WO2018008621A1 - Steel for mechanical structures - Google Patents
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
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- B22D11/124—Accessories for subsequent treating or working cast stock in situ for cooling
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
Definitions
- the present invention relates to steel, and more particularly to steel for machine structural use.
- 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, heat treatment (normalization, etc.), surface hardening heat treatment (induction hardening, etc.) or quenching and tempering may be performed on the machine parts.
- Heat treatment normalization, etc.
- surface hardening heat treatment induction hardening, etc.
- quenching and tempering may be performed on the machine parts.
- 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.
- Patent Document 1 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%
- Pb 0 ⁇ 0.12%
- Zr 0 to less than 0.04%
- Nd 0 to 0.05%
- Nb 0 to 0.1%
- 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.
- 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 ⁇ 0.001 ⁇ 2 + 0.13 ⁇ .
- the element symbol in each formula indicates the content in mass% of the element, and ⁇ 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.
- machining such as cutting may be performed by an automated manufacturing facility.
- 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.
- 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.
- 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.
- An object of the present invention is to provide a steel for machine structure that is excellent in machinability, cracking characteristics, and hot workability.
- the steel for machine structural use according to the present invention is, in mass%, C: 0.30 to 0.80%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0.00. 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 ⁇ 0.0030%, Cr: 0 ⁇ 0.70%, Ni: 0 ⁇ 3.50%, B: 0 ⁇ 0.0050%, V: 0 ⁇ 0.70%, Mo: 0 ⁇ 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%
- the balance is composed of Fe and impurities, and has a chemical composition satisfying the formula (1).
- 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 ⁇ m or more is 40 / mm 2. That's it.
- 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, and hot workability.
- 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. 3A is a photographic image of the S distribution obtained by performing EPMA analysis on the machine structural steel of this embodiment.
- FIG. 3B is a photographic image of a Pb distribution obtained by performing EPMA with the same field of view as FIG. 3A.
- FIG. 3C is a composite image of FIGS. 3A and 3B.
- FIG. 4 is a cross-sectional view of the cast material.
- FIG. 5 is a schematic diagram of a cutting test machine for explaining a cutting test.
- FIG. 6A is a
- the present inventors investigated and examined the machinability, the cracking characteristics, and the hot workability of the steel for machine structures.
- C 0.30 to 0.80%, 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 to 0.70%, 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%, with the balance being Fe and It was thought that excellent machinability and excellent hot workability could be obtained with a machine structural steel having a chemical composition consisting of impur
- 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.
- 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.
- 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.
- 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.
- “MnS inclusions”, “Pb inclusions”, and “composite inclusions” are specified by the method described in the item “Method of measuring the number TN of specific inclusions and the composite ratio RA” described later.
- 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.
- 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.
- the composite inclusions enhance the machinability of steel more than the MnS inclusions and the Pb inclusions alone.
- a crack occurs around the composite inclusion, liquefied Pb enters the opened crack.
- 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.
- 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.
- the steel for machine structural use containing MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions is prone to wrinkling.
- 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.
- 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.
- the steel is more likely to start.
- 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.
- 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.
- 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.
- the Mn content and the S content satisfy the following formula (1).
- the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).
- 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.
- 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.
- the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions can be sufficiently reduced, and the steel firing characteristics are enhanced.
- inclusions that are any of MnS inclusions, Pb inclusions, and composite inclusions and having an equivalent circle diameter of 5 ⁇ m or more are defined as “specific inclusions”.
- 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.
- 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).
- 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.
- 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.
- the mechanical structural steel according to the present embodiment completed based on the above knowledge is, in mass%, C: 0.30 to 0.80%, 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: 0.00.
- the balance is Fe and impurities, and has a chemical composition satisfying the formula (1).
- 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 ⁇ m or more is 40 / mm 2. That's it.
- the content (mass%) of the corresponding element is substituted for each element in the formula (1).
- the chemical composition of the steel for mechanical structure is as follows: Cr: 0.10 to 0.70%, 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: One or more selected from the group consisting of 0.003 to 0.100% may be contained.
- the chemical composition of the steel for machine structural use may contain Ca: 0.0001 to 0.0030%.
- the number ratio of the composite inclusions to the specific inclusions may be 40% or more.
- C 0.30 to 0.80% Carbon (C) increases the strength of the steel.
- heat treatment normalization, etc.
- surface hardening heat treatment induction hardening, etc.
- quenching and tempering is performed as necessary after forging the machine structural steel.
- C increases the strength of the steel. If the C content is less than 0.30%, sufficient strength cannot be obtained.
- the C content exceeds 0.80%, a large amount of retained austenite is generated after tempering. In this case, not only the increase in strength is saturated, but hard cementite is generated, and the machinability of the steel is reduced. Therefore, the C content is 0.30 to 0.80%.
- the preferable lower limit of the C content in the case of using the parts as normal is 0.34%, more preferably 0.40%.
- the preferable upper limit of the C content is 0.70%. In this case, the strength corresponding to the C content is obtained.
- the temperature range which becomes an austenite single phase at the time of quenching is very narrow. Therefore, when mass-producing, the preferable upper limit of the C content is 0.60%.
- Si 0.01 to 0.80% Silicon (Si) deoxidizes steel.
- Si modifies the oxide by adding Si after adding Mn.
- Si added to molten steel modifies an oxide mainly composed of Mn into an oxide mainly composed of Si.
- Al 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.
- 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.
- an oxide mainly composed of Mn is formed in the molten steel.
- Si, Al another strong deoxidizing element
- Mn in the oxide is discharged into the molten steel and the oxide is modified.
- 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.
- 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.
- 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.
- MnS suppresses tool wear. If the S content is less than 0.010%, MnS cannot be sufficiently crystallized, and composite inclusions containing MnS and Pb are hardly generated. 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%.
- the preferred lower limit of the S content when giving priority to mechanical properties is 0.015%
- 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%
- 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.
- deoxidation with Al kill is performed in order to suppress the formation of vacancies and surface defects during solidification.
- 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.
