Basic knowledge of continuous casting and 11 issues for improving the quality of continuous casting slab

1. What are the causes of longitudinal cracks on the surface of continuous casting slabs and their prevention methods?

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 300mm and a depth of 2.5mm leaves a 1125mm layered defect on the rolled plate. Severe longitudinal cracks will cause leaks and rejects.

The study pointed out that the longitudinal crack originated from the unevenness of the thickness of the primary blank shell on the meniscus of the mold. The tensile stress acting on the billet shell exceeds the allowable strength of the steel, and stress concentration occurs 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 fissure can be summarized as follows:

1) Misalignment of the nozzle and the crystallizer causes a bias current to scour the solidified shell.

2) Poor melting performance of mold powder, and excessively thick or thin liquid slag layer causes uneven slag film thickness, which makes the local solidified shell too thin. If the liquid slag layer is less than 10mm, the longitudinal cracks increase obviously.

3) The liquid level of the crystallizer fluctuates. If the liquid level fluctuates more than 10mm, the probability of longitudinal fissure is 30%.

4) S+P content in steel. In steel, S>0.02%, P>0.017%, the high-temperature strength and plasticity of steel are obviously reduced, and longitudinal cracks tend to increase. 5) If C in steel is 0.12～0.17%, the tendency of longitudinal cracking will increase.

The measures to prevent the occurrence of longitudinal fissures are:

1) The nozzle should be aligned with the crystallizer.

2) The liquid level fluctuation of the crystallizer is stable at ±10mm.

3) Proper immersion nozzle insertion depth.

4) Appropriate crystallizer taper.

5) The arc alignment between the mold and the upper part of the secondary cooling zone must be accurate.

6) Appropriate mold flux performance.

7) The use of hot-top crystallizer, that is, inserting stainless steel and other materials with poor thermal conductivity in the 75mm copper plate in the meniscus area, which reduces the heat flow in the meniscus area by 50-70%, delays the shrinkage of the blank shell and reduces the depression Reduce the chance of occurrence of longitudinal fissure.

2. What are the causes of transverse cracks on the surface of continuous casting slabs and their prevention methods?

The transverse cracks are located at the troughs of the vibration marks on the inner arc surface of the cast slab, which are usually hidden and invisible. After pickling inspection, it was pointed out that the crack depth could reach 7mm and the width 0.2mm. The crack is located in the ferrite network zone, and the network zone happens to be the primary austenite grain boundary. And there are fine particles (such as AlN) precipitation on the grain boundaries. Especially C-Mn-Nb(V) steel is more sensitive to cracks.

Causes of transverse cracks:

1) Too deep vibration marks are the origin of transverse cracks.

2) The content of A1 and Nb in steel increases, which promotes the precipitation of particles (A1N) at the grain boundaries and induces transverse cracks.

3) The cast slab is straightened at a brittle temperature of 900～700℃.

4) The secondary cooling is too strong.

Measures to prevent the occurrence of transverse cracks:

1) The mold adopts high frequency (200～400 times/min) and small vibration amplitude (2～4mm) is an effective way to reduce the depth of vibration marks.

2) The secondary cooling zone adopts stable and weak cooling, so that the surface temperature of the cast slab during straightening is greater than 900°C.

3) The liquid level of the crystallizer is stable, and the mold powder with good lubricating performance and low viscosity is used.

4) Use flame to clean up surface cracks.

3. What are the causes of net-like cracks on the surface of continuous casting slabs and their prevention methods?

This kind of crack can only be found after pickling the surface of the cast slab, and the depth can reach 5mm.

Causes:

(1) The surface of the high-temperature cast slab absorbs the copper of the mold, and the copper becomes liquid and then penetrates along the austenite grain boundary.

(2) The selective oxidation of iron on the surface of the cast slab makes the residual elements (such as Cu, Sn, etc.) in the steel remain on the surface and penetrate along the grain boundary to form cracks.

Studies have shown that the crack zone is enriched with elements such as Cu, Sn, Sb, and the Cu content in steel is greater than 0.1%, the crack is aggravated; the Al content in the steel is increased, and the network crack is aggravated.

Preventive measures:

1) The surface of the mold is plated with Cr or Ni to increase the hardness.

2) Appropriate amount of secondary cooling water.

3) Control residual elements in steel such as Cu<0.2%.

4) Control Mn/S>40.

4. What are the causes and preventive measures for the formation of longitudinal cracks at the corners of continuous casting slabs?

Longitudinal cracks at the corners may be located near the edge of the border between the wide surface and the narrow surface. Some are 10 to 15 mm away from the edge, and some are just on the edge. In severe cases, it will cause steel breakout.

