High carbon cold rolled steel sheets, high carbon hot rolled steel sheets and spiral springs

Abstract

This high-carbon cold-rolled steel sheet has a chemical composition, in mass%, of 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% but 0.60% or less, Ca: 0.0005 to 0.0030%, O: 0.0040% or less, N: 0.0100% or less, and the balance: Fe. and impurities, the surface Vickers hardness is 530Hv or more, the area fraction of pearlite structure is 95% or more, the lamellar spacing of the pearlite structure is 20 to 50nm, and when the plate thickness is t, the number density of single inclusions of oxides, sulfides and nitrides with an average particle size of 1.0 to 10.0μm or composite inclusions consisting of two or more of the above single inclusions is 3.0 pieces / mm 2 The following is the result.

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 spiral spring. This application claims priority based on Japanese Patent Application No. 2024-077352, filed on May 10, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] Conventionally, high-carbon steel sheets with a C content of 0.60 mass% or more have been widely used as materials for spring products such as spiral springs (power springs).

[0003] Because spring products are subject to high stresses, the high-carbon steel sheets used as raw materials must have high strength and fatigue resistance. Furthermore, if the high-carbon steel sheets contain coarse inclusions that can act as initiation points for fatigue cracks, these can cause premature fracture. Therefore, it is necessary to control the amount and shape of the inclusions.

[0004] Patent Document 1 discloses a cold-rolled carbon steel containing C: 0.63-0.85%, Si: max 0.40%, Mn: 0.20-0.90%, P: max 0.035%, S: max 0.035%, Al: max 0.060%, Cr: max 0.40%, N: 0.003-0.010% (preferably 0.005-0.008%), at least one micro-alloying element (Ti, Nb, V and Zr) having a content of max 0.12%, and the remainder being iron and impurities.

[0005] Patent Document 2 discloses a method for producing a high-carbon steel sheet, including the steps of: hot-rolling, cold-rolling, and annealing a steel material having predetermined chemical compositions 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 a solder bath temperature of 500°C to 530°C. Patent Document 2 also discloses that the patenting heat treatment may further include a cooling step and a cold rolling step of 85% or more, and that the temperature range for heating the steel sheet before the patenting heat treatment is 800°C to 1100°C. The manufacturing method disclosed in 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 lamellar carbides is 0.5 μm or less and the ratio of the major axis to the minor axis is 10:1 or more. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japan Special Publication No. 2012-518723 [Patent Document 2] Japan Special Publication No. 2010-528189 Summary of the Invention [Problem to be solved by the invention]

[0007] Generally, a known technique 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 hot-rolled steel sheets to a patenting treatment using a lead (Pb) bath before cold rolling.

[0008] However, in recent years, there has been a demand to reduce heat treatment, including patenting, from the perspective of reducing environmental impact (CO2 reduction, no use of Pb) and improving productivity by reducing processes.However, in order to omit heat treatment, it is necessary to refine the pearlite structure at the stage of hot-rolled steel sheet, which is an intermediate product of high-carbon cold-rolled steel sheet.

[0009] It is thought that the refinement of the pearlite structure at the hot-rolled steel sheet stage can be achieved by lowering the coiling temperature CT (increasing the degree of supercooling).However, lowering the coiling temperature could cause problems such as deformation of the coil shape after coiling or deterioration of the sheet shape due to the expansion and heat generated by the transformation that accompany the transformation.

[0010] In addition, the prior art in Patent Document 1 considers that the addition of micro-alloying elements is intended to refine the structure, thereby suppressing cracks that may form due to the notch effect of coarse structures, but does not fully consider inclusions that serve as crack initiation points. Furthermore, the manufacturing method disclosed in Patent Document 1 requires a patenting process, including cold rolling, after the hot rolling step.

[0011] Furthermore, Patent Document 2 examines the morphology of pearlite from the perspective of suppressing the propagation of fatigue cracks, but does not fully address inclusions that serve as the starting points for fatigue cracks. Furthermore, patenting treatments including cold rolling and the like are required after the hot rolling process, which increases the number of manufacturing steps and poses a problem in productivity.

[0012] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-carbon cold-rolled steel sheet, a high-carbon hot-rolled steel sheet, and a spiral spring that have high strength (torque characteristics) and excellent fatigue durability. [Means for solving the problem]

