Steel for machine structure

Abstract

The purpose of the present invention is to provide a steel for a machine structure, the steel being capable of suppressing chevron cracks during extrusion by controlling the degree of segregation, inclusions, and dislocation density in the steel after hot rolling.　The steel for a machine structure has a specific chemical composition, in which: the average value of the degree of segregation, which is obtained by dividing the maximum C, Si, and Mn contents as measured by an electron probe micro analyzer (EPMA), by the respective average C, Si, and Mn contents as measured by the EPMA, satisfies a predetermined formula; the ID, which is the predicted maximum diameter of inclusions obtained by an extreme value statistical method, satisfies a predetermined formula; and the dislocation density satisfies a predetermined formula.

Description

Machine structural steel

[0001] The present invention relates to steel for machine structures that is subjected to extrusion processing and is used in the fields of construction industry machinery and automobiles.

[0002] Power transmission parts such as drive shafts and axle shafts used in automobiles and construction machinery are made of carbon steel for mechanical structures, including JIS S45C. The shafts of these parts are formed by extrusion. In recent years, as part shapes have become more complex, shafts have begun to have large steps, requiring more severe extrusion processes. In extrusion, if a large amount of extrusion is applied in a single process, the tensile stress generated inside the shaft increases, resulting in internal cracks known as chevron cracks. To prevent this, conventional extrusion processes have had to be repeated multiple times, minimizing the amount of extrusion per process, which increases manufacturing costs.

[0003] In response to such problems, Patent Document 1 proposes a method for manufacturing high-strength thick steel plates with good internal quality that do not crack during welding or bending, even when the steel is made of B-containing steel, by optimizing the casting conditions.

[0004] Furthermore, Patent Document 2 proposes a method for manufacturing a high-strength shaft part that has excellent cold workability and induction hardenability by optimizing the steel composition and rolling conditions, without performing spheroidizing annealing and tempering treatment.

[0005] Japanese Patent Application Laid-Open No. 2002-316240 Japanese Patent Application Laid-Open No. 2001-192731

[0006] In order to enable extrusion molding with a large processing amount, a better ability to suppress chevron cracks than conventional techniques is required. The technology of Patent Document 1 is capable of suppressing internal cracks in the slab, but is insufficient in suppressing chevron cracks during extrusion molding.

[0007] Furthermore, while the technology of Patent Document 2 was able to suppress chevron cracks during wire drawing, there was a problem in that sufficient chevron crack suppression was not achieved in extrusion molding, which involves a large amount of processing.

[0008] The present invention was developed in view of the above circumstances, and aims to provide a steel for machine structural use that can suppress chevron cracks during extrusion by controlling the degree of segregation, inclusions, and dislocation density of the steel after hot rolling.

[0009] The present inventors have investigated various factors affecting chevron cracking during extrusion and found that controlling the segregation degree, inclusion density, and dislocation density of the steel after hot rolling is an important factor. They have also found that when the segregation degree, inclusion density, and dislocation density of the steel after hot rolling are all within suitable ranges, the ability to suppress chevron cracking during extrusion is improved.

[0010] The present invention is based on the above findings and has the following gist: [1] A steel sheet having a composition containing, in mass%, C: 0.10 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.05 to 1.80%, P: 0.050% or less (including 0%), S: 0.050% or less (including 0%), Al: 0.001 to 0.090%, N: 0.0015 to 0.0150%, with the balance consisting of Fe and unavoidable impurities, wherein the C, Si, and Mn concentrations measured using an EPMA (Electron Probe Micro Analyzer) satisfy the following formula (1), and wherein ID (μm), which is the predicted maximum diameter of inclusions obtained by the extreme value statistics method, satisfies the following formula (2), and wherein the dislocation density ρ (m －２ ) satisfies the following formula (3). Ｃ１ / C Ｃ０ +C Ｓｉ１ / C Ｓｉ０ +C Ｍｎ１ / C Ｍｎ０ ) / 3≦5.0 (1) where C Ｃ１ , C Ｓｉ１ , C Ｍｎ１ are the maximum values ​​of C, Si, and Mn measured using EPMA, and C Ｃ０ , C Ｓｉ０ , C Ｍｎ０ indicates the average values ​​of C, Si, and Mn measured using EPMA. ID (μm)≦77 ... (2) Here, ID is the predicted maximum diameter of inclusions, √Area, obtained by the extreme value statistics method. Dislocation density ρ (m －２ ) ≦ 5 × 10１５ ...(3) [2] The steel for machine structural use according to [1], wherein the chemical composition further contains, in mass%, one or more selected from one or more groups consisting of the following groups A to D: Group A: Cr: 0.70% or less, Mo: 0.50% or less, Cu: 1.00% or less, Ni: 1.00% or less, and B: 0.0050% or less Group B: Se: 0.3% or less, Ca: 0.05% or less, Pb: 0.3% or less, Bi: 0.3% or less, Mg: 0.05% or less, Zr: 0.2% or less, REM: 0.01% or less, and O: 0.025% or less Group C: Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.3% or less Group D: Sn: 0.1% or less, and Sb: 0.1% or less

