Abstract
To obtain appropriate chemical composition and thermo-mechanical schedules for processing the V-N microalloyed 600 MPa grade high strength rebar, the microstructure analysis during dynamic continuous cooling and tensile tests of three experimental steels with different nitrogen contents were conducted. The results show that increasing nitrogen content promotes ferrite transformation and broadens the bainite transformation interval, when the nitrogen content is in the range of 0.019–0.034 mass%. Meanwhile, the martensite start temperatures decrease and the minimal cooling rate to form martensite increases. To achieve a good combination of strength and ductility, the cooling rates should be controlled in the range of 0.5–3°C/s, leading to the microstructure of ferrite, pearlite and less than 10% bainite (volume fraction). Furthermore, all the experimental steels satisfy the performance requirement of 600 MPa grade rebar and the rebar with nitrogen content of 0.034 mass% shows the highest strength through systematically comparative investigation.
Introduction
With the rapid development of urbanization and construction industry, 500 MPa grade rebar can no longer well meet the requirement of architecture. The 600 MPa grade high strength rebar with a good combination of strength, ductility, and weldability are largely needed in order to satisfy the modern manufacturing applications and the harsh service environment [1]. Nowadays, the 600 MPa grade high strength rebars are still being developed and the design of microalloy elements and parameters for thermo-mechanical controlled process is incomplete. Furthermore, the comprehensive mechanical properties can be achieved by optimizing the chemical composition and the thermo-mechanical controlled process (TMCP) [2–6]. Among them, an appropriate addition of nitrogen in the microalloyed steel promotes the precipitation of V(C,N) which stimulates the contributions of grain refinement and precipitation strengthening to the yield strength of steel [7–10]. When the steel is lacking in nitrogen, part of the vanadium is existed in the form of solid solution, which causes the waste of microalloy element. Oppositely, the excess nitrogen content brings about the free nitrogen, which is harmful to the ageing properties of the steel. With reference to the chemical composition of V-N microalloyed steels, the nitrogen content is about 0.015–0.030 mass% [3, 11–14], and we design the nitrogen content in the range of 0.019–0.034 mass%. Besides, the mechanical properties are also governed by the microstructure while a proper design of the controlled cooling, especially optimal cooling rate, results in a desirable microstructure after transformation [15–18].
As to low-carbon microalloyed steel, the transformed microstructures are mainly composed of following phases, such as ferrite (F), pearlite (P), bainite (B), martensite (M) and precipitate particles, according to the cooling rate [19, 20]. In addition, the retained austenite (γR), more or less, exists in the final microstructure because martensitic transformation is directly related to the carbon content of austenite [21]. For the high strength rebar, martensite greatly deteriorates the ductility in spite of its phase transformation strengthening; and it should be pointed out that the yield plateau is obviously destroyed when the volume fraction of bainite exceeds 10%, which is detrimental to the performance and stability of the rebar [22]. Therefore, the cooling rate leading to the preferred microstructure of fine ferrite, pearlite and less than 10% bainite (volume fraction) in the V-N microalloyed 600 MPa high strength rebar steel is intrinsically important, in order to improve the mechanical properties. In an industrial view, the result of a specific microstructure in steel, especially long products, under dynamic continuous cooling conditions, is prior to that obtained from static continuous cooling or isothermal condition.
In the present work, we investigated the transformation behavior under dynamic continuous cooling condition and tensile properties at room temperature of the V-N microalloyed 600 MPa high strength rebar steel. The results provide applicable information to design appropriate nitrogen content and cooling rate.
Experimental
Three experimental steels with different contents of nitrogen were molten in a medium frequency vacuum induction furnace. The chemical compositions of the steels are presented in Table 1.
