On the Transition of Internal to External Selective Oxidation on CMnSi TRIP Steel

The selective oxidation of a CMnSi transformation-induced plasticity (TRIP) steel during intercritical annealing (IA) in a N₂ + 10 pct H₂ gas atmosphere with a dew point (DP) in the range from 213 K to 278 K (?60 °C to +5 °C) was investigated by transmission electron microscopy. The decarburization during IA resulted in a fully ferritic matrix at the TRIP steel surface. Annealing in high DP gas atmospheres resulted in a reduction of the oxide layer thickness at the surface and an increase of the depth of the subsurface internal oxidation. The experimental results were compared to the calculations of the DP for the transition from internal to external oxidation based on the Wagner model. The evolution of the surface oxide composition during annealing was analyzed thermodynamically by means of the chemical potential diagram for the surface oxides. In the high DP atmosphere conditions, mainly, Mn-rich xMnO·SiO₂ (1 < x < 2) oxides were formed at the surface, while Si-rich xMnO·SiO₂ (x < 1) oxides were formed by internal oxidation. The use of a high DP gas atmosphere is therefore advantageous to induce internal selective oxidation and reduce the amount of surface oxides. It also leads to the formation of Mn-rich xMnO·SiO₂ (1 < x < 2) oxides.     

Influence of Gas Atmosphere Dew Point on the Selective Oxidation and the Reactive Wetting During Hot Dip Galvanizing of CMnSi TRIP Steel

The selective oxidation and reactive wetting of intercritically annealed Si-bearing CMnSi transformation-induced plasticity steels were investigated by high-resolution transmission electron microscopy. In a N₂ + 10 pct H₂ gas atmosphere with a dew point (DP) ranging from 213 K to 278 K (?60 °C to 5 °C), a continuous layer of selective oxides was formed on the surface. Annealing in a higher DP gas atmosphere resulted in a thinner layer of external oxidation and a greater depth of internal oxidation. The hot dipping was carried out in a Zn bath containing 0.22 mass pct Al, and the bath temperature was 733 K (460 °C). Coarse and discontinuous Fe₂Al_5?x Zn _x grains and Fe-Zn intermetallics (?? and ??) were observed at the steel/coating interface after the hot dip galvanizing (HDG) of panels were annealed in a low DP atmosphere 213 K (?60 °C). The Fe-Zn intermetallics were formed both in areas where the Fe₂Al_5?x Zn _x inhibition layer had not been formed and on top of non-stoichiometric Fe-Al-Zn crystals. Poor wetting was observed on panels annealed in a low DP atmosphere because of the formation of thick film-type oxides on the surface. After annealing in higher DP gas atmospheres, i.e., 263 K and 278 K (?10 °C and 5 °C), a continuous and fine-grained Fe₂Al_5?x Zn _x layer was formed. No Fe-Zn intermetallics were formed. The small grain size of the inhibition layer was attributed to the nucleation of the Fe₂Al_5?x Zn _x grains on small ferrite sub-surface grains and the presence of granular surface oxides. A high DP atmosphere can therefore significantly contribute to the decrease of Zn-coating defects on CMnSi TRIP steels processed in HDG lines.     

Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel

The Fe-Zn reaction occurring during the galvannealing of a Si-bearing transformation-induced plasticity (TRIP) steel was investigated by field-emission electron probe microanalysis and field-emission transmission electron microscopy. The galvannealing was simulated after hot dipping in a Zn bath containing 0.13 mass pct Al at 733 K (460 °C). The galvannealing temperature was in the range of 813 K to 843 K (540 °C to 570 °C). The kinetics and mechanism of the galvannealing reaction were strongly influenced by the gas atmosphere dew point (DP). After the galvannealing of a panel annealed in a N₂+10 pct H₂ gas atmosphere with low DPs [213 K and 243 K (?60 °C and ?30 °C)], the coating layer consisted of δ (FeZn₁₀) and η (Zn) phase crystals. The Mn-Si compound oxides formed during intercritical annealing were present mostly at the steel/coating interface after the galvannealing. Galvannealing of a panel annealed in higher DP [263 K and 273 K, and 278 K (?10 °C, 0 °C, and +5 °C)] gas atmospheres resulted in a coating layer consisting of δ and Г (Fe₃Zn₁₀) phase crystals, and a thin layer of Г ₁ (Fe₁₁Zn₄₀) phase crystals at the steel/coating interface. The Mn-Si oxides were distributed homogeneously throughout the galvannealed (GA) coating layer. When the surface oxide layer thickness on panels annealed in a high DP gas atmosphere was reduced, the Fe content at the GA coating surface increased. Annealing in a higher DP gas atmosphere improved the coating quality of the GA panels because a thinner layer of oxides was formed. A high DP atmosphere can therefore significantly contribute to the suppression of Zn-alloy coating defects on CMnSi TRIP steel processed in hot dip galvanizing lines.     

