V–N microalloying means that adding small amounts of V and N elements enables the strength index of steels to reach a new high level. However, the precipitation of VC1–xNx is detrimental to the hot ductility of steels. Hence, considerable scientific research has been stimulated on the influence of V–N microalloying on the high-temperature mechanical behavior of steels. C. Offerman  believed that V performed in a same manner like Nb but had a reduced effect on the hot ductility due to the high solubility of [%V][%N] in austenite. Z. Mohamed  indicated that raising V content from 0.009% to 0.1% deteriorated the hot ductility, while the lowest ductility obtained for plain carbon steels didn’t approach the value presented by Nb-containing steel. N. E. Hannerz  claimed that nitrogen had little detriment when there were no microalloying additions. Nitrogen affected the hot ductility via the integration with C, N. B. Minz  found that the ductility of plain C–Mn steel could be deteriorated to almost same degree as 0.03%Nb-containing steel when the level of [%V][%N] was as high as 1.2 × 10−3. V–N microalloying is always considered as a preference to improve the strength of non-quenched and tempered steel for its less detriment to hot ductility. Previous research was mainly focused on the influence of V–N microalloying on the plain C–Mn steel. Whereas the application of V–N microalloying has been involved in weathering steel area, and increasingly high level of [%V][%N] causes frequent occurrence of crack defect in recent years, which arouses public concern about the influence of V–N microalloying on the high-temperature mechanical behavior of weathering steels.
In this paper, the influence of V–N microalloying on the high-temperature mechanical behavior of high strength weathering steel (containing 0.3%Cu, [%V][%N] > 1.2 × 10−3) was investigated, and the net crack defect of bloom was improved through the control of N content based on the research results.
Hot tensile test has been proved to be very useful in assessing the high-temperature mechanical behavior of steels. The samples of 10 mm diameter and 120 mm gauge length were machined from blooms obtained from one steel plant. The compositions of samples were shown in Table 1. Samples 1–3 were high strength weathering steel containing [%V][%N] 1.5 × 10−3, 1.7 × 10−3, 2.02 × 10−3, respectively, and sample 4 containing 0.029% Nb was used for comparison .
All tensile tests were conducted on a Gleeble 1500 thermomechanical simulator. A shielding gas atmosphere of 1.0 L min−1 argon was supplied to avoid oxidation during full test process. The tensile samples were heated to 1,350℃ at the rate of 10℃ s−1, held for 1 min and then cooled to the test temperature at the rate of 1℃ s−1. After held for 1 min at the test temperature, the samples were strained to break down at a strain rate of 3 × 10−3 s−1. The fractures were quenched immediately after breakdown in order to observe fracture appearances.
Results and discussion
The relationship between stress and elongation can be obtained directly through the thermomechanical simulator test, the connection between the reduction of area (RA) and temperature can be acquired through the measurements of diameters before and after breakdown and the fracture mode of samples can be confirmed through scanning electron microscope (SEM) observation.
Stress–elongation curves of samples 1–3 are shown in Figure 1. In the case of sample 1, in the temperature range of 650–900℃, the stress decreases rapidly after increasing to peak value, which indicates that the cracks extend rapidly after nucleation. When the temperature elevates as high as 950℃, the appearance of dynamic recrystallization can be determined by the curves of abrupt oscillations , and the stress declines slowly after reaching the peak value. As the deformation temperature is higher than 950℃, the occurrence of dynamic recrystallization improves the ability of steel to resist cracks propagation and the elongation keeps high. For samples 2 and 3, the stress–elongation curves show almost the same variation rule as sample 1, while the onset temperature of dynamic recrystallization rises from 950℃ to 1,000℃ with the increase of [%V][%N].
Hot strength curves and hot ductility curves
The variation rule of high-temperature mechanical behavior can be manifested through hot strength curves and hot ductility curves, which characterize the dependence of hot strength and the RA on the deformation temperature, respectively.
