US20190169723A1 - Steel for Machine Structural Use - Google Patents

Steel for Machine Structural Use Download PDF

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US20190169723A1
US20190169723A1 US16/320,651 US201716320651A US2019169723A1 US 20190169723 A1 US20190169723 A1 US 20190169723A1 US 201716320651 A US201716320651 A US 201716320651A US 2019169723 A1 US2019169723 A1 US 2019169723A1
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steel
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Masayuki Hashimura
Makoto Egashira
Shouji Toudou
Takanori Iwahashi
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Nippon Steel Corp
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Nippon Steel Corp
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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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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
    • 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
    • B22D11/12—Accessories for subsequent treating or working cast stock in situ
    • B22D11/124—Accessories for subsequent treating or working cast stock in situ for cooling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0006—Adding metallic additives
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/06—Deoxidising, e.g. killing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10—Handling in a vacuum
    • 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

Definitions

  • the present invention relates to steel, and more particularly relates to a steel for machine structural use.
  • Excellent rolling contact fatigue properties may be required for machine components to be used for structural use and power transmission use, such as components of general machinery and automobiles.
  • An example of a method for producing such kind of machine components is as follows. A steel for machine structural use is subjected to hot working (hot forging or the like) to produce an intermediate product. The intermediate product is subjected to machining (cutting or grinding) to produce a machine component. As necessary, a case hardening treatment may also be performed on the machine component. The case hardening treatment is, for example, a carburizing.
  • a steel for machine structural use for producing such kind of machine components is required to be excellent not only in hot workability, but also in machinability.
  • a steel for machine structural use that is excellent in machinability is also called a “free-cutting steel”, and is defined in JIS G 4804 (2008) (Non Patent Literature 1).
  • the machinability of a free-cutting steel is enhanced by containing Pb.
  • Patent Literature 1 A steel for machine structural use that contains Pb is disclosed, for example, in Japanese Patent Application Publication No. 2000-282172 (Patent Literature 1).
  • the steel material for machine structural use disclosed in Patent Literature 1 has a chemical composition which contains, in 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% or less, Ni: 0 to 2.0%, Mo: 0 to 1.5%, B: 0 to 0.01%, Bi: 0 to 0.10%, Ca: 0 to 0.05%, Pb: 0 to 0.12%, Ti: 0 to less than 0.04%, Zr: 0 to less than 0.04% and Ti (%)+Zr (%): 0 to less than 0.04%, Te: 0 to 0.05%, Nd: 0 to
  • a proportion that a ferritic phase occupies in the micro-structure is, with respect to the area ratio, from 10 to 80%, and Hv hardness is in a range from 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 ⁇ .
  • a symbol of an element in the respective formulas represents the content in mass % of the corresponding element, and a represents the area ratio (%) of the ferritic phase in the micro-structure. It is described in Patent Literature 1 that the steel material for machine structural use is excellent in machinability and toughness.
  • machining such as cutting is performed using an automated equipment system.
  • an automated equipment system in the case of producing a large amount of machine components by cutting intermediate products, such as producing several hundred or more machine components per day, excellent chip treatability is required. It is preferable that chips that are to be discharged accompanying cutting are split into small pieces and discharged. If the chips remain connected in a long length, the chips are liable to become entwined around the intermediate product, and defects are liable to arise on the surface of the machine component after cutting. If a chip is entwined around a machine component, it is also necessary to temporarily stop the production line to remove the chip that is entwined around the machine component.
  • chip treatability affects both the quality of the machine components and the production cost.
  • productivity decreases as tool wear increases. Therefore, a steel for machine structural use is required to have high machinability, such as being capable of suppressing tool wear and being excellent in chip treatability.
  • rust occurs in the machine component.
  • a water-soluble cutting oil is utilized from the viewpoint of performing unattended operations. Consequently, in some cases rust occurs in the machine components. Rust is not only a cause of the occurrence of shape errors, but is also a cause of quality defects when performing a plating treatment on the machine component.
  • the machine components are sometimes stored in a bucket or the like for a long time period until undergoing the next process after the cutting process.
  • a steel for machine structural use is required to be not only excellent in machinability, but also to have characteristics that suppress the occurrence of rust (hereunder, referred to as “rusting characteristics”).
  • An objective of the present invention is to provide a steel for machine structural use that is excellent in machinability, rusting characteristics and hot workability, and with which a machine component that is excellent in rolling contact fatigue properties is obtained.
  • a steel for machine structural use according to the present invention has a chemical composition which consists of, in mass %, C: 0.15 to less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0.030% or less, S: 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al: 0.010 to 0.050%, N: 0.015% or less, O: 0.0005 to 0.0030%, Cr: 0.50 to 2.00%, Ni: 0 to 3.50%, B: 0 to 0.0050%, V: 0 to 0.70%, Mo: 0 to 0.70%, W: 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 impurities, the chemical composition satisfying Formula (1).
  • a total number of specific inclusions which are any of MnS inclusions, Pb inclusions, and composite inclusions, the
  • a steel for machine structural use according to the present invention is excellent in machinability, rusting characteristics and hot workability, and is a steel with which a machine component that is excellent in rolling contact fatigue properties is obtained.
  • FIG. 1A is a schematic diagram illustrating an S distribution in an observation surface, that was obtained by EPMA analysis.
  • FIG. 1B is a schematic diagram illustrating a Pb distribution in the same observation surface as in FIG. 1A , that was obtained by EPMA analysis.
  • FIG. 1C is a schematic diagram of an image obtained by combining FIG. 1A and FIG. 1B .
  • FIG. 2 is a schematic diagram for describing a criterion for determining whether or not to regard adjacent inclusions as a single inclusion.
  • FIG. 3 is a transverse sectional view of a starting material that was cast.
  • FIG. 4 is a schematic diagram of a cutting test machine for describing a cutting test.
  • FIG. 5A is a perspective view of a chip.
  • FIG. 5B is a planar photographic view of a chip.
  • FIG. 6 shows a front view and a side view of a rolling contact fatigue test specimen used in a rolling contact fatigue test.
  • FIG. 7 is a schematic diagram of a thrust-type rolling contact fatigue test machine for describing a rolling contact fatigue test.
  • the present inventors conducted investigations and studies regarding the machinability, rusting characteristics and hot workability of steels for machine structural use. As a result, the present inventors found that if a steel for machine structural use has a chemical composition consisting of, in mass %, C: 0.15 to less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0.030% or less, S: 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al: 0.010 to 0.050%, N: 0.015% or less, O: 0.0005 to 0.0030%, Cr: 0.50 to 2.00%, Ni: 0 to 3.50%, B: 0 to 0.0050%, V: 0 to 0.70%, Mo: 0 to 0.70%, W: 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 impurities
  • Mn in the steel combines with S to form MnS.
  • the MnS is divided into MnS inclusions and MnS precipitates according to the formation process.
