Priya Manohar

Design of an expert system for the optimization of steel compositions and process route

Abstract This paper describes a new integrated approach to build an expert system to generate and evaluate alternative steelmaking aim compositions that not only meet the customer requirements but also suit the established rolling schedules. The methodology developed is based on the hybrid approach combining knowledge-bases as well as mathematical modelling and is applicable for C–Mn steel grades. The expert system consists of two modules. The first module utilizes various empirical models for the relationship between mechanical properties and the elements in steelmaking aim compositions, along with knowledge-bases containing expert and heuristic knowledge of expert metallurgists to generate a list of alternative steelmaking aim compositions. The second module uses the output of the first module and computes the microstructural evolution during processing depending on steel composition and the known values of processing conditions such as strain, strain rate, temperature, interpass time and plate cooling rate. Calculated values of the metallurgical parameters are then utilized to estimate realistically the achievable mechanical properties in the final hot rolled product using knowledge-bases. The output of this expert system is expected to assist the product development metallurgists in the selection of appropriate steelmaking aim composition for any combination of property specifications required by the customer.

Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar 3, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar 3, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar 3 temperature, decreased the time of transformation, and increased the hardness.

Thermomechanical Process Innovation via Computer Modeling and Simulation

A new approach is proposed in this paper that describes the development of flexible rolling technology in industrial processing of C – Mn and Low C - microalloyed steels. Scientific knowledge of industrially-significant processes for these materials is presently fragmented and scattered in published literature, which causes impediment to process innovation and optimization. In the current work, it is demonstrated that new process sequences could be developed by breaking down existing process routes in to key elements and then by recombining them to generate novel alternative and more efficient hot processing sequences. The proposed methodology establishes a platform for a more realistic assessment of existing process routes and the development of new hybrid process routes that combine ideas from alternative processes. This enables the identification of an optimal process sequence for specified steel compositions that also satisfies simultaneous design criteria such as process feasibility and property maximization. Application of the proposed algorithm in industrial-scale rod rolling of a medium C-Mn steel is demonstrated and discussed.

Simulation of Microstructural Evolution in Rod Rolling of a Medium C-Mn Steel

An ‘Expert System’ is proposed in this work to conduct computational exploration of the deformation and restoration behavior of a medium C-Mn steel under high strain rate conditions, at elevated temperatures and complex strain paths that occur in rod rolling process. The expert system computes appropriate thermomechanical parameters necessary for describing rod rolling process in detail and then utilizes these parameters in mathematical models to determine microstructure evolution during a typical industrial-scale rod rolling process. Microstructure simulation in rod rolling is a challenging problem due to the fact that several softening mechanisms may operate sequentially or concurrently during each deformation step. Different softening mechanisms have very different impact on microstructure development and therefore it is important to investigate the particular combinations of processing conditions under which transition of operating softening mechanisms occurs. In the present work, the transition from dynamic to metadynamic recrystallization is studied in detail based on the criteria of critical strain, austenite grain size and Zener-Hollomon parameter when the interpass (interdeformation) time is very short of the order of few milliseconds during the later stages of rod rolling. Computational results are subsequently validated by comparing the program output to in-plant measured microstructure data. The proposed expert system is designed as an off-line simulation tool to examine and assess the various options for thermomechanical process optimization. Introduction Optimization of the industrial rod rolling process presents a formidable challenge as this process is characterized by continuous multi-pass deformation (up to 30 deformation passes) at high strain rate in the range 0.4 – 3000s, at elevated temperatures in the range 1173 – 1373 K, and very short interpass times of the order 0.015 – 1.0s. These processing conditions make it virtually impossible to study experimentally the microstructural evolution during the intermediate stages of hot rolling. On the other hand, knowledge of the in-process microstructural evolution is important for both the optimization of the process schedule and to adjust the properties of the hot rolled product [1,2]. For example, a fine austenite grain size is desirable at the end of rod rolling to decreases its hardenability to obtain a fine ferrite + pearlite structure via controlled cooling, to eliminate or reduce the necessity of post-rolling annealing treatment, and to improve the mechanical properties of the as-rolled products. Previous efforts [3-5] to simulate microstructural evolution in wire rod rolling have concentrated mainly on calculating evolution of the mean austenite grain size in medium C-Mn steels. In the present work, the focus is on the fundamental aspects of microstructure development mechanisms such as static, dynamic and metadynamic recrystallization (SRX, DRX and MDRX respectively) and their kinetics, how to resolve the boundary conditions when they operate concurrently and finally their impact on microstructure evolution in continuous processing. Expert System Development The flow chart for the expert system is given in Figure 1 located at the end of the text. The initial (i.e. as-reheated) microstructure and expected rolling schedule are the basic inputs to the system. The program then calculates the deformation conditions such critical and peak strains, and Zener – Hollomon parameter based on initial grain size, pass strain, strain rate and temperature. The program subsequently computes the evolution of microstructure by computation of the recrystallized grain size, fraction recrystallized and recrystallization kinetics for a given rolling pass based on the mathematical models listed in Table 1. Table 1: Mathematical models describing kinetics of relevant softening mechanisms. Parameter DRX MDRX SRX Fraction Recrystallized (Fx) [6-8] ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − k