Reasons for formation: For a square shape, the thickness of the water gaps along the height of the mold may be uneven, causing poor cooling of the corners of the mold; the mold taper is too small, and the mold fillet radius is too small.

For slabs, it may be due to

(1) Improper support on the narrow surface causes bulging on the narrow surface. There is a 6-12mm bulge on the narrow surface, accompanied by longitudinal corner cracks, resulting in a steel breakout.

(2) The taper is inappropriate.

(3) Insufficient cooling water on the narrow surface.

ways to improve:

For billet

1) Control the geometry of the mold well to prevent deformation.

2) Appropriate fillet radius.

3) When assembling the crystallizer, keep the thickness of the cooling water gap consistent to make the cooling uniform. For the slab 1) Adjust the gap between the narrow-faced foot rollers so that it is l～2mm inward to limit the bulge. 2) Appropriate taper (1.0%/m). 3) Appropriate amount of cooling water.

4) Do not offset the flow between the nozzle and the crystallizer.

5. What are the causes and preventive measures for the formation of transverse cracks at the corners of continuous casting slabs?

This is a small transverse crack located at the corner of the cast slab.

The reasons may be:

1) The taper of the crystallizer is too large.

2) The surface of the mold is scratched.

3) Inaccurate arc alignment between mold outlet and zero section.

ways to improve:

Adjust the crystallizer taper, strictly align the arc, and adjust the secondary cooling so that the slab corner temperature cannot be less than 800°C during straightening.

6.How are the subcutaneous bubbles of the continuous casting billet formed?

Below the surface of the cast slab, there are large bubbles growing toward the columnar crystals with a diameter and a length of 1 mm and 10 mm or more. For example, these bubbles are called surface bubbles that are exposed, subcutaneous bubbles that are not exposed, and small holes that are smaller and denser than bubbles are called hypodermic pinholes.

In the heating furnace, the surface bubbles of the cast slab or the inner surface of the subcutaneous bubbles are oxidized to form a decarburized layer, which cannot be welded after rolling and forms surface defects. Shallow buried bubbles can be removed by grinding wheels, wind shovel and flame cleaning. Deep-buried bubbles are difficult to find, which can cause cracks in the product.

Insufficient deoxidation of molten steel is the main reason for the generation of bubbles. If enhanced deoxidation is used to reduce the oxygen content in the steel, the aluminum content in the molten steel will reach 0.01% to 0.015%, thereby eliminating the bubbles. In addition, the gas content (especially hydrogen) in molten steel is also an important reason for the formation of bubbles. Therefore, all materials added to the molten steel (such as ferroalloys, slag powder, etc.) should be dried, ladle and tundish should be baked, the amount of lubricating oil should be appropriate, and the use of protective pouring for injection has a significant effect on reducing bubbles.

7. What is the surface folding defect of continuous casting slab?

There are horizontal folding marks on the surface of the cast slab, which may be accompanied by horizontal cracks in severe cases. Reasons for formation:

(1) The suspension in the mold causes the solidification shell to tear. Due to the strong cooling of the mold, the molten steel leaking at the tearing site immediately solidifies on the surface to form fold marks;

(2) Improper adjustment of mold vibration parameters;

(3) Poor arc alignment between the mold outlet and the secondary cooling section;

(4) The mold is poorly lubricated, and the billet shell adheres to the copper wall.

8. What is the cause of the "cold mole" on the surface of the cast slab?

The metal lump or slag embedded under the surface of the cast slab is called "cold mole". The reasons are:

(1) The cold steel that is stuck to the surface of the mold by the splash of steel flow during open pouring is embedded in the solidified shell;

(2) The liquid level of the mold fluctuates too much, and the insoluble matter in the slag is drawn into the solidified shell.

9. What is the heavy skin defect on the surface of the continuous casting slab?

There are lateral discontinuities on the surface of the cast slab, and there are obvious traces of incomplete welding called heavy skin. cause:

(1) The injection of the mold suddenly stops pouring, or the drawing stops instantaneously. If the pouring stop time is too long, an obvious reconnection will be formed on the surface of the cast slab;

(2) The molten steel is too viscous, the temperature is too low, the nozzle is blocked, the injection deviation, etc., may cause heavy skin.

10.Why is the surface of the continuous casting slab sometimes concave?

This defect is common on the narrow side of the billet or slab. Reasons for formation:

(1) The cone of the mold is too large;

(2) Uneven cooling in the secondary cooling zone. The use of suitable crystallizer taper and uniform secondary cooling can be prevented.

11.Why is the surface of the continuous casting slab sometimes convex?

This defect is common in rectangular blanks and narrow faces of slabs. The reason is that the foot roller or the supporting roller of the mold is poorly arced and the cooling is too weak, which causes the solid shell to swell.