Mn对节Ni型双相不锈钢热变形及耐蚀性的影响机制研究

The low nickel type duplex stainless steels (DSS) are to stabilize the austenite phase by substitution Mn element for Ni element, possessing good application prospect, but it is key to expand their application fields by improving hot working performance. The stacking fault energy difference between austenite and ferrite phases increase thermal deformation instability of DSS, and different effect of austenite stability brought by Mn and Ni austenite stabilized elements will affect thermal deformation behavior. Furthermore, the change of two phase compositions and austenite stacking fault energy caused by Mn contents variation can result in the difference of two phase volume fraction, the amount of grain boundaries and dislocations distribution after thermal deformation, thus influence corrosion resistance of DSS. For high and low Cr contents low nickel type duplex stainless steels for engineering application, this project intends to study the influencing mechanism of Mn on thermal deformation behavior and corrosion resistance. The following studies will be conducted, including the effect of Mn contents variation on hot compression and high temperature tensile deformation behaviors, microstructure evolution and two phase volume fraction ratios, the influencing mechanism of Mn contents variation on dynamic recrystallization kinetics and single-pass hot compression thermodynamics, the effect of Mn contents variation on hot ductility and mechanical properties during high temperature tensile deformation. Meanwhile, the influencing rules and mechanism of Mn contents variation on pitting corrosion and intergranular corrosion will be investigated after hot deformation, so as to establish corresponding physical model of corrosion. Therefore, the research can provide scientific criterion for microstructure optimization of low nickel type DSS during thermal deformation, which can improve hot working performance and is beneficial to establish theoretical basis for development and application.

节Ni型双相不锈钢以Mn代Ni稳定奥氏体相，具有良好应用前景，提高热加工性能是扩大其应用领域的关键。双相不锈钢两相层错能差异增加了热变形不稳定性，Mn对Ni稳定奥氏体的不同作用将影响其热变形行为。且Mn含量变化影响两相成分分布、奥氏体层错能变化，所导致热变形两相体积分数、晶界数量和位错分布变化将影响耐蚀性。对工程用高低Cr含量节Ni型双相不锈钢，本项目拟对其热变形行为及耐蚀性的Mn影响机制进行研究。研究Mn含量变化对热压缩和高温拉伸变形行为、组织演变和两相体积分数比例的影响；探讨Mn含量变化对热变形过程动态再结晶动力学和单道次热压缩热力学的影响机理；分析Mn含量变化对高温拉伸塑性和力学性能的影响；研究Mn含量变化对热变形点蚀、晶间腐蚀和应力腐蚀的影响规律和机理，并建立相应的腐蚀物理模型。为节Ni型双相不锈钢热加工过程组织调控优化提供科学判据，以提高热加工性能，为其研制和应用提供理论基础。

稀缺资源Ni价格远高于Mn，钢铁生产中增N困难，Mn能有效取代部分Ni稳定奥氏体相，且能提高N在钢中固溶度。双相不锈钢两相结构不同导致其热加工时变形差异大，故Mn相对Ni稳定奥氏体能力不同影响其热变形行为。为提高以Mn代Ni节约型双相不锈钢的热加工性能，扩大其工程应用领域，针对23%Cr和18%Cr高低Cr系列节Ni型双相不锈钢，在不同热变形参数条件下采用物理模拟的方法研究了Mn含量对其热压缩和高温拉伸热行为影响机制。获得了Mn含量对800-1150℃/0.01-10s-1变形条件下热压缩流变曲线软化特征的影响规律，发现相对2304商业钢，增加Mn能促进23%Cr实验钢在50%变形量变形的较低变形速率和温度的动态再结晶软化，且变形量由50%增至70%促进了18%Cr较高Mn含量实验钢流变再结晶软化效果。获得了Mn含量变化对两相热变形组织的影响规律，发现较高Mn添加在热压缩变形过程中能有效稳定奥氏体相，热变形晶粒细化以奥氏体相再结晶为主，但大变形量有利于铁素体相在较低温度发生动态再结晶。探讨了Mn含量对单道次热压缩热力学影响过程和机理，拟合得到实验钢的热变形方程和激活能，且变形量增加明显降低了10.3%中Mn含量23%Cr实验钢激活能；相对高Ni钢种，Mn含量增加使节Ni钢的Z参数值降低，且热加工图表明较高Mn添加有利于提高大变形时的加工稳定性。探讨了Mn含量对热压缩变形再结晶动力学影响机理，50%变形量变形时，0.1和1s-1变形时Mn增加明显减小了23%Cr实验钢较低变形温度时的临界应变，并一定程度降低初始加工硬化率。获得了Mn含量对高温拉伸力学性能和组织演变影响规律和机理，发现高Mn添加有效提高23%Cr实验钢550℃塑性与变形过程中位错胞组织演变有关，且Mn增加能有效提高18%Cr实验钢的热塑性和抗拉强度。实验钢耐点蚀性主要受Mn添加导致的点蚀当量变化和热压缩导致的再结晶和硬化程度影响；较低Mn含量范围耐晶间腐蚀敏感性比较低，高Mn含量添加使晶间腐蚀敏感性略有提高。建立了不同热变形条件下实验钢的电化学阻抗模型。