（1）AISI standard expression of steel grade
For steel of forging and casting, the grade is formed of 3 digits; the first digit means type of steel, and the other two digits are simply the order. The specific serial numbers are:
2×× ——chromium-manganese-nickel-nitrogen austenitic steel.
×× is the order (the same below).
For example, 202 is the austenitic stainless heat resistant steel with C≤0.15%, Mn≤10.0%, Si≤1.0%, Cr17%～19%, Ni4%～6%, N≤0.25%.
3××——Nickel-chromium austenitic steel.
For example, 302 corresponds to austenitic stainless steel of 1Cr18Ni9 in our national standard of YB.
4××——High chromium martensite and low-carbon-high-chromium ferritic steel.
For example, 403 corresponds to the steel 1Cr13 in our national standard of YB.
430 corresponds to the steel Cr17 in the YB national standard.
5××——Low chromium martensite steel.
For example, 501 corresponds to the heat resistant steel Cr5Mo in our national YB standard.
(2）SAE standard of steel grading
Steel grade is formed of a 5-digit: the first three digits represent type of steel, the other 2 represents order (the same with order in AISI).
The specific serials are:
302××——Nickel-chromium austenitic stainless heat resistant steel (forging steel).
For example, 30316 corresponds to steel 316 in the standard of AISI.
514××——High-chromium martensite steel and low-carbon-high-chromium ferritic stainless heat resistant steel (forging steel).
For example, 51414 corresponds to the 414 steel of AISI.
515××——Low chromium martensite steel (forging steel).
For example, 51501 corresponds to the steel 501 of AISI.
60×××——heat resistant steel (cast steel) used below 650℃,
××× is the same number order with AIAI. For example, 70334 corresponds to steel 316 of AIAI.
70×××——heat resistant steel (cast steel) used over650℃
××× is the same number order with AIAI. For example, 70334 corresponds to steel 334 of AIAI.
It can be seen from the above that the last 3 digits of SAE consists of the last three of AISI serial system.
3）ACI standard of steel grading
Steel grade is formed of only 2 letters or followed by a number which represents carbon content and letters of alloying elements.
Usually the first letter of steel grade is C or H.
Steel of C type means the heat resistant steel used below 650℃, and Steel of H type is the heat resistant steel used over 650℃.
The second letter of C type in the grading, A,B,C,D,……, represents different nickel contents (refer to the table).
And the number following the letters means carbon contents by ten thousandth, plus a hyphen between numbers and letters.
For example, CE-30 is the acid-resisting steel of C＜0.30, Cr26%～30%, Ni8%～11%.
The second letter of steel grade of H type, A, B, C, D….. , represents different nickel contents (refer to the table). Generally, it does not show the number of carbon content.
For example, HC represents the heat resistant steel of C＜0.50%, Cr 26.0%～30.0%, Ni＜4.0%.
Some steel grades (mainly C type) has their numbers followed by addition letters, such as C means Cb (Nb) is added, M means Mo is added, and F means the feature of free-cutting.
For example: CF-8C, CF-16F.
（4）UNS system of steel grading
Stainless heat resistant steel could apply S××××× digital grading.
The first three numbers basically take the AISI stainless steel grading numbers.
The last two digits are used to distinguish steels which have the same main ingredient and different individual ingredients or specific elements.
But the main difference between UNS and AISI are:
① S1×××× series of UNS are now precipitation hardening stainless steel.
AISI grading does not include 1×× series, but including the expression of 63× series which represent precipitation hardening stainless steel.
② AISI 3×× series are all nickel-chromium austenite steel, and 4×× series are all the high-chromium martensite steel and low-carbon-high-chromium ferritic steel.
UNS system have exceeded the range with these two series of digital grading numbers, and included some precipitation hardening stainless steel, which are expressed with general commercial digital grading numbers.
For example, AM-350, AM-355 in UNS system are called S35000, S35500, S45500.
（5）ASTM standard expression of steel grading
ASTM standard expression of steel grading are the same with AISI standard.
（6）FS standard of steel grading
FS standard of steel gradinghave the same numbering system with SAE, while prefixed the grading number with FS.
出典： 中国特钢企业协会不锈钢分会
http://www.cssc.org.cn/page148?article_id=67
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