ISIJ International
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Forming Processing and Thermomechanical Treatment
Effects of Coiling Temperature on Microstructure and Precipitation Behavior in Nb–Ti Microalloyed Steels
Lixiong XuHuibin WuQibo Tang
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2018 Volume 58 Issue 6 Pages 1086-1093

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Abstract

This paper presents our latest studies on the effects of coiling temperature on the microstructure, precipitation behavior, and mechanical properties of an Nb–Ti microalloyed steel produced by endless strip processing (ESP) and coiled at different temperatures. The amounts of soluble elements were measured using inductively coupled plasma optical emission spectrometry (ICP-OES). The microstructure and precipitates were analyzed using SEM, EBSD, TEM, and electrolytic dissolution and filtration tests. The results revealed that large amounts of microalloying elements were still in solution before coiling. As the coiling temperature decreased from 600°C to 560°C, the content of acicular ferrites (AF) increased and the average ferritic grain size was refined from 2.01 µm to 1.29 µm, the yield strength and tensile strength of the tested steel increased by 22 MPa and 20 MPa, respectively, under the effect of microstructural strengthening. As the coiling temperature increased from 600°C to 640°C, the mass fraction of precipitates increased from 0.083% to 0.110% and the percentage of fine precipitates (smaller than 18 nm) increased from 12.2% to 14.7%; the intense precipitation strengthening effect increased the yield strength and tensile strength by 35 MPa and 42 MPa, respectively. Therefore, as the coiling temperature decreased from 640°C to 560°C, the strength of the tested steel decreased first and then increased while the elongation decreased steadily from 18.9% to 14.1% due to the increasing content of AF.

1. Introduction

Traditional thermo-mechanical controlled processing (TMCP) and microalloying technology are two major methods for refining microstructure and improving the mechanical properties of HSLA steels.1) The microstructure and mechanical properties of thermo-mechanically processed hot rolled strip steels are significantly influenced by the process parameters such as rolling ratio, rolling temperature, cooling pattern, cooling rate, and coiling temperature. Controlling the coiling temperature is the most economical and efficient way to improve the properties of steels.2,3) Zhang found that the microstructure of an X70 pipeline steel transformed form granular ferrite to bainite ferrite strip as the coiling temperature decreased.4) Xu et al. studied the influence of coiling temperature on the microalloying and properties of an (Nb, V, Ti)-containing HSLA steel, and concluded that the grain size was refined when coiling temperature dropped from 570°C to 450°C, which improved the yield strength of about 100 MPa.5)

Microalloying elements such as Nb, Ti, and V have been recognized as crucial strengthening elements in HSLA steels due to the multipath strengthening mechanism during TMCP.6,7) Nb usually facilitates grain refinement through the precipitation of carbides or carbonitrides in austenite thereby inhibiting the static recrystallisation of austenite, resulting in a fine final microstructure.8) Ti has frequently been added to HSLA steels to enhance the control of austenite and transformed ferrite grain sizes during deformation and subsequent heat treatment.9) Precipitation of Nb and Ti in Nb–Ti microalloyed steels is particularly sensitive to the coiling temperature. Militzer et al.10) clarified that precipitation is accelerated at higher coiling temperature; however, coarsening of precipitates may occur during high-temperature coiling, which leads to a decrease in strength.

The endless strip processing (ESP) has been the only fully endless hot strip production line since its first operation in 2009, and the detailed information about ESP is introduced elsewhere.11) In strip rolling based on ESP, the finish rolling process completes in tens of seconds due to the short process and the fast strip speed; hence, there is insufficient time for the microalloying elements dissolved during reheating to precipitate fully before coiling.12) The remaining dissolved microalloying elements contribute to solution strengthening and play a role in microstructural strengthening by decreasing the phase transformation temperature.13) The precipitates formed in the subsequent coiling and coiling stages contribute to precipitation hardening.14) Hence, it is crucial to optimize the microstructure and mechanical properties of microalloyed steels on the ESP production line through the control of coiling temperature.

This paper presents our latest results on the effects of coiling temperature on microstructure, precipitation behavior, and mechanical properties of Nb–Ti microalloyed HSLA steels produced on an ESP production line and coiled at three different temperatures. The findings from the present study are expected to provide theoretical guidance for the development of high strength hot-rolled steels.

