WO2011155605A1 - High-machinability high-strength steel and manufacturing method therefor - Google Patents

High-machinability high-strength steel and manufacturing method therefor Download PDF

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Publication number
WO2011155605A1
WO2011155605A1 PCT/JP2011/063393 JP2011063393W WO2011155605A1 WO 2011155605 A1 WO2011155605 A1 WO 2011155605A1 JP 2011063393 W JP2011063393 W JP 2011063393W WO 2011155605 A1 WO2011155605 A1 WO 2011155605A1
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less
steel
strength
excluding
bainite
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PCT/JP2011/063393
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French (fr)
Japanese (ja)
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武広 土田
智一 増田
睦久 永濱
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株式会社神戸製鋼所
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Publication of WO2011155605A1 publication Critical patent/WO2011155605A1/en

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    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/30Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for crankshafts; for camshafts
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/002Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32Ferrous alloys, e.g. steel alloys containing chromium with boron
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/002Bainite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/005Ferrite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/009Pearlite

Definitions

  • the present invention relates to steel for manufacturing steel parts by cutting and a method for manufacturing the same.
  • Machine structural parts such as shafts and connecting rods are usually manufactured by subjecting hot-worked steel (for example, hot rolling or hot forging) to a final shape (part shape) by cutting.
  • This steel part is required to have high strength, but in order to increase the strength of the steel part, if the strength of the steel before cutting is increased, cutting becomes difficult. Since the cost required for cutting is high in the entire part production cost, the steel before cutting is required to have good machinability. Therefore, the steel before cutting improves the machinability by reducing its hardness, and increases the strength of steel parts by performing heat treatment such as quenching and tempering (tempering) and carburizing and quenching after cutting. Has been done.
  • the cutting process will be described in detail.
  • the cutting process for manufacturing gears among the above-mentioned mechanical structural parts it is common to perform gear cutting with a hob.
  • the cutting process is called intermittent cutting.
  • a high-speed tool steel coated with AlTiN or the like hereinafter sometimes abbreviated as “high-speed tool”.
  • gear cutting by hobbing (intermittent cutting) using a high-speed tool is low speed (specifically, cutting speed of about 150 m / min or less) and low temperature (specifically, about 200 to 600 ° C.) Because of the intermittent cutting, the tool can easily come into contact with air and be easily oxidized and worn. For this reason, steel used for intermittent cutting such as hobbing is particularly required to extend the tool life.
  • Patent Document 1 The applicant has proposed steels for machine structural use with improved machinability (particularly, tool life) in interrupted cutting in Patent Documents 1 and 2.
  • the machinability in intermittent cutting with a high-speed tool is improved by appropriately adjusting each component of the oxide inclusions so that the entire inclusion is easily deformed at a low melting point.
  • Patent Document 2 by adding an element that has a greater tendency to oxidize than Fe to solid solution, the mechanical structure steel is prevented from being rapidly oxidized during intermittent cutting, and oxidative wear of the tool is prevented. Suppresses and improves the machinability of steel.
  • Patent Documents 1 and 2 in order to increase the strength of the steel part, it is necessary to perform heat treatment such as quenching and tempering (tempering) or carburizing and quenching after cutting.
  • Patent Documents 3 and 4 are known as techniques for increasing the strength of non-heat treated steel.
  • the strength of non-tempered steel is increased by making the structure after hot forging and cooling be bainite or a mixed structure of bainite and martensite that does not contain pro-eutectoid ferrite.
  • the critical cooling rate Vc calculated based on the composition of steel is an average cooling rate at 800 to 500 ° C. in air cooling or blast cooling performed after hot forging.
  • the component composition of the steel is adjusted so as to be Va or less (Vc ⁇ Va).
  • this technique improves the strength and toughness of non-tempered steel, and no consideration is given to machinability (particularly machinability when intermittently cut).
  • Patent Document 4 discloses a technique for increasing the strength of non-heat treated steel and improving fatigue strength and machinability.
  • This non-tempered steel contains 0.0005 to 0.050% of Al, and the structure ratio f of bainite is 1.4C + 0.4 ⁇ f ⁇ 1.4C with respect to the amount of C contained in the steel. There is a feature.
  • the present invention has been made paying attention to such circumstances, and the purpose thereof is to cut steel parts into steel parts without cutting and tempering (tempering) or carburizing and quenching.
  • An object of the present invention is to provide a high-strength steel that can ensure the required strength and is excellent in machinability at the time of cutting, and a method for producing the same.
  • the high-strength steel according to the present invention has C: 0.20 to 0.70% (meaning mass%, the same applies hereinafter), Si: 0.03 to 2%, Mn: 0.2 to 1.8%, P: 0.03% or less (not including 0%), S: 0.10% or less (not including 0%), Al: 0.12 to 0.5%, B: 0.0005 to 0.008 %, N: 0.002 to 0.030%, and O: 0.002% or less (not including 0%), Al and N satisfy the relationship of the following formula (1), and the balance is Steel made of iron and inevitable impurities.
  • the metal structure is a mixed structure of ferrite, pearlite, and bainite
  • the area ratio f (B) of bainite is
  • the metal structure is a mixed structure of ferrite and pearlite, or a mixed structure containing bainite.
  • the gist of the present invention is that the area ratio f (B) of bainite satisfies the following formula (3).
  • [] indicates the content (% by mass) of each element.
  • the high-strength steel of the present invention is still another element, (A) Cr: 1.5% or less (excluding 0%), (B) Mo: 1% or less (excluding 0%), (C) Ti: 0.005% or less (not including 0%), Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), Ta: At least one element selected from the group consisting of 0.02% or less (excluding 0%) and Nb: 0.15% or less (not including 0%); (D) V: 0.5% or less (excluding 0%), Cu: 3% or less (not including 0%), and Ni: 3% or less (not including 0%) It may contain at least one element.
  • the present invention also includes a steel part formed of the above-described high-strength steel, and the metal structure of the steel part is the same as the high-strength steel used.
  • the temperature range from 800 ° C. to 500 ° C. is cooled at an average cooling rate Va satisfying the following formula (4).
  • Va 0.1 ⁇ Vc ⁇ Va ⁇ 0.9 ⁇ Vc (4)
  • Vc 10 k (5)
  • k 4.05- ⁇ 4.5 ⁇ [C] + [Mn] + 0.5 ⁇ [Ni] + 0.8 ⁇ [Cr] + 1.6 ⁇ [Mo] + 9.0 ⁇ [Nb] ⁇ (6)
  • the steel part of the present invention can be manufactured by cutting the high-strength steel obtained by the above-described manufacturing method without heating to a temperature of 850 ° C. or higher.
  • the high-strength steel excellent in machinability by prescribing the component composition of the steel and appropriately controlling the area ratio of bainite contained in the metal structure according to the amount of C contained in the steel.
  • the high-strength steel of the present invention has good machinability when it is cut, in particular, the tool life when it is cut intermittently, and the steel part formed by cutting is still in the state of cutting. Since the strength required for steel parts can be secured, heat treatment such as quenching and tempering (tempering) and carburizing and quenching after cutting can be omitted.
  • the present inventors have repeatedly studied to provide high-strength steel that has good machinability at the time of cutting and that can ensure the strength required as a steel part in a state of being cut into a part shape. It was.
  • the metal structure is appropriately controlled according to the amount of C contained in the steel. Specifically, the amount of C is 0.20% or more and less than 0.35%. Is a mixed structure of ferrite, pearlite and bainite, and when the C content is 0.35% or more and 0.70% or less, it is a mixed structure of ferrite and pearlite, or a mixed structure containing bainite.
  • strength can be provided by controlling appropriately the area ratio of the bainite which occupies for a mixed structure, and completed this invention. *
  • the present inventors considered that if the steel metal structure is a mixed structure of ferrite and pearlite, the machinability when cutting (particularly, the tool life when cutting intermittently) can be improved.
  • the metal structure of steel is a mixed structure of ferrite and pearlite
  • the strength of the steel may decrease, and the strength required for steel parts may not be ensured.
  • bainite may be contained in the mixed structure, and if the area ratio of bainite is controlled in accordance with the C content of steel, it is found that the strength can be improved without degrading the machinability. It was.
  • the metal structure is a mixed structure of ferrite, pearlite, and bainite, and the area ratio f (B) of bainite. It is important to adjust so as to satisfy the following formula (2).
  • [C] indicates the amount of C (% by mass) contained in the steel. ⁇ 60 ⁇ [C] +21 ⁇ f (B) ⁇ 60 ⁇ [C] +50 (2)
  • the area ratio f (B) is “ ⁇ 60 ⁇ [C] +21” or less, the amount of bainite is too small, and the strength of the steel cannot be secured. Accordingly, the area ratio f (B) is more than “ ⁇ 60 ⁇ [C] +21”, preferably “ ⁇ 60 ⁇ [C] +25” or more, more preferably “ ⁇ 60 ⁇ [C] +30” or more. However, if the area ratio f (B) is “ ⁇ 60 ⁇ [C] +50” or more, the strength of the steel becomes too high and the machinability deteriorates. Therefore, the area ratio f (B) is less than “ ⁇ 60 ⁇ [C] +50”, preferably “ ⁇ 60 ⁇ [C] +45” or less, more preferably “ ⁇ 60 ⁇ [C] +40” or less.
  • the amount of C contained in the steel is 0.35% or more and 0.70% or less, the strength required for steel parts can be ensured even with a mixed structure of ferrite and pearlite, so bainite is not necessarily generated. You don't have to.
  • bainite may be included in order to further increase the strength. That is, when the amount of C contained in the steel is in the above range, the metal structure may be a mixed structure of ferrite and pearlite, or a mixed structure containing bainite.
  • the area ratio f (B) may be 0 area%, but in order to further increase the strength, it is preferably “ ⁇ 60 ⁇ [C] +5” or more, more preferably “ ⁇ 60 ⁇ [C] +10” or more. . However, when the area ratio f (B) becomes “ ⁇ 60 ⁇ [C] +50” or more, as described above, the strength of the steel becomes too high and the machinability deteriorates instead. Accordingly, the area ratio f (B) is less than “ ⁇ 60 ⁇ [C] +50”, preferably “ ⁇ 60 ⁇ [C] +45” or less, more preferably “ ⁇ 60 ⁇ [C] +40” or less.
  • the area ratio f (B) of bainite may be measured by observing with a scanning electron microscope (SEM) or an optical microscope after repeller corrosion.
  • Patent Document 4 the machinability of non-tempered steel is improved by positively generating bainite as the amount of C increases, whereas in the present invention, the case of low C In this case, bainite is produced as an essential structure, and in the case of high C, bainite is not made an essential structure.
  • the said patent document 4 and this invention differ in the component composition (specifically Al amount) of steel.
  • the present invention is different in that pearlite is an essential organization, and it is considered that such a difference is a difference in technical idea.
  • a metal structure that satisfies the above requirements can be manufactured by appropriately controlling the average cooling rate Va when passing through a temperature range from 800 ° C. to 500 ° C. after processing at a temperature of 850 ° C. or higher.
  • the method for controlling the metal structure will be described in detail later.
  • the high-strength steel of the present invention has its metal structure controlled as described above, and the composition of the steel is as follows.
  • the high-strength steel of the present invention contains 0.12 to 0.5% Al, 0.0005 to 0.008% B, 0.002 to 0.030% N, and Al and N are represented by the following formula ( Satisfies 1).
  • Al, B, and N are all elements that contribute to improving the machinability of steel (particularly, the tool life when cut intermittently).
  • [] indicates the content (% by mass) of each element. 0.10 ⁇ [Al] -1.9 ⁇ [N] (1)
  • Al 0.12 to 0.5%
  • Al is an element necessary for improving the machinability when intermittent cutting is performed by suppressing the wear on the tool surface by being present in a solid solution state in steel.
  • Al is an element that binds to N and precipitates AlN to prevent the crystal grains from growing abnormally during processing and lowering the strength.
  • Al also acts as a deoxidizer.
  • Al is made 0.12% or more, preferably 0.16% or more, more preferably 0.20% or more.
  • Al is 0.5% or less, preferably 0.4% or less, more preferably 0.3% or less.
