JP5314509B2 - Steel for machine structure - Google Patents

Description

  The present invention relates to a machine structural steel material, and in particular, to a machine structural steel material 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 differentials are generally hot-worked such as forging steel. After finishing, the final shape is finished by cutting. The machine structure parts finished in the final shape are subjected to surface hardening treatment such as carburizing and carbonitriding (including atmospheric pressure, low pressure, vacuum, and plasma atmosphere), and further quenching and tempering as necessary. And induction hardening are 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. In this way, the manufacture of gears requires a great number of processes, including many cutting and grinding processes, so that machinability can be improved particularly for mechanical structural steels used as gear materials. It has been demanded.

  As one of the deterioration factors of machinability, for example, there is abrasive wear due to hard inclusions in the work material, which is particularly noticeable in continuous cutting. On the other hand, in intermittent cutting, such as hobbing, where the tool is not continuously in contact with the work material, there is a moment when the tool is idling, that is, the work material is not in contact with the tool, and the steel material attached to the tool The new surface is exposed to air at this time, and is rapidly oxidized because it generates heat during cutting. As a result, there is a problem that tools used for interrupted cutting are subject to oxidative wear and have a short life. 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, Patent Document 1 generates sulfide by adding B, S, and Ca to JIS steel, and precipitates fine BN using this sulfide as a precipitation nucleus, thereby improving 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 discloses a steel material in which AlN is precipitated by controlling the contents of Al and N and the ratio between the two, and the machinability during intermittent cutting is improved by the lubricating effect of AlN. . Patent Document 4 discloses a solid solution Al that improves impact characteristics by uniformly dispersing MnS by adding a large amount of Al, and improves machinability by high temperature embrittlement, and a crystal of high temperature embrittlement effect and cleavage. A steel material that secures an appropriate amount of AlN that improves machinability depending on the structure, further increases the yield ratio by fine precipitation of AlN and solute Al, while keeping solid solution N low, which degrades machinability and impact properties. Is disclosed.

JP-A-6-145890 (paragraph 0009) JP-A-3-10050 (Claim 1) Japanese Patent No. 392691 (paragraphs 0018 to 0023) JP 2008-13788 A (Claim 1, paragraph 0016)

  However, the steel materials disclosed in Patent Documents 1 and 2 correspond to turning with a carbide tool. Further, not only steel materials but also materials that do not contain Pb (Pb-free) have been demanded in recent years. 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. In addition, the steel material disclosed in Patent Document 4 is compatible with continuous cutting, and when applied to intermittent cutting, the solid solution Al activated to fix oxygen during idling of the tool is suppressed to some extent. However, since the remaining solid solution N is easily bonded to oxygen or more, the effect of fixing oxygen is lost. Furthermore, in the steel materials disclosed in Patent Documents 3 and 4, AlN is decomposed into solute Al and solute N by heating (800 to 1200 ° C.) during hot working. Thereafter, when hot working is performed, AlN is segregated at the grain boundary due to solute Al and solute N, and the grain boundary strength is remarkably deteriorated.

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

  The inventors of the present invention have a tendency to oxidize more than Fe, that is, steel for machine structure in intermittent cutting by adding and solid-solving Al to which O (oxygen) is easy to bind compared to Fe. It was decided to prevent rapid oxidation of the new surface of the tool and suppress oxidative wear of the tool. On the other hand, in order to prevent AlN from segregating during hot working and deteriorating hot ductility (hot workability) due to the addition of Al, precipitation of AlN is caused by adding Ti and bonding with N. In addition, the present inventors have found that the formed TiN prevents the coarsening of crystal grains and improves the mechanical properties.