[0013] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0014] (1) A high-carbon cold-rolled steel sheet according to one aspect of the present invention comprises, in mass%, The chemical composition is, in 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%, less than 0.60%, 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: 0-0.50% Ni: 0 to 0.50% B: 0~0.010%, 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: 0~0.05%, REM: 0 to 0.0100%, and The balance is Fe and impurities. The surface Vickers hardness is 530Hv or more, The area fraction of pearlite structure is 95% or more, The lamellar spacing of the pearlite structure is 20 to 50 nm, When the plate thickness is t, the number density of single inclusions of oxides, sulfides and nitrides with an average particle size of 1.0 to 10.0 μm, or composite inclusions consisting of two or more of the above single inclusions, is 3.0 pieces / mm 2 The following is the case. (2) The high-carbon cold-rolled steel sheet according to (1) above has a chemical composition in mass% of: 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% It may contain one or more of the above. (3) The high-carbon cold-rolled steel sheet according to (1) above has a chemical composition, in mass%, of: Al: 0.010 to 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: May be 0 to 0.0050%. (4) The high-carbon cold-rolled steel sheet according to any one of (1) to (3) above has a number density of coarse inclusions having an average grain size exceeding 10.0 μm of 0 pieces / mm 2 may be. (5) A high-carbon hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in 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%, less than 0.60%, 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: 0-0.50% Ni: 0 to 0.50% B: 0~0.010%, 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: 0~0.05%, REM: 0 to 0.0100% or less, and The balance is Fe and impurities. The surface Vickers hardness is 305Hv or more, The area fraction of pearlite structure is 95% or more, The average lamellar spacing of the pearlite structure is 70 to 200 nm, When the plate thickness is t, the number density of single inclusions of oxides, sulfides and nitrides with an average particle size of 1.0 to 10.0 μm, or composite inclusions consisting of two or more of the above single inclusions, is 3.0 pieces / mm 2 The following is the result. (6) In the high carbon hot rolled steel sheet described in (5) above, the Vickers hardness of the surface may be 400 Hv or less. (7) The high carbon hot rolled steel sheet according to (5) or (6) above, wherein the chemical composition is, in 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: 0.0001~0.010%, 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:0.0001~0.0100% or less It may contain one or more of the above. (8) The high carbon hot rolled steel sheet according to (5) above, wherein the chemical composition is, in mass%, Al: 0.010 to 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: 0-0.05%, REM: 0-0.0050% may be. (9) The high carbon hot rolled steel sheet according to any one of (5) to (8) above has a number density of coarse inclusions having an average grain size exceeding 10.0 μm of 0 pieces / mm 2 may be. (10) 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 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%, less than 0.60%, 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: 0-0.50% Ni: 0 to 0.50% B: 0~0.010%, 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: 0~0.05%, REM: 0 to 0.0100% or less, and The balance is Fe and impurities. The area fraction of pearlite structure is 95% or more, The lamellar spacing of the pearlite structure is 20 to 50 nm, When the plate thickness is t, the number density of single inclusions of oxides, sulfides and nitrides with an average particle size of 1.0 to 10.0 μm, or composite inclusions consisting of two or more of the above single inclusions, is 3.0 pieces / mm 2 is as follows: The surface hardness is 530Hv or more. (11) In the spiral spring described in (10) above, the surface hardness may be 580 Hv or more. (12) The spiral spring according to (10) or (11) above, wherein the chemical composition is, in 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 ~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:0.0001~0.0100% or less It may contain one or more of the above. (13) The spiral spring according to (10) above, wherein the chemical composition is, in mass%, Al: 0.010 to 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% may be. (14) The spiral spring according to any one of (10) to (13) above has a number density of coarse inclusions having an average particle size exceeding 10.0 μm of 0 pieces / mm 2 may be. [Effects of the Invention]

[0015] According to the above aspects of the present invention, it is possible to provide a high-carbon cold-rolled steel sheet, a high-carbon hot-rolled steel sheet, and a spiral spring that have high strength (torque characteristics) and excellent fatigue durability. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a high-carbon cold-rolled steel sheet (hereinafter also referred to as cold-rolled steel sheet), a high-carbon hot-rolled steel sheet, and a spiral spring according to one embodiment of the present invention will be described. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention. Furthermore, numerical ranges described below with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" do not include the value in the numerical range.

[0017] <High carbon cold rolled steel sheet> The high-carbon cold-rolled steel sheet according to this embodiment has a predetermined chemical composition described later, 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 with an average particle size of 1.0 μm or more and 10.0 μm or less, or composite inclusions comprising two or more types of single inclusions, is 3.0 pieces / mm 2 The following is the case. In this embodiment, "high carbon" means that the carbon content in the steel is 0.65% or more.

[0018] [Chemical composition] The chemical composition of the high carbon cold rolled steel sheet according to this embodiment will be described below. In the following description, the unit of chemical composition is "mass %."

[0019] C: 0.65 to 0.80% 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 ensure a sufficient 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.

[0020] Si: 0.15 to 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 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. If this substance is formed, red scale will form on the surface of the resulting hot-rolled steel sheet, which may degrade the surface quality of the final cold-rolled steel sheet. Therefore, the Si content is set to 0.50% or less. Preferably, the Si content is 0.40% or less, more preferably 0.30% or less.

[0021] Mn: 0.40 to 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.

[0022] P:0.020% 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 the lower limit of the P content may be 0.001%, 0.003%, or 0.005% to reduce refining costs.

[0023] S: 0.0015% or less Sulfur (S) is an impurity element. S forms nonmetallic inclusions such as MnS. These nonmetallic 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.

[0024] Al: 0.010 to 0.065% Aluminum (Al) acts as a deoxidizer during the steelmaking process and remains in steel. When the Al content is 0.010% or more, a sufficient deoxidizing effect can be obtained. Preferably, the Al content is 0.015% or more, more preferably 0.020% or more. On the other hand, when the Al content exceeds 0.050%, inclusions are likely to form in the steel. When such inclusions are formed, they may become the starting point for fatigue crack initiation, resulting in reduced fatigue durability. Therefore, the Al content is 0.065% or less. Preferably, the Al content is 0.050% or less, more preferably 0.045% or less, and even more preferably 0.040% or less.

[0025] Cr: more than 0.40%, less than 0.60% Chromium (Cr) is an element that has the effect of refining the lamellar spacing of pearlite structures, thereby improving the strength of steel sheets. To fully obtain this effect, the Cr content is greater than 0.40%. Preferably, the Cr content is 0.41% or more, more preferably 0.42% or more. On the other hand, if the Cr content exceeds 0.60%, the strength of the steel may become excessively high, resulting in reduced cold rolling properties. Therefore, the Cr content is 0.60% or less. Preferably, the Cr content is 0.58% or less, more preferably 0.55% or less.

