Classifications

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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
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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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  • 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
  • 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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  • 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
  • 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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  • 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
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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
  • C21D8/0263—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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  • 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
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C—ALLOYS
  • 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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  • C22C—ALLOYS
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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C—ALLOYS
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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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  • 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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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  • 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
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/60—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 cold-rolled steel sheets, and more particularly to cold-rolled steel sheets from which steel parts having excellent toughness can be produced.
• the present invention also relates to a steel part using the cold-rolled steel sheet, a method for manufacturing the cold-rolled steel sheet, and a method for manufacturing the steel part.
• Cold-rolled steel sheets are widely used as materials for manufacturing various steel parts.
• cold-rolled steel sheets made of high-carbon steel have high hardness, so they are used in applications that require wear resistance, such as parts for textile machinery, bearing parts, and cutlery for machinery and household use.
• steel parts such as parts for textile machinery, bearing parts, and knives for machines and households are subjected to repeated impacts due to reciprocating motion during use. Therefore, steel parts are also required to have excellent toughness in order to prevent breakage due to impact due to reciprocating motion.
• Patent Documents 1 and 2 disclose techniques for improving the toughness of high-carbon cold-rolled steel sheets by utilizing the grain refinement effect of adding Nb.
• Patent Document 3 by dispersing coarse Nb-containing carbides in a matrix consisting of a ferrite phase at a high density, the wear resistance of a cold-rolled steel sheet is improved, and the grain refinement effect due to the addition of Nb is used. Techniques have been proposed to improve the toughness of steel.
• Patent Document 4 proposes a technique for improving the wear resistance and toughness of cold-rolled steel sheets by dispersing coarse Nb/Ti-based carbides in a matrix at high density and reducing the number density of voids.
• Patent Document 5 a steel sheet containing 0.5 to 0.7% by mass of carbon is annealed before final quenching and tempering to improve the spheroidization rate of carbides such as cementite. As a result, techniques for improving toughness have been proposed.
• Patent Document 6 in the stage immediately before final quenching and tempering, the material is annealed to increase the number density of generated voids contained in the material. Techniques for producing carbon steel sheets have been proposed.
• Patent Document 7 in a high-carbon steel sheet, impact toughness and resistance are improved by controlling the formation of cementite carbides that do not contain niobium, titanium, and vanadium carbides, and setting the spheroidization rate and number density of cementite carbides to desired values. Techniques for improving abrasion resistance have been proposed.
• Patent Document 4 also utilizes the effect of improving wear resistance by dispersing hard Nb/Ti-based carbides at high density.
• the Nb/Ti-based carbides are dispersed at a high density, voids are generated between the matrix and the carbides during cold rolling, resulting in a decrease in toughness. Therefore, in Patent Document 4, the generation of voids is suppressed by limiting the rolling reduction in cold rolling.
• this method limits the rolling reduction, it inevitably limits the thickness and mechanical properties of the cold-rolled steel sheets that can be produced, and cannot be said to be an essential solution.
• the present invention has been made in view of the above circumstances, and an object of the present invention is to achieve even better toughness in a cold-rolled steel sheet whose hardness is improved by using carbides such as Nb.
• the size and density of the Nb-Ti-V-based carbides in the cold-rolled steel sheet can be appropriately controlled.
• the present invention has been completed based on the above findings, and the gist thereof is as follows.
• the carbide containing at least one of Nb, Ti, and V present in the ferrite grains has an average grain size of 0.1 ⁇ m or more, and A cold-rolled steel sheet, wherein the number density of carbides having a grain size of 0.1 ⁇ m or more is 100 pieces/mm 2 or more.
• the component composition in mass%, 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, 2.
• the heated steel slab is hot-rolled under the conditions of finish rolling entry temperature: Ac3 point or higher to obtain a hot-rolled steel sheet,
• the hot-rolled steel sheet is cooled under the conditions that the time from the end of the hot rolling to the start of cooling: 2 seconds or less, the average cooling rate: 25 ° C./s or more, and the cooling stop temperature: 720 ° C. or less, Winding the cooled hot-rolled steel sheet,
• the coiled hot-rolled steel sheet is subjected to a first annealing under conditions of an annealing temperature of 650° C. or more and 780° C.
