Steel plate

It is a flat steel that is cast with molten steel and pressed after cooling.
It is flat, rectangular and can be directly rolled or cut from wide steel strips.
The steel plate is divided according to the thickness, the thin steel plate is less than 4 mm (the thinnest is 0.2 mm), the medium-thick steel plate is 4-60 mm, and the extra-thick steel plate is 60-115 mm.
Steel sheets are divided into hot-rolled and cold-rolled according to rolling.
The width of the thin plate is 500~1500 mm; the width of the thick sheet is 600~3000 mm. Sheets are classified by steel type, including ordinary steel, high-quality steel, alloy steel, spring steel, stainless steel, tool steel, heat-resistant steel, bearing steel, silicon steel and industrial pure iron sheet, etc.; Enamel plate, bulletproof plate, etc. According to the surface coating, there are galvanized sheet, tin-plated sheet, lead-plated sheet, plastic composite steel plate, etc.
Low alloy structural steel
(also known as ordinary low alloy steel, HSLA)
1. Purpose
Mainly used in the manufacture of bridges, ships, vehicles, boilers, high-pressure vessels, oil and gas pipelines, large steel structures, etc.
2. Performance requirements
(1) High strength: generally its yield strength is above 300MPa.
(2) High toughness: the elongation is required to be 15% to 20%, and the impact toughness at room temperature is greater than 600kJ/m to 800kJ/m. For large welded components, high fracture toughness is also required.
(3) Good welding performance and cold forming performance.
(4) Low cold-brittle transition temperature.
(5) Good corrosion resistance.
3. Ingredient characteristics
(1) Low carbon: Due to the high requirements for toughness, weldability and cold formability, the carbon content does not exceed 0.20%.
(2) Add manganese-based alloying elements.
(3) Adding auxiliary elements such as niobium, titanium or vanadium: a small amount of niobium, titanium or vanadium forms fine carbides or carbonitrides in steel, which is beneficial to obtain fine ferrite grains and improve the strength and toughness of steel .
In addition, adding a small amount of copper (≤0.4%) and phosphorus (about 0.1%) can improve the corrosion resistance. Adding a small amount of rare earth elements can desulfurize and degas, purify steel, and improve toughness and process performance.
4. Commonly used low alloy structural steel
16Mn is the most widely used and most productive type of low-alloy high-strength steel in my country. The structure in use state is fine-grained ferrite-pearlite, and its strength is about 20% to 30% higher than that of ordinary carbon structural steel Q235, and its atmospheric corrosion resistance is 20% to 38% higher.
15MnVN is the most used steel in medium-strength steels. It has high strength, and good toughness, weldability and low temperature toughness, and is widely used in the manufacture of large structures such as bridges, boilers, and ships.
After the strength level exceeds 500MPa, the ferrite and pearlite structures are difficult to meet the requirements, so low carbon bainitic steel is developed. The addition of Cr, Mo, Mn, B and other elements is beneficial to obtain bainite structure under air cooling conditions, so that the strength is higher, the plasticity and welding performance are also better, and it is mostly used in high-pressure boilers, high-pressure vessels, etc.
5. Characteristics of heat treatment
This type of steel is generally used in a hot-rolled and air-cooled state and does not require special heat treatment. The microstructure in use state is generally ferrite + sorbite.
Alloy carburized steel
1. Purpose
It is mainly used in the manufacture of transmission gears in automobiles and tractors, camshafts, piston pins and other machine parts on internal combustion engines. Such parts suffer from strong friction and wear during work, and at the same time bear large alternating loads, especially impact loads.
2. Performance requirements
(1) The surface carburized layer has high hardness to ensure excellent wear resistance and contact fatigue resistance, as well as appropriate plasticity and toughness.
(2) The core has high toughness and sufficiently high strength. When the toughness of the core is insufficient, it is easy to break under the action of impact load or overload; when the strength is insufficient, the brittle carburized layer is easily broken and peeled off.
(3) Good heat treatment process performance Under the high carburizing temperature (900℃～950℃), the austenite grains are not easy to grow and have good hardenability.
3. Ingredient characteristics
(1) Low carbon: the carbon content is generally 0.10% to 0.25%, so that the core of the part has sufficient plasticity and toughness.
(2) Add alloying elements to improve hardenability: Cr, Ni, Mn, B, etc. are often added.
(3) Add elements that hinder the growth of austenite grains: mainly add a small amount of strong carbide forming elements Ti, V, W, Mo, etc. to form stable alloy carbides.
4. Steel grade and grade
20Cr low hardenability alloy carburized steel. This type of steel has low hardenability and low core strength.
