Coupling Effect of Prior Austenite Grain Size and Inclusion Characteristics on Acicular Ferrite Formation in Ti-Zr Deoxi
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ACICULAR ferrite (AF) as the optimum microstructure feature in heat affected-zone (HAZ)[1] and high strength low alloy steel (HSLA)[2] was generally accepted, due to its nature of small grain size and chaotic arrangement that hindered the propagation of cleavage cracks.[3,4] Generally speaking, AF nucleated on some intragranular effective non-metallic inclusions
YONGKUN YANG, DONGPING ZHAN, HONG LEI, RONGJIAN WANG, and ZHOUHUA JIANG are with the School of Metallurgy, Northeastern University, NO. 3-11, Wenhua Road, Heping District, Shenyang 110819, P.R. China. Contact e-mail: [email protected] YULU LI is with the School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China GUOXING QIU is with the School of Materials Science and Engineering, Northeastern University and also with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China. HUISHU ZHANG is with the Metallurgical Engineering College, Liaoning Institute of Science and Technology, Benxi 117004, China. Manuscript submitted September 5, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
during c/a transformation in solidification,[5] welding and thermal cycling.[6,7] The main factors affecting AF formation[8] were inclusions characteristics[9] (e.g., inclusion composition, inclusion number density, and inclusion size, etc.), cooling rate of transformation from austenite to ferrite,[10] and the prior austenite grain size,[11] etc. For the types of effective non-metallic inclusion, whether it was the single inclusion (e.g., Ti2O3,[12,13] TiN,[14] CuS,[15] etc.) or multiphase inclusion (e.g., TiN-MnS,[16] MgO-Al2O3-SiO2-MnO[17], Ti3O5-Al2O3-MnO[18]), they were likely to be the nucleation site for AF. Among of them, Ti oxides were commonly used to induce AF nucleation through producing a Mn-depletion zone (MDZ) around that.[12,13] Zr, as a congener of Ti, had many similar properties.[19] Treatment of Ti-bearing steel with Zr could refine and modify the Ti oxides. The modified Ti-Zr composite oxides would become the nucleation core of MnS and induce AF nucleation.[20,21] Cooling rate, it was generally believed determining the microstructure types after the austenite decomposition. For low carbon steel, if the cooling rate was higher, the austenite would transform as bainite or martensite, while
if the cooling rate was lower, the austenite would transform as polygonal ferrite. Therefore, controlling the appropriate cooling rate was very important for obtaining more AF formation.[22,23] On the basis of proper cooling rate, the prior austenite grain size indirectly affected the volume fraction of intragranular ferrite by determining the volume fraction of ferrite nucleated at grain boundary.[24] Barbaro et al.,[25] Zhang et al.,[26] and Song et al.,[27] had studied the effect of austenite grain size on AF formation by heating to different target temperatures or holding different times at the target heating temperature. From their research results, the optimal austenite grain size of AF formation was diff
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