Slit rolling for bar production


Slit rolling for bar production

In merchant bar mills with conventional rolling, rounds and rebars (both of them commonly known as bars) are produced from billets which are rolled in a sequence of passes through a multi stand rolling mill. Each of the stands is having a set of two rolls, grooved on the circumference to give together a desired pass shape. Cross section of the billet is gradually reduced and shaped in a sequence of passes till the final shape and cross sectional size is obtained.

While rolling bars in a merchant bar mill, the productivity of smaller diameter bars is generally lower than the bigger diameter bars. To improve such productivity, one of the most significant developments in the area of bars production in recent years is the slit rolling. This method of rolling bars is also called “Multi Slit Rolling (MSR)”. The process of slit rolling is to roll two or more bars simultaneously from a single billet. When compared with the conventional single strand continuous rolling, MSR has reduced number of passes.

The multi slit rolling technology use a combination of special roll pass design and designated guide equipment to prepare, shape and longitudinally separate the incoming billet into two or more individual strands for further rolling into the finished size. The bar is slit by rolls without any other special slitting devices. The numbers of slits can be selected by the appropriate design of the roll grooves. A bar can be separately split into two, three or four pieces in one pass. By selecting the number of slits according to the product size, the same productivity of all the products can be achieved in the bar mill. The slitting is done by applying axial force which is done by using special pass design.

Historical development

The production of multiple sections from a single metal billet dates back to 1910 when a method was developed to produce wires from sheets of soft metals. This early method gave a general concept of longitudinally dividing a metal piece into individual section. This method was not suitable for steel because of various problems which included high hardness of the steel and high rolling temperature. Another method of rolling metal sections in multiple of two or more units, all formed at the same time from a billet, was developed in 1934. In this method the sections were separated into individual units in final pass by means of rolls arranged to vertically shear and thereby cut the thin web of metal joining the section. This method has many problems which included different tensional stresses between inner and outer sections, high wear of costly cutting rolls and problems related to the speed differentials etc. During 1980 one more method was proposed. This proposed method was for slitting of billets which are rolled to produce only a double and triple stranded bars. This method was patented but was limited to three strands.

Process of slitting

The following are the steps in the process of slit rolling.

  • Reducing the billet conventionally through the roughing and intermediate rolling mill to produce an acceptable section for the first special shaping pass at the forming stand.
  • Precise guidance of this stock to the forming stand where it is reduced and shaped to form a symmetrical “forming section”.
  • Further close guidance and control of the “dog bone (DB)“ through the separating stand, were the stock is reduced and shaped into a “slit pass (SP)”, designed to be easily separated into two equal sections of rough shaped round.
  • A special guide on the delivery side of the separating stand ensures a clean slitting of the bar and now delivers multiple strands of equal sections to their respective finishing lines.

In the multi slit rolling process, the roll pass design needs special attention. Roll pass design is based on the application of special shaping passes (cutting in passes or slitting passes) in the final stage of rolling process. Rolling in the slitting passes is of great significance. In these passes the rolling stock is subjected to deformations considerably differing from those occurring in conventional stretching or shaping passes. The most important issue during the pass design with the multi slit rolling is the shape determination of the grooves called “dog bone” and “slit pass” (Fig 1). The remaining passes, before and after slitting, are most often standard stretching passes typically used for round rolling in merchant mills. The construction of the first shaping pass (dog bone pass) is characterized by the symmetrical “knives” of quite large height. The essence of rolling in this pass is to provide precise dividing of square into two equal parts. Precise inserting of a bar into the pass and holding it in a right position is achieved with application of rolling guide system. It is not possible to correct any irregularities such as asymmetrical slitting in subsequent rolling passes and it leads to rejects.

Dog bone and slit process

Fig 1 Dog bone and slit passes

The slit pass is the final pass dividing the bar into two strands. In the axis of the pass very high and narrow knives are situated and minimum distance between them is set. The material leaving the rolls should consist of two almost perfectly equal parts connected to each other with a narrow and thin web of a maximum thickness of 1 mm. These strands are separated from each other and are further rolled into finished sizes.

In case of MSR the leader sections (square, rectangular or round) before the dog bone and slit passes have to be of very close tolerance. This is achieved by the following.

  • Accurate pass profile in the rolls
  • Accurate roll gap to ensure no overfill
  • Accurate pass alignment between top and bottom rolls
  • Sound roll neck bearings and positive roll location to avoid axial flow
  • Rigid rolling mill stands providing rigid screw down and roll balance
  • Sound roll quality and good pass conditions
  • Uniform temperature of rolling stock

Further the guiding of the leader section to the forming pass and on to the separating pass in the separating stand is critical in order to ensure equally balanced strands. The following features are desired.

  • Sound and robust guides
  • Secure and rigid rest bars
  • Precise guide equipment
  • Positive, adjustable and secure alignment
  • Guiding of the bars close to the stand
  • Correctly set up and maintained guides

Typical roll pass deign with application of dog bone (DB) and slitting (SP) passes for various profile dimensions is given in Fig 2

Typical roll pass design with slit passes

Fig 2 Typical roll pass design with dog bone and slit passes

Advantage of slit rolling

The slit rolling has the following advantages over conventional rolling in a merchant bar mill.

  1. The number of rolling stands get reduced
  2. Reduction in the capital cost
  3. There is reduction in specific power consumption, specific fuel consumption and specific roll consumption.
  4. There is increased in rolling yield
  5. The reduction in production cost
  6. There is Increase in the productivity level of the mill (Fig 3). With the same output rolling speed the productivity is increased linearly with increase in the number of strands

Productivity comparison with MSR

Fig 3 Higher productivity compared to conventional rolling