QP980-CR vs QP980-HDG – 成分、熱処理、特性、および用途

Introduction

QP980-CR and QP980-HDG are two product forms of the same high-strength Quenching-and-Partitioning (Q&P) steel chemistry developed for structural automotive and high-performance structural applications. Engineers and procurement specialists commonly weigh trade-offs between corrosion protection, surface condition, and downstream processing requirements when choosing between a cold-rolled (CR) bare coil and a hot-dip galvanized (HDG) coil of the same strength level. Typical decision contexts include: whether corrosion resistance should be provided by an integral zinc coating or by separate surface treatments, and whether coating presence will affect welding, formability, and paint-adhesion during assembly.

The principal practical difference between the two is surface protection: QP980-CR is supplied as bare cold‑rolled steel (no factory-applied sacrificial coating), while QP980-HDG is delivered with a continuous hot‑dip zinc (or Zn–Fe) coating for corrosion protection. Because the underlying base chemistry and Q&P processing route are similar, many mechanical attributes are comparable, but the coating state drives differences in corrosion performance, pre- and post‑processing handling, and some fabrication considerations.

1. Standards and Designations

• Common standard systems in which QP-type steels appear:
• GB (China): QP980 is used in 国内 standards and manufacturer specifications.
• EN (Europe): Equivalent steels are typically referenced as advanced high-strength steels (AHSS) — not a single EN grade.
• JIS (Japan): Similar concepts exist (Q&P steels), but exact QP980 designation may be manufacturer-specific.
• ASTM/ASME: No single ASTM grade; QP steels are supplied to manufacturer/automotive specifications and test certificates.
• Classification: QP980 family is a high-strength low-alloy (HSLA) / advanced high-strength steel (AHSS) produced by quenching and partitioning metallurgy. It is not a stainless, tool, or high-alloy grade — base chemistry is low‑carbon, microalloyed/controlled alloying.

Note: Exact designation formats (CR = cold‑rolled, HDG = hot‑dip galvanized) reflect product form/coating rather than a separate metallurgical grade.

2. Chemical Composition and Alloying Strategy

The QP980 family targets a chemistry optimized for Q&P processing: low carbon for weldability and ductility recovery, controlled Mn and Si for hardenability and partitioning behavior, and microalloying (e.g., Nb, Ti, V) in some variants to refine grain size and provide precipitation strengthening. Coating issues (for HDG) may impose additional limits on Si and P content to ensure good galvanizing behavior.

Element	Typical role / notes
C	Low to moderate; balances strength and ductility and controls martensite fraction after Q&P.
Mn	Principal austenite stabilizer and hardenability aid; also contributes to solid solution strengthening.
Si	Promotes carbon partitioning during Q&P; can affect galvanizing (high Si can produce poor coating).
P	Typically minimized; higher values can worsen corrosion and galvanizing quality.
S	Kept low for toughness and surface quality.
Cr, Ni, Mo	May be present in small amounts in some recipes to tailor hardenability and tempering; not primary alloying elements.
V, Nb, Ti	Microalloying additions for grain refinement and precipitation strengthening; influence processing windows.
B	When used in ppm levels, enhances hardenability.
N	Controlled; interacts with Ti/Nb for precipitation control.

Note: Specific mass fractions vary by producer and product specification. For HDG product forms, steelmakers often control the "Si-equivalent" (Si + 2.5P) or other parameters to ensure consistent zinc coating formation.

How alloying affects behavior: - Strength and hardenability: Mn, microalloying elements, and low levels of Cr/Ni/Mo increase hardenability and capability to form martensite during Q&P, raising ultimate tensile strength (UTS). - Partitioning and ductility: Si is used to retard carbide formation and favor carbon partitioning to retained austenite, improving work-hardening and ductility. - Corrosion and coating compatibility: P and Si must be balanced to avoid adverse galvanizing intermetallic layers; surface condition matters for paint adhesion.

