The pig iron with 40% ~ 100% of the above components and less than 60% of the return material are melted in a 150kg medium frequency furnace. Strengthen the operation process to ensure that the spheroidizing temperature of nodular cast iron reaches 1500 ~ 1550 ℃, and the time from molten iron discharging to pouring is controlled within 10min to ensure the spheroidizing effect of nodular cast iron. Use customized spheroidizing agent with the following composition: 38% ～ 48% Si, 5.0% ～ 6.5% Mg, re ≤ 1.0%, Al ≤ 1.0%. Particle size: 5 ~ 40mm.
Compared with the domestic commonly used rare earth ferrosilicon magnesium alloy or rare earth ferrosilicon magnesium calcium alloy spheroidizing agent, the magnesium content of this spheroidizing agent is lower than the former and higher than the latter. The spheroidizing reaction of nodular cast iron is relatively smooth, and on the premise of ensuring the spheroidizing effect of nodular cast iron, it can better control the addition amount of spheroidizing agent, ensure the absorption rate of spheroidizing agent, and better control the decrease of molten iron temperature. In addition, the content of rare earth elements in the spheroidizing agent used is very low, so that the residual rare earth in the molten iron of nodular cast iron after spheroidizing treatment is not too high and the formation of cementite is reduced; The trace rare earth elements remaining in the molten iron can not only inhibit the effect of spheroidizing interference elements of nodular cast iron, but also improve the nucleation rate during inoculation.
Through a series of tests, it is found that the addition amount of nodular cast iron nodularizing agent required by different V / O ratio (ratio of section area to perimeter) of piston ring is different; The amount of spheroidizing agent required for different sulfur content of molten iron is also different. The addition amount of spheroidizing agent is the sum of burning loss, desulfurization magnesium consumption and residual magnesium.
Since the discharge temperature of molten iron, spheroidizing treatment method of nodular cast iron and residence time will affect the loss of magnesium, in order to correctly control the addition amount of spheroidizing agent and control the factors with great changes in production, the spheroidizing temperature of nodular cast iron should be controlled at 1500 ~ 1550 ℃ (preferably 1530 ~ 1550 ℃), and the pit bottom punching method is adopted for the addition of spheroidizing agent, The pouring time of nodular cast iron after nodularization treatment shall not exceed 8min (the best control is within 6min). Under the condition that the operation cycle and process are basically unchanged, it is considered that the burning loss of magnesium is basically the same.
Due to the high sulfur content of raw molten iron, it is impossible to meet the requirement of < 0.018%. The addition amount of nodularizing agent needs to consider the consumption of magnesium for sulfur removal. The rapid determination of carbon sulfur analyzer can provide the sulfur content of raw molten iron in time. In the test, the sulfur content of molten iron is 0.02% ～ 0.03%, and the magnesium consumption for sulfur removal fluctuates up and down at 0.0076%, which is acceptable in the process. The amount of spheroidizing agent directly affects the amount of residual magnesium. If the amount of spheroidizing agent is too much, it will cause too much residual magnesium. Although it will not affect the spheroidizing qualification rate of nodular cast iron (the ball diameter is still normal), it will increase shrinkage and cause brittleness. At the same time, due to the high manganese content of the original molten iron and the excessive addition of spheroidizing agent, carbides are easy to appear and the tendency of white mouth is heavy, resulting in the decline of the qualified rate of the number of balls; If the addition amount of spheroidizing agent is too small, it will lead to insufficient residual magnesium, affect the qualified rate of spheroidization of nodular cast iron, and the qualified rate of ball number will also be reduced. ① When the amount of spheroidizing agent is 1.4%, vermicular graphite appears due to the lack of residual magnesium; ② When the addition amount is 1.8%, carbides appear due to the excessive amount of residual magnesium, which leads to the decrease of the number of balls, but the ball diameter of the two still meets the requirements; ③ Spheroidizing qualified rate and non spheroidizing rate of nodular cast iron.