Chapter 9 – Steel Products Key: carbon content: –Steel – alloy consisting mostly of iron with a little carbon (0.2% % by weight) –Cast iron = carbon content between 2.1% - 4.0% –Iron = iron-carbon alloy with less than 0.005% carbon. –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 (12 th 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 a relatively inexpensive mass-produced good

The “abc’s” of Steel Making: Raw Material: –Carbon in the form of coke –Iron ore (Fe 2 O 3 ) –Limestone (CaCO 3 ) –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 (Fe 3 O 4 ) or hematite (Fe 2 O 3 )magnetitehematite 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 m 3 /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: –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: –Two important chemical reactions: Oxidation of the carbon from coke: Reduction of iron ore:

The “abc’s” of Steel Making: Step 1 – The Blast Furnace: –Products from the blast furnace: Iron stored 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: –Much smaller furnace –More impurities removed (oxidised) –Calculated amount of carbon added to make steel! –Poured into molds to form ingots

Steel Ingots

Manufacturing of Steel 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

Continuous Casting

The “abc’s” of Steel Making: Final steps – making useful products:

Figure 9-12: processing of refined steel into products.

F 9-13 – The whole spectrum of steel products!

Steel Types (Brief Overview)

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 –1 st number is the major alloying agent –2 nd 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 carbonYield strength 55ksi 1045 –Medium carbonYield strength 70ksi ASTM A36 or A37 – aka structural steel –Low carbonYield strength 36ksi 12L14 –Low carbonYield strength 70ksi 1144 –Medium carbonYield 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

Alloying Elements used in Steel Sulfur (S) (11xx) Imparts brittleness Improves machining Okay if combined with Mn Some free-machining steels contain 0.08% to 0.15% S

Alloying Elements used in Steel Nickel (Ni) (2xxx) Increase toughness Increase impact resistance 2% to 5% 12% to 20% with low amounts of C possess great corrosion resistance Invar –contains 36% Ni –virtually no thermal expansion –used for sensitive measuring devices

Alloying Elements used in Steel Chromium (Cr) (5xxx) Usually < 2% increase hardenability and strength typically used in combination with Ni and Mo 10.5% < Cr < 27% = stainless steel – used for corrosion resistance Molybdenum (Mo) (4xxx) Usually < 0.3% increase hardenability and strength Mo-carbides help increase creep resistance at elevated temps –typical application is hot working tools

Alloying Elements used in Steel Boron (B) (14xx) for low carbon steels, can drastically increase hardenability improves machinablity and cold forming capacity Aluminum (Al) deoxidizer 0.95% to 1.30% produce Al-nitrides during nitriding

Alloying Elements used in Steel Manganese (Mn) combines with sulfur to prevent brittleness >1% – increases hardenability 11% to 14% –increases hardness –good ductility –high strain hardening capacity –excellent wear resistance Ideal for impact resisting tools

Alloying Elements used in Steel Vanadium (V) Usually 0.03% to 0.25% increase strength –without loss of ductility Tungsten (W) helps to form stable carbides increases hot hardness –used in tool steels

Alloying Elements used in Steel Copper (Cu) 0.10% to 0.50% increase corrosion resistance Reduced surface quality and hot-working ability used in low carbon sheet steel and structural steels Silicon (Si) About 2% increase strength without loss of ductility enhances magnetic properties

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