Steel material

Abstract

A steel material according to the present disclosure has a chemical composition containing 0.01-0.20 mass% C, 0.001-0.350 mass% Si, 0.80-2.00 mass% Mn, more than 0.030 mass% and 0.100 mass% or less P, more than 0.150 mass% and 0.800 mass% or less S, 0.01-0.30 mass% Cu, 0.01-0.30 mass% Ni, 0.01-0.50 mass% Cr, 0.01-0.50 mass% Mo, 0.001-0.100 mass% Sn, 0.0200 mass% or less N, and 0.0350 mass% or less O, with the remainder consisting Fe and impurities. The Mn content is 10.0% or more, the S content is 5.0% or more, the average equivalent circle diameter of Cu-containing MnS inclusions having a Cu content of more than 2.0% is less than 2.0 μm, and the Cu-containing MnS inclusions have a number density of 40 / mm2 or more.

Description

steel material

[0001] This disclosure relates to steel materials, and more specifically, to steel materials suitable for use as materials for machine parts.

[0002] Automobiles, office automation equipment, electrical equipment, etc., use mechanical parts. Examples of mechanical parts include brake parts for automobiles and shafts for office automation equipment. Sulfur free-cutting steel materials, such as SUM23 and SUM23L as specified in JIS G 4804 (2021), are used as materials for mechanical parts.

[0003] Machine parts made from these steel materials are manufactured, for example, through the following process: Cold working is performed on the steel material. Subsequently, cutting is performed to shape it into the desired part shape. Through the above manufacturing process, machine parts are produced.

[0004] As mentioned above, in the manufacturing process of machine parts, steel materials undergo cold working followed by machining. Therefore, steel materials require excellent machinability.

[0005] A steel material with improved machinability is proposed in International Publication No. 2016 / 199843 (Patent Document 1).

[0006] The steel material disclosed in Patent Document 1 contains, by mass%, C: 0.005 to 0.150%, Si: less than 0.010%, Mn: 1.02 to 2.00%, P: 0.010 to 0.200%, S: 0.350 to 0.600%, Pb: 0.010 to 0.100%, N: 0.004 to 0.015%, O: 0.0080 to 0.0250%, Al: 0 to 0.003%, one or more elements selected from the group consisting of Ca, Mg and Zr: totaling 0 to 0.0005%, and B: 0 to 0.0200%, with the remainder being Fe and impurities, satisfying formula (1) (Mn / S ≥ 2.90). Patent Document 1 states that by satisfying formula (1) in this steel material, MnS inclusions, Pb inclusions, and composite inclusions containing MnS inclusions and Pb can be obtained, resulting in excellent machinability in the steel material.

[0007] International Publication No. 2016 / 199843

[0008] However, excellent machinability of steel materials may be obtained by means other than those proposed in Patent Document 1.

[0009] The purpose of this disclosure is to provide a steel material that exhibits excellent machinability.

[0010] The steel material disclosed herein has a chemical composition in mass percent of: C: 0.01 to 0.20%, Si: 0.001 to 0.350%, Mn: 0.80 to 2.00%, P: greater than 0.030 to 0.100%, S: greater than 0.150 to 0.800%, Cu: 0.01 to 0.30%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, Sn: 0.001 to 0.100%, N: 0.0200% or less, O: 0.0350% or less, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.020%. It contains Te: 0-0.050%, Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.200%, Al: 0-0.010%, Co: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, B: 0-0.0050%, Se: 0-0.100%, Pb: 0-0.090%, Bi: 0-0.100%, and rare earth elements: 0-0.020%, with the remainder being Fe and impurities, and the mass percent of the Mn content is 10.0% or more, the S content is 5.0% or more, and the Cu content is more than 2.0%, with the average equivalent circle diameter of the Cu-containing MnS inclusions being less than 2.0 μm. The number density of the Cu-containing MnS inclusions is 40 particles / mm². ２ That's all.

[0011] The steel material according to this disclosure provides excellent machinability.

[0012] The inventors first investigated steel materials that can be obtained with excellent machinability from the perspective of chemical composition. As a result, the inventors found that a chemical composition of the following proportions in mass percent is desirable: C: 0.01 to 0.20%, Si: 0.001 to 0.350%, Mn: 0.80 to 2.00%, P: greater than 0.030 to 0.100%, S: greater than 0.150 to 0.800%, Cu: 0.01 to 0.30%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, Sn: 0.001 to 0.100%, N: 0.0200% or less, O: 0.0350% or less, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%. We considered that a steel material containing Zr: 0-0.020%, Te: 0-0.050%, Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.200%, Al: 0-0.010%, Co: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, B: 0-0.0050%, Se: 0-0.100%, Pb: 0-0.090%, Bi: 0-0.100%, and rare earth elements: 0-0.020%, with the remainder being Fe and impurities, could potentially provide excellent machinability.

[0013] However, even with steel materials having the above-mentioned chemical composition, sufficient machinability was sometimes not achieved. Therefore, the inventors further investigated means to improve machinability. As a result, the inventors obtained the following findings.

[0014] In steel materials that satisfy the above-mentioned chemical composition, MnS inclusions are formed within the steel. In this case, during machining, stress concentrates at the interface between the MnS inclusions and the matrix phase, causing cracks to form. Furthermore, these cracks propagate, and multiple cracks connect. As a result, chips become easier to separate from the steel. Therefore, the formation of MnS inclusions in steel improves its machinability.

[0015] Here, the inventors focused on the elements contained in MnS inclusions and investigated their effect on the machinability of steel materials. As a result, the inventors found that Cu contained in MnS inclusions may contribute to further improving machinability. Specifically, it was found that if the Cu content in MnS inclusions exceeds 2.0% by mass, the machinability of steel materials increases. In this specification, MnS inclusions having a Mn content of 10.0% or more, an S content of 5.0% or more, and a Cu content of more than 2.0% are also referred to as Cu-containing MnS inclusions. The inventors believe that the mechanism by which Cu-containing MnS inclusions improve machinability is as follows.

[0016] During the machining of steel materials, processing heat is generated in the steel. When copper (Cu) is present in MnS inclusions, the melting point of the MnS inclusions decreases. As a result, the Cu-containing MnS inclusions soften due to the processing heat. Softened Cu-containing MnS inclusions tend to become stress concentration points during machining. Therefore, compared to MnS inclusions with a Cu content of 2.0% or less, cracks are more likely to occur at the interface between Cu-containing MnS inclusions and the matrix phase. Furthermore, cracks that occur at the interface between Cu-containing MnS inclusions and the matrix phase are more likely to propagate and connect.

