Estimation of Ferrite Grain Size and Mechanical Properties of a 22MnVNb6 Microalloyed Low Carbon Cast Steel

The ferrite grain size of a 22MnVNb6 microalloyed steel can be estimated by developing a relationship between ferrite grain size, austenitising temperature and cooling rate from austenitising time, temperature. An extended Hall-Petch relationship was used to estimate the yield stress from the estimation ferrite grain size. Heat treatment as the annealing was used to improve the tensile and hardness properties of the steel. It was shown that the best combination of tensile and hardness properties were achieved when a higher austenitising temperature was used. Transmission electron and optical microscopy were used to study the morphology of ferrite and pearlite formed by the heat treatments. The microstructural studies showed that partition of grain by as-cast state was probably the reason for austenite and ferrite grain size improvement. Considering the experimental results, maximum errors of 14.5% and 7.5% were found in the estimation of ferrite grain size and tensile strength, respectively.


Introduction
The name "Microalloyed Steels" was first applied to a class of higher strength low carbon containing small additions of niobium and/ or vanadium. Any attempt at a rational definition of microalloying based on the increases in strength produced by small additions would now include aluminium, vanadium, titanium and of course niobium-treated steels. The effects of the microalloying elements are also strongly influenced by thermal treatments [1,2]. The transformation product grain size control is an important parameter in thermal and mechanical processing of microalloyed steels. It has been well recognized that the tensile and hardness properties of steels are strongly related to the microstructure, specially the grain size [3,4]. In the heat treatment of microalloyed low carbon cast steels involving the austenitising and cooling treatments, the parameters such as cooling rate and austenitising temperature and time have considerable influence on the ferrite grain size and tensile and hardness properties of the steels. It has been shown that accelerated cooling treatment refines the ferrite grain size while increasing the austenitising time and temperature increases the ferrite grain size. Several studies on accelerated cooling have been carried out, considering the relation of processing parameters and tensile and hardness properties to the microstructures [5]. In those studies the relationships presented for estimating ferrite grain size were deduced experimentally to ralate the ferrite grain size to the cooling rate and prior austenite grain size [6][7][8][9]. The grain size estimation model presented by Umemoto et al., is extended in this research to develop a relationship between ferrite grain size, cooling rate, austeniting time, and temperature. The following Hall-Petch relationship is utilized to relate the yield stress to the ferrite grain size [10][11][12][13][14] σ σ α y where σ y is the yield stress, σ o and K are constants, d α the ferrite grain size. The grain size of a microalloyed steel has an important effect on its properties. This phenomenon is known in the literature as the grain size effect. As far as the tensile and hardness properties of the steel is concerned (such as yield and flow stress, ductility, hardness and fatigue limit), a refinement Where R o is friction stress, DR a is related to solid solution strengthening being linearly proportional to the concentration of alloying elements, DR g is a grain size contribution, DR p is precipitation strengthening and is the proportional to precipitation potential, DR t is matrix phase transformation strength.

Determination of austenite grain size distribution
Austenite grain size distribution can be recorded by photographing the microscopic picture and the determination requires measurement of each individual grain. Austenite grain size distribution in one dimension is the distribution of the lengths of linear intercepts through the grains. Metallographic techniques for measuring austenite grain size were originally base on point counting or linear intersection lengths (chords) distrbutions, the derivation by Spektor appears to be the first [17,18]. The working formula is obtained [12] as below , is the number of chords per unit length of test line. The austenite grain size microstructures of entire the relevant samples representative photomicrographs were taken. In order to determine the austenite grain size, the samples were specially etched in a saturated piric acid solution at about 80°C [19,20]. The austenite grain size was then measured at magnification 25× using a filler eyepiece. The degree of magnification will be limited by the fact that the picture must include a sufficient number of grain [21][22][23].

Estimation of Ferrite Grain Size
When ferrite nucleation occurs homogeneously in the austenite matrix, ferrite grain is independent of austenite grain size. In the case that ferrite nucleation occurs at austenite grain corners, ferrite grain size is proportional to d g . The ferrite grain size is determined by the number of ferrite nucleations at austenite grain surfaces until these surfaces are completely occupied by ferrite grains. During cooling, ferrite grains are nucleated and grown at each temperature with corresponding nucleation and growth rates. On the temperature T i ferrite nucleates at the rate I s (T i ) and grows at the rate α (T i ), where I s (T i ) and α (T i ) are the nucleation rate and the parabolic rate constant in the isothemal transformation at temperature Ti [24]. Ferrite grains number n a nucleated at temperature T a during cooling was calculated by Umemoto et al. [25] as is the cooling rate. Assuming that, ferrite nucleation occurs at n α is proportional to CR n 1 , where CR and n 1 are the cooling rate. If ferrite nucleation occurs at austenite grain edges & corners, n α is proportional to CR n 2 and CR n 3 . Ferrite grain size can be calculated as follows [25,26] where S gb is the austenite grain surface area per unit volume.
As increasing the austenite grain size and decreasing the cooling rate, ferrite grain size is increased. Determining the prior austenite grain size in heat treatment of microalloyed steels, one may substitute by the austenitising time t and temperature T as below [7,27] where positive constants are k 1 , k 2 , & Q. Therefore, the ferrite grain size is derived from Eq. (10) and (9) as follows

