Round Steel
Steel Round Bar: Global Specifications, Material Classifications, and Technical Selection Encyclopedia
1. Definition and Core Overview
A Round Bar is a solid long steel product with a circular cross-section. Its specification is expressed by its diameter in millimeters (mm). For example, "50mm" represents a round bar with a diameter of 50mm.
Round bars are highly versatile in industrial applications, ranging from basic structural components to high-precision aerospace parts. Their uniform cross-section provides excellent torsional strength and machining flexibility.
Read more:Common Round Bar Materials and Their Applications
2. Production Processes and Surface Treatment
Based on environmental requirements and precision needs, round bars are produced through four core processes:
- Hot-Rolled Round Bar: Performed under GB/T 702. Available in sizes 5.5mm–380mm. It features a surface oxide scale (mill scale) and is suitable for construction structures and general mechanical parts.
- Forged Round Bar: Performed under GB/T 908. The forging process results in a denser internal grain structure, primarily used for heavy-duty components subject to extreme stress.
- Cold-Drawn Round Bar: Performed under GB/T 3078 / GB/T 905. It offers a bright finish and extremely high dimensional accuracy (e.g., H7–H11 tolerances). It can often be used directly in precision shaft machining, saving costs on preliminary turning/cutting.
- Galvanized Round Bar: Produced by applying a Hot-Dip or Electro-galvanized coating to hot-rolled or cold-drawn bars.
Deep Dive: Multidimensional Applications of Galvanized Round Bars
Beyond simple electrical fittings, galvanized round bars are the backbone of "structural corrosion protection" in outdoor and harsh environments:
- Power and Lightning Protection: Used as jumpers between metal conduits (common Φ6, Φ8), lightning protection belts, and down-conductors for lightning protection grounding grids.
- Construction and Facades: Support members for glass curtain wall systems, hangers for dry-hanging stone, and tension rods or diagonal bracing in light steel structures.
- Transportation Infrastructure: Guardrail posts for highways, signpost brackets, railway catenary pillars, and tunnel support systems.
- Modern Agriculture: Skeleton frames for vegetable greenhouses, greenhouse shading system beams, and fencing for automated livestock equipment.
- Municipal and Photovoltaic (PV): Anchor bolts for street light foundations, connection rods for solar PV brackets, park landscape fencing, and outdoor fitness equipment.
- Marine and Petrochemical: Jetty bridge supports for coastal docks and outdoor pipe rack supports (serving as a cost-effective alternative to stainless steel).
3. Analysis of Core Technical Standards
Based on the official technical documentation, the execution standards for round bars are categorized into four dimensions:
| Standard Category | Typical Standard | Scope of Application | Mandatory Attribute |
| Basic Standard | GB/T 702 | General requirements for hot-rolled steel bars (5.5-380mm) | Recommended |
| Material Standard | GB/T 699 | Quality carbon structural steels | Recommended |
| Professional Standard | GB/T 3078 | Technical conditions for cold-drawn steel bars (High precision) | Recommended |
| Safety Standard | GB 1499.2 | Requirements for steel for the reinforcement of concrete | Mandatory |
Key Points of Precision and Tolerance:
- Dimensional Permissible Variation: Taking hot-rolled material as an example (GB/T 702-2017), the permissible variation for diameter d≤40mm is ±0.4mm; for d>40–80mm, the tolerance is widened to ±0.6mm. For cold-drawn bars (3–6mm), the tolerance is as tight as±0.05mm.
- Straightness/Curvature Requirements: Curvature per meter shall not exceed 6mm, and the total curvature shall not exceed 0.6% of the total length.
- Decarburization Depth (Cold-drawn): For d≤16mm, it shall not exceed 0.2mm; ford>16mm, it shall not exceed 1.5% of the diameter.
- Surface Quality: Cold-drawn bars feature a surface roughness of Ra≤1.6μm.
- Delivery Condition: Usually delivered in straight lengths; however, delivery in coils can be negotiated. The delivery quantity of short lengths must not exceed 10% of the total weight of the batch.
