Classifications

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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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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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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • 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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  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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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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• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
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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
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/003—Cementite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

Definitions

• hot-rolled steel sheets which are carbon steels for machine structural use specified in JIS G4051 and alloy steels for machine structural use into desired shapes through cold working and then subjecting the resultants to quenching treatment to ensure the desired hardness.
• the hot-rolled steel sheets used as materials are required to have high cold workability and high hardenability, and various steel sheets have previously been proposed.
• Patent Literature 1 discloses a high-carbon steel sheet for fine blanking.
• the steel sheet has a chemical composition containing, by wt %, C: 0.15% to 0.9%, Si: 0.4% or less, Mn: 0.3% to 1.0%, P: 0.03% or less, T.
• Patent Literature 3 discloses a B-alloyed steel that contains, by mass %, C: 0.20% or more and 0.45% or less, Si: 0.05% or more and 0.8% or less, Mn: 0.5% or more and 2.0% or less, P: 0.001% or more and 0.04% or less, S: 0.0001% or more and 0.006% or less, Al: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.2% or less, B: 0.001% or more and 0.01% or less, and N: 0.0001% or more and 0.01% or less, and, furthermore, one or more components selected from Cr: 0.05% or more and 0.35% or less, Ni: 0.01% or more and 1.0% or less, Cu: 0.05% or more and 0.5% or less, Mo: 0.01% or more and 1.0% or less, Nb: 0.01% or more and 0.5% or less, V: 0.01% or more and 0.5% or less, Ta: 0.01% or more and 0.5% or less, W: 0.01% or
• Al 0.10% or less, N: 0.005% or less, and B: 0.0005% to 0.0050%, further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% to 0.03% in total, has a microstructure composed of ferrite and cementite, with the density of cementite in ferrite grains being 0.10/ ⁇ m 2 or less, and has a hardness of 75 or less in terms of HRB and a total elongation of 38% or more.
• Al 0.10% or less, N: 0.005% or less, and B: 0.0005% to 0.0050%, further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% to 0.03% in total, has a microstructure composed of ferrite and cementite, with the density of cementite in ferrite grains being 0.10/ ⁇ m 2 or less, and has a hardness of 65 or less in terms of HRB and a total elongation of 40% or more.
• Patent Literature 7 discloses a high-carbon hot-rolled steel sheet that contains, by mass %, C: 0.20% to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.
• Al 0.10% or less, N: 0.005% or less, and B: 0.0005% to 0.0050%, further contains one or more of Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% to 0.03% in total, with the proportion of the amount of solute B to the B content being 70% or more, has a microstructure composed of ferrite and cementite, with the density of cementite in ferrite grains being 0.08/ ⁇ m 2 or less, and has a hardness of 73 or less in terms of HRB and a total elongation of 39% or more.
• Patent Literature 8 discloses a high-carbon hot-rolled steel sheet that has a composition containing, by mass %, C: 0.15% to 0.37%, Si: 1% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.03% or less, sol.
• Al 0.10% or less
• N 0.0005% to 0.0050%
• B 0.0010% to 0.0050%
• Patent Literature 1 relates to fine blanking properties, and the influence of the dispersion morphology of carbide on the fine blanking properties and hardenability is described. Specifically, Patent Literature 1 states that a steel sheet with improved fine blanking properties and improved hardenability can be obtained by controlling the average carbide grain size to 0.4 to 1.0 ⁇ m and the spheroidization ratio to 80% or more. However, Patent Literature 1 does not discuss cold workability and does not describe carburizing hardenability.
• Patent Literature 2 focuses on the fact that not only the average carbide grain size but fine carbide grains having a size of 0.3 ⁇ m or less have an influence on workability, and controls the average carbide grain size to 1.0 ⁇ m or less and also controls the proportion of carbide grains having a size of 0.3 ⁇ m or less to 20% or less.
• Patent Literature 2 states that this control provides a steel sheet with improved workability and discloses a steel sheet further containing Ti and B and having high hardenability.
• Patent Literature 2 does not describe, for example, solute B which influences hardenability and does not state that the quenching hardness is determined in what area of the steel sheet.
• Patent Literature 3 According to the technique described in Patent Literature 3, a steel with improved cold workability and improved decarbonization resistance can be obtained by adjusting the chemical composition.
• Patent Literature 3 does not describe immersion-quench hardenability or carburizing hardenability.
