Steel component and method for manufacturing same

Abstract

A steel component that has excellent wear resistance and surface characteristics, and for which slab cracking does not occur during continuous casting. The steel component has a defined chemical composition. A number density of cementite particles having a particle size of 0.090 µm or more is 200,000 / mm2 or more, and a Cr amount in the cementite particles having a particle size of 0.090 µm or more is 2.5 mass% or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to steel components, and more particularly to a steel component having excellent wear resistance and surface characteristics. Further, the present disclosure relates to a method of producing the steel component.

[0002] Carbon steel, a steel containing a high concentration of carbon, has high hardness and is therefore widely used as a material for textile machinery components, bearing components, machine blades, and other steel components that require wear resistance.

[0003] In typical steel component production, a cold-rolled steel sheet as a raw material is worked into a component shape, followed by quenching treatment and tempering treatment. While quenching treatment increases hardness and decreases toughness, subsequent tempering treatment may improve toughness. However, tempering decreases the hardness, which results in a decrease in wear resistance.

[0004] Therefore, various techniques have been proposed to achieve excellent wear resistance even when the hardness of a steel component is decreased.

[0005] For example, in Patent Literature (PTL) 1, a steel sheet is proposed that contains 0.10 mass% to 0.40 mass% of C and has a ferrite-cementite microstructure. In PTL 1, formability and wear resistance are improved by increasing ferrite grain size in the steel sheet, spheroidizing carbides (mainly cementite) to an appropriate particle size, and decreasing pearlitic microstructure.

[0006] Further, in PTL 2, formation of a pearlitic microstructure having an interlamellar spacing of 0.3 µm or less is proposed, by annealing a cold-rolled steel sheet containing 0.40 mass% to 0.90 mass% of C under specific conditions.

[0007] In PTL 3, precipitation of coarse Nb-Ti carbides having a circle equivalent diameter of 0.5 µm or more is proposed, in the ferrite phase, which is the matrix microstructure, in a steel sheet containing 0.60 mass% to 1.25 mass% of C.

[0008] In PTL 4, precipitation of coarse carbides having a particle size of 2 µm or more is proposed, in the matrix microstructure of steel containing 0.6 mass% to 1.0 mass% of C.

[0009] Further, adding alloying elements to steels having high carbon concentrations decreases the toughness of the steel. As a result, cracks are more likely to occur when the slab is cooled during the production of the steel slab, so-called "delayed cracking". When delayed cracking occurs, the slab may fracture during transportation, making it impossible to use the slab for hot rolling. Even when the slab does not fracture, cracks may open during hot rolling, causing the hot-rolled steel sheet to fracture. Further, when the cracks are small, surface defects such as scabs and slivers occur on the steel sheet or steel component after cold rolling.

[0010] Cracks on a steel slab surface are normally removed with a grinder. However, the toughness of steel slabs is decreased due to high alloying, and therefore stress generated when cracks are removed with a grinder may instead cause the cracks to propagate. In such a case, completely removing the cracks is difficult. On the other hand, small cracks may be overlooked and appear as surface defects on steel sheets or steel components after cold rolling. For these reasons, suppressing delayed cracking in steel slabs is necessary.

[0011] Accordingly, various techniques have been proposed to prevent the occurrence of delayed cracking in high alloy steel slabs.

[0012] For example, in PTL 5 is a proposal that a steel slab containing 0.16 % to 0.35 % of C is cast, cut, and then three or more cut steel slabs are stacked in a defined shape and cooled. By cooling using the above method, the steel slab is slowly cooled in a temperature range where austenite transforms to ferrite (700 °C to 500 °C), and a bainite / martensite transformation is suppressed. As a result, stress caused by transformation expansion is decreased, and delayed cracking of the slab can be prevented.

[0013] Further, in PTL 6 is a proposal that a steel slab containing 0.020 mass% to 0.600 mass% of C is produced by continuous casting, and then gradually cooled at an average cooling rate of 20 °C / h or less in a temperature range from 700 °C to 500 °C. This helps prevent delayed cracking due to stress during cooling. Further, occurrence of quality defects such as scabs during hot rolling can also be prevented.CITATION LISTPatent Literature

[0014] PTL 1: WO 2016 / 204288 A1 PTL 2: JP 2020-132953 A PTL 3: JP 2017-190494 A PTL 4: JP 2010-138453 A PTL 5: JP 2020-139209 A PTL 6: JP 2019-167560 A SUMMARY(Technical Problem)

[0015] According to conventional technologies, such as those proposed in PTL 1 to 4, there is some improvement in the hardness and wear resistance of steel. However, the inventors have found that steel components produced from conventional steel material may not have sufficient wear resistance in actual use. Further, regarding PTL 3 and PTL 4, there have been cases where delayed cracking has occurred in the slab, and surface defects such as scabs have occurred in steel components due to the delayed cracking.

[0016] Further, according to PTL 5 and PTL 6, it is stated that by gradually cooling the cast slab in the temperature range of 700 °C to 500 °C, delayed cracking of the slab can be prevented. However, it has been found that even this method cannot sufficiently prevent the occurrence of delayed cracking in the slab when the C content is 0.80 % or more.

[0017] The present disclosure is made in view of the above circumstances, and it would be helpful to provide a steel component that has excellent wear resistance and surface characteristics, and for which slab cracking does not occur during continuous casting.(Solution to Problem)

[0018] As a result of studies, the inventors arrived at the following discoveries. (1) Wear resistance can be improved by appropriately controlling a number density of cementite in steel components and a Cr amount in the cementite. (2) By appropriately controlling the chemical composition of the steel slab used and production conditions in continuous casting, it is possible to suppress delayed cracking in the steel slab and to suppress surface defects in steel components. (3) By appropriately controlling the chemical composition of the steel slab used and production conditions of processes such as hot rolling, annealing, cold rolling, quenching, and tempering, it is possible to appropriately control a number density of cementite particles in steel components and a Cr amount present in the cementite particles.

[0019] The present disclosure is based on the discoveries described above, and primary features of the present disclosure are as described below. 1. A steel component comprising a chemical composition containing (consisting of), in mass%, C: 0.80 % to 1.25 %, Si: 0.10 % to 1.0 %, Mn: 0.20 % to 2.5 %, P: 0.0005 % to 0.05 %, S: 0.03 % or less, Al: 0.001 % to 0.1 %, N: 0.001 % to 0.01 %, O: 0.0100 % or less, Cr: 0.56 % to 1.6 %, and at least one selected from the group consisting of Nb: 0.029 % to 0.21 %, Ti: 0.01 % to 0.21 %, and V: 0.01 % to 0.21 %, with the balance being Fe and inevitable impurity, and the total content of Nb, Ti, and V being 0.21 mass% or less, wherein a number density of cementite particles having a particle size of 0.090 µm or more is 200,000 / mm 2< or more, and a Cr amount in the cementite particles having a particle size of 0.090 µm or more is 2.5 mass% or more. 2. The steel component according to 1, above, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of: Ta: 0.10 % or less, W: 0.10 % or less, B: 0.0100 % or less, Mo: 1.00 % or less, Co: 1.00 % or less, Ni: 1.00 % or less, Cu: 1.00 % or less, Sn: 0.200 % or less, Sb: 0.200 % or less, Ca: 0.0100 % or less, Mg: 0.0100 % or less, REM: 0.0100 % or less, Zr: 0.100 % or less, Te: 0.100 % or less, Hf: 0.10 % or less, and Bi: 0.200 % or less. 3. The steel component according to 1 or 2, above, wherein the steel component is any one of a component for textile machinery, a bearing component, or a blade for machinery. 4. A method of producing a steel component, the method comprising: cooling a steel slab produced by continuous casting and having the chemical composition according to 1 or 2, above, under conditions such that, at a transverse direction center of the steel slab and 10 mm from a surface, a residence time in a temperature range from 1200 °C to 1400 °C is 130 s or shorter, and an average cooling rate in a temperature range from 550 °C to 700 °C is 10 °C / h or less; heating the steel slab after the cooling at a slab heating temperature of 1100 °C or higher for a slab heating time of 60 min or longer; hot rolling the heated steel slab under conditions including a rolling finish temperature exceeding Tc, as defined by the following expression (1), and 950 °C or lower, to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet under conditions including an average cooling rate of 20 °C / s or more and a cooling stop temperature of Tc or lower; coiling the cooled hot-rolled steel sheet at a coiling temperature of 530 °C or higher and Tc or lower; subjecting the hot-rolled steel sheet after coiling to first annealing once or more, under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer; subjecting the hot-rolled steel sheet after the first annealing to cold rolling at a rolling ratio of 15 % or more and second annealing at an annealing temperature of 600 °C or higher and Tc or lower for an annealing time of 3 h or longer, twice or more, to obtain a cold-rolled steel sheet; working the cold-rolled steel sheet into a component shape; quenching under conditions including a quenching temperature of Tc or higher and 1000 °C or lower and a holding time of 1.0 min or longer and 60 min or shorter, and then carrying out a heat treatment including tempering, where the element symbols in expression (1) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0. (Advantageous Effect)

[0020] According to the present disclosure, it is possible to provide a steel component that has excellent wear resistance and surface characteristics, and for which delayed cracking of a slab does not occur during continuous casting. The steel component of the present disclosure can be suitably used for a variety of applications including textile machinery components, bearing components, and machine blades.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the accompanying drawings: FIG. 1 is a schematic diagram illustrating shape of a wear test piece; FIG. 2 is a schematic diagram of a wear test apparatus; and FIG. 3 is a schematic diagram illustrating wear depth of a test piece after a wear test. DETAILED DESCRIPTION

[0022] A detailed description is provided below. The present disclosure is not limited to the following embodiments.[Chemical composition]

[0023] The cold-rolled steel sheet according to the present disclosure has the chemical composition described above. The reasons for the above limitations are described below. Hereinafter, "%" as a unit of content indicates "mass%" unless otherwise specified.C: 0.80 % to 1.25 %

[0024] Carbon is an essential element for various steel components, including textile machinery components, bearing components, machine blades, and household blades, in order to improve hardness after quenching. Further, C is an element necessary for producing cementite. When C content is less than 0.80%, the required number density of cementite particles cannot be obtained. The C content is therefore 0.80 % or more. The C content is preferably 0.85 % or more. The C content is more preferably 0.90 % or more. On the other hand, when the C content exceeds 1.25 %, surface scale becomes firm during slab heating, resulting in degradation of surface characteristics. Further, when the C content exceeds 1.25 %, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The C content is therefore 1.25 % or less. The C content is preferably 1.20 % or less. The C content is more preferably 1.10 % or less.Si: 0.10 % to 1.0 %

[0025] Si is an element having an effect of increasing strength of the cold-rolled steel sheet by solid solution strengthening. To obtain the above effect, Si content is 0.10 % or more. The Si content is preferably 0.15 % or more. The Si content is more preferably 0.20 % or more. On the other hand, an excess of Si degrades surface characteristics as a result of surface scale becoming firm during heating. Further, when the Si content exceeds 1.0 %, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Si content is therefore 1.0 % or less. The Si content is preferably 0.80 % or less. The Si content is more preferably 0.55 % or less.Mn: 0.20 % to 2.5 %

[0026] Mn is an element that has an effect of improving the hardness of the steel component by promoting quenching. Further, Mn delays the dissolution of cementite during quenching, thereby increasing the number density of cementite particles in the steel component. To obtain these effects, Mn content is 0.20 % or more. The Mn content is preferably 0.30 % or more. The Mn content is more preferably 0.40 % or more. The Mn content is even more preferably 0.50 % or more. On the other hand, when the Mn content exceeds 2.5 %, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Mn content is therefore 2.5 % or less. The Mn content is preferably 2.0 % or less. The Mn content is more preferably 1.8 % or less. The Mn content is even more preferably 1.5 % or less.P: 0.0005 % to 0.05 %

[0027] The addition of a trace amount of P has a strength improving effect on the steel component due to solid solution strengthening. To achieve this effect, P content is 0.0005 % or more. The P content is preferably 0.001 % or more. On the other hand, when the P content exceeds 0.05 %, toughness of the slab is decreased due to grain boundary embrittlement. As a result, cracks may occur during casting and cooling of the slab and during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The P content is therefore 0.05 % or less. The P content is preferably 0.04 % or less.S: 0.03 % or less

[0028] S is an element that causes grain boundary embrittlement during slab casting. When S is excessive, cracks may occur during casting of the slab, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. S content is therefore 0.03 % or less. The S content is preferably 0.02 % or less. On the other hand, from the viewpoint of cracking during casting of the slab, the lower the S content, the better, and therefore there is no particular lower limit for the S content. However, excessively reducing the S content causes an increase in production costs. Therefore, from a production cost viewpoint, the S content is preferably 0.0001 % or more. The S content is more preferably 0.0005 % or more.Al: 0.001 % to 0.1 %

[0029] Al is an element necessary for deoxidation during steelmaking. Al content is therefore 0.001 % or more. On the other hand, when Al is excessive, nitrides and oxides are formed, which decreases the toughness of the slab. As a result, cracks may occur during casting and cooling of the slab and during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Al content is therefore 0.1 % or less. The Al content is preferably 0.08 % or less.N: 0.001 % to 0.01 %

[0030] N is an element that refines grain size by forming fine nitrides, thereby improving the strength of the steel component. N content is therefore 0.001 % or more. On the other hand, when N is excessive, N combines with Al to form nitrides, which decreases the toughness of the steel slab, and this may cause cracks during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The N content is therefore 0.01 % or less. The N content is preferably 0.008 % or less.O: 0.0100 % or less

[0031] O is present as an oxide and is an element that causes embrittlement of the slab. O decreases the toughness of the slab, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. O content is therefore 0.0100 % or less. The O content is preferably 0.0050 % or less. A lower limit of the O content is not particularly limited, and may be 0 %. However, in view of production technology constraints, the O content is preferably 0.0001 % or more.Cr: 0.56 % to 1.6 %

[0032] Cr is an element that dissolves in cementite, and improves the hardness of cementite and the wear resistance of the steel component. Further, Cr delays the dissolution of cementite during quenching, thereby improving the number density of cementite in the steel component. To obtain the above effects, Cr content is 0.56 % or more. The Cr content is preferably 0.60 % or more. The Cr content is more preferably 0.70 % or more. On the other hand, when the Cr content exceeds 1.6 %, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Cr content is therefore 1.6 % or less. The Cr content is preferably 1.5 % or less. The Cr content is more preferably 1.3 % or less.

[0033] The chemical composition described above contains at least one element selected from the group consisting of Nb, Ti, and V. In order to obtain a desired wear resistance, it is necessary to contain at least one of Nb, Ti, or V in the following amounts.Nb: 0.029 % to 0.21 %

[0034] Nb is an element that forms carbides and can improve wear resistance. When Nb is contained, in order to obtain these effects, Nb content is 0.029 % or more. On the other hand, when Nb is excessive, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Nb content is therefore 0.21 % or less. The Nb content is preferably 0.15 % or less. The Nb content is more preferably 0.10 % or less.Ti: 0.01 % to 0.21 %

[0035] Ti is an element that forms carbides and can improve wear resistance. When Ti is contained, in order to obtain these effects, Ti content is 0.01 % or more. On the other hand, when Ti is excessive, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Ti content is therefore 0.21 % or less. The Ti content is preferably 0.15 % or less. The Ti content is more preferably 0.10 % or less.V: 0.01 % to 0.21 %

[0036] V is an element that forms carbides and can improve wear resistance. When V is contained, in order to obtain these effects, V content is 0.01 % or more. On the other hand, when V is excessive, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The V content is therefore 0.21 % or less. The V content is preferably 0.15 % or less. The V content is more preferably 0.10 % or less.Nb + Ti + V: 0.21 % or less

[0037] When the total content of Nb, Ti, and V in the chemical composition exceeds 0.21 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. Therefore, the total content of Nb, Ti, and V is 0.21 % or less. On the other hand, a lower limit of the total content is not particularly limited, but 0.01 % is a practical lower limit. The lower limit corresponds to a case where only either Ti or V is contained in an amount of 0.01 %. The total content may be 0.020 % or more. The total content may be 0.025 % or more. The total content may be 0.030 % or more.

