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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
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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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
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • 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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  • 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
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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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
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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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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
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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
  • C21D2211/001—Austenite
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  • 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

Definitions

• the present invention relates to a hot-rolled steel sheet, and specifically, to a hot-rolled steel sheet that is used after being molded into various shapes by press processing or the like, and particularly, to a hot-rolled steel sheet having high strength and little deterioration in the crack arresting property after plastic deformation.
• Non-Patent Document 1 describes that plastic deformation becomes more difficult as the strength of the steel sheet increases, and the crack arresting property generally deteriorates.
• Patent Document 1 discloses a steel sheet for large structures having an excellent crack arresting property after plastic deformation by strictly controlling impurity elements and also setting the ferrite grain size in the surface layer to 3 ⁇ m or less.
• Patent Document 2 discloses a steel sheet for large structures having an excellent crack arresting property after plastic deformation wherein the steel sheet has a structure containing ferrite crystal grains having a flatness of 2 or more and a minor axis diameter of 5 ⁇ m or less and subgrains having an equivalent circle diameter of 3 ⁇ m or less in the ferrite crystals.
• Non-Patent Document 1 Yuzo Takahashi, Osamu Kono, Kosaku Ushioda, Shuji Awaihara: Iron and Steel, 99, (2013), 4, 312-321
• Patent Documents 1 and 2 are both techniques related to a steel sheet for large structures and are not intended for hot-rolled steel sheets. In addition, both have a structure design mainly composed of a ferrite structure, and the steel sheet has a strength of 450 to 700 MPa, and thus it may be difficult to apply the techniques disclosed in Patent Documents 1 and 2 to high-strength hot-rolled steel sheets of 980 MPa or more, which are mainly composed of bainite and martensite.
• the present invention has been made in view of the above circumstances in the related art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and little deterioration in the crack arresting property after plastic deformation.
• the gist of the present invention made based on the above findings is as follows.
• the hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile members, mechanical structural members and also building members.
• the hot-rolled steel sheet according to the present embodiment contains, in mass%, C: 0.040 to 0.400%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, and O: 0.0100% or less, with the remainder: Fe and impurities.
• C 0.040 to 0.400%
• Si 0.05 to 3.00%
• Mn 1.00 to 4.00%
• Al 0.001 to 0.500%
• P 0.100% or less
• S 0.0300% or less
• N 0.1000% or less
• O 0.0100% or less
• the C increases the fraction of the hard phase and lowers the transformation point of the hard phase, and thus increases the strength of the hot-rolled steel sheet. If the C content is less than 0.040%, it becomes difficult to obtain a desired strength. Therefore, the C content is 0.040% or more.
• the C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably 0.080% or more.
• the C content is 0.400% or less.
• the C content is preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.150% or less.
• Si has a function of solid-solution strengthening at room temperature and increasing the strength of the hot-rolled steel sheet and a function of solid-solution softening at a low temperature and improving the toughness of the hot-rolled steel sheet.
• Si has a function of minimizing flaws in steel (minimizing the occurrence of defects such as blowholes in steel) by deacidification. If the Si content is less than 0.05%, it is not possible to obtain the effect of the above function. Therefore, the Si content is 0.05% or more.
• the Si content is preferably 0.50% or more, and more preferably 0.80% or more.
• the Si content is 3.00% or less.
• the Si content is preferably 2.70% or less and more preferably 2.50% or less.
• Mn has a function of inhibiting ferrite transformation and increasing the strength of the hot-rolled steel sheet and a function of solid-solution softening at a low temperature and improving the toughness of the hot-rolled steel sheet. If the Mn content is less than 1.00%, it is not possible to obtain a desired tensile strength. Therefore, the Mn content is 1.00% or more.
• the Mn content is preferably 1.30% or more and more preferably 1.50% or more.
• the Mn content is 4.00% or less.
• the Mn content is preferably 3.70% or less, and more preferably 3.50% or less.
• Al has a function of deacidifying steel and minimizing flaws in steel and a function of exhibiting solid-solution softening at a low temperature and increasing the toughness of the hot-rolled steel sheet. If the sol. Al content is less than 0.001%, it is not possible to obtain the effect of the above function. In addition, if the sol. Al content is less than 0.001%, it is not possible to obtain a desired Rcf value. Therefore, the sol. Al content is 0.001% or more. The sol. Al content is preferably 0.010% or more.
