production, the end temperature of the finish rolling has to be Ar3+30° C. or more. If the end temperature of the finish rolling is less than Ar3+30° C., if ferrite is formed in part of the structure due to variations in the constituents and rolling temperature in the steel sheet, the ferrite is liable to be worked. The worked ferrite sometimes becomes a cause of a drop in the ductility. In addition, if the end temperature of the finish rolling is less than Ar3+30° C., sometimes the ratio of crystal grains with an orientation difference in grains of 5 to 14° will become more than 60% or excessively high. In the present method of production, Ar3 (° C.) is found by the following formula 4 based on the chemical composition of the hot rolled steel sheet:Ar⁢3=901-325×[C]+33×[Si]+287×[P]+40×[sol. Al]-92×([Mn]+[Mo]+[Cu])-46×([Cr]+[Ni])formula⁢ 4

[0180] where [C], [Si], [P], [sol. Al], [Mn], [Mo], [Cu], [Cr], and [Ni] are the contents (mass %) of the elements in the steel. Cases where the elements are not contained are indicated as 0.[(B) Cooling Step]

[0181] In the present method of production, the finish rolled steel sheet is cooled in two stages in the next cooling step. Specifically, first, the finish rolled steel sheet is primary cooled by an average cooling speed of 10° C. / s or more down to a temperature region of 650 to 750° C., is held at the temperature region for 3.0 to 10.0 seconds, then is secondary cooled down to 100° C. or less by an average cooling speed of 30° C. / s or more. By combining with the conditions of (A2) and (A3), etc., in the hot rolling step and performing such two stage cooling, transformation by paraequilibrium occurs at the desired relatively low temperature region and thereby it becomes possible to reliably control the ratio of crystal grains with an orientation difference in grains of 5 to 14° to within a range of, by area %, 10 to 60%. As opposed to this, if the average cooling speed of the primary cooling is less than 10° C. / s or the cooling stop temperature of primary cooling is more than 750° C., transformation occurs by paraequilibrium at a relatively high temperature and the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. Further, if the cooling stop temperature of primary cooling is less than 650° C., transformation occurs by paraequilibrium at a temperature lower than the desired temperature region and similarly the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. Furthermore, even if the holding time at 650 to 750° C. is less than 3.0 seconds, similarly the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. On the other hand, if the holding time at 650 to 750° C. becomes more than 10.0 seconds or the average cooling speed of the secondary cooling is less than 30° C. / s, the cementite harmful to the stretch flangeability is easily formed. Further, if the cooling stop temperature of the secondary cooling is more than 100° C., the area ratio of martensite becomes less than 2%. The upper limits of the average cooling speeds of the primary and secondary cooling are not particularly prescribed, but, for example, the average cooling speeds of the primary and secondary cooling may be 200° C. / s or less considering the capacity of the cooling facilities.

[0182] According to the hot rolled steel sheet produced by the above method of production, it is possible to obtain a microstructure containing, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10% and, when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, having a ratio of the crystal grains with an orientation difference in the grains of 5 to 14°, by area %, 10 to 60%. As a result, despite being high in strength, it becomes possible to remarkably improve the stretch flangeability, ductility, and notch fatigue property. In addition, in the obtained hot rolled steel sheet, since the surface roughness Ra is controlled to less than 1.50 μm and the difference of the maximum value and minimum value in the surface roughness Ra is controlled to 0.50 μm or less, it is possible to remarkably suppress the formation of the Si scale pattern and, along with this, achieve an excellent post-painting corrosion resistance. Therefore, according to the hot rolled steel sheet produced by the above method of production, it is possible to reliably achieve both the contradictory properties of high strength and excellent workability. Further, the steel sheet is also excellent in post-painting corrosion resistance. Therefore, it is particularly useful in usage in the automobile field where these properties are sought.

[0183] Below, examples will be illustrated to explain the present invention more specifically, but the present invention is not limited to these examples in any way.EXAMPLES

[0184] In the following examples, steel sheets according to one embodiment of the present invention were produced under various conditions and the obtained steel sheets were investigated for tensile strength, stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance.

[0185] First, slabs obtained by casting molten steel by the continuous casting method and having the various chemical compositions shown in Tables 1 and 2 were formed. These slabs were heated under the conditions shown in Table 3, then were hot rolled. The hot rolling was performed by rough rolling, high pressure water descaling, and finish rolling. The rough rolling was performed under the same conditions in all of the invention examples and comparative examples. In each, the high pressure water descaling was performed using sprayed high pressure water with a water pressure of 15 MPa. The highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before finish rolling was as shown in Table 3. Further, the finish rolling was performed using a tandem rolling machine comprised of five rolling stands. The cumulative strain (εeff.) of the later three stages of finish rolling and the end temperature of the finish rolling were as shown in Table 3. Next, each finish rolled steel sheet was primary cooled and secondary cooled under the conditions shown in Table 3 to obtain a hot rolled steel sheet having a sheet thickness of 2.9 mm.TABLE 1Steel Chemical composition (mass %), balance: Fe and impuritiesno.CSiMnTisol.A1PSNONbVCrNiCuA0.0420.2511.080.1210.2830.0120.0020.00330.00280.0160.09B0.0480.2911.360.1550.3200.0120.0030.00280.00300.0150.10C0.0680.1201.070.1220.0720.0150.0020.00290.0032D0.0610.3081.210.1100.2370.0110.0020.00240.00270.12E0.0260.4931.080.1100.3010.0130.0030.00280.0031F0.0481.5411.040.0970.5510.0180.0030.00300.00300.0170.09G0.0521.8421.080.1020.0350.0120.0030.00300.00330.020H0.0460.1071.860.0990.3440.0140.0020.00290.00310.022I0.0490.3040.680.1010.1860.0190.0030.00310.00330.0210.11J0.0510.2641.110.0980.8200.0100.0030.00340.0036K0.0370.2601.030.1920.3210.0150.0020.00340.00340.120.420.08L0.0470.1650.990.0160.2700.0160.0020.00240.00260.0390.08M0.0410.1361.230.1240.3520.0160.0020.00240.00270.090N0.0420.1480.970.1020.2360.0190.0020.00300.00300.0180.32O0.0410.3210.990.1100.3240.0140.0030.00270.00260.09P0.0810.3401.020.1520.3500.0140.0030.00290.00350.0230.10Q0.0191.0121.040.1050.3180.0120.0030.00290.00290.0210.10R0.0420.4402.340.1200.2490.0200.0030.00260.00330.018S0.0511.0410.480.1120.0220.0200.0020.00300.00280.0180.21T0.0430.1460.990.1011.1670.0180.0020.00260.00270.0210.11U0.0451.7071.210.1500.9250.0080.0030.00290.0031V0.0360.1021.350.2090.2140.0150.0020.00350.00340.019W0.0480.7081.240.0120.2050.0210.0020.00250.00330.020Underlines indicate outside scope of present invention.TABLE 2Chemical composition (mass %), balance: Fe and impuritiesSteelSi + sol. no.MoBSbCaMgHfREMBiAsZrCoZnWSnA1A0.534B0.611C0.08700.192D0.0020.545E0.00300.794F0.152.092G0.181.877H0.020.451I0.030.490J0.00110.00071.084K0.581L0.435M0.040.488N0.1060.00220.384O0.00430.00210.645P0.690Q1.330R0.689S1.063T1.313V0.316W0.913Underlines indicate outside scope of present invention.TABLE 3Hot rolling stepCooling stepHighest heating temp. from endof rough rolling Secondary Solu-to start of high FinishPrimary coolingcoolingbilizationSlabpressure water rollingAverageCooling650~AverageCoolingtemp.heatingdescaling before Ar3 + end coolingstop 750° C.coolingstop TestSteelSRTmintemp.Tfinish rolling30temp.speedtemp.holdingspeedtemp.no.no.(° C. )(° C. )(° C. )(° C. )εeff.(° C. )(° C. )(° C. )(° C. )time (s)(° C. / s)(° C. )Remarks 1A11151223108011250.56837891497295.23567Inv. ex. 2B11621228108811150.54812920756764.44241Inv. ex. 3B11621232108811120.54812954686529.75452Inv. ex. 4B11621165108811100.51812914587104.74763Comp. ex. 5B11621233108810800.53812919487074.25367Comp. ex. 6B11621229108811160.63812919447114.54266Comp. ex. 7B11621232108811140.48812901616964.54536Comp. ex. 8B11621219108811200.56812811577344  4562Comp. ex. 9B11621222108811240.57812891 87384.14572Comp. ex.10B11621222108811230.57812910457587.14537Comp. ex.11B11621233108811250.57812917346444.14645Comp. ex.12B11621230108811280.54812906307262.84242Comp. ex.13B11621248108811290.54812904507264.659105 Comp. ex.14C11761235101610620.56822905386975.25565Inv. ex.15C11761256101611300.55822952487354  5284Inv. ex.16C11761250101611250.55822980726533.55690Inv. ex.17D11491229108410900.56817966546884.64255Inv. ex.18E10491225110711280.54855874637094.33342Inv. ex.19F11041208119011980.55894898517226.25457Inv. ex.20G11201237116511700.57880916697035.95152Inv. ex.21G11201242116511720.56880885357309.55250Inv. ex.22H11021221105210820.56766904726804.44435Inv. ex.23I11121226108010920.54870888407114.55750Inv. ex.24J11131247111911350.54857949727415.25549Inv. ex.25K11561253108511120.53818910757084.34230Inv. ex.26L 9191197106311350.56842895566834.34029Inv. ex.27M11151221106311150.53828894397104.94034Inv. ex.28N10951227105411350.56823955747094.35541Inv. ex.29O11011254109211400.57850904557153.74545Inv. ex.30P12291238109611160.54835906277155.54056Comp. ex.31Q10121231114511520.54874890447246.44367Comp. ex.32R11141234109911020.57732919787255.43455Comp. ex.33S11291208109411270.56902917506754.34660Comp. ex.34T109612161146Cracks formed during rollingComp. ex.35U11501270121312150.54901952467074.25352Comp. ex.36V11631209103110850.55811920667054.13634Comp. ex.37W 8961199111811240.56839935487176.34151Comp. ex.Underlines indicate outside scope of present invention or production conditions not preferable.The properties of each obtained steel sheet were measured and evaluated by the following[Tensile Strength (TS) and Total Elongation (El)]The tensile strength (TS) and total elongation (El) were measured by taking a JIS No. 5 test piece from an orientation (C direction) of the long direction of the test piece becoming parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.[Evaluation of Stretch Flangeability]

[0188] The stretch flangeability was evaluated by the saddle shaped stretch flangeability test method using a saddle shaped article. Specifically, a saddle shaped article simulating a stretched flange shape comprised of a straight part and curved part such as shown in FIG. 1 was press-formed. The stretch flangeability was evaluated by the limit shaped height at that time. In the saddle shaped stretch flangeability test method, a saddle shaped article having a radius of curvature R of the corners of 50 to 60 mm and an opening angle θ of 120° was used to measure the limit shaped height H (mm) when making the clearance 11% at the time of punching the corner parts. Here, the “clearance” indicates the ratio of the gap between the punching die and punch and the thickness of the test piece. The clearance is actually determined by the combination of the punching tool and sheet thickness, so “11%” means the 10.5 to 11.5% range is satisfied. The limit shaped height H was judged by examining for the presence of any cracks having lengths of ⅓ or more of the sheet thickness by visual observation after shaping. This was made the limit shaped height with no cracks present. The product (TS×H) of the tensile strength TS (MPa) and limit shaped height H (mm) was evaluated as an indicator of the stretch flangeability. If TS×H≥19500 MPa·mm, it was evaluated that the stretch flangeability was improved.[Evaluation of Ductility]

[0189] If the product of TS (MPa) and El (%) (TS×El) satisfies TS×El≥13500 MPa·%, it was evaluated that the ductility was improved.[Evaluation of Notch Fatigue Property]

[0190] The notch fatigue property was evaluated as follows: Specifically, a fatigue test piece of the shape shown in FIG. 2 was taken from a position similar to the position of taking the tensile test piece so that the orientation (C direction) parallel to the rolling perpendicular direction became the long side. The fatigue test piece was ground down to a depth of 0.05 mm or so from the surface most layer. A stress controlled fatigue test was performed by a stress ratio R=0.1 and frequency 5 Hz. The stress with no breakage after 10 million cycles was defined as the notch fatigue limit (FL) and used for evaluation of the notch fatigue property. If as a result of the test, FL / TS≤0.25 was satisfied, it was evaluated that the notch fatigue property was improved.[Evaluation of Chemical Convertibility]

[0191] The chemical convertibility was evaluated in the following way. Specifically, first, the produced hot rolled steel sheet was pickled, then was treated by phosphate conversion forming a 2.5 g / m2 zinc phosphate coating. At this stage, the chemical convertibility was evaluated by the presence of bald spots and measurement of the P ratio. A “bald spot” means a part where no chemical conversion coating is attached. The “P ratio” means the value shown by P / (P+H) of the X-ray diffraction strength P of the (100) plane of phosphophyllite (FeZn2(PO4)2·4H2O) measured using an X-ray diffraction apparatus and the X-ray diffraction strength H of the (020) plane of the hopite (Zn3(PO4)2·4H2O).

