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 steel sheets and their manufacturing methods.
• steel sheets and high-carbon steel sheets are used as materials for structural and mechanical parts in various machines and devices, such as automobiles. These steel sheets are processed into a predetermined shape and then subjected to heat treatments such as quenching and tempering to become various parts.
• Patent Document 1 proposes a medium carbon steel cold-rolled steel sheet having a chemical composition consisting of C: 0.2 to 0.8 wt%, Si: 0.35 wt% or less, Mn: 0.90 wt% or less, P: 0.03 wt% or less, S: 0.02 wt% or less, Cr: 0.2 wt% or less, Ca: 0.001 to 0.02 wt%, the balance Fe and unavoidable impurities, and the cleanliness of the steel (JIS G0555) is dA (60 ⁇ 400): 0.015% or less, dT (60 ⁇ 400): 0.030% or less, and having excellent press workability and a ferrite-pearlite mixed structure.
• Patent Document 1 has good workability and wear resistance, it cannot be said to be sufficiently hardened, and therefore, in order to ensure the strength required for parts, it must be heat-treated after processing to increase the strength.
• the present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a steel sheet that has high hardness, allows for the omission of heat treatment after processing, and has excellent uniform elongation and wear resistance, and a manufacturing method thereof.
• the present invention provides a steel sheet having a composition containing, on a mass basis, C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities;
• the average grain size of ferrite is 15.0 ⁇ m or less
• the area ratio of pearlite is 90% or more
• the area ratio of crystal grains having a KAM value of 5° or less and a crystal grain size of 1.0 to 10.0 ⁇ m is 20% or more
• the present invention relates to a steel sheet having a KAM value of 5° or less and a number density of crystal grains having a grain size of 20.0 ⁇ m or more of 500 grains/mm 2 or less.
• the present invention also provides a hot rolling process in which a slab having a composition containing, by mass, 0.35 to 0.90% C, 0.01 to 0.50% Si, 0.20 to 1.30% Mn, 0.100% or less P, 0.100% or less S, 0.100% or less Al, 0.50% or less Cr, and 0.0150% or less N, with the balance being Fe and impurities, is hot rolled under conditions where the rolling temperature in the final stage is 830 to 950°C and the rolling reduction is 10 to 25%, and then cooled to a coiling temperature of 525 to 700°C at an average cooling temperature of 7 to 100°C/second and coiled at the coiling temperature;
• the present invention relates to a method for producing a steel sheet, which includes a cold rolling step of cold rolling the hot-rolled steel sheet obtained in the hot rolling step at a rolling ratio of 25 to 60%, and does not include an annealing step after the cold rolling step.
• the present invention provides a steel sheet and its manufacturing method that has high hardness, allows for the elimination of post-processing heat treatment, and is excellent in uniform elongation and wear resistance.
• a steel sheet according to an embodiment of the present invention has a composition containing C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities.
• the term "steel plate” refers to a plate-shaped (including strip-shaped) material made of steel.
• impurities refers to components that are mixed in during industrial production of steel sheets due to raw materials such as ores and scraps, or various factors in the manufacturing process, and are acceptable to the extent that they do not adversely affect the present invention.
• impurities include unavoidable impurities.
• impurities include Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca.
• xx% or less means that it is xx% or less, but includes an amount exceeding 0% (particularly, exceeding the impurity level).
• a numerical range expressed using "to” means a range that includes the numerical values before and after "to" as the lower and upper limits.
• the steel sheet according to the embodiment of the present invention may further contain one or more elements selected from the group consisting of Ni: 1.000% or less, Mo: 0.500% or less, V: 0.500% or less, Nb: 0.500% or less, Ti: 0.150% or less, and B: 0.0100% or less.
• C 0.35-0.90%
• the C content is set to 0.35% or more, preferably 0.38% or more, more preferably 0.40% or more, and even more preferably 0.45% or more.
• the C content is set to 0.90% or less, preferably 0.88% or less, and more preferably 0.85% or less.
• the above-mentioned numerical ranges of the C content may be any combination of the numerical ranges.