  That is, the steel for machine structure according to claim 1 is C: 0.05 to 1.2 mass%, Si: 0.03 to 2 mass%, Mn: 0.2 to 1.8 mass%, Cr: 0 0.1 to 3% by mass, Al: 0.06 to 0.5% by mass, Ti: 0.01 to 0.1% by mass, N: 0.02% by mass or less, O: 0.003% by mass or less Furthermore, it contains at least one of Ca: 0.0005 to 0.02 mass%, Mg: 0.0001 to 0.005 mass%, and regulates P and S to 0.03 mass% or less. When the balance is composed of Fe and inevitable impurities, and the respective contents (mass%) of Cr, Al, O are expressed as [Cr], [Al], [O], (0.1 × [Cr] + [Al]) / [O] ≧ 150, and when the respective contents (mass%) of N and Ti are expressed as [N] and [Ti], [N] −0.3 × [Ti ] ≦ 0.0005 Satisfied, N solid solution amount is equal to or less than 0.0005 wt%.

  Thus, by adding Al, which has a higher oxidation tendency than Fe, it is possible to prevent the new surface of the steel for machine structural use from being rapidly oxidized in intermittent cutting. Then, by adding Ti, N (nitrogen) is combined with Ti instead of Al to suppress the formation of AlN, while the crystal grains are refined by the formed TiN. Further, by limiting the content of N with respect to Ti to a certain value or less, the solid solution amount of N is suppressed to a predetermined value or less. As a result, the effect of suppressing oxidative wear can be sufficiently exhibited without Al being bonded to the solute N during intermittent cutting. Furthermore, segregation of AlN during hot working due to AlN or solute N in steel can be prevented. On the other hand, by adding at least one of Ca and Mg, it is possible to prevent the Al oxide from causing abrasive wear as a hard inclusion. Moreover, the production | generation of a coarse oxide type inclusion can be suppressed by restrict | limiting content of O (oxygen) with respect to Cr and Al below fixed.

  In addition, the steel for machine structure according to claim 2 is characterized in that the steel for machine structure according to claim 1 further contains Mo: 1% by mass or less. By adding Mo, 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 claim 3 is characterized in that the steel for machine structure according to claim 1 or 2 further contains Nb: 0.2% by mass or less. By adding Nb, abnormal growth of crystal grains in the carburizing process can be effectively prevented.

  Further, 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 includes V: 0.5 mass% or less, Cu: 3 mass%. Hereinafter, one or more of Ni: 3% by mass or less and B: 0.005% by mass or less are contained. By adding V and B, abnormal growth of crystal grains in the carburizing process can be effectively prevented. Moreover, by adding Cu and Ni, 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 hot workability, 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.

It is a top view of the test piece in a tension test.

Hereinafter, the form which carries out steel for machine structure concerning the present invention is explained.
The steel for machine structure according to the present invention has C: 0.05 to 1.2 mass%, Si: 0.03 to 2 mass%, Mn: 0.2 to 1.8 mass%, Cr: 0.1 to 0.1 mass%. 3% by mass, Al: 0.06 to 0.5% by mass, Ti: 0.01 to 0.1% by mass, N: 0.02% by mass or less, O: 0.003% by mass or less, , Ca: 0.0005 to 0.02% by mass, Mg: 0.0001 to 0.005% by mass, one or more of them are contained, P and S are restricted to 0.03% by mass or less, and the balance is It is composed of Fe and inevitable impurities, and the N solid solution amount is 0.0005 mass% or less.

And the steel for machine structure which concerns on this invention limits the ratio with respect to O content of each content of Cr and Al which is easy to form an oxide inclusion among said each component to more than predetermined value. is there. That is, when the contents (mass%) of Cr, Al, and O are expressed by [Cr], [Al], and [O], the contents of these components are adjusted so that the following formulas are satisfied. Is.
(0.1 × [Cr] + [Al]) / [O] ≧ 150

Furthermore, in the steel for machine structure according to the present invention, among the above components, the content of N is limited to a predetermined value or less with respect to the content of Ti that is easy to bond N as compared with Al. . That is, when the contents (mass%) of N and Ti are represented by [N] and [Ti], the contents of these components are adjusted so as to satisfy the following formula.
[N] −0.3 × [Ti] ≦ 0.0005

  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: 0.05-1.2% 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, the hardness becomes excessive and the machinability, toughness, and workability deteriorate. Therefore, the C content is 1.2% by mass or less, preferably 1.0% by mass or less, and more preferably 0.8% by mass or less.

