B. Mintz

Hot ductility of TWIP steels

Abstract The hot ductility of twin induced plasticity (TWIP), 0·6 wt-%C steels containing 18–22 wt-%Mn with N levels in the range 0·005–0·023 wt-% and the Al additions either low (<0·05 wt-%) or high (1·5 wt-%) has been examined. Little change in ductility occurred in the temperature range 1100–650°C as the structure was always fully austenitic. Ductility was generally poor (<40%), reduction of area values, the best ductility at the higher Al level being given by the steel with the lowest N and S levels. Because the steel is fully austenitic, the ductility is solely dependent on that for unrecrystallised austenite. Therefore, to avoid transverse cracking the volume of second phase particles should be kept to a minimum, i.e. the N should be low to reduce the amount of AlN that can be precipitated out and the S level should be as low as possible to limit the amount of MnS inclusions. Metallographic and TEM studies were carried out and the poor ductility was found to be due to extensive precipitation of AlN at the austenite grain boundaries. Increasing the cooling rate from the melting point to the test temperature from 60 to 180°C min−1 or introducing an undercooling step both led to even worse ductility.

Influence of S and AlN on hot ductility of high Al, TWIP steels

Abstract The hot ductility of a high Al, twin induced plasticity austenitic steel with S levels of 0·003, 0·010 and 0·023% has been examined after heating to 1250°C and cooling at 60 K min−1 to test temperatures in the range of 700–1100°C. Ductility as measured by reduction of area (R of A) was very poor, (∼20%R of A) in the two higher S steels in the temperature range of 950–1100°C but was better at lower temperatures of 700–900°C reaching 30–40%R of A. For the very low S steel, ductility was similar for the temperature range of 700–900°C but improved in the higher temperature range of 950–1100°C to 50–55%R of A. In the higher S steels (0·01 and 0·023%S), ductility was poor because of the presence of long, coarse dendritic AlN rods situated at both the dendrite, but, more importantly, the austenite grain boundaries, the latter being particularly detrimental for encouraging intergranular failure. AlN seems to need MnS inclusions as nuclei in the melt for it to precipitate in the form of these detrimental long rods, and a reasonable volume fraction of MnS (equivalent to having more than 0·003%S present) is required for this to happen. If the S level is low (⩽0·003%), there are only a few MnS nucleation sites, and the AlN precipitates out in the form of coarse hexagonal plates. These precipitates end up mainly within the austenite matrix and have little influence on the hot ductility. It is therefore very important commercially in these high Al steels to limit the S level to ⩽0·003% to avoid transverse cracking.

The hot ductility of Nb/V containing high Al, TWIP steels

Abstract The hot ductility of Nb/V containing high Al, twin induced plasticity (TWIP) steels has been examined over the temperature range 650–1150°C after melting and after ‘solution treatment’. Previous work had shown that the hot ductility is poor for the 1·5 mass-%Al, TWIP steel due to precipitation of AlN at the austenite grain boundaries, the depth of the trough being similar to that for an X65 grade pipeline steel but with the trough covering a much wider temperature range. Adding Nb and V made the ductility even worse due to the additional precipitation of NbCN and VN. Very low reduction of area values, 10–20% were obtained in the temperature range 700–900°C. Increasing the cooling rate to the test temperature resulted in even worse ductility. The ductility of these steels after ‘solution treatment’ is similar to that obtained after melting but when the cast was hot rolled followed by ‘solution treatment’ and cooling to the test temperature ductility improved due to grain refinement.

Influence of thermal history on hot ductility of steel and its relationship to the problem of cracking in continuous casting

Abstract 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 800°C 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 ∼1300°C 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 >800°C, 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.

Influence of Ti on hot ductility of Nb containing HSLA steels

Abstract 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.

Influence of B and Ti on hot ductility of high Al and high Al, Nb containing TWIP steels

Abstract The addition of ∼0·002%B and ∼0·04%Ti as microalloying additions to improve the poor hot ductility and high risk of cracking on continuous casting of high Al containing twinning induced plasticity (TWIP) steels has been examined. Tensile specimens were either cast in situ or heated to 1250°C before cooling at 60 K min−1 to test temperatures in the range 700–1100°C and strained to failure at 3×10−3 s−1. For tensile specimens reheated to 1250°C, the presence of B with sufficient Ti to combine with all the N improved ductility over the temperature range of 700–950°C, the reduction in area (RA) values being >40%. For the higher strength more complex high Al, TWIP steels having Nb present, there was no improvement in ductility with a similar B and Ti addition, when the average cooling rate after melting to the test temperature was 60 K min−1. Reducing the cooling rate to 12 K min−1 resulted in the RA values being close to the minimum required to avoid transverse cracking throughout the temperature range 800–1000°C. Using these additions of B and Ti, transverse cracking was found not to be a problem when continuously casting these high Al containing TWIP steels.

Regression equation for Ar3 temperature for coarse grained as cast steels

Abstract A regression equation for the Ar3 temperature for as cast ferrite/pearlite steels has been obtained. At these coarse grain sizes, very little influence of grain size on the Ar3 is observed. Out of all the elements examined, C, Mn and Nb had the major influence in decreasing the Ar3. A change in cooling rate from 10 to 200 K min−1 results in only a small decrease of around 20°C. Of particular interest is the very marked effect of Nb in reducing the Ar3, an addition of 0·03%Nb causing a decrease in the Ar3 of 50°C.

Influence of B on hot ductility of high Al, TWIP steels

Abstract The influence of B on the hot ductility of high Al, Ti containing twinning induced plasticity (TWIP) steels has been examined. It was established that provided the B was fully protected by adding sufficient Ti to combine with all the N, then B could segregate to the austenite grain boundaries and improve ductility. This improvement was particularly marked for the temperature range of 700–900°C, the range in which the straightening operation often takes place in continuous casting. Of most importance in the present work has been the detection of B at the boundaries using a secondary ion mass spectrometry technique. The cooling rate from the reheating temperature of 1250°C to the tensile testing temperature range of 700–1200°C was 60 K min−1, but it is likely that slower cooling rates ≤25 K min−1, more in keeping with the secondary cooling rate on continuous casting, will give even better ductility. Ti additions in themselves are beneficial to the hot ductility of these steels as precipitation of AlN at the austenite boundaries is avoided, but only if the cooling rate is sufficiently slow to allow the TiN particles to coarsen. However, to ensure freedom from cracking, an addition of B is also required.

Influence of V and Ti on hot ductility of Nb containing steels of peritectic C contents

Abstract 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.

Influence of P and N on hot ductility of high Al, boron containing TWIP steels

Abstract The influence of P in the range ∼0·01 to 0·07% with high Ti and N additions on the hot ductility of 1·5%Al, boron treated twinning induced plasticity (TWIP) steels has been examined. P, even at the 0·02% level, has a small detrimental influence on the hot ductility, and ductility decreases progressively as the P content is increased. Low melting point Fe(Mn)phosphide phases were found at the austenite grain boundaries accounting for this deterioration in ductility. As it is difficult to cast these steels, without cracks forming, P levels should be as low as possible, preferably ∼0·01%. High Ti and N additions (≧0·07%Ti, 0·01%N) to these B treated, TWIP steels gave rise to good ductility throughout the straightening temperature range 800–950°C. It is suggested that the high N level results in good ductility because although there is a large volume fraction of TiN, the high Ti/N ratio encourages growth of the TiN precipitates, and the high [Ti][N] product causes the precipitation to take place at high temperatures so that it is too coarse to influence the hot ductility.