Chemical composition of the experimental steels (mass%).
| Materials | C | Si | Mn | P | S | V | N |
| VN1 | 0.23 | 0.75 | 1.55 | 0.007 | 0.004 | 0.23 | 0.019 |
| VN2 | 0.23 | 0.74 | 1.54 | 0.007 | 0.004 | 0.22 | 0.024 |
| VN3 | 0.24 | 0.72 | 1.42 | 0.007 | 0.004 | 0.22 | 0.034 |
Dilatometry tests were conducted on Gleeble-1500 thermal simulator to study the phase transformation under dynamic continuous cooling condition. Specimens were heated at 1,150°C, held for 5 min, then cooled to 1,100°C and experienced three passes of hot compression, and finally cooled to room temperature at several cooling rates between 0.5°C/s and 20°C/s. The specific thermo-mechanical schedule is shown in Figure 1. In addition, the critical phase transformation temperatures (Ac1, Ac3) were also measured at the heating rate of 0.05°C/s.
Schematic illustration of thermo-mechanical simulation schedule.
The micro-hardness of the specimens was measured by Vickers hardness using 0.2 kg load. The microstructure of the specimens was observed by use of optical microscope (OM, 9XB-PC) and scanning electron microscope (SEM, FEI MLA 250) after being etched with 4% nital. The chemical composition of precipitate particles was measured using energy dispersive spectrometer (EDS). Image analysis for evaluation of grain size and volume fraction of micro-constituents was carried out by using the mean linear intercept method and Image Pro-Plus software.
In order to illustrate the effect of nitrogen content on the mechanical properties, the air cooled samples of the experimental steels were used to replace the dilatometry specimens to do the tensile tests since the dimension of dilatometry specimen is inadequate to the above test. Moreover, the microstructure of air cooled samples is close to that of dilatometry specimens obtained at the cooling rate of 0.5°C/s. Tensile specimens with a dimension of 10 mm diameter and 50 mm original gauge length were machined from samples in longitudinal direction. The tensile tests were conducted at room temperature using a CMT4105 universal testing machine. The tensile fracture surfaces of specimens were observed by SEM.
The thermodynamic calculations including evolution of various phases as a function of temperature, such as V(C,N), body-centered cubic (α and δ), face-centered cubic (γ), cementite, and MnS were studied, as well as the start (Ae3) and finish (Ae1) temperatures of austenite to ferrite transformation and the composition of V(C,N), using the Thermo-Calc software combined with TCFE6 database.
Results and discussion
Thermodynamic analysis of secondary phases
Taking steel VN3 as an example, Figure 2(a) and 2(b) illustrate the mole fractions of phase precipitates and the weight percentage of each element in V(C,N) precipitates as a function of temperature. The mole fraction of V(C,N) precipitates in austenite is more than 90% of total mole fraction in ferrite. The chemical composition of V(C,N) precipitates, especially the carbon and nitrogen content, is related to the temperature range in which they form. Figure 2(b) shows that the V(C,N) particles forming in the whole temperature range are nitrogen-rich V(C,N). Compared to steel VN3, the experimental steels VN1 and VN2 have the similar results about the mole fraction of phase precipitates with temperature, whereas the V(C,N) precipitates forming in the high temperature austenitic region are nitrogen-rich V(C,N) and then enriched with C in the low-temperature ferritic region, as shown in Figure 2(c). Furthermore, the precipitation temperatures of V(C,N) are 1,210°C, 1,230°C and 1,260°C, respectively, for the three experimental steels. According to the above calculation of secondary phases, the V(C,N) precipitates in austenite largely affect the nucleation of ferrite and bainite. The start and finish temperatures of austenite to ferrite transformation (Ae1, Ae3) and the critical phase transformation temperatures (Ac1, Ac3) are listed in Table 2. It is obvious that the calculated equilibrium temperatures are always lower than the critical phase transformation temperatures due to the higher heating rate of 0.05°C/s.
(a) The calculated mole fraction of phases for the experimental steel VN3, 2(b) and 2(c) weight percent of elements in V(C,N) for the experimental steels VN3 and VN1 as a function of temperature.
