Quy trình cán thép.pdf

Quy trình cán thép.

ME 4563 Fall ’05 Dr S Haran

(10.06.2005)

Fall 2005 College of Engineering Arkansas State University

Fall 2005 College of Engineering Arkansas State University

ME 4563 Fall ’05 Dr S Haran

Rolling

What is Rolling ?

ƒ A process of reducing the thickness

(or changing the cross-section of long

work-piece

ƒ By applying compressive forces

through a set of rolls

ME 4563 Fall ’05 Dr S Haran

Some Rolling Facts …

ƒ Process developed in late 1500’s

ƒ Accounts for over 90% of all metals produced by

metalworking process by volume

ƒ Various cross-sectional shapes are achieved

ƒ Major metal that is rolled is Steel, followed by

Aluminum

ƒ Can be carried out Hot or Cold

ƒ Machinery for Rolling (called “Mills”) have a variety

of configurations

ƒ Several parameters to be considered in practice, in

the design and selection of machines

• Pipes and tubes

• Shape rolled products such as channel, I, L, T,

etc.

• Other structural shapes also made

Rolling – Products include

Major Metals that are Rolled include:

• Steel

• Aluminum

• Others like Copper, Brass, Lead, etc.

• Special applications – Titanium and

other exotic metals

Rolling

• Construction, etc etc.

Classifications of Rolling Material

Rolling material is separated into classifications based

on the material's cross-sectional dimensions

Rolling

And the range of applications …

Rolling

Two Major types of rolling processes

-• Flat rolling – to produce Plates

and Sheets of metal

• Profile (or Shape) rolling – to

produce all other possible shapes

Rolling

(process in which new equiaxed / strain free grains are formed, replacing old grains)

Carried out a room temperature;

better finish, mechanical properties

Two or more layers of metals are rolled together; improves

productivity (Al foil)

Rolling

in the sheets are removed (sheets undergo yield point elongation

0.5 to 1.5% reduction in thickness

To improve flatness Sheets may not be sufficiently flat due to various reasons such as roll gap and process parameters

section; various lengths

Thinner rectangular cross section; various lengths (thinner version of a bloom)

Smaller square cross sections;

Continuous Flat Rolling Process – A Schematic

Casting

ME 4563 Fall ’05 Dr S Haran

Slabs

Hot Strip Mill

Directly to Rolling

Rods

Tube Rounds

Wire & wire products

Seamless Tubes

Cold-drawn Bars

Flat Rolling Process - Products

Rolling

ME 4563 Fall ’05 Dr S Haran

Blooms

Structural Shapes

ME 4563 Fall ’05 Dr S Haran

cross sections, channel sections, I-beams, and rails

sets of rolls specially designed for these

applications

takes experience and care, as material failure is

fairly frequent in case of poor designs

Various other structural sections, such as channels and I-beams, are also rolled by this kind of process

Shape Rolling

Shape Rolling

A Shape Rolling

ME 4563 Fall ’05 Dr S Haran

Rolling out a V-Section

Shape Rolling

A Shape Rolling

ring is expanded into a

larger diameter thinner

ring

the rolling and the other

to support and guide

ME 4563 Fall ’05 Dr S Haran

ring-rolling operation Thickness reduction results in an increase in the part diameter

that can be formed by ring rolling

Ring Rolling

ƒ Variety of cross sections can be

ring-rolled with shaped rolls

ƒ Short production runs, material

savings, closer tolerances, favorable

grain flow

ƒ Rockets, turbines, gearwheel rims,

bearing races, etc.

B Ring Rolling

Ring Rolling

screws, bolts

regular thread cutting process

Thread Rolling

by these processes, at high rates of

cuts through the

grains of the metal,

the rolling of threads

causes improved

strength, because of

cold working and

Thread Rolling

ME 4563 Fall ’05 Dr S Haran

C Thread Rolling – Material is not wasted

Cut Thread Rolled Thread

Thread Rolling

C Thread Rolling

Thread Rolling

ME 4563 Fall ’05 Dr S Haran

wall seamless tubing

tensile stress develops in the center of the rod

D Rotary Tube Piercing

using a round eraser and rolling it on the table !

Rolling

cavity begins to form in the center

required diameter, the hole size is expanded

the mandrel are used

must be used for this

process

D Rotary Tube Piercing

Rolling

ME 4563 Fall ’05 Dr S Haran

Rolls

Rotary Tube Piercing – A Schematic

Rolls are normally arranged in a skewed manner (axes at an angle)

Rotary Tube piercing is

also called the

“Mannesmann process”

Rolling

Rotary Tube Piercing

Rolling

Szekely, Scientific American, July 1987.

