Effect of Starting As-cast Structure on the Microstructure–Texture Evolution During Subsequent Processing and Finally Ridging Behavior of Ferritic Stainless Steel

Metallurgical and Materials Transactions A - Effect of the initial as-cast structure on the microstructure–texture evolution during thermomechanical processing of 409L grade ferritic...     

Effect of Rolling and Subsequent Annealing on Microstructure,Microtexture, and Properties of an Experimental Duplex Stainless Steel

A lean duplex stainless steel (LDSS) has been prepared with low-N content and processed by different thermo-mechanical schedules, similar to the industrial processing that comprised hot-rolling, cold-rolling, and annealing treatments. The microstructure developed in the present study on low-N LDSS has been compared to that of high-N LDSS as reported in the literature. As N is an austenite stabilizer, lower-N content reduced the stability of austenite and the austenite content in low-N LDSS with respect to the conventional LDSS. Due to low stability of austenite in low-N LDSS, cold rolling resulted in strain-induced martensitic transformation and the reversion of martensite to austenite during subsequent annealing contributed to significant grain refinement within the austenite regions. δ-ferrite grains in low-N LDSS, on the other hand, are refined by extended recovery mechanism. Initial solidification texture (mainly cube texture) within the δ-ferrite region finally converted into gamma-fiber texture after cold rolling and annealing. Although MS-brass component dominated the austenite texture in low-N LDSS after hot rolling and cold rolling, that even transformed into alpha-fiber texture after the final annealing. Due to the significant grain refinement and formation of beneficial texture within both austenite and ferrite, good combination of strength and ductility has been achieved in cold-rolled and annealed sample of low-N LDSS steel.     

Tensile Behavior of Ferrite-Carbide and Ferrite-Martensite Steels with Different Ferrite Grain Structures

Ferrite-carbide and ferrite-martensite dual-phase microstructures have been produced in a low-carbon steel with different ferrite grain structures such as, uniform distribution of coarse- and very fine-ferrite grains, and bimodal distribution of ferrite grain sizes comprising of coarse grains (~12 μm) and very fine grains (<2 μm). Very fine-grained dual-phase structure offered the best combination of tensile-strength and ductility among all the samples. The above microstructures have been compared in terms of their strain-hardening rate and the mechanism of plastic deformation.     

Microstructural engineering and strength-impact toughness prediction in ultra-low carbon bainitic steel

The strength–toughness combination of an ultra-low carbon, low alloyed ‘air-cooled’ steel has been enhanced by developing and refining the granular bainite (GB) structure. Suitable alloy design and thermo-mechanical treatment involving forging, austenitisation (950–1150°C) and air-cooling resulted in the formation of GB matrix along with small islands of martensite–austenite constituents and allotriomorphic ferrite grains. Microstructural characterisation and phase transformation behaviour were studied by scanning electron microscopy and electron backscattered diffraction analysis. The effect of microstructural parameters on the strength-impact toughness combination was investigated. The steel showed an exceptionally low yield ratio, which is beneficial for seismic resistance application.     

Orientation Dependence of Deformation-Induced Martensite Transformation During Uniaxial Tensile Deformation of Carbide-Free Bainitic Steel

Metallurgical and Materials Transactions A - The present study investigates the orientation dependence of deformation-induced martensite (DIM) transformation in carbide-free bainitic steel using...     

Comparison Between Different Processing Schedules for the Development of Ultrafine-Grained Dual-Phase Steel

A comparative study was carried out on the development of ultrafine-grained dual-phase (DP) (ferrite–martensite) structures in a low-carbon microalloyed steel processed using two thermomechanical processing routes, (i) intercritical deformation and (ii) warm-deformation and intercritical annealing. The samples were deformed using Gleeble3500^® simulator, maintaining a constant total strain (ε = 1) and strain rate ( $ \dot \varepsilon $  = 1/s). Evolution of microstructure and micro-texture was investigated by SEM, TEM, and EBSD. Ultrafine-grained DP structures could be formed by careful selection of deformation temperature, T _def (for intercritical deformation) or annealing temperature, T _anneal (for warm-deformation and annealing). Overall, the ferrite grain sizes ranged from 1.5 to 4.0 μm, and the sizes and fractions of the uniformly distributed fine-martensitic islands ranged from 1.5 to 3.0 μm and 15 to 45 pct, respectively. Dynamic strain-induced austenite-to-ferrite transformation followed by continuous (dynamic) recrystallization of the ferrite dictated the grain refinement during intercritical deformation, while, continuous (static) recrystallization by pronounced recovery dictated the grain refinement during the warm-deformation and the annealing. Regarding intercritical deformation, the samples cooled to T _def indicated finer grain size compared with the samples heated to T _def, which are explained in terms of the effects of strain partitioning on the ferrite and the heating during deformation. Alpha-fiber components dominated the texture in all the samples, and the fraction of high-angle boundaries (with >15 deg misorientation) increased with the increasing T _def or T _anneal, depending on the processing schedule. Fine carbide particles, microalloyed precipitates and austenitic islands played important roles in defining the mechanism of grain refinement that involved retarding conventional ferrite recrystallization and ferrite grain growth. With regard to the intercritical deformation, warm-deformation followed by annealing is a simpler process to control in the rolling mill; however, the need for high-power rolling mill and controlled annealing facility imposes industrial challenges.     

Effect of Alloying and Coiling Temperature on the Microstructure and Bending Performance of Ultra-High-Strength Strip Steel

Metallurgical and Materials Transactions A - Two different high-strength B-containing microalloyed steel strips produced in industrial processing conditions, one treated with Ti and the other...     

Prediction of Inhomogeneous Distribution of Microalloy Precipitates in Continuous-Cast High-Strength,Low-Alloy Steel Slab

Spatial distribution in size and frequency of microalloy precipitates have been characterized in two continuous-cast high-strength, low-alloy steel slabs, one containing Nb, Ti, and V and the other containing only Ti. Microsegregation during casting resulted in an inhomogeneous distribution of Nb and Ti precipitates in as-cast slabs. A model has been proposed in this study based on the detailed characterization of cast microalloy precipitates for predicting the spatial distribution in size and volume fraction of precipitates. The present model considers different models, which have been proposed earlier. Microsegregation during solidification has been predicted from the model proposed by Clyne and Kurz. Homogenization of alloying elements during cooling of the cast slab has been predicted following the approach suggested by Kurz and Fisher. Thermo-Calc software predicted the thermodynamic stability and volume fraction of microalloy precipitates at interdendritic and dendritic regions. Finally, classical nucleation and growth theory of precipitation have been used to predict the size distribution of microalloy precipitates at the aforementioned regions. The accurate prediction and control over the precipitate size and fractions may help in avoiding the hot-cracking problem during casting and selecting the processing parameters for reheating and rolling of the slabs.