JPWO2018078844A1 - Method of manufacturing steel parts and steel parts - Google Patents

Method of manufacturing steel parts and steel parts Download PDF

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JPWO2018078844A1
JPWO2018078844A1 JP2018547070A JP2018547070A JPWO2018078844A1 JP WO2018078844 A1 JPWO2018078844 A1 JP WO2018078844A1 JP 2018547070 A JP2018547070 A JP 2018547070A JP 2018547070 A JP2018547070 A JP 2018547070A JP WO2018078844 A1 JPWO2018078844 A1 JP WO2018078844A1
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steel
less
carburized layer
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steel plate
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JP6835095B2 (en
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力 岡本
力 岡本
啓達 小嶋
啓達 小嶋
匹田　和夫
和夫 匹田
紀之 前川
紀之 前川
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Nippon Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0257—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/78—Combined heat-treatments not provided for above
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0457—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0081—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for slabs; for billets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/28—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in one step
    • C23C8/30—Carbo-nitriding
    • C23C8/32—Carbo-nitriding of ferrous surfaces
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

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Abstract

鋼部品の製造方法は、質量％で、Ｃ：０．０００５〜０．１％、Ｓｉ：０．０１〜２．０％、Ｍｎ：０．０５〜３．０％、Ａｌ：０．９％以下、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ｔｉ：０．０〜０．２％、Ｎｂ：０．０〜０．１％、Ｃｒ：０〜２％、Ｍｏ：０．０〜０．２％、Ｂ：０．０００〜０．００５％、かつ、残部：Ｆｅおよび不純物、で表される化学組成を有し、フェライトの面積分率が７０％以上、で表される鋼組織を有する鋼板を、浸炭雰囲気中で加熱して前記鋼板の表面に浸炭層を形成する工程と、金型を用いて前記鋼板を成形し、前記金型に収めたまま前記鋼板の焼き入れを行って、前記浸炭層をマルテンサイトに変態させるとともに、前記鋼板の前記浸炭層よりも内側を面積分率でフェライトが５０％以上で表される鋼組織にする工程と、を有する。The production method of steel parts is, by mass%, C: 0.0005 to 0.1%, Si: 0.01 to 2.0%, Mn: 0.05 to 3.0%, Al: 0.9% Hereinafter, P: 0.05% or less, S: 0.01% or less, Ti: 0.0 to 0.2%, Nb: 0.0 to 0.1%, Cr: 0 to 2%, Mo: 0 And B: 0.000 to 0.005% and the balance: Fe and impurities, and the area fraction of ferrite is 70% or more. Heating the steel plate having the steel structure in a carburizing atmosphere to form a carburized layer on the surface of the steel plate, forming the steel plate using a mold, and baking the steel plate while being contained in the mold The carburized layer is transformed to martensite by inserting, and the area fraction of the inner side of the carburized layer of the steel plate is 50% or more of ferrite. And a step of the steel structure to, the.

Description

本発明は、鋼部品の製造方法及び鋼部品に関する。   The present invention relates to a method of manufacturing a steel part and a steel part.

トルクコンバータ等に代表される自動車に搭載される機械や産業機械は、一般的に、種々の形状を有する複数の鋼部品を含んで構成される。そして、このような鋼部品の多くは、プレス加工により成形される。   In general, a machine or industrial machine mounted on an automobile represented by a torque converter or the like includes a plurality of steel parts having various shapes. And many such steel parts are shape | molded by press work.

このような鋼部品は、鋼部品を構成する鋼板のＣ含有量が低くなると靱性が高くなる。そこで、高い靱性が要求される鋼部品には、Ｃ含有量が低い鋼板が用いられることがある。一方で、鋼部品を構成する鋼板のＣ含有量が低くなると、鋼部品の疲労強度や耐摩耗性が低下する。そこで、靱性を低下させることなく疲労強度や耐摩耗性を高めるため、鋼部品の表層部を硬化させる方法が用いられる。鋼部品の表層部を硬化させる方法としては、浸炭焼き入れが知られている。   Such a steel part becomes high in toughness when the C content of the steel plate constituting the steel part is reduced. Therefore, a steel sheet having a low C content may be used for steel parts that require high toughness. On the other hand, when the C content of the steel plate constituting the steel part is lowered, the fatigue strength and the wear resistance of the steel part are lowered. Therefore, in order to improve the fatigue strength and the wear resistance without reducing the toughness, a method of hardening the surface portion of the steel component is used. Carburizing and quenching is known as a method of hardening the surface portion of steel parts.

しかしながら、プレス加工された鋼板を浸炭焼き入れすると、焼き入れの際に生じる変態ひずみによって変形が生じることがある。このため、浸炭焼き入れでは、鋼部品の形状凍結性が低下する。特許文献１には、プレス加工された鋼板を焼き入れするに際し、金型で拘束しながら冷却することにより、良好な形状を確保する方法が開示されているが、Ｃ量が高い鋼板を用いるため、焼入後の靭性は十分でなかった。   However, when the pressed steel plate is carburized and quenched, deformation may occur due to transformation strain generated during quenching. For this reason, in the carburizing and quenching, the shape freezeability of the steel parts is reduced. Patent Document 1 discloses a method for securing a good shape by quenching while holding a pressed steel plate while constraining with a die, but using a steel plate having a high C amount The toughness after quenching was not sufficient.

特開平１０−９６０３１号公報Japanese Patent Application Laid-Open No. 10-96031

本発明の目的は、高い靱性と耐摩耗性と疲労強度と形状凍結性を有する鋼部品の製造方法と鋼部品を提供することである。   An object of the present invention is to provide a method of producing a steel component and a steel component having high toughness, wear resistance, fatigue strength and shape freezeability.

本願発明者は、鋭意検討の結果、以下に示す発明の諸態様に想到した。   The inventors of the present invention have arrived at the following aspects of the invention as a result of intensive studies.

（１）
質量％で、
Ｃ：０．０００５〜０．１％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．０５〜３．０％、
Ａｌ：０．９％以下、
Ｐ：０．０５％以下、
Ｓ：０．０１％以下、
Ｔｉ：０．０〜０．２％、
Ｎｂ：０．０〜０．１％、
Ｃｒ：０〜２％、
Ｍｏ：０．０〜０．２％、
Ｂ：０．０００〜０．００５％、かつ、
残部：Ｆｅおよび不純物、
で表される化学組成を有し、
フェライトの面積分率が７０％以上、
で表される鋼組織を有する鋼板を浸炭雰囲気中で加熱して、前記鋼板の表面に浸炭層を形成する工程と、
金型を用いて前記鋼板を成形し、前記金型に収めたまま前記鋼板の焼き入れを行って、前記浸炭層をマルテンサイトに変態させるとともに、前記鋼板の前記浸炭層よりも内側を面積分率でフェライトが５０％以上で表される鋼組織にする工程と、
を有することを特徴とする鋼部品の製造方法。(1)
In mass%,
C: 0.0005 to 0.1%
Si: 0.01 to 2.0%,
Mn: 0.05 to 3.0%,
Al: 0.9% or less,
P: 0.05% or less,
S: 0.01% or less,
Ti: 0.0 to 0.2%,
Nb: 0.0 to 0.1%
Cr: 0 to 2%,
Mo: 0.0 to 0.2%,
B: 0.000 to 0.005%, and
Remainder: Fe and impurities,
Has a chemical composition represented by
70% or more area fraction of ferrite,
Heating in a carburizing atmosphere a steel plate having a steel structure represented by and forming a carburized layer on the surface of the steel plate;
The steel sheet is formed using a mold, and the steel sheet is quenched while stored in the mold to transform the carburized layer to martensite, and the area inside the carburized layer of the steel sheet is divided into areas Forming a steel structure having a ferrite content of 50% or more
A method of manufacturing a steel component, comprising:

（２）
前記鋼板の表面に浸炭層を形成する工程においては、前記鋼板を８２０℃以上の温度で均熱処理することを特徴とする（１）に記載の鋼部品の製造方法。(2)
In the step of forming a carburized layer on the surface of the steel sheet, the steel sheet is subjected to soaking at a temperature of 820 ° C. or higher, and the method for producing a steel component according to (1) is characterized.

（３）
前記鋼板の表面に浸炭層を形成する工程においては、前記鋼板を１時間以上均熱処理することを特徴とする（１）又は（２）に記載の鋼部品の製造方法。(3)
In the step of forming a carburized layer on the surface of the steel sheet, the steel sheet is subjected to a soaking treatment for one hour or more, and the method for producing a steel component according to (1) or (2).

（４）
前記鋼板の表面に浸炭層を形成する工程よりも前に、
前記鋼板を５％未満または２０％以上の相当ひずみで加工する工程をさらに有することを特徴とする（１）から（３）のいずれかに記載の鋼部品の製造方法。(4)
Prior to the step of forming a carburized layer on the surface of the steel plate,
The method for producing a steel part according to any one of (1) to (3), further comprising the step of processing the steel sheet with a equivalent strain of less than 5% or 20% or more.

（５）
質量％で、
Ｃ：０．０００５〜０．１％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．０５〜３．０％、
Ａｌ：０．９％以下、
Ｐ：０．０５％以下、
Ｓ：０．０１％以下、
Ｔｉ：０．０〜０．２％、
Ｎｂ：０．０〜０．１％、
Ｃｒ：０〜２％、
Ｍｏ：０．０〜０．２％、
Ｂ：０．０００〜０．００５％、かつ、
残部：Ｆｅおよび不純物、
で表される化学組成を有し、
面積分率で、
フェライト：５０％以上、
で表される鋼組織を有する母材と、
マルテンサイトからなり前記母材の表面に存在する浸炭層と、
を有することを特徴とする鋼部品。(5)
In mass%,
C: 0.0005 to 0.1%
Si: 0.01 to 2.0%,
Mn: 0.05 to 3.0%,
Al: 0.9% or less,
P: 0.05% or less,
S: 0.01% or less,
Ti: 0.0 to 0.2%,
Nb: 0.0 to 0.1%
Cr: 0 to 2%,
Mo: 0.0 to 0.2%,
B: 0.000 to 0.005%, and
Remainder: Fe and impurities,
Has a chemical composition represented by
In area fraction,
Ferrite: 50% or more,
A base material having a steel structure represented by
A carburized layer made of martensite and present on the surface of the base material,
A steel part characterized by having.

（６）
前記マルテンサイトからなる浸炭層は、質量％で０．５〜０．９％のＣを含有し、
前記鋼部品の厚さをｔとした場合の前記鋼部品の表面からｔ／４〜３ｔ／４の範囲において、円相当径で２００μｍ以上の粒径を有するフェライト粒の総面積分率が５％以下であることを特徴とする（５）に記載の鋼部品。(6)
The martensite carburized layer contains 0.5 to 0.9% C by mass%,
When the thickness of the steel part is t, the total area fraction of ferrite grains having a circle equivalent diameter of 200 μm or more is 5% in the range of t / 4 to 3 t / 4 from the surface of the steel part The steel part as described in (5) characterized by being the following.

