Type of Longitudinal Rolling Mills | Metals | Metallurgy

The names of longitudinal rolling mills are generally derived from the products they roll. Some of the names are, bloom rolling mill, billet rolling mill, merchant section rolling mill or simply merchant mill, rail and heavy section rolling mill.

For getting a rolled product different types of rolling stands and several other pieces of equipment are employed. Besides, there are other technological features which are important and are taken care of for getting the desired product quality.

One or more rolling stands laid out in proper manner along with material handling equipment and other auxiliary equipment required for rolling of a product constitute a rolling mill.

Since the maximum tonnage rolled is that of steel, therefore, the developments in rolling mills are also influenced by it. Steel plants generally have separate set-ups for rolling of flat products like strips, sheets and plates and for rolling of structural products like bars, angle sections beams, rails etc. In each of these set-ups there are separate groups of rolling stands called rolling mills.

Type # 1. Bloom Rolling Mill:

In the set-up used for rolling of structural sections, the ingot which has cross-section of about 740 x 740 mm² is rolled into blooms which have cross section in the range of 150 x 150 mm² to 330 x 330 mm². The bloom cross-section depends on the requirements and capacities of the mills following the bloom rolling mill.

Bloom rolling mill feeds directly to billet rolling mill and heavy rail and structural mill. Layout of plant for rolling of structural sections is shown in Fig. 8.12. The plant comprises of bloom rolling mill, billet rolling mill, and rail and structure rolling mill and heating furnaces.

Bloom rolling mill is generally a single stand mill which is a two high reversing stand having roll diameter in the range of 1150 mm to 1600 mm and roll length of 2800 to 3000 mm. Each roll is powered by a separate speed controlled DC motor of nearly 5000 HP or more. The upper roll is counter balanced for easier up and down motion for adjusting the roll gap so that different reductions can be given.

Generally screw down mechanism is used for adjusting top roll height. Two manipulators, one on each side of the stand, are provided for moving and turning the ingot on feed table. Many steel plants have only one bloom rolling mill. The entire steel production of the plant has to pass through this mill, therefore, the blooms should have as large a section as possible so as to reduce the number of passes (reductions) in bloom rolling and save the time of mill.

Before rolling the ingot is heated in soaking pit furnaces to rolling temperature which for low carbon steels is nearly 1250°C.

After giving two/three passes on one side of ingot, it is turned by 90° to give reductions on the other side (Fig. 8.13). Again after two or three passes on this side the ingot is turned again by 90°. After these passes the ingot is sufficiently worked out and heavy reductions may be given in subsequent passes which are carried out in rectangular grooves (Fig. 8.14).

The number of passes should be odd so that the finished bloom ends up on the forward side of the mill. Let A x B be the initial cross section of ingot and a x b the cross section of bloom to be rolled. The number of passes N may be calculated as given below-

The widening may be taken from 10 % to 12% of reduction. Average reduction per pass has to be calculated considering the life of rolls. After rolling a specified tonnage, the rolls are machined, thus there is decrease in roll diameter. Therefore, the maximum possible reduction also reduces. The rolls have to be discarded when its diameter goes below a minimum limit. For calculation of reductions in various passes, therefore, an average diameter of rolls over their life time is taken.

Example 1:

Calculate the number of passes for rolling ingots of 760 x 700 mm² cross-section to blooms of 250 x 250 mm² in a blooming mill of initial roll diameter of 1100 mm. Maximum machining allowance is 12% of initial diameter. Widening may be taken as 12% of reduction. Co-efficient of friction is 0.45.

Solution:

Type # 2. Billet Rolling Mill:

Figure 8.12 also shows the arrangement of a modern billet rolling mill. It is located just after bloom rolling mill, so that the hot bloom coming from blooming mill can be rolled directly into billets of cross sections ranging from 55 x 55 mm² to 170 x 170 mm² without reheating it.

The mill consists of two groups of rolling stands with 5 to 6 rolling stands in each group. In the first group of stands the bloom is rolled into billets of 120 x 120 mm² cross section and in the second group it is further rolled to 55 x 55 mm². There is arrangement of transfer tables to take out the bigger sections from the first group of stands without going through the second group.

Each group consists of 2 to 3 vertical and three horizontal stands so that rolling can be done in the height as well as in width direction without turning the billets. Because the billet is rolled simultaneously through several stands precise control of roll speed of each stand is essential.

The diameter of rolls in the two groups varies because billets of different dimensions are rolled in them. In the first group of stands the roll diameter ranges from 750 to 650 mm while in the second it is in the range of 600 to 450 mm. After finish rolling the billet is cut into pieces of suitable lengths with the help of a flying shear which cuts the billet in motion.

The billets are generally rolled in grooves. In most of the plants rectangular type of grooves shown in Fig. 8.15 are used. In a few plants square-diamond type grooves are also employed. These passes have the advantage that heavy reductions are possible, at the same time, there is a disadvantage that material at the corners cools fast and in case of alloy steels cracks may develop. The square-diamond type grooves are generally used for rolling billets of mild steels.

