Materials Science Forum Vols. 783-786

Abstract: An experimental method was developed to examine oxidations of austenitic and martensitic stainless steels. The results show that the surface roughness along both rolling and transverse directions decreases with an increase of reduction. When the reheating time is increased, the average thickness of oxide scales of stainless steels increases, which results in relatively rough surface after hot rolling. The effects of oxide scale on the friction condition and surface roughness transfer in hot rolling depend on the oxide scale generated during reheating. The calculated surface roughness is close to the experimental results, which verifies the developed FEM model.

Abstract: By processing steels in the semi-solid state it is possible to achieve unconventional structures even with commonly used steels. This can be demonstrated on X210Cr12 tool steel. After semi-solid state processing, 96% of the microstructure can consist of metastable austenite. In the microstructure, there are polyhedral grains embedded in a ledeburitic network. A combination of semi-solid state processing and rapid solidification is a new method for modifying the microstructure more substantially. In the present experiment, two tool steels, CPM 15V and CPM S30V, were processed by an unconventional method. Since the steels are made by powder metallurgy, their initial structure contains globular carbides in ferritic matrix. Both materials have high levels of carbon and alloying elements, namely vanadium and chromium. The unconventional processing was carried out by mini-thixoforming which enables the use of a small amount of metal. After heating into the semi-solid region, the material was rapidly forced by lateral extrusion into a cavity of a metal die where rapid solidification and rapid cooling took place. Two cooling schedules were employed. In the first case, the die was at room temperature, whereas in the second one it was pre-cooled to-196°C using liquid nitrogen. Since the test sample was cooled from both sides and its thickness was 3 mm, immensely high cooling rates were achieved. The influence of the cooling rate was also noticeable in the microstructures containing high fractions of metastable austenite, martensite and carbides.

Abstract: High-chromium ferritic stainless steels have been developed for applications such as exhaust systems that require good formability. To improve formability, continuous yielding is preferred. However, in high-chromium ferritic stainless steels an upper yield point is often present as a result of free interstitials and Cottrell atmospheres. The upper yield point can be removed by temper rolling but it would be better to avoid it via a suitable heat treatment. This paper describes how this can be done in the case of a ferritic stainless steel containing 0.011%C, 0.012%N, 18%Cr, 2,1%Mo, 0.33%Nb, 0.15Ti%. Despite the presence of Nb and Ti, which should bind the free carbon and nitrogen as carbides and nitrides, an upper yield point was still observed. Previously it has been suspected that this is due to an intermetallic Laves phase present in this steel depleting the Nb in the matrix so that some carbon remains free. A series of short-term annealing experiments showed that the upper yield point diminishes, when the annealing temperature increases above 550 °C, finally disappearing after a heat treatment at 750 °C. On the basis of Thermo-Calc calculations and EDS analyses, free interstitials in the matrix could be related to depletion of MX or insufficient time to reach the equilibrium state.

Abstract: In this work, the design of a new technology for nanoparticle addition into molten steel that improves the mechanical properties of the material, as well as the upscale of the process, was pursued. The process was scaled from laboratory to industrial level starting from first experiments carried out in pure iron bath, in order to analyze the behavior of nanoparticles in molten metal environment, and finishing with the addition of nanoparticles into microalloyed steel bath as industrial trials. The first steps of the research were performed in the levitation furnace at Tecnalia R&I installations that can cast samples up to 1 kg, continuing with a high vacuum furnace with a capacity up to 35 Kg. By the end of the investigation, the process was scaled up to industrial level at Gerdau facilities.

