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

Introduction

SPHC and SPHD are two JIS-designated hot-rolled steel grades commonly specified in sheet and strip for structural and forming operations. Engineers, procurement managers, and production planners frequently face the choice between them when balancing cost, formability, and downstream processing requirements. Typical decision contexts include whether to prioritize deep-draw performance and tight surface quality (forming-intensive parts) versus general structural applications where cost and availability are primary drivers.

The core distinction is that SPHD is produced and specified for improved drawability and forming characteristics relative to SPHC, which is a general-purpose commercial hot-rolled grade. Because both are used for similar part families, direct comparison is frequent in design reviews and procurement specifications.

1. Standards and Designations

• JIS (Japanese Industrial Standards):
• SPHC — Hot-rolled steel plates, sheets and strips for general forming (commercial quality).
• SPHD — Hot-rolled steel plates, sheets and strips for drawing (improved formability).
• ASTM/ASME: There is no direct one-to-one ASTM equivalent; the closest analogs are generic hot-rolled mild steels covered under ASTM A1011 / A1008 families (commercial quality vs drawing quality), but specification differences vary by chemistry and allowable tolerances.
• EN: EN 10025 family covers structural steels and other EN standards cover drawing/deep-drawing grades; equivalence requires checking chemistry and mechanical properties case by case.
• GB (China): Chinese hot-rolled commercial and drawing steels (e.g., Q235 series for general grades) may serve similar functions but are not direct equivalents without cross-comparison.

Classification: Both SPHC and SPHD are carbon (low-carbon) steels rather than alloy, tool, stainless, or HSLA steels. Some mill variants may include microalloying elements at trace levels for property control, but both are principally low-carbon hot-rolled steels.

2. Chemical Composition and Alloying Strategy

Notes: The table gives qualitative composition strategy rather than specific weight-percent limits. Both grades are intentionally low-alloy/low-carbon; SPHD places stronger emphasis on minimizing elements and impurities that degrade surface quality and formability (P, S, free N, and high C).

How alloying affects properties: - Carbon increases strength and hardenability but reduces ductility and weldability. Lower carbon in SPHD improves formability. - Manganese raises strength and hardenability and helps deoxidation; excessive Mn can slightly reduce formability. - Silicon is used for deoxidation; high Si can reduce surface quality and coatability. - Microalloying (V, Nb, Ti) when present in small amounts may refine grain size and increase strength without large losses of ductility, but such elements are usually avoided for deep-drawing grades.

3. Microstructure and Heat Treatment Response

Typical microstructures: - SPHC: Ferrite–pearlite microstructure produced by hot rolling and controlled cooling. Grain size and pearlite fraction depend on cooling rate; generally balanced for moderate strength and ductility. - SPHD: Also ferrite–pearlite but with process control tuned to produce a finer ferrite grain size and lower pearlite fraction (or finer lamellar spacing) to enhance formability and uniform elongation.

Heat treatment and processing: - Both grades are normally supplied in the as-hot-rolled condition. They are not intended for quench-and-temper treatment typical of higher-strength structural steels. - Normalizing can refine grain size and modestly increase strength and toughness for both grades, but is rarely applied to SPHD because it can alter formability. - Thermo-mechanical controlled processing (TMCP) is not typical for commodity SPHC/SPHD, though some mills may apply controlled rolling and accelerated cooling to achieve narrower property ranges. - Cold working (e.g., pickling, skin-pass) affects surface finish and mechanical properties; SPHD often receives tighter surface processing for drawing applications.

4. Mechanical Properties

Explanation: SPHD is formulated and processed to yield superior ductility and consistent forming behavior; SPHC emphasizes broad applicability and cost competitiveness. Mechanical test values are thickness- and mill-dependent; when exact numbers are needed, refer to mill certification or JIS tables for the specific lot.

5. Weldability

Weldability in low-carbon hot-rolled steels is generally good, but it depends on carbon content, alloying, and impurity levels. Two commonly used weldability indices are shown below.

• Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• International/Pcm index: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Both SPHC and SPHD are low-carbon and therefore generally easy to weld with common processes (MIG/MAG, SMAW, TIG). - SPHD’s lower carbon and tighter control of impurities often make it slightly more forgiving during welding (lower cold-cracking susceptibility and lower tendency to form hard HAZ microstructures). - Microalloying in some variants (Nb, V, Ti) can raise $CE_{IIW}$ and $P_{cm}$ marginally and may necessitate preheat or controlled interpass temperatures for thicker sections. - For critical fabrications, follow manufacturer’s mill certificates and pre/post-weld heat treatment guidelines and perform hydrogen control, particularly on thicker sections and multilayer welds.

6. Corrosion and Surface Protection

• Neither SPHC nor SPHD is stainless; corrosion resistance is that of plain carbon steel and must be managed for service life.
• Typical protective measures:
• Hot-dip galvanizing for atmospheric corrosion protection.
• Electrogalvanizing, coil coating, or painting for improved aesthetic and corrosion resistance.
• Conversion coatings (phosphating) before painting or forming to aid adhesion.
• PREN (Pitting Resistance Equivalent Number) applies to stainless grades and is not relevant to SPHC or SPHD, but for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index is not applicable to these non-stainless steels.

When specifying surface finish and protection, consider forming-induced damage to coatings: for deep drawing (SPHD), select coatings and pretreatments designed for high-strain forming to avoid cracking or delamination.

7. Fabrication, Machinability, and Formability

• Cutting: Plasma, laser, or mechanical shearing are all used. Both grades cut readily; SPHD’s improved surface quality can reduce secondary finishing.
• Bending and forming: SPHD outperforms SPHC in deep drawing and high-strain forming due to lower carbon, tighter impurity control, and process-milled surface finish; it typically tolerates smaller bend radii and more complex geometries without cracking.
• Machinability: Both grades are machinable as mild steels; machinability factors are mainly influenced by carbon, sulfur additions (free-cutting variants), and hardness. SPHC and SPHD are not optimized for free-machining operations.
• Finishing: Surface quality more consistent on SPHD; SPHC may exhibit more scale and surface irregularities requiring pickling/shot-blasting for critical finishes.

8. Typical Applications

Selection rationale: - Use SPHC when cost, wide availability, and acceptable forming performance suffice. - Use SPHD when designs require high-drawability, tight surface control, or more severe forming operations.

9. Cost and Availability

• SPHC is typically the less expensive and more widely available commodity hot-rolled sheet across thickness ranges and service centers.
• SPHD commands a modest premium due to tighter chemistry and surface control and may have more constrained supply in certain regions or thicknesses.
• Product forms: coil, sheets, and blanks; both are commonly produced as coils. For just-in-time manufacturing, confirm local mill stocks and lead times for SPHD, as longer procurement lead times may offset material benefits.

10. Summary and Recommendation

Recommendations: - Choose SPHC if you need a cost-effective, readily available hot-rolled steel for general structural, welded, or lightly formed components where extreme formability is not required. - Choose SPHD if your application demands superior deep-drawing performance, higher uniform elongation, and tighter surface quality after forming (e.g., automotive inner panels, complex stamped parts), and you are willing to accept a modest material premium and potentially longer lead times.

Final note: Always confirm the exact mill chemical and mechanical certificates for the lot you intend to use. Because both SPHC and SPHD are defined by process intent and mill practice rather than high-alloy chemistry, property spreads can vary by supplier and thickness; material selection should combine specification review, forming trials, and supplier collaboration to ensure nominal behavior in production.