WO2018021452A1 - Steel for machine structures - Google Patents

Abstract

A steel for machine structures is provided which has excellent machinability, rusting characteristics and hot rolling properties and from which carburized components with excellent rolling fatigue characteristics can be obtained. This steel for machine structures has a chemical composition which satisfies expression (1) and contains, in mass%, C: 0.15 to less than 0.30%, Si: 0.01-0.80%, Mn: 0.20-2.00%, P: 0.030 or less, S: 0.010-0.100%, Pb: 0.010-0.100%, Al: 0.010-0.050%, N: 0.015% or less, O: 0.0005-0.0030% and Cr: 0.50%-2.00%, the remainder being Fe and impurities. The total number of specific inclusions included in the steel that are MnS inclusions, Pb inclusions, or composite inclusions containing MnS and Pb and that have a circle equivalent diameter of greater than or equal to 5μm is greater than or equal to 40 inclusions/mm². Mn/S ≥ 8.0 (1) Here, each element in formula (1) is substituted with the amount contained (mass%) of the corresponding element.

Description

Steel for machine structure

The present invention relates to steel, and more particularly to steel for machine structural use.

¡Excellent rolling fatigue characteristics may be required for mechanical parts used for structural and power transmission of general machines and automobile parts. An example of a method for manufacturing such a machine part is as follows. Machine structural steel is hot-worked (hot forging, etc.) to produce intermediate products. Intermediate parts are machined (cutting, grinding) to produce machine parts. If necessary, surface hardening treatment may be performed on the machine part. The surface hardening process is, for example, a carburizing process. Machine structural steel for producing such machine parts is required to have not only excellent hot workability but also excellent machinability.

Machine structural steel with excellent machinability is also called free-cutting steel, and is defined in JIS G 4804 (2008) (Non-patent Document 1). Free-cutting steel increases the machinability by containing Pb.

A machine structural steel containing Pb is disclosed in, for example, Japanese Patent Laid-Open No. 2000-282172 (Patent Document 1). The steel for machine structure described in Patent Document 1 is mass%, C: 0.05 to 0.55%, Si: 0.50 to 2.5%, Mn: 0.01 to 2.00%, S : 0.005 to 0.080%, Cr: 0 to 2.0%, P: 0.035% or less, V: 0 to 0.50%, N: 0.0150% or less, Al: 0.04% Hereinafter, Ni: 0 to 2.0%, Mo: 0 to 1.5%, B: 0 to 0.01%, Bi: 0 to 0.10%, Ca: 0 to 0.05%, Pb: 0 ~ 0.12%, Ti: 0 to less than 0.04%, Zr: 0 to less than 0.04%, and Ti (%) + Zr (%): 0 to less than 0.04%, Te: 0 to 0.05%, Nd: 0 to 0.05%, Nb: 0 to 0.1%, Cu: 0 to 1.5%, Se: 0 to 0.5%, represented by the following formula The value of fn1 is 100 or less, fn represented by the following formula Value is 0 or more, satisfying the value of fn3 represented by the following formula is 3.0 or more, having a chemical composition the balance being Fe and impurities. Further, the ratio of the ferrite phase to the structure in the area ratio is 10 to 80%, and the Hv hardness is 160 to 350. Here, fn1 = 100C + 11Si + 18Mn + 32Cr + 45Mo + 6V, fn2 = −23C + Si (5-2Si) -4Mn + 104S-3Cr-9V + 10, fn3 = 3.2C + 0.8Mn + 5.2S + 0.5Cr−120N + 2.6Pb + 4.1Bi−0.001α ² + 0.13α . The element symbol in each formula indicates the content in mass% of the element, and α indicates the area ratio (%) of the ferrite phase in the structure. Patent Document 1 describes that this steel for machine structure is excellent in machinability and toughness.

JP 2000-282172 A

By the way, machining such as cutting may be performed by an automated manufacturing facility. When a machine part is manufactured by cutting a large amount of intermediate products such as several hundred or more in an automated manufacturing facility per day, excellent chip disposal is required. It is preferable that the chips discharged with the cutting are divided into small pieces and discharged. When the chips remain connected for a long time, the chips are entangled with the intermediate product, and wrinkles are likely to occur on the surface of the machine part after cutting. When chips are entangled with machine parts, it is further necessary to temporarily stop the production line in order to remove the entangled chips. In this case, unmanned manufacturing becomes difficult, and personnel assignment for monitoring is required. As described above, the chip disposability affects both the quality of the machine part and the manufacturing cost. Further, in an automated manufacturing facility, if the tool is worn much, the productivity is lowered. Therefore, the steel for machine structure is required to have high machinability such that tool wear can be suppressed and chip disposal is excellent.

In addition, there is a case where wrinkles are generated in machine parts in the cutting process using the automated manufacturing equipment. In an automated manufacturing facility, water-soluble cutting oil is used from the viewpoint of unattended operation. For this reason, mechanical parts may be generated. The wrinkle not only causes a shape error, but also causes a quality defect when plating a machine part. Furthermore, the machine parts after cutting may wait for a long time in a bucket or the like before the next process after cutting. For example, when cutting in Japan and the next process is processed in another factory in another country, a period of several days to several months may elapse after cutting until the next process is performed. Therefore, not only machinability but also a characteristic for suppressing the generation of wrinkles (hereinafter referred to as a wrinkling characteristic) is required for steel for machine structural use.

An object of the present invention is to provide a steel for machine structure that provides a machine part having excellent machinability, cracking characteristics, hot workability, and excellent rolling fatigue characteristics.

The steel for machine structural use according to the present invention is, in mass%, C: 0.15 to less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0 0.030% or less, S: 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al: 0.010 to 0.050%, N: 0.015% or less, O: 0.0. 0005 to 0.0030%, Cr: 0.50 to 2.00%, Ni: 0 to 3.50%, B: 0 to 0.0050%, V: 0 to 0.70%, Mo: 0 to 0 70%, W: 0 to 0.70%, Nb: 0 to less than 0.050%, Cu: 0 to 0.50%, Ti: 0 to 0.100%, and Ca: 0 to 0.0030 %, And the balance is composed of Fe and impurities, and has a chemical composition satisfying the formula (1). In steel, the total number of specific inclusions that are any of MnS inclusions, Pb inclusions, and composite inclusions containing MnS and Pb and having an equivalent circle diameter of 5 μm or more is 40 / mm ^2. That's it.
Mn / S ≧ 8.0 (1)
Here, the content (mass%) of the corresponding element is substituted for each element in the formula (1).

The machine structural steel according to the present invention is excellent in machinability, cracking characteristics, hot workability, and mechanical parts having excellent rolling fatigue characteristics.

FIG. 1A is a schematic diagram showing an S distribution in an observation surface obtained by EPMA analysis. FIG. 1B is a schematic diagram showing a Pb distribution in the same observation surface as that of FIG. 1A obtained by EPMA analysis. FIG. 1C is a schematic diagram of an image obtained by combining FIGS. 1A and 1B. FIG. 2 is a schematic diagram for explaining a criterion for determining whether or not adjacent inclusions are regarded as one inclusion. FIG. 3 is a cross-sectional view of the cast material. FIG. 4 is a schematic diagram of a cutting test machine for explaining a cutting test. FIG. 5A is a perspective view of chips. FIG. 5B is a plan photograph of chips. FIG. 6 is a front view and a side view of a rolling fatigue test piece used in the rolling fatigue test. FIG. 7 is a schematic diagram of a thrust type rolling fatigue tester for explaining the rolling fatigue test.

The present inventors investigated and examined the machinability, the cracking characteristics, and the hot workability of the steel for machine structures. As a result, in mass%, C: 0.15 to less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0.030% or less, S : 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al: 0.010 to 0.050%, N: 0.015% or less, O: 0.0005 to 0.0030% Cr: 0.50 to 2.00%, Ni: 0 to 3.50%, B: 0 to 0.0050%, V: 0 to 0.70%, Mo: 0 to 0.70%, W: Contains 0 to 0.70%, Nb: 0 to less than 0.050%, Cu: 0 to 0.50%, Ti: 0 to 0.100%, and Ca: 0 to 0.0030%, the balance If it is a steel for mechanical structures having a chemical composition consisting of Fe and impurities, excellent machinability and excellent hot workability can be obtained, and excellent rolling fatigue characteristics after carburizing treatment. Thought that there may be obtained.