[0017] Based on the above findings, the inventors investigated the appropriate size and number density of Cu-containing MnS inclusions to improve the machinability of steel materials. As a result, the inventors found that in steel materials satisfying the above chemical composition, the average equivalent circle diameter of Cu-containing MnS inclusions is less than 2.0 μm, and the number density of Cu-containing MnS inclusions is 40 pieces / mm². ２ We found that the machinability of the steel material improves under these conditions.

[0018] The steel material of this embodiment was completed based on the above technical concept and has the following configuration.

[0019] The steel material of the first configuration has a chemical composition in mass %, C: 0.01 to 0.20%, Si: 0.001 to 0.350%, Mn: 0.80 to 2.00%, P: over 0.030 to 0.100%, S: over 0.150 to 0.800%, Cu: 0.01 to 0.30%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, Sn: 0.001 to 0.100%, N: 0.0200% or less, O: 0.0350% or less, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.020%, Te: 0 to 0.050%, Ti: 0 to 0.050%, Nb: 0 to 0.050%, V: 0 to 0.200%, Al: 0 to 0.010%, Co: 0 to 0.100%, Sb: 0 to 0.050%, As: 0 to 0.050%, B: 0 to 0.0050%, Se: 0 to 0.100%, Pb: 0 to 0.090%, Bi: 0 to 0.100%, and rare earth elements: 0 to 0.020%, and the balance consists of Fe and impurities, and in mass %, the Mn content is 10.0% or more, the S content is 5.0% or more, and the average equivalent circle diameter of the Cu-containing MnS inclusions with a Cu content of more than 2.0% is less than 2.0 μm, and the number density of the Cu-containing MnS inclusions is 40 pieces / mm ２ or more.

[0020] The steel material of the second configuration is the steel material of the first configuration, and the chemical composition in mass % contains one or more selected from the group consisting of Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Zr: 0.001 to 0.020%, Te: 0.001 to 0.050%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, V: 0.001 to 0.200%, Al: 0.001 to 0.010%, Co: 0.001 to 0.100%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, B: 0.0001 to 0.0050%, Se: 0.001 to 0.100%, Pb: 0.001 to 0.090%, Bi: 0.001 to 0.100%, and rare earth elements: 0.001 to 0.020%.

[0021] Hereinafter, the steel material according to this embodiment will be described in detail. In addition, "%" regarding an element means "mass%" unless otherwise specified.

[0022] [Characteristics of the steel material of this embodiment] The steel material of this embodiment includes the following characteristics. (Characteristic 1) The chemical composition is, by mass%, C: 0.01 to 0.20%, Si: 0.001 to 0.350%, Mn: 0.80 to 2.00%, P: more than 0.030 to 0.100%, S: more than 0.150 to 0.800%, Cu: 0.01 to 0.30%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, Sn: 0.001 to 0.100%, N: 0.0200% or less, O: 0.0350% or less, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.020%, Te: 0 to 0.050%, Ti: 0 to 0.050%, Nb: 0 to 0.050%, V: 0 to 0.200%, Al: 0 to 0.010%, Co: 0 to 0.100%, Sb: 0 to 0.050%, As: 0 to 0.050%, B: 0 to 0.0050%, Se: 0 to 0.100%, Pb: 0 to 0.090%, Bi: 0 to 0.100%, and rare earth elements: 0 to 0.02%, and the balance consists of Fe and impurities. (Characteristic 2) In terms of mass%, the average equivalent circle diameter of Cu-containing MnS inclusions with a Mn content of 10.0% or more, an S content of 5.0% or more, and a Cu content of more than 2.0% is less than 2.0 μm, and the number density of Cu-containing MnS inclusions is 40 pieces / mm ２ or more. Hereinafter, Characteristic 1 and Characteristic 2 will be described.

[0023] [(Characteristic 1) Regarding the chemical composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0024] C: 0.01-0.20% Carbon (C) increases the strength of machine parts manufactured from steel. If the C content is less than 0.01%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content exceeds 0.20%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel decreases. Therefore, the C content is 0.01-0.20%. The preferred lower limit of the C content is 0.02%, more preferably 0.03%, and still more preferably 0.04%. The preferred upper limit of the C content is 0.18%, more preferably 0.16%, still more preferably 0.13%, and still more preferably 0.10%.

[0025] Si: 0.001 to 0.350% Silicon (Si) increases the strength of machine parts manufactured from steel. If the Si content is less than 0.001%, the above effect cannot be sufficiently obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 0.350%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel decreases. Therefore, the Si content is 0.001 to 0.350%. The preferred lower limit of the Si content is 0.003%, more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.040%. The preferred upper limit of the Si content is 0.300%, more preferably 0.250%, and even more preferably 0.200%.

[0026] Mn: 0.80-2.00% Manganese (Mn), together with S, forms MnS inclusions, which improve the machinability of the steel. If the Mn content is less than 0.80%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.00%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel decreases. Therefore, the Mn content is 0.80-2.00%. The preferred lower limit of the Mn content is 0.85%, more preferably 0.90%, and still more preferably 0.95%. The preferred upper limit of the Mn content is 1.90%, more preferably 1.80%, and still more preferably 1.70%.

[0027] P: Greater than 0.030% to 0.100% Phosphorus (P) embrittles steel and improves its machinability. If the P content is 0.030% or less, the above effect cannot be fully obtained even if the content of other elements is within the range of this embodiment. However, if the P content exceeds 0.100%, the steel becomes excessively embrittle. Therefore, even if the content of other elements is within the range of this embodiment, the hot workability of the steel decreases. Accordingly, the P content is greater than 0.030% to 0.100%. The preferred lower limit of the P content is 0.031%, more preferably 0.035%, more preferably 0.040%, and still more preferably 0.045%. The preferred upper limit of the P content is 0.095%, more preferably 0.090%, and still more preferably 0.085%.

[0028] S: Greater than 0.150% to 0.800% Sulfur (S) forms MnS inclusions, improving the machinability of steel. If the S content is 0.150% or less, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the S content exceeds 0.800%, even if the content of other elements is within the range of this embodiment, excessive MnS inclusions are generated. In this case, the MnS inclusions become the starting point for cracks during hot working. Therefore, even if the content of other elements is within the range of this embodiment, the hot workability of the steel decreases. Accordingly, the S content is greater than 0.150% to 0.800%. The preferred lower limit of the S content is 0.151%, more preferably 0.200%, more preferably 0.250%, and still more preferably 0.300%. The preferred upper limit of the S content is 0.750%, more preferably 0.700%, and still more preferably 0.650%.

