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Effects of Rare Earth on Mechanical Properties of LZ50 Axle Steels and Its Formation Mechanism

  • Jin-ling Zhang , Yan-chong Yu , She-bin Wang EMAIL logo , Yang Hou and Shu-chun Yin
Published/Copyright: March 23, 2017
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High Temperature Materials and Processes
From the journal High Temperature Materials and Processes Volume 37 Issue 6

Abstract

To study the mechanism of rare earth (RE) elements on the mechanical properties of axle steels, trace RE were added to LZ50 axle steel, which was melted in vacuum induction furnace. By calculating the thermodynamic and kinetic of RE inclusions, the formation mechanism of inclusions was discussed. And the effects of RE elements on the microstructures and mechanical properties were investigated. The results show that RE and O elements in the molten steels diffused into the interface and increased the thickness of the liquid film. The inclusions transform into 1–3 μm spherical RE compound inclusions instead of 5 μm Al2O3-SiO2 with sharp edges. The grain sizes of the steels containing 0.0010 %～0.0026 % RE were decreased, impact toughness and fatigue resistance were improved greatly, compared with the steel without RE. The impact toughness and fatigue resistance of the LZ50 steel with the addition of 0.0010 % RE were increased by 1.65 and 2 times, respectively.

Keywords: LZ50 steel; rare earth; inclusion; formation mechanism

Introduction

As raw materials for locomotives and vehicle axles, LZ50 axle steels chronically work under alternating stress conditions [1]. Their quality and force situation are highly related to the safety of the railway transportation. In order to satisfy the imperative demands of railways, including high speed, heavy load and safety, the properties of LZ50 steels, such as impact toughness and fatigue resistance, must be improved. Enhancing the purity of steels, refining microstructures and reducing the amount of inclusions in the steels are considered to be effective ways to increase the strength and toughness [2, 3]. Nevertheless, the Al, Si and Mn deoxidation products and sulfides with an average size of less than 5 μm, which are formed during the alloying and refining processes, are difficult to be removed efficiently thus remain in the steels as inclusions. These inclusions have the features of minute extension and low floating-up rate. The compounds with high melting points are formed by high-temperature chemical reaction. Although their sizes are only about 5 μm, their morphologies, amount, and distributions may seriously affect the impact toughness and fatigue resistance of steels owing to their irregular edges and corners.

Adding rare earth (RE) elements into the molten steels during the refining process can refine the microstructures and modify the properties, morphologies and distribution of inclusions. Therefore, it is one of the effective ways of improving the comprehensive mechanical performances of steels [4]. Wenbin Xin [5] et al. found that as Ce content increased from 0.037 to 0.095 wt%, the dominant inclusion in the melt was changed from the Ce-S-O inclusion fully coated by Ce-S-As inclusion to the Ce-S-As inclusion completely covered by Ce-As inclusion. Xiaocong Ma [6] et al. found that RE can reduce the mismatch between ferrite and austenite. Yanchong Yu [7] et al. found that adding 0.048 % yttrium had a substantial improvement in the hot ductility of Fe-36Ni invar alloy over the whole testing temperature range especially at 950–1000 ℃.

However, little information has been obtained about the detailed study on raising the comprehensive properties of steels by adding trace RE elements. In this study, LZ50 steel samples with different contents of RE elements were prepared by adding RE alloy into LZ50 axle steels (originally produced by a Chinese factory) molten in a vacuum induction furnace to investigate the effects of RE elements on the fatigue resistance of axle steels. In addition, the effects of trace RE elements on the microstructures and mechanical properties of axle steels and the mechanisms were discussed. Basic experimental data were provided for improving the mechanical properties of LZ50 axle steels and realizing the application of trace RE elements in axle steels.

