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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals

Definitions

• the present invention relates to a hot-rolled steel sheet, and specifically, to a hot-rolled steel sheet having high strength and excellent fatigue properties and shear processability.
• Patent Document 1 proposes a hot-rolled steel sheet having excellent fatigue properties at sheared edges by increasing the volume proportion of martensite and decreasing the volume proportion of pearlite.
• Patent Document 2 proposes a steel sheet that is mainly composed of ferrite and bainite structure, and has excellent fatigue properties at a punched shear section by reducing the maximum height of the steel sheet surface.
• Patent Document 3 proposes a steel sheet with a fatigue crack occurrence lifespan prolonged by securing ⁇ 011> and ⁇ 111> of ferrite and martensite and minimizing ⁇ 001>.
• Patent Document 4 proposes a method of controlling crystal orientation of a ferrite or bainite main phase structure by controlling the shape ratio up to the final pass in finish rolling.
• the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent fatigue properties and shear processability.
• hard fresh martensite is a structure that inhibits plastic deformation, during strong processing such as punching, voids are formed around fresh martensite and cracks easily occur in the punched sheared surface. Therefore, in a hot-rolled steel sheet having a dual-phase structure utilizing ferrite and fresh martensite, shear processability generally deteriorates.
• the inventors analyzed each deformation mechanism in detail. As a result, the inventors found that, when the crystal orientation of ferrite and bainite in a desired region is strictly controlled, it is possible to improve shear processability while securing fatigue properties of the hot-rolled steel sheet. That is, it was found that, when area proportions of hard fresh martensite and tempered martensite, which is the main phase, are controlled, fatigue properties are secured, and when the crystal orientations of ferrite and bainite undergoing large crystal rotation due to punching are appropriately formed in a desired region, it is possible to achieve both fatigue properties and shear processability of the hot-rolled steel sheet at a high level.
• the present invention has been made based on the above findings and the gist of the present invention is as follows.
• the hot-rolled steel sheet of the present invention it is possible to provide a hot-rolled steel sheet having high strength and excellent fatigue properties and shear processability. According to the hot-rolled steel sheet of the present invention, it is possible to integrally mold parts for reducing the weight of a vehicle body of an automobile or the like, shorten the processing process, improve fuel efficiency, and reduce production costs.
• a hot-rolled steel sheet according to one embodiment of the present invention (sometimes referred to as a hot-rolled steel sheet according to the present embodiment) will be described.
• the present invention is not limited only to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the gist of the present invention.
• the hot-rolled steel sheet according to the present embodiment has a chemical composition containing, in mass%, C: 0.02 to 0.30%, Si: 0.10 to 2.00%, Mn: 0.5 to 3.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.10 to 1.00%, N: 0.0100% or less, and Ti: 0.06 to 0.20%, with the remainder: Fe and impurities.
• the C is an element important for improving the strength of the hot-rolled steel sheet. If the C content is less than 0.02%, it is not possible to obtain a desired strength. Therefore, the C content is 0.02% or more, and preferably 0.04% or more, 0.06% or more, or 0.10% or more.
• the C content is 0.30% or less, and preferably 0.25% or less, or 0.20% or less.
• Si is an element that has an effect of inhibiting formation of carbides during ferrite transformation and improving the fatigue properties of the hot-rolled steel sheet. If the Si content is less than 0.10%, it is not possible to obtain this effect. Therefore, the Si content is 0.10% or more, and preferably 0.20% or more, 0.30% or more, or 0.50% or more.
• the Si content is 2.00% or less, and preferably 1.80% or less, 1.60% or less, or 1.50% or less.
• Mn is an element effective in improving the strength of the hot-rolled steel sheet according to improvement in hardenability and solid-solution strengthening. If the Mn content is less than 0.5%, it is not possible to obtain this effect. Therefore, the Mn content is 0.5% or more, and preferably 0.7% or more, or 1.0% or more.
• the Mn content is 3.0% or less and preferably 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.