[0192] The phosphate conversion treatment is treatment using a chemical mainly comprised of phosphoric acid and Zn ions. It is a chemical reaction forming crystals called phosphophyllite with the Fe ions eluted from the steel sheet. In phosphate conversion treatment, (1) causing elution of Fe ions and promoting a reaction and (2) densely forming phosphophyllite crystals at the steel sheet surface are important. In particular, regarding (1), if oxides due to formation of Si scale at the steel sheet surface remain, elution of Fe is obstructed and bald spots appear or Fe is not eluted and hopite or another abnormal chemical conversion coating not containing Fe such as not inherently forming due to Fe not being eluted and, as a result, the post-painting corrosion resistance falls. The presence of any bald spots was judged by examination under a scan type electron microscope. Specifically, about 20 fields were examined by a power of 1000×. The case where oxides uniformly became attached to the entire surface and no bald spots could be confirmed was evaluated as “A” measuring no bald spots. Further, the case where the fields where bald spots are confirmed is 5% or less was evaluated as “B”. The case of more than 5% was evaluated as “C” meaning presence of bald spots.

[0193] On the other hand, the P ratio was measured using an X-ray diffraction apparatus. The P ratio expresses the ratio between the hopite and phosphophyllite in the coating obtained by performing phosphate conversion, therefore the higher the P ratio, the greater the phosphophyllite included and the more densely phosphophyllite crystals are formed at the steel sheet surface. Therefore, if the P ratio is 0.80 or more and evaluation of bald spots is A or B, the steel sheet was evaluated as being excellent in chemical convertibility.[Evaluation of Post-Painting Corrosion Resistance]

[0194] The post-painting corrosion resistance was evaluated in the following way. Specifically first, the chemically converted steel sheet was painted by electrodeposition to a 25 μm thickness, then the paint was baked on at 170° C.×20 minutes. A sharp edged knife was used to make a length 130 mm cut into the electrodeposited coating until reaching the base iron (base material). This steel sheet was sprayed with 5% salt water at a 35° C. temperature under the salt spray conditions shown in JIS Z 2371: 2015 continuously for 700 hours. After spraying by salt water, the cut part was covered by a width 24 mm tape (Nichiban 405A-24 JIS Z 1522: 2009) parallel to the cut part to a 130 mm length. The maximum paint peeling width when peeling this off was measured. If the maximum paint peeling width was 4.0 mm or less, the steel sheet was evaluated as excellent in post-painting corrosion resistance.

[0195] If the tensile strength TS was 540 MPa or more and TS×H≥t19500 MPa·mm, TS×El≥13500 MPa·%, FI / TS≥0.25, and the greatest paint peeling width≤4.0 mm, the steel sheet was evaluated as a hot rolled steel sheet which is high in strength, yet improved in stretch flangeability, ductility, and notch fatigue property and excellent in post-painting corrosion resistance. The results are shown inTABLE 4Ferrite Ratio of Difference and crystal grainsof bainite with orientation maximum Ferrite Bainite total Martensite difference invalueand area area area areagrains of minimumTest Steel ratioratioratioratio5 to 14°Ravalue in Rano.no.(%)(%)(%)(%)(%)(μm)(μm)Remarks 1A7813915230.850.21Inv. ex. 2B2462864210.860.34Inv. ex. 3B7814927380.840.31Inv. ex. 4B2663895271.230.55Comp. ex. 5B2467915312.121.62Comp. ex. 6B2466908 70.850.20Comp. ex. 7B2865935 90.870.23Comp. ex. 8B6819877710.910.15Comp. ex. 9B83 8915 80.820.28Comp. ex.10B6127887 60.840.16Comp. ex.11B1672886 60.780.21Comp. ex.12B1274867 70.790.24Comp. ex.13B2968971200.720.29Comp. ex.14C12758710 150.690.20Inv. ex.15C1182936180.420.09Inv. ex.16C 0878710 120.410.09Inv. ex.17D2168895230.940.09Inv. ex.18E7025952190.880.18Inv. ex.19F3355888171.230.30Inv. ex.20G81 7885191.420.45Inv. ex.21G87 0878131.350.31Inv. ex.22H10798910 201.030.20Inv. ex.23I6526912250.960.24Inv. ex.24J7517923160.850.29Inv. ex.25K2364874250.940.17Inv. ex.26L2764915271.000.19Inv. ex.27M2662884201.000.26Inv. ex.28N2263859251.020.20Inv. ex.29O1570855220.880.19Inv. ex.30P17658213 240.910.21Comp. ex.31Q87 9961230.840.20Comp. ex.32R 9647322 110.900.16Comp. ex.33S905953241.020.18Comp. ex.34TCracks formed during rollingComp. ex.35U91 6971301.380.48Comp. ex.36V2166875220.790.27Comp. ex.37W7218909220.780.27Comp. ex.Underlines indicate outside scope of present invention or production conditions not preferable.TABLE 5Post-paintingcorrosion Notch resistanceTensile Total Limit fatigueChemical Maximum strengthelongation shape TS × El T × SH limit convertibility peeling Test Steel TS El height H(MPa ·(MPa ·FL FL / Bald P width no.no.(MPa) (%) (mm)%)mm)(MPa) TSspots ratio(mm) Remarks 1A 61129.239.2 17841 23951 220 0.36 A 0.951.4 Inv. ex. 2B 806 20.8 26.9 16765 21681 251 0.31 A 0.911.3Inv. ex. 3 B 624 27.1 35.6 16910 22214 200 0.32 A 0.91 1.2 Inv. ex. 4 B 794 18.3 26.9 14530 21359 235 0.30 C0.75 4.9 Comp. ex. 5 B 798 18.3 27.1 14603 21626 236 0.30 C 0.76 4.9Comp. ex. 6 B 789 18.8 22.8 14833 17989 234 0.30 A 0.91 1.3Comp. ex. 7 B 802 18.4 22.4 14757 17965 235 0.29 A 0.93 1.1 Comp. ex. 8 B 613 21.6 31.9 13241 19555 217 0.35 A 0.96 1.1Comp. ex. 9 B 598 27.8 31.1 16624 18598 196 0.33 A 0.92 1.2Comp. ex.10B60530.6 30.9 18513 18695 210 0.35 A 0.971.7Comp. ex.11 B 61622.6 30.6 13922 18850 210 0.34 A 0.95 1.5Comp. ex.12 B 64228.6 28.2 18361 18104 214 0.33 A 0.91 1.5 Comp. ex.13 B 53232.3 37.3 17184 19844 128 0.24 A 0.97 1.4 Comp. ex.14 C 84724.1 25.1 20413 21260 260 0.31 A 0.971.4 Inv. ex.15 C 845 24.3 25.0 20534 21125 260 0.31 A 0.98 1.1nv. ex16 C 885 20.1 23.9 17789 21152 275 0.31 A 0.98 1.1 Inv. ex17 D 869 25.1 26.1 21812 22681 270 0.31 A 0.93 1.4Inv. ex18 E 587 32.2 38.5 18901 22600 172 0.29 A 0.931.6 Inv. ex19 F 807 19.2 26.5 15494 21386 241 0.30 A 0.87 1.6Inv. ex20 G 632 26.9 33.8 17001 21362 198 0.31B0.82 1.1Inv. ex21 G 609 29.0 35.2 17661 21437 202 0.33 B 0.85 1.1Inv. ex22 H 829 18.5 24.1 15337 19979 262 0.32 A 0.94 1.5 Inv. ex23 I 604 28.8 37.7 17395 22771 177 0.29 A 0.92 1.4Inv. ex24 J 725 21.8 30.3 15805 21968 218 0.30 A 0.95 1.3 Inv. ex25 K 83518.5 27.4 15448 22879 247 0.30 A 0.94 1.7 Inv. ex26 L 758 21.9 28.3 16600 21451 227 0.30 A 0.971.2Inv. ex27 M81119.0 28.2 15409 22870 242 0.30 A 0.96 1.3 Inv. ex28 N 815 18.9 27.6 15404 22494 243 0.30 A 0.91 1.7 Inv. ex29 O 832 18.7 27.4 15558 22797 246 0.30 A 0.911.3 Inv. ex30 P 856 18.7 19.7 16007 16863 262 0.31 A 0.95 1.7 Comp. ex31 Q 506 35.7 48.3 18064 24440 117 0.23 A 0.91 1.1 Comp. ex32 R 870 15.9 18.2 13833 15834 262 0.30A 0.96 1.3 Comp. ex33 S 531 34.2 44.5 18160 23630 201 0.38 A 0.95 1.1 Comp. ex34 T Cracks formed during rolling Comp. ex35 U 517 28.3 32.3 14631 16699 114 0.22 A 0.891.9Comp. ex36 V 802 15.1 27.0 12110 21654 251 0.31 A 0.941.1Comp. ex37 W59231.0 32.1 18352 19003 169 0.28 A 0.911.5 Comp. exUnderlines indicate outside scope of present invention or production conditions not preferable.Referring to Tables 1 to 5, in Comparative Example 4, the heating temperature of the slab was low, therefore removal of scale by high pressure water descaling became uneven and formation of the Si scale pattern could not be sufficiently suppressed. As a result, the difference of the maximum value and minimum value in the surface roughness Ra became more than 0.50 μm and the chemical convertibility and post-painting corrosion resistance fell. In Comparative Example 5, the highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before finish rolling was low, therefore it was not possible to sufficiently remove scale even by subsequent high pressure water descaling. As a result, the surface roughness Ra became 1.50 μm or more, the difference of the maximum value and minimum value in the surface roughness Ra became more than 0.50 μm, and the chemical convertibility and post-painting corrosion resistance fell. In Comparative Example 6, the cumulative strain (εeff.) of the later three stages of the finish rolling was high, therefore recrystallization of austenite occurred during hot rolling and it is believed the cumulative dislocation density fell at the time of transformation. As a result, the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 7, the εeff. was low, therefore it is believed the dislocation density of the austenite introduced was not sufficient. As a result, similarly the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 8, the end temperature of the finish rolling was low, therefore the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° exceeded 60% and the ductility fell. In Comparative Example 9, the average cooling speed of the primary cooling in the cooling step was low, therefore it is believed that transformation by paraequilibrium occurred at a relatively high temperature. As a result, the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 10, the cooling stop temperature of the primary cooling was high, therefore similarly it is believed that transformation by paraequilibrium occurred at a relatively high temperature. As a result, the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 11, the cooling stop temperature of the primary cooling was low, therefore it is believed that transformation by paraequilibrium occurred at a lower temperature than the desired temperature region. As a result, similarly the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 12, the holding time at 650 to 750° C. in the primary cooling was short, therefore similarly the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 13, the cooling stop temperature of the secondary cooling in the cooling step was high, therefore the area ratio of martensite became less than 2%. As a result, the TS and notch fatigue property fell.

[0197] In Comparative Examples 30 and 32, the C and Mn contents were high, therefore the stretch flangeability fell. In Comparative Examples 31 and 33, the C and Mn contents were low, therefore sufficient strength could not be obtained. In Comparative Example 34, the sol. Al content was high, therefore cracking occurred during rolling and the subsequent test could not be performed. In Comparative Example 35, the total content of Si and sol. Al was high, therefore formation of ferrite was promoted and the area ratio of martensite became less than 2%. As a result, the TS fell. In Comparative Example 36, the Ti content was high, therefore the carbides (TiC) became coarser and the ductility fell. In Comparative Example 37, the Ti content was low, therefore it is believed that formation of cementite could not be sufficiently suppressed and as a result the stretch flangeability fell.

[0198] In contrast to this, in the hot rolled steel sheets according to all of the invention examples, it was possible to obtain the hot rolled steel sheet having a predetermined chemical composition and a microstructure obtained by further suitably controlling the conditions in the method of production, the microstructure containing, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, wherein a ratio of the crystal grains with an orientation difference in the grains of 5 to 14° is, by area %, 10 to 60%, a surface roughness Ra is less than 1.50 pin, and the difference of the maximum value and minimum value in the surface roughness Ra is 0.50 μm or less. Further, as a result, despite having a high strength of a tensile strength of 540 MPa or more, it was possible to remarkably improve the stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance. Further, in all of the invention examples, the area ratio of the balance structures was 10% or less. In the presence of balanced structures, the balance structures included at least one of retained austenite and pearlite.

Examples

examples

[0184]In the following examples, steel sheets according to one embodiment of the present invention were produced under various conditions and the obtained steel sheets were investigated for tensile strength, stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance.

[0185]First, slabs obtained by casting molten steel by the continuous casting method and having the various chemical compositions shown in Tables 1 and 2 were formed. These slabs were heated under the conditions shown in Table 3, then were hot rolled. The hot rolling was performed by rough rolling, high pressure water descaling, and finish rolling. The rough rolling was performed under the same conditions in all of the invention examples and comparative examples. In each, the high pressure water descaling was performed using sprayed high pressure water with a water pressure of 15 MPa. The highest heating temperature from the completion of rough rolling to the start of high pressure w...

FIELD

[0001] The present invention relates to a hot rolled steel sheet.BACKGROUND

[0002] In recent years, in the automotive industry, lighter weight of car bodies has been sought from the viewpoint of improvement of fuel efficiency. To achieve both lighter weight of car bodies and collision safety, increasing the strength of the steel sheet used is one effective method. A high strength steel sheet is being developed from this background. On the other hand, along with higher strength, the workability of a steel sheet generally falls. For this reason, in development of a high strength steel sheet, it is important to secure a certain level or more of workability while raising the strength.

