Effect of aluminum content on austenite-ferrite transformation temperature in low carbon (Si-Al) hot rolled GNO electrical steels

Abstract

The aim of the present study is to investigate the synergistic effect of silicon and aluminum content on the austenite-ferrite transformation temperatures on cooling (Ar3, Ar1) in non-oriented (GNO) Al-Si-low carbon electrical steel strips. Two specimens with different Al contents: A=0.22 wt% and B=0.61 wt%Al, were analyzed by "in-situ" high temperature X-ray diffraction experiments. The samples were austenitized at 1050 °C for 5 minutes and sequentially cooling in a stepwise manner by steps of 10 °C inside an environmental chamber installed in a Philips X'Pert multi-purpose diffractometer. X-ray diffraction patterns were recorded every 10 °C during cooling from 1000 to 720 °C. The austenite to ferrite transformation temperatures on cooling, Ar3 and Ar1, were estimated from changes in the intensities of the (110)-α and (111)-γ peaks as a function of temperature. The results of the experiments show that the transformation temperatures increase with increasing aluminum content from 0.22 to 0.61 wt%. In addition, the two-phase field (austenite + ferrite in the system Fe-C decreases with increasing silicon and aluminum contents for these GNO steels. X-Ray diffraction results were supported by microstructural observations of quenched samples of steel B which were subjected to similar heat treatment conditions than those applied in the X-ray diffractometer experiments. Thin ferrite films (∼ 4-10 um) were observed in the microstructure of specimens of steel B quenched from temperatures close to the experimental Ar3.