Steel parts and their manufacturing method

Abstract

The present invention provides a steel part that does not suffer from slab cracking during continuous casting and has excellent wear resistance and surface properties. The steel part has a specified composition and contains cementite particles with a particle diameter of 0.090 μm or more, with a number density of 200,000 particles / mm 2 or more, and the cementite particles having a particle diameter of 0.090 μm or more have a Cr content of 2.5 mass % or more.

Description

[Technical Field]

[0001] The present invention relates to a steel part, particularly to a steel part having excellent wear resistance and surface quality, and also to a method for manufacturing said steel part.

[0002] Carbon steel, which is steel with a high carbon content, has high hardness and is therefore widely used as a material for steel parts that require wear resistance, such as parts for textile machinery, bearing parts, and cutting tools for machinery.

[0003] In the manufacture of typical steel parts, cold-rolled steel sheets are processed into the part shape and then quenched and tempered. While increasing hardness through quenching reduces toughness, subsequent tempering can improve toughness. However, tempering also reduces hardness, which ultimately reduces wear resistance.

[0004] Therefore, various techniques have been proposed to achieve excellent wear resistance even if the hardness of the steel parts is reduced.

[0005] For example, Patent Document 1 proposes a steel sheet containing 0.10 to 0.40 mass% C and having a ferrite-cementite structure. In Patent Document 1, the formability and wear resistance are improved by increasing the ferrite grain size in the steel sheet, spheroidizing carbides (mainly cementite) to an appropriate grain size, and reducing the pearlite structure.

[0006] Furthermore, Patent Document 2 proposes that a pearlite structure with a lamellar spacing of 0.3 μm or less be formed by annealing a cold-rolled steel sheet containing 0.40 to 0.90 mass % of C under specific conditions.

[0007] Patent Document 3 proposes that in a steel sheet containing 0.60 to 1.25 mass % of C, coarse Nb·Ti carbides with an equivalent circle diameter of 0.5 μm or more are precipitated in the ferrite phase, which is the matrix structure.

[0008] Patent Document 4 proposes that coarse carbides having a particle size of 2 μm or more be precipitated in the matrix structure in a steel containing 0.6 to 1.0 mass % of C.

[0009] Furthermore, adding alloying elements to steel containing a high concentration of carbon reduces the toughness of the steel. As a result, cracks during slab cooling, so-called "delay cracks," are more likely to occur during the manufacturing process of steel slabs. If delay cracks occur, the slab may break during transportation, making it impossible to use the slab for hot rolling. Even if the slab does not break, the cracks may open during hot rolling, causing the hot-rolled steel sheet to break. Furthermore, if the cracks are small, surface defects such as scabs and slivers may occur on steel sheets and steel parts after cold rolling.

[0010] Cracks on the surface of steel slabs are usually removed with a grinder. However, because the toughness of steel slabs has decreased due to the use of high alloys, the stress generated when removing cracks with a grinder can actually cause the cracks to grow. In such cases, it is difficult to completely remove the cracks. On the other hand, small cracks can be overlooked and appear as surface defects on steel plates and steel parts after cold rolling. For these reasons, it is necessary to suppress cracks in steel slabs.

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

[0012] For example, Patent Document 5 proposes casting a steel slab containing 0.16 to 0.35% C, cutting it, and then stacking three or more of the cut steel slabs in a predetermined shape and cooling them. By cooling using this method, the steel slab is slowly cooled in the temperature range (700 to 500°C) where austenite transforms to ferrite, and the bainite / martensite transformation is suppressed. As a result, stress caused by transformation expansion is reduced, and it is possible to prevent the slab from cracking during heating.

[0013] Patent Document 6 proposes that a steel slab containing 0.020 to 0.600 mass% C is produced by continuous casting, and then slowly cooled at an average cooling rate of 20°C / hr or less in a temperature range of 700 to 500°C. This prevents cracking due to stress during cooling, and also prevents the occurrence of quality defects such as scabs during hot rolling. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] International Publication No. 2016 / 204288 [Patent Document 2] Japanese Patent Publication No. 2020-132953 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-190494 [Patent Document 4] Japanese Patent Application Laid-Open No. 2010-138453 [Patent Document 5] Japanese Patent Application Publication No. 2020-139209 [Patent Document 6] Japanese Patent Application Publication No. 2019-167560 Summary of the Invention [Problem to be solved by the invention]

[0015] According to the conventional techniques proposed in Patent Documents 1 to 4, certain improvements have been observed in the hardness and wear resistance of steel materials. However, according to the investigations of the present inventors, when steel parts manufactured using conventional steel materials are actually used, it has been found that sufficient wear resistance may not be obtained. Furthermore, with regard to Patent Documents 3 and 4, there have been cases where surface defects such as cracks in the slab and scabs due to the cracks have occurred in the steel parts.

[0016] Furthermore, Patent Documents 5 and 6 state that the occurrence of cracking in a slab can be prevented by slowly cooling the cast slab in a temperature range of 700 to 500° C. However, it has been found that even this method cannot sufficiently prevent the occurrence of cracking in a slab when the C content is 0.80% or more.

[0017] The present invention has been made in view of the above circumstances, and has as its object to provide a steel part which does not cause slab cracking during continuous casting and which has excellent wear resistance and surface properties. [Means for solving the problem]

[0018] The present inventors have conducted research into methods for solving the above problems and have come to the following findings.

[0019] (1) Wear resistance can be improved by appropriately controlling the number density of cementite in steel parts and the Cr content in the cementite.

[0020] (2) By appropriately controlling the component composition of the steel slab used and the manufacturing conditions during continuous casting, it is possible to suppress cracks in the steel slab and to suppress surface defects in steel parts.

[0021] (3) By appropriately controlling the chemical composition of the steel slab used and the manufacturing conditions such as hot rolling, annealing, cold rolling, quenching, and tempering, it is possible to appropriately control the number density of cementite particles in steel parts and the amount of Cr present in the cementite particles.

[0022] The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

[0023] 1. By mass%, C: 0.80~1.25%, Si: 0.10 to 1.0% Mn: 0.20 to 2.5% P: 0.0005~0.05%, 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%, The balance consists of Fe and unavoidable impurities, and The total content of Nb, Ti, and V is 0.21% by mass or less. The number density of cementite particles with a particle diameter of 0.090 μm or more is 200,000 particles / mm 2 or more, and the cementite particles having a particle diameter of 0.090 μm or more have a Cr content of 2.5 mass % or more.

