Jiří Kliber

Recrystallization/precipitation behaviour in microalloyed steels

Microalloyed high-strength low-alloy (HSLA) steels contain additions of Nb, V, Ti, or in combination, in amounts of 0.01 to 0.1 weight percent to improve mechanical properties, which are strongly dependent on the thermomechanical interaction taking place in the course of rolling mill processes. The recrystallizatian of hat-twisted austenite has been investigated in a cylindrical specimen (f 6×50 mm) machined from hat rolled plates of 0,052 wt % Niobium microalloyed steel. Continuous and interrupted torsion test were carried out in the temperature range 1123 K to 1173 K after a solution treatment of 1.5 minutes at 1423 K and torque-twist data were analysed. The various methods were discussed for obtaining results from torsion tests. The effect of precipitation kinetics was appreciated by way of connection tp/tp(red), where tp is the experimental measured time for the peak stress and tp(red) is the newly defined reduced time. The softening ratio X and time t0.05R for start of static recrystallization were established. The correlation between precipitation and recrystallization is presented as a graphs for chosen requirements (temperature of austenitization, carbon and niobium content and strain rate). If temperature goes below 850°C, the restoration processes are hardly suppressed, both are limited by diffusion and Nb(CN) precipitation, which are extended dynamically in the range of strains rates 10−2 to 1 s−1. In the present paper, an attempt is made to derive the PRTT diagram and to define all mathematical equations for describing recrystallization times t0.05R, t0.5R, t0.95R and t0.05P for the start of precipitation. In real metal forming processes such as the hot rolling of plates or strips the knowledge of these parameters and results is extremely important for the the correct microstructure and sheet quality to be obtained.

Effects of Thermomechanical Processing on Microstructure and Mechanical Properties Multiphase Steels Exhibiting a TRIP Effect

Effects of hot-rolling conditions on these steels are much less studied than their importance for practice would suggest. It should be emphasized that bainite transformation is the key reaction to enrich non-transformed austenite with carbon. This study was carried out in order to gain understanding of the effect of thermomechanical hot rolling on final microstructure and mechanical properties of C-Mn-Si TRIP steel. Fundamental of the transformation induced plasticity effect – TRIP is the stabilization of substantial amount of retained austenite down to the ambient temperature by thermomechanical processing and its subsequent transformation into strain induced martensite as a consequence of applied plastic deformation. The special prepared stepped specimens were rolled on laboratory tandem mill. The effects of finish rolling temperature, strain and isothermal bainite transformation temperature on mechanical properties of mentioned TRIP steel were evaluated (mechanical properties were examined with tension test). Major deformation, higher finishing rolling temperature and higher temperature of bainite hold result in drop in strength. Proportionately to the drop in strength, the ductility grows in the TRIP steel. Microstructures were examined with X-ray diffraction (retained austenite). Image analysis software was used to process SEM micrographs of structure (ferrite, bainite assessment). Plastometric testing was conducted on GLEEBLE 3800 thermo-mechanical simulator. First stage of experiment yielded stress-strain curves for various temperatures and strain rates. Gleeble 1500 was used for the remaining plastometric simulation. Specimens were reheated to austenitization temperature of 1100°C and soaked. Then they were cooled to the temperature of deformation and subsequently cooled at higher rate down to the bainitic transformation temperature (400 – 550 °C). Specimens were held at the bainitic transformation temperature and then air-cooled. Final microstructures were evaluated with respect to transformation diagrams and optical microscopy findings. Higher bainite volume fraction was found in the specimens cooled at higher cooling rate as compared with more slowly cooled specimens.

Hot deformation behaviour and grain refinement of intermetallic compound Ni3Al

The paper describes the preparation of Ni3Al-based intermetallic compounds and investigations of their structures in the ascast, worked, and annealed states. The performed experimental works approved a possibility of hot working of the highly brittle polycrystalline intermetallic compound Ni3Al with large grain cast structure. The facility of originating the recrystallization seems to be surprising in such a material. On the other hand, the growth of recrystallization nuclei is strongly retarded. The case is further complicated by the fact of heterogeneous deformation in the microvolume. Microalloying with boron or zirconium influenced the original structure parameters as well as the optimum deformation temperature. Simultaneously the probability of a transgranular cleavage fracture increased. For the stoichiometric Ni3Al intermetallic compound the optimum deformation temperature 1150 °C has been assessed. At temperature 1250 °C, the alloy Ni3Al + 0,1 at. % B manifested the best plasticity. The special type of rotary working combined with friction heating gave the best results from point of view of obtaining a fully recrystallized region.

Study of concast steels plasticity in low-temperature region

At bending and straightening the continuous cast steel semi-products, there can transverse cracking occure. The relation between strand temperature and course of repeated austenite/ferrite transformation is one of the most important circumstances influencing that phenomenon. At plain carbon Al-killed steel, deformation taking place by strain rate of order 10−4 s−1 was simulated using the computer-controlled torsion plastometer. There was compared its plasticity at constant deformation temperatures and/or at temperatures cycling within ± 50 °C (by help of special software, the combined effects of water spray cooling and heat conduction inside the strand were simulated). There was established that such temperature regions exist, in which a temperature oscillation and thus induced periodical course of transformation processes improve the resistance of material to transverse cracking. If do not holding to some specific conditions, temperature oscillation on the contrary deteriorates plasticity.

The Microstructural Properties of Microalloyed Steel after Plastometric Testing

With the aid of an ASC-type finishing line (Automatic System Control) and continuous cooling of the rolled stock in water boxes, modern continuous light mills enable thermomechanical (controlled) rolling of SBQ-type rods (Special Bar Quality). During the thermomechanical forming (at different temperatures or cooling rates etc.), one can analyze the quality parameters of roundsection steel bars. In order to obtain better understanding to the forming conditions (temperature, strain and strain rate) and the possibility of cooling (related to its rate and time), an experiment involving continuous and discontinuous testing by means of a Gleeble plastometer was suggested. After its implementation, the attention was focused on the mathematical interpretation of the stressstrain characteristics of the steel and on metallographic observation of microstructural changes. The results achieved are of universal meaning and contribute to the theoretical understanding of controlled forming.

Transformation Induced Plasticity (TRIP) Effect Used in Forming of Carbon CMnSi Steel

Transformation induced plasticity (TRIP) steel combines high strength and high ductility that makes it particularly suitable for forming. Martensite within a ferrite matrix is usually obtained either by continuous casting of slabs followed by hot rolling (which is the fastest method, hence the most economical one, producing, however, relatively thick products) or by the continuous casting of slabs followed by hot rolling, cold rolling and annealing (the method used for thin products). High cooling rates, low coiling temperatures and low reduction during hot deformation were generally found to suppress the formation of polygonal ferrite and promote the presence of retained austenite. This paper focuses on development and modifications of two CMnSi-based TRIP steels with 0,23 % C;1,4 % Mn; 1,9 % Si; ( 0,08 % Nb) by means of laboratory thermomechanical processing. Description of experimental devices for the analysis of transformation plasticity under tensioncompression loading is given. Experiments were carried out on the simulator for thermaldeformation cycles SMITWELD and TANDEM was used for thermomechanical processing on the laboratory rolling mill. The maximum volume fraction of retained austenite and the resulting optimum combination of tensile strength and ductility were achieved in testing heats. Special attention was paid to volume fraction changes of single phases and to changes in morphology of phases. The results suggest that rather short isothermal bainite transformation times are sufficient to obtain TRIP microstructure. The influence of parameters of thermomechanical processing such as the amount of strain, forming temperature and austenitization time and temperature on microstructures of TRIP steels were evaluated.