Effects of reduced (– 40 °C), ambient (20 °C), and elevated (200 °C) deformation temperatures on the microstructure evolution and strain hardening behavior of two low-C thermomechanically processed high-manganese steels were studied. The microstructure was characterized by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) techniques. The temperature-dependent tendency of austenite to strain-induced ε/α′-martensitic transformation and mechanical twinning was qualitatively and quantitatively assessed using the EBSD technique. The steel containing 26 wt% of Mn showed the beneficial strength–ductility balance at reduced deformation temperature -40 °C due to the intense Transformation-Induced Plasticity (TRIP) effect which resulted in the formation of significant ε- and α′-martensite fractions during tensile deformation. The mechanical properties of steel containing 27 wt% of Mn were more beneficial at elevated deformation temperature 200 °C due to the occurrence of intense Twinning-Induced Plasticity (TWIP) effect expressed by the presence of significant fraction of mechanical twins. Moreover, at the highest deformation temperature 200 °C, the evidence of thermally activated processes affecting the mechanical behavior of the higher Mn steel was identified and described.

The aim of this manuscript was to study the influence of alloying elements on the phase transformation behavior in advanced high-strength multiphase steels. Continuous cooling transformation (CCT) and time–temperature–transformation (TTT) diagrams were calculated to analyze the stability of phases at variable time–temperature processing parameters. The analyzed materials were lean-alloyed transformation induced plasticity (TRIP) medium manganese steels. The simulations of the phase diagrams, the stability of the phases during simulated heat treatments, and the chemical composition evolution diagrams were made using Thermo-Calc and JMatPro material simulation softwares. The influence of alloying elements, i.e., Mn and C, were studied in detail. The computational and modelling results allowed the influence of alloying elements on equilibrium and non-equilibrium phase diagrams and microstructural and chemical composition evolutions to be studied. Good symmetry and correlation between computational softwares were achieved. The study allows for future optimization of the heat-treatment temperature and time conditions of modern medium-Mn automotive sheet steels.

The thermal and mechanical stabilities of retained austenite in aluminum-containing medium-Mn 0.16C–4.7Mn–1.6Al–0.2Si sheet steel were investigated. The strain-induced martensitic transformation in Mn TRIP steel was studied at different temperatures. Static tensile tests were carried out at the temperature ranging from − 60 to 200 °C. The tests allowed to study the influence of the temperature on austenite-to-martensite transformation kinetics. The interrupted tensile tests and corresponding X-ray measurements of retained austenite amount were performed to determine the mechanical stability of retained austenite using the Sugimoto model. The microstructure changes were investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Observed results reflected the effects of deformation temperature on the mechanical stability of retained austenite and the corresponding response of this phase to martensitic transformation. It was found that an increase in the deformation temperature resulted in the reduced intensity of the TRIP effect due to the higher mechanical stability of retained austenite. At the highest deformation temperature (200 °C), the evidence of thermally activated processes affecting the mechanical behavior was identified.

The strain-aging of low alloyed, multiphase high-strength steels with strain-induced austenite to martensite transformation was studied. The influence of prestrain, aging time, and temperature dependence of the static strain aging was carried out. Ageing temperatures between 60 and 220 ∘ C and aging times from 20 to 10,000 min were investigated. The choice of steel composition allowed studying the influence of alloying elements, such as Si and Al, on the static strain aging behavior. Samples after aging were studied using light-optical microscopy, X-ray diffraction, and in-depth transmission electron microscopy (TEM). The Harper model was used to describe the precipitation mechanisms occurring during aging. The study of thin foils after aging using TEM showed the precipitation of low temperature transition carbides in the microstructure, which was observed between 60 and 5000 min. By using X-ray diffraction, it was revealed that aging at 170 ∘ C for a long time caused a slight decrease of the retained austenite volume fraction, but the C content remained constant.

In the present study four different density reduced TRIP steel concepts with varying Mn and Al contents were investigated with regard to their microstructure, mechanical properties and retained austenite (RA) stability. For the hot rolled strips, scanning electron microscopy (SEM) revealed a microstructure consisting of ferrite and two types of carbides. Using both X-ray diffraction (XRD) and SEM with backscattered electron (BSE)-detection, the carbides were identified as cementite for the steel grades containing 2.8 and 3.3 wt% Al and kappa-carbides for the compositions with 5.2 wt% Al. The microstructure of the laboratory continuously annealed cold rolled sheets consisted of numerous inclusions of bainite and RA, embedded in a ferritic matrix. As an aftermath of the increased Al content, resulting in an increased ferrite content, the bainitic transformation was significantly reduced, which led to a destabilization of RA and in turn to the formation of martensite upon final cooling to room temperature (RT). With increasing Al- and Mn-contents the tensile strength rose from 720 to 1050 MPa, whereas the total elongation decreased from 39% to 17%. Both highest mechanical and chemical RA stability were found for the steel grades containing 2.8 and 3.3 wt% Al, resulting in the superior combination of strength and ductility, namely R(m)XA(80) of almost 30,000 MPa% by a density reduction up to almost 5%.

