Secondary refining is the process of transferring part or all of the refining tasks completed in conventional steelmaking furnaces, such as removing impurities, adjusting and homogenizing composition and temperature, to a ladle or other special container for refining. It is also called secondary refining or ladle metallurgy.
What is the purpose of refining outside the furnace?
The purpose of refining outside the furnace is to carry out deep decarburization, desulfurization, deoxidation, degassing, adjusting composition, microalloying and adjusting temperature and making it homogenized, removing inclusions, changing the morphology and composition of inclusions, etc. under vacuum, inert atmosphere or controlled atmosphere.
Refining of molten steel outside the furnace is to adapt to the improvement of steel quality, production of new steel grades and rationalization of production process, and to meet the strict requirements of continuous casting on molten steel composition, temperature and purity. It is an indispensable process for the connection between steelmaking and continuous casting processes and has become an important link in modern steelmaking production.
In order to create the best metallurgical reaction conditions, the basic means of refining outside the furnace include stirring, slag washing, heating, vacuum, blowing, etc.
In actual production, one or several means of refining technology combined can be selected according to different purposes to complete the required refining tasks.
What is the basis for selecting equipment for refining molten steel outside the furnace?
The selection of molten steel refining equipment is mainly based on the following.
① The final product produced and its quality requirements.
② The technical parameters and smelting process characteristics of the primary refining furnace, as well as the connection and coordination methods with the off-furnace processing equipment. If the off-furnace processing equipment is added to the old production workshop, the specific conditions of the workshop should also be considered, such as the height of the workshop, the capacity of the lifting equipment, the layout of the original equipment and the process flow.
③ The specific conditions of the location of the off-furnace processing workshop, such as the characteristics and sources of raw materials, energy and its supply methods, transportation methods, etc.
④ The quality requirements of the continuous casting machine for molten steel, and the operation coordination requirements of the converter and continuous casting machine production.
⑤ Economic and social benefits, such as infrastructure and equipment investment, operating costs and production costs, environmental protection and market demand.
How does off-furnace refining technology meet the demand for high-quality steel grades?
The refining technology outside the furnace guarantees the demand for high-quality steel grades from the following aspects.
① Accurately control the chemical composition to ensure stable mechanical properties.
② Reduce the phosphorus and sulfur content in the steel to improve the impact performance, lamellar crack resistance, hot brittleness, and reduce center segregation and prevent surface defects of continuous casting billets.
③ Reduce the oxygen, hydrogen, and nitrogen content in the steel to reduce ultrasonic flaw detection defects, strip cracks, etc., and improve the pipe-making performance of the steel.
④ Use advanced technology to refine the molten steel to meet various special needs for steel quality. For example, control the morphology of sulfide inclusions to prevent hydrogen-induced cracks.
⑤ Control the shape of inclusions to improve the deep drawing performance and processing performance of steel.
What are the process characteristics of off-furnace refining?
The processes of various off-furnace refining methods are different, and their common characteristics are as follows.
The ideal refining atmosphere conditions are usually to stir the molten steel with vacuum, inert atmosphere or reducing gas, which can be done by electromagnetic stirring, inert gas stirring or mechanical stirring; heating facilities are used to compensate for the temperature loss of the molten steel during the refining process, such as arc heating, plasma heating or increasing chemical heat in the molten steel.
What are the metallurgical functions of off-furnace refining technology?
① It can change the metallurgical reaction conditions. The reaction products of deoxidation, decarburization and degassing in steelmaking are gases. Refining can be carried out under vacuum conditions, which is conducive to the forward reaction. Usually the working pressure is >50Pa, which is suitable for degassing molten steel.
② It can speed up the mass transfer rate of the molten pool. The liquid phase mass transfer rate determines the speed of the metallurgical reaction. The refining process uses a variety of stirring forms (gas stirring, electromagnetic stirring, mechanical stirring) to make the melt in the system flow, accelerate the heat transfer and mass transfer process in the melt, and achieve the purpose of uniform mixing.
③ It can increase the area of slag steel reaction. Various refining equipment has a stirring device. During the stirring process, the steel slag can be emulsified. The alloy and steel slag will melt, dissolve and polymerize during the floating process of bubbles. Usually, the slag steel reaction area of 1t of molten steel is 0.8~1.3mm2. When the slag amount is 6% of the original, the steel slag will form a droplet with a radius of 0.3mm after emulsification, and the reaction interface will increase by 1000 times. Microalloying and modification treatment use this principle to improve the refining effect.
④ It can play a buffering role between the electric furnace (converter) and continuous casting. The refining furnace is flexible, which makes the operation time and temperature control more coordinated, forming a smoother production process with continuous casting.
