steel material

Abstract

A steel capable of suppressing surface roughness after peeling is provided. The chemical composition of the steel according to this embodiment, by mass%, contains: C: 0.50–0.80%, Si: 1.20–2.90%, Mn: 0.25–1.00%, Cr: 0.40–1.90%, V: 0.05–0.60%, P: ≤0.020%, S: ≤0.020%, N: ≤0.0100%, Mo: 0–0.50%, Nb: 0–0.050%, W: 0–0.60%, Ni: 0–0.50%, Co: 0–0.30%, B: 0–0.0050%, Cu: 0–0.050%, Al: 0–0.0050%, and Ti: 0–0.050%, with the balance consisting of Fe and impurities. In the microstructure of steel, pearlite has an area ratio of over 90%, and within the ferrite in the pearlite, the number density of V-series precipitates with a maximum diameter of 2–20 nm is 3000–80000 per μm. 3 .

Description

Technical Field

[0001] This invention relates to steel, and more specifically, to steel used as blanks for springs such as damping springs or valve springs. Background Technology

[0002] Springs are widely used in automobiles and general machinery. Among the springs used in automobiles and general machinery, damping springs absorb shocks or vibrations from the outside. Damping springs are used, for example, in torque converters that transmit power from a car to a transmission. High fatigue strength is required for damping springs. Additionally, valve springs are used in automobiles and general machinery to regulate the opening and closing of valves within the machinery. Valve springs are used, for example, to control the opening and closing of the intake and exhaust valves in an automobile's internal combustion engine. Therefore, like damping springs, high fatigue strength is also required for valve springs.

[0003] The manufacturing process of springs, such as damping springs and valve springs, is as follows: First, prepare steel (wire) as the blank for the spring. Then, perform a peeling (skinning) process on the steel. Here, peeling refers to the process of passing the steel (wire) through a peeling die to remove the entire surface (circumference) of the steel. Through this peeling process, surface defects and decarburized layers are removed from the steel.

[0004] The steel, after being peeled, is drawn into steel wire. The steel wire is then subjected to quenching and tempering. The quenched and tempered steel wire is cold-wound to produce coiled intermediate steel. The intermediate steel is then subjected to stress-relief annealing. If necessary, the stress-relief annealed intermediate steel undergoes surface hardening heat treatment (nitriding, etc.). The intermediate steel after stress-relief annealing or surface hardening heat treatment is then shot-peened to apply compressive residual stress to the surface. Through these manufacturing processes, a spring is produced.

[0005] Japanese Patent Application Publication No. Hei 7-173577 (Patent Document 1) and Japanese Patent Application Publication No. 2007-327084 (Patent Document 2) disclose technologies related to steel as the blank material for such springs.

[0006] Patent Document 1 discloses a spring steel containing, by mass percent, 0.3–0.6% C, 1.0–3.0% Si, 0.1–0.5% Mn, and 0.5–1.5% Cr, further containing less than 1.0% Ni (excluding 0) and / or 0.1–0.5% Mo, with the balance consisting of Fe and unavoidable impurities. Furthermore, Patent Document 1 describes that in this steel, FP (=(0.23[C]+0.1)×(0.7[Si]+1)×(3.5[Mn]+1)×(2.2[Cr]+1)×(0.4[Ni]+1)×(3[Mo]+1)) is 2.5–4.5. Therefore, the above-mentioned steel, after quenching and tempering, possesses a high strength of over 1900 MPa and excellent corrosion resistance.

[0007] Patent Document 2 discloses a wire containing, by mass percent, 0.6–1.1% C, 0.1–2.0% Si, 0.1–1% Mn, 0.020% or less (excluding 0%) P, 0.020% or less (excluding 0%) S, 0.006% or less (excluding 0%) N, 0.03% or less (excluding 0%) Al, and 0.003% or less (excluding 0%) O, with the balance consisting of Fe and unavoidable impurities. This wire also possesses a pearlitic structure with a second-phase ferrite area fraction of 11.0% or less and a pearlite lamellar spacing of 120 μm or more. Patent Document 2 describes that, with the above-described structure, this wire is not constrained by increases in drawing speed or cross-sectional area reduction, making wire breakage less likely, and extending the life of the dies used in the wire drawing process.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 7-173577

[0011] Patent Document 2: Japanese Patent Application Publication No. 2007-327084 Summary of the Invention

[0012] The problem the invention aims to solve

[0013] On the other hand, as mentioned above, during the spring manufacturing process, the steel used as the spring blank is subjected to a peeling process. In this case, the surface of the peeled steel sometimes develops defects caused by peeling, such as "burrs," "scratches," and "crazing" (hereinafter referred to as peeling-induced defects). Here, "burrs" refers to defects caused by a portion of the cutting debris generated during the peeling process remaining on the steel surface. "Scratches" refers to defects caused by a portion of the steel surface near the root of the cutting debris being peeled away when it detaches from the steel surface. "Cracks" refers to defects caused by cracks forming on a portion of the steel surface near the root of the cutting debris when it detaches from the steel surface. The smoothness of the steel surface with peeling-induced defects decreases, resulting in a rough surface. Springs manufactured using steel with a rough surface exhibit reduced fatigue strength. Therefore, it is required to suppress the surface roughness of the steel surface after the peeling process for the spring blank.

[0014] Patent documents 1 and 2 mentioned above do not disclose any technology for suppressing the surface roughness of steel after peeling.

[0015] The purpose of this disclosure is to provide a steel that can suppress surface roughness when a peeling process is performed.

[0016] Solution for solving the problem

[0017] The chemical composition of the steel in this embodiment, expressed in % by mass, contains...

[0018] C: 0.50-0.80%

[0019] Si: 1.20~2.90%

[0020] Mn: 0.25~1.00%

[0021] Cr: 0.40–1.90%

[0022] V: 0.05~0.60%

[0023] P: below 0.020%

[0024] S: Below 0.020%

[0025] N: below 0.0100%

[0026] Mo: 0–0.50%

[0027] Nb: 0~0.050%

[0028] W: 0–0.60%

[0029] Ni: 0-0.50%

[0030] Co: 0-0.30%

[0031] B: 0~0.0050%

[0032] Cu: 0–0.050%

[0033] Al: 0–0.0050%, and

[0034] Ti: 0~0.050%,

[0035] The balance consists of Fe and impurities.

[0036] In the microstructure of the aforementioned steel, the pearlite area ratio is over 90%.

[0037] In the aforementioned pearlite, ferrite

[0038] The number density of V-series precipitates with a maximum diameter of 2–20 nm is 3,000–80,000 per μm. 3 .

[0039] The effects of the invention

[0040] The steel disclosed herein can suppress surface roughness when the steel surface has undergone a peeling process. Attached Figure Description

[0041] Figure 1 This is an example of a transmission electron microscope (TEM) image of ferrite in pearlite of a thin film sample.

[0042] Figure 2 This is a flowchart illustrating the manufacturing process of the steel in this embodiment.

[0043] Figure 3 It means Figure 2 A diagram illustrating an example of the temperature history of steel during the finishing rolling process.

[0044] Figure 4 It means Figure 3 The graph shows the continuous cooling phase transformation curve (CCT curve) of the steel cooling process in this embodiment.

[0045] Figure 5 This is a flowchart illustrating the manufacturing process of a spring using the steel described in this embodiment. Detailed Implementation

[0046] The inventors first investigated the chemical composition and microstructure of steel suitable as blanks for springs, such as damping springs and valve springs. When manufacturing springs from steel blanks, a steel with a chemical composition suitable for spring applications and exhibiting high fatigue strength can be obtained. Furthermore, during the peeling process in spring manufacturing, the steel's microstructure can be peeled to obtain a structure suitable for spring applications. The study concluded that if the chemical composition of steel, by mass%, contains C: 0.50–0.80%, Si: 1.20–2.90%, Mn: 0.25–1.00%, Cr: 0.40–1.90%, V: 0.05–0.60%, P: less than 0.020%, S: less than 0.020%, N: less than 0.0100%, Mo: 0–0.50%, Nb: 0–0.050%, W: 0–0.60%, Ni: 0–0.50%, Co: 0–0.30%, B: 0–0.0050%, Cu: 0–0.050%, Al: 0–0.0050%, and Ti: 0–0.050%, with the balance consisting of Fe and impurities, and the pearlite area ratio in the microstructure is above 90%, then the steel is suitable for spring applications.

[0047] Therefore, the inventors have studied methods for suppressing surface roughness on the surface of steel having the above-described chemical composition and microstructure after peeling. As a result, the following insights were obtained.

[0048] As described above, peeling (skinning) is a process in which the entire surface of steel (wire) is peeled (cut) using a peeling die. During peeling, since the entire surface of the steel is peeled, oxide scale, decarburized layers, and defects such as rolling imperfections can be removed. As a result, the surface of the steel becomes smooth after peeling. However, if the steel surface is not smoothly cut during peeling, defects such as burrs, peeling, or cracking will occur on the surface after peeling. These defects reduce the smoothness of the steel surface, creating a rough surface. Therefore, the inventors investigated the cutting mechanism of the steel surface during peeling. The following insights were obtained as a result.

[0049] In the peeling process, the surface of steel is cut using a peeling die. The cut portion of steel is severed from the steel surface in the form of cutting chips. If the cutting chips from the steel surface using the peeling die are difficult to cut completely off, some of the chips may remain on the steel surface, the steel near the root of the chips may peel off, or cracks may form on the steel surface near the root of the chips. This residue of cutting chips, the peeling at the root of the chips, and the cracking become defects caused by peeling, resulting in surface roughness of the steel. Therefore, the inventors believe that if the cutting chips generated by the peeling die are easily cut off from the steel surface during the peeling process, the chips can be cut shorter. If the chips are cut shorter, the shorter chips are more easily separated from the steel surface. Therefore, it can suppress the presence of cutting debris remaining on the steel surface, or the partial peeling of steel near the root of the cutting debris, or the formation of cracks on the steel surface near the root of the cutting debris. As a result, defects caused by peeling are suppressed, surface roughness of the steel surface is reduced, and the smoothness of the steel surface is ensured.

