The molten steel is injected into the mold through the ladle and solidified to form a steel ingot. After the molten steel is smelted in the steelmaking furnace, it must be cast into a certain shape of the ingot or billet for processing. The process of casting a steel ingot by a mold is simply referred to as an ingot.
Steel ingots are still the main raw material for rolling steel production. The quality of the ingot, the condition of the ingot type and its weight play a very important role in rolling steel production. In the steel rolling production, in addition to various types of roughing mills, steel ingots can be used as raw materials. Some special-purpose rolling mills and some medium-thick plate mills also use steel ingots as raw materials. In many local small and medium-sized enterprises, the mills for rolling mills also use small steel ingots as raw materials. . The selection of ingots is mainly to correctly select the type of ingot, its weight, and the shape and size of the section.
Steel ingots can generally be divided into the following categories:
The ingots have only undergone the most basic (lower limit) deoxidation treatment. When the ingot is solidified, a layer of metal close to pure iron is formed around the bottom and bottom of the ingot mold, and carbon, sulfur and phosphorus are deflected near the central shrinkage hole. Oxygen in the metal creates a carbon monoxide roll that creates pores in the metal, but the pores disappear during the hot rolling process. The advantage of uncleaned steel is that it can obtain several flawless steel surfaces - the surface is pure iron. Most of the unpurified steel has a carbon content of less than 0.1%.
Cap steel (capped steel)
The oxygen in the liquid causes the deflection of the steel ingot near the center of shrinkage to be lower than that of the unpurified steel, the surface is beautiful, the internal chemical composition is uniform, and the mechanical properties are better than those of the unpurified steel.
Full net steel
The oxygen in the liquid is completely removed before it is solidified to be limited to the lower limit to avoid the edge effect of unclean steel. In the deoxidation treatment, an iron-silicon alloy is added to the molten steel to cause oxygen in the molten steel to be reversely generated into slag to obtain a uniform molten metal.
Semi-clean steel is between unpurified steel and all-purified steel. A small amount of iron-silicon or aluminum is added as an oxygen scavenger in the molten steel. The amount added is just enough to remove the edge effect of unpurified steel and make oxygen. Get a full breakdown.
Vacuum deoxidized steel (vacuum degassing steel)
By vacuum treatment, it is not necessary to add any oxygen scavenging element, so that the molten steel can obtain a sufficient oxygen scavenging effect and no non-metal intervening material is formed in the steel. The treatment process is to first increase the carbon content in the steel, and then the molten steel is vacuum casted, at which time carbon and oxygen will react to form carbon monoxide to gradually reduce the carbon and oxygen in the molten steel to a prescribed level. The steel is very clean because no deoxidizing elements that form solid oxides are added during the process.
In addition, due to the different oxygen content in the molten steel before casting, the steel ingot is divided into three basic types: killed steel, boiling steel and semi-killed steel.
Also known as fully deoxidized steel, it is a steel in which the oxygen content in the molten steel is low enough to not react with carbon in the steel to form carbon monoxide bubbles. The molten steel before casting must be fully deoxidized. For example, deoxidation with silicon and aluminum, the silicon content of the steel is about 0.3%, and the aluminum content is (0.02-0.06%). The calmed steel ingots have shrinkage holes and must be cast with an ingot with a heat insulating cap. After the rolling, the ingot is cut, and the ingot yield is 85 to 89%. Steel ingots are required for steels with uniform composition and compact structure. The killed steel is made of a mold with a large upper and lower insulation cap. In recent years, heat insulation caps and heat insulation board insulation caps have been widely used to increase the blanking rate.
The molten steel has a high oxygen content (0.02-0.04%), and a strong carbon-oxygen reaction occurs in the ingot mold to generate carbon monoxide bubbles, which is named after the molten steel boils in the mold. At the beginning of this steel solidification, bubbles form and float. The steel ingot skin is solidified into a relatively pure shell layer containing iron. When the surface layer reaches the required thickness, a cover plate is added on the top of the ingot to solidify the top to prevent the bubbles from continuing to escape; or a de-oxygenation of ferrosilicon, aluminum or the like may be added at the top for chemical capping; Capping. Another method is to add aluminum to the whole molten steel after the solidification of the molten steel into a surface layer. This steel ingot is called an outer boiling inner steel. Boiling steel generally adopts a bottle opening mold with a small upper and a large open type. The boiling steel ingot has a blanking rate of 90-92%, which is mainly used for low carbon steel.
A steel grade between calm steel and boiling steel. This steel has less internal gas and is structurally close to killed steel. In the initial stage of semi-killed steel casting, no bubbles are generated. When the top is naturally solidified and capped (the bottle die can be used to promote the capping), due to the enrichment of carbon and oxygen in the molten steel and the temperature decrease, a small amount of carbon monoxide bubbles are generated on the top of the ingot, filling the whole. The solidification shrinkage space of molten steel. Therefore, the billet ratio of the steel ingot similar to that of the boiling steel can be obtained. Semi-killed steel is mainly used for medium-carbon and medium-quality structural steels. The molds used are generally open, small, and large.
Demoulding The cast ingot must be demolded after the interior is completely solidified. For alloy ingots with strong crack sensitivity, after demolding, they should be placed in a slow cooling pit in a hot state (>900 °C) for slow cooling, or sent to a soaking furnace in a rolling mill at a temperature not lower than 750 °C or Heating furnace.