无氟和低氟连铸保护渣生成区域的研究
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摘要
连铸保护渣对连铸工艺的顺行和提高铸坯表面质量具有重要影响,是连铸过程中关键性的辅料。由于保护渣中氟对其高温物化性能具有重要而又特殊的调节作用,所以一般保护渣都含有2~15%的氟。但由于氟的存在会导致环境污染等一系列问题,因此保护渣的无氟或低氟化是环保的客观要求。论文在综述无氟保护渣国内外发展现状的基础上,分析了保护渣无氟化后面临的主要问题,即如何确保高碳钢保护渣高温时粘度-温度性能的稳定以及包晶钢(亚包晶钢)用保护渣结晶性能得到有效控制;同时指出已有的研究工作虽然解决了降氟的局部问题,但可能带来保护渣使用中的更多其它问题。因此,对与氟相关的保护渣各项性能进行系统评价以及寻求与高氟渣系性能相近的保护渣组成是保护渣无氟、低氟化研究的迫切需要。
     论文用粘度、熔点、转折温度以及结晶体比例等性能指标对目前方坯、低合金钢板坯以及亚包晶钢板坯等常用高氟保护渣的特点进行了总结和归纳,并采用单纯形法设计了无氟和低氟渣系,在实验室对其相关的高温物化性能进行了测试和分析,得出如下主要结论:
     ①在设计渣系范围内,得到无氟保护渣的生成区域。其中适合板坯低碳钢保护渣的有以下区域:TiO_2=6%, Li_2O=2%,B_2O_3小于3%;TiO_2=6%, Li_2O=0%,MnO含量大于3.3%,小于5%时,B_2O_3含量大于3%;TiO_2=4%, Li_2O=0%,B_2O_3含量大于2%,小于5%。未发现板坯亚包晶钢板坯连铸的无氟渣生成区域。此外,该渣系范围内存在大量适合小方坯高碳钢保护渣的区域。
     ②在设计渣系范围内,得到低氟保护渣生成区域。其中适合板坯低碳钢保护渣的有以下区域:TiO_2=6%, Li_2O=0%,当B_2O_3含量大于2%,小于8%的区域;TiO_2=4%, Li_2O=0%,在整个区域内,TiO_2=0%, Li_2O=2%,当渣中B_2O_3含量大于2%,小于5%的区域。适合板坯低合金高强度钢保护渣的是TiO_2=6%, Li_2O=0%,在B_2O_3含量大于2%,MnO含量大于4%,小于5.2%的区域。适合板坯亚包晶钢保护渣:TiO_2=0%, Li_2O=4%,Na_2O含量为12%。同样该渣系也存在大量适合小方坯保护渣的区域。
     ③矿相研究结果表明,高氟渣析出的矿相主要是枪晶石(3CaO·2SiO_2·CaF_2)。无氟渣主要的析出矿相为Na_2O·2CaO·3SiO_2、2CaO·SiO_2。Na_2O·2CaO·3SiO_2熔点为1250℃,但2CaO·SiO_2熔点高,达到2130℃,可能导致在熔渣冷却过程中该物相析出温度较高,差热分析表明,无氟条件下保护渣在保证其它性能指标前提下,结晶温度难以协调控制在合适范围内。因此对裂纹敏感性亚包晶钢板坯保护渣的无氟化,除了在无氟渣系范围内继续搜寻外,对可能与枪晶石具有类似结晶行为的矿相还有待进一步研究和探索。
Mould fluxes is a very important and critical material during continuous casting of steel, takes a key role to improve quality of strands. Mould fluxes usually contain 2~15% fluorine, due to its special influence on the high temperature physicochemical properties of mould fluxes. However, Fluoride can cause a series of environmental problems, so it’s very important to reduce fluorine content in mould fluxes. On the base of research of development of non–fluorine mould fluxes both home and abroad, the problem of mould fluxes without fluorine was analyzed, that is how to keep the viscosity-temperature property at high temperature while casting high carbon steel and how to control the crystallizing property while casting peritectic steel or hypo-peritectic steel. The present research can deal with part of the problems after reducing fluorine in mould fluxes, but can cause other more problems. So, it is necessary to systematically estimate all the properties of mould fluxes and to seek the component area close to the mould fluxes bearing high fluorine for the research of non-fluorine and low-fluorine mould fluxes.
     Viscosity, melting temperature, breaking temperature and crystallizing temperature have been used as property index to estimate mould fluxes used in billet, low alloy high-intensity slab and hypo-peritectic steel slab casting. The slag system of non-fluorine and low-fluorine slag was designed by the simplicity grid method, the physiochemical properties at high temperature were testing and analyzed, the conclusions were as follows:
     ①The new producing zones of non-fluorine mould fluxes are found in the simplicity grid designed area. The zones of slag system fit for low carbon slab mould fluxes are: TiO_2=6%, Li_2O=2%, B_2O_3 content are less than 3%;TiO_2=6%, Li_2O=0%,MnO content are more than 3.3%,less than 5%,B_2O_3 content are more than 3%;TiO_2=4%, Li_2O=0%,B_2O_3 content are more than 2%,less than 5%. Fluorine free slag fit for hypo-peritectic slab hasn’t been found in this area. Other more, large quantity of slag fit for high carbon billet mould fluxes were found in the designed area.
     ②The new producing zones of low-fluorine mould fluxes are found in the simplicity grid designed area. The zones of slag system fit for low carbon slab mould fluxes are TiO_2=6%, Li_2O=0%, B_2O_3 content are more than 2%, less than 8%; TiO_2=4%, Li_2O=0%,in the whole area of contour chart; TiO_2=0%, Li_2O=2%, B_2O_3 content are more than 2%,less than 5%.The zones of slag system fit for low alloy high-intensity steel slab mould fluxes are TiO_2=6%, Li_2O=0%, B_2O_3 content are more than 2%, MnO content are more than 4%,less than 5.2%.
     ③Crystalline phase of mould fluxes bearing high fluorine is cuspidine (3CaO·2SiO_2·CaF_2). Crystalline phase of non-fluorine are Na_2O·2CaO·3SiO_2 and 2CaO·SiO_2. Melting temperature of Na_2O·2CaO·3SiO_2 is 1250℃, while melting temperature of 2CaO·SiO_2 is 2130℃, and the crystallizing temperature of this crystalline phase may be high in the cooling process. DSC results show that, the crystallizing temperature of non-fluorine slag is not in the proper range. It’s effective to seek a crystalline phase which has a crystallizing property close to cuspidine in the coming research.
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