The influence of a low Ti addition (∼0·01%) on the hot ductility of Nb containing HSLA steels has been examined. For conventional cooling conditions in which an average cooling rate from the melting point to the test temperature was used, the ductility decreased markedly with the addition of Ti. However, when cooling conditions after melting were more in accord with the thermal heat treatment undergone by the strand during continuous casting, i.e. cooling is fast to begin with, reaches a minimum and then reheats, after which the temperature falls more slowly to the test temperature, the Ti addition was found to be beneficial.

A variety of heating and cooling programmes have been examined for plain C-Mn and high strength low alloy steels to examine their suitability in a hot tensile test for assessing the likelihood of transverse cracking occurring in the straightening operation. A tensile test temperature of 800uC was chosen for comparison, this being the temperature that generally results in poor ductility. For steels with 1?4-1?75%Mn, the simple procedure of heating to y1300uC to take all the microalloying additions into solution, followed by cooling to the test temperature, was found to be the easiest and most suitable. For high strength low alloy steels containing Ti and low Mn, high S steels, melting is required. For these steels, it is also advisable to have both primary rapid cooling followed by a slower secondary cooling stage, simulating more accurately the actual industrial operation. The addition of thermal oscillations to simulate slab roll contact makes the cycle even closer to the commercial process and generally led to a small decrease in reduction in area values. Because of its complexity, this latter method would not be generally recommended for steels showing wide trough behaviour (high Mn, peritectic C steels), and melting followed by primary and secondary cooling is sufficient. For narrow troughs (low Mn, low C steels), which require melting and where the minimum ductility will be at a temperature of .800uC, the more complex procedure will be required. It will be necessary to obtain the full hot ductility curve using a cycle that, as well as melting and having primary and secondary cooling, also incorporates commercial thermal oscillations or at least some limited thermal oscillations.

The as cast hot ductility of low C -V, low C -V -Nb, and niobium microalloyed steels has been investigated using in situ melted tensile specimens, which were subjected to cooling rates and strain rates found typically in thin slab casting. Stress relaxation tests were performed on in situ melted specimens to monitor the kinetics of strain induced precipitation in the above steels. Although the addition of niobium to low C -V and low C -V -N steels increased the temperature at which ductility began to deteriorate, ductility was improved in the low temperature region of the austenite. This was attributed to a delay in NbV(C,N) precipitation in V -Nb steels. Increasing the nitrogen content did not influence the temperature at the onset of ductility loss in low C -V and low C -V -Nb steels, but did increase the temperature marking the onset of the ductility trough. Faster cooling rates led to a deterioration of ductility in low C -V -N and low C -V -Nb -N steels. The as cast ductility of low C -V and low C -V -Nb steels was superior to that as of cast peritectic C -Nb steel as well as reheated peritectic C -V and peritectic C -V -Nb steels. MST/4988Dr Banks

A regression equation for the Ar 3 temperature for as cast ferrite/pearlite steels has been obtained. At these coarse grain sizes, very little influence of grain size on the Ar 3 is observed. Out of all the elements examined, C, Mn and Nb had the major influence in decreasing the Ar 3 . A change in cooling rate from 10 to 200 K min 21 results in only a small decrease of around 20uC. Of particular interest is the very marked effect of Nb in reducing the Ar 3 , an addition of 0?03%Nb causing a decrease in the Ar 3 of 50uC.

The hot ductility of in situ melted tensile specimens of Ti–Nb containing steels having C contents in the peritectic C range 0·12–0·17 with and without V has been examined over the temperature range 700–1000°C. An improved testing regime for simulating the continuous casting process was used, which takes into account both primary and secondary cooling conditions. For the Nb containing steels, the ductility improved in the temperature range 750–850°C as the Ti/N ratio increased. However, ductility at 800°C was still below the 35–40 reduction in area values required to avoid transverse cracking. This was attributed to the copious precipitation of sub 40 nm NbTi(CN) precipitates along the grain boundaries and finer precipitates within the grains. Adding V to the Ti–Nb containing steels resulted in significantly improved ductility with reduction in area values at 800°C in excess of 45. This improvement was due to a decrease in the fraction of fine particles, and in accord with this better ductility, transverse cracking of industrial slabs was avoided.