JP3907986B2 - Method for producing case-hardened steel with excellent cold workability and grain size characteristics - Google Patents
Method for producing case-hardened steel with excellent cold workability and grain size characteristics Download PDFInfo
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Description
【0001】
【発明の属する技術分野】
本発明は冷間加工性、結晶粒度特性に優れた肌焼き鋼の製造方法に関する。
【0002】
【従来の技術】
鋼材の冷間加工性を良好に保つためには、冷間加工前の組織を球状化組織にすることが一般的である。しかし、そうした場合に冷間加工後、焼きならしなどの熱処理を行わないまま950℃で浸炭すると、オーステナイト初期粒度がおおきくなり混粒が発生し、熱処理歪みが生じる。そこで、結晶粒度調整のため、冷間加工前の組織をフェライト+パーライト組織にすると、今度は冷間加工性が問題となる。そこで冷間加工性および結晶粒度特性ともに良好に保つためには、冷鍛前素材の硬さを85HRB以下、ミクロ組織をオーステナイト初期結晶粒度番号が11以下で球状化の進んでいないフェライト+パーライト組織にする必要がある。特願2000−344694では鋼素材を840〜930℃に加熱し、730〜650℃の温度域を15〜30℃/hの冷却速度で冷却することで組織をフェライト+パーライトとして結晶粒度特性を向上しているが、この方法では熱処理時間が遅い。
【0003】
【発明が解決しようとする課題】
本発明の解決しようとする課題は、より短い熱処理時間で結晶粒度特性と、冷間加工性を向上させることを目的とし、上記した冷間加工性、結晶粒度特性ともに良好に保つための優れた肌焼き鋼の製造方法を提供することである。すなわち、冷間加工性も良好で、冷間加工ままで浸炭温度としては高温の950℃で浸炭しても混粒の発生しない肌焼き鋼の製造方法を提供することである。
【0004】
【課題を解決するための手段】
上記課題を解決するためのこの発明の手段は、質量%で、C:0.10〜0.25%、Si:0.03〜0.35%、Mn:0.20〜2.0%、Cr:0.1〜2.0%、N:0.010〜0.025%、Al:0.010〜0.050%、Nb:0.03〜0.10%を含有し、残部がFe及び不可避不純物からなる鋼を、1150〜1250℃に加熱後熱間圧延を行って素材とし、この素材を870〜950℃に加熱保持後、空冷した後、650〜700℃で焼鈍することを特徴とする冷間加工性と結晶粒度特性に優れた肌焼き鋼の製造方法である。
【0005】
請求項2の発明では、質量%で、C:0.10〜0.25%、Si:0.03〜0.35%、Mn:0.20〜2.0%、Cr:0.1〜2.0%、N:0.010〜0.025%、Al:0.010〜0.050%、Nb:0.03〜0.10%を含有し、さらにMo:≦0.35%、Ni:≦3.0%からから選択した1種または2種を含有し、残部がFe及び不可避不純物からなる鋼を、1150〜1250℃に加熱後熱間圧延を行って素材とし、この素材を870〜950℃に加熱保持後、空冷した後、650〜700℃で焼鈍することを特徴とする冷間加工性と結晶粒度特性に優れた肌焼き鋼の製造方法である。
【0006】
請求項3の発明では、請求項1または2の手段の鋼成分に、さらに、質量%で、Pb:0.01〜0.30%、Bi:0.01〜0.20%、Te:0.001〜0.05%、Ca:0.0005〜0.003%、Se:0.003〜0.05%から選択した1種または2種以上を含有し、残部がFe及び不可避不純物からなる鋼を、1150〜1250℃に加熱後熱間圧延を行って素材とし、この素材を870〜950℃に加熱保持後、空冷した後、650〜700℃で焼鈍することを特徴とする冷間加工性と結晶粒度特性に優れた肌焼き鋼の製造方法。
