STEELCH MILL CONTROLLED ROLLING AND MICROSTRUCTURE PROPERTIES OF X80 PIPELINE STEEL

Abstract: The controlled rolling technology of X80 pipeline steel produced by steckel mill , the microstructure, precipitated phase and mechanical properties of X80 steel plate/coil, as well as the mechanical properties of longitudinal welded pipe and spiral welded pipe were analyzed and studied. The results show that the X80 pipeline steel plate / coil of the steckel mill has obtained higher strength and toughness, in which the average yield strength and tensile strength are above 575 MPa and 665 MPa respectively , and the impact energy at -20 ° C is higher than 330J, FATT50CVNV — 60°C ; the pipeline steel is a typical fine acicular ferrite structure, and the ferrite matrix is dispersed with nanoscale carbonitride precipitates of Nb and Ti , and there are mainly two kinds of precipitated particles : one with an average size of About 20 nm , it is the Nb(Ti)C precipitate phase dominated by NbC ; the other kind of particle size is mostly in the range of 50-200nm, which is the composite precipitate phase of Ti and Nb(NC) mainly TiN ; Longitudinal welded pipes and spiral welded pipes made of steel plates and steel coils have high comprehensive mechanical properties.

Key words: pipeline steel; X80 ; acicular ferrite; precipitated phase; controlled rolling

Pipeline transportation is an efficient and economical way of long-distance transportation of oil and natural gas . With China's increasing exploitation and application of oil and natural gas , the next few years will be the climax of China's oil and gas pipeline construction , especially represented by the Second West-to-East Gas Pipeline Project , which will be China's gas transmission pressure after completion The highest natural gas transmission pipeline with the largest gas transmission capacity and the longest distance . In order to improve transmission efficiency , reduce energy consumption , reduce investment and reduce pipeline operating costs , high steel grade and large diameter have become the general trend of pipeline steel pipe development . Industrial developed countries generally list X80 as the preferred steel grade for natural gas transmission pipelines in the 21st century . X80 is also the preferred steel grade 13 for the trunk line of the Second West-East Gas Pipeline Project in China . Therefore , the market prospect of X80 high-grade pipeline steel is very broad .
four-in-one high-efficiency and practical coil rolling medium and heavy plate production line of advanced steelmaking , external refining , continuous casting and steckel rolling mill. Its main advantages are compact structure, small investment, and thick slabs can be used to produce thin and wide steel coils . This paper analyzes and researches the controlled rolling technology of X80 pipeline steel produced by the steckel mill , the microstructure , precipitated phase , mechanical properties of X80 steel plate / coil, and the mechanical properties of the produced longitudinal welded pipe and spiral welded pipe .
Composition characteristics and production process of X80 pipeline steel
According to the process characteristics of the steckel mill , the production and smelting of X80 is designed with high niobium composition. Table 1 shows the main design composition of X80 , in which the carbon content is 0.06 %, the niobium content is about 0.1 %, and the molybdenum content is about 0.2% . . Niobium can achieve ferrite grain refinement and precipitation strengthening through various mechanisms in controlled rolling . Molybdenum is the main element that promotes the formation of acicular ferrite, that is, low-carbon bainite . The two are important elements for X80 to obtain high strength and toughness [ 4-5 ] .
Steckel mill X80 pipeline steel production process is usually: high-quality hot metal , hot metal pretreatment , top-bottom combined blowing converter fRH vacuum degassing treatment * LF furnace refining * slab casting * walking heating furnace f high pressure water descaling - furnace Coil mill rolling - controlled cooling .
The technical key in smelting is to use a variety of refining methods and technical means to purify molten steel , In continuous casting, light reduction technology is used to improve slab composition segregation , Finally, enter the steckel rolling mill to realize the acicular ferrite structure of X80 pipeline steel through controlled rolling technology , In order to achieve the matching of high strength and high toughness .

Controlled Rolling and Controlled Cooling Technology for Production of X80 Pipeline Steel by Steckel Mill
X80 Pipeline Steel Heating Technology
/ coil controlled rolling process for pipelines . Whether the slab heating temperature is reasonable will directly affect the initial austenite grain size and the solid solution degree of microalloying elements of pipeline steel , which will also affect the recrystallization process of austenite during rolling and the deformation after deformation. The state of austenite grains and the precipitation state and quantity of carbonitrides will affect the comprehensive mechanical properties of steel sheets / coils for hot-rolled pipelines .
At the same time , The control of heating temperature and heating time has an important impact on on-site production rhythm control and production cost control . Considering various factors , the heating system of high-niobium X80 steel billet is as follows: the tapping temperature is 1200 °C , and the heating time is about 180min . _

