Experimental validation and thermodynamics analysis of (Nb, V)(C, N) coupling precipitation in high-strength spring steel FAS3550

Abstract

Based on the thermodynamic coupling model of multiple microelements in steels, the solid solution and coupling precipitation behavior of Nb-V-N-C system quaternary secondary phase was further verified in microalloyed steels, and combining with the engineering practice in high-strength spring steel FAS3550 containing nanoparticles. According to the thermodynamic calculations, the dissolved V (or Nb) content increased with the increasing addition V (or Nb), while decreased with the increase of addition C, N or Nb (or V). The complete dissolution temperature was 1314 °C, and the effective precipitation temperature of carbonitride was 1060 °C in 0.49%C-0.0060%N-0.03%Nb-0.12%V system spring steel FAS3550, the grains in steel began to coarsen above 1100 °C during heat treatment, and the (Nb, V)(C, N) nanoprecipitates were smaller than 100 nm. The strength and plasticity have been improved simultaneously, and its fatigue life has been increased by 13% after optimizing the heat treatment process.

Precipitate behaviour of large-sized Nb(C, N) in micro-alloyed steel

“…Through microalloying (vanadium, titanium, niobium), and controlling the rolling and cooling process, it is widely used in automobile connecting rods, crankshaft and other parts. 1–5 Adding an appropriate amount of vanadium, titanium and niobium can form fine carbonitride fixed on the austenite grain boundary, preventing austenite grain growth, so vanadium, titanium and niobium elements play the role of refining grain and precipitates strengthening. 6–8 However, large-sized and angular carbonitride, not only plays an adverse role in grain refinement but also destroys the continuity of the steel matrix, easily becoming a crack source under the action of external forces, seriously deteriorating the performance of steel.…”