【作者】 王楠；

【导师】 陈永楠；

【作者基本信息】 长安大学 ， 机械工程材料， 2023， 博士

【摘要】 贝氏体/铁素体(B/F)双相钢具有强度高、应变硬化率高、强韧性匹配好等优点,已成为主要的抗变形管线钢及结构钢。然而管道在成型及服役中易发生局部塑性应变,内部微结构演变产生的应变硬化效应,增加了可移动位错滑移阻力,促进了裂纹萌生和扩展速率,导致管道力学性能稳定性降低。因此,深入研究引发应变硬化的微结构演变特征,揭示其过程主控因素,对于双相钢的安全评价具有十分重要的意义。本文以贝氏体/铁素体双相钢为研究对象,采用不同拉伸预应变(预应变量为1～5%)来模拟塑性变形过程,研究应变硬化期间微结构演变及对力学性能的影响规律。基于代表性体积元模型,分析应变硬化对双相钢应变局部化、应变分配行为及铁素体变形能力的影响;研究应变硬化过程中位错组态演变规律及界面背应力对晶粒变形抗力的作用机理;构建铁素体内部微结构演变模型,研究预应变期间亚结构对流动应力的作用规律;研究双相钢预应变后的应变硬化行为对强韧性和断裂行为的影响,并结合微结构演变机制建立屈服应力强化模型和预测公式。取得的主要结论如下:随着预应变程度增加,铁素体中应变局部化逐渐加剧,应变局部化因子由0.023增加到0.061,两相间的变形差异程度由6.71%增加到10.93%,应变分配系数从7.11降低到3.07,导致双相钢中铁素体硬化和两相间的应变协调性降低。硬化后的铁素体对应变的贡献率迅速降低,对应力的贡献率则逐渐增加,使得流动应力不能通过塑性变形来释放或传递,导致了双相钢的塑性变形能力降低。预应变期间铁素体中几何必要位错(GNDs)率先被激活并滑移,形成了由位错壁组成且沿剪切方向分布的滑移带,滑移带内分布着滑移性较好的统计存储位错(SSDs),随着预应变增加,双相钢中多滑移和交滑移相继被激活,使得GNDs和SSDs密度快速增加,位错组态由平行的位错壁向胞状结构转变。界面背应力与GND密度相关,预应变程度越大,积累的背应力越强,使得双相钢中的铁素体呈压应力状态,具有明显的应变硬化效应。双相钢在预应变期间首先会形成较强的型织构,由(113[1(?)0向(223[1(?)0织构转变,最终演变为强烈的型织构,形成了明显的(111[1(?)0变形织构,导致Taylor取向硬化。同时双相钢中GNDs-低角度晶界(LAGBs)-高角度晶界(HAGBs)之间的亚稳态结构演变导致应变梯度形成和铁素体硬化,预应变程度越高,应变响应时间越长,GNDs向LAGBs演变以及LAGBs向HAGBs的转变过程越充分,使得铁素体内部形成的亚晶密度越高,从而对流动应力的强化效果越显著。双相钢预应变后产生的界面背应力、存储的固有位错以及形成的亚晶结构可以显著增加再次变形时的力学性能和断裂行为,屈服强度随着预应变程度增加从610 MPa增加到723 MPa,均匀延伸率从7.12%快速降低至1.02%,应变硬化能力显著降低。微裂纹由铁素体滑移带形核转变为界面形核和贝氏体撕裂形核方式,预应变程度越大,微裂纹沿着亚晶界扩展速率越快,导致断裂机制由微孔聚集型为主的韧性断裂向准解理型为主的脆性断裂转变。更多还原

