1. Carbon Steel Basics
Carbon steel, an iron-carbon alloy with carbon content [C] < 2%, typically contains minor amounts of silicon, manganese, sulfur, and phosphorus.

• By Application:
  • Structural carbon steel
  • Tool carbon steel
  • Free-cutting structural steel
  (Structural carbon steel further divides into building structural steel and machinery structural steel)

• By Carbon Content:
  • Low-carbon steel ([C] ≤ 0.25%)
  • Medium-carbon steel ([C] = 0.25%-0.6%)
  • High-carbon steel ([C] > 0.6%)

• By Impurity Levels:
  • Ordinary carbon steel (higher P/S)
  • Quality carbon steel (lower P/S)
  • High-quality carbon steel (lowest P/S)
  General rule: Higher [C] increases hardness and strength but reduces ductility.

2. Structural Carbon Steel
Guaranteed mechanical properties define this category. Steel grades reflect yield strength using "Q" (from Chinese "屈" / qu, meaning yield) + numeric value.

• Examples:
  • Q275: Yield strength = 275 MPa
  • Suffixes denote quality tiers (A/B/C/D):
  • Lower S/P content ascending from A to D
    • Process markers:
  • "F" = rimmed steel
  • "b" = semi-killed steel
  • No marker = killed steel
    • Example grades:
  • Q235-AF: Grade A rimmed steel, σ_y = 235 MPa
  • Q235-C: Grade C killed steel, σ_y = 235 MPa

Usage: Typically used as-supplied without heat treatment.

• Low-[C] grades (Q195/Q215/Q235): Excellent weldability, ductility, and toughness. Rolled into sheet, rebar, pipe for bridges, buildings, rivets, bolts.
• Higher-[C] grades (Q255/Q275): Suitable for welded structural sections (bars/plates), plus simple machinery parts (links, gears, pins).

3. Quality Structural Steel
Requires guaranteed chemical composition AND mechanical properties. Grades indicate mean carbon content in 0.01% units (e.g., 45 = 0.45% [C]).

Primary Use: Machine components (generally heat-treated). Applications by [C]:

• 08/08F/10/10F: High formability/weldability → cold-rolled sheet for auto bodies, dashboards.
• 15/20/25: Carburized parts with light loads (piston pins, gauges).
• 30-50: Quenched + tempered → balanced strength/ductility → shafts (crankshafts, spindles).
• 55-65: Quenched + medium-tempered → springs ≤15mm (valve springs, coil springs).

4. Carbon Tool Steel
High-carbon steel ([C] = 0.65%-1.35%) without alloying elements. Features low cost, easy machining, and high hardness/wear resistance post-heat treatment. Widely used for cutting tools, dies, and gauges.

Limitations:

• Poor red-hardness (>250°C → sharp hardness drop)
• Limited hardenability in large sections → warping/cracking risks.