High-carbon cold-rolled steel sheet, high-carbon hot-rolled steel sheet, and coil spring

Abstract

The high-carbon cold-rolled steel sheet has a chemical composition comprising, in mass%, 0.65-0.80% of C, 0.15-0.50% of Si, 0.40-0.80% of Mn, 0.020% or less of P, 0.0015% or less of S, 0.010-0.065% of Al, more than 0.40% and 0.60% or less of Cr, 0.0005-0.0030% of Ca, 0.0040% or less of O, and 0.0100% or less of N, with the balance being Fe and impurities, and has a surface Vickers hardness of 530 Hv or more, an area fraction of a pearlite structure of 95% or more, a lamellar spacing of the pearlite structure of 20-50 nm, and a thickness t in a 1 / 4 t plane, where t is a sheet thickness, and t is a thickness of the sheet. The number density of individual inclusions of oxides, sulfides, and nitrides having an average particle diameter of 1.0-10.0 [mu] m, or composite inclusions formed by compounding two or more individual inclusions is 3.0 / mm2 or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a high-carbon cold-rolled steel sheet, a high-carbon hot-rolled steel sheet, and a coil spring.

[0002] This application claims priority based on Japanese Patent Application No. 2024-077352 filed on May 10, 2024 in Japan, and the contents thereof are hereby incorporated by reference. BACKGROUND

[0003] In the past, as a raw material of a spring product such as a coil spring (clock spring), a high-carbon steel sheet having a C content of 0.60 mass% or more has been widely used.

[0004] Since such a spring product generates a high stress, a high-carbon steel sheet as a raw material is required to have high strength and fatigue durability. In addition, when a coarse inclusion that becomes a starting point of a fatigue crack is contained in the high-carbon steel sheet, the coarse inclusion becomes a main cause of early fracture, and thus the generation amount, the morphology, and the like of the inclusion are required to be controlled.

[0005] Patent Literature 1 discloses a cold-rolled carbon steel in which C: 0.63 to 0.85%, Si: maximum 0.40%, Mn: 0.20 to 0.90%, P: maximum 0.035%, S: maximum 0.035%, Al: maximum 0.060%, Cr: maximum 0.40%, N: 0.003 to 0.010% (preferably 0.005 to 0.008%), at least one microalloying element (Ti, Nb, V, and Zr) having a content of maximum 0.12%, and the balance being iron and impurities.

[0006] Patent Literature 2 discloses a manufacturing method of a high-carbon steel sheet, which includes a stage of manufacturing a steel sheet having a structure of spheroidized cementite and initial ferrite by performing a hot-rolling process, a cold-rolling process, and an annealing process on a steel material having a prescribed composition, and a stage of performing lead quenching heat treatment for 60 seconds or more while maintaining the temperature of a solder bath at 500°C or higher and 530°C or lower after heating the steel sheet. In addition, Patent Literature 2 discloses that a stage of cooling and a stage of cold-rolling by 85% or more can be included after the lead quenching heat treatment, and the temperature range when heating the steel sheet before the lead quenching heat treatment is 800°C or higher and 1100°C or lower. According to the manufacturing method disclosed in Patent Literature 2, a high-carbon steel sheet in which the interlaminar spacing between lamellar carbides is 0.5 μm or less, and a fine pearlite phase having a lamella structure in which the ratio of the major axis to the minor axis is 10:1 or more is contained by 90% or more in terms of volume fraction can be obtained.

[0007] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent Application Laid-Open No. 2012-518723 Patent Literature 2: Japanese Patent Application Laid-Open No. 2010-528189 SUMMARY

[0008] Problem to be solved by the invention

[0009] Generally, as a main method for improving the strength and fatigue durability of a high carbon steel sheet as a raw material of a coil spring or the like, a technique of making pearlite microfine by subjecting a hot-rolled steel sheet before cold rolling to lead quenching treatment using a lead (Pb) bath or the like is known.

[0010] However, in recent years, from the viewpoint of reduction of environmental load (CO2 reduction effect, non-use of Pb) and productivity improvement due to process reduction, reduction of heat treatment including the lead quenching treatment is required. On the other hand, in order to omit the heat treatment, it is necessary to make the pearlite microfine at the stage of a hot-rolled steel sheet which is an intermediate product of a high carbon cold-rolled steel sheet.

[0011] It is considered that the microfining of the pearlite at the stage of the hot-rolled steel sheet can be achieved by lowering the coiling temperature CT (increasing the undercooling degree). On the other hand, when the coiling temperature is lowered, there is a concern that the shape of the coiled material after coiling collapses, the shape of the sheet deteriorates, and the like due to the influence of the heat generation caused by the expansion accompanying the phase change, the phase change.

[0012] Further, in the above Patent Literature 1 which is a prior art, the microfining of the structure by the addition of a microalloying element is studied, whereby the crack which can be formed due to the notch effect of the coarse structure is suppressed, but the inclusion which becomes a starting point of the crack is not sufficiently studied. Further, in the manufacturing method disclosed in Patent Literature 1, the lead quenching treatment including cold rolling or the like is also required after the hot-rolling step.

[0013] Further, in Patent Literature 2, the form of the pearlite is studied from the viewpoint of suppressing the propagation of the fatigue crack, but it cannot be said that the inclusion which becomes a starting point is sufficiently mentioned. Further, the lead quenching treatment including cold rolling or the like is also required after the hot-rolling step, the manufacturing man-hours are many, and the productivity is problematic.

[0014] The present application was completed in view of the above circumstances, and aims to provide a high carbon cold-rolled steel sheet, a high carbon hot-rolled steel sheet, and a coil spring which have high strength (torque characteristics) and excellent fatigue durability.

[0015] Means for solving the problem

[0016] The present application was completed based on the above insight, and the gist thereof is as follows.

[0017] (1) A high carbon cold-rolled steel sheet of one embodiment of the present application contains, in mass%, the chemical composition contains, in mass%, C: 0.65 to 0.80%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.80%, P: 0.020% or less, S: 0.0015% or less, Al: 0.010 to 0.065%, Cr: more than 0.40% and 0.60% or less, Ca: 0.0005 to 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 to 0.05%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, As: 0 to 0.05%, REM: 0 to 0.0100%, and balance: Fe and impurities, the Vickers hardness of the surface of the high-carbon cold-rolled steel sheet is 530 Hv or more, the area fraction of the pearlite structure is 95% or more, the lamellar spacing of the pearlite structure is 20 to 50 nm, the number density of the oxide, sulfide, and nitride individual inclusions having an average particle size of 1.0 to 10.0 μm, or the complex inclusions formed by the combination of two or more of the individual inclusions, is 3.0 pieces / mm 2 hereinafter.

[0018] (2) The high-carbon cold-rolled steel sheet according to the above (1), wherein the chemical composition contains, by mass%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10%, V: 0.01 to 0.10%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, B: 0.0001 to 0.010%, Mo: 0.001 to 0.10%, W: 0.001 to 0.05%, Ta: 0.001 to 0.05%, Mg: 0.001 to 0.05%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, REM: 0.0001 to 0.0100% one or two or more kinds thereof.

[0019] (3) The high-carbon cold-rolled steel sheet described in the above (1) can be such that the chemical composition contains, by mass%, Al: 0.010 to 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: 0 to 0.0050%.

[0020] (4) The high-carbon cold-rolled steel sheet described in any one of the above (1) to (3) can be such that the number density of coarse inclusions having an average particle diameter exceeding 10.0 μm is 0 pieces / mm 2 .

[0021] (5) The high-carbon hot-rolled steel sheet of one embodiment of the present application, the chemical composition contains, by mass%, C: 0.65 to 0.80%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.80%, P: 0.020% or less, S: 0.0015% or less, Al: 0.010 to 0.065%, Cr: more than 0.40% and 0.60% or less, Ca: 0.0005 to 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 to 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 balance: Fe and impurities, the Vickers hardness of the surface of the high-carbon hot-rolled steel sheet is 305 Hv or more, the area fraction of the pearlite structure is 95% or more, the average lamellar spacing of the pearlite structure is 70 to 200 nm, the number density of the oxide, sulfide, and nitride individual inclusions having an average particle size of 1.0 to 10.0 μm, or the complex inclusions formed by the combination of two or more of the individual inclusions, is 3.0 pieces / mm 2 hereinafter.

[0022] (6) The high-carbon hot-rolled steel sheet according to the above (5), wherein the Vickers hardness of the surface is 400 Hv or less.

[0023] (7) The high-carbon hot-rolled steel sheet according to the above (5) or (6), wherein the chemical composition contains, by mass%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10%, V: 0.01 to 0.10%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, B: 0.0001 to 0.010%, Mo: 0.001 to 0.10%, W: 0.001 to 0.05%, Ta: 0.001 to 0.05%, Mg: 0.001 to 0.05%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, REM: 0.0001 to 0.0100% or less of one or two or more kinds.

[0024] (8) The high-carbon hot-rolled steel sheet described in the above (5) can be such that the chemical composition includes, by mass%, Al: 0.010 to 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: 0 to 0.0050%.

[0025] (9) The high-carbon hot-rolled steel sheet described in any one of the above (5) to (8) can be such that the number density of coarse inclusions having an average particle diameter exceeding 10.0 μm is 0 pieces / mm 2 .

