Effect of Vanadium on the Microstructure and Mechanical Properties of 2100 MPa Ultra-High Strength High Plasticity Spring Steel Processed by a Novel Online Rapid-Induction Heat Treatment

Advanced automotive industries generate large demand for the next generation of high strength and high toughness spring steels. Vanadium-containing 55SiCrV spring steels subjected to rapid-induction heating treatment can fulfil such requirements. However, the effect of vanadium microalloying under online rapid-induction heat treatments is rarely reported. A comparative study of the microstructure and tensile properties of 55SiCr and 55SiCrV spring steel wires subjected to a novel online rapid induction heat treatment has been demonstrated herein. It is found that the tensile strength of the 55SiCr spring wire decreases with the decrease in the wire speed in online rapid-induction heating, and the plasticity increases. Whereas, the tensile strength of the 55SiCrV steel wire increases with the decrease in the wire speed with the retained high plasticity, which is attributed to the strengthening effect of the dislocations. Through the optimized rapid-induction heating/cooling thermal cycles and intermediate-temperature tempering treatment, the tensile strength of the 55SiCrV steel wire approaches 2106 MPa with total elongation of 9.7%. Compared with the 55SiCr spring steel, the addition of V in 55SiCrV spring steel changes the strengthening and toughening mechanisms via the grain refinement and enhancement in the hardenability and tempering resistance. The finely dispersed V-containing secondary phases are rarely found in the matrix, which indicates that the precipitation effect stemming from the addition of V is not the dominant strengthening factor in the online rapid-induction heat process. The proposed novel online rapid-induction heat treatment provides a promising pathway for the mechanical property improvement of the spring steel.


Introduction
As automotive and transportation industries develop more, the demand for spring steels with high design stress and low weight increases. This requires quenched and tempered spring steel wires with high strength and plasticity [1][2][3][4][5]. Currently, the main base material used in the manufacture of automobile suspension springs is medium-carbon Si-Cr martensitic steels. Through rapid induction heating and cooling followed by intermediate-temperature tempering, the strength can reach 2000 MPa. The strength of such steel can be increased further by lowering the tempering temperature but with reduced plasticity and toughness [6][7][8][9]. Thus, commercial mediumcarbon Si-Cr martensitic steels cannot be used in the automotive industry because they cannot reach a high strength beyond 1 3 2000 MPa to satisfy the toughness and plasticity requirements for fabricating suspension springs [10,11].
The successful application of high-strength spring steels depends on the effects of their chemical composition and thermal processing to achieve the desired strength/plasticity ratio [12][13][14]. A large number of studies have shown that the addition of V can increase the strength of spring steel. Strengthening is realized through grain refinement and precipitation, combined with suitable quenching and tempering process [15][16][17][18][19][20][21]. Chen [16] studied the effect of quenching-tempering heat treatment on the microstructure and properties of 55SiCrVNb suspension spring steel and observed that a considerable quantity of needle-shaped M 2.5 C carbides and fine spherical MC carbides precipitated inside martensite laths and plates when the optimum tempering temperature was approximately 400 °C. Dong et al. [17] investigated the changes in the precipitates at temperatures ranging from 200 to 700 °C, showing that MC carbides remained stable under different tempering temperatures. It was noted that these studies on the effect of the microalloying element of V on the structure and properties of the material are based on the typical quenchingtempering heat treatment process, which is carried out under the conditions of longer heating austenitization and preservation of tempering. However, in the online rapid-induction heat treatment of automobile suspension spring steel, the austenite formation and tempering time of the spring steel material does not exceed 2 min. Under such conditions, studies on the effect of V on the microstructure and properties of spring steel are rarely reported despite the distinct heating and cooling environment. The online rapid-induction heat treatment of suspension spring wire goes through three stages, namely, rapid heating, cooling, and rapid tempering. In the case of rapid heating, the microstructural evolution of low-or medium-carbon steel during the rapid (20-100 °C/s) or ultra-fast (> 100 °C/s) heating cycle has been studied in several previous works [22][23][24][25][26][27][28]. The final microstructure showed considerable heterogeneity and consisted of undissolved cementite or carbides, martensite, bainite, and retained austenite, which resulted in insufficient time for cementite or carbide dissolution, carbon diffusion, and homogenization of chemical composition during rapid or ultra-fast austenitization.
In this study, the effect of vanadium microalloying on the microstructure and tensile properties of 55SiCr spring steel by online rapid-induction heating treatment was investigated. The spring steel was produced without (55SiCr) and with (55SiCr-V) vanadium additive to compare the effects of microalloying and process. Comparison of this analysis with commonly used 55SiCr spring steel was imperative to better understand the phenomena that take place during continuous online rapid-induction heating treatment.