Rolling – Spray Casting

ME 4563 Fall ’05 Dr S Haran

Rolling Mills

using diverse rolls arrangement

same, except the process parameters, roll materials,

lubricants and cooling system

Rolling Mills

Rolling Mill

Rolling Mills

mill used as a two stage rolling process

Rolling Mills

rolls lower roll forces and power,

as well as spreading of sheets;

easily replaced when broken or worn

more; have to be supported by other rolls as in the four-high and cluster arrangements

suited for cold rolling

The stiffness of the

housing, the rolls, and

the roll bearings are

ME 4563 Fall ’05 Dr S Haran

Schematic illustration of various roll arrangements: (a) two-high; (b)

three- high; (c) four-high; (d) cluster (Sendzimir) mill

Roll Arrangements

Rolling Mills

Tandem Mills

stands to smaller gauges with each pass

Roll Arrangements

Rolling Mills

ƒ Strength and Resistance to wear

ƒ Made of Cast iron, cast steel & forged steel

Tungsten carbide for smaller diameter rolls

ƒ Forged steel among the best materials – greater

strength, stiffness and toughness

ƒ Rolls are ground to fine finish for cold rolling

Polished for special applications

ƒ Rolls for hot and cold rolling are not

interchangeable

Rolling Mills - Rolls

ME 4563 Fall ’05 Dr S Haran

• Because of forces acting on them, rolls

undergo geometric changes

• As a result, there are problems with the

resulting rolled product

Problems with Rolls in Practice

Rolling

• The forces result from the

resistance of the material to

deformation, and cause

ME 4563 Fall ’05 Dr S Haran

the size of the equipment required

Roll Force Compensation

•Lowering the friction

•Friction is lower for cold rolling than for hot With effective

lubricant:

Cold Rolling - friction = 0.2 to 0.3 Hot Rolling - friction = 0.2 to 0.7

•Choose the right material for rolls

•Modify the lubrication parameters ( Type, Amount, Method of

Application)

• Reduce the Roll Radius

•This also lowers the energy

•Reducing the roll radius shortens the length of contact

•However, rolls will be more prone to deflection (see defects)

Reducing the Roll Force

ME 4563 Fall ’05 Dr S Haran

• Use Lower Reductions

This may increase the number of required passes, thus requiring:

More time Possibly more equipment requirements Possibly more energy

• Increase the Work piece Temperature

This lowers the yield point This allow recrystallization and reduces strain hardening

• Apply Tension

Either side will reduce the force

If applied at the entrance, termed back tension

If applied at the exit, termed front tension

Reducing the Roll Force

Lubricants …

may be used sometimes

lubricants such as oils and fatty acids

Rolling Mills - Lubricants

ME 4563 Fall ’05 Dr S Haran

other processes (extrusion, drawing, machining, etc.)

•variation in thickness, poor surface finish, high levels of

scrap

frequencies of the system

width of job, speed of rolling, increasing damping, etc

Rolling – Vibration and Chatter

• The forces of rolling will flatten the rollers elastically –

similar to tires on the road surface (Roll radius,

Modulus of elasticity, roll forces)

area

increase

elasticity

Rolling – Problems with Rolls

Roll Flattening

ME 4563 Fall ’05 Dr S Haran

Roll Deflection

• Roll forces tend to bend the rolls: leading to a strip

that is thicker at the center than at the edges

• Solution: Grind the rolls such that the diameter at

the center is slightly larger than at the edges

• This is known as giving them CAMBER– In

practice, this would generally be around 0.25 mm

or less on the radius Camber is only correct for a

• given load on the roll

• thickness of the strip

Hence normally done for a large batch, due to

economic reasons

Rolling – Problems with Rolls

(a) Bending of straight cylindrical rolls, caused by the roll force

(b) Bending of rolls ground with camber, producing a strip with uniform thickness

Roll

Deflection

Rolling – Problems with Rolls

ME 4563 Fall ’05 Dr S Haran

Spreading

width to increase considerably

Rolling – Problems with Rolls

Successful Rolling depends upon various factors

including material properties, process variables, and

lubrication

prior treatment and working of the material

Defects due to Rolling

ME 4563 Fall ’05 Dr S Haran

the rolled product

caused by bending of the rolls

are restrained from expanding freely, they

“buckle”

low ductility and barreling (in the edges & middle)

material deformation during rolling and defects in

the original cast ingot itself

Defects due to Rolling

ME 4563 Fall ’05 Dr S Haran

Defects due to Rolling

Defects due to Rolling

ME 4563 Fall ’05 Dr S Haran

Taking care of wavy edges

Defects due to Rolling

Defects due to Rolling

Taking care of wavy

edges

ME 4563 Fall ’05 Dr S Haran

Hot Rolling

• Hot Rolling is done between 100 and 200 degrees F

above the re-crystallization temperature for

most metals

• Hot rolling gives a final product with a fine grain

size and little strain hardening

• This provides increased material yield strength

• This also provides increased ductility, by breaking

grain boundaries, closing internal defects and boundaries, closing internal defects and