• the hot-rolled steel sheet after the first annealing is subjected to cold rolling at a rolling reduction of 15% or more and second annealing at an annealing temperature of 600 to 800 ° C. twice or more, and then further A method for producing a cold-rolled steel sheet, wherein final cold rolling is performed at a rolling reduction of 20% or more.
• the cold-rolled steel sheet manufactured by the manufacturing method described in 5 or 6 above is quenched under the conditions of quenching temperature: 700 ° C. or more and 800 ° C. or less, holding time: 1 minute or more and less than 60 minutes, and then tempering temperature : 150 to 300°C, holding time: 20 minutes or more and 3 hours or less.
• the cold-rolled steel sheet of the present invention can be very suitably used as a material for various steel parts such as parts for textile machinery, bearing parts, and cutlery for machinery and household use. Moreover, according to the present invention, it is possible to provide a steel component using the cold-rolled steel sheet.
• carbides containing at least one of Nb, Ti, and V which are present in ferrite grains. Therefore, in the following description, "carbides containing at least one of Nb, Ti, and V present in ferrite grains” may be simply referred to as “carbides.”
• the cold-rolled steel sheet of the present invention has the chemical composition described above. The reason for the limitation will be described below. In the following description, “%” as a unit of content indicates “% by mass” unless otherwise specified.
• C 0.6-1.25%
• C is an element necessary for improving the hardness after quenching and tempering.
• C is also an element necessary for forming cementite, Nb, Ti, V, and other elements and carbides.
• the C content In order to generate necessary carbides and obtain strength after quenching and tempering, the C content must be 0.6% or more. Therefore, the C content should be 0.6% or more, preferably 0.7% or more.
• the C content when the C content exceeds 1.25%, the hardness excessively increases and embrittlement occurs.
• the C content exceeds 1.25%, the surface texture becomes deteriorated as a result of hardened surface scales during heating. Therefore, the C content should be 1.25% or less, preferably 1.20% or less.
• Si 0.10-0.55%
• Si is an element that has the effect of increasing strength through solid solution strengthening.
• the Si content should be 0.10% or more, preferably 0.12% or more, and more preferably 0.14% or more.
• Si content should be 0.10% or more, preferably 0.12% or more, and more preferably 0.14% or more.
• Si content is excessive, Si oxides are formed and the toughness is lowered.
• the Si content is excessive, the formation of ferrite and grain growth are promoted, the precipitation of carbides at grain boundaries is promoted, and the precipitation of carbides within grains is suppressed.
• the Si content should be 0.55% or less, preferably 0.50% or less, and more preferably 0.45% or less.
• Mn 0.20-2.0%
• Mn is an element that has the effect of improving hardness by promoting quenching and suppressing temper softening. In order to suppress temper softening, it is necessary to suppress the formation of C as cementite or delay the recovery of dislocations. A hard structure with a high dislocation density can be maintained. In order to obtain the above effects, the Mn content should be 0.20% or more, preferably 0.25% or more. On the other hand, when the Mn content exceeds 2.0%, a band-like structure is generated due to segregation of Mn. In particular, abnormal grain growth and structural heterogeneity are likely to occur in MnS segregation parts, and local precipitation at ferrite grain boundaries occurs, thereby suppressing intragranular carbide formation. Moreover, it causes cracks and shape defects during processing. Therefore, the Mn content should be 2.0% or less, preferably 1.95% or less.
• the P content should be 0.0005% or more, preferably 0.0008% or more.
• the P content should be 0.05% or less, preferably 0.045% or less.
• the S content should be 0.03% or less, preferably 0.02% or less.
• the lower limit of the S content is not particularly limited, and may be 0%.
• the S content is preferably 0.0005% or more, more preferably 0.001% or more.
• Al 0.001-0.1%
• Al is an element necessary for deoxidation during steelmaking. Therefore, the Al content is set to 0.001% or more.
