Effect of Chemical Composition and Structure on Mechanical Properties of Low-Alloy Weldable Steels After Thermomechanical Treatment

Abstract

On the basis of analyzing 22 melts of high-strength steel (σ_0.2 = 428–886 MPa) of different grades 3–25 mm thick produced within countries of the European Union by thermomechanical rolling technology, the effect of chemical composition and structure on a set of mechanical properties is studied. It is shown that yield strength of high-strength steels is due to titanium, niobium, and vanadium carbonitrides. The contribution of titanium to steel strength is greater than that of niobium by a factor of three, and by a factor of 16.5 for vanadium. An increase in steel yield strength above 750 MPa leads to its inclination towards an increase in impact strength anisotropy coefficient K_a. In the concentration range 0.001–0.08 wt.% Ti, the value of K_a = 1–2 is independent of Ti content, but increases sharply to K_a = 3.8 with a Ti content in steel of 0.09 to 0.14 wt.% giving rise to a requirement for limiting Ti content to not more than 0.08 wt.%. A marked effect of Ti and Al concentration on high-strength steel impact strength is established with entirely ductile failure KCV^max. According to x-ray-spectral analysis data, coarse (with a size of 2.2–2.8 μm) inclusions have a complex structure. During formation, inclusions contain up to 15–17 wt.% Al, and then an outer layer forms upon them having up to 29–42 wt.% Ti.