• the C content may be 0.35 to 0.90%, 0.35 to 0.88%, 0.35 to 0.85%, 0.38 to 0.90%, 0.38 to 0.88%, 0.38 to 0.85%, 0.40 to 0.90%, 0.40 to 0.88%, 0.40 to 0.85%, 0.45 to 0.90%, 0.45 to 0.88%, or 0.45 to 0.85%.
• Si 0.01-0.50%
• Si is an element necessary for deoxidation.
• the Si content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more.
• the Si content is set to 0.50% or less, preferably 0.48% or less, and more preferably 0.45% or less.
• Mn is an element that affects the strength, toughness, and other properties of steel sheets. If the Mn content is too low, ferrite is more likely to form, reducing the strength of the steel sheet and making it impossible to ensure wear resistance and punchability. Therefore, the Mn content is set to 0.20% or more, preferably 0.22% or more, and more preferably 0.25% or more. On the other hand, if the Mn content is too high, the steel sheet becomes hard, reducing toughness and uniform elongation. Therefore, the Mn content is set to 1.30% or less, preferably 1.28% or less, and more preferably 1.25% or less.
• the P content is set to 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.
• the P content can be set to, for example, 0.001% or more.
• S forms MnS, which easily becomes the starting point of fracture and reduces the workability of the steel sheet. Therefore, the S content is set to 0.100% or less, preferably 0.095% or less, and more preferably 0.090% or less. On the other hand, the lower the S content, the better, so there is no particular lower limit. However, since excessive reduction of the S content leads to increased costs, the S content can be set to, for example, 0.001% or more.
• Al 0.100% or less
• Al is an element that forms AlN and suppresses coarsening of crystal grains through a pinning effect.
• the Al content is set to 0.100% or less, preferably 0.098% or less, and more preferably 0.095% or less.
• the Al content may be low, and the lower limit is not particularly limited.
• the Al content can be set to, for example, 0.001% or more, 0.003% or more, or 0.005% or more.
• Cr 0.50% or less
• Cr is an element effective in forming a predetermined metal structure.
• the Cr content is set to 0.50% or less, preferably 0.48% or less, and more preferably 0.47% or less.
• the Cr content may be low, and the lower limit is not particularly limited.
• the Cr content can be, for example, 0.01% or more, 0.05% or more, 0.10% or more, 0.15%, or 0.20%.
• N is an element that forms AlN and suppresses coarsening of crystal grains through a pinning effect.
• the N content is set to 0.0150% or less, preferably 0.0140% or less, and more preferably 0.0135% or less.
• the N content may be low, and there is no particular lower limit.
• the N content can be set to, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more.
• Ni is an element that dissolves in steel to improve strength without impairing toughness.
• Ni is an expensive element, and excessive Ni content increases costs. Therefore, the Ni content is set to 1.000% or less, preferably 0.950% or less, and more preferably 0.900% or less.
• the Ni content may be low, and the lower limit is not particularly limited. However, from the viewpoint of obtaining the above-mentioned effects of Ni, the Ni content can be set to, for example, 0.001% or more, 0.010% or more, or 0.050% or more.
• Mo 0.500% or less
• Mo is an element that improves the strength of steel sheet.
• the Mo content is set to 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.
• the Mo content may be low, and the lower limit is not particularly limited.
• the Mo content can be set to, for example, 0.001% or more, 0.005% or more, or 0.010% or more.
• V, Nb, and Ti are all elements that improve the strength of steel sheets by carbide precipitation. However, if the contents of these elements are too high, a large amount of carbides are formed, resulting in a decrease in the toughness of the steel sheets. Therefore, the V content is set to 0.500% or less, preferably 0.480% or less, and more preferably 0.460% or less; the Nb content is set to 0.500% or less, preferably 0.480% or less, and more preferably 0.460% or less; and the Ti content is set to 0.150% or less, preferably 0.148% or less, and more preferably 0.145% or less.
• the contents of these elements may be small, and there is no particular lower limit.
• the contents of V, Nb, and Ti may all be set to, for example, 0.001% or more, 0.003% or more, or 0.005% or more.