Hot strength curves
Figure 2 shows the effect of deformation temperature on the tensile strength for samples 1–3. During the entire range of test temperature, the tensile strength increases with the decrease of temperature. When the temperature decreases from 1,100℃ to 700℃, the tensile strength increases slowly and keeps almost the same for samples with various levels of [%V][%N]. However, the tensile strength grows sharply below 700℃ and the higher level of [%V][%N] is, the more intensely strength increases. Ar3 and Ar1 temperatures are determined to be 755℃ and 610℃ (shown in Figure 3) for this high strength weathering steel through thermal expansion test, respectively. Considering this behavior of tensile strength, it is concluded that the variation of [%V][%N] has no significant influence on the tensile strength at austenite single-phase region and the difference of strengthening reflects only after proeutectoid phase change, which can be interpreted that increasing the level of [%V][%N] promotes the precipitation at the interface between austenite and ferrite, more effectively preventing the grain growth and strengthening ferrite.
H. Masuoka  has noted that there is a fluctuation of tensile strength caused by the strength difference between ferrite and austenite for Cu- and Sn-bearing steel just around A3 (shown in Figure 4). Obviously, there isn’t such a temperature range for this high strength weathering steel (containing 0.3%Cu, [%V][%N] > 1.2 × 10−3). It can also be inferred that when the temperature is below A3, prenucleated carbonitride along the boundaries contributes to the nucleation , refinement and reinforcement of ferrite structure so that the fluctuation of tensile strength is disappeared.
Hot ductility curves
The dependence of RA on the deformation temperature for all samples examined is shown in Figure 5. For this high strength weathering steel, when the temperature is above 1,000℃, the RA almost exceeds 80% because of the appearance of dynamic recrystallization. In the temperature range of 700–1,000℃, it is clear that steel presents a ductility trough in which the RA is below 60%, moreover, the higher level of [%V][%N] is, the wider and deeper the trough tends to be. Sample 1 with the lowest level of [%V][%N] (1.5 × 10−3) appears as the best ductility with a ductility trough in the range of 700–917℃ and the minimum RA of 27.9% at 750℃. When the level of [%V][%N] increases to 2.02 × 10−3, the temperature range of ductility trough expands to 700–970℃, and the minimum RA declines to 23.9% at 850℃. As shown in Figure 5, the level of [%V][%N] more intensely affects the high-temperature end of hot ductility so that the trough tends to skew toward bottom right with the increase of [%V][%N]. It can be interpreted that the greater degree of precipitation along the boundaries, the more intensely the recrystallization is retarded, causing greater damage to the hot ductility at the high-temperature end of ductility trough.
As can be seen from Figure 5, the RA of sample 2 with the level of [%V][%N] 1.7 × 10−3 is in agreement with that of Nb-containing steel (sample 4) in the temperature range of 800–900℃; however, the low-temperature stage of sample 2 is wider, stretching to 750℃ than 800℃ before ductility rebounds. It has been suggested that the low-temperature end of ductility trough depends on being able to form sufficient amounts of ferrite (above 40%) to prevent strain concentration . For this high strength weathering steel, Ae3 and Ar3 temperatures are determined to be 813℃ and 755℃, respectively. If deformation-induced ferrite readily spreads into the interior of austenite, the ductility can improve just below Ae3, however, when this doesn’t happen the low-temperature stage can extend from Ae3 to just below Ar3 until enough normal transformation ferrite forms. Considering the ductility behavior of this high strength weathering steel, it can be concluded that massive alloying elements, such as Cu, Cr and Ni, improve the stability of super-cooled austenite and retard the ferrite transformation. Thus, for this steel, thin films of ferrite appear along the boundaries of austenite at a wider low-temperature range than sample 4, causing a more severe detriment to the ductility at low-temperature stage of trough.
Fracture appearance examinations
The fracture mode above 1,000℃ for samples 1–3 is typically transgranular fracture just as shown in Figure 6. The fractures present lots of voids with different depths indicating the happening of dynamic recrystallization during deformation. The migration of grain boundaries separates the originally intergranular cracks forming as a consequence of grain boundary slipping or stress concentration into the inner of grain. Then the original microcracks grow into large voids until breakdown by necking during the following deformation. Figure 7 shows the appearance of fractures at 950℃ for samples 1–3. The fracture mode transforms from transgranular fracture to intergranular fracture, the voids gradually disappear and the boundaries of grains tend to be obvious. These also manifest that the onset temperature of dynamic recrystallization improves with the increase of [%V][%N].