• Patent Literature 4 specifies the hydrogen concentration in an atmosphere in the annealing step as 95% or more and does not describe whether nitrogen absorption can be suppressed to ensure solute B in an annealing step in a nitrogen atmosphere.
• Patent Literatures 5 to 7 the incorporation of B and one or more of Sb, Sn, Bi, Ge, Te, and Se in an amount of 0.002% to 0.03% in total is highly effective in preventing nitrogen infiltration, and, for example, even when annealing is performed in a nitrogen atmosphere, nitrogen infiltration is prevented, and a predetermined amount of solute B is maintained, thus enhancing hardenability.
• Patent Literatures 5 to 7 describe the quenching hardness in a surface layer.
• Patent Literature 8 a steel that contains C: 0.15% to 0.37%, B, and at least one of Sb and Sn and hence has high hardenability is proposed.
• Patent Literature 8 does not discuss higher hardenability, such as carburizing hardenability.
• the present inventors have conducted intensive studies on the relationship among conditions for the production of a high-carbon hot-rolled steel sheet having a steel chemical composition containing B and one or two selected from Sn and Sb, cold workability, and hardenability (immersion-quench hardenability and carburizing hardenability) and obtained the following findings.
• annealing When annealing is performed in a nitrogen atmosphere, nitrogen in the atmosphere is infiltrated and concentrated into a steel sheet and binds to B and Al in the steel sheet to form boron nitride and aluminum nitride in a surface layer. This may reduce the amount of solute B in the steel sheet, or the presence of aluminum nitride may decrease the austenite grain size during heating in the austenite range before quenching, thus resulting in insufficient quenching.
• at least one of Sb and Sn is added in a predetermined amount into a steel sheet required to have higher hardenability (high carburizing hardenability).
• the desired microstructure can be ensured as follows: after hot rough rolling, finish rolling is performed at a finishing temperature equal to or higher than an Ar 3 transformation temperature, and then cooling is performed to 650° C. to 750° C. at an average cooling rate of 20° C./sec to 100° C./sec; coiling is performed at a coiling temperature of 500° C. to 700° C., and the coil is cooled to normal temperature to obtain a hot-rolled steel sheet; the hot-rolled steel sheet is then heated between 450° C. and 600° C.
• two-stage annealing that involves holding at a temperature equal to or higher than an Ac 1 transformation temperature and equal to or lower than an Ac 3 transformation temperature for 0.5 h or more, followed by cooling to a temperature lower than an Ar 1 transformation temperature at an average cooling rate of 1° C./h to 20° C./h, and holding at a temperature lower than the Ar 1 transformation temperature for 20 h or more is performed.
• a high-carbon hot-rolled steel sheet has a chemical composition containing, by mass %, C: 0.20% or more and 0.50% or less, Si: 0.8% or less, Mn: 0.10% or more and 0.80% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.01% or less, Cr: 1.0% or less, B: 0.0005% or more and 0.005% or less, and one or two selected from Sb and Sn in an amount of 0.002% or more and 0.1% or less in total, with the balance being Fe and unavoidable impurities.
• the steel sheet has a microstructure including ferrite, cementite, and pearlite that accounts for 6.5% or less of the entire microstructure by area fraction.
• the proportion of the number of cementite grains having an equivalent circle diameter of 0.1 ⁇ m or less to the total number of cementite grains is 20% or less, the average cementite grain size is 2.5 ⁇ m or less, and the cementite accounts for 3.5% or more and 10.0% or less of the entire microstructure by area fraction.
• the average concentration of solute B in a region extending from a surface layer to a depth of 100 ⁇ m is 10 mass ppm or more.
• the average concentration of N present as AlN in the region extending from the surface layer to the depth of 100 ⁇ m is 70 mass ppm or less.
• the high-carbon hot-rolled steel sheet according to [1] has a tensile strength of 480 MPa or less and a total elongation of 33% or more.
• the ferrite has an average grain size of 4 to 25 ⁇ m.
• the chemical composition further contains, by mass %, one or two groups selected from Group A and Group B.
• Group A Ti: 0.06% or less
• a method for manufacturing the high-carbon hot-rolled steel sheet according to any one of [1] to [4] includes subjecting a steel having the chemical composition to hot rough rolling and then performing finish rolling at a finishing temperature equal to or higher than an Ar 3 transformation temperature; then performing cooling to 650° C. to 750° C. at an average cooling rate of 20° C./sec to 100° C./sec; performing coiling at a coiling temperature of 500° C. to 700° C. to obtain a hot-rolled steel sheet; then heating the hot-rolled steel sheet in a temperature range from 450° C. to 600° C.
