Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

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 °C/s to about 100 °C/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 (>0.45) 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.

Transition between static and metadynamic recrystallization kinetics in coarse Nb microalloyed austenite

Abstract: The recrystallization behaviour of a Nb microalloyed steel, under deformation conditions corresponding to the initial stands of a thin slab hot rolling process which is characterized by coarse initial austenite grain sizes and large strains per pass, has been investigated. A range of strain levels were studied ranging from below the critical strain for the onset of dynamic recrystallization, e c , well into the steady state for dynamic recrystallization, e > e ss . There is a transition strain, e c e T e ss , separating the post-dynamic softening strain-dependent range from that which is strain-independent. At strains above e T , the kinetics of recrystallization is independent of strain but strongly dependent on strain rate. Under these conditions, metadynamic recrystallization operates as the prime softening mechanism and the kinetics have been derived. At strains between e c and e T , both metadynamic and classical static recrystallization are involved in post-dynamic softening. A model is derived to predict the evolution of softening as a function of the holding time for different strain ranges, based on the contributions of the individual components to the overall softening.

Carbide precipitation kinetics in austenite of a Nb–Ti–V microalloyed steel

Abstract: The isothermal precipitation kinetics of carbides in both strain-free and strained austenite (γ) of a microalloyed steel were quantitatively investigated through the electrical resistivity and transmission electron microscopy. The (Nb,Ti)C carbides at the interfaces of the undissolved (Ti,Nb)(C,N) carbonitrides were observed at all temperatures in strain-free γ. However, for strain-induced precipitation, above 950 °C, the precipitation of (Nb,Ti)C carbides near the undissolved (Ti,Nb)(C,N) carbonitrides was predominant due to the recrystallization of strained γ. Meanwhile, the fine (Nb,Ti,V)C carbides were homogeneously precipitated in non-recrystallized γ at 850 °C and 900 °C, as well as near the undissolved (Ti,Nb)(C,N) particles. The electrical resistivity method was successfully used to quantitatively measure the isothermal precipitation kinetics of carbides in both strain-free and strained γ. The precipitation-time–temperature diagrams of the carbide in strain-free and strained γ, with nose temperatures of 950 °C, were generated and the precipitation kinetics were greatly accelerated by the applied strain.