【作者】 钟宁；

【导师】 戎咏华；

【作者基本信息】 上海交通大学 ， 材料物理与化学， 2009， 博士

【摘要】 获得高强度高韧性材料一直是材料工作者们追求的目标。目前我国钢材年产量已逾四亿吨,但大量生产和应用的结构钢材强度一般400MPa~800MPa。为节约能源、减少环境污染、节约矿产资源,迫切需要发展高强度和超高强度钢,以实现器件的轻量化,达到节能减排的目的。为研制具有高强度(1500MPa-2000MPa),兼具一定的塑性和韧性且成本低廉的结构钢,本文根据徐祖耀等提出的高强度钢成分和组织设计思想和淬火-碳分配-回火(沉淀)(Q-P-T)热处理新工艺,结合近年来Speer等提出的淬火-碳分配(Q&P)热处理工艺,设计出1500MPa级别和2000MPa级别的Q-P-T高强度钢。利用光学金相(OM)、X射线衍射(XRD)、透射电镜(TEM)、扫描电镜(SEM)等多种方法研究了Q-P-T钢的显微组织,揭示了其具高强度和塑性的原因;为在热处理中对Q&P钢和Q-P-T钢的组织进行控制,本文研究了淬火-碳分配过程中残余奥氏体的稳定性。本文的主要研究内容如下:首先,设计成分为0.2C-1.53Si-1.46Mn的Q&P钢,并籍J-Mat Pro软件和Speer等提出的约束碳平衡(CCE)模型设计其相应的热处理工艺(初始淬火温度为250℃,碳分配温度为480℃)。经不同Q&P处理工艺后其抗拉强度约1000MPa,延伸率10~20%。其中,一步法(初始淬火温度和碳分配温度相同)处理Q&P钢的强度比两步法(初始淬火温度小于碳分配温度)处理Q&P钢高约200MPa,但其塑性仅约为两步法处理Q&P钢的一半。通过OM、SEM、TEM和XRD表征了两步法处理Q&P钢微观组织随碳分配时间的变化,研究结果表明Q&P钢组织由板条马氏体基体和条间残余奥氏体复合组成,其强度主要取决于组织中马氏体含量和马氏体中的碳含量,而残余奥氏体则起着提高塑性和韧性的作用。实验表明随碳分配时间的增加,碳化物析出增多,残余奥氏体含量减少。残余奥氏体含量减少的原因主要有:1)在Q&P处理过程中马氏体/奥氏体相界面的迁移;2)在较高温度和较长时间碳分配的过程中奥氏体分解成渗碳体。通过与其他先进结构钢(如双相钢,相变诱发塑性钢,马氏体钢等)比较,Q&P钢具有高强度和良好塑性相结合的优秀综合力学性能。为进一步提高钢的强度,根据徐祖耀等提出的超高强度钢成分和组织设计思想,通过在原有的Q&P钢成分基础上添加Nb、Mo等碳化物形成元素,研制出强度级别为1500MPa的低碳超高强度淬火-碳分配-回火(沉淀)钢(低碳Q-P-T钢),其成分设计为:Fe-0.2C-1.5Si-1.5Mn-0.053Nb-0.13Mo;热处理工艺设计为:初始淬火温度QT=220℃,碳分配和回火温度为400℃。低碳Q-P-T钢经两步法处理后其抗拉强度可达1500MPa,延伸率大于15%。通过OM、SEM、TEM和XRD等测试手段对其微观组织进行表征,结果表明,由于Nb的合金化使低碳Q-P-T钢较类似成分的Q&P钢具有更为细小的原始奥氏体晶粒尺寸,由此淬火后获得更为细小的马氏体领域和马氏体板条,马氏体板条内含高密度位错并弥散分布着几个纳米的复杂碳化物,而且由于Si的加入使马氏体条间存在一定量的薄膜状残余奥氏体(体积分数4～6%)。上述组织结构的特征正是由徐祖耀提出的高强度钢组织设计思想所预期的。为了获得更高强度级别,即抗拉强度达到2000MPa的超高强度钢,根据超高强度钢组织设计原理,通过在低碳Q-P-T钢成分基础上适当增加碳含量(降低Ms温度以细化组织,同时增加碳原子的固溶强化效应)和合金元素含量(增加弥散沉淀强化效应),设计出含Nb、Ni的中碳Q-P-T钢,设计成分为: Fe-0.485C-1.2Si-1.2Mn-0.21Nb-0.98Ni;相应的热处理工艺为:初始淬火温度QT=95℃,碳分配和回火温度为400℃。实验结果表明:研制的中碳Q-P-T钢经两步法处理后其抗拉强度大于2000MPa,总延伸率大于10%。通过SEM、TEM、XRD和EDS揭示了中碳Q-P-T钢的微观组织为:高密度位错的马氏体、马氏体条间的薄膜状残余奥氏体和马氏体内的弥散分布的复杂碳化物,其中马氏体板条宽度为几十纳米,残余奥氏体厚度和复杂碳化物的尺寸均为几个纳米。XRD衍射分析测定出中碳Q-P-T钢的残余奥氏体含量为4～6%。因此,本论文研究所获得组织全部为纳米级的马氏体高强钢首次被报道。研究表明中碳Q-P-T钢的高强度取决于马氏体组织的细化和马氏体中弥散析出的复杂碳化物,塑性主要取决于残余奥氏体和马氏体基体的软化(经Q-P-T处理后马氏体中碳浓度下降,碳的固溶强化效应降低)。本文研究验证了Q-P-T工艺的可行性和体现出其比Q&P工艺可获得更高强度结构钢和更广泛的含Si钢的应用。本文研究的Q-P-T钢(低碳和中碳)是继TRIP、TWIP和Q&P钢后,成为一种新型先进结构钢。在淬火-碳分配过程中马氏体/奥氏体相界面的迁移直接影响奥氏体的稳定性及其含量,从而影响Q&P钢的微观组织和力学性能。本文通过实验观察到0.2C-1.45Mn-1.53Si钢在Q&P处理过程中马氏体/奥氏体相界面发生了迁动,并从理论上证明在Q&P处理过程中存在界面上铁原子扩散的热力学条件,从而否定了Speer等提出的Q&P处理的CCE热力学模型的限制条件(马氏体/奥氏体相界面不移动)。通过热力学计算出相界面迁移的速度为3.08nm/s;随后通过数值模拟方法对Q&P过程中碳分配和马氏体/奥氏体相界面迁移进行耦合模拟,结果表明马氏体/奥氏体相界面在Q&P处理过程中可以产生数十纳米的位移。更多还原

【Abstract】 Currently in China, the annual output of steels is more than 400 million tons, but the general strengths of structural steel in production and application are between 400MPa ~ 800MPa. In order to save energy and raw materials as well as on environment protection, there is an urgent need to develop high or ultra-high strength steel. For gainning higher strength (1500MPa-2000MPa) accompanying adequate plasticity, toughness and low-cost structural steel, the principle of microstructure design of high-strength steel and a new Quenching-Partitioning-Tempering(Q-P-T ) process was proposed by Xu Zuyao(T. Y. Hsu)according to the principle of microstructure design of high-strength steel based on the recently developed Quenching and Partitioning(Q&P) treatment by