Materials Science Forum Vol. 762

Abstract: A model describing the kinetics of static recrystallization and the kinetics of Nb and Ti precipitation has been reconsidered for modern steels. The effects of alloying and microalloying elements have been introduced into the model. The comparison of the modeling with the experimental softening shows good agreement.

Abstract: The combined effect of Al and Nb additions on the static softening behavior of C-Mn steels was investigated. The compositions of the steels studied in this work are representative of the recently developed TRIP-assisted steels: a base composition of 0.2wt.%C, 2wt.%Mn, 50ppmN, three different Al levels, 0.03 (base steel), 1 and 2wt.%, and two Nb contents of 0.03 and 0.07wt.%. Double-hit torsion tests were performed at different deformation temperatures (925-1065oC) and pass-strains (=0.2 and 0.35). It was found that solute Al produced a significant retardation on the static softening kinetics, this effect being enhanced by the addition of Nb. Additionally, below 1000oC the addition of 2 wt.%Al promotes the γα phase transformation to occur concurrently with softening. For the Nb microalloyed steels strain induced precipitation also occurred, resulting in a complex interaction between softening, phase transformation and strain induced precipitation.

Abstract: The present study highlights the approach to multiscale analysis of the grain refinement during thermomechanical processing of microalloyed steels. HSLA steels for pipe-line use are introduced in which strain-induced precipitation and strain-induced transformations are controlled to produce ultra-fine grained microstructures. Multiscale modeling combining Digital Material Representation and Crystal Plasticity enables to gain a better fundamental understanding of mechanical response and microstructure evolution of precipitation strengthened austenite and ferrite of three microalloyed steel grades. The opportunities for the development of new thermomechanical processing schedules, focused on the grain refinement, are enhanced and the proposed models identifications procedure on the basis of the torsion tests at various forming conditions are presented and discussed. In the pipe-line grade steels, the combination of microalloying elements, accelerated cooling and transformation temperature, has led to much higher grain refinements and increased amount of bainitic ferrite microstructures than in the standard thermomechanical processing. Sensitivity analysis of studied microalloyed steel grades in the light of the discussed problems using microstructural analysis of obtained microstructures was also performed. Finally, the main challenges related to the multiscale modeling of proposed ideas are addressed.

Abstract: In order to promote the production of lightweight automobiles and enhance the strength grade of the automobile wheel steel, a nanoscale precipitation strengthened hot-rolled 590MPa grade wheel steel was developed to replace the Q235 steel, which was originally use for the wheel. The new wheel steel type was based on a C-Mn steel composition, microalloyed with Nb, V and Ti, thus making full use of the strengthening from both grain refinement as well as nanoscale precipitation. The microstructure of the wheel steel was composed of fine-grained ferrite and pearlite and carbides distributed along the ferrite grain boundaries. The yield and tensile strengths of the strip were about 550 MPa and 620 MPa, respectively. The value of hole expansion rate was 90%. The strip has shown high low temperature impact toughness and cold formability. The strengthening mechanisms of the strip were mainly governed by grain refinement and nanoscale precipitation (Nb,V,Ti)C, and the amount of the contribution from precipitation strengthening alone was about 215 MPa. The finishing rolling start temperature was about 950°C and the finish rolling temperature was between 790~830°C. Coiling temperature of 650°C gave the best combination of mechanical properties.

Abstract: By using physical thermal simulation technology, combined with metallographic analysis, tensile tests, impact and hardness tests, effects of heating temperature and cooling speed on microstructure and mechanical properties of X80 induction heating bends were investigated. The results show that as the heating temperature rises, TS of X80 induction heating bends increases gradually. However, when the heating temperature rises above 1100°C, plasticity and toughness of the bends begin to decrease, and grain growth tends to be obvious. When the heating temperature is 1050°C, X80 induction heating bends have a good strength and toughness. As the cooling rate increases, strength and toughness of X80 bends are improved considerably. In the cooling rate range between 20°C/s and 30°C/s, the microstructure of X80 bends is mainly composed of polygonal ferrite and granular bainite. Due to the very high dislocation density inside granular bainite and the fine and dispersed M-A constituents, X80 induction heating bends have a very good strength and toughness.

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. The effects of reheat soaking time on shear fracture area and impact toughness are also described.

Abstract: The effect of different cooling paths on the microstructure and properties of a plain carbon steel was carefully investigated by thermal simulation, hot rolling, tensile tests and quantitative metallography. Experimental results indicate that the more rapid the cooling rate is, the smaller the average ferritic grain size is and the higher the mechanical properties are. Both ultra fast cooling method and ultra fast cooling+accelerated controlled cooling method could refine grain size and improve mechanical properties. Without any alloy addition, using the ultra fast cooling immediately after hot rolling process, the yield strength of the plain carbon steel could reach 360 MPa and the elongation is 32%.

Abstract: In this work, the structural behaviour during tempering of two different heats of 16Cr-5Ni supermartensitic stainless steel has been studied by means of dilatometry, transmission electron microscopy and X-ray diffraction. A thermomechanical simulator (Gleeble 3800) has been also used to characterize the effects on final mechanical properties of different tempering temperatures in the range 600 °C to 700 °C and the influence of sub-zero cooling on industrial double tempering treatments. It has been found that the pre-existence of retained austenite in as-quenched conditions can induce significant differences in the microstructural evolution during tempering and on the final mechanical properties of industrial components, thus inducing problems in controlling final maximum hardness allowable by normative requirements.

Abstract: AISI 316L steel, subjected to a low temperature carburizing treatment (kolstering), has been examined by Mechanical Spectroscopy (MS) and nanoindentation to determine the Youngs modulus of the surface hardened layer (S phase). MS results showed that the average value of elastic modulus of S phase is 202 GPa, a little higher than that of the untreated material.Nanoindentation tests, carried out with loads of 5, 15 and 30 mN, evidence a modulus profile vs depth: E is ~ 400 GPa at a distance from the surface of ~ 110 nm, then decreases to reach the value of the steel substrate (190 GPa) at 33 μm.These results, together with X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) measurements of carbon concentration profile, can be explained by considering the presence of a very thin surface layer, different from S phase and consisting of a mixed structure of Diamond-like carbon (DLC) and tetrahedral carbon (taC).Furthermore, the same experiments have been carried out also after heat treatments at 450 °C to correlate the modulus change to the decomposition of the metastable S phase leading to the formation of (Cr,Mo)C and Cr₂₃C₆ carbides in a Cr-depleted austenitic matrix.

Abstract: Hot compression experiments were carried out on rare earth (RE) added and RE-free Nb-containing steels by using a Gleeble simulator. Stress-strain curves obtained at various temperatures were analyzed to investigate the dynamic recovery and dynamic recrystallization softening behaviours. Morphology, size and number of precipitates in the both steels were examined by means of transmission electron microscopy (TEM). The results showed that, for the experimental Nb-containing steel, the grain size was fined by the RE addtion. In general, dynamic recrystallization cant occur in two steel under 40% deformation rates, and the deformation resistance of RE-containing steel is higher than that of RE-free steel in both the the austenite and ferrite temperatures range.While under the higher deformation rate, the dynamic recovery starting strains of the RE addition steel are higher than that of RE-free steel.It is also shown that the number of precipitate in the RE-containing steel more than that in the RE-free steel, which is due to the RE increasing nucleation rate and promoting Nb carbonitrides precipitation growth in the austenite region. Furthermore, the carbon activity may change by the RE addition, and thereby promote the precipitation strengthening of Nb-microalloyed steel.