Research and development of P9 seamless steel pipe for petrochemical plant

The domestic research and development process of P9 hot-rolled seamless steel pipes for petrochemical plants is introduced . Through reasonable steelmaking , rolling and heat treatment process control, the chemical composition and various properties of the developed P9 hot-rolled seamless steel pipe meet the requirements of the ASME SA 335/SA 335M standard. The microstructure is tempered martensite, brittle The transformation temperature is around -48°C , and the short-term high-temperature performance meets the requirements of relevant ASME specifications. At 600 °C , the yield strength can still reach 270 MPa , which can replace imported ones and realize the localization of P9 seamless steel pipes .
Key words: P9 seamless steel pipe; heat-resistant steel; production process; brittle transition temperature; short-term high-temperature performance

P9 steel is a martensitic heat -resistant steel, by adding 9% Cr and about 1 % M. , so that the steel has strong oxidation resistance and high temperature strength; it can be applied to superheater tubes , reheater tubes, and steam pipes at a temperature of W 650 C , and can also be applied to highly corrosive oil at a temperature of W 550 C Chemical equipment ⑴. At present, the P9 seamless steel pipes used in the domestic petrochemical industry are basically purchased from abroad, but the delivery time is long and the price is expensive, which seriously restricts the rapid development of China's petrochemical industry . To this end, Tianjin Steel Pipe Group Co., Ltd. (Tianjin Steel Pipe for short) has carried out localized research and development of P9 seamless steel pipes .
1 Production process of P9 seamless steel pipe
According to the current equipment status of TPCO, combined with the characteristics of P9 steel, the trial production process of P9 seamless steel pipe is designed : electric arc furnace steelmaking - ladle refining - vacuum degassing - die casting - tube billet forging - tube billet annealing - tube billet heating ——Skew rolling and piercing L-PQF continuous pipe mill rolling - three-roll sizing - cooling bed cooling - pipe end sawing - pipe straightening - magnetic flux leakage detection - heat treatment - straightening - ultrasonic flaw detection - hydraulic test - dimensions And visual inspection - steel pipe storage .

1. Steelmaking and Alloy Composition Control

Since 9% Cr is added to P9 steel , in order to ensure that the temperature of molten steel does not drop too much in the later stage of smelting, and to shorten the steelmaking cycle, the induction furnace is used to melt the alloy mother liquor in advance, thereby avoiding the traditional process of adding a large amount of alloy to the molten steel Adverse effects of temperature , treatment cycle and carburization of molten steel .
C content is too low , the high temperature strength of P9 seamless steel pipe will be reduced, and 8 ferrite may be formed at the same time ; but if the C content is too high, the weldability will be reduced, and the room temperature hardness will be too high . By using high-quality low-carbon scrap steel, increasing the proportion of molten iron, and strictly controlling the C content, the C content is finally designed to be 0.08% to 0.12% .
S and sulfides are easy to segregate at the grain boundaries, thereby weakening the grain boundary strength and forming holes, leading to grain boundary embrittlement and creep embrittlement. Therefore, strict control is required, and the S content is generally controlled below 0.005 % .
Al is a good deoxidizer, it will also significantly reduce the high-temperature durability of P9 steel, so it is controlled below 0.015% .
In addition, P9 steel has the effect of reducing the austenite zone due to its high content of ferrite-forming elements such as Cr , Mo , and Si . Due to the expansion of the ferrite phase region and the shrinkage of the austenite phase region, the 8 -ferrite that originally existed in the high-temperature region above 1 400 Wu may be retained at room temperature, and the 8 -ferrite has a significant effect on the room temperature of the steel. The influence of plasticity is quite obvious, and at the same time, the impact on the durable strength of steel is also quite significant ; therefore, it is necessary to suppress the formation of 8 ferrite during the cooling process of steelmaking, that is, strictly control the content of ferrite forming elements such as Cr , Mo , and Si .
P9 seamless steel pipe specification is ① 219.1 mm x 10.31 mm ) measured value of chemical composition .

Table 1 Chemical composition (mass fraction) of P9 seamless steel pipe %

project	C	Si	mn	P	S	Ni	Cr	Mo	Cu	Al
Measured 1	0.11	0.40	0.41	0.013	0.002	0.07	9.07	1.01	0.06	0.014
Measured 2	0.11	0.41	0.42	0.013	0.002	0.07	8.99	1.01	0.06	0.014
Standard requirements [ 5 ]	W0.15	0.25 ~ 1.00	0.30 ~ 0.60	W 0.025	W 0.025	-	8.00 ~ 10.00	0.90 ~ 1.10	-	-

1. rolled tube

P9 seamless steel pipe has the characteristics of high content of Cr and Mo alloy elements, high deformation resistance at high temperature, and difficult rolling . TPCO has ensured that the steel grade is rolled in the best plastic deformation zone by setting reasonable ring furnace heating temperature and rolling process parameters .

1. heat treatment

P9 seamless steel pipe can be heat treated by annealing or normalizing + tempering process . The cooling speed of the annealing process is slow, which affects the production rhythm. It is difficult to control the production process and the cost is high; therefore, the annealing heat treatment process is rarely used in actual production, and the normalizing + tempering heat treatment method is often used instead of the annealing process to achieve industrialization . produce .
P9 steel does not contain microalloying elements such as V and Nb , it is positive
The fire temperature is lower than that of P91 steel, which is 950 ~ 1050 °C , and the heat preservation

1. when normalizing in this process, most of the carbides dissolve but no obvious grain growth occurs, but if the normalizing temperature is too high, austenite grains tend to be coarse (6); the tempering temperature is 740 ~ 790 C , in order to obtain For lower hardness, the tempering holding time should be extended appropriately .

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