Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • 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
  • C21D2211/004—Dispersions; Precipitations
• • 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
  • C21D2211/009—Pearlite

Definitions

• the present disclosure relates to a high-carbon cold-rolled steel sheet, a high-carbon hot-rolled steel sheet, and a spiral spring.
• high-carbon steel plate with a C content of 0.60% by mass or more has been widely used as the material for spring products such as spiral springs.
• the high-carbon steel plate from which they are made must have high strength and fatigue resistance. Furthermore, if the high-carbon steel plate contains coarse inclusions that can act as starting points for fatigue cracks, these can cause premature fracture, so it is necessary to control the amount and shape of the inclusions.
• Patent Document 1 discloses a cold-rolled carbon steel containing C: 0.63-0.85%, Si: maximum 0.40%, Mn: 0.20-0.90%, P: maximum 0.035%, S: maximum 0.035%, Al: maximum 0.060%, Cr: maximum 0.40%, N: 0.003-0.010% (preferably 0.005-0.008%), at least one micro-alloying element (Ti, Nb, V, and Zr) with a maximum content of 0.12%, and the remainder being iron and impurities.
• Ti, Nb, V, and Zr micro-alloying element
• Patent Document 2 discloses a method for manufacturing a high-carbon steel sheet, including the steps of: hot-rolling, cold-rolling, and annealing a steel material having a predetermined composition to produce a steel sheet having a structure of spheroidized cementite and primary ferrite; and heating the steel sheet and then subjecting it to a patenting heat treatment for 60 seconds or more while maintaining the temperature of the solder bath at 500°C or higher and 530°C or lower.
• Patent Document 2 also discloses that the patenting heat treatment may further include a cooling step and a cold rolling step of 85% or higher, and that the temperature range for heating the steel sheet before the patenting heat treatment is 800°C or higher and 1100°C or lower.
• Patent Document 2 produces a high-carbon steel sheet containing 90% or more by volume of a fine pearlite phase having a lamellar structure in which the interlayer spacing between layered carbides is 0.5 ⁇ m or less and the ratio of major axis to minor axis is 10:1 or higher.
• the main manufacturing method for improving the strength and fatigue durability of high-carbon steel sheets used in spiral springs and other products is to refine the pearlite structure by subjecting the hot-rolled steel sheet to a patenting process using a lead (Pb) bath before cold rolling.
• Pb lead
• Patent Document 1 which is a conventional technology, considers suppressing cracks that can form due to the notch effect of coarse structures by adding micro-alloying elements to refine the structure, but does not fully consider inclusions that serve as crack initiation points. Furthermore, the manufacturing method disclosed in Patent Document 1 requires patenting treatments, including cold rolling, after the hot rolling process.
• Patent Document 2 examines the morphology of pearlite from the perspective of suppressing the propagation of fatigue cracks, it does not adequately address the inclusions that serve as the starting points for crack initiation. Furthermore, patenting processes including cold rolling are required after the hot rolling process, which increases the number of manufacturing steps and poses productivity challenges.
• the present invention was made in consideration of the above circumstances, and aims to provide high-carbon cold-rolled steel sheets, high-carbon hot-rolled steel sheets, and spiral springs that have high strength (torque characteristics) and excellent fatigue durability.
• the present invention was made based on the above findings, and its gist is as follows:
• a high carbon cold rolled steel sheet comprises, in mass%,
• the chemical composition, in mass% is C: 0.65-0.80%, Si: 0.15-0.50%, Mn: 0.40-0.80%, P: 0.020% or less, S: 0.0015% or less, Al: 0.010-0.065%, Cr: more than 0.40%, less than 0.60%, Ca: 0.0005-0.0030%, O: 0.0040% or less, N: 0.0100% or less, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V: 0 to 0.10%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, B: 0 to 0.010%, Mo: 0 to 0.10%, W: 0 to 0.05%, Ta: 0 to 0.05%, Mg: 0-0.05%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, As: 0 to 0 to
• the compound may contain one or more of the above.
• the high carbon cold rolled steel sheet according to (1) above wherein the chemical composition is, in mass%, Al: 0.010-0.050%, Ti: 0 to 0.02%, Nb: 0 to 0.05%, V: 0 to 0.05%, Cu: 0 to 0.05%, Ni: 0 to 0.05%, Mo: 0 to 0.05%, REM: May be 0 to 0.0050%.
• the high-carbon cold-rolled steel sheet according to any one of (1) to (3) above may have a number density of coarse inclusions having an average grain size exceeding 10.0 ⁇ m of 0 pieces/ mm2 .
• a high-carbon hot-rolled steel sheet has a chemical composition, in mass%, of: C: 0.65-0.80%, Si: 0.15-0.50%, Mn: 0.40-0.80%, P: 0.020% or less, S: 0.0015% or less, Al: 0.010-0.065%, Cr: more than 0.40%, less than 0.60%, Ca: 0.0005-0.0030%, O: 0.0040% or less, N: 0.0100% or less, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V: 0 to 0.10%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, B: 0 to 0.010%, Mo: 0 to 0.10%, W: 0 to 0.05%, Ta: 0 to 0.05%, Mg: 0-0.05%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, As: 0 to 0.05%, REM: 0 to 0.0100% or less,
• the Vickers hardness of the surface may be 400 Hv or less.
• a spiral spring according to one aspect of the present invention has a chemical composition, in mass%, of: C: 0.65-0.80%, Si: 0.15-0.50%, Mn: 0.40-0.80%, P: 0.020% or less, S: 0.0015% or less, Al: 0.010-0.065%, Cr: more than 0.40%, less than 0.60%, Ca: 0.0005-0.0030%, O: 0.0040% or less, N: 0.0100% or less, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V: 0 to 0.10%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, B: 0 to 0.010%, Mo: 0-0.10%, W: 0 to 0.05%, Ta: 0 to 0.05%, Mg: 0-0.05%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, As: 0 to 0.05%, REM: 0 to 0.0100% or less, and the balance: Fe and impur
• the surface hardness may be 580 Hv or more.
• the above aspects of the present invention make it possible to provide high-carbon cold-rolled steel sheets, high-carbon hot-rolled steel sheets, and spiral springs that have high strength (torque characteristics) and excellent fatigue durability.