• the P is an impurity, and a lower P content is desirable, and the P content is preferably 0%. If the P content is more than 0.100%, the processability and weldability of the hot-rolled steel sheet significantly deteriorate and the fatigue properties also deteriorate. Therefore, the P content is 0.100% or less, and preferably 0.070% or less, 0.050% or less, or 0.030% or less.
• the P content may be 0.001% or more in consideration of refining costs.
• S is an impurity, and a lower S content is desirable, and the S content is preferably 0%. If the S content is more than 0.010%, a large amount of inclusions such as MnS are formed, and the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the S content is 0.010% or less and preferably 0.008% or less, or 0.007% or less. If better shear processability is required, the S content is preferably 0.006% or less.
• the S content may be 0.001 % or more in consideration of refining costs.
• Al is an element important to control ferrite transformation. If the Al content is less than 0.10%, it is not possible to preferably control the area proportion of ferrite. Therefore, the Al content is 0.10% or more, and preferably 0.20% or more, 0.30% or more, or 0.40% or more.
• the Al content is 1.00% or less, and preferably 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.
• N is an impurity, and a lower N content is desirable and the N content is preferably 0%. If the N content is more than 0.0100%, coarse Ti nitrides are formed at a high temperature, and the shear processability of the hot-rolled steel sheet deteriorates. Therefore, the N content is 0.0100% or less, and preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less.
• the N content may be 0.0001 % or more in consideration of refining costs.
• Ti is an element that strengthens precipitation of ferrite, and is an element important for controlling ferrite transformation to obtain a desired amount of ferrite. If the Ti content is less than 0.06%, it is not possible to obtain an effect of precipitation strengthening and ferrite transformation control. Therefore, the Ti content is 0.06% or more, and preferably 0.08% or more, or 0.10% or more.
• the Ti content is 0.20% or less, and preferably 0.18% or less, or 0.16% or less.
• the hot-rolled steel sheet according to the present embodiment may have the above chemical composition, with the remainder being made up of Fe and impurities.
• impurities are elements that are mixed in from raw materials such as ores and scrap, and other factors when steel materials are industrially produced, and/or are allowable as long as they do not adversely affect the hot-rolled steel sheet according to the present embodiment.
• the chemical composition of the hot-rolled steel sheet according to the present embodiment may contain the following optional elements that are not essential for satisfying required properties. However, since none of the following optional elements is essential to satisfy required properties, the lower limits of the contents thereof are 0%.
• Nb is an element that has an effect of increasing the strength of the hot-rolled steel sheet according to refining the crystal grain size and strengthening precipitation of NbC.
• the Nb content is preferably 0.01 % or more.
• the Nb content is more than 0.10%, the above effect is maximized. Therefore, even if Nb is contained, the Nb content is 0.10% or less, and preferably 0.06% or less.
• Ca is an element that fixes S in steel as spherical CaS, inhibits formation of elongated inclusions such as MnS, and improves hole expandability of the hot-rolled steel sheet.
• the Ca content is preferably 0.0005% or more.
• the Ca content is more than 0.0060%, the above effect is maximized. Therefore, even if Ca is contained, the Ca content is 0.0060% or less, and preferably 0.0040% or less.
• Mo is an element effective in improving the strength of the hot-rolled steel sheet according to strengthening precipitation of ferrite.
• the Mo content is preferably 0.02% or more and more preferably 0.10% or more.
• the Mo content is more than 1.00%, cracking sensitivity of the slab increases and it is difficult to handle the slab. Therefore, even if Mo is contained, the Mo content is 1.00% or less, and preferably 0.60% or less, 0.50% or less, or 0.30% or less.
• the Cr is an element effective in improving the strength of the hot-rolled steel sheet.
• the Cr content is preferably 0.02% or more and more preferably 0.10% or more.
• the Cr content is more than 1.00%, the ductility of the hot-rolled steel sheet deteriorates. Therefore, even if Cr is contained, the Cr content is 1.00% or less, and preferably 0.80% or less.
• V is an element that improves the strength of the hot-rolled steel sheet by dislocation strengthening according to precipitation strengthening and recrystallization inhibition.