[0038] The steel component according to an embodiment of the present disclosure has a chemical composition consisting of the above components, with the balance being Fe and inevitable impurity. The inevitable impurity includes H.

[0039] Further, the chemical composition of the steel component according to another embodiment of the present disclosure may optionally further contain at least one of the following elements.Ta: 0.10 % or less

[0040] Ta has an effect of further improving the wear resistance of the steel component by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when Ta content exceeds 0.10 %, a large amount of coarse precipitates and inclusions are formed, and the toughness of the steel slab decreases. The Ta content is therefore 0.10 % or less. The Ta content is preferably 0.08 % or less. On the other hand, a lower limit of the Ta content is not particularly limited. However, from the viewpoint of enhancing the effect of Ta addition, the Ta content is preferably 0.01 % or more.W: 0.10 % or less

[0041] W has an effect of further improving the wear resistance of the steel component by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when W content exceeds 0.10 %, a large amount of coarse precipitates and inclusions are formed, and the toughness of the steel slab decreases. The W content is therefore 0.10 % or less. The W content is preferably 0.08 % or less. On the other hand, a lower limit of the W content is not particularly limited. However, from the viewpoint of enhancing the effect of W addition, the W content is preferably 0.01 % or more.B: 0.0100 % or less

[0042] B segregates at austenite grain boundaries during hot rolling or annealing, and has an effect of further improving hardenability. However, when B content exceeds 0.0100 %, the toughness of the steel slab decreases. The B content is therefore 0.0100 % or less. The B content is more preferably 0.0080 % or less. On the other hand, a lower limit of the B content is not particularly limited. From the viewpoint of enhancing the effect of B addition, the B content is preferably 0.0003 % or more.Mo: 1.00 % or less

[0043] Mo is an element that has an effect of further improving hardenability. However, when Mo content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Mo content is therefore 1.00 % or less. The Mo content is preferably 0.80 % or less. On the other hand, a lower limit of the Mo content is not particularly limited. However, from the viewpoint of enhancing the effect of Mo addition, the Mo content is preferably 0.01 % or more.Co: 1.00 % or less

[0044] Co is an element that has an effect of further improving hardenability. However, when Co content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Co content is therefore 1.00 % or less. The Co content is preferably 0.80 % or less. On the other hand, a lower limit of the Co content is not particularly limited. From the viewpoint of enhancing the effect of Co addition, the Co content is preferably 0.001 % or more.Ni: 1.00 % or less

[0045] Ni is an element that has an effect of further improving hardenability. However, when Ni content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Ni content is therefore 1.00 % or less. The Ni content is preferably 0.80 % or less. On the other hand, a lower limit of the Ni content is not particularly limited. However, from the viewpoint of enhancing the effect of Ni addition, the Ni content is preferably 0.01 % or more.Cu: 1.00 % or less

[0046] Cu is an element that has an effect of further improving hardenability. However, when Cu content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Cu content is therefore 1.00 % or less. The Cu content is preferably 0.80 % or less. On the other hand, a lower limit of the Cu content is not particularly limited. However, from the viewpoint of enhancing the effect of Cu addition, the Cu content is preferably 0.01 % or more.Sn: 0.200 % or less

[0047] Sn is an element that has an effect of further improving hardenability. However, when Sn content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Sn content is therefore 0.200 % or less. The Sn content is preferably 0.100 % or less. On the other hand, a lower limit of the Sn content is not particularly limited. However, from the viewpoint of enhancing the effect of Sn addition, the Sn content is preferably 0.001 % or more.Sb: 0.200 % or less

[0048] Sb is an element that suppresses decarburization and enables strength adjustment of the steel sheet. However, when Sb content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Sb content is therefore 0.200 % or less. The Sb content is preferably 0.100 % or less. On the other hand, a lower limit of the Sb content is not particularly limited. However, from the viewpoint of enhancing the effect of Sb addition, the Sb content is preferably 0.001 % or more.Ca: 0.0100 % or less

[0049] Ca is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Ca content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Ca content is therefore 0.0100 % or less. The Ca content is preferably 0.0050 % or less. On the other hand, a lower limit of the Ca content is not particularly limited. However, from the viewpoint of enhancing the effect of Ca addition, the Ca content is preferably 0.0005 % or more.Mg: 0.0100 % or less

[0050] Mg is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Mg content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Mg content is therefore 0.0100 % or less. The Mg content is preferably 0.0050 % or less. On the other hand, a lower limit of the Mg content is not particularly limited. However, from the viewpoint of enhancing the effect of Mg addition, the Mg content is preferably 0.0005 % or more.REM: 0.0100 % or less

[0051] REM (rare earth metals) are elements that have an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when REM content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The REM content is therefore 0.0100 % or less. The REM content is preferably 0.0050 % or less. On the other hand, a lower limit of the REM content is not particularly limited. However, from the viewpoint of enhancing the effect of REM addition, the REM content is preferably 0.0005 % or more.Zr: 0.100 % or less

[0052] Zr is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Zr content exceeds 0.100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Zr content is therefore 0.100 % or less. The Zr content is preferably 0.080 % or less. On the other hand, a lower limit of the Zr content is not particularly limited. However, from the viewpoint of enhancing the effect of Zr addition, the Zr content is preferably 0.001 % or more.Te: 0.100 % or less

[0053] Te is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Te content exceeds 0.100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Te content is therefore 0.100 % or less. The Te content is preferably 0.080 % or less. On the other hand, a lower limit of the Te content is not particularly limited. However, from the viewpoint of enhancing the effect of Te addition, the Te content is preferably 0.001 % or more.Hf: 0.10 % or less

[0054] Hf is an element that has an effect of spheroidizing the shape of nitrides and sulfides and improving ultimate deformability of the steel sheet. However, when Hf content exceeds 0.10 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. Th Hf content is therefore 0.10 % or less. The Hf content is preferably 0.08 % or less. On the other hand, a lower limit of the Hf content is not particularly limited. However, from the viewpoint of enhancing the effect of Hf addition, the Hf content is preferably 0.01 % or more.Bi: 0.200 % or less

[0055] Bi is an element that mitigates segregation. However, when Bi content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Bi content is therefore 0.200 % or less. The Bi content is preferably 0.100 % or less. On the other hand, a lower limit of the Bi content is not particularly limited. However, from the viewpoint of enhancing the effect of Bi addition, the Bi content is preferably 0.001 % or more.

[0056] The elements from Ta to Bi have effects of further improving the properties of the cold-rolled steel sheet of the present disclosure, and can be contained in the steel sheet of the present disclosure. In the above description, the lower limit values of the preferred content of these optionally-contained elements are indicated. However, when the content of these elements is lower than the preferred lower limit, the effect of adding the elements is decreased, but the properties of the cold-rolled steel sheet still satisfy requirements. Accordingly, containing these elements is not essential, and the lower limit of content may be 0 %.[Cementite]

[0057] The following describes cementite contained in the steel component of the present disclosure.Number density of cementite particles: 200,000 / mm 2< or more

[0058] According to the present disclosure, the wear resistance can be improved by controlling the number density of cementite particles. In order to obtain the desired wear resistance, it is necessary to control the number density of cementite particles having a particle size of 0.090 µm or more to 200,000 / mm 2< or more. Therefore, the number density of cementite particles having a particle size of 0.090 µm or more contained in the steel component is 200,000 / mm 2< or more. The number density is preferably 250,000 / mm 2< or more. The number density is more preferably 300,000 / mm 2< or more. The number density is even more preferably 400,000 / mm 2< or more. The number density is most preferably 500,000 / mm 2< or more. On the other hand, an upper limit of the number density is not particularly limited, but may be, for example, 2,000,000 / mm 2< or less, 1,500,000 / mm 2< or less, or 1,200,000 / mm 2< or less. Hereinafter, the "number density of cementite particles having a particle size of 0.090 µm or more" may be referred to simply as the "number density of cementite" or the "number density".Cr amount in cementite particles: 2.5 % or more

[0059] When Cr is dissolved in cementite, the hardness of the cementite increases, and the harder the cementite, the higher the resistance to wear and the more wear resistance improves. However, when the Cr amount in the cementite particles is less than 2.5 %, the desired wear resistance cannot be obtained. The Cr amount in the cementite particles is therefore 2.5 % or more. The Cr amount in the cementite particles is preferably 2.7 % or more. The Cr amount in the cementite particles is more preferably 3.0 % or more. The Cr amount in the cementite particles is even more preferably 4.0 % or more. On the other hand, an upper limit of the Cr amount in the cementite particles is not particularly limited. The Cr amount in the cementite particles is preferably 10 % or less.[Method of producing steel component]

[0060] The following describes a method of producing the steel component according to an embodiment. In the following explanation, unless otherwise specified, temperature indicated in "°C" indicates a surface temperature (a temperature at a surface of the steel slab, the steel sheet, or the like).

[0061] The steel component can be produced by sequentially carrying out the following processes on a steel slab having the above-mentioned chemical composition and produced by continuous casting. (1) Cooling (2) Heating (3) Hot rolling (4) Cooling (5) Coiling (6) First annealing (7) Cold rolling (8) Second annealing (9) Working and heat treatment (1) Cooling

[0062] First, a steel slab having the above-mentioned chemical composition and produced by continuous casting is cooled.Residence time in temperature range from 1200 °C to 1400 °C: 130 s or shorter

[0063] During the cooling, the residence time in the temperature range from 1200 °C to 1400 °C at a transverse direction center position of the steel slab 10 mm from a surface is 130 s or shorter. The reasons for this are described below.

[0064] Prior austenite grain size is a factor that determines a fracture unit, and the larger the prior austenite grain size, the lower the toughness. The austenite grain size of the steel slab is determined by the residence time in the temperature range from 1200 °C to 1400 °C, and the longer the residence time in this temperature range, the coarser the prior austenite grain size becomes. When the residence time exceeds 130 s, the prior austenite grain size becomes coarse, which may cause slab cracking. Therefore, according to the present disclosure, the residence time in the temperature range from 1200 to 1400 °C is 130 s or shorter. The residence time is preferably 120 s or shorter. The residence time is more preferably 110 s or shorter. The residence time is even more preferably 100 s or shorter.

[0065] On the other hand, although there is no particular lower limit to the residence time in the temperature range, when the residence time is too short, the risk of breakout occurring due to non-uniform solidification increases. Here, breakout refers to a phenomenon in which a part of the solidified shell breaks during continuous casting, causing molten steel inside to leak out. The residence time is therefore preferably 50 s or longer. The residence time is more preferably 70 s or longer.Average cooling rate in temperature range from 550 °C to 700 °C: 10 °C / h or less

[0066] Further, in the cooling, the average cooling rate in the temperature range from 550 °C to 700 °C is 10 °C / h or less at a temperature at a transverse direction center of the steel slab and 10 mm from a surface. The reasons for this are described below.

[0067] The temperature range is a temperature range in which the microstructure of the steel slab transforms into pearlite. When the cooling rate in this temperature range is 10 °C / h or less, bainite transformation can be suppressed, and the microstructure of the steel slab can be made mainly pearlite. As a result, internal stress is decreased and cracks in the slab can be prevented. Therefore, the average cooling rate in the temperature range from 550 °C to 700 °C is 10 °C / h or less. The average cooling rate is preferably 8 °C / h or less. The average cooling rate is more preferably 6 °C / h or less. A lower limit of the average cooling rate is not particularly limited. However, a separate energy source is required for control, and therefore the average cooling rate is preferably 1 °C / h or more. The average cooling rate is more preferably 2 °C / h or more.

[0068] Here, the temperature of the steel slab is the temperature at a transverse direction center of the steel slab at a position 10 mm from a surface. Actually measuring the temperature is difficult, and therefore the temperature is calculated by thermal analysis. The temperature at the transverse direction center of the steel slab is used because this position is the position in the slab where the residence time in temperature range described above is the longest.

[0069] In the cooling, the steel slab may be cooled to 550 °C or lower. In other words, the cooling end temperature is 550 °C or lower. The cooling end temperature is preferably 300 °C or lower. The cooling end temperature is more preferably 100 °C or lower. The cooling end temperature is even more preferably 50 °C or lower. Typically, it is sufficient to cool to atmospheric temperature. For example, when a steel slab is allowed to naturally cool outdoors, the steel slab can be cooled to ambient temperature. On the other hand, a lower limit of the cooling end temperature is not particularly limited, but for example, when the steel slab is allowed to naturally cool outdoors, the cooling end temperature depends on the ambient temperature. Typically, the cooling end temperature is preferably -10 °C or higher. The cooling end temperature is more preferably 0 °C or higher. The cooling end temperature is even more preferably 10 °C or higher.

[0070] The method of the cooling is not particularly limited and may be any method. For example, in a continuous casting line, it is common for the steel to be cooled in a mold and then spray cooled. According to the present disclosure, cooling within a mold and spray cooling are carried out in the continuous casting line, and it is preferable that the residence time in the temperature range from 1200 °C to 1400 °C is 130 s or shorter.

[0071] On the other hand, cooling in the temperature range from 550 °C to 700 °C is preferably carried out by allowing the continuously cast slab to cool naturally after cutting. For example, cut steel slabs can be stacked and allowed to naturally cool. At this time, the average cooling rate in the temperature range from 550 °C to 700 °C is preferably adjusted to 10 °C / h or less by covering the steel slab with a cover.(2) Heating

[0072] Next, the steel slab after the cooling is heated.Slab heating temperature: 1100 °C or higher

[0073] Heating is carried out prior to hot rolling to homogenize the components and dissolve carbides such as cementite and segregations present in the steel slab. However, when the heating temperature of the steel slab (slab heating temperature) during the heating is lower than 1100 °C, cementite cannot be sufficiently dissolved, and as a result, the number density of cementite in the finally obtained steel component cannot be set within the desired range. The slab heating temperature is therefore 1100 °C or higher. The slab heating temperature is preferably 1150 °C or higher. On the other hand, although there is no particular upper limit to the slab heating temperature, when the slab heating temperature is excessively high, the surface characteristics may deteriorate. Therefore, from the viewpoint of further improving the surface characteristics, the slab heating temperature is preferably 1350 °C or lower.Slab heating time: 60 min or longer

[0074] When the heating time of the steel slab during the heating (slab heating time) is shorter than 60 min, carbides such as cementite cannot be sufficiently dissolved, and as a result, the number density of cementite in the finally obtained steel component cannot be set within the desired range. The slab heating time is therefore 60 min or longer. The slab heating time is preferably 90 min or longer. On the other hand, an upper limit of the slab heating time is not particularly limited. The slab heating time is preferably 300 min or shorter. The slab heating time is more preferably 200 min or shorter.

