The objective of investigation was to determine the influence of chosen cooling rates after finish rolling on final microstructural and mechanical properties of the laboratory rolled products from steel 42CrMo4. Metallographic analysis showed that microstructure of rolled products, which were after finish rolling cooled down in accelerated manner by water sprays, was composed mostly by hardening phases and by smaller amount of ferrite. Microstructure of the rolled product cooled down in furnace in decelerated manner was formed by pearlitic blocks with minority occurrence of ferrite. Laboratory rolled products cooled down by more complicated modes, which consisted of combination of their cooling by water sprays with subsequent annealing in furnace, showed different phase morphology and distinct band structure. Results of tensile test have proved that the applied modes of cooling of rolled products by accelerated manner had only very small influence on their final mechanical properties. Considerably cooling the rolled product by decelerated manner in furnace resulted in drop yield strength, but on the other hand in an increase of ductility. More complicated modes of cooling the rolled products, which comprised also their annealing, showed the possibilities of significant influencing the strength, and particularly the plastic properties of the steel 42CrMo4.
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
A physical simulation of the thermomechanical processing of the Mn-Ni-Cr-Mo low-alloy steel was performed in the laboratory rolling mill Tandem in the Institute of Modelling and Control of Forming Processes at VŠB – Technical University of Ostrava. The task was to determine the influence of the finish rolling temperature on the structural and mechanical properties of the rolled products. After different modes of rolling and slow cooling in the furnace, the final structure of the tested samples was in all cases composed of ferrite, bainite and islands of martensite. The finish rolling temperature markedly influenced a part by volume of the individual phases as well as the structure homogeneity. The results of the tensile tests at room temperature indicated that the studied steel did not show any pronounced dependence of the yield stress on the finish rolling temperature in the investigated range of values (750 – 1000 °C). On the other hand, the closely corresponding dependences of the ultimate tensile stress and elongation exhibited a considerable and very complex course, which can be explained mainly by the martensite fraction originating during the last stage of the final air cooling from temperature 600 °C.
Newly developed methods for heat treatments and any necessary thermomechanical working have led to the achievement of excellent properties for new types of multiphase structures. One of these new methods is the Q-P (Quenching and Partitioning) process. The result is usually martensitic structure with a defined proportion of retained austenite. This process can be used for different kinds of steels with a tensile strength about 2000 MPa with retention of good ductility of more than 10%. The Q-P process was applied to experimental steel whose main alloying elements were silicon, manganese and chrome. The resulting specimens were evaluated metallographically, the proportion of retained austenite was ascertained by Xray analysis and mechanical properties were found using tension tests.
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