C.M. Sellars

Application of the Generalized Pareto Distribution to the estimation of the size of the maximum inclusion in clean steels

Abstract Predicting the maximum inclusion size in a large volume of clean steel from observations on a small volume is a key problem facing the steel industry. The maximum inclusion size controls fatigue behaviour and other mechanical properties. A statistical method based on the Generalized Pareto Distribution, which has not previously been applied to inclusions, is described and used to analyse results from two steels with different cleanness levels. The predictions of maximum size in larger volumes are compared with those from extrapolating the log-normal curve which fits the size distribution of inclusions observed on the surface of cold crucible remelted buttons of the steels. If the predicted maximum inclusion size is plotted against increasing volume of steel, there is a continuous increase for the log-normal extrapolation but for the Generalized Pareto Distribution method the curve tends towards an upper limit. This is of great significance to steelmakers because the existence of an upper limit is more in accord with the expectation from steelmaking practice.

Microstructural development during warm rolling of an IF steel

Abstract Wedge-shaped slab rolling was employed to investigate the microstructural evolution of an interstitial-free (IF) steel during warm working in the temperature range 500–800°C. Mean flow stress-strain curves calculated from load-time data of rolling tests reasonably correspond to work hardening and dynamic recovery behaviour. The development of substructures in the deformed material was investigated using optical and electron microscopy. A close correlation was observed between mechanical behaviour and microstructural development during deformation. Microbands in directions of ± 35° with respect to the rolling direction, independent of strain, temperature and initial grain orientations are the most noticeable features in the microstructural observations. The sequences of substructural changes from the appearance of early microbands at very low strains, their development with strain, to the formation of equiaxed subgrains at higher strains and temperatures were followed by TEM.

Comparison of extreme value statistics methods for predicting maximum inclusion size in clean steels

AbstractThe prediction of the maximum inclusion size in a large volume of clean steel from data on small specimens is a key issue for steelmakers and users. The statistics of extremes has recently emerged as a powerful tool for this purpose. Murakami and coworkers have applied one branch of the theory to steels, based on measuring the maximum inclusion size in a series of areas on the polished surface of the specimen. The present authors have recently reported on the application to steels of another branch of the theory, using the Generalised Pareto Distribution (GPD), and in this paper the two methods are compared using data on oxide inclusions obtained by quantitative image analysis on polished cross-sections. The most important feature of the GPD method is that it predicts an upper limit to the inclusion size, in contrast to the method of Murakami and coworkers and indeed the more basic route of simply extrapolating the log- normal distribution where, as the volume of steel is increased, the size of th...

Microstructural evolution of austenite under conditions simulating thin slab casting and hot direct rolling

Abstract The hot direct rolling (HDR) of thin slabs introduces some new microstructural phenomena with respect to conventional hot rolling of steels. This paper aims to investigate the microstructural changes of as cast austenite under these conditions. Current laboratory techniques for HDR simulation require a freshly cast slab for every experiment and a perfect link between casting and hot deformation. The present work adopted a new approach; the C–Mn steel is substituted by austenitic Fe–30Ni alloy, Conventional reheating before rolling replaces the direct link. The experimental ingot casting of Fe–30Ni alloy resulted in a solidification structure in good agreement with that of thin slabs of C–Mn steels. From metallographic observations, a mixed softening process and a strong grain refinement and homogenisation characterise the microstructural changes during HDR simulation. The microstructural behaviour and the grain refinement measured for the Fe–30Ni alloy is closely comparable with that predicted for C–Mn steels for the same conditions. The steel substitution appears to constitute a suitable and advantageous experimental approach for HDR simulation.

Maximum inclusion size in two clean steels Part 1 Comparison of maximum size estimates by statistics of extremes and generalised Pareto distribution methods

Abstract Estimation of the maximum size of inclusions in a large volume of steel is an important issue in relation to clean steels. Statistics of extremes can be used for the estimation, and one such method, developed by Murakami and coworkers, is now used by the steel industry for characterising steel specimens. An alternative method, based on the generalised Pareto distribution, has recently been developed and has advantages. A key factor in such methods is the extent to which they can discriminate between steels of different cleanness levels. In the present paper, this discrimination is demonstrated using data from polished metallographic specimens. The confidence limit intervals are estimated using the maximum likelihood method.

From trial and error to computer modelling of thermomechanical processing

Abstract Since the 1950s, thermomechanical processing in the iron and steel industry has progressed from being skills based, using results from trial and error development work, to being science based, using computer modelling for process optimisation and control, and prediction of product properties. Some of the steps in this evolution are illustrated in the lecture, using hot rolling, as the main example, because The University of Sheffield has been involved in research in this field over the whole of the period. As basic understanding of the physics of the microstructural changes, and the speed of computing have increased, there has been a continuing trend for more complex models to be used for offline optimisation and for online control and property prediction. Such models are now accepted as an economically valuable tool for thermomechanical processing.

Maximum inclusion size in two clean steels Part 2 Use of data from cold crucible remelted samples and polished optical cross-sections

Abstract For two different steels, the characteristic size of the maximum inclusion in a large volume has been estimated by the generalised Pareto distribution (GPD)method using inclusion data from two measurement procedures: first, optical metallography of polished sections, and second, scanning electron microscopy of the surfaces of cold crucible remelted samples. The confidence intervals were determined using the maximum likelihood method. The results from the two data sources are consistent, which is an important conclusion for practical application of the GPD method.

Quantitative characterisation of substructural development during warm working of an interstitial free steel

Abstract A Ti containing interstitial free steel was warm rolled in the temperature range 500–800°C, using wedge shaped slabs to produce a range of strains in a single rolling test. Some plane strain compression tests, under similar conditions, were carried out to obtain accurate stress–strain data. Variations in substructural features including subgrain sizes, subgrain aspect ratios, and misorientations between subgrains were quantitatively measured by TEM. Close correlation was observed between mechanical behaviour and variations in the substructure at different temperatures. At the lower temperature (500°C), the material showed cold worked characteristics, but as the deformation temperature was increased the effects of recovery became more pronounced, and hot working behaviour was obvious in the flow stress as well as in the substructural observations at 800°C.

Static restoration processes in warm rolled interstitial free steel

Abstract Microstuctural changes after warm rolling of an interstitial free steel have been studied by annealing deformed specimens in a salt bath in the ferrite temperature range. Static recovery and recrystallisation were followed by Knoop hardness measurements, optical microscopy, and TEM. The results show that the kinetics of the static restoration processes are systematically dependent on deformation variables. However, occurrence of some precipitation during static restoration affects the kinetics of the process and the extent to which they proceed.

Effect of conditions of finish rolling of steel rod on mechanical descalability

Abstract The effects of position of specimens in coils of low carbon steel rod, laying temperature, cooling conditions, aging time, relative humidity and temperature on scale cracking and removal behaviour in bending tests of mechanical descalability have been studied. It was found that laying temperature has a larger effect on scale thickness than cooling rate. Thicker scale results in better descalability. Aging time under ambient conditions after rolling results in little systematic effect on descalability, but considerable variability is attributed to variability in ambient relative humidity. Aging under controlled humidity conditions showed that low humidity is beneficial to descalability and that aging for more than 5 days at ambient temperature has little further effect. Higher aging temperature with constant high relative humidity has a small, but beneficial effect on descalability.