2. Experimental

2.1. Materials

The chemical composition of the studied steel (in wt%) was 0.05C, 1.65Mn, 0.25Si, 0.025Al, 0.06Ti, 0.05Nb, 0.012P, 0.0008S, and Fe (balance). Nb and Ti were added for precipitation strengthening and grain refinement. The microalloyed steel discussed here was industrially produced on an endless strip processing. The cast billets were directly rough-rolled through three continuous mills with average reduction of 50% and deformation temperature in the range 1150–1050°C. After cooling to 900°C, the intermediate billets were immediately reheated to 1150°C and held for 15 s, and then continuously rolled to the desired thin strips through five finishing mills with average reduction of 30% and deformation temperature in the range 1000–850°C. Next, the strips were rapidly cooled to three different coiling temperatures (560°C, 600°C, and 640°C) and then air-cooled to ambient temperature. The technological process of the tested steel produced on the ESP line is illustrated in Fig. 1.

Fig. 1.

Schedule of the technological process of the tested steel produced on the ESP line.

Standard tensile tests were conducted at room temperature using a computerized tensile test machine (CMT5105). Longitudinal specimens were machined according to the GB/T 228.1-2010 specification to dimensions of 225 mm × 12.5 mm and gauge length of 50 mm. The mechanical properties of samples coiled at different temperatures are listed in Table 1.

Table 1. Mechanical properties of samples coiled at different temperatures.
Coiling temperature (°C)Yield strength (MPa)Tensile strength (MPa)Yield ratio (YS/TS)Elongation (%)
5606717230.9314.1
6006497030.9217.2
6406847450.9218.9

2.2. Measurement of the Soluble Elements

The amounts of microalloying elements in solution were quantitatively measured by inductively coupled plasma optical emission spectrometry (ICP-OES, Varian 715-ES). Metal filings weighing M (g) were cut from the samples and dissolved in solution of 46/2/52 (v/v/v) hydrochloric acid, stannous chloride, and water at 70°C for 3–4 h. The filtered solutions were transferred to a flask and diluted with distilled water to volume of L (mL). Based on the spectral intensities of the solutions, the mass concentrations (X, μg/mL) of microalloying elements in solution were measured by ICP-OES to within 0.005 mg·mL−1. Finally, the mass fractions (w,%) of microalloying elements in solution were calculated by the formula below:   

w= X×L M× 10 6 ×100% (1)

2.3. Microstructure and Grain Size Analysis

A field-emission scanning electron microscope (SEM, Zeiss Ultra 55) was used to observe the microstructures of the longitudinal specimens, which were polished and etched using 4% nital solution for approximately 5–10 s. The substructure and dislocation morphology were observed by an energy-filtered transmission electron microscope (TEM, FEI Tecnai F20). The foil specimens for TEM were acquired from the 1/4 center of the strip steel samples thinned to 50 ± 10 μm using silicon carbide paper, and polished in a twin-jet electro-polisher using a solution of 15/85 (v/v) perchloric acid and ethanol at 20 V and −20°C for 20–25 s. The final grain sizes were measured with a Link Opal electron backscatter diffraction (EBSD) device (Oxford Instrument HKL, Channel 5 software) at an acceleration voltage of 15 kV with step size of 0.1 μm. The specimens prepared for EBSD were sectioned longitudinally and electro-polished for 15 s at a voltage of 20 V and current of 1.5 A in an electrolyte solution of 20/80 (v/v) perchloric acid and ethanol.

2.4. Precipitate Analysis

Qualitative analysis and morphological observation of precipitates were conducted on the above-mentioned TEM equipped with energy disperse spectroscopy (EDS). The quantitative analysis and grading analysis of precipitates were conducted via electrolytic dissolution and filtration tests.15) Precipitates were extracted from the specimens (dimensions 100 × 22 × 2 mm3) by electrolysis and separation. The micrographs of these extracted precipitates were observed by SEM; chemical analysis of the precipitates was performed using an inductively coupled plasma atomic emission spectrometer (ICP-AES, TJA-IRIS). Particle size distribution of the precipitates was examined using an X-ray diffraction spectrometer equipped with a Kratky-small angle goniometer. The statistic of the particle size distribution excludes the contribution of particles larger than 300 nm.

