Steel for machine structures

Abstract

An object of the present invention is to provide a steel for machine structures capable of suppressing Chevron cracks during extrusion molding by controlling the segregation degree, inclusions, and dislocation density of steel after hot rolling. A steel for machine structures having a specific component composition, wherein the average value of the segregation degree, which is the value obtained by dividing the maximum values of the amounts of C, Si, and Mn measured using an EPMA (Electron Probe Micro Analyzer) by the average values of the amounts of C, Si, and Mn measured using the EPMA, respectively, satisfies a predetermined formula, and the ID, which is the predicted maximum diameter of inclusions obtained by the extreme value statistical method, satisfies a predetermined formula, and the dislocation density satisfies a predetermined formula.

Description

Technical Field

[0001] The present invention relates to a steel for mechanical structures to be subjected to extrusion processing, which is used in the fields of construction industry machinery and automobiles.

Background Art

[0002] For power transmission parts such as drive shafts and axle shafts used in automobiles and construction machinery, carbon steel for mechanical structures such as S45C according to JIS standards is used. In this type of part, the shaft portion is formed by extrusion processing. With the recent complication of part shapes, large steps have come to be provided in the shaft portion, and more severe extrusion processing has come to be performed. In extrusion processing, if a large amount of processing is given in a single forming, the tensile stress generated inside the shaft increases, and internal cracks called chevron cracks occur. To prevent this, conventionally, it has been necessary to repeat the extrusion processing a plurality of times and keep the amount of processing per time small, which has led to an increase in manufacturing cost.

[0003] In response to such problems, Patent Document 1 proposes a method for manufacturing a high-strength thick steel plate with good internal quality in which even a B-containing steel does not crack during welding or bending by optimizing casting conditions.

[0004] Further, Patent Document 2 proposes a method for manufacturing a high-strength shaft component excellent in cold workability and high-frequency hardenability by optimizing the steel component composition and rolling conditions and without performing spheroidizing annealing and tempering treatment.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In order to enable extrusion molding with a large processing amount, a chevron crack suppression ability superior to the conventional one is required. In the technology of Patent Document 1, although it is possible to suppress internal cracks in the slab, it is insufficient for suppressing chevron cracks during extrusion molding.

[0007] Also, in the technology of Patent Document 2, although it was possible to suppress chevron cracks during wire drawing, there was a problem that sufficient chevron crack suppression ability could not be obtained in extrusion molding with a large processing amount.

[0008] The present invention has been developed in view of the above actual situation, and an object thereof is to provide a steel for machine structures capable of suppressing chevron cracks during extrusion molding by controlling the segregation degree, inclusions, and dislocation density of steel after hot rolling.

Means for Solving the Problems

[0009] As a result of examining various influencing factors regarding chevron cracks during extrusion molding, the present inventors have found that controlling the segregation degree, inclusions, and dislocation density of steel after hot rolling is an important factor. And it has been found that when the segregation degree, inclusions, and dislocation density of steel after hot rolling are all within a suitable range, the chevron crack suppression ability during extrusion molding is improved.

[0010] The present invention is based on the above findings, and its main configuration is as follows. [1] By 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%, It contains [specific content] and the balance has a component composition consisting of Fe and inevitable impurities. The C, Si, and Mn concentrations measured using EPMA (Electron Probe Micro Analyzer) satisfy the following formula (1), and ID (μm), which is the predicted maximum diameter of inclusions obtained by the extreme value statistical method, satisfies the following formula (2), and The dislocation density ρ (m -2 ) satisfies the following formula (3). Steel for mechanical structures. (C C1 / C C0 +C Si1 / C Si0 +C Mn1 / C Mn0 ) / 3 ≦ 5.0 ···(1) Here, C C1 , C Si1 , C Mn1 represent the maximum values of C, Si, and Mn measured using EPMA, and C C0 , C Si0 , C Mn0 represent the average values of C, Si, and Mn measured using EPMA. ID (μm) ≦ 77 ···(2) Here, ID is the predicted maximum diameter √Area of inclusions obtained by the extreme value statistical method. The dislocation density ρ (m -2 ) ≦ 5×10 15 ···(3) [2] The component composition further contains, by mass%, one or more selected from one or more groups consisting of the following A to D groups in the steel for mechanical structures described in [1]. 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: below 0.05%, Zr: below 0.2%, REM: below 0.01% and O: below 0.025% Group C: Ti: below 0.10%, Nb: below 0.10% and V: below 0.3% Group D: Sn: below 0.1% and Sb: below 0.1%

