Study On Preparation And Performance Of Mn -Series Low & Medium Carbon Bainitic Structural Steels

Non-quenched and tempered steel can have coequal strength withquenched and tempered steel by air-cooling after hot-working treatment. Butplastic property, especially fracture toughness property of non-quenched andtempered steel is so low that it can't be widely used. Tested steels of this workhave a good combinability of σb and Aku. It is vital for extensive use ofnon-quenched and tempered steel.The relation among composition, structure, heat treatment andperformance of low carbon and medium carbon bainitic steel was investigatedby microstructure and general mechanical properties etc. The difference ofstructure and mechanical properties was studied though the increasing Sicontent and a little content of Mo. The results showed that granular bainitemicrostructure could be obtained by air-cooling after austenization in lowcarbon bainitic steel and bainite/martensite mixed microstructure could beobtained by air-cooling after austenization in medium carbon bainitic steel.Temperature of tempering brittleness happening increased and valley value ofcurve reduced though the increasing content of Si. Aku after high temperaturetempering was lower than non-tempered steel when the content of Si is 1.7%.σb and Aku have a good combinability between 0.7~1.4% Si content when thesteel was tempered at 580℃, σb>850Mpa and Aku>160J. Temperature oftempering brittleness happening increased when a few content of Mo wasadded. σ b of Mo steel is lower than free Mo steel at high temperingtemperature. Figure of M/A island transformed from massive island tobanding island to massive island through the increasing content of Si. Studies of alloying(Si and Mo) agent's effect on transition temperatureof bainite by determining CCT curve were carried out in low carbon steel. Theresults showed that the beginning transition temperature of bainite can befurther depressed though the increasing content of Si. And Si had week effecton the beginning transition temperature of bainite when the content of Si wasabove 1.4%. Critical cooling velocity falled down when more Si was added inlow content of Si steel. The curve of high temperature transition in CCTfigure was postponed and the beginning transition temperature of martensitewas depressed though the adding of Mo. Cooling velocity domain of bainitetransition of Mo steel is wider than Mo free steel. All of the above results hadlaid a foundation for the extensive use Mn-bainitic steel as structural steel.