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23.03.2016

"Under the High Star". NTZMK develops import substitution technologies

"Lakhta Center" is a unique and innovative project under a variety of indicators. The height record for the Russian and original architecture solutions of the skyscraper require the use of the most advanced construction technologies and materials, borrowing of the advanced foreign experience of building construction.

The application of the steel grades new for the Russian market is one of the innovations of the project. So, to produce high-strength columns and beams for the "Lakhta" Multipurpose Center, NTZMK will have to master the technologies how to work with unique HiSTAR® 460.

The HiSTAR® grade has a little more than 20 years of the history. Anyone who knows a little English would decide that the name is translated into the Russian language as “high star”, in fact it is a complex abbreviation of HIgh STrength ArcelorMittal. i.e. the name of one of the largest transcontinental steel companies in the world. For the first time, Arcelor Mittal produced its low-alloyed grades of structural steels with the yield point of 355 and 460 miliPascal (MPa) under HiSTAR® for the design and construction of buildings and structures in 1993. Over the past two decades HiSTAR has proven its highest performance in a number of major projects, making it a truly "high star". This low alloy steel has earned its “high strength” reputation in a special building: in the construction of earthquake-resistant structures, sea platforms, including in the Far North.

Head of the NTZMK Central Laboratory A.Iu. Bushuev speaks of the peculiarities of HiSTAR steel:

In the building almost half a kilometer high, what Lakhta will be, each load bearing frame will have to simply bear enormous loads only rugged steel can cope with. There are actually not so many ways to create new grades of high-strength steel.

Up to a certain point, increase of the percentage of alloying elements in the steel was a key method to improve it in our country. This made it possible to get grades with excellent properties. However, chromium, nickel, molybdenum, titanium used in this case are rare and expensive chemicals, the use of which considerably increases the cost of the materials, so high-alloyed steel has already been left in the history.

The second method of steel strengthening is improved overall chemical composition (decrease in the content of harmful impurities such as phosphorus or sulfur) and improved steel structure in terms of rolling thickness. It was the way chosen by the manufacturers of HiSTAR. Its chemical composition is not unusual. But it is very clean, free from unwanted impurities, the steel which production technologies have allowed it to significantly reduce the possibility of such problem as large thickness, as structural heterogeneity when mechanical properties of the surface and deep layers may significantly differ.

ArcelorMittal says that "excellent weldability of HiSTAR steel without the need for pre-heating" is one of the most important arguments of their superiority over similar grades of steel. This property in 1998 was confirmed by the American Welding Society after the successful docking of two HiSTAR 460 brand large-sized profiles (400x1086 kg/m) by unheated automatic welding. To perform such docking, the laboratory welder had to perform 140 sets, but the resulting seam meet all quality requirements.

NTZMK engineers look skeptical at such extreme experiments.

Deputy Chief Technologist - Chief Welding M.Iu.Koltunov says:

- HiSTAR460 weldability ratio lies on the border of the indicator, which allows welding steel without heating. Given the substantial thickness of rolling, with which we have to work on this project, we cannot do without a set of additional measures for the thermal preparation of metal for welding. If you start welding structures made of steel 460 without warming up to its pre-design temperature, there is a high probability of cracking.

Another feature of this brand is that it requires not only carefully-designed welding technology, but also strict adherence to this technology. To get to a high quality welded seam on HiSTAR, the process must be continuous and continuously monitored. The welder simply has no right to leave work halfway and go for lunch: this can lead to the most serious defects in the weld and the entire structure as a whole.

I must say that the complexity of the manufacture of steel structures for "Lakhta" lies not only in the brand properties of steel, but also in the design features of nodes that we have to make. The solutions applied by designers give a rise to a number of difficulties for welding and non-destructive testing: closely spaced stitches that can create a dangerous violation of metal structures and knots tied in a tight loop, which is fraught with excessive internal stress and de-formation of constructions, etc.

NTZMK experts did not exaggerate talking about the difficulties of working with innovative HiSTAR in Russia. In a very short time engineers and factory workers will have to resolve in practice the issues the theoretical understanding of which only begins in our country.

- Up to date, specifications and technical documents for the construction do not contain a single example of assessment of stitches with thicknesses over 60 mm. We have to weld thicknesses up to 120 mm for Lakhta. Especially for this project, SIC "Construction" of CNIISK named afterVA Kucherenko - one of the key research and development institutions in our country - developed special technical regulations the effectiveness of which we still have to check in practice. While working on the Sochi stadium, it seemed that nothing could be more difficult - now I doubt it - concludes M.Iu.Koltunov.

"Lakhta Center" is not the first project in Russia with the use of HiSTAR460. Back in 2005, it was applied in the construction of three business center skyscrapers "Moscow City". Over the past 10 years more than 20 thousand tons of HiSTAR metal structures have been installed in this project. However, since Russian companies lack experience in working with this innovative material, all designs were imported to the construction site as a finished product after their manufacturing by Turkish contractors. The underlying principles of the guidelines of the Russian industrial policy - the import substitution concept - requires the development of this technology by domestic producers of building materials. NTZMK will be the first one to develop it.