Microstructural analysis of cracking phenomenon occurring during cold rolling of (0.1~0.7)C-3Mn-5Al lightweight steels

An investigation was conducted into the cracking phenomenon occurring during the cold rolling of lightweight steel plates. Four steels of varying C contents were fabricated and steel plates containing C contents of 0.5wt% or higher were cracked during the initial stage of the cold rolling. The steels were basically composed of ferrite grains and -carbides in a band shape, but the volume fraction and thickness of κ-carbide band increased as the C content increased. Microstructural observation of the deformed region of fractured tensile specimens revealed that deformation bands were homogeneously formed in wide areas of ferrite matrix in the steels containing C contents of 0.3 wt% or lower, while κ-carbide bands were hardly deformed or cracked. In the steels containing high C contents of 0.5 wt% or higher, on the other hand, microcracks were initiated mostly at fine proeutectoid ferrite located within κ-carbide bands, and were grown further to coalesce with other microcracks to form long cracks. To prevent the cracking, thus, the proeutectoid ferrite should be minimized by the hot rolling in the (α+γ) two phase region. As practical methods, the content of C below 0.5% or Al above 5% was suggested to expand the (α+γ) phase region.     

Effect of annealing temperature on microstructural modification and tensile properties in 0.35 C–3.5 Mn–5.8 Al lightweight steel

The microstructural modifications occurring during annealing treatment of an Fe–0.35 C–3.5 Mn–5.8 Al ferrite-based lightweight steel and its effects on the tensile properties were investigated with respect to (α + γ) duplex microstructures. Steels annealed above the dissolution finishing temperature of κ-carbides (795 °C) were basically composed of ferrite band and austenite band in a layered structure. As the annealing temperature was increased the tensile strength increased, while the yield strength and elongation decreased. This could be explained by a decrease in the mechanical as well as thermal stability of austenite with increasing size and austenite volume fraction. In the 980 °C annealed steel in particular, whose mechanical stability due to austenite was lowest, cracks were readily formed at ferrite/austenite (or martensite) interfaces with little deformation, thereby leading to the least tensile elongation. In order to obtain the best combination of strength and ductility the formation of austenite having an appropriate mechanical stability was essentially needed, and could be achieved when 22–24 vol.% fine austenite was homogeneously distributed in the ferrite matrix, as in the 830 °C or 880 °C annealed steels.     

Development of a recovered/recrystallized multilayered microstructure in Al alloys by accumulative roll bonding

A high-purity Al alloy and a supersaturated Al–0.3 wt.% Sc alloy (Al(Sc)) were accumulative roll bonded at 200 °C to generate sheet material consisting of alternating layers of Al and Al(Sc). The deformation structure within these layers consisted of lamellar bands aligned parallel to the rolling direction. Compared with those bands in Al(Sc), the bands in the Al layers were less refined but contained a larger fraction of high angle grain boundaries (HAGBs). Subsequent annealing at 350 °C generated alternating layers of coarse grains (Al layers) and a recovered substructure (Al(Sc) layers); the latter were stabilized by the precipitation of Al₃Sc particles. Within the Al layers, annealing did not significantly alter the rolling texture (β-fibre), although the strong Brass component was largely eliminated; this behaviour has been explained using the “ReNuc” model of recrystallization whereby nucleation is deemed to occur on HAGBs between the β-fibre components of the lamellar bands in conjunction with orientation dependent recovery.     

Low-density low-carbon Fe–Al ferritic steels

Fe–Al solid-solution alloys, with interstitial-free matrix to avoid detrimental κ-carbides, have been investigated in the context of low-density steels for automotive applications. The mechanical properties of the 6.8 wt.% Al-containing alloy were found to be comparable to those of dual-phase steel but with the benefit of reduced density and better formability. Future work on these alloys should concentrate on improving the Young’s modulus and the deep drawability.     

