S420 steel plate refers to a high-strength low-alloy (HSLA) structural steel plate that conforms to the European standard EN 10025. This standard specifies the technical delivery conditions for non-alloy structural steels used in construction and engineering applications.

S420 is designed for structural steel applications, such as heavy machinery, bridges, and buildings. The 420 refers to a minimum yield strength of 420 megapascals (MPa). These plates are available in different grades designated as S420N, S420NL and S420MC. These grades determine the performance and suitability for different types of construction projects.  The S420 steel plate is typically supplied in the normalized or thermomechanically rolled condition.