[0003] In relation to this, PTL 1 describes a hot rolled steel sheet having a predetermined chemical composition and having a structure containing, by area ratio, a total of 80 to 98% of ferrite, and bainite and 2 to 10% of martensite and, when deeming a boundary with an orientation difference of 15° or more in that structure as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, having a ratio of crystal grains with an orientation difference in the grains of 5 to 14° of an area ratio of 10 to 60%. Further, PTL 1 teaches that by making the ratio of crystal grains with an orientation difference in the grains of 5 to 14° an area ratio of 10 to 60%, it is possible to improve the stretch flangeability and ductility while being high in strength and further that by controlling the total area ratio of ferrite and bainite and the area ratio of martensite in the structure to within predetermined ranges, it is possible to improve the notch fatigue property.CITATIONS LISTPatent Literature

[0004] [PTL 1] WO20161133222SUMMARYTechnical Problem

[0005] For example, to make a steel sheet high in strength, sometimes Si is included in the steel sheet in a relatively large amount. However, if including Si in steel in a relatively large amount, sometimes a tiger stripe shaped scale pattern called “Si scale” is formed at the steel sheet surface. If such a scale pattern is formed, the chemical convertibility deteriorates and sometimes the post-painting corrosion resistance is lowered. For this reason, PTL 1 teaches that by limiting the Si content in the hot rolled steel sheet to 0.100% or less, a drop in post-painting corrosion resistance due to formation of the scale pattern is suppressed.

[0006] On the other hand, in the automobile industry, etc., further lighter weight of steel materials is being sought. To achieve such lighter weight, a need arises for further raising the strength of steel materials even more than up to now. Therefore, there is a high need for a high strength steel sheet which, even if containing a relatively large amount of Si for further raising the strength, is improved in stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance as described in PTL 1.

[0007] Therefore, the present invention has as its object the provision of a hot rolled steel sheet which, despite being high strength, is improved in stretch flangeability, ductility, and notch fatigue property and is excellent in post-painting corrosion resistance.Solution to Problem

[0008] The inventors engaged in studies to achieve the above object focusing in particular on the microstructure and surface properties of a hot rolled steel sheet. As a result, the inventors discovered that by configuring the microstructure of a hot rolled steel sheet having a predetermined chemical composition by at least one of ferrite and bainite and martensite in specific ratios and further controlling the ratio of crystal grains to within a predetermined range, it is possible to improve the stretch flangeability, ductility, and notch fatigue property and that by additionally controlling the surface roughness Ra of the steel sheet and its variation to within predetermined ranges, it is possible to improve the post-painting corrosion resistance and thereby completed the present invention.

[0009] The present invention able to achieve this object is as follows:

[0010] (1) A hot rolled steel sheet having a chemical composition comprising, by mass %,

[0011] C: 0.020 to 0.070%,

[0012] Si: more than 0.100 to 2.000%,

[0013] Mn: 0.60 to 2.00%,

[0014] Ti: 0.015 to 0.200%,

[0015] sol. Al: 0.010 to 1.000%,

[0016] P: 0.100% or less,

[0017] S: 0.030% or less,

[0018] N: 0.0060% or less,

[0019] O: 0.0100% or less,

[0020] Nb: 0 to 0.050%,

[0021] V: 0 to 0.300%,

[0022] Cr: 0 to 2.00%,

[0023] Ni: 0 to 2.00%,

[0024] Cu: 0 to 2.00%,

[0025] Mo: 0 to 1.000%,

[0026] B: 0 to 0.0100%,

[0027] Sb: 0 to 1.00%,

[0028] Ca: 0 to 0.0100%,

[0029] Mg: 0 to 0.0100%,

[0030] Hf: 0 to 0.0100%,

[0031] REM: 0 to 0.1000%,

[0032] Bi: 0 to 0.0100%,

[0033] As: 0 to 0.0100%,

[0034] Zr: 0 to 1.00%,

[0035] Co: 0 to 1.00%,

[0036] Zn: 0 to 1.00%,

[0037] W: 0 to 1.00%,

[0038] Sn: 0 to 1.00%, and

[0039] balance: Fe, and impurities, and

[0040] satisfying 0.110<[Si]+[sol. Al]≤2.500, wherein [Si] and [sol. Al] are the contents (mass %) of the elements, and

[0041] a microstructure comprising, by area %,

[0042] at least one of ferrite and bainite: 80 to 98% in total, and

[0043] martensite: 2 to 10%, wherein

[0044] when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area %, 10 to 60%, and

[0045] a surface roughness Ra is less than 1.50 μm and a difference of a maximum value and minimum value in the surface roughness Ra is 0.50 μm or less.

[0046] (2) The hot rolled steel sheet according to the above (1), wherein the chemical composition contains, by mass %, at least one of

[0047] Nb: 0.001 to 0.050%,

[0048] V: 0.001 to 0.300%,

[0049] Cr: 0.01 to 2.00%,

[0050] Ni: 0.01 to 2.00%,

[0051] Cu: 0.01 to 2.00%,

[0052] Mo: 0.001 to 1.000%,

[0053] B: 0.0001 to 0.0100%,

[0054] Sb: 0.01 to 1.00%,

[0055] Ca: 0.0001 to 0.0100%,

[0056] Mg: 0.0001 to 0.0100%,

[0057] Hf: 0.0001 to 0.0100%,

[0058] REM: 0.0001 to 0.1000%,

[0059] Bi: 0.0001 to 0.0100%,

[0060] As: 0.0001 to 0.0100%,

[0061] Zr: 0.01 to 1.00%,

[0062] Co: 0.01 to 1.00%,

[0063] Zn: 0.01 to 1.00%,

[0064] W: 0.01 to 1.00%, and

[0065] Sn: 0.01 to 1.00%.

[0066] (3) The hot rolled steel sheet according to the above (1) or (2), wherein the hot rolled steel sheet is a painted steel sheet provided with a paint layer on at least one surface.

[0067] (4) A part including the hot rolled steel sheet according to any one of the above (1) to (3).ADVANTAGEOUS EFFECTS OF INVENTION

[0068] According to the present invention, it is possible to provide a hot rolled steel sheet which, despite being high strength, is improved in stretch flangeability, ductility, and notch fatigue property and is excellent in post-painting corrosion resistance.BRIEF DESCRIPTION OF DRAWINGS

[0069] FIG. 1 is a view showing a shape of a saddle shaped article used in a saddle type stretch flanging test method.

[0070] FIG. 2 is a view showing a shape of a fatigue test piece used for evaluating a notch fatigue property.DESCRIPTION OF EMBODIMENTS<Hot Rolled Steel Sheet>

[0071] The hot rolled steel sheet according to an embodiment of the present invention is characterized by having a chemical composition comprising, by mass %,

[0072] C: 0.020 to 0.070%,

[0073] Si: more than 0.100 to 2.000%,

[0074] Mn: 0.60 to 2.00%,

[0075] Ti: 0.015 to 0.200%,

[0076] sol. Al: 0.010 to 1.000%,

[0077] P: 0.100% or less,

[0078] S: 0.030% or less,

[0079] N: 0.0060% or less,

[0080] O: 0.0100% or less,

[0081] Nb: 0 to 0.050%,

[0082] V: 0 to 0.300%,

[0083] Cr: 0 to 2.00%,

[0084] Ni: 0 to 2.00%,

[0085] Cu: 0 to 2.00%,

[0086] Mo: 0 to 1.000%,

[0087] B: 0 to 0.0100%,

[0088] Sb: 0 to 1.00%,

[0089] Ca: 0 to 0.0100%,

[0090] Mg: 0 to 0.0100%,

[0091] Hf: 0 to 0.0100%,

[0092] REM: 0 to 0.1000%,

[0093] Bi: 0 to 0.0100%,

[0094] As: 0 to 0.0100%,

[0095] Zr: 0 to 1.00%,

[0096] Co: 0 to 1.00%,

[0097] Zn: 0 to 1.00%,

[0098] W: 0 to 1.00%,

[0099] Sn: 0 to 1.00%, and

[0100] balance: Fe, and impurities, and

[0101] satisfying 0.110<[Si]+[sol. Al]≤2.500, wherein [Si] and [sol. Al] are the contents (mass %) of the elements, and

[0102] a microstructure comprising, by area %,

[0103] at least one of ferrite and bainite: 80 to 98% in total, and

[0104] martensite: 2 to 10%, wherein

[0105] when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area %, 10 to 60%, and

[0106] a surface roughness Ra is less than 1.50 μm and a difference of a maximum value and minimum value in the surface roughness Ra is 0.50 μm or less.

[0107] As explained above, it is known that the stretch flangeability and other properties fall along with higher strength of a steel sheet and that due to the formation of a scale pattern due to Si scale (below, also referred to as an “Si scale pattern”), sometimes the post-painting corrosion resistance falls. First, in an embodiment of the present invention, by making the microstructure of the hot rolled steel sheet having a predetermined chemical composition include at least one of ferrite and bainite and martensite in specific ratios, more specifically by making it include, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, it is possible to improve the strength and the stretch flangeability, ductility, and notch fatigue property with a good balance. In addition, if defining a region surrounded by grain boundaries with an orientation difference of 15° or more and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, crystal grains with an orientation difference in grains of 5 to 14° are effective for improving the strength and the stretch flangeability and ductility. For this reason, by suitably controlling the ratios of the crystal grains, more specifically by controlling it to within a range of, by area %, 10 to 60%, it becomes possible to further improve the balance of the strength and the stretch flangeability and ductility.

[0108] On the other hand, if explaining post-painting corrosion resistance in more detail, a steel sheet is generally treated by zinc phosphate or other chemical conversion, then painted. In chemical conversion, a chemical reaction is caused between the treatment solution and the Fe eluted from the steel sheet to cause the formation of a dense chemical conversion coating comprised of chemically converted crystals on the steel sheet surface and thereby improve the post-painting corrosion resistance. However, if the steel sheet surface has oxides derived from the formation of the Si scale pattern remaining on it, elution of Fe is obstructed and portions called “bald spots” where the chemical conversion coating is not formed appear or Fe is not eluted and therefore a chemical conversion coating not including Fe which originally should not be formed is formed and as a result, sometimes the post-painting corrosion resistance falls. Therefore, the inventors engaged in studies to realize improvement of the post-painting corrosion resistance by suppressing the formation of the Si scale pattern in addition to improvement of the stretch flangeability, ductility, and notch fatigue property, in particular focusing on the surface properties of a hot rolled steel sheet. As a result, the inventors discovered that by controlling the surface roughness Ra of the hot rolled steel sheet to less than 1.50 μm and controlling the difference of the maximum value and minimum value in the surface roughness Ra to 0.50 μm or less, it is possible to suppress the formation of the Si scale pattern and thereby possible to remarkably improve the post-painting corrosion resistance of the hot rolled steel sheet.

[0109] Explained more specifically, in a steel sheet containing a relatively large amount of Si, Si oxides are formed at the interface of the Fe oxides (also called “scale”) formed at the steel sheet surface at the time of the hot rolling step and the steel. The Si oxides cause the Fe oxides to be strongly attached to the steel, therefore sometimes the scale cannot be sufficiently removed even by subsequent descaling using high pressure water, etc. In such a case, the scale not sufficiently removed is pressed into the steel sheet surface due to the subsequent finish rolling or other rolling. As a result, asperities are formed at the steel sheet surface and the surface properties deteriorate. Due to such poor descaling and deterioration of the surface properties due to the subsequent rolling, the Si scale pattern is formed at the hot rolled steel sheet after pickling.

[0110] Therefore, the inventors discovered that, as explained later in detail relating to the method of production of a hot rolled steel sheet, by suitably controlling the temperature conditions between the rough rolling and the finish rolling, it is possible to sufficiently or completely remove scale by descaling before finish rolling and that, along with this, it is possible to remarkably keep scale from being pressed into the steel sheet surface and forming asperities at the steel sheet surface at the time of the subsequent finish rolling. The inventors further studied the relationship between the surface properties of a steel sheet and formation of the Si scale pattern. As a result, the inventors discovered that by sufficiently or completely removing the scale by descaling before the finish rolling so that the surface roughness Ra of the finally obtained hot rolled steel sheet becomes less than 1.50 μm and the difference of the maximum value and minimum value in the surface roughness Ra becomes 0.50 μm or less, it is possible to remarkably suppress the formation of the Si scale pattern and along with that obtain a hot rolled steel sheet having excellent post-painting corrosion resistance. That is, according to an embodiment of the present invention, despite being high strength, for example, a high strength of a tensile strength of 540 MPa or more, it is possible to achieve an improvement of the stretch flangeability, ductility, and notch fatigue property and realize excellent post-painting corrosion resistance. Therefore, since the hot rolled steel sheet according to an embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent workability and is also excellent in post-painting corrosion resistance, it is particularly useful in use in the automobile field where achievement of both of these properties is sought.

