Comprehensive improvement of strength and ductility in CLAM steel subjected to TEU: The role of heterogeneous microstructure

Abstract

To improve the comprehensive properties of CLAM steel and expecting good performance in service conditions, twist extrusion and upsetting (TEU) based thermo-mechanical treatment (TMT) was introduced into the steel at 600 °C, 670 °C, and 740 °C. Optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) was used to characterize the microstructural evolution during the deformation. Uniaxial tension test was used to evaluate the tensile mechanical property. The results show that the grain size and dislocation density of the processed samples decrease from the as-received condition, but are higher than that in the as-tempered conditions. The particle sizes of M23C6 and MX in the TEUed samples are lower than that in the as-tempered case, which contribute a lot to the yield strength. The yield strength of the TEUed CLAM steel decrease with the increase of deformation temperature and a good match in strength and ductility, relating to heterogeneous microstructure, has been achieved at the deformation temperature of 670 °C (yield stress: 853.59 MPa, ultimate tensile stress: 992.4 MPa, elongation: 15.7%). The K value and exponent in the Hall-Petch relationship should be modified owing to the volume fraction of martensite, which is harder than ferrite and causes interface elastic mismatch stress, is different at different temperatures.