【作者】 张君；

【导师】 丁桦；

【作者基本信息】 东北大学 ， 材料加工工程， 2015， 博士

【摘要】 随着全球范围内的能源紧张和环境恶化,减重、节能、环保和提高安全性已成为现在汽车工业的主要发展趋势。汽车制造商对车用钢铁材料的性能、成本和轻量化也提出了更高的要求。美国科罗拉多矿业学院的Speer教授等人提出了一种制备超高性能汽车用钢的工艺——淬火配分(Quenching and Partitioning, Q&P)。该工艺最早以低合金钢为研究对象,通过热处理工艺的设计,使钢铁材料的最终室温组织为板条马氏体和富碳的残余奥氏体。利用残余奥氏体的相变诱发塑性效应,实现提高钢材强度的同时而不降低其塑性的目的。为了进一步研究合金成分和微观组织对Q&P钢性能的影响,本文设计了四种不同成分的低碳Si-Mn钢,通过单一或者复合添加Nb、Ni、Cr等元素来实现实验钢组织的细化和析出强化,进一步提高钢的强度。在Q&P工艺的基础之上,通过改变热处理工艺,得到不同组织配比、不同残余奥氏体含量的实验钢。通过对实验微观组织和力学性能的分析,揭示了实验钢成分和热处理工艺对Q&P钢组织演变的影响规律,同时研究了不同组织所带来的微观变形机制的不同。主要的研究内容和结果如下：(1)设计了四种不同Nb、Ni、Cr含量的低碳Si-Mn基Q&P钢。利用Speer教授等人提出的“约束碳准平衡”(CCE)模型,对室温下实验钢中的残余奥氏体含量进行理论预测,从而提出合理的Q&P工艺参数。实验结果表明,普通的低碳Si-Mn基Q&P钢呈现了高强度和良好塑性的结合,其抗拉强度均达到1000MPa以上,对应的延伸率和强塑积分别为15%和16GPa%。而经过Nb微合金化的低碳Si-Mn基Q&P钢,其抗拉强度超过1100MPa(最高可达1220MPa),对应的延伸率和强塑积分别为18%和20GPa%,性能远远高于不含微合金元素的低碳Si-Mn基Q&P钢。但是在低碳Q&P钢中Nb的添加并不是越多越好,过多的Nb含量虽然能够提高实验钢的强度,但由于会降低钢中残余奥氏体含量而最终导致塑性和强塑积的下降。(2)Cr和Ni的添加能够显著提高低碳Q&P钢的抗拉强度(约为400～500MPa),但会导致延伸率的小幅度减少(约为2-5%)。实验钢的显微组织观察表明,Cr和Ni的添加能够明显地细化实验钢组织,减小马氏体领域尺寸。TEM观察结果显示,Cr和Ni的添加导致钢中出现较多的马氏体孪晶和Cr的碳化物析出颗粒。由于固溶的Cr和Ni能够提高奥氏体的稳定性,而Cr的析出却降低奥氏体中的碳含量,因此,两种实验钢中的残余奥氏体含量变化不大。(3)系统的研究了配分后的冷却方式对实验钢组织性能的影响。实验证明,配分后空冷处理有利于无碳化物贝氏体的产生,使未转变的奥氏体的富碳程度提高,提高了室温下钢中的残余奥氏体含量。配分后水冷的实验钢组织为板条马氏体+残余奥氏体；而配分后空冷的实验钢组织为板条马氏体+残余奥氏体+无碳化物贝氏体。与水冷的实验钢相比,空冷的实验钢抗拉强度下降约为200MPa,而总延伸率的提高将近一倍,综合力学性能得到显著提高。由于钢中残余奥氏体含量的提高,空冷的实验钢表现出更优异的加工硬化能力。(4)对实验钢进行了不同退火温度的Q&P工艺处理,通过扫描电镜、电子探针以及拉伸性能测试,对不同退火温度下实验钢的组织和性能进行了系统的研究,并采用Hollomon公式对实验钢的加工硬化行为进行对比分析。实验结果显示,经两相区(部分奥氏体化)退火Q&P工艺处理后,实验钢的组织为铁素体、马氏体和残余奥氏体。且在两相区退火过程中,钢中的C和Mn元素向奥氏体的富集提高了奥氏体的稳定性,有利于提高室温下钢中残余奥氏体的含量。两相区退火虽然使实验钢的强度降低,但在很大程度上改善了低碳Q&P钢的塑性,其最高延伸率可达24%以上,强塑积远远超过完全奥氏体化退火的实验钢。(5)利用马氏体时效钢的循环相变理论,对实验钢进行预先的完全奥氏体化淬火处理,研制出具有高强度和高延伸率的Q&Q-P钢。研究结果显示,经Q&Q-P和两相区Q&P工艺处理后,实验钢的基体组织均为铁素体加板条马氏体。不同的是,Q&Q-P钢中的板条马氏体领域尺寸较两相区Q&P钢的有所减小,且铁素体不是呈块状,而是呈条状。经Q&Q-P处理的实验钢,其残余奥氏体含量较两相区Q&P处理的实验钢有显著的提高。且经Q&Q-P处理的实验钢的强度较两相区Q&P处理的也有所提高,最高强度可达1150MPa,且延伸率均在20%以上,其最高强塑积可达到30GPa%。在多种Q&P处理的微合金化低碳Si-Mn基Q&P钢中凸显出优异的综合性能。(6)采用XRD实验对不同应变条件下的低碳Si-Mn基Q&P钢进行了测定,并结合对变形过程中实验钢瞬时加工硬化指数的分析,证实了实验钢中残余奥氏体的TRIP效应。通过对不同热处理工艺下残余奥氏体的含量以及变形过程中残余奥氏体TRIP效应的研究发现,低碳Si-Mn基Q&P钢中残余奥氏体的含量取决于残余奥氏体的成分、尺寸和形貌。而在变形过程中残余奥氏体的TRIP效应除了取决于上述因素外,还取决于实验钢的基体组织。在以马氏体为基体组织的实验钢中,当应变较小时,钢中残余奥氏体就开始大量地向马氏体转变,导致实验钢的延伸率不高。而在以铁素体和马氏体混合组织为基体的实验钢中,由于铁素体在变形早期承受了大量变形,延缓了应力集中,推迟了实验钢中残余奥氏体的TRIP效应。因此,在应变较大时,仍有残余奥氏体发生马氏体相变,提高了实验钢的延伸率。更多还原

【Abstract】 With the deterioration of environment and energy crisis in the world, the main trend of modern car industry is to reduce weight, conserve energy, protect environment and improve safety. The automakers propose higher demands on the performance, costs and lightweight of the automotive steels. For this reason, Professor Speer, from Colorado School of Mines, put forward a novel process for producing the automotive steels with excellent performance, named quenching and partitioning (Q&P). Through the heat treatment design, this process was first carried out on the low alloying steels to obtain the steels with lath martensite and carbon-enriched retained austenite. In view of the transformation induced plasticity effect of retained austenite, the simultaneous enhancement of strength and ductility can be realized in Q&P steels.In this regard, four kinds of low carbon Si-Mn steels are designed in this study. In order to increase the strength of the experimental steels by microstructural refinement and carbide precipitation, the single or combined addition of Nb, Ni and Cr have been adopted. The Q&P processes with different heat treatment parameters are performed to obtain the steels with different microstructures and different volume fraction of retained austenite. By the combined analysis of the microstructures and the mechanical properties of the experimental steels, the influences of the compositions and heat treatments on the microstructural evolution of the Q&P steels are revealed. Also, the differences on the deformation mechanism of the experimental steels with different microstructures are investigated. The main achievements are expressed below.