Advantages of the Invention

[0011] According to the present invention, it is possible to provide a steel for mechanical structures that is excellent in the ability to suppress Chevron cracks during extrusion molding.

Modes for Carrying Out the Invention

[0012] In the following, one embodiment for carrying out the present invention will be described in terms of the component composition of the steel for mechanical structures of the present invention. Note that “%” indicating the content of each element represents “mass %” unless otherwise specified.

[0013] C: 0.10 - 0.60% C needs to be contained at least 0.10% or more in order to ensure the strength of the hardened layer after high-frequency quenching. Therefore, the C content is 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 quenching, so the C content is 0.60% or less. In addition, in order to improve the balance between cold workability and strength after high-frequency quenching, the C content is preferably 0.55% or less.

[0014] Si: 0.01 - 0.90% Si is effective as a deoxidizer, but if it exceeds 0.90%, the cold workability deteriorates, 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 0.01% or more. The Si content is preferably 0.10% or more.

[0015] Mn: 0.05 to 1.80% Mn may be contained to improve hardenability and adjust the strength after high-frequency hardening. However, if it exceeds 1.80%, it will cause a decrease in cold workability. Therefore, the Mn content should be 1.80% or less. Preferably, it is 1.50% or less, and more preferably 1.00% or less. On the other hand, if the Mn content is too low, the hardenability will be insufficient. Therefore, the Mn content should be 0.05% or more. The Mn content is preferably 0.15% or more.

[0016] P: 0.050% or less (including 0%) P segregates at the prior austenite grain boundaries after high-frequency hardening and deteriorates the fatigue characteristics of the hardened layer. Therefore, it is preferably suppressed to as small an amount as possible. For the above reasons, the P content is defined in the range of 0.050% or less. More preferably, it is 0.015% or less. It may be 0%.

[0017] S: 0.050% or less (including 0%) S exists as sulfide inclusions and is an element effective in improving machinability. However, if the content exceeds 0.050%, it will reduce the cold workability. Therefore, the S content should be 0.050% or less. The S content is preferably 0.035% or less. If improvement of machinability is required, 0.010% or more may be added, and the S content is preferably 0.010% or more.

[0018] Al: 0.001 to 0.090% Al is an element effective for deoxidation. Also, it has the effect of improving the fatigue strength by combining with N to form fine nitrides and refining the crystal grain size. In order to obtain these effects, a content of 0.001% or more is required. The Al content is preferably 0.005% or more, and more preferably 0.015% or more. On the other hand, even if it is contained in excess of 0.090%, those effects are only saturated. Therefore, the Al content should be 0.090% or less. Preferably, it is 0.075% or less, and more preferably 0.065% or less.

[0019] N: 0.0015 to 0.0150% N has the effect of refining the crystal grain size and improving the fatigue strength by combining with Al to form fine nitrides. To obtain this effect, a content of 0.0015% or more is required. The N content is preferably 0.0020% or more, 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 should be 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 selected from one or more groups consisting 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 high-frequency hardening. However, if it exceeds 0.70%, it will cause a decrease in cold workability. Therefore, when Cr is contained, the Cr content should be 0.70% or less. The lower limit is not particularly limited, but in order to obtain the hardenability improvement effect, the Cr content is preferably 0.10% or more.

