Study On The Cause Of Plastic Die Steel Cracking And Product Process Optimization

Combined with the cooperative subject with Nanjing Iron& Steel Group Corp Research on the Cause of Nanjing Iron Mold Steel Products’ Cracking in Processing and the Optimization of Manufacturing Process and based on the failure analysis of cracked sample of the relevant series products(NSM30、P20、718)of cracked die steel by saw cutting, the paper has proposed the solutions to the cracking issue. And through a series of experimental research, it has proposed some measures for the component and process optimization to improve the quality of series mold steel product. I have made the thermal normalizing and tempering process experiment and Gleeble thermal simulation research on the typical Mold Steel NSM30 and the study of internal friction of tempering after rolling and quenching and made a research on the structure and performance of testing steels by using optical microscopy, scanning electron microscopy, transmission electron microscopy, Rockwell hardness tester, shock testing machine etc.The failure analysis results of cracked sample of die steel have shown that, the crack is originated from the center part of the cracked steel plate, near the micro cracks. There are many coarse titanium nitride or carbide networks, the structure of steel plate especially the thick one is asymmetrical. The center parts even have some pearlite which leads to the non-uniform hardness. Therefore, the major reason for the cracking of saw cutting is the existence of exceeding titanium nitride among the steel plate and the inhomogeneity of the structure and hardness of steel plate’s cross section.The CCT software simulation shows, when the cooling velocity of three die steel is within 0.1℃/s~10℃/s, we mainly obtained the structure of a certain content of ferrite and bainite. When the cooling velocity is within 10℃/s~100℃/s we can obtain bainite structure and martensitic structure. And when the cooling velocity is over100℃/s we will obtain martensite. NSM30 is obtained by minus a proper Mo on P20 component and then add Mn and 718 mode steel is obtained by add 1% Ni into the P20 component. With the increasing of the content of alloying element, the hardenability of the mode steel’s bainite enhanced accordingly. In actual production, we can make the die steel plate that major constructed by bainite through the air cooling or controlled cooling.The experiment of thermal treatment processing have shown that, after the austenitizing and air cooling, those three experimental steel all mainly obtained a mixed structure of bainite and a certain amount of martensite. After the normalizing, the hardness range of those three experimental steel is within 49 ~52 HRC. If tempered in different temperature, with the temperature of tempering rising, the hardness value will decrease gradually and with the temperature that is greater than 400℃, the tempering hardness value decreased rapidly. The range of the hardness value with 600 ℃tempering can meet the pre-hardening requirement of mode steel as 29 HRC ~38.5 HRC.The Gleeble thermal simulation test of NSM30 steel have shown that, after heat the austenite, in the 0.1℃/s cooling velocity, the structure mainly is fine pearlite and a small amount of ferrite, the hardness value is 40 HRC and the room temperature impact toughness is 35 J. When the cooling velocity is greater than 0.5℃/s, there will be a great number of bainite structure existed in the structure with the hardness value of 51. 7HRC and room temperature impact toughness is 24 J. when the cooling velocity is greater than 1℃/s, there will be a mixed structure of bainite structure and a certain number of martensite with the hardness value of 55 HRC~58 HRC and the room temperature impact toughness is within 13 J~22 J, with the increase of cooling velocity, the hardness value doesn’t change significantly but the tenacity reduce continually.The Gleeble thermal simulation test also reveals that, the hot rolling NSM3 die steel sample with different cooling velocity after tempered in 600 ℃environment, the microscopic structure does not change and the obtained structure is the tempered lath bainite and martensitic structure, many carbide are precipitate out in bainitic ferrite, among martensitic lathes and between lathes. The hardness value is within 32~34 HRC, which illustrated that after the tempering, the cooling velocity has little impact on the hardness. The impact toughness gradually increased to 27 J at the cooling velocity of 10℃/s from 19 J at the cooling velocity of 0.1℃/s, which means the cooling velocity after tempering definitely can affect the impact toughness. Therefore, in actual production process, it will be beneficial to impact toughness by improving the cooling velocity after tempering appropriately.Experimental internal friction results of rolled NSM30 sample shows that, in the internal friction spectrum, there is only one significant SKK peak in 225℃,after the point of 225℃, the internal friction value rapidly reduced, in heat preservation at the temperature of 200℃、300℃、400℃, the internal friction value continually reduce and then gradually tends to be stable. To increase the tempering temperature and extend the tempering time will all be good for decreasing the residual stress in the steel. The quenched NSM30 sample has two peaks in the internal friction spectrum, an internal friction peak(Snoke peak)at 50 and a significant SKK peak at 200℃ ℃. With THE tempering process of different temperature, the internal friction peak gradually lower as the tempering temperature rise, in tempering at400℃, the SKK peak value is very low and even has no internal friction peak after tempering at 600℃.Which illustrated the sample that tempered at 600 has little Supersaturated and solid solute C, a great deal of C mainly exists in the ℃form of carbide.Through the optimization of mode steel’s component and relevant production process, the inclusions of the produced die steel is controlled well, the uniformity of the structure and hardness increased significantly and Cracking rate reduced to 0.1% from 1.6% before improvement under the control of the cracking phenomenon, improved the entire quality level of die steel products.