What is the difference between Q345B steel and Q355B steel plate

Q355B steel is an upgrade of Q345B steel. Their differences are as below.

Differences in implementation standards of Q345B steel and Q355B steel plate:
Q345B steel executive standard: GB/T1591-2008 "low alloy high strength structural steel".
GB/T 1591-2008 Q345B steel can not be synchronized with the European Union. Now a new regulation GB/T 1591-2018 is issued, which changes Q345B to Q355B.

Q345B steel and Q355B steel plate are the same in tensile strength, elongation after fracture and impact temperature, but the yield strength is different.
Tensile strength: 450-630MPa
Elongation: ≥21%.
Impact temperature: 20 ℃

Differences in yield strength of Q345B steel and Q355B steel plate:

Q345B steel and Q355B steel plate are delivered in the state of controlled rolling, normalizing, normalizing + tempering, and thermal mechanical rolling (TMCP). If necessary, the Z-direction performance test shall be conducted according to the customer's needs, such as Q345D-Z15, Q345D-Z25, Q345D-Z35, etc.