Mechanical properties and strengthening mechanism of V-containing weathering steel

In order to insure atmospheric resistance and weldability, the carbon content of weathering steel was decreased. At the same time, increases in Mn content and V-N micro-alloying were adopted to increase the strength of weathering steel in this paper. The results indicate that when the mass fraction of Mn is 1.36%, the yield strength increases to 435 MPa, and the contribution of fine grain strengthening and dislocation strengthening to the yield strength is 59.8%. When the yield strength reaches 555 MPa, V-N alloyed weathering steel has good precipitation strengthening and fine grain strengthening effects, and the sum of the two mechanisms contributes more than 70% of the yield strength of the two groups of V-N alloyed weathering steel.


Introduction
Weathering steel is one type of low alloy structural steel with good comprehensive performance and lower cost, which has good atmospheric corrosion resistance, mechanical properties and weldability [1,2]. The development of weathering steel is facing decreasing carbon content and increasing strength to obtain a higher combination of strength-toughness and weldability [3,4].
Vanadium in steel mostly exists in solid solution state. The precipitation temperature of V (C, N) is low, therefore, the grain refinement effect of V is weak and its precipitation strengthening effect is generally concerned [5][6][7]. The interface energy and nucleation driving force between different precipitates and ferrite were calculated by Kimura [8], the results show that V-N particles are favorable for ferrite nucleation and grain refinement [9]. The application of V-N alloying in C-Mn steel was studied by Gong [10,11] and it was found that increasing nitrogen can strongly accelerate the precipitation process of V (C, N) after comparing 0.1% V ∼ 0.014% N (mass fraction) steel with 0.1% V (mass fraction) steel. The N content in steel (generally 0.01% ∼ 0.02%) can change the precipitation thermodynamic conditions, the precipitation temperature, and hightemperature precipitation ability. Modassir Akhtar investigated the numerical modelling for evolution of nanosized particles, and found that inclusion of boron decelerates phase transformations in P91B steel, which delays ferrite/martensite nucleation on cooling [12]. Similarly, Modassir Akhtar found that boron in small amounts was found to be effective due to the decreasing nature of the multiplication factors of analytical models [13].
At present, V-N alloying has been applied in long profiles, such as steel bars. However, few reports on the application of V-N micro-alloying technology in the field of weathering steel have been discussed. Therefore, the strengthening mechanism of V-N micro-alloying steel for weathering steel is studied in this paper.

Materials and experimental methods
The smelting of weathering steel was carried out in a 10 Kg vacuum medium frequency induction furnace, the chemical compositions of the experimental steels is shown in table 1. The steel ingots were heated to 1200°C and the forging temperature was about 1150°C. After forging, the ingot was cut into a 30 mm (h) × 100 mm (w) × 150 mm (l) billet. Then the billet was put into the heating furnace and started rolling after holding at 1200°C for 1 h, and finally a plate thickness of 4 mm was achieved after 8 passes of rolling. Taking tensile samples from 4 mm steel plates of each group of experimental steels along the rolling direction, the uniaxial tensile tests were carried out on a CMT5105 multifunctional tensile testing machine using a displacement rate of 0.25 mm min −1 . A minimum of three tests were performed for each condition. Characterization of the Scanning Electron Microscopy microstructure was performed with a ZEISS-SUPRA-55 field emission scanning electron microscope. Transmission Electron Microscopy (TEM) was carried out on a JEM-2010F electron transmission microscope at an acceleration voltage of 200 kV. The specimens were mechanically grounded to 40 μm thickness and punched out into a diameter of 3 mm, and then etched by double-jet polishing using a solution of 1000 ml methanol and 500 ml HNO 3 at 12 V voltage and −30°C temperature.

Results and discussion
3.1. Mechanical properties and strengthening mechanism Figure 1 shows the engineering stress-strain curves of the experimental steel. Figure 2 is the comparison chart of mechanical properties. The strength and plasticity of high Mn steel and V-N alloyed steel has obvious difference compared with 1# standard weathering steel.
According to the literature report [14,15], the yield strength of low alloy steel with carbon content in the range of 0.05 ∼ 0.25 wt% can be estimated by the following empirical formula: 2# steel: σ s (MPa) = 380.3+σ d . The experimental measurement of the yield strength of 2# steel is obtained as 435 MPa, and precipitation strengthening is also omitted. The Hall-Petch formula term of 2# steel provides a yield strength of 205 MPa, the dislocation entanglement phenomenon provides a yield strength of 55 MPa. The total contribution ratio of yield strength of fine grain strengthening and dislocation strengthening is 59.8%.