• the V content is preferably 0.01% or more.
• the V content is more than 0.40%, a large amount of carbonitrides are precipitated and the moldability of the hot-rolled steel sheet decreases. Therefore, the V content is 0.40% or less and preferably 0.20% or less.
• Ni is an element that inhibits phase transformation at a high temperature and improves the strength of the hot-rolled steel sheet.
• the Ni content is preferably 0.01% or more.
• the Ni content is more than 0.40%, the weldability of the hot-rolled steel sheet decreases. Therefore, the Ni content is 0.40% or less and preferably 0.20% or less.
• the B is an element that inhibits phase transformation at a high temperature and improves the strength of the hot-rolled steel sheet.
• the B content is preferably 0.0001 % or more.
• the B content is 0.0020% or less, and preferably 0.0005% or less.
• Cu is element that is present in steel in the form of fine particles and improves the strength of the hot-rolled steel sheet.
• the Cu content is preferably 0.02% or more.
• the Cu content is 1.00% or less, and preferably 0.80% or less.
• Sn is an element that inhibits coarsening of crystal grains and improves the strength of the hot-rolled steel sheet.
• the Sn content is preferably 0.01% or more.
• the Sn content is more than 0.50%, steel becomes brittle and easily broken during rolling. Therefore, the Sn content is 0.50% or less, and preferably 0.30% or less.
• Zr is an element that contributes to improving the moldability of the hot-rolled steel sheet.
• the Zr content is preferably 0.001% or more.
• the Zr content is 0.050% or less, and preferably 0.030% or less.
• the chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method.
• ICP-AES inductively coupled plasma-atomic emission spectrometry
• C and S may be measured using a combustion-infrared absorption method
• N may be measured using an inert gas fusion-thermal conductivity method.
• the microstructure of the hot-rolled steel sheet according to the present embodiment contains, in area proportion, a total amount of ferrite and bainite: 30 to 47%, tempered martensite: 50 to 70%, and fresh martensite: 3 to 10%, and within regions obtained by dividing a sheet thickness cross section parallel to a rolling direction into three parts in a sheet thickness direction, when a pole density of the ⁇ 001 ⁇ plane of ferrite and bainite in a center region is P i and a pole density of the ⁇ 001 ⁇ plane of ferrite and the bainite in a surface layer region is P s , P i /P s is 1.2 to 2.0.
• the hot-rolled steel sheet according to the present embodiment preferably has a microstructure composed of only ferrite, bainite, tempered martensite and fresh martensite. That is, the hot-rolled steel sheet according to the present embodiment preferably has a microstructure composed of only, in area proportion, a total amount of ferrite and bainite: 30 to 47%, tempered martensite: 50 to 70%, and fresh martensite: 3 to 10%.
• the microstructure in this region is a typical microstructure of the hot-rolled steel sheet.
• Ferrite and bainite improve the shear processability of the hot-rolled steel sheet. If the total area proportion of ferrite and bainite is less than 30%, the shear processability of the hot-rolled steel sheet may deteriorate or the fatigue strength of the hot-rolled steel sheet may deteriorate. Therefore, the total area proportion of ferrite and bainite is 30% or more, and preferably 33% or more, 35% or more, or 37% or more.
• the total area proportion of ferrite and bainite is 47% or less, and preferably 45% or less, or 43% or less.
• Tempered martensite 50 to 70%
• tempered martensite which has a high martensite formation temperature and is formed by tempering during cooling.
• the area proportion of tempered martensite is 50% or more, and preferably 53% or more, or 55% or more.
• the area proportion of tempered martensite is 70% or less, and preferably 65% or less, or 60% or less.
• Fresh martensite improves the fatigue strength of the hot-rolled steel sheet. If the area proportion of fresh martensite is less than 3%, the fatigue strength of the hot-rolled steel sheet may deteriorate and/or the strength of the hot-rolled steel sheet may deteriorate. Therefore, the area proportion of fresh martensite is 3% or more, and preferably 4% or more, or 5% or more.
• the area proportion of fresh martensite is 10% or less, and preferably 9% or less, or 8% or less.
