Study On Control Of Non-metallic Inclusions In 20CrMnTi Gear Steel

The study background of this paper is the quality defects of 20CrMnTi gear steel in further deep processing,such as cracking during forging and cracks or holes occurred during turning.The Al₂O₃ inclusions,calcium aluminate inclusions,CaO-Al₂O₃-CaS and other nonmetallic inclusions were the main factors limiting the improvement of product quality.Based on the theoretical basis of inclusion generation,collision-aggregation and absorption by slag and lining refractory,the control of typical nonmetallic inclusions in gear steel was studied by thermodynamic equilibrium calculation,kinetic calculation,physical simulation,numerical simulation and industrial test.The main contents and conclusions are as follows.First,the investigation of the present situation of production process and quality status of gear steel in a plant was carried out and some improvement advices were proposed to solve the problems found in production,for example,low self-starting pouring ratio of ladle,frequent adjustment of casting speed,serious secondary oxidization and so on.The“Al deoxidation+Ca treatment,process has the following problem:due to low concentration of Al₂O₃ inclusions in the calcium treatment stage,the collision between calcium bubbles and fine Al₂O₃ inclusions is the restrictive step of calcium treatment.Aiming at this problem,this paper proposed a complex deoxidation process with Al-Ca,which makes Al-Ca alloy react directly with[O]to form calcium aluminate.The evolution history of inclusions in molten steel were predicted by thermodynamic calculation of Al-Ca deoxidation equilibrium.The results show that,assuming the yield of Ca and A1 is 100%,the evolution process of deoxidation products is Al₂O₃→CaO·6Al₂O₃→CaO·2Al₂O₃→CaO-Al₂O₃→12CaO·7Al₂O₃→Cao·Al₂O₃→12CaO·7Al₂O₃ when Al-Ca alloy with Ca/Al mass ratio of 1/15 or 1/20 is used to deoxidize 20CrMnTi gear steel.It is also confirmed that the deoxidation system has the best purification effect on steel,which makes the solid deoxidation products form first and then the liquid deoxidation products form in the steel.The formation conditions of CaO-Al₂O₃ inclusions and CaS inclusions in 20CrMnTiS steel were analyzed by thermodynamic equilibrium calculation and kinetic analysis for the reaction between molten steel and inclusions.Combined with industrial test,the reasonable refining process of 20CrMnTiS steel is determined as follows:hot metal pretreatment→converter refining→LF refining→Ca treatment→soft blowing→RH refining→soft blowing（sulfur wire feeding）→billet continuous casting.Due to low wettability,the liquid inclusions are difficult to separate from molten steel and enter slag at the steel-slag interface,and easily to return into steel again,which leads to the large spherical calcium aluminate inclusions remaining in steel.The two-step Ca-treatment process can improve the cleanliness of steel by promoting the growth of calcium aluminate inclusions in molten steel and separation at steel-slag interface.The first Ca treatment step takes place after LF refining and the second step is carried out in the middle-later stage of soft blowing after RH refining.No inclusion with size greater than 20 μm was detected in tundish with two-step Ca treatment process.The Ds inclusions were rated as 0,and the maximum rating of D inclusions was reduced from 1.5 to 1.0.Based on the simple combination model for characterization of continuous reactors,an RTD curve analysis model for multi-strand tundish is proposed,which fully considers the influence of flow rate on the flow characteristics in the vessel.Then based on the new model,a novel eight-strand tundish internal structure with channel filter is designed,and the flow fields in the new tundish were studied by water model experiment and numerical simulation.The present dissertation mainly studied the effects of installation positions of channel filter and dam,opening angle of channel filter on steel fluid flow in the tundish.The results show that,in addition to the adsorption effect on inclusions,the channel filter is beneficial to the collision removal of small-size inclusions by prolonging the residence time of molten steel in the casting area and increasing the turbulent collision probability of inclusions.At the same time,it has little influence on the removal of large-size inclusions.