A brief onCold Rolling of Steel

in a 4 Hi Mill

By Hussam AdeniLubricant Specialist

Cold rolling of Steel strips

The raw material for Cold rolling is typically a Hot rolled coil (HRC). At the CR Mill it undergoes the following operations:

1. Raw material – Hot rolled coils – slitting2. Pickling or acid wash with Hydrochloric Acid3. Water wash4. Short term rust protection with emulsion5. Sizing to get idea coil OD.6. Cold rolling using emulsion7. Annealing8. Final rolling – “Skin Pass” and rust protection9. Slitting sizing of CR strip10.Packing

1. Raw Material slitting

A Cold Rolling Mill, is typically a Sendzimer type mill and is PLC controlled. CRM’s may be 4 Hi Reversible Mill or 8 Hi reversible. A 4 Hi mill, consists of two work rolls and two back up rolls and rolling operations can be reversed, hence the nomenclature. Sendzimer is a generic name and it may have been supplied, erected and commissioned by mill suppliers, like Mecon Wean-United.

The raw material is a hot rolled strip and is available in varying widths and thickness; ranging from 900 mm to 1600 mm in width. CR mills can accommodate fixed width, depending on the maximum OD of the rolls. If it is 720 mm, the coils have to be slit or sized to this parameter. The hot rolled strip received as raw material varies in thickness from 2 mm to 5 mm

The main purpose of a Cold Rolling Mill is to

To reduce the thickness Impart better metallurgical properties. Make sheet suitable for forming and drawing. Make sheets ideal for Deep drawing (DD) or Extra Deep Drawing (EDD) Widely used in Automotive component and White Goods & Engineering components

manufacture.

During a normal cold rolling operation the strip is reduced between 60 to 70% depending on the end use. The finished CR strip produced attains IS:513 specifications and can be supplied between 0.3 mm to 3.5 mm thickness and between 25 mm to 720 mm widths.

Bulk of the raw material is imported from Korea, France, Germany, Japan, Brazil etc.. Indigenous producers are Tisco, Rourkela, Bokharo etc. Over the years, imported HRC were preferred over indigenous as they were “softer. In the 1990’s it was observed that imported sheets were marginally softer, hence preferred. It requires lesser power and production costs, as compared to indigenous HR sheets. From the financial angle too, they were “softer” as 365 days plus credit was offered from some of the suppliers.

Tisco also supplies what is known as “Skelp” these HR coils have widths ranging from 350 mm to 200 mm and are available in standard thickness of 2.8 mm, 2.0 mm, 3.25 mm, 4.0 mm, and 4.5 mm

2. Pickling or Acid wash

The purpose of the pickling operation is to remove traces of rust stains, oxide layers, dirt and oil layers normally present in HR sheets/coils.

Pickling can be done in batches or by continuous process. In batch processing the sized coils are placed in cages and dipped in pickling tanks. The typical pickling time is as follows:

Batch time Tonnage Acid concentrations

25 minutes 1st 35 Tons 15%35 minutes 2nd 35 Tons 10%45 minutes 3rd 25 Tons 5%

Hydrochloric acid is received at 35% concentration and diluted to 15% concentration for pickling. During use, the concentration of the acid gets depleted, hence subsequent batches are kept at pickling tanks, for longer periods. The discard limit of the acid is 105 Tons of 5% concentration. The approximate consumption of acid is between 25 to 35 Tons per month typically for a 2000 TPM CRM.

A typical unit may have two pickling tanks of 4 KL capacity. Protective covers - with sliding cover arrangements – are placed on the pickling tanks. Fumes emitted during pickling are drawn out with the aid of strategically positioned exhaust fans.

The HR coils are placed in cages with the help of EOT cranes and dipped into acid (pickling) tank. The cages have an ID of 900 mm and may often be charged with 2 x 350 mm coils. If larger widths are available they may be loaded accordingly. Cages have gear arrangement such that the coils inside the cages, can be rotated. During pickling when the coils are rotated the coils become un-winded or uncoiled. This is done twice; once clock-wise and the second time in the anti clock-wise direction. This increases the surface area for the acid to react and cleanse.

