Materials Science Forum Vols. 706-709

Abstract: The effect of alloying elements and rough rolling condition on the microstructure evolution of ferritic stainless steel has been investigated in order to understand the recrystallization and precipitation behaviour during hot rolling. In the present study, a series of high temperature compression tests with plane strain deformation mode were conducted for Nb added and Ti+Nb added ferritic stainless steels. Compression tests then were subjected to various conditions of deformation temperature, reduction ratio and holding time. After the tests, EBSD mapping and SEM observation were performed to analyze the recrystallization and precipitation behaviour. Nb added and Ti + Nb added steels show an increasing tendency of recrystallization with an elevation of deformation temperature, holding time and reduction ratio. An increase of holding temperature and holding time enlarges the recrystallized regime due to a decrease of activation energy for recrystallization and a growth of recrystallized grain. A higher reduction ratio also increases the recrystallized regime due to a rise of stored energy for recrystallization. Nb added steel, however, is more resistant to recrystallization because most of Nb (C, N) particles in Nb added steel are finely dispersed in the matrix.

Abstract: 690 MPa grade HSLA steel has wide application in engineering, such as mechanical equipment and oceanographic platform. Qualified joint is crucial for structure safety. Many factors, such as preheating temperature and interpass temperature, welding heat input and plate thickness, may influence the welding thermal cycle so that microstructure and mechanical properties of weld metal are changed. In this paper, the effect of interpass temperature on microstructure and mechanical properties of weld metal of 690 MPa grade steel are studied. Gas metal arc welding method was used to weld the joint. Four interpass temperatures, 80°C, 120°C, 160°C and 200°C are adopted during welding. Optical microscope, scanning electron microscope, transmission electron microscope and electron back-scattered diffraction (EBSD) were used to analyze the microstructure of weld metal. Tensile test and impact test were used to measure its mechanical properties. Research results show that the weld metal are composed of lath bainite and granular bainite. M-A constituents are found on grain boundary and inside grains. The shape and size of bainite structure and M-A constituent are detailed. The distribution and quantity of residual austenite are also detected. The relationship between microstructure and mechanical properties is discussed. Experiment results show that when interpass temperature is 80°C the yield strength and impact toughness are higher than the other three cases.

Abstract: Thermal desorption spectroscopy (TDS) is a very important tool in hydrogen related research. It allows to distinguish between the different types of microstructural hydrogen traps based on the analysis of the different temperatures at which hydrogen desorbs from the material during heating. These peak temperatures depend on the metallurgical and microstructural characteristics of the steel under investigation and provide important information on the possible mechanisms for hydrogen embrittlement (HE). In the present work, multiple TDS experiments and an in-depth study of the microstructure were performed on a TRIP steel (TRIP700) that was previously cold deformed in order to make a correlation between the microstructural features of this material, e.g. grain boundaries, dislocations, martensite formation and the peaks that became visible during TDS. The results obtained for the TRIP grade were compared with those obtained for electrolytic pure iron, which only contained a limited amount of possible trap sites such as grain boundaries and an increasing amount of dislocations due to previous application of cold deformation. Significant differences between both materials and a significant impact of the degree of cold deformation for TRIP steels were observed.

Abstract: In this investigation a new low carbon and low alloy (LCLA) (Third generation AHSS) steel with exceptional combination of yield strength and fracture toughness has been developed. The steel has been processed using a novel two step austempering process. The investigation also examined the influence of a two-step austempering process on the microstructure and mechanical properties of this steel. Test results show that by two-step austempering process significant improvement in yield strength and fracture toughness can be obtained in this steel.

Abstract: Thermo-mechanical process followed by accelerated cooling and high temperature tempering was applied to investigate the microstructure evolution and mechanical properties of a high strength crack-free steel. Optical microscopy, transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD) technique were employed to analyze the complex microstructural characteristics of the steel. The results indicated that the precipitation strengthening effect played an important role in optimizing the tempered strength. According to EBSD results, the average grain size of as-rolled specimens was about 3.2 μm, and it increased slightly with the tempering temperature. Therefore, the grain refinement wasthe major reason for the good mechanical properties of the crack-free steel.

Abstract: A duplex steel with 26Mn-9.7Al-0.47C (wt.%) was investigated for its suitability as an automotive material. The material was processed in the laboratory following standard processing route for steel sheets. It is found that this duplex steel achieves promising mechanical properties at about 22–28 vol.% of ferrite in the microstructure. Control of ferrite volume fraction by optimising annealing temperature appears to be key to monitor the mechanical properties. A tendency of the alloying elements Mn, C and Al to be partitioned in austenite/ferrite bands is observed irrespective of the annealing temperature. The formability (stretchability, bendability and hole expansion coefficient) of the material is better/comparable than conventional automotive steels with similar strength levels.

Abstract: Industrial thin slab casting and direct rolling processing started in 1989 with the world’s first CSP® plant at Crawfordsville (USA). Since this time CSP® and competing thin slab casting and direct rolling concepts have been developed to a standard process for hot strip production [1]. Typical features of the CSP® process are the homogeneous structural and mechanical properties all along the strip. Direct hot rolling of thin slabs may be followed by a well defined cooling pattern to produce hot strip from high strength multiphase steel, like dualphase (DP) grades, on the runout table. These steel grades are characterized by a favorable combination of strength and ductility based on hard martensitic particles embedded in a ductile ferritic matrix. This paper highlights the mechanical properties of hot rolled DP steel from CSP® production. To this purpose, multiple tests and modeling have been applied to determine e.g. r-values, forming limit curves and yield locus. In addition, forming simulation as well as laboratory and industrial deep drawing tests have been performed.

Abstract: Hot processing of austenitic steels is an essential step in the production of steel products. However its microstructural characterization is at best indirect and reliant only on the processing parameters. This work describes a novel method to use stress-strain data at various temperatures and strain rates to extract microstructural parameters. Two low-carbon steels with a magnitude difference in carbon content are compared.

Abstract: Annealing of martensite/austenite microstructures leads to the partitioning of carbon from martensite to austenite until the chemical potential of carbon equilibrates in both phases. This work calculates the volume change associated with this phenomenon using theoretical models for the carbon partitioning from martensite to austenite. Calculations are compared with experimentally determined volume changes. This comparison reveals that in the case of steels with higher contents of austenite-stabilizing elements, reported volume changes are satisfactory predicted assuming a low mobilily martensite/austenite interface. In the case of a steel with lower additions of austenite-stabilizing elements, experimentally measured expansions are considerably larger than predicted ones. The large measured volume expansions probably reflect the decomposition of the austenite.

Abstract: An investigation concerning laser beam – GMA – hybrid welding of high strength steels has been completed for a crane plant. Materials, welding procedure qualification tests and the course of action during welding a demonstrator were studied [1].