Classifications

• • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/18—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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/003—Cementite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• This disclosure relates to steel sheets, and more specifically, to steel sheets that can be used as materials for mechanical parts, such as automotive parts.
• Steel plates with a high C content are used as materials for mechanical parts, such as automobile parts.
• the method for manufacturing these mechanical parts using such steel plates as materials is as follows.
• the steel plate is cold worked to form it into the shape of the mechanical part. After cold working, the steel plate is quenched and tempered. Through the above manufacturing process, high-strength mechanical parts are manufactured.
• Patent Document 1 In order to increase the strength of machine parts, it is necessary to improve the hardenability of the steel plate that is the material for the machine parts. Therefore, steel plates with improved hardenability are proposed in Patent Document 1 and Patent Document 2.
• the steel plate disclosed in Patent Document 1 contains, in 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: 0.0050% or less, and B: 0.0005 to 0.0050%, and further contains at least one of Sb, Sn, Bi, Ge, Te, and Se in a total amount of 0.002 to 0.030%, with the balance being Fe and unavoidable impurities.
• the proportion of the amount of solid-solution B in the B content is 70% or more.
• the microstructure is composed of ferrite and cementite.
• the cementite density in the ferrite grains is 0.08 pieces/ ⁇ m 2 or less.
• the amount of solid-solution B is secured, thereby improving the hardenability.
• the steel plate disclosed in Patent Document 2 contains, by mass%, C: 0.10% to 0.33%, Si: 0.01% to 0.50%, Mn: 0.40% to 1.25%, P: 0.03% to 0.01%, sol. Al: 0.10% to 0.01%, N: 0.01% to 0.50%, and Cr: 0.50% to 1.50%, with the remainder being Fe and unavoidable impurities, and has a microstructure containing ferrite and carbides.
• the volume ratio of ferrite and carbides to the entire microstructure is 90% or more, and the volume ratio of pro-eutectoid ferrite to the entire microstructure is 20% to 80%.
• the Mn concentration in the carbides is 0.10 mass% or more and 0.50 mass% or less, and the ratio of the number of carbides with a grain size of 1 ⁇ m or more to the total number of carbides is 30% or more and 60% or less.
• the Mn concentration in the carbides is reduced. This makes it easier for the carbides to dissolve during quenching. As a result, the hardenability is improved.
• steel sheets with a C content of 0.50% or more are often used as materials for mechanical parts such as springs and washers.
• mechanical parts such as springs and washers.
• the steel sheets as materials are subjected to bending processing. Therefore, the steel sheets as materials are required to have high bendability.
• the mechanical parts have high strength after quenching. Therefore, the mechanical parts are required to have high hydrogen embrittlement resistance.
• Patent Document 1 nor Patent Document 2 discusses bendability and hydrogen embrittlement resistance.
• the objective of this disclosure is to provide a steel sheet that has excellent bendability and excellent hardenability, and that has excellent resistance to hydrogen embrittlement when used as a mechanical part.
• the steel sheet according to the present disclosure has In mass percent, C: 0.50 to 0.90%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, Cu: 0 to 0.15%, W: 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, Ca:
• the total area ratio of ferrite and cementite particles is 95% or more,
• the average grain size of the ferrite is 20.0 ⁇ m or less,
• the average particle size of the cementite particles is 1.50 ⁇ m or less.
• a spheroidization rate which is a ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 85% or more.
• the steel plate disclosed herein has excellent bendability and excellent hardenability, and when used as a mechanical part, it has excellent resistance to hydrogen embrittlement.
• the inventors have conducted research into steel plates that have excellent hardenability and bendability, and that provide excellent resistance to hydrogen embrittlement when used in machine parts. As a result, the inventors have made the following findings.
• the inventors investigated the improvement of hardenability and bendability in steel plate with a C content of 0.50% or more, and the improvement of hydrogen embrittlement resistance when used as a mechanical part, from the viewpoint of chemical composition.
• the inventors found that the following composition, by mass%, was obtained: C: 0.50-0.90%, Si: 0.10-0.50%, Mn: 0.20-1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01-1.20%, N: 0.0150% or less, Mo: 0-0.500%, Ni: 0-1.000%, B: 0-0.0100%, V: 0-0.500%, Nb: 0-0.500%, Ti: 0-0.150%, Cu: 0-0.15%, W: 0-0.15%, T ...
• the inventors first investigated means for improving the hardenability of the microstructure of steel sheet during quenching.
• the microstructure of steel sheet having the above-mentioned chemical composition is substantially composed of ferrite and cementite particles.
• the cementite particles in the steel sheet are easily dissolved during quenching.
• the particle diameter of the cementite particles is small. In the case of steel sheet having the above-mentioned chemical composition, it is effective to make the average particle diameter of the cementite particles 1.50 ⁇ m or less.
• the inventors further investigated means for improving the bendability of the microstructure of the steel sheet.
• it is effective to increase the spheroidization rate of the cementite particles and to make the average grain size of the ferrite an appropriate size. Therefore, in the steel sheet of this embodiment, the spheroidization rate of the cementite particles is set to 85% or more, and the average grain size of the ferrite is set to 20.0 ⁇ m or less.
• the present inventors have further studied and found that, when the C content at a depth of 50 ⁇ m from the surface of a steel plate in the plate thickness direction is [C]s and the C content at the center position in the plate thickness direction of the steel plate is [C]c, if the C concentration ratio F1 defined by formula (1) is 0.50 or less, the steel plate can have excellent bendability, and a mechanical part manufactured using the steel plate as a material can have excellent hydrogen embrittlement resistance.
• F1 [C]s / [C]c (1)
• the steel plate of this embodiment was completed based on the above-mentioned technical concept and has the following configuration.
• the steel plate of the first configuration is In mass percent, C: 0.50 to 0.90%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01 to 1.20%, N: 0.0150% or less, Mo: 0 to 0.500%, Ni: 0 to 1.000%, B: 0 to 0.0100%, V: 0 to 0.500%, Nb: 0 to 0.500%, Ti: 0 to 0.150%, Cu: 0 to 0.15%, W: 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, Ca:
• the total area ratio of ferrite and cementite particles is 95% or more,
• the average grain size of the ferrite is 20.0 ⁇ m or less,
• the average particle size of the cementite particles is 1.50 ⁇ m or less.
• a spheroidization rate which is a ratio of the total number of the spherical cementite particles to the total number of the cementite particles, is 85% or more.
• the steel plate of the second configuration is A steel plate having a first configuration, Mo: 0.001 to 0.500%, Ni: 0.001 to 1.000%, and B: 0.0001 to 0.0100%.