(Si: 0.03 to 2% 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 0.03% by mass or more, preferably 0.05% by mass or more, and more preferably 0.1% 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% by mass or less, preferably 1.8% by mass or less, and more preferably 1.5% by mass or less.

(Mn: 0.2 to 1.8% 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 0.2% by mass or more, preferably 0.3% by mass or more, and more preferably 0.5% 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 1.8% by mass or less, preferably 1.6% by mass or less, and more preferably 1.4% by mass or less.

(Cr: 0.1 to 3% 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, if Cr is added excessively, hardenability becomes excessive and a supercooled structure develops, coarse carbides are formed at the grain boundaries, and machinability deteriorates. Becomes excessive and the toughness decreases. Therefore, the Cr content is 3% by mass or less, preferably 2% by mass or less, and more preferably 1.6% by mass or less.

(Al: 0.06-0.5% by mass)
Al has a strong deoxidation effect and improves the internal quality of the steel for machine structural use. Further, since O is more easily bonded to Al than Fe, Al has an effect of suppressing oxidative wear of the tool during intermittent cutting. In addition, although Al is a trace amount, it combines with N in steel to form AlN, and this AlN has an effect of suppressing abnormal growth of crystal grains (abnormal grain growth) in carburizing treatment. In order to make these effects sufficient, the Al content is 0.06% by mass or more, preferably 0.1% by mass or more, and more preferably 0.15% by mass or more. On the other hand, if Al is added excessively, the proportion of alumina (Al ₂ O ₃ ) being excessively formed and becoming hard inclusions increases, and the machinability decreases, or after heat treatment (quenching) in carburizing. The amount of residual austenite (residual γ phase) increases, and sufficient hardness cannot be obtained for the carburized phase. Therefore, the Al content is 0.5% by mass or less, preferably 0.45% by mass or less, and more preferably 0.4% by mass or less.

(Ti: 0.01 to 0.1% by mass)
Ti combines with N inevitably contained in steel to reduce the amount of N solid solution in the product. Further, the formed TiN has an effect of refining crystal grains in the heat treatment, and improves the mechanical characteristics of the steel for machine structure. In order to make these effects sufficient, the Ti content is set to 0.01% by mass or more, preferably 0.02% by mass or more, and more preferably 0.03% by mass or more. Furthermore, TiN has an effect of suppressing abnormal grain growth during the carburizing process. On the other hand, when Ti is added excessively, the size of TiN becomes large and mechanical properties such as workability and machinability deteriorate. Therefore, the Ti content is 0.1% by mass or less, preferably 0.09% by mass or less, and more preferably 0.08% by mass or less.

(N: 0.02 mass% or less)
N (nitrogen) is an element that is inevitably mixed in the melting process of steel, and when coexisting with Ti or the like in the steel, it binds to this to form a nitrogen compound, or dissolves in the steel. As will be described later, when the solute N coexists with Al in the steel, it is necessary to reduce it as much as possible in order to reduce the effect of suppressing oxidative wear during intermittent cutting by Al and to deteriorate the mechanical properties. . When a large amount of N is contained in the steel, the amount of N dissolved in the steel increases accordingly. Therefore, the content is limited, and it is preferable to fix N as a nitrogen compound as much as possible. However, even if a large amount of N in the steel is a nitrogen compound, if it contains a large amount of N, it is excessively formed and its size increases, so the machinability and the effect of suppressing abnormal grain growth during carburizing treatment deteriorate. . Therefore, the N content is 0.02% by mass or less, preferably 0.018% by mass or less, and more preferably 0.015% by mass or less.

