X80 pipeline steel produced by hot continuous rolling

The static recrystallization behavior of the deformed austenite of the thick gauge X80 pipeline steel produced by the 2160 hot continuous rolling was studied through double-pass compression experiments , and the production process was improved and optimized according to the experimental rules . Through mechanical stretching , impact and drop weight tests, the comprehensive performance of the X80 steel produced after the improved process was tested , and the microstructure of the X80 steel coil was observed and analyzed by optical microscope, scanning electron microscope and transmission electron microscope. The results show that the deformation temperature is the most important factor The main factor of the static recrystallization behavior of austenite; the precipitation of microalloy carbonitride inhibits the progress of recrystallization, so that the softening rate curve appears a plateau; the static recrystallization activation energy of X80 pipeline steel is calculated by regression of experimental results as 380 kJ.mor \ and discussed the rationality of the results based on literature research. Through process improvement and optimization, the microstructure of the X80 steel coil produced is fine and uniform, showing typical acicular ferrite characteristics; the precipitated phase mainly contains composite ( TiNb )(GN) and a single NbC; the tensile properties of X80 steel coil rod specimens have a large margin compared with the relevant standards, especially in terms of impact and drop weight performance, it shows good low temperature toughness .

Key words pipeline steel ; continuous rolling ; static recrystallization ; microstructure ; mechanical properties

As another major project of our country in the 21st century, the launch of the second pipeline project of the West-to-East Gas Pipeline has raised the development of high-grade pipeline steel in China to a new level . The design pressure of the pipeline is as high as 12MPa , and the main line of the 4945 Ridge is all made of X80 high-strength pipeline steel . At present , the X80 spiral submerged arc welding adopts the thickest specification of hot-rolled coil ( 18.4mm), and the comprehensive mechanical properties are strictly required. Product specifications and performance Once completed, the Pakistan pipeline, which represents the highest level of X80 hot-rolled coils , will become the world's longest single-phase project with the largest gas transmission volume, and will become the main beam of China's increasingly perfect natural gas pipeline network.
In the previous literature, although the trial production of X80 pipeline steel in the laboratory and the field production of thin gauge products ( 16mm ) Partial research has been carried out on the microstructure and properties of the steel, but the thick specification X80 pipeline steel (18.4mm) must not only ensure the strength of the steel, but also ensure excellent low-temperature toughness and dynamic tear resistance . Control, and post-rolling cooling control put forward more stringent requirements, it is very difficult to achieve stable production in large quantities . The law of recrystallization plays a vital role in the final structure and performance of the product, but it has not been paid enough attention in the previous research on the structure and performance of pipeline steel . Therefore, this paper combines the process of producing X80 pipeline steel with 2160 wide-band steel hot rolling production line , based on the previous research, and improved the experimental parameters such as pass deformation and deformation rate, which is more in line with the actual production conditions on site . The research on the recrystallization law and the optimization of the process have significantly improved the microstructure and properties of the produced X80 pipeline steel. At present, 350,000 tons of products have been supplied to the second line of the West-East Gas Pipeline. The products have good performance and have brought significant economic benefits to the enterprise. and social benefits .
1 Experimental materials and methods
the 18.4mm X80 pipeline steel coil produced by the 2160 hot rolling production line of Shougang Qiangang Company . It is designed with low carbon, low phosphorus and microalloyed niobium and titanium . : C 004 ~ 0.06 ; Si 0.15 ~ 0.25 ; Mn 1.7 ~ 1.9; pC 0.012 ; S < 0.003 ; Nb + Ti + vW 0.15 ; carbon equivalent P m C0.21 ; , phosphorus , sulfur and nitrogen content should be as low as possible . At the same time, in order to ensure the strength and toughness of X80 pipeline steel , alloy elements such as copper and nickel are appropriately added. After the molten iron is desulfurized, it is smelted in a converter and treated with lf + rh vacuum. It is continuously cast into a billet with a thickness of 230mm . After heating, two-stage rolling and Cooling is controlled after rolling , and X80 steel coils with a thickness specification of 18.4mm are produced by coiling .
The static recrystallization behavior of deformed austenite in the interval between passes was studied by using the experimental method of double-pass compression . The sample is processed into a cylinder of 8mm X 12mm , and the thermal simulation experiment is carried out on the G leeble 2000 thermal simulation testing machine. After the sample is heated to 1200C at a speed of 20°C , it is kept for 5m n and then heated at 5°C.sT Cool rapidly to temperature T and keep it warm for 1mn, then perform two deformations , with a certain time interval in between , and finally quench the sample to room temperature.

Fig . 1 Process route of double-pass compression experiment
F ig* 1 Double _ hit ccmpiession testng scheme

, impact and drop hammer properties of the mass-produced products after process improvement were tested using API standard samples . Sampling on finished rolls , After grinding and polishing, it is etched with 4 % nitric acid alcohol solution , The microstructure at 1/4 of the plate thickness was observed with an optical microscope . The transmission samples were prepared by electrolytic double spraying and carbon replica extraction respectively . The microstructure and the morphology and distribution of precipitated particles were observed under a JSM 2000 transmission electron microscope .