[0026] (10) A coil spring of one embodiment of the present application, the chemical composition includes, by mass%, C: 0.65 to 0.80%, Si: 0.15 to 0.50%, Mn: 0.40 to 0.80%, P: 0.020% or less, S: 0.0015% or less, Al: 0.010 to 0.065%, Cr: more than 0.40% and 0.60% or less, Ca: 0.0005 to 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 to 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 balance: Fe and impurities, the area fraction of the pearlite structure of the disc spring is 95% or more, the lamellar spacing of the pearlite structure is 20 to 50 nm, in the case where the plate thickness is t, the number density of the single inclusions of oxides, sulfides and nitrides having an average particle diameter of 1.0 to 10.0 μm, or the composite inclusions formed by the combination of two or more of the single inclusions, is 3.0 pieces / mm 2 Hereinafter, the hardness of the surface is 530 Hv or more.

[0027] (11) The disc spring according to the above (10), wherein the hardness of the surface is 580 Hv or more.

[0028] (12) The disc spring according to the above (10) or (11), wherein the chemical composition contains, by mass%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10%, V: 0.01 to 0.10%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, B: 0.0001 to 0.010%, Mo: 0.001 to 0.10%, W: 0.001 to 0.05%, Ta: 0.001 to 0.05%, Mg: 0.001 to 0.05%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, REM: 0.0001 to 0.0100% or less one or two or more of them.

[0029] ​​​​​​​​​​​​​​​​​​​​​​​​​​​​(13) The disc spring according to any one of (10) to (13) above, wherein the chemical composition can contain, by mass%, Al: 0.010 to 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: 0 to 0.0050%.

[0030] (14) The disc spring according to any one of (10) to (13) above, wherein the number density of coarse inclusions having an average particle diameter exceeding 10.0 μm is 0 inclusions / mm 2 .

[0031] Effects of the invention

[0032] According to the above-described aspect of the present application, a high-carbon cold-rolled steel sheet, a high-carbon hot-rolled steel sheet, and a disc spring having high strength (torque characteristics) and excellent fatigue durability can be provided. DETAILED DESCRIPTION

[0033] Hereinafter, a high-carbon cold-rolled steel sheet (hereinafter, also referred to as a cold-rolled steel sheet), a high-carbon hot-rolled steel sheet, and a disc spring according to one embodiment of the present application will be described. However, the present application is not limited only to the configuration disclosed in the present embodiment, and various modifications can be made within the scope of the gist of the present application. In addition, the numerical range indicated below with "to" includes the lower limit value and the upper limit value within the range. In the numerical value indicated as "less than" or "exceeding", the value is not included in the numerical range.

[0034] [High-carbon cold-rolled steel sheet] The high-carbon cold-rolled steel sheet according to the present embodiment has the prescribed chemical composition described later, the hardness of the surface of the steel sheet is 530 Hv or more in Vickers hardness, the area fraction of the pearlite structure is 95% or more, the lamellar spacing of the pearlite structure is 20 nm or more and 50 nm or less, and the number density of the single inclusions of oxides, sulfides, and nitrides having an average particle diameter of 1.0 μm or more and 10.0 μm or less, or the complex inclusions formed by the combination of two or more of the single inclusions is 3.0 inclusions / mm 2 or less in the region of 1 / 8 depth from the surface to 3 / 8 depth from the surface, that is, the 1 / 4 region.

[0035] Note that "high-carbon" in the present embodiment means that the carbon content in the steel is 0.65% or more.

[0036] [Chemical composition] The chemical composition of the high-carbon cold-rolled steel sheet of the present embodiment will be described. Note that, in the following description, the unit of the chemical composition is set to a unit indicating "mass%".

[0037] C: 0.65 to 0.80% Carbon (C) is an element that contributes to high strength by increasing the area fraction of cementite phase and making the lamellar spacing of pearlite microstructure fine. When the C content is less than 0.65%, it is difficult to sufficiently secure the pearlite microstructure as the main microstructure. Therefore, the C content is 0.65% or more. The C content is preferably 0.70% or more, and more preferably 0.72% or more. On the other hand, when the C content exceeds 0.80%, pro-eutectoid cementite phase is sometimes precipitated. In this case, cold rolling becomes difficult, and fatigue durability also decreases. Therefore, the C content is 0.80% or less. The C content is preferably 0.79% or less, and more preferably 0.78% or less.

[0038] Si: 0.15 to 0.50% Silicon (Si) is an element contained for deoxidization. When the Si content is less than 0.15%, deoxidization is insufficient, and oxidized inclusions remain. Therefore, in the case where cold rolling is performed in such a manner that the hot-rolled steel sheet is made extremely thin, it becomes a cause of cracking on the surface and decrease in fatigue durability. Therefore, the Si content is 0.15% or more. The Si content is preferably 0.18% or more, and more preferably 0.20% or more or 0.24% or more. On the other hand, in the case where the Si content exceeds 0.50%, the generation of a substance that promotes the induction of red scale is promoted at the time of reheating the slab for hot rolling. When this substance is generated, red scale is sometimes generated on the surface of the obtained hot-rolled steel sheet, and the surface quality of the final cold-rolled steel sheet decreases. Therefore, the Si content is 0.50% or less. The Si content is preferably 0.40% or less, and more preferably 0.30% or less.

[0039] Mn: 0.40 to 0.80% Manganese (Mn) is an element effective for improving the hardenability of steel to increase strength. When the Mn content is less than 0.40%, the hardenability of the steel is insufficient, and sufficient strength is sometimes not obtained. Therefore, the Mn content is 0.40% or more. The Mn content is preferably 0.45% or more, and more preferably 0.55% or more. On the other hand, when the Mn content exceeds 0.80 mass%, the strength of the steel excessively increases, and sometimes leads to a decrease in toughness. Therefore, the Mn content is 0.80% or less. The Mn content is preferably 0.75% or less, and more preferably 0.70% or less.

[0040] P: 0.020% or less Phosphorus (P) is an impurity element. P makes the grain boundary brittle, and reduces the cold-rolling property. Therefore, the P content is 0.020% or less. The P content is preferably as low as possible, and is preferably 0.018% or less, more preferably 0.015% or less. However, when the P content is excessively reduced, the smelting cost significantly increases, and therefore the P content can be 0.0010% or more. The lower limit of the P content is 0%, but in order to reduce the smelting cost, the lower limit of the P content can be set to 0.001%, 0.003%, or 0.005%.

[0041] S: 0.0015% or less Sulfur (S) is an impurity element. S forms non-metallic inclusions such as MnS. Such non-metallic inclusions become the starting point of cracking in cold-rolling and the starting point of the generation of cracks, and sometimes cause a reduction in 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%. When the S content is excessively reduced, the smelting cost significantly increases, and therefore the S content can be 0.0001% or more, 0.0003% or more, 0.0005% or more, or 0.0010% or more.

[0042] Al: 0.010 to 0.065% Aluminum (Al) is an element that remains in the steel as a deoxidizer in the steelmaking process. When the Al content is 0.010% or more, sufficient deoxidation effect is obtained. The Al content is preferably 0.015% or more, more preferably 0.020% or more. On the other hand, when the Al content exceeds 0.050%, inclusions are easily formed in the steel. When such inclusions are formed, sometimes the starting point of fatigue cracks is formed, and fatigue durability is reduced. Therefore, the Al content is 0.065% or less. The Al content is preferably 0.050% or less, more preferably 0.045% or less, and further more preferably 0.040% or less.

[0043] Cr: more than 0.40% and 0.60% or less Chromium (Cr) has the effect of refining the lamellar spacing of the pearlite structure, and is thus an element that can increase the strength of the steel sheet. In order to sufficiently obtain such an effect, the Cr content is more than 0.40%. The Cr content is preferably 0.41% or more, more preferably 0.42% or more. On the other hand, when the Cr content exceeds 0.60%, the strength of the steel becomes too high, and sometimes the cold-rolling property is reduced. Therefore, the Cr content is 0.60% or less. The Cr content is preferably 0.58% or less, more preferably 0.55% or less.

[0044] Ca: 0.0005 to 0.0030% Calcium (Ca) is an element effective for the morphology control of sulfides. Specifically, Ca has an effect of reducing coarse and elongated inclusions. In the case where the Ca content is less than 0.0005%, the effect of the morphology control of sulfides is not obtained. Therefore, the Ca content is 0.0005% or more. The Ca content is preferably 0.0006% or more, and more preferably 0.0007% or more. On the other hand, when the Ca content exceeds 0.0030%, the effect of the morphology control of sulfides is saturated, and sometimes, sulfides are aggregated or coarsened to reduce the fatigue durability. Therefore, the Ca content is 0.0030% or less. The Ca content is preferably 0.0025% or less, and more preferably 0.0020% or less.

[0045] O: 0.0040% or less Oxygen (O) forms oxides in steel. When oxides are aggregated and coarsened, or the number density of oxides increases in steel, sometimes, the cold-rolling property is reduced, and the fatigue durability of a coil spring as a final product is reduced. Therefore, the O content is 0.0040% or less. The O content is preferably 0.0030% or less, and more preferably 0.0025% or less. The O content is more preferably less, and the lower limit thereof is 0%. On the other hand, it is practically difficult to excessively reduce the O content. Therefore, the O content can be 0.0001% or more. In the present embodiment, the O content is in the range of 0.0001 to 0.0040%.