(N solid solution amount: 0.0005 mass% or less)
N dissolved in machine structural steel itself has the effect of improving the machinability. However, when coexisting with Al in the steel, it combines with the solute Al by the heat generated during cutting, so that AlN is dissolved. Form and adhere to the tool surface. As a result, the bonding force of Al to O during intermittent cutting is impaired, and the effect of suppressing oxidative wear of the tool is reduced. Further, solute N is combined with Al even during hot working, and segregates as AlN at the austenite grain boundaries, thereby significantly reducing the grain boundary strength. Further, when N is excessively dissolved, age hardening may proceed excessively and ductility and toughness may be reduced. Therefore, the N solid solution amount is reduced as much as possible to 0.0005% by mass or less, preferably 0.0004% by mass or less, and more preferably 0.0003% by mass or less. Such an N solid solution amount is controlled by satisfying the above-described limitation of the N content and the relational expression between the N content and the Ti content described later. In addition, the value of the N (nitrogen) solid solution amount in the present invention is based on JIS G 1228, and is the difference between the total nitrogen amount (N content) of the steel for machine structural use and the nitrogen amount in the nitrogen compound. Below, the method to measure each nitrogen amount in steel is demonstrated.

  The total amount of nitrogen in the steel is measured by an inert gas melting method-thermal conductivity method. In this method, a sample cut from a test steel material is put into a crucible, the sample is melted in an inert gas stream, a gas containing nitrogen is extracted, and the gas is conveyed to a thermal conductivity cell. The amount of nitrogen is determined by measuring the change in thermal conductivity.

  The amount of nitrogen in the nitrogen compound in the steel is measured by ammonia distillation separation indophenol blue absorptiometry. This method is as follows. First, an approximately 0.5 g sample cut from the test steel material is a 10% AA electrolyte solution (a non-aqueous solvent electrolyte solution that does not generate a passive film on the surface of the steel material. % Acetylacetone, 10% tetramethylammonium chloride, balance: methanol). The dissolved sample (and electrolyte solution) is filtered through a polycarbonate filter having a mesh size of 0.1 μm, and separated into an insoluble residue (nitrogen compound) and a filtrate. The insoluble residue is heated and decomposed in sulfuric acid, potassium sulfate, and pure Cu chips, and then mixed with the filtrate. The mixed solution is alkalized with sodium hydroxide and then steam distilled to absorb the distilled ammonium by dilute sulfuric acid. The solution is added with phenol, sodium hypochlorite, and sodium pentacyanonitrosyl iron (III) to form a blue complex. The absorbance of this blue complex is measured using a photometer, and the amount of nitrogen in the nitrogen compound is determined from this absorbance.

(Ca: 0.0005 to 0.02 mass%, Mg: one or more of 0.0001 to 0.005 mass%)
Since Ca and Mg each have an action of softening hard inclusions such as alumina, tool wear due to the hard inclusions is suppressed. Moreover, since Ca and Mg each have the effect | action which spheroidizes MnS inclusion, it suppresses the deterioration of toughness by this MnS inclusion. In order to make these effects sufficient, the Ca content is set to 0.0005% by mass or more, preferably 0.0007% by mass or more, and more preferably 0.001% by mass or more. Similarly, the Mg content is 0.0001% by mass or more, preferably 0.0002% by mass or more, and more preferably 0.0003% by mass or more. Ca, Mg can obtain the effect of suppressing the tool wear if only one of the specified contents is contained, and the other is not contained or contained less than the specified content. It may be. Further, both Ca and Mg may contain a specified content. On the other hand, when both Ca and Mg are added excessively, inclusions such as CaO and MgO increase, and the ductility and toughness decrease due to these inclusions. Therefore, the Ca content is 0.02 mass% or less, preferably 0.015 mass% or less, and more preferably 0.01 mass% or less. Similarly, the Mg content is 0.005% by mass or less, preferably 0.004% by mass or less, and more preferably 0.003% 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.

