The influence of final coiling temperature on the microstructure and mechanical properties of high Ti-V HSLA steels

. In general, the strength-to-weight ratio is a marked property of high-strength low-alloy steel. The coiling temperature is an important process parameter in the control and improvement of final microstructure and mechanical properties. In this work, the effect of deformation and coiling temperature on microstructure and mechanical properties of high Ti-V microalloyed HSLA steels were investigated using the Gleeble 1500 TM thermomechanical processing simulator. The samples were austenitised at 1150 ˚C for 300 s, then deformed in 4 passes at 1050, 1000, 950 and 900 ˚C. After final pass, the samples were cooled and isothermally held for 1800 s to simulate the coiling process at 550 and 650 ˚C. The results showed that at low coiling temperature (LCT) of 550 ˚C, the microstructures consisted of ferrite and bainite and also had higher hardness (304 HV).


Introduction
High-strength low-alloy (HSLA) steels are a type of low-carbon steels that exhibit a combination of high strength, excellent toughness, weldability, and good corrosion resistance [1,2]. These alloys have been applied to oil and gas transportation, automotive industry, navy vessels, and even nuclear fission power plant components due to their excellent mechanical properties [3,4].
The microalloying elements, such as Ti, V, Nb, and Mo elements are usually added in small amount for the production of high-strength steels. These elements can improve the strength of steels by grain refining, solid solution, dislocation and precipitation hardening mechanisms [5][6][7][8]. In addition to control of chemical composition, another effective way to obtain desired mechanical properties is applied thermo-mechanical controlled process (TMCP) on steels [9]. Therefore, traditional thermo-mechanical controlled processing is regularly used to refine the microstructure of HSLA steels. In TMCP hot rolled steel strips, the microstructure and mechanical properties are significantly influenced by the process parameters, such as rolling ratio, rolling temperature, cooling pattern, cooling rate and the coiling temperature. Among them, the influence of coiling temperature is considered to be significant [10,11].
In this study, a low carbon high Ti -V microalloyed steel was hot rolled and coiled at two different temperatures. The effect of coiling temperature on the microstructure, and the mechanical properties, was investigated.

Materials
The chemical composition of the tested steel is given in Table 1. Samples were machined from forged ingots into Փ10 mm and 15 mm.

Experimental methods
Thermomechanical simulation experiments were conducted on a Gleeble 1500TM thermal simulation machine. As shown in Figure 1, the specimens were reheated to 1150 ℃ at a heating rate of 10 ℃/s and held for 300 s to make sure that maximum micro-alloyed elements dissolved into austenite. The specimens were then cooled at a cooling rate of 10 ℃/s to1050, 1000, 950 and 900 ℃ and subjected to 0.25, 0.2, 0.2 and 0.25 strain respectively at a strain rate of 5/s. After four deformation passes, the specimens were cooled continuously to coiling temperatures of 650 and 550 ℃ at a cooling rate of 20 ℃/s (simulating the run-out-table cooling rate) and then held at these temperatures for 1800 s to simulate the coiling process. The specimens were finally cooled to room temperature at a cooling rate of 10 ℃/s. The deformed samples were sectioned along the vertical plane at 1/2 of the diameter for optical metallography. The cross-section surfaces of the samples were mounted, ground, polished and etched with 4 vol. % Nital. Finally, the microstructures were observed by an Olympus BX51MTM optical microscope (OM).
The precipitation behaviour was studied by using scanning electron microscopy (SEM) equipped with the energy dispersive spectroscopy (EDS). The electron back scattered diffraction (EBSD) analysis was carried out using a JSM-IT300 JEOL SEM at a step size of 0.25 μm. The acquired data was analysed with the help of TANGO software provided in AZtec HKL system to generate Inverse Pole Figures (IPF), grain size and misorientation angle distribution graphs.
The hardness was determined by Struers hardness testing machine (duramin-40) using test load of 10 kg.
The samples were mounted and polished to 1 μm for the instrumented indentation plastometry tests. The specimens were placed on the baseplate of the plastometer and indented with a spherical indenter of radius 1 mm, made of WC-Co cemented carbide. Force was applied and the profile being measured after each loading operation, and the nominal uniform strain-stress curves were obtained from FEM simulation.

