Classification and Standard of Steel In China

Explanation of the Classification Method of Steel Number in China

1. Carbon Structural Steel

① Composed of Q + number + quality grade symbol + deoxidation method symbol. The steel number is prefixed with “Q”, representing the yield point of the steel, and the number that follows indicates the value of the yield point in MPa. For example, Q235 indicates carbon structural steel with a yield point (σs) of 235 MPa.

② If necessary, symbols indicating the quality grade and deoxidation method can be marked after the steel number. Quality grade symbols are A, B, C, D. Deoxidation method symbols: F represents boiling steel; b represents semi-killed steel; Z represents killed steel; TZ represents special killed steel. The killed steel can be unmarked, that is, both Z and TZ can be unmarked. For example, Q235-AF represents grade A boiling steel.

③ Carbon steel for special purposes, such as bridge steel, marine steel, etc., basically adopts the expression method of carbon structural steel, but a letter indicating the purpose is added at the end of the steel number.

2. High-quality Carbon Structural Steel

① The first two digits of the steel number indicate the carbon content of the steel, which is expressed in ten-thousandths of the average carbon content. For example, for steel with an average carbon content of 0.45%, the steel number is “45”, which is not a serial number, so it cannot be read as number 45 steel.

② High-quality carbon structural steel with high manganese content should mark out the manganese element, such as 50Mn.

③ Boiling steel, semi-killed steel, and high-quality carbon structural steel for special purposes should be specifically marked out at the end of the steel number. For example, the steel number for semi-killed steel with an average carbon content of 0.1% is 10b.

3. Carbon Tool Steel

① The steel number is prefixed with “T” to avoid confusion with other types of steel.

② The number in the steel number indicates the carbon content, which is expressed in thousandths of the average carbon content. For example, “T8” indicates an average carbon content of 0.8%.

③ If the manganese content is high, “Mn” is marked at the end of the steel number, such as “T8Mn”.

④ The phosphorus and sulfur content of high-quality carbon tool steel is lower than that of general high-quality carbon tool steel. The letter “A” is added at the end of the steel number to indicate the difference, such as “T8MnA”.

4. Free-Cutting Steels

① The steel grade starts with “Y” to distinguish it from high-quality carbon structural steel.

② The number following the letter “Y” indicates the carbon content, represented as a percentage in ten-thousands of the average carbon content. For example, for free-cutting steel with an average carbon content of 0.3%, the steel grade would be “Y30”.

③ For those with higher manganese content, “Mn” is also indicated after the steel grade, such as “Y40Mn”.

5. Alloy Structural Steels

① The first two digits of the steel grade represent the steel’s carbon content, expressed as a percentage in ten-thousands of the average carbon content, such as 40Cr.

② The major alloy elements in the steel, except for a few microalloy elements, are generally represented as a percentage. When the average alloy content is <1.5%, the steel grade typically only marks the element symbol without indicating the content. However, in special cases where confusion may arise, the symbol can be followed by the number “1”, for example, “12CrMoV” and “12Cr1MoV”. The former has a chromium content of 0.4-0.6%, while the latter has a content of 0.9-1.2%, with all other components being the same. When the alloy element’s average content is ≥1.5%, ≥2.5%, ≥3.5%, etc., the content should be indicated after the element symbol, which can be represented as 2, 3, 4, etc. For instance, 18Cr2Ni4WA.

③ The alloy elements in the steel, such as vanadium (V), titanium (Ti), aluminum (Al), boron (B), and rare earth (RE), are all considered microalloy elements. Even though their contents are very low, they should be indicated in the steel grade. For example, in 20MnVB steel, the vanadium content is 0.07-0.12%, and boron is 0.001-0.005%.

④ High-grade quality steel should have an “A” added at the end of the steel grade to distinguish it from general-quality steel.

⑤ For specialized-purpose alloy structural steels, the steel grade should be prefixed (or suffixed) with a symbol representing the steel’s purpose. For example, 30CrMnSi steel specifically used for rivet screws would be denoted as ML30CrMnSi.

6. Low-Alloy High-Strength Steel

① The method of denoting the steel number is fundamentally similar to that of alloy structural steel.

② For specialized low-alloy high-strength steel, the designation should be appended at the end of the steel number. For example, “16Mnq” is the specific grade for bridge construction, “16MnL” for automotive beams, and “16MnR” for pressure vessels, all derived from 16Mn steel.