(O: 0.003 mass% or less)
O (oxygen) is an element inevitably mixed in the steel melting process. When the O content is excessive, coarse oxide inclusions are generated, and the machinability, ductility, toughness, and hot workability of steel are reduced by the oxide inclusions. Accordingly, the O content is 0.003% by mass or less, preferably 0.002% by mass or less, and more preferably 0.0015% by mass or less.

Among the components of the mechanical structural steel according to the present invention, Cr and Al have a tendency to oxidize more than Fe, that is, they are more likely to bond to O compared to Fe. On the other hand, a hard oxide inclusion Cr ₂ O ₃ , Al ₂ O ₃ or the like is formed. If the sum of 1/10 of the Cr content and the Al content in which O is more easily bonded than Cr is less than 150 times the O content, the effect of suppressing the above-mentioned oxidation wear cannot be obtained sufficiently, and intermittent cutting is performed. While the machinability at the time deteriorates, hard oxide inclusions are excessively formed, the abrasive wear becomes remarkable, and the machinability at the time of continuous cutting also deteriorates. Accordingly, when the contents (% by mass) of Cr, Al, and O are expressed by [Cr], [Al], and [O], the contents of these components are adjusted so as to satisfy the following formula. .
(0.1 × [Cr] + [Al]) / [O] ≧ 150

Moreover, in each component of the steel for machine structure according to the present invention, as described above, Ti is combined with N to form TiN to reduce solid solution N, and further to Al during hot working. By preferentially bonding with solid solution N, it has an effect of suppressing segregation of AlN to the grain boundary. Therefore, if the N content is excessive with respect to the Ti content, specifically, if the N content exceeds the amount of Ti that can be bound to Ti, these effects cannot be obtained sufficiently, and a part of N is dissolved. A state is formed and AlN is formed. As a result, the effect of suppressing oxidative wear during intermittent cutting is reduced, and the hot ductility is deteriorated. The amount of N that can be bonded to Ti is approximately 0.3 times the amount of Ti in terms of mass, and it can be estimated that N that is not bonded to Ti becomes solid solution N. That is, when the contents (% by mass) of N and Ti are expressed by [N] and [Ti], the contents of these components are adjusted so as to satisfy the following formula.
[N] −0.3 × [Ti] ≦ 0.0005

  The machine structural steel according to the present invention may further contain Mo: 1% by mass or less. Moreover, the steel for machine structure which concerns on this invention may further contain Nb: 0.2 mass% or less. The mechanical structural steel according to the present invention is further one of V: 0.5% by mass or less, Cu: 3% by mass or less, Ni: 3% by mass or less, and B: 0.005% by mass or less. You may contain the above.

(Mo: 1% 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. In order to obtain this effect, the Mo content is preferably 0.005% by mass or more, more preferably 0.008% by mass or more, and further preferably 0.01% by mass or more. 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% by mass or less, preferably 0.9% by mass or less, and more preferably 0.8% by mass or less.

(Nb: 0.2% by 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. Nb has the effect of preventing this abnormal grain growth. In order to obtain this effect, the Nb content is preferably 0.001% by mass or more, and more preferably 0.002% by mass or more. On the other hand, when Nb is added excessively, hard carbides are generated and mechanical properties such as workability and machinability deteriorate. Therefore, the Nb content is 0.2% by mass or less, preferably 0.15% by mass or less, and more preferably 0.1% by mass or less.

(V: 0.5 mass% or less, Cu: 3 mass% or less, Ni: 3 mass% or less, B: 0.005 mass% or less)
V and B have the effect of preventing abnormal grain growth during the carburizing process, like Nb. In order to obtain this effect, the V content is preferably 0.001% by mass or more, and more preferably 0.002% by mass or more. Similarly, the B content is preferably 0.0004% by mass or more, more preferably 0.0008% by mass or more. Further, B has an effect of improving machinability. On the other hand, when these elements are added excessively, hard carbides are generated, and mechanical properties such as workability and machinability deteriorate. Therefore, the V content is 0.5% by mass or less, preferably 0.4% by mass or less, and more preferably 0.3% by mass or less. The B content is 0.005% by mass or less, preferably 0.003% by mass or less, and more preferably 0.001% by 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 3% by mass or less, preferably 2% by mass or less, and more preferably 1% by mass or less.

