Journal Article

Effects of Al addition on tensile properties of partially recrystallized austenitic TRIP/TWIP steels

04 Mar 2021--Vol. 806, pp 140823
Abstract: Austenitic high-Mn steels are regarded as a promising candidate for high-strength cold-rolled steels because their combination of strength and ductility improves greatly by combined effects of twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). Although it is well known that Al plays key roles in tensile properties of austenitic TWIP steels, its effects in austenitic TRIP or TRIP/TWIP steel are still unclear yet. In this study, three austenitic steels (composition; 0.4C–15Mn–1Si-(0,0.5,1)Al-0.3Mo-0.5V (wt.%)) were fabricated, and the effect of Al alloying on microstructures and tensile properties were investigated in relation to the deformation behavior with TRIP and TWIP mechanisms. A partial recrystallization was conducted for enhancing the yield strength, which was characterized with electron backscatter diffraction (EBSD) grain orientation spread maps. The present steels showed 1 GPa of yield strength achieved by partial recrystallization with the precipitation of (V + Mo) complex carbides. Particularly in the non-Al-alloyed steel, the e-martensite formed in the early deformation stage, and the martensitic transformation continued until the failure, thereby resulting in the highest tensile strength (1.5 GPa) along with the highest strain hardening.

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7 results found

Open accessJournal Article
Chih-Yuan Chen1, Ya-Hui Lin1, Po-Han Chiu1, Zhen-Wei Chen1  +1 moreInstitutions (1)
Abstract: A dual phase structure consisting of ferrite and martensite has been achieved in Ti-bearing steels microalloyed with Mo or Cu, respectively, which were treated under different austenitizing and hot deformation conditions to investigate the effects of second microalloying elements and thermomechanical processes on the microstructural evolution and mechanical properties. The results showed the largest microhardness values, 343 HV0.1 for Ti–Mo steel and 307 HV0.1 for Ti–Cu steel, in microalloyed steels austenitized at 1200 °C with 30% deformation strain. Furthermore, critical patent analysis revealed that higher tensile strength is achieved in a Ti–Mo steel having a larger Ti/Mo atomic ratio, but no such relationship between tensile strength and Ti/Cu atomic ratio applies to Ti–Cu steel. This finding can be ascribed to the different metallurgical effects of Mo and Cu in the Ti-bearing steels. For example, the addition of Mo in a Ti-bearing steel leads to tiny carbides due to the decrease in coarsening via the synergistic effect of Ti and Mo. On the other hand, the separate precipitation of TiC and Cu in the Ti-bearing steels is consistent with the results of patent analysis. That is, the tensile strength of these steels is not associated with the Ti/Cu atomic ratio.

Journal Article
Hai-lian Wei1, Hong-bo Pan2, Hong-wei Zhou1Institutions (2)
Abstract: The flow stress curves of a Nb–Ti microalloyed C–Mn–Al high strength steel were obtained by hot compression experiment with Gleeble-1500 thermal simulator at the temperatures from 900 to 1100 °C and strain rates from 0.01 to 30 s−1. Firstly, strain-compensated physical constitutive models considering the temperature dependence of Young's modulus (E) and austenite self-diffusion coefficient (D) with creep stress exponent 5 and variable stress exponent $$n^{\prime}$$ were established, respectively. Secondly, the traditional apparent Arrhenius constitutive model was established to compare the accuracy of the models. The results show that the physical constitutive model with exponent $$n^{\prime}$$ has higher accuracy to predict the flow stress of the experimental steel (correlation coefficient R = 0.992, average absolute relative error δ = 3.83%) than the model with exponent 5 (correlation coefficient R = 0.990, average absolute relative error δ = 10.54%). This is because when the stress exponent in the physical constitutive model is 5, it means that the deformation mechanism considered is only slip and climb, and the constitutive model with variable stress exponent $$n^{\prime}$$ considers all the deformation mechanisms comprehensively, so the prediction accuracy is higher. The correlation coefficient R of the Arrhenius constitutive model is 0.991, and the average absolute relative error δ is 4.58%. Therefore, the physical constitutive model with exponent $$n^{\prime}$$ has the highest prediction accuracy in this study, thus can be an alternative way to predict the flow curves of steels.

