【作者】 姜雯；

【导师】 赵昆渝；

【作者基本信息】 昆明理工大学 ， 材料物理与化学， 2014， 博士

【摘要】 超级马氏体不锈钢的成分特点是在传统马氏体不锈钢的基础上,降低C含量至≤0.02%,增加Ni含量为4-6%,Mo含量为0.5-2.5%。超级马氏体不锈钢的成分特点就决定了它是一种具有高强度、高韧性以及良好的耐蚀等性能的不锈钢材料,该材料优异的综合性能与其回火显微组织中具有逆变奥氏体这一韧性相有关。因此,控制回火过程中逆变奥氏体的形成对和发展具有良好强韧性的超级马氏体不锈钢具有重要的意义。本文采用多种实验方法对三种不同成分的超级马氏体不锈钢展开基础性研究工作,研究了热处理制度对不同Cr含量试验钢组织性能的影响并对比了两试验钢中逆变奥氏体形成规律及机制的异同,还探索研究了合金元素(Ni和Cu)的添加对逆变奥氏体形成的作用。13Cr和15Cr两种超级马氏体不锈钢经900-℃温度淬火后其显微基体组织都为淬火马氏体,而15Cr钢中由于添加有较多的Cr含量导致其淬火组织中还含有少量的残余奥氏体和δ-铁素体。两试验钢的奥氏体晶粒尺寸均随着淬火温度的升高不断增大,当淬火温度小于1050℃时,13Cr钢中奥氏体晶粒长大速度较快,而淬火温度高于1050℃时,15Cr钢由于解除碳化物及第二相的钉扎作用更明显,使其奥氏体晶粒长大速度迅速增加。在整个淬火温度范围内两试验钢的洛氏硬度变化趋势相似,但15Cr钢具有更高的硬度且变化趋势相对平缓。两试验钢经1050℃淬火,400-750℃回火后,回火显微组织都为回火马氏体、逆变奥氏体和ε-Cu。两试验钢中逆变奥氏体含量随着回火温度的升高都是先增加再减少,并在650-700℃时达到最大值,但15Cr钢中逆变奥氏体的含量多于13Cr钢,在基体中的分布也更密集。由于15Cr钢中具有较高的合金总量及较多的逆变奥氏体,因此它的硬度、强度和延伸率都高于13Cr钢,并且随着回火温度的升高各项性能的变化相对平缓。超级马氏体不锈钢中逆变奥氏体的形成机制为扩散机制,Ni元素的扩散对逆变奥氏体的形核长大起到重要作用。但13Cr钢和15Cr钢中逆变奥氏体的形成规律较相似,但仍存在差异。两者的相同之处包括：(1)逆变奥氏体含量和尺寸变化趋势相同,即随着回火温度的升高,两试验中逆变奥氏体的含量和尺寸均先增加再减小,并在650-700℃达到最大值。(2)两试验钢中逆变奥氏体的形成规律相同：即在低温回火时,逆变奥氏体开始形核；随着回火温度的升高,Ni元素在逆变奥氏体内的富集程度不断提高,导致逆变奥氏体尺寸逐渐长大；当回火温度继续升高后,逆变奥氏体由于稳定性下降而导致回溶,其尺寸逐渐变小,而马氏体板条内部的逆变奥氏体在回溶过程中,变细的逆变奥氏体会将原来较宽的马氏体板条分割成两条或是若干条,从而导致对基体组织的细化。(3)随着逆变奥氏体含量的变化,两试验钢回火后力学性能变化趋势相同,即两钢的抗拉强度随着回火温度的升高均先减小再增加,与逆变奥氏体含量变化趋势相反,而延伸率随着回火温度的升高先增加再减小,与逆变奥氏体含量变化趋势基本相同。13Cr钢和15Cr钢中逆变奥氏体的形成规律存在的不同之处包括：(1)形成温度不同。经测试分现,13Cr钢中逆变奥氏体在550℃回火时开始形成,15Cr钢在450℃回火时逆变奥氏体开始形核。(2)形核位置存在差异：13Cr钢中逆变奥氏体的主要形核位置主要集中在马氏体板条边界处和马氏体板条内部,而15Cr钢除了以上两个形核位置外,其逆变奥氏体还会附着在碳化物M23C6处形核。(3)含量与尺寸不同：15Cr钢中由于含有较高的Cr和Ni,其回火组织中逆变奥氏体的含量和尺寸明显大于13Cr钢。(4)Ni元素富集程度不同：在逆变奥氏体形核长大过程中,15Cr钢中逆变奥氏体内的Ni的富集程度大于13Cr钢,从而导致15Cr钢中逆变奥氏体更稳定。(5)细化基体的程度不同：当回火温度超过650-700℃时,逆变奥氏体将重新转变为马氏体,15Cr钢在回溶过程中对基体组织的细化作用更明显。经过对逆变奥氏体的形成过程分析发现,Cu与Ni对促进其形核长大具有一定的协同作用。对比1.5Cu钢和3Cu钢逆变奥氏体的形成发现：当Cu添加量较多时,会增加逆变奥氏体中Ni的富集程度和富集区域,从而促进逆变奥氏体含量；此外,在逆变奥氏中Ni浓度高的区域,Cu元素的富集程度也较大,并且此区域附近ε-Cu析出的也较多。即Cu和Ni在促进逆变奥氏体形核长大方面具有协同作用。这一协同作用使钢中逆变奥氏体含量增加,从而对改善超级马氏体不锈钢的力学性能起到促进作用。更多还原

