Effect of V-N alloying on mechanical properties of HRB400E

V-N alloying is used to improve the mechanical properties of HRB400E steel for construction. The research shows that the adding number of 0.025% V and 0.0080% N can achieve the solidification micro-structure of rolled steel, significantly improve the strength of steel, but also bring the toughening effect, which can reach this yield strength of 430 MPa and its tensile strength of 650 Pa.


Introduction
The traditional view is usually that nitrogen is a harmful impurity element in steel, which can cause aging embrittlement of steel, and in the process of steel-making, vacuum degassing, and refining processes should be used to remove nitrogen in steel as far as possible [1][2][3][4] .However, in vanadium steel, the properties of N change.A large number of research results show that nitrogen can promote the precipitation of V carbonitride in vanadium steel, refine the structure of steel, produce strong precipitation strength, and improve the comprehensive properties of steel.While the content of nitrogen and vanadium in this steel is properly increased, the ferritic grains of steel can be refined, and the comprehensive properties of vanadium-containing steel can be improved [5][6][7][8] .In this paper, by adding a little number of V and controlled N to HRB400E, V-N alloying can achieve solidification microstructure refinement and mechanical properties of rolled materials.

Experimental process
In this paper, the HRB400E produced by the continuous casting machine in a steel mill is studied, and the components of the steel are shown in Table 1.The casting billets with different process components are transferred to the rolling mill and rolled according to the rolling process in Table 2. To obtain the rolled sample in the rolling stable period, the rolled sample was sampled in the middle of each rolling cycle to detect the mechanical properties of the rolled material respectively, while the rolled sample was made into the metallographic structure sample to be detected by cutting, rough grinding, fine grinding, polishing, and corrosion.

Results and analysis
The rolled materials with different V-N alloying were prepared, and the mechanical properties (yield strength and tensile strength) of the rolled materials were measured.
As can be seen from Figure 1, compared with rolled materials with conventional components, grain refinement can be achieved by adding a small amount of V and N, and the grain refinement effect increases with the enhancement of N content.From Figure 2 and Figure 3, it can be seen that with this increase of nitrogen content, when this nitrogen content in steel increases from 30 ppm to 80 ppm, the yield strength of steel increases by 20 MPa, and the tensile strength increases by 40 MPa while other components remain unchanged, that is, the increase of N content in steel has an obvious strengthening effect.V has a significant refining effect on grain size.In Figure 4, with the enhancement of V content, the grain size of the rolled structure is significantly refined.When V content reaches 0.025%, the grain size is only 1/4-1/5 of the original size.
Among all kinds of micro-alloying elements, vanadium is the most suitable element for stable and strong precipitation.Nitrogen in steel can form a big number of dispersed fine vanadium carbonitride particles with vanadium.Through precipitation strength and grain refining strengthening, the strength of steel is significantly improved, and the good plasticity and toughness of steel are improved or maintained.With this same nitrogen content, the yield strength and tensile strength of steel increase significantly with the enhancement of vanadium content, as shown in Figure 5   V-N micro-alloying could significantly improve the strength of steel.As shown in Figure 7, precipitation strength and grain refinement strength contribute greatly to this yield strength, among which this contribution velocity of precipitation strength to the yield strength is 32%, the contribution rate of grain refinement strengthening to the yield strength is 41%, and the combined contribution rate of the two is 73%.So, the precipitation strength of steel can significantly improve this strength of steel, but also reduce the toughness of steel.However, in the case of V-N micro-alloyed steel, the grain refinement strengthening effect is stronger than the precipitation strengthening effect, and the result is that the grain refinement effect offsets the reduction of toughness caused by precipitation strengthening.Among the micro-alloying techniques to obtain grain refining and precipitation strength, the V-N micro-alloying method is ideal.

Conclusion
V-N micro-alloying can realize the grain refinement of the rolled steel structure, and realize the simultaneous improvement of the yield strength and tensile strength of the steel.In HRB400E steel, only 80 ppm of N and 0.025% of V need to be added to achieve a yield strength of more than 430 MPa and a yield strength of more than 650 MPa.V-N micro-alloying is an ideal and practical alloy-strengthening method.

Figure 3 .
Figure 3. Tensile strength of rolled material with different N content.
and Figure6.Adding 0.025% V and 80 ppm N can increase this yield strength by 45 MPa and tensile strength by 80 MPa.

Figure 7 .
Figure 7. Contribution rate of V-N alloying strengthening mechanism.