• the high-carbon cold-rolled steel sheet according to this embodiment has a predetermined chemical composition described below, in which the surface hardness of the steel sheet is 530 Hv or more in Vickers hardness, the area fraction of pearlite structures is 95% or more, the lamellar spacing of the pearlite structures is 20 nm or more and 50 nm or less, and in a 1/4 region that is a region from 1/8 depth of the sheet thickness from the surface to 3/8 depth of the sheet thickness from the surface, the number density of single inclusions of oxides, sulfides and nitrides having an average particle size of 1.0 ⁇ m or more and 10.0 ⁇ m or less, or composite inclusions comprising two or more types of single inclusions, is 3.0 pieces/ mm2 or less.
• "high carbon” means that the carbon content in the steel is 0.65% or more.
• Carbon (C) is an element that contributes to high strength by increasing the area fraction of the cementite phase and refining the lamellar spacing of the pearlite structure. If the C content is less than 0.65%, it becomes difficult to sufficiently secure the pearlite structure as the main microstructure. Therefore, the C content is 0.65% or more. Preferably, the C content is 0.70% or more, more preferably 0.72% or more. On the other hand, if the C content exceeds 0.80%, pro-eutectoid cementite phase may precipitate. In this case, cold rolling becomes difficult and fatigue durability decreases. Therefore, the C content is 0.80% or less. Preferably, the C content is 0.79% or less, more preferably 0.78% or less.
• Si 0.15-0.50%
• Silicon (Si) is an element contained for deoxidation. If the Si content is less than 0.15%, deoxidation will be insufficient, resulting in residual oxidized inclusions. Therefore, when a hot-rolled steel sheet is cold-rolled to an extremely thin thickness, cracks will form on the surface, causing reduced fatigue durability. Therefore, the Si content is set to 0.15% or more. Preferably, the Si content is set to 0.18% or more, more preferably 0.20% or more or 0.24% or more. On the other hand, if the Si content exceeds 0.50%, the formation of a substance that induces red scale will be promoted when the slab is reheated for hot rolling.
• the Si content is set to 0.50% or less.
• the Si content is set to 0.40% or less, more preferably 0.30% or less.
• Mn 0.40-0.80%
• Manganese (Mn) is an element effective in improving the hardenability of steel and increasing its strength. If the Mn content is less than 0.40%, the hardenability of the steel may be insufficient and sufficient strength may not be obtained. Therefore, the Mn content is 0.40% or more. Preferably, the Mn content is 0.45% or more, more preferably 0.55% or more. On the other hand, if the Mn content exceeds 0.80 mass%, the strength of the steel may increase excessively, resulting in a decrease in toughness. Therefore, the Mn content is 0.80% or less. Preferably, the Mn content is 0.75% or less, more preferably 0.70% or less.
• Phosphorus (P) is an impurity element. P embrittles grain boundaries and reduces cold rolling properties. Therefore, the P content is 0.020% or less.
• the P content is preferably as low as possible, preferably 0.018% or less, and more preferably 0.015% or less. However, excessive reduction of the P content significantly increases refining costs, so the P content may be 0.0010% or more.
• the lower limit of the P content is 0%, but in order to reduce refining costs, the lower limit of the P content may be 0.001%, 0.003%, or 0.005%.
• S 0.0015% or less Sulfur (S) is an impurity element.
• S forms non-metallic inclusions such as MnS. Such non-metallic inclusions can become the starting points for crack initiation and fracture initiation during cold rolling, resulting in reduced fatigue durability. Therefore, the S content is 0.0015% or less.
• the S content is preferably as low as possible, and the S content is preferably 0.0010% or less.
• the lower limit of the S content is 0%. Since excessive reduction of the S content significantly increases refining costs, the S content may be 0.0001% or more, 0.0003% or more, 0.0005% or more, or 0.0010% or more.
• Al 0.010-0.065%
• Aluminum (Al) acts as a deoxidizer during the steelmaking process and remains in steel.
• the Al content is 0.010% or more, a sufficient deoxidizing effect is obtained.
• the Al content is 0.015% or more, more preferably 0.020% or more.
• the Al content exceeds 0.050%, inclusions are likely to form in the steel. The formation of such inclusions may serve as the initiation point for fatigue cracks, resulting in reduced fatigue durability. Therefore, the Al content is 0.065% or less.
• the Al content is 0.050% or less, more preferably 0.045% or less, and even more preferably 0.040% or less.
• Chromium (Cr) is an element that has the effect of refining the lamellar spacing of pearlite structures, thereby improving the strength of steel sheets.
• the Cr content is more than 0.40%.
• the Cr content is 0.41% or more, more preferably 0.42% or more.
• the Cr content is 0.60% or less.
• the Cr content is 0.58% or less, more preferably 0.55% or less.
• Ca 0.0005-0.0030%
• Calcium (Ca) is an element effective in controlling the morphology of sulfides. Specifically, Ca has the effect of reducing coarse and elongated inclusions. If the Ca content is less than 0.0005%, the effect of controlling the morphology of sulfides cannot be obtained. Therefore, the Ca content is 0.0005% or more. Preferably, the Ca content is 0.0006% or more, more preferably 0.0007% or more. On the other hand, if the Ca content exceeds 0.0030%, the effect of controlling the morphology of sulfides becomes saturated, and the sulfides may aggregate or coarsen, resulting in a decrease in fatigue durability. Therefore, the Ca content is 0.0030% or less. Preferably, the Ca content is 0.0025% or less, more preferably 0.0020% or less.
• O 0.0040% or less
• Oxygen (O) forms oxides in steel. If the oxides in the steel aggregate and become coarse, or if the number density of the oxides increases, this may result in a decrease in cold rolling ability and a decrease in fatigue durability of the final product, the spiral spring. Therefore, the O content is 0.0040% or less. Preferably, the O content is 0.0030% or less, more preferably 0.0025% or less. A lower O content is preferable, with the lower limit being 0%. On the other hand, it is practically difficult to reduce the O content excessively. Therefore, the O content may be 0.0001% or more. In this embodiment, an O content within the range of 0.0001 to 0.0040% is acceptable.
