Materials Science Forum Vol. 782

Abstract: The work experimentally analyses the effect of various factors on hardness measured values on thin steel sheets using Ultrasonic Contact Impedance (UCI) technique. The conditions experimentally used are compared with that according the ASTM A 1038-08 standard. UCI is an experimental technique for indirect hardness measurement. The equipment uses a Vickers indenter and the hardness measurement is based on the change of the resonance frequency during indenter ́s penetration [.The UCI hardness may depend on some factors, therefore optimal measurement conditions must be determined. The effect of distance between indents, zinc coating, sample weight, sample mounting and adhesive material for sample fixation were determined.

Abstract: The paper deals with investigation into segregation behavior of selected elements in longitudinal cut of continuous steel slab, in the breakout area. The breakout occurred after a flying change of tundish in order to begin casting of another steel grade. Altogether 11 samples were taken from the part of a solidified slab. Concentrations of selected elements (Al, Si, P, S, Cr, Mn, Ni and Mo) were measured in these samples using scanning electron microscope and energy dispersive spectroscopy. Using the original mathematical models the basic micro-segregation characteristics and the parameter of macro-heterogeneity were further determined for each analyzed element. Then a quantitative measurement of inclusions ("micro-purity") in the samples was performed using a metallographic microscope. Then method of differential thermal analysis was used for the measurements of temperatures of phase transformations. The following main results were found: - magnitude of micro-segregation of the analyzed elements in the measured sections of 1000 μm is approximately the same in all the analyzed samples, - chemical macro-heterogeneity is very high across the analyzed slab section, - very uneven mixing of melts of both steels was probably one of the main causes of formation of the breakout.

Abstract: The aim of this work was to analyze the morphology and distribution of the microalloy precipitates in the slab surface zone of ULC/IF steel microalloyed with titanium. The slab was made by continuous casting using two different slab pulling rates. Transient slabs were pulled with pulling rate 0.4 m/min at the start and 0.8 m/min at the end of the slab. It was confirmed that morphology of the particles evaluated in the surface areas of slab were globular, cubical or elliptical shape and have been identified as of TiS, TiN and TiC. At the lower drawing rate particles from the middle cut-out from an area with coarse ferritic grains at the slab surface attained an mean size of 2r = 41.8 nm, and from an area with fine ferritic grains they attained an mean size of 2r = 32.5 nm. At the higher drawing rate particles in the middle cut-out attained an mean size of 2r= 63.5 nm. The coarser particles were found in areas with coarse ferrite grains and at higher pulling speed.

Abstract: Two slabs of Ti-Nb microalloyed steel were analysed in this work. The first slab was transitional with the initial pulling rate 0.43 m.min^-1 and the final pulling rate 0.9 m.min^-1. The second slab was cast at the real production pulling rate 1.03 m.min^-1. The presence of larger amounts of oscillation marks was observed on the first slab at both pulling rates. The second slab showed no oscillation marks. At the lowest pulling rate, cracks were discovered below the slab surface, often below oscillation marks. Cracks were seldom observed at pulling rate 0.9 m.min^-1. In the first slab, especially at the low pulling rate, the presence of cracks and pores was found. Pores were observed often with clusters of aluminum oxides. The microstructure of the slab surface zone was characterised by heterogeneity of ferrite grain sizes at all three pulling rates. This heterogeneity was manifested mainly in the marginal cut-outs at all pulling rates. In the marginal cut-outs the microstructure was granulometrically finer at all three pulling rates than in the central cut-outs. Non-equilibrium microstructure in the marginal cut-outs was also observed.

Abstract: 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.

Abstract: The kinetics of austenite recrystallization was evaluated in range of typical forging temperatures (850 - 1250) °C. The steels SA-508 and 3.5Ni-1.5Cr were compared. The laboratory one step deformation was applied. The effect of selected chemical elements in evaluated steels on grain growth and recrystallization kinetics of austenite and precipitation was monitored using metallographic methods. The retarding of static recrystallization was proved and no abnormal grain coarsening at defined condition was observed.

Abstract: The paper presents results of in-situ neutron diffraction experiments aimed on monitoring the phase evolution and load distribution in TRIP steel when subjected to tensile loading. Tensile deformation behaviour of TRIP steel with different initial microstructures showed that the applied tensile load is redistributed at the yield point and the harder retained austenite (Feγ) bears larger load then ferrite (Feα) matrix. After load partioning is finished, macroscopic yielding comes through simultaneous activity of the martensite transformation (in the austenite) and plastic deformation process in ferrite. The steel with higher volume fraction of retained austenite and less stronger ferrite appears to be a better TRIP steel having efficient structure for better plasticity purpose.

Abstract: The work presents the results on grains refinement of steel containing 0,45 wt pct carbon resulted from severe plastic deformation (SPD). Different steel structures from prior solutioning and/or thermomechanical treatment were prepared for deformation experimental. A coarse grain ferrite-pearlite structure was achieved applying solutioning. By application of thermomechanical (TM) controlled forging process, performing multistep open die forging, the refined ferrite-pearlite mixture was prepared. Final structure refinement of steel, having different initial structure, was then accomplished applying warm Equal Channel Angular Pressing (ECAP) at 400°C. Employment of this processing route resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. Applying the highest shear stress (ε_ef - 4) the mixed structure of subgrains and ultrafine grains was present within the ferrite phase. In pearlite grains, modification of cementite lamellae due to shearing, bending, twisting and breaking was found efficient. The coarse cementite lamellae spheroidization was more efficient in prior TM treated steel. The tensile deformation records confirmed strength increase and diversity in strain hardening behaviour.

Abstract: The aim of present article was to consider the influence of annealing parameters on evolution of microstructure and mechanical properties of dual phase steel. Dual phase steel was annealed according to the three chosen cycles of annealing: into intercritical region (780°C), into austenite region (920°C) and into austenite region (920°C) by subsequently cooling into intercritical region (780°C) with the hold at the temperature of 495°C. Tensile tests of the heat-treated specimens were carried out. The obtained microstructure consists from three phases: ferritic matrix, austenite and martensite. Nanoindentation experiments were performed with the peak load of 19.62 mN for ferrite grains and 0.981 mN for austenite and martensite grains, using a Berkovich tip as an indenter. The nanohardness for ferrite and martensite was 2.5 ±1 GPa and 7.1 ±1 GPa and for austenite the nanohardness varied from 4.1 to 4.5 GPa.

Abstract: Pearlite spheroidisation of 100CrMn6 steel was investigated. This process is well known and studied during conventional soft annealing. Presented paper describes cementite lamellae fragmentation during accelerated carbide spheroidisation. Mechanism of cementite lamellae fragmentation during conventional soft annealing depends on carbon and iron diffusion in ferrite-cementite system. On the other hand, accelerated carbide spheroidisation relies on partial pearlite austenitization and backward austenite decomposition. Aim of presented experiments was to examine shape evolution of cementite particles during transition from lamellar to globular form. Pearlite spheroidisation is normally quantified by image analysis of 2D metallographic section. Conventional metallographic observation was used for globular-lamellar particle ratio estimation. However, whole lamellae observation is necessary for spheroidisation process revelation. Ferrite matrix deep etching and cementite separation was performed to study morphological aspects of acceolerated carbide spheroidisation.