Research and development of grade thick X80 pipeline steel

Abstract : This paper introduces the key control technology and process route of developing hot-rolled coils for X80 steel grade petroleum pipelines by adopting low-carbon composition design and TMCP process . Through the composition design of micro-alloying such as low-carbon multi-alloy strengthening , controlling the purity of molten steel and slab segregation, and adopting a reasonable two-stage controlled rolling and controlled cooling process, acicular ferrite + bainite structure can be obtained . The test indicators show that the mechanical properties such as strength index and low-temperature toughness are good, which meet the technical conditions of hot-rolled coils for the " West Second Line " pipeline project of PetroChina , and the indicators of the pipe body sampling after pipe making are good .

Keywords :

X80 pipeline steel; acicular ferrite; low carbon microalloy strengthening; high strength

In recent years, with the development of the national economy, energy utilization and development have become increasingly important . The petroleum industry is a pillar industry of the country. With the completion of the " Second West Line " , the energy shortage in economically developed regions such as the Yangtze River Delta has been greatly alleviated, and vitality has been added to economic development . In order to meet the ever-increasing oil and gas demand, the development of energy is extended to remote areas, the transportation distance of oil and gas pipelines is increased, and the geological and climatic conditions are more complex. And high-strength pipeline steel with stronger acid corrosion resistance . In the process of pipeline compression deformation, the critical failure is buckling, so the pipeline has sufficient buckling resistance, and the ratio of steel pipe diameter to wall thickness is
D/t, with the increase of D/t , the buckling strain will be reduced. In order to improve the buckling resistance, the wall thickness of the steel pipe must be increased. With the construction of long-distance pipelines, the corresponding delivery pressure has changed from 4.5 in the 1960s MPa has been increased to the current 15MPa , steel grades have been raised from X42 to X70 , X80 or higher, and long-distance transmission pipelines are developing toward high steel grades and thick walls . With the " West Line " put into use, it marks that China has stepped into the world's advanced industry in the field of oil pipeline construction, and has greatly stimulated the research and development progress of iron and steel enterprises . At the end of the last century, several major domestic iron and steel enterprises already possessed mature production technology of X70 steel grade. With the construction of the "West Second Line", several major domestic iron and steel enterprises have mastered the research and production control technology of X80 steel grade or higher steel grade Gradually mature , the localization rate of X80 steel grade is gradually increasing .
has reached a number of strategic cooperation agreements on the introduction of oil and gas with Russia, Central Asia and other countries. The design of the " Third West Line " has been completed, and the preliminary work for the " Fourth West Line " has begun . The sources come from the five Central Asian countries, the pressure design is greater, the transportation distance and geological conditions are more severe, and the pipe body material is mainly X80 steel grade . Under the background of mastering the X70 production technology and mature batch supply , Bayi Iron and Steel started to study the hot-rolled coils for X80 steel grade petroleum steel pipes. It is increasingly important to meet the needs of the northwest region .

The article mainly introduces the experimental research and
Trial production process, and discuss the relationship between material structure and performance .

