Ð/Ð-Constituent in Bainitic Low Carbon High Strength Steel Structure. Part 1

Results are provided for experimental studies of conditions for the formation of various types of M/A-constituent (structural component consisting of martensite and austenite) in the microstructure of low-carbon pipe steel specimens produced by rolling with thermomechanical treatment (controlled rolling and accelerated cooling). As a result of experiments different types of M/A constituent are obtained and the temperature-time conditions for their formation are determined. A research procedure is developed making it possible with maximum confidence to determine the type, size and volume fraction of M/A-constituent in the microstructure. Depending on cooling conditions and exposure temperature, two critical states of the M/A-constituent are identified: at high exposure temperature, predominantly twinned martensite is formed, and austenite is formed at a low temperature. At intermediate temperatures, various combinations of these phases are observed, while the fraction of martensite in the composition of M/A "islands" decreases with decreasing exposure temperature, but the fraction of austenite increases.