有効硬化層深さは０．０５〜０．５ｍｍであることを特徴とする（５）又は（６）に記載の鋼部品。   The steel part according to (5) or (6), wherein the effective hardened layer depth is 0.05 to 0.5 mm.

本発明の鋼部品の製造方法によれば、高い靱性と耐摩耗性と疲労強度と形状凍結性とを有する鋼部品を製造できる。また、本発明の鋼部品は、高い靱性と耐摩耗性と疲労強度と形状凍結性を有する。   According to the method for producing a steel component of the present invention, it is possible to produce a steel component having high toughness, wear resistance, fatigue strength and shape freezeability. In addition, the steel component of the present invention has high toughness, wear resistance, fatigue strength and shape freezeability.

図１は、実施例で使用した、打ち抜き加工後の鋼板の平面図及び側面図である。FIG. 1: is the top view and side view of the steel plate after punching which were used in the Example. 図２は、実施例で製造した、鋼部品の平面図及び側面図である。FIG. 2 is a plan view and a side view of a steel part manufactured in the example.

以下、本発明の実施形態について詳述する。なお、以下の説明において、鋼部品およびその製造に用いられる鋼板に含まれる各元素の含有量の単位である「％」は、特に断りがない限りは「質量％」を意味する。   Hereinafter, embodiments of the present invention will be described in detail. In the following description, “%”, which is a unit of the content of each element contained in a steel part and a steel plate used in the manufacture thereof, means “mass%” unless otherwise noted.

［第１の実施の形態］
［鋼部品の製造方法］
第１の実施形態に係る鋼部品の製造方法は、鋼板を浸炭雰囲気中で加熱して鋼板の表面にオーステナイトの浸炭層を形成する工程（以下、「浸炭工程」と記す）と、オーステナイトが存在する状態で金型を用いて鋼板を成形し、金型に収めたまま鋼板の焼き入れを行って、オーステナイトをマルテンサイトに変態させるとともに、鋼板の浸炭層よりも内側を面積分率でフェライトが５０％以上で表される鋼組織にする工程（以下、「ホットスタンプ工程」と記す）とを有する。First Embodiment
[Method of manufacturing steel parts]
The method of manufacturing a steel component according to the first embodiment includes the steps of heating a steel plate in a carburizing atmosphere to form a carburized layer of austenite on the surface of the steel plate (hereinafter referred to as "carburizing step"), and austenite is present The steel sheet is formed using a mold in the forming state, and the steel sheet is quenched while being contained in the mold to transform austenite to martensite, and ferrite with an area fraction inside the carburized layer of the steel sheet And a step of forming a steel structure represented by 50% or more (hereinafter referred to as a "hot stamping step").

［鋼板の化学組成と鋼組織］
まず、本実施形態に係る鋼部品の製造方法に用いる鋼板の化学組成について説明する。鋼板は、質量％で、Ｃ：０．０００５〜０．１％、Ｓｉ：０．０１〜２．０％、Ｍｎ：０．０５〜３．０％、Ａｌ：０．９％以下、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ｔｉ：０．０〜０．２％、Ｎｂ：０．０〜０．１％、Ｃｒ:０〜２％、Ｍｏ：０．０〜０．２％、Ｂ：０．０００〜０．００５％、並びに、残部：Ｆｅおよび不純物で表される化学組成を有している。不純物としては、鉱石やスクラップ等の原材料に含まれるもの、製造工程において含まれるもの、が例示される。[Chemical composition and steel structure of steel plate]
First, the chemical composition of the steel plate used in the method of manufacturing a steel component according to the present embodiment will be described. The steel plate is, by mass%, C: 0.0005 to 0.1%, Si: 0.01 to 2.0%, Mn: 0.05 to 3.0%, Al: 0.9% or less, P: 0.05% or less, S: 0.01% or less, Ti: 0.0 to 0.2%, Nb: 0.0 to 0.1%, Cr: 0 to 2%, Mo: 0.0 to 0 .2%, B: 0.000 to 0.005%, and the balance: chemical composition represented by Fe and impurities. Examples of the impurities include those contained in raw materials such as ore and scrap, and those contained in the production process.

（Ｃ：０．０００５〜０．１％）
Ｃは、靱性および引張強度に影響する。Ｃ含有量が高すぎれば、焼き入れ時に鋼板内部にマルテンサイトが生じ硬化するため靭性が低下する。一方、Ｃ含有量を低くし過ぎることはコスト増を招く。したがって、鋼板のＣ含有量は、０．０００５〜０．１％以下である。(C: 0.0005 to 0.1%)
C affects toughness and tensile strength. If the C content is too high, martensite is generated inside the steel sheet at the time of quenching to harden and the toughness decreases. On the other hand, excessively lowering the C content leads to an increase in cost. Therefore, the C content of the steel plate is 0.0005 to 0.1% or less.

（Ｓｉ：０．０１〜２．０％）
Ｓｉは、固溶強化により強度を上昇させる。Ｓｉ含有量が２．０％超では、Ａｃ３変態点が上昇しすぎ、浸炭層をオーステナイト化させるのが困難になり、焼き入れ時にマルテンサイトが得られない。したがって、Ｓｉ含有量は、２．０％以下である。一方、Ｓｉ含有量を低下させすぎるとコスト増を招く。したがって、Ｓｉ含有量は、０．０１〜２．０％である。(Si: 0.01 to 2.0%)
Si raises the strength by solid solution strengthening. If the Si content is more than 2.0%, the Ac3 transformation point rises too much, making it difficult to austenitize the carburized layer, and martensite can not be obtained during quenching. Therefore, the Si content is 2.0% or less. On the other hand, if the Si content is reduced too much, the cost increases. Therefore, the Si content is 0.01 to 2.0%.

（Ｍｎ：０．０５〜３．０％）
Ｍｎは、固溶強化により強度を上昇させる元素である。また、焼入性を向上させる元素であるので、Ｍｎ含有量が３．０％超では、Ｃ含有量が０．１％以下であっても、焼入時に鋼板内部がマルテンサイトになり、靭性を劣化させる。したがって、Ｍｎ含有量は３．０％以下である。一方、Ｍｎ含有量が０．０５％未満に低減するとコストが著しく上昇する。したがって、Ｍｎ含有量は０．０５％以上である。(Mn: 0.05 to 3.0%)
Mn is an element which raises the strength by solid solution strengthening. In addition, since it is an element that improves hardenability, when the Mn content is over 3.0%, the inside of the steel sheet becomes martensite at the time of quenching even if the C content is 0.1% or less, and the toughness is Degrade the Therefore, the Mn content is 3.0% or less. On the other hand, when the Mn content is reduced to less than 0.05%, the cost is significantly increased. Therefore, the Mn content is 0.05% or more.

（Ａｌ：０．９％以下）
Ａｌは、必須元素ではなく、例えば鋼板中に不純物として含有される。また、固溶強化により強度を上昇させる元素であるので任意添加してもよい。ＡｌＮ形成により浸炭層のオーステナイト粒径が細粒になり焼入性が低下するので、固溶強化を必要としないときには、Ａｌ含有量は低ければ低いほどよい。特にＡｌ含有量が０．９％超では、Ａｃ３変態点が上昇しすぎ、浸炭層をオーステナイト化させるのが困難になる。したがって、Ａｌ含有量は、０．９％以下とする。ただし、Ａｌ含有量の低減にはコストがかかり、０．００２％未満まで低減しようとすると、コストが著しく上昇する。このため、Ａｌ含有量は０．００２％以上としてもよい。(Al: 0.9% or less)
Al is not an essential element, and is contained, for example, as an impurity in a steel plate. Moreover, since it is an element which raises strength by solid solution strengthening, it may be added arbitrarily. Since the austenite grain size of the carburized layer becomes fine grains and the hardenability decreases due to the formation of AlN, the lower the Al content, the better, when solid solution strengthening is not required. In particular, when the Al content is more than 0.9%, the Ac3 transformation point is excessively increased, which makes it difficult to austenitize the carburized layer. Therefore, the Al content is 0.9% or less. However, the reduction of the Al content is costly, and if it is attempted to reduce it to less than 0.002%, the cost is significantly increased. Therefore, the Al content may be 0.002% or more.

（Ｐ：０．０５％以下）
Ｐは、必須元素ではなく、例えば鋼板中に不純物として含有される。焼入後の浸炭層の靭性の観点から、Ｐ含有量は低ければ低いほどよい。特にＰ含有量が０．０５％超では、靭性の低下が著しい。したがって、Ｐ含有量は０．０５％以下とする。ただし、Ｐ含有量の低減にはコストがかかり、０．００１％未満まで低減しようとすると、コストが著しく増加する。このため、Ｐ含有量は０．００１％以上であってもよい。(P: 0.05% or less)
P is not an essential element, and is contained, for example, as an impurity in a steel plate. From the viewpoint of the toughness of the carburized layer after quenching, the lower the P content, the better. In particular, when the P content is more than 0.05%, the decrease in toughness is remarkable. Therefore, the P content is 0.05% or less. However, reduction of the P content is costly, and if it is attempted to reduce it to less than 0.001%, the cost is significantly increased. Therefore, the P content may be 0.001% or more.

（Ｓ：０．０１％以下）
Ｓは、必須元素ではなく、例えば鋼板中に不純物として含有される。ＭｎＳを形成して焼入後の浸炭層の靭性を低下させるから、Ｓ含有量は低ければ低いほどよい。特にＳ含有量が０．０１超では、靭性の低下が著しい。したがって、Ｓ含有量は０．０１％以下とする。ただし、Ｓ含有量の低減にはコストがかかり、０．０００５％未満まで低減しようとすると、コストが著しく増加する。このため、Ｓ含有量は０．０００５％以上であってもよい。(S: 0.01% or less)
S is not an essential element, and is contained, for example, as an impurity in a steel plate. The lower the S content, the better, as it forms MnS and reduces the toughness of the carburized layer after quenching. In particular, when the S content is more than 0.01, the toughness is significantly reduced. Therefore, the S content is 0.01% or less. However, reduction of the S content is costly, and if it is attempted to reduce it to less than 0.0005%, the cost is significantly increased. Therefore, the S content may be 0.0005% or more.

Ｔｉ、Ｎｂ、Ｃｒ、Ｍｏ、Ｂは、必須元素ではなく、鋼板に所定量を限度に適宜含有されてもよい任意元素である。   Ti, Nb, Cr, Mo, and B are not essential elements, but arbitrary elements which may be suitably contained in the steel sheet within the predetermined amount.