1. High Strength and High Tenacity X80 Coil and Technical Requirements

The technological process of Bayi Iron and Steel’s high-grade steel pipeline: blast furnace hot metal — — — hot metal pretreatment — oxygen bottom-blown converter — LF refining — RH refining — — — continuous slab casting — — — billet finishing — — — heating — ——Descaling ———— Rolling ———— Laminar cooling ———— Coiling _ _ _ _ _ _ _
120t nominal capacity bottom-blown converter; 120tLF ladle refining furnace; double-station RH vacuum treatment; single-strand slab continuous casting machine; 1750mm hot continuous rolling; intensive laminar cooling device .
Main testing equipment: electronic universal testing machine 5589 (600kN ) ; ZBC - 500 Charpy impact testing machine; ZCJ2404 drop weight testing machine .
X80 steel grade hot-rolled coil adopts the hot-rolled coil technical conditions ( Q/SY GJX 0101-2010 ) for PetroChina's " West Second Line " pipeline project , which puts forward strict requirements on the strength and toughness of raw materials . The strength index is high, and there are strict regulations on the toughness, yield ratio , hardness, and DWTT index of the material . In order to meet its requirements, a series of measures should be taken from steelmaking , continuous casting , controlled rolling and cooling . In order to improve the toughness and DWTT index of the steel plate, the industry is constantly exploring to reduce the carbon content, and the resulting strength loss is realized by adding a variety of microalloys, compensating and refining grains , and improving the precipitation effect of carbonitrides . Strengthen the control of phase transformation function, improve the internal quality of the slab at the same time, reduce the content of inclusions and denaturation treatment through RH and LF furnace refining, control the segregation of the slab by using continuous casting pouring temperature , casting speed and light reduction technology degree etc. The composition of X80 steel products is shown in Table 1 , and the requirements for mechanical properties are shown in Table 2 .

Table 1 X 80 steel grade coil product composition mass fraction ( 3 " % , not greater than )

C Si Mn P S Nb V Ti Cu Cr			Mo		Ni	N Pcm
0.09 0.42 1.85 0.022 0.005 0.11 0.06 0.025 0.3 0.45			0. 35		0.5	0. 008 0. 23
Note : ( 1 ) For every 0.00 % reduction in carbon content compared with the specified maximum carbon content , the content is allowed to increase by 0.05 % compared with the specified maximum manganese content , but the maximum content is not allowed to exceed 1.95 % ; ( 2 ) Nb +V + TiW 0.15 % . Table 2 X 80 Steel Grade Coil Mechanical Properties Requirements
in test direction , ran Ro® MPa Rm, MPa Rtas/R.					A,%	wxya
Rolled with coil to 30°	15. 3 555 ~ 690 625 ~ 825	0.93 _		API Spec 5L			W265
	A 20 °C Charpy impact test			T5TDWTT test
	Shear area SA, % impact energy ( 10nmX10nmX55nim), A k „ J			Shear area SA, %
	single value average single value	average value		single value			average value
	M80 M90 N160	N220		N70			M85

1. Chemical Composition Design

- rolled ferritic ———— pearlite type pipeline steel, also known as less pearlite type steel, is the first generation of pipeline steel developed and perfected in the early 1970s. This steel can guarantee high toughness and good Under the condition of welding performance, the strength limit level is 500 ~ 550MPa, so it is mainly used for steels below X70 . Acicular ferritic pipeline steel is the second generation developed and perfected in the late 1980s, and is a typical structure of X80 steel . This kind of steel has better weldability than ferrite-pearlite steel ( P 4 0.20 % ) , HIC resistance and fairly high impact toughness, and the rapid work hardening during tube forming counteracts the Bauschinger effect, There is little or no drop in yield strength . At the same time, it has quite superior welding performance, which is very beneficial) for welded pipes and on-site laying pipeline projects .
The development of pipeline steel is a process of continuously reducing carbon content. The reduction of carbon content can help improve the toughness of steel on the one hand, and improve the welding performance of steel on the other hand . Alloy composition design revolves around the requirements of performance indicators and organizational structure .
Acicular ferrite has irregular ferrite grains with a high density of dislocations distributed in the matrix, formed by the decomposition of austenite in the temperature interval between the polygonal ferrite and bainite transformation temperatures . From the chemical composition of steel, Mn is the cheapest element to inhibit the formation of polygonal ferrite, and Mo is an element that further supports this effect . In order to obtain high toughness, the austenite grains can be refined as much as possible by adding Nb , and the mass fraction of C should be kept at 0.06 % or lower. Therefore, the acicular ferritic X80 grade pipeline steel should use low carbon Mn - Design of Mo-Nb microalloying system, the specific composition is shown in Table 3 .

