The influence of composition and cooling rate on the hot ductility of steels has been reviewed. Models to predict hot ductility behaviour have been discussed and the parts of the trough which can be used to predict the likelihood of cracking occurring are highlighted. On tensile testing both deformation induced ferrite in sufficient quantity to improve ductility and dynamic recrystallisation occur but not when straightening during continuous casting; the strain being too low. This limits the use of the hot ductility curve in predicting cracking behaviour. The temperature range in which straightening of the continuously cast strand should be carried out is either 30uC below the Ar 3 when there is a large amount of ferrite (y40%) present before deformation or above the T d , the temperature at which dynamic recrystallisation starts to take place in a tensile test; this being when the ferrite film no longer forms and precipitates are sufficiently coarse and few in number to influence the ductility. The influence of C,Mn,N, Nb,V,B,Ti,Al and the residuals Cu and Sn on hot ductility are examined and recommendations made with regard to the amounts and cooling conditions required to give freedom from transverse and edge cracks. The hot ductility of the new transformation induced plasticity (TRIP) and twin induced plasticity (TWIP) steels has also been reviewed. P has been shown to have little influence on the hot ductility, but for Al based TRIP and TWIP steels, Al levels need to be closely controlled and high Al levels, (1?5 to 2%Al) are favoured.

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.

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 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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The microstructural characteristics and mechanical properties, including micro-hardness, tensile properties, three-point bending properties and Charpy impact toughness at different test temperatures of 8 mm thick S960 high strength steel plates were investigated following their joining by multi-pass ultra-narrow gap laser welding (NGLW) and gas metal arc welding (GMAW) techniques. It was found that the microstructure in the fusion zone (FZ) for the ultra-NGLW joint was predominantly martensite mixed with some tempered martensite, while the FZ for the GMAW joint was mainly consisted of ferrite with some martensite. The strength of the ultra-NGLW specimens was comparable to that of the base material (BM), with all welded specimens failed in the BM in the tensile tests. The tensile strength of the GMAW specimens was reduced approximately by 100 MPa when compared with the base material by a broad and soft heat affected zone (HAZ) with failure located in the soft HAZ. Both the ultra-NGLW and GMAW specimens performed well in three-point bending tests. The GMAW joints exhibited better impact toughness than the ultra-NGLW joints.

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A study has been undertaken of four vanadium based steels which have been processed by a simulated direct charging route using processing parameters typical of thin slab casting, where the cast product has a thickness of 50 to 80mm ( in this study 50 mm) and is fed directly to a furnace to equalise the microstructure prior to rolling. In the direct charging process, cooling rates are faster, equalisation times shorter and the amount of deformation introduced during rolling less than in conventional practice. Samples in this study were quenched after casting, after equalisation, after 4 th rolling pass and after coiling, to follow the evolution of microstructure. The mechanical and toughness properties and the microstructural features might be expected to differ from equivalent steels, which have undergone conventional processing. The four low carbon steels (~0.06wt%) which were studied contained 0.1wt%V (V-N), 0.1wt%V and 0.010wt%Ti (V-Ti), 0.1wt%V and 0.03wt%Nb (VNb), and 0.1wt%V, 0.03wt%Nb and 0.007wt%Ti (V-Nb-Ti). Steels V-N and V-Ti contained around 0.02wt% N, while the other two contained about 0.01wt%N. The as-cast steels were heated at three equalising temperatures of 1050°C, 1100°C or 1200°C and held for 30-60 minutes prior to rolling. Optical microscopy and analytical electron microscopy, including parallel electron energy loss spectroscopy (PEELS), were used to characterise the 2 precipitates. In the as-cast condition, dendrites and plates were found. Cuboid particles were seen at this stage in Steel V-Ti, but they appeared only in the other steels after equalization.In addition, in the final product of all the steels, fine particles were seen, but it was only in the two titanium steels that cruciform precipitates were present. PEELS analysis showed that the dendrites, plates, cuboids, cruciforms and fine precipitates were essentially nitrides. The two Ti steels had better toughness than the other steels but inferior lower yield stress values. This was thought to be, in part, due to the formation of cruciform precipitates in austenite, thereby removing nitrogen and the microalloying elements which would have been expected to precipitate in ferrite as dispersion hardening particles.Dr Li is now with Vanitec,