Integrated process-metallurgy modelling for niobium microalloyed hot rolled steels

The prediction of microstructures and mechanical properties is an important point for the control of steel properties and quality. Numerous models, either physical or more phenomenological, developed by Arcelor R&D laboratories are being assembled in a through-process modeling scheme, to simulate the production process, the thermomechanical history of a steel product, its metallurgical evolution and final properties. In the case of a niobium microalloyed steel, a model for predicting the precipitation state of Nb as a function of time is used. It describes the full precipitation kinetics from the nucleation stage to the Oswald ripening stage, solving numerically the set of coupled equations for nucleation, growth and coarsening. It gives the volume fraction, mean radius and number of NbC precipitates, and consequently, the niobium and carbon remaining in solid solution. This precipitation state is of prime importance to control the final mechanical properties of hot rolled HSLA steels. Other microstructural factors play a key role on the final properties, especially the grain size and the phase fractions that depend on the thermomechanical history of the strip. The physical precipitation model has thus been introduced in an integrated model to predict the microstructure and mechanical properties of hot rolled steel, including the microstructure of C–Mn and C–Mn–Nb steels, the strength, the cooling curve affected by heat evolution due to transformation and the precipitation strengthening. On-line prediction of microstructure and mechanical properties has been applied to three hot strip mills in Arcelor equipped with advanced process computer systems. One of the main applications of this computer metallurgy is to provide assistance for designing metallurgical routes, since the metallurgical model is a good tool to analyze the different sources of strengthening (grain size, alloying, phase fraction) in relation to chemical compositions and process conditions. Computing mechanical properties of the strip over the entire strip length is another feature of the on-line system when conventional quality controls are usually only available at the strip head and tail. As an example, this modeling is applied to evaluate the distributions of tensile strengths in strips, which are different between a Nb-microalloyed steel and a non-microalloyed steel. (AU)