• the steel plate of the third configuration is A steel plate having a first or second configuration, V: 0.001 to 0.500%, Nb: 0.001 to 0.500%, and Ti: 0.001 to 0.150%.
• the steel plate of the fourth configuration is A steel plate having any one of the first to third configurations, Cu: 0.01 to 0.15%, W: 0.01 to 0.15%, Ta: 0.01 to 0.15%, Sn: 0.001 to 0.050%, Sb: 0.001 to 0.050%, Co: 0.001 to 0.050%, As: 0.001 to 0.050%, Mg: 0.001 to 0.050%, Y: 0.001 to 0.050%, Zr: 0.001 to 0.050%, La: 0.001 to 0.050%, Ce: 0.001 to 0.050%, and Ca: 0.001 to 0.050%.
• the steel plate of this embodiment satisfies the following features 1 to 6.
• the chemical composition is, in mass%, C: 0.50-0.90%, Si: 0.10-0.50%, Mn: 0.20-1.30%, P: 0.100% or less, S: 0.100% or less, Al: 0.100% or less, Cr: 0.01-1.20%, N: 0.0150% or less, Mo: 0-0.500%, Ni: 0-1.000%, B: 0-0.0100%, V: 0-0.500%, Nb: 0-0.500%, Ti: 0- 0.150%, Cu: 0-0.15%, W: 0-0.15%, Ta: 0-0.15%, Sn: 0-0.050%, Sb: 0-0.050%, Co: 0-0.050%, As: 0-0.050%, Mg: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, Ce: 0-0.050%, Ca: 0-0.050%, and the balance
• the total area ratio of ferrite and cementite particles is 95% or more.
• the average grain size of ferrite is 20.0 ⁇ m or less.
• the average particle size of the cementite particles is 1.50 ⁇ m or less.
• the spheroidization rate which is the ratio of the total number of spherical cementite particles to the total number of cementite particles, is 85% or more.
• C 0.50 to 0.90% Carbon (C) enhances the hardenability of steel plate.
• C Carbon
• the lower limit of the C content is preferably 0.52%, more preferably 0.55%, and further preferably 0.60%.
• the upper limit of the C content is preferably 0.88%, more preferably 0.85%, and further preferably 0.80%.
• the C content is preferably in the range of, for example, 0.52 to 0.88%, more preferably 0.55 to 0.85%, and even more preferably 0.60 to 0.80%.
• Si 0.10 to 0.50%
• Silicon (Si) deoxidizes steel during the steelmaking stage of the steel sheet manufacturing process.
• Si also increases the temper softening resistance of the steel sheet when tempering is performed in the process of manufacturing machine parts using the steel sheet as a raw material.
• Si also promotes decarburization of the steel sheet surface during cold-rolled sheet annealing. If the Si content is less than 0.10%, the above effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 0.50%, the strength of the steel sheet becomes excessively high due to solid solution strengthening. Therefore, the bendability of the steel sheet decreases. Therefore, the Si content is 0.10 to 0.50%.
• the lower limit of the Si content is preferably 0.12%, more preferably 0.15%, and further preferably 0.20%.
• the upper limit of the Si content is preferably 0.48%, more preferably 0.44%, and further preferably 0.40%.
• the Si content is preferably in the range of, for example, 0.12 to 0.48%, more preferably 0.15 to 0.44%, and further preferably 0.20 to 0.40%.
• Mn 0.20 to 1.30%
• Manganese (Mn) enhances the hardenability of steel plate.
• Mn Manganese
• the lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, and further preferably 0.35%.
• the upper limit of the Mn content is preferably 1.25%, more preferably 1.20%, and further preferably 1.15%.
• the Mn content is preferably in the range of, for example, 0.25 to 1.25%, more preferably 0.30 to 1.20%, and further preferably 0.35 to 1.15%.
• Phosphorus (P) is an impurity.
• the P content may be 0%. If the P content exceeds 0.100%, the toughness of the steel plate decreases. Therefore, the P content is 0.100% or less.
• the P content is preferably as low as possible. In other words, the P content is preferably 0%. However, excessive reduction in the P content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the P content is preferably 0.090%, more preferably 0.080%, and further preferably 0.050%.
• the P content is preferably in the range of, for example, 0.001 to 0.090%, more preferably 0.003 to 0.080%, and even more preferably 0.005 to 0.050%.
• S 0.100% or less Sulfur (S) is an impurity.
• the S content may be 0%. If the S content exceeds 0.100%, S forms an excessive amount of sulfides. As a result, the bendability of the steel sheet decreases. Therefore, the S content is 0.100% or less.
• the S content is preferably as low as possible. In other words, the S content is preferably 0%. However, excessive reduction in the S content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the S content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the S content is preferably 0.090%, more preferably 0.080%, and further preferably 0.050%. The preferred range of the S content is, for example, 0.001 to 0.090%, more preferably 0.003 to 0.080%, and even more preferably 0.005 to 0.050%.
• Al 0.100% or less
• Aluminum (Al) is an impurity.
• the Al content may be 0%.
• Al combines with N to form AlN.
• AlN refines austenite grains when heated for quenching in the process of manufacturing mechanical parts using steel sheet as a raw material. Refining austenite grains reduces the hardenability of the steel sheet. If the Al content exceeds 0.100%, the austenite grains are excessively refined when heated for quenching, and the hardenability of the steel sheet is significantly reduced. Therefore, the Al content is 0.100% or less.
• the lower limit of the Al content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Al content is preferably 0.090%, more preferably 0.080%, further preferably 0.070%, and further preferably 0.050%.
• the Al content is preferably in the range of, for example, 0.001 to 0.090%, more preferably 0.005 to 0.070%, and even more preferably 0.010 to 0.050%.
• the Al content means the acid-soluble Al (sol. Al) content.
• Chromium (Cr) enhances the hardenability of steel plate.
• Cr Chromium
• the lower limit of the Cr content is preferably 0.02%, more preferably 0.03%, and further preferably 0.05%.
• the upper limit of the Cr content is preferably 1.15%, more preferably 1.10%, further preferably 1.00%, further preferably 0.70%, and further preferably 0.50%.
• the Cr content is preferably in the range of, for example, 0.02 to 1.15%, more preferably 0.03 to 1.10%, and even more preferably 0.05 to 0.50%.
