JP5318638B2 - Machine structural steel with excellent machinability - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a machine structural steel for producing machine parts to be machined, and more particularly to a machine structural steel having excellent machinability at low-speed intermittent cutting such as hobbing. Is.

  Machine structural parts such as gears, shafts, pulleys, constant velocity joints, etc. used for various gear transmissions including transmissions and differentials for automobiles, as well as crankshafts, connecting rods, etc. are processed by forging. After applying, it is generally finished to a final shape by cutting. Since the cost required for the cutting processing is large in the production cost, the steel material constituting the mechanical structural component is required to have good machinability. Therefore, a technique for improving machinability has been disclosed.

  For example, adding Pb or adding S to produce MnS is typical, but Pb is harmful to the human body and its use has been regulated, and S is a machine caused by sulfides. There is a limit to the use of parts where deterioration of the mechanical characteristics is a problem. In addition, gear cutting is generally performed with a hob, especially in the case of cutting of gears and the like. In this case, cutting is different from continuous cutting such as so-called turning, and is a type called intermittent cutting. Currently, steel materials that improve machinability in hobbing are hardly practically used. The tool material used as the hob is generally a high speed steel and coated with TiAlN or the like. In this case, it is known that the tool surface is worn while being oxidized by repeating cutting and idling in processing at a relatively low speed.

  As a method for improving the intermittent machinability, in Patent Document 1, by including Al: 0.04 to 0.20%, O: 0.0030% or less, at a high speed (cutting speed: 200 m / min or more). A steel material excellent in interrupted cutting (tool life) is described.

  In Patent Document 2, C: 0.05 to 1.2%, Si: 0.03 to 2%, Mn: 0.2 to 1.8%, P: 0.03% or less, S: 0.03 %: Cr: 0.1-3%, Al: 0.06-0.5%, N: 0.004-0.025%, O: 0.003% or less, and Ca: 0 .0005 to 0.02% and Mg: 0.0001 to 0.005%, solute N in the steel is 0.002% or more, the balance is made of iron and inevitable impurities, and ( Mechanical structural steels satisfying 0.1 × [Cr] + [Al]) / [O] ≧ 150 are described.

Patent Document 3 discloses that oxide inclusions present in steel are CaO: 15 to 60%, SiO ₂ : 20% or less when the average composition total of the oxide inclusions is 100% ( _{0%), Al 2 O 3: 20~80} %, MgO: 40% or less (not including 0%) with containing _{respectively, Li 2 O, Na 2 O} , K 2 O, BaO, SrO And steel for machine structural use in which the total content of at least one selected from the group consisting of Ti oxides is 0.5 to 20%.

  In Patent Document 4, as the component composition of steel, the amount of addition of Al and other nitride-forming elements and N is adjusted, and by applying an appropriate heat treatment, a solid solution that is harmful to machinability and impact properties is disclosed. By securing a proper amount of solid solution Al that suppresses N and improves machinability by high temperature embrittlement, and AlN that improves machinability by high temperature embrittlement effect and crystal structure of cracking, low speed to high speed Excellent cutting performance in the cutting speed range up to 1, and by increasing the amount of Al added, MnS (SIMS) has high segregation at the slab stage and high uniform dispersibility compared to conventional Al killed steel. The steel for machine structural use having a high impact property with a large amount of type III MnS) according to the above classification is described.

JP 2001-342539 A Japanese Patent No. 4193998 JP 2009-7643 A JP 2008-13788 A

  However, the steel material described in Patent Document 1 does not target intermittent cutting at a low speed (for example, a cutting speed of about 150 m / min). Further, when the Al content is increased, hot ductility is reduced, and problems such as cracks are likely to occur during hot forging.

  Patent Documents 2 and 3 focus on oxide inclusions. However, oxide inclusions generally have a function of deteriorating continuous machinability, and in fact, oxide inclusions. It is difficult to balance both continuous machinability and intermittent machinability of machine structural steel by controlling inclusions.

  Further, Patent Document 4 describes that the machinability of machine structural steel is improved by securing an appropriate amount of AlN. It doesn't let you.

  The present invention has been made paying attention to the circumstances as described above, and its purpose is not to improve machinability by increasing the amount of S added accompanied by a decrease in mechanical properties, but also to improve Ca. It is not due to the addition of Mg and Mg, and exhibits excellent machinability (especially tool life) in intermittent cutting (for example, hobbing) at a low speed in a high-speed tool while ensuring manufacturability such as hot workability. It is to provide a steel for machine structural use.