  • B is an element that contributes to improving the machinability when securing the solid solution amount of Al and performing intermittent cutting. That is, B binds to N in steel and precipitates BN, thereby suppressing N from binding to Al and precipitating AlN, and thus acts to secure a solid solution Al amount. Further, the precipitated BN contributes to improvement of machinability. B is an element that also acts to improve the hardenability and grain boundary strength to increase the strength of the steel. In order to exert such effects, B is 0.0005% or more, preferably 0.0010% or more, more preferably 0.0025% or more. However, if B is excessive, the steel becomes too hard and the machinability deteriorates. Therefore, B is 0.008% or less, preferably 0.005% or less, more preferably 0.0040% or less.
  • N is an element that precipitates AlN and prevents crystal grains from growing abnormally during processing to lower the strength, and contributes to improving the machinability by depositing BN.
  • N is 0.002% or more, preferably 0.003% or more, more preferably 0.005% or more.
  • N is 0.030% or less, preferably 0.020% or less, more preferably 0.015% or less, and particularly preferably 0.010% or less.
  • Al and N must satisfy the above formula (1), and by satisfying this formula, Al can be present in a solid solution state.
  • the value of “[Al] ⁇ 1.9 ⁇ [N]” is preferably 0.15 or more, more preferably 0.18 or more.
  • Component composition other than Al, B, and N is as follows.
  • C is an element necessary for ensuring strength, and is contained by 0.20% or more.
  • C is preferably 0.30% or more, and more preferably 0.40% or more.
  • the C content is 0.70% or less.
  • the amount of C is preferably 0.60% or less, more preferably 0.55% or less.
  • Si acts as a deoxidizing element and is an element necessary for improving the internal quality of steel.
  • Si is 0.03% or more, preferably 0.10% or more, more preferably 0.20% or more.
  • Si is 2% or less, preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.7% or less.
  • Mn is an element necessary for improving the hardenability and improving the strength of the steel, and is 0.2% or more, preferably 0.5% or more, more preferably 0.90% or more.
  • Mn is 1.8% or less, preferably 1.5% or less, more preferably 1.10% or less.
  • P 0.03% or less (excluding 0%)
  • P is an impurity element that is inevitably contained in steel, and if the amount of P is excessive, it promotes the occurrence of cracks during processing, so it needs to be reduced as much as possible. Therefore, P is 0.03% or less, preferably 0.02% or less, more preferably 0.015% or less. In addition, it is industrially difficult to make P amount 0%.
  • S 0.10% or less (excluding 0%)
  • S is an element that effectively acts to improve the machinability of steel by combining with Mn in steel to form MnS inclusions.
  • the S content is 0.10% or less, preferably 0.08% or less, more preferably 0.05% or less.
  • S is an impurity inevitably contained in steel, it is industrially difficult to make the amount 0%.
  • O is an impurity element inevitably contained in steel, and when the amount of O is excessive, coarse oxide inclusions are generated, and hot workability, ductility, toughness, and machinability deteriorate. Therefore, the O content is 0.002% or less, preferably 0.0018% or less, more preferably 0.0015% or less.
  • the component composition of the high-strength steel according to the present invention is as described above, and the balance is iron and inevitable impurities.
  • inevitable impurities mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed.
  • Cr, Mo, Ti, Zr, Hf, Ta, Nb, V, Cu, Ni, etc. may be positively contained as other elements within the range not impairing the effects of the present invention.
  • Cr 1.5% or less (excluding 0%)
  • Cr is an element that effectively acts to enhance the hardenability of the steel and improve the strength. Moreover, it is an element which acts effectively also for improving the machinability (especially intermittent machinability) of steel by combined addition with Al.
  • Cr is preferably contained in an amount of 0.08% or more, more preferably 0.10% or more, still more preferably 0.2% or more, and particularly preferably 0.7% or more.
  • the Cr amount is preferably 1.5% or less, more preferably. Is 1.3% or less.
  • Mo 1% or less (excluding 0%)
  • Mo is an element that acts to increase the hardenability of steel and suppress the formation of a structure that has not been quenched. Such an action increases as the content increases, but is preferably 0.05% or more, more preferably 0.1% or more, and further preferably 0.15% or more.
  • Mo is excessively contained, a supercooled structure is generated even after normalization and machinability is lowered, so that the content is preferably 1% or less. More preferably, it is 0.8% or less, More preferably, it is 0.5% or less.
  • Ti 0.005% or less (not including 0%), Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), Ta: 0.0. 02% or less (not including 0%), and Nb: at least one element selected from the group consisting of 0.15% or less (not including 0%)]
  • Ti, Zr, Hf, Ta, and Nb are elements that have an effect of preventing abnormal growth of crystal grains during hot working and preventing toughness and fatigue strength of steel from being lowered. Such an effect is exhibited by containing two or more kinds selected arbitrarily.
  • Ti is 0.0003% or more (particularly 0.0005% or more)
  • Zr is 0.002% or more (particularly 0.005% or more)
  • Hf is 0. 0.002% or more (especially 0.005% or more)
  • Ta is preferably 0.002% or more (particularly 0.005% or more)
  • Nb is preferably 0.015% or more (particularly 0.05% or more).
  • Ti is 0.005% or less (particularly 0.003% or less) and Zr is 0.02 or less.
  • Hf is 0.02% or less (particularly 0.015% or less)
  • Ta is 0.02% or less (particularly 0.015% or less)
  • Nb is 0.15% or less. (Especially 0.14% or less) is preferable.
  • V at least 1 selected from the group consisting of 0.5% or less (not including 0%), Cu: 3% or less (not including 0%), and Ni: 3% or less (not including 0%) Species element
  • V, Cu, and Ni are elements that effectively act to improve the hardenability and increase the strength. These effects increase as the content of these elements increases.
  • V is 0.05% or more
  • Cu is 0.1% or more
  • Ni is 0.1% or more. It is preferable. More preferably, V is 0.1% or more, Cu is 0.2% or more, and Ni is 0.5% or more.
  • a supercooled structure is formed and ductility and toughness are lowered.
  • V is preferably 0.5% or less
  • Cu is 3% or less
  • Ni is preferably 3% or less. More preferably, V is 0.3% or less, Cu is 2% or less, and Ni is 2% or less.
  • V, Cu, and Ni may each be contained alone, or two or more selected arbitrarily may be contained.
  • Such a high-strength steel of the present invention is obtained by processing a steel satisfying the above component composition at a temperature of 850 ° C. or higher, and then, in the temperature range from 800 ° C. to 500 ° C., with an average cooling rate Va satisfying the following formula (4). It can be manufactured by cooling. 0.1 ⁇ Vc ⁇ Va ⁇ 0.9 ⁇ Vc (4)
  • the metal structure becomes an austenite single phase, and then cooling by appropriately controlling the average cooling rate Va in the temperature range from 800 ° C. to 500 ° C. Can be generated.
  • the processing performed at 850 ° C. or higher is a work heat treatment accompanied by heating, such as hot rolling or hot forging, and may be plastic processing.
  • hot rolling steel that satisfies the above component composition is melted and cast, and then hot rolled at a temperature of 850 ° C. or higher.
  • hot forging a steel satisfying the above component composition is prepared, and this steel may be hot forged at a temperature of 850 ° C. or higher.
  • the processing temperature is preferably 950 ° C. or higher. Although the upper limit of processing temperature is not specifically limited, For example, it is about 1050 degreeC.
  • Vc indicates a critical cooling rate.
  • Vc is calculated by the following formula (5)
  • k is calculated by the following formula (6).
  • [] indicates the content (% by mass) of each element.
  • Ni, Cr, Mo, and Nb are selective elements in the high-strength steel of the present invention. Therefore, when not added, it may be calculated that there is no term.
  • Vc 10 k (5)
  • k 4.05- ⁇ 4.5 ⁇ [C] + [Mn] + 0.5 ⁇ [Ni] + 0.8 ⁇ [Cr] + 1.6 ⁇ [Mo] + 9.0 ⁇ [Nb] ⁇ (6)
  • the average cooling rate Va When the average cooling rate Va becomes 0.1 ⁇ Vc or less, the amount of bainite produced decreases and the strength becomes insufficient. Therefore, the average cooling rate Va is more than 0.1 ⁇ Vc, preferably 0.2 ⁇ Vc or more, more preferably 0.3 ⁇ Vc or more. However, when the average cooling rate Va is 0.9 ⁇ Vc or more, the amount of bainite generated increases and the strength becomes too high, so that machinability deteriorates. Therefore, the average cooling rate Va is less than 0.9 ⁇ Vc, preferably 0.8 ⁇ Vc or less, more preferably 0.7 ⁇ Vc or less.
  • the high-strength steel of the present invention obtained by cooling under the above conditions has the strength required for steel parts despite excellent machinability. Therefore, if the high-strength steel of the present invention is cut into a part shape, it has the strength required as a steel part even in the state of cutting. That is, if a steel part is manufactured using the high-strength steel of the present invention, heat treatment such as quenching and tempering (tempering) and carburizing and quenching, which has been conventionally performed to improve the strength after cutting, can be omitted.
  • the intermittent cutting process may be performed at a low speed (specifically, a cutting speed of about 150 m / min or less) and a low temperature (specifically, about 200 to 600 ° C.).
  • the high-strength steel When cutting the high-strength steel into a part shape, it is necessary to perform cutting without heating to a temperature of 850 ° C. or higher.
  • the high-strength steel When the high-strength steel is heated to a temperature of 850 ° C. or higher, all the appropriately adjusted metal structures are cancelled, so that machinability deteriorates.
  • the high-strength steel may be cut after being heated in a range where the temperature does not become 850 ° C. or higher before being cut into a part shape.
  • the high-strength steel may be cold-worked and / or warm-worked as necessary before cutting, and at this time, it is necessary to control the temperature not to exceed 850 ° C. *
  • the steel part of the present invention is formed of the above-described high-strength steel, it satisfies the composition and metal structure of the high-strength steel and has the strength required as a steel part. In order to further improve the wear resistance, surface hardening treatment may be performed.
  • the surface hardening treatment should not increase the temperature of the entire steel part as in the case of quenching and tempering (tempering), but should adopt a method of hardening only the surface of the steel part so as not to reduce the strength of the steel part. .
  • Examples of the surface hardening treatment method include induction hardening and nitriding treatment.
  • Induction hardening is a method in which the surface of a steel part is sequentially heated for a short time with an induction hardening apparatus, and the heated portion is immediately cooled and quenched.
  • a cooling method water cooling (for example, jet water cooling) is employed.
  • the heating temperature is, for example, A C3 transformation point or more, it may be set to 950 ° C. or less.
  • the heating temperature is preferably AC3 transformation point + 30 ° C. or higher and 920 ° C. or lower.
  • Tempering may be performed after induction hardening.
  • the tempering temperature may be, for example, 100 ° C. or more and 250 ° C. or less. Preferably it is 120 degreeC or more and 230 degrees C or less.
  • nitriding method methods such as ion nitriding (plasma nitriding) and radical nitriding can be applied.
  • Preferred conditions for the nitriding treatment are a treatment temperature: 500 to 650 ° C. (more preferably 500 to 575 ° C.), and a treatment time: 4 to 12 hours (more preferably 6 to 10 hours). If the treatment temperature during the nitriding treatment exceeds 650 ° C., the steel is easily softened, and if it is lower than 500 ° C., the nitriding depth (hardened layer depth) becomes shallow and the surface is not sufficiently cured.
  • the steel parts of the present invention are used as mechanical structural parts such as gears, shafts, pulleys, constant velocity joints, etc., as well as crankshafts, connecting rods, etc. used in various gear transmissions including transmissions and differentials for automobiles. It can be used suitably.
  • Table 2 below shows the content of the components shown in Table 1 below, the k value calculated from the above formulas (5) and (6), the Vc value (value of 10 k ), and “0.1 ⁇ Vc”. ”And“ 0.9 ⁇ Vc ”, respectively.
  • the metal structure of the plate material was subjected to repeller corrosion at the center of the plate thickness, and the area ratio of each structure was measured by image analysis of a photograph taken with an optical microscope at an observation magnification of 200 times.
  • the bainite area ratio f (B) is shown in Table 2 below. It was confirmed that the structures other than bainite were ferrite and pearlite.
  • Table 2 below shows values obtained by calculating the left side and the right side of Formula (2) or Formula (3) based on the C amount shown in Table 1 below. No. For No. 22, formula (2), no. For Formula 23, Formula (3) was followed.
  • the pass / fail result is shown in the following table, where the bainite area ratio f (B) satisfies the relationship of the above formula (2) or formula (3) according to the amount of C, and the case where it is not satisfied is ⁇ . It is shown in 2.
  • the Vickers hardness Hv of the plate material was measured.