• the Al content should be 0.1% or less, preferably 0.08% or less, more preferably 0.06% or less.
• N 0.001 to 0.009% Nitrogen is an element that refines grain size and improves toughness by forming fine nitrides. Therefore, the N content is made 0.001% or more. On the other hand, when N is excessive, it combines with Al to form nitrides, promoting the formation of cracks and voids originating from the nitrides, resulting in a decrease in toughness. Therefore, the N content should be 0.009% or less, preferably 0.008% or less.
• Cr 0.1-1.0% Cr is an element that enhances the hardenability of steel and improves strength.
• the Cr content should be 0.1% or more, preferably 0.12% or more.
• the Cr content should be 1.0% or less, preferably 0.95% or less.
• the above component composition contains one or more selected from Ti: 0.01 to 1.0%, Nb: 0.05 to 0.5%, and V: 0.01 to 1.0%. To obtain the desired carbide number density, at least one of Ti, Nb, and V should be added in the amounts described above.
• Ti 0.01-1.0%
• Ti is an element that forms carbides in grains and has the effect of improving toughness.
• the Ti content should be 0.01% or more, preferably 0.015% or more, in order to obtain the above effect.
• the austenitizing temperature becomes high, so ferrite tends to form on the surface of the steel sheet due to the temperature drop during hot rolling. The ferrite formed on the surface remains even after subsequent cold rolling and annealing, and as a result of preferentially forming carbides at grain boundaries, formation of intragranular carbides is suppressed. Therefore, the Ti content should be 1.0% or less, preferably 0.9% or less.
• Nb 0.05-0.5%
• Nb is an element that forms carbides in grains and has the effect of improving toughness.
• Nb is an element that is highly effective in refining crystal grains.
• the Nb content is made 0.05% or more in order to obtain the above effect.
• the Nb content should be 0.5% or less, preferably 0.45% or less.
• V 0.01-1.0%
• V is an element having the effect of forming carbides in grains and improving toughness. V also has the effect of improving the hardenability, thereby improving the strength of the steel.
• temper softening it is necessary to suppress the formation of cementite by C or delay the recovery of dislocations. The processed structure can be maintained even afterward, and the toughness is improved.
• V content is made 0.01% or more in order to obtain the above effect.
• the V content should be 1.0% or less, preferably 0.95% or less.
• a cold-rolled steel sheet in an embodiment of the present invention has a chemical composition consisting of the above components and the balance of Fe and unavoidable impurities.
• the above component composition is optionally Sb: 0.1% or less, Hf: 0.5% or less, REM: 0.1% or less, Cu: 0.5% Below, 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 It can further contain one or more selected from the group.
• Sb 0.1% or less Sb is an element effective in improving corrosion resistance. scratches). Therefore, the Sb content is set to 0.1% or less.
• the lower limit of the Sb content is not particularly limited, the Sb content is preferably 0.0003% or more from the viewpoint of increasing the effect of addition.
• Hf 0.5% or less Hf is an element that is effective in improving corrosion resistance. scratches). Therefore, the Hf content is set to 0.5% or less.
• the lower limit of the Hf content is not particularly limited, but from the viewpoint of enhancing the effect of addition, the Hf content is preferably 0.001% or more.
• REM 0.1% or less REM (rare earth metal) is an element that improves the strength of steel. However, excessive addition of REM retards the spheroidization of cementite and promotes uneven deformation during cold working, which may deteriorate the surface properties. Therefore, the REM content is set to 0.1% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of increasing the effect of addition, the REM content is preferably 0.005% or more.
• Cu 0.5% or less Cu is an element effective in improving corrosion resistance. scratches). Therefore, the amount of Cu to be added is set to 0.5% or less.
• the lower limit of the Cu content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Cu content is preferably 0.01% or more.
• Ni 3.0% or less
• Ni is an element that improves the strength of steel. However, excessive addition may promote uneven deformation during cold working, degrading the surface properties. Therefore, the Ni content is set to 3.0% or less.
• the lower limit of the Ni content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Ni content is preferably 0.01% or more.