Topics: Flow stress (53%), , Exponent (52%)

Journal Article
Junwei Fu1, Junwei Fu2, Jiangchun Wang1Institutions (2)
Abstract: In this work, effect of Mo content on the microstructure, thermal conductivity, and corrosion resistance of the die steels was investigated by optical microscopy, scanning electron microscopy, x-ray diffraction, laser thermal conductivity meter, electrochemical experiments, and pitting tests. The microstructure of the die steels is mainly composed of lath-shaped tempered martensite. At all the tested temperatures, the thermal conductivity of the die steels is decreased with the increase in Mo content from 1.2 to 5.0 wt.%. However, electrochemical experiments indicate that increase in Mo content in the die steels can reduce the corrosion current density and increase the charge transfer resistance in 0.5 mol·L−1 HCl solution. Furthermore, it was found that Mo in the die steels is beneficial to decrease weight loss and pitting corrosion rate, which improves the pitting corrosion resistance of the die steels.

Topics: Pitting corrosion (66%), Corrosion (57%), Microstructure (54%)

Journal Article
Weijie Pan1, Yumin Mao1, Zhang Min, Yang Chen1  +3 moreInstitutions (1)
Abstract: The fast hydration rate of aluminate makes the slurry of CaO–Al2O3-based mold flux coagulates fast, which hinders the production and large-scale application in steel plants. This study selected four dispersants and investigated their mechanism and influence on the dispersibility of CaO–Al2O3-based mold powder slurry. Main results showed that the stability of CaO–Al2O3-based mold powder slurry was poorer than CaO–SiO2-based mold powder slurry. The addition of non-ionic dispersant, OP-10 or glycerol, could make the viscosity of the CaO–Al2O3-based slurry become larger, and the macromolecule dispersant, Poly Vinyl Pyrrolidone, resulted in unstable variation of the viscosity of the mold powder slurry. The anionic dispersant, tannic acid, performed well, as it was able to release negative charges covering the powder particles, and the electrostatic repulsion made the aggregation among particles difficult to occur. The 5 wt% addition of tannic acid was found to effectively improve the dispersion of the CaO–Al2O3-based slurry.

Topics: Slurry (59%), Dispersant (55%)

Journal Article
Shuaishuai Du1, Huijie Liu1, Minghao Jiang1, Yanying Hu2  +1 moreInstitutions (2)
Abstract: To widen the narrow friction stir welding (FSW) process window of TA5 alloy, titanium alloy supporting friction stir welding (TSFSW) is proposed. Both conventional FSW (CFSW) and TSFSW methods were employed to reveal the formation mechanism of cavity defect, clarify the microstructure evolution in the weld and investigate the performance of TSFSW. Results showed that the prior β grains below the cavity defect were much smaller than the above and a clear converging line of material flow was observed between the varied grains, indicating the cavity defect was flow-related and concerned with the insufficient heat input at weld bottom. It was found that the stir zone (SZ) could be divided into the contaminated zone (CZ) and the non-contaminated zone (NCZ) according to whether the β-stable tool elements were introduced during welding. The transformed microstructure consisting of α laths, equiaxed α grains and retained β was detected in the CZ, resulting in extremely high microhardness. The NCZ, where the peak temperature was below α-β transus temperature, was composed of refined equiaxed α grains due to the occurrence of dynamic recrystallization. The cavity defect downgrades the mechanical properties of joint by determining the fracture mode. Without complicated devices and preparations, the TSFSW method is capable of obtaining defect-free joint under higher welding speed by reducing the heat dissipation from weld bottom, thereby improving mechanical properties and widening the parameter range.

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43 results found

Journal Article
Abstract: The production of high strength steel components with desired properties by hot stamping (also called press hardening) requires a profound knowledge and control of the forming procedures. In this way, the final part properties become predictable and adjustable on the basis of the different process parameters and their interaction. In addition to parameters of conventional cold forming, thermal and microstructural parameters complicate the description of mechanical phenomena during hot stamping, which are essential for the explanation of all physical phenomena of this forming method. In this article, the state of the art in the thermal, mechanical, microstructural, and technological fields of hot stamping are reviewed. The investigations of all process sequences, from heating of the blank to hot stamping and subsequent further processes, are described. The survey of existing works has revealed several gaps in the fields of forming-dependent phase transformation, continuous flow behavior during the whole process, correlation between mechanical and geometrical part properties, and industrial application of some advanced processes. The review aims at providing an insight into the forming procedure backgrounds and shows the great potential for further investigations and innovation in the field of hot sheet metal forming.