• annealing that involves holding at a temperature equal to or higher than an Ac 1 transformation temperature and equal to or lower than an Ac 3 transformation temperature for 0.5 h or more, followed by cooling to a temperature lower than an Ar 1 transformation temperature at an average cooling rate of 1° C./h to 20° C./h, and holding at a temperature lower than the Ar 1 transformation temperature for 20 h or more.
• the C content is an element important to provide the strength after quenching. If the C content is less than 0.20%, a desired hardness is not provided by heat treatment after forming, and thus the C content needs to be 0.20% or more. However, a C content of more than 0.50% causes hardening, leading to deterioration of toughness and cold workability. Thus, the C content is 0.20% or more and 0.50% or less. When the steel sheet is used for cold working of a part having a complex shape and difficult to form by pressing, the C content is preferably 0.45% or less, more preferably 0.40% or less.
• Si is an element that increases strength through solid-solution strengthening.
• a higher Si content results in a higher hardness to deteriorate cold workability, and thus the Si content is 0.8% or less, preferably 0.65% or less, more preferably 0.50% or less.
• the Si content is preferably 0.30% or less.
• the Si content is preferably 0.1% or more, more preferably 0.2% or more.
• Mn is an element that improves hardenability and increases strength through solid-solution strengthening. If the Mn content is less than 0.10%, both immersion-quench hardenability and carburizing hardenability begin to deteriorate, and thus the Mn content is 0.10% or more.
• the Mn content is preferably 0.25% or more, more preferably 0.30% or more. If the Mn content exceeds 0.80%, a banded structure due to Mn segregation develops, resulting in an inhomogeneous microstructure, and the steel becomes hard through solid-solution strengthening, resulting in low cold workability. Thus, the Mn content is 0.80% or less. In the case of a material for a part required to have formability, a certain level of cold workability is necessary, and thus the Mn content is preferably 0.65% or less, more preferably 0.55% or less.
• S is an element that needs to be minimized because S forms sulfides and reduces the cold workability and the toughness after quenching of the high-carbon hot-rolled steel sheet. If the S content exceeds 0.010%, the cold workability and the toughness after quenching of the high-carbon hot-rolled steel sheet deteriorate significantly. Thus, the S content is 0.010% or less. To provide high cold workability and high toughness after quenching, the S content is preferably 0.005% or less. Since S reduces the cold workability and the toughness after quenching, the S content is preferably as low as possible. However, an excessive reduction in S leads to an increase in refining cost, and thus the S content is preferably 0.0005% or more.
• sol. Al 0.10% or less
• the sol. Al content exceeds 0.10%, AlN is formed during heating in quenching treatment, resulting in excessively fine austenite grains. This promotes the formation of a ferrite phase during cooling to form a microstructure composed of ferrite and martensite, resulting in low hardness after quenching.
• the sol. Al content is 0.10% or less, preferably 0.06% or less.
• sol. Al has a deoxidation effect, and to achieve sufficient deoxidation, the sol. Al content is preferably 0.005% or more.
• the N content exceeds 0.01%, the formation of AlN leads to the formation of excessively fine austenite grains during heating in quenching treatment, which promotes the formation of a ferrite phase during cooling, resulting in low hardness after quenching.
• the N content is 0.01% or less, preferably 0.0065% or less, more preferably 0.0050% or less.
• N is an element that forms AlN, Cr-based nitride, and B nitride and thus moderately inhibits the growth of austenite grains during heating in quenching treatment to improve the toughness after quenching.
• the N content is preferably 0.0005% or more, more preferably 0.0010% or more.
• Cr is an important element that enhances hardenability. If the Cr content in the steel is 0%, ferrite is readily formed in a surface layer particularly during carburizing and quenching, and a completely quenched microstructure is not obtained, which may increase the likelihood of a decrease in hardness. Thus, when the steel sheet is used in applications where hardenability is important, the Cr content is preferably 0.05% or more, more preferably 0.10% or more, still more preferably 0.20% or more. If the Cr content exceeds 1.0%, the steel sheet before quenching becomes hard to have impaired cold workability. Thus, the Cr content is 1.0% or less. When a part difficult to form by pressing and requiring high workability is processed, even higher cold workability is required, and thus the Cr content is preferably 0.7% or less, more preferably 0.5% or less.