  • MnS inclusions crystallize in the molten steel before solidification.
  • MnS precipitates precipitate in the steel after solidification.
  • the MnS inclusions form in the molten steel. Therefore, the size of the MnS inclusions tends to be large in comparison to the MnS precipitates that form after solidification.
  • Pb in the steel does not dissolve in the steel, and is present as Pb inclusions (Pb grains).
  • Pb inclusions Pb grains.
  • the MnS inclusions and Pb inclusions each enhance the machinability of the steel.
  • composite inclusions means inclusions that contain MnS and Pb, with the balance being impurities. More specifically, there are cases where composite inclusions are composed by MnS and Pb that are adjacent to each other, and there are also cases where Pb dissolves into MnS to form a composite inclusion.
  • MnS inclusions refers to inclusions that contain Mn and S and do not contain Pb.
  • Pb inclusions refers to inclusions which are composed of Pb and impurities and which do not contain Mn.
  • composite inclusions refers to inclusions that contain Mn, S and Pb.
  • MnS inclusions are known as inclusions that enhance machinability.
  • the fusing point of Pb inclusions is lower than the fusing point of MnS inclusions. Therefore, Pb inclusions exert a lubricating action during cutting, and as a result the machinability of the steel is enhanced.
  • composite inclusions enhance the machinability of steel more than individual MnS inclusions and Pb inclusions.
  • a fissure has arisen at the periphery of a composite inclusion
  • liquefied Pb enters into the open crack.
  • propagation of the crack is promoted and machinability is enhanced.
  • the machinability is enhanced further.
  • the mechanism by which composite inclusions are formed is considered to be as follows. It is easier for Pb to move in liquid phase than in solid phase. Therefore, almost no composite inclusions can be formed from MnS precipitates which form after solidification of the steel, and the composite inclusions are instead formed by adherence of Pb to MnS inclusions that are formed in the molten steel before solidification. Accordingly, in order to form a large number of composite inclusions, it is desirable to form a large number of MnS inclusions in the molten steel rather than forming MnS precipitates after solidification.
  • MnS inclusions are formed in molten steel by crystallization.
  • the greater the number of MnS inclusions that are present the greater the number of composite inclusions that will be formed. Therefore, it is considered that the machinability of the steel is enhanced by causing a large number of MnS inclusions to crystallize in the molten steel.
  • MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions become starting points for rust.
  • the susceptibility to rusting depends more on the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions than the size of the MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions.
  • the susceptibility to rusting of the steel increases as the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions increases. Based on the above finding, the present inventors concluded that in order to suppress rusting while obtaining excellent machinability, it is effective to decrease the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions. Therefore, the present inventors studied methods for decreasing the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions.
  • MnS inclusions that are formed by crystallization in molten steel are liable to grow (coarsen) in the molten steel. Therefore, the size of MnS inclusions is larger than the size of MnS precipitates that are formed by precipitation in the steel after solidification. That is, the MnS precipitates precipitate more finely than the MnS inclusions. Therefore, in a steel having a certain Mn content and S content, if a case in which MnS inclusions are caused to crystallize and a case in which MnS precipitates are caused to precipitate are supposed, the number of MnS precipitates that are formed by precipitation will be noticeably greater than the number of MnS inclusions that are formed by crystallization. Accordingly, to improve the rusting characteristics of a steel, it suffices to suppress precipitation of MnS precipitates by crystallizing MnS inclusions in the molten steel and causing the MnS inclusions to grow (coarsen).
  • MnS inclusions In order to cause MnS inclusions to crystallize and grow in molten steel and suppress precipitation of MnS precipitates and, as a result, decrease the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions, it suffices to significantly increase the Mn content in comparison to the S content. If the Mn content is sufficiently higher than the S content, coarse MnS inclusions are likely to form in the molten steel. In this case, because S is consumed by the crystallization of the coarse MnS inclusions, the amount of dissolved S in the steel after solidification is lowered. Consequently, precipitation of MnS precipitates can be suppressed, and the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions can be decreased. As a result, excellent rusting characteristics are obtained.
  • the Mn content and S content satisfy the following Formula (1).
  • F1 Mn/S. If F1 is less than 8.0, it is difficult for MnS inclusions to adequately crystallize in the molten steel. Therefore, the amount of dissolved S in the steel after solidification cannot be adequately decreased, and a large number of fine MnS precipitates are formed after solidification. In this case, because the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions cannot be decreased, the rusting characteristics of the steel decline. On the other hand, if F1 is 8.0 or more, the Mn content is adequately high in comparison with the S content. In this case, by using an appropriate production method, MnS inclusions in the molten steel adequately crystallize and grow.
  • the amount of dissolved S in the steel after solidification is adequately decreased, 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 can be adequately reduced, and the rusting characteristics of the steel are enhanced.
  • inclusions which are any of MnS inclusions, Pb inclusions and composite inclusions and which have an equivalent circular diameter of 5 ⁇ m or more are defined as “specific inclusions”.
  • the term “equivalent circular diameter” means the diameter of a circle in a case where the area of an inclusion or a precipitate that is observed during micro-structure observation is converted into a circle having the same area.
  • the total number of specific inclusions is 40 per mm 2 or more.
  • the total number of specific inclusions in the steel is 40 per mm 2 or more, coarse MnS inclusions adequately crystallize and formation of MnS precipitates can be suppressed. As a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions which become starting points for rusting can be adequately reduced. Therefore, excellent machinability and excellent rusting characteristics can both be realized in a compatible manner.
  • the total number of specific inclusions in the steel is less than 40 per mm 2 , MnS inclusions do not adequately crystallize, and a large number of MnS precipitates form. As a result, the formation of MnS precipitates can be suppressed. Consequently, the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions which become starting points for rusting cannot be adequately reduced. Therefore, although excellent machinability is obtained, adequate rusting characteristics are not obtained.
  • a steel for machine structural use according to the present embodiment that was completed based on the above findings has a chemical composition which consists of, in mass %, C: 0.15 to less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0.030% or less, S: 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al: 0.010 to 0.050%, N: 0.015% or less, O: 0.0005 to 0.0030%, Cr: 0.50 to 2.00%, Ni: 0 to 3.50%, B: 0 to 0.0050%, V: 0 to 0.70%, Mo: 0 to 0.70%, W: 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 impurities, the chemical composition satisfying Formula (1).
  • the total number of specific inclusions which are any of MnS inclusions, Pb
  • the chemical composition of the steel for machine structural use that is described above may contain one or more types of element selected from a group consisting of Ni: 0.02 to 3.50%, B: 0.0005 to 0.0050%, V: 0.05 to 0.70%, Mo: 0.05 to 0.70%, W: 0.05 to 0.70%, Nb: 0.001 to less than 0.050%, Cu: 0.05 to 0.50% and Ti: 0.003 to 0.100%.
