Petr Opěla

Transformation diagrams of selected steel grades with consideration of deformation effect

The aim of this article was to assess the effect of previous plastic deformation on the transformation kinetics of selected steels with a wide range of chemical composition. Transformation (CCT and DCCT) diagrams were constructed on the basis of dilatometric tests on the plastometer Gleeble 3800 and metallographic analyses supplemented by measurements of HV hardness. Effect of previous deformation on transformation was evaluated of the critical rate of formation of the individual structural components (ferrite, pearlite and bainite) in the case of formation of martensite respect to Ms temperature. Previous plastic deformation accelerated especially diffuse transformations (ferrite and pearlite), temperature of Ms was lower after previous plastic deformation and bainitic transformation was highly dependent on the chemical composition of steel.

The combined effect of chemical composition and cooling rate on transformation temperatures of hypoeutectoid steels

The transformation temperatures Ar3 and Ar1 of four unalloyed hypoeutectoid steels with a carbon content of 0.029–0.73 % were determined using dilatometric tests. Unusually high cooling rates of 2 and 8◦C s−1 were used intentionally, corresponding to the conditions in the wire rod rolling mills. The developed regression models are phenomenological and allow a simple prediction of transformation temperatures, depending only on the cooling rate and the chemical composition of the steel represented by the carbon equivalent (in the case of Ar1), respectively by the Ac3 temperature (for Ar3). When calculating the Ac3 temperature, it was worth considering its non-linear dependence on carbon content. It has been verified that the derived equations are applicable even at relatively low cooling rates when the austenite decomposes exclusively on ferrite and pearlite. K e y w o r d s: hypoeutectoid steel, carbon equivalent, dilatometer test, cooling rate, decomposition of austenite, transformation temperature

Effect of hot rolling history and cooling rate on the phase transformation of plain carbon steel

Bars from plain carbon steel were hot rolled in the new laboratory semi-continuous mill with the application of various total deformation. The phase transformation temperatures were determined from the surface temperature-time curves registered in the course of free cooling of the rolled products. The Ar3 temperature was influenced by the total amount of the previous deformation, and it increased with the descending rolling temperature. The obtained results were verified by metallographic analyses, and they were compared with the CCT and DCCT diagrams, constructed from the results of dilatometric tests. The performed experiments illustrate the useful cooperation of plastometric and laboratory hot rolling experiments, as well as the potentiality of a simple study of the non-isothermal decomposition of austenite using analysis of the cooling curves. K e y w o r d s: plain carbon steel, transformation temperature influenced by deformation, (D)CCT transformation diagrams, laboratory hot rolling, microstructure, grain size

Laboratory Controlled Rolling of Microalloyed Steel for Production of Seamless Tubes

Using the laboratory rolling mill with smooth rolls, piercing, as well as rolling in a pilger mill of the seamless tubes with diameter 273 mm from the HSLA steel microalloyed with vanadium steel was simulated. Influence of the wall thickness (6.3 – 40 mm) and finish rolling temperature on the final structural and mechanical properties was investigated. Necessary temperatures of the phase transformations in the course of cooling were determined by dilatometric tests. Based on the dilatometry results, finish rolling temperatures were reduced. Lower rolling temperatures yielded in a relative grain refinement. Effect of the finish rolling temperature did not have any marked impact on the tensile tests results. Strength properties decreased only slightly with the increasing wall thickness and the plastic properties were not influenced significantly by this parameter. The positive effect of the reduced finishing temperature appeared markedly in the results of impact tests performed at room temperature only. Notch toughness was increased by approx. 25 % in the case of the wall thickness of not less than 20 mm.

Study of Phase Transformation of Medium Carbon Steel after Hot Plastic Deformation

The CCT and DCCT diagrams of steel C60 (with approx. 0.6 % C) were constructed on the basis of dilatation tests with and/or without an influence of the previous deformation and they were then compared, order to make an evaluation of the influence of the previous deformation on the phase transformation kinetics. For the execution of the experiment, the dilatation module of the plastometer Gleeble 3800 was used. The accuracy of the diagrams was faced with metallographic analyses and measurements. The previous deformation expressly retarded a bainite transformation and slightly accelerated ferrite and pearlite transformations. The martensite start temperature was practically not influenced by the previous deformation; however, the applied deformation caused the creation of the martensite at lower cooling rates.