难除鳞合金钢坯高温多功能防护涂层
文献类型：学位论文
| 作者 | 周旬 |
| 学位类别 | 博士 |
| 答辩日期 | 2012-06-06 |
| 授予单位 | 中国科学院研究生院 |
| 导师 | 陈运法 |
| 关键词 | 高温防护涂层 抗氧化性能 除鳞率 元素贫化 表面质量 |
| 其他题名 | Study on High Temperature Multifunction Protective Coatings for Difficult-to-descale Alloy Steel and Their Mechanisms |
| 学位专业 | 化学工程 |
| 中文摘要 | 大部分难除鳞合金钢坯在热轧前都需要在加热炉内的高温氧化性气氛下加热到1200℃以上。在加热过程中，金属表面和氧化性气氛发生反应生成氧化皮，严重的氧化烧损不仅造成大量的能源和钢（1-2%）的浪费，而且氧化层在炉内钢坯移动的过程中会掉落炉底，腐蚀炉底的耐火材料。同时由于氧化皮结构和成分的差异，部分钢种在出加热炉后，高压水仅能除掉部分初生氧化皮，残留的氧化皮被压入钢材表面，造成压入式缺陷，影响后期产品质量。过度氧化也导致表面附近的非Fe元素贫化。元素贫化影响钢材表面附近元素分布，从而影响钢材表面质量。为了解决这些问题，本文采用涂层的方式，控制加热炉内氧化皮的生成速率和生成量，降低氧化烧损。通过改变氧化层的结构以及控制合金元素在界面处的富集量，提高初生氧化皮的去除率，从而降低表面缺陷，改善后期产品质量。具体工作如下： 研究了难除鳞合金钢坯的高温氧化行为及伴生问题。计算了X80 和20Cr2Ni4A在1100℃，1200℃和1250℃的表观氧化速率，并分别计算了两种钢在此温度区间的表观活化能。研究了20Cr2Ni4A的高温氧化导致的除鳞问题。分析了1200℃时，9Ni钢表面因非Fe元素选择性氧化导致的合金元素贫化问题。 提出了高温防护涂层的设计原则。对目前热轧用的高温防护涂层进行重新分类，即玻璃基屏蔽型涂层和陶瓷基反应型涂层。描述了屏蔽型涂层和反应型涂层的高温反应过程，分析不同涂层的温度适用范围。从相图出发，结合加热制度，钢材成分，离子半径，提出了陶瓷基反应涂层的详细设计选材原则。 在遵循高温防护涂层设计原则的基础上，设计了系列陶瓷基尖晶石反应涂层，采用共沉淀-煅烧法制备了一系列Mg系涂层。分析了涂层的物性参数，包括粘度，线膨胀系数，烧结点和柠檬酸活性等参数。并且提出综合评价涂层防护效果的方案，包括连续热重测试防氧化性能，金相法测试元素贫化问题，同时改进了实验室除鳞率检测方法：采用热压缩法，通过分析表面残留氧化层占总面积的比例来反应除鳞率。 针对微合金钢X80制备的高温MgO防护涂料，1300℃时最大防护效果为59.36%. 涂料高温防护寿命长于8h。 针对20Cr2Ni4A的高温Mg-Ni-O固溶体涂层，既提高了高温抗氧化性能，又抑制了FeO-Fe2SiO4共晶产物，压缩了松散层在总氧化层中所占的比例，从而提高了钢材后期表面的表面除鳞率。针对9Ni钢研发的Mg-Fe-O尖晶石涂层，可以有效提高钢材1100℃-1250℃之间5h内高温抗氧化腐蚀性能50%以上。同时涂层有效减薄贫化层厚度46.5%，降低酸洗时间，提高后期产品表面质量。 分析了以MgO涂层为代表的高温防护涂层在钢坯表面的反应过程，指出当温度超过570℃时，FeO开始形成，温度800℃时，FeO通过塑性形变包裹MgO颗粒，在1100℃发生固溶反应生成内层的MgxFe(1-x)O和外层的MgxFe(1-x)Fe2O4尖晶石。铁氧化物随着温度升高外扩散速率增加，1200℃时，内层没有发现MgxFe(1-x)O，可能因为MgxFe(1-x)O被氧化为MgxFe(1-x)Fe2O4。而外层生成符合完美比例的Mg-Fe尖晶石。1300℃时，Fe3O4 和Fe2O3 无限溶于MgxFe(1-x)Fe2O4。Mg在外层中的浓度被不断稀释，可以推测，随着时间的延长，涂覆样的氧化速率逐渐接近空白样的氧化速率。 探讨了高温防护涂层的作用机理。Mg系涂料对合金钢中Fe的防护主要基于以下两个行为：⑴ Mg2+填充内层wusitite的空位，降低空位浓度，减小阳离子穿过wusitite的扩散速率，从而降低氧化速率。⑵ Mg2+取代外层Fe3O4中氧八面体中心的Fe2+，生成MgxFe(1-x)Fe2O4尖晶石，通过提高八面体扩散能垒，降低阳离子穿过该层的扩散速率。高温防护涂层通过控制合金钢中Fe的氧化速率，控制钢的整体氧化速率，从而也抑制合金元素的氧化。涂层中加入的助剂，降低初始氧化阶段界面处的富集物浓度梯度，从而减轻非Fe元素在界面处的富集。涂层也直接参与氧化层的反应，通过竞争反应，改变氧化层的结构，抑制非Fe元素的富集。 |
| 英文摘要 | Most of alloy steel slabs are heated in a gas fired reheating furnace until the temperature reaches 1200℃ before hot rolling. During the reheating operation, steel surfaces react with the oxidizing atmosphere resulting in the formation of scale. Oxidation leads to the loss of energy and marketable steel (1~2% of the total). Descaling ability, the ability to detach the oxide scale from the substrate during hot rolling process, greatly influences the surface quality of the steel. Most of the difficult-to-descale steels are alloy steels. During reheating process, parts of scale remain on the substrate even its main part was removed by high-pressure water sprays. The remaining scale is impressed into the substrate during the subsequent rolling process, forming micro defects on the surface. At the same time, several oxidation leads to the depletion of alloy elements in surface alloyed layer. To minimize oxidation and reduce surface deterioration during reheating, coating is used to limit the oxidation rate of steel and decrease the metal loss during oxidation. By using the coating, the structure of the oxide scale could be changed, and the enrichment of alloy at the interface of scale and substrate could be controlled. As a result, the descaling ability could be enhanced, the micro defect on the surface could be reduced, and the quality of later product could be improved. The oxidation behavior and associated problems of several difficult-to-descale alloy steels have been investigated. The oxide scale formation process and oxidation kinetic of X80 steel during1100℃~1250℃ have been studied. The descaling ability of 20Cr2Ni4A and the depletion of alloy elements of 9Ni steel have also been researched. The design principal of high temperature protective coating was proposed. The high temperature protective coatings were classified into glass base molten layer and ceramic based reaction layer. The