[0111] Below, the hot rolled steel sheet according to an embodiment of the present invention will be explained in more detail. In the following explanation, the “%” of the units of contents of the elements, unless otherwise indicated, means “mass %”. Further, in this Description, the “to” showing a numerical range, unless otherwise indicated, is used in the sense of the numerical values described before and after the same being included as the lower limit value and the upper limit value.[C: 0.020 to 0.070%]

[0112] C is an element effective for raising the strength of a steel sheet. Further, C forms carbides and / or carbonitrides with Ti and Nb in the steel and also contributes to the precipitation strengthening based on the precipitates formed and the refinement of the structure by the pinning effect of the precipitates. To sufficiently obtain these effects, the C content is 0.020% or more. The C content may also be 0.022% or more, 0.025% or more, 0.028% or more, or 0.030% or more. On the other hand, if excessively containing C, sometimes the stretch flangeability and weldability fall. Therefore, the C content is 0.070% or less. The C content may also be 0.065% or less, 0.060% or less, 0.055% or less, or 0.050% or less.[Si: More than 0.100 to 2.000%]

[0113] Si is an element effective for raising strength as a solid solution strengthening element. To sufficiently obtain such an effect, the Si content is more than 0.100%. The Si content may 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 excessively containing Si, even if suitably controlling the surface roughness Ra of the steel sheet and the difference between the maximum value and minimum value, sometimes formation of the Si scale pattern cannot be sufficiently suppressed. Therefore, the Si content is 2.000% or less. The Si content may also be 1.800% or less, 1.600% or less, 1.400% or less, or 1.200% or less.[Mn: 0.60 to 2.00%]

[0114] Mn is an element effective for hardenability and for raising strength as a solid solution strengthening element. To sufficiently obtain these effects, the Mn content is 0.60% or more. The Mn content may also be 0.70% or more, 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if excessively containing Mn, sometimes the stretch flangeability falls. Therefore, the Mn content is 2.00% or less. The Mn content may also be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.[Ti: 0.015 to 0.200%]

[0115] Ti is an element finely precipitating in steel as a carbide (TiC) and improving the strength of the steel by precipitation strengthening. Further, Ti is an element forming carbides to fix C and suppress the formation of the cementite harmful to the stretch flangeability. To sufficiently obtain these effects, the Ti content is 0.015% or more. The Ti content may also be 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, if excessively containing Ti, the carbides become coarser and sometimes the ductility falls. Therefore, the Ti content is 0.200% or less. The Ti content may also be 0.180% or less, 0.170% or less, 0.150% or less, or 0.120% or less.[sol. Al: 0.010 to 1.000%]

[0116] sol. Al is an element acting as a deoxidizer of molten steel. To sufficiently obtain such an effect, the sol. Al content is 0.010% or more. The sol. Al content may also be 0.012% or more, 0.015% or more, or 0.020% or more. On the other hand, if excessively containing sol. Al, coarse oxides are formed and the toughness and ductility fall sometimes leading to fracture during rolling. Therefore, the sol. Al content is 1.000% or less. The sol. Al content may also be 0.800% or less, 0.600% or less, or 0.400% or less. Note that “sol. Al” means acid soluble Al and indicates the dissolved Al present in the steel in a dissolved state.[P: 0.100% or Less]

[0117] P, if excessively contained, sometimes disadvantageously affects the weldability, etc. Therefore, the P content is 0.100% or less. The P content may 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 prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the P content may also be 0.001% or more, 0.003% or more, or 0.005% or more.[S: 0.030% or Less]

[0118] S, if excessively contained, sometimes forms a large amount of MnS and lowers the toughness. Therefore, the Si content is 0.030% or less. The S content may 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 prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the S content may also be 0.001% or more, 0.002% or more, or 0.003% or more.[N: 0.0060% or Less]

[0119] N forms precipitates with Ti more preferentially than C and sometimes causes a reduction in the Ti effective for fixing the C. Therefore, the N content is 0.0060% or less. The N content may 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 prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the N content may also be 0.0001% or more or 0.0005% or more.[O: 0.0100% or Less]

[0120] O is an element entering in the production process. If excessively containing 0, coarse inclusions are formed and sometimes the toughness of the steel sheet is lowered. Therefore, the O content may also be 0.0100% or less. The O content may 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 prescribed and may also be 0%, but for reduction to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the O content may also be 0.0001% or more or 0.0005% or more.

[0121] The basic chemical composition of the hot rolled steel sheet according to an embodiment of the present invention is as explained above. Further, the hot rolled steel sheet may, in accordance with need, contain at least one of the following optional elements in place of part of the balance of Fe.[Nb: 0 to 0.050%]

[0122] Nb is an element forming carbides, nitrides, and / or carbonitrides in steel to contribute to refinement of the structure due to the pinning effect and in turn higher strength of the steel sheet. Further, Nb is an element forming carbides and / or carbonitrides to thereby fix C and suppress the formation of the cementite harmful to stretch flangeability. The Nb content may also be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may also be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes the ductility of the steel sheet falls. Therefore, the Nb content is preferably 0.050% or less. The Nb content may also be 0.040% or less, 0.030% or less, or 0.020% or less.[V: 0 to 0.300%]

[0123] V is an element contributing to improvement of strength by precipitation strengthening, etc. The V content may also be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if excessively including V, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the V content is preferably 0.300% or less. The V content may also be 0.200% or less, 0.100% or less, or 0.080% or less.[Cr: 0 to 2.00%]

[0124] Cr is an element raising the hardenability of steel and contributing to improvement of strength. The Cr content may also be 0%, but to obtain such an effect, the Cr content is preferably 0.01% or more. The Cr content may also be 0.03% or more or 0.05% or more. On the other hand, even if excessively including Cr, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the Cr content is preferably 2.00% or less. The Cr content may 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.[Ni: 0 to 2.00%][Cu: 0 to 2.00%]

[0125] Ni, and Cu are elements contributing to improvement of strength by precipitation strengthening or solid solution strengthening. The Ni and Cu contents may also be 0%, but to obtain such an effect, the contents of these elements are preferably 0.01% or more and may also be 0.03% or more or 0.05% or more. On the other hand, even if excessively including these elements, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the Ni and Cu contents are preferably respectively 2.00% or less and may 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.[Mo: 0 to 1.000%]

[0126] Mo is an element raising the hardenability of steel and contributing to improvement of the strength. The Mo content may also be 0%, but to obtain such an effect, the Mo content is preferably 0.001% or more. The Mo content may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if excessively containing Mo, the deformation resistance at the time of hot working increases and sometimes the load on the facilities becomes greater. Therefore, the Mo content is preferably 1.000% or less. The Mo content may also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.[B: 0 to 0.0100%]

[0127] B segregates at the grain boundaries and raises the intergranular strength to raise the low temperature toughness. The B content may also be 0%, but to obtain such an effect, the B content is preferably 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, even if excessively including B, the effect becomes saturated and a rise in production costs is liable to be invited. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.[Sb: 0 to 1.00%]

[0128] Sb is an element effective for improvement of the corrosion resistance. The Sb content may also be 0%, but to obtain such an effect, the Sb contents is preferably 0.01% or more. The Sb content may also be 0.02% or more or 0.05% or more. On the other hand, if excessively Sb, sometimes a drop in toughness is invited. Therefore, Sb content is preferably 1.00% or less. The Sb content may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.[Ca: 0 to 0.0100%][Mg: 0 to 0.0100%][Hf: 0 to 0.0100%]

[0129] Ca, Mg, and Hf are elements enabling control of the form of the nonmetallic inclusions. The Ca, Mg, and Hf contents may also be 0%, but to obtain such an effect, the contents of these elements are preferably respectively 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 excessively containing these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Ca, Mg, and Hf contents are preferably respectively 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.[REM: 0 to 0.1000%]

[0130] An REM is an element enabling control of the form of nonmetallic inclusion. The REM content may be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing a REM, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the REM content is preferably 0.1000% or less. The REM content may also be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The “REM” in this Description is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.[Bi: 0 to 0.0100%][As: 0 to 0.0100%]

[0131] Bi and As are elements effective for improvement of the corrosion resistance. The Bi and As contents may also be 0%, but to obtain such an effect, the contents of these elements are preferably respectively 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 excessively including these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Bi and As contents are preferably respectively 0.0100% or less and may also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.[Zr: 0 to 1.00%]

[0132] Zr is an element enabling control of the form of nonmetallic inclusions. The Zr content may also be 0%, but to obtain such an effect, the Zr content is preferably 0.01% or more. The Zr content may also be 0.05% or more or 0.10% or more. On the other hand, even if excessively containing Zr, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Zr content is preferably 1.00% or less. The Zr content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.[Co: 0 to 1.00%]

[0133] Co is an element contributing to improvement of the hardenability and / or heat resistance. The Co content may also be 0%, but to obtain these effects, the Co content is preferably 0.01% or more. The Co content may also be 0.05% or more or 0.10% or more. On the other hand, if excessively containing Co, the hot workability sometimes falls. This also leads to an increase in material costs. Therefore, the Co content is preferably 1.00% or less. The Co content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.[Zn: 0 to 1.00%]

[0134] Zn is an element effective for control of the shape of the inclusions. To obtain such an effect, the Zn content is preferably 0.01% or more. The Zn content may also be 0.05% or more or 0.10% or more. On the other hand, even if excessively including Zn, the effect becomes saturated and a rise in production costs is invited. Therefore, the Zn content is preferably 1.00% or less. The Zn content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.[W: 0 to 1.00%]

[0135] W is an element raising the hardenability of steel and contributes to improvement of strength. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.01% or more. The W content may also be 0.05% or more or 0.10% or more. On the other hand, if excessively containing W, sometimes the weldability falls. Therefore, the W content is preferably 1.00% or less. The W content may also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.[Sn: 0 to 1.00%]

[0136] Sn is an element effective for improvement of the corrosion resistance. The Sn content may also be 0%, but to obtain such an effect, the Sn content is preferably 0.01% or more. The Sn content may also be 0.02% or more or 0.05% or more. On the other hand, if excessively Sn, sometimes a drop in toughness is invited. Therefore, Sn content is preferably 1.00% or less. The Sn content may also be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[0137] In the hot rolled steel sheet according to an embodiment of the present invention, the balance aside from the above elements is comprised of Fe and impurities. The “impurities” are constituents, etc., entering due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing the hot rolled steel sheet.[0.110<[Si]+[sol. Al]≤2.500]

[0138] The chemical composition of the hot rolled steel sheet according to an embodiment of the present invention has to satisfy the following formula:0.11<[Si]+[sol. Al]≤2.5

[0139] wherein [Si] and [sol. Al] are the contents (mass %) of the elements. As explained previously, if defining a region surrounded by boundaries with orientation differences of 15° or more and with a circle equivalent diameter of 0.3 μm or more as a crystal grain, crystal grains with orientation differences in the grains of 5 to 14° are effective for improvement of the strength and stretch flangeability. For this reason, in the hot rolled steel sheet according to an embodiment of the present invention, as explained in detail later, by controlling the ratio of these crystal grains to within a range of, by area %, 10 to 60%, the balance of the strength and stretch flangeability is improved. Si and sol. Al are elements effective for controlling the ratio of crystal grains with an orientation difference in the grains of 5 to 14° to within a range of 10 to 60% in addition to the effects explained regarding the individual elements. This is believed to be because by inclusion of Si and sol. Al, the temperature of the Ar3 point rises and the transformation strain introduced into the grains becomes smaller. To sufficiently obtain these effects, the chemical composition of the hot rolled steel sheet according to an embodiment of the present invention is controlled so that the total contents of Si and sol. Al becomes more than 0.110%, i.e., satisfies [Si]+[sol. Al]>0.110. From the viewpoint of further enhancing these effects, the total content of Si and sol. Al is preferably 0.120% or more and may also 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, formation of ferrite is promoted and sometimes the strength falls. Therefore, the total content of Si and sol. Al is 2.500% or less, i.e., [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.

[0140] The chemical composition of the hot rolled steel sheet according to an embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the hot rolled steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-nondispersive type infrared absorption method.[Microstructure][At Least One of Ferrite and Bainite: 80 to 98% in Total and Martensite: 2 to 10%]

[0141] The microstructure of the hot rolled steel sheet according to an embodiment of the present invention includes, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%. By the microstructure of the hot rolled steel sheet being comprised of these structure, it is possible to improve the strength, stretch flangeability, ductility, and notch fatigue property with a good balance. If the total area ratio of the at least one of ferrite and bainite is low or the area ratio of the martensite is high, in particular the balance of the strength and stretch flangeability falls and the desired properties cannot be obtained. Therefore, the total area ratio of the at least one of ferrite, and bainite is 80% or more and, for example, may also be 82% or more, 85% or more, 88% or more, or 90% or more. Similarly, the area ratio of martensite is 10% or less and, for example, may 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 the at least one of ferrite and bainite is high or the area ratio of martensite is low, sometimes in particular the balance of strength and notch fatigue property fall and the desired properties cannot be obtained. Therefore, the total area ratio of the at least one of ferrite and bainite is 98% or less and, for example, may also be 96% or less, 94% or less, or 92% or less. Similarly, the area ratio of martensite is 2% or more and may also be, for example, 3% or more, 4% or more, or 5% or more.

