Effect of Finish Rolling Temperature on Microstructure and Mechanical Properties of High Grade Pipeline Steel
TANG Xing-chang, KANG Yong-lin, BO Yan-yan
(School of Materials Science and Engineering, State Key Laboratory for Advanced Metals and Materials,
University of Science and Technology Beijing, Beijing 100083, China)
Abstract: The correlation among finish rolling temperature (FRT), microstructure and mechanical property of the high grade pipeline steel was investigated in this study. The microstructure of the steels with different finish rolling temperatures was observed with scanning electronic microscope (SEM) and transmission electronic microscope (TEM). The martensite/ austenite (M/A) islands distribution was fixed by colour metallography, and the mechanical properties of the steels were tested with quasi-static tensile testing machine. The result shows that the fraction of M/A island increased with the finish rolling temperature decreasing, and when the finish rolling temperature is 800℃, the mechanical properties are the best. Key words:high grade pipeline steel; finish rolling temperature; martensite/austenite island
1 Introduction
Long distance gas and oil transmission pipelines have been widely used all over the world and also such pipelines expand rapidly in China [1]. But now it is under a strong pressure to reduce its investment and operation costs. The widely accepted way to achieve this goal is to increase the operation pressure. When the pressure increase, the high grade pipeline steels should be used [2-3]. In recent years, the requirements of pipeline steel and steel grade enhance unceasingly. For the purpose of safety and economy, the grade above X100 pipeline was developed, and X120 production testing also started [4-5]. In modern pipeline steel technology, the target is to obtain excellent comprehensive properties through innovations and optimizations on chemical composition design, microstructure design and related thermo-mechanical controlled processing (TMCP), etc [6].
2 Material and Experimental Methods
2.1 Material
The experimental melt of low-carbon microalloyed steel was prepared by a 50-kg vacuum induction melting furnace, and the cast ingot was forged to 95mm (thick)×100mm (wide)×110mm (long). The chemical compositions are listed in Table 1. The steels were reheated at 1200℃, and then rolled in two stages. The First-rolling stage for the steels was started at 1150℃; then the second-rolling stage was started at 930℃ and finished rolling at 780℃, 800℃ and 830℃. The steels were cooled to 360℃ at 40℃/s and finally air cooled to room temperature. The rolling process is listed in Table 2.
2.2 Experimental methods
The specimens for tensile and impact tests were cut from the middle of the rolled plates, for the finish rolling temperature were different in the transversal direction. Tensile tests were operated at room temperature with the tensile speed of 5mm/min in an CMT-4305machine. The test of Charpy impact energy, subsize specimens (5×10×55mm) were used in accordance with the standard method of ASTM E8M-04. The microstructures of hot rolled plates were examined by scanning electronic microscope (SEM) and transmission electron microscopy (TEM). The samples for SEM were mechanically polished and etched in 4%nital, and then examined. For TEM observation, thin foils were prepared from 400μm thick discs, which were firstly mechanically thinned to about 50μm and then electropolished by a twin-jet electropolisher in a solution of 10% perchloric acid. Thin foil samples were observed under a JEM 2100TEM.
Table 1 Chemical compositions of the steels investigated (wt%)
C Si Mn S P Cr Cu+Mo+Ni Ti+Nb B Fe
Ceq Pcm
0.06 0.25 1.7-2 0.006 0.007 0.2-0.3
0.93 0.094 0.002 Balanced 0.47 0.21
Table 2 The rolling process of the steels investigated
Reheat temperature
Start rolling temperature
Non-Recrystallization start rolling
temperature
FRT CT
Cooling rate
1200℃ 1150℃ 930℃ 780/800/830℃℃℃ 360℃ 40/s ℃
3 Result and Discussion
3.1 Microstructures
The microstructures of the steels with different
finish roll temperatures are shown in Fig.1 (a)-(c), respectively. It could be found that different micro-
structures contained GB and BF was available. The average grain size decreases with the decreasing finish
rolling temperature for rolling processes used in the present work. And it also found that the proportion of
the M/A island increased with the finish rolling tem- perature decreasing. The reason for such variation of
grain size with rolling temperatures could be related to the recrystallization of the austenite grains. The ferrite grains are mostly the result of the austenite deformation
below the recrystallization temperature, which increase the nucleation of ferrite phase. So the average grain
(a) 830℃; (b) 800℃; (c) 780℃. Fig. 1 SEM micrographs of the three steels
size is obtained by controlling the rolling temperature. The addition of a little microalloy elements like Nb and Ti result in the formation of precipitates, which could effectively prevent the grain growth [7].
