Carbon Steel (Mild Steel) Basics You Should Know

Basic concepts

Alloy elements: chemical elements specially are added to steel to ensure the required structural, physical, chemical and mechanical properties.

Impurities: chemical elements brought in by raw materials, smelting methods and process operations during smelting.

Carbon steel: iron carbon alloy with carbon content in the range of 0.0218-2.11%.

Alloy steel: steel added with a certain amount of alloy elements on the basis of carbon steel.

Low alloy steel: generally refers to steel with total alloy element content less than or equal to 5%.

Medium alloy steel: generally refers to steel with a total alloy element content of 5% – 10%.

High alloy steel: generally refers to steel with a total content of alloy elements of more than 10%.

Microalloyed steel: steel with alloy elements (such as V, Nb, Ti, Zr, B) content less than or equal to 0.1%, which can significantly affect the structure and properties.

Carbon Steel (Mild Steel) Basics You Should Know 1

Steelmaking line

Introduction to carbon steel

1. Common impurities in carbon steel

1. Manganese (MN) and silicon (SI)

It enters the steel with deoxidizer or residual pig iron during steelmaking.

Mn: the content in carbon steel is generally less than 0.8%.

It can be solid solution, and can also form high melting point MNS (1600 ° ℃) inclusions.

MnS has certain plasticity at high temperature and will not cause hot embrittlement of steel. After processing, manganese sulfide is distributed along the rolling direction in strip shape.

Si: the content in steel is usually less than 0.5%.

It can be solid solution, also can form SiO, inclusions.

Mn and Si are beneficial impurities, but the inclusions MnS and SiO will reduce the fatigue strength, plasticity and toughness of steel.

2. Sulfur (S) and phosphorus (P)

S: The solubility in solid iron is very small, S and Fe can form FeS, and it is easy to form low melting point eutectic.

Thermal embrittlement (cracking) occurs.

P: Soluble in α- Iron, however, strongly reduces the toughness of steel, especially at low temperature, which is called cold embrittlement.

Phosphorus can improve the corrosion resistance of steel in the atmosphere.

S and P are harmful impurities, but they can improve the machinability of steel.

3. Nitrogen (N), hydrogen (H), oxygen (O)

N: Soluble in a-iron, steel containing supersaturated n precipitates nitrides after cold deformation – Mechanical aging or strain aging.

N can form stable nitrides with vanadium, titanium, sharp, etc., with grain refinement and precipitation strengthening.

H: The combined action of neutralizing stress in steel will cause hydrogen embrittlement of metal materials.

O: Silicate 2MnO · SiO2, MnO · SiO, or composite oxides MgO · Al2O3, MnO · Al2O3 are formed in steel.

N, H and O are harmful impurities.

2. Classification of carbon steel

1. According to the carbon content in steel

1) Classification according to Fe Fe3C phase diagram

Hypoeutectoid steel: 0.0218% ≤ WC ≤ 0.77%

Eutectoid steel: Wc=0.77%

Hypereutectoid steel: 0.77% < WC ≤ 2.11%

Classification of carbon steel

2) Classification by carbon content in steel

Low carbon steel: WC ≤ 0.25%

Medium carbon steel: 0.25% < WC ≤ 0.6%

High carbon steel: WC > 0.6%

2. According to the quality of steel (quality)

(1) Ordinary carbon steel: WS ≤ 0.05%, WP ≤ 0.045%.

(2) High quality carbon steel:

wS≤0.035%,wP≤0.035%。

(3) High quality carbon steel:

wS≤0.02%,wP≤0.03%。

(4) Super quality carbon steel:

wS≤0.015%,wP≤0.025%。

3. Classification according to the use of steel

(1) Carbon structural steel:

It is mainly used for various engineering components, such as bridges, ships, building components, etc.

It can also be used for less important parts.

(2) High quality carbon structural steel:

It is mainly used to manufacture various machine parts, such as shafts, gears, springs, connecting rods, etc.

