含Nb低合金高强度耐磨钢强韧化与冲击磨损性能研究

发布时间：2018-07-04 07:41

本文选题：Nb微合金化 + NM500耐磨钢　；参考：《北京科技大学》2015年博士论文

【摘要】：NM500低合金高强度耐磨钢适用于低、中冲击载荷工况,服役环境要求其不但具有较高的硬度和强度,同时还需较高的冲击韧性。目前国内生产的该级别耐磨钢冲击韧性普遍较低,从而导致耐磨性能较差,如何在保证国产NM500耐磨钢硬度、强度的前提下,提高其冲击韧性,进一步提高其使用寿命,是目前国产NM500的主要研发方向。针对上述问题,本论文工作在国产NM500化学成分的基础上添加不同含量的微合金元素Nb,系统研究了Nb含量变化对实验钢的析出相转变热力学、相变动力学、热处理工艺优化、强韧化机制及抗冲击磨粒磨损性能等方面的影响,获得了具备高硬度、高强韧性及抗冲击磨损性能的新型低合金高强度耐磨钢化学成分及相应的热处理工艺。基于Thermo-calc热力学软件对含Nb NM500耐磨钢中析出相的类型、析出温度及析出量进行了计算,结果表明：实验钢中随着Nb的含量由0.018%增加到0.078%,富含Nb的MC型碳化物的析出温度显著提高,由1150℃提高到1300℃,同时析出量也明显增加,这有利于通过细晶强化提高实验钢的冲击韧性。在低温回火条件下,MC相、M7C3相、MC_ETA相和MC SHP相碳氮化物析出相会在淬火马氏体基体上弥散析出,产生二次硬化,使实验钢因回火而造成的硬度下降得到一定的补偿,为较高硬度的获得提供一定保障。实验钢不同奥氏体化温度下的奥氏体晶粒长大规律研究表明,随着Nb质量分数的增加,当奥氏体化温度在850～950℃温度区间时,Nb的细化晶粒作用比较明显,当奥氏体化温度在1000～1200℃温度区间时,Nb的细化晶粒作用减弱,实验钢的奥氏体粗化温度为950℃。奥氏体化时间对淬火组织硬度的研究表明,奥氏体化时间在10～20min区间时,随奥氏体化时间的增加,淬火组织硬度增加,当奥氏体化时间超过20min时,淬火组织硬度随奥氏体化时间的增加而逐渐降低,为使实验钢热处理后的组织硬度达到NM500的级别要求,奥氏体化时间应低于40min。实验钢动态连续冷却转变曲线(CCT曲线)的研究结果表明,在冷却过程中,实验钢主要存在先共析铁素体、珠光体+贝氏体、马氏体等三个转变区域,为使实验钢获得适当的马氏体+贝氏体比例,在获得较高硬度、强度的同时,也具有较高的冲击韧性,冷却速度选择在5～8℃/s比较合适。实验钢在850～890℃范围内保温20～40min后水淬,并在200～250℃范围内进行30～60min的低温回火后空冷,获得的组织为回火板条马氏体+少量残余奥氏体,可以使实验钢获得优良的硬度和强韧性配合。在此热处理工艺条件下,4组实验钢均达到国外企业生产的该级别耐磨钢的综合性能:含Nb量为0.043%的2#实验钢经850℃保温30min后水淬,再经250℃回火60min后空冷,获得的组织为回火板条马氏体+少量残余奥氏体,组织布氏硬度值为484、抗拉强度Rm=1652MPa、屈服强度Rp=1412MPa、断后延长率δ=10.8%、室温和-40℃冲击功值分别为53.3J和51.3J,达到了NM500低合金高强度耐磨钢的标准要求,并具有优良的冲击韧性,超过了国外厂家生产的同级别耐磨板的冲击韧性,为该淬火与低温回火热处理工艺下的最佳成分和热处理方案。实验钢经等温淬火与低温回火后的组织为回火马氏体+黑色针状下贝氏体。实验钢在850～930℃范围保温20-40min后淬入温度为330～370的50%NaNO2+50%KNO3熔盐中保温5-20s后水淬,之后在200℃回火30min后空冷,均可获得优良的硬度和强韧性配合,并表现出比淬火与低温回火热处理工艺更好的综合性能。在此热处理工艺参数条件下,含Nb质量分数为0.043%的实验钢不但综合性能好,而且各性能指标非常稳定,波动较小,易于热处理过程中的控制,因此,添加0.043%Nb为最佳选择。通过对实验钢经热处理后的试样模拟挖掘机铲斗等的冲击磨粒磨损实验的研究表明,在中等冲击载荷附加磨粒磨损的条件下,塑性变形区域占磨损表面的90%左右,由塑性变形造成的塑变疲劳和剥落是造成失重的主要原因。因此,在具备较高的硬度和强度的前提下,具备较高的冲击韧性,能表现出更好的抗冲击磨粒磨损能力。不同组织在冲击磨粒磨损下的磨损结果表明,在中等冲击磨粒磨损条件下,回火板条马氏体+黑色针状下贝氏体比回火板条马氏体+少量残余奥氏体具有更加优越的抗冲击磨粒磨损性能。在本实验钢成分条件下,含Nb量为0.043%的2#实验钢无论是采用淬火+低温回火还是等温淬火+低温回火热处理工艺,都表现出最佳的硬度和强韧性配合及性能稳定性,能表现出最佳的抗冲击磨损性能,所以添加0.043%的Nb为最佳选择。
[Abstract]:NM500 low alloy high strength wear resistant steel is suitable for low and medium impact load conditions. The service environment requires not only the high hardness and strength, but also the high impact toughness. At present, the impact toughness of this grade wear resistant steel is generally low, which leads to poor wear resistance, and how to ensure the hardness and strength of the domestic NM500 wear resistant steel. On the premise of degree, improving its impact toughness and further improving its service life is the main research and development direction of domestic NM500. In this paper, we add different content of microalloyed element Nb on the basis of the chemical composition of domestic NM500, and systematically study the thermodynamics and phase transition of Nb content change on the precipitation phase of experimental steel. The chemical composition and heat treatment process of a new low alloy and high strength wear resistant steel with high hardness, high strength and toughness and impact wear resistance are obtained by the dynamics, the optimization of heat treatment process, the mechanism of strengthening and toughening and the abrasion resistance of the impact abrasive.
Based on the Thermo-calc thermodynamic software, the precipitation temperature and precipitation of the precipitated phase in Nb NM500 resistant steel were calculated. The results showed that as the content of Nb increased from 0.018% to 0.078% in the experimental steel, the precipitation temperature of MC carbide rich in Nb increased significantly from 1150 to 1300, and the precipitation increased obviously. It is beneficial to improve the impact toughness of the experimental steel through fine grain strengthening. Under the condition of low temperature tempering, the precipitation of MC phase, M7C3 phase, MC_ETA phase and MC SHP phase carbonitride will disperse and precipitate on the quenched martensite matrix, producing two times of hardening, which makes the hardness decrease of the experimental steel due to tempering, and the higher hardness is obtained. For a certain guarantee.