[0023] Mo: 0.50% or less Mo has the effect of improving hardenability and may be contained to adjust the strength after high-frequency hardening. However, if it exceeds 0.50%, it will cause a decrease in cold workability. Therefore, when Mo is contained, the Mo content should be 0.50% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving hardenability, the Mo content is preferably 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 high-frequency hardening. However, if it exceeds 1.00%, it will promote surface cracking in the hot rolling process and lead to an increase in maintenance cost. Therefore, when Cu is contained, the Cu content should be 1.00% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving hardenability, the Cu content is preferably 0.10% or more.

[0025] Ni: 1.00% or less Ni has the effect of improving hardenability and may be contained to adjust the strength after high-frequency hardening. However, if it exceeds 1.00%, it will cause a decrease in cold workability. Therefore, when Ni is contained, the Ni content should be 1.00% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving hardenability, the Ni content is preferably 0.10% or more.

[0026] B: 0.0050% or less B has the effect of improving hardenability and may be contained to adjust the strength after high-frequency hardening. However, if it exceeds 0.0050%, it will cause an increase in internal defects during casting. Therefore, when B is contained, the B content should be 0.0050% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving hardenability, the B content is preferably 0.0003% or more.

[0027] Group B Se: 0.3% or less Se combines with Mn to form MnSe, which is an element effective in improving machinability and may be contained as required. However, if the content exceeds 0.3%, it will reduce the cold workability. Therefore, when Se is contained, the Se content should be 0.3% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving machinability, the Se content is preferably 0.01% or more.

[0028] Ca: 0.05% or less Ca combines with S to form CaS, which is an element effective in improving machinability and may be contained as required. However, if the content exceeds 0.05%, it will reduce the cold workability. Therefore, when Ca is contained, the Ca content should be 0.05% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving machinability, the Ca content is preferably 0.0010% or more, and more preferably 0.03% or more.

[0029] Pb: 0.3% or less Pb disperses Pb particles in steel, which is an element effective in improving machinability and may be contained as required. However, if the content exceeds 0.3%, it will reduce the cold workability. Therefore, when Pb is contained, the Pb content should be 0.3% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving machinability, the Pb content is preferably 0.01% or more.

[0030] Bi: 0.3% or less Bi disperses Bi particles in steel, which is an element effective in improving machinability and may be contained as required. However, if the content exceeds 0.3%, it will reduce the cold workability. Therefore, when Bi is contained, the Bi content should be 0.3% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving machinability, the Bi content is preferably 0.01% or more.

[0031] Mg: 0.05% or less Mg combines with S to form MgS, which is an element effective in improving machinability and may be contained as required. However, if the content exceeds 0.05%, it will reduce the cold workability. Therefore, when Mg is contained, the Mg content should be 0.05% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving machinability, the Mg content is preferably 0.01% or more.

[0032] Zr: 0.2% or less Zr combines with O to form ZrO2, which is an element effective in improving machinability by increasing the precipitation nuclei of MnS and promoting the fine dispersion of MnS. It may be contained for this purpose. However, if the content exceeds 0.2%, it will reduce the cold workability. Therefore, when Zr is contained, the Zr content should be 0.2% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving machinability, the Zr content is preferably 0.01% or more.

[0033] REM: 0.01% or less REM is an element that contributes to the improvement of machinability by refining carbides and may be contained as required. However, if the content exceeds 0.01%, it will reduce the cold workability. Therefore, when REM is contained, the REM content should be 0.01% or less. The lower limit is not particularly limited, but in order to obtain the effect of improving machinability, the REM content is preferably 0.001% or more.

[0034] O: 0.025% or less O is an element effective in improving machinability by solid solution strengthening of MnS and may be contained as required. However, if the content exceeds 0.025%, it will reduce the cold workability. Therefore, when O is contained, the O content should be 0.025% or less. The lower limit is not particularly limited, but when a normal degassing process is carried out, it should contain at least 0.001% or more. Further deoxidation will lead to an increase in refining cost. Therefore, the O content is preferably 0.001% or more.