Mn in steel combines with S to produce MnS. MnS is divided into MnS inclusions and MnS precipitates depending on the production process. MnS inclusions crystallize in the molten steel before solidification. On the other hand, MnS precipitates precipitate in the steel after solidification. MnS inclusions are generated in the molten steel. Therefore, the size of the MnS inclusion tends to be larger than the MnS precipitate generated after solidification.

On the other hand, Pb in steel hardly dissolves in steel and exists as Pb inclusions (Pb grains). Both MnS inclusions and Pb inclusions enhance the machinability of steel.

Further, when Mn and Pb are present in the steel, Mn and Pb are not only MnS inclusions and Pb inclusions mentioned above, but also composite inclusions containing MnS and Pb (hereinafter simply referred to as “composite inclusions”). Formed). The composite inclusions contain MnS and Pb, and the balance means inclusions made of impurities. More specifically, the composite inclusion may be configured such that MnS and Pb are adjacent to each other, or Pb may be dissolved in MnS to form a composite inclusion. In this specification, “MnS inclusions”, “Pb inclusions”, and “composite inclusions” are specified by the method described in the item “Method for measuring number TN and RA” described later. In the present specification, the MnS inclusion is an inclusion containing Mn and S and not containing Pb. The Pb inclusion is an inclusion made of Pb and impurities and not containing Mn. The composite inclusion is an inclusion containing Mn, S, and Pb.

MnS inclusions are known as inclusions that enhance machinability. On the other hand, the melting point of the Pb inclusion is lower than the melting point of the MnS inclusion. Therefore, the Pb inclusion exhibits a lubricating action during cutting, and as a result, improves the machinability of the steel.

Furthermore, it is considered that the composite inclusions enhance the machinability of steel more than the MnS inclusions and the Pb inclusions alone. When a crack occurs around the composite inclusion, liquefied Pb enters the opened crack. Thereby, progress of a crack is accelerated | stimulated and machinability increases. Therefore, not only MnS inclusions and Pb inclusions are generated, but if composite inclusions are generated, machinability is further enhanced.

The mechanism by which complex inclusions are generated is considered as follows. Pb is more mobile in the liquid phase than in the solid phase. Therefore, the composite inclusion can hardly be generated from the MnS precipitate generated after the solidification of the steel, and is generated when Pb adheres to the MnS inclusion generated in the molten steel before the solidification. Therefore, in order to generate a large amount of composite inclusions, it is desirable to generate a large amount of MnS inclusions in the molten steel rather than to generate MnS precipitates after solidification.

As described above, in order to improve the machinability of steel, a large number of MnS inclusions, Pb inclusions, and composite inclusions may be generated. As described above, MnS inclusions are produced in molten steel by crystallization. Furthermore, as described above, the more complex inclusions are generated the more MnS inclusions. Therefore, it is considered that the machinability of the steel increases if a large amount of MnS inclusions are crystallized in the molten steel.

On the other hand, the steel for machine structural use containing MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions is prone to wrinkling. However, no detailed examination has been made so far on the mechanism of the mechanical structural steel. Therefore, the present inventors conducted investigations and studies on the mechanism of the germination. As a result, the present inventors obtained the following knowledge.

MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions themselves are the starting point of soot. Here, the easiness of generation is MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions rather than the size of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions. Depends on the total number of Specifically, as the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions increases, the steel is more likely to start. Based on the above findings, the present inventors have determined the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions in order to obtain excellent machinability and suppress rusting. We thought that it was effective to reduce. Therefore, the present inventors examined a method for reducing the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions.

As described above, MnS inclusions generated by crystallization in molten steel are likely to grow (coarse) in molten steel. Therefore, the MnS inclusion is larger in size than the MnS precipitate produced by precipitation in the solidified steel. That is, the MnS precipitate precipitates finer than the MnS inclusion. Therefore, in a steel with a constant Mn content and S content, when assuming the case where MnS inclusions are crystallized and the case where MnS precipitates are precipitated, rather than the number of MnS inclusions generated by crystallization, The number of MnS precipitates generated by precipitation is significantly increased. Therefore, in order to enhance the cracking characteristics of steel, MnS inclusions may be crystallized and grown (coarse) in the molten steel to suppress the precipitation of MnS precipitates.

By crystallizing and growing MnS inclusions in molten steel, precipitation of MnS precipitates is suppressed, and as a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions is reduced. For this purpose, the Mn content may be sufficiently increased as compared with the S content. If the Mn content is sufficiently higher than the S content, coarse MnS inclusions are easily generated in the molten steel. In this case, since S is consumed for crystallization of coarse MnS inclusions, the amount of solute S in the steel after solidification becomes low. Therefore, precipitation of MnS precipitates can be suppressed, and the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions can be reduced. As a result, excellent firing characteristics can be obtained.

Specifically, the Mn content and the S content satisfy the following formula (1).
Mn / S ≧ 8.0 (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

Defined as F1 = Mn / S. If F1 is less than 8.0, MnS inclusions are not easily crystallized in the molten steel. Therefore, the amount of solute S in the steel after solidification cannot be sufficiently reduced, and many fine MnS precipitates are produced after solidification. In this case, since the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions cannot be reduced, the cracking characteristics of the steel are deteriorated. On the other hand, if F1 is 8.0 or more, the Mn content is sufficiently higher than the S content. In this case, by using an appropriate manufacturing method, MnS inclusions are sufficiently crystallized and grown in the molten steel. As a result, the amount of dissolved S in the steel after solidification is sufficiently reduced, and precipitation of MnS precipitates in the steel after solidification can be suppressed. As a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions can be sufficiently reduced, and the steel firing characteristics are enhanced.

Here, inclusions that are any of MnS inclusions, Pb inclusions, and composite inclusions and having an equivalent circle diameter of 5 μm or more are defined as “specific inclusions”. In this specification, the equivalent circle diameter means the diameter of a circle when the area of inclusions or precipitates observed in the microstructure observation is converted into a circle having the same area. In this case, in the present embodiment, the total number of specific inclusions is 40 pieces / mm ² or more in the mechanical structural steel having the above chemical composition and satisfying the formula (1).

If the number of specific inclusions in the steel is 40 pieces / mm ² or more, coarse MnS inclusions are sufficiently crystallized, and generation of MnS precipitates can be suppressed. As a result, it is possible to sufficiently reduce the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions, which are the starting points of rusting. Therefore, it is possible to achieve both excellent machinability and excellent cracking characteristics. On the other hand, if the number of specific inclusions in the steel is less than 40 / mm ² , MnS inclusions are not sufficiently crystallized, and a large number of MnS precipitates are generated. As a result, generation of MnS precipitates can be suppressed. As a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions that are the starting points of the cracking cannot be sufficiently reduced. As a result, although excellent machinability can be obtained, sufficient cracking characteristics cannot be obtained.

The steel for machine structural use according to the present embodiment completed based on the above knowledge is, in mass%, C: 0.15 to less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20. ~ 2.00%, P: 0.030% or less, S: 0.010 ~ 0.100%, Pb: 0.010 ~ 0.100%, Al: 0.010 ~ 0.050%, N: 0 .015% or less, O: 0.0005 to 0.0030%, Cr: 0.50 to 2.00%, Ni: 0 to 3.50%, B: 0 to 0.0050%, V: 0 to 0 70%, Mo: 0 to 0.70%, W: 0 to 0.70%, Nb: 0 to less than 0.050%, Cu: 0 to 0.50%, Ti: 0 to 0.100%, And Ca: 0 to 0.0030% is contained, the balance is made of Fe and impurities, and has a chemical composition satisfying the formula (1). In steel, the total number of specific inclusions that are any of MnS inclusions, Pb inclusions, and composite inclusions containing MnS and Pb and having an equivalent circle diameter of 5 μm or more is 40 / mm ^2. That's it.
Mn / S ≧ 8.0 (1)
Here, the content (mass%) of the corresponding element is substituted for each element in the formula (1).

