Study On Die Galling Behavior In Hot Stamping Process Of Uncoated 22Mn B5 Boron Steel

With the rapid development of the automobile industry,energy saving and emission reduction has been paid more and more attention by the state.The emergence of ultra-high-strength steel meets the requirements of automobile safety while meeting the requirements of vehicle lightweight.Therefore,the hot forming technology of ultra-high-strength steel came into being and has been widely used in the automotive industry.As 22Mn B5 is the most commonly used high-strength steel material in the automotive industry,the wear in hot stamping will seriously affect the quality of the forming parts and damage the mold.Therefore,it is particularly important and urgent to explore the wear behavior of boron steel and the mechanism of mold galling wear.Based on the high-temperature sliding-on-sheet-strip tribo-tester made by the research group,the high-temperature friction behavior of uncoated 22Mn B5 boron steel under different austenitizing temperature,initial friction temperature,friction sliding speed,pressure and mold temperature was comparatively studied in this paper.On this basis,hot stamping die and galling simulation test device were designed to further study the galling mechanisms of H13 hot stamping die.The results are shown as follows.（1）The high temperature friction behavior of uncoated 22Mn B5 steel under different process parameters was compared.Under the condition of changing the austenitizing temperature,as the austenitizing temperature rose from 870°C to 900°C and 930°C,the friction coefficient between the uncoated boron steel and H13 steel increased from 0.474 to 0.508 and 0.525,and the adhesive wear increased.At an austenite temperature of 930°C,the effects of different initial friction temperatures（700°C,770°C,800°C）were studied.It was found that when the initial friction temperature was 770°C,the friction coefficient was the lowest,corresponding to 0.506,the initial friction temperature had little effect on the wear morphology.Under the condition of changing the pressure,when the pressure increased from 2 MPa to 3 MPa,the friction coefficient was basically unchanged,respectively 0.503 and 0.506,the friction coefficient decreased to 0.474 while the pressure increased to 4 MPa,and the wear of boron steel increased gradually with the increase of the pressure.What’s more,the presence of micro-cracks could be observed at a pressure of 4 MPa.Under the condition of changing the sliding speed,as the sliding speed increased from 10 mm/s to 20 mm/s and 30 mm/s,the friction coefficient decreased from 0.537 to 0.506 and0.496,and the galling wear was the lightest when the sliding speed was 30 mm/s.（2）The effects of mold temperature on the friction behavior and mechanism of uncoated boron steel were studied.The friction coefficient between the uncoated boron steel and the H13 steel was basically stable at 0.5 when the temperature of the mold was lower,and the wear mechanisms were mainly classified to abrasive wear and adhesive wear.Besides,when the mold temperature exceeded 100°C,the friction coefficient of the uncoated boron steel decreased from 0.474 to 0.414 with the increase of temperature from 150°C to 200°C,inferred that the adhesive wear was weakened.The hardness of the boron steel matrix was approximately close to 430 HV from room temperature to 100°C.Moreover,with the temperature further rised to 150°C and200°C,the hardness decreased to 413.5 HV and 399.7 HV respectively.（3）The correlation between die galling and stamping times was studied.The increase in the number of stamping times does not change the galling of the punch significantly,but had different effects on the different positions of the die.Compared the straight wall,the die-binder area and the die round corner,it was found that with the increase of the stamping times,the galling of the round corner of the die changed obviously,and its Ra and Ry value showed a tendency to increase first,then decrease,and then gradually increase.The galling of the die straight wall and the die-binder area does not change significantly.The impact of the number of stamping times on the different position of die corner was also different.The galling at the 85°position of the die corner was the most serious,after 180 times of stamping,the Ry value of this position reached 10μm.The galling at the 45°position was the least,the value of Ra does not exceed 0.5μm,and the Ry value does not exceed 4μm.（4）The galling damage mechanisms of the die corner were explored.The wear mechanism of the die corner at 0°-30°was basically dominated by abrasive wear.At the position of the die corner of 30°-50°,in addition to abrasive wear,the adhesive wear gradually appeard with the increase of stamping times.The main wear mechanism of the 50°-70°position of the die corner was abrasive wear when the stamping times were less,and then the fatigue wear gradually manifested and played a leading role.The 70°-90°position of the die fillet showed the existence of scratches and furrows when there were fewer stamping times,and was accompanied by the presence of adhesive tumors,at this time,abrasive wear and adhesive wear played an important role.When the number of stamping was more,adhesive wear dominated,which was the main wear mechanism.When the stamping times increased further,abrasive wear dominated.According to the energy spectrum analysis,adhesive tumors were mainly Fe O,Fe₂O₃and Fe₃O₄ which were composed of Fe and O elements.