Transformation-Induced Ductility of Reverse Austenite Evolved by Low-Temperature Tempering of Martensite

Abstract

A novel steel combining the “quenching and tempering (Q&T)” process was exploited that can achieve the enhancement of austenite by interface migration during tempering the martensitic matrix mixed with films of austenite. A high uniform elongation (12%) combined with high yield tensile strength (1500 MPa) was obtained, which showed distinct advantages over all the other advanced high strength steels under development for a lightweight car body. Furthermore, the effect of austenite on enhancement of ductility in “Q&T” steels with a martensite matrix was elucidated, which suggested that the ductility was promoted by enhancing boundary sliding and delaying work hardening of the martensitic matrix.

Recent Progress of Low and Medium-Carbon Advanced Martensitic Steels

“…The heat-treatment process of AMS is as simple as the Q&T process of the conventional martensitic steels. In addition, the process after austenitizing is conducted at relatively low temperature, compared with those of TBF, Q&P, and CFB steels [58][59][60][61][62][63][66][67][68][69][70][71][72][73]. This means that quenching in an oil bath can be used for the heat treatment, not a salt bath.…”

“…To optimize the microstructure and mechanical properties of the AMS, ausforming at temperatures between Ac3 and Ms is carried out before the IT, DQ, or DQ-P process [68,[81][82][83], in the same way as TBF [84] and CFB steels [49,85]. Heat treatment diagrams of (a) the isothermal transformation (IT) process below M f and direct quenching to room temperature (RT) followed by partitioning (DQ-P) process in a case of RT < M f [58][59][60][61][62][63] and (b) the DQ and DQ-P process in a case of RT > M f [67][68][69]. RT: room temperature, T IT : isothermal transformation temperature, T Q : quenching temperature, T P : partitioning temperature.…”

“…On the other hand, when the M f of the AMS is lower than room temperature such as for M-MMn steel with a relatively high Mn content of about 10 mass %, the DQ or DQ-P process is carried out [67][68][69] (Figure 2b). For the DQ-P process, He et al [67] adopt the partitioning (tempering) at 300 to 400 • C for 10 min in 0.2%C-10%Mn-2%Al-0.1%V steel.…”

“…(IV). Third-generation AHSS (Type B): TRIP-aided martensitic (TM) steel [58][59][60][61][62][63] and martensite-type medium manganese (M-MMn) steel [53,[64][65][66][67][68][69], which are called advanced martensitic steel (AMS).…”

“…In connection with this, the IT process at low temperatures during martensitestart temperature (M s ) and martensite-finish temperature (M f ) is recently applied to TBF, one-step Q&P, and CFB steels [30][31][32][34][35][36][37][38][39][40][41]43,47,48,[58][59][60][61], which achieve high tensile strength and mechanical properties due to bainitic ferrite/martensite (BF/M) structure matrix. To obtain the tensile strength higher than 1.5 GPa, TM and M-MMn steels with martensitic structure matrix are recently developed [58][59][60][61][62][63][64][65][66][67][68][69]. These steels are classified as the third-generation AHSSs (Type B).…”