Hot extrusion + cold rolling manufacturing 022Cr25Ni7Mo4N

Abstract: Super duplex stainless steel 22Cr25Ni7Mo4N seamless steel pipes have long relied on imports. In order to achieve the goal of localization of such steel pipes, BBN STEEL Co., Ltd. The hot working, cold working and solution heat treatment processes were studied. After years of hard work , 022Cr25Ni7Mo4N seamless steel pipes with all properties meeting the relevant technical requirements have been produced , which is a successful step in localization.
Key words: super duplex stainless steel pipe; 022Cr25Ni7Mo4N ; hot extrusion; cold rolling; process technology
Ferritic-austenitic duplex stainless steel pipe, which combines the advantages of ferritic stainless steel and austenitic stainless steel, has excellent stress corrosion cracking resistance, high mechanical strength, excellent pitting and crevice corrosion resistance, and low thermal expansion coefficient. Impact resistance, fatigue corrosion resistance, low service cycle cost. In recent years, with China's substantial increase in oil and gas production, the ability to refine sour crude oil has been continuously improved, the downstream products that use ethylene as raw materials have been deeply developed, the fertilizer production process has been optimized, and the exploration and development of offshore oil and gas has been vigorously promoted. Therefore, the pitting corrosion resistance index The super duplex stainless steel of PREN M 40 is widely used.
022Cr25Ni7Mo4N (ASTM grade UNS S32750 , referred to as 2507) is a typical representative of the current super duplex stainless steel pipe series. Compared with 2205 duplex stainless steel, its Cr, Mo and N contents have increased to a certain extent, especially the N content reaches About 0.28%, PREN reaches 43, so it has higher strength, better resistance to chloride ion pitting corrosion and stress corrosion resistance. The use of super duplex stainless steels often results in lower life-cycle costs compared to other materials. In the material selection process, it is not enough to only consider the purchase cost. In terms of short-term behavior, 2507 is more expensive than 304L, 316L and ordinary duplex stainless steel, but when considering the ratio of performance to cost, expected service life, maintenance costs, safety and reliability, whether it is possible to save materials or reduce process control, etc. When considering various factors, it is found that the use of super duplex stainless steel will reduce the overall cost, sometimes even a lot. It is foreseeable that 2507 super duplex stainless steel seamless steel pipes will be reused more and more widely in harsh chlorine-containing environments, chemical and processing industries .
Compared with austenitic stainless steel, 2507 duplex stainless steel seamless pipe has poor hot working plasticity and high cold working deformation resistance. In view of this, the company developed this product by using the "hot extrusion + cold rolling" process technology. The hot extrusion process has the characteristics of three-dimensional compressive stress deformation and large deformation, which is better than the two-roll cross-rolling commonly used in China. The piercing process has a small probability of defects; the cold rolling process equipment selects a long-stroke ring-type cold rolling mill to strictly control the dimensional accuracy, the chemical composition, mechanical properties, process properties, corrosion resistance, metallographic structure and high temperature of the finished steel pipe. Performance and so on have reached the specified technical requirements.

1. Technical conditions of steel pipe
   1. technical standard

( 1 ) GB/T 21833 — 2008 "Austenitic - ferritic duplex stainless steel seamless pipe".
( 2 ) Technical agreement: Technical conditions for the development of 2507 duplex stainless steel heat exchange tubes.

1. 1. technical indicators

( 1 ) See Table 1 for chemical composition requirements .
( 2 ) See Tables 2-3 for mechanical performance requirements .