• N 0.0150% or less Nitrogen (N) is an impurity that is inevitably contained. In other words, the N content is more than 0%. N combines with Al to form AlN. AlN refines austenite grains when heated for quenching in the process of manufacturing mechanical parts using steel plate as a raw material. Refining austenite grains reduces the hardenability of the steel plate. If the N content exceeds 0.0150%, the austenite grains are excessively refined when heated for quenching, and the hardenability of the steel plate is significantly reduced. Therefore, the N content is 0.0150% or less.
• the lower limit of the N content is preferably 0.0001%, and more preferably 0.0005%.
• the upper limit of the N content is preferably 0.0140%, and more preferably 0.0130%.
• the N content is preferably in the range of, for example, 0.0001 to 0.0140%, and more preferably, 0.0005 to 0.0130%.
• the remainder of the chemical composition of the steel plate according to this embodiment is composed of Fe and impurities.
• impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of the steel plate, and are acceptable to the extent that they do not adversely affect the steel plate according to this embodiment.
• the chemical composition of the steel sheet of the present embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of first to third groups.
• First Group Mo: 0 to 0.500%, Ni: 0 to 1.000%, and B: 0 to 0.0100%, one or more selected from the group consisting of [Group 2] V: 0 to 0.500%, Nb: 0 to 0.500%, and Ti: 0 to 0.150%, one or more selected from the group consisting of [Group 3] Cu: 0 to 0.15%, W: 0 to 0.15%, Ta: 0 to 0.15%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Co: 0 to 0.050%, As: 0 to 0.050%, Mg: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, and Ca: 0
• the chemical composition of the steel sheet according to the present embodiment may further contain, instead of a portion of Fe, one or more elements selected from the group consisting of Mo, Ni, and B. All of these elements are optional elements and may not be contained. When contained, Mo, Ni, and B improve the hardenability of the steel sheet.
• Mo 0 to 0.500%
• Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0%.
• Mo When Mo is contained, that is, when the Mo content is more than 0%, Mo enhances the hardenability of the steel sheet. Therefore, by performing hardening in a process for manufacturing a mechanical part using the steel sheet as a raw material, the strength of the mechanical part is enhanced. Furthermore, when tempering is performed in a process for manufacturing a mechanical part using the steel sheet as a raw material, Mo enhances the temper softening resistance of the steel sheet. The above effect can be obtained to some extent even if even a small amount of Mo is contained. However, if the Mo content exceeds 0.500%, the strength of the steel sheet becomes excessively high.
• the Mo content is 0 to 0.500%.
• the lower limit of the Mo content is preferably 0.001%, more preferably 0.003%, further preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Mo content is preferably 0.450%, more preferably 0.400%, further preferably 0.350%, and further preferably 0.300%.
• the Mo content is preferably in the range of, for example, 0.001 to 0.450%, more preferably 0.003 to 0.400%, still more preferably 0.005 to 0.350%, and still more preferably 0.010 to 0.300%.
• Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%.
• Ni enhances the hardenability of the steel plate. Therefore, by performing hardening in a process for manufacturing a mechanical part using the steel plate as a raw material, the strength of the mechanical part is enhanced.
• Ni further enhances the temper softening resistance of the steel plate when tempering is performed in a process for manufacturing a mechanical part using the steel plate as a raw material. The above effect can be obtained to some extent even if even a small amount of Ni is contained. However, if the Ni content exceeds 1.000%, the strength of the steel plate becomes excessively high.
• the Ni content is 0 to 1.000%.
• the lower limit of the Ni content is preferably 0.001%, more preferably 0.005%, further preferably 0.007%, and further preferably 0.010%.
• the upper limit of the Ni content is preferably 0.950%, more preferably 0.900%, further preferably 0.800%, further preferably 0.700%, and further preferably 0.600%.
• the Ni content is preferably in the range of, for example, 0.001 to 0.950%, more preferably 0.005 to 0.900%, still more preferably 0.007 to 0.800%, still more preferably 0.010 to 0.700%, and still more preferably 0.010 to 0.600%.
• B 0 to 0.0100%
• Boron (B) is an optional element and may not be contained, that is, the B content may be 0%.
• B When B is contained, that is, when the B content is more than 0%, B enhances the hardenability of the steel plate. Therefore, by carrying out hardening in a process for manufacturing a mechanical part using the steel plate as a raw material, the strength of the mechanical part is increased. Even if even a small amount of B is contained, the above effect can be obtained to a certain extent. However, if the B content exceeds 0.0100%, B compounds are generated. In this case, the strength of the steel plate becomes excessively high. Therefore, the bendability of the steel plate decreases. Therefore, the B content is 0 to 0.0100%.
• the lower limit of the B content is preferably 0.0001%, more preferably 0.0003%, and further preferably 0.0005%.
• the upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, still more preferably 0.0070%, still more preferably 0.0060%, and still more preferably 0.0050%.
• the preferred range of the B content is, for example, 0.0001 to 0.0090%, more preferably 0.0003 to 0.0080%, still more preferably 0.0005 to 0.0070%, still more preferably 0.0005 to 0.0060%, and still more preferably 0.0005 to 0.0050%.
• the chemical composition of the steel plate according to the present embodiment may further contain one or more elements selected from the group consisting of V, Nb, and Ti instead of a portion of Fe. All of these elements are optional elements and may not be contained. When contained, V, Nb, and Ti form carbides and suppress coarsening of austenite grains during quenching heating in the process of manufacturing mechanical parts using the steel plate as a raw material. Therefore, the toughness of the mechanical parts is improved.
• V 0 to 0.500%
• Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%.
• V When V is contained, that is, when the V content is more than 0%, V forms carbides and suppresses the coarsening of austenite grains during quenching and heating in the process of manufacturing mechanical parts using the steel plate as a raw material. Therefore, the toughness of the mechanical parts is improved. The above effect can be obtained to a certain extent even if even a small amount of V is contained.
• the V content exceeds 0.500%, excessive carbides are formed and the steel plate is precipitation strengthened. Therefore, the bendability of the steel plate is reduced. Therefore, the V content is 0 to 0.500%.
• the lower limit of the V content is preferably 0.001%, more preferably 0.003%, and further preferably 0.005%.
• the upper limit of the V content is preferably 0.480%, more preferably 0.450%, and further preferably 0.400%.
• the V content is preferably in the range of, for example, 0.001 to 0.480%, more preferably 0.003 to 0.450%, and further preferably 0.005 to 0.400%.
• Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%.
• Nb When Nb is contained, that is, when the Nb content is more than 0%, Nb forms carbides and suppresses the coarsening of austenite grains during quenching heating in the process of manufacturing mechanical parts using the steel plate as a raw material. Therefore, the toughness of the mechanical parts is improved.
