钨钼复合二次硬化超高强度钢析出相及热变形行为研究

发布时间：2018-05-07 19:42

本文选题：超高强度钢 + 二次硬化　；参考：《燕山大学》2016年博士论文

【摘要】：超高强度钢广泛运用于航空航天领域。本文以多组元碳化物(Mo,Cr,W)_2C为主要强化相,设计并试制了一种强度级别为2200MPa级的钨钼复合二次硬化超高强度钢,系统研究了热处理工艺对其组织与性能的影响,从而确定了最佳热处理工艺;分析并讨论了主要强化相M_2C碳化物在回火过程中的析出长大机制;运用第一性原理计算的方法预测了不同Cr、Mo、W含量条件下M_2C碳化物的结构稳定性、弹性性能以及电学性能;建立热变形方程,提出了该钢的最佳热加工工艺参数。取得如下主要研究成果:钨钼复合二次硬化超高强度钢经950℃~1100℃保温1h淬火,随淬火温度的升高,强度和韧性均先升高后降低,在1000℃~1050℃达到最大值。淬火温度为950℃和1000℃时,存在未溶的M6C碳化物;淬火温度为1050℃时,M6C碳化物全部溶解;淬火温度高于1050℃时,晶粒发生异常长大。钨钼复合二次硬化超高强度钢经200℃~700℃范围内回火5小时,在200℃回火时,析出较多的ε-碳化物;在300℃~440℃回火时,析出大量粗大的层片状渗碳体,导致强度和韧性不断下降,在440℃达到最低值。回火温度高于470℃时,马氏体板条内析出大量均匀弥散的M_2C碳化物及少量的Laves相,使实验钢出现明显的二次硬化现象,抗拉强度、屈服强度分别在490℃和530℃回火时达到最大值,冲击功在510℃时达到最大值,此时具有较好的综合力学性能。回火温度高于490℃时,逆转变奥氏体含量不断增加。回火温度在560℃附近时,晶界上出现大量碳化物并富集了大量的N、P、S元素,引起沿晶断裂,韧性下降。钨钼复合二次硬化超高强度钢在510℃回火时,随回火时间的延长,M_2C依次出现形核、长大及熟化三个过程。当回火10min~30min时,有渗碳体存在,伴随M_2C碳化物的析出,渗碳体逐渐溶解,晶内M_2C碳化物处于形核阶段,完成由G-P区到新相晶核的转变;在晶界处聚集了大量的M_2C碳化物,且随着回火时间的延长碳化物晶界聚集状态逐渐减弱。回火5h时,晶内M_2C碳化物处于长大阶段,渗碳体全部溶解,晶界M_2C碳化物聚集现象消失。回火100h时,M_2C碳化物处于熟化阶段,碳化物尺寸出现大小两类,大尺寸碳化物继续长大而小尺寸碳化物逐渐溶解。在形核和长大阶段,随回火时间的延长合金元素扩散距离逐渐变长,M_2C碳化物数量变多、尺寸增大,且碳化物内部C、Cr、Mo、W元素含量逐渐上升。在熟化阶段,M_2C碳化物尺寸及体积含量继续增大,而数量减少,受合金元素扩散和碳化物稳定性共同影响,与C原子结合能力更强的Mo和W元素逐渐取代Cr原子进入M_2C碳化物,使碳化物心部C、Mo、W元素含量逐渐上升,而Cr元素含量则逐渐下降。运用密度泛函理论,系统地研究了不同Mo、Cr、W含量的多组元(Mo,Cr,W)_2C型碳化物结构稳定性、弹性性能、热物理性能以及电学性能等。发现Mo6W1Cr1C4拥有最高的稳定性和体积模量、剪切模量、弹性模量及硬度,具有优良的抗变形能力。Mo7Cr1C4的各向异性最强,而Mo_8C_4的各向异性最弱。从电子性质分析中发现,掺杂Cr原子和W原子不仅能使自身轨道杂化,而且能增强Mo原子与C原子之间的轨道杂化,增强原子间相互作用。在变形温度为850℃~1200℃、应变速率为0.01 s-1~10s-1的变形条件下,钨钼复合二次硬化超高强度钢的热变形激活能为471.28kJ/mol,建立了热变形本构方程和动态再结晶晶粒平均晶粒尺寸D(um)与Z参数的定量关系。基于动态材料模型,建立了该钢的热加工图,确定了其最佳热加工工艺条件。
[Abstract]:Ultra high strength steel is widely used in the field of Aeronautics and Astronautics. In this paper, Mo, Cr, W _2C as the main strengthening phase, a tungsten molybdenum composite two hardening super strength steel with a strength grade of 2200MPa is designed and tested. The effect of heat treatment process on its microstructure and properties is studied systematically, and the optimum heat treatment process is determined. The mechanism of precipitation and growth of the main enhanced phase M_2C carbides in the process of tempering was analyzed and discussed. The structural stability, elastic properties and electrical properties of M_2C carbides in different Cr, Mo and W content were predicted with the method of first principle calculation, and the thermal deformation equation was established, and the optimum processing parameters of the steel were put forward. The main results are as follows: tungsten molybdenum composite two hardening super strength steel is quenched at 950 C ~1100 C for 1H. With the increase of quenching temperature, the strength and toughness increase first and then decrease and reach the maximum value at 1000 C ~1050 C. When the quenching temperature is 950 C and 1000 C, there is an undissolved M6C carbides; when the quenching temperature is 1050 C, all M6C carbides dissolve When the quenching temperature is higher than 1050 C, the grain has abnormal growth. The tungsten molybdenum composite two hardening ultra high strength steel is tempered for 5 hours in the range of 200 c ~700 C, and more epsilon carbides are precipitated when tempering at 200 c. A large amount of thick layer like carburizing body is precipitated when tempering at 300 centigrade, causing the strength and toughness to decrease and reach 440 C. Minimum value. When the tempering temperature is higher than 470, the martensitic slab can precipitate a