Influence of thermomechanical processing parameters on critical temperatures to develop an Advanced High-Strength Steel microstructure

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.

A new approach to etching low‐carbon microalloyed steels to reveal prior austenite grain boundaries and the dual‐phase microstructure

“…Samples of a steel with relative similar addition of carbide‐former elements with a similar heat treatment but more than the double carbon addition 18 were used to observe the effectiveness of the solution E2 on higher carbon steels. By contrasting the images with the results of etching E2 on V, CrV and CrNbV steels, it was determined a reduction in the amount of HCl was required in solution E2 to obtain good micrographs.…”

Microstructural Evolution in a 0.09% Niobium Low Carbon Steel during Controlled Hot Deformation

“…It is important to highlight the drop in the initial flow stress in the second and mostly the third finishing pass, this behaviour in the flow curves indicates that important restoration took place in the microstructure during the holding time between the finishing passes. The amount of softening obtained in the second and third finishing passes was estimated using the parameter XA, where this softening parameter was developed by Kwon and DeArdo [30] and subsequently used by Romano-Acosta et al [31] to calculate the fractional softening from the area under the flow curves and is expressed as below:…”

“…Evidence of this, can be clearly observed in the second holding time in which the fractional softening was remarkable despite the static recrystallisation of PAGs was negligible (the analysis of recrystallisation is presented in the following section). The amount of softening obtained in the second and third finishing passes was estimated using the parameter X A , where this softening parameter was developed by Kwon and DeArdo [30] and subsequently used by Romano-Acosta et al [31] to calculate the fractional softening from the area under the flow curves and is expressed as below:…”

Microstructural Evolution in a 0.09% Niobium Low Carbon Steel During Controlled Hot Deformation

“…It is important to highlight the drop in the initial flow stress in the second and mostly the third finishing pass, this behaviour in the flow curves indicates that important restoration took place in the microstructure during the holding time between the finishing passes. The amount of softening obtained in the second and third finishing passes was estimated using the parameter X A , where this softening parameter was developed by Kwon and DeArdo [30] and subsequently used by Romano-Acosta et al [31] to calculate the fractional softening from the area under the flow curves and is expressed as below:…”