Phase Steel Research Articles

Effect of alloying elements on microstructure and mechanical properties of hot rolled multiphase steels

The compositional development and thermomechanical processing of multiphase steels – high strength microalloyed steels with good formability – are reviewed. Optimisation of austenite grain size with titanium or Ti + B additions combined with appropriate controlled rolling and cooling during runout and coiling allow martensitic steels with tensile strengths > 1200 MPa to be obtained. Silicon additions have been found to reduce sensitivity to variations in coiling temperature and hence scatter in mechanical properties. These steels have found application in safety critical automotive components where their higher strength can allow the use of thinner, lighter sections. Martensitic grades can reduce production costs by eliminating the need for an additional heat treatment step. The influence of niobium and titanium on the properties of dual phase and TRIP steels are also discussed.

Development of Dual Phase and Multiphase High Strength Steels by Using a New Cooling Technology: The Howaq-Twice Process

In order to develop low alloy dual and multiphase high strength steels, CRM and Arcelor implemented the Howaq-twice process in a continuous annealing line. This new equipment combines a slow primary cooling with a fast secondary cooling. Various cooling strategies were investigated in regard to microstructure and mechanical properties of low alloyed steels. Depending on the cooling rate and the quenching temperature, the microstructure varied between fully martensitic, ferritic-martensitic, and ferritic-martensitic-bainitic. The associated tensile strengths reached from 720 to 1100 MPa.

Application of the diode laser to welding on tailored blanks

Summary Over recent years, one of the most interesting developments in the automotive industry has been the weight reduction of the automobile, thereby bringing about a reduction in fuel consumption. Among other requirements, automobile weight reduction and the maintenance of high performance levels necessitates the use of bodywork materials that have a high level of mechanical resistance. In this work, the authors present a study of the viability of using the diode laser to weld high-resistance steel sheets, more specifically microalloyed ferritic (ZStE) steel, dual phase (DP) steel and transformation induced plasticity (TRIP) steel. All these steels are materials that can be employed to form part of ‘tailored blanks’, which are defined as parts made through the combination, by welding, of steel sheets of different qualities and thicknesses. In this study the authors examine the metallographic characteristics of welds beads and the mechanical behaviour of the welds obtained, carrying out tests on microhardness, drawing, tension and fatigue.

Sintering products molded by injecting ceramic and metal powders

The injection molding products with different volume ratios of ZrO2 ceramic powder to 316L stainless steel powder were prepared. Properties and structure of the products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the compressive stress exists in the products and the bend strength reaches 300 MPa. ZrO2 phase and stainless steel phase are uniform in samples. The toughness of ceramic increases with the increasing the content of stainless steel. Through TEM study of the interface, some crystalline orientation relationships are determined.

The effect of microstructure

Abstract The influence of microstructural parameters on low cycle fatigue damage in some single phase and biphasic stainless steels has been studied. In a single phase austenitic steel, the fatigue life increase with decreasing grain size is attributed to the positive effect of microstructural barriers in the bulk. In a metastable austenitic steel, the grain size of the initial microstructure has a combined influence on the volume fraction of martensite formed during cyclic loading and on the fatigue life. In this case, the beneficial effect of grain refinement is indirect because all cracks grow in the martensite. Possible damage patterns in a cluster of grains with different properties are discussed using the example of a duplex austenitic-ferritic steel. It is demonstrated that the propagation behaviour of short cracks related to the resistance of microstructural barriers in the bulk has a decisive influence on the fatigue life. The macroscopic fatigue resistance is thus controlled by damage events at the microstructure size scale.