3. Results

3.1. Solubility of Nb and Ti before Coiling

During TMCP, the precipitation rate in steels is affected by the rolling temperature, reduction, cooling rate, and the concentrations of microalloying elements in solution.16) Hence, the dissolution of Nb and Ti before finish rolling is essential for the formation of fine particles re-precipitated in the later rolling, cooling, and coiling stages, which causes intense effects in grain refining and precipitation hardening in steels. To maximize the effects of Nb and Ti and to fully dissolve the coarse precipitates, induction reheating was introduced between rough rolling and finish rolling on the ESP production line. But due to the characteristics of short process and fast strip speed, the re-precipitation behavior of the dissolved elements before coiling is still unclear.

To determine the states of Nb and Ti before coiling and to clarify the influence of coiling temperature on the amounts of the microalloying elements in solution, the tested steel was rolled on the ESP production line with interruptions and subsequently water quenched to room temperature. Samples were then cut from the strip steel after induction reheating, finish rolling, and laminar cooling with coiling temperatures of 560°C, 600°C, and 640°C (named IRH, FR, LC-560, LC-600, and LC-640, respectively), which are marked with imaginary line in Fig. 1. The mass fractions of Nb, Ti, and C in solution of these five samples were measured by ICP-OES, with the quantitative results being presented in Fig. 2.

Fig. 2.

Mass fractions of Nb, Ti, and C in solution of samples at different positions on the ESP line. (Online version in color.)

As shown in Fig. 2, precipitates in the tested steel dissolved mostly into the matrix after induction reheating (IRH), with 77.0% of the total Nb, 60.5% of the total Ti, and 88.8% of the total C in solution. After finish rolling (FR) with average reduction of 30%, Nb and Ti partially precipitated in steel by strain induced precipitation (SIP) and the mass fractions of Nb, Ti, and C in solution decreased to 64.8%, 46.5%, and 78.4%. During laminar cooling (LC), almost no Nb and Ti precipitated at the coiling temperature of 560°C due to the fast strip speed (about 3 m/s) and the fast cooling speed (about 20°C/s), so 63.4% of Nb, 44.0% of Ti, and 74.8% of C still remained in the matrix before coiling. When the coiling temperature increased to 600°C and 640°C, the amounts of Nb, Ti, and C in solution decreased slightly due to the increased amount of fine precipitates formed during laminar cooling at a slower cooling rate. Abundant Nb, Ti, and C in solution before coiling will exert a great influence on both the phase transformation process during laminar cooling and the subsequent precipitate behavior controlled by coiling temperature.

3.2. Microstructure Response

In general, the austenite will transform into polygonal ferrite (PF), quasi-polygonal ferrite (QF), and granular ferrite (GF) successively, with the decreasing of the phase transformation temperature. PF is normally formed by diffusion mechanism at a relatively high temperature, characterized by equiaxed grains with clear and straight boundaries, which is suitable for ductility but detrimental to the strength of steels. QF and GF, distinguished by anisometric grains with irregular boundaries and spherical grains with a very small size respectively, are known as acicular ferrites (AF) which is normally considered to be formed by diffusion mechanism at a low temperature slightly higher than that for upper bainite.17)

SEM images of the microstructures acquired from the 1/4 center of the longitudinal specimens, which were cut off from the finished strip steels coiled at 560°C, 600°C, and 640°C, are presented in Fig. 3. The microstructures of these three specimens at room temperature were all single-phase ferrite. Judging from the morphology of the ferrite grains, the microstructure coiled at 560°C mainly consisted of QF and GF, and polygonal ferrite (PF) was rare (Fig. 3(a)). As the coiling temperature increased to 600°C, the amount of PF increased and the amount of GF decreased (Fig. 3(b)). When the coiling temperature was 640°C, the microstructure was largely composed of PF with a small amount of QF, and the GF disappeared almost (Fig. 3(c)). The TEM micrographs of PF, QF, and GF obtained from the sample coiled at 600°C are presented in Fig. 4. Because the dislocation annihilation caused by static recovery was restrained due to the low average kinetic energy of atoms at a relatively low phase transformation temperature, there are apparent dislocation tangles and dislocation cells in the grains of QF and GF (Figs. 4(b) and 4(c)), which can significantly increase the tensile strength but decrease the ductility of steels.