Experimental

The experimental specimens were prepared through adding RE alloys into the LZ50 axle steels. The RE alloy comprises 2.3 % RE (La, Ce), 2.0 % Ca, 4.0 % Ba, 10% Al, 25 % Si, and 58.7 % Fe (weight percentage). The preparation process of specimens was as follows: Firstly, 2.5 kg billet steels were melted in a ZG0.25-100-2.5 vacuum induction furnace under the Ar atmosphere of 0.0001 MPa, followed by adding different amounts of RE alloys into the molten steels through a vacuum feeding tank attached to the furnace. Secondly, the molten steels were refined at 1873 K for 15 min. When the temperature was decreased to 1823 K, the molten steels were poured into a sand mold, which was pre-placed in the furnace, and cast into a rod bar (Φ 40 mm×300 mm), followed by cooling with the furnace to the room temperature. Thirdly, the steel was heated to (1373 ± 20) K and then forged into Φ 20 mm rods. Finally, the compositions, microstructures and mechanical properties of the specimens were investigated. In this study, four types of steels with different compositions were smelted. The contents of major elements were analyzed by SPARKLAB spark atomic emission spectrometer, while the concentrations of RE elements were detected by 721 Spectrophotometer. The results are listed in Table 1.

Table 1:

Chemical compositions of four steels (mass fraction, %).

SteelsRECSiMnSPCrNiCuNOFe
No. 100.520.290.800.00340.0170.0640.0120.0190.00140.0027Balance
No. 20.00100.520.260.790.00330.0170.0650.0140.0170.00180.0028Balance
No. 30.00170.510.280.800.00360.0180.0670.0130.0180.00180.0031Balance
No. 40.00260.491.140.780.00430.0190.0680.0110.0200.00200.0009Balance

Specimens (8 mm long) were cut laterally from the Φ 20 mm steel rods. The lateral cross-sections were grinded, polished progressively on emery papers up to 2000 level and etched in a solution of picric acid. The NIKON L150 optical microscope (OM) was used to observe the morphological characterization and JSM-6700F scanning electron microscope (SEM) was employed for the microanalysis. According to GB/T229-2007 national standards of China, the impact specimens with U notches (10 mm × 10 mm × 55 mm) were prepared and examined using JB-30B impact testing machine. According to GB/T15248-2008 standards, fatigue specimens with a diameter of 5 mm were prepared, and their fatigue resistance was investigated using PIG-200D high-frequency fatigue-testing machine, according to the staircase method recommended by HB5287-96 under the following conditions: (1) the stress ratio of 0.16, (2) at room temperature, (3) with a resonant frequency 134.2~140.3 Hz, and (4) the moisture of 30 %. In addition, during the refining process, a thermocouple was employed to detect the refining and casting temperatures. MC016-WGG2-323 optical pyrometer was used to monitor the temperature of molten metal. Over the whole smelting process, the temperature was controlled by adjusting the output power.

Results and discussion

Effects of RE elements on the morphologies of inclusions

Figure 1 shows the SEM images and EDS spectra of typical inclusions in specimens with various content of RE. It is clear that in the specimen without RE, there exists 5 μm SiO2-Al2O3 inclusions with an irregular shape, as denoted by the arrow in Figure 1(a). In the specimens with RE, the inclusions are 1–3 μm spherical SiO2-Al2O3-La2O3 composite inclusions, as denoted by the arrow Figure 1(b), and 2 μm ellipsoidal Al2O3-MnS compound inclusions as denoted by the arrow in Figure 1(c). Besides, many dot-shaped RE oxides are observed with an average size of less than 100 nm in the No. 4 sample, as denoted by the arrow in Figure 1(d).

Figure 1: SEM images and EDS spectra of typical inclusions in steels without RE (a and a′), with 0.0017 % RE (b and bnd EDS spectra of  % RE (c, d, cnd EDS sp.
Figure 1:

SEM images and EDS spectra of typical inclusions in steels without RE (a and a′), with 0.0017 % RE (b and bnd EDS spectra of  % RE (c, d, cnd EDS sp.

Figure 2: OM images of No. 1(a), No. 2(b), No. 3(c) and No. 4(d) steels.
Figure 2:

OM images of No. 1(a), No. 2(b), No. 3(c) and No. 4(d) steels.