• N 0.0100% or less Nitrogen (N) combines with Al in steel to form AlN.
• AlN has a pinning effect that inhibits the coarsening of pearlite block diameter and improves cold rolling properties.
• the lower limit of the N content is 0%, but to obtain this effect, the N content may be 0.0020% or more. However, if the N content is excessively high, the effect saturates and cold rolling properties may be reduced. Therefore, the N content is 0.0100% or less.
• the N content is 0.0080% or less, more preferably 0.0070% or less, and even more preferably 0.0060% or less.
• the cold-rolled steel sheet according to this embodiment may contain one or more of Ti, Nb, V, Cu, Ni, B, Mo, W, Ta, Mg, Sn, Sb, As, and REM.
• these optional elements By containing these optional elements, more preferable properties can be obtained for the cold-rolled steel sheet according to this embodiment.
• the lower limit for these optional elements is 0%.
• Titanium (Ti) has the effect of increasing strength by forming carbonitrides, so it may be contained within a range of 0.10% or less as necessary. On the other hand, if the Ti content exceeds 0.10%, coarse, angular carbonitrides are likely to be formed, resulting in significant deterioration of workability. Therefore, the Ti content is 0.10% or less. The Ti content may be 0.06% or less, 0.04% or less, or 0.02% (0.020%) or less. The lower limit of the Ti content may be 0.001% or more, or 0.003% or more.
• Niobium has the effect of increasing strength by forming carbonitrides, so it may be contained within a range of 0.10% or less as necessary.
• the Nb content exceeds 0.10%, coarse angular carbonitrides are likely to be formed, and deterioration of workability becomes apparent, so the Nb content is 0.020% or less.
• the Nb content may be 0.07% or less or 0.05% (0.050%) or less.
• the lower limit of the Nb content may be 0.001% or more or 0.003% or more.
• V 0 to 0.10%
• Vanadium (V) has the effect of increasing strength by forming carbonitrides, so it may be contained within a range of 0.10% or less as necessary.
• the V content exceeds 0.10%, coarse angular carbonitrides are likely to be formed, and deterioration of workability becomes apparent, so the V content is 0.10% or less.
• the V content may be 0.05% or less, or 0.05% (0.050%) or less.
• the lower limit of the V content may be 0.01% or more.
• Cu 0-0.50% Copper (Cu) has the effect of improving the strength (hardness) of the steel sheet. Therefore, Cu may be contained within a range of 0.50% or less, as necessary. On the other hand, if the Cu content exceeds 0.50%, there is a risk of hot working cracks occurring during hot rolling due to molten metal embrittlement (Cu embrittlement), so the Cu content is 0.50% or less.
• the Cu content may be 0.30% or less, 0.10% or less, or 0.05% (0.050%) or less.
• the lower limit of the Cu content may be 0.01% or more.
• Ni 0-0.50%
• Nickel (Ni) has the effect of preventing molten metal embrittlement (Cu embrittlement) when Cu is contained. Therefore, Ni may be contained within a range of 0.50% or less as necessary.
• the Ni content if the Ni content exceeds 0.50%, the cost increases while the above effect saturates, so the Ni content is 0.50% or less.
• the Ni content may be 0.30% or less, 0.10% or less, or 0.05% (0.050%) or less.
• the lower limit of the Ni content may be 0.01% or more.
• B 0-0.010% Boron (B) has the effect of improving the hardenability and improving the strength of the steel sheet. Therefore, B may be contained within a range of 0.010% or less as needed. On the other hand, if the B content exceeds 0.010%, B-based compounds are generated, which reduces the workability of the steel sheet. Therefore, the B content is 0.010% or less.
• the lower limit of the B content may be 0.0001% or more or 0.0003% or more.
• Mo 0-0.10% Molybdenum (Mo) has the effect of increasing the strength of steel sheet by forming carbonitrides. Therefore, Mo may be contained within a range of 0.10% or less, as necessary. On the other hand, if the Mo content exceeds 0.10%, coarse carbonitrides are likely to be formed, and deterioration of workability becomes apparent. Therefore, the Mo content is 0.10% or less. The Mo content may be 0.07% or less or 0.05% (0.050%) or less. The lower limit of the Mo content may be 0.001% or more or 0.003% or more.
• W 0 to 0.05% Tungsten (W) has the effect of increasing the strength of steel sheet by forming carbonitrides. Therefore, W may be contained within a range of 0.05% or less as necessary. On the other hand, if the W content exceeds 0.05%, coarse carbonitrides are likely to be formed, and deterioration of workability becomes apparent. Therefore, the W content is 0.05% or less. The W content may be 0.03% or less or 0.02% or less. The lower limit of the W content may be 0.001% or more or 0.003% or more.
• Tantalum (Ta) has the effect of increasing the strength of the steel sheet by forming carbonitrides, so it may be contained within a range of 0.05% or less as necessary. On the other hand, if the Ta content exceeds 0.050%, coarse carbonitrides are likely to be formed, and deterioration of workability becomes apparent, so the Ta content is 0.05% or less.
• the Ta content may be 0.03% or less or 0.02% or less.
• the lower limit of the Ta content may be 0.001% or more or 0.003% or more.
• Mg 0-0.05%
• Magnesium (Mg) is an element that can control the morphology of sulfides and also contributes to fatigue durability. To fully obtain the above effects, the Mg content is preferably 0.0001% or more. The lower limit of the Mg content may be 0.002% or more or 0.003% or more. On the other hand, if the Mg content exceeds 0.05%, the steel sheet may become embrittled and the ductility may decrease. Therefore, the Mg content is 0.05% or less. The Mg content may also be 0.03% or less or 0.02% or less.
• Tin is contained when scrap is used as a steel raw material and is an element that strongly segregates at grain boundaries. Therefore, the lower the Sn content, the better, and it may be 0%. If the Sn content exceeds 0.05%, the steel sheet may become embrittled and the ductility may decrease. Therefore, the Sn content is 0.05% or less. The Sn content may be 0.03% or less or 0.02% or less. The lower limit of the Sn content may be 0.001% or more or 0.003% or more.
