Crocodile cracking is a phenomenon that occurs in hot-rolled steel during the rolling process, particularly in the head of billets. It is characterized by a series of interconnected cracks that run parallel to the rolling direction and along the edges of the steel.

Root Causes:

1. Residual stresses: Residual stresses are stresses that remain in the steel after it has been deformed and then relaxed. These stresses can cause cracking in the steel, especially when it is subjected to high stresses or strains.
2. MnS inclusions: MnS is a type of carbide phase that forms in steel during the rolling process. These inclusions can be the root cause of crocodile cracking because they can produce residual stresses that cause the steel to crack.
3. Microstructure: The microstructure of the steel can also contribute to crocodile cracking. For example, a steel with a high proportion of MnS inclusions or a Cold Worked microstructure may be more susceptible to crocodile cracking.

Metallurgy Causes:

1. Carbon content: The carbon content of the steel can affect the susceptibility to crocodile cracking. High-carbon steels are more susceptible to crocodile cracking than low-carbon steels because the carbon content can lead to the formation of MnS inclusions.
2. Alloying elements: The presence of certain alloying elements, such as silicon, molybdenum, and tungsten, can also affect the susceptibility to crocodile cracking. These elements can increase the formation of MnS inclusions, which can contribute to the phenomenon.
3. Rolling procedures: The rolling procedures used during hot rolling can also affect the susceptibility to crocodile cracking. For example, the rolling speed, temperature, and pressure can all impact the formation of MnS inclusions and the resulting cracking.

Rolling Causes:

1. Rolling speed: The rolling speed can affect the formation of MnS inclusions in the steel. Higher rolling speeds can lead to the formation of larger MnS inclusions, which can increase the susceptibility to crocodile cracking.
2. Temperature: The temperature during the rolling process can also impact the formation of MnS inclusions and the resulting cracking. Higher temperatures can lead to the formation of larger MnS inclusions, which can increase the susceptibility to crocodile cracking.
3. Pressure: The rolling pressure can also affect the susceptibility to crocodile cracking. Higher rolling pressures can lead to the formation of MnS inclusions, which can increase the susceptibility to cracking.

In summary, crocodile cracking in hot-rolled steel is a complex phenomenon that can be caused by a combination of factors, including residual stresses, MnS inclusions, microstructure, metallurgy, and rolling procedures. Understanding these factors is essential for avoiding or mitigating crocodile cracking and optimizing the rolling process for hot-rolled steel.

Crocodile cracking, also known as alligator cracking due to its resemblance to the scales on a reptile's back, is a type of defect that occurs in the head of a steel billet during the hot rolling process. This phenomenon is often seen in the manufacturing of round bars and can be caused by a variety of factors. Understanding these causes requires knowledge in both metallurgy and the rolling process.

Metallurgical Causes:

• Segregation: If there are non-uniform distributions of alloying elements or impurities within the billet, it can lead to areas with varying melting points and properties, which under thermal and mechanical stress can cause cracks.

• Inclusion Content: High inclusion content, particularly from deoxidation products such as alumina or silica from the steelmaking process, can lead to stress concentrations within the billet that crack under the strain of rolling.

• Microstructural Weaknesses: The presence of certain phases or microstructural weaknesses due to improper heating or cooling before rolling can lead to cracks initiating and propagating during deformation.

• Precipitation of Brittle Phases: At high temperatures, certain elements may form brittle intermetallic compounds that can crack under the pressure of rolling.

• Non-metallic Inclusions and Segregations: These can act as stress concentrators and crack initiation sites.

Rolling Process Causes:

• Rolling Temperature: If the steel is rolled at an improper temperature that is too low, it will not have enough ductility to withstand the deformation, leading to cracking.

• Rolling Speed and Reduction Ratios: Both too high or too low rolling speeds, as well as improper reduction ratios, can induce tensile stresses that exceed the ductility limit of the material at certain temperatures, leading to cracks.

• Rolling Force and Roll Gap: An excessive rolling force or incorrect roll gap can impose a strain on the material that's higher than what it can sustain, especially if the billet has internal weaknesses.

• Misalignment of Rolls: Misalignment can lead to uneven deformation, creating tensile stresses on the bar surface that can promote the initiation and propagation of cracks.

• Insufficient Descale: Prior to rolling, scale (the layer of oxides formed on the surface of hot steel) needs to be removed. If not removed correctly, the scale can be rolled into the surface, causing weaknesses where cracks can initiate.

Combined Factors:

The phenomena of crocodile cracking can be a complex interaction between material properties and the mechanical stresses imposed during rolling:

• Impacts of Billet Shaping: Prior operations such as the continuous casting process can introduce internal defects that become the source of cracks when the billet is shaped under rolling stresses.

• Thermal-Mechanical Conditions: Hot rolling combines high temperatures with severe mechanical deformation. Any deviation from the optimal range of processing parameters for the specific steel composition can create conditions ripe for cracking.

Preventative Measures:

To prevent crocodile cracking, it is crucial to control the chemical composition, ensure proper homogenization of the billet, maintain a suitable rolling temperature and speed, provide sufficient reduction per pass, and ensure good alignment and condition of the mill rolls. Additionally, monitoring the descaling process and controlling the Mn/S ratio, the degree of superheat, and the casting speed are important steps as suggested by the research mentioned.

In industrial practice, a combination of these factors is typically at play and the exact cause could be ascertained through a detailed analysis of the rolling process parameters, the microstructure of the material, and the defects observed post-process.