The inclusions with evident edges and corners, presented in Figure 1(a), may deteriorate the mechanical properties of the steels, owing to their poor cohesive performance with the steel matrix. It is hard for inclusions to deform during hot working process so that micro-cracks may emerge at the interface. In contrast, the surface of the spherical inclusions, observed in Figure 1(b), is smooth, which makes it difficult for micro-cracks to form in the corresponding position during hot working. As a result, the strength and toughness of steels are enhanced. Furthermore, the thermal expansion coefficient of RE inclusions approximates to that of the steel, which can reduce the stress concentration around the inclusions during heating or cooling processes, leading to the increase of the fatigue resistance [8]. Figure 1(c) shows the inclusion is composed of Al2O3 and MnS, both with high melting temperature. After RE addition, the elongated MnS inclusion, which is commonly observed in steels, is replaced by ellipsoidal MnS inclusion. In Figure 1(d), as the concentration of oxygen in the steel is low, the dystectic RE oxides are formed, which could act as heterogeneous nucleating particles, increasing nucleation rate during solidification and refining the grains of the matrix. In addition, these small particles could hinder the movements of dislocations, leading to the improvement of the strength and toughness of the steels.

Effects of trace RE elements on the microstructures of LZ50 steels

OM images of specimens, including No. 1, 2, 3, and 4, after forging and air cooling are presented in Figure 2. It is noted that the microstructures of the specimens consist of dark flocculent pearlites and bright ferrites. The grain sizes of pearlites in No. 2, No. 3, and No. 4 specimens are smaller than those in No. 1 sample. Among these specimens, No. 3 specimen shows the smallest grain size and the largest amount of ferrites (bright region). This can be attributed to the fact that the No. 3 specimen has the largest solid solubility of RE. The interaction between RE and C decreases the activity of C, improves the solubility of C in ferrite [9] and reduces the amount of pearlites. Moreover, the critical nucleation energy is reduced and the amount of nucleation in unit volume is increased by RE elements, in line with the reduced grain size of pearlites. When the content of RE elements reaches 0.0017 %, the pearlites in the steel matrix have the minimum grain size, while the ferrites not only grow in quantity but also almost surround the pearlites in net work form.

Effects of trace RE elements on the mechanical properties of LZ50 steels

Effects of trace RE elements on the impact toughness of LZ50 steels

The variation of the impact energy with RE contents at room temperature is plotted in Figure 3. It can be seen that the impact energy increases first then declines with the increasing of RE contents. The maximum value is about 200 J/cm2 at the RE content of 0.0010 %. The impact energy of No. 2 specimen increases by 65 % in comparison with that of No. 1 specimen.

Figure 3: Variation of impact energy with RE content.
Figure 3:

Variation of impact energy with RE content.

Effects of trace RE elements on the fatigue resistance of LZ50 steels

Table 2 lists the results of the fatigue stress, fatigue life and sample states of the specimens with different contents of RE. The fatigue strength presents an upward trend first and downward trend later with the rise of RE content. Compared with No. 1 specimen (fatigue strength: 230.28 MPa), the fatigue strength of No. 2 (450.48 MPa), No. 3 (390.64 MPa) and No. 4 (288.55 MPa) specimens increases by 96 %, 75 % and 25 %, respectively. This demonstrates the significant influences of RE elements on the fatigue resistance of steels, i. e., refining the microstructures of steel matrix and modifying the morphologies of inclusions.

Table 2:

Fatigue test results of four steels.

SteelsFatigue stress, MPaFatigue life N, ×103cycState of Sample
No. 1230.285011unbroken
No. 2450.485391unbroken
No. 3390.645028unbroken
No. 4288.555100unbroken

Mechanisms of formation and deformation of the RE compound inclusions

Under the refining conditions at 1873 K, the molten LZ50 steels consist of multi-components. No. 1 specimen contains elements such as Fe, C, Mn, Si, Al, O, S, and particles like solid Al2O3-SiO2. After adding RE alloys, elements La and Ce are included in the system. Whether there are new types of inclusion forming can be evaluated by calculating the Gibbs free energy of the pyrometrical reactions. The RE alloy used in this study is a mixture of La and Ce. These two elements are similar in chemical properties, and since the element Ce has detailed thermodynamic data, all the thermodynamic calculations are based on the statistical data of the element Ce.

Thermodynamic analysis of the reaction system

Regarding the typical inclusions, shown in Figure 1, it involves five chemical reactions during the refining process at 1873 K in the system [10, 11, 12].