• the sol. Al content is more than 0.500%, since the above effect is maximized and this is economically unfavorable, the sol. Al content is 0.500% or less.
• the sol. Al content is preferably 0.300% or less, and more preferably 0.100% or less.
• sol. Al is acid-soluble Al, and indicates solid solution Al present in steel in a solid solution state.
• P is an element that is generally contained as an impurity, and is an element having a function of increasing the strength of the hot-rolled steel sheet according to solid-solution strengthening. Therefore, P may be actively contained, but P is an element that easily segregates, and if the P content is more than 0.100%, the grain boundary strength decreases significantly due to grain boundary segregation, and grain boundary fracture is likely to occur. Therefore, the P content is 0.100% or less.
• the P content is preferably 0.030% or less.
• the lower limit of the P content is preferably 0.001% or more in consideration of refining costs.
• S is an element contained as an impurity, and forms a sulfide-based inclusion in steel and promotes the occurrence of cracks. If the S content is more than 0.0300%, the occurrence of cracks during plastic deformation becomes significant, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the S content is 0.0300% or less. The S content is preferably 0.0050% or less.
• the lower limit of the S content is preferably 0.0001% or more in consideration of refining costs.
• N is an element that is contained in steel as an impurity and has a function of promoting the occurrence of cracks starting from impurities. If the N content is more than 0.1000%, the occurrence of cracks during plastic deformation becomes significant, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the N content is 0.1000% or less.
• the N content is preferably 0.0800% or less, more preferably 0.0700% or less, and still more preferably 0.0100% or less.
• the lower limit of the N content may be 0.0001% or more.
• the N content is preferably 0.0010% or more and more preferably 0.0020% or more.
• the O content is 0.0100% or less.
• the O content is preferably 0.0080% or less, and more preferably 0.0050% or less.
• the O content may be 0.0005% or more, or 0.0010% or more.
• the remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be composed of Fe and impurities.
• impurities are elements that are mixed in from ores or scraps as raw materials or a production environment or the like and/or are allowable as long as they do not adversely affect the hot-rolled steel sheet according to the present embodiment.
• the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements in place of some Fe.
• the lower limit of the content when the optional elements are not contained is 0%.
• the optional elements will be described in detail.
• Ti, Nb and V are elements that precipitate finely in steel as carbides and nitrides and improve the strength of steel according to precipitation strengthening. Therefore, one, two or more of these elements may be contained. In order to obtain this effect more reliably, each content of Ti, Nb and V is preferably 0.010% or more. Here, it is not necessary to contain all of Ti, Nb and V, and any one of them may have a content of 0.010% or more. Each content of Ti, Nb and V is preferably 0.060% or more, and more preferably 0.080% or more.
• each content of Ti, Nb and V is 1.000% or less, preferably 0.800% or less and more preferably 0.500% or less.
• Cu, Cr, Mo, Ni and B all have a function of improving hardenability of the hot-rolled steel sheet.
• Ni has a function of effectively minimizing grain boundary cracks of the slab caused by Cu. Therefore, one, two or more of these elements may be contained.
• the Cu content is preferably 0.01% or more and more preferably 0.05% or more.
• the Cu content is 2.00% or less.
• the Cu content is preferably 1.50% or less, and more preferably 1.00% or less.
• the Cr content is preferably 0.01% or more and more preferably 0.05% or more.
• the Cr content is 2.00% or less.
• Mo has a function of improving hardenability of the hot-rolled steel sheet and a function of precipitating in steel as carbides and increasing the strength of the hot-rolled steel sheet.
• the Mo content is preferably 0.01% or more and more preferably 0.02% or more.
• the Mo content is 1.00% or less.
• the Mo content is preferably 0.50% or less and more preferably 0.20% or less.
• Ni has a function of improving hardenability of the hot-rolled steel sheet.
• Ni has a function of effectively minimizing grain boundary cracks of the slab caused by Cu.
• the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically unfavorable to contain a large amount of Ni. Therefore, the Ni content is 2.00% or less.
• B has a function of improving hardenability of the hot-rolled steel sheet.
• the B content is preferably 0.0001% or more and more preferably 0.0002% or more.
• the B content is 0.0100% or less.
• the B content is preferably 0.0050% or less.
• Ca, Mg and REMs all have a function of improving the crack arresting property of the hot-rolled steel sheet by adjusting the shape of inclusions in steel to a preferable shape.
• Bi has a function of improving the crack arresting property of the hot-rolled steel sheet according to refining of the solidification structure. Therefore, one, two or more of these elements may be contained.
