B. C. De Cooman

State-of-the-knowledge on TWIP steel

Abstract High Mn twinning induced plasticity (TWIP) steel is a new type of structural steel, characterised by both high strength and superior formability. TWIP steel offers an extraordinary opportunity to adjust the mechanical properties of steel by modifying the strain hardening. The use of TWIP steel may therefore lead to a considerable lightweighting of steel components, a reduction of material use and an improved press forming behaviour. These key advantages will help implement current automotive vehicle design trends which emphasise a reduction of greenhouse gas emissions and lowering of fuel consumption. In addition, high strength TWIP steel will effectively contribute to weight containment in vehicles equipped with hybrid and electric motors, as these are considerably heavier than conventional motors. The present review addresses all aspects of the physical metallurgy of the high strength TWIP steel with a special emphasis on the properties and key advantages of TWIP sheet steel products relevant to automotive applications.

High Mn TWIP Steels for Automotive Applications

Modern car design puts an increasing emphasis on the notion that a material used in building the body-in-white (BIW) should be selected on the basis of how well it helps achieving specific engineering targets such as low vehicle weight, high passive safety, stability, stiffness, comfort, acoustics, corrosion, and recycling. Steel is at present still the material of choice for car bodies, with 99% of the passenger cars having a steel body, and 6070% of the car weight consisting of steel or steel-based parts. The automotive industry is however continuously making excursions in the area of light materials applications. At present, most car makers are routinely testing multi-materials concepts, which are not limited to the obvious use of light materials for closures, e.g. the use of Al for the front lid or thermosetting resins for trunk lids. The steel industry has made a sustained effort to innovate and create advanced steels and original steel-based solutions and methods in close collaboration with the manufacturers by an early involvement in automotive projects, but also by involving automakers in their own developments. Carmakers have increasingly built passenger cars with body designs which emphasize passenger safety in the event of a collision, and most passenger cars currently achieve high ratings in standardized crash simulations such as the EURO NCAP or the North American NHST tests. The safety issue directly related to the BIW materials is passive safety. High impact energy absorption is required for frontal crash and rear collision, and anti-intrusion properties are required in situations when passenger injury must be avoided, i.e. during a side impact and in case of a roll over, with its associated roof crush. Increased consumer expectations have resulted in cars which have steadily gained in weight as illustrated in figure 1. This weight spiral is a direct result of improvements in vehicle safety, increased space, performance, reliability, passenger comfort and overall vehicle quality. This trend has actually resulted in an increased use of steel in car body manufacturing in absolute terms, and this increase may in certain cases be as high as 25%. The weight issue is therefore high on the agenda of BIW design, as it is directly related to environmental concerns, i.e. emissions of CO2, and the economics of the gas mileage. Reports on weight saving resulting from the use of Advanced High Strength Steels (AHSS) are difficult to evaluate as these tend to focus on the use of advanced steels and improved designs for a single part, rather than the entire car body. The use of Dual Phase (DP) and Transformation-Induced Plasticity (TRIP) steels has been

State-of-the-Science of High Manganese TWIP Steels for Automotive Applications

Recent trends in automotive industry towards improved passenger safety and reduced weight have led to a great interest in AHSS (Advanced High Strength Steel), and DP, TRIP, CP, MA and high-Mn TWIP (TWinning Induced Plasticity) steels are particularly promising due to their superior toughness and ductility. The properties of low SFE (Stacking Fault Energy) austenitic high Mn FeMnC steel exhibiting twinning-induced plasticity have recently been analyzed in detail. It is argued that although the mechanical properties of TRIP and TWIP steels are often assumed to be solely due to effects related to straininduced transformation and deformation twinning, respectively, other mechanisms may also play an essential role such as point-defect cluster formation, planar glide, pseudo-twinning, short range ordering, and dynamic strain ageing, e.g. in the case of TWIP steel. At low strain rates, the plastic deformation of TWIP steels is often controlled by the movement of very few well-defined localized deformation bands. The formation and propagation of these Portevin-LeChatelier (PLC) bands lead to serrated stress-strain curves, exhibiting a small negative strain rate sensitivity.

Hydrogen Effects in Prestrained Transformation Induced Plasticity Steel

Thermal desorption analysis (TDA) was performed on laboratory heat-treated transformation induced plasticity (TRIP) steel with 14.5xa0pct retained austenite (RA), ultimate tensile strength (UTS) of 880xa0MPa, and elongation to failure of 33xa0pct. Samples were tensile prestrained 5xa0pct at 253xa0K (–20xa0°C), 296xa0K (23xa0°C), and 375xa0K (102xa0°C) to generate different amounts of deformation-induced martensite, 10.5, 5.5, and 0.5xa0pct, respectively, prior to cathodically charging to a hydrogen content of 1 to 2 ppm. TDA was performed on charged samples to determine the location and strength of hydrogen trapping sites. TDA results suggest that dislocations were the main trapping sites in prestrained TRIP steel. The TDA peak intensity increased with prestrain, suggesting that the quantity of hydrogen trap sites increased with deformation. Tensile tests were performed on the four hydrogen-charged TRIP steel conditions. As confirmed with transmission electron microscope images, samples with more homogeneous dislocation distributions (i.e., prestrained at 375xa0K (102xa0°C)) exhibited greater resistance to hydrogen embrittlement than samples that included a high dislocation density adjacent to the formations of strain-induced martensite (i.e., samples prestrained at 253xa0K (–20xa0°C) and 296xa0K (23xa0°C)).

