【作者】 张文凤；

【导师】 杨柯； 单以银；

【作者基本信息】 中国科学技术大学 ， 材料加工， 2014， 博士

【摘要】 本文以目前超(超)临界火电机组中实际使用的蠕变性能优异的耐热钢P91、P92钢以及未来核聚变反应堆用候选结构材料中国低活化马氏体耐热钢CLAM钢为研究对象,研究了几种常用高铬马氏体耐热钢的失效方式。并在P92钢基础上进行成分优化,以提高组织的高温稳定性,增强材料的热强性。同时结合弥散强化合金在不同条件下的不同蠕变机制,提出了多尺度碳氮化物强化马氏体耐热钢的概念,建立了热处理态及蠕变过程中的组织模型,并通过热变形及后续热处理的方法获得了多尺度碳氮化物强化马氏体耐热钢,蠕变性能优于P92钢。首先,通过调整钢的化学成分,主要包括降C,以降低M23C6的含量,从动力学上降低其粗化速率；去B,以防止形成脆性BN成为裂纹源；去Mo,以避免形成粗化速率较高的Laves相。然后通过调整钢的热变形参数,控制诱变铁素体的体积分数及分布进而控制诱变析出相的尺寸及分布。主要的实验结果是,通过精确定位各种软化机制的开始位置,如动态回复、动态再结晶、准动态再结晶、诱导相变、静态再结晶等,确定各软化机制的发生条件及各软化机制对组织演变的影响,进而调整变形参数以获得目标组织。研究结果显示,在低Zener-Hollomon(Z)条件下(高温低应变速率),动态再结晶及诱导相变的快速进行导致了近等轴晶组织的形成。随着Z值增加,动态再结晶及诱导相变的形核过程减慢,但诱导相变铁素体的长大速度较大,形成条状铁素体和马氏体组织。同时铁素体的长大消耗了大部分的储存能,使其成为维持良好加工性的主要因素。但当Z值继续增加时,动态再结晶和诱导铁素体晶粒的长大速率也大幅降低,但准动态再结晶发生,使动态再结晶晶粒快速长大,导致铁素体和马氏体混合晶组织的出现。由于铁素体中的Cr、Nb、V等合金元素固溶量小于奥氏体中的含量,且合金元素在铁素体中的扩散系数高于在奥氏体,因而高温下诱变铁素体会更利于析出相的诱导析出及长大,铁素体的分布及形态决定着诱导析出相的分布。因此可以通过控制诱变铁素体的含量及分布来调整诱导析出相的分布及体积分数。而变形条件为1000-1100℃C温度区间及0.01-1/s应变速率时,诱变铁素体的形态为条状,与马氏体相间分布,且诱导铁素体的体积分数约占50%,为最有利于析出相析出及均匀分布的变形条件。在随后的变形后的弛豫实验中,确定了析出相开始析出位置为弛豫曲线中的应力突增位置。实验结果发现,在不同变形条件及弛豫温度下,诱导析出相的析出行为不同,例如在连续变形后的弛豫过程中,Nb(C,N)析出相在940℃C变形并弛豫时大量析出；在变温连续变形后的弛豫过程中,M23C6在800℃C变形并弛豫时大量析出；在变温非连续变形后的弛豫过程中,除了上述两种析出相外,在750℃C变形并弛豫时,(Nb,V)(C,N)大量析出。而且变形量及初始变形温度也影响析出相的析出行为。前者通过影响位错密度,进而影响位错节数量,即析出相的形核位置,最终影响析出相的析出行为；后者通过影响该温度下的组织形态,尤其是诱变铁素体分布及含量,最终影响析出相的分布及数量。而析出相的尺寸是弛豫温度和弛豫时间的函数,在高温时,合金元素扩散速率较大,有利于析出相的析出；时间延长,析出相的扩散距离增大,有利于析出相的长大。对于在940℃C鼻尖温度析出的Nb(C,N)粒子,其弛豫1000s时,析出相的尺寸最大在120nm左右；在800℃C鼻尖温度析出的M23C6粒子,弛豫1000s时,尺寸最大在230nm左右；在750℃C鼻尖温度析出的(Nb,V)(C,N),弛豫1000s时,尺寸最大在30nm左右。最后通过控制后续热处理工艺参数,实现多尺度但碳化物强化马氏体耐热钢的制备。其中,后续热处理主要涉及奥氏体化及回火过程,热变形后的试样经奥氏体化后,初始的诱变铁素体+马氏体双相组织均转变为奥氏体,并在空冷后切变为单一马氏体组织。随保温时间的延长,晶粒的均匀性提高。变形过程中诱变析出的碳氮化钒、M23C6在奥氏体化过程中全部重新溶入基体,而Nb(C,N)则由于在奥氏体中的固溶度积小而溶解的较少。回火过程中,合金元素在未溶的析出相与奥氏体界面上偏聚,导致非均质形核,形成较大尺寸(200nm)的析出相,稳定了晶界及亚晶界。同时,位错节上形成弥散细小(<20nm)的析出相,钉扎位错。最终获得稳定性较高,符合设计的组织模型：多尺度碳氮化物强化的单一马氏体组织。研发成功的多尺度碳氮强化马氏体耐热钢在600℃C时效时表现出优良的组织稳定性,在650℃C时效时,组织发生再结晶,稳定性急剧降低,但再结晶发生开始时间由单尺度析出相强化时的500h延长至3000h。通过组织观察发现,650℃C时效时发生再结晶的原因与晶界上200nm左右析出相的重溶有关。新钢种在600℃C蠕变时,随应力的增加,位错密度增加,组织得到细化。其在600℃C的持久性能优于P92钢,且随应力的增加,其持久性能的优越性更加突出,到210MPa时是P92的2倍以上。尽管调控后的组织初步达到设计的目标,但200nm左右的析出相分布不均匀,且蠕变／时效过程中析出的Laves相易于连成条状,失去了阻碍晶界运动作用的同时,成为裂纹的萌生的优选位置。后续研究应该重点放在200nmm析出相的分布及Laves相的长大方式等方向上。更多还原