	Table 1 2507 duplex stainless steel seamless steel pipe Chemical composition ( mass fraction ) requirements			%
C	Si	mn	P	S
W 0.030	W 0.80	W 1.20	W 0.035	W 0.020
Cr	Ni	Mo	N	Cu
24.00 ~ 26.00	6.00 ~ 8.00	3.0 ~ 5.0	0.24 ~ 0.32	W 0.50

Table 2 Room temperature mechanical property requirements of 2507 duplex stainless steel seamless pipe
Rp0.2 /MPa Rm /MPa A / % hardness HRC
M 550 M 800 M 15 W 32

Table 3 Requirements for high temperature mechanical properties of 2507 duplex stainless steel seamless steel pipes
Test temperature / seven 50 100 150 200 250 300 350
Ra/MPa M 530 M 480 M 445 M 420 M 405 M 396 M 393
( 3 ) The metallographic structure of the finished pipe should be austenite and ferrite, the mass fraction of ferrite is 40 %~ 60% , and no o phase is allowed in the structure. The grain size is not lower than grade 6 .
( 4 ) Carry out corrosion tests, including pitting corrosion tests, crevice corrosion tests, chloride ion stress corrosion cracking tests, intergranular corrosion tests and harmful precipitate phase tests.

1. Research and analysis of pipe making technology
   1. Pipe making process

Smelting T forging round steel T tube billet processing T hot extrusion parent tube T cold rolling (finished product) T solution treatment T finishing T pickling T inspection T packaging storage.

1. 1. Condition of the tube blank The tube blank is provided by Shanxi Taigang Stainless Steel Co., Ltd.

by electric furnace smelting + out-of-furnace refining. After entering the factory for re-inspection, the chemical composition of the tube blank meets the technical requirements; the inclusion content of the tube blank is 0.5 for class A , 0.5 for class B , 0.5 for class C , and 1.0 for class D. The total non-metallic inclusions are Grade 2.5 , which meets the requirements of technical conditions; the metallographic structure of the tube blank is an austenite - ferrite two-phase structure, the mass fraction of ferrite is 46% , and there is no O phase and other harmful phases, which meets the requirements of technical conditions.

1. 1. Tube blank processing technology

The tube blanks processed by the long material peeling machine have eliminated all the defects (scale, forging marks, cracks, etc.) on the surface of forged and rolled round steel. The diameter tolerance of the bar after peeling is 0.5 mm , and the surface roughness of the processed tube blank is W 1.6 " m . The tube blank after segmentation, drilling, and end face processing is subject to dimensional tolerance and surface inspection and cleaning according to the process requirements.

1. 1. Tube blank heating process

The tube blank enters the continuous degreasing cleaning device for cleaning and degreasing before heating , completely removes the grease and dirt on the inner and outer surfaces of the tube blank , prevents the tube blank from being polluted during the heating process, and ensures the quality of the tube blank.
The heating of the tube billet before extrusion adopts a three-stage heating process:
One is low temperature preheating. Fully preheat in the annular furnace with natural gas as the medium to ensure uniform preheating of the tube blank at low temperature, prevent secondary pollution at high temperature, and reduce oxidation on the surface of the tube blank.
The second is to carry out two high-temperature rapid heating. In the controllable power frequency induction furnace, it is quickly and accurately heated to the temperature of hole expansion and hot extrusion respectively, so as to avoid staying in the brittle transition zone of the material for a long time. Make sure that the extruded tube has a reasonable structure, good surface quality and uniform wall thickness.

1. 1. hot extrusion process

The thermal deformation process uses two deformation processes. Firstly, thermal expansion of the tube blank is carried out by using a movable reaming head and a special lubrication method to obtain a reamed tube blank with uniform wall thickness and good surface. Then, under the action of three-dimensional compressive stress, the tube blank completes the deformation process of the hot extrusion parent tube.