[0046] N: 0.0100% or less Nitrogen (N) combines with Al in steel to form AlN. AlN hinders the coarsening of the diameter of pearlite blocks by pinning effect, and has an effect of improving the cold-rolling property. The lower limit of the N content is 0%, but in order to obtain such an effect, the N content can be 0.0020% or more. However, when the N content is too much, the effect is saturated, and in addition to this, there is a case where the cold-rolling property is reduced. Therefore, the N content is 0.0100% or less. It is preferable that the N content is 0.0080% or less, more preferably 0.0070% or less, and further preferably 0.0060% or less.

[0047] The cold-rolled steel sheet of the present embodiment can contain one or two or more of Ti, Nb, V, Cu, Ni, B, Mo, W, Ta, Mg, Sn, Sb, As, and REM in addition to the above-described elements. By containing any of these elements, more preferable characteristics of the cold-rolled steel sheet of the present embodiment can be obtained. However, even if the cold-rolled steel sheet does not contain any of these components, the cold-rolled steel sheet of the present embodiment can obtain desired characteristics, and therefore, the lower limit value of any of these elements is 0%. Hereinafter, any of these elements will be described in detail.

[0048] Ti: 0 to 0.10% Titanium (Ti) has the effect of increasing strength by forming carbonitrides, and therefore can be contained in a range of less than 0.10% as needed. On the other hand, when the Ti content exceeds 0.10%, coarse, angular carbonitrides are easily formed, resulting in significant deterioration of processability. Therefore, the Ti content is 0.10% or less. The Ti content can be less than 0.06%, less than 0.04%, or less than 0.02% (0.020%). The lower limit of the Ti content can be more than 0.001% or more, or more than 0.003%.

[0049] Nb: 0~0.10% Niobium (Nb) has the effect of increasing strength by forming carbonitrides, and therefore can be contained in the range of 0.10% or less as needed. On the other hand, when the Nb content exceeds 0.10%, it is easy to form coarse, angular carbonitrides, which significantly deteriorates the workability; therefore, the Nb content is 0.020% or less. The Nb content can be 0.07% or less or 0.05% (0.050%) or less. The lower limit of the Nb content can be 0.001% or more or 0.003% or more.

[0050] V: 0~0.10% Vanadium (V) has the effect of increasing strength by forming carbonitrides, and therefore can be contained in amounts up to 0.10% as needed. On the other hand, when the V content exceeds 0.10%, coarse, angular carbonitrides are easily formed, significantly deteriorating processability; therefore, the V content is 0.10% or less. The V content can be 0.05% or less (0.050%). The lower limit for V content can be 0.01% or more.

[0051] Cu: 0~0.50% Copper (Cu) has the effect of increasing the strength (hardness) of steel sheets. Therefore, it can be contained in the range of 0.50% or less, depending on the requirements. On the other hand, when the Cu content exceeds 0.50%, there is a risk of hot working cracking during hot rolling due to molten metal embrittlement (Cu embrittlement), so the Cu content is 0.50% or less. The Cu content can be 0.30% or less, 0.10% or less, or 0.05% (0.050%) or less. The lower limit of Cu content can be 0.01% or more.

[0052] Ni: 0~0.50% Nickel (Ni) has an effect of preventing the embrittlement of molten metal when Cu is contained (Cu embrittlement). Therefore, Ni can be contained in a range of 0.50% or less as necessary. On the other hand, when the Ni content exceeds 0.50%, the cost increases, on the other hand, the above effect is saturated, and therefore the Ni content is 0.50% or less. The Ni content can be 0.30% or less, 0.10% or less, or 0.05% (0.050%) or less. The lower limit value of the Ni content can be 0.01% or more.

[0053] B: 0 to 0.010% Boron (B) has an effect of improving the hardenability to increase the strength of the steel sheet. Therefore, B can be contained in a range of 0.010% or less as necessary. On the other hand, when the B content exceeds 0.010%, a B-based compound is generated, and the workability of the steel sheet is reduced, and therefore the B content is 0.010% or less. The lower limit value of the B content can be 0.0001% or more or 0.0003% or more.

[0054] Mo: 0 to 0.10% Molybdenum (Mo) has an effect of increasing the strength of the steel sheet by forming a carbonitride. Therefore, Mo can be contained in a range of 0.10% or less as necessary. On the other hand, when the Mo content exceeds 0.10%, a coarse carbonitride is easily formed, and the degradation of the workability is significantly increased, and therefore the Mo content is 0.10% or less. The Mo content can be 0.07% or less or 0.05% (0.050%) or less. The lower limit value of the Mo content can be 0.001% or more or 0.003% or more.

[0055] W: 0 to 0.05% Tungsten (W) has an effect of increasing the strength of the steel sheet by forming a carbonitride. Therefore, W can be contained in a range of 0.05% or less as necessary. On the other hand, when the W content exceeds 0.05%, a coarse carbonitride is easily formed, and the degradation of the workability is significantly increased, and therefore the W content is 0.05% or less. The W content can be 0.03% or less or 0.02% or less. The lower limit value of the W content can be 0.001% or more or 0.003% or more.

[0056] Ta: 0 to 0.05% Tantalum (Ta) has an effect of increasing the strength of the steel sheet by forming a carbonitride, and therefore can be contained in a range of 0.05% or less as necessary. On the other hand, when the Ta content exceeds 0.050%, a coarse carbonitride is easily formed, and the degradation of the workability is significantly increased, and therefore the Ta content is 0.05% or less. The Ta content can be 0.03% or less or 0.02% or less. The lower limit value of the Ta content can be 0.001% or more or 0.003% or more.

[0057] Mg: 0 to 0.05% Magnesium (Mg) is an element that can control the morphology of sulfides, and is also an element that contributes to fatigue durability. In order to sufficiently obtain the above effects, the Mg content is preferably 0.0001% or more. The lower limit value of the Mg content can be 0.002% or more or 0.003% or more. On the other hand, when the Mg content exceeds 0.05%, the steel sheet becomes brittle, and sometimes the ductility decreases. Therefore, the Mg content is 0.05% or less. The Mg content can be 0.03% or less or 0.02% or less.

[0058] Sn: 0 to 0.05% Tin (Sn) is contained when scrap is used as a steel raw material, and is an element that strongly segregates at grain boundaries. Therefore, the Sn content is more preferably less, and can also be 0%. When the Sn content exceeds 0.05%, the steel sheet becomes brittle, and sometimes the ductility decreases. Therefore, the Sn content is 0.05% or less. The Sn content can be 0.03% or less or 0.02% or less. The lower limit value of the Sn content can be 0.001% or more or 0.003% or more.

[0059] Sb: 0 to 0.05% Antimony (Sb) is contained when scrap is used as a steel raw material, and is an element that strongly segregates at grain boundaries, like Sn. Therefore, the Sb content is more preferably less, and can also be 0%. When the Sb content exceeds 0.05%, the steel sheet becomes brittle, and sometimes the ductility decreases. Therefore, the Sb content is 0.05% or less. The Sb content can be 0.03% or less or 0.02% or less. The lower limit value of the Sb content can be 0.001% or more or 0.003% or more.

[0060] As: 0 to 0.05% Arsenic (As) is contained when scrap is used as a steel raw material, and is an element that strongly segregates at grain boundaries, like Sn. Therefore, the As content is more preferably less, and can also be 0%. When the As content exceeds 0.05%, the steel sheet becomes brittle, and sometimes the ductility decreases. Therefore, the As content is 0.05% or less. The As content can be 0.03% or less or 0.02% or less. The lower limit value of the As content can be 0.001% or more or 0.003% or more.

[0061] REM: 0 to 0.0100% REM is an element effective in reducing MnS, controlling the morphology of inclusions, and improving the workability of the steel sheet. In order to obtain the above effects, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.003% or more or 0.0005% or more. On the other hand, when the REM content exceeds 0.0100%, nozzle clogging during continuous casting is likely to occur. In addition, when the REM content exceeds 0.0100%, the number density of REM-based inclusions (oxides, sulfides) generated becomes relatively high, and thus these REM-based inclusions are likely to accumulate on the lower surface side of the curved cast slab during continuous casting of the cast slab. This has the risk of causing internal defects in the steel sheet obtained by rolling the cast slab, and further deteriorating 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.

[0062] Note that REM (Rare Earth Metal) refers to rare earth elements, and is a general term for 17 elements of scandium Sc (atomic number 21), yttrium Y (atomic number 39), and lanthanoid elements (15 elements from lanthanum (atomic number 57) to lutetium (atomic number 71)). In the cold-rolled steel sheet of the present embodiment, at least one or more elements selected from among them are contained. As a method of containing, for example, a mixed rare earth metal in which these elements are mixed is contained in the steel. The main components of the mixed rare earth metal are Ce, La, Nd, and Pr. In the present embodiment, the total amount of the rare earth elements is set as the REM content.