Alloy Design,Combinatorial Synthesis,and Microstructure–Property Relations for Low-Density Fe-Mn-Al-C Austenitic Steels

We present recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and <4–5 wt.% Al or even <8 wt.% Al when naturally aged. The second one is a class of κ-carbide strengthened austenitic steels with even higher Al content. Here, κ-carbides form either at 500–600°C or even during quenching for >10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels, and their susceptibility to hydrogen embrittlement.     

Experimental evidence and thermodynamics analysis of high magnetic field effects on the austenite to ferrite transformation temperature in Fe–C–Mn alloys

The non-isothermal decomposition of austenite into ferrite and pearlite in Fe–xC–1.5 wt.% Mn steels with x = 0.1, 0.2 and 0.3 wt.% C is investigated by in situ dilatometry and microstructure characterization in magnetic fields up to 16 T. The global shift towards higher temperatures of the respective austenite, ferrite + austenite and ferrite + pearlite stability regions is experimentally quantified. A systematic increase in the ferrite area fraction and proportional reduction of the Vickers hardness values with the magnetic field intensity are also reported. Moreover, the steels’ magnetizations, measured up to 3.5 T and 1100 K, are used to calculate the magnetic contribution to the free energy of the transformation and to account thermodynamically for the field dependence of the transformation temperature. The impact of magnetic field is found to be greater with increasing carbon content in the steels.     

Pre-yielding and pre-yield cracking in TiAl-based alloys

Samples of Ti44Al8Nb1B (at.%) and Ti46Al8Nb (700 wt ppm oxygen) and of the alloy K5 (Ti-46.5Al-2Cr-3Nb-0.2W-0.15B-0.4C (800 wt ppm oxygen) have been heat treated to produce duplex, near fully lamellar and fully lamellar microstructures. These samples have been tested in tension, using detectors to detect acoustic events during pre-yielding. Many acoustic emission events and corresponding cracking, occur well below the 0.2% proof stress in the near fully lamellar samples of Ti44Al8Nb1B, in fully lamellar Ti46Al8Nb and fully lamellar K5. In fully lamellar Ti44Al8Nb1B alloy some samples show no acoustic events before yield and others show only single acoustic emission events. Duplex samples generate no signals until the applied stress exceeds the macroscopic yield point. The stress–strain curves from Ti448Nb1B, Ti46Al8Nb and from K5 show that in near fully lamellar and fully lamellar samples significant pre-yielding occurs, but none is obvious in the duplex samples. These observations are discussed in terms of the factors contributing to pre-yield plasticity and to pre-yield cracking in these alloys and with reference to earlier acoustic emission work where the reported behaviour is very different. It is concluded that yield, within sufficiently long lamellae or within gamma grains, can give rise to pre-yield cracking and thus to acoustic emission in lamellar samples.     

Microstructural study of pre-yielding and pre-yield cracking in TiAl-based alloys

Samples of Ti44Al8Nb1B (at.%), Ti46Al8Nb (700 ppm oxygen) and of the alloy K5 (Ti–46Al–2Cr–3Nb–0.2W–0.15B–0.4C (800 ppm oxygen) which have been tested in tension at stresses below their macroscopic yield stresses, have been examined using transmission electron microscopy. In lamellar samples it has been shown that dislocation multiplication takes place at stresses from about 400 MPa, well below their 0.2% proof stress. In samples with a duplex microstructure no dislocation activity is observed until the applied stress exceeds the macroscopic yield stress. Deformation twinning is initially observed in lamellar samples at stresses just below the 0.2% proof stress. No acoustic emission events are observed corresponding to the twinning seen in the fully lamellar samples. These observations are discussed in terms of the different pre-yielding behaviour of lamellar and duplex samples and in terms of acoustic emission signals, pre-yield cracking and pre-yield twinning. It is concluded that pre-yield cracking is caused by slip in gamma grains in near fully lamellar samples and by slip in lamellae longer than about 70 μm in fully lamellar samples. It is further concluded that all pre-yield acoustic emission is caused by cracking.     

