Microstructural Control For Enhancing The Mechanical Properties Of High Mn Austenitic Steel At Ambient And Cryogenic Temperatures

In view of energy saving and high safety requirements,the development of high strength and high toughness steels has always been the hot research topic in cryogenic materials science.9%Ni steel possesses excellent ambient and cryogenic temperature properties(the main properties of 9%Ni steel in GB 24510-2009 provisions include:yield strength?590 MPa,tensile strength 680~820 MPa and elongation?18%at room temperature,Charpy impact energy?80 J at-196 ?)and has been used successfully in the storage and transportation of liquefied natural gas(LNG),such as building LNG tanks.Because of the high content Ni and expensive manufacturing processes,the cost of 9%Ni steel is very high.Furthermore,the 9%Ni steel used in China to date is mainly imported.In recent years,great effort has been made to develop high manganese(Mn)twinning-induced plasticity(TWIP)austenitic steels,which have relatively low cost,for cryogenic-temperature applications.The typical compositions of such TWIP steels are Fe-(15~25)%Mn-Al-C.The TWIP effect caused by the low stacking fault energy in these steels results in good tensile elongation at ambient and cryogenic temperature(50%),higher than that of 9%Ni steel.But the yield strength of these steels is only about 450 MPa at room temperature.Furthermore,at low temperature or under plastic deformation,martensite will form within austenite grains and at grain boundaries,which greatly reduces the impact performance at low temperatures(-196 ? Sharpy impact energy of about 40 J).It is apparent that these two properties are lower than that of 9%Ni steel.On the other hand,previous studies on binary Fe-Mn austenitic steels have shown that when the Mn content increases to 30~40%,the dominant plastic deformation mechanism is dislocation slip and the martensite transformation is suppressed.The tensile elongation of these steels is more than 40%and the impact energy is higher than 200 J at-196 ?,which are better than that of 9%Ni steel.However,the yield strength of this kind of high Mn austenite steels is typically low(<200 MPa),which cannot meet the requirements for the design of cryogenic applications such as LNG tanks.The objective of the present research is thus to enhance the mechanical properties of high Mn steel(30~40%)at ambient and cryogenic temperatures through microstructural control to the level comparable to that of 9%Ni steel.The precondition of the microstructural control in this project is to strengthen the material without sacrificing its ductility and toughness.For the high Mn steel,microstructural refinement is an approach that not only increases the strength,but also suppresses the martensitic transformation that is beneficial for maintaining high toughness.Recent studies have also reported that steels with ultrafine lamellar composite structures have improved strength,ductility and toughness.Therefore,two basic strategies will be applied for the microstructural control in the present study:refinement of microstructure and design of ultrafine lamellar composite.A high Mn austenitic steel with a simple composition,Fe-34.5Mn-0.04C,and good cold-rollability has been developed independently.By combining 90%cold rolling with annealing treatments,three kinds of specimens with different microstructures(different grain morphologies and grain sizes)have been successfully prepared:(1)specimens with a full lamellar structure where the lamellar boundary spacings vary within the range of 0.047~0.067mm;(2)specimens with a laminated composite structure composited of recovered and recrystallized layers where the grain sizes vary within the range of 0.45~2.0mm;(3)specimens with a fully recrystallized equiaxed grain structure where the average grain sizes change from 2.3 to21.0mm.When the samples were tensile tested at room temperature,it was found that the yield strength decreases and the elongation increases with increasing grain size.The yield strength of the full lamellar structure is 890~1060 MPa,but the elongation is less than13%.The yield strength of the laminated composite structure is 460~783 MPa,and the elongation is as large as 20~40%.The yield strength of the fully recrystallized microstructure is 210~440 MPa,and the elongation reaches 30~40%.The microstructure has an important influence on the tensile behavior.The tensile curves showed a continuous flow when the grain sizes are less than 2.0mm.When the grain size is the range of 2.0~3.8mm the tensile curves showed a discontinuous feature.When the grain sizes are larger than3.8mm the tensile curves return to a continuous flow.Tensile tests at low temperatures and corresponding microstructural characterization of the tensile-tested samples were carried out.It was found that the yield strength increases with decreasing temperature,but the tensile ductility of samples with different morphologies and grain sizes does not show monotonic change with temperature,and premature failure occurs when the temperature is below a critical temperature for a given grain size.The martensitic transformation has been effectively suppressed for the full lamellar structure and the laminated composite structure with grain sizes less than 2?m,at the same time the strength and ductility at low temperatures have been greatly improved.For example,the laminated composite structure specimen with an average grain size of 2?m has a yield strength of 600 MPa and an elongation of 50%at-180 ?.The product of uniform elongation and tensile strength is more than 50 GPa·%.When the specimens with grain sizes larger than 3.8?m were tested at-180 ?,it was found that martensite forms along grain boundaries,causing a significant decrease in tensile elongation.The martensite formation was not continuous at the grain boundary,a mixed fracture was observed(fractographs showed a mixture of dimple,cleavage and intergranular fractures).When the grain size was increased to 21?m,martensitic transformation occurred both in austenite grain interiors and at grain boundaries with a layer thickness of about 1?m when tensile tested at-180 ?,which caused transgranular fracture and intergranular fracture of the material.Accordingly the elongation decreased to less than 10%at-180 ?.The Hall-Petch relationship between the yield strength and the grain size was analyzed.It was found that the Hall-Petch slope for the samples showing yield plateau is higher than that for the samples of coarser grain sizes.This means a positive deviation of the Hall-Petch slope in the fine grained samples,which is similar to the previous observations in fine-grained Al,Ti and IF steel.To eliminate the yield plateau of the sample with an ultrafine laminated composite structure(2?m in average grain size),an additional cold rolling of 3~5%was applied.Such a slight deformation has introduced dislocation structures in the grain interior,which not only removed the yield plateau but also increased the yield strength while ensuring good ductility.The Hall-Petch relationships for the samples with the full lamellar structure and the fully recrystallized structure were established.The strength of a laminated composite structure consisting of recrystallized grains and recovered lamellar structures was calculated using the rule of mixture to evaluate the contributions of constraint effect and dislocation source strengthening effect.It was found that for the laminated composite structure(average grain size of 2?m)with a yield strength of 461 MPa at room temperature,the strength contribution originated from the constraint effect is about 90MPa and the dislocation source strengthening is 50 MPa.These results confirmed the existence of strengthening effect of constraint in the single phase laminated composite structure.In summary,through structural refinement,design of laminated composite structure design and additional deformation treatment,excellent tensile properties at ambient and cryogenic temperatures have been achieved in the Fe-34.5Mn-0.04C steel studied which has a relatively simple composition and low cost.The obtained good combinations are the yield strength of 600~783 MPa and tensile elongation of 20~35%at room temperature,which are comparable or even better than the conventional 9%Ni steel.The future research will focus on the preparation of large scale samples with similar microstructures to evaluate the low temperature impact properties,and pave a way for industrial applications of this cryogenic steel.