TMCP Simulation for Hot Rolling of P91 Seamless Steel Pipe

 However, the rolling process of steel tube is complex and difficult to control, which results in narrow technological window of piercing, rolling and sizing, which limits the application of TMCP technology in seamless steel pipe production. The United States and other developed countries have carried out systematic research on TMCP of steel pipes [9, 10, 11, 12, 13], but the main equipment used is expensive, and they have imposed export restrictions and technical confidentiality on China. Only a few scholars in China have conducted preliminary studies on TMCP of steel tubes such as on-line normalization, on-line quenching and on-line accelerated cooling [14, 15, 16, 17, 18, 19, 20]. Because the control mechanism of microstructure is not clear, it is difficult to get the ideal structure with good strength and toughness, and it is difficult to improve the comprehensive performance of steel tube. In view of this, based on the advanced PQF (premium quality finishing) tube rolling process and the dynamic transformation law of P91 steel, the TMCP of P91 steel pipe is realized by means of hot mechanical simulation technology, the deformation behavior, transformation behavior and the second phase precipitation law of P91 steel pipe are studied, the structure genetic law is explored, and the feasibility of TMCP of P91 steel pipe is verified.

Experimental method

The sample is taken from P91 forged tube blank of a factory, and its chemical composition is listed in Table 1. The dynamic transformation point test sample and TMCP hot rolling deformation simulation sample were cut from the forging by molybdenum wire cutting machine, and their sizes were 6 mm × 90 mm and 8 mm × 15 mm respectively.

Table 1  Chemical composition of P91 (mass fraction, %)

C	Si	Mn	P	S	Cr	Mo	V	Nb	N	Ni
0.09	0.32	0.49	0.018	0.003	9.00	0.90	0.20	0.10	0.03	0.02

The experimental scheme of dynamic phase change law research: heating the test sample of dynamic phase change point on gleeble-1500d thermal simulation test machine under vacuum condition, setting the heating rate: 20 ℃ / min at 30 ~ 650 ℃, 2 ℃ / min at 650 ~ 1060 ℃. The austenitizing condition is 1060 ℃ for 30 min. Then it is reduced to 1040 ℃ and 990 ℃ at the speed of 2 ℃ / min. the true strain is equivalent to the total deformation of sizing, taking 0.2, and the cooling speed is 0.5 ℃ / s and 1.0 ℃ / s respectively.

The thermal expansion curves under different deformation conditions were measured by dilatometer, and the transformation point MS was determined. In order to improve the accuracy of testing three times under each condition, the average value of MS point is calculated. The samples with different cooling rates were cut along the cross section, and then milled, polished and corroded by ferric chloride hydrochloric acid solution. The evolution of microstructure was observed by SEM.
TMCP hot rolling deformation simulation experiment scheme: Based on the production process of PQF of seamless steel pipe in the field: two roll tapered roll piercing, three roll limit mandrel PQF continuous rolling, micro tension sizing, using the multiple hot compression experiment of gleeble-1500d hot simulation testing machine to simulate TMCP hot rolling deformation. As P91 steel belongs to high alloy steel, its high-temperature deformation resistance is large, so when TMCP process parameters are formulated, the heating temperature is 1290 ℃, and the high-temperature large deformation with true strain of 1.8 is proposed to be used for recrystallization controlled rolling in perforation and continuous rolling stage. The TMCP process parameters in sizing stage refer to the research results of dynamic transformation law of P91 steel, and hot rolling is carried out on gleeble-1500d thermal simulation test machine The process of piercing (1 pass) – continuous rolling (5 passes) – sizing (7 passes) was simulated by thermo mechanical test. The technological parameters of piercing, continuous rolling and sizing deformation are listed in Table 2. After sizing, cooling is controlled to room temperature with cooling speed of 0.5 ℃ / s and 1.0 ℃ / s respectively.

Table 2  TMCP parameters of piercing, PQF continuous rolling and sizing of P91 pipe

Pass	Equivalent strain	Temperature /℃	Strain rate /s-1	Interval time/s
Heating	–	1290	–	–
Piercing	1.303	1250	2	50
PQF1	0.153	1125	3	0.853
PQF2	0.15	1112	4	1.004
PQF3	0.104	1098	4	0.678
PQF4	0.056	1088	3	0.645
PQF5	0.012	1080	2	50
Sizing1	0.019	1040	2	2
Sizing 2	0.015	1036	2	2
Sizing 3	0.016	1025	2	2
Sizing 4	0.015	1018	2	2
Sizing 5	0.015	1004	2	2
Sizing 6	0.008	996	2	2
Sizing 7	0.008	990	2	–

