Rolls, Roll Cooling and Roll Management

Introduction

Rolls are the most expensive tools of the rolling mill trade and are used to execute the duty of deforming steel.

Turning of a Roll

Fig. 3.1 Turning of a Roll.

There are Three Parts in a Roll, Namely

  • (a) The Body: The part on which rolling is made.
  • (b) The Neck: Which support the body and withstand the rolling pressure.
  • (c) The Wobbler: From where driving force is applied through loose fitting spindles and boxes, which together form a sort of coupling.

A plant surface rolls are used (in pairs) for rolling sheets, flats and plates, while for bar and shaped profiles, gr ooved or shaped rolls are used.

Classification of Rolls

Generally, rolls are classified according to the purpose or to the specific duty they have to perform, as explained below:

Classification wrt Purpose

Classification according to the purpose, for which they are used and categorized into the following groups:

(a) Rolls for Longitudinal Rolling: These rolls are cylindrical in shape. Plain and groove rolls are used for flat and section mill rolling respectively.

Rolls for Longitudinal Rolling

Fig. 3.2 Rolls for Longitudinal Rolling.

Rolls for longitudinal rolling comprises several elements (Fig. 3.2), 1-barrel,its diameter-/) and length L; 2-neck, neck diameter-^ and length-/, on both sides of barrel, mounted in the bearing and 3-tang, having a fork shape and seiving to link the roll to the spindle, while other end of roll is either cylindrical or notched as a wobbler for rigging the hoisting rope, when handling with a crane or for driving the roll during redressing in a roll lathe.

(b) Rolls for Cross / Spiral Rolling: These rolls are used in the production of tubes, hollow profiles and for machine parts viz. barrels, disc, cone and special shapes.

Rolls for Tube Rolling

Fig. 3.3 Rolls for Tube Rolling.

Classification wrt Specific Duty

The major qualities required in rolls for rolling mill are:

  • (u) Strength/toughness: Ability to withstand shock load, including thermal shock.
  • (b) Hard Wearing: Good surface quality to ensure long life in term of tonnage rolling.

On the basis of hardness, rolls can be categorized into four types as given below:

Table 3.1

Types of Rolls

Hardness

Use

Material

Brinnel

“Shore

Soft

150-250

25-35“

  • - Primary Mill
  • - Roughing stands of Heavy section mill
  • - Cast steel
  • - Forged Carbon Steel
  • - Grey Cast Iron

Semi Hard Rolls

250-400

35-60“

  • - Roughing stand of Rail & Structural Mill
  • - Section Mill
  • - Flat Mill
  • - Finishing Stands of Section Mill
  • - Semi Chilled Iron
  • - Cast steel
  • - Forged steel

Hard Rolls

400-600

60-85“

  • - Finishing stands of strip, sheet, rail and section mill.
  • - Back up rolls of for 4-high stand
  • - Chilled iron
  • - Alloy steel

Very Hard Rolls

600-800

85-100°

Cold Rolling Mill

- Forged Alloy steel with Cr

Rolls Qualities

Steel and iron rolls are two main categories of rolls. Their use in any particular purpose depends upon the specific duty which they have to perform. Accordingly, mechanical property of a roll is the most important for that particular application i.e., toughness, resistance to thermal cracking, shock loading or wear out. The classification of rolls depends upon the carbon percentage in the roll.

Effect of Alloying Elements in Iron and Steel Rolls

The carbon content of iron or steel has an important bearing on these properties as shown in table 3.2. In general, greater the carbon content, the harder will be the roll and conversely with lower carbon content, the roll shall be more tougher and softer.

Table 3.2

Iron Rolls

Effect of Elements

Steel Rolls

Increases hardness, wear resistance brittleness and ductility of chill

Carbon

Increases hardness, brittleness and wear resistance.

Increase content of graphite, adds cleanness

Silicon

Cleanses steel, if added in proportion of 0.2-0.35, adds to hardness, deoxidizer and promotes sound casting.