[0063] The cold-rolled steel sheet of the present embodiment contains the above-described elements, and the balance is Fe and impurities. Here, the impurities are elements that are mixed due to various reasons of raw materials such as ores, waste materials, and manufacturing processes when the steel is industrially manufactured, and are elements that are allowed to exist within a range that does not hinder the characteristics of the cold-rolled steel sheet of the present embodiment.

[0064] The above-described chemical composition can be measured by a general analysis method. For example, measurement can be performed using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using a combustion-infrared absorption method, and N can be measured using a non-active gas melting-thermal conductivity method. In a case where a ladle analysis value at the time of manufacturing a slab for a cold-rolled steel sheet or an analysis value in the slab is obtained, it is not necessary to separately measure the chemical composition of the cold-rolled steel sheet, and the ladle analysis value or the like thereof can be used. In the chemical composition of the hot-rolled steel sheet or the coil spring described later, the ladle analysis value or the like can also be used.

[0065] [Microstructure] Next, the microstructure (metallic structure) of the high-carbon cold-rolled steel sheet of this embodiment will be described.

[0066] (Area fraction of pearlite tissue: 95% or more) Pearlite microstructure exhibits higher work hardening capacity compared to ferrite and bainite, making it an effective microstructure for achieving high strength (torsional characteristics) and excellent fatigue durability. When the area fraction of microstructures other than pearlite, such as ferrite and bainite, exceeds 5%, strength decreases. Furthermore, under heavy processing of cold-rolled steel sheets, differences in hardness (strength) between microstructures can become the initiation point for cracks, sometimes resulting in reduced fatigue durability. Therefore, a pearlite area fraction of 95% or more is effective for achieving good torque characteristics and fatigue durability. Preferably, the pearlite area fraction is 98% or more. It should be noted that the pearlite area fraction can be 100%. The microstructure other than pearlite (the remaining microstructure) can be ferrite, bainite, or martensite.

[0067] In addition, the area ratio of each structure was determined by the following method. It should be noted that in components using hot-rolled steel sheets and cold-rolled steel sheets (such as disc springs), which are described later, the area ratio of each structure was also determined by the following method.

[0068] Test pieces are taken in a manner that allows observation of the thickness section of the cold-rolled steel sheet. Preferably, the test piece is taken from the 1 / 4 position along the width direction of the cold-rolled steel sheet. After etching the observation surface with nitric acid and ethanol to reveal the microstructure, a scanning electron microscope is used to take microstructure photographs at 5000x magnification, with a field of view of 20 μm in the thickness direction and 30 μm perpendicular to the thickness, centered at 30 μm in the rolling direction and 1 / 4 depth from the surface of the steel sheet. Five fields of view are taken for microstructure photographs. In each microstructure photograph, regions with lamellar carbides are identified as pearlite, and the average area fraction of these areas is calculated to obtain the pearlite area fraction.

[0069] In addition, the rolling direction of cold-rolled steel sheets is determined by the following method.

[0070] A test piece is taken from a cold-rolled steel sheet in a manner that enables observation of a plate thickness section. After the plate thickness section of the taken test piece is finished by mirror polishing, an optical microscope is used to observe at magnifications of 100x, 200x, 500x, and 1000x, respectively. The observation results at appropriate magnifications that enable measurement of the size of the inclusions are selected according to the size of the inclusions. The observation range is set to a range of 500 μm or more in width and the entire thickness of the plate thickness, and regions that are dark in brightness are determined to be inclusions. The observation can also be performed in multiple fields of view during observation. Next, for a plane parallel to the plane that is rotated at a 5° scale in the range of 0° to 180° with the plate thickness direction as the axis with respect to the plate thickness section that is first observed by the above-described method, the cross section is observed by the same method as described above. The average of the lengths of the long axes of the inclusions in each cross section is calculated for each cross section. The cross section for which the average of the lengths of the long axes of the inclusions is the largest is determined. The direction parallel to the direction of the long axes of the inclusions in this cross section is identified as the rolling direction.

[0071] For a member (a coil spring or the like) that uses a hot-rolled steel sheet and a cold-rolled steel sheet, the rolling direction is also identified by the same method as described above. Note that in a case where the rolling direction can be grasped in advance (for example, in a case where a test piece is taken from a steel strip or the like), the identification of the rolling direction is not required, and can be omitted.

[0072] (Interlamellar spacing of pearlite structure: 20 to 50 nm) When the interlamellar spacing of the pearlite structure is less than 20 nm, the cold workability at the time of forming sometimes decreases. Therefore, the interlamellar spacing of the pearlite structure is 20 nm or more. It is preferable that the interlamellar spacing of the pearlite structure be 22 nm or more, 25 nm or more, or 27 nm or more. On the other hand, when the interlamellar spacing of the pearlite structure exceeds 50 nm, a desired strength (torque characteristic) is sometimes not obtained. Therefore, the interlamellar spacing of the pearlite structure is 50 nm or less. It is preferable that the interlamellar spacing of the pearlite structure be 48 nm or less, 45 nm or less, 42 nm or less, or 40 nm or less.

[0073] Here, the interlamellar spacing of the pearlite structure refers to the average distance between the center points in the thickness direction of each of the adjacent ferrite phase and cementite phase that constitute the lamella. The above-described interlamellar spacing can be grasped as one set of layers of 1 layer of ferrite phase and 1 layer of cementite phase, for example, and can be obtained by cutting several sets of layers by a line segment of a prescribed length in a direction perpendicular to the extension direction of the layers in the observation of the structure. Note that layers in which both ends of the line segment are not completely cut by the line segment are excluded from the measurement target.

[0074] Specifically, in the present embodiment, the lamella spacing of the pearlite structure is calculated as follows. First, a sample is taken from a position at 1 / 4 of the thickness of the steel sheet from the surface of the steel sheet, with a cross section of the steel sheet parallel to the rolling direction and the thickness direction being taken as the observation surface. Next, mirror polishing is performed on the observation surface, and after etching with a picral etching solution, observation of the structure is performed using a scanning electron microscope (SEM). The magnification is set to 5000x (measurement area: 80 μm x 150 μm), and 10 points at which the cementite layers are perpendicularly cut with respect to the paper surface of the structure photograph are selected. By performing measurement using picral etching solution, information in the depth direction is obtained, and thus the points at which the cementite layers are perpendicularly cut are known. By selecting 10 or more such points for measurement, the lamella spacing S is calculated at each point, and by taking the average of these, the "lamella spacing" is obtained. The method of measuring the lamella spacing at each point is as follows. First, a straight line is drawn perpendicularly with respect to the cementite layers so as to cut 10 to 30 cementite layers, and the length of this straight line (the length of the line segment) is set to L. In addition, the number of groups of layers composed of one layer of ferrite phase and one layer of cementite phase that are cut by this straight line is set to N. At this time, the lamella spacing S at this point is calculated as S = L / (N x 2).

[0075] Note that in the components (coil springs and the like) described later that use hot-rolled steel sheets and cold-rolled steel sheets, the lamella spacing of the pearlite structure can also be measured by the above method.

[0076] That is, the lamella spacing of the present embodiment is calculated by the following formula: Lamella spacing S = length of line segment L ÷ (number of groups N x 2) (1 / 4t In-plane number density of inclusions: 3.0 pieces / mm 2 The following) When the number density of inclusions having an average particle size of 1.0 to 10.0 μm (1.0 μm or more and 10.0 μm or less) exceeds 3.0 pieces / mm 2 in the cold-rolled steel sheet, the inclusions become the starting points and become the main cause of early fracture, and sometimes high fatigue durability cannot be obtained. Therefore, the number density of inclusions in the cold-rolled steel sheet is set to 3.0 pieces / mm 2 or less. Thereby, high fatigue durability is obtained. It is preferable that the number density of inclusions be 2.5 pieces / mm 2 or less, 2.0 pieces / mm 2 or less, or 1.5 pieces / mm 2 or less. The number density of inclusions is more preferably smaller, and thus the lower limit value is 0 pieces / mm 2 . Note that inclusions having an average particle size of less than 1.0 μm have no effect on fatigue durability, and thus are not counted as inclusions specified in the present embodiment.

[0077] In addition, a coarse inclusion having an average particle diameter exceeding 10.0 μm can become a starting point of fracture. That is, a coarse inclusion having an average particle diameter exceeding 10.0 μm early fractures regardless of the number, and desired fatigue durability is not obtained. Therefore, in the present embodiment, the number density of coarse inclusions having an average particle diameter exceeding 10.0 μm is preferably 0 pieces / mm 2 It should be noted that, in the case where the number density of coarse inclusions having an average particle diameter exceeding 10.0 μm obtained by the calculation method of the number density of inclusions described later is less than 0.5 pieces / mm 2 , the number density of coarse inclusions having an average particle diameter exceeding 10.0 μm is judged to be 0 pieces / mm 2 The number density of coarse inclusions having an average particle diameter exceeding 10.0 μm can be 0.4 pieces / mm 2 or less, 0.3 pieces / mm 2 or less, or 0.2 pieces / mm 2 or less.

[0078] The "inclusion" in the present embodiment means a single inclusion of an oxide, a sulfide, and a nitride, and a composite inclusion in which two or more of the single inclusions are combined.

