EP3480333A1 - Stahl für mechanische strukturen - Google Patents

Stahl für mechanische strukturen Download PDF

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Publication number
EP3480333A1
EP3480333A1 EP17824224.4A EP17824224A EP3480333A1 EP 3480333 A1 EP3480333 A1 EP 3480333A1 EP 17824224 A EP17824224 A EP 17824224A EP 3480333 A1 EP3480333 A1 EP 3480333A1
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Prior art keywords
inclusions
steel
mns
content
composite
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English (en)
French (fr)
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EP3480333A4 (de
Inventor
Masayuki Hashimura
Makoto Egashira
Takanori IWAHASHI
Shouji TOUDOU
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal 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
    • C22C38/18—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
    • 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
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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 a steel, and more particularly relates to a steel for machine structural use.
  • Machine components that are to be used for structural use and power transmission use such as general machine components and automobile components are produced using a steel for machine structural use.
  • 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. There are also cases in which, as necessary, the machine component is subjected to a heat treatment (normalizing or the like), a case hardening heat treatment (induction hardening or the like), or quenching and tempering.
  • 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 symbol of an element in the respective formulas represents the content in mass% of the corresponding element, and ⁇ 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.
  • Patent Literature 1 Japanese Patent Application Publication No. 2000-282172
  • Non Patent Literature 1 Japanese Industrial Standards Committee, Standard No.: JIS G 4804 (2008), Standard Name: Free-cutting Steels
  • 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.
  • a steel for machine structural use according to the present invention has a chemical composition which consists of, 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: 0.015% 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 impurities, the chemical composition satisfying Formula (1).
  • a total number of specific 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 is 40 per mm 2 or more.
  • a steel for machine structural use according to the present invention is excellent in machinability, rusting characteristics and hot workability.
  • 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: 0.30 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 impurities, there is
  • 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 particles Pb inclusions
  • 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.
  • 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: 0.30 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 impurities, the chemical composition satisfying Formula (1).
  • the total number of specific 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 is 40 per mm 2 or more.
  • 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 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: 0.003 to 0.100%.
  • 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) enhances the strength of the steel.
  • a heat treatment normalizing or the like
  • a case hardening heat treatment induction hardening or the like
  • quenching and tempering are performed after forging the steel for machine structural use.
  • C increases the strength of the steel. If the C content is less than 0.30%, sufficient strength will not be obtained.
  • the C content is more than 0.80%, a large amount of retained austenite will form after tempering. In such a case, not only will an increase in the strength be saturated, but hard cementite will form and the machinability of the steel will decrease. Accordingly, the C content is in a range of 0.30 to 0.80%.
  • Si deoxidizes the steel.
  • Si deoxidizes the steel.
  • Si modifies oxides.
  • Si added to molten steel modifies oxides that are mainly composed of Mn into oxides that are mainly composed of Si.
  • Al By adding Si, composite oxides containing Si and Al form in the steel.
  • the composite oxides serve as nuclei for crystallization of MnS inclusions. Therefore, the composite 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 modified.
  • the modified oxides are referred to as "composite oxides".
  • the Mn that is discharged into the molten steel from the oxides combines with S to form MnS inclusions.
  • composite oxides formed by modification of oxides easily become nuclei for crystallization of MnS inclusions. Therefore, if composite oxides are formed, crystallization of MnS inclusions is promoted. The MnS inclusions formed by crystallization easily form composite inclusions also.
  • 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 particles) 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.
  • oxides in the steel are modified and composite oxides containing Si and Al are formed.
  • the composite 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 composite oxides are liable to form.
  • the composite oxides are liable to become coarse. If coarse composite oxides are formed in the steel, surface defects are liable to occur on the steel. If coarse composite 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.
  • 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 composite oxides that serve as nuclei for crystallization of MnS inclusions. If the O content is less than 0.0005%, the formed amount of composite 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 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.
  • Chromium (Cr) is an optional element and need not be contained. If contained, Cr dissolves in the steel and increases the hardenability and temper softening resistance of the steel and enhances the steel strength. In a case where nitriding is performed as a case hardening treatment, Cr also deepens the hardened layer depth. The aforementioned effects are obtained to a certain extent if even a small amount of Cr is contained. On the other hand, if the Cr content is more than 0.70%, if quenching and tempering are performed, cementite in the steel will coarsen. In addition, if the Cr content is more than 0.70%, if induction hardening is performed, cementite in the steel will not dissolve.
  • the Cr content is in a range from 0 to 0.70%.
  • a preferable lower limit of the Cr content for increasing hardenability is 0.10%, and more preferably is 0.30%.
  • a preferable upper limit of the Cr content is 0.60%.
  • 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%.
  • 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 A 1 , 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 structure 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 structure 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 structure 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 nitrides, carbides, or 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 structure is liable to form and consequently the hot workability of the steel will decrease. Accordingly, 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.
  • F1 Mn/S.
  • 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 and pearlite. Specifically, a total area fraction of ferrite and pearlite 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 and pearlite 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 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 and pearlite 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 and pearlite.
  • 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 and a Pb inclusion.
  • FIG. 3A is a photographic image of an S distribution obtained by performing EPMA analysis on the steel for machine structural use of the present embodiment
