Ironmaking & Steelmaking: Processes,Products and Applications

The 18^th Annual Conference of the Sheffield Metallurgical and Engineering Association was held on the 7^th and 8^th July 2009 at the Endcliffe Conference Centre, University of Sheffield and attracted 98 delegates. The aim of the conference was to explore recent developments in the manufacture, evaluation and application of alloy steels, nickel-based superalloys and light metal components for critical engineering applications. Five technical sessions focused on nuclear energy, aerospace, energy supply chain and transport applications, and then considered some advanced technologies.

In recent years, as the demand for steels of high added value, such as the ultralow carbon (ULC) steels, the demands on the Ruhrstahl–Heraeus-oxygen blowing (RH-OB) process, have increased. In order to meet these strong demands, the RH-OB equipment at Taranto Works has been revamped, particularly by means of the enlargement of the snorkel diameter. This has led not only to increase in the decarburisation rate due to the improvement of the metal recirculation, but also to an increase in productivity, improved process control and savings in electrical power and process gas consumption.

The 5^th European Rolling Conference was held in London, at the Carlton House Terrace headquarters of the Institute of Materials, Minerals and Mining, on 23–25 June 2009. The conference comprised sixty five oral presentations, with plenary sessions on each of the three mornings followed by parallel sessions covering hot strip, cold strip, long products, aluminium and plate rolling as well as miscellaneous themes. The event was kindly sponsored by Siemens VAI Ltd.

The original remit of this paper was a review of how capital investment in new rolling technology is reducing the variable costs of the metals industry. The whole industry is in the same economic boat at the moment, however, and few of us are going to be persuaded to spend a lot of money to recoup even more. Since we're in this boat, a more appropriate emphasis is to review the technology base we share, and what's being achieved through co-operation. In large part, this common base is not in manufacturing assets, but rather in organisations and in technical communities.

The Italian steel producer Acciaieria Arvedi SpA operates a minimill for flat rolled products with a single casting line (maximum width, 1300 mm) in Cremona, Italy, with an annual output of 1·3 million tonnes per year of flat products using inline strip production (ISP) technology, patented by Arvedi. Following this positive experience with its continuous thin slab casting and rolling process, improved in several stages over the years and resulting in a stable production unit of high quality hot rolled strip, it was decided to invest in a new line adopting endless strip production (ESP) technology, also patented by Arvedi, a direct evolution of ISP. Owing to its highly compact layout with a total length of ∼180 m, lower investment costs are incurred for an Arvedi ESP line compared with conventional thin slab casting and direct rolling plants. Production costs are also noticeably lower than those of other thin slab plants and also of the ISP plant already operating in Cremona. Productivity of the ESP plant, based on a single casting line, will be ∼2 million tonnes per year for the first phase and a possible increase in mass flow from the caster may take capacity up to 3 million tonnes per year. Total energy and water consumption of the Arvedi ESP process is very low compared with that of a conventional casting and rolling process: depending on the final product, energy consumption will be 50–70% lower. Owing to endless rolling operations, the production of strip with uniform and repeatable mechanical properties will be possible along the entire strip width and length and with the line's ability to produce ultrathin hot rolled strip gauges of 0·8 mm (and lower), a subsequent cold rolling process will no longer be necessary for many strip applications. The Arvedi ESP plant is the first example of fully continuous production from liquid steel to hot rolled coil on the downcoiler in 4·5 min, and the Cremona plant is the reference plant (master plant) for sales of future ESP lines. The aim of this presentation is to provide data relative to the new plant and the first production results of the endless casting and rolling line. A contract was signed in 2006 with Siemens VAI as principal process and plant supplier of the new plant, and construction was completed in Cremona in autumn 2008. Start-up was completed in the first half of 2009.