  • the Vickers hardness was measured as a load: 200 g at the center position of a cross section of thickness: 30 mm ⁇ width: 155 mm.
  • the measurement results are shown in Table 2 below.
  • the case where the Vickers hardness is Hv230 or higher is regarded as acceptable (high strength), and the case where it is less than Hv230 is regarded as unacceptable (low strength).
  • flank wear amount Vb was measured to obtain an average value.
  • Table 2 the flank wear amount in the present invention, those having a flank wear amount Vb of 100 ⁇ m or less after intermittent cutting were evaluated as “excellent machinability during intermittent cutting”.
  • the plate material was subjected to surface hardening treatment, and the Vickers hardness after the surface hardening treatment was measured.
  • nitriding treatment or induction hardening was performed.
  • gas soft nitriding treatment was performed at a treatment temperature of 530 ° C. and a treatment time of 2 hours.
  • Induction hardening was performed at a heating temperature of 850 ° C. and cooling was performed by water cooling.
  • the Vickers hardness of the plate material after the surface hardening treatment was measured as a load: 200 g at the center position of a cross section of thickness: 30 mm ⁇ width: 155 mm. As a result, the Vickers hardness after the surface hardening treatment was not changed from the Vickers hardness before the surface hardening treatment.
  • Table 1 and Table 2 can be considered as follows. No. Nos. 1 to 21 are examples that satisfy the requirements defined in the present invention, and a steel having both desired hardness (strength) and machinability can be realized.
  • No. 22 to 27 are examples that do not satisfy any of the requirements defined in the present invention, and at least one of strength and machinability cannot be improved.
  • No. No. 22 is an example in which the amount of C is too small, bainite is not generated, and hardness is not secured. Therefore, the strength is insufficient.
  • No. 23 is an example in which the amount of C is excessive and the average cooling rate Va exceeds a predetermined range. Therefore, bainite is generated excessively, becomes too hard, the flank wear amount Vb increases, and the machinability deteriorates.
  • No. 24 is an example in which there is too little Al, and the relationship between Al and N does not satisfy the above formula (1). Therefore, since the amount of dissolved Al is insufficient, the flank wear amount Vb is increased, and the machinability cannot be improved.
  • No. No. 25 is an example in which B is too small, and since the amount of dissolved Al is insufficient, the flank wear amount Vb is increased and the machinability cannot be improved.
  • No. No. 26 is an example in which the average cooling rate Va falls below the range defined in the present invention, and since the amount of bainite produced is too small, the hardness cannot be ensured and the strength is insufficient.
  • No. No. 27 is an example in which the average cooling rate Va exceeds the range defined in the present invention. Since bainite is excessively generated, the flank wear amount Vb increases and the machinability cannot be improved.
  • the high-strength steel of the present invention can ensure the strength required as a steel part without performing heat treatment such as quenching and tempering (tempering) or carburizing and quenching after cutting into the shape of the part. Because of its excellent performance, it is useful for mechanical structural parts such as gears, shafts, pulleys, constant velocity joints, crankshafts, connecting rods, etc., used in various gear transmissions including transmissions for high-strength automobiles and differentials.

Abstract

The disclosed high-strength steel has the following properties: (a) said steel contains specific amounts of carbon, silicon, manganese, phosphorus, sulfur, aluminum, boron, nitrogen, and oxygen; (b) the relationship between the aluminum content and the nitrogen content satisfies relation (1); (c) if the carbon content is less than 0.35% and no less than 0.20%, the structure of the disclosed steel is a mixture of ferrite, pearlite, and bainite, with the area ratio of the bainite (f(B)) satisfying relation (2); and (d) if the carbon content is between 0.35% and 0.70% inclusive, the structure of the disclosed steel either is a mixture of ferrite and pearlite or also includes bainite with an area ratio (f(B)) satisfying relation (3). (1) 0.10 < [Al] - 1.9×[N] (2) -60×[C] + 21 < f(B) < -60×[C] + 50 (3) 0 ≤ f(B) < -60×[C] + 50

Description

被削性に優れた高強度鋼、およびその製造方法High-strength steel with excellent machinability and method for producing the same
 本発明は、切削加工して鋼部品を製造するための鋼、およびその製造方法に関するものである。 The present invention relates to steel for manufacturing steel parts by cutting and a method for manufacturing the same.
 自動車や各種機械類に用いられる鋼部品(具体的には、自動車用変速機や差動装置をはじめとする各種歯車伝達装置に利用される歯車、シャフト、プーリーや等速ジョイント等、更にはクランクシャフト、コンロッド等の機械構造部品)は、通常、熱間加工(例えば、熱間圧延や熱間鍛造など)した鋼に、切削加工を施して最終形状(部品形状)に仕上げて製造される。この鋼部品は、高強度であることが求められるが、鋼部品の強度を高めるために、切削加工前の鋼の強度を高めると切削加工が困難となる。切削加工に要するコストは、部品製作費全体中に占める割合が高いことから、切削加工前の鋼は被削性が良いことが要求される。そこで、切削加工前の鋼は、その硬さを低くして被削性を改善し、切削加工後に、焼入れ焼戻し(調質)や浸炭焼入れ等の熱処理を行うことによって鋼部品の強度を高めることが行われている。 Steel parts used in automobiles and various machines (specifically, gears, shafts, pulleys, constant velocity joints, etc. used in various gear transmissions including transmissions and differentials for automobiles, as well as cranks) Machine structural parts such as shafts and connecting rods are usually manufactured by subjecting hot-worked steel (for example, hot rolling or hot forging) to a final shape (part shape) by cutting. This steel part is required to have high strength, but in order to increase the strength of the steel part, if the strength of the steel before cutting is increased, cutting becomes difficult. Since the cost required for cutting is high in the entire part production cost, the steel before cutting is required to have good machinability. Therefore, the steel before cutting improves the machinability by reducing its hardness, and increases the strength of steel parts by performing heat treatment such as quenching and tempering (tempering) and carburizing and quenching after cutting. Has been done.
 ここで切削加工について詳しく説明すると、上記機械構造部品のうち特に歯車を製造するときの切削加工においては、ホブによる歯切りを行うのが一般的であり、この場合の切削加工は断続切削と呼ばれている。ホブ加工に用いられる工具としては、高速度工具鋼にAlTiNなどのコーティングを施したもの(以下、「ハイス工具」と略称することがある)が現状の主流である。しかしハイス工具を用いたホブ加工(断続切削)による歯切りは、低速(具体的には、切削速度150m/分程度以下)、低温(具体的には、200~600℃程度)であるが、断続切削のため工具が空気と触れ易く、酸化、摩耗し易くなる。そのためホブ加工等の断続切削に供される鋼は、特に工具寿命を伸ばすことが求められている。 Here, the cutting process will be described in detail. In the cutting process for manufacturing gears among the above-mentioned mechanical structural parts, it is common to perform gear cutting with a hob. In this case, the cutting process is called intermittent cutting. It is. As a tool used for hobbing, a high-speed tool steel coated with AlTiN or the like (hereinafter sometimes abbreviated as “high-speed tool”) is the current mainstream. However, gear cutting by hobbing (intermittent cutting) using a high-speed tool is low speed (specifically, cutting speed of about 150 m / min or less) and low temperature (specifically, about 200 to 600 ° C.) Because of the intermittent cutting, the tool can easily come into contact with air and be easily oxidized and worn. For this reason, steel used for intermittent cutting such as hobbing is particularly required to extend the tool life.
 本出願人は、断続切削における被削性(特に、工具寿命)を向上させた機械構造用鋼を特許文献1、2に提案している。これらのうち特許文献1では、酸化物系介在物の各成分を適切に調整して介在物の全体が低融点で変形し易くすることによってハイス工具での断続切削における被削性を改善している。一方、特許文献2では、Feより酸化傾向の大きい元素を機械構造用鋼に添加して固溶させることによって、断続切削における機械構造用鋼の急速な酸化を防止して、工具の酸化摩耗を抑制し、鋼の被削性を改善している。しかし上記特許文献1、2では、上述したように、鋼部品の強度を高めるために、切削加工後に焼入れ焼戻し(調質)や浸炭焼入れ等の熱処理を行う必要がある。 The applicant has proposed steels for machine structural use with improved machinability (particularly, tool life) in interrupted cutting in Patent Documents 1 and 2. Among these, in patent document 1, the machinability in intermittent cutting with a high-speed tool is improved by appropriately adjusting each component of the oxide inclusions so that the entire inclusion is easily deformed at a low melting point. Yes. On the other hand, in Patent Document 2, by adding an element that has a greater tendency to oxidize than Fe to solid solution, the mechanical structure steel is prevented from being rapidly oxidized during intermittent cutting, and oxidative wear of the tool is prevented. Suppresses and improves the machinability of steel. However, in Patent Documents 1 and 2, as described above, in order to increase the strength of the steel part, it is necessary to perform heat treatment such as quenching and tempering (tempering) or carburizing and quenching after cutting.
 ところで、近年では、地球環境への負荷を低減すると共に、作業環境を改善するために、焼入れ焼戻し(調質)や浸炭焼入れ等の熱処理を省略することが求められている。ところが、上記特許文献1、2において、切削加工後の熱処理を省略すると、鋼部品の強度が低下してしまう。 Incidentally, in recent years, in order to reduce the burden on the global environment and improve the working environment, it is required to omit heat treatments such as quenching and tempering (tempering) and carburizing and quenching. However, in Patent Documents 1 and 2, if the heat treatment after cutting is omitted, the strength of the steel part is lowered.
 非調質鋼の強度を高める技術として、特許文献3、4が知られている。これらのうち特許文献3には、熱間鍛造・冷却後の組織を、初析フェライトを含まないベイナイトまたはベイナイトとマルテンサイトの混合組織にすることによって、非調質鋼の強度を高めている。この文献では、上記混合組織を得るために、鋼の成分組成に基づいて算出される臨界冷却速度Vcが、熱間鍛造後に実施される空冷もしくは衝風冷却での800~500℃における平均冷却速度Va以下(Vc≦Va)になるように、鋼の成分組成を調整している。しかし、この技術は、非調質鋼の強度と靱性を改善するものであり、被削性(特に、断続切削したときの被削性)については全く考慮されていない。  Patent Documents 3 and 4 are known as techniques for increasing the strength of non-heat treated steel. Among these, in Patent Document 3, the strength of non-tempered steel is increased by making the structure after hot forging and cooling be bainite or a mixed structure of bainite and martensite that does not contain pro-eutectoid ferrite. In this document, in order to obtain the above mixed structure, the critical cooling rate Vc calculated based on the composition of steel is an average cooling rate at 800 to 500 ° C. in air cooling or blast cooling performed after hot forging. The component composition of the steel is adjusted so as to be Va or less (Vc ≦ Va). However, this technique improves the strength and toughness of non-tempered steel, and no consideration is given to machinability (particularly machinability when intermittently cut).
 これに対し、特許文献4には、非調質鋼の強度を高めると共に、疲労強度および被削性を向上させる技術が開示されている。この非調質鋼は、Alを0.0005~0.050%含有すると共に、ベイナイトの組織率fを鋼に含まれるC量に対して、1.4C+0.4≧f≧1.4C、としているところに特徴がある。 On the other hand, Patent Document 4 discloses a technique for increasing the strength of non-heat treated steel and improving fatigue strength and machinability. This non-tempered steel contains 0.0005 to 0.050% of Al, and the structure ratio f of bainite is 1.4C + 0.4 ≧ f ≧ 1.4C with respect to the amount of C contained in the steel. There is a feature.
日本国特開2009-30160号公報Japanese Unexamined Patent Publication No. 2009-30160 日本国特開2009-287111号公報Japanese Unexamined Patent Publication No. 2009-287111 日本国特開平6-299285号公報Japanese Unexamined Patent Publication No. 6-299285 日本国特開平7-109545号公報Japanese Unexamined Patent Publication No. 7-109545
 本発明はこのような事情に着目してなされたものであって、その目的は、部品形状に切削加工した後に、焼入れ焼戻し(調質)や浸炭焼入れ等の熱処理を行わなくても鋼部品として要求される強度を確保でき、しかも切削加工時には被削性に優れた高強度鋼、およびその製造方法を提供することにある。 The present invention has been made paying attention to such circumstances, and the purpose thereof is to cut steel parts into steel parts without cutting and tempering (tempering) or carburizing and quenching. An object of the present invention is to provide a high-strength steel that can ensure the required strength and is excellent in machinability at the time of cutting, and a method for producing the same.
