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 590980MPa 980MPa 980MPa R TRIP 1 TRIP 234567 TRIP TRIP TDP 23 TRIP TBF 45 TRIP TAM 67 980MPa TBF 8910 1112 TBF 11 1.110-5 /s 0.4%C-TBF TBF R 19 8 31 TBF R TBF R 0.2%C-1.5%Si-1.5%Mn R TBF TBF 0.20C-1.50Si-1.50Mn-0.015P-0.0024S -0.039Almass% 1.2mmFig. 1 a 30mm 3.2mm 1.2mm 900 250350 010000s Fig. 1b TBF M S 1 13 M S ()=550-361(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 =3.5780+0.033C +0.00095Mn +0.005Al +0.0220N 2 Mn Al N R mass% JIS14B 15mm 6mm 1.2mm 25 1mm/min 8.3 10-4 /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 4 16 65mm 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.
bf m R Table 2 TBF R TBF R f 0 1.5 4.2vol%C 0 0.271.01mass% T A f 0 C 0 t A f 0 C 0 TBF TS 12221565MPa TEl 10.116.9% Fig. 3 TBF H T TSTBF 300 Fig. 4 TBF 50 200 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 1617 9 17 18 19 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.
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 1.5 4.2vol 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 9 1.67/min 8792 127 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 1.54.2vol R R
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. 11 21 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.
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