Dagoberto Brandão Santos

Mechanical properities of an HSLA bainitic steel subjected to controlled rolling with accelerated cooling

Controlled rolling followed by accelerated cooling was utilised in laboratory simulations to study the microstructure and mechanical properties of an HSLA low carbon bainitic steel. The effects of processing parameters, such as cooling start temperature and cooling rates, on the final microstructure and mechanical properties were studied. Optical microscopy and transmission electron microscopy were used to evaluate the complex microstructures consisting of polygonal ferrite, pearlite, bainite and martensite/retained austenite constituent. The use of the multiple regression analysis allowed establishment of the relationships between mechanical properties and accelerated cooling variables: cooling rates and cooling start temperatures.

Microstructural Characterization of Bainitic Steel Submitted to Torsion Testing and Interrupted Accelerated Cooling

HSLA low-carbon bainitic steel containing B was submitted to torsion tests to simulate controlled rolling, followed by interrupted accelerated cooling. Microstructural characteristics and the mechanisms for the refinement of structure were evaluated using light microscopy, scanning electron microscopy, transmission electron microscopy, and Vickers hardness testing. The final microstructure was found to contain complex mixture of granular bainite, small islands of MA constituent, bainitic ferrite, and polygonal ferrite. Increasing the cooling rate or decreasing the finish cooling temperature resulted in a decrease in the volume fraction and average size of the MA islands and the polygonal ferrite. A finish cooling temperature of 400°C produced a microstructure consisting of fine laths of bainitic ferrite with an interlath MA constituent. A quantitative relationship between the accelerated cooling variables and the ferrite grain size was developed.

Simulation of the controlled rolling and accelerated cooling of a bainitic steel using torsion testing.

Controlled rolling, followed by accelerated cooling, was simulated by means of torsion tests. High-strength low-alloy (HSLA) lowcarbon (0.08%) bainitic steel containing B, recently developed by the industry as a bainitic steel grade of the API X80 class, was examined. The influence of cooling rate and finish-cooling temperature on the microstructure and mechanical properties were studied. The final microstructure was predominantly bainitic. For a finish-cooling temperature of 4008C the microstructure consists of fine laths of bainitic ferrite with interlath MA constituent, and increase in the cooling rate leads to a continuous increase of the tensile and yield strengths of 158 and 183 MPa, respectively. The analysis of the results enabled the establishment of quantitative relationships between the accelerated cooling variables and the mechanical properties of steel. # 2000 Elsevier Science S.A. All rights reserved.

DETERMINATION OF CCT DIAGRAMS BY THERMAL ANALYSIS OF AN HSLA BAINITIC STEEL SUBMITTED TO THERMOMECHANICAL TREATMENT

CCT diagrams are broadly used to predict the microstructure and mechanical properties after thermal treatments. Most of these curves are determined by dilatometry with or without deformation prior to cooling. Dilatometry, however, even when straining is present, applies little deformation to the sample as compared to that imposed during an industrial process such as hot rolling. An alternative and attractive technique, however, is torsion testing, and this has ben successfully used to simulate industrial rolling schedules. The use of torsion testing and thermal analysis may yield cooling curves from a deformed austenite, which may be more suitable for the rolling conditions. Therefore, it seems preferable to apply the last technique to obtain information about the behavior of the austenite transformation after the samples had been deformed by torsion according to a rolling schedule. Among the industrial rolling processes available for simulation, controlled rolling of microalloyed steels remains great technological interest because it results in a fine grain microstructure, providing a high strength and good material toughness. HSLA low carbon steels with bainitic or polyphase microstructure have been the subject of countless scientific works in the last decades.

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.

A dual analysis for recycled particulate composites: linking micro- and macro-mechanics

The large amount of disposable bottles produced nowadays makes imperative the search for alternative procedures for recycling them since they are not biodegradable. This paper takes into consideration the thermomechanical recycling of post-consumed plastic bottles, especially the ones made of polyethylene terephthalate (PET) and high-density polyethylene (HDPE), and their use as composite materials for engineering applications. As changes on the composites microstructure can have an influence on macroscopic behavior, a new type of analysis is needed. To be able to evaluate the composite performance, a dual analysis procedure was developed. It consists of a micro-mechanical analysis where the microstructure is observed by optical microscopy, and variations in morphology are related to composite overall mechanical behavior. The macro-mechanical analysis is performed by ASTM D 3039/3039 M tensile tests. By doing this, the composite effective moduli can be determined. The new composite seems to be encouraging, i.e., an HDPE/PET composite with 40:60 ratio, in weight, experiments a stiffness recovery from the third to the fourth recycle. Moreover, the dual analysis was able to capture this variation.

