Research regarding oxygen content reduction in steels

Actual paperwork reflects obtained results of technological analyse of reducing oxygen content in steel for pipe manufacturing, enterprised in a steel furnance equipped with a triple aggregate EAF-LF-VD. Oxygen content was measured during manufacturing process in Loading Furnance and Vacuum Degasing and has been analysed in comparison with several parameters. The results are presented using Matlab through regression surfaces and correlation equations and highlights the influence of analysed factors on oxygen content variation in steel.


Introduction
Steel deoxiding is made by three methods: deoxidation by precipitation, deoxidation by diffusion and vacuum deoxidation.
Main characteristics of deoxidizing treatment is bubbling molten steel bath with argon.The scope is homogenizing molten bath by non-metallic impurities and gases removal together with oxygen, hydrogen and nitrogen.Excess H2 may cause pinhole formation which conduct to porosity in solidified steel.Lower levels of H2 causes blistering which conducts to losses of tensile strength.Excess N2 in solidified steel causes formation of blow holes and nitrides that conducts to embrittlement of heat during welding of steels and also affects cold formability of steel.Oxygen removal refers to deoxidation.After carbon monoxide consolidates with oxygen is removed in the similar way as hydrogen and nitrogen from molten steel bath.By removing oxygen steel cleanliness is improved [1], [2].
Oxygen content reduction must be correlated with influence that it has on steel quality, apreciated through qualitative charactheristic values, repectively by comparing with technical practice.
In case of steel deoxidation by precipitation, the minimum oxygen content that ca be in equilibrium relation with deoxidizing element depends on the following factors [3], [4], [5]: temperature of metallic bath, thermodynamic acivity of deoxidizer and thermodinaic activity of deoxidizing element.When complex deoxidizers are used, complex compounds are formed as deoxidation products in which the thermodynamic activity of each deoxidizer is less than one.Deoxidizers must have specific weight and large granulation to be able to penetrate molten steel bath and react with steels dissolved oxygen.A good practice to reduce loss of deoxiding elements is adding them to the bottom of the ladle as well when stell is casted in steel jet.Another technological solution is immersing the ferroalloy in steel bath as wires.
If in case of steel deoxidations with manganese and silicon, deoxidation products cannot be formed by homogeneous germination but only by inhomogeneous germination, in case of aluminium deoxidation process sufficient heat is released therefore homogeneous germination is present [6], [7], [8].Although small dexidation products are formed, they are advanced removed form molten bath to high interphace tension.Nevertheless sufficient small particles remain in steel bath that act as crystallization centers and result in fine austenite grains.Combined deoxidation can be achieved by simultaneous addition of simple deoxidizers or addition of a complex deoxidizer, the effect being higher in the second case because as favorable conditions are created for complex products formation (higher decantation capacity) [1], [6], [9].
Vacuum deoxidation is one of the highly efficient steel deoxidation processes with multiple applications in high purity steel manufacturing.The scope of vacuum deoxidation is oxygen transfer, dissolved in molten bath in gas phase, based on autodeoxidation reaction with carbon [1], [8], [10].The fact that on low pressures carbon becomes a strong deoxidizer determins that vacuum deoxidation in steelmaking practice as being a particular importance by reducing consumption of ferroalloys, reducing elements loss through combustion and volatilization, simultaneously with sample casting duration, as well as an improvement in purity [11], [12], [13], [14].
In industrial practice it is recomended that for completion of deoxidation by precipitation, steel alloying to be done after vacuum treatment.In this way dissolved oxygen is mostly removed in CO form, also when ferroalloys are added deoxidation and alloying, steel is practically deoxidized and therefore a very small amount of non-metallic inclusions are formed.Vacuum deoxidation is potentiated by the fact that metallic bath is constantly stirred by bubbling with argon (electromagnetic stirring) which is favouring homogeneity of metallic bath both thermally and chemically.
In terms of oxygen content (total and dissolved) steel quality manufactured in vacuum degasing aggregates depends on the following factors: vacuum total duration, vacuum duration under advanced vacuum (pressure desirably under 1 torr), pressure in vacuum aggregate, initial-final steel temperature in vacuum aggregate, pressure and flow rate of argon used for bubbling.