• the content of any one or more of Ca, Mg, REM and Bi is preferably 0.0005% or more.
• the Ca content or the Mg content is more than 0.0200% or the REM content is more than 0.1000%, inclusions are excessively formed in steel, and thus the crack arresting property of the hot-rolled steel sheet may deteriorate.
• the Ca content and the Mg content are 0.0200% or less
• the REM content is 0.1000% or less
• the Bi content is 0.020% or less.
• the Bi content is preferably 0.010% or less.
• REM refers to a total of 17 elements composed of Sc, Y and lanthanides, and the REM content refers to a total amount of these elements.
• lanthanides they are industrially added in the form of misch metals.
• the inventors confirmed that, even if a total amount of 1.00% or less of these elements is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. Therefore, a total amount of 1.00% or less of one, two or more of Zr, Co, Zn and W may be contained.
• the inventors confirmed that, even if a small amount of Sn is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. However, if a large amount of Sn is contained, flaws may occur during hot rolling and thus the Sn content is 0.05% 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 plasmaatomic emission spectrometry
• sol. Al may be measured using a filtrate after thermally decomposing a sample with an acid through ICP-AES.
• C and S may be measured using a combustion-infrared absorption method
• N may be measured using an inert gas fusion-thermal conductivity method
• O may be measured using an inert gas fusion-nondispersive infrared absorption method.
• the microstructure contains, in area%, less than 3.00% of retained austenite, and the Rcf value indicating a ratio between average fracture surface units before and after plastic deformation is 2.0 or more.
• the hot-rolled steel sheet according to the present embodiment can have high strength and an excellent crack arresting property after plastic deformation.
• the structure fraction and the Rcf value in the microstructure at a depth of 1/4 of the sheet thickness from the surface are specified.
• the reason for this is that the microstructure at that position is a typical microstructure of the steel sheet.
• Retained austenite is a microstructure that is present as fcc at room temperature. Retained austenite has concentrated carbon in the surrounding structure, transforms to hard martensite during plastic deformation, and thus can become a starting point for the occurrence of cracks. If the area proportion of retained austenite is 3.00% or more, the above function becomes apparent, and the crack arresting property after plastic deformation significantly deteriorates. Therefore, the area proportion of retained austenite is less than 3.00%.
• the area proportion of retained austenite is preferably 2.00% or less, less than 1.50% or 1.00% or less, and more preferably less than 1.00% or less than 0.50%. Since it is preferable that the amount of retained austenite be as small as possible, the area proportion of retained austenite may be 0.00%.
• Methods of measuring the area proportion of retained austenite include X-ray diffraction, electron back scattering diffraction pattern (EBSP) analysis, and magnetic measurement methods.
• the area proportion of retained austenite is measured by X-ray diffraction.
• the microstructure of the hot-rolled steel sheet according to the present embodiment may contain ferrite, martensite, bainite and pearlite in addition to retained austenite.
• the area proportion of ferrite may be 60.00% or less, 50.00% or less, or 45.00% or less. In addition, the area proportion of ferrite may be 0.00% or more, or 0.05% or more.
• the area proportion of martensite may be 100.00% or less, or 99.00% or less. In addition, the area proportion of martensite may be 0.00% or more, 1.00% or more, or 1.50% or more.
• the area proportion of bainite may be 100.00% or less, or 96.00% or less. In addition, the area proportion of bainite may be 0.00% or more, or 0.01% or more.
• the area proportions of the above retained austenite, ferrite, bainite and martensite are area proportions that apply to all three steel types to be described below (DP steel, bainite steel and martensite steel).
• the microstructure contains, in area proportion, less than 3.00% of retained austenite, and ferrite, martensite and a very small amount of the residual structure, preferably, the area proportion of ferrite is 15.00 to 60.00%, the area proportion of martensite is 40.00 to 85.00%, and the area proportion of the residual structure is less than 45.00%. If the area proportion of each structure is set as above, it is possible to improve the strength and ductility in a well-balanced manner.
• Ferrite is a structure that is formed when fcc transforms to bcc at a relatively high temperature. Since ferrite has a high processing hardening rate, it has a function of improving strength-ductility balance of the hot-rolled steel sheet. If the area proportion of ferrite is 15.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of ferrite is preferably 15.00% or more. The area proportion of ferrite is more preferably 20.00% or more, 25.00% or more, or 30.00% or more.