(1)Al+RE+3O=REAlO3ΔGθ=−1414050+305.53T
(2)3/4SiO2+RE=1/2RE2O3+3/4SiΔGθ=−191091+26.75T
(3)RE+3/2O=1/2RE2O3ΔGθ=−725991+179.8T
(4)RE+O+1/2S=1/2RE2O2SΔGθ=−675700+165.5T
(5)Mn+S=MnSΔGθ=−167000+88.68T

where ΔGθ is the standard Gibbs free energy (J/mol), and T is the reaction temperature (K). The values of ΔGθ for the five reactions are calculated assuming T  =  1873 K and the results are as follows: −322.00,−56.45,−109.74,−105.22, and 89.77 kJ/mol, respectively. It is noticed that the ΔGθ values of reactions (1, 2, 3), and (4) are all negative, and the absolute values are relatively large, which means all these four reactions may occur. On the contrary, the ΔGθ value of reaction (5) is positive, which means this hyperthermal reaction is unable to proceed. As is seen from Figure 1(b), the inclusions consist of Al, Si, O and RE elements. EDS analysis shows that the intensity of La element is rather small. According to reactions (1, 2), the EDS analysis and the phase diagram of Al2O3-SiO2, the inclusion is considered to be mainly composed of compound Al2O3-SiO2 and REAlO3 or RE2O3.

Kinetic analysis of the reaction system

Without RE element addition, the element O exists in forms of dissolved oxygen and compound Al2O3-SiO2 in the molten steels. It is obtained from Al2O3-SiO2 phase diagram [13], the binary eutectic temperature is 1873 K, rationalizing the hypothesis that Al2O3 and SiO2 exist in the form of little liquid and massive solid, respectively, and they are surrounded by elements Si, Al and O, which are in equilibrium with them. After RE elements addition, the inclusion with irregular shape, as shown in Figure 1(a), is modified as spherical inclusion, presented in Figure 1(b). The kinetic process is as follows: Initially, the RE elements diffuse into the interface between inclusions and molten steels to react with the active sites on the inclusions. The products attach to the surface of original solid inclusions and grow. Then, the newly formed RE2O3 reacts with the molten silica and aluminum oxides, lowering the melting points of the oxides. These oxides are attached to the solid Al2O3-SiO2 in the form of liquid film. As RE and O elements in the molten steels continuously diffuse into the interface, the reactions (1)–(4) proceed smoothly, increasing the thickness of the liquid film. Once the film reaches a certain thickness, it will solidify and aggregate into spherical shape on the basis of lowest surface energy principle [14]. In this way, the compound inclusions, which contain a core of Al2O3-SiO2 and a surface of RE, Si, and Al compound oxides, are formed, as displayed in Figure 1(b). In addition, the molten steel keeps rotating in the induction furnace under the electromagnetic force. During this process, the RE atoms react with encountered O atoms, producing RE2O3 compound, as shown in Figure 1(d). Figure 4 shows the diagrammatic sketch of the inclusion evolution process. The thermodynamic calculation suggests that the ΔGθ of reaction (3) is smaller than the ΔGθ of reaction (4), which means the products in the system should be RE2O3 rather than RE2O2S.

Figure 4: The diagrammatic sketch of inclusion evolution process.
Figure 4:

The diagrammatic sketch of inclusion evolution process.

The following relationship may exist, provided that the reaction (5) could proceed within the system:

lgaMn⋅aS≥ΔG(MnS)Θ19.147T=−8722T+4.63

where aMn stands for the activity of Mn,aS for the activity of S, and T for temperature (K). When the temperature decreases to a certain degree during solidification, the value of ΔG(MnS) is negative, the segregated S would increase aS, resulting in the precipitation of MnS. Since the Al2O3 particles, as marked by the arrow in Figure 1(b), may act as the nucleating points, the MnS inclusions can rely on them and grow up into ellipsoidal shape, as marked by the arrow in Figure 1(c).

Refining the microstructure of LZ50 steel by RE elements

The grain size depends on the quantity of grains in unit volume, which can be described as Zv =  0.9(n/v)3/4 [15], where n is nucleation rate, and v is coarsening rate. Therefore, under fixed composition and cooling conditions, the grain size is determined by n and v.

