Sharifi Saham Sadat, Bakhtiari Saeid, Shahryari Esmaeil, Sommitsch Christof, Poletti Maria Cecilia
Institute of Materials Science, Joining and Forming at Graz University of Technology, Kopernikusgasse 24/I, 8010 Graz, Austria.
K1-MET GmbH, 4020 Linz, Austria.
Materials (Basel). 2024 Sep 16;17(18):4551. doi: 10.3390/ma17184551.
Continuous casting is the most common method for producing steel into semi-finished shapes like billets or slabs. Throughout this process, steel experiences mechanical and thermal stresses, which influence its mechanical properties. During continuous casting, decreased formability in steel components leads to crack formation and failure. One reason for this phenomenon is the appearance of the soft ferrite phase during cooling. However, it is unclear under which conditions this ferrite is detrimental to the formability. In the present research, we investigated what microstructural changes decrease the formability of microalloyed steels during continuous casting. We studied the hot compression behaviour of microalloyed steel over temperatures ranging from 650 °C to 1100 °C and strain rates of 0.1 s-1 to 0.001 s-1 using a Gleeble 3800 (Dynamic Systems Inc, Poestenkill, NY, USA) device. We examined microstructural changes at various deformation conditions using microscopy. Furthermore, we implemented a physically-based model to describe the deformation of austenite and ferrite. The model describes the work hardening and dynamic restoration mechanisms, i.e., discontinuous dynamic recrystallisation in austenite and dynamic recovery in ferrite and austenite. The model considers the stress, strain, and strain rate distribution between phases by describing the dynamic phase transformation during the deformation in iso-work conditions. Increasing the strain rate below the transformation temperature improves hot ductility by reducing dynamic recovery and strain concentration in ferrite. Due to limited grain boundary sliding, the hot ductility improves at lower temperatures (<750 °C). In the single-phase domain, dynamic recrystallisation improves the hot ductility provided that fracture occurs at strains in which dynamic recrystallisation advances. However, at very low strain rates, the ductility decreases due to prolonged time for grain boundary sliding and crack propagation.
连铸是将钢生产成方坯或板坯等半成品形状的最常用方法。在整个过程中，钢会经历机械应力和热应力，这会影响其机械性能。在连铸过程中，钢部件的可成形性降低会导致裂纹形成和失效。这种现象的一个原因是冷却过程中出现了软铁素体相。然而，目前尚不清楚在何种条件下这种铁素体会对可成形性产生不利影响。在本研究中，我们调查了在连铸过程中哪些微观结构变化会降低微合金钢的可成形性。我们使用Gleeble 3800（美国纽约州波斯滕基尔市动态系统公司）设备研究了微合金钢在650℃至1100℃温度范围和0.1 s-1至0.001 s-1应变速率下的热压缩行为。我们使用显微镜检查了各种变形条件下的微观结构变化。此外，我们建立了一个基于物理的模型来描述奥氏体和铁素体的变形。该模型描述了加工硬化和动态回复机制，即奥氏体中的不连续动态再结晶以及铁素体和奥氏体中的动态回复。该模型通过描述等功条件下变形过程中的动态相变来考虑各相之间的应力、应变和应变速率分布。在转变温度以下提高应变速率可通过减少铁素体中的动态回复和应变集中来提高热延展性。由于晶界滑动有限，在较低温度（<750℃）下热延展性会提高。在单相区域，如果断裂发生在动态再结晶进行的应变下，动态再结晶会提高热延展性。然而，在非常低的应变速率下，由于晶界滑动和裂纹扩展的时间延长，延展性会降低。
