Ronaldo Barbosa

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.

Laboratory simulation of seamless-tube rolling

Abstract In the manufacture of high-strength seamless tubing at the Algoma Steel Corporation, a post-hot deformation quench and temper treatment is employed to produce the higher yield-strength grades. In this laboratory study, the aim was to determine the microstructures and associated strength levels that can be achieved in the as-hot-rolled condition using TiVN microalloyed steel chemistries. The benefits are significant savings in handling and energy costs and a substantial increase in productivity. The # seamless-tube mill, which consists of a piercer, a multi-pipe mill (MPM) and a stretch-reducing mill (SRM), was characterized in terms of the key hot-working variables: strain, strain rate, temperature and interpass delays. Torsional simulations of this process were carried out. The dependent variables of interest were the flow resistance during the simulated passes, the final microstructure and the room temperature yield strength. The results indicate that the candidate steels can be readily rolled in the mill without exceeding the force limitations. The as-hot-rolled microstructures were composed of fine grained polygonal ferrite-pearlite. The yield strengths associated with these microstructures varied from 553 to 629 MPa (77 to 91 ksi): these cover much of the strength range of current quench and temper seamless products.

EFFECT OF THE CYCLIC STRAIN AMPLITUDE ON THE HOT DYNAMIC RESTORATION OF COPPER

Federal University of MinasGerais, Department of Metallurgical and Materials Engineering, Rua Espi´rito Santo 35, s206,Centro, 30160-030, Belo Horizonte, Minas Gerais, Brazil(Received May 18, 2000)(Accepted in revised form July 20, 2000)Keywords: Dynamic phenomena; Recrystallization and recovery; Copper; Hot cyclic strainingIntroductionThe microstructures developed during hot and warm working of metals are profoundly affected byvariables such as the prevailing temperatures, strain rates and strains [1,2]. Another variable that hasbeen receiving attention is the strain path [3,4], with special emphasis on cyclic and multiaxial straining.This can be of particular importance in the industrial rolling of flat and long products. For both cases,there is a cyclic superficial shearing associated to the change of direction of the friction shear stressesas the material traverses the rolling gap [5]. For long products, there is also a 45° or 90° change in thecompression direction from one rolling pass to the next one. The situation is particularly relevant to thefinishing stands of wire rod mills, where interpass times are very low [6] and softening between passesis probably not important.Cyclic cold straining of metals leads to lower work hardening rates than monotonic deformation. Thedegree of hardening decreases as the strain amplitude is lowered [7]. It has also been shown [3] that theflow stress of copper in hot cyclic torsion is substantially lower than under monotonic torsion. Theeffect of strain amplitude was the same as for cold working, and the dynamic recrystallization peak waseliminated. These facts are illustrated in Figure 1.The effects of hot cyclic straining depend on the strain amplitude (De) in the cycle. The availabledata for copper at 500°C and a strain rate of 0.1s

Simulation of the Process of Hot Rolling of Seamless Tubes

Seamless tubes are manufactured, as it is well known, via continuous mandrel rolling process. A billet is first pierced, then the hollow is reduced and rolled with mandrel at temperatures higher than 1000oC. After mandrel extraction, the hollow can either cool down to room temperature or be directly charged in an intermediate furnace for re-austenitization. Finally, the tube is finished in a stretch-reducing mill to several gauges. The thermomechanical process is complex allowing little flexibility along the line. The aims of this paper are a) to describe the industry schedule in terms of simple process variables and b) to simulate this process via numerical and physical models. The latter uses torsion testing as experimental technique. It is shown that good agreement is obtained between industry results and predictions from the numerical model. Torsion experiments have produced somewhat larger predictions for ferrite grains sizes, however. This is mostly attributed to the necessary simplifications made to the mechanical testing experiments due to restrictions in maximum strain rates and shortest dwell times achievable with thermomechanical simulators.

Physical Metallurgy Related to the Thermomechanical Process of Hot Rolling of Nb Microalloyed Steel Beams: Prediction of Mechanical Properties

The main physical metallurgy processes controlling final mechanical properties in as hot rolled steel products are basically recrystalization, grain growth, precipitation and austenite to ferrite phase transformation. Knowledge of how these processes interact during an industry hot rolling schedule is the key to understand how to obtain certain mechanical properties. This know how has been routinely applied to the manufacturing of flat products such as plates and strips. Less has been reported for non flat products. When even there are such reports, these usually do not present any comparison between predictions and industry real data. This paper addresses the case of rolling of structural beams at an industry trial. Emphasis is put on how the final mechanical properties were obtained and how they can be predicted. An ordinary microstructure model was used and then the predictions were checked by comparing them to industry data. After validation, suggestions were made for improvements in the schedule aiming at having superior mechanical properties at the end product. Austenite grain size evolution is important in defining final properties, as expected. The presence of Nb as microalloyed element can enhance such properties and its role to this particular industry process was discussed.

