Q355 Steel Grades: Q355B, C, D, E and F Impact Levels with GB/T 1591 Data

How the Q355 Designation Works

Q355 is defined in GB/T 1591, the Chinese standard for high-strength low-alloy structural steel. The letter Q stands for the yield strength property, and 355 is the minimum yield strength in MPa for material up to 16 mm thick. The 2018 edition of GB/T 1591 replaced the earlier Q345 designation with Q355 at the same strength level, so old drawings marked Q345 are generally satisfied by Q355 material provided the impact and chemistry requirements match. The standard covers Q355 together with the higher strength grades Q390, Q420, Q460, Q500, Q550, Q620 and Q690.

Quality Grades and Impact Temperature

The letter that follows the strength figure identifies the impact quality grade and therefore the minimum service temperature for which the steel is certified. All grades are tested with Charpy V-notch specimens.

Cold-region structures, offshore equipment and crane components normally specify D, E or F quality, while general building work is often satisfied by grade B. Correcting the impact temperature is the single most common change a buyer has to make when a drawing carries an old grade designation.

Chemical Composition

The C, D, E and F quality grades carry tighter phosphorus and sulfur limits than grade B, because low-temperature toughness depends on keeping segregation and inclusion content down. The exact limits for each quality grade are tabulated in GB/T 1591, and the certified values on the mill certificate should be checked against that table rather than against the maximum values shown here for grade B.

Mechanical Properties by Thickness

Yield strength decreases with thickness in the same way as in other structural steel standards. Table 3 lists the minimum yield strength of Q355 by thickness and the tensile strength range that applies.

Delivery Conditions and Applications

Q355 is rolled in the as-rolled condition or with a suffix that records the delivery condition, N for normalised or normalising rolled and M for thermomechanical rolling. Buildings, bridges, transmission towers, cranes, mining equipment and general plant structures are the main outlets. Where atmospheric corrosion resistance is needed the weathering grades of GB/T 4171, such as Q355NH, are specified instead, and dedicated bridge steel grades such as Q345qD and Q345qE are applied to highway and railway structures under GB/T 714. For all of these, the plate or section thickness must be stated, because thickness fixes the yield strength that the mill is required to certify.

Frequently Asked Questions

Q: What does Q355 mean?
A: Q indicates the yield strength property and 355 is the minimum yield strength in MPa for thickness up to 16 mm.

Q: Is Q355 the same as the former Q345?
A: GB/T 1591-2018 raised the strength level of the old Q345 designation and renamed it Q355, so material ordered to Q355 is normally accepted where Q345 was called for.

Q: What is the difference between Q355B, Q355C, Q355D, Q355E and Q355F?
A: The letter gives the Charpy impact test temperature: plus 20 C for B, 0 C for C, minus 20 C for D, minus 40 C for E and minus 60 C for F, with the corresponding tightening of phosphorus and sulfur limits.

Q: What is the maximum carbon content of Q355B?
A: 0.24% by heat analysis, with silicon limited to 0.55% and manganese to 1.60%.

Q: Can Q355 be welded?
A: Yes, with procedures qualified for the thickness and heat input involved; the carbon equivalent calculated from the certified chemistry determines the preheat and interpass temperature.

Q: Which grade should be used for low-temperature service?
A: Grade D for minus 20 C, grade E for minus 40 C and grade F for minus 60 C minimum design metal temperature.