Thermoplasticity of duplex stainless steel is poor due to the different deformation behaviors of the austenite phase and the ferrite phase. The stress and strain distribution between the two phases during hot processing is uneven, which can easily induce the nucleation and expansion of cracks at the phase boundary. Brittle cracking occurs when steel pipes are hot perforated, rolled, hot extruded in the deformation zone. At the same time, due to the high yield strength of duplex stainless steel and its great resistance to cold working deformation, the amount of cold working deformation is strictly limited, and problems such as cracking and roughening of steel pipes may occur during cold working deformation. In addition, duplex stainless steel is prone to separate harmful phases during hot working, which will seriously affect the toughness, plasticity and corrosion resistance of steel. A company in China has developed a method of manufacturing large-diameter thin-walled duplex stainless steel seamless pipes that are economical, flexible, have a high yield rate and high manufacturing precision through the organic combination of hot rolling, hot expansion, and cold drawing processes. This method effectively solves many problems in the manufacturing of large-diameter thin-walled duplex stainless steel seamless pipes.
 
1. Developing manufacturing processes of large-diameter thin-walled duplex stainless steel seamless pipes
1.1 Manufacturing process plans
The overall manufacturing process of large-diameter thin-walled duplex stainless steel seamless pipes are as follows: billet refining and continuous casting, forging billet opening, turning and perforation of billets, hot rolling, hot expansion, cold drawing, solution treatment and fine finishing.
 
1.2 Refining processes of steel billets
S32205 duplex stainless steel is a low-carbon, high-chromium, nitrogen-controlled stainless steel. Adding an appropriate amount of N to stainless steel can not only improve the strength of the steel but also improve the corrosion resistance of the steel, including the ability to resist pitting corrosion and intergranular corrosion between products." To achieve the required N content, Chromium nitride is added for adjustment in the later stage of vacuum refining. However, chromium nitride contains a small amount of C, which will affect the precise control of C content. Therefore, the C content in chromium nitride should be controlled to not exceed 0.30%, or nitrogen charging should be used to reduce the amount of chromium nitride added.
 
Electric furnace steelmaking, refining outside the furnace, vacuum oxygen decarburization and continuous casting are used to prepare duplex stainless steel billets. Electric furnaces (EF) are used for the primary refining of molten steel, decarburization and phosphorus removal, and strict control of the phosphorus content during liquid discharge. Leave room for the phosphorus content to increase during the subsequent LF alloying for phosphorus; then it is refined in the LF furnace, desulfurized to w (S) less than and equal to 0.005%, and deoxidized to w ( O) 0.0025%; alloy the molten steel. Adjust the composition of the molten steel, and discharge the liquid to the VOD furnace after passing the spectral sampling analysis; use the vacuum oxygen blowing decarburization to further decarburize, and adjust the oxygen blowing flow rate according to the degree of vacuum in the vacuum oxygen blowing process. As the degree of decarburization deepens, increase the degree of vacuum to below 100Pa to increase the affinity between carbon and oxygen and reduce the oxidation of chromium. In addition, fill the molten steel with nitrogen or use ferrochromium nitride alloy FeNCr3-A with a carbon content of less than 0.03%, achieving the purpose of increasing nitrogen while preventing the carbon content from increasing. Through the two-step refining process of the LF furnace and VOD furnace, the purity of the molten steel can be improved, and the proportion of ferrite and austenite phases can be ensured by precisely controlling the chemical composition.
 
After the chemical composition is qualified, the continuous casting billet is tapped and poured. Before the molten steel is poured, the mold cavity is purged with argon gas. When the actual oxygen concentration is lower than 4%, pouring begins. In the pouring process, oxygen is continuously blown for protection. To prevent secondary oxidation of molten steel, appropriately speed up the cooling rate of molten steel to avoid harmful phases such as σ phases, X phases, and intermetallic compounds in the solidification process of molten steel.
 
1.3 The ultimate blanking process
Duplex stainless steel pipes serving in marine environments sometimes experience lower ambient temperatures. Since the ferrite phase in duplex stainless steel has a body-centered cubic structure, there is a ductile-brittle transition in low-temperature environments, and the ferrite and austenite phases have a band-like structure distributed between them; the anisotropy of the material is great. The transverse low-temperature impact performance is poor, and brittle fractures are extremely prone to occur in low-temperature environments. Therefore, higher requirements are put forward for the transverse low-temperature impact toughness in the technical agreement. To give full play to the excellent toughness of the austenite phase in duplex stainless steel, to take advantage of the excellent toughness of the austenite phase in duplex stainless steel and improve the toughness of the ferrite phase, the upsetting and elongation forging was used in the development process.
 