The chemical composition of the mechanical structural steel is as follows: Ni: 0.02 to 3.50%, B: 0.0005 to 0.0050%, V: 0.05 to 0.70%, Mo: 0.05 to 0 .70%, W: 0.05 to 0.70%, Nb: 0.001 to less than 0.050%, Cu: 0.05 to 0.50%, and Ti: 0.003 to 0.100% You may contain 1 type, or 2 or more types selected from the group which consists of.

The chemical composition of the steel for machine structural use may contain Ca: 0.0001 to 0.0030%.

In the machine structural steel, the number ratio of the composite inclusions to the specific inclusions may be 40% or more.

Hereinafter, the steel for machine structure of the present embodiment will be described in detail. “%” In the chemical composition means mass% unless otherwise specified.

[Chemical composition]
The chemical composition of the steel for machine structural use of this embodiment contains the following elements.

C: 0.15 to less than 0.30% Carbon (C) increases the strength of steel. When parts are manufactured using steel for machine structure, carburizing treatment may be performed after forging the steel for machine structure. In this case, C increases the strength of the steel surface layer. If the C content is less than 0.15%, the steel cannot provide sufficient strength. In soft steel, chips are connected at the time of cutting, and may wrap around tools and materials and break them. If the C content is less than 0.15%, the cutting resistance of the steel is further increased. On the other hand, if the C content is 0.30% or more, the hardness of the core part of the part after carburizing treatment increases, and the balance of toughness with respect to the strength of the surface layer decreases. Therefore, the C content is 0.15 to less than 0.30%. The minimum with preferable C content is 0.16%, More preferably, it is 0.18%. The upper limit with preferable C content is 0.25%, More preferably, it is 0.23%.

Si: 0.01 to 0.80%
Silicon (Si) deoxidizes steel. In the deoxidation treatment, Si modifies the oxide by adding Si after adding Mn. Specifically, Si added to molten steel modifies an oxide mainly composed of Mn into an oxide mainly composed of Si. After adding Si, by adding Al, a composite oxide containing Si and Al is generated in the steel. The composite oxide serves as a nucleus from which MnS inclusions crystallize. Therefore, the composite oxide enhances the glazing characteristics of the steel. Si further increases temper softening resistance and strength. If the Si content is less than 0.01%, the above effect cannot be obtained.

On the other hand, Si is a ferrite forming element. If the Si content exceeds 0.80%, the steel surface layer may be decarburized. If the Si content exceeds 0.80%, the ferrite fraction may increase and the strength may decrease. Therefore, the Si content is 0.01 to 0.80%. The minimum with preferable Si content for raising temper softening resistance is 0.10%, More preferably, it is 0.20%. The upper limit with preferable Si content for suppressing a ferrite fraction is 0.70%, More preferably, it is 0.50%.

Mn: 0.20 to 2.00%
Manganese (Mn) generates MnS inclusions and composite inclusions containing MnS and Pb, and improves the machinability of steel.

Mn further deoxidizes steel. The deoxidizing power of Mn is weak compared to Si and Al. Therefore, a large amount of Mn may be contained. When no other strong deoxidizing element is present in the molten steel, an oxide mainly composed of Mn is formed in the molten steel. Thereafter, when another strong deoxidizing element (Si, Al) is added to the molten steel, Mn in the oxide is discharged into the molten steel and the oxide is modified. Hereinafter, the modified oxide is referred to as a composite oxide. Mn discharged from the oxide into the molten steel combines with S to form MnS inclusions. In addition, the composite oxide produced | generated by modification | reformation of an oxide tends to become the nucleus which MnS inclusion crystallizes. Therefore, when a composite oxide is produced, crystallization of MnS inclusions is promoted. Further, the MnS inclusions generated by crystallization are likely to form composite inclusions.

When the Mn content is less than 0.20%, MnS inclusions are not easily crystallized. Therefore, a large number of MnS precipitates are generated in the solidified steel. In this case, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions increases. As a result, the galling characteristics of the steel are reduced. On the other hand, if the Mn content exceeds 2.00%, the hardenability of the steel becomes too high, and as a result, the hardness of the steel becomes too high. In this case, the machinability of steel decreases. In this case, the hot workability of the steel further decreases. Therefore, the Mn content is 0.20 to 2.00%. A preferable lower limit of the Mn content is 0.50%. The upper limit with preferable Mn content is 1.50%, More preferably, it is 1.20%.

P: 0.030% or less Phosphorus (P) is unavoidably contained. P embrittles steel and improves machinability. On the other hand, if the P content exceeds 0.030%, the hot ductility decreases. In this case, productivity is reduced, such as the occurrence of rolling wrinkles. Therefore, the P content is 0.030% or less. The minimum with preferable P content for improving machinability is 0.005%. In this case, machinability, in particular, chip disposal is improved. The upper limit with preferable P content is 0.015%.

S: 0.010 to 0.100%
Sulfur (S) generates MnS in steel and improves machinability. In particular, MnS suppresses tool wear. If the S content is less than 0.010%, MnS does not crystallize sufficiently, and composite inclusions containing MnS and Pb are difficult to generate. As a result, the sprout characteristics are degraded. On the other hand, if the S content exceeds 0.100%, S segregates at the grain boundaries, the steel becomes brittle, and the hot workability of the steel decreases. Accordingly, the S content is 0.010 to 0.100%. Among machinability and mechanical properties, the preferred lower limit of the S content when giving priority to mechanical properties is 0.015%, and the preferred upper limit is 0.030%. The preferable lower limit of the S content when priority is given to machinability is 0.030%, and the preferable upper limit is 0.050%.

Pb: 0.010 to 0.100%
Lead (Pb) alone generates Pb inclusions (Pb grains) and improves the machinability of steel. Furthermore, Pb combines with MnS inclusions to form composite inclusions, which enhances the machinability of steel, and in particular improves chip disposal. If the Pb content is less than 0.010%, the above effect cannot be obtained. On the other hand, if the Pb content exceeds 0.100%, the machinability increases, but the steel becomes brittle. As a result, the hot workability of the steel is reduced. If the Pb content exceeds 0.100%, the Pb inclusions are excessively increased, so that the galling property of the steel is deteriorated. Therefore, the Pb content is 0.010 to 0.100%. The preferable lower limit of the Pb content for promoting the formation of composite inclusions and enhancing the machinability is 0.020%, more preferably 0.025%. The upper limit with preferable Pb content for improving the glazing property is 0.050%.

Al: 0.010 to 0.050%
Aluminum (Al) deoxidizes steel. In the steel for machine structure according to the present invention, deoxidation with Al kill is performed in order to suppress the formation of vacancies and surface defects during solidification. As will be described later, when deoxidation is performed by adding Al after Mn and Si in the molten steel, the oxide in the steel is modified to produce a composite oxide containing Si and Al. The composite oxide tends to become a crystallization nucleus of MnS inclusions. Therefore, MnS inclusions disperse and crystallize, grow and coarsen easily, and complex inclusions containing MnS and Pb are easily generated. In this case, the machinability of the steel is increased. When the MnS inclusions are dispersed and crystallized, the precipitation of fine MnS precipitates is further suppressed. In this case, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions increases. As a result, the glazing characteristics of the steel are enhanced. Al further combines with N to form AlN, and suppresses austenite grain coarsening in various heat treatments. If the Al content is less than 0.010%, the above effect cannot be obtained.

On the other hand, if the Al content exceeds 0.050%, a coarse composite oxide is likely to be generated. When coarse complex oxide is produced in steel, surface flaws are likely to occur in the steel. When coarse complex oxide is formed in steel, the fatigue strength of the steel is further reduced. If the Al content exceeds 0.050%, deoxidation proceeds excessively, and the amount of oxygen in the molten steel decreases. In this case, MnS inclusions are not easily formed, and the machinability (particularly, tool wear suppression) of steel is reduced. In this case, furthermore, it becomes difficult to produce composite inclusions in which Pb is bonded to MnS inclusions, and many Pb inclusions remain alone in the steel. As a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions increases, and the firing characteristics deteriorate. Therefore, the Al content is 0.010 to 0.050%. The preferable lower limit of the Al content for further obtaining the effect of suppressing the coarsening of crystal grains due to the generation of AlN is 0.015%, and more preferably 0.020%. The upper limit with preferable Al content is 0.035%. The Al content referred to in this specification means the content of acid-soluble Al (sol. Al).