3# steel: σ s (MPa) = 337+σ p . The dislocation is pinned by the second phase particles, and the dislocation entanglement is not obvious, so the dislocation strengthening term σ d is omitted. The actual yield strength of 3# steel is 555 MPa. Compared with 1# steel, the Hall-Petch formula term of 3# steel provides 198.7 MPa, and precipitation strengthening provides 218 MPa. The total contribution ratio of the two terms to the yield strength is 75.1%. Figure 2(b) shows the contribution of each strengthening mechanism to the yield strength of the three groups of experimental steels. The solution strengthening has low strengthening efficiency, while fine grain strengthening and precipitation strengthening can greatly improve the yield strength.
The change law of the macro mechanical properties of the experimental steel corresponds to the microphysical process. Through the detailed study of the macro mechanical parameters, the micro-physical mechanism can be revealed. The study of the true stress and true strain curve reveals the difference between the energy distribution and the work hardening rate of the experimental steel in the tensile process. Figure 3 is the true stress-true strain curve based on the above two formulas. The true yield stress(σ TY ), maximum load true stress (σ TU ), fracture true stress (σ TF ) and corresponding true yield strain(ε TY ), true strain corresponding to uniform plastic deformation(ε TU ) and fracture true strain (ε TF ) can be obtained from the true stress-true strain curves. The values are listed in table 2.
Since the section from point u to point k is the necking deformation section, the stress is changed from a unidirectional stress state to a three-dimensional stress state. Therefore, the fracture true stress obtained from the true stress-true strain curve needs Bridgman correction [16] to obtain .  In which, According to the Hollomon equation, the relationship between true stress and true strain in the stage of uniform plastic deformation from point d to point u can be expressed by the following formula: Where K is a constant, n is the strain hardening index.
The fitting curve and the parameter values of K and n are obtained using Origin software. Figure 4 shows the true stress-true strain curve and fitting curve in the uniform deformation stage. The fitting curve is in good agreement with the experimental curve, and the fitting variance is less than 3%. The fitting K and n values are listed in table 2.
The deformation process of the experimental steel under the action of external force can be understood as the process of accepting external force for work. Under the condition of uniaxial tension, the deformation curve can be divided into three parts. On the left side of the yield point, it is in an elastic deformation state, and the energy consumed is elastic deformation energy, expressed by E e . From yield point d to necking point u is the stage of uniform plastic deformation, the energy absorbed in this stage is the uniform plastic deformation energy, expressed by E p . After the stress exceeds σ TU , the crack begins to expand and the specimen breaks. The energy from the necking point u to the fracture point K is the crack propagation energy, E d .   (1) E n is the sum of elastic strain energy(E e ) and plastic strain energy(E P ), which was consumed for the formation of micro-cracks. Its mathematical significance is the integral area from the origin of the true stress-true strain curve to the curve segment at point u, in which the area of the curve segment from yield point d to necking point u is the main component. Therefore, the value of E n is mainly related to the yield strength, tensile strength and plastic strain capacity, that is, the crack formation energy E n is the comprehensive embodiment of the strength and plasticity. 1# reference steel and 2# steel containing 1.36 wt% Mn take second place, which is 134 and 142 respectively. V-N alloyed 3# steel has smaller crack formation energies, which is 129. The σ TY and σ TU values of 2# steel are both higher than that of 1# steel, however, its ε TU value is lower than that of 1# steel, and its E n value is the same as that of 1# steel. The σ TY and σ TU values of 3# steel are at a high level, and the ε TU value decreases obviously, so the crack formation energy is the lowest.  (2) The crack propagation energy(E d ) is the energy consumed by the propagation of micro-cracks. The formula (7) shows that the value of E d is related to σ TU , σ′ TF and ε TF , therefore, the crack propagation energy is a comprehensive embodiment of the strength and necking deformation ability. Figure 5 and the values in table 2 show that the strength and necking strain capacities of 2# steel and 3# steel are at a high level, and the highest crack propagation energy is obtained, which are 568 and 579 respectively. 1# steel shows low strength and high necking strain rate, while the 3# steel has high strength and low necking strain rate. However, the crack propagation energy is close to each other after balancing the two mechanical properties.
(3) According to the analysis of total energy consumption(E k ), 2# steel and 3# steel have the largest total energy consumption, and the strength and plasticity are balanced.
