EP4324953A1 - Pièce en acier et procédé de fabrication de pièce en acier - Google Patents

Pièce en acier et procédé de fabrication de pièce en acier Download PDF

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Publication number
EP4324953A1
EP4324953A1 EP22824938.9A EP22824938A EP4324953A1 EP 4324953 A1 EP4324953 A1 EP 4324953A1 EP 22824938 A EP22824938 A EP 22824938A EP 4324953 A1 EP4324953 A1 EP 4324953A1
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EP
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carbides
comparative example
annealing
steel
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EP22824938.9A
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German (de)
English (en)
Inventor
Mayumi Ojima
Yoshimasa Funakawa
Yasuhiro Sakurai
Akimasa Kido
Hideyuki Kimura
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JFE Steel Corp
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JFE Steel Corp
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    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • 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 disclosure relates to a steel component and in particular to a steel component having excellent wear resistance. Further, the present disclosure relates to a method of producing the steel component.
  • Carbon steel a steel containing a high concentration of carbon, has high hardness and is therefore widely used as a material for textile machinery components, bearing components, machine blades, and other steel components that require wear resistance.
  • a cold-rolled steel sheet as a raw material is worked into a component shape, followed by quenching treatment and tempering treatment. While quenching treatment increases hardness and reduces toughness, subsequent tempering treatment may improve toughness. However, there is a problem that tempering treatment reduces hardness.
  • Patent Literature 1 a technology is described for improving formability and wear resistance in a steel sheet having a ferrite-cementite microstructure by increasing the grain size of ferrite, spheroidizing carbides (mainly cementite) of appropriate particle size, and reducing pearlite microstructure.
  • a technology for improving wear resistance of a cold-rolled steel sheet by annealing the steel sheet under specific conditions to make the metal microstructure a pearlitic microstructure, which is a layered microstructure of hard cementite and soft ferrite.
  • PTL 3 a technology is described for improving wear resistance of a steel sheet by precipitating coarse Nb, Ti carbides having an equivalent circular diameter of 0.5 ⁇ m or more in the ferrite phase matrix microstructure.
  • a technology is proposed to improve a spheroidization rate of carbides such as cementite and to improve toughness of a steel sheet containing C: 0.5 mass% to 0.7 mass% by bringing the steel sheet to an annealing finishing state in a stage immediately before final quenching and tempering.
  • 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 a high-carbon steel sheet 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.
  • a technology is proposed to improve toughness by adjusting the particle size of cementite, not including niobium, titanium, or vanadium carbides, and the grain size of retained austenite and prior austenite, by bringing the material to an annealing finishing state in a stage immediately before final austempering, and further, by obtaining a bainitic microstructure instead of a martensitic tempered microstructure obtainable in a typical heat treatment of quenching and tempering.
  • the present disclosure provides a steel component having excellent wear resistance.
  • the steel component according to the present disclosure exhibits excellent wear resistance not only under static conditions, but also under conditions where temperature rises due to friction, and is therefore suitable for use for various applications including components for textile machinery, bearing components, and blades for machinery.
  • 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 hardness and wear resistance 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.
  • the C content exceeds 1.25 %, surface scale becomes firm during heating, resulting in deterioration of surface characteristics, and the surface becomes prone to cracking during subsequent cold rolling, as well as cracking when quenching, resulting in reduced wear resistance.
  • the C content is therefore 1.25 % or less.
  • the C content is preferably 1.20 % or less.
  • Si is an element that has an effect of increasing strength through solid solution strengthening, and an increase in strength also improves wear resistance.
  • Si content is 0.10 % or more.
  • the Si content is preferably 0.12 % or more.
  • the Si content is excessive, coarse ferrite is formed on the steel sheet surface during hot working, which inhibits the formation of carbides necessary for improving wear resistance in subsequent working.
  • 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.
  • Mn also has an effect of inhibiting dislocation recovery caused by friction heat during use of a steel component.
  • 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 microstructure nonuniformity are likely to occur at MnS segregations, which inhibit carbide formation and are a cause of cracking and shape defects during component 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.
  • the strength of grain boundaries decreases and embrittlement occurs.
  • the P content is therefore 0.05 % or less.
