Materials Science Forum Vols. 783-786

Abstract: Traditional TRIP steels have been developed for several decades, however, when thestrength reaches 1000MPa, the elongation rate is difficult to be raised over 20%. In the light ofthermodynamics, phase diagram of TRIP steel containing rather high amount of Al is optimized andthe attractive function of Al, which largely increases carbon solubility in austenite is found. As highercarbon content leads to higher stability of austenite and better TRIP effect of TRIP steel, newcomposition of high strength TRIP steel with high amount Al is designed as well as heat treatmentprocesses. The newly developed TRIP steel exhibits superior mechanical properties and the productof its strength and plasticity is higher than 30000 MPa%, i.e., the target of the mechanical propertiesof 3th generation automotive steel.

Abstract: Key parameters for thermomechanical control process (TMCP) and integrated welding operations have been determined to industrialize extra high strength micro-alloyed low carbon SiMnCrMoNiCu steel plates for bridge applications. Confocal Scanning Microscope was used to make In-situ observation on austenite grain growth during reheating. A Gleeble 3800 thermomechanical simulator was employed to investigate transformation behavior of the TMCP conditioned austenite. Integrated industrial rolling trial was conducted to correlate the laboratory observations and commercial production of the plates. Microstructure factors affecting the toughness of the steel were analyzed. Submerged-Arc Welding (SMAW) trails were conducted and the structures and mechanical properties of the weld joints characterized. The representative plate with thickness of 60 mm consisted of acicular ferrite (AF) + refined polygonal ferrite (PF) + granular bainite (GB) across the entire thickness section exhibit yield strength (YS) greater than 560 MPa in transverse direction and excellent Charpy V Notch (CVN) impact toughness greater than 100 J at-40 °C in the parent metal and the weld joints. These provide useful integrated database for producing advanced high strength steel plates via TMCP. Keywords: Thermo-Mechanical Control Process;Weathering Steel Plate for Bridge; Submerged-Arc Welding without Preheating

Abstract: An alloy chemical composition of a new ultra-high strength stainless has been successfully designed through strengthening mechanisms of carbide and intermetallic compounds. The forging round bar with diameter of 200 mm has been manufactured by means of ultra-high purity smelting and the whole process technologies of micro-unit refinement and phase change control. It was revealed that the developed steel assumes tensile strength of 1960MPa, elongation of 13.5%, fracture toughness K_IC greater than 90MPa·m^1/2, and K_ISCC greater 60MPa·m^1/2. Based on the microstructural observation and phase identification, it was found that the M₂C and the Laves phase were precipitated in the martensitic laths and a small amount of austenite phase was retained in between the Martensitic lath interfaces, which were related to the improved strength and toughness of the developed steel. Furthermore, it was indicated that the steel also presents high fatigue properties and good high-temperature mechanical properties. The corrosion resistance of the steel is equivalent to that of 15-5PH stainless steel but much better than that of the Aermet100 steel under the condition of the marine atmosphere and sea water immersion. This developed steel can be applied in marine corrosive environment without the surface protection and thus can save the expensive maintenance costs and avoid environmental pollution. Based on the promising properties, it was concluded that the developed steel has wide application prospects in the fields of aviation, aerospace, ships, advanced machinery, and advanced machinery manufacturing and other high-tech.

Abstract: Void nucleation behavior of ferrite/martensite dual phase steels varying martensite fraction was investigated. As easily recognized, void fraction was increased with strain induced, and more voids were nucleated in the sample with higher martensite fraction. On the other hands, void fraction at ductile fracture was decreased with increasing martensite fraction. Void nucleation was observed to occur due to the fracture of martensite in DP steels. In the sample with high martensite fraction, many micro voids were nucleated at initial deformation with small strain and lead to ductile fracture. Inter-voids distance at the fracture was almost same among the DP steels with various martensite fractions. It is considered that the most effective factor on ductile fracture of DP steels was not 2^nd phase fraction but the distance between 2^nd phases which caused micro voids.

Abstract: A one step quenching and partitioning process was applied to a 0.2%C-2.0%Mn-0.5%Cr-1.5%Si steel by quenching austenitised samples to several different temperatures below the experimentallydetermined martensite start temperature of 397 °C and isothermally partitioning them beforequenching to room temperature using a quenching deformation dilatometer. These treatmentsyielded predominantly martensitic microstructures containing 5.6 vol.% to 7.5 vol.% retained austenite,as measured by x-ray diffraction. In each treatment, strong dilation was recorded during isothermalpartitioning, with little indication of phase transformation during subsequent cooling to room temperature.This behaviour lends weight to the idea that an isothermal phase transformation occurred duringpartitioning, and that the final microstructure is a mixture of athermally and isothermally formed constituents.These results also suggest that the final microstructure of this steel is mostly formed beforeand during partitioning.

