SD390 vs SD490 – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
SD390 and SD490 are two widely used high-strength structural steel grades encountered in construction, reinforcing bars, and some cold-formed structural members. Engineers, procurement managers, and manufacturing planners routinely face a selection dilemma between these grades: balancing higher strength against weldability, ductility, fabrication cost, and availability. Typical decision contexts include whether to specify the higher yield strength to reduce section sizes and weight, or to prioritize improved weldability and formability for complex fabrication.
The principal distinguishing factor between the two is the target minimum yield strength: SD390 is specified around a 390 MPa yield class and SD490 around a 490 MPa class under the relevant Japanese-style designation practice. Because both steels are intended primarily as structural carbon/HSLA-type steels rather than stainless or tool steels, they are commonly compared when designers must choose a strength level without moving to alloyed or stainless categories.
1. Standards and Designations
- Common standards where SD-series grades appear or are referenced:
- JIS (Japanese Industrial Standards) – SD grades are commonly associated with JIS designations for reinforcing and structural steels.
- GB/T (Chinese national standards) and other regional standards sometimes use similar strength-class nomenclature for reinforcing bars and structural steels.
- EN and ASTM do not use the SD prefix directly but have analogous strength-class grades (e.g., S355, GRADE 50 rebar equivalents).
- Classification by metallurgy:
- SD390: Low‑to‑mid-carbon structural/HSLA class steel (carbon/HSLA).
- SD490: Low‑to‑mid-carbon structural/HSLA class steel (higher-strength carbon/HSLA).
- Neither grade is a stainless, tool, or high-alloy steel; they are typically plain carbon steels modified by controlled chemistry and, often, microalloying and thermomechanical processing to achieve target properties.
2. Chemical Composition and Alloying Strategy
Note: Exact chemical limits and compositions vary by standard edition and supplier. The table below summarizes typical chemical characteristics found in supplier data sheets for SD-series structural steels. Always confirm composition with mill certificates for critical applications.
| Element | Typical content in SD390 | Typical content in SD490 |
|---|---|---|
| C (carbon) | Low carbon, controlled to improve weldability (typical: ≤ ~0.25%) | Low carbon, often similarly controlled (typical: ≤ ~0.22%) |
| Mn (manganese) | Medium levels for strength and hardenability (typical range) | Medium to slightly higher to assist higher strength |
| Si (silicon) | Small amounts (deoxidation; ~0.1–0.6%) | Similar levels as SD390 |
| P (phosphorus) | Kept low for toughness (trace; e.g., ≤ ~0.04–0.05%) | Kept low |
| S (sulfur) | Kept low for ductility/machinability (trace; e.g., ≤ ~0.04–0.05%) | Kept low |
| Cr, Ni, Mo | Usually minimal or trace; present only if required for specific mixes | May have small additions in some mills to reach higher strength without excessive carbon |
| V, Nb, Ti (microalloying) | Often present in trace to low amounts to refine grain and improve strength/toughness | Frequently used to raise yield strength through precipitation strengthening |
| B, N | Trace levels controlled to avoid embrittlement; N controlled for toughness | Same considerations apply |
How alloying affects properties: - Carbon increases strength and hardenability but reduces weldability and ductility if raised excessively. - Manganese contributes to strength and toughness; it also raises hardenability. - Silicon is a deoxidizer and contributes to strength; excessive Si can impair certain coatings. - Microalloying elements (V, Nb, Ti) enable higher yield strengths via precipitation strengthening and grain refinement with only small reductions in weldability compared with increasing carbon.
3. Microstructure and Heat Treatment Response
- Typical microstructure: Both SD390 and SD490 are manufactured to produce ferrite–pearlite or fine-grained ferrite with dispersed pearlite and/or microalloy precipitates. Thermomechanical rolling (controlled rolling) and accelerated cooling produce fine-grained ferritic/pearlitic or bainitic-like structures depending on cooling rate and chemistry.
- SD390: With lower strength targets, microstructure is commonly ferrite–pearlite with controlled grain size for good ductility and toughness under ambient conditions.
- SD490: To achieve higher yield values, manufacturers often rely on a combination of slightly higher Mn and microalloying plus thermomechanical processing to produce a finer microstructure and precipitation strengthening; some commercial products can have transitional bainite or finer pearlite.
- Heat-treatment response:
- Normalizing/refining: Both grades respond to normalizing or controlled rolling with grain refinement and improved toughness.
- Quenching and tempering: Not typically applied to as-supplied deformed steels used in reinforcing bars or standard structural sections; Q&T can raise strength and tailor toughness but changes cost and availability.
- Thermo‑mechanical processing: Common route to achieve SD490-class strength without increasing carbon significantly, preserving better weldability than carbon-increased approaches.
4. Mechanical Properties
The defining mechanical distinction is minimum yield stress. Other mechanical properties depend strongly on processing, product form, and testing temperature. The table below gives typical or standardized minimum values where applicable and usual ranges.
| Property | SD390 (typical/minimum) | SD490 (typical/minimum) |
|---|---|---|
| Minimum Yield Strength (MPa) | ~390 MPa (design class) | ~490 MPa (design class) |
| Tensile Strength (MPa) | Typical range dependent on processing (often ~520–680 MPa) | Typical range dependent on processing (often ~560–760 MPa) |
| Elongation (Lo = specified gauge) | Moderate (commonly ≥10–18% depending on section and standard) | Typically lower than SD390 at similar product forms (commonly ≥8–16%) |
| Impact Toughness | Good at ambient when grain refined; varies with section thickness | Can be good if processed thermomechanically and microalloyed; may require tighter control for low-temperatures |
| Hardness | Moderate (depends on tensile) | Higher than SD390 for comparable forms |
Which is stronger, tougher, or more ductile: - Strength: SD490 > SD390 by design (higher yield and usually higher tensile). - Ductility/toughness: SD390 generally offers higher ductility and may show better low-temperature toughness in equivalent product forms unless SD490 is specifically processed to enhance toughness (e.g., careful TMCP and microalloy use). - Trade-off: Achieving SD490-class strength without compromising toughness typically requires microalloying and controlled processing rather than simply adding carbon.