  • the P content is preferably 0.045 % or less.
  • S consumes Mn by forming sulfides with Mn, and therefore reduces hardenability. As hardenability decreases, strength of steel decreases, resulting in lower wear resistance. S content is therefore 0.01 % or less. From the viewpoint of improving wear resistance, 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 results in the formation of coarse nitrides. The nitrides are often formed on the surface of steel, and promote formation of cracks and voids initiating from the nitrides, thus reducing wear resistance. 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 0.001 % or more.
  • an excess of N combines with Al to form coarse nitrides.
  • the nitrides are often formed on the surface of steel, and promote formation of cracks and voids initiating from the nitrides, thus reducing wear resistance.
  • the N content is therefore 0.009 % or less.
  • the N content is preferably 0.008 % or less.
  • Cr is an element that has an effect of increasing hardenability of steel and improving hardness, and therefore the addition of Cr improves wear resistance.
  • Cr content is 0.05 % or more.
  • the Cr content is preferably 0.12 % or more.
  • 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 performance of steel components. Further, as a result of the formation of Cr carbides, the formation of carbides effective in improving wear resistance is inhibited.
  • the Cr content is therefore 0.55 % 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: 0.05 % to 1.0 %, Nb: 0.1 % to 0.5 %, and V: 0.01 % to 1.0 %.
  • Ti is an element that has an effect of forming fine carbides and inhibiting both static wear and thermal wear. Further, Ti has an effect of improving wear resistance by refining prior austenite grains during quenching and inhibiting dislocation recovery.
  • Ti content is 0.05 % or more.
  • the Ti content is preferably 0.015 % or more.
  • excessive addition of Ti causes carbides to become coarser than necessary, and the carbides become initiation points for voids and cracking, which reduces the workability of steel sheets when worked into component shapes.
  • the Ti content is therefore 1.0 % or less.
  • the Ti content is preferably 0.9 % or less.
  • Nb is an element that has an effect of forming fine carbides and inhibiting both static wear and thermal wear. Further, Nb has an effect of improving wear resistance by refining prior austenite grains during quenching and inhibiting dislocation recovery.
  • Nb content is 0.1 % or more.
  • excessive addition of Nb causes carbides to become coarser than necessary, and the carbides become initiation points for voids and cracking, which reduces the workability of steel sheets when worked into component shapes.
  • the Nb content is therefore 0.5 % or less.
  • the Nb content is preferably 0.45 % or less.
  • V 0.01 % to 1.0 %
  • V is an element that has an effect of forming fine carbides and inhibiting both static wear and thermal wear. Further, V has an effect of improving wear resistance by refining prior austenite grains during quenching and inhibiting dislocation recovery.
  • V content is 0.01 % or more.
  • excessive addition of V causes carbides to become coarser than necessary, and the carbides become initiation points for voids and cracking, which reduces the workability of steel sheets when worked into component shapes.
  • 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 impurity.
  • 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 carbides, 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 grain size of prior austenite grains 25 ⁇ m or less
  • a grain boundary strengthening effect is improved by refining prior austenite grains.
  • dislocation recovery during frictional heat generation is inhibited, and therefore hardness may be maintained even under a hot environment, improving wear resistance.
  • the average grain size of prior austenite grains is 25 ⁇ m or less.
  • the steel component according to the present disclosure contains carbides including at least one of Nb, Ti, or V.
  • carbides containing at least one of Nb, Ti, or V are harder than cementite, and therefore precipitation of carbides containing at least one of Nb, Ti, or V is able to improve wear resistance more than conventionally.
  • coarse carbides have the effect of inhibiting static wear
  • fine carbides have the effect of inhibiting wear at elevated temperatures caused by friction. Therefore, by appropriately controlling particle sizes of coarse carbides and fine carbides, respectively, the wear resistance of steel components in actual use may be effectively improved.
  • carbides having a particle size of 0.1 ⁇ m or more are defined as coarse carbides and carbides having a grain size of less than 0.1 ⁇ m are defined as fine carbides.
  • the coarse carbides act to inhibit static wear.
  • the presence of the coarse carbides may reduce wear caused by abrasion with fibers and foreign matter such as grit attached to the fibers.