【Abstract】 On the basis of traditional martensitic stainless steel, super martensitic stainless steel (SMSS) decreases the content of carbon (wt.%≤0.02%) and increases the content of Ni (4%～6%) and Mo (0.5%～2.5%). Because of the feature of composition and application, super martensitic stainless steel has high strength, high toughness and good corrosion resistance and other properties.The excellent of properties of this steel is related with the reversed austenite. In this thesis, three kinds of SMSS are designed. The effect of heat treatment processing on the microstructures and mechanical properties of different content of Cr tested steels have been researched. The formation rules and transformation mechanism of reversed austenite in tested steels have been focused. The influence of alloying elements (Ni and Cr) on formation of reversed austenite have been studied.The matrix microstructure of13Cr and15Cr tested steels is lath martensite after900-1100℃quenching, while there are retained austenite and δ-ferrite in15Cr because of higher Cr content. The sizes of austenite grain increases with quenching temperature increasing. When the quenching temperature is below1050℃, the growth of austenite grain13Cr is faster. However, when the quenching temperature is above1050℃, the growth of austenite grain in15Cr increases rapidly due to dissolution of the carbide and the second phase. The variation of HRC in two tested steels is similar after quenching. HRC of15Cr is bigger and more gently than13Cr. The microstructures are tempered martensite, reversed austenite and ε-Cu after tempering at400-750℃in two tested steels. The volume fraction of reversed austenite increases first, and then decreases with the increasing tempering temperature and reaches the maximum value at650-700℃. Reversed austenite in15Cr is more than that in13Cr and distributes more dense in the matrix. Since there are more amount of alloying element and reversed austenite, so HRC, tensile strength and elongation are higher than those in13Cr steel. Moreover, the variation of the mechanical properties in15Cr is more smooth with the increasing tempering temperature.The formation mechanism of reversed austenite is diffusion in super martensitic stainless steel. Ni diffusion plays an important role in nucleation and growth of reversed austenite. The formation of reversed austenite in13Cr and15Cr steels is similar, but there are still some difference. The sameness:(1) Both the quantity and the size of reversed austenite increases first, and then decreases with the increasing tempering temperature and reacher the maximum value at650-700℃(2) The formation rule of reversed austenite in two tested steels in the same. Reversed austenite nucleates at low tempering temperature; With the increasing tempering temperature, the degree of Ni enrichment in reversed austenite increases constantly, so that the reversed austenite size becomes bigger; When the tempering temperature continues to rise, reversed austenite dissolved back and its size becomes smaller. The dissolution of reversed austenite in the martensitic lath will separate the lath into two or several, which leading to refinement of matrix.(3) The variation trend of mechanical properties is the same. The tensile decreases first, and then increases with the tempering temperature, which is opposite with the variation of the reversed austenite quantity. However, elongation and the amount of reversed austenite are both increase first, and then decrease with the increasing tempering temperatureThe formation of reversed austenite in13Cr and15Cr steels still exist some differents:(1) The reversed austenite in13Cr forms at550℃, while forms at450℃in15Cr.(2) The nucleation site of reversed austenite are at the boundary of martensitic lath of in the lath in both13Cr and15Cr. In addition, reversed austenite in15Cr also can attach to the M23C6and nucleate.(3) Due to the more content of Cr and Ni, the amount and size of reversed austenite in15Cr are bigger than those in13Cr.(4) The degree of Ni enrichment in15Cr reversed austienite is bigger than that in13Cr.(5) When the reversed austenite dissolve, the refinement effect on matrix in15Cr is more obvious than that in13Cr.This paper compares two tested steel with1.5Cu and3Cu. The results show that there is a synergistic effect between Ni and Cu on formation of the reversed austenite. When Cu content is higher, it can increase the degree of Ni enrichment in reversed austenite and the quantity of enrichment areas. Thus it also can promote the increasing of amount of reversed austenite. Moreover, in the area of high Ni concentration, there also is the high degree enrichment of Cu and ε-Cu precipitation. This synergy can make reversed austenite content increased and the mechanical properties of super martensitic stainless steel improved.更多还原