The simulated samples of different hot deformation stages (respectively water quenched after perforation and continuous rolling, and controlled cooling after sizing) were cut along the cross section, and observed by laser co aggregation microscope and scanning electron microscope (SEM) after grinding, polishing and corrosion by ferric chloride hydrochloric acid solution. Then, it was cut into 0.4mm thick thin plates by wire cutting, mechanically thinned to 30-50 μ m, and then thinned by electrolytic double spraying. The fine substructure was observed by jeol transmission electron microscopy (TEM) with an accelerating voltage of 200kV, and the rule of tissue transmission in different deformation stages of P91 steel tube under TMCP was studied.

Results and discussion

Dynamic phase transformation law

Figure.1 Shows the thermal expansion curve of P91 Steel under the conditions of 1 ℃ / s cold speed and different sizing deformation. It can be seen from figure 1A that the average value of MS point of P91 steel is 431 ℃ without applying deformation. It can be seen from Fig. 1b that the average value of martensitic transformation point ms of P91 steel can be increased to about 442 ℃ by applying sizing deformation with equal effect changing to 0.2 at 1040 ℃. The reason is that deformation can not only refine grains, but also introduce deformation dislocation, increase martensite nucleation point, promote martensite nucleation and increase MS point. Compared with FIG. 1b and C, when the sizing temperature decreased from 1040 ℃ to 990 ℃, the average value of MS point increased from 442 ℃ to 452 ℃. The reason is that the temperature is the sensitive parameter of dislocation movement. When the deformation temperature is low, the dislocation movement can be restrained, so that the deformation band formed in the process of deformation can better segment the original austenite grains, increase the grain boundary area of martensite nucleation, and increase the MS point.

Fig.1 Thermal expansion curves of P91 steel under different deformation conditions (cooling rate is 1℃/s) (a) 1040℃, ε=0, (b) 1040℃, ε=0.2, (c) 990℃, ε=0.2

Fig. 2 shows the SEM microstructure under different deformation and cooling conditions. It can be seen that the deformation not only improves the MS point, but also refines the lath martensite. The comparison between Fig. 2a and Fig. B shows that when the deformation temperature is 1040 ℃, the deformation with true strain of 0.2 can refine the martensitic lath bundle from 3.0-4.0 μ m to 1.5-3.0 μ m; the smaller the deformation temperature is, the smaller the martensitic lath bundle is. Compared with FIG. 2B and C, it can be seen that the deformation temperature is further reduced from 1040 ℃ to 990 ℃ and the lath bundle is further refined to 1.0-1.5 μ m; the lath martensite can also be significantly refined by increasing the cooling rate after deformation. Compared with figure 2C and figure D, when the cooling rate after deformation increases from 0.5 ℃ / s to 1 ℃ / s, the martensitic lath bundle is further refined to 0.6-1.0 μ M. Based on the above results, in order to obtain the fine lath martensite, the TMCP process parameters of P91 steel tube were determined, the final rolling temperature was 990 ℃, the equivalent true strain was 0.2, and the cooling rate of 1 ℃ / s was used to control the cooling after the sizing deformation.

Fig.2 SEM microstructures of P91 under different deformation and cooling conditions (a) 1040℃, ε=0, cooling rate=0.5℃/s, (b) 1040℃, ε=0.2, cooling rate=0.5℃/s, (c) 990℃, ε=0.2, cooling rate=0.5℃/s, (d) 990℃, ε=0.2, cooling rate=1℃/s

Recrystallization behavior of TMCP

The true stress-true strain curve of TMCP of P91 steel tube with single pass piercing, 5 passes continuous rolling and 7 passes sizing deformation is shown in Figure 3. It can be seen from Fig. 3A that there is an obvious stress peak value at the perforation stage, RP is about – 81.933 MPa, and the corresponding strain ε P is – 0.349; after the stress drops, a stable platform appears, indicating that P91 steel has fully dynamic recrystallization during the perforation deformation, which greatly refines the billet grain. In theory, the critical strain ε C of dynamic recrystallization is about 0.83 ε P (ε P is the strain corresponding to the peak stress RP) [21,22,23]. It can be seen that the critical strain ε C of dynamic recrystallization in the perforation stage of P91 steel is about – 0.29, while the actual perforation strain is as high as – 1.303, so sufficient dynamic recrystallization is inevitable. The stress decreased rapidly after the perforation, which indicated that almost complete static recrystallization occurred in the gap time after the perforation. At the same time, the recrystallization of TMCP with large deformation will soften the billet structure, reduce the deformation resistance and improve the plasticity. It is very beneficial to improve the hot working performance of P91 steel with high deformation resistance in the subsequent continuous rolling and sizing process.

