Research on the Characterization of Ti Inclusions and Their Precipitation Behavior in Tire Cord Steel

ARCHIVES of ISSN (2299-2944) Volume 19 Issue 3/2019 FOUNDRY ENGINEERING 33 – 37 10.24425/afe.2019.127135 6/3 Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences Research on the Characterization of Ti Inclusions and Their Precipitation Behavior in Tire Cord Steel Jialiu Lei *, Dongnan Zhao, Yongjun Fu, Xianfeng Xu Hubei Polytechnic University 16 Guilin N Rd, Xialu Qu, Huangshi Shi, Hubei Sheng, Chiny * Corresponding author. E-mail address: Received 14.05.2019; accepted in revised form 03.06.2019 Abstract In the present investigation, the morphology of Ti inclusions in high strength tire cord steel was investigated and their precipitation behavior was discussed using a precipitation and growth model. The results show that Ti inclusions mainly exist in the form of TiN. The two-dimensional characterization of Ti inclusions is square-like with sharp edges and corners, while its three-dimensional shape exhibits a cubic or rectangular-prism morphology. The Ti inclusions do not precipitate when the solid fraction of tire cord during solidification is less than 0.987, and their final radius is closely related to the cooling rate and initial concentration product. The higher the cooling speed, the smaller the final radius, when the cooling speed is constant, the final radius of Ti inclusions is mainly determined by the initial concentration product, w[N]0×w[Ti]0. In order to retard the precipitation and growth of Ti inclusions in tire cord steel, the cooling rate and initial concentration product can be taken into consideration. Keywords: Characterization, Precipitation behavior, Ti inclusions, Concentration product, Tire cord steel 1. Introduction As a superior quality product of wire rod, tire cord steel is mainly used in radial ply tyres. Its outstanding mechanical performances, such as its excellent elasticity, high strength, long service life, and impact resistance, are necessary [1,2]. It is a pivotal mission to improve the steel purity in the production. Before tire cord is made, the steel wire is drawn from 5.5 mm to 0.15 mm in diameter and subjected to cyclic stress in the drawing and twisting process. Therefore, the breakage of steel wire during fabrication is a crucial issue. It has been shown that if the size of non-metallic inclusions in steel cord is greater than 2% of the diameter of the processing wire, it causes breakage of the steel wire. This filament break is especially sensitive with the existence of angular and non-deformable Ti inclusions, such as TiN or Ti(CN), which act as cleavage initiators [3,4]. This causes fracture delamination in the drawing and twisting process for steel wire or decreases the fatigue performance and seriously affects traffic safety [5]. It has been one of the main challenges in tire cord steel production, especially for high-strength or ultra-highstrength tire cord steel [6,7]. Therefore, internationally recognized companies, such as Bekaert, proposed the penalty point specification of Ti inclusions to judge wire rods [8]. With the improvement of strength grade for tire cord steel, the effective control of Ti inclusions has become more and more crucial. In this study, grade-80 tire cord steel is researched by an industrial experiment combined with the formation thermodynamics and dynamics. Then the characterization of Ti inclusions and their precipitation behavior are further revealed. ARCHIVES of FOUNDRY ENGINEERING Volume 19, Issue 3/2019, 33 -37 33 2. Experimental Aspects The chemical composition of grade-80 tire cord steel slab is 0.82 wt% C, 0.19 wt% Si, 0.5 wt% Mn, 0.009 wt% P, 0.009 wt% S, 0.0011 wt% Als, 0.0017 wt% O, 0.0039 wt% N, 0.0008 wt% Ti and balance Fe, respectively. In order to gain a greater understanding of Ti inclusions, rectangular pieces (20 mm×20 mm×20 mm thickness) and cylindrical samples 10 mm in diameter and 120 mm high were cut from the billet, which was not rolled. The former were employed for two-dimensional (2D) observation and the latter for three-dimensional (3D) observation. After polishing, the characterization of Ti inclusions was observed by scanning electron microscopy (SEM) equipped with energy dispersive (EDS). In order to clearly observe the 3D morphology of Ti inclusions, the polished cylindrical samples (φ10×120 mm) were placed in the device as shown in Figure 1, for the nonaqueous solution electrolysis experiment after cleaning. N Ti N Ti Fig. 2. Characterization of Ti inclusions (a) 2D morphology of Ti inclusions, (b) 3D morphology of Ti inclusions 3.2 Thermodynamic Calculations Inclusion Precipitation Behaviour of Ti To provide a better clarity of the precipitation behavior of Ti inclusions, thermodynamic calculations for tire cord steel have been performed. The above characterization studies indicate that Ti inclusions mainly exist in the form of TiN, and this was taken into account for the calculations. According to the chemical composition of the steel, the solidus and liquidus temperatures (K) can be obtained by Equations (1) and (2) [9], where w[i] is the mass percentage of solute element i (wt%) . Fig. 1. Schematic diagram of non-aqueous solution electrolysis (1 sample, 2 cathode, 3 thermometer, 4 electrolyte, 5 electrolytic cell, 6 stents) The electrolytic temperature was 273-278 K and current density was 50 mA/cm2. In the experimental process, Argon was used to stir the electrolyte. After 4 hours of electrolysis, the inclusions from the electrolytic sample were separated via ultrasonic cleaning. With this method, the 3D morphology of Ti inclusions can be obtained. 3. Results and Discussion TL=1809-65w[C]-8w[Si]-5w[Mn]-30w[P]-25w[S]-3w[Al]20w[Ti]-90w[N]-80w[O] (1) TS=1809-175w[C]-20w[Si]-30x[Mn]-280w[P]-575w[S]7,5w[Al]-40w[Ti]-160w[O] (2) Thermodynamic calculation of the precipitation of TiN inclusions in tire cord steel is given as follows [10]: [Ti] + [N] = TiN(s) ∆𝐺 0 = −291000 + 107.91𝑇(J/mol) During the precipitation of TiN inclusions, the actual change of Gibbs free energy can be calculated by the following formula: ∆𝐺 = ∆𝐺 0 + 𝑅𝑇ln 3.1 Characterization of Ti Inclusions The typical morphology of Ti inclusions in tire cord steel is shown in Figure 2. The 2D characterization of Ti inclusions shows that they have a square shape with sharp edges and corners and the 3D shape of Ti inclusions extracted by the non-aqueous solution electrolytic method exhibits a cubic or rectangular-prism morphology. The Ti inclusions are of a block type with a size less than 5 μm. The mapping analysis results indicated that the Ti inclusion mainly consisted of N and Ti elements. 34 (3) 𝑎TiN 𝑎N 𝑎Ti = ∆𝐺 0 + 𝑅𝑇ln 1 𝑓N 𝑤[N]𝑓Ti 𝑤[Ti] (4) Where 𝑎TiN , 𝑎N , and 𝑎Ti denote the activities of TiN, N, and Ti, respectively. For pure TiN inclusions, 𝑎TiN = 1. R is the ideal gas constant, whose value is 8.314 J/(mol·K). The activity coefficients 𝑓N and 𝑓Ti in liquid steel can be calculated by 𝑗 Equation (5) and the interaction coe (...truncated)