Two Base Grades, One Coating System
Q235B and Q355B are the two structural carbon steel grades that appear most often on galvanized fabrication drawings in Asia and on imported projects that follow Chinese design practice. Q235B is specified in GB/T 700 with a minimum yield strength of 235 MPa, and Q355B is specified in GB/T 1591 with a minimum yield strength of 355 MPa; the letter B in both cases denotes an impact-test quality with a test at 20 degrees Celsius. Galvanizing is a coating process applied to either base grade, so the two materials share the whole of the corrosion protection system and differ only in the load they can carry for a given section.
That distinction matters commercially because galvanizing is priced by surface area and steel by mass. A designer who needs the corrosion durability of a coating but not the strength of a 355 MPa grade can often save material cost by using Q235B with a heavier section, while a designer working to a fixed member size must use Q355B to reach the required capacity.
Composition and Mechanical Requirements of Both Grades
The two grades differ in chemistry as well as strength: the higher grade is cleaner in phosphorus and sulphur, which improves toughness and weld quality as well as raising yield.
| Requirement | Q235B (GB/T 700) | Q355B (GB/T 1591) |
|---|---|---|
| Carbon, max | 0.20% | 0.24% |
| Silicon, max | 0.35% | 0.55% |
| Manganese, max | 1.40% | 1.60% |
| Phosphorus, max | 0.045% | 0.035% |
| Sulphur, max | 0.045% | 0.035% |
| Yield strength, min (16 mm and under) | 235 MPa | 355 MPa |
| Tensile strength | 370 - 500 MPa | 470 - 630 MPa |
| Impact test | 20 degrees Celsius | 20 degrees Celsius |
The practical consequence is that Q355B offers roughly fifty per cent more yield strength for a modest increase in alloy content and price, and its lower phosphorus and sulphur ceilings make it the better choice where the fabrication involves heavy multi-pass welding or low-temperature service. Q235B remains the default for light structures, supports, frames and general fabrication where stiffness rather than stress governs.
Physical and Electrochemical Protection in Service
A galvanized coating protects steel in two ways at once. The zinc layer is first a barrier: it seals the steel from air, water and chloride-bearing deposits, and its corrosion products are compact and adherent, so they slow further attack. The layer is also a sacrificial anode: zinc is less noble than iron, so where the coating is scratched or cut back at a drilled hole or a sheared edge, the exposed zinc around the damage corrodes preferentially and keeps the steel beneath it cathodic. That second mechanism is why a galvanized member tolerates minor site damage that a painted member does not.
Both base grades behave identically in this respect, because the coating reacts to its own environment rather than to the strength of the substrate. Where the two differ is at the design stage: galvanizing is done at about 450 degrees Celsius, and if the steel has been cold-worked or heavily welded, residual stress may be relieved during the dip and cause distortion. That risk is a function of the fabrication detail and section thickness rather than of the grade, so both Q235B and Q355B fabrications need the same venting, draining and distortion-control measures.
Coating Thickness Requirements and Service Life
Coating thickness, not the steel grade, sets the corrosion life of a galvanized member. For products galvanized after fabrication, ISO 1461 gives minimum local and average coating thicknesses that rise with the steel thickness, because a thicker section holds more heat and reacts longer with the zinc bath.
| Steel thickness | Local minimum coating thickness | Average minimum coating thickness |
|---|---|---|
| 6 mm and over | 70 micrometres | 85 micrometres |
| 3 mm up to 6 mm | 55 micrometres | 70 micrometres |
| 1.5 mm up to 3 mm | 45 micrometres | 55 micrometres |
| Under 1.5 mm | 35 micrometres | 45 micrometres |
Continuously galvanized coil and sheet follow a different route: the coating is ordered as a coating mass per unit area under the continuous coating standards, where the designation states the total mass on both surfaces in grams per square metre. Corrosion rate in the intended environment then converts coating thickness into service life: a rural or indoor atmosphere consumes roughly a fraction of a micrometre per year, an urban or industrial atmosphere more, and a severe marine or chemical atmosphere considerably more. Uncoated structural steel in a damp outdoor environment typically shows visible rust within a few years, while a properly specified galvanized member will hold for decades.
Fabrication, Welding and Post-Galvanizing Practice
Both grades weld readily by the common arc processes. Q355B benefits from its lower carbon, phosphorus and sulphur levels where thick sections are joined with restrained joints, and consumables should be matched to the higher strength so the joint does not under-match the member. Welding before galvanizing leaves flux residue and spatter that must be removed, because the pickling stage will not fully clean heavy deposits and the coating will be locally thin or absent over them.
Three detailing rules apply to any grade that will be dipped: provide vent and drain holes so air can escape and zinc can flow out of closed sections, avoid overlapping plates that trap pickling acid, and avoid sealed enclosures entirely. After galvanizing, do not weld the coated member without grinding back the coating in the weld zone and repairing it afterwards with a zinc-rich repair material, and do not cut it with a thermal process without the same repair.
Which Grade to Choose
Choose Q355B where member size is constrained, where stress rather than deflection governs, or where the project is executed to a design code that allows the higher yield to be used in the calculation. Choose Q235B where stiffness governs, where the section is sized by handling or connection geometry, or where a large number of small members makes material price the dominant cost.
In both cases, write the galvanizing requirement into the specification as a coating thickness or coating mass with the applicable standard and the acceptance method, state whether the coating is applied after fabrication or supplied on continuously coated coil, and state the repair procedure for cut edges and weld zones. A galvanized specification without a coating number is not a specification.
Frequently Asked Questions
Q: Do Q235B and Q355B galvanize equally well?
A: Yes. Both are plain carbon-manganese structural steels with silicon and phosphorus levels that suit the conventional hot-dip process; coating quality depends on surface preparation, bath control and venting, not on the grade.
Q: How much stronger is Q355B than Q235B?
A: Q355B has a specified minimum yield strength of 355 MPa against 235 MPa for Q235B for thickness up to 16 mm, an increase of about fifty per cent, with tensile strengths of 470 to 630 MPa and 370 to 500 MPa respectively.
Q: What coating thickness should be specified?
A: For batch galvanizing after fabrication, ISO 1461 minima run from 35 micrometres local and 45 micrometres average on thin steel to 70 micrometres local and 85 micrometres average on steel 6 mm and over. For continuously coated material, specify the coating mass designation instead.
Q: How long does a galvanized coating last?
A: Service life is a function of coating thickness and environment. In mild rural or indoor conditions a coating can last for decades; in severe marine or chemical exposure the same coating lasts far less, so the thickness must be chosen for the site rather than by habit.
Q: Can galvanized steel be welded?
A: Yes, but the zinc must be ground back from the weld zone, appropriate fume extraction must be used, and the coating must be repaired after welding with a zinc-rich material compatible with the original coating.
Q: Which grade is used for transmission towers and heavy structures?
A: Higher-strength grades such as Q355B are used where stress governs and sections are kept small; Q235B remains common for secondary members, bracing and light frames.
Q: Will galvanizing distort a fabrication?
A: It can, because the dip temperature is about 450 degrees Celsius and relieves residual stress. Distortion is controlled by balanced welding sequence, venting and draining design, and by supporting thin panels during cooling.