Hot compression experiments conducted on a Gleeble-3500thermo-mechanical simulator and metallographic observation tests were employed to study the critical conditions of dynamic recrystallization（DRX）of 316 LN austenitic stainless steel.The true stress-true strain curves of 316 LN were obtained at deformation temperatures ranging from 900℃to 1 200℃and strain rates ranging from 0.001s-1 to 10s-1.Based on the above tests,the critical conditions of DRX were determined and compared with those obtained from work-hardening theory and the Cingara-McQueen flow stress model.Furthermore,the microstructure was observed to validate the calculated results.The ratio of critical strain to peak strain（εc/εp）for 316 LN was determined,and the quantitative relationship between the critical strain and the deformation parameters of 316 LN was elucidated.The results demonstrated that the onset of DRX corresponds to the constant normalized strain hardening rate（Γ）,namely,the critical strain hardening rateΓcfor316LN is equal to 0.65.
The effect of rare earth（RE） on Mo partitioning and resultant mechanical and microstructural behavior of a duplex stainless steel during hot working condition was investigated. It was found that RE effect was sensitive to temperature. At the high temperature, the development of dynamic recovery（DRV） in α phase was slowed down while the dynamic recrystallization（DRX） process in γ phase was accelerated by RE, whereby both work hardening rate（at low strain） and dynamic softening rate（at high strain） increased and moreover, the discrepancy on the hardness of the both phase reduced. Whereas at the low temperature, the effect of RE was opposite as compared with those in the high temperature. Mo partitioning analysis by EPMA indicated that RE enhanced the partitioning of Mo in α phase while reduced Mo concentration in γ phase at higher temperature whereby the mismatch between two phases could be improved indicated by the elimination of voids and cracks at α/γ interface, but it was contrary to that at the low temperature. Mo partitioning was believed to be an important cause for the RE effect on the differences of mechanical and microstructural behavior. Also this result provided a reasonable evidence for micro-alloying of RE in DSSs.
In order to reveal the mechanism and condition of void closure in large diameter steel rod during horizontal- vertical （H-V） groove rolling process, a three-dimensional thermomechanicaily coupled finite element model was es- tablished for 9-stand H-V groove rolling process aiming at a 4150 mm steel rod production line. A spherical hole with diameter from 2 to 10 mm was preset into the center of continuous casting billet with a rectangle cross section of 300 mmX 360 mm in this model to simulate the void defect, and then finite element analyses were carried out to observe and quantify the void shape evolution in each pass on the three orthogonal coordinate plane sections. The re- suits showed that the void was formed roughly in the reduction and extension directions, and crushed gradually from spherical shape to an approximate ellipsoid, micro-crack and finally to be closed. A quantitative analysis was carried out by using elliptic radii and closure ratio to describe this evolution process; it indicated that the longest axis of the ellipsoid coincided with the rolling line, and the second and third axes were alternatively ihorizontal and vertical on the exit cross section according to change of the reduction direction in H-V groove. The void closure behavior during H- V rolling was more complicated than that of common horizontal rolling, and the influence of groove type and the ex- tension coefficient on the void closure ratio was presented. Finally, a pilot rolling experiment was performed on a 5- stand H-V experimental mill to verify the numerical simulation results, and the experimental results are in good agree- ment with the numerical simulation results.
通过热压缩实验研究了21Cr^(-1)1Ni-N-RE节镍型奥氏体耐热钢的热力学行为和微观组织演变过程。通过对实验数据的回归分析,得到实验钢热变形激活能为451 k J/mol,应力指数为5.12。建立了热变形方程,确定了最大变形抗力和动态再结晶临界应变预测模型。通过对微观组织演变过程的分析,得到了实验钢获得均匀细小的完全动态再结晶组织的热变形条件为1150℃和10 s^(-1)。