  • the average particle size of the coarse carbides is therefore 0.15 ⁇ m or more.
  • the average particle size of the coarse carbides is therefore 2.5 ⁇ m or less.
  • the number density of the coarse carbides is not particularly limited. The higher the number density, the better, and 250/mm 2 or more is preferred.
  • the coarse carbides may be present at crystal grain boundaries or within crystal grains.
  • the fine carbides stabilize dislocation microstructure and help prevent dislocation recovery when temperature rises due to frictional heat. Accordingly, by precipitating the fine carbides, softening due to frictional heat may be inhibited and wear resistance under a hot environment may be improved.
  • the effect of inhibiting dislocation recovery increases with a smaller average particle size of the fine carbides.
  • the average particle size of the fine carbides is therefore 0.05 ⁇ m or less.
  • excessively fine carbides lead to excessive hardness and embrittlement.
  • the average particle size of the fine carbides is therefore 0.005 ⁇ m or more.
  • the fine carbides are more effective when formed within crystal grains than at crystal grain boundaries.
  • the number density of the fine carbides is not particularly limited. The higher the number density, the greater the effect, and 0.11/ ⁇ m 2 or more is preferred.
  • the microstructure of the steel component according to the present disclosure is not particularly limited. Desired properties are obtainable as long as the conditions described above are met.
  • the microstructure of the steel component according to the present disclosure may consist of tempered martensite, cementite, and carbides containing at least one of Nb, Ti, or V.
  • the cementite and the carbides containing at least one of Nb, Ti, or V are preferably spheroidized.
  • La is obtained by dividing the sum of the major axis length of all carbides in a 100 ⁇ m 2 range by the number of such carbides. Further, Lb is obtained by dividing the sum of the minor axis length of all carbides in a 100 ⁇ m 2 range by the number of such carbides.
  • 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 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 steel slab is heated under a set of conditions including: a slab heating temperature of 1,100 °C or more and a holding time of 1.0 h or more, in order to homogenize the steel microstructure and to allow some carbides in the steel to be solid-dissolved and the rest to be precipitated.
  • Some C combined with Nb, Ti, V to form the coarse carbides needs to be precipitated at the slab heating stage described above, while other undissolved carbides are dissolved at the slab heating stage in order to precipitate to desired dimensions at a later annealing stage.
  • the slab heating temperature is lower than 1,100 °C or when the holding time is shorter than 1 h, coarse Nb, Ti, V carbides are not precipitated, and later, coarse Nb, Ti, V carbides to increase resistance to static wear are not obtainable.
  • the slab heating temperature is too high, Nb, Ti, V solid-solubilize, decreasing a precipitation amount, and therefore the slab heating temperature is preferably 1,380 °C or less.
  • 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.
  • Finishing start temperature Ac3 or more
  • finishing start 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 finishing start temperature of the hot rolling is therefore Ac3 or more.
  • An upper limit of the finishing start temperature is not particularly limited.
  • the finishing start 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 2.0 s or less
  • the hot-rolled steel sheet is then cooled.
  • coarse ferrite grains are formed in a surface layer of the steel sheet and remain until later processing.
  • the time between the end of hot rolling and the start of cooling is therefore 2.0 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 during the cooling is less than 25 °C/s, coarse ferrite grains are formed in a surface layer of the steel sheet, and precipitation of carbides that are effective in improving wear resistance is inhibited.
  • the average cooling rate in the cooling 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 640 °C to 720 °C
  • the cooling stop temperature is therefore 720 °C or less.
  • the cooling stop temperature is too low, coiling shape defects occur due to volume expansion caused by transformation during coiling. As a result, non-uniform strain is introduced into the steel sheet during subsequent cold rolling, and therefore carbides of the desired particle size are not obtained, and wear resistance is not improved.
  • the cooling stop temperature is therefore 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 700 °C.
  • 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 720 °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 cementite and ferrite.
  • strain is not introduced stably during subsequent cold rolling, and the cold-rolled steel sheet may be defective in shape. Accordingly, breaking up the pearlitic microstructure and spheroidizing the carbides is necessary.
  • the pearlitic microstructure is stable to heat, and therefore tends to maintain plate shapes.