 本発明に係る高強度鋼は、C:0.20~0.70%(質量%の意味。以下同じ。)、Si:0.03~2%、Mn:0.2~1.8%、P:0.03%以下(0%を含まない)、S:0.10%以下(0%を含まない)、Al:0.12~0.5%、B:0.0005~0.008%、N:0.002~0.030%、およびO:0.002%以下(0%を含まない)を含有すると共に、AlとNが下記式(1)の関係を満足し、残部が鉄および不可避不純物からなる鋼である。そして、前記鋼に含まれるC量が0.20%以上、0.35%未満の場合は、金属組織がフェライト、パーライト、およびベイナイトの混合組織であり、且つベイナイトの面積率f(B)が下記式(2)を満足しており、C量が0.35%以上、0.70%以下の場合は、金属組織がフェライトとパーライトの混合組織であるか、更にベイナイトを含む混合組織であり、且つベイナイトの面積率f(B)が下記式(3)を満足しているところに要旨を有している。なお、下記式(1)~式(3)において、[]は、各元素の含有量(質量%)を示している。
0.10<[Al]-1.9×[N] ・・・(1)
-60×[C]+21<f(B)<-60×[C]+50 ・・・(2)
0≦f(B)<-60×[C]+50 ・・・(3) 
The high-strength steel according to the present invention has C: 0.20 to 0.70% (meaning mass%, the same applies hereinafter), Si: 0.03 to 2%, Mn: 0.2 to 1.8%, P: 0.03% or less (not including 0%), S: 0.10% or less (not including 0%), Al: 0.12 to 0.5%, B: 0.0005 to 0.008 %, N: 0.002 to 0.030%, and O: 0.002% or less (not including 0%), Al and N satisfy the relationship of the following formula (1), and the balance is Steel made of iron and inevitable impurities. When the amount of C contained in the steel is 0.20% or more and less than 0.35%, the metal structure is a mixed structure of ferrite, pearlite, and bainite, and the area ratio f (B) of bainite is When the following formula (2) is satisfied and the C content is 0.35% or more and 0.70% or less, the metal structure is a mixed structure of ferrite and pearlite, or a mixed structure containing bainite. In addition, the gist of the present invention is that the area ratio f (B) of bainite satisfies the following formula (3). In the following formulas (1) to (3), [] indicates the content (% by mass) of each element.
0.10 <[Al] -1.9 × [N] (1)
−60 × [C] +21 <f (B) <− 60 × [C] +50 (2)
0 ≦ f (B) <− 60 × [C] +50 (3)
 本発明の高強度鋼は、更に他の元素として、
(a)Cr:1.5%以下(0%を含まない)、
(b)Mo:1%以下(0%を含まない)、
(c)Ti:0.005%以下(0%を含まない)、Zr:0.02%以下(0%を含まない)、Hf:0.02%以下(0%を含まない)、Ta:0.02%以下(0%を含まない)、およびNb:0.15%以下(0%を含まない)よりなる群から選ばれる少なくとも1種の元素、
(d)V:0.5%以下(0%を含まない)、Cu:3%以下(0%を含まない)、およびNi:3%以下(0%を含まない)よりなる群から選ばれる少なくとも1種の元素、等を含有してもよい。
The high-strength steel of the present invention is still another element,
(A) Cr: 1.5% or less (excluding 0%),
(B) Mo: 1% or less (excluding 0%),
(C) Ti: 0.005% or less (not including 0%), Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), Ta: At least one element selected from the group consisting of 0.02% or less (excluding 0%) and Nb: 0.15% or less (not including 0%);
(D) V: 0.5% or less (excluding 0%), Cu: 3% or less (not including 0%), and Ni: 3% or less (not including 0%) It may contain at least one element.
 本発明には、上記高強度鋼で形成された鋼部品も包含され、この鋼部品の金属組織は、用いた高強度鋼と同じになっている。 The present invention also includes a steel part formed of the above-described high-strength steel, and the metal structure of the steel part is the same as the high-strength steel used.
 本発明の高強度鋼は、上記成分組成を満足する鋼を、温度850℃以上で加工した後、800℃から500℃までの温度域を、下記式(4)を満たす平均冷却速度Vaで冷却することによって製造できる。
0.1×Vc<Va<0.9×Vc ・・・(4)
In the high-strength steel of the present invention, after processing a steel satisfying the above component composition at a temperature of 850 ° C. or higher, the temperature range from 800 ° C. to 500 ° C. is cooled at an average cooling rate Va satisfying the following formula (4). Can be manufactured.
0.1 × Vc <Va <0.9 × Vc (4)
 上記式(4)において、Vcは下記式(5)で示され、kは下記式(6)で示される。なお、下記式(6)において[]は、各元素の含有量(質量%)を示している。
Vc=10 ・・・(5)
k=4.05-{4.5×[C]+[Mn]+0.5×[Ni]+0.8×[Cr]+1.6×[Mo]+9.0×[Nb]}・・・(6)
In the above formula (4), Vc is represented by the following formula (5), and k is represented by the following formula (6). In addition, in following formula (6), [] has shown content (mass%) of each element.
Vc = 10 k (5)
k = 4.05- {4.5 × [C] + [Mn] + 0.5 × [Ni] + 0.8 × [Cr] + 1.6 × [Mo] + 9.0 × [Nb]} (6)
 本発明の鋼部品は、上述した製造方法で得られた高強度鋼を、温度850℃以上に加熱することなく切削加工することによって製造できる。 The steel part of the present invention can be manufactured by cutting the high-strength steel obtained by the above-described manufacturing method without heating to a temperature of 850 ° C. or higher.
 本発明によれば、鋼の成分組成を規定すると共に、鋼に含まれるC量に応じて金属組織に含まれるベイナイトの面積率を適切に制御することによって、被削性に優れた高強度鋼を提供できる。即ち、本発明の高強度鋼は、切削加工したときの被削性、特に、断続切削したときの工具寿命が良好であり、しかも切削加工して形成された鋼部品は、切削加工ままの状態で鋼部品として要求される強度を確保できているため、切削加工後の焼入れ焼戻し(調質)や浸炭焼入れ等の熱処理を省略できる。 According to the present invention, the high-strength steel excellent in machinability by prescribing the component composition of the steel and appropriately controlling the area ratio of bainite contained in the metal structure according to the amount of C contained in the steel. Can provide. That is, the high-strength steel of the present invention has good machinability when it is cut, in particular, the tool life when it is cut intermittently, and the steel part formed by cutting is still in the state of cutting. Since the strength required for steel parts can be secured, heat treatment such as quenching and tempering (tempering) and carburizing and quenching after cutting can be omitted.
 本発明者らは、切削加工時の被削性が良好で、しかも部品形状に切削加工したままの状態で鋼部品として要求される強度を確保できる高強度鋼を提供するために検討を重ねてきた。その結果、鋼の成分組成を適切に調整したうえで、鋼に含まれるC量に応じて金属組織を適切に制御、具体的には、C量が0.20%以上、0.35%未満の場合は、フェライト、パーライト、およびベイナイトの混合組織とし、C量が0.35%以上、0.70%以下の場合は、フェライトとパーライトの混合組織であるか、更にベイナイトを含む混合組織とし、且つ、混合組織に占めるベイナイトの面積率を適切に制御することによって、被削性と強度を兼ね備えた鋼を提供できることを見出し、本発明を完成した。    The present inventors have repeatedly studied to provide high-strength steel that has good machinability at the time of cutting and that can ensure the strength required as a steel part in a state of being cut into a part shape. It was. As a result, after appropriately adjusting the component composition of the steel, the metal structure is appropriately controlled according to the amount of C contained in the steel. Specifically, the amount of C is 0.20% or more and less than 0.35%. Is a mixed structure of ferrite, pearlite and bainite, and when the C content is 0.35% or more and 0.70% or less, it is a mixed structure of ferrite and pearlite, or a mixed structure containing bainite. And the steel which combines machinability and intensity | strength can be provided by controlling appropriately the area ratio of the bainite which occupies for a mixed structure, and completed this invention. *
 即ち、本発明者らは、鋼の金属組織を、フェライトとパーライトの混合組織とすれば、切削加工したときの被削性(特に、断続切削したときの工具寿命)を改善できると考えた。ところが、鋼の金属組織をフェライトとパーライトの混合組織にすると、鋼の強度が低下することがあり、鋼部品として要求される強度を確保できない場合があることが分かった。そこで、強度を高めるために、上記混合組織にベイナイトを含有させればよく、鋼のC量に応じてベイナイトの面積率を制御すれば、被削性を劣化させることなく強度を向上できることを見出した。 That is, the present inventors considered that if the steel metal structure is a mixed structure of ferrite and pearlite, the machinability when cutting (particularly, the tool life when cutting intermittently) can be improved. However, it has been found that if the metal structure of steel is a mixed structure of ferrite and pearlite, the strength of the steel may decrease, and the strength required for steel parts may not be ensured. Thus, in order to increase the strength, bainite may be contained in the mixed structure, and if the area ratio of bainite is controlled in accordance with the C content of steel, it is found that the strength can be improved without degrading the machinability. It was.
 具体的には、鋼に含まれるC量が0.20%以上、0.35%未満の場合は、金属組織を、フェライト、パーライト、およびベイナイトの混合組織とし、ベイナイトの面積率f(B)は、下記式(2)を満足するように調整することが重要となる。下記式(2)において、[C]は、鋼に含まれるC量(質量%)を示している。
-60×[C]+21<f(B)<-60×[C]+50 ・・・(2) 
Specifically, when the amount of C contained in the steel is 0.20% or more and less than 0.35%, the metal structure is a mixed structure of ferrite, pearlite, and bainite, and the area ratio f (B) of bainite. It is important to adjust so as to satisfy the following formula (2). In the following formula (2), [C] indicates the amount of C (% by mass) contained in the steel.
−60 × [C] +21 <f (B) <− 60 × [C] +50 (2)
 上記面積率f(B)が「-60×[C]+21」以下になると、ベイナイト量が少な過ぎるため、鋼の強度を確保できなくなる。従って面積率f(B)は、「-60×[C]+21」超、好ましくは「-60×[C]+25」以上、より好ましくは「-60×[C]+30」以上とする。しかし上記面積率f(B)が「-60×[C]+50」以上になると、鋼の強度が高くなり過ぎて被削性が劣化する。従って面積率f(B)は、「-60×[C]+50」未満、好ましくは「-60×[C]+45」以下、より好ましくは「-60×[C]+40」以下とする。  When the area ratio f (B) is “−60 × [C] +21” or less, the amount of bainite is too small, and the strength of the steel cannot be secured. Accordingly, the area ratio f (B) is more than “−60 × [C] +21”, preferably “−60 × [C] +25” or more, more preferably “−60 × [C] +30” or more. However, if the area ratio f (B) is “−60 × [C] +50” or more, the strength of the steel becomes too high and the machinability deteriorates. Therefore, the area ratio f (B) is less than “−60 × [C] +50”, preferably “−60 × [C] +45” or less, more preferably “−60 × [C] +40” or less.
 一方、鋼に含まれるC量が0.35%以上、0.70%以下の場合は、フェライトとパーライトの混合組織であっても鋼部品に要求される強度を確保できるため、ベイナイトは必ずしも生成させなくてもよい。但し、更なる高強度化を目指してベイナイトを含有させてもよい。即ち、鋼に含まれるC量が上記範囲の場合は、金属組織を、フェライトとパーライトの混合組織とするか、更にベイナイトを含む混合組織とすればよい。 On the other hand, when the amount of C contained in the steel is 0.35% or more and 0.70% or less, the strength required for steel parts can be ensured even with a mixed structure of ferrite and pearlite, so bainite is not necessarily generated. You don't have to. However, bainite may be included in order to further increase the strength. That is, when the amount of C contained in the steel is in the above range, the metal structure may be a mixed structure of ferrite and pearlite, or a mixed structure containing bainite.
 このときベイナイトの面積率f(B)は、下記式(3)を満足するように調整する。下記式(3)において、[C]は、鋼に含まれるC量(質量%)を示している。
0≦f(B)<-60×[C]+50 ・・・(3)
At this time, the area ratio f (B) of bainite is adjusted so as to satisfy the following formula (3). In the following formula (3), [C] indicates the amount of C (% by mass) contained in the steel.