（Ｔｉ：０．０〜０．２％）
Ｔｉは、フェライト粒の微細化に寄与し、所定の鋼組織を得るために用いられる。また、Ｔｉは、析出強化により強度を上昇させる。したがって、Ｔｉが含有されていてもよい。ただし、Ｔｉ含有量が０．２％超では、浸炭処理前の鋼板の延性を劣化させる。したがって、Ｔｉ含有量は０．２％以下である。(Ti: 0.0 to 0.2%)
Ti contributes to the refinement of ferrite grains and is used to obtain a predetermined steel structure. Moreover, Ti raises the strength by precipitation strengthening. Therefore, Ti may be contained. However, if the Ti content exceeds 0.2%, the ductility of the steel sheet before carburizing treatment is deteriorated. Therefore, the Ti content is 0.2% or less.

（Ｎｂ：０．０〜０．１％）
Ｎｂは、フェライト粒の微細化に寄与し、所定の鋼組織を得るために用いられる。また、Ｎｂは、析出強化により強度を上昇させる。したがって、Ｎｂが含有されていてもよい。ただし、Ｎｂ含有量が０．１％超では、浸炭処理前の鋼板の延性を劣化させる。したがって、Ｎｂ含有量は０．１％以下である。(Nb: 0.0 to 0.1%)
Nb contributes to the refinement of ferrite grains and is used to obtain a predetermined steel structure. Also, Nb raises the strength by precipitation strengthening. Therefore, Nb may be contained. However, if the Nb content exceeds 0.1%, the ductility of the steel sheet before carburizing treatment is deteriorated. Therefore, the Nb content is 0.1% or less.

（Ｃｒ:０〜２％）
Ｃｒは、固溶強化元素であるので、強度上昇のため適宜添加してもよい。ただし、焼入性を向上させる元素であるので、Ｃｒ含有量が２％超では、Ｃ含有量が０．１％以下であっても、焼入時に鋼板内部がマルテンサイトになり、靭性を劣化させる。したがって、Ｃｒ含有量は２％以下である。(Cr: 0 to 2%)
Since Cr is a solid solution strengthening element, it may be added as appropriate to increase the strength. However, since it is an element that improves hardenability, if the Cr content is more than 2%, the inside of the steel sheet becomes martensite at the time of quenching even if the C content is 0.1% or less, and the toughness is degraded Let Therefore, the Cr content is 2% or less.

（Ｍｏ：０．０〜０．２％）
Ｍｏは、焼入性を向上させる元素であるので、Ｍｏ含有量が０．２％超では、Ｃ含有量が０．１％以下であっても、焼入時に鋼板内部がマルテンサイトになり、靭性を劣化させる。したがって、Ｍｏ含有量は０．２％以下である。(Mo: 0.0 to 0.2%)
Since Mo is an element improving the hardenability, when the Mo content is more than 0.2%, the inside of the steel sheet becomes martensite at the time of quenching even if the C content is 0.1% or less, Decrease the toughness. Therefore, the Mo content is 0.2% or less.

（Ｂ：０．０００〜０．００５％）
Ｂは、鋼板の焼き入れ性を高め、鋼組織の制御に用いる元素である。したがって、Ｂが含有されていてもよい。ただし、Ｂ含有量が０．００５％超では、効果が飽和する。したがって、Ｂ含有量は０．００５％以下である。(B: 0.000 to 0.005%)
B is an element used to improve the hardenability of the steel sheet and to control the steel structure. Therefore, B may be contained. However, when the B content is more than 0.005%, the effect is saturated. Therefore, the B content is 0.005% or less.

また、鋼板は、フェライトの面積分率が７０％以上で表される鋼組織を有していることが好ましい。これは、ベイナイト、マルテンサイト、パーライト等の硬質な組織が多く存在すると、鋼板の成形性が劣化するためである。したがって、フェライトの面積分率は７０％以上が好ましい。   Moreover, it is preferable that the steel plate has a steel structure represented by an area fraction of ferrite of 70% or more. This is because the formability of the steel sheet is degraded if a large amount of hard structure such as bainite, martensite, and pearlite is present. Therefore, the area fraction of ferrite is preferably 70% or more.

鋼板の板厚ｔは特に限定されない。また、鋼板の引張強度は６３０ＭＰａ以下であることが好ましく、５９０ＭＰａ以下であることがより好ましく、４４０ＭＰａ以下であることがさらに好ましい。本実施形態では、製造される鋼部品の耐摩耗性および疲労強度を、表層部の浸炭層によって確保する。したがって、鋼部品の製造に用いる鋼板それ自体の強度を抑制することにより、製造される鋼部品の靱性を高めることができる。成形性の観点から、強度は低いことが好ましい。   The thickness t of the steel plate is not particularly limited. The tensile strength of the steel plate is preferably 630 MPa or less, more preferably 590 MPa or less, and still more preferably 440 MPa or less. In the present embodiment, the wear resistance and fatigue strength of the steel component to be manufactured are secured by the carburized layer in the surface layer portion. Therefore, by suppressing the strength of the steel plate itself used for producing the steel component, the toughness of the produced steel component can be enhanced. The strength is preferably low from the viewpoint of formability.

また、鋼部品の製造に用いる鋼板は、鋼部品の高い靱性が要求される部分が、５％以上２０％未満の範囲の相当ひずみで塑性加工されていないものであることが好ましい。これは、鋼板において５％以上２０％未満の範囲の相当ひずみで加工された部分は、浸炭工程における均熱処理の際に、結晶粒が粗大化し、製造される鋼部品の靱性が低下するためである。なお、高い靱性が要求されない部分は、５％以上２０％未満の相当ひずみで塑性加工されていてもよい。   Moreover, it is preferable that the steel plate used for manufacture of a steel component is a part by which the high toughness of a steel component is requested | required is not plastic-worked by equivalent distortion of the range of 5% or more and less than 20%. This is because, in the steel sheet, a portion processed at equivalent strain in the range of 5% or more and less than 20% causes coarsening of crystal grains during soaking treatment in the carburizing step, and the toughness of the manufactured steel part decreases. is there. The portion where high toughness is not required may be plastically worked at an equivalent strain of 5% or more and less than 20%.

次に、本実施形態に係る鋼部品の製造方法について説明する。   Next, a method of manufacturing a steel component according to the present embodiment will be described.

［浸炭工程］
この工程では、前述の化学組成および鋼組織を有する鋼板に浸炭を実施するとともに、鋼板の表層部の鋼組織をオーステナイトにする。本発明における「浸炭」には浸炭窒化も含まれる。したがって、形成される浸炭層には、浸炭窒化層も含まれる。浸炭には、例えば、ガス浸炭や真空浸炭が適用できる。[Carburizing process]
In this step, carburizing is performed on the steel plate having the above-described chemical composition and steel structure, and the steel structure of the surface layer portion of the steel plate is austenite. "Carburizing" in the present invention also includes carbonitriding. Therefore, the carburized layer to be formed also includes a carbonitriding layer. For carburizing, for example, gas carburizing or vacuum carburizing can be applied.

浸炭工程における鋼板の均熱温度（浸炭温度）は、例えば、８２０〜１１００℃である。浸炭後の鋼板は、そのまま金型でプレス加工されるとともに焼き入れされる。そこで、この浸炭工程において、浸炭により炭素が浸入および拡散した表層部の鋼組織をオーステナイト化する。例えば、均熱温度が８２０℃以上であれば、鋼板の表層部の鋼組織はオーステナイト化する。なお、均熱温度は、８４０℃以上であることがより好ましい。一方、浸炭温度が１１００℃超では、結晶粒が粗大化し、靭性が低下する場合がある。したがって、均熱温度の好ましい上限は１１００℃である。浸炭雰囲気のカーボンポテンシャルにより、表層部のＣ含有量は調整され、例えば、０．５％以上０．９％以下になる。   The soaking temperature (carburizing temperature) of the steel plate in the carburizing step is, for example, 820 to 1100 ° C. The steel sheet after carburizing is directly pressed and hardened with a mold and hardened. Therefore, in this carburizing process, the steel structure of the surface layer portion where carbon has infiltrated and diffused by carburizing is austenitized. For example, if the soaking temperature is 820 ° C. or more, the steel structure of the surface layer portion of the steel sheet austenitizes. The soaking temperature is more preferably 840 ° C. or higher. On the other hand, if the carburizing temperature is higher than 1100 ° C., the crystal grains may be coarsened to lower the toughness. Therefore, the preferred upper limit of the soaking temperature is 1100 ° C. The carbon content of the surface layer portion is adjusted by the carbon potential of the carburizing atmosphere, and becomes, for example, 0.5% or more and 0.9% or less.

上記均熱温度での均熱時間は、例えば、１．０時間以上、５．０時間以下である。均熱時間が短すぎれば、浸炭層の有効硬化層深さが所定の深さ、例えば０．０５ｍｍ以上になりにくい。したがって、均熱時間は１．０時間以上であることが好ましく、１．５時間以上であることがより好ましい。一方、均熱時間が長すぎれば、浸炭層の有効硬化層深さが大きくなりすぎ、所定の深さ、例えば０．５ｍｍを超える深さとなる。したがって、例えば、均熱時間は５．０時間以下であることが好ましい。   The soaking time at the soaking temperature is, for example, 1.0 hour or more and 5.0 hours or less. If the soaking time is too short, the effective hardened layer depth of the carburized layer does not easily become a predetermined depth, for example, 0.05 mm or more. Therefore, the soaking time is preferably 1.0 hour or more, and more preferably 1.5 hours or more. On the other hand, if the soaking time is too long, the effective hardened layer depth of the carburized layer becomes too large, and the depth exceeds a predetermined depth, for example, 0.5 mm. Therefore, for example, the soaking time is preferably 5.0 hours or less.

なお、鋼板の鋼組織のオーステナイト化が開始する温度は、Ｃ含有量が高くなるにしたがって低くなる。このため、この浸炭工程における均熱温度が、鋼板の鋼組織のオーステナイト化が開始する温度よりも低い場合には、炭素の浸入により鋼板の表面からＣ含有量が高くなっていき、それに伴ってオーステナイト化が開始する温度が低くなっていく。この場合、オーステナイト化が開始する温度が均熱温度よりも低くなった部分において、オーステナイト化が開始する。このため、この場合には、時間の経過に伴って浸炭層の厚さが厚くなっていくとともに、浸炭層の厚さの増加に伴い、オーステナイト化した部分の厚さも表面から厚くなっていく。これに対して、この浸炭工程における均熱温度が、鋼板の鋼組織のオーステナイト化が開始する温度よりも高い場合には、鋼板の鋼組織がオーステナイト化し、オーステナイト化した部分に炭素が浸入していく。   The temperature at which austenitization of the steel structure of the steel sheet starts becomes lower as the C content becomes higher. For this reason, when the soaking temperature in this carburizing process is lower than the temperature at which austenitization of the steel structure of the steel sheet starts, the carbon content increases from the surface of the steel sheet due to the infiltration of carbon, The temperature at which austenitization starts becomes lower. In this case, austenitization starts in the portion where the temperature at which austenitization starts becomes lower than the soaking temperature. Therefore, in this case, the thickness of the carburized layer increases with the passage of time, and the thickness of the austenitized part also increases from the surface as the thickness of the carburized layer increases. On the other hand, when the soaking temperature in this carburizing step is higher than the temperature at which austenitization of the steel structure of the steel sheet starts, the steel structure of the steel sheet austenitizes and carbon infiltrates into the austenitized part Go.

