Notch-Fatigue Properties of Advanced TRIP-Aided Bainitic Ferrite Steels
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NTRODUCTION
FOR the past decade, the 2nd- and 3rd-generation advanced high-strength steels such as 5 to 25 pct Mn transformation-induced plasticity (TRIP)/twinninginduced plasticity (TWIP) steels,[1–3] quench and partitioning steel,[4] and TRIP-aided bainitic ferrite (TBF) steel[5–8] have been developed to reduce the weight of the automotive body in white and to improve impact safety. The TBF steel is characterized by mixed structure of bainitic ferrite matrix and metastable retained austenite films and particles. It is produced by austempering at temperatures above the Ms temperature. However, its mixed microstructure contains some amount of martensite when austempered at temperatures below the Ms temperature. Because of the characteristic microstructure, the TBF steels possess a good combination of tensile strength and stretch flangeability.[6–8] Recently, 980 to 1180 MPa grade TBF steels have been applied to some automotive parts in Japan. The TBF steel also exhibits high fatigue limit[9] and low notch-sensitivity for fatigue,[10] as well as an excellent impact toughness[11] and high delayed fracture strength.[12] So, some applications to the diesel engine common rail system, which needs high inner pressure above 300 MPa, can be expected if the TBF steel has a high hardenability and high notch-fatigue limit. However, there is not any research on notch-fatigue properties of the TBF steel with high hardenability. In the current study, Cr, Mo and/or Ni were added into a 0.2 pct C-1.5 pct Si-1.5 pct Mn-0.05 pct Nb TBF NOBUO YOSHIKAWA and JUNYA KOBAYASHI, Graduate Students, are with the Graduate School, Shinshu University, 4-17-1, Wakasato, Nagano 380-8553, Japan. KOH-ICHI SUGIMOTO, Professor, is with the Department of Mechanical Systems Engineering, Shinshu University. Contact e-mail: [email protected] Manuscript submitted December 18, 2011. Article published online June 27, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
base steel. The notch-fatigue properties of these TBF steels were investigated and compared with those of commercial Cr-Mo–bearing structural steels (SCM420, SCM435, and SCM440 steels), which are used as the base steels for the practical common rail. In addition, the notch-fatigue limit and notch sensitivity were related to metallurgical factors such as microstructure and retained austenite characteristics.
II.
EXPERIMENTAL PROCEDURE
In the current work, five kinds of steel bars A through E with different Cr, Mo, and Ni contents were prepared by vacuum melting, followed by hot forging and hot rolling. The chemical compositions of steels A through E are listed in Table I, where Nb of 0.05 pct is added to refine the prior austenitic grain. For comparison, commercial SCM420 (F) and SCM435 (G) steels and vacuum-melted SCM440 (H) steel were used. After smooth and notched specimens for tension and fatigue tests (Figure 1) were machined from the steel bars, heat treatment illustrated by Figure 2 was conducted in salt baths for steels A through E. For steels F, G, and H, quenching in oil after austenit
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