Ultrahigh Ductility, High-Carbon Martensitic Steel

The twice softening of martensitic matrix in Q–P–T steels and its effect on ductility

“…With the decrease of partitioning time, the tensile strength increases accompanied by a reduction of elongation. The mechanical properties of Q-P-T samples with different partitioning/tempering times at 400°C are shown in Figure 7 [13]. The tensile strength and elongation are 1860 MPa and 28.9% for partitioning time with 600 s, 1910 MPa and 21.7% for 300 s, and 1920 MPa and 18.8% for 60 s, respectively.…”

“…Since in a high-carbon Q-P-T steel twin-type martensite will partially replace dislocation-type martensite, whether DARA effect is suppressed is in question. For the sake, we recently measured the average dislocation densities in martensite and retained austenite in Fe-0.6C-0.03Nb Q-P-T samples and Q&T samples for comparison [13].…”

“…Since retained austenite in Q-P-T steel has more than 0.8 wt% carbon content, the straininduced martensite is twin-type martensite. Figure 9 shows TEM micrographs of strain-induced twinning martensite in deformed Q-P-T specimen after fracture [13]. Based on the following fact, although the V RA (29.1%) in the high carbon Q-P-T martensitic steel is much higher than 16% in medium carbon Q-P-T martensitic steel, the elongation (12.4%) is much less than 20% of the medium carbon Q-P-T martensitic steel.…”

“…This means that the detrimental effect of SIMT on ductility is greater than the enhancing effect of SIMT on ductility in this high carbon Q-P-T martensitic steel. In view of the detrimental effect of SIMT on ductility, we proposed the design idea of Anti-TRIP [13]. We designed another process in which the normalization (NOR) process was taken as a pretreatment for the Q-P-T process [13].…”

“…Matlock and Speer [12] demonstrated that, in ferritic matrix plus metastable retained austenite steel, if the mechanical stability of retained austenite is higher, the ductility is better. Recently, we found the strong negative effect of SIMT in study of high carbon Q-P-T steel, and proposed the design idea of 'Anti-TRIP' [13], from which the ductility of high carbon Q-P-T steel was outstandingly raised by enhancing the mechanical stability of retained austenite. Our studies indicate that the high ductility of martensitic steels is mainly attributed to the dual softening of martensitic matrix caused by partitioning of carbon during Q-P-T process and the DARA effect during deformation, rather than the TRIP effect.…”