［ホットスタンプ工程］
この工程では、浸炭工程の後、鋼板にオーステナイトが存在する状態で、例えば、鋼板の温度が８００℃以下になるよりも前に、鋼板をホットスタンプする。なお、浸炭工程からホットスタンプの間には、鋼板を焼き入れしない。浸炭工程の終了時には、鋼板は８２０℃以上の温度に均熱処理されており、鋼板の表層部の鋼組織はオーステナイトになっている。さらに、鋼板の表層部のＣ含有量は、例えば、０．５％以上０．９％以下になっている。このため、この鋼板に対してホットスタンプを実施すれば、鋼板の表層部に存在するオーステナイトはマルテンサイトになる。また、鋼板の浸炭層よりも内側は、面積分率でフェライトが５０％以上で表される鋼組織になる。このホットスタンプ工程では、例えば、冷却媒体が循環する金型を用いる。そして、オーステナイトが存在する状態の鋼板を、この金型を用いてプレス成形しながら抜熱して焼き入れする。[Hot stamp process]
In this step, after the carburizing step, the steel plate is hot-stamped in the presence of austenite in the steel plate, for example, before the temperature of the steel plate becomes 800 ° C. or less. The steel plate is not hardened between the carburizing process and the hot stamping. At the end of the carburizing step, the steel plate is subjected to soaking at a temperature of 820 ° C. or higher, and the steel structure of the surface layer portion of the steel plate is austenite. Furthermore, the C content of the surface layer portion of the steel plate is, for example, 0.5% or more and 0.9% or less. For this reason, if hot stamping is performed on this steel plate, austenite present in the surface layer of the steel plate becomes martensite. Moreover, inside the carburized layer of a steel plate, it becomes a steel structure where a ferrite is represented by 50% or more by area fraction. In this hot stamping process, for example, a mold in which a cooling medium circulates is used. Then, the steel sheet in a state in which austenite exists is removed by heat extraction while being press-formed using this mold and is quenched.

以上のとおり、本実施形態に係る鋼部品の製造方法では、Ｃ含有量が低い鋼板、例えばＣ含有量が０．００５〜０．１％の鋼板に対し、表層部がオーステナイト化する温度での均熱処理の後に、この均熱処理で生じたオーステナイトが存在する状態で、プレス加工を実施する。このため、製造された鋼部品の母材のＣ含有量を、低Ｃ含有量、例えば０．００５〜０．１％にできる。また、母材の鋼組織を、フェライトの面積分率で５０％以上にできる。このため、高い靭性を有する鋼部品を製造できる。また、浸炭工程及びホットスタンプ工程により、鋼部品の表層部にマルテンサイトからなる浸炭層が形成されるため、表層部を内部（すなわち母材）に比較して硬くできる。したがって、鋼部品の表層部に存在するマルテンサイトからなる浸炭層によって、耐摩耗性および疲労強度を確保できる。さらに、金型を用いて鋼板を成形し、鋼板をこの金型に収めたまま焼き入れするため、焼き入れにおいて鋼部品の変形を抑制できる。したがって、優れた形状凍結性を有する鋼部品を製造できる。以上のとおり、に係る鋼部品の製造方法によれば、高い靱性と耐摩耗性と疲労強度と形状凍結性を有する鋼部品を製造できる。   As described above, in the method of manufacturing a steel component according to the present embodiment, a steel sheet having a low C content, such as a steel sheet having a C content of 0.005 to 0.1%, at a temperature at which the surface layer portion is austenitized. After soaking, pressing is carried out in the presence of austenite produced by the soaking. For this reason, C content of a base material of manufactured steel parts can be made into low C content, for example, 0.005-0.1%. Also, the steel structure of the base material can be 50% or more in area fraction of ferrite. For this reason, steel parts having high toughness can be manufactured. In addition, since a carburized layer made of martensite is formed in the surface layer portion of the steel component by the carburizing step and the hot stamping step, the surface layer portion can be made harder than the inside (that is, the base material). Therefore, the wear resistance and the fatigue strength can be secured by the carburized layer made of martensite existing in the surface layer portion of the steel part. Furthermore, since a steel plate is formed using a die and quenched while the steel plate is stored in the die, it is possible to suppress deformation of steel parts in quenching. Therefore, steel parts having excellent shape freezing can be manufactured. As described above, according to the method of manufacturing a steel component according to the present invention, it is possible to manufacture a steel component having high toughness, wear resistance, fatigue strength and shape freezeability.

なお、鋼板が、高い靱性が要求される部分が５％以上２０％未満の範囲の相当ひずみで塑性加工されていないものであれば、当該部分において、フェライト粒が異常粒成長して粗大化すること（例えば、粒径が円相当径で２００μｍ以上になること）を抑制できる。したがって、鋼板に存在するフェライト粒がオーステナイト化せずに鋼部品に残存する場合では、高い靱性が要求される部分において、粗大化したフェライト粒による靱性の低下を抑制できる。なお、鋼板の均熱処理においてフェライト粒がすべてオーステナイト化した場合には、オーステナイトから生成したフェライト粒は粗大化しないため、フェライト粒の粗大化による靱性の低下は生じない。   If the steel sheet is not plastically worked at a portion requiring high toughness but at an equivalent strain of 5% or more and less than 20%, the ferrite grains grow abnormally and coarsen in that portion. (For example, it can suppress that a particle size becomes 200 micrometers or more by equivalent circle diameter). Therefore, in the case where ferrite grains present in the steel sheet remain in the steel component without austenitizing, it is possible to suppress a decrease in toughness due to coarsened ferrite grains in portions where high toughness is required. In the case where all ferrite grains are austenitized in the soaking treatment of the steel sheet, the ferrite grains generated from austenite are not coarsened, so that the decrease in toughness due to the coarsening of ferrite grains does not occur.

［鋼部品］
以上の製造方法により、本発明の実施形態に係る鋼部品が製造される。本実施形態に係る鋼部品は、母材と、母材の表面に存在するマルテンサイトからなる浸炭層とを備える。[Steel parts]
The steel component according to the embodiment of the present invention is manufactured by the above manufacturing method. The steel component according to the present embodiment includes a base material and a carburized layer made of martensite present on the surface of the base material.

母材のＣ含有量は０．０００５〜０．１％であり、その化学組成は鋼板の化学組成と同じである。母材のＣ含有量は、鋼部品の板厚中央位置において、ＥＰＭＡにより分析可能である。母材の鋼組織は、フェライトを含有する。特に、母材の鋼組織は、フェライトの面積分率が５０％以上であることが好ましい。母材の鋼組織のフェライト以外の部分は、例えば、パーライトやベイナイトである。   The C content of the base material is 0.0005 to 0.1%, and the chemical composition is the same as the chemical composition of the steel sheet. The C content of the base material can be analyzed by EPMA at the center position of thickness of the steel part. The steel structure of the base material contains ferrite. In particular, in the steel structure of the base material, the area fraction of ferrite is preferably 50% or more. The portion other than the ferrite of the steel structure of the base material is, for example, pearlite or bainite.

母材において、円相当径で２００μｍ以上の粒径を有するフェライト粒の総面積分率（以下、粗大フェライト率という）は、５％以下であることが好ましい。このように、本実施形態の製造方法で製造された鋼部品は、異常粒成長により粗大化したフェライト粒（円相当径で２００μｍ以上の粒径を有するフェライト粒）が少ないことが好ましい。   In the base material, the total area fraction (hereinafter referred to as coarse ferrite percentage) of ferrite particles having a circle equivalent diameter of 200 μm or more is preferably 5% or less. As described above, it is preferable that the steel parts manufactured by the manufacturing method of the present embodiment have a small amount of ferrite grains (ferrite grains having a circle equivalent diameter of 200 μm or more) coarsened by abnormal grain growth.

なお、母材の粗大フェライト率は、次の方法で測定される。ここでは、鋼部品の板厚をｔ（ｍｍ）と定義する。鋼部品の表面からｔ／４〜３ｔ／４の範囲の任意の位置から、サンプルを採取する。サンプルの表面（観察面）をナイタルで腐食させる。腐食させた観察面における任意の５視野（各視野面積＝２ｍｍ×２ｍｍ）を、１００倍の光学顕微鏡で観察して写真画像を生成する。得られた各視野の写真画像において、フェライト粒を特定する。特定された各フェライト粒の円相当径を求める。円相当径（μｍ）は、各フェライト粒を、同じ面積を有する円に換算した場合の円の直径で定義される。各フェライト粒の円相当径を求めた後、円相当径が２００μｍ以上の粗大フェライト粒を特定する。各視野での粗大フェライト粒の総面積を求め、測定した全視野の総面積で除して、粗大フェライト率（面積％）を求める。フェライトの特定、円相当径の算出、及び粗大フェライト率は、汎用の画像処理アプリケーションを用いて実行できる。   The coarse ferrite ratio of the base material is measured by the following method. Here, the thickness of the steel part is defined as t (mm). A sample is taken from any position in the range of t / 4 to 3 t / 4 from the surface of the steel part. The surface of the sample (viewing surface) is corroded with natal. Arbitrary 5 views (each view area = 2 mm × 2 mm) in the corroded viewing plane are observed with a 100 × optical microscope to generate a photographic image. Ferrite grains are identified in the obtained photographic image of each field of view. The equivalent circle diameter of each of the identified ferrite particles is determined. The equivalent circle diameter (μm) is defined by the diameter of a circle when each ferrite particle is converted to a circle having the same area. After determining the circle equivalent diameter of each ferrite grain, coarse ferrite grains having a circle equivalent diameter of 200 μm or more are specified. The total area of coarse ferrite grains in each field of view is determined and divided by the total area of all fields of view to obtain the coarse ferrite percentage (area%). The specification of the ferrite, the calculation of the equivalent circle diameter, and the coarse ferrite ratio can be performed using a general purpose image processing application.