[0050] Therefore, the inventors have further investigated a means to cut the cutting debris during the peeling process into shorter pieces in steel having the above-described chemical composition and microstructure.

[0051] Generally, it is known that MnS, as an inclusion in steel, can improve the machinability of steel. Therefore, the inventors first considered suppressing the surface roughness of the steel surface after peeling by controlling the morphology of MnS, which is an inclusion in steel.

[0052] However, in springs, MnS, as an inclusion, can reduce fatigue characteristics. Therefore, the inventors believe that utilizing MnS in steel to suppress the surface roughness of steel after peeling is inappropriate. Therefore, the inventors have considered using other methods, different from MnS, to suppress the surface roughness of steel after peeling.

[0053] Here, the inventors have focused on the microstructure of the steel used as a blank for springs. As described above, the microstructure of the steel used as a blank for springs is a pearlite-based structure with a pearlite area ratio of 90% or more. Pearlite consists of ferrite and cementite. Since ferrite is softer than cementite, it is more difficult to cut during the peeling process. Therefore, the inventors have focused on the ferrite in the pearlite structure and have studied methods to facilitate the cutting of ferrite.

[0054] As a result of their research, the inventors conceived of utilizing precipitates formed in ferrite during the peeling process, rather than inclusions like MnS, to facilitate the cutting of ferrite. Therefore, they realized that intentionally generating a large number of fine, nano-sized V-based precipitates in ferrite would make the ferrite easier to cut during the peeling process. Here, fine V-based precipitates refer to V-containing precipitates with a maximum diameter of 2–20 nm in the field of view of a TEM (transmission electron microscope), as described later. Examples of V-containing precipitates include V carbides and V carbonitrides.

[0055] V-based precipitates can form in ferrite through phase interface precipitation. Furthermore, V-based precipitates are extremely fine compared to MnS. Specifically, even fine MnS precipitates are around 1 μm in size, while V-based precipitates can form at sizes of 2–20 nm. The smaller the precipitate size, the less likely it is to become the initiation point for fatigue fracture. Therefore, V-based precipitates are less likely to reduce the fatigue strength of springs. On the other hand, if a large number of V-based precipitates are formed in the ferrite, the ferrite can be easily cut short during the stripping process. As a result, defects caused by peeling, such as residual cutting debris, spalling, and cracking, become less likely to occur.

[0056] It can be argued that V-series precipitates can also inhibit wear on peeling dies. V-series precipitates have high hardness. During peeling, some of the V-series precipitates contained in the cutting chips adhere to the tip of the peeling die. The adhered V-series precipitates increase the wear resistance of the peeling die tip. If the wear resistance of the peeling die tip is increased, the cutting force (peeling force) of the peeling die can be maintained. Therefore, the ease of cutting cutting chips from the steel surface can also be maintained.

[0057] That is, by generating multiple V-series precipitates in ferrite, not only can the ease of cutting the steel's own cutting chips be improved, but the ease of cutting the cutting chips, achieved by maintaining the wear resistance of the cutting die tip, can also be improved. As a result, it is believed that the surface roughness of the steel after peeling can be significantly suppressed.

[0058] Based on the above technical concept, the inventors conducted a further detailed study on the number density of V-series precipitates in steel that can effectively suppress surface roughness during the peeling process. As a result, the inventors found that in steel with the content of each element in the chemical composition within the above-mentioned range and a pearlite area ratio of 90% or more in the microstructure, if the number density of V-series precipitates with a maximum diameter of 2-20 nm in the ferrite within the pearlite is 3000-80000 precipitates / μm... 3 This can effectively suppress the surface roughness of steel after peeling.

[0059] Although the above mechanism is speculative, the examples described later demonstrate that if the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of pearlite is 3,000–80,000 per μm... 3 This can effectively suppress the surface roughness of steel after peeling.

[0060] The steel in this embodiment is made based on the above-described technical concept. The steel in this embodiment has the following characteristics. [1]

[0062] A type of steel,

[0063] Its chemical composition, expressed in % by mass, contains

[0064] C: 0.50-0.80%

[0065] Si: 1.20~2.90%

[0066] Mn: 0.25~1.00%

[0067] Cr: 0.40–1.90%

[0068] V: 0.05~0.60%

[0069] P: below 0.020%

[0070] S: Below 0.020%

[0071] N: below 0.0100%

[0072] Mo: 0–0.50%

[0073] Nb: 0~0.050%

[0074] W: 0–0.60%

[0075] Ni: 0-0.50%

[0076] Co: 0-0.30%

[0077] B: 0~0.0050%

[0078] Cu: 0–0.050%

[0079] Al: 0–0.0050%, and

[0080] Ti: 0~0.050%,

[0081] The balance consists of Fe and impurities.

[0082] In the microstructure of the aforementioned steel, the pearlite area ratio is over 90%.

[0083] In the aforementioned pearlite, ferrite

[0084] The number density of V-series precipitates with a maximum diameter of 2–20 nm is 3,000–80,000 per μm. 3 . [2]

[0086] According to the steel described in [1], wherein,

[0087] The aforementioned chemical composition contains selected free radicals.

[0088] Mo: 0.01–0.50%

[0089] Nb: 0.001~0.050%

[0090] W: 0.01~0.60%

[0091] Ni: 0.01~0.50%

[0092] Co: 0.01-0.30%, and

[0093] B: One or more species in a group consisting of 0.0001% to 0.0050%.

[0094] The steel used in this embodiment will now be described in detail. Unless otherwise specified, "%" for elements refers to mass percentage.

[0095] [Chemical Composition of Steel]

[0096] The chemical composition of steel contains the following elements.

[0097] C: 0.50–0.80%

[0098] Carbon (C) improves the fatigue strength of springs manufactured from steel blanks. If the C content is less than 0.50%, the above-mentioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the C content exceeds 0.80%, coarse cementite will be produced even if the contents of other elements are within the range of this embodiment. Coarse cementite will reduce the ductility of the steel. Coarse cementite will also reduce the fatigue strength of springs manufactured from steel blanks. Therefore, the C content is 0.50 to 0.80%. The preferred lower limit of the C content is 0.51%, more preferably 0.52%, more preferably 0.53%, and more preferably 0.54%. The preferred upper limit of the C content is 0.79%, more preferably 0.78%, more preferably 0.76%, more preferably 0.74%, more preferably 0.72%, more preferably 0.70%, and more preferably 0.68%.

[0099] Si: 1.20–2.90%

[0100] Silicon (Si) improves the fatigue strength of springs manufactured from steel blanks. Si also deoxidizes steel. Si further increases the tempering softening resistance of steel. Therefore, even after tempering (quenching and tempering) during the spring manufacturing process, the fatigue strength of the spring can be maintained at a high level. If the Si content is less than 1.20%, the aforementioned effects cannot be sufficiently obtained even with the contents of other elements within the range of this embodiment. On the other hand, if the Si content exceeds 2.90%, the ductility of the steel used as the spring blank will decrease excessively, even with the contents of other elements within the range of this embodiment. Consequently, the fatigue strength of the spring manufactured from the steel blank will decrease. Therefore, the Si content is 1.20% to 2.90%. The preferred lower limit for the Si content is 1.25%, more preferably 1.30%, more preferably 1.35%, more preferably 1.40%, more preferably 1.45%, more preferably 1.50%, more preferably 1.55%, and more preferably 1.60%. The preferred upper limit for the Si content is 2.85%, more preferably 2.80%, more preferably 2.75%, more preferably 2.70%, more preferably 2.65%, and more preferably 2.60%.

[0101] Mn: 0.25~1.00%

[0102] Manganese (Mn) improves the hardenability of steel and enhances the fatigue strength of springs manufactured from steel blanks. If the Mn content is less than 0.25%, the aforementioned effects cannot be sufficiently achieved even with the contents of other elements within the range of this embodiment. On the other hand, if the Mn content exceeds 1.00%, the strength of the steel becomes excessively high during the spring manufacturing process, even with the contents of other elements within the range of this embodiment, resulting in decreased workability of the steel. Therefore, the Mn content is 0.25% to 1.00%. The preferred lower limit for the Mn content is 0.28%, more preferably 0.30%, more preferably 0.35%, more preferably 0.40%, more preferably 0.45%, more preferably 0.50%, and more preferably 0.55%. The preferred upper limit for the Mn content is 0.95%, more preferably 0.90%, more preferably 0.85%, more preferably 0.80%, and more preferably 0.75%.

[0103] Cr: 0.40–1.90%

[0104] Chromium (Cr) improves the hardenability of steel and enhances the fatigue strength of springs manufactured from steel blanks. If the Cr content is less than 0.40%, the aforementioned effects cannot be sufficiently achieved even with the contents of other elements within the range of this embodiment. On the other hand, if the Cr content exceeds 1.90%, coarse Cr carbides will be excessively formed even with the contents of other elements within the range of this embodiment. Coarse Cr carbides reduce the fatigue strength of the spring. Therefore, the Cr content is 0.40% to 1.90%. The preferred lower limit of the Cr content is 0.45%, more preferably 0.50%, more preferably 0.55%, more preferably 0.60%, more preferably 0.65%, more preferably 0.70%, more preferably 0.75%, and more preferably 0.80%. The preferred upper limit of the Cr content is 1.85%, more preferably 1.80%, more preferably 1.75%, more preferably 1.70%, more preferably 1.65%, and more preferably 1.60%.