0 ≦ f (B) <− 60 × [C] +50 (3)
 上記面積率f(B)は0面積%でもよいが、強度を一層高めるには、好ましくは「-60×[C]+5」以上、より好ましくは「-60×[C]+10」以上とする。しかし上記面積率f(B)が「-60×[C]+50」以上になると、上述したように、鋼の強度が高くなり過ぎて被削性が却って劣化する。従って面積率f(B)は、「-60×[C]+50」未満、好ましくは「-60×[C]+45」以下、より好ましくは「-60×[C]+40」以下とする。 The area ratio f (B) may be 0 area%, but in order to further increase the strength, it is preferably “−60 × [C] +5” or more, more preferably “−60 × [C] +10” or more. . However, when the area ratio f (B) becomes “−60 × [C] +50” or more, as described above, the strength of the steel becomes too high and the machinability deteriorates instead. Accordingly, the area ratio f (B) is less than “−60 × [C] +50”, preferably “−60 × [C] +45” or less, more preferably “−60 × [C] +40” or less.
 ベイナイトの面積率f(B)は、レペラー腐食した後、走査型電子顕微鏡(SEM)や光学顕微鏡で観察することによって測定すればよい。 The area ratio f (B) of bainite may be measured by observing with a scanning electron microscope (SEM) or an optical microscope after repeller corrosion.
 なお、上記特許文献4においては、C量の増加に伴ってベイナイトを積極的に生成させることによって非調質鋼の被削性を改善しているのに対し、本発明では、低Cの場合にはベイナイトを必須組織として生成させ、高Cの場合にはベイナイトは必須組織としていない点で相違している。このように上記特許文献4と本発明の技術的思想は全く逆になっているのであるが、上記特許文献4と本発明は、鋼の成分組成(具体的には、Al量)が相違しており、しかも本発明ではパーライトを必須組織としている点で相違しており、こうした相違点が技術的思想の違いになっていると考えられる。 In Patent Document 4, the machinability of non-tempered steel is improved by positively generating bainite as the amount of C increases, whereas in the present invention, the case of low C In this case, bainite is produced as an essential structure, and in the case of high C, bainite is not made an essential structure. Thus, although the technical idea of the said patent document 4 and this invention is completely reverse, the said patent document 4 and this invention differ in the component composition (specifically Al amount) of steel. Moreover, the present invention is different in that pearlite is an essential organization, and it is considered that such a difference is a difference in technical idea.
 上記要件を満足する金属組織は、温度850℃以上で加工した後、800℃から500℃までの温度域を通過するときの平均冷却速度Vaを適切に制御することによって製造できる。金属組織の制御方法については、後で詳述する。 A metal structure that satisfies the above requirements can be manufactured by appropriately controlling the average cooling rate Va when passing through a temperature range from 800 ° C. to 500 ° C. after processing at a temperature of 850 ° C. or higher. The method for controlling the metal structure will be described in detail later.
 本発明の高強度鋼は、その金属組織を上記のように制御したものであり、鋼の成分組成は次の通りである。 The high-strength steel of the present invention has its metal structure controlled as described above, and the composition of the steel is as follows.
 本発明の高強度鋼は、Alを0.12~0.5%、Bを0.0005~0.008%、Nを0.002~0.030%含有し、AlとNが下記式(1)を満足している。Al、B、Nは、いずれも鋼の被削性(特に、断続切削したときの工具寿命)を改善するのに寄与する元素である。下記式(1)において、[]は、各元素の含有量(質量%)を示している。
0.10<[Al]-1.9×[N] ・・・(1) 
The high-strength steel of the present invention contains 0.12 to 0.5% Al, 0.0005 to 0.008% B, 0.002 to 0.030% N, and Al and N are represented by the following formula ( Satisfies 1). Al, B, and N are all elements that contribute to improving the machinability of steel (particularly, the tool life when cut intermittently). In the following formula (1), [] indicates the content (% by mass) of each element.
0.10 <[Al] -1.9 × [N] (1)
[Al:0.12~0.5%] 
 Alは、鋼中に固溶状態で存在させることによって断続切削したときの被削性を向上させる(工具表面の酸化摩耗を抑制する)ために必要な元素である。また、AlはNと結合してAlNを析出し、加工時に結晶粒が異常成長して強度が低下するのを防止する元素である。また、Alは、脱酸剤としても作用する。こうした効果を発揮させるためには、Alは、0.12%以上、好ましくは0.16%以上、より好ましくは0.20%以上とする。しかしAlが過剰になると、AlNが多量に析出して加工性を低下させる。従ってAlは0.5%以下、好ましくは0.4%以下、より好ましくは0.3%以下とする。
[Al: 0.12 to 0.5%]
Al is an element necessary for improving the machinability when intermittent cutting is performed by suppressing the wear on the tool surface by being present in a solid solution state in steel. Further, Al is an element that binds to N and precipitates AlN to prevent the crystal grains from growing abnormally during processing and lowering the strength. Al also acts as a deoxidizer. In order to exert such an effect, Al is made 0.12% or more, preferably 0.16% or more, more preferably 0.20% or more. However, when Al is excessive, a large amount of AlN precipitates and the workability is lowered. Therefore, Al is 0.5% or less, preferably 0.4% or less, more preferably 0.3% or less.
[B:0.0005~0.008%]
 Bは、Alの固溶量を確保して断続切削したときの被削性を向上させるのに寄与する元素である。即ち、Bは、鋼中のNと結合してBNを析出させることによって、NがAlと結合してAlNを析出するのを抑制するため、固溶Al量を確保するのに作用する。また、析出したBNは、被削性の向上に寄与する。また、Bは、焼入れ性や粒界強度を向上させて鋼の強度を高めるのにも作用する元素である。こうした効果を発揮させるには、Bは、0.0005%以上、好ましくは0.0010%以上、より好ましくは0.0025%以上とする。しかしBが過剰になると、鋼が硬くなり過ぎて被削性が却って劣化する。従ってBは0.008%以下、好ましくは0.005%以下、より好ましくは0.0040%以下とする。
[B: 0.0005 to 0.008%]
B is an element that contributes to improving the machinability when securing the solid solution amount of Al and performing intermittent cutting. That is, B binds to N in steel and precipitates BN, thereby suppressing N from binding to Al and precipitating AlN, and thus acts to secure a solid solution Al amount. Further, the precipitated BN contributes to improvement of machinability. B is an element that also acts to improve the hardenability and grain boundary strength to increase the strength of the steel. In order to exert such effects, B is 0.0005% or more, preferably 0.0010% or more, more preferably 0.0025% or more. However, if B is excessive, the steel becomes too hard and the machinability deteriorates. Therefore, B is 0.008% or less, preferably 0.005% or less, more preferably 0.0040% or less.
[N:0.002~0.030%] 
 Nは、AlNを析出して加工時に結晶粒が異常成長して強度が低下するのを防止する他、BNを析出して被削性を向上させるのに寄与する元素である。こうした効果を発揮させるには、Nは0.002%以上、好ましくは0.003%以上、より好ましくは0.005%以上とする。しかしNが過剰になると、AlNが多量に析出して加工性を低下させる。従ってNは、0.030%以下、好ましくは0.020%以下、より好ましくは0.015%以下、特に好ましくは0.010%以下とする。
[N: 0.002 to 0.030%]
N is an element that precipitates AlN and prevents crystal grains from growing abnormally during processing to lower the strength, and contributes to improving the machinability by depositing BN. In order to exert such an effect, N is 0.002% or more, preferably 0.003% or more, more preferably 0.005% or more. However, when N is excessive, a large amount of AlN precipitates and the workability is lowered. Therefore, N is 0.030% or less, preferably 0.020% or less, more preferably 0.015% or less, and particularly preferably 0.010% or less.
 上記AlとNは、上記式(1)を満足している必要があり、この式を満足することによってAlを固溶状態で存在させることができる。「[Al]-1.9×[N]」の値は、好ましくは0.15以上、より好ましくは0.18以上である。 The Al and N must satisfy the above formula (1), and by satisfying this formula, Al can be present in a solid solution state. The value of “[Al] −1.9 × [N]” is preferably 0.15 or more, more preferably 0.18 or more.
 Al、B、N以外の成分組成は次の通りである。  Component composition other than Al, B, and N is as follows.
[C:0.20~0.70%] 
 Cは、強度を確保するために必要な元素であり、0.20%以上含有する。Cは、好ましくは0.30%以上であり、より好ましくは0.40%以上である。しかしC量が過剰になると、鋼が硬くなり過ぎて被削性や靱性が劣化する。従ってC量は0.70%以下とする。C量は、好ましくは0.60%以下であり、より好ましくは0.55%以下である。
[C: 0.20 to 0.70%]
C is an element necessary for ensuring strength, and is contained by 0.20% or more. C is preferably 0.30% or more, and more preferably 0.40% or more. However, if the amount of C is excessive, the steel becomes too hard and the machinability and toughness deteriorate. Therefore, the C content is 0.70% or less. The amount of C is preferably 0.60% or less, more preferably 0.55% or less.
[Si:0.03~2%]
 Siは、脱酸元素として作用し、鋼の内部品質を向上させるのに必要な元素である。Siは、0.03%以上、好ましくは0.10%以上、より好ましくは0.20%以上とする。しかしSi量が過剰になると、温度850℃以上の加工時に異常組織が生成したり、このときの加工性が劣化する。従ってSiは、2%以下、好ましくは1.5%以下、より好ましくは1.0%以下、更に好ましくは0.7%以下とする。
[Si: 0.03 to 2%]
Si acts as a deoxidizing element and is an element necessary for improving the internal quality of steel. Si is 0.03% or more, preferably 0.10% or more, more preferably 0.20% or more. However, when the amount of Si becomes excessive, an abnormal structure is generated during processing at a temperature of 850 ° C. or higher, and the workability at this time deteriorates. Therefore, Si is 2% or less, preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.7% or less.
[Mn:0.2~1.8%]
 Mnは、焼入れ性を向上させて鋼の強度を向上させるのに必要な元素であり、0.2%以上、好ましくは0.5%以上、より好ましくは0.90%以上とする。しかしMnが過剰になると、焼入れ性が向上し過ぎて過剰にベイナイトが生成したり、マルテンサイトが生成し易くなり、被削性が低下する。従ってMnは、1.8%以下、好ましくは1.5%以下、より好ましくは1.10%以下とする。
[Mn: 0.2 to 1.8%]
Mn is an element necessary for improving the hardenability and improving the strength of the steel, and is 0.2% or more, preferably 0.5% or more, more preferably 0.90% or more. However, when Mn is excessive, the hardenability is excessively improved and bainite is excessively generated or martensite is easily generated, and the machinability is lowered. Therefore, Mn is 1.8% or less, preferably 1.5% or less, more preferably 1.10% or less.
[P:0.03%以下(0%を含まない)] 
 Pは、鋼に不可避的に含まれる不純物元素であり、P量が過剰になると加工時に割れが発生するのを助長するので、できるだけ低減する必要がある。従ってPは、0.03%以下、好ましくは0.02%以下、より好ましくは0.015%以下とする。なお、P量を0%とすることは工業的に困難である。
[P: 0.03% or less (excluding 0%)]
P is an impurity element that is inevitably contained in steel, and if the amount of P is excessive, it promotes the occurrence of cracks during processing, so it needs to be reduced as much as possible. Therefore, P is 0.03% or less, preferably 0.02% or less, more preferably 0.015% or less. In addition, it is industrially difficult to make P amount 0%.
[S:0.10%以下(0%を含まない)] 
 Sは、鋼中のMnと結合してMnS介在物を形成し、鋼の被削性を向上させるのに有効に作用する元素である。しかしS量が過剰になると、MnS系介在物量が増大し、この介在物が加工時(例えば、熱間圧延や熱間鍛造など)に加工方向に伸展するため、加工方向に直角な方向の靱性(横目靱性)が劣化する原因となる。従ってS量は0.10%以下、好ましくは0.08%以下、より好ましくは0.05%以下とする。なお、Sは、鋼に不可避的に含まれる不純物であるため、その量を0%とすることは工業的に困難である。
[S: 0.10% or less (excluding 0%)]
S is an element that effectively acts to improve the machinability of steel by combining with Mn in steel to form MnS inclusions. However, if the amount of S becomes excessive, the amount of MnS inclusions increases, and the inclusions extend in the processing direction during processing (for example, hot rolling or hot forging), so the toughness in the direction perpendicular to the processing direction. (Train toughness) will be deteriorated. Therefore, the S content is 0.10% or less, preferably 0.08% or less, more preferably 0.05% or less. In addition, since S is an impurity inevitably contained in steel, it is industrially difficult to make the amount 0%.
