Additions of 3 and 5 wt.% Al have been investigated as a low-cost method for transformation acceleration in nano-bainitic steels. For both Al contents, two groups of steels with C-content in the range ~0.7 to ~0.95 wt.% were studied. Thermodynamic and physical simulations were used in alloy and heat treatment design. Characterization was performed via dilatometry, scanning and transmission electron microscopy, Synchrotron X-ray diffraction, and tensile and impact testing. Fast bainitic-transformation time-intervals ranging from 750–4600 s were recorded and tensile strengths up to 2000 MPa at a ductility of ~10 elongation percent were attainable for the 3 wt.% Al group at an austempering temperature of 265 °C. Higher Al additions were found to perform better than their lower Al counterparts as the austempering temperature is dropped. However, Al lowered the austenite stability, increased the martensite start temperature, austenitization temperatures and, consequently, the prior austenite grain size, as well as limiting the austempering temperatures to higher ones. Additionally, the lowered austenite stability coupled with higher additions of hardenability elements (here carbon) to maintain the martensite start at around 300 °C, causing the 5 wt.% Al group to have a large amount of low stability retained austenite (and consequently brittle martensite) in their microstructure, leading to a low elongation of around 5%.

中文翻译：

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纳米贝氏体钢：高铝添加加速转变及其对力学性能的影响

添加 3 和 5 重量％的铝已被研究作为一种低成本的纳米贝氏体钢相变加速方法。对于这两种 Al 含量，研究了 C 含量在 ~0.7 至 ~0.95 wt.% 范围内的两组钢。在合金和热处理设计中使用了热力学和物理模拟。通过膨胀计、扫描和透射电子显微镜、同步加速器 X 射线衍射以及拉伸和冲击测试进行表征。记录了 750-4600 秒的快速贝氏体转变时间间隔，并且在 265°C 的奥氏体回火温度下，3 wt.% 的 Al 组在延展性为~10% 的情况下可获得高达 2000 MPa 的拉伸强度。随着等温回火温度的降低，发现添加量较高的铝比添加量较低的铝表现更好。然而，Al 降低了奥氏体稳定性，增加了马氏体起始温度、奥氏体化温度，从而增加了原始奥氏体晶粒尺寸，并将奥氏体回火温度限制在更高的温度。此外，降低的奥氏体稳定性加上增加淬透性元素（此处为碳）以保持马氏体起始温度在 300 °C 左右，导致 5 wt.% Al 组具有大量低稳定性残余奥氏体（因此变脆）马氏体）在其微观结构中，导致约 5% 的低延伸率。