δ-铁素体对钠冷快堆用316KD奥氏体不锈钢热变形行为和动态再结晶的影响

doi:10.11900/0412.1961.2022.00039

金属学报

2024, Vol. 60

Issue (3): 367-376 DOI: 10.11900/0412.1961.2022.00039

研究论文

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δ-铁素体对钠冷快堆用316KD奥氏体不锈钢热变形行为和动态再结晶的影响

陈胜虎¹, 王琪玉¹^,², 姜海昌¹, 戎利建¹(

)

¹中国科学院金属研究所中国科学院核用结构材料与安全评价重点实验室沈阳 110016
²中国科学技术大学材料科学与工程学院沈阳 110016

Effect of δ-Ferrite on Hot Deformation and Recrystallization of 316KD Austenitic Stainless Steel for Sodium-Cooled Fast Reactor Application

CHEN Shenghu¹, WANG Qiyu¹^,², JIANG Haichang¹, RONG Lijian¹(

)

¹CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
²School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China

引用本文:

陈胜虎, 王琪玉, 姜海昌, 戎利建. δ-铁素体对钠冷快堆用316KD奥氏体不锈钢热变形行为和动态再结晶的影响[J]. 金属学报, 2024, 60(3): 367-376.
Shenghu CHEN, Qiyu WANG, Haichang JIANG, Lijian RONG. Effect of δ-Ferrite on Hot Deformation and Recrystallization of 316KD Austenitic Stainless Steel for Sodium-Cooled Fast Reactor Application[J]. Acta Metall Sin, 2024, 60(3): 367-376.

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摘要:

奥氏体不锈钢中δ-铁素体的存在显著影响热加工过程中奥氏体晶粒度的控制，造成晶粒不均匀现象，关于δ-铁素体对奥氏体动态再结晶行为的影响机制尚不清楚。本工作利用Gleeble-3800热力模拟试验机在1423 K、0.1 s^-1条件下进行了铸态样品和均质化处理态样品的热压缩实验，结合SEM、EBSD和TEM等研究了δ-铁素体对热变形行为和动态再结晶的影响。结果表明：1473 K均质化处理14 h可基本消除δ-铁素体，同时伴随着奥氏体晶粒的显著长大，计算表明δ-铁素体向奥氏体的转变速率主要受控于Cr在奥氏体中的扩散。铸态样品变形过程中，δ-铁素体内部、δ-铁素体/奥氏体界面处的塑性变形先于奥氏体相，高温下较软δ-铁素体的存在，造成铸态样品的流变应力明显低于均质化处理态无δ-铁素体样品。随着变形的进行，δ-铁素体易发生动态回复，动态回复引起的软化导致流变应力显著下降。均质化处理态样品的动态再结晶机制是原始奥氏体晶界弓出形核的不连续动态再结晶。而δ-铁素体的存在促进了δ-铁素体/奥氏体界面附近奥氏体的动态再结晶，其机制为连续动态再结晶。铸态样品中的原始奥氏体晶界为不连续动态再结晶的形核位置，2种动态再结晶机制的耦合作用使得铸态样品的动态再结晶程度显著高于均质化处理态样品。

关键词 ：奥氏体不锈钢, δ-铁素体, 均质化处理, 热变形, 动态再结晶

Abstract：

The sodium-cooled fast reactor is the most mature reactor among generation-IV nuclear reactors. A carbon/nitrogen-controlled 316KD austenitic stainless steel has been developed for the construction of pressure vessels and internals in Chinese CFR600 demonstration reactor. During their industrial production, δ-ferrite is present in large-scale billets because of the combined effect of non-equilibrium segregation and low cooling rate. For large-scale billets containing δ-ferrite, inhomogeneous grain-size distributions are observed in the product after hot working. Extensive studies on the recrystallization of the austenite phase in austenitic stainless steels during hot deformation were conducted. However, the effect of δ-ferrite on the recrystallization behavior of the austenite phase remains unclear. In this study, uniaxial hot compression tests of 316KD austenitic stainless steels involving as-cast and homogenized conditions were conducted at 1423 K and 0.1 s^-1 using a Gleeble-3800 thermal-mechanical simulator, and the effect of δ-ferrite on hot deformation and recrystallization was analyzed by SEM, EBSD, and TEM. Results showed that δ-ferrite could be nearly eliminated through δ-ferrite→austenite transformation after homogenization at 1473 K for 14 h, whereas austenite grain showed evident growth. The elimination of δ-ferrite was a Cr-diffusion-controlled process through kinetic analysis. Plastic deformation occurred preferentially in δ-ferrite and at the δ-ferrite/austenite interface, and subsequently in austenite during hot deformation. The flow stress of as-cast samples was much lower than that of homogenized samples at the same strain because of the presence of soft δ-ferrite. Dynamic recovery occurred easier in δ-ferrite, and the resulting dynamic softening remarkably reduced flow stress with an increase in strain. Discontinuous dynamic recrystallization characterized by original austenite grain boundary bulging was the dominant mechanism in homogenized samples. However, the presence of δ-ferrite promoted the occurrence of continuous dynamic recrystallization in austenite near the δ-ferrite/austenite interface in as-cast samples. Compared with the homogenized samples, a higher degree of recrystallization was observed in as-cast samples because of the combined effects of continuous dynamic recrystallization and discontinuous dynamic recrystallization.

Key words： austenitic stainless steel δ-ferrite homogenization treatment hot deformation dynamic recrystallization

收稿日期: 2022-01-27

ZTFLH:

TG142.1

基金资助:国家自然科学基金项目(51871218);中科院青年创新促进会项目(2018227);中核集团青年英才计划项目

通讯作者: 戎利建，ljrong@imr.ac.cn，主要从事特种合金研究

Corresponding author: RONG Lijian, professor, Tel: (024)23971979, E-mail: ljrong@imr.ac.cn

作者简介: 陈胜虎，男，1986年生，研究员，博士

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https://www.ams.org.cn/CN/10.11900/0412.1961.2022.00039 或 https://www.ams.org.cn/CN/Y2024/V60/I3/367

图1 180 mm厚316KD奥氏体不锈钢铸坯中δ-铁素体含量沿厚度方向的分布曲线和心部位置微观组织

图2 铸坯心部样品的δ-铁素体含量与均质化处理温度和保温时间的关系曲线

图3 铸坯心部样品经1473 K均质化处理2、4和14 h后的微观组织

图4 lg{ln[1 / (1 - X(t))]}与lg{lnt}以及lnk与1 / T之间的拟合关系曲线

表1 δ-铁素体等温转变动力学方程中时间指数(n)和反应速率常数(k)的计算值

图5 铸态和均质化处理态样品经1423 K、0.1 s-1压缩变形时的真应力-真应变曲线

图6 铸态样品经不同变形量压缩后的EBSD晶界特征、局域取向差分布和晶粒取向图

图7 均质化处理态样品经不同变形量压缩后的EBSD晶界特征分布

图8 铸态样品经应变为0.36压缩变形后的EBSD晶界特征分布

图9 铸态样品经应变为0.36压缩变形后的TEM像及元素分布图

图10 δ-铁素体/奥氏体界面处奥氏体的动态再结晶过程示意图

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