• the area proportion of ferrite is preferably 60.00% or less.
• the area proportion of ferrite is more preferably 55.00% or less, and still more preferably 50.00% or less.
• Martensite is a structure that is formed when fcc transforms to bcc at a relatively low temperature. Martensite is a structure composed of fine crystal grains with a high dislocation density and has a function of increasing the strength of the hot-rolled steel sheet. If the area proportion of martensite is 40.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of martensite is preferably 40.00% or more. The area proportion of martensite is preferably 50.00% or more.
• the area proportion of martensite is preferably 85.00% or less.
• the area proportion of martensite is more preferably 80.00% or less, and still more preferably 75.00% or less, or 70.00% or less.
• a total amount of less than 45.00% of pearlite and bainite may be contained as the residual structure.
• the area proportion of the residual structure may be 10.00% or less, or 5.00% or less.
• the residual structure may not be contained and a total area proportion may be 0.00%.
• the microstructure contains, in area proportion, less than 3.00% of retained austenite, and bainite and a very small amount of the residual structure, preferably, the area proportion of bainite is 50.00% or more, and the area proportion of the residual structure is less than 50.00%. If the area proportion of each structure is set as above, it is possible to improve the strength, ductility and hole expandability at the same time.
• Bainite is a structure that is formed when fcc transforms to bcc at a low temperature. Bainite is a structure composed of fine crystal grains and carbides and has a function of improving the strength, ductility and hole expandability of the hot-rolled steel sheet in a well-balanced manner. If the area proportion of bainite is 50.00% or more, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of bainite is preferably 50.00% or more. The area proportion of bainite is more preferably 80.00% or more, 85.00% or more, or 90.00% or more.
• the upper limit is not particularly specified, and may be 100.00% or less.
• the residual structure less than 50.00%
• a total amount of less than 50.00% of pearlite, ferrite and martensite may be contained as the residual structure.
• the area proportion of the residual structure is more preferably 20.00% or less, 15.00% or less, or 10.00% or less.
• the residual structure may not be contained and a total area proportion may be 0.00%.
• a total area proportion of martensite is 85.00% or more, and the area proportion of the residual structure is preferably less than 15.00%. If the area proportion of each structure is set as above, it is possible to improve the strength and hole expandability at the same time.
• martensite has a function of increasing the strength of the hot-rolled steel sheet.
• martensite since martensite has a random crystal orientation structure, it has a function of improving hole expandability of the hot-rolled steel sheet. If the area proportion of martensite is more than 85.00%, the effect of the above function can be sufficiently obtained. Therefore, the area proportion of martensite is preferably more than 85.00%.
• the area proportion of martensite is more preferably 90.00% or more, 93.00% or more, or 95.00% or more.
• the upper limit is not particularly specified, and may be 100.00% or less.
• the residual structure less than 15.00%
• a total amount of less than 15.00% of pearlite, ferrite and bainite may be contained as the residual structure.
• the area proportion of the residual structure is more preferably 10.00% or less, 7.00% or less, or 5.00% or less.
• the residual structure may not be contained and a total area proportion may be 0.00%.
• Pearlite is a lamellar microstructure in which cementite precipitates in layers between ferrites, and is a softer microstructure than bainite and martensite. If the area proportion of pearlite is 5.00% or more, carbon is consumed by cementite contained in pearlite, the strength of martensite and bainite decreases, and it may not be possible to obtain a tensile strength of 980 MPa or more. Therefore, in any of the aspects, the area proportion of pearlite may be less than 5.00%. The area proportion of pearlite is more preferably 3.00% or less. In order to improve the elongation-flangeability of the hot-rolled steel sheet, it is preferable that the area proportion of pearlite be reduced as much as possible, and the area proportion of pearlite is more preferably 0.00%.
• the area proportion of pearlite here is an area proportion that applies to all of the above three steel types (DP steel, bainite steel and martensite steel).
• Structures other than retained austenite are measured by the following method.
• the area proportion of ferrite and of pearlite is measured by the following method.
• a sample is collected so that the 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 the center position in the sheet width direction can be observed.
• the cross section of the sample parallel to the rolling direction is mirror-finished and polished with colloidal silica containing no alkaline solution at room temperature for 8 minutes to remove the strain introduced into the surface layer of the sample.
• a region with a length of 50 ⁇ m and a position of a depth of 1/4 of the sheet thickness from the surface (region of 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) and a center position in the sheet width direction is measured at 0.1 ⁇ m measurement intervals by an electron back scattering diffraction method to obtain crystal orientation information.