[0024] 2. The component composition is, in mass%, 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, The steel part according to claim 1, further comprising at least one selected from the group consisting of:

[0025] 3. A steel part according to 1 or 2 above, wherein the steel part is any one of a textile machine part, a bearing part, and a machine blade.

[0026] 4. A steel slab produced by continuous casting and having the chemical composition described in 1 or 2 above is cooled at a temperature at the center of the width direction of the steel slab and 10 mm from the surface under the conditions of a residence time of 130 seconds or less in a temperature range of 1200 to 1400°C and an average cooling rate of 10°C / hr or less in a temperature range of 550 to 700°C, The cooled steel slab is heated at a slab heating temperature of 1100°C or more for a slab heating time of 60 minutes or more. The heated steel slab is hot-rolled under the condition of a finish rolling end temperature: higher than Tc defined by the following formula (1) and 950°C or less to obtain a hot-rolled steel sheet; The hot-rolled steel sheet is cooled under the conditions of an average cooling rate of 20°C / s or more and a cooling stop temperature of Tc or less, The cooled hot-rolled steel sheet is coiled at a coiling temperature of 530°C or higher and Tc or lower, The hot-rolled steel sheet after coiling is subjected to first annealing at least once under the conditions of an annealing temperature of 600°C or higher and Tc or lower, and an annealing time of 3 hours or longer, The hot-rolled steel sheet after the first annealing is subjected to cold rolling at a rolling reduction rate of 15% or more and second annealing at an annealing temperature of 600°C or more and Tc or less for an annealing time of 3 hours or more, repeated at least twice to obtain a cold-rolled steel sheet; processing the cold-rolled steel sheet into a part shape; A manufacturing method for steel parts, which involves quenching under the conditions of a quenching temperature of Tc or higher and 1000°C or lower, and a holding time of 1.0 minute or higher and 60 minutes or lower, and heat treatment including tempering. Tc(℃)=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr+6.38×W…(1) Here, the element symbols in the above formula (1) represent the content (mass %) of each element, and are set to zero when the element is not contained. [Effects of the Invention]

[0027] According to the present invention, it is possible to provide a steel part that is free from cracks during slab placement during continuous casting and that has excellent wear resistance and surface quality. The steel part of the present invention can be suitably used for a variety of applications, including textile machine parts, bearing parts, and machine blades. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 2 is a schematic diagram showing the shape of a wear test piece. [Figure 2] FIG. 1 is a schematic diagram of an abrasion test device. [Figure 3] FIG. 2 is a schematic diagram showing the wear depth of a test piece after a wear test. DETAILED DESCRIPTION OF THE INVENTION

[0029] The present invention will be described in detail below, but the present invention is not limited to the embodiments.

[0030] [Component composition] The cold-rolled steel sheet of the present invention has the above-mentioned chemical composition. The reasons for the limitations will be explained below. In the following explanation, "%" as a unit of content refers to "mass%" unless otherwise specified.

[0031] C: 0.80 to 1.25% Carbon (C) is an essential element for improving hardness after quenching, making it essential for various steel parts, including textile machinery parts, bearing parts, and machine and household cutlery. Carbon is also essential for cementite formation. A C content of less than 0.80% makes it difficult to obtain the required cementite particle density. Therefore, the C content must be 0.80% or more, preferably 0.85% or more, and more preferably 0.90% or more. On the other hand, a C content of more than 1.25% strengthens the surface scale during slab heating, resulting in poor surface quality. Furthermore, a C content of more than 1.25% reduces the toughness of the slab, potentially causing cracks during casting and cooling or heating, significantly reducing productivity. Furthermore, surface defects may occur in steel parts. Therefore, the C content must be 1.25% or less, preferably 1.20% or less, and more preferably 1.10% or less.

[0032] Si: 0.10 to 1.0% Silicon is an element that has the effect of increasing the strength of cold-rolled steel sheets through solid-solution strengthening. To achieve this effect, the Si content is set to 0.10% or more, preferably 0.15% or more, and more preferably 0.20% or more. On the other hand, excessive silicon strengthens the surface scale during heating, resulting in poor surface quality. Furthermore, if the Si content exceeds 1.0%, the toughness of the slab decreases, which can lead to cracks during casting and cooling of the slab or during heating of the slab, significantly reducing productivity. Furthermore, surface defects occur in steel parts. Therefore, the Si content is set to 1.0% or less, preferably 0.80% or less, and more preferably 0.55% or less.

[0033] Mn: 0.20 to 2.5% Mn is an element that promotes quenching and thereby improves the hardness of steel parts. Furthermore, Mn delays the dissolution of cementite during quenching, thereby increasing the number density of cementite particles in steel parts. To achieve this effect, the Mn content is set to 0.20% or more, preferably 0.30% or more, more preferably 0.40% or more, and even more preferably 0.50% or more. On the other hand, if the Mn content exceeds 2.5%, the toughness of the slab decreases, which can lead to cracks during casting and cooling of the slab or during heating of the slab, significantly reducing productivity. Furthermore, surface defects occur on steel parts. Therefore, the Mn content is set to 2.5% or less, preferably 2.0% or less, more preferably 1.8% or less, and even more preferably 1.5% or less.

[0034] P: 0.0005 to 0.05% Adding trace amounts of P improves the strength of steel parts through solid solution strengthening. To achieve this effect, the P content is set to 0.0005% or more, preferably 0.001% or more. On the other hand, if the P content exceeds 0.05%, grain boundary embrittlement occurs, reducing the toughness of the slab. As a result, cracks may occur during the casting and cooling of the slab and during heating of the slab, significantly reducing productivity. Furthermore, surface defects may occur on the steel parts. Therefore, the P content is set to 0.05% or less, preferably 0.04% or less.

[0035] S: 0.03% or less S is an element that causes grain boundary embrittlement during slab casting. Excessive S can cause cracks during slab casting, significantly reducing productivity. It also causes surface defects on steel parts. Therefore, the S content is set to 0.03% or less, preferably 0.02% or less. On the other hand, from the viewpoint of cracks during slab casting, the lower the S content, the better, so there is no particular lower limit for the S content. However, excessive reduction in the S content leads to increased manufacturing costs. Therefore, from the viewpoint of manufacturing costs, the S content is preferably set to 0.0001% or more, and more preferably 0.0005% or more.

[0036] Al: 0.001 to 0.1% Al is an element necessary for deoxidation during steelmaking. Therefore, the Al content should be 0.001% or more. On the other hand, excessive Al will form nitrides and oxides, reducing the toughness of the slab. As a result, cracks may occur during the casting and cooling of the slab and during heating of the slab, significantly reducing productivity. Furthermore, surface defects will occur on steel parts. Therefore, the Al content should be 0.1% or less, preferably 0.08% or less.

