Reasons for head / Tail splitting during rolling of cast steel blooms in to bars & Preventive measures

Head and tail splitting during the rolling of cast steel blooms into bars is a common issue that can affect product quality/ Productivity and increased mill down time due to cobble.. The causes of this phenomenon can be traced to various factors related to material properties, rolling conditions, and process control. Here are some key reasons:

1. Non-Uniform Temperature Distribution:

If the bloom is not uniformly heated before rolling, it leads to temperature gradients between the core and the surface or between different sections of the bloom (head, tail, and middle). The hotter and softer parts may deform more easily, causing the metal to split, especially at the head and tail where temperature differences are often more pronounced. This keads to  non-uniform deformation, causing cracks or splits at the head and tail of the rolled bars.

2. Casting Defects in the Blooms:

Blooms that have internal defects from the casting process, such as segregation, porosity, or cracks, are prone to splitting during rolling. Segregation of impurities at the head and tail of the bloom can weaken these areas. These defects become stress concentration points, leading to splits during rolling.

3. Inadequate Shearing of Blooms:

If the bloom head or tail has been improperly cut during cropping or shearing (which happens before rolling), the irregular surface may lead to stress concentrations during the rolling process, resulting in splits.

4. Improper Rolling Speed and Pass Design:

Incorrect rolling speeds or improper pass designs (rolling sequence) can lead to excessive deformation in certain areas of the bloom. For instance, if the head and tail enter the rolling stands at a different speed than the body, it can lead to excessive strain on these sections, resulting in splits.

5. Excessive Reduction or Draft in Initial Passes:

 If the initial passes in the rolling mill apply too much reduction or draft to the bloom, the material at the head and tail may not be able to handle the strain and could split.The head and tail are more susceptible to high strain and may fail under the excessive stress, leading to splitting.

6. Metallurgical Inhomogeneity:

The head and tail sections of cast blooms are often more prone to metallurgical inhomogeneities, such as segregation of alloying elements, impurities, or different grain structures due to uneven cooling during casting. These inhomogeneities create weak points in the material.This weaker head and tail sections may fail under the rolling stresses, leading to splitting or cracking.

7. Residual Stresses from Casting:

Residual stresses are often present in cast blooms due to uneven cooling after solidification. These stresses are typically higher in the head and tail regions of the bloom, When these residual stresses combine with the stresses of rolling, they can lead to head and tail splitting.

8. Surface Defects or Scale on Blooms:

 Blooms may develop surface defects or oxidation scale during reheating or handling. These defects are more pronounced at the head and tail, leading to weak points during rolling.

9. Excessive Strain Rate:

 If the bloom is rolled at a high strain rate without proper control, especially in the head and tail areas, the material can fail due to insufficient ductility or the inability to accommodate such rapid deformation.The material may split at the head and tail because these areas are more vulnerable to high strain rates.

10. Insufficient Descaling:

 If the bloom's surface scale is not properly removed before rolling, it can cause stress concentrations during deformation, particularly at the head and tail where scale may be thicker.

Preventive Measures:

• Uniform Heating: Ensure uniform reheating of blooms before rolling to avoid thermal gradients.
• Proper Cropping: Ensure proper shearing or cropping of the bloom's head and tail.
• Optimized Rolling Parameters: Adjust rolling speeds, reduction ratios, and pass designs to prevent excessive strain.
• Inspection of Cast Blooms: Conduct thorough inspection for casting defects such as cracks, segregation, and porosity before rolling.
• Controlled Cooling: Implement controlled cooling techniques during casting to reduce residual stresses.
• Effective Descaling: Use effective descaling methods to remove surface scale before rolling.

These steps help reduce the likelihood of head and tail splitting and ensure higher quality rolled products.

#SteelRolling #QualityControl #SteelBlooms #RollingDefects #Metallurgy #SteelBars #ProcessOptimization