(1) Four kinds of low carbon Si-Mn based Q&P steels with different contents of Nb, Ni and Cr were designed in the present study. In order to design the reasonable heat treatment parameters, the volume fraction of retained austenite in the experimental steels was predicted by the carbon constrained equilibrium (CCE) model. The experimental results indicated that the low carbon Si-Mn based Q&P steels exhibited a good combination of high strength and ductility. The ultimate tensile strength of the experimental steels was more than 1000MPa and the optimal elongation and product of strength and elongation were 15% and 16GPa%, respectively. However, the mechanical properties of the steels with Nb were significantly improved compared with the steel without Nb. The ultimate tensile strength exceeded 1100MPa (1220MPa for the highest strength) and the optimal elongation and product of strength and elongation were 18% and 20GPa%, respectively. However, the Nb-content in low carbon Q&P steels was not the more the better. Due to the carbon consumption of precipitation of NbC, the excessive Nb-content in steels could result in low carbon enrichment in austenite, which decreased the stability of austenite and, therefore, leading to the decrease in volume fraction of retained austenite. Finally, the total elongation and the product of strength and elongation of the experimental steels decreased.(2) The experimental steel with Cr and Ni exhibited significant improvement in the ultimate tensile strength (400-500MPa) of the experimental steels with a few decrease in elongation (2-5%). The addition of Cr and Ni could refine the original austenite grain size of the heat treated steel and resulted in the decrease in the size of martensite packets. The precipitation of Cr and the twinned martensite were observed in the experimental steel by TEM analysis. Due to the co-work of soluted Cr and Ni increasing austenite stability and precipitated CrC decreasing carbon content in austenite, there was no obvious change in the volume fraction of retained austenite in these two steels.(3) The effects of cooling styles after partitioning on the microstructures and mechanical properties of the experimental steels were investigated. The results showed that air-cooling after partitioning was beneficial to obtain carbide free bainite, which increased the carbon enrichment of austenite and increased the volume fraction of retained austenite in steels at room temperature. The microstructures of the experimental steels quenched after partitioning consisted of lath martensite and retained austenite. In contrast, except for the two above phases, the carbide free bainite was observed in the experimental steels air-cooled after partitioning. Compared with the quenched steels, the elongation of the air-cooled steels was remarkably improved with a certain decrease in the ultimate tensile strength (200MPa drop) and the optimal product of strength and elongation reached 24.33GPa%. The steels after these two cooling styles exhibited a two-stage work hardening behavior. Due to the high volume fraction of retained austenite, the air-cooled steels exhibited more excellent work hardening behavior than the quenched ones.