X70 pipeline steel coil

successfully developed X70 pipeline steel in its 150t converter - 1780nun hot continuous rolling production line by designing reasonable chemical composition and production process . The product has an acicular ferrite structure, and its mechanical properties meet the standard requirements.
Key words: pipeline steel ; chemical composition; production process; acicular ferrite; mechanical properties

Technical requirements for X70 pipeline steel
Due to the special use environment of pipeline steel, it is not only required to have high strength, but also to have excellent toughness, fatigue resistance, fracture resistance, corrosion resistance, welding performance and processing performance. Tables 1 to 3 are the main technical indicators of the X70 pipeline steel used in the Central Asia Natural Gas Pipeline Project .

Table 1 Tensile property requirements

RtO. 5	/MPa	Heart / MPa	RtO. 5 / Rm	A50/ %
485	~ 630	570 ~ 760	<0.90	API SPEC 5L

5% of the heat yield ratio Rms/RmWO. 92° is allowed . According to the principle of consensus, the manufacturer can adjust the yield strength of the pipeline steel, but it must ensure that the yield strength after the pipe is made conforms to the spiral submerged arc welded steel pipe technology conditions stipulated.
Table 2 Charpy V -type impact toughness requirements

Test temperature /°c	Shear area SA/%	Impact energy /J
	Min Average	minimum value	mean direction
-10	$75 M85	$130	$170 at 30° to rolling direction

In addition , the maximum allowable hardness value on the transverse section of X70 pipeline steel coil is HV1O26O ; the grades of A , B , C and D inclusions are not greater than 2.0 .

 

Table 3 DWTT test temperature and shear area

Test temperature /°c	DWTT shear area minimum percentage/%
	minimum value	average value	Specimen direction
-15	$70	$85	30° to rolling direction

Production process route
The X70 pipeline steel coil of Anyang Iron and Steel Co., Ltd. is produced in the 150t converter- 1780mm hot continuous rolling production line of the second rolling mill . The process flow is as follows:
Hot metal pretreatment - *150t converter smelting - LF refining VD/RH vacuum degassing - double-strand slab continuous casting - slab heating - descaling f rough rolling - 7 -stand finishing mill continuous rolling - laminar cooling ~ coiling f pumping Roll inspection- * sampling-packing, printing, marking~finished product storage.
3 Key technologies and process points of production
3.1 Composition Design
While ensuring the good strength and toughness of X70 pipeline steel, it must also ensure its good welding performance. Therefore, the composition design adopts C reduction to improve the performance and toughness of DWTT .

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for the strength loss caused by C reduction by adding microalloying elements such as Mn , Nb , V , Ti , Al , Mo , Cr , etc., while strictly controlling the content of harmful elements such as S , P , and making full use of controlled rolling and controlled cooling Technology to obtain acicular ferrite structure, so as to ensure the mechanical properties of X70 pipeline steel Table 4 is the chemical composition of X70 pipeline steel trial produced by Anyang Iron and Steel Co., Ltd.

Table 4 Chemical composition of X70 pipeline steel (mass·percentage) /%

project	C	Si	mn	P	S	Alt	Nb	V	Ti
control	<0.07	0. 10 ~ 0. 30	1. 50 ~ h 70	<0.015	W0.003	0. 02 ~ 0. 06	0. 05 ~ 0. 08	0. 02 ~ 0. 06	0. 010—0. 020
standard	<0. 09	<0.35	<1.65	<0.020	W0.005	<0. 06	=0.11	<0.08	Wo. 025
project	Cr	Ni	Cu	Mo	N	B			Pcm
control	0. 20 — 0. 30	< 0.30	£0.30	0. 18 ~ 0・30	<0. 008	CO. 0005		<0.42	€0.20
standard	< 0.35	< 0.30	<0.30	W0. 30	0.010	<0.0005			W0. 23

Note: V+Nb + TiWO. 15%; B and rare earth elements are not allowed to be intentionally added ; the C content is 0.01% lower than the specified maximum content , and the Mn content is allowed to be higher than the specified maximum
0.05%, but shall not exceed 1.85 %.