  • the chemical composition of the steel for machine structural use that is described above may contain Ca: 0.0001 to 0.0030%.
  • a ratio of the number of the composite inclusions to the specific inclusions may be 40% or more.
  • the chemical composition of the steel for machine structural use of the present embodiment contains the following elements.
  • Carbon (C) increases the strength of steel.
  • a carburizing is performed after forging of the steel for machine structural use.
  • C increases the strength of the outer layer of the steel. If the C content is less than 0.15%, sufficient strength is not obtained in the steel. If a soft steel is used, in some cases chips connect during cutting, and the chips become entwined around the tools or material and cause damage thereto. In addition, if the C content is less than 0.15%, the cutting resistance of the steel increases.
  • the C content is from 0.15 to less than 0.30%.
  • a preferable lower limit of the C content is 0.16%, and more preferably is 0.18%.
  • a preferable upper limit of the C content is 0.25%, and more preferably is 0.23%.
  • Si deoxidizes the steel.
  • Si deoxidizes the steel.
  • Si deoxidizes oxides.
  • Si added to molten steel reforms oxides that are mainly composed of Mn into oxides that are mainly composed of Si.
  • Al By adding Si, complex oxides containing Si and Al form in the steel.
  • the complex oxides serve as nuclei for crystallization of MnS inclusions. Therefore, the complex oxides enhance the rusting characteristics of the steel. Si also enhances temper softening resistance and raises the strength. The aforementioned effects are not obtained if the Si content is less than 0.01%.
  • Si is a ferrite forming element. If the Si content is more than 0.80%, the outer layer of the steel may be decarburized. Furthermore, if the Si content is more than 0.80%, the ferrite fraction may increase and the strength decrease in some cases. Accordingly, the Si content is from 0.01 to 0.80%.
  • a preferable lower limit of the Si content for increasing the temper softening resistance is 0.10%, and more preferably is 0.20%.
  • a preferable upper limit of the Si content for keeping the ferrite fraction low is 0.70%, and more preferably is 0.50%.
  • Manganese (Mn) forms MnS inclusions and composite inclusions containing MnS and Pb, and enhances the machinability of the steel.
  • Mn also deoxidizes the steel.
  • the deoxidizing power of Mn is weak compared to Si or Al. Therefore, a large amount of Mn may be contained.
  • oxides that are mainly composed of Mn form in the molten steel.
  • the Mn contained in the oxides is discharged into the molten steel, and the oxides are reformed.
  • the reformed oxides are referred to as “complex oxides”.
  • the Mn that is discharged into the molten steel from the oxides combines with S to form MnS inclusions.
  • the Mn content is from 0.20 to 2.00%.
  • a preferable lower limit of the Mn content is 0.50%.
  • a preferable upper limit of the Mn content is 1.50%, and more preferably is 1.20%.
  • Phosphorus (P) is unavoidably contained. P embrittles the steel and enhances the machinability. On the other hand, if the P content is more than 0.030%, hot ductility decreases. In such a case, rolling defects and the like occur, and the productivity decreases. Accordingly, the P content is 0.030% or less. A preferable lower limit of the P content for enhancing the machinability is 0.005%. In this case, the machinability, particularly the chip treatability, is enhanced. A preferable upper limit of the P content is 0.015%.
  • S Sulfur
  • MnS Sulfur
  • S forms MnS in the steel and enhances the machinability.
  • MnS suppresses tool wear. If the S content is less than 0.010%, MnS will not crystallize adequately and it will be difficult for composite inclusions containing MnS and Pb to form. As a result, the rusting characteristics will decrease. On the other hand, if the S content is more than 0.100%, S will segregate at grain boundaries and the steel will become brittle, and the hot workability of the steel will decrease. Accordingly, the S content is from 0.010 to 0.100%.
  • a preferable lower limit of the S content is 0.015%, and a preferable upper limit is 0.030%.
  • a preferable lower limit of the S content is 0.030%, and a preferable upper limit is 0.050%.
  • Pb forms Pb inclusions (Pb grains) by itself, and enhances the machinability of the steel.
  • Pb also combines with MnS inclusions to form composite inclusions and enhance the machinability of the steel, and in particular enhance the chip treatability.
  • the aforementioned effects are not obtained if the Pb content is less than 0.010%.
  • the Pb content is more than 0.100%, although the machinability will be enhanced, the steel will become brittle. As a result, the hot workability of the steel will decrease.
  • the Pb content is more than 0.100%, because the Pb inclusions will excessively increase, the rusting characteristics of the steel will decrease. Accordingly, the Pb content is from 0.010 to 0.100%.
  • a preferable lower limit of the Pb content for promoting the formation of composite inclusions and enhancing the machinability is 0.020%, and more preferably is 0.025%.
  • a preferable upper limit of the Pb content for enhancing the rusting characteristics is 0.050%.
  • deoxidation is performed by aluminum killing.
  • Al As described later, if deoxidation is performed by adding Al into the molten steel after Mn and Si, oxides in the steel are reformed and complex oxides containing Si and Al are formed.
  • the complex oxides easily become nuclei for crystallization of MnS inclusions. Therefore, it is easy for MnS inclusions to disperse and crystallize, and to grow and coarsen, and it is also easy for composite inclusions containing MnS and Pb to form. In this case, the machinability of the steel improves.
  • the Al content is more than 0.050%, coarse complex oxides are liable to form. If coarse complex oxides are formed in the steel, surface defects are liable to occur on the steel. If coarse complex oxides are formed in the steel, the fatigue strength of the steel will also decrease.
  • the Al content is more than 0.050%, deoxidation will proceed excessively, and the amount of oxygen in the molten steel will decrease. In this case, it will be difficult to form MnS inclusions, and the machinability (particularly, suppression of tool wear) of the steel will decrease. In such a case, in addition, it will be difficult for composite inclusions in which Pb is combined with MnS inclusions to form, and a large number of independent Pb inclusions will remain in the steel.
  • the Al content is from 0.010 to 0.050%.
  • a preferable lower limit of the Al content for obtaining a further effect of suppressing the coarsening of grains by formation of AlN is 0.015%, and more preferably is 0.020%.
  • a preferable upper limit of the Al content is 0.035%.
  • Al content means the content of acid-soluble Al (sol. Al).
  • N Nitrogen
  • N is unavoidably contained. N combines with Al to form AlN to thereby suppress coarsening of austenite grains during heat treatment and enhance the strength of the steel.
  • the N content is more than 0.015%, the cutting resistance of the steel increases and the machinability decreases. If the N content is more than 0.015%, the hot workability also decreases. Accordingly, the N content is 0.015% or less.
  • a preferable lower limit of the N content is 0.002%, and more preferably is 0.004%.
  • a preferable upper limit of the N content is 0.012%, and more preferably is 0.008%.
  • the term “N content” means the total content of N (t-N).
