连铸特厚板坯二冷强冷及表层组织控制研究

发布时间：2018-07-29 10:21

【摘要】：以微合金化钢S355及新钢特厚板坯连铸机为研究对象,首先通过热膨胀试验研究了冷却速度对组织转变规律的影响、通过高温拉伸试验研究了温度履历对钢种热塑性的影响,分析得到了 S355钢第三脆性区的高温脆性机理、明确了冷却速率及冷却温度对高温组织及塑性的影响规律;其次通过冷却后返温再降温试样的高温拉伸试验研究了返温制度下温度履历对钢种热塑性的改善效果,提出了特厚板坯连铸强冷最佳冷却速率及冷却温度;最后在钢种连续冷却过程中的相变规律及高温塑性特征的研究基础上,以铸坯凝固传热计算分析为基础,提出了特厚板坯连铸机垂直段与弯曲段强冷的新工艺,并以铸机二冷喷淋系统的结构分析为基础提出了强冷区域的喷嘴选型优化,制定了特厚板坯连铸二冷强冷工艺试验方案并组织了生产验证试验,获得了细化铸坯表层组织、降低表面裂纹率的效果。以微合金钢S355为研究对象,利用热膨胀分析仪、金相显微镜、扫描电镜、透射电镜及高温激光共聚焦显微镜等设备研究了微合金钢S355在连续冷却过程中微观组织的转变规律,研究表明,微合金钢S355发生贝氏体转变的临界冷却速率为2℃/s、发生马氏体相变的临界冷却速率为15℃/s,而当冷却速率达到7℃/s以上时不发生珠光体相变;原始奥氏体晶粒尺寸不均匀很容易造成混晶组织的出现。利用Gleeble热模拟设备系统研究了不同温度履历条件下微合金钢S355的高温塑性行为、利用扫描电镜和透射电镜观察了断口组织与析出物分布,研究表明:原始奥氏体晶界处形成的薄膜状铁素体是导致S355钢第三脆性区塑性低谷的首要因素;经强冷-返温-再冷却过程后,材料的热塑性明显提高,且断口附近网状薄膜铁素体明显减少、奥氏体晶粒内部形成了铁素体;大多数析出物在晶内均匀分布,位错交叉位置析出物尺寸较大。应用数学模型模拟分析了强冷工艺对连铸坯表面温度的影响,校核了现有的扇形段喷淋系统、优化了强冷区的喷嘴选型,确定了连铸机内强冷的最佳位置及冷却速度等强冷参数,提出了二冷强冷工艺方案并组织了生产试验。试验结果证明,铸坯表面温度在强冷后达到了 720℃以下,铸坯的表层组织得到了细化、晶粒度均匀性得到提高,基本消除了粗大的枝晶结构,强冷铸坯的表层晶粒度达到了 11级、表层细晶区深度达到了 5mm,同时铸坯的表面裂纹率由原来的7.53%降低到了 3.29%,实现了铸坯表层组织和表面裂纹控制的目的。强冷铸坯的对比、热送轧制试验结果说明,特厚板坯及钢板表层晶粒度得以细化、特厚钢板的心部质量得到了显著改善,说明强冷工艺的开发及强冷区的喷淋系统改造取得了成功。形成的创新点如下:1)提出并验证了利用二冷局部强冷工艺,细化铸坯表面组织、降低裂纹发生率的技术思想;2)揭示了 S355钢先共析铁素体膜及其厚度是影响钢种第三脆性区塑性的关键因素,通过铸坯热履历的控制,实现了铸坯表面铁素体膜及析出物的有效控制;3)以S355钢特厚板坯连铸为对象,开展了工业规模的二冷局部强冷试验,证明了特厚板表面铁素体化控制工艺的可行性。
[Abstract]:The effect of cooling rate on the law of microstructure transformation was studied by thermal expansion test. The effect of temperature record on the thermal plasticity of steel was studied by thermal expansion test. The mechanism of high temperature brittleness in third brittle zone of S355 steel was obtained, and the cooling rate was clarified. The effect of rate and cooling temperature on the microstructure and plasticity of high temperature was studied. Secondly, the improvement effect of temperature on the thermal plasticity of steel was studied by the high temperature tensile test of the back temperature and temperature drop after cooling, and the best cooling rate and cooling temperature of the heavy plate slab continuous casting were put forward. On the basis of the study of the phase transition and high temperature plastic characteristics, based on the calculation and analysis of the solidification heat transfer of the cast billet, a new technology for the strong cold of the vertical section and the bending section of the thick slab caster is put forward. Based on the structure analysis of the two cold spray system of the casting machine, the selection and optimization of the nozzle of the strong cold zone are put forward, and the two cold strength of the thick slab continuous casting is formulated. The cold process test scheme and the production verification test were organized. The effect of refining the surface structure and reducing the surface crack rate was obtained. Microalloy steel S355 was used as the research object. The microalloy steel S355 was studied by the thermal expansion analyzer, metallographic microscope, scanning electron microscope, transmission electron microscope and high temperature laser confocal microscope. However, the transformation law of microstructures in the process shows that the critical cooling rate of bainite transformation in microalloy steel S355 is 2 /s, and the critical cooling rate of martensitic transformation is 15 C /s, while the phase transformation of pearlite does not