A twinning induced plasticity (TWIP) steel, developed for automotive applications, was characterized down to the nanoscale to investigate the nature of the strengthening mechanisms. Both tensile-deformed and non-deformed materials were studied by light optical microscopy, X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. The investigated TWIP steel showed extensive twinning upon deformation. With high-resolution transmission electron microscopy, nano-twins as small as 3 nm in width were observed, and large-angle convergent-beam electron diffraction identified the pole mechanism as one of the twinning mechanisms in this TWIP steel. This study emphasizes that a thoughtful combination of techniques is necessary to fully capture the microstructure of this TWIP steel and explain the origin of superior mechanical properties compared to other TWIP steel grades.

Hot-dip galvanized (HDG) 2nd generation advanced high strength steel (AHSS), nano-TWIP (twinning induced plasticity) with 15.8 wt.% Mn, 0.79 wt.% C, was analyzed at the interface between steel and zinc by scanning Auger electron microscopy (AES) in order to confirm and improve an existing model of additional pre-oxidation treatment before annealing and immersion into the hot zinc bath. Furthermore these steel samples were fractured in the analysis chamber of the AES and analyzed without breaking vacuum. In these measurements the results of an aluminothermic reduction of the manganese and iron surface oxides on the steel could be confirmed by AES. (C) 2015 Elsevier Ltd. All rights reserved.

Abstract Medium-Mn steels are a promising candidate to fulfil the requirements of the 3rd generation of AHSS (Advanced High Strength Steels) which combine high strength and high ductility. A new approach is to add micro alloying elements, like niobium, to affect microstructural development during recrystallization and phase transformations. Using a quenching dilatometer, the kinetics of the phase transformations from austenite (810 °C and 840 °C) to its transformation-products by varying different cooling rates (100 K/s to 0.03 K/s) were investigated for two low carbon medium-manganese steels with different contents of niobium. Based on the dilatometric curves, the results of light optical microscopy (LOM) and quantitative metallography, measurements of the retained austenite (RA) content by X-ray diffractometry (XRD) and hardness measurements, continuous-cooling-transformation (CCT) diagrams were constructed. Furthermore, the influence of the element niobium on the microstructure after intercritical annealing was investigated by similar methods. Different etching-methods (Nital, Klemm-I) were applied to reveal different microstructural aspects. Besides considerable grain-refinement effects with increased niobium content, the phase-transformations were accelerated in the CCT-diagrams. The microstructure varies from 100 % martensite to certain amounts of bainite, but also ferrite at lower cooling rates. Intercritical annealing results in different amounts of retained austenite.

Die Inhalte dieser Arbeit konzentrieren sich auf die Herstellung von Stahl-Keramik-Verbunden mit reduzierter Dichte, bei gleichzeitiger Erhöhung der Bauteilsteifigkeit. Als Randbedingung galt ein Anteil von maximal 20 Vol.-% an keramischen Partikeln mit annähernd globularer Geometrie, bei einem maximalen mittleren Durchmesser von 10 µm. Das Verhalten der Partikel in der Matrix wurde über ein modifiziertes pulvermetal­lurgisches Versuchskonzept betrachtet und die Dispersion der Partikel in Abhängigkeit von der Zugabe unterschiedlicher Legierungselemente bewertet. Es zeigte sich, dass sowohl die Benetzung zwischen Keramik und Metall als auch die thermodynamische Stabilität der Partikel in der flüssigen Stahlmatrix hierbei ein wesentliches Kriterium für die Ausbildung eines Verbundwerkstoffes darstellen.

