Mechanism of effect of calcium on impact absorbing energy of H11 die steel
Impact absorbing energy is an important feature in evaluating the service performance of H11 die steel.Aiming at the phenomenon of large fluctuation of impact absorbing energy of H11 die steel produced by a domestic iron and steel company,combined with microstructure characterization,nonmetallic inclusion characterization and comparative analysis and thermodynamic theoretical calculation,the main reasons leading to the fluctuation of impact absorbing energy of H11 die steel and the transformation mechanism of Ca to MgO·Al2O3 inclusions were explored.The results showed that the large size of Ds inclusions was the main factor leading to the low and fluctuat-ing impact absorbing energy of H11 die steel(the average value of unnotched impact absorbing energy at room tem-perature decreased from(327±3.4)J to(233±26.1)J).It was determined that the Ca content in steel was the main reason cause of the exceeding of D/Ds inclusions.Combined with the calculation results of commercial thermo-dynamic software FactSage 8.2,it can be seen that when the Ca in the experimental steel is low(<0.000 5%),the inclusions in the H11 die steel are dominated by high melting point MgO·Al2O3 inclusions,which are difficult to polymerize and grow.However,when the Ca in the experimental steel is high(0.000 9%),the inclusions in the steel are dominated by low melting point CaO-MgO-Al2O3 inclusions,which are easy to polymerize and grow due to the modifying effect of Ca on MgO·Al2O3inclusions.Furthermore,it is difficult to remove the inclusions from the molten steel,which leads to excessive Ds inclusions in H11 die steel.It was found that under impact loading,large-size inclusions(>10 μm)would first debonding from the matrix and produce microcracks,and the local concentra-tion of internal stress around the micropores would lead to crack initiation and crack propagation,thus reducing the impact absorbing energy of the experimental steel.Finally,it is proposed that reducing the Ca content by optimizing the smelting process is the main research direction to solve the problem of large size inclusions exceed in H11 die steel.
H11 die steelimpact absorbing energynonmetallic inclusioncrack initiationcalcium treatment
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