Experimental validation and thermodynamics analysis of (Nb, V)(C, N) coupling precipitation in high-strength spring steel FAS3550

Based on the thermodynamic coupling model of multiple microelements in steels, the solid solution and coupling precipitation behavior of Nb-V-N-C system quaternary secondary phase was further verified in microalloyed steels, and combining with the engineering practice in high-strength spring steel FAS3550 containing nanoparticles. According to the thermodynamic calculations, the dissolved V (or Nb) content increased with the increasing addition V (or Nb), while decreased with the increase of addition C, N or Nb (or V). The complete dissolution temperature was 1314 °C, and the effective precipitation temperature of carbonitride was 1060 °C in 0.49%C-0.0060%N-0.03%Nb-0.12%V system spring steel FAS3550, the grains in steel began to coarsen above 1100 °C during heat treatment, and the (Nb, V)(C, N) nanoprecipitates were smaller than 100 nm. The strength and plasticity have been improved simultaneously, and its fatigue life has been increased by 13% after optimizing the heat treatment process.


Introduction
The improvement of comprehensive properties of metallic materials is the durable objective that scientists and engineers are pursuing [1,2]. With the growing development of the automobile industry, high strength and high ductility and fatigue resistance of the spring steel are urgently needed. Effective heat treatment and the corresponding microstructure greatly influence the comprehensive properties [3].
Different microelements have different effects on properties of special steels at different temperatures [4][5][6]. At high temperature a large number of fine carbides, nitrides or carbonitrides are precipitated in microalloyed steel. This precipitation can retard recrystallization and austenite grain coarsening. At low temperature, the comprehensive properties of steel can be improved by precipitation strengthening in the ferrite phase [7,8]. There are two main effects of microelements in steel, one is to exert strong precipitation strengthening effect, and the other is to hinder the effect of grain coarsening. Both effects are due to carbides, nitrides or carbonitride precipitation [9][10][11][12]. Thermodynamic analysis and its applications in terms of the secondary phase control have been a challenging research topic [13][14][15][16]. For unary or binary secondary phase, the law of equilibrium solid solubility as a function of temperature can be determined experimentally. Much research focused on thermodynamics of the secondary phases below ternary, but only a few reports carried out rigorous theoretical calculations of the quadratic multiple secondary phases. Calculations are greatly simplified by assuming that the solid solution of a certain element in the multi-component secondary phase is close to zero in a certain temperature range [17][18][19]. In spring steel manufacture, quality problems caused by heat treatment are common [20,21]. Therefore, it is important to study the effect of multiple secondary precipitates on grain coarsening of high-strength spring steel flat bar FAS3550 at different temperatures [22,23]. A thermodynamic analysis model of the multi-component secondary phase equilibrium solution in special steels was developed using mass balance and solubility product equations [24][25][26], and thermodynamics of quaternary phase equilibrium solution in Nb-V-N-C system was studied in this paper, along with analysis of microelements impacts on solid solution and precipitation temperature. In conjunction with heat treatment, the effects of secondary phase precipitates on grain coarsening in high-strength spring steel flat bar FAS3550 at various temperatures were investigated, and the microstructure of secondary phase in steel was viewed. Through this thermodynamic analysis, the solid solution and precipitation behavior of microelements in spring steel can be determined, according to the quantitative relationship between the content of microelements and the secondary phase at different temperatures, and the heat treatment schedule of spring steel can be scientifically formulated to prevent the grain size of spring steel from coarsening during the heat treatment process, thus improving its mechanical properties and fatigue life etc, which provides scientific basis for heat treatment process development. In turn, scientific design of components in multi-element alloying steel could be conducted based on the specific heat treatment process.

Thermodynamic analysis
Based on Wagner's theory and mass balance, a thermodynamic coupling model has been developed previously [27,28], which allows the composition and relative amounts of the equilibrium multiple secondary phases to be calculated as a function of steel composition and temperature. For the Nb-V-N-C system quaternary secondary phase in microalloyed steels, and A Nb = 92.9, A V = 50.9, A N = 14, A C = 12, [Nb], [V], [C] and [N] are the concentrations (in wt%) of the respective elements dissolved in the solution, respectively, T is the temperature, and t is the total molar fraction of the carbonitride Nb (k1+m1) V (k2+m2) C (k1+k2) N (m1+m2) formed in steel, so these various secondary phases can be thought as a medley of the following amounts of pure carbides and nitrides: k 1 t mole NbC, k 2 t mole VC, m 1 t mole NbN, and m 2 t mole VN. The equilibrium matrix composition, precipitate composition, and volume fraction of the precipitate may all be calculated for a given steel at any suitable temperature.