• Sn 0.5% or less Sn is an element that is effective in improving corrosion resistance. scratches). Therefore, the Sn content is set to 0.5% or less.
• the lower limit of the Sn content is not particularly limited, but from the viewpoint of enhancing the effect of addition, the Sn content is preferably 0.0001% or more.
• Mo 1% or less Mo is an element that improves the strength of steel. However, excessive addition delays cementite spheroidization, promotes uneven deformation during cold working, and sometimes deteriorates the surface properties. Therefore, the Mo content is set to 1% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of increasing the effect of addition, the Mo content is preferably 0.001% or more.
• Zr 0.5% or less
• Zr is an element that is effective in improving corrosion resistance. scratches). Therefore, the Zr content should be 0.5% or less.
• the lower limit of the Zr content is not particularly limited, but from the viewpoint of enhancing the effect of addition, the Zr content is preferably 0.01% or more.
• B 0.005% or less
• B is an element that has the effect of improving hardenability and can be optionally added.
• the B content is set to 0.005% or less.
• the lower limit of the B content is not particularly limited, but from the viewpoint of enhancing the addition effect, when B is added, the B content is preferably 0.0001% or more.
• W 0.01% or less W is an element that has the effect of improving hardenability and can be optionally added. However, if the W content exceeds 0.01%, the surface tends to crack during quenching. Therefore, the W content is set to 0.01% or less. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of enhancing the effect of addition, when W is added, the W content is preferably 0.001% or more.
• Average grain size 0.10 ⁇ m or more Number density: 100 pieces/mm 2 or more
• This structure increases the resistance to strain introduced by cyclic deformation and improves the toughness of the final product.
• the average grain size of carbide containing at least one of Nb, Ti, and V present in ferrite grains must be 0.10 ⁇ m or more. For the same reason, it is necessary to set the number density of carbides having a particle size of 0.10 ⁇ m or more to 100/mm 2 or more.
• the average grain size of carbides is less than 0.10 ⁇ m, the amount of fine Nb, TI, and V carbides precipitated after quenching and tempering treatment is insufficient, and a high effect of improving toughness cannot be obtained. Further, when the number density of carbides is less than 100/mm 2 , the amount of fine Nb, TI, and V carbides precipitated after quenching and tempering treatment is insufficient, as in the case of the average grain size, resulting in a high toughness improvement. No effect.
• the thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness, but is preferably 0.1 mm or more, more preferably 0.2 mm or more.
• the upper limit of the plate thickness is not particularly limited, but it is preferably 2.5 mm or less, more preferably 1.6 mm or less, and even more preferably 0.8 mm or less.
• the plate thickness is 0.2 mm or more and 0.8 mm or less, it can be particularly suitably used as a material for textile machine parts such as knitting needles.
• the cold-rolled steel sheet can be produced by sequentially subjecting a steel slab having the chemical composition described above to the following steps. (1) heating (2) hot rolling (3) cooling (4) coiling (5) first annealing (6) cold rolling (7) second annealing (8) final cold rolling and the above ( Steps 6) and (7) are repeated two more times. Each step will be described below.
• the steel slab can be manufactured by any method without particular limitation.
• the composition adjustment of the steel slab may be performed by a blast furnace converter method or by an electric furnace method.
• Casting of molten steel into slabs may be performed by continuous casting or by blooming.
• the heating can be performed by any method, but is preferably performed using a heating furnace.
• the temperature inside the heating furnace is not particularly limited, but from the viewpoint of homogenizing the steel components and dissolving the segregation and undissolved carbides in the steel slab, the temperature is 1100 ° C. or higher. It is preferable to
• the holding time for the heating is not particularly limited, but from the viewpoint of sufficiently dissolving the undissolved carbide, the holding time is preferably 1 hour or longer.
• Hot rolling Next the heated steel slab is hot rolled to form a hot rolled steel sheet.
• rough rolling and finish rolling can be carried out according to a conventional method.