3. Water wash

The cages containing the pickled coils are then placed into a water tank mainly to neutralize acid carry-over from the pickling process. Here again, to increase surface area for water wash, the coils are uncoiled and coiled and fresh water is continuously circulated. Carry-over from the pickling process results in the formation of sludge, this makes the water turbid. Water is completely replenished every two days. A typical water wash takes 3 to 4 minutes.

4. Rust protection – using emulsion

For facilitating storage of coils after the pickling and the subsequent water wash and also to protect the sheet from rust, the coils are dipped in a solution of coolant or soluble oil emulsion. The purpose of this step is to provide an alkaline environment, to further neutralize acid carryover and pre-empt rusting. The oily layer also provides temporary rust protection. A soluble cutting oil ( Servocut S, Cooledge SL) at 5 to 8% concentration is often used as the coolant, mentioned above.

The coolant discard period is around 240 Tons of coil. The coolant is changed if the emulsion “splits.” Top up requirement is 75 to 100 Liters of coolant per day. Typical monthly requirement is 10 to 15 barrels of soluble cutting oil.

Here again the coils are rotated or coiled and uncoiled in their cages. The dipping time is 4 to 5 minutes.

Most CR mills consume 60 to 70% of their Soluble oil requirement in this application. In some instances, or in units located in coastal areas or during the monsoon or rainy seasons, the coils may be dipped directly in the “soluble oils” (without making emulsion) for extended rust protection.

Selection of ideal coolant: CR Mills normally like to reduce “Coolant” requirement at this stage and employ a simple test. 5 to 8% emulsion is formed in the lab, to which hydrochloric acid of 15% concentration is added. The volume of acid required to split acid is noted. Soluble Cutting oils/Coolants requiring larger volumes of acid to split the emulsion is the “preferred supplier.”

5. Sizing – to get ideal OD

Between 7 to 8 meters, of the ends of coil are normally scrapped. This is actually the distance between the delivery reel (or feed reel) to the work roll. Thus if the coils have smaller lengths or smaller OD, there is more scrap. To increase productivity and reduce scrap, a head-to-tail welding is normally carried out . The optimal OD is 1200 mm and is called ideal OD. Incidentally, the feed reel or the delivery reel can accommodate coils upto 1250 mm OD.

6. Cold rolling - using Roll Coolant - emulsion

In this CRM, Cold rolling is done on a Mecon Weanunited PLC controlled 3000 tons/m 4 Hi reversible mill. As the name suggests the 4 Hi mill employs 4 rolls, two these are the work rolls and the other two are the backup rolls. The rolls between which the cold rolling actually takes lace is called the work roll. The OD of the work rolls is between 208 to 225 mm or 145 to 155 mm, while the backup roll may be 575 to 610 mm. The length of both rolls are 700 mm, hence it can accommodate coils having width upto 630 mm. The mill is reversible and rolling takes place at speeds of 300 to 350 m/sec.

The work rolls for primary rolling have a rough finish. As a 5 to 6 micron finish is desirable during the final rolling or “skin pass” the work rolls are changed for the ‘skin pass” operation. Typical regrinding time for work roll is 100 passes.

The first step in cold rolling is alignment, during which the centre line of feed stock is aligned with the centre line of the mill. This is achieved by using the “pay-off reel.” Alignment ensures uniformity in rolling and also increases the life of work roll.

During the process of cold rolling, the sheet is released by the pay-off reel and passes between the work rolls and is coiled on the delivery reel. This is designated as one pass. Maximum reduction takes place during the first pass, as the sheet is relatively softer. Sheets get progressively harder as the number of passes increase.

In order to have uniform thickness, back tension is applied to the delivery reel. This pulling action combined with the reduction results in increase only of the length of the coil. The increase in width is only marginal. And may vary between 2 to 1.5 mm. This increase of the width, i.e. to the edges is removed by slitting the edges often called “edge cuts.”