母材のビッカース硬さ（ＨＶ）は２５０以下であることが好ましい。これは、硬さが高すぎる靭性が劣化するためである。また、母材のビッカース硬さ（ＨＶ）の下限は、８０であることが好ましく、１５０であることがより好ましい。これは、機械部品としての強度は表層ではなく母材の硬さで決まるためである。母材のビッカース硬さ（ＨＶ）は次の方法で求められる。鋼部品の板厚中央位置の任意の３点を選択する。選択された位置に、ＪＩＳＺ ２２４４（２００９）に準拠したビッカース硬さ試験を実施する。このとき試験力を２．９４２Ｎ（０．３ｋｇｆ）とする。得られた硬さの平均値を、母材のビッカース硬さ（ＨＶ）と定義する。   The Vickers hardness (HV) of the base material is preferably 250 or less. This is because the toughness which is too high in hardness deteriorates. The lower limit of the Vickers hardness (HV) of the base material is preferably 80, and more preferably 150. This is because the mechanical component strength is determined not by the surface layer but by the hardness of the base material. The Vickers hardness (HV) of the base material is determined by the following method. Select any 3 points at the thickness center position of steel parts. Conduct Vickers hardness test according to JIS Z 2244 (2009) at the selected position. At this time, the test force is set to 2.942 N (0.3 kgf). The average value of the obtained hardness is defined as the Vickers hardness (HV) of the base material.

浸炭層のＣ含有量は、例えば、０．５％以上、０．９％以下である。浸炭層のＣ含有量は、ＥＰＭＡ（電子線マイクロアナライザ）により分析可能である。浸炭層の鋼組織はマルテンサイトからなる。   The C content of the carburized layer is, for example, 0.5% or more and 0.9% or less. The C content of the carburized layer can be analyzed by EPMA (electron beam microanalyzer). The steel structure of the carburized layer is made of martensite.

浸炭層のビッカース硬さ（ＨＶ）の下限は、５００であることが好ましく、６００であることがより好ましい。これは、耐摩耗性、疲労強度を確保するためである。浸炭層のビッカース硬さ（ＨＶ）は次の方法で求められる。鋼部品の表面から０．０５ｍｍ深さ位置の浸炭層部分において、任意の３点を選択する。選択された位置に、ＪＩＳＺ ２２４４（２００９）に準拠したビッカース硬さ試験を実施する。このとき試験力を２．９４２Ｎ（０．３ｋｇｆ）とする。得られた硬さの平均値を、浸炭層のビッカース硬さ（ＨＶ）と定義する。   The lower limit of the Vickers hardness (HV) of the carburized layer is preferably 500, and more preferably 600. This is to ensure wear resistance and fatigue strength. The Vickers hardness (HV) of the carburized layer is determined by the following method. Arbitrary three points are selected in the carburized layer portion of 0.05 mm deep from the surface of the steel part. Conduct Vickers hardness test according to JIS Z 2244 (2009) at the selected position. At this time, the test force is set to 2.942 N (0.3 kgf). The average value of the obtained hardness is defined as the Vickers hardness (HV) of the carburized layer.

以上のとおり、本実施形態に係る鋼部品の母材のＣ含有量は、例えば０．０００５〜０．１％である。さらに、母材において、フェライトの面積分率は５０％以上であり、粗大フェライト率は例えば５％以下である。このため、母材が高い靱性を有することから、このような母材を有する鋼部品は、高い靱性を有する。また、鋼部品の母材の表面には、浸炭層が存在する。この浸炭層は、マルテンサイトからなり、優れた硬さを有する。したがって、鋼部品は優れた耐摩耗性及び疲労強度を有する。そして、本実施形態に係る鋼部品は、金型を用いて鋼板を成形するとともに、鋼板をこの金型に収めたまま焼き入れすることにより製造される。このため、本実施形態に係る鋼部品は、優れた形状凍結性を有する。このように、本実施形態に係る鋼部品は、高い靱性と耐摩耗性と疲労強度と形状凍結性を有する。   As described above, the C content of the base material of the steel component according to the present embodiment is, for example, 0.0005 to 0.1%. Furthermore, in the base material, the area fraction of ferrite is 50% or more, and the coarse ferrite ratio is 5% or less, for example. For this reason, since the base material has high toughness, a steel component having such a base material has high toughness. In addition, a carburized layer exists on the surface of the base material of the steel part. This carburized layer is made of martensite and has excellent hardness. Thus, steel parts have excellent wear resistance and fatigue strength. And while the steel components which concern on this embodiment shape | mold a steel plate using a metal mold | die, it is manufactured by hardening while holding a steel plate in this metal mold | die. For this reason, the steel component according to the present embodiment has excellent shape freezing. Thus, the steel component according to the present embodiment has high toughness, wear resistance, fatigue strength, and shape freezeability.

［第２の実施の形態］
次に、本発明の第２の実施形態に係る鋼部品の製造方法について説明する。第２の実施の形態に係る鋼部品の製造方法は、第１の実施の形態と比較すると、浸炭工程よりも前の段階において、鋼板に加工ひずみを導入する工程（以下、「ひずみ導入工程」と称する）をさらに有する。なお、他の工程（浸炭工程、ホットスタンプ工程）は、第１の実施の形態と同じである。Second Embodiment
Next, a method of manufacturing a steel component according to a second embodiment of the present invention will be described. The method of manufacturing a steel component according to the second embodiment is, compared to the first embodiment, a step of introducing a processing strain to a steel plate at a stage prior to the carburizing step (hereinafter, “strain introduction step” Furthermore, the The other steps (carburizing step, hot stamping step) are the same as in the first embodiment.

［ひずみ導入工程］
ひずみ導入工程では、前述の化学組成および鋼組織を有する鋼板のうち、鋼製品に成形された状態で高い靱性が要求される部分に対して、５％未満または２０％以上の相当ひずみで加工を実施する。なお、塑性加工の方法は限定されるものではなく、例えば、プレス加工でもよく、圧延でもよい。[Strain introduction process]
In the strain introduction step, the steel plate having the above-described chemical composition and steel structure is processed at an equivalent strain of less than 5% or 20% or more with respect to a portion required to have high toughness in a state formed into a steel product. carry out. In addition, the method of plastic processing is not limited, For example, you may be press processing and rolling may be sufficient.

鋼板の相当ひずみが５％以上２０％未満となるように塑性加工された部分は、浸炭工程において加熱されると、結晶粒の異常粒成長が生じる。一方、５％未満または２０％以上の相当ひずみで塑性加工された部分は、浸炭工程において加熱されても、結晶粒の異常粒成長が生じにくい。したがって、浸炭工程よりも前の段階において鋼板を加工する場合（事前加工）には、鋼製品に成形された状態で高い靱性が要求される部分に対して、５％未満または２０％以上の相当ひずみで加工する。これにより、鋼板に存在するフェライト粒の異常粒成長が抑制され、製造された鋼部品の靱性の低下を抑制できる。   When the portion plastically worked so that the equivalent strain of the steel plate is 5% or more and less than 20%, abnormal grain growth of crystal grains occurs when it is heated in the carburizing step. On the other hand, a portion plastically processed with a equivalent strain of less than 5% or 20% or more is unlikely to cause abnormal grain growth of crystal grains even if it is heated in the carburizing step. Therefore, when processing a steel plate at a stage prior to the carburizing step (pre-processing), it is equivalent to less than 5% or 20% or more with respect to a part requiring high toughness in a state formed into a steel product Process with strain. Thereby, abnormal grain growth of the ferrite grain which exists in a steel plate is controlled, and a fall of toughness of a manufactured steel part can be controlled.

次に、本発明の実施例について説明する。実施例での条件は、本発明の実施可能性および効果を確認するために採用した条件であり、本発明はこれらの条件例に限定されない。本発明は、その趣旨を逸脱せず、目的を達成する限りにおいて、種々の条件を採用しうるものである。   Next, examples of the present invention will be described. The conditions in the examples are conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these conditions. The present invention can adopt various conditions as long as the purpose is achieved without departing from the gist of the invention.

この実施例では、表１に示す化学組成を有する鋼種を用いた。鋼種Ａ〜Ｄは、Ｃ含有量が本発明の範囲内であり、鋼種Ｅは、本発明の範囲外である。引張強度は、常温、大気中の引張試験により測定した。いずれの鋼種も、フェライト分率は７０％以上であった。   In this example, steel types having the chemical compositions shown in Table 1 were used. Steel types A to D have a C content within the scope of the present invention, and steel type E is outside the scope of the present invention. The tensile strength was measured by a tensile test at normal temperature and in the air. The ferrite fraction of each of the steel types was 70% or more.

Figure 2018078844
Figure 2018078844

各鋼種の板厚３．２ｍｍの鋼板を打ち抜き加工して、図１に示す円板状の鋼板１０を製造した。図１に示すように、鋼板１０は、中央に貫通孔２０を有する円板である。鋼板１０の外径は２００ｍｍであり、内径は５０ｍｍである。   A steel plate having a thickness of 3.2 mm of each steel type was punched to produce a disk-shaped steel plate 10 shown in FIG. As shown in FIG. 1, the steel plate 10 is a disk having a through hole 20 at the center. The outer diameter of the steel plate 10 is 200 mm, and the inner diameter is 50 mm.

そして、表２に示す試験番号１〜１５の製造方法を用い、鋼板１０から鋼部品３０を製造した。図２は、製造した鋼部品３０を示す。鋼部品３０は、周縁部に底部５０を有し、中央部に凸部４０を有する。凸部４０の外径は１２０ｍｍであり、凸部４０の底部５０からの高さは３ｍｍである。   And using the manufacturing method of the test numbers 1-15 shown in Table 2, the steel components 30 were manufactured from the steel plate 10. FIG. 2 shows the manufactured steel part 30. The steel part 30 has a bottom 50 at the periphery and a protrusion 40 at the center. The outer diameter of the projection 40 is 120 mm, and the height from the bottom 50 of the projection 40 is 3 mm.

