Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
  • 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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to a hot-rolled steel sheet having a yield strength of 700 MPa or more and excellent ductility and shear properties, and to a method for manufacturing the same.
• the hot-rolled steel sheet of the present invention is suitable as a material for a wide range of applications, including automotive parts and non-automotive parts.
• hot-rolled steel sheets are required to have both ductility and shear strength.
• formability and shear strength are essential to further expand the use of high-strength hot-rolled steel sheets.
• hot-rolled steel sheets with good galvanizability are also desired from the perspective of extending the life of components and improving their appearance.
• the combined volume fraction of the ferrite phase and bainite phase which have a small difference in hardness, is set to 95% or more, the volume fraction of the ferrite phase is set to 50-90%, and a specified amount of precipitates containing Ti and smaller than 20 nm in size are precipitated. This is said to result in a hot-rolled steel sheet with a tensile strength of 780 MPa or more and excellent stretch-flange formability.
• Patent Document 2 Ti carbide with an average grain size of less than 6 nm and TiS with an average grain size of 0.5 ⁇ m or less are dispersed and precipitated in a metal structure in which 95% or more of the area is made up of ferrite crystal grains. This is said to result in a hot-rolled steel sheet with excellent bending workability and a tensile strength of 780 MPa or more.
• Patent Document 3 a hot-rolled steel sheet is obtained that has a chemical composition in which the mass ratio of the Ti content to the C content, Ti/C, is 0.625 to 3.000, the dislocation density is 1 ⁇ 10 14 to 1 ⁇ 10 16 m -2 , and TiC precipitates with an average diameter of 2.0 nm or less are precipitated within crystal grains at a predetermined density or more. This results in a hot-rolled steel sheet with small punched edge damage and a tensile strength of 780 MPa or more.
• Patent Document 1 does not achieve sufficient uniform elongation because it utilizes a bainite structure obtained by low-temperature coiling. Furthermore, many of the steels disclosed in Patent Document 1 are silicon-added steels, which reduces galvanizability, so there is also the problem that hot-rolled steel sheets with good galvanizability cannot be obtained. Even in steels with low silicon additions, the composition and manufacturing method are inappropriate, so it is not possible to achieve a metal structure or properties that combine yield strength, excellent ductility, and shear strength.
• Patent Document 2 has a high ferrite area ratio, and in such a metal structure, the distribution of crystal grain sizes is narrow, resulting in poor shear strength.
• Patent Document 3 contains a large amount of dislocations, making it impossible to obtain the desired yield strength and uniform elongation.
• Patent Documents 1 and 3 require complex runout control before winding, which results in poor manufacturing stability.
• the present invention was developed in consideration of the above-mentioned problems with conventional technology, and aims to provide a high-strength hot-rolled steel sheet with a yield strength of 700 MPa or more and excellent ductility and shear properties, as well as a method for manufacturing the same.
• the thickness of the hot-rolled steel sheet targeted by this invention is in the range of 1.0 mm to 3.6 mm. To obtain good ductility in a hot-rolled steel sheet, it is effective to impart high uniform elongation. High uniform elongation characteristics in a hot-rolled steel sheet are easily obtained with a metal structure that is coiled at 600°C or above, at which dislocation recovery is facilitated.
• the metal structure of steel coiled at temperatures above 600°C was primarily ferrite.
• the crystal grains are uniformly sized with a narrow distribution, and the crystal strain is distributed throughout the entire steel sheet. This results in unstable crack propagation when the steel sheet is sheared, making it difficult to achieve the desired shear strength.
• many prior art technologies utilize Si to harden ferrite, which is inherently soft. However, the inclusion of Si in steel sheet reduces galvanizability.
• Shear strength is improved by dispersing a large number of crystal grains with a high KAM value, which stabilizes crack propagation during shear. Furthermore, crystal grains with a high KAM value have superior ductility properties compared to bainite or martensite, which have a lath-shaped structure. Therefore, it is possible to provide steel sheets that combine excellent shear strength and ductility.
