Chapter 9 – Steel Products

Report
Chapter 12 – Steel Products
• Key: carbon content:
– Steel – alloy consisting mostly of iron with a little
carbon (0.05% - 2.04% by weight)
– Also have:
• Iron = iron-carbon alloy with less than 0.005% carbon.
• Cast iron = carbon content between 2.1% - 4.0%
• Wrought iron – contains 1 – 3% by weight of slag in the form of
particles elongated in one direction – more rust resistant than steel
and welds better
Brief History:
• Iron age (12th century BC) (mostly wrought iron) – weapons made
with inefficient smelting methods. The best weapons? When iron
combined with carbon!
• Became more common after more efficient production methods
were devised in the 17th century.
• With invention of the Bessemer process in the mid-19th century,
steel became relatively inexpensive, easily mass-produced and high
quality.
• Blast Furnace then Bessemer Furnace
Low cost method for removing
carbon and impurities
The “abc’s” of Steel Making:
• Raw Material:
– Carbon in the form of coke
– Iron ore (Fe2O3)
– Limestone (CaCO3)
– Air (lots of it!!)
The “abc’s” of Steel Making:
• Coke
– Solid residue product from the destructive
distillation of coal.
– About 80 to 95% C.
– Made by heating black coal in small ovens at
300 C for 24 hours in a coke plant.
The “abc’s” of Steel Making:
• The iron ore
– Consists of oxides in nature of iron and
oxygen
• Primarily magnetite (Fe3O4) or hematite (Fe2O3)
• The blast furnace basically separates the iron from
the oxygen in a reduction process
– Mined primarily in Australia, Brazil and
Canada.
The “abc’s” of Steel Making:
• The limestone
– Acts as a flux – converts impurities in the ore
into a fuseable slag
The “abc’s” of Steel Making:
• Air
– Preheated by fuel gas from the coke ovens to
about 1000 C.
– Delivered to the blast furnace at 6,000 m3/min
– Passes through furnace and burns the coke to
produce heat required and also generates the
carbon monoxide.
The “abc’s” of Steel Making:
• Typical blast furnace:
– 1.6 tons of iron ore
– 0.18 tons of limestone
– 0.6 tons of coke
– 2 -3 tons of preheated air
The “abc’s” of Steel Making:
• Step 1 – The Blast Furnace:
– Stands 300 feet tall
– Designed to run continuously for 4 -5 years
before being relined.
– Heat generated by burning coke in the
preheated air.
– Coke acts as reducing agent and changes to
carbon monoxide (the reducing agent) which
removes the oxygen from the iron oxide.
The “abc’s” of Steel Making:
• Step 1 – The Blast Furnace:
– Two important chemical reactions:
• Oxidation of the carbon from coke:
2C  O2  2CO
• Reduction of iron ore:
Fe3O3  3CO  2Fe  3CO2
The “abc’s” of Steel Making:
• Step 1 – The Blast Furnace:
– Four primary zones – the bottom zone (zone
4) reaches temperature of 1800 C – this is
where iron is tapped off.
– The top zone (zone 1) – where coke is burned
and moisture driven off.
– Zone 2 – slag coagulates and is removed.
The “abc’s” of Steel Making:
• Step 1 – The Blast Furnace:
– Products from the blast furnace:
• Iron transported in steel shelled ladles
• Pig iron (brittle w/ 4% carbon)
Step 2: Manufacturing of Steel
from Iron
• Two common methods:
– Bessemer Furnace = Ingots = molten steel
poured into molds to create ingots which then
go through forging press and roughing mill to
create billet, bloom or slab, OR:
– Continuous cast – continuous process to
again create a billet, bloom, slab or “as cast
semis”
• Step 2 – The Bessemer converter:
– Used for REFINEMENT:
• Takes pig iron with high C content and removes C.
• Removes impurities such as Si and Mn (via
oxides)
– Much smaller furnace (vs. Blast furnace)
– Lowered cost of steel making
– Poured into molds to form ingots
Replaced by basic oxygen process
and electric arc furnace.
