Processing, microstructure, and mechanical properties of medium-Mn steels containing Cu and Ni

Xu, Zigan; Bleck, Wolfgang (Thesis advisor); Wenwen, Song (Thesis advisor); Ding, Hua (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022, Kumulative Dissertation

Abstract

The development of advanced high strength steels (AHSS) is important for the scientific and engineering field of metallic materials. Their superior mechanical performance enables weight reduction for automotive structural components, contributing to lower CO2 emissions and a more environmentally and economically sustainable future. The medium-Mn steels, as key candidates for the third generation of AHSS, exhibit a superior combination of strength and ductility owing to their unique ultra-fine grained (UFG) microstructure. The primary focus of this thesis is to investigate how processing variants can influence the microstructure characteristics of medium-Mn steels containing Cu and Ni, such as the austenite fraction and stability, nano-precipitation, elemental partitioning, and dislocation densities, as well as their synergic contribution to the tensile properties. To characterize the UFG microstructure, electron backscatter diffraction (EBSD) and high-energy synchrotron X-ray diffraction (SY-XRD) were used. To characterize the element partitioning, electron probe microanalysis was used. The nano-size Cu-rich precipitates were characterized using atom probe tomography (APT).This thesis has identified the microstructure evolution and resulting tensile properties during typical processing routes of medium-Mn steels. The annealing temperature during intercritical annealing can significantly control the volume fraction and mechanical stability of austenite, resulting in distinguished strain hardening behavior by activating the transformation-induced plasticity (TRIP) effect. A two-step short intercritical annealing (2min) and tempering (3h) heat treatment was developed to achieve a balanced austenite reversion and nano-precipitation by controlling the partitioning of Cu and Ni. During a warm rolling in the intercritical temperature range, a significant amount of austenite can be retained at room temperature by the dynamic partitioning of Mn and C, resulting in excellent tensile properties with a compact processing route. These findings shed light on the new possibilities for microstructure adjustments for tailoring mechanical properties in medium-Mn steels.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Materials Engineering of Metals and Department of Ferrous Metallurgy [522110]