Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

Influence of cooling paths on microstructural characteristics and precipitation behaviors in a low carbon V–Ti microalloyed steel

Abstract: Based on ultra fast cooling, the microstructural characteristics, precipitation behaviors and mechanical properties of a low-carbon V–Ti microalloyed steel were investigated in details using optical microscope, electron back-scattered diffraction and transmission electron microscope. The results show that the ferrite grains can be slightly refined, the sheet spacings of interphase precipitation can be also slightly reduced and the number fraction of ferrite grains with higher precipitation hardening can be significantly enhanced by increasing cooling rate (by comparisons of air cooling and furnace cooling), and a ferritic steel precipitation-strengthened by nanometer-sized carbides was developed to produce hot rolled high strength steel with the tensile strength of ~810 MPa, elongation of ~24% and yield ratio of ~0.82. While for furnace cooling after ultra fast cooling, its tensile strength, elongation and yield ratio is only ~750 MPa, ~22% and ~0.84, respectively. The interphase precipitation in V–Ti microalloyed steel was observed, and these nanometer-sized carbides were detected as (V, Ti)C using energy dispersive X-ray spectroscopy spectra. In addition, the precipitation hardening was estimated as ~313 MPa and ~293 MPa for air cooling and furnace cooling after ultra fast cooling, respectively.

Precipitation behavior in ultra-low-carbon steels containing titanium and niobium

Abstract: This work revealed the basic mechanism for the stabilization of carbon in ultra-low-carbon (ULC) steels that contain moderate S (0.004 to 0.010 wt pct), adequate Ti (0.060 to 0.080), and low Mn (≤0.20). During cooling through the austenitic region to the ferritic, the initially formed sulfide particles (TiS) undergo an in situ transformation into carbosulfides (H-Ti4C2S2) by absorbing C and Ti. The transformation from TiS to H may be considered as a hybrid of shear and diffusion, i.e., faulted Ti8S9 (9R)+10[Ti]+9[C] → 4 1/2Ti4C2S2 (H). At low temperature (≤930 °C), the stabilization process continues through epitaxial growth of carbides on H phase, i.e., [M]+x[C]+H → epitaxial MCx (on H). This mechanism differs from the traditional view of stabilization, where the carbon is removed from solution by the formation of free-standing or independently nucleated H and/or MCN precipitates. While these two forms of carbon stabilization are now well known, this article presents a method of predicting which mechanism of stabilization will be operative in a given steel based on its bulk composition. Implications bearing upon new ULC steel design, considering the role of S, will be discussed.

The effect of niobium on the hardenability of microalloyed austenite

Abstract: The powerful effect that varying the extent of niobium-carbide dissolution has on the “hardenability” of microalloyed austenite is demonstrated using dilatometric measurement of the critical cooling rate required to from microstructures containing >95 Pct martensite. The results can be rationalized on the hypothesis that the hardenability of austenite is enhanced by niobium in solid solution, possibly by its segregation to austenite grain boundaries, but is decreased by precipitation of niobium-carbide particles. This effect appears analogous to that of boron in steels and is found to be independent of variations in the austenite grain size.