Cold-rolled SPCC is a grade that is well known to those in the steel trade, and is often labelled as ‘cold-rolled sheet’, ‘general-purpose’ or ‘run-of-the-mill’.
However, you may not be aware that the SPCC standard also includes nuances such as ‘1/2 hard’, ‘annealed only’, and ‘rough or smooth surface’. As for questions like ‘What is the difference between SPCC S D and SPCCT?’, you might be even more baffled.
As we always say, in the steel trade, ‘buying the wrong product means losing money’. Today, we’ve invited an expert to provide a detailed breakdown for you.
The Origins of the SPCC Grade
The SPCC grade originates from JIS, which stands for Japanese Industrial Standards.
SPCC is listed in JIS G 3141, a standard entitled ‘Cold-rolled steel sheets and strips’, which includes five grades—SPCC, SPCD, SPCE, SPCF and SPCG—each of which is suitable for different applications.
Different Tempering Grades of SPCC
We often say that a grade cannot stand alone; it is only when combined with the standard number, grade designation and suffix that it constitutes a complete specification.
Naturally, this principle also applies to SPCC; in the JIS standard, different suffixes represent different products, the most important of which is the tempering grade code.
Tempering Grade Codes
A——Annealed only
S——Standard tempering
8 — 1/8 hardened
4 — 1/4 hardened
2 — 1/2 hardened
1 — hardened
What do ‘annealed only’ and ‘standard temper’ mean?
‘Standard temper’ may be unclear to some; here, it typically refers to the annealing and levelling process (for an explanation of levelling, see our previous article: ‘What on earth is levelling?’).
If no levelling is carried out, it is referred to as 【Annealed Only】.
However, as steel mills’ annealing processes are now all equipped with levelling machines, and failure to level the material cannot guarantee flatness, products that are not levelled are rarely seen nowadays; consequently, products such as SPCC A are uncommon.
The performance requirements for standard tempering or annealed-only grades are as follows:
Why are there no requirements for yield strength, tensile strength or elongation?
This is because the original JIS standard for SPCC does not specify these requirements. If tensile test values are to be guaranteed, a ‘T’ must be added after SPCC, resulting in SPCCT.
Many domestic steel mills’ in-house standard grades designated as SPCC are actually revised versions of the JIS SPCCT standard.
The mechanical property requirements for SPCCT apply specifically to [standard tempering or annealing processes]; this table cannot be used as a reference for products requiring different levels of hardness.
What do the designations ‘8’, ‘4’, ‘2’ and ‘1’ in the standard refer to in terms of hardness?
By adjusting the annealing process, products with different degrees of hardness can be obtained, such as 1/8 hardness or 1/4 hardness.
Note: It should be noted that the suffix ‘1’ here, which denotes ‘hardness’, does not refer to what is commonly known as ‘cold-rolled coils’; it still requires low-temperature annealing.
Some may wonder: since products of different hardness grades do not conform to that yield and tensile strength table, how are their properties evaluated?
What are the performance requirements for hardened materials?
Everything is laid out in the standard
For products of different hardness grades, only the hardness value is guaranteed; other properties such as yield strength, tensile strength, elongation, and even chemical composition are not guaranteed.
Useful Information
- In trade, we often see domestic enterprise standard warranty certificates where certain SPCC grades do not include the suffix ‘S’; this is generally taken to mean standard temper.
Due to domestic application practices and equipment configurations, annealing and levelling are considered standard processes and are therefore not specifically mentioned.
- Surface finish is also a very important parameter; this standard specifies two types of surface finish.
Surface finish codes
D – Rough finish
B – Smooth finish
Smooth and rough finishes are primarily achieved by means of the rolling mill (levelling rolls). The surface roughness of the rolls is transferred to the steel plate during the rolling process.
Rolling with rough-surfaced rolls produces a rough finish, whilst rolling with smooth-surfaced rolls produces a smooth finish. Smooth and rough finishes have different effects on further processing; an inappropriate choice may lead to processing issues.
- Finally, we shall examine some typical examples of specifications found in quality assurance certificates, such as:
JIS G 3141 2015 SPCC 2 B: A 1/2-hard, smooth-surfaced SPCC that complies with the requirements of the 2015 edition of the JIS standard. This product guarantees hardness only; other parameters such as chemical composition, yield strength, tensile strength and elongation are not guaranteed.
Q/BQB 408 2014 SPCC D: A mill-finished SPCC with standard quenched and tempered hardness, conforming to the 2014 edition of the Baosteel corporate standard. Its mechanical properties meet the standard requirements, and the average surface roughness Ra is controlled within the range of 0.6 μm < Ra ≤ 1.9 μm.