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.     

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.     

Iso‐Work‐Rate Weighted‐Taylor Homogenization Scheme for Multiphase Steels Assisted by Transformation‐induced Plasticity Effect

A simple homogenization scheme for multiphase microstructure (composite) is developed. This scheme is based on the classical Taylor (iso‐strain‐rate) averaging scheme. Scalar multipliers are introduced as weighting parameters in order to relax the condition of uniform strain distribution used in the classical Taylor scheme. In the present work, the scalar weighting parameters are determined by satisfying the iso‐work‐rate condition, i.e., work is equally distributed in all constituent phases. In combination with micromechanical models developed for transformation‐induced plasticity (TRIP) effect, the iso‐work‐rate weighted‐Taylor scheme is applied for simulating the effective mechanical behaviour of multiphase TRIP‐assisted steel. The predictions of the iso‐work‐rate weighted‐Taylor scheme are compared with the result of the corresponding simulation with the direct finite‐element method (FEM).     

Spline‐Interpolation and Calculation of Machine Parameters for the Three‐Roll‐Pushbending of Spline‐Contours

Today, bending tasks become more and more complex. Not even constant bending radii are required in the industrial practice. There is a growing demand for bending spline‐contours, too. Such geometries are often produced with Freeform‐Bending procedures like Three‐Roll‐Pushbending. This paper presents a method to interpolate a given spline bending‐contour (by CAD data), in order to calculate its radii distribution, which is needed to determine the machine parameters in certain points for the Three‐Roll‐Pushbending. For the determination of the machine parameters one has to consider the different influences on the bending process. The material springback and the deflection of the bending machine per radius need to be compensated to reach a near net shape bending result. Nevertheless deviations cannot be avoided. To improve the results, a possibility to adjust the pre‐calculated machine parameters is shown. For the investigations tube profiles with constant wall thicknesses were considered. The corresponding plasticity calculations refer to tube cross‐sections. The results were validated by bending a representative spline contour on the bending machine of the Chair of Forming Technology at the University of Siegen.     

A Study of Microstructure and Performance of Quenching Fe‐V‐W‐Mo Alloy

The critical points and time temperature transformation (TTT) curves of Fe‐5%V‐5%W‐5%Mo‐5%Cr‐3%Nb‐2%Co (Fe‐V‐W‐Mo) were measured, and the effects of quenching temperature and cooling modes on the microstructure and performance of Fe‐V‐W‐Mo alloy were investigated. The results showed that the hardness of Fe‐V‐W‐Mo alloy increased until the quenching temperature reached 1025°C and dropped down as the quenching temperature exceeded 1050°C in oil cooling. The hardness obtained in air cooling and spray cooling exhibited a similar tendency as that in oil cooling, but the temperature at which the highest hardness was obtained in these slower cooling processes changed to a higher range. The hot hardness and toughness of Fe‐V‐W‐Mo alloy increased with rising quenching temperature until it reached 1150°C, and from then on the toughness began to drop. The main reasons why the structures and properties of Fe‐V‐W‐Mo alloy obviously change under different quenching conditions are particularly analysed at last.     

An Intercomparison Study of Analytical Methods for the Determination of Magnesium in Low Alloy Steel

In an intercomparison study three low alloy steel materials were analyzed on their content of the trace element Mg, and five different analytical techniques were used, namely spark‐OES, inductively coupled plasma optical emission spectrometry (ICP‐OES), inductively coupled plasma time of flight mass spectrometry (ICP‐TOFMS), inductively coupled plasma quadrupole mass spectrometry (ICP‐QMS), and glow discharge mass spectrometry (GD‐MS). Solid steel discs were used for analysis with spark‐OES and GD‐MS. For the analyses with ICP‐OES, ICP‐TOFMS, and ICP‐QMS steel chips were wet‐digested in aqua regia, and the wet‐digestion was performed either in polypropylene tubes placed in a heating block or in Teflon pressure vessels using a microwave assisted system. The Mg concentrations obtained for the three steel materials were: 2.0, 2.8, and 10.3 µg g^?1, respectively, and the spread in results was acceptable, giving RSD values in the range of 20–30%.     

