Plastic- Case Study 3: Rod Rolling Flashcards
Order of stages in rod mill process route
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
Difference between billets and blooms
Billets are square or circular
Blooms and rectangular
Reheat furnace
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
Breakdown mill
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
Diverter and tunnel furnace
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
Roughing mills
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
Intermediate mills
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
Loopers and prefinishing mills
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
No twist mill
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
Controlled water and air cooling
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.
Why is the rod deposited like a big slinky in stelmor controlled cooling?
Improved mechanical and microstructural properties.
Improved round wrap tensile properties
Coil reforming
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.
Quality control
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
Range of grades and properties produced by rod mills
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.
Process and product factors affecting as-rolled rod tensile properties
Process: reheat temperature, laying temperature, laying pattern, cooling rate.
Product: composition, rod diameter.
Environment and plant factors that affect as-rolled rod tensile properties
Environment: ambient temperature (different in winter and summer).
Plant: slot condition, conveyor speed, fan speed.
For a given steel composition which two main factors controlled by the rod mill affect the microstructure and tensile properties? What affects these?
Prior austenite grain size- lay temperature.
Cooling rate through transformation: fan speed, conveyor speed, laying pattern, slot condition, ambient temperature, rod diameter.
Effect of prior austenite grain size
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?
What do applications of low carbon wire rod require?
Good drawability/formability.
This is ability to be deformed without needing intermediate heat treatment to restore ductility
How is low carbon wire rod strengthened?
Product strength imparted by cold work during the forming process.
Wire drawing and cold heading
Ways of improving drawability/formability of low carbon wire rod
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)
What is added to medium carbon wire rod and what does it do?
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%
Applications of medium carbon wire rod
Springs for bedding and seating.
Involve high drawing reductions to produce the wire. Wire breaks common unless microstructure optimised
Spring steel features and requirements and difficulties
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.
