【作者】 王璐；

【导师】 孙玉福；

【作者基本信息】 郑州大学 ， 材料加工工程， 2011， 硕士

【摘要】 碳钢及低合金钢在低温工况下的开裂问题与钢材的强度和低温韧性有关,而材料的强度与低温韧性与材料的化学成分、显微组织和钢液质量有关,目前世界各国通过合金化、热处理及控轧控冷等措施来开发高强高韧性低温钢。课题在LCC基础上,优化主要合金元素、添加0～0.08%的微合金元素铌,设计淬火+调质、退火+调质、正火+调质三种热处理工艺,通过金相组织观察、SEM扫描及能谱分析、XRD分析、低温冲击试验和拉伸试验,研究了铌和热处理工艺对低温钢组织和性能的影响规律。本课题低温钢化学成分为C0.17%～0.19%,Si0.5%～0.6%,Mn1.1%～1.2%, P≤0.02%,S≤0.02%,Cr0.5%～0.6%,Ni0.98%～1.00%,Mo0.5%～0.6%, Nb0.03%～0.05%.该低温钢铸态组织为片状珠光体+少量块状铁素体,随着铌含量的增加,珠光体片间距逐渐减小,块状铁素体减少,组织细化。热处理后的试样组织均为回火索氏体,淬火+调质态回火索氏体中铁素体基本为多边形状,大小均匀；退火+调质态回火索氏体中铁素体多为板条状,多边形铁素体较少；正火+调质态为多边形铁素体加少量板条铁素体,板条长度比退火+调质态的短。该低温钢淬火+调质态低温冲击功随着含铌量的增加先升高后降低,-40℃比-20℃下降的程度更快。含铌0.039%时,-20℃、-40℃AKv达最高值,分别为45.7J和29.9J。试样-40℃冲击断口形貌以解理为主,同时有少量的准解理,解理台阶上主要为扇状花样,内部有撕裂岭。随着含铌量的增加,撕裂岭的长度和数量也逐渐增加,韧窝带加大,当含铌量超过0.039%时,解理面上出现河流花样。该低温钢三种热处理状态低温冲击功的变化规律基本相似,含铌量相同时淬火+调质态低温冲击功最高,正火+调质态次之,退火+调质态较低。含铌0.039%时三种热处理状态试样AKV均达最高值,-20℃时分别为45.7J、42.3J和44.4J,-40℃分别为29.9J、27.6J和28.4J。淬火+调质态和正火+调质态-40℃冲击断口形貌中撕裂岭数量明显大于退火+调质态,退火+调质态断口形貌解理台阶上为扇状花样,解理面上为河流花样。低温冲击功和断口形貌均表明含铌0.039%时淬火+调质、退火+调质和正火+调质工艺均能满足低温钢对组织和性能的要求,生产上可采用正火+调质工艺。该低温钢正火+调质态的抗拉强度、屈服强度以及断后伸长率均随着含铌量的增加呈现先增加后降低的变化规律。抗拉强度和屈服强度的变化规律非常相似,含铌量为0.057%时抗拉强度和屈服强度达到最大值1110MPa和990MPa,含铌量为0.039%时断后伸长率达到最大值17.0%。拉伸试样断口宏观形貌为典型的杯锥状断口,微观断口形貌为等轴韧窝,呈现韧性断裂的典型特征。随着铌含量的增加,粗大韧窝数量先减少后增加。该低温钢含铌0.039%经正火+调质工艺处理后,获得了强度、塑性和低温冲击韧性的较好匹配,-20℃、-40℃的冲击功为44.4J、28.4J,抗拉强度和屈服强度分别为995MPa、885MPa,断后伸长率为17.0%。该低温钢综合性能良好,可应用于石油天然气管道输送系统中的低温阀门零部件上,具有良好的发展前景。更多还原

【Abstract】 The cracking problems of carbon steels and low alloy steels at low temperature conditions are relevant to strength and low temperature toughness of steels, whereas the properties of the materials depend on the composition, microstructure and quality of liquid steel, and therefore all countries in the world develop high strength and high toughness cryogenic steels by optimizing the composition and microstructure. In this paper,0～0.08% weight percentage of niobium was added into LCC steel, the main alloying elements were optimized, and quenching+quenching and tempering, annealing+quenching and tempering, and normalizing+quenching and tempering three heat treatment processes were designed to develop high strength and toughness cryogenic steels. Microstructure observasion, scanning electron microscopy, spectrum analysis, X-ray diffraction analysis, low-temperature impact toughness test, and tensile strength test were done to research the effect of niobium and heat treatments on microstructure and mechanical properties of cryogenic steels.The chemical composition of the cryogenic steels were, carbon 0.17%～0.19%, silicon 0.5%～0.6%, manganese 1.1%～1.2%, phosphatides≤0.02%, sulphur≤0.02%, chromium 0.5%～0.6%, nickel 0.98%～1.00%, molybdenum 0.5%～0.6%, niobium 0.03%～0.05%.The as-cast microstructure of the cryogenic steels were pearlite+blocky ferrite. With the increase of the content of niobium in the samples, the interlamellar spacing of pearlite decreased gradually, and the microstructure was refined. The microstructure was tempered sorbite after three heat treatment processes. The microstructure was mainly homogeneous and fine polygon ferrite after quenching+ quenching and tempering, mostly lath ferrite and little polygon ferrite after annealing +quenching and tempering, and polygon ferrite and little lath ferrite and the length of lath was short after normalizing+quenching and tempering.The impact works of the cryogenic steels after quenching+quenching and tempering changed from increase to decrease with the increase of the content of niobium in the samples, and decreased faster at-40℃.When the content of niobium was 0.039%, the impact works peaked, respectively,45.7J at-20℃,29.9J at -40℃.The impact fracture morphology was brittle cleavage and little quasi-cleavage, composed of cleavage plane, tearing ridge and dimple with small size. When the content exceeded 0.039%, river pattern existed on the cleaved surfaces.The change of the impact works after three heat treatments was similar. The low-temperature impact toughness of the steels containing the same amount of Nb was best after quenching+quenching and tempering and better after normalizing+ quenching and tempering. The impact works peaked at 0.039% niobium, respectively, 45.7J,42.3J,44.4J at -20℃, and 29.9J,27.6J,28.4J at -40℃. The impact works and impact fracture morphology showed that the microstructure and mechanical properties of the cryogenic steels containing 0.039% niobium could be met after three heat treatment processes. The process of normalizing+quenching and tempering could be used in practice.The tensile strength, yield strength and percentage elongation after fracture of the cryogenic steels after normalizing+quenching and tempering first increased then decreased with the increase of the content of niobium. The variation between tensile strength and yield strength was very similar. When the content of Nb was 0.057%, tensile strength and yield strength had the maximum value 111 OMPa and 990MPa. When the content of Nb was 0.039%, percentage elongation after fracture was 17.0%. Macroscopic fracture morphology of tensile specimens was typical cup cone fracture, micro-fracture morphology was equiaxed dimples. With the increase of the content of niobium, the amount of coarsed individual dimples first decreased and then increased.When the cryogenic steels contained 0.039% niobium after normalizing+ quenching and tempering, the steels had good comprehensive performance. The impact works were 44.4J at -20℃, and 28.4J at -40℃, and tensile strength, yield strength and percentage elongation after fracture were 995MPa,885MPa,17.0%. The steels might be widely applied to the valve parts of petroleum and natural gas transmission pipeline system, with good development prospects.更多还原