Formation Mechanism and Control of Ultrasonic Testing Defects in Large Forged Cr5 Steel Rolls  

Introduction  

Cr5 series forged steel rolls serve as critical components in modern rolling mills, prized for their:  

•   High surface hardness   (typically 5565 HRC). 
•   Exceptional wear resistance. 
•   Deep hardened layers   (50100mm).  
•   Superior core toughness.    

These properties make them indispensable in steel and aluminum rolling industries, where roll quality directly impacts:  

• Production efficiency.  
• Product dimensional accuracy.
• Surface finish quality.  

Key Quality Challenges  

Modern highspeed rolling mills impose stringent manufacturing standards:  

Common defect types include:  

1.   Cracks   (thermal fatigue or forginginduced).  

2.   Gas pores   (from improper degassing).

3.   Shrinkage cavities   (solidificationrelated).  

4.   Macroinclusions   (most prevalent quality issue).  

Metallurgical Root Causes  

Inclusion Formation Mechanisms  

Primary sources of macroinclusions:    

Al₂O₃ clusters   from deoxidation.  

MgO·Al₂O₃ spinels   (melting point >2000°C).  

Reoxidation products during casting. 

Critical control points:    

mermaid.

graph TD.

    A[Electric Arc Furnace] > B[LF Refining]

    B > C[VD/RH Degassing]

    C > D[Casting]

    D > E[Forging]

Process Optimization Strategies  

1.   Alloying sequence optimization    

    Complete Al addition before 15min of LF refining . 

    Final Ca treatment at RH degassing stage.  

2.   Slag composition control    

    Maintain basicity (CaO/SiO₂) at 2.53.5.  

    Al₂O₃ content <20% to avoid viscous slag . 

3.  Magnesium treatment effects    

    Reduces inclusion size to 03μm range.

    Lowers inclusion aspect ratio by 4060%.  

 Industrial Implementation  

 Proven solutions from steel plants:    

   Prevention of nozzle clogging:    

   Maintain [Mg] >5ppm in liquid steel.  

   Use submerged entry nozzles with ZrO lining.  

UT defect reduction case study:    

Conclusion  

Effective control of Cr5 roll defects requires:  

1.   Metallurgical process synchronization from EAF to forging.  

2.   Inclusion morphology control through Mg/Ca treatment. 

3.   Strict reoxidation prevention during casting.  

These measures can reduce ultrasonic testing rejections by   6075%  , significantly improving production efficiency and product quality in rolling applications.  

Further reading suggestions:

 [Advanced inclusion analysis techniques].  

 [Case studies on roll failure analysis]. 

 [Latest innovations in steel cleanliness control].  

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