JP2009287111A - Steel for machine structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a machine structure having excellent machinability in intermittent machining such as hob working. <P>SOLUTION: Disclosed is a steel for a machine structure having a composition comprising, by mass, 0.05 to 0.65% C, 0.1 to 2.0% Si, 0.5 to 2.0% Mn, 0.1 to 3.0% Cr, ≤1.2% Mo, ≤0.06% Al, ≤0.02% N and 0.0005 to 0.01% Ca, further comprising one or more kinds selected from ≤0.005% Mg and ≤0.005% REM, and in which the contents of P and S are regulated to ≤0.03%, respectively, and the balance Fe with inevitable impurities, and, provided that when the contents (mass%) of the Si, Mn, Cr, Ca, Mg and REM are expressed respectively as [Si], [Mn], [Cr], [Ca], [Mg] and [REM], 2.25≤[Si]+[Mn]+[Cr]+100×([Ca]+[Mg]+[REM])≤7.0 is satisfied. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel material for machine structure, and in particular, to a steel for cold work subjected to intermittent cutting.

  Machine structural parts such as gears, shafts, pulleys, constant velocity joints, crankshafts, and connecting rods used in various gear transmissions such as automobile transmissions and differential gears are generally subjected to forging and other processing. After that, it is finished to the final shape by cutting. Then, the machine structural parts finished in the final shape are subjected to surface hardening treatment such as carburizing and carbonitriding (including atmospheric pressure, low pressure, vacuum, plasma atmosphere), and quenching-tempering and high frequency as necessary. Quenching or the like is performed to ensure a predetermined strength. In the manufacture of such machine structural parts, the cost required for cutting is large, and therefore the steel (machine structural steel) constituting the machine structural parts is required to have good machinability. .

  The manufacturing method of gears, which are one of the parts for machine structure, is generally forged steel for machine structure, rough cutting (hobbing) by hobbing, finishing to the final shape by shaving, carburizing etc. Then, the polishing process (honing process) is performed again. Further, in recent years, in order to improve the dimensional accuracy of gears, a grinding process (hard finish) may be performed before the polishing process in order to completely correct the distortion of the shape due to the heat treatment. Thus, the production of gears requires a great number of processes, including many cutting and grinding processes. Therefore, improvement of machinability is desired especially for the steel for machine structure constituting the gear.

  The hobbing corresponds to intermittent cutting. Interrupted cutting is a cutting mode in which the tool is not continuously in contact with the work material. As a tool used for this hobbing, a high-speed tool steel coated with AlTiN or the like (hereinafter referred to as “high-speed tool”) is the current mainstream. Other cutting tools include a cemented carbide alloy coated with AlTiN or the like (hereinafter referred to as “carbide tool”). This tool is often used for continuous cutting such as turning.

  Since the cutting mechanism differs between intermittent cutting and continuous cutting, a tool corresponding to each cutting is used. Accordingly, the machine structural steel as the work material is required to have good machinability in any cutting. However, in intermittent cutting, as described above, there is a period when the tool is idling, that is, there is a period when the work material is not in contact with the tool. At this time, the new steel surface adhering to the tool is exposed to air and further generates heat during cutting. It oxidizes rapidly. As a result, the tool is subject to oxidative wear, and there is a problem that the tool life is short in intermittent cutting. Furthermore, since the tool used for hobbing is expensive, the steel for machine structure used for intermittent cutting such as hobbing is required to improve machinability, particularly the tool life.

Therefore, for example, in Patent Document 1, by adding B, S, and Ca to JIS steel, sulfide is generated, and by using this sulfide as a precipitation nucleus, fine BN is precipitated, machinability, fatigue strength, And steel materials with improved toughness. Further, Patent Document 2 discloses a steel material that has improved machinability while maintaining toughness by adding Pb and Ca instead of S, which is an element that improves machinability (free-cutting element). Yes. Patent Document 3 does not add free-cutting elements such as S, Pb, Ca, etc., and controls the content of Al and N and the ratio between the two to precipitate AlN, and intermittent cutting is performed by the lubricating effect of AlN. Steel materials with improved machinability are disclosed.
JP-A-6-145890 (paragraph 0009) JP-A-3-10050 (Claim 1) Japanese Patent No. 392691 (paragraphs 0018 to 0023)