  • the plate-like microstructure needs to be broken up by holding at a high temperature for a long time to increase interface area.
  • the first annealing is therefore performed at a temperature of 650 °C or more for 3 h or more. After the first annealing, further cold rolling and second annealing helps to break up plate-like cementite.
  • the annealing temperature is higher than 720 °C, microstructure change proceeds preferentially from one portion, resulting in a mixed microstructure of coarse and fine microstructures, and ultimately carbides of the desired size are not obtained, and wear resistance is not improved.
  • 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.
  • Cold rolling is an important process for improving wear resistance, as the plate-like carbides broken up in the first annealing process are further broken up and dispersed throughout the steel sheet, and dispersed at the desired dimensions by the second annealing process.
  • Plate-like carbides formed after hot rolling and coiling are stable, and therefore tend to remain until later stages of production. Plate-like carbide formation may cause void formation and cracking. Further, wear progresses unilaterally in portions of the microstructure where carbide formation does not occur due to not being dispersed throughout the steel sheet.
  • the hot-rolled steel sheet after the first annealing is subjected to two or more cycles of cold rolling and second annealing.
  • the cold rolling is used to break up the plate-like carbides formed in the steel sheet
  • the second annealing is used to distribute the carbides of the desired dimensions throughout the steel sheet.
  • the rolling ratio in the cold rolling is 15 % or more and the annealing temperature in the second annealing is 600 °C or more.
  • the annealing temperature is higher than 800 °C, prior austenite grains in the finally obtained microstructure become coarser, resulting in decreased wear resistance during friction heat generation.
  • the annealing temperature is therefore 800 °C or less.
  • the heating rate in the second annealing is too slow, local coarsening of the carbides occurs, and the fine carbides necessary to inhibit the softening of the steel sheet as temperature rises during friction are not obtained.
  • the heating rate is therefore 50 °C/h or more.
  • An upper limit of the heating rate is not particularly limited.
  • the heating rate is preferably 200 °C/s or less.
  • the rolling ratio is 65 % or more, the shape of a resulting cold-rolled steel sheet may become unstable.
  • the rolling ratio is therefore preferably less than 65 %.
  • 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 refines the microstructure and distributes carbides throughout the steel sheet to achieve the final desired carbide sizes. A large number of cycles is preferable to consistently obtain good steel sheet shape and thickness accuracy, and an upper limit of the number of cycles is not particularly limited. However, when the number of cycles exceeds five, the effect saturates, and therefore the number of cycles is preferably five or less.
  • final cold rolling at a rolling ratio of 30 % or more is further applied.
  • the fine carbides are produced during final quenching and tempering, which improves wear resistance during frictional heat generation.
  • final cold rolling at a rolling ratio of 30 % or more refines prior austenite grain size, and therefore further improves wear resistance.
  • the rolling ratio is therefore preferably less than 65 %.
  • the final cold-rolled steel sheet may be subjected to further optional surface treatment.
  • the resulting cold-rolled steel sheet is then machined into component shapes and heat-treated to produce the final steel component.
  • the method of machining is not particularly limited and any method may be applied.
  • the machining may be, for example, at least one of blanking, cutting work, drawing, bending, or polishing.
  • the heat treatment includes quenching under a set of conditions including: a quenching temperature of 700 °C or more and 950 °C or less and a holding time of 1.0 min or more to 60 min or less, and tempering under a set of conditions including: a tempering temperature of 100 °C to 400 °C and a holding time of 20 min or more to 3 h or less.
  • the quenching and tempering conditions are important to control carbide particle size and prior austenite grain size, in order to obtain excellent wear resistance.
  • the quenching temperature (heating temperature during quenching) needs to be high.
  • the quenching temperature is therefore 700 °C or more.
  • the quenching temperature is preferably 720 °C or more.
  • the quenching temperature is therefore 950 °C or less.
  • the quenching temperature is preferably 920 °C or less.
  • cooling in the quenching process is preferably performed by cooling to room temperature using oil or other coolant.
  • tempering temperature In order to improve hardness and obtain high wear resistance, tempering temperature needs to be low.
  • the tempering temperature is therefore 400 °C or less.
  • the tempering temperature is preferably 380 °C or less.
