Internal cracks in steel billets are common quality defects in continuous casting production. They not only affect the mechanical properties and processing properties of steel, but may also cause scrap in the subsequent rolling process, causing huge economic losses to steel companies.
Hazards of internal cracks in steel billets
Internal cracks in steel billets refer to cracks formed inside the ingot, which can be divided into middle cracks, center cracks, subcutaneous cracks, etc. according to their positions. The hazards of this defect run through the entire process of steel production.
In the rolling process, internal cracks can cause problems such as fracture and folding of steel. For example, the internal cracks of 20 steel continuous casting tube billets will evolve into external transverse cracks and external folds during the perforation process, increase the grinding amount of rough pipes or rough pipes, and reduce the metal recovery rate. Data show that severe internal cracks may reduce the first-time qualified rate of rolled pipes by 10%-30%.
From the perspective of use, internal cracks will reduce the mechanical properties of steel. Stress concentration is easily formed at the cracks, which becomes the starting point of fracture when subjected to force, resulting in a significant decrease in the strength and toughness of the steel, which cannot meet the engineering requirements. For steel billets used to manufacture seamless steel pipes, internal cracks can also cause safety hazards such as leakage during use.
In addition, internal cracks increase production costs. Billets containing internal cracks require additional testing and refining, and some severely substandard billets must be scrapped, wasting resources and reducing production efficiency.
Causes of internal cracks in steel billets
The formation of internal cracks in steel billets is the result of the combined action of multiple factors, involving multiple aspects such as molten steel composition, process parameters, and equipment status. They can be mainly summarized into the following categories:
(I) Influence of molten steel composition
The chemical composition in steel has a significant effect on crack sensitivity. Sulfur (S) is a typical crack-sensitive element. When the S content in steel is too high, it will form low-melting-point FeS (melting point 989°C) with iron, which is distributed at the grain boundaries, causing intergranular brittleness and becoming a path for crack propagation. Studies have shown that when [S]＞0.025%, the ductility of steel decreases significantly and the probability of internal cracks increases significantly.
The ratio of manganese (Mn) to sulfur (Mn/S) is also crucial. When Mn/S＞25, high-melting-point MnS (melting point 1600°C) can be formed, which is dispersed in the austenite matrix in the form of rods to reduce grain boundary brittleness; if Mn/S＜25, the harm of sulfur cannot be effectively suppressed, and internal cracks are easily caused.
(II) Unreasonable process parameters
Molten steel superheat: Molten steel superheat refers to the difference between the temperature of molten steel and the temperature of liquidus. The higher the superheat, the more developed the columnar crystals of the ingot, and the thinner the shell is, which is easy to deform under the static pressure of molten steel. At the same time, the “notch effect” of the columnar crystals will aggravate stress concentration and cause internal cracks. Tests show that when the superheat exceeds 35°C, the frequency of internal cracks increases significantly.
Pulling speed control: Pulling speed is a key parameter in continuous casting production. Too high a pulling speed will lead to insufficient shell thickness, unable to withstand the pressure of molten steel, and prone to bulging deformation; too low a pulling speed will cause the ingot to stay in the secondary cooling zone for too long, the surface temperature will rise too much, and large thermal stress will be generated.
Secondary cooling system: The cooling intensity and uniformity of the secondary cooling zone directly affect the formation of internal cracks. High specific water volume will increase the temperature difference between the inside and outside of the ingot and aggravate thermal stress; unreasonable water distribution, such as excessive concentration in a certain area, will cause the ingot to return to temperature after rapid cooling, causing cracks. In addition, poor nozzle atomization and uneven cooling can cause partial overcooling or overheating of the ingot, resulting in thermal stress concentration.
(III) Equipment and operation factors
Crystallizer state: The crystallizer is a key equipment for the formation of ingots. Its uneven cooling, inverted taper deviation or copper plate deformation can cause air gaps between the ingot shell and the crystallizer wall, affecting heat transfer, making the thickness of the ingot shell uneven and prone to internal cracks. For example, poor processing quality of the crystallizer copper tube (dimensional deviation, insufficient finish) can lead to uneven cooling and increase the risk of internal cracks.
Clamping and straightening: If there is a lack of effective clamping after the ingot leaves the crystallizer, it is easy to generate mechanical stress due to bulging deformation; during the bending and straightening process of the arc continuous casting machine, if the straightening temperature is too low (in the brittle zone) or the straightening force is too large, additional stress will be generated inside the ingot, causing internal cracks. Casting machines with single-point straightening are more prone to such problems due to stress concentration.
Improper operation: Insufficient operator skills and untimely parameter adjustment can also lead to internal cracks. For example, insufficient baking temperature in the tundish will cause the molten steel to cool down too quickly, causing uneven crystallization; inadequate protective pouring measures will lead to secondary oxidation of the molten steel, increase the inclusion content, and promote crack formation.
Improvement Measures for Internal Cracking in Steel Billets
To address the causes of internal cracking, comprehensive measures must be implemented across multiple aspects, including molten steel control, process optimization, equipment improvements, and operational standards. Specifically, the following are available:
(I) Optimizing Molten Steel Composition and Purity
Strictly Control Sulfur and Phosphorus Contents: Utilize processes such as hot metal pretreatment desulfurization and the converter duplex method to control [S] below 0.020% and [P] below 0.030%, while ensuring a Mn/S ratio greater than 25 to mitigate the effects of sulfur hot brittleness.
Rationally Adjust Carbon Content: Depending on the steel grade requirements, maintain the carbon content within the lower and middle limits. For example, for Q235 steel, maintain a [C] of 0.15%-0.20% to reduce the phase transformation stress generated by the peritectic reaction.
Enhance the Deoxidation Process: Utilize a silicon-manganese alloy and aluminum composite deoxidation process, maintaining a Mn/Si ratio between 3 and 6 to promote the floating of deoxidation products and reduce inclusions. For example, optimizing the deoxidation process for HPB300 steel increases the particle size of inclusions, making them easier to remove.
(II) Optimizing Continuous Casting Process Parameters
Controlling Molten Steel Superheat: Through measures such as tapping temperature adjustment and tundish temperature compensation, the tundish superheat is maintained at 20-30°C.
Stabilizing Casting Speed: Establish a reasonable casting speed range based on the cross-section and steel grade characteristics of the ingot, and maintain a stable speed to avoid frequent fluctuations.
Improving the Secondary Cooling System: Adopt an appropriate water ratio to avoid rapid cooling and heating. Simultaneously, optimize water distribution, employing a “dense top, sparse bottom” spraying method to ensure a uniform and gradual decrease in the ingot surface temperature, keeping the recovery temperature below 100°C/min. For example, Anshan Iron and Steel effectively reduced internal cracking by replacing three stages in the secondary cooling section with four and adjusting the nozzle layout.
(III) Equipment Improvement and Maintenance
Optimizing Mold Design: Adopting new mold copper tubes ensures precise back taper and uniform water gaps, improving cooling efficiency.
Strengthening Ingot Clamping and Guiding: Installing foot rollers and clamping rollers at the mold outlet reduces ingot bulging. For example, Anshan Iron and Steel installs three clamping rollers after exiting the mold, increasing the ovality qualification rate of round billets to over 95%.
Improving the straightening system: For arc-shaped continuous casting machines, multi-point straightening technology is used to reduce straightening stress and ensure the straightening temperature is above 920°C, avoiding the brittle zone of the steel.
By controlling molten steel composition, optimizing process parameters, improving equipment performance, and standardizing operations, the incidence of internal cracks can be significantly reduced, thereby improving billet quality.