- Specialized Alloy Requirements: Alloy structural steel 20Cr follows GB/T 3077-2018 (C: 0.18%-0.24%, Cr: 0.70%-1.00%). Low alloy steel Q345D follows GB/T 1591-2018, requiring a −20℃ low-temperature impact energy ≥34J.
4. Material Classification and Performance Encyclopedia
We provide round bars across all major global grades, including GB (China), ASTM/AISI (USA), and EN/DIN (Europe).
4.1 Carbon Structural Steels
Classified by carbon content, which determines strength and machinability:
1.Low Carbon Steel (C: 0.10%--0.30%): Also known as Mild Steel.
- Grades: Q195, Q215, Q235, Q235B, S235JR, A36, 1018, A500,SS490.
- Features: Excellent weldability, forgeability, and machinability with high plasticity.
- Applications: Construction rebar, scaffolding, tie rods, rivets, bolts, deep-drawn parts (e.g., 08F, 08Al), and boiler materials (20G).
2.Medium Carbon Steel (C: 0.25%--0.60%):
- Grades: 35#, 45#, 50#, 1045, HG3.
- Features: Exceptional comprehensive mechanical properties after Quenching and Tempering (Q&T), hardness up to HRC55 (HB538), Tensile Strength (σb) 600–1100MPa.
- Applications: Transmission shafts, gears, machine tool spindles, piston pins. Q235 is the recommended baseline for general procurement.
3.High Carbon Steel (C: 0.60%--1.70%): Commonly known as Tool Steel.
- Grades: T8, T10, 65Mn, 50Mn.
- Features: Extremely high hardness and wear resistance; can be through-hardened and tempered.
- Applications: Hammers, crowbars, drill bits, taps, reamers, springs.
4.2 Alloy Structural Steels
Enhanced with Cr, Mo, Mn, Ni, etc., for superior hardenability and toughness.
1.Low-Alloy High-Strength (HSLA): Q345 (16Mn), Q345B, Q345D.
Applications: Structural parts for heavy loads and impact, such as bridges and heavy machinery.
2.Cr-Mo / Ni-Cr-Mo Series: 20Cr, 40Cr, 20CrMo, 35CrMo, 42CrMo, 4340, 4140, 4130, 40CrNiMo, 50Cr, 30CrMnSi.
Technical Spotlight 20Cr: Tensile≥835MPa, Yield≥540MPa, Elongation≥10%.
Applications: Heavy-duty gear shafts, oil drilling rigs, engine connecting rods, tire bolts.
3.Specialty Alloy Steels: 12Cr1Mov, 15CrMo, 20CrMnTi (Essential carburizing steel), 5CrMnMo, 3Cr2W8V.
Applications: Automotive gearboxes, large forging dies, heat-resistant pressure vessels.
4.3 Stainless Steel Round Bars
- Austenitic: 304, 304L, 316, 316L, 321, 1Cr18Ni9Ti.
- Features: Superior corrosion resistance and low-temperature toughness. Used in food, medical, and chemical sectors.
- Martensitic / Precipitation Hardening: 2Cr13, 3Cr13, 17-4PH (630).
- Applications: High-strength pump shafts, cutting tools, aerospace components.
4.4 Other Specialty Grades
- Bearing Steel (GCr15): High carbon chromium steel for rolling elements and precision spindles.
- Spring Steel (60Si2Mn): High elastic limit for automotive and industrial springs.
- Free-Cutting & Cold-Heading Steels: Optimized for high-speed automated lathe machining and cold forging of fasteners.
- High-Speed Steel (HSS) (W6Mo5Cr4V2): For drill bits, milling cutters, and high-speed cutting tools.
5. Dimensions and Theoretical Weight Calculation
The theoretical weight of round bars is calculated based on a standard density of 7.85g/cm3.
Calculation Formula:
W(kg/m)=0.006165×d2(where d is diameter in mm)
Reference Weights for Typical Specifications (GB/T 702):
Φ10mm: 0.617 kg/m
Φ20mm: 2.47 kg/m
Φ80mm: 39.45 kg/m
Φ100mm: 61.7 kg/m
6. Expert Selection Guide (Buyer's Checklist)
Strength vs. Toughness: For environments with impact loads, prioritize Q345D or 42CrMo.