It is noted that [16] the surface active elements RE reduce the surface tension and critical nucleation energy of hyperthermal melts, decrease the under-cooling of embryo transforming into nucleus and increase the value of n. As is known, in the high temperature melts, the RE elements react with oxygen, producing RE oxides, which have a similar density with that of molten steels. These oxides with larger size may float up to the surface of molten steel, being removed by the absorption of the crucible wall under electromagnetic stirring. The critical energy of heterogeneous nucleation is determined by the similarity between the heterogeneous nucleus and the matrix [17]. Because the difference between the structure of δ-Fe and the dystectic RE oxides, such as Ce2O3, is small [18], the lattices of them match well. The well-distributed small-scale inclusions, as it were, may act as the substrate for heterogeneous nucleation, increasing the value of n. During solidification and crystallization, most RE atoms gather on the interface of solid/liquid, since the covalent radius of RE atom (0.1877 nm) is larger than that of Fe atom (0.1210 nm) by 55 % [19]. The RE atoms can be easily pushed to the interdendrite or grain boundary, preventing the Fe atom from transferring to Fe crystal lattice and thus reducing the value of v. Hence, a conclusion can be drawn that the RE elements added into the molten steel increase the amount of grains in unit volume and decrease the grain sizes of pearlites.

Figure 5 shows the variation of average grain size of pearlites with RE contents. As is seen from Figure 5, when the RE content increases from 0 to 0.0017 %, the grain size decreases from 105 μm to 17 μm. Nevertheless, when the RE content increases to 0.0026 %, the grain size ascends to 57 μm. In line with the thermodynamic theory and the analysis of kinetic phenomena, these results suggest that trace RE elements can refine the grain sizes of LZ50 steels, and reveal the basic principle of RE elements refining the steel matrix.

Figure 5: Variations curve of average grain size with RE content.
Figure 5:

Variations curve of average grain size with RE content.

Figure 6(a) and 6(b) shows the SEM images of pearlites in No. 1 and No. 2 specimens, respectively. Apparently, the cemenite lath, shown in Figure 6(a), presents a clear directionality with larger size, along with larger pearlite particle. On the contrary, most cemenites, shown in Figure 6(b), presents a short-bar shape, and the size of pearlites is relatively small. Because of the segregation of RE atoms at the interdendrites or grain boundaries, the interfacial energy of grain or phase boundaries is reduced, and the value of n for pearlite is increased. Meanwhile, the RE atom, with large radius, impedes C atoms from diffusing into austenite, hindering the growth of pearlite grains. The interaction between the segregated and dissolved RE atoms effectively controls the pearlite grain size in steel matrix, and the amount and distribution of ferrites, which lays the foundation for refining the microstructures and enhancing the mechanical performances of LZ50 steels.

Figure 6: SEM images of pearlite in the steels (a) without RE and (b) with 0.0010 % RE.
Figure 6:

SEM images of pearlite in the steels (a) without RE and (b) with 0.0010 % RE.

3.6 Influencing factors for impact toughness and fatigue strength

Influencing factors for impact toughness

Impact toughness is dependent on the factors such as grain size and interior defects (e. g. inclusion) in the materials. Adding RE into the steels changes the grain size of pearlites and modifies the size and morphology of inclusions.

Figure 7 shows the SEM images of typical impact fracture. River patterns appear on the fracture surface of the specimen, as is shown in Figure 7(a) and 7(b). The impact fracture is composed of many facets, i. e., cleavage faces, which reflect the size of the grain [20]. In line with the results obtained by Wang et al. [20], the grain size of the specimen with 0.0026 % RE is smaller than that without RE, and the impact toughness of the specimen in Figure 7(b) is evidently higher in comparison with that in Figure 7(a). This indicates that the impact toughness is enhanced by RE elements, which decrease the grain size.

Figure 7: SEM images of the impact fracture surface in the steels (a) without RE and (b) with 0.0026 % RE.
Figure 7:

SEM images of the impact fracture surface in the steels (a) without RE and (b) with 0.0026 % RE.

