超细贝氏体组织演变及相变加速技术研究

发布时间：2018-06-06 23:58

本文选题：超级贝氏体 + 相变动力学　；参考：《北京科技大学》2016年博士论文

【摘要】：将成分为0.98C-1.59Si-1.94Mn-1.33Cr-0.30Mo-0.02Ni-0.11V的合金,在200-300℃等温数十小时可以获得超薄贝氏体铁素体片与残留奥氏体片层叠结构(Slim Bainitic ferrite-Austenite,SBA)的超细贝氏体组织。SBA钢以其超细(片厚100nm)的显微结构、低廉的原料成本、简单易行的制备工艺和超高强韧性能(抗拉强度2.5GPa,硬度600HV,韧性30-40MPa·m1/2)受到学术界和工业界的广泛关注。然而,数十小时的相变时间限制了其工业应用。本文从合金优化和预应变加速相变两方面入手,结合经典热力学计算和系统的实验研究,成功将制备SBA组织(抗拉强度2154MPa,延伸率13%)的时间缩短至1-3小时,主要工作和结果如下。建立了贫碳区切变形核的热力学模型,利用超组元模型修正后的KRC和LFG热力学模型计算了低温贝氏体相变形核自由能,确定了实验钢在300℃以下温度的贝氏体形核驱动力为-1900J·mol-1到-2000J·mol-1,即在热力学条件上相变形核可能以切变机制发生。结合MUCG83软件(基于Russell的经典形核理论计算孕育期)进行了辅助成分设计与优化。设计了三个成分体系的富硅(1.5～2.5 wt%)合金,包括不同C含量(0.6、0.8、0.9)的0.5Mn-0Cr系列合金,不同Cr、Mn含量的0.8C-1Mn-0Cr和0.8C-1Mn-1Cr合金以及对比合金0.9C-2Mn-1.5Cr-1.5Co-1Al系列,共八个成分。利用DIL805淬火相变膨胀仪测定了以上合金的等温相变(TTT)曲线,精确测定了不同温度下的贝氏体相变孕育期,实验结果与理论分析一致。增加C元素含量能显著降低Ms点温度和Bs点温度,并且能增大两个相变点温度区间,Mn元素是推迟相变开始和延长相变完成时间的主要元素,同时添加锰、铬元素对贝氏体相变孕育期的推迟作远大于单一添加锰或铬。利用Gleelbe热模拟实验机研究了预应变作用下贝氏体相规律,结果表明,过冷奥氏体预应变对后续等温贝氏体相变加速效果显著,且在合理控制预应变温度和应变量的情况下,最终贝氏体转变量略有增加。在大于600℃的较高温度区间施加单道次50%预应变,可明显阻滞贝氏体相变导致最终转变量的降低。在600℃进行小应变量(20%)单轴压缩,虽不能显著加速贝氏体相变,但能缩短孕育期,加速贝氏体形核。在低于600℃且高于马氏体转变温度区间进行预应变可有效加速低温贝氏体等温相变,相变孕育期和完成时间随着预应变温度的降低和应变量的增加而减小。预应变温度为300℃,应变量为20%时,过冷奥氏体在230℃的等温贝氏体相变孕育期可由5小时缩短至30分钟。在冷轧试验机上完成了多步形变热处理及快速制备SBA组织的控轧控冷工艺研究。利用改进的多步形变热处理工艺,在单次形变量不大于10%,总应变量不大于30%的预应变条件下,通过控制形变间隔时间,即控制形变奥氏体回复时间,使形变过冷奥氏体在2小时内完成等温贝氏体相变,并且不降低贝氏体最终转变量。经多步形变热处理后的贝氏体铁素体(BF)片层厚度约50-70nnm,抗拉强度2154MPa。残留奥氏体片的厚度因形变奥氏体机械稳定性而增加到100nnm,使该超细贝氏体组织在具备超高强度的同时,拥有高达13%的总延伸率。利用控制轧制控制冷却工艺将0.8C-2.5Si-0.5Mn-1Al合金进行温轧后在空气缓慢冷却,可在不用等温的情况下获得SBA组织。控轧控冷SBA钢抗拉强度在2600MPa时,延伸率为7%,强塑积18.2GPa·%,在2000MPa时,延伸率为13%,强塑积26GPa·%。阐明了SBA组织中块状残留奥氏体的形成机理,提出了消除块状残留奥氏体的解决方案。多道次小应变量的预变形条件下,过冷奥氏体进行单滑移系塑性应变,通过塑性协调产生取向择优,进而减少单个奥氏体晶粒中可能出现的Packet(一系列互相平行的BF束包)数量,使得残留奥氏体只能以片层状存在。预应变为BF提供更多形核位置,从而细化高温区形成的BF的厚度,在300℃等温1小时形成的SBA组织强度与200℃等温数十小时的强度相同。这一发现打破了抗拉强度超过2GPa的SBA组织仅能通过低温(250℃)长时间等温热处理获得的传统观念,使工业推广成为可能。
[Abstract]:The ultra-fine bainite ferrite sheet and residual austenite layer structure (Slim Bainitic ferrite-Austenite, SBA) of ultra-fine bainite microstructure of.SBA steel can be obtained from the alloy of 0.98C-1.59Si-1.94Mn-1.33Cr-0.30Mo-0.02Ni-0.11V at 200-300 degrees C for dozens of hours. The microstructure of ultra-fine (100nm).SBA steel with its ultra-fine (slice thickness 100nm) can be obtained. The simple and easy preparation process and super high strength and toughness (tensile strength 2.5GPa, hardness 600HV, ductile 30-40MPa. M1/2) are widely concerned in the academic and industrial circles. However, for decades, the phase transition time restricts its industrial application. This paper, starting with two aspects of alloy optimization and prestrain plus phase transition, combines classical thermodynamic calculation and In the experimental study of the system, the time of preparing SBA tissue (tensile strength 2154MPa, elongation 13%) is shortened to 1-3 hours. The main work and results are as follows. A thermodynamic model for the shear nucleation of the lean carbon region is established. The nuclear free energy of the low temperature bainite phase is calculated by using the modified KRC and LFG thermodynamic model. The driving force of the bainite nucleation drive at the temperature below 300 C is -1900J mol-1 to -2000J mol-1. That is, the phase deformation nucleus may occur in the shear mechanism on the thermodynamic condition. The auxiliary components are set up and optimized with the MUCG83 software (based on the classical nucleation theory calculation based on Russell). The silicon rich (1.) is designed. (1. 5 to 2.5 wt%) alloys, including 0.5Mn-0Cr series alloys with different C content (0.6,0.8,0.9), 0.8C-1Mn-0Cr and 0.8C-1Mn-1Cr alloys with different Cr and Mn content, and 0.9C-2Mn-1.5Cr-1.5Co-1Al series of contrast alloys, are eight components. The isothermal phase transition (TTT) curve of the upper alloy was measured by the DIL805 quenching phase change instrument, and the different temperatures were accurately measured. The experimental results are consistent with the theoretical analysis. The increase of C element content can significantly reduce the temperature of Ms point and Bs point, and can increase the temperature range of two phase transition points. The