• Nb combines with N to suppress the formation of nitrides by the solute B. This improves the hardenability of the steel plate due to the solute B. If even a small amount of Nb is contained, the above effect can be obtained to a certain extent.
• the Nb content is 0 to 0.500%.
• the lower limit of the Nb content is preferably 0.001%, more preferably 0.003%, and further preferably 0.005%.
• the upper limit of the Nb content is preferably 0.480%, more preferably 0.450%, further preferably 0.400%, further preferably 0.350%, and further preferably 0.300%.
• the Nb content is preferably in the range of, for example, 0.001 to 0.480%, more preferably 0.003 to 0.450%, still more preferably 0.005 to 0.400%, still more preferably 0.005 to 0.350%, and still more preferably 0.005 to 0.300%.
• Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%.
• Ti When Ti is contained, that is, when the Ti content is more than 0%, Ti forms carbides and suppresses the coarsening of austenite grains during quenching heating in the process of manufacturing mechanical parts using the steel plate as a raw material. Therefore, the toughness of the mechanical parts is improved.
• Ti combines with N to suppress the formation of nitrides by the solute B. This improves the hardenability of the steel plate due to the solute B. If even a small amount of Ti is contained, the above effect can be obtained to a certain extent.
• the Ti content is 0 to 0.150%.
• the lower limit of the Ti content is preferably 0.001%, more preferably 0.003%, and further preferably 0.005%.
• the upper limit of the Ti content is preferably 0.145%, more preferably 0.130%, further preferably 0.120%, further preferably 0.100%, and further preferably 0.080%.
• the Ti content is preferably in the range of 0.001 to 0.145%, more preferably 0.003 to 0.130%, still more preferably 0.005 to 0.120%, still more preferably 0.005 to 0.100%, and still more preferably 0.005 to 0.080%.
• the chemical composition of the steel sheet according to the present embodiment may further contain, instead of a part of Fe, one or more elements selected from the group consisting of Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca. All of these elements are optional elements and may not be contained. In other words, the content of these elements may be 0%.
• the Cu content is 0-0.15%
• the W content is 0-0.15%
• Sb 0-0.050%
• Ca 0-0.050%.
• the lower limit of the Cu content is preferably 0.01%, and more preferably 0.03%.
• the upper limit of the Cu content is preferably 0.13%, and more preferably 0.10%.
• the Cu content is preferably in the range of, for example, 0.01 to 0.13%, and more preferably, 0.03 to 0.10%.
• the lower limit of the W content is preferably 0.01%, and more preferably 0.03%.
• the upper limit of the W content is preferably 0.13%, and more preferably 0.10%.
• the W content is preferably in the range of, for example, 0.01 to 0.13%, and more preferably, 0.03 to 0.10%.
• the lower limit of the Ta content is preferably 0.01%, and more preferably 0.03%.
• the upper limit of the Ta content is preferably 0.13%, and more preferably 0.10%.
• the preferred range of the Ta content is, for example, 0.01 to 0.13%, and more preferably, 0.03 to 0.10%.
• the lower limit of the Sn content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Sn content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the preferred range of the Sn content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.
• the lower limit of the Sb content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the preferred range of the Sb content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.
• the lower limit of the Co content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Co content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the preferred range of the Co content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and even more preferably 0.010 to 0.035%.
• the lower limit of the As content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the As content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the As content is preferably in the range of, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and further preferably 0.010 to 0.035%.
• the lower limit of the Mg content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Mg content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the preferable range of the Mg content is, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and further preferably 0.010 to 0.035%.
• the lower limit of the Y content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Y content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the Y content is preferably in the range of, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and further preferably 0.010 to 0.035%.
• the lower limit of the Zr content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Zr content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the Zr content is preferably in the range of, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and further preferably 0.010 to 0.035%.
• the lower limit of the La content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the La content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the La content is preferably in the range of, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and further preferably 0.010 to 0.035%.
• the lower limit of the Ce content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Ce content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the Ce content is preferably in the range of, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and further preferably 0.010 to 0.035%.
• the lower limit of the Ca content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%.
• the upper limit of the Ca content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.
• the Ca content is preferably in the range of, for example, 0.001 to 0.045%, more preferably 0.005 to 0.040%, and further preferably 0.010 to 0.035%.
• the chemical composition of the steel plate of this embodiment can be measured by a known compositional analysis method. Specifically, chips are collected from the inside of the steel plate to a depth of 0.1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by a known inert gas fusion-thermal conductivity method.
• ICP-AES Inductively Coupled Plasma Atomic Emission Spectrometry
• the content of each element is determined by rounding off the measured value based on the significant figures specified in this embodiment to the lowest digit of the content of each element specified in this embodiment.
• the C content of the steel plate in this embodiment is specified as a value to two decimal places. Therefore, the C content is determined as a value to two decimal places obtained by rounding off the measured value to two decimal places.
• the contents of other elements than the C content of the steel plate of this embodiment are determined by rounding the measured value to the smallest digit specified in this embodiment. Rounding means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.
• the total area ratio of ferrite and cementite particles is 95% or more, that is, the microstructure of the steel sheet of the present embodiment is substantially composed of ferrite and cementite particles.
• the structure other than the ferrite and cementite particles is, for example, one or more types selected from the group consisting of bainite, martensite, and pearlite.
• the total area ratio of ferrite and cementite particles in the microstructure is 96% or more, more preferably 97% or more, even more preferably 98% or more, and even more preferably 99% or more.
• the microstructure may be a structure consisting of ferrite and cementite particles.
• the preferred range of the total area ratio of ferrite and cementite particles is 96-100%, more preferably 97-100%, even more preferably 98-100%, and even more preferably 99-100%.
• the total area ratio of ferrite and cementite particles is 95% or more, assuming that characteristic 1 and characteristics 3 to 6 are satisfied, excellent bendability will be obtained, and high resistance to hydrogen embrittlement will be obtained when the material is made into a mechanical part.
• the total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method.
• a test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate.
• the surface of the test piece measuring 15 mm in the rolling direction and thickness is defined as the observation surface.
• the observation surface of the test piece is mirror-polished.
• the mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etching solution). Secondary electron images are observed with a 1000x scanning electron microscope (SEM) for any five observation fields of the etched observation surface. Each observation field is a rectangle measuring 100 ⁇ m x 120 ⁇ m.
• ferrite and cementite particles show different contrast and morphology than other structures (bainite, martensite, pearlite, etc.). Therefore, ferrite and cementite particles are identified in the observation field based on the contrast and morphology.
• the total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ferrite and cementite particles in the five observation fields and the total area of the five observation fields.