Fig. 3.

Microstructures of finished strip steels coiled at (a) 560°C, (b) 600°C, and (c) 640°C.

Fig. 4.

TEM micrographs of (a) PF, (b) QF, and (c) GF obtained from the sample coiled at 600°C. (Online version in color.)

The EBSD micrographs containing grain boundary maps and band contrast maps of the microstructures coiled at 560°C, 600°C, and 640°C are shown in Figs. 5(a), 5(c), and 5(e), respectively, where the black lines denote ferrite grain boundaries that are greater than 10°. The corresponding grain size distribution maps are exhibited in Figs. 5(b), 5(d), and 5(f) with valid grain diameter ranging from 0.2 μm to 15 μm and interval increment 0.2 μm. The relative frequency (%) refers to the percentage of grains within one interval relative to the total number of grains. As shown in Figs. 5(a) and 5(b), when coiled at 560°C, the grain diameter was mainly distributed (95.3%) in the range less than 3.0 μm, and the mean diameter and medium diameter were 1.29 μm and 1.05 μm, respectively. As the coiling temperature increased to 600°C, the amount of large grains (PF) increased and small grains (GF) decreased (Figs. 5(c) and 5(d)), which reduced the total number of grains from 685 to 541 in the same windows and increased the mean diameter and medium diameter to 2.01 μm and 1.78 μm, respectively. As the coiling temperature increased to 640°C, the ferritic grains became larger, with the average grain diameter becoming 2.97 μm (Figs. 5(e) and 5(f)).

Fig. 5.

EBSD micrographs (grain boundary map and band contrast map, black lines indicate the ferrite grain boundary (θ>10°)) and grain size distribution maps of finished strip steels coiled at (a), (b) 560°C, (c), (d) 600°C, and (e), (f) 640°C. (Online version in color.)

3.3. Precipitation Behavior

Figures 6 and 7 show TEM micrographs of precipitates in samples coiled at 640°C and 600°C, respectively. After coiling at 640°C followed by air-cooling to room temperature, many fine precipitates were observed in foils. The spherical precipitates in Fig. 6(a) and the cubic precipitates in Fig. 6(c) were confirmed by EDS analysis to be (Ti, Nb)(C, N) and Ti(C, N) (as shown in Figs. 6(b) and 6(d)), respectively. The appearance of (Ti, Nb)(C, N) results from the interchangeability of Ti and Nb in the precipitate lattice because of their similar crystal structures and lattice parameters.18) Figures 6(e)–6(h) show high resolution transmission electron microscopy (HRTEM) images coupled with the corresponding inverse fast Fourier transform (IFFT) patterns of (Ti, Nb)(C, N) and Ti(C, N) precipitates. The diffraction patterns in Figs. 6(f) and 6(h) reveal that TEM observation was along the [111] zone axis of ferrite. As the coiling temperature decreased to 600°C, both the amounts of spherical (Ti, Nb)(C, N) and the cubic Ti(C, N) appear to be fewer relative to that at the coiling temperature of 640°C (Fig. 7). The difference in precipitation behavior can be attributed to the kinetics of precipitation.

Fig. 6.

TEM micrographs of precipitates in sample coiled at 640°C showing (a) spherical (Ti, Nb)(C, N) and (c) cubic Ti(C, N); (b) and (d) EDS spectra from precipitates in (a) and (c); (e) and (g) are the HRTEM images of (Ti, Nb)(C, N) and Ti(C, N), (f) and (h) are the corresponding IFFT patterns, respectively. (Online version in color.)

Fig. 7.

TEM micrographs of precipitates in sample coiled at 600°C showing (a) spherical (Ti, Nb)(C, N) precipitates, and (b) cubic Ti(C, N) precipitates. (Online version in color.)