• the number of measurement points is at least 500 points.
• an EBSD 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.
• the degree of vacuum in the EBSD device is 9.6 ⁇ 10 -5 Pa or less
• the acceleration voltage is 15 kV
• the irradiation current level is13
• the electron beam irradiation level is 62.
• a reflected electron image is captured in the same field of view.
• crystal grains in which ferrite and cementite precipitate in layers are identified from the reflected electron image, and when the area proportion of the crystal grains is calculated, the area proportion of pearlite is obtained.
• crystal orientation information for crystal grains other than crystal grains determined as pearlite, in the obtained crystal orientation information, using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" bundled in the EBSD analysis device, a region in which the Grain Average Misorientation value is 1.0° or less is determined as ferrite. If the area proportion of the region determined as ferrite is determined, the area proportion of ferrite is obtained.
• nital corrosion is performed, and using an optical microscope and a scanning electron microscope (SEM), at least three 30 ⁇ m ⁇ 30 ⁇ m regions to a position of 1/4 of the sheet thickness from the surface (region of a depth of 1/8 in the sheet thickness direction from the surface to a depth of 3/8 in the sheet thickness direction from the surface) are observed.
• Image analysis is performed on the structure image obtained by this structure observation, and thus the area proportion of bainite is obtained.
• repeller corrosion is performed on the same observation position, structure observation is then performed using an optical microscope and a scanning electron microscope, image analysis is performed on the obtained structure image, and thus the area proportion of martensite is calculated.
• Martensite is a structure having a high dislocation density and substructures such as blocks and packets within grains, and can be distinguished from other microstructures according to an electron channeling contrast image using a scanning electron microscope.
• Bainite is an aggregate of lath-shaped crystal grains, which is a non-martensite structure among structures that do not contain Fe-based carbides having a major diameter of 20 nm or more inside the structure or a structure which contains Fe-based carbides having a major diameter of 20 nm or more inside the structure, and in which the Fe-based carbides have a single variant, that is, Fe-based carbides extend in the same direction.
• Fe-based carbides elongated in the same direction are Fe-based carbides with a difference of 5° or less in the elongation direction.
• the rolling direction of the hot-rolled steel sheet is determined by the following method.
• a test piece is collected so that the sheet thickness of the hot-rolled steel sheet cross section can be observed.
• the cross section of the collected test piece in the sheet thickness is mirror-polished, and then observed using an optical microscope.
• the observation range is the entire sheet thickness, and the direction parallel to the elongation direction of crystal grains is determined as the rolling direction.
• the Ref value indicates a ratio between average fracture surface units before and after plastic deformation, and the following formula is represented using the Cf1 value, which is the average fracture surface unit before undergoing plastic deformation, and the Cf2 value, which is the average fracture surface unit after undergoing plastic deformation.
• the crack arresting property deteriorates.
• the crack arresting property is also affected by the fracture surface unit, and as the fracture surface unit is finer, the propagation path is bent, and the crack propagation is minimized. Therefore, in order to exhibit a favorable crack arresting property even after plastic deformation, it is necessary to increase the Ref value.
• the Rcf value is 2.00 or more, and preferably 2.20 or more, and more preferably 2.30 or more.
• the upper limit is not particularly specified, and may be 5.00 or less, 4.00 or less, or 3.00 or less.
• the Cf1 value and the Cf2 value can be obtained by the following method.
• brittle fracture in order to calculate the Cf1 value and the Cf2 value, it is necessary to cause brittle fracture.
• a test method for causing brittle fracture for example, according to JIS Z 2242: 2018, a 2.5 mm sub-sized V notch test piece in which the width direction (C direction) of the hot-rolled steel sheet is the longitudinal direction of the test piece is prepared, and a Charpy impact test may be performed at -196°C. If the sheet thickness of the hot-rolled steel sheet is less than 2.5 mm, the test may be performed on the entire thickness.
• the rolling direction of the hot-rolled steel sheet is determined by the above method, and the direction perpendicular to the rolling direction is determined as the width direction of the hot-rolled steel sheet.
• the SEM image capturing region that is captured in order to calculate the Cf1 value and the Cf2 value is a position from the surface of the steel sheet to a depth of 1/4 of the sheet thickness (region of 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) and a center position in the sheet width direction in the cross section parallel to the rolling direction.