[0011] According to the present invention, it is possible to provide a steel for machine structural use that is excellent in the ability to suppress chevron cracks during extrusion.

[0012] The present invention will be described below with reference to an embodiment thereof, starting with the chemical composition of the steel for machine and structure of the present invention. Note that "%" indicating the content of each element indicates "% by mass" unless otherwise specified.

[0013] C: 0.10 to 0.60% At least 0.10% of C is required to ensure the strength of the hardened layer after induction hardening. Therefore, the C content is set to 0.10% or more. The C content is preferably 0.15% or more, and more preferably 0.35% or more. On the other hand, if it exceeds 0.60%, cracks are likely to occur during hardening, so the C content is set to 0.60% or less. Note that in order to improve the balance between cold workability and strength after induction hardening, the C content is preferably 0.55% or less.

[0014] Si: 0.01 to 0.90% Si is effective as a deoxidizer, but if it exceeds 0.90%, cold workability decreases, so it is limited to 0.90% or less. Preferably, it is 0.75% or less. On the other hand, if the Si content is too low, deoxidation becomes difficult, so the Si content is set to 0.01% or more. The Si content is preferably 0.10% or more.

[0015] Mn: 0.05 to 1.80% Mn has the effect of improving hardenability and may be added to adjust the strength after induction hardening. However, if the Mn content exceeds 1.80%, it will result in a decrease in cold workability, so the Mn content is set to 1.80% or less. It is preferably set to 1.50% or less, and more preferably set to 1.00% or less. On the other hand, if the Mn content is too low, hardenability will be insufficient, so the Mn content is set to 0.05% or more. The Mn content is preferably set to 0.15% or more.

[0016] P: 0.050% or less (including 0%) P segregates at prior austenite grain boundaries after induction hardening and reduces the fatigue properties of the hardened layer, so it is preferable to keep it as small as possible. For the above reasons, the P content is set to a range of 0.050% or less. It is more preferably 0.015% or less. It may also be 0%.

[0017] S: 0.050% or less (including 0%) S exists as sulfide-based inclusions and is an element effective in improving machinability, but a content exceeding 0.050% reduces cold workability, so the S content is set to 0.050% or less. The S content is preferably 0.035% or less. Note that if improvement in machinability is required, S may be added in an amount of 0.010% or more, and the S content is preferably set to 0.010% or more.

[0018] Al: 0.001 to 0.090% Al is an element effective for deoxidation. It also bonds with N to form fine nitrides, thereby refining the grain size and improving fatigue strength. To achieve these effects, an Al content of 0.001% or more is necessary. The Al content is preferably 0.005% or more, and more preferably 0.015% or more. On the other hand, even if the Al content exceeds 0.090%, these effects will simply saturate. Therefore, the Al content is set to 0.090% or less. It is preferably 0.075% or less, and more preferably 0.065% or less.

[0019] N: 0.0015 to 0.0150% N combines with Al to form fine nitrides, thereby refining the grain size and improving fatigue strength. To achieve this effect, a content of 0.0015% or more is required. The N content is preferably 0.0020% or more, and more preferably 0.0035% or more. On the other hand, a content exceeding 0.0150% promotes surface cracking during continuous casting, so the N content is set to 0.0150% or less. The N content is preferably 0.0110% or less. The N content is more preferably 0.0100% or less.

[0020] The balance consists of Fe and unavoidable impurities.

[0021] Furthermore, in the present invention, one or more elements selected from one or more of the following groups A to D can be optionally contained. Group A: Cr: 0.70% or less, Mo: 0.50% or less, Cu: 1.00% or less, Ni: 1.00% or less, and B: 0.0050% or less Group B: Se: 0.3% or less, Ca: 0.05% or less, Pb: 0.3% or less, Bi: 0.3% or less, Mg: 0.05% or less, Zr: 0.2% or less, REM: 0.01% or less, and O: 0.025% or less Group C: Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.3% or less Group D: Sn: 0.1% or less, and Sb: 0.1% or less

[0022] Group A Cr: 0.70% or less Cr has the effect of improving hardenability and may be contained to adjust the strength after induction hardening, but if it exceeds 0.70%, it will cause a decrease in cold workability, so if Cr is contained, the Cr content should be 0.70% or less. There is no particular lower limit, but in order to obtain the effect of improving hardenability, the Cr content should preferably be 0.10% or more.

