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Work performed from the beginning of the project
Micro-alloyed steels with different additions of Carbon, Boron, Titanium, Manganese, Sulphur, Aluminium, Copper and Nitrogen were studied. The steels were manufactured in the Partner’s research laboratories, in the Pilot plant and in the industrial steel plants.
The investigation work covered the following areas:
- Industrial, pilot plant and laboratory-scale manufacturing of Boron- and other micro-alloyed steel grades with varying Mn and S contents.
- Thermodynamic and kinetic simulations of the precipitation processes followed by metallographic examination of steel samples to verify the simulation results.
- Hot ductility tests, simulation of the secondary- and tertiary cooling and investigation of the bending-unbending conditions during continuous casting. Metallographic examination of samples and analysis of fractures and the mechanism of fracture.
- Investigation of the relationship between steel susceptibility to cracking and the conditions affecting cracking: segregation of precipitates, cooling rates, deformations.
- In-field investigation of solidification and secondary- and tertiary cooling conditions and their effect on the ductility of crack-sensitive billet steel grades
The main results obtained
Steel chemistry
It was shown that the worst combination for the hot ductility was obtained when there was no Ti added to protect against precipitation of BN and when the proportions between Manganese and Sulphur contents exceeded critical values. Binding Boron to Nitrogen strongly decreased precipitation of iron boride Fe2B that stabilizes the prior austenite grain boundaries and improves hot ductility. Ti addition is beneficial, only if the content is high enough to prevent BN precipitation. It cannot be too high, to avoid excessive TiNbCN formation. The carbo-nitride precipitation makes a high Ti-content more detrimental for the hot ductility in alloys with high C- and Nb-contents. The beneficial effect of Fe2B on the hot ductility has limits when the detrimental effect of too high amounts of TiN and BN cannot be compensated by Fe2B.
The effect of critical MnS-values on hot ductility cannot be counteracted by Fe2B at temperatures above approximately 1000°C. However, the safe Mn and S chemistry window can be used to solve this problem. High Carbon contents (0.25 – 0.40) impair the efficiency of using Ti and B to improve the hot ductility of steels.
The investigation results show that the hot ductility can be controlled by following the chemistry-based criteria developed in the project. Higher ductility is easier to obtain in steels with lower Carbon contents like 0.10 wt.%. More restrictions concerning the contents of micro-alloy elements are valid for steels with higher Carbon.
Bending/unbending process
Together with optimization of steel chemistry, the effect of continuous casting conditions on the steel hot ductility during the bending /unbending procedure was investigated. The study was focused on parameters that can be controlled during the process, i.e. the cooling rate, the temperature at which the bending/unbending takes place and the strain. Relations between crack formation and strain or temperature were investigated using industrial data and the laboratory tests. The main conclusions can be summarized as follows:
- The industrial bending/unbending conditions did not lead to a surface cracking at laboratory trials. No cracking has been observed on the specimen surface. It was the cooling process that was considered as the additional source that could increase strains and affect cracking.
- Laboratory simulations demonstrated that the strains associated with the secondary cooling can be of the same order of magnitude as the strains related to the unbending process of the continuous casting process.
- The oxidation of the steel surface enhanced cracking susceptibility due to the hot shortness mechanism related to the Cu, and to fine MnS precipitation close to the oxide layer.
- Fracture occurred by an intergranular cracking which was closely related to the presence of fine carbonitrides and MnS at austenite grain boundaries.
Tertiary Cooling of Crack Sensitive Billets
A comprehensive industrial study of the continuous casting tertiary cooling conditions for billet casting have been undertaken. Temperature measurement results were used and compared with temperature predictions of the billet surface based on DISTEMP and IDS mathematical modelling. The simulation work revealed that small primary austenite grains, high volumes of proeutectoid ferrite, and relatively slow cooling rates are beneficial for the reduction of the thermal expansion of the billet cooling. If these conditions are met the internal stresses within the billet will be minimized, resulting in a reduced risk of internal transformation stress cracking.
The effect of a cooling rate on the ductility of crack-sensitive billet steel grades was investigated. The work revealed that the resistance to transformation stress cracking is a complicated one. Within the cooling billet, temperature gradients are set up with the centre of the billet being hotter than the surface. This results in differences in cooling rates, thermal contractions, and expansions. Stress will undoubtedly concentrate in localized weaker areas of the microstructure, such as at thin ferrite networks that are densely populated with MnS and other micro-alloy precipitates. If stress build-up continues micro-voids around the precipitates will start to propagate as micro-cracks. If these micro-cracks grow and coalesce the material can then start to crack on a more macro-scale. This will give rise to transformation stress cracking that predominantly will remain internal to the billet but in extreme cases can reach the surface.
Thermal Stress Cracking from Slab Torch Cutting
A comprehensive industrial study of the tertiary cooling conditions for slab continuous casting have been undertaken. This work revealed that there were no issues with the tertiary cooling conditions for crack sensitive steel grades provided the slabs were charged to the slow cooling hot boxes at high enough temperatures and allowed to cool through the range 800 °C to 500 °C at a rate of 0.02 °C/s. Thermal stress cracking was not experienced when the slabs were subdivided at a temperature of approximately 400 °C, followed by slow cooling in the hot boxes prior to rolling.
To complement the industrial slab trials, two Pilot Plant mini slab trials were completed. As-cast mini slab semis were flame cut from the continuously cast strand and charged to a slow cooling hot box at temperatures in excessive of the trial minimum of 700 °C. During the slow cooling process mini slabs were taken out of the slow cooling hot box and subdivided at 500 °C and 300 °C, using manually operated oxy-propane gas torch cutting. Additional mini slabs were then subdivided at ambient temperature both for the slow cooled material and from part of the remainder of the strand that had been allowed to cool quickly on the continuous caster runout table.
Based on this work it was concluded that the combination of the steel grade and the cooling rate plays a key role in the susceptibility to thermal stress cracking. A finer grained, harder microstructure was found more prone to thermal stress cracking than a softer dendritic microstructure.
Transformation Stress Cracking in Slabs
A factorial experimental investigation in which three mini slab heats of micro-alloyed composition were manufactured with different casting speeds, secondary cooling, and tertiary cooling conditions, was successfully completed.
Transformation stress cracking and cracked segregation features were only identified in air cooled mini slabs that exhibited a microstructure of relatively fine grained equiaxed ferrite and pearlite, with evidence of proeutectoid ferrite networks, and pockets of bainite. Mini slabs which had air cooled at a faster cooling rate were found to be fully bainitic and devoid of proeutectoid ferrite networks, but free of transformation stress cracking. The mini slabs that had all slow cooled in a hot box were free of transformation cracking and were found to exhibit either a relatively coarse-grained ferrite and pearlite microstructure or a had a dendritic microstructure, due to very slow cooling conditions.
Based on the factorial experiments undertaken on the Pilot plant continuous caster, it can be concluded that the combination of the steel grade, cooling rate and through thickness thermal gradients play a key role in the susceptibility to transformation stress cracking. A microstructure of fine-grained mixed ferrite and pearlite and thin ferrite networks rich in MnS and micro-alloying precipitates is more prone to transformation stress cracking than a harder bainitic microstructure.
Whilst these findings appear to be applicable to a mini slab of section size 142 x 300 mm, replicating these conditions on a conventional continuous caster is far more difficult. Therefore, the current practice of charging material into stacks, pits/boxes, or hoods should continue to be the preferred practice for the successful manufacture of steel grades that are at risk of transformation stress cracking.