Classifications

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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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/58—Oils
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys 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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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0268—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment between cold rolling steps
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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
  • 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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  • C22C—ALLOYS
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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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
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  • 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 disclosure relates to a cold-rolled steel sheet, and in particular to a cold-rolled steel sheet for use in producing a steel component having excellent toughness. Further, the present disclosure relates to a steel component using the cold-rolled steel sheet, a method for producing the cold-rolled steel sheet, and a method for producing the steel component.
• Cold-rolled steel sheets are widely used as a material for producing various steel components.
• Cold-rolled steel sheets made of high-carbon steel have high hardness and are used for applications requiring wear resistance, including components for textile machinery, bearing components, machine blades, and household knives.
• steel components such as components for textile machinery, bearing components, machine blades, and household knives are repeatedly subjected to impacts from reciprocating motion when in use. Therefore, steel components are also required to have excellent toughness to help prevent damage due to impact caused by reciprocating motion.
• Patent Literature (PTL) 1 and PTL 2 describe technologies to improve the toughness of high-carbon cold-rolled steel sheets by utilizing a crystal grain refinement effect due to Nb addition.
• PTL 3 a technology is proposed to improve the wear resistance of cold-rolled steel sheets by densely dispersing coarse Nb-containing carbides in a matrix consisting of ferrite phase, and to improve toughness by utilizing the crystal grain refinement effect due to Nb addition.
• PTL 4 a technology is proposed to improve the wear resistance and toughness of cold-rolled steel sheets by densely dispersing coarse Nb, Ti carbides in a matrix and reducing the number density of voids.
• PTL 6 a technology is proposed to produce a soft high-carbon steel sheet having excellent blanking properties by increasing the number density of generated voids in the material by bringing the material to an annealing finishing state in a stage immediately before final quenching and tempering.
• PTL 7 a technology is proposed to improve impact toughness and wear resistance in high-carbon steel sheets by controlling the formation of cementite, not including niobium, titanium, or vanadium carbides, and by achieving desired values for the spheroidization rate and number density of cementite.
• the toughness of high-carbon cold-rolled steel sheets is improved by utilizing the crystal grain refinement effect due to Nb addition.
• the crystal grain refinement effect of Nb saturates at a Nb content of about 0.1 mass%, so the required toughness is not obtainable from the crystal grain refinement effect alone.
• the technology proposed in PTL 4 also utilizes the effect of improving wear resistance by densely dispersing hard Nb, Ti carbides.
• Nb, Ti carbides are densely dispersed, voids form between the matrix and the carbides during cold rolling, resulting in reduced toughness. Therefore, in PTL 4, the generation of voids is inhibited by limiting the rolling ratio in cold rolling.
• this method limits the rolling ratio, and therefore inevitably limits the thickness and mechanical properties of the cold-rolled steel sheet that may be produced, and is therefore not really a solution.
• the cold-rolled steel sheet having a hardness improved by use of carbides such as Nb carbides. Therefore, the cold-rolled steel sheet is very well suited as a material for various steel components, including components for textile machinery, bearing components, machine blades, and household knives. Further, a steel component made using the cold-rolled steel sheet is also provided.
• the cold-rolled steel sheet according to the present disclosure has the chemical composition described above. The reasons for the above limitations are described below. Hereinafter, “%” as a unit of content indicates “mass%” unless otherwise specified.
• C is an element necessary to improve hardness after quenching and tempering. Further, C is an element necessary to form cementite and carbides of elements such as Nb, Ti, V, and the like. To produce the required carbides and to obtain strength after quenching and tempering, C content needs to be 0.6 % or more. The C content is therefore 0.6 % or more. The C content is preferably 0.7 % or more. On the other hand, when the C content exceeds 1.25 %, hardness increases excessively and embrittlement occurs. Further, when the C content exceeds 1.25 %, surface scale becomes firm during heating, resulting in degradation of surface characteristics. The C content is therefore 1.25 % or less. The C content is preferably 1.20 % or less.
