【作者】 孙桂芳；

【导师】 刘常升； Jyoti Mazumder； 陶兴启；

【作者基本信息】 东北大学 ， 材料学， 2009， 博士

【摘要】 冶金轧辊是使金属产生塑性变形的工具,是决定轧机效率和轧材质量的重要大型消耗性部件。其质量和使用寿命,直接关系到轧制生产的效率、产品质量、生产成本及钢材品种结构。因此,提高轧辊耐磨性、延长轧辊的使用寿命对降低辊耗至关重要。除了改进轧辊制造技术和优化轧辊材质外,近年来,钢铁公司广泛采用感应加热淬火、堆焊、热喷焊、热喷涂等方法提高轧辊表面耐磨性,但制备的耐磨涂层存在一定的不足。激光表面合金化和激光直接金属沉积作为一种新兴的表面技术,最大优点是可以制备致密的冶金结合涂层,从而改善基体的耐磨性。本文旨在利用激光合金化和激光直接金属沉积技术在轧辊表面制备具有冶金结合界面、组织致密和耐磨性能优良的涂层,为利用激光合金化技术修复和强化轧辊表面,利用直接金属沉积技术制备轧辊工作层提供有效的理论指导,并为其今后发展应用提供一定的技术支持。本文以高镍铬无限冷硬铸铁、球墨铸铁和70MnV铸钢热轧辊作为研究对象,探索利用激光表面合金化技术对其表面进行强化处理的可行性;以1018低碳钢为基体,探索采用激光直接金属沉积的方法沉积热轧辊工作层材料的可行性。通过激光工艺参数的优化,制备出具有良好冶金结合的耐磨涂层。利用光学显微镜（OM）、附带能谱仪（EDAX）的扫描电镜（SEM）、X-射线衍射仪（XRD）、表面光度仪、显微硬度计以及常温和高温摩擦磨损试验机等测试分析设备,对所制备涂层的微观组织、成分、裂纹分布及扩展、相组成、硬度和磨损行为进行了系统的研究。采用高功率CO₂激光器对预涂C-B-W-Cr粉末的高镍铬无限冷硬铸铁轧辊进行激光表面合金化处理。结果表明,合金化层与基体形成了冶金结合,部分区域存在裂纹。在激光功率、光斑直径、搭接率一定的条件下,合金化层厚度随扫描速度变化不大;裂纹率随扫描速度增加而增加;合金化层硬度随扫描速度的增加先增加后降低。当激光功率为7.2kW,光斑直径为0.8～3mm,搭接率为33.3%时,最佳扫描速度为11m/min。此时,合金化层平均厚度为0.29mm,平均显微硬度为1001HV_0.05,是基体材料（656HV）的1.53倍。球墨铸铁轧辊表面激光合金化C-B-W-Cr后,得到和基体呈冶金结合的合金化层;合金化层厚度为0.33～0.37mm;随着激光比能量的增加,气孔数减少、裂纹率降低,合金化层硬度提高;优化工艺参数为:激光功率4kW、扫描速度4m/min、光斑直径1.5mm、搭接率33.3%;此时得到的激光合金化层的厚度为0.37mm,平均硬度为1201HV_0.05,是基体(500HV_0.05)的2.4倍;合金化层组织为先共晶碳化物+枝晶间分布的细小片层状莱氏体（M+A_R+Fe₃C）;500℃空气中磨损实验结果表明,合金化层耐磨性是球墨铸铁基体的1.6倍;合金化层的磨损机制是粘着磨损、磨粒磨损和氧化磨损的混合磨损。在70MnV铸钢轧辊上进行激光表面合金化NiCr-Cr₃C₂粉末得到与基体呈冶金结合的合金化层;合金化层结构致密、无孔洞和裂纹;随着扫描速度的增加,合金化层和热影响区厚度减小,残余奥氏体含量增加,合金化层硬度和耐磨性提高;相组成受扫描速度影响很小,含量稍微变化;当激光功率为4kW、扫描速度为2.2 m/min、光斑直径为1.5mm、搭接率为33.3%时,合金化层具有最高的硬度和耐磨性;合金化层平均厚度和硬度分别为0.48mm和858HV_0.1;500℃空气中干滑动摩擦磨损904.32m后,合金化层的耐磨性是同条件下基体耐磨性的8.8倍;合金化层耐磨性的提高是细晶强化、固溶强化以及韧性γ-Fe基体、硬质相Cr₇C₃、Fe₃C和马氏体,以及基体和硬质相之间良好的结合等综合作用的结果。利用激光直接金属沉积的方法,在1018低碳钢基体上分别沉积了Co-285高温合金粉末、Co-285+30wt%WC混合粉末和（Co-285+30wt%WC）+0.8wt%Y混合粉末。当沉积粉末为Co-285高温合金粉末时,优化工艺参数为:激光功率为0.8kW,光斑直径为0.5mm,搭接率50%,粉末质量流率为8.6g/min,扫描速度为0.375m/min。优化工艺参数下涂层的硬度为420HV_0.5。涂层中出现了α-Co固溶体,Cr₂₃C₆和Co₃W。激光直接金属沉积制得的大面积无裂纹沉积层,在室温下空气中与Al₂O₃刚玉球磨损60min后,涂层体积损失为1.4mm³,磨损机制为粘着磨损、塑性变形、磨料磨损和氧化磨损。当沉积粉末为Co-285+30wt%WC和Co-285+30wt%WC+Y时,优化工艺参数为:激光功率为1kW,光斑直径为0.5mm,粉末质量流率为8.5g/min,搭接率为50%,扫描速度为0.3m/min。优化工艺参数下沉积层中未熔WC除外的基体的平均硬度为751HV_0.5,是Co-285涂层(420 HV_0.5)的1.83倍。Co-285+30wt%WC涂层中出现了WC,NiCoCr固溶体,W₂C,CoC_x,Cr₃C₂和Cr₂₃C₆相。Co-285+30wt%WC+Y涂层中存在WC、NiCoCr固溶体、W₂C、CoC_x、Cr₇C₃、Cr₂₃C₆和Co₆W₆C。在和Co-285涂层相同的磨损实验条件下,Co-285+WC和Co-285+WC+Y沉积层的磨损体积分别为0.18mm³和0.17mm³,Co-285沉积层磨损体积分别为它们的7.8倍和8.2倍。对Co-285+30wt%WC沉积层进行激光重熔,重熔层组织更加均匀,无显微缺陷,WC溶解更加充分,重熔层硬度平均硬度由重熔前的770 HV_0.5提高至963HV_0.5;可见,激光重熔对改善沉积层综合性能有益;添加的0.8wt%Y降低了Co-285+30wt%WC沉积层的残余应力、裂纹和气孔率、硬度;增加了Co-285+WC沉积层的韧性;稍微增加了Co-285+30wt%WC沉积层的耐磨性;并未改变其磨损机制。利用激光表面纳米陶瓷合金化技术可显著提高球墨铸铁和铸钢轧辊表面的磨损抗力,尤其对碳和合金含量低的轧辊强化效果更加明显。利用激光辅助直接金属沉积技术得到的Co-285沉积层致密、沉积性能很好,但是其硬度和耐磨性有待进一步提高;添加WC显著增加了Co-285沉积层的耐磨性;激光重熔提高了Co-285+WC沉积层显微组织的分布均匀性和显微硬度;添加Y明显降低了Co-285+WC沉积层的裂纹率,稍微降低了其显微硬度,提高了Co-285+WC沉积层的耐磨性。更多还原

【Abstract】 Rolls are the main consumptive parts which can deform the metals and determine the efficiency of the rolling mills and the quality of the mill bars. The quality and lives of them have an important effect on the efficiency of rolling production, product quality and manufacturing costs. How to improve the wear resistance and elongate the service life of rolls matters a lot in reducing the consumption of rolls.In recent years, in addition to the improvement in the manufacture technique and materials of rolls, induction heat quenching, puddle-welding and thermal spraying are widely used to improve the wear resistance of rolls. But there are some drawbacks in the obtained coatings. Laser surface alloying （LSA） and laser-aided direct metal deposition （LADMD） can fabricate dense coatings with a metallurgical bonding with substrates, thus improving the wear