Figure 2018078844
Figure 2018078844

表２中の「工程」には、各試験番号の製造工程が記載されている。「工程」欄の「Ａ」は、鋼板１０を浸炭雰囲気中で均熱処理して浸炭させ、その後、鋼板１０にオーステナイトが存在する状態で金型を用いてプレス成形するとともに焼き入れすることにより鋼部品３０を製造する製造方法であることを示す。「工程」欄の「Ｂ」は、鋼板１０をプレス加工して図２に示す形状に成形し、成形した鋼板１０を浸炭雰囲気中で均熱処理して浸炭させ、その後、油焼き入れすることにより、鋼部品３０を製造する工程であることを示す。すなわち、工程「Ａ」においては、鋼部品３０の凸部４０は、浸炭雰囲気中での均熱処理（浸炭工程）の後に、焼き入れとともに成形されたものである。一方、工程「Ｂ」においては、鋼部品３０の凸部４０は、浸炭雰囲気中での均熱処理（浸炭工程）およびその後の焼き入れよりも前の段階でプレス加工により成形されたものである。   The “process” in Table 2 describes the manufacturing process of each test number. “A” in the “process” column is a steel sheet by carrying out soaking heat treatment in a carburizing atmosphere to carburize, and then performing press forming and hardening using a mold in a state where austenite exists in the steel sheet 10 It shows that it is a manufacturing method for manufacturing the part 30. “B” in the “process” column is formed by pressing the steel plate 10 and forming it into the shape shown in FIG. 2 and subjecting the formed steel plate 10 to soaking and carburizing in a carburizing atmosphere, and then oil quenching , Is a process of manufacturing the steel part 30. That is, in the step "A", the convex portion 40 of the steel part 30 is formed along with quenching after soaking (carburizing step) in a carburizing atmosphere. On the other hand, in the process "B", the convex portion 40 of the steel part 30 is formed by pressing at a stage prior to soaking (carburizing process) in a carburizing atmosphere and subsequent quenching.

「事前加工工程での相当ひずみ率」欄は、事前加工工程で鋼板に導入された相当ひずみの最大値を示す。事前加工工程は、浸炭雰囲気下で均熱処理するよりも前の段階で、鋼板１０にひずみを導入する加工である。この事前加工工程においては、円板形状に打ち抜く前に、冷間圧延により、鋼板１０にひずみを導入し、プレス成形加工を模擬した。「均熱温度」欄は、浸炭雰囲気下での均熱温度（浸炭温度、単位は℃）を示す。「均熱時間」欄は、浸炭雰囲気下での鋼板１０の均熱時間（ｈｒ）を示す。   The "equivalent strain rate in the pre-machining step" column indicates the maximum value of the equivalent strain introduced to the steel plate in the pre-machining step. The pre-processing step is processing for introducing strain into the steel plate 10 at a stage prior to soaking in a carburizing atmosphere. In this pre-machining step, strain was introduced into the steel plate 10 by cold rolling before punching into a disk shape to simulate press forming. The "soaking temperature" column indicates the soaking temperature (carburizing temperature, unit: ° C) under a carburizing atmosphere. The "soaking time" column shows the soaking time (hr) of the steel sheet 10 under a carburizing atmosphere.

［評価試験］
［ミクロ組織観察試験］
製造された各試験番号の鋼部品３０の浸炭層及び母材の鋼組織を、次の方法で観察した。鋼部品３０を半径方向に切断し、底部５０の切断面を含むサンプルを樹脂埋めし、切断面を研磨した、研磨した切断面をナイタル腐食した。そして、腐食させた切断面のうち、表層部における表面から０．０５ｍｍ深さ位置の浸炭層と、板厚中央部の母材とを、１００〜５００倍の光学顕微鏡で観察し、鋼組織を特定した。さらに、表層部と板厚中央部において、ＥＰＭＡを実施してＣ含有量を特定した。[Evaluation test]
[Microstructure observation test]
The steel structures of the carburized layer and the base material of the manufactured steel part 30 of each test number were observed by the following method. The steel part 30 was cut in the radial direction, the sample including the cut surface of the bottom 50 was filled with resin, the cut surface was polished, and the polished cut surface was subjected to natal corrosion. Then, among the corroded cut surfaces, the carburized layer at a position 0.05 mm deep from the surface in the surface layer portion and the base material in the central portion of the plate thickness are observed with an optical microscope of 100 to 500 times, Identified. Furthermore, EPMA was performed in the surface layer part and the thickness center part, and C content was specified.

［ビッカース硬さ試験］
各試験番号の鋼部品のうち、浸炭層（表層部から０．０５ｍｍ深さ）及び母材（板厚中央部）のビッカース硬さ（ＨＶ）を、上述の方法により求めた。なお、ビッカース硬さ（ＨＶ）は、底部５０を用いて求めた。[Vickers hardness test]
The Vickers hardness (HV) of the carburized layer (0.05 mm depth from the surface layer portion) and the base material (middle portion of the plate thickness) of the steel parts of each test number was determined by the method described above. The Vickers hardness (HV) was determined using the bottom portion 50.

［粗大フェライト率］
各試験番号の鋼部材３０のうち、底部５０において、表面からｔ／４〜３ｔ／４位置にてミクロ組織観察用のサンプルを採取した。採取されたサンプルを用いて、上述の方法により粗大フェライト率を求めた。[Coarse ferrite rate]
Of the steel members 30 of each test number, at the bottom 50, samples for microstructure observation were taken at positions t / 4 to 3 t / 4 from the surface. The coarse ferrite ratio was determined by the method described above using the collected samples.

［形状凍結性］
「形状凍結性」の欄は、鋼部品３０の形状凍結性の評価結果を示す。「〇」は形状凍結性が高いことを示し、「×」は形状凍結性が低いことを示す。形状凍結性は、鋼部品３０の凸部４０及び底部５０の平面度により評価した。平面度は、３次元形状測定機を用いて測定した各試験番号の凸部４０及び底部５０の形状から求めた。そして、凸部４０及び底部５０の平面度のいずれかもが０．２ｍｍ以下である場合には、形状凍結性が高いと評価した（表２において「○」印で示す）。一方、凸部４０及び底部５０の平面度のいずれかが０．２ｍｍを超えた場合、形状凍結性が低いと評価した（表２において「×」印で示す）。[Shape freezeability]
The column of “shape freezeability” shows the evaluation result of the shape freezeability of the steel part 30. "O" indicates that the shape freezeability is high, and "x" indicates that the shape freezeability is low. The shape freezeability was evaluated by the flatness of the convex portion 40 and the bottom portion 50 of the steel part 30. The flatness was determined from the shapes of the convex portion 40 and the bottom portion 50 of each test number measured using a three-dimensional shape measuring machine. And when any one of the flatness of the convex part 40 and the bottom part 50 is 0.2 mm or less, it evaluated that shape freezing property is high (it shows by "(circle)" mark in Table 2). On the other hand, when any of the flatness of the convex part 40 and the bottom part 50 exceeded 0.2 mm, it evaluated that shape freezing property was low (it shows by "x" mark in Table 2).

［靱性］
「靱性」の欄は、鋼部品３０の靱性の評価結果を示す。「◎」は靱性が高いことを示し、「○」は靱性がやや低いことを示し、「×」は靱性が低いことを示す。ただし、「○」の評価も実施例に含まれる。靱性は、シャルピー衝撃試験の結果に基づいて評価した。[Toughness]
The column of “toughness” shows the evaluation results of the toughness of the steel component 30. "◎" indicates that the toughness is high, "や や" indicates that the toughness is somewhat low, and "×" indicates that the toughness is low. However, the evaluation of “o” is also included in the examples. The toughness was evaluated based on the results of Charpy impact test.

製造された鋼部品３０の底部５０からシャルピー衝撃試験片を採取し、採取した試験片を用いて、ＪＩＳ Ｚ２２４２（２００５）に準拠して、−２０℃と常温でシャルピー衝撃試験を実施した。試験後、試験片に割れの貫通が確認されたか否かを判断した。−２０℃と常温のいずれにおいても割れが貫通していない場合、靱性が高いと判断した（表２において「◎」印で示す）。一方、−２０℃と常温のいずれにおいても、試験後に試験片に割れが貫通して試験片が２分割された場合、靱性が低いと判断した（表２において「×」印で示す）。また、−２０℃では貫通し、常温では貫通しなかった場合、靱性がやや低いと判断した（表２において「○」印で示す）。   Charpy impact test specimens were collected from the bottom 50 of the manufactured steel part 30, and Charpy impact tests were conducted at normal temperature of -20 ° C according to JIS Z2242 (2005) using the collected test specimens. After the test, it was judged whether penetration of a crack was confirmed in the test piece. When the crack did not penetrate at any of -20 ° C and normal temperature, it was judged that the toughness was high (indicated by "◎" in Table 2). On the other hand, when the crack penetrated the test piece after the test and the test piece was divided into two at both -20 ° C and normal temperature, it was judged that the toughness was low (indicated by "x" mark in Table 2). Moreover, when it penetrated at -20 degreeC and did not penetrate at normal temperature, it was judged that toughness was somewhat low (it shows by "(circle)" mark in Table 2).

［試験結果］
次に、試験結果について説明する。[Test results]
Next, test results will be described.

試験番号１〜５，９〜１１は、鋼板１０のＣ含有量と製造工程とが、いずれも本発明の範囲内にある例である。特に、試験番号１，２，４，５は、事前加工工程での相当ひずみ率と均熱温度と均熱時間とがより好ましい範囲内にある例である。これらの試験番号１、２、４、５の鋼部品３０は、いずれも、マルテンサイトからなる浸炭層と、フェライトを含有する母材とからなり、浸炭層のＣ含有量は０．５〜０．９％の範囲内であり、母材のＣ含有量は０．１％以下であった。また、これらの試験番号１，２，４，５の鋼部品３０の母材の粗大フェライト率は、５％以下であった。さらに、母材のビッカース硬さ（ＨＶ）は、表層部に存在する浸炭層よりも低く、かつ、２５０以下であった。そして、シャルピー衝撃試験では試験片に割れの貫通が観察されず、優れた靱性を示した。さらに、浸炭層の硬さは５００以上であり、高い硬さを有した。さらに、浸炭工程の後にホットスタンプ工程を実施したため、これらの試験番号１，２，４，５の鋼部品３０は、優れた形状凍結性を有した。   Test numbers 1 to 5 and 9 to 11 are examples in which the C content of the steel plate 10 and the manufacturing process are both within the scope of the present invention. In particular, test numbers 1, 2, 4, and 5 are examples in which the equivalent strain rate, the soaking temperature, and the soaking time in the pre-processing step are in more preferable ranges. The steel parts 30 of these test numbers 1, 2, 4, and 5 are each composed of a carburized layer made of martensite and a base material containing ferrite, and the C content of the carburized layer is 0.5 to 0. The C content of the base material was within 0.1%. Moreover, the coarse ferrite ratio of the base material of steel parts 30 of these test numbers 1, 2, 4, 5 was 5% or less. Furthermore, the Vickers hardness (HV) of the base material was lower than that of the carburized layer present in the surface layer and 250 or less. And in a Charpy impact test, penetration of a crack was not observed in a specimen but showed the outstanding toughness. Furthermore, the hardness of the carburized layer was 500 or more, and had high hardness. Furthermore, since the hot stamping process was performed after the carburizing process, the steel parts 30 of these test numbers 1, 2, 4 and 5 had excellent shape freezeability.