Stress‐Temperature‐Transformation and Deformation‐Temperature‐Transformation Diagrams for an Austenitic CrMnNi as‐cast Steel

Stress‐Temperature‐Transformation (STT) and Deformation‐Temperature‐Transformation (DTT) diagrams are well‐suited to characterize the TRIP (transformation‐induced plasticity) and TWIP (twinning‐induced plasticity) effect in steels. The triggering stresses for the deformation‐induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain‐induced twins and martensite formations are shown in the DTT diagram. The microstructure evolution of a novel austenitic Cr‐Mn‐Ni (16%Cr, 6% Mn, 6% Ni) as‐cast steel during deformation was investigated at various temperatures using static tensile tests, optical microscopy and the magnetic scale for the detection of ferromagnetic phase fraction. At the temperatures above 250 °C the steel only deforms by glide deformation of the austenite. Strain‐induced twinning replaces the glide deformation at temperatures below 250 °C with increasing strain. Below 100 °C, the strain‐induced martensite formation becomes more pronounced. The kinetics of the α'‐martensite formation is described according to stress and deformation temperatures. The STT and DTT diagrams, enhanced with the kinetics of the martensite formation, are presented in this paper.     

Metallurgical Phenomena during Processing of Cold Rolled TRIP Steel

Metallurgical phenomena taking place during processing of TRIP Steel are investigated and described with the aim of achieving better understanding of the microstructure development throughout the entire integrated processing routes. Different TRIP steel structure sizes were created by controlling the hot rolling process prior to cold rolling. After that the specimens were intercritically annealed under different conditions to obtain prescribed austenite fractions, and subsequently quenched in salt bath at the bainite transformation temperature. The microstructures had been investigated using light optical microscopy (LOM) and the amount of retained austenite was determined by magnetometry.     

Deformation Behaviour of Iron‐Rich Iron‐Aluminium Alloys with Ternary Transition Metal Additions

The hardness and yield stress at room temperature and the brittle‐to‐ductile transition temperature of Fe‐Al alloys with 16 at.% Al, which is in the range of the so‐called K‐state with possible short‐range ordering reactions, and ternary additions of 0.5 and 4 at.% of the transition metals Cr, Mo, Mn, V, Ti and Ni were studied with respect to possible hardening effects of the ternary additions. The addition of Cr, Mo and Mn to the Fe‐Al alloys produce solid‐solution hardening which corresponds to the hardening effect of Al. Only Ti, V and Ni produce extra hardening effects which cannot be related to solid‐solution hardening. This extra hardening is attributed to possible fine NiAl precipitates in the Fe‐Al‐Ni case and to possible enhanced short‐range ordering and/or fine carbide precipitates in the cases of Fe‐Al‐V and Fe‐Al‐Ti.     

Study on the Formation of Open‐Eye and Slag Entrainment in Gas Stirred Ladle

Laboratory experiments were carried out to study the phenomena related to open‐eye formation in ladle treatment. Ga‐In‐Sn alloy with a melting temperature of 283 K was used to simulate the liquid steel, while MgCI₂‐Glycerol(87%) solution as well as HCl solution were used to simulate the ladle slag. No open‐eye was formed at lower gas flow rates, but, occurred when gas flow reached a critical rate. This critical gas flow rate was found to depend significantly on the height of the top liquid. No noticeable amount of top liquid was observed in any of the samples taken from the metal bulk during gas stirring. To confirm this aspect, samples of slag‐metal interface were taken around the open‐eye in an industrial gas stirred steel ladle. No entrapped slag droplet was found in the solidified steel within the region between the interface and 2 cm from the interface. The accordance of the laboratory and industrial results suggests that the entrainment of slag into the steel bulk around the open‐eye cannot be considered as the major contribution to inclusion formation.     

