| As national economy is rapidly developing, steel products used in architectural construction are required to have enhanced mechanical properties such as high strength, good ductility, toughness and weldability due to the appearance of high-rise, large-span and anti-seismic building structure, as well as its safety. Taking energy saving, emission reduction and low-carbon environmental protection into consideration, the ordinary grade steel bars could no longer well reach the demand of civil engineering. Under the premise of securing building safety, the usage of 600MPa grade steel bars not only decreases the amount of steel but increases the available area. Therefore, it is necessary for steel enterprise to develop higher grade steel bars. Currently, the relationship among composition, microstructure evolution, precipitation behavior and thermo-mechanical controlled process have not been deeply investigated, which brings about the waste of expensive alloying elements and the problem that mechanical properties do not meet the requirement. In this work, three cooling processes after hot rolling and eight kinds of steel bars with different vanadium and nitrogen contents are designed, based on the compositions of HBR400 and HRB500. Simultaneously, microstructural evolution, V(C,N) precipitation and mechanical properties variation have been systematically analyzed.Thermo-Calc software is used to calculate the equilibrium phases and their composition change versus temperature in the studied steels. JMatPro software and isothermal heat treatment are applied to investigate the isothermal transformation rule, kinetics and formation mechanism of intragranular ferrite. The effects of cooling rate and finish cooling temperature on microstructural characterization, V(C,N) precipitation behavior and mechanical properties have been investigated on thermal simulation samples. Under the pilotscale experimental condition, the cooling processes after hot rolling and V/N ratio have been optimized.Thermodynamic calculation results by Thermo-Calc software show that primary equilibrium phases in experimental steels are austenite, ferrite, cementite, MnS, M7C3, M3P and V(C,N). Notably, when V/N ratio is less than 5, vanadium content in steels is insufficient, which leads to the appearance of free nitrogen. It would combine with solide dissolved Si atoms, and concequently, Si3N4 phase forms. All the above cases cause the decrease of solide solution strengthening and ageing of rebar steel. Moreover, with increase of vanadium and nitrogen contents (vanadium increases from 0.06wt% to 0.23wt% and nitrogen increases from 0.009wt% to 0.043wt%), the precipitation temperature of V(C,N) increases from 1100℃ and 1140℃ to 1210℃ and 1280℃, respectively. When V/N ratio is less than 6.5, nitrogen-rich V(C,N) forms in the whole existed temperature range. In another situation, when V/N ratio is above 9, nitrogen-rich V(C,N) precipitates at high temperatures of austenitic region and carbon-enriched V(C,N) forms at relatively lower temperatures of (α+γ) and ferrite regions.Temperature-Time-Transformation curves calculated by JMatPro software and experimental results obtained from isothermal heat treatment at different temperatures show that microstructure is mainly composed of bainite and some acicular ferrite at temperatures of 450℃ and 500℃. When isothermal temperature reaches 550℃, grain boundary ferrite could be observed and some pearlitic transformation occurs around it. At temperature of 600℃, microstructure consists of grain boundary ferrite, intragranular polygonal ferrite, and pearlite. The morphology of the intragranular ferrite changes from acicular to polygonal ferrite as isothermal temperature increases. Furthermore, the preferred formation temperature of intragranular polygonal ferrite is about 600℃; most of the IPF transformation occurs at this temperature.The isothermal transformation at 600℃ shows that ferrite mainly transforms at the prior austenite grain boundaries, in terms of steel with vanadium content of 0.11wt% and 0.18wt%. In contrast, a large amount of intragranular polygonal ferrite nucleats on nitrogen-rich carbonitrides and its content increases, as vanadium and nitrogen contents increase. This is because V(C,N) particles have a Baker-Nutting orientation relationship with the nucleated ferrite, and lattice mismatch is likely to be very small, which largely decreases the activation energy barrier for ferrite nucleation.Thermal simulation tests show that the cooling rate after hot rolling should be controlled in the range of 0.5℃/s～3℃/s, leading to the microstructure of refined ferrite, pearlite and less than 10% bainite, to achieve a good combination of strength and ductility. As cooling rate increases from 0.5℃/s to 3℃/s, the number density, particle size and sheet spacing of V(C,N) precipitates decrease in ferrite matrix. Through HRTEM observation, it is obvious that ferrite and nanoscale V(C,N) precipitate are connected by coherent interface, and the lower coherent interfacial energy promotes the formation of V(C,N). Besides, the nano-hardness and Young’s modulus of ferrite also decrease from 5.25GPa and 321.2GPa to 4.46GPa and 275.1GPa, which is directly related to number density of nanoscale V(C,N) precipitates.Effect of finish cooling temperature on microstructure, hardness and tensile strength has been comparatively analyzed and results show that ferrite grains are refined and their contents decrease, as the temperature decreases from 760℃ to 550℃. Pearlite content increases at first and then decreases, and the maximum value is obtained at 600℃. Bainite transformation happens at the temperature of 600℃ and its content is lower than 10%. When temperature decreases to 550℃, more than 30% bainite appears. With the decrease of temperature, hardening constituent increases and the corresponding hardness and tensile strength increase. To improve strength and ductility of 600 MPa grade rebar steel, finish cooling temperature should be controlled in the range from 600℃ to 625℃.Under pilotscale experimental condition, micro structures are all composed of polygonal ferrite and pearlite for three cooling processes after hot rolling (direct air cooling, water cooling to 750℃ and 600℃ followed by air cooling). Taking 3# experimental steel (0.23V-0.019N) for example, compared to the microstructure obtained from traditionally direct air cooling, the grain size of ferrite is refined from 6.5μm to 4.6μm and the interlaminar spacing of pearlite decreases from 136nm to 45nm, respectively, by the application of accelerated cooling and lower finish cooling temperature. The number fraction of high misorientation angle boundaries increases from 44% to 51%. Meanwhile, the sheet spacing of interphase precipitates decreases from (23nm-26nm) to (14nm-17nm) and the size of dispersively precipitated V(C,N) particles reduces from (5nm-8nm) to (2nm-5nm), coupled with higher number density. Through comparative analysis, the optimal mechanical properties are obtained in the steel water cooled to 600℃.Effect of vanadium and nitrogen contents on microstructure and mechanical properties has been systematically investigated and experimental results show that the best comprehensive mechanical properties far exceeding the requirement of 600 MPa grade high strength rebar is obtained at vanadium and nitrogen contents of 0.22wt% and 0.024wt%, in which synergistic effect of strengthening mechanisms, especially grain refinement and precipitation strengthening, has been exerted in the largest degree. Therefore, the optimal V/N ratio is 9 to well match the controlled cooling process. |
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