Optimalizace termomechanického zpracováni oceli 42CrMo4 v podmínkách profilové válcovny

Abstract

Continuously rising requirements to service properties of steels, together the current tendency aimed at reduction of manufacturing costs requires systematic scientific and research activity dealing with an optimisation of conditions of production and following processing of these materials. The presented dissertation thesis deals with optimisation of thermal-mechanical treatment of the steel grade 42CrMo4 in conditions of section mill. The investigated steel is broadly used namely in machine-building and in automotive industries. For this reason it is very important to investigate intensively not only deformation behaviour at hot forming, but also structure forming processes, which influence the resulting final properties of investigated steel. For this purposes particularly physical modelling of forming processes was used on several laboratory devices, which was supplemented by mathematical modelling in the program QTSteel 3.1. On the basis of results of continuous torsion tests performed on plastometer SETARAM, an activation energy of the given steel was determined, which, together with other material constants, was afterwards used for mathematical description of kinetics of dynamic recrystallisation and for prediction of flow stress of the given steel. Transformation diagrams CCT and DCCT for the steel 42CrMo4 were determined by dilatometric analysis and mathematical modelling in the program QTSteel 3.1. The corresponding temperature conditions at finish rolling, coiling and namely cooling of investigated steel, which is at the continuous small-section mill in Třinecké železárny a.s. (TŽ a.s.) coiled into coils, were simulated by laboratory rolling on the Tandem mill. The obtained findings were then used at industrial scale experiment, aimed at verification of possibility of accelerated cooling of coils made of the steel 42CrMo4, which were coiled in Garret’s coilers. The purpose was to achieve such micro-structural state of coiled material, which would enable shortening of duration of subsequent heat treatment in comparison with the existing mode of spheroidising annealing, which ensures structure homogenisation and simplifies the next process of bar drawing that is coiled into a coil.