• B (B: 0.0100% or less) B is an element that segregates at grain boundaries and improves grain boundary strength. However, if the B content is too high, the effect saturates and raw material costs increase. Therefore, the B content is set to 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less. On the other hand, the B content may be low, and the lower limit is not particularly limited. However, from the viewpoint of obtaining the above-mentioned effect of B, the B content can be set to, for example, 0.0001% or more, 0.0003% or more, or 0.0005% or more.
• These elements may be contained as impurities either alone or in combination of two or more.
• the content of each of Cu, W, and Ta is 0 to less than 0.15%, preferably 0 to 0.14%.
• the contents of Sn, Sb, Co, As, Mg, Y, Zr, La, Ce and Ca are all 0 to less than 0.050%, preferably 0 to 0.045%.
• a steel sheet according to an embodiment of the present invention has a metal structure mainly composed of pearlite.
• the pearlite has a eutectoid structure in which fine cementite is uniformly dispersed in ferrite.
• the steel sheet according to an embodiment of the present invention has an average grain size of ferrite of 15.0 ⁇ m or less, an area ratio of pearlite of 90% or more, an area ratio of grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 ⁇ m of 20% or more, and a number density of grains having a KAM value of 5° or less and a grain size of 20.0 ⁇ m or more of 500 grains/ mm2 or less.
• the average grain size of ferrite is 15.0 ⁇ m or less, preferably 13.0 ⁇ m or less, and more preferably 11.0 ⁇ m or less. It should be noted that the ferrite specifying the average grain size here refers to the ferrite in a mixed structure containing ferrite and pearlite, and not the ferrite that constitutes pearlite.
• the lower limit of the average grain size of ferrite is not particularly limited.
• the ferrite grains may be fine immediately after generation, the lower limit of the average grain size of ferrite may be 0.1 ⁇ m, 0.5 ⁇ m, or 0.9 ⁇ m.
• the steel sheet according to the embodiment of the present invention may be composed of a pearlite single phase structure, in which case ferrite is not present and the average grain size of ferrite can be considered to be 0 ⁇ m.
• the average grain size of ferrite is determined in accordance with JIS G0551:2020. Specifically, a cross section (L cross section) parallel to the rolling direction of a test piece cut from a steel sheet is polished, and then immersed in a 3% nital etchant to reveal the structure. In this cross section, the position corresponding to 1 ⁇ 4 of the thickness of the steel sheet is set as the center of the field of view, and the structure is observed at any five points using a scanning electron microscope (SEM). The magnification is set to 500 to 3000 times depending on the size of the crystal grains. The average crystal grain size is determined from the obtained microstructure photograph using a cutting method. The average crystal grain size is the average of the measurement results from the five points.
• SEM scanning electron microscope
• the steel sheet according to the embodiment of the present invention has a metallographic structure mainly composed of pearlite, thereby improving wear resistance and punchability.
• the area fraction of pearlite is set to 90% or more, preferably 91% or more, more preferably 92% or more, and even more preferably 93% or more.
• the upper limit of the area fraction of pearlite is not particularly limited, and may be 100% (i.e., the steel sheet may have a pearlite single-phase structure).
• the area ratio of pearlite is determined as follows. First, a cross section (L cross section) parallel to the rolling direction of a test piece cut from a steel plate is polished, and then immersed in a 3% nital etchant to reveal the structure. In this cross section, the position corresponding to 1 ⁇ 4 of the thickness of the steel plate is set as the center of the field of view, and the structure is observed at any five points using a scanning electron microscope (SEM). The magnification is set to 500 to 3000 times depending on the size of the crystal grains.
• SEM scanning electron microscope
• the area where cementite is uniformly dispersed in ferrite is recognized as pearlite, and the area ratio of pearlite (the area ratio of pearlite to the entire measurement area) is calculated using image analysis software.
• the area ratio of pearlite is taken as the average value of the measurement results from the five points.
• the KAM (Kernel Average Misorientation) value is a parameter that indicates the magnitude of strain, and a larger KAM value indicates a larger strain.
• the area ratio of grains with a KAM value of 5° or less and a grain size of 1.0 to 10.0 ⁇ m is related to the toughness and uniform elongation of the steel sheet. By setting the area ratio of these grains to 20% or more, a sufficient area with small strain is secured, thereby improving the toughness and uniform elongation of the steel sheet.