[0029] Cu: 0.01-0.30% Copper (Cu) is contained in MnS inclusions, lowering the melting point of the MnS inclusions. As a result, the Cu-containing MnS inclusions soften due to the heat generated during machining of the steel material. Consequently, the Cu-containing MnS inclusions become stress concentration points during machining, improving the machinability of the steel material. If the Cu content is less than 0.01%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cu content exceeds 0.30%, the strength of the steel material becomes excessively high, even if the content of other elements is within the range of this embodiment. Consequently, the machinability of the steel material decreases. Therefore, the Cu content is 0.01-0.30%. The preferred lower limit of the Cu content is 0.04%, more preferably 0.06%, more preferably 0.07%, and still more preferably 0.10%. The preferred upper limit for the Cu content is 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0030] Ni: 0.01 to 0.30% Nickel (Ni) increases the strength of machine parts manufactured from steel. If the Ni content is less than 0.01%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ni content exceeds 0.30%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel decreases. Therefore, the Ni content is 0.01 to 0.30%. The preferred lower limit of the Ni content is 0.02%, more preferably 0.03%, and still more preferably 0.04%. The preferred upper limit of the Ni content is 0.25%, more preferably 0.23%, still more preferably 0.20%, and still more preferably 0.15%.

[0031] Cr: 0.01 to 0.50% Chromium (Cr) increases the strength of machine parts manufactured from steel. If the Cr content is less than 0.01%, the above effect cannot be sufficiently obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content exceeds 0.50%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel decreases. Therefore, the Cr content is 0.01 to 0.50%. The preferred lower limit of the Cr content is 0.04%, more preferably 0.07%, and even more preferably 0.10%. The preferred upper limit of the Cr content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0032] Mo: 0.01 to 0.50% Molybdenum (Mo) increases the strength of machine parts manufactured from steel. If the Mo content is less than 0.01%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content exceeds 0.50%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel decreases. Therefore, the Mo content is 0.01 to 0.50%. The preferred lower limit of the Mo content is 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the Mo content is 0.40%, more preferably 0.35%, and even more preferably 0.30%.

[0033] Sn: 0.001 to 0.100% Tin (Sn) embrittles the steel material and improves its machinability. If the Sn content is less than 0.001%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. However, if the Sn content exceeds 0.100%, the steel material becomes excessively embrittle. Therefore, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material decreases. Accordingly, the Sn content is 0.001 to 0.100%. The preferred lower limit of the Sn content is 0.004%, more preferably 0.007%, and even more preferably 0.010%. The preferred upper limit of the Sn content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0034] N: 0.0200% or less. Nitrogen (N) is an unavoidable impurity. That is, the lower limit of the N content is greater than 0%. N increases the strength of machine parts manufactured using steel as a material. If the N content exceeds 0.0200%, the strength of the steel will become excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel will decrease. Therefore, the N content is 0.0200% or less. It is preferable to have as low an N content as possible. However, if the N content is reduced excessively, the manufacturing cost will increase. Therefore, considering normal industrial production, the preferred lower limit of the N content is 0.0001%, more preferably 0.0010%, more preferably 0.0020%, and still more preferably 0.0040%. The preferred upper limit of the N content is 0.0180%, more preferably 0.0150%, and still more preferably 0.0120%.

[0035] O: 0.0350% or less. Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. O combines with other elements in the steel to form oxides. If the O content exceeds 0.0350%, coarse oxides are excessively produced. Coarse oxides become the starting point for cracks during hot working. Therefore, even if the content of other elements is within the range of this embodiment, the hot workability of the steel decreases. Accordingly, the O content is 0.0350% or less. It is preferable to have as low an O content as possible. However, if the O content is reduced excessively, the manufacturing cost will increase. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit of the O content is 0.0300%, more preferably 0.0250%, and even more preferably 0.0200%.

[0036] The remainder of the chemical composition of the steel material in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of steel material, and are acceptable within a range that does not adversely affect the steel material in this embodiment.

[0037] [Regarding Optional Elements] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.020%, Te: 0-0.050%, Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.200%, Al: 0-0.010%, Co: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, B: 0-0.0050%, Se: 0-0.100%, Pb: 0-0.090%, Bi: 0-0.100%, and rare earth elements: 0-0.020%. These elements are all optional and do not need to be included. The following describes these optional elements.

[0038] [Group 1: Ca, Mg, Zr, and Te] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, Zr, and Te in place of a portion of Fe. All of these elements control the morphology of inclusions and suppress the occurrence of cracks originating from inclusions.

[0039] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. If Ca is present, that is, if the Ca content is greater than 0%, Ca controls the morphology of inclusions, causing them to become spheroidal. Therefore, the occurrence of cracks originating from inclusions is suppressed. Even if only a small amount of Ca is present, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse oxides will be formed. In this case, even if the content of other elements is within the range of this embodiment, coarse oxides will become the starting point for cracks. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit of the Ca content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0040] Mg: 0-0.0050% Magnesium (Mg) is an optional element and may not be present. In other words, the Mg content may be 0%. If Mg is present, that is, if the Mg content is greater than 0%, Mg controls the morphology of inclusions, causing them to become spheroidal. Therefore, the occurrence of cracks originating from inclusions is suppressed. Even if only a small amount of Mg is present, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, the above effect becomes saturated. Therefore, the Mg content is 0-0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit of the Mg content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0041] Zr: 0-0.020% Zirconium (Zr) is an optional element and may not be present. In other words, the Zr content may be 0%. If it is present, that is, if the Zr content is greater than 0%, Zr controls the morphology of the inclusions, causing them to become spheroidal. Therefore, the occurrence of cracks originating from the inclusions is suppressed. Even if only a small amount of Zr is present, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.020%, the above effect saturates. Therefore, the Zr content is 0-0.020%. The preferred lower limit of the Zr content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Zr content is 0.018%, more preferably 0.016%, and even more preferably 0.014%.

[0042] Te: 0-0.050% Tellurium (Te) is an optional element and may not be present. In other words, the Te content may be 0%. If Te is present, that is, if the Te content is greater than 0%, Te controls the morphology of inclusions, causing them to become spheroidal. As a result, the occurrence of cracks originating from inclusions is suppressed. Even if only a small amount of Te is present, the above effect can be obtained to some extent. However, if the Te content exceeds 0.050%, the above effect saturates. Therefore, the Te content is 0-0.050%. The preferred lower limit of the Te content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Te content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0043] [Second group: Ti, Nb, V, and Al] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Ti, Nb, V, and Al in place of a portion of Fe. All of these elements form precipitates and suppress grain coarsening during hot working of the steel material by pinning effect.

