Quality problem of steel strip

Surface Porosity :
The formation & precipitation of H2 & N2 out of liquid steel during solidification  and the evaporation of moisture & excessive lubricant oil at the miniscus can lead to surface porosity. The resultant gas may be engulfued by the solidifying shell so that open and also closed pores appear at the surface, with depth ranging up to 5.0 mm.

Pinholes :
In contrast pinholes caused by the carbon monoxide precipitation resulting from insufficient deoxidation, may assume depth up to 25 mm. In general, contents of 0.08 Si, 0.01 Al are sufficient to kill the liquid steel completely.

Oxidic inclusion :

These are of two type- endogenous (deoxidation product) & exogenous (caused by refractory material, reoxidation product)
If there is mold level fluctuation or turbulence at meniscus level, slag particles are detached from the casting flux rim & drawn into the molten bath & thus causing surface defect.

 Longitudinal crack :
Longitudinal crack propagation occur due to several reasons
1) due to improper water flow pattern in mould, it causes uneven cooling & thus uneven shell formation, due to which the ferrostatic force acting inside the shell increase at the weak shell & compressive force from outside can't balance it, lead to LC.
2) improper taper in mould
3) heat flux difference
4) casting powder, if it is not providing proper lubrication, then also it can cause LC (Longitudinal stresses occur during withdrawal of the strand shell & if the frictional forces were not reduced, the strand would tear in inside the mold).
5) S & P content (see longitudinal crack during high speed casting..)
6) Mn - S ratio : it should be above 20, it provide hot shortness like FeS.
7) C content, if carbon is on higher side then there is possibility of propagation of Lc as it provide higher strength.
8) high casting speed : at higher casting speed, the strand shell formed is weaker, so it can cause LC as it can't sustain the strain acting on strain shell.
9) if there arise any dent mark in mold, then also strand shell formation is nonuniform & causes LC. 

Transverse Crack :
To reduce friction mold is oscillated along the strand in accordance with a sinusoidal velocity pattern.During the stroke, the strand shell is placed under tension, recovering,& healing & in following compression phase brought about by down-stroke.
So if there is improper negative strip, then it can cause transverse crack.

Star Crack :
Abrasion of cu liners at exit end of the mold can cause star crack, with heavy cu pick up,may extend longitudinally in addition to there transverse orientation.The cause lies in the phenomenon of hot shortness, in which a low melting metal penetrates casting through the grain boundary. in view of high strand shell temp., the penetrating metal remains liquid & prevents the formation of a film inter granular bond.

to be continue......

U may also like : 
Reduction of Slivers DueTo Nonmetallic Inclusions inContinuous Casting 
Erosion of SEN 
Tundish preparation for casting.. 
Tundish Systems 
Mold oscillation

 tramadol hcl


This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 2.5 India License.