[0142] The microstructure of the hot rolled steel sheet may contain either of ferrite and bainite, preferably includes both of ferrite and bainite. Therefore, either of the area ratios of ferrite and bainite may be 0%, for example, they may also respectively be 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more. Similarly, the area ratios of ferrite and bainite may also be, for example, respectively 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less. From the viewpoint of improvement of the ductility of the hot rolled steel sheet, the area ratio of bainite is preferably 80% or less, more preferably is 70% or less.[Balance Structures]

[0143] The balance structures other than the ferrite, bainite, and martensite may also be an area % of 0%, but if there are balance structures present, the balance structures may be at least one of retained austenite and pearlite. The area ratio of the balance structures is not particularly limited, but, for example, may be 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flangeability, the area ratio of the balance structures is preferably, for example, 10% or less and may also be 8% or less, 6% or less, or 5% or less.[Identification of Microstructure and Calculation of Area Ratios]

[0144] The microstructure at the hot rolled steel sheet is identified and the area ratios are calculated by observation under an optical microscope after corrosion using a Nital reagent or LePera solution and by X-ray diffraction. The structure is observed under an optical microscope at a sheet thickness cross-section in a direction parallel to the rolling direction and vertical to the sheet surface. Specifically, first, a sample is taken from the hot rolled steel sheet and the observed surface of the sample is etched by Nital. Next, an optical microscope is used to obtain a structural photograph of a 300 μm×300 μm field at a ¼ depth position in sheet thickness. The image is analyzed to calculate the area ratios of the ferrite and pearlite and the total area ratio of bainite and martensite. Crystal grains not including sub structures equiaxially can be identified as ferrite while those including sub structures can be identified as bainite and martensite. Next, the observed surface of the sample is corroded by a LePera solution. In the same way, an optical microscope is used to obtain a structural photograph of a 300 μm×300 μm field at a ¼ depth position in sheet thickness. The image is analyzed to calculate the total area ratio of the retained austenite and martensite. The image analysis is performed using the “Analyze” function of the image analysis software “ImageJ”. Due to this, it is possible to calculate the individual area ratios and total area ratio. Here, the “Imager” is an open source public domain image processing software which is widely utilized among persons skilled in the art. Next, the sample is face cut from the rolling surface normal direction down to a ¼ depth in the sheet thickness. This is then measured by X-ray diffraction to calculate the volume ratio of the retained austenite. The volume ratio of retained austenite is equivalent to the area ratio, so this is made the area ratio of the retained austenite. The area ratio of the retained austenite obtained is subtracted from the total area ratio of the residual austenite and martensite calculated previously to calculate the area ratio of the martensite. Finally, the area ratio of the obtained martensite is subtracted from the total area ratio of the bainite and martensite calculated previously in the same way to calculate the area ratio of the bainite.[Ratio of Crystal Grains With Orientation Difference in Grains of 5 to 14°: Area % of 10 to 60%]

[0145] In the microstructure of the hot rolled steel sheet according to an embodiment of the present invention, if deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is controlled to within a range of, by area %, 10 to 60%. Crystal grains having such an orientation difference in grains are effective for raising the strength and the stretch flangeability. While not intending to be constrained to any specific theory, it is believed that the crystal orientation difference in the grains is correlated with the dislocation density contained in the crystal grains. In general, an increase in the dislocation density in grains causes improvement of strength, but lowers the workability. However, in crystal grains with an orientation difference in grains controlled to 5 to 14°, it is believed possible to raise the strength without lowering the workability. As opposed to this, crystal grains with an orientation difference in grains of less than 5° are excellent in workability, but difficult to make high in strength. On the other hand, crystal grains with an orientation difference in grains of more than 14° differ in deformation ability in the crystal grains, therefore do not necessarily contribute to improvement of the stretch flangeability. Therefore, in the hot rolled steel sheet according to an embodiment of the present invention, it becomes possible to suitably control the ratio of crystal grains with an orientation difference in grains of 5 to 14°, more specifically to control it to within a range of, by area %, 10 to 60%, to achieve the desired steel sheet strength while improving the stretch flangeability and to further improve the balance of strength and the stretch flangeability. If the ratio of crystal grains with an orientation difference in grains of 5 to 14° is small, sometimes the stretch flangeability falls. Therefore, from the viewpoint of improvement of the stretch flangeability, the ratio of crystal grains with an orientation difference in grains of 5 to 14° may also be 15% or more, 18% or more, or 20% or more. On the other hand, if the ratio of crystal grains with an orientation difference in grains of 5 to 14° is large, sometimes the ductility falls. Therefore, from the viewpoint of improvement of ductility, the ratio of crystal grains with an orientation difference in grains of 5 to 14° may also be 55% or less, 50% or less, 45% or less, or 40% or less.[Measurement of Ratio of Crystal Grains with Orientation Difference in Grains of 5 to 14°]

[0146] The ratio of crystal grains with an orientation difference in grains of 5 to 14° is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from a steel sheet so that a sheet thickness cross-section in a direction parallel to the rolling direction and vertical to the sheet surface becomes the examined surface. Next, a region of 200 μm in the rolling direction of the steel sheet and 100 μm in the rolling surface normal direction at a ¼ depth position in sheet thickness from the steel sheet surface is analyzed by EBSD at 0.2 μm measurement intervals so as to acquire crystal orientation information. Here, the EBSD analysis is performed using an apparatus comprised of a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and an EBSD detector (HIKARI detector made by TSL) by a 50 to 300 points / s analysis speed. Next, for the obtained crystal orientation information, a region with an orientation difference of 15° or more and a circle equivalent diameter of 0.3 μm or more is defined as a crystal grain, the average orientation difference in the grain of each crystal grain is calculated, and the ratio of crystal grains with an orientation difference in grains of 5 to 14° is found. Such a defined crystal grain or average orientation difference in grains can be calculated using the software “OIM Analysis™” attached to the EBSD analysis apparatus. In the present invention, the “orientation difference in grains” expresses the grain orientation spread (GOS). The value of the orientation difference in grains, as described in “Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-Ray Diffraction Method”, Hidehiko Kimura et al., Transactions of the JSME (A Edition), vol. 71, no. 712, 2005, p. 1722-1728, is found as the average value of the misorientation among all measurement points from the crystal orientation becoming the reference in the same crystal grain. In the embodiment of the present invention, the reference crystal orientation is the orientation of the average of all measurement points in the same crystal grain. The value of GOS can be calculated using the software “OIM Analysis™ Version 7.0.1” attached to the EBSD analysis apparatus.[Surface Roughness Ra: Less Than 1.50 μm and Difference of Maximum Value and Minimum Value in Surface Roughness Ra: 0.50 μm or Less]

[0147] In the hot rolled steel sheet according to an embodiment of the present invention, the surface roughness Ra is controlled to less than 1.50 μm and the difference of the maximum value and minimum value in the surface roughness Ra is controlled to 0.50 μm or less. As explained later in detail relating to the method of production of the hot rolled steel sheet, by scale being sufficiently or completely removed by descaling before finish rolling, it is possible to keep asperities from being formed at the steel sheet surface due to scale being pressed into the steel sheet surface at the time of the later finish rolling. In an embodiment of the present invention, by realizing surface properties giving a surface roughness Ra of the finally obtained hot rolled steel sheet of less than 1.50 μm and a difference of the maximum value and minimum value in the surface roughness Ra of 0.50 μm or less, the formation of the Si scale pattern due to the scale being pressed into the steel sheet surface can be remarkably suppressed and, along with this, excellent post-painting corrosion resistance can be achieved. If it is not possible to sufficiently suppress the formation of the Si scale pattern and the surface roughness Ra of the hot rolled steel sheet becomes 1.50 μm or more and / or the difference of the maximum value and minimum value in the surface roughness Ra becomes more than 0.50 μm, as a result, the post-painting corrosion resistance of the hot rolled steel sheet falls. For example, even if the surface roughness Ra is less than 1.50 μm, if the difference of the maximum value and minimum value in the surface roughness Ra is more than 0.50 μm, the variation in the surface roughness is great, so it cannot be said that formation of an Si scale pattern was sufficiently suppressed. In this case, for example, it becomes no longer possible to realize uniform chemical conversion after that and as a result the post-painting corrosion resistance falls. Therefore, in an embodiment of the present invention, it is not sufficient to just control the surface roughness Ra to less than 1.50 μm. In addition to this, controlling the difference of the maximum value and minimum value in the surface roughness Ra to 0.50 μm or less is extremely important.

[0148] From the viewpoint of suppressing the formation of the Si scale pattern to improve the post-painting corrosion resistance, the lower the surface roughness Ra and the difference of the 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, most preferably 0.80 μm or less. Similarly, the difference of the maximum value and minimum value in the surface roughness Ra is preferably 0.40 μm or less, more preferably is 0.30 μm or less.

[0149] The lower limits of the surface roughness Ra and the difference of the maximum value and minimum value in the same are not particularly prescribed, but, for example, the surface roughness Ra may 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 of the maximum value and minimum value in 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.[Measurement of Surface Roughness Ra and Difference of Maximum Value and Minimum Value in Same]

[0150] The surface roughness Rain the hot rolled steel sheet and the difference of the maximum value and minimum value in the same are measured in the following way. More specifically, first, if the hot rolled steel sheet has scale on the surface, the sample is pickled, then sent on for measurement of roughness. The pickling is performed under conditions of dipping in hydrochloric acid with a hydrochloric acid concentration of 3 to 10 vol % at a 85 to 98° C. temperature for 20 to 300 seconds. Further, the pickling may be performed at one time or may be performed divided into several times in accordance with need. The above pickling time (20 to 300 seconds) means the duration of the pickling in the case of performing the pickling just one time and means the total duration of the pickling if performing the pickling several times. By making the pickling temperature 85° C. or more, it is possible to sufficiently remove the oxides at the surface layer. Next, the sample surface of the pickled hot rolled steel sheet is measured for roughness of the surface in the rolling direction at different measurement locations using locations at 50 mm intervals along the width direction as the measurement locations. Ten or more points are preferably measured, but if the sheet width is insufficient, positions 50 mm apart in the rolling direction are similar measured and 10 or more points measured. The measurement length at each measurement location is 5 mm. Contour curve filters with cutoff values λc and λs are successively applied to the measurement cross-section curve obtained by measurement to obtain a roughness curve. Specifically, the component with a wavelength λc of 0.8 mm or less and component with a wavelength λs of 2.5 μm or more are removed from the obtained measurement results to obtain a roughness curve. Based on the obtained roughness curve, the arithmetic average roughnesses of the individual measurement locations are calculated based on JIS B 0601: 2013. The average value of all of the obtained arithmetic average roughnesses is determined as the surface roughness Ra. Similarly, the difference of the maximum value and minimum value in the surface roughness Ra is determined from all of the obtained arithmetic average roughnesses.

[0151] If the hot rolled steel sheet is provided with a plating layer and paint or other surface coating on its surface, the base iron surface obtained after removing the surface coating is measured. The method of removing the surface coating can be suitably selected in accordance with the type of the surface coating within a range not affecting the surface roughness of the base iron. For example, if the surface coating is formed by electrogalvanization, electric Zn—Ni alloy plating, hot dip galvanization, hot dip galvannealing, hot dip Zn—Al alloy plating, hot dip Zn—Al—Mg alloy plating, hot dip Zn—Al—Mg—Si alloy plating, or other zinc plating layer, dilute hydrochloric acid to which an inhibitor is added may be used to dissolve the galvanized layer. Due to this, it is possible to peel off only the galvanized layer from the steel sheet. An “inhibitor” is an additive used for suppressing changes in roughness by prevention of over dissolution of the base iron. For example, it is possible to use hydrochloric acid diluted to 5 vol % to which is added an “ibit No. 700BK” corrosion inhibitor for hydrochloric acid pickling made by Asahi Chemical Co., Ltd. to a 0.6 g / L concentration. Further, if the surface coating is hot dip aluminum plating or other aluminum plating layer, based on the description in JIS G 3314: 2019, the steel sheet is successively dipped in a sodium hydroxide aqueous solution and dilute hydrochloric acid aqueous solution to which hexamethylene tetramine is added. The Al plating is made to dissolve by dipping the steel sheet until the foaming due to the dissolution of the plating dies down. Further, if the surface coating was formed by electroplating, a release agent (Neo-Rever SP-751: made by Sansaikako) is used to peel off the electrodeposited paint.[Sheet Thickness]

[0152] The hot rolled steel sheet according to an embodiment of the present invention is not particularly limited, but in general has a sheet thickness of 1.0 to 6.0 mm. For example, the sheet thickness may also be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and / or may also be 5.0 mm or less or 4.0 mm or less.

[0153] The hot rolled steel sheet according to an embodiment of the present invention may also be a painted steel sheet provided with at paint layer on at least one surface. If the hot rolled steel sheet is provided with a plating layer and / or a chemical conversion coating, the paint layer can be formed on the plating layer and / or chemical conversion coating. The paint layer is not particularly limited and many be any suitable paint layer known to persons skilled in the art. The thickness of the paint layer is not particularly limited. The paint layer can have any suitable thickness. A paint layer generally includes an electrodeposited paint layer and may also include an intermediate paint layer, base coat layer, and clear coat layer on top of that.

