Improvement of non-metallic inclusions
The main cause of fatigue cracks is oxide inclusions in the steel, and D-type inclusions are more harmful to fatigue life than Class B inclusions. Therefore, foreign steel mills and automobile plants put forward higher requirements for oxide inclusions in spring steels. For example, the Swedish SKF standard requires that the oxygen content in steel is less than 15×10-6, and the D-type inclusions are lower than the B-type inclusions. . In particular, Al2O3 and TiN inclusions are the most damaging to the fatigue life of the spring. In order to produce high-quality spring steel, electric furnace-electroslag remelting or vacuum arc remelting and other special smelting methods have been used in the past.
With the development of the refining technology outside the furnace, the use of the refining process outside the furnace can significantly reduce the inclusions in the steel, such as the use of RH vacuum degassing in Japan to achieve ultra-low oxygen steel (ULO) or ultra-pure steel (UCS) production. Silica-deoxidized killed steels are refined by the use of refined synthetic slag with strictly controlled alkalinity to denature the undeformed Al2O3-rich inclusions while stirring the molten steel, allowing the inclusions to float and remove, reducing the inclusion content and making Residual inclusions are harmless, etc., thereby obtaining ultra-pure spring steel. It was confirmed that low oxygen steel (LO) with an oxygen content of less than 15×10-6 can meet the requirements for using 200MPa high stress springs. The fatigue limit of ULO+ULTiN steel produced by ultra-low oxygen plus ultra-low titanium nitride process is the same as that of vacuum arc remelted steel. Steel can be used to manufacture high-strength valve springs . Xining Special Steel Plant and Jiangsu Huaiyin Steel Plant (ultra) high-power EAF initial refining + LF refining + alloy steel bloom or billet continuous casting production of spring steel, deoxidation of niobium alloy in 60 t and 80 t LF, respectively The test of inclusion and inclusion denaturation shows that the oxygen content in the steel is reduced to less than 15×10-6, the residual aluminum content is controlled at 0.020 to 0.025 %, and the proportion of Al2O3 in the oxide inclusions is less than 40 %, and the residual inclusions are small. The uniform, dispersive plastic inclusions are distributed. Qingdao Iron and Steel Co., Ltd. adopts carbon smelting, argon blowing at the bottom of the ladle, feeding, continuous casting using protective casting, reasonable control of superheat, casting speed and crystallizer parameters, and secondary cooling water gas-water atomization cooling method. Produce 60Si2Mn spring steel that meets the requirements of GB1222-84, effectively solves the quality problems in the production process, and reduces the oxygen content and inclusion level in the steel.