Phase transformation temperatures (°C).
| Materials | Ac1 | Ac3 | Ae1 | Ae3 |
| VN1 | 734 | 838 | 701 | 819 |
| VN2 | 736 | 841 | 700 | 814 |
| VN3 | 746 | 845 | 701 | 814 |
Microstructural characterization
The transformed microstructures under different cooling rates (CR) were shown in Figure 3. For steel VN1, at the slowest cooling rate of 0.5°C/s, the microstructure was composed of polygonal ferrite and pearlite. When the cooling rate increased to 3°C/s, some bainite was observed and the volume fraction of bainite was less than 10%. For the faster cooling rate of 10°C/s, the phase of martensite was observed. In the range of cooling rate from 10°C/s to 20°C/s, the relevant microstructure was netting grain boundary ferrite, bainite and martensite. Increasing the cooling rate, the dominant phase of the microstructure progressively changed from bainite to martensite. Both of VN1 and VN2 basically have the similar phenomenon, but for the steel VN3, the formation of martensite was observed when the cooling rate increased to 15°C/s. From the above microstructure analysis, the cooling rate should be controlled in the range of 0.5°C/s to 3°C/s to achieve the dominated phases of ferrite and pearlite in the microstructure.
The microstructures of experimental steels obtained in the dilatometry specimens observed by optical microscopy: (a)–(d) VN1; (e)–(h) VN2; (i)–(l) VN3.
In the three experimental steels, for the cooling rate of 3°C/s, the significant refinement of ferrite grain is observed in comparison to that at the cooling rate of 0.5°C/s, as shown in Figure 4. As the ferrite transformation is diffusion type, with the rise of cooling rate, the nucleation of ferrite is promoted and the grain boundary movement is restrained, resulting in the decrease of growth rate. For all the aforementioned reasons, the size of ferrite grain decreased. Moreover, when the nitrogen content is in the range of 0.019–0.034 mass%, the grain size of ferrite is decreased by adding nitrogen because the precipitation of vanadium carbonitrides is promoted and V(C,N) particles contribute to the nucleation of ferrite and then the ferrite grains are refined.
The size variation of ferrite grain with cooling rate in VN1 (0.23 V–0.019 N), VN2 (0.22 V–0.024 N) and VN3 (0.22 V–0.034 N). The error bars represent the standard deviation of grain sizes.
Dynamic continuous cooling transformation curves
Depending on the transformation temperatures obtained from the dilatation curves, the dynamic CCT diagrams of three experimental steels were plotted, as shown in Figure 5. With the rise of cooling rate, the start and finish temperatures of austenite to ferrite transformation decreased because the ferrite transformation is diffusion type and the diffusion rate of Fe and C atoms became slow. The martensite start point (Ms) of the experimental steels increased as the cooling rate increased since bainite transformation makes lower segregation degree of carbon-rich region forming around super-cooled austenite during bainite transformation at faster cooling rate.
The dynamic continuous cooling transformation diagrams of the experimental steels: (a) VN1 (0.23 V–0.019 N); (b) VN2 (0.22 V–0.024 N); (c) VN3 (0.22 V–0.034 N).
HV0.2 values changed with the microstructure and increased with the rise of cooling rate. The highest hardness values between 407 HV0.2 and 420 HV0.2 were displayed by the microstructure consisting of bainite, martensite and some grain boundary ferrite, forming at the cooling rate of 20°C/s in the three experimental steels. Besides the lowest hardness values between 235 HV0.2 and 243 HV0.2 were presented by the microstructure of ferrite and pearlite, generating at the cooling rate of 0.5°C/s.
In Figure 6, the dynamic CCT diagrams for both VN1 (0.23 V–0.019 N) and VN3 (0.22 V–0.034 N) are drawn together for comparison of nitrogen. It is clear that the increase of nitrogen content evidently broadened the pearlite and bainite transformation intervals and brought about the increase of ferrite transformation start and finish temperatures. Meanwhile, the martensite start point decreased and the minimal cooling rate to generate martensite increased from 10°C/s to 15°C/s.
