Papers by Keyword: TRIP Steel

Abstract: The authors have developed the in situ neutron diffraction technique focusing on bainite transformation during austempering. Thanks to the features of time-of-flight type neutron diffraction, textures, phase fractions and lattice parameters can be simultaneously measured at high temperature. In this paper, the design of experimental equipment and analytical approach are mainly described.

Abstract: Despite that the conventional CSiMn TRIP steel has a promissing mechanical attributes, it has a limitations on the galvanizability of such grades of steel due to Silicon. Thus, aluminum as a strong candidate for substituting silicon has been introduced in this study accompanied by vanadium as a microalloying element. Microstructure of the studied steel was observed by using OM, and SEM. X-ray diffraction analysis, and tent-etching technique carried out on the studied steel to identify the fractions of the retained Austenite after thermal mechanical process, as well as its morphology. In addition, the mechanical properties in term of strength, ductility, strain hardening, and the rate of strain hardening were studied to define the influenced parameters throughout this alteration. The results refer to the possibility of complete replacement of silicon in TRIP steel with aluminum at the presence of vanadium as a micro alloying element.

Abstract: The paper describes effect of hydrogen on mechanical properties and fracture characteristics of two types of C-Mn-Si TRIP steel; laboratory prepared steel TRIP 800 and commercially manufactured steel TRIP 780. TRIP steels are very promising materials thanks to their combination of a very good strength and toughness. However, these steels can be embrittled by hydrogen during technological operations related to galvanizing. That is why the knowledge of effects of hydrogen on the properties and fracture characteristics of the TRIP steels is of particular importance. In the presented study, effects of hydrogen were studied by tensile tests after electrolytical hydrogen charging. Electrolytical hydrogen charging was performed in 0.05 M solution of sulfuric acid with addition of potassium thiocyanate to promote hydrogen absorption. Hydrogen provoked embrittlement in both steel variants and changed their fracture micromechanism. Hydrogen embrittlement manifested itself mainly by a loss of plasticity. Index of hydrogen embrittlement, expressed on the basic of a relative drop of elongation to fracture, reached values about 77 % for the steel variant TRIP 800, resp. 83 % for the steel variant TRIP 780. No significant difference was observed between two steel variants studied. Concerning fractographic characteristics, steels containing hydrogen displayed quasi-cleavage fracture mostly on the edges of the sample and around elongated non-metallic inclusions.

Abstract: Atmospheric corrosion test of TRIP steels was conducted in laboratory. The surface morphologies of the specimens were analyzed by scanning electron microscope (SEM), X-ray diffraction (XRD) and electro-probe microanalysis (EPMA). Corrosion performance of TRIP steels under atmospheric environment was investigated by discussing the protective mechanism. The corrosion rates of steel A are significantly greater than steel B in atmospheric environment tests. The enhancement of corrosion performance of TRIP steel is attributed to the additions of alloying elements, such as P, Cr, Cu, and Ni etc.. The alloying elements increase the compactness and densification of rust layers. Electrochemical characteristic of TRIP steel is improved by means of the enhancement of the thermodynamic stability.

Abstract: TRIP steel shows excellent mechanical properties such as greatly high strength, ductility and toughness by means of the appropriate combination of the strain-induced martensitic transformation (SIMT) behavior and the deformation behavior of each phase at crystal scale. In the past, the effect of grain size in the austenite on the deformation behavior of TRIP steel is investigated by introducing the grain size into a generalized model for the kinetics of SIMT. In order to validate the size-dependent kinetics modelling, it is necessary to simulate the deformation and SIMT behavior of the polycrystalline for the different grain size at the crystal scale. This study focuses on an investigation of SIMT behavior in polycrystalline TRIP steel by finite element simulation. The constitutive formula for monocrystalline TRIP steel including transformation strain in each variant system derived on the basis of the continuum crystal plasticity theory is applied. For the polycrystalline model, Voronoi tessellation is employed. The deformation behavior with a patterning process of martensitic phase in two different numbers of grains with initial crystal orientations for describing the deformation-related length scale is simulated under plane strain condition with two planar slip systems by a cellular automata approach.