N: 0.015% or less Nitrogen (N) is inevitably contained. N combines with Al to form AlN, suppresses coarsening of austenite grains during heat treatment, and increases the strength of the steel. On the other hand, if the N content exceeds 0.015%, the cutting resistance of the steel increases and the machinability decreases. If N content exceeds 0.015%, hot workability will fall further. Therefore, the N content is 0.015% or less. The minimum with preferable N content is 0.002%, More preferably, it is 0.004%. The upper limit with preferable N content is 0.012%, More preferably, it is 0.008%. As used herein, the N content means the total N (tN) content.

O: 0.0005 to 0.0030%
Oxygen (O) is included not only in the oxide but also in MnS inclusions. O produces | generates the complex oxide used as the crystallization nucleus of a MnS inclusion. If the O content is less than 0.0005%, the amount of complex oxide produced is insufficient, and MnS inclusions are difficult to crystallize in the molten steel. In this case, the machinability of steel decreases. In this case, a large number of fine MnS precipitates are further formed after solidification. As a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions increases, and the firing characteristics deteriorate. If the O content exceeds 0.0030%, coarse alumina-based oxides are generated, and the wear of the cutting tool is promoted, so that the machinability of the steel is lowered. Therefore, the O content is 0.0005 to 0.0030%. A preferable lower limit of the O content for further enhancing the machinability of the steel and the cracking characteristics of the steel is 0.0007%, more preferably 0.0010%. The upper limit with preferable O content is 0.0025%, More preferably, it is 0.0020%. As used herein, the O content means the total oxygen (t—O) content.

Cr: 0.50 to 2.00%
Chromium (Cr) dissolves in the steel to increase the hardenability and temper softening resistance of the steel and increase the strength of the steel. As a result, the rolling fatigue characteristics of the steel are enhanced. Further, when the carburizing treatment is performed as the surface hardening treatment, Cr deepens the hardened layer depth. If the Cr content is less than 0.50%, the above effect cannot be obtained. On the other hand, if the Cr content exceeds 2.00%, the hardenability becomes too high, and a supercooled structure (martensite) is generated during cooling, and the steel becomes too hard. In this case, the machinability of steel decreases. If the Cr content exceeds 2.00%, the austenite may be stabilized even at a low temperature, and the steel may become brittle. Therefore, the Cr content is 0.50 to 2.00%. The minimum with preferable Cr content is 0.70%, More preferably, it is 0.90%. The upper limit with preferable Cr content is 1.80%, More preferably, it is 1.60%.

The remainder of the chemical composition of the machine structural steel according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from ore as a raw material, scrap, or the manufacturing environment when industrially producing steel for machine structural use, and have an adverse effect on the steel for machine structural use of the present embodiment. It means that it is allowed in the range that does not give.

[About optional elements]

The chemical composition of the steel for machine structure of the present embodiment may further contain one or more selected from the group consisting of Ni, B, V, Mo, W, Nb, Cu, and Ti. .

Ni: 0 to 3.50%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni dissolves in the steel to increase the hardenability of the steel and increase the strength of the steel. Ni further increases the ductility of the matrix. Ni further increases the toughness of the steel. Ni further enhances the corrosion resistance of the steel. If Ni is contained even a little, the above effect can be obtained to some extent. On the other hand, if the Ni content exceeds 3.50%, a large amount of retained austenite remains. In this case, part of the retained austenite is transformed into martensite due to the processing-induced transformation, and the ductility of the steel is lowered. Therefore, the Ni content is 0 to 3.50%.

The preferable lower limit of the Ni content for stably obtaining the above effect is 0.02%, more preferably 0.05%. The upper limit with preferable Ni content for further suppressing a retained austenite is 2.50%, More preferably, it is 2.00%. When giving priority to toughness, the preferable lower limit of the Ni content is 0.20%. Ni detoxifies Cu and increases toughness. When steel contains Cu, the minimum with preferable Ni content is more than Cu content.

B: 0 to 0.0050%
Boron (B) is an optional element and may not be contained. When contained, B increases the hardenability of the steel and increases the strength of the steel. B further suppresses segregation of P and S to the grain boundary, which lowers toughness, and improves fracture characteristics. If B is contained even a little, the above effect can be obtained to some extent. On the other hand, if the B content exceeds 0.0050%, a large amount of BN is generated and the steel becomes brittle. Therefore, the B content is 0 to 0.0050%. The preferable lower limit of the B content when Ti or Nb which is a nitride-forming element is contained is 0.0005%. The upper limit with preferable B content is 0.0020%.

V: 0 to 0.70%
Vanadium (V) is an optional element and may not be contained. When contained, V precipitates as V carbide, V nitride, or V carbonitride during tempering and nitriding, and increases the strength of the steel. V precipitates (V carbide, V nitride and V carbonitride) further suppress the austenite grain coarsening and increase the toughness of the steel. V further dissolves in the steel and increases the temper softening resistance of the steel. If V is contained even a little, the above effect can be obtained to some extent.

On the other hand, V content if it exceeds 0.70%, V precipitates to produce even _three or more points _A. A V precipitates generated at ₃ points or more are not easily dissolved in steel and remain in the steel as undissolved precipitates. When undissolved precipitates remain, the amount of solid solution V decreases. Therefore, the tempering softening resistance of steel falls. In the case where undissolved precipitates remain, fine V precipitates are hardly deposited by subsequent heat treatment. In this case, the strength of the steel decreases. Therefore, the V content is 0 to 0.70%. The minimum with preferable V content for acquiring the said effect stably is 0.05%, More preferably, it is 0.10%. The upper limit with preferable V content is 0.50%, More preferably, it is 0.30%.

Mo: 0 to 0.70%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo precipitates as Mo carbide in heat treatment at a low temperature of A ₁ point or less such as tempering or nitriding. Therefore, the strength and temper softening resistance of the steel are increased. Mo further dissolves in the steel to enhance the hardenability of the steel. If Mo is contained even a little, the above effect can be obtained to some extent. On the other hand, if the Mo content exceeds 0.70%, the hardenability of the steel becomes too high. In this case, a supercooled structure is likely to be generated by rolling, softening heat treatment before wire drawing, or the like. Therefore, the Mo content is 0 to 0.70%.

The preferable lower limit of the Mo content for stably obtaining the above effect is 0.05%, more preferably 0.10%, and still more preferably 0.15%. A preferable upper limit of the Mo content for stably obtaining ferrite, pearlite, and bainite in the steel microstructure is 0.40%, and more preferably 0.30%.

W: 0 to 0.70%
Tungsten (W) is an optional element and may not be contained. When contained, W precipitates as W carbide in the steel, increasing the strength and temper softening resistance of the steel. W carbide, generates at a low temperature of less than _three points A. Therefore, unlike V, Nb, Ti, etc., W hardly generates undissolved precipitates. As a result, W carbide increases the strength and temper softening resistance of the steel by precipitation strengthening. W further dissolves in the steel to increase the hardenability of the steel and increase the strength of the steel. If W is contained even a little, the above effect can be obtained to some extent.

On the other hand, if the W content exceeds 0.70%, a supercooled structure is likely to be generated, and the hot workability of steel decreases. Therefore, the W content is 0 to 0.70%. A preferable lower limit of the W content for stably increasing the temper softening resistance of the steel is 0.05%, more preferably 0.10%. A preferable upper limit of the W content for stably obtaining ferrite, pearlite, and bainite in the steel microstructure is 0.40%, and more preferably 0.30%.

W and Mo hardly generate nitrides. Therefore, these elements can increase the temper softening resistance of steel without being affected by the N content. The preferred total content of W and Mo for obtaining high temper softening resistance is 0.10 to 0.30%.