3. Microstructure and Heat Treatment Response

QP980 steels are typically produced via a Q&P route (continuous annealing lines in the case of cold-rolled coil, with an optional galvanizing step for HDG), giving a mixed microstructure consisting of tempered martensite, fresh martensite, and retained austenite stabilized by carbon partitioning. Key process steps and their microstructural effects:

• Intercritical anneal and quench: Partially transforms austenite to martensite; the degree controls martensite fraction.
• Partitioning hold: Carbon transfers from martensite to remaining austenite, stabilizing retained austenite and improving ductility/strain hardening.
• Cooling and coiling: Final microstructure is a fine distribution of martensitic matrix with islands or films of retained austenite.

Effect of processing routes: - Cold-rolled (QP980-CR): Typically done on continuous annealing lines with precise temperature control for Q&P; yields the intended microstructure with limited additional surface reactions. - Hot-dip galvanized (QP980-HDG): Typically requires either in-line galvanizing after annealing (galvannealing or HDG), which can slightly alter the surface thermal history and may produce thin iron-zinc intermetallics; these surface reactions do not fundamentally change the bulk Q&P microstructure but can affect surface decarburization or interfacial chemistry.

Alternative heat treatments: - Full quenching & tempering (Q&T) produces a more uniformly martensitic and tempered microstructure, often with higher toughness after tempering but different elongation behaviour; QP processing specifically targets retained austenite to enhance ductility at very high strengths. - Thermo-mechanical control processing (TMCP) can be applied prior to Q&P in plate products to refine grains and influence precipitation strengthening.

4. Mechanical Properties

QP980 is named for an approximate minimum tensile strength level around 980 MPa; exact guaranteed mechanicals depend on supplier and processing (and whether the product is CR or HDG). The presence of a zinc coating does not meaningfully change bulk tensile strength but can influence surface-sensitive properties (e.g., fatigue initiation sites, bending performance).

Property	QP980-CR (bare cold‑rolled)	QP980-HDG (hot‑dip galvanized)
Tensile strength (UTS)	Designed for ≈ 980 MPa (by grade designation)	Designed for ≈ 980 MPa (coating does not change UTS)
Yield strength (YS)	High (depends on temper and martensite fraction)	Similar to CR in bulk; surface treatment does not substantially alter YS
Elongation	Moderate but limited vs mild steels (typical AHSS behaviour)	Comparable bulk elongation; coating can influence bendability and edge cracking susceptibility
Impact toughness	Dependent on processing and thickness; Q&P aims to retain reasonable toughness at high strength	Comparable to CR in the core; surface intermetallics can affect notch performance in coated edges
Hardness	High (martensitic matrix)	Same bulk hardness; surface hardness influenced by Zn intermetallics at the interface

Explanation: QP980’s high strength is due to a controlled fraction of martensite plus carbon-enriched retained austenite. Ductility and toughness are improved relative to purely martensitic steels by the stability of retained austenite and transformation-induced plasticity (TRIP)-like mechanisms. The coating state (CR vs HDG) does not alter these bulk mechanisms but can affect surface fracture initiation and localized deformation during forming.

5. Weldability

Weldability of QP980 steels depends on base chemistry (especially carbon equivalent and hardenability) and on surface condition (presence of zinc).

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

Interpretation (qualitative): - Base carbon is kept low in QP steels to limit CE and facilitate weldability. However, Mn and microalloying can increase hardenability, raising the risk of hard martensitic HAZ and hydrogen-induced cold cracking if not managed. - For HDG material, the zinc coating introduces additional welding hazards: Zn boils at welding temperatures, causing porosity, increased spatter, and toxic fumes; zinc at the weld root can promote hydrogen pickup unless coatings are removed or appropriate welding procedures (preheating, controlled heat input, backing gas, consumable selection) are used. - Practical guidance: remove coating at butt joints where possible, use controlled preheat and interpass temperatures for thick sections or high CE, employ low-hydrogen consumables, and validate procedures with welding procedure qualification tests.

6. Corrosion and Surface Protection

• QP980-CR (bare): Requires external corrosion protection for most exposed applications — paint systems, cathodic protection, or aftermarket coatings are typical. Bare cold‑rolled surface finish provides a clean substrate for e-coat and paint but requires pretreatment.
• QP980-HDG (hot-dip galvanized): Provides galvanic (sacrificial) protection via a continuous zinc layer; for many environments, HDG will significantly extend service life without immediate painting. HDG may be supplied as pure Zn (galvanized) or galvannealed (Zn–Fe intermetallic) which improves paint adhesion.