  However, the steel materials disclosed in Patent Documents 1 and 2 correspond to turning with a carbide tool. In particular, the steel material disclosed in Patent Document 2 contains Pb, which has been pointed out to be harmful to the human body, so it is not preferable to use this. In addition, not only steel materials but in recent years, Pb-free (Pb-free) materials have been demanded. Moreover, although the steel material disclosed by patent document 3 respond | corresponds to intermittent cutting, it improves machinability by the precipitate in steel materials similarly to the steel materials disclosed by patent documents 1 and 2. is there. Such inclusions and precipitates in the steel material have a problem that the mechanical properties of the steel material are easily deteriorated.

  The present invention has been made in view of the above problems, and in order to combine the mechanical properties such as toughness with the means for improving the machinability, during intermittent cutting regardless of the inclusions and precipitates in the steel material. An object of the present invention is to provide a steel for machine structure that improves the machinability of the steel, in particular, the tool life.

  The present inventors have added a solid oxidation solution to an element having a tendency to oxidize more than Fe, that is, O (oxygen) is more easily bonded to Fe than Fe. It was decided to prevent rapid oxidation of the new surface of the tool and suppress oxidative wear of the tool. As a result of studying Si, Mn, Cr, Ca, Mg, and REM (rare earth metal elements) as such elements, the present inventors have found that Si, which can be added in a large amount but has a small oxidation inhibiting effect alone. Oxidation of tools in interrupted cutting while maintaining mechanical properties by adding Mn, Cr and Ca, Mg, REM, which have large oxidation inhibition effect but deteriorate mechanical properties when added in a large amount. It has been found that wear can be suppressed.

  That is, the steel for machine structure according to claim 1 is C: 0.05 to 0.65 mass%, Si: 0.1 to 2.0 mass%, Mn: 0.5 to 2.0 mass%, Cr : 0.1-3.0 mass%, Mo: 1.2 mass% or less, Al: 0.06 mass% or less, N: 0.02 mass% or less, Ca: 0.0005-0.01 mass% And P and S are each regulated to 0.03% by mass or less, the balance is Fe and inevitable impurities, and each content (mass%) of Si, Mn, Cr, Ca is [Si], When expressed as [Mn], [Cr], [Ca], 2.25 ≦ [Si] + [Mn] + [Cr] + 100 × [Ca] ≦ 7.0 is satisfied.

  Moreover, the steel for machine structure which concerns on Claim 2 is C: 0.05-0.65 mass%, Si: 0.1-2.0 mass%, Mn: 0.5-2.0 mass%, Cr : 0.1-3.0 mass%, Mo: 1.2 mass% or less, Al: 0.06 mass% or less, N: 0.02 mass% or less, Ca: 0.0005-0.01 mass% Further, Mg: 0.005% by mass or less, REM: 0.005% by mass or less is contained, P and S are restricted to 0.03% by mass or less, and the balance is Fe and It consists of inevitable impurities, and each content (mass%) of the Si, Mn, Cr, Ca, Mg, and REM is changed to [Si], [Mn], [Cr], [Ca], [Mg], [REM]. ] 2.25 ≦ [Si] + [Mn] + [Cr] + 100 × ([Ca] + [Mg] + [REM]) ≦ 7.0 And satisfying.

  Thus, by newly adding Si, Mn, and Cr, which have a higher oxidation tendency than Fe, and Ca, Mg, and REM, which have a higher oxidation tendency, the new surface of the machine structural steel is rapidly oxidized in intermittent cutting. Can be prevented.

  In the steel for machine structure according to claim 3, the steel for machine structure according to claim 1 or 2, further, B: 0.005 mass% or less, Ti: 0.2 mass% or less, Nb: 0 .2% by mass or less, and V: one or more of 0.5% by mass or less.

  By adding these elements, the occurrence of abnormal grain growth in the carburizing process can be effectively prevented.

