Four steels, C-Mn-0.05V, C-Mn-0.11V, C-Mn and C-Mn-0.03Nb, all essentially boron-free were subjected to processing to simulate the microstructure of a coarse grained heat affected zone (GC HAZ) and an intercritically reheated coarse grained HAZ (IC GC HAZ). This involved reheating to 1 350°C, rapid cooling (Dt 8/5 ϭ24 s) to room temperature and then reheating to either 750°C or 800°C. The toughness of the simulated GC HAZ and IC GC HAZ was assessed using both Charpy and CTOD tests and the hardness of both zones was also measured. A detailed assessment of the size and area fraction of martensite-austenite (M-A) phase in the IC GC HAZ in the steels was obtained from a combination of Scanning Electron Microscopy (SEM) and Image Analysis of the resultant SEM micrographs. In addition, the distribution of the M-A phase was examined by observing 250 fields at a magnification of 2 500 times in the SEM for each of the steels.It is clear that the alloying addition has a significant effect on the amount and size of the M-A phase. The addition of 0.05% V to the C-Mn steel resulted in the lowest IC GC HAZ Charpy 50J impact transition temperature and the 0.1 mm CTOD transition temperature. The corresponding size and area fraction of the M-A phase were the smallest of the four steels. Raising the level of vanadium to 0.11% caused a deterioration in IC GC HAZ toughness, which was reflected in a greater area fraction of M-A phase, larger mean and maximum sizes of M-A particles and significantly more fields containing M-A phase. The addition of 0.03%Nb produced poorer IC GC HAZ toughness data than C-Mn-V and C-Mn steels and this was related to the large size and area fraction of M-A phase quantified in the Nb steel. The presence of M-A phase is considered to be the dominant factor in determining the toughness of IC GC HAZ.KEY WORDS: V-microalloyed steel; Nb-microalloyed steel; coarse grained heat affected zone; intercritically reheated coarse grained heat affected zone; martensite-austenite phase; heat affected zone toughness; welding.effect on the HAZ toughness, although its effect is strongly dependent on heat input. At medium to high heat input, and quite apart from a precipitation hardening effect via Nb (C, N), niobium has a detrimental influence on the fracture toughness of coarse grained HAZs. 8,9) Niobium reduces the grain boundary ferrite and promotes formation of a coarse structure of ferrite with aligned M-A-C (martensite-austenite-carbide) resulting in increased hardness. A small addition of niobium (ϳ0.02 %) is known to suppress ferrite nucleation at prior austenite grain boundaries and increase the volume fraction of martensite or bainite.10,11) Lee et al. 6) reported that the major advantages of a niobium addition, i.e. the grain refinement and the resultant improvement of base metal mechanical properties, appear to be outweighed by the detrimental effects of martensite formation, when the steel plates are welded.Vanadium gives grain refinement and precipitation strengthening to HSLA steels. The effect of vanadiu...

The evolution of precipitation and microstructure during a simulation of the thin slab direct rolling process, in six vanadium based, low carbon, steels with V, V-N, V-Ti-N, V-Nb, V-Nb-Ti and V-Zr additions was studied by optical microscopy, analytical transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and parallel electron energy loss spectroscopy (PEELS). Tensile properties and Charpy vee-notch toughness of the final strip were also determined. The effects of microalloying additions and processing conditions, including equalisation temperature (1 200°C, 1 100°C and 1 050°C) and end water cool temperature, on the austenite and ferrite grain sizes, as well as the type and composition of the precipitates, were determined. The relationship between the microstructure and the properties in the steels was also ascertained.

The evolution of microstructure during a simulation of the thin slab direct rolling process has been studied on two low carbon steels, microalloyed with V-N and V-Ti-N. The steels were examined using optical microscopy, analytical transmission electron microscopy (TEM) and energy dispersive X-ray (EDAX).After the 4th rolling pass, in a five pass schedule, the initial coarse austenite grain size (ഠ1 mm) was reduced to about 50 mm in Steel V-N and 22 mm in Steel V-Ti-N. The average ferrite grain size in the final strip was slightly smaller in Steel V-Ti-N (4.8-6.6 mm) than in Steel V-N (5.3-7.2 mm). For Steel V-N, VN was only observed after 1 050°C equalization, but it was not found after 1 200°C and 1 100°C equalisation. For Steel V-Ti-N, V-Ti(N) particles formed during casting and during equalization for all the equalization temperatures (1 200°C, 1 100°C and 1 050°C). AlN particles precipitated in Steel V-N only during 1 050°C equalization and were often associated with MnS or MnS and VN. No AlN was detected in Steel V-Ti-N. Fine V containing precipitates (Ͻ10 nm) were observed in the final strip for both of the steels, but the frequency of the fine particles was lower in Steel V-Ti-N than in Steel V-N. The fine precipitates in the final strip make a major contribution to dispersion strengthening. High strength (LYSഠ460-560 MPa) with good toughness and good ductility were achieved in the steels, which are competitive to similar products made by conventional controlled rolling. However, the addition of Ti to the V-N steel decreased the yield strength due to formation of V-Ti(N) particles in austenite, which reduced the amounts of V and N available for subsequent V rich fine particle precipitation in ferrite.KEY WORDS: vanadium and vanadium-titanium microalloyed steel; thin slab direct rolling; equalization temperature; microstructure, properties.rolling is generated at a temperature in excess of 1 450°C, while the equilibrium solubility of microalloy carbonitrides is very much greater than that at the soaking temperature used in the CCR process. Most TSDR processing chooses steels with carbon content less than 0.065 wt% to avoid the peritectic reaction and subsequent segregation. Also in the TSDR process, the as-cast austenite prior to rolling may be more highly supersaturated with respect to microalloying elements than the reheated austenite in the CCR process. This can affect subsequent microstructural development during processing.The addition of Nb to HSLA steel can give considerable strengthening, but when Nb is present in continuously cast HSLA steels, slab surface cracking, especially in the transverse direction, is a well documented observation.9) Attempts to produce acceptable surface finishes in Nb microalloyed steels have not been completely successful to date.7,10) This is associated with the precipitation of Nb compounds in a manner similar to that responsible for the ductility trough found during hot tensile testing of CCR processed steels in the temperature range from 750 to 925°C. 1,11) For this ...

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