Materials Science Forum Vol. 753

Abstract: An investigation into the influence of the reheat temperature and the austenite deformation temperature on Nb precipitation and recrystallisation kinetics was carried out for a steel containing 0.081C–0.021Ti–0.064Nb (wt. %). Thermo-mechanical processing was carried out using a Gleeble 3500 simulator. The austenite grain structure was correlated to the dispersive properties of Nb atom clustering and precipitation. Irrespective of the reheat temperature, deformation to 0.75 strain at 1075 °C produced a fully recrystallised austenitic microstructure. After deformation at 975 °C, only partial recrystallisation was observed in the samples reheated to the higher temperature, whereas samples reheated to the lower temperature were fully recrystallised. The influence of solute drag and particle pinning effects on the recrystallisation rate is discussed.

Abstract: Under certain conditions of temperature, time and deformation, static recrystallization of austenite in microalloyed steels can be temporarily inhibited by means of the strain-induced precipitation of nanoparticles that cause a pinning effect on austenite grain boundaries in motion. This inhibition can be seen by the formation of a “plateau” in the curves of static recrystallization of austenite obtained from double-deformation tests carried out under isothermal conditions. In this work, several microalloyed steels with different compositions are studied by hot torsion tests in order to characterize the kinetics of recrystallization and its inhibition. The precipitation state in austenite is studied in several samples by means of transmission electron microscopy. The influence of the type of microalloying element (Al, Nb, V) and the mean size of the precipitates on the duration time of the plateau is studied and relationships between these variables can be obtained. Particularly, it is seen that Al-alloyed steels present a much coarser particle size and a considerably shorter plateau compared to Nb and V-microalloyed steels.

Abstract: The austenite recrystallization and grain growth during thermo-mechanical control processing (TMCP) of a pipeline steel grade is described and analysed in terms of precipitation state progress. The influences of finish rolling temperature and cooling rate on the link between microstructure-precipitation evolution and their consequent effect on the mechanical properties were examined. Two stage controlled rolling (roughing and finishing) was carried out on a laboratory rolling mill for a set of completed and interrupted schedules. Subsequent to rolling, two different cooling routes were used (air-cooling and accelerated water cooling (ACC) together with coiling simulation). From the combination of transmission electron microscopy (TEM) observations, detailed texture analysis and inductively coupled plasma-mass spectroscopy (ICPMS) precipitates quantification, consistent correlations between precipitation state and microstructure at every stage of TMCP can be recognized. The role of grain size and precipitation on final mechanical properties was discussed based on different strengthening mechanisms.

Abstract: The softening processes that take place after hot deformation in two high Mn steels, one of them microalloyed with 0.1%Nb, have been investigated. Double hit torsion tests were carried out in order to determine the mechanical softening at temperatures ranging from 900 to 1100°C. In addition, the applicability of different parameters obtained by means of the Electron Back Scattering Diffraction (EBSD) technique to estimate the recrystallized fraction was investigated. It has been found that the Grain Orientation Spread (GOS) is the parameter that best allows quantifying the recrystallized fraction under the conditions investigated. The correlation between the mechanical softening and the recrystallized fraction measured by EBSD depends on the material and deformation conditions. A good correlation between both values is observed for the base steel at all the temperatures. However, for the Nb microalloyed steel, although at high temperatures a good correlation is also observed, at low temperatures the mechanical softening fraction tends to be larger than the recrystallized fraction denoting that recovery has an important contribution.

Abstract: The effects of slab reheat temperature and soaking time are studied to characterize austenite grain growth, microstructure homogeneity and dissolution of precipitates in linepipe X80 grade steel. It is shown that the uniformity of austenite microstructure strongly depends on the slab reheat temperature and soaking time. With increasing reheat temperature an abnormal growth of individual grains is observed that stems from gradual dissolution of microalloy carbonitrides. As the result, individual grain boundaries become unpinned and mobile thus "nucleating" secondary recrystallization. The highest reheat temperature at which the dissolution kinetics of precipitates is still slow enough to prevent the onset of secondary recrystallization within long soaking times is 1160°C. The as reheated austenite microstructure and the character of austenite grain size distribution are inherited throughout the entire roughing rolling sequence and even further downstream to the finishing rolling entry.