[0023] Mo: 0.50% or less Mo has the effect of improving hardenability and may be added to adjust the strength after induction hardening, but if it exceeds 0.50%, it will cause a decrease in cold workability, so if Mo is contained, the Mo content is set to 0.50% or less. There is no particular lower limit, but in order to obtain the effect of improving hardenability, the Mo content is preferably set to 0.03% or more.

[0024] Cu: 1.00% or less Cu has the effect of improving hardenability and may be contained to adjust the strength after induction hardening, but if it exceeds 1.00%, it promotes surface cracking in the hot rolling process, resulting in increased maintenance costs. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. Although there is no particular lower limit, the Cu content is preferably set to 0.10% or more in order to obtain the effect of improving hardenability.

[0025] Ni: 1.00% or less Ni has the effect of improving hardenability and may be added to adjust the strength after induction hardening, but if it exceeds 1.00%, it will cause a decrease in cold workability, so if Ni is contained, the Ni content is set to 1.00% or less. There is no particular lower limit, but in order to obtain the effect of improving hardenability, the Ni content is preferably set to 0.10% or more.

[0026] B: 0.0050% or less B has the effect of improving hardenability and may be added to adjust the strength after induction hardening, but if it exceeds 0.0050%, it will lead to an increase in internal defects during casting, so if B is contained, the B content is set to 0.0050% or less. There is no particular lower limit, but in order to obtain the effect of improving hardenability, the B content is preferably set to 0.0003% or more.

[0027] Group B Se: 0.3% or less Se is an element that combines with Mn to form MnSe, which is effective in improving machinability, and may be contained as needed, but a content exceeding 0.3% reduces cold workability, so when Se is contained, the Se content is set to 0.3% or less. There is no particular lower limit, but in order to obtain the effect of improving machinability, the Se content is preferably set to 0.01% or more.

[0028] Ca: 0.05% or less Ca is an element that combines with S to form CaS, thereby effectively improving machinability, and may be contained as needed. However, a content exceeding 0.05% reduces cold workability, so when Ca is contained, the Ca content is set to 0.05% or less. There is no particular lower limit, but in order to obtain the effect of improving machinability, the Ca content is preferably 0.0010% or more. A content of 0.03% or more is more preferable.

[0029] Pb: 0.3% or less Pb is an element that is effective in improving machinability by dispersing Pb particles in the steel, and may be contained as needed, but a content of more than 0.3% reduces cold workability, so when Pb is contained, the Pb content is set to 0.3% or less. There is no particular lower limit, but in order to obtain the effect of improving machinability, the Pb content is preferably set to 0.01% or more.

[0030] Bi: 0.3% or less Bi is an element that is effective in improving machinability by dispersing Bi particles in the steel, and may be contained as needed, but a content of more than 0.3% reduces cold workability, so when Bi is contained, the Bi content is set to 0.3% or less. There is no particular lower limit, but in order to obtain the effect of improving machinability, the Bi content is preferably set to 0.01% or more.

[0031] Mg: 0.05% or less Mg is an element that is effective in improving machinability by bonding with S to form MgS, and may be contained as needed, but a content exceeding 0.05% reduces cold workability, so when Mg is contained, the Mg content should be 0.05% or less. There is no particular lower limit, but in order to obtain the effect of improving machinability, the Mg content should preferably be 0.01% or more.

[0032] Zr: 0.2% or less Zr combines with O to form ZrO ２Zr is an element that is effective in improving machinability by increasing the number of MnS precipitation nuclei and promoting the fine dispersion of MnS by forming Zr. Zr may be contained for this purpose, but a content exceeding 0.2% reduces cold workability, so if Zr is contained, the Zr content should be 0.2% or less. There is no particular lower limit, but in order to obtain the effect of improving machinability, it is preferable that the Zr content be 0.01% or more.

[0033] REM: 0.01% or less REM is an element that contributes to improving machinability by refining carbides, and may be contained as needed, but a content exceeding 0.01% reduces cold workability, so when REM is contained, the REM content is set to 0.01% or less. There is no particular lower limit, but in order to obtain the effect of improving machinability, the REM content is preferably set to 0.001% or more.