Speer et al. Two strength level steels: 1500MPa level and 2000 MPa level have been designed, respectively. The microstructure of the Q-P-T steels were characterized by means of optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy(TEM) and the origin of good combination of high strength and adequate ductility of Q-P-T steels has been revealed. Furthermore, the stability of retained austenite during the Quenching and Partitioning process has been studied in order to control the microstructure of the Q&P and Q-P-T steel.The Q&P steel with composition of 0.2C-1.53Si-1.46Mn has been designed and developed, and the heat treatment process parameters have been proposed as following: the initial quenching temperature is calculated as 250℃and the carbon partitioning temperature is calculated as 480℃in Constrain Paraequelibrium (CCE) conditions by using the J-Mat Pro software. Tensile strengths of the 0.2C-1.53Si-1.46Mn steel developed are about 1000MPa, elongation of 10 ~ 20% after the different Q&P treatments. The tensile strength of one-step (the initial quenching temperature and the carbon partitioning temperature are the same) treated Q&P steel is higher than that treated by two-step process(the initial quenching temperature is lower than carbon partitioning temperature), but the elongation of one-step Q&P steel is only about half of two-step’s. The microstructure of Q&P steel has been characterized by OM, SEM, TEM and XRD, and the results show that the Q&P steel consists of martensite and retained austenite, and its tensile strength depends primarily on the volume fraction and the carbon content of martensite, the retained austenite plays an important role in plasticity and toughness. The experimental result indicates that the volume fraction of cementite and transition carbides (unstable) precipitated in martensite matrix increase, while the volume fraction of the retained austenite decreases with increasing partitioning time. The main reasons are: 1) the migration of martensite / austenite interface during the Q&P process; 2) the decomposition of austenite into cementite at relatively high temperature and longer period during the Q&P process. By comparing with other advanced structural steels (such as dual-phase steel, TRIP steel, martensitic steel, etc.), Q&P steel has a better combination of high strength and good plasticity.In order to further raise the strength of steels, based upon modifying the recently developed Q&P treatment, a Quenching-Partitioning-Tempering process is proposed for ultra-high strength steels containing certain amount of carbide forming elements such as Nb and Mo. A designed and developed steel with composition Fe-0.2C-1.5Si-1.5Mn-0.053Nb-0.13Mo shows tensile strength about 1500MPa and total elongation 15% after subjected to Q-P-T treatment(the initial quenching temperature is designed to be 220℃and the partitioning/tempering temperature is designed to be 400℃) . The microstructure characterization shows the parent austenite grains are much finer than that of Q&P steel with similar composition. The addition of Nb results in the existence of finer martensitic packets and