COMPARATIVE ANALYSIS BETWEEN CUTTING NOZZLES FOR A CONTINUOUS CAST OXYCUT MACHINE

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COMPARATIVE ANALYSIS BETWEEN CUTTING NOZZLES FOR A CONTINUOUS CAST OXYCUT MACHINE*

Allison de Sá Barreto Ferreira1

Neiclésio Ibiapina2

Vinícius dos Santos Gonçalves3

Rafaela Dutra Boaventura4

Dimereis José Filho5

Alexandre Alves de Mendonça6

Abstract

Oxyfuel is a technique widely used in most industries for cutting steel due to the fact that it has the lowest cost, and its configuration is quick and simple. The cutting nozzle is one of the main components of this process, the cutting oxygen speed, its outlet pressure and the shape of the jet are directly related to the geometry of the cutting nozzle. The objective of the work is to compare, through computer simulation, analytical calculations and experimental tests, two cutting nozzles for the Sinobras Continuous Casting Oxyfuel machine, aiming to choose the one that will bring higher quality with lower cost for the cutting process. The difference between the speeds was proportional to the two analyzes, analytical and numerical, an increase of approximately 30% in the speed of the cutting oxygen with test nozzle. The test nozzle range, according to the numerical simulation, was also 50mm higher. However, there were no significant changes in the quality of the billet cut in the hypothesis test performed. There was a reduction in gas consumption, but in relation to the cost of the test nozzle, standardization is not feasible.

Keywords: Cutting Nozzle; Continuous Casting Oxyful Machine; Computer Simulation.

1. Mechanical Engineering, Graduate, Mechanical Engineer, Steelmaking, Siderúrgica Norte Brasil (Sinobras S.A), Marabá, Pará, Brazil.
2. Materials Engineering, Graduate, Process Engineer, Steelmaking, Siderúrgica Norte Brasil (SINOBRAS S.A), Marabá, Pará, Brazil.
3. Mechanical Engineering, Graduate, Mechanical Engineer, Maintenance, JBS S.A. Marabá, Pará, Brazil.
4. Mechanical Engineering, Graduate, Mechanical Engineer, Steelmaking, Siderúrgica Norte Brasil (Sinobras S.A), Marabá, Pará e Brazil.
5. Materials Engineering, Graduate, Continuous Casting Leader, Steelmaking, Siderúrgica Norte Brasil (SINOBRAS S.A), Marabá, Pará e Brazil.
6. Metallurgical Engineering, Master's, Steelmaking Manager, Steelmaking, Siderúrgica Norte Brasil (SINOBRAS S.A), Marabá, Pará e Brazil.

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1 INTRODUCTION

Continuous steel casting is the process used to solidify approximately 750 million tons of steel produced worldwide each year [1], and in 2004 it was responsible for 92.7% of the production of 32.9 million tons of steel gross in Brazil [2]. The continuous casting process aims to solidify the liquid steel that comes from the secondary refining and presents considerable advantages, among which we can mention: lower operating cost, higher yield, greater flexibility of operation, and higher quality of the final products [3]. The most important application of the continuous casting process refers to the production of slabs and billets, constituting an essential part of the modern steel industry.

Right after the last roll of the continuous casting machine, the oxyfuel machine is located, which cuts the billet into lengths normally 12 meters. The cutting of the billet occurs due to the melting of a thin layer of steel due to the heating caused by the chemical reactions between the iron and the oxygen [4]. Several factors are important to ensure quality in billet cutting and reliability for rolling it, for example: quality of cutting oxygen, pressure of cutting and preheating gases, temperature of billet, type, distance and condition of the nozzle cut, among others.

Oxyfuel is the technique used in most industries for cutting steel since it has the lowest cost, and its configuration is quick and simple [5]. The cutting oxygen gas causes a chemical oxidation reaction with the base metal at high temperatures, expelling the liquid metal from the site of interest at high speeds. The ignition temperature is maintained by a previously heated flame that is the source of the combustion of the combustible gas with pure oxygen. This reaction is strongly exothermic, generating a high amount of heat that can sustain the process itself in the sequence. Figure 02 shows a generic scheme of the process [6].

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Figure 01: Scheme of operation of the oxyfuel process. Source: Guerra, 2015 [7].

The correct selection of the nozzle for the process is very important. The design of the nozzle, in particular the design of the orifice, determines the shape of the gas jet and therefore the quality of the cut. If the nozzle diameter is large, the cutting gas consumption increases and provides an insufficient gas flow to expel the molten material. When the nozzle diameter is small, the cut will have its quality reduced, resulting in burrs at the bottom and may sometimes not be completed [8].

The nozzles have three different sections - convergent section, throat and divergent section. The point where the nozzle diameter is the smallest is called the throat. The throat can be a single point or it can be elongated. The upstream section of the throat is the converging section and the downstream section of the throat is the diverging section. The area of the converging section decreases as the nozzle profile goes from the tube to the beginning of the throat. The area of the diverging section increases as the nozzle profile extends from the end of the throat to the tube.

The objective of the work is to compare through computer simulation, analytical calculations and hypothesis testing of two cutting nozzles for the Sinobras Continuous Casting Oxyfuel machine, aiming to choose the one that will bring higher quality with lower cost for the cutting process.

2 FLUIDODYNAMIC MODELING FOR A CONVERGENT-DIVERGENT CUTTING NOZZLE

The two nozzles analyzed are of the convergent-divergent type, also known as Laval. In Figure 02. This modeling was performed based on the book by Zucrow & Hoffman, 1976 [9] and Zuker & Biblarz, 2002 [10].

When the flow is treated as compressible, the dimensionless number of Mach (Ma), which relates the local speed (v) in the flow with the speed of sound in the medium (a), is an important analysis parameter. Saad (1993) [11] explains that the flow passage area decreases in the converging section to a minimum cross section, known as the throat, with an acceleration of the fluid to Mach equal to 1 and an increase in pressure. Then, when the divergent section increases to the exit, there is a decrease in pressure and an increase in speed to supersonic levels. The supersonic flow at maximum pressure is reached at the point of the critical area, at which point the speed is equal to the critical speed of sound, with the terms "*" to characterize this region.