[0044] Ti: 0-0.050% Titanium (Ti) is an optional element and may not be included. In other words, the Ti content may be 0%. If Ti is included, that is, if the Ti content is greater than 0%, Ti forms Ti precipitates such as Ti carbides and Ti carbonitrides. The Ti precipitates suppress grain coarsening during hot working of steel materials through a pinning effect. As a result, the strength of machine parts manufactured using steel materials is increased. Even if only a small amount of Ti is included, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.050%, the Ti precipitates coarseen. In this case, grain coarsening during hot working cannot be sufficiently suppressed. Therefore, even if the content of other elements is within the range of this embodiment, the strength of the steel material decreases. Accordingly, the Ti content is 0-0.050%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the Ti content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0045] Nb: 0-0.050% Niobium (Nb) is an optional element and may not be present. In other words, the Nb content may be 0%. If Nb is present, that is, if the Nb content is greater than 0%, Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides. The Nb precipitates suppress grain coarsening during hot working of steel materials by a pinning effect. As a result, the strength of machine parts manufactured using steel materials is increased. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.050%, the Nb precipitates will coarse. In this case, grain coarsening during hot working cannot be sufficiently suppressed. Therefore, even if the content of other elements is within the range of this embodiment, the strength of the steel material will decrease. Accordingly, the Nb content is 0-0.050%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the Nb content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0046] V: 0 to 0.200% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V forms V precipitates such as V carbides and V carbonitrides. The V precipitates suppress grain coarsening during hot working of steel materials through a pinning effect. As a result, the strength of machine parts manufactured using steel materials is increased. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.200%, the V precipitates coarseen. In this case, grain coarsening during hot working cannot be sufficiently suppressed. Therefore, even if the content of other elements is within the range of this embodiment, the strength of the steel material decreases. Accordingly, the V content is 0 to 0.200%. The preferred lower limit of the V content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the V content is 0.180%, more preferably 0.160%, and even more preferably 0.140%.

[0047] Al: 0-0.010% Aluminum (Al) is an optional element and may not be present. In other words, the Al content may be 0%. If Al is present, that is, if the Al content is greater than 0%, Al forms Al precipitates such as Al oxide and Al nitride. The Al precipitates suppress grain coarsening during hot working of steel materials through a pinning effect. As a result, the strength of machine parts manufactured using steel as a material is increased. Even if only a small amount of Al is present, the above effect can be obtained to some extent. However, if the Al content exceeds 0.010%, the Al precipitates become coarser. In this case, grain coarsening during hot working cannot be sufficiently suppressed. Therefore, even if the content of other elements is within the range of this embodiment, the strength of the steel material decreases. Accordingly, the Al content is 0-0.010%. The preferred lower limit of the Al content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Al content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0048] [Group 3: Co, Sb, and As] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Co, Sb, and As in place of a portion of Fe. All of these elements enhance the corrosion resistance of the steel material.

[0049] Co: 0 to 0.100% Cobalt (Co) is an optional element and may not be included. In other words, the Co content may be 0%. If it is included, that is, if the Co content is greater than 0%, Co enhances the corrosion resistance of the steel. Even if only a small amount of Co is included, the above effect can be obtained to some extent. However, if the Co content exceeds 0.100%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Co content is 0 to 0.100%. The preferred lower limit of the Co content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Co content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0050] Sb: 0-0.050% Antimony (Sb) is an optional element and may not be present. In other words, the Sb content may be 0%. If it is present, that is, if the Sb content is greater than 0%, Sb enhances the corrosion resistance of the steel. Even if only a small amount of Sb is present, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.050%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is 0-0.050%. The preferred lower limit of the Sb content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Sb content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0051] As: 0-0.050% Arsenic (As) is an optional element and may not be present. In other words, the As content may be 0%. If it is present, that is, if the As content is greater than 0%, As enhances the corrosion resistance of the steel. Even if only a small amount of As is present, the above effect can be obtained to some extent. However, if the As content exceeds 0.050%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the As content is 0-0.050%. The preferred lower limit of the As content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the As content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0052] [Group 4: B, Se, Pb, Bi, and rare earth elements] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of B, Se, Pb, Bi, and rare earth elements in place of a portion of Fe. All of these elements enhance the machinability of the steel material.

[0053] B: 0 to 0.0050% Boron (B) is an optional element and may not be included. In other words, the B content may be 0%. If it is included, that is, if the B content is greater than 0%, B forms BN together with N, improving the machinability of the steel. Even if only a small amount of B is included, the above effect can be obtained to some extent. However, if the B content exceeds 0.0050%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0054] Se: 0 to 0.100% Selenium (Se) is an optional element and may not be present. In other words, the Se content may be 0%. If Se is present, that is, if the Se content is greater than 0%, Se improves the machinability of the steel. Even if only a small amount of Se is present, the above effect can be obtained to some extent. However, if the Se content exceeds 0.100%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Se content is 0 to 0.100%. The preferred lower limit of the Se content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Se content is 0.095%, more preferably 0.090%, and even more preferably 0.085%.

[0055] Pb: 0 to 0.090% Lead (Pb) is an optional element and may not be present. In other words, the Pb content may be 0%. If Pb is present, that is, if the Pb content is greater than 0%, Pb improves the machinability of the steel. Even if only a small amount of Pb is present, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.090%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Pb content is 0 to 0.090%. The preferred lower limit of the Pb content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Pb content is 0.085%, more preferably 0.080%, and even more preferably 0.075%.

[0056] Bi: 0 to 0.100% Bismuth (Bi) is an optional element and may not be present. In other words, the Bi content may be 0%. If it is present, that is, if the Bi content is greater than 0%, Bi improves the machinability of the steel. Even if only a small amount of Bi is present, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.100%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Bi content is 0 to 0.100%. The preferred lower limit of the Bi content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit of the Bi content is 0.095%, more preferably 0.090%, and even more preferably 0.085%.

[0057] Rare Earth Elements: 0-0.020% Rare earth elements (REM) are optional elements and do not need to be included. In other words, the REM content may be 0%. If REM is included, that is, if the REM content is greater than 0%, REM improves the machinability of the steel. Even if only a small amount of REM is included, the above effect can be obtained to some extent. However, if the REM content exceeds 0.020%, the hot workability of the steel will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the REM content is 0-0.020%. The preferred lower limit of the REM content is 0.001%, more preferably 0.002%, and still more preferably 0.005%. The preferred upper limit of the REM content is 0.018%, more preferably 0.016%, and still more preferably 0.014%.

[0058] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and lanthanides, lanthanum (La), atomic number 57 to lutetium (Lu), atomic number 71. In this specification, REM content refers to the total content of these elements.

