Effect of cooling temperature on microstructure and mechanical properties of Ti microalloyed steel

Ti is a relatively inexpensive alloying element. Ti micro-alloying enables steel to have higher strength, which has already been applied to steel products in many domains, including engineering machinery. In this work, the effects of cooling temperature upon the mechanical properties and microstructure of Ti microalloyed steel were researched by OM, TEM, tensile and impact tests. The consequences show that as the final cooling temperature (FCT) decreased, the steel plate microstructure changed from PF + P + B (less) to GB + PF (less)+ P (less), and the grain size decreased obviously. As the temperature further decreased to 607°C, the intragranular dislocation tangles and subgrains increased significantly. The size and number of large TiC precipitates in the steel plate decreased with the decrease of the final cooling temperature. Additionally, the mean size of precipitates decreases gradually. As the FCT varied between 672°C and 645°C, the steel plate strength and low temperature toughness were significantly improved. As the temperature is further reduced to 607°C, the strength increase was reduced. Due to the improvement of grain uniformity, the low temperature toughness was further enhanced to 156 J.


Introduction
In mining and mineral processing, chemical metallurgy, construction and transportation industries, highstrength steel is extensively applied to the manufacture of engineering machinery and vehicle structural parts.At present, in the domain of engineering machinery, high-strength steel mainly uses Nb microalloying as the main technical means to ensure strength and toughness.However, with the intensification of market competition, the cost of steel needs to be further reduced, alloy reduction has become the focus of research, and Ti has been widely considered as a cheaper microalloying element.In recent years, the influence of Ti on phase transition, precipitates and properties have been widely studied [1][2][3] .
The cooling process exerts a prominent effect on precipitates and properties of titanium-containing steels.The distribution and morphology of precipitated phases in as-cast steel (0.08C-0.2Mo-0.37Ti)were investigated at disparate cooling rates [4] .These results showed that as the subcooling degree increases, the size of TiC particles decreases, and the TiC nucleation location grain boundaries shift to the grain interior.García-Sesma et al. [5] investigated the influence of final cooling temperature (FCT)/coiling temperature on the organization and properties of Nb microalloyed steels by adding different contents of Ti.These results illustrated that the steel hardness increased significantly with increasing Ti content, which was mainly due to vast quantities of small-sized (< 3 nm) precipitated phases in the steel.Meanwhile, the coiling temperature affects the steel strength by influencing the level of precipitation strength.
To further reduce the production cost of steel, the micro-alloying element Ti was added to study the influence of FCT on phase transformation, precipitation and mechanical capacities of steel plates.

Test materials and methods
Steel ingots are melted in one lab vacuum induction melting stove with ingot dimensions of 120 mm × 120 mm × 250 mm.Table 1 shows the chemical component.The ingot was heated in a muffle stove at 1200°C for 1 h.The plate was rolled on a φ550 two-roll test mill, using a two-stage rolling method.As for the intermediate billet, its thickness reached 48 mm, while the thickness of the rolled steel plate reached 12 mm.After rolling, the steel plate entered laminar cooling.After cooling, the steel plate was immediately put into the preheating muffle furnace and slowly cooled to room temperature.Table 2 is the test process.The tensile test was carried out on a Zwick / Roell Z600 tensile tester, and the sample was a φ10 ratio sample.The -20°C KV2 impact test was carried out on the SANS ZBC2602 impact tester, and the sample size was 10 mm × 10 mm × 55 mm.The metallographic structure was observed under Zeiss optical microscope.The fine structure and carbon extraction replica precipitates were observed under the Tecnai G 2 20 transmission electron microscope.

Microstructure
Figure 1 displays the test steel microstructure.It illustrates that the grain size declines significantly as the FCT decreases.By comparing Specimens 1 #, 2 # and 3 # of the steel plates, the microstructures vary from PF + P + B (less) to GB + PF (less) + P (less) when the FCT is lowered from 672°C to 645°C.As temperature further decreases, the microstructure does not change significantly and remains GB + PF (less) + P (less), indicating that 607 ~ 645°C is in granular bainite transformation.According to the fine structure of the test steel shown in Figure 2, when the FCT is 672°C, the grain boundaries of Sample 1 # are relatively flat, with lamellar organization at the boundaries of the grain and low dislocation density in grains.While the FCT was lowered to 645°C, Sample 2# showed a large quantity of irregularly shaped grain boundaries with vast quantities of dislocations distributed within the grains.
While the FCT was lowered to 607°C, a large number of dislocation entanglements and high-density dislocation subgrains appeared in the microstructure of Sample 3#.Also, subgrain boundaries of slaty bainite ferrite were observed.The dislocation density of Specimen 3# significantly exceeded that of Specimens 1 # and 2 #.As the FCT decreases, the dislocation density increases and the dislocation content increases significantly.This is because temperature affects the thermal motion of atoms, in turn, the driving force of dislocation motion is affected.If the temperature is higher, the driving force of dislocation motion will be greater, and the speed of dislocation motion will be faster.Under the same rolling conditions, as the FCT of the steel plate decreases, the lower the slow cooling temperature is, the lower the driving force of dislocation motion within the grain is, because more dislocations are pinned and entangled in the precipitated phase to be retained [6] .At the same time, the rate of dislocation movement from the grain to the grain boundaries slowly decreases due to the decrease in driving force, and more dislocations are retained inside the grain.

