【作者】 傅卫；

【导师】 方洪渊；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2018， 博士

【摘要】 铜结晶器作为连铸设备中的核心部件,其性能和寿命对连铸生产的稳定顺行至关重要。在结晶器铜板表面制备涂层是延长结晶器使用寿命的重要途径之一,但当前生产中广泛应用的结晶器镀层因局部过早失效主要表现为上部涂层热裂甚至剥落、下部涂层磨损以及铜板变形等,已越来越无法满足高效连铸生产的性能需求及环保要求。激光熔覆技术被认为是制备冶金结合且高质量结晶器铜板表面涂层的一种新兴绿色环保工艺,但是由于铜合金的高热导率及对激光的高反射率等特性,使得如何大面积制备与铜基材界面相容性好、可靠冶金界面连接、无裂纹等缺陷且具有良好使用性能的涂层仍是目前需要解决的难题。结晶器铜板的首要功能是作为冷凝器的结晶器系统的一个部件,铜板表面涂层的制备势必影响整个冷却系统的传热和受力状态。因此,研究了解涂层本身对系统热力行为的影响是结晶器铜板涂层制备的前提,基于研究结果反过来指导涂层的设计。课题建立铜结晶器温度场和应力场有限元耦合计算数值模型,研究不同涂层材质、涂层厚度以及非等厚涂层设计对结晶器铜板传热及热应力的影响规律,结果表明,结晶器铜板工作涂层表面具有不均匀的温度场和应力分布,弯月面附近承受最高的表面温度和热应力。结晶器铜板涂层表面温度随涂层厚度增加而明显增加,且导热性能越差的材质表面温度随厚度增加增长越快。等效热应力最大值出现在涂层表面,涂层表面承受热应力要高于涂层与基体结合面的热应力;且在弯月面以下随着高度的降低,等效应力值随之发生显著降低。根据结晶器铜板工作涂层表面温度场及应力场的分布特点以及涂层结构设计对其的影响规律,结合结晶器表面不同区域表现的不同失效形式,提出对结晶器铜板热面沿拉坯方向进行分区涂层设计,使涂层硬度与厚度梯度变化,以使各区域获得大致相同使用寿命的设计思想。结晶器热面上部区域制备0.6-0.8 mm厚度的低硬度Co基合金涂层,可保证铜结晶器弯月面附近区域良好的传热,热面最高温度在350℃以下;中部区域制备1 mm厚度的中等硬度Co基合金涂层,在铜结晶器热面高度方向上形成良好的传热及耐磨性过渡;下部区域制备2 mm厚度的高硬度Co基合金涂层,保证下部涂层的高耐磨性能。通过对铜合金表面激光熔覆制备涂层的材料、工艺方法和涂层结构进行设计解决了目前在研发及生产中难于在铜合金表面大面积制备可靠冶金界面连接且无缺陷的激光熔覆涂层的难题。采用光纤耦合输出半导体激光器,常温下对铜合金表面预置的0.4 mm纯镍镀层进行激光重熔,功率4200 W,扫描速度10 mm/s,搭接率30%时可获得较优的无缺陷且可靠冶金结合的涂层。镍镀层的预置和半导体激光的应用降低了铜基表面涂层制备的难度;预置镍镀层由重熔前的γ-Ni转变为重熔后的（Ni,Cu）固溶体是保证新涂层与铜合金基体良好的界面相容性和可靠界面冶金结合的基础。此外,激光重熔层硬度约为135 HV_0.05,稍高于CuCrZr基体的硬度,这种硬度平滑过渡的分布有利于缓解熔合界面的应力,为后续梯度强化涂层的制备奠定了基础。在激光重熔打底层上采用激光熔覆同步送粉法依次制备钴基过渡层和工作层获得无缺陷的梯度复合涂层。涂层组织成分和硬度的梯度变化缓解了涂层激光熔覆制备过程的热应力,避免了激光熔覆层裂纹的产生。所获激光熔覆梯度复合涂层具备良好的抗热疲劳及高温热稳定性能。其常温及高温销盘式摩擦磨损性能均远高于工业中成熟应用的结晶器铜板NiCo镀层,相对耐磨性为其10倍以上。激光熔覆涂层磨损机制表现为微切削“犁沟”状的磨粒磨损。为降低多层多道激光熔覆过程中的应力水平以避免涂层制备中的开裂现象,研究了熔覆工艺路径、单层熔覆厚度等对平板激光熔覆涂层应力的影响。并采用单元生死法数值模拟分析多层多道激光熔覆过程的应力场,研究分析了多层多道激光熔覆过程中热应力演变、分布与变形情况。结果表明,激光熔覆层残余应力为拉应力,且沿熔覆焊道方向残余拉应力远大于垂直焊道方向的残余拉应力;试件背部残余应力同样为拉应力。单层激光熔覆厚度的增加导致涂层及背部基材残余应力均明显增大。激光熔覆前对基材进行约2 mm拱度的预变形对涂层残余应力影响并不明显,但显著降低了试件背部残余应力。五种多层多道激光熔覆路径设计方案中,对待熔覆区分区堆焊且各分区间及子区域内多层熔覆层扫描路径垂直交叉熔覆的情况下,可有效降低激光熔覆层的残余应力,所得激光熔覆涂层残余应力水平最低。熔覆结束并充分冷却后板材产生沿长度中线方向的向上翘曲变形;熔覆层表面纵向拉应力大于横向拉应力及厚度方向应力,纵向塑性变形是产生熔覆层裂纹的主要原因。基于以上的涂层设计思想、新型涂层制备工艺及优化的熔覆路径,制作连铸铜结晶器实物,并进行工程上机验证,其过钢量从当前钢铁行业广泛应用的NiCo镀层结晶器的5万吨提高到18万吨,大幅度降低了生产成本。更多还原

