Effect of Vanadium on the Hot Deformation Behavior of Vanadium-Microalloyed Steel for Thin Slab Direct Rolling
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INTRODUCTION
THE thin slab casting and direct rolling (TSDR) process is a very efficient process to produce hot-rolled steel strip with low capital investment and higher productivity. The essential difference between TSDR and conventional hot strip mill (HSM) is that the entire casting and rolling process is run continuously in TSDR. In addition, TSDR provides ecological advantages related to a lower energy consumption and reduced CO2 greenhouse gas emissions.[1,2] In terms of liquid metal sourcing, the TSDR technologies can be combined with the blast furnace (BF) and basic oxygen furnace (BOF) steel plant technologies or the scrapbased electric arc furnace (EAF) technologies.[3] Nb and Ti provide a pronounced precipitation hardening effect and austenite grain refinement. Nb may cause the transverse cracking at the slab surface of continuously cast high strength low alloy steel.[4] In the case of steel from scrap-based EAF, the nitrogen level is relatively higher than the steel from the BF-BOF route. Ti additions result in the formation of TiN, 200 to 2000 nm in size, which do not contribute to strength.[5] When Ti is added in combination with other microalloying elements (V or Nb), it suppresses the formation of fine distributed precipitates and forms coarser coprecipitates.[6,7] This usually results in a decrease of the yield strength. CHANG WOOK LEE, Graduate Student, and BRUNO C. DE COOMAN, Professor, are with Graduate Institute for Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Republic of Korea. Contact e-mail: [email protected] HWAN GOO SEONG, Senior Researcher, is with the Technical Research Laboratories, POSCO, Gwangyang, 545-090, Republic of Korea. Manuscript submitted March 28, 2015. Article published online April 21, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
V alloying additions are very compatible with the TSDR technology. The higher N content in the steel is beneficial for precipitation strengthening by V-nitrides. V-added steel grades are less susceptible to transverse cracking because VN is highly soluble in austenite. The high solubility of VN in austenite also reduces roll forces. This in turn leads to a reduction of the roll wear and strip shape-control difficulties.[8,9] Precipitation of V-bearing compounds occurs mainly in the run out table and it is the coiled strip. The present work was therefore focussed on identifying the maximum content of V which could be added to TSDR steel grades without having an impact on the materials flow stress and recrystallization behavior. Dynamic recrystallization (DRX) is the most efficient method for controlling microstructure and mechanical properties during hot deformation of austenite.[10] DRX is explained in the schematic hot deformation stress– strain curves shown in Figure 1. Hot deformation flow curves can be categorized into three types: strain hardening type, dynamic recovery (DRC) type, and DRX type. In the strain hardening type flow curves, the stress increases continuously with deformation. DRC type behavi
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