【作者】 行舒乐；

【导师】 余圣甫；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2013， 博士

【摘要】 轧辊是轧钢和有色金属轧制的一项重要的大型消耗工具。大型热轧支撑辊售价在百万以上。支撑辊长期在严酷环境中服役,由于磨损造成的支撑辊的辊身直径低于其使用下限,或者在其服役期间出现的剥落等损伤现象,支撑辊就会失去使用功能。如何采用新材料、新技术、新方法对已失去使用功能的大型支撑辊进行修复再制造,恢复使用功能,这是钢铁冶金企业非常关心、重视的课题。使用ANSYS模拟了支撑辊在工况下的受力状态,并用采力可夫公式和赫兹理论计算了轧制力、最大接触应力、最大剪应力等数据。计算得到的轧制力大小为5.799MN,使用ANSYS得到轧制力大小为5.33MN。根据赫兹原理得到的最大接触应力和最大剪应力分别为1031.7MPa和313MPa,最大剪应力的最大值距离轧辊表面6.26mm。根据ANSYS模拟的结果,接触应力的最大值为1158MPa,最大剪应力的最大值为348MPa,最大剪应力最大值出现的位置为距离表面5mm处,这与轧辊实际发生接触疲劳片状剥落的位置近似。这些研究为硬面层材料的制备提供了目标和方向。使用ANSYS模拟了轧辊堆焊修复过程中轧辊的温度场和应力场,模拟得到的焊接热循环曲线与熔池尺寸与实测数据非常吻合。残余拉应力的实测值比模拟值低,压应力比模拟值高,这是由于模拟未考虑焊后材料的相变应力。模拟结果表明环向应力对焊缝的影响大于横向应力,环向应力在焊后最初表现为压应力,冷却后由于塑变表现为拉应力,拉伸应力最大值出现在焊缝中心和焊趾附近,拉应力模拟最大值为1086MPa,实测值为805MPa。横向应力主要表现为压应力,模拟压应力最大值为531MPa,实测值为354MPa,因此残余应力主要为较大的环向拉伸应力。在不同预热温度下模拟残余应力场,结果表明未预热的焊后残余应力的最大值为1190MPa,300℃预热残余应力最大值为801MPa,400℃预热残余应力最大值为642MPa,500℃预热残余应力最大值为491MPa。可见焊前预热可以有效减小焊后残余应力。通过优化实验制备了两种埋弧焊用硬面药芯焊丝,考察了硬面层硬度在不同回火热处理温度下的高温稳定性,两组硬面合金的最佳热处理温度为480℃,且高合金钢配方硬面合金的硬度值高于马氏体不锈钢配方硬面合金。并分析了Ni和Mo对硬面层高温稳定性的影响,根据硬度测试结果并考虑制备药芯焊丝的经济性决定钼和镍的添加量均为4%。氮合金化使残余的奥氏体更加稳定并且尽量保留固溶的Cr,从而延迟了M23C6及M6C型析出物的析出,因此氮合金化硬面合金的二次硬化温度为520℃,高于碳合金化的480℃。经强碳氮化物形成元素Nb、V、Ti的合金化后,硬面层的硬度明显得到提高。研究强碳氮化合金元素在堆焊冶金反应过程中的析出特点,碳氮化物主要分为两种,第一类为尺寸较大的富Ti的碳氮化物,在焊缝凝固初期形成,碳氮化物生长时间较长。第二类为小尺寸圆形颗粒的碳氮化物,此类碳氮化物应是在低温凝固过程和回火热处理过程中析出的,由于此类碳氮化物析出后生长时间较短,新的析出相没有足够的时间依附其上,所以尺寸较小,析出温度在520℃以上。这些析出的小颗粒碳氮化物成为堆焊合金回火热处理过程中二次硬化的重要原因。高温磨损实验表明氮合金化可以提高硬面合金的高温耐磨性。高温磨损的主要失效形式为高温氧化皮的剥落和磨粒磨损两种形式。硬面合金组织主要由马氏体和残余奥氏体组成。氮合金化具有稳定奥氏体区的作用,残余奥氏体含量增加,使得碳氮合金化硬面合金的韧性比碳合金化硬面合金的韧性高。塑性较差的碳合金化基体会更容易产生疲劳裂纹并更容易使裂纹扩展,产生的裂纹会使基体连同其上的氧化膜一起剥落,使得硬面合金的磨损率提高。在高温环境中硬面合金基体表面析出的的较细小的碳氮化物可以降低磨粒磨损引起的磨损失效。研究了稀土Ce对硬面合金断裂韧性和磨粒磨损行为的影响。Nb和稀土氧化物可以细化组织,从而提高堆焊金属的硬度,硬面合金的组织主要包括板条马氏体,残余奥氏体和点状碳化物。添加稀土后,柱状晶晶粒得到细化而且碳化物在基体中的分布也更加均匀弥散。Nb的添加对提高堆焊金属的断裂韧性有一定的影响,但是效果不如稀土氧化物。稀土氧化物仅细化了组织,而且可以净化晶界,使碳化物球化,这些都为提高堆焊层的韧性做出了贡献。添加稀土氧化物后硬面合金的耐磨性得到提高。“犁”作用和显微切削是其主要的磨损机理。硬度的提高能减小由微观切削形成的划痕的深度。同时断裂韧性的提高能增加塑性变形的抗力,所以由“犁”作用形成的犁沟两侧和前部材料的剥落数量减少。所以添加稀土氧化物后硬面合金的耐磨性由此得到提高。更多还原