Constitutive Modeling of High Temperature Mechanical Behavior of a Medium C - Mn Steel

1) Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA. 2) Plate and Rod research group, POSCO Technical Research Laboratory, Pohang, P.O. Box 36, KOREA. 3) Department of Materials Science and Engineering, Inha University, 253 Yongheondong Namgu, Inchun 402-751, KOREA. 4) Dynamic Materials Properties: Testing and Modeling, Los Alamos National Laboratory, MS G755, Los Alamos, NM 87545, USA.

Recrystallization of Ferrite and Austenite

A combination of recrystallization controlled processing, optimization of steel chemical composition is leading the way for achieving fine-grained and ultrafine-grained steels. These steels have desired combination of most sought-after properties such as ultra high strength, high toughness, good formability, and weldability at a reasonable cost. An understanding of recrystallization of ferrite and austenite is thus of crucial importance as these phases undergo thermomechanical processing (TMP) of steel. As steel is deformed at high temperature internal stresses of deformation are relieved through several competing softening (restoration) processes. This article summarizes the concepts involved, and the kinetics, characteristics, and industrial applications of recrystallization as it occurs in ferrite and austenite as a function of complex combinations of several of these factors.

Failure Analysis of a Hammer Drill Shaft under Complex Loading Paths and Severe Environmental Conditions

This paper describes the failure investigation of a tubular shaft that is part of a hammer drill assembly. The failure investigation was particularly challenging as the fracture surfaces were completely damaged during and subsequent to the failure process. However, careful examination of the component and its assembly revealed many clues that pointed to the root causes of failure. It was determined that the shaft was subjected to impact, fatigue, bending and torsional loads simultaneously at elevated temperatures. The basic failure mode was identified as a combination of torsional fatigue and rotating bending fatigue failure that originated on the inside diameter of the shaft. The root causes were determined to be operational overload in combination with rough machining marks on the bore surface and higher than necessary operating torque required to overcome the dry adhesive friction in the system. The preventative measures recommended were many-fold including improving surface finish on the bore diameter, reducing dry sliding friction, decreasing the overall level of dynamic loads by appropriate design changes and adding a surface strengthening heat treatment

Stress corrosion cracking failure of jackbolts for die casting press

Several prematurely failed jack bolts were analyzed to determine the root causes of failure. Bolts were employed to ensure that die halves do not separate during casting of high pressure die castings of light metals. Fractography of jack bolts revealed unusual morphology consisting of both circumferential and longitudinal cracking. The basic fracture type was identified as transgranular cleavage (brittle) fracture mode. SEM / EDS analysis of the fracture surface revealed the presence of, to varying degrees, chemical species containing sulfur (S), oxygen (O) and chlorine (Cl). Material composition, heat treatment, microstructure and hardnesses of the jack bolts were found to be in agreement with the expected steel grade and properties. It was concluded that the failure of the bolts was due to a combination of inappropriately chosen mechanical properties of the bolts, operating stress, and the presence of corrosive environmental materials leading to ideal conditions that promoted stress corrosion cracking failures. Suitable remedial actions to alleviate the risk of SCC failure of the bolts are presented and discussed in the paper.

Development of an integrated system for designing steelmaking aim compositions

A new integrated approach is proposed in this paper to generate and evaluate the alternative steelmaking aim compositions which not only meet the customer requirements but also suit the established rolling schedules. The methodology developed is based on hybrid approach combining knowledge-bases as well as mathematical modelling and is applicable for C-Mn steel grades. The system consists of two modules. The first module utilises various empirical models for the relationship between mechanical properties and the elements in steelmaking aim compositions, along with knowledge bases containing expert and heuristic knowledge of expert metallurgists to generate a list of alternative steelmaking aim compositions. The second module uses the output of the first module and computes the microstructural evolution during processing depending on steel composition and known processing conditions such as strain, strain rate, temperature, interpass time and plate cooling rate. Calculated values of the metallurgical parameters are then used to estimate the achievable mechanical properties in the final hot rolled product using knowledge-bases. System output is expected to assist product development metallurgists in the selection of appropriate steelmaking aim compositions for any combination of property specifications required by the customer.