The Effect Of Sulfide On The Microstructure Of Non-Quenched And Tempered Medium Carbon Steel

Non-quenched and tempered microalloyed steels have been used more and more widely due to their advantages of more energy saving, shorter production period and lower cost in comparison with quenched and tempered steels. This work is done as one part of the"10.5"National Science and Technology Program"Research and Development of High Performance Non-quenched and Tempered Steels", focusing on controlling of the morphology and size of sulfides and its effect on microstructure and mechanical properties.The effect of sulfides on microstructure refining has been studied for a medium carbon high sulfide steel in industrial production route of 30t EAF(Electric Arc Furnace) + LFV(Ladle Furnace Vacuum) + MC(Mould Casting) + CR(Continuous Rolling). Results show that sulfides can be refined and disperse evenly after continuous multi-pass rolling, with the average width of sulfides being 4.91μm, the average length being 14.27μm, the average aspect ratio being 3.03, and the average area being 44.72μm2. The sulfides can refine the microstructure due to their pinning effect, and the reefing effect shows Zener relation.De-oxidation process and sulfide modification process have been studied in a 10 kg non-vacuum melting furnace for a non-quenched and tempered microalloyed steel 38MnVS6, which is used as crankshaft of cars. Results show that Al de-oxidation and Si-Ca-RE treatment can result in fine and dispersedly distributed oxide and sulfide inclusions. Sulfides with oxides inside can act as the nucleation site for intra-ferrite and ferrite can form in the Mn-poor region around MnS, which can also play a role in refining ferrite. Simulated studies on controlled forging and controlled cooling have been carried out for the 38MnVS6 steel produced in the route of 30t EAF + LFV + MC. Results show that vanadium and titanium cabonitrides formed on sulfides during cooling after forging can act as nucleation site of intra-granular ferrite, resulting in an increase in the amount and a decrease in the average size of ferrites. The Mn-poor region formed in the vicinity of MnS during reheating can further increase the amount and decrease the average size of ferrites. On the effect of microstructure refinement by pinning austenite grain boundary and formating intra-grain ferrite, the 38MnVS6 steel has a good impact flexibility at high and low temperature.