(4) Annealing with different temperatures were carried out on the experimental steels to investigate the effect of annealing temperature on the microstructure and mechanical properties by SEM, EMPA and tensile tests. Also, the work hardening behavior was analyzed by Hollomon equation. The results showed that the microstructure of the intercritical annealed steels consisted of ferrite, martensite and retained austenite. During intercritical annealing process, austenite stabilizing elements, such as C and Mn, enriched in austenite, which played an important role in stabilizing austenite and, finally, increased the volume fraction of retained austenite in steels at room temperature. Compared with the complete austenitizing samples, the ultimate tensile strength of the intercritical annealed samples decreased due to the exsitence of ferrite, but the total elongation had been significantly improved and the optimal total elongation reached 24%. The product of strength and elongation for the intercritical annealed samples was much higher than that for the complete austenitizing samples.(5) In the consideration of the theory of cyclic phase transformation in the maraging martensitic steels, a novel design, the pre-quenching from complete austenitization before Q&P treatment, was performed on the low carbon Si-Mn based steels to produce the Q&Q-P steels with high strength and high ductility. The microstructures of the Q&Q-P and Q&P steels both consisted of ferrite, lath martensite and retained austenite. The differences were the the size of martensite packets in the Q&Q-P samples were reduced compared with the intercritically annealed Q&P steels and ferrite in the Q&Q-P samples exhibited lath shape instead of blocky shape in the intercritical annealed Q&P samples. XRD results showed that the volume fraction of retained austenite in the Q&Q-P samples was higher that that in the intercritical annealed Q&P samples. Also, the strength of the Q&Q-P samples had been improved and the optimal ultimate tensile strength reached 1150MPa. The total elongation of all the Q&Q-P samples excessed 20% and the optimal product of strength and elongation reached 30GPa%, which exhibited excellent performance amount the low carbon Si-Mn based Q&P steels.(6) Combined with the changes in the volume fraction of retained austenite and the instantaneous work hardening exponents during strain, the TRIP effect of retained austenite in the experimental steels was explicitly proved. The volume fraction of retained austenite and the TRIP effect of retained austenite during deformation in the steels by different Q&P processes were investigated. The results showed that the retained austenite fraction in the experimental steels depended on its compositions, size and shapes. However, not only was the TRIP effect of retained austenite depended on the above factors, but also on the matrix of the steels. For the steels with martensitic matrix, a large amount of retained austenite transformed to martensite at a small strain stage, which led to the unsatisfactory elongation of the experimental steels. In contrast, for the steels with complex matrix with martensite and ferrite (or carbide free bainite), the transformation of retained austenite was postponed due to the delayed stress concentration caused by the compatible deformation capability of ferrite. At the large strain stage, there was still some retained austenite which contributed to the TRIP effect and, finally, led to the enhanced elongation.更多还原