[0026] Ca: 0.0005 to 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.

[0027] O: 0.0040% or less Oxygen (O) forms oxides in steel. If the oxides in 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, a 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.

[0028] 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 set to 0.0020% or more. However, if the N content is excessively high, the effect saturates and the cold rolling properties may deteriorate. Therefore, the N content is set to 0.0100% or less. Preferably, the N content is 0.0080% or less, more preferably 0.0070% or less, and even more preferably 0.0060% or less.

[0029] In addition to the above elements, 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. By including these optional elements, the cold-rolled steel sheet according to this embodiment can obtain more preferable properties. However, since the cold-rolled steel sheet according to this embodiment can obtain desired properties even if it does not contain these optional elements, the lower limit of these optional elements is 0%. These optional elements will be described in detail below.

[0030] Ti: 0 to 0.10% 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 form, 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.

[0031] Nb: 0 to 0.10% Niobium (Nb) 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 Nb content exceeds 0.10%, coarse, angular carbonitrides are likely to be formed, resulting in significant deterioration of workability, 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.

[0032] 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. On the other hand, if the V content exceeds 0.10%, coarse, angular carbonitrides are likely to be formed, resulting in significant deterioration of workability, 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.

[0033] Cu: 0 to 0.50% Copper (Cu) has the effect of improving the strength (hardness) of steel sheet. Therefore, Cu may be contained within a range of 0.50% or less, if 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.

[0034] Ni: 0 to 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 needed. On the other hand, if the Ni content exceeds 0.50%, the cost increases and 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.

[0035] B: 0 to 0.010% Boron (B) has the effect of improving the hardenability and strength of the steel sheet. Therefore, B may be contained in 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 and the workability of the steel sheet is reduced, so 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.

[0036] Mo: 0 to 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, resulting in a significant deterioration in workability. Therefore, the Mo content is set to 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.

[0037] 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 needed. 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.

[0038] Ta: 0 to 0.05% Tantalum (Ta) has the effect of increasing the strength of 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.

[0039] Mg: 0 to 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.

[0040] 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 lower the Sn content, the better, and it may even be 0%. If the Sn content exceeds 0.05%, it may cause embrittlement of the steel sheet and reduce ductility. 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.

[0041] Sb: 0 to 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 even be 0%. If the Sb content exceeds 0.05%, it may cause embrittlement of the steel sheet and reduce ductility. 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.

[0042] As: 0 to 0.05% Arsenic (As), 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 even be 0%. If the As content exceeds 0.05%, it may cause embrittlement of the steel sheet and reduce ductility. 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.

[0043] REM: 0 to 0.0100% REM is an element effective in reducing MnS, controlling the morphology of inclusions, and improving the workability of steel sheets. To achieve 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, if the REM content exceeds 0.0100%, nozzle clogging during continuous casting becomes more likely. Furthermore, if the REM content exceeds 0.0100%, the number density of the resulting REM inclusions (oxides and oxysulfides) becomes relatively high, leading to accumulation of these REM 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.

[0044] REM (Rare Earth 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.

[0045] The cold-rolled steel sheet according to this embodiment contains the above elements, with the balance being Fe and impurities. Here, the impurities are elements that are mixed in during industrial steel production due to raw materials such as ore and scrap, and various factors in the production process, and whose presence is permitted to the extent that they do not impair the properties of the cold-rolled steel sheet according to this embodiment.

[0046] The above-mentioned chemical compositions may be measured by a common analytical method. For example, they may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. When ladle analysis values ​​or slab analysis values ​​are obtained during the production of slabs of cold-rolled steel sheets, it is not necessary to separately measure the chemical composition of the cold-rolled steel sheets, and the ladle analysis values ​​may be used. Ladle analysis values ​​may also be used for the chemical composition of hot-rolled steel sheets or spiral springs, which will be described later.

[0047] [Microstructure] Next, the microstructure (metal structure) of the high carbon cold rolled steel sheet according to this embodiment will be described.

[0048] (Area fraction of pearlite structure: 95% or more) Because pearlite has a higher work hardening capacity than ferrite and bainite, it is an effective structure for achieving high strength (torque characteristics) and excellent fatigue durability. When structures other than pearlite, such as ferrite and bainite, are contained at an area fraction of more than 5%, strength decreases. Furthermore, when a cold-rolled steel sheet is subjected to severe processing, differences in hardness (strength) occur between microstructures, and these hardness differences can become the starting point for crack initiation, resulting in reduced fatigue durability. For these reasons, it is effective to set the area fraction of pearlite to 95% or more in order to achieve torque characteristics and fatigue durability. Preferably, the area fraction of pearlite is 98% or more. The area fraction of pearlite may be 100%. The structure other than pearlite (the remaining structure) may be ferrite, bainite, or martensite.

[0049] 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. Preferably, the test pieces are taken from a quarter position in the width direction of the cold-rolled steel sheet. After the observation surface is etched with nital to reveal the structure, a scanning electron microscope is used to take a structural photograph at a magnification of 5000x, covering a field of view of 20 μm in the thickness direction and 30 μm perpendicular to the thickness direction, centered at a depth of 1 / 4 of the thickness from the surface of the steel sheet. Five structural photographs are taken. In each structural photograph, regions containing lamellar carbides are identified as pearlite, and the area ratio of pearlite is determined by calculating the average of their area ratios.

