Q235 Carbon Steel: Specifications and Applications

Q235 carbon steel is a widely used carbon structural steel, recognized for its reliability in various fabrication and construction applications. This overview details its technical specifications according to relevant standards.

1. Understanding Q235 carbon steel Classification

Q235 steel is defined under the Chinese standard GB/T 700-2006. The designation signifies:

• “Q”: Represents the yield strength.
• “235”: Indicates a minimum yield strength of 235 MPa.

2. Q235 carbon steel Chemical Composition

The chemical composition varies slightly between the quality grades defined by GB/T 700-2006. For example, Q235A typically contains:

• Carbon (C):14%−0.22%
• Silicon (Si):≤0.30%
• Manganese (Mn):≤1.40%
• Phosphorus (P):≤0.045% (Note: Different sources might list slightly varied max P/S limits for Grade A)
• Sulfur (S):≤0.050% (Note: Different sources might list slightly varied max P/S limits for Grade A)
• Nitrogen (N):≤0.008% (For Q235A, B, C)
• Residual elements like Nickel (Ni), Copper (Cu), and Arsenic (As) also have specified limits.

Grades Q235B, C, and D have progressively tighter controls on carbon and impurity contents.

3. Q235 carbon steel Mechanical Properties

Mechanical properties for Q235, according to GB/T 700-2006, are dependent on the material thickness. Key properties include:

• Yield Strength (ReH​): Thickness ≤16 mm is 235 MPa, 16-40 mm is 225 MPa, and >40 mm is 215 MPa.
• Tensile Strength (Rm​): Typically ranges from 315−490 N/mm², varying with thickness.
• Elongation (A%): Varies with thickness and grade.

4. Q235 carbon steel Heat Treatment and Surface Enhancement

While often used in its as-delivered state, Q235 can undergo heat treatment for specific requirements:

• Normalizing: Commonly performed around 900-920℃.
• Surface Hardening: For applications needing enhanced wear resistance (e.g., simple molds, tools for softer materials), techniques like carburizing, carbonitriding, or specialized quenching processes can be applied. With specific treatments, surface hardness can reach up to 62−65 HRC or higher.
• Quenching: Direct quenching (sometimes without tempering) has achieved hardness levels around 36−40 HRC for components like punches.

5. Applications

Q235’s versatility and cost-effectiveness lead to its use in diverse applications:

• General Structural Steel: Widely used in buildings, bridges (designated Q235q), and other structures.
• Weathering Steel: Grade Q235NH offers improved atmospheric corrosion resistance.
• Components: Used for agricultural tools, cold-formed tubes, bicycle parts (e.g., axle tubes via cold extrusion), and as a base material for composite plates or surface-hardened tools.
• Boiler/Pressure Vessels: Specific grades exist, though Q235R was not detailed in the source text.

6. International Standard Equivalents

Q235 steel grades have equivalents in other major international standards. For example:

• Q235A: Similar to ISO E235A, JIS SM400B, ASTM Grade 65 (240), EN S235JR (1.0038), DIN S235JR, ГОСТ СТЗКП2.
• Q235B: Similar to EN S235JR (1.0038), ASTM A36.
• Q235C: Similar to EN S235J0 (1.0114).
• Q235D: Similar to EN S235J2 (1.0117).

Note that while often considered equivalent, slight variations in chemical composition limits and mechanical property requirements between standards can exist. Always verify against the specific standard required for your application.

7. Processing Considerations

Proper processing is essential for Q235. For instance, incorrect annealing can lead to brittle phases (tertiary cementite) forming at grain boundaries, negatively impacting ductility and toughness. Adhering to appropriate processing parameters is crucial for achieving the desired material performance.