In order to facilitate rolling, provide lubricity as well as dissipate heat, an emulsion is used as a “roll coolant.’ The emulsion is often at around 3 to 5% concentration. The emulsion is circulated through an elaborate filtration system, consisting of a paper filter, magnetic drum filter, edge filter and finally a skimmer on the top of the emulsion tank. The emulsion tank is typically 12,000 liters and requires daily top up of 30 to 40 Litres of “Soluble Oil.” Monthly consumption of soluble oil or Rolling oils isaround 10 barrels. The emulsion is discarded after 100 Tons of rolling or after 15 days. If the emulsion splits, it is discarded immediately.

After the first pass, the mill is reversed and the delivery reel becomes the feed reel for the second pass. A sheet may undergo upto 10 passes depending upon the final thickness required.

7. Annealing

Annealing is a process requiring heating at controlled conditions and it is followed up by slow cooling. The objective of annealing is to improve the strength of the metal. Annealing occurs in three stages:

Heat build-up - 8 to 10 hours Soaking - 8 to 16 hours at 640 to 720 C Cooling - 25 to 36 hours

In the first step the coils are placed in a Bell type furnace. Inert conditions are created by charging with Nitrogen gas. Large blowers located at the base of the furnace provide desired circulation of heat. The heat build-up take up 8 to 10 hours.

In the second step, the coils are allowed to soak in the heat at temperatures between 640 to 720 degrees centigrade. For 8 to 16 hours.

The final and most critical stage is controlled cooling and this too occurs In inert conditions, to prevent oxidation. This process may take 25 to 36 hours.

The annealed coils are then unloaded at a “safe point” which is 100 degrees centigrade. At the safe point, “scales” do not form. The coils are then left to open atmosphere to cool till about 40 degrees centigrade.

After cooling the sheets become fairly hard and typical values may vary between 400 to 500 VPN. Annealing makes the metal soft and post annealing values for hardness vary between 80 to 90 VPN.

8. Final rolling – Skin pass

The final rolling is also called “skin pass” as a very nominal reduction of about 5% takes place. The skin pass is carried out without a roll coolant at about 40 degree C. This temperature is critical, as it was observed the rusting begins to start on the annealed sheet at temperatures below 40 degrees C.

The purpose of skin pass is s follows:

Stage hardening Improve hardness Provide uniform surface

The skin pass is normally carried out with “super ground: work rolls of 5 to 6 micron finish.

To protect the sheet from rusting, the coil is sprayed ( at a point just before the delivery is coiled )with rust protective. Castrol’s Rustillo 630, an oil and lanoline based rust protection oil was used, till a few years ago.

9. Slitting/sizing of SR strip

The first stage of sizing consists of edge cuts. About 5 mm fro the edge of the sheet is trimmed. This is necessary as the edges may not have uniform thickness.

The second stage is called sizing, wherein the cold rolled sheets are cut as per customers requirements. This is called slitting and consists mainly of slitting to widths desired by the customers. This CRM can offer widths ranging from 25 mm to 700 mm and thickness between 0.3 to 3.5 mm. The product is available in a bright or mat finish. For mat finish, the work roll having a mat finishd is used.

For customers who require the CR Sheet is specified lengths, the coils undergo a straightening process after which it is cut to length, as desired by the customer.

A second round of rust protection treatment is often applied prior to packing.

10. Packing

The finished coils are wrapped in wax paper, followed by jute or hessian wrapping. Cut-to-length sizes undergo similar packing or as desired by customer. Prior to transportation the sheets are palletised.

The finished cold rolled strip is ideal for DD ( Deep drawing) and often used in the following industries.

Compressor manufacture Refrigerator manufacture Automobile components Bearing manufacture Container manufacture Transformer fabrication Heat exchanger manufacture.

Value added products: Some of the CRM manufacture also manufacture and supply ready to use value added products which can be used directly. These are formed in the CRM in the Cold rolled section and cut to desired length

Guards rails used on highways and roads are an example..