5. Weldability
Weldability is a critical selection factor. Key considerations include carbon equivalent and the presence of elements that increase hardenability.
Useful predictive formulas (interpret qualitatively; do not substitute for welding procedure qualification): - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm for weldability assessment: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Higher $CE_{IIW}$ or $P_{cm}$ values indicate increased risk of hard, brittle weld‑heat‑affected zones and a higher need for preheat, controlled interpass temperatures, or post‑weld heat treatment. - SD490, by virtue of higher strength and often higher alloy content or microalloying, tends to have higher hardenability than SD390 for similar carbon levels. Therefore, SD490 products may require more careful welding procedures (preheat, lower heat input control, qualified consumables) especially in thicker sections. - Microalloy elements (V, Nb, Ti) increase precipitation strengthening but also can raise hardenability; careful heat input control mitigates HAZ hardness.
6. Corrosion and Surface Protection
- Both SD390 and SD490 are non‑stainless carbon/HSLA steels and do not offer inherent corrosion resistance beyond plain carbon steel.
- Common protection methods:
- Hot-dip galvanizing for long-term atmospheric protection.
- Epoxy or zinc-rich primers and coatings for aggressive environments.
- Paint systems and cathodic protection where appropriate.
- PREN is not applicable to these non‑stainless steels. For reference, the PREN index for stainless alloys is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ but this does not apply to SD-series carbon steels.
- Selection guidance: If corrosion resistance is a primary driver, consider specifying protective coatings or switching to a corrosion-resistant alloy instead of relying on SD390/SD490.
7. Fabrication, Machinability, and Formability
- Formability/bending:
- SD390 generally offers better bending and forming capacity for a given product form because of its lower yield strength and higher ductility.
- SD490 requires tighter bend radii control, potentially higher springback forces, and more careful process planning when cold forming.
- Machinability:
- Both are readily machinable with appropriate tooling; SD490 can be somewhat more abrasive or harder to machine at higher strength levels.
- Cutting and welding:
- SD490 may require higher forces for shearing and more robust cutting/welding parameters.
- Surface finishing:
- Both accept common finishing operations; high-strength microalloyed variants may show slightly different grinding/polishing responses.
8. Typical Applications
| SD390 – Typical Uses | SD490 – Typical Uses |
|---|---|
| Reinforcing bars and mesh for concrete where standard strength is sufficient | Reinforcing bars and structural components where higher yield strength reduces section size |
| Structural members in buildings with standard loadings where ductility and ease of welding are prioritized | Bridges, high‑strength structural sections, and projects where weight reduction is critical |
| Cold-formed sections and general structural applications where forming and bending are frequent | Applications requiring higher load capacity per cross-section, or where design codes call for higher strength grades |
| Fabrication that emphasizes weldability and low preheat requirements | Fabrication where advanced thermomechanical processing gives both higher strength and acceptable toughness |
Selection rationale: - Choose SD390 when fabrication complexity, high ductility, and easier welding are prioritized and when the 390 MPa class meets structural requirements. - Choose SD490 when the design requires higher yield strength to reduce section thickness or when project specifications call for the higher class, provided the procurement and fabrication teams can manage the welding/forming implications.
9. Cost and Availability
- Cost: SD490 typically commands a higher unit cost than SD390 due to additional processing (TMCP, microalloying, tighter controls) and sometimes due to smaller production volumes. However, material cost per member can be offset by reduced weight or smaller sections.
- Availability: SD390 is usually more widely available in a variety of product forms (rebar, bars, certain structural shapes). SD490 availability depends on regional market demand and mill capabilities; it is commonly available for rebar and certain structural products but may have lead times or minimum order considerations.
- Product form: Both grades are commonly sold as reinforcing bars, merchant bars, and sometimes as hot-rolled sections; check local mill inventories and certifications.
10. Summary and Recommendation
| Attribute | SD390 | SD490 |
|---|---|---|
| Weldability | Better (lower CE) | Acceptable but requires more control |
| Strength–Toughness balance | Good ductility and toughness for general use | Higher strength; toughness depends on processing |
| Cost | Lower (material basis) | Higher (processing and tighter controls) |
Recommendation: - Choose SD390 if: - Your design loads can be met by ~390 MPa yield material. - Ease of welding, forming, and higher ductility are priorities. - You prefer broader availability and lower material cost. - Choose SD490 if: - Structural optimization demands higher yield strength to reduce section sizes or weight. - The project tolerates tighter fabrication controls (welding, bending) or the supplier provides TMCP/microalloyed product with proven toughness. - You have qualified welding procedures and experienced fabricators to manage HAZ concerns.
Final note: SD390 and SD490 are both useful classes within the family of structural carbon/HSLA steels. The right choice depends on a holistic assessment of structural requirements, fabrication capability, welding procedures, coating needs, and total project cost. For critical projects, always obtain mill certificates, welding procedure specifications (WPS), and, if necessary, consult with steel producers to select the exact chemistry and processing route that delivers the required strength–toughness balance.