API 5L Pipe Spec Clarification

Hello,

Pipe Standard:- API 5L/ ISO 3183

Table-1 Pipe grades,Steel Grades and acceptable delivery conditions

Pipe Spec:- PSL-2, L450 Q or X65 Q

PSL- Product Specification Level (Level-1 and Level-2 available in API5L/ISO3183)

65 = 65000 Psi or 65Ksi

Q = Quenched and Tempered

By the way,What is this L450 ? From where I can get the defenition of this term ?In API 5L/ISO 3183 where it is mentioned?I am confused...

Thanks in advance