[0075] The heating may be carried out by any method, but use of a heating furnace is preferred.(3) Hot rolling

[0076] The heated steel slab is then hot rolled to obtain a hot-rolled steel sheet. In the hot rolling, rough rolling and finishing rolling may be carried out according to conventional methods.Rolling finish temperature: higher than Tc, 950 °C or lower

[0077] When the rolling finish temperature in the hot rolling is Tc or lower, as defined by the following expression (1), a portion of austenite is transformed into pearlite during rolling, which promotes non-uniform deformation and degrades the surface characteristics. The rolling finish temperature is therefore higher than Tc. The rolling finish temperature is preferably Tc + 50 °C or higher. The rolling finish temperature is more preferably Tc + 100 °C or higher. On the other hand, when the rolling finish temperature exceeds 950 °C, the surface scale becomes firm and the surface characteristics degrade. The rolling finish temperature is therefore 950 °C or lower. The rolling finish temperature is preferably 930 °C or lower. The rolling finish temperature is more preferably 910 °C or lower.

[0078] Here, Tc is a value that can be used as an index of the temperature at which a transformation involving cementite occurs, and is defined by the following expression (1).

[0079] Here, the element symbols in expression (1) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0.(4) CoolingAverage cooling rate: 20 °C / s or more

[0080] After the finishing rolling is completed, cooling is started, then stopped at a cooling stop temperature. When the average cooling rate in the cooling is less than 20 °C / s, the number density of cementite in the finally obtained steel component cannot be set within the desired range. This is because when the average cooling rate is less than 20 °C / s, coarse cementite is formed and this coarse cementite remains in the steel component. The average cooling rate is therefore 20 °C / s or more. The average cooling rate is preferably 30 °C / s or more. The average cooling rate is more preferably 50 °C / s or more. On the other hand, although there is no particular upper limit to the average cooling rate, when the cooling rate is excessively high, controlling the cooling stop temperature becomes difficult. The average cooling rate is therefore preferably 500 °C / s or less.Cooling stop temperature: Tc or lower

[0081] When the cooling stop temperature in the cooling is higher than Tc, the number density of cementite in the finally obtained steel component cannot be set within the desired range. This is because when the cooling stop temperature is higher than Tc, coarse cementite is formed and this coarse cementite remains in the steel component. The cooling stop temperature is therefore Tc or lower. The cooling stop temperature is preferably Tc - 20 °C or lower. On the other hand, a lower limit of the cooling stop temperature is not particularly limited. However, when the cooling stop temperature is lower than 530 °C, the volume expansion caused by transformation during the subsequent coiling results in a poor coil shape. Therefore, from the viewpoint of improving the coiling shape, the cooling stop temperature is preferably 530 °C or higher. The cooling stop temperature is more preferably 550 °C or higher. The cooling stop temperature is even more preferably 600 °C or higher.(5) CoilingCoiling temperature: 530 °C or higher and Tc or lower

[0082] After the cooling is stopped, the cooled hot-rolled steel sheet is coiled. At this time, when the coiling temperature is higher than Tc, the number density of cementite in the finally obtained steel component cannot be set within the desired range. This is because when the coiling temperature is higher than Tc, coarse cementite is generated and this coarse cementite remains in the steel component. The coiling temperature is therefore Tc or lower. The coiling temperature is preferably Tc - 20 °C or lower. The coiling temperature is more preferably Tc - 40 °C or lower. On the other hand, when the coiling temperature is lower than 530 °C, volume expansion due to transformation during coiling results in a poor coiling shape. The coiling temperature is therefore 530 °C or higher. The coiling temperature is preferably 550 °C or higher. The coiling temperature is more preferably 600 °C or higher.(6) First annealing

[0083] Annealing temperature: 600 °C or higher and Tc or lower Annealing time: 3 h or longer

[0084] The hot-rolled steel sheet after the coiling is subjected to the first annealing under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer. The microstructure of the hot-rolled steel sheet after the coiling is a pearlitic microstructure lined with plate-like cementite and ferrite. The pearlitic microstructure is stable, and therefore does not homogenize without prolonged holding at a high temperature. In order to break up the pearlitic microstructure and allow the subsequent cold rolling and annealing process to produce the desired cementite, the annealing temperature needs to be 600 °C or higher and the annealing time needs to be 3 h or longer. The annealing temperature is preferably 620 °C or higher. Further, the annealing time is preferably 4 h or longer. On the other hand, when the annealing temperature is higher than Tc, phase transformation starts preferentially from one portion, and a locally coarse microstructure is formed. As a result, the microstructure becomes non-uniform, and the desired number density of cementite particles cannot be obtained. The annealing temperature is therefore Tc or lower. The annealing temperature is preferably Tc - 20 °C or lower. The annealing temperature is more preferably Tc - 40 °C or lower. On the other hand, although there is no particular upper limit to the annealing time, when too long, productivity decreases. The annealing time is therefore preferably 50 h or shorter. The annealing time is more preferably 30 h or shorter.

[0085] By carrying out the first annealing under the above conditions, the pearlitic microstructure can be broken up, and the desired cementite can be more easily formed in the subsequent cold rolling and annealing processes.

[0086] The first annealing can be carried out any number of times, but from the viewpoint of enhancing the effect of the annealing, the first annealing is preferably carried out two or more times. On the other hand, there is no particular upper limit to the number of times the first annealing is carried out, but even when the first annealing is carried out more than three times, the effect is saturated. The first annealing is therefore preferably carried out three times or less. When the first annealing is carried out two or more times, each annealing may be carried out under the above conditions. The annealing conditions for each time may be the same or different from each other.

[0087] Prior to the first annealing, the hot-rolled steel sheet is preferably pickled.(7) Cold rolling(8) Second annealing

[0088] Plate-like cementite is formed in the steel sheet after hot rolling. This plate-like cementite is stable and therefore prone to remain for a long time. The plate-like cementite is coarse, and therefore when the cementite remains, the number density of the cementite particles in the final steel component cannot be set within the desired range. As a result, the desired wear resistance is not obtainable. Therefore, in order to refine and spheroidize the plate-like cementite by heating during annealing, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and second annealing at least once.Rolling ratio: 15 % or more

[0089] The cold rolling causes the cementite to be deformed, broken up, and fragmented, thereby making it possible to obtain the desired number density of cementite particles. When the rolling ratio in the cold rolling is less than 15 %, the effect cannot be obtained. The rolling ratio is therefore 15 % or more. The rolling ratio is preferably 25 % or more. The rolling ratio is more preferably 35 % or more. The rolling ratio is even more preferably 45 % or more. On the other hand, an upper limit of the rolling ratio is not particularly limited. The rolling ratio is preferably 85 % or less. The rolling ratio is more preferably 80 % or less.Annealing temperature: 600 °C or higher and Tc or lowerAnnealing time: 3 h or longer

[0090] In the second annealing, the cementite that has been deformed, broken up, and fragmented by the cold rolling can be refined and spheroidized. At the same time, Cr can be concentrated in the cementite. To obtain these effects, the annealing temperature in the second annealing needs to be 600 °C or higher, and the annealing time needs to be 3 h or longer. The annealing temperature is preferably 620 °C or higher. Further, the annealing time is preferably 4 h or longer. On the other hand, when the annealing temperature is higher than Tc, phase transformation starts preferentially from one portion, and a locally coarse microstructure is formed. As a result, the microstructure becomes non-uniform, and the desired number density of cementite particles cannot be obtained. The annealing temperature is therefore Tc or lower. The annealing temperature is preferably Tc - 20 °C or lower. The annealing temperature is more preferably Tc - 40 °C or lower. On the other hand, although there is no particular upper limit to the annealing time, when too long, productivity decreases. The annealing time is therefore preferably 30 h or shorter. The annealing time is more preferably 20 h or shorter.

[0091] By carrying out the cold rolling and the second annealing under the above conditions, it is possible to promote the refinement of cementite and the concentration of Cr in the cementite.

[0092] The cold rolling and the second annealing can be carried out any number of times, but from the viewpoint of enhancing the above-described effects, the process is preferably carried out two or more times. When the cold rolling and the second annealing are carried out two or more times, the cold rolling and the second annealing may be repeated alternately. On the other hand, there is no particular upper limit to the number of times that the cold rolling and the second annealing are carried out, but even when the number of times is more than five, the effect is saturated. The cold rolling and the second annealing are therefore preferably carried out five times or less. When the cold rolling and the second annealing are carried out two or more times, each iteration of the cold rolling and the second annealing may be carried out under the above conditions. The conditions for each iteration may be the same or different from each other.

[0093] When the cold rolling and the second annealing are carried out only once, the rolling ratio of the cold rolling is preferably 70 % or more from the viewpoint of reliably refining cementite.

[0094] It is preferable to carry out cold rolling (final cold rolling) after carrying out cold rolling and second annealing, before carrying out subsequent working and heat treatment. By carrying out the final cold rolling, it is possible to improve workability in the subsequent process of forming the steel component, particularly blanking workability. To obtain this effect, the rolling ratio of the final cold rolling is preferably 20 % or more. The rolling ratio is more preferably 25 % or more. The rolling ratio is even more preferably 30 % or more.

[0095] Through the above procedure, a cold-rolled steel sheet is obtained. The sheet thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness. The sheet thickness is preferably 0.1 mm or more. The sheet thickness is more preferably 0.2 mm or more. Further, an upper limit of the sheet thickness is not particularly limited. The sheet thickness is particularly 2.5 mm or less. The sheet thickness is more preferably 1.6 mm or less. The sheet thickness is even more preferably 0.8 mm or less. When the sheet thickness is 0.2 mm or more and 0.8 mm or less, the cold-rolled steel sheet is particularly suitable for use as a material for knitting needles and the like.

[0096] Further, the final cold-rolled steel sheet may be subjected to further optional surface treatment.(9) Working and heat treatment

[0097] Next, the obtained cold-rolled steel sheet is worked into a component shape and subjected to heat treatment. The working and the heat treatment can be carried out in any order. For example, the heat treatment may be carried out after the working, or during the working.

[0098] The method of the working is not particularly limited and any method may be applied. The working may be, for example, at least one selected from the group consisting of blanking, cutting, wire drawing, bending, and polishing.

[0099] The heat treatment includes quenching and tempering. Conditions are explained below.• Quenching

[0100] By carrying out quenching, the strength of the component can be increased, the number density of cementite particles can be controlled, and excellent wear resistance can be obtained. To obtain these effects, quenching temperature and holding time need to satisfy the following conditions.• Quenching temperature: Tc or higher and 1000 °C or lower

[0101] In the quenching, first, the cold-rolled steel sheet worked into a component shape is heated to a quenching temperature and held at the quenching temperature for a holding time described later. This transforms the microstructure into austenite. Then, by cooling, the austenite transforms into martensite, improving the strength. At this time, when the quenching temperature (heating temperature during quenching) is low, the transformation to austenite is insufficient. As a result, hard martensite is not obtained after cooling, and the desired wear resistance is not obtained. The quenching temperature is therefore Tc or higher. The quenching temperature is preferably Tc + 20 °C or higher. The quenching temperature is more preferably Tc + 50 °C or higher. On the other hand, when the quenching temperature is too high, cementite dissolves. As a result, the desired number density of cementite cannot be obtained, and the wear resistance decreases. The quenching temperature is therefore 1000 °C or lower. The quenching temperature is preferably 950 °C or lower. The quenching temperature is more preferably 900 °C or lower. The quenching temperature is even more preferably 870 °C or lower. The quenching temperature is most preferably 840 °C or lower.• Holding time: 1.0 min or longer and 60 min or shorter

[0102] During the quenching, in order to transform the microstructure into austenite, holding at the heating temperature for 1.0 min or longer is required. The holding time is therefore 1.0 min or longer. The holding time is preferably 2 min or longer. The holding time is more preferably 5 min or longer. On the other hand, when the holding time exceeds 60 min, dissolution of cementite proceeds, the desired cementite number density cannot be obtained, and the wear resistance decreases. The holding time is therefore 60 min or less.

[0103] Cooling in the quenching process is preferably performed by cooling to 150 °C or lower using oil or other coolant. It is also preferable to cool the steel component by pressing with a mold, or by die quenching, press quenching, or the like to a temperature of 150 °C or lower.• Tempering

[0104] The steel component after quenching is then tempered. By carrying out the tempering, the hardness and toughness of the steel component can be adjusted. The tempering can be carried out under any conditions without any particular limitations. Typically, it is preferable that the tempering temperature is 100 °C to 400 °C and the holding time is 10 min or longer and 180 min or shorter.

[0105] The heat treatment may include any other heat treatment in addition to quenching and tempering. According to an embodiment of the present disclosure, the heat treatment may consist of quenching and tempering.

[0106] According to the above method, the steel component having excellent wear resistance and surface characteristics may be produced. Applications of the steel component are not particularly limited. The steel component is particularly suitable for applications requiring wear resistance, such as components for textile machinery, bearing components, and blades for machinery.EXAMPLES

[0107] In order to confirm the effects of the present disclosure, steel components were produced according to the following procedure.

[0108] First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then subjected to cooling, heating, hot rolling, cooling, coiling, first annealing, cold rolling, and second annealing in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Table 2, and the first annealing, cold rolling, and second annealing were carried out the number of times listed in Table 2.

[0109] For some examples, the cold-rolled steel sheet after the second annealing was further subjected to final cold rolling. The rolling reduction in the final cold rolling is listed in Tables 2 and 3.

[0110] The resulting cold-rolled steel sheet was then worked into component shapes and heat-treated to obtain the final steel component.

[0111] In working the component shape, a plurality of steel components were obtained from one cold-rolled steel sheet by wire electric discharge machining. The wire electric discharge machining was carried out from a position 5 mm inward from a transverse direction end of the cold-rolled steel sheet at intervals of about 15 mm in the transverse direction and at intervals of about 100 mm in the length direction (rolling direction). The dimensions of the steel component were 10 mm wide × 80 mm long.

[0112] In the heat treatment, quenching and tempering were carried out. The quenching was carried out under the conditions listed in Table 2. The tempering was carried out under conditions such that the Vickers hardness of the steel component after the tempering was 710 ± 10.

[0113] According to the above procedure, a plurality of steel components were produced for each example.

[0114] Next, for each of the obtained steel components, the number density of cementite particles and the Cr amount in the cementite particles were measured by the following procedures. The measurement results are listed in Tables 4 and 5.(Number density of cementite)

[0115] Test pieces for microstructure observation were taken from the obtained steel components. For each test piece for microstructure observation, a rolling direction cross-section (L-section) was polished to a mirror finish, and then etched with 1 vol% to 3 vol% nital to reveal the microstructure. Thereafter, the etched surface of the test piece for microstructure observation was imaged at ten locations at a 1 / 2 thickness position using a scanning electron microscope (SEM) at an accelerating voltage of 15 keV and a magnification of 3000 times to obtain microstructure images. From the obtained microstructure images, cementite particles having a particle size of less than 0.090 µm were removed by image processing, and the number of cementite particles having a particle size of 0.090 µm or more was counted. The number density of cementite was calculated by dividing the obtained number of particles by the area of the microstructure image. The number density was calculated for each of the ten microstructure images using the same procedure, and an average value was taken as the number density of cementite having a particle size of 0.090 µm or more.(Cr amount in cementite particles)

[0116] Test pieces for microstructure observation were taken from the obtained steel components. L-sections of the test pieces for microstructure observation were each polished to a mirror surface. Thereafter, the polished surface of the test piece for microstructure observation was observed by SEM to identify cementite particles. Next, using an electron probe micro analyzer (EPMA), the Cr amount was measured at three locations with respect to one cementite particle having an area of 0.06 µm 2< or more under the conditions of an electron beam accelerating voltage of 15 keV, an irradiation current of 1.0 × 10 -8< A, and a measurement time of 1000 ms, and an average value of the three locations was calculated. The Cr amount was similarly measured for ten cementite particles each having a particle size of 0.090 µm or more, and an average value for the ten particles was taken as the Cr amount in the cementite particles.

