【作者】 李阳；

【导师】 吕宇鹏；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 钢铁材料经淬火-分配(Q-P)处理可获得优异的综合力学性能。近年来,该工艺在基础理论和应用研究方面都取得了明显进展。目前应用研究主要集中于开发新一代先进高强度钢,对于传统钢的Q-P热处理强韧化研究比较匮乏。在理论研究方面,分配过程中奥氏体的富碳化机制、变形时残余奥氏体(RA)起作用的增塑机制等问题有待进一步深入和系统研究。本文选用商品化合金钢27SiMn,35CrMnSi,35CrMo,35CrMoSi,38CrMoAl,60Si2Mn,9SiCr为试验材料,进行Q-P热处理试验,优化其工艺参数,并与传统调质、等温淬火热处理工艺进行对比。通过深冷和回火试验,研究了Q-P处理钢的组织和力学性能稳定性。同时,采用声发射(AE)技术和分子动力学(MD)模拟方法,研究了RA提高钢塑性的机理。对27SiMn,35CrMnSi,38CrMoAl,35CrMoSi钢分别进行不同淬火温度、分配温度和分配时间的Q-P处理。结果表明,仅35CrMoSi钢组织中RA含量与“限制条件碳”(CCE)模型预测的规律基本一致。相反,分配温度对RA含量的影响规律在所有试验钢中一致,即在350～420℃分配处理时,获得的RA含量最高；在低温或高温分配处理时,RA含量均较低。Q-P处理35CrMnSi和35CrMoSi钢中,RA含量在2～30min的分配时间内,RA量持续降低,但能保持在10vol.%左右。结合对工程应用中工件尺寸、操作规程等因素的考虑,认为淬火获得约75%的马氏体再在350～420℃温度范围内分配处理30min以内,可获得较理想的Q-P组织。通过对Q-P处理35CrMnSi和35CrMoSi钢光学显微组织的分析,得出贝氏体转变是分配处理时奥氏体富碳化的重要机制,可以作为对碳原子分配CCE模型的补充,解释了分配温度和时间对RA含量的影响规律。分配温度和时间适中(350-420℃处理30min以内)时,贝氏体转变和碳原子分配同时进行,贝氏体转变过程中碳原子自贝氏体铁素体向奥氏体中扩散作用与CCE模型描述的碳原子分配行为共同起作用,获得最多的RA。分配温度较低时,仅CCE机制起作用；分配温度过高或时间过长时,贝氏体转变完成(获得贝氏体铁素体和Fe3C),二者均获得较少的RA。根据Q-P原理,提出了“循环淬火-分配”(M-Q-P)工艺,可实现在更大范围内调控Q-P组织中RA的含量,并在35CrMnSi钢中成功应用。结果表明,五次M-Q-P处理35CrMnSi钢中RA的含量是单次Q-P处理组织中的2倍。随着RA量的提高,钢的抗拉强度降低,延伸率升高,强塑积基本不变。与传统调质和等温淬火热处理相比,Q-P处理后35CrMnSi钢的综合性能最好,强塑积远高于另外两种热处理得到的性能。将Q-P处理后的试样再高温回火,得到的组织和力学性能与传统调质处理的试样相近。在Q-P和等温淬火处理的试样拉伸过程中,产生了部分具有较高能量和持续时间的特殊声发射信号,在Q-P处理后又高温回火的试样中没有这种信号。结合对断口和微观组织的分析,确定这部分信号是由组织中的RA在变形过程中发生马氏体相变产生的,为TRIP机制提高Q-P钢的塑性提供了证据。MD模拟结果表明,采用M势函数对纯Fe双晶Bain模型进行准静态拉伸和压缩时,FCC晶体的应变诱发FCC→BCC相变行为导致拉伸过程中屈服的产生,相变以均匀形核、非均匀性长大的方式进行,新生相与母相之间具有K-S位向关系。压缩时导致相变产生的临界应力大于拉伸时的应力,且二者新生相的形态不同。采用A势函数模拟纯Fe三晶体K-S模型准静态拉伸和压缩过程。拉伸时,FCC晶体内自相界面处的位错萌生和滑移导致拉伸过程中屈服的产生,随着形变量的增加,在滑移面的交界处产生BCC相形核并逐渐长大,快速完成相变。压缩时,FCC相在应变作用下也向BCC相转变,但转变是以相界面迁移的方式完成,且在屈服发生前已完成相变。采用不同应变率进行拉伸变形时,屈服强度随着应变率的升高而增加,FCC相在较高应变率下更稳定。深冷和回火试验结果表明,Q-P组织中RA的含量在-80℃以上深冷和400℃以下回火时变化不大,表现了良好的热稳定性。经低温和中温回火后,Q-P处理钢的强度和塑性均下降,可能是由于回火改变了RA的力学稳定性；高温回火后,强度继续下降,塑性略有回升,综合力学性能与传统调质处理的力学性能相近。更多还原

【Abstract】 Quenching and partitioning (Q-P) heat treatments can be used to generate good combination of high strength and good plasticity in steels, and encouraging progress has been made in theoretical and applied studies of the process in the past few years. However, most of the studies are focused on the development of a new generation of advanced high strength steels (AHSS), while less attention are paid on the application of Q-P in traditional steels to enhance their mechanical properties. Additionally, theoretical perspectives such as the mechanism of carbon-enrichment of the austenite during partitioning treatment, the mechanism of the enhancement of plasticity and toughness works during deformation, etc. remain to be further developed. In this work, the experiments of the Q-P heat treatments are carried out on several traditional steels, e.g.27SiMn,35CrMnSi,35CrMo,35CrMoSi,38CrMoAl,60Si2Mn,9SiCr (in Chinese grade), and the results are compared with traditional quenching and tempering (Q-T) and austempering (AT) heat treatments after the optimization of the Q-P processing parameters. Moreover, the stability of retained austenite (RA) in the Q-P treated steels is studied systematically by cryogenic and tempering experiments. Meanwhile, acoustic emission (AE) technology and molecular dynamic (MD) simulations are applied to investigate the mechanism of the enhancement of plasticity and toughness by RA.Various Q-P heat treatments with different quenching temperature (QT), partitioning temperature (PT) and partitioning time (tp), are carried out on27SiMn,35CrMnSi,38CrMoAl,35CrMoSi steels. Firstly, the RA fraction follows the relationship with QT predicted by the CCE model qualitatively in35CrMoSi steel while different laws are found in other steels; Secondly, the variation of RA fraction versus the PT are similar in all of the experimental steels, i.e. the highest fraction of RA are attained in the range of350～420℃while a decreased RA fraction attained at lower or higher temperatures; Thirdly, the RA fraction decreases as the tp increases in the range of2～30min for35CrMnSi and35CrMoSi steels when partitioned at420℃maintaining at the level of10vol.