• the KAM value is an index that represents the plastic strain gradient in a microscopic region.
• the hot-rolled steel sheet according to the present invention which was developed based on the above findings, has the following configuration.
• C 0.06% or more and 0.18% or less
• Si less than 0.15%
• Mn more than 0.7% and 2.5% or less
• P 0.05% or less
• S 0.010% or less
• Al 0.005% or more and 0.080% or less
• N 0.0060% or less
• Ti 0.08% or more and 0.20% or less
• the hot-rolled steel sheet has a chemical composition
• the metal structure has a ratio of an area percentage where the KAM value is 1.0 or more and 4.0 or less to an area percentage where the KAM value is less than 1.0 of 0.05 or more, a coefficient of variation of crystal grain size of 0.55 or more, an amount of dissolved Ti present in the steel of 0.03% or less, an average particle size of Ti-containing carbides of 10 nm or less, a yield strength of 700 MPa or more, and a uniform elongation of 8.5% or more.
• Group A at least one selected from V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less;
• Group B at least one selected from Cu: 0% or more and 1.0% or less, Ni: 0% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, and B: 0% or more and 0.010% or less;
• Group C at least one selected from Ca: 0% or more and 0.01% or less, Mg: 0% or more and 0.01% or less, REM: 0% or more and 1.0% or less, and Co: 0% or more and 0.01% or less, and
• Group D at least one selected from Sb: 0% or more and 0.01% or less, Sn: 0% or more and 0.1% or less, As: 0% or more and 0.01%
• the method for producing a hot-rolled steel sheet according to the present invention is configured as follows. [3] In mass%, C: 0.06% or more and 0.18% or less, Si: less than 0.15%, Mn: more than 0.7% and 2.5% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0060% or less, and Ti: 0.08% or more and 0.20% or less, and optionally contains at least one component selected from the following groups A to D, with the balance being Fe and
• the method includes a rough rolling step of rough rolling a steel material having a component composition consisting of steel and unavoidable impurities to form a sheet bar, a finish rolling step of finish rolling the sheet bar to form a hot rolled steel sheet, a cooling step of cooling the hot rolled steel sheet, a coiling step of winding the hot rolled steel sheet to form a coil, and a coil cooling step of cooling the coil,
• a rolling speed at the finish rolling exit side is 500 m/min or more; a mean cooling rate of the hot-rolled steel sheet is 40°C/s or more to a cooling stop temperature of 600°C or more and 700°C or less; a coiling temperature of the hot-rolled steel sheet is 600°C or more and 700°C or less; a coiling temperature of the hot-rolled steel sheet is 600°C or more and 700°C or less; a coiling temperature of the hot-rolled steel sheet is 600°C or more and 700°C or less; a coiling speed of the coil is 50°C or more to a cooling stop temperature of 500°C or less; Group A: at least one selected from V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less; Group B: at least one selected from
• the method for producing a hot-rolled steel sheet includes a joining step of joining the roughly rolled sheet bar and a preceding sheet bar at 1000°C or higher between the rough rolling step and the finish rolling step, and in the finish rolling step, the joined sheet bar is finish-rolled.
• the method for producing a hot-rolled steel sheet further includes a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720°C or less to form a hot-rolled annealed sheet, and a plating step of plating the hot-rolled annealed sheet.
• the present invention it is possible to manufacture a high-strength hot-rolled steel sheet having a yield strength (YS) of 700 MPa or more, a uniform elongation of 8.5% or more, excellent ductility, and good shear properties.
• YS yield strength
• Use of the hot-rolled steel sheet according to the present invention makes it possible to reduce the thickness compared to conventional materials, thereby contributing to the reduction of CO2 emissions.
• the chemical composition of the hot-rolled steel sheet contains, in mass%, C: 0.06% or more and 0.18% or less, Si: less than 0.15%, Mn: more than 0.7% and 2.5% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0060% or less, and Ti: 0.08% or more and 0.20% or less.