The Development of a Processing Window for the Continuous Galvanizing of a Mn–Cr–Si Martensitic Steel

Martensitic or complex phase steels are leading candidates for automotive impact management applications. However, achieving high strengths while obtaining high quality coatings via continuous galvanizing is a challenge due to cooling rate limitations of the processing equipment and selective oxidation of alloying elements such as Cr, Mn, and Si adversely affecting reactive wetting. The galvanizability of a Cr? Mn? Si steel with a target tensile strength above 1250 MPa was investigated within the context of the continuous galvanizing line. The continuous cooling transformation behavior of the candidate alloy was determined, from which intercritical and austenitic annealing thermal cycles were developed. The evolution of substrate surface chemistry and oxide morphology during these treatments and their subsequent effect on reactive wetting during galvanizing were characterized. The target strength of 1250 MPa was achieved and high quality coatings produced using both intercritical (75% γ) and austenitic (100% γ) annealing using a conventional 95%N₂–5%H₂, ?30°C dew point process atmosphere and 0.20 wt% dissolved (effective) Al bath, despite the presence of significant Mn and Cr oxides on the substrate surfaces. It is proposed that complete reactive wetting by the Zn(Al, Fe) bath was promoted by in situ aluminothermic reduction of the Mn and Cr‐oxides by the dissolved bath Al.     

Selective Oxidation and Surface Segregation in High Strength Steels during Short Term Annealing in H2‐N2 ‐ Influence of B on Surface Chemistry

In order to achieve appropriate mechanical properties, new high strength steels aimed for the car industry have to be alloyed with solution strengthening elements. The annealing treatment undergone on cold rolled sheets induces the selective oxidation of alloying elements such as Al, Mn, Si and Cr. The formed oxides exhibit a poor wetting by the Zn bath during hot dip galvanising, thus deteriorating the properties of the zinc coating. While surface‐segregating elements get oxidised, they interact with each other through the formation of spinels and/or mixed oxides during annealing and oxides which have a deleterious effect on wetting can be formed. The formation of (Mn, B) oxide was observed on alloys containing even very small amounts of B and this oxide is almost not wetted at all by Zn. Boron is added to interstitial‐free steels to improve the cold work embrittlement, by replacing phosphorus at the grain boundaries. In this paper, the selective oxidation of steels with and without B, in 5 vol. % H₂‐N₂ atmosphere at 820°C and different dew points was investigated. We found a very strong effect of segregation and oxidation of B on Si and S segregation and oxidation behaviour.     

Influence of Al,Si and P on the kinetics of intercritical annealing of TRIP‐aided steels: thermodynamical prediction and experimental verification

TRIP‐aided steels offer an excellent combination of strength and formability, which makes them particularly interesting for use in automotive applications. Recent investigations have shown that while the typical high CMnSi TRIP‐aided steel composition offers good mechanical properties, alloying with other elements or a modification of the processing are required to make this steel readily galvanizable without loss of the TRIP properties. Al‐alloying seems especially promising to realize this goal and P could also be an alternative. Due to the very specific thermal processing needed to obtain a TRIP microstructure, it is important to know the influence of these alloying elements on the re‐austenitization kinetics during the annealing. This paper aims at identifying the differences in the influence of Si, Al and P on the intercritical annealing of TRIP‐aided steels. The equilibrium thermodynamics calculations and diffusion‐controlled transformation simulations were used in order to predict the transformation behaviour, and experimental verification was done based on dilatometric experiments.     

Galvanizing and Galvannealing Behavior of CMnSiCr Dual-Phase Steels

Alloying elements, such as Mn, Mo, Si, and Cr, are commonly used to enhance the strength of advanced high-strength steels. Those elements also play an important role in the hot-dip galvanizing (GI) and galvannealing (GA) process. In this study, two kinds of CMnSiCr dual-phase steels were galvanized and galvannealed using a hot-dip simulator to investigate the effect of the alloying elements on the microstructure of the GI and GA coatings. The results showed that the dual-phase steels had good galvanizability because no bare spots were observed and the Fe-Zn phases were readily formed at the interface. However, the alloying reaction during the GA process was significantly hindered. XPS analysis showed that external oxidation occurred under an extremely low dew point [213 K to 203 K (?60 °C to ?70 °C)] atmosphere during the annealing prior to hot dipping. However, most of the oxides were reduced during the GI process. After the GI process, the Al was present as solid solutes in the Fe-Zn phase, suggesting that the Fe-Zn phase was formed from the transformation of the Fe-Al inhibition alloy. Meanwhile, the solubility of Si in the ζ phase was extremely low. With continued GA reaction, the ζ phase transformed into the δ phase, which contained approximately 1.0 at.pct Si. The Si also diffused into the Zn layer during the GA reaction. Hence, the ζ phase did not homogeneously nucleate at the steel substrate/Zn coating interface, but was found at the area away from the interface. Therefore, the Fe-Zn phases on the CMnSiCr dual-phase steels were relatively non-uniform compared to those on interstitial-free steel.     

