SU1036434A1 - Exothermic slag forming mixture - Google Patents

Abstract

EXOTHERMAL CLASTER MIXTURE MIXTURE, mainly for continuous casting of electrical steel with a residual content of T l / d% lx: g, {: -. alumina 0.2-0 ,, including silicocalcium, iron oxide, sodium nitrate, cryolite, a boron-containing substance. The trap, silicate lump, characterized in that, in order to reduce non-metallic inclusions on the surface of the elite billet by reducing the thermal conductivity of the mixture, it additionally contains soda ash in the following ratio of ingredients, wt .: 30-36 Silicocalcium 10-18 Iron scale 3- 8 Sodium nitrate 12-18 Cryolite i 6-8 Silicate lump 2-7 Graphite Calcined k-} Q soda Boron-containing Remaining K substance

Description

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4 The invention relates to the casting of steel, 8 in particular, to mixtures for protecting metal from oxidation in crystalline congestion of continuous machines (PUR billet). ; When steel is cast, including on a continuous casting machine, the metal is oxidized again by direct contact with air, which leads to the appearance of defects (captivity, cracks, twists, etc.) on ingots and uninterrupted castings. | To protect the metal mirror from secondary oxidation during continuous casting, slag-forming mixtures of different composition are poured in the mold. After such mixtures melt, a layer of liquid slag forms on the surface of the metal, which prevents steel from oxidizing with air and at the same time lubricates the working walls of the crystallizer, preventing the molten metal from sticking to the cooled walls of the crystallizer. Exothermic slag-forming mixtures are known, containing combustible components (aluminum silicocalcium, etc.), oxidizing agents (sodium nitrate, oxides of iron and manganese, etc.), slag-forming components (fluorine swords, cryolite silicate block, boron-containing compounds, etc. ), as well as fillers (graphite, mica, etc.). These mixtures are ignited upon contact with the liquid metal in the mold. Selected u. During the oxidation of the combustible components of the mixture, an additional amount of heat contributes to the rapid formation of a liquid-mobile slalon on the metal scale. However, the use of these exothermic slag-forming mixtures is not effective enough, and because of a number of inherent flaws. So, in a number of cases, the slag formed during the combustion of the mixture has a high melting point and low liquid mobility, it is pelletized ... and does not spread well over the metal mirror, does not enter the gap between the forming ingot and the wall 1 of the crystallizer, but now it sticks to the walls mold above metal level. In this case, a reliable protection of the metal in the mold from secondary oxidation is not ensured, j-effects appear on the surface of the continuously cast billet and, moreover, the resistance of the continuous-casting molds decreases sharply. An exothermic slag-forming agent is also known, which includes a combustible component, oxidizing agents, slag-forming and calcined. However, this cake has a high melting point, which does not provide a sufficient quality of the metal surface. Closest to the invention in its technical essence and the effect achieved is an exothermic slag-forming mixture 4, comprising, in May, L: Silicocalcium Base Saltpetre sodium 5-15 Iron scale 15-30 Cryolite15 30 A substance containing boron oxides 2-10 Graphite5-15. Silicate block 5-15 The disadvantage of this mixture is that the slag formed during its combustion has a high thermal conductivity, low liquid-mobile pelletizing on the metal surface in the crystallizer, sticking to the crystallizer wall above the metal level. This slag does not serve as an effective lubrication of the working walls of the mold and therefore does not provide a high quality surface of cast billets. The aim of the invention is to reduce non-metallic inclusions on the surface of the cast billet by reducing the thermal conductivity of the mixture. To achieve this goal, the exothermic slag-forming mixture, mainly for continuous casting, electrical steel with a residual aluminum content of 0.2-0,%, including silicocalcium, iron oxide, sodium nitrate, cryolite, a boron-containing substance, graphite, silicate lump, additionally contains calcined sodium in the following ratio of ingredients, wt.