[0079] The number density of inclusions in the cold-rolled steel sheet is calculated by the following method. First, five test pieces are each taken from the cold-rolled steel sheet.

[0080] The taken test pieces are resin-embedded. The test pieces embedded with resin are polished in the sheet thickness direction, and, in the case where the sheet thickness is t, mirror polishing is performed so that a 1 / 4t surface (a surface parallel to the surface of the cold-rolled steel sheet at a distance of 1 / 4t from the surface of the cold-rolled steel sheet) becomes the surface. A scanning electron microscope (SEM) having a composition analysis function is used to observe an observation region in the polished surface. The observation region is set to a region of 1.2 mm x 1.6 mm. The long side of the observation region corresponds to the rolling direction of the cold-rolled steel sheet. At the time of observation, 48 fields of view of 200 μm x 200 μm, which do not overlap, are selected, and each field of view is observed at a magnification of 500 times.

[0081] In the observation area, particles (precipitates or inclusions) having an equivalent circle diameter of 1.0 μm or more were determined on the basis of Z-contrast in a reflection electron image. Note that in the reflection electron image, the particles are represented in contrast darker than the matrix. In addition, the equivalent circle diameter refers to the diameter of a circle having the same area as the area of the particle. For each of the determined particles, EDX analysis (elemental concentration analysis) using energy dispersive X-ray spectroscopy (EDX) was performed, and a standardless method was used. In addition, the acceleration voltage was set to 20 kV, and the quantified elements were Si, Mn, P, S, Cr, Ti, Nb, Cu, Ni, Ca, N, O, and Al, and Mg.

[0082] In the EDX analysis results of each particle, when the total content of the above-mentioned quantified elements in mass% was set to 100%, a substance in which the total of the O content, the S content, and the N content was 10% or more in mass% was determined to be a single inclusion and a complex inclusion of an oxide, a sulfide, and a nitride.

[0083] On the basis of the total number of the single inclusions and the complex inclusions of the oxide, the sulfide, and the nitride having an equivalent circle diameter of 1.0 μm or more, which were determined in each observation area of the five test pieces, and the total area of each observation area of the five test pieces, the number density (pieces / mm 2 ).

[0084] Note that in the components (coil springs and the like) using the hot-rolled steel sheet and the cold-rolled steel sheet described later, the number density of inclusions can also be measured by the above-mentioned method.

[0085] (Vickers hardness: 530 Hv or more) In the cold-rolled steel sheet of the present embodiment, the Vickers hardness of the steel sheet surface is 530 Hv or more. Thereby, excellent torque characteristics can be obtained. It is preferable that the Vickers hardness of the steel sheet surface be 535 Hv or more, 540 Hv or more, or 550 Hv or more. It is not necessary to determine the upper limit of the Vickers hardness, but it can be set to 700 Hv or less, 670 Hv or less, 650 Hv or less, or 620 Hv or less, for example.

[0086] Hardness is determined by Vickers hardness testing with a test force of 9.807 N, according to JIS Z 2244:2009. The preferred testing location is at least 50 mm from the end of the cold-rolled steel sheet. Hardness is measured at any five locations, and the average hardness is calculated to obtain the Vickers hardness. It should be noted that Vickers hardness can also be determined using the above method in components using both hot-rolled and cold-rolled steel sheets (such as coil springs), as described later. However, if the testing location cannot be more than 50 mm from the end, the Vickers hardness should, in principle, be measured at the center of the width.

[0087] The thickness of cold-rolled steel sheets is not particularly limited, and can range from 0.100 to 0.400 mm. The preferred lower limit for the thickness of cold-rolled steel sheets is 0.140 mm, 0.160 mm, or 0.180 mm. The preferred upper limit for the thickness of cold-rolled steel sheets is 0.350 mm, 0.300 mm, 0.250 mm, or 0.230 mm.

[0088] High-carbon hot-rolled steel plate Next, the high-carbon hot-rolled steel sheet of this embodiment will be described.

[0089] The high-carbon hot-rolled steel sheet (hereinafter also referred to as hot-rolled steel sheet) of this embodiment has a specified chemical composition. The Vickers hardness of the surface of the hot-rolled steel sheet is 320 Hv or higher. In the microstructure of a region from 1 / 8 of the thickness of the hot-rolled steel sheet to 3 / 8 of the thickness of the sheet from the surface, i.e., a 1 / 4 region, the area fraction of pearlite is 95% or higher, and the average lamellar spacing of the pearlite is 80 to 200 nm. Assuming a sheet thickness of t, in the 1 / 4 t thickness plane, the number density of individual inclusions of oxides, sulfides, and nitrides with an average particle size of 1.0 μm or higher and 10.0 μm or lower, or composite inclusions composed of two or more of the aforementioned individual inclusions, is 3.0 particles / mm. 2 the following.

[0090] It should be noted that the thickness of hot-rolled steel sheets is not particularly limited, and can be, for example, 1.60 to 4.80 mm. The preferred lower limit for the thickness of hot-rolled steel sheets is 2.00 mm or 2.20 mm, and the preferred upper limit is 4.00 mm or 3.80 mm.

[0091] [Chemical Composition] The hot-rolled steel sheet of this embodiment is the same as the hot-rolled steel sheet used for the high-carbon cold-rolled steel sheet described in this embodiment. Therefore, the chemical composition of the hot-rolled steel sheet is the same as that of the cold-rolled steel sheet described above.

[0092] [Microstructure] Next, the microstructure (metallic structure) of hot-rolled steel sheets will be explained.

[0093] The microstructure of the hot-rolled steel sheet in the present embodiment has a pearlite structure of 95% or more in area ratio, and the average lamellar spacing of the pearlite structure is 80 to 200 nm.

[0094] (Area fraction of pearlite structure: 95% or more) By the pearlite structure being the main structure is meant that the area fraction of the pearlite structure with respect to the entire structure is 95% or more. It is preferably 98% or more. When the area fraction of the pearlite structure is less than 95%, the desired hardness is not obtained, and when the area fraction of the structure other than the pearlite structure (bainite, ferrite, martensite) is more than 5%, the structure becomes a concentration site of strain, and thus the structure becomes a starting point of fracture, and the cold-rolling property is reduced. In addition, when the area fraction of the pearlite structure is less than 95%, the fatigue durability of a coil spring as a final product is sometimes reduced. The area fraction of the pearlite structure can be 100%. The structure other than the pearlite structure (the balance) can be ferrite, bainite, or martensite.

[0095] (Lamellar spacing of pearlite structure: 70 to 200 nm) From the viewpoint that the pearlite structure has a high work hardening ability in a cold-rolled steel sheet, the initial lamellar spacing before cold-rolling, that is, the lamellar spacing in the hot-rolled steel sheet is important. When the average lamellar spacing of the pearlite structure in the hot-rolled steel sheet is more than 200 nm, even if the hot-rolled steel sheet is subjected to cold-rolling at a high cold-rolling rate, it is sometimes difficult to sufficiently increase the hardness of the obtained cold-rolled steel sheet. Therefore, the average lamellar spacing of the pearlite structure in the hot-rolled steel sheet is 200 nm or less. It is preferably 170 nm or less, or 150 nm or less, and further preferably 130 nm or less, or 120 nm or less. On the other hand, when the initial lamellar spacing before cold-rolling is less than 70 nm, the strength is significantly increased by cold-rolling at a high cold-rolling rate, and it is sometimes not possible to ensure good cold-rolling property. Therefore, the average lamellar spacing of the pearlite structure in the hot-rolled steel sheet is 70 nm or more. It is preferably 75 nm or more, 80 nm or more, 85 nm or more, or 90 nm or more. Adjustment of the lamellar spacing can be performed by adjusting the cooling rate from the austenite phase, and the coiling temperature described later, in addition to adjustment of the chemical composition described above.

[0096] (Number density of inclusions in 1 / 4t in-plane: 3.0 pieces / mm 2 and the following) The number density of inclusions having an average particle diameter of 1.0 to 10.0 μm (1.0 μm or more and 10.0 μm or less) is 3.0 pieces / mm 2 and the following, high durability can be obtained. When it is more than 3.0 pieces / mm 2 , the inclusions become a starting point and a major cause of early fracture, and high fatigue durability is not obtained. Therefore, the number density of inclusions is set to 3.0 pieces / mm2 The following parameters result in high durability. A density of 2.5 particles / mm is preferred. 2 Below, 2.0 pieces / mm 2 Below or 1.5 pieces / mm 2 the following.

[0097] Large inclusions with an average particle size exceeding 10.0 μm may become the starting point for fracture. Therefore, in this embodiment, the number density of large inclusions with an average particle size exceeding 10.0 μm is preferably 0 inclusions / mm. 2 It should be noted that the number density of coarse inclusions with an average particle size exceeding 10.0 μm is less than 0.5 inclusions / mm. 2 In this case, the number density of coarse inclusions with an average particle size exceeding 10.0 μm is determined to be 0 inclusions / mm. 2 The number density of coarse inclusions with an average particle size exceeding 10.0 μm can reach 0.4 inclusions / mm. 2 Below, 0.3 pieces / mm 2 Below or 0.2 pieces / mm 2 the following.