• Sb and Sn are elements effective in suppressing nitrogen infiltration through the steel sheet surface layer. If the total content of one or more of these elements is less than 0.002%, the effect is not sufficiently produced. Thus, the total content of one or more of these elements is 0.002% or more, more preferably 0.005% or more. If one or more of these elements are contained in an amount of more than 0.1% in total, the nitrogen infiltration prevention effect plateaus. In addition, these elements tend to segregate at grain boundaries, and thus if these elements are contained in an amount of more than 0.1% in total, grain boundary embrittlement may occur due to the excessively high content. Thus, the total content of one or two selected from Sb and Sn is 0.1% or less, preferably 0.03% or less, still more preferably 0.02% or less.
• the balance is Fe and unavoidable impurities.
• Nb is an element that forms a carbonitride and is effective in preventing exaggerated grain growth during heating before quenching, improving toughness, and improving temper softening resistance.
• the Nb content is less than 0.0005%, the effect of addition is not sufficiently produced.
• the lower limit is preferably 0.0005%, more preferably 0.0010% or more.
• the Nb content exceeds 0.1%, the effect of addition plateaus, and, in addition, a niobium carbide increases the tensile strength of the base metal to decrease elongation.
• the upper limit is preferably 0.1%, more preferably 0.05% or less, still more preferably less than 0.03%.
• Mo is an element effective in improving hardenability and temper softening resistance.
• the Mo content is less than 0.0005%, the effect of addition is small.
• the lower limit is preferably 0.0005%, more preferably 0.0010% or more.
• the upper limit is preferably 0.1%, more preferably 0.05% or less, still more preferably less than 0.03%.
• Ta 0.0005% or more and 0.1% or less
• Ta is an element that forms a carbonitride similarly to Nb and is effective in preventing exaggerated grain growth during heating before quenching, preventing coarsening of grains, and improving temper softening resistance.
• the lower limit is preferably 0.0005%, more preferably 0.0010% or more.
• the upper limit is preferably 0.1%, more preferably 0.05% or less, still more preferably less than 0.03%.
• Ni 0.0005% or more and 0.1% or less
• Ni is an element highly effective in improving toughness and hardenability.
• the lower limit is preferably 0.0005%, more preferably 0.0010% or more.
• the upper limit is preferably 0.1%, more preferably 0.05% or less.
• Cu is an element effective in ensuring hardenability.
• the lower limit is preferably 0.0005%, more preferably 0.0010% or more.
• the upper limit is preferably 0.1%, more preferably 0.05% or less.
• V 0.0005% or more and 0.1% or less
• W 0.0005% or more and 0.1% or less
• the microstructure includes ferrite and cementite.
• the proportion of the number of cementite grains having an equivalent circle diameter of 0.1 ⁇ m or less to the total number of cementite grains is 20% or less, the average cementite grain size is 2.5 ⁇ m or less, and the cementite accounts for 3.5% or more and 10.0% or less of the entire microstructure by area fraction.
• the average concentration of solute B in a region extending from a surface layer to a depth of 100 ⁇ m is 10 mass ppm or more.
• the average concentration of N present as AlN in the region extending from the surface layer to the depth of 100 ⁇ m is 70 mass ppm or less.
• the average grain size of the ferrite is preferably 4 to 25 ⁇ m, more preferably 5 ⁇ m or more.
• pearlite may be formed in addition to the ferrite and cementite described above. Pearlite may be contained as long as the area fraction thereof in the entire microstructure is 6.5% or less because pearlite in such an amount does not impair the advantageous effects according to aspects of the present invention.
• the proportion of the number of cementite grains having an equivalent circle diameter of 0.1 ⁇ m or less to the total number of cementite grains is 20% or less. This can further achieve a tensile strength of 480 MPa or less and a total elongation (El) of 33% or more.
• the proportion of the number of cementite grains having an equivalent circle diameter of 0.1 ⁇ m or less to the total number of cementite grains is preferably 10% or less.
• the proportion the number of cementite grains having an equivalent circle diameter of 0.1 ⁇ m or less to the total number of cementite grains is 10% or less, a tensile strength of 440 MPa or less and a total elongation (El) of 36% or more can be achieved.
• cementite grains having an equivalent circle diameter of 0.1 ⁇ m or less have a dispersion strengthening ability, and an increase in the number of cementite grains having such a size impairs cold workability.
• cementite grains present before quenching have an equivalent circle diameter of about 0.07 to 3.0 ⁇ m.
• the dispersion state of cementite grains before quenching having an equivalent circle diameter of more than 0.1 ⁇ m, which is a size not affecting precipitation strengthening much, is not particularly specified in accordance with aspects of the present invention.