• Si is an element having an effect of increasing strength by solid solution strengthening. To obtain the above effect, Si content is 0.10 % or more. The Si content is preferably 0.12 % or more. The Si content is more preferably 0.14 % or more. However, excessive Si content leads to Si oxide formation and a decrease in toughness. Further, excessive Si content promotes ferrite formation and grain growth, promotes carbide precipitation to grain boundaries, and inhibits intragranular carbide precipitation. Further, an excess of Si degrades surface characteristics as a result of surface scale becoming firm during heating. The Si content is therefore 0.55 % or less. The Si content is preferably 0.50 % or less. The Si content is more preferably 0.45 % or less.
• Mn is an element having an effect of improving hardness by promoting quenching and inhibiting temper softening.
• temper softening inhibiting the formation of C as cementite or delaying dislocation recovery is necessary, and Mn has both of these effects. Accordingly, the addition of Mn may maintain a high dislocation density and high hardness microstructure even after tempering.
• Mn content is 0.20 % or more.
• the Mn content is preferably 0.25 % or more.
• the Mn content exceeds 2.0%, a banded microstructure is formed due to Mn segregation.
• abnormal grain growth and microstructural nonuniformity are likely to occur at MnS segregations, and local precipitation to ferrite grain boundaries inhibits intragranular carbide formation. Further, this is a cause of cracking and shape defects during machining.
• the Mn content is therefore 2.0 % or less.
• the Mn content is preferably 1.95 % or less.
• P content is 0.0005 % or more.
• the P content is preferably 0.0008 % or more.
• toughness is reduced due to grain boundary embrittlement.
• the P content is therefore 0.05 % or less.
• the P content is preferably 0.045 % or less.
• S causes a decrease in toughness by forming sulfides with Mn.
• S content is therefore 0.03 % or less.
• the S content is preferably 0.02 % or less. From the viewpoint of improving toughness, the lower the S content, the better, and therefore a lower limit of S content is not particularly limited and may be 0 %. However, excessive reduction leads to increased production costs, and therefore from the viewpoint of industrial production, the S content is preferably 0.0005 % or more.
• the S content is more preferably 0.001 % or more.
• Al is an element necessary for deoxidation during steelmaking. Al content is therefore 0.001 % or more. On the other hand, an excess of Al causes nitrides to form and promotes the formation of cracks and voids initiating from the nitrides, resulting in a decrease in toughness. Al content is therefore 0.1 % or less. The Al content is preferably 0.08 % or less. The Al content is more preferably 0.06 % or less.
• N content is therefore 0.001 % or more.
• an excess of N combines with Al to cause nitrides to form and promotes the formation of cracks and voids initiating from the nitrides, resulting in a decrease in toughness.
• the N content is therefore 0.009 % or less.
• the N content is preferably 0.008 % or less.
• Cr is an element that increases hardenability and improves strength of steel. To achieve the effects, Cr content is 0.1 % or more. The Cr content is preferably 0.12 % or more. On the other hand, an excess of Cr causes formation of coarse Cr carbides and Cr nitrides, and voids forming around the Cr carbides and Cr nitrides results in reduced toughness. The Cr content is therefore 1.0 % or less. The Cr content is preferably 0.95 % or less.
• the chemical composition described above contains at least one element selected from the group consisting of Ti: 0.01 % to 1.0 %, Nb: 0.05 % to 0.5 %, and V: 0.01 % to 1.0 %. To obtain the desired number density of carbides, at least one of Ti, Nb, and V needs to be added in the amount described above.
• Ti is an element that has an effect of forming carbides in grains and improving toughness.
• Ti content is 0.01 % or more.
• the Ti content is preferably 0.015 % or more.
• excessive addition of Ti increases the austenitization temperature, making ferrite more likely to form on the surface of the steel sheet due to the lower temperature during hot rolling. The ferrite formed on the surface remains after subsequent cold rolling and annealing, and carbide formation at grain boundaries is prioritized, resulting in inhibition of intragranular carbide formation.
• the Ti content is therefore 1.0 % or less.
• the Ti content is preferably 0.9 % or less.