- the precipitation of fine MnS precipitates is further suppressed.
- the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions decreases.
- 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.
- the Al content exceeds 0.050%, a coarse composite oxide is likely to be generated. Complex oxides tend to be coarse. 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.
- 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%.
- the N content means the total N (tN) content.
- Oxygen (O) is included not only in the oxide but also in MnS inclusions. O produces
- 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%.
- the O content means the total oxygen (t—O) content.
- the remainder of the chemical composition of the machine structural steel according to the present embodiment consists of Fe and impurities.
- 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.
- the chemical composition of the steel for machine structure of the present embodiment further includes one or more selected from the group consisting of Cr, Ni, B, V, Mo, W, Nb, Cu, and Ti. Also good.
- Chromium (Cr) is an optional element and may not be contained. When contained, Cr dissolves in the steel to increase the hardenability and temper softening resistance of the steel and increase the strength of the steel. Furthermore, Cr increases the depth of the hardened layer when performing nitriding as surface hardening. If Cr is contained even a little, the above effect can be obtained to some extent. On the other hand, if the Cr content exceeds 0.70%, cementite in the steel becomes coarse when performing quenching and tempering. Further, when the Cr content exceeds 0.70%, cementite in the steel does not dissolve in the case of induction hardening. Further, if the Cr content exceeds 0.70%, the austenite is stabilized even at a low temperature.
- the Cr content is 0 to 0.70%.
- the minimum with preferable Cr content for improving hardenability is 0.10%, More preferably, it is 0.30%.
- the upper limit with preferable Cr content is 0.60%.
- 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%.
- the preferable lower limit of the Ni content is 0.20%.
- Ni detoxifies Cu and increases toughness.
- 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.
- 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.
- 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 Vanadium
- V is an optional element and may not be contained. When contained, V precipitates as carbide, nitride, or carbonitride during tempering and nitriding, and increases the strength of the steel. V precipitates (nitrides, carbides and carbonitrides) further suppress the coarsening of austenite grains 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.
- 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.
- the amount of solid solution V decreases. Therefore, the tempering softening resistance of steel falls.
- 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 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 a ferrite / pearlite structure is 0.40%, and more preferably 0.30%.
- W 0 to 0.70% Tungsten (W) is an optional element and may not be 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.
- 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.
- 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 W content for stably obtaining a ferrite / pearlite structure 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%.
- Niobium (Nb) is an optional element and may not be contained. When contained, Nb produces nitrides, carbides, or carbonitrides, 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.
- Titanium 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.
- Ti 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.
- Ca generates CaS or (Mn, Ca) S to spheroidize the MnS inclusions and reduce the amount of tool wear.
- the machinability of the steel is increased. If Ca is contained even a little, the above effect can be obtained to some extent.
- 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%.
- the preferable upper limit of Ca content is 0.0015%, more preferably 0.0003%.
- 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.
- F1 is 8.0 or more
- the Mn content is sufficiently higher than the S content.
- MnS inclusions are sufficiently crystallized and grow in the molten steel.
- 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.
- the microstructure of the machine structural steel according to the invention consists mainly of ferrite and pearlite.
- the mechanical structural steel having the above chemical composition has a total area ratio of ferrite and pearlite in the microstructure of 99% or more.
- the total area ratio of ferrite and pearlite in the microstructure can be measured by the following method.
- a sample is taken from the machine structural steel.
- 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.
- R / 2 portion the surface perpendicular to the central axis of the machine structural steel is taken as the observation surface.
- 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.
- each phase such as ferrite, pearlite, and bainite has a different contrast for each phase. Therefore, each phase is specified based on the contrast.
- the total area ( ⁇ m 2 ) of ferrite and pearlite 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 and pearlite.
- 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 ⁇ m or more ( That is, the total number TN of specific inclusions is 40 / mm 2 or more.
- the number of specific inclusions TN is 40 / mm 2 or more, coarse MnS inclusions having an equivalent circle diameter of 5 ⁇ m 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.
- 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 ⁇ m 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.
- 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 .
- the upper limit of the circle equivalent diameter of a specific inclusion is not specifically limited, For example, it is 200 micrometers.
- the steel is likely to start.
- 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%.
- 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.
- SEM scanning electron microscope
- specific inclusions any of MnS inclusions, Pb inclusions, and composite inclusions with an equivalent circle diameter of 5 ⁇ m or more
- Specific inclusions and other inclusions can be distinguished by contrast.
- MnS inclusions, Pb inclusions, and composite inclusions are specified by the following methods, respectively.
- FIG. 1A is a schematic diagram showing the S distribution in the observation surface obtained by EPMA analysis
- 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
- Reference numeral 20 in FIG. 1B is an area where Pb exists.
- Pb may be divided by rolling or the like and arranged in the rolling direction as indicated by reference numeral 20A.
- S the same applies to S.
- 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.
- Pb inclusion 20 overlaps with MnS inclusion 10
- the inclusion is recognized as composite inclusion 30.
- MnS inclusion 10 and Pb inclusion 20 do not overlap (region A1, region A2, etc. in FIG. 1C)
- these inclusions are MnS inclusions and Pb inclusions. Identifies it.
- FIG. 3A is a photographic image of S distribution obtained by carrying out EPMA analysis on the steel for mechanical structure of this embodiment
- FIG. 3B is a photographic image of Pb distribution
- FIG. 3C is a photographic image obtained by duplicating FIGS. 3A and 3B.
- MnS inclusions 10 are observed in region A10 of FIG. 3A
- Pb inclusions 20 are observed in region A10 of FIG. 3B. Therefore, it can be recognized that the composite inclusion 30 exists in the region A10 of FIG. 3C.
- the MnS inclusion 10 is not observed in the region A20 of FIG. 3A
- the Pb inclusion 20 is observed in the region A20 of FIG. 3B. Therefore, the inclusion existing in the region A20 of FIG. 3C can be recognized as the Pb inclusion 20.
- 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 ( ⁇ m) of each inclusion.
- 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.
