13- Metal-Rolling Processes And Equipment

Front 1

forming

Back 1

indicates changing the shape of an existing solid body

Front 2

shaping

Back 2

involves molding and casting of soft or molten materials, finished product is usually at or near the desired shape, forcing molten polymer into a mold and letting it solidify

Front 3

continuous products

Back 3

made by rolling, extrusion, drawing processes

Front 4

discrete products

Back 4

made by forging, forming, ...

Front 5

ingot

Back 5

metal cast into a block shape for convenience, to be further processed

Front 6

how are cast structures converted to wrought structures?

Back 6

by plastic-deformation

Front 7

sinter

Back 7

heat without melting

Front 8

forming and shaping processes

Back 8

rolling (flat, shape), forging, extrusion, drawing, sheet-metal forming, powder metallurgy, processing of plastics and composite materials, forming and shaping of ceramics

Front 9

flat rolling

Back 9

high speed foil production

Front 10

shape rolling

Back 10

high speed production of i-beams and rails

Front 11

forging

Back 11

production of discrete parts using dies

Front 12

extrusion

Back 12

production of long, constant shapes; cut into desired lengths

Front 13

drawing

Back 13

production of long rod and wire

Front 14

sheet-metal forming

Back 14

production of simple or complex shapes with thin walls

Front 15

powder metallurgy

Back 15

production of simple or complex shape by compacting and sintering metal powders

Front 16

processing of plastics and composite materials

Back 16

production of a wide variety of continuous or discrete products by extrusion, molding, casting, and fabricating processes

Front 17

forming and shaping of ceramics

Back 17

production of discrete products by various shaping, dring, firing processes

Front 18

rolling

Back 18

process of reducing the thickness or changing the cross section of a long workpiece by compressive forces applied through a set of rolls

Front 19

which forming and shaping process accounts for 90% of all metals produced by metalworking processes?

Back 19

rolling

Front 20

hot rolling

Back 20

first stage of rolling, rolling carried out at high temps, coarse-grained, brittle, and porous structure of ingot is broken down into a wrought structure with finer grain size and better properties

Front 21

cold rolling

Back 21

subsequently, rolling carried out at room temp
-good: improves strength and hardness and surface finish
-bad: causes anisotropy

Front 22

why does cold rolling require more energy than hot rolling?

Back 22

because of higher strength of the material at room temp

Front 23

anisotropic

Back 23

has different properties when measured in different directions

Front 24

plates

Back 24

high thickness (ship hulls, boilers, bridges, ...), more than 6mm

Front 25

sheets

Back 25

low thickness (automobile bodies), less than 6mm

Front 26

foil

Back 26

used for wrapping candy, thickness of .008mm

Front 27

flat-rolling process

Back 27

metal strip with thickness no enters roll gap and is reduced to thickness nf by rotating rolls (surface speed of rolls =Vr), velocity of strip increases from Vo (enters roll gap) to Vf (exits roll gap, highest)

Front 28

roll gap

Back 28

space between rolls

Front 29

neutral point / no-slip point

Back 29

-point along contact length where the velocity of the strip is the same as the roll
-left of neutral point, roll is faster
-right of "", strip is faster

Front 30

maximum draft (eq)

Back 30

different between initial and final thickness, ho-hf, function of roll radius (R) and coeff. of fric (u) s.t. ho-hf=(u^2)R

Front 31

roll force (eq)

Back 31

F=Lw(Yavg), F=roll force, L=roll-strip contact length, w=width of strip, Yavg=avg true stress of strip in roll gap

Front 32

how do you modify roll force eq to get ACTUAL roll force?

Back 32

add 20%

Front 33

torque

Back 33

F*a, a=L/2

Front 34

Power (kW)

Back 34

=(2piFLN)/60,000, N=revolutions/min of roll, F=newtons, L=meters

Front 35

Power (hp)

Back 35

=(2piFLN)/33,000, F=lbs, L=ft

Front 36

roll stand

Back 36

includes housing, chocks, bearings

Front 37

ways to reduce roll forces

Back 37

reduce friction at roll-workpiece interface, use smaller diameter rolls to reduce contact area, take smaller reductions per pass to reduce contact area, rolling at elevated temps to lower strength of the material, apply front or back tension to strip

Front 38

back tension

Back 38

tension at the entry zone, applied to sheet by applying a braking action to the reel that supplies the sheet into the roll gap

