Investigation of RE-O-S-As Inclusions in High Carbon Steels
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RESIDUAL elements, such as As, Cu, and Sn, are difficult to remove from steel in the current steelmaking process. These elements have proved to be very detrimental to the mechanical properties of most steel products.[1,2] Arsenic usually segregates at grain boundaries, dramatically decreasing toughness and hot workability of steel products.[3–5] As-bearing iron ores and steel scrap are main sources of the increasing arsenic content in steel products. On the one hand, to cut cost, As-bearing iron ores have been used frequently in southern China.[6] On the other hand, every single recycling use of steel scrap contributes to the enrichment of arsenic in steel products. There are three main ways to lighten the harm of arsenic to steel products. The first way is to remove arsenic from the ironmaking and steelmaking processes. The concept of removing arsenic from As-bearing iron ores has been presented before, but it is hard to apply in mass production because of its high costs, the limited removal rate of arsenic, and the emission of toxic gases.[7,8] Liu and Sun[9] successfully decreased arsenic HONGPO WANG, LU XIONG, LEI ZHANG, and YU WANG are with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. Contact email: wanghp@ cqu.edu.cn YIYI SHU and YUANHUA ZHOU are with Chongqing Iron and Steel Co. Ltd., Chongqing 401258, China. Manuscript submitted March 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
content to 0.010 wt pct by using SiCa alloy and CaF2; however, this process caused fluoride pollution simultaneously. Although vacuum refining has the function of removing arsenic from steel, it usually leads to a lot of iron loss because of the close volatilization coefficients of Fe and As. In summary, currently, there is no economical or suitable method to remove arsenic from steel. Instead, diluting arsenic concentration in steel has become the most widely used method.[10] China imports tremendous amounts of As-free iron ores from Australia and Brazil every year, which puts great pressure on China because the import of iron ores accounted for near 80 wt pct of the total consumption in 2016. The third way is adding elements, such as nickel, to steel to modify the existing state of arsenic in certain steel products.[11] In the past decades, rare earth (RE) elements have been widely used in the steelmaking process because of their strong ability to refine grains, modify inclusions, and purify molten steel.[12,13] RE elements are the few elements that react with arsenic in steel.[14] When added to steel, RE elements usually first react with O and S because of their strong reactivity, then react with arsenic if the RE content is in excess. Therefore, RE-O-S inclusions need to be investigated before studying the effect of RE on arsenic. It is important to clarify the formation mechanism of RE-O-S-As inclusions, including binary, ternary, and quaternary inclusions, so as to modify the existing state of arsenic and use RE elements efficiently. There is some initial research focusing
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