• Nb 0.05 % to 0.5 %
• Nb is an element that has an effect of forming carbides in grains and improving toughness. Further, Nb is also a highly effective element for crystal grain refinement. When Nb is added, in order to obtain these effects, Nb content is 0.05 % or more. On the other hand, excessive addition of Nb results in the formation of carbides at grain boundaries and a decrease in the number density of carbides formed in grains. Carbides formed at grain boundaries are initiation points for voids and cracking, reducing toughness. The Nb content is therefore 0.5 % or less. The Nb content is preferably 0.45 % or less.
• V is an element that has an effect of forming carbides in grains and improving toughness. Further, V has an effect of improving hardenability and improving strength of steel. Further, in order to inhibit temper softening, inhibiting the formation of C as cementite or delaying dislocation recovery is necessary, and V has both of these effects. The addition of V may maintain deformed microstructure even after tempering, improving toughness. When V is added, to obtain these effects, the V content is 0.01 % or more. On the other hand, excessive addition of V causes coarsening of carbides formed at grain boundaries, and carbides formed at grain boundaries become initiation points for voids and cracking, resulting in a decrease in toughness. The V content is therefore 1.0 % or less. The V content is preferably 0.95 % or less.
• the cold-rolled steel sheet according to an embodiment of the present disclosure has a chemical composition consisting of the above components, with the balance being Fe and inevitable impurities.
• the chemical composition described above contains at least one selected from the group consisting of Sb: 0.1 % or less, Hf: 0.5 % or less, REM: 0.1 % or less, Cu: 0.5 % or less, Ni: 3.0 % or less, Sn: 0.5 % or less, Mo: 1 % or less, Zr: 0.5 % or less, B: 0.005 % or less, and W: 0.01 % or less.
• Sb is an effective element for improving corrosion resistance, but when added in excess, a rich Sb layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling.
• Sb content is therefore 0.1 % or less.
• a lower limit of the Sb content is not particularly limited. From the viewpoint of increasing the effect of Sb addition, the Sb content is preferably 0.0003 % or more.
• Hf is an effective element for improving corrosion resistance, but when added in excess, a rich Hf layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling.
• Hf content is therefore 0.5 % or less.
• a lower limit of the Hf content is not particularly limited. From the viewpoint of increasing the effect of Hf addition, the Hf content is preferably 0.001 % or more.
• REM rare earth metals
• REM content is therefore 0.1 % or less.
• a lower limit of the REM content is not particularly limited. From the viewpoint of increasing the effect of REM addition, the REM content is preferably 0.005 % or more.
• Cu is an effective element for improving corrosion resistance, but when added in excess, a rich Cu layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling.
• Cu content is therefore 0.5 % or less.
• a lower limit of the Cu content is not particularly limited. From the viewpoint of increasing the effect of Cu addition, the Cu content is preferably 0.01 % or more.
• Ni is an element that improves strength of steel. However, excessive addition may promote non-uniform deformation during cold working and degrade surface characteristics. Ni content is therefore 3.0 % or less. A lower limit of the Ni content is not particularly limited. From the viewpoint of increasing the effect of Ni addition, the Ni content is preferably 0.01 % or more.
• Sn is an effective element for improving corrosion resistance, but when added in excess, a rich Sn layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling.
• Sn content is therefore 0.5 % or less.
• a lower limit of the Sn content is not particularly limited. From the viewpoint of increasing the effect of Sn addition, the Sn content is preferably 0.0001 % or more.
• Mo is an element that improves strength of steel. However, excessive addition of Mo may retard the spheroidization of cementite, promote non-uniform deformation during cold working, and degrade surface characteristics.
• the Mo content is therefore 1 % or less.
• a lower limit of the Mo content is not particularly limited. From the viewpoint of increasing the effect of Mo addition, the Mo content is preferably 0.001 % or more.
• Zr is an effective element for improving corrosion resistance, but when added in excess, a rich Zr layer is formed under scale generated during hot rolling, causing surface defects (scratches) on the steel sheet after hot rolling.
• Zr content is therefore 0.5 % or less.
• a lower limit of the Zr content is not particularly limited. From the viewpoint of increasing the effect of Zr addition, the Zr content is preferably 0.01 % or more.