Analysis and Discussion of Experimental Results
Static recrystallization behavior of X80 pipeline steel
2.1.1 Static recrystallization percentage The stress - strain curves under different experimental parameters were obtained from the double-pass compression experiment , And determine the softening rate in the interval time of the pass at the corresponding temperature . The research methods mainly include unloading method , back-calculation method and average flow stress method . In this paper, the stress compensation method is adopted , and the true strain value is taken as 0.02
The static recrystallization percentage X s is calculated using the stress compensation method :
Nine - 6
In the formula, % is the stress at the end of the first loading, 6 is the yield stress at the first loading, . 2 is the yield stress at the second loading . According to the above formula, the static percentage of each temperature in different pass intervals is calculated , and the static kinetic curve is drawn accordingly .
different process parameters on the static recrystallization percentage of X80 pipeline steel is shown in Figure 2 . From the experimental results , it can be seen that In the same interpass time , as the temperature rises , the recrystallization fraction increases significantly. It is generally believed that recrystallization begins to occur when the static percentage X s =15%~ 20% , and the static softening rate X s >90% complete recrystallization can be considered complete . Therefore, at lower temperatures , For example , at 950°C and 1000C , complete recrystallization was not achieved within 100 s of intermittent time . The softening rate curve at 950C showed a plateau, indicating that the precipitation of microalloying element carbonitride precedes the recrystallization process , which leads to the formation of dislocations. Migration and the migration of subgrain boundaries and grain boundaries are hindered , which hinders the progress of recrystallization [8] - At higher temperatures, such as 1050C and 11O0C , the softening rate increases rapidly, and the softening rate increases rapidly in a very short interval between passes. Complete recrystallization was achieved .

Static recrystallization kinetic model and activation energy
The dynamics of austenite static recrystallization of X80 steel can be described by Avvam equation [9] :
X s =1 — exp[—0-693(14. 5 ) n ] (2)
In the formula, X s is the recrystallization percentage , t is the time , 05 is the time when the recrystallization percentage is 50% , n is a constant related to the material . 0.5 can be expressed by the following formula :
0.5=AepeqDsexp(QSRX Rt) (3)
In the formula, D is the grain size, Mm ; Q S rx is the static recrystallization activation energy, J · mol 1 R is the gas constant, with a value of 8.314 J.mol 1 • K —1 ; A pq and s are constants •
Take the logarithm on both sides of formula (3) , have
Jn0.5 _ = hA+plnE+qln e + slnD + Q SRX Rt ( 4 ) It can be seen from the above formula that the recrystallization activation energy Qsrx R is the slope of the relationship between nt . Regression " = 0443 correlation coefficient R =0.925, static recrystallization activation energy Q S rx ^380 kJ-mo f 1 of X80 pipeline steel .
The static recrystallization activation energy Qsrx and the deformation condition of the metal are realized through the influence of the chemical composition on the activation energy Qsrx . The static recrystallization activation energy Qsrx is calculated by the empirical formula of the chemical composition . The relationship between the recrystallization activation energy and the chemical composition of the steel is as follows show :
Q sRX =124714+28385.68[Mn]+
64716.68[si]+7277.540[Mo]+
76830.32[Ti] 0.123 +121100.37[Nb] 0.1 (5)
The calculated static recrystallization activation energy Qsrx of X80 pipeline steel is about 330 kj · mo , which is slightly lower than the experimental results . In the study of [12] , the composition is C 0.024% , The static recrystallization activation energy of high-niobium pipeline steel with Nb 0.099 % is 338 kJ-mo f 1 ; and the research in Zhou Xiaofeng [3] showed that the increase of C content can increase the static recrystallization activation energy . The experimental material in this paper The Nb content is close to 0.99% , while the C content is about 0.06%, so the static recrystallization activation energy is slightly higher than 338 kJ-mo f 1 , indicating that the experimental
The result is reasonable .
Microstructure and properties of X80 pipeline steel
During the controlled rolling process of pipeline steel, the occurrence of static recrystallization will lead to the disappearance of dislocations generated in the early deformation process, which is not conducive to the accumulation of deformation . Therefore, in the rolling process, especially the finish rolling process

This is irrelevant, but the chemical composition of the steel affects the recrystallization activation energy of the static recrystallization behavior of the deformed austenite in the steel, which affects the structure
The chemical composition of the species has a significant impact on static recrystallization, mainly because it is very important to improve the degree of homogenization and avoid structural defects such as mixed crystals. In the early stage of development of X80 pipeline steel, the structure of mixed crystals often appeared in the trial - produced products Phenomenon , resulting in low temperature toughness, especially drop hammer performance unqualified . Through the above research , corresponding adjustments were made to the on-site process system , such as reducing the rolling temperature in the second stage , speeding up the rolling rhythm and shortening the interval time between passes . The microstructure and properties of the product have been significantly improved .

2.2.2 Mechanical properties of X80 pipeline steel
53 batches of X80 -grade coils produced on-site have 30 sampling directions of bar-shaped specimen tensile test statistical results . The results show that : 
The average yield strength of the coil is 574MPa , which is nearly 20MPa richer than the standard requirement of 555MPa . The average tensile strength of 676MPa is more than 50MPa ; compared with the standard requirement of 0.94 , the yield strength ratio is larger, and the average value is about 0.85 .

Statistical results of the Charpy impact toughness at 20°C in the 30 direction of the coil and the drop hammer performance at T5°C . The lowest impact energy is 293 J and the highest sectional area is 85% except for a few , most of the values have reached 100% . The low temperature of the product Toughness is very good .

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