• the area ratio of crystal grains having a KAM value of 5° or less and a crystal grain size of 1.0 to 10.0 ⁇ m is determined as follows. First, a cross section (L cross section) parallel to the rolling direction of a test piece cut from a steel sheet was polished, and then electron backscattering diffraction (EBSD) analysis was performed at a 0.2 ⁇ m pitch over three or more fields of view, with a range of 200 ⁇ m in the thickness direction and 200 ⁇ m in the longitudinal direction, with the center of the field of view being a position corresponding to 1/4 of the thickness of the steel sheet. The grain size of the crystal grains was analyzed using the analysis software OIM Analysis (manufactured by TSL Solutions Co., Ltd.) based on the EBSD data.
• OIM Analysis manufactured by TSL Solutions Co., Ltd.
• the grain size was determined by defining the boundary where the orientation difference between adjacent measurement points was 5° or more as the grain boundary, and the crystal grain was calculated using the same software. Furthermore, based on the EBSD data, the local orientation difference around each measurement point was mapped using the KAM method, and the KAM value was calculated.
• the KAM method is a method for calculating for each pixel by averaging the orientation differences between the six adjacent pixels (first approximation) of a certain regular hexagonal pixel in the measurement data, the 12 pixels outside of that (second approximation), and the 18 pixels outside that (third approximation), and using this value as the local orientation difference (KAM value) of the central pixel.
• crystal grains with a KAM value of 5° or less and a grain size of 1.0 to 10.0 ⁇ m are identified, and their area ratio (the area ratio of the crystal grains in the entire measurement area) is calculated. This area ratio of the crystal grains is taken as the average value for each measurement field.
• Crystal grains with a KAM value of 5° or less and a grain size of 20.0 ⁇ m or more are related to the wear resistance of the steel sheet. If the number density of these crystal grains is 500 grains/ mm2 or less, it can be said that there are few coarse crystal grains present in the steel sheet, and the wear resistance of the steel sheet can be improved.
• the number density of crystal grains having a KAM value of 5° or less and a crystal grain size of 20.0 ⁇ m or more can be determined as follows. First, a backward electron diffraction analysis is performed in the same manner as above to calculate the grain size and KAM value of the grains. Next, the number of grains with a KAM value of 5° or less and a grain size of 20.0 ⁇ m or more is identified and divided by the area ( mm2 ) of the measurement region to calculate the number density of these grains. The number density of these grains is the average value of each measurement field.
• the steel sheet according to the embodiment of the present invention has the composition and metal structure described above, and therefore can have the following properties.
• the steel sheet according to the embodiment of the present invention preferably has a Vickers hardness of 300 to 500 Hv, more preferably 310 to 480 Hv, and even more preferably 315 to 460 Hv.
• a Vickers hardness within this range can be said to ensure workability (uniform elongation and toughness) while being hardened. Therefore, heat treatment after working the steel sheet can be omitted.
• the Vickers hardness is determined by polishing a cross section (L cross section) parallel to the rolling direction of a test piece cut out from a steel sheet, and then performing a Vickers hardness test at a position corresponding to 1 ⁇ 4 of the thickness of the steel sheet in accordance with JIS Z2244: 2009. In the Vickers hardness test, a measurement load of 1 kgf (9.807 N) is used, and measurements are taken at three arbitrary locations, with the average value being the measurement result.
• the steel sheet according to the embodiment of the present invention preferably has a uniform elongation of 1.5% or more.
• the uniform elongation is determined by preparing a 13B test piece in accordance with JIS Z2241:2023 and conducting a tensile test with the tensile axis in the rolling direction of the steel sheet. Three 13B test pieces are taken from the steel sheet, and measurements are performed on these three 13B test pieces, with the average value being the measurement result.
• the steel sheet according to the embodiment of the present invention may be either a hot-rolled steel sheet or a cold-rolled steel sheet, but is preferably a cold-rolled steel sheet.
• the thickness of the steel sheet is not particularly limited, but can be, for example, 10.0 mm or less, 8.0 mm or less, or 6.0 mm or less.
