2205 duplex stainless steel sheet and coil development and application

2205 duplex stainless steel is one of the most widely used duplex stainless steel, accounting for duplex stainless steel 80% or so, it is widely used in petroleum, chemical, salt, water conservancy, power plant flue gas desulphurization (FGD), papermaking and other fields, but its hot working plasticity is poor, which restricts the large-scale production and development of it, this paper, through the thermal simulation of compression experiments, the optimization of the solution treatment process, has successfully developed the 2205 duplex stainless steel plate coil, and the product has been successfully applied to chemical ships, pressure vessels and other industries.

Keywords: duplex stainless steel; thermal simulation compression experiments; solution treatment process; application

2205 stainless steel is a duplex stainless steel composed of austenitic and ferrite organization, the duplex stainless steel both ferrite and austenitic stainless steel performance advantages, with excellent corrosion resistance, mechanical properties, welding properties [1-2]. 2205 duplex stainless steel because of its low carbon, N, high chromium, molybdenum has austenitic stainless steels than 304, 316L, etc. better than 304, gap resistance Corrosion, Cl-pitting resistance, resistance to grain boundary corrosion. Excellent toughness, yield strength of 300 series stainless steel is 2 times; excellent welding performance, compared with ferritic stainless steel and austenitic stainless steel, the weld zone is not like ferritic such as a significant drop in plasticity, but also not like austenitic stainless steel, the weld heat cracking is more sensitive. Because of its excellent comprehensive performance, widely used in petroleum, chemical, salt, water conservancy, power plant flue gas desulfurization (FGD), paper and other fields [3-4].

However, due to the differences in austenite and ferrite organization composition, strength, elongation, hot working deformation is easy to produce stress concentration at the phase boundary, resulting in cracks on the edge or surface, which seriously affects the quality of its plate and the rate of formation. In this paper, through thermal simulation compression experiments, solution treatment process optimization, successfully developed 2205 duplex stainless steel plate coil, and the product has been successfully applied to chemical ships, pressure vessels and other industries.

1、Thermal simulation compression experiment

High-temperature hot compression experiments were carried out on a Gleeble 3500 thermal/force simulation testing machine, and the chemical composition of the specimens is shown in Table 1.The high-temperature compression specimens with a diameter of 10 mm × 15 mm were heated to 1200°C and then held for 5 min, and then cooled down to 1100°C at a rate of 5°C/s and held for 20 s before high-temperature compression deformation was carried out under the condition of a strain rate of 5 s-1 , and the specimens were quenched to room temperature after deformation was completed. The specimens were quenched to room temperature at the end of the deformation. The thermal simulation process route is schematically shown in Fig. 1, and the corresponding true stress-strain curves are shown in Fig. 2. From the rheological curves in Fig. 2, it can be seen that the coordinated deformation of ferrite matrix and austenite occurs due to the coexistence of two phases, ferrite and austenite. In the multi-channel rheological curve, the initial part of the first curve also appeared in a similar "yield-like platform" phenomenon. The microstructure evolution of duplex stainless steel during high temperature deformation is mainly affected by the strain distribution and the nature of the phase or grain boundaries. Compared with single-phase ferritic or austenitic phase, the high temperature microstructure evolution of duplex stainless steel is very different from it. For the ferrite phase, the initial stage of deformation, the dislocation density in the ferrite increases to produce work hardening. Subsequently, the ferrite occurs dynamic recovery, due to the strain is mainly distributed in the softer ferrite matrix, delaying the strain transfer to the austenite process through the phase boundary, thus impeding the continuity of deformation and the "yield-like platform" softening phenomenon. With increasing deformation, strain is transferred from ferrite to austenite through phase boundaries in order to maintain the continuity of material deformation. The strain transfer leads to an increase in the austenite rheological stress and work hardening occurs again. With further increase in strain, continuous dynamic recrystallization of ferrite and austenite occurs simultaneously. At the same time, the grain boundaries with Σ3 orientation relationship in the austenite phase gradually disappear, and the rheological curve softens again. It can be seen that the coupling effect of the dual-phase microstructure evolution mechanism in the high-temperature deformation process jointly determines the characteristics of the rheological curve [5-6].

The deformed sample was cut along the axis and the metallographic organization was observed, see Fig. 3. It can be seen in the figure that the brighter organization is austenite and the darker organization is ferrite. From the metallographic organization, it can be seen that with the increase of deformation, the streamline austenite organization along the rolling direction in the metallographic phase is more and more obvious and more elongated.

2、 Solid solution treatment experiment

2.1 Effect of solid solution treatment temperature on the mechanical properties of 2205 duplex stainless steelThree tensile and hardness specimens were taken for each solution treatment temperature and the experimental results were averaged and the test results are shown in Fig. 3. As can be seen from Figure 4 from 1020 ~ 1250 ℃ range of strength, hardness decreases first and then increases, 1050 ℃ when the hardness and strength values are smaller. Because the solution treatment process on the mechanical properties of 2205 duplex stainless steel is mainly affected by two factors: the degree of completion of tissue recrystallization and the degree of transformation of the two-phase organization. The higher the degree of completion of recrystallization, the smaller the strength and hardness; and the higher the ferrite content at room temperature, the greater the hardness and strength, mainly at room temperature to body-centered cubic ferrite organization strength is higher than the face-centered cubic for the organization of austenitic organization. Thus, with the solid solution temperature and the occurrence of austenite organization to ferrite organization will make the steel strength and hardness increase.

