Q235B vs Q235C – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Q235 is a widely used Chinese structural carbon steel family. The suffixes B and C designate variants of the same base grade that are commonly compared when engineers and procurement specialists select material for plates, bars, and structural sections. Typical decision contexts include balancing cost versus low-temperature toughness, prioritizing weldability and ease of fabrication versus the need for verified impact performance at reduced temperatures.
The principal distinction between Q235B and Q235C is the verified impact toughness at a lower test temperature for Q235C. Both grades share essentially the same chemical specification and general mechanical behavior, but the acceptance criteria for impact testing differ — which affects application choices in colder environments or where fracture toughness control is critical.
1. Standards and Designations
- GB/T 700 — Chinese national standard for carbon structural steels (Q235 family).
- Comparable international designations (for general reference):
- EN: S235 series (roughly comparable in use, not identical in chemistry or testing).
- ASTM/ASME: A36 (serves similar structural functions; not a direct one-to-one).
- JIS: Equivalent designations vary (no single exact match).
Classification: - Q235B and Q235C are plain low-carbon structural steels (carbon steels), not alloy, tool, stainless, or HSLA grades. They are intended for general structural use with straightforward production routes and high formability.
2. Chemical Composition and Alloying Strategy
Both Q235B and Q235C have the same nominal chemical specification under GB/T 700; the difference between the suffixes is in impact-test temperature and acceptance, not chemistry. Minor differences between mills can exist, but intentional alloying additions are minimal — the grade is designed as a low‑carbon, low‑alloy structural steel.
Table: Typical composition limits (wt%) per GB/T 700 for Q235 (B/C)
| Element | Typical limit or range (wt%) |
|---|---|
| C (Carbon) | ≤ 0.22 |
| Mn (Manganese) | ≤ 1.40 |
| Si (Silicon) | ≤ 0.35 |
| P (Phosphorus) | ≤ 0.045 |
| S (Sulfur) | ≤ 0.045 |
| Cr (Chromium) | Not intentionally added; typically ≤ 0.30 (impurity) |
| Ni (Nickel) | Not intentionally added; typically ≤ 0.30 (impurity) |
| Mo (Molybdenum) | Not intentionally added; typically ≤ 0.10–0.30 trace |
| V, Nb, Ti, B | Not intentionally added (microalloying not a defining feature) |
| N (Nitrogen) | Controlled as part of steelmaking; not a specified alloying addition |
How the alloying strategy affects properties: - Low carbon (≤0.22%) keeps the steel readily weldable and ductile. - Manganese provides deoxidation and a modest strengthening effect, also improving hardenability slightly. - Silicon is a deoxidizer and contributes to strength when present at moderate levels. - Phosphorus and sulfur are controlled low because they embrittle grain boundaries and reduce toughness. - Absence of intentional microalloying (V, Nb, Ti) means limited strengthening via precipitation and modest hardenability; Q235 behaves like a classic mild structural steel.
3. Microstructure and Heat Treatment Response
Microstructure under typical processing: - As-rolled or hot-rolled Q235 (both B and C) is primarily ferrite with polygonal ferrite and some pearlite islands. Grain size and banding depend on rolling schedule and cooling rate. - No significant volume fractions of bainite or martensite are intended under standard processing.
Heat treatment response: - Q235 is not a heat-treatable grade in the sense of achieving high strength via quench-and-temper because its low carbon and lack of alloying limit hardenability. Normalizing can refine grain size and slightly raise strength and toughness. - Typical production routes: - Hot-rolled + controlled cooling → typical microstructure with good ductility. - Normalizing (if applied) → slightly finer ferrite–pearlite, modest toughness improvement. - Quenching and tempering is not generally used because deep hardening requires higher carbon and alloy content; attempts yield only modest benefits and risk distortion. - Thermo-mechanical controlled processing (TMCP) used by modern mills can improve strength–toughness balance by grain refinement and controlled transformation, but the grade remains a low-carbon structural steel.
4. Mechanical Properties
Table: Typical mechanical properties (representative ranges)
| Property | Q235B (typical) | Q235C (typical) |
|---|---|---|
| Yield strength (Rp0.2) | ≈ 235 MPa (nominal design value) | ≈ 235 MPa (nominal design value) |
| Tensile strength (Rm) | ≈ 370–500 MPa (depends on thickness/processing) | ≈ 370–500 MPa (similar) |
| Elongation (A) | ≥ 20–26% (depending on thickness) | ≥ 20–26% (similar) |
| Impact toughness (Charpy V-notch) | ≥ 27 J at +20°C (typical acceptance for "B") | ≥ 27 J at 0°C (typical acceptance for "C") |
| Hardness (HB) | ~120–170 HB (dependent on processing) | ~120–170 HB (similar) |
Interpretation: - Strength: Both grades are specified to have the same nominal yielding behavior (the “235” designator). Actual tensile and yield values vary with section size and mill process, but there is no intrinsic strength advantage between B and C. - Toughness: The defining difference is the verified impact performance at a lower temperature for Q235C. That means Q235C must demonstrate acceptable energy absorption at reduced temperature, reducing the risk of brittle fracture in colder service. - Ductility: Both grades retain high ductility consistent with low-carbon steels.