• a steel sheet manufacturing method The method for manufacturing a steel sheet according to an embodiment of the present invention is not particularly limited as long as it can manufacture a steel sheet having the above-described characteristics.
• a steel sheet according to an embodiment of the present invention can be produced by a method including a hot rolling step in which a slab having the composition described above is hot rolled under conditions where the rolling temperature in the final stage is 830 to 950°C and the rolling reduction is 10 to 25%, and then cooled to a coiling temperature of 525 to 700°C at an average cooling temperature of 7 to 100°C/second and coiled at that coiling temperature, and a cold rolling step in which the hot-rolled steel sheet obtained in the hot rolling step is cold-rolled at a rolling reduction of 25 to 60%, but which does not include an annealing step after the cold rolling step.
• a hot rolling step in which a slab having the composition described above is hot rolled under conditions where the rolling temperature in the final stage is 830 to 950°C and the rolling reduction is 10 to 25%, and then
• Hot rolling process In hot rolling, the rolling of a slab is performed in multiple stages. Usually, the final stage of rolling is called finish rolling, and the other stages of rolling are called rough rolling.
• the slab is preferably heated to 1100°C or higher to sufficiently redissolve Ti carbonitrides and the like. Rough rolling is performed on the slab to adjust the plate thickness, etc.
• the conditions for rough rolling are not particularly limited as long as the desired dimensions can be secured.
• Finish rolling final stage rolling is performed under conditions of a rolling temperature of 830 to 950°C and a rolling reduction of 10 to 25%. If the rolling temperature is less than 830°C and the rolling reduction is more than 25%, a great burden is placed on the rolling mill, which may cause equipment trouble.
• the rolling temperature is more than 950°C and the rolling reduction is less than 10%, the crystal grains become coarse, making it difficult to control the number density of crystal grains having a KAM value of 5° or less and a crystal grain size of 20.0 ⁇ m or more to 500 grains/ mm2 or less.
• the rolling temperature is preferably 840 to 940°C and the rolling reduction is preferably 11 to 24%.
• the steel After hot rolling, the steel is cooled to a coiling temperature of 525-700°C at an average cooling rate of 7-100°C/s, and then coiled at that temperature. If the coiling temperature is below 525°C, pearlite will not form and a pearlite-based structure will not be obtained, while if it exceeds 700°C, pickling properties will be impaired. Furthermore, if the average cooling rate is below 7°C/s, too much ferrite will precipitate, making it impossible to obtain a pearlite-based structure, and if it exceeds 100°C/s, it will be difficult to obtain a flat steel sheet.
• the hot-rolled steel sheet obtained in the hot rolling process is cold-rolled at a rolling reduction of 25 to 60%. If the rolling reduction is less than 25%, the cementite is not sufficiently refined, making it difficult to control the number density of crystal grains having a KAM value of 5° or less and a grain size of 20.0 ⁇ m or more to 500 grains/ mm2 or less. On the other hand, if the rolling reduction exceeds 60%, it becomes difficult to control the area ratio of crystal grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 ⁇ m to 20% or more.
• known steps may be performed. For example, a pickling step may be performed between the hot rolling step and the cold rolling step. The conditions for these steps are not particularly limited, and these steps may be performed in accordance with the conditions of known steel sheet manufacturing methods.
• a 250 mm thick slab having the composition shown in Table 1 (the remainder being Fe and impurities other than the elements shown in Table 1) was produced using a continuous casting method.
• the slab was then heated to 1100-1250°C and held there for 1 hour, after which it was subjected to a hot rolling process.
• Hot rolling was performed under the conditions of the rolling temperature and reduction ratio in the final stage shown in Table 2.
• the slab was then cooled to the coiling temperature shown in Table 2 at the average cooling temperature shown in Table 2, and coiled at that coiling temperature.
• the coiled hot-rolled steel sheet was then pickled to remove oxide scale, and then cold-rolled at the reduction ratio shown in Table 2 to obtain a 2 mm thick cold-rolled steel sheet.
• annealing was performed at 710°C for 40 hours after cold rolling.