• C 0.06% or more and 0.18% or less
• Si less than 0.15%
• Mn more than 0.7% and 2.5% or less
• P 0.05% or less
• S 0.010% or less
• Al 0.005% or more and 0.080% or less
• N 0.0060% or less
• Ti 0.08% or more and 0.20% or less.
• C 0.06% or more and 0.18% or less C is an element that combines with Ti to increase the strength of the steel sheet and contribute to delaying transformation during coiling.
• a C content of 0.06% or more is required.
• the C content is set to a range of 0.06% or more and 0.18% or less.
• the C content is preferably set to a range of 0.07% or more and 0.15% or less.
• Si Less than 0.15% Si is a harmful element that reduces platability. Furthermore, Si increases the driving force for the austenite-to-ferrite transformation and impairs the delay effect of the austenite-to-ferrite transformation during coiling, so in this embodiment, it is an element that needs to be reduced as much as possible. Therefore, the Si content is less than 0.15%. The Si content is preferably less than 0.10%. Note that even if the Si content is 0%, the effects of the present invention are not impaired.
• Mn more than 0.7% and not more than 2.5%
• Mn is an element that contributes to delaying transformation during coiling.
• the Mn content is set to more than 0.7%.
• the Mn content exceeds 2.5%, the austenite-to-ferrite transformation does not proceed during coiling, making it impossible to obtain the desired structure and properties.
• the Mn content is set to a range of more than 0.7% and not more than 2.5%, and preferably to a range of 0.8% to 2.0%.
• P 0.05% or less
• P is a harmful element that segregates at grain boundaries and reduces shear strength, so it is preferable to reduce it as much as possible.
• the P content can be tolerated up to 0.05%.
• the P content is preferably 0.04% or less.
• P may be unavoidably mixed in, with the lower limit set to 0.002%.
• S 0.010% or less S forms coarse sulfides in steel, which elongate and become wedge-shaped inclusions during hot rolling, adversely affecting shear properties. Therefore, since S is also a harmful element, it is preferable to reduce its content as much as possible.
• S is tolerable up to 0.010%, so the upper limit of the S content is set to 0.010%.
• the S content is preferably 0.003% or less. For use in environments requiring stricter toughness, it is more preferable to suppress the S content to 0.002% or less.
• the lower limit of the S content is 0.0001%, and S may be unavoidably mixed in.
• Al 0.005% or more and 0.080% or less
• the Al content is 0.005% or more.
• Al reduces ductility and shear strength by forming oxides. Therefore, the Al content is set to 0.080% or less.
• the Al content is set to a range of 0.010% or more and 0.070% or less.
• N 0.0060% or less
• N is a harmful element that combines with Ti to form coarse TiN, reducing strength and shear resistance. Therefore, it is preferable to reduce N as much as possible.
• the N content can be up to 0.0060%.
• the N content is more preferably 0.0050% or less.
• N may be unavoidably mixed in, with the lower limit being 0.0005%.
• Ti 0.08% or more and 0.20% or less.
• Ti combines with C to form fine Ti-containing carbides, thereby contributing to the strength of the steel sheet. Furthermore, Ti precipitates as TiC at the interface between austenite and ferrite during the transformation from austenite to ferrite, thereby pinning the interface and delaying the transformation. To obtain the desired yield strength and metal structure, the Ti content is set to 0.08% or more. On the other hand, if the Ti content exceeds 0.20%, the coarse Ti-containing carbides cannot be dissolved in the heating process before hot rolling, resulting in a saturation of the strength-enhancing effect and adversely affecting ductility and shear strength. Therefore, the Ti content is set to 0.08% or more and 0.20% or less. The Ti content is preferably set to 0.09% or more and 0.18% or less.
• [%C], [%Mn], [%N] and [%Ti] are the C content, Mn content, N content and Ti content in mass %, respectively.
• the above is the basic composition of the hot-rolled steel sheet according to this embodiment.
• it may further contain at least one element from Groups A to D below.
• Group A at least one element selected from V: 0% to 0.2%, Nb: 0% to 0.07%, Mo: 0% to 0.15%, Zr: 0% to 0.1%, Hf: 0% to 0.1%, and W: 0% to 0.1%.
