Papers by Author: Hana Jirková

Abstract: Three low alloyed transformation induced plasticity (TRIP) steels with 0.2%C were used in this work. The first one was based on the most common and popular 0.2%C - 1.5%Mn - 1.8%Si concept and was used as a reference material. The second steel was further micro-alloyed by 0.06% of Niobium. The third steel was designed with lower manganese content of 0.6% and additional alloying by 0.8% of Chromium. Thermo-mechanical processing with incorporated incremental deformation was applied to each steel. Various cooling rates and numbers of deformation steps were tested with regard to final microstructure and properties. After this optimization, microstructures with the potential to utilize TRIP effect were achieved for all steels. Very good mechanical properties were obtained with ductility typically in the interval of 30-40% and the tensile strengths in the range of 680-835 MPa.

Abstract: Semi-solid processing of steels is typically studied using high-alloy steels with higher carbon levels, as those offer a long freezing range which is favourable for conducting the process. The drawback to their application is their microstructure which typically consists of austenite grains embedded in ledeburitic network. This type of microstructure typically fails in brittle manner by fracturing along the interface of the hard network and ductile austenite grains. This is why a way was sought to altering or even inverting the configuration of the microstructure. Eventually, suitable steel chemistries were found which allow the inverted microstructure to be obtained. With regard to the high content of alloy additions, these steels have to be made by powder metallurgy methods. Five different steels of this kind were selected for the experimental programme. All contained high amounts of alloying elements and a large fraction of carbides. Their carbon content was taken into account as well, ranging from 0.55 to 3.4 %. Differences between the steels consisted in the levels of major alloying elements, namely chromium, vanadium, molybdenum, tungsten and cobalt. After suitable process parameters were found, semi-solid processing was used to prepare demonstration products. The transition through semi-solid state transformed the ferritic matrix to austenitic-martensitic one, in which the high-stability carbides were retained. The resulting microstructures were of unconventional nature where carbide particles were embedded in tough metal matrix. Their configuration was thus inverted in contrast to the ones typically obtained by semi-solid processing of tool steels.

Abstract: The cold formability of ferritic-pearlitic steels is one of the base parameters for material choice for different forming parts. One of the key factors is the pearlite morphology, which is strongly dependent on chemical composition and previous treatment history. The carbides in pearlite occur mainly in the lamellar form. One of the ways of improving the ductility along with formability is the change of lamellar carbides to globular carbides. This can be conventionally done by soft annealing, which is characterised by long processing times and high energy costs. This paper presents a new processing modification which can lead on the one hand to significant shortening of carbide spheroidization times and on the other hand to intensive refinement of grain size even for low-carbon steels. Low temperature thermomechanical treatment with variation of the heating temperature around A_c1 and incremental deformation was examined on low carbon plain RSt-32 steel. After the thermomechanical treatment conditions were optimized, the refinement of the ferritic grains from an initial 30 μm to circa 5 μm took place, and the time necessary for carbide spheroidization was shortened from several hours to several seconds.

Abstract: The use of the combined influence of retained austenite and bainitic ferrite to improve strength and ductility has been known for many years from the treatment of multiphase steels. Recently, the very fine films of retained austenite along the martensitic laths have also become the centre of attention. This treatment is called the Q-P process (quenching and partitioning). In this experimental program the quenching temperature and the isothermal holding temperature for diffusion carbon distribution for three advanced high strength steels with carbon content of 0.43 % was examined. The alloying strategies have a different content of manganese and silicon, which leads to various martensite start and finish temperatures. The model treatment was carried out using a thermomechanical simulator. Tested regimes resulted in a tensile strength of over 2000MPa with a ductility of above 14 %. The increase of the partitioning temperature influenced the intensity of martensite tempering and caused the decrease of tensile strength by 400MPa down to 1600MPa and at the same time more than 10 % growth of ductility occurred, increasing it to more than 20%.

Abstract: Semi-solid forming processes belong to the progressive technologies which can be exploited for the manufacturing products with complicated shapes and special material and utility properties. Semi-solid metal forming takes place at temperatures between solidus and liquidus combining the advantages of metal forging and metal casting processes. The paper is focused on the analysis of microstructures and phase composition of the X210Cr12 steel identified in the thin-walled products after forming in semi-solid state. Microstructure of the X210Cr12 steel after rapid cooling and solidification from semi-solid state is formed by quasi-globular grains of metastable austenite surrounded by carbides and fine eutectics. The fraction of metastable austenite in structure was found to be 96 % exhibiting the thermal stability up to the temperature of 500 °C. Along with experimental study, the numerical analysis of the material flow during a die cavity filling was carried out using developed 2D simulation model taking into account one-phase non-newtonian flow.

Abstract: Material-technological modelling has made great progress over recent years, thanks to the new possibilities opened up by developments in sensor technology, and especially in new methods of control, supported by innovative electronic elements and electronic circuits. One such device, developed for material-technological modelling, is the thermomechanical simulator which was established in the laboratories of the Research Centre of Forming Technologies FORTECH, in Pilsen, in the Czech Republic. Thanks to new knowledge and technical equipment the majority of technological processes or even technological chains can be modelled. The most considerable and most important innovation in the material-technological modelling process is the significant acceleration and increased precision of the modelling process. The present technology even allows modelling of highly dynamic processes, such as wire rolling including all thermodynamical effects. This paper presents the broad possibilities of the most modern material-technological modelling. The process of detecting technical and manufacturing problems during rolling and the possibilities of failure elimination are introduced in a practical example.

Abstract: The concepts new types of materials are, for economic reasons, focused mainly on low alloyed steels with a good combination of strength and ductility. Suitable heat and thermo-mechanical treatments play an important role for the utilization of these materials. Different alloying strategies are used to influence phase transformations. The quenching and partitioning process (Q-P Process) is one of the heat treatment methods which can result in a high ultimate strength as well as a good ductility. However, these good properties can be obtained only if a sufficient amount of retained austenite is stabilized. The influence of different contents of manganese, silicon and chromium on microstructural development and mechanical properties were experimentally tested. Alloying elements were used to stabilize the retained austenite in the final microstructure and also to strengthen the solid solution. Ultimate strengths of over 2000MPa with ductility over 10% were reached after the optimization of the Q-P Process. The microstructures were analyzed using several microscopic methods; mechanical properties were determined by a tensile test and the volume fraction of the retained austenite was established by X-ray diffraction phase analysis.