Materials and heat treatment
The high-strength spring steel flat bar FAS3550 developed by FangDa special steel technology Co., Ltd, this product is widely used in automotive springs, and its chemical composition is listed in table 2. The steel was fabricated through converter smelting, LF refining, VD vacuum processing, continuous casting combined with electromagnetic stirring technique, continuous rolling and testing processes etc The size of the rolling slab was 180 mm × 180 mm × 7950 mm, and the rolling temperature was about 1000°C. The investigated steel was hotrolled into required product specification (89 × 33 mm) via rough, middle and finishing rolling procedures, then experienced air cooling to room temperature. The finishing rolling temperature was about 900°C. After rolling, the rolled steels were heat-treated. The sample was placed in the intermediate frequency furnace after the furnace heated to the specified temperature (at 850°C, 950°C, 1050°C and 1100°C) for 60 min in furnace, -: Values not found in the reference; they are supposed to be zero in current calculation.
respectively, then the grain coarsening should be examined at various temperatures after being finally cooled to room temperature in the furnace.

Microstructural observations
The hot-rolled microstructures in high-strength spring steel flat bar FAS3550 are obtained using optical microscopy (OM) and Zeiss Supra 55 field emission scanning electron microscopy (SEM), and the specimens of microstructure observations were ground by SiC sandpapers to 5000 grit, mechanically polished, and then etched with 4% nital solution. Transmission electron microscopy was used to better study the microstructure and nanoparticles (TEM, JEOL JEM-2100, worked at 200 kV). The carbon extraction replicas and twin-jet electropolished thin foils served as the TEM samples. At a low temperature, thin TEM foils were thinned in a 10 vol.% HClO4 methanol electrolyte.

Mechanical testing
After heat treatments, the microhardness of the samples was calculated on a Vickers micro-hardness tester (LEICA VMHT30 M) with a load of 50 g and a span of 15 s. Uniaxial tensile tests for the samples were completed at room temperature using a model CMT 4204 TMS instrument at a strain rate of 10 −4 s −1 . The YYU-4/10 clipon extensometer was used to evaluate the tensile strain upon loading. The samples were cut by electrical discharge machining (EDM) into flat tensile specimens in the form of a dog-bone with gauge dimensions of 15 mm, 4 mm, and 1 mm. For the mechanical properties, each sample underwent more than three tensile tests. For the fatigue properties tests of automobile plate spring, which were performed using an Instron 8801 fatigue testing system under a frequency of 1 Hz and a stress of 833 MPa [24].

Results and discussion
3.1. The coupling precipitation of Nb-V-N-C system secondary phase For the Nb-V-N-C system microelements in steels, the chemical formula of quaternary phase precipitates is The influence of different microelements on the complete dissolution temperature in Nb-V-N-C system quaternary secondary phase was analyzed, and the results are shown in table 3. With the increase of the mass fraction of either C, N, Nb or V, the complete dissolution temperature increased, and the influence of Nb and C mass fraction on its complete dissolution temperature are expected to be stronger compared with N and V, the change range of the complete dissolution temperature for the secondary phase in Nb-V-N-C system was smaller than that in the Ti-V-N-C and Ti-Nb-N-C systems [24,25].
The results of solid solution and precipitation change of the secondary phase in Nb-V-N-C system special steel with different microelements at different temperatures are shown in figures 1-4. The solid solution Nb content in steel increased with Nb content ( figure 3(a)), but decreased with the increase of C ( figure 1(a)), N (figure 2(a)) or V ( figure 4(a)) contents. The solid solution V content of the Nb-V-N-C system steel increased with V content ( figure 4(b)) and decreased with the increase of C ( figure 1(b)), N (figure 2(b)) or Nb ( figure 3(b)) contents. The solid solution V content decreased rapidly below 1050°C. Also, solid solution content of Nb declined rapidly in the range of complete dissolution temperature T AS ∼ 1000°C, so the solid solution of each microelement in the Nb-V-N-C system varied with temperature and mass fraction. Their clear solid solution and precipitate rules provides scientific basis for the design of microelements in steel according to the specific production process.

Thermodynamic analysis of high-strength spring steel FAS3550
The 0.49%C-0.0060%N-0.03%Nb-0.12%V system microelements in spring steel flat bar FAS3550 was thermodynamically analyzed, and its complete dissolution temperature T AS was 1313.54°C, its solidus temperature T S was 1319.67°C [29]. The solid solution and precipitation of different elements in steel at different temperatures (figure 5), coefficients like k 1 , k 2 , m 1 , m 2 , and the total molar amount of quaternary phase precipitates (t) with temperature ( figure 6) were illustrated. The solute content of different microelements decreased with the temperature decrease, especially Nb element. The changes of k 1 , k 2 , m 1 and m 2 coefficients with temperature were more complex. The proportion of secondary phase precipitated in steel during the cooling process changed with time. The proportion of NbC in the quaternary phase precipitates was more obvious at high temperature, but the proportion of VC in the precipitates was more obvious at low temperature. With a decrease in temperature, the total molar amount of quaternary secondary phase precipitates increased,