The distribution of M/A islands under the diffident finish rolling temperatures are shown in Fig.2 (a)-(c), respectively. It could be found that the proportion and the average size of M/A islands was increased with the decreasing of finish rolling temperatures from Fig.2 (a)-(c). The contents of M/A islands are 0.67%, 0.94%, 1.82% and the average sizes of M/A islands are about 0.5μm, 0.7
μm, 1
μ
m with finish rolling temperature of 830℃, 800℃ and 780℃.
(a) FRT 830℃; (b) FRT 800℃; (c) FRT 780℃. Fig. 2 Distribution of M/A island of the different finish
rolling temperature
The TEM micrographs of the steel with the finish rolling temperature of 800℃ are shown in Fig.3 (a)-(f),
temperature 800℃
respectively. Fig.3 (a) shows a bright-field TEM image of highly dislocation density near the grain boundary. Fig.3 (b) shows the width of the bainite ferrite (BF) lath is 200nm on average. The second phases are presented along bainite ferrite lath boundary, the reason for which is thought to be that carbon was diffused into austenite after the ferrite nucleation, which increased the carbon concentration in austenite and made the temperature of γ→α fall. Some austenite transformed into martensite as continuous cooled to lower temperature, and some untransformed austenite was retained 0. Fig.3 (c) shows the M/A island in the lath. The selected area diffraction pattern (SADP) of M/A island is shown in Fig.3 (d). The high grade pipeline steel contains several microalloy elements which can form various types of precipitates. The Nb and Ti have an affinity for the formation of carbonitrides. Fig.3 (e) shows the microstructure of the precipitates for the sample which finish rolling temperature is 800℃. The precipitates were found to be in size range of 20nm around. The chemical constitution of these precipitates was investigated by the EDS analysis. The precipitates were identified as complex cabinetries mixed with Nb and Ti
as shown in Fig.1 (h).
3.2 Mechanical properties
The mechanical properties of experiment steel test results are given in Table 3. The sample tempered at 550℃ is designated as T (tempered). At the finish rolling temperature of 800℃, the strength value obtained is 850MPa YS, and 950MPa TS, with 18% Elongation, without tempered. The results show that the optimal mechanical property was obtained in the experiment steel, which finish rolled at 800℃ and tempered at 550℃. At the same time, the more quantity
of sulfur in the steel, the lower impact ductility of the steel. In general, the sulfur content of high grade pipeline steel which is produced in Steel mills is lower than 10ppm. And the sulfur content of the experiment steel is 60ppm because of the limited conditions. The impact ductility of the high grade pipeline steel from the steel works would be greatly improved. The grain size decreased with the finish rolling temperatures decreasing. The size and proportion of M/A islands increased with the finish rolling temperatures decreasing. The strength of test steel depends on microstructure, grain size and M/A islands content.
Table 3 Mechanical property of the steel investigated of
different finish rolling temperature
FRT/℃Yield
strength
/MPa
Tensile
strength
/MPa
YS/TS Elongation
/%
CVN
(-20)/J
℃
780 810 1030 0.79 16 106 800 850 950 0. 18 92 830 800 1010 0.79 17 104 780(T)870 930 0.94 17 96 800(T)900 980 0.92 17 90 830(T)860 930 0.92 18 90 T-tempered at 550℃.
4 Conclusions
1) The results indicate that the steel with low finish rolling temperature has the smaller grains and
M/A island size.
2) The strength of pipeline steel depends on microstructure, grains size and the size and distribution M/A islands.
3) The optimal mechanical property was obtained in the experiment steel , which finish rolled at 800℃and tempered at 550℃. Therefore, the appropriate finish rolling temperature should be determined at about 800℃ in industrial production.
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