Grade System and Analysis of Low-Alloy High-Strength Structural Steels

The grading and classification of low-alloy high-strength structural steels vary by national/regional standards, with the core being to reflect their "low alloy content" and "high strength" characteristics through chemical composition and mechanical property indicators. Below are the grade rules and typical examples from major standards:

I. Chinese Standard (GB/T 1591-2018)

Grade Composition

Q + Yield Strength Value + Quality Grade + Deoxidation Method (optional)

 

Q: Pinyin initial for "yield," representing yield strength.

Yield Strength Value: Unit in MPa (e.g., Q355 indicates yield strength ≥355 MPa).

Quality Grades: Classified by impact toughness into A, B, C, D, E (impact temperature decreases sequentially, with higher toughness requirements):

Grade A: No impact test required;

Grade B: +20°C impact test;

Grade C: 0°C impact test;

Grade D: -20°C impact test;

Grade E: -40°C impact test.

Deoxidation Method: F (rimmed steel), Z (killed steel), TZ (special killed steel), often omitted.

Typical Grades

Grade	Yield Strength (MPa)	Alloy Element Features	Application Scenarios
Q345	≥345 (formerly 16Mn)	Mn-based (1.0%–1.6%), with Si, V, Nb, Ti	Bridges, vehicles, pressure vessels
Q355	≥355 (replaces Q345)	Optimized alloy ratio, improved corrosion resistance	Steel structures, wind turbine towers
Q420	≥420	Added Mo, Cr for strengthening, with Nb/V for grain refinement	High-rise buildings, heavy machinery
Q690	≥690	Quenched and tempered, with Ni, Cr, Mo	Construction machinery, mining equipment, offshore platforms

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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