[0050] 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 inclusion dimensions can be measured is selected depending on the size of the inclusion. The observation range is 500 μm or more in width and across the entire thickness of the sheet, and dark areas are determined to be inclusions. Observation may be performed from multiple fields of view. Next, using the thickness cross section initially observed using the above method as a reference, the sheet is rotated in 5° increments between 0° and 180° around the thickness direction, and the cross sections parallel to the plane are observed using the same method as above. The average length of the major axes of the multiple inclusions in each cross section is calculated for each cross section. The cross section with the largest average value of the major axis length of the inclusions obtained is identified. The direction parallel to the major axis of the inclusions in that cross section is determined to be the rolling direction. The rolling direction of parts (spiral springs, etc.) made from hot-rolled and cold-rolled steel sheets is also determined using the same method as above. However, if the rolling direction can be determined in advance (for example, when test pieces are taken from steel strips), this determination of the rolling direction is not necessary and can be omitted.

[0051] (Lamellar spacing in pearlite structure: 20 to 50 nm) If the lamellar spacing of the pearlite structure is less than 20 nm, cold workability during forming may decrease. Therefore, the lamellar spacing of the pearlite structure is 20 nm or more. Preferably, the lamellar spacing of the pearlite structure is 22 nm or more, 25 nm or more, or 27 nm or more. On the other hand, if the lamellar spacing of the pearlite structure exceeds 50 nm, the desired strength (torque characteristics) may not be obtained. Therefore, the lamellar spacing of the pearlite structure is 50 nm or less. Preferably, 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.

[0052] Here, the lamellar spacing of the pearlite structure refers to the average distance between the center points of adjacent ferrite and cementite phases in the thickness direction that make up the lamellae. 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 predetermined length perpendicular to the direction of layer extension during structural observation. Layers that are not completely cut by the line segment at both ends of the line segment are excluded from the measurement.

[0053] Specifically, in this embodiment, 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). The magnification is 5000 times (measurement area: 80 μm × 150 μm), and 10 locations where the cementite layer crosses perpendicularly to the paper surface of the structure photograph are selected. By etching with picral etchant and measuring, information in the depth direction can be obtained, and the locations where the cementite layer crosses perpendicularly can be identified. By selecting and measuring 10 or more such locations, 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, draw a straight line perpendicular to the cementite layer so as to cross 10 to 30 cementite layers, and let the length of the line (line segment length) be L. Also, let N be the number of layers in a set consisting of one ferrite phase layer and one cementite phase layer that the line crosses. In this case, the lamellar spacing S at that point can be calculated by S = L / (N × 2). It is to be noted that 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.

[0054] That is, the lamellar spacing according to this embodiment is calculated by the following formula: Lamellar spacing S = line length L ÷ (number of pairs cut by line segments N × 2)

[0055] (Inclusion density within the 1 / 4t plane: 3.0 pieces / mm 2 below) In cold-rolled steel sheets, the number density of inclusions with an average grain size of 1.0 to 10.0 μm (1.0 μm or more and 10.0 μm or less) is 3.0 pieces / mm 2 If the density exceeds 3.0 / mm, the inclusions may become the starting point and cause early fracture, making it difficult to obtain high fatigue durability. 2 This ensures high fatigue durability. Preferably, the number density of inclusions is 2.5 pieces / mm 2 Below, 2.0 pieces / mm 2 Less than or equal to 1.5 pieces / mm 2 The smaller the density of inclusions, the better, so the lower limit is 0 pieces / mm 2 Inclusions with an average particle size of less than 1.0 μm do not affect fatigue durability, and therefore are not counted as inclusions as defined in this embodiment. Furthermore, coarse inclusions with an average grain size exceeding 10.0 μm can become the starting point of fracture. In other words, coarse inclusions with an average grain size exceeding 10.0 μm will fracture early regardless of the number, and the desired fatigue durability will not be obtained. Therefore, in this embodiment, the number density of coarse inclusions with an average grain size exceeding 10.0 μm is set to 0 pieces / mm 2 It is preferable that the number density of coarse inclusions having an average particle size exceeding 10.0 μm, which is obtained by the method for calculating the number density of inclusions described later, is 0.5 pieces / mm 2 If the average grain size is less than 10.0 μm, the number density of coarse inclusions with an average grain size exceeding 10.0 μm is 0 pieces / mm 2 The number density of coarse inclusions with an average grain size exceeding 10.0 μm is set to 0.4 pieces / mm 2 Below, 0.3 pieces / mm 2 Less than or equal to 0.2 pieces / mm 2 The following may also be used.

[0056] In this embodiment, the term "inclusions" refers to single inclusions of oxides, sulfides and nitrides, as well as composite inclusions in which two or more of these single inclusions are combined.

[0057] 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 embedded test specimens are polished in the thickness direction, and then mirror-polished so that the 1 / 4t plane (the plane parallel to the surface of the cold-rolled steel plate, 1 / 4t away from the surface of the cold-rolled steel plate, where t is the plate 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 1.2 mm x 1.6 mm. The long side of the observation area corresponds to the rolling direction of the cold-rolled steel plate. During observation, 48 non-overlapping fields of 200 μm x 200 μm are selected, and each field is observed at 500x magnification.

[0058] In the observation area, particles (precipitates or inclusions) with a circle-equivalent diameter of 1.0 μm or more are identified based on the Z-contrast of the backscattered electron image. In the backscattered electron image, particles appear with a dark contrast compared to the matrix. The circle-equivalent diameter refers to the diameter of a circle when the area of ​​a particle is converted to a circle with the same area. Each identified particle is analyzed for elemental concentration using energy dispersive X-ray spectroscopy (EDX). The standardless method is used for EDX analysis (elemental concentration analysis). The accelerating voltage is 20 kV, and the quantified elements are Si, Mn, P, S, Cr, Ti, Nb, Cu, Ni, Ca, N, O, Al, and Mg.