  As mentioned above, although the form for implementing this invention has been described, the Example which confirmed the effect of this invention is demonstrated concretely compared with the comparative example which does not satisfy | fill the requirements of this invention below. 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 about 1100 ° C. × 1 hr, and cut to a length of about 100 mm. The cut square material was hot forged into a plate material of 30 mm thickness × 155 mm width and a round bar material of φ80 mm at about 1100 ° C. × 1 hr. The plate material was cut to a length of 100 mm, and the round bar material was cut to a length of 300 mm. These plate materials and round bar materials were normalized (after heat treatment at 900 ° C. × 2 hr and then allowed to cool), and the test material No. 1 to 70 (Examples and Comparative Examples). Relational expression of content of Cr, Al, O “(0.1 × [Cr] + [Al]) / [O]” and relational expression of content of N, Ti “[N] −0.3 × [ “Ti]” is calculated from the chemical composition and listed in Tables 1 and 2. Moreover, the amount of N solid solution measured by the inert gas melting method-thermal conductivity method and ammonia distillation separation indophenol blue absorptiometry in the sample cut out from the test material (plate material) is shown in Tables 1 and 2. It is written together. The following measurements and evaluations were performed on the prepared specimens.

[Measurement and evaluation]
(Machinability)
In order to evaluate the machinability during interrupted cutting, tool wear was observed after a simulated evaluation of hobbing by interrupted cutting 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 (CP): 1.0% target) → 900 ° C. × 90 min (CO ₂ concentration: 0.17%, CP: 0.8% target) ) → 840 ° C. × 60 min (CO ₂ concentration: 0.39%, CP: 0.8% target)

(Hot workability)
As the hot workability, the hot ductility of the test material was evaluated. The test material (round bar) was made into a test piece having the shape shown in FIG. This test piece was subjected to a tensile test at 0.01 mm / s in a state heated to 900 ° C. by a hot working reproduction test apparatus (manufactured by Fuji Denpa Kogyo Co., Ltd.). %). The gauge distance of the test piece is 15 mm which is the same as the length of the parallel part. The measured cross-sectional reduction rate is shown in Tables 1 and 2. The acceptance criterion for hot workability was a cross-section reduction rate of 40% or more.
(Cross section reduction rate) = {(Cross sectional area between gauge points) − (Cross sectional area)} / (Cross sectional area between gauge points) × 100

(Evaluation)
As shown in Table 1, the test material No. Since 1-227 and 61-70 are examples in which the content and correlation of each component are within the scope of the present invention, the amount of tool wear after the intermittent cutting test is small, and the machinability during intermittent cutting is excellent. Further, the toughness and hot workability of the transverse eye were also good.

  On the other hand, as shown in Table 2, the test material No. 28 to 60 are comparative examples in which at least one of the content and correlation of each component is outside the scope of the present invention. Specimen No. No. 28 had an excessive C content, so that machinability, transverse toughness, and hot workability deteriorated. Specimen No. In No. 29, the machinability decreased because the Si content was insufficient and the O content was excessive. On the other hand, the test material No. Since No. 30 had an excessive Si content, the effect of improving the strength by carburizing treatment was insufficient, and the toughness of the transverse line was lowered. Specimen No. No. 31 had insufficient Mn content, so the hardenability was insufficient and the toughness of the transverse eye was lowered. On the other hand, the test material No. In No. 32, the machinability was lowered due to the excessive Mn content, and the toughness of the transverse eye was lowered due to the excessive P content. Specimen No. No. 33 had an excessive S content, so machinability was improved, but the toughness and hot workability of the transverse eye were lowered.

