PROPERTIES THROUGH PRIOR AUSTENITE OPTIMIZATION. Lieven Bracke, Elke Leunis, Laura Moli Sanchez. ArcelorMittal Global R&D Ghent, Belgium.
Contributed Papers from Materials Science and Technology (MS&T) 2015 October 4 – 8, 2015, Greater Columbus Convention Center, Columbus, Ohio, USA Copyright © 2015 MS&T15®
THE EFFECT OF Nb ON LOW-CARBON MARTENSITIC STEEL PROPERTIES THROUGH PRIOR AUSTENITE OPTIMIZATION. Lieven Bracke, Elke Leunis, Laura Moli Sanchez ArcelorMittal Global R&D Ghent, Belgium Steven G. Jansto CBMM North America, Inc. Bridgeville, PA 15017, USA Keywords: martensite, prior austenite condition, Nb additions, mechanical properties Abstract The strength and the toughness of martensite are known to depend strongly on the prior austenite state, in re-austenitizing and quench (RAQ) as well as in direct quench (DQ) processes. Process and various Nb concentrations in a low-carbon martensitic steel have been evaluated to create different prior austenite conditions in order to quantify the grain size and morphology effect and to find an optimized strength-toughness balance. The prior austenite grain size is the main factor to improve the toughness in both the RAQ and the DQ processes. An in-house developed austenite reconstruction routine was used to characterize the prior austenite condition in detail. For the DQ processes, an increased dislocation density in the austenite increases the strength. Excessive strain-induced NbC precipitation reduces the toughness in the DQ condition for the highest Nb additions. Introduction Development of hot rolled martensitic steels with ultra-high strength and toughness remains an important field for steel development, with typical applications in equipment for mining, earth moving, loading and lifting, shipbuilding, automotive and general construction. The conventional way of producing martensitic steels is via reheating in the fully austenitic regime and quenching to martensite. This re-austenitize and quench (RAQ) process is a separate production step after hot rolling, leading to additional production costs and energy consumption. To overcome these issues, hot rolling processes in which martensite is formed through quenching directly after hot rolling (DQ) have been subject of development in the last decades [1]. Despite the fact that there is debate about which martensite microstructural features drive its properties, it is well-known that a finer martensite structure is one of the key factors to obtain better strength and toughness [1, 2, 3, 4, 5]. In DQ processes, the martensite structure can be refined by quenching from austenite containing residual strain from hot rolling [6], i.e. finish rolling below the temperature of no-recrystallization (Tnr). More deformation below Tnr can be expected to further refine the martensite structure and improve the strength-toughness balance [7], although it has also been reported that this effect may saturate quite quickly [2]. These contradictory reports suggest that the optimum amount of rolling below Tnr might depend on the chemical composition of the steel. In the assumption that more deformation below Tnr is indeed beneficial, the addition of Nb should be considered since Nb is the most effective element in increasing Tnr [8].
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For the RAQ process, the austenite grains are always recrystallized prior to quenching. As such, the only way to improve the strength-toughness balance is to reduce the prior austenite grain size [4, 5, 9]. There are three main parameters that can be optimized for this. Firstly, the austenitization temperature can be lowered and the time can be shortened [10]. Limiting factor, however, is that the complete material has to be transformed to austenite in a homogeneous way. Secondly, alloying elements in solid solution reduce grain growth by solute drag [10]. In this case, the limits are typically imposed by alloying cost and weldability of the material. A third way to reduce the grain growth is by grain boundary pinning by particles, i.e. Zener-Smith pinning [11]. In Nb-added steels, NbC-precipitates have been reported to be very effective in limiting austenite grain growth and as such in enhancing the strength-toughness balance of martensitic steels [12, 13]. In the current study, the effect of Nb additions on the strength and toughness of DQ and RAQ low C martensite will be studied. The results will be interpreted based on a detailed characterization of the precipitation state and the prior austenite condition. Materials and experimental procedures Four low carbon steels were cast under protective atmosphere as 80kg ingots in an induction lab furnace. The chemical composition is shown in Table 1. After reheating at 1250°C, two plates of each composition were hot rolled in a lab hot rolling mill. The final thickness was 5mm and the finish rolling temperature was 870°C for all. The Tnr of the 0Nb alloy has been estimated around 930°C [1]. For each composition, one of the plates was direct quenched in water to room temperature (DQ plates), whereas the second plate was air cooled to room temperature. The air cooled plates were subject to re-austenitization at 880°C for 15min., followed by water quenching to room temperature (RAQ plates). Table 1 Chemical composition of the lab alloys (all in wt%, balance: Fe+impurities, Ti* = Ti – 3.42N) 0Nb 200Nb 600Nb 1000Nb
C 0.120 0.119 0.121 0.120
Mn