Steel Ingots (after step 2)
Figure 9-12: processing of refined steel into products.
F 9-13 – The whole spectrum of steel products!
Optional Step 2 (directly
from blast furnace)
Step 2 w/ Continuous Casting
• Overcomes the ingot related difficulties of:
– Piping and entrapped slag
– More cost effective
• Process
– molten metal continuously flows from the ladle
into a tundish
– through a bottomless,water-cooled mold
– temp controlled water spray  not fully cooled
– Straightened, reheated, sized, and cut-off
– Advantages
– Common for Structural Shapes
Continuous Casting
Steel Types
(Brief
Overview)
Much more
detail in
Chapter 14
Cast Iron Types (remember
carbon > 2%)
•
•
•
•
•
Gray iron
Ductile iron
Austempered ductile iron
White iron
Malleable iron
• Much more will be said about cast irons
later!
HRS vs. CRS
• HRS
– AKA hot finishing – ingots
or continuous cast shapes
rolled in the “HOT”
condition to a smaller
shape.
– Since hot, grains
recrystallize without
material getting harder!
– Dislocations are annihilated
(recall dislocations impede
slip motion).
• HRS Characterized by:
– Extremely ductile (i.e. %
elongation 20 to 30%)
– Moderate strength (Su
approx 60 – 75 ksi for
1020)
– Rough surface finish –
black scale left on surface.
HRS vs. CRS
• CRS
– AKA cold finishing – coil of
HRS rolled through a
series of rolling mills AT
ROOM TEMPERATURE.
– Since rolled at room
temperature, get crystal
defects called dislocations
which impede motion via
slip!
– AKA work hardening
– Limit to how much you can
work harden before too
brittle.
– How reverse? Can
recrystallize by annealing.
• CRS Characterized by:
– Less ductlie – almost brittle
(i.e. % elongation 5 to
10%)
– High strength (Su approx
120 ksi for 1020)
AISI - SAE
Classification System
American Iron and Steel Institute (AISI)
• classifies alloys by chemistry
• 4 digit number
– 1st number is the major alloying agent
– 2nd number designates the subgroup
alloying agent
– last two numbers approximate amount of
carbon
(expresses in 0.01%)
Plain Carbon Steel vs.
Alloy Steel
Plain Carbon Steel (10xx)
• Lowest cost
• Should be considered first in most
application
• 3 Classifications
– Low Carbon Steel
– Medium Carbon Steel
– High Carbon Steel
Plain Carbon Steel (10xx)
• 1018
– Low carbon
Yield strength 55ksi
• 1045
– Medium carbon
Yield strength 70ksi
• ASTM A36 or A37 – aka structural steel
– Low carbon
Yield strength 36ksi
• 12L14
– Low carbon
Yield strength 70ksi
• 1144
– Medium carbon
Yield strength 95ksi
Plain Carbon Steel
vs. Alloy Steel
Alloy Steel
• > 1.65%Mn, > 0.60% Si, or >0.60% Cu
• Most common alloy elements:
– Chromium, nickel, molybdenum, vanadium,
tungsten, cobalt, boron, and copper.
• Added in small percents (<5%)
– increase strength and hardenability
• Added in large percents (>20%)
– improve corrosion resistance or stability at high
or low temps
Corrosion Resistant Steel
• Stainless Steel
• 10.5% < Cr < 27% = stainless steel – used
for corrosion resistance
• AISI assigns a 3 digit number
– 200 and 300 … Austenitic Stainless Steel
– 400 … Ferritic or Martensitic Stainless Steel
– 500 … Martensitic Stainless Steel
Tool Steel
• Wear Resistant, High Strength and
Tough
• High Carbon steels
• Modified by alloy additions
• AISI-SAE Classification
– Letter & Number Identification
Tool Steel
Classification
• Letters pertain to significant characteristic
– W,O,A,D,S,T,M,H,P,L,F
– E.g. A is Air-Hardening medium alloy
• Numbers pertain to material type
– 1 thru 7
– E.g. 2 is Cold-work

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