The Effect of Cooling Rate, and Cool Deformation Through Strain-Induced Transformation, on Microstructural Evolution and Mechanical Properties of Microalloyed Steels

Abstract

In this article, a detailed study was conducted to evaluate the microstructural evolution and mechanical properties of microalloyed steels processed by thermomechanical schedules incorporating cool deformation. Cool deformation was incorporated into a full scale simulation of hot rolling, and the effect of prior austenite conditioning on the cool deformability of microalloyed steels was investigated. As well, the effect of varying cooling rate, from the end of the finishing stage to the cool deformation temperature, 673 K (400°C), on mechanical properties and microstructural evolution was studied. Transmission electron microscopy (TEM) analysis, in particular for Nb containing steels, was also conducted for the precipitation evaluation. Results show that cool deformation greatly improves the strength of microalloyed steels. Of the several mechanisms identified, such as work hardening, precipitation, grain refinement, and strain-induced transformation (SIT) of retained austenite, SIT was proposed, for the first time in microalloyed steels, to be a significant factor for strengthening due to the deformation in ferrite. Results also show that the effect of precipitation in ferrite for the Nb bearing steels is greatly overshadowed by SIT at room temperature.

Superior mechanical properties of microalloyed steels processed via a new technology based on austenite conditioning followed by warm deformation

“…Tensile stress-strain curves of the two studied steels in two processing conditions Comparison of mechanical properties obtained in this work to those after contemporary processing showed significant advantages of the AC 2 WD-technology. For the Ti-steel, similar or slightly higher strength and ductility were obtained here for up to 6.6 times lower Ti content in steel composition after a more simple processing schedule, namely reduced number of deformation passes (compare this work with [29,31] in Table 1). For the same number of deformation passes, the AC 2 WD-technology resulted in 150-200 MPa higher YS and UTS with similar El for a 5 times lower Ti content (compare this work with [30] in Table 1).…”

Characterisation on Al-bearing hot-rolled TRIP steel produced through isothermal bainite transformation

“…As mentioned above, Nb refined prior austenite and proeutectoid ferrite, where grain refinement was beneficial to obtain higher tensile strength and total elongation. In addition, the solid solution of Nb in matrix microstructures also contributed to the increase in tensile strength [27]. The TEM observations confirmed the presence of Nb(C,N) precipitates in Nb-alloyed steel.…”

High strength, low carbon, Cu containing steel plates with tailored microstructure and low yield ratio

“…The solution temperature of Nb(C, N) in the current steel was estimated as 1124°C. 9 This was carried out based on equations (1) and (2) by which the presence of Mn and Si (in wt-%) on the solubility of Nb(C,N) was taken into account 11 In the production line, equipped with a 5 m wide rolling mill as described elsewhere, 12 the slab was reheated at 1200°C for 120 min where Nb(C, N) was taken fully into solution, and then rolled by rough rolling in the austenite recrystallisation region, then finish rolling in the austenite non-recrystallisation region. Key processing parameters are listed in Table 2.…”