Delayed static failure of twinning-induced plasticity steels

Delayed static failure of high-Mn twinning-induced plasticity (TWIP) steels containing various Al contents (0–3.5 wt.%) was studied in the context of hydrogen embrittlement. The roles of residual stress and texture on hydrogen embrittlement were discussed. In the deformed state, Al-added TWIP steels exhibited much better delayed fracture resistance as compared to 0 Al steel, owing to the decrease in the number of diffusible hydrogen-trapping sites coupled with smaller residual stress and strong 〈1 1 1〉 and 〈1 0 0〉 textures.     

Rapid alloy prototyping: Compositional and thermo-mechanical high throughput bulk combinatorial design of structural materials based on the example of 30Mn–1.2C–xAl triplex steels

We introduce a new experimental approach to the compositional and thermo-mechanical design and rapid maturation of bulk structural materials. This method, termed rapid alloy prototyping (RAP), is based on semi-continuous high throughput bulk casting, rolling, heat treatment and sample preparation techniques. 45 Material conditions, i.e. 5 alloys with systematically varied compositions, each modified by 9 different ageing treatments, were produced and investigated within 35 h. This accelerated screening of the tensile, hardness and microstructural properties as a function of chemical and thermo-mechanical parameters allows the highly efficient and knowledge-based design of bulk structural alloys. The efficiency of the approach was demonstrated on a group of Fe–30Mn–1.2C–xAl steels which exhibit a wide spectrum of structural and mechanical characteristics, depending on the respective Al concentration. High amounts of Al addition (>8 wt.%) resulted in pronounced strengthening, while low concentrations (<2 wt.%) led to embrittlement of the material during ageing.     

A novel high-strength,high-ductility and high-corrosion-resistance FeAlMnC low-density alloy

The as-quenched Fe–8.68 wt.% Al–30.5 wt.% Mn–1.85 wt.% C alloy is plasma-nitrided at 500 °C for 8 h. The nitrided layer obtained is 40 μm thick and composed predominantly of AlN, with a small amount of Fe₄N. The resultant surface hardness (1860 Hv), substrate hardness (550 Hv), ductility (33.6%) and corrosion resistance in 3.5% NaCl solution in the present nitrided alloy are far superior to those obtained previously in optimally nitrided high-strength alloy steels, as well as martensitic and precipitation-hardening stainless steels.     

Microstructure and stress rupture property of titanium-containing 11Cr ferritic/martensitic steels

The microstructure and stress rupture behavior of 11Cr ferritic/martensitic steels with 0.02 wt.%Ti (low Ti) and 0.14 wt.%Ti (high Ti) have been studied. The steels are prepared by vacuum induction melting followed by hot forging and rolling into plates. The results show that titanium is easy to combine with oxygen and other elements to form complex inclusions. Large MX particles with 1–3 μm are found in the high titanium steel. Most of the large MX particles have a TiO₂ cored structure. After normalizing at 1100 °C for 1 h, cooled in air and tempering at 750 °C for 1 h, nano-sized MX precipitates distribute densely near martenstic lath boundaries in the high titanium steel. The large MX particles cannot be dissolved even at austenitizing temperature up to 1300 °C. Creep cracks nucleate at the interface between matrixes and the large MX particles or titanium-containing oxide inclusions.     

Effect of C,Ti, Zr and B alloying on fracture mechanisms in hot-rolled Fe–40 (at.%)Al

This paper reports a study of fracture behavior of FeAl-based intermetallic alloys with the addition of carbon, titanium, zirconium and boron (Fe–40Al–1C, Fe–40Al–1Ti and Fe–40Al–Zr–B). The alloys were prepared by modified processing technology of vacuum induction melting and hot rolling in special stainless steel sheath. Tensile and fracture toughness tests were carried out at 20 °C, 400 °C, 600 °C, 700 °C and 800 °C. The alloy showed best fracture toughness and tensile properties with Zr and B addition. The fracture toughness at 600 °C was comparable with values in stainless steels and nickel-based superalloys. The fractographic analysis revealed the change of fracture micromechanisms with temperature. Moreover, under specific conditions, the fracture micromechanisms were different in tensile and fracture toughness specimens.     