• B is an element that has an effect of improving hardenability and may be added. However, when B content exceeds 0.005 %, surface cracking is likely to occur during quenching. The B content is therefore 0.005 % or less. A lower limit of the B content is not particularly limited. From the viewpoint of increasing the effect of B addition, when B is added, the B content is preferably 0.0001 % or more.
• W is an element that has an effect of improving hardenability and may be added. However, when W content exceeds 0.01 %, surface cracking is likely to occur during quenching. The W content is therefore 0.01 % or less. A lower limit of the W content is not particularly limited. From the viewpoint of increasing the effect of W addition, when W is added, the W content is preferably 0.001 % or more.
• Average particle diameter 0.10 ⁇ m or more
• the average particle size of carbides containing at least one of Nb, Ti, and V in ferrite grains needs to be 0.10 ⁇ m or more.
• the number density of the carbides having a particle size of 0.10 ⁇ m or more needs to be 100/mm 2 or more.
• the average particle size of the carbides is less than 0.10 ⁇ m, the amount of fine Nb, TI, and V carbides precipitating after quenching and tempering treatment is insufficient to achieve a high toughness improvement effect.
• the number density of carbides is less than 100/mm 2 , as in the case of insufficient average particle size, the amount of fine Nb, TI, and V carbides precipitating after quenching and tempering treatment is insufficient to achieve a high toughness improvement effect.
• the sheet thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness.
• the sheet thickness is preferably 0.1 mm or more.
• the sheet thickness is more preferably 0.2 mm or more.
• an upper limit of the sheet thickness is not particularly limited.
• the sheet thickness is particularly 2.5 mm or less.
• the sheet thickness is more preferably 1.6 mm or less.
• the sheet thickness is even more preferably 0.8 mm or less.
• the cold-rolled steel sheet is particularly suitable for use as a material for textile machinery components such as knitting needles and the like.
• the following describes a method for producing a cold-rolled steel sheet according to an embodiment.
• the cold-rolled steel sheet may be produced by performing the following processes in sequence, starting with a steel slab having the chemical composition described above.
• a method for producing the steel slab is not particularly limited, and any method may be used.
• composition adjustment of the steel slab may be performed by a blast furnace converter steelmaking process or by an electric furnace steelmaking process.
• casting from molten steel into a slab may be done by continuous casting or by blooming.
• the heating may be performed by any method, but use of a heating furnace is preferred.
• the furnace temperature is not particularly limited. From the viewpoint of homogenizing the steel composition and dissolving segregation and unsolved carbides in the steel slab, the temperature is preferably 1,100 °C or more.
• the holding time in the heating is not particularly limited. From the viewpoint of sufficient dissolving of unsolved carbides, the holding time is preferably 1 h or more.
• the heated slab is then hot rolled to obtain a hot-rolled steel sheet.
• rough rolling and finishing rolling may be performed according to conventional methods.
• Finisher entry temperature Ac3 or more
• the finisher entry temperature of the hot rolling is less than Ac3
• stretched ferrite is formed in the steel sheet after hot rolling, and this stretched ferrite remains in the finally obtainable cold-rolled steel sheet.
• the finisher entry temperature of the hot rolling is Ac3 or more.
• An upper limit of the finisher entry temperature is not particularly limited.
• the finisher entry temperature is preferably 1,200 °C or less.
• the Ac3 temperature (°C) is obtained by the following Formula (1).
• Ac3 ⁇ C 910 ⁇ 203 ⁇ C 1/2 + 44.7 ⁇ Si ⁇ 30 ⁇ Mn ⁇ 11 ⁇ Cr + 400 ⁇ Ti + 460 ⁇ Al + 700 ⁇ P + 104 ⁇ V + 38
• Time from end of hot rolling to start of cooling 2 s or less
• the hot-rolled steel sheet is then cooled.
• coarse ferrite is formed and carbides containing at least one of Ti, Nb, and V precipitate non-uniformly at grain boundaries.
• This non-uniform microstructure does not homogenize in subsequent cold rolling and annealing and hinders intragranular carbide formation.