[0059] In the EDX analysis results of each particle, when the total content of the above quantified elements in mass% is taken as 100%, particles with a total O content, S content, and N content of 10% or more in mass% are identified as single inclusions and composite inclusions of oxides, sulfides, and nitrides.

[0060] Based on the total number of oxide, sulfide, and nitride single inclusions and composite inclusions with a circle equivalent diameter of 1.0 μm or more identified in each observation area of ​​the five test specimens and the total area of ​​each observation area of ​​the five test specimens, the number density (pieces / mm 2) is found. 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.

[0061] (Vickers hardness: 530Hv or more) In the cold-rolled steel sheet of this embodiment, the Vickers hardness of the steel sheet surface is 530 Hv or more. This allows for excellent torque characteristics to be obtained. Preferably, 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.

[0062] 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 least 50 mm away from the edge of the cold-rolled steel plate. Hardness is measured at any five points, and the Vickers hardness is obtained by calculating the average of the obtained hardness values. Note that the Vickers hardness can also be measured using the above method for parts (spiral springs, etc.) made from hot-rolled steel plate and cold-rolled steel plate, as described below. However, if the measurement point cannot be at least 50 mm away from the edge, Vickers hardness is generally measured at the center of the width. 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.

[0063] <High carbon hot rolled steel sheet> Next, the high carbon hot rolled steel sheet according to this embodiment will be described. The high-carbon hot-rolled steel sheet (hereinafter also referred to as hot-rolled steel sheet) of this embodiment has a predetermined chemical composition, and the Vickers hardness of the surface of the hot-rolled steel sheet is 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 when the sheet thickness is t, the number density of single inclusions of oxides, sulfides and nitrides with an average particle size of 1.0 μm or more and 10.0 μm or less, or composite inclusions comprising two or more of the single inclusions, is 3.0 pieces / mm 2 The following is the result. 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.

[0064] [Chemical composition] 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.

[0065] [Microstructure] Next, the microstructure (metal structure) of the hot-rolled steel sheet will be described. The hot-rolled steel sheet in this embodiment has a microstructure in which the pearlite structure accounts for 95% or more in area ratio, and the average lamellar spacing of the pearlite structure is 80 to 200 nm.

[0066] (Area fraction of pearlite structure: 95% or more) 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%, strain will concentrate and these structures will become fracture initiation points, 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.

[0067] (Lamellar spacing of pearlite structure: 70 to 200 nm) From the viewpoint of utilizing the high work hardening ability of the pearlite structure in a cold-rolled steel sheet, the initial lamellar spacing before cold rolling, i.e., the lamellar spacing in the hot-rolled steel sheet, is important. If 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. It is preferably 170 nm or less or 150 nm or less, more preferably 130 nm or less or 120 nm or less. On the other hand, if the initial lamellar spacing before cold rolling is less than 70 nm, cold rolling at a high cold reduction ratio significantly increases the strength, and good cold rolling properties may not be ensured. Therefore, 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. In addition to adjusting the chemical components, the lamellar spacing can be adjusted by adjusting the cooling rate from the austenite phase and the coiling temperature, which will be described later.

[0068] (Inclusion density within the 1 / 4t plane: 3.0 pieces / mm 2 below) The average particle size of the inclusions is 1.0 to 10.0 μm (1.0 μm or more, 10.0 μm or less) and the number density is 3.0 pieces / mm 2 High durability is achieved at 3.0 pieces / mm or less. 2 If the number density of inclusions is over 3.0 pieces / mm, the inclusions become the starting point and cause early fracture, and high fatigue durability cannot be obtained. 2 High durability can be obtained by limiting the number of particles to 2.5 or less. 2 Below, 2.0 pieces / mm 2 Less than or equal to 1.5 pieces / mm 2 The following is the result. Coarse inclusions with an average grain size exceeding 10.0 μm can become the starting point of fracture. Therefore, in this embodiment, the number density of coarse inclusions with an average grain size exceeding 10.0 μm is set to 0 pieces / mm 2 It is preferable that the number density of coarse inclusions having an average particle size exceeding 10.0 μm is 0.5 pieces / mm 2 If the average grain size is less than 10.0 μm, the number density of coarse inclusions with an average grain size exceeding 10.0 μm is 0 pieces / mm 2 The number density of coarse inclusions with an average grain size exceeding 10.0 μm is set to 0.4 pieces / mm 2 Below, 0.3 pieces / mm 2 Less than or equal to 0.2 pieces / mm 2 The following may also be used.

[0069] (Vickers hardness: 305Hv or more) In the hot-rolled steel sheet of this embodiment, the Vickers hardness of the surface is 305 HV or more. By increasing the hardness of the hot-rolled steel sheet, the strength of the cold-rolled steel sheet after cold rolling and the final product (spiral spring) can be sufficiently increased. If necessary, 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.

[0070] The method for measuring the hardness of the hot-rolled steel sheet is the same as the above-mentioned method for measuring the hardness of the cold-rolled steel sheet.

