W. Borek

Thermo-mechanical processing of high-manganese austenitic TWIP-type steels

The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on recrystallization behaviour was investigated. On the basis of initial investigations realized by hot upsetting the thermo-mechanical conditions resulting in a fine-grained structure were designed. The σ-e curves and identification of thermally activated processes controlling work-hardening by the use of the Gleeble simulator were determined. It was found that the thermo-mechanical treatment conditions influence a phase composition of the investigated steels after solution heat treatment.

Hot-Rolling of Advanced High-Manganese C-Mn-Si-Al Steels

The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on a recrystallization behaviour was investigated. Flow stresses during the multistage compression test were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steel was compressed to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light microscopy. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn steels and slightly higher compared to Fe-(15-25) Mn alloys. Making use of dynamic and metadynamic recrystallization, it is possible to refine the microstructure and to decrease the flow stress during the last deformation at 850°C. Applying the true strains of 0.23 and 0.19 requires the microstructure refinement by static recrystallization. The obtained microstructure – hot-working relationships can be useful in the determination of powerful parameters of hot-rolling and to design a rolling schedule for high-manganese steel sheets with fine-grained austenitic structures.

Hot-Working Behaviour of Advanced High-Manganese C-Mn-Si-Al Steels

The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on a recrystallization behaviour was investigated. Flow stresses during the multistage compression test were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steel was compressed to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light microscopy. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn steels and slightly higher compared to Fe-(15-25) Mn alloys. Making use of dynamic and metadynamic recrystallization, it is possible to refine the microstructure and to decrease the flow stress during the last deformation at 850°C. Applying the true strains of 0.23 and 0.19 requires the microstructure refinement by static recrystallization. The obtained microstructure – hot-working relationships can be useful in the determination of powerful parameters of hot-rolling and to design a rolling schedule for high-manganese steel sheets with fine-grained austenitic structures.

Microstructure Evolution of C-Mn-Si-Al-Nb High-Manganese Steel during the Thermomechanical Processing

The aim of the paper is to determine the influence of hot deformation conditions on σ-ε curves and microstructure evolution of new-developed high-manganese C-Mn-Si-Al-Nb austenitic steel. The force-energetic parameters of hot-working were determined in continuous and multi-stage compression tests performed in a temperature range of 850 to 1100°C by the use of the Gleeble 3800 thermomechanical simulator. Evaluation of processes controlling work-hardening were identified by microstructure observations of the specimens water-quenched after various conditions of plastic deformation. Multi-stage compression tests with true strain of 0.29 permit to use the dynamic and metadynamic recrystallization for forming the fine-grained, austenite microstructure of steel in the whole range of deformation temperature.

2.10 – Structure and Mechanical Properties of High-Manganese Steels

This chapter presents results from investigations about high-manganese austenitic steels, which have gained increasing importance over the recent two decades. Intense research into this group of steels indicates the need for employing them in the global automotive sector, especially for constructional components of cars absorbing impact energy in road collisions. The examples of extensive research over this group of high-manganese austenitic steels have been pursued by the authors at the Division of Materials Processing Technology, Management and Computer Techniques in Materials Science, the Institute of Engineering Materials and Biomaterials of the Silesian University of Technology, Gliwice, Poland. The essence of the research concerns the designing of thermo-mechanical processing of selected high-manganese austenitic steels with carefully selected but diversified chemical composition in order to refine their structure and improve mechanical properties. Thermo-mechanical processing was performed using a thermo-mechanical simulator DSI (Dynamic System Inc.) Gleeble 3800. The results of the investigations performed with the DSI simulator allowed to design different variants of hot rolling consisting of several stages for the investigated high-manganese austenitic steels. Different processes controlling the curve of strain hardening could be used by applying the varied conditions of thermo-mechanical processing, and especially a diversified degree of deformation and isothermal heat treatment after finishing plastic working, such as dynamic recovery, dynamic recrystallization, and static recrystallization.

Mechanical Properties of High-Manganese Austenitic TWIP-Type Steel

Taking into consideration increased quantity of accessories used in modern cars, decreasing car’s weight can be achieved solely by optimization of sections of sheets used for bearing and reinforcing elements as well as for body panelling parts of a car. Application of sheets with lower thickness requires using sheets with higher mechanical properties, however keeping adequate formability. The goal of structural elements such as frontal frame side members, bumpers and the others is to take over the energy of an impact. Therefore, steels that are used for these parts should be characterized by high value of UTS and UEl, proving the ability of energy absorption. Among the wide variety of recently developed steels, high-manganese austenitic steels with low stacking faulty energy are particularly promising, especially when mechanical twinning occurs. Beneficial combination of high strength and ductile properties of these steels depends on structural processes taking place during cold plastic deformation, which are a derivative of SFE of austenite, dependent, in turn on the chemical composition of steel and deformation temperature. High-manganese austenitic steels in effect of application of proper heat treatment or thermo-mechanical treatment can be characterized by different structure assuring the advantageous connection of strength and plasticity properties. Proper determinant of these properties can be plastic deformation energy supply determined by integral over surface of cold plastic deformation curve. Obtaining of high strength properties with retaining the high plasticity has significant influence for the development of high-manganese steel groups and their significance for the development of materials engineering.