• SU-6600 Schottky-emission electron gun commercially available from Hitachi High-Tech Corporation
• a tungsten emitter are used, and in a vacuum of 9.6 ⁇ 10 -5 Pa or less, the acceleration voltage is 1.5 kV.
• the imaging magnification is 1,000x, and the number of imaging fields is 3 or more.
• a ductile fracture part called a tear ridge is imaged with a bright contrast.
• the region surrounded by the tear ridge is defined as one cleavage facet, and the equivalent circle diameter is obtained from the area of each cleavage facet, and is used as the fracture surface unit of each cleavage facet. From the obtained fracture surface unit, the area average diameter weighted by the area of each cleavage facet is determined and used as the average fracture surface unit.
• the above processing is performed on the hot-rolled steel sheet before plastic deformation and the hot-rolled steel sheet after plastic deformation, and thus the Cf1 value and the Cf2 value are calculated.
• a JIS No. 5 tensile test piece in which the width direction (C direction) of the hot-rolled steel sheet is the longitudinal direction of the test piece is prepared, a compressive pre-strain of 10% is applied to the steel material in the longitudinal direction of the test piece and various test pieces are then collected.
• the standard deviation of the Mn concentration may be 0.60 mass% or less.
• the standard deviation of the Mn concentration is preferably 0.50 mass% or less and more preferably 0.47 mass% or less. In order to minimize a decrease in fracture energy, a smaller value of the lower limit of the standard deviation of the Mn concentration is more desirable, and due to restrictions on the producing process, 0.10 mass% is the substantial lower limit.
• a position from the surface of the steel sheet to a depth of 1/4 of the sheet thickness (region of 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) and a center position in the sheet width direction are measured with an electron probe micro-analyzer (EPMA), and the standard deviation of the Mn concentration is measured.
• the acceleration voltage is 15 kV and the magnification is 5,000x (the region of a depth of 1/8 from the surface to a depth of 3/8 from the surface), and a distribution image in a range of 20 ⁇ m in the sheet thickness direction of the sample is measured. More specifically, measurement intervals are 0.1 ⁇ m, and the Mn concentration is measured at 40,000 or more points. Then, the standard deviation is calculated based on the Mn concentrations obtained from all the measurement points, and thus the standard deviation of the Mn concentration is obtained.
• the tensile property (tensile strength) is evaluated according to JIS Z 2241: 2011.
• the test piece is a No. 5 test piece according to JIS Z 2241: 2011.
• the tensile test piece is collected at a position a quarter from the edge in the sheet width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
• the tensile (maximum) strength of the hot-rolled steel sheet according to the present embodiment is 980 MPa or more, and preferably 1000 MPa or more. If the tensile strength is less than 980 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 1,780 MPa or less, 1,500 MPa or less, or 1,300 MPa or less in order to reduce mold wear.
• the sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, and may be 1.20 to 8.00 mm. If the sheet thickness of the hot-rolled steel sheet is less than 1.20 mm, it may difficult to secure the rolling completion temperature, the rolling load may become excessive, and hot rolling may become difficult. Therefore, the sheet thickness of the hot-rolled steel sheet according to the present embodiment may be 1.20 mm or more, and is preferably 1.40 mm or more. On the other hand, if the sheet thickness is more than 8.00 mm, the effect of the standard deviation of the Mn concentration becomes significant, and it becomes difficult to obtain a desired Rcf value. Therefore, the sheet thickness may be 8.00 mm or less, and is preferably 6.00 mm or less, or 3.00 mm or less.
• 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.
• a preferable method of producing the hot-rolled steel sheet having the above chemical composition and microstructure according to the present embodiment is as follows.
• the slab is heated under predetermined conditions and then hot-rolled, and accelerated cooling to a predetermined temperature range is performed, and then slow-cooling is performed as necessary, and it is effective to control a cooling history until coiling.
• the following processes (1) to (7) are sequentially performed.
• the temperature of the slab and the temperature of the steel sheet in the present embodiment are the surface temperature of the slab and the surface temperature of the steel sheet.
• a slab to be hot-rolled For a slab to be hot-rolled, a slab obtained by continuous casting or a slab obtained by casting and blooming can be used, and as necessary, one obtained by performing hot processing or cold processing on a slab can be used.
• a slab to be hot-rolled is preferably heated and held in a temperature range of 1,100°C or higher for 6,000 sec or longer.
• the steel sheet temperature may vary in a temperature range of 1,100°C or higher or may be constant.