  Furthermore, in the steel for machine structure according to claim 4, the steel for machine structure according to any one of claims 1 to 3, further Cu: 5.0 mass% or less, and Ni: 5. 1 type or more is contained among 0 mass% or less, It is characterized by the above-mentioned.

  By adding these elements, the hardenability of the steel for machine structure can be improved and the hardness after quenching can be improved.

  The steel for machine structure according to the present invention has sufficient mechanical properties such as toughness and has improved machinability. The steel for machine structure according to the present invention has improved machinability in interrupted cutting, particularly as a steel material constituting machine structural parts such as gears, thereby suppressing oxidative wear of the cutting tool. Life can be extended.

Hereinafter, the best mode for carrying out the steel for machine structure according to the present invention will be described.
The steel for machine structural use according to the present invention has C: 0.05 to 0.65 mass%, Si: 0.1 to 2.0 mass%, Mn: 0.5 to 2.0 mass%, Cr: 0.00. 1 to 3.0% by mass, Mo: 1.2% by mass or less, Al: 0.06% by mass or less, N: 0.02% by mass or less, Ca: 0.0005 to 0.01% by mass, P and S are each regulated to 0.03% by mass or less, and the balance is composed of Fe and inevitable impurities. In addition, the steel for mechanical structure may further contain one or more of Mg: 0.005 mass% or less and REM: 0.005 mass% or less.

And the steel for machine structure which concerns on this invention is the sum total of each content of Si, Mn, and Cr with which the tendency of oxidation is larger than Fe among each said component, ie, O is easy to couple | bond compared with Fe, Si. , Mn, and Cr, the total sum of the contents of Ca, Mg, and REM, which are more easily combined with O, is limited to a predetermined range. That is, when the contents (mass%) of Si, Mn, Cr, Ca, Mg, and REM are represented by [Si], [Mn], [Cr], [Ca], [Mg], and [REM], respectively. The contents of these components are adjusted so as to satisfy the following formula.
2.25 ≦ [Si] + [Mn] + [Cr] + 100 × ([Ca] + [Mg] + [REM]) ≦ 7.0
Below, the numerical range of content of each component which comprises the steel for machine structure which concerns on this invention, and the reason for limitation of the numerical range are demonstrated.

(C: 0.05 to 0.65% by mass)
C is an element that has an effect of improving the strength of steel for machine structural use and is effective for ensuring the hardness of the core part necessary for machine structural parts. In order to make the mechanical structural steel sufficiently hard, the C content is 0.05% by mass or more, preferably 0.10% by mass or more, and more preferably 0.15% by mass or more. On the other hand, when C is added excessively, hardness becomes excessive and machinability and toughness are lowered. Therefore, the C content is 0.65% by mass or less, preferably 0.60% by mass or less, and more preferably 0.55% by mass or less.

(Si: 0.1 to 2.0% by mass)
Si has a deoxidizing effect and reduces the oxide inclusions in the steel for machine structural use to improve the internal quality. In order to make this effect sufficient, the Si content is 0.10% by mass or more, preferably 0.12% by mass or more, and more preferably 0.15% by mass or more. Further, since O is more easily bonded to Si than Fe, Si has an effect of suppressing oxidative wear of the tool during intermittent cutting. On the other hand, if Si is added excessively, an abnormal structure is generated during carburizing, or the amount of retained austenite (residual γ phase) after heat treatment (quenching) increases, so that sufficient hardness cannot be obtained in the carburized phase. . Therefore, the Si content is 2.0% by mass or less, preferably 1.8% by mass or less, and more preferably 1.5% by mass or less.

(Mn: 0.5 to 2.0% by mass)
Mn has the effect of improving the hardenability and improves the hardness of the steel for machine structure after quenching. In order to make this effect sufficient, the Mn content is 0.5% by mass or more, preferably 0.6% by mass or more, and more preferably 0.7% by mass or more. Further, since O is more easily bonded to Mn than Fe, Mn has an effect of suppressing oxidative wear of the tool during intermittent cutting. On the other hand, when Mn is added excessively, hardenability becomes excessive, and a supercooled structure is generated even after normalization, thereby reducing machinability. Therefore, the Mn content is 2.0% by mass or less, preferably 1.9% by mass or less, and more preferably 1.8% by mass or less.