[0105] V: 0.05~0.60%

[0106] Vanadium (V) bonds with C and / or N to form V-based precipitates with a maximum diameter of 2–20 nm in the ferrite of pearlite. During the peeling process, these V-based precipitates readily cut through the ferrite. Therefore, during peeling, the V-based precipitates readily cut the cutting debris into shorter pieces. As a result, the surface roughness of the steel surface after peeling can be suppressed, improving the smoothness of the steel surface. If the V content is less than 0.05%, even if the contents of other elements are within the range of this embodiment, the above-mentioned effect cannot be sufficiently obtained. On the other hand, if the V content exceeds 0.60%, even if the contents of other elements are within the range of this embodiment, a large number of coarse V-based precipitates with a maximum diameter exceeding 20 nm will be generated in the steel. These coarse V-based precipitates reduce the fatigue strength of the spring. Therefore, the V content is 0.05–0.60%. The preferred lower limit for the V content is 0.06%, more preferably 0.07%, more preferably 0.08%, more preferably 0.10%, more preferably 0.15%, more preferably 0.17%, more preferably 0.18%, and more preferably 0.20%. The preferred upper limit for the V content is 0.58%, more preferably 0.57%, more preferably 0.55%, more preferably 0.53%, more preferably 0.50%, more preferably 0.45%, more preferably 0.40%, more preferably 0.35%, and more preferably 0.30%.

[0107] P: below 0.020%

[0108] Phosphorus (P) is an impurity. P segregates at grain boundaries, reducing the fatigue strength of springs manufactured from steel blanks. Therefore, the P content is 0.020% or less. The preferred upper limit for P content is 0.018%, more preferably 0.016%, more preferably 0.014%, more preferably 0.012%, and more preferably 0.010%. The P content is preferably as low as possible, and most preferably 0%. However, excessively low P content increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit for P content is greater than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0109] S: below 0.020%

[0110] Sulfur (S) is an impurity. S either segregates at grain boundaries like P or bonds with Mn to form MnS, thereby reducing the fatigue strength of springs made from steel. Therefore, the S content is 0.020% or less. The preferred upper limit for the S content is 0.018%, more preferably 0.016%, more preferably 0.014%, more preferably 0.012%, and more preferably 0.010%. The S content is preferably as low as possible, and most preferably 0%. However, excessively low S content increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit for the S content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0111] N: below 0.0100%

[0112] Nitrogen (N) is an impurity. N bonds with Al or Ti to form AlN and TiN, which reduces the fatigue strength of springs made from steel. Therefore, the N content is 0.0100% or less. The preferred upper limit for the N content is 0.0095%, more preferably 0.0090%, more preferably 0.0085%, more preferably 0.0080%, more preferably 0.0075%, more preferably 0.0070%, more preferably 0.0065%, and more preferably 0.0060%. The N content is preferably as low as possible, and most preferably 0%. However, excessively low N content increases manufacturing costs. Therefore, the preferred lower limit for the N content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0007%, and more preferably 0.0010%.

[0113] The steel in this embodiment has a chemical composition consisting of Fe and impurities. Here, impurities refer to substances that may be introduced into the steel during industrial manufacturing from raw materials such as ore, waste, or the manufacturing environment, and are permissible within a range that does not adversely affect the steel of this embodiment.

[0114] [About optional elements]

[0115] The chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of Mo, Nb, W, Ni, Co, and B to replace a portion of the Fe. These elements are arbitrary and all will improve the fatigue strength of the springs manufactured using the steel of this embodiment.

[0116] Mo: 0–0.50%

[0117] Molybdenum (Mo) can be any element, or it may be absent. That is, the Mo content can be 0%. When it is present, that is, when the Mo content exceeds 0%, Mo increases the hardenability of the steel and improves the fatigue strength of the spring manufactured from the steel blank. Mo also increases the tempering softening resistance of the steel. Therefore, even after tempering treatment is performed in the spring manufacturing process, the fatigue strength of the spring can be maintained at a high level. The above effects can be obtained to a certain extent with just a small amount of Mo. However, if the Mo content exceeds 0.50%, the strength of the steel used as the spring blank will become too high, even if the contents of other elements are within the range of this embodiment, and the workability of the steel will decrease. Therefore, the Mo content is 0 to 0.50%, and when Mo is present, the Mo content is more than 0 and less than 0.50%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit of the Mo content is 0.45%, more preferably 0.40%, more preferably 0.35%, and more preferably 0.30%.

[0118] Nb: 0~0.050%

[0119] Niobium (Nb) is an arbitrary element, or it may be absent. That is, the Nb content can be 0%. When it is present, that is, when the Nb content exceeds 0%, Nb bonds with C and / or N to form carbides or carbonitrides (hereinafter referred to as Nb carbonitrides, etc.). Nb carbonitrides, etc., refine the austenite grains. Therefore, the fatigue strength of springs made of steel can be improved. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Nb is present. However, if the Nb content exceeds 0.050%, coarse Nb carbonitrides, etc., will be formed even if the contents of other elements are within the range of this embodiment. Coarse Nb carbonitrides, etc., will reduce the fatigue strength of the spring. Therefore, the Nb content is 0 to 0.050%, and when Nb is present, the Nb content is more than 0% and less than 0.050%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit for Nb content is 0.045%, more preferably 0.040%, more preferably 0.035%, more preferably 0.030%, and more preferably 0.025%.

[0120] W: 0–0.60%

[0121] Tungsten (W) can be any element, or it may be absent. That is, the W content can be 0%. When present, i.e., when the W content exceeds 0%, W increases the hardenability of the steel and improves the fatigue strength of springs made from the steel. W also increases the tempering softening resistance of the steel. Therefore, even after tempering treatment is performed during the spring manufacturing process, the fatigue strength of springs made from the steel can be maintained at a high level. The above effects can be achieved to a certain extent with just a small amount of W. However, if the W content exceeds 0.60%, the strength of the steel becomes excessively high, even if the contents of other elements are within the range of this embodiment, and the workability of the steel decreases. Therefore, the W content is 0 to 0.60%, and when W is present, the W content is more than 0 but less than 0.60%. The preferred lower limit of the W content is 0.01%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit for the W content is 0.55%, more preferably 0.50%, more preferably 0.45%, more preferably 0.40%, more preferably 0.35%, and more preferably 0.30%.

[0122] Ni: 0-0.50%

[0123] Nickel (Ni) can be any element, or it may be absent. That is, the Ni content can be 0%. When present, i.e., when the Ni content exceeds 0%, Ni improves the hardenability of the steel and increases the fatigue strength of springs made from the steel. Even a small amount of Ni can achieve these effects to some extent. However, if the Ni content exceeds 0.50%, the strength of the steel becomes excessively high, even if the contents of other elements are within the range of this embodiment, and the workability of the steel decreases. Therefore, the Ni content is 0 to 0.50%, and when Ni is present, the Ni content exceeds 0 but is less than 0.50%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.02%, more preferably 0.03%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit of the Ni content is 0.45%, more preferably 0.40%, and more preferably 0.35%.

[0124] Co: 0-0.30%

[0125] Cobalt (Co) can be any element, or it may be absent. That is, the Co content can be 0%. When it is present, that is, when the Co content exceeds 0%, Co increases the tempering softening resistance of the steel. Therefore, even after tempering treatment is performed in the spring manufacturing process, the fatigue strength of the spring made of steel can be maintained at a high level. The above-mentioned effect can be obtained to a certain extent as long as a small amount of Co is present. However, if the Co content exceeds 0.30%, the strength of the steel will become too high, and the workability of the steel will decrease, even if the contents of other elements are within the range of this embodiment. Therefore, the Co content is 0 to 0.30%, and when Co is present, the Co content is more than 0 and less than 0.30%. The preferred lower limit of the Co content is 0.01%, more preferably 0.05%, and more preferably 0.10%. The preferred upper limit of the Co content is 0.28%, more preferably 0.26%, more preferably 0.24%, more preferably 0.22%, and more preferably 0.20%.

[0126] B: 0~0.0050%

[0127] Boron (B) can be any element, or it may be absent. That is, the B content can be 0%. When present, that is, when the B content exceeds 0%, B improves the hardenability of the steel and increases the fatigue strength of springs made from the steel. Even a small amount of B can achieve these effects to some extent. However, if the B content exceeds 0.0050%, the strength of the steel becomes excessively high, even if the contents of other elements are within the range of this embodiment, and the workability of the steel decreases. Therefore, the B content is 0 to 0.0050%, and when B is present, the B content exceeds 0 but is less than or equal to 0.0050%. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0015%, and more preferably 0.0020%. The preferred upper limit for the B content is 0.0049%, more preferably 0.0048%, more preferably 0.0047%, more preferably 0.0045%, more preferably 0.0043%, and more preferably 0.0040%.

[0128] [Regarding impurity elements]

[0129] It should be noted that the chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of Cu, Al, and Ti, in the form of impurities, to replace a portion of the Fe. If the content of these elements is within the ranges described below, the aforementioned effects of the steel in this embodiment can be sufficiently obtained.

[0130] Cu: 0–0.050%

[0131] Copper (Cu) is an impurity, and it is preferable that this element is not present. That is, the Cu content can be 0%. If the Cu content exceeds 0.050%, the workability of the steel will significantly decrease even if the contents of other elements are within the range of this embodiment. Therefore, the Cu content is 0 to 0.050%. The preferred upper limit of the Cu content is 0.045%, more preferably 0.040%, more preferably 0.038%, and more preferably 0.036%. As mentioned above, the Cu content is preferably as low as possible. However, excessive reduction of the Cu content will increase manufacturing costs. Therefore, the preferred lower limit of the Cu content is more than 0%, more preferably 0.001%, and more preferably 0.002%.