• V, Nb, Mo, Zr, Hf, and W are elements that contribute to strengthening the steel sheet by forming precipitates. Therefore, the content of one or more elements selected from V, Nb, Mo, Zr, Hf, and W is preferably 0% or more.
• each element is contained in an amount exceeding the upper limit, TiC cannot be dissolved during slab reheating, and sufficient fine TiC cannot be obtained during hot rolling, resulting in the failure to obtain the desired metal structure.
• Group B at least one selected from Cu: 0% to 1.0%, Ni: 0% to 1.0%, Cr: 0% to 1.0%, and B: 0% to 0.010%.
• Cu, Ni, Cr, and B are elements that mainly change the transformation behavior from austenite to ferrite. Therefore, the content of one or more elements selected from Cu, Ni, Cr, and B is preferably 0% or more.
• each element is contained in excess of the upper limit, the interfacial migration speed between austenite and ferrite during the transformation from austenite to ferrite decreases, making it impossible to obtain a metal structure with a high KAM value.
• Group C at least one selected from Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM: 0% to 1.0%, and Co: 0% to 0.01%.
• Ca, Mg, REM, and Co are elements that can be expected to change the morphology of inclusions and improve shear strength. Therefore, the content of one or more elements selected from Ca, Mg, REM, and Co is preferably 0% or more.
• the upper limits of the contents of Ca, Mg, REM, and Co are specified as above.
• REM is a collective term for 17 elements, including Sc, Y, and lanthanides, and is expressed as the total content of each element.
• Group D at least one selected from Sb: 0% or more and 0.01% or less, Sn: 0% or more and 0.1% or less, As: 0% or more and 0.01% or less, Ta: 0% or more and 0.01% or less, Pb: 0% or more and 0.01% or less, Cs: 0% or more and 0.01% or less, Te: 0% or more and 0.01% or less, Bi: 0% or more and 0.01% or less, Zn: 0% or more and 0.01% or less, Ge: 0% or more and 0.01% or less, and Sr: 0% or more and 0.01% or less.
• Sb, Sn, As, Ta, Pb, Cs, Te, Bi, Zn, Ge, and Sr are elements mixed in as impurities.
• the upper limit of the content of each element is specified as above as a range that does not affect the properties of the hot-rolled steel sheet according to this embodiment.
• the chemical composition of the hot-rolled steel sheet according to this embodiment contains the above elements, with the remainder being Fe and unavoidable impurities.
• the metal structure of the hot-rolled steel sheet according to this embodiment has a ratio (area ratio of KAM values of 1.0 or more and 4.0 or less)/(area ratio of KAM values less than 1.0) of 0.05 or more, a coefficient of variation of crystal grain size of 0.55 or more, an amount of solute Ti present in the steel of 0.03% or less, and an average particle size of Ti-containing carbides of 10 nm or less.
• the KAM value of a steel sheet obtained by electron backscatter diffraction (EBSD) analysis represents the strain distribution of crystal grains.
• EBSD electron backscatter diffraction
• the (Area ratio of KAM value of 1.0 or more and 4.0 or less) / (Area ratio of KAM value less than 1.0) (hereinafter also referred to as the "KAM ratio") is 0.05 or more.
• the KAM ratio is 0.07 or more. While there is no upper limit, the KAM ratio is preferably 0.20 or less from the viewpoint of ensuring sufficient ductility.
• Coefficient of variation of grain size 0.55 or more
• the grain size of the steel sheet can be obtained by EBSD analysis. By mixing grains with different grain sizes, the shear properties become good.
• the desired shear properties are obtained when the coefficient of variation of grain size given by the following formula (2) is 0.55 or more.
• a preferable coefficient of variation of grain size is 0.65 or more.
• the coefficient of variation of grain size is preferably 0.75 or less.
• Amount of solute Ti present in steel 0.03% or less
• the amount of strengthening due to Ti-containing carbides depends not only on the average particle size but also on the amount of precipitation.