Fig.3 True stress-true strain curve of (a) piercing, continuous rolling and sizing, (b) continuous rolling and sizing (magnification) of P91 TMCP
It can be seen from the true stress-true strain curve (Fig. 3b) in the continuous rolling stage that the strain accumulated in the first two passes, and the stress in the second pass was as high as – 94.5 MPa, but then the stress decreased rapidly, indicating that from the third pass, since the strain value in each pass decreased significantly compared with the first two passes, the static recrystallization softening occurred before the strain accumulated. Because of the high deformation temperature (1250-1100 ℃) and large deformation amount (true strain up to 1.8) in the piercing continuous rolling stage of P91 steel, the recrystallization control rolling can be used to refine the austenite grains before sizing.
From the true stress-true strain curve of sizing stage (Fig. 3b), it can be seen that the first five passes of sizing deformation achieve the accumulation of stress variables, forming a certain degree of work hardening, the peak stress reaches – 54.535 MPa, the sixth and seventh passes are close to the finished rolling passes, the stress variables are small, and the accumulation effect of stress is no longer obvious. Because the total strain of TMCP of P91 steel does not reach the critical strain of dynamic recrystallization and is in the strain accumulation in the non recrystallization area, the TMCP sizing process can realize the controlled rolling of non recrystallization type, the deformation characteristics make the material in the high energy state with a lot of “defects”, increase the nucleation core of martensite, make the subsequent TMCP controlled cooling process easier to induce martensitic transformation, and greatly Refine martensitic lath.

Genetic rule of TMCP

Figure 4 shows the microstructure of P91 steel TMCP after perforation, continuous rolling and sizing. It can be seen from Fig. 4 that the grain refinement tendency is more obvious with the increase of deformation amount and the decrease of deformation temperature. It can be seen from FIG. 4A that there are obvious long fiber structures after perforation deformation, and dynamic recrystallization core can also be observed at the grain boundary of deformation grain, indicating that dynamic recrystallization has taken place; with continuous rolling deformation, static recrystallization grains gradually increase and continuously devour deformation grains, and grow up, until all the structures are transformed into recrystallization grains, gradually replacing the original coarse grains After continuous rolling, the equiaxed grain size is about 40 μ m, as shown in Fig. 4B. Because the TMCP of P91 steel is easy to move and annihilate due to the high temperature during the process of piercing and rolling, the metal is easy to form nucleus and grow up in the process of deformation, so as to refine the austenite grain. Figure 4C and figure d show the microstructure of P91 steel TMCP with cooling rate of 0.5 ℃ / s and 1 ℃ / s respectively after sizing. It can be seen that the coarse deformed grains disappear completely, and the cooling rate increases from 0.5 ℃ / s to 1 ℃ / s, and the grain size is refined from about 30 μ m to about 20 μ M. Because the TMCP of P91 steel uses the lower sizing and finishing temperature of 990 ℃, the movement speed of dislocation is lower, the deformation band formed in sizing process can better segment austenite grain, deformation-induced martensite transformation, promote martensite nucleation, combined with the controlled cooling of 1 ℃ / s, retain the original microstructure before fine phase transformation, at the same time, it can greatly refine the transformed martensite structure.

Fig.4 Microstructures of the P91 TMCP after piercing, continuous rolling and sizing (a) quenching after piercing, (b) quenching after continuous rolling, (c) cooling at 0.5℃/s after sizing, (d) cooling at 1℃/s after sizing

Fig. 5A and Fig. b show the SEM structure of P91 steel TMCP with controlled cooling rate of 0.5 ℃ / s and 1.0 ℃ / s respectively. Compared with FIG. 5A and Fig. B, when the cooling rate is increased from 0.5 ℃ / s to 1.0 ℃ / s after sizing, the width of martensitic lath bundle is refined from 1.0 ~ 1.5 μ m to 0.6 ~ 1.0 μ m; at the same time, a large number of fine precipitates are seen in Fig. 5B. EDS test results show that the precipitates mainly contain Cr, Fe and Mo elements, which can be determined as (Cr, Fe, Mo) 23c6. Therefore, the fine lath martensite structure strengthened by precipitates can be obtained by using 1.0 ℃ / s cooling rate after TMCP sizing of P91 steel.