(3) Carbon tool steel:

It is mainly used for manufacturing various tools, such as cutting tools, molds, measuring tools, etc.

(4) Cast carbon steel for general engineering:

It is mainly used for manufacturing parts with complex shape and requiring certain strength, plasticity and toughness.

4. Classification according to deoxidation degree during steel smelting

(1) Rimmed steel: refers to steel with incomplete deoxidization, code F.

(2) Killed steel: refers to steel with complete deoxidization, code named Z.

(3) Semi killed steel: refers to the deoxidization degree between rimming steel and killed steel, code b.

(4) Special killed steel: refers to the steel with special deoxidation, code named TZ.

3. Use of carbon steel

1. Ordinary carbon structural steel

a. It is mainly used for general engineering structures and common parts.

It is usually rolled into steel plates, steel strips, steel tubes, wire rods, section steels, bar steels or various profiles (round steel, square steel, I-beam, reinforcement, etc.), which can be used for welding, riveting, bolting and other structural parts.

It is widely used (more than 70% of the total steel output).

b. Air cooling after hot rolling is the usual supply state of this kind of steel.

Users generally do not need to carry out heat treatment, but use it directly.

wC =0.06%~0.38%.

When the quality grade is “A” and “B”, the chemical composition can be adjusted appropriately according to the requirements of the demander under the guarantee of mechanical properties.

c. Designation of ordinary carbon structural steel

It is composed of letters (Q) representing yield point, yield point value, quality grade symbols (A, B, C, D) and deoxidation method symbols (F, b, Z, TZ) in sequence.

The yield point value is divided into five strength grades: 195, 215, 235, 255 and 275;

Grade symbol refers to the quality grade symbol used solely for this kind of steel, which is also divided according to the number of impurities S and P. four symbols A, B, C and D represent four grades, among which:

Class A wS ≤ 0.05%, WP ≤ 0.045%,

Class B wS ≤ 0.045%, WP ≤ 0.045%,

Class C wS ≤ 0.04%, WP ≤ 0.04%,

Level D wS ≤ 0.035%, WP ≤ 0.035%

Among them, the highest quality grade is grade D, which reaches the high-quality grade of carbon structural steel. Grades A, B and C all belong to the range of ordinary grades.

The symbol of deoxidation method can be omitted from the grades of killed steel and special killed steel.

d. Typical brand, performance and Application

Q195, Q215:

The carbon content is very low and the strength is not high, but it has good plasticity, toughness and welding performance.

It is often used as workpieces with low strength requirements, such as nails, iron wires, steel windows and various thin plates.

Q235A, Q255A:

It is used for pull rods, small shafts, chains, etc. in agricultural machines and tools.

It is also used for building reinforcement, steel plate, section steel, etc;

Q235B, Q255B:

It is used as welded structural parts with high quality requirements in construction projects, and general rotating shafts, hooks, bicycle frames, etc. in machinery;

Q235C, Q235D:

The quality is good, and it can be used as some important welding structural parts and machine parts.

Q255, Q275:

The strength is high, among which Q275 is medium carbon steel, which can be used to manufacture friction clutch, brake steel belt, etc.

2. High quality carbon structural steel

(1) For important mechanical parts, the mechanical properties of parts can be adjusted through various heat treatments.

(2) The supply state can be air cooling after hot rolling, annealing, normalizing and other states, which generally depends on the needs of users.

(3) The brand is generally represented by two digits.

These two figures represent ten thousand times the average mass fraction of carbon in steel, such as 20 steel and 45 steel.

a. Three grades of high-quality carbon structural steel are rimmed steel, which are 08F, 10F and 15F.

Semi killed steel is marked with “b”, and killed steel is generally not marked with symbols.

b. High quality carbon structural steel is marked with “A” after the brand, and super grade carbon structural steel is marked with “E”.

c. For special high-quality carbon structural steel, a symbol representing the purpose of the product should also be added to the head (or tail) of the brand, such as boiler steel with an average carbon content of 0.2%, whose brand is “20g”, etc.

d. High quality carbon structural steel is divided into two groups: ordinary manganese content and higher manganese content according to the different manganese content.

