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Basic knowledge of steel

Attention:     Issuing time:2019-11-15 10:51

 Basic knowledge of steel
1. Definition of carbon steel and five elements in steel
Iron carbon alloy containing less than 2% carbon is called steel.
The five elements in carbon steel refer to the main components in chemical composition, i.e. C, Si, Mn, s, P (carbon, silicon, manganese, sulfur, phosphorus). Secondly, gas will inevitably be mixed in the steel-making process, containing o, h, n (oxygen, hydrogen, nitrogen). In addition, in the process of deoxidization and sedation with Al Si, there must be al in the molten steel, when als (acid soluble aluminum) is ≥ 0. At 020%, it can refine the grains.
2. How is steel made?
The main task of steelmaking is to adjust the content of carbon and alloy elements in the steel to the specified range according to the quality requirements of the steel, and reduce the content of P, s, h, O, N and other impurities to the allowable limit.
The steelmaking process is essentially an oxidation process, in which the excess carbon in the charge is oxidized and burned into CO gas to escape, and other Si, P, Mn, etc. are oxidized and then enter the slag. Part s enters the slag and part generates SO2 for discharge. When the composition and temperature of the molten steel meet the process requirements, the steel can be tapped. In order to remove the excess oxygen and adjust the chemical composition, Deoxidizer and ferroalloy or alloy elements can be added.

3. Introduction to converter steelmaking
The molten iron transported from the torpedo car can be poured into the converter as the main charge after desulfuration, slag blocking and other treatment, plus less than 10% scrap. Then, oxygen is blown into the converter for combustion, the excessive carbon in the molten iron is oxidized and a large amount of heat is released. When the probe reaches the predetermined low carbon content, the oxygen blowing is stopped and the steel is tapped. Generally, deoxidation and composition adjustment are required in the ladle, and then carbonized rice husk is thrown on the surface of the molten steel to prevent the molten steel from being oxidized, which can be sent to the continuous casting or die casting area.
For the steel with high requirements, bottom blowing argon, RH vacuum treatment and powder spraying treatment (spraying Si-Ca powder and denatured lime) can effectively reduce the gas and inclusions in the steel, and further reduce carbon and sulfur. After these measures, the composition can be adjusted to meet the demand of high quality steel.
4. Blooming
The ingot of mould casting adopts the new process of hot charging and hot delivery, enters the soaking pit for heating, and then is rolled into slab, tube billet, billet and other products through the blooming mill and billet continuous rolling mill. After the head cutting, tail cutting and surface cleaning, (flame cleaning and grinding) the high-quality products also need to be peeled and inspected, and they will be put into storage after passing the inspection.
At present, the products of blooming mill include bloom, rolled billet, billet for oxygen cylinder, billet for gear, billet for railway vehicle axle and steel for plastic mould, etc.
The blooms are mainly supplied by the hot rolling mill as raw materials; the rolled billets are mainly supplied by the high-speed wire rod mill as raw materials.
Due to the advanced nature of continuous casting slabs, the demand for blooming slabs has been greatly reduced, so the other products have been turned to other products.
5. Hot rolling
The continuous casting slab or blooming slab is used as raw material, heated by the walking heating furnace, descaled by high-pressure water, and then enters the roughing mill. The roughing mill enters the finishing mill through the cutting head, tail and then the finishing mill, and the computer-controlled rolling is carried out. After finishing rolling, the roughing mill passes through the laminar cooling (computer-controlled cooling rate) and coiler coiling to become a straight coil. The head and tail of the straight hair curl are usually in the shape of tongue and fish tail, and the accuracy of thickness and width is poor. The coil is heavy and its inner diameter is 760mm. (generally, the industry likes to use it. )
After cutting the head, tail, edge and several times of straightening, leveling and other finishing line treatment, the straight hair coil can be cut or rewound, namely: hot-rolled steel plate, flat hot-rolled steel coil, longitudinal cut strip and other products.
If the oxide scale is removed and oiled by acid pickling, the hot-rolled finished coil will become hot-rolled acid pickling coil. The product has the trend of replacing cold rolled sheet locally, and the price is moderate, which is deeply loved by the majority of users A new hot rolling pickling line invested by Baosteel is under construction.