- a steel making process in which molten steel is refined and cast to manufacture a material (slab or ingot)
- a hot working process in which the material is hot-worked to manufacture steel for machine structural use.
- 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.
- Mn is added to the molten steel that has been removed from the converter.
- an oxide mainly composed of Mn is generated in the molten steel.
- Si having a stronger deoxidizing power than Mn is added.
- the oxide mainly composed of Mn is modified to an oxide mainly composed of Si.
- Al having a stronger deoxidizing power than Si is added.
- 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 ⁇ m 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.
- a well-known removal treatment is performed.
- secondary refining is performed.
- composite refining is performed.
- LF Laser Furnace
- VAD Vauum Arc Degassing
- RH Rasterstahl-Hausen vacuum degassing treatment
- 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.
- 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.
- 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.
- 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.
- 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.
- the total number of MnS inclusions, MnS precipitates, Pb inclusions, and composite inclusions can be sufficiently reduced, and the steel firing characteristics are enhanced.
- 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. 4 is a cross-sectional view of the cast material.
- 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 ⁇ 2 ( ⁇ m) in the thickness direction of the solidified tissue at the point P1 is measured. Using the measured value ⁇ 2, the solidification cooling rate RC (° C./min) is obtained based on the following equation (3).
- RC ( ⁇ 2 / 770) ⁇ (1 / 0.41) (3)
- the secondary dendrite arm interval ⁇ 2 depends on the solidification cooling rate RC. Therefore, the solidification cooling rate RC can be obtained by measuring the secondary dendrite arm interval ⁇ 2.
- 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.
- 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.
- 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
- 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.
- deoxidation treatment was performed by adding Mn, Si, and Al in that order after the steel from the converter.
- Mn, Si, and Al were added in this order and deoxidation was performed.
- Mn, Al, and Si were added in this order to perform deoxidation treatment.
- 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.
- a Vickers hardness test was carried out in accordance with JIS Z 2244 (1981) at any five points of R / 2 part of the steel bar of each test number.
- the test force was 100N.
- the average of the five values obtained was defined as the Vickers hardness (HV) of the steel bar of that test number. If the Vickers hardness was HV160 or more, it was judged to have sufficient strength. On the other hand, when the Vickers hardness was less than HV160, it was determined that the strength was insufficient. Table 2 shows the results. In any of the test numbers, the Vickers hardness was HV160 or more, indicating a sufficient strength.
- Machinability evaluated the tool abrasion amount (micrometer) and the chip disposal property. Specifically, 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. 5 was implemented with respect to the cutting test piece. Table 3 shows the peripheral turning conditions.
- a P20 class cemented carbide tool was used as the tool 50.
- 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: 200 m / min, a feed speed V2: 0.2 mm / rev, a cutting depth 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.
- Tool life evaluation The tool wear amount (mm) of the front flank was measured for the tool 50 after the turning of the 1000th test piece was completed. The measurement results are shown in the “Tool wear amount” column of Table 2. When the amount of tool wear was 200 ⁇ m or less, it was judged that the tool life was excellent. On the other hand, when the tool wear amount exceeded 200 ⁇ m, it was determined that the tool life was not excellent.
- 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 “Spring characteristics” 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.
- Hot ductility evaluation test A hot tensile test by conducting heating was performed to evaluate hot ductility. 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 was excellent. On the other hand, when the aperture value was less than 70%, it was evaluated that the hot ductility was not excellent.
- test results In test numbers 1 to 26, 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, the amount of tool wear was 200 ⁇ m or less, 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 the hot ductility evaluation test, the drawing value was 70% or more, and excellent hot ductility was obtained.
- 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 points were all less than 10 points, and even better sprinkling characteristics were obtained as compared with test numbers 7 to 16, 18, 20, 21, 23, 24, and 26.
- test numbers 27 to 35 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.
- test numbers 36 and 37 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.
- test number 38 the chemical composition was appropriate and the deoxidation order 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.
- test number 39 the Mn content was too high. As a result, the amount of tool wear exceeded 200 ⁇ m, and excellent machinability was not obtained.
- test number 40 the Mn content was too low. Furthermore, 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. Furthermore, the aperture value was less than 70%, and excellent hot ductility was not obtained.
- test number 41 the S content was too high. As a result, the aperture value was less than 70%, and excellent hot ductility was not obtained.
- test number 42 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.
- test number 43 the Pb content was too high. As a result, excellent ripening characteristics could not be obtained.
- test number 44 the Pb content was too low. Furthermore, 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, the amount of tool wear exceeded 200 ⁇ m, and further excellent chip disposal was not obtained. That is, excellent machinability was not obtained.
- test number 45 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.
- test number 46 the N content was too high. As a result, the amount of tool wear exceeded 200 ⁇ m, and excellent machinability was not obtained. Furthermore, the aperture value was less than 70%, and excellent hot ductility was not obtained.
- test number 47 the O content was too high. Furthermore, 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, the amount of tool wear exceeded 200 ⁇ m, and excellent machinability was not obtained.
- test number 48 the O content was too low. Furthermore, 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, the amount of tool wear exceeded 200 ⁇ m, and further excellent chip disposal was not obtained. That is, excellent machinability was not obtained.
- test number 49 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. Therefore, the number TN of specific inclusions was less than 40 / mm 2 . As a result, excellent ripening characteristics could not be obtained.
- test number 50 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. Therefore, the number TN of specific inclusions was less than 40 / mm 2 . As a result, excellent ripening characteristics could not be obtained.
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Abstract
Description
Mn/S≧8.0 (1)
ここで、式(1)中の各元素には、対応する元素の含有量(質量%)が代入される。 The steel for machine structural use according to the present invention is, in mass%, C: 0.30 to 0.80%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0.00. 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 ~ 0.0030%, Cr: 0 ~ 0.70%, Ni: 0 ~ 3.50%, B: 0 ~ 0.0050%, V: 0 ~ 0.70%, Mo: 0 ~ 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% 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).
Mn/S≧8.0 (1)
ここで、式(1)中の各元素記号には、対応する元素の含有量(質量%)が代入される。 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).