Nb: 0 to less than 0.050% Niobium (Nb) is an optional element and may not be contained. When Nb is contained, Nb produces Nb nitride, Nb carbide, or Nb carbonitride, and suppresses austenite grain coarsening during quenching and normalization. Nb further increases the strength of the steel by precipitation strengthening. If Nb is contained even a little, the above effect can be obtained to some extent. On the other hand, if the Nb content exceeds 0.050%, undissolved precipitates are generated and the toughness of the steel is lowered. If the Nb content exceeds 0.050%, a supercooled structure is likely to be generated, and the hot workability of steel is reduced. Therefore, the Nb content is 0 to less than 0.050%. The minimum with preferable Nb content for acquiring the said effect stably is 0.001%, More preferably, it is 0.005%. The upper limit with preferable Nb content is 0.030%, More preferably, it is 0.015%.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be contained. When contained, Cu prevents decarburization. Cu further enhances corrosion resistance like Ni. If Cu is contained even a little, the above effect can be obtained to some extent. On the other hand, if the Cu content exceeds 0.50%, the steel becomes embrittled and rolls are likely to occur. Therefore, the Cu content is 0 to 0.50%. The minimum with preferable Cu content for acquiring the said effect stably is 0.05%, More preferably, it is 0.10%. When Cu is contained in an amount of 0.30% or more, hot ductility can be maintained if the Ni content is higher than the Cu content.

Ti: 0 to 0.100%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti produces nitrides, carbides, or carbonitrides, and suppresses austenite grain coarsening during quenching and normalization. Ti further increases the strength of the steel by precipitation strengthening. Ti further deoxidizes the steel. When Ti further contains B, it combines with solute N to maintain the amount of solute B. In this case, hardenability increases. If Ti is contained even a little, the above effect can be obtained to some extent.

On the other hand, since Ti generates the nitrides and sulfides, it affects MnS inclusions and composite inclusions. Specifically, when the Ti content exceeds 0.100%, the crystallization amount of MnS inclusions decreases and the generation of composite inclusions also decreases. In this case, the cracking characteristics of the steel are reduced. If the Ti content is too high, further, nitrides and sulfides are generated and the fatigue strength decreases. Therefore, the Ti content is 0 to 0.100%. A preferable lower limit of the Ti content for effectively obtaining the above effect is 0.003%. In particular, when B is contained, the preferable lower limit of the Ti content for reducing the solid solution N is 0.005%. The upper limit with preferable Ti content for improving corrosion resistance is 0.090%, More preferably, it is 0.085%.

The mechanical structural steel of this embodiment may further contain Ca.

Ca: 0 to 0.0030%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca generates CaS or (Mn, Ca) S to spheroidize the MnS inclusions and reduce the amount of tool wear. As a result, the machinability of the steel is increased. If Ca is contained even a little, the above effect can be obtained to some extent. On the other hand, if the Ca content exceeds 0.0030%, the oxide inclusions become coarse and the fatigue strength of the steel decreases. Therefore, the Ca content is 0 to 0.0030%. A preferable lower limit of the Ca content for further improving the machinability is 0.0001%. When giving priority to fatigue strength over machinability, the preferable upper limit of Ca content is 0.0015%, more preferably 0.0003%.

[Regarding Formula (1)]
The chemical composition of the machine structural steel of this embodiment further satisfies the formula (1).
Mn / S ≧ 8.0 (1)
Here, the content (mass%) of the corresponding element is substituted for each element in the formula (1).

Defined as F1 = Mn / S. F1 means Mn content with respect to S content. If F1 is less than 8.0, MnS inclusions are not sufficiently crystallized. Therefore, the amount of solute S in the steel after solidification cannot be sufficiently reduced, and many fine MnS precipitates are produced after solidification. In this case, since the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions cannot be reduced, the galling property of steel is deteriorated. When the amount of solid solution S in the steel after solidification cannot be reduced sufficiently, the solid solution S after solidification remains at the grain boundaries. As a result, the hot workability of steel may be reduced.

On the other hand, if F1 is 8.0 or more, the Mn content is sufficiently higher than the S content. In this case, MnS inclusions are sufficiently crystallized and grow in the molten steel. As a result, the amount of dissolved S in the steel after solidification is sufficiently reduced, and precipitation of MnS precipitates in the steel after solidification can be suppressed. Therefore, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions in the steel can be sufficiently reduced, and the firing characteristics of the steel are enhanced. A preferable lower limit of F1 for enhancing the cracking characteristics of steel is 10.0, and more preferably 20.0.

[About the microstructure of steel]
The microstructure of the mechanical structural steel according to the present invention is mainly composed of ferrite, pearlite, and bainite. Specifically, the total area ratio of ferrite, pearlite, and bainite in the microstructure of the mechanical structural steel having the above chemical composition is 99% or more.

The total area ratio of ferrite, pearlite, and bainite in the microstructure can be measured by the following method. A sample is taken from the machine structural steel. For example, when the machine structural steel is a steel bar or a wire rod, a sample is taken from the central portion of the radius R (hereinafter referred to as R / 2 portion) connecting the surface and the central axis in the cross section (surface perpendicular to the axial direction). Collect. Of the cross section (surface) of the R / 2 part sample, the surface perpendicular to the central axis of the machine structural steel is taken as the observation surface. After the observation surface is polished, it is etched with 3% nitric acid alcohol (nitral etchant). The etched observation surface is observed with a 200 × optical microscope, and photographic images with arbitrary five fields of view are generated.

In each field of view, each phase such as ferrite, pearlite, and bainite has a different contrast for each phase. Therefore, each phase is specified based on the contrast. Among the identified phases, the total area (μm ² ) of ferrite, pearlite, and bainite in each visual field is obtained. The total area in each field is summed for all fields (5 fields), and the ratio to the total area of all fields is determined. The obtained ratio is defined as the total area ratio (%) of ferrite, pearlite, and bainite.

[Number of specific inclusions TN]
The steel for machine structural use according to the present invention is one of MnS inclusions, Pb inclusions, and composite inclusions containing MnS and Pb in steel, and an inclusion having an equivalent circle diameter of 5 μm or more ( That is, the total number TN of specific inclusions is 40 / mm ² or more.

When the number of specific inclusions TN is 40 / mm ² or more, coarse MnS inclusions having an equivalent circle diameter of 5 μm or more are sufficiently crystallized. As a result, MnS inclusions, MnS precipitates, Pb inclusions The total number of objects and composite inclusions can be sufficiently reduced. Therefore, it is possible to achieve both excellent machinability and excellent cracking characteristics. On the other hand, if the number TN of specific inclusions in the steel is less than 40 / mm ² , coarse MnS inclusions having an equivalent circle diameter of 5 μm or more are not sufficiently crystallized, and as a result, MnS inclusions are obtained. The total number of MnS precipitates, Pb inclusions, and composite inclusions cannot be sufficiently reduced. For this reason, sufficient firing characteristics cannot be obtained. The minimum with the preferable number TN of specific inclusions is 80 pieces / mm < ² >, More preferably, it is 150 pieces / mm < ² >. A preferable upper limit of the number TN of specific inclusions is 300 / mm ² . In addition, although the upper limit of the circle equivalent diameter of a specific inclusion is not specifically limited, For example, it is 200 micrometers.

[Ratio of the number of composite inclusions among specific inclusions (composite ratio) RA]
Preferably, circle the total number of equivalent diameter of the composite inclusions is 5μm or more (number / mm ^2), the ratio of the number (pieces / mm ²⁾ for the particular inclusions (hereinafter, also referred to as "composite ratio") RA is 40% or more.

As described above, as the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions increases, the steel is likely to start. Here, as the MnS inclusions and the Pb inclusions generate more complex inclusions, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions can be reduced. In particular, the total number of Pb inclusions in the steel can be reduced. In particular, Pb inclusions tend to deteriorate the glazing characteristics. If the composite ratio is 40% or more, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions can be reduced, and the number of Pb inclusions present alone can also be reduced. As a result, the starting characteristics of the steel are further enhanced. Therefore, the composite ratio RA is preferably 40% or more. In this case, the cracking characteristics of the steel can be further enhanced. The minimum with more preferable composite ratio RA is 60%, More preferably, it is 75%.

[Measurement Method of Number of Specific Inclusions TN and Composite Ratio RA]
The number TN of specific inclusions and the composite ratio RA can be measured by the following method. A sample is taken from the machine structural steel in the manner described above. 20 fields of view are randomly observed at a magnification of 1000 times using a scanning electron microscope (SEM) on the cross section (surface) of the R / 2 part sample. In each field of view (referred to as an observation surface), specific inclusions (any of MnS inclusions, Pb inclusions, and composite inclusions with an equivalent circle diameter of 5 μm or more) are specified. Specific inclusions and other inclusions can be distinguished by contrast. Furthermore, among the specific inclusions, MnS inclusions, Pb inclusions, and composite inclusions are specified by the following methods, respectively.