%: Silicocalcium 30-36 Iron oxide 10-18 Sodium nitrate Cryolit12-18 Silicate lump 6-8 Graphite2-7 Soda ash -10 Boron containing substance The remaining Introduction of soda ash to the mixture provides an accelerated formation of a metal in the mold on the mirror: a homogeneous liquid-mobile and sufficiently heat-conducting slag. This is facilitated by the thermal dissociation of soda Na2 proceeding at a temperature of about. At E. THOMA, the release of carbon dioxide loosens the mixture and intensifies its combustion, while sodium oxide is a good melt for the oxidation products of silicoalkanes and CaO. As a result, the burning rate of the mixture and the rate of slag formation increase and a homogeneous formation is formed on the liquid metal mirror (without lumps and a highly mobile slag, which spreads well over the metal surface and fills the gap between the forming and discontinuous casting and the walls of the crystallizer. This slag protects the metal in the crystallizer against oxidation and at the same time is an effective lubrication of the working walls of the crystallizer, which provides an improvement in the surface quality of the continuous cast The proposed exothermic slag-forming mixture contains a combustible component (silicocalcium), oxidizing agents (sodium nitrate and iron scale), slag-forming materials (cryolite or fluorides of other metals, silicate lump, datolite or other boron compounds, calcined soda), and CP Fit. The content in the mixture of a combustible component — silicocalcium should ensure, when it is burned, the release of a certain amount of heat sufficient to quickly melt the slag-forming components of the mixture and form Fluidization of liquid slag. The content of silicocalcium in the mixture should be in the range of 30-36. When the silicocalcium content is less than 30, the heat released during its combustion is not enough to quickly form slag on the metal surface in the crystallizer. At the same content of silicocalcium in a mixture of more than 36, the physicochemical properties of the resulting slag deteriorate (its viscosity and melting point increase), since an excess amount of refractory silico-oxidation products — CaO and SIO oxides passes into the slag. the mixtures in the recommended range 30-36 are determined mainly by the calcium content of silicocalcium; at elevated levels of calcium in silicocalcium, a minimum amount (30-31%) of this alloy can be introduced into the mixture and, conversely, when using silicocalcium with a low calcium content, this fuel component in the mixture should be increased to 35-36%. For the preparation of the mixture, it is recommended to use silicocalcium SC-15 with calcium content from k q 18%. The content of silicocalcium in the mixture is determined by the content of oxidizers in it — sodium nitrate and iron oxide: it is in the range of 10–18%, respectively. In this case, sodium nitrate, as a more active oxidizing agent, ensures a rapid start of the mixture burning, i.e. used only to ignite the mixture. Therefore, its content in the mixture is small (within 3-8%), and its specific content depends on the quantity and composition of the silicocalc used in the mixture: with its increased consumption and low content in calcium alloy, the fraction of nitrate can be 7-8%, and with a reduced consumption of silicocalcium and a high content of calcium in it can be limited by the content of nitrate in the mixture at the lower limit (). The main oxidizing agent in the mixture is iron scale. The lower limit of its content in the mixture (10%) corresponds to the lower limit of the consumption of silicocalcium - 30%. With a lower scale content (less than 10%), the speed of the mixture itself and, consequently, the rate of slag formation on the metal surface in the mold slows down significantly, which adversely affects the quality of the workpiece cast in the caster. The upper limit of the scale content in the mixture (18%) corresponds to the upper limit of the consumption of silicocalcium - 36% and is determined by the following circumstance. When the upper limit of the scale in the mixture is exceeded

(18%) in the forming slag the proportion of oxides of iron significantly increases. (it can exceed the value of 9-10), which leads to the development of the processes of secondary oxidation of cast steel with slag, especially its active components such as aluminum, titanium, etc.

Graphite, which is part of the slag-forming mixture, is not assimilated by the liquid slag formed during its combustion and improves the properties of this slag, like lubricants of the working walls of the crystallizer. The content of graphite in the proposed mixture should be in the range of 2-7%. When the graphite content is less than 2%, its effect on improving the lubricating properties of the liquid slag is not felt. When the content. however, more than 7% of graphite shows a noticeable transition to the cast one. steel. When casting low carbon steels, such as dynamo steels, in order to avoid their carbonization, the content of graphite in the mixture should be at the lower limit (2-3%). When casting medium-carbon steel, the content of graphite in the mixture can reach the upper limit of 7%.