Abstract: Use of ultra-high-strength steels (UHSS) in weight critical constructions is an effective way to save energy and minimize carbon footprint in the end use. On the other hand, the demands for reducing manufacturing costs and energy consumption of the steelmaker are increasing. This has led to development of energy efficient direct quenching (DQ) steelmaking process as an alternative to the conventional quenched and tempered or thermomechanical rolling and accelerate cooled processes. Ruukki has employed thermomechanical rolling and direct quenching process (TM + DQ) for a novel type of ultra-high-strength strip and plate steels since 2001. Advantages of the ultra-high-strength level (>900MPa) can be fully utilized only if fabricated properties are on a sufficient level. Bending is one of the most important workshop processes and a good bendability is essential for a structural steel. Hence, the metallurgy and bendability of Ruukki ́s TM + DQ strip steel Optim^® 960QC have been investigated closely. It was found that by optimizing process parameters and chemical composition, a good combination of strength and ductility can be achieved by a modification of martensitic-bainitic microstructure. Despite of smaller total elongation, the bendability of Optim^® 960QC is at least on the same level as on conventionally manufactured 960MPa steels. However, it is important to pay special attention to bending process (tool parameters, springback, bending force, material handling) when bending UHSS. It was also found that the bendability of Optim^® 960QC can be significantly enhanced by local laser heat treatments or roll forming.

Abstract: In order to seek the proper soaking condition of a high carbon chromium bearing steel, 100Cr6, a new approach was investigated considering the diffusion of chromium atoms. Although it is true that the large carbides are bigger with the worse degree of center segregation of continuously cast blooms, the size of center segregation band in blooms has a more accurate relationship with the degree of center segregation. Therefore, on behalf of the large carbide size in the conventional method, the size of center segregation band in continuously cast blooms of the steel has been used for the new approach. As a result, the center segregation and large carbides in them were removed completely by the new soaking condition.

Abstract: The formation of nanocrystalline structure in a 304-type austenitic stainless steel during multidirectional forging (MDF) at room temperature was investigated. Initial coarse austenite grains with an average size of 50 μm were refined to about 80 nm by martensitic transformation during MDF to a total true strain of 2 and remained unchanged upon further deformation up to a strain of 4. The volume fraction of martensite achieved ~0.9 after forging to a strain of 1.6. The MDF at room temperature was accompanied by a significant hardening of the 304-type steel. The microhardness and the flow stress increased during forging and approached their saturations on the levels of about 5 GPa and 1.7 GPa, respectively, after total true strain of 2. The structural mechanisms responsible for microstructure evolution during severe deformation are discussed.

Abstract: A F138 austenitic stainless steel was solution heat treated, deformed by equal-channel angular pressing (ECAP) at 25 and 300°C. The equivalent strain was ~0.7 per pass and the applied equivalent strain varied from 0.7 to 2.8. Microstructure evolution was observed by transmission electron microscopy (TEM) electron back-scattered diffraction (EBSD) and X –ray diffraction. Work hardening behavior was studied by making use of Kocks-Mecking plots and hardness measurements, the influence of deformation on corrosion resistance was evaluated recording anodic polarization curves in 0.9% NaCl solution.

Abstract: Nowadays, the strategy for improving of mechanical properties in metals is not oriented to alloying followed by heat treatment. An effective way how to improve the mechanical properties of metals is focused on the research looking for some additional structural abilities of steels. Structural refinement is one of the ways. Refinement of the austenitic grain size (AGS) carried out through plastic deformation in a spontaneous recrystallization region of austenite, formation of AGS by plastic deformations in a non-recrystallized region of austenite will be considered as potential ways for AGS refinement. After classic methods of plastic deformations, next structure refinement can be obtained by an application of severe plastic deformation (SPD) methods.

Abstract: Stabilized austenitic stainless steels are widely used in nuclear and oil industries. The 316 Nb steel grade presented in this study holds a small amount of delta ferrite in the austenitic matrix which tends to transform into sigma phase during prolonged exposures in the temperature range of 600-1000°C. Sigma phase is promoted by ferritic elements such as chromium, molybdenum, niobium and silicon. Time-Temperature-Transformation (TTT) diagram of the δ-ferrite evolution is established thanks to DSC experiments and quantitative metallographic analysis. It is observed that the highest sigma phase formation rate occurs between 800 and 900°C, and that the transformation of ferrite begins after a few minutes of exposure in this temperature range. The microstructure of transformed δ-ferrite is mostly dominated by the eutectoid mixture σ + γ₂. Tensile tests were performed for three different cooling conditions: a significant embrittlement attributed to the δ-ferrite transformation is measured by a ductility loss for the lowest cooling rate.