Neural Network Based Adaptation Algorithm for Online Prediction of Mechanical Properties of Steel

After production of a steel product in a steel plant, a sample of the product is tested in a laboratory for its mechanical properties like yield strength (YS), ultimate tensile strength (UTS) and percentage elongation. This paper describes a mathematical model based method which can predict the mechanical properties without testing. A neural network based adaptation algorithm was developed to reduce the prediction error. The uniqueness of this adaptation algorithm is that the model trains itself very fast when predicted and measured data are incorporated to the model. Based on the algorithm, an ASP.Net based intranet website has also been developed for calculation of the mechanical properties. In the starting Furnace Module webpage, austenite grain size is calculated using semi-empirical equations of austenite grain size during heating of slab in a reheating furnace. In the Mill Module webpage, different conditions of static, dynamic and metadynamic recrystallization are calculated. In this module, austenite grain size is calculated from the recrystallization conditions using corresponding recrystallization and grain growth equations. The last module is a cooling module. In this module, the phase transformation equations are used to predict the grain size of ferrite phase. In this module, structure-property correlation is used to predict the final mechanical properties. In the Training Module, the neural network based adapation algorithm trains the model and stores the weights and bias in a database for future predictions. Finally, the model was trained and validated with measured property data

Niobium in Microalloyed Rail Steels

Rail steels rely primarily on possessing adequate wear and rolling contact fatigue resistance. These properties, together with the toughness, can in principle be optimized by implementing thermomechanical processing assisted by controlled niobium additions. The purpose of the current work is to develop a Nb-microalloying strategy in the context of high-carbon pearlitic and cementite-free bainitic steels. The conventional methods do not leave the critical regions of a rail section in a suitably processed state. An attempt has been made for the first time, to create a pancaked austenite grain structure, with an examination of the consequences on the final product. One of the major difficulties is to ensure that niobium does not segregate during manufacturing, since niobium is a strong carbide former and rail steels traditionally contain large carbon concentrations. Niobium solubility in austenite has been assessed critically and thermodynamic calculations for microsegregation have been taken into account. The aim is to ensure that any primary niobium carbide precipitated from solute-enriched liquid during non-equilibrium solidification, can be taken into solution in austenite during reheating, to mitigate potential effects of coarse precipitates on the final mechanical properties. Rail steels containing 0.01-0.02 wt% Nb have been designed and characterised. In as-cast condition, primary niobium carbides as large as ~10 µm can be observed, which dissolve slowly during reheating. An attempt has been made to develop a model to estimate the dissolution kinetics of the carbides. Dissolved niobium in reheated austenite precipitates during hot deformation as fine niobium carbides (<50 nm) which inhibit austenite recrystallisation by pinning the austenite grain boundaries. Nb-microalloying increases the ‘no-recrystallisation temperature’ of deformed austenite during multi-pass compression tests. The topology of grain deformation has been analysed in terms of stereological calculations and dilatometric experiments have shown that transformation kinetics tend to accelerate when the austenite is deformed below the no-recrystallisation temperature, however the effect is relatively small. The microstructure and mechanical properties of the as-rolled Nb-microalloyed steels have been characterised along with their rolling-sliding wear performance and compared with their non-microalloyed counterparts. Increased austenite grain boundary area and increased dislocation activity due to pancaking, hinder bainite growth which leads to an increased retained austenite volume fraction. This in turn, leads to slightly improved ductility, improved toughness and improved wear resistance in Nb-microalloyed bainitic alloys. Microstructural refinement in Nbmicroalloyed pearlitic alloys does not have any significant effect on tensile and toughness properties, but wear resistance improves significantly. A Bayesian neural network model has been developed to estimate the wear of rails. Predicted trends have been found consistent with metallurgical experience and the perceived noise levels are consistent with reasonable repeatability of the wear testing method used. The model can be applied widely to estimate wear because of its capacity to indicate uncertainty, including both the perceived level of noise in the output, and an uncertainty associated with fitting the function in the local region of input space

