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:

COOC 22 2

• Reduction of iron ore:

233 323 COFeCOOFe

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