  Specimen No. Since the Cr content of 34 was insufficient, machinability decreased. On the other hand, the test material No. Since the Cr content of 35 was excessive, machinability and transverse toughness were reduced. Specimen No. As for No. 36, since the Al content is insufficient, the machinability is lowered, and the abnormal grain growth in the carburizing process is not suppressed, and the toughness of the transverse eye is lowered. In addition, specimen No. In No. 37, the Al content itself was insufficient, and the Cr and Al contents were insufficient with respect to the O content. On the other hand, the test material No. In No. 38, the Al content was excessive, so that the effect of improving the strength by carburizing treatment was insufficient, and the toughness of the transverse eye was lowered.

  Specimen No. In No. 39, the machinability decreased due to an excessive N content, and solid solution N also increased, resulting in a decrease in the toughness and hot workability of the transverse eye. Specimen No. In No. 40, since the Ti content was insufficient (no addition), the N content was relatively excessive, so that the solid solution N was excessive and the hot workability was lowered. On the other hand, the test material No. Since No. 41 had an excessive Ti content, machinability and hot workability decreased.

  Specimen No. As for 42 and 43, since each content of Ca and Mg was insufficient (no addition), machinability fell, respectively. On the other hand, the test material No. No. 44 had an excessive Ca content, so that machinability and toughness of the transverse eye were lowered.

  Specimen No. No. 45 has an excessive Mo content, so the machinability was lowered. Specimen No. No. 46 has an excessive Nb content. Since No. 47 had an excessive V content, machinability and hot workability were reduced. Specimen No. No. 48 had an excessive B content, so that machinability was improved but hot workability was lowered.

  Specimen No. No. 49, the content of each component is within the scope of the present invention, but the machinability deteriorated because the Cr and Al contents were insufficient with respect to the O content. Specimen No. 50-60, the content of each component is within the scope of the present invention, but since the Ti content is insufficient with respect to the N content, the solid solution N becomes excessive and both have hot workability. Declined. Furthermore, the test material No. In 51-60, the improvement effect of the machinability by Al fell. On the other hand, the test material No. No. 50 has an N content in the vicinity of the upper limit within the range of the present invention, and an excessive amount with respect to the Ti content is particularly large. Therefore, machinability was improved by the action of solid solution N alone, but the toughness of the transverse eye was lowered.

Claims (4)

C: 0.05-1.2 mass%, Si: 0.03-2 mass%, Mn: 0.2-1.8 mass%, Cr: 0.1-3 mass%, Al: 0.06- 0.5% by mass, Ti: 0.01 to 0.1% by mass, N: 0.02% by mass or less, O: 0.003% by mass or less, and Ca: 0.0005 to 0.02 Mass%, Mg: containing at least one of 0.0001 to 0.005 mass%, regulating P and S to 0.03 mass% or less, the balance consisting of Fe and inevitable impurities,
When each content (mass%) of the Cr, Al, O is expressed as [Cr], [Al], [O], (0.1 × [Cr] + [Al]) / [O] ≧ 150 satisfied,
When each content (% by mass) of N and Ti is expressed as [N] and [Ti], [N] −0.3 × [Ti] ≦ 0.0005 is satisfied,
A machine structural steel, wherein the N solid solution amount is 0.0005 mass% or less.   Furthermore, Mo: 1 mass% or less is contained, Steel for machine structure of Claim 1 characterized by the above-mentioned.   Furthermore, Nb: 0.2 mass% or less is contained, Steel for machine structure of Claim 1 or Claim 2 characterized by the above-mentioned.   Furthermore, it contains 1 or more types among V: 0.5 mass% or less, Cu: 3 mass% or less, Ni: 3 mass% or less, and B: 0.005 mass% or less. The steel for machine structure of any one of thru | or 3.

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