• Sb 0-0.05% Antimony (Sb), like Sn, is contained when scrap is used as a steel raw material and is an element that strongly segregates at grain boundaries. Therefore, the lower the Sb content, the better, and it may be 0%. If the Sb content exceeds 0.05%, the steel sheet may become embrittled and the ductility may decrease. Therefore, the Sb content is 0.05% or less. The Sb content may be 0.03% or less or 0.02% or less. The lower limit of the Sb content may be 0.001% or more or 0.003% or more.
• Arsenic like Sn, is contained when scrap is used as a steel raw material and is an element that strongly segregates at grain boundaries. Therefore, the lower the As content, the better, and it may be 0%. If the As content exceeds 0.05%, the steel sheet may become embrittled and the ductility may decrease. Therefore, the As content is 0.05% or less.
• the As content may be 0.03% or less or 0.02% or less.
• the lower limit of the As content may be 0.001% or more or 0.003% or more.
• the REM content is preferably 0.0001% or more.
• the REM content is more preferably 0.003% or more or 0.0005% or more.
• the REM content exceeds 0.0100%, nozzle clogging during continuous casting becomes more likely.
• the REM content exceeds 0.0100%, the number density of the resulting REM-based inclusions (oxides and oxysulfides) becomes relatively high, leading to accumulation of these REM-based inclusions on the underside of the slab that curves during continuous casting. This may cause internal defects in the steel sheet obtained by rolling the slab and further deteriorate the workability of the steel sheet. Therefore, the REM content is preferably 0.0100% or less. The REM content is more preferably 0.0050% or less.
• REM Radar Metal refers to rare earth elements and is a collective term for 17 elements: scandium Sc (atomic number 21), yttrium Y (atomic number 39), and lanthanides (15 elements ranging from lanthanum with atomic number 57 to lutetium with atomic number 71).
• the cold-rolled steel sheet according to this embodiment contains at least one element selected from these elements. For example, these elements are often contained in the steel as misch metal, which is a mixture of these elements.
• the main components of misch metal are Ce, La, Nd, and Pr. In this embodiment, the total amount of rare earth elements is referred to as the REM content.
• the cold-rolled steel sheet according to this embodiment contains the above elements, with the remainder being Fe and impurities.
• impurities are elements that are mixed in during the industrial production of steel due to various factors in the raw materials, such as ore and scrap, and in the manufacturing process, and are elements whose presence is permitted to the extent that they do not impair the properties of the cold-rolled steel sheet according to this embodiment.
• the above-mentioned chemical compositions can be measured using common analytical methods. For example, they can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method. If ladle analysis values or slab analysis values are available during the production of cold-rolled steel plate slabs, there is no need to measure the chemical composition of the cold-rolled steel plate separately, and the ladle analysis values can be used. Ladle analysis values can also be used for the chemical composition of hot-rolled steel plate or spiral springs, as described below.
• ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
• microstructure metal structure of the high carbon cold rolled steel sheet according to this embodiment.
• the fraction of pearlite structure 95% or more
• the structure other than pearlite may be ferrite, bainite, or martensite.
• the area ratio of each structure is measured by the following method. Note that the area ratio of each structure is also measured by the following method for parts (spiral springs, etc.) using hot-rolled steel sheets and cold-rolled steel sheets described below.
• Cold-rolled steel sheet Test pieces are taken so that the thickness cross section of the cold-rolled steel sheet can be observed. In this case, it is preferable to take test pieces from a 1/4 position in the width direction of the cold-rolled steel sheet.
• a structural photograph is taken using a scanning electron microscope at a magnification of 5000x, with a field of view of 20 ⁇ m in the thickness direction ⁇ 30 ⁇ m in the direction perpendicular to the thickness, centered at a depth of 1/4 of the thickness from the surface of the steel sheet, and 30 ⁇ m in the thickness direction.
• Five structural photographs are taken. In each structural photograph, regions having lamellar carbides are identified as pearlite, and the area ratio of pearlite is obtained by calculating the average value of their area ratios.
• the rolling direction of the cold-rolled steel sheet is determined by the following method. Test pieces are taken from cold-rolled steel sheets so that the thickness cross section can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed at magnifications of 100x, 200x, 500x, and 1000x using an optical microscope. An appropriate magnification at which the dimensions of the inclusions can be measured is selected according to their size. The observation range is 500 ⁇ m or more in width and across the entire thickness of the sheet, and areas with dark brightness are determined to be inclusions. Observation may be performed from multiple fields of view.
• the lamellar spacing of the pearlite structure is 20 nm or more.
• the lamellar spacing of the pearlite structure is 22 nm or more, 25 nm or more, or 27 nm or more.
• the lamellar spacing of the pearlite structure is 50 nm or less.
• the lamellar spacing of the pearlite structure is 48 nm or less, 45 nm or less, 42 nm or less, or 40 nm or less.
• the lamellar spacing of the pearlite structure refers to the average distance between the center points of adjacent ferrite and cementite phases that make up a lamella in the thickness direction.
• the lamellar spacing can be determined, for example, by treating one ferrite phase layer and one cementite phase layer as a set of layers and measuring how many sets of layers can be cut by a line segment of a given length in a direction perpendicular to the direction of layer extension during structure observation. Note that layers that are not completely cut by the line segment at both ends of the line segment are excluded from the measurement.
• the lamellar spacing of the pearlite structure is determined as follows. First, a sample is taken from a position 1/4 of the sheet thickness from the surface of the steel sheet so that the cross section parallel to the rolling direction and thickness direction of the steel sheet serves as the observation surface. Next, the observation surface is mirror-polished and etched with a picral etchant, and then the structure is observed using a scanning electron microscope (SEM). At a magnification of 5000x (measurement area: 80 ⁇ m ⁇ 150 ⁇ m), 10 locations where the cementite layer crosses perpendicularly to the paper surface of the structure photograph are selected.
• SEM scanning electron microscope
• the lamellar spacing S is determined at each location, and the average of these values is used as the "lamellar spacing.”
• the lamellar spacing at each location is measured as follows. First, a straight line is drawn perpendicular to the cementite layer so as to cross 10 to 30 cementite layers, and the length of the straight line (line segment length) is defined as L. Furthermore, the number of layers crossed by the straight line, each consisting of one ferrite phase layer and one cementite phase layer, is defined as N.
