On the “oxide metallurgy” technology

 The article "Smelting of steel with specified characteristics of nonmetallic inclusions. Review" devoted to "oxide metallurgy" problems had published in Russian “Electrometallurgiya” journal, 2 (2009). You can find the paper on English at page «“Library”.

A. Alexeenkp1 and D. Ponomarenko2
1 Lasmet Co. (Laboratory of Special Metallurgy)
2 IBMT Co. (Innovation Bureau of Metallurgical Technology)

Summary:

Some steel grades, e.g. rail, cord, deep drawing, pipe steels, etc. are strictly controlled on non-metal inclusions characteristics (composition, size, quantity per unit).
It is necessary to guarantee a good workability during manufacturing and high mechanical properties of products. They are, for instance:
- good metal castability (especially of Al-killed steels);
- breakless wire drawing of cord steels;
- defectless cold rolling and deep drawing of LCAK-, ULC-, IF-steels;
- high level of mechanical properties, contact fatigue resistance (of rail and wheel steels), and pitting corrosion resistance (of pipeline steels).
It is known that the influence of nonmetallic inclusions on steel properties may be direct and indirect.
Direct detrimental effect of inclusions is connected with breakage in wire drawing, appearance of surface defects during cold rolling or deep drawing, pitting growth, decrease of contact fatigue resistance, etc.
Indirectly, through interconnection with the microstructure, inclusions influence steel mechanical properties. For example, inclusions of a certain composition evenly dispersed through the bulk of metal facilitate grain size reduction and consequently to increase of strength and toughness.
In some cases, the “oxide metallurgy” methods are permit to exclude a heat treatment after forging or considerable increase strength and toughness of heat affected zones (HAZ).
The oxide inclusion control pursues the tasks: a) to limit maximum inclusion size (it may vary from 15 to 100 µm for different steel grades) and b) to ensure presence of microinclusions of required types only and restricted in number per unit.
Macroinclusions observed in crude steel as a rule are products of microinclusions coagulation or reoxidation. Also they may have exogenous nature, e.g. refractory or slag particles. Large deoxidation products are successfully removed during secondary steelmaking operations, therefore they are usually absent.
Measures to decrease contamination of continuous casting billets or slabs with macroinclusions are given in the article.
Microinclusions have another origin than macroinclusions. They nucleate and grow in melt during secondary treatment and casting or during crystallization process.
For manufacture of steel with specified non-metallic inclusions characteristics, it is necessary to provide control of inclusions nucleation and transformation processes during secondary steelmaking. Such control is very important for preparation of ‘clean’ steel free from detrimental non-metallic inclusions before the ladle is sent for casting. Besides, for aluminum non-killed steel grades (e.g. cord steel, rail steel, wheel and tire steel, etc.) it is important that a certain chemical composition of metal before casting would ensure minimal amount of crystallization inclusions, their composition and dispersion being optimal.
It is known that microinclusions are removed from metal bath much worse than macroinclusions. Therefore the minimization of microinclusions nucleation during secondary steelmaking (especially during deoxidation) is important for ‘clean’ steel production. Main ways of this task decision are considered in the article.
Another actual problem is composition control of inclusions in aluminum non-killed high quality steel grades. The chemical interaction of oxide inclusions with liquid aluminum non-killed steel has one peculiarity. It is a strong variation of inclusions composition due to small change of amounts of high oxygen affinity elements or changing of oxygen activity in the melt.
The influence of steel chemical composition, temperature and pressure above the melt during secondary steelmaking on composition and amount of oxide inclusions has a complex character. The computer thermodynamic simulation permits to take into account the most significant relationships in the “melt – oxide inclusions” system and influence of main factors on the occurring conversions. One of thermodynamic models developed for that purpose is represented on web-site http://www.steelmaker.ru. The internet version of the program “Oxide inclusions” is available ibid.
This program was used for simulation of inclusions transformation during secondary treatment of various steel grades. The investigations were executed at steel plants of such companies us CherMK, ZapSib and Amurmetal. The received computation results in combination with SEM tests of the probes have permitted to considerably increase efficiency of investigations and to improve secondary treatment technologies.
Application of the program in combination with GIBBS® melt control system gives even more advantages. In this case the control of inclusions characteristics during secondary steelmaking and casting is a real task.
GIBBS® control system is based on models of liquid metal treatment (which are developed for specific steel grades) and an application program package to perform thermodynamic computation, material balance and energy balance computation taking into account peculiarities of the aggregates.
The precise thermodynamic computation of “metal – slag – gas” system taking into consideration kinetics of real processes is the basis of GIBBS® models package. It should be noted that interactions in the “metal – slag – gas” system are essential to all steelmaking processes.
After adjustment for a particular steelmaking unit, the GIBBS® package precisely computes the whole complex of mass- and heat transfer processes. It permits to simulate the technological process course and to predict changes of all basic parameters of a melt from its start to the finish, such us melt temperature and chemical composition of melt, slag and waste gas.
The operation of the secondary steelmaking GIBBS® control system at Belorusskiy metallurgical works (RUP “BMZ”, Zhlobin city) has shown a high accuracy of prediction of steel chemical composition and temperature. It ensured trouble-free work in the automatic process control mode. Implementation of the system has allowed to stabilize the process and improve its monitoring.
Precise design of the trajectory of steel chemical composition and temperature change during secondary steelmaking and casting as well as the possibility of reliable application of the worked out technology in practice thought the instrumentality of GIBBS® control system, gives way to effective control of inclusions characteristics in such steel grades as cord, rail, pipe steel, etc.
In the basis of an on-line computation of current inclusions composition and amount in liquid steel there lies a simulation of “metal – slag – gas – inclusions” system changing. It’s executed for a particular steel ladle during all secondary treatment from taping up to casting.
During this simulation, the control system also computes the amount of inclusions removed from the melt.
The available knowledge base and simulators of metallurgical processes permit to considerably increase both efficiency of steelmaking process design and reliability of the process implementation by means of nonmetallic inclusions control in the course of secondary treatment and casting.

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