Microstructures and mechanical properties of TRIP steel produced by strip casting simulated in the laboratory

Abstract

Conventional transformation induced plasticity (TRIP) steel (0.17C-1.52Si-1.61Mn-0.03Al, wt%) was produced via strip casting technology simulated in the laboratory. Effects of holding temperature, holding time and cooling rate on ferrite formation were studied via analysis of the continuous cooling transformation diagram obtained here. A typical microstructure for conventional TRIP steels consisting of ~ 0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite was obtained. However, coarse prior austenite grain size of ~80 μm led to large polygonal ferrite grain size of ~17 μm, coarse second phase regions of ~21 μm size, small amount of retained austenite (0.02-0.045) and the presence of Widmanstätten ferrite. Optimisation of the microstructure-property relationship was reached via a variation in the isothermal bainite transformation temperature. The highest retained austenite fraction of 0.045±0.003 with medium carbon content of 1.23±0.01 wt% was obtained after holding at 400 °C, resulting in the highest ultimate tensile strength of 590±35 MPa and largest total elongation of 0.27±0.05. The presence of TRIP effect in the studied steel was revealed through the analysis of strain hardening exponent and modified Crussard-Jaoul model. Effect of processing parameters on retained austenite retention and stress-strain behaviour was discussed. Conventional transformation induced plasticity (TRIP) steel (0.17C-1.52Si-1.61Mn-0.03Al, wt. %) was produced via strip casting technology simulated in the laboratory. Effects of holding temperature, holding time and cooling rate on ferrite formation were studied via analysis of the continuous cooling transformation diagram obtained here. A typical microstructure for conventional TRIP steels consisting of ~ 0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite was obtained. However, coarse prior austenite grain size of ~ 80 μm led to large polygonal ferrite grain size of ~ 17 μm, coarse second phase regions of ~ 21 μm size, small amount of retained austenite (0.02 -0.045) and the presence of Widmanstätten ferrite. Optimisation of the microstructure-property relationship was reached via a variation in the isothermal bainite transformation temperature. The highest retained austenite fraction of 0.045±0.003 with medium carbon content of 1.23±0.01 wt. % was obtained after holding at 400 °C, resulting in the highest ultimate tensile strength of 590±35 MPa and largest total elongation of 0.27±0.05. The presence of TRIP effect in the studied steel was revealed through the analysis of strain hardening exponent and modified Crussard -Jaoul model. Effect of processing parameters on retained austenite retention and stress-strain behaviour was discussed.

Microstructure and mechanical properties of strip cast TRIP steel subjected to thermo-mechanical simulation

“…14, the combination of strength and ductility after holding at 400 °C was the highest, whether the deformation was applied or not. Without deformation, the best combination of properties after holding at 400 °C was ascribed to the highest RA fraction in the previous study [28]. This phenomenon after holding at different IBT temperatures was reported by many researchers [1,51,52].…”

“…Based on the above discussion, it means that the RA fraction after deformation may only change marginally in the studied TRIP steel; the RA fraction in the non-deformed samples was reported to be between 0.02 and 0.05 [28].…”

“…These samples are denoted as T 350, T 400 and T 450, corresponding to isothermal holding temperatures of 350, 400 and 450 °C, respectively. They are discussed in details elsewhere [28]. As shown in Table 1, deformation resulted in a decrease in the average ferrite grain size from 17±10 to 13±7 μm and the average size of second phase regions from 21±24 to 18±16 μm, indicating a refined and more homogenised microstructure after deformation.…”

“…Some samples were directly water quenched from different T IC to room temperature in order to study the effect of T IC on ferrite formation. Holding at 1250 °C for 300 s was determined in order to simulate the prior austenite grain structure (the average grain size of 83±31 μm) inherent for strip casting, as described in the previous study [28]. Plain strain compression during hot rolling was simulated in the thermo-mechanical experiments.…”

“…For TRIP steels, researchers focus on further development of hot rolling and cold rolling followed by annealing. Except our previous paper, which presented the microstructure and mechanical properties of TRIP steels obtained via the simulation of strip casting without deformation [28], there is no other published study on the production of TRIP steels using strip casting technology. This paper presents a further step in our investigation, namely the effect of austenite deformation on the microstructure and mechanical properties.…”