Q355NH and Q415NH are Chinese High-Strength Low-Alloy (HSLA) weathering (atmospheric corrosion-resistant) steel plates, differing primarily in strength, with Q355NH offering good strength (yield ~355 MPa) and Q415NH being stronger (yield ~415 MPa), both achieving corrosion resistance by forming a protective rust layer, ideal for containers, bridges, and structures exposed to atmosphere, containing alloying elements like Copper (Cu), Chromium (Cr), and Nickel (Ni).
Chemical Composition
| Element | Q355NH (typical strategy) | Q415NH (typical strategy) |
|---|---|---|
| C | Low carbon to moderate carbon - balanced for weldability and strength | Low-to-moderate carbon, sometimes slightly higher than Q355NH to assist strength |
| Mn | Moderate level for strengthening and deoxidation | Moderate to somewhat higher to support strength and hardenability |
| Si | Present as deoxidizer (trace to small %) | Similar role; not a strength driver |
| P | Controlled low levels (residual) | Controlled low levels |
| S | Controlled low levels (residual) | Controlled low levels |
| Cr | Usually low or absent; sometimes trace to improve hardenability | Often low or trace - used to tune strength/hardenability |
| Ni | Typically minimal or absent | Typically minimal or absent |
| Mo | Trace if present to improve hardenability/toughness in heavier sections | May be used in small amounts in some mills for hardenability |
| V, Nb, Ti | Microalloying elements used in small quantities to refine grain and improve strength/toughness | Microalloying present to achieve higher strength and stable properties |
| B | Not a principal alloying strategy; trace in some formulations | Same |
| N | Low levels; important where copper or weathering elements are present | Low levels; contributes to precipitation strengthening when controlled |
Mechanical Properties
| Property | Q355NH (typical) | Q415NH (typical) |
|---|---|---|
| Yield strength (min) | ~355 MPa (by grade name) | ~415 MPa (by grade name) |
| Tensile strength | Moderate - sufficient for structural use (typical mid-range) | Higher - increased ultimate strength consistent with higher yield |
| Elongation | Good ductility; useful for forming and absorbing overloads | Slightly lower elongation than Q355NH due to higher strength |
| Impact toughness | Usually specified at low temperatures (e.g., –20°C) and generally strong | Toughness is engineered but may be marginally lower, depending on processing |
| Hardness | Moderate; consistent with structural plate | Higher but still within weldable structural limits |
Key Differences & Similarities
Strength: The "355" and "415" in their names denote their minimum yield strength in MPa (megapascals), meaning Q415NH is stronger than Q355NH.
Corrosion Resistance: Both are weathering steels (designated by 'NH' in GB/T 4171 standard) that develop a stable, protective patina (rust layer), reducing further corrosion and eliminating the need for painting.
Alloying Elements: They contain specific elements like Copper (Cu), Nickel (Ni), Chromium (Cr), and Phosphorus (P) to enhance mechanical properties and corrosion resistance.
Applications: Used in similar environments: vehicles, bridges, towers, offshore containers, shipping containers, and structures where atmospheric exposure is a concern.
