What Q235B Carbon Steel Is
Q235B is a carbon structural steel grade in the Chinese national standard GB/T 700. The designation is built from three parts: Q for the Chinese word for yield, 235 for the specified minimum yield strength of 235 MPa, and the letter B for the quality class. Grade B is the second of four quality classes A, B, C and D, and it is supplied as killed steel, which means the melt was fully deoxidised so that the material has a sound, uniform structure and a controlled impact requirement.
Relative to the basic Grade A, Grade B adds a guaranteed Charpy V-notch impact test at 20 °C with a minimum absorbed energy of 27 J on longitudinal specimens, and it applies tighter control on phosphorus and sulfur. That combination makes Q235B the standard choice for load-bearing structures, mechanical parts and welded pipe that must work reliably at ambient temperature.
Chemical Composition and Mechanical Properties
The table below gives the GB/T 700 requirements for Q235B together with the typical production ranges achieved by Chinese mills. Where a value is stated as a maximum, the standard sets the limit and the mill aims below it.
| Element | GB/T 700 requirement for Q235B | Typical production range |
|---|---|---|
| Carbon (C) | 0.20% max | 0.12 to 0.20% |
| Manganese (Mn) | 1.40% max | 0.30 to 0.70% |
| Silicon (Si) | 0.35% max | 0.15 to 0.30% |
| Phosphorus (P) | 0.045% max | 0.030% max |
| Sulfur (S) | 0.045% max | 0.030% max |
| Property | Q235B requirement |
|---|---|
| Minimum yield strength | 235 MPa for thickness up to 16 mm, reducing with thickness |
| Tensile strength | 370 to 500 MPa |
| Elongation | Defined by the standard for each thickness range |
| Impact test | Charpy V-notch at 20 °C, 27 J minimum longitudinal |
Q235A, Q235B, Q235C and Q235D
All four quality classes share the same strength level, so the difference lies in toughness and impurity control rather than in yield or tensile strength.
| Class | Deoxidation | Impact test | Typical use |
|---|---|---|---|
| Q235A | Rimmed or semi-killed permitted | Not required | Non load-bearing parts, brackets, guardrails |
| Q235B | Killed steel | 20 °C, 27 J minimum | Load-bearing structures, mechanical parts, welded pipe |
| Q235C | Killed steel | 0 °C, 27 J minimum | Outdoor structures in cold climates |
| Q235D | Killed steel | Minus 20 °C, 27 J minimum | Low temperature and heavy welded structures |
Q235B Pipe Production Range and Specification
SSAW spiral welded pipe: outside diameter 219 mm to 3120 mm, wall thickness 3 mm to 25 mm.
LSAW longitudinal submerged arc welded pipe: outside diameter 16 in to 64 in, wall thickness up to 50 mm.
ERW pipe: outside diameter 1/8 in to 24 in, wall thickness up to about 26.5 mm.
Seamless pipe: outside diameter 1/8 in to 36 in, wall thickness 1.25 mm to 50 mm.
Dimensions to ASME B36.10M and B36.19M, schedules from SCH 5 through SCH XXS.
Ends plain, bevelled or threaded; lengths random, fixed, single random or double random up to 18 m.
Testing includes chemical analysis, tensile, flattening, bend, impact, hydrostatic and non-destructive electric testing.
Equivalent Grades and Selection Notes
Q235B is commonly cross-referenced with ASTM A36, EN 10025-2 S235JR and JIS G3101 SS400. These grades are close in strength level but they are not interchangeable on paper: the phosphorus and sulfur limits, the impact test temperature and the delivery condition all differ between standards. Where a project specifies an ASTM, EN or JIS grade, that grade and its test regime should be ordered, and where Q235B is offered as an alternative the substitution should be approved against the applicable design code.
For welding, Q235B is readily joined by all common arc processes. Because carbon is capped at 0.20% and the steel is fully killed, hot cracking risk is low, but heat input should still be controlled on heavy sections and consumables should match the base metal strength.
Inspection Points and Common Pitfalls
Quoting a 0.22% carbon maximum for Q235B; grade B is limited to 0.20% carbon in GB/T 700.
Accepting a mill certificate that shows no impact test result when grade B is specified.
Treating Q235B as interchangeable with S235JR or A36 without checking phosphorus, sulfur and impact requirements.
Using Q235A where a killed, impact tested grade B is required by the design.
Overlooking the reduction in minimum yield strength as wall thickness increases.
FAQ
Q: Is Q235B carbon steel?
Yes. Q235B is a plain carbon structural steel grade in GB/T 700 with carbon limited to 0.20%, manganese up to 1.40% and no deliberate alloying additions.
Q: What is the yield strength of Q235B?
The specified minimum yield strength is 235 MPa for the thinnest section range, and the tensile strength is 370 to 500 MPa. The yield value decreases as thickness increases, as defined in GB/T 700.
Q: What is the difference between Q235A and Q235B?
Both have the same strength level. Grade B is fully killed steel with a 20 °C Charpy impact requirement of 27 J and tighter impurity control, while grade A may be rimmed or semi-killed and requires no impact test.
Q: Is Q235B equivalent to ASTM A36?
They are close in strength level and are often compared, but the chemical limits and testing requirements are not identical. For code-governed work, order the grade the specification calls for.
Q: Can Q235B be used for pipe?
Yes. Q235B is produced as spiral welded, longitudinal welded, ERW and seamless pipe for structural, mechanical and low pressure conveying service. It is not intended for high pressure or elevated temperature pipelines.
Q: Is Q235B good for welding?
Yes. With carbon capped at 0.20% and a killed steel structure, Q235B has good weldability by shielded metal arc, gas metal arc and submerged arc welding, provided heat input and consumable selection are controlled on thick sections.