At each observation surface, an image of the S distribution and the Pb distribution in the observation surface is obtained by a wavelength dispersive X-ray analyzer (EPMA). FIG. 1A is a schematic diagram showing the S distribution in the observation surface obtained by EPMA analysis, and FIG. 1B is a schematic diagram showing the Pb distribution in the same observation surface obtained by EPMA analysis as in FIG. 1A. is there.

Numeral 10 in FIG. 1A is an area where S exists. Since S exists almost as MnS, it can be considered that MnS exists in the code | symbol 10 in FIG. 1A. Reference numeral 20 in FIG. 1B is an area where Pb exists.

As shown in FIG. 1B, Pb may be divided by rolling or the like and arranged in the rolling direction as shown by reference numeral 20A. The same applies to S. As shown in FIG. 2, in the image obtained by the EPMA analysis, when all the adjacent inclusions IN have an equivalent circle diameter of 5 μm or more, if the interval D between the adjacent inclusions IN is within 10 μm, these The inclusion IN is regarded as one inclusion. As described above, the equivalent circle diameter means the diameter of a circle when the area of each inclusion or each precipitate is converted into a circle having the same area. Even in the inclusion group defined as one inclusion, the equivalent circle diameter is the diameter of the same circle as the total area of the inclusion group.

FIG. 1C is an image obtained by synthesizing FIG. 1B with FIG. 1A. Referring to FIG. 1C, when Pb inclusion 20 overlaps with MnS inclusion 10, the inclusion is recognized as composite inclusion 30. On the other hand, referring to FIG. 1C, when MnS inclusion 10 and Pb inclusion 20 do not overlap (region A1, region A2, etc. in FIG. 1C), these inclusions are MnS inclusion 10 and Pb inclusion. 20 is specified.

By the above method, MnS inclusions, Pb inclusions, and composite inclusions are specified using a scanning microscope and EPMA. The area of each specified inclusion is obtained, and the diameter of a circle having the same area is obtained as the equivalent circle diameter (μm) of each inclusion.

Among each inclusion, a specific inclusion having an equivalent circle diameter of 5 μm or more is specified. Obtains the total number of the identified specific inclusions (number at 20 fields), is converted into 1 mm ² number per TN (pieces / mm ^2). The number TN of specific inclusions is obtained by the above method. Further, among the specified inclusions, the number MN (pieces / mm ² ) of composite inclusions having an equivalent circle diameter of 5 μm or more is obtained, and the composite ratio RA (%) is calculated based on the following equation (2). Ask.
RA = MN / TN × 100 (2)

[Production method]
An example of a method for producing machine structural steel according to the present invention will be described. In the present embodiment, a method for manufacturing a steel bar or wire will be described as an example of steel for machine structural use. However, the machine structural steel according to the present invention is not limited to steel bars or wires.

An example of a manufacturing method includes a steel making process in which molten steel is refined and cast to manufacture a material (slab or ingot), and a hot working process in which the material is hot-worked to manufacture steel for machine structural use. Hereinafter, each process will be described.

[Steel making process]
The steel making process includes a refining process and a casting process.

[Refining process]
In the refining process, refining in the converter (primary refining) is first performed on the hot metal produced by a well-known method. Secondary refining is performed on the molten steel produced from the converter. In secondary refining, alloy addition for component adjustment is performed to produce molten steel having the above chemical composition.

Specifically, Mn is added to the molten steel that has been removed from the converter. As a result, an oxide mainly composed of Mn is generated in the molten steel. After completing the addition of Mn, Si having a stronger deoxidizing power than Mn is added. As a result, the oxide mainly composed of Mn is modified to an oxide mainly composed of Si. After completing the addition of Si, Al having a stronger deoxidizing power than Si is added. As a result, the oxide mainly composed of Si is modified into a complex oxide containing Si and Al (hereinafter also simply referred to as “composite oxide”).

The composite oxide generated by the above refining process becomes a crystallization nucleus of MnS inclusions. Therefore, by producing the composite oxide, MnS inclusions are sufficiently crystallized and grow coarse. That is, when the composite oxide is generated, specific inclusions, which are inclusions having an equivalent circle diameter of 5 μm or more, are easily generated, and the number TN of the specific inclusions is 40 / mm ² or more. As a result, the amount of dissolved S in the steel after solidification is sufficiently reduced, and precipitation of MnS precipitates in the steel after solidification can be suppressed. As a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions can be sufficiently reduced, and the steel firing characteristics are enhanced.

After performing the deoxidation treatment, a well-known removal treatment is performed. After the removal process, secondary refining is performed. In the secondary refining, for example, composite refining is performed. For example, first, a refining process using LF (Laddle Furnace) or VAD (Vacuum Arc Degassing) is performed. Further, RH (Ruhrstahl-Hausen) vacuum degassing treatment may be performed. In secondary refining, Mn, Si, and other elements are added as necessary to adjust the components of the molten steel. After adjusting the components of the molten steel, the casting process is performed.

[Casting process]
A raw material (slab or ingot) is manufactured using the molten steel manufactured by the refining process. Specifically, a slab is manufactured by continuous casting using molten steel. Or you may manufacture an ingot by an ingot-making method using molten steel. Hereinafter, slabs and ingots are collectively referred to as materials. The cross-sectional area of the material here is, for example, 200 to 350 mm × 200 to 600 mm.

Solidification cooling rate RC at the time of casting is 100 degrees C / min or less. When the solidification cooling rate RC is 100 ° C./min or less, MnS inclusions are sufficiently crystallized and grow in the molten steel. Therefore, specific inclusions are easily generated, and the number TN thereof is 40 pieces / mm ² or more. As a result, the amount of dissolved S in the steel after solidification is sufficiently reduced, and precipitation of MnS precipitates in the steel after solidification can be suppressed. As a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions can be sufficiently reduced, and the steel firing characteristics are enhanced.

On the other hand, if the solidification cooling rate RC exceeds 100 ° C./min, the MnS inclusions are not sufficiently crystallized, and the MnS inclusions are not sufficiently grown. Therefore, specific inclusions are not easily generated, and the number TN of specific inclusions is less than 40 / mm ² . In this case, the amount of dissolved S in the steel after solidification cannot be sufficiently reduced, and many fine MnS precipitates are formed after solidification. As a result, since the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions cannot be reduced, the steel firing characteristics are deteriorated. Therefore, the solidification cooling rate RC is 100 ° C./min or less.

A preferable solidification cooling rate RC is 8 to less than 50 ° C./min. In this case, MnS inclusions are more likely to crystallize and grow. If the solidification cooling rate RC is less than 8 to 50 ° C./min, the time until solidification is further long, so that a sufficient time for Pb to move through the molten steel and adhere to the MnS inclusions can be secured. Therefore, composite inclusions containing MnS and Pb are easily generated, and the composite ratio RA is 40% or more. A more preferable upper limit of the solidification cooling rate RC is 30 ° C./min. The more preferable lower limit of the solidification cooling rate RC is 10 ° C./min, and more preferably 15 ° C./min.

The solidification cooling rate RC can be obtained from the cast material. FIG. 3 is a cross-sectional view of the cast material. Among the materials of thickness W (mm), the cooling rate from the liquidus temperature to the solidus temperature at the point P1 at the position W / 4 from the surface toward the material center is the solidification cooling rate RC in the casting process. (° C / min). The solidification cooling rate RC can be obtained by the following method. Cut the solidified material in the transverse direction. Of the cross section of the material, the secondary dendrite arm interval λ2 (μm) in the thickness direction of the solidified tissue at the point P1 is measured. Using the measured value λ2, the solidification cooling rate RC (° C./min) is obtained based on the following equation (3).
RC = (λ2 / 770) ^{− (1 / 0.41)} (3)

The secondary dendrite arm interval λ2 depends on the solidification cooling rate RC. Therefore, the solidification cooling rate RC can be obtained by measuring the secondary dendrite arm interval λ2.

[Hot working process]
In the hot working step, one or more hot workings are usually performed. The material is heated before each hot working. Thereafter, hot working is performed on the material. Hot working is, for example, hot forging or hot rolling. In the case where a plurality of hot workings are performed, the first hot working is, for example, block rolling or hot forging, and the next hot working is finish rolling using a continuous rolling mill. In a hot rolling mill, horizontal stands having a pair of horizontal rolls and vertical stands having a pair of vertical rolls are alternately arranged in a line. The material after hot working is cooled by a known cooling method such as air cooling.