Figure 8 shows the dependence of the impact energy and fatigue strength on the grain size. The impact energy decreases with the increase of grain size in the range of larger than 43 μm, otherwise it shows an opposite trend. This may be attributed to the variation of RE contents, leading to the changes of the distribution of ferrites in the matrix, the grain size of pearlites as well as the morphology of inclusions. The stress required for the Cottrell’s cleavage fracture [21] can be expressed by σc = [4Gγ/d]1/2, where G is shear elastic modulus, γ is the material’s surface energy, and d is the grain size. Usually, the smaller the value of d is, the higher stress is needed for the formation of cleavage fracture. Furthermore, the finer grain and more twisted boundaries would be unfavorable for the dissemination of the fracture. Therefore, the specimen may possess higher impact toughness since more energy needs to be absorbed during fracture process. However, the experimental results show that the impact toughness of the specimen with an average grain size of 20 μm is obviously lower than that of the specimen with an average grain size of 43 μm. This may be attributed to the influences of the morphology of inclusions in the steel.

Figure 8: Variations of the impact energy and stress Smax with 5×106 cyclic loading with average grain size.
Figure 8:

Variations of the impact energy and stress Smax with 5×106 cyclic loading with average grain size.

Figure 9 shows the SEM images of typical inclusions on the fracture surface of impact specimens. Together with EDS spectra, it can be seen that the inclusions in the steel without RE have a size about 5 μm Al2O3-SiO2 inclusion with edges and corners and barely plasticity, as marked by the arrow in Figure 9(a). In contrast, the inclusions in the steel with 0.0026 % RE are fine and smooth spherical inclusion with a size of smaller than 2 μm, as marked by the arrow in Figure 9(b), and ellipsoidal RE compound inclusion, as marked by the arrow in Figure 9(c). Under shearing stress, the dislocations move along the glide plane. When the dislocations encounter with the inclusions, shown in Figure 9(a), an area with high stress concentration is formed at the front of the edge, leading to the formation of microcracks at the interface between steel matrix and inclusions. As a result, the impact toughness is decreased. When the dislocations encounter with the inclusions with small size and large curvature radius, as the ones shown in Figure 9(b) and 9(c), the stress concentration is reduced. And the fracture is retarded, with the impact toughness improved. It is noticed from Figure 9, there exists cracks between the substrate and the inclusions in Figure 9(b) and 9(c). Over the fracture process under the impact force, a certain kind of deformation is generated at the matrix/inclusion interface, which absorbs the impact energy and enhances the impact toughness. Obviously, after adding appropriate amount of RE into the steels, not only the microstructure is refined but also the morphology of inclusions is modified, which produces relatively deep dimples around the small-scale inclusions during the fracture process. Hence, improving the impact energy lies in two possible factors. One relates to the increase of the radius of the matrix/spherical inclusion interface, while the other is the deformation at the interface.

Figure 9: SEM images of inclusions on the impact fracture surface in the steels without RE (a) and with 0.0026 % RE (b) and (c).
Figure 9:

SEM images of inclusions on the impact fracture surface in the steels without RE (a) and with 0.0026 % RE (b) and (c).

From the analysis above, it can be concluded that the impact properties of LZ50 steels with RE are superior to that without RE. And the grain size, morphology of inclusions and distribution of ferrites are all influenced by the concentration of RE elements, which directly affects the impact performances of steels. Specifically, LZ50 steel with 0.0010 %RE has the highest impact energy, in line with the result obtained by Wang et al. [20]. The experimental result is in accordance with the theory of impact fracture.

Influences of morphology of inclusions on the fatigue strength

As is known from the previous studies, the grain size of the steel matrix is controlled within the range of 40～60 μm and the fatigue strength can reach 350～450 MPa when the RE content is within the range of 0.0010 % ~ 0.0026 %, while when no RE is added, the grain size is 105 μm, cyclic loading stress is 271 MPa and the specimen is destructed only for 5×105～8×105 circles. As can be seen from Figure 8, when the grain size is larger than 43 μm, the fatigue strength shows an upward trend with the decrease of grain size, otherwise it shows an opposite trend. According to the theory of the fatigue fracture [22], cycle and cyclic loading stress would significantly increase with the decrease of the grain size of pearlites. The present study indicates that the fatigue strength is not improved evidently with the decrease of grain size when the RE content is more than 0.0010 %. This suggests that the effects of the size and morphology of inclusions on the fatigue strength of steels cannot be neglected provided that the grain size and ferrite-to-pearlite ratio are fixed.

