固溶溫度對Mn-N型雙相不銹鋼拉伸變形行為的影響

固溶溫度對Mn-N型雙相不銹鋼拉伸變形行為的影響Effect of Solution Temperature on Tensile Deformation Behavior of Mn-N Bearing Duplex Stainless Steel

在Gleeble-3800試驗機上進行了一種新型Mn-N合金化雙相不銹鋼的拉伸變形試驗，獲得了不同固溶溫度下（1000-1200℃）試驗鋼的力學性能指標及加工硬化規律。利用光學顯微鏡（OM），掃描電鏡（SEM）和電子背散射衍射技術（EBSD）研究了固溶溫度對試驗鋼的形變亞結構及斷裂特征的影響，探討了固溶溫度影響加工硬化的機理。結果表明，隨著固溶溫度的升高，試驗鋼屈服強度與抗拉強度均逐漸降低，而延伸率（均勻延伸率和斷裂延伸率）則先升高后降低。其中，1100℃固溶時試驗鋼的塑性最佳，均勻延伸率可達46.7%，且綜合力學性能優異，強塑積達44.6GPa·%。不同固溶溫度下，試驗鋼的加工硬化率隨應變的增加均表現為開始迅速下降，經再次升高后再下降的“三階段”特征，但隨著固溶溫度升高，加工硬化率升高的趨勢減弱。經EBSD分析發現，試驗鋼中奧氏體相發生了形變誘導馬氏體相變，且相變表現為γ→ε→α′和γ→α′兩種演化機制，從而形成TRIP效應使得加工硬化率升高、塑性增加，但較高的固溶溫度會使馬氏體轉變受到抑制。斷口結果表明，不同固溶溫度下鐵素體與形變誘導馬氏體均表現出解理斷裂特征，而殘余奧氏體則主要為韌性斷裂。經計算，隨著固溶溫度增加（1000-1200℃），奧氏體相的Md30值從81℃降到38℃，即奧氏體穩定性增加，減弱了TRIP效應，進而導致試驗鋼加工硬化和增塑效果降低。

Advanced duplex stainless steels (DSSs) in which Ni is mostly or completely replaced by Mn and N have newly been developed in recent years. Such Mn-N bearing DSSs can readily achieve exceptional room-temperature tensile properties through the transformation-induced plasticity (TRIP) effect of metastable austenite. During the processing of DSSs, solution treatment is a critical step that tailors the phase fraction and the overall properties. In particular, the phase chemistry can change due to different element partitioning between the two constituents, resulting in a different TRIP kinetics, when DSS is annealed at different temperature. In this work, the effect of solution temperature on tensile deformation behavior of a new Mn-N bearing DSS is studied. Tensile deformation tests were carried out on a Gleeble-3800 testing machine. The mechanical properties and work-hardening characteristic of the steels annealed at different solution temperature (1000-1200℃) were obtained. The effects of solution temperature on the deformation substructure and fracture characteristics were investigated by optical microscope (OM), scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD). The results show that as the solution temperature increases, the yield strength and tensile strength of the steels decrease, while the elongation (uniform elongation and total elongation) increases firstly and then decreases. The steel annealed at 1100℃ shows the optimum uniform elongation with 46.7%, and a better combination of ultimate tensile strength and ductility with approximately 44.6 GPa·%. The work-hardening rate of the steel shows a three-stage characteristic, namely it declines firstly and then increases and subsequently declines again as the strain increases. However, the increasing extent of the work-hardening rate decreases as the solution temperature increases. The EBSD analysis shows that the strain-induced martensitic transformation (SIMT) of metastable austenite which causes the transformation induced plasticity (TRIP) have two evolution mechanisms of γ→ε→α' and γ→α'. But SIMT can be suppressed when the solution temperature increases. The fracture surfaces of specimens annealed at different temperature show a quasi-cleavage mode, in which both ferrite and strain-induced martensite exhibit cleavage fracture while the residual austenite displays a dimple-mode fracture. Furthermore, the Md30 which can characterize the stability of metastable austenite was calculated. It decreases from 81℃ to 38℃ as the solution temperature increases from 1000℃ to 1200℃. This indicates that the TRIP effect gets weakening at a higher solution temperature, the work-hardening and plasticity therefore decrease.

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