Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels

Abstract

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of low-and ultralow-carbon microalloyed steels containing additions of boron and/or molybdenum to enhance hardenability. Each alloy was subjected to simulated recrystallization and nonrecrystallization rolling schedules, followed by controlled cooling at rates from 0.1 ЊC/s to about 100 ЊC/s, and the corresponding continuous-cooling-transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element contents, through to bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. Combined additions of boron and molybdenum were found to be most effective in increasing steel hardenability, while boron was significantly more effective than molybdenum as a single addition, especially at the ultralow carbon content. Severe plastic deformation of the parent austenite (Ͼ0.45) markedly enhanced PF formation in those steels in which this microstructural constituent was formed, indicating a significant effective decrease in their hardenability. In contrast, in those steels in which only nonequilibrium ferrite microstructures were formed, the decreases in hardenability were relatively small, reflecting the lack of sensitivity to strain in the austenite of those microstructural constituents forming in the absence of PF.

Modeling of CCT Diagrams and Ferrite Grain Size Prediction in Low Carbon Nb–Mo Microalloyed Steels

“…(9) and (10) are compared to experimental values. In this case the hardness values reported by Cizek et al 13) for a Nb microalloyed steel (steel A, 0.046%C-0.212%Si-1.64%Mn-0.045%Mo-0.029%Nb-0.015%Ti) and a Nb-Mo microalloyed steel (steel B, 0.043%C-0.21%Si-1.67%Mn-0.26%Mo-0.022%Nb-0.016%Ti) are also included in the figure in terms of model validation. These steels were submitted to two thermomechanical cycles designed to produce different austenite conditions: non-deformed austenite and deformed austenite.…”

“…On one hand, it seems reasonable that the terms %C, %Mn, %Nb and %Mo contain a negative sign, denoting the retardation effect on the transformation associated with these elements in solid solution. Cizek et al 13) found that microalloying elements cause a decrease in transformation start temperature. Nb and Mo tend to segregate at the austenite boundaries and deformation-induced boundaries.…”

Low-Carbon Bainite Steel with High Strength and Toughness Processed by Recrystallization Controlled Rolling and Ultra Fast Cooling (RCR+UFC)

“…6, showing microstructure consisting of granular bainite (GB), upper bainite (UB) and lath bainite (LB) distinguished based on bainite features. 21,22) The granular bainite (denoted by white arrow in Fig. 6(a)) consists of lath ferrite with low misorientations, containing equiaxed M/A constituents, but these lath boundaries can not be observed in OM pictures.…”

Influence of Deformation Temperature on γ-α Phase Transformation in Nb–Ti Microalloyed Steel during Continuous Cooling

“…[2][3][4][5][6][7][8][9][10][11][12] These studies mainly focus on effect of chemical compositions, austenite state (undeformed or recrystallized austenite and deformed austenite) or cooling rate on phase transformation behaviors, however, systematic work on influence of deformation temperature on Ar3 temperature and quantitative relationship between Ar3 temperature, deformation temperature and cooling rates is lack. Bengochea et al 4) and Eghbali et al 5) mainly indicated influence of deformation temperature and prior austenite microstructure on ferrite grain refinement, respectively.…”