Computational Design of a Novel Medium-Carbon, Low-Alloy Steel Microalloyed with Niobium
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Computational Design of a Novel Medium-Carbon, Low-Alloy Steel Microalloyed with Niobium Vahid Javaheri, Tuomo Nyysso¨nen, Bjørnar Grande, and David Porter (Submitted September 24, 2017; in revised form March 29, 2018) The design of a new steel with specific properties is always challenging owing to the complex interactions of many variables. In this work, this challenge is dealt with by combining metallurgical principles with computational thermodynamics and kinetics to design a novel steel composition suitable for thermomechanical processing and induction heat treatment to achieve a hardness level in excess of 600 HV with the potential for good fracture toughness. CALPHAD-based packages for the thermodynamics and kinetics of phase transformations and diffusion, namely Thermo-Calc and JMatPro, have been combined with an interdendritic segregation tool (IDS) to optimize the contents of chromium, molybdenum and niobium in a proposed medium-carbon low-manganese steel composition. Important factors taken into account in the modeling and optimization were hardenability and as-quenched hardness, grain refinement and alloying cost. For further investigations and verification, the designed composition, i.e., in wt.% 0.40C, 0.20Si, 0.25Mn, 0.90Cr, 0.50Mo, was cast with two nominal levels of Nb: 0 and 0.012 wt.%. The results showed that an addition of Nb decreases the austenite grain size during casting and after slab reheating prior to hot rolling. Validation experiments showed that the predicted properties, i.e., hardness, hardenability and level of segregation, for the designed composition were realistic. It is also demonstrated that the applied procedure could be useful in reducing the number of experiments required for developing compositions for other new steels. Keywords
CALPHAD, computational design, homogenization, IDS, JMatPro, microsegregation, prior austenite grain size, Thermo-Calc, wear resistance steel
1. Introduction Steel has played and will continue to play a major role in the development of modern societies (Ref 1-3). Materials scientists continue to devote themselves to the development of new steels in order to meet the ever-increasing demands for better performing steels (Ref 4-7). The design of new steels with desired combinations of sometimes contradictory properties is a challenging task as many variables and their complex interactions should be taken into account. For example, in the case of steel pipeline for the transport of slurries and cement in the mining and construction industries, pipe service life benefits from high resistance to erosion (wear) and corrosion, combined with sufficient toughness. In many slurry transport applications, the main failure mechanism determining the slurry equipment life is erosion rather than corrosion (Ref 8-11) and it has been shown that longer service lives are obtained with higher hardness on the working (inner) surface (Ref 12-15). Also, for a given hardness and chemical composition, a tougher microstructure could have a b
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