  • Oxygen (O) is contained not only in oxides, but also in MnS inclusions. O forms complex oxides that serve as nuclei for crystallization of MnS inclusions. If the O content is less than 0.0005%, the formed amount of complex oxides will be insufficient, and it will be difficult for MnS inclusions to crystallize in the molten steel. In such a case, the machinability of the steel will decrease. Furthermore, in such a case, a large number of fine MnS precipitates will form after solidification. As a result, the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions will increase, and rusting characteristics will decrease.
  • the O content is more than 0.0030%, coarse alumina-based oxides will form and promote cutting wear of the tools, and hence the machinability of the steel will decrease. Accordingly, the O content is from 0.0005 to 0.0030%.
  • a preferable lower limit of the O content for further improving the machinability of the steel as well as the rusting characteristics of the steel is 0.0007%, and more preferably is 0.0010%.
  • a preferable upper limit of the O content is 0.0025%, and more preferably is 0.0020%.
  • the term “O content” means the total content of oxygen (t-O).
  • Chromium (Cr) dissolves in the steel and enhances the hardenability and temper softening resistance of the steel to thereby increase the strength of the steel. As a result, the rolling contact fatigue properties of the steel are enhanced. If a carburizing is performed as a case hardening treatment, Cr also deepens the hardened layer depth. The aforementioned effects are not obtained if the Cr content is less than 0.50%. On the other hand, if the Cr content is more than 2.00%, the hardenability will be too high and a supercooled microstructure (martensite) will form during cooling and the steel will become too hard. In such a case, the machinability of the steel will decrease.
  • the Cr content is more than 2.00%, in some cases austenite may stabilize at even a low temperature and the steel will become brittle. Accordingly, the Cr content is from 0.50 to 2.00%.
  • a preferable lower limit of the Cr content is 0.70%, and more preferably is 0.90%.
  • a preferable upper limit of the Cr content is 1.80%, and more preferably is 1.60%.
  • the balance of the chemical composition of the steel for machine structural use according to the present embodiment is Fe and impurities.
  • impurities refers to elements which, during industrial production of the steel for machine structural use, are mixed in from ore or scrap that is used as a raw material, or from the production environment or the like, and which are allowed within a range that does not adversely affect the steel for machine structural use of the present embodiment.
  • the chemical composition of the steel for machine structural use of the present embodiment may further contain one or more types of element selected from the group consisting of Ni, B, V, Mo, W, Nb, Cu and Ti.
  • Nickel (Ni) is an optional element and need not be contained. If contained, Ni dissolves in the steel and increases the hardenability of the steel, and enhances the steel strength. Ni also improves the ductility of the matrix. In addition, Ni increases the toughness of the steel. Furthermore, Ni increases the corrosion resistance of the steel. The aforementioned effects are obtained to a certain extent if even a small amount of Ni is contained. On the other hand, if the Ni content is more than 3.50%, a large amount of retained austenite will remain. In such a case, a part of the retained austenite will transform into martensite by strain induced transformation, and the ductility of the steel will decrease. Accordingly, the Ni content is from 0 to 3.50%.
  • a preferable lower limit of the Ni content for stably obtaining the aforementioned effects is 0.02%, and more preferably is 0.05%.
  • a preferable upper limit of the Ni content for further suppressing the formation of retained austenite is 2.50%, and more preferably is 2.00%.
  • a preferable lower limit of the Ni content is 0.20%. Note that, Ni detoxifies Cu and enhances the toughness. If the steel contains Cu, a preferable lower limit of the Ni content is equal to or more than the Cu content.
  • B Boron
  • B is an optional element and need not be contained. If contained, B increases the hardenability of the steel and increases the steel strength. B also suppresses segregation at the grain boundaries of P and S that decrease toughness, and thus enhances the fracture characteristics. The aforementioned effects are obtained to a certain extent if even a small amount of B is contained. On the other hand, if the B content is more than 0.0050%, a large amount of BN will be formed and the steel will become brittle. Accordingly, the B content is from 0 to 0.0050%. A preferable lower limit of the B content in a case where Ti or Nb that are nitride-forming elements is contained is 0.0005%. A preferable upper limit of the B content is 0.0020%.
  • Vanadium (V) is an optional element and need not be contained. If contained, V precipitates as V carbides, V nitrides, or V carbo-nitrides during tempering and during nitriding, and enhances the strength of the steel.
  • the V precipitates (V carbides, V nitrides and V carbo-nitrides) also suppress coarsening of austenite grains and increase the toughness of the steel.
  • V dissolves in the steel and thereby increases the temper softening resistance of the steel. The aforementioned effects are obtained to a certain extent if even a small amount of V is contained.
  • V content is from 0 to 0.70%.
  • a preferable lower limit of the V content for stably obtaining the aforementioned effects is 0.05%, and more preferably is 0.10%.
  • a preferable upper limit of the V content is 0.50%, and more preferably is 0.30%.
  • Molybdenum (Mo) is an optional element and need not be contained. If contained, Mo precipitates as Mo carbides during a heat treatment at a low temperature that is not more than the Ai, such as a heat treatment for tempering or nitriding. Therefore, the strength and temper softening resistance of the steel increase. Mo also dissolves in the steel and increases the hardenability of the steel. The aforementioned effects are obtained to a certain extent if even a small amount of Mo is contained. On the other hand, if the Mo content is more than 0.70%, the hardenability of the steel will be too high. In such a case, a supercooled microstructure is liable to form during rolling or a softening heat treatment before wire drawing or the like. Accordingly, the Mo content is from 0 to 0.70%.
  • a preferable lower limit of the Mo content for stably obtaining the aforementioned effects is 0.05%, more preferably is 0.10%, and further preferably is 0.15%.
  • a preferable upper limit of the Mo content for stably obtaining ferrite, pearlite and bainite in the micro-structure of the steel is 0.40%, and more preferably is 0.30%.
  • Tungsten is an optional element and need not be contained. If contained, W precipitates as W carbides in the steel and enhances the strength and temper softening resistance of the steel. W carbides form at a low temperature that is not more than the A 3 point. Therefore, unlike V, Nb, Ti and the like, it is difficult for W to form insoluble precipitates. Consequently, W carbides increase the strength and temper softening resistance of the steel by precipitation strengthening. W also dissolves in the steel and thereby increases the hardenability of the steel and increases the steel strength. The aforementioned effects are obtained to a certain extent if even a small amount of W is contained.
  • the W content is from 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%, and more preferably is 0.10%.
  • a preferable upper limit of the W content for stably obtaining ferrite, pearlite and bainite in the micro-structure of the steel is 0.40%, and more preferably is 0.30%.
  • a preferable total content of W and Mo for obtaining a high temper softening resistance is from 0.10 to 0.30%.