[0117] Further, the wear resistance and the surface characteristics of each of the obtained steel components were evaluated by the following procedures. The evaluation results are listed in Tables 4 and 5.(Surface characteristics)

[0118] The appearance of the steel components was visually inspected to evaluate the surface characteristics. Specifically, the surface characteristics were evaluated as "good" when no surface defects caused by scabs or slivers on the cold-rolled steel sheet were observed on any of the steel components, and the surface characteristics were evaluated as "poor" when at least one surface defect caused by scabs or slivers on the cold-rolled steel sheet was observed on any of the steel components.(Wear resistance)

[0119] The wear resistance of the steel components was evaluated by the following procedure. First, a wear test piece 10 having the shape illustrated in FIG. 1 was taken from the steel component. Each of the wear test pieces 10 was provided with four holes 11 for threading.

[0120] Wear tests were conducted using the wear test pieces 10 and a wear test apparatus 20 illustrated in FIG. 2. Specifically, a wear amount d 1 was measured by running a yarn S fed from a yarn unwinder 21 for 5000 m per hole with the yarn S in contact with the side of the hole 11 of the wear test piece 10. Thereafter, the yarn S was again run at the same position for another 15,000 m per hole (total 20,000 m) while in contact with the side of the hole 11 of the wear test piece 10, and a wear amount d 2 was measured. Full dull polyester knitting yarn was used as the yarn S. The running speed of the yarn S was 200 m / min. Further, the tension of the yarn was adjusted to 20 ± 2 N / cm using a tension regulator 22.

[0121] As illustrated in FIG. 3, a groove 12 was formed by wear at a point where the hole 11 was in contact with the yarn. After running the yarn for 5000 m and 20,000 m, the running was stopped and a depth d (wear depth) of the groove 12 was measured using optical microscopy.

[0122] The wear depth when the yarn had run 5000 m was defined as d 1 , and the wear depth when the yarn had run a further 15,000 m (total 20,000 m) was defined as d 2 . The difference between d 1 and d 2 was defined as Δd, which was used as an index of wear resistance.

[0123] The same test was carried out on each of the four holes 11 and an average value of the four wear depths obtained was taken as the wear depth Δd of the wear test piece 10. When the wear depth Δd was 10 µm or less, the wear resistance was rated as "good", and when more than 10 µm, the wear resistance was rated as "poor". The evaluation results are listed in Tables 4 and 5.[Table 1]

[0124] Table 1Steel sample IDChemical composition (mass%)Nb + Ti + V (mass%)Tc (°C)RemarksCSiMnPSAlNOCrNbTiVOtherA0.910.210.510.0080.00360.0090.00340.00050.700.080---0.080735Disclosed steelB0.850.331.120.0050.00370.0080.00300.00210.560.0390.02--0.059730Disclosed steelC1.100.100.520.0070.00400.0040.00350.00071.800.101-0.08-0.181751Comparative steelD1.250.320.820.0080.00330.0040.00290.00100.750.0950.060.04-0.195736Disclosed steelE1.050.150.500.0050.00130.0040.00330.00081.08-0.12--0.120740Disclosed steelF1.080.320.280.0090.00390.0920.00420.00061.31--0.19-0.190751Disclosed steelG0.980.230.750.0040.00070.0050.00330.00150.61-0.100.09-0.190732Disclosed steelH1.070.201.130.0110.00280.0030.00320.00051.420.094--Ta: 0.05, Mo: 0.4, Cu: 0.12, Sb: 0.010.094741Disclosed steelI0.970.551.010.0090.00190.0070.00450.00100.730.095---0.095741Disclosed steelJ1.130.150.350.0100.00040.0040.00290.00051.110.062--Mo: 0.120.062742Disclosed steelK0.860.182.980.0060.00180.0050.00360.00100.930.136---0.136712Comparative steelL0.920.801.070.0090.00380.0090.00520.00050.820.121--Zr: 0.06, Te: 0.040.121749Disclosed steelM1.080.200.700.0450.02810.0420.00340.00111.590.0290.05--0.079748Disclosed steelN0.870.990.900.0080.00340.0090.00500.00120.870.078--Hf: 0.05, Bi: 0.080.078757Disclosed steelO1.090.242.450.0060.00230.0100.00340.00941.270.046--W: 0.04, Mg: 0.005, REM: 0.0050.046725Disclosed steelP1.050.220.730.0360.01520.0080.00290.00130.750.192---0.192734Disclosed steelQ1.080.121.930.0040.00290.0710.00300.00091.460.097--Cu: 0.15, Sb: 0.010.097731Disclosed steelR0.850.340.490.0090.00200.0080.00940.00061.100.077--Ni: 0.05, Cu: 0.120.077745Disclosed steelS0.870.251.200.0050.00090.0030.00740.00090.750.140--Ca: 0.0050.140730Disclosed steelT0.870.290.710.0040.00390.0030.00350.00120.990.031--B: 0.00150.031741Disclosed steelU1.090.310.620.0090.00190.0090.00100.00060.750.184--Co: 0.006, Sn: 0.050.184738Disclosed steela0.750.281.020.0050.00280.0070.00350.00050.720.033---0.033732Comparative steelb0.870.240.150.0080.00230.0050.00380.00070.590.053---0.053738Comparative steelc0.890.350.350.0130.00210.0040.00380.00060.420.048---0.048737Comparative steeld0.810.260.450.0050.00210.0080.00350.00100.590.020---0.020736Comparative steele1.060.331.020.0090.00160.0100.00220.00161.090.261---0.261740Comparative steelf0.980.291.710.0090.00340.0080.00140.00121.050.1910.08--0.271731Comparative steelg1.100.330.770.0100.00180.0060.00320.00091.48-0.150.20-0.350749Comparative steelh1.081.341.720.0090.00320.0060.00430.00061.230.152---0.152764Comparative steeli1.020.423.540.0130.00420.0080.00520.00091.050.123---0.123715Comparative steelj1.050.531.940.0110.00180.0070.00350.00072.360.112---0.112758Comparative steel [Table 2]

[0125] Table 2NoSteel sample IDCoolingHeatingHot rollingCoolingCoilingFirst annealingCold rollingSecond annealingNumber of times cold rolling and second annealingFinal cold rollingQuenchingRemarks1400 °C to 1200 °C Residence time (s)700 °C to 550 °C Average cooling rate (°C / h)Slab heating temp. (°C)Time (min)Rolling finish temp. (°C)Average cooling rate (°C / s)Cooling stop temp. (°C)Coiling temp. (°C)Heating temp. (°C)Annealing time (h)TimesRolling ratio (%)Heating temp. (°C)Annealing time (h)Rolling ratio (%)Quenching temp. (°C)Holding time (min)1A1205118070900306806607107245680423579017Example2B1134120080910806506206708230630103408109Example3B11341200809108065062067081506301023082012Example4B1134120080910806506206708275640111-80015Example5B11341200809108065062067082306301034079013Example6C10341220908754060058069062506301022077055Comparative Example7D1175117080860206205706702122564075-83015Example8E10561150708505064059070021245640623581010Example9F10031120658453066062069016240670142458007Example10G109611708088035700660680202256301151580012Example11H12831200908657559055071082506601323084010Example12I9541220909005066065068010245640112358305Example13I95412209090050660650680102706401113581019Example14I95410303080060660650710102457001123579010Comparative Example15-1I9541220907008066065071010245700112358205Comparative Example15-2I95412209098040660650710102457001123583014Comparative Example16I9541220908001066065070010245700112357907Comparative Example17I9541220909008080077070010245700112358008Comparative Example18I95412209090050660650550224560042358408Comparative Example19I95412209090050660650770102456901123582013Comparative Example20I95412209090050660650620314555022358109Comparative Example21I9541220909005066065068010245770112358505Comparative Example22I95412209090050660650710102107001126580019Comparative Example23I95412209090050660650680102456401123570020Comparative Example24I95412209090050660650680102456401123510508Comparative Example25J1187123010095040630610670112506501322079011Example [Table 3]

[0126] Table 3NoSteel sample IDCoolingHeatingHot rollingCoolingCoilingFirst annealingCold rollingSecond annealingNumber of times cold rolling and second annealingFinal cold rollingQuenchingRemarks1400 °C to 1200 °C Residence time (s)700 °C to 550 °C Average cooling rate (°C / h)Slab heating temp. (°C)Time (min)Rolling finish temp. (°C)Average cooling rate (°C / s)Cooling stop temp. (°C)Coiling temp. (°C)Heating temp. (°C)Annealing time (h)TimesRolling ratio (%)Heating temp. (°C)Annealing time (h)Rolling ratio (%)Quenching temp. (°C)Holding time (min)26K112811406086030650600700162356501225081018Comparative Example27L9131190909203557052068092506601323081016Example28M102711808087525730710700624063073-8006Example29N10841150708704570067067052306401034081014Example30O12351220909005060057071021245660112359601Example31P9651290708504560059068010250640142258309Example32P1505slab crack occurred during continuous castingComparative Example33P9621slab crack occurred during continuous castingComparative Example34Q115210702108454058054070025235650725083014Example35R105312501809107062059068024230660132608105Example36S1131115010093010055051068019250630112-81018Example37T123311707087035580580700132506701223085019Example38U96411601009003057053065020250660112-78014Example39a11241190809054567064071072356901125082010Comparative Example40b1053117080900507106607001324564042359103Comparative Example41c113412001109104564062068011230650626081020Comparative Example42d10241120557302564059067016225640726582019Comparative Example43e1123slab crack occurred during continuous castingComparative Example44f1054slab crack occurred during continuous castingComparative Example45g1215slab crack occurred during continuous castingComparative Example46h1055slab crack occurred during continuous castingComparative Example47i986slab crack occurred during continuous castingComparative Example48j1205slab crack occurred during continuous castingComparative Example [Table 4]

[0127]

[0132] Table 4No.Steel sample IDCementiteSurface characteristicsWear resistanceRemarksNumber density *1 ( / mm 2< )Cr amount *2 (mass%)1A3110003.1goodgoodExample2B5840002.8goodgoodExample3B4880002.9goodgoodExample4B5520002.8goodgoodExample5B6880002.7goodgoodExample6C13050008.3poorgoodComparative Example7D3650003.9goodgoodExample8E5670005.3goodgoodExample9F5300006.3goodgoodExample10G5560002.9goodgoodExample11H5410008.1goodgoodExample12I5430003.9goodgoodExample13I5000003.7goodgoodExample14I1770003.6goodpoorComparative Example15-1I2990003.8poorgoodComparative Example15-2I2040003.9poorgoodComparative Example16I1930003.6goodpoorComparative Example17I1820003.7goodpoorComparative Example18I1260003.9goodpoorComparative Example19I1020003.8goodpoorComparative Example20I1630003.6goodpoorComparative Example21I1030004.2goodpoorComparative Example22I1660003.7goodpoorComparative Example23I11400002.8goodpoorComparative Example24I0-goodpoorComparative Example25J6300005.2goodgoodExample*1: Number density of cementite particles having particle size of 0.090 µm or more *2: Cr amount in cementite particles having particle size of 0.090 µm or more [Table 5]

[0128] Table 5No.Steel sample IDCementiteSurface characteristicsWear resistanceRemarksNumber density *1 ( / mm 2< )Cr amount *2 (mass%)26K8910005.9poorgoodComparative Example27L4710004.3goodgoodExample28M10130007.9goodgoodExample29N5550004.4goodgoodExample30O5640008.0goodgoodExample31P4340003.9goodgoodExample32Pslab crack occurred during continuous castingComparative Example33Pslab crack occurred during continuous castingComparative Example34Q7010008.5goodgoodExample35R5410005.5goodgoodExample36S6250003.9goodgoodExample37T297000 5.5goodgoodExample38U5080003.4goodgoodExample39a1810003.8goodpoorComparative Example40b1210002.8goodpoorComparative Example41c1840001.7goodpoorComparative Example42d2600002.7goodpoorComparative Example43eslab crack occurred during continuous castingComparative Example44fslab crack occurred during continuous castingComparative Example45gslab crack occurred during continuous castingComparative Example46hslab crack occurred during continuous castingComparative Example47islab crack occurred during continuous castingComparative Example48jslab crack occurred during continuous castingComparative Example*1: Number density of cementite particles having particle size of 0.090 µm or more *2: Cr amount in cementite particles having particle size of 0.090 µm or more

[0129] As can be seen from the results listed in Tables 1 to 5, steel components satisfying the conditions of the present disclosure can be produced without the occurrence of delayed cracking during continuous casting, and have excellent surface characteristics and wear resistance.REFERENCE SIGNS LIST

[0130] 10wear test piece 11hole 12groove 20wear test apparatus 21yarn unwinder 22tension adjuster 23yarn winder Syarn dwear depth

Examples

examples

[0107]In order to confirm the effects of the present disclosure, steel components were produced according to the following procedure.

[0108]First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then subjected to cooling, heating, hot rolling, cooling, coiling, first annealing, cold rolling, and second annealing in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Table 2, and the first annealing, cold rolling, and second annealing were carried out the number of times listed in Table 2.

[0109]For some examples, the cold-rolled steel sheet after the second annealing was further subjected to final cold rolling. The rolling reduction in the final cold rolling is listed in Tables 2 and 3.

[0110]The resulting cold-rolled steel sheet was then worked into component shapes and heat-treat...

TECHNICAL FIELD

[0001] The present disclosure relates to steel components, and more particularly to a steel component having excellent wear resistance and surface characteristics. Further, the present disclosure relates to a method of producing the steel component.

[0002] Carbon steel, a steel containing a high concentration of carbon, has high hardness and is therefore widely used as a material for textile machinery components, bearing components, machine blades, and other steel components that require wear resistance.

[0003] In typical steel component production, a cold-rolled steel sheet as a raw material is worked into a component shape, followed by quenching treatment and tempering treatment. While quenching treatment increases hardness and decreases toughness, subsequent tempering treatment may improve toughness. However, tempering decreases the hardness, which results in a decrease in wear resistance.

[0004] Therefore, various techniques have been proposed to achieve excellent wear resistance even when the hardness of a steel component is decreased.

[0005] For example, in Patent Literature (PTL) 1, a steel sheet is proposed that contains 0.10 mass% to 0.40 mass% of C and has a ferrite-cementite microstructure. In PTL 1, formability and wear resistance are improved by increasing ferrite grain size in the steel sheet, spheroidizing carbides (mainly cementite) to an appropriate particle size, and decreasing pearlitic microstructure.

[0006] Further, in PTL 2, formation of a pearlitic microstructure having an interlamellar spacing of 0.3 µm or less is proposed, by annealing a cold-rolled steel sheet containing 0.40 mass% to 0.90 mass% of C under specific conditions.

[0007] In PTL 3, precipitation of coarse Nb-Ti carbides having a circle equivalent diameter of 0.5 µm or more is proposed, in the ferrite phase, which is the matrix microstructure, in a steel sheet containing 0.60 mass% to 1.25 mass% of C.

[0008] In PTL 4, precipitation of coarse carbides having a particle size of 2 µm or more is proposed, in the matrix microstructure of steel containing 0.6 mass% to 1.0 mass% of C.