[0098] (Vickers hardness: 305Hv or higher) In this embodiment, the Vickers hardness of the hot-rolled steel sheet is 305 HV or higher. By increasing the hardness of the hot-rolled steel sheet, the strength of the cold-rolled steel sheet and the final product (coil spring) can be significantly improved. Depending on the requirements, the Vickers hardness of the surface can be 310 or higher, 320 HV or higher, or 330 HV or higher. It is not necessary to determine an upper limit for the Vickers hardness, but it can be set to, for example, below 450 HV, below 420 HV, below 400 HV, below 380 HV, or below 370 HV.

[0099] The method for determining the hardness of hot-rolled steel sheets is the same as that for cold-rolled steel sheets.

[0100] <Coil Spring> The coil spring of this embodiment is obtained by forming the cold-rolled steel sheet of this embodiment into a spring shape. Specifically, the area fraction of pearlite is 95% or more, the lamellar spacing of the pearlite is 20-50 nm, and the number density of individual inclusions (or composite inclusions consisting of two or more individual inclusions) of oxides, sulfides, and nitrides with an average particle size of 1.0-10.0 μm within a 1 / 4 t thickness plane is 3.0 inclusions / mm². 2 The surface hardness of the spring is above 530Hv.

[0101] In this embodiment, "coil spring" refers to "coil spring No. 3400" as defined in JIS B 0103:2015, which is a spring that is spiral-shaped in a plane.

[0102] Note that, in the disc spring of the present embodiment as a final product (disc spring), after the cold-rolled steel sheet is cold-formed into a spring shape, heat treatment (strain aging) is preferably performed at a temperature region of 200 to 300°C for 15 to 45 minutes in order to increase the elastic limit. By going through such a heat treatment process, the hardness of the disc spring can be increased. For example, by the above heat treatment, the Vickers hardness of the surface of the disc spring can be 580 Hv or more. However, the above heat treatment is not essential, and the Vickers hardness of the surface of the disc spring is the same as that of the above cold-rolled steel sheet (specifically, the lower limit of the Vickers hardness is 530 HV).

[0103] Note that, regardless of the implementation of the above heat treatment, the chemical composition, the microstructure (metal structure), the lamellar spacing of the pearlite structure, and the number density of inclusions (including coarse inclusions exceeding 10.0 pm) of the disc spring of the present embodiment are basically the same as those of the above cold-rolled steel sheet as a raw material. Therefore, the description of these requirements is omitted.

[0104] <Method for manufacturing high-carbon cold-rolled steel sheet> Next, a preferred manufacturing method of the high-carbon cold-rolled steel sheet of the present embodiment will be described.

[0105] The manufacturing method of the high-carbon cold-rolled steel sheet of the present embodiment can be manufactured by a manufacturing method including the following processes.

[0106] (I) Steel manufacturing (smelting · casting) process, manufacturing a slab having the above chemical composition.

[0107] (II) Slab heating process, heating the slab to 1100°C or higher.

[0108] (III) Hot rolling process, performing finish rolling of the heated slab at a finish rolling exit temperature of 820 to 920°C to obtain a high-carbon hot-rolled steel sheet.

[0109] (IV) Cooling process, including: a first cooling process 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 / sec; and a second cooling process of subsequently cooling the high-carbon hot-rolled steel sheet from T1 to a coiling temperature at an average cooling rate of less than 20°C / sec.

[0110] (V) Coiling process, coiling the high-carbon hot-rolled steel sheet at a coiling temperature of 560 to 700°C.

[0111] (VI) Pickling process, removing the scale on the surface of the high-carbon hot-rolled steel sheet after the coiling process.

[0112] (VII) Cold rolling process, cold-rolling the high-carbon hot-rolled steel sheet at a total reduction of 90% or more to obtain a high-carbon cold-rolled steel sheet.

[0113] Generally, a manufacturing method of a high-carbon cold-rolled steel sheet applied to a disc spring or the like is known to include, after hot rolling, sequentially performing pickling, spheroidizing annealing, primary cold rolling, lead quenching treatment, and secondary cold rolling. On the other hand, in the manufacturing method of the present embodiment including the above-described processes (I) to (VII), by producing a pearlite structure at the stage of the hot rolling process and the coiling process, it is possible to provide a high-carbon cold-rolled steel sheet that can omit the spheroidizing annealing, the primary cold rolling, and the lead quenching treatment conventionally performed, and that has high strength (torque characteristics) and excellent fatigue durability. Furthermore, the high-carbon steel sheet obtained by such a manufacturing method can be applied to a disc spring or the like.

[0114] Hereinafter, preferred conditions of each process will be described. As for processes and conditions not described below, publicly known conditions can be employed.

[0115] [Steel manufacturing (smelting, casting) process] As with a general steel sheet, for example, a molten iron produced in a blast furnace is used as a raw material, and a molten steel produced by performing converter smelting and secondary smelting is subjected to continuous casting to obtain a slab or the like. Then, the slab is subjected to the hot rolling, cold rolling, and the like described below to obtain a cold-rolled steel sheet.

[0116] In the present embodiment, after the decarburization treatment in the converter, in the secondary smelting, the composition adjustment of the steel is performed, and the inclusion control based on Ca addition is performed. From the viewpoint of reducing the number density of oxide, sulfide, and nitride individual inclusions, and composite inclusions composed of two or more of the individual inclusions, the total oxygen amount (T.O amount) in the molten steel is adjusted to 0.0040% or less by mass%. Thereby, it is possible to make the number density of inclusions 3.0 pieces / mm 2 Hereinafter, the T.O amount is preferably 0.0030% or less, and more preferably 0.0020% or less.

[0117] In the inclusion control, first, the composition of the addition elements other than Ca is adjusted. At this time, the time for allowing Al2O3 produced by Al deoxidation to float is sufficiently ensured. After Al2O3 is sufficiently allowed to float, Ca is added to the molten steel. When Al2O3 remains in the molten steel in a large amount, Ca is consumed by reduction by Al2O3. Therefore, the amount of Ca used in the fixation of S decreases, and it is not possible to sufficiently suppress the generation of MnS. From such a viewpoint, it is necessary to sufficiently ensure the time for allowing Al2O3 to float before Ca addition. This time (necessary floating time) also depends on the size of the ladle for adjusting the composition of the addition elements other than Ca, other factors (for example, in the case where the composition adjustment is performed in vacuum degassing, the capacity of the vacuum degassing device, and the like). Therefore, it is not possible to uniformly determine the required floating time. However, a person skilled in the art can easily determine the required floating time for each smelting device or the like including the ladle by performing analysis of a cold-rolled steel sheet or the like obtained by a test in which the floating time is changed.

[0118] On the other hand, when Ca is added to molten steel having an S concentration of 0.0015% or more, CaS is generated and aggregated in the molten steel, and sometimes, a coarse inclusion is generated. Therefore, the S concentration of the molten steel is preferably set to less than 0.0015%.

[0119] Note that Ca has a high vapor pressure, and therefore, in the case where Ca is added, in order to improve the yield, a Ca-Si alloy, an Fe-Ca-Si alloy, a Ca-Ni alloy, or the like can be added. These alloy additions can also use respective alloy wires.

[0120] [Slab heating step] In the slab heating step, the slab having the above-described chemical composition is heated to 1100°C or higher before the hot rolling step. In order to sufficiently re-dissolve the carbonitride, the heating temperature of the slab is set to 1100°C or higher. Note that when the heating temperature of the slab exceeds 1300°C, the effect saturates, and therefore, it is preferably 1300°C or lower.

[0121] In addition, the heating time in the slab heating step is preferably 30 minutes or more. When the heating time is less than 30 minutes, the temperature cannot be uniformly raised to the inside of the slab, and sometimes, the carbonitride cannot be sufficiently re-dissolved. In the case where the re-dissolution of the carbonitride is insufficient, sometimes, a coarse carbonitride remains, which functions as a starting point of cracking at the time of cold rolling, or the fatigue properties of the obtained cold-rolled steel sheet deteriorate. On the other hand, when the heating time exceeds 120 minutes, the effect of sufficiently re-dissolving the carbonitride saturates. In addition, excessively prolonging the heating time leads to a decrease in productivity and an increase in manufacturing cost. Therefore, the heating time is preferably 120 minutes or less.

[0122] Note that from the viewpoint of productivity, the heated slab is preferably manufactured by a continuous casting method, but can also be manufactured by other casting methods (for example, an ingot casting method).

[0123] [Hot rolling step] The hot rolling step of the present embodiment is roughly divided into rough rolling and finish rolling.

[0124] (Rough rolling) The heated slab is subjected to rough rolling in order to adjust the plate thickness or the like. The rough rolling is only required to obtain a desired size and shape of a rough bar, and the conditions are not particularly limited. Note that in the rough rolling, in order to homogenize the temperature of the rough bar, the rough bar can be heated using an induction heating device such as a bar heater or an edge heater.