[0034] O: 0.025% or less O is an element effective in improving machinability by solid solution strengthening MnS, and may be contained as needed, but a content exceeding 0.025% reduces cold workability, so when O is contained, the O content is set to 0.025% or less. There is no particular lower limit, but when a normal degassing process is performed, it should contain at least 0.001% or more, and since lowering the oxygen content any further increases refining costs, the O content is preferably set to 0.001% or more.

[0035] Group C Ti: 0.10% or less Ti is an element that inhibits grain growth and is effective in improving toughness by forming carbonitrides, and may be contained as needed. However, a content exceeding 0.10% promotes surface cracking during the casting process, resulting in increased maintenance costs. Therefore, when Ti is contained, the Ti content is set to 0.10% or less. There is no particular lower limit, but in order to obtain the grain growth inhibiting effect, the Ti content is preferably set to 0.001% or more. 0.01% or more is more preferable.

[0036] Nb: 0.10% or less Nb is an element that inhibits grain growth by forming carbonitrides and is effective in improving toughness, and may be contained as needed. However, a content exceeding 0.10% promotes surface cracking during the casting process, resulting in increased maintenance costs. Therefore, when Nb is contained, the Nb content is set to 0.10% or less. There is no particular lower limit, but in order to obtain the grain growth inhibiting effect, the Nb content is preferably set to 0.003% or more. 0.01% or more is more preferable.

[0037] V: 0.3% or less V is an element that is effective in improving strength through precipitation strengthening by forming carbonitrides, or in improving toughness by inhibiting grain growth, and may be contained as needed. However, a content exceeding 0.3% promotes surface cracking during the casting process, resulting in increased maintenance costs. Therefore, when V is contained, the V content is set to 0.3% or less. Although there is no particular lower limit, the V content is preferably set to 0.003% or more in order to obtain the grain growth inhibiting effect. 0.01% or more is more preferable.

[0038] Group D Sn: 0.1% or less Sn is an element that affects scale formation behavior and is effective for suppressing decarburization, and may be contained as needed. However, a content exceeding 0.1% reduces cold workability, so when Sn is contained, the Sn content is set to 0.1% or less. There is no particular lower limit, but in order to obtain the decarburization suppression effect, the Sn content is preferably set to 0.0010% or more. 0.01% or more is more preferable.

[0039] Sb: 0.1% or less Sb is an element that is effective in suppressing decarburization by inhibiting the diffusion of carbon, and may be contained as needed. However, a content exceeding 0.1% reduces cold workability, so the Sb content is set to 0.1% or less. There is no particular lower limit, but in order to obtain the decarburization suppression effect, the Sb content is preferably 0.0010% or more. 0.01% or more is more preferable.

[0040] Next, the provisions of the present invention regarding the degree of segregation, inclusions, and dislocation density of the steel after hot rolling will be explained.

[0041] The average value of the segregation degree, which is the value obtained by dividing the maximum values ​​of C, Si, and Mn measured using an EPMA (Electron Probe Micro Analyzer) by the average values ​​of C, Si, and Mn measured using the EPMA, satisfies formula (1) (C Ｃ１ / C Ｃ０ +C Ｓｉ１ / C Ｓｉ０ +C Ｍｎ１ / C Ｍｎ０ ) / 3≦5.0 (1) where C Ｃ１ , C Ｓｉ１ , C Ｍｎ１ are the maximum values ​​of C, Si, and Mn measured using EPMA, and C Ｃ０ , C Ｓｉ０ , C Ｍｎ０ indicates the average values ​​of C, Si, and Mn measured using EPMA. The left side of equation (1) represents the segregation degree C of C, Si, and Mn, respectively. Ｃ１ / C Ｃ０ , C Ｓｉ１ / C Ｓｉ０ , C Ｍｎ１ / C Ｍｎ０ This index indicates the non-uniformity of the alloy concentration distribution in the steel. In other words, the higher the value, the greater the non-uniformity. The ductility of steel changes depending on the alloy concentration, and when the alloy concentration is non-uniform, there are areas with high ductility and areas with low ductility, resulting in local differences in ductility. It is believed that such local differences in ductility cause stress concentration, making internal cracks more likely to occur. For this reason, the average segregation degree, which is the value obtained by dividing the maximum values ​​of C, Si, and Mn amounts measured using EPMA by the average values ​​of C, Si, and Mn amounts measured using the EPMA, must satisfy formula (1). There is no particular lower limit for the value of the left side of formula (1), but when the alloy concentration distribution is most uniform, the left side of formula (1) will be 1.0. (C Ｃ１ / C Ｃ０ +C Ｓｉ１ / C Ｓｉ０ +C Ｍｎ１ / C Ｍｎ０ ) / 3≦5.0 (1) where C Ｃ１ , C Ｓｉ１ , C Ｍｎ１ are the maximum values ​​of C, Si, and Mn measured using EPMA, and CＣ０ , C Ｓｉ０ , C Ｍｎ０ indicates the average values ​​of C, Si, and Mn measured using EPMA.