  • FIG. 3B is a photographic image of the Pb distribution
  • FIG. 3C is a photographic image obtained by superposing the images in FIG. 3A and FIG. 3B .
  • the MnS inclusions 10 are observed in an area A10 in FIG. 3A
  • the Pb inclusions 20 are observed in the area A10 in FIG. 3B
  • the composite inclusions 30 are present in the area A10 in FIG. 3C
  • the MnS inclusions 10 are not observed in an area A20 in FIG. 3A
  • the Pb inclusions 20 are observed in the area A20 in FIG. 3B . Therefore, it can be recognized that the inclusions present in the area A20 in FIG. 3C are the Pb inclusions 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 modified 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 modified to composite oxides containing Si and Al (hereinafter, also referred to simply as "composite oxides").
  • the composite oxides that were formed by the above described refining process serve as nuclei for crystallization of MnS inclusions. Therefore, by forming the composite oxides, MnS inclusions adequately crystallize and grow coarse. That is, if composite 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.
  • a well-known removing slag is performed after performing the deoxidation.
  • secondary refining is performed.
  • composite refining is performed as the secondary refining.
  • LF ladle furnace
  • VAD vacuum 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.
  • 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. 4 is a transverse sectional view of a starting material that was cast. In the starting material having a thickness W (mm), at a point P1 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 secondary dendrite arm spacing ⁇ 2 depends on the solidification cooling rate RC. Accordingly, the solidification cooling rate RC can be determined by measuring the secondary dendrite arm spacing ⁇ 2.
  • 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 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 treatment in a range of 800 to 950°C was performed on the produced steel bar. The cooling method adopted in the normalizing treatment 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 and pearlite 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" 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 Vickers hardness test was performed in conformity with JIS Z 2244 (1981) at an arbitrary five points of the R/2 part of the steel bar of each test number.
  • the test force was set to 100 N.
  • the average of the obtained five values was defined as the Vickers hardness (HV) of the steel bar of the relevant test number. If the Vickers hardness was HV 160 or more, it was determined that the steel bar had sufficient strength. On the other hand, if the Vickers hardness was less than HV 160, it was determined that the steel bar had insufficient strength.
  • Table 2 The results show that the Vickers hardness was HV 160 or more for each test number, indicating sufficient strength.
  • the machinability was evaluated by evaluating the amount of tool wear ( ⁇ m) and the chip treatability. Specifically, a steel bar having a diameter of 50 mm was cut to a predetermined length and adopted as a cutting test specimen. The cutting test specimen was subjected to outer circumferential lathe turning as illustrated in FIG. 5 . The conditions of the outer circumferential lathe turning are shown in Table 3.
  • a P20 cemented carbide tool was used as a tool 50.
  • the nose radius 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 V1: 200 m/min; feed speed V2: 0.2 mm/rev; depth-of-cut amount D1: 2 mm; and longitudinal direction cutting length L1: 200 mm. After cutting the outer circumference, cutting lathe turning was repeated again so as to obtain a small diameter of D1: 2 mm, and with respect to test specimen 5, a lathe turning test was performed under the aforementioned conditions for four minutes.
  • the amount of tool wear (mm) of the minor flank was measured with respect to the tool 50 after lathe turning of the 1,000 th test specimen was completed. The measurement results are shown in the "amount of tool wear" column in Table 2. The service life was determined as excellent if the amount of tool wear was 200 ⁇ m or less. On the other hand, if the amount of tool wear was more than 200 ⁇ m, the service life was determined as being not excellent.
  • a chip as illustrated in FIG. 6A and FIG. 6B was obtained.
  • a length L20 and a diameter D20 of the chip were measured.
  • the chip treatability was evaluated as follows. If the chip was a coil shape of not more than 30 mm in diameter, or if the chip length 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 chip was not a coil shape of not more than 30 mm in diameter, and the chip length was also 50 mm or more, the chip treatability was determined as being poor (" ⁇ " in Table 2).
  • a hot tension test was performed by electrical heating, and the hot ductility 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 screw machining 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 was evaluated as excellent. On the other hand, if the reduction of area was less than 70%, the hot ductility was evaluated as not excellent.
  • 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 per mm 2 or more. As a result, the amount of tool wear was 200 ⁇ m or less, 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. 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 7 to 16, 18, 20, 21, 23, 24 and 26 were obtained.
  • Test Number 39 the Mn content was too high. As a result, the amount of tool wear was more than 200 ⁇ m, and thus excellent machinability was not obtained.
  • Test Number 40 the Mn content was too low.
  • the solidification cooling rate RC was more than 100°C/min. Therefore, the number TN of specific inclusions was less than 40 per mm 2 . As a result, excellent rusting characteristics were not obtained.
  • the point value was less than 70%, and thus excellent hot ductility was not obtained.
  • Test Number 41 the S content was too high. As a result, the point value was less than 70%, and thus excellent hot ductility was not obtained.
  • Test Number 42 the S 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.
  • the Pb content was too low.
  • the solidification cooling rate RC was more than 100°C/min. Therefore, the number TN of specific inclusions was less than 40 per mm 2 .
  • the amount of tool wear was more than 200 ⁇ m and, furthermore, excellent chip treatability was also 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 per mm 2 . As a result, excellent rusting characteristics were not obtained.
  • Test Number 46 the N content was too high. As a result, the amount of tool wear was more than 200 ⁇ m, and thus excellent machinability was not obtained. In addition, the point value was less than 70%, and thus excellent hot ductility was not obtained.
  • Test Number 47 the O content was too high.
  • the solidification cooling rate RC was more than 100°C/min. Therefore, the number TN of specific inclusions was less than 40 per mm 2 . As a result, the amount of tool wear was more than 200 ⁇ m, and thus excellent machinability was not obtained.
  • Test Number 48 the O content was too low.
  • the solidification cooling rate RC was more than 100°C/min. Therefore, the number TN of specific inclusions was less than 40 per mm 2 .
  • the amount of tool wear was more than 200 ⁇ m and, furthermore, excellent chip treatability was also not obtained. That is, excellent machinability was not obtained.
  • Test Number 49 although the chemical composition was appropriate and 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.
  • Test Number 50 although the chemical composition was appropriate and 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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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Treatment Of Steel In Its Molten State (AREA)
EP17824224.4A 2016-07-04 2017-07-04 Stahl für mechanische strukturen Withdrawn EP3480333A4 (de)

