Poor metallurgical quality of steel has the greatest impact on the quality of steel. Some defects may even lead to the scrapping of steel, and some defects may cause rolling failures. The quality of steel billets directly affects product quality, metal consumption, and mill operation rate. High-speed wire mills have strict requirements on the composition of steel due to the large number of rolling passes, high rolling speed, controlled rolling, and controlled cooling. Many metallurgical quality defects of steel billets cannot be judged by macroscopic inspection. Controlling the metallurgical process, improving the quality control capabilities of smelting, casting and other processes, ensuring tooling, and ensuring the process are the final ways to ensure the quality of billets.
Therefore, in organizing the production of high-speed rolling mill wire, the quality of steel billets should be grasped from the smelting process. It is necessary to put forward control quality requirements for the previous process for different varieties, and reflect these requirements in the process specifications of each process.
In order to enable steel mills that use medium frequency furnaces as their main production equipment to produce steel billets that meet the requirements without being restricted by the quality of scrap steel, our company independently developed the IF+LOD+LRF steelmaking process to make up for the lack of refining capacity of the medium frequency furnace itself.
Requirements for smelting molten steel
- Accurately control the content of C, S, H2, O2, N2, and P in the molten steel. The content fluctuation of major elements such as C, Mn, and Si should be small, and the fluctuation of carbon generally does not exceed 0.04%. At present, my country’s standards stipulate that the allowable fluctuation range of carbon content is 0.08%. The actual quality of some foreign products sampled in the sampling inspection shows that the fluctuation range of carbon content in each batch is only 0.02%.
- Extremely high cleanliness. After smelting, the molten steel must be separated from the slag. First, the slag must be blocked or the slag must be removed. Steel and slag are not allowed to be flushed into the ladle at the same time. Slag inclusion will cause inclusions.
- Small temperature fluctuation range, such as superheat less than 15 degrees Celsius.
In order to achieve the above requirements, ladle refining is often used. Ladle refining is also called off-furnace refining or secondary steelmaking. Secondary steelmaking is a means of providing clean steel.
The main functions of secondary steelmaking
- Even the chemical composition and temperature of molten steel;
- Remove non-metallic inclusions;
- Control the morphology of non-metallic inclusions;
- Deoxidation and desulfurization;
- Dehydrogenation (denitrogenation);
- Carbon addition and alloying;
- Heating molten steel; deoxidation or vacuum decarburization.
These functions can be completed separately or in combination by stirring, spraying, vacuum treatment and heating.
- Stirring Blow inert gas argon (Ar) into the ladle to stir the molten steel, or electromagnetic stirring to even the chemical composition and temperature, and induce inclusions to float and gas to escape.
- Spraying Use a spray gun to spray powdered deoxidizer and alloying material into the ladle, or put alloy wire or alloying material (granular) into the ladle while stirring with gas to achieve the purpose of deoxidation, desulfurization and alloying.
- Vacuum treatment places the ladle in a vacuum chamber to reduce the partial pressure of gas to induce the escape of harmful gases.
Discussion of secondary steelmaking must also involve injection protection, which plays a great role in controlling the cleanliness of molten steel by preventing secondary oxidation and nitrogen absorption of molten steel. In the production of ultra-clean molten steel, the role of injection protection is more prominent.
The following points should also be noted during the transfer and injection of molten steel
- Slag-free steelmaking – strictly control with plug rods, slag balls, siphons or other methods to ensure that the molten steel poured from the steelmaking furnace into the ladle is absolutely slag-free.
- Tundish – plug rods and slag dams to block slag.
- Ladle, tundish and crystallizer – protective slag and inert gas liquid surface protection are applied to the surface of the molten steel. These are absolutely important means for preparing clean steel.