【0007】
本発明は上記のように、鋼すなわち肌焼き鋼を素材とし、冷間加工前の組織を85HRB以下のフェライト+パーライト組織でオーステナイト初期結晶粒度番号を11以下にすることで、冷間加工性、結晶粒度特性に優れた鋼の製造方法に関するものであり、1150〜1250℃に加熱後熱間圧延を行った素材を、本発明では、870〜950℃に加熱後、空冷した後、650〜700℃の焼鈍を行うものである。
【0008】
本発明における鋼の組成割合の限定理由を述べる。
【0009】
C:Cは機械構造用部品として浸炭処理後の芯部強度を確保するために必要な元素であり、0.10%未満ではその効果が十分に得られず、反対に0.25%を超えると芯部が硬化し過ぎて冷間加工性を低下させる。そのため含有量を0.10〜0.25%とした。
【0010】
Si:Siは0.03%未満では脱酸効果が十分に得られず、0.35%を超えて過剰に含有させると加工性を低下させるとともに浸炭時の粒界酸化層の形成を助長し疲労特性についても低下させる。そのため含有量を0.03〜0.35%とした。
【0011】
Mn:Mnは焼入れ性を確保するのに必要な元素であるが0.20%未満ではその効果が十分に得られず、2.0%を超えると加工性を低下させる。そのため含有量を0.20〜2.0%とした。
【0012】
Cr:Crは焼入性、靱性、疲労寿命の向上に効果のある元素で少な過ぎると効果がなく、多過ぎると効果は飽和する。そこため含有量を0.1〜2.0%とする。
【0013】
N:NはAlNを形成し、AlNには結晶粒度粗大化防止の機能がある。0.010%未満ではその効果がなくなり、0.025%を超えると、化合物が増え疲労強度に悪影響を及ぼす。そのため含有量を0.010〜0.025%とした。
【0014】
Al:AlはAlNを形成し、AlNには結晶粒度粗大化防止の機能がある。0.010%未満ではその効果がなくなり、0.050%を超えると、アルミナ系酸化物が増加し疲労特性、加工性を低下させる。そのため含有量を0.010〜0.050%とした。
【0015】
Nb:Nbは炭化物、あるいは炭窒化物を形成して結晶粒度特性を向上させる。0.03%未満ではその効果がなく、0.10%を超えると、効果が飽和する。そのため含有量を0.03〜0.10%とした。
【0016】
Mo:Moは焼入れ性、靭性、疲労強度の向上に効果のある元素で、次のNiとともに必要に応じて選択的に1種または2種添加し得る。そのためMoの含有量を0.35%以下とした。
【0017】
Ni:Niは焼入性および靱性を向上させる元素であるが、3.0%を超えて含有させると圧延あるいは鍛造後にベイナイトやマルテンサイト組織となり、加工性を著しく低下させる。そのためその含有量を3.0%以下とした。
【0018】
Pb、Bi、Te、Ca、Se:Pb、Bi、Te、Ca、Seは被削性を向上させる働きがあり、選択的に添加し得る。Pb0.01%未満、Bi0.01%未満、Te0.001%未満、Ca0.0005%未満、Se0.003%未満では、その効果が少ない。一方、Pb0.30%超、Bi0.20%超、Te0.05%超、Ca0.003%超、Se0.05%超では、熱間加工性が悪化する。そのため含有量をPb:0.01〜0.30%、Bi:0.01〜0.20%、Te:0.001〜0.05%、Ca:0.0005〜0.003%、Se:0.003〜0.05%から選択した1種または2種以上とした。
【0019】
本発明では、上記化学成分を含有する鋼およびこれに任意添加元素を加えた鋼を対象とし、冷間加工性、結晶粒度特性の良好な肌焼き鋼を製造するために、1150〜1250℃に加熱後、熱間圧延を行い、870〜950℃に加熱後、空冷した後、650〜700℃の焼鈍を行うものである。
【0020】