[O:0.002%以下(0%を含まない)] 
 Oは、鋼に不可避的に含まれる不純物元素であり、O量が過剰になると、粗大な酸化物系介在物が生成し、熱間加工性、延性、靱性、および被削性が劣化する。従ってO量は0.002%以下、好ましくは0.0018%以下、より好ましくは0.0015%以下とする。
[O: 0.002% or less (excluding 0%)]
O is an impurity element inevitably contained in steel, and when the amount of O is excessive, coarse oxide inclusions are generated, and hot workability, ductility, toughness, and machinability deteriorate. Therefore, the O content is 0.002% or less, preferably 0.0018% or less, more preferably 0.0015% or less.
 本発明に係る高強度鋼の成分組成は上記の通りであり、残部は、鉄および不可避不純物である。不可避不純物としては、原料、資材、製造設備等の状況によって持ち込まれる微量元素(例えば、As、Sb、Snなど)の混入が許容される。 The component composition of the high-strength steel according to the present invention is as described above, and the balance is iron and inevitable impurities. As inevitable impurities, mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed.
 また、本発明の効果を損なわない範囲で、更に他の元素として、Cr、Mo、Ti、Zr、Hf、Ta、Nb、V、Cu、Niなどを積極的に含有させてもよい。 Further, Cr, Mo, Ti, Zr, Hf, Ta, Nb, V, Cu, Ni, etc. may be positively contained as other elements within the range not impairing the effects of the present invention.
[Cr:1.5%以下(0%を含まない)]
 Crは、鋼の焼入れ性を高め、強度を向上させるために有効に作用する元素である。また、Alとの複合添加によって、鋼の被削性(特に、断続切削性)を高めるのにも有効に作用する元素である。こうした効果を発揮させるには、Crは0.08%以上含有することが好ましく、より好ましくは0.10%以上、更に好ましくは0.2%以上、特に好ましくは0.7%以上である。しかし、Cr量が過剰になると、粗大な炭化物が生成するか、或いは過冷組織が生成して被削性を却って劣化させるので、Cr量は1.5%以下とすることが好ましく、より好ましくは1.3%以下である。
[Cr: 1.5% or less (excluding 0%)]
Cr is an element that effectively acts to enhance the hardenability of the steel and improve the strength. Moreover, it is an element which acts effectively also for improving the machinability (especially intermittent machinability) of steel by combined addition with Al. In order to exert such an effect, Cr is preferably contained in an amount of 0.08% or more, more preferably 0.10% or more, still more preferably 0.2% or more, and particularly preferably 0.7% or more. However, if the amount of Cr is excessive, coarse carbides are generated, or a supercooled structure is generated and the machinability is deteriorated, so the Cr amount is preferably 1.5% or less, more preferably. Is 1.3% or less.
[Mo:1%以下(0%を含まない)]
 Moは、鋼の焼入れ性を高め、焼入れされていない組織が生成するのを抑制するのに作用する元素である。こうした作用は、その含有量が増加するにつれて増大するが、好ましくは0.05%以上、より好ましくは0.1%以上、更に好ましくは0.15%以上である。しかしMoを過剰に含有すると、焼きならし後でも過冷組織が生成して被削性が低下するため、1%以下とすることが好ましい。より好ましくは0.8%以下であり、更に好ましくは0.5%以下である。
[Mo: 1% or less (excluding 0%)]
Mo is an element that acts to increase the hardenability of steel and suppress the formation of a structure that has not been quenched. Such an action increases as the content increases, but is preferably 0.05% or more, more preferably 0.1% or more, and further preferably 0.15% or more. However, when Mo is excessively contained, a supercooled structure is generated even after normalization and machinability is lowered, so that the content is preferably 1% or less. More preferably, it is 0.8% or less, More preferably, it is 0.5% or less.
[Ti:0.005%以下(0%を含まない)、Zr:0.02%以下(0%を含まない)、Hf:0.02%以下(0%を含まない)、Ta:0.02%以下(0%を含まない)、およびNb:0.15%以下(0%を含まない)よりなる群から選ばれる少なくとも1種の元素]
 Ti、Zr、Hf、Ta、およびNbは、熱間加工時に結晶粒が異常成長するのを防止し、鋼の靱性や疲労強度を低下するのを防止する作用を有する元素であり、1種または任意に選択される2種以上を含有することによってこうした作用が発揮される。こうした作用は、その含有量が増加するにつれて増大するが、Tiは0.0003%以上(特に0.0005%以上)、Zrは0.002%以上(特に0.005%以上)、Hfは0.002%以上(特に0.005%以上)、Taは0.002%以上(特に0.005%以上)、Nbは0.015%以上(特に0.05%以上)含有することが好ましい。しかし、これらの元素を過剰に含有すると、硬質の炭化物が生成して鋼の被削性が却って低下するので、Tiは0.005%以下(特に0.003%以下)、Zrは0.02%以下(特に0.015%以下)、Hfは0.02%以下(特に0.015%以下)、Taは0.02%以下(特に0.015%以下)、Nbは0.15%以下(特に0.14%以下)とすることが好ましい。
[Ti: 0.005% or less (not including 0%), Zr: 0.02% or less (not including 0%), Hf: 0.02% or less (not including 0%), Ta: 0.0. 02% or less (not including 0%), and Nb: at least one element selected from the group consisting of 0.15% or less (not including 0%)]
Ti, Zr, Hf, Ta, and Nb are elements that have an effect of preventing abnormal growth of crystal grains during hot working and preventing toughness and fatigue strength of steel from being lowered. Such an effect is exhibited by containing two or more kinds selected arbitrarily. These effects increase as the content increases, but Ti is 0.0003% or more (particularly 0.0005% or more), Zr is 0.002% or more (particularly 0.005% or more), and Hf is 0. 0.002% or more (especially 0.005% or more), Ta is preferably 0.002% or more (particularly 0.005% or more), and Nb is preferably 0.015% or more (particularly 0.05% or more). However, if these elements are contained excessively, hard carbides are generated and the machinability of the steel is lowered, so that Ti is 0.005% or less (particularly 0.003% or less) and Zr is 0.02 or less. % Or less (particularly 0.015% or less), Hf is 0.02% or less (particularly 0.015% or less), Ta is 0.02% or less (particularly 0.015% or less), and Nb is 0.15% or less. (Especially 0.14% or less) is preferable.
[V:0.5%以下(0%を含まない)、Cu:3%以下(0%を含まない)、およびNi:3%以下(0%を含まない)よりなる群から選ばれる少なくとも1種の元素]
 V、Cu、およびNiは、焼入れ性を向上させて強度を高めるのに有効に作用する元素である。こうした作用は、これらの元素の含有量が増加するにつれて増大するが、有効に発揮させるには、Vは0.05%以上、Cuは0.1%以上、Niは0.1%以上含有させることが好ましい。より好ましくは、Vは0.1%以上、Cuは0.2%以上、Niは0.5%以上である。しかし過剰に含有させると過冷組織が生成し、延性や靭性が低下するので、Vは0.5%以下、Cuは3%以下、Niは3%以下とすることが好ましい。より好ましくは、Vは0.3%以下、Cuは2%以下、Niは2%以下である。なお、V、Cu、およびNiは、夫々、単独で含有させてもよいし、任意に選ばれる2種以上を含有させてもよい。 
[V: at least 1 selected from the group consisting of 0.5% or less (not including 0%), Cu: 3% or less (not including 0%), and Ni: 3% or less (not including 0%) Species element]
V, Cu, and Ni are elements that effectively act to improve the hardenability and increase the strength. These effects increase as the content of these elements increases. However, in order to effectively exhibit them, V is 0.05% or more, Cu is 0.1% or more, and Ni is 0.1% or more. It is preferable. More preferably, V is 0.1% or more, Cu is 0.2% or more, and Ni is 0.5% or more. However, if it is excessively contained, a supercooled structure is formed and ductility and toughness are lowered. Therefore, V is preferably 0.5% or less, Cu is 3% or less, and Ni is preferably 3% or less. More preferably, V is 0.3% or less, Cu is 2% or less, and Ni is 2% or less. V, Cu, and Ni may each be contained alone, or two or more selected arbitrarily may be contained.
 こうした本発明の高強度鋼は、上記成分組成を満足する鋼を、温度850℃以上で加工した後、800℃から500℃までの温度域を、下記式(4)を満たす平均冷却速度Vaで冷却することによって製造できる。
0.1×Vc<Va<0.9×Vc ・・・(4) 
Such a high-strength steel of the present invention is obtained by processing a steel satisfying the above component composition at a temperature of 850 ° C. or higher, and then, in the temperature range from 800 ° C. to 500 ° C., with an average cooling rate Va satisfying the following formula (4). It can be manufactured by cooling.
0.1 × Vc <Va <0.9 × Vc (4)
 上記温度850℃以上で加工することによって、金属組織をオーステナイト単相にし、その後、800℃から500℃までの温度域における平均冷却速度Vaを適切に制御して冷却することによって、所定量のベイナイトを生成させることができる。 By processing at a temperature of 850 ° C. or higher, the metal structure becomes an austenite single phase, and then cooling by appropriately controlling the average cooling rate Va in the temperature range from 800 ° C. to 500 ° C. Can be generated.
 上記850℃以上で行う加工は、熱間圧延や熱間鍛造など加熱を伴う加工熱処理であり、塑性加工であればよい。850℃以上で行う加工が熱間圧延の場合は、上記成分組成を満足する鋼を溶製し、鋳造した後、温度850℃以上で熱間圧延すればよい。一方、850℃以上で行う加工が熱間鍛造の場合は、上記成分組成を満足する鋼を用意し、この鋼を温度850℃以上で熱間鍛造すればよい。 The processing performed at 850 ° C. or higher is a work heat treatment accompanied by heating, such as hot rolling or hot forging, and may be plastic processing. When the processing performed at 850 ° C. or higher is hot rolling, steel that satisfies the above component composition is melted and cast, and then hot rolled at a temperature of 850 ° C. or higher. On the other hand, when the processing performed at 850 ° C. or higher is hot forging, a steel satisfying the above component composition is prepared, and this steel may be hot forged at a temperature of 850 ° C. or higher.
 上記加工温度は、好ましくは950℃以上である。加工温度の上限は特に限定されないが、例えば、1050℃程度である。 The processing temperature is preferably 950 ° C. or higher. Although the upper limit of processing temperature is not specifically limited, For example, it is about 1050 degreeC.
 上記平均冷却速度Vaは、上記式(4)で規定する範囲を満足している必要があり、Vcは、臨界冷却速度を示している。Vcは下記式(5)で算出され、kは下記式(6)で算出される。下記式(6)において[]は、各元素の含有量(質量%)を示している。なお、Ni、Cr、Mo、Nbは、本発明の高強度鋼においては選択元素であるため、添加しない場合は、その項がないものとして計算すればよい。
Vc=10 ・・・(5)
k=4.05-{4.5×[C]+[Mn]+0.5×[Ni]+0.8×[Cr]+1.6×[Mo]+9.0×[Nb]}・・・(6) 
The average cooling rate Va needs to satisfy the range defined by the above formula (4), and Vc indicates a critical cooling rate. Vc is calculated by the following formula (5), and k is calculated by the following formula (6). In the following formula (6), [] indicates the content (% by mass) of each element. Ni, Cr, Mo, and Nb are selective elements in the high-strength steel of the present invention. Therefore, when not added, it may be calculated that there is no term.
Vc = 10 k (5)
k = 4.05- {4.5 × [C] + [Mn] + 0.5 × [Ni] + 0.8 × [Cr] + 1.6 × [Mo] + 9.0 × [Nb]} (6)
 上記平均冷却速度Vaが0.1×Vc以下になると、ベイナイトの生成量が少なくなり、強度不足となる。従って平均冷却速度Vaは0.1×Vc超、好ましくは0.2×Vc以上、より好ましくは0.3×Vc以上とする。しかし上記平均冷却速度Vaが0.9×Vc以上になると、ベイナイトの生成量が多くなり、強度が高くなり過ぎるため、被削性が劣化する。従って平均冷却速度Vaは0.9×Vc未満、好ましくは0.8×Vc以下、より好ましくは0.7×Vc以下とする。 When the average cooling rate Va becomes 0.1 × Vc or less, the amount of bainite produced decreases and the strength becomes insufficient. Therefore, the average cooling rate Va is more than 0.1 × Vc, preferably 0.2 × Vc or more, more preferably 0.3 × Vc or more. However, when the average cooling rate Va is 0.9 × Vc or more, the amount of bainite generated increases and the strength becomes too high, so that machinability deteriorates. Therefore, the average cooling rate Va is less than 0.9 × Vc, preferably 0.8 × Vc or less, more preferably 0.7 × Vc or less.
