Brief introduction to modification of cast iron used in Ma steel pipe fittings

There are three ways to obtain vermicular graphite in cast iron.

1、 The sulfur content of hot metal is reduced to less than 0.002% by strong desulfurization, and the hot metal is cooled rapidly;

2、 A small amount of spheroidizing elements (Mg, CE, etc.) are used to treat the molten iron, which makes the graphite metamorphism but not enough for spheroidization;

3、 The spheroidized and anti spheroidized elements are treated simultaneously.

It is also mentioned that vermicular graphite can be obtained by adding nitrogen to molten iron. However, this method is only the experimental results under certain conditions and has not been reported in practical application.

Among the three methods mentioned above, the first method of desulfurization and fast cooling has strict requirements on production conditions and control, which is difficult to be applied in actual production and has certain limitations. The latter two methods are easy to operate and have no special requirements for production conditions, so they are widely used in practical production.

At present, in the production of Maanshan Iron and Steel Co., Ltd., the method of adding a certain amount of modified elements into molten iron is adopted. The operation process is similar to that of nodular cast iron. The range of upper and lower limits of modified elements required by Maanshan Iron and Steel Co., Ltd. is very sensitive to the fluctuation of process factors, which is more difficult to control than spheroidizing treatment, which is one of the most difficult problems in stable production of Masteel. There are two ways to solve this problem: on the one hand, it is necessary to develop suitable modified alloy to make it have a large appropriate range. On the other hand, measures should be taken to make the process factors as stable as possible, the absorption rate of the modified elements should be as high and uniform as possible, and the fluctuation should be reduced as much as possible.

As for the process to achieve the above purpose, it is similar to the type of Vermicular Agent. For Mg based vermicularizing agents, the boiling point (1120 ℃) of magnesium is lower than the temperature of hot metal, so it is easy to vaporize and escape and burn out during the treatment; however, the vermicularizing agent mainly composed of rare earth is not easy to dissolve rapidly and evenly in molten iron due to no boiling and stirring effect. In this section, some treatment processes currently used in Masteel production will be introduced according to the above two main types of vermicularizing agents.