Table 3 The chemical composition mass fraction of the trial steel coil 3 " %
C	Si	mn	P S Al Nb> V. Ti Mo 、 Ni 、 Cu> Cr
0.03 ~ 0.07	0. 20 30	1. 65 ~ 1.85	WO. 015 W0.004 0.02~0.04 W0.15 Appropriate compound addition

1. Steel purity control

The mass fraction of phosphorus in the finished steel coil of the trial production is controlled below 0.015 % to reduce the influence of phosphorus segregation, and the phosphorus content of the raw material is required to be below a certain level . The converter dephosphorization process is adopted in the smelting process, and the basicity of the final slag is strictly controlled. In order to reduce phosphorus return, the slag blocking operation is adopted in the tapping process, and the amount of slag is strictly controlled. The sulfur content is controlled below 0.004%, and the sulfide content is reduced to control Slab segregation caused by sulfide . The control of sulfur content in steel is mainly to control the sulfur content of molten iron, and it is realized through desulfurization pretreatment of molten iron , control of converter sulfur return, and deep desulfurization of LF furnace refining. It is necessary to control the sulfur content of converter, otherwise it will increase the load of LF furnace and cause time Nitrogen increase in molten steel over a long period of time .
The continuous casting link focuses on controlling liquid level fluctuations. Excessive liquid level fluctuations will cause non-metallic inclusions to be involved. Maintaining constant casting speed and pouring temperature control is an effective means to reduce slab segregation. Billet segregation is favorable .
4 TMCP process
220mm slab was used for trial production and rolled into a 15.3mm steel coil. Firstly, a reasonable heating system was selected and the solid solution of the microalloy was considered . Nb dissolved in austenite interacts with defects in austenite to prevent grain boundary migration, delay recrystallization, and expand the non-recrystallized region; strain-induced Nb(CN) in the non-recrystallized region is fully precipitated, inhibiting Austenite recrystallization causes a large number of cake-shaped grains to form in the steel, which can be transformed into fine ferrite after cooling . At the same time, vanadium precipitates during and after the ferrite transformation, which plays a role in strengthening the ferrite matrix. Therefore, these alloying elements are dissolved in the austenite as much as possible during the heating process to ensure the desired organization . It is key to consider the dissolution temperature of different microalloying elements when formulating the heating regime .
Controlling the total compression ratio and starting rolling temperature in the non-recrystallized zone is a necessary condition to obtain dispersed fine ferrite grains. For X80 pipeline steel, the finishing rolling is generally carried out at 950 r , which can increase the slip band in the austenite grains. and dislocation density, increase the effective area of the grain boundary, and create conditions for the ferrite phase deformation nucleus. The total compression ratio in the finishing rolling area is considered according to the installed capacity of the equipment and the temperature drop capability of the finishing rolling area, because the finishing rolling temperature is very The key parameter, which not only directly affects the microstructure of the rolled steel, is generally slightly higher than the Ar3 temperature .
The laminar flow cooling after rolling adopts the forced cooling method in the front stage, which can suppress the growth of austenite grains after rolling, and suppress or reduce the formation of secondary bands. Accelerated cooling at temperatures above Ar3 can increase the proportion of acicular ferrite , pearlite disappears or decreases, and a certain proportion of bainite is strengthened .

1. Production of X80 Pipeline Steel Hot Rolled Coil

5.1 Organization
For X80 pipeline steel coils, the grades of A, B, C, and D non-metallic inclusions in the steel are tested according to the ASTM E45A method. All types of inclusions are less than level 2 , and the steel is pure . Meet the requirements of technical conditions . The microstructure of X80 pipeline steel is composed of acicular ferrite + a small amount of irregular equiaxed ferrite + a small amount of MA components, and there is no pearlite band structure, showing the typical characteristics of acicular ferrite pipeline steel. Figure 1 shows the optical microstructure and banded structure of X80 pipeline steel coil .
Figure 2 Ribbon Organization
5.2 Mechanical properties
5.2.1 Tensile properties
a comparative test was carried out on the same steel plate at the end of the steel coil with a direction of 30 ° and a direction of 90 ° to the rolling direction. The specific detection indicators are shown in Table 4 , and the values of the yield strength are R 0.5, gauge length 50.8mm, sampling position is 1/4 of coil width . The indicators show that the strength shows a strong and obvious trend in the 90 ° direction, and all batches show that the yield and tensile strength are 30 to 40 MPa higher than those in the 30 ° direction .

