THE EFFECTS OF SMALL TITANIUM ADDITIONS ON THE MECHANICAL-PROPERTIES AND THE MICROSTRUCTURES OF CONTROLLED ROLLED NIOBIUM-BEARING HSLA PLATE STEELS

The effects of small titanium additions (0.010% and 0.022%) on the mechanical properties and the microstructures of niobium-bearing HSLA plate steels under two different rolling schedules have been investigated. For comparison, a Ti-free steel was controlled rolled to 814-degrees-C. A side effect of the small titanium additions was observed on the strength of the niobium-bearing steels. The lower yield stress values and hardness levels for all Ti-Nb steels were reduced both in as-rolled and normalized conditions, depending on Ti/N ratio and the details of the thermomechanical process. However, the Ti-Nb steels with a lower finish rolling temperature (FRT), 800-degrees-C, showed better toughness properties than the Ti-free control steel. For Ti-Nb steels, the toughness was improved significantly by lowering the FRT from 940 to 800-degrees-C, owing mainly to a considerable refinement of the ferrite grain sizes and fewer fine carbides available for dispersion hardening. No advantages have been observed for an over-stoichiometric titanium addition (Ti/N = 4.4). The steel with an under-stoichiometric titanium addition (Ti/N = 2) and a lower FRT (800-degrees-C) showed the best overall mechanical and toughness properties among all the processed Ti-Nb steels. The precipitation of niobium nitrides and carbides in the niobium steels was changed by titanium additions which led to the formation of complex Ti-Nb nitrides and carbonitrides, i.e. Ti-rich plates and cuboids. Moreover, these plates and cuboids acted as nucleating cores on which pure carbides formed, i.e. niobium carbide in the 0.01%Ti steels whereas Nb-rich Ti-Nb carbide in the 0.022%Ti steels. In the as-rolled samples, no aluminium was found in the complex particles or as individual AlN while in the normalized condition, fine AlN precipitates were detected frequently with the under-stoichiometric titanium addition (0.01%Ti), whereas the formation of AlN was suppressed by the over-stoichiometric titanium addition (in 0.022%Ti). The Ti/N ratio, therefore, has a strong influence on the type of precipitation and the size distribution of Ti-Nb particles. With the higher Ti/N ratio, coarse Ti-rich plates and cuboids were observed at a much higher frequency. As a result, a greater amount of niobium was used in forming the Ti-Nb compounds for a given Ti/N ratio, thus, the volume fraction of fine carbides precipitated in ferrite was reduced proportionally, which would be responsible for the loss of yield strength. However, titanium additions can produce a more uniform ferrite grain size and reduce dispersion hardening by fine niobium carbides, which led to excellent toughness properties when a lower FRT was utilized.