試験番号１，２，４，５は、均熱時間が１時間未満の例である試験番号９や、均熱温度が８２０℃未満の例である試験番号１０や、均熱時間が１時間未満でかつ均熱温度が８２０℃未満の例である試験番号１１と比較すると、表層部の炭素濃度が高く、有効硬化層が厚く、表層部硬さが高い。また、試験番号１０，１１では、表面の炭素濃度が低く、オーステナイト化が充分なためマルテンサイトの面積率が低いが、試験番号１，２，４，５では、表層部においてマルテンサイトの面積率が１００％の組織が得られた。   The test numbers 1, 2, 4 and 5 are the test number 9 which is an example where the soaking time is less than 1 hour, the test number 10 which is an example where the soaking temperature is less than 820 ° C., the soaking time is less than 1 hour Compared with Test No. 11 which is an example in which the soaking temperature is less than 820 ° C., the carbon concentration in the surface layer is high, the effective hardened layer is thick, and the surface layer hardness is high. In Test Nos. 10 and 11, the carbon concentration in the surface is low and the area ratio of martensite is low because austenitizing is sufficient, but in Test Nos. 1, 2, 4, and 5, the area ratio of martensite in the surface layer portion There was 100% tissue.

これらの試験番号１，２，４，５の鋼部品３０の粗大フェライト率は、事前加工工程での相当ひずみ率が５％未満または２０％以上ではない例である試験番号３の鋼部品に比較して、低くなっている。   The coarse ferrite ratio of steel parts 30 of these test numbers 1, 2, 4 and 5 is compared with the steel parts of test number 3 which is an example in which the equivalent strain rate in the pre-machining process is less than 5% or more than 20%. Then it's getting lower.

試験番号６〜８は、製造方法の工程が本発明の要件を充足していない例である。ホットスタンプされていないので形状凍結性が良好ではない。また、試験番号７は、事前加工工程での相当ひずみ率が、５％未満または２０％以上である条件を充足しない例である。試験番号７では、板厚中央部に粗大フェライトが発生し、靭性が劣化している。   Test Nos. 6 to 8 are examples in which the steps of the manufacturing method do not satisfy the requirements of the present invention. Shape freezeability is not good because it is not hot stamped. Moreover, the test number 7 is an example which does not satisfy the conditions whose equivalent distortion rate in a pre-processing process is less than 5% or 20% or more. In Test No. 7, coarse ferrite is generated at the center of the plate thickness, and the toughness is deteriorated.

試験番号１２〜１５は、鋼板１０のＣ含有量が０．１％を超えており、本発明の要件を充足しない例である。板厚中央部のＣ含有量が高く、フェライト分率が５０％以下のため、靭性が劣っている。   The test numbers 12 to 15 are examples in which the C content of the steel sheet 10 exceeds 0.1% and does not satisfy the requirements of the present invention. The toughness is inferior because the C content at the central portion of the plate thickness is high and the ferrite fraction is 50% or less.

試験番号１３は、鋼板１０のＣ含有量が０．１％を超えているとともに、均熱時間が１時間未満であり、鋼板１０のＣ含有量と均熱時間とが本発明の要件を充足しない例である。   The test No. 13 shows that the C content of the steel plate 10 exceeds 0.1%, the soaking time is less than 1 hour, and the C content of the steel plate 10 and the soaking time satisfy the requirements of the present invention Not an example.

試験番号１４は、鋼板１０のＣ含有量が０．１％を超えているとともに、均熱温度が８２０℃未満であり、鋼板１０のＣ含有量と均熱温度が本発明の要件を充足しない例である。   Test number 14 is that the C content of the steel plate 10 exceeds 0.1%, the soaking temperature is less than 820 ° C., and the C content of the steel plate 10 and the soaking temperature do not satisfy the requirements of the present invention It is an example.

試験番号１５は、鋼板１０のＣ含有量が０．１％を超えており、均熱温度が８２０℃未満であり、均熱時間が１時間未満であり、鋼板１０のＣ含有量と均熱温度と均熱時間とが本発明の要件を充足しない例である。   The test number 15 shows that the C content of the steel plate 10 exceeds 0.1%, the soaking temperature is less than 820 ° C., the soaking time is less than 1 hour, and the C content of the steel plate 10 and soaking Temperature and soaking time are examples that do not satisfy the requirements of the present invention.

以上、本発明の実施の形態を説明した。しかしながら、上述した実施の形態は本発明を実施するための例示に過ぎない。したがって、本発明は上述した実施の形態に限定されることなく、その趣旨を逸脱しない範囲内で上述した実施の形態を適宜変更して実施することができる。   The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the scope of the invention.

本発明は、例えば、鋼部品のプレス成形に関連する産業に利用可能である。

The present invention is applicable to, for example, the industry related to press forming of steel parts.

（Ｓ：０．０１％以下）
Ｓは、必須元素ではなく、例えば鋼板中に不純物として含有される。ＭｎＳを形成して焼入後の浸炭層の靭性を低下させるから、Ｓ含有量は低ければ低いほどよい。特にＳ含有量が０．０１％超では、靭性の低下が著しい。したがって、Ｓ含有量は０．０１％以下とする。ただし、Ｓ含有量の低減にはコストがかかり、０．０００５％未満まで低減しようとすると、コストが著しく増加する。このため、Ｓ含有量は０．０００５％以上であってもよい。 (S: 0.01% or less)
S is not an essential element, and is contained, for example, as an impurity in a steel plate. The lower the S content, the better, as it forms MnS and reduces the toughness of the carburized layer after quenching. In particular, when the S content is more than 0.01 % , the decrease in toughness is remarkable. Therefore, the S content is 0.01% or less. However, reduction of the S content is costly, and if it is attempted to reduce it to less than 0.0005%, the cost is significantly increased. Therefore, the S content may be 0.0005% or more.

以上のとおり、本実施形態に係る鋼部品の製造方法では、Ｃ含有量が低い鋼板、例えばＣ含有量が０．００５〜０．１％の鋼板に対し、表層部がオーステナイト化する温度での均熱処理の後に、この均熱処理で生じたオーステナイトが存在する状態で、プレス加工を実施する。このため、製造された鋼部品の母材のＣ含有量を、低Ｃ含有量、例えば０．００５〜０．１％にできる。また、母材の鋼組織を、フェライトの面積分率で５０％以上にできる。このため、高い靭性を有する鋼部品を製造できる。また、浸炭工程及びホットスタンプ工程により、鋼部品の表層部にマルテンサイトからなる浸炭層が形成されるため、表層部を内部（すなわち母材）に比較して硬くできる。したがって、鋼部品の表層部に存在するマルテンサイトからなる浸炭層によって、耐摩耗性および疲労強度を確保できる。さらに、金型を用いて鋼板を成形し、鋼板をこの金型に収めたまま焼き入れするため、焼き入れにおいて鋼部品の変形を抑制できる。したがって、優れた形状凍結性を有する鋼部品を製造できる。以上のとおり、本実施形態に係る鋼部品の製造方法によれば、高い靱性と耐摩耗性と疲労強度と形状凍結性を有する鋼部品を製造できる。 As described above, in the method of manufacturing a steel component according to the present embodiment, a steel sheet having a low C content, such as a steel sheet having a C content of 0.005 to 0.1%, at a temperature at which the surface layer portion is austenitized. After soaking, pressing is carried out in the presence of austenite produced by the soaking. For this reason, C content of a base material of manufactured steel parts can be made into low C content, for example, 0.005-0.1%. Also, the steel structure of the base material can be 50% or more in area fraction of ferrite. For this reason, steel parts having high toughness can be manufactured. In addition, since a carburized layer made of martensite is formed in the surface layer portion of the steel component by the carburizing step and the hot stamping step, the surface layer portion can be made harder than the inside (that is, the base material). Therefore, the wear resistance and the fatigue strength can be secured by the carburized layer made of martensite existing in the surface layer portion of the steel part. Furthermore, since a steel plate is formed using a die and quenched while the steel plate is stored in the die, it is possible to suppress deformation of steel parts in quenching. Therefore, steel parts having excellent shape freezing can be manufactured. As described above, according to the method of manufacturing a steel component according to the present embodiment , it is possible to manufacture a steel component having high toughness, wear resistance, fatigue strength, and shape freezeability.

なお、母材の粗大フェライト率は、次の方法で測定される。ここでは、鋼部品の板厚をｔ（ｍｍ）と定義する。鋼部品の表面からｔ／４〜３ｔ／４の範囲の任意の位置から、サンプルを採取する。サンプルの表面（観察面）をナイタルで腐食させる。腐食させた観察面における任意の５視野（各視野面積＝２ｍｍ×２ｍｍ）を、１００倍の光学顕微鏡で観察して写真画像を生成する。得られた各視野の写真画像において、フェライト粒を特定する。特定された各フェライト粒の円相当径を求める。円相当径（μｍ）は、各フェライト粒を、同じ面積を有する円に換算した場合の円の直径で定義される。各フェライト粒の円相当径を求めた後、円相当径が２００μｍ以上の粗大フェライト粒を特定する。各視野での粗大フェライト粒の総面積を求め、測定した全視野の総面積で除して、粗大フェライト率（面積％）を求める。フェライトの特定、円相当径の算出、及び粗大フェライト率を求めることは、汎用の画像処理アプリケーションを用いて実行できる。 The coarse ferrite ratio of the base material is measured by the following method. Here, the thickness of the steel part is defined as t (mm). A sample is taken from any position in the range of t / 4 to 3 t / 4 from the surface of the steel part. The surface of the sample (viewing surface) is corroded with natal. Arbitrary 5 views (each view area = 2 mm × 2 mm) in the corroded viewing plane are observed with a 100 × optical microscope to generate a photographic image. Ferrite grains are identified in the obtained photographic image of each field of view. The equivalent circle diameter of each of the identified ferrite particles is determined. The equivalent circle diameter (μm) is defined by the diameter of a circle when each ferrite particle is converted to a circle having the same area. After determining the circle equivalent diameter of each ferrite grain, coarse ferrite grains having a circle equivalent diameter of 200 μm or more are specified. The total area of coarse ferrite grains in each field of view is determined and divided by the total area of all fields of view to obtain the coarse ferrite percentage (area%). The specification of the ferrite, the calculation of the equivalent circle diameter, and the determination of the coarse ferrite ratio can be performed using a general-purpose image processing application.