• the lamellar spacing of the pearlite structure can also be measured by the above method in parts (such as spiral springs) made from hot-rolled steel sheets and cold-rolled steel sheets, which will be described later.
• Lamellar spacing S line segment length L ⁇ (number of sets cut by line segments N ⁇ 2)
• the number density of inclusions in the cold-rolled steel sheet is set to 3.0 pieces/ mm2 or less. This results in high fatigue durability.
• the number density of inclusions is 2.5 pieces/ mm2 or less, 2.0 pieces/ mm2 or less, or 1.5 pieces/ mm2 or less.
• the lower limit is 0 pieces/ mm2 .
• inclusions with an average grain size of less than 1.0 ⁇ m do not affect fatigue durability and are therefore not counted as inclusions as defined in this embodiment.
• coarse inclusions with an average particle size exceeding 10.0 ⁇ m can become the starting point of fracture. In other words, coarse inclusions with an average particle size exceeding 10.0 ⁇ m will fracture prematurely regardless of the number, making it impossible to obtain the desired fatigue durability. Therefore, in this embodiment, it is preferable that the number density of coarse inclusions with an average particle size exceeding 10.0 ⁇ m is 0 pieces/ mm2 .
• the number density of coarse inclusions with an average particle size exceeding 10.0 ⁇ m obtained by the method for calculating the number density of inclusions described below is less than 0.5 pieces/ mm2
• the number density of coarse inclusions with an average particle size exceeding 10.0 ⁇ m is determined to be 0 pieces/ mm2 .
• the number density of coarse inclusions with an average particle size exceeding 10.0 ⁇ m may also be set to 0.4 pieces/ mm2 or less, 0.3 pieces/ mm2 or less, or 0.2 pieces/ mm2 or less.
• inclusions refers to single inclusions of oxides, sulfides, and nitrides, as well as composite inclusions that are combinations of two or more of these single inclusions.
• the number density of inclusions in a cold-rolled steel sheet is calculated by the following method: First, five samples are taken from each cold-rolled steel sheet. The collected test specimens are embedded in resin. The resin-embedded test specimens are polished in the thickness direction, and then mirror-polished so that the 1/4t plane (a plane parallel to the surface of the cold-rolled steel sheet, 1/4t away from the surface of the cold-rolled steel sheet, where t is the thickness) becomes the surface. The observation area within the polished surface is observed using a scanning electron microscope (SEM) equipped with composition analysis capabilities. The observation area is an area of 1.2 mm x 1.6 mm. The long side of the observation area corresponds to the rolling direction of the cold-rolled steel sheet. During observation, 48 non-overlapping fields of 200 ⁇ m x 200 ⁇ m are selected as the observation area, and each field is observed at a magnification of 500x.
• SEM scanning electron microscope
• particles precipitates or inclusions
• an equivalent circle diameter refers to the diameter of a circle when the area of a particle is converted into a circle with the same area.
• Each identified particle is analyzed for elemental concentration using energy dispersive X-ray spectroscopy (EDX).
• EDX energy dispersive X-ray spectroscopy
• the standardless method is used for EDX analysis (elemental concentration analysis).
• the accelerating voltage is set to 20 kV, and the quantified elements are Si, Mn, P, S, Cr, Ti, Nb, Cu, Ni, Ca, N, O, Al, and Mg.
• particles with a total O, S, and N content of 10% or more by mass, where the total mass content of the above quantified elements is taken as 100%, are identified as single inclusions or complex inclusions of oxides, sulfides, and nitrides.
• the number density (pieces/ mm2 ) of oxide, sulfide and nitride single inclusions and composite inclusions with an equivalent circle diameter of 1.0 ⁇ m or more was calculated.
• the number density of inclusions can also be measured by the above method in parts (such as spiral springs) made from hot-rolled steel sheets and cold-rolled steel sheets, which will be described later.
• the Vickers hardness of the steel sheet surface is 530 Hv or more. This allows for excellent torque characteristics to be obtained.
• the Vickers hardness of the steel sheet surface is 535 Hv or more, 540 Hv or more, or 550 Hv or more. There is no need to set an upper limit for the Vickers hardness, but it may be, for example, 700 Hv or less, 670 Hv or less, 650 Hv or less, or 620 Hv or less.
• the hardness is determined by Vickers hardness measurement in accordance with JIS Z 2244:2009, using a test force of 9.807 N.
• the measurement point is preferably at a position at least 50 mm away from the edge of the cold-rolled steel sheet.
• the hardness is measured at any five positions, and the Vickers hardness is obtained by calculating the average of the obtained hardness values.
• the Vickers hardness can also be measured by the above method for parts (spiral springs, etc.) using hot-rolled steel sheets and cold-rolled steel sheets, as described below. However, if the measurement point cannot be at least 50 mm away from the edge, the Vickers hardness is measured at the center of the width as a general rule.
• the thickness of the cold-rolled steel sheet is not particularly limited, but may be, for example, 0.100 to 0.400 mm.
• the preferred lower limit of the thickness of the cold-rolled steel sheet is 0.140 mm, 0.160 mm, or 0.180 mm.
• the preferred upper limit of the thickness of the cold-rolled steel sheet is 0.350 mm, 0.300 mm, 0.250 mm, or 0.230 mm.
• the high-carbon hot-rolled steel sheet (hereinafter also referred to as hot-rolled steel sheet) of this embodiment has a predetermined chemical composition, a Vickers hardness of the surface of the hot-rolled steel sheet of 320 Hv or more, and in a microstructure in a 1/4 region, which is a region from the surface of the hot-rolled steel sheet to a depth of 1/8 of the sheet thickness to a depth of 3/8 of the sheet thickness from the surface, the area fraction of pearlite structures is 95% or more, the average lamellar spacing of the pearlite structures is 80 to 200 nm, and, where the sheet thickness is t, the number density of single inclusions of oxides, sulfides, and nitrides having an average particle size of 1.0 ⁇ m or more and 10.0 ⁇ m or less, or composite inclusions comprising two or more of the single inclusions, is 3.0 pieces/ mm2 or less
• the thickness of the hot-rolled steel sheet is not particularly limited, but may be, for example, 1.60 to 4.80 mm.