• the time between the end of hot rolling and the start of cooling is therefore 2 s or less.
• the shorter the time between the end of hot rolling and the start of cooling the better, and therefore a lower limit is not particularly limited.
• the time may be 0.5 s or more, or even 0.8 s or more.
• Average cooling rate 25 °C/s or more
• the average cooling rate in the cooling is less than 25 °C/s, ferrite grains become coarse and carbides formed become localized, and therefore when subsequent cold rolling and annealing are repeated, carbide formation is concentrated at grain boundaries and the formation of intragranular carbides is inhibited.
• the average cooling rate is therefore 25 °C/s or more.
• An upper limit of the average cooling rate is not particularly limited.
• the average cooling rate is preferably 160 °C/s or less.
• the average cooling rate is more preferably 150 °C/s or less.
• Cooling stop temperature 720 °C or less
• the cooling stop temperature in the cooling is too high, ferrite grains coarsen and carbide formation into grains is inhibited when cold rolling and annealing is repeated.
• the cooling stop temperature is therefore 720 °C or less.
• a lower limit of the cooling stop temperature is not particularly limited.
• the cooling stop temperature is therefore preferably 620 °C or more.
• the cooling stop temperature is more preferably 640 °C or more.
• the cooled hot-rolled steel sheet is coiled.
• the coiling temperature is not particularly limited.
• the coiling temperature is preferably 600 °C to 730 °C. This temperature stabilizes the coiling shape by precipitating plate-like cementite.
• the hot-rolled steel sheet after the coiling is subjected to the first annealing under a set of conditions including: an annealing temperature of 650 °C or more and 780 °C or less, and an annealing time of 3 h or more.
• the microstructure of the hot-rolled steel sheet after the coiling is a pearlitic microstructure lined with plate-like carbides and ferrite.
• the pearlitic microstructure is stable, and therefore does not homogenize without prolonged holding at a high temperature.
• the annealing temperature needs to be 650 °C or more and the annealing time needs to be 3 h or more.
• the annealing temperature is more than 780 °C
• phase transformation begins preferentially from one portion, resulting in a locally coarse and non-uniform microstructure, making obtaining intragranular carbides difficult, such that the desired carbide number density may not be obtained.
• An upper limit of the annealing time is not particularly limited. An excessively long annealing time reduces productivity and also saturates the effect. Therefore, the annealing time is preferably 20 h or less.
• the hot-rolled steel sheet Prior to the first annealing, the hot-rolled steel sheet is preferably pickled.
• Plate-like carbides are formed in steel sheets after hot rolling. Such plate-like carbides are stable, and therefore tend to remain until later stages. Plate-like carbides that finally remain may cause void formation and cracking, reducing toughness. Therefore, in order to re-dissolve plate-like carbides into particle shapes by annealing heating and to cause intragranular precipitation of the carbides, the hot-rolled steel sheet after the first annealing is subjected to two or more cycles of cold rolling and second annealing.
• the rolling ratio in the cold rolling is less than 15 %, carbides at grain boundaries become coarser, and therefore the number density of intragranular carbides formed decreases, and the particle size of intragranular carbides becomes smaller.
• the rolling ratio is therefore 15 % or more.
• An upper limit of the rolling ratio is not particularly limited.
• the rolling ratio is preferably 70 % or less.
• Annealing temperature 600 °C to 800 °C
• the annealing temperature in the second annealing is more than 800 °C, carbides at grain boundaries become coarser, and therefore the number density of intragranular carbides formed decreases, and the particle size of intragranular carbides becomes smaller.
• the annealing temperature is therefore 800 °C or less.
• the annealing temperature is less than 600 °C, the formation of intragranular carbides is inhibited and the desired particle size is not obtainable.
• the annealing temperature is therefore 600 °C or more.
• the heating rate in the second annealing is not particularly limited.
• the heating rate in the second annealing is preferably 50 °C/h or more.
• An upper limit of the heating rate is also not particularly limited.
• the heating rate is preferably 200 °C/s or less.
• the number of cycles of the cold rolling and the second annealing is two or more. Two or more cycles of the cold rolling and the annealing promotes carbide formation and leads finally to achieving the desired intragranular carbide size and number density.