Mn/S≧8.0 (1)
ここで、式(1)中の各元素には、対応する元素の含有量(質量%)が代入される。 The mechanical structural steel according to the present embodiment completed based on the above knowledge is, in mass%, C: 0.30 to 0.80%, 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: 0.00. 015% or less, O: 0.0005 to 0.0030%, Cr: 0 to 0.70%, 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%, the balance is 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).
本実施形態の機械構造用鋼の化学組成は、次の元素を含有する。 [Chemical composition]
The chemical composition of the steel for machine structural use of this embodiment contains the following elements.
炭素(C)は、鋼の強度を高める。機械構造用鋼を用いて部品を製造する場合、機械構造用鋼を鍛造後、必要に応じて熱処理(焼準等)、表面硬化熱処理(高周波焼入れ等)、又は、焼入れ焼戻しが実施される。この場合、Cは鋼の強度を高める。C含有量が0.30%未満であれば、十分な強度が得られない。一方、C含有量が0.80%を超えれば、焼戻し後に残留オーステナイトが多く生成する。この場合、強度の上昇が飽和するだけでなく、硬質なセメンタイトが生成し、鋼の被削性が低下する。したがって、C含有量は0.30~0.80%である。 C: 0.30 to 0.80%
Carbon (C) increases the strength of the steel. When manufacturing parts using machine structural steel, heat treatment (normalization, etc.), surface hardening heat treatment (induction hardening, etc.), or quenching and tempering is performed as necessary after forging the machine structural steel. In this case, C increases the strength of the steel. If the C content is less than 0.30%, sufficient strength cannot be obtained. On the other hand, if the C content exceeds 0.80%, a large amount of retained austenite is generated after tempering. In this case, not only the increase in strength is saturated, but hard cementite is generated, and the machinability of the steel is reduced. Therefore, the C content is 0.30 to 0.80%.
シリコン(Si)は、鋼を脱酸する。脱酸処理時において、Mnを添加した後にSiを添加することにより、Siは酸化物を改質する。具体的に、溶鋼中に添加されたSiは、Mnを主体とする酸化物を、Siを主体とする酸化物に改質する。Siを添加した後、Alを添加することにより、鋼中にSi及びAlを含有する複合酸化物が生成する。複合酸化物は、MnS介在物が晶出する核となる。そのため、複合酸化物は鋼の発銹特性を高める。Siはさらに、焼戻し軟化抵抗を高め、強度を高める。Si含有量が0.01%未満であれば、上記効果が得られない。 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.
マンガン(Mn)はMnS介在物と、MnS及びPbを含有する複合介在物とを生成し、鋼の被削性を高める。 Mn: 0.20 to 2.00%
Manganese (Mn) generates MnS inclusions and composite inclusions containing MnS and Pb, and improves the machinability of steel.
りん(P)は、不可避に含有される。Pは鋼を脆化し、被削性を高める。一方、P含有量が0.030%を超えれば、熱間延性が低下する。この場合、圧延疵が発生する等、生産性が低下する。したがって、P含有量は0.030%以下である。被削性を高めるためのP含有量の好ましい下限は0.005%である。この場合、被削性、特に、切り屑処理性が高まる。P含有量の好ましい上限は0.015%である。 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%.
硫黄(S)は、鋼中でMnSを生成し、被削性を高める。MnSは特に、工具摩耗を抑制する。S含有量が0.010%未満であれば、MnSを十分に晶出せず、MnSとPbとを含有する複合介在物が生成しにくい。その結果、発銹特性が低下する。一方、S含有量が0.100%を超えれば、Sが粒界に偏析して、鋼が脆化し、鋼の熱間加工性が低下する。したがって、S含有量は0.010~0.100%である。被削性及び機械特性のうち、機械特性を優先する場合のS含有量の好ましい下限は0.015%であり、好ましい上限は0.030%である。被削性を優先する場合のS含有量の好ましい下限は0.030%であり、好ましい上限は0.050%である。 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 cannot be sufficiently crystallized, and composite inclusions containing MnS and Pb are hardly generated. 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%.
鉛(Pb)は単独でPb介在物(Pb粒)を生成し、鋼の被削性を高める。Pbはさらに、MnS介在物と結合して複合介在物を生成し、鋼の被削性を高め、特に切り屑処理性を高める。Pb含有量が0.010%未満であれば、上記効果が得られない。一方、Pb含有量が0.100%を超えれば、被削性は高まるものの、鋼が脆化する。その結果、鋼の熱間加工性が低下する。Pb含有量が0.100%を超えればさらに、Pb介在物が過剰に増加するため、鋼の発銹特性が低下する。したがって、Pb含有量は0.010~0.100%である。複合介在物の生成を促進し、被削性を高めるためのPb含有量の好ましい下限は0.020%であり、より好ましくは0.025%である。発銹特性を高めるためのPb含有量の好ましい上限は0.050%である。 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%.
アルミニウム(Al)は、鋼を脱酸する。本発明による機械構造用鋼では、凝固時の空孔及び表面疵の生成を抑制するため、Alキルドによる脱酸を実施する。後述のとおり、溶鋼中にMn、Siに次いでAlを添加して脱酸を行えば、鋼中の酸化物が改質され、Si及びAlを含有する複合酸化物が生成する。複合酸化物はMnS介在物の晶出核になりやすい。そのため、MnS介在物が分散して晶出し、成長して粗大化しやすく、かつ、MnS及びPbを含有する複合介在物が生成しやすい。この場合、鋼の被削性が高まる。MnS介在物が分散して晶出した場合はさらに、微細なMnS析出物の析出が抑制される。この場合、MnS介在物、MnS析出物、Pb介在物、及び複合介在物の総個数が低下する。そのため、鋼の発銹特性が高まる。Alはさらに、Nと結合してAlNを形成して、各種の熱処理におけるオーステナイト粒の粗大化を抑制する。Al含有量が0.010%未満であれば、上記効果が得られない。 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 decreases. 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.
