Ferrochromium is an important alloying additive for steelmaking. Chromium in steel can significantly improve the corrosion resistance and oxidation resistance of steel, improve wear resistance and maintain high temperature strength. In addition to the main components of chromium and iron, ferrochromium also contains carbon, silicon and small amounts of other elements. According to the different carbon content, it can be divided into high carbon ferrochrome (carbon ferrochrome), medium carbon ferrochrome, low carbon ferrochrome and micro carbon ferrochrome. Commonly used in chromium-based ferroalloys are silicon-chromium alloys, metallic chromium, and ferrochromium nitride.
High carbon ferrochrome
It is mainly used as an alloying agent for ball steel, tool steel and high-speed steel with high carbon content, as an additive for cast iron, as a chromium-containing raw material for the production of silicon-chromium alloys and medium, low and micro-carbon ferrochrome by the slag-free method, and for the production of metallic chromium by electrolysis Chromium-containing raw materials, raw materials for smelting stainless steel by oxygen blowing method.
The smelting methods of high-carbon ferrochromium mainly include blast furnace method and submerged arc furnace method. The raw materials used are chrome ore, coke and silica. High-quality metallurgical grade chrome ore requires Cr2O3>48%, S<0.1%, P<0.1%, CrzO3/FeO>3. The blast furnace method can only produce special pig iron with a chromium content of about 30%. High-carbon ferrochromium with high chromium content is mostly smelted in submerged arc furnaces by flux method. Due to the high melting point of ores and alloys, the required furnace temperature is also high, so high secondary voltage and electrode current density are used for smelting.
Medium, low and micro carbon ferrochrome
Medium and low-carbon ferrochrome is used to produce medium and low-carbon structural steel, chrome steel and alloy structural steel, etc. Micro-carbon ferrochrome is mainly used to produce stainless steel, heat-resistant steel and acid-resistant steel.
Medium, low, and micro-carbon ferrochrome (collectively referred to as refined ferrochrome) is smelted with silicon-chromium alloy, chromite and lime as raw materials, and is smelted by electrosilicon thermal reduction. Use 1500~6000 kVA electric furnace for refining and desiliconization, and use high alkalinity slag for operation (CaO/SiOz is 1.6~1.8). The lower the carbon content of these ferrochrome, the higher the requirements for raw materials.
The smelting of medium and low carbon ferrochrome also adopts the oxygen blowing method, that is, oxygen is directly blown into liquid high carbon ferrochrome to decarburize it. A small amount of lime and fluorite are added to the molten pool during blowing to form slag, and silicon-chromium alloy or ferrosilicon is added before iron tapping to recover chromium in the slag. The oxygen blowing method is to put the liquid high-carbon ferrochrome out of the electric furnace and put it into the converter for oxygen blowing decarburization refining, so it is also called the converter method.
Low and micro-carbon ferrochrome smelting is also produced on a large scale by the heat transfer method, that is, two electric furnaces are used during production, one furnace smelts silicon-chromium alloy; the other furnace melts slag composed of chromium ore and lime. The desiliconization and refining reaction is carried out in two stages in two ladles: after the slag of the national slag furnace is injected into the first ladle, the silicon-chromium alloy that has been initially desiliconized in the other ladle is blended in. There is a large excess of oxidant and sufficient desiliconization, and micro-carbon ferrochrome with a silicon content of less than 0.8% and a carbon content of as low as 0.02% can be obtained. ② After the reacted slag (containing about 15% of Cr203) in the first tank is moved to the second tank, the silicon-chromium alloy (containing 45% of silicon) smelted in the silicon-chromium electric furnace is heated into the slag, and the reaction The silicon-chromium alloy (containing about 25% of silicon) obtained after preliminary desiliconization is blended into the first ladle for further desiliconization, and the slag containing Cr2O3 can be discarded below 3%.
Micro-carbon ferrochromium smelting also has a vacuum solid-state decarburization method, that is, high-carbon ferrochrome is ground into powder, mixed with an appropriate oxidant, and produced by mixing, pressing, drying and vacuum smelting to obtain micro-carbon with low carbon content. ferrochrome. The equipment used is a vacuum resistance furnace.