Improving measures for impact performance of S55J2 thin wear-resistant steel plate
S355J2 is a kind of low-alloy wear-resistant steel plate exported by European standard. When rolling 6mm thin wear-resistant steel plate, the impact toughness of the product is prone to unqualified phenomenon. By means of metallographic analysis, the causes of substandard impact properties of wear-resistant steel plates are analyzed. Mixed crystal and large particle TiN inclusion are the direct causes of substandard impact properties of wear-resistant steel plates at low temperature. Relevant improvement measures are:
1, Nb microalloying
On the basis of the original composition system, adding an appropriate amount of Nb element to the steel can effectively inhibit austenite recrystallization, and Nb atoms in the solid solution state will delay the occurrence of austenite recrystallization, and the precipitation of carbon nitride in the rolling process can strongly inhibit the occurrence of recrystallization and increase the recrystallization temperature of steel. At high rolling temperature, Nb steel can be rolled into the non-recrystallized zone, which makes the austenite grains extend along the rolling direction, the deformation zone within the deformed grains divides the austenite grains, increases the grain boundary area of austenite, increases the nucleation points during the transition from austenite to ferrite, thus refining the ferritic grains and reducing the incidence of mixed crystals. The strength and toughness of the wear-resistant steel plate are improved, and the impact work is higher at low temperature.
2. Rolling process
The rolling strategy of high temperature and high pressure is adopted in rough rolling stage to shorten the 10 passes in rough rolling stage to 8 passes to ensure that the reduction rate of two passes is above 15% and the reduction amount of single pass is above 30mm. Through strong and large pressing in rough rolling stage, the deformation on the full-thickness section of the slab is promoted, and then the recrystallization of the slab core is promoted, which ensures the uniform and fine grain of the intermediate slab. In this way, in the process of the final rolling of the finished wear-resistant steel plate with thin specifications, the conditions are laid for the uniform and fine grains of the finished wear-resistant steel plate to avoid serious mixed crystal phenomenon and the appearance of abnormal grains.
3, chemical composition adjustment
In the original composition system, the function of Ti element is to inhibit the occurrence of defects such as star cracking on the surface of wear-resistant steel plate. When its mass fraction is below 0.030%, it has little effect on the strength improvement of steel, but the large particle block inclusion formed by TiN element in steel can significantly affect the low temperature impact performance of steel. For 6mm thick wear-resistant steel plate, the effect of Ti element on steel strength is as following: The compression ratio of the billet to the finished product is more than 30 times, and the probability of defects on the surface of the wear-resistant steel plate is greatly reduced, so the absence of Ti element in the steel will not have a greater impact on the strength and surface quality of the steel.
In conclusion, the low temperature impact performance of 6mm thick S355J2 wear-resistant steel plate is effectively improved by eliminating mixed crystal and TiN inclusion through technical measures such as Nb microalloying, reducing rolling passes and not adding Ti elements.