Abstract: Thin slab direct rolling (TSDR) technology has some important metallurgical differences compared to conventional routes. One of these differences deals with the combination of coarse as-cast austenite grains and the possibility of limited solubility of microalloyed addition at the entry of the rolling mill. The industrial tendency of increasing the strength and toughness of steel grades produced by TSDR route requires increased amounts of Nb addition to achieve a proper austenite conditioning prior to transformation. Nevertheless, an increase in the Nb content rises the risk of premature Nb(C,N) precipitation during early rolling stages. In order to avoid this, one possibility is to add Mo and limit the amount of Nb. This study analyzes the influence of different Mo contents on static recrystallization of Nb microalloyed steels. The analysis includes the evaluation of the incidence of the combined effect of Nb and Mo solute drag retardation in both heterogeneous recrystallization evolution and grain size distribution for the case of initial coarse austenite grain sizes. To do that, partially recrystallized microstructures have been analyzed and microstructural parameters of interest quantified.

Abstract: Hot deformation of pearlitic steel was carried out to examine the overall deformation response to micro structural and texture evolution. To understand the mechanisms operative during hot deformation, compression tests were carried out at various temperatures in the range 400 - 600o C and strain rates in the range 0.001-10 s-1. The flow curves were analyzed to examine the occurrence of dynamic recrystallization. The evolution of microstructure and texture of hot deformed sample is analysed using EBSD and X-ray texture goniometer respectively.

Abstract: The development of laminated composite Mg alloy sheets, prepared by solid diffusion and roll bonding, is an effective way of improving the stiffness and surface properties of these materials while retaining their lightness. Laminated composites consisting of a core of Mg alloy between sheets of A5083 alloy as the coating material with Ti foil interlayers were prepared by solid diffusion and roll bonding. The laminated material had a strength and was resistant to cracking during deformation. Compounds that were formed and dispersed at the bonding interface between the Al and Mg alloys subjected to grain refinement improved the fracture toughness and strength of the composites, and it was important that these compounds were formed discontinuously. The fracture toughness of the laminated composite was twice that of the base Mg alloy, and its Young's modulus was 57 GPa.

Abstract: Equal channel angular pressing (ECAP) of magnesium alloys dramatically effects the texture and structure of the alloys due to processes of severe plastic deformation and dynamic recrystallization. Changes in texture, structure and tensile properties of AZ41 alloy after ECAP by Bc regime with 4 passes at a temperature of 245° C and subsequent annealing were studied. Evaluation of texture and structure was performed using the orientation distribution function (ODF) restore from the X-ray direct pole figures and the large number of individual orientations measured by the EBSD method. After ECAP the structure of the dynamically recrystallized fine grains with an average size of about 2,9 μm was revealed. The ratio of the volume fraction of low angle grain boundaries (LAGBs) to the high angle grain boundaries (HAGBs) constituted as 1:3. The formation of few basal texture, tilted by 45 ÷ 55 ° to the pressing direction was also found. Subsequent annealing increases the grain size to 10 μm, the fraction of HAGBs - up to 85% and practically does not change the texture type. These changes in structural parameters improve considerably the low-temperature ductility of AZ41 alloy.

Abstract: In the present study, the grain refinement, grain growth behavior, and tensile properties of rolled and annealed AZX311 Mg alloys were investigated. The yield strength and ultimate tensile strength of the rolled material were 360 MPa and 370 MPa, respectively, and the total elongation was 5%. When annealing was performed at 423 K for 1hr, the yield strength and ultimate tensile strength were unchanged, but the elongation increased to 10%. Furthermore, the strength and elongation did not change for annealing temperatures of 473–673 K owing to Al₂Ca precipitations.