[0009] Further, adding alloying elements to steels having high carbon concentrations decreases the toughness of the steel. As a result, cracks are more likely to occur when the slab is cooled during the production of the steel slab, so-called "delayed cracking". When delayed cracking occurs, the slab may fracture during transportation, making it impossible to use the slab for hot rolling. Even when the slab does not fracture, cracks may open during hot rolling, causing the hot-rolled steel sheet to fracture. Further, when the cracks are small, surface defects such as scabs and slivers occur on the steel sheet or steel component after cold rolling.

[0010] Cracks on a steel slab surface are normally removed with a grinder. However, the toughness of steel slabs is decreased due to high alloying, and therefore stress generated when cracks are removed with a grinder may instead cause the cracks to propagate. In such a case, completely removing the cracks is difficult. On the other hand, small cracks may be overlooked and appear as surface defects on steel sheets or steel components after cold rolling. For these reasons, suppressing delayed cracking in steel slabs is necessary.

[0011] Accordingly, various techniques have been proposed to prevent the occurrence of delayed cracking in high alloy steel slabs.

[0012] For example, in PTL 5 is a proposal that a steel slab containing 0.16 % to 0.35 % of C is cast, cut, and then three or more cut steel slabs are stacked in a defined shape and cooled. By cooling using the above method, the steel slab is slowly cooled in a temperature range where austenite transforms to ferrite (700 °C to 500 °C), and a bainite / martensite transformation is suppressed. As a result, stress caused by transformation expansion is decreased, and delayed cracking of the slab can be prevented.

[0013] Further, in PTL 6 is a proposal that a steel slab containing 0.020 mass% to 0.600 mass% of C is produced by continuous casting, and then gradually cooled at an average cooling rate of 20 °C / h or less in a temperature range from 700 °C to 500 °C. This helps prevent delayed cracking due to stress during cooling. Further, occurrence of quality defects such as scabs during hot rolling can also be prevented.CITATION LISTPatent Literature

[0014] PTL 1: WO 2016 / 204288 A1 PTL 2: JP 2020-132953 A PTL 3: JP 2017-190494 A PTL 4: JP 2010-138453 A PTL 5: JP 2020-139209 A PTL 6: JP 2019-167560 A SUMMARY(Technical Problem)

[0015] According to conventional technologies, such as those proposed in PTL 1 to 4, there is some improvement in the hardness and wear resistance of steel. However, the inventors have found that steel components produced from conventional steel material may not have sufficient wear resistance in actual use. Further, regarding PTL 3 and PTL 4, there have been cases where delayed cracking has occurred in the slab, and surface defects such as scabs have occurred in steel components due to the delayed cracking.

[0016] Further, according to PTL 5 and PTL 6, it is stated that by gradually cooling the cast slab in the temperature range of 700 °C to 500 °C, delayed cracking of the slab can be prevented. However, it has been found that even this method cannot sufficiently prevent the occurrence of delayed cracking in the slab when the C content is 0.80 % or more.

[0017] The present disclosure is made in view of the above circumstances, and it would be helpful to provide a steel component that has excellent wear resistance and surface characteristics, and for which slab cracking does not occur during continuous casting.(Solution to Problem)

[0018] As a result of studies, the inventors arrived at the following discoveries. (1) Wear resistance can be improved by appropriately controlling a number density of cementite in steel components and a Cr amount in the cementite. (2) By appropriately controlling the chemical composition of the steel slab used and production conditions in continuous casting, it is possible to suppress delayed cracking in the steel slab and to suppress surface defects in steel components. (3) By appropriately controlling the chemical composition of the steel slab used and production conditions of processes such as hot rolling, annealing, cold rolling, quenching, and tempering, it is possible to appropriately control a number density of cementite particles in steel components and a Cr amount present in the cementite particles.

[0019] The present disclosure is based on the discoveries described above, and primary features of the present disclosure are as described below. 1. A steel component comprising a chemical composition containing (consisting of), in mass%, C: 0.80 % to 1.25 %, Si: 0.10 % to 1.0 %, Mn: 0.20 % to 2.5 %, P: 0.0005 % to 0.05 %, S: 0.03 % or less, Al: 0.001 % to 0.1 %, N: 0.001 % to 0.01 %, O: 0.0100 % or less, Cr: 0.56 % to 1.6 %, and at least one selected from the group consisting of Nb: 0.029 % to 0.21 %, Ti: 0.01 % to 0.21 %, and V: 0.01 % to 0.21 %, with the balance being Fe and inevitable impurity, and the total content of Nb, Ti, and V being 0.21 mass% or less, wherein a number density of cementite particles having a particle size of 0.090 µm or more is 200,000 / mm 2< or more, and a Cr amount in the cementite particles having a particle size of 0.090 µm or more is 2.5 mass% or more. 2. The steel component according to 1, above, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of: Ta: 0.10 % or less, W: 0.10 % or less, B: 0.0100 % or less, Mo: 1.00 % or less, Co: 1.00 % or less, Ni: 1.00 % or less, Cu: 1.00 % or less, Sn: 0.200 % or less, Sb: 0.200 % or less, Ca: 0.0100 % or less, Mg: 0.0100 % or less, REM: 0.0100 % or less, Zr: 0.100 % or less, Te: 0.100 % or less, Hf: 0.10 % or less, and Bi: 0.200 % or less. 3. The steel component according to 1 or 2, above, wherein the steel component is any one of a component for textile machinery, a bearing component, or a blade for machinery. 4. A method of producing a steel component, the method comprising: cooling a steel slab produced by continuous casting and having the chemical composition according to 1 or 2, above, under conditions such that, at a transverse direction center of the steel slab and 10 mm from a surface, a residence time in a temperature range from 1200 °C to 1400 °C is 130 s or shorter, and an average cooling rate in a temperature range from 550 °C to 700 °C is 10 °C / h or less; heating the steel slab after the cooling at a slab heating temperature of 1100 °C or higher for a slab heating time of 60 min or longer; hot rolling the heated steel slab under conditions including a rolling finish temperature exceeding Tc, as defined by the following expression (1), and 950 °C or lower, to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet under conditions including an average cooling rate of 20 °C / s or more and a cooling stop temperature of Tc or lower; coiling the cooled hot-rolled steel sheet at a coiling temperature of 530 °C or higher and Tc or lower; subjecting the hot-rolled steel sheet after coiling to first annealing once or more, under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer; subjecting the hot-rolled steel sheet after the first annealing to cold rolling at a rolling ratio of 15 % or more and second annealing at an annealing temperature of 600 °C or higher and Tc or lower for an annealing time of 3 h or longer, twice or more, to obtain a cold-rolled steel sheet; working the cold-rolled steel sheet into a component shape; quenching under conditions including a quenching temperature of Tc or higher and 1000 °C or lower and a holding time of 1.0 min or longer and 60 min or shorter, and then carrying out a heat treatment including tempering, where the element symbols in expression (1) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0. (Advantageous Effect)

[0020] According to the present disclosure, it is possible to provide a steel component that has excellent wear resistance and surface characteristics, and for which delayed cracking of a slab does not occur during continuous casting. The steel component of the present disclosure can be suitably used for a variety of applications including textile machinery components, bearing components, and machine blades.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the accompanying drawings: FIG. 1 is a schematic diagram illustrating shape of a wear test piece; FIG. 2 is a schematic diagram of a wear test apparatus; and FIG. 3 is a schematic diagram illustrating wear depth of a test piece after a wear test. DETAILED DESCRIPTION

[0022] A detailed description is provided below. The present disclosure is not limited to the following embodiments.[Chemical composition]

[0023] The cold-rolled steel sheet according to the present disclosure has the chemical composition described above. The reasons for the above limitations are described below. Hereinafter, "%" as a unit of content indicates "mass%" unless otherwise specified.C: 0.80 % to 1.25 %

[0024] Carbon is an essential element for various steel components, including textile machinery components, bearing components, machine blades, and household blades, in order to improve hardness after quenching. Further, C is an element necessary for producing cementite. When C content is less than 0.80%, the required number density of cementite particles cannot be obtained. The C content is therefore 0.80 % or more. The C content is preferably 0.85 % or more. The C content is more preferably 0.90 % or more. On the other hand, when the C content exceeds 1.25 %, surface scale becomes firm during slab heating, resulting in degradation of surface characteristics. Further, when the C content exceeds 1.25 %, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The C content is therefore 1.25 % or less. The C content is preferably 1.20 % or less. The C content is more preferably 1.10 % or less.Si: 0.10 % to 1.0 %

[0025] Si is an element having an effect of increasing strength of the cold-rolled steel sheet by solid solution strengthening. To obtain the above effect, Si content is 0.10 % or more. The Si content is preferably 0.15 % or more. The Si content is more preferably 0.20 % or more. On the other hand, an excess of Si degrades surface characteristics as a result of surface scale becoming firm during heating. Further, when the Si content exceeds 1.0 %, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Si content is therefore 1.0 % or less. The Si content is preferably 0.80 % or less. The Si content is more preferably 0.55 % or less.Mn: 0.20 % to 2.5 %

[0026] Mn is an element that has an effect of improving the hardness of the steel component by promoting quenching. Further, Mn delays the dissolution of cementite during quenching, thereby increasing the number density of cementite particles in the steel component. To obtain these effects, Mn content is 0.20 % or more. The Mn content is preferably 0.30 % or more. The Mn content is more preferably 0.40 % or more. The Mn content is even more preferably 0.50 % or more. On the other hand, when the Mn content exceeds 2.5 %, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Mn content is therefore 2.5 % or less. The Mn content is preferably 2.0 % or less. The Mn content is more preferably 1.8 % or less. The Mn content is even more preferably 1.5 % or less.P: 0.0005 % to 0.05 %

[0027] The addition of a trace amount of P has a strength improving effect on the steel component due to solid solution strengthening. To achieve this effect, P content is 0.0005 % or more. The P content is preferably 0.001 % or more. On the other hand, when the P content exceeds 0.05 %, toughness of the slab is decreased due to grain boundary embrittlement. As a result, cracks may occur during casting and cooling of the slab and during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The P content is therefore 0.05 % or less. The P content is preferably 0.04 % or less.S: 0.03 % or less

[0028] S is an element that causes grain boundary embrittlement during slab casting. When S is excessive, cracks may occur during casting of the slab, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. S content is therefore 0.03 % or less. The S content is preferably 0.02 % or less. On the other hand, from the viewpoint of cracking during casting of the slab, the lower the S content, the better, and therefore there is no particular lower limit for the S content. However, excessively reducing the S content causes an increase in production costs. Therefore, from a production cost viewpoint, the S content is preferably 0.0001 % or more. The S content is more preferably 0.0005 % or more.Al: 0.001 % to 0.1 %

[0029] Al is an element necessary for deoxidation during steelmaking. Al content is therefore 0.001 % or more. On the other hand, when Al is excessive, nitrides and oxides are formed, which decreases the toughness of the slab. As a result, cracks may occur during casting and cooling of the slab and during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Al content is therefore 0.1 % or less. The Al content is preferably 0.08 % or less.N: 0.001 % to 0.01 %

[0030] N is an element that refines grain size by forming fine nitrides, thereby improving the strength of the steel component. N content is therefore 0.001 % or more. On the other hand, when N is excessive, N combines with Al to form nitrides, which decreases the toughness of the steel slab, and this may cause cracks during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The N content is therefore 0.01 % or less. The N content is preferably 0.008 % or less.O: 0.0100 % or less

[0031] O is present as an oxide and is an element that causes embrittlement of the slab. O decreases the toughness of the slab, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. O content is therefore 0.0100 % or less. The O content is preferably 0.0050 % or less. A lower limit of the O content is not particularly limited, and may be 0 %. However, in view of production technology constraints, the O content is preferably 0.0001 % or more.Cr: 0.56 % to 1.6 %

[0032] Cr is an element that dissolves in cementite, and improves the hardness of cementite and the wear resistance of the steel component. Further, Cr delays the dissolution of cementite during quenching, thereby improving the number density of cementite in the steel component. To obtain the above effects, Cr content is 0.56 % or more. The Cr content is preferably 0.60 % or more. The Cr content is more preferably 0.70 % or more. On the other hand, when the Cr content exceeds 1.6 %, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Cr content is therefore 1.6 % or less. The Cr content is preferably 1.5 % or less. The Cr content is more preferably 1.3 % or less.

[0033] The chemical composition described above contains at least one element selected from the group consisting of Nb, Ti, and V. In order to obtain a desired wear resistance, it is necessary to contain at least one of Nb, Ti, or V in the following amounts.Nb: 0.029 % to 0.21 %

[0034] Nb is an element that forms carbides and can improve wear resistance. When Nb is contained, in order to obtain these effects, Nb content is 0.029 % or more. On the other hand, when Nb is excessive, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Nb content is therefore 0.21 % or less. The Nb content is preferably 0.15 % or less. The Nb content is more preferably 0.10 % or less.Ti: 0.01 % to 0.21 %

[0035] Ti is an element that forms carbides and can improve wear resistance. When Ti is contained, in order to obtain these effects, Ti content is 0.01 % or more. On the other hand, when Ti is excessive, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The Ti content is therefore 0.21 % or less. The Ti content is preferably 0.15 % or less. The Ti content is more preferably 0.10 % or less.V: 0.01 % to 0.21 %

[0036] V is an element that forms carbides and can improve wear resistance. When V is contained, in order to obtain these effects, V content is 0.01 % or more. On the other hand, when V is excessive, the toughness of the slab decreases, which may cause cracks to occur during casting and cooling of the slab or during slab heating, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. The V content is therefore 0.21 % or less. The V content is preferably 0.15 % or less. The V content is more preferably 0.10 % or less.Nb + Ti + V: 0.21 % or less

[0037] When the total content of Nb, Ti, and V in the chemical composition exceeds 0.21 %, the toughness of the steel slab decreases, which may cause cracks to occur during the production of the steel slab or during slab heating before hot rolling, resulting in a significant decrease in productivity. Further, surface defects occur on the steel component. Therefore, the total content of Nb, Ti, and V is 0.21 % or less. On the other hand, a lower limit of the total content is not particularly limited, but 0.01 % is a practical lower limit. The lower limit corresponds to a case where only either Ti or V is contained in an amount of 0.01 %. The total content may be 0.020 % or more. The total content may be 0.025 % or more. The total content may be 0.030 % or more.

[0038] The steel component according to an embodiment of the present disclosure has a chemical composition consisting of the above components, with the balance being Fe and inevitable impurity. The inevitable impurity includes H.

