Q235B
Non-alloy carbon structural steel · GB/T 700-2006 — Carbon structural steels
Q235B is a general-purpose Chinese carbon structural steel defined by GB/T 700-2006. Its designation indicates a 235 MPa minimum upper yield strength at thickness or diameter up to 16 mm and quality grade B, which carries a longitudinal Charpy V-notch requirement of 27 J at +20 °C when impact testing applies. Strength and elongation decrease with section thickness, so “Q235B” must never be specified without the product form, dimensions, delivery condition and applicable product standard.
- GB/T 700-2006Current governing edition · Current as of October 1, 2026; a replacement standard project was in the approval stage, so contracts should state the required edition.
- U12355Unified numerical code · Applies to killed Q235B; the deoxidation symbol Z is normally omitted from the designation.
- 235 MPa at t or d ≤16 mmMinimum yield strength · Falls to 185 MPa for thickness or diameter over 150 mm through 200 mm.
- 370–500 MPaTensile strength · For material over 100 mm, the standard permits the lower limit to be reduced by 20 MPa.
- Average ≥27 J at +20 °CImpact quality · Longitudinal Charpy V-notch specimens; testing applicability and permitted waivers depend on product size.
- C ≤0.20%; Mn ≤1.40%; Si ≤0.35%Chemistry basis · Heat-analysis limits; C up to 0.22% is permitted only with purchaser agreement.
Overview
- Designation system
- Chinese GB/T yield-strength and quality-grade designation
- Product forms
- Hot-rolled plate and sheet, Hot-rolled strip, Hot-rolled sections and profiles, Hot-rolled bars
- Condition
- Hot rolled, Controlled rolled, Normalized
- Density
- 7.85 g/cm³ (Representative density used for ordinary carbon-steel weight calculations; GB/T 700 does not define density as a grade requirement.)
What the designation defines
The designation consists of Q, the nominal yield-strength level and the quality grade. “Q” identifies yield strength, “235” is the minimum upper yield strength ReH in MPa for material no thicker than 16 mm, and “B” identifies the quality level. For Q235B, grade B principally adds tighter sulfur control than Q235A and a +20 °C impact-toughness requirement. It is not a statement that every thickness has a 235 MPa minimum yield strength.
Standard Q235B is killed steel, with unified numerical code U12355. The deoxidation symbol Z may be omitted, so Q235BZ is normally written Q235B. Q235BF is a distinct rimmed-steel variant with code U12353; its F suffix must not be silently omitted when that route is intended.
GB/T 700 covers structural hot-rolled plates, strip, sections and bars used generally in welded, riveted and bolted construction. Its chemistry also applies to associated ingots and billets, but that does not turn a billet into a finished structural product complying with all product requirements.
Chemical composition and steelmaking controls
| Element | Maximum mass fraction | Qualification |
|---|---|---|
| Carbon, C | 0.20% | May be increased to 0.22% only with purchaser agreement |
| Silicon, Si | 0.35% | Maximum |
| Manganese, Mn | 1.40% | Maximum |
| Phosphorus, P | 0.045% | Maximum |
| Sulfur, S | 0.045% | Maximum |
| Chromium, Cr | 0.30% | Residual-element maximum |
| Nickel, Ni | 0.30% | Residual-element maximum |
| Copper, Cu | 0.30% | Residual-element maximum |
| Nitrogen, N | 0.008% | General maximum, subject to the standard's nitrogen/phosphorus and nitrogen-fixing provisions |
| Arsenic, As | 0.080% | General maximum; special agreement applies to steel made using arsenic-bearing ore routes |
Values are standard requirements for heat analysis, not typical mill aim values. Finished-product analysis tolerances are governed by GB/T 222.
The chemistry table contains maximum values rather than a narrowly prescribed composition range. Consequently, Q235B from different mills can have materially different carbon, manganese and microalloy contents while remaining compliant. The standard does not specify a carbon-equivalent limit, Pcm limit or mandatory weldability value.
Mechanical requirements are thickness-dependent
| Thickness or diameter | ReH minimum | Rm | Elongation A minimum |
|---|---|---|---|
| ≤16 mm | 235 MPa | 370–500 MPa | 26% for t or d ≤40 mm |
| >16–40 mm | 225 MPa | 370–500 MPa | 26% |
| >40–60 mm | 215 MPa | 370–500 MPa | 25% |
| >60–100 mm | 215 MPa | 370–500 MPa | 24% |
| >100–150 mm | 195 MPa | 370–500 MPa; lower limit may be reduced to 350 MPa | 22% |
| >150–200 mm | 185 MPa | 370–500 MPa; lower limit may be reduced to 350 MPa | 21% |
The upper tensile-strength limit is not a delivery condition for wide strip, including plate cut from wide strip. GB/T 700 also permits elongation to be two percentage points below the tabulated value under its specimen provisions.
Tensile and bend specimens are normally transverse to the rolling direction for plate and strip, and longitudinal for sections and bars. Impact specimens are longitudinal. Sampling location and preparation are controlled through the referenced sampling and test-method standards and may also be refined by the applicable product standard.
A drawing or calculation that assumes fy = 235 MPa for thick Q235B is incorrect. The applicable minimum must be selected from the actual thickness or diameter range and from the governing design code, which may apply its own nominal strengths or resistance factors.
