Study On Making Tool And Die Steels By Direct Reduction With Scheelite And Molybdenum Oxide Completely

Making tool and die steels by direct reduction of oxide ores is to eliminate the process of making ferrous-alloys, to decrease the environmental pollution and to economize resources and energy, which is of a steelmaking technology with sustainable development and substantial economical benefit. Some metallurgists have investigated direct reduction and alloying of scheelite and molybdenum oxide. However, lots of technical problems such as excessive slag amount, strong boiling, slow reduction, bad volatilizing of molybdenum oxide and low yields of tungsten and molybdenum, have restricted the technological development of direct alloying of scheelite and molybdenum oxide.In this article, by theoretic analysis and calculation of thermodynamics and kinetics and relative experiment, systematic study on basic theory was conducted for making alloy steels by direct alloying with scheelite and molybdenum oxide. The technology of making tool and die steels of high tungsten content and high molybdenum content with scheelite and molybdenum oxide completely instead of ferrotungsten and ferromolybdenum was initiated successfully in 20t AC EARThe reducing process of scheelite and molybdenum oxide can be classified into solid-solid reaction, liquid-solid reaction, reaction in molten steel and slag-melt reaction according to reaction temperature. The thermodynamic data about seven reducing reagents(carbon, silicon, manganese, iron, aluminum, calcium carbide and silicon carbide)reducing WO3, CaWO4, MoO3 and CaMoO4 was deduced, and relative â–³G⁰-T diagrams were also drawn. Activity calculation models of CaO-SiO2-FeO-MoO3 and CaO-SiO2-FeO-WO3 slag were established for calculating real Gibbs energy of reducing reaction of scheelite and molybdenum oxide. Reducing ability of silicon carbide is better than that of silicon and carbon at high temperature, however, the ability is worse than that of silicon when the temperature is low. Oxidizability of slag has considerable effect on content of tungsten in molten steel, so measures such as low oxidizability of slag and low basicity of slag (<1.5) should be taken for preventing tungsten in molten steel oxidizing. Molybdenum in molten steel is difficult to be oxidized by FeO.Molybdenum oxide badly volatilizes in air. The volatilization velocity of Molybdenum oxide will be very fast relating to the greater surface area and higher temperature. The reducing reagents and liquid steel can restrained its volatilization in acertain extent, so in the process of making tool and die steels by direct reducing molybdenum oxide, its volatilization can be controlled efficiently.Kinetics analysis shows that reducing scheelite mainly completed in the process of ferrous bathing and liquid-liquid. And scheelite melts very soon due to high temperature electric arc. However, apart from the high temperature area, scheelite will hard to melt and be carried into the top slag. The formula of scheelite ferrous bathing reduced by [Si] in the molten steel can be expressed as follows:The diffusion of CaWO4 is the restrictive step of reduction process. The reduction velocity is related to coefficient of mass transfer of CaWO4 and area of reaction interface. Strengthening stirring of bath can improve kinetics condition of slag-melt reaction. Solid molybdenum oxide can be easily reduced by carbon at low temperature. The low temperature reduction of molybdenum oxide not only occurred during the process of the reduction between carbon and solid molybdenum oxide but also during the process of the direct reduction between gaseous molybdenum oxide and carbon. Kinetics condition of high-temperature reaction of CaMoO4 is more advantageous to that of CaWO4. Kinetics condition of high-temperature reaction of carborundum is superiority to silicon and carbon and less influence of blowing condition. Making alloy steels of high tungsten content with scheelite completely instead of ferrotungsten should use carborundum as main reagent. And making alloy steels of molybdenum content with molybdenum oxide should...