The most active slag-forming component of the mixture is cryolite (compound 3NaF, ACFj), which provides a rapid liquefaction with a forming state — when the slag mixture is burned. The content of this substance is 12-18% provides the optimal values of the rate of formation of the slag melt on the surface of the metal in the mold. At the same time, the upper limit of the content of cryolite in the mixture (18%) was chosen, based on the need to ensure satisfactory sanitary and hygienic conditions for the personnel of the CCM, since large amounts of harmful compounds are released into the atmosphere of the casting span at elevated levels of cryolite. fluorides. When the cryolite content is less than 12%, even the addition of another sufficiently effective diluent slag to the mixture — datolite — does not provide for the rapid formation of active and liquid mobile slag. Cryolite in the composition of the mixture can be partially or even completely replaced by fluorides of other metals - fluorspar, flufit concentrate, and t, p.

The slag-forming material is datolite (compound from the group of silicates with a content,%: B, 15-17; CaO 35 ifO ;.Si FeO, MnO, MgO,), the rest has a low melting point (950-1100 ° C) and successfully replaces composition of slag-forming mixtures of fluoride compounds. Datolite is added to the mixture in an amount of 8-18% in order to replace part of the cryolite. At the same time, the rate of slag formation remains rather high, and at the same time the emission of harmful fluoride compounds into the atmosphere of the workshop is noticeably reduced. The upper limit of the content of datolite in the mixture (18%) is due to the need to limit the uncontrolled transition of boron to steel as a result of the reaction of its reduction from the compounds in the slag using silicocalcium. When the content of datolite in the mixture is less than 8%, even at the maximum content of cryolite in it (18% The formation of an active liquid slag slows down significantly, and this leads to a deterioration in the quality of the cast billet.

The contents of cryolite and datolite in the mixture are interconnected, with an increased content of cryolite (17-18%), the content of datolite is usually selected at the lower 5 limit (8-10%) and, conversely, at the lower consumption of cryolite (12-13%) the content Datolite should increase to 17-18%

In addition to datolite, other boron compounds can be used in the mixture, for example, borax (compound Ma, Vd 0, 10). However, additional amount of heat is used to decompose the crystallization moisture contained in the storm, therefore the formation of liquid slag goes more slowly than application of datolite. G ..