窒素(N)は不可避に含有される。NはAlと結合してAlNを形成し、熱処理時のオーステナイト粒の粗大化を抑制し、鋼の強度を高める。一方、N含有量が0.015%を超えれば、鋼の切削抵抗が高まり、被削性が低下する。N含有量が0.015%を超えればさらに、熱間加工性が低下する。したがって、N含有量は0.015%以下である。N含有量の好ましい下限は0.002%であり、より好ましくは0.004%である。N含有量の好ましい上限は0.012%であり、より好ましくは0.008%である。本明細書でいうN含有量は、全N(t-N)の含有量を意味する。 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.
酸素(O)は酸化物中に含まれるだけでなく、MnS介在物にも含まれる。Oは、MnS介在物の晶出核となる複合酸化物を生成する。O含有量が0.0005%未満であれば、複合酸化物の生成量が不足し、溶鋼中でMnS介在物が晶出しにくくなる。この場合、鋼の被削性が低下する。この場合さらに、凝固後に微細なMnS析出物が多数生成する。その結果、MnS介在物、MnS析出物、Pb介在物、及び、複合介在物の総個数が増加し、発銹特性が低下する。一方、O含有量が0.0030%を超えれば、粗大なアルミナ系酸化物が生成し、切削工具の摩耗を促進するため、鋼の被削性が低下する。O含有量が0.0030%を超えればさらに、破壊の起点となる粗大な酸化物が生成する場合がある。この場合、機械部品の転動疲労特性が低下する。したがって、O含有量は0.0005~0.0030%である。鋼の被削性及び鋼の発銹特性をさらに高めるためのO含有量の好ましい下限は0.0007%であり、より好ましくは0.0010%である。O含有量の好ましい上限は0.0025%であり、より好ましくは0.0020%である。本明細書でいうO含有量は、全酸素(t-O)の含有量を意味する。 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. On the other hand, 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 steel is lowered. If the O content exceeds 0.0030%, a coarse oxide serving as a starting point of destruction may be further generated. In this case, the rolling fatigue characteristics of the machine parts are reduced. 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.
本実施形態の機械構造用鋼の化学組成はさらに、Cr、Ni、B、V、Mo、W、Nb、Cu、及び、Tiからなる群から選択される1種又は2種以上を含有してもよい。 [Arbitrary elements]
The chemical composition of the steel for machine structure of the present embodiment further includes one or more selected from the group consisting of Cr, Ni, B, V, Mo, W, Nb, Cu, and Ti. Also good.
クロム(Cr)は任意元素であり、含有されなくてもよい。含有される場合、Crは鋼中に固溶して、鋼の焼入れ性及び焼戻し軟化抵抗を高め、鋼の強度を高める。Crはさらに、表面硬化処理として窒化処理を実施する場合、硬化層深さを深くする。Crが少しでも含有されれば、上記効果がある程度得られる。一方、Cr含有量が0.70%を超えれば、焼入れ焼戻しを実施する場合、鋼中のセメンタイトが粗大化する。Cr含有量が0.70%を超えればさらに、高周波焼入れを実施する場合、鋼中のセメンタイトが固溶しない。Cr含有量が0.70%を超えればさらに、オーステナイトが低温でも安定化する。この場合、鋼が脆化する。したがって、Cr含有量は0~0.70%である。焼入れ性を高めるためのCr含有量の好ましい下限は0.10%であり、より好ましくは0.30%である。Cr含有量の好ましい上限は0.60%である。 Cr: 0 to 0.70%
Chromium (Cr) is an optional element and may not be contained. When contained, Cr dissolves in the steel to increase the hardenability and temper softening resistance of the steel and increase the strength of the steel. Furthermore, Cr increases the depth of the hardened layer when performing nitriding as surface hardening. If Cr is contained even a little, the above effect can be obtained to some extent. On the other hand, if the Cr content exceeds 0.70%, cementite in the steel becomes coarse when performing quenching and tempering. Further, when the Cr content exceeds 0.70%, cementite in the steel does not dissolve in the case of induction hardening. Further, if the Cr content exceeds 0.70%, the austenite is stabilized even at a low temperature. In this case, the steel becomes brittle. Therefore, the Cr content is 0 to 0.70%. The minimum with preferable Cr content for improving hardenability is 0.10%, More preferably, it is 0.30%. The upper limit with preferable Cr content is 0.60%.
ニッケル(Ni)は任意元素であり、含有されなくてもよい。含有される場合、Niは鋼に固溶して鋼の焼入れ性を高め、鋼の強度を高める。Niはさらに、マトリクスの延性も高める。Niはさらに、鋼の靭性を高める。Niはさらに、鋼の耐食性を高める。Niが少しでも含有されれば、上記効果がある程度得られる。一方、Ni含有量が3.50%を超えれば、残留オーステナイトが多く残存する。この場合、加工誘起変態により、残留オーステナイトの一部がマルテンサイトに変態し、鋼の延性が低下する。したがって、Ni含有量は0~3.50%である。 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%.
ボロン(B)は任意元素であり、含有されなくてもよい。含有される場合、Bは鋼の焼入れ性を高め、鋼の強度を高める。Bはさらに、靭性を低下するP、Sの粒界への偏析を抑制し、破壊特性を高める。Bが少しでも含有されれば、上記効果がある程度得られる。一方、B含有量が0.0050%を超えれば、BNが多量に生成して鋼が脆化する。したがって、B含有量は0~0.0050%である。窒化物生成元素であるTi又はNbを含有した場合のB含有量の好ましい下限は0.0005%である。B含有量の好ましい上限は0.0020%である。 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%.
バナジウム(V)は任意元素であり、含有されなくてもよい。含有される場合、Vは焼戻し時及び窒化処理時に炭化物、窒化物、又は炭窒化物として析出し、鋼の強度を高める。V析出物(窒化物、炭化物及び炭窒化物)はさらに、オーステナイト粒の粗大化を抑制し、鋼の靭性を高める。Vはさらに、鋼に固溶して、鋼の焼戻し軟化抵抗を高める。Vが少しでも含有されれば、上記効果がある程度得られる。 V: 0 to 0.70%
Vanadium (V) is an optional element and may not be contained. When contained, V precipitates as carbide, nitride, or carbonitride during tempering and nitriding, and increases the strength of the steel. V precipitates (nitrides, carbides and carbonitrides) further suppress the coarsening of austenite grains 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.