Mn element is the main element to postpone the start of phase transition and prolong the completion time of the phase transition, and the addition of manganese and chromium element to the bainite phase transition period. The postponement was far greater than a single addition of manganese or chromium. The bainite phase law under pre strain was studied by the Gleelbe thermal simulation test machine. The results showed that the pre strain pre strain of the supercooled austenite has a significant effect on the subsequent isothermal bainite transformation, and the final bainite transformation amount is slightly increased in the case of reasonable control of the pre strain temperature and the strain. The application of single pass 50% pre strain at a higher temperature range greater than 600 degrees C can obviously block the reduction of the final transformation of bainite phase transition. The small strain (20%) compression at 600 C can not significantly accelerate the bainite phase transition, but it can shorten the inoculation period and accelerate the bainite nucleation at a temperature of lower than 600 and higher than the martensite transition temperature zone. The pre strain can effectively accelerate the isothermal phase transition of low temperature bainite, and the phase transition period and completion time decrease with the decrease of pre strain temperature and the increase of the strain. The pre strain temperature is 300 C and the strain is 20%, the isothermal bainite phase transition period of supercooled austenite can be shortened from 5 hours to 30 minutes at 230. The process of controlled rolling and controlled cooling of SBA tissue with multi step deformation heat treatment and rapid preparation was studied on the machine. The modified multistep deformation heat treatment process was used to control the deformation interval time, that is, to control the deformation austenite recovery time, to make the deformation supercooled austenite under the pre strain condition with the single shape variable not greater than 10% and the total strain less than 30%. The isothermal bainite phase transition is completed within 2 hours, and the final transformation of bainite is not reduced. The thickness of the bainite ferrite (BF) lamellae after multistep heat treatment is about 50-70nnm, and the thickness of the retained austenite fragment of tensile strength 2154MPa. is added to 100nnm because of the mechanical stability of the deformation austenite, so that the ultrafine bainite structure is in the possession of superfine bainite. At the same time, it has a total elongation of up to 13%. Using controlled rolling control cooling process, the 0.8C-2.5Si-0.5Mn-1Al alloy is cooled slowly in the air, and SBA can be obtained without constant temperature. The tensile strength of the controlled rolling and controlled cold SBA steel is 7%, the strong plastic product 18.2GPa%, and the elongation at 2000MPa, when the tensile strength of the controlled rolling and controlled cold steel is at 2600MPa. The rate is 13%, the strong plastic product 26GPa%. Clarifies the formation mechanism of the massive retained austenite in the SBA tissue, and puts forward the solution to eliminate the massive retained austenite. Under the predeformation condition of the multipass small strain, the supercooled austenite is made by the plastic strain of the single slip system, and the orientation of the austenite is produced by the plastic coordination, and the single austenite grain is reduced. The possible presence of Packet (a series of parallel BF bundles) makes the retained austenite only in lamellar. The prestrain provides more nucleation positions to BF, thus refining the thickness of BF formed by the high temperature region. The strength of the SBA tissue formed at 300 degrees centigrade for 1 hours is the same as that of 200 hours at 200. The SBA structure with tensile strength exceeding 2GPa can only be obtained by the traditional idea of low temperature (250 degree) long time isothermal heat treatment, so that industrial extension can be made possible.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.1

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