Topics: Hot stamping (68%), Mechanical Phenomena (52%), Sheet metal (50%)

1,193 Citations

Open accessJournal Article
Georg Frommeyer1, Udo Brüx1, Peter Neumann1Institutions (1)
15 Mar 2003-Isij International
Abstract: The microstructural properties of advanced high strength and supra-ductile TRIP and TWIP steels with high-manganese concentrations (15 to 25 mass%) and additions of aluminum and silicon (2 to 4mass%) were investigated as a function of temperature (−196 to 400°C) and strain rate (10−4≤e≤103 s−1). Multiple martensitic γfcc (austenie)→ehcpMs (hcp-martensite)→αbccMs (bcc-martensite)-transformations occurred in the TRIP steel when deformed at higher strain rates and ambient temperatures. This mechanism leads to a pronounced strain hardening and high tensile strength (>1 000 MPa) with improved elongations to failure of >50%. The austenitic TWIP steel reveals extensive twin formation when deformed below 150°C at low and high strain rates. Under these conditions extremely high tensile ductility (>80%) and energy absorption is achieved and no brittle fracture transition temperature occurs. The governing microstructural parameter is the stacking fault energy Γfcc of the fcc austenite and the phase stability determined by the Gibbs free energy ΔGγ→e. These factors are strongly influenced by the manganese content and additions of aluminum and silicon.The stacking fault energy Γfcc and the Gibbs free energy G were calculated using the regular solution model. The results show that aluminum increases Γfcc and suppresses the γfcc→ehcpMs transformation, whereas silicon sustains the γfcc→ehcpMs transformation and decreases the stacking fault energy. At the critical value of Γfcc≈25 mJ/mol and for ΔGγ→e>0, the twinning mechanism is favored. At lower stacking fault energy of (Γfcc 0, martensitic phase transformation will be the governing deformation mechanism.The excellent ductility and the enhanced impact properties enable complex deep drawing or stretch forming operations of sheets and the fabrication of crash absorbing frame structures.

804 Citations

Journal Article
Abstract: Precipitation hardening has long been used to increase the strength of commercial alloys, such as quenched and tempered steels and the duralumin type aluminium alloys. The theoretical treatments of precipitation hardening are briefly considered. The equations for strengthening by ‘hard’ indeformable particles and by ‘soft’ deformable particles are presented, and the implications are discussed. These lead to the concept of an optimum particle size for a given system, but the optimum can vary from system to system depending upon the particle characteristics. A broad comparison is made between the increments in strength that occur due to precipitation in commercial alloys and the predictions of the theories; an important contribution to these increments in strength is shown to derive from variations in the volume fraction of precipitated particles that can be employed in the various systems.

577 Citations

Journal Article
Gregory B. Olson1, Morris Cohen1Institutions (1)
Abstract: Consideration of the martensitic nucleation process as a sequence of steps which take the particle from maximum to minimum coherency leads to the hypothesis that the first step in martensitic nucleation is faulting on planes of closest packing. It is further postulated that the faulting displacements are derived from an existing defect, while matrix constraints cause all subsequent processes to occur in such a way as to leave the fault plane unrotated, thus accounting for the observed general orientation relations. Using basic concepts of classical nucleation theory, the stacking fault energy is shown to consist of both volume energy and surface energy contributions. When the volume energy contribution is negative, the fault energy decreases with increasing fault thickness such that the fault energy associated with the simultaneous dissociation of an appropriate group of dislocations (e.g. a finite tilt boundary segment) can be zero or negative. This condition leads to the spontaneous formation of a martensitic embryo. For the specific case of the fcc → hcp martensitic transformation in Fe-Cr-Ni alloys, the defect necessary to account for spontaneous embryo formation at the observedM s temperatures may consist of four or five properly spaced lattice dislocations. Such defects are considered to be consistent with the known sparseness of initial martensitic nucleation sites.

557 Citations

Journal Article
01 Jan 2018-Acta Materialia
Abstract: This article reviews original work and important new developments in the field of deformation behavior of high manganese face-centered cubic γ-Fe alloys. Owing to their exceptional mechanical properties, these alloys, referred to as twinning-induced plasticity, or TWIP, steels, have come to the fore as prime candidate materials for light-weight applications, notably in automotive, shipbuilding, and oil and gas industries. It is established that a superior combination of strength and ductility exhibited by TWIP steels is associated with a specific character of the variation of the dislocation density. The defining feature of TWIP steels is the small magnitude of the intrinsic stacking fault energy. In addition to limiting the dynamic recovery rate, the low stacking fault energy of TWIP steels results in the formation of isolated stacking faults and deformation twins, which reduces the dislocation mean free path. Both effects lead to an increased strain hardening rate. Despite the progress made, there are still considerable differences between the models proposed for the microstructural evolution during the deformation of TWIP steels and the concomitant strain hardening behavior. The review surveys the experimental literature, summarizes the current modeling concepts, and identifies the outstanding issues with TWIP steels that require the attention of the materials science community. Suggestions for the directions of future research on twinning-induced plasticity steels are offered.