Effect Of Heat Treatment Process On Microstructure And Mechanical Properties Of 30MnSi Steel Bars

With the development of large-scale and light-weight construction steel structures,prestressed concrete steel bars(PC steel bars)with high strength,high toughness,and low relaxation properties have been widely used in prestressed concrete structures.The higher requirement was put forward to the quality of 30MnSi hot-rolled steel and PC steel bars.Through the investigation of the production site,it was found that individual PC steel bars would occur brittle fracture after leaving the production line or being stored for some time.This type of brittle fracture accident not only affects the product quality of the manufacturer but also affects the user’s trust in the product.At present,there is no detailed research on this issue in domestic and foreign literature.In this study,through comparative analysis of the microstructure and mechanical properties of hot-rolled,cold-drawn,and PC steel bars,the cause of the failure of the PC steel bars has been investigated.By comparing the microstructure and mechanical properties of 30MnSi steel bars with different heat treatment processes,the best heat treatment process parameters were selected,the kinetics of tempering was sy stematically studied,the tempering equation was established,which provided technical data for the production enterprise to optimize the process.The main conclusions are as follows:(1)Comparing the microstructure,mechanical properties,and fracture characteristics of the samples with different quenching and tempering temperatures,the best heat treatment system for 30MnSi steel bars is austenitizing at 920℃ for 15 minutes and water cooling,then tempering at 400℃ for 20 minutes and air cooling.The martensite structure was obtained in the quenching process and then transformed into tempered troostite after tempering.After the heat treatment,the mechanical properties of the PC steel bars met the requirements of relevant national standards.(2)There was a small amount of banded ferrite in the core of the PC steel bars along the rolling direction,similar to the microstructure of the steel bars austenitized at 840℃.The reason for the appearance of banded ferrite was that,the original cold-drawn structure was not completely austenitized at 840℃,which was just below the Ac3 temperature of 30MnSi steel.Similarly,when the PC steel bars were inductively heated,the heat energy input to the core structure was insufficient,the heating temperature is within the temperature of the γ/α two-phase region.And the originally banded ferrite was not fully austenitized,remained in the final microstructure after quenching and tempering.To avoid the appearance of abnormal ferrite bands,the austenitizing temperature or induction heating intensity should be appropriately increased.(3)The tensile strength and elongation on the core region of the PC steel rod were significantly lower than that on the edge region,which was related to the band ferrite in the core region on the steel bars.Due to the large difference in performance between ferrite and tempered troostite,during deformation the stress would be transferred to ferrite through tempered troostite,microcracks were easy to form at the ferrite/tempered troostite phase boundary and grow in the ferrite with relatively low strength.Also,there were defects such as larger-sized sulfides,spherical MnO-SiO2,CaO-Al2O3-SiO2 with sharp ends,blocky CaO-SiO2,etc.in the microstructure,which might caused the PC steel bars to break and failure.(4)According to the P and X parameters corresponding to the tempering process parameters of 30MnSi steel and the Vickers hardness H,the P parameter tempering equation was constructed as H=991.49-0.043T(19.56+logt),and the λ parameter tempering equation as H=-42.07lot+378669.75/T-154.61.The linear relationship between the calculated hardness value and the actual measured value were 0.9765 and 0.9750,respectively.The correlation was very high,which showed that the mechanical properties of steel bars can be predicted by this relationship among the hardness,tempering temperature,and time,which can also provide a reference for the optimization of the induction tempering process.