• An upper limit of the number of cycles is not particularly limited. The number of cycles is preferably five or less, as the effect saturates when the number of cycles is more than five.
• final cold rolling at a rolling ratio of 20 % or more is further applied.
• toughness improves due to precipitation of carbides at the desired number density into grains during quenching and tempering.
• the rolling ratio is therefore preferably less than 65 %.
• a cold-rolled steel sheet having excellent toughness after quenching and tempering may be produced.
• the final cold-rolled steel sheet may be subjected to further optional surface treatment.
• a steel component may be produced by quenching and tempering a cold-rolled steel sheet produced according to the method described above.
• the quenching and tempering conditions are not particularly limited. In order to obtain higher toughness, quenching is preferably performed under a set of conditions including: a quenching temperature of 700 °C or more and 900 °C or less, a holding time of 1 min or more to less than 60 min, followed by tempering preferably performed under a set of conditions including: a tempering temperature of 150 °C to 400 °C and a holding time of 20 min or more to 3 h or less.
• the quenching temperature is more preferably 750 °C or more.
• the quenching temperature is more preferably 850 °C or less.
• the tempering temperature is more preferably 200 °C to 300 °C.
• Cooling in the quenching is not particularly limited and may be performed by any method.
• the cooling may be, for example, air cooling, water quenching, or oil quenching.
• the cold-rolled steel sheet Prior to the quenching and tempering, the cold-rolled steel sheet may be optionally worked to a desired shape.
• cold-rolled steel sheets were produced according to the procedures described below, and the toughness of the resulting cold-rolled steel sheets after quenching and tempering was evaluated.
• Test pieces for microstructure observation were taken from the obtained cold-rolled steel sheets. For each test piece, after polishing a cross-section in the rolling direction (L-section) of the test piece for microstructure observation, the polished surface was corroded with 1 vol% to 3 vol% nital solution to reveal the microstructure. Next, the surface of the test piece for microstructure observation was imaged using a scanning electron microscope (SEM) at a magnification of 3,000 ⁇ to obtain a microstructure image. From the obtained microstructure image, the particle size of Nb, Ti, and V carbides formed in grains was measured by a cutting method, and the number density was calculated by counting the carbides in the measurement field of view. The average of three fields of view was calculated and used as particle size and number density.
• SEM scanning electron microscope
• Nb, Ti, and V carbides were identified using SEM energy dispersive X-ray spectroscopy (EDS) analysis. Elemental mapping was performed with respect to the observed fields of view to separate cementite from other carbides, and the other carbides were considered to be Nb, Ti, V carbides.
• EDS energy dispersive X-ray spectroscopy
• the following procedure was used to conduct tests and measure impact values using the Charpy impact test.
• the obtained cold-rolled steel sheets were quenched and tempered.
• the quenching was performed by holding the cold-rolled steel sheet in a furnace preheated to 800 °C for 10 min, followed by oil quenching at 80 °C.
• the tempering was performed by holding the quenched cold-rolled steel sheet in a furnace preheated to 250 °C for 1 h and then air cooling.
• Charpy impact tests were then performed to measure impact values. The measurement results are listed in Tables 4 and 5.
• test pieces were used having a notch depth of 2.5 mm and a notch radius of 0.1 mm (notch width 0.2 mm) taken from cold-rolled steel sheets after quenching and tempering.
• a U-notch was formed by electric discharge machining. According to the present disclosure, the toughness after quenching and tempering was judged to be excellent when the impact value was 8 J/cm 2 or more.
• cold-rolled steel sheets meeting the conditions of the present disclosure have excellent toughness after quenching and tempering. According to the present disclosure, both high hardness and excellent toughness is obtainable due to Nb, Ti, V carbides, and therefore the cold-rolled steel sheet according to the present disclosure may be used to produce a steel component that has a high level of both hardness and toughness. Therefore, the cold-rolled steel sheet according to the present disclosure is very well suited as a material for various steel components, including components for textile machinery, bearing components, blades, and the like.

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• 2022
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