• the austenite grains can be made uniform when the slab is heated.
• the slab is held in a temperature range of 700 to 850°C for 900 sec or longer, and then additionally heated, and may be held in a temperature range of 1,100°C or higher for 6,000 sec or longer.
• the steel sheet temperature may vary in a temperature range or may be constant.
• Mn is distributed between ferrite and austenite, and when its transformation time is prolonged, Mn can diffuse in the ferrite region. Thereby, Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. If the standard deviation of the Mn concentration is large, a region in which Mn is locally concentrated and the fracture energy decreases develops, the occurrence of cracks during plastic deformation is promoted, and thus it may not possible to obtain a desired Rcf value.
• a reverse mill or a tandem mill for multipass rolling.
• the total rolling reduction rate in a temperature range of 850 to 1,100°C is less than 90%, the standard deviation of the Mn concentration increases, it is not possible to inhibit development of a region in which Mn is locally concentrated and fracture energy decreases, and the occurrence of cracks during plastic deformation may not be promoted. Thereby, it may not be possible to obtain a desired Ref value.
• the sheet thickness reduction in a temperature range of 850 to 1,100°C can be represented by ⁇ (t0-t1)/t0 ⁇ 100(%) where the inlet sheet thickness before first rolling in rolling in this temperature range is t0, and the outlet sheet thickness after final stage rolling in rolling in this temperature range is t1.
• Hot rolling start temperature 850°C or higher and lower than 930°C
• rolling temperature from the first stage of hot rolling to the stage two stages before the final stage 850°C or higher and lower than 950°C
• rolling reduction rate for the rolling less than 30%
• the hot rolling start temperature is 850°C or higher and lower than 930°C
• the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is 850°C or higher and lower than 950°C
• the rolling reduction rate from the first stage of hot rolling to the stage two stages before the final stage is less than 30%.
• the hot rolling start temperature is set to a relatively low temperature
• the temperature of the first stage of hot rolling is set to low, and rolling is performed at a low rolling reduction rate, it is possible to minimize recrystallization at the first stage of hot rolling and accumulate strains in the austenite grains.
• unrecrystallized austenite with a high dislocation density can be maintained within grains up to the latter stage of rolling.
• the rolling start temperature is 930°C or higher
• the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is 950°C or higher, or the rolling reduction rate for the rolling is 30% or more, it is not possible to minimize recrystallization of austenite at the first stage to the stage two stages before the final stage of hot rolling, and as a result, it may not be possible to obtain a desired Ref value.
• the hot rolling start temperature is lower than 850°C, or the rolling temperature at the first stage of hot rolling to the stage two stages before the final stage is lower than 850°C, it is difficult to set the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage to 930°C or higher, and as a result, it may not be possible to obtain a desired Rcf value.
• the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage is 930°C or higher and lower than 1,010°C
• the rolling reduction rate at the final stage and the stage one stage before the final stage is 50% or more
• the rolling completion temperature is 950°C or higher.
• Recrystallization at the first stage of rolling is minimized, recrystallization of crystal grains with a high dislocation density is caused at the latter stage of rolling, and thus orientation difference between structures generated from the recrystallized austenite increases.
• the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage is lower than 930°C, the rolling reduction rate at the final stage and the stage one stage before the final stage is less than 50%, or the rolling completion temperature is lower than 950°C, recrystallization of austenite is insufficient, and it may not be possible to obtain a desired Rcf value.
• the rolling temperature at the final stage of hot rolling and the stage one stage before the final stage and the rolling completion temperature are lower than 1,010°C, it is possible to refine the structure by preventing coarsening of the austenite grain size. Thereby, it is possible to minimize the occurrence of cracks during plastic deformation and to increase the Ref value.
• cooling is performed at an average cooling rate of 50°C/sec or faster within 1.0 sec after hot rolling is completed, and the cooling start temperature is 850°C or higher and lower than 960°C.
• cooling is performed at a high average cooling rate immediately after hot rolling is completed, for example, cooling water may be sprayed onto the surface of the steel sheet.
• the cooling start temperature is 850°C or higher and lower than 960°C and cooling is performed at an average cooling rate of 50°C/sec or faster within 1.0 sec after hot rolling is completed. Thereby, it is possible to minimize the occurrence of cracks during plastic deformation and to increase the Ref value.
• the cooling start temperature used here is a temperature immediately before cooling is performed at an average cooling rate of 50°C/sec or faster, for example, a temperature immediately before cooling water is sprayed onto the surface of the steel sheet.