[0037] N: 0.001 to 0.01% N is an element that refines grain size by forming fine nitrides, thereby improving the strength of steel parts. Therefore, the N content is set to 0.001% or more. On the other hand, excessive N combines with Al to form nitrides, which reduces the toughness of the slab, which can cause cracks during casting and cooling of the slab or during heating of the slab, significantly reducing productivity. Furthermore, surface defects occur on steel parts. Therefore, the N content is set to 0.01% or less, preferably 0.008% or less.

[0038] O: 0.0100% or less O exists as an oxide and is an element that embrittles slabs. A decrease in slab toughness can lead to cracks during casting and cooling of the slab or during heating of the slab, significantly reducing productivity. It also causes surface defects on steel parts. Therefore, the O content is set to 0.0100% or less, preferably 0.0050% or less. Meanwhile, the lower limit of the O content is not particularly limited and may be 0%. However, due to constraints on production technology, the O content is preferably set to 0.0001% or more.

[0039] Cr: 0.56~1.6% Cr is an element that dissolves in cementite, improving the hardness of cementite and the wear resistance of steel parts. Cr also delays the dissolution of cementite during quenching, thereby improving the cementite density of steel parts. To achieve this effect, the Cr content is set to 0.56% or more, preferably 0.60% or more, and more preferably 0.70% or more. On the other hand, if the Cr content exceeds 1.6%, the toughness of the slab decreases, which can lead to cracks during casting and cooling of the slab or during heating of the slab, significantly reducing productivity. Furthermore, surface defects occur on steel parts. Therefore, the Cr content is set to 1.6% or less, preferably 1.5% or less, and more preferably 1.3% or less.

[0040] The above-mentioned component composition contains at least one selected from the group consisting of Nb, Ti, and V. To obtain the desired wear resistance, it is necessary to contain at least one of Nb, Ti, and V in the following amount.

[0041] Nb: 0.029 to 0.21% Nb is an element that forms carbides and can improve wear resistance. When Nb is contained, the Nb content is set to 0.029% or more to obtain the above effect. On the other hand, excessive Nb content reduces the toughness of the slab, which can cause cracks during casting and cooling of the slab or during heating of the slab, significantly reducing productivity. In addition, surface defects occur on steel parts. Therefore, the Nb content is set to 0.21% or less, preferably 0.15% or less, and more preferably 0.10% or less.

[0042] Ti: 0.01 to 0.21% Ti is an element that forms carbides and can improve wear resistance. When Ti is contained, the Ti content is set to 0.01% or more to obtain the above effect. On the other hand, excessive Ti content reduces the toughness of the slab, which can cause cracks during casting and cooling of the slab or during heating of the slab, significantly reducing productivity. It also causes surface defects on steel parts. Therefore, the Ti content is set to 0.21% or less, preferably 0.15% or less, and more preferably 0.10% or less.

[0043] V: 0.01 to 0.21% V is an element that forms carbides and can improve wear resistance. When V is contained, the V content is set to 0.01% or more to obtain the above effect. On the other hand, excessive V content reduces the toughness of the slab, which can cause cracks during casting and cooling of the slab or during heating of the slab, significantly reducing productivity. It also causes surface defects on steel parts. Therefore, the V content is set to 0.21% or less, preferably 0.15% or less, and more preferably 0.10% or less.

[0044] Nb+Ti+V: 0.21% or less If the total content of Nb, Ti, and V in the above-mentioned chemical composition exceeds 0.21%, the toughness of the steel slab decreases, which may cause cracks during the steel slab manufacturing process or when the slab is heated before hot rolling, significantly reducing productivity. Furthermore, surface defects may occur in steel parts. Therefore, the total content of Nb, Ti, and V is set to 0.21% or less. While the lower limit of the total content is not particularly limited, 0.01% is the practical lower limit. The lower limit corresponds to the case where only either Ti or V is contained in an amount of 0.01%. The total content may be 0.020% or more, 0.025% or more, or 0.030% or more.

[0045] A steel part according to one embodiment of the present invention has a composition comprising the above components with the balance being Fe and unavoidable impurities, such as H.

[0046] Furthermore, the component composition of the steel part in another embodiment of the present invention may optionally further contain at least one of the following elements.

[0047] Ta: 0.10% or less Ta has the effect of further improving the wear resistance of steel parts by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, if the Ta content exceeds 0.10%, a large amount of coarse precipitates and inclusions are formed, reducing the toughness of the steel slab. Therefore, the Ta content is set to 0.10% or less, preferably 0.08% or less. On the other hand, there is no particular lower limit for the Ta content. However, from the viewpoint of enhancing the effect of adding Ta, it is preferable that the Ta content be 0.01% or more.

[0048] W: 0.10% or less W has the effect of further improving the wear resistance of steel parts by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, if the W content exceeds 0.10%, a large amount of coarse precipitates and inclusions are formed, reducing the toughness of the steel slab. Therefore, the W content is set to 0.10% or less, preferably 0.08% or less. On the other hand, there is no particular lower limit for the W content. However, from the viewpoint of enhancing the effect of adding W, it is preferable that the W content be 0.01% or more.

[0049] B: 0.0100% or less B segregates at austenite grain boundaries during hot rolling or annealing, and has the effect of further improving hardenability. However, if the B content exceeds 0.0100%, the toughness of the steel slab decreases. Therefore, the B content is set to 0.0100% or less, more preferably 0.0080% or less. On the other hand, there is no particular lower limit for the B content. However, from the viewpoint of enhancing the effect of adding B, the B content is preferably set to 0.0003% or more.

[0050] Mo: 1.00% or less Mo is an element that has the effect of further improving hardenability. However, if the Mo content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Mo content is set to 1.00% or less, preferably 0.80% or less. On the other hand, there is no particular lower limit for the Mo content. However, from the viewpoint of enhancing the effect of adding Mo, it is preferable that the Mo content be 0.01% or more.

[0051] Co: 1.00% or less Co is an element that has the effect of further improving hardenability. However, if the Co content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Co content is set to 1.00% or less, preferably 0.80% or less. On the other hand, there is no particular lower limit for the Co content. However, from the viewpoint of enhancing the effect of adding Co, it is preferable that the Co content be 0.001% or more.

[0052] Ni: 1.00% or less Ni is an element that has the effect of further improving hardenability. However, if the Ni content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Ni content is set to 1.00% or less, preferably 0.80% or less. On the other hand, there is no particular lower limit for the Ni content. However, from the viewpoint of enhancing the effect of adding Ni, it is preferable that the Ni content be 0.01% or more.

