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Split Ends and Cracking Problem During Hot Rolling of Continuously Cast Steel Billets

Technical Article---Peer-Reviewed

Abstract

The paper presents a detailed analysis of split end and cracking problems that occurred during hot rolling of thermomechanically treated (TMT) grade steel bars. The major analytical tool was a multiscan computer-controlled ultrasonic image analysis system for analysis of the defects observed on transverse cut slices of the billets under investigation. The possible reasons for billet cracking during hot rolling such as a low Mn/S ratio, high casting speed, high degree of superheat, and high inclusion content in steel were analyzed. Additionally in the process parameters such as low roll diameter, high friction, and low reduction ratio were also analyzed. The cracking appeared to be associated with large surface/internal defects, axial porosity, off-centered cavities/porosities, off-centered cracks, and inclusion bands. The analysis results suggested methods to prevent such defects in continuously cast steel billets and to reduce/eliminate split end problems. The casting parameters should include: (a) Mn/S ratio >35, (b) degree of superheat <60 °C, and (c) casting speed: <3.0 m/min, where roll radius and initial thickness of the workpiece should be optimized.

Keywords

Split end Continuous casting of steel Thermomechanically treated (TMT) steel bars 

Nomenclature

hf, ho

Strip thickness

J*

Total power per unit width

j*

Total relative power

\( j_{\Upgamma }^{ * } \)

Relative shear power

\( k \)

Yield stress in shear

\( \bar{k} \)

Mean value of yield stress

ld

Length of the projection of the arc of contact

\( m \)

Constant friction factor

Ra

Distance (see Fig. 1)

Ro

Roll radius

\( R_{\Upgamma } \)

Discontinuity surface radius

{v}

Velocity vector

vf, vo

Velocity of workpiece

vr

Roll surface velocity

Xa, Ya

x, y coordinate of point A

Ya

Yield stress

αc

Contact angle

ε

Relative reduction

εi

Effective strain rate

εv

Volumetric strain rate

σv

Vertical component of stress

ξb, ξf

Back and front relative stress

\( \Upphi_{1}, \Upphi_{2} \)

Angles, (see Fig. 1)

ω, ωo

Zone II and roll rotational velocity

Subscripts

i

Serial, i = 1, 2, 3

opt

Optimal

se, sf

Split ends, sound flow

Notes

Acknowledgment

The authors are thankful to the management of Tata Steel for giving permission to publish this work.

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Copyright information

© ASM International 2008

Authors and Affiliations

  1. 1.R&D and SS Division, Tata Steel Ltd.Jamshedpur India
  2. 2.Merchant Mill, Tata Steel Ltd.Jamshedpur India

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