[0039] Further, the chemical composition of the steel component according to another embodiment of the present disclosure may optionally further contain at least one of the following elements.Ta: 0.10 % or less

[0040] Ta has an effect of further improving the wear resistance of the steel component by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when Ta content exceeds 0.10 %, a large amount of coarse precipitates and inclusions are formed, and the toughness of the steel slab decreases. The Ta content is therefore 0.10 % or less. The Ta content is preferably 0.08 % or less. On the other hand, a lower limit of the Ta content is not particularly limited. However, from the viewpoint of enhancing the effect of Ta addition, the Ta content is preferably 0.01 % or more.W: 0.10 % or less

[0041] W has an effect of further improving the wear resistance of the steel component by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when W content exceeds 0.10 %, a large amount of coarse precipitates and inclusions are formed, and the toughness of the steel slab decreases. The W content is therefore 0.10 % or less. The W content is preferably 0.08 % or less. On the other hand, a lower limit of the W content is not particularly limited. However, from the viewpoint of enhancing the effect of W addition, the W content is preferably 0.01 % or more.B: 0.0100 % or less

[0042] B segregates at austenite grain boundaries during hot rolling or annealing, and has an effect of further improving hardenability. However, when B content exceeds 0.0100 %, the toughness of the steel slab decreases. The B content is therefore 0.0100 % or less. The B content is more preferably 0.0080 % or less. On the other hand, a lower limit of the B content is not particularly limited. From the viewpoint of enhancing the effect of B addition, the B content is preferably 0.0003 % or more.Mo: 1.00 % or less

[0043] Mo is an element that has an effect of further improving hardenability. However, when Mo content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Mo content is therefore 1.00 % or less. The Mo content is preferably 0.80 % or less. On the other hand, a lower limit of the Mo content is not particularly limited. However, from the viewpoint of enhancing the effect of Mo addition, the Mo content is preferably 0.01 % or more.Co: 1.00 % or less

[0044] Co is an element that has an effect of further improving hardenability. However, when Co content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Co content is therefore 1.00 % or less. The Co content is preferably 0.80 % or less. On the other hand, a lower limit of the Co content is not particularly limited. From the viewpoint of enhancing the effect of Co addition, the Co content is preferably 0.001 % or more.Ni: 1.00 % or less

[0045] Ni is an element that has an effect of further improving hardenability. However, when Ni content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Ni content is therefore 1.00 % or less. The Ni content is preferably 0.80 % or less. On the other hand, a lower limit of the Ni content is not particularly limited. However, from the viewpoint of enhancing the effect of Ni addition, the Ni content is preferably 0.01 % or more.Cu: 1.00 % or less

[0046] Cu is an element that has an effect of further improving hardenability. However, when Cu content exceeds 1.00 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Cu content is therefore 1.00 % or less. The Cu content is preferably 0.80 % or less. On the other hand, a lower limit of the Cu content is not particularly limited. However, from the viewpoint of enhancing the effect of Cu addition, the Cu content is preferably 0.01 % or more.Sn: 0.200 % or less

[0047] Sn is an element that has an effect of further improving hardenability. However, when Sn content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Sn content is therefore 0.200 % or less. The Sn content is preferably 0.100 % or less. On the other hand, a lower limit of the Sn content is not particularly limited. However, from the viewpoint of enhancing the effect of Sn addition, the Sn content is preferably 0.001 % or more.Sb: 0.200 % or less

[0048] Sb is an element that suppresses decarburization and enables strength adjustment of the steel sheet. However, when Sb content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Sb content is therefore 0.200 % or less. The Sb content is preferably 0.100 % or less. On the other hand, a lower limit of the Sb content is not particularly limited. However, from the viewpoint of enhancing the effect of Sb addition, the Sb content is preferably 0.001 % or more.Ca: 0.0100 % or less

[0049] Ca is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Ca content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Ca content is therefore 0.0100 % or less. The Ca content is preferably 0.0050 % or less. On the other hand, a lower limit of the Ca content is not particularly limited. However, from the viewpoint of enhancing the effect of Ca addition, the Ca content is preferably 0.0005 % or more.Mg: 0.0100 % or less

[0050] Mg is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Mg content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Mg content is therefore 0.0100 % or less. The Mg content is preferably 0.0050 % or less. On the other hand, a lower limit of the Mg content is not particularly limited. However, from the viewpoint of enhancing the effect of Mg addition, the Mg content is preferably 0.0005 % or more.REM: 0.0100 % or less

[0051] REM (rare earth metals) are elements that have an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when REM content exceeds 0.0100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The REM content is therefore 0.0100 % or less. The REM content is preferably 0.0050 % or less. On the other hand, a lower limit of the REM content is not particularly limited. However, from the viewpoint of enhancing the effect of REM addition, the REM content is preferably 0.0005 % or more.Zr: 0.100 % or less

[0052] Zr is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Zr content exceeds 0.100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Zr content is therefore 0.100 % or less. The Zr content is preferably 0.080 % or less. On the other hand, a lower limit of the Zr content is not particularly limited. However, from the viewpoint of enhancing the effect of Zr addition, the Zr content is preferably 0.001 % or more.Te: 0.100 % or less

[0053] Te is an element that has an effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, when Te content exceeds 0.100 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases instead. The Te content is therefore 0.100 % or less. The Te content is preferably 0.080 % or less. On the other hand, a lower limit of the Te content is not particularly limited. However, from the viewpoint of enhancing the effect of Te addition, the Te content is preferably 0.001 % or more.Hf: 0.10 % or less

[0054] Hf is an element that has an effect of spheroidizing the shape of nitrides and sulfides and improving ultimate deformability of the steel sheet. However, when Hf content exceeds 0.10 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. Th Hf content is therefore 0.10 % or less. The Hf content is preferably 0.08 % or less. On the other hand, a lower limit of the Hf content is not particularly limited. However, from the viewpoint of enhancing the effect of Hf addition, the Hf content is preferably 0.01 % or more.Bi: 0.200 % or less

[0055] Bi is an element that mitigates segregation. However, when Bi content exceeds 0.200 %, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab decreases. The Bi content is therefore 0.200 % or less. The Bi content is preferably 0.100 % or less. On the other hand, a lower limit of the Bi content is not particularly limited. However, from the viewpoint of enhancing the effect of Bi addition, the Bi content is preferably 0.001 % or more.

[0056] The elements from Ta to Bi have effects of further improving the properties of the cold-rolled steel sheet of the present disclosure, and can be contained in the steel sheet of the present disclosure. In the above description, the lower limit values of the preferred content of these optionally-contained elements are indicated. However, when the content of these elements is lower than the preferred lower limit, the effect of adding the elements is decreased, but the properties of the cold-rolled steel sheet still satisfy requirements. Accordingly, containing these elements is not essential, and the lower limit of content may be 0 %.[Cementite]

[0057] The following describes cementite contained in the steel component of the present disclosure.Number density of cementite particles: 200,000 / mm 2< or more

[0058] According to the present disclosure, the wear resistance can be improved by controlling the number density of cementite particles. In order to obtain the desired wear resistance, it is necessary to control the number density of cementite particles having a particle size of 0.090 µm or more to 200,000 / mm 2< or more. Therefore, the number density of cementite particles having a particle size of 0.090 µm or more contained in the steel component is 200,000 / mm 2< or more. The number density is preferably 250,000 / mm 2< or more. The number density is more preferably 300,000 / mm 2< or more. The number density is even more preferably 400,000 / mm 2< or more. The number density is most preferably 500,000 / mm 2< or more. On the other hand, an upper limit of the number density is not particularly limited, but may be, for example, 2,000,000 / mm 2< or less, 1,500,000 / mm 2< or less, or 1,200,000 / mm 2< or less. Hereinafter, the "number density of cementite particles having a particle size of 0.090 µm or more" may be referred to simply as the "number density of cementite" or the "number density".Cr amount in cementite particles: 2.5 % or more

[0059] When Cr is dissolved in cementite, the hardness of the cementite increases, and the harder the cementite, the higher the resistance to wear and the more wear resistance improves. However, when the Cr amount in the cementite particles is less than 2.5 %, the desired wear resistance cannot be obtained. The Cr amount in the cementite particles is therefore 2.5 % or more. The Cr amount in the cementite particles is preferably 2.7 % or more. The Cr amount in the cementite particles is more preferably 3.0 % or more. The Cr amount in the cementite particles is even more preferably 4.0 % or more. On the other hand, an upper limit of the Cr amount in the cementite particles is not particularly limited. The Cr amount in the cementite particles is preferably 10 % or less.[Method of producing steel component]

[0060] The following describes a method of producing the steel component according to an embodiment. In the following explanation, unless otherwise specified, temperature indicated in "°C" indicates a surface temperature (a temperature at a surface of the steel slab, the steel sheet, or the like).

[0061] The steel component can be produced by sequentially carrying out the following processes on a steel slab having the above-mentioned chemical composition and produced by continuous casting. (1) Cooling (2) Heating (3) Hot rolling (4) Cooling (5) Coiling (6) First annealing (7) Cold rolling (8) Second annealing (9) Working and heat treatment (1) Cooling

[0062] First, a steel slab having the above-mentioned chemical composition and produced by continuous casting is cooled.Residence time in temperature range from 1200 °C to 1400 °C: 130 s or shorter

[0063] During the cooling, the residence time in the temperature range from 1200 °C to 1400 °C at a transverse direction center position of the steel slab 10 mm from a surface is 130 s or shorter. The reasons for this are described below.

[0064] Prior austenite grain size is a factor that determines a fracture unit, and the larger the prior austenite grain size, the lower the toughness. The austenite grain size of the steel slab is determined by the residence time in the temperature range from 1200 °C to 1400 °C, and the longer the residence time in this temperature range, the coarser the prior austenite grain size becomes. When the residence time exceeds 130 s, the prior austenite grain size becomes coarse, which may cause slab cracking. Therefore, according to the present disclosure, the residence time in the temperature range from 1200 to 1400 °C is 130 s or shorter. The residence time is preferably 120 s or shorter. The residence time is more preferably 110 s or shorter. The residence time is even more preferably 100 s or shorter.

[0065] On the other hand, although there is no particular lower limit to the residence time in the temperature range, when the residence time is too short, the risk of breakout occurring due to non-uniform solidification increases. Here, breakout refers to a phenomenon in which a part of the solidified shell breaks during continuous casting, causing molten steel inside to leak out. The residence time is therefore preferably 50 s or longer. The residence time is more preferably 70 s or longer.Average cooling rate in temperature range from 550 °C to 700 °C: 10 °C / h or less

[0066] Further, in the cooling, the average cooling rate in the temperature range from 550 °C to 700 °C is 10 °C / h or less at a temperature at a transverse direction center of the steel slab and 10 mm from a surface. The reasons for this are described below.

[0067] The temperature range is a temperature range in which the microstructure of the steel slab transforms into pearlite. When the cooling rate in this temperature range is 10 °C / h or less, bainite transformation can be suppressed, and the microstructure of the steel slab can be made mainly pearlite. As a result, internal stress is decreased and cracks in the slab can be prevented. Therefore, the average cooling rate in the temperature range from 550 °C to 700 °C is 10 °C / h or less. The average cooling rate is preferably 8 °C / h or less. The average cooling rate is more preferably 6 °C / h or less. A lower limit of the average cooling rate is not particularly limited. However, a separate energy source is required for control, and therefore the average cooling rate is preferably 1 °C / h or more. The average cooling rate is more preferably 2 °C / h or more.

[0068] Here, the temperature of the steel slab is the temperature at a transverse direction center of the steel slab at a position 10 mm from a surface. Actually measuring the temperature is difficult, and therefore the temperature is calculated by thermal analysis. The temperature at the transverse direction center of the steel slab is used because this position is the position in the slab where the residence time in temperature range described above is the longest.

[0069] In the cooling, the steel slab may be cooled to 550 °C or lower. In other words, the cooling end temperature is 550 °C or lower. The cooling end temperature is preferably 300 °C or lower. The cooling end temperature is more preferably 100 °C or lower. The cooling end temperature is even more preferably 50 °C or lower. Typically, it is sufficient to cool to atmospheric temperature. For example, when a steel slab is allowed to naturally cool outdoors, the steel slab can be cooled to ambient temperature. On the other hand, a lower limit of the cooling end temperature is not particularly limited, but for example, when the steel slab is allowed to naturally cool outdoors, the cooling end temperature depends on the ambient temperature. Typically, the cooling end temperature is preferably -10 °C or higher. The cooling end temperature is more preferably 0 °C or higher. The cooling end temperature is even more preferably 10 °C or higher.

[0070] The method of the cooling is not particularly limited and may be any method. For example, in a continuous casting line, it is common for the steel to be cooled in a mold and then spray cooled. According to the present disclosure, cooling within a mold and spray cooling are carried out in the continuous casting line, and it is preferable that the residence time in the temperature range from 1200 °C to 1400 °C is 130 s or shorter.

[0071] On the other hand, cooling in the temperature range from 550 °C to 700 °C is preferably carried out by allowing the continuously cast slab to cool naturally after cutting. For example, cut steel slabs can be stacked and allowed to naturally cool. At this time, the average cooling rate in the temperature range from 550 °C to 700 °C is preferably adjusted to 10 °C / h or less by covering the steel slab with a cover.(2) Heating

[0072] Next, the steel slab after the cooling is heated.Slab heating temperature: 1100 °C or higher

[0073] Heating is carried out prior to hot rolling to homogenize the components and dissolve carbides such as cementite and segregations present in the steel slab. However, when the heating temperature of the steel slab (slab heating temperature) during the heating is lower than 1100 °C, cementite cannot be sufficiently dissolved, and as a result, the number density of cementite in the finally obtained steel component cannot be set within the desired range. The slab heating temperature is therefore 1100 °C or higher. The slab heating temperature is preferably 1150 °C or higher. On the other hand, although there is no particular upper limit to the slab heating temperature, when the slab heating temperature is excessively high, the surface characteristics may deteriorate. Therefore, from the viewpoint of further improving the surface characteristics, the slab heating temperature is preferably 1350 °C or lower.Slab heating time: 60 min or longer

[0074] When the heating time of the steel slab during the heating (slab heating time) is shorter than 60 min, carbides such as cementite cannot be sufficiently dissolved, and as a result, the number density of cementite in the finally obtained steel component cannot be set within the desired range. The slab heating time is therefore 60 min or longer. The slab heating time is preferably 90 min or longer. On the other hand, an upper limit of the slab heating time is not particularly limited. The slab heating time is preferably 300 min or shorter. The slab heating time is more preferably 200 min or shorter.

[0075] The heating may be carried out by any method, but use of a heating furnace is preferred.(3) Hot rolling

[0076] The heated steel slab is then hot rolled to obtain a hot-rolled steel sheet. In the hot rolling, rough rolling and finishing rolling may be carried out according to conventional methods.Rolling finish temperature: higher than Tc, 950 °C or lower

[0077] When the rolling finish temperature in the hot rolling is Tc or lower, as defined by the following expression (1), a portion of austenite is transformed into pearlite during rolling, which promotes non-uniform deformation and degrades the surface characteristics. The rolling finish temperature is therefore higher than Tc. The rolling finish temperature is preferably Tc + 50 °C or higher. The rolling finish temperature is more preferably Tc + 100 °C or higher. On the other hand, when the rolling finish temperature exceeds 950 °C, the surface scale becomes firm and the surface characteristics degrade. The rolling finish temperature is therefore 950 °C or lower. The rolling finish temperature is preferably 930 °C or lower. The rolling finish temperature is more preferably 910 °C or lower.

[0078] Here, Tc is a value that can be used as an index of the temperature at which a transformation involving cementite occurs, and is defined by the following expression (1).