(Cr: 0.1-3.0% by mass)
Cr has the effect of improving hardenability and improves the hardness of the steel for machine structure after quenching. In order to make this effect sufficient, the Cr content is 0.1% by mass or more, preferably 0.3% by mass or more, and more preferably 0.7% by mass or more. Further, since O is more easily bonded to Cr than Fe, Cr has an effect of suppressing oxidative wear of the tool during intermittent cutting. On the other hand, when Cr is added excessively, the hardenability becomes excessive and a supercooled structure develops, and coarse carbides are generated at the grain boundaries to deteriorate the machinability. Therefore, the Cr content is 3.0% by mass or less, preferably 2.5% by mass or less, and more preferably 2.0% by mass or less.

(Mo: more than 0% by mass and 1.2% by mass or less)
Mo is dissolved in steel to ensure hardenability and has the effect of suppressing the formation of an incompletely hardened structure, and this effect increases as the Mo content increases. The lower limit of the Mo content is not particularly specified, but in order to obtain this effect, 0.005% by mass or more is preferable, 0.008% by mass or more is more preferable, and 0.01% by mass or more is more preferable. On the other hand, when Mo is added excessively, the hardenability becomes excessive, and even after normalization, a supercooled structure is generated and the machinability is lowered. Therefore, the Mo content is 1.2% by mass or less, preferably 1.1% by mass or less, and more preferably 1.0% by mass or less.

(Al: more than 0% by mass and 0.06% by mass or less)
Al has a deoxidizing effect and improves the internal quality of steel for machine structural use. Further, Al combines with N to form AlN, and this AlN has an effect of suppressing abnormal growth of crystal grains in the carburizing process. The lower limit of the Al content is not particularly specified, but in order to obtain these effects, 0.0005% by mass or more is preferable, 0.0008% by mass or more is more preferable, and 0.001% by mass or more is more preferable. Note that O is easily bonded to Al like Cr and the like, but when Al is bonded to O, hard alumina is formed. And when alumina increases with an increase in Al content and the Al content exceeds 0.06 mass%, abrasive wear becomes remarkable, and machinability, particularly machinability in continuous cutting, decreases. Therefore, the Al content is 0.06% by mass or less, preferably 0.05% by mass or less, and more preferably 0.04% by mass or less.

(N: more than 0% by mass and 0.02% by mass or less)
N (nitrogen) is an element inevitably mixed in the steel melting step. N has the effect of suppressing the oxidation reaction of the new surface of the machine structural steel in interrupted cutting, and extends the tool life in interrupted cutting. The lower limit of the N content is not particularly defined, but 0.002% by mass or more is preferable and 0.004% by mass or more is more preferable in order to obtain this effect. On the other hand, when N is added excessively, ductility and toughness are reduced by age hardening. Therefore, the N content is 0.02% by mass or less, and preferably 0.015% by mass or less.

(P: 0.03 mass% or less)
P is an element (impurity) inevitably contained in steel. P is preferably reduced as much as possible because it promotes cracking during hot working. Therefore, the P content is 0.03% by mass or less, preferably 0.025% by mass or less, and more preferably 0.02% by mass or less.

(S: 0.03 mass% or less)
S is an element (impurity) inevitably contained in steel. S has the effect of improving machinability, but on the other hand, reduces ductility and toughness. Furthermore, S reacts with Mn to form MnS inclusions. When the inclusions extend in the rolling direction during rolling, the toughness in a direction perpendicular to the rolling direction of the steel material (this direction is generally referred to as “horizontal”) deteriorates. Therefore, the S content is 0.03% by mass or less, preferably 0.025% by mass or less, and more preferably 0.02% by mass or less.