The effects of rolling and sensitization treatments on the stress corrosion cracking of 304L stainless steel in salt-spray environment

U-bent and notched tensile tests in a 80 °C salt-spray environment were conducted to evaluate the effect of cold rolling at room temperature (CR), warm rolling at 150 °C (WR), and a sensitization at 650 °C/10 h (CRS and WRS) on the hydrogen embrittlement (HE) susceptibility of the 304L stainless steel. The CR specimen exhibited the highest crack growth rate with a greater number of short cracks found in the CRS specimen in U-bent tests. The CR specimen was resistant to HE in notched tensile tests relative to other specimens. Cracking in these specimens was more likely to initiate at the slip bands.     

Effect of deformation and annealing on the formation and reversion of ε-martensite in an Fe–Mn–C alloy

Microstructure and texture evolution during cold rolling and subsequent annealing were studied in an Fe–22 wt.% Mn–0.376 wt.% C alloy. During rolling the deformation mechanisms were found to be dislocation slip, mechanical twinning, deformation-induced ε-martensite transformation and shear banding. At higher strains, the brass-type texture with a spread towards the Goss-type texture dominated. A decrease in the Cu- and S- components was attributed to the preferential transformation to ε-martensite in Cu- and S-oriented grains. The texture of ε-martensite was sharp and could be described as {1 1 2 9}〈3 3 6 2〉. The orientation relationship {1 1 1}^γ//{0 0 0 1}^ε and 〈110〉^γ//〈1 1 –2 0〉^ε between ε-martensite and austenite was observed but only certain variants were selected. On subsequent annealing, the ε-martensite transformed reversely to austenite by a diffusionless mechanism. Changes in length along rolling, normal and transverse directions on heating were anisotropic due to a combination of volume expansion and shape memory effects. The S-texture component increased significantly due to transformation from the ε-martensite.     

Linking recovery and recrystallization through triple junction motion in aluminum cold rolled to a large strain

Recovery mechanisms and kinetics have been studied in commercial purity aluminum (AA1050) cold rolled to a true strain of 5.5 (99.6% thickness reduction) and annealed at low temperatures from 140 to 220 °C. Transmission electron microscopy, electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) are used to characterize the microstructural evolution during annealing. The microstructural characterization shows that a deformed lamellar structure coarsens uniformly during annealing by triple junction motion while maintaining the lamellar morphology, leading to a gradual transition into a more equiaxed structure, where recrystallization nuclei start to evolve. The apparent activation energy for the microstructural coarsening is estimated separately for different stages characterized by an increase in the lamellar boundary spacing measured by EBSD and ECC. The apparent activation energy increases during annealing, from 110 kJ mol^?1 at the beginning to 230–240 kJ mol^?1 at the end of uniform coarsening, linking the recovery stages to recrystallization. The increase in activation energy underpins operation of different diffusion mechanisms for migration of boundaries and their junctions during coarsening, and solute drag may become increasingly important as the structure coarsens. These findings form the basis for a discussion of the thermal behavior of a fine lamellar structure produced by cold rolling to a large strain of both scientific and applied interest.     