The content of soda ash in the mixture should be in the range of -10%. When the content of soda in the mixture is less than 4%, its role as an intensifier of the processes of mixture combustion and slag formation is significantly weakened. When the content of soda is more than 10%, the dust emission from the surface of the mixture poured onto the metal mirror in the mold noticeably increases. The consumption of soda ash in the selected interval () is directly proportional to the consumption of silicocalcium and oxidizing agents (sodium nitrate and iron oxide). Compared to cryolite and datolite, the effectiveness of silicate lump (compound, SiO) as a slag-forming component is lower. However, the silicate block lacks non-. the wealth inherent in the compositions of mixtures with cryolite (the presence of harmful emissions of fluoride) and datolite (the development of uncontrolled transition of boron to steel). Therefore, the use of silicate: lump in the composition of the proposed mix of SI in the amount of 6-18% is appropriate. Due to the noted limitations on the content of cryolite and datolite in the mixture, the silicate block serves as an additional slag-forming material, and the choice of its content in a particular mixture (in the specified range of 6-18%) is made taking into account the consumption of all other components of the mixture. The proposed exothermic mixture is prepared by simple mechanical mixing of its initial components. For fractional composition requirements for all source materials. they are the same - they must be of a size (particles no more than 1 mm. To provide such fractional composition, it is necessary to organize grinding of silicocalcium, iron scale and silicate lump, as well as sodium nitrate. which coagulates during long-term storage due to hydration. Other materials (cryolite, datolite) , graphite and soda ash are used to prepare the mixture throughout delivery, i.e., no prior grinding is required. The prepared mixture is powdered to a metal mirror in a crystallized glass. The mixture consumption is 1.1, 6 kg / ton of steel. Upon contact with the liquid metal in the crystallizer, the mixture immediately ignites. As the mixture burns, liquid slag forms, which protects the open surface of the liquid metal in the crystallizer from oxidation, and also serves as a lubricant between the walls of the mold and the solid side surface of the cast billet.As a result of the elimination of the secondary oxidation of the metal and the absence of inhibition of shrinkage, a defect-free crust of a continuously cast billet is formed, The quality of the metal surface is good. Example 1. When casting on a double-strand continuous casting machine of a vertical type of the Cherepovetsky metallurgical plant, slabs with a cross section of 1200–200 mm from dynamo steel 2013 along rOCt. Tc content of less than 0.03 C and 0.2-0, AC), an exothermic slag-forming mixture of the following composition is applied to the mold evenly along the casting, the following composition, wt .: silicon silicate 33, iron slag 12, sodium SELITRA 5, cryolite H, silicate lump T , datolite 13, graphite k and soda ash 5. Mixture consumption 1, A-1.6 kg / t steel. . Example 2. When casting on a double-strand continuous caster of a vertical type of slab with a section of 1070-200 mm from steel grade 25, an exothermic slag-forming mixture of the following composition, in wt.%: Silicocalcium 30, iron oxide 10, sodium nitrate 3, cryolite 12, silicate lump 18, datolite 18, graphite 5 and soda ash. Mixture consumption 1,, 6 kg / t steel. Example 3. When casting on a double-strand continuous caster of a vertical type slab with a section of 1070-200 mm from grade 5 steel, an exothermic slag-forming mixture of the following composition, wt.%: Silicocalcium 33, scale 16, sodium nitrate 6, cryolite 18 , silicate clay Dolite 8, graphite 7 and soda ash 6. Mixture consumption 1, -1, 6 kg / t of steel. Example k. When casting on a double-strand continuous casting machine of a vertical type of slab with a cross section of 1200-200 mm from 2011 dynamo-steel according to GOST 21427. 2-75 (with a content of less than 0.03% C), an exothermic slag-forming mixture of the following composition is applied to the mold evenly during casting %: silicocalcium 3. Iron dross 18, sodium nitrate 8, cryolite 13, silicate lump 6, datolite 9, graph 11t 2, soda ash 10,

The results of screening of continuously cast billets (with a section of 2DO-1070 mm) of the experimental heats are given in the table,.

The application of the proposed exothermic slag-forming mixture during continuous casting of steel will provide improved technical and economic indicators of production associated with the improvement of the quality of the surface of continuously cast billets. The economic efficiency of the application of the proposed mixture consists of increasing the yield of metal from continuously cast billets by 0.5-0.7 and reducing the cost of steel processing with a slag-forming mixture.

For the Cherepovets plant, the specific plant application efficiency of the proposed mixture will be 0.6-0.7 rubles / ton of cast steel. When casting in the electric steel-smelting shop 400 thousand tons of steel, the annual economic effect will be 260-300 thousand rubles. ,

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Claims (1)

EXOTHERMIC SLAG-FORMING MIXTURE, mainly for continuous casting of electrical steel with a residual aluminum content of 0.2-0.4%, including silicocalcium, iron oxide, sodium nitrate, cryolite, boron-containing substance, Trafite, silicate block, characterized in that In order to reduce non-metallic inclusions on the surface of the cast billet by reducing the thermal conductivity of the mixture, it additionally contains soda ash in the following ratio of ingredients, wt.%: Silicocalcium 30-36 Iron oxide 10-18 Sodium Nitrate 3-8 Cryolite 12-18 Silicate block 6-8 G raffite 2-7 calcined soda • 4-10 Boron-containing substance Rest J036434. <1036434