Table 4 Properties of steel coils in different directions

Volume number	Sampling direction	Rb», MPa	Rto.5, MPa	4, %	RtO.s/Ra
1#	30°	658	588	30. 58	0.90
	90°	696	634	33. 04	0.91
2#	30°	639	564	38. 26	0. 88
	90°	687	617	33.40	0.90
3#	30°	643	576	41.24	0.90
	90°	682	613	39.52	0. 89
4#	30°	652	578	38. 20	0.88
	90°	690	622	31.08	0.90

The impact performance is different according to - 10 ° C , - 20 ° C , - 30 ° C , - 40 ° C with V -shaped notch

Testing the impact indicators of different batches of steel coils shows that the impact test is required in the agreement, and the sample size is 10 x 10 x 55 (mm ) , and the results are shown in

300J under the calculated direction and temperature, and there is a large margin. Table 5 shows that the V- shaped shock is in two directions under the test condition of 40 °C.

amount, indicating that its toughness is good . For further verification material in different directions
And its toughness performance under different low temperature conditions, for one of the indicators that the steel coil direction is excellent, it shows that the low temperature toughness of the material is excellent, completely
It can fully meet the technical requirements of the pipe body raw materials of the " West Second Line " project .

Table 6 Drop hammer results at different temperatures

Volume number	Test temperature, "C	SA,%
1#	-15	99 , 99 ,	97
	-25	89 , 100 ,	100
2#	T5	95 , 90 ,	91
	-25	97、 100、	92
3#	-15	97、 100、	100
	-25	92、 97、	85
4#	_15	96、 86 、	97
	-25	87、 87、	85

After sampling at the tail of the steel coil in two directions of 90 ° and 30 ° , under the condition of -15 ° C , the indicators are all above 90% , and some samples have separation fractures, indicating that the material does not show obvious directionality at this temperature . In order to further verify the low temperature resistance of the DWTT index of the material, the verification comparison of the temperature group interval of -25 ° C has been added, showing that the DWTT index of some batches has a downward trend under the condition of -25 ° C in the sampling direction of 30 ° angle, However, the downward trend is obvious, and it can still meet the technical requirements of the process .

1. user effect

A pipe manufacturing company in PetroChina uses Bayi Steel to produce X80 with a specification of 15.3x 1550 (mm) . The quality of the pipe factory’s inspection surface and specifications is good, and the pipe specification is (? 1016 x 15.3mm , the sampling direction of the steel pipe is that the helix angle of the pipe body is 30 ° ), the Charpy impact sample is the standard sample, and the various indicators of the steel coil and steel pipe sampling and testing are shown in Table 7 . General technical requirements for steel pipes for natural gas transmission pipelines " Q/SYGJX0101-2010 specifies the technical requirements after pipe making .

