Materials Science Forum Vols. 706-709

Abstract: Microstructual observations indicate that the recrystallization of cold rolled extralow carbon steel sheets occurs due to the abnormal growth of selected subgrains in recovered subgrain microstructures. The authors measured the orientations of a recovered microstructure of a cold rolled extralow carbon steel sheet by SEM-EBSP and classified the deformed grains into several types due to the orientation and its scattering degree. In this study, a model for predicting the recrystallization behavior of cold rolled extralow carbon steels has been developed by applying the model of Humphreys modified to the grain of each type.A good agreement between the experimental and calculated results using the model developed was obtained.

Abstract: With the increasing demand for high performance engine or suspension components, bainitic steels are receiving interest as potential replacement of their quench and tempered counterparts. Indeed, for a number of mechanical components, ferrite pearlite microstructures are no longer sufficient in terms of mechanical properties. Bainitic steel grades allow production of hot-rolled bars or forged components exhibiting a homogeneous bainitic microstructure and achieving UTS up to 1200 MPa without the need for additional heat-treatments [1]. During tempering, these V-microalloyed bainitic steels exhibit unusual yield strength variations, with a very pronounced increase around 250-300 °C followed by the better known secondary hardening peak for temperatures around 600-650 °C. Indeed, after tempering at 250-300 °C, some of these steels exhibit an increase in yield strength of up to 200 MPa, concurrent with an increase in impact toughness of up to 25%. This, however, goes unnoticed if hardness measurements are used to characterize tempering. In the following, results are presented for three different bainitic steel grades, and the origins of the changes in mechanical properties are discussed.

Abstract: The influence of time and temperature of ageing on the σ-phase morphology in ferritic-austenitic cast steel have been investigated in this study. The morphology and quantity of the σ-phase were analyzed by using optical microscope, X-ray diffractometry, scanning electron microscope and image analysis, respectively. The annealing temperature has a significant influence on the morphology, volume fraction and the dynamics of σ-phase precipitation. Surface development of the σ-phase decreases from R=4.3, for the cast steel annealed at 700°C to R=2.4 for the cast steel annealed at 900°C, and the volume fraction Vv_σ from 50.8% to 26.5%, respectively. The most rapid progress of δ → σ + γ’ transformation was observed at 800°C.

Abstract: Thermo-mechanical Controlled Processing (TMCP) is one of the greatest achievements in steel industry in the 20th century, which, however, depends too much upon low temperature rolling for the refinement of austenite grains, causing great loss in terms of productivity. To overcome this disadvantage, a new processing route with ultra fast cooling as the core has been proposed, and pilot rolling and industrial trials were carried out. As compared to conventional accelerated cooling, the ultra fast cooling can achieve cooling rate up to 300°C/s for 3 mm thick strip and highly homogeneous cooling by the pressurized water spraying. In the present work, the metallurgical backgrounds for ultra fast cooling in thermo-mechanical processing were studied and elucidated. By the pilot hot rolling experiments with a lean composition of a typical 600 MPa grade high strength steel, it has been found that the application of ultra fast cooling (UFC) at the exit of hot rolling mills can improve the strength by as much as 100 MPa as compared to the conventional TMCP. The strengthening mechanism lies in that the ultra fast cooling immediately after hot rolling may further improve the strengthening effects by precipitation, grain refinement, and dislocation hardening. The theoretical calculations and experiments indicate that the grain refinement, dislocation hardening, and precipitation in the TMCP with in-front UFC have caused the strength increments of 36, 34 and 54 MPa over the conventional TMCP with ACC, respectively. The microstructure characterization showed that the density of high angle grain boundaries had been increased, and the average size of precipitates had been reduced from about 34 nm to 10 nm with the cooling pattern changing from ACC to the application of UFC. The theoretical estimation indicates that when the cooling profile is changed from the conventional ACC to UFC+ACC, and to UFC, precipitation strengthening accounts for more and more strength increment in the improved strength of hot rolled micro-alloyed steels.

Abstract: The dynamic process of grain evolution in an S304H-type austenitic stainless steel was studied in multiple forging tests at temperatures of 500°C, 600°C and 700°C. The deformation microstructure with a grain size of about 100 to 400 nm resulted from continuous dynamic recrystallization. The size of new grains and the recrystallization kinetics decreased with decreasing the deformation temperature. The dynamically equilibrium grain size evolved at large strains followed a power law function of the flow stress with a grain size exponent of about-0.2. The formation of new fine grains was assisted by dynamic recovery, which leads to an apparent steady state flow at large total strains.