5. Weldability
Weldability of Q235B and Q235C is generally good because of the low carbon content and lack of strong hardenability alloying elements. Several measures and formulas help engineers assess weldability qualitatively.
Common carbon-equivalent indices: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (IIW-derived more conservative): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both grades typically have low $CE_{IIW}$ and $P_{cm}$ values compared to higher-strength, higher-alloy steels, which implies straightforward welded practice with little preheat for common thicknesses. - The primary practical welding consideration is that Q235C has validated toughness at a lower temperature; welders and engineers should ensure that the weld zone and heat-affected zone meet any impact-test requirements for the component as a whole, especially in cold climates. - For thicker sections or complex welded assemblies, implement standard controls: preheat, interpass temperature, post‑weld heat treatment (if required for geometry), and qualified welding procedures.
6. Corrosion and Surface Protection
- Q235B and Q235C are non-stainless plain carbon steels; corrosion resistance is limited to that inherent in mild steel.
- Typical protection strategies:
- Hot-dip galvanizing for atmospheric corrosion protection.
- Zinc-rich primers, painting, powder coating for aesthetic and protective layers.
- Corrosion allowances in design or use of sacrificial coatings.
- PREN (pitting resistance equivalent number) is not applicable to these non‑stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Use of PREN is relevant only for stainless steels; for Q235 variants, selection of protective systems and coatings is the correct approach for corrosion control.
7. Fabrication, Machinability, and Formability
- Formability: Excellent formability for both grades due to low carbon content and ductile ferritic microstructure. Suitable for bending, stamping, and moderate cold forming. Q235C's verified toughness at lower temperature does not materially change forming behavior at ambient temperatures.
- Machinability: Typical of mild carbon steels. Machinability can be further optimized with appropriate tooling, feeds, and coolants; low alloy content simplifies cutting; free‑machining variants are different and not part of Q235 B/C.
- Cutting and drilling: No special requirements beyond standard practices for mild steels. Thermal cutting, plasma, and oxy-fuel cutting are commonly used for plates.
- Finishing: Weld spatter removal, grinding, and surface treatments follow normal procedures. If welding to meet impact requirements, control of HAZ and post-weld inspection may be necessary.
8. Typical Applications
| Q235B (common uses) | Q235C (common uses) |
|---|---|
| General structural sections (I‑beams, channels), building frames, welded steel structures in temperate environments | Structural components and plates for outdoor equipment in colder climates or where low-temperature impact performance is required |
| Fabricated parts where weldability and cost are primary drivers (supports, brackets, general fabrication) | Pressure vessel supports, offshore or elevated structures where impact resiliency at reduced temperature is a specified requirement |
| Machinery parts, general-purpose plates and bars | Cold-storage equipment frames, transport equipment exposed to seasonal low temperatures |
Selection rationale: - Choose the variant that provides sufficient verified toughness at the lowest expected service temperature while balancing cost. Q235B is appropriate where ambient temperatures and service conditions do not approach the lower threshold; Q235C is selected when the design or regulation requires validated impact resistance at reduced temperature.
9. Cost and Availability
- Cost: Q235B and Q235C are manufactured from the same base chemistry and similar processes; the cost differential is usually small. Q235C may carry a modest premium due to additional testing and inspection required to verify impact performance at the lower temperature.
- Availability: Both grades are widely available in China and in global supply chains that source Chinese structural steel. Availability in specific product forms (plates, coils, bars, welded sections) depends on local mill production and inventory. For specialized sizes, lead times can increase.
10. Summary and Recommendation
Table: Quick comparison
| Attribute | Q235B | Q235C |
|---|---|---|
| Weldability | Excellent | Excellent |
| Strength–Toughness balance | Standard structural balance at ambient temperatures | Improved verified low-temperature toughness |
| Cost | Slightly lower (fewer low-temp tests) | Slightly higher (additional testing/certification) |
Concluding recommendations: - Choose Q235B if: the component will operate in normal temperate environments, weldability and lowest material cost are priorities, and there is no regulatory or project requirement for verified impact toughness at sub-ambient temperatures. - Choose Q235C if: the part will be exposed to lower service temperatures (seasonal cold, refrigerated environments, or cold climates), project specifications mandate impact testing at a lower temperature, or a higher margin against brittle fracture is required.
Both grades serve as practical, economical structural steels. The decision between Q235B and Q235C is primarily governed by the required verified low-temperature toughness rather than by differences in chemistry or baseline mechanical strength. In practice, align material selection with service temperature, applicable codes/specifications, and qualification requirements for welded assemblies.