[0132] Al: 0~0.0050%

[0133] Aluminum (Al) is an impurity, and it is preferable that this element is not present. That is, the Al content can be 0%. Al forms coarse non-metallic inclusions, reducing the fatigue strength of springs made of steel. If the Al content exceeds 0.0050%, the fatigue strength of the spring will decrease significantly even if the contents of other elements are within the range of this embodiment. Therefore, the Al content is 0 to 0.0050%. The preferred upper limit of the Al content is 0.0045%, more preferably 0.0040%, more preferably 0.0035%, more preferably 0.0032%, and more preferably 0.0030%. As mentioned above, the Al content is preferably as low as possible. However, excessive reduction of the Al content will increase manufacturing costs. Therefore, the preferred lower limit of the Al content is more than 0%, more preferably 0.0001%, and more preferably 0.0005%.

[0134] Ti: 0~0.050%

[0135] Titanium (Ti) is an impurity, and it is preferable that this element is not present. That is, the Ti content can be 0%. Ti forms coarse TiN. TiN is prone to becoming the starting point of fracture. Therefore, TiN reduces the fatigue strength of springs made of steel. If the Ti content exceeds 0.050%, the fatigue strength of the spring will decrease significantly even if the contents of other elements are within the range of this embodiment. Therefore, the Ti content is 0 to 0.050%. The preferred upper limit of the Ti content is 0.045%, more preferably 0.040%, more preferably 0.035%, more preferably 0.032%, and more preferably 0.030%. As mentioned above, the Ti content is preferably as low as possible. However, excessive reduction of the Ti content will increase manufacturing costs. Therefore, the preferred lower limit of the Ti content is more than 0%, more preferably 0.001%, more preferably 0.003%, and more preferably 0.005%.

[0136] [Microstructure of steel]

[0137] The microstructure of the steel in this embodiment is predominantly pearlite. Here, "microstructure predominantly pearlite" means that the pearlite area fraction in the microstructure is 90% or more. It should be noted that phases other than pearlite include precipitates, inclusions, ferrite, and hard phases (martensite and / or bainite). It should also be noted that the area fraction of precipitates and inclusions is negligible compared to other phases.

[0138] [Method for determining pearlite area ratio]

[0139] The pearlite area ratio can be calculated using the following method.

[0140] The cross-section (surface) cut perpendicular to the length of the steel, i.e., along its diameter, is used as the observation surface. The observation surface is mirror-polished. Etching based on 5% picric acid alcohol (picric acid etching solution) is then performed on the mirror-polished observation surface. A depth position, radially at a distance of 1 / 4 of the diameter from the steel surface (outer periphery of the observation surface), is designated as the observation field. Ten observation fields are observed using a 2000x scanning electron microscope (SEM), generating photographic images of all ten fields. Each field is set to a size of 40 μm × 60 μm.

[0141] In each field of view, the contrast and morphology of each phase—pearlite, ferrite, hard phase, precipitates, and inclusions—are different. Therefore, pearlite is determined based on its contrast and morphology. Pearlite is a layered morphology of alternating layers of cementite and ferrite. Therefore, anyone skilled in the art can easily distinguish pearlite from other phases based on its contrast and morphology. The total area (μm) of pearlite in each field of view is calculated. 2 The total area of ​​pearlites in the entire field of view is compared to the total area of ​​the entire field of view (24000 μm). 2 The proportion of pearlite area is defined as the pearlite area ratio (%). The pearlite area ratio is obtained by rounding the first decimal place (i.e., an integer).

[0142] The pearlite area fraction in the microstructure of the steel in this embodiment is 90% or more. Therefore, compared with steel mainly composed of hard phases (martensite and / or bainite), the steel surface is easier to grind during the peeling process, and its cold workability is also high. As described above, in the spring manufacturing process, the steel is peeled and then drawn. Therefore, the steel of this embodiment is suitable for spring manufacturing. It should be noted that in the steel of this embodiment, the preferred lower limit of the pearlite area fraction is 91%, and more preferably 92%.

[0143] [Number density of V series precipitates]

[0144] In the steel of this embodiment, the number density of V-series precipitates with a maximum diameter of 2 to 20 nm in the ferrite within the pearlite is 3,000 to 80,000 per μm. 3 In this specification, the number density of V-series precipitates refers to the number density per unit area (1 μm in this specification). 3 The number of V-series precipitates.

[0145] In this specification, V-based precipitates refer to precipitates containing V. V-based precipitates may contain Cr along with V. Examples of V-based precipitates are V carbides and V carbonitrides. V-based precipitates can be complex precipitates containing V carbides and elements other than V, Cr, and C, or they can be complex precipitates containing V carbonitrides and elements other than V, Cr, C, and N. V-based precipitates are extremely fine compared to Fe carbides such as cementite. Therefore, they can be easily distinguished from Fe carbides such as cementite, and V-based precipitates can be identified. It should be noted that, as will be discussed later, regarding the V-series precipitates, V, or V and Cr, were detected by elemental analysis using energy dispersive X-ray spectroscopy (EDS), and the crystal structure was determined to be cubic with lattice constants a, b, and c of 0.4167 nm ± 5% (according to ICDD (International Center for Diffraction Data) No. 065-8822).

[0146] In the steel of this embodiment, multiple fine V-series precipitates with a maximum diameter of 2 to 20 nm are precipitated within the ferrite in the pearlite. When the steel of this embodiment is subjected to a peeling process, these fine V-series precipitates readily cut off the ferrite in the cutting debris generated from the steel surface by the peeling process. Therefore, the cutting debris is easily cut into shorter pieces. As a result, it is possible to suppress the residual cutting debris on the steel surface, the partial peeling of the steel near the root of the cutting debris, or the formation of cracks on the steel surface near the root of the cutting debris. That is, the generation of defects caused by peeling is suppressed, and the surface roughness of the steel surface is suppressed.

[0147] In the ferrite within pearlite, if the number density of V-series precipitates with a maximum diameter of 2–20 nm is less than 3000 precipitates / μm. 3 If the number density of V-series precipitates is insufficient, the surface roughness of the steel surface after peeling cannot be adequately suppressed. In the ferrite within pearlite, if the number density of V-series precipitates with a maximum diameter of 2–20 nm is 3000 precipitates / μm... 3The above results in a sufficiently high number density of V-series precipitates in the ferrite within the pearlite. Therefore, in the process of manufacturing springs from steel blanks, when the steel is peeled, the surface roughness of the steel surface after peeling can be sufficiently suppressed, and the smoothness of the steel surface can be improved. Therefore, in the steel of this embodiment, based on the content of each element in the chemical composition within the range of this embodiment, the number density of V-series precipitates with a maximum diameter of 2-20 nm in the ferrite within the pearlite is 3000 precipitates / μm. 3 The preferred lower limit for the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of pearlite is 3500 precipitates / μm. 3 Further optimized to 4000 / μm 3 Further optimized to 4500 / μm 3 Further preferred is 5000 / μm 3 Further preferred is 5500 / μm 3 Further optimized to 6000 / μm 3 Further optimized to 6500 / μm 3 Further optimized to 7000 / μm 3 Further optimized to 8000 / μm 3 Further optimized to 9000 / μm 3 Further preferred is 10,000 / μm 3 Further optimized to 15,000 per μm 3 .

[0148] It should be noted that the upper limit of the number density of V-series precipitates with a maximum diameter of 2–20 nm is not particularly limited. However, when the content of each element in the chemical composition of the steel is within the range of this embodiment, the upper limit of the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of pearlite is 80,000 precipitates / μm. 3 The preferred upper limit for the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite within pearlite is 75,000 precipitates / μm. 3 Further optimized to 72,000 / μm 3 .

[0149] [Method for determining the number density of V-series precipitates]

[0150] Number density (numbers / μm) of V-series precipitates with a maximum diameter of 2–20 nm in ferrite within pearlite. 3The thickness can be determined by the following method: Cut the steel (wire) along its diameter direction. Collect a circular plate with a cross-section in the diameter direction and a thickness of 0.5 mm in the central axis direction of the steel. Grind the circular plate from both sides using sandpaper to set the thickness of the circular plate to 60 μm. Then, collect a sample with a diameter of 3 mm from the circular plate. Immerse the sample in a 10% perchloric acid-glacial acetic acid solution and perform electrolytic grinding to prepare a thin film sample with a thickness of 100 nm.

[0151] The prepared thin film sample was observed using a transmission electron microscope (TEM). Specifically, observations were conducted at five locations (fields of view) on the surface (observation plane) along the diameter direction of the thin film sample, with a magnification of 200,000x and an accelerating voltage of 200 kV. The field of view was then selected within the ferrite within the pearlite. Each field of view was set to 0.09 μm × 0.09 μm.

[0152] As described above, pearlite can be easily distinguished from other phases based on contrast and morphology. Specifically, in TEM observation, pearlite can be identified as a layered structure, which is a striped pattern of white and black regions. Here, in the layered structure, the white regions are ferrite, and the black regions are cementite. Therefore, by using contrast, the ferrite in pearlite can be easily distinguished from the cementite in pearlite. Therefore, five observation fields were selected within the ferrite of pearlite based on contrast.

[0153] Furthermore, the precipitates can be determined by contrast within each observation field. Thus, from the identified precipitates, those with a maximum diameter of 2–20 nm are determined. Here, the maximum diameter refers to the maximum length of a line segment connecting any two points at the interface between the precipitate and the parent phase, where the entire segment is contained within the precipitate.

