Transformation behaviour of Si and Mn bearing low carbon steels

摘要

The transformation behaviour of two low carbon steels was studied by means of isothermal heat treatments. Mass contents of silicon and manganese in these steels were: 2.03 and 0.70% for one of them, and 0.77 and 1.66% for the other. The critical transformation temperatures were determined by dilatometric analysis, the isothermal treatments were designed from these data. Samples of the steel with the higher manganese content were heated for periods of times of 5 and 10 minutes at temperatures of 760 to 780℃, and later were held at 410℃ for periods of time ranging from 10 to 600 seconds. Specimens cut from the steel with the higher silicon content were held at 860℃ for 10 minutes before being treated at 410℃ for periods of time ranging from 10 to 1800 seconds. Progress of the isothermal transformation reactions were studied by X-ray diffraction and metallographic means, a colouring etchant was used to quantify the structures present in the material. It was found that the amount of retained austenite, as well as its carbon content, in the steel with the higher amount of manganese varied with time, independently of the temperature used for the first stage of treatment, with the higher values encountered towards the shorter times. In contrast, for the steel containing the higher silicon levels neither the amount of retained austenite, nor its carbon content were found to vary.