Formation Mechanism
1. Intrinsic Core Factors: Compositional Segregation and High-Temperature Brittleness
(a) Segregation of impurity elements is the fundamental internal cause. Sulfur, phosphorus, carbon, and other elements tend to accumulate at grain boundaries during solidification, forming low-melting-point phases (such as FeS, Fe(S,O)), reducing grain boundary strength and inducing brittle fracture.
(b) Specific high-temperature brittleness zones exist in steel grades. From the solidus temperature to 1200℃ (Zone I), grain boundary melting causes brittle fracture; from 1200℃ to 900℃ (Zone II), impurity melting leads to grain boundary cracking; from 900℃ to 700℃ (Zone III), carbonitride precipitation or ferrite network formation reduces ductility.
(c) Alloying elements have a significant impact. Excessive manganese content easily induces interdendritic segregation. The precipitation of carbonitrides from elements such as Nb, Al, and V exhibits a “nose point” temperature (820-950℃), during which grain boundary embrittlement is pronounced. During hot transport, insufficient plasticity accompanying the γ→α phase transformation easily leads to cracking.
2. External inducing factors: Process and equipment conditions
(1) Uneven secondary cooling, with large differences in cooling width or inner and outer arcs of the billet, leads to an imbalanced temperature gradient, generating thermal stress and inducing cracks in the brittle zone. Localized stress concentration is also a contributing factor. Rapid reheating in the first and second cooling zones (with a maximum reheating rate of 217.48℃/m) can cause stress and strain exceeding the limit in the 15-30mm layer of the billet surface. When the temperature at the corner of the straightening zone falls into the third brittle zone (around 700℃), the reduction of area decreases, exacerbating crack initiation.
(2) Inappropriate process parameters: High drawing speed and high superheat will exacerbate columnar crystal growth and central segregation; excessive or improperly positioned roll gap shrinkage will cause mechanical stress concentration.
(3) Insufficient equipment precision: Roll gap deviation and poor guide roller alignment will generate additional stress; nozzle blockage will exacerbate uneven cooling, all of which will promote crack formation and propagation.
Control Measures
1.Composition Optimization: Reducing the Risk of Intrinsic Brittleness
Strictly control impurity content; sulfur and phosphorus mass fractions must both be below 0.015%, and carbon content must be controlled at the lower limit of the composition specification (preferably <0.15%); optimize alloy ratios to ensure a manganese-to-sulfur ratio (w(Mn)/w(S)) ≥ 25; appropriately control the content of microalloying elements such as vanadium and niobium to avoid excessive precipitation; improve the purity of molten steel, reduce oxygen and nitrogen content, reduce inclusions through refining processes, and improve the uniformity of solidification structure.
2.Process Control: Reducing External Stress and Segregation
Optimize secondary cooling, adopt a uniform cooling regime, and rationally control the specific water volume. Maintain the transverse temperature difference on the billet surface within 20~40℃, and keep the inner and outer arc water volume ratio at 1~1.7. Stabilize key process parameters, implement constant casting speed operation, and control the superheat of molten steel within 15~25℃ to avoid large fluctuations. Rationally set the roll gap, with a shrinkage peak <0.4 mm/m and a total shrinkage of approximately 4 mm, compensating for solidification shrinkage while avoiding excessive extrusion.
3.Equipment Support: Enhancing Precision and Stability
Regularly calibrate equipment, controlling roll gap deviation within 0.2 mm and guide roller alignment deviation ≤0.3 mm to ensure uniform force distribution; strengthen nozzle maintenance, regularly clean clogged nozzles to ensure uniform spraying and avoid localized insufficient or excessive cooling.
4.Hot Charging Process Optimization:
Control the hot charging temperature of microalloyed steel slabs at ≥700℃ to avoid the carbonitride precipitation and embrittlement range, achieving fully continuous hot charging production and reducing the risk of cracking during cooling and reheating; implement heat preservation measures during hot charging, using heat insulation covers or hot charging furnaces to reduce surface temperature fluctuations of the slab and avoid structural stress caused by secondary phase transformation.
FAQ
Why do internal cracks in slabs typically occur at the end of solidification?
A: Internal cracks are essentially a type of “hot crack,” usually occurring in the liquid-solid phase region at the solidification front. When the slab passes through the secondary cooling zone or the straightening zone, the shell deforms due to bulging or mechanical stress, concentrating stress in the weakest dendrite intergranular spaces. If the molten steel has low plasticity in this region and cannot withstand tensile strain, it will crack along the dendrites, forming an internal crack.
What is the key role of the manganese-sulfur ratio (Mn/S ratio) in preventing internal cracks in slabs?
A: Sulfur (S) in steel forms low-melting-point FeS, which remains in the liquid state for a long time at grain boundaries, easily inducing cracks. By increasing the Mn/S ratio (usually >25 or higher), sulfur can be converted into MnS inclusions with higher melting points, thereby improving the high-temperature plasticity of the steel. In modern high-efficiency continuous casting, strictly controlling the Mn/S ratio is fundamental to reducing internal cracks in segregation-sensitive grades (such as medium carbon steel and low alloy steel).
Why is temperature reheating in the secondary cooling zone a major cause of cracking?
A: Uneven spraying or abrupt changes in cooling intensity in the secondary cooling zone can lead to a drastic temperature reheating on the slab surface. Surface thermal expansion generates tensile stress in the not-yet-fully-solidified central region. According to industry standards, slab surface temperature fluctuations should be controlled within 100℃/m to prevent excessive internal stress due to thermal stress accumulation.