モリブデン(Mo)は任意元素であり、含有されなくてもよい。含有される場合、Moは焼戻しや窒化処理等のA1点以下の低温での熱処理において、Mo炭化物として析出する。そのため、鋼の強度及び焼戻し軟化抵抗が高まる。Moはさらに、鋼に固溶して、鋼の焼入れ性を高める。Moが少しでも含有されれば、上記効果がある程度得られる。一方、Mo含有量が0.70%を超えれば、鋼の焼入れ性が高くなりすぎる。この場合、圧延や、伸線前の軟化熱処理等で過冷組織が生じやすくなる。したがって、Mo含有量は0~0.70%である。 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%.
タングステン(W)は任意元素であり、含有されなくてもよい。含有される場合、Wは鋼中でW炭化物として析出し、鋼の強度及び焼戻し軟化抵抗を高める。W炭化物は、A3点以下の低温で生成する。そのため、Wは、VやNb、Ti等とは異なり、未溶解析出物を生成しにくい。その結果、W炭化物は、析出強化により鋼の強度及び焼戻し軟化抵抗を高める。Wはさらに、鋼に固溶して鋼の焼入れ性を高め、鋼の強度を高める。Wが少しでも含有されれば、上記効果がある程度得られる。 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.
ニオブ(Nb)は任意元素であり、含有されなくてもよい。含有される場合、Nbは窒化物、炭化物、又は炭窒化物を生成し、焼き入れ時や焼準時においてオーステナイト粒の粗大化を抑制する。Nbはさらに、析出強化により鋼の強度を高める。Nbが少しでも含有されれば、上記効果がある程度得られる。一方、Nb含有量が0.050%を超えれば、未固溶析出物が生成して鋼の靭性が低下する。Nb含有量が0.050%を超えればさらに、過冷組織が生成しやすくなり、鋼の熱間加工性が低下する。したがって、Nb含有量は0~0.050%未満である。上記効果を安定して得るためのNb含有量の好ましい下限は0.001%であり、より好ましくは0.005%である。Nb含有量の好ましい上限は0.030%であり、より好ましくは0.015%である。 Nb: 0 to less than 0.050% Niobium (Nb) is an optional element and may not be contained. When contained, Nb produces nitrides, carbides, or carbonitrides, 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%.
銅(Cu)は任意元素であり、含有されなくてもよい。含有される場合、Cuは脱炭を防止する。Cuはさらに、Niと同様に耐食性を高める。Cuが少しでも含有されれば、上記効果がある程度得られる。一方、Cu含有量が0.50%を超えれば、鋼が脆化して圧延疵が発生しやすくなる。したがって、Cu含有量は0~0.50%である。上記効果を安定して得るためのCu含有量の好ましい下限は0.05%であり、より好ましくは0.10%である。Cuを0.30%以上含有する場合、Ni含有量がCu含有量よりも高ければ、熱間延性を維持できる。 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.
チタン(Ti)は任意元素であり、含有されなくてもよい。含有される場合、Tiは窒化物、炭化物、又は炭窒化物を生成し、焼き入れ時や焼準時においてオーステナイト粒の粗大化を抑制する。Tiはさらに、析出強化により鋼の強度を高める。Tiはさらに、鋼を脱酸する。Tiはさらに、Bを含有する場合、固溶Nと結合して固溶B量を維持する。この場合、焼入れ性が高まる。Tiが少しでも含有されれば、上記効果がある程度得られる。 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.
カルシウム(Ca)は任意元素であり、含有されなくてもよい。含有される場合、CaはCaS又は(Mn,Ca)Sを生成してMnS介在物を球状化し、工具摩耗量を低減する。その結果、鋼の被削性が高まる。Caが少しでも含有されれば、上記効果がある程度得られる。一方、Ca含有量が0.0030%を超えれば、酸化物系介在物が粗大化し、鋼の疲労強度が低下する。したがって、Ca含有量は0~0.0030%である。被削性をより高めるためのCa含有量の好ましい下限は0.0001%である。被削性よりも疲労強度を優先する場合、Ca含有量の好ましい上限は0.0015%であり、より好ましくは0.0003%である。 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%.
本実施形態の機械構造用鋼の化学組成はさらに、式(1)を満たす。
Mn/S≧8.0 (1)
ここで、式(1)中の各元素には、対応する元素の含有量(質量%)が代入される。 [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).
本発明による機械構造用鋼のミクロ組織は、主としてフェライト及びパーライトからなる。具体的に、上記化学組成の機械構造用鋼は、ミクロ組織におけるフェライト及びパーライトの合計面積率は、99%以上である。 [About the microstructure of steel]
The microstructure of the machine structural steel according to the invention consists mainly of ferrite and pearlite. Specifically, the mechanical structural steel having the above chemical composition has a total area ratio of ferrite and pearlite in the microstructure of 99% or more.
本発明による機械構造用鋼は、鋼中において、MnS介在物、Pb介在物、及び、MnS及びPbを含有する複合介在物のいずれかであって、円相当径が5μm以上である介在物(つまり、特定介在物)の総個数TNが40個/mm2以上である。 [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.
好ましくは、円相当径が5μm以上である複合介在物の総個数(個/mm2)の、特定介在物に対する個数(個/mm2)の比(以下、「複合比率」ともいう)RAが40%以上である。 [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.
特定介在物の個数TN及び複合比率RAは次の方法で測定できる。上述の方法で、機械構造用鋼からサンプルを採取する。R/2部のサンプルの横断面(表面)に対して、走査型電子顕微鏡(SEM)を用いて1000倍の倍率でランダムに20視野観察する。各視野(観察面という)において、特定介在物(MnS介在物、Pb介在物、及び、複合介在物のいずれかであり、円相当径が5μm以上である)を特定する。特定介在物と他の介在物とは、コントラストで区別可能である。さらに、特定介在物のうち、MnS介在物、Pb介在物、及び、複合介在物は、それぞれ次の方法で特定する。 [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.