[0053] Cu: 1.00% or less Cu is an element that has the effect of further improving hardenability. However, if the Cu content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Cu content is set to 1.00% or less, preferably 0.80% or less. On the other hand, there is no particular lower limit for the Cu content. However, from the viewpoint of enhancing the effect of adding Cu, it is preferable that the Cu content be 0.01% or more.

[0054] Sn: 0.200% or less Sn is an element that has the effect of further improving hardenability. However, if the Sn content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Sn content is set to 0.200% or less, preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sn content. However, from the viewpoint of enhancing the effect of adding Sn, it is preferable that the Sn content be 0.001% or more.

[0055] Sb: 0.200% or less Sb is an element that suppresses decarburization and enables adjustment of the strength of steel sheets. However, if the Sb content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Sb content is set to 0.200% or less, preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sb content. However, from the viewpoint of enhancing the effect of adding Sb, it is preferable that the Sb content be 0.001% or more.

[0056] Ca:0.0100% or less Ca is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the Ca content exceeds 0.0100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the Ca content is set to 0.0100% or less, preferably 0.0050% or less. On the other hand, there is no particular lower limit for the Ca content. However, from the viewpoint of enhancing the effect of adding Ca, it is preferable that the Ca content be 0.0005% or more.

[0057] Mg: 0.0100% or less Mg is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the Mg content exceeds 0.0100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0050% or less. On the other hand, there is no particular lower limit for the Mg content. However, from the viewpoint of enhancing the effect of adding Mg, it is preferable that the Mg content be 0.0005% or more.

[0058] REM: 0.0100% or less REM (rare earth metal) is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the REM content exceeds 0.0100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the REM content is set to 0.0100% or less, preferably 0.0050% or less. On the other hand, there is no particular lower limit for the REM content. However, from the viewpoint of enhancing the effect of adding REM, it is preferable that the REM content be 0.0005% or more.

[0059] Zr: 0.100% or less Zr is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the Zr content exceeds 0.100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the Zr content is set to 0.100% or less, preferably 0.080% or less. On the other hand, there is no particular lower limit for the Zr content. However, from the viewpoint of enhancing the effect of adding Zr, it is preferable that the Zr content be 0.001% or more.

[0060] Te: 0.100% or less Te is an element that has the effect of spheroidizing the shape of nitrides and sulfides and further improving the toughness of the slab. However, if the Te content exceeds 0.100%, the amount of coarse precipitates and inclusions increases, and the toughness of the steel slab actually decreases. Therefore, the Te content is set to 0.100% or less, preferably 0.080% or less. On the other hand, there is no particular lower limit for the Te content. However, from the viewpoint of enhancing the effect of adding Te, it is preferable that the Te content be 0.001% or more.

[0061] Hf: 0.10% or less Hf is an element that has the effect of spheroidizing the shape of nitrides and sulfides and improving the ultimate deformability of steel sheets. However, if the Hf content exceeds 0.10%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Hf content is set to 0.10% or less, preferably 0.08% or less. On the other hand, there is no particular lower limit for the Hf content. However, from the viewpoint of enhancing the effect of adding Hf, it is preferable that the Hf content be 0.01% or more.

[0062] Bi:0.200% or less Bi is an element that reduces segregation. However, if the Bi content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the toughness of the steel slab. Therefore, the Bi content is set to 0.200% or less, preferably 0.100% or less. On the other hand, there is no particular lower limit for the Bi content. However, from the viewpoint of enhancing the effect of adding Bi, it is preferable that the Bi content be 0.001% or more.

[0063] The elements Ta to Bi have the effect of further improving the properties of the cold-rolled steel sheet of the present invention and can be optionally contained in the steel sheet of the present invention. The above description indicates the preferred lower limits of the contents of these optionally contained elements. However, if the contents of these elements are lower than the preferred lower limits, the effects of adding these elements will be reduced, but the properties of the cold-rolled steel sheet will still meet the required level. Therefore, the inclusion of these elements is not essential, and the lower limit of the content may be 0%.

[0064] [Cementite] Next, cementite contained in the steel part of the present invention will be described.

[0065] Cementite particle density: 200,000 particles / mm 2 End In the present invention, the wear resistance can be improved by controlling the number density of cementite particles. In order to obtain the desired wear resistance, the number density of cementite particles having a particle diameter of 0.090 μm or more is set to 200,000 particles / mm 2Therefore, it is necessary to control the number density of cementite particles with a particle diameter of 0.090 μm or more contained in steel parts to 200,000 particles / mm 2 More than 250,000 pieces / mm 2 More preferably, 300,000 pieces / mm 2 More preferably, 400,000 pieces / mm 2 More than 500,000 particles / mm 2 On the other hand, the upper limit of the number density is not particularly limited, but is, for example, 2,000,000 pieces / mm 2 may be less than 1,500,000 pieces / mm 2 It may be less than 1,200,000 pieces / mm 2 In this specification, the "number density of cementite particles having a particle size of 0.090 μm or more" may be simply referred to as the "number density of cementite" or "number density".

[0066] Cr content in cementite particles: 2.5% or more The solid solution of Cr in cementite increases the hardness of the cementite, and the harder the cementite, the higher the resistance to wear and the improved wear resistance. However, if the Cr content in the cementite particles is less than 2.5%, the desired wear resistance cannot be obtained. Therefore, the Cr content in the cementite particles is set to 2.5% or more, preferably 2.7% or more, more preferably 3.0% or more, and even more preferably 4.0% or more. On the other hand, the upper limit of the Cr content in the cementite particles is not particularly limited, but it is preferably 10% or less.

[0067] [Manufacturing method for steel parts] Next, a method for manufacturing a steel part according to an embodiment of the present invention will be described. In the description, unless otherwise specified, the unit "°C" used for temperature indicates the surface temperature (the temperature on the surface of a steel slab, steel plate, etc.).

[0068] The steel part can be produced by sequentially carrying out the following steps on a steel slab having the above-mentioned composition produced by continuous casting. (1) Cooling (2)Heating (3) Hot rolling (4) Cooling (5) Winding (6) First annealing (7) Cold rolling (8) Second annealing (9) Processing and heat treatment

[0069] (1) Cooling First, a steel slab having the above-mentioned composition produced by continuous casting is cooled.

[0070] Residence time in the temperature range of 1200-1400°C: 130 seconds or less In the cooling, the residence time in the temperature range of 1200 to 1400°C at the center of the width direction of the steel slab and at a position 10 mm from the surface is set to 130 seconds or less. The reason for this will be explained below.

