Study On The Smelting Process Of High Grade Free-Cutting Non-Quenched And Tempered Steel

With the increasing concern about energy and environmental protection issues,the development of energy-saving,environmentally-friendly,low-cost non-tempered steel instead of high-energy,long-cycle modulated steel to produce automotive crankshafts has become an inevitable trend in the development of automotive steel.The typical nontempered crankshaft steel has the following advantages:the use of microalloying technology improves the strength of the steel;the non-tempered steel eliminates the quenching and tempering heat treatment,and the energy saving effect is remarkable;by controlling the sulfide in the steel,Steel has good processing properties.However,due to its chemical composition,it is difficult to smelt in the production of such varieties,which restricts the low-cost production of such steels.The metallurgical difficulties include:stable control of sulfur mass fraction in steel,and poor castability of continuous casting.It is easy to cause nodulation of the nozzle,and the surface of the continuous casting blank and the subcutaneous crack tend to be serious.When the company produces non-tempered crankshaft steel in Fushun Special Steel Co.,Ltd.,it adopts the short process production route of EAF-LF-VD-CC.The production process is prone to unstable sulfur yield and continuous casting of molten steel.Poorness,many defects on the surface of the rolled material,and the tendency of the submerged crack of the cast blank to be serious.In this paper,through the study of such varieties on the smelting process,the following conclusions are drawn:In the smelting process,the sulfur core-containing wire is used,the sulfur element yield is good,and the molten steel temperature is lowered,which is favorable for stably controlling the mass fraction of sulfur in the steel;Through tracer calibration,when Fugang produces free-cutting non-quenched and tempered steel,the large-sized B-type coarse inclusions appearing in the steel are caused by the slag-crushing during the refining process.By reducing the amount of ladle slag and controlling the soft blowing operation,Reduce the chance of large-sized B-type coarse inclusions in steel;By reducing the oxygen content of the end point of the electric furnace,strengthening the deoxidation,optimizing the amount of refining top slag,soft blowing after vacuum,and reducing the secondary oxidation of molten steel,the purity of the molten steel can be effectively improved,and the non-tempered steel joint can be effectively solved.The problem of flocculation in the casting process,the floc flow rate is reduced to less than 3%;The surface quality of the free-cutting non-quenched and tempered steel slab is poor.The special slag for free cutting steel and the thickness of the stable slag layer can be used to improve the quality of the slab.The pass rate of the black skin of the steel can be steadily increased from 67%to 85%.When using a curved continuous casting machine to produce sulfur-containing freecutting steel,choose a crystallizer copper tube with a fillet radius of 8 mm,reduce the crystallizer water flow to 2100 L/min,improve the uniformity of the cooling energy of the second cold water,and ensure the casting.Machine-to-arc precision can effectively control the incidence of subcutaneous cracks in continuous casting blanks to within 7%and subcutaneous crack lengths<5 mm.