Critical Assessment of the Micromechanical Behaviour of Dual Phase and Trip-Assisted Multiphase Steels

The strength and formability properties of high performance steels have been recently improved thanks to the combination of several strengthening mechanisms such as dislocation strengthening and mechanically-induced martensitic transformation within complex microstructures. These finely grained metastable microstructures are generated during controlled multistage thermomechanical treatments involving several phase transformations. However, the way the different active mechanisms combine in order to improve the properties of these steels is not yet clearly established. The present study proposes an overview of an experimental program devoted to the micromechanical characterisation of Dual Phase and TRIP-assisted multiphase steels. Thanks to different techniques such as X-Ray and neutron diffraction, strain mapping and transmission electron microscopy (TEM), it was possible to characterize the flow behaviour of the different constitutive phases and to measure the critical parameters of the microstructures responsible for the work hardening capabilities of the TRIP-assisted multiphase steels. Comparison of the experimental stress and strain partitioning within the heterogeneous microstructures with the predictions of different modelling assumptions was also carried out.La résistance mécanique et les propriétés de formabilité des aciers à haut performance ont été améliorées récemment grâce à la combinaison de plusieurs mécanismes de renforcement comme le renforcement par dislocations et la transformation martensitique induite mécaniquement dans des microstructures complexes. Ces microstructures métastables à grains fins sont engendrées lors de traitements thermomécaniques contrôlés, à plusieurs étapes, impliquant plusieurs transformations de phase. Cependant, la manière dont les différents mécanismes actifs se combinent pour améliorer les propriétés de ces aciers n’est pas encore clairement établie. Cette présente étude propose une revue du programme expérimental dévoué à la caractérisation micromécanique des aciers biphasés et des aciers multiphasés à effet TRIP. Grâce à différentes techniques comme la diffraction des rayons X et des neutrons, la cartographie de la déformation et le TEM, il a été possible de caractériser les propriétés d’écoulement des différentes phases constitutives et de mesurer les paramètres microstructuraux critiques responsables de la capacité d’écrouissage des aciers multiphasés à effet TRIP. On effectue également une comparaison du partitionnement expérimental de la contrainte et de la déformation à l’intérieur des microstructures hétérogènes avec les prédictions de différentes hypothèses de modélisation.

Rapid cooling in continuous annealing and galvanizing lines

The production of new steel grades - such as dual phase and TRIP steels - requires improvements to both process and equipment of continuous galvanizing lines. In particular, progress has to be obtained in cooling technology in order to get the desired mechanical properties. This paper presents a study of design parameters allowing the optimization of fast gas multi-jet cooling systems. The thermal study involves the application of infrared thermography and three-dimensional numerical simulations. Furthermore, a dynamic study is performed in order to reduce steel strip vibration.

선상가열 판변형 해석을 위한 고유변형도 영역의 결정법

선상가열에 의한 판 변형 예측은 고유변형도법에 의해 효율적으로 예측할 수 있다. 종래의 용접에서의 고유변형도 결정 방법은 용접 실험을 통하여 온도분포와 강의 상변태 영역(Ac3)을 시편을 절단하여 계측하고 이를 고유변형도 영역으로 간주하는 것이었다. 선상가열의 현상은 용접과 유사하므로, 용접과 같은 조건 하에서 얻어진 결과를 그대로 선상가열 해석에 이용하여 왔으나 이 결과는 가열 패턴이나 판 두께에 제한을 가지고 있다. 또한 현장에서는 선상가열 후 수냉 처리하는데 그 과정에서 강이 원래의 상으로 돌아가지 않고 마르텐사이트가 되면서 전단 소성 변화를 일으킨다는 점에 착안하여, 본 연구에서는 종래에 시편의 온도계측과 상변태 영역을 직접 계측하는 파괴검사법을 FEM을 이용한 이론해석으로 대체하였다. 즉 임의의 적절한 열속 모델에 대한 온도 분포를 얻고, 조직변화에 따라 추가적으로 발생하는 소성 영역을 고려하기 위하여 공석 온도 영역까지 포함시켜 온도계측과 파괴실험 없이 순수한 이론만으로 고유변형도 영역을 결정하는 새로운 방법을 제안하였다. 이 방법으로 결정한 영역을 이용하여 판의 변형을 예측하고 실험 결과와 비교하여 잘 일치함으로써 본 논문에서 제안한 방법의 유효성을 입증하였다. The inherent strain method is known to be very effective in predicting the plate deformation by line heating. Traditionally the inherent strain regions have been determined from the temperature distribution and the phase transformation regions(Ac3) of welding experiments. Since the phenomena of line heating are similar to those of welding, the experimental results under the same welding conditions have been applied directly to line heating analysis. The results cannot, however, reflect the effect of heating pattern and plate thickness. Besides, water-cooling in the actual heating process can alter the steel's phase to martensite and shear plastic deformation occurs during the transformation. In this study, the experimental measurement of temperature distribution was substituted with a transient heat transfer analysis using FEM so that we could obtain the temperature distribution according to heat flux models of the heating pass. In order to consider plastic strains occurring additionally under phase transformation, inherent strain regions were assumed to be limited to the eutectoid temperature(Ac1). Using the regions, plate deformations could be predicted to validate our method and the results were in good agreement with the experimental ones