X-ray photoelectron spectroscopy study of passive layers formed on Pb-Sn and Pb-Sb alloys

In this article, passivated Pb-Sn and Pb-Sb alloys with different compositions were studied by X-ray photoelectron spectroscopy (XPS). The passive films were found to have a double-layer structure. Both Sn and Sb were found to play an important role during the growth of passive film, which resulted in different compositions of passive films. The Pb oxides, Pb sulfate, Sn/Sb oxides, and Pb-Sn/Sb complex oxides (PbO _x ·Sn_1−x O₂, PbSb₂O₆) were present in the passive films. Both Sn and Sb facilitated PbO _x (1<x<2) formation, which was considered to be very helpful to optimize the conductivity of passive films. It was also shown that the positions of peak signals of alloying Sn/Sb, Pb, and O were almost fixed, even though the contents of alloying Sn/Sb oxides were quite different in different specimens.     

Analysis of the Strain‐Induced Martensitic Transformation of Retained Austenite in Cold Rolled Micro‐Alloyed TRIP Steel

The stability of retained austenite and the kinetics of the strain‐induced martensitic transformation in micro‐alloyed TRIP‐aided steel were obtained from interrupted tensile tests and saturation magnetization measurements. Tensile tests with single specimens and at variable temperature were carried out to determine the influence of the micro‐alloying on the M_s^σ temperature of the retained austenite. Although model calculations show that the addition of the micro‐alloying elements influences a number of stabilizing factors, the results indicate that the stability of retained austenite in the micro‐alloyed TRIP‐aided steels is not significantly influenced by the micro‐alloying. The kinetics of the strain‐induced martensitic transformation was also not significantly influenced by addition of the micro‐alloying elements. The addition of micro‐alloying elements slows down the autocatalytic propagation of the strain‐induced martensite due to the increase of the yield strength of retained austenite. The lower uniform elongation of micro‐alloyed TRIP‐aided steel is very likely due to the presence of numerous precipitates in the microstructure and the pronounced ferrite grain size refinement.     

Microstructure and Hardness Properties Investigation of Ti and Nb Added FeNiAlCuCrTi x Nb y High Entropy Alloys

Properties of pure metals can be enhanced by alloying with other metallic or non-metallic elements according to the need. However, as multiple alloying elements in an alloy may lead to the formation of many intermetallic compounds with complex microstructures and poor mechanical properties, new types of metallic alloys called high entropy alloys with at least five elements with equimolar ratios were developed. In this study, FeNiAlCuCrTi _x Nb _y (x, y = 0, 0.5, 1.0, 1.5) alloys have been prepared using Ar arc melting technique. Microstructural studies using scanning electron microscope and XRD showed that Ti addition promoted secondary BCC₂ phase whereas, Nb acted as FCC stabilizer. Samples with combined Nb and Ti addition showed FCC₁ and FCC₂ structure with Nb-rich FCC₂ dendritic phase as dominant phase. Though, individual Nb and Ti additions have resulted in increased hardness, combined additions have resulted in highest hardness of 797 HV under 1 kg load.     

Electron Microscopic Study on the Suction Cast In Situ Ti-Fe-Sn Ultrafine Composites

