Primarily ferrite-pearlite microstructure having coarse ferrite grain size (24 µm) and high pearlite fraction (42%) offered YS ∼ 575 MPa with poor impact properties such as, upper shelf energy (USE) of only 30 J and ductile brittle transition temperature (DBTT) as high as 27 °C in an industrially hot-rolled plate of 0.25 wt% C steel. In order to improve the strength along with the impact properties by developing ferrite-bainite microstructures, two different types of heat-treatments, namely step-cooling (SC) and intermediate cooling (IC) treatments, were carried out on the as-received material. Significant improvement in strength along with the impact toughness (with YS of 740 MPa, USE of 222 J and DBTT of − 57 °C) has been achieved by developing fibrous microstructure, with alternate thin-films (2–4 µm thick) of ferrite and bainite through intermediate cooling (IC) treatment. Fine film-like structure with large orientation difference across the ferrite-bainite interface boundaries not only increased the strength but also resulted in frequent deflection in cleavage crack propagation path which improved the low-temperature impact toughness and reduced the DBTT.

主要具有铁素体晶粒尺寸（24 µm）和高珠光体比例（42％）的铁素体-珠光体显微组织，其YS〜575 MPa，具有较差的冲击性能，例如仅30 J的上层储能（USE）和易碎的脆性转变温度（在0.25 wt％C钢的工业热轧钢板中，DBTT的温度高达27°C。为了通过开发铁素体-贝氏体微结构来提高强度和冲击性能，对收到的材料进行了两种不同类型的热处理，即分步冷却（SC）和中冷（IC）处理。通过开发纤维微结构，强度和冲击韧性（YS为740 MPa，USE为222 J，DBTT为− 57°C）得到了显着改善，通过中间冷却（IC）处理，交替涂有铁素体和贝氏体交替的薄膜（厚度为2-4 µm）。在铁素体-贝氏体界面边界处取向差较大的薄膜状结构，不仅提高了强度，而且在解理裂纹扩展路径中产生了频繁的挠曲，从而改善了低温冲击韧性并降低了DBTT。