[0059] [(Characteristic 2) Regarding Cu-containing MnS inclusions] In the steel material of this embodiment, furthermore, in terms of mass%, the Mn content is 10.0% or more, the S content is 5.0% or more, and the average equivalent circle diameter of Cu-containing MnS inclusions with a Cu content exceeding 2.0% is less than 2.0 μm, and the number density of Cu-containing MnS inclusions is 40 pieces / mm ２ or more. In this specification, the average equivalent circle diameter of Cu-containing MnS inclusions is also referred to as "average equivalent circle diameter D ＡＶＥ ". Furthermore, in this specification, the number density of Cu-containing MnS inclusions is also referred to as "number density ND". Cu-containing MnS inclusions are, for example, inclusions in which a part of Mn in MnS inclusions is substitutionally dissolved by Cu. However, the specific form of Cu-containing MnS inclusions is not particularly limited as long as, in the elemental concentration analysis by mass% using EDX described later, the Mn content is 10.0% or more, the S content is 5.0% or more, and the Cu content exceeds 2.0%.

[0060] Cu-containing MnS inclusions have a lower melting point compared to MnS inclusions with a Cu content of 2.0% or less. Therefore, due to the heat generation during machining of the steel material, Cu-containing MnS inclusions soften. The softened Cu-containing MnS inclusions tend to become stress concentration parts during machining. Therefore, cracks are likely to occur at the interface between Cu-containing MnS inclusions and the matrix phase. Furthermore, cracks generated at the interface between Cu-containing MnS inclusions and the matrix phase are likely to propagate or connect. As a result, the machinability of the steel material is enhanced.

[0061] Average equivalent circle diameter D ＡＶＥ When a large number of fine Cu-containing MnS inclusions with a diameter less than 2.0 μm are distributed in the steel material, cracks are likely to occur, propagate, and connect during machining. As a result, the machinability of the steel material is enhanced. That is, in the steel material satisfying Characteristic 1, if the number density ND of Cu-containing MnS inclusions with an average equivalent circle diameter D ＡＶＥ less than 2.0 μm is 40 pieces / mm ２ or more, excellent machinability of the steel material can be obtained. On the other hand, even if the average equivalent circle diameter D ＡＶＥ of Cu-containing MnS inclusions is less than 2.0 μm, if the number density ND of Cu-containing MnS inclusions is 40 pieces / mm ２If it is less than this, crack initiation, propagation, and coupling are less likely to occur during cutting. As a result, the excellent machinability of the steel material cannot be obtained. Furthermore, the average equivalent circle diameter D of the Cu-containing MnS inclusions ＡＶＥ If the particle size is 2.0 μm or larger, the number density of Cu-containing MnS inclusions does not increase. Therefore, the number density ND of Cu-containing MnS inclusions is 40 particles / mm³. ２ It becomes less than [value]. As a result, excellent machinability of the steel material cannot be obtained. Therefore, the average equivalent circle diameter D of the Cu-containing MnS inclusions is [value]. ＡＶＥ The particle size is less than 2.0 μm, and the number density ND is 40 particles / mm². ２ That concludes the explanation. Note that the average circle equivalent diameter D ＡＶＥ Cu-containing MnS inclusions having a size of less than 2.0 μm are also referred to as fine Cu-containing MnS inclusions in this specification.

[0062] Average circular equivalent diameter D of Cu-containing MnS inclusions ＡＶＥ The preferred upper limit is 1.9 μm, more preferably 1.8 μm, even more preferably 1.7 μm, and even more preferably 1.6 μm. Average equivalent circle diameter D of Cu-containing MnS inclusions ＡＶＥ The lower limit is not particularly limited. When the chemical composition of the steel material satisfies characteristic 1, the average circular equivalent diameter D of the Cu-containing MnS inclusions is... ＡＶＥ The lower limit is, for example, 1.0 μm, for example, 1.1 μm, for example, 1.2 μm.

[0063] The preferred lower limit for the number density ND is 45 particles / mm². ２ And more preferably 50 pieces / mm ２ More preferably 55 pieces / mm ２ The upper limit of the number density ND is not particularly limited. If the chemical composition of the steel material satisfies characteristic 1, the upper limit of the number density ND is, for example, 350 pieces / mm ２ For example, 300 pieces / mm ２ For example, 200 pieces / mm ２ That is the case.

[0064] [Average circular equivalent diameter D of Cu-containing MnS inclusions] ＡＶＥ [Method for measuring the number density ND] In this embodiment, the average circular equivalent diameter D of the Cu-containing MnS inclusions ＡＶＥ(μm) and number density ND (pieces / mm) ２ The centroid can be determined by the following method: If the cross-section perpendicular to the rolling direction of the steel material is circular with radius R, five test specimens are taken from the R / 2 position, which is the midpoint of the line segment connecting the surface of the steel material to the central axis, within a cross-section that includes the central axis of the steel material and is parallel to the central axis. If the cross-section perpendicular to the rolling direction of the steel material is not circular, the central axis of the steel material is defined as the axis parallel to the rolling direction that passes through the centroid of the cross-section perpendicular to the rolling direction of the steel material. Let R be the length of the shortest line segment connecting the central axis to the surface of the steel material. Five test specimens are taken from the R / 2 position, which is the midpoint of the shortest line segment, within a cross-section that includes the shortest line segment and the central axis. The centroid of the cross-section is defined as the position of the centroid when the cross-section is considered as a thin, plate-like object with uniform density.

[0065] Of the surfaces of the collected test specimens, the surface corresponding to the above cross-section is defined as the observation surface. After mirror polishing the observation surface, a scanning electron microscope (SEM) is used to observe the observation region containing the R / 2 position on the observation surface to obtain a backscattered electron image. The observation region is a rectangle of 1600 μm × 1200 μm centered on the R / 2 position. The long side of the observation region corresponds to the central axis direction of the steel material. During observation, the observation region is divided into 60 non-overlapping fields of view of 160 μm × 200 μm, and each field of view is observed at a magnification of 500x.

[0066] In the observation area, particles (precipitates or inclusions) that may contain Cu-containing MnS are identified based on the Z contrast of the backscattered electron image. In the backscattered electron image, particles that may contain Cu-containing MnS are shown with a darker contrast compared to the matrix. The longest length of a straight line passing through any two points on the outer circumference of the identified particle is taken as the major axis (μm) of that particle. From the multiple identified particles, particles with a major axis of 1.0 μm or more are selected. Note that particles with a major axis of less than 1.0 μm are difficult to confirm with a 500x magnification SEM. Therefore, the particles to be selected are limited to those with a major axis of 1.0 μm or more.