Microstructure
The Temperature-Time-Transformation (TTT) and Continuous Cooling Transformation (CCT) diagrams of the studied steel, as calculated by JMatPro are given in Figure 2. JMatPro assumes grain size of 20 µm after austenitization at 1150 °C. It can be seen that the bainite start temperature (Bs) and martensite Ms are 575 °C and 406 °C, respectively. Figure 2 predicts that coiling at 650 and 550 °C would result in ferrite (or ferrite-pearlite) depending on the cooling rate and bainite, respectively. OM images of the microstructures of the specimens after hot strip rolling and coiling at 650 and 550 °C are presented in Figure 3. The microstructure of high coiling temperature (HCTS) sample mainly consists of polygonal ferrite (PF), pearlite and granular ferrite (GF), which was rarely found. When the coiling temperature was reduced to 550 °C, accicular ferrite (AF), PF, and bainite (B) were observed. It was also observed that the size of PF grains was significantly decreased with the decrease in the coiling temperature, Figure 3.  Figures 4 a & b show the morphology, distribution and composition of precipitates of hot rolled samples after coiling at 650 and 550 ℃ respectively. As may be seen from Figure 4, six different types of precipitates were observed namely: Type 1 precipitates were cuboidal titanium nitrides (TiN); Type 2 precipitates were the medium size manganese-titanium carbosulfides ((Mn-Ti) (C, S)); Type 3 precipitates were the manganese sulfides (MnS); Type 4 and 5 precipitates were the fine spherical titanium carbosulfide (Ti4C2S2) and titanium carbide (TiC) respectively, and Type 6 was cementite (FeC).

EBSD Analysis
EBSD mapping was conducted to characterize the grain size and grain boundary misorientation after hot deformation and coiling simulations. Figure 5 shows the EBSD inverse pole figure (IPF) images. In this figure low angle grain boundaries (LAGB) were marked by grey lines and high angle grain boundaries (HAGB) were marked by black lines. It is clearly seen that the grain size is very fine with random orientation distribution. The nonuniform grain size is attributed to the inhomogeneous nucleation and growth of ferrite grains during austenite to ferrite transformation [12]. Figure 6 (a) and (b) show the grain size distribution and the distribution of the grain misorientation angle respectively. With decrease in coiling temperature from 650 to 550 ℃, a significant refinement in grain size can be clearly observed. The average grain size reduced from 5.6 µm to 3.7 µm with decrease in coiling temperature. There is a reduction in low angle grain boundaries (LAGB) fraction from 13% to 7 % with increase in coiling temperature.

Mechanical Properties
The hardness increased sharply from 247 HV to 304 HV with decrease in coiling temperature from 650 to 550 °C. Figure 7 shows the stress-strain curves that were extracted from IIP test and the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (%EL) are presented in Table 2. The YS and UTS of 800 and 1000 MPa respectively were observed for the LCTS as opposed to the 590 and 800 MPa respectively for the HCTS.

Discussion
Based on CCT and TTT diagrams (Fig. 2), bainite transformation occur at 575 ℃. Therefore, ferrite in the steel coiled at 550 ℃ must have formed during the pre-coiling process, while in the case of steel coiled at 650 ℃, the PF nucleated during the coiling process. Also, the optical micrograph in Figure 3, clearly indicates that when the coiling temperature was 650 °C, the microstructure was largely composed of PF while at 550 °C it was AF and both phase Transformations are diffusion controlled [13]. The difference in the morphology of the ferrite can be attributed to the difference in undercooling and faster diffusion rate at higher coiling temperature.
In HSLA steels, Ti is added for grain refinement and precipitation strengthening [14], so dissolution of Ti in the austenite phase during the reheating process and reprecipitation during deformation and coiling process to form fine precipitates is very critical. SEM micrograph in figures 7 shows coarse TiN particles with average diameter of 4.5 μm. The TiN's role could be negligible as a strengthening mechanism because of its large size and is stable at reheating temperatures. However, in this work it was found that the TiC and Ti (S, C) precipitates responsible for the strengthening of the steel. The mechanical properties results show that LCTS exhibited better mechanical properties than HCTS except for the elongation. Several strengthening mechanisms are active in HSLA steel such as solid solution strengthening, grain boundary strengthening, dislocation strengthening and precipitation hardening [15]. The strength of microalloyed steel can be represented by equation1: Where σys is a yield strength (MPa), σ0 is the lattice friction stress (MPa), Δσss is solid solution contribution (MPa), Δσgb is grain boundary strengthening contribution (MPa), Δσppt is precipitation strengthening contribution (MPa), Δσdis is the dislocation strengthening contribution (MPa). In this study, lattice friction stress and solid solution contribution could be considered same for both samples. As mentioned above, microstructure of LCTS consisted of AF, which beneficial microstructure for strengthening due to its high-density dislocation. In other words, Δσdis increased with the decrease in the coiling temperature. In addition, both the EBSD results and optical micrograph showed that the average grain size decreased with the decrease in coiling temperature. Therefore, through the Hall-Petch relationship, Δσgb increased with the decrease in the grain size. In the present study, effect of coiling temperature on microstructure and mechanical properties was investigated and the main observation are presented below.

Conclusion
• The coiling simulation at 550 °C resulted in accicular ferrite and bainite while at 650 °C resulted in polygonal ferrite. • The average grain size decreased from 5.6 to 3.7 µm when the coiling simulation temperature was decreased from 650 ℃ to 550 ℃. • The increase in strength at low coiling temperature was attributed to grain refinement and dislocation hardening.