SUS304 vs SS304: Standards, Composition and Selection Guide

SUS304 and SS304 appear side by side in drawings, purchase orders and mill certificates, and buyers reasonably ask whether one is better than the other. The short answer is that they describe the same 18/8 austenitic stainless steel under two different standards systems: SUS304 belongs to the Japanese Industrial Standards, while SS304 is the everyday name for grade 304 as defined in American standards. There is no quality hierarchy between them. What does differ is the designation, the certificate that accompanies the material, and the ordering rules that go with it.

Where the Two Designations Come From

SUS is the abbreviation used in JIS for stainless steel grades, so SUS304 is the JIS designation, defined for flat product in JIS G4304 and JIS G4305. SS304 is not a formal standards body name at all; it is shorthand for AISI 304, the grade listed in ASTM A240 for chromium and nickel stainless plate, sheet and strip and recognised worldwide.

Designation	Standards system	Principal standard	Notes
SUS304	JIS, Japan	JIS G4304 / JIS G4305	Equivalent in composition and properties to AISI 304
SS304 / AISI 304	ASTM and AISI, United States	ASTM A240	Most widely quoted grade for international trade
1.4301 / X5CrNi18-10	EN, Europe	EN 10088-2	European designation for the same grade
06Cr19Ni10	GB/T, China	GB/T 20878	Current Chinese designation

Because the four designations describe one grade family, a sheet marked SUS304 from a Japanese mill, a coil marked 1.4301 from a European mill and a plate marked SS304 from an American stockist can all meet the same technical requirement. The designation alone never proves it: the mill test certificate and the material itself do.

Chemical Composition of SUS304 and SS304

The composition limits in JIS G4305 and ASTM A240 for the 304 grade are effectively identical, and the familiar heat analysis figures below apply to both designations.

Element	SUS304 / SS304 limits	Practical effect
Chromium	18.0 - 20.0 %	Forms the passive chromium oxide film that provides corrosion resistance
Nickel	8.0 - 10.5 %	Stabilises the austenitic structure and improves toughness and ductility
Carbon	0.08 % max	Higher carbon raises strength but increases sensitisation risk in welds
Manganese	2.00 % max	Austenite former, also permitted in place of some nickel
Silicon	1.00 % max	Deoxidation during melting
Phosphorus	0.045 % max	Impurity controlled for weldability
Sulfur	0.030 % max	Impurity; low sulfur improves hot workability and weld soundness

Where welded construction or elevated service temperature is involved, the low-carbon version 304L with 0.030 % maximum carbon, or a stabilised grade such as 321, is the safer selection for both designations.

Mechanical and Physical Properties

Typical room temperature values for annealed product are the same under either designation, so a specification that quotes SUS304 can be satisfied by material certified to ASTM A240 and vice versa.

Property	Typical value
Tensile strength	520 - 750 MPa
Yield strength, 0.2 % offset	205 MPa minimum
Elongation	40 % minimum
Hardness	90 HRB maximum
Density	7.93 g/cm³
Magnetic response	Non-magnetic when annealed, light magnetism after cold work

Because 304 work hardens rapidly, forming operations such as deep drawing or cold rolling raise both strength and magnetic permeability, and may require interstage annealing. This behaviour is identical for SUS304 and SS304 and does not reflect a difference in grade quality.

Supplying, Specifying and Verifying

For procurement teams, the practical questions are which designation to put on the purchase order and what documentation to demand.

State both the designation and the standard. A line such as SUS304 to JIS G4305 or 304 to ASTM A240 removes ambiguity, and many mills will dual certify the same heat.

Require a mill test certificate to EN 10204 3.1 showing heat number, chemical analysis and mechanical results for the actual heat supplied.

Confirm the form and finish. Sheet and coil are commonly supplied in 2B or BA finish, with hair line, No.4 and mirror finishes available; tube and pipe follow separate standards such as ASTM A312 or ASTM A269.

Check the marking. Traceability from the plate or bundle back to the heat number is what allows a discrepancy to be resolved after delivery.

Verify when the application is critical. Portable X-ray fluorescence or optical emission spectrometry identifies a grade in seconds and detects mislabelled or substituted material.

A price difference between the two designations, when it appears, is normally driven by origin, surface finish, thickness tolerance and order quantity rather than by the material itself.

Frequently Asked Questions

Q: Is SUS304 better than SS304?
No. They are the same grade described by different standards systems. SUS304 is the JIS designation and SS304 is the common name for AISI or ASTM 304, so a correct comparison is not between the two names but between the certificates and tolerances offered by each supplier.

Q: Is SUS304 exactly the same as 304?
In composition and mechanical properties, yes. JIS G4305 and ASTM A240 set the same chromium range of 18 to 20 per cent and nickel range of 8 to 10.5 per cent for the grade, along with the same carbon, manganese and impurity limits.

Q: Is SUS304 food grade?
Yes. SUS304 is widely used for kitchen equipment, sinks, cookware and food processing lines, including in Japan and across Asia. For food contact applications, confirm that the material and finish meet the applicable food contact requirements, such as GB 4806.9 for metal materials and articles.

Q: Is SUS304 magnetic?
In the annealed condition it is essentially non-magnetic. Cold working such as bending, drawing or shearing induces martensite and produces light magnetism. Strong magnetism is unusual and should be investigated as a possible wrong-grade supply.

Q: Why is SUS304 sometimes more expensive than SS304?
Any price gap reflects commercial factors rather than metallurgy. Origin of the mill, surface finish, thickness tolerance, quantity and delivery terms move the price more than the designation printed on the certificate.

Q: Can SUS304 and SS304 be used interchangeably?
Yes, provided the ordered specification, finish and tolerances are met. The designation on the purchase order still governs the documentation, so buyers who need a particular standards route for a project should state it explicitly and ask for the corresponding mill test certificate.