Decomposition kinetics of supercooled austenite in moderate-carbon pipe steel

Abstract

The mechanical properties of hot-rolled pump, compressor, and casing pipe made from moderate-carbon steel are largely determined by the content of various decomposition products of supercooled austenite (pearlite, excess ferrite, bainite) in the structure and by their morphology [1‐4]. The type and quantity of structural components depends on the composition of the steel (specifically, on the stability of the supercooled austenite), which, in turn, is affected by the rolling conditions, especially in the final stages of pipe production (the initial and final temperatures of reduction, the postdeformation cooling rate, etc.). In recent years, a group of dispersion-hardening Cr‐Mn‐Mo steels has been developed for the production of high-strength hotrolled pump and compressor pipe at OAO Sinarskii Trubnyi Zavod (SinTZ), in collaboration with the Russian Research Institute of the Pipe Industry and Ural State Technical University [4]. To this end, thermokinetic decomposition diagrams of supercooled austenite must be plotted both for the 37 E2e and 48 E2Aa steel traditionally used at SinTZ and for new steel. The influence of the austenitization temperature and hot plastic deformation on this diagram must be investigated. We consider the group of moderate-carbon pipe steels based on the compositions Mn‐Si and Mn‐Cr, microalloyed with vanadium (0.05‐0.10%), niobium (around 0.04%), and molybdenum (0.10‐0.20%). The thermokinetic decomposition diagrams of supercooled austenite are plotted from dilatometric, microstructural, and durometric data. To study the influence of various technological factors on the decomposition kinetics of supercooled austenite, 13 × 13 mm rods of the given steels are subjected to hot rolling on a laboratory mill, with 15% reduction. The laboratory conditions approximate the final operations in the production of hotrolled pipe. As is evident from Fig. 1, the decomposition of supercooled austenite in the given steels is characterized by clear separation of both decomposition stages and increased stability of the supercooled austenite in the region of diffusional transformations. It follows from direct measurements in the SinTZ shop [3] and from combined analysis of the microstructure and hard