Static strain ageing behaviour of dual phase steels

In this work an investigation was conducted into the cold deformation ageing susceptibility of a carbon steel and a microalloyed steel, both with dual phase micro-structure. Ageing experiments after different prestrains were carried out at temperatures ranging from 25 to 250 °C. It was found that yield strength (YS) and tensile strength (UTS) of the steels with different dual phase micro-structures exhibit maximum values at ageing temperature of 100 °C after different prestrains. It is assumed that the first rise is based on the formation of solute atom atmospheres around dislocations and the further strengthening in the second step is caused by the low-temperature carbide precipitation in ferrite. When the ageing temperature increased to 150, 200 or 250 °C, YS decreased due to tempering effect in martensite. It was also found that the ageing of the microalloyed steel occurred more slowly than that of the carbon steel. The slow occurrence of ageing was clearly observed at temperatures of 100, 150, 200 and 250 °C and was attributed to the chemical composition of the steels.     

Prediction model for the austenite grain growth in a hot rolled dual phase steel

The main purpose of this work is to develop a pragmatic model to predict isothermal austenite grain growth in a hot rolled dual phase steel. Austenite grain growth kinetics have been studied in different heating conditions, involving soaking time and soaking temperature as well as heating rate. The contribution of the initial grain size to the time exponent in Beck equation was analyzed mathematically. The time exponent and initial grain size were also quantitatively described by models proposed. It was found that the time exponent value which had a wide range increased when the temperature decreased. No meaningful activation energy can be obtained for the grain growth process when the time exponent varies. The initial grain size increases with the soaking temperature. The model predictions present a good agreement with experimental austenite grain growth data.     

Calculation of the ferrite volume in some dual phase steels

The relation between the γ/γ + boundary temperature, T, and the equivalent values of [Cr] and [Ni], as well as the variation of the ferrite volume, V_f, with the temperature in + γ dual-phase steels have been studied. With the aid of a computer, the regressive expressions derived from the experimental results are: T (°C) = T₃ + 21.2 [Cr] − 15.8 [Ni] + 223; V_f (%) = 0.715 exp [0.015(T − T_δ)] − exp[0.015(T_c − T_δ)] + 1.85 exp [0.0083(T − T_c]).     

Simple model for austenite grain growth in microalloyed steels

Abstract

Previous models for grain growth are usually based on Beck's formula, which are inadequate for quantitative prediction of austenite grain growth during reheating of as cast microstructures in microalloyed steels. The applications of these empirical grain growth models are limited to some particular categories of steels, such as Nb, Nb–Ti and Ti–V microalloyed steels, etc. In this study, a metallurgically based model has been developed to predict the austenite grain growth kinetics in microalloyed steels. This model accounts for the pinning force of second phase particles on grain boundary migration, in which the mean particle size with time and temperature is calculated on the basis of the Lifshitz–Slyozov–Wagner (LSW) particle coarsening theory. The volume fraction of precipitates is obtained according to the thermodynamic model. The reliability of the model is validated by the agreement between theoretical predictions and experimental measurements in the literature.     

Crystal plasticity-based modelling of grain size effects in dual phase steel

With decreasing grain size, the strength of steel increases due to the well-known Hall–Petch type effects, which is generally neglected in the classical crystal plasticity-based models. In the present work, the classical crystal plasticity-based model has been modified to incorporate the grain size effect. Validation of the present model was carried out with the published experimental results of a dual phase steel and, it was found to be possible to predict the grain size effects quite accurately using the model. The proposed model was used to carry out a parametric study for effects of grain size and was further used to predict the influence of grain size on cross effects during orthogonal loading.     

On the grain boundary network characteristics in a dual phase steel

Journal of Materials Science - The relative areas of interfaces in dual-phase steel containing an equal fraction of ferrite and martensite have been measured and classified according to five...     

Damage in dual phase steels and its constituents studied by X-ray tomography

Damage in a dual phase steel was measured using in situ high-resolution X-ray absorption tomography. A comparison with the behavior of its two constituents ferrite and martensite, taken separately, was also achieved in the present work. The method was particularly useful for analyzing the respective contribution of nucleation and growth of voids in the studied materials. Quantitative analysis of the damage events was carried out on a same 3D region inside the reconstructed volumes at different deformation steps for different samples cut from the three kinds of materials. Void number prediction and growth model, based on local stress triaxiality, show a good agreement with the experimental data.     

Development in high-grade dual phase steels with low C and Si design

Cold rolled dual phase steels with low C and Si addition were investigated in terms of combination of composition and processing in order to improve mechanical properties and workability including welding and galvanizing. Mo and Cr could be used as alloying elements to partially replace C and Si to assure enough hardening ability of the steels and also give solute-hardening. Mo addition is more effective than Cr addition in terms of obtaining the required volume fraction of martensite and mechanical strength. The ferrite grain was effectively refined by addition of Nb microalloying, which gives optimized mechanical properties. The experimental results show that it is possible to obtain the required mechanical properties of high grade 800 MPa dual phase steel, i.e., tensile strength > 780 MPa, elongation > 15%, and yield/tensile strength ratio < 0.6 in the condition of low carbon (C < 0.11 wt.%) and low silicon design (Si < 0.05 wt.%) through adequate combination of composition and processing.     