Experimental Procedure
With different austenitising times, temperatures, and cooling rates from the tests of heat treatment were carried out to determine of values a, A, n, & m from Eq. (7). A commercial strip of 22MnVNb6 microalloyed low carbon cast steel 30.00 mm long, with a rectangular cross section, 20.00 mm wide by 5.00 mm thick was applied on the tests [28][29][30][31]. The research material having chemical composition showed in Table 1. Surface processing operations for property enhancing to determine the d α , the test of heat treatment programmes are showed in Table 2 and have been utilized. Using thermocouple to determine the cooling rate of each specimen, it was utilized to record the data of temperature at austenitising time 1 s. Transmission electron microscopy (TEM) was the most widely used technique for studying all aspects of phase transformations in steels over the length scale range 1-100 nm [32]. TEM data was used as a starting point for the proposed model in micromagnetic simulation for electrodeposited nickel nanowires [33]. Defects created by the superelastic cycling in thin Ni-Ti wires were analyzed by TEM [34]. TEM studies were carried out on the some samples of the as-cast state and the heat treatment test. Optical microscopy was used to examine the microstructure of the microalloyed steel specimen in as-cast state and the heat treatment test. In order to determine the microstructure, the samples are specially etched in 5% nital solution [9]. The concentration of the nital solution has to be adjusted for different samples. The microstructure is then measured at magnification 50x using a filler eyepiece. It was observed that the microstructures consisted of ferrite and a small amount of pearlite. The classic data in the literature for the grain size dependence of the strength in many metals are reviewed [13]. The ferrite microstructure determination should be done in a magnification suited to the size of the grain so that small grains may not be lost. The degree of magnification will be limited by the fact that the picture must include sufficient ferrite grain size. The ferrite grain size of each specimen was measured according to ISO 643 standard. To determine yield strength of the as-cast state steel of a 22MnVNb6 microalloyed, the different ferrite grain size microstructures of the steel were determined and calculated by using the Spektor equation and extended Hall-Petch relationship. After calculating a, A, n, and m in Eq. (7) and the extended Hall-Petch relationships for the low carbon microalloyed steel, some heat treatment as the annealing was carried out in order to compare estimated ferrite grain size and strength data.

Result and Discussion
The annealing was already used to improve the tensile and hardness properties of the 22MnVNb6 microalloyed low carbon cast steel in as-cast state. Since the austenite and ferrite grain size of microalloyed steel are important factor, they were necessary to show the grain size with austenite and ferrite as in Fig. 1 and ferrite-pearlite micrograph of heat treated as annealing in Fig. 2. The microstructures were examined using light microscopy and by transmission electron microscopy of thin foils. Representative TEM images of a 22MnVNb6 microalloyed steel are given on as-cast state in Fig. 3 and on the austenitising temperature at 1000 °C in Fig. 4 [35,36]. The austenite grain size of the as-cast microalloyed steels were investigated using Spektor's analysis in order to determine the size distribution. The prior austenite grain boundaries were revealed by etching the specimens for evaluated the mean grain size. Table 3 shows size distribution of austenite grain and Table 4, there is comparison between the mean austenite grain size of Spektor's method and the values of G, đ & l of International standard [10,[37][38][39][40]. The microalloyed steel was studied investigated to optimize the influence of single and multiple microalloying additions on the ferrite grain size microstructure in as-cast state. Ferrite grain size distributions can be recorded by photographing the microscopic picture and the determination requires measurement of each individual grain. Ferrite grain size distribution in one dimension is the distribution of the lengths of linear intercepts through the grains. Metallographic techniques for measuring ferrite grain size were originally based on point counting or linear intersection length distribution. The ferrite grain size of microalloyed steel is important factors, on Table 5 shows size distribution and grain size contribution to yield stress. Table 6 shows estimation of   Where d α , CR, t, and T are expressed in μm, °C/s, s, and K, respectively.
Considering the experimental results showed in Fig. 4 and 5 and Eq. (8), by increasing the austenitising time and temperature and decreasing the cooling rate, the ferrite grain size transforming from austenite is increased. In fact, by increasing the austenitising time and temperature, the prior austenite grain size is increased, and nucleation sites for ferrite formation are decreased. Atom possessing an energy level of the reactants and activation energy will have sufficient energy to react spontaneously to reach the reacted state of energy of the products [41]. A basic procedure for determining the activation energy was developed by Conrad and Wiedersich [42]. By decreasing the cooling rate, the activation energy for nucleation is decreased and a smaller number of ferrite nucleation sites produce large ferrite grain size.

Conclusions
The Hall-Petch relationship can be utilized to examine the dependence of yield stress on ferrite grain size of the as-cast state steel, but extended Hall-Petch relationship can be utilized of the betterment that on the test of heat treatment programmes. When increased by the austenitising temperature, the effects of austenitising time and cooling rate on ferrite grain size are decreased. When d α formed by continuous cooling transformation, is decreased with an increase in cooling rate or decrease in austenitising time and temperature, the d α can be shown as in Eq. (8). Also, it's good correlation between experimental and estimated ferrite grain size, yield stress of the material studied of this research work.