20# Carbon Steel: Low-Carbon Structural Steel for General Engineering & Machining Applications
20# carbon steel (GB/T 699-2015 standard, equivalent to AISI 1020) is a low-carbon, non-alloy structural steel widely used in general engineering, machining, and fabrication due to its excellent weldability, cold formability, and balanced mechanical properties. With a nominal carbon content of 0.17-0.23%, this grade offers moderate strength (tensile strength ≥ 410 MPa) while maintaining superior ductility and machinability. This article explores its chemical composition, mechanical properties, manufacturing processes, and industrial applications.
1. Chemical Composition (GB/T 699-2015 Standard)
| Element | Content Range | Function |
|---|---|---|
| Carbon (C) | 0.17-0.23% | Primary strengthening element; low content ensures weldability and ductility |
| Manganese (Mn) | 0.35-0.65% | Enhances hardenability and deoxidation during steelmaking |
| Silicon (Si) | 0.17-0.37% | Improves strength and deoxidation; controlled to avoid brittleness |
| Phosphorus (P) | ≤ 0.035% | Impurity element; minimized to prevent cold brittleness |
| Sulfur (S) | ≤ 0.035% | Impurity element; controlled to improve machinability and ductility |
| Chromium (Cr) | ≤ 0.25% | Residual element; no intentional addition in low-carbon grades |
| Nickel (Ni) | ≤ 0.25% | Residual element; no intentional addition in standard composition |
| Copper (Cu) | ≤ 0.25% | Residual element; excessive content may affect surface quality |
2. Mechanical Properties (Room Temperature)
- Tensile Strength (σb): ≥ 410 MPa (hot-rolled), ≥ 390 MPa (cold-drawn)
- Yield Strength (σs): ≥ 245 MPa (hot-rolled), ≥ 235 MPa (cold-drawn)
- Elongation (δ5): ≥ 25% (hot-rolled), ≥ 22% (cold-drawn)
- Impact Toughness (Akv): ≥ 55 J at 20°C (charpy V-notch test)
- Hardness (HB): ≤ 156 (Brinell hardness, annealed condition)
- Reduction of Area (ψ): ≥ 50% (indicates excellent ductility for forming operations)
3. Manufacturing Process & Heat Treatment
- Steelmaking: Basic oxygen furnace (BOF) or electric arc furnace (EAF) with ladle refining to control carbon and impurity levels. Continuous casting produces billets with uniform grain structure.
- Hot Rolling: Heated to 1150-1250°C and rolled to final dimensions (bars, plates, or sections). Controlled cooling prevents surface defects and internal stresses.
- Cold Drawing: For precision bars/wires, cold drawing through dies improves dimensional accuracy and surface finish (tolerance ≤ ±0.1mm).
- Normalizing: Heated to 890-940°C, air-cooled to refine grain structure and improve machinability (typical hardness: 120-140 HB).
- Annealing: Full annealing at 870-920°C followed by furnace cooling to ≤ 156 HB for maximum ductility in cold-forming applications.
- Surface Treatment: Pickling (sulfuric/hydrochloric acid) to remove oxide scale; phosphating or oiling for temporary corrosion protection during storage.
4. Key Industrial Applications
Machined Components
Gears, shafts, bolts, and fasteners for automotive and machinery industries. Low carbon content ensures excellent machinability (cutting speed 120-150 m/min for turning operations).
Structural Fabrication
Welded frames, brackets, and supports in construction and equipment manufacturing. High weldability (preheat not required for sections < 20mm) reduces fabrication costs.
Cold-Formed Parts
Stamped or deep-drawn components for appliances, furniture, and electrical enclosures. Elongation ≥ 25% enables complex shapes without cracking.
Pipeline Systems
Low-pressure fluid transportation pipes and fittings (water, oil, or gas). Compatible with threading, bending, and flanging operations for custom installations.
Automotive Industry
Chassis components, suspension parts, and non-critical engine parts. Often carburized (case-hardened) to achieve surface hardness > 50 HRC for wear resistance.
General Engineering
Jigs, fixtures, and tooling for manufacturing processes. Easily welded to other low-carbon steels (e.g., Q235) using SMAW, GMAW, or FCAW methods.
5. Comparison with Similar Carbon Steel Grades
| Grade | Carbon Content | Key Properties | Typical Applications |
|---|---|---|---|
| 20# (GB/T 699) | 0.17-0.23% | Balanced strength/ductility; excellent weldability | General machining, cold-forming, welded structures |
| 45# (GB/T 699) | 0.42-0.50% | Higher strength; lower weldability; heat-treatable | Shafts, axles, and heat-treated components |
| Q235 (GB/T 700) | ≤ 0.22% | Structural grade; higher P/S limits; cost-effective | Construction beams, plates, and profiles |
| AISI 1018 | 0.15-0.20% | Similar to 20#; tighter composition control | Precision machined parts for export markets |
| AISI 1020 | 0.18-0.23% | Direct equivalent; identical properties | Interchangeable with 20# in global supply chains |
6. Selection Guidelines & Processing Recommendations
- Machining: Use high-speed steel (HSS) or carbide tools with cutting speeds of 120-150 m/min for turning. Coolant recommended to extend tool life and improve surface finish (Ra ≤ 3.2 μm achievable).
- Welding: Suitable for all arc welding processes (SMAW, GMAW, FCAW). ER70S-6 filler metal recommended; preheat not required for sections < 20mm. Post-weld stress relief at 600-650°C for critical applications.
- Heat Treatment: For carburizing, heat to 900-950°C in carbon-rich atmosphere, quench in oil/water, then temper at 150-200°C to achieve case hardness of 55-62 HRC with core toughness retained.
- Cold Working: Maximum cold reduction of 70% without intermediate annealing. Annealing required after severe deformation to restore ductility (heat to 870-920°C, furnace cool).
- Corrosion Protection: Not corrosion-resistant; requires painting, galvanizing, or phosphating for outdoor/exposed applications. Hot-dip galvanizing (ASTM A123) adds 50-80 μm zinc coating for 20+ years service life.
- Quality Certification: Verify mill test reports (MTR) for chemical composition, mechanical properties, and ultrasonic testing (UT) results per GB/T 699-2015 or equivalent ASTM standards.
7. Request a Carbon Steel Quote
For customized 20# carbon steel products—including hot-rolled bars, cold-drawn wires, plates, or machined components—contact our team with your specifications (dimensions, tolerances, and quantity). We provide competitive pricing, technical support, and global shipping for bulk orders.