A Material Model for TRIP‐Steels and its Application to a CrMnNi Cast Alloy

A material model is presented that accounts for strain rate dependent inelastic deformation and strain‐induced phase transformation in TRIP‐steels. Modifications for the kinetics equations of the strain‐induced phase transformation, introduced by Stringfellow, are proposed to overcome a drawback of Stringfellow's model. A parameter identification strategy that relies on Gauss‐Markov estimates is used to determine the model parameters from experimental data of a recently developed cast TRIP‐steel. Good agreement is observed between experimental results of the compression test and the corresponding finite element simulation employing the proposed model. This forms the basis for future applications of the material model in the design of composites and structures.     

Strain Rate Dependent Flow Stress and Energy Absorption Behaviour of Cast CrMnNi TRIP/TWIP Steels

Modern steel developments often use additional deformation mechanisms like the deformation induced martensitic transformation (TRIP‐effect) and mechanical twinning (TWIP‐effect) to enhance elongation and strength. Three high‐alloyed cast CrMnNi‐steels with different austenite stabilities were examined. Dependent on the austenite stability, TRIP‐effect and TWIP‐effect were found. A low austenite stability causes a distinctive formation of deformation induced α'‐martensite and therefore a strong strain hardening. The increase of strain rate leads to an increase in yield strength and flow stress, but also to a counteractive adiabatic heating of the specimen. Dependent on the degree of deformation, low austenite stabilities and high strain rates lead to excellent values in specific energy absorption.     

Strain Rate Effect on Material Behavior of TRIP‐Steel/Zirconia Honeycomb Structures

A pure TRIP‐steel alloy and a novel zirconia reinforced TRIP‐steel matrix composite were implemented in a 2D square‐celled honeycomb structure fabricated by a paste extrusion method, respectively. In terms of a series of compression tests in out‐of‐plane loading direction the buckling and the pronounced strain hardening behavior of the honeycomb structures are described with regard to different material compositions and varied nominal strain rates. Both the compressive flow behavior and the microstructure evolution in the crushed zones are controlled by the rate of formation of strain‐induced martensite and the ceramic particle/steel matrix interactions. The insertion of magnesia partially‐stabilized zirconia (Mg‐PSZ) particles in the austenitic steel matrix cause an increased yield strength and higher compression stresses up to certain deformations degrees. The limited ductility of the composite materials is a consequence of the rearrangement and fracture of zirconia particles initiating cracks and shear bands during deformation. Consistently, the visible strain rate effects on the mechanical responses of the honeycomb structures are similar to AISI 304L austenitic stainless steel specimens in the form of compact rods. However, at high local strain rates generated in drop weight impact tests a micro‐inertia factor support the failure behavior of the cellular structures.     

STT and DTT Diagrams of Austenitic Cr–Mn–Ni As‐Cast Steels and Crucial Thermodynamic Aspects of γ → α′ Transformation

The mechanical behavior and microstructure evolution during deformation of novel austenitic Cr–Mn–Ni as‐cast steels with varied Ni content were investigated at various temperatures using static tensile tests, optical microscopy, and the magnetic scale for the detection of ferromagnetic phase fraction. To summarize all knowledge about the deformation‐induced processes, the STT and DTT diagrams were developed for Cr–Mn–Ni steels. The diagrams illustrate the different deformation mechanisms depending on temperature and tension load, and quantify the elongation of the deformation mechanisms. The deformation‐induced ε‐ and α' martensite formation and twinning – the TRIP and TWIP effects – occur in the Cr–Mn–Ni steels depending on the chemical composition and temperature. The differences of deformation‐induced processes depend on thermodynamics and are confirmed by thermodynamic calculations. The nucleation threshold of γ → α′ transformation was determined for the investigated Cr–Mn–Ni steels.     

Galvannealing of (High‐)Manganese‐Alloyed TRIP‐ and X‐IP®‐Steel

In this study the influence of Mn on galvannealed coatings of 1.7% Mn‐1.5% Al TRIP‐ and 23% Mn X‐IP®‐steels was investigated. It is shown that the external selective oxides like Mn, Al and Si of the TRIP steel which occur after annealing at 800 °C for 60 s at a dew point (DP) of ‐25 °C (5% H₂) hamper the Fe/Zn‐reaction during subsequent galvannealing. Preoxidation was beneficially utilized to increase the surface‐reactivity of the TRIP steel under the same dew point conditions. The influence of Mn on the steel alloy was investigated by using a 23% Mn containing X‐IP®‐steel which was bright annealed at 1100 °C for 60 s at DP ‐50 °C (5% H₂) to obtain a mainly oxide free surface prior to hot dip galvanizing (hdg) and subsequent galvannealing. As well known from the literature Mn alloyed to the liquid zinc melt stabilizes δ‐phase at lower temperatures by participating in the Fe‐Zn‐phase reactions, it was expected that the metallic Mn of the X‐IP®‐steel increases the Fe/Zn‐reactivity in the same manner. The approximation of the effective diffusion coefficient (D_eff(Fe)) during galvannealing was found to be higher than compared to a low alloyed steel reference. Contrary to the expectation no increased Fe/Zn‐reaction was found by microscopic investigations. Residual η‐ and ζ‐phase fractions prove a hampered Fe/Zn‐reaction. As explanation for the observed hampered Fe/Zn‐reaction the lower Fe‐content of the high‐Mn‐alloyed X‐IP®‐steel was suggested as the dominating factor for galvannealing.     