X80 Controlled Rolling Technology
The X80 steel plate / coil is descaled by high-pressure water after being heated , and then enters the coil rolling mill . In addition to the main parameters such as pass deformation control , the important technical parameters in the rolling process are the starting rolling temperature and finishing rolling temperature in the first stage , and the second rolling temperature. The starting rolling temperature , coiling furnace temperature , second- stage finish rolling temperature , post-rolling cooling rate , final cooling temperature , coiling temperature , etc. are also important technical parameters. The total deformation of the stage, the cooling rate after rolling and the final cooling temperature are the key parameters to determine the final performance of X80 steel plate / coil .
The most important reference factor for determining the starting rolling temperature and finishing rolling temperature in the first stage is the recrystallization termination temperature T nr ( non-recrystalliza tion temperature ) , that is, the final rolling temperature must be higher than the minimum temperature of complete recrystallization of austenite , so that a complete recrystallization process occurs , and at the same time comprehensively consider factors such as production process , equipment load , production efficiency ; the second stage of rolling temperature and final rolling temperature It is necessary to ensure that the austenite does not recrystallize, especially the partial recrystallization process . At the same time, in the second stage, the large deformation control rolling technology is implemented in the austenite non-recrystallized area , so that the deformed and non-recrystallized austenite The ferrite retains a large number of crystal defects , which significantly increases the nucleation rate of ferrite when the ferrite transformation occurs in subsequent cooling , and is also conducive to the deformation-induced precipitation of carbonitrides of microalloying elements Nb , V , Ti , so as to achieve fine The purpose of hardening , toughening and strengthening ferrite ; the cooling rate and final cooling temperature after rolling determine the type and degree of transformation of ferrite , and also affect the thickness of the microstructure . The ideal microstructure of pipeline steel is acicular ferrite that is fully refined and has no obvious directionality. At the same time , retaining a small amount of polygonal ferrite plays a certain role in ensuring the toughness index of pipeline steel , and the formation of acicular ferrite requires a certain under cooling rate conditions .
Through experimental research and production practice , usually the first stage rolling temperature of X80 steel plate/coil is about 1100C, the final rolling temperature is higher than 1050C , and the total reduction in the first stage exceeds 50% ; the second stage rolling temperature is about 950 C, the final rolling temperature is higher than 850 C, and the total reduction exceeds 70%; the cooling rate is controlled at 20 ~ 30C/s, and the final cooling temperature is about 500 C.

Organizational structure of X80 pipeline steel
The important feature of the microstructure of high-strength pipeline steel is that it is based on acicular ferrite, that is, low-carbon bainite or ultra-low-carbon bainite. Since the phase transformation is realized by a mixed mechanism of diffusion and shear, the product has Strong phase transformation substructure strengthening and fine grain strengthening ; at the same time, high-strength pipeline steel uses microalloying components to obtain ideal precipitation strengthening effects through the precipitation of strong carbonitrides during controlled rolling . Therefore, whether high-strength and high-toughness high-strength pipeline steel can be finally obtained in actual production mainly depends on the type , grain size , substructure and precipitated phase status of the final ferrite formed .
Substructure of X80 Pipeline Steel
the X80 pipeline steel of the steckel mill was analyzed under the transmission electron microscope , and the substructure of the X80 pipeline steel plate under the transmission electron microscope . The substructure of acicular ferrite under the transmission electron microscope presents two characteristics :
One has typical fine lath ferrite subgrains .
There are two types of lath ferrite observed in practice , A ferrite lath is parallel to each other to form a lath bundle , Different crystallographic orientations among the slab bundles , Interlaced ; another kind of lath crystal does not form a lath bundle , the laths are interlaced , and the crystallographic directions are different . Lath ferrite with these two morphologies can improve the crack arrest performance of the material at different scales;
The second has a high density of dislocations . High-density dislocations can improve the strength of materials on the one hand , On the other hand, the dislocation of the cellular structure can be regarded as the further refinement of the grain , It is not only beneficial to the strength but also can improve the toughness of the material . At the same time, it increases the nucleation position of the second phase particles , which is beneficial to the dispersion distribution of the second phase particles . Therefore , lath ferrite with different crystallographic orientations and high-density dislocations work together to It has become an important reason for the high strength and high toughness of X80 pipeline steel .

