Showing papers by "Steel Authority of India published in 2004"

Age replacement of components during IFR delay time

Abstract: This paper proposes two alternative policies for preventive replacement of a component, which shows sign of occurrence of a fault, and operates for some random time with degraded performance, before its final failure. The time between fault occurrence and component failure is termed as delay time. The first policy, namely age replacement during delay time policy (ARDTP), recommends replacement of a faulty component on failure or preventive replacement of the same after a fixed time during its delay time. It considers the performance degradation during delay time to develop an age replacement policy. It is also shown that the policy is a feasible proposition for a component that has positive (nonnegative) performance degradation during its CFR (IFR) delay time. The second policy, OARDTP, extends ARDTP to opportunistic age replacement policy where a faulty component is replaced at the first available randomly occurring maintenance opportunity, after a fixed time from occurrence of fault, or on failure. The time between opportunities (TBO) is considered to be exponentially distributed. This policy reduces the number of forced shutdowns, which is essential to ARDTP. It is shown that the second policy is superior to the first policy if the cost of a preventive replacement with forced shutdown is more than the preventive replacement cost during an opportunity. The policies are appropriate for complex process plants, where the tracking of the entire service life of each component is difficult. Their implementation requires tracking of components' delay time only, and estimation of mean time to occurrence of faults. The policies are relatively insensitive to estimation error in failure replacement cost. As their implementation requires immediate capturing of fault occurrence information, they are particularly attractive to organizations where operators are involved in the maintenance of machines.

Predicting microstructural evolution and yield strength of microalloyed hot rolled steel plate

Abstract: A mathematical model has been developed to optimise process parameters for production of API grade steel plates by thermomechanical controlled processing at a plate mill in Bhilai Steel Plant, India. The model comprises the prediction of the microstructural evolution during hot rolling, the subsequent phase transformation, and, finally, the mechanical properties of microalloyed steels. Effects of chemistry and mill parameters on recrystallisation, grain growth, and precipitation kinetics were taken into consideration to describe the metallurgical processes. . The model has been validated through laboratory experiments as well as full-scale rolling at the plate mill.

Metallurgical investigation of prematurely failed hot-strip mill work-rolls: Some microstructural observations

Abstract: A metallurgical investigation of failed samples of hot-strip mill work-rolls used in an integrated steel plant was made to determine the influence of microstructural characteristics on failure susceptibility and roll life. The samples investigated pertained to prematurely failed indefinite chill double-poured (ICDP) iron work-rolls, which exhibited varying roll lives under similar mill operating environments. Although microstructures of all the investigated rolls showed similar graphite morphologies irrespective of their mill performance, discernible differences in carbide characteristics could be observed between high and low life rolls. Microstructural observation of nital-etched roll specimens revealed that lower life rolls were characterized by carbide microcracking. The propensity for cracking was particularly high in carbides exhibiting microhardness greater than 1020 VPN. Electron-probe microanalysis (EPMA) indicated that carbides in the spalled rolls were mostly of M3C type, where M was Fe and Cr. Quantitative image analysis of phases in the investigated rolls revealed that while graphite volume fraction in the range of 4.0 to 6.4% did not significantly affect roll life, carbide content higher than 28.5 vol% was found detrimental. In fact, a carbide content in the range of 24.0 to 28.50 vol% was found to be desirable for higher roll life. The study thus revealed that although carbides are indispensable for high hardness, resistance to wear, and thermal cracking, an excessive volume fraction (>30 vol%) of high hardness (microhardness > 1020 VPN) carbides accentuated microcracking, which ultimately induced premature spalling of hot-strip mill work-rolls.

