20G and 12Cr1MoVG Seamless Steel Tubes Study on Low - temperature Thermo

The microstructure, grain size, non-metallic inclusion and room-temperature mechanical properties of 20G and 12Cr1MoVGhot-expanded steel tubes afterlow-temperature thermo-mechanical annealing treatment were investigated, which were compared with crude seamless steel tubes and the tubes with heat treatment later. The results indicate that the microstructure, grain size, non-metallic inclusion and room-temperature mechanical properties of 20G and 12Cr1MoVG hot-expanded steel tubes and the tubes with heat treatment later, fully meet the needs of GB5310-2008 to 20G and 12Cr1MoVG. It is important to popularize the simplified production flow, steady quality, economical and practical

hot-expanding process.
Key words：20G; 12Cr1MoVG; seamless steel tube; hot-expanded steel tube
20G and 12Cr1MoVG , as representatives of high-quality carbon steel and alloy steel for boilers , have been widely used and developed in practice . 12Cr1MoVG is a low-alloy pearlitic heat-strength steel that was successfully developed earlier and more maturely applied. It has low carbon content , good manufacturability and thermal conductivity , and has high durable strength and durable plasticity . It is generally used to manufacture headers and steam pipes with a steam temperature below 540 °C , superheaters , reheaters and some castings and forgings with a metal wall temperature not exceeding 580 °C . It is currently the most commonly used in high-parameter thermal power plants. One of the steels used for main steam pipes and reheater pipes . According to statistics , the 12Cr1MoVG steel pipe used in a high-pressure boiler accounts for about 80% of all alloy steel pipes [1-8] . On the one hand , with the operating parameters of power plant units in China and the
low-temperature thermo-mechanical annealing treatment;
The sharp increase in capacity and the need for the development of high-temperature boiler pipes to large diameters have promoted further research on high-temperature and high-strength boiler pipes. At present, the maximum outer diameter of hot-rolled ( drawn ) seamless steel pipes that can be produced in China is 720mm [9] , and more Large-diameter seamless steel pipes can only be realized by thermal expansion processes and the like . On the other hand , compared with hot-rolled seamless steel pipes with high production costs and expensive production equipment , the push-type thermal expansion process with the characteristics of simple production process , stable quality , economical and practical can make steel pipes in a short period of time. Increased diameter , low cost and high production efficiency . Because of this , the push-type thermal expansion steel pipe as the extension of the main line of smelting and rolling pipes fills the gap in the production of non-standard and special large-diameter seamless steel pipes by major steel mills at home and abroad , and solves the special needs of users. .
The essence of the push-type thermal expansion steel pipe process is a special deformation heat treatment . In this paper, the microstructure and mechanical properties at room temperature of 20G and 12Cr1MoVG seamless steel pipes , original pipes and steel pipes after subsequent heat treatment treated by low temperature deformation annealing were compared and analyzed , and evaluated with reference to the GB5310-2008 standard .
1 Experimental materials and methods
The chemical composition of the materials used in the experiment meets the requirements of GB5310-2008 ( see Table 1) . The main parameters of the standard low-temperature deformation annealing standard are : 20G ( expansion temperature 80 ~ 730 °C , expansion ratio 1.30 , expansion speed
300 mm/min , coil size 750 mm , coil frequency 2500 Hz and normalizing heat treatment at 910 °C ) ; 12Cr1MoVG ( expansion temperature 750 ~ 800 °C , expansion ratio 1.27 , expansion speed 170mm/min , coil size 750 mm , coil frequency 2500 Hz and normalizing at 990 °C + tempering at 740 °C ) . The 20G , 12Cr1MoVG original pipes, heat expanded pipes and heat treated steel pipes were sampled and numbered respectively by means of on-site sampling ( see table 2) .
Table 1 Chemical composition of samples ( mass fraction, % )
Tab.1 Chemical compositions of species(wt, % )

	\	C	Si	mn	Cr	Mo	V	P	S
20G	GB5310-2008	0.17 ~	0.17 ~	0.35 ~	-	-	-	w	w
	standard value	0.23	0.37	0.65				0.025	0.015
	test value	0.21	0.21	0.54	-	-	-	0.013	0.010
12Cr1MoVG	GB5310-2008	0.08 ~	0.17 ~	0.35 ~	0.90 ~	0.25 ~	0.15 ~	w	w
	standard value	0.15	0.37	0.65	1.20	0.35	0.30	0.025	0.010
	test value	0.13	0.25	0.55	1.01	0.27	0.22	0.014	0.010

