What Q355C Means in the GB/T 1591-2018 System
Q355C is a low alloy high strength structural steel defined by GB/T 1591-2018. The letter Q denotes minimum yield strength and 355 is that value in megapascals for the thinner product range. The trailing letter is a quality class, not a carbon level: it fixes the temperature at which Charpy impact toughness is verified. Q355B is tested at plus 20 degrees C, Q355C at 0 degrees C, Q355D at minus 20 degrees C and Q355E at minus 40 degrees C, so the class should always be selected from the lowest service temperature rather than from habit.
Strength is developed through micro-alloying with niobium, vanadium and titanium together with controlled rolling, rather than through high carbon. Carbon is typically limited to about 0.20 % maximum, manganese to about 1.60 %, silicon to about 0.55 %, and phosphorus and sulphur are each held to around 0.030 % or tighter for the C class. Minimum yield strength is 355 MPa, tensile strength falls in the 470-630 MPa range and elongation is at least 22 % for the common thickness range, with a minimum Charpy absorbed energy of 34 J at 0 degrees C. For readers comparing international specifications, Q355C is broadly comparable in strength and impact class to the S355J0 designation in EN 10025-2, although the two standards are not identical and substitution should be confirmed against the design basis.
Steelmaking and Casting Route
The process route starts with iron and steelmaking in a basic oxygen converter or an electric arc furnace, followed by secondary metallurgy in the ladle. Ladle refining is where the C class is actually created: desulphurisation lowers sulphur to the level the impact class demands, alloy additions trim carbon, manganese and the micro-alloying elements to the target heat analysis, and vacuum degassing removes hydrogen so that the cast product is not prone to internal cracking. Aluminium or a similar deoxidiser is used to control oxygen and grain size.
The refined steel is continuously cast into square billets of the size suited to the rolling mill. Casting temperature, casting speed and secondary cooling are controlled so that the billet has a sound, fine grained structure with minimal segregation in the centre. Billets are then inspected, dressed where necessary to remove surface defects, and identified by heat number so that the finished angle can be traced back to the cast and to its test results.
Reheating and Rolling of Angle Steel
Billets are reheated in a walking beam or pusher furnace to a controlled austenitising temperature and held long enough to equalise the temperature through the cross section without excessive grain growth or surface decarburisation. The heated billet then passes through a roughing train that reduces the section, followed by finishing passes in grooved rolls that gradually form the L profile. The legs of an angle taper from the thicker root to the thinner toe, and the 90 degree corner is filled to a controlled radius, which is why angle rolling depends on careful pass design rather than on a single forming step.
For Q355C the finishing temperature and reduction schedule are adjusted to exploit the micro-alloying additions: controlled rolling refines the ferrite grain size and lets the steel reach the 355 MPa yield class with good toughness rather than by adding more carbon. This is the essential difference between rolling a plain carbon structural angle and rolling a micro-alloyed one.
Cooling, Straightening, Cutting and Downstream Processing
After the last pass the angle moves onto a cooling bed, where the cooling rate must be even so that the legs do not distort or develop residual stresses. Straightening follows on a roller straightening machine, after which the material is cut to ordered lengths and the ends are finished. Dimensional checks cover leg width, leg thickness, root radius, corner angle, length and straightness, all measured against the requirements of the hot rolled section standard.
Downstream operations depend on the application. Punching and drilling for bolted connections are usually carried out before coating so that no bare steel is left after protection. Hot dip galvanizing to ASTM A123 or EN ISO 1461 is the most common corrosion protection for angles used outdoors, and the process order matters: cutting, punching and drilling should be finished before dipping, and any site modification afterwards must be repaired with a zinc rich coating. Painting, shot blasting and cutting to short lengths are also typical services supplied on angle steel.
Inspection, Tolerances and Traceability
Quality control for a Q355C angle order combines the heat analysis with product analysis, tensile testing to confirm yield strength, tensile strength and elongation, and Charpy impact testing at 0 degrees C to confirm the class. Because the impact requirement is the whole reason for choosing the C class over B, temperature controlled impact specimens and correctly calibrated testing equipment are essential. Dimensional and visual inspection then checks the profile, straightness and surface condition for defects such as rolled-in scale, laps or seams.
Finished material should be marked with the grade, heat number and size, and delivered with an inspection document that links the marking to the test results, typically a mill certificate to the level agreed with the buyer. This traceability is what allows a fabricator to demonstrate that the angle installed in a cold climate structure genuinely meets the 0 degrees C impact requirement.
Frequently Asked Questions
Q: What is the difference between Q355B and Q355C angle steel?
They are the same strength class and differ in impact test temperature. Q355B is verified at plus 20 degrees C and Q355C at 0 degrees C, so the C class is chosen for colder service conditions.
Q: Does the C in Q355C mean a carbon content class?
No. The trailing letter denotes the quality or impact class. Carbon is separately limited, at about 0.20 % maximum for the usual Q355 chemistry.
Q: What is the yield strength of Q355C?
355 MPa minimum in the thinner product range, with a tensile strength of about 470-630 MPa and at least 22 % elongation. As with all structural steels, the guaranteed yield value decreases as thickness increases.
Q: How is a Q355C angle steel made?
Converter or electric furnace steelmaking, ladle refining and vacuum degassing, continuous casting into billets, controlled reheating and rolling into the angle profile, then controlled cooling, straightening and cutting to length.
Q: Why is controlled rolling important for Q355C?
Micro-alloyed steel reaches its strength through fine grain structure produced by controlled rolling. Without it, the same chemistry would need more carbon, which would reduce toughness and weldability.
Q: Should holes be drilled before or after galvanizing?
Before galvanizing wherever possible, so that the coating covers the full surface. Any hole or cut made afterwards exposes bare steel and must be repaired with a zinc rich coating.