Study On Structure Evolution And Toughness Mechanism Of Advanced High Strength Steel Based On Carbon Partitioning

In the development of automobile industry,lightweight has become the main theme.The application of advanced high strength steel is the main technical means of automobile lightweight.In 1970 s,European scientists put forward HS(Hot Stamping)technology,which solved the problem of high strength steel forming to a large extent.The hot stamping steel has high mechanical properties,generally up to 1500 MPa,it can fully meet the needs of modern automobile industry.However,the product of strength and elongation of hot stamping steel is only 5 ~ 10 GPa·%.Q&P process has been the key development direction of advanced high strength steel(AHSS)since it was put forward in 2003.The carbon partitioning technology of Q&P process can significantly improve the elongation of the test steel under the condition of only slightly reducing the strength of the test steel.In this paper,HS technology and Q&P process are combined to put forward HS-Q&P process.In this process,high strength and high elongation steel can be obtained by Multi links joint carbon partitioning in hot stamping process.In this paper,the effects of different parameters of carbon partitioning in three stages of HS-Q&P process on the microstructure evolution and mechanical properties of the tested steel were studied.Through OM,SEM,XRD,universal electronic testing machine and hardness tester,the effects of temperature parameters,hot stamping pressure parameters,holding time of hot stamping and temperature parameters of secondary carbon partitioning on the microstructure evolution and mechanical properties of the steel were studied.HS-Q&P process was compared with direct quenching process,dual phase zone quenching process,traditional Q&P process and dual phase zone Q&P process to explore the influence of different carbon composition conditions on the microstructure and mechanical properties of the test steel.The results are as follows:(1)The microstructure of HS-Q&P is martensite,bainite and retained austenite.There are two forms of Martensite: strip and block.The retained austenite is thin film or granular.Compared with the structure treated by HS-Q&P,the structure treated by HS-I-Q&P has more ferrite with irregular block,strip or broken band.Compared with the two processes,HS-Q&P process has higher tensile strength,but lower elongation after fracture,resulting in lower product of strength and elongation.(2)Under different hot stamping pressure,the composition of the whole structure is basically the same,but the morphology,proportion and distribution are different.With the increase of hot stamping pressure,the whole structure tends to be flat and fragmented.HS-Q&P process performance has a floating of up to 250 Mpa.The best performance is at a pressure of 50 kN,with the tensile strength of 1600 MPa,the elongation of 9.72% and the product of strength and elongation of 15.55 GPa ·%.HS-I-Q&P process has a small strength fluctuation of only 80 MPa.However,the highest elongation is 1.33 times of the lowest,and the best performance is obtained at the pressure of 90 kN,with the tensile strength of 1370 mpa,the elongation after fracture of 15.84% and the product of strength and elongation of 21.70GPA·%.The hardness is basically the same as that of the product of strength and elongation.(3)Under different carbon partition time,the composition of the whole structure is the same,but the morphology,proportion and distribution are different.The austenite content increases first and then decreases with the increase of blending time,and the change range of test steel in HS-I-Q&P process is relatively small.The tensile strength of HS-Q&P process decreased with the increase of time,and the elongation after fracture decreased first and then increased.The best comprehensive performance appeared in 30 s,with tensile strength of1740 mpa,elongation after fracture of 10.04% and product of strength and elongation of 17.47 GPA ·%.The best comprehensive performance of HS-I-Q&P process appears in 90 s,with the tensile strength of 1350 MPa,the elongation of 14.24% after fracture and the product of strength and elongation of 19.22 GPA ·%.In the aspect of hardness,the hardness of fully austenitized carbon is similar to that of the product of strength and elongation,and the maximum value appears at 60 s with dual phase austenitized carbon partitioning.(4)At different secondary carbon partitioning temperature,the composition of the whole structure is the same,but the morphology,proportion and distribution are not the same.With the increase of the secondary carbon partitioning temperature,the bainite content began to increase and the primary martensite production decreased.The content of retained austenite in the process of HS-Q&P process reaches the maximum of 6.03% at 350 ?.And the content of retained austenite in the process of HS-I-Q&P process reaches the maximum of 4.22% at300 ?.The comprehensive mechanical properties of the steel decreased with the increase of temperature.The strength decreases with the increase of temperature under the condition oftwo-phase austenite.However,due to the improvement of plastic toughness,the comprehensive properties have increased with temperature.(5)The secondary carbon partitioning has a great influence on the structure of the test steel,and a certain amount of bainite and secondary martensite will be introduced into the structure of the steel after secondary carbon partitioning.At the same time,The austenite content has obviously increased after the secondary carbon partitioning.After the secondary carbon partitioning,the similar rule appeared,that is,the strength decreased slightly.However,the elongation increased significantly(67.64%).Furthermore,the comprehensive properties of the tested steel after secondary carbon partitioning are relatively excellent.In the aspect of hardness,there will be a certain decline in the test steel after the second carbon partitioning,which proves the effectiveness of carbon partitioning.