Table 2 Specimen Specifications and Numbers
Tab.2 Size and number of specimens

Sample steel number	20G			12Cr1MoVG
Specimen Specifications /mm	Standard 273x 15	Standard 355.6x _ 12.7	Standard 355.6x 12.7	Standard 480x 40	Standard 610x 35	Standard 610x _ 35
Sample status	Original tube	thermal expansion tube	heat treatment tube	Original tube	thermal expansion tube	heat treatment tube
Sample No	Y1	K1	T1	Y2	K2	T2

After cutting , inlaying , grinding and polishing the sample, and eroding it with 4% nitric acid alcohol solution, the microstructure observation and grain size test were carried out according to GB/T13298-1991 and GB/T6394-2002 respectively , referring to GB5310-2008 for 20G , 12Cr1MoV The requirements are compared , and the microstructure of the treated sample is observed under a metallographic microscope . Tensile specimens were prepared according to GB/T228-2002 . The tensile test was carried out on the WAW-600 micro-controlled electro-hydraulic servo universal testing machine produced by Changchun Kexin Testing Instrument Co., Ltd. , and the yield strength , tensile strength and elongation after breaking of the sample were measured , and compared with GB5310-2008 Ask for a comparison . According to the provisions of GB/T10561-2005 , the sample is inspected for non-metallic inclusions and evaluated according to the general requirements of GB5310-2008 for boiler steel pipes .
Figure 2 shows the microstructure morphology of 20G and 12Cr1MoVG tubes in different states . From the photos of the microstructure of the sample , the 20G and 12Cr1MoVG seamless original pipes were subjected to normalizing and normalizing + tempering heat treatment samples before and after low-temperature deformation annealing at 725 °C and 770 °C , respectively, and after low-temperature deformation annealing The microstructures are fine and uniform ferrite + pearlite structure . 20G has a 2.5 -level banded structure , which is directly related to the fact that 20G has a certain amount of sulfide inclusions and the dendrite segregation of the original tube is easily left over by thermal expansion [10] , but after subsequent normalizing After that, it can be eliminated and meet the requirements of GB5310-2008 .
Table 3 shows the detection results of non-metallic inclusions and grain size of the 20G , 12Cr1MoVG seamless original pipe before and after low-temperature deformation annealing and after subsequent heat treatment . It can be seen that the inclusion of various non-metallic objects remains consistent before and after low-temperature deformation annealing of the 20G seamless original pipe , the grain size level increases , and the average grain size becomes smaller . After the normalizing heat treatment of 20G thermal expansion steel pipe , the grade of sulfide inclusions decreases , and the average grain size basically remains unchanged . This is because the non-metallic inclusions are partially dissolved after normalizing , the internal stress of the material is eliminated , and the grains are refined and homogenized. The result of the effect , which is consistent with the test results of the mechanical properties at room temperature , meets the requirements of GB5310-2008 . 0 12Cr1MoVG seamless original pipe has less non-metallic inclusions , and normalizes before and after low-temperature deformation annealing and after low-temperature deformation annealing + All kinds of non-metallic inclusions in the tempered samples are consistent , and the grains are refined after low-temperature deformation annealing , which meets the requirements of GB5310-2008 .

Fig. 2 Microscopic texture of 2 0 GG2Cr IMo V G G tubes in the same state xl 00
Fig. 2 Micro ctr uc uce of 2fGan d I2C2 IM0 M G G e el Ui be at d i f d ^ reer S ta s xlW
Table 3 20G and 12Cr1MoVG non-metallic inclusions, grain size test results
Tab.3 The testing results of non - metallic inclusion and grain size of 20G and 12Cr1MoVG

Sample No		Sulfides	Alumina	Silicates	spherical oxides	grain size	Microstructure
20G	YC	C	0.5	0.5	0	6.5	ferrite + pearlite
	KC	C	0.5	0.5	0	7.0	ferrite + pearlite
	TC	0.5	0.5	0.5	0	7.5	ferrite + pearlite
C2CrCM2VG	Y2	0.5	0.5	0.5	0	7.0	ferrite + pearlite
	K2	0.5	0.5	0.5	0	8.0	ferrite + pearlite
	T2	0.5	0.5	0.5	0	8.0	ferrite + pearlite
GB53C0-2008	-	W 2.5 W 2.5 W 2.5 W 2.5 M 4					ferrite + pearlite
common enquiries	-	The total number of fine line grades of various inclusions W 65					( including granular bainite )

The room temperature tensile test results of the samples are shown in Table 4 . It can be seen that 20G
When the strength of the seamless original pipe after the low-temperature deformation annealing treatment is consistent with that before the treatment , the elongation after fracture increases , which is mainly related to the expansion
Table 4 Test results of longitudinal mechanical properties of 20G and 12Cr1MoVG at room temperature
Tab.4 The testing results of longitudinal mechanical properties at room - temperature of 20G and 12Cr1MoVG