Conference proceedings: Thermo-mechanical processing of Steels & 5th Gleeble User Workshop India

To bring together the national experts, academia, R&D establishments, industries and students on a common platform for learning, sharing and updating the latest developments in the area of thermo-mechanical processing of steels. To provide a platform for Gleeble users in India to discuss the Gleeble related applications, operations and maintenance issues

Advances in Low-carbon and Stainless Steels

This Special Issue of Metals was dedicated to recent advances in low-carbon and stainless steels. Although these types of steels are not new, they are still receiving considerable attention from both research and industry sectors due to their wide range of applications and their complex microstructure and behavior under different conditions. The microstructure of low-carbon and stainless steels resulting from solidification, phase transformation, and hot working is complex, which, in turn, affect their performance under different working conditions. A detailed understanding of the microstructure, properties, and performance for these steels has been the aim of steel scientists for a long time. This Issue received quality papers on different aspects of these steels including their solidification, thermomechanical processing, phase transformation, texture, etc., and their mechanical and corrosion behaviors

The behaviour of advanced quenched and tempered steels during arc welding and thermal cutting

Quenched & tempered (Q&T) wear-resistant plate steels with martensitic microstructures have been in use for many years in the mining, defence, and construction industry due to their excellent mechanical properties (up to 1700 MPa of tensile strength and \u3e10% elongation to failure). These mechanical properties are achieved by utilisation of up to 0.4 wt.% Carbon (C), \u3c1.5 wt.% Manganese (Mn), microalloying with Molybdenum (Mo), Chromium (Cr), Nickle (Ni), Titanium (Ti), and sometimes Boron (B), and a combination of carefully designed thermomechanical processing schedule and post rolling heat treatment. In the last 10 years addition of \u3c1.5 % Ti was shown to provide superior wear resistance at a moderate C content. Improvement in the wear resistance was achieved via the formation of TiC hard particles embedded in the tempered martensite matrix. Moderation of the C content in Ti-alloyed steels allowed to obtain steels with relatively low hardness, high toughness, and enhanced weldability (due to the low carbon equivalent of the steel composition). A combination of moderate hardness and high toughness positively influenced the wear resistance. Fabrication of tools and equipment from the Q&T steels is carried out using conventional fusion arc welding and thermal cutting with oxy-fuel or plasma jet. The main problem, in this case, is the formation of an edge microstructure highly susceptible to cold cracking or hydrogen-induced cracking (HIC), which results in deterioration of mechanical properties, making steel unsuitable for the required application. In the case of Ti-alloyed steels, the heat input associated with thermal cutting and welding alters the TiC particle size distribution, in addition to the tempering of the martensitic microstructure, occurring in conventional Q&T steels. However, fabrication parameters may be controlled to avoid catastrophic microstructure deterioration and product failure. Generally, a type of welding process, environment, alloy composition, joint geometry, and size are the main causes of cracking after cutting and welding. Cracking susceptibility increases as the weld metal hydrogen content, material strength, and thickness increase. Cold cracking will occur if three conditions are satisfied: susceptible microstructure; type and magnitude of residual stresses; and importantly, the level of diffusible hydrogen that enters the weld pool. Cold cracking can be avoided through the selection of controlled heat input (depends upon current, voltage, and travel speed of welding) and preheating temperature

A Study of Processing, Microstructure and Mechanical Properties of Ultra-High Strength Microalloyed Steel Hot Band Coils for Automotive Applications

