After the occurrence of bad nodularity, our plant strengthened the detection of sulfur in the original molten iron and the molten iron after spheroidizing treatment, as well as the control of the use of recycled materials to prevent such problems.
In order to produce high quality nodular iron castings, the spheroidizing rate must be above 90%. Through long-term production practice and referring to relevant literature, we think that the main factors affecting the spheroidization rate are as follows.
(1) If the amount of residual magnesium or rare earth is too low, there should be enough amount of spheroidizing agent. In spheroidizing treatment, the burning loss of magnesium should be reduced as much as possible, and the absorptivity of spheroidizing agent should be increased, such as using cover spheroidizing.
(2) The reason of weak inoculation or decline is mainly due to the low amount of inoculant added or the imperfect inoculation process. Because the existence of magnesium is a necessary condition for spheroidization, and adequate inoculation is the guarantee to promote graphitization. Therefore, it is impossible to produce high-quality nodular cast iron by only focusing on spheroidizing treatment without paying attention to inoculation treatment.
Measures to prevent inoculation decline: the selection and addition amount of inoculant should be appropriate, the long-term inoculant containing barium and calcium should be used, and the compound measures of secondary inoculation and stream inoculation should be adopted.
(3) The content of interfering elements is too high
1) The content of s in molten iron is high: s is an anti spheroidizing element, which consumes mg and re in the nodularizing agent, hinders graphitization and reduces the spheroidization rate. If the sulfide slag can not be removed completely, it will lead to re sulfur in molten iron, consume nodularized elements again, accelerate the spheroidization decline, and further affect the nodularization rate. Therefore, in the actual production process, the operating procedures should be strictly implemented.
Preventive measures: in order to achieve high spheroidizing rate, WS of molten iron should be controlled below 0.02%.
2) Excessive anti spheroidizing elements: strictly control the content of anti spheroidizing elements, especially titanium and lead.
Prevention measures: select high purity pig iron and control anti spheroidizing elements in scrap and pig iron.
(4) If the temperature of molten iron is too high or too low, the temperature of nodularized iron is too high, the covering agent can not be compacted, resulting in too fast melting speed of nodularizing agent, resulting in excessive burning loss. However, when the temperature of nodularized molten iron is lower than 1390 ℃, the alloy is not easy to melt, and the spheroidizing reaction is not complete, and the spheroidizing grade is difficult to meet the requirements. In addition, due to the low temperature of molten iron, the nodularizing agent can not be melted and absorbed rapidly during the floating process of nodularizing agent, which causes the spheroidizing agent to float up to the liquid iron level for melting and burning.
Preventive measures: the temperature of molten iron should be controlled within the range of process requirements.