Control Of Banded Structure For 20CrMnTiH Series Gear Steel

In recent years,with the increase of the output of gear steel,the quality of gear steel is also improving.The hardenability,gas content,non-metallic inclusions and other aspects have been significantly improved.However,they are still affected by banded defects and other problems.Therefore,it is of great significance for the production and use of gear steel to understand the hazards of belt structure,analyze the causes of formation of belt structure and find out the control methods.In this paper,20CrMnTiH series gear steel produced by Laiwu Iron and Steel Co.Ltd.is taken as the research object.By means of statistical analysis of production history data,metallographic analysis and thermal simulation experiment,the influencing factors,mechanism and control measures of 20CrMnTiH series gear steel strip structure in the process of smelting,continuous casting,rolling and heat treatment are discussed.The main research contents are as follows:(1)Based on the statistical analysis of the continuous casting process and banded structure grade of 20CrMnTiH series gear steels,three main factors influencing the banded microstructure in the continuous casting process,such as superheat,billet drawing speed and hot loading and hot delivery,are determined.Under the condition of keeping the original production process basically unchanged,the superheat should be controlled at the level of<25?,and the casting speed should be less than 0.65 m/min,and the slab should be slowly cooled on the ground before being loaded into the heating furnace.For some stoves with superheat degree of molten steel(>25?),low drawing speed and slow cooling on the ground can effectively reduce the grade of banded structure.If the superheat,drawing speed and charging mode can be controlled simultaneously,the qualified rate of 20CrMnTiH steel with banded structure(<2 grade)will be over 92%.(2)The macro-morphology and micro-structure of 20CrMnTiH steel billet are analyzed.It is found that the macro-structure of 20CrMnTiH steel billet can be divided into fine grain zone in quenching layer,columnar grain zone and equiaxed grain zone.The micro-structure mainly consists of coarse network ferrite,massive ferrite,westernized structure,acicular ferrite and pearlite.The more Westernized content near the core,the larger the size of network ferrite and acicular ferrite.The needle is longer.Normalizing,forging and rolling tests were carried out on the edge and core of the billet.It was found that the structure of the edge of the billet was uniform,and it was not easy to form banded structure after forging or rolling.Increasing the forging or rolling ratio could reduce the banded structure in the center region.(3)Gleeble 3800 thermal simulation experiment machine,forging experiment,metallographic observation,electron probe composition analysis and other methods were used to study the effect of heating temperature,deformation temperature,deformation amount and cooling rate on the banded structure of 20CrMnTiH series gear steel.The results show that the composition segregation in the slab is the main reason for the formation of banded structure.The element segregation in the quench zone of continuous casting is small,and it is not easy to form banded structure.The composition segregation in the inner zone of billet is obvious,and it is easy to form banded structure under appropriate conditions.Increasing heating temperature can reduce the trend of banded structure formation.When the heating temperature reaches 1250 C,the band segregation in the thermal simulated structure of the sample decreases obviously.When the deformation is small,with the increase of the deformation,the dendrite distribution along the rolling direction increases,and the banded structure grade increases.When the deformation increases to a certain extent,for example,when the compression ratio is 9 in forging test,the original dendritic structure is destroyed,segregation bands become discontinuous and the banded structure level decreases.Therefore,measures such as increasing heating temperature and deformation can effectively reduce the banded structure in steel.When the deformation is small,the effect of cooling rate on banded structure is not obvious.Deformation temperature and cooling rate are related to the recovery and recrystallization of steel,and are also important parameters affecting the banded structure.(4)Through a series of heat treatment experiments,the effect of heat treatment process on banded structure grading was studied.The results show that bainite often exists in the rolled gear steel,which has a certain effect on the banded structure rating.The bainite in steel can be partly eliminated by normalizing treatment.Wide ferrite bands were obtained by complete annealing,and the results of tissue grading were the highest among all states.The bainite in the microstructure can be completely eliminated by isothermal annealing.