Isothermal Microstructural Transformations of the Heat‐treatable Steel 42CrMo4 during Heat‐treatment following Hot‐forming

Apart from reducing the processing energy, hardening and tempering of near‐net shape forged components from their forging heat primarily promises shortened conventional process sequences with reduced manufacturing costs. In this case, the time‐temperature‐transformation diagrams (T‐T‐T diagrams) found in the literature can only be used to a limited extent for determining the microstructural transformations during the heat‐treatment. The reasons for this are that firstly, the deformation influences the transformation kinetics and secondly, the forming temperatures at which austenitising takes place are comparatively high. For this reason, isothermal deformation T‐T‐T diagrams for forging temperatures from 1200 °C and deformation levels of 0.3 and 0.7 were determined for the heat‐treatable steel 42CrMo4 (1.7225). These diagrams were subsequently modelled for simulating the heat‐treatment and implemented in the FE‐software ANSYS®.     

Local Equilibrium and Nodule Formation on Al‐TRIP‐Aided Steel Surfaces During Intercritical Annealing

Several advanced high strength steels were intercritically annealed at a dew point of ?5°C. Afterwards, surface morphology was investigated by scanning and transmission electron microscopy (TEM). In Al alloyed TRIP‐aided steels, nodules of metallic Fe containing traces of Mn could be proven by TEM analysis. To clarify the mechanism of origin, several further annealing trails were done and finally a new component of the occurring heterogeneous gas/metal reaction during intercritical annealing was postulated. It is concluded that neither the diffusion mechanism of the alloying elements nor the stress gradient between stress‐free surface region of the internal oxidation front nor the oxygen partial pressure dominates alone the selective oxidation process. It is suggested that the reactivity of the considered surface, its local surface chemistry and the local thermodynamic equilibrium should be taken into account in greater detail. Preferred dissociation points of absorbed water vapor could lead to a local increased oxygen partial pressure. With this, a nanoscaled oxidation/reduction process could be initiated.     

A Study of Microstructure and Performance of Tempered Fe‐V‐W‐Mo Alloy

Influences of tempering temperature, holding time and tempering times on the microstructure and performance of Fe‐5%V‐5%W‐5%Mo‐5%Cr‐3%Nb‐2%Co(Fe‐V‐W‐Mo) were investigated by means of metallography, optical microscopy, hardness measurements, impact tester and pin abrasion tester. The results show that the hardness of Fe‐V‐W‐Mo alloy remains constant when tempered below 350°C. The hardness decreases gradually as the tempering temperature increase until around 475°C and then it increases again to a peak at 525°C. The hardness of Fe‐V‐W‐Mo alloy reaches nearly the highest value after the first tempering and decreases after triple‐tempering. The toughness of Fe‐V‐W‐Mo alloy increases until the tempering temperature reaches 475°C and then decreases until the temperature reaches 525°C. However, it increases again when tempering is beyond that temperature. The excellent wear resistance can be obtained by tempering at 500‐550°C.     

Deformation Induced Martensite Formation and its Effect on Transformation Induced Plasticity (TRIP)

The knowledge of the stress‐ and deformation‐induced martensite formation in metastable austenitic steels including the formation temperatures and amounts formed is of considerable importance for the understanding of the transformation induced plasticity. For this purpose a stress‐temperature‐transformation (STT) and a deformation‐temperature‐transformation (DTT) diagram have been developed for the steel X5CrNi 18 10 (1.4301, AISI 304). It is shown that the M_d‐temperature for γ→?, ?→α', γ→?→α’ and γ→α’ martensite formation is defined by two stress‐temperature curves which show a different temperature dependence. They specify the beginning and the end of the deformation‐induced martensite formation in the range of uniform elongation. The intersection point defines the corresponding M_d‐temperature. The stress difference which results from the stresses for the end and the beginning of the martensite formation shows positive values below the M_d‐temperature. It defines the amount of martensite being formed. When the M_d^γ→? temperature is reached and the formation of the first deformation‐induced amount of ?‐martensite appears, an anomalous temperature dependence of the maximum uniform elongation starts. The highest values of the maximum uniform elongation are registered for the tested steel in the immediate vicinity of the M_d^γ→α' or the M_d^γ→?→α' temperature ‐ similar as in other metastable austenitic CrNi steels. At this temperature the highest amount of deformation‐induced ?‐phase exists. The transformation plasticity in the test steel is considerably caused by the deformation‐induced ? and α’ martensite formation. Using the new evaluation method, the increase of plasticity ΔA (TRIP‐effect) and strength ΔR can be quantified.