• the amount of precipitation is greatly affected by the coiling temperature, and when the coiling temperature is below 600°C, Ti does not precipitate but remains in a solid solution state, making it impossible to obtain the desired yield strength. Therefore, to obtain a yield strength of 700 MPa or more, the amount of solute Ti is limited to 0.03% or less. More preferably, the amount of solute Ti is 0.02% or less.
• the amount of solute Ti may be 0.
• Average particle size of Ti-containing carbides 10 nm or less
• the steel sheet is strengthened by Ti-containing carbides.
• the average particle size of Ti-containing carbides dispersed in the steel is set to 10 nm or less.
• the average particle size of Ti-containing carbides is set to 5 nm or less.
• the average particle size of Ti-containing carbides is 1 nm or more.
• the hot-rolled steel sheet according to this embodiment preferably has a plating layer on its surface. Even if a plating layer is formed, the functionality of the hot-rolled steel sheet is not impaired.
• the composition of the plating layer is preferably at least one selected from Zn, Si, Al, Ni, and Mg.
• the plated steel sheet according to this embodiment includes steel that has been subjected to hot-dip galvanizing (hereinafter also referred to as GI), steel that has been subjected to hot-dip galvanizing followed by alloying (hereinafter also referred to as GA), and steel that has been subjected to electro-galvanizing (hereinafter also referred to as EG).
• the method for producing a hot-rolled steel sheet according to this embodiment includes a rough rolling process in which a steel material having the specified composition for the hot-rolled steel sheet is rough-rolled to form a sheet bar, a finish rolling process in which the sheet bar is finish-rolled to form a hot-rolled steel sheet, a cooling process in which the hot-rolled steel sheet is cooled, a winding process in which the hot-rolled steel sheet is wound into a coil, and a coil cooling process in which the coil is cooled.
• the steel material is heated to 1200°C or higher, or the steel material is held at 1200°C or higher after casting and then rough-rolled, and the temperature of the sheet bar at the completion of rough rolling is set to a range of 1000°C to 1100°C.
• the temperature of the rolled material at the start of finish rolling is set to 950°C or higher
• the total reduction rate for the first and second passes is set to 70% or less
• the temperature of the rolled material at the finish rolling exit is set to 850°C or higher
• the rolling speed at the finish rolling exit is set to 500 m/min or higher.
• the average cooling rate of the hot-rolled steel sheet is 40°C/s or more to a cooling stop temperature in the range of 600°C to 700°C.
• the coiling temperature of the hot-rolled steel sheet is 600°C to 700°C.
• the average cooling rate of the coiled coil is 50°C/h or more to 500°C.
• hot-rolled steel sheet is manufactured by casting a slab (steel material), then loading the slab (steel material) into a heating furnace after its temperature has dropped to below 1000°C, where it is heated for a short period of time, then reducing its thickness to a predetermined thickness on a hot rolling line and winding it into a coil.
• the slab (steel material) is once cooled to room temperature and heated for a long period of time in a heating furnace, then reducing its thickness to a predetermined thickness on a hot rolling line and winding it into a coil.
• the hot-rolled steel sheet manufacturing method according to this embodiment can be applied not only to the process of heating the steel material after casting, but also to the process of directly sending the steel material to a hot rolling line after casting without heating it.
• Steel material temperature heated to 1200°C or higher, or maintained at 1200°C or higher after casting. If coarse carbides containing Ti are present during finish rolling, ductility and shear strength decrease. If the slab temperature after casting drops below 1200°C, carbides containing Ti precipitate in the slab, and the grains grow to form coarse carbides. Therefore, if the slab temperature drops below 1200°C, it is necessary to heat the slab to 1200°C or higher to dissolve the carbides containing Ti. A preferred heating temperature is 1220°C or higher. There is no particular upper limit to the heating temperature, but 1300°C is a manufacturing constraint to avoid thermal damage in the annealing furnace. If the slab temperature after casting is maintained at 1200°C or higher, hot rolling can be performed without heating. A preferred slab temperature is maintained at 1220°C or higher. Due to manufacturing constraints during slab casting, it is preferable to set the upper limit of the slab temperature to approximately 1300°C.