Figure 8 indicates the fracture appearance at 900℃ and 750℃, respectively. The fracture modes are all typically intergranular fracture but appear differently. At 900℃, the surface of grain shows smooth and the interfaces among grains exist obviously. At 750℃, the surface of fracture appears as dimples. It can be seen in Figure 9(a) that thin films of ferrite distribute along the austenite boundaries at 750℃. Ferrite is softer than austenite so that strain concentrates in these thin films during deformation, and voids nucleate around the second-phase particles or inclusions within the films until the sample breaks by voids necking.
When the temperature decreases to 700℃, the dimples become more apparent (shown in Figure 10). The appearance of intragranular ferrite eliminates the net characteristic of thin films as shown in Figure 9(b) and weakens the strain concentration in boundaries so that the ductility rebounds.
The influence of V–N microalloying on net crack defect
Net crack defect of bloom is affected by many factors, such as equipment, process system, operations and so on. Nevertheless, the high-temperature mechanical behavior of steel is the most intrinsic factor. It is always an effective way to prevent cracks formation through proper control of process based on the fully understanding of variation rule of the high-temperature mechanical behavior [10, 11].
Net crack defect profile of high strength weathering steel
As is known to us, weathering steel always presents a high sensitivity of hot shortness because Cu readily enriches progressively in the subscale layer under oxidation conditions and tends to penetrate into the grain boundary of austenite to decrease binding force [12, 13]. During actual production process, high level of residual elements, uneven cooling of molten steel in mold and abrasion of mold plating coat are always responsible for net crack of weathering steel.
It is found that the problem of subsurface net crack is serious for bloom of this high strength weathering steel (containing 0.3%Cu, [%V][%N] > 1.2 × 10−3) at one steel plant (shown in Figure 11). Much work such as the control of residual elements contents, the adjustment of primary cooling, the alternation of mold taper and the optimization of slag performance have been conducted, however, little obvious progress has been obtained [14, 15].
The control of net crack defect of high strength weathering steel
SEM examination indicates that there is a more evident concentration phenomenon of microalloy elements than Cu along boundaries, and composition analysis shows the level of V, C and N are 10 times, 23 times and 18 times matrix content in the microvoids, respectively (shown in Figure 12 and Table 2). Hence, it can be confirmed that the brittleness of boundaries is more relevant to the precipitation of carbonitride than the concentration of Cu in the subscale layer .
Figure 13 shows that original control range of N for this high strength weathering steel varies from 0.01% to 0.02%, sometimes even to 0.03%. Through the above analysis of the influence of V–N microalloying on the high-temperature mechanical behavior of this steel, the hot ductility is notably susceptible to the fluctuation of N content. When the level of [%V][%N] comes to 1.7 × 10−3, this weathering steel has presented extremely high crack sensitivity. What’s more, the level of [%V][%N] in actual control range even can reach 3 × 10−3, which intensely increases the possibility of cracking.
Therefore, under the precondition of this high strength weathering steel with sufficient strength, the N content is suggested to be stability controlled within the low range of 0.01–0.014% (shown in Figure 13). As expected, the subsurface net crack defect is effectively improved and it can be seen in Figure 14 that the occurrence ratio of crack is stabilized around 1% after optimization.
This paper focuses on the research of the influence of V–N microalloying on the high-temperature mechanical behavior of high strength weathering steel through thermosimulation experiments, and the net crack defect of bloom is improved based on the research results. Key conclusions are as follows:
The difference of strengthening caused by variation of [%V][%N] only reflects after proeutectoid phase change. Increasing the level of [%V][%N] causes the ductility trough to be deeper and wider, and the level of [%V][%N] more intensely affects the ductility at the high-temperature end.
When the level of [%V][%N] reaches to 1.7 × 10−3, the depth and the high-temperature end of ductility trough of high strength weathering steel are similar to 0.03%Nb-containing steel, while the low-temperature end of ductility trough stretches more lowly.
The hot ductility of the high strength weathering steel is notably susceptible to the fluctuation of N content. It is significant for the improvement of net crack defect of bloom to stability control the N content in low range under the precondition of high strength weathering steel with sufficient strength.
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About the article
Published Online: 2015-07-25
Published in Print: 2016-06-01
Funding: The authors would like to acknowledge the independent research and development fund of State Key Laboratory of Advanced Metallurgy (no. 41602023) for financial supports.