Multiple grain growth events in liquid phase sintering

Sintered tungsten heavy alloys consist of a solidified liquid alloy matrix phase which interpenetrates a solid tungsten skeletal structure. A consequence of liquid phase sintering is considerable grain growth while the compact densifies. The driving force for grain growth is a decrease in the interfacial surface energy, and the process itself is the combined result of liquid diffusion, solid diffusion, and vapor diffusion if porosity is present. In this study, we utilized microgravity sintered samples to avoid solid-liquid segregation to study the multiple diffusion processes. Coupled with the diffusion event through the liquid phase, there is simultaneous solid-state sintering such as coalescence. The dihedral angle determines the contiguity and the grain growth rate. The liquid diffusion grain growth rate constant is at least one order of magnitude larger than the solid diffusion grain growth rate constant. As composition changes, the ratio of grain growth contributions from these three components also changes, which, in turn, causes grain size, grain size distribution, and contiguity variations.     

Effect of martensite strength on the tensile strength of dual phase steels

Fe-2% Si-1.5% Mn steels with three levels of carbon content (0.10, 0.14 and 0.19 wt%) were intercritically annealed followed by water quenching to obtain dual phase (martensite plus ferrite) structure. It is found that the ultimate tensile strength of dual phase steels increased with increasing the volume fraction as well as the tensile strength of martensite. The tensile strength of dual phase steel can be predicted using the law of mixtures although the predicted tensile strength is slightly higher than the experimental one. It is suggested that martensite never reaches its ultimate tensile strength when the necking of dual phase steels occurs.     

Computer simulation of grain growth by grain boundary migration during liquid phase sintering

From many experiments with mixtures of small and large particles, it can be concluded that during liquid phase sintering, smaller particles partially dissolve and a solid phase precipitates on the larger particles. Therefore, the number of smaller particles decreases due to coarsening. The growth rate can be controlled either by the solid-liquid phase boundary reaction or by diffusion through the liquid phase. This dissolution-reprecipitation process leads to further densification by rearrangement of smaller and larger particles. The microstructure may change either by larger particles growing during the Ostwald ripening process or by shape accommodation. In this study, two-dimensional simulation of grain growth by grain boundary migration based on such a physical and corresponding numerical modeling of liquid phase sintering was considered. The simulation method developed is based on the defined submodels for model system definition, for solution-precipitation, and for grain coarsening process.     

碳锰钢中亚微米和微米尺寸铁素体晶粒的长大行为

碳锰钢中有亚微米和微米两种晶粒尺寸的铁素体,在亚临界时效热处理中具有不同的晶粒长大行为.亚微米尺寸的铁索体在亚临界时效热处理中逐渐长大,材料的硬度逐渐降低;与亚微米尺寸的铁索体不同,微米尺寸的铁素体在亚临界热处理中具有很高的稳定性,晶粒尺寸和材料的硬度基本保持不变.铁素体晶粒的长大行为与晶粒的表面能和晶粒中的第二相有关.     

Characterisation of bimodal grain structures in HSLA steels

High strength low alloy (HSLA) steels can show varying degrees of bimodality in their grain size distributions following rolling and also in the reheated condition, which can have significant effects on their toughness. Current methods of measuring bimodality work well for distinguishing between structures with significantly different levels and/or types of bimodality. However, these are not as good at consistently quantifying small differences between microstructures of, for example, steels processed under different conditions. This paper suggests a new method to construct the grain size distributions (in area-percent versus linear scale of equivalent circle diameter grain size) and to quantify bimodality in HSLA steels based on two parameters (peak height ratio, PHR, and peak grain size range, PGSR) measured from such distributions. The parameters were found to be simple, easy to measure, less subjective and more consistent for these steels compared to the standard and non-standard parameters used in the literature.     

Shape accommodation during grain growth in the presence of a liquid phase

The shape change of large tungsten spheres in a matrix of small tungsten particles and nickel-rich melt was studied. The formation of polyhedral particle shapes, which require less interspace to be filled with melt or pores may be caused by shape accommodation or contact flattening. Calculations support that shape accommodation during Ostwald ripening occurs, due to the small cross sections for diffusion in the melt of the neck areas of the large growing particles. It was shown that shape changes caused by shape accommodation may occur even at low stresses in flat neck areas in contrast to the contact flattening mechanism which require high stresses in these regions.     

Effect of grain size and second phase particles on the hydrogen occlusivity of iron and steels

The trapping of hydrogen by various interfaces in iron and steels has been studied. The hydrogen content of pure iron specimens was found to depend more on hydrogen than those of low angle, and thus specimens with low angle grain boundaries were less susceptible to hydrogen damage. Inclusions and thermo-mechanical treatments were also significant in determining the hydrogen pick-up in iron. A critical volume fraction of inclusions was detected, below which no hydrogen- induced cracking occurred. Ferrite/pearlite and pearlite/pearlite colony interfaces in steel were found to trap hydrogen, whereas the ferrite/comentite interface within the pearlitic colonies had little effect on the hydrogen occlusivity. Received: 26 May 1999 / Reviewed and accepted: 28 October 1999