Ivo Schindler

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.

A new model describing the hot stress–strain curves of HSLA steel at high deformation

Abstract The torsion test is often applied for the determination of the hot flow stress of steels. An originally derived system of equations has been used to describe the basic parameters of the torsion test and has enabled precision to be obtained in the simulation of flat rolling. Because of the relatively low strain rates obtained with a torsion plastometer, it is necessary to extrapolate the laboratory results into the range of real working conditions. In this case, only a limited number of demanding and expensive interrupted tests have to be done. The more important role is then played by computer prediction based on equations that incorporate the results of simple, but mainly continuous tests. Examples of such mathematical models are presented, including the influence of dynamic or static softening in hot rolling.

FeAl-based alloys cast in an ultrasound field

Abstract An ultrasonic device was designed to fabricate relatively small (about 0.7 kg) vacuum chill castings of FeAl-based alloys with improved microstructure. The influence of ultrasound on the solidification microstructure of the Fe-40 at.% Al based castings was investigated. The subsequent hot-rolling of such an as-cast alloy was made using a special stainless steel bandage of the casting. This effectively prevents fraying and cracking caused by the thermal shock arising at the surface of the hot casting, when it comes in contact with a cold roller. The efficiency of ultrasonic vacuum casting, based on the influence of the acoustic cavitation on the solidification of the melt, is manifested by a refined microstructure of the prepared iron aluminides Fe-40 at.% Al with addition of C or Zr and B. First results of improved mechanical properties of these alloys are presented and discussed.

Microstructure and mechanical properties of cold rolled, annealed HSLA strip steels

A large testing programme of a strip steel grade, microalloyed by vanadium, titanium and niobium, was conducted. The experiment was based on combination of cold rolling, recrystallization annealing, mechanical testing, metallographic examinations, SEM and TEM analyses. Samples in the form of stripes with dimensions of 3.9×25×500 mm were rolled in several passes with the total height reduction of 5 to 75 %. Particular partial strains were realized at room temperature in the housingless, hydraulically prestressed laboratory mill. Afterwards the laboratory mill products were annealed in the vacuum furnace with the protective gas atmosphere consisting of N2+H2. The annealed samples underwent the tensile test at the room temperature and the Brinell hardness test. The gained results – hardness, yield stress, tensile strength and their ratio, as well as elongation A80, were summarized in dependence on cold deformation before annealing.

Recrystallization in as-cast polycrystalline intermetallic compound Ni3Al

Abstract The Ni 3 Al-based alloys were hot formed and their dynamic as well as postdynamic recrystallization capability studied. In the as-cast material, the large original grain size and inhomogeneous strain distribution complicate the softening processes. It is very easy to provoke recrystallization in such a material. Strains ranging from 0.05 to 0.2 lead to the nucleation of dynamically recrystallized grains along the primary grain boundaries and twins. On the other hand, to achieve thorough recrystallization is often difficult. It depends chiefly on previous deformation, less on annealing time, temperature or microalloying. In some cases, annealing does not lead to any significant increase of recrystallization fraction but only to the growth of selected grains. The localization of strain plays a dominant role.

Complex Flow Stress Model for a Magnesium Alloy AZ31 at Hot Forming

Abstract Compression tests of magnesium alloy Mg-3Al-1Zn (AZ31) at different temperatures and strain rate were made on plastometer Gleeble 3800. Deformation behaviour and particularly shape of stress-strain curves of the alloy AZ31 differ significantly at low and high values of Zener–Hollomon parameter Z. The border between these areas was determined mathematically as Z = 2.9E+13 s–1. While the calculated activation energy Q was for both these areas practically identical (157 or 155 kJ mol–1), mathematical description of coordinates of the peak stress differs considerably. Regression and statistical analysis of experimental data have confirmed unequivocally, that it was impossible to describe by a uniform equation the whole set of data (i.e. traditional stress-strain curves, as well as those with atypical initial stage, given by the massive twinning). Thats why two mathematical models were developed enabling prediction of the flow stress of investigated magnesium alloy in dependence on temperature, strain and strain rate, with inclusion of the influence of dynamic recrystallisation.

Homogenization of Microstructure of Castings from the Alloy Fe-40at.%Al-Zr-B

The samples intended for the study of static recrystallization on plastometer Gleeble were prepared from the laboratory castings of iron aluminide containing 24.6 Al 0.17 Mn 0.16 Zr 0.026 B 0.004 C (in wt. %, remainder Fe). Nevertheless, the structure analysis discovered that the results were excessively influenced by the huge heterogeneity of the as-cast microstructure, mostly of the grain size. Combination of the hot forming and recrystallization process during the long-term high-temperature annealing was selected for the necessary structure homogenization. As the tested intermetallic alloy is extremely brittle and susceptible to surface cracking, the original method was applied for its processing. The method consists in hot rolling in the protective capsules welded from the ferritic stainless steel sheet. The castings were rolled to 2/3 of their thickness by 4 reductions with the inter-stage heating, and then annealed at the temperature of 1200 °C in the vacuum furnace for several periods. Metallographic analysis revealed that annealing lasting 7 hours was essential for the uniform coarsening of the recrystallized grains. Material processed in this way proved successful for the subsequent metallographical study of static recrystallization.

Influence of Finish-Rolling Conditions on Microstructure and Mechanical Properties of Low-Alloy Mn-Ni-Cr-Mo Steel Grade

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.

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.