Through the above process, the steel for machine structure according to the present embodiment is manufactured. The machine structural steel is, for example, a steel bar or a wire rod.

Machine structural steel manufactured by the above method is excellent in machinability and cracking characteristics. Manufacture of machine structural steel to machine parts is performed, for example, by the following method.

熱 Hot forging is performed on machine structural steel to produce a rough intermediate product. Normalize the intermediate product as necessary. Further, machining is performed on the intermediate product. The machining is, for example, cutting. A tempering treatment (quenching and tempering) may be performed on the intermediate product subjected to machining. When the tempering treatment is performed, machining such as cutting may be performed on the intermediate product after the tempering treatment. A machine part is manufactured by the above process. Machine parts may be manufactured by cold forging instead of hot forging.

The molten steel which has the chemical composition shown in Table 1 was manufactured.

The molten steel of each test number was manufactured by the following method. The primary refining in the converter was produced under the same conditions for the hot metal produced by a known method.

For the molten steels with test numbers other than test numbers 65 and 66, Mn, Si, and Al were added in this order after the steel was removed from the converter and subjected to deoxidation treatment. For the molten steel of test number 65, after the steel was removed from the converter, it was added in the order of Si, Al, and Mn and deoxidized. For the molten steel of test number 66, after the steel was removed from the converter, Mn, Al, and Si were added in this order to perform deoxidation treatment.

滓 After the deoxidation treatment, the removal treatment was performed. After the removal process, a refining process using VAD was performed, and then an RH vacuum degassing process was performed. After the RH vacuum degassing treatment, final adjustment of the alloy elements was performed. Through the above steps, molten steel having the chemical composition shown in Table 1 was manufactured.

The molten steel was cast to produce a rectangular parallelepiped experimental ingot. The cross shape of the ingot was rectangular and was 190 mm × 190 mm. The solidification cooling rate RC (° C./min) of each test number was as shown in Table 2. The solidification cooling rate RC was obtained from the above equation (3) by measuring the secondary dendrite arm interval of the ingot.

The steel bar was manufactured by carrying out hot working twice on the manufactured experimental ingot. In hot working, partial rolling was performed, and then finish rolling (bar rolling) was performed. The manufactured experimental ingot was hot forged to produce a steel bar having a diameter of 50 mm. Alternatively, a block rolling was performed on the experimental ingot, and then finish rolling was performed to produce a steel bar having a diameter of 50 mm. A normalizing treatment at 800 to 950 ° C. was performed on the manufactured steel bar. The cooling method in the normalizing treatment was left to cool. A steel bar (steel for machine structure) having a diameter of 50 mm was manufactured by the above manufacturing process.

[Evaluation test]
[Microstructure observation]
A specimen for observing the structure was collected from R / 2 part of the steel bar of each test number. Of the surface of the test piece, a cross section parallel to the longitudinal direction of the steel bar (that is, the rolling direction or the stretching direction) was defined as an observation surface. Based on the above method, the total area ratio (%) of ferrite, pearlite, and bainite was obtained. All the microstructures of the steel bars of each test number had a total area ratio of 99% or more. The microstructure with a total area ratio of 99% or more is shown in Table 2 as “F + P + B”.

[Number of specific inclusions TN and composite ratio RA]
A specimen for observing the structure was collected from R / 2 part of the steel bar of each test number. Of the surface of the test piece, a cross section parallel to the longitudinal direction of the steel bar (that is, the rolling direction or the stretching direction) was defined as an observation surface. Based on the above-described method, the number of specific inclusions TN (pieces / mm ² ) and the composite ratio RA (%) were determined for the observation surface of the test piece for tissue observation of each test number. The results are shown in Table 2.

[Machinability]
As for machinability, tool life characteristics and chip disposal with a normal drill were evaluated.

[Tool life characteristics CL1000]
A steel bar having a diameter of 50 mm was cut to a length of 20 mm to obtain a perforated test piece. Perforation was performed on the perforated specimen. Table 3 shows the conditions for drilling.

Concretely, the accumulated hole depth until the drill breakage was measured by changing the drill outer peripheral speed. A high-speed steel straight drill was used as the drill. The nose radius of the drill was 3 mm and the tip angle was 118 °. Drilling was performed at a cutting speed of 10 to 70 m / min, a feed rate of 0.25 mm / rev, and a hole depth of 9 mm. When the accumulated hole depth (hole depth x number of drill holes) reached 1000 mm, drilling with one drill was completed. In this case, the drill was replaced, and the test was repeated until breakage at a higher drill peripheral speed. The maximum drill peripheral speed at which a cumulative hole depth of 1000 mm can be drilled was defined as CL1000 (m / min) and used as an index of machinability. The results are shown in the column “CL1000” in Table 2. When CL1000 was 50 m / min or more, it was judged that the tool life characteristics were excellent. On the other hand, when CL1000 was less than 50 m / min, it was judged that the tool life characteristics were not excellent.

[Evaluation of chip disposal]
A steel bar having a diameter of 50 mm was cut at a predetermined length to obtain a cutting test piece. The outer periphery turning shown in FIG. 4 was implemented with respect to the cutting test piece. Table 4 shows the peripheral turning conditions.

Specifically, the tool 50 was a P20 class cemented carbide tool. The nose R of the tool 50 was 0.4, and the rake angle was 5 °. The peripheral turning was carried out at a cutting speed V1: 250 m / min, a feed speed V2: 0.2 mm / rev, a cutting amount D1: 2 mm, and a longitudinal cutting length L1: 200 mm. After cutting the outer periphery, the cutting turning was repeated so that the diameter again decreased by D1: 2 mm, and the test piece 5 was subjected to a turning test under the above conditions for 4 minutes.

In turning the 1000th test piece, the chips shown in FIGS. 5A and 5B were obtained. Therefore, the length L20 of the chips and the diameter D20 were measured. Based on the measurement results, the evaluation was as follows. When the chip diameter D20 has a coil shape of 30 mm or less, or when the chip length L20 is less than 50 mm even when the chip has no coil shape, it is determined that the chip disposal is excellent (see “ ○ "). On the other hand, when the chip diameter D20 was not a coil shape of 30 mm or less and the chip length L20 was 50 mm or more, it was determined that the chip disposal was not excellent (“×” in Table 2). .

[Evaluation test of rust characteristics (corrosion resistance)]
A cracking test piece was prepared by cutting a steel bar having a diameter of 50 mm into a predetermined length. The sputter test piece was turned under the same conditions as in the above cutting test. Thereafter, the test piece was stored in an atmosphere of 70% humidity and 20 ° C. for 1 hour while spraying tap water on the cut surface. After storage, the cut surface of the test piece was observed and the number of saddle points was measured. The measurement results are shown in the “Identification” column of Table 2. When the saddle point is less than 10 points (“」 ”in Table 2) and when the saddle point is 10 points or more and less than 20 points (“ ◯ ”in Table 2), the glazing characteristics are excellent. It was judged. On the other hand, when the saddle point was 20 points or more (“×” in Table 2), it was determined that the glazing characteristics were not excellent.

[Rolling fatigue test]
The rolling fatigue life was evaluated by the Mori-type thrust rolling fatigue test. Ten disc-shaped rolling fatigue test pieces 100 having a diameter of 60 mm and a thickness of 5 mm shown in FIG. 6 were collected from R / 2 part of the steel bars of each test number. The rolling fatigue test specimen 100 was carburized to harden the surface. The effective hardened layer depth was 0.8 mm or more.

The carburizing conditions were as follows. Cp = 0.7 to 0.8, 930 ° C. × 2 hours, 870 ° C. × 1 hour, oil quenching, washing, and low temperature tempering at 170 ° C. × 2 hours. The surface of the rolling fatigue test piece 100 subjected to the carburizing treatment was ground to remove the carburized abnormal layer, and the effective hardened layer depth was set to 0.7 mm. The effective hardened layer depth was set to the depth from the surface of the position which becomes HV550. The removal by grinding was 0.2 mm or less. The hardness distribution and effective hardened layer depth in each rolling fatigue test piece were adjusted by adjusting Cp and the amount of grinding removal.

