Purpose: The aim of the paper is to determine the influence of cooling conditions on a structure and a shape ofCCT diagrams of TRIP aided steels.Design/methodology/approach: The diagrams of undeformed supercooled austenite transformations forlow carbon and medium carbon steels were determined. The specimens were austenitized at a temperature of1100°C and cooled from a temperature of 900°C with a rate in a range from 1 to 300°Cs1. The dilatometric testswere carried out by the use of the DIL805A/D dilatometer with a LVDT type measuring head.Findings: It was found that obtained CCT diagrams of low carbon and medium carbon steels are favourable formanufacturing TRIP type steels with multiphase structures. The steels are characterized by large ferritic and bainiticfields and a right displaced pearlitic range. However, a ferrite fraction obtained after cooling with an optimum rate froma temperature of 900°C is low. Increasing the fraction of the α phase requires two stage cooling after austenitizing.Research limitations/implications: To obtain the optimum ferrite fraction, it is necessary to modify acooling course in a range of γ→α transformation. It should result in an effective utilization of the time for thetransformation of austenite into the fine grained ferrite.Practical implications: The obtained diagrams of supercooled austenite transformations can be useful in adetermination of a cooling course from a finishing rolling temperature for sheets with a multiphase structure.Originality/value: The diagrams of the undeformed supercooled austenite for the low carbon and mediumcarbon steels containing Nb and Ti microadditions were obtained
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