Through the comparison of morphologies of inclusions, it is obtained that there exists about 5 μm silicon and aluminum oxides with sharp edges in the steels without RE. At the sharp edges, the curvature radius is very small, which decreases the fatigue strength. However, in the steels with RE addition, there exists 0.05～3 μm and spherical RE, Si and Al oxides and sulfides. The location, size as well as morphology of the oxide inclusions together remarkably affect the fatigue strength of axle steels.

Figure 10 presents SEM images of the typical fracture surface of fatigue specimens. Figure 10(a) is the fatigue fracture, provided that σmax = 500.14 MPa and N = 972.643×103 cycle. Figure 10(b) is the fatigue fracture, provided that σmax = 449.91 MPa and N = 330.373×103 cycle. It turns out that inclusions in No. 2 specimen are spherical, as shown in Figure 10(a), with diameters between 1.6 and 3.3 μm, and an average size of 2.48 μm. Inclusions in No. 3 specimen have ellipsoidal shape, as shown in Figure 10(b). The size of major axis is between 0.58 and 10 μm (mainly in the range of 1~3.6 μm) and the average size is 3.68 μm. Inclusions in No. 4 specimen are spherical oxides and ellipsoidal sulfides, and the diameter is between 1 and 16 μm (the size of major axis is in the range of 1~3.6 μm), the average size is 5.1 μm. The results have demonstrated [23] that the crack emergence of low strength steels mainly originates from the surface slip bands, grain boundaries and positions where interior non-metallic inclusions exist. According to the estimation [24] of the critical size of spherical inclusions which locate in the three positions mentioned above under the high-frequency fatigue condition, the sizes of inclusions at the fatigue fracture surface are smaller than critical fatigue fracture size, i. e., 4.1 μm (Figure 10(a)). In other words, the fatigue cracks derive from surface slip bands and grain boundaries. The base strength or surface state determines the fatigue strength of the steel, which reaches 450 MPa. The average size of inclusions in No. 3 specimen is 3.68 μm, though smaller than the size range of 4.1–4.7 μm, large inclusions whose major axis size is larger than critical fatigue fracture size, appear in the matrix (Figure 10(b)), which indicates that the fatigue cracks derive from the interface of interior ellipsoidal inclusion/steel matrix, and the steel has a fracture owing to the development of cracks during cyclic loading process, together with a decreased fatigue strength of 391 MPa. The fatigue strength of No. 1 and No. 2 specimens decreases to 288 and 230 MPa, respectively. In addition, it can be seen from Figures 1 and 10 that the inclusions are small-scale compound oxides with a shell containing RE elements. These inclusions possess good compatability with steel matrix and similar thermal expansion coefficients and elastic modulus of elasticity compared to the steel matrix, decreasing the degree of the stress concentration at the interface during cyclic loading process. Consequently, the morphology and size of inclusions vary with RE contents. When the major axis size of inclusions is smaller than that under the critical fatigue fracture, the fatigue strength of the steels is determined by the base strength. Otherwise, it is dependent on the development of cracks at the interface of substrate/inclusion. Therefore, the fatigue strength of LZ50 steels with RE is superior to that without RE.

Figure 10: SEM images of fatigue fracture in steels (a) with 0.0010 % RE and (b) with 0.0017 % RE.
Figure 10:

SEM images of fatigue fracture in steels (a) with 0.0010 % RE and (b) with 0.0017 % RE.

In summary, controlling the RE contents at the level of 0.0010–0.0026 % and sizes of pearlite grains in the range of 40–60 μm produces spherical compound oxides (RE, Si and Al oxides) and ellipsoidal sulfides inclusions, with a size of about 0.05–4 μm. These are the basic conditions to optimize the mechanical properties of LZ50 steels. In present study, it is found that the optimum fatigue strength for LZ50 steel is 450.3 MPa at the RE content of 0.0010 %.