Increases hardness and brittleness

Phosphorous

Increases hardness and brittleness. Decreases ductility.

Increases hardness and brittleness depths of chill.

Sulphur

Increases hardness and brittleness. Decreases ductility, must be used with discretions.

Reduce chill in lower range increase chill in higher range, increase hardness in combination with nickel and chromium, increase brittleness.

Manganese

Increases hardness and brittleness cleanser for oxides and sulphides, increases tensile strength and wear resistance.

Increase tensile strength and wear resistance, decrease depth of chill.

Nickel

Increase strength and resistance to fire cracking in combination with others.

Increase strength makes fine-grained structure; increase strength and resistance to fire cracking.

Molybdenum

Increase strength and hardness.

Hardener used in combination with Ni or Mo or both increase depth of chill.

Chromium

Hardener used in combination with Ni or Mo or both.

Similar to nickel

Copper

Similar to nickel.

Hardener used with discretion.

Boron

Increase hardness.

Effect of Casting Practice

The classification of rolls is also based upon the casting practice adopted for the manufacture of the roll i.e., the form and percentage in which the free carbon is present in the structure of the roll as shown in table 3.3 below:

Table 3.3

Rolls

Steel Rolls

Steel Base Roll

Cast-Iron Rolls

Micro

structure

Ferrite & Pearlite

Pearlite & Cementite

Cementite spheriodical & pearlite {i.e. heat treated)

Cementite or graphite + pearlite sorbite or martensite

C%

0.80

1.25

> 2.5

3.3 The distinctive line between cast iron and steel base rolls is about 2.5%. The variation in hardness and tensile strength varies directly with respect to the carbon percentage as indicated in Table 3.4 below:

Table 3.4

Types of Rolls

C %

0.60

0.80

1.0

1.20

1.40

1.60

1.8

2.0

2.2

2.4

HARDNESS Sh °c

34

40

41

37

42

45

46

47

47

48

Tensile strength N/mm2

74

76

77

79

74

69

66

55

50

35

Quality Classification of Rolls

Table 3.5

Rolls

Quality

Hardness 0 Shore

Characteristics

Typical Application

Cast

Steel

Alloy Steel (0.5-08%)

25-40

Good Machinability

Cogging, slabbing and Blooming Mill.

Steel Base (0.8-1.4% C)

40-50

High tensile strength, Elongation

Roughing stand of Medium, heavy & light Section mill.

Adamite (1.4-2.2 % C)

50-80

Resistance against fire cracking and breakages. High Bending Strength

Finishing stand of heavy flange Profile

Forged

Steel

Carbon Steel (0.35-0.70%C)

25-35

High Tensile Strength

Cogging and Blooming mill.

Alloy Steel (0.35-0.70%C)

  • 50-60
  • 80-90

Resistance against Fire Cracking, Breakages and spalling.

Slabbing mill, large Backup rolls of plate mill, Backup and Work roll of cold rolling mill.

Mo. chill

60-70

Resistance against Fire Cracking, Breakages and spalling.

Large Backup rolls of plate mill, Backup and work roll of cold rolling mill.

Cast

Iron

Grain Type (1.5-2.5%C)

  • 40-50
  • 50-60

Uniform hardness Penetration and wear resistance with tough core and neck.

Roughing stand of heavy and medium structural mill Finisher of billet mill. Intermediate of light Structural mill.

Indefinite Chill (3.0-3.5%C)

  • 50-60
  • 60-70
  • 70-80
  • 80-90

Equal and gradual hardness penetration with high and uniform wear resistance

Intermediate and Finisher of medium And light structural Mill. Finisher of Wire Rod, Hot strip, and Merchant mill.

Spheroidal Graphite (3.0-3.5% C)

  • 40-50
  • 50-60

60-70

70-80

Combination of wear resistance and high tensile strength allowing rolls to use as substitute to the steel as well as to cast iron rolls.

Roughing, intermediate and finishing rolls of heavy medium and light structural mills. Use as a finisher of Billet, Plate and sheet mill.

 
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