ASTM A242 Type 1 High Strength Low-alloy Steel

ASTM A242 Type 1 Steel is a high strength low-alloy structural steel specification with improved atmospheric corrosion resistance. The ASTM A242 specification is normally applicable to steel plates through one-half inch in thickness. ASTM A242 HSLA steel is produced in steel shapes, plates, bars and angles.
ASTM A242 Type 1 high strength low-alloy steel has a 50ksi minimum yield strength and a 70ksi minimum tensile strength.

ASTM A242 Type 1 Steel Chemical Analysis:

C Max at 0.15%
Mn Mx at 1.00%
P Mx 0.15%
S Mx 0.05%
Cu Min 0.20%
If using other alloy elements, following elements are to be considered to add as required: Cr, Ni, Si, V, Ti and Zr.

ASTM A242 Type 1 Steel Plate Mechanical Properties

Different thickness of ASTM A242 Type 1 plates and bars has different requirements for tensile strength and yield strength.
For tensile strength:
20 mm below: 70 Ksi or 480 Mpa
20 mm to 40 mm: 67 Ksi or 460 Mpa
40 mm to 100 mm: 63 Ksi or 435 Mpa
For yield strength:
345, 315 and 290 Mpa respectively.
Elongation for 50 mm specie is 21% and for 200 mm is 18%.
ASTM A242 & ASTM A36 Steel Difference 
The atmospheric corrosion resistance of ASTM A242 steel is substantially better than that of carbon steels such as A36 and A572-50 with or without copper addition. When properly exposed to the atmosphere, A242 is suitable for many applications in the unpainted condition.