Texture and mechanical properties evolution of a deep drawing medium carbon steel during cold rolling and subsequent recrystallization

Medium carbon steels are mostly used for simple applications; however, new applications have been developed for which good sheet metal formability is required. These types of steels have an inherent low formability. A medium-carbon hot-rolled SAE 1050 steel was selected for this study. It has been cold rolled with thickness reductions varying between 7 and 80%. The samples obtained were used to evaluate the strain hardening curve. For samples with a 50 and 80% thickness reduction, an annealing heat treatment was performed to achieve recrystallization. The material was characterized in the “as-received”, cold rolled and annealed conditions using several methods: optical metallography, X-ray diffraction (texture), Vickers hardness, and tensile testing. For large thickness reductions, the SAE 1050 steel presented low elongation, less than 2%, and yield strength (YS) and tensile strength (TS) around 1400 MPa. Texture in the “as-received” condition showed strong components on the {0 0 1} plane, in the 〈1 0 0〉, 〈2 1 0〉 and $〈1\overline{1}0〉$ directions. After cold rolling, the texture did not present any significant changes for small thickness reductions, however. It changed completely for large ones, where gamma, 〈1 1 1〉//ND, alpha, 〈1 1 0〉//RD, and gamma prime, 〈2 2 3〉//ND, fibres were strengthened. After annealing, the microstructure of the SAE 1050 steel was characterized by recrystallized ferrite and globular cementite. There was little change in the alpha fibre for the 50% reduction, whereas for the 80% reduction, its intensity increased. Both gamma and gamma prime fibres vanished upon annealing for 50 and 80% reductions alike.     

The effect of pH on the selective dissolution of Zn and Al from Zn–Al coatings on steel

Selective dissolution of Al and Zn from 5 wt.% Al–Zn (Galfan) and 55 wt.% Al–Zn (Galvalume) galvanized steel coatings was investigated by comparing Zn and Al dissolution rates in 30 mM NaCl at pH from 2 to 12 using in situ kinetic analysis (atomic emission spectroelectrochemistry, AESEC) and in a 5 day immersion test. The selective dissolution of Zn occurred at low pH and selective dissolution of Al at high pH. Results from AESEC and from the immersion test were compared and interpreted in terms of the inhibiting and passivating effect of corrosion product films.     

The dynamic transformation of deformed austenite at temperatures above the Ae3

Recent observations regarding the dynamic transformation of deformed austenite at temperatures above the Ae₃ are reviewed. Experimental results obtained on four different steels over the temperature range from 743 to 917 °C and at strains up to ε = 5 are described. It is shown that there is a critical strain for the formation of superequilibrium ferrite and that the volume fraction of transformed ferrite increases with the strain. The structures observed are Widmanstätten in form and appear to have nucleated displacively. The effect of deformation on the Gibbs energy of austenite is estimated by assuming that the austenite continues to work-harden after initiation of the transformation and that its flow stress and dislocation density can be derived from the experimental flow curve by making suitable assumptions about two-phase flow. By further taking into account the inhomogeneity of the dislocation density, Gibbs energy contributions (driving forces) are derived that are sufficient to promote transformation as much as 100 °C above the Ae₃. The C diffusion times required for the dynamic formation of the cementite particles observed are estimated. These range from ～25 to 100 μs and are therefore consistent with the times available during rolling. The Gibbs energy calculations suggest that growth of the Widmanstätten ferrite is followed by C diffusion at the lower carbon contents, while it is accompanied by C diffusion at the higher carbon levels.     

Grain refinement in γ-TiAl-based alloys by solid state phase transformations

The colony size of a fully lamellar Ti–46Al–2Cr–2Mo–0.25Si–0.3B ingot was refined from 120 to 30–65 μm by well defined heat-treatments which exploit the suppression of the α → α + γ transformation or involve cyclic annealing around the α-transus temperature. In addition, an extremely fine lamellar spacing in the range of 30–40 nm was obtained. For coarse-grained fully lamellar Ti–46Al–9Nb a massive phase transformation was used for microstructural refinement. The thermal stability of the massively transformed material was tested by annealing treatments and characterized by hardness measurements and the variation of the c/a-ratio of the tetragonal γ-TiAl cell as obtained from X-ray diffraction. After annealing at 1200 °C α₂-Ti₃Al lamellae appear within the former massively transformed γ-TiAl grains parallel to all four (111)_γ-planes causing an increase in hardness.