Reading designations
How to read JIS steel designations (S45C, SCM440, SNCM439, SUS304, SS400, SM490, SD390)
Japanese JIS steel symbols start with S for steel and add letters for the steel family or its use, but the number means different things in different standards. In machine-structural steels such as S45C and SCM440 it shows carbon content, in SUS304 it follows the AISI stainless type, and in SS400, SM490 and SD390 it gives a minimum strength in N/mm².
- First letter
- S = steel (other JIS materials start with F for iron, A for aluminium, C for copper)
- Carbon steels
- S + carbon × 100 + C (JIS G 4051): S45C ≈ 0.45% C
- Alloy steels
- SCr, SCM, SNC, SNCM + three digits (JIS G 4053): last two digits = carbon × 100
- Stainless
- SUS + type number following AISI: SUS304, SUS316, SUS430
- Strength-named
- SS400 and SM490 = minimum tensile strength; SD390 = minimum yield point, all in N/mm²
- Old symbols
- SCM4 is now SCM440; SNCM8 is now SNCM439
The letters
A JIS steel symbol starts with S for steel. The next letters name either the alloy family (C, Cr, CM, NC, NCM) or the product and use (US for stainless, S for general structure, M for welded structure, D for deformed bar).
| Symbol | Meaning | Standard |
|---|---|---|
| S●●C | Carbon steel for machine structural use | JIS G 4051 |
| SCr | Chromium steel | JIS G 4053 |
| SCM | Chromium-molybdenum steel | JIS G 4053 |
| SNC / SNCM | Nickel-chromium / nickel-chromium-molybdenum steel | JIS G 4053 |
| SUS | Stainless steel (steel use stainless) | JIS G 4303 (bars), G 4304 / G 4305 (plate and sheet) |
| SS | Rolled steel for general structure | JIS G 3101 |
| SM | Rolled steel for welded structure | JIS G 3106 |
| SR / SD | Round / deformed bar for concrete reinforcement | JIS G 3112 |
Carbon content or strength?
The number's meaning depends on the standard. Machine-structural steels, which are bought for heat treatment, carry a carbon figure. Structural and reinforcing steels, which are bought for strength, carry a strength figure.
In S45C the 45 is 100 times the carbon content, so about 0.45% C, and the final C means carbon steel. In SS400 the 400 is the minimum tensile strength, 400 N/mm²; the steel's carbon content is not in the name. In SD390 the 390 is the minimum yield point, not the tensile strength.
Alloy steels: the three digits in SCM440 and SNCM439
JIS G 4053 alloy steels carry three digits after the letters. The last two are the mean carbon content times 100: SCM440 has 0.38–0.43% C and SNCM439 has 0.36–0.43% C.
The first digit is a code for the alloy-content level within the family, not a percentage. SCM415, SCM420, SCM435 and SCM440 share the same Cr-Mo code and differ mainly in carbon.
Older JIS editions used short symbols such as SCM4 and SNCM8. These were replaced by the three-digit symbols SCM440 and SNCM439, and the old names still appear on older Japanese drawings. An H suffix (SCM440H, JIS G 4052) marks a steel bought to a hardenability band.
Structural and reinforcing steels
SS steels to JIS G 3101 are general-purpose structural steels named by minimum tensile strength. SM steels to JIS G 3106 are for welded structures and add chemistry and toughness controls. SM490 has a tensile strength of 490–610 N/mm².
A letter after an SM grade sets the impact requirement: A has none, while B and C add Charpy impact tests. A Y before it marks a higher-yield version, as in SM490YA and SM490YB.
Reinforcing bars to JIS G 3112 use SR for round bars and SD for deformed bars, followed by the minimum yield point. SD390 has a yield point of 390–510 N/mm² and a tensile strength of at least 560 N/mm².