Precipitates
The morphology of the precipitated phases of the test steel is shown in Figure 3. Besides, the TiC precipitated phases of those test steels are spherical at different FCTs.When the FCT is 672°C, the mean size of the TiC precipitated phase within Sample 1 # is about 18 nm, containing a large-size precipitated phase with a diameter of 30 nm.When the FCT was lowered to 645°C, the average size of TiC precipitated phases in Samples 2 # and 3 # was about 11 nm, with large-size precipitated phases with a diameter of about 15 nm.When the FCT was lowered to 607°C, the diameters of the large-size precipitates were still about 15 nm.Although the larger-sized precipitated phase still exists in the steel plate, the quantity is greatly reduced, and the mean size reaches about 9 nm.As the FCT decreases, the size and the quantity of the larger TiC precipitated phase in the steel plate decrease, and the mean size of the precipitated phase decreases step by step.This is because when the FCT is lower, and the degree of austenite subcooling is greater, the driving force of precipitated phase nucleation will be greater, the rate of precipitated phase nucleation will be higher, and the size of the precipitated phase will be smaller.Synchronously, if the slow cooling temperature of the cooled steel plate is lower, the amount of dislocations will retain greater.These large amounts of dislocations provide a lower energy nucleation location for the precipitated phase, which further increases the nucleation rate of the precipitated phase [7] .In addition, the lower the slow cooling temperature of the steel plate is, the smaller the range of atomic diffusion is, and the smaller the number of atoms available for the formation and growth of the precipitated phase is.The increase in the nucleation rate exacerbates the competition for the atoms needed for the precipitation process, bringing about a decline in the size of the grain and the average grain size of the large-size precipitated phase.

Mechanical properties
In terms of the tested steels, their mechanical capacities are shown in Table 3.When the FCT is lowered from 672°C to 645°C, the yield strength varies between 572 MPa and 668 MPa, and the tensile strength varies between 653 MPa and 772 MPa.Besides, the impact energy at 20°C is increased from 53 J to 118 J, which is a significant improvement in the mechanical properties.This is on account of the FCT reduction.In terms of the steel plate, the grain size is greatly refined, the size of the precipitation phase is reduced, and the dislocation content increases, which enhances the fine grain reinforcement, precipitation reinforcement and dislocation reinforcement, so that the strength of the steel plate is greatly improved.At the same time, grain refinement is capable of effectively improving the crack extension work, and the reduction of the size of the precipitated phase can reduce its harm to the toughness of the steel plate, thus improving the low-temperature toughness.When the FCT continues to reduce to 607℃, the yield strength continues to increase to 692 MPa, tensile strength increases to 807 MPa, and the strength increase is reduced.This is due to the reduction in the FCT from 645°C to 607°C.The reduction in grain refinement and precipitation phase refinement of the steel plate provides a limited strength increment.The grain refinement was more uniform in Specimen 3 # as compared to Specimen 2 # and hence the impact energy was further increased to 156 J at -20°C.

Conclusion
In this paper, the impact of cooling conditions upon the properties of Ti microalloyed steel was detected by analyzing microstructure and precipitation phases.The conclusions of this research are listed.1) With the decrease of FCT, the grain size decreases significantly.When the FCT was lowered from 672°C to 645°C, the steel plate organization changed from PF + P + B (less) to GB + PF (less) + P (less).As the temperature deeply decreases to 607°C, the phase composition does not change significantly, but the intracrystalline dislocation entanglement and subgranular grains increase significantly.
2) As the FCT decreases, the size and number of larger TiC precipitated phases in the steel plate decrease, and the mean size of the precipitated phases is reduced step by step.
3) When the FCT was lowered from 672°C to 645°C, the strength and toughness improved significantly.As the temperature is deeply lessened to 607°C, the increase in strength decreases.The low temperature toughness was further increased to 156 J due to the improvement of grain uniformity.

Table 3 .
Mechanical properties of test steels.