Effect of Ultra Fast Cooling Coiling Temperature on Microstructures and Properties of X80 Pipeline Steel

Effect of ultra fast cooling coiling temperature on microstructures and properties of X80 pipeline steel was studied adopting the detection means of energy dispersive spectrometer, scanning and transmission electron microscopy. Result shows that the toughness increases with the increasing of ultra fast cooling coiling temperature. The yield strength decreases firstly and then increases with the increasing of ultra fast cooling coiling temperature. The yield strength reaches the minimum value at the temperature of around 350 °C.


Introduction
There is a great demand on pipeline steel for oil and gas transmission, especially in P.R.China [1][2][3] due to its high strength and good toughness [4]. At the present, thermo-mechanical controlled processing (TMCP) is the main manufacturing technology to obtain better performance of X80 pipeline steel with refining grains [5][6][7][8][9][10][11][12][13][14].The basic mean to controlled rolling is that the big pressure rate rolling at low temperature which may beyond the equipment capability.
The core concept in ultra fast cooling which is also called NG-TMCP (new generation thermomechanical controlled processing) includes [15]: 1) achieving the strain accumulation in austenite during the continuous rolling at relatively high rolling temperatures; 2) ultra fast cooling after rolling to keep the work hardening of austenite; 3) stop cooling temperature near dynamic phase transition temperature; 4) cooling route control according to the requirements for microstructure and properties of the steel. The present work aims to utilize ultra fast cooling process to produce the X80 pipeline steel and investigate its microstructure characteristics and mechanical properties.

Experimental Procedures
The specimens were 17.5 mm-thick plates. The chemical compositions are listed in Table 1. The technical parameters of TMCP for pipeline steel are shown in Table 2. Specimens for tensile tests were taken from the rolled plates in the transverse direction. Tensile tests were conducted at room temperature at a crosshead speed of 5 mm/min on a SHT4106-1000KN servo-hydraulic machine. Drop weight tear test (DWTT) specimens were machined into the dimensions of 75 mm× 17.5mm× 305 mm (T-L). The DWTT test was performed at -15°C using a JL-40000 impact machine. Full size Charpy impact specimens were machined from the rolled plates in the transverse direction with their notch parallel to the rolling direction (T-L), and impact tests were done at tempreatures from 20°C to -100 using a ZBC- mechanically ground to about 50 μm in thickness. Then, these specimens were twin-jet electropolished at a voltage of 40 V. The electrolyte consisted of 10 vol% perchloric acid and 90 vol% glacial acetic acid.

Mechanical Properties
Mechanical properties are shown in Table 3. It is seen that the mechanical properties of all specimens meet the requirements of X80 pipeline steel standard. The yield ratio of Specimen 3# is the lowest. Result of drop weight tear test are shown in Table 4. It can be seen that all specimens' DWTT datum meet the requirements of X80 standard. Tear area of Specimen 4 # is relatively small. Full size Charpy impact test results are shown in Table 5. It is concluded that all specimens' mechanical properties meet the requirements of X80 standard.     Figure 3 gives the microstructure morphology of Specimens2# and 4# for X80 pipeline steel using TEM observation.
It is seen that Specimen 2 # dislocation is developed, while precipitation is less. The size precipitation is relatively fine and uneven distributed. The types of precipitation is Ti, Nb carbonitride precipitation. It can be known that Ti content is higher in rectangular particle, Nb content is higher in round particles. Nb/Ti ratio is 0.3.

Figure 4. Specimen 4 # TEM observations
The grain boundary of Specimen 4 # has plenty of dislocation which is widely distributed in the range of observation. Figure 4(c) shows that most precipitation particle size is small (around 30nm), and Figure  4 (d) shows a single round particle whose types of precipitation are Ti, Nb carbonitride precipitation composite. Most precipitation particle size is smaller, most typical particles as shown in the picture on the right around 30 nm,A single round particles can be observed, types of precipitation mostly Ti, Nb carbonitride precipitation composite, Through the EDS analysis It can be known that Nb content is high in circular particle Ti content is low and Nb/Ti ratio of 15.1.

Relationship between X80 Pipeline Steel Coiling Temperature and Mechanical Properties
The coiling temperature of the samples are shown in table 2. It is seen toughness increases with the increase of coiling temperature, and the yield strength decreases firstly and then increases for X80 pipeline steel. Moreover, at about 350 °C , the lowest yield strength presents, yield ratio firstly decreases and then increases with the rising of coiling temperature. Comprehensive analysis shows that coiling temperature at about 400 °C can deliver better comprehensive mechanical properties.

Relationship between X80 Pipeline Steel Cooling Rate and Mechanical Properties
The cooling speed is greater than 30 degrees per second to ensure the passing through in austenite region in a tiny period of time, which results in work hardened austenite before transformation, which can lay a solid foundation for the control of phase transformation in work hardened state. The faster the cooling rate, the finer the microstructure can be obtained. Microstructure 1 # relatively is bulky.
As compared with the microstructure observation, it is seen that the M/A island near dislocation plug and launch dislocation phenomenon, dislocation in grain boundary caused the grain of work hardening and grain boundary of stress concentration. In addition, precipitation particle counterpoint dislocation pinning effect is significant. And with the increase of coiling temperature, precipitation particles get more and more, and also gradually become evenly distributed. But with the increase of coiling temperature, precipitation particle size increases gradually.

Conclusions
The relation between microstructure characteristics and mechanical properties of X80 pipeline steel by ultra fast cooling process was researched, and the following conclusions are drawn. a) As the coiling temperature increases, so does the impact toughness of X80. Its yield strength decreases first and then decreases, the lowest point presents at around 350 °C , and yield ratio first decreases and then increases with the rising of coiling temperature. Comprehensive analysis shows that, at coiling temperature of about 400 °C , better comprehensive mechanical properties can be achieved. b) Cooling speed is greater than 30°C /s to ensure the passing through in austenite region in a tiny period of time, resulting in work hardened austenite before transformation, which can lay a solid foundation for the control of phase transformation in work hardened state. The faster cooling rate, the finer microstructure. c) Types of precipitation are mostly Ti, Nb carbonitride precipitation composite. The EDS analysis shows that Ti content is higher in rectangular particle,Nb content is higher in round particles. d) With the increasing of coiling temperature, precipitation particles get more and more, and also gradually become evenly distributed. But with the increase of coiling temperature, precipitation particle size increases gradually.

Acknowledgments
The work was financially supported by Natural Science Foundation of Guangdong Province (no.2017A030313276) and Major Project of Department of Education of GuangDong Province (no.2016KZDXM046)