For further quantitative analysis and grading analysis, precipitates were extracted entirely from the specimens by electrolysis and separation. SEM images of the partial precipitates extracted from the samples coiled at 640°C and 600°C are shown in Fig. 8. The mass fractions of Ti, Nb, C, and N present in these extracted precipitates, measured by ICP-AES, are presented in Table 2. As the coiling temperature increased from 600°C and 640°C, the mass fractions of Ti and Nb in the extracted precipitates increased by 18.3% and 16.0%, respectively, and the total mass fraction of the precipitates increased from 0.083% to 0.110%. This result is in accordance with the observations in Figs. 6 and 7.

Fig. 8.

SEM images of precipitates extracted from samples coiled at (a) 640°C and (b) 600°C.

Table 2. Chemical analyses of the precipitates extracted from samples coiled at (a) 640°C and (b) 600°C.
Coiling temperature (°C)Mass fraction of elements contained in the extracted precipitates (wt%)
TiNbCN
6400.0470.0390.0190.0050.110
6000.0360.0310.0120.0040.083

According to previous studies,19) higher coiling temperature can promote precipitation and also lead to a higher coarsening rate of precipitates. The particle size distribution of precipitates extracted from samples coiled at 640°C and 600°C is exhibited in Fig. 9, where the frequency distribution (f(D)) refers to the average mass percentage of precipitates included in each nanometer interval. As shown in Fig. 9(a), when the coiling temperature increased from 600°C to 640°C, the percentage of precipitates larger than 96 nm increased from 19.3% to 24.5%, increasing the average size of precipitates from 70.3 nm to 71.1 nm. Meanwhile, the percentage of precipitates smaller than 18 nm also increased from 12.2% to 14.7%; this result is in agreement with the observation of increased number of fine precipitates in Fig. 8(a) relative to Fig. 8(b). In addition, Fig. 9(b) reveals that the precipitates were generally fine in the tested steel, with 80.7% and 75.5% of total precipitates smaller than 96 nm when coiled at 600°C and 640°C, respectively.

Fig. 9.

Frequency distribution (a) and cumulative mass fraction (b) of precipitates extracted from samples coiled at 640°C and 600°C. (Online version in color.)

The fine nanosized precipitates nucleated during the coiling process are expected to induce significant strengthening in steels,20) which can be quantified by the Ashby-Orowan model according to the theory of Gladman21) and described as   

σ= 10μb 5.72 π 3 2 r f 1 2 ln( r b ) (2)
where σ denotes the strength addition attributed to precipitation strengthening (MPa); r is the radius of precipitates (nm); μ is the shear modulus (equal to 80.26 × 103 MPa for ferrite low carbon steels); b is the Burgers vector (equal to 2.48 × 10−4 μm); and f represents the volume fraction of precipitates. According to Eq. (2), the strengthening effect of precipitates is proportional to the square root of the volume fraction and inversely proportional to the particle size of precipitates. The dependences of strength addition on the volume fraction and the particle size of precipitates for Nb–Ti microalloyed HSLA steel are exhibited in Fig. 10. The strength addition increased slowly with increasing volume fraction of precipitates when the particle size was larger than 20 nm, indicating that the precipitation strengthening effect is essentially attributed to precipitates smaller than 20 nm.
Fig. 10.

Dependence of the strength addition on the volume fraction and particle size of precipitates. (Online version in color.)

4. Discussion

Above all, the improvement of mechanical properties for the present ultra-low carbon Nb–Ti microalloyed steel, produced on the ESP production line, depends mainly on the combined effects of microstructural strengthening and precipitation strengthening, which are very sensitive to the coiling temperature and the amount of microalloying elements such as Nb and Ti in solution before coiling.

4.1. Microstructural Strengthening at Low Coiling Temperature

Due to the characteristics of the ESP, most precipitates dissolved into the matrix after induction reheating, while small quantities of the dissolved elements re-precipitated during the subsequent finish rolling and laminar cooling process, especially at fast cooling rates. It is known that higher coiling temperature corresponds to lower cooling rate.22) Based on the process parameters of the ESP production line, the laminar cooling rates for the strip steels coiled at 560°C, 600°C, and 640°C were calculated to be 18.5°C/s, 15.3°C/s, and 12.4°C/s, respectively. when the coiling temperature decreased from 600°C to 560°C, the tested steel contained more dissolved microalloying elements during laminar cooling (as shown in Fig. 2) due to the higher cooling rate. Among them, the soluble Nb can decrease the temperature of the γα phase transformation by retarding the movement of phase interface by solute drag effect.23,24,25) Moreover, the γα phase transformation temperature will also be decreased due to the greater degree of undercooling at a higher cooling rate.