【Abstract】 The thesis proposed a method to improve the microstructure stability at high temperature in order to increase the heat resistant steels. The new method was brought up based on the study of the failure mechanisms of the most currant used steels. Meanwhile, the concept of the new multi-size carbonitrides strengthened heat resistant (NS) steel was also, for the first time, brought up, which also provided the schematic of the multi-size carbontrides strengthened microstructure. The microstructure was experimentally gained through proper heat deformation and the following-up heat treatment. The NS steel with the desired microstructure showed better creep properties than the P92steel.Firstly, the chemical composition was modified by reducing the carbon content to decrease the volume fraction of M23C6particles and their coarsening rate through decreasing the driving force. Meanwhile, the boron was eliminated to avoid the formation of BN, which is brittle and would cause the cracking. Additionally, the Mo content was reduced to nearly zero in order to decrease the coarsening rate of Laves phase of Fe2Mo.During the hot deformation, the softening mechanisms, including dynamic recovery, dynamic recrystallization, metadynamic recrystallization, dynamic phase transformation and static recrystallization, were precisely located. The thesis also mentioned the conditions for the occurrence of all the above softening mechanisms and how these processes affected the microstructure evolution. Therefore, the desired microstructure could be formed by controlling the deformation conditions. For instance, at the low Zener-Hollomon (Z) value, i.e., high deformation temperature and low strain rate, both the dynamic recrystallization and the dynamic phase transformation took place, which resulted in the approximate equal-axial grains. With increase of the Z value, the process of dynamic recrystallization slowed down and the growth of strain-induced ferrite grains consumed most of the stored energy. Therefore, the growth of the ferrite made the most of the effort in maintaining the ductility. However, the growth rate of both dynamic recrystallization grains and the ferrite grains decreased, leading to the blend microstructure of ferrite and martensite with no obvious phase boundaries. Actually, the strain-induced ferrite played the most important role in the formation of precipitates, due to its low solute contents of alloying elements and high diffusion rates of alloying elements in it. Therefore, the volume fraction and distribution of ferrite determined the amount and the distribution of the precipitates. The optimum deformation for the formation of ferrite lied in the range of1000-1100℃with strain rate of0.01-1/s. Meanwhile, the ferrite preferred to form and grew along the prior austenite boundaries and inflated faster with increase of the temperature.In the stress relaxation curves, the stress abruption was determined to be the starting of the precipitation. However, different kinds of particles preferred to be precipitated under different conditions. For instance, the Nb(C,N) tended to formed at940℃under the condition of continuous deformation, while the M23C6particles preferred to precipitate during the relaxation after the second deformation at800℃followed by the primary deformation at900℃without interval relaxation. However, when200-second interval relaxation took place between the two passes, the (Nb,V)(C,N) particles bloomed at the temperature of750℃. Besides the factors mentioned above, the amount of reduction and the primary deformation temperature affected the precipitation behavior as well. The former one would prompt the precipitation by increasing the dislocation density with larger reduction. The number of nods of dislocations, which was the initial sites of precipitates, was exaggerated when the dislocation density was increased. The latter one would change the distribution and volume fraction of the precipitates through altering the distribution and volume fraction of the ferrite. As mentioned earlier, the nucleation and the growth of the strain-induced ferrite were closely depended on the deformation conditions. With extension of the relaxation time, the particles grew larger. As for the Nb(C,N) particles, which tended to precipitated at940℃, it would take them about1000s to achieve120nm maximum in size. While the M23C6particles at800℃grew up to230nm after1000s relaxation. However, the largest (Nb,V)(C,N) particles were only30nm after relaxation for1000s at750℃.The followed-up heat treatment, mainly austenizing and tempering, was to modify the microstructure. The dual phase of martensite and ferrite evolved into single martensite due to austenizing, while most of the precipitates resolved into the matrix except for the Nb(C,N) particles, which had small solution content in the austenite and would partially dissolved into the matrix. Therefore, the remained particles became the nuclei and the alloying element tended to integrate on them during tempering. The heterogeneous precipitation during the tempering led to uneven size of particles. The200nm-size precipitates among them were associated with the nucleus of remained particles and would contribute to the stabilization of the boundaries. Another one was that the under-20nm particles, which were attributed to the homogeneous formation of the precipitates without nucleus, would stump the dislocation movement effectively.The microstructure of muli-size carbonitrides strengthened heat resistant steel exhibited excellent stability at the aging temperature of600℃. Recrystallization took place in the microstructure after aging at650℃for3000h, latter than the single-size carbonitrides strengthened heat resistant steel at650℃for500h. The recrystallization was found to be associated to the resolution of the200nm-size particles at boundaries. The microstructure of the newly developed muli-size carbonitride strengthened steel got refined with the increase of stress at the creep tests. This refinement was mainly attributed to the augment of dislocations density. But when crept at650℃, with the increase of stress, besides the increase of the dislocations density, the speed of the dislocations movement was also advocated, resulting to the more refined microstructure. The new steel showed better creep resistance than the P92steel at600℃, as almost twice the creep rupture time as P92steel at210MPa.However, the200nm-size carbonitrides distributed unevenly and the Laves phase formed during aging/creeping tended to grow into chains, although the modified microstructure mainly met the required items. The coarse Laves phase could not abrupt the movement of boundaries efficiently and would highly stimulate the cracking. Therefore, the emphasis would be laid on the distribution of200nm-size particles and the growth behavior of Laves phase in the future work.更多还原