CR4 vs CR5 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
CR4 and CR5 are cold‑rolled low‑carbon steel grades used widely in sheet applications where forming quality, surface finish, and dimensional control are critical. Engineers, procurement managers, and manufacturing planners often face a choice between them when specifying materials for stamped automotive panels, deep‑drawn appliance shells, or precision fabricated enclosures. The selection typically balances formability versus strength and cost, as well as downstream process constraints such as welding and coating.
The principal practical distinction between the two grades lies in their relative suitability for severe drawing operations: one grade is optimized for very high deep‑draw formability (improved drawability and flangeability), while the other provides a slightly higher strength/ductility balance for applications where drawing severity is moderate. Because both grades are cold‑rolled and annealed to produce a fine ferritic microstructure and controlled surface condition, they are frequently compared during material selection for forming‑intensive production.
1. Standards and Designations
Cold‑rolled general‑purpose and drawing quality steels are covered by multiple national and international standards. While CR4/CR5 nomenclature is commonly used in commercial and supplier catalogs, equivalent or related grades are found in these specifications:
- ASTM / ASME: A1008 / A1008M (cold‑rolled, commercial quality, drawing quality variants)
- EN: EN 10130 (cold‑reduced, non‑alloy steels for cold forming)
- JIS: G3141 (commercially available cold‑rolled steels and deep‑drawing grades)
- GB/T (China): Various cold‑rolled sheet standards covering low‑carbon drawing steels
Classification: Both CR4 and CR5 are non‑alloy, low‑carbon cold‑rolled steels (not stainless, not tool steel, not HSLA) typically intended for cold forming and exposed‑surface applications.
2. Chemical Composition and Alloying Strategy
The chemistry of CR4 and CR5 is intentionally simple: very low carbon, tightly controlled silicon, manganese, and minimal impurities (P, S). Microalloying additions (Ti, Nb, V, B) are usually absent or at low levels in standard drawing steels; when present they are carefully controlled because they affect grain size, recrystallization, and formability.
| Element | CR4 (typical strategy) | CR5 (typical strategy) |
|---|---|---|
| C | Very low — kept minimal to maximize formability and weldability | Extremely low — optimized further to improve drawability and reduce hardenability |
| Mn | Controlled (moderate) to aid strength and rolling | Controlled; often similar to CR4 |
| Si | Low — deoxidation control; limited to avoid embrittlement | Low — similar rationale |
| P | Strictly limited (impurity) | Strictly limited |
| S | Controlled (often ≤0.01–0.02%) | Controlled; may be lower for improved surface quality |
| Cr, Ni, Mo | Typically trace or absent in standard drawing grades | Typically trace or absent |
| V, Nb, Ti | Generally not deliberately added for standard CR grades; if present, at very low microalloy levels | May be engineered at trace levels in specialty variants to tailor texture, but not common in plain CR5 |
| B | Not typical in drawing quality CR grades | Not typical |
| N | Low and controlled | Low and controlled |
How alloying affects properties: - Carbon: primary control on strength and hardenability; lower carbon improves ductility and weldability but reduces as‑rolled strength. - Manganese and silicon: provide deoxidation and moderate strengthening; excessive Mn can raise CE and impair weldability. - Microalloying and impurities: trace microalloying can refine grain size but must be balanced against texture effects that influence deep‑drawability.
3. Microstructure and Heat Treatment Response
Microstructure: - Both CR4 and CR5 are processed to produce a predominantly ferritic (α‑iron) microstructure with fine, equiaxed grains after annealing and recrystallization. The final grain size and crystallographic texture (plane and directionality of grains) are crucial for deep drawing performance.
Typical processing routes and effects: - Recrystallization anneal (continuous or batch anneal) followed by controlled cooling and possible skin‑pass: Produces a uniform, fine ferritic microstructure and a crystallographic texture that governs planar anisotropy (r‑value) and deep‑drawability. - Normalizing: Not typical for cold‑rolled drawing steels; used rarely when a different microstructure is required. - Quench & temper: Not applicable — CR grades are non‑heat‑treatable steels intended for cold forming. - Thermo‑mechanical processing: In tailored variants, controlled rolling and annealing cycles can refine grain size and control texture to enhance drawability and stretch flangeability.
CR5 variants intended for extreme deep drawing often receive annealing cycles and rolling schedules optimized to produce a higher average r‑value and more favorable planar anisotropy, thereby reducing local thinning during severe drawing.
4. Mechanical Properties
For cold‑rolled drawing steels the absolute values vary by supplier, temper, and final anneal. The meaningful comparison is relative performance for strength, ductility, and toughness.
| Property | CR4 (typical) | CR5 (typical) |
|---|---|---|
| Tensile strength | Moderate — conforms to low‑carbon cold‑rolled sheet ranges | Similar or slightly lower (tradeoff to gain formability) |
| Yield strength | Moderate | Often slightly lower to improve drawability |
| Elongation (ductility) | Good | Higher — optimized for deep drawing |
| Impact toughness | Good at room temperature | Comparable; focus is on ductility rather than improved impact |
| Hardness | Low to moderate (soft anneal) | Low — softer to permit more deformation before necking |
Which is stronger/tougher/more ductile and why: - CR4 provides a balanced combination of strength and formability for many stamping operations. - CR5 is tuned for higher formability (greater elongation and better resistance to localized thinning) and therefore typically exhibits slightly lower yield and tensile strengths in the annealed condition, while offering improved ductility for severe forming.