• the area proportion of each structure can be obtained by the following method.
• a test piece is collected from the hot-rolled steel sheet so that the microstructure can be observed in the sheet thickness cross section parallel to the rolling direction at a depth of 1/4 of the sheet thickness from the surface (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface) and a center position in the sheet width direction.
• the cross section of the test piece is polished using silicon carbide paper of #600 to #1500 and then mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 ⁇ m in a diluted solution such as an alcohol or pure water. Next, polishing is performed using colloidal silica containing no alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample.
• a region with a length of 50 ⁇ m and to a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface is measured by a back scattering electron beam diffraction method at 0.1 ⁇ m measurement intervals to obtain crystal orientation information.
• an EBSD analysis device composed of a thermal field emission scanning electron microscope (JSM-7001F commercially available from JEOL) and an EBSD detector (DVC5 type detector commercially available from TSL) is used.
• JSM-7001F thermal field emission scanning electron microscope
• DVC5 type detector commercially available from TSL
• the degree of vacuum in the EBSD analysis device is 9.6 ⁇ 10 -5 Pa or less
• the acceleration voltage is 15 kV
• the irradiation current level is 13
• the electron beam irradiation level is 62.
• fresh martensite and tempered martensite are distinguished by the following method.
• a region having a substructure within grains and in which cementites with a plurality of variants are precipitated is determined as tempered martensite.
• a region in which the luminance is large and the substructure is not exposed by etching is determined as fresh martensite.
• a method such as buff polishing using alumina particles having a particle size of 0.1 ⁇ m or less or Ar ion sputtering may be used.
• P i /P s is less than 1.2, which indicates that the ⁇ 001 ⁇ planes are uniformly distributed from the surface of the hot-rolled steel sheet.
• P i /P s is 1.2 or more, and preferably 1.3 or more, 1.4 or more, or 1.5 or more.
• P i /P s is more than 2.0, it indicates that the ⁇ 001 ⁇ planes are excessively concentrated in the center region. In this case, there are many ⁇ 001 ⁇ planes, which are brittle fracture surfaces, on the fracture surface, and cracks are likely to occur in the punched sheared surface, and as a result, the shear processability of the hot-rolled steel sheet deteriorates. Therefore, P i /P s is 2.0 or less and preferably 1.9 or less, 1.8 or less, or 1.7 or less.
• the center region is a region of a depth of 1/3 of the sheet thickness from the surface to a depth of 2/3 of the sheet thickness from the surface within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction.
• the surface layer region is a region from the surface to a depth of 1/3 of the sheet thickness from the surface, or a region from the surface to a depth of 2/3 of the sheet thickness to the back surface (another surface different from the surface) within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction, and in the present embodiment, it is not particularly limited to any region.
• ⁇ hkl ⁇ indicates a crystal plane parallel to the rolled plane. That is, ⁇ hkl ⁇ indicates that the rolling direction and the ⁇ hkl ⁇ plane are parallel.
• the pole density of the ⁇ 001 ⁇ plane of ferrite and bainite is measured using a device in which a scanning electron microscope and an EBSD analysis device are combined and OIM Analysis (registered trademark, commercially available from TSL).
• the pole density can be obtained from the crystal orientation distribution function (ODF) that represents a 3D texture calculated using orientation data measured by an electron back scattering diffraction (EBSD) method and spherical surface harmonics.
• OIM crystal orientation distribution function
• EBSD electron back scattering diffraction
• the measurement ranges are the center region (a region of a depth of 1/3 of the sheet thickness from the surface to a depth of 2/3 of the sheet thickness from the surface within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction) and the surface layer region (a region from the surface to a depth of 1/3 of the sheet thickness from the surface, or a region of a depth of 2/3 of the sheet thickness from the surface to the back surface (another surface different from the surface) within regions obtained by dividing the sheet thickness cross section parallel to the rolling direction into three parts in the sheet thickness direction).
• the pole density of the regions identified as ferrite and bainite is measured by the same method as in the above EBSD measurement.
