Alloy steel is a type of steel that is produced by adding one or more alloying elements (such as chromium, nickel, molybdenum, titanium, etc.) to carbon steel in order to improve its properties.

Alloy steel has the following characteristics:

1. Performance improvement: Compared to carbon steel, its strength, hardness, toughness, wear resistance, corrosion resistance, etc. are often significantly enhanced or improved.

2. Special properties: Various special properties can be obtained according to needs to meet different working environments and requirements.

3. Wide application: It is widely used in fields such as aerospace, automotive manufacturing, mechanical engineering, petrochemicals, etc., where the performance of materials is highly demanded.

There are numerous types of alloy steel, and different combinations and contents of alloying elements give alloy steel different characteristics and uses. 

Alloy steel refers to steel that has some alloy elements added to it on the basis of carbon steel, such as Si, Mn, W, V, Ti, Cr, Ni, Mo, etc. These alloy elements can improve the strength, toughness, hardenability, and weldability of the steel. Alloy steel can be classified according to the content of alloy elements. In industrial production and life, alloy steel is specifically applied to specific industries, and it is also common to classify it according to its usage. 

Classified by the amount of alloying elements:

1. Low alloy steel: The total amount of alloying elements is less than 5%;

2. Medium alloy steel: The total amount of alloying elements is between 5% and 10%;

3. High alloy steel: The total amount of alloying elements is more than 10%. 

Classified by application:

1. Alloy structural steels: Low alloy structural steels (also known as ordinary low alloy steels); Alloy carburized steels, alloy quenched and tempered steels, alloy spring steels; Ball bearing steels

2. Alloy tool steels: Alloy cutting tool steels (including low alloy cutting tool steels, high-speed steels); Alloy die steels (including cold die steels, hot die steels); Measuring tool steels

3. Special performance steels: Stainless steels, heat-resistant steels, wear-resistant steels, etc. 

Alloy steel designation

1. Low-alloy high-strength structural steel

Its designation consists of three parts: the Chinese pinyin letter representing the yield point (Q), the yield limit value, and the quality grade symbol (A, B, C, D, E), arranged in sequence. For example, Q390A indicates a low-alloy high-strength structural steel with a yield strength σs = 390 N/mm2 and quality grade A. 

2. Alloy structural steel

Its grade is composed of three parts: "two-digit number + element symbol + number". The first two digits represent the ten-thousand times of the average carbon mass fraction in the steel, the element symbol indicates the alloying element contained in the steel, and the number following the element symbol represents the hundred times of the average mass fraction of that element. When the average mass fraction of the alloying element is less than 1.5%, it is generally only indicated by the element without the value; when the average mass fraction is ≥1.5%, ≥2.5%, ≥3.5%,... , then the corresponding number 2, 3, 4, ... is marked after the alloying element. 。 For example, 40Cr has an average carbon mass fraction Wc = 0.4% and an average chromium mass fraction WCr < 1.5%. If it is a high-quality steel, an "A" is added at the end of the designation. For instance, 38CrMoAlA steel belongs to high-quality alloy structural steel. 

3. Rolling bearing steel

Add "G" (the initial letter of the Chinese pinyin for "rolling") before the grade number, and the subsequent digits represent the thousandfold quality fraction of chromium. The carbon quality fraction is not indicated. For example, GCr15 steel is a rolling bearing steel with an average chromium quality fraction of WCr = 1.5%. If other alloy elements besides chromium are present in the chromium bearing steel, their representation methods are the same as those of general alloy structural steels. Rolling bearing steel is all high-grade high-quality steel, but no "A" is added after the grade number. 

4. Alloy Tool Steel

The numbering method for this type of steel differs from that of alloy structural steel in the following way: when Wc is less than 1%, a single digit is used to represent the thousandfold of the carbon mass fraction; when the carbon mass fraction is 1% or more, it is not indicated. For example, Cr12MoV steel has an average carbon mass fraction of Wc = 1.45% to 1.70%, so it is not indicated; the average carbon mass fraction of Cr is 12%, and the mass fractions of Mo and V are both less than 1.5%. Another example is 9SiCr steel, with an average Wc of 0.9% and an average WCr of less than 1.5%. However, high-speed tool steel is an exception; the average carbon mass fraction of it is not indicated regardless of its value. Since both alloy tool steel and high-speed tool steel are high-quality advanced steels, there is no need to indicate "A" after their brand names. 

5. Stainless Steel and Heat-resistant Steel

The numbers preceding the steel grade indicate the thousandfold of the carbon mass fraction. For example, 3Crl3 steel indicates an average carbon mass fraction of Wc = 0.3% and an average chromium mass fraction of WCr = 13%. When the carbon mass fraction Wc is ≤ 0.03% and ≤ 0.08%, respectively, the prefix "00" and "0" are added before the grade, such as 00Cr17Ni14Mo2 and 0Cr19Ni9 steel, etc.