The mechanical properties of 20# round bar are primarily determined by its heat treatment state. The two most common states are hot-rolled (as-delivered) (the default for most applications) and annealed (for enhanced ductility in precision forming). All properties below comply with GB/T 699-2015 and are tested via standard methods (tensile tests per GB/T 228.1, impact tests per GB/T 229, hardness tests per GB/T 231.1).
Hot-Rolled State (As-Delivered)
Hot-rolled 20# steel is cooled naturally after rolling, with a microstructure of ferrite + pearlite (ferrite dominates, accounting for 70–80%). This state balances machinability, ductility, and moderate strength, making it suitable for most general-purpose applications (e.g., non-high-load shafts, pipe fittings).
| Mechanical Property | Test Standard | Typical Value (Hot-Rolled) | Minimum Requirement (GB/T 699-2015) |
|---|---|---|---|
| Yield Strength (YS, Rₑₗ) | GB/T 228.1 | 220–280 MPa | ≥210 MPa (for diameter ≤160mm) |
| Tensile Strength (TS, Rₘ) | GB/T 228.1 | 380–460 MPa | ≥375 MPa (for diameter ≤160mm) |
| Elongation After Fracture (A) | GB/T 228.1 | 25–30% | ≥25% (for diameter ≤160mm) |
| Reduction of Area (Z) | GB/T 228.1 | 55–65% | ≥55% (for diameter ≤160mm) |
| Brinell Hardness (HBW) | GB/T 231.1 | 110–150 HBW | ≤156 HBW (no minimum requirement) |
| Charpy Impact Toughness (αₖ) | GB/T 229 | 45–60 J/cm² (20℃) | No mandatory requirement (typical value for hot-rolled state) |
Annealed State
Annealing (heating to 880–920℃, holding, then slow cooling) refines the microstructure of 20# steel to coarse ferrite + fine pearlite, further reducing hardness and improving ductility. This state is used for components requiring severe forming (e.g., deep drawing, cold bending) or precision machining (e.g., complex threaded parts).
| Mechanical Property | Test Standard | Typical Value (Annealed) | Key Advantage for Applications |
|---|---|---|---|
| Yield Strength (YS, Rₑₗ) | GB/T 228.1 | 190–240 MPa | Lower yield strength allows easier plastic deformation (e.g., cold bending without cracking). |
| Tensile Strength (TS, Rₘ) | GB/T 228.1 | 350–420 MPa | Moderate strength is maintained while ductility is enhanced. |
| Elongation After Fracture (A) | GB/T 228.1 | 28–35% | Higher elongation enables deep drawing (e.g., manufacturing seamless pipe blanks). |
| Brinell Hardness (HBW) | GB/T 231.1 | 95–130 HBW | Lower hardness reduces tool wear during precision machining (e.g., turning small-diameter threads). |
| Charpy Impact Toughness (αₖ) | GB/T 229 | 55–70 J/cm² (20℃) | Enhanced toughness is critical for components subject to mild impact (e.g., mechanical fasteners). |