[0071] The prior austenite grain size is a factor that determines the fracture unit, and the larger the prior austenite grain size, the lower the toughness. The austenite grain size of a steel slab is determined by the residence time in the temperature range of 1200 to 1400°C, and the longer the residence time in this temperature range, the coarser the prior austenite grain size becomes. If the residence time exceeds 130 seconds, the prior austenite grain size will coarsen, and slab cracking may occur. Therefore, in the present invention, the residence time in the temperature range of 1200 to 1400°C is set to 130 seconds or less, preferably 120 seconds or less, more preferably 110 seconds or less, and even more preferably 100 seconds or less.

[0072] On the other hand, although there is no particular lower limit for the residence time in the temperature range, an excessively short residence time increases the risk of breakout due to uneven solidification. Here, breakout refers to a phenomenon in which part of the solidified shell breaks during continuous casting, causing the molten steel inside to leak out. Therefore, the residence time is preferably 50 seconds or more, and more preferably 70 seconds or more.

[0073] Average cooling rate in the temperature range of 550-700°C: 10°C / hr or less Furthermore, during the cooling, the average cooling rate in the temperature range of 550 to 700°C at a position 10 mm from the surface and at the center in the width direction of the steel slab is set to 10°C / hr or less. The reason for this will be explained below.

[0074] The above temperature range is the temperature range in which the structure of the steel slab transforms to pearlite. If the cooling rate in this temperature range is 10°C / hr or less, bainite transformation can be suppressed, and the microstructure of the steel slab can be made mainly of pearlite. As a result, internal stress is reduced and cracking of the slab can be prevented. Therefore, the average cooling rate in the temperature range of 550 to 700°C is set to 10°C / hr or less, preferably 8°C / hr or less, and more preferably 6°C / hr or less. On the other hand, there is no particular restriction on the lower limit of the average cooling rate. However, because a separate energy source is required for control, it is preferably set to 1°C / hr or more, and more preferably 2°C / hr or more.

[0075] Here, the temperature of the steel slab is the temperature at the center of the width of the steel slab and 10 mm from the surface. Since it is difficult to actually measure the temperature, it is calculated by heat transfer analysis. The temperature at the center of the width of the steel slab is used because this position is the longest in the slab in the temperature range.

[0076] In the cooling process, the steel slab may be cooled to 550°C or less. In other words, the cooling end temperature may be 550°C or less. The cooling end temperature is preferably 300°C or less, more preferably 100°C or less, and even more preferably 50°C or less. Typically, the cooling may be performed to ambient temperature. For example, when the steel slab is allowed to cool outdoors, the cooling may be performed to ambient temperature. On the other hand, there is no particular restriction on the lower limit of the cooling end temperature, but when the steel slab is allowed to cool outdoors, the cooling end temperature depends on the ambient temperature. Typically, the cooling end temperature is preferably -10°C or more, more preferably 0°C or more, and even more preferably 10°C or more.

[0077] The cooling method is not particularly limited, and any method can be used. For example, in a continuous casting line, cooling in a mold is generally followed by spray cooling. In the present invention, cooling in a mold and spray cooling are also performed in a continuous casting line, and it is preferable that the residence time in the temperature range of 1200 to 1400°C is 130 seconds or less.

[0078] On the other hand, cooling in the temperature range of 550 to 700°C is preferably carried out by cutting the continuously cast slab and then allowing it to cool naturally. For example, multiple cut steel slabs can be stacked and allowed to cool naturally. In this case, it is preferable to cover the steel slabs so that the average cooling rate in the temperature range of 550 to 700°C is adjusted to 10°C / hr or less.

[0079] (2)Heating Next, the cooled steel slab is heated.

[0080] Slab heating temperature: 1100℃ or higher Heating prior to hot rolling homogenizes the components and dissolves carbides such as cementite and segregations present in the steel slab. However, if the heating temperature of the steel slab (slab heating temperature) during heating is less than 1100°C, cementite cannot be sufficiently dissolved, and as a result, the cementite number density in the final steel part cannot be within the desired range. Therefore, the slab heating temperature is set to 1100°C or higher, preferably 1150°C or higher. On the other hand, although there is no particular upper limit for the slab heating temperature, an excessively high slab heating temperature may deteriorate the surface quality. Therefore, from the viewpoint of further improving the surface quality, it is preferable that the slab heating temperature be 1350°C or lower.

[0081] Slab heating time: 60 minutes or more If the heating time of the steel slab during the heating (slab heating time) is less than 60 minutes, carbides such as cementite cannot be sufficiently dissolved, and as a result, the number density of cementite in the finally obtained steel part cannot be set within the desired range. Therefore, the slab heating time is set to 60 minutes or more, preferably 90 minutes or more. On the other hand, there is no particular upper limit to the slab heating time, but it is preferably set to 300 minutes or less, and more preferably set to 200 minutes or less.

[0082] The heating can be carried out by any method, but is preferably carried out using a heating furnace.

[0083] (3) Hot rolling The heated steel slab is then hot-rolled to form a hot-rolled steel sheet. In the hot-rolling, rough rolling and finish rolling can be performed according to a conventional method.

[0084] Finishing rolling temperature: above Tc, below 950°C If the finish rolling temperature in the hot rolling is equal to or lower than Tc, as defined by the following formula (1), a portion of the austenite transforms into pearlite during rolling, promoting inhomogeneous deformation and degrading surface quality. Therefore, the finish rolling temperature is set to be higher than Tc, preferably Tc + 50°C or higher, and more preferably Tc + 100°C or higher. On the other hand, if the finish rolling temperature exceeds 950°C, the surface scale becomes strong and the surface quality deteriorates. Therefore, the finish rolling temperature is set to be 950°C or lower, preferably 930°C or lower, and more preferably 910°C or lower.

[0085] Here, Tc is a value that can be used as an index of the temperature at which transformation involving cementite occurs, and is defined by the following formula (1). Tc(℃)=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr+6.38×W…(1) Here, the element symbols in the above formula (1) indicate the content (mass %) of each element, and are set to zero when the element is not contained.

[0086] (4) Cooling Average cooling rate: 20℃ / s or more After the finish rolling is completed, cooling is started and stopped at the cooling stop temperature. If the average cooling rate during the cooling is less than 20°C / s, the cementite number density in the finally obtained steel part cannot be set within the desired range. This is because, if the average cooling rate is less than 20°C / s, coarse cementite is formed, and this coarse cementite remains in the steel part. Therefore, the average cooling rate is set to 20°C / s or more, preferably 30°C / s or more, and more preferably 50°C / s or more. On the other hand, although there are no particular limitations on the upper limit of the average cooling rate, if the cooling rate is too high, it becomes difficult to control the cooling stop temperature. Therefore, the average cooling rate is preferably set to 500°C / s or less.