Experimental researches
Experimental research contains in analyzing 22 steel samples ST52-A hot rolled steel round bar with diameter 280mm.Deoxidation is being made by all three deoxidizing methods: deoxidation by precipitation (evacuation from EBT), deoxidation by difusion with sinthetic slags in LF and VD, deoxydation under vacuum.This study is focused on VD deoxidation therefore the scope is obtaining correlations between total oxygen content after vacuum (independet parameter) and the other vacuum parameters taken in consideration: vacuum duration, vacuum duration on advanced vacuum, initial and final steel temperature on vacuum degassing process, temperature reduction during vacuum, argon flow and pressure in vacuum aggregate.
The results were analyzed using Matlab program obtaining 8 double correlations, represented graphical and analytical.By analyzing from technological point of view the influence of vacuum parameters it is found a decrease of oxygen content.As a result by technological analyze can be established optimal variation domains for technological parameters by obtaining a lower oxygen content.Following is presented technological analyze for the 8 groups of double correlations.As reference value for oxygen content was chosen 20 ppm.
For graphical and analitical representations were used following notations: -T -total vacuum duration (min); -tv -advanced vacuum duration (min); -pv -pressure on advanced vacuum duration (mBar); -pAv -Argon pressure during bubbling treatment (atm); -∆T -temperature reduction during vacuum degassing ( 0 C); -Ti -initial temperature at the beginning of vacuum degassing ( 0 C); -Tf -final temperature at the end of vacuum degassing ( 0 C); -Q -Argon flow (l/t.min).Multiple correlations processed in Mathlab were established using the same parameters as in previous case of simple correlation.In that reason is used the equation ( 1). (1) In figures 1-8 are presented regression surfaces and contour lines for analyzed parameters.IOP Publishing doi:10.1088/1742-6596/2714/1/0120246 differences of -1,6ppm, 1,0ppm and -0,3ppm [O]fVD by allowable reference value of 20ppm which is accepted as maximum allowed value is 26ppm.
-for correlation 8, [O]fVD=f(∆T, Q) represented in fig.8 shows that very good results are obtained for temperature reduction between 40 -70 0 C and vacuum duration under advanced vacuum between 12 -20min.From coordinate points A(52; 2,7; 20,60); B(52; 2,7; 21,40) and C(52; 2,7; 20,80) shows differences of 0,60ppm, 1,40ppm și 0,80ppm [O]fVD by allowable reference value of 20ppm which is accepted as maximum allowed value is 26ppm.From all presented correlations, it can be chosen values for independent parameters, respectively the range of variation, so that for the final oxygen content (dependent parameter) to obtain a closer value to the one established as reference, of 20ppm in this case.
Analyzing from technological point of view the influence of vacuum parameters is observed an amplification of oxygen content reduction effect.
Obtained correlations have industrial practice applicability by allowing and correlating independent parameters, so as obtaining semi-finished casted products with the lowest possible oxygen content and concomitant lowest H and N content.
Input and output steel temperature measured on vacuum aggregate, as well as temperature reduction during vacuum degasing treatment represent another three important parameters that have an important impact on steel quality.It is observed during experiment that a input steel temperature closer to the upper limit results in a lower oxygen content in steel.Technological explanation is that by ensuring optimal fluidity conditions for oxygen diffusion by bubbling with argon the molten steel bath and in that reason removing the inclusions.Obtained correlations are representative by the point of view of regression coefficients and also technologically.Desirably the temperature should be between 1660-1700 0 C.
For obtaining good results output temperature desirably should be between 1600-1630 0 C.

Conclusions
By analyzing results of experimental research regarding vacuum effect on steel deoxidizing it can be concluded that by processing the data with Matlab program were obtained correlation equations and technological parameters of vacuum and oxygen content from semi-finished steel product were represented in by technological and analytical point of view.Obtained values for oxygen contented resulting from correlation equations are located in proportion of 91,66% below 21ppm (maximum allowed content 26ppm), confirming validations of obtained data on one side and on the other side the possibility of using this procedure in current practice.Obtained correlations are useful for current practice because they allow choosing and correlating independent parameters so that obtaining semi-finished steel products with as much as reduced oxygen content (and by the same time reduced content of H and N).
By analysed correlations resulted that very good results for oxygen content were achieved on vacuum durations between 22 -28min, vacuum duration under advanced vacuum between 15 -17min, temperature reduction during vacuum treatment with values between 45 -70 0 C, argon flow between 2.5 -3,5l/t.min the obtained oxygen content have values between 18.2 -23.48ppm.