SPHC vs SPHD – Composition, Heat Treatment, Properties, and Applications

Introduction

SPHC and SPHD are two commonly specified JIS (Japanese Industrial Standard) hot-rolled steel grades used across fabrication, automotive components, general structure, and light manufacturing. Engineers and procurement teams often choose between them when balancing cost, formability, weldability, and the mechanical performance required for stamped, bent, or welded parts.

The principal technical distinction relevant to component selection is that SPHD is produced and specified to deliver higher plasticity (ductility and formability) relative to SPHC, which is a general-purpose hot-rolled commercial-quality grade. Because both are low‑carbon, low‑alloy steels, the selection dilemma typically centers on forming performance (deep drawing, extensive bending) versus the broad availability and lower cost of a commercial-quality product.

1. Standards and Designations

• JIS: SPHC and SPHD are JIS-designated grades in the family of hot-rolled mild steels.
• Other standards:
• ASTM/ASME: Rough equivalents are general-purpose hot-rolled low-carbon steels (e.g., ASTM A1011 commercial grades) but direct one-to-one equivalence should not be assumed without reference to specific property requirements and certification.
• EN: Similar roles are played by EN steels such as S235JR/S235J0 for structural or general-quality hot‑rolled products; again, crosswalks must be validated by chemical and mechanical requirements.
• GB (China): Various Q235 family steels serve similar markets.
• Classification: Both SPHC and SPHD are low‑carbon, non‑stainless carbon steels (not HSLA, not tool steel, not stainless). They are intended for forming and general structural uses rather than for high‑strength or corrosion‑resistant applications.

2. Chemical Composition and Alloying Strategy

Both SPHC and SPHD are designed as low‑carbon, low‑alloy steels. They rely on minimal deliberate alloying; the alloying strategy is to keep carbon and residual elements low to ensure good cold formability, weldability, and low cost.

Explanation: The alloying intent for both grades is minimal additions: enough Mn and Si for deoxidation and basic strength, while keeping elements that increase hardenability or reduce ductility (C, Cr, Mo, etc.) to low levels. SPHD specifications and mill processing target improved plasticity through tighter limits and process control rather than through significant chemical alloying.

3. Microstructure and Heat Treatment Response

Microstructure: - As‑rolled microstructure for both grades is typically ferrite with pearlite pockets (typical of low‑carbon hot‑rolled steels). The volume fraction of pearlite is small because carbon is low. - Inclusion morphology and grain size depend on steelmaking practice and rolling/annealing schedules.

Processing response: - SPHC: Produced as a general hot‑rolled product with standard controlled cooling. Microstructure is generally coarse ferrite/pearlite. Normalizing is rarely applied for these commercial grades; mechanical property enhancements via heat treatment are limited because they are not intended for quenched/tempered use. - SPHD: While chemically similar, SPHD is processed and specified for improved formability. This can include tighter control of hot rolling finish temperature, controlled cooling to refine grain structure, and potential light annealing to improve ductility. The effect is a finer ferritic microstructure and cleaner inclusion population that enhances formability.

Effects of common heat treatments and thermo‑mechanical routes: - Annealing (intercritical or full) will increase ductility for both grades, but SPHD is more likely to be supplied with processing history aimed at preserving ductility. - Quenching & tempering or heavy thermal treatments are not typical for these grades; they are not formulated for martensitic hardening responses because of low carbon and lack of hardenability elements. - Thermo‑mechanical control (controlled rolling and accelerated cooling) can modestly increase strength without sacrificing ductility — usually a route for HSLA steels rather than SPHC/SPHD.

4. Mechanical Properties

Below is a qualitative comparison reflecting typical behavior of these JIS hot‑rolled commercial grades. Specific mill certificates and purchase specifications should be used for design calculations.

Explanation: The critical selection factor is formability: SPHD is targeted to give higher elongation and superior plastic deformation behavior (deep drawing, severe bending) compared with the general-purpose SPHC. Strength differences are usually small and overlapping; choosing SPHD is rarely about strength increase but rather about predictable and improved plastic behavior during forming.

5. Weldability

Weldability of both SPHC and SPHD is generally good due to low carbon and low alloying content, but microalloying and processing can influence susceptibility to cold cracking and heat‑affected zone (HAZ) hardening.

Common weldability assessment formulas: - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (weldability parameter): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - For these low‑carbon grades, $CE_{IIW}$ and $P_{cm}$ values are typically low, indicating good general weldability with standard carbon‑steel welding consumables and preheat procedures. - SPHD's improved ductility reduces the risk of weld‑induced distortion and cracking during forming operations, but because SPHD may be supplied with slightly lower yield and higher ductility, welding practice should consider the potential for residual distortion in thin gauges. - Preheat and post‑weld heat treatment are generally not required for moderate thicknesses, but always follow welding procedure specifications (WPS) based on thickness, restraint, and service environment.

6. Corrosion and Surface Protection

• Neither SPHC nor SPHD are stainless steels; they are subject to corrosion in atmospheric and industrial environments.
• Standard protections:
• Hot‑dip galvanizing, electrogalvanizing, or pre‑painted/coated systems provide sacrificial or barrier protection.
• Organic coatings (paints, powder coats) are common for finished parts.
• Oil or temporary rust inhibitors may be used during storage and shipment.
• PREN is not applicable to these non‑stainless steels. If assessing stainless alternatives, the PREN index can be used: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Corrosion allowances (material thickness, coatings selection) are based on environment and expected life; galvanizing is a common economical choice for structural exposures.

7. Fabrication, Machinability, and Formability

• Formability: SPHD is optimized for forming (drawing, deep drawing, multiple bending) and will show fewer splits, better wrinkle resistance, and higher uniform elongation than SPHC at equivalent processing histories.
• Bending: SPHD tolerates tighter bend radii and deeper draws with reduced risk of edge cracking.
• Machinability: Both grades have moderate machinability; SPHC with slightly higher sulfur content (if specified to enhance machinability) could be easier to machine, but standard practice is to select machinability-enhanced subgrades when needed.
• Cutting and stamping: Both are readily stamped; SPHD may run better on high‑speed presses when complex draws are required.
• Secondary operations (forming after welds, heat straightening) should account for residual stresses; SPHD's higher ductility often reduces springback variability in forming.

8. Typical Applications

Selection rationale: - Choose SPHC for broad availability, lower cost, and when forming operations are simple or when higher yield may be acceptable. - Choose SPHD when the process involves deep drawing, severe bending, or other high‑strain forming operations where predictable plastic behavior and higher elongation are required.

9. Cost and Availability

• Cost: SPHC is typically the lower‑cost, general‑purpose product due to broader production volumes and less stringent process control. SPHD may carry a modest premium because of tighter control of chemistry, processing, and guaranteed formability metrics.
• Availability: SPHC is widely available in many thicknesses and coils from multiple mills. SPHD availability may be slightly more limited by mill and region, but it is usually stocked for automotive and appliance supply chains. Product form (coil, sheet, plate) availability varies by mill and market; always confirm lead time with suppliers.

10. Summary and Recommendation

Summary table

Recommendation: - Choose SPHC if your requirements prioritize availability and cost for general structural and stamped parts where deep forming and maximum ductility are not critical. - Choose SPHD if your parts undergo significant plastic deformation (deep drawing, severe bending, multi‑stage forming) and you require predictable, higher elongation and improved formability even at a modest premium.

Concluding note: SPHC and SPHD are sibling low‑carbon hot‑rolled grades with overlapping strengths. The primary engineering decision turns on formability—choose the grade matched to the forming severity, verify mill certificates for chemical and mechanical limits, and validate forming and welding procedures on representative material batches before full production.