Continuous casting is a core process in steel production, and its product quality directly determines the success or failure of subsequent rolling processes. Despite continuous technological advancements, internal cracks induced by thermal stress and mechanical stress remain a major challenge in the industry.
I. The ductility trap of continuous casting billets: Three brittle zones
During the cooling and solidification process of steel, its ductility changes nonlinearly with temperature, and there are three main dangerous brittle zones:
The first brittle zone (from 1350°C to the melting point): At this point, there is still a liquid phase between the dendrites, and the material’s strength and ductility are extremely low, making it highly prone to solidification cracks.
The second brittle zone (900–1200°C): The ductility is greatly influenced by the Mn/S ratio. Increasing the manganese content can inhibit the formation of low-melting-point FeS, thereby expanding the safe processing window.
The third brittle zone (600–900°C): The carbides and nitrides of microalloying elements (such as Nb, V, Ti) precipitate along the grain boundaries, combined with the formation of pre-austenite, result in significant loss of ductility.
II. Analysis of the Root Causes of Internal Cracks
Research shows that the fracture of a plate without surface defects during the heating process usually results from hidden internal damages.
1. Equipment and process factors
Improper cooling mechanism: Inadequate spray distribution in the secondary cooling zone results in a significant temperature gradient on the surface and subsequent rapid re-heating (re-warming), which causes thermal stress cracks between dendrites.
Mechanical stress overload: In the bending and straightening sections of the continuous casting machine, if the ingot is in the low ductility zone and is subjected to tensile stress exceeding the critical value, lateral cracks are highly likely to form internally.
Excessive temperature rise: If the temperature of the molten steel is too high, it will significantly expand the zone of columnar crystals, reduce the proportion of central equiaxed crystals, and make brittle cracks more likely to propagate along the columnar dendrites.
2. Material and Component Influence
Microalloying Sensitivity: Steel types with only vanadium (V) microalloying are more prone to cracking compared to V-Nb-Ti composite alloying steel.
The “trap zone” of carbon content: For eutectic steel with a carbon content ranging from 0.09% to 0.14%, due to the intense phase transformation stress, its crack sensitivity is the highest.
III. Experimental Research and Failure Verification
By sampling the broken billets and analyzing them using the GLEEBLE thermal simulation equipment, it was found that:
Macroscopic appearance: The acid etching results show that the cracks are mainly located in the columnar crystal zone 30-50mm away from the bottom surface, extending in a wavy pattern perpendicular to the casting direction.
Microscopic features: The cracks mainly extend between the dendrites. The microstructure exhibits the typical characteristics of ferrite surrounding pearlite, indicating that the cracks originated during the high-temperature solidification stage.
Thermal mechanical behavior: Simulation results show that when the temperature drops below 1000°C, the cross-sectional contraction rate sharply decreases to below 40%, entering the period of high crack occurrence.
IV. Process Improvement Measures and Countermeasures
To eliminate internal defects in the billets and prevent failures in the heating furnace, the following countermeasures are recommended:
Cooling and mechanical control optimization
Balancing secondary cooling: Prevent the spray pipes from getting clogged or leaking, optimize the water distribution scheme for secondary cooling, and avoid excessive reheating cycles on the surface of the cast billet.
Mechanical precision maintenance: Regularly inspect the operation status of the upper section of the continuous casting machine to ensure precise alignment and reduce unnecessary mechanical stress caused by bending.
2. Component and Metallurgical Adjustment
Optimize carbon content: Try to avoid the sensitive range of 0.09% – 0.14%, and adjust the carbon content to above 0.15% to reduce the tendency of cracking.
Composite microalloying: By using a scientific combination of V, Nb, and Ti, the toughness of the material is enhanced in the critical temperature range.
Control Mn/S ratio: Increase the manganese-sulfur ratio to reduce the thermal brittleness caused by sulfur segregation.
3. Heat Treatment and Subsequent Operations
Controlled Cooling: For sensitive steel grades, implement slow stacking cooling of the billets (slow cooling pit).
Temperature-controlled heating: Before entering the heating furnace for hot rolling, a slow heating curve is adopted to prevent the rapid expansion of existing micro-cracks due to sudden changes in thermal stress.
