Rolling Mills

ROLLING BASICS

 Topics • Rolling Terminology • Types of Mills and Layouts

 Topics • Rolling Terminology • Types of Mills and Layouts

ROLLING BASICS Rolling Terminology • WHAT IS ROLLING ? • Rolling is the process of  plastically deforming metal for getting a desired shape by passing it between two rolls.

ROLLING BASICS Rolling Terminology • HOT & COLD ROLLING ? • RECRYSTALLISATION TEMPERATURE

ROLLING BASICS Rolling Terminology LONG PRODUCTS

FLAT PRODUCTS

ROLLING BASICS Rolling Terminology •Draft (H-h)

h

H B

• Spread (b-B)

b

ROLLING BASICS Rolling Terminology • Elongation (l-L) • Reduction (A-a) • Coeff of Reduction A/a • Rolling Constance • V1= V2= V3

ROLLING BASICS Rolling Terminology • ANGLE OF CONTACT •ANGLE OF BITE

Terminology used in monitoring critical parameters of mill efficiency • Mill Availability • Mill Utilisation • Rolling Rate • Hot hours •  Yield

Terminology used in monitoring critical parameters of mill efficiency • Mill Availability

• Calendar Hours= 365 or 366 *24 Calendar Hours-(Repairs+Capital Repairs) Mill Availability= Calendar Hours

•Mill Utilisation

• Available Hours= Calendar Hours – (Repairs & Capital repairs) Available Hours-(Delays & planned stoppages) Mill Utilisation =

Available Hours

Terminology used in monitoring critical parameters of mill efficiency • Yield

• It is the ratio of useful output to input expressed as percentage • Rolling rate

• It is tonnage rolled in an hour. It is a measure of speed of rolling and its unit is tons/hr

PRODUCTS OF ROLLING MILLS OF SAIL Bhilai Steel Plant • Semis (Blooms, Billets, Slabs and Narrow width slabs) • Rails • Heavy Structurals (Beams, Channels, Angles, Crossing Sleepers) • Merchant Products (Angles, Channels, Rounds and TMT Bars) • Wire Rods (TMT, Plain and Ribbed) • Plates Bokaro Steel Plant • HR coils, sheets, plates • CR coils, sheets • Galvanised plain and corrugated sheets • Tin mill black plates

PRODUCTS OF ROLLING MILLS OF SAIL Durgapur Steel Plant • Semis (Blooms, Billets, Slabs) • Merchant Products (Bars, Rods, Rebars) • Medium Structurals (Joists, Channels, Angles) • Wheel and axle • Skelps Rourkela Steel Plant • Plate Mill Plates, Special plates • HR Plates, Coils • CR Coils and Sheets

PRODUCTS OF ROLLING MILLS OF SAIL IISCO Steel Plant,Burnpur • Structurals • Merchant and Rod Products Alloy Steel Plant,Durgapur • Alloy and stainless steel Slabs, Blooms, Billets, Bars, Plates Salem Steel Plant • Hot rolled carbon and stainless steel flat products • Cold rolled stainless steel sheets and coils Visweswaraya Iron and Steel Plant, Bhadrawati • Semis, Bars

WHAT IS A FURNACE? A furnace is an equipment to melt metals for casting or heat materials for change of shape (rolling, forging etc) or change of properties (heat treatment)

Characteristics of an Efficient Furnace Furnace should be designed so that in a given time, as much of material as possible can be heated to an uniform temperature as possible with the least possible fuel and labour.

SELECTION OF FURNACES • •

• • •

Achieving the desired minimum discharge temperature for the desired production rate. Normally, discharge temperatures vary from 1150 to 1300 0C. Heat the material within a maximum permissible temperature gradient. a) For rolling of rounds a temperature difference of 50-80 0C from surface to core or from top to bottom is tolerable. b) For rolling sections like “H” Beams temperature difference should be less than 30 0C. Minimum fuel consumption. Minimum oxidation of metal. Avoiding thermal stresses.

Classification of Furnaces Based on the movement of the stock (i) Batch Type Furnace (ii) Pusher type furnaces (iii)

Rotary hearth furnaces

(iv) Walking beam furnaces.

Advantages of the pusher-type furnaces • Low installation cost, which means high production rate per Rupee of investment. • Low maintenance cost and ease in charging and drawing steel. • Can be built for any reasonable length of piece to be heated, resulting in higher mill yield. • In the top fired furnaces charging of any shorter lengths is possible, resulting in higher mill yield

Disadvantages of pusher-type continuous furnaces • In the top fired pusher furnaces heat transfer takes place from the top surface only, resulting in high temperature gradient from top to bottom. For proper heating of bottom surface top surface is to be overheated, resulting in excess oxidation metal on the top surface. • Length of the furnace is to limited for preventing of  piling of the charge. • It is not possible to empty the furnace everyday. For small scale re-rollers, who operate the mills for 10-16 hours, the entire material inside the furnace is cooled during idling time and then heated next day resulting in higher fuel consumption and oxidation of metal

Disadvantages of pusher-type continuous furnaces • Lack of flexibility for heating efficiently small orders of different grades of steel or different thicknesses continuously. • In case of top and bottom fired pusher furnaces there will be additional cost towards water cooling system and its maintenance. • Trouble from building up of scale on solid hearth at soaking zone, expensive to empty furnace at end of schedule. • Difficulty in pushing mixed sizes through furnace.