Materials and Online Rapid-Induction Heat Treatment
The chemical composition and mechanical properties of the investigated spring steels are given in Table 1. The two alloys underwent the same refining + vacuum degassing smelting process. Base material wire rods with a diameter of φ15 mm were fabricated. The base material structures had a pearlite structure. Before online rapid-induction heat treatment, the surface oxide scale removal and wire diameter drawing treatments were performed on the two different alloys. The wire diameter was drawn from φ15 to φ13.6 mm. and the wire with a diameter of 13.6 mm will be directly fabricated to a suspension spring after on-line rapid-induction heat treatment. As shown in Fig. 1, the rapid-induction heat treatment process of automobile suspension spring steel wire consists of three steps, viz., rapid heating austenitization, cooling, and intermediate-temperature tempering treatment, which divides the suspension spring steel wire into three continuous zones: rapid heating zone, quenching zone, and rapid tempering zone. The length of each zone is fixed. Many induction coils connected in series are used for heating and constant-temperature circulating water is used for cooling. The 55SiCr and 55SiCrV wire rods passed continuously through the three zones at travel speeds of 18 m/min, 21 m/min, and 23 m/min, respectively. At each travel speed, the austenitizing temperature of the material was 940 °C, while the tempering temperature was 410 °C. Since the length of the three zones is fixed, the time required to pass through each zone is different as the wire travel speed of the material is different. The rapid-induction heating treatment curves at the three travel speeds in the required times to pass through the three zones are shown in Fig. 2.

Mechanical testing and microstructure characterizations
After completing the rapid-induction heat treatment of the two alloy materials at different wire travel speeds, the tensile strength of the specimens under each parameter was tested. Tensile tests were conducted using an MTS system at a strain rate of 5 × 10 −3 s −1 . The tensile samples were round bars with the size φ13.6 mm × 350 mm, and three samples were used for each parameter. The average of the three measurements characterizes the tensile properties of one specific parameter combination.
The samples for microstructure analysis were cut along the transverse direction of the round bar, and the observations were made near a point at half the radius of the round bar. The prior austenite grain size was determined by the oxidation method and the grain boundary morphology of prior austenite was observed by optical microscopy (OM). A transmission electron microscope (TEM, FEI Talos F200×) was used to analyze the microscopic morphology and precipitates of the samples. Specimens for the TEM examination were sliced to 0.5 mm thickness, thinned to 50 μm by mechanical polishing, and punched into standard 3 mm discs. Electropolishing was done using a twin jet

Mechanical Properties
Tensile properties of 55SiCr and 55SiCrV spring steels under rapid-induction heat treatment with different wire travel speeds extracted from the engineering strain-stress plots ( Fig. 3) are listed inTable 2. It can be seen that as the travel speed of 55SiCr spring steel increases and correspondingly the tempering holding time decreases, the tensile strength shows an increasing trend. However, the total elongation decreases gradually. When the travel speed increased from 18 m/min to 23 m/min, its ultimate tensile strength was increased from approximately 1907 MPa to nearly 2012 MPa, while the total elongation is reduced from 9.5 to 7.6%. With the addition of V, the mechanical properties of the 55SiCrV spring steel wire show a trend that completely opposes that of the 55SiCr spring steel wire. As the travel speed of 55SiCrV spring steel wire increases, the ultimate tensile strength shows a downward trend. However, the the total elongation does not increase significantly. When the travel speed increases from 18 m/min to 23 m/ min, its ultimate tensile strength decreases from about 2106 to 2017 MPa, and the total elongation changes from approximately 9.7%-9.2%. The specific reasons will be explained in the discussion section. It should be pointed out that although the tensile strength of 55SiCrV spring steel wire increases with the decrease in travel speed without compromising plasticity, the further reduction of travel speed is restricted because, in practice, as the travel speed is reduced further, the time for the spring wire to pass through the water-cooling zone will increase. As a result, quenching cracks and breakage occur easily in the water tank, thereby stopping the continuous advancement of the steel wire. Figure 4 shows the analysis result of the austenite grain size of the base materials and the austenite grain size of 55SiCr and 55SiCrV spring steels under different travel speeds of rapid-induction heat treatment. Figure 4a and b show the austenite grain size of 55SiCr and 55SiCrV base material, respectively. It can be seen that, due to the addition of V element in 55SiCr steel, the austenite grain size of the material is significantly refined, and the average grain size is refined from 50.3 to 29.4 μm. When the material undergoes rapid induction heating heat treatment, for the same material, as the travel speed increases, the austenite grain size does not change significantly. It can be seen from Fig. 4c and e that for 55SiCr spring steel, when the travel speed increased from 18 to 23 m/min, the average austenite grain size was 42.8 μm and 41.6 μm, respectively. For 55SiCrV spring steel, as shown in Fig. 4d and f, when the travel speed was increased from 18 to 23 m/min, the average austenite grain size was 22.1 μm and 21.2 μm, respectively. This is because the austenite grain size is related to the heating temperature and high temperature holding time. In this study, the rapid austenitization temperature was 940 °C and, with the change in travel speed, the difference in the high temperature holding time was less than 2 s, which was not enough to cause any significant change in grain size. Therefore, as the travel speed increases, the austenite grain size of the material does not change significantly. However, due to the addition of V, the austenite grain size of 55SiCrV spring steel was significantly refined compared to 55SiCr spring steel. The austenite grain size was refined from 42 μm to about 21 μm, as shown in Fig. 4. Additionally, the austenite grain size of materials could be refined after rapid-induction heating heat treatment, as seen in Fig. 4b and d, the average austenite grain size was refined from 29.4 to 22.1 μm. This was due to the rapid heating shifts the recrystallization temperature to higher values and results in grain refinement [22,25,28].