• the cementite In quenching, the cementite needs to be wholly dissolved to ensure a desired amount of solute C in the ferrite. If the average cementite grain size exceeds 2.5 ⁇ m, the cementite cannot be completely dissolved during holding in the austenite range, and thus the average cementite grain size is 2.5 ⁇ m or less, more preferably 2.0 ⁇ m or less. If the cementite is excessively fine, precipitation strengthening of the cementite reduces cold workability, and thus the average cementite grain size is preferably 0.1 ⁇ m or more, more preferably 0.15 ⁇ m or more.
• cementite grain size refers to an equivalent circle diameter of a cementite grain
• the equivalent circle diameter of a cementite grain is a value obtained by measuring the major axis and the minor axis of the cementite grain and converting them into an equivalent circle diameter.
• average cementite grain size refers to a value determined by dividing the sum of equivalent circle diameters of all cementite grains by the total number of cementite grains.
• the proportion of the cementite to the entire microstructure exceeds 10.0%, the number of cementite grains of 0.1 ⁇ m or less contributing to precipitation strengthening is also increased, and the steel becomes hard.
• the proportion of the cementite is 10.0% or less, preferably 9.5% or less.
• this proportion is less than 3.5%, the substantial C content does not reach 0.20%, and a desired hardness cannot be provided after heat treatment.
• the proportion is 3.5% or more, more preferably 4.0% or more.
• the average grain size of the ferrite is less than 4 ⁇ m, the strength before cold working may increase to deteriorate press formability, and thus the average grain size of the ferrite is preferably 4 ⁇ m or more. If the average grain size of the ferrite exceeds 25 ⁇ m, the strength of the base metal may decrease. In the field where the steel sheet is formed into an intended product shape and then used without quenching, the base metal needs to have some degree of strength. Thus, the average grain size of the ferrite is preferably 25 ⁇ m or less. The average grain size of the ferrite is more preferably 5 ⁇ m or more, still more preferably 6 ⁇ m or more, and more preferably 20 ⁇ m or less, still more preferably 18 ⁇ m or less.
• the equivalent circle diameter of a cementite grain, the average cementite grain size, the proportion of the cementite to the entire microstructure, the area fraction of the ferrite, the average grain size of the ferrite, etc. described above can be measured by methods described in EXAMPLES described later.
• solute B and N present as AlN in the steel sheet surface layer portion are closely related to the manufacturing conditions in each step including heating conditions, coiling conditions, and annealing conditions and that these manufacturing conditions need to be optimized.
• the reasons necessary for achieving the amounts of solute B and N present as AlN in each step will be described later.
• the high-carbon hot-rolled steel sheet according to aspects of the present invention is used to form automotive parts such as gears, transmissions, and seat recliners by cold pressing and thus is required to have high cold workability. In addition, it is necessary to impart wear resistance by increasing the hardness through quenching treatment.
• the high-carbon hot-rolled steel sheet according to aspects of the present invention has a reduced tensile strength (TS) of 480 MPa or less and an increased total elongation (El) of 33% or more and hence can achieve both high cold workability and high hardenability (immersion-quench hardenability and carburizing hardenability). More preferably, the TS is 460 MPa or less, and the El is 35% or more.
• the tensile strength of the steel sheet is further reduced to a TS of 440 MPa or less, and the total elongation of the steel sheet is further increased to an El of 36% or more, whereby both high cold workability and high hardenability (immersion-quench hardenability and carburizing hardenability) can be achieved.
• the TS is 410 MPa or less, and the El is 38% or more.
• TS tensile strength
• El total elongation
• the high-carbon hot-rolled steel sheet according to aspects of the present invention is manufactured in the following manner using, as a material, a steel having a chemical composition as described above.
• the material (steel material) is subjected to hot rough rolling, and then finish rolling is performed at a finishing temperature equal to or higher than an Ar 3 transformation temperature.
• cooling is performed to 650° C. to 750° C. at an average cooling rate of 20° C./sec to 100° C./sec.
• Coiling is performed at a coiling temperature of 500° C. to 700° C., and the coil is cooled to normal temperature to obtain a hot-rolled steel sheet.
• the hot-rolled steel sheet is then heated in a temperature range from 450° C. to 600° C. at an average heating rate of 15° C./h or more.
• Annealing that involves holding at an annealing temperature lower than an Ac 1 transformation temperature for 1.0 h or more is performed.
