Materials Science Forum Vols. 638-642

Abstract: The design of new steel grades and microstructures is mostly motivated by the necessity of steel industry to process always better suited high strength steel with low production costs. Automotive customers are asking for more steel options to meet increased specifications for strength, crash worthiness, energy absorption, part complexity, and dent resistance. To meet these requirements, new developed types of steel known as Advanced High-Strength Steels (AHSS) were introduced (e.g.: DP steel "Dual Phase", TRIP steel "Transformation Induced Plasticity",…etc). This paper presents a case study for producing DP600 dual phase steel in EZDK company through building up an integrated model to predicting both final austenite grain size after finishing rolling and the final ferrite grain size after cooling.

Abstract: Two kinds of ULCB steel were designed with different Nb content. The yield strength of the steel which was made by adding 0.09%Nb, TMCP (Thermo Mechanical Controlling Processing) and tempering can achieve more than 900MPa. As compared with 0.05%Nb steel, adding 0.09%Nb can refine the austenite grain significantly. Through the analysis by TEM and EDX, a large amount of Nb precipitation particles can be observed in the steel with 0.09% Nb tempered at 650°C for 1h and these particles are generally small than 20nm.

Abstract: In the present study, the micro-sized cantilever-beam type specimens containing only one block of lath martensite were fabricated, and change in deformation microstructure inside a block with strain was observed directly by scanning electron microscopy and transmission electron microscopy. A number of slip bands appeared in the fixed end of the specimen by deformation. The propagations of slip bands, however, terminate due to the gradient of strain inside the specimen. The direction of slip bands changed during the propagation by the low angle boundaries inside the block. The shear localized region becomes narrow with an increase in strain. Furthermore, the width of laths increases greatly at the large strain region. The increase in width of laths is attributed to the disappearance of some initial lath boundaries by deformation.

Abstract: Although relatively simple in its chemical composition, low-alloy steel can form in a wide variety of microstructures, which directly implies that the (mechanical) properties of the material can vary strongly. Mankind has been using this to his advantage for ages, but the requirements for modern production and use of the material necessitate an ever better insight in the formation of these microstructures. Newly developed steel grades like DP-steel (Dual-Phase) or TRIP-steel (Transformation-Induced Plasticity) consist of several of the well-known phases ferrite, bainite, martensite, austenite, which need to be carefully balanced in their amount, composition and morphology to attain the desired material properties. An overview is given of the basic principles of microstructure formation in low-alloy steel, and the implications for several types of multiphase steel microstructures, in relation to the mechanical properties, are discussed.

Abstract: Low alloy transformation-induced plasticity aided (TRIP) steels have attracted much interest over the last years. TRIP steels were initially developed for automotive applications as they offer an excellent combination of strength and ductility at reasonable costs. These excellent mechanical properties mainly arise from a complex multiphase microstructure of a ferrite matrix and a dispersion of multiphase grains of bainite, martensite and metastable retained austenite. The relevant influence of microstructure on physical and mechanical properties makes metallographic study essential for an appropriate understanding and improvement of the mechanical behavior. An accurate microstructural characterization and quantification of the amount of the different constituents is indispensable to know how the stresses and strains are distributed within the different microstructural constituents. Among the different characterization methods commonly used electron backscatter diffraction (EBSD) appears to be the unique technique able to observe retained austenite grains often no larger than 1 μm. The present work shows the evolution of retained austenite while straining. Microstructural and textural evolution after different strains was examined by optical microscopy OM, EBSD and XRD techniques on TRIP800 steel. EBSD technique appears as a powerful tool for characterizing the complex multiphase steel microstructure and provides an accurate evaluation of the local crystallographic texture. It allows to measure orientation gradients within individual grains of each different phase. The distinction between some phases is observed.