Grain Evolution
The retained austenite in the microstructure after rapidinduction heat treatment was analyzed using EBSD, as shown in Fig. 5. Figure 5a, c, and e are the phase distribution diagrams when the travel speed of 55SiCrV spring steel varied from 18 m/min to 23 m/min. It can be seen from the figure that as the travel speed increases and the time to pass the water-cooling zone became smaller, the amount of retained austenite in the microstructure gradually increases. When the speed is 18 m/min, the proportion of retained austenite is 0.506%. When the speed increased to 23 m/min, the amount of retained austenite increased to 4.03%. Figure 5b, d, and f are the phase distribution diagrams when the travel speed of 55SiCr spring steel increases from 18 to 23 m/min. It can be seen that the amount of retained austenite in the microstructure also increases with the travel speed increasing. With the increase in travel speed, the amount of retained austenite increase rapidly from 2.07 to 8.3% at 18 mm/min and 23 mm/min, respectively. It shows that for spring steels of two different alloys, as the travel speed of the wire decreases, as shown in Fig. 2, its material surface temperature varies in the range of 41-68 °C due to different wire speeds. The material wire speed slows down, and the time after the cooling zone is extended, then the temperature of the material after quenching is relatively low. For 55SiCr and 55SiCrV martensitic spring steels, the martensitic transformation finished temperature (Mf) is close to room temperature, as can be reasonably inferred from the carbon content and alloying element content [29]. The amount of martensite transformation increases and the amount of retained austenite decreases relatively as the material varies within 41-68℃ after passing through the cooling zone. However, the amount of retained austenite of 55SiCrV steel wire under the same process was significantly lower than that of 55SiCr spring steel. It is defined that grain boundaries with a misorientation less than 15° are small-angle grain boundaries, and grain boundaries with a misorientation greater than 15° are high-angle grain boundaries. At the same time, smallangle grain boundaries with a misorientation of less than 5° can reflect the distribution of dislocation [30,31]. In this study, the spring steel will enter the rapid tempering zone after rapid cooling. With the difference in travel speed, the time it takes for the sample material to pass EBSD analysis of the distribution of misorientation in the microstructure of 55SiCrV and 55SiCr spring steel after heat treatment is carried out, and the results are shown in Fig. 6 and Table 3. Figure 6a, c, and e respectively show the distribution of misorientation when the travel speed of 55SiCrV spring steel increases from 18 m/min to 23 m/min. It can be seen from the figures that, as the travel speed reduces and the tempering time increases, the proportion of the misorientation from 2° to 5° in the microstructure tended to increase. When the speed is 23 m/ min, the proportion is 22.7%. When the speed is reduced to 18 m/min, the proportion is 25.8%. Figure 6b, d, and f are the distribution of misorientation when the travel speed of 55SiCr spring steel increases from 18 to 23 m/min. It can be seen from the figures that as the travel speed reduced and the tempering time increased, the proportion of the misorientation from 2° to 5° in the microstructure shows a significant decrease. When the speed is 23 m/min, the proportion is 21.7%. When the speed is reduced to 18 m/ min, the proportion is reduced to 17.7%.

