Slit Rolling for Bar Production
Slit Rolling for Bar Production
In merchant bar mills with conventional rolling, rounds and rebars (both of them normally 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 shape of the pass. 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 normally lower than the bigger diameter bars. To improve the mill productivity to a high level in case of smaller diameter bars, one of the most significant developments in the area of bars production in recent years is the slit rolling. Fig 1 shows the improvement of productivity by slit rolling. The slit rolling technology is a combination of special roll pass design with designated guide equipment to shape and longitudinally separate the incoming billet into two or more individual strands which then is further rolled into finished sizes. In principle the billet is rolled in the same way as in conventional rolling upto the intermediate rolling mill. Here, after an acceptable section is produced, the slitting operation will start.
Fig 1 Improvement of productivity by slit rolling
Slit rolling is the most economical rolling process which allows high production rates also for small size bars and a significant reduction in the production cost. The slit rolling process enables production of two, three, four or even five bars from one billet. The slitting process uses special passes and guides to prepare, shape and longitudinally separate the incoming material into two or more individual strands, which is then further rolled into the finished sizes. This process 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 process technology has reduced the number of passes. The MSR process technology is very frequently employed during the rolling of the ribbed reinforcement bars. Fig 2 shows the technique of slitting and slitting of bar in two, three, four, and five strands.
Fig 2 Technique of slitting and multi slit rolling
Roll pass design in case of bar rolling with longitudinal slitting is based on the application of special shaping passes, so-called cutting-in passes, in the final stage of the rolling process. In these passes, the rolling stock is subjected to deformations considerably differing from those occurring in conventional stretching or shaping passes. Rolling in multi-strand passes can be conducted on the existing rolling mills without having to incur any considerable investment outlays or install any special equipment. It allows a considerable increase in the production capacity of the rolling mill.
The separating pass in the slit rolling consists of two or more major sections, which are held together by small strips of material. During the slitting or separating process these sections are divided into separate bars of false round. Also, during the slitting operation the divided bars have a little fin or fins of material either on the left, right or on both sides of the slit round stock (Fig 2). During the transformation of the slit round stock into an oval, these fins are elongated. The rolled-in fin on the final product has a depth which is accepted by the standards.
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 temperatures. 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 had 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.
Multi slit rolling process
The slit rolling process differs from conventional continuous rolling by the use of special roll passes and guides to prepare, shape and longitudinally separate the incoming billet into two or more individual strands for further rolling into the finished size. In principal, this process is achieved by (i) reducing the billet conventionally through the roughing and intermediate rolling mill stands to produce an acceptable section for the first special shaping pass at the forming stand, (ii) precise guidance of this rolling stock to the forming stand where it is reduced and shaped to form a symmetrical ‘forming section’ normally in the shape of a dog bone, (iii) further close guidance and control of the dog bone through the separating stand, where the rolling stock is reduced and shaped into a ‘slit pass’, designed to be easily separated into two equal sections of false round, and (v) a special guide on the delivery side of the separating stand ensures a clean slitting of the bar and now delivers multiple strand of equal sections to their respective finishing lines. Typical roll pass design of bar slitting for two, three, four, and five strands is shown in Fig 3.
Fig 3 Typical roll pass design of bar slitting for two, three, four, and five strands
In the multi slit rolling process, the roll pass design needs special attention. Roll pass design (Fig 4 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 the dog bone pass and the slit pass. The remaining passes, before and after slitting, are most often standard stretching passes typically used for round rolling in a merchant bar mill. 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 / rectangular / round/ oval shape into two equal parts. Precise inserting of a bar into the pass and holding it in a right position is achieved with application of proper rolling stock guide system. The rolling stock system is important, since it is not possible to correct any irregularities such as asymmetrical slitting in subsequent rolling passes and it leads to rejects.
Fig 4 Typical design of dog bone and slit grooves
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 is to 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.
Typically, the slit breakdown sequence is performed in two passes (Fig 5). Herein, a first pass is performed to adapt the incoming work piece shape (normally a bastard oval) to an 8-shaped (dog bone) semi-finished piece. Fig 5 also shows a schematic of a split pass. The rolls involved in the first and the second passes have relatively similar grooves consisting of a double-round shape with a central protrusion which produces progressive separation of the work piece. The inclination of the protrusion ‘X’ shown in Fig 5 in the rolls involved in the first pass is generally much higher than that of the second pass to increase its stiffness. The rolls involved in the two passes have different objectives. In the first pass, the roll protrusion exerts a rolling-indentation effect on the work piece, which can be assumed as a deep roll coining. In the second pass, the side edges of the protrusion (rather than its apex) are used to separate the work piece in developing of the round bars. The key design parameters for designing a slit pass are represented in Fig 5. As can be noted, the slit pass is significantly different from the common concave calibers used for rod rolling. Actually, the slit rolls are characterized by the protrusion (nose) that results in a mixed concave-convex configuration.
Fig 5 Typical two pass slit breakdown sequence and schematics of a split pass
Since the aim of the slit breakdown sequence is parallel splitting, hence, normally the maximum clearance of the rolls is such that the height of the groove is almost equal to the height of the incoming work piece. Only slight adjustments are carried out to the rolls clearance in order to compensate possible design errors, as shown in Fig 6. Actually, an incorrect design of the slit pass or an inappropriate choice of the rolls clearance can lead to some effects on the final work piece. Fig 6a shows schematically the case in which an excessive value of roll clearance C1 (i.e., the roll groove is oversized compared with the incoming work piece) is adopted. In this case, the rolled bar product can be affected by an excessive lack of circularity for subsequent passes. On the other hand, an excessively low value of the roll clearance C3 (i.e., an undersized roll groove with respect to the incoming work piece), as shown in Fig 6c, can determine an excessive roll groove fulfilling with production of lateral burrs on the work piece and a sudden increase in the rolls separation force in addition to other possible rolls damage.
Fig 6 Filling behaviours of rolling stock in roll grooves
In case of MSR process, the leader sections (square, rectangular, round, or oval) before the dog bone and slit passes have to be of very close tolerance. This is achieved by (i) accurate pass profile in the rolls, (ii) accurate roll gap to ensure no overfill, (iii) accurate pass alignment between top and bottom rolls, (iv) sound roll neck bearings and positive roll location to avoid axial flow, (v) rigid rolling mill stands providing rigid screw down and roll balance, (vi) sound roll quality and good pass conditions, and (vii) 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 features which are desired for the proper guidance of the leader section include (i) sound and robust guides, (ii) secure and rigid rest bars, (iii) precise guide equipment, (iv) positive, adjustable and secure alignment, (v) guiding of the bars close to the stand, and (vi) correctly set up and maintained guides. Typical roll pass deign with application of dog bone and slitting passes for different bar sections is given in Fig 7.
Fig 7 Typical dog bone and slit pass design for different bar sections
Advantage of slit rolling
The different advantages slit rolling over the conventional rolling in a merchant bar mill are (i) the number of rolling stands get reduced, (ii) reduction in the capital cost, (iii) there is reduction in specific power consumption, specific fuel consumption, and specific roll consumption, (iv) there is increased in rolling yield, (v) the reduction in the production cost, and (vi) there is increase in the productivity level of the mill. With the same output rolling speed, the productivity is increased linearly with increase in the number of strands.