The size and morphology of sulfides are important for both machinability and mechanical properties of free-cutting steels.In order to improve the level of sulfide control,characteristics(length,aspect ratio and area)of sulfide in Ca treated free-cutting steel before and after hot rolling were analyzed systematically by scanning electron microscope equipped with an automatic inclusion analyzing system.There were two kinds of sulfides in Ca treated free-cutting steel,pure MnS and CaS-MnS.The proportion of CaS-MnS was 13%-21%,and the core was an oxide.The energy spectrum distribution indicated that CaS-MnS was a solid solution phase with uniform composi-tion.For pure MnS,the average and maximum aspect ratios in continuous casting billets were 2.191 and 26.87,respectively.After hot rolling,the average and maximum aspect ratios of pure MnS were 4.583 and 60.83,respec-tively.The average aspect ratio increased by 107.12%.For CaS-MnS,the length,aspect ratio and area in continu-ous casting billets were significantly smaller than that of pure MnS.After hot rolling,the deformation of CaS-MnS was slight,with an average aspect ratio of 1.598 and a maximum aspect ratio of 14.42,respectively.The average aspect ratio increased by 8.48%.The area had a significant impact on the deformation of pure MnS.For MnS with area no more than 30 μm2,the aspect ratios in both continuous casting billet and hot rolled rod were relatively small(<4).For MnS with area larger than 30 μm2,the aspect ratio significantly increased after hot rolling.The forma-tion mechanism of sulfides was explained.The inclusions after Ca treatment were CaO-Al2O3.After sulfur addi-tion,CaO was transformed into CaS because the S content in the steel was much higher than that of O content.CaS and MnS had mutual solubility,which can promote the heterogeneous nucleation of MnS and form fine CaS-MnS.At the end of solidification,the supersaturation of Mn and S increased sharply,which makes a large amount of MnS to precipitate in a short period of time.It helps to better understand the effect of Ca treatment on sulfur-containing free cutting steels and provides guidance for engineering applications.
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