上記において、1150〜1250℃に加熱して熱間圧延を行う理由は、圧延時にNbの炭化物、炭窒化物を完全固溶させ、微細なNbの炭化物、炭窒化物を析出させるためである。1050℃以上で1150℃以下では、Nbの炭化物、炭窒化物は十分に固溶せず、析出物の成長が起こり、浸炭時のオーステナイト結晶粒度特性を低下させる。また、1050℃未満ではPbの影響で圧延時にキズの発生が多くなる。1250℃を超えると、粒度特性は回復するが、熱間圧延後ベイナイト組織になりやすくなり加工性が低下する。そのため熱間圧延時の加熱温度は1150〜1250℃とした。
【0021】
本発明において、870〜950℃に加熱後、空冷した後、650〜700℃の焼鈍を行う理由は、870〜950℃に加熱後空冷することにより、フェライト+パーライト組織にし、650〜700℃の焼鈍を行うことにより、オーステナイト初期結晶粒度番号が11以下のフェライト+パーライト組織のまま硬さを85HRB以下に低下させるためである。加熱温度870℃未満では、炭化物の溶け込みが不十分で、一部球状化したフェライト+パーライト組織になり、950℃を超えると、焼入れ性があがり、空冷時ベイナイトが析出する可能性がある。そのため、加熱温度を870〜950℃にした。また、焼鈍温度が650℃未満では、硬さが85HRB以下にならず、700℃を超えると硬さは85HRB以下になるが、炭化物の溶け込みが始まり、パーライトが一部球状化した組織となる。そのため、焼鈍温度を650〜700℃にした。
【0022】
【発明の実施の形態】
本発明の実施の形態を以下の実施例を通じて説明する。先ず、本発明は鋼組成自体の発明でなく、熱間加工後の熱処理特に焼鈍による製造方法の発明であり、冷間加工性、結晶粒度特性の良好な肌焼き鋼を製造するために、1150〜1250℃に加熱後、熱間圧延を行い、870〜950℃に加熱後、空冷した後、650〜700℃の焼鈍を行うものである。そこで本発明の実施の形態における代表的な1例としての鋼組成を表1に示す。なお、表1においてP、S、Cuは不純物として含有されるものである。
【0023】
【表1】
【0024】
本発明の実施の形態では、表1に示す化学成分(質量%)の肌焼き鋼を電気溶解炉で溶製し、1180℃に加熱後、熱間圧延し、900℃に加熱して2時間保持後、空冷し、600℃、650℃、670℃、700℃、720℃の各温度で焼鈍した。得られた鋼素材の硬度測定、ミクロ組織観察、オーステナイト初期結晶粒度測定を行った結果を表2に示す。
【0025】
【表2】
【0026】
さらに、上記5条件で低温焼戻しした素材から14mmφ×21mmHの円柱試験片をそれぞれ作製し、端面拘束型圧縮試験機を使用し加工率70%で加工し試験片を作製した。その試験片を用い、930℃、950℃、970℃の各温度に6時間保持後水冷したときのJIS G0551による混粒発生有無の判定結果を表3に示す。
【0027】
【表3】
【0028】
【実施例】
実施例は、表2、3に示すように、焼鈍温度が600℃では硬さが85HRB以上で冷間加工性が悪くなっており、焼鈍温度が720℃では、フェライト+パーライトが一部球状化した組織となっているので、オーステナイト初期粒度が11以上となり、950℃で混粒が発生しているが、焼鈍温度が650、670、700℃では、硬さ85HRB以下、フェライト+パーライト組織、オーステナイト初期粒度番号11以下が得られ、950℃で混粒が発生していない。
【0029】
【発明の効果】
以上説明したように、本発明方法により肌焼き鋼を製造すると、硬さ85HRB以下でオーステナイト初期粒度番号が11以下のフェライト+パーライト組織の冷間加工性、結晶粒度特性に優れた肌焼き鋼を得ることができ、本発明は従来にない優れた効果を奏するものである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a case hardening steel excellent in cold workability and crystal grain size characteristics.