• the average cooling rate is a value obtained by dividing the temperature drop range of the steel sheet from when accelerated cooling starts (when the steel sheet is introduced into the cooling facility) until accelerated cooling is completed (when the steel sheet is taken out of the cooling facility) by the time required from when accelerated cooling starts until accelerated cooling is completed.
• Cooling is performed to a temperature range of 600 to 730°C at an average cooling rate of 50°C/sec or faster, and slow cooling with an average cooling rate of slower than 5°C/s is performed in a temperature range of 600 to 730°C for 2.0 sec or longer. Then, cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/s or faster.
• slow cooling may not be performed, and when slow cooling is not performed, cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/s or faster.
• the average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from the cooling stop temperature of accelerated cooling to the slow cooling stop temperature by the time required from when accelerated cooling stops until slow cooling stops.
• the average cooling rate from the cooling stop temperature of slow cooling to the coiling temperature is preferably 50°C/sec or faster.
• the average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from the cooling stop temperature of slow cooling with an average cooling rate of slower than 5°C/s to the coiling temperature by the time required from when slow cooling with an average cooling rate of slower than 5°C/s stops until coiling.
• the upper limit of the time for which slow cooling is performed is determined by the facility layout, and may be generally shorter than 10.0 sec.
• the lower limit of the average cooling rate for slow cooling is not particularly set, but it may be 0°C/s or faster because increasing the temperature without cooling involves a large investment in facility.
• slow cooling may not be performed.
• accelerated cooling is performed to a temperature range of 350°C or lower at an average cooling rate of 50°C/sec or faster without performing slow cooling, it is possible to inhibit formation of ferrite and pearlite with a low strength and promote formation of martensite.
• the structure is refined, it is possible to stably produce the above martensite steel, and it is possible to achieve both the strength and the crack arresting property of the hot-rolled steel sheet.
• the average cooling rate herein is a value obtained by dividing the temperature drop range of the steel sheet from when accelerated cooling starts (when the steel sheet is introduced into the cooling facility) until accelerated cooling is completed (when the steel sheet is taken out of the cooling facility) by the time required from when accelerated cooling starts until accelerated cooling is completed.
• the upper limit value of the cooling rate is not particularly specified, but if the cooling rate increases, the cooling facility will be large-scaled and the facility cost increases. Therefore, 300°C/sec or slower is preferable in consideration of facility costs.
• the coiling temperature is 350°C or lower.
• the coiling temperature is 350°C or lower, it is possible to reduce the amount of iron carbides precipitated and reduce the variation in hardness distribution in the hard phase.
• martensite steel was obtained by performing cooling to a temperature range of 350°C or lower at a desired average cooling rate without performing slow cooling.
• DP steel was obtained by performing slow cooling in a high temperature range (660 to 730°C) within the temperature range of 600 to 730°C
• bainite steel was obtained by performing slow cooling in a low temperature range (600°C or higher, lower than 660°C) within the temperature range of 600 to 730°C.
• the area proportion of the microstructure, the Rcf value, the standard deviation of the Mn concentration and the tensile strength TS of the obtained hot-rolled steel sheets were obtained by the above methods.
• the obtained measurement results are shown in Table 5A and Table 5B.
• the tensile strength TS was 980 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 980 MPa, it was determined as unsatisfactory because the hot-rolled steel sheet did not have high strength.
• the crack arresting property was evaluated by a Charpy impact test.
• a Charpy impact test According to JIS Z 2242: 2018, a 2.5 mm sub-sized V notch test piece in which the width direction (C direction) of the hot-rolled steel sheet was the longitudinal direction of the test piece was prepared, and the Charpy impact test was performed at -196°C. If the sheet thickness of the hot-rolled steel sheet was less than 2.5 mm, the test was performed on the entire thickness.
• a JIS No. 5 B tensile test piece in which the width direction (C direction) of the hot-rolled steel sheet was the longitudinal direction of the test piece was prepared, a compressive pre-strain of 10% was applied to the steel material in the longitudinal direction of the test piece and the V notch test piece was then collected.
• the test piece was subjected to the Charpy impact test at -196°C by the above method and thus the absorbed energy after plastic deformation was obtained.
• the hot-rolled steel sheets according to examples of the present invention had high strength and little deterioration in the crack arresting property after plastic deformation.
• the hot-rolled steel sheets according to the comparative examples did not have one or more of the above properties.

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