breaking inclusions

Hot Rolling

™ Large deformations possible

™ Breaks up the cast structure into preferable forms

™ Rolls need to be cooled

™ Material handling difficult

™ Personnel must be protected from heat

Hot Rolling

Disadvantages

ME 4563 Fall ’05 Dr S Haran

• Causes changes in the grain structure of cast or of

large-grain wrought metals

• Hot rolling is an effective way to reduce grain size in

metals, for improved strength and ductility

Hot Rolling

Hot Rolling – some points

•If there are non-metallic inclusions, they will not recrystallizeand

may impart directionality

may cause the same effect

complicated shapes (I-beam, Flanges do this)

high-temperature oxides (termed mill scale), tolerances can

Hot Rolling Mill Design

• Basically the same as cold mills, but designed for

higher temperatures and are reversible

• Typically have edging rolls to control thickness

• Often have a warm mill at the end

ME 4563 Fall ’05 Dr S Haran

Hot Rolling Mill Design

• Basically the same as cold mills, but designed for

higher temperatures and are reversible

• Typically have edging rolls to control thickness

• Often have a warm mill at the end

Slab

Hot mill

Roller table

Warm mill Take-up roll

Hot Rolling Process - Example

• Start with 12in thick slab, 20 feet long

• Hot mill converts to 2 in thick slab, 120 feet long with

11 passes (six forward and 5 reverse)

• Front end sheared off to eliminate alligatoring

• Warm mill converts this to roll of sheet 0.25 in thick,

960 feet long

• Rolls more easily handled by overhead cranes to

transport it to cold mills than 1000 ft long slabs

ME 4563 Fall ’05 Dr S Haran

Overall Process for Sheet and Plate

• Cast ingot

• Scalp (not always)

• Reheat and homogenize (uniform)

• Hot roll in reversing mill to ~1-2 in thick

• Cut off ends (alligators)

• Warm roll to 0.1-2 in thick

• Heat treat to recrystallize (sometimes)

• Cool

• Cold roll to finish thickness

• Heat treat (where appropriate)

• Results in material which must be further worked

Hot Rolling

ME 4563 Fall ’05 Dr S Haran

Hot Rolling

Rolling Mill

Hot Rolling

Casting Line rolling out billets

Hot Rolling

ME 4563 Fall ’05 Dr S Haran

Continuous Rolling of Billets

Hot Rolling

Billets Casting

Hot Rolling

Hot Rolling

ME 4563 Fall ’05 Dr S Haran

Hot Cutting the rolled Rails

Hot Rolling

Section of the Mill where rolling is taking place

Hot Rolling

ME 4563 Fall ’05 Dr S Haran

Profiled coated steel sheets

color-U-section long bars

Rolling – Further Processing

Wire rods in

a coil

U-section long bars

Rolling – Further Processing

series of rolling mill stands which apply pressure to the strip and

progressively reduce its thickness -down to as low as 0.15 mm down to as low as 0.15 mm

quality of the steel

reduced strip is subsequently annealed: a process of very

carefully controlled heating and cooling to soften it

Cold Rolling

ME 4563 Fall ’05 Dr S Haran

• Advantages

– Deformations of ~ 50 to 80%

– Work hardening increases strength

– Excellent surface finish

– Excellent tolerance on thickness and shape

• Disadvantages

– High forces

– Small reductions give rise to surface stresses and

non-uniform stress distributions

– ?

Cold Rolling

• Input and output coils with tensioning control

• Shape meters consisting of segmented coils

measuring force

• Tandem mills allow multiple reductions in one

operation

• Laser inspection systems for surface finish inspection

becoming popular (especially for canstock)

• Surface condition of rolls critical to maintain surface

ME 4563 Fall ’05 Dr S Haran

Cold Rolling - Schematic

Cold Rolling

such as skin-rolled, quarter hard, half hard, full hard

depending on how much cold work has been performed

results in a surface that is smooth and the yield point

phenomenon excessive stretching and wrinkling in

subsequent operations, is eliminated

and stretching operations

Cold Rolling

ME 4563 Fall ’05 Dr S Haran

material properties assume different properties along the

grain orientation

decreases

direction of rolling) on itself without fracturing Half hard

involving great amounts of bending and deformation,

without fracturing

Cold Rolling

of dies which progressively reduce the rod's circumference to

produce wire

tensile strength

support the world's largest suspension bridges

Cold Drawing

Cold Rolling

ME 4563 Fall ’05 Dr S Haran

Independent Variables for Rolling

For a given rolling set-up:

• Speed of exiting strip

• Final strip temperature

• Maximum draft

• Roll strip contact length

• Sheet or plate shape

Dependent Variables for Rolling

ME 4563 Fall ’05 Dr S Haran

Control of Overall Properties

• Thickness

of the mill frame

• Width (due to spreading of slab)

• Length

• Mechanical Properties

rolling parameters (reduction, temperature, etc)

• Tolerance

– Thickness

– Gauges measure thickness

of input and output

– Computers control roll gap

– Camber

– Special segmented tension

meters measure "shape"

– Computers bend the rolls