• Finish rolling entry temperature Ac 3 point or higher
• the finish rolling entry temperature in the hot rolling is less than Ac 3 point
• expanded ferrite is generated in the steel sheet after hot rolling, and this expanded ferrite is finally It also remains in the cold-rolled steel sheet obtained in As a result, the formation of grain boundary carbides is promoted and the formation of intragranular carbides is suppressed, resulting in a decrease in toughness. Therefore, the temperature at the entry side of the finish rolling in the hot rolling is set to Ac3 or higher.
• the upper limit of the finish rolling entry side temperature is not particularly limited, it is preferably 1200° C. or less.
• the Ac3 point (° C.) is obtained by the following formula (1).
• Ac3 (°C) 910 - (203 * C1/2) + (44.7 x Si) - (30 x Mn) - (11 x Cr) + (400 x Ti) + (460 x Al) + (700 x P ) + (104 ⁇ V) + 38 ...
• the element symbol in the above formula (1) indicates the content (% by mass) of each element, and is zero when the element is not contained.
• Average cooling rate 25° C./s or more If the average cooling rate in the cooling is less than 25° C./s, the ferrite grains become coarse and the carbides formed are localized, so subsequent cold rolling and annealing are repeated. At this time, carbide formation concentrates on the grain boundaries and the formation of intragranular carbides is suppressed. Therefore, the average cooling rate is set to 25° C./s or higher.
• the upper limit of the average cooling rate is not particularly limited, but if the cooling rate is excessively high, the coiled shape becomes defective due to volume expansion due to transformation during subsequent coiling. Therefore, from the viewpoint of improving the winding shape, the average cooling rate is preferably 160° C./s or less, more preferably 150° C./s or less.
• Cooling stop temperature 720°C
• the cooling stop temperature in the cooling is set to 720° C. or lower.
• the lower limit of the cooling stop temperature is not particularly limited. Therefore, the cooling stop temperature is preferably 620° C. or higher, more preferably 640° C. or higher.
• the cooled hot-rolled steel sheet is wound into a coil.
• the winding temperature is not particularly limited, but it is preferably 600 to 730°C. At this temperature, plate-shaped cementite is precipitated to stabilize the winding shape of the coil.
• First annealing Annealing temperature 650°C or higher and 780°C or lower
• Annealing time 3 hours or longer
• Annealing temperature 650°C or higher and 780°C or lower
• annealing time 3 hours or longer
• a first annealing is performed under the conditions of
• the structure of the hot-rolled steel sheet after coiling is a pearlite structure in which carbides and ferrite formed in a plate shape are arranged side by side. Since the pearlite structure is stable, it will not be homogenized unless it is held at a high temperature for a long time. In order to destroy the pearlite structure and form the desired carbides in the grains in the subsequent cold rolling and annealing processes, the annealing temperature must be 650° C.
• the annealing time must be 3 hours or longer.
• the annealing temperature is higher than 780 ° C., the phase transformation starts preferentially from one part, resulting in a locally coarse structure and a non-uniform structure. No number density is obtained.
• the upper limit of the annealing time is not particularly limited. Therefore, it is preferable to set the time to 20 hours or less.
• Second Annealing Plate-like carbides are formed in the steel sheet after hot rolling. Since these plate-like carbides are stable, they tend to remain until later, and the plate-like carbides that finally remain cause void formation and cracking, and reduce toughness. Therefore, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and second annealing in order to make the plate-like carbides into particles and re-melt them by heating by annealing and to precipitate the carbides in the grains. , is repeated two or more times.
• the rolling reduction in the cold rolling is less than 15%, the carbides at the grain boundaries are coarsened, so the number density of the carbides formed in the grains decreases and the grain size of the intragranular carbides. becomes smaller. Therefore, the said rolling rate shall be 15% or more.
• the upper limit of the rolling reduction is not particularly limited, it is preferably 70% or less.
• Annealing temperature 600-800°C If the annealing temperature in the second annealing is higher than 800° C., the grain boundary carbides are coarsened, so that the number density of the carbides formed in the grains decreases and the grain size of the intragranular carbides decreases. Therefore, the annealing temperature is set to 800° C. or lower. On the other hand, if the annealing temperature is less than 600° C., formation of intragranular carbides is suppressed, and a desired grain size cannot be obtained. Therefore, the annealing temperature is set to 600° C. or higher.
