Materials Science Forum Vols. 706-709

Abstract: In the present study non-recrystallisation (Tnr) and Ar3 temperatures have been determined for the C-Mn steels from multi-pass hot torsion experiments with continuous cooling in the temperature range of 1260°C to 600°C. Results show that Tnr decreases with increasing strain/pass, strain rate or interpass time. An alternative approach based on the work-hardening rate is proposed for the determination of Tnr and is shown to be more suitable in case the usual mean flow stress method does not provide a clear Tnr value.

Abstract: This paper reports the framework of a computer model that calculates the precipitation kinetics of MX type carbides or carbonitrides from austenite matrix in microalloyed steels during hot rolling. The kinetic model is based on the classical Johnson-Mehl-Avrami theory adapted to include the saturation of nucleation sites. The effect of deformation on precipitation kinetics is quantitatively described through its effect on flow stress, from which the number of potential nucleation sites can be estimated. The time-temperature precipitation diagram can then be calculated for a given alloy chemistry and deformation conditions. A preliminary study has been carried out to test its performance in both undeformed and deformed conditions.

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: Recent observations regarding the transformation of deformed austenite are reviewed. It is shown that superequilibrium ferrite and pearlite can be formed at temperatures well above the Ae₃ and Ae₁, respectively. The role of the stored energy associated with the introduction of the dislocations introduced by the deformation is discussed. It is shown that the forward dynamic transformation into ferrite and pearlite is several orders of magnitude faster than the reverse static transformation back into austenite. The retarding effect of alloying additions such as niobium is also outlined. The results are interpreted in terms of the effect of deformation on the modified phase diagrams pertaining to the transformation of deformed austenite.

Abstract: A mathematical model was developed predicting the effects of alloying and thermomechanical processing on the final microstructure of steel. Various factors influencing transformation kinetics, including microalloying with Nb and plastic deformation of austenite, are considered. Subsequent stages of the model development and calibration are described.

Abstract: The use of CSP^® thin slab casting followed by direct thermomechanical rolling is well placed for the production of low-carbon Nb microalloyed steels. In this process thin slabs of between 48 and 90 mm thickness are cast and directly hot rolled to hot strip typically between 1 and 12 mm thick. To obtain optimum strength and toughness property combinations in a direct rolling process, hot rolling has to compact the dendritic as-cast microstructure and to achieve a fine-grained microstructure. This affords a two-stage rolling strategy with start rolling above the recrystallization stop temperature and finish rolling in the non-recrystallization temperature range. Temperature and deformation in the first stand should be as high as possible in order to delete the initial as-cast microstructure by complete recrystallization. Based on these considerations, SMS Siemag further developed the CSP^® concept including features allowing isothermal rolling in the first stands of the finishing mill. The present contribution gives the results of a laboratory study of this innovative approach. The report concludes with resulting new plant configurations for improved high strength and API linepipe grade production.

Abstract: Stress-relaxation method was first used by Liu and Jonas in 1980s for following the kinetics of strain induced precipitation of carbonitrides in austenite. Since then, the method has been widely employed for studying not only the precipitation kinetics but also the recovery and recrystallization processes in steels and other alloys. In the present paper, the principle and the worldwide usage of the method were reviewed. The dependence of stress relaxation rate on the occurrence of precipitation, recovery and recrystallization was interpreted based on a dislocation model. The capabilities and the limitations of the method were discussed and ways to overcome the existing limitations were also proposed.

Abstract: The effect of Al addition on the static softening behavior of C-Mn steels was investigated. The compositions of the steels studied are representative of the recently developed TRIP-assisted steels: a base composition of 0.2%C, 2%Mn, 50ppm N and three different Al levels, 0.03 (base steel), 1 and 2%. Double-hit torsion tests were performed at different deformation temperatures, in the range 950°C to 1100°C, and pass-strains, =0.2 and 0.35. It was found that solute Al produced a significant retardation on static recrystallization kinetics, equivalent to that exerted by 0.026%Nb for the 1%Al steel and to 0.05%Nb for the 2%Al steel. Additionally, at the lowest temperatures (950-1000°C) and 2%Al level, austenite to ferrite phase transformation was found to be concurrent with softening, enhancing retardation in the mechanical softening.

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: After 1780, wrought iron (WI) provided a structural material and steel was cherished for its hardness and cutting qualities. When available in quantity after ~1860, steel’s structural strength and wear resistance were recognized in normalized condition in armor plates, rails and drawn wire. The responsible microstructure component was pearlite in which the lamellar spacing of ferrite and carbides could be refined by simple bulk heat treatments that are practiced with small modification until today. The strength and toughness rose as the layer thickness decreased the ferrite slip length and the carbide cracking. In hot working, the strength rises as much as 200% (while ductility falls) with fraction of pearlite; below the transus compared to austenite just above it, strengths are equal at about 0.7C (ductilities equal at 0.35C).