Precipitated Phases in X80 Pipeline Steel
The acicular ferrite precipitates of the X80 pipeline steel in the steckel mill were analyzed , and Fig . 3 shows the morphology of the precipitates of the X80 pipeline steel plate under the transmission electron microscope . It can be seen that there are two main types of precipitated particles : one is relatively large in number , nearly spherical , and the distribution is more diffuse , uniform , and fine , with an average size of about 20 nm . The analysis shows that Nb(Ti)C precipitates mainly composed of NbC phase, as shown in (a) and c ) in Figure 3 ; the other type of particles is in a regular square shape, relatively small in number, and larger in size than NbC , mostly around 50-100 nm , and a small number of particles reach 200 nm in size . It is Ti and Nb (NC) composite precipitates mainly composed of TiN , as shown in (a) and b ) in Figure 3 . In austenite, TiN has a higher solid solution temperature than Nb(C,N) and has stronger stability 67 〕 , because the test steel contains a small amount of titanium, nitrogen is mainly combined with titanium to precipitate in the form of Ti(Nb)N at high temperature , while The Nb compound takes Ti(Nb)N as the core at a lower temperature and grows on one side of it. It can be seen from Figure 3(b) that the elliptical Nb(Ti)C particles are formed by the square Ti(Nb)N particles . The morphology of the growth of the substrate, the whole particle is composite precipitation , so it can be considered as Ti.Nb(NC) composite precipitation phase .

Properties of X80 pipeline steel
the X80 pipeline steel composition in the actual production of the steel coil mill show that the chemical composition of the X80 steel coil and steel plate meets the design requirements, and both meet the technical requirements for the steel plate/coil of the Second West-East Gas Pipeline .
X80 steel plate produced to make straight seam steel pipe , The produced X80 steel coils are made into spiral welded steel pipes, and samples are taken from the produced X80 longitudinally welded pipes and spiral welded pipes for transverse and longitudinal mechanical property tests at different parts . Mechanical property test of manufactured longitudinal welded pipe and spiral welded pipe
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Table 2 Mechanical properties of X80 steel plate

mechanical properties	R eL /MPa	R m /MPa	A 50 / %	R eL /R m	CV N (—20C)		DWT T (—15C)
					A KV /J	SA/ %	SA/ %
average value	575	665	40	0. 86	345	100	92
standard requirement	>555	>625	>16	C0.94 _	>240	>90	>85

Table 3 Mechanical properties of X80 ERW pipes
CVN(—10C) DWT T(0C)

mechanical properties	R eL /MPa	R m /MPa	A 50 / %	R eL /R m	A KV /J	SA/ %	SA/ %
average value	591	686	26	0.87	334	96	91
standard requirement	>555	>625	>16	C0.94 _	>220	>90	>85

Table 4 Mechanical properties of X80 steel coil
CV N (—20C) DWT T (—15C)

mechanical properties	R eL /MPa	R m /MPa	A 50 / %	R eL /R m	A KV /J	SA/ %	SA/ %
average value	599	682	26	0.87	334	96	91
standard requirement	>555	>625	>16	C0.94 _	>240	>90	>85

Table 5 Mechanical properties of X80 spiral welded pipe
Table 5 Mechanical properties of X80 SSAW pipes
CV N (—10C) DWT T (—0C)

mechanical properties	R eL /MPa	R m /MPa	A 50 / %	R eL /R m	A KV /J	SA/ %	SA/ %
average value	572	690	26	0.83	352	100	90
standard requirement	>555	>625	>16	C0.94 _	>220	>90	>85

Note : The tensile test uses a round bar sample

From the test results , the X80 pipeline steel plate / coil produced by the steckel mill through the controlled rolling technology has obtained higher strength and toughness, and the average yield strength and tensile strength have reached 575 MPa and 665 MPa respectively . The work force is higher than 330J, and the average shear area of —15C DWTT is higher than 90 %; the comprehensive properties of the produced longitudinal welded pipe and spiral welded pipe, including strength , plasticity , toughness and other performance indicators, all meet the technical requirements of the second line of West-East Gas Pipeline Pipeline . Among them, the average yield strength and tensile strength of the longitudinal seam welded pipe body reached 591 MPa and 686 MPa respectively, and the average impact energy at T0°C was 337J; the average yield strength and tensile strength of the spiral welded pipe body were 572 and 690MPa, respectively, The average impact energy of -10C is 352J, and the two kinds of welded pipes have high elongation and proper yield ratio . In the test , the ductile-brittle transition temperature of X80 steel plate and steel coil was also measured , and FATT50CVN was all lower than -60 °C ; the test results of the welding performance of the welded pipe weld zone were also well satisfied. According to the technical requirements of the steel pipe , each performance value has a certain margin compared with the standard value, which shows that the actual production of the X80 steel plate/coil of the steckel mill has high strength and low temperature toughness .

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