[0042] Inclusion ID (μm)≦77 ID is the predicted maximum diameter √Area of ​​the inclusion obtained by the extreme value statistics method. After determining the maximum grain size of oxide-based, TiN-based, and MnS-based inclusions for each field area of ​​10 mm × 10 mm, the maximum grain size was calculated by the extreme value statistics method. ２ The predicted maximum grain size was estimated. The grain size was evaluated using √Area, i.e., the square root of the product of the major and minor diameters of the inclusion. This extreme value statistics method is performed in accordance with Reference 1. It is believed that the larger the inclusion diameter √Area in the steel, the greater the stress concentration around the inclusion, making it more likely to develop internal cracks. For these reasons, ID is set to 77 μm or less. There is no particular lower limit, but since excessive improvement in cleanliness leads to increased manufacturing costs, it is preferable that ID be 5 μm or more. [Reference 1] Murakami, Takayoshi et al.: Transactions of the Japan Society of Mechanical Engineers, Vol. 83, No. 853, 1 (2017)

[0043] Dislocation density ρ(m －２ ) ≦ 5 × 10 １５ In the present invention, it is also important to control the dislocation density of the steel material. By satisfying a predetermined dislocation density, it is possible to ensure the ductility of the material, which is thought to contribute to the suppression of chevron cracks. For this purpose, the dislocation density is set to 5 × 10 １５ (m －２ ) or less, but preferably 1 × 10 １５ (m －２ ) or less, and more preferably 5 × 10 １４ (m －２ The lower limit is not particularly limited, but even when pure iron is annealed and dislocations are sufficiently removed, the lower limit is 2 × 10 １１ (m －２ ) dislocations are considered to be included, so the lower limit of the dislocation density is set to 2 × 10 １１ (m －２ )

[0044] There are various methods for controlling dislocation density, and the following can be used alone or in combination. For example, there are methods such as reducing the C content, suppressing martensitic and bainite transformations by controlling the cooling rate during the cooling process after hot rolling, and eliminating or simplifying correction processing by improving dimensional accuracy after hot rolling. Of course, other methods may also be used to control dislocation density. Here, reducing the C content is effective in reducing dislocation density when martensitic and bainite structures are present by reducing the amount of solute C in the martensitic and bainite structures. Furthermore, using an insulating cover or the like to prevent heat dissipation in the cooling bed after hot rolling can reduce the cooling rate of the steel, thereby suppressing martensitic and bainite transformations. Furthermore, online dimensional measurement during hot rolling allows for appropriate adjustment of the gap between the rolling rolls, thereby improving dimensional accuracy after hot rolling. A desired dislocation density can be achieved by using these methods alone or in combination.

[0045] Regarding the segregation degree, inclusion density, and dislocation density of steel after hot rolling, satisfying only one or two of these criteria will not provide sufficient chevron crack suppression; all of them must be satisfied. Although the detailed mechanism is not entirely clear, it is believed that there is some interrelationship between these criteria, which brings about a synergistic effect that is effective in suppressing cracking.

[0046] There are no particular limitations on the manufacturing method, and manufacturing steps not specifically mentioned in this specification may be the same as those for ordinary steel for machine structural use.

[0047] The constitution and effects of the present invention will be specifically explained below with reference to examples. However, the present invention is not limited to the following examples, and appropriate modifications can be made within the scope of the gist of the present invention, and all such modifications are included in the technical scope of the present invention.

[0048] Steels having the chemical compositions shown in Table 1 were melted and continuously cast at various casting speeds to obtain billets, which were then hot-rolled into round bars of various diameters. Some of the hot-rolled materials were then straightened using a two-roll straightener at various roll gaps to obtain straight steel bars.