• the average grain size of ferrite is 20.0 ⁇ m or less. If the average grain size of ferrite exceeds 20.0 ⁇ m, the bendability is reduced. If the average grain size of ferrite exceeds 20.0 ⁇ m, the annealing time is lengthened due to the coarsening of the ferrite grains. In this case, the alloy elements are concentrated in the cementite, and the hardenability is reduced. Therefore, the average grain size of ferrite is 20.0 ⁇ m or less.
• the preferred upper limit of the average grain size of ferrite is 19.5 ⁇ m, more preferably 19.0 ⁇ m, even more preferably 18.5 ⁇ m, and even more preferably 18.0 ⁇ m.
• the lower limit of the average grain size of ferrite is not particularly limited. However, excessive refinement of ferrite excessively increases the yield strength of the steel sheet. In this case, the processing load during bending increases excessively. Therefore, the preferred lower limit of the average grain size of ferrite is 5.0 ⁇ m.
• the more preferred lower limit of the average grain size of ferrite is 5.2 ⁇ m, more preferably 5.5 ⁇ m, more preferably 5.7 ⁇ m, more preferably 6.0 ⁇ m, and even more preferably 6.5 ⁇ m.
• the preferred range of the average particle size of ferrite is, for example, 5.0 to 20.0 ⁇ m, more preferably 5.2 to 19.5 ⁇ m, even more preferably 5.5 to 19.0 ⁇ m, even more preferably 5.7 to 18.5 ⁇ m, even more preferably 6.0 to 18.0 ⁇ m, even more preferably 6.5 to 18.0 ⁇ m.
• the average grain size of ferrite can be measured by the following method.
• a test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate.
• the surface of the test piece measuring 15 mm in the rolling direction and thickness is defined as the observation surface.
• the observation surface of the test piece is mirror-polished. After mirror-polishing, etching is performed with a 3% nital etching solution. In the etched observation surface, a secondary electron image is taken using a scanning electron microscope (SEM) at any five observation fields at a depth position of thickness/4 from the surface of the steel plate.
• SEM scanning electron microscope
• the grain size number of ferrite is obtained by a cutting method.
• the magnification of the SEM is selected in the range of 500 to 3000 times so that the number of ferrite grains cut by one line segment is at least 10 or more in one field of view.
• the cutting length is obtained for five observation fields of view.
• the grain size number of ferrite is obtained from the arithmetic average value of the cutting lengths of the five observation fields of view. From the obtained grain size number, the average grain size ( ⁇ m) of ferrite is determined.
• the average grain size of ferrite is a value obtained by rounding off the obtained value to one decimal place (i.e., the value to the first decimal place).
• the average particle size of the cementite particles is 1.50 ⁇ m or less. If the average particle size of the cementite particles is large, the bendability of the steel sheet is reduced. If the average particle size of the cementite particles is large, the cementite particles do not dissolve sufficiently when heated in the quenching process in the process of manufacturing a mechanical part using the steel sheet as a material. In this case, the steel sheet does not have sufficient quenchability. As a result, the mechanical part manufactured using the steel sheet as a material does not have sufficient strength.
• the cementite particles are sufficiently small. Therefore, the steel plate has sufficient bendability. Furthermore, when heated in the hardening process, the cementite particles dissolve sufficiently, improving the hardenability of the steel plate.
• the preferred upper limit of the average particle size of the cementite particles is 1.45 ⁇ m, more preferably 1.40 ⁇ m, even more preferably 1.35 ⁇ m, and even more preferably 1.30 ⁇ m.
• the average particle size of the cementite particles is small. However, if the average particle size of the cementite particles is too small, the hardness of the steel sheet becomes too high and the cold workability decreases. Therefore, the preferred lower limit of the average particle size of the cementite particles is 0.05 ⁇ m, more preferably 0.10 ⁇ m, even more preferably 0.15 ⁇ m, and even more preferably 0.20 ⁇ m.
• the preferred range of the average particle size of the cementite particles is, for example, 0.05 to 1.50 ⁇ m, more preferably 0.10 to 1.45 ⁇ m, even more preferably 0.15 to 1.40 ⁇ m, even more preferably 0.20 to 1.35 ⁇ m, and even more preferably 0.20 to 1.30 ⁇ m.
• the average particle size of the cementite particles can be determined by the following method.
• a test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate.
• the surface of the test piece measuring 15 mm in the rolling direction and thickness is defined as the observation surface.
• the observation surface of the test piece is mirror-polished. After mirror-polishing, the observation surface is etched using picral liquid. Secondary electron images are taken of five observation fields of interest at any position located at a depth of 4/th the plate thickness from the surface of the steel plate on the etched observation surface. Specifically, a scanning electron microscope (SEM) is used to observe the five observation fields at a magnification of 2000 times, and the above-mentioned secondary electron images are taken. Each observation field is a rectangle of 50 ⁇ m x 60 ⁇ m.
• cementite particles are identified based on the contrast.
• the area of each identified cementite particle is calculated, and the circle-equivalent diameter of each cementite particle is calculated based on the area.
• the circle-equivalent diameter thus calculated is regarded as the particle diameter of the cementite particle.
• the particle diameter is calculated using well-known image processing software.
• the arithmetic mean value of the particle diameters of the cementite particles obtained in the five observation fields is defined as the average particle diameter ( ⁇ m) of the cementite particles.
• the average particle size of the cementite particles is a value obtained by rounding off the obtained value to two decimal places (i.e., the value to one decimal place).
• cementite particles having an aspect ratio of 3.0 or less are defined as spherical cementite particles.
• the spheroidization rate which is the ratio of the total number of spherical cementite particles to the total number of the plurality of cementite particles, is 85% or more.
• the steel plate will have excellent bendability, provided that Features 1 to 4 and 6 are satisfied. Therefore, in the steel plate of this embodiment, the spheroidization rate is 85% or more.
• the preferred lower limit of the spheroidization rate is 87%, more preferably 89%, even more preferably 91%, and even more preferably 95%.
• the preferred range of the spheroidization rate is, for example, 85 to 100%, more preferably 87 to 100%, even more preferably 89 to 100%, even more preferably 91 to 100%, and even more preferably 95 to 100%.
• the spheroidization rate can be measured by the following method.
• the aspect ratio is determined for each of the cementite particles identified in the five observation fields by the above-mentioned [Method for measuring the average particle size of cementite particles].
• the maximum distance obtained when the contour of the cementite particle is sandwiched between two parallel line segments is defined as the major axis.
• the distance between the two line segments when the contour of the cementite particle is sandwiched between two line segments parallel to the major axis i.e., the width perpendicular to the major axis
• minor axis is defined as the minor axis.