Machining Efficiency: Need to reduce machining steps? Choose Cold-Drawn Round Bar (tighter tolerances).
Corrosion Protection Life: For outdoor structural use, choose Hot-Dip Galvanized Round Bar; for acidic/alkaline environments, choose 316L Stainless Steel.
Weldability: Q235 and low-carbon steels are easiest to weld; medium/high carbon steels require pre-heating.
Quality Assurance
Our round bar products strictly comply with the GB, ASTM, and DIN standards mentioned above. Every batch is accompanied by a full MTC (Mill Test Certificate), covering chemical composition (e.g., precise Cr content for 20Cr) and physical properties (Tensile, Yield, Elongation, and Impact Energy), ensuring your engineering projects are safe and compliant.
[Contact our experts for the latest price list for round bar]
Common Steel Grades Used in CNC Machining
Selecting the right steel grade is essential for successful Steel CNC Machining. Each grade differs in composition, strength, machinability, and corrosion resistance. Understanding these characteristics helps engineers and buyers balance performance, cost, and precision in every machining project.
| Steel Grade | Composition & Type | Key Properties | Machinability / Weldability | Typical Applications | TiRapid Notes & Example |
| 1018 (Low Carbon Steel) | Fe, ~0.18% C, Mn | High ductility, easily carburized, good toughness | Good machinability, excellent weldability | Fixtures, shafts, bushings, carburized parts | Used for locating fixtures and shafts requiring H7 tolerance (±0.01 mm). |
| 1215 (Free-Machining Steel) | Fe, C, Mn, high S | Generates small chips, supports high cutting speed, lower strength | Excellent machinability, poor weldability | Fittings, pins, screws, automatic lathe parts | Ideal for mass-production screws; saves 25% cycle time. |
| 1045 (Medium Carbon Steel) | Fe, ~0.45% C, Mn, Si | Tough, affordable, moderately hardenable | Moderate machinability, fair weldability | Shafts, gears, bolts, pins | Machined hydraulic rods, surface ground to Ra ≤ 0.4 µm. |
| 4130 (Cr-Mo Alloy Steel) | Fe, Cr, Mo, Mn | High strength-to-weight ratio, heat-treatable | Average machinability, good weldability | Welded frames, aircraft parts | Used in welded control arms; bores kept ≤ 0.01 mm coaxiality. |
| 4140 (Cr-Mo Alloy Steel) | Fe, Cr, Mo, Mn | Excellent fatigue and wear resistance; pre-hard machinable | Good machinability, moderate weldability | Couplings, spindles, gears | Pre-hardened 4140 hubs ±0.01 mm; induction-hardened teeth. |
| 304 Stainless Steel | 18 Cr – 8 Ni (Austenitic) | Excellent corrosion resistance, high formability | Fair machinability, excellent weldability | Food equipment, housings, clamps | Enclosures with 0.03 mm flatness over 100 mm. |
| 316 / 316L Stainless Steel | 17 Cr – 12 Ni – 2.5 Mo | Superior corrosion resistance in marine or chemical environments | Fair machinability, excellent weldability | Marine valves, chemical equipment | 316L valve blocks, Ra ≤ 0.8 µm after grind, leak-free. |
| 17-4PH Stainless Steel | Fe, Cr, Ni, Cu | High strength and corrosion resistance, age-hardenable | Moderate machinability, good weldability | Aerospace parts, pump shafts | Machined then H900-aged pins, positional tol ≤ 0.02 mm. |
| Tool Steels (D2, H13, O1, A2) | High C + Cr / Mo | Extreme wear and heat resistance, stable at high temp | Difficult machinability, poor weldability | Dies, molds, cutting tools | H13 mold cavities EDM-finished Ra ≤ 0.4 µm, corner radius ≤ 0.2 mm. |
Selection Guide
| Purpose / Requirement | Recommended Grade | Key Advantage |
| Rapid machining, prototypes | 1215 / 1018 | High cutting efficiency, low tool wear |
| High strength & fatigue parts | 4140 / 4130 | Excellent mechanical endurance |
| Corrosion-critical environments | 316L / 17-4PH | Superior corrosion resistance |
| Food & medical components | 304 / 316L | Hygienic and easy to weld |
| Wear-resistant molds & dies | D2 / H13 | High hardness and heat stability |
| Economic general-purpose shafts | 1045 | Tough and cost-effective |
GB/T 702-2017: Hot-rolled steel bars — Dimensions, shape, weight and permissible variations
3.2 Dimensions and Permissible Variations
3.2.1 Permissible variations in dimensions for hot-rolled round and square bars shall comply with the provisions in Table 1. The tolerance group shall be specified in the corresponding product standard or the purchase contract; if not specified, Group 3 permissible variations shall be applied.