[0079] Here, the element symbols in expression (1) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0.(4) CoolingAverage cooling rate: 20 °C / s or more

[0080] After the finishing rolling is completed, cooling is started, then stopped at a cooling stop temperature. When the average cooling rate in the cooling is less than 20 °C / s, the number density of cementite in the finally obtained steel component cannot be set within the desired range. This is because when the average cooling rate is less than 20 °C / s, coarse cementite is formed and this coarse cementite remains in the steel component. The average cooling rate is therefore 20 °C / s or more. The average cooling rate is preferably 30 °C / s or more. The average cooling rate is more preferably 50 °C / s or more. On the other hand, although there is no particular upper limit to the average cooling rate, when the cooling rate is excessively high, controlling the cooling stop temperature becomes difficult. The average cooling rate is therefore preferably 500 °C / s or less.Cooling stop temperature: Tc or lower

[0081] When the cooling stop temperature in the cooling is higher than Tc, the number density of cementite in the finally obtained steel component cannot be set within the desired range. This is because when the cooling stop temperature is higher than Tc, coarse cementite is formed and this coarse cementite remains in the steel component. The cooling stop temperature is therefore Tc or lower. The cooling stop temperature is preferably Tc - 20 °C or lower. On the other hand, a lower limit of the cooling stop temperature is not particularly limited. However, when the cooling stop temperature is lower than 530 °C, the volume expansion caused by transformation during the subsequent coiling results in a poor coil shape. Therefore, from the viewpoint of improving the coiling shape, the cooling stop temperature is preferably 530 °C or higher. The cooling stop temperature is more preferably 550 °C or higher. The cooling stop temperature is even more preferably 600 °C or higher.(5) CoilingCoiling temperature: 530 °C or higher and Tc or lower

[0082] After the cooling is stopped, the cooled hot-rolled steel sheet is coiled. At this time, when the coiling temperature is higher than Tc, the number density of cementite in the finally obtained steel component cannot be set within the desired range. This is because when the coiling temperature is higher than Tc, coarse cementite is generated and this coarse cementite remains in the steel component. The coiling temperature is therefore Tc or lower. The coiling temperature is preferably Tc - 20 °C or lower. The coiling temperature is more preferably Tc - 40 °C or lower. On the other hand, when the coiling temperature is lower than 530 °C, volume expansion due to transformation during coiling results in a poor coiling shape. The coiling temperature is therefore 530 °C or higher. The coiling temperature is preferably 550 °C or higher. The coiling temperature is more preferably 600 °C or higher.(6) First annealing

[0083] Annealing temperature: 600 °C or higher and Tc or lower Annealing time: 3 h or longer

[0084] The hot-rolled steel sheet after the coiling is subjected to the first annealing under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer. The microstructure of the hot-rolled steel sheet after the coiling is a pearlitic microstructure lined with plate-like cementite and ferrite. The pearlitic microstructure is stable, and therefore does not homogenize without prolonged holding at a high temperature. In order to break up the pearlitic microstructure and allow the subsequent cold rolling and annealing process to produce the desired cementite, the annealing temperature needs to be 600 °C or higher and the annealing time needs to be 3 h or longer. The annealing temperature is preferably 620 °C or higher. Further, the annealing time is preferably 4 h or longer. On the other hand, when the annealing temperature is higher than Tc, phase transformation starts preferentially from one portion, and a locally coarse microstructure is formed. As a result, the microstructure becomes non-uniform, and the desired number density of cementite particles cannot be obtained. The annealing temperature is therefore Tc or lower. The annealing temperature is preferably Tc - 20 °C or lower. The annealing temperature is more preferably Tc - 40 °C or lower. On the other hand, although there is no particular upper limit to the annealing time, when too long, productivity decreases. The annealing time is therefore preferably 50 h or shorter. The annealing time is more preferably 30 h or shorter.

[0085] By carrying out the first annealing under the above conditions, the pearlitic microstructure can be broken up, and the desired cementite can be more easily formed in the subsequent cold rolling and annealing processes.

[0086] The first annealing can be carried out any number of times, but from the viewpoint of enhancing the effect of the annealing, the first annealing is preferably carried out two or more times. On the other hand, there is no particular upper limit to the number of times the first annealing is carried out, but even when the first annealing is carried out more than three times, the effect is saturated. The first annealing is therefore preferably carried out three times or less. When the first annealing is carried out two or more times, each annealing may be carried out under the above conditions. The annealing conditions for each time may be the same or different from each other.

[0087] Prior to the first annealing, the hot-rolled steel sheet is preferably pickled.(7) Cold rolling(8) Second annealing

[0088] Plate-like cementite is formed in the steel sheet after hot rolling. This plate-like cementite is stable and therefore prone to remain for a long time. The plate-like cementite is coarse, and therefore when the cementite remains, the number density of the cementite particles in the final steel component cannot be set within the desired range. As a result, the desired wear resistance is not obtainable. Therefore, in order to refine and spheroidize the plate-like cementite by heating during annealing, the hot-rolled steel sheet after the first annealing is subjected to cold rolling and second annealing at least once.Rolling ratio: 15 % or more

[0089] The cold rolling causes the cementite to be deformed, broken up, and fragmented, thereby making it possible to obtain the desired number density of cementite particles. When the rolling ratio in the cold rolling is less than 15 %, the effect cannot be obtained. The rolling ratio is therefore 15 % or more. The rolling ratio is preferably 25 % or more. The rolling ratio is more preferably 35 % or more. The rolling ratio is even more preferably 45 % or more. On the other hand, an upper limit of the rolling ratio is not particularly limited. The rolling ratio is preferably 85 % or less. The rolling ratio is more preferably 80 % or less.Annealing temperature: 600 °C or higher and Tc or lowerAnnealing time: 3 h or longer

[0090] In the second annealing, the cementite that has been deformed, broken up, and fragmented by the cold rolling can be refined and spheroidized. At the same time, Cr can be concentrated in the cementite. To obtain these effects, the annealing temperature in the second annealing needs to be 600 °C or higher, and the annealing time needs to be 3 h or longer. The annealing temperature is preferably 620 °C or higher. Further, the annealing time is preferably 4 h or longer. On the other hand, when the annealing temperature is higher than Tc, phase transformation starts preferentially from one portion, and a locally coarse microstructure is formed. As a result, the microstructure becomes non-uniform, and the desired number density of cementite particles cannot be obtained. The annealing temperature is therefore Tc or lower. The annealing temperature is preferably Tc - 20 °C or lower. The annealing temperature is more preferably Tc - 40 °C or lower. On the other hand, although there is no particular upper limit to the annealing time, when too long, productivity decreases. The annealing time is therefore preferably 30 h or shorter. The annealing time is more preferably 20 h or shorter.

[0091] By carrying out the cold rolling and the second annealing under the above conditions, it is possible to promote the refinement of cementite and the concentration of Cr in the cementite.

[0092] The cold rolling and the second annealing can be carried out any number of times, but from the viewpoint of enhancing the above-described effects, the process is preferably carried out two or more times. When the cold rolling and the second annealing are carried out two or more times, the cold rolling and the second annealing may be repeated alternately. On the other hand, there is no particular upper limit to the number of times that the cold rolling and the second annealing are carried out, but even when the number of times is more than five, the effect is saturated. The cold rolling and the second annealing are therefore preferably carried out five times or less. When the cold rolling and the second annealing are carried out two or more times, each iteration of the cold rolling and the second annealing may be carried out under the above conditions. The conditions for each iteration may be the same or different from each other.

[0093] When the cold rolling and the second annealing are carried out only once, the rolling ratio of the cold rolling is preferably 70 % or more from the viewpoint of reliably refining cementite.

[0094] It is preferable to carry out cold rolling (final cold rolling) after carrying out cold rolling and second annealing, before carrying out subsequent working and heat treatment. By carrying out the final cold rolling, it is possible to improve workability in the subsequent process of forming the steel component, particularly blanking workability. To obtain this effect, the rolling ratio of the final cold rolling is preferably 20 % or more. The rolling ratio is more preferably 25 % or more. The rolling ratio is even more preferably 30 % or more.

[0095] Through the above procedure, a cold-rolled steel sheet is obtained. The sheet thickness of the cold-rolled steel sheet is not particularly limited and may be any thickness. The sheet thickness is preferably 0.1 mm or more. The sheet thickness is more preferably 0.2 mm or more. Further, an upper limit of the sheet thickness is not particularly limited. The sheet thickness is particularly 2.5 mm or less. The sheet thickness is more preferably 1.6 mm or less. The sheet thickness is even more preferably 0.8 mm or less. When the sheet thickness is 0.2 mm or more and 0.8 mm or less, the cold-rolled steel sheet is particularly suitable for use as a material for knitting needles and the like.

[0096] Further, the final cold-rolled steel sheet may be subjected to further optional surface treatment.(9) Working and heat treatment

[0097] Next, the obtained cold-rolled steel sheet is worked into a component shape and subjected to heat treatment. The working and the heat treatment can be carried out in any order. For example, the heat treatment may be carried out after the working, or during the working.

[0098] The method of the working is not particularly limited and any method may be applied. The working may be, for example, at least one selected from the group consisting of blanking, cutting, wire drawing, bending, and polishing.

[0099] The heat treatment includes quenching and tempering. Conditions are explained below.• Quenching

[0100] By carrying out quenching, the strength of the component can be increased, the number density of cementite particles can be controlled, and excellent wear resistance can be obtained. To obtain these effects, quenching temperature and holding time need to satisfy the following conditions.• Quenching temperature: Tc or higher and 1000 °C or lower

[0101] In the quenching, first, the cold-rolled steel sheet worked into a component shape is heated to a quenching temperature and held at the quenching temperature for a holding time described later. This transforms the microstructure into austenite. Then, by cooling, the austenite transforms into martensite, improving the strength. At this time, when the quenching temperature (heating temperature during quenching) is low, the transformation to austenite is insufficient. As a result, hard martensite is not obtained after cooling, and the desired wear resistance is not obtained. The quenching temperature is therefore Tc or higher. The quenching temperature is preferably Tc + 20 °C or higher. The quenching temperature is more preferably Tc + 50 °C or higher. On the other hand, when the quenching temperature is too high, cementite dissolves. As a result, the desired number density of cementite cannot be obtained, and the wear resistance decreases. The quenching temperature is therefore 1000 °C or lower. The quenching temperature is preferably 950 °C or lower. The quenching temperature is more preferably 900 °C or lower. The quenching temperature is even more preferably 870 °C or lower. The quenching temperature is most preferably 840 °C or lower.• Holding time: 1.0 min or longer and 60 min or shorter

[0102] During the quenching, in order to transform the microstructure into austenite, holding at the heating temperature for 1.0 min or longer is required. The holding time is therefore 1.0 min or longer. The holding time is preferably 2 min or longer. The holding time is more preferably 5 min or longer. On the other hand, when the holding time exceeds 60 min, dissolution of cementite proceeds, the desired cementite number density cannot be obtained, and the wear resistance decreases. The holding time is therefore 60 min or less.

[0103] Cooling in the quenching process is preferably performed by cooling to 150 °C or lower using oil or other coolant. It is also preferable to cool the steel component by pressing with a mold, or by die quenching, press quenching, or the like to a temperature of 150 °C or lower.• Tempering

[0104] The steel component after quenching is then tempered. By carrying out the tempering, the hardness and toughness of the steel component can be adjusted. The tempering can be carried out under any conditions without any particular limitations. Typically, it is preferable that the tempering temperature is 100 °C to 400 °C and the holding time is 10 min or longer and 180 min or shorter.

[0105] The heat treatment may include any other heat treatment in addition to quenching and tempering. According to an embodiment of the present disclosure, the heat treatment may consist of quenching and tempering.

[0106] According to the above method, the steel component having excellent wear resistance and surface characteristics may be produced. Applications of the steel component are not particularly limited. The steel component is particularly suitable for applications requiring wear resistance, such as components for textile machinery, bearing components, and blades for machinery.EXAMPLES

[0107] In order to confirm the effects of the present disclosure, steel components were produced according to the following procedure.

[0108] First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then subjected to cooling, heating, hot rolling, cooling, coiling, first annealing, cold rolling, and second annealing in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Table 2, and the first annealing, cold rolling, and second annealing were carried out the number of times listed in Table 2.

[0109] For some examples, the cold-rolled steel sheet after the second annealing was further subjected to final cold rolling. The rolling reduction in the final cold rolling is listed in Tables 2 and 3.

[0110] The resulting cold-rolled steel sheet was then worked into component shapes and heat-treated to obtain the final steel component.

[0111] In working the component shape, a plurality of steel components were obtained from one cold-rolled steel sheet by wire electric discharge machining. The wire electric discharge machining was carried out from a position 5 mm inward from a transverse direction end of the cold-rolled steel sheet at intervals of about 15 mm in the transverse direction and at intervals of about 100 mm in the length direction (rolling direction). The dimensions of the steel component were 10 mm wide × 80 mm long.

[0112] In the heat treatment, quenching and tempering were carried out. The quenching was carried out under the conditions listed in Table 2. The tempering was carried out under conditions such that the Vickers hardness of the steel component after the tempering was 710 ± 10.

[0113] According to the above procedure, a plurality of steel components were produced for each example.

[0114] Next, for each of the obtained steel components, the number density of cementite particles and the Cr amount in the cementite particles were measured by the following procedures. The measurement results are listed in Tables 4 and 5.(Number density of cementite)

[0115] Test pieces for microstructure observation were taken from the obtained steel components. For each test piece for microstructure observation, a rolling direction cross-section (L-section) was polished to a mirror finish, and then etched with 1 vol% to 3 vol% nital to reveal the microstructure. Thereafter, the etched surface of the test piece for microstructure observation was imaged at ten locations at a 1 / 2 thickness position using a scanning electron microscope (SEM) at an accelerating voltage of 15 keV and a magnification of 3000 times to obtain microstructure images. From the obtained microstructure images, cementite particles having a particle size of less than 0.090 µm were removed by image processing, and the number of cementite particles having a particle size of 0.090 µm or more was counted. The number density of cementite was calculated by dividing the obtained number of particles by the area of the microstructure image. The number density was calculated for each of the ten microstructure images using the same procedure, and an average value was taken as the number density of cementite having a particle size of 0.090 µm or more.(Cr amount in cementite particles)

[0116] Test pieces for microstructure observation were taken from the obtained steel components. L-sections of the test pieces for microstructure observation were each polished to a mirror surface. Thereafter, the polished surface of the test piece for microstructure observation was observed by SEM to identify cementite particles. Next, using an electron probe micro analyzer (EPMA), the Cr amount was measured at three locations with respect to one cementite particle having an area of 0.06 µm 2< or more under the conditions of an electron beam accelerating voltage of 15 keV, an irradiation current of 1.0 × 10 -8< A, and a measurement time of 1000 ms, and an average value of the three locations was calculated. The Cr amount was similarly measured for ten cementite particles each having a particle size of 0.090 µm or more, and an average value for the ten particles was taken as the Cr amount in the cementite particles.

[0117] Further, the wear resistance and the surface characteristics of each of the obtained steel components were evaluated by the following procedures. The evaluation results are listed in Tables 4 and 5.(Surface characteristics)

[0118] The appearance of the steel components was visually inspected to evaluate the surface characteristics. Specifically, the surface characteristics were evaluated as "good" when no surface defects caused by scabs or slivers on the cold-rolled steel sheet were observed on any of the steel components, and the surface characteristics were evaluated as "poor" when at least one surface defect caused by scabs or slivers on the cold-rolled steel sheet was observed on any of the steel components.(Wear resistance)

[0119] The wear resistance of the steel components was evaluated by the following procedure. First, a wear test piece 10 having the shape illustrated in FIG. 1 was taken from the steel component. Each of the wear test pieces 10 was provided with four holes 11 for threading.

[0120] Wear tests were conducted using the wear test pieces 10 and a wear test apparatus 20 illustrated in FIG. 2. Specifically, a wear amount d 1 was measured by running a yarn S fed from a yarn unwinder 21 for 5000 m per hole with the yarn S in contact with the side of the hole 11 of the wear test piece 10. Thereafter, the yarn S was again run at the same position for another 15,000 m per hole (total 20,000 m) while in contact with the side of the hole 11 of the wear test piece 10, and a wear amount d 2 was measured. Full dull polyester knitting yarn was used as the yarn S. The running speed of the yarn S was 200 m / min. Further, the tension of the yarn was adjusted to 20 ± 2 N / cm using a tension regulator 22.