(Ca: 0.0005 to 0.01% by mass)
Ca has an action of softening hard inclusions such as alumina, and therefore suppresses tool wear due to hard inclusions. In order to make this effect sufficient, the Ca content is set to 0.0005 mass% or more, preferably 0.0007 mass% or more, and more preferably 0.001 mass% or more. Moreover, since Ca is easily combined with O, it has an effect of suppressing oxidative wear during intermittent cutting. On the other hand, when Ca is added excessively, inclusions such as CaO increase, and the inclusions reduce ductility and toughness. Therefore, the Ca content is 0.01% by mass or less, preferably 0.009% by mass or less, and more preferably 0.008% by mass or less.

(Mg: 0.005 mass% or less, REM: 0.005 mass% or less)
Since Mg and REM (rare earth metal elements) are easily combined with O like Ca, they have the effect of suppressing oxidative wear during intermittent cutting. Moreover, since Mg and REM have the effect | action which softens hard inclusions, such as an alumina, it suppresses tool wear. Specific examples of the rare earth metal element include Ce, La, Nd and the like, and the content of REM in this specification refers to the total content of all these rare earth metal elements. In order to acquire the said effect, each content of Mg and REM has preferable 0.0001 mass% or more, and 0.0002 mass% or more is further more preferable. On the other hand, when both Mg and REM are added excessively, inclusions such as MgO and CeO ₂ are increased, and ductility and toughness are reduced by these inclusions. Therefore, each content of Mg and REM is set to 0.005 mass% or less, preferably 0.004 mass% or less, and more preferably 0.003 mass% or less.

Among the components of the steel for machine structure according to the present invention, the oxidation tendency is greater than that of Fe, that is, the sum of the contents of Si, Mn, and Cr in which O is easy to bond as compared with Fe, and further O is The sum total of 100 times the content of each of Ca, Mg, and REM that can be easily combined is defined as [A]. That is,
[A] = [Si] + [Mn] + [Cr] + 100 × ([Ca] + [Mg] + [REM])
It is represented by At this time, when the parameter [A] is less than 2.25, the oxidation of the mechanical structural steel is not sufficiently suppressed, and the oxidative wear of the tool is likely to occur. Accordingly, the parameter [A] is set to 2.25 or more, preferably 2.35 or more, and more preferably 2.45 or more. On the other hand, when the parameter [A] exceeds 7.0, the hardness becomes excessive due to an increase in the solid solution element in the steel for machine structural use, and the machinability is lowered. Moreover, the inclusion in machine structural steel increases and machinability deteriorates. Therefore, the parameter [A] is set to 7.0 or less, preferably 6.5 or less, and more preferably 6.0 or less. Each content of Si, Mn, Cr, Ca, Mg, and REM of the steel for machine structure according to the present invention is adjusted so that the parameter [A] falls within the above range.

  The mechanical structural steel according to the present invention is further selected from among B: 0.005 mass% or less, Ti: 0.2 mass% or less, Nb: 0.2 mass% or less, and V: 0.5 mass% or less. You may contain 1 or more types in Cu: 5.0 mass% or less and Ni: 5.0 mass% or less.

(B: 0.005 mass% or less, Ti: 0.2 mass% or less, Nb: 0.2 mass% or less, V: 0.5 mass% or less)
Among machine structural steels, case-hardened steel is usually carburized for surface hardening. During this carburizing process, abnormal grain growth may occur depending on the processing temperature, processing time, heating rate, and the like. Since B, Ti, Nb, and V have the effect of preventing this abnormal grain growth, it is effective to add these elements. In order to obtain this effect, the B content is preferably 0.0001% by mass or more, more preferably 0.0003% by mass or more, and further preferably 0.0005% by mass or more. Similarly, each content of Ti, Nb, and V is preferably 0.001% by mass or more, and more preferably 0.002% by mass or more. On the other hand, when these elements are added excessively, hard carbides are generated and the machinability deteriorates. Therefore, the B content is 0.005 mass% or less, preferably 0.003 mass% or less, and more preferably 0.001 mass% or less. Similarly, each content of Ti and Nb is 0.2% by mass or less, preferably 0.15% by mass or less, and more preferably 0.1% by mass or less. And V content shall be 0.5 mass% or less, 0.4 mass% or less is preferable, and 0.3 mass% or less is further more preferable.