[0067] For selected particles with a major axis of 1.0 μm or larger, elemental concentration analysis using energy-dispersive X-ray spectroscopy (EDX) is performed to identify Cu-containing MnS inclusions. The EDX-Standardless method is used for the EDX analysis (elemental concentration analysis). The acceleration voltage is set to 20 kV, and the elements to be quantified are C, Si, Mn, P, S, Cu, Ni, Cr, Sn, N, O, Ca, Ti, and Al.

[0068] In the EDX analysis results for each particle, if the total mass percentage content of the above-mentioned quantitative elements is set to 100%, and the mass percentages show that the Mn content is 10.0% or more, the S content is 5.0% or more, and the Cu content is greater than 2.0%, then that particle is identified as a Cu-containing MnS inclusion. Note that the Mn, S, and Cu content values ​​are rounded to the second decimal place (i.e., to the first decimal place).

[0069] The equivalent circular diameter (μm) of the Cu-containing MnS inclusions identified in each observation area of ​​the five test specimens is determined. Here, the equivalent circular diameter refers to the diameter of a circle when the area of ​​the particle is converted to a circle of the same area. The arithmetic mean of the obtained equivalent circular diameters is defined as the average equivalent circular diameter D of the Cu-containing MnS inclusions. ＡＶＥ Let (μm) be the equivalent diameter of the circle D. ＡＶＥ This value is obtained by rounding the second decimal place of the resulting number (i.e., the value to one decimal place).

[0070] Furthermore, the total number of Cu-containing MnS inclusions identified in each observation area of ​​the five test specimens is determined. By dividing the obtained total number of Cu-containing MnS inclusions by the total area of ​​each observation area in the five test specimens, the number density ND (inclusions / mm²) of the Cu-containing MnS inclusions is calculated. ２ The number density ND is calculated by rounding the obtained value to the first decimal place (i.e., an integer value).

[0071] [Effects of the steel material of this embodiment] The steel material of this embodiment satisfies features 1 and 2. Therefore, the steel material of this embodiment has excellent machinability.

[0072] [Regarding the microstructure of the steel material] The microstructure of the steel material in this embodiment consists of ferrite and pearlite.

[0073] [Method for measuring ferrite and pearlite in the microstructure] The ferrite and pearlite in the microstructure of the steel material of this embodiment are measured by the following method. If the cross section perpendicular to the rolling direction of the steel material is circular with radius R, a test piece is taken from the cross section perpendicular to the central axis direction of the steel material, including the R / 2 position. If the cross section perpendicular to the rolling direction of the steel material is not circular, as described above, the central axis of the steel material is defined as the axis parallel to the rolling direction passing through the centroid of the cross section perpendicular to the rolling direction of the steel material. The length of the shortest line segment connecting the central axis to the surface of the steel material is defined as R. A test piece is taken from the R / 2 position, which is the midpoint of the shortest line segment in the cross section perpendicular to the central axis direction of the steel material. The surface of the test piece, specifically the cross section perpendicular to the central axis direction of the steel material, is used as the observation surface.

[0074] The observation surface of the test specimen is polished to a mirror finish. The polished observation surface is etched using 2% nitric acid alcohol (Nital etching solution). Three observation fields centered around the R / 2 position on the etched observation surface are observed using a 400x optical microscope. The area of ​​the observation field is 500 μm × 500 μm.

[0075] The tissue within the observation field is identified by the following method: In the observation field, inclusions such as MnS, oxides, and nitrides are observed in granular or rod-like forms stretched in the rolling direction. In the tissue region excluding these inclusions, areas that are brighter than the inclusions and observed as white are identified as ferrite. Furthermore, in the tissue region excluding the inclusions and ferrite, areas that are darker than the ferrite and observed as black are marked by methods such as creating indentations around the periphery. If there are five or more such black areas in one observation field, any five of these areas are marked. The marked areas are observed using a scanning electron microscope (SEM) at an observation magnification of 2000x. If all of the marked areas in the three observation fields are tissues with a lamellar structure, the black areas are identified as pearlite. In this case, it is further determined that the microstructure of the steel material consists of ferrite and pearlite.

[0076] [Shape and Use of Steel Material in This Embodiment] The shape of the steel material in this embodiment is not particularly limited and may be, for example, a steel bar or wire. The steel material in this embodiment may further have the shape of any mechanical part used in automobiles, office automation equipment, electrical equipment, etc. Examples of mechanical parts include brake parts for automobiles and shafts for office automation equipment. However, the steel material in this embodiment can also be applied to uses other than mechanical parts.

[0077] [Method for Manufacturing Steel Materials] An example of a method for manufacturing steel materials according to this embodiment will be described. The method for manufacturing steel materials described below is just one example for manufacturing the steel materials of this embodiment. Therefore, steel materials having the above-described configuration may be manufactured by other manufacturing methods other than the method described below. However, the method described below is a preferred example of a method for manufacturing steel materials according to this embodiment.

[0078] An example of a manufacturing method for steel materials according to this embodiment includes the following steps: (Step 1) Steelmaking process (Step 2) Hot rolling process Each step will be described below.

[0079] [(Process 1) Steelmaking Process] In the steelmaking process, bloom having a chemical composition that satisfies characteristic 1 above is produced. Specifically, molten steel is prepared in which the content of each element in the chemical composition satisfies characteristic 1. The refining method is not particularly limited, and any well-known method may be used. For example, molten iron produced by a well-known method is subjected to refining in a converter (primary refining). A well-known secondary refining is performed on the molten steel tapped from the converter. Primary refining may be performed using, for example, an electric furnace. Molten steel having a chemical composition that satisfies characteristic 1 is produced through the above process. Bloom is produced using the produced molten steel by continuous casting. Bloom is produced through the above process.

[0080] [(Process 2) Hot Rolling Process] In the hot rolling process, the bloom is hot-rolled to produce steel. The hot rolling process includes the bract rolling process and the product rolling process.

[0081] [(Process 21) Ball-forming Rolling Process] In the ball-forming rolling process, first, the bloom is heated in a heating furnace. After heating, the bloom is hot-rolled (ball-forming) using a ball-forming mill to produce a billet. Furthermore, the billet is hot-rolled using a continuous rolling mill, which is located downstream of the ball-forming mill and has multiple rolling stands arranged in a line, to further reduce the size of the billet. The billet produced in the ball-forming rolling process is allowed to cool to room temperature (air-cooled) before the product rolling process. The ball-forming rolling process also satisfies the following condition 1. (Condition 1) The heating temperature T1 is 1100°C or less. Condition 1 will be explained below.