【Abstract】 Dual-phase steel with bainite/ferrite(B/F[ has become the major deformation resistant pipeline steel and structural steel due to its high strength,high work hardening rate and good strength toughness matching.However,it can be easy to occur the plastic strain during the forming and service,which can reduce the stability of mechanical properties for the pipe.Due to the strain hardening effect produced by microstructure evolution,it increases the sliding resistance of movable dislocation and promotes the crack initiation and propagation rate.Therefore,it is great significance for the safety evaluation of dual-phase steel to deeply reveal the main factors that cause the microstructure evolution and strain hardening.In this work,the bainite/ferrite steels are used as test materials,and the pre-tensile experiments(1～5%[ are used to simulate the process of plastic deformation,so as to reveal the evolution of microstructure during strain hardening and the influence on mechanical properties.Based on the representative volume element model,it can analyze the effects of strain hardening on strain localization,strain distribution and ferrite deformation ability in dual-phase steel.It studies the evolution of dislocation configuration during strain hardening and the mechanism of interface back stress on grain deformation resistance.Based on a microstructure evolution model in dual-phase steel,it can analyze the effect of substructure on flow stress during prestrain.It reveals the effect of strain hardening behavior on mechanics and fracture behavior after pre-strain,and establishes yield stress strengthening model and prediction formula based on microstructure evolution mechanism.The strain localization gradually intensifies with the increase of pre-strain degree in ferrite,the strain localization factor increases from 0.023 to 0.061,the deformation difference between the two phases increases from 6.71% to 10.93%,and the strain distribution coefficient decreases from 7.11 to 3.07,resulting in the ferrite hardening and strain coordination reduction in dualphase steel.The contribution rate of hardened ferrite to strain decreases rapidly,while the contribution rate of that to stress increases gradually,which makes the flow stress cannot be released or transferred through plastic deformation,resulting in the reduction of plastic deformation ability of dual-phase steel.During pre-strain,geometrically necessary dislocations(GNDs[ are first activated and slip in ferrite,forming a slip band composed of dislocation walls and distributed along the shear direction,in which statistical storage dislocations(SSDs[ are distributed with good slip ability.With the increase of pre-strain,the multi-slip and cross-slip are activated successively in dualphase steel,resulting in a rapid increase in the density of GNDs and SSDs,which leads to the transformation of dislocation configuration from parallel dislocation wall to cellular structure.The interface back stress is related to GND density,and the greater the degree of prestrain,the stronger the accumulated back stress,resulting in a compressive stress state for ferrite,with obvious strain strengthening effect in dual-phase steel.The strong -fibre will first be formed,which can transform from(113 [ 1(?)0 to(223 [ 1(?)0,and finally evolve into the -fibre,forming the deformation texture of the(111 [ 1(?)0 after pre-strain,leading to Taylor orientation hardening in dual-phase steel.The metastable structure evolution between GNDs-low angle grain boundaries(LAGBs[-high angle grain boundaries(HAGBs[ leads to the formation of strain gradient and ferrite hardening in dual-phase steel.The higher the degree of prestrain,the longer the strain response time,and the more fully the evolution of GNDs to LAGBs and the transformation of LAGBs to HAGBs.the higher the density of subgrains formed in ferrite,and the more obvious the strengthening effect of hardened ferrite on flow stress.It can cause the high sub-grain density formed in the ferrite,which leads to the significant strengthening effect on flow stress in dual-phase steel.During the re-deformation,the mechanical properties and fracture behavior are increased due to the interface back stress,the stored inherent dislocation and the sub-grain structure formed after the pre-strain.With the increase of prestrain,the yield strength increases from 610 MPa to 723 MPa,and the uniform elongation decreases rapidly from 7.12% to 1.02%,resulting in the significant decrease in strain hardening capacity of dual-phase steel.The initiation of microcrack changes from ferrite slip band to interface and bainite tearing nucleation,and then the greater the degree of prestrain,the faster the propagation rate of microcracks along the subgrain boundary.It causes the fracture mechanism of dual-phase steel to change from ductile fracture dominated by micro-pore aggregation to brittle fracture dominated by quasi-cleavage.更多还原