[0154] Within the observed field of view, precipitates with a maximum diameter of 2–20 nm are classified as V-based precipitates. Therefore, precipitates with a maximum diameter of 2–20 nm are identified as V-based precipitates. It should be noted that the classification of precipitates with a maximum diameter of 2–20 nm as V-based precipitates can be confirmed using EDS and NBED. Specifically, each precipitate with a maximum diameter of 2–20 nm is irradiated with a beam of light to detect characteristic X-rays, and elemental analysis is performed on the precipitates. Furthermore, for each precipitate with a maximum diameter of 2–20 nm, nanobeam diffraction (NBED) is used to obtain nanobeam diffraction patterns. The obtained nanobeam diffraction patterns are analyzed to determine the crystal structure and lattice constant of the precipitate. If V, or V and Cr, are detected by EDS, and the NBED analysis shows a cubic crystal structure with lattice constants a, b, and c all of 0.4167 nm ± 5%, then the precipitate is classified as a V-based precipitate.

[0155] Figure 1 This is an example of a TEM image of ferrite in pearlite of a thin film sample. Figure 1 In the TEM image, reference numeral 10 represents V-series precipitates.

[0156] Using the above method, the total number of V-series precipitates with a maximum diameter of 2–20 nm was determined in five observation fields. Based on the determined total number of V-series precipitates and the total volume of the five observation fields, the number density (numbers / μm) of V-series precipitates with a maximum diameter of 2–20 nm was calculated. 3 ).

[0157] As described above, the chemical composition of the steel in this embodiment contains elements within the range specified in this embodiment, and the pearlite area ratio in the microstructure is 90% or more. Furthermore, the number density of V-series precipitates with a maximum diameter of 2 to 20 nm in the ferrite within the pearlite is 3,000 to 80,000 precipitates / μm. 3 Therefore, in the spring manufacturing process, the steel of this embodiment can effectively suppress defects such as burrs, peeling, and cracking on the surface of the steel after the peeling process. As a result, the surface roughness of the steel surface after the peeling process can be effectively suppressed, and the smoothness of the steel surface can be significantly improved.

[0158] [Method for manufacturing steel according to this embodiment]

[0159] The following describes an example of a method for manufacturing the steel according to this embodiment. It should be noted that the manufacturing method is not limited to the method described below, as long as the steel of this embodiment possesses the above-described characteristics. However, the manufacturing method described below is a suitable example for manufacturing the steel of this embodiment.

[0160] Figure 2 This is a flowchart illustrating an example of the steel manufacturing process according to this embodiment. (See reference) Figure 2 The steel manufacturing method of this embodiment includes a billet preparation process (S110), a rough rolling process (S120), and a finish rolling process (S130). Each process will be described in detail below.

[0161] [Burnt material preparation process (S110)]

[0162] In the billet preparation step (S110), a billet having the aforementioned chemical composition is manufactured. The billet referred to here is a large billet or an ingot. In the billet preparation step (S110), firstly, molten steel with the content of each element in its chemical composition within the range of this embodiment is manufactured using a known refining method. The manufactured molten steel is then used to manufacture a billet (large billet or ingot). Specifically, the large billet is manufactured using molten steel via continuous casting. Alternatively, an ingot is manufactured using molten steel via ingot casting.

[0163] [Rough rolling process (S120)]

[0164] In the roughing rolling process (S120), the billet is hot-rolled to produce small square billets. Specifically, in the roughing rolling process (S120), the billet is first heated. The billet is heated using a heating furnace or a soaking furnace. The billet is heated to 1200–1300°C in the heating furnace or soaking furnace. For example, the billet is held at a furnace temperature of 1200–1300°C for 1.5–50.0 hours. The heated billet is then removed from the heating furnace or soaking furnace and hot-rolled. The hot rolling in the roughing rolling process (S120) is performed, for example, using a primary rolling mill. The billet is first rolled using a primary rolling mill to produce small square billets. If a continuous rolling mill is provided downstream of the primary rolling mill, the billet after primary rolling can be further hot-rolled using the continuous rolling mill to produce even smaller square billets. In a continuous rolling mill, a rolling mill with a pair of horizontal rolls (horizontal rolling mill) and a rolling mill with a pair of vertical rolls (vertical rolling mill) are arranged alternately in a row. Through the above processes, the billet (large billet or ingot) is manufactured into a small billet in the rough rolling process (S120).

[0165] [Finishing rolling process (S130)]

[0166] In the finishing rolling process (S130), the small square billet is hot-rolled to produce steel (wire). In the finishing rolling process (S130), the small square billet after the rough rolling process (S120) is first heated in a heating furnace.

[0167] [Regarding heating in the finishing rolling process]

[0168] The heating temperature of the furnace in the finishing rolling process is set to 1050℃ or higher. The holding time at a heating temperature of 1050℃ or higher is set to, for example, 0.5 to 5.0 hours.

[0169] In small square billets manufactured by the roughing rolling process (S120), V-series precipitates may sometimes form due to cooling after hot rolling. If the small square billets containing residual V-series precipitates are then subjected to finishing rolling, excessive amounts of coarse V-series precipitates will form in the ferrite within the pearlite of the steel after the finishing rolling process. As a result, the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite within the pearlite will become less than 3000 precipitates / μm.3 .

[0170] If the heating temperature in the finishing rolling process (S130) is above 1050°C, the V-series precipitates that may remain in the small square billet after the rough rolling process (S120) can be fully dissolved. Therefore, under the premise of meeting other manufacturing conditions, the formation of coarse V-series precipitates with a maximum diameter exceeding 20 nm is suppressed, and the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the finishing rolling process becomes 3000 precipitates / μm. 3 above.

[0171] [Regarding finishing rolling]

[0172] Hot rolling (finishing) is performed on heated small square billets using a finishing mill to produce wire rod as steel. The outer diameter of the wire rod is not particularly limited; for example, it can be 5–10 mm. The outer diameter of the steel (wire rod) is determined based on the wire diameter of the spring, the final product. The finishing mill and hot rolling using a finishing mill will be described in detail below.

[0173] [Regarding the finishing mill train]

[0174] A finishing mill line comprises multiple mills arranged in a row from upstream to downstream. Each mill includes multiple rolls arranged around a conveyor line. Dies are formed on the rolls of each mill. Small square billets are hot-rolled by passing them through the dies formed by the multiple rolls of each mill, gradually reducing the cross-section of the billets to produce steel (wire rod).

[0175] The multiple mill groups arranged consecutively from the upstream mill in a finishing mill train are called the "roughing mill train". The multiple mill groups arranged consecutively downstream of the roughing mill train are called the "intermediate mill train". The single or consecutive mill groups arranged downstream of the intermediate mill train are called the "final mill train". In short, for convenience, the finishing mill train is divided into three mill groups from upstream to downstream: the roughing mill train, the intermediate mill train, and the final mill train. The number of mills in the roughing mill train, the intermediate mill train, and the final mill train is not particularly limited. For the sake of explaining the temperature history of the steel in the finishing milling process described later, the finishing mill train is divided into three mill groups (roughing mill train, intermediate mill train, and final mill train). It should be noted that water-cooling devices for cooling the steel are installed between the mills in the roughing mill train, the intermediate mill train, and the final mill train. Water cooling devices, for example, cool the steel leaving the preceding mill and entering the following mill, i.e., the portion of steel between the preceding and following mills, thereby reducing the steel temperature.

[0176] [Temperature history of steel in finishing rolling]

[0177] Figure 3This is a schematic diagram of the temperature history of steel during finishing rolling. (Reference) Figure 3 Interval S131 is the temperature history of the steel from the furnace to the first mill of the roughing mill train in the finishing mill train. Interval S132 is the temperature history of the steel in the roughing mill train. Interval S133 is the temperature history of the steel in the intermediate mill train. Interval S134 is the temperature history of the steel in the finishing mill train. Interval S135 is the temperature history of the steel after it exits the last mill of the finishing mill train. In the finishing rolling process, the temperature of the steel is as follows... Figure 3 The steel of this embodiment is manufactured using the temperature history shown in S131 to S135. Hereinafter, each interval S131 to S135 will be explained.

[0178] First, the typical temperature history of steel (wire rod) during the finishing rolling process will be explained. From the time the steel is drawn from the furnace until it reaches the first mill in the roughing mill train, the steel temperature remains at the furnace's heating temperature. Then, when hot rolling begins in the roughing mill train, the steel loses heat to the mill rolls. This phenomenon is called roll heat dissipation. During rolling in the roughing mill train, the steel temperature decreases over time due to roll heat dissipation. However, when the steel is rolled in stages by multiple mills, and the cumulative reduction rate increases to a certain level, processing exothermics are generated in the steel. In the steel that generates processing exothermics, the steel temperature rises. Therefore, in typical finishing rolling, the steel temperature rises again during rolling in the later mills of the roughing mill train or during rolling in the intermediate mill train. The steel temperature continues to rise in the final mill train as well.

[0179] Therefore, in this embodiment, in order to keep the number density of V-series precipitates with a maximum diameter of 2 to 20 nm in the ferrite of the pearlite of the steel after the finishing rolling process within a suitable range, the temperature history of the steel during finishing rolling is adjusted as follows.

[0180] In the finishing rolling process (S130) of this embodiment, the period during which the surface temperature of the small square billet is continuously maintained at 950–850°C (referred to as the specific temperature dwell time) is set to 5–100 seconds. The specific temperature dwell time affects the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the finishing rolling process (S130). Specifically, if the specific temperature dwell time in the finishing rolling is set to 5–100 seconds, then, under the premise of satisfying other manufacturing conditions, the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the finishing rolling process becomes 3000–80000 precipitates / μm. 3 .

[0181] When the dwell time at a specific temperature is set to 5–100 seconds, under the premise of meeting other manufacturing conditions, the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the finishing rolling process is 3,000–80,000 precipitates / μm. 3 The reason is not yet certain, but it can be considered as follows: If the dwell time at a specific temperature is 5 to 100 seconds, then, under the premise that other manufacturing conditions are met, clusters of V atoms, or V atoms and Cr atoms, will form in the small square billet during finishing rolling. Here, V atoms, or clusters of V atoms and Cr atoms (hereinafter referred to as clusters), refer to the aggregate of atoms formed in the stage prior to the formation of V-based precipitates. If multiple clusters are formed before the formation of V-based precipitates, the phase interface precipitation of V-based precipitates will be promoted during the cooling treatment after finishing rolling (described later). As a result, the V-based precipitates in the ferrite of the pearlite of the steel, with a maximum diameter of 2 to 20 nm, become 3,000 to 80,000 per μm. 3 .