Second generation advanced high strength steel is one promising material of choice for modern automotive structural parts because of its outstanding maximal elongation and tensile strength. Nonetheless there is still a lack of corrosion protection for this material due to the fact that cost efficient hot dip galvanizing cannot be applied. The reason for the insufficient coatability with zinc is found in the segregation of manganese to the surface during annealing and the formation of manganese oxides prior coating. This work analyses the structure and chemical composition of the surface oxides on so called nano-TWIP (twinning induced plasticity) steel on the nanoscopic scale after hot dip galvanizing in a simulator with employed analytical methods comprising scanning Auger electron spectroscopy (SAES), energy dispersive X-ray spectroscopy (EDX), and focused ion beam (FIB) for cross section preparation. By the combination of these methods, it was possible to obtain detailed chemical images serving a better understanding which processes exactly occur on the surface of this novel kind of steel and how to promote in the future for this material system galvanic protection.

A new high-manganese, low-silicon TWIP steel was studied to evaluate austenite stability after different heat treatment conditions. To determine the phase transformations, dilatometric experiments were performed, and the microstructure was characterized by light optical microscopy, X-ray diffraction, and transmission electron microscopy. Precipitation of lamellar cementite was observed in the microstructure for extended treatment times at 823 K (550 °C). Long isothermal holding at this temperature also caused epsilon martensite formation during cooling, resulting from a decrease in austenite stability due to carbon depletion in the matrix when a quantifiable amount of cementite is formed.

Electro hydraulic forming of a range of different sheet steels was studied experimentally and with finite element methods. Four carbon and stainless sheet materials were studied. In this paper we present results on a mild steel (IF210), two high strength steels (DPX800 and TRIP700) and one stainless steel (1.4509). The flow properties of the materials were evaluated at a range of strain rates up to 1000/s. These were typical strain rates in the FE simulations. The flow properties were characterized with the Johnson Cook model. Electro hydraulic forming trails were performed with a chamber of water with a pair of electrodes on one side of the sheet. In one case free forming was performed and in the other case forming was performed into a truncated conical die. Geometrical shapes and strain distributions were evaluated after forming. A finite element model was formulated in ABAQUS explicit. The model takes the chamber filled with water into account and the effect of the electrical discharge is modeled as a pressure wave originating from the location of the electrodes. The sheet is given the properties defined by the Johnson Cook model and stiff tools are used. The forming of the sheet is described including rebound effects at the tools. The model shows satisfactory results in relation to the experimental trials regarding both shape and strains of the pressed sheets.

The aim of the contribution is to review the structure/property relationship of a high-performance FeMnN steel composition. In this work, the influence of the Mn content ranging from 12 wt. % to 24 wt. % on the FeC phase diagram was calculated by thermodynamic simulations. The influence of coiling temperature on the microstructure of the FeMnN steel was studied by annealing treatments from 350°C up to 750°C. It showed the possible presence of cementite and martensite/pearlite after long annealing treatments for temperatures close to 500°C. The steel was observed to exhibit outstanding combinations of tensile strength and ductility. The FeMnN steel exhibited at room temperature a remarkably high total elongation of about 100% with a high tensile strength level of 1100 MPa. This product, proportional to toughness, achieved an outstanding value of about 110000 %MPa. The extended tensile ductility of the high-performance FeMnN steel was attributed to the nano-size twinning induced plasticity (NS-TWIP), which was observed by in-depth transmission electron microscopy study.

The aim of the contribution is to review the structure/property relationship of a high-performance FeMnN steel composition. In this work the influence of the Mn content ranging from 12 wt.-% to 24 wt.-% on the Fe-C phase diagram was calculated by thermodynamic simulations. The influence of coiling temperature on the microstructure of the FeMnN steel was studied by annealing treatments from 350 °C up to 750 °C. It showed the possible presence of cementite and martensite/pearlite after long annealing treatments at temperatures close to 500 °C. The steel was observed to exhibit outstanding combinations of tensile strength and ductility. The FeMnN steel exhibited at room temperature a remarkably high total elongation of about 100 % with a high tensile strength level of 1100 MPa. This product, proportional to toughness, achieved an outstanding value of about 110000 %MPa. The extended tensile ductility of the high-performance FeMnN steel was attributed to the nano-size twinning induced plasticity (NS-TWIP), which was observed by in-depth transmission electron microscopy study.

The car industry is facing pressure because of the growing demand for more fuel-efficient passenger cars.In order to limit energy consumption and air pollution the weight of the carbody has to be reduced.At the same time, high levels of safety have to be guaranteed.In this situation, the choice of material becomes a key decision in car design.As a response to the requirements of the automotive sector, high strength steels and advanced high strength steels have been developed by the steel industry.These modern steel grades offer an excellent balance of low cost, light weight and mechanical properties.