The Difference Between Q235 Steel And Q355 Steel And The Classification Method Of Steel Grades

The difference between Q235 steel and Q355 steel and the classification method of steel grades
I. The main difference between Q235 steel and Q355 steel

Chemical composition

Q235: It is an ordinary carbon structural steel with a low carbon content (0.14%-0.22%) and a high content of phosphorus and sulfur impurities;
Q355: It is a low-alloy high-strength steel with stricter carbon content control (≤0.18%), and alloy elements such as manganese (≤1.60%) and silicon (≤0.55%) are added, and the phosphorus and sulfur content are lower.

Mechanical properties

Strength: The yield strength of Q235 is 235MPa, and the tensile strength is 375-500MPa; the yield strength of Q355 is 355MPa, and the tensile strength is 470-630MPa;
Plasticity: The elongation of the two is similar (Q235≥21% vs. Q355≥20%), but Q355 still maintains better plasticity under high strength.

Application areas‌

Q235‌: light buildings, general mechanical parts, low-strength bridges, etc.;

Q355‌: heavy-duty structures (such as high-rise buildings, heavy bridges), marine engineering, chemical equipment, etc., which require higher strength and corrosion resistance.

Economic and design considerations‌

Q355 is 3%-8% more expensive than Q235, but its higher strength can save 15%-20% of steel;
Q355 is preferred for strength-dominated designs, while Q235 is more economical for stability-controlled designs.

Identification method‌

The difference in appearance color (Q235 turns blue, Q355 turns red) may be related to the rolling cooling process, which is not an essential difference and needs to be confirmed by chemical analysis or mechanical testing.
2. Classification of steel grades

Carbon structural steel (such as Q235)‌

The quality grades range from A to D. Grade A has no impact toughness requirements. Grades B/C/D need to pass impact tests at room temperature, 0℃, and -20℃ respectively, and the phosphorus and sulfur content decreases step by step.

Low alloy high-strength steel (such as Q355/Q345)‌

Grades are divided into A/B/C/D/E, based on the impact test temperature (room temperature to -40℃) and phosphorus and sulfur content, with Grade E having the most stringent requirements.

Grade identification rules‌

Take Q235B as an example, "Q" represents the yield strength value (235MPa), and the last letter corresponds to the quality grade.

3. Summary

The core difference between the two lies in alloy composition and strength. Q355 achieves higher performance by adding alloy elements, but the cost increases accordingly. The steel grade classification is mainly based on impact toughness, impurity content and applicable temperature. The material and grade need to be selected in combination with specific scenarios.