【作者】 陈鹏；

【导师】 张晓明；

【作者基本信息】 东北大学 ， 材料加工工程， 2021， 硕士

【摘要】 经济型双相不锈钢是一种高性能低成本的双相不锈钢,在降低成本的同时获得了优异的性能,但是与传统双相不锈钢相比,在轧制过程中更易产生开裂。为解决经济型双相不锈钢热塑性的问题,本文采用在铁素体单相区热轧,再采用大压下量冷轧后进行热处理的工艺路线,设计出了具有较好热塑性的新型资源节约型Fe-Cr-Mn-Al双相不锈钢。主要研究了不同退火工艺和Cu元素对于Fe-Cr-Mn-Al双相不锈钢组织、力学性能和耐腐蚀性能的影响,主要工作内容如下:(1)实验室热轧试验结果表明,实验钢的热轧组织为铁素体单相组织,热轧板未产生裂纹,具有较好的热成形性能。实验钢经80%的冷轧变形后,粗大的铁素体晶粒破碎,组织内部出现大量位错和亚晶,为奥氏体形核提供了有利条件。(2)实验钢轧制退火后研究表明,随着退火时间的增加,铁素体晶粒尺寸不断变小,而奥氏体晶粒尺寸则缓慢增加,在退火2h后两相比例基本达到平衡状态。在退火过程中,率先发生了铁素体相向奥氏体相的转变,均匀分布在铁素体晶界处的奥氏体相抑制了铁素体相的再结晶和长大,从而获得均匀细小的两相组织。随着退火温度的提高,两相的晶粒尺寸增大,奥氏体的体积分数减少。在退火过程中,铬和铝元素主要富集在铁素体相中,镍和锰元素主要富集在奥氏体相中。(3)在退火750℃时,在铁素体和奥氏体两相的晶界处出现σ相的析出,由于σ相的存在导致实验钢表现出高强度,低塑性;退火800℃和退火900℃时组织中两相的比例不佳,而在退火850℃时,组织分布均匀,两相的比例和晶粒尺寸最优,其屈服强度、抗拉强度和伸长率分别为440MPa、750MPa和35%,此时具有良好的强度和塑性的结合。在拉伸试验基础上,结合真应力真应变曲线和加工硬化曲线,明确了实验钢在变形过程中的强化机理,除退火750℃外,其他退火温度均发生TRIP效应。(4)动电位极化测试表明,随着退火温度的升高,两组实验钢的点蚀电位表现出先增加再降低的趋势;电化学阻抗谱结果表明,不锈钢在NaCl溶液中形成的钝化膜表现为双极性半导体。XPS分析结果表明,在含Al的双相不锈钢中钝化膜表面的Al2O3可以有效提高钝化膜的稳定性。铁素体相的点蚀当量高于奥氏体相的点蚀当量,点蚀坑会优先在奥氏体相或两相的晶界处形核,随着两相PREN值差值的减小,钢种的耐点蚀性能表现的越好。(5)添加Cu促进了奥氏体相形成,铁素体相向奥氏体相发生转变转变时间加快,组织中的奥氏体相明显增多,铁素体含量随着退火温度的增加而增加,但是随着Cu的加入而减少。加入Cu元素后,钢种的强度变低,延伸率变高。试验所添加0.5%Cu对于耐点蚀性能的影响并不大,但明显提高了双相不锈钢耐均匀腐蚀的能力。更多还原

【Abstract】 Lean duplex stainless steel is a kind of high-performance and low-cost duplex stainless steel.It has achieved excellent performance while reducing costs.However,compared with traditional duplex stainless steel,it is more prone to cracking during the rolling process.In order to solve the problem of lean duplex stainless steel thermoplasticity,this paper adopts the process route of hot rolling in the ferrite single-phase zone and then heat treatment after cold rolling with large reduction,and a new resource-saving type with better thermoplasticity is designed.Fe-Cr-Mn-Al duplex stainless steel.Mainly studied effects of different annealing processes and Cu elements on the structure,mechanical properties and corrosion resistance of Fe-Cr-Mn-Al duplex stainless steel.The main contents of the work are as follows:(1)The hot-rolling test of the laboratory results show that the hot-rolled structure of the experimental steel is a ferrite single-phase structure,and the hot-rolled sheet has no cracks and has good hot forming properties.After 80%of the experimental steel is deformed by cold rolling,the coarse ferrite grains are broken,and a large number of dislocations and subcrystals appear in the structure,which provides favorable conditions for the nucleation of austenite.(2)Research after rolling and annealing of the experimental steel shows that with the annealing time increases,the ferrite grain size keeps getting smaller,while the austenite grain size increases slowly.The phase ratio is basically in equilibrium after annealing for 2h.During the annealing process,the transformation from ferrite phase to austenite phase took the lead.The austenite phase uniformly distributed at the ferrite grain boundary inhibits the recrystallization and growth of the ferrite phase,so as to obtain uniform fineness.As the annealing temperature increases,the grain size of the two phases increases,and the volume fraction of austenite decreases.During the annealing process,chromium and aluminum are mainly concentrated in the ferrite phase,and nickel and manganese are mainly concentrated in the austenite phase.(3)When annealing at 750℃,the precipitation of σ phase appears at the grain boundary of the two phases of ferrite and austenite.Due to the existence of σ phase,it exhibits high strength and low plasticity;the ratio of the two phases at 800℃ and 900℃ is not Good.when annealed at 850℃,the uniform structure,the ratio of the two phases and the grain size are optimal,and the yield strength,tensile strength and elongation are 440 MPa,750 MPa and 35%,with good strength and plasticity The combination.Based on the tensile test,combined with the true stress true strain curve and the work hardening curve,the strengthening mechanism of the experimental steel in the deformation process is clarified.The TRIP effect occurs at all annealing temperatures except for annealing at 750℃.(4)The potentiodynamic polarization test showed that with the increase of annealing temperature,the pitting corrosion performance of the two groups of experimental steels showed a trend of first increasing and then decreasing;the results of electrochemical impedance spectroscopy showed that the passive film formed by stainless steel in NaCl solution behaved as a bipolar semiconductor.XPS analysis results show that the passivation film of Al-containing duplex stainless steel does contain A12O3,which can improve the stability of the passivation film.The pitting equivalent of the ferrite phase is higher than the pitting equivalent of the austenite phase,and the pitting pits will preferentially nucleate at the austenite phase or the grain boundary of the two phases.As the difference between PREN value of the two phases decreases,the better the pitting corrosion resistance of the steel.(5)The addition of Cu promotes the formation of the austenite phase,the transformation time of the ferrite phase to the austenite phase is accelerated,and the austenite phase in the structure increases significantly.The ferrite content increases with the increase of the annealing temperature,but it decreases with the addition of Cu.After adding the Cu element,the strength of the steel grade becomes lower and the elongation becomes higher.The addition of 0.5%Cu has little effect on pitting corrosion performance,but it significantly improves the uniform corrosion resistance of duplex stainless steel.更多还原