【Abstract】 Copper crystallizer is a core component of continuous casting equipment,and its performance and service life are critical to the stable operation during the continuous casting.The preparation of coating on the surface of the crystallizer copper plate is one of the important ways to prolong the service life of the crystallizer.However,the premature failure of the crystallizer coatings which are widely used is mainly caused by local thermal cracking or peeling of the coating,wear and deformation of the copper plate,etc..Based on this,the current coating cannot meet the performance requirements and environmental requirements of high-efficiency continuous casting production.Laser cladding technology is considered as an emerging environment-friendly technology for preparing coatings with metallurgical bonding and high quality on the mold copper surface.However,due to the high thermal conductivity and the high reflectivity of copper alloys to laser,it is still difficult to realize large area preparation of coatings with good interfacial compatibility,reliable metallurgical bonding,free of cracks and other defects on copper substrate.The coating prepared on the surface of copper plate will affect the heat transfer and stress state of the copper cyrstallizer.Therefore,researches to understand the influences of the coating itself on the thermo-mechnical behavior of the crystallizer are the prerequisite for coating preparation on the copper crystallizer surface.The effects of coating material,coating thickness and design of coating structure with unequal thickness on the heat transfer and thermal stress of the mold copper plate were studied.The results show that the surface of the working layer had uneven temperature and stress distribution.The highest surface temperature and thermal stress occur near the mould meniscus.The temperature on the coating surface increases significantly with the thicker of the coating and the lower of the thermal conductivity of the coating material.The maximum value of the equivalent thermal stress appear on the surface of the coating near the mould meniscus.The thermal stress on the surface was higher than that on the bonding surface between the coating and substrate on the same thickness section.The equivalent stress decrease significantly with the decrease of height below the meniscus.According to the distribution characteristics of the surface temperature field and stress field of the working copper plate and the influence of the coating structure design,combined with the different failure modes of different regions on the surface of the crystallizer,a coating design idea is proposed to prepare the coating of which the hardness and thickness are gradiently increased along the direction of the mold blanking,so that each region of the hot surface of copper plate can obtain the same service life.The coating designed will gain better heat conduction in the upper region of copper plate and higher wear resistance in the lower part.In order to reduce the cracking risk of the coating during the multi-layer and multi-passlaser laser cladding process,the influences of the cladding process path and the thickness of the single-layer on the stress of the samples were studied.And the stress field of multi-layer laser cladding process was analyzed by numerical simulation with life-and-death cell method.The results show that the residual stress of the laser cladding layer is tensile stress,and the residual stress along the longitudinal direction of the cladding layer is much larger than that in the direction of the vertical direction.The residual stress on the back of the specimen is also tensile stress.The residual stresses in coating and back surface of substrate are both increased obviously when increasing the thickness of the laser cladding single-layer.Though the pre-deformation of the substrate with a crown of about 2 mm before laser cladding has no obvious effect on the residual stress of the coating,but significantly reduces the residual stress on the back of the specimen.In the five kinds of schemes about multi-layer and multi-pass laser cladding path,the residual stress of the laser cladding coating is the lowest in the case where sub-regional preparation of laser cladding coatings and the scanning paths of the cladding layers in each region intersect vertically.The plate has an upward warping deformation along the length of the center line after the completion and sufficiently cooled of the cladding.The longitudinal plastic deformation was the main cause to produce cracking during the laser cladding process.By designing the material,process and coating structure of the coating on the surface of copper alloy by laser cladding,the problem that it is difficult to prepare a reliable metallurgical interfacial bonding and no defect on the surface of copper alloy by laser cladding is solved.A superior defect-free and reliable metallurgically bonded coating can be obtained by laser remelting a 0.4 mm thickness of pure nickel plating on the surface of the copper alloy at room temperature with a fiber-coupled-output semiconductor laser equipment.The remelting parameters were as below:laser power 4200 W,the scanning speed 10mm/s,and overlap ratio 30%.The preset of nickel plating and application of semiconductor laser greatly increase the absorption rate of laser by copper substrate.The preset nickel plating converted fromγ-Ni before remelting to re-melted（Ni,Cu）solid solution to ensure a good metallurgical bonding.In addition,the hardness of the laser remelting layer was about 135 HV_0.05,slightly higher than the hardness of the CuCrZr matrix.The smooth transition of the hardness was beneficial to alleviate the residual stress at the fusion interface.The cobalt-based transition layer and the working layer are sequentially prepared on the laser remelting primer layer by laser cladding synchronous powder feeding,to obtain a defect-free gradient composite coating.The gradient of the composition and hardness of the coating mitigate the thermal stress during the laser cladding preparation process and avoid the cracking in the layer.At the same time,the obtained gradient composite laser cladding coating had good resistance to thermal fatigue and good thermal stability at high temperatures.Its pin-disk friction and wear performance at both room temperature and high temperature is much higher than that of NiCo plating for crystallizer,and its relative wear resistance is more than 10 times that of the NiCo plating.The wear mechanism of the laser cladding coating was characterized by micro-cutting"rough"-like abrasive wear.Based on the above coating design idea,new coating preparation process and optimized cladding path,the continuous casting copper crystallizer with laser cladding coating is made and verified by engineering.Its over-steel capacity has increased from 50,000 tons of NiCo plating cyrstallizers widely used in the current steel industry to 180,000 tons,significantly reducing production costs.更多还原