| The classical armor steels are usually alloyed with Cr,Ni and Mo,and have the martensitic matrix sometimes having a small fraction of retained austenite.Although they have ultrahigh strength and hardness,they often exhibit poor plasticity and toughness,thus being fragmentation when impacted by bullet.Moreover,the expensive alloying of Ni and Mo increase the cost of armor steel.In this thesis,I present how to develop new hot-rolled medium-Mn(9Mn and 7Mn)steels possessing ultrahigh strength and high toughness steels at the reduced density.A new alloying strategy of Si-Mn-Al-V-Nb compositional system is employed to design the medium-Mn steel for the reduced density and alloyed cost.The designed steels have been successfully manufactured and their mechanical properties were measured.The developed 7Mn steel indeed met the requirement of mechanical properties at the reduced density of 7.39 g/cm3 and successfully resist the shot of 53 type steel-core bullet with 7.62 mm diameter.By examining the microstructural evolution during hot rolling and tempering,the strengthening,toughening and bulletproof mechanism on this steel are also proposed.The main results in this thesis can be summarized as follows.(1)Since the mechanical properties of medium-Mn steels strongly depend on the quantity and stability of retained austenite,it is then necessary to predict the retained austenite fraction accurately for designing both composition and processing route.In this case,Ms temperature has to be first calculated for medium-Mn steels by optimizing the empirical model using the existing experimental data,leading to the precision of prediction significantly improved to less than 5%.Furthermore,on the basis of new Ms and the optimized models for estimating the lattice constants of martensite and austenite,a new model to calculate density is established with the deviation from the measured values less thaną1%.(2)The developed 9Mn hot-rolled steel exhibits 1000 MPa yield strength(YS),1100 MPa ultimate tensile strength(UTS)and more than 28%total elongation(TEL)after the intercritical annealing at 700? for 5h.The retained austenite(RA)fraction decreases from 50%to 25%during the tensile deformation to fracture.In contrast,1100 MPa YS,1300 MPa UTS and 30%TEL were obtained after the short-time annealing at 700? for 10min,and the RA fraction decreases from 50%to 13%during the deformation.The much short annealing led to retarded recovery due to the insufficient coarsening of carbide precipitates;consequently,dislocation hardening together with solid solution and precipitation strengthening all resulted in the increase of YS by 130 MPa.Moreover,the insufficient partition of C and Mn to austenite during the short annealing leads to the reduced stability of RA grains;thus,more RA grains transformed during deformation and contributed to enhanced work hardening via TRIP effect,finally leading to much higher UTS with the similar elongation when compared to the long-annealed specimen.Although this steel has the UTS 1300 MPa and TEL 30%,it fails to resist the bullet shot,indicating that it is strength who governs the bulletproof performance rather than the plasticity.(3)According to the drawback of 9Mn steel,we designed and manufactured the hot-rolled 7Mn stee with a unique microstructure,which is composed of martensite matrix,isolated ?-ferrite with the size of 3 ?m and 15%volume fraction laminated along the rolling direction and the dispersed RA grains with the size of?1?m and 16%volume fraction.After tempering at 150-200?,this steel exhibits 1300 MPa YS,2120 MPa UTS,13%TEL,16-24J impact toughness at both room temperature and-40?,and can be successfully cold bended to 900 at the reduced density of 7.39 g/cm3.These mechanical properties are significantly superior to the present armor steels in addition to the advantages of low cost and low density.(4)The new mechanism on strengthening and plasticizing 7Mn steel was revealed by the in-situ synchrotron tensile test.RA grains transformed gradually after the macroscopic yielding,and the stress was released and transferred to martensite and ?-ferrite.The yielding sequence in three phases is from martensite,austenite to the final ?-ferrite.It is surprising that the ?-ferrite,which is usually considered as the softest phase in low-density steels and determines the low YS,was not to yield first in this steel,which then led to the much higher YS than the reported low-density ?-ferrite medium-Mn steels.In this steel,?-ferrite is strengthened by refined grain,fine VC precipitates and dislocation,all formed during hot rolling.Moreover,the strengthened ?-ferrite grains are isolated and embedded in the martensitic matrix so that they are capable to deform compatibly with martensite and undertake load partition.The gradual transformation of RA during the deformation will release the stress concentration continually and provide sustainable work hardening,leading to improved UTS and plasticity.(5)The toughening and bulletproofing mechanism in 7Mn steel were examined and clarified.The 7Mn steel could successfully resist the bullet penetration of 53 type steel core cartridge with 7.62 mm diameter.Due to lamellar microstructure in 7Mn steel,the cracks propagated continuously and alternately along the prior austenite grain boundary(PAGB)and the laminated ?-ferrite/martensite phase boundaries,instead of crack propagation in the thickness direction leading to the quick penetration.In other words,the cracking path is changed to propagate along ?-ferrite lamellas together with the plastic deformation in ?-ferrite and almost the full transformation of RA to martensite near cracks all contribute to much higher energy required to fracture or penetrate the steel specimens during the impact and the bulletproof tests.In this case,?-ferrite,which is generally considered to be soft phase then decrease the yield strength,is used to improve toughness and bulletproof performance.This is an innovation on designing microstructures for ultrahigh strength steel,to my knowledge. |
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