• the cold-rolled steel sheets and cold-rolled annealed steel sheets obtained in the above examples were evaluated as follows.
• the average grain size of ferrite was measured according to the method described above.
• the test specimens had dimensions of 15 mm in the rolling direction, 10 mm in the width direction, and 2 mm in thickness.
• the area ratio of the specified crystal grains and the number density of the specified crystal grains were determined according to the above-mentioned method.
• the test specimen had a size of 15 mm in the rolling direction, 10 mm in the width direction, and 2 mm in thickness.
• the condition A is that the KAM value is 5° or less and the area ratio of crystal grains with a grain size of 1.0 to 10.0 ⁇ m is 20% or more, and those that satisfy this condition are indicated as OK, and those that do not satisfy this condition are indicated as NG.
• the condition B is that the KAM value is 5° or less and the number density of crystal grains with a grain size of 20.0 ⁇ m or more is 500 grains/ mm2 or less, and those that satisfy this condition are indicated as OK, and those that do not satisfy this condition are indicated as NG.
• Vickers hardness Vickers hardness was measured according to the method described above.
• the uniform elongation was measured according to the method described above. In this evaluation, a uniform elongation of 1.5% or more was evaluated as OK (good uniform elongation), and a uniform elongation of less than 1.5% was evaluated as NG (insufficient uniform elongation).
• a pin-on-disk test was performed under dry conditions at room temperature using abrasive paper (SiC with a grit size of #800) as a disk, and the wear volume was measured.
• the cold-rolled steel sheet and the cold-rolled annealed steel sheet were processed into pin test pieces with a diameter of 5 mm.
• the pin-on-disk test was performed under a load of 20 N, a friction speed (rotational speed) of 0.66 m/s, and a friction time of 2.5 minutes (friction distance of 100 m), and the wear volume was measured from a friction distance of 50 m to 100 m.
• a wear amount of 35 ⁇ 10 ⁇ 5 mm 3 /Nm or less was designated as OK (good wear resistance)
• a wear amount exceeding 35 ⁇ 10 ⁇ 5 mm 3 /Nm was designated as NG (insufficient wear resistance).
• V-notch test specimens were taken from the cold-rolled steel sheets and the cold-rolled annealed steel sheets, and subjected to a Charpy impact test at 150° C. This test was performed in accordance with JIS Z2242:2023, and the test specimens were V-notched, had a thickness of 2.0 mm (dimensional tolerance ⁇ 0.05 mm), and were taken so that the length direction was parallel to the rolling direction. In this evaluation, a specimen having a brittle fracture rate of 70% or less was designated as OK (good toughness), and a specimen having a brittle fracture rate of more than 70% was designated as NG (insufficient toughness).
• the cold-rolled steel sheets of Examples 1 to 35 had suitable compositions and metal structures, and therefore, the results of each characteristic evaluation were good.
• the cold-rolled steel sheet of Comparative Example 1 had an excessively high C content, resulting in high strength and insufficient uniform elongation and toughness.
• the cold-rolled steel sheet of Comparative Example 2 had insufficient strength due to an excessively low C content, and also had a small area ratio of pearlite, resulting in insufficient wear resistance and punchability.
• the cold-rolled steel sheet of Comparative Example 3 had an excessively high Si content, and therefore had a small pearlite area ratio, resulting in insufficient wear resistance and punchability.
• the cold-rolled steel sheet of Comparative Example 4 had an excessively low Mn content, and therefore had a small pearlite area ratio, resulting in insufficient wear resistance and punchability.
• the cold-rolled steel sheet of Comparative Example 5 had an excessively high Mn content, and therefore had high strength but insufficient uniform elongation and toughness.
• the cold-rolled steel sheet of Comparative Example 6 had an excessively high Cr content, and therefore had high strength but insufficient uniform elongation and toughness.
• the rolling reduction ratio during cold rolling was too high, so that the area ratio of crystal grains having a KAM value of 5° or less and a crystal grain size of 1.0 to 10.0 ⁇ m could not be made 20% or more, resulting in insufficient uniform elongation and toughness.