  • Niobium (Nb) is an optional element and need not be contained. If contained, Nb forms Nb nitrides, Nb carbides, or Nb carbo-nitrides and suppresses coarsening of austenite grains during quenching or during normalizing. Nb also increases the strength of the steel by precipitation strengthening. The aforementioned effects are obtained to a certain extent even if a small amount of Nb is contained. On the other hand, if the Nb content is more than 0.050%, insoluble precipitates form and the toughness of the steel decreases. In addition, if the Nb content is more than 0.050%, a supercooled microstructure is liable to form and consequently the hot workability of the steel will decrease.
  • the Nb content is from 0 to less than 0.050%.
  • a preferable lower limit of the Nb content for stably obtaining the aforementioned effects is 0.001%, and more preferably is 0.005%.
  • a preferable upper limit of the Nb content is 0.030%, and more preferably is 0.015%.
  • Copper (Cu) is an optional element and need not be contained. If contained, Cu prevents decarburization. Cu also increases corrosion resistance, similarly to Ni. The aforementioned effects are obtained to a certain extent if even a small amount of Cu is contained. On the other hand, if the Cu content is more than 0.50%, the steel will become brittle and rolling defects are liable to arise. Accordingly, the Cu content is from 0 to 0.50%. A preferable lower limit of the Cu content for stably obtaining the aforementioned effects is 0.05%, and more preferably is 0.10%. In a case where 0.30% or more of Cu is contained, the hot ductility can be maintained if the Ni content is higher than the Cu content.
  • Titanium (Ti) is an optional element and need not be contained. If contained, Ti forms nitrides, carbides or carbo-nitrides, and suppresses coarsening of austenite grains during quenching and during normalizing. Ti also increases the strength of the steel by precipitation strengthening. Ti also deoxidizes the steel. In addition, in a case where B is contained, Ti combines with dissolved N and maintains the amount of dissolved B. In this case, the hardenability increases. The aforementioned effects are obtained to a certain extent if even a small amount of Ti is contained.
  • Ti influences MnS inclusions and composite inclusions. Specifically, if the Ti content is more than 0.100%, the crystallized amount of MnS inclusions decreases, and formation of composite inclusions also decreases. In this case, the rusting characteristics of the steel decrease. In addition, if the Ti content is too high, the Ti forms nitrides and sulfides, and the fatigue strength decreases. Accordingly, the Ti content is from 0 to 0.100%. A preferable lower limit of the Ti content for effectively obtaining the aforementioned effects is 0.003%. In particular, in a case where B is contained, a preferable lower limit of the Ti content for reducing dissolved N is 0.005%. A preferable upper limit of the Ti content for increasing corrosion resistance is 0.090%, and more preferably is 0.085%.
  • the steel for machine structural use of the present embodiment may further contain Ca.
  • Ca is an optional element and need not be contained. If contained, Ca forms CaS or (Mn, Ca)S and spheroidizes MnS inclusions, and reduces the amount of tool wear. As a result, the machinability of the steel increases. The aforementioned effects are obtained to a certain extent if even a small amount of Ca is contained. On the other hand, if the Ca content is more than 0.0030%, oxide-based inclusions coarsen and the fatigue strength of the steel decreases. Accordingly, the Ca content is from 0 to 0.0030%. A preferable lower limit of the Ca content for further enhancing the machinability is 0.0001%. When prioritizing fatigue strength over machinability, a preferable upper limit of the Ca content is 0.0015%, and more preferably is 0.0003%.
  • F1 means the Mn content relative to the S content. If F1 is less than 8.0, it will be difficult for MnS inclusions to adequately crystallize. Consequently, the amount of dissolved S in the steel after solidification will not adequately decrease, and a large number of fine MnS precipitates will form after solidification. In such a case, the rusting characteristics of the steel will decrease because the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions cannot decrease. If the amount of dissolved S in the steel after solidification cannot be adequately decreased, dissolved S will remain at crystal grain boundaries after solidification. As a result, in some cases the hot workability of the steel will decrease.
  • F1 is 8.0 or more
  • the Mn content will be adequately high in comparison to the S content.
  • MnS inclusions in the molten steel will adequately crystallize and grow.
  • the amount of dissolved S in the steel after solidification will be adequately decreased, 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 adequately reduced, and the rusting characteristics of the steel improve.
  • a preferable lower limit of F1 for improving the rusting characteristics of the steel is 10.0, and more preferably is 20.0.
  • the micro-structure of the steel for machine structural use according to the present invention is mainly composed of ferrite, pearlite and bainite. Specifically, a total area fraction of ferrite, pearlite and bainite in the micro-structure of the steel for machine structural use having the aforementioned chemical composition is 99% or more.
  • the total area fraction of ferrite, pearlite and bainite in the micro-structure can be measured by the following method.
  • a sample is taken from the steel for machine structural use.
  • the steel for machine structural use is a steel bar or a wire rod
  • a sample is taken from a middle part of a radius R (hereunder, referred to as “R/2 part”) that links the external surface and the central axis.
  • R/2 part a radius of a radius that links the external surface and the central axis.
  • a surface that is perpendicular to the central axis of the steel for machine structural use is adopted as an observation surface.
  • the observation surface After polishing the observation surface, the observation surface is subjected to etching using 3% nitric acid-alcohol (nital etching reagent). The etched observation surface is observed with an optical microscope having a magnification of ⁇ 200, and photographic images of an arbitrary five visual fields are generated.
  • 3% nitric acid-alcohol nital etching reagent
  • the contrast differs for each of the respective phases of ferrite, pearlite, bainite and the like. Accordingly, the respective phases are identified based on the contrast.
  • the total area ( ⁇ m 2 ) of ferrite, pearlite and bainite among the identified phases is determined for each visual field.
  • the total area in the respective visual fields is totaled for all of the visual fields (five visual fields), and the ratio relative to the gross area of all the visual fields is determined. The determined ratio is defined as the total area fraction (%) of ferrite, pearlite and bainite.
  • a total number TN of inclusions which are any of MnS inclusions, Pb inclusions and composite inclusions containing MnS and Pb and which have an equivalent circular diameter of 5 ⁇ m or more in the steel is 40 per mm 2 or more.
  • the number TN of specific inclusions is 40 per mm 2 or more, coarse MnS inclusions having an equivalent circular diameter of 5 ⁇ m or more will adequately crystallize, and as a result the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions can be adequately reduced. Therefore, excellent machinability and excellent rusting characteristics can both be realized in a compatible manner.
  • the number TN of specific inclusions in the steel is less than 40 per mm 2 , coarse MnS inclusions having an equivalent circular diameter of 5 ⁇ m or more do not adequately crystallize, and as a result the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions cannot be adequately reduced.
  • a preferable lower limit of the number TN of specific inclusions is 80 per mm 2 , and more preferably is 150 per mm 2 .
  • a preferable upper limit of the number TN of specific inclusions is 300 per mm 2 . Note that, although an upper limit of the equivalent circular diameter of the specific inclusions is not particularly limited, for example, the upper limit is 200 ⁇ m.