The above calculation and analysis show that the energy consumption and distribution in each stage of the tensile process is the comprehensive response of the strength and plasticity, and is an effective means to investigate the comprehensive mechanical properties of the designed weathering steel.
Work hardening refers to the phenomenon that the external stress required for continuous plastic deformation of steel increases after plastic deformation. Its essence is the result of the interaction among dislocations, particles (precipitated particles or carbides), and dislocations in steel. The uniaxial tensile energy analysis shows that each experimental steel exhibits different energy consumption in the plastic deformation stage, which is caused by different work hardening properties.
After fitting the plastic deformation section of the true stress-true strain curve according to the Hollomon equation, the work hardening index n (table 2) of each experimental steel is obtained, and the definition formula of n is [16]: The formula shows that n represents the average work-hardening effect of steel in the stage of uniform deformation, while n is a changing value in the actual tensile process which cannot reflect the work-hardening characteristics in different strain stages. The value of n is closely related to the work-hardening rate. Therefore, the Crussard-Jaoult analysis method is usually used to reflect the changes of strain-hardening mechanism in different deformation stages.
According to the Hollomon equation, the C-J analysis equation can be obtained as The true stress and true strain in the stage of uniform plastic deformation are analyzed using this equation, and figure 6 is obtained. Figure 6 shows that the work-hardening rate decreases rapidly at a small strain rate (about < 0.1), and slows down when the strain rate is greater than 0.1. The work-hardening rate of 1# reference steel is at a relatively low level during the whole plastic deformation process, and the final value is 482 MPa. The initial processing rate of 2# steel is much higher than that of the other two experimental steels and reaching 4200 MPa. 3# V-N alloyed steel has almost the same wor-hardening rate and maintains the final value of the highest work-hardening rate (1200 Mpa). Figure 7 shows the relationship curve between work-hardening rate and true stress. The work-hardening rate of 3# steel remains high at a high strength level, followed by 2# steel, while the work-hardening rate of 1# steel has decreased to a low level at low strength.
The above analysis shows that 2# steel and 3# steel have high work-hardening rates in the whole stage of uniform plastic deformation.
In the process of uniform plastic deformation of the tensile specimen after yield deformation, there are two processes at the same time. One is the work-hardening process, which increases the bearing capacity of the specimen. Another is the elongation and the decrease of the cross-sectional area so that the bearing capacity decreases. Work-hardening is dominant before necking, while the work-softening effect caused by the decrease of cross-sectional area is greater after necking. The critical condition for plastic instability is that the workhardening is equal to work-softening. The occurrence conditions of plastic instability are judged by the Kangsid-Criterion formula, can well reflect the work-hardening characteristics, which is called the work-hardening coefficient. In its physical sense, the work-hardening coefficient reflects the ratio of the effect of work-hardening and work-softening at different strain rates in the stage of uniform plastic deformation.

Morphology of fracture and fine microstructure
From the fracture morphology shown in figure 8, the three groups of experimental steels show micro-porous aggregation fracture, and the micro-pits show equiaxial dimples which belong to ductile fractures. The formation of equiaxed dimples is closely related to the dislocation entanglement and the interaction between the hard point phase and dislocation in. According to the metal tensile fracture theory [17,18], the micro-cracks are formed and the dislocation aggregation are rapidly occured at the micro-cracks to form micropores when the load of the sample exceeds the dislocation accumulation or hard point deformation resistance. The pores are deformed plastically along the normal stress direction and shear stress direction, and finally the equiaxial dimples are formed. Therefore, the size, depth, and number of dimples characterize the plasticity of the sample. 1# ∼ 3# steels show a decreasing trend of dimple size, indicating that the increased content of Mn or Cr in steel increases the nucleation rate of micro-pores. The SEM images show that the dimple of 1# steel is unevenly distributed, and the deformation zone around the larger dimples is thick. In particular, 1# steel shows elongated dimples locally, which can consume more energy in necking deformation. The dimple size of 2# steel is small and uniform, resulting in uniform energy consumed on the whole fracture surface and obtaining high plasticity. Comparing 3# steel with 1# steel and 2# steel respectively, the dimple size distribution of 3# steel with precipitated particles is uneven with small average size, and the dimple is shallow, indicating that the plastic deformation during dimple formation is small.   