Fig.5 SEM microstructures of the P91 steel in the TMCP controlled cooling after sizing (a) 0.5℃/s, (b) 1℃/s

Microstructure characteristics of TMCP

Figure 6 shows the substructure and precipitate observed by TEM after TMCP sizing of P91 steel with controlled cooling rate of 1 ℃ / s. It can be seen from Fig. 6A that the lath martensite of P91 steel after TMCP controlled cooling is straight and complete with lath width of 0.1-0.5 μ M. according to the national standard GB / t6394-2002, the average width of lath is 0.28 μ m by using the method of cut-off. A large number of fine dislocation networks are distributed inside the lath. In addition, micro twins with size of about 2-20 nm are also found in the martensite lath, as shown in Fig. 6B. It can be seen that in the process of TMCP controlled cooling of P91 steel tube, the twin is the first step to coordinate the strain, while the large deformation of TMCP through piercing and rolling greatly refines the original austenite structure, and the accumulated sizing deformation further strengthens the deformed austenite, and a large number of dislocations are produced in the matrix at the same time of twin growth. Therefore, in the process of transformation, martensitic lath can continue to form through dislocation slip to provide plastic coordination, thus forming a special substructure of coexistence of twin and high-density dislocation. The fine distribution of dislocation network is located at the edge of martensitic lath, and the twin structure can be seen in the middle of martensitic lath. As shown in Fig. 6C, there are a large number of fine strip precipitates between martensitic laths, with the size of about 20 nm × 100 nm. As the precipitates can only aggregate and grow between martensitic laths, their morphology is short rod. The precipitate is M23C6 with complex cubic structure, calibrated by diffraction spots (diffraction band axis [1-1-1], Ao = 1.064 nm). According to the EDS test results shown in Fig. 6D, it can be seen that the precipitate mainly contains Cr, Fe, Mo elements, so it can be determined that such carbides are (Cr, Fe, Mo) 23c6. These results show that after TMCP large deformation and controlled cooling, P91 steel can obtain ultra-fine lath martensite with high density dislocation, micro twin and nano carbide, which will greatly improve the mechanical properties of rolled P91 steel.

Fig.6 The substructure and precipitates of the P91 steel at the TMCP controlled cooling rate of 1℃/s after sizing (a) martensite laths and dislocations, (b) twins, (c) precipitates, (d) EDS of precipitates
In order to further clarify the carbide precipitation rule of P91 steel in TMCP cooling process, the curve of carbide quantity changing with temperature in P91 steel at high temperature was calculated by thermo Calc software, as shown in Fig. 7a, it can be seen that M6C and M23C6 carbide were mainly precipitated during sizing cooling of P91 steel. The precipitation temperature of M6C carbide is 500 ℃ ~ 370 ℃, and the precipitation amount is very small. The initial precipitation temperature of M23C6 carbide is about 860 ℃, which is mainly CR carbide, as well as Fe, Mo, V and other elements. The content of each element changes with temperature as shown in Figure 7b. It can be seen that with the decrease of precipitation temperature, the content of Cr and Mo increases, while the content of Fe decreases. The atomic fraction of each element in M23C6 at 860 ℃ and room temperature is listed in Table 3. Because the precipitation is promoted by the sizing deformation, a large number of dislocations produced by the sizing cumulative deformation provide more favorable nucleation points for carbide precipitation. At this time, due to the higher temperature, the solute atoms such as Cr, Fe, Mo and C diffuse more quickly, so the carbide precipitates directly inside the austenite grain. In addition, because P91 steel is rolled under TMCP control, it gets fine austenite grain, and there are many dislocations and grain boundaries in the grain, in order to reduce the free energy, solute atoms such as Cr, Fe and Mo tend to occupy the vacancy, dislocation and grain boundary defects So as to accelerate the diffusion and precipitation of M23C6 carbide. When the temperature is lower than 800 ℃, the diffusion of Cr, Fe, Mo and other alloy elements slows down, which slows down the growth rate of M23C6 phase and increases the growth resistance of carbides in the grains. Finally, M23C6 carbides are dispersed and fine precipitated in the deformed austenite grains. The calculation results show that the precipitation amount is basically kept at about 1.8%. In order to retain the nano scale precipitates, TMCP controlled cooling was carried out at the above cooling rate of 1 ℃ / s, and finally the nano scale carbides dispersed among the martensitic laths were obtained.

