The bending tests reveal a significant influence of Th at the critical bending temperature of 900 °C. For that reason, a sample was produced for each Al content, where the high temperature oxidation is mostly suppressed. The samples stay in the mould for 60 s until a surface temperature of ~1050 °C. Afterwards, the sample is cooled immediately to 900 °C and held isothermally until the start of the bending test at 700 s.
Table 3 lists the total number of IG cracks for all samples with Tb = 900 °C. The first notable point is the high value with Th = 1200 °C for both Al contents. The cracks are partially in networks. During holding at 1200 °C, notches form at the austenite grain boundaries due to selective grain boundary oxidation. The mechanism for the easier formation of cracks during tensile load is the stress concentration at these notches. As an example, a network of cracks and notches can be seen in Fig. 4c (steel 0Al). At this point it should be mentioned that the austenite grain size was measured for all of the samples and there is no clear correlation between the grain size and the crack appearance in the present study.
For the samples at Th = 1050°C, the effect of grain boundary oxidation is already critical in the first seconds of oxidation. The cause of the higher number of IG cracks in steel 0Al happens right after the mould, where the surface temperature is ~1180 °C. In this case, the surface of steel 0Al is more damaged than with Al deoxidation, which can be seen in Fig. 3. The austenite grain boundaries are clearly visible in Fig. 3b (003Al), but the notches are very shallow. Fig. 3c shows steel 0Al, where deep notches are broken up and turned into cracks.
For a longer residual time in the mould, the sample temperature in the first contact with the atmosphere is ~1050 °C. The number of cracks for 0Al drops from 383 to 50. The same cooling strategy for steel 003Al now leads to more cracks than for steel 0Al. The selective grain boundary oxidation is suppressed for both samples, but at this stage, other damage mechanisms are getting more active for steel 003Al. Calculations show high amounts of AlN precipitates, especially for this temperature in the areas with higher strains, which indicates the harmful effect of deformation-induced AlN precipitates.
To classify the crack formation in the bending test with regard to the continuous casting process, a critical strain εcrit is defined. It represents the first strain value, dependent on the crack position and the distance to the bending axis, where the number of cracks rises to more than 2. Fig. 5 shows the values for the tests at 900 °C (Table 3) depending on Th. Additionally, the reproduced tests with Th = 1050 °C are plotted. At Th = 1200 °C, εcrit drops for both Al contents to values in a critical strain range (~1.7%), which is already close to straightening conditions.