Carbon Steel

Carbon steel (referred to as carbon steel) is an iron-carbon alloy with a mass fraction of C between 0.2% and 2.06% and a small amount of impurities such as Si, Mn, P, and S.

Classification of carbon steel

1. Ordinary carbon structural steel

According to the provisions of GB700-88, the grade of carbon structural steel is expressed by the minimum yield strength of the steel. Although this type of steel contains a lot of harmful impurities and non-metallic inclusions, its smelting method is simple, the manufacturability is good, the price is low, and its performance can also meet the requirements of general engineering structural parts and ordinary parts, so it is used in a large amount. , accounting for about 80% of the total steel.

The grade of steel is composed of the first letter “Q” in the Chinese pinyin of the word “Qu” representing the yield point, the yield point value (number), the quality grade symbols A, B, C, D, symbols such as deoxidation methods (using deoxidation methods, etc. The first letter of the name in Chinese Pinyin indicates, such as boiling steel (F), semi-killed steel (b), killed steel (Z), special killed steel (TZ)). The “Z” and “TZ” symbols can be omitted. For example, Q235-A·F stands for carbon structural steel, the yield point value is 235MPa, and it is grade A boiling steel.

2. High quality carbon structural steel

High-quality carbon structural steel contains less harmful impurities P and S and less non-metallic inclusions, and its uniformity and surface quality are relatively good, and the chemical composition and mechanical properties of the steel must be guaranteed at the same time. This type of steel has large output, low price and good mechanical properties, and is widely used in the manufacture of various mechanical parts and structural parts. These parts are usually used after heat treatment.

1）The number and composition characteristics of high-quality carbon structural steel

The grade of high-quality carbon structural steel is a two-digit representation of the mass fraction of C in the steel, expressed in ten thousandths. For example, “40 steel” means high-quality carbon structural steel with an average C mass fraction of 0.40%. If there are less than two digits, add 0 in front. Starting from 10 steel, increase by one steel grade with the number “5” as the change range. If there is an “F” after the number (such as 08F), it means boiling steel. High-quality carbon structural steel is divided into two groups according to manganese content: ordinary manganese content (ω=0.35%~0.8%) and higher manganese content (ω=0.7%~1.2%). For a group with higher manganese content, “Mn” is added after the steel grade, such as 15 Mn, 20 Mn, etc.

2）Heat treatment and application of commonly used high-quality carbon structural steel

The properties and application scope of commonly used high-quality carbon structural steel are as follows.
08 steel and 10 steel have very low carbon content, low strength, good plasticity, and good welding performance and rolling performance. They are usually rolled into thin plates or steel strips. Mainly used for the manufacture of cold stamping parts, such as various instrument panels, containers and gaskets and other parts.

15 steel, 20 steel and 25 steel also have good weldability and calendering properties, and are often used in the manufacture of structural parts and parts with little stress and high toughness, such as welding containers, manufacturing nuts, screws, etc., as well as manufacturing strength Carburizing parts that are not too demanding, such as cams, gears, etc. The heat treatment of carburized parts is generally a quenching (840~920℃) and low temperature tempering after carburizing.
35 steel, 40 steel, 45 steel, 50 steel, 55 steel are quenched and tempered steels, which can be used to manufacture parts with high performance requirements, such as gears, connecting rods, shafts, etc. Quenched and tempered steel is generally quenched and tempered to obtain comprehensive mechanical properties with good strength and toughness. For workpieces that do not require high comprehensive mechanical properties or have large cross-sectional dimensions and poor quenching effect, normalizing can be used instead of quenching and tempering.
60 steel, 65 steel, 70 steel, 75 steel, 80 steel, and 85 steel belong to spring steel. After proper heat treatment, they can be used to manufacture parts that require good elasticity and high strength, such as springs, spring hot coils, etc., and can also be used for Manufacture of wear parts. Cold-formed springs are generally only subjected to low temperature stress relief treatment. Hot-formed springs are generally quenched (~850℃) and tempered at low temperature (200~250℃).

3. Carbon Tool Steel

In the machinery manufacturing industry, the steel used to manufacture various cutting tools, dies and measuring tools is called tool steel. Since tools require high hardness and high wear resistance, and most cutting tools are also hot-hardened, the carbon content of tool steels is high. Tool steel usually adopts a heat treatment process of quenching + low temperature tempering to ensure high hardness and wear resistance.

The mass fraction of C in carbon tool steel is 0.65% to 1.35%. According to the content of S and P, carbon tool steel can be divided into two categories: high-quality tool steel and high-grade high-quality tool steel.

1. The number and composition characteristics of carbon tool steel
The steel number of carbon tool steel is represented by the prefix “T” in the Chinese pinyin of the word “carbon”, followed by a sequential number. The number represents the average mass fraction of C of the steel, expressed in thousandths, such as high-quality carbon For plain tool steel, add the word “A” after the number. For example, T8 steel means high-quality carbon tool steel with an average mass fraction of C of 0.8%. T12A steel means high-quality carbon tool steel with an average mass fraction of C of 1.2%. Those with higher manganese content are marked with “Mn” after the steel number, such as T8Mn. The advantages of carbon tool steel are that it is easy to forge, has good processing performance, and is cheap. The production volume of paper accounts for about 60% of all tool steels. The disadvantage is that the hardenability is low, and the content of Si and Mn is slightly changed, which will have a greater impact on the hardenability. Therefore, it is easy to produce quenching deformation and quenching cracking for carbon tool steel, especially for tools with complex shapes, and its tempering resistance is also poor. In order to improve the segmentability of carbon tool steel and reduce its tendency to quench cracking, its sulfur and phosphorus content should be more strictly limited than that of high-quality carbon structural steel.

2. Heat treatment and application of carbon tool steel
The blank of tool steel is generally formed in segments, and then machined into tool products from the blank. After the carbon tool steel segment is manufactured, due to its high hardness, it is not easy to carry out cutting processing, there is a large stress, and the structure does not meet the quenching requirements, so spheroidizing annealing should be carried out to improve the machinability and prepare the structure for the final quenching. The annealed structure should be spherical pearlite, and its hardness is generally less than 217HBS.
The quenching heating temperature should be determined according to the steel bell, and at the same time, the performance requirements, the shape and size of the workpiece and the cooling medium should also be considered. Due to its low hardenability during quenching and cooling, in order to obtain martensitic structure, in addition to cooling in oil for small cutting tools with complex shapes, effective thickness or diameter less than 5mm, cooling media with strong cooling capacity (such as water, water, etc.) are generally used. brine, alkaline water, etc.). It should also be pointed out that during quenching, the strong cooling medium will increase the quenching stress, so it may cause large deformation or even cracking, which is a significant weakness of carbon tool steel.

4. Cast steel

Some large-scale parts with complex shapes and high comprehensive mechanical properties require the use of steel castings because they are difficult to form by forging in terms of technology and cannot be made of cast iron with low mechanical properties in terms of performance. At present, cast steel is widely used in heavy machinery manufacturing, transportation machinery, and national defense industries, such as rolling mill frames, hydraulic press beams and cylinders, locomotive frames, bolsters on railway vehicle bogies, and wheel forks for automobiles and tractors. , Lifting truck wheels, large gears, etc.
The mass fraction of C of carbon cast steel used in engineering is 0.2% to 0.6%. If the carbon content is too high, the plasticity is not good, and cracks are prone to occur during solidification.
To improve the mechanical properties of carbon cast steel, alloy cast steel can be formed by adding alloying elements.