Abstract: Dual Phase steels are increasingly selected for structural applications including parts in automotive industry because of their interesting mechanical properties and their good formability. This work presents an experimental analysis of the evolution of microstructure of a DP1000 alloy submitted to thermomechanical loadings. Monotonous tensile tests were performed at various plastic strain levels up to fracture for temperatures ranging between 25 °C and 440 °C. A strong degradation of the mechanical properties is observed for temperatures higher than 275 °C. The evolution with plastic strains and temperature of the microstructure was studied by scanning and transmission electron microscopy. Different microstructure parameters such as volume fraction of martensite and carbide precipitation were taken into account in order to understand the mechanical behavior of DP1000 steels tested in this temperature range. The microstructural observations indicate that diminution of carbon in martensite, due to its diffusion to form carbides, could partially explain the drop in mechanical properties at around 275 °C.

Abstract: Recrystallization-precipitation-time-temperature (RPTT) diagrams were experimentally determined for two microalloyed steels with V and Nb, respectively, at a strain of 0.35 and a strain rate of 3.63 s^-1. From the RPTT diagrams, and applying the classic theory of nucleation, the nucleation rate was calculated for both steels. In order to determine the mentioned magnitudes, several parameters were calculated, such as: the Zeldovich factor (Z), the energy of formation of the nucleus (ΔG), the driving force for precipitation (ΔGv), the critical radius for nucleation (Rc), and the dislocation density at the start of precipitation (ρ), among others. The calculated data has made it possible to clarify the shape of precipitation start and finish curves and to plot the nucleation rate as a function of temperature. The number of precipitates was calculated by integration of the nucleation rate expression. In this way, substantial differences were established between the two types of microalloyed steels, including the final size of the V(C, N) and Nb (C, N) precipitates.

Abstract: In this study, samples of stainless steel AISI 316L have been processed by selective laser melting, a layer-by-layer near-net-shape process allowing for an economic production of complex parts. The resulting microstructures have been characterised in details in order to reach a better understanding of the solidification and consolidation processes. The influence of the processing parameters on the mechanical properties was investigated by means of uniaxial tensile testing performed on samples produced with different main orientations with respect to the building direction. A strong anisotropy of the mechanical behaviour was thus interpreted in relation with the microstructures and the processing conditions.

Abstract: The effects of temperature and strain rate on the stress–strain relationship and deformation twinning, during tensile tests, in polycrystalline Fe–5%Si alloy were studied. We performed tensile tests over a wide range of temperatures from 173 to 273 K, and strain rates ranging from 0.0001 to 0.1 /s, to clarify the relationship between total elongation and ratio of deformation twins, and the dependence of deformation twinning on grain orientation. All tensile specimens were fractured in a completely brittle manner. The total elongation decreased as the temperature decreased and the strain rate increased. The presence of deformation twins, in all fractured specimens, was confirmed by scanning electron microscope–electron backscattering diffraction analyses. The area fraction of the deformation twins increased as the total elongation decreased. However, a strong influence from the grain orientation on twinning activity was not observed for all temperatures and strain rates. A previous study on Fe–Si alloy single crystals showed that deformation twins form easily in <001>-oriented single crystals, but not in <111>-oriented single crystals. Our observations, on the dependence of deformation twinning on grain orientation in polycrystalline Fe–5%Si alloy, did not agree with those from single crystals. The present findings suggest that grain orientation does not play an important role in determining the occurrence of deformation twinning; not even in polycrystals. It is believed that the stress concentration, due to piled-up dislocations, during tensile deformation, cannot be relieved by the slip at low temperatures or high strain rates, and thus significantly affects deformation twinning.

Abstract: The present study aims to clarify the development of blocks and packets in lath martensite in Fe–18Ni maraging steel using three-dimensional observations. The specimens were step-quenched in order to clarify the sequential development of the three-dimensional morphology in a prior austenite grain. In a prior austenite grain, we found that five independent packets formed during the early stage of martensitic transformation. Four of the packets exist along the prior austenite grain boundaries and one packet grows from the boundary edge into the prior austenite grain. Each packet consists of a single block, although the fraction of martensitic transformation is 50.6%. The observed rules of the block-selection are as follows: (1) the blocks have near Kurdjumov–Sachs orientation relationship with adjacent austenite grains and elongated directions of the laths are parallel to adjacent grain boundaries and (2) transformation shear directions of the laths are parallel to adjacent grain boundaries.