5. Weldability
Weldability of cold‑rolled drawing steels is generally good due to very low carbon content. Assessments typically use carbon‑equivalent formulas to estimate susceptibility to cold cracking in the heat‑affected zone.
Common weldability indices: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International formula for weldability (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: - Lower $CE_{IIW}$ and $P_{cm}$ values indicate better weldability and lower preheat requirements. CR4 and CR5 both have low carbon and minimal alloying, so their carbon equivalents are typically low and they are readily welded by common methods (MIG/MAG, TIG, resistance welding) with standard practices. - CR5’s slightly lower carbon and controlled impurities generally give it a small edge in weldability for complex, thin‑gauge applications where minimizing HAZ embrittlement risk is important. However, because CR5 may be softer, weld distortion and heat input should still be managed.
6. Corrosion and Surface Protection
Neither CR4 nor CR5 is stainless; corrosion protection strategies are therefore required for exposed applications.
- Common protections: hot‑dip galvanizing, electrogalvanizing, coil coatings, conversion coatings, and organic paint systems.
- Surface preparation: Pickling and phosphating or pre‑treatment are commonly used to promote coating adhesion.
When stainless‑type indices are relevant: - For stainless steels the pitting resistance equivalent number (PREN) is used: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - PREN does not apply to CR grades because Cr and Mo are present at negligible levels. Corrosion resistance of CR4/CR5 must be achieved through coatings and design (e.g., drainage, edge protection).
7. Fabrication, Machinability, and Formability
- Cutting: Both grades machine and shear similarly; burr formation is controlled by sheet quality and tooling.
- Bending: CR5 will generally tolerate tighter bend radii and more severe stretch bending without cracking because of superior ductility and anisotropy control.
- Drawing/forming: This is the decisive area. CR5 is engineered for higher deep‑drawability — better draw‑bead performance, improved stretch flangeability, and reduced earing tendency in cup drawing. CR4 offers good formability for standard stamping operations but may require modified tooling or lubrication for high draw ratios.
- Finish: Surface finish and skin‑pass control are similar; CR5 variants intended for critical visible panels may receive tighter surface quality tolerances and lower inclusions.
Practical note: Tooling, lubrication, blank holder force, and anneal temper all interact with material selection. A material with very high drawability can reduce springback and forming defects, but process settings still must be optimized.
8. Typical Applications
| CR4 — Typical Uses | CR5 — Typical Uses |
|---|---|
| General automotive outer panels with moderate drawing | Highly complex automotive inner panels and severe‑draw body components |
| Appliance panels (washers, dryers) with moderate forming | Deep‑drawn appliance shells and components with demanding geometry |
| Electrical enclosures and fabricated cabinets | Complex cookware/houseware blanks and multi‑draw components (where food‑safe coatings are applied) |
| General fabrication where cost balance matters | High‑volume stamping processes requiring minimal rejection from thinning or wrinkling |
Selection rationale: - Choose CR4 when the forming severity is moderate, when a balance of strength and formability is needed, and when cost is a primary concern. - Choose CR5 for parts that require multiple draws, high draw ratios, minimal thinning, and tight appearance/functional tolerances.
9. Cost and Availability
- Cost: CR5 typically commands a modest premium relative to CR4 because of tighter control of chemistry, anneal cycles, and texture control required to achieve very high drawability.
- Availability: CR4 is widely available from major mills and service centers in multiple product forms (coils, cut sheets). CR5 may be less ubiquitous and in some regions provided as a specialty or premium drawing product; lead times can vary depending on coating and temper requirements.
- Product forms: Both grades are commonly supplied in cold‑rolled coils and cut lengths; galvanizing and prepainted forms are widely available for CR4 and CR5 when corrosion protection is requested, though specific coated CR5 lots may be more limited.
10. Summary and Recommendation
| Metric | CR4 | CR5 |
|---|---|---|
| Weldability | Good | Very good (slightly better due to lower C/impurities) |
| Strength–Toughness balance | Balanced | Slightly lower strength but higher ductility for forming |
| Cost | Lower | Higher (premium for enhanced formability) |
Recommendation: - Choose CR4 if you need a cost‑effective cold‑rolled sheet for standard stamping, light structural panels, and applications where forming severity is moderate and where availability and supply security are priorities. - Choose CR5 if your parts require severe or multi‑stage deep drawing with tight tolerances on thinning and earing, if you need the highest planar ductility and drawability from a non‑stainless cold‑rolled sheet, or if minimizing part rejection from formability issues is critical despite a modest material premium.
Final practical guidance: specify required temper, coating, and surface finish in procurement documents; request supplier data on r‑value (planar anisotropy) and draw test performance for both CR4 and CR5 to validate real‑world formability for the intended die and lubrication system.