Static Recrystallization Kinetics and Crystallographic Texture of Nb-Stabilized Ferritic Stainless Steel Based on Orientation Imaging Microscopy

In the present study, Nb-stabilized ferritic stainless steel was prepared with annealing (430-A) and without annealing (430-NA) annealing, and the microstructure of the resulting samples was examined. The steel was then subjected to cold rolling and isothermal annealing in order to analyze its recrystallization kinetics and texture evolution. Microstructural characterization was performed by scanning and transmission electron microscopies. Recrystallization kinetics were evaluated by measuring the microhardness of the samples, and analyzing their kernel average misorientation and grain orientation spread via electron backscatter diffraction. The Avrami exponent data revealed that one-dimensional grain growth occurred owing to the migration of high-angle grain boundaries. The mean activation energies for recrystallization for 430-NA and 430-A was found to be 365 and 419 kJ mol−1, respectively. The recrystallization texture was influenced by oriented nucleation and selected growth mechanisms, as well as by the Nb carbonitride distribution and grain boundary energy. The recrystallized and growing grains with the {554}〈225〉 orientation showed a dimensional advantage over the other recrystallized components. The coincident site lattice boundaries were attributed to the progression of recrystallization since the CSL numeric fraction increased as the temperature increased. The {554}〈225〉 component was associated with the ∑19a boundary, which exerted a significant control on the selective growth during the recrystallization.

Martensite Formation and Recrystallization Behavior in 17Mn0.06C2Si3Al1Ni TRIP/TWIP Steel after Hot and Cold Rolling

This work evaluates the evolution of the microstructure and its influence on the mechanical behavior of steel containing 17% Mn, 0.06% C, 2% Si, 3% Al, and 1% Ni after hot rolling at 1070°C, cold rolling with 44% reduction, and annealing at 700°C for different time periods. The resultant athermal, strain-induced martensite and austenite grains were analyzed by optical and scanning electron microscopy (SEM). The volume fractions of the g, e, and α’ phases of martensite were confirmed by X-ray diffraction, dilatometry, and SEM-electron backscatter diffraction (EBSD) techniques. It was found that cold reduction results in the formation of more a’ martensite. The Vickers microhardness values were higher for the cold-rolled condition and lower for recrystallized samples, as expected. However, this reduction is counterbalanced by the formation of athermal e and a’ martensite during the cooling process. The sizes of the recrystallized grains change exponentially during their growth and remain within 1–3 mm. The yield and tensile strength of the hot-rolled steel reach values close to 250 and 800 MPa, respectively, with a total elongation of 40%, which demonstrates the high work-hardening rate of the steel.

High cycle rotating bending fatigue property in high strength casting steel with carbide free bainite

Abstract The present work shows a steel structure with bainitic ferrite dispersed on a matrix of carbon enriched retained austenite. The steel was produced using an air melting technique, and it was austempered at 200°C for 240 h. The steel presents tensile strength of ∼2 GPa. The authors report the new results of resistance to high cycle rotating fatigue in high strength bending life limit 107 cycles. A fatigue strength of 593 MPa was obtained, a result that is higher than that presented by important engineering materials such as forged steel and austempered ductile iron, even with the presence of fracture type ‘fish eye’, which nucleates mainly on shrinkage defects.

Comparison of Microstructure and Mechanical Behavior of the Ferritic Stainless Steels ASTM 430 Stabilized with Niobium and ASTM 439 Stabilized with Niobium and Titanium

The use of Ferritic Stainless Steels has become indispensable due its lower cost and the possibility to replace austenitic stainless steels in many applications. In this study, cold rolled sheets of two stabilized ferritic stainless steels with 85% thickness reduction were annealed by applying a heating rate of 24 oC/s and a soaking time of 24 s. The niobium stabilized ferritic stainless steel type ASTM 430 (430Nb) was annealed at 880 oC while the niobium and titanium bi-stabilized steel ASTM 439 was annealed at 925 oC. The annealed samples were tensile tested and due to the smaller grain size, steel 430Nb, showed a higher yield stress and a higher total elongation. Concerning drawability the steel ASTM 439 presented a better performance with higher average R-value, lower planar anisotropy coefficient and a greater value for Limit Drawing Ratio (LDR). These results are explained in terms of the differences in size and volume fraction of precipitates between the two steels.