In 1020 ~ 1050 ℃ specimens of recrystallization completion of tensile strength and hardness is greater than the effect of tissue transformation on the strength and hardness, so the strength and hardness decrease. In 1080 ~ 1250 ℃ recrystallization of the specimen on the strength and hardness of the impact of less than the impact of tissue transformation on the strength and hardness, so the strength and hardness with the increase in temperature and increase.

2.2 Effect of solid solution temperature on the ferrite content of 2205 duplex stainless steel

The effect of different solid solution temperature on the ferrite content of 2205 duplex stainless steel is shown in Figure 5.From the figure, it can be seen that the ferrite content increases with the rise of temperature. It has been pointed out that when the phase ratio of duplex stainless steel is about 1:1, its comprehensive performance is better, through the 1000 ~ 1250 ℃ solid solution experiments, to determine the 2205 duplex stainless steel coil solid solution treatment temperature between 1050 ~ 1100 ℃, which is the ferrite content of about 50%.

2.3 Effect of solid solution temperature on the pitting performance of 2205 duplex stainless steel

GB / T 17897-1999 standard for different solid solution temperature treatment of the specimen after pitting corrosion experiments, the experimental temperature of 35 ℃, take the average value of corrosion rate, corrosion rate and solid solution temperature of the relationship between the temperature shown in Figure 6. Corrosion rate in 1020 ~ 1250 ℃ corrosion rate with the increase in temperature first reduced and then increased trend, in 1020 ~ 1120 ℃ corrosion rate tends to flatten out, the value of the difference is not large, and in 1150 ~ 1250 ℃ solid solution treatment, the corrosion rate is a sharp up and down trend, mainly due to the solid solution temperature of 1020 ~ 1250 ℃, the ability to resist pitting corrosion with the pitting corrosion equivalence of two phases between the two have a Close relationship, with the rise in temperature, dual-phase steel ferrite pitting resistance equivalent gradually decreased, while the austenite pitting resistance equivalent gradually increased.

In stainless steel, chromium, molybdenum, nitrogen is the main pitting resistance elements, of which chromium, molybdenum belongs to the ferrite generating elements, mainly present in the ferrite organization, while nitrogen belongs to the austenite forming elements, mainly solid solution in the face-centered cubic austenite organization. With the solid solution temperature continues to rise, the ferrite content of steel gradually increased, while the austenite content continues to decrease, resulting in dilution of chromium and molybdenum in the ferrite, leading to a gradual decline in ferrite pitting resistance equivalent, austenite nitrogen concentration gradually increased, and pitting corrosion occurs first in the pitting resistance of the more vulnerable phase.

3. Industrial trial production

3.1 Smelting of 2205 duplex stainless steel

2205 duplex stainless steel smelting process is: electric furnace → AOD → LF → continuous casting. Continuous casting process in the intermediate package to take the composition analysis specimen, the specimen after grinding in the direct reading spectral analysis instrument for composition analysis. The chemical composition of the continuous casting billet is shown in Table 2. from the smelting composition, it meets the design requirements. Fig. 7 shows the low times organization of continuous casting billet, in which the proportion of equiaxial crystals reaches 80%.

3.2 Rolling of 2205 duplex stainless steel

The rolling process of 2205 duplex stainless steel is as follows: step heating furnace → high-pressure water dephosphorization → rough rolling → finishing rolling → coiling. Its heating temperature is 1250 ℃, roughing passes for 5 passes, finishing passes for 7 passes, the final rolling temperature is 1000 ℃, Figure 8 is a photo of the hot rolled coil. As can be seen from the figure, the hot rolled coil has good shape and meets the design requirements.

3.3 Solution treatment of 2205 duplex stainless steel

The solution treatment process of 2205 duplex stainless steel is as follows: uncoiling → heating → shot blasting → electrolysis → pickling → curling. Among them, the solution treatment temperature is 1040～1080℃, and the cooling method is water cooling.

3.4 Properties of 2205 duplex stainless steel sheets

Table 3 is 2205 duplex steel mechanical properties, ferrite content and intermediate harmful phase detection, all the test values are used in three specimens to take the average value, from the test performance has reached the standard requirements.

3.5 Weldability of 2205 duplex stainless steel sheets

Table 4 is the welding process related parameters of duplex stainless steel. The experiment uses gas shielded welding, after welding to measure the mechanical properties of the weld and heat-affected zone, the mechanical properties are shown in Table 5. from the test data, the properties after welding are up to the standard requirements.

4. Main applications

Dongfang Special Steel 2205 duplex stainless steel has passed the certification of six classification societies in China, the United States, France, the United Kingdom, Germany, Norway and the certification of special equipment manufacturing license, which can be widely used in chemical ships, pressure vessels and other industries.

5. Conclusion

(1) The coupling of dynamic restitution and dynamic recrystallization evolution mechanisms during high-temperature deformation of dual-phase steels determines the characteristics of the rheological profile.

(2) tensile and hardness with the solid solution temperature first decreased and then increased, pitting resistance with (α + γ) two-phase pitting resistance equivalent change and change in the temperature range of 1020 ~ 1120 ℃ pitting rate change is not significant, while 1150 ~ 1250 ℃ in the temperature range, the pitting rate is a rapid increase in the trend.

(3) 2205 duplex stainless steel plate type is good, no edge crack, after testing, its composition, mechanical properties and corrosion properties are in line with the relevant standards.

(4) 2205 duplex stainless steel has passed the classification society certification and special equipment manufacturing license certification, and can be widely used in pressure vessels and chemical ships.

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