The steel for machine structural use of the present invention capable of achieving the above object is
C: 0.05 to 0.8% (meaning of mass%, hereinafter the same), Si: 0.03 to 2%, Mn: 0.2 to 1.8%, P: 0.03% or less (0% S: 0.006 to 0.03%, Al: 0.1 to 0.5%, N: 0.002 to 0.015%, O: 0.003% or less (including 0%) The remainder is composed of iron and inevitable impurities, and there are 30 or less (not including 0) inclusions with an equivalent circle diameter of 2 μm or more appearing on the steel cross section per square millimeter (mm ² ), of which Further, composite inclusions of oxide and sulfide that satisfy the following formula (1) are present in a number ratio of 50% or more.

However, in the above formula (1), both the mass of oxide and the mass of sulfide are calculated only for the portion appearing on the steel cross section.

  In the steel for machine structural use, Cr: 3% or less (not including 0%), Mo: 1.0% or less (not including 0%), or Nb: 0.15% or less (0) as necessary. %), Or Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), and Ta: 0.02% or less (not including 0%) 1) or more selected from the group consisting of: V: 0.5% or less (not including 0%), Cu: 3% or less (not including 0%), and Ni: 3% or less (including 0%) 1) or more selected from the group consisting of Ca: 0.005% or less (not including 0%) and / or Mg: 0.005% or less (not including 0%), or Ti: 0.00. 05% or less (not including 0%) and / or B: 0.008% or less (not including 0%) It may be in.

  According to the present invention, in mechanical structural steel to which a relatively large amount of Al is added, the number of oxides and sulfide composite inclusions that have a certain ratio of sulfide to oxide is increased. Makes it possible to achieve excellent machinability (especially tool life) in both intermittent cutting with a high-speed tool and continuous cutting with a carbide tool while satisfying the strength characteristics of a machine structural steel. Can be obtained.

FIG. 1 is a cross-sectional image of the steel for mechanical structure of the present invention observed with a scanning electron microscope. FIG. 2 is a graph showing the average flank wear width during the continuous cutting test.

  The present inventors have studied from various angles in order to improve machinability in intermittent cutting at low speed. As a result, in steel for machine structural use in which a large amount of Al is added, a large amount of hard oxide inclusions (mainly alumina) are produced and the continuous machinability is deteriorated. It was found that the adverse effect can be minimized by compounding with the sulfide of the present invention, and the present invention has been completed. The reasons for limiting the range of the chemical composition of the mechanical structural steel defined in the present invention are as follows.

[C: 0.05 to 0.8%]
C is an essential element for ensuring the strength required for mechanical structural parts, and therefore needs to be 0.05% or more. However, if the C content is excessive, the hardness will increase too much, and the machinability and toughness will decrease, so it is necessary to make it 0.8% or less. The preferable lower limit of the C content is 0.10% (more preferably 0.15%), and the preferable upper limit is 0.6% (more preferably 0.5%).

[Si: 0.03 to 2%]
Si is an element effective for improving the internal quality of steel as a deoxidizing element, and in order to exert such an effect effectively, the Si content needs to be 0.03% or more, preferably It is desirable that the content be 0.07% or more (more preferably 0.1% or more). Further, if the Si content is excessive, an abnormal structure at the time of carburization is generated, or hot and cold workability is impaired, so it is necessary to be 2% or less, preferably 1.7% or less (more preferably Is preferably 1.5% or less).

[Mn: 0.2 to 1.8%]
Mn is an effective element for improving the hardenability and improving the strength of the steel material. In order to exhibit such an effect effectively, it is contained 0.2% or more (preferably 0.4% or more, more preferably 0.5% or more). However, if the Mn content is excessive, the hardenability is excessively increased, and a supercooled structure is generated even after normalization and the machinability is lowered. Therefore, it is 1.8% or less (preferably 1.6%) Hereinafter, more preferably 1.5% or less.

[P: 0.03% or less (excluding 0%)]
P is an element (impurity) inevitably contained in the steel material, and promotes cracking during hot working, so it is preferably reduced as much as possible. Therefore, the P content is determined to be 0.03% or less (more preferably 0.02% or less, still more preferably 0.01% or less). It is industrially difficult to make the amount of P 0%.