[0071] <Spiral spring> The spiral spring according to this embodiment is obtained by forming the cold-rolled steel sheet according to this embodiment into a spring shape. That is, the area fraction of pearlite structure is 95% or more, the lamellar spacing of the pearlite structure is 20 to 50 nm, and the number density of single inclusions of oxides, sulfides, and nitrides with an average particle size of 1.0 to 10.0 μm or composite inclusions consisting of two or more types of single inclusions is 3.0 pieces / mm within the plane of the sheet thickness 1 / 4t. 2 Hereinafter, the hardness of the spring surface is 530Hv or more. Here, the "spiral spring" in this embodiment is a "number 3400 spiral spring" defined in JIS B 0103:2015, and is a spring that has a spiral shape in a plane.

[0072] In the spiral spring according to this embodiment, which is the final product (spiral spring), after cold-forming the cold-rolled steel sheet into a spring shape, it is preferable to perform heat treatment (strain aging) at a temperature range of 200 to 300°C for 15 to 45 minutes to increase the elastic limit. By undergoing such a heat treatment process, the hardness of the spiral spring can be increased. For example, the above-mentioned heat treatment can increase the Vickers hardness of the spiral spring surface to 580 Hv or more. However, the above-mentioned heat treatment is not essential, and the Vickers hardness of the spiral spring surface can be the same as that of the above-mentioned cold-rolled steel sheet (specifically, the lower limit of the Vickers hardness is 530 HV).

[0073] Regardless of whether the heat treatment is performed or not, the chemical composition, microstructure (metal structure), lamellar spacing of the pearlite structure, and number density of inclusions (including coarse inclusions exceeding 10.0 μm) of the spiral spring of this embodiment are basically the same as those of the cold-rolled steel sheet used as the raw material. Therefore, the description of these requirements will be omitted.

[0074] <Manufacturing method for high carbon cold rolled steel sheet> Next, a preferred method for producing the high carbon cold rolled steel sheet of this embodiment will be described. The method for producing a high carbon cold rolled steel sheet according to this embodiment can be achieved by a production method including the following steps.

[0075] (I) Steelmaking (refining and casting) process to produce slabs having the above chemical composition. (II) A slab heating process in which the slab is heated to 1100°C or higher. (III) A hot rolling process 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 sheet. (IV) A cooling process including a primary cooling step in which the high-carbon hot-rolled steel sheet is cooled to a cooling stop temperature T1 at an average cooling rate of 30 to 80°C / second, and a secondary cooling step in which the high-carbon hot-rolled steel sheet is subsequently cooled from point T1 to a coiling temperature at an average cooling rate of less than 20°C / second. (V) A coiling process in which high-carbon hot-rolled steel sheets are coiled at a coiling temperature of 560 to 700°C. (VI) A pickling process to remove scale from the surface of the high carbon hot rolled steel sheet after the coiling process. (VII) A cold rolling process in which a high-carbon hot-rolled steel sheet is cold-rolled at a total reduction rate of 90% or more to obtain a high-carbon cold-rolled steel sheet.

[0076] Generally, 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. On the other hand, the manufacturing method of this embodiment, including the above steps (I) to (VII), creates a pearlite structure in the hot rolling and coiling stages, 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. Furthermore, 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.

[0077] [Steelmaking (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.

[0078] In this embodiment, after decarburization in a converter, in secondary refining, the steel composition is adjusted and inclusions are controlled by adding Ca. From the viewpoint of reducing the number density of single inclusions such as oxides, sulfides, and nitrides, as well as composite inclusions consisting of two or more of these single inclusions, the total oxygen content (TO content) in the molten steel is adjusted to 0.0040% by mass or less. This reduces the number density of inclusions to 3.0 pieces / mm 2 The TO content is preferably 0.0030% or less, and more preferably 0.0020% or less.

[0079] Inclusion control involves first adjusting the composition of additive elements other than Ca. At this time, sufficient time is required for Al2O3, which is generated by Al deoxidation, to float up. After Al2O3 has sufficiently floated up, 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 up before Ca is added. This time (required floating time) also 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, those 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 with 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 in the molten steel is preferably less than 0.0015%. Since the vapor pressure of Ca is high, when Ca is added, it is better to add it as a Ca-Si alloy, Fe-Ca-Si alloy, Ca-Ni alloy, etc., in order to increase the yield. When adding these alloys, the respective alloy wires may be used.

[0080] [Slab heating process] In the slab heating step, a slab having the above 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. However, if the heating temperature of the slab exceeds 1300°C, the effect is saturated, so the heating temperature is preferably 1300°C or lower.

[0081] In addition, 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 becomes saturated. Moreover, an excessively long heating time may result in a decrease in productivity and an increase in manufacturing costs. Therefore, the heating time is preferably 120 minutes or less.

[0082] From the viewpoint of productivity, the slab to be heated is preferably produced by a continuous casting method, but may be produced by other casting methods (for example, an ingot casting method).

[0083] [Hot rolling process] The hot rolling process in this embodiment is roughly divided into rough rolling and finish rolling.

[0084] (rough 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. In the rough rolling, 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.

[0085] (Finish rolling) Next, 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 in 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 in the finish rolling is 920°C or lower, preferably 900°C or lower, and more preferably 880°C or lower. On the other hand, the lower limit of the exit temperature in the finish rolling may be equal to or higher than the Ar3 point in order to suppress coarsening of the austenite phase. However, if the finishing temperature is too low, the deformation resistance of the steel sheet increases, which places a heavy load on the rolling mill and causes equipment troubles. Therefore, the lower limit of the exit temperature in the finish rolling is preferably 820°C or higher.

[0086] The thickness of the hot-rolled steel sheet is determined in consideration of the reduction rate during subsequent cold rolling, and may be, for example, 2.0 mm or more and 4.0 mm or less.