[0087] Cooling stop temperature: Tc or less If the cooling stop temperature in the cooling is higher than Tc, the cementite number density in the finally obtained steel part cannot be set within the desired range. This is because, if the cooling stop temperature is higher than Tc, coarse cementite is generated, and this coarse cementite remains in the steel part. Therefore, the cooling stop temperature is set to Tc or lower, preferably Tc - 20°C or lower. On the other hand, there is no particular restriction on the lower limit of the cooling stop temperature. However, if the cooling stop temperature is lower than 530°C, the volume expansion due to transformation during the subsequent coiling will result in a poor coiled shape. Therefore, from the viewpoint of improving the coiled shape, the cooling stop temperature is preferably set to 530°C or higher, more preferably 550°C or higher, and even more preferably 600°C or higher.

[0088] (5) Winding Winding temperature: 530°C or higher, Tc or lower After the cooling is stopped, the cooled hot-rolled steel sheet is coiled. If the coiling temperature is higher than Tc, the cementite number density in the finally obtained steel part cannot be set within the desired range. This is because if the coiling temperature is higher than Tc, coarse cementite is generated, and this coarse cementite remains in the steel part. Therefore, the coiling temperature is set to Tc or lower, preferably Tc - 20°C or lower, and more preferably Tc - 40°C or lower. On the other hand, if the coiling temperature is lower than 530°C, volume expansion due to transformation during coiling will result in a poor coiled shape. Therefore, the coiling temperature is set to 530°C or higher, more preferably 550°C or higher, and even more preferably 600°C or higher.

[0089] (6) First annealing Annealing temperature: 600°C or higher, Tc or lower Annealing time: 3 hours or more The hot-rolled steel sheet after coiling is subjected to first annealing under the conditions of an annealing temperature of 600°C or higher and Tc or lower, and an annealing time of 3 hours or longer. The structure of the hot-rolled steel sheet after coiling is a pearlite structure in which cementite and ferrite formed in a plate shape are aligned. Because the pearlite structure is stable, it does not become homogenized unless it is held at a high temperature for a long time. Furthermore, in order to break down the pearlite structure and generate the desired cementite in the subsequent cold rolling and annealing processes, the annealing temperature must be 600°C or higher and the annealing time must be 3 hours or longer. The annealing temperature is preferably 620°C or higher. Furthermore, the annealing time is preferably 4 hours or longer. On the other hand, if the annealing temperature is higher than Tc, phase transformation will preferentially begin in one part, resulting in the formation of a locally coarse structure. As a result, the structure will become non-uniform, and the desired cementite particle number density will not be obtained. Therefore, the annealing temperature is set to Tc or lower, preferably Tc - 20°C or lower, and more preferably Tc - 40°C or lower. On the other hand, although there is no particular upper limit to the annealing time, if it is too long, productivity decreases, so the annealing time is preferably 50 hours or less, and more preferably 30 hours or less.

[0090] By carrying out the first annealing under the above conditions, the pearlite structure can be broken down, and the desired cementite can be more easily formed in the subsequent cold rolling and annealing processes.

[0091] The first annealing can be performed one or more times, but from the viewpoint of enhancing the effect of the annealing, it is preferable to repeat it two or more times. On the other hand, there is no particular upper limit on the number of times the first annealing is performed, but the effect saturates even if it is performed more than three times. Therefore, it is preferable to perform the first annealing three or less times. When the first annealing is repeated two or more times, it is sufficient that each annealing is performed under the above-mentioned conditions. The annealing conditions for each time may be the same or different.

[0092] It is also preferable to subject the hot-rolled steel sheet to pickling prior to the first annealing.

[0093] (7) Cold rolling (8) Second annealing Plate-like cementite is formed in the steel sheet after hot rolling. This plate-like cementite is stable and therefore likely to remain for a long time. Because the plate-like cementite is coarse, if this cementite remains, the number density of cementite particles in the final steel part cannot be set within the desired range. As a result, the desired wear resistance cannot be obtained. 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 one or more times.

[0094] Rolling ratio: 15% or more The cold rolling deforms, fragments, and decomposes cementite, thereby achieving a desired cementite particle number density. If the rolling reduction ratio in the cold rolling is less than 15%, the effect cannot be obtained. Therefore, the rolling reduction ratio is set to 15% or more, preferably 25% or more, more preferably 35% or more, and even more preferably 45% or more. On the other hand, the upper limit of the rolling reduction ratio is not particularly limited, but is preferably 85% or less, and more preferably 80% or less.

[0095] Annealing temperature: 600°C or higher, Tc or lower Annealing time: 3 hours or more The second annealing can refine and spheroidize the cementite deformed, fragmented, and decomposed by the cold rolling. At the same time, it can concentrate Cr in the cementite. To achieve these effects, the annealing temperature in the second annealing must be 600°C or higher and the annealing time must be 3 hours or longer. The annealing temperature is preferably 620°C or higher. The annealing time is preferably 4 hours or longer. On the other hand, if the annealing temperature is higher than Tc, phase transformation begins preferentially in a certain region, resulting in the formation of a locally coarse structure. As a result, the structure becomes nonuniform, and the desired cementite particle number density cannot be obtained. Therefore, the annealing temperature is set to Tc or lower, preferably Tc - 20°C or lower, and more preferably Tc - 40°C or lower. On the other hand, although there is no particular upper limit for the annealing time, an excessively long annealing time reduces productivity. Therefore, the annealing time is preferably set to 30 hours or lower, and more preferably 20 hours or lower.

[0096] By carrying out the cold rolling and second annealing under the above conditions, it is possible to refine the cementite and promote the concentration of Cr in the cementite.

[0097] The cold rolling and second annealing can be performed one or more times, but from the viewpoint of enhancing the above-mentioned effects, it is preferable to repeat them two or more times. When the cold rolling and second annealing are repeated two or more times, the cold rolling and the second annealing can be alternately repeated. On the other hand, there is no particular upper limit on the number of times that the cold rolling and the second annealing are performed, but the effect saturates even if the cold rolling and the second annealing are repeated more than five times. Therefore, it is preferable to perform the cold rolling and the second annealing five or less times. When the cold rolling and the second annealing are repeated two or more times, it is preferable that the cold rolling and the second annealing are performed under the above-mentioned conditions for each time. The conditions for each time may be the same or different.