Specimen direction	Sample No	R p0.2 / MPa	Rm / MPa	A (%)
20G	YC	345	495	36
	KC	3C0	450	39
	TC	325	475	4C
	GB53C0-2008 requirements	M 245	4C0~550	M 24 ( portrait )
C2CrCM2VG	Y2	420	550	35.5
	K2	4C0	540	34
	T2	455	6C0	3C
	GB53C0-2008 requirements	M 255	470~640	M 21 ( portrait )

The fine grains in the microstructure of the expanded tube are related to the good coordination of the strength and toughness of the steel . Its comprehensive performance is equivalent to that of the expanded tube after subsequent normalizing treatment. After 20G thermal expansion, the banded structure is eliminated after subsequent normalizing treatment . The properties of the expanded steel pipes meet the requirements of GB53C0-2008 for 20G . The mechanical properties of C2CrCM2VG seamless raw pipes at room temperature are basically unchanged before and after low-temperature deformation annealing . have good overall performance , Meet the requirements of GB53C0-2008 for C2CrCM2VG , From the perspective of economic cost 2 energy saving and emission reduction and the realistic requirements of promoting a low-carbon economy today , The subsequent heat treatment process of 12Cr1MoVG can be completely omitted .
To sum up , low-temperature deformation annealing is a special deformation heat treatment process in which the steel is heated to a certain temperature near or below Aa for pressure processing , and then slowly cooled to a certain temperature with the mandrel and then air-cooled to room temperature . The low-temperature deformation annealing process is below the austenite recrystallization temperature of the steel , and the metal carbides are difficult to dissolve , which increases the strength of the material . Because it is an annealing process at the same time , it eliminates or reduces the internal stress in the steel , reduces the hardness , and improves the cutting and heating . Expansion and other processing properties [10] . The 20G seamless steel pipe is subjected to normalizing heat treatment after low-temperature deformation annealing. The microstructure of the heat-treated pipe is ferrite + pearlite , the grain size is 7.5 , and the longitudinal mechanical properties at room temperature are : yield strength 325MPa , tensile strength 475MPa , elongation after fracture The length is 41% , which meets the requirements of GB5310-2008 for 20G . After low-temperature deformation annealing of 12Cr1MoVG seamless steel pipe, the microstructure of heat-expanded pipe is ferrite + pearlite , the longitudinal tensile strength and yield strength at room temperature are 540 and 410MPa , and the elongation after fracture is 34% , which meets the requirements of GB5310-2008 for 12Cr1MoVG .
3 Conclusion
20G and 12Cr1MoVG seamless heat-expanded steel pipes produced by push-type low-temperature deformation annealing retain the ferrite + pearlite structure of the original pipe , and its grain size , non-metallic inclusions and room temperature mechanical properties all meet the requirements of GB5310-2008 . From the perspective of economic cost , energy saving and emission reduction, and the realistic requirements of promoting low-carbon economy today , the subsequent heat treatment of 12Cr1MoVG can be omitted . The banded structure produced after low-temperature deformation annealing of 20G seamless original tube can be eliminated by subsequent normalizing heat treatment , and the subsequent heat treatment tube has good comprehensive performance , which meets the requirements of GB5310-2008 .
References :
( Continued from page 174 ) After treatment , the 4032 alloy will have different effects . Plastic deformation increases the subsequent modification effect of all modifiers . Solid solution aging can improve the morphology of silicon phase and other phases in the alloy. It has a certain subsequent modification effect on some modifiers , and can partially passivate primary silicon , eutectic silicon and other alloy phases , and has a certain degree of refinement and spheroidization. However , for some modifiers , the passivation effect of heat treatment is not very significant . On the contrary , if the solid solution temperature and time are not well controlled , coarsening will occur. For example , the unmodified aluminum-silicon alloy is difficult to remove Improve the shape of eutectic silicon , and the Al-Si alloy modified by Sr + Y + La can improve the shape of eutectic silicon and bulk phase well through solution treatment , and the effect of Sr + Y + La modification is through solid solution treatment. Solution aging treatment is the best .
3 Conclusion

1. Plastic deformation will improve the subsequent modification effect of all modifiers . Alloys can make up for the coarse structure caused by insufficient modification after rolling . Primary silicon , eutectic silicon , dendrites, etc. can be further refined , and the refining effect varies with deformation. increased by volume increase .
2. Homogenizing annealing can make the distribution of silicon phase more uniform , and solution aging process can affect the morphology of eutectic silicon .

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