Microstructure defects of tube blank
Generally speaking, the billet standards do not specify the standards for billets, but there are corresponding requirements for the microstructure of alloy steel pipes and high alloy steel pipe billets with special requirements. As long as these requirements are reflected in the composition of the blank tube, the uniformity of the composition and structure of the tube blank, and the content and distribution of non-metallic inclusions.
When the tube blank contains harmful elements such as As, Pb, Sn, Bi, etc., because these elements are almost completely insoluble in the intragranular distribution of the crystalline zone on the grain boundary, after the tube blank is heated to a certain temperature, these have a lower melting point than steel Much more elements begin to melt in advance, thereby weakening the relationship between the grains. When the tube is thermally deformed, it is easy to produce metal cracks, resulting in a large number of quality defects. When the composition and structure of the tube blank are not uniform and serious segregation occurs, the steel tube will have a serious band structure, which will seriously affect the mechanical properties and corrosion resistance of the steel tube. Too many inclusions in the tube blank, especially after clustering and forming large inclusions, will not only seriously affect the performance of the steel pipe, but also may cause cracks in the steel pipe during the production process.
Harmful elements in steel are often difficult to remove in molten steel due to smelting technology or cost. When electric furnace steelmaking, the harmful elements of scrap steel tend to be higher. The smelting of steel generally adopts the policy of refined materials.
The composition and segregation of the billet is often due to insufficient smelting time of molten steel (it is often more prominent when steel is made with a fully electric furnace) or because the mixing effect of molten steel is not good. When smelting alloy steel, especially high-alloy steel, the ferroalloy can be melted in a power frequency furnace first, and then added to the refining furnace instead of directly adding the ferroalloy to the refining furnace. The smelting time of molten steel in the refining furnace can be shortened, and the composition of molten steel can be homogenized.
Non-metallic inclusions in the tube blank are inevitable. It is a compound formed by the action of iron in steel and oxygen, sulfur, nitrogen, etc.; it may also be refractory fragments mixed in steelmaking and pouring. Oxides of Si, Al, Fe, Cr, Ca, Mg, etc. The presence of metal inclusions can cause deterioration of metal plasticity, inconsistent deformation of steel during processing, or cracking of steel pipe. Non-metallic inclusions in steel include endogenous non-metallic inclusions and foreign non-metallic inclusions. Endogenous non-metallic inclusions are caused by molten steel deoxidation, secondary oxidation, or the cooling and solidification of molten steel. There are mainly the following three sources
a) Deoxidation products formed after adding deoxidizers, secondary oxides formed by oxidation of exposed molten steel by the atmosphere and oxidation by refractory materials;
b) Inclusions of the source refractory material, which are the products generated due to chemical corrosion or mechanical erosion of the refractory slag line or inner lining;
c) Inclusions caused by slag entrapment, which are caused by the high flow rate of molten steel at the slag-steel interface and the emulsification of the slag, which causes the liquid slag to drip into the molten steel.
Methods of restraining molten steel vortex:
a) Optimize the slag retaining structure of the molten steel outlet;
b) Insert a baffle near the molten steel outlet;
c) Temporarily closing the molten steel outlet can effectively suppress the whirlpool;
d) Blowing can delay the generation of molten steel vortex
Preventing the secondary oxidation of molten steel is mainly to control the secondary oxidation of molten steel by slag; control the atmosphere to prevent secondary oxidation of molten steel; watch the secondary oxidation of molten steel by refractory materials. The use of ladle refining technology and tundish metallurgical technology can reduce non-metallic inclusions in steel. All ladle refining furnaces have the function of removing non-metallic inclusions. Its main function is to reduce the secondary oxidation of molten steel through degassing, and there is enough time during the refining process to make non-metallic inclusions in molten steel float up into the slag.