Applications Claiming Priority (2)

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JP2016132902 2016-07-04
PCT/JP2017/024442 WO2018008621A1 (ja) 2016-07-04 2017-07-04 機械構造用鋼

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EP3480333A4 EP3480333A4 (de) 2019-11-20

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KR (1) KR20190027848A (de)
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WO (1) WO2018008621A1 (de)

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KR20220143735A (ko) * 2020-02-21 2022-10-25 닛폰세이테츠 가부시키가이샤 강선
CN112048659B (zh) * 2020-09-11 2021-08-10 上海大学 一种高强度高塑韧性钢板及其制备方法
WO2022234319A1 (en) * 2021-05-04 2022-11-10 Arcelormittal Steel sheet and high strength press hardened steel part and method of manufacturing the same
WO2022234320A1 (en) * 2021-05-04 2022-11-10 Arcelormittal Steel sheet and high strength press hardened steel part and method of manufacturing the same
CN115449704B (zh) * 2022-07-29 2023-07-25 江阴兴澄特种钢铁有限公司 一种新能源汽车轮毂轴承用钢及其生产方法

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JPS5585658A (en) * 1978-12-25 1980-06-27 Daido Steel Co Ltd Free cutting steel
JPS6148557A (ja) * 1984-08-16 1986-03-10 Daido Steel Co Ltd 機械構造用鋼
JP2567630B2 (ja) * 1987-10-15 1996-12-25 愛知製鋼株式会社 高疲労強度快削鋼及びその製造方法
JPH06114500A (ja) * 1992-10-05 1994-04-26 Nippon Steel Corp 低炭硫黄系快削鋼の製造方法
JP3437079B2 (ja) * 1998-02-05 2003-08-18 株式会社神戸製鋼所 切りくず処理性に優れた機械構造用鋼
JPH11293391A (ja) * 1998-04-13 1999-10-26 Kobe Steel Ltd 切屑処理性に優れた低炭素快削鋼およびその製造方法
JP3680674B2 (ja) 1999-01-28 2005-08-10 住友金属工業株式会社 被削性と靱性に優れた機械構造用鋼材及び機械構造部品
EP1264909B1 (de) * 2000-03-06 2005-11-30 Nippon Steel Corporation Stahl mit ausgezeichneter eignung für schmieden und bearbeitung
JP3468239B2 (ja) * 2001-10-01 2003-11-17 住友金属工業株式会社 機械構造用鋼及びその製造方法
KR20120049405A (ko) * 2008-02-26 2012-05-16 신닛뽄세이테쯔 카부시키카이샤 파단 분리성 및 피삭성이 우수한 열간 단조용 비조질강과 열간 압연 강재 및 열간 단조 비조질강 부품
JP5655986B2 (ja) * 2012-06-08 2015-01-21 新日鐵住金株式会社 鋼線材又は棒鋼

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JP6760375B2 (ja) 2020-09-23
CN109477174A (zh) 2019-03-15
KR20190027848A (ko) 2019-03-15
US20190233927A1 (en) 2019-08-01
WO2018008621A1 (ja) 2018-01-11
JPWO2018008621A1 (ja) 2019-05-16

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