Based on earlier studies,26) the AF structure contributes to the tensile strength and grain refinement increases yield strength of steels. The decrease of the phase transformation temperature increased the content of AF and refined the average grain size of ferrite from 2.01 μm to 1.29 μm. Hence, under the intense microstructural strengthening effects, the yield strength and tensile strength of the tested steel increased by 22 MPa and 20 MPa, respectively, as the coiling temperatures decreased from 600°C to 560°C (Table 1). However, the increasing content of AF deteriorated the ductility of tested steel, so the elongation decreased steadily from 18.9% to 14.1% as the coiling temperatures decreased from 640°C to 560°C.

4.2. Precipitation Strengthening at High Coiling Temperature

When the coiling temperatures increased from 600°C to 640°C, the diffusion rate of microalloying elements increased and the cooling rates during both laminar cooling and coiling process slowed down, providing advantageous condition and additional time for the formation of precipitates, so the total mass fraction of precipitates increased from 0.083% to 0.110% and the average size of the precipitates increased from 70.3 nm to 71.1 nm. Since there were sufficient microalloying elements still in solution before coiling, the higher coiling temperature not only coarsened the size of the existing precipitate, but also increased the nucleation rate of the new precipitates due to the higher diffusivity of microalloying elements. So as coiling temperatures increased from 600°C to 640°C, the percentage of the coarse precipitates (larger than 96 nm) increased from 19.3% to 24.5%, and that of fine precipitates (smaller than 18 nm), which provides an essential contribution to the precipitation strengthening effect (according to Fig. 10), increased from 12.2% to 14.7% simultaneously.

Natarajan et al.27) reported that the microstructure of a Ti–Nb microalloyed steel coiled at lower temperature predominantly consisted of bainitic ferrite with lower yield strength, while steel coiled at higher temperature consisted of polygonal ferrite and extensive precipitation of carbides, and was characterized by higher yield strength. This indicates that the effect of precipitation strengthening is more intense than the microstructural strengthening for HSLA steels. Hence, even though the higher coiling temperature reduced the proportion of AF and coarsened the ferrite grains, the intense precipitation strengthening effect increased the yield strength and tensile strength by 35 MPa and 42 MPa, respectively, as the coiling temperature increased from 600°C to 640°C.

5. Conclusions

(1) Due to the short process and fast strip speed of the ESP production line, 63.4% of Nb, 44.0% of Ti, and 74.8% of C remained in solution before coiling at the coiling temperature of 560°C.

(2) As the coiling temperature decreased from 600°C to 560°C, the more soluble Nb and the greater degree of undercooling decreased the γα phase transformation temperature, which increased the content of AF and refined the average ferritic grain size from 2.01 μm to 1.29 μm. The yield strength and tensile strength of the tested steel increased by 22 MPa and 20 MPa, respectively, under the effect of microstructural strengthening.

(3) As the coiling temperature increased from 600°C to 640°C, the cooling rates during both laminar cooling and coiling process slowed down and the nucleation rate of precipitates increased due to the higher diffusivity of the sufficient dissolved elements. Hence, the total mass fraction of precipitates increased from 0.083% to 0.110% and the percentage of fine precipitates (smaller than 18 nm) increased from 12.2% to 14.7%. The intense precipitation strengthening effect increased the yield strength and tensile strength by 35 MPa and 42 MPa, respectively.

(4) Under the combined effects of microstructural strengthening and precipitation strengthening, the yield strength and tensile strength of the tested steel decreased first and then increased, as the coiling temperatures decreased from 640°C to 560°C, while the increasing content of AF reduced the elongation of steel monotonously from 18.9% to 14.1%.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project no. 51474031).

References
 
© 2018 by The Iron and Steel Institute of Japan
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