[0121] As illustrated in FIG. 3, a groove 12 was formed by wear at a point where the hole 11 was in contact with the yarn. After running the yarn for 5000 m and 20,000 m, the running was stopped and a depth d (wear depth) of the groove 12 was measured using optical microscopy.

[0122] The wear depth when the yarn had run 5000 m was defined as d 1 , and the wear depth when the yarn had run a further 15,000 m (total 20,000 m) was defined as d 2 . The difference between d 1 and d 2 was defined as Δd, which was used as an index of wear resistance.

[0123] The same test was carried out on each of the four holes 11 and an average value of the four wear depths obtained was taken as the wear depth Δd of the wear test piece 10. When the wear depth Δd was 10 µm or less, the wear resistance was rated as "good", and when more than 10 µm, the wear resistance was rated as "poor". The evaluation results are listed in Tables 4 and 5.[Table 1]

[0124] Table 1Steel sample IDChemical composition (mass%)Nb + Ti + V (mass%)Tc (°C)RemarksCSiMnPSAlNOCrNbTiVOtherA0.910.210.510.0080.00360.0090.00340.00050.700.080---0.080735Disclosed steelB0.850.331.120.0050.00370.0080.00300.00210.560.0390.02--0.059730Disclosed steelC1.100.100.520.0070.00400.0040.00350.00071.800.101-0.08-0.181751Comparative steelD1.250.320.820.0080.00330.0040.00290.00100.750.0950.060.04-0.195736Disclosed steelE1.050.150.500.0050.00130.0040.00330.00081.08-0.12--0.120740Disclosed steelF1.080.320.280.0090.00390.0920.00420.00061.31--0.19-0.190751Disclosed steelG0.980.230.750.0040.00070.0050.00330.00150.61-0.100.09-0.190732Disclosed steelH1.070.201.130.0110.00280.0030.00320.00051.420.094--Ta: 0.05, Mo: 0.4, Cu: 0.12, Sb: 0.010.094741Disclosed steelI0.970.551.010.0090.00190.0070.00450.00100.730.095---0.095741Disclosed steelJ1.130.150.350.0100.00040.0040.00290.00051.110.062--Mo: 0.120.062742Disclosed steelK0.860.182.980.0060.00180.0050.00360.00100.930.136---0.136712Comparative steelL0.920.801.070.0090.00380.0090.00520.00050.820.121--Zr: 0.06, Te: 0.040.121749Disclosed steelM1.080.200.700.0450.02810.0420.00340.00111.590.0290.05--0.079748Disclosed steelN0.870.990.900.0080.00340.0090.00500.00120.870.078--Hf: 0.05, Bi: 0.080.078757Disclosed steelO1.090.242.450.0060.00230.0100.00340.00941.270.046--W: 0.04, Mg: 0.005, REM: 0.0050.046725Disclosed steelP1.050.220.730.0360.01520.0080.00290.00130.750.192---0.192734Disclosed steelQ1.080.121.930.0040.00290.0710.00300.00091.460.097--Cu: 0.15, Sb: 0.010.097731Disclosed steelR0.850.340.490.0090.00200.0080.00940.00061.100.077--Ni: 0.05, Cu: 0.120.077745Disclosed steelS0.870.251.200.0050.00090.0030.00740.00090.750.140--Ca: 0.0050.140730Disclosed steelT0.870.290.710.0040.00390.0030.00350.00120.990.031--B: 0.00150.031741Disclosed steelU1.090.310.620.0090.00190.0090.00100.00060.750.184--Co: 0.006, Sn: 0.050.184738Disclosed steela0.750.281.020.0050.00280.0070.00350.00050.720.033---0.033732Comparative steelb0.870.240.150.0080.00230.0050.00380.00070.590.053---0.053738Comparative steelc0.890.350.350.0130.00210.0040.00380.00060.420.048---0.048737Comparative steeld0.810.260.450.0050.00210.0080.00350.00100.590.020---0.020736Comparative steele1.060.331.020.0090.00160.0100.00220.00161.090.261---0.261740Comparative steelf0.980.291.710.0090.00340.0080.00140.00121.050.1910.08--0.271731Comparative steelg1.100.330.770.0100.00180.0060.00320.00091.48-0.150.20-0.350749Comparative steelh1.081.341.720.0090.00320.0060.00430.00061.230.152---0.152764Comparative steeli1.020.423.540.0130.00420.0080.00520.00091.050.123---0.123715Comparative steelj1.050.531.940.0110.00180.0070.00350.00072.360.112---0.112758Comparative steel [Table 2]

[0125] Table 2NoSteel sample IDCoolingHeatingHot rollingCoolingCoilingFirst annealingCold rollingSecond annealingNumber of times cold rolling and second annealingFinal cold rollingQuenchingRemarks1400 °C to 1200 °C Residence time (s)700 °C to 550 °C Average cooling rate (°C / h)Slab heating temp. (°C)Time (min)Rolling finish temp. (°C)Average cooling rate (°C / s)Cooling stop temp. (°C)Coiling temp. (°C)Heating temp. (°C)Annealing time (h)TimesRolling ratio (%)Heating temp. (°C)Annealing time (h)Rolling ratio (%)Quenching temp. (°C)Holding time (min)1A1205118070900306806607107245680423579017Example2B1134120080910806506206708230630103408109Example3B11341200809108065062067081506301023082012Example4B1134120080910806506206708275640111-80015Example5B11341200809108065062067082306301034079013Example6C10341220908754060058069062506301022077055Comparative Example7D1175117080860206205706702122564075-83015Example8E10561150708505064059070021245640623581010Example9F10031120658453066062069016240670142458007Example10G109611708088035700660680202256301151580012Example11H12831200908657559055071082506601323084010Example12I9541220909005066065068010245640112358305Example13I95412209090050660650680102706401113581019Example14I95410303080060660650710102457001123579010Comparative Example15-1I9541220907008066065071010245700112358205Comparative Example15-2I95412209098040660650710102457001123583014Comparative Example16I9541220908001066065070010245700112357907Comparative Example17I9541220909008080077070010245700112358008Comparative Example18I95412209090050660650550224560042358408Comparative Example19I95412209090050660650770102456901123582013Comparative Example20I95412209090050660650620314555022358109Comparative Example21I9541220909005066065068010245770112358505Comparative Example22I95412209090050660650710102107001126580019Comparative Example23I95412209090050660650680102456401123570020Comparative Example24I95412209090050660650680102456401123510508Comparative Example25J1187123010095040630610670112506501322079011Example [Table 3]

[0126] Table 3NoSteel sample IDCoolingHeatingHot rollingCoolingCoilingFirst annealingCold rollingSecond annealingNumber of times cold rolling and second annealingFinal cold rollingQuenchingRemarks1400 °C to 1200 °C Residence time (s)700 °C to 550 °C Average cooling rate (°C / h)Slab heating temp. (°C)Time (min)Rolling finish temp. (°C)Average cooling rate (°C / s)Cooling stop temp. (°C)Coiling temp. (°C)Heating temp. (°C)Annealing time (h)TimesRolling ratio (%)Heating temp. (°C)Annealing time (h)Rolling ratio (%)Quenching temp. (°C)Holding time (min)26K112811406086030650600700162356501225081018Comparative Example27L9131190909203557052068092506601323081016Example28M102711808087525730710700624063073-8006Example29N10841150708704570067067052306401034081014Example30O12351220909005060057071021245660112359601Example31P9651290708504560059068010250640142258309Example32P1505slab crack occurred during continuous castingComparative Example33P9621slab crack occurred during continuous castingComparative Example34Q115210702108454058054070025235650725083014Example35R105312501809107062059068024230660132608105Example36S1131115010093010055051068019250630112-81018Example37T123311707087035580580700132506701223085019Example38U96411601009003057053065020250660112-78014Example39a11241190809054567064071072356901125082010Comparative Example40b1053117080900507106607001324564042359103Comparative Example41c113412001109104564062068011230650626081020Comparative Example42d10241120557302564059067016225640726582019Comparative Example43e1123slab crack occurred during continuous castingComparative Example44f1054slab crack occurred during continuous castingComparative Example45g1215slab crack occurred during continuous castingComparative Example46h1055slab crack occurred during continuous castingComparative Example47i986slab crack occurred during continuous castingComparative Example48j1205slab crack occurred during continuous castingComparative Example [Table 4]

[0127]

[0132] Table 4No.Steel sample IDCementiteSurface characteristicsWear resistanceRemarksNumber density *1 ( / mm 2< )Cr amount *2 (mass%)1A3110003.1goodgoodExample2B5840002.8goodgoodExample3B4880002.9goodgoodExample4B5520002.8goodgoodExample5B6880002.7goodgoodExample6C13050008.3poorgoodComparative Example7D3650003.9goodgoodExample8E5670005.3goodgoodExample9F5300006.3goodgoodExample10G5560002.9goodgoodExample11H5410008.1goodgoodExample12I5430003.9goodgoodExample13I5000003.7goodgoodExample14I1770003.6goodpoorComparative Example15-1I2990003.8poorgoodComparative Example15-2I2040003.9poorgoodComparative Example16I1930003.6goodpoorComparative Example17I1820003.7goodpoorComparative Example18I1260003.9goodpoorComparative Example19I1020003.8goodpoorComparative Example20I1630003.6goodpoorComparative Example21I1030004.2goodpoorComparative Example22I1660003.7goodpoorComparative Example23I11400002.8goodpoorComparative Example24I0-goodpoorComparative Example25J6300005.2goodgoodExample*1: Number density of cementite particles having particle size of 0.090 µm or more *2: Cr amount in cementite particles having particle size of 0.090 µm or more [Table 5]

[0128] Table 5No.Steel sample IDCementiteSurface characteristicsWear resistanceRemarksNumber density *1 ( / mm 2< )Cr amount *2 (mass%)26K8910005.9poorgoodComparative Example27L4710004.3goodgoodExample28M10130007.9goodgoodExample29N5550004.4goodgoodExample30O5640008.0goodgoodExample31P4340003.9goodgoodExample32Pslab crack occurred during continuous castingComparative Example33Pslab crack occurred during continuous castingComparative Example34Q7010008.5goodgoodExample35R5410005.5goodgoodExample36S6250003.9goodgoodExample37T297000 5.5goodgoodExample38U5080003.4goodgoodExample39a1810003.8goodpoorComparative Example40b1210002.8goodpoorComparative Example41c1840001.7goodpoorComparative Example42d2600002.7goodpoorComparative Example43eslab crack occurred during continuous castingComparative Example44fslab crack occurred during continuous castingComparative Example45gslab crack occurred during continuous castingComparative Example46hslab crack occurred during continuous castingComparative Example47islab crack occurred during continuous castingComparative Example48jslab crack occurred during continuous castingComparative Example*1: Number density of cementite particles having particle size of 0.090 µm or more *2: Cr amount in cementite particles having particle size of 0.090 µm or more

[0129] As can be seen from the results listed in Tables 1 to 5, steel components satisfying the conditions of the present disclosure can be produced without the occurrence of delayed cracking during continuous casting, and have excellent surface characteristics and wear resistance.REFERENCE SIGNS LIST

[0130] 10wear test piece 11hole 12groove 20wear test apparatus 21yarn unwinder 22tension adjuster 23yarn winder Syarn dwear depth

Examples

examples

[0107]In order to confirm the effects of the present disclosure, steel components were produced according to the following procedure.

[0108]First, steels having the chemical compositions listed in Table 1 were melted in a converter and made into steel slabs by continuous casting. Each steel slab was then subjected to cooling, heating, hot rolling, cooling, coiling, first annealing, cold rolling, and second annealing in sequence to produce a cold-rolled steel sheet having a final sheet thickness of about 0.4 mm. Each process was carried out under the conditions listed in Table 2, and the first annealing, cold rolling, and second annealing were carried out the number of times listed in Table 2.

[0109]For some examples, the cold-rolled steel sheet after the second annealing was further subjected to final cold rolling. The rolling reduction in the final cold rolling is listed in Tables 2 and 3.

[0110]The resulting cold-rolled steel sheet was then worked into component shapes and heat-treat...

Claims

1. A steel component comprising a chemical composition containing, in mass%, C: 0.80 % to 1.25 %, Si: 0.10 % to 1.0 %, Mn: 0.20 % to 2.5 %, P: 0.0005 % to 0.05 %, S: 0.03 % or less, Al: 0.001 % to 0.1 %, N: 0.001 % to 0.01 %, O: 0.0100 % or less, Cr: 0.56 % to 1.6 %, and at least one selected from the group consisting of Nb: 0.029 % to 0.21 %, Ti: 0.01 % to 0.21 %, and V: 0.01 % to 0.21 %, with the balance being Fe and inevitable impurity, and the total content of Nb, Ti, and V being 0.21 mass% or less, wherein a number density of cementite particles having a particle size of 0.090 µm or more is 200,000 / mm2 or more, and a Cr amount in the cementite particles having a particle size of 0.090 µm or more is 2.5 mass% or more.

2. The steel component according to claim 1, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of: Ta: 0.10 % or less, W: 0.10 % or less, B: 0.0100 % or less, Mo: 1.00 % or less, Co: 1.00 % or less, Ni: 1.00 % or less, Cu: 1.00 % or less, Sn: 0.200 % or less, Sb: 0.200 % or less, Ca: 0.0100 % or less, Mg: 0.0100 % or less, REM: 0.0100 % or less, Zr: 0.100 % or less, Te: 0.100 % or less, Hf: 0.10 % or less, and Bi: 0.200 % or less.

3. The steel component according to claim 1 or 2, wherein the steel component is any one of a component for textile machinery, a bearing component, or a blade for machinery.

4. A method of producing a steel component, the method comprising: cooling a steel slab produced by continuous casting and having the chemical composition according to claim 1 or 2, under conditions such that, at a transverse direction center of the steel slab and 10 mm from a surface, a residence time in a temperature range from 1200 °C to 1400 °C is 130 s or shorter, and an average cooling rate in a temperature range from 550 °C to 700 °C is 10 °C / h or less; heating the steel slab after the cooling at a slab heating temperature of 1100 °C or higher for a slab heating time of 60 min or longer; hot rolling the heated steel slab under conditions including a rolling finish temperature exceeding Tc, as defined by the following expression (1), and 950 °C or lower, to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet under conditions including an average cooling rate of 20 °C / s or more and a cooling stop temperature of Tc or lower; coiling the cooled hot-rolled steel sheet at a coiling temperature of 530 °C or higher and Tc or lower; subjecting the hot-rolled steel sheet after coiling to first annealing once or more, under conditions including an annealing temperature of 600 °C or higher and Tc or lower and an annealing time of 3 h or longer; subjecting the hot-rolled steel sheet after the first annealing to cold rolling at a rolling ratio of 15 % or more and second annealing at an annealing temperature of 600 °C or higher and Tc or lower for an annealing time of 3 h or longer, twice or more, to obtain a cold-rolled steel sheet; working the cold-rolled steel sheet into a component shape; quenching under conditions including a quenching temperature of Tc or higher and 1000 °C or lower and a holding time of 1.0 min or longer and 60 min or shorter, and then carrying out a heat treatment including tempering, where the element symbols in expression (1) denote the content in mass% of the respective elements, and the content of any element not contained is assumed to be 0.

Images