occur when the cooling rate is above 7 C /s, and the unhomogeneous grain size of the original austenite is easy to cause the mixed crystal group. The Gleeble thermal simulation equipment system was used to study the high temperature plastic behavior of microalloy steel S355 under different temperature records. The distribution of fracture tissue and precipitates was observed by scanning electron microscope and transmission electron microscope. The study showed that the thin film ferrite formed at the grain boundary of the original austenite resulted in the low plasticity of the third brittle zone in the S355 steel. The main factor of the valley is that the thermal plasticity of the material is obviously improved after the strong cold return temperature recooling process, and the ferrite in the net film near the fracture surface is obviously reduced. The ferrite is formed inside the austenite grain; most of the precipitates are evenly distributed in the crystal and the dislocations are larger in the intersection position. The strong cold work is simulated and analyzed by the mathematical model. The effect of art on the surface temperature of continuous casting billet is checked. The existing fan section spray system is checked, the nozzle selection of the strong cold zone is optimized, the best position and cooling speed of the strong cooling in the continuous casting machine are determined. The two cold strong cooling process scheme is put forward and the production test is organized. The test results show that the surface temperature of the slab is reached after the strong cold. Under 720 centigrade, the surface structure of the slab is refined, the grain degree uniformity is improved, the coarse dendrite structure is basically eliminated, the surface grain size of the strong cold cast billet reaches 11, the depth of the surface fine crystal area reaches 5mm, and the surface crack rate of the slab is reduced from 7.53% to 3.29%, and the surface structure of the slab is realized and the surface structure of the cast billet is realized. The purpose of control of surface crack. Contrast of strong cold cast billet, the result of hot rolling test shows that the grain size of the thick slab and steel plate can be refined, and the quality of the heart of the special thick steel plate has been greatly improved. It shows that the development of the strong cold process and the renovation of the spray system in the strong cold zone have been successful. The innovation points are as follows: 1) put forward and verified. Two cold local strong cooling technology is used to refine the surface structure of the slab and reduce the rate of crack occurrence. 2) it is revealed that the preeutectoid ferrite film and its thickness of S355 steel are the key factors affecting the ductility of the steel third brittle zone, and the effective control of the ferrite film and precipitates on the surface of the billet is realized through the control of the heat record of the billet. 3) 55 steel thick slab continuous casting is taken as an example. An industrial scale two cold local cold test is carried out, which proves the feasibility of the ferrite control technology on the surface of the heavy plate.
【学位授予单位】：钢铁研究总院
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TF777.1

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