In the automotive industry, thin gauge high strength steels require not only good tensile ductility, but also good sheared edge ductility or a good hole expansion ratio (HER) value. These properties can be achieved through producing a microstructure consisting of a single-phase ferrite and strengthened by microalloyed precipitates. The objective of this current study was to develop a hot band steel with a ferrite-based microstructure with a tensile strength higher than 1200MPa, total elongation larger than 10%, good HER values and low temperature toughness. The steel being studied in this project has the following nominal composition (wt %): 0.14C, 0.35Mo, 0.163Ti and 0.294V. In this investigation, the relationship between the microstructure and mechanical properties with different finish rolling and coiling temperatures were explored. It was found that the finish rolling temperature did not have an obvious influence on either microstructure or mechanical properties. However, the coiling temperatures strongly affected both microstructure and mechanical properties. Steels with a coiling temperature of 610°C, exhibited predominately polygonal ferrite and a few acicular ferrite grains. The corresponding tensile strength was over 1200MPa, total elongation of about 20%; however, the low temperature toughness and HER were rather low. The fracture surface from broken CVN specimens shows nearly pure brittle fracture. When the coiling temperature was reduced to 530°C, the microstructure appears to be a mixture of granular bainite and coarse quasi-polygonal ferrite grains. Here, the tensile strength drops to under 1000MPa, but the low temperature toughness and HER improve greatly. With a further lower coiling temperature of 450°C, the microstructure is a mixture of granular bainite, quasi-polygonal ferrite and upper bainite. In this case, the tensile strength increases to about 1100MPa, and the steels have intermediate low temperature toughness and HER. Studies showed that the high strength of the steels with the highest coiling temperature were due to the excessively formed fine precipitates at the coiling temperature of 610°; while the strength of steels with lower coiling temperatures originates predominately from dislocation strengthening. Because of the very high percentage of Ti and low amount of N, coarser TiN inclusions were formed in the liquid and in the interdendritic pools separating the dendritic δ ferrite grains. These hyper-stoichiometric TiN particles with a size larger than 3 microns can be observed throughout the steels. The relatively low toughness and HER values can be attributed, at least partially, to the large amount of coarse TiN inclusions found in all steel conditions. In addition, the low values found with 610°C coiling can be partly attributed to excessive precipitation hardening, while the low values found with 450°C coiling is the result of high levels of MA microconstituent. The scientific hypothesis guiding this study is that excessively large amounts of TiN particles are detrimental to toughness and sheared edge ductility (HER) in high strength steels

Processing and Structure of High Strength Steels

There is currently an extensive effort to develop new steels with a better combination of strength and ductility, the so-called Ultra-High Strength Steels (UHSS) and Advanced High Strength Steels (AHSS). The new steels must be able to be manufactured using existing plant and at the same time meet ever decreasing cost demands. This study looks at developing AHSS, with a focus primarily on the microalloying precipitation behaviour and microstructural evolution during thermomechanical processing of transformation induced plasticity (TRIP) assisted steels. This approach combines two mechanisms that are known to enhance properties, namely the TRIP effect and precipitation hardening arising from strain induced precipitation. In respect of the TRIP effect, a detailed investigation was undertaken to look at the effect of the intercritical anneal on strength and ductility. For the strain induced precipitation, the effect of combined microalloy additions was investigated. Initially, the effect of intercritical annealing was studied using a trial as-cold rolled multiphase V microalloyed TRIP assisted steel with a polygonal ferrite matrix (1.5Mn C+Si+Al+N≈1.0 (C 0.2) and V˂0.2 wt%) focusing on the isothermal bainite transformation at 460°C. The V(C,N) precipitates were extensively observed in all phases (ferrite, retained austenite, martensite and bainitic ferrite) after isothermal bainite transformation in a random manner. The tensile testing suggested considerable improvements in yield, ultimate tensile strengths and elongation after 180s isothermal bainite holding time. According to SEM/TEM observations, these effects were attributed to the evolution of retained austenite from block to film morphology, and possible interaction between microalloying precipitates and newly formed dislocations as a result of bainite transformation. The average density of V(C,N) slightly increased after intercritical annealing, though it did not show a significant variation during the isothermal bainite transformation. Nevertheless, the size distribution of V(C,N) precipitates did not change significantly. The effect of a combined addition of Nb, Mo and V was investigated in TRIP multiphase steel (0.12C 1.50Mn 1.50Si and NbVTiMo≤0.20 wt%), which was compared to a V single addition steel with otherwise matched composition. A comparative study was undertaken using intercritical annealing after ~20% cold-rolled deformation of the rough rolled specimens. The resulting microstructure in both alloys comprised acicular/bainitic ferrite with an uneven proportion of allotriomorphic ferrite and retained austenite and martensite. An average density of precipitates increased in both alloys after intercritical annealing, with an average of 137 and 219 precipitate/µm2 in NbVMo and V steels, respectively. NbV(C,N), NbVMo(C,N) and V(C,N) were observed in the NbVMo steel. In both alloys, precipitates with different morphologies were observed, located in the matrix, on dislocations and at grain boundaries. The results suggested that the NbMo addition retarded the growth/coarsening of precipitates with a size of lower than 15nm. Also, much greater precipitation strengthening was observed in the NbVMo steel after intercrital annealing compared to the V steel. The last part of the project was to systematically study the effect of Nb and Mo on the V steel as a function of thermomechanical processing. Laboratory simulations were developed by plane strain compression testing which accurately replicate the whole thermomechanical process route from the hot rolling through intercritical annealing, followed by the bainite transformation. After hot and controlled rolling at intercritical annealing range the resulting microstructure was acicular/bainitic ferrite, retained austenite and martensite surrounded by allotriomorphic ferrite in both alloys. The TEM observations suggested that a noticeable number of precipitates were formed in the NbVMo steel up to the finishing stage (i.e. average of 112 precipitate/µm2 in NbVMo containing steels). It was also found that the V(C,N) precipitation occurred in austenite and ferrite below the finishing stage (i.e. ≤830˚C) with an average cooling rate of ~12˚C/s. The overall findings suggest that the high dislocation density in the ~20% cold rolled acicular/bainitic ferrite could lead to an intense precipitation and coarsening of V(C,N) during the intercritical annealing