[0154] The hot rolled steel sheet according to an embodiment of the present invention, as explained above, despite being high in strength, is improved in stretch flangeability, ductility, and notch fatigue property and achieves excellent post-painting corrosion resistance. For this reason, the hot rolled steel sheet according to an embodiment of the present invention can realize both the contradictory properties of high strength and excellent workability and, further, is excellent in post-painting corrosion resistance, therefore is particularly useful for use in parts, etc., of technical fields in which these features are sought. In a preferable embodiment, an auto part containing the hot rolled steel sheet according to an embodiment of the present invention, in particular a part selected from the suspension, chassis, bumpers, etc., of an automobile, is provided. Such an auto part need only include the hot rolled steel sheet according to an embodiment of the present invention in a portion of the same. Therefore, it satisfies the features of the chemical compositions, microstructure, and surface properties explained earlier in at least a portions of the part. At portions of the hot rolled steel sheet which do not directly contact the die in press-forming or other shaping and which are low in degree of working, the features of the microstructure and surface properties do not particularly change before and after shaping.[Mechanical Properties][Tensile Strength: TS]

[0155] According to the hot rolled steel sheet having the above chemical composition and microstructure, it is possible to achieve a high tensile strength, specifically a 540 MPa or more tensile strength. The tensile strength is preferably 600 MPa or more, 700 MPa or more, 780 MPa or more, or 850 MPa or more. According to the hot rolled steel sheet according to an embodiment of the present invention, despite having such an extremely high tensile strength, by the specific combination of the chemical composition, microstructure, and surface toughness Ra explained above, it is possible to remarkably improve the stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance. The upper limit of the tensile strength is not particularly prescribed, but for example the tensile strength of the hot rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, or 1180 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.[Total Elongation: El]

[0156] According to the hot rolled steel sheet having the above-mentioned chemical composition and microstructure, in addition to the high tensile strength, it is possible to improve the total elongation, more specifically possible to achieve a 15.0% or more total elongation. The total elongation is preferably 18.0% or more, more preferably 20.0% or more, most preferably 22.0% or more. The upper limit is not particularly prescribed, but, for example, the total elongation is 40.0% or less or 35.0% or less. The total elongation is measured by taking a JIS No. 5 test piece from an orientation (C direction) where the longitudinal direction of the test piece becomes parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.<Method of Production of Hot Rolled Steel Sheet>

[0157] Next, a preferable method of production of the hot rolled steel sheet according to an embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for production of the hot rolled steel sheet according to an embodiment of the present invention and is not intended to limit the hot rolled steel sheet to one produced by the method of production explained below.

[0158] The method of production of the hot rolled steel sheet according to an embodiment of the present invention comprises:

[0159] (A) hot rolling including heating a slab having a chemical composition explained above in relation to the hot rolled steel sheet, then rough rolling, descaling by high pressure water, and finish rolling it, and satisfying the following (A1) to (A4) conditions:

[0160] (A1) a heating temperature of the slab is a solubilization temperature (SRTmin) ° C. expressed by the following formula 1 or more and 1170° C. or more,

[0161] (A2) a highest heating temperature from completion of rough rolling to the start of high pressure water descaling before finish rolling is T° C. expressed by the following formula 2 or more,

[0162] (A3) a cumulative strain (εeff.) of the later three stages of finish rolling expressed by the following formula 3 is 0.50 to 0.60,SRT⁢min=7000 / {2.75-log⁡([Ti]×[C])}-273formula⁢ 1

[0163] where [Ti] and [C] are the contents (mass %) of the elements in the steel

[0164] (A4) an end temperature of finish rolling is Ar3+30° C. or more,T=1081+63×[Si]+27×[sol. Al]+10×[sol. Al]⁢ / [Si]-8⁢ / [Si]-1⁢ / [sol. Al]formula⁢ 2

[0165] where [Si] and [sol. Al] are the contents (mass %) of the elements in the steelε⁢eff. =∑ε⁢i⁡(t,T)formula⁢ 3whereε⁢i⁡(t,T)=ε⁢i⁢0 / exp⁢{(t / τ⁢R)2 / 3}τ⁢R=τ0·exp⁡(Q / RT)τ0=8.46×10-6Q=183200⁢JR=8.314J / K·mol

[0166] εi0 indicates the logarithmic strain at the time of rolling reduction, “t” indicates the cumulative time (s) up right before the cooling of the pass, and T indicates the rolling temperature (° C.) at the pass, and

[0167] (B) cooling including primary cooling the finish rolled steel sheet by an average cooling speed of 10° C. / s or more down to a temperature region of 650 to 750° C., holding in the temperature region for 3.0 to 10.0 seconds, then secondary cooling down to 100° C. or less by an average cooling speed of 30° C. / s or more. Below, the steps will be explained in detail.[(A) Hot Rolling Step][(A1) Heating Temperature of Slab]

[0168] First, a slab having the chemical composition explained above in relation to the hot rolled steel sheet is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be produced by an ingot making method or a thin slab casting method. The heating temperature of the slab has to be the solubilization temperature (SRTmin) ° C. expressed by the following formula 1 or more.SRT⁢min=7000 / {2.75-log⁡([Ti]×[C])}-273formula⁢ 1

[0169] where [Ti] and [C] are the contents (mass %) of the elements in the steel.

[0170] The slab used contains relatively large amounts of alloy elements, in particular, contains Ti. For this reason, the alloy elements have to be made to dissolve in the slab. In particular, Ti has to be made to sufficiently dissolve. If the heating temperature of the slab is less than the solubilization temperature (SRTmin° C.), Ti will not sufficiently dissolve. If Ti does not sufficiently dissolve at the time of slab heating, it becomes difficult to make Ti finely precipitate as carbides (TiC) in the steel in the cooling step after the hot rolling step, etc., to improve the strength of the steel by precipitation strengthening. In addition, it also becomes difficult to form carbides (TiC) to fix C and suppress the formation of the cementite harmful to stretch flangeability. Further, the heating temperature of the slab has to be 1170° C. or more. The scale formed at the steel sheet surface at the time of heating has to be removed at the descaling after heating, but if less than 1170° C., the scale is unevenly removed by descaling and sometimes formation of the Si scale pattern cannot be sufficiently suppressed.[(A2) Highest Heating Temperature From Completion of Rough Rolling to Start of High Pressure Water Descaling Before Finish Rolling: T° C. or More]

[0171] In the present method of production, the heated slab is adjusted in sheet thickness, etc., by rough rolling before finish rolling, then the highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before the finish rolling is controlled to T° C. or more shown in the following formula 2.T=1081+63×[Si]+27×[sol. Al]+10×[sol. Al]⁢ / [Si]-8⁢ / [Si]-1⁢ / [sol. Al]formula⁢ 2

[0172] where [Si] and [sol. Al] are the contents (mass %) of the elements in the steel.

[0173] In a steel sheet contain a relatively large amount of Si, more specifically a steel sheet containing Si in a more than 0.100% amount, Si oxides are formed at the interface of the Fe oxides formed at the steel sheet surface (also called “scale”) at the time of the hot rolling step and the steel. The Si oxides strongly fix the Fe oxides at the steel, so sometimes the scale cannot be sufficiently removed even by subsequent descaling using high pressure water, etc. In such a case, the not sufficiently removed scale is pressed into the steel sheet surface by the subsequent finish rolling and as a result asperities are formed at the steel sheet surface and the surface properties deteriorate. Therefore, controlling the highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before finish rolling to the T° C. expressed by the above formula 2 becomes important. Due to such temperature control, it is believed possible to change to properties of scale facilitating removal by the subsequent high pressure water descaling. It is then possible to easily remove the scale by subsequent high pressure water descaling or possible to completely remove it. In this case, it becomes possible to remarkably suppress poor descaling and formation of the Si scale pattern due to scale being pressed into the steel sheet surface in the subsequent finish rolling. Along with this, in the finally obtained hot rolled steel sheet, it is possible to realize surface properties giving a surface roughness Ra of less than 1.50 μm and a difference of the maximum value and minimum value in the surface roughness Ra of 0.50 μm or less and becomes possible to achieve excellent post-painting corrosion resistance.

[0174] Rough rolling secures the desired sheet bar dimensions. Not only that, sometimes suitable temperature control is required in the relationship with the high pressure water descaling before finish rolling. For example, if reheating is not performed using a bar heater or other facility between the rough rolling and finish rolling, the highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before finish rolling corresponds to the exit side temperature of the rough rolling. Therefore, in such a case, it becomes necessary to control the exit side temperature of the rough rolling to the T° C. expressed by the above formula 2 or more. On the other hand, if reheating is performed using a bar heater or other facility between the rough rolling and finish rolling, the highest heating temperature at any point from completion of rough rolling to high pressure water descaling before finish rolling may be controlled to the T° C. expressed by the above formula 2 or more. Therefore, the exit side temperature of the rough rolling is not necessarily limited. A suitable temperature may be suitably selected.

[0175] In the present method of production, after the completion of the rough rolling and before the finish rolling, the rolled material is descaled by high pressure water. High pressure water descaling is performed using sprayed high pressure water with a water pressure of 10 to 50 MPa. By using such high pressure water, it becomes possible to sufficiently or completely remove scale from the rough rolled material. Scale grows in the hot rolled steel sheet after finish rolling. However, by sufficiently or completely removing the scale before the finish rolling, as explained above, it is possible to keep scale from being pressed into the steel sheet surface at the finish rolling. For this reason, even if the scale subsequently grows, scale cannot be uniformly formed over the steel sheet surface as a whole. Formation of such scale does not affect the formation of the Si scale pattern in any way. Further, such scale can be relatively easily removed by suitable pickling after the hot rolling.[(A3) Cumulative Strain (εeff) of Later Three Stages of Finish Rolling: 0.50 to 0.60]

[0176] The rough rolled then high pressure water descaled slab is next finish rolled. In the present method of production, for example, a tandem rolling machine comprised of four or more rolling stands is preferably used for finish rolling. In the present method of production, to control the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° to within a range of, by area %, 10 to 60%, in the finish rolling performed on a heated slab, the later explained cooling step has to be performed after making the cumulative strain (εeff.) of the later three stages (final three stages) 0.50 to 0.60. This is due to the following reason. The crystal grains with an orientation difference in the grains of 5 to 14° are formed by transformation in the paraequilibrium state by a relatively low temperature. For this reason, by limiting the dislocation density of austenite before transformation in the hot rolling step to a certain range and limiting the cooling speed in the subsequent cooling step to a certain range, it becomes possible to control the formation of the crystal grains with an orientation difference in the grains of 5 to 14°. That is, by controlling the cumulative strain of the later three stages of the finish rolling and the subsequent cooling, it is possible to control the frequency of formation of nuclei of the crystal grains with an orientation difference in the grains of 5 to 14° and the subsequent growth speed. As a result, it is possible to control the area ratio of crystal grains with an orientation difference in the grains of 5 to 14° in the hot rolled steel sheet obtained after cooling. More specifically, the dislocation density of austenite introduced by the finish rolling is mainly related to the formation of nuclei while the cooling speed after finish rolling is mainly related to the speed of growth.

[0177] If the cumulative strain of the later three stages of finish rolling is less than 0.50, the dislocation density of austenite introduced is not sufficient and the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° becomes less than 10%. On the other hand, if the cumulative strain of the later three stages of finish rolling is more than 0.60, during hot rolling, recrystallization of austenite occurs and the cumulative dislocation density at the time of transformation falls. As a result, similarly the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° becomes less than 10%. In the present method of production, the cumulative strain (εeef.) of the later three stages of finish rolling is found by the following formula 3:ε⁢eff. =∑ε⁢i⁡(t,T)formula⁢ 3whereε⁢i⁡(t,T)=ε⁢i⁢0 / exp⁢{(t / τ⁢R)2 / 3}τ⁢R=τ0·exp⁡(Q / RT)τ0=8.46×10-6Q=183200⁢JR=8.314J / K·mol

[0178] εi0 indicates the logarithmic strain at the time of rolling reduction, “t” indicates the cumulative time (s) up right before the cooling of the pass, and T indicates the rolling temperature (° C.) at the pass[(A4) End Temperature of Finish Rolling: Ar3+30° C. or More]

[0179] In the present method of production, the end temperature of the finish rolling has to be Ar3+30° C. or more. If the end temperature of the finish rolling is less than Ar3+30° C., if ferrite is formed in part of the structure due to variations in the constituents and rolling temperature in the steel sheet, the ferrite is liable to be worked. The worked ferrite sometimes becomes a cause of a drop in the ductility. In addition, if the end temperature of the finish rolling is less than Ar3+30° C., sometimes the ratio of crystal grains with an orientation difference in grains of 5 to 14° will become more than 60% or excessively high. In the present method of production, Ar3 (° C.) is found by the following formula 4 based on the chemical composition of the hot rolled steel sheet:Ar⁢3=901-325×[C]+33×[Si]+287×[P]+40×[sol. Al]-92×([Mn]+[Mo]+[Cu])-46×([Cr]+[Ni])formula⁢ 4

[0180] where [C], [Si], [P], [sol. Al], [Mn], [Mo], [Cu], [Cr], and [Ni] are the contents (mass %) of the elements in the steel. Cases where the elements are not contained are indicated as 0.[(B) Cooling Step]

[0181] In the present method of production, the finish rolled steel sheet is cooled in two stages in the next cooling step. Specifically, first, the finish rolled steel sheet is primary cooled by an average cooling speed of 10° C. / s or more down to a temperature region of 650 to 750° C., is held at the temperature region for 3.0 to 10.0 seconds, then is secondary cooled down to 100° C. or less by an average cooling speed of 30° C. / s or more. By combining with the conditions of (A2) and (A3), etc., in the hot rolling step and performing such two stage cooling, transformation by paraequilibrium occurs at the desired relatively low temperature region and thereby it becomes possible to reliably control the ratio of crystal grains with an orientation difference in grains of 5 to 14° to within a range of, by area %, 10 to 60%. As opposed to this, if the average cooling speed of the primary cooling is less than 10° C. / s or the cooling stop temperature of primary cooling is more than 750° C., transformation occurs by paraequilibrium at a relatively high temperature and the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. Further, if the cooling stop temperature of primary cooling is less than 650° C., transformation occurs by paraequilibrium at a temperature lower than the desired temperature region and similarly the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. Furthermore, even if the holding time at 650 to 750° C. is less than 3.0 seconds, similarly the ratio of crystal grains with an orientation difference in grains of 5 to 14° becomes less than 10%. On the other hand, if the holding time at 650 to 750° C. becomes more than 10.0 seconds or the average cooling speed of the secondary cooling is less than 30° C. / s, the cementite harmful to the stretch flangeability is easily formed. Further, if the cooling stop temperature of the secondary cooling is more than 100° C., the area ratio of martensite becomes less than 2%. The upper limits of the average cooling speeds of the primary and secondary cooling are not particularly prescribed, but, for example, the average cooling speeds of the primary and secondary cooling may be 200° C. / s or less considering the capacity of the cooling facilities.