[S: 0.006 to 0.03%]
S is an element that forms MnS and improves machinability. In particular, in the present invention, by adding a necessary amount of S to steel for machine structural use, a sulfide is generated around an oxide containing Al as a nucleus, and a composite inclusion can be formed. The characteristics of this composite inclusion will be described later. In order to produce a necessary amount of sulfide, S is, for example, 0.006% or more (preferably 0.007% or more, more preferably 0.008%). Or more) On the other hand, when S is contained excessively, the ductility and toughness of the steel material are lowered. Therefore, the upper limit of the amount of S is set to 0.03% (more preferably 0.02%, still more preferably 0.015%). In particular, when the S content is excessive, the amount that forms MnS inclusions by reacting with Mn increases, and these inclusions extend in the rolling direction during rolling, and the toughness in the direction perpendicular to the rolling (toughness of the transverse direction) is increased. Deteriorate.

[Al: 0.1 to 0.5%]
Al is required more than conventional case-hardened steel in order to improve the intermittent machinability, and it is particularly preferable that Al is present in a solid solution state of 0.05% or more. A part of Al is combined with N to suppress abnormal grain growth during the carburizing process, and also has a role as a deoxidizing agent. Therefore, the total Al is 0.1% or more (preferably 0.15% or more). And more preferably 0.2% or more). On the other hand, if there is too much Al, it will bind to N at a high temperature and AlN will be easily formed to reduce hot workability, so the upper limit is 0.5% (preferably 0.45%, more preferably 0.4). %).

[N: 0.002 to 0.015%]
N combines with Al to suppress grain growth and exerts the effect of improving the strength of the steel for mechanical structures. In order to effectively exhibit such an effect, N: 0.002% or more (preferably 0.003% or more, more preferably 0.004% or more) is contained. On the other hand, if the amount of N is too large, AlN is generated at a high temperature and the hot workability is lowered, so the content was made 0.015% or less (preferably 0.013% or less, more preferably 0.011% or less).

[O: 0.003% or less (not including 0%)]
When the O content is excessive, coarse oxide inclusions are generated, which adversely affects machinability, ductility / toughness, hot workability and ductility of steel. Therefore, the upper limit of the O content is set to 0.003% (preferably 0.002%, more preferably 0.0015%).

  The basic components of the machine structural steel used in the present invention are as described above, and the balance is substantially iron. Of course, inevitable impurities are allowed to be contained in the machine structural steel. Of course, it is also possible to use a steel for mechanical structure in which other elements are positively contained within a range that does not adversely affect the action of the present invention, and these optional elements will be described later.

In the present invention, in addition to adjusting the chemical composition of steel for machine structural use to the specified range, inclusions having an equivalent circle diameter of 2 μm or more (hereinafter referred to as “target inclusions”) have a steel cross section. There are 30 or less (not including 0) per square millimeter (mm ² ), of which 50% or more of composite inclusions of oxide and sulfide satisfying the following formula (1) are present. Since this is an important point, it will be described in detail below.

(Target inclusions are 30 / mm ² or less)

Oxide inclusions present in the steel deteriorate continuous machinability, and sulfide inclusions improve machinability but deteriorate mechanical properties. Oxide inclusions are inevitably generated during the deoxidation of steel. Further, S present as an impurity combines with Mn and the like, and sulfide is inevitably generated. However, if the amount of inclusions is too large, the machinability of the machine structural steel deteriorates or the mechanical properties deteriorate, so it is necessary to provide an upper limit for the inclusion density. Therefore, in the present invention, the number of target inclusions is limited to 30 or less (not including 0) per square millimeter (mm ² ) of the steel material cross section. Preferably it is 20 or less, more preferably 10 or less.

