Commonly used materials Q235, 20 steel, 45 steel, 40Cr, Q355, what type of steel are they? What is the distinction between ordinary carbon structural steel and high-quality carbon structural steel? What do the numbers in steel grades mean? The correct reading and writing of steel?

1. Carbon steel

Steel is a general term for iron-carbon alloys with carbon content between 0.02% and 2.11% by mass percentage.
The chemical composition of steel can have great changes, only carbon steel is called carbon steel (carbon steel) or ordinary steel, carbon content of about 0.05% ~ 0.70%, individual up to 0.90%, does not contain other alloying elements (except residual elements).
Iron alloys containing more than 2% carbon are cast iron.

2. Carbon steel classification

Plain steel can be divided into three categories according to different carbon content:
Mild steel (C≤0.25%)
Medium carbon steel (C 0.25-0.6%)
High carbon steel (C > 0.6%)
Carbon steel can be divided into:
Ordinary carbon steel
High quality carbon steel
Ordinary carbon steel: contains more impurities, low price, used in places with low performance requirements. Many uses, the amount is large, the manufacture of various metal components under static load and unimportant mechanical parts and general welding parts that do not require heat treatment.
High-quality carbon structural steel: pure steel, less impurities, good mechanical properties, can be used after heat treatment.

3. The distinction between carbon steel and high-quality carbon steel

Carbon content is not the index to distinguish between carbon steel and high-quality carbon steel, that is, carbon steel and high-quality carbon steel have low carbon steel, medium carbon steel and high carbon steel three categories. Carbon structural steel and high-quality carbon structural steel are distinguished by impurity content, not carbon content. Then another question arises, which elements are called impurities? The answer is sulfur and phosphorus, the amount of harmful impurities P and S and non-metallic inclusions.
Sulfur: Sulfur is usually a harmful element that makes steel hot brittle and limited to less than 0.05%.
Phosphorus: it is a harmful element, which reduces the plasticity and toughness of steel, appears cold brittleness, and the content should be limited to less than 0.05%.
The sulfur and phosphorus content of high-quality carbon structural steel is less than 0.035%.

4. Name of carbon steel and high-quality carbon steel

The carbon steel grade reflects its mechanical properties, expressed by Q+ numbers, in which "Q" is the Chinese pinyin prefix of the yield point "Qu" word, the number represents the yield point value, and the letters A, B, C, D are marked behind the grade, indicating that the quality of steel is different, the amount of S and P is reduced in turn, and the quality of steel is increased in turn. If the letter "F" is marked after the grade, it is boiling steel, the mark "b" is semi-killed steel, and the one who does not mark "F" or "b" is killed steel. For example:
Q235A·F: Grade A boiling steel with yield point of 235MPa,
Q235B: Grade B killed steel or special killed steel with a yield point of 235MPa.
The grade of high-quality carbon structural steel is represented by two digits, that is, the 10,000 digits of the average carbon content in the steel.
For example:
20 Steel: high-quality carbon steel with an average carbon content of 0.20%.
45 steel: high-quality carbon steel with an average carbon content of 0.45%, the steel number is "45", it is not a serial number, so it can not be read as 45 steel.
It can be seen from this analysis that the number of high-quality carbon steel indicates the average carbon content, rather than the serial number, so it is "45 steel", read as "four or five steel", not "45 steel" or "45# steel", and can not be read as "45 steel".

5. Comparison and selection

5.1 Q235 and 45 steel
Q235 is a carbon structural steel that only guarantees mechanical properties and does not guarantee chemical composition. 45 steel High-quality carbon structural steel, not only to ensure mechanical properties, but also to ensure the chemical composition. If the parts do not need heat treatment, choose carbon structural steel, provided that the mechanical strength meets the requirements. If the parts need heat treatment, you must choose high-quality carbon structural steel, chemical composition instability is no way to determine the heat treatment process. The use of high-quality carbon structural steel 45 steel, must be heat treatment (such as: tempering), otherwise it is a waste.
5.2 45 steel with 40Cr
45 steel and 40Cr are commonly used materials for shafts, 45 steel belongs to (high-quality) carbon structural steel, and 40Cr belongs to alloy structural steel. The carbon content of the two is similar, and the heat treatment hardness is similar, but 40Cr is wear-resistant than 45 steel, and the wear resistance of 40Cr is more than 1.7 times that of 45 steel. 45 steel has high strength, good strength and toughness coordination, low hardenability, and easy to crack when water quenching.
40Cr: alloy structural steel with carbon content of 0.40%, it is on the basis of carbon structural steel No.40, the appropriate addition of cr element, Cr as the main addition element, after tempering, with good comprehensive performance, low temperature impact toughness and low notch sensitivity.
After the tempering treatment, 40Cr has good comprehensive mechanical properties, low temperature impact toughness and low notch sensitivity, good hardenability, oil quenching can get higher fatigue strength, water quenching complex shape parts are easy to crack, there is a tempering brittleness tendency. The cold deformation plasticity is medium, and the machinability is good after normalization or tempering, but the workpiece requiring high surface hardness is generally treated after cutting and tempering. It is generally used in the tempered state, and can also be carbonitriding and high-frequency induction hardening treatment.
In the field of electromechanical special equipment, most of the ordinary shafts are processed with 45 steel, and individual shaft parts with larger forces are processed with 40Cr.

6. Summarize

Q235: ordinary carbon steel, low carbon steel, a kind of steel with the largest amount in electromechanical special equipment, such as the main beam and leg of lifting machinery.
20 Steel: high-quality carbon steel, low carbon steel, mainly seamless steel pipe material, usually used in such as amusement equipment leg posts.
45 steel: high-quality carbon steel, medium carbon steel, the drive shaft between the motor reducer in the crane, forged wheels with tread diameter not greater than 400mm, etc.
40Cr: not carbon steel, is low alloy structural steel, in the manufacturing of amusement facilities, small parts using tempering treatment, large parts should be normalized treatment, mainly used for manufacturing high strength moving parts, such as shaft, gear, rack, worm and so on.
Q355: not carbon steel, is low alloy high strength structural steel, used for low and medium pressure vessels, cranes, such as spherical tanks, the main beam of large cranes.