Effects of Retained Austenite Characteristics on Delayed Fracture Properties of Ultra High-Strength TBF Steels

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1 Effects of Retained Austenite Characteristics on Delayed Fracture Properties of Ultra High-Strength TBF Steels Tomohiko HOJO To develop the ultra high-strength steels having excellent delayed fracture resistance, effects of retained austenite characteristics on hydrogen absorption properties and delayed fracture properties of ultra high strength low alloy TRIP-aided steels with bainitic ferrite matrix (TBF steels) were investigated. The hydrogen concentration of TBF steels were increased with increasing volume fraction of retained austenite. It was considered that the hydrogen which charged to the TBF steels was mainly trapped in retained austenite and/or on retained austenite/matrix interface. The TBF steel with high carbon concentration of retained austenite indicated high delayed fracture strength. This was expected that (1) hydrogen-assisted and/or stress-assisted martensite transformation of retained austenite was suppressed by existence of the stabilized or carbon enriched retained austenite, (2) hydrogen was trapped in refined interlath retained austenite films and (3) localized stress concentration was relaxed by strain induced plasticity of retained austenite. : TRIP-aided steel, ultra high-strength steel, retained austenite, hydrogen, delayed fracture MPa 980MPa 980MPa R TRIP 1 TRIP TRIP TRIP TDP 23 TRIP TBF 45 TRIP TAM MPa TBF TBF /s 0.4%C-TBF TBF R TBF R TBF R 0.2%C-1.5%Si-1.5%Mn R TBF TBF 0.20C-1.50Si-1.50Mn-0.015P S Almass% 1.2mmFig. 1 a 30mm 3.2mm 1.2mm s Fig. 1b TBF M S 1 13 M S ()= (C)-39(%Mn)-0(%Si) +30(%Al) 1 CMnSiAl mass R f γ0 Mo-Kα

Fig. 1 Hot and cold rolling and heat treatment process of TBF steels. 200211200220 311 14 R C 0 Cu-Kα 200220311 a 10-1 nm2 15 a =3.

3 10-4 /s 16 Table 1 Table 1 Hydrogen charging conditions.

$2% offset proof stress, TS (MPa): tensile strength, TEl (%): total elongation, f 0 (vol%): volume fraction of R, C 0 (mass%): carbon$

2 Fig. 1 Hot and cold rolling and heat treatment process of TBF steels R C 0 Cu-Kα a 10-1 nm2 15 a = C Mn Al N 2 Mn Al N R mass% JIS14B 15mm 6mm 1.2mm 25 1mm/min /s 16 Table 1 Table 1 Hydrogen charging conditions. Table 2 Tensile properties and retained austenite characteristics of TBF steels. T A (): austempering temperature, t A (s): austempering time, YS (MPa): yield stress or 0.2% offset proof stress, TS (MPa): tensile strength, TEl (%): total elongation, f 0 (vol%): volume fraction of R, C 0 (mass%): carbon concentration of R mm 10mm 1.2mm 25 5 DFL Fig. 2 TBF SEM TBF M S Fig. 2 Typical scanning electron micrographs of TBF steels austempered at (a) T A =250 for 10s, (b) T A =350 for 10s and (c) T A =350 for 10000s.

3 bf m R Table 2 TBF R TBF R f vol%C mass% T A f 0 C 0 t A f 0 C 0 TBF TS MPa TEl % Fig. 3 TBF H T TSTBF 300 Fig. 4 TBF Fig. 3 Variations in total charged hydrogen concentration (H T ) as a function of tensile strength (TS) in TBF steels. T A =350t A 10s TBF TBF Fig. 5 σ A t f TBF DFL TS Fig. 6 TBF TS1400MPa TBF SEM Fig. 7 T A =250t A =10s TBF T A =350t A =10s t A =10000s TBF TBF Fig. 5 Typical applied bending stress ( A ) - time to fracture (t f ) curves of TBF steels. Fig. 4 Comparison of hydrogen evolution curves of TBF Fig. 6 Variations in delayed fracture strength (DFL) as a steels. function of tensile strength (TS) in TBF steels.

$and c) fracture surface and (d, e and f) cross$ $sectional area of fracture region of typical TBF$ 10s, (b and e) T A =350 for 10s and (c and f) T

$concentration (H T ) and initial volume fraction$

4 Fig. 7 Scanning electron micrographs of (a, b and c) fracture surface and (d, e and f) cross sectional area of fracture region of typical TBF steels austempered at (a and d) T A =250 for 10s, (b and e) T A =350 for 10s and (c and f) T A =350 for 10000s. Fig. 2 TBF vol R 9 R fcc R bcc Chan 20 R R R Fig. 8 TBF R f 0 H T TBF R 1112 TRIP R T A =350 t A 10s TBF TBF /min R R 12/min R TBF Fig. 8 Relationship between total charged hydrogen concentration (H T ) and initial volume fraction of retained austenite (f 0 ) of TBF steels. TBF R R R TBF TBF vol R R