Abstract: The influence of pre-straining and bake-hardening on the mechanical properties of thermomechanically processed 0.2C-1.5Si-1.5Mn-0.2Mo-0.004Nb (wt%) steel was analysed using tensile test, transmission electron microscopy (TEM) and atom probe tomography (APT). This steel after processing had high strength (~1200MPa) and good ductility (~20%) due to the formation of fully bainitic microstructure with nanolayers of bainitic ferrite and retained austenite. The bake hardening (BH) of pre-strained (PS) samples increased the yield strength of steel up to 690MPa and showed the bake-hardening response of 220MPa due to the operation of several strengthening mechanisms such as transformation induced plasticity during pre-straining and pinning the dislocations by carbon during bake-hardening treatment. The carbon content of the bainitic ferrite and retained austenite before and after bake-hardening treatment, the solute distribution between these phases and the local composition of fine Fe-C clusters and particles formed during bake-hardening treatment was calculated using APT. The bainitic ferrite and retained austenite microstructural characteristics such as thickness of the layers and their dislocation density before and after bake-hardening treatment were studied using TEM.

Abstract: Quenching and partitioning (Q&P) treatment was applied to a commercial low carbon martensitic stainless steel, AISI Type 410 (Fe-12Cr-0.1C). The condition of partial quenching and partitioning was optimized with consideration of the untransformed austenite fraction and stability of austenite (carbon concentration in solid solution). As a result, the amount of retained austenite could be increased up to approximately 15 vol%. Tensile testing revealed that the specimens after Q&P heat treatment exhibited lower yield stress and larger work hardening rate compared with quench-and-tempered (Q&T) specimens under the same tensile strength level, resulting in a significantly better strength-ductility balance. It was confirmed that the TRIP effect had contributed to the mechanical property.

Abstract: The microstructure of abrasion resistant steel plate usually consists of as quenched martensite, because harder matrix structure improves abrasion resistance of a steel plate. However hard martensitic material has lower formability, which is the important material property for fabricating machine parts. In this paper, a new type of abrasion resistant technique for steel plate is introduced. The steel provides good combination of high abrasion resistance and excellent formability without increasing hardness. The key technology to balance abrasion resistance and formability is the microstructural control of the ferritic matrix structure with dispersed extremely hard carbide particles. Basic research of abrasion mechanism revealed that abrasion resistance is strongly affected by the kind of carbides and the dispersed condition.

Abstract: No clear-cut information is available with regard to the effect of foreign atoms on the solubility limit of C in b.c.c. iron despite many previous studies. Against this backdrop, the influence of substitutional atoms (Mn, Cr, P, Si, Al) on the solubility limit of C in b.c.c. iron in equilibrium with cementite was investigated in low-carbon steels at a temperature of 700°C. In detail, the C solubility limit was determined from internal friction measurements combined with infrared analysis of C using a high-frequency combustion technique. It has been clarified that Mn, Cr, and Al hardly change the C solubility limit, whereas P and Si increase it. The thermodynamical calculation indicates that, under para equilibrium Si increases the C solubility limit and Mn hardly changes it, while under ortho equilibrium Mn and Si decrease it. However, the present experimental condition was verified to be close to ortho form. The discrepancy between the experiments and the calculations seems to come from the fact that: 1) single solute C atoms and the C atoms combined as Substitute-C complex are not distinguished experimentally, and 2) in the regular solution model, the non-uniform distribution of C atoms around alloying atoms is not introduced into the entropy term, which is something that should be studied further in the future.

Abstract: Thermal desorption spectroscopy (TDS) is a very important tool in hydrogen embrittlement (HE) related research and has been applied on many different materials over the last decades in order to improve knowledge on the HE phenomenon. TDS provides the opportunity to distinguish between different types of hydrogen traps based on the analysis of a spectrum with different peak temperatures each corresponding to hydrogen desorption from a specific trap. These peak temperatures, and consequently the different traps in a material, arise from the various microstructural characteristics of the material. However, TDS results are also influenced by many other parameters, such as the sample surface preparation, the electrolytes used for hydrogen charging, sample geometry, charging time, current density, charging temperature. Even though the use of thermal desorption to evaluate hydrogen-metal interactions has increased over the past years, a careful evaluation of the effect of these other parameters was not yet performed. In this work, the impact of some of the above mentioned parameters was studied. It was demonstrated that the sample geometry, the surface roughness, and the initial total pressure of the TDS chamber influenced significantly the obtained TDS spectrum.