(The number of composite inclusions satisfying the formula (1) is 50% or more)
As mentioned above, by combining oxides and sulfides generated in steel in inclusions, the adverse effects of oxides (decrease in continuous machinability) can be reduced, and the balance between continuous machinability and intermittent machinability. Can be achieved. Such an effect appears when 50% or more (preferably 55% or more, more preferably 60% or more) of target inclusions satisfy the above formula (1) among target inclusions. In addition, the value of the ratio represented by the left side of the above equation (1) can be calculated by the following procedure, for example. That is, first, the composition of inclusions in a steel cross section is measured by EPMA, and values such as [S], [Mn], [Al] are measured (however, in the present invention, [X] is in the cross section of the inclusions). Meaning the total mass of the X element). Since it is not possible to measure the value of [O] in EPMA, as an approximate calculation, it is assumed that the total amount of S in inclusions combines with Mn to form MnS, and the amount of MnS produced from the total amount of Mn in inclusions. Assuming that the remaining Mn and other alloy elements after subtracting all form oxides, the value of the oxide / sulfide ratio is ([MO] + [MnO]) / [MnS] Calculated (“M” indicates an alloy element other than Mn). In addition, since the mass of each element measured by EPMA is the mass in the cross section of the inclusion as described above, in the formula (1), both the mass of the oxide and the mass of the sulfide appear only in the steel cross section. Is the object of calculation.

  In the present invention, as described above, the composite inclusion of oxide and sulfide is used to balance the continuous machinability and interrupted machinability of machine structural steel, and sulfide and oxide form separate inclusions. However, the above effects cannot be obtained. In particular, oxides in steel deteriorate the continuous machinability of steel for machine structures. In the present invention, since it contains a large amount of Al, a large amount of Al in a solid solution exists and the intermittent machinability is good, and even if the above-described composite inclusion increases, the intermittent machinability does not deteriorate.

Even if the steel for machine structural use is melted by an ordinary method, oxides and sulfides are easily generated separately, and the composite inclusion of the present invention defined by the above formula (1) is not generated. In order to produce the steel for machine structure of the present invention, it is necessary to use, for example, the following method. First, S in the molten steel is once reduced to 0.005% or less, and then Al ₂ O ₃ is precipitated by adding Al. Thereafter, the required amount of S is added. By passing through such an order, first, an oxide containing Al is generated, and a sulfide is generated so as to cover the periphery with this oxide as a nucleus, whereby the present invention defined by the above formula (1) is obtained. Composite inclusions can be formed. The order of addition of other alloy elements such as Mn, Si, C, etc. is not particularly limited.

  When adding Mg and Ca, which will be described later, Al is added after decreasing the amount of S, then Mg and Ca are added, and finally S is added. Thereby, the inclusion which the oxide and sulfide compounded by the above-mentioned (1) formula can be generated.

FIG. 1 is a cross-sectional image of the steel for mechanical structure of the present invention observed with a scanning electron microscope. As shown in FIG. 1, sulfide (MnS) is formed so as to cover the periphery with oxide (Al ₂ O ₃ ) as a nucleus, and a composite oxide is generated.

  The mechanical structural steel of the present invention has been described above. Hereinafter, elements added as necessary for the purpose of further improving the characteristics of the mechanical structural steel of the present invention will be described.

[Cr: 3% or less (excluding 0%)]
Cr is an effective element for enhancing the hardenability of the steel material and increasing the strength of the steel for machine structural use. Moreover, it is an element effective in improving the intermittent machinability of steel materials by compound addition with Al. In order to exert such effects, the Cr content is, for example, 0.1% or more (more preferably 0.3% or more, further preferably 0.7% or more). However, if the Cr content is excessive, the machinability deteriorates due to the formation of coarse carbides or the development of a supercooled structure, so 3% or less (more preferably 2% or less, more preferably 1.6% or less). Is desirable.

[Mo: 1.0% or less (excluding 0%)]
Mo is an element effective for securing the hardenability of the base material and suppressing the formation of an incompletely quenched structure, and may be contained in the steel for machine structure as necessary. In order to effectively exhibit such effects of Mo, for example, 0.05% or more (more preferably 0.1% or more, further preferably 0.15% or more) is contained. Such effects increase as the Mo content increases. However, if excessively contained, an undercooled structure is formed even after normalization, and the machinability of the steel for machine structures is lowered. It is desirable to make it 0% or less (more preferably 0.8% or less, and still more preferably 0.6% or less).

[Nb: 0.15% or less (excluding 0%)]
Among machine structural steels, especially case-hardened steel, the carburizing treatment is usually performed to harden the surface. During this treatment, abnormal growth of crystal grains may occur depending on the carburizing temperature, time, heating rate, etc. is there. Nb has an effect of suppressing such a phenomenon. In order to effectively exhibit such effects of Nb, for example, 0.01% or more (more preferably 0.03% or more, still more preferably 0.05% or more) is contained. Such an effect of Nb increases as the Nb content is increased. However, if it is excessively contained, hard carbides are generated and the machinability is lowered, so that it is less than 0.15% (more preferably 0.12%). In the following, it is more preferable that the content is 0.1% or less.

[Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), Ta: 0.02% or less (not including 0%) Contains above]
Zr, Hf and Ta have the effect of suppressing the abnormal growth of crystal grains as in the case of Nb, and may be contained in steel as necessary. These effects increase as the content of these elements (total amount of one or more elements) increases, but if included excessively, hard carbides are generated and the machinability of the steel for machine structural use decreases. Each of the above amounts is preferably the upper limit, and more preferably 0.02% or less in total.

[V: 0.5% or less (not including 0%), Cu: 3% or less (not including 0%), and Ni: 3% or less (not including 0%) that's all]
These elements are effective for improving the hardenability of the steel material to increase the strength, and may be contained in the machine structural steel as necessary. On the other hand, these effects increase as the content of these elements increases, but if they are contained excessively, a supercooled structure is formed or ductility and toughness are lowered. Is preferred.

[Ca: 0.005% or less (not including 0%) and / or Mg: 0.005% or less (not including 0%)]
Ca and Mg improve the intermittent machinability by softening the oxide inclusions by forming CaO—Al ₂ O ₃ and MgO—Al ₂ O _3. Good. On the other hand, if it is contained excessively, it may combine with S to form a high-melting sulfide, which may cause nozzle clogging during casting.

[Ti: 0.05% or less (not including 0%) and / or B: 0.008% or less (not including 0%)]
When Ti is added, TiN is generated and contributes to grain growth suppression. More specifically, most of the added Ti combines with N to suppress the solid solution amount of N and improve the hot workability of the steel, and partly by entering into oxide inclusions It reduces the melting point of inclusions and contributes to improved machinability.

  When B is added, BN is generated and contributes to improvement of machinability. More specifically, B combines with N to generate BN and suppresses the solid solution amount of N to improve the hot workability of the steel material, and BN also has a machinability improving effect.

  As described above, both Ti and B have the effect of being able to improve the hot workability of the steel material because the solid solution amount of N is suppressed by bonding with N and AlN at high temperature is suppressed. In order to improve the intermittent machinability, Ti and / or B may be contained.

  In order to effectively exhibit the above-described effect of Ti, it is desirable that the Ti amount is, for example, 0.001% or more (preferably 0.005% or more, more preferably 0.009% or more). On the other hand, when Ti is added excessively, coarse TiN reduces the machinability of the steel for machine structure. Therefore, it is desirable that the Ti content be 0.05% or less (preferably 0.045% or less, more preferably 0.04% or less).

  In order to effectively exhibit the above-described effect of B, it is desirable that the B amount is 0.0005% or more (preferably 0.0006% or more, more preferably 0.0007% or more). On the other hand, when B is added excessively, the hardenability becomes higher than necessary, the hardness of the steel for machine structural use becomes high, and the machinability decreases. Therefore, it is desirable that the B content is 0.008% or less (preferably 0.0075% or less, more preferably 0.007% or less).

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can be adapted to the above-described purpose. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Create specimen]
150 kg of chemical components shown in Table 1 were melted in a vacuum induction furnace and cast into a substantially cylindrical ingot having a diameter of 245 mm on the upper surface, a diameter of 210 mm on the lower surface, and a length of 480 mm. In Table 1, “-” means that the corresponding element is not added (no addition).

Subsequently, this ingot is forged (soaking: about 1250 ° C. for about 3 hours, forging heating: about 1100 ° C. for about 1 hour), and cut into a square material shape with a side of 150 mm × 150 mm × length of 680 mm. The following two types of forging materials (a) and (b) were processed.
(A) Plate material having a thickness of 30 mm, a width of 155 mm, and a length of 100 mm (b) a round bar material having a diameter of 80 mm and a length of 100 mm

  The obtained plate and round bar were heated at 900 ° C. for 1 hour and then allowed to cool. Then, the plate (forged material (a)) was used as an end mill cutting test piece, and the round bar (forged material (b) ) Was a turning specimen.