• the preferred lower limit of the thickness of the hot-rolled steel sheet is 2.00 mm or 2.20 mm, and the preferred upper limit is 4.00 mm or 3.80 mm.
• the hot-rolled steel sheet of this embodiment is the hot-rolled steel sheet for the high-carbon cold-rolled steel sheet of this embodiment described above, and therefore the chemical composition of the hot-rolled steel sheet is the same as that of the cold-rolled steel sheet described above.
• the hot-rolled steel sheet in this embodiment has a microstructure having a pearlite structure with an area ratio of 95% or more, and the average lamellar spacing of the pearlite structure is 80 to 200 nm.
• Having a pearlite structure as the main structure means that the area fraction of the pearlite structure relative to the entire structure is 95% or more. Preferably, it is 98% or more. If the area fraction of the pearlite structure is less than 95%, the desired hardness cannot be obtained. If the area fraction of structures other than pearlite (bainite, ferrite, martensite) exceeds 5%, these structures become the origin of fracture due to strain concentration, resulting in reduced cold rolling properties. Furthermore, if the area fraction of the pearlite structure is less than 95%, the fatigue durability of the final spiral spring product may be reduced. The area fraction of the pearlite structure may be 100%. The structure other than pearlite (the remaining structure) may be ferrite, bainite, or martensite.
• the initial lamellar spacing before cold rolling i.e., the lamellar spacing in the hot-rolled steel sheet.
• the average lamellar spacing of the pearlite structure in the hot-rolled steel sheet exceeds 200 nm, it may be difficult to sufficiently increase the hardness of the resulting cold-rolled steel sheet even if the hot-rolled steel sheet is subjected to cold rolling at a high cold reduction ratio. Therefore, the average lamellar spacing of the pearlite structure in the hot-rolled steel sheet is 200 nm or less.
• the average lamellar spacing of the pearlite structure in the hot-rolled steel sheet is 70 nm or more.
• the lamellar spacing is preferably 75 nm or more, 80 nm or more, 85 nm or more, or 90 nm or more.
• the lamellar spacing can be adjusted by adjusting the cooling rate from the austenite phase and the coiling temperature, which will be described later.
• Coarse inclusions having an average particle size exceeding 10.0 ⁇ m can become the starting point of fracture. Therefore, in this embodiment, it is preferable that the number density of coarse inclusions having an average particle size exceeding 10.0 ⁇ m is 0 pieces/ mm2 . If the number density of coarse inclusions having an average particle size exceeding 10.0 ⁇ m is less than 0.5 pieces/ mm2 , the number density of coarse inclusions having an average particle size exceeding 10.0 ⁇ m is determined to be 0 pieces/ mm2 . The number density of coarse inclusions having an average particle size exceeding 10.0 ⁇ m may be set to 0.4 pieces/ mm2 or less, 0.3 pieces/ mm2 or less, or 0.2 pieces/ mm2 or less.
• the Vickers hardness of the surface is 305 HV or more.
• the strength of the cold-rolled steel sheet after cold rolling and the final product (spiral spring) can be sufficiently increased.
• the Vickers hardness of the surface may be 310 HV or more, 320 HV or more, or 330 HV or more. There is no need to set an upper limit for the Vickers hardness, but it may be, for example, 450 Hv or less, 420 Hv or less, 400 Hv or less, 380 Hv or less, or 370 Hv or less.
• the method for measuring the hardness of hot-rolled steel sheets is the same as that for cold-rolled steel sheets described above.
• the spiral spring according to this embodiment is obtained by forming the cold-rolled steel sheet according to this embodiment described above into a spring shape, which has an area fraction of pearlite of 95% or more, a lamellar spacing of pearlite of 20 to 50 nm, a number density of single inclusions of oxides, sulfides, and nitrides with an average particle size of 1.0 to 10.0 ⁇ m or a composite inclusion of two or more types of single inclusions of 3.0 pieces/mm2 or less within the plane of the sheet thickness 1 ⁇ 4t , and a hardness of the spring surface of 530 Hv or more.
• the "spiral spring” in this embodiment is a "number 3400 spiral spring” defined in JIS B 0103:2015, and is a spring having a spiral shape in a plane.
• the spiral spring according to this embodiment which is the final product (spiral spring)
• the hardness of the spiral spring can be increased.
• the aforementioned heat treatment can increase the Vickers hardness of the spiral spring surface to 580 Hv or more.
• the aforementioned heat treatment is not required, and the Vickers hardness of the spiral spring surface can be the same as that of the aforementioned cold-rolled steel sheet (specifically, the lower limit of Vickers hardness is 530 HV).
• the chemical composition, microstructure (metal structure), lamellar spacing of the pearlite structure, and number density of inclusions (including coarse inclusions exceeding 10.0 ⁇ m) of the spiral spring of this embodiment are basically the same as those of the aforementioned cold-rolled steel sheet that is the raw material. Therefore, description of these requirements will be omitted.
• a steelmaking (refining and casting) process for producing slabs having the above chemical composition (I) A slab heating step in which the slab is heated to 1100°C or higher. (III) A hot rolling step in which the heated slab is finish-rolled at an outlet temperature of 820 to 920°C to obtain a high-carbon hot-rolled steel plate. (IV) A cooling process including a primary cooling step of cooling the high-carbon hot-rolled steel sheet to a cooling stop temperature T1 at an average cooling rate of 30 to 80°C/second, and a secondary cooling step of cooling the high-carbon hot-rolled steel sheet from the T1 point to a coiling temperature at an average cooling rate of less than 20°C/second.
• (V) A coiling process in which the high carbon hot rolled steel sheet is coiled at a coiling temperature of 560 to 700°C.
• (VI) A pickling step for removing scale from the surface of the high carbon hot rolled steel sheet after the coiling step.
• (VII) A cold rolling step of cold rolling the high-carbon hot-rolled steel sheet at a total reduction rate of 90% or more to obtain a high-carbon cold-rolled steel sheet.