Front 39

front tension

Back 39

tension at the exit zone, applied to sheet by increasing rotational speed of the take-up reel

Front 40

steckel rolling

Back 40

process carried out by front tension only, with no power supplied to the rolls

Front 41

roll bending

Back 41

tendency of rolls to bend during rolling, caused by the difference between forces required to bend strip at center and edges

Front 42

crown

Back 42

tendency of rolled strip to be thicker at center than edges, result of roll bending

Front 43

camber

Back 43

way to counteract crown by grinding rolls in such a way that their diameter at center is slightly larger than at edges, so that strip has contant uniform thickness

Front 44

what is the difference between the radius of the max camber point and and the edges of the roll

Back 44

radius of max camber point is generally .25 mm greater than that of the edges

Front 45

thermal camber

Back 45

tendency of rolls to become barrel shaped because of the heat generated by plastic deformation during rolling

Front 46

what happens to the width of the strip when rolling plates and sheets have high width-to-thickness ratios?

Back 46

the width of the strip remains effectively constant

Front 47

spreading

Back 47

with small width-to-thickness ratios, width increases significantly as it passes through the rolls, like rolling dough with a rolling pin

Front 48

spreading increases with:

Back 48

a. decreasing width-to-thickness ratios
b. increasing friction
c. decreasing ratio of the roll radius to the strip thickness

Front 49

how do you prevent spreading?

Back 49

using additional rolls in contact with the edges of the rolled product in the roll gap (edger mills)

Front 50

chatter

Back 50

-self-excited vibration
-in rolling it leads to periodic variations in the thickness of the rolled sheet and in its surface finish, and consequently, leads to excessive scrap

Front 51

tandem mills

Back 51

where chatter in rolling have been found to occur predominantly

Front 52

how much does chatter slow rolling?

Back 52

by 50%

Front 53

cause of chatter?

Back 53

interaction between structural dynamics of the mill stand and the dynamics of the rolling operation

Front 54

ways to reduce chatter

Back 54

increasing dist between the stands of the rolling mill, increasing the strip width, decreasing the reduction per pass (draft), increasing roll radius, increasing strip-roll friction, incorporating dampers in the roll supports

Front 55

cast structure

Back 55

dendritic with coarse and nonuniform grains, brittle and porous

Front 56

dendritic

Back 56

having a branching structure similar to a tree

Front 57

flat rolling process

Back 57

1. break down material by hot rolling, converts cast structure to wrought structure-->resulting wrought structure is bloom, slab, or billet
2. cold rolling

Front 58

wrought structure

Back 58

finer grains and better ductility by breaking up brittle grain boundaries and closing up internal defects (porosity)

Front 59

bloom

Back 59

square cross section, becomes i-beams and railroad rails

Front 60

slab

Back 60

rectangular cross section, becomes plates and sheets

Front 61

billet

Back 61

square with smaller cross section than bloom, rods and bars

Front 62

conditioned

Back 62

prepared for subsequent operation; surface of bloom, slab, and billet is usually conditioned before rolling

Front 63

scarfing

Back 63

conditioning by torch

Front 64

pickling

Back 64

acid etching, by blasting with water or grinding

Front 65

pack rolling

Back 65

flat-rolling operation in which 2 or more layers of metal are rolled together to increase productivity
ex: foil is pack rolled in 2 layers, only the top and bottom outer layers have been in contact with rolls, foil-to-foil side is matte, foil-to-roll side is shiny

Front 66

yield-point elongation

Back 66

phenomenon that causes surface irregularities called stretcher strains or luder's bands, happens to rolled mild steel when subsequently stretched during sheet-forming operations

Front 67

how do you correct yield-point elongation?

Back 67

sheet metal undergoes a final light pass of 1% reduction known as temper rolling or skin pass

Front 68

leveling rolls

Back 68

to improve flatness, rolled strip goes through series of "", the workpiece is flexed in opposite directions as it passes through the sets of rollers

Front 69

(T/F) Each roller is usually powered individually by a separate motor.