[0035] Group C Ti: 0.10% or less Ti is an element that suppresses grain growth and is effective in improving toughness by forming carbonitrides. It may be contained as needed, but if the content exceeds 0.10%, it promotes surface cracking in the casting process and increases the maintenance cost. Therefore, when Ti is contained, the Ti content should be 0.10% or less. The lower limit is not particularly limited, but in order to obtain the grain growth suppression effect, the Ti content is preferably 0.001% or more, and more preferably 0.01% or more.

[0036] Nb: 0.10% or less Nb is an element that suppresses grain growth and is effective in improving toughness by forming carbonitrides. It may be contained as needed. However, if the content exceeds 0.10%, it promotes surface cracking in the casting process and increases the maintenance cost. Therefore, when Nb is contained, the Nb content should be 0.10% or less. The lower limit is not particularly limited, but in order to obtain the grain growth suppression effect, the Nb content is preferably 0.003% or more, and more preferably 0.01% or more.

[0037] V: 0.3% or less V is an element that is effective in improving strength by precipitation strengthening or suppressing grain growth and improving toughness by forming carbonitrides. It may be contained as needed. However, if the content exceeds 0.3%, it promotes surface cracking in the casting process and increases the maintenance cost. Therefore, when V is contained, the V content should be 0.3% or less. The lower limit is not particularly limited, but in order to obtain the grain growth suppression effect, the V content is preferably 0.003% or more, and more preferably 0.01% or more.

[0038] Group D Sn: 0.1% or less Sn affects the scale generation behavior and is an effective element for suppressing decarburization. It may be contained as necessary, but if the content exceeds 0.1%, the cold workability will be reduced. Therefore, when Sn is contained, the Sn content should be 0.1% or less. The lower limit is not particularly limited, but in order to obtain the decarburization suppression effect, the Sn content is preferably 0.0010% or more. More preferably, it is 0.01% or more.

[0039] Sb: 0.1% or less Sb is an effective element for suppressing decarburization by inhibiting the diffusion of carbon. It may be contained as necessary, but if the content exceeds 0.1%, the cold workability will be reduced. Therefore, the Sb content should be 0.1% or less. The lower limit is not particularly limited, but in order to obtain the decarburization suppression effect, the Sb content is preferably 0.0010% or more. More preferably, it is 0.01% or more.

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

[0041] The average value of the segregation degree, which is the value obtained by dividing the maximum values of the amounts of C, Si, and Mn measured using EPMA (Electron Probe Micro Analyzer) by the average values of the amounts of C, Si, and Mn measured using the same EPMA, satisfies Equation (1). (C C1 / C C0 + C Si1 / C Si0 + C Mn1 / C Mn0 ) / 3 ≦ 5.0 ··· (1) Here, 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 represent the average values of C, Si, and Mn measured using EPMA. The left side of Equation (1) is the segregation degree C C1 / C C0 , C Si1 / C Si0 and C Mn1 / C Mn0 is the average value. This index indicates the non-uniformity of the alloy concentration distribution in steel. That is, the higher the above value, the higher the non-uniformity. The ductility of steel changes under the influence of the alloy concentration. However, when the alloy concentration is non-uniform, there are regions with high ductility and low ductility, resulting in a local difference in ductility. Due to such a local ductility difference, stress concentration occurs, and it is considered that internal cracks are likely to occur. Therefore, the average value of the segregation degree, which is the value obtained by dividing the maximum values of the amounts of C, Si, and Mn measured using EPMA by the average values of the amounts of C, Si, and Mn measured using the same EPMA, respectively, needs to satisfy Equation (1). Although there is no particular limitation on the lower limit of the value on the left side of Equation (1), when the alloy concentration distribution is most uniform, the left side of Equation (1) becomes 1.0. (C C1 / C C0 + C Si1 / C Si0 + C Mn1 / C Mn0 ) / 3 ≤ 5.0 ··· (1) Here, 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 represent 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 inclusions obtained by the extreme value statistics method. After obtaining the maximum particle sizes of oxide-based, TiN, and MnS-based inclusions for each 10 mm × 10 mm field of view area, the predicted maximum particle size at 30000 mm 2 was estimated by the extreme value statistics method. The particle size is evaluated by √Area, that is, the value obtained by taking the square root of the product of the major axis and minor axis of the inclusion. This extreme value statistics method is carried out in accordance with Reference 1. It is considered that as the inclusion diameter √Area in steel increases, the stress concentration around the inclusions increases, and internal cracks are likely to occur. From the above, the ID shall be 77 μm or less. Although the lower limit is not particularly limited, since excessive improvement in cleanliness leads to an increase in manufacturing cost, the ID is preferably 5 μm or more. [Reference 1] Keigi Murakami et al.: Transactions of the Japan Society of Mechanical Engineers, vol. 83, No. 853, 1 (2017)