Physical Metallurgy of Multi‐Phase Steel for Improved Passenger Car Crash‐Worthiness

The dynamic testing of high strength automotive steel grades is of great practical importance if their crash‐worthiness is to be evaluated. During forming operations, steels are processed in a controlled dynamic manner. In collisions, the deformation is different in the sense that the deformation is not controlled, i.e. both strain and strain rate are not pre‐determined. No clear standard testing procedures are currently available to test high strength steels dynamically, in order to evaluate their performance during car crashes. High tensile strength TRIP‐aided steels have been developed by the steel industry because of their promising high strain rate performance. The present contribution focuses on the effect of the strain rate and temperature on the mechanical behaviour of the low alloy high strength TRIP steel. The tests were carried out on the separated phases in order to determine their specific high strain rate deformation response. The temperature‐dependence of the transformation rate of the retained austenite is presented. It is argued that the adiabatic conditions present during high strain rate deformations have a beneficial effect on the behaviour of TRIP steel.     

Strain‐Rate‐Dependent Flow Stress and Failure of an Mg‐PSZ Reinforced TRIP Matrix Composite Produced by Spark Plasma Sintering

A composite consisting of 5 vol% MgO‐partially stabilized ZrO₂ particles (Mg‐PSZ) and a TRIP‐steel‐matrix (CrNiMn steel; transformation induced plasticity) was produced through Spark Plasma Sintering. The processed material was tested under compression at various nominal strain rates (4 × 10^?4 s^?1; 10^?3 s^?1; 1 s^?1, 10² s^?1). Both, the pure steel and the composite showed a considerable plasticity and high strength due to the very fine grained steel matrix. The addition of 5 vol% ceramic particles led to a rise in the offset yield strength of 60 MPa till 90 MPa according to the applied strain rate. Up to a strain rate of 1 s^?1, no change in offset yield strength was measured. A strain‐rate of 100 s^?1 leads to a rise in the offset yield strength of approx. 100 MPa. Both, the ceramic and an increase in the strain rate implicate to an early generation of microdeterioration. Limited by the interfacial strength of steel and Mg‐PSZ, failure occurs early at the interfaces, which is shown in a decrease in the work hardening. During the compression, especially at higher strain‐rates, adiabatic heating occurred and counteracted to the martensitic transformation.     

High Temperature Deformation Behavior of 4340 Steel: Activation Energy Calculation and Modeling of Flow Response

The 4340 steel is extensively utilized in several industries including automotive and aerospace for manufacturing a large number of structural components. Due to the importance of thermo-mechanical processing in the production of steels, the dynamic recrystallization (DRX) characteristics of 4340 steel were investigated. Namely, hot compression tests on 4340 steel have been performed in a temperature range of 900–1200 °C and a strain rate range of 0. 01–1 s^?1 and the strain of up to 0. 9. The resulting flow stress curves show the occurrence of dynamic recrystallization. The flow stress values decrease with the increase of deformation temperature and the decrease of strain rate. The microstructure of 4340 steel after deformation has been studied and it is suggested that the evolution of DRX grain structures can be accompanied by considerable migration of grain boundaries. The constitutive equations were developed to model the hot deformation behavior. Finally based on the classical stress-dislocation relations and the kinematics of the dynamic recrystallization; the flow stress constitutive equations for the dynamic recovery period and dynamic recrystallization period were derived for 4340 steel, respectively. The validity of the model was demonstrated by demonstrating the experimental data with the numerical results with reasonable agreement.     

Strain‐Induced Precipitation Behavior of a Nb–Ti–V Steel in the Austenite Phase Field

The precipitation behavior of a Nb (0.041 wt%), V (0.11 wt%), and Ti (0.018 wt%) micro‐alloyed steel influenced by a deformation process in the austenite region has been investigated by transmission electron microscopy and atom probe. After solution annealing at 1250°C, nearly the entire Nb and V amount is in solid solution. Ti does not go into in solid solution and stays stable as TiN precipitates. During the cooling process to deformation temperature, these TiN precipitates act as nucleation sites for Nb and V. Precipitates with a TiN rich core and a V‐rich shell are formed. Nb is homogenously distributed over the precipitate. After deformation, strain induced precipitates occur, which consist mainly of Nb, V, and only a little Ti. Smaller strain induced precipitates have a higher N‐content than C, with growth the C‐content increases.     

Influence of Manganese and Nickel on the α´ Martensite Transformation Temperatures of High Alloyed Cr‐Mn‐Ni Steels

The martensite start temperature (M_s), the martensite austenite re‐transformation start temperature (A_s) and the re‐transformation finish temperature (A_f) of six high alloyed Cr‐Mn‐Ni steels with varying Ni and Mn contents in the wrought and as‐cast state were studied. The aim of this investigation is the development of the relationships between the M_s, A_s, A_f, T₀ temperatures and the chemical composition of a new type of Cr‐Mn‐Ni steels. The investigations show that the M_s, A_s and A_f temperatures decrease with increasing nickel and manganese contents. The A_f temperature depends on the amount of martensite. Regression equations for the transformation temperatures are given. The experimental results are based on dilatometer tests and microstructure investigations.