Reading Japanese JIS Grade Names: S45C, SCM440, SUS304 and SKD11

JIS names compress family, carbon content and use into a few letters and digits. This guide decodes the common prefixes, explains why structural numbers mean tensile strength, and maps the tool steel families.

The letter skeleton

JIS names start with S (steel) followed by letters for the family or application:

SS — rolled steel for general structure (SS400).
SM — weldable structural steel (SM490).
SCM / SCr / SNCM — chromium-molybdenum, chromium, and nickel-chromium-molybdenum steels for machine structural use.
SUS — stainless steel; SUJ — bearing steel; SUP — spring steel; SUH — heat-resistant steel; SUM — free-cutting steel.
SK — tool steels, subdivided by a third letter: SKD die steels, SKH high-speed steels, SKS alloy tool steels, SKT forging-die steels.

The carbon number in the middle

For machine structural steels the digits encode carbon:

S45C = steel with 0.45% carbon; the closing C marks a carbon steel for machine parts.
SCM440 = Cr-Mo steel in which the last two digits give carbon (0.40%) and the first digit identifies the alloy series. The same pattern gives SCr415 (chromium steel, 0.15% C, a carburizing grade) and SNCM439 (Ni-Cr-Mo, 0.39% C).
An H suffix (SCM440H) guarantees hardenability to a standard band.

Structural grades: tensile, not yield

SS400 and SM490 are named by minimum tensile strength (400 and 490 MPa) — the opposite convention from EN and GB structural names, which use yield. Comparing "355" with "490" as if they were the same quantity is a recurring sourcing error. Within SM grades, suffix letters A, B and C step up the requirements: SM490C adds mandatory Charpy impact testing (at 0 °C), while plain SS400 carries no guaranteed impact value at all unless separately agreed. Variants such as SM490YA/YB denote higher-yield versions.

Stainless and tool steels borrow other systems

SUS304 mirrors AISI numbering directly, including suffix logic: SUS316L is the low-carbon version, SUS304H the high-carbon one.
Tool steel numbers are serials within the family, not chemistry: SKD11 is the cold-work die steel corresponding to AISI D2, SKD61 corresponds to H13, and SKH51 to M2 high-speed steel. There is no way to read composition out of "11" or "61" — the governing JIS standard (G4404, G4403) holds the analysis.
Bearing and spring grades behave the same way: SUJ2 corresponds to 52100-type bearing steel, SUP grades are serials for spring steels.

Sheet and strip follow a parallel pattern: SPCC is cold-rolled commercial-quality sheet (P = plate, C = cold rolled, final C = commercial quality), SPHC its hot-rolled counterpart, and the final letter steps through D (drawing quality) and E (deep-drawing quality). The habit is the same throughout JIS — the letters first classify the product and its duty, the numbers then grade it.

Quick decode table

Name	Reading
S45C	Machine carbon steel, 0.45% C
SCM440	Cr-Mo machine steel, 0.40% C (4140 counterpart)
SUS304	Stainless steel, AISI 304 counterpart
SKD11	Cold-work die steel (D2 counterpart)
SM490C	Weldable structural steel, 490 MPa tensile, impact grade C
SUJ2	Bearing steel (52100 counterpart)