Materials Science Forum Vols. 638-642

Abstract: A novel carbide free bainite/martensite (CFB/M) high strength steel of 1500MPa grade is micro-alloyed with Nb in order to refine the microstructure and improve the toughness. After the steel containing Nb was forged, coarse microstructure and strong transmissibility of structure were found. It is suggested that the large numbers of precipitates of Nb(CN) in the steel makes the structure transmissibility more serious. After solution treatment at 1200°C, both complete annealing twice and isothermal treatment at 600°C twice can eliminate the structure transmissibility and refine the structure. Investigation on refinement of cycle phase transformation shows that excellent effect of the grain refinement is obtained when the heating velocity exceeds some threshold. In addition, the temperature of austenitizing is of little influences on the size of refined austenite grain.

Abstract: During the life cycle of an API steel there are occasions when it will be submitted to thermal cycle. For tubes produced by the UOE process, for the oil and gas industry, severe thermal cycles occur during the welding procedure and when the tube requires hot induction bending. It is therefore of interest to learn about the influence of different cooling rates on the microstructure of these steels, produced by TMCP (Thermomechanical Controlled Process), since these changes can be reflected in a variation in the mechanical properties and hence in the performance of the pipeline. In this work three steels of class API 5L X80, with additions of, Nb-Cr, Nb-Cr-Mo and Nb-Cr-Mo-V, and otherwise similar alloy content, produced by TMCP without accelerated cooling were investigated. These alloys were submitted to austenitization at 900oC for 1 hour followed by three different cooling rates of 1.5oC/s, 30oC/s, 115oC/s, in air, oil and water respectively. It is well known that the main strengthening mechanism in TMCP, which allows a simultaneous increase in strength and toughness, is the reduction of the grain size. Other mechanisms such as solid solution hardening, precipitation and increasing dislocation density also contribute. For the steels in this study, all having a similar base chemical composition and yield strength, all the different thermal cycles applied promoted a reduction in the grain size, and only for the higher cooling rate (115oC/s) were there significant observable phase transformations. The Nb-Cr-Mo-V system exhibited a higher percentage of martensite than the Nb-Cr and Nb-Cr-Mo systems.

Abstract: The steel production from scrap using continuous cast technology has increased in last decades. Sometimes, steels processed via this route display poor ductility at high temperature. This feature is associated to cooling conditions and chemical composition, which in turns affect the segregation pattern and vary the transformation temperatures and the phase transformation kinetics. The material under study was a C40 steel with a dendrite solidification microstructure coming from an industrial continuous casting plant. The high temperature ductility was evaluated by means of tensile tests up to fracture at strain rate of 0.001 s-1 in a temperature range of 1100 to 710°C. The reduction in area at fracture as a function of temperature graphs show a clear reduction of the steel ductility in the intercritical region, but also after the pearlite transformation. Single deformation compression tests were also carried out on the steel in the austenitic temperature domain, 900 to 1100°C, and at strain rate of 0.001 to 1 s-1. A modification of the Garofalo hyperbolic sine equation has been employed to derive the peak and steady stresses of the flow curve. The work hardening, U, and dynamic recovery, Ω, parameters which describe the flow curve before dynamic recrystallization takes place and the k and t50 parameters, based on the JMAK model, to describe the recrystallization kinetics were also calculated for every test and expressed as a function of the Zener Hollomon parameter, Z.

Abstract: The unbending operation is a critical stage of steel continuous casting because it is carried out at thermomechanical conditions for which embrittlement mechanisms can appear leading to transverse cracking. The hot tensile test is commonly used to simulate such thermomechanical conditions, at the surface of the slab, and, the reduction in area of the samples tested to fracture is taken as a measure of the susceptibility to cracking of the steel. However, a further metallographic and fractographic evaluation of the samples is required in order to identify the embrittlement mechanisms. These mechanisms are usually related to transformations in the microstructure, such as precipitation or the appearance of deformation induced ferrite, which imply changes in the strength of the material and should therefore be detectable in the flow curves. However, the features of tensile curves are not usually analyzed when evaluating the hot ductility because necking makes the interpretation of the curves complicated. In this work the hot ductility of a C-Mn steel will be discussed by means of hot tensile and compression tests. The embrittlement mechanism identified for this steel is the appearance of a ferrite layer at austenite grain boundaries. The effect of this mechanism on the features of the tensile curves will be discussed. Moreover, these curves will be compared to compression curves obtained under the same testing conditions to see whether transformation induced ferrite can be detected by means of hot compression testing. The possibility of assessing the ductile behavior of different steel grades through hot compression, which requires less material and is easier to control, will be discussed.