Conclusions

(1) After adding RE elements into the steels, inclusions transform into 1–3 μm spherical RE compound inclusions instead of 5 μm Al2O3-SiO2 with sharp edges, and the elongated MnS inclusions transform into about 2 μm ellipsoidal inclusions with small length-width ratio. When the steels contain extremely low content of oxygen (like No. 4 specimen only 0.0009 % O), nanoscale (＜100 nm) dot-shaped RE oxides inclusions are found in the steel.

(2) When the content of RE in steels is in the range of 0.0010–0.0026 %, the effect of grains refinement is evident. The minimum grain size is 17 μm in the steels with 0.0017 % RE.

(3) When the content of RE in steels is in the range of 0.0010–0.0026 %, fatigue strength and impact toughness are improved and these properties of steels with RE addition have increased by 2 and 1.65 times, respectively, compared with those of RE-free steels.

Funding statement: This research was supported by the National Natural Science Foundation of China (No. 51371122), Shanxi Province Science Foundation (No. 2015011068) and The Open Project of State Key Laboratory of New Technology in Iron and Steel Metallurgy (No. KF13-06).

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Received: 2016-6-21
Accepted: 2017-1-12
Published Online: 2017-3-23
Published in Print: 2018-6-26

© 2018 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Articles in the same Issue

  1. Frontmatter
  3. Effects of Rare Earth on Mechanical Properties of LZ50 Axle Steels and Its Formation Mechanism
  4. Formation of Non-metallic Inclusions of Si-killed Stainless Steel during GOR Refining Process
  5. Application of Direct Resistance Heating in Hot Forging and Analysis of Processing Parameters based on Thermo-electro-mechanical Coupling FEM
  6. Effect of Long-Term Ageing at 600 °C on Microstructure of ZG1Cr10MoWVNbN Martensitic Heat-Resistant Steel
  7. Plasma-Assisted Nitriding in Low-Frequency Inductively Coupled Plasma Enhanced with Ferromagnetic Cores
  8. Modeling of the Hot Flow Behaviors for Ti-6Al-4V-0.1Ru Alloy by GA-BPNN Model and Its Application
  9. Preparation and Oxidation Behavior of Metallic Nickel Containing MgAlON Composite
  10. Numerical Investigation on the Strain Evolution of Ti-6Al-4V Alloy during Multi-directional Forging at Elevated Temperatures
  11. Study of the High-temperature Synthesis of MgAl2O4 Spinel Refractory Raw Materials from Chromium Slag
  12. Hardness Evolution and High Temperature Mechanical Properties of Laser Welded DP980 Steel Joints
  13. Microstructure and Properties of Si3N4 Ceramics and 304 Stainless Steel Brazed Joint with Cu/Ag-Cu/Ti Laminated Filler Metal
https://doi.org/10.1515/htmp-2016-0122
Keywords for this article
LZ50 steel; rare earth; inclusion; formation mechanism
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  3. Effects of Rare Earth on Mechanical Properties of LZ50 Axle Steels and Its Formation Mechanism
  4. Formation of Non-metallic Inclusions of Si-killed Stainless Steel during GOR Refining Process
  5. Application of Direct Resistance Heating in Hot Forging and Analysis of Processing Parameters based on Thermo-electro-mechanical Coupling FEM
  6. Effect of Long-Term Ageing at 600 °C on Microstructure of ZG1Cr10MoWVNbN Martensitic Heat-Resistant Steel
  7. Plasma-Assisted Nitriding in Low-Frequency Inductively Coupled Plasma Enhanced with Ferromagnetic Cores
  8. Modeling of the Hot Flow Behaviors for Ti-6Al-4V-0.1Ru Alloy by GA-BPNN Model and Its Application
  9. Preparation and Oxidation Behavior of Metallic Nickel Containing MgAlON Composite
  10. Numerical Investigation on the Strain Evolution of Ti-6Al-4V Alloy during Multi-directional Forging at Elevated Temperatures
  11. Study of the High-temperature Synthesis of MgAl2O4 Spinel Refractory Raw Materials from Chromium Slag
  12. Hardness Evolution and High Temperature Mechanical Properties of Laser Welded DP980 Steel Joints
  13. Microstructure and Properties of Si3N4 Ceramics and 304 Stainless Steel Brazed Joint with Cu/Ag-Cu/Ti Laminated Filler Metal
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