Abstract: Steel sheets for automobiles are usually formed into various parts by cold working. Therefore, plastic strain introduced by the cold working must be considered as a factor affecting the hydrogen embrittlement in addition to the applied stress and the content of diffusible hydrogen entered into steels, which are considered as factors in the studies of high strength steel bolts. However, there are few detailed reports investigating the influence of these factors on hydrogen embrittlement of steel sheets quantitatively. In this study, the influence of plastic strain, as well as stress and diffusible hydrogen content, on hydrogen embrittlement of steel sheet was quantitatively studied to evaluate the hydrogen embrittlement susceptibility of steel sheet by using an 1180 MPa grade cold rolled dual phase steel sheet. Plastic strain was introduced by U-shape bending, and stress was applied by tightening the bent specimen with a bolt. Then, hydrogen was introduced by dipping in hydrochloric acid, and the time to fracture and the content of diffusible hydrogen entered into steel during dipping were investigated. The fracture was promoted by severe deformation near the bending limit, and it seemed to be caused by the presence of micro cracks and/or micro voids. The hydrogen cracking conditions region of the steel sheet were mapped in the three-dimensional space with the axes of applied strain, applied stress and diffusible hydrogen content. It was considered that the evaluation of the risk of delayed fracture of automotive parts made of the steel sheet under service environment was possible by a comparison of the 3D map and the service conditions of the parts.

Abstract: Ultrafine grain refinement to 1 m deteriorates the uniform elongation in the tensile tests of steels. Such loss of ductility has been argued to be an inherent feature of the ultrafine-grained steels. While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. Ultrafine grained steels with different carbon contents from ultralow carbon to high carbon were produced through warm caliber rolling and evaluated for their stress-strain behavior along with the reduction in area. It was found that the reduction in area- tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to bainitic steels for all materials tested in the present study. Formability of ultrafine grained steel is examined by applying to make a micro screw. Good formability was verified by this process.

Abstract: The Fe-25Cr-1N alloy produced by solution nitriding possesses extremely high yield strength owing to the solid solution strengthening by nitrogen. However, it was found that the steel exhibited an insufficient elongation because of the brittle intergranular fracture caused during the uniform tensile deformation. This is due to the marked stress concentration at grain boundaries, which is derived from the grain coarsening caused during long time solution nitriding and the development of planar dislocation structure characteristic of high nitrogen austenitic steels. The most effective way to reduce the stress concentration at grain boundary during deformation should be grain refinement. In this study, grain refinement was attempted by the two-step heat treatment for the Fe-25Cr-1N(-Mn) alloy, and then the mechanical properties were investigated by means of tensile tests and fatigue tests. The two-step heat treatment resulted in the grain refinement of austenite to 20 microns in diameter. The intergranular fracture was greatly suppressed from 70% (as-solution-nitrided) to 10% (grain-refined) in area fraction by the grain refinement. In addition, elongation was markedly increased with local necking. The yield stress and tensile strength were also increased, and thus, the fatigue limit is also raised by more than 30%.

Abstract: High strength steel plates with 780MPa in tensile strength, suitable for building construction use, have been developed. The steel plates provide excellent combination of high strength, toughness, deformability and weldability. The key technology to obtain the excellent combination in mechanical properties of the steel is the microstructural control of M-A (martensite-austenite constituent) and the bainitic ferrite dual-phase structure, through the on-line heat treatment immediately after the accelerated cooling in Thermo-mechanical control process (TMCP). The developed steel plates have microstructure of fine M-A dispersed in the bainitic ferrite matrix. Basic metallurgical research revealed that the transformation behavior and microstructural morphologies were varied with the cooling stop temperature before the on-line heating, and the on-line heating temperature itself. Trial production of the developed 780MPa grade steel plates was also carried out with the plate mill. The obtained plates showed the satisfactory combination of high strength, low yield ratio, toughness.

Abstract: The crystallographic texture and grain size have a strong influence on the magnetic properties of FeSi alloys. These microstructural parameters are determined by the thermo-mechanical processing of the material. Here, some recent results on FeSi-alloys with variable Si-content and without phase transformation are presented. Hot rolling conditions were varied in broad interval of parameters and afterwards, the samples were cold rolled and annealed. After the different processing steps, the samples were characterized by optical microscopy, X-ray diffraction and Electron BackScatter Diffraction (EBSD) in order to evaluate the texture, grain size and the homogeneity of the structure through the thickness. This allowed to study the evolution of the intensity of the favourable magnetic texture components during processing.