In order to obtain the required shape of hot rolled steel, it is necessary to permanently deform it. The technology adopted can be local or whole bending, deep drawing, stretching or the combination of these molding methods.
Hot strip mill steel and strip products are made from slabs (mainly continuous casting billets) as raw materials, which are heated and then made into strips by rough rolling units and finishing units. The hot steel strip from the last mill of the finishing mill is cooled to the set temperature by laminar flow and rolled into steel strip coils by the coiler. The cooled steel strip coils are processed into steel plates, flat coils and longitudinal steel strip products through different finishing lines (leveling, straightening, cross-cutting or longitudinal cutting, inspection, weighing, packaging and marking, etc.) according to different needs of customers. Due to the high strength, good toughness, easy processing and good weldability of hot rolled steel products, they are widely used in ship, automobile, bridge, construction, machinery, pressure vessel and other manufacturing industries. With the increasing maturity of new technologies for controlling the dimensional accuracy, plate shape and surface quality of hot-rolled products and the introduction of new products, hot rolled steel plates and strips have been more and more widely used and have stronger and stronger competitiveness in the market. The general description of hot rolled steel products, a wide variety of steel specifications, a wide range of uses, from general engineering structures to automotive, bridges, ships, boilers, pressure vessels and other manufacturing, have been used in large quantities. A variety of different uses, the material properties of steel plate, surface quality and size, shape accuracy and other requirements are different, therefore, must have an understanding of the varieties of hot rolled steel products, materials, characteristics and their uses, in order to achieve economic and reasonable use.
(1) The yield strength of thin steel plate indicates the formability and strength after forming. For the forming of ordinary carbon steel plate, the yield point value is too high, there is often a possibility of excessive rebound, easy to break when forming, fast wear of abrasives and defects due to poor plasticity. However, the yield point of the material is less than 140Mpa, and may not be able to withstand the stress applied during the forming process, for more complex or complex forming process or stamping processing of the steel plate, usually requires a relatively low yield strength value, and the smaller the yield ratio, the better the forming properties of the steel plate.
(2) the cold formability of the thick plate and the material yield strength and elongation are directly related. The lower the yield strength value, the smaller the stress required to produce permanent deformation; the higher the elongation value, the higher the ductility can be allowed to withstand large deformations without fracture.Selection of hot rolled steel products
(3) for building structures, bridges and mechanical structural components of the steel plate, in order to prevent fracture of the component, the steel material has the characteristics of tensile strength, and to prevent deformation of the component, but also requires a certain yield strength of the steel material, so the steel for such purposes are required to specify the minimum value of tensile strength, yield strength or range of values.
(4) for impact load deformation, such as ships, bridges, oil and gas pipelines with steel plate, in order to prevent the use of brittle fracture, but also requires a sufficiently high impact toughness – impact power value.Selection of hot rolled steel products
Mechanical properties terminology
( 1 ) mechanical properties: the mechanical properties of the steel plate refers to the steel plate under the action of force and the elastic or inelastic reaction analysis off or involved in the stress – strain relationship properties. Tensile strength, yield point, elongation half and impact absorption work are the main indicators of the mechanical properties of hot-rolled steel. The size of the steel resistance to various effects of the size of the ability to assess the quality of steel material is the main criterion, but also the design of steel plate parts and the main basis for strength calculations.
( 2) mechanical properties of the experiment: the determination of the mechanical properties of hot-rolled steel plate experiments are mainly tensile test and impact test. ( 3) yield strength: specimen in the tensile process, the load does not increase or begin to decrease and the specimen can continue to grow (deformation) when the stress. The lower the yield strength of steel, the smaller the force required to produce permanent deformation, that is, the easier to shape processing.
(4) tensile strength: the specimen is stretched, the maximum stress sustained before pulling off. When the material is subjected to external stress greater than its tensile strength, will rupture, therefore, the greater the tensile strength of the steel material, the more it can withstand the large external stress without fracture.
( 5) elongation: specimen after pulling off, the length of the marker part of the increase and the original marker length of the percentage. The greater the ratio of the elongation rate, it means that the material can withstand permanent deformation before the destruction of the force (plasticity) the better; vice versa, the worse the plasticity.
( 6 ) impact work (impact absorbed work): impact test, the specified shape and size of the specimen in the impact of a break when the work absorbed, the size of the impact work, that the metal material resistance to impact loads. The higher the impact work, the stronger the ability of the material to resist sudden brittle fracture.Selection of hot rolled steel products