5 TBF R Fig. 9 Fig. 10 TBF DFL R C 0 Fig. 9 TBF Fig. 9 Relationship between ratio of applied stress to yield stress ( A /YS) and volume fraction ratio (f /f 0 ) of TBF steels. Fig. 10 Relationship between total charged hydrogen concentration (H T ) and initial volume fraction of retained austenite (f 0 ) of TBF steels. R R TBF R R TBF R Fig. 10 TBF R TRIP R Fig R T A =250t A =10s TBF 1 R M S T A =350 t A =10s TBF TBF R M S Fig. 11 (b)t A =250t A =10s TBF R T A =350t A =10s TBF R M S T A =250t A =10s TBF M S R TBF R TBF R TBF R R R R Fig. 11 Influence of temperature on (a) martensite transformation mechanisms and (b) free energy change.

6 Hashimoto, T. Kashima and S. Ikeda: The Effects of Heattreatment Conditions on Stretch-flangeability and Bendability of High-strength Low Alloy TRIP-aided Sheet Steels with Annealed Martensitic Matrix, Tetsu-to-Hagane, 89 (2003), TBF R TBF 7K. Sugimoto, A. Kanda, R. Kikuchi, S. Hashimoto, T. Kashima and S. Ikeda: Ductility and formability of Newly Developed High Strength Low Alloy TRIP-aided Sheet Steels with Annealed Martensite 1TBF Matrix, ISIJ Int., 42 (2002), :,, (1989), 18. T A =350t A 10s TBF TBF 9, :, R R,, (1997), T. Hojo, S. Song, K. Sugimoto, A. Nagasaka, S. Ikeda, H. Akamizu and M. Mayuzumi: Hydrogen Embrittlement of Ultra High Strength Low Alloy 2TBF TS1400MPa TRIP-aided Steels, Tetsu-to-Hagane, 90 (2004), R 11T. Hojo, K. Sugimoto, Y. Mukai, H. Akamizu and S. R R Ikeda: Hydrogen Embrittlement Properties of Ultra High-Strength Low Alloy TRIP-Aided Steels with Bainitic Ferrite Matrix, Tetsu-to-Hagane, 92 (2006), T. Hojo, K. Sugimoto, Y. Mukai and S. Ikeda: Effects of Aluminum on Delayed Fracture Properties of Ultra High-strength Low Alloy TRIP-aided Steels, Tetsu-to-Hagane, 93 (2007), :,, (1970), H. Maruyama: X, J. Jpn. Soc. Heat Treat., 17 (1977), D. J. Dyson and B. Holmes: Effect of Alloying Additions on the Lattice Parameter of Austenite, J. Iron Steel Inst., 208 (1970), T. Tsuchida, T. Hara and K. Tsuzaki: Relationship between Microstructure and Hydrogen Absorption Behavior in a V-bearing High Strength Steel, Tetsu-to-Hagane, 88 (2002), A. Kimura and H. Kimura: Effect of Carbon on the 1V. F. Zackay, E. R. Parker, D. Fahr and R. Bush: The Hydrogen Induced Grain Boundary Fracture in Iron, Enhancement of Ductility in High-Strength Steels, J. Jpn. Inst. Met., 47 (1983), Trans. Am. Soc. Met., 60 (1967), J. L. Gu, K. D. Chang, H. S. Fang and B. Z. Bai: 2 K. Sugimoto, M. Kobayashi and S. Hashimoto: Delayed Fracture Properties of 1500MPa Ductility and Strain-Induced Transformation in a Bainite/Martensite Dual-phase High Strength Steel High-Strength Transformation-Induced Plasticity and Its Hydrogen Traps, ISIJ Int., 42 (2002), -Aided Dual-Phase Steel, Metall. Trans. A, 23A (1992), : 3O. Matsumura, Y. Sakuma, Y. Ishii and J. Zhao: Effect, of Retained Austenite on Formability of High,, (1997), Strength Sheet Steels, ISIJ Int., 32 (1992), 20S. L. I. Chan, H, L, Lee and J. R. Yang: Effect of Retained Austenite on the Hydrogen Content and 4K. Sugimoto, J. Sakaguchi, T. Iida and T. Kashima: Effective Diffusitivity of Martensitic Structure, Stretch-flangeability of a High-strength TRIP Type Metal. Trans. A, 22A (1997), Bainitic Sheet Steel, ISIJ Int., 40 (2000), G. B. Olson and M. Arzin: Transformation Behavior 5K. Sugimoto, M. Tsunezawa, T. Hojo and S. Ikeda: of TRIP Steels, Metall. Trans. A, 9A (1978), Ductility of C-1.5Si-1.5Mn Ultra High-strength TRIP-aided Sheet Steels with Bainitic Ferrite Matrix, ISIJ Int., 44 (2004), K. Sugimoto, R. Kikuchi, M. Tsunezawa, S.

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