• the high-carbon hot-rolled steel sheet according to aspects of the present invention is manufactured in the following manner using, as a material, a steel having a chemical composition as described above.
• the material (steel material) is subjected to hot rough rolling, and then finish rolling is performed at a finishing temperature equal to or higher than an Ar 3 transformation temperature.
• cooling is performed to 650° C. to 750° C. at an average cooling rate of 20° C./sec to 100° C./sec.
• Coiling is performed at a coiling temperature of 500° C. to 700° C., and the coil is cooled to normal temperature to obtain a hot-rolled steel sheet.
• the hot-rolled steel sheet is then heated in a temperature range from 450° C. to 600° C.
• Two-stage annealing that involves holding at a temperature equal to or higher than an Ac 1 transformation temperature and equal to or lower than an Ac 3 transformation temperature for 0.5 h or more, followed by cooling to a temperature lower than an Ar 1 transformation temperature at an average cooling rate of 1° C./h to 20° C./h, and holding at a temperature lower than the Ar 1 transformation temperature for 20 h or more is performed.
• the expression “° C.” regarding temperature indicates a temperature at a steel sheet surface or a surface of a steel material.
• the steel material may be produced by any method.
• a converter or an electric furnace can be used to prepare a molten high-carbon steel according to aspects of the present invention.
• the molten high-carbon steel prepared by a known method, for example, using a converter is formed into, for example, a slab (steel material) by ingot making and blooming or continuous casting.
• the slab is heated and then subjected to hot rolling (hot rough rolling and finish rolling).
• the heating temperature of the slab is preferably 1280° C. or lower in order to avoid deterioration of the surface state due to scales.
• the lower limit of the heating temperature of the slab is preferably 1100° C., more preferably 1150° C., still more preferably 1200° C. or higher.
• the material to be rolled may be heated by heating means such as a sheet bar heater in order to ensure the finishing temperature.
• the Ar 3 transformation temperature described above can be determined by actual measurement such as thermal expansion measurement or electrical resistance measurement during cooling using, for example, Formaster testing.
• the average rate cooling to 650° C. to 750° C. greatly affects the size of spheroidized cementite grains after annealing. If the average cooling rate after the finish rolling is less than 20° C./sec, a microstructure before annealing is composed of an excessive ferrite microstructure and a pearlite microstructure, and thus a desired cementite dispersion state and a desired cementite size are not provided after annealing. Thus, the cooling needs to be performed at 20° C./sec or more.
• the average cooling rate is preferably 25° C./sec or more. If the average cooling rate exceeds 100° C./sec, cementite grains having a desired size are not readily formed after annealing, and thus the average cooling rate is 100° C./sec or less, preferably 75° C./sec or less.
• the hot-rolled steel sheet after the finish rolling is wound into a coil shape. If the coiling temperature is excessively high, the hot-rolled steel sheet has excessively low strength and may be deformed by its own weight when wound into a coil shape. This is not preferable from the viewpoint of operation. Thus, the upper limit of the coiling temperature is 700° C., preferably 690° C. or lower. If the coiling temperature is excessively low, the hot-rolled steel sheet disadvantageously becomes hard. Thus, the coiling temperature is 500° C., preferably 530° C. or higher.
• the annealing temperature is lower than the Ac 1 transformation temperature, preferably (Ac 1 transformation temperature ⁇ 10° C.) or lower.
• the lower limit of the annealing temperature is not particularly specified, and to provide a desired cementite dispersion state, the annealing temperature is preferably 600° C. or higher, more preferably 700° C. or higher.
• an atmospheric gas any of nitrogen, hydrogen, and a gas mixture of nitrogen and hydrogen can be used.
• the holding time at the annealing temperature is 1.0 hour (h) or more.
• the holding time at the annealing temperature is 1.0 hour or more, preferably 5 hours or more, more preferably more than 20 hours. If the holding time at the annealing temperature exceeds 40.0 hours, the productivity decreases, resulting in an excessively high manufacturing cost.
• the holding time at the annealing temperature is preferably 40.0 hours or less, more preferably 35 hours or less.
• the following two-stage annealing may be performed instead of the above-described annealing.
• the high-carbon hot-rolled steel sheet can also be manufactured as follows: after coiling and cooling to normal temperature are performed to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is heated in a temperature range from 450° C. to 600° C.
• first-stage annealing that involves holding at a temperature equal to or higher than the Ac 1 transformation temperature and equal to or lower than the Ac 3 transformation temperature for 0.5 h or more (first-stage annealing), followed by cooling to a temperature lower than an Ar 1 transformation temperature at an average cooling rate of 1° C./h to 20° C./h, and holding at a temperature lower than the Ar 1 transformation temperature for 20 h or more (second-stage annealing) is performed.
