Forming Mechanism And Control Of Inclusion Of High Grade Pipeline Steel

Modern pipeline steel belongs to low carbon steel or low carbon microalloyed steel.It is a kind of high technology content and high value-added products.The development of pipeline engineering is trend to large diameter,high pressure rich gas transportation,high cold and corrosion service environment,the thick wall of seabed pipeline.Thus,the modern pipeline steel should have high strength,high toughness and resistance to brittle fracture,low welding carbon and good weldability,and resistance the corrosion of HIC and H2S.The technical difficulties mainly focused on:(1)the control of impurity elements content;(2)inclusions control;(3)the narrow composition control.The control of the inclusions in steel mainly in class B inclusions,which can achieve through calcium treatment,so calcium treatment technology is the key of the modern pipeline steel quality control.This article mainly to study the formation mechanism and control of high grade pipeline steel inclusions,which through the basis investigation of technological process,the quality of the product.And hot simulation experiment in the laboratory.Under field situation,we can find sulfur in the inclusions when the mole ratio of CaO and Al2O3 is 2:1 in the inclusion and the content of sulfur is 0.002%in the steel.Thermodynamic calculation shows when[%Al]s=0.02?0.05,[%Al]T=0.03?0.06,[%Ca]T>0.0023?0.0037 can generate liquid 12CaO·7Al2O3 deoxidizing products;when[%Ca]T>0.0064?0.0101 can generate liquid 3CaO·Al2O3 deoxidizing products.There are two kinds of mode to produce calcium sulfide,one kind of calcium sulfide comes from calcium sulfur directly reaction,another comes from reaction of sulfur at edge of inclusion.The dynamics calculation shows that if there are 15 microns Al2O3 inclusions,bottom blowing 10 min after feed calcium line.When[%Ca]T>0.0014%,inclusions can be modified in the direction of C3A.To make Al2O3 inclusions achieve complete modification,calcium content should be 0.0031%,if less than 0.0031%,the reaction layer of C12A7 is modified to C3A will disappear automatically,Al2O3 inclusions will be modified to C12A7.If the calcium content is higher than 0.0031%,may make inclusion modified excessively.Similarly,if Al2O3 inclusions is modified to C12A-7,calcium content should be greater than 0.0011%.For example,with 15 microns Al2O3 inclusions,10 min static blowing time,to make complete modification,calcium content should be 0.0027%,if less than 0.0027%,the reaction layer of CA is modified to C12A7 will disappear,Al2O3 inclusions will be modified to CA.If the calcium content is lower,it will be modified to appropriate types of calcium aluminate or fail to modify Al2O3 inclusions.If the calcium content is higher than 0.0027%below 0.0031%,the Al2O3 inclusions will be modified to a type of calcium aluminate between C12A7 and C3A.If calcium content is 0.0031%,inclusion will be modified to C3A.If the content is higher than 0.0031%,may make the inclusions modified excessively.According to the research of metallurgical properties of high basicity refining slag:in the range of 6 to 14 of alkalinity,with the increase of alkalinity,refining slag's deoxidation and desulfurization ability gradually enhance.When the basicity refining slag basicity reaches 8,deoxidization ability increases relatively slow.The higher basicity,the smaller inclusion size and the less inclusion quantity.The species of inclusions have no obvious change,mainly spherical aluminum oxide,magnesium aluminate spinel and MgO-Al2O3-MnO-TiO2 composite inclusions.According to calcium treatment study,when Al2O3 inclusions are modified to CA,sulfur content in liquid steel maintains 0.002?0.005%,calcium sulfide inclusion is difficult to generate.When Al2O3 inclusions are modified excessively,even less sulfur content in liquid steel,can cause the generation of CaS.