  • a ratio (hereunder, also referred to as “composite ratio”) RA of the total number (number per mm 2 ) of composite inclusions having an equivalent circular diameter of 5 ⁇ m or more with respect to the number (number per mm 2 ) of specific inclusions is 40% or more.
  • the susceptibility of the steel to rusting increases as the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions increases.
  • the larger the number of composite inclusions that the MnS inclusions and Pb inclusions form the more that the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions can be reduced.
  • the total number of Pb inclusions in the steel can be reduced.
  • Pb inclusions are liable to decrease the rusting characteristics. If the composite ratio is 40% or more, the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions can be reduced, and the number of Pb inclusions that are independently present can also be reduced.
  • the composite ratio RA is preferably 40% or more. In this case, the rusting characteristics of the steel can be further enhanced.
  • a more preferable lower limit of the composite ratio RA is 60%, and further preferably is 75%.
  • the number TN of specific inclusions and the composite ratio RA can be measured by the following methods.
  • a sample is taken from the steel for machine structural use by the method described above.
  • SEM scanning electron microscope
  • 20 visual fields at a transverse section (surface) of the sample of the R/2 part are randomly observed at a magnification of ⁇ 1000.
  • Specific inclusions any of MnS inclusions, Pb inclusions and composite inclusions for which an equivalent circular diameter is 5 ⁇ m or more
  • observation surfaces it is possible to distinguish specific inclusions and other inclusions based on contrast.
  • MnS inclusions, Pb inclusions and composite inclusions are respectively identified by the following method.
  • FIG. 1A is a schematic diagram illustrating the S distribution in an observation surface, which was obtained by EPMA analysis.
  • FIG. 1B is a schematic diagram illustrating the Pb distribution in the same observation surface as in FIG. 1A , which was obtained by EPMA analysis.
  • Reference numeral 10 in FIG. 1A denotes a region in which S is present. Because S is almost entirely present as MnS, MnS can be regarded as being present at the locations indicated by each reference numeral 10 in FIG. 1A .
  • Reference numeral 20 in FIG. 1B denotes a region in which Pb is present.
  • Pb is divided by rolling or the like and is arranged in the rolling direction.
  • S the same applies with respect to S.
  • FIG. 2 in an image obtained by EPMA analysis, in a case where adjacent inclusions IN each have an equivalent circular diameter of 5 ⁇ m or more, if a distance D between the adjacent inclusions IN is not more than 10 ⁇ m, these inclusions IN are regarded as a single inclusion.
  • the term “equivalent circular diameter” means the diameter of a circle in a case where the area of the respective inclusions or respective precipitates is converted into a circle that has the same area. Even when an inclusion group is defined as a single inclusion, the equivalent circular diameter is the diameter of a circle having the same total area as the inclusion group.
  • FIG. 1C is an image obtained by combining FIG. 1B with FIG. 1A .
  • the relevant inclusions are recognized as being composite inclusions 30 .
  • the relevant inclusions are identified as an MnS inclusion 10 and a Pb inclusion 20 .
  • MnS inclusions, Pb inclusions and composite inclusions are identified using a scanning microscope and EPMA.
  • the area of each inclusion that is identified is determined, and the diameter of a circle with the same area is determined as the equivalent circular diameter (m) for each of the inclusions.
  • a method for producing the steel for machine structural use according to the present invention will now be described. According to the present embodiment, a method for producing a steel bar or a wire rod as an example of the steel for machine structural use will be described.
  • a steel for machine structural use according to the present invention is not limited to a steel bar or a wire rod.
  • One example of the production method includes a steel making process of refining and casting molten steel to produce a starting material (a cast piece or an ingot), and a hot working process of subjecting the starting material to hot working to produce a steel for machine structural use.
  • a steel making process of refining and casting molten steel to produce a starting material a cast piece or an ingot
  • a hot working process of subjecting the starting material to hot working to produce a steel for machine structural use.
  • the steel making process includes a refining process and a casting process.
  • Mn is added to the molten steel that was tapped from the converter.
  • oxides that are mainly composed of Mn form in the molten steel.
  • Si which has a stronger deoxidizing power than Mn is added.
  • the oxides that are mainly composed of Mn are reformed to oxides that are mainly composed of Si.
  • Al which has an even stronger deoxidizing power than Si is added.
  • the oxides that are mainly composed of Si are reformed to complex oxides containing Si and Al (hereinafter, also referred to simply as “complex oxides”).
  • the complex oxides that were formed by the above described refining process serve as nuclei for crystallization of MnS inclusions. Therefore, by forming the complex oxides, MnS inclusions adequately crystallize and grow coarse. That is, if complex oxides form, it is easy for specific inclusions that are inclusions having an equivalent circular diameter of 5 ⁇ m or more to form, and the number TN of specific inclusions becomes 40 per mm 2 or more. As a result, the amount of dissolved S in the steel after solidification is adequately 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 can be adequately reduced, and the rusting characteristics of the steel are enhanced.
  • secondary refining is performed.
  • composite refining is performed as the secondary refining.
  • LF ladle furnace
  • VAD vacuum arc degassing
  • RH Rasterstahl-Hausen vacuum degassing
  • Mn, Si, and other elements are added as necessary to adjust the components of the molten steel.
  • a starting material (a cast piece or an ingot) is produced using the molten steel produced by the above described refining process. Specifically, a cast piece is produced by a continuous casting process using the molten steel. Alternatively, an ingot may be produced by an ingot-making process using the molten steel.
  • starting material a cast piece and an ingot are referred to generically as “starting material”.
  • a cross-sectional area of the starting material in this case is, for example, 200 to 350 mm ⁇ 200 to 600 mm.
  • a solidification cooling rate RC during casting is 100° C./min or less. If the solidification cooling rate RC is 100° C./min or less, MnS inclusions adequately crystallize and grow in the molten steel. Therefore, it is easy for specific inclusions to form, and the number TN thereof becomes 40 per mm 2 or more. As a result, the amount of dissolved S in the steel after solidification is adequately 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 can be adequately reduced, and the rusting characteristics of the steel are enhanced.
  • the solidification cooling rate RC is more than 100° C./min, MnS inclusions do not adequately crystallize, and MnS inclusions also do not adequately grow. Therefore, it will be difficult for specific inclusions to be formed, and the number TN of specific inclusions will be less than 40 per mm 2 . In this case, the amount of dissolved S in the steel after solidification cannot be adequately reduced, and a large number of fine MnS precipitates will form after solidification. As a result, since the total number of MnS inclusions, MnS precipitates, Pb inclusions and composite inclusions cannot be reduced, the rusting characteristics of the steel will decline. Accordingly, the solidification cooling rate RC is 100° C./min or less.
