Surface Quality of Continuous Casting Billets

1. Surface cracks
1) Surface longitudinal cracks
Longitudinal cracks on the surface of continuous casting slabs will affect the quality of rolled products. For example, a longitudinal crack with a length of 300 mm and a depth of 2.5 mm leaves a layered defect of 1125 mm on the rolled sheet. When the longitudinal crack is serious, it will cause leakage and scrap.
The research points out that the longitudinal cracks originate from the uneven thickness of the primary shell of the mold. The tensile stress acting on the billet shell exceeds the allowable strength of the steel, resulting in stress concentration at the weak point of the billet shell, which leads to fracture, and expands in the secondary cooling zone after exiting the mold.

The causes of longitudinal cracks are:
A. The nozzle and the crystallizer are not aligned, resulting in a bias current to scour the solidification shell;
B. Poor melting performance of mold slag, too thick or too thin liquid slag layer lead to uneven thickness of slag film, making the local solidification shell too thin. The liquid slag layer is less than 10mm, and the longitudinal cracks increase significantly;
C. The liquid level of the crystallizer fluctuates. Liquid level fluctuation>10mm, the probability of longitudinal cracking is 30%;
D. S+P content in steel. When S>0.02% and P>0.017% in steel, the high temperature strength and plasticity of the steel are obviously reduced, and the longitudinal crack tends to increase;
E. When C in steel is 0.12-0.17%, the tendency of longitudinal cracking increases.
Preventive measures include:
A. The nozzle and the crystallizer should be aligned;
B. The crystallizer liquid level fluctuation is stable at ±10mm;
C. Appropriate immersion nozzle insertion depth;
D. Appropriate mold taper;
E. The arc between the mold and the upper part of the secondary cooling zone should be accurate;
F. Appropriate mold powder performance;
G. Adopt hot top crystallizer;
H. Determine the reasonable pouring temperature and drawing speed according to the type of steel to be poured;
I. The chemical composition of the steel should be controlled within an appropriate range.

2) Surface transverse cracks
Transverse cracks are located at the trough of the vibration marks on the inner arc surface of the slab, and are usually hidden and invisible.

The reasons are:
A. Too deep vibration marks are the origin of transverse cracks;
B. The content of Al and Nb in the steel increases, which promotes the precipitation of particles (AlN) at the grain boundaries and induces transverse cracks;
C. The billet is straightened at the brittle temperature of 900-700℃;
D. Secondary cooling is too strong.
Preventive measures include:
A. The crystallizer adopts high frequency (200-400 times/min) and small amplitude (2-4mm) vibration;
B. The secondary cooling zone adopts stable weak cooling, so that the surface temperature of the billet during straightening is increased by 900 °C;
C. The liquid level of the crystallizer is stable, and the mold slag with good lubricating performance and low viscosity is used;
D. Clean surface cracks with flame.

3) Star cracks (reticular cracks)
Star-shaped cracks generally occur in small cracks between grains, in the shape of stars or nets. It is usually hidden under the iron oxide scale and difficult to find, and only appears on the surface of the slab after pickling or shot peening.

The reasons are:
A. The surface of the high temperature casting billet absorbs the copper of the mold, and the copper becomes liquid and then penetrates along the austenite grain boundary;
B. Selective oxidation of iron on the surface of the slab, so that residual elements (such as Cu, Sn, etc.) in the steel remain on the surface and penetrate along the grain boundaries to form cracks.
Prevention methods are:
A. The surface of the mold is plated with Cr or Ni to increase the hardness;
B. Appropriate amount of secondary cooling water;
C. Control residual elements in steel such as Cu<0.2%;
D. Control Mn/S>40;
E. Control the Al content in the steel.

2. Slag on the surface
Refers to the 2-10mm inlaid large pieces of slag under the surface of the billet.
Slag inclusions on the surface of the billet, if not removed in time, will often cause surface defects in the finished product. In addition, the thermal conductivity of the slag is poor, and the solidified shell at the slag inclusion is thin, which will cause a leakage accident.
Preventive measures:
(1) Minimize the fluctuation of the liquid level of the crystallizer;
(2) The insertion depth of the immersion nozzle should be controlled at the position of (125±25) mm;
(3) The inclination angle of the outlet hole of the submerged nozzle should be selected properly, and the principle of the outlet stream will not stir the meniscus slag layer;
(4) Choose mold powder with good performance.

3. Subcutaneous air bubbles and pores
Below the slab skin, the diameter is about 1mm and the length is about 10mm. The bubbles distributed along the growth direction of the columnar crystals are called subcutaneous bubbles. These bubbles are called surface bubbles if they are exposed outside. A hole that is not exposed is called a subcutaneous bubble, and a hole that is smaller and denser than the bubble is called a subcutaneous pinhole. In the heating furnace, the surface of the subcutaneous bubbles of the billet is oxidized, the rolling process cannot be welded, and the product forms cracks.

Reason: high oxygen and hydrogen content in molten steel.
Preventive measures:
(1) Strengthen deoxidation;
(2) All materials entering the furnace, all refractory materials in direct contact with molten steel, such as ladle, tundish, mold slag, covering agent, etc., must be dried to reduce the source of hydrogen;
(3) The whole process of protection pouring is adopted;
(4) Use appropriate refining methods to reduce the gas content in the steel.

4. Double pour
When the molten steel casting is interrupted, a condensed shell is formed at the meniscus, and it is not easy to melt with the re-cast molten steel.

Reason: The molten steel casting is interrupted due to the blockage of the nozzle and the replacement of the submerged nozzle.
Preventive measures: reduce the interruption time of steel flow.

5. Heavy Leather
On the surface of the slab, there is transverse discontinuity, and there are obvious traces of incomplete welding, which is called heavy skin.

reason:
(1) The injection flow of the mold is suddenly stopped, or the billet drawing is stopped instantaneously;
(2) The molten steel is too viscous, the temperature is too low, the nozzle is blocked, and the injection flow is deviated, etc., which may cause heavy skin.