Effect of particles on the grain growth of austenite in microalloyed low carbon steels

Abstract

Second phase precipitates are effective in refining grain size by pinning the austenite grain boundaries and retarding grain growth. The purpose of this study was to investigate the effect of particles on the austenite grain growth in different compositions of low carbon steels at various reheating temperatures and holding time. To investigate this, low carbon steel of four various compositions were used and these were reheated and held at various temperatures before being quenched in water. Mean austenite grain diameter was determined using the matrox inspector software and grain size distribution was plotted. Grain coarsening temperature of steels was determined by plotting mean austenite grain size against reheating temperature and locating the grain coarsening temperature range. Transmission electron microscopy of carbon extraction replicas was carried out to determine the size, distribution, and composition of particles. Weight fraction of the precipitates in the steel was deduced and related to the Zener’s equation to study the R/r vs fv relationship. The findings of this research suggest that the addition of microalloying niobium and titanium to low carbon steel resulted in the formation of TiN, TiC, and NbC precipitates that inhibit austenite grain growth and refine the grain size. The presence of particles is responsible for initiating abnormal grain growth and as a result the grain size distribution (GSD) shifts from unimodal to bimodal. A higher volume fraction of precipitates is responsible for effectively pinning austenite grain boundaries and shifts the grain coarsening temperature to higher values. On the basis of Zener’s equation $＼frac{R}{r}=＼frac{k}{f_v＾n}$, fine grain size can be achieved either by increasing the volume fraction of precipitates (fv) in the steel matrix, or by decreasing the particle size (r). The study of R/r vs fv relationship showed that an accurate estimation of the weight fraction of precipitates would result in the Zene...