• cementite particles having an aspect ratio of 3.0 or less are specified as "spheroidal cementite particles.”
• the ratio of the total number of spherical cementite particles to the total number of multiple cementite particles in the five observation fields is defined as the spheroidization rate (%).
• the spheroidization rate is a value obtained by rounding off the obtained value to the nearest integer.
• the strength of the surface layer of the steel plate is high, and the strength of the surface layer of the mechanical parts manufactured using the steel plate as a material is also high. In this case, the bendability of the steel plate decreases, and further, the hydrogen embrittlement resistance of the mechanical parts manufactured using the steel plate as a material decreases.
• the steel plate has excellent bendability. Furthermore, mechanical parts manufactured using the steel plate as a material have excellent resistance to hydrogen embrittlement.
• the preferred upper limit of the C concentration ratio F1 is 0.49, more preferably 0.48, even more preferably 0.47, even more preferably 0.46, and even more preferably 0.45.
• the lower limit of the C concentration ratio F1 is not particularly limited. In steel sheets that satisfy characteristics 1 to 6, the lower limit of the C concentration ratio F1 is, for example, 0.05, more preferably 0.10, and even more preferably 0.15.
• the preferred range of the C concentration ratio F1 is, for example, 0.05 to 0.50, more preferably 0.10 to 0.49, even more preferably 0.15 to 0.48, even more preferably 0.15 to 0.47, even more preferably 0.15 to 0.46, and even more preferably 0.15 to 0.45.
• the C concentration ratio F1 can be measured by the following method.
• a test piece measuring 15 mm in the rolling direction of the steel plate, 10 mm in the width direction, and thickness is taken from the center of the steel plate.
• the surface of the test piece measuring 15 mm in the rolling direction and thickness is defined as the observation surface.
• the observation surface of the test piece is mirror-polished.
• the observation surface after mirror polishing is measured at a depth of 50 ⁇ m from the steel plate surface in the thickness direction, and at 300 measurement positions at 1 ⁇ m pitch in the longitudinal direction of the steel plate, using an electron beam microanalyzer (EPMA).
• EPMA electron beam microanalyzer
• the arithmetic average value of the C content (mass%) at the obtained 300 points is defined as [C]s.
• the C content (mass%) is measured at 300 measurement positions at 1 ⁇ m pitch in the longitudinal direction of the steel plate using an electron beam microanalyzer (EPMA).
• the arithmetic average value of the C content at the obtained 300 points is defined as [C]c.
• the C concentration ratio F1 is calculated from formula (1).
• the EPMA is measured under the following conditions.
• the steel plate of this embodiment which satisfies the above-mentioned features 1 to 6, has excellent bendability. Furthermore, the steel plate of this embodiment has sufficient hardenability during hardening in a process for manufacturing a mechanical component using the steel plate as a material. Furthermore, the mechanical component manufactured using the steel plate of this embodiment as a material has excellent hydrogen embrittlement resistance.
• the bendability is evaluated by the following method.
• [Bendability evaluation method] A plate-shaped test piece is taken from the center of the width of the steel plate. The shape of the plate-shaped test piece is 15 mm in the rolling direction of the steel plate ⁇ 30 mm in the width direction ⁇ plate thickness.
• a 90 ° V bending test is performed on the plate-shaped test piece. Specifically, a 90 ° V bending process is performed at the center position of the plate width (30 mm) of the plate-shaped test piece using a die and a push-in punch. The bending line formed on the plate-shaped test piece by the V bending process is parallel to the rolling direction of the steel plate (L-axis bending).
• the R of the push-in punch used in the 90 ° V bending test is 0.1 mm.
• the presence or absence of cracks on the surface of the plate-shaped test piece after the 90 ° V bending test is visually observed. If no cracks are confirmed, it is determined that excellent bendability is obtained.
• the hardenability is evaluated by the following method.
• [Heatenability evaluation method] (A c1 transformation point measurement) A cylindrical test piece with a diameter of 3 mm and a length of 10 mm is taken from the center of the width of the steel plate. The longitudinal direction of the cylindrical test piece is parallel to the rolling direction of the steel plate. The thermal expansion coefficient during heating is measured using a Formaster testing machine. The A c1 transformation point is calculated from the obtained thermal expansion coefficient.
• a plate-shaped test piece was taken from the widthwise center of the steel plate, and had a shape of 15 mm in the rolling direction of the steel plate, 30 mm in the width direction, and plate thickness.
• the plate test piece is heated at 1000°C for 20 minutes using a salt bath.
• the plate test piece is then immersed in water in a water tank and quenched.
• the quenched plate test piece is cut into two equal parts in the width direction of the steel plate.
• the cut surface is mirror-polished.
• a Vickers hardness test in accordance with JIS Z2244:2009 is carried out at three arbitrary points in the center position in the thickness direction of the cut surface after mirror polishing. At this time, the test force is 98N.
• the arithmetic average value of the obtained Vickers hardness is defined as the maximum quenched hardness HD0 (HV).
• a plate-shaped test piece was taken from the widthwise center of the steel plate, and had a shape of 15 mm in the rolling direction of the steel plate, 30 mm in the width direction, and plate thickness.
• the plate test piece is immersed in a salt bath at A c1 transformation point +80°C for 10 minutes.
• the plate test piece is then removed from the salt bath and immersed in water in a water tank for quenching.
• the quenched plate test piece is cut into two equal parts in the plate width direction.
• the cut surface is mirror-polished.
• a Vickers hardness test in accordance with JIS Z2244:2009 is carried out at three arbitrary points in the center position in the plate thickness direction of the cut surface after mirror polishing. At this time, the test force is 98N.
• the arithmetic mean value of the obtained Vickers hardness is defined as the quenched hardness HD1 (HV). If the obtained quenched hardness HD1 is 0.95 times or more the maximum quenched hardness HD0, it is determined that the steel plate has sufficient hardenability.
• Hydrogen embrittlement resistance is evaluated by the following method.
• [Hydrogen embrittlement resistance evaluation method] A plate-shaped test piece is taken from the center position of the plate width of the steel plate. The shape of the plate-shaped test piece is 30 mm in the rolling direction of the steel plate ⁇ 100 mm in the plate width direction ⁇ plate thickness. The plate-shaped test piece is bent into a U-shape by a press bending method to prepare a U-bend test piece with a curvature radius R of 2.0 mm. Specifically, U-bending is performed at the center position of the plate width (100 mm) of the plate-shaped test piece. The bend line formed in the plate-shaped test piece by U-bending is parallel to the rolling direction of the steel plate (L-axis bending).