3.2 Heating process
Since the content of alloying elements in X70 pipeline steel is relatively high, if the heating temperature is too low, Nb will not be completely dissolved, and the role of Nb in controlled rolling cannot be fully exerted during the rolling process ; and if the heating temperature is too high, it may affect The degree of refinement and uniformity of austenite grain size affects the final structure and performance. Therefore, the basis for formulating the heating temperature is to fully dissolve a large number of alloying elements without making the austenite grains too Coarse. Use Owen’s formula: lgw(Nb)(w(C) + 12/14w(N))=2. 26- 6670/T to calculate the heating temperature, the billet temperature is controlled at 1100 ~ 1150°C, and the furnace temperature is controlled at 1200 ~ 1250°C, considering the thickness of the slab, the heating time is above 200min .
3.3 Controlled rolling process
the main means to refine the grain and ensure the performance of pipeline steel, especially the performance of DWTT . Reducing the entrance temperature of finish rolling can increase the density of deformation bands inside the austenite grains during the D- stage rolling process, increase the nucleation rate during phase transformation, refine the ferrite grains after phase transformation, and improve the low temperature. Purpose of toughness and DWTT performance. However, the grain refinement will obviously increase the yield strength and yield ratio, and reduce the ductile-brittle transition temperature, and the overall performance of the steel will decrease, so the final rolling temperature is set at 820 °C 0
When designing the controlled rolling process of X70 pipeline steel, the total deformation of the recrystallized zone and the uncrystallized zone is determined according to the performance requirements of the steel and its chemical composition. Applying large deformation in the controlled rolling stage can form a large number of deformation bands (4) that are conducive to nucleation, thereby refining the grains. For the X70 pipeline steel with a thickness of 15.9mm , the rough rolling stage is completed above 1000°C , and a large amount of deformation is used in the high-temperature recrystallization zone to roll the cast slab from a thickness of 230mm to 58mm, and the total reduction reaches 172mm. The reduction rate reaches 74.78 %, and the maximum reduction rate in a single pass reaches more than 22% , fully refining the austenite grains; the finishing rolling stage is controlled in the non-recrystallized area rolling and two-phase area rolling, and the rolled piece is 58mm Rolled to 15.9mm, the total reduction at this stage is 42.1mm, the total reduction rate is 72.58%, and the maximum reduction rate of a single stand is 24%, which can "flatten" the austenite grains and increase the grain size. The number of internal deformation bands provides conditions for the formation of finer and uniform ferrite grains.
3.4 Controlled cooling process
Controlling the starting cooling temperature, final cooling temperature and cooling speed in the cooling process has a direct relationship with the final structure and performance of pipeline steel. According to literature, low-carbon micro-alloyed pipeline steel mainly obtains acicular ferrite structure, the cooling rate after final rolling should be controlled at 15 ~ 305, and the final cooling temperature should be controlled at 400 ~ 550°C .
4Product Quality
4.1 Mechanical properties
Figure 1 shows the mechanical properties of the 15.9mm thick X70 pipeline steel coil, and the sampling direction of the sample is 30° to the rolling direction . As can be seen from Figure 1 , the yield strength fluctuation range is 495 ~ 605MPa, mainly distributed between 520 ~ 570MPa , with an average of 544MPa . The fluctuation range of tensile strength is 595 ~ 705MPa, mainly distributed between 625-675 MPa , with an average of 648MPa, and the yield ratio is mainly distributed in 0. 74 - 0. 92, with an average of 0.84 . -10°C Charpy V -type impact energy is 200J at the lowest, 415J at the highest , and 29OJ on average . - 15 ° C drop weight tear test is all qualified. The product strength and yield strength ratio all meet the requirements, and the control range is good, the elongation, impact energy and DWTT performance margin are large, and the overall performance pass rate is 100% .
4.2 Organization inspection
Figure 2 shows the structure under different coiling temperature conditions. When the coiling temperature is 680°C , the structure of the steel plate is F + a small amount of P ; when the coiling temperature is 600°C , the ferrite boundary begins to zigzag, and needle-like The characteristics of ferrite structure, the grain is finer, the corresponding strength is obviously improved during the performance test, and the yield ratio is increased; the coiling temperature is 55O°C

, the microstructure of the steel plate is acicular ferrite + a small amount of granular bainite, the resistant area is also stable at more than 90% , the ductile-brittle transition temperature is low, the comprehensive tensile strength is greatly improved, and the corresponding yield ratio is reduced. The tear joint performance is the best.

epilogue
The production practice shows that Angang X70 pipeline steel has reasonable chemical composition and process formulation, and the performance indicators of the product are good, which meets the requirements of the standard and fully meets the requirements of steel for Central Asia pipeline engineering.

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