194 論 文 SUJ2 材の焼入れ組織に及ぼす高周波誘導加熱条件の影響 1 1 1 Influence of Induction Heating Condition on Quenched Structure of JIS-SUJ2 Steel Hiroshi Yuki, Miyu Sato and Chikara Ohki Synopsis : When JIS-SUJ2 steel is hardened by induction heating, high temperature austenitizing is applied in order to achieve high productivity. In this case, SUJ2 is quenched from the single-phase austenite zone. As a result, the properties of SUJ2 may be different from the conventional furnace process involving SUJ2 quenched from the two-phase austenite / cementite zone. In this study, we experimentally investigated the relation between the amount of undissolved carbide, austenitizing temperature, and duration. Additionally, we developed a prediction formula for undissolved carbide in the single-phase austenite zone. The formula is consists of the Arrhenius equation and the Kolomogorov-Johnson-Mehl-Avrami equation. Subsequently, the effect of austenitizing temperature and amount of undissolved carbide on hardness, retained austenite content, prior-austenite grain size, and martensite block size were investigated. Key words: heat treatment; bearing steel; induction heating; grain size; retained austenite. 1. 緒言 JIS-SUJ2 SUJ2 SUJ2 SUJ2 1 2 900 SUJ2 SUJ2 900 2 6 2,5,6 X Dilatometer 7 9 28 6 24 28 11 25 28 12 28 J-STAGE Received on Jun. 24, 2016 ; Accepted on Nov. 25, 2016 ; J-STAGE Advance published on Dec. 28, 2016 1 NTN Advanced Technology R&D center, NTN Corporation, 5-105 Hidamarinooka Kuwana Mie 511-0867 Corresponding author : E-mail : hiroshi_yuuki@ntn.co.jp DOI : http://dx.doi.org/10.2355/tetsutohagane.tetsu-2016-058 20
SUJ2 195 2. 実験方法 2 1 均熱温度および残留炭化物量を変更した試験片作製方法 Table 1 SUJ2 Fig.1 60.3 mm 53.7 mm 15.3 mm 103 mm 74 mm 22 mm 80 khz 1300 Arms Fig.2 900 57 s 70 H 0.14 cm 1 100 180 2 h 240 43 s 900 950 1000 3 3.5 12.1% 900 11 316 s 950 3 65 s 1000 0.7 10.3 s 2 2 組織観察方法 Fig.3 SEM 15 2000 ASTM 10 FE-SEM/EBSD 2 3 残留オーステナイト測定 0.1 mm X α 211 γ 220 2 4 脱炭量測定 Fig.3 0.5 mm EPMA 2 m 2 m Table 1. Chemical Composition of the SUJ2 steel used in the present study (mass%). C Si Mn P S Ni Cr Mo Cu 1.02 0.31 0.41 0.008 0.003 0.07 1.33 0.03 0.08 Fig. 2. Temperature variation during induction heating. Fig. 1. Photograph of a test piece. (Online version in color.) Fig. 3. Schematic illustration of the observed cross-section. 21
196 3. 実験結果 3 1 均熱温度 保持時間と残留炭化物量の関係 Fig.4 900 70 s Fig.5 11 Kolomogorov-Johnson-Mehl- Avrami 12 1 2 1 2 f K t s T R n A Q 15.6% 7860 kg/m 3 7680 kg/m 3 13 0% Fig.5 1 2 n A Q T t s M % 3 3 Fig.6 3 R 2 0.968 900 1000 3 3 2 均熱温度および残留炭化物量が材料組織に及ぼす影響 900 950 1000 3 4 8 12% 3 9 900 950 2 4 8 12% 3 6 Fig.7 Fig. 4. Undissolved carbide particles observed by SEM. Fig. 6. Comparison between experimental and predicted values of undissolved carbide fraction(%), according to the formula (3). Fig. 5. Undissolved carbide fraction of all tested conditions, as a function of the duration time with various heating temperatures. Fig. 7. Influence of austenitizing temperature and undissolved carbide fraction on retained austenite content. 22