Worked examples
Closest equivalents are in region order (US · EU). They are near matches, not identical steels.
| Symbol | Standard | Reading | The number means | Closest (US · EU) |
|---|---|---|---|---|
| S45C | JIS G 4051 | S steel · 45 · C carbon steel | About 0.45% C | 1045 · C45 |
| SCM440 | JIS G 4053 | S steel · CM chromium-molybdenum · 4 alloy code · 40 | Carbon 0.38–0.43% | 4140 · 42CrMo4 |
| SNCM439 | JIS G 4053 | S steel · NCM nickel-chromium-molybdenum · 4 alloy code · 39 | Carbon 0.36–0.43% | 4340 · — |
| SUS304 | JIS G 4303 | S steel · US use stainless · 304 | AISI type 304 | 304 · X5CrNi18-10 |
| SS400 | JIS G 3101 | S steel · S structure · 400 | Tensile strength ≥400 N/mm² | A36 · S275JR (closest) |
| SM490A | JIS G 3106 | S steel · M welded structure · 490 · A no impact test | Tensile strength 490–610 N/mm² | — |
| SD390 | JIS G 3112 | S steel · D deformed bar · 390 | Yield point 390–510 N/mm² | — |
Standards
Governing standards by region
| Region | System | Designation | Standard | Match |
|---|---|---|---|---|
| Japan | Carbon steels for machine structural use | S●●CNumber = carbon × 100. | JIS G 4051 | Same material |
| Low-alloyed steels for machine structural use | SCr, SCM, SNC, SNCMAlloy code digit + carbon × 100. | JIS G 4053 | Same material | |
| Stainless steel bars | SUSType numbers follow AISI; plate and sheet in JIS G 4304 / G 4305. | JIS G 4303 | Same material | |
| Structural steels | SS, SMNumber = minimum tensile strength in N/mm². | JIS G 3101 / JIS G 3106 | Same material | |
| Reinforcing bars | SR, SDNumber = minimum yield point in N/mm². | JIS G 3112 | Same material |
Domande e risposte
Common questions
What does S45C mean?
S45C is a JIS G 4051 carbon steel for machine structural use. S stands for steel, 45 is 100 times the carbon content (about 0.45% C) and C means carbon steel. Its closest counterparts are 1045 and C45.
What does SCM440 mean?
SCM440 is a JIS G 4053 chromium-molybdenum steel. S is steel, CM is chromium-molybdenum, the 4 is the alloy-content code and 40 means about 0.40% carbon (0.38–0.43%). It was formerly called SCM4, and its closest counterparts are 4140 and 42CrMo4.
What does SUS stand for in SUS304?
SUS is read as steel use stainless. The number follows the AISI type, so SUS304 is the 304-type austenitic stainless steel, the counterpart of 1.4301.
Is the 400 in SS400 the carbon content?
No. In SS400 the 400 is the minimum tensile strength, 400 N/mm². SS steels are named by strength, while machine-structural steels such as S45C are named by carbon content.
What does SD390 mean?
SD390 is a deformed reinforcing bar to JIS G 3112. SD means deformed bar and 390 is the minimum yield point in N/mm²; the specified range is 390–510 N/mm².
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Related references
References
Sources
- The meaning and reading of JIS codes (Brother machine tools) — S prefix, US = use stainless, SS400 tensile, S45C carbon, SM, SCM, SNC
- SCM435 vs SCM440 (kinzoku, note.com, Japanese) — Last two digits are mean carbon × 100
- SCM440 (former SCM4) explained (Lunar Creation, Japanese) — Old symbol SCM4; carbon 0.38–0.43%
- SNCM439 (Hanshin Metalics, Japanese) — SNCM439 carbon 0.36–0.43%
- Steel material properties cross-reference (Airtools) — SCM440 (SCM4) = 4140; SNCM439 (SNCM8) = 4340
- Reinforcing bars to JIS G 3112 (Nippon Steel construction handbook) — SR/SD grades, SD390 yield point 390–510 N/mm²
- Tung Ho hot-rolled steel mechanical specifications (Australian Anti-Dumping Commission record) — SM490 A/B/C and YA/YB tensile and impact requirements
- JIS G 4053 Low-alloyed steels for machine structural use (Japanese Standards Association)
Values summarize published limits and typical data. The governing standard and the current edition control; check them before specifying.