For the group with high manganese content, add “Mn” at the end of its number, such as 15Mn, 45Mn, etc.

e. There are 31 steel grades of high-quality carbon structural steel

08F steel: low mass fraction of carbon, good plasticity and low strength.

It can be used for all kinds of cold deformation forming parts.

10~25 steel: it has good welding and cold stamping properties, and can be used to manufacture standard parts, shaft sleeves, containers, etc.

It can also be used to manufacture wear-resistant and impact resistant parts with high surface hardness and high strength and toughness in the heart.

Such as gears, cams, pins, friction plates, cement nails, etc.

45 medium carbon steel: good comprehensive mechanical properties can be obtained by proper heat treatment.

It can be used for mechanical parts such as transmission shaft, engine connecting rod, machine tool gear, etc.

High carbon structural steel: after proper heat treatment, it can obtain high elastic limit, yield ratio, sufficient toughness and wear resistance.

It can manufacture springs, heavy rails, rollers, shovel, steel wire ropes, etc. with small wire diameters.

3. Carbon tool steel

(1) It is mainly used for making various small tools.

It can be quenched and tempered at low temperature to obtain high hardness and high wear resistance.

It can be divided into high-quality carbon tool steel and high-quality carbon tool steel.

(2) The trademark is generally expressed by the symbolic symbol “T” (the Chinese phonetic prefix of carbon) plus a thousand times the mass fraction of carbon. Such as T10, T12, etc.

Generally, high-quality carbon tool steel does not add the quality grade symbol, while high-quality carbon tool steel adds the word “a” after its number, such as T8A, T12, etc.

(3) The mass fraction of manganese in manganese containing carbon tool steel can be expanded to 0.6%.

At this time, Mn is marked at the end of the brand, such as T8Mn, T8MnA.

(4) Typical carbon tool steel

T7, T8: suitable for manufacturing cutting tools that bear certain impact and require high toughness, such as woodworking axes, bench chisels, etc. the hardness after quenching and low-temperature tempering is 48~54HRC (working part);

T9, T10, T11 steel: it is used to manufacture cutting tools requiring high hardness and wear resistance due to small impact, such as small drill bits, taps, hand saw blades, etc. the hardness after quenching and low temperature tempering is 60~62HRC.

T10A Steel can also be used to manufacture some cold working molds and measuring tools with simple shape and small working load;

T12 and T13 steel: the hardness and wear resistance are the highest, but the toughness is the worst.

It is used to manufacture cutting tools that do not bear impact, such as files, shovels and scrapers. The hardness after quenching and low-temperature tempering is 62~65 HRC.

T12A can also be used to manufacture measuring tools.

T7~t12 and T7A~T12A can also be used for plastic molds with simple shapes.

4. Cast carbon steel for general engineering

(1) It is mainly used for rough parts made of cast iron that cannot guarantee its plasticity, and its shape is complex, which is not convenient for forging.

Its carbon content is generally less than 0.65%.

(2) The brand is represented by the symbol “ZG” (the Chinese phonetic prefix of cast steel) plus the minimum yield point value – the minimum tensile strength value.

For example, ZG340-640 refers to cast steel with yield strength not less than 340MPa and tensile strength not less than 640MPa.

(3) Typical carbon cast steel

(4) Other types of steel castings include:

Carbon steel castings for welded structures

(GB/T7659-1987), such as ZG230-450H;

Low alloy steel castings

(GB/T14408-1993), such as ZGD535-720;

Heat resistant steel castings

(GB/T8492-1987), such as ZG40Cr30Ni20;

Stainless and acid resistant steel castings

(GB2100-1980), such as ZG1Cr18Ni9Ti;

Medium and high strength stainless steel castings (GB6967-1986), such as ZG10Cr13Ni1Mo, etc.

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