[0087] [Cooling process] After finish rolling, hot-rolled steel sheets undergo controlled cooling before coiling. Controlled cooling is carried out in two stages: a primary cooling process and a secondary cooling process.

[0088] (Primary cooling process) In the primary cooling step, 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 preventing a pearlite area fraction of 95% or more in the quarter region. Therefore, the average cooling rate CR1 is preferably 30°C / s or more, more preferably 40°C / s or more. On the other hand, if the average cooling rate CR1 exceeds 80°C / s, the hot-rolled steel sheet may not be uniformly cooled, potentially resulting in variations in the material properties. Therefore, 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).

[0089] Ae1(℃)=723-10.7×[Mn]+29.1×[Si]+16.9×[Cr] ···(1)

[0090] In the formula (1), the [element symbols] respectively indicate the content of each element in mass %.

[0091] (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. 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. Therefore, 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. By making the average cooling rate CR2 in the secondary cooling step slower than the average cooling rate CR1 in the primary cooling step, it is possible to suppress variations in the lamellar spacing of the pearlite structure and improve cold rollability. The smaller the average cooling rate CR2, the better, so there is no particular lower limit to the rate, but it may be 1.0°C / sec or more.

[0092] 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. In this embodiment, the film is immediately wound up after the secondary cooling step is completed.

[0093] [Winding process] After the cooling process, 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, the structure is appropriately transformed during coiling, making it possible to refine the average lamellar spacing of the pearlite structure. As a result, a lamellar spacing of 80 nm or more and 200 nm or less can be obtained before cold rolling, and the Vickers hardness of the surface of the hot-rolled steel sheet can be made 320 Hv or more. Then, by subjecting such a hot-rolled steel sheet to cold rolling, as described below, a high-carbon cold-rolled steel sheet with a surface hardness of 530 Hv or more can be obtained.

[0094] If the coiling temperature CT is less than 560°C, structures other than pearlite structures, such as bainite structures, will appear, making it difficult to achieve an area fraction of pearlite structures of 95% or more. Therefore, the coiling temperature CT is preferably 560°C or higher, more preferably 580°C or higher, and even more preferably 600°C or higher. On the other hand, if the coiling temperature exceeds 700°C, the average lamellar spacing of the pearlite structures will become large, and it may become impossible to ensure good fatigue durability and / or torque characteristics. For this reason, the coiling temperature CT is preferably 700°C or lower, more preferably 680°C or lower, and even more preferably 660°C or lower. Through the above steps, the hot-rolled steel sheet according to this embodiment is obtained.

[0095] [Pickling process] Next, the produced hot-rolled steel sheet is subjected to pickling to remove scale (surface oxide layer) from the surface of the steel sheet.

[0096] [Cold rolling process] After the pickling step, 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. By setting the reduction to 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. Furthermore, by controlling the lamellar spacing to 20 nm or more and 50 nm or less, the Vickers hardness of the surface of the cold-rolled steel sheet can be set to 530 Hv or more.

[0097] 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.

[0098] From the viewpoint of suppressing strain aging during cold rolling, it is preferable to perform cold rolling at a steel sheet temperature of 110° C. 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. For example, in order to suppress an increase in the steel sheet temperature, it is preferable to set the reduction rate per pass to 5% or less.

[0099] The effect of suppressing strain aging during cold rolling is to suppress the increase in strength due to strain aging, which results in improved cold workability when cold-rolled steel sheets are formed into springs, and the same hardness can be obtained in the subsequent aging treatment.

[0100] <Manufacturing method for high carbon hot rolled steel sheet> The method for producing a high-carbon hot-rolled steel sheet according to this embodiment can employ the above steps (I) to (VI). When the pickling step is omitted, the high-carbon hot-rolled steel sheet may be produced by the above steps (I) to (V).

[0101] <Spiral spring manufacturing method> The spiral spring according to this embodiment can be manufactured by the following method. First, the cold-rolled steel sheet obtained by the above-mentioned 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. [Example]

[0102] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0103] Slabs having the chemical compositions (ladle analysis values) shown in Tables 1A and 1B (however, the floating time in the steelmaking process was as shown in Tables 2A and 2B) were hot rolled under the conditions shown in Tables 2A and 2B and coiled to obtain hot-rolled steel sheets having the thicknesses shown in Tables 2A and 2B. The surface Vickers hardness, the area fraction of pearlite structures in the 1 / 4 region, and the number density of inclusions were determined for the obtained hot-rolled steel sheets using the methods described above. The results are shown in Tables 5A and 5B. In Tables 2A and 2B, values ​​that deviate from the preferred manufacturing method for the high-carbon hot-rolled steel sheet of this embodiment are underlined. With the equipment of this example, when the floating time was 10 minutes or more, the number density of coarse inclusions with an average particle size exceeding 10.0 μm was reduced to 0.5 pieces / mm 2 It was possible to make it less than that.

[0104] The obtained hot-rolled steel sheet was then cold-rolled under the conditions shown in Table 3 to obtain a cold-rolled steel sheet. The Vickers hardness of the surface, the area fraction of pearlite structures in the 1 / 4 region, and the number density of inclusions were determined for the obtained cold-rolled steel sheet using the methods described above. The results are shown in Tables 4A and 4B. In Table 3, values ​​outside the range of the preferred manufacturing method for the high-carbon cold-rolled steel sheet of this embodiment are underlined.

[0105] In both the hot-rolled steel sheets shown in Tables 5A and 5B and the cold-rolled steel sheets shown in Tables 4A and 4B, the remaining microstructures other than the pearlite structure were ferrite, bainite, and martensite.