[0002]
[Prior art]
In order to keep the cold workability of the steel material good, it is common to make the structure before cold working into a spheroidized structure. However, in such a case, if carburizing at 950 ° C. without performing heat treatment such as normalizing after cold working, the austenite initial grain size becomes large, mixed grains are generated, and heat treatment distortion occurs. Therefore, if the structure before cold working is made a ferrite + pearlite structure for adjusting the grain size, cold workability becomes a problem. Therefore, in order to keep both the cold workability and the grain size characteristics good, the hardness of the material before cold forging is 85 HRB or less, the microstructure is an austenite initial grain size number of 11 or less, and the ferrite + pearlite structure that is not spheroidized. It is necessary to. In Japanese Patent Application No. 2000-344694, the steel material is heated to 840-930 ° C., and the temperature range of 730-650 ° C. is cooled at a cooling rate of 15-30 ° C./h. However, the heat treatment time is slow in this method.
[0003]
[Problems to be solved by the invention]
The problem to be solved by the present invention is to improve the crystal grain size characteristics and cold workability in a shorter heat treatment time, and is excellent for maintaining both the above-mentioned cold workability and crystal grain size characteristics. It is to provide a method for producing case-hardened steel. That is, an object of the present invention is to provide a method for producing a case-hardened steel which has good cold workability and does not generate mixed grains even when carburized at a high temperature of 950 ° C. while being cold worked.
[0004]
[Means for Solving the Problems]
Means of the present invention for solving the above problems are, in mass%, C: 0.10 to 0.25%, Si: 0.03 to 0.35%, Mn: 0.20 to 2.0%, Cr: 0.1 to 2.0 % , N: 0.010 to 0.025%, Al: 0.010 to 0.050%, Nb: 0.03 to 0.10%, the balance being Fe And steel made of inevitable impurities is heated to 1150 to 1250 ° C. and then hot-rolled to obtain a raw material. This material is heated to 870 to 950 ° C., air cooled, and then annealed at 650 to 700 ° C. Is a method for producing a case-hardened steel excellent in cold workability and grain size characteristics.
[0005]
In the invention of claim 2, in mass%, C: 0.10 to 0.25%, Si: 0.03 to 0.35%, Mn: 0.20 to 2.0%, Cr: 0.1 to 0.1% 2.0 % , N: 0.010 to 0.025%, Al: 0.010 to 0.050%, Nb: 0.03 to 0.10%, Mo: ≦ 0.35%, Ni: containing one or two selected from ≦ 3.0%, the remainder comprising Fe and unavoidable impurities, heated to 1150 to 1250 ° C. and hot-rolled as a raw material, It is a method for producing a case-hardened steel excellent in cold workability and crystal grain size characteristics, characterized by annealing at 650 to 700 ° C. after heating and holding at 870 to 950 ° C., followed by air cooling.
[0006]
In the invention of claim 3, the steel component of the means of claim 1 or 2 is further added in terms of mass%, Pb: 0.01 to 0.30%, Bi: 0.01 to 0.20%, Te: 0. 0.001 to 0.05%, Ca: 0.0005 to 0.003%, Se: One or more selected from 0.003 to 0.05%, with the balance being Fe and inevitable impurities The steel is heated to 1150 to 1250 ° C. and then hot-rolled to obtain a raw material. This material is heated to 870 to 950 ° C., air cooled, and then annealed at 650 to 700 ° C. Method of case-hardened steel with excellent properties and grain size characteristics.
[0007]
As described above, the present invention uses steel, that is, case-hardened steel as a raw material, and the structure before cold working is a ferrite + pearlite structure of 85 HRB or less and an austenite initial grain size number of 11 or less, thereby providing cold workability, The present invention relates to a method for producing steel having excellent grain size characteristics. In the present invention, a material that has been hot-rolled after heating to 1150 to 1250 ° C. is heated to 870 to 950 ° C., air-cooled, and then 650 to 700 Annealing is performed at ℃.