  • the tempering temperature when the tempering temperature is too low, the fine carbides do not grow to the desired dimensions. Further, hardness becomes too high and the material is embrittled.
  • the tempering temperature is therefore 100 °C or more.
  • the tempering temperature is preferably 130 °C or more.
  • the holding time during the tempering is less than 20 min, the fine carbides do not grow to the desired dimensions, and hardness increases too much, causing embrittlement, and therefore the holding time is 20 min or more.
  • the holding time exceeds 3 h, the fine carbides become too coarse to achieve the desired dimensions. The holding time is therefore 3 h or less.
  • the heat treatment may be performed after the machining or during the machining.
  • a steel component having excellent wear resistance may be produced.
  • Applications of the steel component are not particularly limited.
  • the steel component is particularly suitable for applications requiring wear resistance, such as components for textile machinery, bearing components, and blades for machinery.
  • the average grain size of prior austenite grains, the average particle size of the coarse carbides, and the average particle size of the fine carbides were measured by the following procedures.
  • Test pieces for microstructure observation were taken from the samples obtained. For each test piece, after polishing a rolling direction cross section (L-section) of the test piece for microstructure observation, final polishing was performed with colloidal silica, and electron backscatter diffraction (EBSD) measurements were performed to identify prior austenite grain boundaries. After identifying prior austenite grain boundaries, individual grain sizes and the number of grains were determined, and the equivalent circular diameter was calculated and used as the average grain size. The evaluation results are listed in Table 4.
  • Test pieces for carbide observation were taken from the samples obtained. For each test piece, after polishing a rolling direction cross section (L-section) of the test piece for carbide observation, the polished surface was corroded with 1 vol% to 3 vol% nital solution to reveal the microstructure. The surface of the test piece for carbide observation was then imaged using scanning electron microscopy (SEM) at a magnification of 3,000 ⁇ to obtain a microstructure image. The particle size of each carbide containing at least one of Nb, Ti, or V in the microstructure image obtained was measured by a cutting method, and the average particle size of the carbides was calculated. Nb, Ti, V carbides were identified using SEM energy dispersive X-ray spectroscopy (EDS) analysis.
  • EDS SEM energy dispersive X-ray spectroscopy
  • 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.
  • the evaluation results are listed in Table 4. The column was left blank (-) when no coarse carbides were observed.
  • Test pieces for carbide observation were taken from the samples obtained, thinned to a thickness of about 70 ⁇ m, and then observation samples were prepared by electropolishing.
  • carbides containing at least one of Nb, Ti, or V were observed by transmission electron microscopy (TEM) at 150,000 ⁇ to 250,000 ⁇ magnification and analyzed by TEM-EDS.
  • the diameter of each carbide was determined by the cutting method, and the arithmetic mean of the obtained diameters was calculated to obtain the average particle size of the fine carbides.
  • the evaluation results are listed in Table 4. The column was left blank (-) when no fine carbides were observed.
  • the wear resistance of the resulting steel sheets after quenching and tempering was evaluated under the following two conditions.
  • a wear test piece 10 was taken having the shape illustrated in FIG. 1 .
  • Each of the wear test pieces 10 was provided with four holes 11 for threading.
  • Wear tests were conducted using the wear test pieces 10 and a wear test apparatus 20 illustrated in FIG. 2 . Specifically, an amount of wear was measured by running a yarn S fed from a yarn unwinder 21 for 100,000 m per hole with the yarn S in contact with the side of the hole 11 of the wear test piece 10. Full dull polyester knitting yarn was used as the yarn S. The running speed of the yarn S was 5 m/min. Further, the tension of the yarn was adjusted to 10 ⁇ 2 N/cm using a tension regulator 22.
  • a groove 12 was formed by wear at a point where the hole 11 was in contact with the yarn. After running the yarn 100,000 m, the running was stopped and a depth d (wear depth) of the groove 12 was measured using optical microscopy.

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  • Organic Chemistry (AREA)
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  • Crystallography & Structural Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Heat Treatment Of Steel (AREA)
  • Powder Metallurgy (AREA)
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EP22824938.9A 2021-06-18 2022-06-10 Pièce en acier et procédé de fabrication de pièce en acier Pending EP4324953A1 (fr)

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