7. Spring Steel

Spring steel, based on its chemical composition, can be divided into carbon spring steel and alloy spring steel. The representation of their steel numbers is akin to high-quality carbon structural steel and alloy structural steel, respectively.

8. Rolling Bearing Steel

① The steel number is prefixed with the letter “G,” indicating a category of rolling bearing steel.

② The carbon content of high-carbon chromium bearing steel is not indicated in the steel number, while the chromium content is expressed in per mille. For example, GCr15. The representation method for carburizing bearing steel’s steel number is essentially the same as that of alloy structural steel.

9. Alloy Tool Steel and High-Speed Tool Steel

① When the average carbon content of alloy tool steel is ≥1.0%, it is not indicated; when it is <1.0%, it is expressed in per mille. For instance, Cr12, CrWMn, 9SiCr, 3Cr2W8V.

② The representation method for the content of alloy elements in the steel is fundamentally similar to that of alloy structural steel. However, for alloy tool steel with lower chromium content, its chromium content is expressed in per mille and the number indicating the content is prefixed with “0” to distinguish it from the generally represented percentage of other elements. For example, Cr06.

③ The steel number of high-speed tool steel generally does not indicate carbon content, but just the average percentage of various alloy elements. For example, the designation for tungsten high-speed steel is “W18Cr4V”. A steel number prefixed with the letter “C” denotes that its carbon content is higher than the general steel number without a “C” prefix.

10. Stainless Steel and Heat-Resistant Steel

① The carbon content in the steel is represented in thousandths. For example, the average carbon content of “2Cr13” steel is “0.2%. If the carbon content in the steel is ≤0.03% or ≤0.08%, it is indicated by “00” and “0” respectively before the steel number, such as 00Cr17Ni14Mo2, 0Cr18Ni9, etc.

② The main alloy elements in the steel are represented in percentages, while titanium, niobium, zirconium, nitrogen, etc. are marked according to the method of indicating micro alloy elements in the aforementioned alloy structural steel.

11. Welding Electrode Steel

The letter “H” is prefixed to its steel number to differentiate it from other types of steel. For example, stainless steel welding wire is “H2Cr13”, which can be distinguished from stainless steel “2Cr13.

12. Electrical Silicon Steel

① The steel number is composed of letters and numbers. The letters at the beginning of the steel number, DR stands for hot-rolled silicon steel for electrical use, DW stands for cold-rolled non-oriented silicon steel for electrical use, and DQ stands for cold-rolled grain-oriented silicon steel for electrical use.

② The numbers following the letters represent 100 times the iron loss value (w/kg).

③ If the letter “G” is added at the end of the steel number, it indicates that it is inspected at high frequency; if “G” is not added, it indicates that it is inspected at a frequency of 50 Hz. For example, the steel number DW470 indicates that the maximum unit weight iron loss value of the cold-rolled non-oriented silicon steel product for electrical use at a frequency of 50Hz is 4.7w/kg.

13. Electrical Pure Iron

Its brand is composed of the letters “DT” and numbers. “DT” stands for electrical pure iron, and the number represents the order number of different brands, such as DT3. The letter added after the number represents the electromagnetic performance: A – advanced, E – special, C – super, such as DT8A.

Introduction to Steel Varieties

Sheets: Cold-rolled coils, cold-rolled plates, hot-rolled coils, hot-rolled plates, color-coated coils, color-coated plates, medium and thick plates

Coating: Hot-dip galvanized coil, electro-galvanized coil, hot-dip tinplate coil, electro-tinplate coil, chrome-plated coil, plastic composite steel, other coated steel coils, tinplate

Profiles and Bars: Rebar, wire rod, round bar, angle iron, I-beam, flat bar, H-beam, rails, special profiles, high-quality profiles, other profiles

Stainless Steel: Stainless steel plate, stainless steel coil, stainless steel pipe, stainless steel profile, stainless steel wire, stainless steel billet, stainless steel products, other stainless steel materials

Pipes: Seamless steel pipes, welded steel pipes

Steel Billet: Plate billet, square billet, pipe billet

Ferroalloys: Ferrosilicon, ferromanganese, ferrovanadium, ferrochrome, ferrotitanium

Other Steel: Silicon steel sheet, metal products, others

Steel Billet:

The steel billet is a semi-finished product for steel production and generally cannot be directly used in society. The billet is produced through three process methods: first, direct casting of molten steel into billets using continuous casting equipment in the steelmaking system (see Chapter 4 for details); second, semi-finished steel products processed from steel ingots or continuous casting billets produced by the steelmaking system using the rolling mill system; third, semi-finished products processed from steel ingots produced by the steelmaking system using forging equipment.