[0182] According to the hot rolled steel sheet produced by the above method of production, it is possible to obtain a microstructure containing, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10% and, when deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, having a ratio of the crystal grains with an orientation difference in the grains of 5 to 14°, by area %, 10 to 60%. As a result, despite being high in strength, it becomes possible to remarkably improve the stretch flangeability, ductility, and notch fatigue property. In addition, in the obtained hot rolled steel sheet, since the surface roughness Ra is controlled to less than 1.50 μm and the difference of the maximum value and minimum value in the surface roughness Ra is controlled to 0.50 μm or less, it is possible to remarkably suppress the formation of the Si scale pattern and, along with this, achieve an excellent post-painting corrosion resistance. Therefore, according to the hot rolled steel sheet produced by the above method of production, it is possible to reliably achieve both the contradictory properties of high strength and excellent workability. Further, the steel sheet is also excellent in post-painting corrosion resistance. Therefore, it is particularly useful in usage in the automobile field where these properties are sought.

[0183] Below, examples will be illustrated to explain the present invention more specifically, but the present invention is not limited to these examples in any way.EXAMPLES

[0184] In the following examples, steel sheets according to one embodiment of the present invention were produced under various conditions and the obtained steel sheets were investigated for tensile strength, stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance.

[0185] First, slabs obtained by casting molten steel by the continuous casting method and having the various chemical compositions shown in Tables 1 and 2 were formed. These slabs were heated under the conditions shown in Table 3, then were hot rolled. The hot rolling was performed by rough rolling, high pressure water descaling, and finish rolling. The rough rolling was performed under the same conditions in all of the invention examples and comparative examples. In each, the high pressure water descaling was performed using sprayed high pressure water with a water pressure of 15 MPa. The highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before finish rolling was as shown in Table 3. Further, the finish rolling was performed using a tandem rolling machine comprised of five rolling stands. The cumulative strain (εeff.) of the later three stages of finish rolling and the end temperature of the finish rolling were as shown in Table 3. Next, each finish rolled steel sheet was primary cooled and secondary cooled under the conditions shown in Table 3 to obtain a hot rolled steel sheet having a sheet thickness of 2.9 mm.TABLE 1Steel Chemical composition (mass %), balance: Fe and impuritiesno.CSiMnTisol.A1PSNONbVCrNiCuA0.0420.2511.080.1210.2830.0120.0020.00330.00280.0160.09B0.0480.2911.360.1550.3200.0120.0030.00280.00300.0150.10C0.0680.1201.070.1220.0720.0150.0020.00290.0032D0.0610.3081.210.1100.2370.0110.0020.00240.00270.12E0.0260.4931.080.1100.3010.0130.0030.00280.0031F0.0481.5411.040.0970.5510.0180.0030.00300.00300.0170.09G0.0521.8421.080.1020.0350.0120.0030.00300.00330.020H0.0460.1071.860.0990.3440.0140.0020.00290.00310.022I0.0490.3040.680.1010.1860.0190.0030.00310.00330.0210.11J0.0510.2641.110.0980.8200.0100.0030.00340.0036K0.0370.2601.030.1920.3210.0150.0020.00340.00340.120.420.08L0.0470.1650.990.0160.2700.0160.0020.00240.00260.0390.08M0.0410.1361.230.1240.3520.0160.0020.00240.00270.090N0.0420.1480.970.1020.2360.0190.0020.00300.00300.0180.32O0.0410.3210.990.1100.3240.0140.0030.00270.00260.09P0.0810.3401.020.1520.3500.0140.0030.00290.00350.0230.10Q0.0191.0121.040.1050.3180.0120.0030.00290.00290.0210.10R0.0420.4402.340.1200.2490.0200.0030.00260.00330.018S0.0511.0410.480.1120.0220.0200.0020.00300.00280.0180.21T0.0430.1460.990.1011.1670.0180.0020.00260.00270.0210.11U0.0451.7071.210.1500.9250.0080.0030.00290.0031V0.0360.1021.350.2090.2140.0150.0020.00350.00340.019W0.0480.7081.240.0120.2050.0210.0020.00250.00330.020Underlines indicate outside scope of present invention.TABLE 2Chemical composition (mass %), balance: Fe and impuritiesSteelSi + sol. no.MoBSbCaMgHfREMBiAsZrCoZnWSnA1A0.534B0.611C0.08700.192D0.0020.545E0.00300.794F0.152.092G0.181.877H0.020.451I0.030.490J0.00110.00071.084K0.581L0.435M0.040.488N0.1060.00220.384O0.00430.00210.645P0.690Q1.330R0.689S1.063T1.313V0.316W0.913Underlines indicate outside scope of present invention.TABLE 3Hot rolling stepCooling stepHighest heating temp. from endof rough rolling Secondary Solu-to start of high FinishPrimary coolingcoolingbilizationSlabpressure water rollingAverageCooling650~AverageCoolingtemp.heatingdescaling before Ar3 + end coolingstop 750° C.coolingstop TestSteelSRTmintemp.Tfinish rolling30temp.speedtemp.holdingspeedtemp.no.no.(° C. )(° C. )(° C. )(° C. )εeff.(° C. )(° C. )(° C. )(° C. )time (s)(° C. / s)(° C. )Remarks 1A11151223108011250.56837891497295.23567Inv. ex. 2B11621228108811150.54812920756764.44241Inv. ex. 3B11621232108811120.54812954686529.75452Inv. ex. 4B11621165108811100.51812914587104.74763Comp. ex. 5B11621233108810800.53812919487074.25367Comp. ex. 6B11621229108811160.63812919447114.54266Comp. ex. 7B11621232108811140.48812901616964.54536Comp. ex. 8B11621219108811200.56812811577344  4562Comp. ex. 9B11621222108811240.57812891 87384.14572Comp. ex.10B11621222108811230.57812910457587.14537Comp. ex.11B11621233108811250.57812917346444.14645Comp. ex.12B11621230108811280.54812906307262.84242Comp. ex.13B11621248108811290.54812904507264.659105 Comp. ex.14C11761235101610620.56822905386975.25565Inv. ex.15C11761256101611300.55822952487354  5284Inv. ex.16C11761250101611250.55822980726533.55690Inv. ex.17D11491229108410900.56817966546884.64255Inv. ex.18E10491225110711280.54855874637094.33342Inv. ex.19F11041208119011980.55894898517226.25457Inv. ex.20G11201237116511700.57880916697035.95152Inv. ex.21G11201242116511720.56880885357309.55250Inv. ex.22H11021221105210820.56766904726804.44435Inv. ex.23I11121226108010920.54870888407114.55750Inv. ex.24J11131247111911350.54857949727415.25549Inv. ex.25K11561253108511120.53818910757084.34230Inv. ex.26L 9191197106311350.56842895566834.34029Inv. ex.27M11151221106311150.53828894397104.94034Inv. ex.28N10951227105411350.56823955747094.35541Inv. ex.29O11011254109211400.57850904557153.74545Inv. ex.30P12291238109611160.54835906277155.54056Comp. ex.31Q10121231114511520.54874890447246.44367Comp. ex.32R11141234109911020.57732919787255.43455Comp. ex.33S11291208109411270.56902917506754.34660Comp. ex.34T109612161146Cracks formed during rollingComp. ex.35U11501270121312150.54901952467074.25352Comp. ex.36V11631209103110850.55811920667054.13634Comp. ex.37W 8961199111811240.56839935487176.34151Comp. ex.Underlines indicate outside scope of present invention or production conditions not preferable.The properties of each obtained steel sheet were measured and evaluated by the following[Tensile Strength (TS) and Total Elongation (El)]The tensile strength (TS) and total elongation (El) were measured by taking a JIS No. 5 test piece from an orientation (C direction) of the long direction of the test piece becoming parallel to the rolling perpendicular direction of the hot rolled steel sheet and conducting a tensile test based on JIS Z 2241: 2011.[Evaluation of Stretch Flangeability]

[0188] The stretch flangeability was evaluated by the saddle shaped stretch flangeability test method using a saddle shaped article. Specifically, a saddle shaped article simulating a stretched flange shape comprised of a straight part and curved part such as shown in FIG. 1 was press-formed. The stretch flangeability was evaluated by the limit shaped height at that time. In the saddle shaped stretch flangeability test method, a saddle shaped article having a radius of curvature R of the corners of 50 to 60 mm and an opening angle θ of 120° was used to measure the limit shaped height H (mm) when making the clearance 11% at the time of punching the corner parts. Here, the “clearance” indicates the ratio of the gap between the punching die and punch and the thickness of the test piece. The clearance is actually determined by the combination of the punching tool and sheet thickness, so “11%” means the 10.5 to 11.5% range is satisfied. The limit shaped height H was judged by examining for the presence of any cracks having lengths of ⅓ or more of the sheet thickness by visual observation after shaping. This was made the limit shaped height with no cracks present. The product (TS×H) of the tensile strength TS (MPa) and limit shaped height H (mm) was evaluated as an indicator of the stretch flangeability. If TS×H≥19500 MPa·mm, it was evaluated that the stretch flangeability was improved.[Evaluation of Ductility]

[0189] If the product of TS (MPa) and El (%) (TS×El) satisfies TS×El≥13500 MPa·%, it was evaluated that the ductility was improved.[Evaluation of Notch Fatigue Property]

[0190] The notch fatigue property was evaluated as follows: Specifically, a fatigue test piece of the shape shown in FIG. 2 was taken from a position similar to the position of taking the tensile test piece so that the orientation (C direction) parallel to the rolling perpendicular direction became the long side. The fatigue test piece was ground down to a depth of 0.05 mm or so from the surface most layer. A stress controlled fatigue test was performed by a stress ratio R=0.1 and frequency 5 Hz. The stress with no breakage after 10 million cycles was defined as the notch fatigue limit (FL) and used for evaluation of the notch fatigue property. If as a result of the test, FL / TS≤0.25 was satisfied, it was evaluated that the notch fatigue property was improved.[Evaluation of Chemical Convertibility]

[0191] The chemical convertibility was evaluated in the following way. Specifically, first, the produced hot rolled steel sheet was pickled, then was treated by phosphate conversion forming a 2.5 g / m2 zinc phosphate coating. At this stage, the chemical convertibility was evaluated by the presence of bald spots and measurement of the P ratio. A “bald spot” means a part where no chemical conversion coating is attached. The “P ratio” means the value shown by P / (P+H) of the X-ray diffraction strength P of the (100) plane of phosphophyllite (FeZn2(PO4)2·4H2O) measured using an X-ray diffraction apparatus and the X-ray diffraction strength H of the (020) plane of the hopite (Zn3(PO4)2·4H2O).

[0192] The phosphate conversion treatment is treatment using a chemical mainly comprised of phosphoric acid and Zn ions. It is a chemical reaction forming crystals called phosphophyllite with the Fe ions eluted from the steel sheet. In phosphate conversion treatment, (1) causing elution of Fe ions and promoting a reaction and (2) densely forming phosphophyllite crystals at the steel sheet surface are important. In particular, regarding (1), if oxides due to formation of Si scale at the steel sheet surface remain, elution of Fe is obstructed and bald spots appear or Fe is not eluted and hopite or another abnormal chemical conversion coating not containing Fe such as not inherently forming due to Fe not being eluted and, as a result, the post-painting corrosion resistance falls. The presence of any bald spots was judged by examination under a scan type electron microscope. Specifically, about 20 fields were examined by a power of 1000×. The case where oxides uniformly became attached to the entire surface and no bald spots could be confirmed was evaluated as “A” measuring no bald spots. Further, the case where the fields where bald spots are confirmed is 5% or less was evaluated as “B”. The case of more than 5% was evaluated as “C” meaning presence of bald spots.

[0193] On the other hand, the P ratio was measured using an X-ray diffraction apparatus. The P ratio expresses the ratio between the hopite and phosphophyllite in the coating obtained by performing phosphate conversion, therefore the higher the P ratio, the greater the phosphophyllite included and the more densely phosphophyllite crystals are formed at the steel sheet surface. Therefore, if the P ratio is 0.80 or more and evaluation of bald spots is A or B, the steel sheet was evaluated as being excellent in chemical convertibility.[Evaluation of Post-Painting Corrosion Resistance]

[0194] The post-painting corrosion resistance was evaluated in the following way. Specifically first, the chemically converted steel sheet was painted by electrodeposition to a 25 μm thickness, then the paint was baked on at 170° C.×20 minutes. A sharp edged knife was used to make a length 130 mm cut into the electrodeposited coating until reaching the base iron (base material). This steel sheet was sprayed with 5% salt water at a 35° C. temperature under the salt spray conditions shown in JIS Z 2371: 2015 continuously for 700 hours. After spraying by salt water, the cut part was covered by a width 24 mm tape (Nichiban 405A-24 JIS Z 1522: 2009) parallel to the cut part to a 130 mm length. The maximum paint peeling width when peeling this off was measured. If the maximum paint peeling width was 4.0 mm or less, the steel sheet was evaluated as excellent in post-painting corrosion resistance.

