Papers by Author: Bohuslav Mašek

Abstract: With the prices of raw materials as well as final products growing a significant tendency to search for weight reduction of the machine components through improving their quality can be observed at present. To achieve as good mechanical properties as possible, efforts are made to reach the finest microstructure possible in steels. Besides the fine grain microstructure the required phase volume fraction must also be obtained in case of AHS steels to ensure the possibility of utilizing the TRIP effect during the final cold deformation. Their structure usually consists of ferrite, bainite and retained austenite, which transforms to deformation induced martensite during final cold deformation. With an appropriate volume fraction of these phases the materials show good combination of strength and ductility. In the experiment eight and twenty step incremental deformation was applied within identical thermomechanical sequences in the deformation temperature range of 900 – 720°C. At the same time the influence of twenty step incremental deformation on the development of microstructure was examined down to 600°C. Comparison of the results was carried out by means of light and electron microscopy. The volume fraction of structural phases including the fraction of retained austenite was determined by image analysis.

Abstract: All sectors of industry experience high demand for shaped products with as good mechanical properties as possible at low costs. Automotive industry, in addition, requires that the parts are of lightweight construction. Consequently, new types of materials and processes have to be combined to design new production chains capable to meet this demand. For instance, there are high-strength low-alloyed steels, whose final properties are attained by advanced heat treating techniques. One of such techniques is the Q&P process which can deliver excellent ultimate strengths exceeding 2000 MPa at a sufficient elongation level of 10 %. When combined with an unconventional forming method, it allows complex-shaped parts with outstanding mechanical properties to be made. One example of such combined procedure is the sequence of internal high pressure forming, hot stamping and Q&P processing. In the present study, thin-walled hollow stock was processed using such a combined procedure. After stepwise optimization of processing parameters, products with martensitic structure and a small amount of bainite were obtained. In all locations of the product which were tested, the ultimate strength exceeded 1950 MPa and elongation reached 15 %

Abstract: Thixoforming is an alternative forming method, by which intricate and complex-shaped products can be manufactured using a single production step. This technology allows a material’s microstructure to be altered profoundly. Typical microstructure of steels processed in this manner consists of quasi-polyhedral austenite grains embedded in a ledeburite-carbide network. This type of microstructure was produced by processing the experimental material in this study: the X210Cr12 steel. Since austenite is a metastable component depending on oversaturation with a number of elements, its thermal and mechanical stability needs to be known. This information is required for further modification and enhancment efforts. In previous experiments, the thermal stability was tested by thermal exposure. In the present work, the behaviour of austenite was explored under mechanical load at room temperature in a micro-compression test. A single block of austenitic material was used for making a test specimen with the dimensions of 2.4×2.2×4.9 µm. Its mechanical properties were measured and deformation stability was investigated using compressive deformation.

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: 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.