The continuous casting billet is heated in the heating furnace. The heating process is divided into three stages. The first stage is to heat up to 800℃ at a lower temperature rise rate. Keep the temperature for 1.0 to 1.5 h to make the temperature inside and outside the continuous casting billet uniform; the second stage is to rapidly heat through the sensitive temperature range of 800 to 960℃ to avoid the precipitation of harmful phases; the third stage is to heat to the temperature range of 1080 to 1150℃ and keep 3 to 5 hours; the holding time is determined according to the size of the continuous casting billet. Within the temperature range where harmful phases precipitate, the residence time should be reduced as much as possible, but rapid heating should be avoided to prevent the billet from cracking due to the excessive temperature difference between inside and outside in the forging process. After coming out of the furnace, perform multiple passes of upsetting and lengthening to ensure that the total forging ratio is greater than 5 and the transverse deformation ratio is greater than 0.5 times the longitudinal deformation ratio. By accurately controlling the opening temperature, final forging temperature, deformation amount and transverse and longitudinal deformation ratio of each forging, the as-cast tree structure of the continuous casting billet can be effectively eliminated; the anisotropy of the material can be reduced, and the shape of inclusions can be improved; break inclusions and coarse grains, thereby refining and homogenizing the two-phase structure, improving the plasticity and toughness of the material.
 
When the forging temperature approaches 960°C in the upsetting and drawing process, it should be returned to the furnace for heating. The initial forging temperature of the last forging is controlled at 1050 to 1100℃, and the final forging temperature is 960℃. The continuous casting billet is forged, and then returned to the heating furnace to be heated to 1050 to 1150℃; kept for 1 to 2 hours for stress annealing and spray water for rapid cooling or water quenching after taking out from the furnace. The annealing temperature should be selected to ensure that harmful phases such as σ are fully dissolved but to prevent the temperature from being too high to cause grain growth. The isothermal transformation curve of duplex stainless steel is shown in Figure 1. The selection of an annealing cooling rate should avoid insufficient cooling rate, which will lead to the precipitation of harmful phases such as σ, and prevent the excessive cooling rate from causing quenching cracks.
 
1.4 Hot rolling and hot expansion processes
There is a significant difference in the strength of austenite and ferrite in duplex stainless steel, and the softening mechanism during hot working deformation is also different, causing stress to easily concentrate at the phase boundary and cracks to initiate and expand; cracks on edges and surfaces are prone to occur during hot working. Therefore, when formulating the hot working process, a temperature range with a suitable proportion of ferrite and austenite should be selected. At the same time, the surface temperature drops on the roller table (usually 60 to 80℃) should be considered after the tube blank is taken from the furnace and before hot piercing; the temperature rise in the hot punching process related to the rotation speed of the molybdenum plug, generally 60 to 100℃. In addition, the precipitation of σ phase should be avoided in the heating and cooling stages of thermal processing.
 
The following hot rolling process was determined through experimental research: heating in an annular heating furnace, rapid heating through the sensitive temperature range of 800 to 960°C to avoid harmful phase precipitation, heating to 1150 to 1190°C, holding for 120 to 150 minutes, and then piercing on the conical roller piercing machine. The three-roller limited mandrel PQF continuous rolling pipe unit or the tapered roller cross-rolling pipe unit is used for hot rolling to ensure that the rolling ratio is greater than 3 to produce pipe blanks.
 
The intermediate frequency thermal expansion process is determined as follows: the intermediate frequency heating temperature is between 960 and 1000℃; the diameter expansion ratio is 1.2 to 1.5, and the pipe expansion advancement speed is 30 to 60 mm/min. Ensure that the raw pipe is fully preheated and reaches the specified thermal expansion temperature when entering the deformation zone. In the thermal expansion process, the temperature change in the deformation zone of the pipe blank is stabilized within the range of ±5°C through the intelligent constant temperature system. After the pipe blank passes through the deformation zone, forced air cooling or spray cooling is used to quickly pass through the sensitive temperature range where harmful phases and brittle phases precipitate.
 
1.5 Cold drawing processes and solution treatment
Cold drawing is mainly to reduce the wall thickness, while improving the dimensional accuracy, and internal and external surface quality of the steel pipe. In addition to selecting an appropriate cold drawing mold, the wall thickness reduction in each pass should also be controlled to not exceed 3 mm due to the high hardness of S32205 duplex stainless steel, and the cold drawing speed should not exceed 0.5m/min. The quality of the inner and outer surfaces of the steel pipes must be inspected one by one in the cold drawing process. If there is any roughening, the cold drawing mold must be ground. Intermediate annealing must be performed before each pass of cold drawing. In the intermediate annealing heating process, rapid heating passes through the σ phase precipitation sensitive temperature range of 800 to 960°C. When heated to 1050 to 1120°C, the temperature is maintained for 90 to 120 minutes. Then water is introduced from the furnace and rapidly cooled to recrystallize the structure to fully soften and reduce strength and hardness.
 
The solution heat treatment of duplex stainless steel is to achieve the appropriate ratio of ferrite phase and austenite phase to ensure the mechanical properties and corrosion resistance of the product. Generally, the overall performance of S32205 duplex stainless steel is better when the ferrite phase and austenite phase account for half each. Generally, the solution heat treatment temperature of S32205 duplex stainless steel is 1020 to1100℃. Through heat treatment process evaluation test research, it was determined that the solid solution temperature of S32205 duplex stainless steel seamless pipe is 1040 to 1080℃. During the heating stage of solution treatment of the finished steel pipe, it is rapidly heated through the sensitive temperature range of 800 to 960°C to avoid harmful phase precipitation. It is heated to 1040 to 1080°C and then kept for 1 to 2 hours. After being released from the furnace, it is transferred to the water with an initial temperature not exceeding 35 ℃ within 60 seconds to obtain the best ratio of austenite and ferrite, good pitting corrosion resistance and comprehensive mechanical properties.