The Bainite Cold Work To Strengthen The Organization And Mechanical Properties Of Non-quenched And Tempered Steel

Energy play a strategically important part in the national economy. In the 1970s, the oilcrisis inspired the invention of microalloyed medium carbon steels.As an important foundation part, fasteners is used widely and produced in quantity.The energy saving eff-ect is significant when cold hardening microalloyed steels are applied to produce high strength bolts, quenching, tempering and spheroidizing annealing before cold drawing tr-eatment are omitted, those which have high energy consumption and long period. In chi-na, energy and resources shortage is getting worse, profound social meaning and obvio-us economic benefits would be obtained when cold hardening microalloyed steels are de-veloped and applied widely.Tests of Hot simulation, metallographic microscope, SEM and TEM et al are appli-ed on new kind of low-carbon Mn-B-Ti type cold hardening microalloyed steels, and t-heir microstructures and mechanical properties were studied on process parameters of h-ot temperature, deformation temperature, reduction, cooling rate, carbon content and man-ganese content et al.The results of thermo-simulation test show that alloying element especially microall-oy element can be dissolved adequately and austenitic grain size can be controlled in a proper heating temperature such as 1150℃; The microstructures refine with the reducti-on of deformation temperature, especially, very fine and uniform granular bainite could be obtained when the test steels were deformed in the tempterature of 750℃; The micr-ostructures refine and homogenize intensely with the increase of reduction; The microst-ructures and hardness of the test steels are extremely influenced by cooling rate,a fine and uniform granular bainite could be obtained when the test steels were cooled in the rate of 2～4℃/s.The cold-drawing reductions varying from 5% to 50% were applied on three kinds of low-carbon Mn-B-Ti type cold hardening microalloyed bainite steels with different c-arbon content, and their microstructures and mechanical properties were studied. The re-sults show that a full fine and uniform granular bainite microstructure could be obtaine-d when carbon content is varying from 0.09% to 0.12%, while the formation of block ferrite will occur when carbon content is too high such as 0.14%. With the increase of carbon content at same cold-drawing reduction y, its strength increases and ductility de-creases and this tendency is more significant when y is higher. Strength increase after s-tress relief aging for cold drawn steel significantly decreases when carbon content incre-ases.The microstructure characteristics and mechanical properties of low carbon Mn-B-Ti type cold hardening microalloyed steels with two different manganese content were stu-died. Experimental results show that a mixed microstructure which consisted of ferrite, granular bainite and pearlite was obtained for sample B8 with lower manganese content of 1.60%,whereas full low carbon granular bainite was obtained for sample B6 with a higher manganese content of 2.02%.The variations of both microstructure and tensile pro.-perty with drawing reduction for the two tested steels were almost consistent, however, the yield strength ratio of sample B6 is obvious lower than that of sample B8.After a-geing treatment at 300℃,the strength of sample B6 meets the requirement of 10.9 grade bolt, while the strength of sample B8 just meets the requirement of 9.8 grade bolt. It is suggested that manganese content should be strictly controlled to obtain a full low ca-rbon granular bainitic microstructure to meet the requirement of cold hardening microall-oyed steel for 10.9 grade bolt.