Steel Standards

Carbon Structural Steels GB700-88, replacing GB700-79, this standard is adopted in reference to ISO 630 “Structural Steels”.

1. Scope and Content of this Standard

This standard specifies the technical conditions for carbon structural steels.

This standard is applicable to general structural steels and hot-rolled steel plates, steel strips, profiled steel, and rolled steel for engineering purposes. These products can be used for welding, riveting, and bolting components, generally in the supplied state.

The chemical composition specified in this standard applies to steel ingots (including continuously cast slabs), steel billets, and their products.

2. Referenced Standards

GB222 Sampling method for chemical analysis of steel and allowable deviation of finished product chemical composition

GB223 Methods for chemical analysis of iron, steel and alloy

GB228 Metal tensile testing method

GB232 Metal bending testing method

GB247 General provisions for acceptance, packaging, marking, and quality certificates of steel plates and strips

GB2101 General provisions for acceptance, packaging, marking, and quality certificates of profiled steel

GB2106 V-notch Charpy impact test method for metals

GB2975 Sampling provisions for mechanical and process property testing of steel materials

GB4159 Metal low-temperature Charpy impact testing method

GB6397 Metal tensile test specimens

3. Steel Grade Nomenclature, Codes, and Symbols

3.1 Steel Grade Nomenclature

The steel grade is composed sequentially of a letter representing yield strength, a numerical value for yield strength, quality grade symbol, and deoxidation method symbol.

For example: Q235-A·F

3.2 Symbols

Q – First letter of the Chinese Pinyin for the word “yield” in “yield point” for steel;

A, B, C, D – Represent respective quality grades;

F – First letter of the Chinese Pinyin for the word “boiling” in “boiling steel”;

b – First letter of the Chinese Pinyin for the word “semi” in “semi-killed steel”;

Z – First letter of the Chinese Pinyin for the word “killed” in “killed steel”;

TZ – Initial letters of the Chinese Pinyin for the words “special killed” in “special killed steel”.

In the grade nomenclature, the symbols “Z” and “TZ” are omitted.

4. Dimensions, Shape, Weight, and Permissible Deviations

The dimensions, shape, weight, and permissible deviations of steel should conform to the respective standards.

5. Technical Requirements

5.1 Steel Grade and Chemical Composition

5.1.1 The steel grade and chemical composition (melt analysis) should conform to the stipulations of Table 1.

Table 1

Note: For Q235A and B grade boiling steel, the upper limit of Mn content is 0.60%.

5.1.1.1 The silicon content in boiling steel should be ≤0.07%; in semi-killed steel, it should be ≤0.17%, and the lower limit for silicon content in killed steel is 0.12%.

5.1.1.2 D-grade steel should contain sufficient elements to form a fine grain structure, such as an acid-soluble aluminum content of ≥0.015% or total aluminum content of ≥0.020% in the steel.

5.1.1.3 The residual elements chromium, nickel, and copper in the steel should each be ≤0.30%, and the nitrogen content of oxygen converter steel should be ≤0.008%. If the supplier can guarantee this, no analysis is needed. With the necessary agreement, the copper content in A-grade steel can be ≤0.35%. At this time, the supplier should analyze the copper content and note its amount in the quality certificate.

5.1.1.4 The residual arsenic content in steel should be ≤0.08%. Steel refined from pig iron smelted with arsenic-containing ore should have its arsenic content agreed upon by both supplier and recipient. If the raw materials do not contain arsenic, it is not necessary to analyze the arsenic content in the steel.

5.1.1.5 To ensure the mechanical properties of steel meet this standard, the lower limit of carbon, silicon manganese content in Grade A steel, and the lower limit of carbon, manganese content in other grades of steel may not be used as delivery conditions. However, their content (melt analysis) should be specified in the certificate of quality.

5.1.1.6 When supplying commercial steel ingots (including continuous casting blanks) and steel billets, the supplier should ensure that the chemical composition (melt analysis) complies with Table 1, but to ensure that the performance of the rolled steel meets the requirements of this standard, the chemical composition of Grades A and B steel can be adjusted appropriately as per customer requirements, under a separate agreement.

