Q235, Q345/Q355, 45#, 40Cr …. Issued by SAC. 2 grades and 1 standards are documented so far; each grade links to a full data sheet and a graded cross-reference to the other systems.
The Chinese structural system is direct: Q stands for qu fu dian, the yield point, and the number is the guaranteed minimum yield in MPa. Q235 is a 235 MPa yield structural steel; Q355 is 355 MPa.
That makes GB structural grades map onto EN grades more cleanly than JIS ones do. Q235 sits beside S235, Q355 beside S355. The letter that follows — A, B, C, D, E — is the quality class and corresponds roughly to impact testing: A untested, B at +20 °C, C at 0 °C, D at −20 °C, E at −40 °C.
Q345 became Q355
The 2018 revision of GB/T 1591 replaced Q345 with Q355, raising the guaranteed yield by 10 MPa and aligning the grade with S355. Q345 remains in circulation on older drawings and in stock descriptions, and the two are not identical.
When a drawing says Q345, check which edition it was drawn to. Accepting Q355 against a Q345 requirement is safe; the reverse is not.
Carbon and alloy grades follow the carbon convention
Outside the structural series, GB uses the same arithmetic as everyone else: the number is carbon in hundredths of a per cent. 45# is 0.45 % carbon, equivalent to C45, 1045 and S45C. 40Cr is 0.40 % carbon with chromium, close to 41Cr4 and AISI 5140. GCr15 is the bearing steel equivalent to 100Cr6 and 52100.
faq.h
What does Q235 mean?
Q is the Chinese abbreviation for yield point and 235 is the guaranteed minimum yield strength in MPa. Q235 is the general structural grade broadly comparable to S235 and A36.
What is the difference between Q345 and Q355?
Q355 replaced Q345 in the 2018 revision of GB/T 1591, raising the guaranteed yield from 345 to 355 MPa and aligning it with S355. Q345 still appears on older drawings; substituting Q355 for it is safe, the reverse is not.
What do the letters A, B, C, D mean after a Q grade?
The quality class, defined mainly by impact testing: A has no impact requirement, B is tested at +20 °C, C at 0 °C, D at −20 °C and E at −40 °C. They correspond loosely to the EN JR, J0 and J2 suffixes.
What is 45# steel?
A GB carbon steel with 0.45 % carbon, equivalent to EN C45, AISI 1045 and JIS S45C. The hash symbol simply marks it as a grade number in Chinese usage.
Is Q355 the same as S355?
Very close on yield strength and broadly interchangeable in general fabrication. The standards differ in impact class definitions, chemical limits and test methods, so a code-governed or classed structure should follow the standard it names.