[0182] The specific temperature residence time range (950–850°C) is directly below the solution temperature range of V-based precipitates (approximately 1000–1150°C). Therefore, within the specific temperature residence time range (950–850°C), the driving force for the formation of V-based precipitate nuclei is weaker compared to the solution temperature range, making it difficult to induce the formation of V-based precipitate nuclei. On the other hand, V atoms and / or Cr atoms dissolved in the small billet diffuse sufficiently within the billet. As a result, clusters are formed, serving as a preliminary stage for the formation of V-based precipitate nuclei. By ensuring a specific temperature residence time of 5–100 seconds, multiple clusters can be generated. As a result, the number density of V-based precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the cooling treatment of the finishing rolling process (S130) reaches 3000–80000 per μm. 3 .

[0183] The above mechanism is a hypothetical one. However, the examples described later demonstrate that even if a different mechanism is assumed to be at work, as long as the residence time at a specific temperature is 5 to 100 seconds, the number density of V-series precipitates with a maximum diameter of 2 to 20 nm in the ferrite of the pearlite in the steel after the cooling treatment of the finishing rolling process (S130) will reach 3,000 to 80,000 precipitates / μm. 3 .

[0184] If the dwell time at a specific temperature is less than 5 seconds, insufficient cluster formation occurs. As a result, the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the cooling treatment of the finishing rolling process (S130) becomes less than 3000 precipitates / μm. 3On the other hand, if the residence time at a specific temperature exceeds 100 seconds, precipitates will form from the clusters. Consequently, the V-series precipitates after the finishing rolling process become coarse, with a large number of V-series precipitates having a maximum diameter exceeding 20 nm. As a result, the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the finishing rolling process (S130) becomes less than 3000 precipitates / μm. 3 Therefore, the dwell time at a specific temperature is set to 5–100 seconds.

[0185] The preferred lower limit for the dwell time at a specific temperature is 8 seconds, more preferably 10 seconds, and even more preferably 12 seconds. The preferred upper limit for the dwell time at a specific temperature is 90 seconds, more preferably 80 seconds, even more preferably 70 seconds, and even more preferably 60 seconds.

[0186] refer to Figure 3 During the rolling process in the roughing mill (S132), the heat dissipation generated by the contact between the surface of the roll and the surface of the small square billet (roll heat dissipation) is greater than the heat released during the rolling process. Therefore, during the rolling process in the roughing mill (S132), the surface temperature of the small square billet decreases with each pass through the mill.

[0187] On the other hand, during the rolling process in the intermediate mill (S133), the heat generated during processing tends to be greater than the heat dissipation from the rolls. Therefore, during the rolling process in the intermediate mill (S133), the surface temperature of the small billet tends to rise with each pass through the mill. Therefore, to ensure a specific temperature dwell time of 5 to 100 seconds, the small billet is water-cooled during the rolling process in the intermediate mill to lower its surface temperature. Specifically, a water-cooling device is installed between the mills in the intermediate mill to water-cool the portion of the small billet passing between the mills. Thus, in the finishing mill, the surface temperature of the small billet in the finishing mill is adjusted to ensure a specific temperature dwell time of 5 to 100 seconds.

[0188] It should be noted that, in the above description, the specific temperature dwell time of 5 to 100 seconds is ensured by cooling the small square billet during the rolling process of the intermediate mill. However, the method for adjusting the specific temperature dwell time in finishing rolling is not limited to the above method. The surface temperature of the small square billet during the rolling process of the roughing mill (S132) can be adjusted by water cooling or the like, and the surface temperature of the small square billet during the rolling process of the finishing mill (S134) can also be adjusted by water cooling or the like. In addition, the surface temperature of the small square billet can be adjusted by water cooling or the like in two or more intervals among the rolling processes of the roughing mill (S132), the intermediate mill (S133), and the finishing mill (S134).

[0189] It should be noted that, as mentioned above, the dwell time (specific temperature dwell time) when the surface temperature of the small square billet is 950-850℃ refers to the duration of the state in which the surface temperature of the small square billet is continuously 950-850℃.

[0190] [Regarding final rolling temperature]

[0191] During the rolling process in the final rolling mill (S134), the surface temperature of the small square billet tends to rise due to heat release during processing. Here, the surface temperature of the steel at the exit side of the mill where it is rolled last in the final rolling mill is defined as the "final rolling temperature" (°C). In this embodiment, the final rolling temperature is set to less than 1000°C. If the final rolling temperature is above 1000°C, the austenite grains in the steel will coarsen. In this case, as explained in the cooling process described later, a large number of fine V-series precipitates with a maximum diameter of less than 2 nm will be generated. As a result, the number density of V-series precipitates with a maximum diameter of 2 to 20 nm in the ferrite of the pearlite in the steel after the finishing rolling process (S130) becomes less than 3000 precipitates / μm. 3 .

[0192] For example, such as Figure 3 As shown, during the rolling periods of the roughing mill and intermediate mill (S132 and S133), the specific temperature dwell time can be set to 5 to 100 seconds, and the temperature of the small square billet at the exit side of the last mill of the intermediate mill can be set to below 950°C, while the final rolling temperature during the rolling period of the final mill (S134) can be suppressed to below 1000°C. Alternatively, the surface of the small square billet can be cooled during the rolling period of the final mill (S134), and the final rolling temperature can be set to below 1000°C. It should be noted that the preferred lower limit of the final rolling temperature is 900°C.

[0193] It should be noted that, as Figure 3 As shown, in finishing rolling, the temperature of the steel (the surface temperature of the steel) is maintained at less than 1000°C from the time it has been held at a specific temperature until the end of rolling on the final mill train.

[0194] exist Figure 3 In this context, the entire rolling period of the intermediate mill train is included within the specific temperature dwell time. However, the specific temperature dwell time is not limited to this. A portion of the rolling period of the intermediate mill train can also be equated with the specific temperature dwell time. Furthermore, the specific temperature dwell time is not limited to the rolling period at the intermediate mill train. In short, in finishing rolling, as long as the steel temperature is continuously between 950°C and 850°C for 5 to 100 seconds (specific temperature dwell time) and the final rolling temperature is less than 1000°C, it is acceptable.

[0195] It should be noted that the temperature history of steel in the finishing mill can be measured, for example, by installing thermometers capable of measuring the surface temperature of the steel at the entrance or exit side of each mill in the finishing mill series (roughing mill series, intermediate mill series, and final mill series). The temperature history of the steel in the finishing mill can be determined based on the measurements from these thermometers.

[0196] [Cooling treatment (range S135)]

[0197] The steel after finishing milling (roughing mill, intermediate mill, and final mill) undergoes a cooling treatment (S135). A rapid cooling treatment (RC) is performed in the cooling treatment zone (S135), followed by a slow cooling treatment (SC). Through this cooling treatment, the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the finishing milling process (S130) reaches 3000–80000 precipitates / μm. 3 The following explains the rapid cooling treatment (RC) and the slow cooling treatment (SC).

[0198] [Rapid Cooling Treatment (RC)]

[0199] In the quenching process (RC), steel with a surface temperature of 950–800°C after finishing rolling is quenched. Specifically, in the small square billets after finishing rolling, the average cooling rate at a surface temperature of 950–800°C is set to exceed 1.0°C / second. If the average cooling rate at a surface temperature of 950–800°C is set to less than 1.0°C / second, the austenite grains in the steel will coarsen even if other manufacturing conditions are met. In this case, a large number of V-series precipitates smaller than 2 nm will be generated. As a result, the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the finishing rolling process (S130) becomes less than 3000 precipitates / μm. 3 .

[0200] If the average cooling rate of the steel at a surface temperature of 950–800°C is set to exceed 1.0°C / second, then, under the premise of meeting other manufacturing conditions, the number density of V-series precipitates with a maximum diameter of 2–20 nm in the ferrite of the pearlite in the steel after the finishing rolling process (S130) becomes 3000–80000 precipitates / μm. 3 .

[0201] The reason why suppressing the coarsening of austenite grains in steel can prevent the V-series precipitates in pearlite from becoming too fine is uncertain, but it can be considered to be for the following reasons.

[0202] Figure 4This is a graph showing the continuous cooling phase transformation curve (CCT curve) during the cooling treatment (S135) of the steel in this embodiment. (Reference) Figure 4 , Figure 4 The solid curve in the graph represents the temperature change of the steel relative to the cooling time. Figure 4 The dashed curves Ps (curves Ps1 and Ps2) in the figure represent the phase transformation initiation temperature of pearlite. If the austenite grains in the steel are fine, the curve Ps shifts towards the short-time side (left side of the figure) (equivalent to...). Figure 4 (Curve Ps1). If curve Ps shifts towards the shorter time side, then during the slow cooling (SC) treatment following the rapid cooling (RC) treatment, a pearlitic phase transformation begins at a higher temperature than the pearlite nose PN. If the pearlitic phase transformation begins, V-based precipitates will precipitate at the phase interface within the pearlite ferrite. V-based precipitates are generated at a higher temperature than the pearlite nose PN. Therefore, the generated V precipitates will grow to a certain extent. As a result, the number density of V-based precipitates with a maximum diameter of 2–20 nm in the pearlite ferrite can be set to 3000–80000 precipitates / μm. 3 .

