Study On Technique And Continuous Casting Process Of High Quality GCr15 Bearing Steel Making

The factors influencing the quality of billet,such as the total oxygen content(T.O),acid soluble aluminum and the number and size of inclusions,were comprehensively studied in this paper.The main process of producing GCr15 bearing steel is 120 T converter(LD)? ladle refining(LF)? vacuum degassing(RH)?continuous casting and rolling(CC)in JY steel plant.Thus,the different technical parameters in the smelting process were controlled to analyze their effects on the product quality,and the low oxygen content and good physical quality of bearing steel were obtained by optimizing the converter,refining,continuous casting process operation.Finally,the quality of bearing steel was greatly improved.It was found that the total oxygen content was gradually reduced with the proceeding of the smelting process: the average oxygen content in the refining process was 10.8ppm,the oxygen content at the end of RH treatment was reduced to 7.7ppm,the oxygen content in the tundish was 8.7ppm,the content oxygen in the billet was7.7ppm,the oxygen content in the wire production was 6.8ppm.The increase of oxygen content in the tundish may be due to the secondary oxidation of the molten steel during the ladle to ladle process.The oxygen content in bearing steel was greatly subjected to the LD end-point carbon content,acid soluble aluminum([Al]s),refining time,refining slag basicity and protection for continuous casting.Controlling the end-point carbon content of more than 0.3%,[Al]s content of 100~200ppm,refining time of 50~60min,refining slag basicity of 5.0~7.0,tundish with long water mouth,argon protection pouring and other technical measures can effectively reduce the T.O content in GCr15 Bearing steel.The study on the evolution of content,morphology and size of inclusions in the smelting process was carried out.The quantity of inclusions in molten metal showed a gradual downward trend: the number of inclusions in the LF process was an average of25.3/mm2,the number of inclusions in RH was reduced to 11.6/ mm2,the average number of inclusions in the tundish sample was 12.7/mm2,and the average quantity of inclusions in the wire was 9.8/mm2.The results of scanning electron microscopy(SEM)showed that: LF refining can make the larger size of the oxide and sulfide inclusions(40?m)transformed into the smaller size of composite oxide inclusions(20?m);after the RH treatment,the points of calcium magnesium aluminate inclusions were onlyleaved and their sizes were down to 6?m below;the quantity of inclusions was obviously reduced by using soft blowing treatment.The effects of different aluminum processes on the total oxygen content and inclusions in the steel during the refining process were studied.The results showed that the deoxidation effect was ideal by adding 1.2kg/t and 0.1-0.3kg/t Al particles during tapping and LF refining process,respectively.The foregoing process could keep the [Al]s content steady in 100~200ppm and the wires T.O of more than 6.7ppm.The size of inclusions became small and thus high purity steel was achieved.The analysis of the argon flow in the 120-ton LF refining process showed that:the argon flow could shorten the stirring time during the heating and alloying of the refining stage.When the argon blowing flow increased to 400Nl/min,continuously increasing the flow of mixing time had little effect,which would cause the appearance of molten steel slag.Therefore,the optimal control range of argon blowing is300~400Nl/min in the refining process.The de-inclusion behavior of the soft-blowing process was studied.The influence of argon blowing time and argon blowing amount on the removal of inclusions in ladle was investigated.The results showed that the removal efficiency of argon is controlled at 40 Nl/min and the soft blowing time is controlled at 25~30min.The removal rate of inclusions up to 90%,which in steel was the best.The effects of different pouring temperatures and secondary cooling strength on continuous casting billet loose and segregation were investigated.And results showed that the degree of carbon segregation was lower in the optimum range of 15-25?.At the same time,the second cold intensity was reduced from strong cold to weak cold,i.e.,the secondary cooling water was reduced from 0.5L/min to 0.18L/min.The experimental results showed that the crack rate of billet was reduced and the quality was obviously improved.The study of solidification characteristics on the continuous casting end showed that the comprehensive solidification coefficient of GCr15 bearing steel of 240×240mm section is 26.8mm/min1/2.The liquid core length was 17.2m.The study on producing 240×240mm section of billet suggested that the distance from the F-EMS center to the meniscus was 5.71~6.69 m.The best configuration parameter was380/ 350 A.