  • a preferable solidification cooling rate RC is from 8 to less than 50° C./min. In this case, it is even easier for MnS inclusions to crystallize and grow. Furthermore, if the solidification cooling rate RC is from 8 to less than 50° C./min, because the time period until solidifying is long, a sufficient time period for Pb to move through the molten steel and adhere to MnS inclusions can be secured. Therefore, it is easy for composite inclusions containing MnS and Pb to form, and the composite ratio RA becomes 40% or more.
  • a more preferable upper limit of the solidification cooling rate RC is 30° C./min.
  • a more preferable lower limit of the solidification cooling rate RC is 10° C./min, and further preferably is 15° C./min.
  • the solidification cooling rate RC can be determined based on the starting material that was cast.
  • FIG. 3 is a transverse sectional view of a starting material that was cast. In the starting material having a thickness W (mm), at a point P 1 located at a position at a depth of W/4 towards the center of the starting material from the surface, the cooling rate from the liquidus temperature to the solidus temperature is defined as the solidification cooling rate RC (° C./min) in the casting process.
  • the solidification cooling rate RC can be determined by the following method. After solidification, the starting material is cut in the transverse direction. In the transverse section of the starting material, a secondary dendrite arm spacing 22 ( ⁇ m) in the thickness direction of the solidification structure at the point P 1 is measured. Using the measurement value 22 , the solidification cooling rate RC (° C./min) is determined based on the following Formula (3).
  • the secondary dendrite arm spacing 22 depends on the solidification cooling rate RC. Accordingly, the solidification cooling rate RC can be determined by measuring the secondary dendrite arm spacing 22 .
  • hot working is usually performed one or a plurality of times.
  • the starting material is heated before each hot working operation is performed. Thereafter, the starting material is subjected to the hot working.
  • the hot working is, for example, hot forging or hot rolling.
  • the initial hot working is, for example, blooming or hot forging, and the next hot working is finish rolling using a continuous mill.
  • the hot rolling mill a horizontal stand having a pair of horizontal rolls, and a vertical stand having a pair of vertical rolls are alternately arranged in a row.
  • the starting material after hot working is cooled by a well-known cooling method such as air cooling.
  • the steel for machine structural use according to the present embodiment is produced by the above described processes.
  • the steel for machine structural use is, for example, a steel bar or a wire rod.
  • the steel for machine structural use produced by the above described method is excellent in machinability and rusting characteristics. Production of the steel for machine structural use into a machine component is performed, for example, by the following method.
  • the steel for machine structural use is subjected to hot forging to produce an intermediate product having a rough shape.
  • the intermediate product is subjected to a normalizing.
  • the intermediate product is also subjected to machining.
  • the machining is, for example, cutting.
  • the intermediate product that underwent machining may be subjected to a thermal refining treatment (quenching and tempering). In a case where a thermal refining treatment is performed, the machining such as cutting may be performed on the intermediate product after the thermal refining treatment.
  • a machine component is produced by the above process.
  • a machine component may also be produced by performing cold forging instead of hot forging.
  • the molten steel of each test number was produced by the following method. Hot metals produced by a well-known method were subjected to primary refining under the same conditions using a converter to thereby produce the molten steels of the respective test numbers.
  • Each of the molten steels was cast to produce an ingot for test use that had a rectangular parallelepiped shape.
  • the cross sectional shape of the ingot was a rectangular shape with dimensions of 190 mm ⁇ 190 mm.
  • the solidification cooling rates RC (° C./min) for the respective test numbers were as listed in Table 2.
  • the solidification cooling rate RC was determined by measuring a secondary dendrite arm spacing of the ingot and applying the determined value to the aforementioned Formula (3).
  • the produced ingots for test use were subjected to hot working twice to produce a steel bar. In the hot working, blooming was performed, and thereafter finish rolling (steel bar rolling) was performed.
  • the produced test ingot was subjected to hot forging to produce a steel bar having a diameter of 50 mm.
  • the test ingot was subjected to blooming, and then subjected to finish rolling to produce a steel bar having a diameter of 50 mm.
  • a normalizing in a range of 800 to 950° C. was performed on the produced steel bar. The cooling method adopted in the normalizing was to allow cooling of the steel bar.
  • a steel bar (steel for machine structural use) having a diameter of 50 mm was produced by the above-described production process.
  • a test specimen for micro-structure observation use was taken from the R/2 part of the steel bar of each test number. Of the entire surface of the test specimen, a cross-section parallel to the longitudinal direction (that is, the rolling direction or elongation direction) of the steel bar was defined as the observation surface.
  • the total area fraction (%) of ferrite, pearlite and bainite were determined based on the method described above. The total area fraction was 99% or more in the micro-structure of the steel bar of each test number.
  • a micro-structure in which the total area fraction was 99% or more is shown as “F+P+B” in Table 2.
  • a test specimen for observing the micro-structure was taken from the R/2 part of the steel bar of each test number. Of the entire surface of the test specimen, a cross-section that was parallel to the longitudinal direction (that is, the rolling direction or elongation direction) of the steel bar was defined as the observation surface.
  • the specific inclusions number TN (inclusions/mm 2 ) and the composite ratio RA (%) were determined based on the above described method. The results are shown in Table 2.
  • a service life characteristic in the case of normal drilling, and the chip treatability were evaluated with respect to the machinability.
  • the steel bar having a diameter of 50 mm was cut at a length of 20 mm to make a piercing test specimen.
  • the piercing test specimen was subjected to piercing.
  • the piercing conditions are shown in Table 3.
  • the cumulative hole depth was measured up to drill breakage at different cutting speed of the drill.
  • a high-speed steel straight drill was used as the drill.
  • the nose R of the drill was 3 mm, and the point angle was 118°.
  • Piercing was performed at a cutting speed of 10 to 70 m/min, a feed rate of 0.25 mm/rev, and a hole depth of 9 mm.
  • Piercing with one drill was ended when the cumulative hole depth (hole depth ⁇ number of piercings) reached 1000 mm. In such case, the drill was replaced, and the cutting speed of the drill was raised to a higher speed and the test was repeated until drill breakage.
  • the maximum cutting speed of the drill at which it was possible to pierce a cumulative hole depth of 1000 mm was defined as CL1000 (m/min), and the CL1000 value was adopted as an index of the machinability.
  • CL1000 The results are shown in the “CL1000” column in Table 2. In a case where CL1000 was 50 m/min or more, the service life characteristic was determined as being excellent. On the other hand, when CL1000 was less than 50 m/min, the service life characteristic was determined as being not excellent.
  • a steel bar having a diameter of 50 mm was cut to a predetermined length and adopted as a cutting test specimen.
  • Outer circumferential lathe turning shown in FIG. 4 was performed on the cutting test specimen.
  • the conditions for the outer circumferential lathe turning are shown in Table 4.
  • a P20 cemented carbide tool was used as a tool 50 .
  • the nose R of the tool 50 was 0.4, and the rake angle thereof was 5°.