[0203] On the other hand, when the austenite grains in the steel are coarse, the curve Ps shifts towards the longer time side (right side of the figure) (equivalent to...). Figure 4 (See curve Ps2). In this case, the pearlite phase transition begins at a lower temperature range than in curve Ps1. Therefore, although V-type precipitates are formed, they are difficult to grow. As a result, a large number of V-type precipitates with a maximum diameter of less than 2 nm are formed, and the number density of V-type precipitates with a maximum diameter of 2–20 nm in the ferrite of pearlite becomes less than 3000 precipitates / μm. 3 .

[0204] [Slow Cooling Treatment (SC)]

[0205] Rapid cooling (RC) followed by slow cooling (SC) is performed. In SC, the steel is cooled at an average cooling rate of less than 2.00 °C / sec when the surface temperature is between 600 °C and 800 °C. If the average cooling rate is greater than 2.00 °C / sec when the surface temperature is between 600 °C and 800 °C, the pearlite phase transformation will be insufficient, even if other manufacturing conditions are met. As a result, the pearlite area fraction becomes less than 90%. Consequently, due to the insufficient pearlite phase transformation, the precipitation of V-type precipitates at the phase interface is also insufficient. As a result, the number density of V-type precipitates with a maximum diameter of 2–20 nm in the ferrite within the pearlite becomes less than 3000 precipitates / μm. 3 .

[0206] In slow cooling (SC), if the average cooling rate of the steel surface temperature is less than 2.00 °C / s when it is above 600 °C and below 800 °C, the pearlite phase transformation is sufficiently promoted. As a result, the pearlite surface area is above 90%. Furthermore, since the precipitation of V-type precipitates at the phase interface is also sufficiently promoted, the number density of V-type precipitates with a maximum diameter of 2–20 nm in the ferrite of pearlite becomes 3000–80000 precipitates / μm. 3 .

[0207] Through the above manufacturing process, the steel (wire) of this embodiment can be manufactured. It should be noted that the above manufacturing method is only one example for manufacturing the steel of this embodiment. Therefore, the manufacturing method of the steel of this embodiment is not limited to the above method as long as the following steel can be manufactured: the content of each element in the chemical composition of the steel is within the range of this embodiment; the pearlite area ratio in the microstructure is 90% or more; and the number density of V-series precipitates with a maximum diameter of 2-20 nm in the ferrite of the pearlite is 3000-80000 precipitates / μm. 3 .

[0208] [About springs]

[0209] Springs made from the steel used in this embodiment are, for example, springs used in automobiles and general machinery. Springs used in automobiles and general machinery are, for example, damping springs or valve springs.

[0210] [Method for manufacturing a spring using the steel blank of this embodiment]

[0211] A spring made from the steel of this embodiment is manufactured using a known manufacturing method. For example, a spring made from the steel of this embodiment is manufactured using the following method.

[0212] Figure 5 This is a flowchart illustrating an example of a method for manufacturing a spring using the steel described in this embodiment. (See reference) Figure 5 The spring made from the steel used in this embodiment includes a steel wire preparation process (S200) and a spring manufacturing process (S300).

[0213] In the wire preparation process (S200), spring wire is manufactured using the steel of this embodiment. Here, wire refers to steel that has undergone drawing at least once from hot-rolled steel (wire). The wire preparation process (S200) includes: a peeling process (S210), an annealing process (S220), a drawing process (S230), and a quenching and tempering process (S240).

[0214] [Peeling process (S210)]

[0215] In the peeling process (S210), the entire surface (circumferential surface) of the steel is peeled off (peeling treatment). The peeling treatment can be carried out using known methods. In the peeling treatment, the steel (wire) is passed through a peeling die to remove the steel surface (peeling). Through the peeling treatment, defects and decarburized layers on the steel surface are removed.

[0216] As described above, when the steel of this embodiment undergoes a peeling process, the cutting debris generated on the steel surface during the peeling process becomes easier to cut off in shorter lengths. Therefore, it is possible to sufficiently suppress defects caused by peeling, such as burrs, peeling, and cracking, from forming on the surface of the steel after the peeling process. As a result, the surface roughness of the steel surface after the peeling process can be sufficiently suppressed, and the smoothness of the steel surface can be significantly improved.

[0217] [Annealing process (S220)]

[0218] In the annealing process (S220), the steel after the peeling process (S210) is annealed to remove the stress generated in the steel by the peeling process. The annealing process can be carried out using known methods. The annealing temperature is, for example, 300°C or higher.

[0219] [Wire drawing process (S230)]

[0220] In the wire drawing process (S230), the steel after the annealing process (S220) is drawn into wire. By performing the wire drawing process, a steel wire with a desired outer diameter is manufactured. The wire drawing process (S230) can be performed using known methods. Specifically, the steel is lubricated to form a lubricating coating, such as a phosphate coating or a metal soap layer, on the surface of the steel. The lubricated steel is then drawn into wire at room temperature. A wire drawing machine with a known structure is used in the wire drawing process. The wire drawing machine is equipped with dies for drawing the steel into wire.

[0221] [Temperature treatment process (S240)]

[0222] In the quenching and tempering process (S240), the steel wire after the drawing process (S230) undergoes quenching and tempering treatment. The quenching and tempering process (S240) includes a quenching process and a tempering process. In the quenching process, the steel wire is first heated to above the Ac3 phase transformation point. Heating can be performed, for example, using a high-frequency induction heating device. The heated steel wire is then rapidly cooled. The rapid cooling method can be water cooling or oil cooling. Through the quenching process, the microstructure of the steel wire is transformed into a martensitic microstructure.

[0223] The spring manufacturing process (S300) includes: a cold winding process (S310), a stress-relieving annealing process (S320), a nitriding process (S330) to be performed as needed, and a shot peening process (S340).

[0224] [Cold winding process (S310)]

[0225] In the cold winding process (S310), the steel wire manufactured in the wire preparation process (S200) is cold-wound to produce the intermediate steel for the spring. The cold winding is performed using a known winding apparatus. The winding apparatus includes, for example, multiple sets of conveyor rollers, a wire guide, multiple coil forming clamps (winding pins), and a mandrel with a semi-circular cross-section. Each set of conveyor rollers consists of a pair of rollers facing each other. Multiple sets of conveyor rollers are arranged in a row. Each set of conveyor rollers clamps the steel between a pair of rollers and conveys the steel wire along the direction of the wire guide. The steel wire passes through the wire guide. The steel wire drawn from the wire guide is bent into an arc shape by multiple winding pins and the mandrel, forming a coil-shaped intermediate steel.

[0226] [Stress-relief annealing process (S320)]

[0227] The stress-relief annealing process (S320) is mandatory. In the stress-relief annealing process (S320), annealing is performed to remove residual stress in the intermediate steel caused by the cold winding process (S310). The treatment temperature (annealing temperature) during annealing is, for example, set to 400–500°C. The holding time at the annealing temperature is not particularly limited, for example, set to 10–50 minutes. After the holding time, the intermediate steel is allowed to cool naturally or slowly to room temperature.

[0228] [Nitriding treatment process (S330)]

[0229] The nitriding process (S330) is optional, not mandatory. That is, the nitriding process (S330) may or may not be performed. When performed, the nitriding process (S330) involves nitriding the intermediate steel after it has been formed by the cold winding process (S310) and undergone stress-relief annealing (S320). The nitriding process described here also includes soft nitriding. In the nitriding process, nitrogen penetrates the surface of the intermediate steel, forming a nitrided layer (hardened layer) on the surface of the intermediate steel through solid solution strengthening based on dissolved nitrogen and precipitation strengthening based on nitride formation.

[0230] Nitriding treatment can be carried out under known conditions. In nitriding treatment, in A... c1The treatment is performed at a temperature below the phase transition point (nitriding temperature). The nitriding temperature is, for example, 400–550°C. The holding time at the nitriding temperature is 1.0 to 5.0 hours. The furnace atmosphere for nitriding is not particularly limited as long as the chemical potential of nitrogen becomes sufficiently high. The furnace atmosphere for nitriding can be, for example, an atmosphere mixed with a carburizing gas (RX gas, etc.), as in soft nitriding.

[0231] [Shot peening process (S340)]

[0232] The shot peening process (S340) is mandatory. In the shot peening process (S340), the surface of the intermediate steel after the nitriding process (S330) is shot peened. This applies compressive residual stress to the surface layer of the spring, further improving the fatigue limit of the spring. Shot peening can be performed using known methods. For example, a blasting material with a diameter of 0.01 to 1.5 mm is used. The blasting material can be, for example, steel shot or steel balls; any known material can be used. The compressive residual stress applied to the spring is adjusted according to the diameter of the blasting material, the blasting speed, the blasting time, and the amount of blasted per unit area per unit time.

[0233] Through the above manufacturing process, a spring is manufactured using the steel material of this embodiment as the blank.

[0234] Example

[0235] Hereinafter, the effects of the steel of this embodiment will be further described in detail according to the embodiments. The conditions in the following embodiments are examples of conditions adopted to confirm the feasibility and effects of the steel of this embodiment. Therefore, the steel of this embodiment is not limited to these examples of conditions.

[0236] To manufacture molten steel with the chemical composition shown in Table 1.

[0237] [Table 1]

[0238]

[0239] The blank areas in Table 1 indicate that the corresponding element content is less than the detection limit. Large square billets are manufactured using the above-mentioned molten steel via continuous casting. A rough rolling process (S120) is then performed on the large square billets. Specifically, after heating the large square billets, a preliminary rolling process and subsequent rolling on a continuous rolling mill are performed to produce small square billets with a cross-section perpendicular to the length direction of 162mm × 162mm. The heating temperature in the rough rolling process (S120) is 1200–1250℃, and the holding time at the heating temperature is 2.0 hours.

