Analýza atypického průvalu při radiálním plynulém lití bramy v pásmu rovnání II. Analýza příčin průvalu aplikací teorie fyzikální podobnosti

Zone Analysis of Atypical Breakout during Radial Continuous Casting of a Slab in the Straightening Zone II. Analysis of the Causes of the Breakout by Application of the Theory of Physical Similarity

journal article - other

en

The so called secondary cooling zone, which is divided into thirteen sections, is an important part of the CCM. In this zone, specifically in the area of the beginning of slabs straightening, a breakout may occur due to an increase of the local and temperature heterogeneity of steel, due to resulting from an increase of the stress caused by bending of the slab and by high local concentration of non-metallic slag inclusions. Changes of the chemical composition of the steel during continuous casting are particularly dangerous. In the event that two melts are cast one right after another, i.e. if the melt of steel with chemical composition A ends and it is immediately followed by the steel B, it may automatically stop the CCM and an atypical breakout may take place. It happened during continuous casting of the slab 250x1530 mm in the area of straightening, 20 minutes after flying change of tundish. With use of dimensional analysis altogether 8 criteria of similarity were derived according to the Pi-theorem. Table of 12 technological, geometrical and thermo-physical dimensional quantities was compiled for these criteria, which characterise both steel grades A and B and process of their continuous casting. Thermo-physical properties of both steels for the table were determined with use of commercial software IDS. Important data in temperature field of the slab, such as maximum and minimum length of iso-liquidus and iso-solidus, range of the so called mushy zone and zone of melt at the place of breakout, temperature of the slab surface, etc. were calculated by the original model. Five of these criteria are at the same time functions of thermo-physical parameters, but also of technological and geometrical parameters. Numerical values of eight criteria were determined for the steels A and B and then standard operations were made with the criteria, such as evaluation of partial shares of pairs of the same criteria for both steels. This application of the theory of physical similarity clearly proved markedly increased tendency of the steel B to breakouts in comparison with the steel A. In order to prevent repetition of this accident of CCM (caster) for another pair of steels cast immediately one after another, it is necessary to assess the derived individual criteria of similarity for both steels and other operations with these criteria. The way to reduce the risk of breakouts may be found mainly in the change of thermo-physical properties of both steels, consisting primarily of reduction of differences of chemical composition of both melts, so that partial shares of similarity criteria were close to the value of one.

Důležitou částí ZPO je t.zv. sekundární chladicí zóna, která je rozdělena do třinácti sekcí. V této zóně v místě začátku rovnání bramy může dojít k průvalu zvýšením lokální a teplotní heterogenity oceli, zvýšením napětí způsobeného ohybem bramy a vysokou lokální koncentrací nekovových struskových vměstků. Zvlášť nebezpečné jsou změny v chemickém složení oceli během plynulého lití. V případě že dvě tavby jsou odlity těsně po sobě, t.j. když tavba oceli o chemickém složení A končí a okamžitě následuje tavba oceli B, může dojít k automatickému zastavení stroje ZPO a ke vzniku atypického průvalu. Došlo k němu při plynulém lití bramy 250x1530 mm v místě rovnání za 20 min po letmé výměně mezipánve. Rozměrovou analýzou bylo podle PI teorému odvozeno 8 kritérií podobnosti. Byla sestavena tabulka 12ti technologických, geometrických a termofyzikálních rozměrových veličin do těchto kritérií, které charakterizují obě kvality oceli A a B a průběh jejich plynulého lití. Termofyzikální vlastnosti obou ocelí do tabulky se stanovily komerčním software IDS. Modelem byly vypočteny údaje o teplotním poli bramy, např. maximální a minimální délka izolikvidy a izosolidy, rozsah t.zv. mushy zóny a zóny taveniny v místě průvalu, teplota povrchu bramy atd. Pět z kritérií je současně funkcí termofyzikálních veličin ale také technologických a geometrických parametrů. Byly stanoveny číselné hodnoty osmi kritérií pro ocel A i B a provedeny standardní operace s kritérii, např. vyhodnocení parciálních podílů dvojic těchže kriterií pro obě oceli. Tato aplikace teorie fyzikální podobnosti jednoznačně prokázala výrazně zvýšený sklon oceli B k průvalu ve srovnání s ocelí A. Aby se tato havárie ZPO (caster) neopakovala pro jinou dvojici ocelí odlévanou těsně po sobě, je třeba vyhodnotit jednotlivá odvozená kritéria podobnosti pro obě oceli a další operace s těmito kritérii. Cestu ke snížení rizika průvalu lze hledat ve změně především termofyzikálních vlastností obou ocelí spočívající především v omezení rozdílů v chemickém složení obou taveb tak, aby parciální podíly kritérií podobnosti byly blízké hodnotě jedna.

The so called secondary cooling zone, which is divided into thirteen sections, is an important part of the CCM. In this zone, specifically in the area of the beginning of slabs straightening, a breakout may occur due to an increase of the local and temperature heterogeneity of steel, due to resulting from an increase of the stress caused by bending of the slab and by high local concentration of non-metallic slag inclusions. Changes of the chemical composition of the steel during continuous casting are particularly dangerous. In the event that two melts are cast one right after another, i.e. if the melt of steel with chemical composition A ends and it is immediately followed by the steel B, it may automatically stop the CCM and an atypical breakout may take place. It happened during continuous casting of the slab 250x1530 mm in the area of straightening, 20 minutes after flying change of tundish. With use of dimensional analysis altogether 8 criteria of similarity were derived according to the Pi-theorem. Table of 12 technological, geometrical and thermo-physical dimensional quantities was compiled for these criteria, which characterise both steel grades A and B and process of their continuous casting. Thermo-physical properties of both steels for the table were determined with use of commercial software IDS. Important data in temperature field of the slab, such as maximum and minimum length of iso-liquidus and iso-solidus, range of the so called mushy zone and zone of melt at the place of breakout, temperature of the slab surface, etc. were calculated by the original model. Five of these criteria are at the same time functions of thermo-physical parameters, but also of technological and geometrical parameters. Numerical values of eight criteria were determined for the steels A and B and then standard operations were made with the criteria, such as evaluation of partial shares of pairs of the same criteria for both steels. This application of the theory of physical similarity clearly proved markedly increased tendency of the steel B to breakouts in comparison with the steel A. In order to prevent repetition of this accident of CCM (caster) for another pair of steels cast immediately one after another, it is necessary to assess the derived individual criteria of similarity for both steels and other operations with these criteria. The way to reduce the risk of breakouts may be found mainly in the change of thermo-physical properties of both steels, consisting primarily of reduction of differences of chemical composition of both melts, so that partial shares of similarity criteria were close to the value of one.

Ocelová brama, kontinuální lití, chemická a teplotní heterogenita, průval, bezrozměrná čísla

Steel slab; Continuous casting; Chemical and temperature heterogeneity; Breakout; Dimensionless number.

2013

04.07.2013

0018-8069

LXVI/2013

3

17–25

9