thinner laths in the Q-P-T steel after quenching and fine complex carbide dispersively distributed in martensite matrix with high density dislocation and the existence of a certain amount (about 4～6% volume fraction as XRD measured) of retained austenite between martensite lath due to the addition of Si. The above microstructural features are expected by Hsu’ideal of microstructure design for good combined mechanical properties.In order to attain ultra-high strength steel with the tensile strength level of 2000MPa accompanying the elongation of 10% for the steel containing less than 0.5wt%C , and a medium-carbon Q-P-T steel is designed and developed according to the principle of design of composition and structure of high-strength steel. The corresponding process is designed as: the initial quenching temperature is 95℃and the partitioning/tempering temperature is 400℃. The medium carbon Q-P-T steel exhibits tensile strength as high as over 2000MPa and total elongation over 10%. The microstructure in the Q-P-T steel is determined as follows. The average width of martensite laths are several tens nanometers, and the width of film-like retained austenite around martensite as well as the average size of stable carbides dispercively distributed in martensite matrix are both several nanometers, such an ultra-high strength fully nano-scaled martensitic steel had never been reported before. The present work shows that the tensile strength of medium carbon Q-P-T steel depends on the refinement of packets and laths of martensite and the stable carbide precipitates in the martensite matrix, and the plasticity depends primarily on content of the retained austenite and the softening of martensite matrix (the carbon concentration decreased in matrix and the effect of solid solution strengthening of carbon decreased after Q-P-T process). The investigation of this work verifies the feasibility of Q-P-T process (including both low-carbon and medium-carbon Q-P-T steels) in stead of Q&P process and demonstrates its more extensive application for Si-alloying structural steels than Q&P process. As a results, Q-P-T steels studied in the work is a new family of advanced structural steels whose comprehensive mechanical properties are overwhelming to dual-phase (DP), transformation induced plasticity (TRIP), twinning induced plasticity(TWIP) and Q&P steels.Migration of interface between martensite and austenite directly affects volume fraction of retained-austenite during Q&P process, in turn does the microstructure and mechanical properties of steels. The migration of martensite/austenite interface is observed in this work and the thermodynamic condition allowing iron atom diffusion across the interface is theoretically verified, which argues the constrained condition of interface between martensite and austenite proposed in the“Constrained Carbon Paraequilibrium”(CCE) model by Speer et al. The migration velocity of martensite/austenite interface is thermodynamically calculated as 3.08nm/s, and the migration distance of interface is high up to tens of nanometers during the Q&P process as the numerical calculation.更多还原