6. Cold rolling
The hot-rolled steel coil is used as raw material, the oxide skin is removed by acid pickling and then cold continuous rolling is carried out. The finished product is hard rolled coil. Due to the cold work hardening caused by continuous cold deformation, the strength, hardness and toughness plastic index of hard rolled coil increase, so the stamping performance will deteriorate and can only be used for simple deformed parts The hard rolled coil can be used as the raw material of the hot-dip galvanizing plant, because the hot-dip galvanizing units are equipped with annealing lines. The weight of hard rolled coil is generally 6-13.5 tons, and the inner diameter of steel coil is 610mm.
Generally, the cold work hardening and rolling stress shall be eliminated by continuous annealing (CAPL unit) or bell type furnace annealing to reach the mechanical property index specified in corresponding standards.
The surface quality, appearance and dimension precision of cold-rolled steel plate are better than that of hot-rolled steel plate, and the thickness of the product is about 0.18mm, so it is favored by the majority of users.
Cold rolled steel coil is used as the base plate for deep processing of products, which becomes high value-added products. For example, electro galvanizing, hot galvanizing, fingerprint resistant electro galvanizing, color coated steel coil, vibration damping composite steel plate, PVC composite steel plate, etc., make these products have good quality such as beautiful appearance, high corrosion resistance, etc., and have been widely used.
After annealing, cold rolled steel coil must be finished, including cutting head, tail, cutting edge, leveling, leveling, rewinding, or longitudinal shear plate, etc. Cold rolling products are widely used in automobile manufacturing, home appliances, instrument switches, construction, office furniture and other industries. The weight of each steel plate bundle is 3-5 tons. Generally, the weight of leveling and winding is 3-10 tons / roll. Coil inner diameter 610mm.
7. Mechanical properties of steel
7.1 tensile test
The tensile test is called tensile test, which is prepared according to the standard and installed in the collet of the tensile testing machine, and slowly applies the uniaxial tensile stress to the sample until the sample is broken.
7.1.1 strength
The ability of metal material to resist deformation and fracture under the action of external force is called strength. Strength indexes include: proportional limit, elastic limit, yield strength, tensile strength, etc.

7.1.2 proportional limit
When the tensile force is applied to the metal, there is a stage in which the force and change form a linear proportion, and the maximum ultimate load PP in this stage divided by the original cross-sectional area of the sample is the proportional limit, which is expressed by σ P.
7.1.3 elastic limit
Metal deforms under the action of external force. After the external force is removed, it can completely restore its original shape. This kind of deformation is called elastic deformation. The maximum stress that metal can keep elastic deformation is called elastic limit, which is expressed by σ E.
7.1.4 tensile strength
When the specimen is in tension, the stress obtained by dividing the maximum load before breaking by the original cross-sectional area is called the tensile strength, which is expressed by σ B. When the external stress of the material is greater than its tensile strength, it will break. Therefore, the higher σ B is, the greater the external stress it can bear without fracture.
Foreign standard structural steel is usually classified according to tensile strength, such as SS400, where 400 means that the minimum value of σ B is 400MPa ultra-high strength steel, which means that σ B ≥ 1373 MPa.
Yield strength ratio
Yield strength ratio is the ratio of yield strength to tensile strength (σ s / σ b). The higher the yield ratio, the higher the strength of the material, the lower the yield ratio, the better the plasticity and the better the stamping formability. For example, the yield strength ratio of deep drawing steel plate is ≤ 0.65.
Spring steel is generally in service within the elastic limit, and plastic deformation is not allowed under load. Therefore, it is required that the spring steel has the highest elastic limit and yield strength ratio (σ s / σ B ≥ 0.90) after quenching and tempering. In addition, the fatigue life is closely related to the tensile strength and surface quality.
7.1.6 plasticity
The ability of metal materials to withstand permanent deformation before stress failure is called plasticity. Plasticity is usually expressed by elongation and reduction of area. The higher the elongation and the reduction of area, the better the plasticity.
8. Impact toughness
When a metal specimen of a certain size and shape is broken under the impact load in a specified type of impact test, the impact energy consumed on the unit cross section of the notch of the specimen is called the impact toughness expressed in α K.
At present, 10 × 10 × 55mm Charpy impact specimens with 2 mm deep V-notch are commonly used. In the standard, the impact energy (J Joule value) AK is directly used instead of α K value. Because the impact energy per unit area has no practical significance.
The impact energy is the most sensitive to check the brittleness transformation of metal materials at different temperatures, while the catastrophic fracture accidents in actual service conditions are often related to the impact energy and service temperature of materials Therefore, in the relevant standards, the impact energy value at a certain temperature is often specified, and the technical conditions of fat (fracture area conversion temperature) lower than a certain temperature are also specified. The so-called fat is a group of impact samples at different temperatures, after impact fracture assessment, when the brittle fracture accounts for 50% of the total area of the corresponding temperature.
Due to the influence of steel plate thickness, 3 / 4 small size impact specimens (7.5 × 10 × 55mm) or 1 / 2 small size impact specimens (5 × 10 × 55mm) can be obtained for steel plates with thickness ≤ 10 mm. However, it must be noted that the impact energy values of the same specification and temperature can be compared with each other. Only under the conditions specified in the standard can the impact energy of the standard impact specimen be converted according to the standard conversion method, and then compared with each other.

9. Hardness test
The ability of metal materials to resist indenting surfaces by indenters (hardened steel balls or diamond indenters with 1200 cones or pyramids) is called hardness. According to the different test methods and application scope, hardness can be divided into Brinell hardness, Rockwell hardness, Vickers hardness, shore hardness, microhardness, high temperature hardness, etc.
Brinell hardness and Rockwell hardness are commonly used in metallurgical products.
10. Hot and cold rolling steel plates and strips for structure
Structural steel is generally classified by strength, and the number in the steel grade often represents the lowest value of tensile strength. Because this kind of steel is often used to make structural members, it is called structural steel. Strengthening mechanism of structural steel tends to reduce carbon and increase carbon
The precipitation, precipitation and fine grain strengthening of ferrite, pearlite and microalloy strengthened by manganese solid solution can ensure the good toughness, plasticity and weldability while improving the strength.

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