The process is known as Slit Rolling had its origins in the methods used in the United States to wrap railway lines for rolling into reinforcement bars. The process permits two or more plain round or deformed bars to be rolled in parallel from a single billet on rolling mills whether cross country or continuous, originally designed for single strand operation.

The Process

By arranging the actual pass design to create axial separating forces within the section being rolled the splitting of the two parts is assured and the special slitting or separating device is only involved with handling the head end. On high production mills with capacities up to 1,000 tonnes per shift, any attempts to use a knife to separate the parts would result in excessive wear. The pass design avoids this problem and guide life can be harmonised with shift or size change programmes.

Comparison of Slit Rolling and Conventional Rolling

Reasons for using the Process

The advantages of the Slit Rolling Process depend on local conditions but can be outlined as follows.

On existing rolling mills the full capacity of reheating furnaces, roughing and intermediate trains may not be available on smaller sizes e.g. 8mm through 12mm because of speed limitations imposed by the cooling bed. By introducing the slit rolling process for the smaller sizes, production rates similar to those of the larger sizes can be achieved at reasonable speeds, e.g. two slit 10mm bars have almost the same cross sectional area as a single 14mm bar. A further advantage achieved in this type of application is an increase in mill entry speed when rolling small slit sizes, providing a better temperature profile, better section control and improved roll life.

In certain installations the increase in capacity on the smallest size has been in excess of 50% and depending on product mix this can have considerable effect on overall production.

A second possibility for an existing rolling mill would be an extension of product range into smaller sizes where capital cost or space inhibits the addition of further rolling stands.

The final opportunity for an existing rolling mill is where there is a requirement to increase the billet size. This may be due to a rationalisation of billets between several mills, the introduction of a new continuous caster to replace ingot route cogged billets, or merely a requirement to improve yield. In a similar way to the extension of product range referred to in the previous case, the Slit Rolling Process will allow the increase in billet size to be achieved without any increase in rolling stands.

It is normally possible in all cases outlined to reuse the majority of the existing pass sequence to provide the necessary square or fluted square leader for the process. In this way roll stocks remain unchanged and the cost of implementing the slit rolling is kept low.

Where new mills are proposed, the incorporation of the Slit Rolling Process for certain products will enable fewer stands to be supplied and the capital cost to be reduced.

The lower finishing speed required for two bars rolled simultaneously may also permit a shorter run into the cooling bed and a reduction in building size.

General

Since this process was developed in the early 1970's, the special rolling method and associated equipment has been used on many mills. During the eighties, installations were adapted to combine slitting with a process for the production of quenched and tempered high yield reinforcing bars. Despite the obvious fact that any central segregation or porosity in the cast billet will end up close to the surface of the slit product, operators of the slit rolling process have had no difficulty meeting major national and international testing standards.

It is not normally an advantage nor has the process been applied to the production of wire rod in wit. Two complete cooling and laying lines would still be required whereas a bar mill application uses the same cooling bed and run in to handle the two, three or four slit bars together.

Reinforcing bar whether plain or deformed is the major product from the process and it is difficult to imagine an extension into high quality or engineering steels, because of possible segregation problems arising during subsequent processing.

However an extension into simple structural flats and angles has already taken place and more trials are proposed in this field.

Equipment

The heart of the process is the special pass design, but certain guides in the area of the forming and separating are also crucial to the correct operation. If the leader square and subsequent dog bone shape are not presented to their respective roll bites in the correct attitude then unbalance will occur between the two strands and a cobble will result.

A certain degree of unbalance can be corrected by the lateral adjustment built into the entry guide supports, but this is only designed for normal set up or normal wear.

Most installations use a manual adjustment of the guides based on information derived from the loops formed after the slitting.

Only one loop is used to actually control rolling speed by means of an infrared scanner. The other loop is screened from the scanner, or a scanner, which detects the larger loop, is used.

As mentioned earlier the separating guide does not use knives, but rather rotating wheels do assist in dividing the two parts of the stock.

Summary

In certain conditions this process offers an alternative low cost route for an increased product range, improved production rates on smaller sizes or use of a larger billet.