Thermo-Mechanical Modelling of Hot Strip Coil Cooling Process

Abstract: In hot rolled steel strip production the processed material are after the cooling section coiled into coils. Coils do normally weigh around 20 tons with a diameter of 2 m and heights equal to the strip width e.g. 0.8-1.4m. When processing HSLA (High Strength Low Alloy) steel grades the initial/coiling temperature is about 650°C but to achieve the full strength enhancement potential from the subsequent precipitation hardening the cooling history is crucial to control. A too high cooling rate will disable diffusion of alloying elements before the destined material properties have been achieved. This may result in downgraded or in worst case even scraped material. Having comprehensive knowledge of the thermal history is essential in controlling coiled material properties and in understanding root-causes for different mechanisms. A 2D thermo-mechanical FE model has in this licentiate work been developed to predict the transient temperature distribution as a function of position and stress state in coils when cooling. The model accounts for the imperfect and pressure dependent contact conditions between adjacent laps that cause anisotropic thermal properties in radial and axial directions. The imperfect contact is in a macroscopic level caused by oxide, dust, water and strip shape properties inherited from up-stream processes and in a microscopic level by surface conditions like roughness and slope of asperities etc. The stresses formed under coiling and cooling has a strong influence on the contact conditions and the thermal heat conductive properties in radial direction. In this model the total stress state during cooling is approximated as a combination of initial stresses set by the coiler and the thermally induced stresses caused by thermal gradients. The thermo-mechanical model developed and explained in this licentiate thesis has proven to be able to predict the thermal cooling of coils with good agreement in comparison with the performed measurements campaigns. Furthermore does the model form an accurate platform to be used in a continuing work for further investigations where the thermal trajectory and stresses have an influence e.g. on coil-set effects as length and cross bow or mechanical properties.