Research On Movement And Precipitation Of Inclusions In Steel During Sampling

The technology of controlling non-metallic inclusions in steel is the key content of clean steel production, and the movement and precipitation of inclusions influence the type and distribution of inclusions in steel directly, which will finally impact the grade and property of steel. During actual production, bucket sampling and cake-shape sampling or other methods are usually used to take steel samples and analyse the cleanliness of steel. But the moving and cooling conditions for inclusions are different in different samplers, which makes analysis for inclusion different, especially for the MnS inclusion, whose precipitation is sensitive to cooling condition, the results of sampling cannot accurately reflect the steel cleanliness. Therefore, researching on the influence of different sampling ways on the movement and precipitation of inclusions and try to infer the actual cleanliness from sampling results is a preliminary exploration of making criterion for sampling and an important try on making better clean steel production.Firstly, based on the calculation results by thermodynamics software Factsage, the precipitation condition of MnS and Al2O3 in steel were analysed. Then a lab experiment of high temperature with tube furnace was designed to obtain the connection between sulfide’s equivalent diameter and cooling condition during process of cooling and the model of size and cooling rate was built and verified. Based on this, another experiment was carried out to study the influent factor on the formation of MnS+AlO3 duplex inclusion, and the effect of Al and S content and cooling rate on encapsulation rate of Al2O3 by Mn was obtained. Based on thermodynamic theory and high temperature experiment results and combined with fluid dynamics theory and inclusion collision theory, a model of inclusion movement and precipitation in different samplers was built. And by the model, the influence of different sampling methods on inclusions’ distribution and size was predicted. At last, an industrial trial was carried out to verify the conclusions and further reveal the rule of inclusion movement and precipitation during sampling.The results show that:1) Thermodynamics calculation indicate that content change of Mn and S will change the precipitation temperature of MnS. Al2O3 is solid in molten steel, MnS precipitates at the end of solidification when temperature decreases, which is benefit to the formation of MnS+Al2O3 duplex inclusion that can improve steel properties.2) With the increase of cooling rate, the size of MnS decreases gradually and the distribution of MnS becomes close. When the diameter of Al2O3 inclusion is smaller than 4.2μm, Al2O3 inclusion can be nucleation core of MnS to form duplex inclusion.3) The content of S influences the precipitation of MnS directly, when the content of S is more than 0.02%, all oxide inclusion will combine with MnS.4) Distribution of flow field and temperature field in sampler is decided by molten steel’s inflow manner. The smaller of sampler, the quicker the sampler to be filled, make the more vigorous of velocity field and the quicker uniformity of temperature field. Circle field around wall moves up with increase of liquid level during vigorous filling process. Concentration of inclusions whose size is 45μm and 95 μm decreases 0.78% and 0.65% separately. Smaller size of inclusion leads to bigger percentage of reduced growth due to collision. Concentration of inclusions whose size is 150μm,240μm and 680μm inecreases 1.92%,1.10% and 0.99% separately.5) The cooling rate of four different size bucket samplers is 1.02K/s,8.15K/s,26.13K/s and 41.15K/s. With the dereasing size of bucket samplers, the cooling rate increases gradually during process. Encapsulation of duplex inclusion in four different size bucket samplers is 48.18%,16.28%,4.65% and 0% separately. Compared with size of inclusions in center and bottom, the size of inclusions in top is bigger and its distribution is more disperse, the number of large size inclusion is more and the average size of inclusion in top position is bigger. The results of this study reveal the rule of inclusion movement and precipitation during sampling in somehow, and can provide effective basis knowledge for creating a creterion of sampling.