(Cu: 5.0 mass% or less, Ni: 5.0 mass% or less)
Cu and Ni have the effect of improving the hardenability and improve the hardness of the steel for machine structure after quenching. Furthermore, this effect increases as the content of Cu and Ni increases. In order to acquire this effect, each content of Cu and Ni is preferably 0.1% by mass or more, and more preferably 0.3% by mass or more. On the other hand, when these elements are added excessively, the hardenability increases, a supercooled structure is generated, and the ductility and toughness decrease. Therefore, each content of Cu and Ni is set to 5.0% by mass or less, preferably 4.0% by mass or less, and more preferably 3.0% by mass or less.

  Although the best mode for carrying out the present invention has been described above, examples in which the effects of the present invention have been confirmed will be specifically described in comparison with comparative examples that do not satisfy the requirements of the present invention. . It should be noted that the present invention is not limited by this embodiment, and can be implemented with appropriate modifications within the scope of the claims, all of which are included in the technical scope of the present invention. The

[Sample preparation]
150 kg of steel having the chemical composition shown in Table 1 and Table 2 was melted in a vacuum induction furnace and cast into an ingot having an upper surface of φ245 mm and a lower surface of φ210 mm × height 480 mm. The ingot was soaked at about 1200 ° C. × 3 hr, forged into a square material of 150 mm square × length 680 mm at 1100 ° C. × 1 hr, and cut to a length of about 100 mm. The cut square material was forged into a plate material having a thickness of 30 mm and a width of 150 mm at about 1100 ° C. × 1 hr, and further cut to a length of about 100 mm. Further, the cut square material was forged into a round bar material of about 80 mm × 100 mm in length at about 1100 ° C. × 1 hr. These plate materials and round bar materials were subjected to normalization (after heat treatment at 900 ° C. × 2 hr, and then air-cooled), and produced as test materials (Examples 1 to 25, Comparative Examples 26 to 41). The following measurements and evaluations were performed on the prepared specimens. The parameters [A] calculated from the chemical composition are listed in Tables 1 and 2.

[Measurement and evaluation]
(Machinability)
In order to evaluate the machinability during intermittent cutting, tool wear was observed after an intermittent cutting test with an end mill tool. The test material (plate material) was made into a cut specimen having a thickness of 25 mm, a width of 145 mm, and a length of 100 mm by removing the scale and grinding the surface by 2 mm in the thickness direction. An end mill tool (Mitsubishi Materials High-Speed End Mill, model number K-2SL, outer diameter φ10 mm, TiAlN coating thickness 2.6 μm) is attached to the main spindle of the machining center. Intermittent cutting was performed in an atmosphere. The interrupted cutting conditions are shown below. After intermittent cutting of 200 cuts (cutting distance: about 3000 m), the used end mill tool was observed with an optical microscope, and the average flank wear width (tool wear amount) was measured. The acceptance criteria for machinability was a tool wear amount of 70 μm or less. In addition, the Vickers hardness of the surface of the same cutting test piece was measured. Table 1 and Table 2 show the tool wear amount and Vickers hardness.

(Intermittent cutting conditions)
Axial cut depth: 1.0mm
Radial cut depth: 1.0mm
Feed amount: 0.117 mm / rev
Feeding speed: 558.9 mm / min
Cutting speed: 150 m / min
Rotation speed: 4777 rpm

(Toughness of lateral eye)
As the mechanical properties, the toughness of the cross-section of the specimen after carburization was evaluated. The test material (round bar material) is formed with a notch (R: 10 mm, depth: 2 mm) along a direction (horizontal line) perpendicular to the rolling (forging) direction, and is cut into a size of 10 mm × 10 mm × 55 mm. And made into a Charpy impact test piece. The test piece was carburized under the following conditions, then quenched with 60 ° C. cold oil and then tempered (after heat treatment at 170 ° C. × 120 min, air-cooled). The Charpy impact value (Charpy absorbed energy) was measured on the test piece after the above treatment. The measured Charpy absorbed energy is shown in Tables 1 and 2. The acceptance criterion for the toughness of the transverse eye was a Charpy absorbed energy of 10.0 J or more.