%. Based on these results and combined with the consideration of engineering applications, it is suggested that the proper Q-P parameters acquiring desired microstructures should be quenched to attain about75%martensite and partitioned in the range of350～420℃for less than30min. Optical microstructural evidences that bainite transformation of austenite contributes to the carbon-enrichment of untransformed austenite are found in35CrMnSi and35CrMoSi, which accounts for the influence of PT and tp on the RA fraction along with the CCE model. When the proper partitioning process (partitioned at350～420℃for less than30min) was applied, both of carbon partitioning between martensite and austenite predicted by the CCE model and bainite transformation would take place. The carbide-free bainite is formed due to the effect of alloying elements in the steel, and the most austenite is stabilized by the two mechanisms. Only the carbon partitioning takes effect at lower temperatures and bainite with Fe3C formed at higher temperatures or at350～420℃for increased time, which induces the decrease of the RA fraction.A novel process termed "multi-cyclic quenching and partitioning"(M-Q-P), aiming at tailoring the RA fraction in an enlarged range, is developed based on the Q-P principle and applied in35CrMnSi steel successfully. For35CrMnSi steel,5times of Q-P heat treatment can increase the content of RA from8vol.%to17vol.%. As a result, the ultimate elongation of the steel is improved from17.4%after the typical Q-P heat treatment to27.1%after5times of Q-P treatment. Meanwhile, the improved combination of strength and ductility for steels by typical Q-P heat treatment is retained by the M-Q-P heat treatment.The Q-P heat treatments enhanced the combined mechanical properties of high strength and effective ductility for35CrMnSi steel, as compared with traditional heat treatments such as Q-T and AT. The mechanical properties would degenerate to a lower level as similar to Q-T heat treated steels once the RA degenerated by tempering for the Q-P treated steel. Additional AE signals with high amplitude and high energy were produced during the tensile deformation of the35CrMnSi steel with RA in microstructures (obtained via Q-P and AT heat treatments), and the additional AE signals would not appear again once the Q-P steel is tempered at high temperature. Combined the AE features with the Optical microstructural and fractography analysis, it is found that the additional AE signals are produced by the strain induced martensitic transformation of RA.Strain induced FCC→BCC phase transformation in a bi-crystal model of pure Fe containing interphase boundaries with a Bain orientation is investigated by MD simulation using the Meyer-Entel interaction potential. Under quasi-static tension and compresstion, homogeneous nucleation and heterogeneous growth of the BCC phase in the FCC crystal is observed. The phase transformation behavior induced the yielding and the yielding strength of the compression is higher than that of the tension. Moreover, the new-formed BCC phase has a K-S orientation with the FCC phase and no motion of the original interface is observed.The MD simulations of strain induced FCC→BCC phase transformation in a tri-crystal model of pure Fe containing interphase boundaries with a K-S orientation is also carried out using the Ackland potential. The heterogeneous nucleation of dislocation from the interphase boundaries induced the yielding and the BCC phase nucleates at the fault band as the strain increase. On the contrary, the FCC→BCC phase transformation has accomplished before the yielding via the motion of initial interphase boundaries under the quasi-static compression. When the tension is completed at a constant engineering strain rate, the yielding strength increases as the strain rate increases and the FCC phase is more stable at a higher strain rate.The results of cryogenic and tempering experiments show that the RA in Q-P treated steels are stable in the temperature range of-80～400℃. However, the strength and ductility are both decreased after tempering at low and medium temperatures due to the reduction of mechanical stability of RA in the Q-P treated steels. At the same time, the ductility is picking up while the strength is consistently decreasing as the tempering temperature is elevated continuously and the mechanical properties are at the same level as the Q-P treated steels.更多还原