In this article, a Mo‐Mn dual phase steel and its process parameters in hot rolling are discussed. The process window was derived by combining the experimental work in a hot deformation dilatometer and numerical calculation of the process parameters using rate law models for ferrite and martensite transformation. The ferrite formation model is based on the Leblond and Devaux approach while martensite formation is based on the Koistinen‐Marburger (K‐M) formula. The carbon enrichment during ferrite formation is taken into account for the following martensite formation. After completing the parameter identification for the rate law model, the evolution of phases in this steel can be addressed. Particularly, the simulations allow the prediction of preferable degree of retained strain and holding temperature on the run out table (ROT) for the required ferrite fraction.
The phase transformation kinetics of a hot rolled transformation induced plasticity (TRIP) steel is investigated by means of in situ high energy X-ray diffraction. The samples are deformed and annealed according to the parameters of a hot rolling process in a deformation dilatometer installed in the synchrotron beamline. Simultaneously, the phase fraction and the carbon content in the remaining austenite were measured by X-ray diffraction. It is advantageous that the retained austenite (RA) can be measured without sample preparation, which usually results in unwanted strain-induced martensite transformation. This study shows that enough carbon enrichment of austenite can be achieved by forming carbide free bainite. However, too little fraction of the RA for the TRIP effect can be formed without the pre-deformation. The pre-deformation also promotes the decarburization of the bainitic ferrite especially at high temperature. Fig. 6. The change in austenite fraction during quenching from the bainite formation temperature (a) 500°C (b) 475°C (c) 450°C (d) 425°C (e) 400°C (f) 375°C. The solid symbols represent the cases with pre-deformation of 0.6. P. Suwanpinij et al./In Situ High Energy XRD for Hot Rolled TRIP Steel ADVANCED ENGINEERING MATERIALS 2014, 16, No. 8Fig. 8. The relationship between the linear thermal expansion of austenite and the calculated carbon content. P. Suwanpinij et al./In Situ High Energy XRD for Hot Rolled TRIP Steel ADVANCED ENGINEERING MATERIALS 2014, 16, No. 8
In this article, the optimal control of a cooling line for production of dual phase steel in a hot rolling process is discussed. In order to achieve a desired dual phase steel microstructure an optimal cooling strategy has to be found. The cooling strategy should be such that a desired final distribution of ferrite in the steel slab is reached most accurately. This problem has been solved by means of mathematical control theory. The results of the optimal control of the cooling line have been verified in hot rolling experiments at the pilot hot rolling mill at the Institute for Metal Forming (IMF), TU Bergakademie Freiberg.
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