How Japanese Grade Designations Are Built
The JIS system compresses three pieces of information into a short code. The first letter identifies the material, with S for steel and F for iron. The second letter identifies the shape, type or purpose, such as P for plate, T for tube and K for tool. The trailing characters give a characteristic value, most often the minimum tensile strength in megapascals.
Reading SS400 with that logic: the first S is steel, the second S stands for structure, and 400 is the lower limit of tensile strength, so the grade describes ordinary structural steel with a guaranteed tensile strength of 400 MPa. The same construction applies across the whole family, which is why a buyer only needs to know the governing JIS part to know what properties are guaranteed.
General Structural Plate Under JIS G3101
SS400 is the most widely ordered general structural plate grade in the Japanese system. JIS G3101 fixes a minimum yield strength of 245 MPa for plate up to 16 mm thick, a tensile range of 400 to 510 MPa, and elongation values that reduce as section thickness increases. The standard controls only the elements that affect weldability and toughness, limiting phosphorus to 0.050% and sulfur to 0.050%, which leaves the producer free to meet the strength through carbon and manganese.
That mechanical rather than chemical basis is why SS400 is easy to weld, easy to source and widely interchangeable with EN 10025-2 S275JR and ASTM A36 in structural duties. Where impact toughness at low temperature is required, SS400 alone is not sufficient and a grade with a guaranteed Charpy energy must be specified instead.
Hot-Rolled Commercial and Drawing Grades Under JIS G3131
JIS G3131 covers hot-rolled mild steel plate, sheet and strip. The designation follows the same pattern: S for steel, P for plate, H for hot rolled, and a final letter that sets the drawing duty.
| Grade | Carbon max | Manganese max | Phosphorus max | Sulfur max | Typical duty |
|---|---|---|---|---|---|
| SPHC | 0.15% | 0.60% | 0.050% | 0.050% | Commercial quality for general forming, tube making and light structures |
| SPHD | 0.10% | 0.50% | 0.040% | 0.040% | Drawing quality for stamping and moderate forming |
| SPHE | 0.08% | 0.45% | 0.030% | 0.030% | Deep drawing quality for severe forming |
Lower carbon and tighter residual control raise formability while reducing strength, so the choice between the three is a forming severity decision rather than a strength decision.
Cold-Rolled Grades Under JIS G3141
JIS G3141 governs cold-reduced carbon steel sheet and strip, with the letter C marking the cold-rolled route. SPCC is the general-purpose grade, SPCD is intended for drawing and SPCE for deep drawing. Where a tensile test must be guaranteed, T is added at the end of the designation, giving SPCCT, and where non-ageing behaviour must be guaranteed, N is added, giving SPCEN.
Temper rolling condition is quoted separately because it controls hardness and forming behaviour. Annealed condition is coded A and standard temper-rolled condition S. The hardness grades run from 1/8 hard at HRB 50 to 71 and HV 95 to 130, through 1/4 hard at HRB 65 to 80 and HV 115 to 150, and 1/2 hard at HRB 74 to 89 and HV 135 to 185, to full hard from HRB 84 upward.
Ordering, Tolerances and Verification
Quote the full designation including the temper condition and the surface finish, because a bare SPCC call-out leaves forming behaviour undefined.
Dimensional tolerances follow JIS G3193 for hot-rolled product and the JIS G3141 tables for cold-rolled product, and thickness tolerance must be stated as a percentage band, not as a nominal value.
Mechanical testing is carried out to JIS Z2241 for tensile properties, with hardness checked to JIS Z2245 on the temper-rolled states.
For exported structural material, ask for an EN 10204 3.1 inspection certificate listing heat number, chemistry, tensile results and, where applicable, impact results.
Frequently Asked Questions
Q: What does the 400 in SS400 mean?
A: It is the lower limit of tensile strength, so the grade guarantees a tensile strength of at least 400 MPa with a specified upper limit of 510 MPa.
Q: Is SPHC the same as SS400?
A: No. SPHC is a hot-rolled commercial forming grade under JIS G3131 with a controlled carbon level, while SS400 is a structural grade under JIS G3101 with guaranteed yield and tensile properties and no narrow carbon limit.
Q: What is the equivalent of SPCC in the Chinese system?
A: SPCC is a general-purpose cold-rolled grade broadly comparable to the Q195 to Q215 class of cold-rolled mild steel, while SPCD corresponds more closely to drawing-quality grades and SPCE to deep-drawing grades.
Q: Can SPHC be used for structural welding?
A: It can be welded easily because of its low carbon content, but it does not carry a guaranteed yield strength, so it should not be substituted for SS400 where design calculations rely on a specified minimum yield value.
Q: How is temper-rolled hardness specified?
A: By the hardness state, from 1/8 hard through 1/4 hard and 1/2 hard to full hard, each with its own HRB or HV band. The state must be quoted on the order because it changes both yield strength and elongation.
