Q355GNH vs Q415GNH – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Selecting between Q355GNH and Q415GNH is a common design and procurement dilemma for engineers, procurement managers, and manufacturing planners working with high-strength structural steels. Typical decision contexts include balancing higher load capacity and thinner sections (strength) against weldability, toughness in low temperatures, and overall cost. Fabricators also weigh forming and machining trade-offs vs. performance in service.
At their core these two grades occupy adjacent strength bands within the family of microalloyed high-strength low-alloy (HSLA) steels used for structural plate and sections. The principal functional difference is a step up in guaranteed yield strength for Q415GNH compared with Q355GNH, which drives downstream differences in processing requirements, toughness management, and selection rationale.
1. Standards and Designations
- Major standard families where similar steels appear: GB/Chinese national standards (Q-series grades), EN (European), ASTM/ASME (U.S.), and JIS (Japan). Exact grade names and requirements vary between standards; conversion tables are guideline-only.
- Classification: Both Q355GNH and Q415GNH are non-stainless, low-carbon, microalloyed HSLA structural steels designed for good strength–toughness balance. They are not tool steels or stainless steels.
- Typical product forms: plates, coils, and welded structures; the suffixes (such as GNH) commonly encode process and property qualifiers (e.g., normalized, thermomechanically rolled, and enhanced low-temperature toughness) in manufacturer or national naming schemes. Verify standard text for exact suffix meaning in purchase specifications.
2. Chemical Composition and Alloying Strategy
The microalloyed HSLA strategy for both grades is to keep carbon low to preserve weldability and toughness, while adding small amounts of microalloying elements (Nb, V, Ti) plus controlled N to refine grain and increase strength through precipitation and grain refinement rather than by large carbon increases.
| Element | Q355GNH (typical role) | Q415GNH (typical role) |
|---|---|---|
| C (Carbon) | Low — limits hardenability, aids weldability | Low — may be marginally controlled lower or similar to Q355 to retain toughness |
| Mn (Manganese) | Moderate — solid solution strengthening and deoxidation | Moderate — may be slightly higher to support higher yield |
| Si (Silicon) | Trace–moderate — deoxidizer | Trace–moderate |
| P (Phosphorus) | Kept low — embrittlement control | Kept low |
| S (Sulfur) | Kept low — machinability, cleanliness | Kept low |
| Cr, Ni, Mo | Typically minimal or trace — not primary hardening mechanism | May be present in small amounts in some variants to support strength/hardenability |
| V, Nb, Ti (microalloying) | Present in microalloyed amounts to refine grains and precipitate-strengthen | Present; may be optimized for slightly higher precipitation strengthening |
| B (Boron) | Rare/trace — if used, controlled for hardenability | Rare/trace |
| N (Nitrogen) | Controlled — forms carbonitrides with microalloying metals | Controlled — may be slightly higher for strengthening control |
Note: Exact chemical limits are specified in relevant national or mill standards and vary with grade suffix and product form. The table shows qualitative roles rather than absolute concentrations.
How alloying affects properties: - Low carbon keeps weldability and ductility acceptable. - Mn and controlled Si deliver solid-solution strengthening and processing behaviour. - Microalloying elements (Nb, V, Ti) enable high yield strength through grain refinement and precipitation hardening without large carbon increases, preserving toughness. - Small increases in alloying or processing intensity to meet Q415 requirements can raise hardenability and require additional thermal control.
3. Microstructure and Heat Treatment Response
Typical microstructures for both grades after standard processing are fine-grained ferrite with controlled amounts of tempered bainite or polygonal ferrite and microalloy precipitates, depending on thermo-mechanical routes.
- Q355GNH: Processed to yield a controlled ferrite–pearlite or ferrite–bainite matrix with fine grain size via normalizing or controlled rolling. Microalloy precipitates (Nb/Ti/V carbonitrides) impede grain growth and contribute to yield strength.
- Q415GNH: To achieve the higher guaranteed yield, processing often increases thermomechanical rolling intensity or uses stronger precipitation strengthening. This can produce a higher fraction of bainitic structures or a more refined ferritic matrix with denser precipitates, increasing strength but requiring tighter thermal control.
Heat treatment response: - Normalizing/refining: Both grades benefit from normalization or controlled rolling to refine grain size and improve toughness. - Quenching and tempering: Not typical for these HSLA structural grades in routine supply; would change the product classification. - Thermo-mechanical controlled processing (TMCP): Especially effective for both, and often used to obtain the strength–toughness balance. For Q415GNH, TMCP parameters may be more aggressive to raise yield toward the higher target.
4. Mechanical Properties
One of the few quantitative certainties is that the grade number indicates the nominal minimum yield strength in MPa under the Chinese Q-series convention.
| Property | Q355GNH | Q415GNH |
|---|---|---|
| Minimum yield strength (MPa) | 355 (nominal by designation) | 415 (nominal by designation) |
| Tensile strength | Typically lower than Q415; designed to maintain ductile fracture behaviour | Typically higher than Q355 to match higher yield |
| Elongation (ductility) | Generally more ductile at equivalent thickness and processing | Generally slightly reduced ductility at the higher strength level |
| Impact toughness (low-temperature) | Engineered for good toughness with TMCP and normalized processing | Can achieve comparable toughness but often requires stricter processing and testing |
| Hardness | Lower than Q415 under similar processing | Higher due to elevated strength and precipitate density |
Interpretation: - Q415GNH provides a higher guaranteed yield strength and therefore enables thinner structures for the same load, but may impose tighter control on toughness and weld procedures. - Q355GNH tends to offer better formability and often somewhat higher elongation at fracture for comparable processing routes.
5. Weldability
Weldability depends on carbon equivalent and hardenability. For qualitative assessment, use recognized indices:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
and the more comprehensive:
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both grades are designed with low carbon and HSLA microalloying to keep $CE_{IIW}$ and $P_{cm}$ relatively low compared with medium-carbon quenched steels, which supports good weldability. - Q415GNH may have marginally higher CE and Pcm values than Q355GNH due to increased Mn or microalloy content or stronger processing; this can increase susceptibility to HAZ hardening and hydrogen-assisted cracking in thick sections. - Practically, Q415 often requires stricter control: lower hydrogen welding consumables, preheat or controlled interpass temperature, and post-weld heat treatment for critical thick sections or low-temperature service. - For both grades, follow mill certificates and perform joint qualification testing when in doubt.
6. Corrosion and Surface Protection
- These are non-stainless steels; corrosion resistance is limited to that of low-alloy carbon steels. Selection should assume ambient corrosion unless protected.
- Surface protection options: hot-dip galvanizing, zinc-rich primers, two-part industrial coatings, epoxy systems, or metallurgical claddings where necessary.
- PREN is not applicable because these are not stainless alloys. For stainless materials one