Correlating the Aluminum Content with Ferrite Grain Size and Core Loss in Non-oriented Electrical Steel

Abstract: Core loss property of a magnetic material, is defined as dissipation of electrical energy in the form of heat during magnetization by an alternating current. Therefore, efforts are always made to minimise core loss value of magnetic materials. In case of electrical steels, core loss can be reduced by controlling a number of metallurgical factors that include inter alia ferrite grain size. It has been reported that, for a given silicon content, core loss value in the steel initially decreases with the increase in the ferrite grain size. However, after attaining an optimum grain size, core loss value starts increasing with further increase in ferrite grain size. Such type of distinct influence of ferrite grain size on core loss property is due to the fact that the size of magnetic domains present in the steel, is directly linked with the ferrite grain size. Therefore, an increase in ferrite grain size results in corresponding increase in the size of magnetic domains and hence, there is lesser number of domain walls to move during magnetisation, which results in lowering of core loss. However, when magnetic domain size becomes very large, the domain walls have to move faster to cover the same distance during magnetisation which results in enhancing the core loss value of the steel which contains ferrite grains coarser than an optimum grain size. In industrial practice, ferrite grain size of non-oriented electrical steels is increased by adopting suitable hot strip rolling parameters which include low finishing and high coiling temperatures. Further to this, both hot rolled and cold rolled coils are required to be annealed to promote coarsening of ferrite grains. It has, also, been reported that addition of higher amounts of aluminum facilitates coarsening of ferrite grain size, though aluminum is generally considered as an effective ferrite grain refiner. In this context, the present study was taken up to understand the efficacy of aluminum addition on ferrite grain size and on core loss property in non-oriented electrical steel with silicon content around 1.5%. It may be mentioned that aluminum is added to the nonoriented electrical steels to form AlN by combining with the nitrogen available in the liquid steel and consequently, to minimise the detrimental effect of free nitrogen on the magnetic properties of the steel. Also, aluminum addition lowers core loss value by increasing electrical resistivity of the steel and, under specific processing conditions, by developing favorable texture for magnetisation.

Optimization of annealing parameters for improvement in formability of extra deep drawing quality steel

Abstract: A number of annealing cycles were investigated in an attempt to find the optimum cycle that results in an attractive combination of mechanical and formability properties of an extra deep drawing (EDD) quality steel. It was found that the cycle that involved an intermediate anneal at 600 °C followed by further soaking at 700 °C resulted in the best combination of mechanical and formability properties. It was also found that the rate of heating up to 600 °C can be kept at 50 °C/h while the heating has to be done at a rate of 30 °C/h from 600 °C to the final annealing temperature of 700 °C. The desirable combination of mechanical and formability properties has been correlated with the microstructure that shows pancaking of the annealed grains accompanied by precipitation of carbides. Precipitates of carbides are more in number and smaller in size in the case of samples annealed by the cycle mentioned above compared to the ones annealed by other cycles. They are spherical in shape, which is desirable for forming applications.

Finite Element Method Simulation of the Cooling of Hot-Rolled Steel Rods

Abstract: The paper addresses the development of a heat transfer model, using the finite element method, for the cooling of hot-rolled steel rods. The model was used for simulation studies to analyze alternative cooling strategies for hot-rolled rods. Cooling in stages was found to be preferable to single stage cooling in the context of temperature differences created between the surface and the core and also in regard to the drop in surface temperature.

Premature failure of work-rolls in tandem mill: Some microstructural revelations

Abstract: The microstructural features of prematurely spalled tandem mill work-rolls were examined in an attempt to correlate microstructure with spalling behavior and roll performance. Spalled samples were collected from work-rolls that had shown variations in roll life under similar conditions of mill usage. Optical microscopy revealed that a fine dispersion of spheroidal carbides in a matrix of tempered martensite was conducive to superior performance in terms of roll life (i.e., tonnage rolled), and that coarse angular and irregular shape carbides were detrimental to roll life. Image analysis of roll microstructures indicated that small carbide size, large carbide volume fraction, and high carbide count were characteristic of higher-life rolls, and that large carbide size, low carbide volume fraction, and less carbide density were typical of lower-life rolls. The carbides in both types of microstructure were M7C3 type.

Mathematical model based coking control system

Abstract: A mathematical model based coking control system has been developed and implemented at coke oven battery no. 3 of Bhilai Steel Plant, Steel Authority of India Ltd., India. The mathematical model predicts and controls the thermal regime of coke oven battery. The key components of the control system are mathematical model, coke oven identification system, coking index determination and coke mass temperature measurement system. The application of the system has resulted in energy saving, coke quality improvement and enhanced battery life. This paper describes in details the methodology followed in the development of mathematical model and its implementation in industry.