 上記条件で冷却して得られた本発明の高強度鋼は、被削性に優れているにもかかわらず、鋼部品として要求される強度を有している。従って本発明の高強度鋼を、部品形状に切削加工すれば、切削加工ままの状態でも鋼部品として要求される強度を兼ね備えている。即ち、本発明の高強度鋼を用いて鋼部品を製造すれば、従来切削加工後に強度を向上させるために行っていた焼入れ焼戻し(調質)や浸炭焼入れ等の熱処理を省略することができる。 The high-strength steel of the present invention obtained by cooling under the above conditions has the strength required for steel parts despite excellent machinability. Therefore, if the high-strength steel of the present invention is cut into a part shape, it has the strength required as a steel part even in the state of cutting. That is, if a steel part is manufactured using the high-strength steel of the present invention, heat treatment such as quenching and tempering (tempering) and carburizing and quenching, which has been conventionally performed to improve the strength after cutting, can be omitted.
 上記切削加工としては、断続切削加工(ホブ加工)を採用することによって本発明の効果が充分に発揮される。断続切削加工は、低速(具体的には、切削速度150m/分程度以下)、低温(具体的には、200~600℃程度)で行えばよい。 As the above-mentioned cutting process, the effect of the present invention is sufficiently exhibited by adopting intermittent cutting process (hobbing process). The intermittent cutting process may be performed at a low speed (specifically, a cutting speed of about 150 m / min or less) and a low temperature (specifically, about 200 to 600 ° C.).
 上記高強度鋼を部品形状に切削加工するにあたっては、温度850℃以上に加熱することなく切削加工する必要がある。上記高強度鋼を温度850℃以上に加熱すると、適切に調整した金属組織が全てキャンセルされるため、被削性が劣化する。換言すると、上記高強度鋼は、部品形状に切削加工する前に、温度が850℃以上にならない範囲で加熱してから切削加工を行ってもよい。 When cutting the high-strength steel into a part shape, it is necessary to perform cutting without heating to a temperature of 850 ° C. or higher. When the high-strength steel is heated to a temperature of 850 ° C. or higher, all the appropriately adjusted metal structures are cancelled, so that machinability deteriorates. In other words, the high-strength steel may be cut after being heated in a range where the temperature does not become 850 ° C. or higher before being cut into a part shape.
 また、上記高強度鋼は、切削加工前に、必要に応じて冷間加工および/または温間加工を行ってもよく、このときも温度が850℃以上にならないように制御する必要がある。    The high-strength steel may be cold-worked and / or warm-worked as necessary before cutting, and at this time, it is necessary to control the temperature not to exceed 850 ° C. *
 本発明の鋼部品は、上記高強度鋼で形成されているため、高強度鋼の成分組成と金属組織を満足したものとなり、鋼部品として要求される強度を有しているが、鋼部品表面の耐摩耗性を更に向上させるために、表面硬化処理を行ってもよい。  Since the steel part of the present invention is formed of the above-described high-strength steel, it satisfies the composition and metal structure of the high-strength steel and has the strength required as a steel part. In order to further improve the wear resistance, surface hardening treatment may be performed.
 表面硬化処理は、焼入れ焼戻し(調質)のように、鋼部品全体の温度を高めるものではなく、鋼部品の強度を低下させないように、鋼部品表面のみを硬化させる方法を採用すべきである。  The surface hardening treatment should not increase the temperature of the entire steel part as in the case of quenching and tempering (tempering), but should adopt a method of hardening only the surface of the steel part so as not to reduce the strength of the steel part. .
 表面硬化処理方法としては、例えば、高周波焼入れや窒化処理が挙げられる。  Examples of the surface hardening treatment method include induction hardening and nitriding treatment.
 高周波焼入れは、高周波焼入れ装置により鋼部品表面を順次短時間加熱し、加熱された部分を直ぐに冷却して焼入れを行なう方法である。冷却方法としては、水冷却(例えば、噴射水冷却)が採用される。加熱温度は、例えば、AC3変態点以上、950℃以下とすればよい。加熱温度は、好ましくは、AC3変態点+30℃以上、920℃以下である。 Induction hardening is a method in which the surface of a steel part is sequentially heated for a short time with an induction hardening apparatus, and the heated portion is immediately cooled and quenched. As a cooling method, water cooling (for example, jet water cooling) is employed. The heating temperature is, for example, A C3 transformation point or more, it may be set to 950 ° C. or less. The heating temperature is preferably AC3 transformation point + 30 ° C. or higher and 920 ° C. or lower.
 高周波焼入れ後は、焼戻しを行ってもよい。焼戻し温度は、例えば、100℃以上、250℃以下とすればよい。好ましくは120℃以上、230℃以下である。  Tempering may be performed after induction hardening. The tempering temperature may be, for example, 100 ° C. or more and 250 ° C. or less. Preferably it is 120 degreeC or more and 230 degrees C or less.
 窒化処理の方法としては、イオン窒化(プラズマ窒化)やラジカル窒化などの方法を適用できる。窒化処理の好ましい条件は、処理温度:500~650℃(より好ましくは500~575℃)、処理時間:4~12時間(より好ましくは6~10時間)である。窒化処理時の処理温度が、650℃を超えると鋼が軟化しやすくなり、500℃よりも低くなると、窒化深さ(硬化層深さ)が浅くなって、表面の硬化が不充分となる。 As a nitriding method, methods such as ion nitriding (plasma nitriding) and radical nitriding can be applied. Preferred conditions for the nitriding treatment are a treatment temperature: 500 to 650 ° C. (more preferably 500 to 575 ° C.), and a treatment time: 4 to 12 hours (more preferably 6 to 10 hours). If the treatment temperature during the nitriding treatment exceeds 650 ° C., the steel is easily softened, and if it is lower than 500 ° C., the nitriding depth (hardened layer depth) becomes shallow and the surface is not sufficiently cured.
 本発明の鋼部品は、自動車用変速機や差動装置をはじめとする各種歯車伝達装置に利用される歯車、シャフト、プーリーや等速ジョイント等、更にはクランクシャフト、コンロッド等の機械構造部品として好適に用いることができる。 The steel parts of the present invention are used as mechanical structural parts such as gears, shafts, pulleys, constant velocity joints, etc., as well as crankshafts, connecting rods, etc. used in various gear transmissions including transmissions and differentials for automobiles. It can be used suitably.
 以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。 EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.
 下記表1に示す化学成分組成の鋼(残部は鉄および不可避不純物)150kgを真空誘導炉で溶解し、上面:φ245mm×下面:φ210mm×長さ:480mmのインゴットに鋳造し、熱間鍛造(ソーキング:1250℃×3時間程度、鍛造加熱:1100℃×1時間程度)してφ60mmの丸棒とした。熱間鍛造後、保温材の使用、送風条件などを調整して800℃から500℃までの温度域における平均冷却速度Va(℃/秒)を制御した。下記表2に平均冷却速度Vaを示す。また、下記表2には、下記表1に示す成分の含有量と上記式(5)、式(6)から算出したk値、Vc値(10の値)、並びに「0.1×Vc」の値、「0.9×Vc」の値、を夫々示す。 150 kg of steel with the chemical composition shown in Table 1 below (the balance is iron and inevitable impurities) is melted in a vacuum induction furnace, cast into an ingot with an upper surface of φ245 mm, a lower surface of φ210 mm and a length of 480 mm, and hot forging (soaking) 1250 ° C. × about 3 hours, forging heating: 1100 ° C. × about 1 hour) to obtain a round bar of φ60 mm. After hot forging, the average cooling rate Va (° C./second) in the temperature range from 800 ° C. to 500 ° C. was controlled by adjusting the use of the heat insulating material, the blowing conditions, and the like. Table 2 below shows the average cooling rate Va. Table 2 below shows the content of the components shown in Table 1 below, the k value calculated from the above formulas (5) and (6), the Vc value (value of 10 k ), and “0.1 × Vc”. ”And“ 0.9 × Vc ”, respectively.
 冷却後、切断し、厚さ:30mm×幅:155mm×長さ:100mmの板材を製造した。  After cooling, it was cut to produce a plate material of thickness: 30 mm × width: 155 mm × length: 100 mm.
 得られた板材について、下記に示す手順で金属組織に占めるベイナイト面積率f(B)およびビッカース硬さを測定した。また、下記に示す手順で板材の被削性を評価した。  About the obtained board | plate material, the bainite area rate f (B) and Vickers hardness which occupy for a metal structure were measured in the procedure shown below. Further, the machinability of the plate material was evaluated by the following procedure.
《金属組織の観察》 
 板材の金属組織は、板厚中央部をレペラー腐食し、光学顕微鏡で観察倍率200倍で撮影した写真を画像解析して各組織の面積率を測定した。各組織のうち、ベイナイト面積率f(B)を下記表2に示す。なお、ベイナイト以外の組織は、フェライトとパーライトであることを確認した。
《Observation of metal structure》
The metal structure of the plate material was subjected to repeller corrosion at the center of the plate thickness, and the area ratio of each structure was measured by image analysis of a photograph taken with an optical microscope at an observation magnification of 200 times. Among the structures, the bainite area ratio f (B) is shown in Table 2 below. It was confirmed that the structures other than bainite were ferrite and pearlite.
 下記表2には、下記表1に示したC量に基づき上記式(2)または式(3)の左辺、右辺を算出した値を示す。尚、No.22については式(2)、No.23については式(3)に従った。 Table 2 below shows values obtained by calculating the left side and the right side of Formula (2) or Formula (3) based on the C amount shown in Table 1 below. No. For No. 22, formula (2), no. For Formula 23, Formula (3) was followed.
 また、ベイナイト面積率f(B)が、C量に応じて上記式(2)または式(3)の関係を満足している場合を○、満足していない場合を×として合否結果を下記表2に示す。  The pass / fail result is shown in the following table, where the bainite area ratio f (B) satisfies the relationship of the above formula (2) or formula (3) according to the amount of C, and the case where it is not satisfied is ×. It is shown in 2.
《ビッカース硬さの測定》 
 板材の強度を評価するために、板材のビッカース硬さHvを測定した。ビッカース硬さは、厚さ:30mm×幅:155mm断面の中心位置において、荷重:200gとして測定した。測定結果を下記表2に示す。本発明では、ビッカース硬さがHv230以上の場合を合格(高強度)、Hv230未満の場合を不合格(低強度)とした。
<Measurement of Vickers hardness>
In order to evaluate the strength of the plate material, the Vickers hardness Hv of the plate material was measured. The Vickers hardness was measured as a load: 200 g at the center position of a cross section of thickness: 30 mm × width: 155 mm. The measurement results are shown in Table 2 below. In the present invention, the case where the Vickers hardness is Hv230 or higher is regarded as acceptable (high strength), and the case where it is less than Hv230 is regarded as unacceptable (low strength).
《板材の被削性について(断続切削時の被削性評価)》
 板材の被削性を評価するために、エンドミル切削試験を行い、板材を断続切削したときの工具摩耗量を測定した。エンドミル切削試験では、上記板材をスケール除去した後、表面を約2mm研削したものをエンドミル切削試験片(被削材)として用いた。具体的には、マニシングセンタ主軸にエンドミル工具を取り付け、上記のようにして製造した厚さ:25mm×幅:150mm×長さ:100mmの試験片をバイスにより固定し、乾式の切削雰囲気下(室温)でダウンカット加工を行った。詳細な加工条件を下記表3に示す。断続切削を200カット行った後、工具表面を光学顕微鏡で観察倍率100倍で観察し、逃げ面摩耗量(工具摩耗量)Vbを測定し平均値を求めた。結果を下記表2に示す。本発明では、断続切削後の逃げ面摩耗量Vbが100μm以下のものを、「断続切削時の被削性に優れる」と評価した。
<< Machinability of plate material (Machinability evaluation during intermittent cutting) >>
In order to evaluate the machinability of the plate material, an end mill cutting test was performed, and the amount of tool wear when the plate material was cut intermittently was measured. In the end mill cutting test, after removing the scale of the plate material, the surface was ground by about 2 mm and used as an end mill cutting test piece (work material). Specifically, an end mill tool is attached to the main spindle of the machining center, and a test piece of thickness: 25 mm × width: 150 mm × length: 100 mm manufactured as described above is fixed with a vise, under a dry cutting atmosphere ( Down-cut processing was performed at room temperature). Detailed processing conditions are shown in Table 3 below. After performing 200 intermittent cuttings, the tool surface was observed with an optical microscope at an observation magnification of 100, and the flank wear amount (tool wear amount) Vb was measured to obtain an average value. The results are shown in Table 2 below. In the present invention, those having a flank wear amount Vb of 100 μm or less after intermittent cutting were evaluated as “excellent machinability during intermittent cutting”.