appropriate application temperature range of coatings was given. The detailed design principal was proposed by considering the influences of reheating process, steel species, phase diagram and ion radius. Based on the design principal, a series of high temperature protective coatings were developed. A series of MgO coatings have been prepared by co-precipitation-calcined method. Some parameters of viscosity, line expansion coefficient, Citric acid activation of the coatings have been tested. The evaluation scheme of comprehensive effects of coating protection have been proposed, including evaluating the oxidation resistant ability of the steel by continuous thermal gravimetric experiment, studying the depletion of alloy elements with metallographic method, and assessing the descaling ability of the steel by hot compression method. The protective coating prepared for X80 micro alloy steel could decrease the metal loss at 1300℃ by 59.36%. The life of the coating could last for more than 8 hours. The descaling ability of X80 steel has been improved. Mg-Ni-O coating, developed for 20Cr2Ni4A alloy steel, could enhance the oxidation resistance of the steel by 50%. The FeO-Fe2SiO4was also suppressed by the coating. The proportional value of loose oxide scale in the whole oxide scale has also been diminished. The Mg-Fe-O spinel coating, developed for 9Ni alloy steel, could enhance the oxidation resistance by 50% and alleviated the depletion of alloy elements in the surface layer by 46.5%. The reaction process of the high temperature protective coatings has been studied. MgO particles were distributed in the coating uniformly. FeO was formed at about 570oC and showed better plastic deformation ability above 800oC, which covered the MgO grain and reacted with them gradually from the outside, MgxFe(1-x)Fe2O4 with spinel structure and MgxFe(1-x)O with rock salt structure were formed at 1100oC. At 1200oC, iron oxides continuous diffusion outward led to the reduction of the concentration of MgO in the scale, and Mg2+ inward transport rate can be ignored when compared with Fe3+. MgxFe(1-x)O was not found in wüstite at this temperature, because the MgxFe(1-x)O formed at 1100oC were oxidized to MgxFe(1-x)Fe2O4.When temperature reached 1300oC，new reactant Fe3O4 and Fe2O3 dissolved inMgxFe(1-x)Fe2O4 unlimitedly. Mg concentration was diluted because mass of oxide scale continued increasing. It could be concluded that the specimen with MgO coating have the same oxide rate as the bare specimen after long period. Based on this principal, the mechanism of the protective coating has been given. Mg series protective coating could protect the steel by these two behaviors: (1) Mg2+ could occupy the vacancies in wüstite, and reduce the concentration of vacancies in wüstite, which leaded to the decrease of the velocity of cation diffusion through the oxide scale. (2) Mg2+ could substitute for Fe2+ in inner wüstite and outer spinel layer, form the relative compact MgxFe(1-x)Osolid solution and MgxFe(1-x)Fe2O4spinel respectively. The substitution would reduce the lattice parameter and enhance the energy barrier of which Fe ions diffusion through the layers. So the oxidation resistance of the steel was improved. These two mechanisms work simultaneously. Two influential factors on the application of coatings in industry have been studied. (1) Stuided the heat transfer of difficult-to-descale alloy steel in reheating furnace. Ansys 12.0 was used to simulate the influence of coating on the convection heat transfer rate of steel. (2) The influences of equipment and the technological process on the quality of coatings have been briefly discussed. |
| 语种 | 中文 |
| 公开日期 | 2013-09-25 |
| 源URL | [http://ir.ipe.ac.cn/handle/122111/1782]  |
| 专题 | 过程工程研究所_研究所（批量导入） |
| 推荐引用方式 GB/T 7714 | 周旬. 难除鳞合金钢坯高温多功能防护涂层[D]. 中国科学院研究生院. 2012. |
入库方式： OAI收割
来源：过程工程研究所
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