Procedure. Due to the large amount of extrusion deformation, the structure is dense and the performance differences are small. This process is an internationally accepted high-alloy steel pipe processing method, which avoids the possibility of cracks on the inner surface of the blank pipe produced by the two-roller piercing and rolling process, and provides high-quality parent pipes for cold-rolled steel pipes, and provides A reliable quality assurance. Research and practice have shown that having a fine duplex structure is very important for duplex stainless steel to obtain excellent performance. The main parameters of the extrusion process of 2507 duplex stainless steel seamless steel pipe are shown in Table 4 .
Table 4 Extrusion process parameters of 2507 duplex stainless steel seamless pipe

Blank specification /mm	Extrusion tube specification /mm	extrusion ratio	Extrusion deformation / %
$204	① 89x9.0	14	92.8

1. 1. cold rolling process

The cold rolling of steel pipes is completed on a long-stroke ring-groove cold-rolling mill. Because the yield strength of 2507 material is higher than that of ordinary austenitic stainless steel, it has greater initial deformation resistance, lower elongation, and cold working elongation coefficient is also smaller than that of austenitic stainless steel. A three-pass cold-rolling process route was formulated, the elongation coefficient of each pass was reasonably distributed, and relatively small deformation was adopted (Table 5 ). At the same time, the appropriate rolling speed and feed rate are selected during cold rolling to reduce longitudinal wall thickness unevenness and rolling folds. The test shows that the plasticity of the material meets the requirements of reasonable deformation in the cold rolling process.
Table 5 Cold rolling process parameters of 2507 duplex stainless steel seamless pipe

pass	Steel pipe specification /mm	elongation factor
0	① 89x9.0	-
1	① 57x5.0	2.81
2	① 38x3.5	2.22
3	① 25x2.0	2.28

After each pass of cold working, the material is annealed and heat treated to eliminate work hardening and restore the plasticity of the material.

1. 1. heat treatment process

The heat treatment process is an important link to ensure that the two-phase structure ratio, normal temperature mechanical properties, high temperature mechanical properties, corrosion resistance and welding properties of 2507 duplex stainless steel seamless steel pipes meet the technical requirements .
The steel pipe heat treatment is carried out in a segmented continuous roller bottom solid solution heat treatment furnace using natural gas as fuel. The results of the heat treatment process test show that within a certain heating time (including the holding time), as the solution treatment temperature increases or the holding time increases, the ferrite content of the steel pipe tends to increase, and the grain size is relatively large . According to the technical information of Sandvik Company of Sweden, the solution heat treatment of 2507 duplex stainless steel seamless steel pipes generally selects a heating temperature of 1 050~1 125 7, sometimes even higher. On the basis of a large number of experimental studies, the company has formulated the solution heat treatment heating temperature of 2507 duplex stainless steel seamless steel pipe to be 1 070 ~ 1 120 T. Content, corrosion resistance, etc. can meet the technical requirements.

1. Chemical composition, structure and properties of finished steel pipes
2. chemical composition

The chemical composition analysis results of the finished steel pipes are shown in Table 6 . In Table 6 , the pitting resistance index PREN ( PREN =% Cr + 3.3%Mo+16%N ) of 2507 duplex stainless steel seamless steel pipe is 42.87 , which meets the requirements of the technical condition PREN M 40 and ensures that the material can withstand chlorine in the environment Good pitting corrosion resistance. The w ( N ) content is 0.27% , and the w ( Ni ) content is 7.68% , which is controlled at the upper limit of the technical requirements, which is conducive to the balance of the two-phase ratio of ferrite and austenite in steel pipes.

1. Tensile properties at room temperature The tensile properties at room temperature of steel pipes are shown in Table 7 .

Table 6 Analysis results of conventional chemical composition ( mass fraction ) of 2507 duplex stainless steel seamless pipe %

project	C	Si	mn	P	S	Cr	Ni	Mo	N	Cu
measured value	0.025	0.437	0.94	0.027	0.003	25.18	7.68	4.05	0.27	0.049
Technical condition requirement value	W 0.030	W 0.80	W 1.20	W 0.035	W 0.020	24.00 ~ 26.00	6.00 ~ 8.00	3.0 ~ 5.0	0.24 ~ 0.32	W 0.50

Table 7 Room temperature tensile properties of $ 25 mm x 2.0 mm 2507 duplex stainless steel seamless pipe

project	R p0.2 /MPa	Rm /MPa	A / %	Hardness HRC
Sample 1	625	915	46.5	27.6
Sample 2	630	890	46.0	27.1
Technical condition requirement value	M 550	M 800	M 25.0	W 28.0

1. Metallographic structure and grain size The microstructure of the steel pipe is austenite - ferrite (Figure 1 ), ferrite

The mass fraction of bulk is 43% , and the grain size is 9.5 .