母材のビッカース硬さ（ＨＶ）は２５０以下であることが好ましい。これは、硬さが高すぎる靭性が劣化するためである。また、母材のビッカース硬さ（ＨＶ）の下限は、８０であることが好ましく、１５０であることがより好ましい。これは、機械部品としての強度は表層ではなく母材の硬さで決まるためである。母材のビッカース硬さ（ＨＶ）は次の方法で求められる。鋼部品の板厚中央位置の任意の３点を選択する。選択された位置に、ＪＩＳ Ｚ ２２４４ （２００９）に準拠したビッカース硬さ試験を実施する。このとき試験力を２．９４２Ｎ（０．３ｋｇｆ）とする。得られた硬さの平均値を、母材のビッカース硬さ（ＨＶ）と定義する。 The Vickers hardness (HV) of the base material is preferably 250 or less. This is because the toughness which is too high in hardness deteriorates. The lower limit of the Vickers hardness (HV) of the base material is preferably 80, and more preferably 150. This is because the mechanical component strength is determined not by the surface layer but by the hardness of the base material. The Vickers hardness (HV) of the base material is determined by the following method. Select any 3 points at the thickness center position of steel parts. JIS at the selected position Z 2244 Conduct Vickers hardness test according to (2009). At this time, the test force is set to 2.942 N (0.3 kgf). The average value of the obtained hardness is defined as the Vickers hardness (HV) of the base material.

浸炭層のビッカース硬さ（ＨＶ）の下限は、５００であることが好ましく、６００であることがより好ましい。これは、耐摩耗性、疲労強度を確保するためである。浸炭層のビッカース硬さ（ＨＶ）は次の方法で求められる。鋼部品の表面から０．０５ｍｍ深さ位置の浸炭層部分において、任意の３点を選択する。選択された位置に、ＪＩＳ Ｚ ２２４４ （２００９）に準拠したビッカース硬さ試験を実施する。このとき試験力を２．９４２Ｎ（０．３ｋｇｆ）とする。得られた硬さの平均値を、浸炭層のビッカース硬さ（ＨＶ）と定義する。 The lower limit of the Vickers hardness (HV) of the carburized layer is preferably 500, and more preferably 600. This is to ensure wear resistance and fatigue strength. The Vickers hardness (HV) of the carburized layer is determined by the following method. Arbitrary three points are selected in the carburized layer portion of 0.05 mm deep from the surface of the steel part. JIS at the selected position Z 2244 Conduct Vickers hardness test according to (2009). At this time, the test force is set to 2.942 N (0.3 kgf). The average value of the obtained hardness is defined as the Vickers hardness (HV) of the carburized layer.

「事前加工工程での相当ひずみ率」欄は、事前加工工程で鋼板に導入された相当ひずみの最大値を示す。事前加工工程は、浸炭雰囲気下で均熱処理するよりも前の段階で、鋼板１０にひずみを導入する加工である。この事前加工工程においては、円板形状に打ち抜く前に、冷間圧延により、鋼板１０にひずみを導入し、プレス成形加工を模擬した。「均熱温度」欄は、浸炭雰囲気下での均熱温度（浸炭温度、単位は℃）を示す。「均熱時間」欄は、浸炭雰囲気下での鋼板１０の均熱時間（ｈ）を示す。   The "equivalent strain rate in the pre-machining step" column indicates the maximum value of the equivalent strain introduced to the steel plate in the pre-machining step. The pre-processing step is processing for introducing strain into the steel plate 10 at a stage prior to soaking in a carburizing atmosphere. In this pre-machining step, strain was introduced into the steel plate 10 by cold rolling before punching into a disk shape to simulate press forming. The "soaking temperature" column indicates the soaking temperature (carburizing temperature, unit: ° C) under a carburizing atmosphere. The "soaking time" column shows the soaking time (h) of the steel sheet 10 under a carburizing atmosphere.

［粗大フェライト率］
各試験番号の鋼部品３０のうち、底部５０において、表面からｔ／４〜３ｔ／４位置にてミクロ組織観察用のサンプルを採取した。採取されたサンプルを用いて、上述の方法により粗大フェライト率を求めた。 [Coarse ferrite rate]
Among the steel part article 30 of each test number, at the bottom 50, a sample was taken for microstructure observation at the surface t / 4~3t / 4 position. The coarse ferrite ratio was determined by the method described above using the collected samples.

製造された鋼部品３０の底部５０からシャルピー衝撃試験片を採取し、採取した試験片を用いて、ＪＩＳ Ｚ ２２４２ （２００５）に準拠して、−２０℃と常温でシャルピー衝撃試験を実施した。試験後、試験片に割れの貫通が確認されたか否かを判断した。−２０℃と常温のいずれにおいても割れが貫通していない場合、靱性が高いと判断した（表２において「◎」印で示す）。一方、−２０℃と常温のいずれにおいても、試験後に試験片に割れが貫通して試験片が２分割された場合、靱性が低いと判断した（表２において「×」印で示す）。また、−２０℃では貫通し、常温では貫通しなかった場合、靱性がやや低いと判断した（表２において「○」印で示す）。 A Charpy impact test specimen was collected from the bottom 50 of the manufactured steel part 30, and the collected specimen was used to 2242 In accordance with (2005), Charpy impact test was performed at -20 ° C and normal temperature. After the test, it was judged whether penetration of a crack was confirmed in the test piece. When the crack did not penetrate at any of -20 ° C and normal temperature, it was judged that the toughness was high (indicated by "◎" in Table 2). On the other hand, when the crack penetrated the test piece after the test and the test piece was divided into two at both -20 ° C and normal temperature, it was judged that the toughness was low (indicated by "x" mark in Table 2). Moreover, when it penetrated at -20 degreeC and did not penetrate at normal temperature, it was judged that toughness was somewhat low (it shows by "(circle)" mark in Table 2).

Claims (7)

質量％で、
Ｃ：０．０００５〜０．１％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．０５〜３．０％、
Ａｌ：０．９％以下、
Ｐ：０．０５％以下、
Ｓ：０．０１％以下、
Ｔｉ：０．０〜０．２％、
Ｎｂ：０．０〜０．１％、
Ｃｒ：０〜２％、
Ｍｏ：０．０〜０．２％、
Ｂ：０．０００〜０．００５％、かつ、
残部：Ｆｅおよび不純物、
で表される化学組成を有し、
フェライトの面積分率が７０％以上、
で表される鋼組織を有する鋼板を浸炭雰囲気中で加熱して、前記鋼板の表面に浸炭層を形成する工程と、
金型を用いて前記鋼板を成形し、前記金型に収めたまま前記鋼板の焼き入れを行って、浸炭層をマルテンサイトに変態させるとともに、前記鋼板の前記浸炭層よりも内側を面積分率でフェライトが５０％以上で表される鋼組織にする工程と、
を有することを特徴とする鋼部品の製造方法。In mass%,
C: 0.0005 to 0.1%
Si: 0.01 to 2.0%,
Mn: 0.05 to 3.0%,
Al: 0.9% or less,
P: 0.05% or less,
S: 0.01% or less,
Ti: 0.0 to 0.2%,
Nb: 0.0 to 0.1%
Cr: 0 to 2%,
Mo: 0.0 to 0.2%,
B: 0.000 to 0.005%, and
Remainder: Fe and impurities,
Has a chemical composition represented by
70% or more area fraction of ferrite,
Heating in a carburizing atmosphere a steel plate having a steel structure represented by and forming a carburized layer on the surface of the steel plate;
The steel sheet is formed using a mold, and the steel sheet is quenched while being contained in the mold to transform the carburized layer to martensite, and the area fraction of the steel sheet on the inner side than the carburized layer Forming a steel structure in which ferrite is represented by 50% or more,
A method of manufacturing a steel component, comprising: 前記鋼板の表面に浸炭層を形成する工程においては、前記鋼板を８２０℃以上の温度で均熱処理することを特徴とする請求項１に記載の鋼部品の製造方法。   The method for manufacturing a steel component according to claim 1, wherein in the step of forming a carburized layer on the surface of the steel sheet, the steel sheet is subjected to soaking at a temperature of 820 ° C or higher. 前記鋼板の表面に浸炭層を形成する工程においては、前記鋼板を１時間以上均熱処理することを特徴とする請求項１または２に記載の鋼部品の製造方法。   The method for manufacturing a steel component according to claim 1 or 2, wherein in the step of forming a carburized layer on the surface of the steel sheet, the steel sheet is subjected to a soaking treatment for one hour or more. 前記鋼板の表面に浸炭層を形成する工程よりも前に、
前記鋼板を５％未満または２０％以上の相当ひずみで加工する工程をさらに有することを特徴とする請求項１から３のいずれか１項に記載の鋼部品の製造方法。Prior to the step of forming a carburized layer on the surface of the steel plate,
The method for producing a steel component according to any one of claims 1 to 3, further comprising the step of processing the steel plate with a equivalent strain of less than 5% or 20% or more. 質量％で、
Ｃ：０．０００５〜０．１％、
Ｓｉ：０．０１〜２．０％、
Ｍｎ：０．０５〜３．０％、
Ａｌ：０．９％以下、
Ｐ：０．０５％以下、
Ｓ：０．０１％以下、
Ｔｉ：０．０〜０．２％、
Ｎｂ：０．０〜０．１％、
Ｃｒ：０〜２％、
Ｍｏ：０．０〜０．２％、
Ｂ：０．０００〜０．００５％、かつ、
残部：Ｆｅおよび不純物、
で表される化学組成を有し、
面積分率で、
フェライト：５０％以上、
で表される鋼組織を有する母材と、
マルテンサイトからなり前記母材の表面に存在する浸炭層と、
を有することを特徴とする鋼部品。In mass%,
C: 0.0005 to 0.1%
Si: 0.01 to 2.0%,
Mn: 0.05 to 3.0%,
Al: 0.9% or less,
P: 0.05% or less,
S: 0.01% or less,
Ti: 0.0 to 0.2%,
Nb: 0.0 to 0.1%
Cr: 0 to 2%,
Mo: 0.0 to 0.2%,
B: 0.000 to 0.005%, and
Remainder: Fe and impurities,
Has a chemical composition represented by
In area fraction,
Ferrite: 50% or more,
A base material having a steel structure represented by
A carburized layer made of martensite and present on the surface of the base material,
A steel part characterized by having. 前記マルテンサイトからなる浸炭層は、質量％で０．５〜０．９％のＣを含有し、
前記鋼部品の厚さをｔとした場合の前記鋼部品の表面からｔ／４〜３ｔ／４の範囲において、円相当径で２００μｍ以上の粒径を有するフェライト粒の総面積分率が５％以下であることを特徴とする請求項５に記載の鋼部品。The martensite carburized layer contains 0.5 to 0.9% C by mass%,
When the thickness of the steel part is t, the total area fraction of ferrite grains having a circle equivalent diameter of 200 μm or more is 5% in the range of t / 4 to 3 t / 4 from the surface of the steel part The steel component according to claim 5, characterized in that: 有効硬化層深さは０．０５〜０．５ｍｍであることを特徴とする請求項５または６に記載の鋼部品。

The steel part according to claim 5 or 6, wherein the effective hardened layer depth is 0.05 to 0.5 mm.

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