A review of the effects of chemical and phase segregation on the mechanical behaviour of multi-phase steels

In the drive towards higher strength alloys, a diverse range of alloying elements is employed to enhance their strength and ductility. Limited solid solubility of these elements in steel leads to segregation during casting which affects the entire down-stream processing and eventually the mechanical properties of the finished product. Although it is thought that the presence of continuous bands lead to premature failure, it has not been possible to verify this link. This poses as increasingly greater risk for higher alloyed, higher strength steels which are prone to centre-line segregation: it is thus vital to be able to predict the mechanical behaviour of multi-phase (MP) steels under loading. This review covers the microstructure and properties of galvanised advanced high strength steels with particular emphasis to their use in automotive applications. In order to understand the origins of banding, the origins of segregation of alloying elements during casting and partitioning in the solid state will be discussed along with the effects on the mechanical behaviour and damage evolution under (tensile) loading. Attention will also be paid to the application of microstructural models in tailoring the production process to enable suppression of the effects of segregation upon banding. Finally, the theory and application of the experimental techniques used in this work to elucidate the structure and properties will be examined.Comment: 53pages, 34 figures, 4 table

Critical strain for dynamic recrystallisation. The particular case of steel

The knowledge of the flow behavior of metallic alloys subjected to hot forming operations has particular interest for metallurgists in the practice of industrial forming processes involving high temperatures (e.g., rolling, forging, and/or extrusion operations). Dynamic recrystallisation (DRX) occurs during high temperature forming over a wide range of metals and alloys, and it is known to be a powerful tool that can be used to control the microstructure and mechanical properties. Therefore, it is important to know, particularly in low stacking fault energy materials, the precise time at which DRX is available to act. Under a constant strain rate condition, and for a given temperature, such a time is defined as a critical strain (ec). Unfortunately, this critical value is not always directly measurable on the flow curve; as a result, different methods have been developed to derive it. Focused on carbon and microalloyed steels subjected to laboratory-scale testing, in the present work, the state of art on the critical strain for the initiation of DRX is reviewed and summarized. A review of the different methods and expressions for assessing the critical strain is also included. The collected data are well suited to feeding constitutive models and computational codes.Peer ReviewedPostprint (published version