resistance of the substrates. The aim of the present work is to use laser techniques to fabricate dense, wear resistant coatings which have a metallurgical bonding with the substrates. It can offer theoretical instructions on repair and strengthening of rolls by LSA, fabrication of new work layers by LADMD and technical support for their further application.The substrates for LSA were high-Ni-Cr infinite chilled cast iron rolls, nodular cast iron rolls and 70MnV cast steel rolls. The 1018 mild steel was used as the substrates for the LADMD experiments. High power laser was used to carry the experiment. Coatings with good metallurgical bonding with substrates were fabricated with optimal processing parameters. Optical microscopy （OM）, scanning electron microscopy （SEM） attached with energy dispersive x-ray spectrometry （EDX）, X-ray diffractometer （XRD）, profilometer, microhardness tester and wear tester were used to analyze the microstructure, composition, the distribution and propagation of cracks, phase and wear behavior of the coatings.The laser alloying technique was applied to the high-Ni-Cr infinite chilled cast iron roll coated with C-B-W-Cr nano-carbide ceramics. The results revealed that the alloyed layers combined metallurgically with the substrate with a lot of cracks and pores found in part. With conditions that the laser power, spot diameter and overlap ratio remained unchanged, the thickness of the coating changed slightly with varying scanning speed, but the ratio of cracks increased and the hardness of the alloyed layer increased then decreased with the increase of scanning speed. The optimal scanning speed was 11 m/min, when the laser power, beam diameter and overlapping ratio were 7.2kW, 0.8-3mm and 33.3%, respectively. In this case, the average thickness of alloyed layer was 0.29mm with an average microhardness up to 1001HV_0.05, ie, about 1.53 times as high as that of the substrate of 656HV.C-B-W-Cr powders were used as the alloying powders on nodular cast iron rolls. The fabricated layers had metallurgical bonding with the substrates. The thickness of layers was 0.33-0.37mm. The number of pores and cracks went done and the hardness of the layer increased with the increase of the laser specific energy. The optimal processing parameters were: laser power 4kW, laser spot diameter 1.5mm and overlap ratio 33.3%. The thickness of this coating was 0.37mm and the average microhardness was 1201 HV, which was 1.4 times higher than that of the substrate. The layer was composed of proeutectic carbides and ledeburite eutectic which contained martensite, residual austenite and cementite. Wear test results at 500℃in ambient air showed that the wear resistance of the coating was 0.6 times higher than that of the nodular cast iron substrate. The wear mechanism of the layer was the mixture of adhesive wear, abrasive wear and oxidation wear.Metallurgical bonding was achieved between the 70MnV cast steel roll substrate and the layer with NiCr-Cr₃C₂ powders by LSA. The layer was dense, pore and crack free. As the scanning speed increased, the thickness of the layer and the HAZ decreased, the content of the retained austenite increased and the hardness and wear resistance of the layer increased. The phases were influenced by the scanning speed with only a little variation in the content. When the laser power is 4kW, scanning speed is 2.2m/min, spot diameter is 1.5mm and overlap ratio is 33.3%, the obtained layer had the highest hardness and wear resistance. In this case, the thickness and hardness of the layer is 0.48mm and 858HV_0.1, respectively. Wear test