Abstract
A good selection of the thermomechanical processing parameters will optimize the function of alloying elements to get the most of mechanical properties in Advanced High-Strength Steels for automotive components, where high resistance is required for passenger safety. As such, critical processing temperatures must be defined taking into account alloy composition, in order for effective thermomechanical processing schedules to be designed. These critical temperatures mainly include the recrystallization stop temperature (T5%) and the transformation temperatures (Ar1, Ar3, Bs, etc.). These critical processing temperatures were characterized using different thermomechanical conditions. T5% was determined through the softening evaluation on double hit tests and the observation of prior austenite grain boundaries on the microstructure. Phase transformation temperatures were measured by dilatometry experiments at different cooling rates. The results indicate that the strain per pass and the interpass time will influence the most on the determination of T5%. The range of temperatures between the recrystallized and non-recrystallized regions can be as narrow as 30 °C at a higher amount of strain. The proposed controlled thermomechanical processing schedule involves getting a severely deformed austenite with a high dislocation density and deformation bands to increase the nucleation sites to start the transformation products. This microstructure along with a proper cooling strategy will lead to an enhancement in the final mechanical properties of a particular steel composition.
Highlights
Advanced High-Strength Steels (AHSS) have been introduced in the automotive industry due to their good combination of ductility and resistance
For AHSS, the temperature in the last pass of the thermomechanical rolling schedule should be higher than Ar3 to ensure austenite deformation but low enough to avoid recrystallization
Microstructural analysis of the prior austenite grains from double hit tests is the most trustful method to detect the presence of recrystallization after a deformation pass
Summary
Advanced High-Strength Steels (AHSS) have been introduced in the automotive industry due to their good combination of ductility and resistance. AHSS family includes transformation induced plasticity (TRIP), twinning induced plasticity (TWIP), martensitic steel, dual phase (DP) and complex phase (CP) steels. All AHSS microstructures are produced through a good control on the thermomechanical processing (TMP) design [1, 2]. The good mechanical properties of AHSS are the result of the refined microstructure. The microstructure typically consists of a combination of ferrite, bainite and martensite, and in certain conditions, retained austenite and/ or pearlite may be present in relatively small amounts [3]. The volume fraction of these phases will depend on the thermomechanical processing route applied to the steel
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