Hardening mechanisms and impact toughening of a high-strength steel containing low Ni and Cu additions

Aging treatments at 400-550 degrees C are commonly used to attain a peak strengthening for the Cu-rich nanocluster-strengthened high-strength low-alloy (HSLA) steels. However, these temperatures fall within the dangerous 300-600 degrees C temper-embrittlement regime, leading to poor impact toughness. On the other hand, aging at temperatures above the embrittlement regime can improve the impact toughness but at a great expense of strength. In this work, the strengthening mechanisms as well as the toughening of a low cost weldable HSLA steel with a low content of carbon (C similar to 0.08 wt.%), nickel (Ni = 0.78 wt.%), and copper (Cu = 1.3 wt.%) were carefully investigated. Our findings show that the low-C-Ni-Cu HSLA steel is insensitive to the aging temperatures and can achieve a yield strength (YS) and ultimate tensile strength (UTS) over 1000 and 1100 MPa, respectively, with tensile ductility >10% (reduction of area >60%) at a heat-treat temperature of 640 degrees C through multiple strengthening mechanisms. Besides, a good low-temperature (similar to 40 degrees C) impact performance(similar to 200 J) with high YS (similar to 900 MPa) and UTS (similar to 1000 MPa) can be obtained by seeking a strength balance among the fine grain size (similar to 2.5 gm), medium-sized (similar to 14 nm) overaged Cu-rich precipitates, tempered martensite, and fresh martensite (or carbides). Moreover, a relatively lower YS (similar to 800 MPa) and UTS (similar to 900 MPa) useful for steel manufacturing can be attained by a prolonged aging at 640 degrees C. In addition, the dislocation-precipitate interactions were also explored based on the dislocation theories in this study. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.