Walking Beam Furnace  – Stock placed on stationary ridges  – Walking beams raise the stock and move forwards  – Walking beams lower stock onto stationary ridges at exit  – Stock is removed  – Walking beams return

to furnace entrance

Advantages of Walking Beam Furnaces • Since heat transfer is through four faces, heating is faster than any other type of furnace and temperature gradient from top surface to the bottom of the billet will be lowest. • Productivity will about three times top fire pusher furnace. • With three movable skids and 3 fixed skids, three row charging can be done, facilitating a design of wide and efficient furnace. • The entire furnace can be emptied. • No skid marking.

Disadvantages of Walking Beam Furnaces • Cost of construction is higher due to requirement of water cooling. • Maintenance cost is higher. • Very short lengths of blooms (less than 2500 mm) can not be charged as there is a need for support of the bloom by two fixed skids and two movable skids. With four beams for supporting of short length blooms, bottom heating will be ineffective

Rolling Of Flat Products •

Descaling • Roughing • Finishing • Cooling • Coiling MAJOR Parameters and factors affecting rolled products and their control: The following major factors affect the quality of rolled products a) Temperature: b) Roll conditions:

Major Defects:

Some of the major defects in hot rolled flat products a. Rolled-in-scale b. Roll mark, fire crack mark c. Bad profile d. Poor flatness e. Bad shape

Rolling of Long Products •

Re-heating • Roughing • Intermediate Rolling • Finished Shape Rolling • Cutting & Sampling • Finishing • Inspection • Dispatch

Defects of hot rolled long products Defects due to which hot rolled long products are rejected may be broadly classified as: a. Rolling defects: Defects induced during the rolling process are classified as rolling defects b. Steel defects: Defects resulting from steel making practices and getting carried forward during the process of rolling to the end product may be classified as steel defects

Rolling defects • Fins and overfills are protrusions formed when the section is too large for the pass it is entering. Overfills are broad and less sharp than fins. • Underfills are the reverse of overfills. It is due to incomplete filling of pass. Under filling appears most frequently on rounds and channels. • Slivers are loose or torn segments of steel rolled into the surface. They are caused by a bar shearing against a guide or a collar or incorrect entry in the closed pass. In hot rolled finished products slivers may be carried through from input billets or slabs. • Laps are caused due to rolling in of fins and overfills in the subsequent passes. • Fire cracks and roll marks are impressions of mill rolls on the product. These are caused by overheating, cracking or spalling of the rolls. • Rolled-in scale: During the process of rolling the scale formed may get rolled in subsequent passes. These if not properly eliminated during rolling process result in rolled in scale. • Buckle and kink is a corrugated or wrinkled surface condition of the product. These may be caused either by worn out pinions on a roll stand or uneven cooling beds. Buckle is an up and down wrinkle whereas kink is a side wrinkle.

Rolling defects

Camber is the deviation of the side edge of a bar from a straight line. It is caused by the improper heating of the ingot/bar, uneven dimensions causing differential expansion or contraction. • Twist is a condition where the ends of the bar have been forced to rotate in relatively opposite direction about its longitudinal axis. This is caused due to excess draft, faulty setting of delivery guides or non-uniform temperature of the stock. • Shear distortion is deformed end on a bar caused by improper or defective adjustment of shearing equipment. • Out of square section refers to section having diagonals of unequal length. This is caused due to improper alignment or leveling of rolls along its axis. • Burnt edges appear as a rough area with checked or serrated edges. It is caused by exposure to excessive temperatures during heating.

Steel defects

Scabs are caused by splash of material against the mould wall when an ingot is being teemed. Rapid solidification of the metal on the mould wall causes it to stick to the ingot surface and finally appear scab on the surface of the rolled product. • Pipe is steel making defect carried through from the ingot. The presence of pipe is detected as cavity located in the centre of an end surface. • Seams are crevices in the steel that have been closed but are not welded. This is due to the presence of blowholes and small cracks in original ingots. • Spongy bar appears as deeply serrated at the edges. It is caused due to bad teeming.

Cold Rolling Purpose of cold reduction is to achieve the following: i. a reduction in the thickness of the final product. ii. a designed surface finish. iii. desirable mechanical properties. iv. close dimensional tolerance. v. producing as per customer requirements.

Steps of Cold Rolling •

Hot Rolled Coil • Pickling • Cold Reduction a) Cold Reversing Mill b) Tandem Mill • Electrolytic Cleaning • Annealing

TECHNO ECONOMICS Techno economic parameters to be monitored in mills are • Mill Utilisation • Mill Availability • Yield • Hot Hours • Rolling Rate • Specific Heat Consumption • Specific power consumption