A critical role of aluminium on austenite formation in high aluminium added steels

Novel alloys with high aluminium addition have been developed recently for the new concepts of δ-TRIP, δ-QP and some other high-aluminium low-density steels. The aluminium addition dramatically affects the thermodynamics and kinetics of the formation of austenite. In the present study, the effect of aluminium on the initial microstructure of ferrite and pearlite has been investigated. The equilibrium prediction of phase fraction by thermodynamics calculations is in accordance with the measured austenite fraction during isothermal at intercritical temperature range; both results strongly demonstrate a significant influence of aluminium addition on intercritical region. The isothermal transformation of high aluminium steel during intercritical annealing was delayed, which has an instruction for process design of the industrial continuous annealing and galvanization. The austenite formation during heating in intercritical region was also obviously affected by aluminium addition. The transformation kinetics simulation conducted by DICTRA simulation, as well as the experimental results of dilatometry, indicate a delayed austenite transformation during heating process.

The Influence of Thermomechanical Processing Conditions on the Evolution of Microstructure and Crystallographic Textures and the Mechanical Properties of Deformed Mild Steels in the Intercritical Region

Rolling temperature and rolling reduction intensively influence the formation of Luder lines and fluting marks in mild steels. They govern these effects through control of strain aging. In order to enhance the strain aging resistance and the consequent reduction of yield point elongation and fluting intensity, warm rolling without using the skin pass process is applied. The development of microstructure and crystallographic textures during deformation process and the determination of fluting intensity and mechanical properties consisting of tensile and formability properties in terms of different thermomechanical conditions (RT and RR%) were investigated in this study. These properties are determined through the use of bending, tensile tests, optical microscope, and EBSD analysis.

Relationship of Microstructure and Mechanical Properties of Dual Phase Steel after Various Annealing Conditions

The development of ultrafine ferrite grain size has become one of attractive way how to improve the behavior of dual phase (DP) steels. The other possible way how to enhance mechanical properties of DP steels is to modify the chemical composition. Therefore object of our investigation was the dual phase steel with modified alloying (three times higher Cr content with addition of phosphorus). The dual phase steel was annealed in laboratory conditions in accordance with three specified annealing cycles: into intercritical region (780°C), into austenite region (920°C) and into austenite region (920°C) by subsequently cooling into intercritical region (780°C) with the hold at the temperature of 495°C. The obtained microstructure after selected annealing regimes consists of three phases (ferritic matrix, martensite and martensite/bainite grains) with different size and distribution. For studied annealing regimes were clearly defined mechanical properties such as: YS, UTS, elongation, n-parameter and ratio YS/UTS. It was defined the scheme of microstructure evolution on base of austenite grain size during the continual cooling process with defined three phases: 1) the hard martensite formed on the grain boundary; 2) the soft interior bainite and 3) the hard isolated martensite.

Influence of material inhomogeneity on the mechanical response of a tempered martensite steel

Failure in steel weldments operating at high temperatures often occurs in the heat-affected zone adjacent to the weld. Such failures can be a result of material inhomogeneity within the heat-affected zone and in the case of tempered martensite steels have been linked with regions of untransformed α (ferrite) phase or over-tempered martensite within the intercritical region of the heat-affected zone. In this work, two-dimensional Voronoi tessellation is used to construct polygonal Voronoi cells to represent the microstructure of the heat-affected zone of a weld in a tempered martensite steel. The Voronoi construction is treated as a representative volume element of the material and is discretised by 8-node linear brick elements, with periodic boundary conditions. The lattice orientation at each material point is specified by three Euler angles, which are assumed to be randomly distributed, to represent the initial lack of texture in the intercritical region of the heat-affected zone. The constitutive response is represented by a nonlinear, rate-dependent, finite-strain crystal plasticity model. The results indicate that small amounts of ferrite can induce significant enhancements in stress and inelastic deformation at the interface of the ferrite and martensite grains. This localisation of stress and strain may be critical for microcrack and/or void formation and may be a contributory factor to Type IV cracking.

Microstructural Characterization and Evaluation of Mechanical Properties of Steel with Different Biphasic Microstructures Obtained from LNE 500 Steel

In this work was carried out the microstructural characterization and evaluation of mechanical properties of steel with different microstructures. The intercritical region and the existing phases in function of temperature were determined using the THERMOCALC software. The samples of steel were quenched at different temperatures to obtain differents microstructures consisting of ferrite, pearlite and martensite. The microstructural characterization of the samples was performed by qualitative and quantitative metallography. The determination of volume was performed with the "Image J" software. The mechanical properties were determined by uniaxial stress test, which determined the parameters: yield strength, tensile strength, breaking point and total elongation.

The Influence of Different Microstructures to Fracture Toughness and Hardness of Quenched LNE 380 Steel at Different Temperatures

In this work it was analyzed the evolution of mechanical properties of Dual-Phase steel as a function of volume fractions of ferrite and martensite, obtained from steel type LNE 380. The intercritical region and the existing phases in function of temperature were determined using the THERMOCALC software. The samples of steel were quenched at different temperatures to obtain differents microstructures consisting of ferrite, pearlite and martensite. The microstructural characterization of the samples was performed by qualitative and quantitative metallography. The mechanical properties were determined by hardness and impact tests. It was concluded that the volume fraction of ferrite and martensite calculated experimentally agrees with the simulation and the variation of these fractions affects significantly the hardness of the steel, but does not significantly affect the results of fracture toughness.

High Silicon Addition in 780 MPa Cold-Rolled Dual Phase Steel for Carbon Reduction and Plastic Reinforce

Two types of DP steels which contain high carbon and high silicon respectively were produced on industrial production line. The microstructures and mechanical properties were investigated. Based on the thermodynamics and kinetic analyses, the intercritical austenization was researched. The results show that the high silicon and low carbon composition used to DP steel can avoid martensite band structure and decrease the martensite fraction, which will improve the elongation and work hardening ability. Phase transformation kinetic analysis indicates that high silicon content can make manganese enrich in austenite and stabilize austenite in intercritical region. Assisted by the strengthening role of silicon in austenite, the mechanical properties of high silicon and low carbon DP can fully meet the standard.