(1) Machinability evaluation during intermittent cutting In order to evaluate the machinability during intermittent cutting, tool wear in end milling was evaluated. In order to remove the influence of the scale and the decarburized layer from the forged material (a) (normalized material or hot forged after normalizing), about 2 mm of the surface was removed by cutting to obtain an end mill cutting test piece. Specifically, an end mill tool is attached to the main spindle of the machining center, and the test piece is manufactured as described above, and a test piece having a thickness of 25 mm, a width of 150 mm, and a length of 100 mm after cutting is fixed with a vice. Down-cut processing was performed in a dry cutting atmosphere. Detailed processing conditions are shown in Table 2 below. After performing 200 intermittent cuttings, the average flank wear width (tool wear amount) Vb was measured with an optical microscope. The results are shown in Table 3. The test piece number (No.) corresponds to the test piece number (No.) in Table 1. Those having Vb of 70 μm or less after intermittent cutting were evaluated as being excellent in machinability during intermittent cutting.

(2) Machinability evaluation during continuous cutting In order to evaluate the machinability during continuous cutting, the surface of the round bar (normalized material) with a diameter of 80 mm x length of 350 mm is removed and the surface is cut by about 2 mm. After the outer peripheral turning was performed using the removed sample, the average flank wear width (tool wear amount) Vb was measured with an optical microscope, and a sample having a wear width of 70 μm or less was evaluated as having excellent machinability. . The peripheral turning conditions at this time are as follows. The results are also shown in Table 3 together with the results of the machinability test during the intermittent cutting. FIG. 2 is a graph showing the average flank wear width during the continuous cutting test.

(Outer peripheral turning conditions)
Tool: Cemented carbide P10 (JIS B4053)
Cutting speed: 200 m / min
Feed: 0.25mm / rev
Cutting depth: 1.5mm
Lubrication system: dry

[Discussion]
Specimen No. Nos. 1 to 11 belong to the present invention, and were excellent in machinability during intermittent cutting and machinability during continuous cutting. On the other hand, test piece No. Nos. 12 to 15 deviate from the condition of the composite inclusion in the machine structural steel, and at least one of the machinability during intermittent cutting and the machinability during continuous cutting was inferior. Specifically, no. In 12-14, since S in the molten steel was not reduced to 0.005% or less once when Al was added, the composite inclusions defined in the formula (1) had a predetermined number ratio (of the target inclusions). The machinability during continuous cutting was inferior. No. No. 15, the chemical composition of the steel satisfied the range specified in the present invention, but because there were more than 30 target inclusions, both machinability during intermittent cutting and machinability during continuous cutting. Was inferior.

Claims (8)

C: 0.05 to 0.8% (meaning mass%, the same shall apply hereinafter)
Si: 0.03 to 2%,
Mn: 0.2-1.8%
P: 0.03% or less (excluding 0%),
S: 0.006 to 0.03%,
Al: 0.1 to 0.5%,
N: 0.002 to 0.015%,
O: 0.003% or less (excluding 0%)
The balance consists of iron and inevitable impurities,
There are 30 or less inclusions (not including 0) per square millimeter (mm ² ) of inclusions appearing in the steel cross section with an equivalent circle diameter of 2 μm or more. A machine structural steel excellent in machinability, characterized in that those satisfying 1) are present in a number ratio of 50% or more.
However, in the above formula (1), both the mass of oxide and the mass of sulfide are calculated only for the portion appearing on the steel cross section.   The steel for machine structure according to claim 1, further comprising Cr: 3% or less (not including 0%).   Furthermore, steel for machine structure of Claim 1 or 2 which contains Mo: 1.0% or less (0% is not included).   Furthermore, Nb: 0.15% or less (0% is not included) The rope for machine structures in any one of Claims 1-3.   Furthermore, Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), Ta: 0.02% or less (not including 0%) The steel for machine structure according to any one of claims 1 to 4, which contains one or more selected.   Further, one type selected from the group consisting of V: 0.5% or less (not including 0%), Cu: 3% or less (not including 0%), Ni: 3% or less (not including 0%) The steel for machine structure according to any one of claims 1 to 5, which contains the above.   Furthermore, it contains Ca: 0.005% or less (not including 0%) and / or Mg: 0.005% or less (not including 0%). Steel for machine structure.   Further, Ti: 0.05% or less (not including 0%) and / or B: 0.008% or less (not including 0%) are contained. Steel for machine structure.