At the initial stage of trial production, zg27simn (carbon content 0.24 ~ 0.32%) was taken as the experimental sample to compare and analyze the chemical composition of molten steel during tapping and the chemical composition data of lost steam steel casting body to study the law of carburization. By changing the production process, the probability of slag inclusion pores in lost steam steel casting is reduced, and this variable is integrated into the study of carburization in lost steam steel casting.
To solve the problem of carbon increase, we should control from the source, reduce the carbon entering the molten steel and increase the carbon discharged.
Phase I experiment:
Using different density foam as a model, find out the source term for carburizing.
Magnitude of impact. Conditions: closed casting of steel castings with concealed risers. The model is a foam board 8-18kg/m3 of various densities, casting vacuum 50, and the carbon content in tapping is controlled at the middle and lower line 0.24~0.27. Test the carbon content during tapping and compare the carbon content sampled from the body of the finished part.
The first batch of lost foam steel casting: model foam density 18kg/m32015 A November 25th, B, C three furnace ZG27SiMn C content is 0.24%, 0.25%, 0.22%. The finished product body shall be sampled, polished and leveled, and five points from the surface to the core shall be recorded as a, B, C, D and E by spectral analysis, with a point spacing of 5 to 10mm. The following table:
Conclusion: the carburization is serious, and the components of lost steam steel casting are completely off grid. In lost steam steel casting, the carbon content increases gradually from the core to the surface, with slight Carburization in the core and serious Carburization on the surface. In the process of molten steel instead of foam punching, the surface of lost foam steel casting solidifies under negative pressure. The residual carbonization of residual gas is in lost foam steel casting, the closer to the surface, the more obvious. Even bright carbon defects appear on the surface of lost steam steel casting.
The second batch of lost foam steel casting: model foam density 14kg/m32015 A November 30th, B, C three furnace, the carbon content of the test before tapping is 0.23%, 0.24%, 0.22%. Other test methods are the same as the first batch. The data are as follows:
Conclusion: the phenomenon of carburization is still obvious, the carbon content is still much higher than the qualified components, and there is still bright carbon wrinkle on the surface of lost steam steel casting. In contrast, for the first time, the method of decreasing the density of foam can effectively reduce carbon addition, but it is far from enough.
The third batch of lost foam steel casting: model foam density 8~10kg/m32015 A December 5th, B, C three furnace, the carbon content of the test before tapping is 0.22%, 0.24%, 0.23%. The test method is the same as the previous two times. The data are as follows:
Conclusion: the overall carbon content meets the requirements of zg27simn, the surface carbon content approaches the upper limit of the material, the core composition is good, the carbon content still increases from the core to the surface, and the range is large, and the composition is uneven.
The analysis of three batches of experiments shows that the method of reducing the model density can effectively reduce the range of carburization. However, the density of the model is not as low as possible. The low density foam is low in strength and easy to deform, increasing the difficulty of modeling and coating. Moreover, the gap between low density foam is more, and the coating easily enters the foam, which affects the surface quality of lost foam steel casting. Even part of the coating is mixed in the lost steam steel casting during the scouring process of molten steel to form internal defects.
Three batches of lost steam steel casting were quenched and tempered. It was found that there were cracks in different quantities of lost steam steel casting in each batch, the first and second batches were the most serious, and some lost steam steel casting had serious defects such as fracture and corner loss. The problem lost steam steel casting is cut from the crack. By analyzing the crack state, the fracture shape is determined: the surface network crack, the lost steam steel casting without obvious defects is carburization, resulting in uneven stress, resulting in the occurrence of lost steam steel casting crack. Large cracks on the surface and honeycomb structure inside, caused by slag inclusion and pore defects. The hidden risernot only increases carbon seriously, but also seriously hinders the gas discharge and the floating of steel slag, which affects the quality of lost steam steel casting.