The current investigation reports detailed study on the microstructural evolution in the suction cast hypereutectic Ti₇₁Fe_29?x Sn _x alloys during Sn addition with x = 0, 2, 2.5, 3, 3.85, 4.5, 6, and 10 at. pct and the solidification of these ternary alloys using SEM and TEM. These alloys have been prepared by melting high-purity elements using vacuum arc melting furnace under high-purity argon atmosphere. This was followed by suction casting these alloys in the water-cooled split Cu molds of diameters, ? = 1 and 3 mm, under argon atmosphere. The results indicate the formation of binary eutectic between bcc solid solution ??-Ti and B2 FeTi in all alloys. ??-Ti undergoes eutectoid transformation, ??-Ti ?? ??-Ti + FeTi, during subsequent solid-state cooling, leading to formation of hcp ??-Ti and FeTi. For alloys x < 2, the primary FeTi forms from the liquid before the formation of eutectic with minute scale Ti₃Sn phase. For alloys with 2 ?? x ?? 10, the liquid is found to undergo ternary quasi-peritectic reaction with primary Ti₃Sn, L+Ti₃Sn ?? ??-Ti+FeTi, leading to formation of another kind of FeTi. In all the other alloy compositions (3.85 ?? x ?? 10), Ti₃Sn and FeTi dendrites are observed in the suction cast alloys with profuse amount of Ti₃Sn being formed for alloys with x ?? 4.5. The current study conclusively proves that the liquid undergoes ternary quasi-peritectic reaction involving four phases, L + Ti₃Sn ?? ??-Ti + FeTi, which lies at the invariant point Ti_69.2±0.8Fe_27.4±0.7Sn_3.4±0.2 (denoted by P). Below P, there is one univariant reaction, i.e., L ?? ??-Ti + FeTi for all alloy compositions, whereas above P, liquid undergoes one of the univariant reactions, i.e., L + ??-Ti ?? Ti₃Sn (Sn = 2, 2.5, 3, and 4.5 at. pct) or L + FeTi ?? Ti₃Sn for alloys (Sn = 6, 10 at. pct). For alloy with Sn = 3.85 at. pct, the ternary quasi-peritectic reaction is co-operated by two monovariant eutectic reactions, i.e., L ?? ??-Ti + FeTi below P and L ?? FeTi + Ti₃Sn above P. Detailed microstructural information allows us to construct liquidus projection of the investigated alloys. The results are critically discussed in the light of available literature data.     

Wettability of Mn<Subscript>x</Subscript>Si<Subscript>y</Subscript>O<Subscript>z</Subscript> by Liquid Zn-Al Alloys

The wettability of Mn_xSi_yO_z by liquid Zn-Al alloys was investigated to obtain basic information on the coating properties of high-strength steels with surface oxides in the hot-dip galvanizing process. In this study, the contact angles of liquid Zn-Al alloys (Al concentrations were 0.12 and 0.23 wt pct) on four different Mn_xSi_yO_z oxides, namely MnO, MnSiO₃, Mn₂SiO₄, and SiO₂, were measured with the dispensed drop method. The contact angle did not change across time. With an increasing Al concentration, the contact angle was slightly decreased for MnO and Mn₂SiO₄, but there was no change for MnSiO₃ and SiO₂. With an increasing SiO₂ content, the contact angle gradually increased by 54 wt pct to form MnSiO₃, and for pure SiO₂ substrate, the contact angle decreased again. Consequently, the MnSiO₃ substrate showed the worst wettability among the four tested oxide substrates.     

Galvanizability of Complex Phase High Strength Steel

Complex phase (CP) steels have very high ultimate tensile strengths, resulting from the use of specific alloying elements, which improve the hardenability but cause difficulties when applying a zinc coating by means of continuous hot‐dip galvanizing. The galvanizability of a cold rolled 1000MPa complex phase steel was investigated by monitoring the surface chemistry before dipping and evaluating the quality of the zinc coatings applied by a laboratory hot‐dipping simulator. Two steel compositions with different Cr levels were used. The influence of the most important production parameters, the annealing temperature and the dew point of the annealing atmosphere, was investigated. Both steel compositions were galvanizable, but both the surface appearance and zinc coating adhesion were improved when low Cr contents were used. At a low dew point of ‐30°C, Cr, Mn and Si segregated to the surface and the presence of Mn₂SiO₄ could be demonstrated. At high dew point of +10°C, less oxides were present at the steel surface. There was no effect of the annealing temperature on the coatability.     

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.     

Phase equilibrium studies in the “MnO”-Al2O3-SiO2 system

Phase relations and the liquidus surface in the system “MnO”-Al₂O₃-SiO₂ at manganese-rich alloy saturation have been investigated in the temperature range from 1373 to 1773 K. This system contains the primary-phase fields of tridymite and cristobalite (SiO₂); mullite (3Al₂O₃·2SiO₂); corundum (Al₂O₃); galaxite (MnO·Al₂O₃); manganosite (MnO); tephroite (2MnO·SiO₂); rhodonite (MnO·SiO₂); spessartine (3MnO·Al₂O₃·SiO₂); and the compound MnO·Al₂O₃·2SiO₂.     

