Effect Of Tempforming On Microstructures And Mechanical Properties Of The Low Carbon & Low Alloying Steel

With the implementation of major projects such as high-speed railway and Hong Kong-Zhuhai-Macao bridge in recent years,new requirements on properties have been put forward for the iron&steels.Steels manufactured by traditional process are difficult to meet the needs of modern industry,so new technology are of great significance to develop new steels with both high strength and good toughness.In this study,three types of low carbon and low alloying element steels such as ML08Al,20Mn2Si Cr and 15Mn2Si V were used as starting materials.We studied variation in their microstructures and mechanical properties of the above steels under different tempforming process.Following results were obtained:(1)Microstructures of three types of steels are composed of tempered troostite and ferrite after tempforming at temperatures of 470? to 530?.During rolling,the grains are continuously elongated along the rolling direction,and an apparent fiber microstructure is formed.Compared with the hot-rolled microstructre,grains in the tempformed steels are greatly refined.Introduction of carbon and alloying elements further refines grains in the tempformed steels.Among them,grain size of the 20Mn2Si Cr steel is about 58.5%lower than that of ML08Al,and due to the addition of fine grain strengthening alloying element vanadium the grain size of the 15Mn2Si V steel is about 9.3%lower than that of20Mn2Si Cr.(2)The steels after tempforming at 500? present optimum mechanical properties at ambient temperature to-60?,and the requenched steels after hot rolling show mechanical properties better than those of the direct quenching after hot rolling.Among them,the15Mn2Si V steel requenched at 880? exhibits the best mechanical properties when tempforming at 500?.Its room-temperature yielding strength is about 1236 MPa,tensile strength is about 1267 MPa,elongation is about 19.1%,percentage reduction of area is about 60.8%and impact energy is 172 J.The yielding strength and tensile strength are about 8.1%and 7.7%,respectively,higher than those of the 20Mn2Si Cr steel,and elongation,percentage reduction of area and impact energy are about 5.4%,2.6%and 9%,respectively,lower than the latter.In comparison with those of the ML08Al steel,the yielding strength and tensile strength are increased about 55.2%and 56.0%,respectively,elongation and percentage reduction of area are increased about 38.4%and 40.7%,respectively,except for 7.5%lower for the impact energy.(3)With decrease in measuring temperatures,their strength increases at the expense of slight decrease in plasicity and toughness.The 15Mn2Si V steel presents yielding strength of about 1276 MPa,tensile strength of about 1321 MPa,elongation of about 18.2%,percentage reduction of area of about 58.6%and impact energy of about 133 J at-60?.The yielding strength,tensile strength and impact energy are about 6.1%,6.9%and 9.0%,respectively,higher than those of the 20Mn2Si Cr steel,and elongation and percentage reduction of area are about 5.7%and 2.5%,respectively,lower than the latter.In comparison with those of the ML08Al steel,the yielding strength and tensile strength are increased about 47.9%and 50.0%,respectively,elongation and percentage reduction of area are increased about 43.3%and 48.7%,respectively,except for 9.5%lower for the impact energy.Moreover,both the 15Mn2Si V and 20Mn2Si Cr steels show impact energy higher than 50 J at very low temperature of-150?.(4)Theoretical analysis on yielding strength of the tempformed steels indicates that dislocation strengthening(?49%)and grain boudary strengthening(?26%)are major strengthening methods for the ML08Al steel.Introduction of elements like C,Mn,Si,Cr and V apparently increases solid solution stengthening,concommitant with weakness in dislocation strengthening.Among total yielding strength of the 15Mn2Si V steel,the solid solution stengthening contributes about 27%,grain boudary strengthening about 30%and dislocation strengthening about 35%.All the above results prove that tempforming is a powewerful process for improving both strength and toughness,especially at low temperature,of the low carbon and low alloying steels.