[0195] If the tensile strength TS was 540 MPa or more and TS×H≥t19500 MPa·mm, TS×El≥13500 MPa·%, FI / TS≥0.25, and the greatest paint peeling width≤4.0 mm, the steel sheet was evaluated as a hot rolled steel sheet which is high in strength, yet improved in stretch flangeability, ductility, and notch fatigue property and excellent in post-painting corrosion resistance. The results are shown inTABLE 4Ferrite Ratio of Difference and crystal grainsof bainite with orientation maximum Ferrite Bainite total Martensite difference invalueand area area area areagrains of minimumTest Steel ratioratioratioratio5 to 14°Ravalue in Rano.no.(%)(%)(%)(%)(%)(μm)(μm)Remarks 1A7813915230.850.21Inv. ex. 2B2462864210.860.34Inv. ex. 3B7814927380.840.31Inv. ex. 4B2663895271.230.55Comp. ex. 5B2467915312.121.62Comp. ex. 6B2466908 70.850.20Comp. ex. 7B2865935 90.870.23Comp. ex. 8B6819877710.910.15Comp. ex. 9B83 8915 80.820.28Comp. ex.10B6127887 60.840.16Comp. ex.11B1672886 60.780.21Comp. ex.12B1274867 70.790.24Comp. ex.13B2968971200.720.29Comp. ex.14C12758710 150.690.20Inv. ex.15C1182936180.420.09Inv. ex.16C 0878710 120.410.09Inv. ex.17D2168895230.940.09Inv. ex.18E7025952190.880.18Inv. ex.19F3355888171.230.30Inv. ex.20G81 7885191.420.45Inv. ex.21G87 0878131.350.31Inv. ex.22H10798910 201.030.20Inv. ex.23I6526912250.960.24Inv. ex.24J7517923160.850.29Inv. ex.25K2364874250.940.17Inv. ex.26L2764915271.000.19Inv. ex.27M2662884201.000.26Inv. ex.28N2263859251.020.20Inv. ex.29O1570855220.880.19Inv. ex.30P17658213 240.910.21Comp. ex.31Q87 9961230.840.20Comp. ex.32R 9647322 110.900.16Comp. ex.33S905953241.020.18Comp. ex.34TCracks formed during rollingComp. ex.35U91 6971301.380.48Comp. ex.36V2166875220.790.27Comp. ex.37W7218909220.780.27Comp. ex.Underlines indicate outside scope of present invention or production conditions not preferable.TABLE 5Post-paintingcorrosion Notch resistanceTensile Total Limit fatigueChemical Maximum strengthelongation shape TS × El T × SH limit convertibility peeling Test Steel TS El height H(MPa ·(MPa ·FL FL / Bald P width no.no.(MPa) (%) (mm)%)mm)(MPa) TSspots ratio(mm) Remarks 1A 61129.239.2 17841 23951 220 0.36 A 0.951.4 Inv. ex. 2B 806 20.8 26.9 16765 21681 251 0.31 A 0.911.3Inv. ex. 3 B 624 27.1 35.6 16910 22214 200 0.32 A 0.91 1.2 Inv. ex. 4 B 794 18.3 26.9 14530 21359 235 0.30 C0.75 4.9 Comp. ex. 5 B 798 18.3 27.1 14603 21626 236 0.30 C 0.76 4.9Comp. ex. 6 B 789 18.8 22.8 14833 17989 234 0.30 A 0.91 1.3Comp. ex. 7 B 802 18.4 22.4 14757 17965 235 0.29 A 0.93 1.1 Comp. ex. 8 B 613 21.6 31.9 13241 19555 217 0.35 A 0.96 1.1Comp. ex. 9 B 598 27.8 31.1 16624 18598 196 0.33 A 0.92 1.2Comp. ex.10B60530.6 30.9 18513 18695 210 0.35 A 0.971.7Comp. ex.11 B 61622.6 30.6 13922 18850 210 0.34 A 0.95 1.5Comp. ex.12 B 64228.6 28.2 18361 18104 214 0.33 A 0.91 1.5 Comp. ex.13 B 53232.3 37.3 17184 19844 128 0.24 A 0.97 1.4 Comp. ex.14 C 84724.1 25.1 20413 21260 260 0.31 A 0.971.4 Inv. ex.15 C 845 24.3 25.0 20534 21125 260 0.31 A 0.98 1.1nv. ex16 C 885 20.1 23.9 17789 21152 275 0.31 A 0.98 1.1 Inv. ex17 D 869 25.1 26.1 21812 22681 270 0.31 A 0.93 1.4Inv. ex18 E 587 32.2 38.5 18901 22600 172 0.29 A 0.931.6 Inv. ex19 F 807 19.2 26.5 15494 21386 241 0.30 A 0.87 1.6Inv. ex20 G 632 26.9 33.8 17001 21362 198 0.31B0.82 1.1Inv. ex21 G 609 29.0 35.2 17661 21437 202 0.33 B 0.85 1.1Inv. ex22 H 829 18.5 24.1 15337 19979 262 0.32 A 0.94 1.5 Inv. ex23 I 604 28.8 37.7 17395 22771 177 0.29 A 0.92 1.4Inv. ex24 J 725 21.8 30.3 15805 21968 218 0.30 A 0.95 1.3 Inv. ex25 K 83518.5 27.4 15448 22879 247 0.30 A 0.94 1.7 Inv. ex26 L 758 21.9 28.3 16600 21451 227 0.30 A 0.971.2Inv. ex27 M81119.0 28.2 15409 22870 242 0.30 A 0.96 1.3 Inv. ex28 N 815 18.9 27.6 15404 22494 243 0.30 A 0.91 1.7 Inv. ex29 O 832 18.7 27.4 15558 22797 246 0.30 A 0.911.3 Inv. ex30 P 856 18.7 19.7 16007 16863 262 0.31 A 0.95 1.7 Comp. ex31 Q 506 35.7 48.3 18064 24440 117 0.23 A 0.91 1.1 Comp. ex32 R 870 15.9 18.2 13833 15834 262 0.30A 0.96 1.3 Comp. ex33 S 531 34.2 44.5 18160 23630 201 0.38 A 0.95 1.1 Comp. ex34 T Cracks formed during rolling Comp. ex35 U 517 28.3 32.3 14631 16699 114 0.22 A 0.891.9Comp. ex36 V 802 15.1 27.0 12110 21654 251 0.31 A 0.941.1Comp. ex37 W59231.0 32.1 18352 19003 169 0.28 A 0.911.5 Comp. exUnderlines indicate outside scope of present invention or production conditions not preferable.Referring to Tables 1 to 5, in Comparative Example 4, the heating temperature of the slab was low, therefore removal of scale by high pressure water descaling became uneven and formation of the Si scale pattern could not be sufficiently suppressed. As a result, the difference of the maximum value and minimum value in the surface roughness Ra became more than 0.50 μm and the chemical convertibility and post-painting corrosion resistance fell. In Comparative Example 5, the highest heating temperature from the completion of rough rolling to the start of high pressure water descaling before finish rolling was low, therefore it was not possible to sufficiently remove scale even by subsequent high pressure water descaling. As a result, the surface roughness Ra became 1.50 μm or more, the difference of the maximum value and minimum value in the surface roughness Ra became more than 0.50 μm, and the chemical convertibility and post-painting corrosion resistance fell. In Comparative Example 6, the cumulative strain (εeff.) of the later three stages of the finish rolling was high, therefore recrystallization of austenite occurred during hot rolling and it is believed the cumulative dislocation density fell at the time of transformation. As a result, the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 7, the εeff. was low, therefore it is believed the dislocation density of the austenite introduced was not sufficient. As a result, similarly the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 8, the end temperature of the finish rolling was low, therefore the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° exceeded 60% and the ductility fell. In Comparative Example 9, the average cooling speed of the primary cooling in the cooling step was low, therefore it is believed that transformation by paraequilibrium occurred at a relatively high temperature. As a result, the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 10, the cooling stop temperature of the primary cooling was high, therefore similarly it is believed that transformation by paraequilibrium occurred at a relatively high temperature. As a result, the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 11, the cooling stop temperature of the primary cooling was low, therefore it is believed that transformation by paraequilibrium occurred at a lower temperature than the desired temperature region. As a result, similarly the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 12, the holding time at 650 to 750° C. in the primary cooling was short, therefore similarly the ratio of the crystal grains with an orientation difference in the grains of 5 to 14° became less than 10% and the stretch flangeability fell. In Comparative Example 13, the cooling stop temperature of the secondary cooling in the cooling step was high, therefore the area ratio of martensite became less than 2%. As a result, the TS and notch fatigue property fell.

[0197] In Comparative Examples 30 and 32, the C and Mn contents were high, therefore the stretch flangeability fell. In Comparative Examples 31 and 33, the C and Mn contents were low, therefore sufficient strength could not be obtained. In Comparative Example 34, the sol. Al content was high, therefore cracking occurred during rolling and the subsequent test could not be performed. In Comparative Example 35, the total content of Si and sol. Al was high, therefore formation of ferrite was promoted and the area ratio of martensite became less than 2%. As a result, the TS fell. In Comparative Example 36, the Ti content was high, therefore the carbides (TiC) became coarser and the ductility fell. In Comparative Example 37, the Ti content was low, therefore it is believed that formation of cementite could not be sufficiently suppressed and as a result the stretch flangeability fell.

[0198] In contrast to this, in the hot rolled steel sheets according to all of the invention examples, it was possible to obtain the hot rolled steel sheet having a predetermined chemical composition and a microstructure obtained by further suitably controlling the conditions in the method of production, the microstructure containing, by area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%, wherein a ratio of the crystal grains with an orientation difference in the grains of 5 to 14° is, by area %, 10 to 60%, a surface roughness Ra is less than 1.50 pin, and the difference of the maximum value and minimum value in the surface roughness Ra is 0.50 μm or less. Further, as a result, despite having a high strength of a tensile strength of 540 MPa or more, it was possible to remarkably improve the stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance. Further, in all of the invention examples, the area ratio of the balance structures was 10% or less. In the presence of balanced structures, the balance structures included at least one of retained austenite and pearlite.

Examples

examples

[0184]In the following examples, steel sheets according to one embodiment of the present invention were produced under various conditions and the obtained steel sheets were investigated for tensile strength, stretch flangeability, ductility, notch fatigue property, and post-painting corrosion resistance.

[0185]First, slabs obtained by casting molten steel by the continuous casting method and having the various chemical compositions shown in Tables 1 and 2 were formed. These slabs were heated under the conditions shown in Table 3, then were hot rolled. The hot rolling was performed by rough rolling, high pressure water descaling, and finish rolling. The rough rolling was performed under the same conditions in all of the invention examples and comparative examples. In each, the high pressure water descaling was performed using sprayed high pressure water with a water pressure of 15 MPa. The highest heating temperature from the completion of rough rolling to the start of high pressure w...

Claims

1-4. (canceled)5. A hot rolled steel sheet having a chemical composition comprising, by mass %,C: 0.020 to 0.070%,Si: more than 0.100 to 2.000%,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 to 0.300%,Cr: 0 to 2.00%,Ni: 0 to 2.00%,Cu: 0 to 2.00%,Mo: 0 to 1.000%,B: 0 to 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 to 1.00%,Co: 0 to 1.00%,Zn: 0 to 1.00%,W: 0 to 1.00%,Sn: 0 to 1.00%, andbalance: Fe, and impurities, andsatisfying 0.110<[Si]+[sol. Al]52.500, wherein [Si] and [sol. Al] are the contents (mass %) of the elements, anda microstructure comprising, by area %,at least one of ferrite and bainite: 80 to 98% in total, andmartensite: 2 to 10%, whereinwhen deeming a boundary with an orientation difference of 15° or more as a grain boundary and defining a region surrounded by grain boundaries and having a circle equivalent diameter of 0.3 μm or more as a crystal grain, a ratio of crystal grains with an orientation difference in grains of 5 to 14° is, by area %, 10 to 60%, anda surface roughness Ra is less than 1.50 μm and a difference of a maximum value and minimum value in the surface roughness Ra is 0.50 μm or less.

6. The hot rolled steel sheet according to claim 5, wherein the chemical composition contains, by mass %, at least one ofNb: 0.001 to 0.050%,V: 0.001 to 0.300%,Cr: 0.01 to 2.00%,Ni: 0.01 to 2.00%,Cu: 0.01 to 2.00%,Mo: 0.001 to 1.000%,B: 0.0001 to 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 to 1.00%,Co: 0.01 to 1.00%,Zn: 0.01 to 1.00%,W: 0.01 to 1.00%, andSn: 0.01 to 1.00%.

7. The hot rolled steel sheet according to claim 5, wherein the hot rolled steel sheet is a painted steel sheet provided with a paint layer on at least one surface.

8. The hot rolled steel sheet according to claim 6, wherein the hot rolled steel sheet is a painted steel sheet provided with a paint layer on at least one surface.

9. A part including the hot rolled steel sheet according to claim 5.

10. A part including the hot rolled steel sheet according to claim 6.

11. A part including the hot rolled steel sheet according to claim 7.

12. A part including the hot rolled steel sheet according to claim 8.

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