(Carburizing conditions)
900 ° C. × 90 min (CO ₂ concentration: 0.11%, carbon potential (hereinafter CP): aiming at 1.0%) → 900 ° C. × 90 min (CO ₂ concentration: 0.17%, CP: aiming at 0.8%) ) → 840 ° C. × 60 min (CO ₂ concentration: 0.39%, CP: 0.8% aim)

(Evaluation)
As shown in Table 1, in Examples 1 to 25, since the content of each component and the parameter [A] are within the scope of the present invention, the amount of tool wear after the intermittent cutting test is small, and the amount of wear during intermittent cutting is low. The machinability was excellent and the toughness of the transverse eye was also good.

  On the other hand, as shown in Table 2, in Comparative Example 26, since the C content was insufficient, the amount of tool wear was large, and the Vickers hardness decreased. On the other hand, in Comparative Example 27, since the C content was excessive, the machinability and the toughness of the transverse eye were lowered. Further, since Comparative Example 28 has an excessive Si content, Comparative Example 29 has an excessive Mn content, and therefore, the machinability and the toughness of each side decreased. Since Comparative Example 30 had an excessive Cr content, Comparative Example 31 had an excessive Mo content, and Comparative Example 32 had an excessive Al content, so that the machinability was reduced.

  In Comparative Example 33, the toughness of the lateral eye was lowered because the P content was excessive. Since the comparative example 34 has an excessive S content, the comparative example 35 has an excessive N content. In Comparative Example 36, since the Ca content was excessive, the machinability was improved, but the toughness of the transverse eye was lowered. On the other hand, since the comparative example 37 has insufficient Ca content (it is no addition), machinability fell.

  In Comparative Examples 39 and 40, the content of each component is within the scope of the present invention, but since the parameter [A] is insufficient, Comparative Example 38 is further insufficient in Ca content (no addition). Therefore, the oxidation wear was caused by insufficient suppression of oxidation, and the amount of tool wear increased. On the other hand, in Comparative Example 41, the content of each component is within the scope of the present invention, but since the parameter [A] is excessive, inclusions increase, machinability decreases, and hardness becomes excessive. As a result, the machinability and the toughness of the transverse eye decreased.

Claims (4)

C: 0.05-0.65 mass%, Si: 0.1-2.0 mass%, Mn: 0.5-2.0 mass%, Cr: 0.1-3.0 mass%, Mo: 1.2 mass% or less, Al: 0.06 mass% or less, N: 0.02 mass% or less, Ca: 0.0005-0.01 mass%, P and S are each 0.03 mass% Restricted to the following, the balance consists of Fe and inevitable impurities,
When each content (mass%) of Si, Mn, Cr, and Ca is expressed as [Si], [Mn], [Cr], and [Ca], 2.25 ≦ [Si] + [Mn] + [Cr] + 100 × [Ca] ≦ 7.0 satisfies the structural structural steel. C: 0.05-0.65 mass%, Si: 0.1-2.0 mass%, Mn: 0.5-2.0 mass%, Cr: 0.1-3.0 mass%, Mo: 1.2 mass% or less, Al: 0.06 mass% or less, N: 0.02 mass% or less, Ca: 0.0005-0.01 mass%, and Mg: 0.005 mass% or less REM: containing one or more of 0.005% by mass or less, P and S are restricted to 0.03% by mass or less, and the balance is Fe and inevitable impurities,
When each content (mass%) of the Si, Mn, Cr, Ca, Mg, and REM is expressed as [Si], [Mn], [Cr], [Ca], [Mg], and [REM], 2.25 ≦ [Si] + [Mn] + [Cr] + 100 × ([Ca] + [Mg] + [REM]) ≦ 7.0 satisfying the mechanical structural steel.   Further, B: 0.005% by mass or less, Ti: 0.2% by mass or less, Nb: 0.2% by mass or less, and V: 0.5% by mass or less, The steel for machine structure according to claim 1 or 2.   The mechanical structure according to any one of claims 1 to 3, further comprising at least one of Cu: 5.0 mass% or less and Ni: 5.0 mass% or less. Steel.