5.1.2 The allowable deviations in chemical composition of finished steel and commercial billets should comply with Table 1 of GB222. No guarantee is given for the deviation in chemical composition of boiling steel finished products and commercial billets.

5.2 Smelting Method

Steel is smelted in an oxygen converter, open-hearth furnace, or electric furnace, unless the customer has special requirements, which should be stated in the contract. The smelting method is usually decided by the supplier.

5.3 Delivery Status

Steel is generally delivered in a hot-rolled (including controlled rolling) condition. Upon the client’s request and by mutual agreement, it can also be delivered in a normalizing treatment condition (excluding Grade A steel).

5.4 Mechanical Properties

5.4.1 The tensile and impact tests of the steel should conform to the specifications in Table 2, and the bending test should conform to the regulations in Table 3.

Table 2: Tensile and Impact Testing of Steel

Table 3: Steel Bending Test

Note: B refers to the sample width, and a refers to the thickness (diameter) of the steel.

5.4.1.1 The yield point of the Q195 grade is only for reference and should not be considered as a delivery condition.

5.4.1.2 For tensile and bending tests, steel plates and strips should use transverse samples, and the elongation rate is allowed to decrease by 1% (absolute value) compared to Table 2. Profile steel should use longitudinal samples.

5.4.1.3 Cold bending tests for all Grade A steel are conducted only if required by the buyer. When the cold bending test is passed, the upper limit of tensile strength can be disregarded as a delivery condition.

5.4.2 The Charpy (V-notch) impact test should comply with the specifications of Table 2.

5.4.2.1 The Charpy (V-notch) impact function value is calculated as the arithmetic mean of a set of three individual sample values, allowing one sample value to be lower than the prescribed value, but not less than 70% of the prescribed value.

5.4.2.2 When conducting an impact test with a small-sized sample of 5mm x 10mm x 55mm, the test result should be ≥50% of the specified value.

5.4.3 Grade B steel made from boiling steel should generally have a thickness (diameter) of ≤25mm.

5.5 Surface Quality

The surface quality of the steel should comply with the relevant standard specifications.

6. Test Methods

6.1 Inspection items, sample quantities, sampling methods, and test methods for each batch of steel should comply with the specifications of Table 4.

6.1.1 When conducting the cold bending test for steel with a thickness base diameter over 20mm, the sample should be planed on one side until its thickness reaches 20mm. The bending core diameter should be determined according to Table 3. During the test, the unprocessed surface should be on the outside. If the sample has not been planed, the bending core diameter should be increased by one sample thickness over ‘a’ than the value listed in Table 3.

6.1.2 The longitudinal axis of the impact sample should be parallel to the rolling direction.

6.1.3 When conducting the impact test for steel plates, steel strips, profiles with thickness ≥12mm, or bar steel with a diameter less than 16mm, a 5mm×10mm×55mm sample should be used. For steel plates, steel strips, profiles with a thickness of 6mm to less than 12mm, or bar steel with a diameter of 12mm to less than 16mm, a 5mm×10mm×55mm small-sized sample should be used. The impact sample can retain one rolling surface.

7. Inspection Rules

7.1 The steel materials shall be inspected and accepted by the technical supervision department.

7.2 The steel materials should be accepted in batches, each batch consisting of the same grade, same furnace mouth, same level, same type, same size, and same delivery status. The weight of each batch must not exceed 60t.

For steel or continuous casting billets smelted in steel furnaces with a nominal capacity of ≤30t, it is permissible to form a mixed batch from A-grade or B-grade steel of the same type, the same smelting and casting method, but different furnace numbers. However, each batch should not have more than six furnace numbers, and the difference in carbon content between the furnace numbers should not exceed 0.02%, and the difference in manganese content should not exceed 0.15%.

7.3 If the results of the Charpy (V-notch) impact test of the steel do not comply with the specifications of section 5.4.2, a set of three samples should be retested from the same batch of steel. The average value of the six samples before and after should not be lower than the specified value, but it is permissible for two samples to be lower than the specified value, and only one sample is allowed to be 70% of the specified value.

7.4 The re-inspection and acceptance rules for other inspection items of the steel should comply with the regulations of GB247 and GB2101.

8. Packaging, Marking, and Quality Certificate

The packaging, marking, and quality certificate of steel should comply with the requirements of GB247 and GB2101.