• the rate of temperature rise in the second annealing is not particularly limited, but if the rate of temperature rise is too slow, carbides tend to form at the ferrite grain boundaries, so the formation of intragranular carbides is suppressed. Therefore, from the viewpoint of further enhancing the effect of improving toughness, it is preferable to set the heating rate in the second annealing to 50° C./hr or more.
• the upper limit of the temperature increase rate is not particularly limited, it is preferably 200° C./s or less.
• the number of repetitions of the cold rolling and the second annealing is two or more. By repeating the cold rolling and annealing two or more times, the formation of carbides can be promoted, and finally the desired intragranular carbide size and number density can be obtained.
• the upper limit of the number of repetitions is not particularly limited, but the effect is saturated even if the number of repetitions is more than 5, so the number of repetitions is preferably 5 or less.
• final cold rolling is further performed at a rolling reduction of 20% or more.
• carbides with a desired number density are precipitated in the grains during quenching and tempering, improving toughness.
• the rolling reduction in the final cold rolling is preferably as large as possible, but if it is 65% or more, the shape of the steel sheet may become unstable. Therefore, the rolling reduction is preferably less than 65%.
• the finally obtained cold-rolled steel sheet may be subjected to any surface treatment.
• a steel component can be manufactured by subjecting the cold-rolled steel sheet manufactured by the above manufacturing method to quenching and tempering.
• the quenching and tempering conditions are not particularly limited, but in order to obtain higher toughness, the quenching temperature is 700° C. or higher and 900° C. or lower, and the holding time is 1 minute or longer and less than 60 minutes. Tempering is preferably performed under the conditions of a return temperature of 150 to 400° C. and a holding time of 20 minutes or more and 3 hours or less. More preferably, the quenching temperature is 750° C. or higher and 850° C. or lower. Further, the tempering temperature is more preferably 200 to 300.degree.
• the cooling in the quenching is not particularly limited, and can 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, can be formed into a desired shape by optionally performing processing.
• cold-rolled steel sheets were manufactured according to the procedure described below, and the toughness of the obtained cold-rolled steel sheets after quenching and tempering was evaluated.
• steel having the chemical composition shown in Table 1 was melted in a converter and made into a steel slab by continuous casting.
• the steel slab is sequentially subjected to heating, hot rolling, cooling, coiling, first annealing, cold rolling, second annealing, and final cold rolling to obtain a final thickness of about 0. .4 mm cold-rolled steel sheet.
• Each step was performed under the conditions shown in Tables 2 and 3, and cold rolling and second annealing were repeated the number of times shown in Tables 2 and 3.
• a test piece for structure observation was taken from the obtained cold-rolled steel sheet. After polishing the rolling direction cross section (L cross section) of the test piece for structure observation, the structure was exposed by corroding the polished surface with a 1 to 3 vol % nital solution. Next, the surface of the test piece for tissue observation was imaged at a magnification of 3000 using a SEM (Scanning Electron Microscope) to obtain a tissue image. From the obtained structure image, the grain size of the Nb, Ti, and V-based carbides generated in the grains was measured by a cutting method, and the number density was calculated by counting the carbides within the measurement field. The average value of 3 fields of view was calculated, and it was set as the particle size and the number density.
• Nb-, Ti-, and V-based carbides were identified using SEM-EDS (Energy Dispersive x-ray Spectroscopy) analysis. Elemental mapping was performed on the observation field to separate cementite from other carbides, and the other carbides were defined as Nb, Ti, and V-based carbides.
• Tables 4 and 5 show the measurement results.
• the U notch of the test piece was formed by electrical discharge machining.
• the cold-rolled steel sheets satisfying the conditions of the present invention are excellent in toughness after quenching and tempering. According to the present invention, it is possible to achieve both high hardness and excellent toughness due to Nb-Ti-V-based carbides. can manufacture parts. Therefore, the cold-rolled steel sheet of the present invention can be very suitably used as a material for various steel parts such as parts for textile machinery, bearing parts, and cutlery.

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