[0125] (Finish rolling) Next, the rough-rolled slab is finish-rolled to produce a hot-rolled steel sheet. The exit temperature (finishing temperature) in this finishing roll is set to 820~920°C. When the exit temperature exceeds 920°C, the austenite phase coarsens, sometimes reducing cold rollability. Therefore, the upper limit of the exit temperature of the finishing roll is 920°C or less, preferably 900°C or less, and more preferably 880°C or less. On the other hand, from the viewpoint of suppressing the coarsening of the austenite phase, the lower limit of the exit temperature of the finishing roll is sufficient to be Ar3 point or above. However, when the finishing temperature is too low, the deformation resistance of the steel sheet increases, placing a huge burden on the rolling mill and becoming a cause of equipment failure. Therefore, the lower limit of the exit temperature of the finishing roll is preferably 820°C or more.

[0126] The thickness of hot-rolled steel sheets is determined by taking into account the reduction rate during subsequent cold rolling. For example, it can be set to be 2.0 mm or more and 4.0 mm or less.

[0127] [Cooling Process] After finishing rolling, the hot-rolled steel sheet undergoes controlled cooling before coiling. Controlled cooling is implemented through two stages: a primary cooling process and a secondary cooling process.

[0128] (First cooling process) In a single cooling process, the hot-rolled steel sheet is cooled from the exit temperature of the finishing mill to a cooling stop temperature T1 below point Ae1 at an average cooling rate CR1 of 30-80°C / second. When the average cooling rate CR1 is less than 30°C / second, proeutectoid ferrite and / or proeutectoid cementite phases precipitate, potentially failing to achieve a pearlite area fraction of 95% or more in the 1 / 4 region. Therefore, the average cooling rate CR1 is preferably set to 30°C / second or more, more preferably 40°C / second or more. On the other hand, when the average cooling rate CR1 exceeds 80°C / second, the hot-rolled steel sheet cannot be cooled uniformly, potentially causing material deviations. Therefore, the average cooling rate CR1 in a single cooling process is preferably set to 80°C / second or less, more preferably 70°C / second or less. The lower limit of the cooling stop temperature T1 is not particularly limited and can be 530°C or more, preferably 560°C or more.

[0129] It should be noted that Ae1 (°C) can be obtained using the following formula (1).

[0130] Ae1 (℃)=723-10.7×[Mn]+29.1×[Si]+16.9×[Cr]…(1) In equation (1), the [element symbols] represent the content of each element by mass%.

[0131] (Secondary cooling process) In the secondary cooling step, 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. When the average cooling rate CR2 from the cooling stop temperature T1 to the coiling temperature CT is fast, the interlamellar spacing in the steel sheet sometimes becomes uneven. In such a case, there are cases where the sheet is broken due to the difference in hardness caused by the interlamellar spacing at the time of cold rolling, and cases where a phase transformation structure such as a bainite structure is generated in a large amount and it is not possible to achieve an area fraction of the pearlite structure of 95% or more. Therefore, the average cooling rate CR2 of the secondary cooling step is 5.0°C / sec or less, preferably 3.0°C / sec or less, and more preferably 2.0°C / sec or less. By thus making the average cooling rate CR2 of the secondary cooling step slower than the average cooling rate CR1 of the primary cooling, it is possible to suppress the deviation of the interlamellar spacing of the pearlite structure, and it is possible to improve the cold rollability.

[0132] The smaller the average cooling rate CR2 is, the more preferable it is, and therefore the lower limit thereof is not particularly limited and can be 1.0°C / sec or more.

[0133] In order to sufficiently suppress the generation of the ferrite phase, the secondary cooling step is preferably performed immediately after the primary cooling step ends.

[0134] Note that the coiling is performed immediately after the secondary cooling step of the present embodiment ends.

[0135] [Coiling Step] After the cooling step, the hot-rolled steel sheet is coiled. The coiling temperature CT of the hot-rolled steel sheet is set to 560 to 700°C. By controlling the coiling temperature CT to 560 to 700°C, it is possible to cause the structure to appropriately transform in the coiling and to make the average interlamellar spacing of the pearlite structure fine. As a result, it is possible to obtain an interlamellar spacing of 80 nm or more and 200 nm or less at a stage before cold rolling, and it is possible to make the Vickers hardness of the surface of the hot-rolled steel sheet 320 Hv or more. Furthermore, by subjecting such a hot-rolled steel sheet to the cold rolling described later, it is possible to obtain a surface hardness of a high-carbon cold-rolled steel sheet of 530 Hv or more.

[0136] In the case where the coiling temperature CT is less than 560°C, other structures than the pearlite structure such as a bainite structure appear, and it is difficult to achieve an area fraction of the pearlite structure of 95% or more. Therefore, the coiling temperature CT is preferably 560°C or more, more preferably 580°C or more, and further preferably 600°C or more. On the other hand, when the coiling temperature exceeds 700°C, the average interlamellar spacing of the pearlite structure becomes large, and sometimes it is not possible to secure good fatigue durability and / or torque characteristics. Therefore, the coiling temperature CT is preferably 700°C or less, more preferably 680°C or less, and further preferably 660°C or less.

[0137] By the above procedure, the hot-rolled steel sheet of the present embodiment is obtained.

[0138] [Pickling step] Next, the hot-rolled steel sheet manufactured is subjected to pickling to remove the scale (surface oxide layer) on the surface of the steel sheet.

[0139] [Cold-rolling step] After the pickling step, the hot-rolled steel sheet is subjected to cold-rolling to obtain a cold-rolled steel sheet. The total reduction rate in the cold-rolling is preferably set to 90.0% or more. By setting to 90.0% or more, the lamella interval of the pearlite structure in the cold-rolled steel sheet can be made to be 20 nm or more and 50 nm or less. The total reduction rate in the cold-rolling is more preferably 91% or more. In addition, by controlling the lamella interval to be 20 nm or more and 50 nm or less, the Vickers hardness of the surface of the cold-rolled steel sheet can be made to be 530 Hv or more.

[0140] Note that the sheet thickness of the cold-rolled steel sheet is preferably set to 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 further preferably 0.180 mm or more and 0.240 mm or less.

[0141] From the viewpoint of suppressing strain aging in the cold-rolling, the steel sheet temperature is preferably set to 110°C or lower for the cold-rolling. The cold-rolling is preferably performed at 100°C or lower, more preferably 80°C or lower, and further preferably 60°C or lower. For example, in order to suppress the rise in the steel sheet temperature, the reduction rate per pass is preferably set to 5% or less.

[0142] As an effect of the suppression of the strain aging in the cold-rolling, the increase in the strength due to the strain aging can be suppressed. As a result, the cold workability at the spring forming of the cold-rolled steel sheet is improved, and the effect of obtaining the same hardness in the subsequent aging treatment can be enjoyed.

[0143] [Manufacturing method of high-carbon hot-rolled steel sheet] The manufacturing method of the high-carbon hot-rolled steel sheet of the present embodiment can employ the above-described procedures (I) to (VI). Note that in the case where the pickling step is omitted, the high-carbon hot-rolled steel sheet can be manufactured by the above-described procedures (I) to (V).

[0144] [Manufacturing method of disc spring] In addition, the manufacturing method of the disc spring of the present embodiment can employ the following method. First, the cold-rolled steel sheet obtained by the above-described method is cold-formed into a spring shape, and then, heat treatment (strain aging) is performed at a temperature in the range of 200 to 300°C for 15 to 45 minutes. By employing such a method, the disc spring of the present embodiment can be appropriately manufactured.

[0145] Example

[0146] Next, an embodiment of the present application will be described, but the conditions in the embodiment are one example of conditions adopted in order to confirm the possibility of implementation and effects of the present application, and the present application is not limited to this one example. Various conditions can be adopted as long as the gist of the present application is not deviated and the object of the present application is achieved.

[0147] The slabs having the chemical compositions (ladle analysis values) described in Tables 1A, 1B (wherein the float-up time in the steelmaking step is as shown in Tables 2A, 2B) were hot-rolled and coiled under the conditions shown in Tables 2A, 2B, whereby hot-rolled steel sheets having the sheet thicknesses shown in Tables 2A, 2B were obtained. For the obtained hot-rolled steel sheets, the surface Vickers hardness, the area fraction of pearlite structure in the 1 / 4 region, and the number density of inclusions were found by the above-described methods. The results are shown in Tables 5A, 5B. Note that in Tables 2A, 2B, values deviating from the preferred manufacturing method of the high-carbon hot-rolled steel sheet of the present embodiment are underlined. In the equipment of the present embodiment, when the float-up time is set to 10 minutes or more, the number density of coarse inclusions having an average particle diameter exceeding 10.0 μm can be made less than 0.5 pieces / mm2in both the hot-rolled steel sheet and the cold-rolled steel sheet. 2 .

[0148] Then, for the obtained hot-rolled steel sheets, cold-rolling was performed under the conditions shown in Table 3, whereby cold-rolled steel sheets were obtained. For the obtained cold-rolled steel sheets, the surface Vickers hardness, the area fraction of pearlite structure in the 1 / 4 region, and the number density of inclusions were found by the above-described methods. The results are shown in Tables 4A, 4B. Note that in Table 3, values deviating from the preferred manufacturing method of the high-carbon cold-rolled steel sheet of the present embodiment are underlined.

