JIS G3106 SM400A High Strength Structural Steel Plate: Properties and Applications

What Is JIS G3106 SM400A Steel Plate?

SM400A is a hot-rolled carbon structural steel plate covered by the Japanese Industrial Standard JIS G3106, the standard that governs hot-rolled steels for welded structures. Because it combines a guaranteed minimum yield strength with low carbon content and dependable weldability, SM400A plate is a baseline material for bridges, building frames, storage tanks, machinery bases, crane girders and general welded fabrication. The steel is normally supplied in the as-rolled condition and can be ordered normalized when heavy sections or low-temperature service require a more uniform microstructure.

The designation itself carries the key information: SM identifies a structural steel for welded structures, 400 marks the 400 MPa class of tensile strength, and the suffix A distinguishes the base quality within the standard. Plate is rolled from continuously cast slab on a plate mill, and mechanical properties are verified on the finished product rather than on the cast analysis alone, so the thickness of the ordered plate directly determines which acceptance values apply.

Minimum Yield Strength by Thickness

Yield strength is specified as a minimum that decreases stepwise as plate thickness increases, reflecting lower rolling reduction and slower cooling in heavy plate. The thickness range must therefore be stated clearly on the enquiry and on the mill test certificate.

Tensile Strength Values

Tensile strength is quoted as a range so that the steel remains ductile while still reaching the required level. Plates up to 100 mm thick are tested against a 400 to 500 MPa window; heavier plate above 100 mm uses a slightly wider window of 400 to 510 MPa.

Chemical Composition (Ladle Analysis, Maximum %)

Two carbon levels apply according to thickness, and manganese is controlled as a ratio to carbon rather than as a fixed range, which keeps the steel weldable across the thickness range while still giving adequate strength.

The 0.035 % ceilings on phosphorus and sulphur are what separate a controlled structural plate from commodity re-rolled material. Low sulphur reduces the risk of lamellar tearing in thick welded joints, while low phosphorus protects toughness and helps the plate meet impact requirements after welding.

Supply Condition, Testing and Documentation

Ladle and product chemical analysis reported against the limits above, heat by heat.

Low-temperature impact testing of the base metal and, where agreed, of the weld and heat-affected zone.

Ultrasonic examination of the plate body to EN 10160 or to ASTM A435, ASTM A577 and ASTM A578 at a level agreed between mill and buyer.

Mill test certificate issued to EN 10204 type 3.1 or 3.2, traceable to heat number, plate number and individual test results.

Shot blasting and shop priming, plus gas cutting, edge preparation, drilling and welding to the fabricator's approved drawings.

Fabrication Behaviour

With carbon capped at 0.23 % and manganese tied to the carbon content, SM400A has a modest carbon equivalent and can be welded with conventional processes such as shielded metal arc, gas metal arc and submerged arc welding. Thick plate and highly restrained joints still call for preheating and controlled interpass temperature, and consumables should match or slightly overmatch the plate strength. Cold forming is practical within generous bend radii; tighter radii or forming at low ambient temperature should be qualified by test. Machining, gas cutting and plasma cutting are all routine, and the plate takes shop primers well after blasting.

How SM400A Compares with Other Structural Grades

SM400A sits at the entry of the 400 MPa tensile class. Within the same standard, SM490A is the higher-strength grade in the 490 MPa class, with correspondingly higher yield values. JIS G3101 SS400 is a general structural steel that controls tensile strength but does not define minimum yield strength per thickness band the way SM400A does, so the two should not be treated as interchangeable where the design depends on yield strength. When a project is written around EN or ASTM documents, strength-comparable alternatives are often considered, but any grade substitution requires approval from the responsible design engineer because toughness, chemistry and weldability all change with the grade.

FAQ

Q: Is SM400A the same as SS400?
No. SS400 is a general structural steel under JIS G3101 that specifies tensile strength only, whereas SM400A belongs to JIS G3106 and additionally controls minimum yield strength per thickness band together with limits on chemistry suited to welded structures.

Q: What is the maximum thickness available?
JIS G3106 covers plate well beyond 160 mm, and the table above lists minimum yield values up to that point, with 205 MPa applying above 100 mm. Availability in the heaviest sizes depends on the mill's rolling programme and should be confirmed before design is frozen.

Q: Can SM400A be welded without preheating?
Thin and moderately thick plate with low restraint can usually be welded without preheat. For heavy sections, high restraint or cold workshop conditions, preheat and interpass control should be specified and the procedure qualified by test.

Q: What certificate is issued with the plate?
A mill test certificate to EN 10204 type 3.1 or 3.2 is issued, and it reports chemical analysis and mechanical results for the specific heat and plate supplied rather than typical values for the grade.

Q: Is impact testing included as standard?
Impact testing is not automatic for the grade; it is ordered as an additional requirement at a stated test temperature, and the acceptance energy should be written into the purchase specification together with the test standard to be used.

Q: Which applications suit SM400A plate best?
Welded structures that need predictable yield strength and good toughness rather than maximum strength: bridge girders and diaphragms, building frames, storage tanks, machine bases, crane runway beams, and general fabrication where a certified structural plate is required.