Phase Morphologies
Both 55SiCrV and 55SiCr spring steels have been subjected to induction heating, quenching, and tempering at the same austenitizing and tempering temperatures. Representative samples with a travel speed of 21 m/min are selected for TEM observations. The detailed martensitic structures of 55SiCr and 55SiCrV spring steels are seen in Fig. 7. Figure 7a and b show the matrix morphology of lath martensite of 55SiCr and 55SiCrV spring steels, respectively. The phase boundaries of the lath martensite of the two spring steels are clear, and high-density dislocations located at the martensite laths can be observed. Also, the retained austenite with different sizes is distributed at the boundary of lath martensite. Figure 7c and d show a detailed view of the martensite, and it can be seen that the parallel inner twin martensite appears in the lath martensite in two kinds of alloy spring steels. There is no obvious difference in the martensite morphology of the two alloy spring steels. However, a very small amount of the second phase of spherical V, which is ca. 100 nm in size, can be seen in the 55SiCrV spring steel wire, as seen in Fig. 8. Figure 8a showed the second phase morphology of spherical V and its EDS analysis in Fig. 8b. Figure 8c showed the SAED pattern of the second phase of V, and the crystal orientation relationship between second phase of V and the matrix was 02 The base material was observed by TEM as shown in Fig. 9. It can be seen that the base material microstructure is mainly pearlite composed of strip cementite and ferrite. Figure 9a shows the strip and granular cementite in the base material and Fig. 9b shows the EDS analysis of cementite in position EDS2 of Fig. 9a. Figure 9c shows that the second phase of approximate width 100 nm was precipitated along the edge of the cementite strip. Figure 9d is the EDS analysis of the precipitated phase in position EDS3 of Fig. 9c, which is the second phase of V. Through the TEM observation of the base material, a large amount of finely dispersed second phase of V which size was smaller than 50 nm was not found in the microstructure, but a small amount of the second V phase of a larger size (approximate or more than 100 nm) was precipitated. It shows that the addition of V in the spring

The Intrinsic Influencing Mechanism of Microstructure on Mechanical Properties
In martensitic steel, the grain size of prior austenite has an important influence on the strength and toughness of the steel. With the refinement of austenite grain size, smaller martensitic packet size or block size can be obtained after quenching and cooling, which can improve the tensile strength and toughness of steels [32][33][34]. With the rapid induction heating process, the austenite grain size of 55SiCr and 55SiCrV spring steel can be refined because of the rapid heating rate, as shown in Fig. 4a and d, It shows that this kind of rapid induction heat treatment has a positive effect on improving the strength and toughness of the materials. Besides, adding V to 55SiCr spring steel can significantly refine the grain size of original austenite, as shown in Fig. 4a and b. Therefore, the strength and plasticity of 55SiCrV quenched-tempered spring steel wire after induction heating treatment are significantly better than that of 55SiCr spring steel wire without V, as shown in Fig. 3. For 55SiCr spring steel without V, during the rapid induction heat treatment process, the tensile strength of the steel wire decreases significantly as the online travel speed decreases. However, due to the addition of the micro-alloy V, the tensile strength of the steel wire gradually increases with the decrease of the online travel speed, and the high plasticity is retained. The essential reason for this phenomenon is related to the role of V in solid solution in the base material during the rapid induction heat treatment.
In the rapid-induction heat treatment of spring steel wire, the austenite heating temperature was approximately 940 °C and the tempering temperature was 410 °C. The rapid heating austenitizing time and holding time together did not exceed 40 s, while the intermediate-temperature tempering time did not exceed 90 s, as seen Fig. 2. Under such conditions, the dissolution and precipitation behavior of the second phase of V in the spring steel base materials will be greatly restricted. A study [16] pointed out that by adding microalloying element V to 55SiCrVNb suspension spring steel, after the conventional 30 min of austenitizing heat preservation and 2 h of 400 °C intermediate-temperature tempering process, the size of the second phase of V dispersed and precipitated in the microstructure was in the In contrast, in this study after the rapid heat treatment, such finely dispersed second V phase was not found in the microstructure, but only a very small amount of the second phase of V with a size of 100 nm or more was found as shown in Fig. 8, and the size is equivalent to the size of the second phase of V in the base material, as shown in Fig. 9. It can be inferred that the second phase of V was not precipitated and grown from the matrix during the rapid tempering process, but was retained from the base material. The above analysis confirms that under normal rapid induction heat treatment process conditions of spring steel, the state of V in the spring steel base material determines the state of V in the induction heated, quenched, and tempered spring steel wire. Moreover, it is difficult to achieve the second phase dissolution and dispersion enhancement effect of V in the base materials through online rapid induction heat treatment.

