The impact of defects in billets on rolling

Before we know the defects in billets, we need to understand the production process and the characteristics of steel blanks

1. Characteristics of primary rolling billets

Primary Steel are made from ingots, which are heated equally and rolled. Therefore, the initial Steel has a wide range of specifications and many steel grades. Ingot after compression ratio of 20-30, rolled into the initial billet compression ratio is large, thereby improving the organization. However, rolling cannot eliminate the ingot surface defects, nor can it eliminate internal defects such as segregation and shrinkage. The surface quality and internal quality of the initial billet depends mainly on the original condition of the ingot.
The initial rolling Steel in the production of burn and to cut the head, cut the tail, so the initial rolling Steel metal yield is about 10% lower than the continuous casting billet.

2. Characteristics of continuous casting slab

Continuous Steel casting is made by direct pouring of steel and drawing and straightening. The continuous casting billet process reduces the reheating of the ingot and the cutting of the head and tail after opening the billet, resulting in high metal yield and low energy consumption. In this regard, it has the advantages of primary billet incomparable.
However, due to the continuous casting of small billet quality is not easy to ensure that the small specifications of the continuous casting billet is not easy to produce, some steel (especially boiling steel and some alloy steel) can not be produced. Therefore, high-speed wire rod plant can not all use continuous casting small square billet as the production of wire rod billet.
Foreign in order to expand the continuous casting billet steel, improve the quality of the billet, some manufacturers first cast a large billet of more than 300mm, and then after the continuous rolling mill rolled into a small billet needed for wire rod rolling mill.

Classification of defects in billets

The main defects in billets its source can be divided into two categories: steelmaking and ingot defects and defects caused by the initial rolling open billet rolling Steel.
From the defective parts can be divided into two kinds of surface defects in billets and internal defects in billets.

Surface defects in billets : scars, cracks, cracks, cracks, teeth, folding, scratches, indentations, dents, corner dissatisfaction, bending (see curvature), blow-ups, shear defects, poor cleaning, splitting, size overruns, etc..

Surface defects in billets

Cracks: Smelting components, heating during opening, rolling may cause cracks in the billet, mostly seen in bearing steel, spring steel and other high carbon steel initial rolling billet, continuous casting billet can also see such defects. Cracks are magnified into fissures in the rolling of open billets, and folding and cracking defects are formed when rolling to finished products.
Blue crack: billet part of the repair and grinding too much lead to local overheating, organizational changes, failure due to stress caused by the billet surface cracking, mostly seen in bearing steel. Generally required to remove the blue cracked part.
Pinhole: Pinhole-sized pores appear on the surface of the billet, mostly seen in continuous casting billets. After re-rolling in the rough rolling will appear after the pulling crack-like surface found defects.
Scarring: metal or non-metal scars on the surface of the billet not welded to the substrate, to the finished surface will form large iron scars.
Repairing craters: deeper defects do not match the height to width ratio after repairing, and are easy to form folds after rolling.
Repair angles: defects caused by billet repair grinding, easy to cause folding after rolling
Off-square: billet part of the regrinding is too heavy to cause off-square, the rolling process is easy to occur backwards steel and folding.

Internal defects in billets

internal defects in billets： inclusions (see non-metallic inclusions), shrinkage residue (see shrinkage), air pockets, air bubbles, air holes, internal cracks, tail holes

Air pockets. A defect similar to a shrinkage in the shear section of boiling Steels. The surface of the billet with this defect is prone to bulging, tearing, and large shear width spreading, etc. The rolling is prone to heavy skin scars, small cracks, and other problems.
Internal cracking. Cracks inside the billet. Steel with this defect are prone to “bursting” in the rolling process, which can easily cause steel piles.
Tail hole. The Steel’s fish tail is not cut net called tail hole, also known as mechanical shrinkage. The end of the billet in the rolling process is easy to split open resulting in the inability to pass the guide pile steel.
Inclusions: non-metallic inclusions mixed in the smelting process, which disrupts the continuity of the metal matrix. Commonly found in stainless steel continuous casting billets. Inclusions and the plasticity of the matrix is different after the initial rolling under large pressure rolling is easily exposed, visible at the rough rolling obvious triangular-shaped cracks, the surface of the finished product visible inclusions. But because the inclusions from the surface of more than a certain distance, so the inspection is more difficult
Internal porosity: mostly seen in continuous casting billet, the surface of the Steel is not seen, after rolling again, closer to the surface, there will be pulling cracks, deeper, may be able to roll into finished products, but in the subsequent drawing, stamping will continue to crack.
Bias is an important quality problem of continuous casting billet, the smaller the continuous casting Steel the more serious the bias.The carbon content of 0.75% ~ 0.83% is greater than 2 * 10-6 degree of segregation can cause by longitudinal cracking

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