• the rolling reduction ratio during cold rolling was too low, so the number density of crystal grains having a KAM value of 5° or less and a grain size of 20.0 ⁇ m or more could not be reduced to 500 grains/ mm2 or less. As a result, the wear resistance was insufficient.
• the rolling reduction ratio in the final stage of hot rolling was too low, so the number density of crystal grains having a KAM value of 5° or less and a grain size of 20.0 ⁇ m or more could not be reduced to 500 grains/ mm2 or less. As a result, the wear resistance was insufficient.
• the rolling temperature in the final stage of hot rolling was too high, so the number density of crystal grains having a KAM value of 5° or less and a grain size of 20.0 ⁇ m or more could not be reduced to 500 grains/ mm2 or less. As a result, the wear resistance was insufficient.
• the rolling ratio in the final stage of hot rolling was too low and the rolling temperature in the final stage of hot rolling was too high, so that the ferrite became coarse and the area ratio of pearlite became small, resulting in insufficient wear resistance and punchability.
• the average cooling rate during cooling after hot rolling was too slow, so that the ferrite became coarse and the area ratio of pearlite became small, resulting in insufficient wear resistance and punchability.
• the cold-rolled steel sheet of Comparative Example 13 did not contain Al or N, and therefore the ferrite was coarsened, resulting in insufficient wear resistance and punchability.
• Comparative Examples 14 to 16 are cold-rolled and annealed steel sheets that were annealed after cold rolling, it was not possible to reduce the number density of crystal grains having a KAM value of 5° or less and a grain size of 20.0 ⁇ m or more to 500 grains/ mm2 or less. Furthermore, in Comparative Example 16, it was not possible to reduce the area ratio of crystal grains having a KAM value of 5° or less and a grain size of 1.0 to 10.0 ⁇ m to 20% or more. For this reason, these cold-rolled and annealed steel sheets were insufficient in strength, and also insufficient in wear resistance and punchability.
• the present invention can provide a steel sheet and its manufacturing method that has high hardness, allows for the elimination of post-processing heat treatment, and is excellent in uniform elongation and wear resistance.
• the present invention can provide a steel sheet and its manufacturing method that has high hardness, allows for the elimination of post-processing heat treatment, and is excellent in uniform elongation and wear resistance.
• the average grain size of ferrite is 15.0 ⁇ m or less,
• the area ratio of pearlite is 90% or more,
• the area ratio of crystal grains having a KAM value of 5° or less and a crystal grain size of 1.0 to 10.0 ⁇ m is 20% or more,
• the impurities include, on a mass basis, one or more selected from the group consisting of Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050%.
• a hot rolling process in which a slab having a composition containing, by mass, C: 0.35 to 0.90%, Si: 0.01 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.50% or less, and N: 0.0150% or less, with the balance being Fe and impurities, is hot rolled under conditions where the rolling temperature in the final stage is 830 to 950°C and the rolling ratio is 10 to 25%, and then cooled to a coiling temperature of 525 to 700°C at an average cooling temperature of 7 to 100°C/second and coiled at the coiling temperature;
• a method for producing a steel plate comprising: a cold rolling step of cold rolling the hot-rolled steel plate obtained in the hot rolling step at a rolling ratio of 25 to 60%, and not including an annealing step after the cold rolling step.
• the impurities include, on a mass basis, one or more selected from the group consisting of Cu: 0 to less than 0.15%, W: 0 to less than 0.15%, Ta: 0 to less than 0.15%, Sn: 0 to less than 0.050%, Sb: 0 to less than 0.050%, Co: 0 to less than 0.050%, As: 0 to less than 0.050%, Mg: 0 to less than 0.050%, Y: 0 to less than 0.050%, Zr: 0 to less than 0.050%, La: 0 to less than 0.050%, Ce: 0 to less than 0.050%, and Ca: 0 to less than 0.050%.

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=96590798

Family Applications (1)

Country Status (2)

Citations (5)

• 2025
  • 2025-01-24 JP JP2025522086A patent/JP7744616B1/ja active Active
  • 2025-01-24 WO PCT/JP2025/002291 patent/WO2025164542A1/ja active Pending

Patent Citations (5)

Also Published As

Similar Documents

Legal Events