Hot Rolled Coil Property Heterogeneities due to Coil Cooling: Impact and Prediction

Abstract

The importance of coil cooling conditions on mechanical properties uniformity of HSLA and AHSS steel grades is discussed. It is namely shown that hot rolled coil under conventional industrial production can be cooled non-uniformly. That is why to predict correctly the final steel microstructure and mechanical properties of hot-rolled products an accurate description of not only run-out table condition but also of coil cooling should be done. Two solutions to provide accurate description of coil cooling were tested. First one is to use 2D finite element (FE) thermal model. When coupled with the ArcelorMittal metallurgical model to predict hot-rolled microstructure and properties (TACSI) it matches well the industrial data within +/-15-20MPa both for the tensile and yield strength. However, this approach is recognized to be heavy and time consuming. A second solution, a new 2D coil cooling simplified model incorporated in TACSI model, proved to be quite efficient, as it leads to performances similar to the more detailed first solution. Moreover, it is able to compute the coupling between the thermal evolution of the hot band and the kinetics of phase transformation during coil coiling and cooling, and will enable a better evaluation of the final mechanical properties especially for the grades for which the phase transformation is not completed before hot band coiling.

Effect of Hot Deformation on Phase Transformation Kinetics in Isothermally Annealed 3Mn-1.6Al Steel

“…The heat treatment cycle in both cases consisted of heating the sample at a rate of 3 °C/s to a temperature of 1100 °C for its austenitization. The holding time at the austenitizing temperature was 300 s. In the case of non-deformed samples the next step was cooling (at a rate of 10 °C/s) to isothermal holding temperature in a range from 750 to 600 °C, which corresponds to the typical temperatures of steel coiling following hot-rolling practices [ 26 , 27 ]. In case of deformed samples the next step was the deformation at 900 °C, to which the samples were cooled at a rate of −10 °C/s.…”

“…In both cases the holding duration was 300 min (5 h). Due to the enormous thermal capacity of a multi-ton coil of sheet metal, even at high temperatures, it cools down very slowly, keeping the high temperature for many hours [27,28]. At the last step all samples were cooled to room temperature at a rate of 10 °C/s.…”

Computer-Integrated Platform for Automatic, Flexible, and Optimal Multivariable Design of a Hot Strip Rolling Technology Using Advanced Multiphase Steels

“…Ferriticpearlitic microstructure was obtained for the CT 700, 650, and 600 °C. Further decrease of the coiling temperature leads to an increase of the bainite volume fraction in the microstructure [26]. Ferrite transformation is faster but the volume fraction of the ferrite is smaller for the not-recrystallized austenite.…”

Temperature Validation of 3D Model for the Reversing Hot Rolling in Connection with a Coil Model

“…In general, the state of the material resulted from its history and interaction with simultaneously running processes during the reversing hot rolling like temperature distribution of a un-or coiled coil is neglected. Hence, the occurrence of the material phenomenon such as the phase transformation or microstructure evolution can take place in a coil [6][7][8]. In reality, each process step influences the materials properties and determines the initial conditions for the next operation.…”