[0098] When the cold rolling and second annealing are carried out only once, the rolling reduction ratio of the cold rolling is preferably 70% or more, from the viewpoint of reliably refining cementite.

[0099] It is also preferable to perform cold rolling (final cold rolling) after cold rolling and second annealing, prior to the next processing and heat treatment. By performing the final cold rolling, it is possible to improve the workability, particularly the punching workability, of the steel part shape that will be obtained thereafter. To achieve this effect, the rolling reduction ratio of the final cold rolling is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more.

[0100] A cold-rolled steel sheet is obtained by the above procedure. The thickness of the cold-rolled steel sheet is not particularly limited and can be any thickness, but is preferably 0.1 mm or more, and more preferably 0.2 mm or more. The upper limit of the thickness is also not particularly limited, but is preferably 2.5 mm or less, more preferably 1.6 mm or less, and even more preferably 0.8 mm or less. When the thickness is 0.2 mm or more and 0.8 mm or less, the steel sheet can be particularly suitably used as a material for knitting needles.

[0101] Furthermore, the finally obtained cold-rolled steel sheet may be further subjected to an optional surface treatment.

[0102] (9) Processing and heat treatment Next, the obtained cold-rolled steel sheet is processed into a part shape and heat-treated. The processing and heat treatment can be performed in any order. For example, the heat treatment may be performed after the processing, or may be performed during the processing.

[0103] The processing method is not particularly limited and can be any method, and may be, for example, at least one selected from the group consisting of punching, cutting, wire drawing, bending, and polishing.

[0104] The heat treatment includes quenching and tempering, and the conditions for these will be explained below.

[0105] Hardening Hardening can increase the strength of parts and control the number density of cementite particles, thereby achieving excellent wear resistance. To achieve this effect, the hardening temperature and holding time must satisfy the following conditions.

[0106] Quenching temperature: Tc or higher, 1000°C or lower During quenching, the cold-rolled steel sheet processed into a part shape is first heated to the quenching temperature and held at the quenching temperature for the holding time described below. This transforms the structure into austenite. Subsequent cooling transforms the austenite into martensite, improving strength. If 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 achieved. Therefore, the quenching temperature is set to Tc or higher, preferably Tc + 20°C or higher, and more preferably Tc + 50°C or higher. On the other hand, if the quenching temperature is too high, cementite dissolves. As a result, the desired cementite number density is not achieved, and wear resistance is reduced. Therefore, the quenching temperature is set to 1000°C or lower, preferably 950°C or lower, more preferably 900°C or lower, even more preferably 870°C or lower, and most preferably 840°C or lower.

[0107] ·Holding time: 1.0 minutes or more, 60 minutes or less During the quenching, the heating temperature must be maintained for 1.0 minute or longer to transform the structure into austenite. Therefore, the holding time is set to 1.0 minute or longer, preferably 2 minutes or longer, and more preferably 5 minutes or longer. On the other hand, if the holding time exceeds 60 minutes, the dissolution of cementite proceeds, the desired cementite number density cannot be obtained, and the wear resistance decreases. Therefore, the holding time is set to 60 minutes or shorter.

[0108] The cooling in the quenching is preferably performed using oil or other cooling medium to a temperature of 150°C or less. It is also preferable to cool the steel part by pressing it with a die, or by die quenching, press quenching, or the like to a temperature of 150°C or less.

[0109] Tempering Next, the quenched steel part is tempered. By tempering, the hardness and toughness of the steel part can be adjusted. The tempering can be performed under any conditions without any particular limitations, but typically, it is preferable that the tempering temperature is 100 to 400°C and the holding time is 10 minutes or more and 180 minutes or less.

[0110] The heat treatment may include any other heat treatment in addition to quenching and tempering. In one embodiment of the present invention, the heat treatment may consist of quenching and tempering.

[0111] The above-described method allows the production of steel parts with excellent wear resistance and surface properties. The steel parts can be used for any purpose without any particular limitation, but are particularly suitable for applications requiring wear resistance, such as textile machine parts, bearing parts, and machine blades. [Example]

[0112] In order to confirm the effects of the present invention, steel parts were manufactured according to the following procedure.

[0113] First, steel having the chemical composition shown in Table 1 was melted in a converter and formed into a steel slab by continuous casting. Next, the steel slab was sequentially subjected to cooling, heating, hot rolling, cooling, coiling, first annealing, cold rolling, and second annealing to obtain a cold-rolled steel sheet with a final thickness of approximately 0.4 mm. Each step was performed under the conditions shown in Table 2, and the first annealing, cold rolling, and second annealing were performed the number of times shown in Table 2.

[0114] In some examples, the cold-rolled steel sheets after the second annealing were further subjected to final cold rolling. The rolling reduction in the final cold rolling is shown in Tables 2 and 3.

[0115] The obtained cold-rolled steel sheets were then processed into part shapes and heat-treated to obtain final steel parts.

[0116] In machining the part shape, multiple steel parts were obtained from a single cold-rolled steel sheet by wire electric discharge machining. The wire electric discharge machining was performed from a position 5 mm inside from the widthwise end of the cold-rolled steel sheet at intervals of approximately 15 mm in the width direction and approximately 100 mm in the lengthwise direction (rolling direction). The dimensions of the steel parts were 10 mm wide x 80 mm long.

[0117] In the heat treatment, quenching and tempering were performed. The quenching was performed under the conditions shown in Table 2. The tempering was performed under conditions such that the Vickers hardness of the steel parts after tempering was 710±10.

[0118] According to the above procedure, a plurality of steel parts were produced for each example.

[0119] Next, for each of the obtained steel parts, the number density of cementite particles and the amount of Cr in the cementite particles were measured by the following procedure. The measurement results are shown in Tables 4 and 5.

[0120] (cementite number density) First, a test piece for microstructure observation was taken from the obtained steel part. The rolling direction cross section (L cross section) of the test piece for microstructure observation was polished to a mirror finish and further etched with 1 to 3 vol.% nital to reveal the microstructure. Thereafter, the etched surface of the test piece for microstructure observation was imaged at 10 locations at 1 / 2 the plate thickness using a scanning electron microscope (SEM) at an acceleration voltage of 15 keV and a magnification of 3000 times to obtain a microstructure image. From the obtained microstructure image, cementite particles with a particle diameter of less than 0.090 μm were excluded by image processing, and the number of cementite particles with a particle diameter of 0.090 μm or more was counted. The number density of cementite was calculated by dividing the obtained particle number by the area of ​​the microstructure image. The number density was calculated for each of the 10 microstructure images using the same procedure, and the average value was taken as the number density of cementite particles with a particle diameter of 0.090 μm or more.