[0106] The obtained cold-rolled steel sheets were also 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 tests in accordance with the "Fatigue Test Method for Thin Plates for Springs" in "Spring Journal Vol. 41 (1996) pp. 53-64" and determining the fatigue limit. 7 The fatigue durability was evaluated according to the following evaluation criteria.

[0107] <Evaluation criteria> A: Fatigue limit 800 MPa or more (pass) B: Fatigue limit 750 MPa or more, less than 800 MPa (pass) C: Fatigue limit less than 750 MPa (failure)

[0108] As can be seen from Tables 4A and 4B and Tables 5A and 5B, the inventive examples had predetermined chemical compositions, and the area integral 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.

[0109] In contrast, the comparative examples had chemical compositions in which at least one of the area integral 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.

[0110] [Table 1A]

[0111] [Table 1B]

[0112] [Table 2A]

[0113] [Table 2B]

[0114] [Table 3]

[0115] [Table 4A]

[0116] [Table 4B]

[0117] [Table 5A]

[0118] [Table 5B] [Industrial Applicability]

[0119] According to the above-mentioned aspects of the present invention, it is possible to provide a high-carbon hot-rolled steel sheet and a high-carbon cold-rolled steel sheet having high strength (torque characteristics) and excellent fatigue durability. Therefore, the high-carbon hot-rolled steel sheet and the high-carbon cold-rolled steel sheet according to the above-mentioned aspects of the present invention can be suitably applied to spiral springs and the like, and therefore the high-carbon hot-rolled steel sheet and the high-carbon cold-rolled steel sheet according to the above-mentioned aspects of the present invention have high industrial applicability.

Claims

1. The chemical composition is expressed in mass percent. 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%, 0.60% or less, 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%, and The remainder consists of Fe and impurities. The surface Vickers hardness is 530 Hv or higher. The area fraction of the perlite structure is 95% or more. The lamellar spacing of the aforementioned pearlite structure is 20 to 50 nm. When the plate thickness is t, the number density of individual inclusions of oxides, sulfides, and nitrides with an average particle size of 1.0 to 10.0 μm, or composite inclusions formed by the combination of two or more such individual inclusions, is 3.0 particles / mm² within a 1 / 4 t plane. ２ A high-carbon cold-rolled steel sheet characterized by the following:

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

3. The aforementioned chemical composition, 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%, The high-carbon cold-rolled steel sheet according to claim 1, characterized in that the REM is 0 to 0.0050%.

4. The number density of coarse inclusions with an average particle size exceeding 10.0 μm is 0 particles / mm³. ２ A high-carbon cold-rolled steel sheet according to any one of claims 1 to 3, characterized in that it is such.

5. The chemical composition is expressed in mass percent. 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%, 0.60% or less, 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 remainder consists of Fe and impurities. The surface Vickers hardness is 305 Hv or higher. The area fraction of the perlite structure is 95% or more. The average lamellar spacing of the aforementioned perlite structure is 70 to 200 nm. When the plate thickness is t, the number density of individual inclusions of oxides, sulfides, and nitrides with an average particle size of 1.0 to 10.0 μm, or composite inclusions formed by the combination of two or more such individual inclusions, is 3.0 particles / mm² within a 1 / 4 t plane. ２ A high-carbon hot-rolled steel sheet characterized by the following:

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

7. The aforementioned chemical composition, in 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: 0.01-0.50%, B: 0.001-0.010%, Mo: 0.001 to 0.10%, W: 0.001-0.05%, Ta: 0.001-0.05%, Mg: 0.001-0.05%, Sn: 0.001-0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, REM: 0.0001 to 0.0100% or less The high-carbon hot-rolled steel sheet according to claim 5, characterized in that it contains one or more of the above.

8. The aforementioned chemical composition, 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: 0~0.0050% The high-carbon hot-rolled steel sheet according to claim 5, characterized in that it is such.

9. The number density of coarse inclusions with an average particle size exceeding 10.0 μm is 0 particles / mm³. ２ A high-carbon hot-rolled steel sheet according to any one of claims 5 to 8, characterized in that it is such.

10. The chemical composition is expressed in mass percent. 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%, 0.60% or less, 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 remainder consists of Fe and impurities. The area fraction of the perlite structure is 95% or more. The lamellar spacing of the aforementioned pearlite structure is 20 to 50 nm. When the plate thickness is t, the number density of individual inclusions of oxides, sulfides, and nitrides with an average particle size of 1.0 to 10.0 μm, or composite inclusions formed by the combination of two or more such individual inclusions, is 3.0 particles / mm² within a 1 / 4 t plane. ２ The following: A spiral spring characterized by having a surface hardness of 530 Hv or higher.

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

12. The aforementioned chemical composition, in 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: 0.01-0.50%, B: 0.001-0.010%, Mo: 0.001 to 0.10%, W: 0.001-0.05%, Ta: 0.001-0.05%, Mg: 0.001-0.05%, Sn: 0.001-0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, REM: 0.0001 to 0.0100% or less The spiral spring according to claim 10, characterized in that it contains one or more of the following.

13. The aforementioned chemical composition, 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: 0~0.05%, REM: 0~0.0050% The spiral spring according to claim 10, characterized in that it is the same as the one described above.

14. The number density of coarse inclusions with an average particle size exceeding 10.0 μm is 0 particles / mm³. ２ A spiral spring according to any one of claims 10 to 13, characterized in that it is the same as described above.