• Rough rolling completion temperature 1000°C or higher and 1100°C or lower
• the rough rolling completion temperature which is the temperature of the rolled material (sheet bar) at the completion of rough rolling of the steel material
• the rough rolling completion temperature is set to a range of 1000°C or higher and 1100°C or lower.
• the rough rolling completion temperature is set to a range of 1010°C or higher and 1080°C or lower.
• the sheet bar temperature at the start of finish rolling should be 950°C or higher, and the total reduction ratio for the first and second passes, where the rolling speed is slower than that at the finish rolling exit side, should be limited to 70% or less.
• the preferred total reduction ratio for the first and second passes is 50% or more and 67% or less.
• the start temperature of finish rolling should be lower than the rough rolling completion temperature.
• Finish rolling exit temperature 850°C or higher
• finish rolling exit rolling speed 500 m/min or higher.
• the amount of Ti-containing carbides precipitated by rolling varies depending on the processing temperature, processing amount, and holding time after processing. To suppress the amount of Ti-containing carbides precipitated in austenite by processing, it is effective to increase the processing temperature to promote the recovery of processed austenite and shorten the holding time after processing.
• the finish rolling exit temperature and rolling speed must be 850°C or higher and 500 m/min or higher, respectively.
• the finish rolling exit temperature and rolling speed are 900°C or higher and 550 m/min or higher, respectively.
• the thickness of the steel sheet after finish rolling is in the range of 1.0 mm to 3.6 mm.
• the upper limit of the temperature on the delivery side of finish rolling is preferably set to about 950° C.
• the upper limit of the rolling speed on the delivery side of finish rolling is preferably set to about 800 m/min.
• Cooling process of hot-rolled steel sheet after finish rolling average cooling rate of 40°C/s or more until cooling stop temperature of 600°C to 700°C. If the cooling stop temperature of the hot-rolled steel sheet after finish rolling exceeds 700°C or the average cooling rate is less than 40°C/s, the temperature at which the austenite-to-ferrite transformation starts becomes too high, and the particle size of Ti-containing carbides precipitated at the interface between austenite and ferrite increases. As a result, the pinning effect at the interface is not sufficiently obtained, and not only is the desired microstructure not obtained, but the amount of strengthening achieved by the precipitation of Ti-containing carbides is reduced, and the yield strength does not reach 700 MPa or more. For this reason, the cooling stop temperature and average cooling rate of the hot-rolled steel sheet after finish rolling are set to 700°C or less and 40°C/s or more, respectively.
• the average cooling rate is forced cooling with a cooling rate faster than air cooling by water cooling or the like, and forced cooling should preferably begin within 3 seconds after the completion of finish rolling. Therefore, the average cooling rate can be calculated by ⁇ (finish rolling completion temperature) - (cooling stop temperature) ⁇ / (cooling time by forced cooling). If the cooling stop temperature is below 600°C, the amount of solute Ti and dislocation density will increase, making it impossible to achieve a yield strength of 700 MPa or more or a uniform elongation of 8.5% or more. For this reason, the cooling stop temperature of hot-rolled steel sheet after finish rolling should be 600°C or higher.
• the preferred cooling stop temperature is in the range of 610°C to 690°C, and the preferred average cooling rate is 50°C/s or higher. While there is no upper limit, considering equipment constraints, it is preferable that the average cooling rate be limited to an upper limit of 200°C/s.
• Coiling process coiling temperature of 600°C or higher and 700°C or lower.
• the coiling temperature of the hot-rolled steel sheet exceeds 700°C, the temperature at which the transformation from austenite to ferrite begins increases, and the particle size of Ti-containing carbides precipitated at the austenite/ferrite interface increases.
• the pinning effect at the interface is not sufficiently obtained, and the desired microstructure is not obtained.
• the strengthening amount obtained by precipitating Ti-containing carbides is reduced, and a yield strength of 700 MPa or higher is not achieved.