• a known method for manufacturing high-carbon cold-rolled steel sheets used in spiral springs and the like involves hot rolling, followed by pickling, spheroidizing annealing, primary cold rolling, patenting, and secondary cold rolling in that order.
• the manufacturing method of this embodiment including the above steps (I) to (VII), creates a pearlite structure in the hot rolling and coiling steps, thereby eliminating the conventionally performed spheroidizing annealing, primary cold rolling, and patenting, and providing a high-carbon cold-rolled steel sheet with high strength (torque characteristics) and excellent fatigue durability.
• the high-carbon steel sheet obtained by this manufacturing method is suitable for use in spiral springs and the like. Preferred conditions for each step will be described below. For steps and conditions not described below, known conditions can be used.
• Stepmaking (refining and casting) process As with general steel sheets, for example, blast furnace molten iron is used as a raw material, and molten steel is produced by converter refining and secondary refining, and then cast pieces such as slabs are obtained by continuous casting. The cast pieces are then subjected to hot rolling or cold rolling, as described below, to obtain cold-rolled steel sheets.
• inclusion control is performed by adding Ca while adjusting the steel composition.
• the total oxygen content (TO content) in the molten steel is adjusted to 0.0040% or less by mass. This makes it possible to reduce the number density of inclusions to 3.0 pieces/ mm2 or less.
• the TO content is 0.0030% or less, more preferably 0.0020% or less.
• Inclusion control involves first adjusting the composition of additive elements other than Ca. At this time, sufficient time is ensured for Al2O3 generated by Al deoxidation to float. After Al2O3 has sufficiently floated, Ca is added to the molten steel. If a large amount of Al2O3 remains in the molten steel, Ca is consumed in the reduction of Al2O3 . As a result, the amount of Ca available for fixing S decreases, and the formation of MnS cannot be sufficiently suppressed. From this perspective, it is necessary to ensure sufficient time for Al2O3 to float before Ca is added.
• This time depends on the dimensions of the ladle used to adjust the composition of additive elements other than Ca and other factors (e.g., the capacity of the vacuum degassing equipment if the composition adjustment is performed during vacuum degassing). For this reason, the required floating time cannot be determined uniformly. However, a person skilled in the art can easily determine the required floating time for each refining equipment, including the ladle, by analyzing cold-rolled steel sheets obtained through tests with different floating times. On the other hand, if Ca is added to molten steel having an S concentration of 0.0015% or more, CaS may be generated in the molten steel, aggregate, and form coarse inclusions. Therefore, the S concentration of the molten steel is preferably less than 0.0015%.
• the vapor pressure of Ca is high, when Ca is added, it is preferable to add it as a Ca-Si alloy, an Fe-Ca-Si alloy, a Ca-Ni alloy, etc., in order to increase the yield.
• the respective alloy wires may be used.
• slab heating process In the slab heating step, a slab having the above-described chemical composition is heated to 1100°C or higher before the hot rolling step.
• the heating temperature of the slab is set to 1100°C or higher in order to sufficiently redissolve the carbonitrides. Note that if the heating temperature of the slab exceeds 1300°C, the effect saturates, so the heating temperature is preferably 1300°C or lower.
• the heating time in the slab heating step is preferably 30 minutes or more. If the heating time is less than 30 minutes, the temperature may not be raised uniformly throughout the slab, and the carbonitrides may not be sufficiently redissolved. If the carbonitrides are not sufficiently redissolved, coarse carbonitrides may remain, which may act as starting points for cracks during cold rolling or may deteriorate the fatigue properties of the resulting cold-rolled steel sheet. On the other hand, if the heating time exceeds 120 minutes, the effect of sufficiently redissolving the carbonitrides will saturate. Furthermore, an excessively long heating time will result in reduced productivity and increased manufacturing costs. Therefore, a heating time of 120 minutes or less is preferable.
• the slab it is preferable to produce the slab to be heated by continuous casting, but it may also be produced by other casting methods (e.g., ingot casting).
• the hot rolling process in this embodiment is roughly divided into rough rolling and finish rolling.
• the heated slab is subjected to rough rolling to adjust the plate thickness, etc.
• the conditions of rough rolling are not particularly limited as long as a rough bar of the desired size and shape is obtained.
• the rough bar may be heated by an induction heating device such as a bar heater or an edge heater to homogenize the temperature of the rough bar.
• the rough-rolled slab is finish-rolled to obtain a hot-rolled steel sheet.
• the exit temperature (finishing temperature) in the finish rolling is 820 to 920°C. If the exit temperature of the finish rolling exceeds 920°C, the austenite phase may coarsen, resulting in a decrease in cold rolling ability. Therefore, the upper limit of the exit temperature of the finish rolling is 920°C or less, preferably 900°C or less, and more preferably 880°C or less.
• the lower limit of the exit temperature of the finish rolling may be Ar3 or higher from the viewpoint of suppressing coarsening of the austenite phase. However, if the finishing temperature is too low, the deformation resistance of the steel sheet increases, placing a great burden on the rolling mill and causing equipment trouble. Therefore, the lower limit of the exit temperature of the finish rolling is preferably 820°C or higher.
• the thickness of hot-rolled steel sheet is determined taking into account the reduction rate during subsequent cold rolling. For example, it may be between 2.0 mm and 4.0 mm.
• Controlled cooling is carried out in two stages: a primary cooling process and a secondary cooling process.
• the hot-rolled steel sheet is cooled from the finish-rolling delivery temperature to a cooling stop temperature T1 below the Ae1 point at an average cooling rate CR1 of 30 to 80°C/s. If the average cooling rate CR1 is less than 30°C/s, pro-eutectoid ferrite and/or pro-eutectoid cementite may precipitate, potentially making it impossible to achieve an area fraction of pearlite in the quarter region of 95% or more. Therefore, the average cooling rate CR1 is preferably 30°C/s or more, more preferably 40°C/s or more.
• the average cooling rate CR1 in the primary cooling step is preferably 80°C/s or less, more preferably 70°C/s or less.
• the lower limit of the cooling stop temperature T1 is not particularly limited, but may be 530°C or more, and is preferably 560°C or more.
• Ae1 (°C) can be calculated using the following formula (1).
