Svätoboj Longauer

Shape memory effect in a Cu–Zn–Al alloy with dual phase α/β microstructure

Abstract The paradoxical decrease of the one way (OWSME) and the two-way shape memory effect (TWSME) with increasing volume fraction of martensite in dual phase microstructure obtained by quenching from dual phase equlibrium field α/β in a Cu–Zn–Al alloy presented at Icomat ’95 is re-evaluated. It is also found that TWSME degradation by thermal cycling increases with decreasing volume fraction of martensite. Both phenomena are accompanied by changes in degree of order in martensite, solution hardening of the α-phase and the stacking fault energy of martensite. They seem to be caused by the synergetic effect of changes in the stacking fault energy of martensite and solid solution hardening of the α-phase.

Relationship of Microstructure and Mechanical Properties of Dual Phase Steel after Various Annealing Conditions

The development of ultrafine ferrite grain size has become one of attractive way how to improve the behavior of dual phase (DP) steels. The other possible way how to enhance mechanical properties of DP steels is to modify the chemical composition. Therefore object of our investigation was the dual phase steel with modified alloying (three times higher Cr content with addition of phosphorus). The dual phase steel was annealed in laboratory conditions in accordance with three specified annealing cycles: 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. The obtained microstructure after selected annealing regimes consists of three phases (ferritic matrix, martensite and martensite/bainite grains) with different size and distribution. For studied annealing regimes were clearly defined mechanical properties such as: YS, UTS, elongation, n-parameter and ratio YS/UTS. It was defined the scheme of microstructure evolution on base of austenite grain size during the continual cooling process with defined three phases: 1) the hard martensite formed on the grain boundary; 2) the soft interior bainite and 3) the hard isolated martensite.

Interrelationship between Hot Ductility and Particle Size in TiNb IF Steel

The aim of this work was to observe the relationship between hot ductility and morphology, distribution and size of particles in TiNb IF steel after hot torsion testing at the critical temperatures of deformation with low as well as maximum values of plasticity. Transmission electron microscopy showed that the particles at all temperatures of deformation with minimum number of turns to failure e.g. 1132°C (33.72 rev.), 946°C (6.24 rev.), 637°C (5.54 rev.) as well as with maximum value of plasticity at 844°C (1726 rev.) were of globular, cuboid or elliptical shape. EDX analysis revealed that there were different types of particles such as carbides, sulfides, and carbonitrides of Ti and Nb, Al and Si oxides, Mn sulfides, and phosphides. Quantitative evaluation of particle size in the carbon extracted replicas showed 20% of total number of particles with size 2r = 30-39 nm and an average linear dimension = 42 nm at the deformation temperature of 1132°C. There were 28% of the particles with size 2r = 20-29nm and = 41nm at the temperature of 946°C while at the temperature of 637°C there were 29% of particles with size 2r = 20-29 and 26% with size 2r=10-19 nm and = 41 nm. In the case of maximum plasticity (1726 rev.) at 844°C, the presence of large particles was confirmed with = 105 nm size and 9% distribution in three size categories of 20-29, 30-39, 90-99 nm.

Effect of Sample Thickness on Slab Surface Zone Toughness in IF and Microalloyed Steels

The aim of this work was to analyze the effect of sample thickness on toughness of the slab surface zone at ambient temperature in IF and microalloy steels. Transitional slabs with different pulling rates at the start and the end of the slab were used as well as slabs with standard pulling rates. Samples of standard size 10x10x55 mm and also non-standard size 5x10x55 mm were used for Charpy impact testing. It was confirmed that higher toughness values were found in thin non-standard size samples (5x10x55 mm), or more precisely for a larger portion of non-standard samples with toughness values higher than 50 J.cm-2. Using non-standard size samples confirmed the distinct stability of toughness values, as well as their smaller spread between minimum and maximum values. The transitional slabs had very distinct and non-uniform spread of toughness values across the slab width. This suggests greatly deteriorated quality of these slabs. During stable casting of TiNb microalloyed steel at the standard conventional higher slab pulling rate, uniform toughness was confirmed across the whole slab width in both sample types.

Local Notch Toughness of Slab Surface Zone in ULC/IF and HSLA Steels

The aim of this work was to analyze changes in local toughness KCV using Charpy V-notch impact tests in the slab surface zone in relation to the microstructure in ULC/ IF steel and TiNb HSLA steel. Marked heterogeneity in KCV values was confirmed in the surface zone across the width of transitional slabs. Distinct local differences in notch toughness across the slab width were found to be linked primarily with changes in ferrite grain size. Low KCV values in the analyzed steels were linked with coarse grain structure, while much finer ferrite structure was identified in tough samples. The heterogeneity of KCV vales in the analyzed steels may be influenced by differences in thickness of the fine-grain slab surface zone, and by the presence of tertial cementite and (in HSLA steel) of pearlite as well.

Precipitation in Surface Zone of Continuously-Cast Slab of ULC/IF Steel Microalloyed with Titanium

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.