As shown in FIG. 7, the rolling fatigue test piece 100 is immersed in a lubricating oil 102 of 70% oil and 30% water, and water in the lubricating oil evaporates due to heat generation, so 30 ml of water is once a day. Added. The test surface pressure was constant at 4 kN. As the hard sphere, a Si ₃ N ₄ ceramic hard sphere was used. The number of hard balls in contact with the rolling fatigue test piece 100 was 3, and the rotation speed was 1200 rpm. As a measure of the rolling fatigue life, “time until occurrence of pitching at a cumulative failure probability of 10% obtained by plotting test results on Weibull probability paper (time)” was used as the durability life. The results are shown in the “Rolling fatigue life” column of Table 2. When the rolling fatigue life was 4.0 hours or longer, it was judged that the rolling fatigue characteristics were excellent. On the other hand, when the rolling fatigue life was less than 4.0 hours, it was determined that the rolling fatigue characteristics were not excellent.

[Hot ductility (hot workability) evaluation test]
A hot tensile test by electric heating was performed to evaluate hot ductility (hot workability). Specifically, a round bar test piece having a diameter of 10 mm and a length of 100 mm and having both ends threaded was produced from the slabs of each test number. The round bar test piece was heated to 1100 ° C. by electric heating and held for 3 minutes. Then, the round bar test piece was cooled to 900 degreeC by standing_to_cool. A tensile test was performed with the round bar test piece at 900 ° C., and a drawing value (%) at the time of breakage was obtained. Tensile tests were performed with three round bar specimens for each test number, and the average of the three values was defined as the squeeze value (%) for that test number. The aperture value is shown in the “hot ductility” column of Table 2. When the aperture value was 70% or more, it was evaluated that the hot ductility (hot workability) was excellent. On the other hand, when the drawing value was less than 70%, it was evaluated that the hot ductility (hot workability) was not excellent.

[Test results]
In test numbers 1 to 31, the chemical composition was appropriate, F1 was 8.0 or more, the deoxidation order was appropriate, and the solidification cooling rate RC was 100 ° C./min or less. Therefore, the number TN of specific inclusions was 40 / mm ² or more. As a result, CL1000 was 50 m / min or more, and excellent chip disposal was obtained. That is, excellent machinability was obtained. Furthermore, in the glazing characteristic evaluation test, the scoring point was less than 20 points, and excellent scouring characteristics were obtained. Furthermore, in all the rolling fatigue tests, the rolling fatigue life was 4.0 hours or more, and excellent rolling fatigue characteristics were obtained. Furthermore, in the hot ductility evaluation test, the drawing value was 70% or more, and excellent hot ductility was obtained.

In test numbers 1 to 7, 26, 30, and 31, the solidification cooling rate RC was 8 to 50 ° C./min. Therefore, not only the number TN of specific inclusions is 40 / mm ² or more, but also the composite ratio RA is 40% or more. As a result, the scoring point was less than 10 in all cases, and even better sprinkling characteristics were obtained as compared with test numbers 8 to 25 and 27 to 29.

On the other hand, in test numbers 32 to 42, the chemical composition was appropriate, F1 was 8.0 or more, and the deoxidation order was appropriate, but the solidification cooling rate RC exceeded 100 ° C./min. Therefore, the number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained.

In test numbers 43 and 44, the chemical composition was appropriate, the order of deoxidation was appropriate, and the solidification cooling rate RC was 100 ° C./min or less, but F1 was less than 8.0. Therefore, the number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained. Furthermore, the aperture value was less than 70%, and excellent hot ductility was not obtained.

In test number 45, the chemical composition was appropriate and the order of deoxidation was appropriate, but the solidification cooling rate RC exceeded 100 ° C./min, and F1 was less than 8.0. Therefore, the number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained. Furthermore, the aperture value was less than 70%, and excellent hot ductility was not obtained.

In test numbers 46 and 47, the Mn content was too high. As a result, CL1000 was less than 50 m / min, and excellent machinability was not obtained.

In test numbers 48 and 49, the Mn content was too low. Therefore, the number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained. Furthermore, the aperture value was less than 70%, and excellent hot ductility was not obtained.

In test number 50, the S content was too high. As a result, the aperture value was less than 70%, and excellent hot ductility was not obtained.

In test number 51, the S content was too high. Furthermore, F1 was less than 8.0. Therefore, the number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained. Furthermore, the aperture value was less than 70%, and excellent hot ductility was not obtained.

In test numbers 52 and 53, the S content was too low. Therefore, the number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained.

In test numbers 54 and 55, the Pb content was too high. As a result, excellent ripening characteristics could not be obtained. Furthermore, the aperture value was less than 70%, and excellent hot ductility was not obtained.

In test numbers 56 and 57, the Pb content was too low. As a result, CL1000 was less than 50 m / min, and excellent chip disposal was not obtained. That is, excellent machinability was not obtained.

In test number 58, the Al content was too low. Therefore, the number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained.

In test number 59, the N content was too high. As a result, CL1000 was less than 50 m / min, and excellent machinability was not obtained. Furthermore, the aperture value was less than 70%, and excellent hot ductility was not obtained.

In test numbers 60 and 61, the O content was too high. As a result, CL1000 was less than 50 m / min, and excellent machinability was not obtained. Furthermore, the rolling fatigue life was less than 4.0 hours, and excellent rolling fatigue characteristics were not obtained.

In test number 62, the O content was too low. Therefore, the number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained. Furthermore, CL1000 was less than 50 m / min, and excellent chip disposal was not obtained. That is, excellent machinability was not obtained.

In test number 63, the Cr content was too high. As a result, CL1000 was less than 50 m / min, and excellent machinability was not obtained.

In test number 64, the Cr content was too low. As a result, the rolling fatigue life was less than 4.0 hours, and excellent rolling fatigue characteristics were not obtained.

In test numbers 65 and 66, the chemical composition was appropriate, F1 was 8.0 or more, and the solidification cooling rate RC was 100 ° C./min or less, but the deoxidation order was inappropriate. The number TN of specific inclusions was less than 40 / mm ² . As a result, excellent ripening characteristics could not be obtained.

The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

10 MnS inclusions 20 Pb inclusions 30 Composite inclusions

Claims (4)

1. % By mass
   C: 0.15 to less than 0.30%,
   Si: 0.01-0.80%
   Mn: 0.20 to 2.00%,
   P: 0.030% or less,
   S: 0.010 to 0.100%,
   Pb: 0.010 to 0.100%,
   Al: 0.010 to 0.050%,
   N: 0.015% or less,
   O: 0.0005 to 0.0030%,
   Cr: 0.50 to 2.00%,
   Ni: 0 to 3.50%,
   B: 0 to 0.0050%,
   V: 0 to 0.70%,
   Mo: 0 to 0.70%,
   W: 0 to 0.70%,
   Nb: 0 to less than 0.050%,
   Cu: 0 to 0.50%,
   Ti: 0 to 0.100%, and
   Ca: 0 to 0.0030% is contained, the balance consists of Fe and impurities, and has a chemical composition satisfying the formula (1),
   In steel, the total number of specific inclusions that are any of MnS inclusions, Pb inclusions, and composite inclusions containing MnS and Pb and having an equivalent circle diameter of 5 μm or more is 40 / mm ^2. This is steel for machine structural use.
   Mn / S ≧ 8.0 (1)
   Here, the content (mass%) of the corresponding element is substituted for each element in the formula (1).
2. The mechanical structural steel according to claim 1,
   The chemical composition is
   Ni: 0.02 to 3.50%,
   B: 0.0005 to 0.0050%,
   V: 0.05 to 0.70%,
   Mo: 0.05 to 0.70%,
   W: 0.05-0.70%
   Nb: 0.001 to less than 0.050%,
   Cu: 0.05 to 0.50%, and
   Ti: Steel for machine structure containing one or more selected from the group consisting of 0.003 to 0.100%.
3. The steel for machine structure according to claim 1 or 2,
   The chemical composition is
   Ca: Steel for machine structure containing 0.0001 to 0.0030%.
4. A machine structural steel according to any one of claims 1 to 3,
   Machine structural steel, wherein the number ratio of the composite inclusions to the specific inclusions is 40% or more.

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