at 500℃in ambient air showed that the wear resistance of the layer was 8.8 times of that of the substrate after sliding for 904.32m. The improvement of wear resistance was contributed to the co-effect of the grain refinement, solution strengthening, the toughγ-Fe matrix,Cr₇C₃, Fe₃C and martensite hard phases and the good bonding between them.LADMD was used to fabricate Co-285, Co-285+30wt% and （Co-285+30wt%WC） +0.8wt%Y coatings on 1018 mild steel substrates.The optimal parameters for the Co-285 coating were laser power 0.8kW, spot diameter 0.5mm, powder flow rate 8.6g/min and the scanning, overlapping ratio 50% and scanning speed 0.375m/min. The coating was pore and crack free with a microhardness of 420HV_0.5 with the optimal parameters. The coating was composed ofα-Co solid solution, Cr₂₃C₆ and Co₃W. Volume loss of the coating after wear against the Al₂O₃ ball in the ambient air in the room temperature for 60min was 1.4mm3. Wear mechanism of the coating was a mixture of adhesive wear, plastic deformation, abrasive wear and oxidation wear.The optimal parameters for the Co-285+30wt% and Co-285+30wt%WC+Y coatings were: laser power 1kW, spot diameter 0.5mm, powder flow rate 8.5g/min, overlapping ratio 50% and scanning speed 0.3m/min. The average hardness of the coating without the undissolved WC was 751HV_0.5, which was 1.83 times of that of the Co-285 coating (420HV_0.5). Co-285+30wt%WC+Y coating was pore and crack free while the Co-285+30wt%WC coating had some micro-cracks and pores. XRD results indicated that the Co-285+30wt%WC coating was composed of WC, NiCoCr solid solution, W₂C, CoC_x, Cr₃C₂ and Cr₂₃C₆, while Co-285 +30wt%WC+Y coating was composed of WC, NiCoCr solid solution, W₂C, CoC_x, Cr₇C₃, Cr₂₃C₆ and Co₆W₆C. Volume loss of the Co-285+WC and Co-285 +WC+Y coatings after wear against the Al₂O₃ ball in the ambient air in the room temperature for 60min was 0.18mm³ and 0.17mm³, respectively. The wear volume loss of the Co-285 coating was 7.8 times of that of the Co-285+WC coating and 8.2 times of that of the Co-285+WC+Y coating. Wear mechanism of the two coatings was also a mixture of adhesive wear, plastic deformation, abrasive wear and oxidation wear, but the adhesive wear and abrasive wear of them were greatly suppressed. The Co-285+WC coating was remelted by laser to investigate the remelting effect on the microstructure and hardness of the coating. Results indicated that the microstructure in the remelting layer was uniform without pores or cracks and the WC had a large dissolution. Furthermore, the microhardness was improved from the 770HV_0.5 in the Co-285+WC coating to 963HV_0.5 in the remelting layer. Laser remelting is good to improve the quality of the deposited coatings. The addition of 0.8wt% Y reduced the residual stress, crack or pore ratio and hardness in the Co-285+WC coating, whilst improved the toughness and wear resistance a little without changing the wear mechanism.Laser surface alloying improved the wear resistance of the nodular cast iron and cast steel rolls. The effect is especially obvious when the roll material is of low carbon and alloying elements content. The deposited Co-285 coating is dense and of good properties, but the hardness and wear resistance of it need to be improved. The addition of WC can improve the wear resistance of the Co-285 coating and laser surface re-melting improved the distribution of microstructure and the microhardness of the Co-285+WC coatings. The addition of Y decreased the crack rate and the microhardness of the Co-285+WC coatings while improved the wear resistance of it at the same time.更多还原