[0008]
The reason for limiting the composition ratio of steel in the present invention will be described.
[0009]
C: C is an element necessary for securing the core strength after carburizing treatment as a machine structural component. If it is less than 0.10%, the effect cannot be sufficiently obtained, and conversely, it exceeds 0.25%. And a core part hardens | cures too much and cold workability is reduced. Therefore, the content was made 0.10 to 0.25%.
[0010]
Si: If Si is less than 0.03%, a sufficient deoxidation effect cannot be obtained. If it exceeds 0.35%, the processability is lowered and the formation of a grain boundary oxide layer during carburization is promoted. It also reduces the fatigue properties. Therefore, the content is set to 0.03 to 0.35%.
[0011]
Mn: Mn is an element necessary for ensuring hardenability, but if it is less than 0.20%, the effect cannot be sufficiently obtained, and if it exceeds 2.0%, workability is lowered. Therefore, the content is set to 0.20 to 2.0%.
[0012]
Cr: Cr is an element effective in improving hardenability, toughness, and fatigue life, and if it is too little, it is ineffective, and if it is too much, the effect is saturated. Therefore, the content is set to 0.1 to 2.0%.
[0013]
N: N forms AlN, and AlN has a function of preventing grain size coarsening. If it is less than 0.010%, the effect is lost, and if it exceeds 0.025%, the compound increases and adversely affects the fatigue strength. Therefore, the content was made 0.010 to 0.025%.
[0014]
Al: Al forms AlN, and AlN has a function of preventing grain size coarsening. If it is less than 0.010%, the effect is lost, and if it exceeds 0.050%, the alumina-based oxide increases and fatigue characteristics and workability are deteriorated. Therefore, the content was made 0.010 to 0.050%.
[0015]
Nb: Nb forms carbides or carbonitrides to improve the grain size characteristics. If it is less than 0.03%, the effect is not obtained, and if it exceeds 0.10%, the effect is saturated. Therefore, the content is set to 0.03 to 0.10%.
[0016]
Mo: Mo is an element effective in improving hardenability, toughness, and fatigue strength, and can be selectively added as needed together with the following Ni. Therefore, the Mo content is set to 0.35% or less.
[0017]
Ni: Ni is an element that improves hardenability and toughness. However, when it exceeds 3.0%, it becomes a bainite or martensite structure after rolling or forging, and the workability is significantly reduced. Therefore, the content was made 3.0% or less.
[0018]
Pb, Bi, Te, Ca, Se: Pb, Bi, Te, Ca, Se have a function of improving machinability and can be selectively added. When Pb is less than 0.01%, Bi is less than 0.01%, Te is less than 0.001%, Ca is less than 0.0005%, and Se is less than 0.003%, the effect is small. On the other hand, when Pb exceeds 0.30%, Bi exceeds 0.20%, Te exceeds 0.05%, Ca exceeds 0.003%, and Se exceeds 0.05%, hot workability deteriorates. Therefore, the contents are Pb: 0.01 to 0.30%, Bi: 0.01 to 0.20%, Te: 0.001 to 0.05%, Ca: 0.0005 to 0.003%, Se: One or more selected from 0.003 to 0.05% were used.
[0019]
In the present invention, in order to produce a case-hardened steel having good cold workability and crystal grain size characteristics, the steel containing the above chemical components and the steel added with an optional additive element are used. After heating, it is hot-rolled, heated to 870-950 ° C., air-cooled, and then annealed at 650-700 ° C.