• the tensile strength of the hot-rolled steel sheet according to the present embodiment is 950 MPa or more, and preferably 1,000 MPa or more. If the tensile strength is less than 950 MPa, application parts are limited, and contribution to vehicle body weight reduction is small.
• the upper limit is not necessarily particularly limited and may be 1,500 MPa or less, or 1,300 MPa or less in order to reduce mold wear.
• the fatigue limit ratio (fatigue strength/tensile strength) of the hot-rolled steel sheet according to the present embodiment may be 0.35 or more.
• the tensile strength is evaluated by performing a tensile test according to JIS Z 2241: 2011.
• the test piece is the No. 5 test piece according to JIS Z 2241: 2011.
• a position at which the tensile test piece is collected may be a quarter from the edge in the sheet width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
• the fatigue strength is measured by collecting No. 1 test piece from the hot-rolled steel sheet according to JIS Z 2275: 1978 using a Schenck plane bending fatigue testing machine. For the stress load during measurement, the test speed is set to 30 Hz in both swings and the fatigue strength is measured over 10 7 cycles. Then, the fatigue strength over 10 7 cycles is divided by the tensile strength measured by the above tensile test to calculate the fatigue limit ratio (fatigue strength/tensile strength).
• the sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, and may be 1.2 to 8.0 mm. If the sheet thickness of the hot-rolled steel sheet is less than 1.2 mm, it may become difficult to secure the rolling completion temperature, the rolling load may become excessive, and hot rolling may become difficult. On the other hand, if the sheet thickness is more than 8.0 mm, it may become difficult to obtain the above microstructure after hot rolling.
• the hot-rolled steel sheet having the above chemical composition and microstructure according to the present embodiment may be a surface-treated steel sheet that has a plating layer on the surface in order to improve the corrosion resistance.
• the plating layer may be an electroplating layer or a melting plating layer.
• electroplating layers include zinc electroplating and electro Zn-Ni alloy plating.
• melting plating layers include melting zinc plating, alloying melting zinc plating, melting aluminum plating, melting Zn-Al alloy plating, melting Zn-Al-Mg alloy plating, and melting Zn-Al-Mg-Si alloy plating.
• the amount of plating adhered is not particularly limited, and may be the same as in the related art.
• it is possible to further improve the corrosion resistance by applying appropriate chemical conversion (for example, applying a silicate-based chromium-free chemical conversion solution and drying) after plating.
• the hot-rolled steel sheet according to the present embodiment has the above chemical composition and microstructure, the effect can be obtained regardless of the production method. However, according to the following production method, this is preferable because the hot-rolled steel sheet according to the present embodiment can be stably obtained.
• hot rolling conditions and subsequent cooling conditions are strictly controlled.
• details will be described.
• the heating temperature of the slab has a great effect on solutionization and elimination of element segregation. If the heating temperature of the slab is lower than 1,100°C, solutionization and elimination of element segregation become insufficient, it is not possible to obtain sufficient precipitation strengthening of the finally obtained product, and the tensile strength deteriorates. In addition, if the heating temperature of the slab is higher than 1,350°C, not only is the effect of solutionization and elimination of element segregation maximized, but also the average grain size of austenite becomes coarse, which causes non-uniformity in crystal rotation during rolling, and it is difficult to obtain a desired texture. Therefore, the heating temperature of the slab is preferably 1,100 to 1,350°C and more preferably 1,150 to 1,300°C.
• the temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
• finish rolling rolling is performed by continuously passing the slab through rolling stands for finish rolling a plurality of times.
• rolling conditions at the rolling stands of the final three stands (the final rolling stand, the rolling stand one stand before the final stand, and the rolling stand two stands before the final stand) satisfy the following Formula (1) and Formula (2).
• the average value of the final three stands satisfy the following Formula (1) and Formula (2).
• the middle side of Formula (1) is a formula for obtaining a rolling shape ratio.
