Diagrams of supercooled austenite transformations of low-carbon and medium-carbon TRIP-steels
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Purpose: The aim of the paper is to determine the influence of cooling conditions on a structure and a shape of CCT-diagrams of TRIP-aided steels. Design/methodology/approach: The diagrams of undeformed supercooled austenite transformations for low-carbon and medium-carbon steels were determined. The specimens were austenitized at a temperature of 1100°C and cooled from a temperature of 900°C with a rate in a range from 1 to 300°Cs-1. The dilatometric tests were 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 for manufacturing TRIP-type steels with multiphase structures. The steels are characterized by large ferritic and bainitic fields and a right-displaced pearlitic range. However, a ferrite fraction obtained after cooling with an optimum rate from a 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 a cooling course in a range of γ→α transformation. It should result in an effective utilization of the time for the transformation of austenite into the fine-grained ferrite. Practical implications: The obtained diagrams of supercooled austenite transformations can be useful in a determination 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 medium-carbon steels containing Nb and Ti microadditions were obtained.
Bibliogr. 16 poz.
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