High-strength structural steels with a structure of low-carbon bainite subjected to thermomechanical treatment

Conclusions The development of TMT for low-pearlite and pearlite-free steels has led to the development of steels containing 0.06% C, 1.8% Mn, 0.3% Mo, and 0.05–0.09% Nb. Depending on the type of TMT, the most important parameters of which are the final rolling temperature and the total deformation at temperatures below 900°, the transformation occurs partially or completely in the intermediate region. The high dislocation density of the bainite has a substantial effect on the increase of the yield strength.Large rolling reductions at temperatures below 870° favor the formation of ferrite nuclei. The polygonal ferrite formed in this case has a characteristic grain size of grade 13–14.Rolling under laboratory conditions showed that by selecting the conditions of TMT one can control the mechanical properties of the steel. A guaranteed yield strength of 70 kgf/mm² and transition temperature T₅₀=–25° can be obtained on plates 13 mm thick, while after rolling under other conditions the ductile-brittle transition temperature is –125° and the yield strength 45 kgf/mm². Numerous production tests indicate that a yield strength of at least 50 kgf/mm² in the hot rolled condition can be obtained for plates 20 mm thick. In rolling mills permitting substantial reduction at low temperatures these values can be improved. For example, a yield strength of 56 kgf/mm² and transition temperature of –60° can be obtained for plates 30 mm thick. The steel has substantial reserves of strength due to precipitation hardening, which amount to 8–12 kgf/mm² after tempering at 600–625°. The weldability of the steel is good due to its low carbon content.Stahl und Eisen,93, No. 22, 1041 (1973).Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 11–18, December, 1975.