Raw materials:

Iron ore — pelletized, ~65% iron.
Limestone — combines with impurities to form floating slag.
Coke — from high-quality coal; (a) generates heat, (b) removes oxygen from iron-oxide as CO. (Residual oxygen severely lowers steel toughness.)

Ironmaking: the three are charged into a blast furnace; hot air drives C+O reactions that strip oxygen; slag is removed. Output = pig iron (~4% C, 1.5% Si, 1% Mn, 0.04% S, 0.4% P, rest Fe).

Steelmaking: refine pig iron — reduce Mn/Si/C and add elements + limestone. Furnaces: electric and basic-oxygen; scrap can be added; a vacuum furnace removes gaseous impurities for high-quality steel.

Ingots vs Continuous Casting

Traditional ingots: rejected oxygen (as CO) causes porosity. Fully deoxidized = killed; partially = semi-killed. Drawbacks: surface correction, porosity, cost.
Continuous casting: molten metal → ladle → water-cooled copper mold → solidified skin + liquid core; pulled ~25 mm/s, shell 12–18 mm, bar ≈ 250 mm. Cheaper, more uniform; eliminates porosity/segregation/shrinkage. Then cold-rolled, annealed, coated (galvanized/aluminized).
- Too fast → metal not solidified → spills out; too slow → mold freezes up.

Effects of Alloying Elements

Carbon: ↑strength, hardness, wear resistance; ↓ductility, weldability, toughness.
Chromium: ↑corrosion & wear resistance, toughness, hardenability, high-T strength.
Nickel: ↑strength, toughness, corrosion resistance, hardenability.
Sulfur: ↑machinability (with Mn); ↓impact strength, ductility, surface quality, weldability.

Elements that favor a given property (alphabetical):

Hardenability	Strength	Toughness	Machinability
Boron, Carbon, Chromium, Manganese, Molybdenum, Phosphorus, Titanium	Carbon, Chromium, Cobalt, Copper, Manganese, Molybdenum, Nickel, Niobium, Phosphorus, Silicon, Tantalum, Tungsten, Vanadium	Calcium, Cerium, Chromium, Magnesium, Molybdenum, Nickel, Niobium, Tantalum, Tellurium, Vanadium, Zirconium	Lead, Manganese, Phosphorus, Selenium, Sulfur, Tellurium

Residual (Trace) Elements — generally unwanted

Hydrogen: severely embrittles steel (mostly driven off if heated).
Nitrogen: ↑strength/hardness; ↓ductility/toughness.
Oxygen: slightly ↑strength; severely ↓toughness.

Steel Designation (AISI / SAE)

Four digits: first two = alloy type/percentages, last two = carbon content (in 0.01%).

1020 — plain carbon, 0.20% C
1040 — plain carbon, 0.40% C
1112 — resulfurized, 0.12% C
1220 — rephosphorized + resulfurized, 0.20% C (great machinability)

Series: 1xxx plain carbon, 13xx Mn, 2xxx Ni, 3xxx Ni–Cr, 4xxx Mo, 41xx Cr–Mo, 43xx Cr–Mo–Ni, 5xxx Cr, 6xxx Cr–V, 86xx Ni–Cr–Mo, 92xx Mn–Si.

Carbon Steel Groups

Low-carbon (mild): < 0.30% C — tubes, bolts, nuts.
Medium-carbon: 0.30–0.60% C — machinery, automotive.
High-carbon: > 0.60% C — cutting tools, cable, springs.

Alloy & Specialty Steels

Alloy steels: significant alloying → better properties, more expensive. Normalizing ↓strength/hardness & ↑ductility; full annealing more so.
Stainless steels: > 10% Cr → chromium-oxide passivation film; high corrosion resistance, strength, ductility. Higher C → lower corrosion resistance. Cutlery, kitchen, surgical, food/chemical.
Precipitation-hardening (PH) steels: Cr, Ni, Cu, Al, Ti, Mo — corrosion resistance + high-T strength; aircraft/space structures.
Tool & die steels: high strength, impact toughness, wear resistance. HSS (W/Mo) hold hardness at high T; H = hot-work; A/D/O = cold-work; S = shock-resisting.

Key Trade-offs

High hardness ↔ low toughness; high wear resistance ↔ low machinability. High-carbon steels harden more than stainless → better edge retention (chefs’ knives).