Microstructural characterization
The hot-rolled microstructures in high-strength spring steel flat bar FAS3550 are consisted of bainite, pearlite and martensite, and the averaged grain size was No.9.0 grade (see figure 7), The pearlite lamella spacing was about 165 ± 17 nm obtained by SEM (see table 4).   The grain morphology and grain coarsening after heat treatments at different temperatures was also investigated. The results of the morphology and analysis of the microstructure are shown in figures 8 and 9. The heat-treated microstructure was consisted mainly of pearlite and ferrite. The grain sizes were No.9.0, No.9.0, No.8.0 and No.6.0 grades under the heat treatment temperature of 850°C, 950°C, 1050°C and 1100°C, respectively, as indicated in figure 9, and the proportion of ferrite increased, i.e., the decarbonization exacerbated, with temperature increase.
The research showed that when the microelements are added into the spring steel, only when the microelements form the secondary phase at high temperature stage, then the grain coarsening can be prevented, and grain refinement be achieved. However, when the microelements exist in the form of solid solution at high temperature stage, it can't play a role in preventing grain coarsening. The dispersed secondary phase particles in steel can control the grain size during the heat treatment process, that is, provide indirect grain refinement. Therefore, based on thermodynamic analysis, it's necessary to master the accurate solid solution amount of microelements in spring steel, and the amount of corresponding secondary phase formed at the heat treatment process, which is very important to scientifically formulate the heat treatment schedule of spring steel. The above thermodynamic analysis results showed that the carbonitride complete dissolution temperature of the products FAS3550 was 1314°C, and its effective precipitation temperature was about 1060°C. The results of heat treatment showed that the grain coarsening was obvious at 1100°C, which provides scientific basis for optimizing the microelements composition. In turn, providing technical support for the enterprise to design the   heat treatment process based on the precipitation behavior of microelements in high-strength spring steel FAS3550. The transmission electron microscopy (TEM) results indicate that the secondary phase precipitated in the hot rolled spring steel flat bar FAS3550 was smaller than 100 nm (see figure 10). Thermodynamic analysis showed that the complete dissolution temperature was relatively low, so no liquid phase existed. Detection results were consistent with the results of thermodynamic analysis. Figure 10(a) shows that there was a large amount of twins in steel, which contributes to the improved performance [30,31]. Figure 10(b) indicates that a large number of dislocations bypassed or cut around the secondary phase. The nano-particles with the diameter of 50 nm were mainly (Nb, V)(C, N) phase according to the energy dispersive spectroscopy (EDS) results (see figures 10(c), (d)). These (Nb, V)(C, N) nano-particles that hinders the grain coarsening during heat treatment  process [32]. The numerous dislocations are distributed dispersedly in the matrix, as indicated by the arrows in figure 10(c), which could contribute to the strengthening [33].
3.4. Effects of (Nb, V) (C, N) precipitates on high-strength spring steel FAS3550 The quenching temperature of high-strength spring steel flat bar FAS3550 is 950 ∼ 1050°C, and the tempering temperature is 450 ± 20°C before this study in FangDa special steel technology Co., Ltd, then according to the above thermodynamic analysis and experimental results, the heat treatment process of spring steel flat bar FAS3550 was optimized, and the quenching temperature of spring steel flat bar FAS3550 is 880 ± 10°C, and the tempering temperature is 420 ± 20°C. The matrix microstructure of high-strength FAS3550 is the fine tempered troostite after heat treatment, as shown in figure 11, and the polygonal ferrites are indicated by the arrows. The mechanical properties and fatigue life are improved after optimization, the performance comparison results before and after optimization as shown in table 5 (F-0: before optimization; F-1: after optimization), the results show that the strength and plasticity have been improved simultaneously after   optimizing the heat treatment process, and the fatigue life of spring steel has been increased by 13%. The microstructure optimization, e.g., the grain refinement and nano-precipitation, can improve the mechanical properties [34], the fatigue life [35], hydrogen embrittlement resistance [36] and stress corrosion cracking [37] properties. Therefore, the thermodynamic analysis is beneficial for designing microelement systems and their contents in special steels. According to the smelting and heat treatment process of high-strength spring steel flat bar FAS3550 in the enterprise, then the microelements can be controlled in the right range in steel through thermodynamic analysis (previously, the microelements hadn't been attached importance in the enterprise), and then combined with the chemical component standard requirements to design the content of microelements in spring steel flat bar FAS3550, so the grain size of this product after heat treatment process is better, and then the mechanical properties and fatigue life are also improved significantly.

Conclusions
(1) The results of the thermodynamic analysis present that the complete dissolution temperature rises with increasing level of either C, N, Nb or V for various special steels, especially Nb and C.
(2) The content of solid solution V in Nb-V-N-C system steel increases with the increase of V content and decreases with the increase of C, N or Nb content. The solid solution V decreases rapidly below 1050°C. The solid solution content of Nb increases with the increase of Nb content, but decreases with the increase of C, N or V contents. Also, solid solution content of Nb declines rapidly in the range of complete dissolution temperature T AS ∼ 1000°C. The solid solution content of V and Nb in special steel decreased monotonously with decreasing temperature.
(3) The complete dissolution temperature in 0.49% C-0.0060% N-0.03% Nb-0.12% V system high-strength spring steel flat bar FAS3550 is 1314°C, and its effective precipitation temperature is about 1060°C. The grains begin to coarsen obviously at 1100°C during heat treatment. TEM analysis shows the nano-particles precipitated were smaller than 100 nm. The mechanical properties and fatigue life of spring steel flat bar FAS3550 are improved simultaneously after optimizing the heat treatment process with the designing of (Nb, V)(C, N).