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

Abstract

Instead of hot rolling and cold rolling followed by annealing, strip casting is a more economic and environmentally friendly way to produce transformation-induced plasticity (TRIP) steels. According to industrial practice of strip casting, rapid cooling in this work was achieved using a dip tester, and a Gleeble 3500 thermo-mechanical simulator was used to carry out the processing route. A typical microstructure of TRIP steels, which included ~0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite, was obtained. The effects of deformation (0.41 reduction) above non-recrystallisation temperature, isothermal bainite transformation temperature and the size of second phase region on microstructure and mechanical properties were studied. The steel isothermally transformed at 400 °C had the best combination of ultimate tensile strength (UTS) and total elongation (TE), whether deformation was applied or not. The deformation resulted in the improvement of mechanical properties after holding at 400 °C: the UTS increased from 590 to 696 MPa and TE decreased from 0.27 only to 0.26. It was predominantly ascribed to grain size refinement and dislocation strengthening. The studied TRIP steel had comparable mechanical properties with TRIP 690 produced commercially. Abstract: Instead of hot rolling and cold rolling followed by annealing, strip casting is a more economic and environmentally friendly way to produce transformation-induced plasticity (TRIP) steels. According to industrial practice of strip casting, rapid cooling in this work was achieved using a dip tester, and a Gleeble 3500 thermo-mechanical simulator was used to carry out the processing route. A typical microstructure of TRIP steels, which included ~ 0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite, was obtained. The effects of deformation (0.41 reduction) above non-recrystallisation temperature, isothermal bainite transformation temperature and the size of second phase region on microstructure and mechanical properties were studied. The steel isothermally transformed at 400 °C had the best combination of ultimate tensile strength (UTS) and total elongation (TE), whether deformation was applied or not. The deformation resulted in the improvement of mechanical properties after holding at 400 °C: the UTS increased from 590 to 696 MPa and TE decreased from 0.27 only to 0.26. It was predominantly ascribed to grain size refinement and dislocation strengthening. The studied TRIP steel had comparable mechanical properties with TRIP 690 produced commercially.

The Effect of Two-Step Heat Treatment Parameters on Microstructure and Mechanical Properties of 42SiMn Steel

“…The optimal annealing hold temperature of thermo-mechanically processed 42SiMn steel would therefore be in the interval of 350-425 • C, depending wheatear higher strength or higher total elongation is desired. The highest formability (RmxA product) was obtained for the hold at 425 • C. This finding corresponds to a previous work on a low carbon 0.17C-1.5Si-1.6Mn TRIP steel, which confirmed the bainitic hold temperature of 400 • C as an optimal choice for both, heat and thermo-mechanical treatment [34]. It should be however noted that unlike the case of low carbon steel, where thermo-mechanical treatment improved mechanical properties of the steel, thermo-mechanical treatment of 42SiMn steel resulted in the formation of a significant amount of pearlite, which deteriorated the mechanical properties in comparison to those that were obtained by heat treatment.…”

Effect Of The Application Of Constant And Variable Temperatures at the Overaging Stage in the Mechanical Properties of Trip780 Steel

“…These values are well above other high strength steels such as Dual Phase (UTS = 900 MPa, UTSxTE = 11.6 GPa%) and Bainitic (UTS = 650MPa, UTSxTE = 8.0 GPa%) and are aligned with TRIP multiphase steels, as shown in the studies by Sugimoto et al 7 In Figures 6 are illustrated the stain hardening exponent results in function of true strain to the 6 conditions tested. The conditions presented typical strain hardening exponent curves of TRIP steels according to Xiong et al 8 , except the condition 6.…”

Microstructure Evolution during the Production of Dual Phase and Transformation Induced Plasticity Steels Using Modified Strip Casting Simulated in the Laboratory

“…This technique has been applied in industry for the production of aluminium, lead, carbon steel, silicon steel and stainless steel [2][3][4]. In laboratory the ferrite-martensite dual-phase (DP) steel [5][6][7], multi-phase transformation-induced plasticity (TRIP) steel [8,9] and twinning-induced plasticity steel [10] have also been successfully produced by strip casting. In comparison with a traditional way to produce steels by the sequence of steel-making, continuous casting, hot rolling and cold rolling, the strip casting technology requires a shorter production line, due to the strips being directly manufactured from liquid metals.…”

“…Recent studies carried out by the present authors produced low-alloyed ferrite-martensite DP [8] and multi-phase TRIP steels [8] using strip casting simulated in the laboratory. Although tensile properties comparable with industrially hot/cold rolled DP and TRIP steels have been achieved, the microstructure is not uniform through strip thickness, due to the cooling rate gradient [8,9]. To avoid this problem, a furnace was introduced into the processing line of strip casting (Figure 1) for the first time in this work, namely the modified strip casting.…”