German experts have repeatedly stressed that “the key to the production of nodular cast iron piston rings lies in raw materials”. It is difficult to produce nodular cast iron piston rings without qualified raw materials. They emphasized the use of high-purity pig iron or synthetic pig iron. Its composition is: 3.8% ～ 4.3% C, 0.2% ～ 0.4% Si, 0.04% ～ 0.15% Mn, P < 0.045%, s < 0.01%, and other elements shall not exceed 0.03%.
There is no such new iron resource in China. The possible components of pig iron in China are: 3.8% ～ 4.3% C, 0.8% ～ 1.3% Si, 0.2% ～ 0.5% Mn, P < 0.07%, s < 0.03%. This kind of pig iron affects the ultra-high of the key elements Mn, P and s of spheroidizing treatment, which brings great adverse factors to the production of nodular cast iron piston rings. At present, it is very difficult for China to find a source of raw materials or synthetic pig iron that meets the requirements, and it is unrealistic to introduce raw materials. Therefore, the selection of raw materials should meet the requirements of foreign countries as much as possible. The pig iron composition selected in the test: 4.0% ～ 4.2% C, 0.80% ～ 0.9% Si, 0.20% ～ 0.4% Mn, 0.04% ～ 0.06% P, s < 0.026%.
Compared with the pig iron required by foreign countries, this pig iron has the following characteristics: ① the silicon content is ultra-high, 0.4% ～ 0.6%. Due to the use of medium frequency furnace smelting, although the recovery and utilization of returned materials will be limited, it will not pose a great threat. ② The manganese content is 0.14% ～ 0.2%. Higher manganese content will affect the control of as cast structure, which can be solved by subsequent ferrite annealing treatment. However, too much cementite will affect the qualified rate of spheroidization and the size control before and after heat treatment. ③ The phosphorus content is acceptable. Even if the phosphorus content of newborn iron used later is slightly higher (< 0.07%), it is not enough to affect the metallographic structure and mechanical properties. If the phosphorus content is too high, the phosphorus content of the casting will be more than 0.1%, and phosphorus eutectic will be generated at the grain boundary, reducing the strength, toughness and plasticity. ④ The sulfur content is over 0.016%. Higher sulfur content will directly consume the amount of spheroidizing agent, especially the influence of dissolved sulfur on graphite spheroidization during solidification, which will directly affect the effect of spheroidizing treatment, reduce mechanical properties and increase casting defects (slag inclusion, subcutaneous pores, etc.). The use of carbon sulfur analyzer creates conditions for timely mastering the sulfur content of raw molten iron and correctly controlling the addition amount of spheroidizing agent. At the same time, the use of M inoculant also plays an auxiliary role in the production of nodular cast iron piston ring.
Through the above process analysis, it is possible to produce nodular cast iron piston rings that meet the requirements of foreign countries by using domestic raw materials.