[0121] (Cr content in cementite particles) A test piece for microstructure observation was taken from the obtained steel part. The L-section of the test piece for microstructure observation was 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, an electron probe micro analyzer (EPMA) was used to identify the cementite particles. 2 For one cementite particle with an area of ​​1.0×10, the electron beam acceleration voltage was 15 keV and the probe current was 1.0×10. -8 A: The Cr content was measured at three locations over a measurement time of 1000 ms, and the average value of the three locations was calculated. The Cr content was similarly measured for 10 cementite particles with a particle size of 0.090 μm or more, and the average value for the 10 particles was taken as the Cr content in the cementite particles.

[0122] Furthermore, the wear resistance and surface properties of each of the steel parts obtained were evaluated according to the following procedures. The evaluation results are shown in Tables 4 and 5.

[0123] (Surface texture) The surface quality of the steel parts was evaluated by visual inspection. Specifically, if no surface defects caused by scabs or slivers on the cold-rolled steel sheet were observed on any of the steel parts, the surface quality was rated as "good," and if at least one surface defect caused by scabs or slivers on the cold-rolled steel sheet was observed on any of the steel parts, the surface quality was rated as "poor."

[0124] (wear resistance) The wear resistance of the steel parts was evaluated by the following procedure. First, wear test pieces 10 having the shape shown in Figure 1 were taken from the steel parts. Each wear test piece 10 had four holes 11 for threading a thread.

[0125] An abrasion test was conducted using the obtained abrasion test piece 10 and the abrasion test device 20 shown in FIG. 2. Specifically, the yarn S fed from the yarn unwinding device 21 was run for 5,000 m per hole while in contact with the side of the hole 11 of the abrasion test piece 10, and the abrasion amount d1 was measured. Thereafter, the yarn S was run again at the same position while in contact with the side of the hole 11 of the abrasion test piece 10 for an additional 15,000 m per hole (total of 20,000 m), and the abrasion amount d2 was measured. A full-dull polyester knitting yarn was used as the running speed of the yarn S, and the running speed of the yarn S was 200 m / min. The tension of the yarn was adjusted to 20±2 N / cm using the tension adjusting device 22.

[0126] As shown in Figure 3, grooves 12 are formed by wear at the locations of holes 11 that were in contact with the yarn. Therefore, the yarn was stopped when it had run for 5,000 m and 20,000 m, and the depth d of the grooves 12 (wear depth) was measured using an optical microscope.

[0127] The wear depth after the thread had run 5,000 m was defined as d1, and the wear depth after it had run a further 15,000 m (total of 20,000 m) was defined as d2. The difference between d1 and d2 was defined as Δd and used as an index of wear resistance.

[0128] The same test was performed on four holes 11, and the average value of the four obtained wear depths Δd was taken as the wear depth of the wear test piece. If the wear depth Δd was 10 μm or less, the wear resistance was rated as "good," and if it was more than 10 μm, the wear resistance was rated as "poor." The evaluation results are shown in Tables 4 and 5.

[0129] [Table 1]

[0130] [Table 2]

[0131] [Table 3]

[0132] [Table 4]

[0133] [Table 5]

[0134] As can be seen from the results shown in Tables 1 to 5, steel parts that satisfy the conditions of the present invention can be produced without the occurrence of lag cracks during continuous casting, and also have excellent surface properties and wear resistance. [Explanation of symbols]

[0135] 10 Wear test specimens 11 holes 12 grooves 20. Abrasion testing equipment 21 Thread unwinding device 22 Tension adjustment device 23 Thread winding device S thread d Wear depth

Claims

1. In mass%, C: 0.80-1.25%, Si: 0.10-1.0%, Mn: 0.20-2.5%, P: 0.0005-0.05%, S: 0.03% or less, Al: 0.001-0.1%, N: 0.001 to 0.01%, O: 0.0100% or less, Cr: 0.56 to 1.6%, and Nb: 0.029 to 0.21%, Ti: 0.01 to 0.21%, and V: 0.01 to 0.21%; The balance consists of Fe and unavoidable impurities, and The total content of Nb, Ti, and V is 0.21% by mass or less. The number density of cementite particles having a particle diameter of 0.090 μm or more is 200,000 particles / mm ２ The cementite particles have a particle size of 0.090 μm or more and a density of 1,013,000 particles / mm 2 or less, and the Cr content in the cementite particles is 2.5 mass % or more.

2. The component composition is, in mass%, 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, The steel component of claim 1 , further comprising at least one selected from the group consisting of:

3. The steel part according to claim 1 or 2, wherein the steel part is any one of a textile machine part, a bearing part, and a machine blade.

4. A method for manufacturing a steel part according to claim 1 or 2, A steel slab having the above-mentioned component composition produced by continuous casting is cooled at a temperature at the center of the width direction of the steel slab and 10 mm from the surface under the conditions of a residence time of 130 seconds or less in a temperature range of 1200 to 1400°C and an average cooling rate of 10°C / hr or less in a temperature range of 550 to 700°C, The cooled steel slab is heated at a slab heating temperature of 1100°C or more for a slab heating time of 60 minutes or more. The heated steel slab is hot-rolled under the condition of a finish rolling end temperature: higher than Tc defined by the following formula (1) and 950°C or lower to obtain a hot-rolled steel sheet; The hot-rolled steel sheet is cooled under the conditions of an average cooling rate of 20°C / s or more and a cooling stop temperature of Tc or less, The cooled hot-rolled steel sheet is coiled at a coiling temperature of 530°C or higher and Tc or lower. The hot-rolled steel sheet after coiling is subjected to first annealing at least once under the conditions of an annealing temperature of 600°C or higher and Tc or lower and an annealing time of 3 hours or longer, The hot-rolled steel sheet after the first annealing is subjected to cold rolling at a rolling reduction rate of 15% or more and second annealing at an annealing temperature of 600°C or more and Tc or less for an annealing time of 3 hours or more, repeated at least twice to obtain a cold-rolled steel sheet; processing the cold-rolled steel sheet into a part shape; A method for manufacturing a steel part, comprising quenching under the conditions of a quenching temperature of not less than Tc and not more than 1000°C and a holding time of not less than 1.0 minute and not more than 60 minutes, and then performing a heat treatment including tempering. Tc (°C) = 723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr+6.38×W…(1) Here, the element symbols in the above formula (1) represent the content (mass %) of each element, and are set to zero when the element is not contained.

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