Optimization Reasearch On Strength And Toughness Of Low Carbon Mn-series Air Cooling Bainitic Steel

Mn-series banitic steels with carbon content 0.07wt%-0.30wt% have been selected as the test materials in this paper. The methods of optimization on the strenghth and toughness of the steel have been systematically studied. The microstructures of Mn-series banitic steels of different carbon content and Si content and the changing of properties of the steels after air cooling and tempering have been studied. Phase transformation and the morphology of islands of granular structure were studied. The steels with the carbon content of about 0.08% were tested on Gleeble thermal-mechanical simulator machine, and the microstructures of non micro-alloyed and micro-alloyed steels were investigated after deformation at different temperature. The researching results were verified well by means of control rolling on testing rolling mills. The main conclusions of the paper are drawn as follows:1. Si almost has no influence on the microstructures of the steels with the carbon content 0.07～0.08%, but can improve the hardenbility of the steels with the carbon content 0.17%.2. The morphologies of M/A islands in the granular structure are two types: irregular and regular. The parallel islands are sensitive to form when the matrix grow in ledgewise mechanism. The islands parallelly or irregularly distributing are not the criterion of granular bainite and granular structure. The transformation of granular structure can be efficiently inhibited by Cr, so there are no islands in the proeutcoid ferrite.3. The huge macro residual stress will be remained in Mn-series bainitic steels after air cooling, result in low yield strength, and the stress will be efficiently eliminated after tempering at low temperature and the optimum temperature is about 400℃. The strength and toughness of the high Si content of the steels can match better after tempering at low temperature.4. The content of ferrite in low carbon Mn-series increases with the deformation temperature decreased, and the grains and toughness are refined and improved. The microstructures of the Nb micro-alloyed steel are more refined on the same deformation process, and the ferrites keeps a relatively stable content when the deformation temperature decrease to about 760℃. However the grain refined further and the transformation of ferrites turns to nucleation dominated process. The strength and toughness of Nb Micro-alloyed Mn Series Bainitic Steels are more sensitive to the finishing rolling temperature, and the properties of Nb Micro-alloyed steels can be improved more effectively with decreasing the finishing rolling temperature.5. The reinforced bar with the yeild strength 930MPa has been developed and the microstructure is B/M dual phase, the process is simple and cost is low, and successfully industrialized. The non-moderating medium plate with high strength and toughness has been developed, and the microstructure includes FGBA,intragranular ferrite,granular bainite, the properties are: R0.2﹥600MPa, Rm﹥800MPa, AKV(0℃)﹥150J after control rolling and air cooling。...