Effect of Zr Microalloying on Austenite Grain Size of Low-Carbon Steels
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TRODUCTION
REFINEMENT of the steel microstructure can significantly improve mechanical properties, particularly strength, plasticity, and impact toughness. The refinement methods generally involve retarding austenite grain growth during thermo-mechanical processing and developing fine room-temperature microstructure on cooling. It is recognized that oxides, carbides or nitrides can effectively inhibit the growth of austenite grains at high temperatures.[1–5] Medina et al.[6] reported that the most stable grains were obtained for a Ti/N ratio close to 2 in structural steels. The steel with a lower N content and approximately the same Ti content had larger precipitates, which started to coarsen as the temperature increased. Sha et al.[7] studied the austenite grain growth in a Nb-V-Ti microalloyed steel. Ti-rich carbonitrides were found to retain a strong pinning effect on austenite grain boundaries up to 1523 K (1250 °C). The sizes of precipitates and austenite grain
MINGHAO SHI is with the School of Material Science and Engineering, Shenyang University of Technology, Shenyang 110870, P.R. China and also with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton T6G 1H9, Canada. RANGASAYEE KANNAN and LEIJUN LI are with the Department of Chemical and Materials Engineering, University of Alberta. Contact e-mail: [email protected] JIAN ZHANG is with the Ansteel Beijing Research Institute, Beijing 102211, P.R. China. XIAOGUANG YUAN is with the School of Material Science and Engineering, Shenyang University of Technology. Manuscript submitted May 3, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
increased as the austenitization temperature increased. At temperatures higher than 1523 K (1250 °C), the pinning effect by precipitates was lost. He et al.[8] studied the effect of Zr carbonitrides on austenite grain coarsening in microalloyed HSLA steels with the Zr/N ratio varying between 2.8 and 22. Austenite grain coarsening was found to occur around 1323 K to 1373 K (1050 °C to 1100 °C) in the specimens with Zr/N ratios of 2.8 to 6.3. Due to the dissolution of carbonitrides around 1673 K (1400 °C), oxide particles are believed to have the capability to pin the prior austenite grain boundaries at temperatures above 1673 K (1400 °C).[9,10] Li et al.[9] reported that steels treated with magnesium (Mg) were able to keep the fine-grained austenite structure after holding at 1673 K (1400 °C). This refinement of austenite grain size was mainly attributed to the formation of pinning oxide particles containing Mg. Liu et al.[11] investigated the effect of Zr additions on the microstructure and impact toughness in the coarse-grained heat-affected zone of high-strength low-alloy steels subjected to 100 kJ cm1 at peak temperature of 1623 K (1350 °C). They showed that the Zr-Al-Ti complex oxides can inhibit grain growth at 1623 K (1350 °C). The Zener pinning force was believed to be high due to the significant amount of smaller oxide particles presented in their experimental steels. Most research mainly focuse
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