[0240] Using the manufactured small square billets, a finishing rolling process (S130) is performed to produce steel (wire) with a diameter of 6.5 mm. The heating temperature in the finishing rolling process (S130) is the temperature shown in the "Heating Temperature (°C)" column of the "Finishing Rolling Process" section in Table 2. The holding time at the heating temperature for any test number is 1.5 hours. The specific temperature dwell time in the finishing rolling process (continuous dwell time within the range of 950–850°C) is the time shown in the "Specific Temperature Dwell Time (seconds)" column of the "Finishing Rolling Process" section in Table 2. The final rolling temperature (°C) in the finishing rolling process is the temperature shown in the "Final Rolling Temperature (°C)" column of the "Finishing Rolling Process" section in Table 2. It should be noted that in the finishing rolling process, the steel temperature after the specific temperature dwell time for any test number is lower than the final rolling temperature until the end of the finishing rolling process.

[0241] After finishing rolling, the steel undergoes quenching (RC) followed by slow cooling (SC). The average cooling rate during quenching (RC), when the steel surface temperature is between 950 and 800°C, is the average cooling rate (°C / second) shown in the "Average Cooling Rate (°C / second) during Quenching" column of Table 2. The average cooling rate during slow cooling (SC), when the steel surface temperature is between 600°C and 800°C, is the average cooling rate (°C / second) shown in the "Average Cooling Rate (°C / second) during Slow Cooling" column of Table 2.

[0242] [Table 2]

[0243]

[0244] Steel is manufactured through the above manufacturing processes.

[0245] [Evaluation Test]

[0246] Microstructure observation test, V-series precipitate number density test, and surface roughness evaluation test after peeling were carried out on the steels manufactured for each test number.

[0247] [Microscopic Tissue Observation Experiment]

[0248] The pearlite area fraction (%) in the microstructure of steel samples from each test number was determined using the following method. The cross-section (surface) cut along the diameter of each steel sample was used as the observation surface. The observation surface was mirror-polished. The mirror-polished observation surface was then etched using 5% picric acid alcohol (picric acid etching solution). The observation field was defined as a location on the etched observation surface at a radial distance of 1 / 4 of the diameter from the steel surface (outer periphery of the observation surface). Ten observation fields were observed using a 2000x scanning electron microscope (SEM), and photographic images of the ten fields were generated. The size of each field was set to 40 μm × 60 μm.

[0249] Pearlite is determined based on contrast and phase morphology. The total area (μm) of pearlite in each field of view is calculated. 2 The total area of ​​pearlites in the entire field of view is compared to the total area of ​​the entire field of view (24000 μm). 2 The proportion of pearlite area (%) is used as the pearlite area ratio. The calculated pearlite area ratio is shown in the "Pearlite Area Ratio (%)" column of Table 2.

[0250] [Number density determination test of V-series precipitates]

[0251] The number density (numbers / μm) of V-series precipitates with a maximum diameter of 2–20 nm in steel samples from each test number was determined using the following method. 3 The steel (wire) for each test number was cut along the wire diameter. Next, a circular plate with a cross-section along the wire diameter and a thickness of 0.5 mm along the central axis of the steel was collected. Using sandpaper, the circular plate was ground from both sides to a thickness of 60 μm. Then, a sample with a diameter of 3 mm was collected from the circular plate. The sample was immersed in a 10% perchloric acid-glacial acetic acid solution and subjected to electrolytic grinding to prepare a thin film sample with a thickness of 100 nm.

[0252] The prepared thin film sample was observed using a transmission electron microscope (TEM). Specifically, observations were conducted at five locations (fields of view) on the surface (observation plane) along the diameter direction of the thin film sample, with a magnification of 200,000x and an accelerating voltage of 200 kV. The field of view was then selected within the ferrite within the pearlite. Each field of view was set to 0.09 μm × 0.09 μm.

[0253] In each field of view, the precipitates are determined based on contrast. Then, from the determined precipitates, those with a maximum diameter of 2–20 nm are selected. Here, the maximum diameter is defined as the longest line segment length when any two points on the interface between the precipitate and the parent phase are selected, and the entire line segment connecting these two points is contained within the precipitate.

[0254] Precipitates with a maximum diameter of 2–20 nm were identified as V-series precipitates. It should be noted that, based on the results confirmed by the aforementioned EDS and NBED, precipitates with a maximum diameter of 2–20 nm are V-series precipitates.

[0255] Using the methods described above, the total number of V-series precipitates with a maximum diameter of 2–20 nm was determined across five observation fields. Based on the total number of V-series precipitates and the total volume of the five observation fields, the number density (numbers / μm) of V-series precipitates with a maximum diameter of 2–20 nm was calculated. 3 The number densities of the V-series precipitates are shown in Table 2, "Number Density of V-series precipitates (numbers / μm)". 3 As shown in the column “)”.

[0256] [Surface roughness evaluation test after peeling]

[0257] A peeling process was performed on the steel samples for each test number. In the peeling process, a peeling die was used to peel the steel surface to a depth of 0.15 mm. The surface roughness of the peeled steel (hereinafter referred to as the test piece) was measured. Specifically, the ten-point average roughness Rz specified in JIS B 0601 (2013) was calculated. The evaluation length was set to 5 times the reference length (cutoff wavelength). The ten-point average roughness Rz was measured using a stylus-type roughness gauge. The measurement speed was set to 0.5 mm / sec. The measurement results are shown in the "Surface Roughness (μm)" column of Table 2. If the ten-point average roughness Rz is 5.0 μm or less, it is judged that the surface roughness of the peeled steel surface has been sufficiently suppressed (indicated by "E" in the "Evaluation" column of "Surface Roughness Suppression Evaluation" in Table 2). On the other hand, if the ten-point average roughness Rz exceeds 5.0 μm, it is judged that the surface roughness of the steel surface after peeling is not sufficiently suppressed (indicated by "NA" in the "Evaluation" column of "Surface Roughness Suppression Evaluation" in Table 2).

[0258] [Experimental Results]

[0259] The experimental results are shown in Table 2. Referring to Table 2, the chemical compositions and manufacturing processes of experiments 1-15 are suitable. Therefore, the pearlite area ratio in the microstructure of the steel in each experiment is above 90%. Furthermore, the number density of V-series precipitates with a maximum diameter of 2-20 nm is 3000-80000 precipitates / μm. 3 Therefore, even when the steel samples numbered 1 to 15 were subjected to a peeling process, the surface roughness of the steel was below 5.0 μm, which effectively suppressed the surface roughness after peeling.

[0260] In experiments 16 and 22, the heating temperature during finishing rolling was too low. Therefore, the number density of V-series precipitates with a maximum diameter of 2–20 nm was less than 3000 precipitates / μm. 3 As a result, the surface roughness of the steel exceeded 5.0 μm, which could not adequately suppress the surface roughness after the peeling process.

[0261] In experiments 17 and 23, the residence time at specific temperatures was too long. Therefore, the number density of V-series precipitates with a maximum diameter of 2–20 nm was less than 3000 precipitates / μm. 3 As a result, the surface roughness (ten-point average roughness Rz) of the steel exceeded 5.0 μm, which could not sufficiently suppress the surface roughness after the peeling process.

[0262] In experiments 18 and 24, the residence time at the specific temperature was too short. Therefore, the number density of V-series precipitates with a maximum diameter of 2–20 nm was less than 3000 precipitates / μm. 3 As a result, the surface roughness of the steel exceeded 5.0 μm, which could not adequately suppress the surface roughness after the peeling process.

[0263] In experiments 19 and 25, the final rolling temperature was too high. Therefore, the number density of V-series precipitates with a maximum diameter of 2–20 nm was less than 3000 precipitates / μm. 3 As a result, the surface roughness of the steel exceeded 5.0 μm, which could not adequately suppress the surface roughness after the peeling process.

[0264] In experiments 20 and 26, the average cooling rate at 950–800 °C during the quenching (RC) treatment was too slow. Therefore, the number density of V-series precipitates with a maximum diameter of 2–20 nm was less than 3000 precipitates / μm. 3 As a result, the surface roughness of the steel exceeded 5.0 μm, which could not adequately suppress the surface roughness after the peeling process.

[0265] In experiments 21 and 27, the average cooling rate during the slow cooling treatment (SC) at temperatures below 800°C and above 600°C was too rapid. Consequently, the pearlite area fraction was less than 90%. Furthermore, the number density of V-series precipitates with a maximum diameter of 2–20 nm was less than 3000 precipitates / μm. 3 As a result, the surface roughness of the steel exceeded 5.0 μm, which could not adequately suppress the surface roughness after the peeling process.

[0266] The embodiments of this disclosure have been described above. However, the above embodiments are merely illustrative examples for implementing this disclosure. Therefore, this disclosure is not limited to the above embodiments, and appropriate modifications can be made to the above embodiments without departing from its spirit.

Claims

1. A type of steel, Its chemical composition, expressed in % by mass, contains C：0.50～0.80％、 Si: 1.20–2.90% Mn: 0.25~1.00% Cr：0.40～1.90％、 V：0.05～0.60％、 P: below 0.020% S: Below 0.020% N: below 0.0100% Mo: 0–0.50% Nb: 0~0.050% W：0～0.60％、 Ni: 0-0.50% Co: 0-0.30% B：0～0.0050％、 Cu: 0–0.050% Al: 0–0.0050%, and Ti: 0~0.050%, The balance consists of Fe and impurities. In the microstructure of the steel, the pearlite area ratio is over 90%. In the ferrite within the pearlite. The number density of V-series precipitates with a maximum diameter of 2–20 nm is 3500–80000 per μm. 3 .

2. The steel according to claim 1, wherein, The chemical composition contains selected... Mo: 0.01–0.50% Nb: 0.001~0.050% W：0.01～0.60％、 Ni: 0.01~0.50% One or more of the following groups: Co: 0.01–0.30% and B: 0.0001–0.0050%.

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