Figure 9 shows the TEM graphs of the experimental 1# steel. High density dislocation lines exist in ferrite grains, and the contribution of grain size to the strengthening is mainly considered. When the dislocation density in ferrite is large, the effect of dislocation and dislocation cells on the mechanical properties should be considered. The mixed morphology of M-A island and high-density dislocation are observed between ferrite grains. During the transformation from austenite to ferrite, there is a redistribution of Mn, and residual austenite enriched with Mn is formed locally which enhances the hardening ability. Figure 9(c) shows the typical triangular grain boundary morphology of the equiaxed ferrite with high-density dislocation lines in ferrite grains. Figure 9(d) shows the morphology of the dislocation wall formed by dislocation motion entanglement. It can be seen that the dislocation wall is closed to form a dislocation cell structure. The dislocation cell was in a critical state before the formation of ferrite sub-grain. The smaller the size of the dislocation cell, the greater the contribution to the strength. The formation of dislocation cells indicate that the ferrite grains with high Mn content contain high dislocation density. The dislocation rings formed by moving dislocations bypassing the precipitated particles are observed, which indicates that the precipitated particles of VN alloyed weathering steel have higher hardness and larger size. A large number of precipitated second phase particles are observed in figure 9(e). The precipitated positions of the second phase are at dislocation lines and grain boundaries. Due to the pinning effect of precipitated particles, the entanglement of dislocation lines is not very significant. The second phase particles precipitated along the grain boundary can also play a role in pinning the movement of the austenite grain boundary, which can prevent the growth of the austenite grain. The second phase particles increase the nucleation position of ferrite and improve the nucleation rate of ferrite. Figure 9(f) shows that the dislocation ring is formed by moving dislocations bypassing the precipitated phase particles, indicating that the precipitated phase particles have high hardness and size, and the moving dislocations cannot be cut when encountering the second phase particles. The critical size when the strengthening mechanism changes is 4.88 nm for VC and 7.89 nm for VN.
The value of crack formation energy reflects the difficulty of micro-crack initiation in steel during deformation. Two main reasons for the formation of micro-cracks during tensile deformation: one is that the sample load exceeds the dislocation entanglement resistance to form micro-cracks; the other is that the hardness of the hard-point phase (including precipitated particles of the second phase and island martensite) is different from that of ferrite. As a result, there are differences in the coordinated strain capacity to form micro-cracks when the load exceeds the deformation resistance of the hard-points. The existence of dispersed VN particles and island martensite in V-N alloyed 3 # steel increase the probability of micro-crack initiation; the island martensite in 2 # high Mn steel and the dislocation tangle formed in the ferrite grain provide a place for the formation of micro-cracks. Therefore, 2# steel and 3# steel show the characteristics of increasing strength and decreasing plastic strain rate. The grain size has an important influence on the E d value. The smaller the grain size, the larger the grain boundary area in the steel. Due to the different grain orientation on both sides of the grain boundary, when the micro-crack passes through the grain, it needs to break through the grain boundary resistance and change the crack propagation direction. The 2# steel with obvious grain refinement has high crack propagation energy. Although the grain size of 3# steel is smaller than 2# steel, the crack propagation energy value is lower. This is because the crack formation energy of 3# steel is low, and a large number of microcracks will be formed during deformation, resulting in poor necking deformation ability which reduces the crack propagation energy value. The work-hardening rate is closely related to the structural characteristic. High-density dislocations and tough hard-points (second phase particles or carbides) in ferrite will increase the work-hardening rate. According to the dislocation theory, the strengthening of steel mainly comes from the interaction between dislocations and hard-points on the slip surface at small strain, and the initial workhardening rate is high. With the increase in strain rate, the dislocations on different slip surfaces reach the critical stress of cross slip and cross slip occurs, which weakens the interaction between dislocations and particles. As the strain rate continues to increase, the strain coordination dislocation density caused by the non-uniform strain coordination of ferrite and martensite islands or carbides increases, so that the final value of the work-hardening rate remains at a high level.

Conclusions
When the mass fraction of Mn is 1.36%, the steel has high actual dislocation density, and the yield strength increases to 435 MPa. The calculation results show that the contribution of fine grain strengthening and dislocation strengthening to the yield strength is 59.8%. The work-hardening property increases significantly, and a higher tensile strength of 600 MPa is obtained.
The yield strength of V-N alloyed weathering steel reaches 555 MPa. The plasticity and toughness meet the design requirements with the same strength grade. The yield strength calculation shows that V-N alloyed weathering steel has good precipitation strengthening and fine grain strengthening effects, and the sum of the two mechanisms contributes more than 70% of the yield strength.