【Abstract】 Roll is a large and important consumption tool for steel and nonferrous rolling. The selling price of large-scale backup roller for hot rolling is over one million RMB. Because the backup rollers are applied to severe conditions for long time, the decrease of roller diameter caused by abrasion and the spalling caused by high stress cycle are the main reason for the roller failure. How to repair the failure roller with the new material, new technology and new method is a big problem that ferrous metallurgy enterprises pay much attention.The stress state of backup roller during working was simulated by ANSYS. The rolling force (P), the maximal contact stress (σmax) and the maximal shear stress (τ45°) were also calculated with formula. The calculated value of the rolling force is5.799MN while the simulation value is5.33MN. According to the Hertz formula, the values of amax and τ45°respectively are1031.7MPa and313MPa, and the distance between the position of the maximal145°and the surface is6.26mm. According to the result calculated by ANSYS, the value of σmax and τ45°respectively are1158MPa and348MPa, and the distance between the position of the maximal145°and the surface is5mm, which agrees with the practice. These works provide purpose and direction for the preparation of hardfacing materials.The temperature field and stress field of roller during welding repairing process were simulated by ANSYS, and the welding thermal cycle curve and the pool size is quite consistent with the experiment data. The experiment data of residual tensile stress is bigger than the simulation value while the residual compressive stress is smaller than the simulation value. This is because the simulation process didn’t consider the influence of transformation stress. The simulation results show that the influence of circumferential stress on the weld is greater than the transverse stress. The circumferential residual stress after welding appears as compressive stress in initial stage, and then changes to tensile stress because of plastic deformation, the maximal tensile stress appears at the middle of the weld and the weld toe, the value of simulation maximal tensile stress is1086MPa while the experiment data is805MPa. The transverse residual stress appears as compressive stress, the value of simulation maximal tensile stress is531MPa while the experiment data is354MPa, which means the main residual stress after welding appears as circumferential tensile stress. The residual stress field of roller after welding under different preheating temperature was simulated, and the results show that the maximal residual stress is1190MPa without preheating, the maximal residual stress is801MPa while preheating temperature is300℃, the maximal residual stress is642MPa while preheating temperature is400℃, the maximal residual stress is491MPa while preheating temperature is500℃. It is obvious that preheating processing before welding is an effective method to decrease the residual stress.Two kinds of hardfacing flux-cored wire were prepared through optimization experiment. The tempering stability of hardness for the hadfacing alloy under different tempering temperature was studied, and the result shows that the best heat treatment temperature is480℃. The hardness value of hadfacing alloy measured from the high Cr alloy steel series is better than the martensitic stainless series. The influence of Ni and Mo on the tempering stability of hardness alloy was studied. According to the result and economic evaluation, the optimum additive content of Ni and Mo are both4%. Nitrogen alloying make the retained austenite and the Cr dissolves in the base stock more stable, thus the precipitate time of M23C6and M6C is put off, so the secondary hardening temperature of Nitrogen alloying hardfacing alloy is520℃, which is higher than the480℃of carbon alloying hardfacing alloy. The strong carbonitride forming elements, such as Nb, V, and Ti, which added to the flux-cored wire can obviously improve the hardness of hardfacing alloy. Precipitation characteristic of strong carbonitride forming elements was studied, and it is shows that there are two kinds of carbonitride particle. First, these particles are Ti-rich carbonitride with large size, and these particles are formed in the initial stage of weld solidification, thus particles have enough time to grow. The second kind is small and round carbonitride, these particles are precipitated in the low temperature solidification process and the tempering process. Because the growth time of the second kind particles after precipitation is very shot, the new precipitated phase has not enough time to grow on its surface, thus the particle size is very small and the precipitation temperature is above520℃. These small size carbonitride particles are the main reason for the secondary hardening of hardfacing alloy during tempering process.The result of high temperature wear experiment shows that the nitrogen can improve the high temperature wear resistance. The main failure modes of high temperature wear include the spalling of high temperature oxide skin and abrasive wear. There are two main phases in the hardfacing alloys, lath martensite and the residual austenite. Nitrogen alloying make the retained austenite more stable, and the increase of retained austenite proportion is benefit to the toughness of Nitrogen alloying hardfacing alloy. The base of carbon alloying hardfacing alloy with poorer toughness is easier to generate endurance crack and the crack is easier to grow. The crack would cause the base and the oxidation film which is on the base flake off together, and finally cause more wear. The small size carbonitride particles precipitated on the surface of hadfacing alloy under high temperature could reduce the amount of wear caused by abrasive wear.Effect of rare earth Ce on the fracture toughness and abrasive wear behaviour of hardfacing alloy was studied. Nb and RE oxide could refine the microstructure, thus the hardness of hardfacing alloy could also be improved. The hardfacing alloys microstructure in this research were composed of lath martensite, residual austenite and dotted carbides. With the addition of RE oxide, the size of columnar crystal was refined and carbides were distributed more homogeneously in the matrix. The addition of Nb has a certain effect on improving the toughness of hardfacing alloy, but not as effective as RE oxide. The addition of RE oxide not only could refine the microstructure, but also could purify the grain boundary and spheroidize the carbide, and all of these could contribute to improving the toughness of deposit. The wear resistance of hardfacing alloys with RE oxide was improved. Micro-cutting and micro-ploughing are the main abrasive micro-mechanisms.The improvement of hardness can decrease the depth of groove caused by micro-cutting, meanwhile, the improvement of fracture toughness can increase the resistance to plastic deformation and scratch, so that the amount of spalling lips and prows caused by microplugging is decreased. Thus the wear resistance of hardfacing alloys is improved.更多还原