Strength and structure of AlMg3 alloy after ECAP and post-ECAP processing

ABSTRACT Equal-channel angular pressing (ECAP) is a promising method for the processing of metals with an ultrafine-grain (UFG) structure of a few hundred nanometers in size. In this paper, the influence of solution treatment and artificial aging conditions, combined with a severe plastic deformation process on the AlMg3 aluminum alloy, was studied. Samples were processed up to six passes with the application of processing route Bc, in which the sample after each separate pass is rotated clockwise by 90 degrees. Optical microscopy and electron backscattered diffraction were used to investigate the microstructure evolution at particular stages of the investigation. To estimate the effect of the process parameters on the change of the strength of the alloy, tensile properties and hardness measurements were determined.

Structure of MgLiAl alloys after various routes of severe plastic deformation studied by TEM

Abstract Two MgLiAl alloys consisting either of α (hcp) + β (bcc) phases or only β phase were subjected to twist channel angular pressing TCAP (with helical component) or cyclic compression to a total strain of ∊ = 5 in order to study the effectiveness of various deformation modes on grain refinement. After the first TCAP pass grains of α phase were refined from 30 μm down to about 6 μm and of β phase from initial 200 μm down to 8 μm. MAXStrain cycling led to much finer grains of α and β phases in the range 200 – 300 nm causing higher hardening and indicating higher effectiveness of the process. The Li2MgAl precipitates were refined during severe plastic deformation (SPD) processes and in addition fine particles of hexagonal α phase of size below 100 nm were observed within the β phase showing orientation relationship (0001) α ‖ (011) β. Two-phase material after SPD showed deviation of about 1.5° from the ideal Burgers orientation relationship (0001) α ‖ (011) β.

Effect of strain deformation rates on forming the structure and mechanical properties of high-manganese austenitic TWIP steels

Abstract The aim of this paper is to determine the influence of strain rate on mechanical properties and structure of high-Mn austenitic TWIP steels. Deformation rates in a range from 0.001 to 1000 s−1 have a significant effect on forming the structure and mechanical properties of high-manganese austenitic TWIP-type steels. TWIP steels not only show excellent strength, but also have excellent formability due to twinning, thereby leading to excellent combination of strength, ductility and formability over conventional dual phase steels or transformation-induced plasticity steels. Also the strain energy per unit volume of advanced high-Mn TWIP steels containing Mn, Al, SI and some of that steels with Nb and Ti microadditions with various structures after their heat- and thermomechanical treatments increases considerably in dynamic conditions. The microstructure of investigated steels was determined in metallographic investigations using light, scanning and high-resolution transmission electron microscopes. Results obtained in static and dynamic conditions for newly developed high-manganese austenitic steels indicate the possibility and purposefulness of their employment for constructional elements of vehicles, especially of the passenger cars to take advantage of the significant growth of their strain energy per unit volume which guarantee reserve of plasticity in the zones of controlled energy absorption during possible collision resulting from the activation of twinning induced by cold working, which may result in significant growth of the passive safety of these vehicles’ passengers.

Application of the Finite Element Method for Modelling of the Spatial Distribution of Residual Stresses in Hybrid Surface Layers

The presented investigations concern PVD/CVD surface treatment performed on samples of heat treated cast magnesium and aluminium alloys and properties modelling of obtained coatings using the finite element method (FEM). In order to identify the structure and fractures of the analysed surface coatings, investigations using the scanning electron microscope Zeiss Supra 35 were performed. Evaluation of the adhesion of the PVD/CVD coatings was carried out using a scratch test. The obtained coatings—Ti/Ti(C,N)-gradient/CrN; Ti/Ti(C,N)-gradient/(Ti,Al)N; Ti/(Ti,Si)N-gradient/(Ti,Si)N as well coatings: Cr/CrN-gradient/CrN; Cr/CrN-gradient/TiN and Ti/DLC-gradient/DLC are characterized by a clear heterogeneity of the surface associated with the presence of microparticles in the structure in form of droplets broken out of the target during the deposition process, as well immersions occurring in the surface due to the loss of some droplets during solidification. It was also found that the applied coatings are characterized with a mono-, di-, or multi-layer structure according to the applied layer system; the individual layers are applied uniformly and tightly adhere to the substrate and to each other. The obtained results of the numerical FEM analysis, have enabled a full integration of the material engineering knowledge and informatics tools, confirming compliance of the simulation model with the obtained experimental results.