JP3907986B2 - Method for producing case-hardened steel with excellent cold workability and grain size characteristics - Google Patents

Description

【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.

Claims (3)

By mass%, C: 0.10~0.25%, Si : 0.03~0.35%, Mn: 0.20~2.0%, Cr: 0.1~2.0%, N: Steel containing 0.010 to 0.025%, Al: 0.010 to 0.050%, Nb: 0.03 to 0.10%, the balance being Fe and unavoidable impurities, to 1150 to 1250 ° C It is excellent in cold workability and grain size characteristics characterized by performing hot rolling after heating to obtain a raw material, heating and holding the raw material at 870 to 950 ° C., air cooling, and annealing at 650 to 700 ° C. A method for producing case-hardened steel. 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: Contains 0.010 to 0.025%, Al: 0.010 to 0.050%, Nb: 0.03 to 0.10%, Mo: ≤0.35%, Ni: ≤3.0% 1 or 2 selected from the above, and the remainder consisting of Fe and inevitable impurities is heated to 1150 to 1250 ° C. and then hot-rolled to form a material, and this material is heated and maintained at 870 to 950 ° C. Then, after air cooling, it anneals at 650-700 degreeC, The manufacturing method of the case hardening steel excellent in the cold workability and the crystal grain size characteristic characterized by the above-mentioned. The steel component according to claim 1 or 2, further in mass%, 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 two or more selected from 0.003 to 0.05%, with the balance being Fe and inevitable impurities, 1150 to 1250 ° C. It is excellent in cold workability and grain size characteristics characterized by performing hot rolling after heating to make a material, heating and holding this material at 870 to 950 ° C., air cooling, and annealing at 650 to 700 ° C. A method for producing baked steel.