Control Of Inclusions In 304 Stainless Steels

Non-metallic inclusions have a detrimental effect on the property of stainless steels, such as their strength, toughness, fatigability and surface quality. Meanwhile, inclusions may lead to various defects on the stainless steel products. Control of inclusions is one of the main tasks of steelmakers. High grade 304 stainless steels are widly used in the production of mobile phone shell, watch chain, etc. High surface quality of the 304 stainless steel product is greatly required. However, Chinese high grade 304 stainless steels can hardly satisfy the quality standard of customers due to the poor control of inclusions in stainless steels. Therefore, it is significant to investigate inclusions in 304 stainless steels.The target composition of inclusions in 304 stainless steels was determined through literature review and product survey. Spinel inclusions resulted in sliver-like defects on the cold sheet of stainless steels. The Al2O3-riched Al2O3-SiO2-CaO-MnO inclusions had poor deformability, causing spot defects on the surface of stainless steel plate after surface finishing. Thus, Al2O3 content in inclusions in 304 stainless steels should be effectively lowered to decrease the melting temperature of inclusions and improve the deformability of inclusions.Effect of slag composition on inclusions in 304 stainless steels was investigated. A low basicity slag can suppress the formation of Al2O3 in inclusions, lowering the melting temperature of inclusions. Meanwhile, MgO in the initial slag was beneficial to reduce the corrosion of refractory and can hardly influence the composition of inclusions. The Al2O3 in slag should be lowered to a minimum to reduce the Al2O3 content in inclusions in 304 stainless steels. The optimal composition of the ladle slag was suggested that CaO/SiO2<1.75, MgO=10%-15% and Al2O3=0.It is feasible to modify inclusions in 304 stainless steels through optimizing calcium treatment. A model of calcium treatment was developed using FactSage to precisely modify the Al2O3-rich inclusions to liquid ones through calcium treatment. In plant trials, it was verified that calcium treatment was beneficial to lower the melting temperature of inclusions in 304 stainless steels.Behavior of inclusions during the reoxidation of Si-Mn-killed 304 stainless steels in continuous casting tundish was studied. After the reoxidation of Si-Mn-killed 304stainless steels, [Al] and [Ca] in the reoxidized region of the molten steel were oxidized and decreased to extremely low levels. A large number of MnO-rich 1-2 ?m small inclusions as transitional products were generated by the oxidation of high [Mn], and gradually decreased from cast start to steady casting. It was illustrated that the reoxidation increased the MnO in liquid inclusions in Si-Mn-killed 304 stainless steels, while promoting the formation of Al2O3-rich inclusions in Al-killed 304 stainless steels.The evolution of inclusions in Si-Mn-killed 304 stainless steels during the heat treatment at 1373 K under an argon atmosphere was investigated. During the heat treatment, [Cr] transfered to the steel/MnO-SiO2-rich inclusions contacting interface, and formed MnO·Cr2O3 spinel on the surface of MnO-SiO2-rich core phase. Meanwhile, the formed [Si] and [Mn] transferred back to the bulk steel from the opened reaction interface. Finally, MnO-SiO2-rich inclusions were fully modified to pure MnO·Cr2O3 spinel in Si-Mn-killed 304 stainless steels. The transformation rate of inclusions from MnO-SiO2-rich oxide to MnOCr2O3 can be dramatically enhanced by increasing the heat treatment temperature from 1273 K to 1473 K. Moreover, a kinetic model was developed, which is an effective tool to predict the transformation rate of inclusions at various temperature during heat treatment of the solid 304 stainless steels.In the current study, formation mechanisms of various inclusions in 304 stainless steel during slag refining, calcium treatment, reoxidation in tundish and heat treatment were figured out through literature review, laboratory experiments and plant trials. Based on the demand and application of the products, the appropriate production route can be determined to achieve the control of inclusions in 304 stainless steels.