Abstract: Development of high strength or even ultra-high strength steels is mainly driven by the automotive industry which strives to reduce the weight of individual parts, fuel consumption, and CO₂ emissions. Another important factor is the passenger safety which will improve by the use of these materials. In order to achieve the required mechanical properties, it is necessary to use suitable heat treatment in addition to an appropriate alloying strategy. The main problem of these treatments is the isothermal holding time. These holding times are technologically demanding which is why industry seeks new possibilities to integrate new processing methods directly into the production process. One option for making high-strength sheet metals is press-hardening which delivers high dimensional accuracy and a small spring-back effect. In order to test the use of AHSS steels for this technology, a material-technological modelling was chosen. Material-technological models based on data obtained directly from a real press-hardening process were examined on two experimental steels, CMnSi TRIP and 42SiCr. Variants with isothermal holding and continuous cooling profiles were tested. It was found that by integrating the Q&P process (quenching and partitioning) into press hardening, the 42SiCr steel can develop strengths of over 1800 MPa with a total elongation of about 10%. The CMnSi TRIP steel with lower carbon content and without chromium achieved a tensile strength of 1160 MPa with a total elongation of 10%.

Abstract: The effect of strain path change on formability of TRIP590 and TRIP780 was investigated experimentally. Two-step uniaxial tension tests, which consist of the first loading in the rolling direction (RD) and the second loading in the directions varied from RD to transverse direction (TD) in every 15º, were conducted. The evolution of strain rates inside and outside the localized necking zone were inspected by using DIC measuring technique. When the angle between the two loading directions was increased from 0º to 90 º, the subsequent hardening behavior in second step was transited from cross-loading type to Bauschinger type. The total elongation was increased when the two loading directions are close to each other and then it was decreased with the increase of angle. When the angle further increased to 90 º, the total elongation is increased again. It is believed that both of the martensite transformation and Bauschinger type transient has a positive impact on the formability of TRIP steels.

Abstract: Metal matrix composites with ceramic reinforcements such as particles or fibers have come into focus during the past decades due to rising requirements on engineering materials. In this work, composite materials out of high-alloy CrMnNi-steel matrices with varying Ni-contents (3 wt.% and 9 wt.%) and 10 vol.% Mg-PSZ were processed by hot-pressing. The variation in Ni-content resulted in a change in stacking fault energy (SFE) which significantly influenced the deformation mechanisms. The mechanical behavior of the developed composites was investigated in a wide strain rate range between 0.0004 s^-1 and 2300 s^-1 under compressive loading. This was done by a servohydraulic testing system, a drop weight tower, and a Split-Hopkinson Pressure Bar for the high strain rates. To study the influence on the deformation mechanisms such as martensitic transformations and/or twinning, interrupted tests were also carried out at 25 % compressive strain. Subsequent microstructural examinations were done by a magnetic balance to measure the quantity of α’-martensite as well as by scanning electron microscopy (SEM). The results show an increase of strength and strain hardening with decreasing SFE of the matrix due to increased α’-martensite formation. The addition of the Mg-PSZ particles resulted in further strengthening over almost the entire deformation range for all investigated composites. At high strain rates quasi-adiabatic heating suppressed the martensite transformation and reduced the strain hardening capacity of the matrix. Nonetheless the particle reinforcement retains its strengthening effect.

Abstract: Three low alloyed transformation induced plasticity (TRIP) steels with 0.2%C were used in this work. The first one was based on the most common and popular 0.2%C - 1.5%Mn - 1.8%Si concept and was used as a reference material. The second steel was further micro-alloyed by 0.06% of Niobium. The third steel was designed with lower manganese content of 0.6% and additional alloying by 0.8% of Chromium. Thermo-mechanical processing with incorporated incremental deformation was applied to each steel. Various cooling rates and numbers of deformation steps were tested with regard to final microstructure and properties. After this optimization, microstructures with the potential to utilize TRIP effect were achieved for all steels. Very good mechanical properties were obtained with ductility typically in the interval of 30-40% and the tensile strengths in the range of 680-835 MPa.

Abstract: Carrying out fatigue testing of reactor vessel material 15H2MFA acoustic emission sensors were applied to follow changes. It is shown, that observed bursts can be explained only with appearance of acoustic Barkhausen Effect (ABE). Interesting source localization is shown during heat treatment and consecutive stress test, which can be explained acoustic emission due to material transition from martenzit phase to bainite phase. Observed ABE opens the way to apply it in industry using magnetic stresses to provoke acoustic response for characterization of the state of the magnetic materials.