Elasticity of Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>W<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> ≤ 0.1875) Alloys from First Principles Calculations

The elastic properties of Ni _x W_1?x alloys up to x = 0.1875 have been determined from first principles calculations. We have used stress–strain relationships to calculate the C _ij elastic coefficients and the Voigt–Reuss–Hill approximations to determine the bulk and shear moduli of polycrystals. The W alloying increases the compression modulus while the shear modulus remains almost constant. Furthermore, the W alloying has a minor effect on the elastic anisotropy and, therefore, on its contribution to the indentation modulus.     

Oxide Transformation in Cr-Mn-Prealloyed Sintered Steels: Thermodynamic and Kinetic Aspects

The main obstacle for utilization of Cr and Mn as alloying elements in powder metallurgy is their high oxygen affinity leading to oxidation risk during powder manufacturing, handling, and especially during further consolidation. Despite the high purity of the commercially available Cr- and Mn-prealloyed iron powder grades, the risk of stable oxide formation during the sintering process remains. Thermodynamic and kinetic simulation of the oxide formation/transformation on the former powder surface during heating and sintering stages using thermodynamic modeling tools (Thermo-Calc and HSC Chemistry) was performed. Simulation is based on the results from the analysis of amount, morphology, and composition of the oxide phases inside the inter-particle necks in the specimens from interrupted sintering trials utilizing advanced analysis tools (HRSEM + EDX and XPS). The effect of the processing parameters, such as sintering atmosphere composition, temperature profile as well as graphite addition on the possible scenarios of oxide reduction/formation/transformation for Fe-Cr-Mn-C powder systems, was evaluated. Results indicate that oxide transformation occurs in accordance with the thermodynamic stability of oxides as follows: Fe₂O₃ → FeO → Fe₂MnO₄ → Cr₂FeO₄ → Cr₂O₃ → MnCr₂O₄ → MnO/MnSiO _x  → SiO₂. Spinel MnCr₂O₄ was identified as the most stable oxide phase at applied sintering conditions up to 1393 K (1120 °C). Controlled conditions during the heating stage minimize the formation of stable oxide products and produce oxide-free sintered parts.     

Interfacial Reaction Between CaO-SiO2-MgO-Al2O3 Flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) Steel at 1873 K (1600 °C)

This study investigated the interfacial reaction kinetics and related phenomena between CaO-SiO₂-MgO-Al₂O₃ flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) steel, which simulates transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels at 1873 K (1600 °C). It also examines the effect of changes in the composition of the steel and slag phases on the interfacial reaction rate and the reaction mechanisms. The content of Al and Si in the 1 mass pct Al-containing steel was found to change rapidly within the first 15 minutes of the reaction, but then it remained relatively constant. The content of Al and Si in the 3 to 6 mass pct Al-containing steels, in contrast, changed continuously throughout the entire reaction time. In addition, the content of Mn in the 1 mass pct Al-containing steels initially decreased with increasing time, but the content did not change in the 3 to 6 mass pct Al-containing steels. Furthermore, the mass transfer coefficient of Al, k _Al, in the 1 mass pct Al-containing systems was significantly higher than that in other systems; i.e., the k _Al can be arranged such that 1 mass pct Al systems >> 3 mass pct Al systems ≥ 6 mass pct Al systems. The compositions of the final slags were close to the saturation lines of the [Mg,Mn]Al₂O₄ and MgAl₂O₄ spinels when the slags reacted with 1 mass pct Al and 3 to 6 mass pct Al-containing steels, respectively. These results, which show the effect of Al content on the reaction phenomena, can be explained by the significant increase in the apparent viscosity of the slags that reacted with the 3 to 6 mass pct Al-containing steels. This reaction was likely caused by the precipitation of solid compounds such as MgAl₂O₄ spinel and CaAl₄O₇ grossite at locally alumina-enriched areas in the slag phase. This analysis is in good accordance with the combination of Higbie’s surface renewal model and the Eyring equation.     

Improvement of the Galvanized Coating Quality of High Strength Dual Phase Steels by Pre‐Electroplating Nickel Layer

Galvanized dual phase steel sheets are used extensively in the industrial applications because of their excellent mechanical properties and superior corrosion resistance, but the segregation of alloying elements and the formation of oxides on the steel surface often have a deleterious effect on coating adhesion during the galvanizing process. In order to improve the coating quality, a nickel layer was pre‐electroplated on the steel substrate before galvanizing and it's found that there is an improvement in the coating quality. The coating microstructures were investigated by scanning electron microscopy together with energy dispersive X‐ray spectroscope, glow discharge optical emission spectroscope and X‐ray diffractions. The experimental results show that the compact Ni₃Zn₂₂ intermetallic layer formed at the zinc/nickel interface during the galvanizing process, prohibiting the nucleation and the growth of the ζ‐Zn phase layer and resulting in the improvement of the zinc coating adhesion.