• the hot-rolled steel sheet after the primary cooling step is cooled from the cooling stop temperature T1 to the coiling temperature CT (i.e., a temperature range of 560 to 700°C) at an average cooling rate CR2 of 5.0°C/sec or less. If the average cooling rate CR2 in the temperature range from the cooling stop temperature T1 to the coiling temperature CT is fast, the lamellar spacing within the steel sheet may become non-uniform. In such cases, the sheet may break due to hardness differences caused by the lamellar spacing during cold rolling, or a large amount of transformed structures such as bainite may be generated, making it impossible to achieve an area fraction of 95% or more of pearlite.
• the average cooling rate CR2 in the temperature range from the cooling stop temperature T1 to the coiling temperature CT is fast, the lamellar spacing within the steel sheet may become non-uniform. In such cases, the sheet may break due to hardness differences caused by the lamellar spacing during cold rolling, or a large amount of transformed structures such as bainite
• the average cooling rate CR2 in the secondary cooling step is set to 5.0°C/sec or less, preferably 3.0°C/sec or less, and more preferably 2.0°C/sec or less.
• the average cooling rate CR2 in the secondary cooling step slower than the average cooling rate CR1 in the primary cooling step, variation in the lamellar spacing of the pearlite structure can be suppressed, and cold rolling properties can be improved.
• the secondary cooling step is preferably carried out immediately after the completion of the primary cooling step in order to sufficiently suppress the formation of the ferrite phase.
• the film is immediately wound up after the secondary cooling step is completed.
• the coiling temperature CT of the hot-rolled steel sheet is set to 560 to 700°C.
• the coiling temperature CT is set to 560 to 700°C.
• the structure can be appropriately transformed during coiling, thereby making it possible to refine the average lamellar spacing of the pearlite structure.
• a lamellar spacing of 80 nm or more and 200 nm or less can be obtained at the stage before cold rolling, and the Vickers hardness of the surface of the hot-rolled steel sheet can be made 320 Hv or more.
• a high-carbon cold-rolled steel sheet with a surface hardness of 530 Hv or more can be obtained.
• the coiling temperature CT is preferably 560°C or higher, more preferably 580°C or higher, and even more preferably 600°C or higher.
• the coiling temperature CT is preferably 700°C or lower, more preferably 680°C or lower, and even more preferably 660°C or lower.
• the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
• the total reduction in cold rolling is preferably 90.0% or more.
• the lamellar spacing of the pearlite structure in the cold-rolled steel sheet can be set to 20 nm or more and 50 nm or less.
• the total reduction in cold rolling is more preferably 91% or more.
• the Vickers hardness of the surface of the cold-rolled steel sheet can be set to 530 Hv or more.
• the thickness of the cold-rolled steel sheet is preferably 0.180 mm or more and 0.260 mm or less, more preferably 0.160 mm or more and 0.250 mm or less, and even more preferably 0.180 mm or more and 0.240 mm or less.
• cold rolling is preferably performed at 100°C or less, more preferably 80°C or less, and even more preferably 60°C or less.
• the reduction rate per pass it is preferable to set the reduction rate per pass to 5% or less.
• the effect of suppressing strain aging during cold rolling is to suppress the increase in strength due to strain aging.
• cold workability is improved when cold-rolled steel sheet is formed into springs, and the same hardness can be achieved through subsequent aging treatment.
• the method for producing a high-carbon hot-rolled steel sheet according to this embodiment can employ the above steps (I) to (VI).
• the high-carbon hot-rolled steel sheet may be produced by the above steps (I) to (V).
• the spiral spring according to this embodiment can be manufactured by the following method. First, the cold-rolled steel sheet obtained by the above-described manufacturing method is cold-formed into a spring shape, and then heat-treated (strain aging) for 15 to 45 minutes in a temperature range of 200 to 300°C. By applying this manufacturing method, the spiral spring according to this embodiment can be suitably manufactured.
• the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples.
• Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the objectives of the present invention.
• the resulting hot-rolled steel sheet was then cold-rolled under the conditions shown in Table 3 to obtain a cold-rolled steel sheet.
• the surface Vickers hardness, area fraction of pearlite structure in the 1/4 region, and number density of inclusions were determined for the resulting cold-rolled steel sheet using the methods described above. The results are shown in Tables 4A and 4B. In Table 3, values that deviate from the preferred manufacturing method for the high-carbon cold-rolled steel sheet of this embodiment are underlined.
• the remaining microstructures other than the pearlite structure were ferrite, bainite, and martensite.
• the obtained cold-rolled steel sheets were subjected to heat treatment equivalent to that of the final product at a temperature range of 200°C to 260°C for 15 to 45 minutes, and then the Vickers hardness and fatigue durability were evaluated.
• Fatigue durability was evaluated by conducting a test in accordance with the "Fatigue Test Method for Spring Thin Plates" in "Spring Research Papers Vol. 41 (1996) pp. 53-64" and determining the fatigue limit.
• the fatigue limit is the stress at which the specimen does not break after 107 cycles of testing, and fatigue durability was evaluated according to the following evaluation criteria.
• the inventive examples had the specified chemical composition, and the area fraction of the pearlite structure, the lamellar spacing of the pearlite structure, and the number density of inclusions were all within the ranges of the present invention. As a result, high strength (torque characteristics) and excellent fatigue durability were obtained.
• the comparative examples had chemical compositions in which at least one of the area fraction of the pearlite structure, the lamellar spacing of the pearlite structure, and the number density of inclusions was outside the range of the present invention, and were inferior in strength (torque characteristics) and fatigue durability.
• Cold rolling process Nos. C24 and C42 shown in Tables 3, 4A and 4B are examples in which the sheets broke due to a decrease in cold rollability.
• the above-described aspects of the present invention make it possible to provide high-carbon hot-rolled steel sheets and high-carbon cold-rolled steel sheets that have high strength (torque characteristics) and excellent fatigue durability. Therefore, since high-carbon hot-rolled steel sheets and high-carbon cold-rolled steel sheets can be suitably applied to spiral springs and the like, the high-carbon hot-rolled steel sheets and high-carbon cold-rolled steel sheets according to the above-described aspects of the present invention have high industrial applicability.

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