RA=MN/TN×100 (2) 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 specific inclusions, the number MN (number / mm 2 ) of composite inclusions having an equivalent circle diameter of 5 μm or more is obtained, and the composite ratio RA (%) is obtained based on the following equation (2). .
RA = MN / TN × 100 (2)
本発明による機械構造用鋼の製造方法の一例を説明する。本実施形態では、機械構造用鋼の一例として、棒鋼又は線材の製造方法を説明する。しかしながら、本発明による機械構造用鋼は、棒鋼又は線材に限定されない。 [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.
製鋼工程は、精錬工程と、鋳造工程とを含む。 [Steel making process]
The steel making process includes a refining process and a casting process.
精錬工程では、初めに周知の方法で製造された溶銑に対して、転炉での精錬(一次精錬)を実施する。転炉から出鋼した溶鋼に対して、二次精錬を実施する。二次精錬において、成分調整の合金添加を実施して、上記化学組成を有する溶鋼を製造する。 [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.
上記精錬工程により製造された溶鋼を用いて、素材(鋳片又はインゴット)を製造する。具体的には、溶鋼を用いて連続鋳造法により鋳片を製造する。又は、溶鋼を用いて造塊法によりインゴットを製造してもよい。以下、鋳片及びインゴットを総称して素材という。ここでいう素材の横断面積はたとえば、200~350mm×200~600mmである。 [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.
RC=(λ2/770)-(1/0.41) (3) The solidification cooling rate RC can be obtained from the cast material. FIG. 4 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)
熱間加工工程では通常、1又は複数回の熱間加工を実施する。各熱間加工を実施する前に、素材を加熱する。その後、素材に対して熱間加工を実施する。熱間加工はたとえば、熱間鍛造や、熱間圧延である。複数回熱間加工を実施する場合、最初の熱間加工はたとえば、分塊圧延又は熱間鍛造であり、次の熱間加工は、連続圧延機を用いた仕上げ圧延である。熱間圧延機では、一対の水平ロールを有する水平スタンドと、一対の垂直ロールを有する垂直スタンドとが交互に一列に配列される。熱間加工後の素材は空冷等の周知の冷却法により冷却される。 [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.
[ミクロ組織観察]
各試験番号の棒鋼のR/2部から、組織観察用の試験片を採取した。試験片の表面のうち、棒鋼の長手方向(つまり、圧延方向又は延伸方向)と平行な断面を観察面と定義した。上述の方法に基づいて、フェライト及びパーライトの合計面積率(%)を求めた。各試験番号の棒鋼のミクロ組織は、いずれも合計面積率が99%以上であった。合計面積率が99%以上のミクロ組織について、「F+P」として表2に示す。 [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 and pearlite was determined. 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”.
各試験番号の棒鋼のR/2部から、組織観察用の試験片を採取した。試験片の表面のうち、棒鋼の長手方向(つまり、圧延方向又は延伸方向)と平行な断面を観察面と定義した。各試験番号の組織観察用の試験片の観察面について、上述の方法に基づいて、特定介在物個数TN(個/mm2)と、複合比率RA(%)とを求めた。結果を表2に示す。 [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.
各試験番号の棒鋼のR/2部の任意の5点において、JIS Z 2244(1981)に準拠してビッカース硬さ試験を実施した。試験力は100Nとした。得られた5つの値の平均を、その試験番号の棒鋼のビッカース硬さ(HV)と定義した。ビッカース硬さがHV160以上であれば、十分な強度を有すると判断した。一方、ビッカース硬さがHV160未満の場合、強度が不十分と判断した。表2に結果を示す。いずれの試験番号においても、ビッカース硬さはHV160以上であり、十分な強度を示した。 [Vickers hardness test]
A Vickers hardness test was carried out in accordance with JIS Z 2244 (1981) at any five points of R / 2 part of the steel bar of each test number. The test force was 100N. The average of the five values obtained was defined as the Vickers hardness (HV) of the steel bar of that test number. If the Vickers hardness was HV160 or more, it was judged to have sufficient strength. On the other hand, when the Vickers hardness was less than HV160, it was determined that the strength was insufficient. Table 2 shows the results. In any of the test numbers, the Vickers hardness was HV160 or more, indicating a sufficient strength.
被削性は、工具摩耗量(μm)及び切り屑処理性を評価した。具体的には、直径50mmの棒鋼を所定の長さで切断して切削試験片とした。切削試験片に対して、図5に示す外周旋削を実施した。外周旋削の条件を表3に示す。 [Machinability]
Machinability evaluated the tool abrasion amount (micrometer) and the chip disposal property. Specifically, 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. 5 was implemented with respect to the cutting test piece. Table 3 shows the peripheral turning conditions.
1000個目の試験片の旋削が完了した後の工具50について、前逃げ面の工具摩耗量(mm)を測定した。測定結果を表2の「工具摩耗量」欄に示す。工具摩耗量が200μm以下の場合、工具寿命が優れると判断した。一方、工具摩耗量が200μmを超える場合、工具寿命が優れないと判断した。 [Tool life evaluation]
The tool wear amount (mm) of the front flank was measured for the
1000個目の試験片の旋削では、図6A及び図6Bに示す切り屑が得られた。そこで、切り屑の長さL20と、直径D20とを測定した。測定結果に基づいて、次のとおり評価した。切り屑が直径30mm以下のコイル形状である場合、又はコイル形状でなくても切り屑長さが50mm未満であった場合、切り屑処理性に優れると判断した(表2中の「○」)。一方、切り屑が直径30mm以下のコイル形状ではなく、かつ、切り屑長さも50mm以上であった場合、切り屑処理性が低いと判断した(表2中の「×」)。 [Evaluation of chip disposal]
In turning the 1000th test piece, chips shown in FIGS. 6A and 6B 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 has a coil shape with a diameter of 30 mm or less, or when the chip length is less than 50 mm even if it is not a coil shape, it is judged that the chip disposal is excellent (“◯” in Table 2). . On the other hand, when the chip was not a coil shape having a diameter of 30 mm or less and the chip length was 50 mm or more, it was determined that the chip disposal was low (“×” in Table 2).
