Research Of Microstructure Control On Bainite Pre-hardened Plastic Mould Steel

Because of good strength, toughness and machining property，the bainiteprehardened plastic mould steel is popular to customers. With the dimension of plasticproducts increasing gradually and manufacturing capability of the plastic mould beimproved continuously, it is necessary to develop the non-quenched and temperedprehardened plastic mould steel with its thickness over800mm. Therefore, the thesisfocuses on the requirement of plastic mould steel with large section and high hardness,and systematically develops study from chemical composition design, formulatingprocess and manufacturing, performance testing of large block and microstructurecontrol of test steel. The main results are the followings:(1) Optimize the composition of SWFT600, decrease Si by0.3%, increase Mn by0.5%, appropriately lower Mo, and obtain modified SDP1steel. Compared toSWFT600, The hardenability of SDP1is significantly improved. The bainitepre-hardened plastic mould steel large block with size of860mm×1300mm×2690mm was producing by using the more advanced EBT+LF+VD process, controlforging and cooling and then tempering.(2) The basic properties of the large block were tested. The main results are thefollowings:①the large block has good strength and toughness. Hardness on thehead of the large block is35.2~38.2HRC and the tail of it is33.5~36.5HRC. Thefluctuation range of the hardness is within±1.5HRC. This shows the good hardnessuniformity can reach international level. The transverse impact energy can reached16.3J (U2-type rabbet). The aspect ratio are about0.80.②The grain size is at level7.5or above from surface to center on the head and tail section. The flaws (FBH≥3.0mm) are no found and the non-metallic inclusions and composition segregation areless. The carbon segregation rate is20.0%on the head and13.3%on the tail. Andthe variation of hardness is consistent with the carbon distribution.③CCT curveshows SDP1steel has good hardenability. The pro-eutectoid ferrite is stronglyinhibited even in the critical cooling rate of0.015°C. Bainite transformation has a wide temperature range and the cooling rate ranged between0.015~0.1°C/s, whichmakes the whole section to get a uniform bainite microstructure. Retained austeniteafter tempering can be controlled below6.7%.(3) Laboratory simulation of Continuous cooling process of the surface, position1/4and center on the bainite pre-hardened plastic mold steel large block. Then, theperformance, organization, retained austenite after cooling were analyzed. The mainresults are the followings:①the large block has good strength and toughness afteraustenitizing and controlling cooling. Hardness is33.0HRC~35.0HRC and thefluctuation range of the hardness is within±1.1HRC. The impact energy can reached16.3J (U2-type rabbet).②The microstructure from the surface to the core part ofthe large block is granular bainite. The microstructure is mainly divided into two.The one is strip granular bainite (450.5~463.6HV) and the other is blocked-shapedgranular bainite (338.7~363.1HV). Retained austenite can be controlled well.③Thegrain size is at level4.0~7.5after austenitizing and controlling cooling. This showsthe mixed crystal.(4) Controlled forging and cooling simulation results show that the SDP1steelhave excellent hardenability due to the reasonable design of the composition. Whenproducing, even if choosing the solution temperature above1000℃, then coolingwith0.015to0.02°C/s, strength and toughness are still good. Microstructure andretained austenite can be better controlled and the entire cross section can get bainiteto ensure the hardness fluctuations within±1.5HRC. By control of the final forgingtemperature can well control the final grain size to avoid the mixed crystal. Therefore,the actual production, control of forging and cooling process is one of the key processto obtain uniform hardness and microstructure.(5) Tempering is also one of the critical process of the final performance andmicrostructure. So four kinds of tempering equations of SDP1steel large block wereestablished by experimental simulation and they are as follows. T(13.227+logt)=11658600-24031300logH+16535200log2H-3794070log3Hλ, parameter tempering equation is:λ’parameter tempering equation is:The accuracy of P’and λ’parameter tempering equation is better than others. They are suited to determine the combinations of process parameters. But P and λ parameter tempering equation is more suitable to predict the mechanical properties of the steel.