• the hot-rolled steel sheet is heated in a temperature range from 450° C. to 600° C. at an average heating rate of 15° C./h or more, held at a temperature equal to or higher than the Ac 1 transformation temperature for 0.5 h or more to dissolve relatively fine carbide precipitated in the hot-rolled steel sheet into a ⁇ phase, and then cooled to a temperature lower than the Ar 1 transformation temperature at an average cooling rate of 1° C./h to 20° C./h and held at a temperature lower than the Ar 1 transformation temperature for 20 h or more.
• the dispersion morphology of carbide is controlled by performing the two-stage annealing under the predetermined conditions, whereby the steel sheet is softened. For the softening of the high-carbon steel sheet of interest in accordance with aspects of the present invention, it is important to control the dispersion morphology of carbide after the annealing.
• the high-carbon hot-rolled steel sheet is held at a temperature equal to or higher than the Ac 1 transformation temperature and equal to or lower than the Ac 3 transformation temperature (first-stage annealing), whereby fine carbide is dissolved, and at the same time, C is dissolved in ⁇ (austenite).
• first-stage annealing the ⁇ / ⁇ interface and undissolved carbide present in a temperature range of the Ac 1 transformation temperature or higher serve as nucleation sites to precipitate relatively coarse carbide.
• the two-stage annealing will be described below.
• any of nitrogen, hydrogen, and a gas mixture of nitrogen and hydrogen can be used.
• ammonia gas is likely to occur in a temperature range from 450° C. to 600° C., and nitrogen decomposed from the ammonia gas enters the surface of the steel sheet and binds to B and Al in the steel to form nitrides.
• the heating time in the temperature range from 450° C. to 600° C. is set to be as short as possible.
• the average heating rate in this temperature range is 15° C./h or more, preferably 20° C./h or more.
• the upper limit of the average heating rate is preferably 100° C./h, more preferably 90° C./h or less.
• the first-stage annealing temperature is higher than the Ac 3 transformation temperature, a large number of rod-like cementite grains are formed after the annealing, and the desired elongation is not provided.
• the first-stage annealing temperature is equal to or lower than the Ac 3 transformation temperature.
• the holding time at a temperature equal to or higher than the Ac 1 transformation temperature and equal to or lower than the Ac 3 transformation temperature is less than 0.5 h, fine carbide cannot be sufficiently dissolved.
• the steel sheet is held at a temperature equal to or higher than the Ac 1 transformation temperature and equal to or lower than the Ac 3 transformation temperature for 0.5 h or more.
• the holding time is preferably 1.0 h or more.
• the holding time is preferably 10 h or less.
• the heating rate is preferably such that the average heating rate in the temperature range from 450° C. to 600° C. is 15° C./h or more and the upper limit is 100° C./h or less.
• the average cooling rate is 1° C./h or more, preferably 5° C./h or more. If the average cooling rate exceeds 20° C./h, pearlite is precipitated to increase the hardness. Thus, the average cooling rate is 20° C./h or less, preferably 15° C./h or less.
• the steel sheet is cooled at a predetermined average cooling rate and held at a temperature lower than the Ar 1 transformation temperature to cause Ostwald ripening so that the coarse spherical carbide is further grown and fine carbide disappears.
• the holding time at a temperature lower than the Ar 1 transformation temperature is less than 20 h, carbide cannot be sufficiently grown, resulting in an excessively high hardness after the annealing.
• the steel sheet is held at a temperature lower than the Ar 1 transformation temperature for 20 h or more.
• the second-stage annealing temperature is preferably, but not necessarily, 660° C. or higher. From the viewpoint of production efficiency, the holding time is preferably, but not necessarily, 30 h or less.
• the Ac 3 transformation temperature, the Ac 1 transformation temperature, the Ar 3 transformation temperature, and the Ar 1 transformation temperature described above can be determined by actual measurement such as thermal expansion measurement or electrical resistance measurement during heating or cooling using, for example, Formaster testing.
• the average heating rates and the average cooling rates described above are determined by measuring temperatures with a thermocouple mounted in the furnace.
• test pieces were taken from the hot-rolled annealed sheets thus obtained, and the microstructure, the amount of solute B, the amount of N in AlN, the tensile strength, the total elongation, and the quenching hardness (hardness of steel sheet after quenching and hardness of steel sheet after carburizing and quenching) were determined as described below.
