SPHD vs SPHE – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
SPHD and SPHE are two commonly specified hot-rolled structural steel grades used in general engineering, automotive subcomponents, and cold-formed components. Engineers and procurement teams frequently balance trade-offs between cost, formability, weldability, and strength when selecting between them. Typical decision contexts include specifying sheet or strip for cold bending and stamping, choosing plate for welded structures, or selecting material for parts that require subsequent surface treatment.
The principal practical difference between SPHD and SPHE lies in their cold-forming behavior: one grade is typically controlled to provide superior resistance to cracking and better edge/cold-bend performance, while the other is produced with slightly different process targets (yield/strength and surface attributes) that may favor structural load-bearing or lower cost. Because both are low-carbon steels used in similar supply forms, they are commonly compared where forming quality versus mechanical performance matters.
1. Standards and Designations
- JIS (Japan): SPHD, SPHE appear in JIS standards for hot-rolled steels (general structural/shape applications).
- EN/European: There is no direct one-to-one European name; comparable steels fall under EN 10025/EN 10111 product categories for non-alloy structural steels or under EN 10111 for cold-rolled steels depending on processing.
- ASTM/ASME: Direct equivalent designations are not standard in ASTM; ASTM A1011/A1018 cover similar commercial steel strip/sheet classes for hot-rolled and cold-rolled products.
- GB (China): GB standards list commercial hot-rolled steels with different labels; direct equivalence requires chemistry and mechanical comparison.
Classification: Both SPHD and SPHE are plain carbon (low-alloy, non-stainless) steels used primarily as carbon/structural steels rather than alloy, tool, stainless, or HSLA classes. They are intended for forming and general fabrication rather than high-temperature or corrosion-critical applications.
2. Chemical Composition and Alloying Strategy
Note: Exact limits depend on the issuing standard and product form. Typical commercial hot-rolled low‑carbon steel practice emphasizes very low carbon, modest Mn for strength, and tight control of P/S for formability.
| Element | Typical range or role (SPHD / SPHE) |
|---|---|
| C (carbon) | Very low carbon to preserve formability; usually controlled to a low maximum (both grades are low‑C). |
| Mn (manganese) | Moderate Mn for strength and deoxidation; SPHE may have slightly higher or more tightly controlled Mn for consistent properties. |
| Si (silicon) | Small addition as deoxidizer; trace levels typical. |
| P (phosphorus) | Controlled to low maximums to avoid embrittlement and poor formability. |
| S (sulfur) | Kept low; sometimes limited even more for improved bending and surface quality. |
| Cr, Ni, Mo, V, Nb, Ti, B | Generally absent or at trace levels in both grades; if present, microalloying is minimal. |
| N (nitrogen) | Low levels; not a design alloying element for these grades. |
How alloying affects behavior: - Carbon increases strength and hardenability but reduces ductility and cold-forming performance. Both grades maintain low C to favor bending and drawing. - Manganese raises tensile strength and contributes to hardenability; higher Mn improves strength but can reduce formability if excessive. - Very small microalloying additions (Ti, Nb, V) can refine grain size and raise strength with minimal ductility penalty if thermomechanically applied, but SPHD/SPHE are generally plain-carbon products, so significant microalloying is not typical.
Consult the relevant standard sheet for authoritative composition limits; manufacturers may publish exact nominal chemistries per coil or plate batch.
3. Microstructure and Heat Treatment Response
Typical microstructure: - Both SPHD and SPHE, after conventional hot-rolling and air cooling, exhibit ferrite–pearlite microstructures typical of low-carbon steels: a ferritic matrix with isolated pearlite colonies. Grain size and banding depend on rolling schedule and cooling rate.
Processing effects: - Normalizing: Produces finer, more homogeneous ferrite/pearlite and can slightly increase strength and toughness; not commonly applied for commercial hot-rolled sheet unless specific requirements exist. - Quench & temper: Not applicable as a standard route for these grades; they are not designed for hardening through quench/tempering. - Thermo-mechanical control (controlled rolling): If applied (more common in HSLA), it refines grain size and can raise strength with retained ductility. For SPHE, tighter process control in rolling and cooling can yield slightly more uniform microstructure and improved cold-forming performance compared with more basic hot-rolled practice.
Implications for forming: - Finer, more uniform ferrite with lower pearlite fraction generally improves cold-bend performance and reduces edge-cracking risk. Manufacturers producing SPHE often target process control that yields such favorable microstructure for forming applications.
4. Mechanical Properties
Exact guaranteed values are specified in standards and purchaser specifications. Typical comparative tendencies are summarized below.
| Property | SPHD (typical) | SPHE (typical) | Comment |
|---|---|---|---|
| Tensile strength (Rm) | Moderate (suitable for structural sheet) | Moderate to slightly higher or similarly controlled | SPHE is often processed for consistent tensile values with narrower tolerances. |
| Yield strength (Rp0.2) | Moderate | Moderate; may be slightly lower to favor formability in some product lines | Manufacturers may control yield for either grade depending on intended forming use. |
| Elongation (%) | Good | Typically equal or better (higher elongation possible) | SPHE is frequently specified where higher elongation/cold-formability is required. |
| Impact toughness | Typical for low-carbon ferrite–pearlite steels | Comparable; can be improved with controlled rolling | Not a primary differentiator at ambient temperatures unless specified. |
| Hardness | Low–moderate | Low–moderate | Both are soft compared with HSLA or alloy steels. |
Which is stronger/tougher/ductile: - Neither grade is intended to be a high-strength steel; differences are subtle. SPHE is often produced with process conditions that prioritize ductility and consistent elongation for cold forming, so it commonly performs better in demanding bend/form operations. SPHD can be specified where standard structural performance and cost are prioritized.