• the U-bend test piece is immersed in a salt bath at A c1 point + 80 ° C for 10 minutes. After immersion, the U-bend test piece is removed from the salt bath and immersed in a water tank for quenching. The quenched U-bend test piece is tempered at 250 ° C for 1 hour. In the tempered U-bend test piece, a pair of ends of the non-bend part of the U-bend test piece are tightened with bolts to elastically deform the U-bend test piece so that the opposing non-bend parts are parallel to each other.
• a delayed fracture acceleration test is conducted on a U-bend test piece with the non-bend portion parallel. Specifically, the U-bend test piece is immersed in hydrochloric acid of pH 1 for 100 hours. After 100 hours, the presence or absence of cracks in the bent portion of the U-bend test piece is visually confirmed. If no cracks are found, it is determined that excellent hydrogen embrittlement resistance has been obtained.
• the steel sheet of this embodiment is suitable as a material for mechanical parts, such as automobile parts.
• the mechanical parts are, for example, automobile springs, washers, etc.
• the steel sheet of this embodiment may be used for applications other than mechanical parts that require bendability and hydrogen embrittlement resistance.
• An example of a method for manufacturing a steel sheet according to the present embodiment includes the following steps.
• (Step 1) Material preparation step (Step 2) Hot rolling step (Step 3) Hot-rolled sheet annealing step (Step 4) Cold rolling step (Step 5) Cold-rolled sheet annealing step
• Step 1 Material preparation step
• Step 2 Hot rolling step
• Step 3 Hot-rolled sheet annealing step
• Step 4 Cold rolling step
• Step 5 Cold-rolled sheet annealing step
• Step 1 Material preparation process
• a material satisfying Feature 1 is prepared.
• the material is produced, for example, by the following method.
• Molten steel is produced, the content of each element in the chemical composition of which falls within the range of this embodiment.
• the molten steel is used to produce a material (slab or ingot) by a casting method.
• the molten steel is used to produce a slab by a well-known continuous casting method.
• the molten steel is used to produce an ingot by a well-known ingot casting method.
• Step 2 Hot rolling step hot rolling is performed on a prepared material (slab or ingot) to produce a steel plate.
• the hot rolling process includes a rough rolling process in which the material is roughly rolled to produce a rough bar (intermediate steel plate), and a finish rolling process in which the rough bar is finish rolled to produce a steel plate.
• the material (slab or ingot) is heated in a heating furnace.
• the heated material is rolled using a rough rolling mill to produce a rough bar.
• the heating temperature of the material in the rough rolling process is, for example, 1100 to 1250°C.
• the material is left in the heating furnace for 30 minutes or more, and preferably 60 minutes or more. There is no particular limit to the upper limit of the time, but it is, for example, 300 minutes.
• the rough bar is further rolled (finish rolling) using a finishing rolling mill to produce steel plate.
• the finishing rolling mill includes multiple stands arranged in a row. Each stand has a pair of work rolls.
• the surface temperature of the steel plate at the outlet of the stand that rolls down the steel plate last among the multiple stands of the finishing rolling mill is defined as the finishing rolling temperature (°C).
• the finishing rolling temperature is 830 to 950°C.
• the reduction ratio of the stand that rolls down the steel plate at the rearmost among the multiple stands arranged in a row in the finishing rolling mill is defined as the reduction ratio of the final pass (%). In this embodiment, the reduction ratio of the final pass is 5 to 25%.
• the hot-rolled steel plate after finishing rolling is wound up into a coil.
• the winding temperature CT will be described later.
• the coiled hot-rolled steel plate is cooled to room temperature.
• Hot-rolled sheet annealing step In the hot-rolled sheet annealing process, the hot-rolled steel sheet is annealed under known conditions.
• the annealing temperature in the hot-rolled sheet annealing process is, for example, 500 to 770°C, and the holding time at the annealing temperature is, for example, 5 to 80 hours.
• so-called box annealing is performed. Annealing is also performed in a reducing atmosphere.
• Step 4 Cold rolling step
• the steel sheet after the hot-rolled sheet annealing process is cold-rolled.
• the cold rolling is performed using a cold rolling mill.
• the cold rolling reduction ratio CR in the cold rolling process will be described later.
• Step 5 Cold-rolled sheet annealing step
• OCA Open Coil Annealing
• annealing is performed on the cold-rolled steel sheet under conditions that satisfy conditions 3 to 6 described below. This adjusts the degree of recrystallization of ferrite and precipitation of cementite particles. Furthermore, the C concentration ratio F1 is adjusted.
• the coiling temperature CT affects the spheroidization rate of cementite particles. If the coiling temperature CT is 750°C or less, the cementite particles generated in the hot-rolled steel sheet are distributed sufficiently uniformly. In this case, the spheroidization rate of the cementite particles is increased by performing annealing. Therefore, the coiling temperature CT is preferably 750°C or less.
• the lower limit of the coiling temperature CT is not particularly limited. However, due to equipment constraints, the preferable lower limit of the coiling temperature CT is 550°C.
• Cold rolling rate CR (1 - (thickness of cold-rolled steel sheet after cold rolling process / thickness of hot-rolled steel sheet before cold rolling process)) x 100
• the cold rolling rate CR is over 35%, sufficient strain is introduced into the steel sheet. In this case, in the next annealing process, the spheroidization of cementite particles is promoted, and the spheroidization rate becomes 85% or more. On the other hand, if the cold rolling rate CR is 60% or less, the strain introduced into the steel sheet is appropriate. In this case, the ferrite is appropriately refined, and the average grain size of ferrite becomes 5.0 ⁇ m or more.
• the annealing temperature T1 adjusts the spheroidization rate of the cementite particles of the steel sheet. If the annealing temperature T1 is less than 650°C, the spheroidization of the cementite becomes insufficient, and the spheroidization rate of the cementite particles becomes less than 85%. Furthermore, the C concentration ratio F1 exceeds 0.50. On the other hand, if the annealing temperature T1 exceeds 750°C, the annealing temperature is too high. In this case, the spheroidization of the cementite becomes insufficient, and the spheroidization rate of the cementite particles becomes less than 85%.
• the holding time t1 at the annealing temperature T1 affects the size of the ferrite and the size of the cementite particles in the steel sheet. Specifically, if the holding time t1 is less than 5 hours, the cementite particles are not sufficiently spheroidized. On the other hand, if the holding time t1 exceeds 40 hours, the holding time is too long, and the ferrite and cementite particles become coarse. As a result, the average particle size of the ferrite exceeds 20.0 ⁇ m, and the average particle size of the cementite particles exceeds 1.50 ⁇ m.
