Effect of hot-working in the gamma+alpha range on a retained austenite fraction in TRIP-aided steel

Purpose: The aim of the paper is to determine the influence of thermomechanical processing conditions on an austenite stability in a TRIP-aided microalloyed steel. Design/methodology/approach: The heat treatment and thermomechanical processing in a two-phase region to obtain ferritic-bainitic structures with the retained austenite in a low-carbon microalloyed steel were conducted. The heat treatment consisted of austenitizing of specimens at 750 degrees centigrade, oil cooling, isothermal holding in a temperature range from 300 to 500 degrees centigrade and slow cooling to room temperature. A part of the specimens was forged with a degree of deformation of 28% before cooling. Optical and transmission electron microscopy were employed for structure observations. The retained austenite amount was determined by X-ray diffraction method. Findings: It was found that hot-working in the gamma+alpha range contributes to a considerable refinement of a ferritic matrix. The grain size of the alpha phase is about 4 micrometres and its volume fraction equals from 60 to 68%. The optimum structure containing 10% fraction of retained austenite was obtained for the specimen forged in the two-phase region and isothermally held at a temperature of 300 degrees centigrade. Research limitations/implications: To determine with more detail the relationship between hot-working conditions and the stability of retained austenite investigations in a wider deformation temperature range should be carried out. Practical implications: The proposed thermomechanical treatment route can be useful in a development of the technology for TRIP-aided low-carbon microalloyed steels with a reduced silicon content. Originality/value: The design thermomechanical treatment conditions made for obtaining the 10% fraction of retained austenite in a steel containing 0.5% Si only in comparison to 1.5% Si concentration used in TRIP-aided steels usually.