[0082] [Regarding Condition 1] In this embodiment, the heating temperature T1 in the bract rolling process is set to 1100°C or lower. If the heating temperature T1 in the bract rolling process exceeds 1100°C, the precipitation of fine Cu-containing MnS inclusions is suppressed, and MnS inclusions with a Cu content of 2.0% or less precipitate preferentially. In this case, even if conditions 2 to 4 described below are met, the steel material of this embodiment does not satisfy feature 2.

[0083] If the heating temperature T1 in the bract rolling process is 1100°C or lower, fine Cu-containing MnS inclusions will precipitate preferentially. As a result, assuming that conditions 2 to 4 described below are met, the steel material of this embodiment satisfies feature 2. The holding time at heating temperature T1 is not particularly limited, but for example, the holding time is 60 to 300 minutes.

[0084] [(Process 22) Product Rolling Process] In the product rolling process, the steel material of this embodiment is manufactured by hot rolling (product rolling) of the billet. Specifically, the billet after the bract rolling process is heated using a heating furnace. Hot rolling (product rolling) is performed on the heated billet using a continuous rolling mill. The steel material after hot rolling is allowed to cool to room temperature. In the product rolling process, the following conditions 2 to 4 are also satisfied. (Condition 2) The heating temperature T2 is 1100°C or less. (Condition 3) In the range where the steel material temperature is 1050°C or higher, the maximum reduction ratio RR per pass is less than 20%. (Condition 4) In the range where the steel material temperature is less than 1050°C, the number of passes PN for reduction with a reduction ratio of 20% or more is 3 or more. Conditions 2 to 4 will be explained below.

[0085] [Regarding Condition 2] In this embodiment, the heating temperature T2 in the product rolling process is set to 1100°C or lower. If the heating temperature T2 in the product rolling process exceeds 1100°C, the precipitation of fine Cu-containing MnS inclusions is suppressed, and MnS inclusions with a Cu content of 2.0% or less precipitate preferentially. In this case, even if conditions 1, 3, and 4 are met, the steel material of this embodiment does not satisfy feature 2.

[0086] If the heating temperature T2 in the product rolling process is 1100°C or lower, fine Cu-containing MnS inclusions will precipitate preferentially. As a result, assuming that conditions 1, 3, and 4 are met, the steel material of this embodiment satisfies feature 2. The holding time at heating temperature T2 is not particularly limited, but for example, the holding time is 60 to 300 minutes.

[0087] [Regarding Condition 3] In this embodiment, the maximum reduction ratio RR per pass is set to less than 20% in the range where the steel material temperature is 1050°C or higher. Here, "pass" means that in product rolling using a continuous rolling mill, the billet receives one reduction (external force) from the work rolls as it passes through each rolling stand from upstream to downstream. Therefore, if the product does not receive an external force from the work rolls during product rolling, that operation does not constitute a "pass". Also, "steel material temperature" means the surface temperature of the steel material at the entry side of the rolling stand. Furthermore, the reduction ratio (%) in each pass during the product rolling process is calculated using the following formula. The reduction ratio = (1 - area of ​​the cross-section perpendicular to the longitudinal direction of the billet after reduction / area of ​​the cross-section perpendicular to the longitudinal direction of the billet before reduction) × 100 Furthermore, "maximum reduction ratio RR per pass in the range where the steel material temperature is 1050°C or higher" means the reduction ratio in the pass if there is one pass in the range of 1050°C or higher, and the maximum reduction ratio among the reduction ratios in each pass if there are two or more passes in the range of 1050°C or higher.

[0088] When the steel material temperature is in the range of 1050°C or higher and a reduction of 20% or more per pass is applied (i.e., when the maximum reduction RR per pass in the range of 1050°C or higher is 20% or more), MnS inclusions with a Cu content of 2.0% or less precipitate preferentially over fine Cu-containing MnS inclusions. In this case, even if conditions 1, 2, and 4 are met, the steel material of this embodiment does not satisfy feature 2.

[0089] Here, even if the heating temperature T2 in the product rolling process is less than 1050°C, the steel material temperature may exceed 1050°C due to processing heat generated during rolling. In other words, even if the heating temperature T2 in the product rolling process is less than 1050°C, the maximum reduction ratio RR per pass may exceed 20% in the range where the steel material temperature is 1050°C or higher.

[0090] On the other hand, if the maximum reduction ratio RR per pass is less than 20% in the range where the steel material temperature is 1050°C or higher, the precipitation of MnS inclusions with a Cu content of 2.0% or less can be suppressed. As a result, assuming that conditions 1, 2, and 4 are met, the steel material of this embodiment satisfies feature 2.

[0091] [Regarding Condition 4] In this embodiment, the number of passes PN for reduction with a reduction ratio of 20% or more is set to 3 or more, within the range of steel material temperature below 1050°C.

[0092] When steel is reduced by a reduction ratio of 20% or more at a steel temperature below 1050°C, strain acts as the driving force, causing numerous fine Cu-containing MnS inclusions to precipitate. However, if the number of passes PN for reduction by a reduction ratio of 20% or more at a steel temperature below 1050°C is less than 3, sufficient strain due to reduction is not applied to the billet. Therefore, fine Cu-containing MnS inclusions do not precipitate sufficiently. In this case, even if conditions 1 to 3 are met, the steel of this embodiment does not satisfy feature 2.

[0093] If the number of passes PN is 3 or more, a large number of fine Cu-containing MnS inclusions can be obtained. As a result, assuming that conditions 1 to 3 are met, the steel material of this embodiment satisfies feature 2.

[0094] The steel material of this embodiment is manufactured through the above manufacturing process.

[0095] [Method for Manufacturing Machine Parts] The machine parts made from the steel material of this embodiment can be manufactured, for example, by the following manufacturing method.

[0096] An example of a manufacturing method for machine parts includes a cold working process and a cutting process. In the cold working process, the steel material of this embodiment is subjected to a cold working process to produce an intermediate product having a predetermined shape. Cold working is, for example, cold drawing. In the cold working process, the steel material is processed at room temperature without heating. A cutting process is then performed on the intermediate product after cold working. The conditions of the cutting process are not particularly limited.

[0097] The effects of the steel material of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

[0098] Steel materials having the chemical compositions shown in Tables 1A and 1B were manufactured by the following method. In the following description, Tables 1A and 1B will be collectively referred to as "Table 1".

[0099]

[0100]

[0101] Steel bars with the chemical composition shown in Table 1 were manufactured by the following method. Specifically, blooms were produced by continuous casting using molten steel produced in the steelmaking process. Billets were produced by bloom rolling in a bloom rolling process. The holding time at heating temperature T1 in the bloom rolling process was 120 minutes. The manufactured billets were allowed to cool to room temperature.