Impact toughness and cold-bend qualification
| Item | Requirement |
|---|---|
| Test type | Charpy V-notch |
| Specimen orientation | Longitudinal |
| Test temperature | +20 °C |
| Full-size requirement | Average of three specimens ≥27 J |
| Permitted low individual result | One result may be below 27 J but not below 70% of 27 J |
| Normally tested size | Thickness ≥12 mm or diameter ≥16 mm, using 10 × 10 × 55 mm specimens |
| Smaller product | For thickness 6–12 mm or diameter 12–16 mm, subsize testing may be agreed |
| Special waiver | For Q235B below 25 mm, impact testing may be omitted with purchaser agreement if the supplier guarantees compliance |
Subsize impact-energy requirements are reduced in accordance with the normative annex of GB/T 700; they are not automatically the full-size 27 J value.
The +20 °C test distinguishes Q235B from Q235A, but it is not a universal minimum service-temperature rating. Brittle-fracture suitability also depends on section thickness, stress state, weld details, strain rate, restraint, defects and the governing design code. Q235C is tested at 0 °C and Q235D at −20 °C, but grade selection still requires an engineering assessment rather than simply matching test temperature to ambient temperature.
| Specimen direction | t or d ≤60 mm | t or d >60–100 mm |
|---|---|---|
| Longitudinal | Former diameter = 1.0a | Former diameter = 2.0a |
| Transverse | Former diameter = 1.5a | Former diameter = 2.5a |
Here a is specimen thickness or diameter. Requirements above 100 mm are agreed between purchaser and supplier. The supplier may omit routine bend testing if compliance is guaranteed.
The specification bend test is a material acceptance test, not a production forming-radius rule. Actual fabrication limits must account for edge condition, rolling direction, hole proximity, local strain, tooling and the forming process.
Product standards, delivery condition and inspection
GB/T 700 supplies the grade definition, chemistry and principal mechanical requirements. The corresponding product standard controls matters such as available dimensions, dimensional tolerances, flatness or straightness, surface quality, edge condition and product-specific inspection. For hot-rolled plate, sheet and strip, GB/T 3274 is commonly relevant; sections and bars require the applicable section or bar product and dimensional standards. A purchase order citing only “Q235B” is therefore incomplete.
| Topic | GB/T 700 provision or practical consequence |
|---|---|
| Delivery condition | Generally hot rolled, controlled rolled or normalized |
| Lot identity | Same designation, heat number, quality grade, product type, size and delivery condition |
| Maximum batch mass | 60 t |
| Routine sampling per batch | One tensile specimen, one bend specimen and three impact specimens when the respective tests apply |
| Chemical analysis | One heat analysis per heat |
| Surface quality | Controlled by the applicable plate, strip, section or bar product standard |
| Documentation | Packing, marking and quality certificate are handled through GB/T 247 or GB/T 2101 as applicable |
The standard permits restricted mixed-heat batching for some grade A and B products, subject to common manufacturing conditions and limits on carbon and manganese variation.
Hot rolled, controlled rolled and normalized material should not be presumed to have identical through-thickness uniformity, residual stress, grain structure or fabrication response merely because each can satisfy Q235B. Where delivery condition matters to forming, welding, toughness or dimensional stability, it should be explicitly stated and verified on the quality certificate.
Fabrication and service implications
Q235B is widely suited to ordinary welded structural fabrication, but GB/T 700 does not provide a guaranteed carbon equivalent or welding procedure. Welding decisions should use the actual mill chemistry together with thickness, heat input, hydrogen level, joint restraint and required heat-affected-zone toughness. Qualification to the applicable welding code remains necessary; the grade name alone does not justify a blanket no-preheat rule.
The grade has no alloying system intended to provide atmospheric-corrosion resistance. Corrosion protection should therefore be selected for the exposure, usually by coating, galvanizing or a project-specific protection system. Surface preparation, weld geometry and drainage often control coating durability more strongly than the base designation.
Q235B is a general structural grade, not by itself a pressure-vessel, boiler, line-pipe, ship-class, seismic-performance or guaranteed through-thickness grade. Such applications require compliance with the relevant product and design specifications, including any supplementary toughness, weldability, lamellar-tearing, nondestructive-testing or certification requirements.
How to specify and verify Q235B
| Specify or verify | Why it matters |
|---|---|
| Grade and edition | State Q235B to GB/T 700-2006 rather than relying on an unqualified trade description |
| Product standard | Establishes product-specific dimensions, tolerances, surface and inspection requirements |
| Product form and dimensions | Controls the applicable strength and elongation requirements |
| Delivery condition | Distinguishes hot-rolled, controlled-rolled and normalized supply |
| Deoxidation route | Confirm killed Q235B/U12355; do not accept Q235BF unless explicitly suitable |
| Impact testing | State whether actual test results are required and whether the below-25 mm waiver is prohibited |
| Chemical analysis | Request reported heat analysis and any required product analysis |
| Inspection certificate | State the required certificate content and any third-party witnessing |
| Additional requirements | Identify NDT, weldability, through-thickness, low-temperature or project-code requirements separately |
As of October 1, 2026, GB/T 700-2006 remained current, while a proposed general-purpose structural-steel delivery standard intended to replace it was in approval. Long-duration projects should freeze the contractual edition and review any subsequently published replacement before permitting substitutions.
Sources
- GB/T 700-2006 — Carbon structural steels: official standard statusNational Public Service Platform for Standards Information, China
- GB/T 700-2006 — Carbon structural steelsChina steel standardization technical organization
- Structural steels—Part 2: Technical delivery conditions for structural steels for general purposes, Project 20251168-T-605National Public Service Platform for Standards Information, China
- GB/T 3274-2017 — Hot-rolled plates, sheets and strips of carbon structural steels and high-strength low-alloy structural steelsNational Public Service Platform for Standards Information, China
- Professional Guide on Selection of Equivalent Steel Materials to European Steel Materials SpecificationsChinese National Engineering Research Centre for Steel Construction (Hong Kong Branch), The Hong Kong Polytechnic University
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