Design Optimization of Microalloyed Steels Using Thermodynamics Principles and Neural-Network-Based Modeling
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INTRODUCTION
HOT rolling is done with the aim of achieving specific shapes, sizes, and properties. It involves reheating steel slabs for chemical homogenization in the austenitic state and dissolving carbides that form during casting. The material is then rolled until the finish rolling temperature (FRT) followed by controlled cooling and coiling. An appropriate choice of the reheating temperature, FRT, coiling temperature (CT), and cooling conditions leads to the desired properties for a given steel composition. The evolution of the austenite grain structure during hot rolling differentiates microalloyed and other steels that do not contain grain pinning particles.[1] In the former case, solutes, such as niobium, form precipitates at the hot-rolling temperature that restrict austenite grain boundaries and also alter the ability of the deformed microstructure to recrystallize. The tendency of grains to coarsen between rolling passes is prevented by allowing recrystallization of austenite at the first few passes and then stopping recrystallization
ITISHREE MOHANTY, APPA RAO CHINTHA, and SAURABH KUNDU are with Research and Development, Tata Steel Limited, Jamshedpur, Jharkhand 831 007, India. Contact email: [email protected] Manuscript submitted July 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
altogether so that pancaked austenite grains are the ones that transform into ferrite. The ferrite is, therefore, finer, making it stronger and tougher when the microalloyed steel is processed in this manner.[2] Solutes suitable for microalloying include niobium, vanadium, and titanium, added individually or in combination. There may be other additions, such as boron, molybdenum, nickel, chromium, and silicon, to achieve the desired mechanical properties. The purpose of the present work was to create an artificial neural network model that, without compromise, deals with the large number of process and compositional variables, founded on actual industrial data, so that the model can be used to optimize properties. The intention also was to interrogate the model so that some of the predicted trends can be validated using thermodynamic models, unseen data, and metallurgical experience. The strengthening due to addition of microalloying elements is achieved through grain refinement and precipitation hardening. Apart from higher strength, weldability, fatigue life, and wear resistance properties of microalloyed steels are also superior to similar heat-treated steels.[3] However, optimizing the properties in microalloyed grades, particularly when used in combination, is difficult, because the best properties in microalloyed steels are obtained at the right combination of reheating temperature, amount of reduction in
various rolling stands, FRT, and CT and the cooling rate before coiling. It is often the case that a design problem can lead to multiple solutions for a given set of desired properties. This makes the design problem interesting because other parameters, such as cost, can then be used to assess the best strat
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