Ferrite Recrystallization in Cold-Rolled Dual Phase Steel

The ferrite recrystallization of a cold rolled dual phase steel with 0.09%C-2.06%Mn was investigated by means of isothermal treatments at the temperatures of 600, 650, 680 and 710°C as well as during continuous heating. From the isothermal treatments the softening kinetics was mathematically adjusted to the AJMK model and the activation energy associated with the ferrite recrystallization process was estimated as 310 ± 22 kJ/mol. Vickers hardness measurements in the ferrite were carried out on samples subjected to heating at a constant rate from room temperature and quenched at temperatures up to 760°C. The results allowed to observe three main stages of the ferrite softening: (1) from room temperature to about 500°C the ferrite hardness remains constant; (2) between 550°C and 640°C, the hardness systematically increases with temperature. This effect was interpreted as being caused by the increase of carbon dissolved in the ferrite, coming from the dissolution of the adjacent pearlite colonies; (3) above 660°C the ferrite hardness decreased rapidly with temperature due to the recrystallization process. Analysis by means of SEM and EBSD indicated that the recrystallization process started mostly in regions severely deformed, which were predominantly orientated along the γ fiber ( parallel to the sheet normal direction).

Grain Size Modeling during Hot Rolling of a Nb Microalloyed Steel Beam

Hot rolling of beams is carried out essentially in two stages. Roughing is performed in a reversing mill at temperatures in the range of 1100 oC, at relatively low strain rates and with long interpass times. Finishing is carried out in a reversing two stand mill at temperatures in between 1000 and as low as 700 oC considering parts of the web in the last passes. Strain rates are moderate and interpass times are in the range of 5 to 20s. There is, therefore, as it can be seen from the description just made of the rolling schedule, a fair resemblance to deformation in plate mills. Technology for themomechanical processing, TMP, of plates is very well known and disseminated. Application of this technology to beam rolling is, on the other hand, rather seldom known of. This paper addresses an application of TMP plate technology to beam rolling. In particular, austenite grain size evolution is examined. The usage of Nb microalloyed steels to this process is discussed in terms of possible beneficial effects to ferrite grain refinement.

Investigations on Interaction between Recrystallization and Precipitation at Finishing Steps of Seamless Tube Rolling

Conventional controlled rolling is normally used in a hot strip mill in order to refine the microstructure and, consequently, to meet a good combination of mechanical strength and toughness in low carbon steels. Nb microalloying are used for this purpose once this rolling technique requires the occurrence of strain induced precipitation before that any recrystallization process takes place. In a hot strip mill, the transfer time between roughing and finishing steps is considered fast enough to keep all Nb, previously solubilized in plate furnace, in solid solution just before finishing. As a consequence, the thermodynamic driving force required for strain induced precipitation remains high, accelerating its kinetics and preventing the occurrence of recrystallization. On the other hand, in a seamless tube rolling, the transfer time between roughing and finishing steps is about 20 times longer than in a hot strip mill. As a consequence, Nb can precipitate in the recrystallized austenite after roughing, decreasing the driving force for strain induced precipitation at finishing. Transmission electron microscopy investigations on samples simulating the material just before finishing rolling in a seamless tube mill show that both grain boundary precipitation and co-precipitation on TiN particles takes place during transfer time. Based on these findings, this paper discusses the effect of long times after roughing steps on the driving force for strain induced precipitation at finishing rolling in seamless tube. Thermo-Calc software was used to evaluate the possible amount of Nb that remains in solid solution just before finishing. Due to previous precipitation, the driving force to be considered in the Dutta-Sellars equations for onset of precipitation decreases and a comparison between recrystallization and precipitation curves shows that recrystallization takes place before the start of strain induce precipitation.

Mathematical Modeling of Microstructure Evolution of V Steels during Hot Rolling of Seamless Tubes

Torsion and compression testing have been used to simulate microstructure evolution of industry processes. Additionally, mathematical modeling of the industry hot rolling processes has been carried out by several researchers. These models employed equations published in the literature describing kinetics of softening, grain size evolution and grain growth. Validation of the models was carried, in some cases, by comparing the microstructure or the average stress per pass, the latter as calculated from industry rolling mill loads. In the present work, torsion simulation and industry trial results were used to validate the mathematical model presented. Equations used in the model were mostly taken from literature and appropriate modifications were implemented concerning basically two points: a) the transfer time between CMM and SRM, a step in the production line typical for seamless rolling and rather unusual for other industry rolling processes and b) the chemical composition used in tube rolling industry where C equivalent values are usually higher than those used in the rolling of flats.