1. Short time high temperature tensile properties

At 50-350 ° C , a short-term high-temperature tensile test was carried out on 2507 duplex stainless steel seamless steel pipes, and the yield strength values are shown in Table 8 .

1. Flattening and expansion test
2.  

Table 8 Short-term high-temperature tensile yield strength MPa of 2507 duplex stainless steel seamless steel pipe

project	R p0, 2 at different test temperatures
	50C _	100C _	150C _	200C _	250C _	300C _	350C _
Sample 1	705	630	560	510	490	495	520
Sample 2	695	655	555	540	505	490	485
Technical condition requirement value	M 530	M 480	M 445	M 420	M 405	M 396	M 393

The flattening test was carried out on the finished pipe samples, and there were no cracks and cracks in the samples after flattening; after the flaring test, there were no cracks and cracks in the samples.

1. Corrosion resistance

Intergranular corrosion test, harmful phase test, pitting corrosion test and stress corrosion test were carried out on 2507 duplex stainless steel seamless steel pipe. The results are as follows:

1. Intergranular corrosion test. The sample was subjected to the intergranular corrosion test in the standard medium of method E of GB/T 4334-2008 " Metal and Alloy Corrosion Stainless Steel Intergranular Corrosion Test Method" . In the slightly boiling state, after 16 h test, there is no crack.
2. Hazardous phase test. The samples were tested for harmful phases in ASTM A 923-2006 Method C standard medium . The test period was 24 h under the 407 state , and the average corrosion rate was 0.001 04 g/ ( m 2 ·h ).
3. Pitting test. The sample is tested in the standard medium of GB/T 17897-1999 "Test Method for Ferric Chloride Pitting Corrosion of Stainless Steel". After 24 hours , the maximum corrosion rate of the sample is 0.017 g/ ( m 2 -h ), and the average corrosion rate is 0.015 g/ ( m 2 -h ) o
4. Stress corrosion test. The samples were tested according to the standard of YB/T 5362-2006 " Stress Corrosion Test Method for Stainless Steel in Boiling Magnesium Chloride Solution". Under the tensile stress of 80% R p0.2 , the sample was tested for 96 h , and no cracks were found by visual inspection with a 10 times magnifying glass.

4 Conclusion

1. "hot extrusion + cold rolling" process technology is used to produce 2507 duplex stainless steel heat exchange tubes, which is advanced and reasonable. The hot extrusion process adopts two deformation processes of hot hole expansion and hot extrusion. The steel pipe has a good surface quality, uniform wall thickness, and excellent comprehensive performance; the cold rolling adopts three-pass cold rolling process, which improves the uniformity of the steel pipe structure and the surface of the steel pipe is smooth. , High dimensional accuracy.
2. The solution heat treatment system has an effect on the ferrite content and grain size of the steel pipe. With the increase of the solution treatment temperature or the extension of the holding time, the ferrite content of the steel pipe tends to increase, and the grain size is relatively large . Using a heating temperature of 1 070 ~ 1 120 ° C , and adopting a suitable heating rate, holding time and cooling rate, the metallographic structure and grain size of the steel pipe meet the requirements of the technical conditions.
3. Using the tube billet processing, hot working, cold working, and heat treatment processes obtained from the research, the technical properties of the developed 2507 duplex stainless steel seamless steel pipe meet the requirements of the technical conditions.
4. 2507 duplex stainless steel seamless steel pipe will promote the localization process of this type of steel pipe, making it more used in oil refining, petrochemical industry, chlor-alkali chemical industry, seawater environment, chemical fertilizer industry, and heat exchangers in power station flue gas desulfurization devices , coolers, evaporators and pipelines and other fields.

---