[0043] The 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 and is considered to contribute to the suppression of Chevron cracks. For this purpose, the dislocation density may be 5 × 10 15 (m -2 ) or less, preferably 1 × 10 15 (m -2 ) or less, and more preferably 5 × 10 14 (m -2 ) or less. Although the lower limit is not particularly limited, even when pure iron is annealed to sufficiently remove dislocations, 2 × 10 11 (m -2 ) of dislocations are considered to be included. Therefore, the lower limit value of the dislocation density is set to 2 × 10 11 (m -2 ).

[0044] Also, there are various methods for controlling the dislocation density, and the following can be used alone or in combination. For example, there are methods such as reducing the C content, suppressing martensite transformation and bainite transformation by controlling the cooling rate during the cooling process after hot rolling, and omitting or simplifying the straightening process by improving the dimensional accuracy after hot rolling. Of course, the dislocation density may also be controlled by other methods. Here, a decrease in the C content is effective in reducing the dislocation density when the martensite structure and the bainite structure exist by reducing the amount of dissolved C in these structures. Further, it is possible to lower the cooling rate of the steel by using a heat insulating cover or the like that obstructs heat radiation on a cooling bed or the like after hot rolling, and suppress the martensite transformation and the bainite transformation. Further, by performing online dimensional measurement during hot rolling, it is possible to appropriately adjust the gap interval between the rolling rolls, and in this way, the dimensional accuracy after hot rolling can be improved. By using these methods singly or in combination, a predetermined dislocation density can be realized.

[0045] Regarding the segregation degree, inclusions, and dislocation density of the steel after hot rolling, it is not sufficient to satisfy only one or two of them to obtain sufficient Chevron crack suppression ability, and it is necessary to satisfy all of them. Although the detailed mechanism is not necessarily clear, there is some mutual relationship among these indexes, and it is considered that they bring a synergistic effect effective in crack suppression.

[0046] The manufacturing method is not particularly limited, and for manufacturing processes not particularly mentioned in this specification, the same method as that for ordinary steel for machine structures may be used.

Examples

[0047] Hereinafter, according to the examples, the configuration and the operation and effect of the present invention will be specifically described. However, the present invention is not limited by the following examples, and it is also possible to appropriately change within the range that can conform to the gist of the present invention, and all of these are included in the technical scope of the present invention.

[0048] Steel having the component composition shown in Table 1 was melted, and the obtained slab obtained by continuous casting at various casting speeds was processed into a billet and then hot rolled into round bars of various diameters. Thereafter, for a part of the hot rolled material, straightening was performed at various roll gaps using a two-roll straightening machine to obtain straight bar steel.