Unique Response of Mechanical Properties to the Online Rapid-Induction Heat Treatment
As the travel speed decreases, the time for the spring steel wire to pass through the water-cooling zone will be extended, when the wire travel speed is reduced from 23 m/ min to 18 m/min, the cooling time is increased from 12.8 to 16.3 s (see Fig. 2), and the cooling will be improved. The variation in the amount of retained austenite in the 55SiCrV and 55SiCr steel wire matrix versus the travel speed was the same, as shown in Fig. 5. That is, the proportion of retained austenite was gradually reduced. However, the difference was that, under the same process, the amount of retained austenite in 55SiCrV steel wire was lower than that of 55SiCr steel wire. It shows that V dissolved in the base material increased the hardenability of 55SiCrV spring steel. As the travel speed decreases, the amount of retained austenite of the two alloy spring steels gradually decreases. It shows that the amount of martensite after quenching is increased gradually. Earlier studies also suggest that as the amount of martensite increases, the dislocation density also increases [35,36]. 55SiCrV and 55SiCr spring steel will run into the rapid tempering zone after quenching. The process of steel tempering will involve the segregation of the C element and the recovery of the martensite structure [37]. In martensitic steels, the dislocation density decreased during tempering upon the recovery procedure in the martensite lath. When the wire speed decreased from 23 m/min to 18 m/min, the tempering holding time increased from 66 s to 84 s, and the prolonged tempering holding duration was prolonged the martensite recovery and in turn the decrease in the dislocation density, and therefore the proportion of dislocations and small-angle grain boundaries would be reduced. Caron and Krauss [38] studied the variation of the proportion of smallangle grain boundaries in lath martensite in C0.2Ni0.01 steel with tempering time. It was found that the small-angle grain boundary area per unit volume decreased sharply during short-time tempering at high temperature (600-700 °C), and then reached a stable value quickly, followed by a slow decrease with the extension of tempering time. For 55SiCr spring steel, as the travel speed decreased and the tempering time increased, the proportion of dislocations decreased gradually because of the recovery procedure (see Fig. 6). Although the amount of martensite during quenching increased gradually (dislocation density increased), the effect of recovery was dominant. The softening during tempering increased with the extension of tempering time.
The degree of dislocation reduction in the recovery process was much greater than the increase in dislocation density caused by the increase in the amount of martensite after quenching, resulting in a decrease in the strength of the steel wire as the travel speed decreased. On the contrary, for the 55SiCrV spring steel, V in the solid solution has a drag effect on the movement of dislocations during tempering and the also diffusion of carbon in martensite, showing good tempering resistance [39,40]. C atoms in tempered martensite are known to be segregated along grain boundaries or dislocations [29]. When the strong carbide-forming element like V was added, attractive V-C interaction might give rise to their co-segregation at dislocations or the formation of V-C dipoles [41][42][43], triggering some synergic effects on the dislocation recovery behaviors. As a result, the proportion of dislocations in the microstructure did not decrease significantly with the extension of the tempering time, but instead, it tended to increase, as shown in Fig. 6. 55SiCrV spring steel retains this tendency of increasing dislocation density after passing through the tempering zone due to the tempering resistance effect of V. Due to the effect of tempering resistance of 55SiCrV spring steel, as the travel speed decreases during the quenching process, the increase in strength brought about by the gradual increase in the amount of martensite transformation takes the leading role. As a result, the strength of the steel wire increased with the decrease in the travel speed.

Conclusions
A silicon chromium spring steel with and without the addition of vanadium by online rapid-induction heat treatment was investigated. The following conclusions can be drawn: (1) In the rapid-induction heat treatment of spring steel for suspension, the ultimate tensile strength of 55SiCr steel wire without V decreased as the wire travel speed decreased, and the total elongation increased. In contrast, the ultimate tensile strength of 55SiCrV steel wire gradually increased with the decrease of wire travel speed without compromising plasticity. For 55SiCrV steel wire treated with optimized rapid-induction heat treatment conditions, the ultimate tensile strength can reach 2106 MPa, while the total elongation was 9.7%. (2) The addition of V refined the austenite grain size of 55SiCr steel. V in the spring steel base material existed in the matrix in the form of solid solution and a small amount of the spherical second phase. Within the scope of the normal spring steel rapid-induction heat treatment process, it is difficult for the second phase of V in the base material to fully dissolve and it is retained in the quenched and tempered spring steel wire. The solid solution of V in the spring steel material is dominant rather than the second phase precipitation during the rapid-induction intermediate-temperature tempering process. (3) With the gradual decrease in the travel speed, because the addition of V enhances the hardenability and tempering resistance of 55SiCr spring steel, the quantity of quenched martensite gradually increases, and dislocation strengthening promotes an increase in tensile strength, showing an opposite trend for strength variation compared with 55SiCr spring steel without V.
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