  • Outer circumferential lathe turning was performed under the following conditions: cutting speed V 1 : 250 m/min; feed speed V 2 : 0.2 mm/rev; depth-of-cut amount D 1 : 2 mm; and longitudinal direction cutting length L 1 : 200 mm. After cutting the outer circumference, turning was repeated again so as to obtain a small diameter of D 1 : 2 mm, and with respect to test specimen 5 , a lathe turning test was performed under the aforementioned conditions for four minutes.
  • a chip as illustrated in FIG. 5A and FIG. 5B was obtained.
  • a length L 20 and a diameter D 20 of the chip were measured.
  • the chip treatability was evaluated as follows. If the diameter D 20 of the chip was a coil shape of not more than 30 mm, or if the chip length L 20 was less than 50 mm even if the chip was not a coil shape, the chip treatability was determined as being excellent (“ ⁇ ” in Table 2). On the other hand, if the diameter D 20 of the chip was not a coil shape of not more than 30 mm, and the chip length L 20 was also 50 mm or more, the chip treatability was determined as being not excellent (“x” in Table 2).
  • a rust test specimen was prepared by cutting the steel bar having a diameter of 50 mm to a predetermined length.
  • the rust test specimen was subjected to lathe turning under similar conditions as in the aforementioned cutting test. Thereafter, the test specimen was stored for one hour in an atmosphere with a humidity of 70% and a temperature of 20° C. while spraying tap water onto the cut surface. After storage, the cut surface of the test specimen was observed and the number of rust points was measured. The measurement results are shown in the “rusting characteristics” column in Table 2. If the number of rust points was less than 10 (indicated by “ ⁇ ” in Table 2), and if the number of rust points was 10 or more and less than 20 (indicated by “ ⁇ ” in Table 2), the rusting characteristics were determined as being excellent. On the other hand, if the number of rust points was 20 or more (“x” in Table 2), the rusting characteristics were determined as being not excellent.
  • the surface of each rolling contact fatigue test specimen 100 on which the carburizing was performed was subjected to grinding to remove a incomplete hardened surface layer, and the effective hardened layer depth was made 0.7 mm.
  • the depth from the surface to a position at which the hardness became HV550 was adopted as the effective hardened layer depth.
  • the amount removed by grinding was set to 0.2 mm or less.
  • the hardness distribution and effective hardened layer depth in each rolling contact fatigue test specimen were adjusted by adjusting the Cp and the cutting depth.
  • each rolling contact fatigue test specimen 100 was immersed in lubricating oil 102 composed of 70% oil and 30% water, and because the moisture in the lubricating oil evaporated due to heat generation, 30 ml of water was added once per day.
  • the test interfacial pressure was set to a constant pressure of 4 kN.
  • the hard balls that were used were Si 3 N 4 ceramic hard balls.
  • Three hard balls were placed in contact with the top of the rolling contact fatigue test specimen 100 , and the number of revolutions was set to 1200 rpm.
  • time (hours) until producing pitting at a cumulative failure rate of 10% obtained when the test result was plotted on Weibull probability paper” was used as the durable life.
  • a hot tension test was performed by electrical heating, and the hot ductility (hot workability) was evaluated. Specifically, from the cast piece of each test number, a round bar specimen that had a diameter of 10 mm and a length of 100 mm and in which both ends had been subjected to threading was prepared. The round bar specimen was heated to 1100° C. by electrical heating, and held at that temperature for three minutes. Thereafter, the round bar specimen was cooled to 900° C. by being allowed to cool. The tension test was executed in a state in which the temperature of the round bar specimen was 900° C., and the reduction of area (%) at the time of breaking off was determined.
  • the tension test was performed on three round bar specimens for each test number, and the average of the three values was defined as the reduction of area (%) of the relevant test number.
  • the reduction of area is shown in the “hot ductility” column in Table 2. If the reduction of area was 70% or more, the hot ductility (hot workability) was evaluated as excellent. On the other hand, if the reduction of area was less than 70%, the hot ductility (hot workability) was evaluated as not excellent.
  • Test Numbers 1 to 31 the chemical composition was appropriate, F1 was 8.0 or more, the deoxidation order was appropriate, and the solidification cooling rate RC was 100° C./min or less. Therefore, the number TN of specific inclusions was 40 per mm 2 or more. As a result, CL1000 was 50 m/min or more, and excellent chip treatability was obtained. That is, excellent machinability was obtained. In addition, in the rusting characteristics evaluation test, the number of rust points was less than 20 for each of these test numbers, and thus excellent rusting characteristics were obtained. Furthermore, in the rolling contact fatigue test, for each of these test numbers, the rolling contact fatigue life was 4.0 hours or more, and thus excellent rolling contact fatigue properties were obtained. In addition, in the hot ductility evaluation test, the reduction of area was 70% or more, showing that excellent hot ductility was obtained.
  • the solidification cooling rate RC was within the range of 8 to 50° C./min. Therefore, not only was the number TN of specific inclusions 40 per mm 2 or more, but furthermore the composite ratio RA was 40% or more. As a result, for each of these test numbers, the number of rust points was less than 10, and thus rusting characteristics that were even more excellent in comparison to Test Numbers 8 to 25 and 27 to 29 were obtained.
  • Test Numbers 43 and 44 although the chemical composition was appropriate and the deoxidation order was appropriate and the solidification cooling rate RC was not more than 100° C./min, F1 was less than 8.0. Consequently, the number TN of specific inclusions was less than 40 per mm 2 . As a result, excellent rusting characteristics were not obtained. In addition, the reduction of area was less than 70%, and thus excellent hot ductility was not obtained.
  • Test Number 45 although the chemical composition was appropriate and the deoxidation order was appropriate, the solidification cooling rate RC was more than 100° C./min and F1 was less than 8.0. Consequently, the number TN of specific inclusions was less than 40 per mm 2 . As a result, excellent rusting characteristics were not obtained. In addition, the reduction of area was less than 70%, and thus excellent hot ductility was not obtained.
  • Test Number 50 the S content was too high. As a result, the reduction of area was less than 70%, and thus excellent hot ductility was not obtained.
  • Test Number 51 the S content was too high.
  • F1 was less than 8.0. Consequently, the number TN of specific inclusions was less than 40 per mm 2 . As a result, excellent rusting characteristics were not obtained.
  • the reduction of area was less than 70%, and thus excellent hot ductility was not obtained.
  • Test Number 62 the O content was too low. Consequently, the number TN of specific inclusions was less than 40 per mm 2 . As a result, excellent rusting characteristics were not obtained. In addition, CL1000 was less than 50 m/min, and thus excellent chip treatability was also not obtained. That is, excellent machinability was not obtained.
  • Test Numbers 65 and 66 although the chemical composition was appropriate, F1 was 8.0 or more and the solidification cooling rate RC was not more than 100° C./min, the deoxidation order was inappropriate. Consequently, the number TN of specific inclusions was less than 40 per mm 2 . As a result, excellent rusting characteristics were not obtained.

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