Type # 3. Merchant Sections Rolling Mills:

These mills are also called re-rolling mills. A typical layout of the mill is shown in Fig. 8.16. The heated metal is first rolled in two high roughing stands. Many merchant rolling mills use three high rolling stands for intermediate passes.

The three high stands or roughing stands may be followed by a few continuous lines of rolling stands for rolling specific profiles. Other rolling stands are arranged in cross country type layout in order to roll a variety of products like flat and round bars of various dimensions, angle sections, small channels etc.

The grooves on the rolls are so designed that maximum variety of products in shape as well as in dimensions may be rolled. Multiple finishing and pre-finishing grooves are provided for structural sections which are often in demand.

Complete continuous rolling lines may be provided for products having continuous and large demand. The set-up time for such a mill is quite high, therefore, continuous mills are economical only for large quantity production.

Roll Passes for Merchant Mills:

In roll pass design, it is always an endeavor to make the roughing passes common for as many products as possible. For roughing passes either square- diamond or oval-square passes are used (Fig. 8.17).

If it is required to roll rounds of many different dimensions out of alloy steels, the round- oval sequence of passes may be preferred although in these passes heavy reductions cannot be given. The advantage of these passes is that no corners are produced. The material in corners cools quickly and there is possibility of cracking. Hence these passes are very suitable for alloy steels.

Oval-square pass Sequence also does not pose a serious problem of fast cooling of corners because in these passes the corners formed in square pass get mixed in the main body of oval shape and in the next pass new corners are formed. Therefore, the material at corners is always at nearly billet temperature. Also reasonably high reductions can be given. Oval-square sequence of passes is, therefore, the most preferred sequence.

The roll diameter of various stands in merchant rolling mills varies from 450 mm for the roughing stand to 270 mm in the finishing stand. Drive system consists of motor, gear box for speed reduction, followed by pinion box to divide the power to the two rolls. Couplings are used at every connection. Several stands in the cross country type layout may be driven by one motor if they are placed in a line.

Since the cross section of billet after the roughing passes is small it can be easily twisted in the guides before entering into grooves. Rolling speed on the finishing stands may be as high as 16 -20 m/sec. Adequate safety measures should be taken to safe guard the workers, because, if the front end of hot bar gets stuck in a guide or some where it may result in serious accidents and injuries to workers.

Basic Principle of Groove Design:

For finished products such as angle section, bar etc. take care of following points while designing roll passes:

(i) Determine the dimensions of the finished product

(ii) Determine tolerances on various dimensions. Determine minimum sizes.

(iii) Calculate the hot dimensions of the minimum sizes taking into account the co-efficient of expansion and the finishing temperature. These dimensions are the dimensions of finishing groove. Determine the roll parting line such that the main dimension should be least affected by roll wear and should be easily repairable.

(iv) For calculation of dimensions of pre-finishing groove, decide on the sequence of reductions in all passes. In the finishing passes the reduction should not be more than 10% to 15%.

(v) Determine the changes in various dimensions taking into account the widening and reduction in finishing pass.

(vi) Adjust the dimensions to determine the sizes entering the finishing pass. These are the dimensions of pre-finishing pass.

(vii) Repeat the procedure for passes before pre-finishing. 

Type # 4. Rail and Heavy Section Rolling Mill:

These mills are also known as heavy section rolling mills or medium section rolling mills depending upon the equipment installed. The raw material for rail and structural section rolling mill is bloom. A typical layout is shown in Fig. 8.12. In many plants the mill is not directly in line with bloom rolling mill.

In such cases the blooms have to be reheated because they loose heat during transfer. The first rolling stand of the mill is a two high reversing rolling stand having roll diameter of about 900 mm and barrel length of 2300 to 2500 mm. It rolls the bloom to required size for starting the profile passes. Rough pre-forms may also be rolled in this stand.

The billet is then rolled in three high rolling stands. Most of the plants have two/three 3- high stands with roll diameter ranging from 800 to 650 mm and a barrel length of 1600 to 1800 mm. A minimum of three passes are given in a 3-high rolling stand in order to keep the billet on the forward side of the stand.

The final finishing pass is given in a two high rolling stand in which manufacturer’s marking may also be rolled onto the product. A large variety of heavy sections are rolled in this mill. However, some typical pass sequences for rail rolling are illustrated in Fig. 8.18. The rolled products having heavy sections such as rails have to be cooled slowly in order to avoid cracking due to internal pressure of dissolved gases particularly hydrogen.

So the rails and such sections are cooled in pits. Because of variations of thickness in the cross section of profile rolled such as in rails, the rolled products get curved during cooling. They are straightened in straightening machines. In case of rails the ends may also be machined and connecting holes are drilled in a machine shop attached to rail and structural mill.