We present calculations of austenite to bainite phase transformation start for different cooling paths and for different steel compositions and a method to estimate the cooling water required to cool a steel strip to desired temperatures during water cooling line after industrial hot rolling. We also quantitatively compare how different alloying elements affect the phase transformation activation energy and the time required for the transformation to start and proceed to the extent that it can be detected with dilatometer. This analysis can be used for aid when designing suitable cooling paths for hot rolled steel products. The calculations of the activation energy can be used as input in more detailed microstructure models.

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The thermo-mechanical processing history together with the steel composition defines the final microstructure, which in turn produces the macroscopic mechanical properties of the final product. In many industrial processes it is therefore of paramount importance to find the optimal thermal path that produces the desired microstructure. In the current study an optimization method has been developed to calculate the optimal thermal path for producing desired amounts of microstructural constituents (ferrite, bainite, martensite) of a medium carbon, low-alloy steel, and a low carbon microalloyed steel. The optimization is performed for two separate industrial processes: induction hardening of a pipeline steel and a water cooling of hot rolled steel strip. The optimization workflow consists of first setting the desired amounts of microstructural constituents, and subsequent optimization of the thermal path, which produces these desired amounts. For the water cooling of a steel strip we additionally employed previously developed tool to calculate the cooling water fluxes that are needed to realize the optimized cooling path in water cooling line after hot rolling. To demonstrate the applicability of the method, we present results that were obtained for different case studies related to the industrial processes.

Analytical approximations that can be used together with the numerical codes to obtain estimates on the temperature distribution inside of the cooled steel strip/plate are discussed. While numerical simulations can give accurate answer after the time required for calculations, the analytical approximations show how thickness and cooling rate affect the temperature distribution. We also present development of graphical user interface interaction with numerical codes for the use in designing and tuning of water cooling schedule for hot rolling strip and plate mill. Interaction of the numerical codes with user friendly frontends have been developed for the following tools: a heat conduction simulation tool for hot strip mill, a tool for calculating phase transformations for user defined cooling paths and a tool for calculating the required cooling water to cool a steel strip to a desired temperature. The functionality and interaction of the tools with the numerical codes is described.