The study of austenite grain growth at different austenitizing temperatures of experimental steel shows that, with the increase of Nb mass fraction, when the austenitizing temperature is at the temperature range of 850~950 C, the grain refining effect of Nb is more obvious. When the austenitizing temperature is at the temperature range of 1000~1200 C, the refined grain effect of Nb is weakened and the austenite of experimental steel is austenite. The somatic coarsening temperature is 950. The study of austenitizing time on the hardness of quenched tissue shows that when austenitizing time is in the range of 10 ~ 20min, the hardness of the quenched tissue increases with the increase of austenitizing time. When the austenitizing time exceeds 20min, the hardness of the quenched tissue decreases with the increase of austenitizing time, which makes the hot place of the experimental steel. The hardness of the tissue is up to the level of NM500. The austenitizing time should be lower than the dynamic continuous cooling transition curve (CCT curve) of 40min. experimental steel. The results show that in the cooling process, the experimental steel mainly existed three transition regions, such as first eutectoid ferrite, pearlite + bainite, martensite and so on, in order to make the experimental steel suitable for martensite. The ratio of bainite to bainite has higher toughness and higher impact toughness, and the cooling rate is more suitable at 5~8 /s.
The experimental steel is quenched at 850~890 C for 20 ~ 40min, and is cooled at low temperature of 30 ~ 60min at 200~250 C. The obtained microstructure is tempered lath martensite and a small amount of retained austenite. The experimental steel can obtain excellent hardness and strong toughness. In this heat treatment process, 4 groups of experimental steels are reached. The comprehensive properties of the wear-resistant steel produced by foreign enterprises: the 2# experimental steel with Nb 0.043% was quenched at 850 C for 30min and then cooled at 250 C after tempering for 60min. The microstructure obtained was tempered lath martensite and a small amount of retained austenite. The hardness value of the tissue Brinell was 484, the tensile strength was Rm=1652MPa, the yield strength Rp=1412MPa, and the post fracture extension. The long rate Delta =10.8%, the impact work value at room temperature and -40 C are 53.3J and 51.3J, respectively. It has reached the standard requirement of NM500 low alloy high strength wear resistant steel, and has excellent impact toughness. It has exceeded the impact toughness of the same grade wear-resistant plate produced by the foreign manufacturers. It is the best composition and heat treatment scheme for the quenching and low temperature tempering heat treatment process.
The microstructure of experimental steel after isothermal quenching and low temperature tempering is tempered martensite and black acicular bainite. The experimental steel is quenched in 50%NaNO2+50%KNO3 molten salt at 850~930 C for 20-40min after heat preservation and then quenched for 5-20s in the molten salt of 330~370. After tempering at 200 c for 30min, the excellent hardness and toughness can be obtained. It shows better comprehensive performance than quenching and low temperature heat treatment. Under the conditions of heat treatment, the experimental steel with Nb mass fraction of 0.043% is not only good in comprehensive performance, but also very stable in various performance indexes, less fluctuating and easy to control during heat treatment. Therefore, adding 0.043%Nb is the best choice.
The research on the impact abrasive wear test of the experimental excavator shovel after heat treatment of the experimental steel shows that the plastic deformation area accounts for about 90% of the worn surface and the plastic deformation caused by plastic deformation is the main cause of the weight loss under the condition of the medium impact load additional abrasive wear. On the premise of high hardness and strength, it has high impact toughness and can show better impact abrasive wear ability. The wear results of different tissues under impact abrasive wear show that the tempering lath martensite + Black needle like bainite is better than the tempered lath martensite and a small amount of remnant austenite under the medium impact abrasive wear conditions. Under the condition of the steel composition, the 2# experimental steel with the amount of 0.043% Nb has the best combination of hardness and toughness and the stability of strength and toughness, and it can show the best anti impact wear resistance. Yes, so adding 0.043% of Nb is the best choice.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1

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