A Study on Brittle Cleavage Fracture on Ti-Mo Nano-precipitation-Strengthened High-Strength Steel

Abstract

Hot-rolled high-strength steel having ferrite matrix and nano-size precipitation possesses a good combination of strength and ductility, and it finds application in the automobile industry for the manufacture of long member and chassis parts of large-capacity vehicles. Such an application needs steel with high stretch flangeability since there are several drilled or broached holes in the finished component. Apart from an adequate strength level of 800 MPa UTS with a minimum elongation of around 16%, the steel requires a high degree of stretch flangeability to avoid any catastrophic failure during its use. During the commercial production process, sometimes, the steel grade namely NPS800 (Nano-Precipitation Strengthened 800 grade) exhibited brittle fracture in the tensile test specimens as well as during the component manufacture. A detailed study was undertaken to understand the causes of brittle fracture or mixed-mode fracture, despite the steel showing a total elongation of not less than 18%, and the resolution to this problem is explained in this paper. Despite the steel having tensile strength and total elongation as per specification, the performance of steel during component forming was found to be erratic. The steel samples while possessing similar chemical composition and mechanical properties exhibited a variation in stretch formability. The main strengthening mechanism of this new grade being precipitation hardening by nano-precipitates, a detailed study involving microstructural characterization became essential to explain the causes of the incidences of poor stretch flangeability. Our study reveals that the size distribution of nano-precipitate formed at the austenite–ferrite interfaces plays an important role in determining the steel properties such as strength, strain-hardening exponent, stretch flangeability, uniform elongation and post-necking elongation. For the normal finish rolling and coiling temperatures, optimization of chemistry was achieved to obtain the mechanical properties such as strength and ductility in the desired range. It is observed that the fineness of the nano-precipitate is important to achieving a good combination of strength, total elongation, strainhardening exponent, and post-necking elongation to obtain satisfactory stretch formability. The study on the crack tip opening displacement (CTOD) also confirms the significance of finer precipitates on crack propagation and thereby improving hole expansion ratio (HER). By the study of fracture samples using transmission electron microscopy, the role of large-size precipitates in causing the mixed-mode or brittle failure is explained. The significance of the reduction in area (RA), post-necking elongation, and strain rate sensitivity in achieving a satisfactory HER is established.