 なお、下記表2に示すNo.14、15、19、20、22、26、27については、上記板材に表面硬化処理を行い、表面硬化処理後のビッカース硬さを測定した。表面硬化処理は、窒化処理または高周波焼入れを行った。窒化処理は、ガス軟窒化処理を、処理温度530℃、処理時間2時間で行った。高周波焼入れは、加熱温度850℃とし、冷却は水冷で行った。 In addition, No. shown in Table 2 below. For 14, 15, 19, 20, 22, 26, and 27, the plate material was subjected to surface hardening treatment, and the Vickers hardness after the surface hardening treatment was measured. As the surface hardening treatment, nitriding treatment or induction hardening was performed. As the nitriding treatment, gas soft nitriding treatment was performed at a treatment temperature of 530 ° C. and a treatment time of 2 hours. Induction hardening was performed at a heating temperature of 850 ° C. and cooling was performed by water cooling.
 表面硬化処理後の板材のビッカース硬さは、厚さ:30mm×幅:155mm断面の中心位置において、荷重:200gとして測定した。その結果、表面硬化処理後のビッカース硬さは、表面硬化処理前のビッカース硬さと変化が認められなかった。 The Vickers hardness of the plate material after the surface hardening treatment was measured as a load: 200 g at the center position of a cross section of thickness: 30 mm × width: 155 mm. As a result, the Vickers hardness after the surface hardening treatment was not changed from the Vickers hardness before the surface hardening treatment.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
 表1、表2から次のように考察できる。No.1~21は、本発明で規定する要件を満足する例であり、所望の硬さ(強度)と被削性を兼ね備えた鋼を実現できている。 Table 1 and Table 2 can be considered as follows. No. Nos. 1 to 21 are examples that satisfy the requirements defined in the present invention, and a steel having both desired hardness (strength) and machinability can be realized.
 これに対し、No.22~27は、本発明で規定するいずれかの要件を満足していない例であり、強度と被削性の少なくとも一方を改善できていない。No.22は、C量が少な過ぎる例であり、ベイナイトが生成せず、硬さを確保できていない。従って強度不足になっている。No.23は、C量が過剰で、しかも平均冷却速度Vaが所定の範囲を超えている例である。そのため、ベイナイトが過剰に生成し、硬くなり過ぎて逃げ面摩耗量Vbが多くなり、被削性が劣化している。 On the other hand, No. 22 to 27 are examples that do not satisfy any of the requirements defined in the present invention, and at least one of strength and machinability cannot be improved. No. No. 22 is an example in which the amount of C is too small, bainite is not generated, and hardness is not secured. Therefore, the strength is insufficient. No. 23 is an example in which the amount of C is excessive and the average cooling rate Va exceeds a predetermined range. Therefore, bainite is generated excessively, becomes too hard, the flank wear amount Vb increases, and the machinability deteriorates.
 No.24は、Alが少な過ぎる例であり、AlとNの関係が、上記式(1)を満足していない。そのため、固溶Al量が不足したため、逃げ面摩耗量Vbが多くなり、被削性を改善できていない。No.25は、Bが少な過ぎる例であり、固溶Al量が不足したため、逃げ面摩耗量Vbが多くなり、被削性を改善できていない。 No. 24 is an example in which there is too little Al, and the relationship between Al and N does not satisfy the above formula (1). Therefore, since the amount of dissolved Al is insufficient, the flank wear amount Vb is increased, and the machinability cannot be improved. No. No. 25 is an example in which B is too small, and since the amount of dissolved Al is insufficient, the flank wear amount Vb is increased and the machinability cannot be improved.
 No.26は、平均冷却速度Vaが本発明で規定する範囲を下回る例であり、ベイナイトの生成量が少な過ぎるため、硬さを確保できず、強度不足になっている。No.27は、平均冷却速度Vaが本発明で規定する範囲を超える例であり、ベイナイトが過剰に生成しているため、逃げ面摩耗量Vbが多くなり、被削性を改善できていない。 No. No. 26 is an example in which the average cooling rate Va falls below the range defined in the present invention, and since the amount of bainite produced is too small, the hardness cannot be ensured and the strength is insufficient. No. No. 27 is an example in which the average cooling rate Va exceeds the range defined in the present invention. Since bainite is excessively generated, the flank wear amount Vb increases and the machinability cannot be improved.
 本発明を詳細にまた特定の実施態様を参照して説明したが、本発明の精神と範囲を逸脱することなく様々な変更や修正を加えることができることは当業者にとって明らかである。 Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
 本出願は、2010年6月10日出願の日本特許出願(特願2010-133316)に基づくものであり、その内容はここに参照として取り込まれる。 This application is based on a Japanese patent application filed on June 10, 2010 (Japanese Patent Application No. 2010-133316), the contents of which are incorporated herein by reference.
 本発明の高強度鋼は、部品形状に切削加工した後に、焼入れ焼戻し(調質)や浸炭焼入れ等の熱処理を行わなくても鋼部品として要求される強度を確保でき、しかも切削加工時には被削性に優れるため、高強自動車用変速機や差動装置をはじめとする各種歯車伝達装置に利用される歯車、シャフト、プーリーや等速ジョイント、クランクシャフト、コンロッド等の機械構造部品に有用である。 The high-strength steel of the present invention can ensure the strength required as a steel part without performing heat treatment such as quenching and tempering (tempering) or carburizing and quenching after cutting into the shape of the part. Because of its excellent performance, it is useful for mechanical structural parts such as gears, shafts, pulleys, constant velocity joints, crankshafts, connecting rods, etc., used in various gear transmissions including transmissions for high-strength automobiles and differentials.

Claims (8)

  1.  C :0.20~0.70%(質量%の意味。以下同じ。)、
     Si:0.03~2%、
     Mn:0.2~1.8%、
     P :0.03%以下(0%を含まない)、
     S :0.10%以下(0%を含まない)、
     Al:0.12~0.5%、
     B :0.0005~0.008%、
     N :0.002~0.030%、および
     O :0.002%以下(0%を含まない)を含有すると共に、
     AlとNが下記式(1)の関係を満足し、残部が鉄および不可避不純物からなる鋼であり、
     前記鋼に含まれるC量が0.20%以上、0.35%未満の場合、金属組織がフェライト、パーライト、およびベイナイトの混合組織であり、且つベイナイトの面積率f(B)が下記式(2)を満足しており、
     C量が0.35%以上、0.70%以下の場合、金属組織がフェライトとパーライトの混合組織であるか、更にベイナイトを含む混合組織であり、且つベイナイトの面積率f(B)が下記式(3)を満足することを特徴とする被削性に優れた高強度鋼。
    0.10<[Al]-1.9×[N] ・・・(1)
    -60×[C]+21<f(B)<-60×[C]+50 ・・・(2)
    0≦f(B)<-60×[C]+50 ・・・(3)
    [上記式(1)~式(3)において、[ ]は、各元素の含有量(質量%)を示している。]
    C: 0.20 to 0.70% (meaning mass%, the same shall apply hereinafter),
    Si: 0.03 to 2%,
    Mn: 0.2-1.8%
    P: 0.03% or less (excluding 0%),
    S: 0.10% or less (excluding 0%),
    Al: 0.12 to 0.5%,
    B: 0.0005 to 0.008%,
    N: 0.002 to 0.030%, and O: 0.002% or less (excluding 0%),
    Al and N satisfy the relationship of the following formula (1), the balance is steel composed of iron and inevitable impurities,
    When the amount of C contained in the steel is 0.20% or more and less than 0.35%, the metal structure is a mixed structure of ferrite, pearlite, and bainite, and the area ratio f (B) of bainite is expressed by the following formula ( 2)
    When the amount of C is 0.35% or more and 0.70% or less, the metal structure is a mixed structure of ferrite and pearlite, or a mixed structure containing bainite, and the area ratio f (B) of bainite is as follows. A high-strength steel excellent in machinability characterized by satisfying the formula (3).
    0.10 <[Al] -1.9 × [N] (1)
    −60 × [C] +21 <f (B) <− 60 × [C] +50 (2)
    0 ≦ f (B) <− 60 × [C] +50 (3)
    [In the above formulas (1) to (3), [] represents the content (% by mass) of each element. ]
  2.  更に他の元素として、
     Cr:1.5%以下(0%を含まない)を含有するものである請求項1に記載の高強度鋼。
    As other elements,
    The high-strength steel according to claim 1, containing Cr: 1.5% or less (excluding 0%).
  3.  更に他の元素として、
     Mo:1%以下(0%を含まない)を含有するものである請求項1または2に記載の高強度鋼。
    As other elements,
    The high-strength steel according to claim 1 or 2, which contains Mo: 1% or less (not including 0%).
  4.  更に他の元素として、
     Ti:0.005%以下(0%を含まない)、
     Zr:0.02%以下(0%を含まない)、
     Hf:0.02%以下(0%を含まない)、
     Ta:0.02%以下(0%を含まない)、および
     Nb:0.15%以下(0%を含まない)よりなる群から選ばれる少なくとも1種の元素を含有するものである請求項1~3のいずれか1項に記載の高強度鋼。
    As other elements,
    Ti: 0.005% or less (excluding 0%),
    Zr: 0.02% or less (excluding 0%),
    Hf: 0.02% or less (excluding 0%),
    2. At least one element selected from the group consisting of Ta: 0.02% or less (not including 0%) and Nb: 0.15% or less (not including 0%) is included. 4. The high strength steel according to any one of items 1 to 3.
  5.  更に他の元素として、
     V :0.5%以下(0%を含まない)、
     Cu:3%以下(0%を含まない)、および
     Ni:3%以下(0%を含まない)よりなる群から選ばれる少なくとも1種の元素を含有するものである請求項1~4のいずれか1項に記載の高強度鋼。
    As other elements,
    V: 0.5% or less (excluding 0%),
    The element according to any one of claims 1 to 4, which contains at least one element selected from the group consisting of Cu: 3% or less (excluding 0%) and Ni: 3% or less (excluding 0%). 2. The high-strength steel according to item 1.
  6.  請求項1~5のいずれか1項に記載の高強度鋼で形成された鋼部品。 A steel part formed of the high-strength steel according to any one of claims 1 to 5.
  7.  請求項1~5のいずれか1項に記載の成分組成を満足する鋼を、
    温度850℃以上で加工した後、800℃から500℃までの温度域を、下記式(4)を満たす平均冷却速度Vaで冷却することを特徴とする被削性に優れた高強度鋼の製造方法。
    0.1×Vc<Va<0.9×Vc ・・・(4)
    [上記式(4)において、Vcは下記式(5)で示され、kは下記式(6)で示される。なお、下記式(6)において[]は、各元素の含有量(質量%)を示している。
    Vc=10 ・・・(5)
    k=4.05-{4.5×[C]+[Mn]+0.5×[Ni]+0.8×[Cr]+1.6×[Mo]+9.0×[Nb]}・・・(6)]
    A steel satisfying the component composition according to any one of claims 1 to 5,
    After processing at a temperature of 850 ° C. or higher, manufacturing a high-strength steel excellent in machinability characterized by cooling a temperature range from 800 ° C. to 500 ° C. at an average cooling rate Va satisfying the following formula (4). Method.
    0.1 × Vc <Va <0.9 × Vc (4)
    [In the above formula (4), Vc is represented by the following formula (5), and k is represented by the following formula (6). In addition, in following formula (6), [] has shown content (mass%) of each element.
    Vc = 10 k (5)
    k = 4.05- {4.5 × [C] + [Mn] + 0.5 × [Ni] + 0.8 × [Cr] + 1.6 × [Mo] + 9.0 × [Nb]} (6)]
  8.  請求項1~5の何れか1項に記載の高強度鋼を、温度850℃以上に加熱することなく切削加工することを特徴とする鋼部品の製造方法。 A method for producing a steel part, comprising cutting the high-strength steel according to any one of claims 1 to 5 without heating to a temperature of 850 ° C or higher.
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