Table 1: Permissible Variations of Dimensions for Hot-rolled Round and Square Bars
(Unit: mm)
| Nominal Cross-section Dimension (Round bar diameter or Square bar side length) | Group 1 | Group 2 | Group 3 (Default) |
|
>5.5∼20 |
±0.25 |
±0.35 |
±0.40 |
|
>20∼30 |
±0.30 |
±0.40 |
±0.50 |
Round Bar Specification Table
Standards:
GB/T 699-1988 (Quality Carbon Structural Steels)
GB/T 700-1988 (Carbon Structural Steels)
GB/T 3077-1988 (Alloy Structural Steels)
GB/T 702-1986 (Hot-rolled Steel Bars Dimensions & Tolerances)
QJ/HG02.17-1991 (Internal Specialized Standard)
| Steel Grade | Diameter (ϕ, mm) | Steel Grade | Diameter (ϕ, mm) |
| 8# - 10# | ϕ16 - 290 | 65Mn | ϕ40 - 300 |
| 15# | ϕ14 - 150 | 45Mn2 | ϕ18 - 75 |
| 20# | ϕ8 - 480 | 60Si2Mn | ϕ16 - 150 |
| 35# | ϕ8 - 480 | 20CrMnTi | ϕ10 - 480 |
| 45# | ϕ6.5 - 480 | 20CrmnTiB | ϕ16 - 75 |
| Q235B | ϕ6.5 - 180 | GCr15 | ϕ16 - 400 |
| 40Cr | ϕ8 - 480 | ML35 | ϕ8 - 150 |
| 20Cr | ϕ10 - 480 | T8 - T13 | ϕ8 - 480 |
| 42CrMo | ϕ12 - 480 | Cr12 | ϕ16 - 300 |
| 35CrMo | ϕ12 - 480 | Cr12MoV | ϕ16 - 300 |
| 20CrMo | ϕ12 - 300 | 3Cr2W8V | ϕ16 - 300 |
| 38CrMoAL | ϕ20 - 300 | 45Cr / 50Cr | ϕ20 - 300 |
| 5CrMnMo | ϕ20 - 450 | 20CrMnMo | ϕ20 - 300 |
| 16Mn (Q345B) | ϕ14 - 365 | 40Mn2 | ϕ28 - 60 |
| 50Mn | ϕ40 - 200 | 35Cr | ϕ55 |
| 15CrMo | ϕ21, ϕ24, ϕ75 | 15Mn | ϕ32, ϕ170 |
| 25# | ϕ16 - 280 | 40CrMnMo | ϕ80 - ϕ160 |
| YF45MnV | ϕ28 - 80 | 27SiMn | ϕ20 - 350 |
| 30# | ϕ6.5 - 480 | CrWMn | ϕ20 - 300 |
| 30CrMo | ϕ28 | H13 (4Cr5MoSiV1) | ϕ20 - 300 |
| 30CrMnTi | ϕ16 - 300 | 40CrNiMo | ϕ20 - 400 |
| 60# | ϕ210, ϕ260 |
As one of the leading round steel manufacturers and suppliers in China, we warmly welcome you to buy round steel in stock here and get free sample from our factory. All customized products are with high quality and low price.
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