SUJ2 材の焼入れ組織に及ぼす高周波誘導加熱条件の影響 度が高いほど および残留炭化物量が少ないほど残留オー ト中での炭素の拡散係数は非常に大きい 14 ため 固溶炭素 ステナイト量が多くなる傾向となった 同一の残留炭化物 が十分拡散し 均熱温度が 900 950 の場合と同等の硬さ 量の場合 均熱温度が高い方ほど残留オーステナイト量が が得られていると考えられる 多い傾向にあった Fig.9 に均熱温度を 900 とした場合の旧オーステナイト Fig.8 に硬さ測定結果を示す 均熱温度が 900 の場合 結晶粒を例示する Fig.10 に結晶粒度測定結果を示す 残 残留炭化物量が少ないほど硬くなる傾向にある 均熱温度 留炭化物量が多くなるほど結晶粒度番号が大きくなる傾 が 950 の場合 残留炭化物量 12% の場合よりも 8% の場 向にあるが 残留炭化物量 4% の場合でも結晶粒度番号は 9 合の方が硬いが 4% の場合は逆転する これは 残留オー 番程度であり 炭化物のピン止め効果により結晶粒の粗大 ステナイト量の割合が増大することに起因すると考えられ 化を抑制できている 均熱温度が高いほど結晶粒は大きく る 均熱温度 1000 の場合 残留炭化物量が多い すなわ なるが その差は微少であり 本実験範囲内では均熱温度 ち固溶炭素量が少ない場合でも高硬さであった 均熱温度 の結晶粒度に及ぼす影響は小さいと言える が 1000 の場合は均熱時間が非常に短いが オーステナイ Fig.11 に均熱温度 900 とした場合のマルテンサイトの Fig. 8. I nfluence of austenitizing temperature and undissolved carbide fraction on hardness. Fig. 10. I nfluence of austenitizing temperature and undissolved carbide fraction on prior-austenite grain size. Fig. 9. Prior-austenite grain structures of samples heated at 900 C. Dissolved carbide fraction : (a) 4%, (b) 8%, and (c) 12%. Fig. 11. Martensite IPF maps of samples heated at 900 C. Dissolved carbide fraction: (a) 4%, (b) 8%, and (c) 12% (Online version in color.) 23 197
198 5 1 Table 2 Fig.12 8% 4% 12% 3 3 脱炭量 Fig.13 900 EPMA 2 m ε Table 2. Martensite block size determined from IPF maps. Temperature ( C) 900 950 Undissolved carbide (%) Martensite block size (μm) Max Min. Ave. 4 2.49 0.15 0.35 8 1.77 0.15 0.33 12 3.03 0.09 0.34 4 3.39 0.15 0.37 8 2.13 0.09 0.33 12 2.07 0.15 0.35 Fig.14 12% 4% 8% 8% 0.02 mm 4% 0.08 mm 4. 考察 SUJ2 3 1 n n K Kolomogorov-Johnson-Mehl-Avrami 1 2 2 4 ln ln 1/1-f ln t Fig.5 Fig.15 Fig. 12. Influence of austenitizing temperature and undissolved carbide fraction on martensite block size. Fig. 14. Influence of austenitizing temperature and undissolved carbide fraction on maximum decarburization depth. Fig. 13. Carbon concentration profiles of samples heated at 900 C. Dissolved carbide fraction : (a) 4%, (b) 8%, and (c) 12%. (Broken lines indicate average carbon concentration in each case) 24
SUJ2 199 n K Table 3 950 1000 n 900 n 2 900 950 1000 SUJ2 900 1.02 mass%c Acm n FactSage 900 Fig.16 0.2% 900 n 0.4860 950 1000 n n 900 950 1000 2 1 2 1 910 15 910 900 950 1000 900 910 15 n 2 1 n 950 1000 900 Fig.16 900 3 900 5. 結言 JIS-SUJ2 900 1000 3.5 12.1% Table 3. Values of n and K in fomula (2) for different austenitizing temperatures determined from the plots shown in Fig.15. Temperature ( C) n K 900 0.4754 9.52 10 2 950 0.5733 1.41 10 1 1000 0.5689 3.22 10 1 Fig. 16. Equilibrium phase diagram of SUJ2 steel. Fig. 15. Plots of ln ln(1/(1-f)) vs ln t for different austenitizing temperature. (a) 900 C, (b) 950 C, and (c) 1000 C. 25
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