When stainless indices apply: - PREN is not applicable to QP980 grades because they are non‑stainless steels. For reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ — this metric is used only for austenitic stainless alloys and is not relevant to carbon/HSLA steels.

Coating considerations: - For HDG, coating mass (g/m² per side) and galvanizing process control affect durability; galvannealed surfaces are often used when subsequent painting is required because the iron-rich surface promotes adhesion.

7. Fabrication, Machinability, and Formability

• Forming and bending: QP980 steels have less formability than lower-strength steels; retained austenite and TRIP effect improve local formability, but springback and edge cracking are concerns. HDG coatings can crack or flake at tight bend radii; tooling radii, lubrication, and process parameters must be optimized.
• Cutting and punching: High-strength microstructures increase tool wear and may require hardened tooling. Coated material (HDG) can produce buildup on tooling due to zinc transfer; tool coatings or lubricant selection help.
• Machining: Bulk machinability is similar between CR and HDG, but zinc coating generates white zinc oxide fumes when machined at high temperature and can affect chip formation.
• Surface preparation: For painting or adhesive bonding, galvanized substrates may need phosphating or other pretreatments to achieve specified adhesion.

8. Typical Applications

QP980-CR (bare)	QP980-HDG (galvanized)
Structural automotive components where final corrosion protection is applied by OEM (e-coat, paint) and tight surface finish control is required (e.g., inner structural members, energy-absorbing components)	Exterior automotive body panels and structural parts where factory-applied galvanizing reduces corrosion risk and downstream painting is possible (galvannealed for optimal paint adhesion)
High-strength structural parts in non-corrosive environments where coating would interfere with joining or assembly	Infrastructure components exposed to atmosphere where galvanic protection is preferred over frequent maintenance
Components requiring precise surface metrology (pre‑assembly machining) before coating	Cold-formed profiles, electrical enclosures, and appliances requiring superior atmospheric corrosion resistance without additional painting

Selection rationale: - Choose CR when the manufacturing flow includes controlled paint lines, when coating would interfere with downstream joining processes, or when surface cleanliness is essential. - Choose HDG when extended corrosion protection is required out of the box or when minimizing aftermarket coating steps is a priority.

9. Cost and Availability

• Cost: QP980-HDG typically costs more per tonne than QP980-CR due to the added galvanizing operation and materials (zinc). However, total life-cycle cost can be lower for HDG where corrosion protection prevents future recoating or replacement.
• Availability: Both product forms are commonly available in automotive coil markets and from specialty mills. HDG availability depends on regional galvanizing-capable continuous lines and buyer-specified coating weights; lead times for HDG can be longer than for CR because of additional process steps and coating schedules.

10. Summary and Recommendation

Attribute	QP980-CR	QP980-HDG
Weldability	Generally good with proper welding practice; avoids Zn-related welding issues	Requires additional weld procedures/precautions due to zinc (fume, porosity); remove coating at joint when practical
Strength–Toughness balance	High strength with engineered retained austenite for improved ductility	Equivalent bulk strength–toughness; surface condition may affect local fatigue/notch behavior
Cost	Lower initial material cost; requires separate corrosion protection	Higher initial cost; reduces downstream coating or maintenance costs

Recommendation: - Choose QP980-CR if: your manufacturing flow relies on clean, uncoated surfaces for precise forming, coating and bonding steps will be performed in controlled paint/e-coat lines, or if welding operations cannot accommodate zinc-related issues. - Choose QP980-HDG if: you need built‑in atmospheric corrosion protection to reduce total life-cycle costs, want a product that tolerates outdoor storage or exposure before finishing, or require galvannealed surfaces for improved paint adhesion with fewer pretreatment steps.

Final note: QP980 in CR and HDG forms share the same underlying Q&P metallurgy and deliver comparable bulk mechanical performance. The decision between them is primarily driven by surface-protection requirements, downstream joining/processing constraints, and total cost-of-ownership considerations. Always request mill test certificates and perform process-specific qualification (forming, welding, painting) on the exact coil/lot to confirm conformance with functional and assembly requirements.