Fig.7 Thermo-Calc calculation of (a) carbide precipitation curve and (b) M23C6 component curve of P91 Steel

Table 3  Atom ratio of each element in carbide precipitate M23C6 of P91 (atomic fraction, %)

Temperature/℃	Cr	Fe	Mo	V	C
860	49.20	22.27	6.32	1.39	20.69
20	67.75	0.30	10.34	0.91	20.69

Analysis and discussion

The deformation design and cooling control of TMCP for P91 seamless steel pipe are two important factors that determine martensite transformation and morphology. Because P91 steel contains a high content of alloy elements and has a high resistance to hot rolling deformation, the TMCP parameters of P91 steel should be determined with a high heating temperature of 1290 ℃, and the high temperature deformation with a true strain of 1.8 should be selected for piercing continuous rolling. This can not only promote the recrystallization of deformed austenite and refine the grains, but also soften the structure and improve the hot working properties. The prior position of martensite nucleation is near the austenite grain boundary, so the fine recrystallization crystal nucleus will be produced near the austenite grain boundary during the large deformation of piercing continuous rolling, the grain will be significantly refined, and the area of grain boundary will be increased, so the martensite nucleation point will be increased during the cooling after sizing. At the same time, the high density dislocation entanglement produced by large deformation can also block the continuous growth of transformation martensite lath, and obtain the refined martensite lath, which makes the martensite substructure have the mixed characteristics of twin and dislocation.
The TMCP sizing of P91 steel selects the lower deformation temperature of 990 ℃, and inherits the twin and dislocation structure characteristics of the large deformation of piercing continuous rolling, which can realize the small deformation accumulation in the non recrystallization area. It is not only beneficial to obtain the accurate shape and size of the finished steel pipe, but also to form a large number of crystal “defects” such as deformation bands and movable dislocations in the deformed austenite grains [24], which not only greatly strengthens the shape The hardening effect of transformed austenite also increases the deformation nucleation point of subsequent martensitic phase. Combined with TMCP controlled cooling at 1 ℃ / s, the deformed austenite of P91 steel after sizing was restrained from softening and coarsening. The deformed austenite with a large number of high energy “defects” was inherited to the martensitic transformation point, and the martensitic lath was greatly refined to 0.1-0.5 μ m. The M23C6 carbide was controlled to be uniformly dispersed and precipitated in the grain and refined to 20 nm × 100 nm after transformation, Finally, the microstructure of ultra-fine lath martensite with high density dislocation, micro twin and nano carbide was obtained, which realized the control of fine grain strengthening, precipitation strengthening and transformation strengthening of TMCP in P91 steel tube.

In order to verify the reliability of TMCP for P91 steel pipe, the trial production of TMCP was carried out in 460pqf unit of a steel plant by using the process parameters in Table 2. Figure 8 shows the finished pipe and its microstructure after TMCP of P91 steel pipe. It can be seen from Fig. 8b that the grains have been refined to below 20 μ m; FIG. 8C shows that the martensitic lath is refined to 0.1-0.4 μ m, and high-density dislocations can be seen in the lath. In Fig. 8D, twin structure and fine precipitates can be seen. The results of mechanical properties test show that after TMCP controlled rolling and controlled cooling, the average hardness of P91 steel pipe is as high as hrc41.4, and the average yield strength at room temperature is 534 MPa. These results verify the feasibility of TMCP of P91 hot rolled seamless steel pipe in actual production.

Fig.8 Product and microstructures of the P91 pipe in TMCP production (a) product (b) OM, (c) martensite laths and dislocations (TEM), (d) twins and precipitates (TEM)

Conclusion

• (1) The TMCP of P91 steel tube can be rolled by recrystallization control and refine the deformed austenite grain by adopting the high temperature and large deformation of piercing continuous rolling with true strain up to 1.8; the TMCP of P91 steel tube can be rolled by non recrystallization control and strengthen the deformed austenite and induce the martensite transformation by sizing small deformation at 990 ℃; the TMCP of P91 steel tube can be refined by combining the controlled cooling at 1.0 ℃ / s after sizing The feasibility of TMCP is verified.
• (2) The 0.1-0.5 μ m ultra-fine lath martensite can be obtained by TMCP of P91 steel tube. There are substructures and high density dislocations in the lath which are characterized by 2-20 nm micro twin. The (Cr, Fe, Mo) 23c6 nano carbide with size of about 20 nm × 100 nm is found between the lathes. This kind of microstructure inherits the effect of TMCP fine grain strengthening, precipitation strengthening and transformation strengthening, which greatly improves the mechanical properties of P91 steel pipe.

Source: Network Arrangement – China Steel Pipe Manufacturer – Wilson Pipeline Pipe Industry Co., Limited www.wilsonpipeline.com

(Wilson Pipeline Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Wilson Pipeline products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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