• the atmosphere in the open coil annealing furnace in the cold rolled sheet annealing process is a 93 to 97 volume % hydrogen-3 to 7 volume % nitrogen atmosphere.
• the C concentration ratio F1 can be made 0.50 or less.
• the dew point in the atmosphere of the open coil annealing furnace in the cold-rolled sheet annealing process is set to +25 to +65°C. Specifically, the dew point is adjusted to the above range using humidified nitrogen. If the dew point is less than +25°C, the C concentration ratio F1 exceeds 0.50. On the other hand, if the dew point exceeds +65°C, the equipment load becomes excessive. Therefore, the dew point in the cold-rolled sheet annealing process is set to +25 to +65°C.
• annealing is performed in the hot-rolled sheet annealing process so as to satisfy conditions 5 and 6 instead of a reducing atmosphere, and then annealing is performed in a reducing atmosphere in the cold-rolled sheet annealing process, a steel sheet satisfying features 1 to 6 cannot be obtained.
• annealing that satisfies conditions 5 and 6 is performed in the hot-rolled sheet annealing process, a decarburized layer is formed in the steel sheet after the hot-rolled sheet annealing process.
• the decarburized layer is extended by the cold rolling process after the hot-rolled sheet annealing process.
• the decarburized layer becomes thinner by the cold rolling process.
• the cold-rolled sheet annealing process is performed, C in the steel sheet is diffused and recarburized, and the C concentration is uniform throughout the steel sheet. As a result, a steel sheet satisfying feature 6 cannot be obtained.
• the hot-rolled sheet annealing process is performed in a reducing atmosphere, and then in the final process, the cold-rolled sheet annealing process, annealing is performed under conditions that satisfy conditions 3 to 6.
• the effects of the steel plate of this embodiment will be explained in more detail below using examples.
• the conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel plate of this embodiment. Therefore, the steel plate of this embodiment is not limited to this one example of conditions.
• molten steel was continuously cast to produce a slab with a thickness of 250 mm.
• the slab was subjected to a hot rolling process. Specifically, the slab was heated at 1100-1250°C for 120 minutes. The heated slab was rolled in a rough rolling mill to produce a rough bar. The rough bar was then rolled using a finishing rolling mill to produce a hot-rolled steel sheet with a thickness of 3.5 mm.
• the finishing rolling temperature for each test number was 830-950°C. The reduction ratio of the final pass was 5-25%.
• the hot-rolled steel sheet after finishing rolling was wound up and formed into a coil. The coiled hot-rolled steel sheet was allowed to cool to room temperature.
• the coiling temperature CT in the hot rolling process for each test number was as shown in Table 2.
• the hot-rolled steel sheets were subjected to a hot-rolled sheet annealing process.
• the annealing temperature in the hot-rolled sheet annealing process was 500 to 770°C, and the holding time at the annealing temperature was 5 to 80 hours.
• the hot-rolled sheet annealing was performed in a reducing atmosphere.
• the hot-rolled steel sheets after the hot-rolled sheet annealing process were subjected to a cold rolling process to produce cold-rolled steel sheets.
• the cold rolling reduction ratio CR in the cold rolling process was as shown in Table 2.
• the cold-rolled steel sheets after the cold rolling were subjected to a cold-rolled sheet annealing process. In the cold-rolled sheet annealing process, open coil annealing was performed.
• the annealing temperature T1, holding time t1, and dew point in the cold-rolled sheet annealing process were as shown in Table 2.
• the atmosphere in the open coil annealing furnace was 95% hydrogen by volume and 5% nitrogen by volume for all test numbers.
• the steel sheets were produced by the above manufacturing process.
• Test 1 Chemical composition measurement test (Test 2) Total area ratio measurement test of ferrite and cementite (Test 3) Average ferrite grain size measurement test (Test 4) Average grain size measurement test of cementite particles (Test 5) Spheroidization rate measurement test of cementite particles (Test 6) C concentration ratio F1 measurement test (Test 7) Bendability evaluation test (Test 8) Hardenability evaluation test (Test 9) Hydrogen embrittlement resistance evaluation test Tests 1 to 9 are described below.
• the hardenability HD1 is less than the hardenability lower limit, it was determined that sufficient hardenability was not obtained (shown as “B (Bad)" in the "hardenability” column in Table 3). Furthermore, if the maximum hardenability HD0 of the steel plate is less than 600 HV, it was determined that sufficient strength as a mechanical part was not obtained.
• test number 36 the C content was too high. As a result, sufficient bendability and hydrogen embrittlement resistance were not obtained.
• test number 37 the Si content was too low. As a result, decarburization during cold-rolled sheet annealing was insufficient, and the C concentration ratio F1 exceeded 0.50. As a result, sufficient bendability and sufficient hydrogen embrittlement resistance were not obtained.
• test number 38 the Si content was too high. As a result, the strength of the steel plate was excessively high due to solid solution strengthening, and sufficient bendability was not obtained.
• test number 39 the Mn content was too low. As a result, sufficient hardenability was not obtained.
• test number 40 the Mn content was too high. As a result, the strength of the steel plate was excessively high due to solid solution strengthening, and sufficient bendability was not obtained.
• test number 41 the Cr content was too high. As a result, sufficient hardenability was not obtained. Furthermore, sufficient bendability was not obtained.
• test numbers 43 and 44 although the chemical composition was appropriate, the holding time t1 at the tempering temperature T1 in the cold-rolled sheet annealing process was too long. As a result, the average grain size of ferrite exceeded 20.0 ⁇ m, and the average grain size of cementite particles exceeded 1.50 ⁇ m. As a result, sufficient hardenability was not obtained, and sufficient bendability was not obtained.
• test numbers 45 to 47 the chemical composition was appropriate, but the dew point during cold-rolled sheet annealing was below 25°C. As a result, the C concentration ratio F1 exceeded 0.50. As a result, sufficient bendability and hydrogen embrittlement resistance were not obtained.
• test number 51 although the chemical composition was appropriate, the coiling temperature CT was too high. As a result, the spheroidization rate of the cementite particles was too low at less than 85%. As a result, sufficient hardenability and sufficient bendability were not obtained.
• test number 52 although the chemical composition was appropriate, the annealing temperature T1 in the cold-rolled sheet annealing process was too low. As a result, the spheroidization rate of the cementite particles was too low at less than 85%. Furthermore, the C concentration ratio F1 exceeded 0.50. As a result, sufficient bendability and sufficient hydrogen embrittlement resistance were not obtained.

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