Grain Boundary Penetration by Lancet Domains in Fe-3%Si Grain-Oriented Steel

We studied the influence of the grain boundary characteristics on the configuration and behavior of domains in the vicinity of a grain boundary in grain-oriented (GO) electrical steel. We evaluated the boundary misorientation and microstructure with electron backscatter diffraction and transmission electron microscopy. We used magneto-optic Kerr microscopy to investigate the magnetic domain structure and the domain wall motion at the grain boundary at various external field strengths. We found that the grain boundary geometry has a pronounced influence on the ability of domain walls to penetrate the grain boundary during the magnetization process. The results show that different external field strengths are required for the magnetic saturation of the regions within GO steel having grain boundaries with different characteristics. A tilted grain boundary with a boundary plane normal making an angle less than 10° to the rolling direction allowed for the easy penetration of the lancet domains during magnetization, suggesting that this type of grain boundary affects the properties of GO electrical steels less negatively.

Strain Rate Sensitivity of C-alloyed, High-Mn, Twinning-induced Plasticity Steel

The mechanical properties of twinning-induced plasticity (TWIP) steels are often assumed to be solely due to the reduction of the mean free path of glide dislocations resulting from deformation twinning. Other mechanisms may also play an essential role: Mn-C cluster formation, planar glide, pseudo-twinning, short range ordering, and dynamic strain ageing. The present contribution offers a critical analysis of the mechanical properties of high-Mn TWIP steels, especially in terms of Dynamic Strain Aging (DSA) and Static Strain Aging (SSA). The presentation offers new insights into the properties of TWIP steels which were obtained by using new experimental techniques such as in-situ strain analysis and high sensitivity infrared thermo-graphic imaging.

Quenching and Partitioning of Ni-Added High Strength Steels

High strength steels containing significant fractions of retained austenite have been developed in recent years, and are the subject of growing commercial interest when associated with the TRIP phenomenon during deformation. A new process concept “quenching and partitioning” (Q&P) has been proposed by CSM/USA, and the results show the potential to create a new kind of steel microstructure with controlled amounts of retained austenite, enriched by carbon partitioning. Four steels containing C, Si, Mn, Ni, Cr and Mo, were designed with variation in the Ni and C content, aiming to decrease Bs temperature and to suppress carbide formation during the partitioning treatment. Several heat-treatment procedures were performed in specimens previously machined for tensile testing, while x-ray diffraction was used to determine the fraction of retained austenite. The tensile test results showed that except for the high C high Ni alloy, most of the processing conditions resulted in strengths superior to those of advanced high strength steels (AHSS), although it is importantly recognized that higher alloy additions were used in this study, in comparison with conventional AHSS grades.. A variety of strength and ductility combinations were observed, confirming the potential of the Q&P process and illustrating the strong influence of the final microstructure on the mechanical properties. Experimental results for samples partitioned at 400 °C indicate that higher ultimate tensile strength is associated with higher fraction of retained austenite for multiple heat treatments of each alloy investigated. The amount of retained austenite obtained was generally lower than that predicted by the model. Further studies are in progress to understand the influence of alloying and processing parameters (time/temperature) on the partitioning of carbon and precipitation of transition carbides.

Texture evolution in grain-oriented electrical steel during hot band annealing and cold rolling

The optimization of magnetic and physical properties of electrical steel is imperative for many engineering applications. The key factors to improve magnetic properties are the steel composition as well as control of the crystallographic orientation and microstructure of the steel during processing. However, this requires careful control of processing at all stages of production. Under certain conditions of deformation and annealing, electrical steel can be produced to have favourable texture components. For grain‐oriented (GO) electrical steels that are used in most transformer cores, a pronounced {110}〈001〉 Goss texture plays a vital role to achieve low power losses and high permeability. Essentially, Goss texture develops during secondary re‐crystallization in GO electrical steels; however, the mechanism of the abnormal Goss grain growth is still disputed in the literature. In the current study, the influence of the annealing conditions on the development of annealing, cold rolling and re‐crystallization textures of hot‐rolled GO electrical steel were investigated in detail following each processing step. Furthermore, the orientation data from electron backscatter diffraction were used to evaluate the orientation‐dependent stored energy of deformed grains after hot rolling. In the light of new findings in the present study, annealing and deformation texture development mechanisms were critically reviewed.

Influence of Texture on Ridging and Formability of 16%Cr Ferritic Stainless Steel

The influence of the texture on ridging and forming properties of transformable 16mass-%Cr steel was studied for two different specific processing routes. In the first route (HACA), hot strips were annealed prior to cold rolling, whereas in the second route (HCA), un-annealed hot strips were directly subjected to cold rolling. Results indicate that compared to HACA, HCA results in an improved surface smoothness, i.e. reduced roping, but a lower mean r-value, as a result of strengthening of the α-fiber texture components. Differences in the roping and forming properties could also be achieved by compositional differences resulting in differences in the fraction of austenite at hot rolling temperatures.

Oxide formation and alloying elements enrichment on TRIP steel surface during inter-critical annealing

The gas atmosphere in continuous annealing and galvanizing lines alters both composition and microstructure of the surface and sub‐surface of sheet steels. The alloying element enrichments and the oxide morphology on transformation‐induced plasticity steel surfaces are strongly influenced by the dew point of the furnace atmosphere and annealing temperature. The formation of a thin oxide film and enrichment of the alloying elements during annealing may result in surface defects on galvanized sheet products. The present contribution reports on the use of microanalysis techniques such as electron backscatter diffraction, glow discharge optical emission spectroscopy and electron probe micro‐analysis for the detailed surface analysis of inter‐critically annealed transformation‐induced plasticity steel such as oxide phase determination, microstructure and microtexture evolutions.