[0049] The segregation degrees of C, Si, and Mn were determined by analyzing a 10 mm-sided cubic test piece collected from the 1 / 2 diameter part (central part) of the bar steel after embedding it in resin and polishing, using an electron probe microanalyzer (EPMA). The EPMA field of view was 3 mm × 3 mm, the acceleration voltage was 15.0 kV, and the irradiation current was 5.0×10 -7 A. The beam shape was a 10-μm circle, measured in 30-μm steps. The C1 / C0 of C, Si, and Mn was obtained from the average concentration C0 and the maximum concentration C1 within the field of view, and the average value of these was taken as the segregation degree of C, Si, and Mn.

[0050] For inclusions, a 11 mm × 11 mm × 5 mm rectangular parallelepiped test piece collected from the 1 / 4 diameter part of the bar steel (the position in the middle between the outer peripheral surface and the axis center of the bar steel) was embedded in resin and polished to obtain a 11 mm × 11 mm mirror finish. A total of 30 such test pieces were prepared and observed with an optical microscope at a magnification of ×100. After obtaining the maximum particle size (μm) of oxide-based, TiN, and MnS-based inclusions for each 10 mm × 10 mm field of view area, the predicted maximum particle size (μm) at 30000 mm 2 was estimated using the extreme value statistical method. The particle size was evaluated as √Area, that is, the square root of the product of the major axis and the minor axis of the inclusion. Also, the major axis and the minor axis were obtained by photographing the position at a magnification of ×500 and performing image analysis using PRECiV-Imaging software Version 1.1 (manufactured by Evident Corporation).

[0051] For the dislocation density, a 10 mm-sided cubic test piece was collected from the 1 / 2 diameter part of the bar steel. After mirror finishing the cross-section perpendicular to the longitudinal direction of the bar steel by wet polishing and electrolytic polishing, it was measured using an X-ray diffractometer (XRD).

[0052] For the Chevron crack test, a Φ20 mm round bar test piece was used and drawn at a drawing angle of 30° until the total reduction of area reached 85%. After polishing the cross-section parallel to the longitudinal direction passing through the center of the diameter of the test part, a visual inspection of the appearance was performed. When a crack with a width of 0.5 mm or more was observed, it was determined that cracking had occurred.

[0053] These results are shown in Tables 2-1, 2-2, 2-3, and 2-4. As such, according to the above-described invention, a steel for machine structures capable of suppressing Chevron cracks can be obtained.

[0054]

Table 1

[0055]

Table 2-1

[0056]

Table 2-2

[0057]

Table 2-3

[0058]

Table 2-4

Claims

1. By mass percentage, C: 0.10 - 0.60%, Si: 0.01 - 0.90%, Mn: 0.05 - 1.80%, P: 0.050% or less (including 0%), S: 0.050% or less (including 0%), Al: 0.001 - 0.090%, N: 0.0015 - 0.0150%, comprising, with the balance being Fe and inevitable impurities, having a component composition, the C, Si, and Mn concentrations measured using EPMA (Electron Probe Micro Analyzer) satisfy the following formula (1), and ID (μm), which is the predicted maximum diameter of inclusions obtained by the extreme value statistical method, satisfies the following formula (2), and The dislocation density ρ (m －２ ) satisfies the following formula (3): Steel for mechanical structures. (C Ｃ１ / C Ｃ０ +C Ｓｉ１ / C Ｓｉ０ +C Ｍｎ１ / C Ｍｎ０ ) / 3 ≤ 5.0... (1) Here, C Ｃ１ , C Ｓｉ１ , C Ｍｎ１ is the maximum value of C, Si, and Mn measured using EPMA, C Ｃ０ , C Ｓｉ０ , C Ｍｎ０ indicates the average value of C, Si, and Mn measured using EPMA. ID (μm) ≤ 76.5 ··· (2) Here, ID is the predicted maximum diameter √Area of inclusions obtained by the extreme value statistical method. The dislocation density ρ (m －２ ) ≦ 4.8 × 10^15... (3)

2. The component composition further contains, by mass percentage, one or more selected from one or more groups consisting of the following Groups A to D according to Claim 1 of the steel for mechanical structures. 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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