Micromechanical Modeling of Ductile Crack Initiation Behavior of Two Phase Steels.

The effects of the volume fraction and the morphology of a second phase on ductile crack initiation behavior were determined by notched round bar tensile specimens using ferrite–pearlite steels which contain quite small amounts of MnS inclusions. Nominal strain to crack initiation was increased by decreasing pearlite volume fraction, and by the controlled rolling, which produces an elongated microstructure. The Gurson–Tvergaard (G-T) constitutive model was used to investigate the micromechanism of ductile crack initiation behavior. For evaluating the void nucleation strain, an axisymmetric unit cell model based on a Voronoi tessellation of the BCC lattice (V-BCC model) was applied to determine the microscopic strain inside the pearlite phase which controls secondary void nucleation. The parameter representing the volume fraction of nucleated void, fN, was evaluated by fitting the numerical solution to experimental data for nominal stress/nominal strain curves of the notched round bar specimen. It was found that steels with lower pearlite volume fractions or elongated pearlite nodules have lower fN, and the void growth rate is lower for the steels with lower fN, which requires a large amount of plastic strain for void growth. Ductile cracking was initiated in the region having the highest void volume fraction for all steels. It was shown that the critical void volume fraction for ductile crack initiation is independent of stress triaxiality, and the steels withlower fN show smaller critical void volume fractions.

Structural Characterization of Surfaces and Interfaces of Materials by Applying X-ray Total External Reflection Phenomena

An in-house grazing incidence x-ray scattering (GIXS) apparatus has been newly developed for characterizing the surface structure at a microscopic level. Using this apparatus, surface diffraction profiles from the oxidized SUS304 stainless steel were measured in the vicinity of the critical angle with the GIXS geometry. The depth profile was discussed from the integrated intensities of both oxidized phase and stainless steel bulk. An in-house grazing x-ray reflection apparatus (GXR) has also been newly built for studying the liquid surface, and the liquid/solid and liquid/liquid interfaces. This system was tested by measuring x-ray reflectivity profiles from free surface of mercury and the water/mercury interface. In addition, a new method has been proposed for determining the atomic number density of a near-surface element in materials using grazing x-ray reflection and anomalous dispersion effect. The essential equations for analyzing the measured reflection data were given and the capability of this anomalous grazing x-ray reflection (AGXR) method was well-confirmed by obtaining the atomic number densities of near surface elements with some selected examples of ZrO 2 -Y 2 O 3 crystal and the passivated stainless steel.

Surface morphology of nitrogen-alloyed steels using high intensity pulsed plasma beams

Abstract Samples of single phase (AISI 321) steel and two-constituent (AISI 1045) steel were irradiated with intense nitrogen plasma pulses. The action of each pulse resulted in melting of the near surface layer and introducing the dose of about 10 17 N atoms/cm 2 . For initially smooth surfaces, in AISI 321 the main morphological changes occur after the first pulse, whereas in AISI 1045 the roughness increases with the number of pulses. For initially rough AISI 1045 (i.e. R a 0.5 μm) the pulse processing results in a decrease of R a and a significant increase (by a factor of about 2) of microhardness HV 001 . Practical conclusions are drawn from these results.

Metallurgical factors controlling impact properties of two phase steels

Microstructural elements were identified and characterized which have a significant influence on the impact properties of dual-phase alloys. Iron base alloys were used. One of these had a composition of 2.0 wt % Si. (FS)