large number of uniformly dispersed M_2C carbides and a small amount of Laves phase. The tensile strength and yield strength reached the maximum value at 490 and 530 C, and the impact work reached the maximum at 510 degrees C. When the tempering temperature is higher than 490, the reversed austenite content increases continuously. When the tempering temperature is near 560, a large number of carbides appear on the grain boundary and a large number of N, P, S elements have been enriched, causing the intergranular fracture and the toughness decrease. When the tungsten molybdenum compound two hardening ultra high strength steel is tempered at 510 C, with the tempering time prolonged, M_2C depends on the tempering time. There are three processes of nucleation, growth and maturation. When tempering 10min~30min, the carburized body exists, with the precipitation of M_2C carbide, the carburized body gradually dissolves, the intragranular M_2C carbides are in the nucleation stage, and the transition from the G-P zone to the new phase nucleation is completed; the large amount of M_2C carbides are gathered at the grain boundary, and the carbides are prolonged with the tempering time. The grain boundary aggregation state gradually weakened. When tempering 5h, the intragranular M_2C carbide was in the growing stage, the carburized body dissolved and the grain boundary M_2C carbide aggregation disappeared. When tempering 100h, the M_2C carbide was in the ripening stage, the size of the carbide appeared two classes, the large size carbides continued to grow and the small size carbides dissolved gradually. In the nucleation and in the nucleation, In the growing stage, the diffusion distance of alloy elements gradually grows with the time of tempering, the number of M_2C carbide becomes more and the size increases, and the content of C, Cr, Mo and W increases gradually. In the ripening stage, the size and volume content of the M_2C carbide continues to increase, and the quantity decreases, which is influenced by the diffusion of alloy elements and the stability of carbides. The Mo and W elements, which are stronger with the C atom, gradually replace the Cr atoms into the M_2C carbide, which makes the content of C, Mo, W elements in the carbide core gradually increasing, while the Cr element content decreases gradually. It is found that Mo6W1Cr1C4 has the highest stability and bulk modulus, the shear modulus, the modulus of elasticity and the hardness, the excellent anisotropy of the anti deformability of.Mo7Cr1C4, and the weakest anisotropy of Mo_8C_4. It is found that the doping of the Cr atom and the W atom can not only make its own orbit miscellaneous from the analysis of the electron properties. When the deformation temperature is 850 C and the strain rate is 0.01 s-1~10s-1, the thermal deformation activation energy of the tungsten molybdenum composite two hardening ultra high strength steel is 471.28kJ/mol. The thermal deformation constitutive equation and the dynamic recrystallized grain are established under the deformation temperature of 850 C ~1200 C and the strain rate of 0.01 s-1~10s-1. The relationship between average grain size D (UM) and Z parameters is established. Based on the dynamic material model, the hot working diagram of the steel is established, and the best hot working conditions are determined.

【学位授予单位】：燕山大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.1

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