Table 7 Test results of main indicators of pipe body sampling after pipe making

performance	Rto.5, MPa steel coil pipe body	Rm, MPa		Rto.5, Rm		A50, steel coil	%, Tube	CVN (-20°C) Akv, J
		steel coil	Tube	steel coil	Tube			steel coil	Tube
X80	588 559	716	704	0. 82	0. 80	32.5	36.6	280	268

1. Discussion and experience
   1. of banded structure on DWTT and impact High-grade pipeline steel has higher requirements for low-temperature toughness, even at low temperature

The impact index is qualified, but there is still a possibility of failure in the drop weight test (DWTT) . The control of the low temperature toughness standard steel quality is the key. The quality of the steel is measured by the total amount of inclusions and the degree of segregation. The interface between the inclusion and the matrix is the weakest place in the material combination, which aggravates the damage of the band structure, affects the crack arrest toughness of the steel, accelerates the crack expansion and causes the toughness index to decrease, and segregation is the process of forming ferrite In the process, carbon is gradually enriched in the remaining austenite, and most of them will be transformed into island-like M-A components. When the composition segregation in the steel reaches a certain level, these island-like MA components will be distributed into strips, forming a banded structure. The more obvious the carbon segregation, the more serious the banded structure .
The banded structure is easy to cause anisotropy when the material is deformed, and most of the banded structure in the high-grade pipeline steel is the MA component. Due to the high content of carbon and alloy elements in the MA component, its strength and hardness are generally lower. Higher than the matrix, when the material is deformed, the interface between the band structure and the matrix is prone to cracks due to the out-of-sync deformation, which is very unfavorable to the performance and impact of DWTT .
C from the composition design to reduce the potential power of segregation. At the same time, the continuous casting accurately controls the pouring temperature, casting speed and rational use of light reduction to reduce the degree of banding of the slab at one time. The slab heating link Ensuring a certain holding time and rapid cooling of rolled tubes are effective means to control the secondary banding level .

1. 1. Influence of organization type on DWTT

In the acicular ferrite structure, because the organization unit is irregular and non-equiaxed in size, the orientation relationship between grains is indeterminate, and it is in a chaotic distribution state, with smaller effective grains size, the cracks are continuously hindered by interlocking and interlaced needle-like strands during the expansion process , and the cracks are not easy to expand along the grain and within the grain, and finally 7 is a good drop hammer performance. more beneficial . In addition, due to the dislocation substructure with high dislocation density in this structure, the " resistance " to brittle fracture is much higher than that of ferrite / pearlite structure .
High-performance pipeline steel is mostly characterized by low-carbon acicular ferrite structure, which makes it have high strength , high toughness , and high toughness crack arrest performance. The work hardening rate is accelerated to make up for the substantial decrease in yield strength caused by the package lattice .
In order to obtain a high proportion of acicular ferrite, the design and control of low-carbon composition is the premise. It is necessary to ensure a certain total rolling reduction ratio in the austenite non-recrystallized zone, the control of the final rolling temperature and the rapid cooling of the rolled tube. The key to getting a higher proportion of acicular ferrite .
8 Conclusion

1. The X80 pipeline steel hot-rolled coil with a specification of 15.3 x 1550 (mn i) developed by Bayi Iron and Steel Co., Ltd. has reasonable composition and process design . The developed pipeline steel has low carbon content, high manganese content, Nb- V-Ti microalloying Treatment and low alloying of Mo control the composition characteristics of the structure . The inclusion control technology and controlled rolling and controlled cooling technology are adopted . The microstructure is mainly acicular ferrite, and the structure is uniform and fine, with typical acicular ferrite The structural characteristics of pipeline steel , so X80 steel has good comprehensive mechanical properties .
2. Low-carbon multi-micro-alloy composition design, control of inclusion content and slab segregation are the prerequisites to ensure good low-temperature toughness .
3. The spiral welded pipe with a diameter of at least 1016mm manufactured by the developed X80 pipeline steel hot-rolled coil meets the requirements of the API 5L specification .

1. Research on Microstructure and Properties of X80 Pipeline Steel . Journal of Northeastern University, 2008 ( 2 ) .
2. Research on Impact Toughness of High Strength Pipeline Steel with Different Microstructure . Baosteel Technology, 2003 ( 6 ) .
3. General technical requirements for hot-rolled coils for spiral seam submerged arc welded pipes for natural gas transmission pipelines . Q/SY.GJX107-2009.

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