• the coiling temperature is set to a range of 600°C or higher and 700°C or lower.
• the coiling temperature is set to a range of 610°C or higher and 680°C or lower.
• the average cooling rate of the coil is 50°C/h or more until it reaches 500°C. If the cooling rate of the coil after coiling is slow, the KAM ratio and the coefficient of variation of the grain size will decrease during the cooling process after coiling, making it impossible to obtain the structure desired in this embodiment. To avoid this adverse effect, the coil is cooled at an average cooling rate of 50°C/h or more until it reaches 500°C after coiling. Preferably, the average cooling rate is 75°C/h or more. In the temperature range below 500°C, the change in structure is small, and cooling by normal air cooling may be used. Although there is no upper limit, considering the temperature variation within the coil, it is preferable that the average cooling rate of the coil after coiling be 150°C/h or less until it reaches 500°C.
• the rough-rolled sheet bar and the preceding sheet bar are joined at 1000°C or higher. If the joining temperature of the sheet bar falls below 1000°C, it becomes difficult to roll at the finish rolling start temperature of 950°C or higher in the subsequent finish rolling process.
• the preferred joining temperature of the sheet bar during joining is 1100°C or higher. From the viewpoint of optimizing the metal structure, it is preferable that the upper limit of the joining temperature of the sheet bar during joining is 1200°C.
• the method for producing a hot-rolled steel sheet according to this embodiment can employ an annealing process in which the hot-rolled steel sheet is annealed in a continuous annealing line at an annealing temperature of 720°C or less, and a plating process in which the hot-rolled steel sheet is plated in a continuous plating line. Furthermore, an alloying process in which the plated hot-rolled steel sheet is heated to a temperature range of 460°C or more and 600°C or less and subjected to an alloying treatment may be included. This annealing process or this plating process does not affect the material properties of the hot-rolled steel sheet according to this embodiment. Therefore, it is possible to plate the surface of the hot-rolled steel sheet to form a plating layer on the steel sheet surface.
• the plating process and the composition of the plating bath do not affect the material properties of the hot-rolled steel sheet according to this embodiment, and therefore any of hot-dip galvanizing, galvannealed hot-dip galvanizing, and electro-galvanizing processes can be applied as the plating process.
• the composition of the plating bath can contain at least one element selected from Zn, Al, Mg, Si, and Ni.
• the composition of the plating layer formed on the surface of the hot-rolled steel sheet during the plating process can contain at least one element selected from Zn, Si, Al, Ni, and Mg.
• the hot-rolled steel sheets obtained under the conditions shown in Tables 1 to 4 were evaluated using the following methods in terms of metal structure, tensile properties, and shear properties. The results are shown in Table 5.
• (i) Metal structure analysis method The KAM value and the coefficient of variation of the grain size were measured using the EBSD method.
• the field area of the analysis target was 2500 ⁇ m2 , and data was acquired with a step width of 0.5 ⁇ m.
• the obtained image data was analyzed using OIM Analysis software (TSL).
• TTL OIM Analysis software
• the KAM value analysis was performed under the condition of 1st nearest neighbor.
• the KAM (Kernel Average Misorientation) value is the average value of the orientation difference between the measurement point of interest and the adjacent part, and is an index representing the plastic strain gradient in a microregion.
• the grain size distribution was acquired in the "Grain Size (diameter)" mode, and the coefficient of variation of the grain size was calculated from the average grain size and standard deviation using the above-mentioned formula (2).
• the shearability evaluation column was marked "good.” On the other hand, if the sum of the lengths of the abnormalities relative to the sum of the punched end face lengths exceeded 3%, the shearability evaluation column was marked "bad.”

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=96590705

Family Applications (1)

Country Status (2)

Citations (6)

• 2025
  • 2025-01-15 JP JP2025529953A patent/JP7827217B2/ja active Active
  • 2025-01-15 WO PCT/JP2025/001011 patent/WO2025164309A1/ja active Pending

Patent Citations (6)

Also Published As

Similar Documents

Legal Events