Fracture Toughness of an Advanced Ultrahigh-strength TRIP-aided Steel

Abstract

The fracture toughness of an advanced ultrahigh-strength 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.05%Nb (in mass%) transformation-induced plasticity (TRIP)-aided steel with a bainitic ferrite and/or martensite structure matrix was investigated for applications in automobiles, construction machines, and pressure vessels. After the steel was austenitized and isothermally transformed via heat treatment at temperatures between 200°C and 350°C below the martensite-finish temperature, it exhibited a good combination of tensile strength (1.4 GPa) and total elongation (15%). In addition, the steel achieved a much higher plane-strain fracture toughness (KIC = 129-154 MPa m 1/2 ) than conventional structural steel such as SCM420 steel (KIC = 57-63 MPa m 1/2 ). Surprisingly, the fracture toughness was nearly the same as that of a maraging steel. Our results indicate that the high fracture toughness was associated with (1) a softened wide lath-martensite matrix with a low carbide content and carbon concentration and (2) effective plastic relaxation of localized stress concentration by the strain-induced transformation of fine metastable retained austenite in the narrow lath-martensite and retained austenite mixture, which suppresses void formation and cleavage crack initiation at the pre-crack tip.

Microstructure and Mechanical Properties of a TRIP-Aided Martensitic Steel

“…Liquid nitrogen, dry ice, ethyl alcohol, and water were used to cool and warm the specimens. The impact properties were evaluated by determining the Charpy impact absorbed value (CIAV) and ductile-brittle transition temperature (DBTT) of the specimens [15][16][17]. Fatigue tests were carried out using a multi-type fatigue testing machine at 25°C, with a sinusoidal wave of 80 Hz.…”

“…Because TM steel also possesses superior mechanical properties such as toughness [15][16][17], fatigue strength [18,19], and hydrogen embrittlement resistance [20] as compared to conventional structural steel (Fig. 2), application of TM steel to automotive drivetrain components such as gears, drive shafts, CV joints, clutch plates, etc.…”

An Overview of Fatigue Strength of Case-Hardening TRIP-Aided Martensitic Steels

“…C−Si−Mn−Cr−Mo−Ni TM steels with tensile strength higher than 1470 MPa can be produced by the multi-step heat-treatment consisting of austenitization and subsequent isothermal transformation at temperature below martensite-finish temperature (Mf) shown in Figure 1 [11][12][13][14][15][16][17][18][19][20][21]. On cooling to the isothermal transformation temperature, most of the austenite first transforms to soft α′-martensite with wide lath structure, with a small amount of untransformed austenite.…”

“…It is noteworthy that the carbide fraction is much lower than that of the conventional low alloy martensitic steels with low content of Si. Multi-step heat-treatment diagram (austenitizing and subsequent isothermal transforming, followed by partitioning) of TM steel [16]. Isothermal transformation time is tIT = 1000 s. Subscripts "IT" and "P" denote isothermal transforming and partitioning, respectively.…”

“…Recently, four types of transformation-induced plasticity (TRIP)-aided steels with different matrix structures and different chemical compositions, namely "TRIP-aided bainitic ferrite (TBF) steel" [1][2][3][4][5][6][7][8][9][10], "TRIP-aided martensitic (TM) steel" [11][12][13][14][15][16][17][18][19][20][21], "quenching and partitioning (Q&P) steel" [22][23][24][25][26][27][28][29][30][31], and "medium Mn TRIP-aided steel" [32][33][34][35][36][37][38][39] have been developed as third-generation advanced high-strength steels (AHSSs) for automotive sheet products. These steels possess high toughness, fatigue strength and delayed fracture strength, as well as excellent cold formability, especially in TM steels [7,11,[16][17][18][19][20]. Therefore, these steels are exp...…”