20CrMnTi medium hardenability alloy carburized steel. This type of steel has high hardenability, low overheating sensitivity, relatively uniform carburizing transition layer, and good mechanical and technological properties.
18Cr2Ni4WA and 20Cr2Ni4A high hardenability alloy carburized steel. This type of steel contains more elements such as Cr and Ni, has high hardenability, and has good toughness and low-temperature impact toughness.
5. Heat treatment and microstructure properties
The heat treatment process of alloy carburized steel is generally direct quenching after carburizing, and then tempering at low temperature. After heat treatment, the structure of the surface carburized layer is alloy cementite + tempered martensite + a small amount of retained austenite, and the hardness is 60HRC ~ 62HRC. The core structure is related to the hardenability of the steel and the cross-sectional size of the parts. When fully hardened, it is low-carbon tempered martensite with a hardness of 40HRC to 48HRC; in most cases, it is troostite, tempered martensite and a small amount of iron. Element body, hardness is 25HRC ~ 40HRC. The toughness of the heart is generally higher than 700KJ/m2.
Alloy quenched and tempered steel
1. Purpose
Alloy quenched and tempered steel is widely used in the manufacture of various important parts on automobiles, tractors, machine tools and other machines, such as gears, shafts, connecting rods, bolts, etc.
2. Performance requirements
Most of the quenched and tempered parts bear a variety of working loads, the stress situation is relatively complex, and high comprehensive mechanical properties are required, that is, high strength and good plasticity and toughness. Alloy quenched and tempered steel also requires good hardenability. However, the stress conditions of different parts are different, and the requirements for hardenability are different.
3. Ingredient characteristics
(1) Medium carbon: the carbon content is generally between 0.25% and 0.50%, with 0.4% in the majority;
(2) Adding elements Cr, Mn, Ni, Si, etc. to improve hardenability: In addition to improving hardenability, these alloy elements can also form alloy ferrite and improve the strength of steel. For example, the performance of 40Cr steel after quenching and tempering treatment is much higher than that of 45 steel;
(3) Add elements to prevent the second type of temper brittleness: alloy quenched and tempered steel containing Ni, Cr, and Mn, which is prone to the second type of temper brittleness during high temperature tempering and slow cooling. Adding Mo and W to steel can prevent the second type of temper brittleness, and its suitable content is about 0.15%-0.30% Mo or 0.8%-1.2% W.
Comparison of the properties of 45 steel and 40Cr steel after quenching and tempering
Steel grade and heat treatment state Section size/ mm sb/ MPa ss/MPa d5/ % y/% ak/kJ/m2
45 steel 850℃ water quenching, 550℃ tempering f50 700 500 15 45 700
40Cr steel 850℃ oil quenching, 570℃ tempering f50 (core) 850 670 16 58 1000
4. Steel grade and grade
(1) 40Cr low hardenability quenched and tempered steel: The critical diameter of oil quenching of this type of steel is 30mm to 40mm, which is used to manufacture important parts of general size.
(2) 35CrMo medium hardenability alloy quenched and tempered steel: the critical diameter of oil quenching of this type of steel is 40mm to 60mm. The addition of molybdenum can not only improve the hardenability, but also prevent the second type of temper brittleness.
(3) 40CrNiMo high hardenability alloy quenched and tempered steel: the critical diameter of oil quenching of this type of steel is 60mm-100mm, most of which are chromium-nickel steel. Adding appropriate molybdenum to chromium-nickel steel not only has good hardenability, but also eliminates the second type of temper brittleness.
5. Heat treatment and microstructure properties
The final heat treatment of alloy quenched and tempered steel is quenching and high temperature tempering (quenching and tempering). Alloy quenched and tempered steel has high hardenability, and oil is generally used. When the hardenability is particularly large, it can even be air-cooled, which can reduce heat treatment defects.
The final properties of alloy quenched and tempered steel depend on the tempering temperature. Generally, tempering at 500℃-650℃ is used. By choosing the tempering temperature, the required properties can be obtained. In order to prevent the second type of temper brittleness, rapid cooling (water cooling or oil cooling) after tempering is beneficial to the improvement of toughness.
The microstructure of alloy quenched and tempered steel after conventional heat treatment is tempered sorbite. For parts that require wear-resistant surfaces (such as gears and spindles), induction heating surface quenching and low-temperature tempering are performed, and the surface structure is tempered martensite. The surface hardness can reach 55HRC ~ 58HRC.
The yield strength of alloy quenched and tempered steel after quenching and tempering is about 800MPa, and the impact toughness is 800kJ/m2, and the hardness of the core can reach 22HRC~25HRC. If the cross-sectional size is large and not hardened, the performance is significantly reduced.