[0020]
In the above, the reason for performing hot rolling by heating to 1150 to 1250 ° C. is to completely dissolve Nb carbide and carbonitride during rolling and to precipitate fine Nb carbide and carbonitride. When the temperature is 1050 ° C. or higher and 1150 ° C. or lower, Nb carbides and carbonitrides are not sufficiently dissolved, and precipitate growth occurs, which deteriorates austenite grain size characteristics during carburizing. Moreover, if it is less than 1050 degreeC, generation | occurrence | production of a crack will increase at the time of rolling under the influence of Pb. When the temperature exceeds 1250 ° C., the particle size characteristics are recovered, but a bainite structure tends to be formed after hot rolling, and the workability is lowered. Therefore, the heating temperature at the time of hot rolling was set to 1150 to 1250 ° C.
[0021]
In the present invention, after heating to 870 to 950 ° C. and air cooling, the reason for performing annealing at 650 to 700 ° C. is to make ferrite + pearlite structure by heating to 870 to 950 ° C. and then cooling to 650 to 700 ° C. This is because annealing reduces the hardness to 85 HRB or less while maintaining the ferrite + pearlite structure with an austenite initial grain size number of 11 or less. If the heating temperature is less than 870 ° C., the carbides are not sufficiently dissolved, and a partially spheroidized ferrite + pearlite structure is formed. If the heating temperature exceeds 950 ° C., the hardenability is improved, and bainite may be precipitated during air cooling. Therefore, the heating temperature was set to 870 to 950 ° C. Further, when the annealing temperature is less than 650 ° C., the hardness does not become 85 HRB or less, and when it exceeds 700 ° C., the hardness becomes 85 HRB or less, but the carbide starts to be melted and the pearlite is partially spheroidized. Therefore, the annealing temperature was set to 650 to 700 ° C.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described through the following examples. First, the present invention is not an invention of the steel composition itself, but an invention of a heat treatment after hot working, particularly a manufacturing method by annealing. In order to produce a case-hardened steel having good cold workability and crystal grain size characteristics, 1150 After heating to ˜1250 ° C., hot rolling is performed, after heating to 870 to 950 ° C. and air cooling, annealing at 650 to 700 ° C. is performed. Therefore, Table 1 shows a steel composition as a typical example in the embodiment of the present invention. In Table 1, P, S, and Cu are contained as impurities.
[0023]
[Table 1]
[0024]
In the embodiment of the present invention, case hardening steel having chemical components (% by mass) shown in Table 1 is melted in an electric melting furnace, heated to 1180 ° C., hot-rolled, heated to 900 ° C., and 2 hours. After holding, it was air-cooled and annealed at temperatures of 600 ° C., 650 ° C., 670 ° C., 700 ° C., and 720 ° C. Table 2 shows the results of hardness measurement, microstructure observation, and austenite initial grain size measurement of the obtained steel material.
[0025]
[Table 2]
[0026]
Furthermore, 14 mmφ × 21 mmH cylindrical test pieces were respectively produced from the materials tempered at the low temperature under the above five conditions, and processed using a face-constrained compression tester at a processing rate of 70% to produce test pieces. Table 3 shows the result of determination of the presence or absence of mixed grains according to JIS G0551 when the test piece was used and held at 930 ° C., 950 ° C., and 970 ° C. for 6 hours and then cooled with water.
[0027]
[Table 3]
[0028]
【Example】
In the examples, as shown in Tables 2 and 3, when the annealing temperature is 600 ° C., the hardness is 85 HRB or more and the cold workability is poor, and when the annealing temperature is 720 ° C., the ferrite + pearlite is partially spheroidized. Therefore, the austenite initial grain size is 11 or more, and mixed grains are generated at 950 ° C., but at annealing temperatures of 650, 670, and 700 ° C., the hardness is 85 HRB or less, ferrite + pearlite structure, austenite An initial particle size number of 11 or less is obtained, and no mixed particles are generated at 950 ° C.
[0029]
【The invention's effect】
As described above, when the case hardening steel is manufactured by the method of the present invention, the case hardening steel having excellent hardness and grain size characteristics of ferrite + pearlite structure having a hardness of 85 HRB or less and an austenite initial grain number of 11 or less is obtained. The present invention provides an excellent effect that has never been achieved.
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