Oxide scale behaviour in thermomechanical processing has been the subject of intensive research for several years that allowed development of a finite element based model to simulate a range of events of relevance to the process and to the surface quality of the hot rolled product. Oxide scale failure is predicted taking into account the main physical phenomena, such as stress directed diffusion, fracture and adhesion of the oxide scale, strain, strain rate and temperature. The most critical parameters for scale failure are measured during modified hot tensile testing and depend on the morphology of the particular oxide scale and scale growth temperature and are also very sensitive to the chemical composition of the underlying metal. Provided that it is validated, the model can also be used for detailed modelling of the microevents during technological operations. The work integrates finite element analysis with a range of experiments each to provide partial insight into oxide fracture, friction, heat transfer, pick-up and descaling, among others. An overview of this research is presented, revealing a variety of phenomena of considerable technological importance.

Experimental and theoretical research on the wire flat rolling process as well as plane strain compression tests have proven that the rolling of small and thin strip out of wire is affected by size effects on yielding due to a changing ratio of grain size to characteristic specimen dimension and by size effects on friction due to the changing share of open lubricant pockets. These size effects have to be taken into account for future optimisation of the wire flat rolling process deployed for the manufacturing of microstrip.

This paper describes the application of a steady state elastic–viscoplastic finite element model to the prediction of strip flatness and out of bite buckling during cold rolling of thin strips. This model is applied here to the last stand of a tinplate tandem cold mill. Results show that strip buckling reshuffles the non-homogeneous strip velocity profile just at the roll bite exit; strip stresses inside the roll bite are little impacted, so that roll stack deformation and roll force profile are not modified. The paper focuses on 'latent strip flatness', i.e. the profile in the transverse direction of the stress in the rolling direction. The profile predicted by the model far downstream the roll bite is in good agreement with shapemeter measurements only if strip buckling (i.e. manifest flatness) just at the roll bite exit and its interaction with the roll bite are considered, using a strong coupling of bite model and post-bite buckling model.

The drive towards higher contribution free cutting steels with improved machinability and higher surface quality and consistency, together with the potential future implementation of the European legislation to promote lead substitutes (Bi, Te and high S), is leading to a requirement for improved understanding and control of as rolled surface quality, most critically for steels which have low ductility and a narrow temperature range for defect free rolling. Ductility breakup for the range of steel investigated (free cutting steel steels) is intrinsically complex with many interactions existing between casting, reheating and rolling. This is especially important during the transition from as cast to wrought structure, i.e. for thermomechanical and microstructural conditions acting in cogging/roughing mills, where the intrinsic reheated cast ductility of some machining steels can be as low as 10%. This paper builds upon previous studies for developing a more physical approach to predict the mechanisms of high temperature ductile damage acting at surface/subsurface and across the length scale of continuously cast free cutting steel billet feedstock during rolling. A methodology combining mechanical testing, modelling and characterisation has been developed to study effect of thermomechanical conditions such as triaxiality including inversion, principal stress, strain, strain path, strain rate and temperature as well as accounting for local microstructural heterogeneities such as MnS inclusions. This methodology has been used to refine the science of nucleation, growth and coalescence acting at high temperature for this range of low ductility steels during rolling. This knowledge has been put into practice with the establishment of regime maps and guidelines to minimise propensity of cracking during industrial rolling as well as being able to design effective schedules and grooves. Issues and future directions in terms of models and experimental technique development will also be highlighted.

The idea of sleeving is rather old, but the presence of permanent tensile stress in sleeves that may lead to cracks handicapped these types of rolls. The developments in mathematical modelling, metallurgy, non-destructive metal testing and other technologies have enabled the production of sleeved rolls with similar or better properties than the solid ones. The paper deals with techniques that allow reduction in the level of permanent tensile stresses, such as computer simulation. Complex design of sleeved rolls including the shape optimalisation of sleeve and arbour, design of chamfers on back-up rolls reducing Hertzian contact stresses and design of roll grinding will be described. A comparison of measured and computed residual stresses in the sleeve after a thermal treatment will be presented. Finally, practical experience with sleeved rolls in hot rolling mills will be discussed.