Plastic- Case Study 3: Rod Rolling

Question 1

Order of stages in rod mill process route

Answer 1

Reheat furnace
Breakdown mill
Diverter and tunnel furnace
Roughing and intermediate mills
Loopers and prefinishing mills
No twist mills
Water boxes and stelmor cooling
Coil reforming
Quality control

Question 2

Difference between billets and blooms

Answer 2

Billets are square or circular
Blooms and rectangular

Question 3

Reheat furnace

Answer 3

Reheat billets and blooms to 1150-1200C in preparation for the rolling process. Fuel is mixed enhanced gas generated on site. Output rate 200 tonnes per hour

Question 4

Breakdown mill

Answer 4

Reduce stock size of blooms and billets to 123mm rounds. High pressure water descaling before rolling. Blooms cropped and sheared into 2 lengths. Billets cropped

Question 5

Diverter and tunnel furnace

Answer 5

Diverts stock from breakdown mill into one of 4 strands in the tunnel furnace. Tunnel furnace retains heat in the stock to maintain a consistent temperature for feeding into the roughing mill

Question 6

Roughing mills

Answer 6

Roughing mill stands 1 to 7 reduce stock size from 123mm rounds to 44x41.5mm. 4 strands with different possible profile types (circle, oval, square, rectangle). Stock speed at stand 7 is 1.3 m/s

Question 7

Intermediate mills

Answer 7

Intermediate mill stands 8 to 13 reduce stock size from 44x41.5mm to 23mm rounds. 4 strands with different possible profile types (circle, oval, square). Stock speed at stand 13 is 4.7m/s

Question 8

Loopers and prefinishing mills

Answer 8

Stands 14 to 15 reduce stock size from 23mm rounds to 16.9x17.3mm. Loopers balance tension and milk speed. One prefinishing mill per stand. Profile type of circle or oval. Stock speed at stand 15 is 6.6m/s

Question 9

No twist mill

Answer 9

Stands 16 to 25 reduce stock size from 16.9x17.3mm to finished rod sizes between 5.5 and 15mm diameter. One no twist mill per stand. Profile types of circle or oval. Finished speed of rod ranges between 13.5 to 68m/s

Question 10

Controlled water and air cooling

Answer 10

Water boxes: 2 boxes per strand, dynamic temperature control via feedback from laying head pyrometer.
Stelmor conveyors: after water boxes, variable speed conveyors with 6 cooling zones with individual fans, laying temperatures between 760 and 950C, deposited like a big slinky.
Here is where microstructure and properties are defined.

Question 11

Why is the rod deposited like a big slinky in stelmor controlled cooling?

Answer 11

Improved mechanical and microstructural properties.
Improved round wrap tensile properties

Question 12

Coil reforming

Answer 12

Rod goes from stelmor conveyors into vertical reform tubs which form the rod into coils. Ring distributor ensures good presentation of coils. Tipping distended puts coils into blocks.

Question 13

Quality control

Answer 13

Coils are inspected and trimmed and samples are taken for testing in the laboratory. 4 automatic compaction machines compact and strap the coils. Coils are weighed and labelled with bar codes. Robot unloaded offloads the coils from the conveyor hooks. Coils then dispatched to on-site warehousing facilities

Question 14

Range of grades and properties produced by rod mills

Answer 14

Carbon contents from 0.005 to 1% (1% for bridge wire).
As-rolled UTS from 300 to 1350MPa.
Final wire UTS in excess of 4000MPa.

Question 15

Process and product factors affecting as-rolled rod tensile properties

Answer 15

Process: reheat temperature, laying temperature, laying pattern, cooling rate.
Product: composition, rod diameter.

Question 16

Environment and plant factors that affect as-rolled rod tensile properties

Answer 16

Environment: ambient temperature (different in winter and summer).
Plant: slot condition, conveyor speed, fan speed.

Question 17

For a given steel composition which two main factors controlled by the rod mill affect the microstructure and tensile properties? What affects these?

Answer 17

Prior austenite grain size- lay temperature.
Cooling rate through transformation: fan speed, conveyor speed, laying pattern, slot condition, ambient temperature, rod diameter.

Question 18

Effect of prior austenite grain size

Answer 18

Coarser prior austenite grains are more stable than finer.
Transformation to pearlite occurs at lower temperature for coarser.
Coarser leads to reduced tensile strength.
Coarser leads to reduced tensile ductility?

Question 19

What do applications of low carbon wire rod require?

Answer 19

Good drawability/formability.
This is ability to be deformed without needing intermediate heat treatment to restore ductility

Question 20

How is low carbon wire rod strengthened?

Answer 20

Product strength imparted by cold work during the forming process.
Wire drawing and cold heading

Question 21

Ways of improving drawability/formability of low carbon wire rod

Answer 21

Reduce %C (pearlite content) via vacuum decarburisation.
Remove interstitials (slow cooling (C), B additions (BN).
Increase grain size (process route and super-stoichiometric B additions).
Minimise residuals (low residual scrap).
Cleanness (inclusion reduction/modification via steelmaking)

Question 22

What is added to medium carbon wire rod and what does it do?

Answer 22

Addition of excess boron to increase hardenability. Suppressed ferrite formation. Microstructural refinement of the pearlite. Increases tensile strength and ductility.
0.005-0.007% B increases UTS by 70N/mm^2, increases tensile ductility by 10% reduction of area, increases % pearlite by 25%

Question 23

Applications of medium carbon wire rod

Answer 23

Springs for bedding and seating.
Involve high drawing reductions to produce the wire. Wire breaks common unless microstructure optimised

Question 24

Spring steel features and requirements and difficulties

Answer 24

Very hardenable. High risk of martensite in small diameter as-rolled rod. Ferrite/pearlite microstructure required. Demanding target for 5.5mm rod.
Required control of prior austenite grain size and rod mill cooling rate. Still air cooling rate is greater than the continuous cooling rate for martensite. Need slow cooling to bunch waps and need conveyor hoods.

Question 25

How does prior austenite grain size affect CCR for martensite?

Answer 25

Graph of prior γ grain size vs CCR for martensite has line from top left to bottom right. Left of line is pearlite and ferrite. Right is pearlite, ferrite, bainite and martensite. Means CCR for martensite is less for larger prior γ grain size

Question 26

What happens in spring steel when cooling rate is too slow?

Answer 26

Low strength and low ductility. Coarse pearlite. Also cementite forms on prior austenite GBs before pearlite transformation. This is a very brittle phase and deleterious for rod ductility and wire drawability.

Question 27

What happens in spring steel when cooling rate is too fast?

Answer 27

High strength and low ductility. Formation of martensite pools reduce ductility.

Question 28

Optimum cooling for spring steel

Answer 28

Between slow and fast cooling on the CCT diagram. Crosses pearlite start and finish lines lower than slow cooling. Get high strength and high ductility.

Question 29

Groups of alloy additions to steel that have similar effects

Answer 29

C, Mn, Cr, free V and free B.
V as carbide.
Si, P, N.
Al, V as nitride.
P, S, N.

Question 30

Effects of C, Mn, Cr, free V and B

Answer 30

Pearlite hardenability
Effect on transformation temperature
Microstructural refinement

Question 31

Effect of V as carbide

Answer 31

Precipitation strengthening

Question 32

Effect of Si, P, N

Answer 32

Solid solution strengthening

Question 33

Effect if Al, V as nitride

Answer 33

Potential for grain refinement

Question 34

Effect of P, N, S

Answer 34

Detrimental towards ductility