5. Weldability
Both grades are readily weldable using conventional fusion processes; their low carbon content and limited alloying make preheat/post-heat requirements minimal in typical thicknesses.
Useful weldability indices: - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Low $CE_{IIW}$ and low $P_{cm}$ indicate good weldability and low risk of hydrogen-assisted cold cracking. Both SPHD and SPHE generally give low values for these indices because of their low carbon and minimal alloy content. - Differences in weldability between SPHD and SPHE are minor; however, variations in sulfur and residuals, steel cleanliness, and surface condition can influence weld quality and require attention to consumables and welding parameters. - If edge cracking during cold forming is a concern, choosing the grade with superior cold-formability (commonly SPHE) can reduce the need for pre- or post-weld forming adjustments.
6. Corrosion and Surface Protection
- Both SPHD and SPHE are non‑stainless carbon steels and require surface protection in corrosive environments.
- Typical protection methods: hot-dip galvanizing, electro-galvanizing, zinc lamella coatings, zinc/flake coatings, painting with appropriate pretreatment, or corrosion-resistant coatings depending on exposure.
- PREN (Pitting Resistance Equivalent Number) is not applicable because these are non‑stainless steels. For comparison, PREN is used only for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
Selection note: - For outdoor or corrosive service, specify surface treatment and coating system; the base steel selection between SPHD and SPHE does not provide intrinsic corrosion resistance differences.
7. Fabrication, Machinability, and Formability
- Cutting and machining: Both grades machine similar to mild carbon steels. Lower hardness improves tool life and machinability. Lubrication and cutting parameters should be tailored to part complexity and tolerances.
- Cold bending/forming: This is where practical differences appear. SPHE is often offered with tighter control of chemistry and processing to optimize bendability, reduce springback variance, and minimize edge cracking in tight radii. SPHD performs well for general bending but may show slightly reduced resistance to edge fracture when forming to small radii.
- Deep drawing/stamping: SPHE is more commonly specified for deep drawing and severe gauge reduction operations due to higher guaranteed elongation and formability consistency.
- Surface finish: SPHE grades intended for forming generally have stricter surface condition controls to avoid tooling damage and part rejection.
Practical advice: - For stamped parts or tight-bend components, request forming data from the mill and consider trial runs. Specify knockout radii, edge preparation, and lubrication for optimal outcomes.
8. Typical Applications
| SPHD — Typical uses | SPHE — Typical uses |
|---|---|
| General structural sheet and light fabrication where standard formability and cost are priorities | Cold-formed automotive parts, deep-drawn components, and applications requiring improved cold-bend edge performance |
| Low-cost body panels for non-critical forming operations | Pressed parts with tight radii or high stretch requirements (e.g., housings, brackets) |
| Welded structures where standard strength is adequate | Components requiring consistent elongation and surface quality for high-volume forming |
Selection rationale: - Choose SPHE when forming performance, consistent elongation, and reduced risk of edge cracking are primary drivers. - Choose SPHD when cost and general structural performance are primary and forming severity is moderate.
9. Cost and Availability
- Relative cost: Both grades are commercially available and cost-competitive. SPHE may command a modest premium in some markets when mills apply additional process controls or surface quality treatments aimed at forming performance.
- Availability by product form: Both are commonly supplied as hot-rolled coils, sheets, and strips. Availability depends on mill portfolios and regional demand—automotive supply chains often drive larger volumes of SPHE-formulated products.
- Procurement tip: For high-volume programs, negotiate mill rolling schedules and request certified mill test reports (MMTRs) to lock in composition and mechanical tolerances.
10. Summary and Recommendation
| Attribute | SPHD | SPHE |
|---|---|---|
| Weldability | Excellent (low C, low alloy) | Excellent (low C, low alloy) |
| Strength–Toughness balance | Adequate for structural use | Similar strength with generally better ductility/formability |
| Cost | Slightly lower in some markets | Slight premium for controlled processing/forming quality |
Recommendations: - Choose SPHE if: you require superior cold-forming performance, higher guaranteed elongation, tighter control on forming-related properties, or frequent tight-radius bending and deep drawing. - Choose SPHD if: your application is general structural fabrication with moderate forming, where cost and standard mechanical performance are the primary criteria.
Final note: The differences between SPHD and SPHE are subtle and often tied to mill processing and specification tolerances rather than radically different chemistries. Always request the exact standard designation, mill certificates, and forming/welding data for the specific coil or sheet batch to confirm suitability for your intended process.