Back 69

T

Front 70

surface defects

Back 70

scale, rust, scratches, gouges, pits, cracks

Front 71

wavy edges

Back 71

result of roll bending, strip is thinner along edges than at its enter causing the edges to elongate more than the center, the edges buckle because they are constrained by the central region from expanding freely in the longitudinal (rolling) direction

Front 72

alligatoring

Back 72

complex phenomenon and typically is caused by nonuniform bulk deformation of the billet during rolling or by the presence of defects in the original cast material

Front 73

residual stresses in rolled metals

Back 73

small diameter rolls: compressive "" on surfaces and tensile "" in bulk
large diameter rolls: opposite

Front 74

gage number

Back 74

number identifying the thickness of a sheet, smaller=thicker

Front 75

two-high rolling mills

Back 75

used for hot rolling in initial breakdown passes on cast ingots or in continuous casting with roll diameters from .6m to 1.4m

Front 76

three-high mill

Back 76

direction of material movement is reversed after each pass, using elevator mechanism and various manipulators

Front 77

four-high mills and cluster mills (sendzimir or Z mill)

Back 77

based on the principle that small-diameter rolls lower roll forces (why? because of small roll-strip contact area) and power requirements and reduce spreading, also smaller rolls are cheaper to replace

Front 78

tandem rolling

Back 78

strip is rolled continuously through a number of stands to thinner gages with each pass, speed and thickness of strip is critical

Front 79

stand

Back 79

consists of a set of rolls with its own housing and controls

Front 80

train

Back 80

group of stands

Front 81

heat checking

Back 81

cracking from thermal cycling, results from using rolls made for cold rolling for hot rolling

Front 82

spalling

Back 82

cracking or flaking of surface layers

Front 83

why does the bottom surface of an aluminum beverage can have longitudinal scratches on it?

Back 83

because the surface of the can is a replica of the surface finish of the roll, which is produced by grinding, makes it easy to see the rolling direction of the original aluminum sheet

Front 84

are lubricants used in hot rolling?

Back 84

hot rolling of FERROUS alloys is carried out without lubricants although graphite may be used, NONFERROUS alloys are hot rolled with various compounded oils, emulsions, and fatty acids

Front 85

are lubricants used in cold rolling?

Back 85

cold rolling is carried out with water-soluble oils or low-viscosity lubricants, such as mineral oil, emulsions, paraffin, fatty oils

Front 86

shape rolling

Back 86

forms straight and long structural shapes (i-beams, rails), the stock goes through a set of specially designed rolls

Front 87

cold shape rolling

Back 87

can be done with the starting materials in the shape of wire with various cross sections, roll-pass design requires considerable experience

Front 88

roll-pass design

Back 88

the design of a series of rolls, requires considerable experience to avoid external and internal defects, hold dimensional tolerances, and reduce roll wear

Front 89

roll forging (also cross rolling)

Back 89

the cross section of a round bar is shaped by passing it through a pair of rolls with profiled grooves, used to produce tapered shafts and leaf springs, table knives

Front 90

skew rolling

Back 90

similar to roll forging, used for making ball bearings, round wire is fed into the roll gap and spherical blanks are formed by the action of rotating rolls

Front 91

ring rolling

Back 91

a thick ring is expanded into a large-diameter thinner one by placing it between 2 rolls and driving 1 while the other is idle, thickness is reduced by bringing the rolls closer together as they rotate

Front 92

thread rolling

Back 92

cold-forming process by which straight or tapered threads are formed on round rods or wire, threads are formed with each stroke of a pair of flat reciprocating dies, makes screws and bolts

Front 93

thread rolling advantages

Back 93

threads with good strength and without loss of material, smooth surface finish, induces compressive residual stresses on surfaces (thus improving fatigue life)

Front 94

internal thread rolling

Back 94

carried out with a fluteless forming tap, similar to external thread rolling, produces internal threads with good strength

Front 95

lubrication in thread rolling

Back 95

important to obtaining good surface finish and surface integrity

Front 96

rotary tube piercing

Back 96

also mannesmann process, hot-working operation for making long, thick-walled seamless pipe and tubing

Front 97

tube rolling

Back 97

used to reduce diameter and thickness of pipes and tubing

Front 98

integrated mills

Back 98

large facilities that involve complete integration of activities, from production of hot metal in a blast furnace to casting and rolling of finished products

Front 99

minimills

Back 99

scrap metal is:
a. melted in electric-arc furnaces
b. cast continuously
c. rolled directly into specific lines of products
each minimill makes 1 type of product (rod, bar), scrap metal is obtained locally from old machinery (cars)