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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.

Recent trends in the production of high strength steel plate call for increasingly sophisticated thermo-mechanical treatment schedules, including the use of high rate accelerated cooling after finish rolling in order to achieve the desired microstructure and mechanical properties. Achieving the necessary cooling process control accuracy in such cases requires a sound understanding and description of the interactions between external heat transfer processes and changes in internal energy due to phenomena such as solid-state phase transformations. The thermal physical properties of the evolving microstructures of complex phase and martensitic steels vary greatly, and are strongly dependent on temperature and constituent phases. As a result, critical parameters such as thermal diffusivity cannot be accurately estimated without appropriate linkage to both phase transformation kinetics and temperature. In the present study, a numerical simulation has been developed to investigate the unsteady heat transfer and phase transformation behaviour of a moving steel plate during accelerated cooling. The simulation includes semi-empirical microstructure evolution sub-models, fitted to measured CCT data using non-linear regression. These are coupled to thermal-physical properties sub-models and thermal conduction calculations. A comprehensive suite of thermal boundary condition models which account for direct water cooling, forced convection film boiling, air cooling, radiation and heat transfer between plate and transport rollers are also included. The required equations for the plate temperature and microstructure evolution are solved numerically using a cell centred finite volume method, and the model has been validated by comparing simulated cooling stop temperatures with measurements obtained on the plate cooling section of an industrial plate mill. The predicted cooling stop temperatures of steel plates for different thicknesses, velocities and water flow rates are in good agreement with plant operational data.

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.

For conventional reheated and quenched (RQ) steels, the level of alloying necessary to achieve a given hardenability and hardness can be estimated from calculated Jominy curves or ideal critical diameters (DI) given in ASTM A255-10(2014). However, for thermomechanically rolled direct quenched (DQ) steels, little data are available. In this study, the accuracy of the ASTM approach was estimated by designing an experiment to study the main effects of seven alloying elements (C, Mn, Cr, Ni, Mo, Nb, and V) at two levels with eight boron steels based on an eight-run resolution III partial factorial designed experiment. Continuous cooling transformation (CCT) diagrams covering cooling rates of 1.5°C/s–48°C/s were determined using Gleeble simulations with or without controlled deformation below Tnr. The effects of deformation below Tnr and the alloying elements were clearly revealed. In general, low temperature straining of the austenite led to higher levels of hardness than quenching from unstrained austenite. The start of the bainite transformation Ar3 (bainite) was modelled as a function of chemical composition and cooling rate. The data in the CCT diagrams were used to derive equivalent ideal critical diameters (DIB) for strained and unstrained prior austenite. The hardenability index DIB calculated from experimental Jominy curves generally agreed well with those of CCT data for unstrained austenite. In order to apply the ASTM A255 approach to the calculation of DIB, boron factors (BF) for steels with alloy factors (AFs) greater than 26 were estimated by extrapolating the ASTM data. However, this approach did not give satisfactory predictions for either strained or unstrained austenite. Preliminary analysis indicated that Cr, Mo, and V might be less effective at increasing hardenability than implied from their AFs. New formulae were given to allow estimations of the hardness to be expected in connection with direct quenching.