Comparison between the CCT diagrams obtained for chemical compositions: nitrogen content from 0.019 mass% to 0.034 mass%.
The SEM micrograph of V(C,N) precipitates with EDS analysis is shown in Figure 7. From the EDS analysis, the nitrogen content is higher than the content of carbon in V(C,N) particles, which is consistent with the result of thermodynamic calculation. It is well known that V(C,N) precipitates can serve as the nucleation sites of ferrite (as observed in Figure 7(a)), and promote the ferrite transformation [23], displacing the CCT curve to higher temperature. The formation of polygonal ferrite greatly improves the concentration of carbon atoms in the untransformed austenite zone, which leads to the pearlitic transformation. Furthermore, V(C,N) particles also contribute to the bainite formation [13, 14, 24] and extend the bainite region since they can act as the nucleation sites of bainite in the steels containing high content of vanadium, as shown in Figure 7(c). Besides, martensite is a diffusionless transformation of austenite which is competitive with pearlite and bainite [25]; as the transformation of pearlite and bainite was accelerated by the additions of vanadium and nitrogen, it is necessary to increase the cooling rate and decrease the transformation temperature, in order to form martensite.
SEM morphologies and energy spectrums of V(C,N) particles in the ferrite and bainite microstructure: (a)(b) the phase of ferrite; (c)(d) the phase of bainite.
Mechanical properties
The tensile properties of the experimental steels are listed in Table 3, including the yield strength (ReL), tensile strength (Rm), the total elongation (A) and the uniform elongation (Agt). The tensile fracture surfaces of the experimental steels are shown in Figure 8. It is evident that all the experimental steels satisfy the performance requirement of 600 MPa grade rebar (The yield strength, tensile strength, total elongation and uniform elongation are above 600 MPa, 730 MPa, 14% and 7.5%, respectively.). The yield strength and tensile strength are increased with the rise of nitrogen content. Meanwhile, the maximum values of both yield strength and tensile strength are achieved in the steel VN3 with nitrogen content of 0.034 mass %. From Figure 8, it is obvious that the tensile fracture surfaces of the experimental steels are similar and characterized by a ductile morphology with different sizes of dimples which are mainly related to the microstructure, precipitated particles, inclusions and so on. The above three morphologies are exactly consistent with the results of the tensile test and they all have a good ductility.
The results of the tensile tests on the samples.
| Material | ReL (MPa) | Rm (MPa) | A (%) | Agt (%) |
| VN1 | 643±4 | 832±5 | 22.5±1.0 | 10.9±0.5 |
| VN2 | 653±2 | 840±3 | 22.0±0.8 | 10.6±0.6 |
| VN3 | 701±3 | 889±6 | 20.7±1.2 | 10.4±0.5 |
SEM micrographs of the tensile fracture surfaces (×1000): (a) VN1 (0.23 V–0.019 N); (b) VN2 (0.22 V–0.024 N); (c) VN3 (0.22 V–0.034 N).
Conclusions
The results indicate that adding nitrogen content, which promotes the precipitation of V(C,N) particles, can contribute to ferrite transformation and broaden the bainite transformation interval, when the nitrogen content is in the range of 0.019–0.034 mass%. Meanwhile, the martensite start temperatures decrease and the minimal cooling rate to form martensite increases.
The suitable cooling rate to obtain enough strength and ductility should be between 0.5°C/s and 3°C/s, which provides guidance for the controlled cooling process and can be completely achieved in the actual industrial production.
All the experimental steels satisfy the performance requirement of 600 MPa grade rebar and the maximum values of both yield strength and tensile strength are achieved in the steel VN3 with nitrogen content of 0.034 mass%.
Acknowledgement
The author would like to acknowledge the financial support of National Natural Science Foundation of China (Grant No. 51374018, 51174020). The author is also grateful to doctor candidate Wenbin Xin for his helpful discussion in this work.
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