[0102] The manufactured billet underwent a product rolling process. The holding time at heating temperature T2 during the product rolling process was 120 minutes.

[0103] Through the above process, a steel bar with a diameter of 38 mm was manufactured.

[0104] In the manufacturing process, the heating temperature T1 (°C) in the bract rolling process, the heating temperature T2 (°C) in the product rolling process, the maximum reduction ratio RR (%) per pass in the product rolling process when the steel material temperature is 1050°C or higher, and the number of passes PN in the product rolling process when the steel material temperature is below 1050°C and a reduction ratio of 20% or more are as shown in Table 2. The microstructure of the steel material for each test number was measured based on the method described in [Method for measuring ferrite and pearlite in the microstructure] above. As a result, it was determined that the microstructure of the steel material for each test number consisted of ferrite and pearlite.

[0105]

[0106] [Evaluation Test] The following evaluation tests were conducted using the steel materials for each test number. (Test 1) Average equivalent circle diameter D of Cu-containing MnS inclusions ＡＶＥ And measurement test of number density ND (Test 2) Machinability evaluation test The following describes each test.

[0107] [(Test 1) Average circular equivalent diameter D of Cu-containing MnS inclusions ＡＶＥ [Measurement test of number density ND] [Average circular equivalent diameter D of Cu-containing MnS inclusions] ＡＶＥ Based on the method described in [Method for measuring number density ND], the average equivalent circle diameter D of the Cu-containing MnS inclusions in the steel material of each test number ＡＶＥ The number density ND was determined. The average equivalent circular diameter D of the obtained Cu-containing MnS inclusions was determined. ＡＶＥ (μm) and number density ND (pieces / mm) ２ ) is shown in Table 2 as "Average Circle Equivalent Diameter D ＡＶＥ The column for "(μm)" and "Number density ND (pieces / mm)" ２ This is shown in the ) column.

[0108] [(Test 2) Machinability Evaluation Test] The steel material (38 mm diameter steel bar) for each test number was cut perpendicular to the axial direction, and then machined (cut) to obtain cut test pieces with a diameter of 35 mm and a length of 300 mm. In this embodiment, a cut test piece with a diameter of 35 mm was used, but the machinability evaluation test can be carried out under similar conditions as appropriate even when using cut test pieces of different diameters.

[0109] The following machinability evaluation tests were performed on the cutting test pieces for each test number. Specifically, the test pieces were mounted on a general-purpose lathe and an outer circumference turning test was performed. The turning insert used was Sumitomo Electric Industries, Ltd., model number SNMN120408-ST20E. The cutting speed was set to 100 m / min, the feed rate to 0.05 mm / rev, and the depth of cut to 1.0 mm. During turning, a water-soluble cutting oil was used as a lubricant. Under the above conditions, outer circumference turning was performed on the test pieces for each test number, and the turning process was carried out until the cumulative cutting time reached 600 seconds. After that, the lateral relief surface of the turning insert was observed, and the maximum relief surface wear width V of the turning insert was determined. Ｂ The (μm) was measured. The maximum flank wear width V of the turning tip in the test specimen for each test number obtained. ＢIf the (μm) is 100 μm or less, the evaluation is set to "E (Excellent)", and it is judged that excellent machinability has been obtained (see "Maximum flank wear width V" in Table 3). Ｂ (Enter "E" in the column). Maximum flank wear width V of the turning tip in the test piece for each test number obtained. Ｂ If (μm) exceeds 100 μm, the evaluation is "B (Bad)", and it is determined that excellent machinability was not obtained (see "Maximum flank wear width V" in Table 3). Ｂ (Write "B" in the "" column).

[0110] [Evaluation Results] Referring to Tables 1 and 2, in test numbers 1 to 25, the steel material met both characteristic 1 and characteristic 2. Therefore, the maximum flank wear width V of the turning chip. Ｂ The particle size was 100 μm or less, resulting in excellent machinability.

[0111] In tests 26 and 27, the heating temperature T1 during the bract rolling process was too high. As a result, the number density ND of Cu-containing MnS inclusions was 40 particles / mm³. ２ It was less than [a certain value]. As a result, excellent machinability was not achieved.

[0112] In tests 28 and 29, the heating temperature T2 during the product rolling process was too high. As a result, the number density ND of Cu-containing MnS inclusions was 40 particles / mm³. ２ It was less than [a certain value]. As a result, excellent machinability was not achieved.

[0113] In tests 30 and 31, the maximum reduction ratio RR per pass was too high during the product rolling process in the range where the steel material temperature was 1050°C or higher. As a result, the number density ND of Cu-containing MnS inclusions was 40 particles / mm². ２ It was less than [a certain value]. As a result, excellent machinability was not achieved.

[0114] In tests 32 and 33, the number of passes PN used in the product rolling process to reduce the steel material by more than 20% at a temperature below 1050°C was too low. As a result, the number density ND of Cu-containing MnS inclusions was 40 inclusions / mm². ２ It was less than [a certain value]. As a result, excellent machinability was not achieved.

[0115] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. The chemical composition, in mass%, is as follows: C: 0.01-0.20%, Si: 0.001-0.350%, Mn: 0.80-2.00%, P: greater than 0.030-0.100%, S: greater than 0.150-0.800%, Cu: 0.01-0.30%, Ni: 0.01-0.30%, Cr: 0.01-0.50%, Mo: 0.01-0.50%, Sn: 0.001-0.100%, N: 0.0200% or less, O: 0.0350% or less, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.020%, Te: 0-0.050%. It contains Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.200%, Al: 0-0.010%, Co: 0-0.100%, Sb: 0-0.050%, As: 0-0.050%, B: 0-0.0050%, Se: 0-0.100%, Pb: 0-0.090%, Bi: 0-0.100%, and rare earth elements: 0-0.020%, with the remainder being Fe and impurities, and the mass percent of the Mn content is 10.0% or more, the S content is 5.0% or more, and the Cu content is more than 2.0%, with the average equivalent circle diameter of the Cu-containing MnS inclusions being less than 2.0 μm. The number density of the Cu-containing MnS inclusions is 40 particles / mm². ２ That's all, regarding the steel materials.

2. The steel material according to claim 1, wherein the chemical composition is, in mass%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Zr: 0.001 to 0.020%, Te: 0.001 to 0.050%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, V: 0.001 to 0.200%, Al: 0.001 to 0.010%, Co: 0.001 to 0.100%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, B: 0.0001 to 0.0050%, Se: 0.001 to 0.100%, A steel material containing one or more elements selected from the group consisting of Pb: 0.001 to 0.090%, Bi: 0.001 to 0.100%, and rare earth elements: 0.001 to 0.020%.

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