[0049] The segregation degrees of C, Si, and Mn were determined by analyzing a cubic specimen with a side length of 10 mm taken from the center of the steel bar, embedding it in resin, polishing it, and then analyzing it with an electron probe microanalyzer (EPMA). The EPMA field of view was 3 mm × 3 mm, the acceleration voltage was 15.0 kV, and the probe current was 5.0 × 10. －７ A, the beam shape is a 10 μm circle, and measurements are taken at 30 μm intervals. The average density within the field of view is C ０ and the maximum density in the visual field C １ C, Si, Mn C １ / C ０ The average values ​​of these were determined as the segregation degrees of C, Si, and Mn.

[0050] The inclusions were measured by embedding an 11 mm x 11 mm x 5 mm rectangular parallelepiped test piece taken from 1 / 4 of the diameter of the steel bar (the midpoint between the outer circumferential surface of the steel bar and the shaft center) in resin and polishing it to a mirror finish of 11 mm x 11 mm. A total of 30 similar test pieces were prepared and observed with an optical microscope at a magnification of 100x. The maximum particle diameters (μm) of oxide-based, TiN-based, and MnS-based inclusions were determined for each field of view of 10 mm x 10 mm, and then the maximum particle diameters (μm) of the oxide-based, TiN-based, and MnS-based inclusions were measured using the extreme value statistics method. ２ The predicted maximum particle size (μm) at each inclusion was estimated. The particle size was evaluated using √Area, which is the square root of the product of the major and minor diameters of the inclusion. The major and minor diameters were determined by photographing the relevant position at a magnification of ×500 and then analyzing the images using PRECiV-Imaging software Version 1.1 (manufactured by Evident).

[0051] The dislocation density was measured using an X-ray diffractometer (XRD) after a cubic test piece with a side length of 10 mm was taken from half the diameter of the steel bar and the cross section perpendicular to the longitudinal direction of the steel bar was mirror-finished by wet polishing and electrolytic polishing.

[0052] In the chevron crack test, a Φ20 mm round bar test piece was drawn at a drawing angle of 30° to a total area reduction of 85%, and then a cross section passing through the diameter center of the test section and parallel to the longitudinal direction was polished, and then a visual inspection of the appearance was performed. If a crack with a width of 0.5 mm or more was found, it was determined that a crack had occurred.

[0053] These results are shown in Tables 2-1, 2-2, 2-3 and 2-4. As can be seen, according to the above-mentioned invention, steel for machine structural use capable of suppressing chevron cracking can be obtained.

[0054]

[0055]

[0056]

[0057]

[0058]

Claims

1. A steel sheet having a composition containing, in mass%, C: 0.10 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.05 to 1.80%, P: 0.050% or less (including 0%), S: 0.050% or less (including 0%), Al: 0.001 to 0.090%, N: 0.0015 to 0.0150%, with the balance consisting of Fe and unavoidable impurities, wherein the C, Si, and Mn concentrations measured using an EPMA (Electron Probe Micro Analyzer) satisfy the following formula (1), and wherein the predicted maximum diameter of inclusions, ID (μm), obtained by the extreme value statistics method, satisfies the following formula (2), and wherein the dislocation density ρ (m －２ ) satisfies the following formula (3). Ｃ１ / C Ｃ０ +C Ｓｉ１ / C Ｓｉ０ +C Ｍｎ１ / C Ｍｎ０ ) / 3≦5.0 (1) where C Ｃ１ , C Ｓｉ１ , C Ｍｎ１ are the maximum values ​​of C, Si, and Mn measured using EPMA, and C Ｃ０ , C Ｓｉ０ , C Ｍｎ０ indicates the average values ​​of C, Si, and Mn measured using EPMA. ID (μm)≦77 ... (2) Here, ID is the predicted maximum diameter of inclusions, √Area, obtained by the extreme value statistics method. Dislocation density ρ (m －２ ) ≦ 5 × 10 １５ ...(3) 2. The steel for machine structural use according to claim 1, wherein the chemical composition further contains, in mass %, one or more elements selected from one or more of the following groups A to D: Group A: Cr: 0.70% or less, Mo: 0.50% or less, Cu: 1.00% or less, Ni: 1.00% or less, and B: 0.0050% or less; Group B: Se: 0.3% or less, Ca: 0.05% or less, Pb: 0.3% or less, Bi: 0.3% or less, Mg: 0.05% or less, Zr: 0.2% or less, REM: 0.01% or less, and O: 0.025% or less; Group C: Ti: 0.10% or less, Nb: 0.10% or less, and V: 0.3% or less; Group D: Sn: 0.1% or less, and Sb: 0.1% or less.

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