[0149] Note that the hot-rolled steel sheets shown in Tables 5A, 5B and the cold-rolled steel sheets shown in Tables 4A, 4B have, in the microstructure, the balance of the structure other than the pearlite structure as ferrite phase, bainite structure, and martensite structure.

[0150] In addition, for the obtained cold-rolled steel sheets, as a heat treatment corresponding to the final product, the Vickers hardness and fatigue durability were evaluated after a heat treatment was performed at a temperature ranging from 200°C to 260°C for a period ranging from 15 minutes to 45 minutes.

[0151] The fatigue durability was evaluated by performing a test according to "Method for Fatigue Test of Steel Sheet for Springs" of "Journal of the Society of Automotive Engineers of Japan, No. 41 (1996) p. 53-64", finding the fatigue limit, and thereby evaluating the fatigue durability. Note that the fatigue limit is the stress at which the test is performed until 106times without failure, and the fatigue durability is evaluated according to the following evaluation criteria. 7

[0152] <Evaluation Criteria> ​A: fatigue limit 800 MPa or more (pass) B: fatigue limit 750 MPa or more and less than 800 MPa (pass) C: fatigue limit less than 750 MPa (fail)

[0153] As shown in Tables 4A, B, 5A, B, in the inventive examples, the chemical composition, the area fraction of pearlite structure, the lamella interval of pearlite structure, and the number density of inclusions are all within the range of the present application. As a result, high strength (torque characteristics) and excellent fatigue durability are obtained.

[0154] On the other hand, in the comparative examples, at least one of the chemical composition, the area fraction of pearlite structure, the lamella interval of pearlite structure, and the number density of inclusions is outside the range of the present application, and the strength (torque characteristics) and fatigue durability are poor.

[0155] Note that the cold rolling method No. C24 and C42 shown in Table 3, Tables 4A, B are examples in which the sheet is broken due to a decrease in cold rolling property.

[0156]

[0157] Industrial applicability According to the above-described mode of the present application, a high-carbon hot-rolled steel sheet and a high-carbon cold-rolled steel sheet having high strength (torque characteristics) and excellent fatigue durability can be provided. Therefore, the high-carbon hot-rolled steel sheet and the high-carbon cold-rolled steel sheet can be appropriately applied to a disc spring or the like, and thus the high-carbon hot-rolled steel sheet and the high-carbon cold-rolled steel sheet of the above-described mode of the present application have high possibility of industrial application.

Claims

1. A high-carbon cold-rolled steel sheet, characterized by comprising, in mass %, a chemical composition containing, C：0.65~0.80%、 Si: 0.15 to 0.50%, Mn: 0.40 to 0.80%, P: 0.020% or less, S: 0.0015% or less, Al：0.010~0.065%、 Cr: more than 0.40% and 0.60% or less, Ca: 0.0005 to 0.0030%, O: 0.0040% or less, N: 0.0100% or less, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V：0~0.10%、 Cu: 0 to 0.50%, Ni: 0 to 0.50%, B：0~0.010%、 Mo: 0 to 0.10%, W：0~0.05%、 Ta: 0 to 0.05%, Mg: 0 to 0.05%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, As: 0 to 0.05%, REM: 0 to 0.0100%, and the balance: Fe and impurities, a Vickers hardness of a surface of the high-carbon cold-rolled steel sheet being 530 Hv or more, an area fraction of a pearlite structure being 95% or more, a lamellar spacing of the pearlite structure being 20 to 50 nm, In a case where the plate thickness is t, the number density of single inclusions of oxides, sulfides, and nitrides having an average particle diameter of 1.0 to 10.0 μm, or composite inclusions in which two or more of the single inclusions are combined, in a 1 / 4t surface is 3.0 pieces / mm 2 The following.

2. The high carbon cold rolled steel sheet according to claim 1, characterized by, the chemical composition containing, in mass %, one or two or more of, Ti: 0.001 to 0.10%, V：0.01~0.10%、 Nb: 0.001 to 0.10%, Cu: 0.01 to 0.50%, B：0.0001~0.010%、 Ni: 0.01 to 0.50%, W：0.001~0.05%、 Mo: 0.001 to 0.10%, Ta: 0.001 to 0.05%, Mg: 0.001 to 0.05%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, REM: 0.0001 to 0.0100%.

3. The high carbon cold rolled steel sheet according to claim 1, characterized by, the chemical composition containing, in mass %, Al：0.010~0.050%、 Ti: 0 to 0.02%, Nb: 0 to 0.05%, V：0~0.05%、 Cu: 0 to 0.05%, Ni: 0 to 0.05%, Mo: 0 to 0.05%, REM: 0 to 0.0050%.

4. The high carbon cold rolled steel sheet according to any one of claims 1 to 3, characterized in that, The number density of coarse inclusions having an average particle diameter of more than 10.0 μm is 0 per mm2 2 .

5. A high-carbon hot-rolled steel sheet, characterized by comprising, in mass %, a chemical composition containing, C：0.65~0.80%、 Si: 0.15 to 0.50%, Mn: 0.40 to 0.80%, P: 0.020% or less, S: 0.0015% or less, Al：0.010~0.065%、 Cr: more than 0.40% and 0.60% or less, Ca: 0.0005 to 0.0030%, O: 0.0040% or less, N: 0.0100% or less, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V：0~0.10%、 Cu: 0 to 0.50%, Ni: 0 to 0.50%, B：0~0.010%、 Mo: 0 to 0.10%, W：0~0.05%、 Ta: 0 to 0.05%, Mg: 0 to 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 impurities, a Vickers hardness of a surface of the high-carbon hot-rolled steel sheet being 305 Hv or more, an area fraction of a pearlite structure being 95% or more, an average lamellar spacing of the pearlite structure being 70 to 200 nm, In a case where the plate thickness is t, the number density of single inclusions of oxides, sulfides, and nitrides having an average particle diameter of 1.0 to 10.0 μm, or composite inclusions in which two or more of the single inclusions are combined, in a 1 / 4t surface is 3.0 pieces / mm 2 The following.

6. The high carbon hot-rolled steel sheet according to claim 5, characterized by, the Vickers hardness of the surface being 400 Hv or less.

7. The high carbon hot-rolled steel sheet according to claim 5 or 6, characterized in that, the chemical composition containing, in mass %, one or two or more of, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10%, V：0.01~0.10%、 Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, B：0.0001~0.010%、 Mo: 0.001 to 0.10%, W：0.001~0.05%、 Ta: 0.001 to 0.05%, Mg: 0.001 to 0.05%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, REM: 0.0001 to 0.0100% or less of one or two or more kinds.

8. The high carbon hot-rolled steel sheet according to claim 5, characterized by, The chemical composition contains, by mass%, Al：0.010~0.050%、 Ti: 0 to 0.02%, Nb: 0 to 0.05%, V：0~0.05%、 Cu: 0 to 0.05%, Ni: 0 to 0.05%, Mo: 0 to 0.05%, REM: 0 to 0.0050%.

9. The high carbon hot-rolled steel sheet according to any one of claims 5 to 8, characterized by, The number density of coarse inclusions having an average particle diameter of more than 10.0 μm is 0 per mm2 2 .

10. A belleville spring characterized by comprising: The chemical composition contains, by mass%, C：0.65~0.80%、 Si: 0.15 to 0.50%, Mn: 0.40 to 0.80%, P: 0.020% or less, S: 0.0015% or less, Al：0.010~0.065%、 Cr: more than 0.40% and 0.60% or less, Ca: 0.0005 to 0.0030%, O: 0.0040% or less, N: 0.0100% or less, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V：0~0.10%、 Cu: 0 to 0.50%, Ni: 0 to 0.50%, B：0~0.010%、 Mo: 0 to 0.10%, W：0~0.05%、 Ta: 0 to 0.05%, Mg: 0 to 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 balance: Fe and impurities, the area fraction of the pearlite structure of the belleville spring is 95% or more, the lamellar spacing of the pearlite structure is 20 to 50 nm, In the case where the plate thickness is t, the number density of single inclusions of oxides, sulfides, and nitrides having an average particle diameter of 1.0 to 10.0 μm, or composite inclusions formed by combining two or more of the single inclusions, in a 1 / 4t surface is 3.0 pieces / mm 2 Hereinafter, the hardness of the surface is 530 Hv or more.

11. The disc spring according to claim 10, characterized in that the hardness of the surface is 580 Hv or more.

12. The disc spring according to claim 10 or 11, characterized in that The chemical composition contains, by mass%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10%, V：0.01~0.10%、 Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, B：0.0001~0.010%、 Mo: 0.001 to 0.10%, W：0.001~0.05%、 Ta: 0.001 to 0.05%, Mg: 0.001 to 0.05%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, REM: 0.0001 to 0.0100% or less of one or two or more kinds.

13. The disc spring according to claim 10, wherein The chemical composition contains, by mass%, Al：0.010~0.050%、 Ti: 0 to 0.02%, Nb: 0 to 0.05%, V：0~0.05%、 Cu: 0 to 0.05%, Ni: 0 to 0.05%, Mo: 0 to 0.05%, REM: 0 to 0.0050%.

14. The disc spring according to any one of claims 10 to 13, characterized in that The number density of coarse inclusions having an average particle diameter of more than 10.0 μm is 0 per mm2 2 .

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