Study And Application Of Mold Fluxes For Quality Steel Development In ShuiGang

With the development of continuous casting, especially the improvement of casting speed, expanding of steel grade and more requirement on steel quality, the technology of mold fluxes has been one of key parts in modern continuous casting. In this work, practical production survey on quality steel in Shuicheng Iron and Steel (Group) Co.Ltd.(ShuiGang) was did, and systemic statistical analysis was did according on large production data, it was found that: there was serious surface depressions on casting billet when casting wire rod steel during the production process of quality steel and the use of relative mold fluxes, and the breakout rate was nearly 2%; casting Q235 and 20MnSi steel, there are not highlight surface quality defects; there are depressions grooves and scrab defects on the surface of casting billet when casting high carbon steel like 70 and 82B. To improve the quality of billet, high-temperature thermal properties, high temperature mechanical properties, the relationship between components and properties of mold fluxes were studied in the laboratory combined with the continuous casting technology in plant, mold fluxes used in casting quality steel were designed and tested, and the rang of physical chemistry properties of mold fluxes in ShuiGang was comfirmed. It was concluded that:①STA449C thermal analyzer , DIL402C thermal dilatometer, Gleeble-1500D thermal simulation testing machine were employed to test major phase change temperature. The linear expansion coefficient, and mechanic properties at high temperature of quality steel for ShuiGang, results show that: in the temperature range of 1350 ~ 1530℃,there are two exothermic peaks in the cooling process, that is, phase transition occurred. The thermal expansion coefficient of H08 steel is 22.2683E-06 in the temperature range of 1400 ~ 1477℃, temperature range of 1350 ~ 1400℃, its thermal expansion coefficient is 2.98 times when the temperature range 1400 ~ 1477℃. The change of the thermal expansion coefficient between Q235 steel and H08 are similar in cooling process, in temperature range of 1350 ~ 1385℃, its thermal expansion coefficient of 59.7E-06 is 2.6 times when the temperature range 1385 ~ 1478℃. In temperature range of 1350 ~ 1400℃,the tensile strength is from 3 to 9 MPa and the section shrinkage rate is less than 40%. We can see, in the cooling process of molten H08 steel, a large volume contraction occured in the solidification of liquid steel, and the chance of depression and crack be increased. According to the high temperature characteristics of ShuiGang quality steel, the mold flux heat transfer performance regulation and control of welding rods and wire steel should be emphasized, the lubricating properties of 70 and 82B mold fluxes should be enhanced.②Based on the properties requirements of mold fluxes for ShuiGang quality steel, the CaO-SiO2-Na2O-F- -MgO-MnO-Al2O3 slag system has been designed in the laboratory, the main components ranges of the mold fluxes follow as R = 0.65 ~ 1.05, Na2O = 4 ~ 12%, F-= 3 ~ 8%, MnO = 0 ~ 6%, MgO = 4%, Al2O3 = 3.5%, the effect of the major components and content to melting temperature, viscosity and transition temperature has been attained.③Under the existing process conditions, the properties range of mold fluxes is suitable for ShuiGang low-carbon welding rod and wire steel as following: dual basicity R = 0.85 ~ 0.95, melting temperature T半=1150 ~ 1180℃,η1300℃= 0.5 ~ 0.65Pa.S, the break temperature, Tbr = 1150 ~ 1190℃, fluxes consumption at 0.45 Kg per ton steel, the packing dencity in the range of 0.7 ~ 0.75 g/cm3. According to statistics, the number, the depth and length of depression on billet surface have significantly reduced, and the ratio of breakout reduces from 2% to 0.13% after using the mold fluxes optimized.The properties range of 20MnSi and Q235 mold fluxes: dual basicity R=0.7~0.8,melting temperature,T= 1135 ~ 1165℃,η1300℃= 0.45 ~ 0.55Pa.S, fluxes consumption at 0.2Kg per ton steel. The properties range of 70 and 82B mold fluxes: dual basicity R = 0.70 ~ 0.90,η1300℃= 0.15 ~ 0.30Pa.S, melting temperature,T= 1020 ~ 1060℃, fluxes consumption at 0.48Kg per ton steel, which provide an effective basis for mold fluxes and evaluation quality system constuction of ShuiGang mold fluxes in the future.