Introduction
10B21 is a boron-bearing low-carbon steel wire rod primarily used for cold heading and cold extrusion of fasteners and structural components. It combines good plasticity, moderate strength, and excellent cold formability — with the addition of boron (B) significantly improving hardenability through heat treatment.
10B21 is derived from the SAE 1021 base composition with boron added. Per SAE J403, boron (0.0005–0.003%) may be added to standard killed carbon steels to improve hardenability; such grades are identified by inserting "B" between the 2nd and 3rd digits of the grade number (e.g., 10B21). The UNS designation changes the last digit to "1" (e.g., G10211).
10B21 is approximately equivalent to the Chinese grade 20MnB. Per GB/T 3098.1-2010, it can be used to produce bolts up to Grade 9.8 and nuts up to Grade 10. While 10.9 grade bolts are technically achievable, the standard caps the specification at 9.8 as 10.9 represents the material’s limit load.
In China, 10B21 is also governed by steel mill enterprise standards, with Baosteel Q/BQB517-2023 being the most widely referenced domestic authority.
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Chemical Composition
10B21 composition varies slightly across standards. The table below compares the key references:
| Standard | C (%) | Mn (%) | Si (%) | P (%) | S (%) | B (%) |
|---|---|---|---|---|---|---|
| Q/BQB517-2023 (Baosteel) | 0.18–0.23 | 0.70–1.00 | ≤ 0.10 | ≤ 0.020 | ≤ 0.020 | 0.0008–0.0035 |
| BZJ550-2004 (Baosteel) | 0.18–0.23 | 0.70–1.00 | ≤ 0.10 | ≤ 0.030 | ≤ 0.035 | ≥ 0.0005 |
| ASTM A1040 | 0.18–0.23 | 0.60–0.90 | — | ≤ 0.04 | ≤ 0.05 | 0.0005–0.003 |
| SAE J403-2001 | 0.18–0.23 | 0.60–0.90 | — | ≤ 0.03 | ≤ 0.05 | 0.0005–0.003 |
Note: Q/BQB517-2023 is the most widely used and authoritative reference in China. It specifies tighter P and S limits than ASTM/SAE, reflecting higher cleanliness requirements for cold heading wire rod.
Role of Key Alloying Elements
- Carbon (C, 0.18–0.23%): Primary strength determinant. Kept low to maintain good plasticity and cold heading performance. Higher C reduces cold formability.
- Manganese (Mn, 0.70–1.00%): Provides solid solution strengthening and improves the distribution of non-metallic inclusions. Controlled below 1.00% to preserve cold formability; excessive Mn significantly increases strength and impairs cold forming.
- Silicon (Si, ≤ 0.10%): Minimized to preserve plasticity and toughness. Higher Si increases strength and hardness while reducing ductility — detrimental to cold heading performance.
- Boron (B, 0.0008–0.0035%): Trace addition that dramatically improves hardenability, enabling the steel to achieve higher strength after heat treatment with minimal impact on cold formability. This is the defining characteristic of 10B21 vs plain 1021.
- Phosphorus & Sulfur: Tightly controlled (especially in Q/BQB517) to ensure cleanliness and consistent cold heading performance.
Mechanical Properties (Wire Rod)
| Property | Value |
|---|---|
| Tensile Strength | 489 MPa |
| Elongation after fracture (A) | 34.5% |
| Reduction of Area (Z) | 69% |
| Cold upsetting (1/2 height) | Pass |
| Total decarburization layer | ≤ 0.02 mm |
| Hardness | HRB 75 |
Fastener Grade Capability
| Fastener Type | Maximum Grade | Standard |
|---|---|---|
| Bolts / Screws | Grade 9.8 | GB/T 3098.1-2010 |
| Nuts | Grade 10 | GB/T 3098.2 |
Note: While 10.9 grade bolts are technically achievable with 10B21, GB/T 3098.1 caps the specification at 9.8 as 10.9 represents the material’s limit load condition.
Applications
10B21 is the dominant cold heading wire rod for fastener production in China and internationally:
- Fasteners — bolts, screws, nuts, and studs up to Grade 9.8/10; the primary application for 10B21 wire rod
- Automotive structures — body structure and chassis fasteners requiring reliable strength and cold formability
- Mechanical components — cold-extruded precision parts, pins, and structural hardware
- Construction — structural bolts and anchoring hardware
- Welded assemblies — structural weldments where good weldability is required alongside moderate strength
Fabrication Characteristics
Cold Heading
10B21 is specifically designed for cold heading and cold extrusion. Its composition is optimized for this process:
- Low C (≤ 0.23%) and very low Si (≤ 0.10%) ensure low hardness, high plasticity, and easy deformation
- Controlled Mn (≤ 1.00%) balances strength improvement with cold formability
- Grain size, grain morphology, and non-metallic inclusion distribution are also controlled to meet cold heading requirements
- Cold upsetting test (1/2 height reduction) is a standard acceptance criterion
Weldability
10B21 has good weldability as a quality low-carbon steel. Weldability is primarily governed by carbon content — higher carbon increases hardenability and crack susceptibility in the heat-affected zone. With C at 0.18–0.23%, 10B21 falls in the range of good weldability, comparable to 20-grade carbon steel. The carbon equivalent method (Ceq) can be used to estimate weldability for specific applications.
Heat Treatment Response
The boron addition gives 10B21 significantly better hardenability than plain 1021 steel. After quenching and tempering, 10B21 can achieve the mechanical properties required for Grade 8.8 and 9.8 fasteners. This makes it more versatile than non-boron low-carbon steels of similar base composition.
Standards reference: Q/BQB517-2023 (Baosteel) | BZJ550-2004 (Baosteel) | ASTM A1040 | SAE J403-2001 | GB/T 3098.1-2010
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