• the rolling shape ratio is controlled, it is possible to control crystal rotation by rolling, and it is possible to obtain a desired crystal orientation in the desired region. If the average value of the rolling shape ratio of the final three stands is less than 2.0, the compressive strain inside the steel sheet increases due to rolling, and formation of a rolling recrystallized texture causes the pole density of the ⁇ 001 ⁇ plane in the center region to decrease. As a result, P i /P s becomes less than 1.2.
• the average value of the rolling shape ratio of the final three stands is preferably 2.0 to 10.0. That is, the average value of the rolling shape ratio at the final rolling stand, the rolling shape ratio at the rolling stand one stand before the final stand and the rolling shape ratio at the rolling stand two stands before the final stand is preferably 2.0 to 10.0.
• the average value of ⁇ T in the final three stands is preferably 5 to 35. That is, the average value of ⁇ T at the final rolling stand, ⁇ T at the rolling stand one stand before the final stand and ⁇ T at the rolling stand two stands before the final stand is preferably 5 to 35.
• the average cooling rate for primary cooling may be 250°C/s or slower in order to reduce extension of cooling facilities.
• the air cooling time in the temperature range of 600 to 750°C is longer than 6.0 sec, a large amount of ferrite is formed, and the total area proportion of ferrite and bainite may not reach a desired amount. If the air cooling time in the temperature range is shorter than 2.0 sec, the area proportion of tempered martensite may increase, and the area proportion of fresh martensite may not reach a desired amount.
• the air cooling As secondary cooling, it is preferable to cool to a temperature range of 200°C or lower at an average cooling rate of 40°C/sec or faster. If the average cooling rate for secondary cooling is slower than 40°C/sec, since the cooling rate is less than the critical cooling rate required for martensite transformation, it may not be possible to obtain a desired amount of fresh martensite and/or tempered martensite.
• the average cooling rate for secondary cooling may be 250°C/s or slower in order to reduce extension of cooling facilities.
• the average cooling rate is a value obtained by dividing the temperature drop range of the steel sheet from start of cooling to end of cooling by the time required from start of cooling to end of cooling.
• the start of cooling is the time when injection of a cooling medium to a steel sheet starts in the cooling facility
• the end of cooling is the time when the steel sheet is taken out of the cooling facility.
• cooling facilities include a facility having no air cooling section midway and a facility having one or more air cooling sections midway. In the present embodiment, any cooling facility may be used.
• the steel sheet After cooling to a temperature range of 200°C or lower by secondary cooling, the steel sheet is wound into a coil shape. Since the steel sheet is wound immediately after secondary cooling, the coiling temperature is almost equal to the cooling stop temperature during secondary cooling. If the coiling temperature is higher than 200°C, a large amount of ferrite or bainite is formed, and a desired microstructure may not be obtained. Therefore, the coiling temperature, which is the cooling stop temperature, is preferably 200°C or lower.
• the hot-rolled steel sheet may be temper-rolled according to a general method or may be pickled to remove scale formed on the surface.
• a plating treatment such as melting plating and electroplating or a chemical conversion may be performed.
• the area proportion of the microstructure, P i /P s , the tensile strength and the fatigue limit ratio of the obtained hot-rolled steel sheets were obtained by the above methods.
• the obtained measurement results are shown in Table 3.
• the tensile strength TS was 950 MPa or more, it was determined as satisfactory because the hot-rolled steel sheet had high strength. On the other hand, if the tensile strength TS was less than 950 MPa, it was determined as unsatisfactory because the hot-rolled steel sheet did not have high strength.
• the fatigue limit ratio was 0.35 or more, it was determined as satisfactory because the hot-rolled steel sheet had excellent fatigue strength. On the other hand, if the fatigue limit ratio was less than 0.35, it was determined as unsatisfactory because the hot-rolled steel sheet did not have excellent fatigue strength.
• the underline indicates outside the scope of the present invention or indicates that properties are not preferable.
• the present invention it is possible to provide a hot-rolled steel sheet having high strength and excellent fatigue properties and shear processability. According to the hot-rolled steel sheet of the present invention, it is possible to reduce the weight of a vehicle body of an automobile or the like, integrally mold parts, and shorten the processing process, thereby improving fuel efficiency and reducing production costs.

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