Abstract: The influence of methods for converting torsion data (torque, number of turns and speed rotation) into equivalent variables (true stress, true strain and true strain rate) on the Garofalo equation parameters is studied. Torsion tests for a high strength low alloy steel at temperatures in the range 900 to 1150°C and strain rates in the range 0.5 to 30 s-1 were conducted. The flow behavior of the material was analyzed by means of the Garofalo equation which is especially adequate for the correlation of torsion data in wide ranges of temperatures and strain rates. The Garofalo equation was fitted at different strains: from peak strain to a value of strain where a steady-state is assumed. The parameters of this equation, A, Q, n, , were determined by the RCR method which does not need initial values and provides coefficients indicating the quality of the fitting. Finally, the Garofalo equation parameters were related to creep mechanisms operating in this steel.

Abstract: One of the most significant aspects of the axisymmetric drawing operation is the occurrence of non-homogeneous deformation in the cross section of the metal. This phenomenon is associated with an internal distortion process that takes place in the bar as it flows through the die, leading to the development of higher drawing forces and affecting the subsequent mechanical behavior of the material. An adequate analysis of the process and of the work hardening of the drawn metal, therefore, must involve a detailed study of the deformation features in the forming operation. In the present work, the deformation in the single-pass drawing of AISI 304 stainless steel bars was investigated through the evaluation of the relationship between the redundant deformation factor and the parameter . Two experimetal procedures were employed in the study: the visioplasticity and the stress-strain curves superposition techniques. The first one, previously considered as the method leading to the most realistic solutions to various forming processes, allowed the establishment of an increasing linear relationship between de redundant deformation factor and the parameter . A similar behavior was observed through the stress-strain curves superposition technique. In this case, however, the redundant deformation factor values were lower or higher than those obtained through visioplasticity according to the drawing conditions and more sensitive to variations of the parameter . The results were compared to those exhibited by the AISI 420 stainless steel, revealing the influence of the structural features on the behavior of the metal.

Abstract: The numerical modelling technique is successfully used for simulation of steel flow and behaviour of non-metallic inclusions in the tundish. The CFD (Computational Fluid Dynamic) method allows information on steel motion, flow, flow turbulence and steel temperature distribution in the tundish to be obtained. In the continuous steel casting process, where the tundish performs the function of a device batching steel to the mould, the monitoring of steel flow is very essential because steel flow has influence on the behaviour of non-metallic inclusions. The paper presents the results of computing simulation steel flow and flotation of non-metallic inclusions in the six-strand tundish with stopper rod system. The subject of simulation was a six-strand tundish unprovided with flow control devices. The tundish is used in the polish steel mill to process of continuous casting billets. The Fluent program was used for solved mathematical model of casting process. The numerical simulation of steel flow was performed for whole test facility without the “symmetry” boundary condition. As a results of numerical simulation maps of directions flow and temperature of steel, Residence Time Distributions curve type of (F) and (E) and distributions of non-metallic inclusions between slag phase and particular tundish outlets has been obtained.

Abstract: The dissolution of different sulphides, carbides, carbo-sulphides and nitrides during re-heating of hot rolled low carbon, low manganese, titanium added steel have been studied using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analysis. In addition, the chemical composition and size distribution of the different precipitates have been determined before and after reheating to analyze the modification of these precipitates in the course of the reheating cycle. The TEM and EDS analyses showed the presence of a wide variety of simple and/or complex precipitates in as rolled samples. The reheating of these samples to temperatures as high as 1350 °C, caused dissolution of most particles, although titanium nitride (TiN) did not dissolve even after reheating. By decreasing the reheating temperature more and more precipitates remained un-dissolved, but some spherodization occurred at higher temperatures.

Abstract: The relationship between microstructure and cleavage resistance in quenched and tempered high strength bainitic and martensitic steels is investigated by means of Charpy-V three-point bending tests, uniaxial tensile test on unnotched specimens and EBSD. Steels under investigation are low/medium carbon (C=0.10%-0.40%) steels with yield strength in the range YS=500-1000 MPa. Results show that the tensile strength and the cleavage resistance of Q&T steels appear to be controlled by different structural parameters and not, as in the case of polygonal ferritic steels, by the same structural unit. In particular, yield strength is controlled by the mean subgrain size, whereas the structural unit controlling the critical cleavage stress is the covariant (bainitic or martensitic) packet, whose size is slightly lower than the average unit crack path (UCP). The critical stage in the fracture process appears to be the propagation of a Griffith crack from one packet to another, and the resistance offered by high-angle boundaries is approximately the same as that of low-C steels with bainitic or polygonal ferrite microstructure.

Abstract: The creep behavior of oxide-bearing Fe-0.6%O steel was studied in the temperature range of 550-700°C at stresses ranging from 100 to 400 MPa. The creep data showed high values of an apparent stress exponent n close to 16 for power-law creep. In addition the apparent experimental activation energy was much higher than that for the lattice diffusion in -iron. Analysis of creep data revealed that the deformation behavior was strongly affected by the threshold stresses, which are associated with the interaction between moving dislocations and fine incoherent oxide particles. Analysis of deformation behavior in terms of threshold stress leads the true stress exponent of 8; the activation energy for creep became close to value of activation energy for lattice diffusion at 700°C and for pipe-diffusion in the temperature range of 550–650°C.