• the Ac 3 transformation temperature, the Ac 1 transformation temperature, the Ar 1 transformation temperature, and the Ar 3 transformation temperature shown in Table 1 were determined by Formaster testing.
• each annealed steel sheet was determined as follows: a test piece (size: 3 mm thick ⁇ 10 mm ⁇ 10 mm) taken from a central portion in the width direction was cut, polished, and then subjected to nital etching. Images were captured with a scanning electron microscope (SEM) at a magnification of 3000 times at five points at 1 ⁇ 4 from a surface layer in the thickness direction. The captured microstructure images were subjected to image processing to identify phases (e.g., ferrite, cementite, and pearlite).
• phases e.g., ferrite, cementite, and pearlite.
• pearlite area fraction is shown as a microstructure, and steels observed to have a pearlite area fraction of more than 6.5% are represented as Comparative Examples. Steels including pearlite with an area fraction of 6.5% or less, ferrite, and cementite are represented as Examples.
• the SEM images were binarized into ferrite and a non-ferrite region using image analysis software to determine the area fraction (%) of ferrite. Also for cementite, the SEM images were binarized into cementite and a non-cementite region using image analysis software to determine the area fraction (%) of cementite. For pearlite, the area fractions (%) of ferrite and cementite were subtracted from 100(%) to determine the area fraction (%) of pearlite.
• each cementite grain was determined.
• the cementite grain size was determined by measuring the major axis and the minor axis and converting them into an equivalent circle diameter.
• the average cementite grain size was determined by dividing the sum of equivalent circle diameters of all cementite grains by the total number of cementite grains.
• the number of cementite grains whose equivalent circle diameter values were 0.1 ⁇ m or less was determined and defined as the number of cementite grains having an equivalent circle diameter of 0.1 ⁇ m or less.
• the number of all cementite grains was determined and defined as the total number of cementite grains.
• the average grain size of ferrite was determined using a method for evaluation of crystal grain size (intercept method) specified in JIS G 0551.
• ground powder from a region extending from a surface layer to a depth of 100 ⁇ m was collected and measured three times, and the average value was determined as the average concentration of solute B.
• the average concentration of N present as AlN was determined by the same method as described in the following reference.
• Each annealed steel sheet was subjected to a carburizing and quenching treatment including steel soaking, carburizing treatment, and diffusion treatment at 930° C. for 4 hours in total, held at 850° C. for 30 minutes, and then cooled in oil (oil cooling temperature: 60° C.).
• the hardness was measured under a load of 1 kgf from a position 0.1 mm deep from the steel sheet surface to a position 1.2 mm deep at intervals of 0.1 mm to determine the hardness (HV) at 0.1 mm from the surface layer and the effective case depth (mm) after carburizing and quenching.
• the effective case depth is defined as a depth at which the hardness measured from the surface after the heat treatment reaches 550 HV or more.
• Table 4 presents acceptance criteria of hardenability depending on the C content, in which the hardenability can be evaluated as sufficient.
• Annealing conditions Average Average cooling heating rate rate to 650° C. in temper- Finish- to 750° C. ature ing after Coiling range from Annealing temper- finish temper- 450° C. to (annealing Sample Steel ature rolling ature 600° C. temperature- No. No.
• the high-carbon hot-rolled steel sheets of Examples each have a microstructure including ferrite and cementite, the proportion of the number of cementite grains having an equivalent circle diameter of 0.1 ⁇ m or less to the total number of cementite grains being 20% or less, the average cementite grain size being 2.5 ⁇ m or less, the cementite accounting for 3.5% or more and 10.0% or less of the entire microstructure, and have both high cold workability and high hardenability.
• the high-carbon hot-rolled steel sheets of Examples were provided with excellent mechanical properties, i.e., a tensile strength of 480 MPa or less and a total elongation (El) of 33% or more.
• any one or more of the chemical composition, the microstructure, the amount of solute B, and the amount of N in AlN do not satisfy the scope of the present invention, and as a result, the target performance described above cannot be satisfied in any one or more of the cold workability and the hardenability.
• the target properties were not satisfied in one or more of the tensile strength (TS) and the total elongation (El).
• TS tensile strength
• El total elongation
• Steel S has a C content lower than the range according to aspects of the present invention and thus does not satisfy the immersion-quench hardenability.
• Steel T has a C content higher than the range according to aspects of the present invention and thus does not satisfy the TS and total elongation of the steel sheet.

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