Yao Shengjie, Chen Long, Chu Guannan, Zhao Hongyun, Feng Lei, Wang Guodong
School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, Weihai 264-209, China.
Shandong Provincial Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264-209, China.
Materials (Basel). 2020 Apr 8;13(7):1741. doi: 10.3390/ma13071741.
Hot stamping is a well-known process to produce structural automotive parts with an excellent strength-to-weight ratio. However, this process is more expensive due to the lower energy efficiency and operating cost of the traditional roller-hearth furnace. Additionally, lower ductility and toughness are commonly recognized as the main disadvantages of the current hot stamped ultra-high-strength parts. Refinement of austenite grains could be a profitable way to improve the strength of hot stamped parts. In this work, the evolution of reversed transformation in asymmetrically cryogenically rolled samples was studied in order to control the austenite. Thermomechanical simulation and heat treatment in the salt bath were used to investigate the reversed transformation process, and the typical microstructures were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Compared with symmetric prerolling, ferrite recrystallization could be remarkably inhibited by asymmetric rolling at the liquid nitrogen temperature (LNT) during the reheating process. Additionally, the nucleation of the austenite inner grains can also be promoted and the dynamics of the reversed transformation accelerated by asymmetric prerolling. Such phenomena might be very useful to refine the parent austenite grains before press hardening and enhance the new hot stamping strategy by partial fast reheating.
热冲压是一种众所周知的工艺，用于生产具有出色强度重量比的汽车结构部件。然而，由于传统辊底式炉的能源效率较低和运营成本较高，该工艺成本更高。此外，较低的延展性和韧性通常被认为是当前热冲压超高强度部件的主要缺点。细化奥氏体晶粒可能是提高热冲压部件强度的一种有效方法。在这项工作中，研究了非对称低温轧制样品中逆转变的演变，以控制奥氏体。采用热机械模拟和盐浴热处理来研究逆转变过程，并用透射电子显微镜（TEM）和扫描电子显微镜（SEM）对典型微观结构进行了表征。与对称预轧制相比，在再加热过程中，在液氮温度（LNT）下进行非对称轧制可显著抑制铁素体再结晶。此外，非对称预轧制还可以促进奥氏体内部晶粒的形核，并加速逆转变的动力学。这些现象对于在冲压硬化前细化原始奥氏体晶粒以及通过部分快速再加热增强新的热冲压策略可能非常有用。