直径50mmの棒鋼を所定の長さに切断した発銹試験片を作製した。発銹試験片に対して、上述の切削試験と同様の条件で旋削加工を行った。その後、切削面に水道水を噴霧しながら、湿度70%、20℃の雰囲気内に1時間試験片を保管した。保管後、試験片の切削面を観察し、銹点の個数を測定した。測定結果を表2の「発銹特性」欄に示す。銹点が10点未満であった場合(表2中の「◎」)、及び、銹点が10点以上20点未満であった場合(表2中の「○」)、発銹特性が優れると判断した。一方、銹点が20点以上であった場合(表2中の「×」)、発銹特性が優れないと判断した。 [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 “Spring characteristics” 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.
通電加熱による熱間引張試験を実施して、熱間延性を評価した。具体的には、各試験番号の鋳片から、直径10mm、長さ100mmであって、両端がねじ加工された丸棒試験片を作製した。丸棒試験片を通電加熱により1100℃に加熱し、3分保持した。その後、放冷により丸棒試験片を900℃まで冷却した。丸棒試験片が900℃の状態で引張試験を実施し、破断時の絞り値(%)を求めた。各試験番号につき3本の丸棒試験片で引張試験を実施して、3つの値の平均を、その試験番号の絞り値(%)と定義した。絞り値を表2の「熱間延性」の欄に示す。絞り値が70%以上の場合、熱間延性が優れると評価した。一方、絞り値が70%未満の場合、熱間延性が優れないと評価した。 [Hot ductility evaluation test]
A hot tensile test by conducting heating was performed to evaluate hot ductility. 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 was excellent. On the other hand, when the aperture value was less than 70%, it was evaluated that the hot ductility was not excellent.
試験番号1~26では、化学組成が適切であり、F1が8.0以上であり、脱酸順が適切であり、凝固冷却速度RCが100℃/分以下であった。そのため、特定介在物の個数TNが40個/mm2以上であった。その結果、工具摩耗量が200μm以下であり、かつ、優れた切り屑処理性得られた。すなわち、優れた被削性が得られた。さらに、発銹特性評価試験において、いずれも、銹点が20点未満であり、優れた発銹特性が得られた。さらに、熱間延性評価試験において、絞り値が70%以上であり、優れた熱間延性が得られた。 [Test results]
In
20 Pb介在物
30 複合介在物 10
Claims (4)
- 質量%で、
C:0.30~0.80%、
Si:0.01~0.80%、
Mn:0.20~2.00%、
P:0.030%以下、
S:0.010~0.100%、
Pb:0.010~0.100%、
Al:0.010~0.050%、
N:0.015%以下、
O:0.0005~0.0030%、
Cr:0~0.70%、
Ni:0~3.50%、
B:0~0.0050%、
V:0~0.70%、
Mo:0~0.70%、
W:0~0.70%、
Nb:0~0.050%未満、
Cu:0~0.50%、
Ti:0~0.100%、及び、
Ca:0~0.0030%を含有し、残部はFe及び不純物からなり、式(1)を満たす化学組成を有し、
鋼中において、MnS介在物、Pb介在物、及び、MnS及びPbを含有する複合介在物のいずれかであって、円相当径が5μm以上である特定介在物の総個数が40個/mm2以上である、機械構造用鋼。
Mn/S≧8.0 (1)
ここで、式(1)中の各元素には、対応する元素の含有量(質量%)が代入される。 % By mass
C: 0.30 to 0.80%,
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 to 0.70%,
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). - 請求項1に記載の機械構造用鋼であって、
前記化学組成は、
Cr:0.10~0.70%、
Ni:0.02~3.50%、
B:0.0005~0.0050%、
V:0.05~0.70%、
Mo:0.05~0.70%、
W:0.05~0.70%、
Nb:0.001~0.050%未満、
Cu:0.05~0.50%、及び、
Ti:0.003~0.100%からなる群から選択される1種又は2種以上を含有する、機械構造用鋼。 The mechanical structural steel according to claim 1,
The chemical composition is
Cr: 0.10 to 0.70%,
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%. - 請求項1又は請求項2に記載の機械構造用鋼であって、
前記化学組成は、
Ca:0.0001~0.0030%を含有する、機械構造用鋼。 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%. - 請求項1~請求項3のいずれか1項に記載の機械構造用鋼であって、
前記複合介在物の前記特定介在物に対する個数比率が40%以上である、機械構造用鋼。 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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KR1020197003389A KR20190027848A (en) | 2016-07-04 | 2017-07-04 | Steel for machine structural use |
US16/313,931 US20190233927A1 (en) | 2016-07-04 | 2017-07-04 | Steel for Machine Structural Use |
EP17824224.4A EP3480333A4 (en) | 2016-07-04 | 2017-07-04 | Steel for mechanical structures |
JP2018526388A JP6760375B2 (en) | 2016-07-04 | 2017-07-04 | Machine structural steel |
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WO2022234320A1 (en) * | 2021-05-04 | 2022-11-10 | Arcelormittal | Steel sheet and high strength press hardened steel part and method of manufacturing the same |
WO2022234319A1 (en) * | 2021-05-04 | 2022-11-10 | Arcelormittal | Steel sheet and high strength press hardened steel part and method of manufacturing the same |
CN115449704B (en) * | 2022-07-29 | 2023-07-25 | 江阴兴澄特种钢铁有限公司 | New energy automobile hub bearing steel and production method thereof |
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- 2017-07-04 KR KR1020197003389A patent/KR20190027848A/en not_active Application Discontinuation
- 2017-07-04 WO PCT/JP2017/024442 patent/WO2018008621A1/en unknown
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