A steel for coiled oil pipe with high surface quality and tensile strength of 1000 MPa and its manufacturing method

Abstract

The present invention provides a high-surface-quality 1000MPa-grade tensile strength steel for coiled oil pipes and a manufacturing method thereof. The manufacturing method comprises the following steps: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting → slab heating → primary high-pressure water descaling → rough rolling → secondary high-pressure water descaling → finishing rolling → cooling → coiling → uncoiling → tensioning and leveling → pickling → rinsing → drying → leveling → edge trimming → oiling → coiling. The high-surface-quality 1000MPa-grade tensile strength steel for coiled oil pipes manufactured according to the present invention has a yield strength of 786-818MPa, a tensile strength of 1054-1097MPa, an elongation of 12%-17%, a yield strength ratio of 0.74-0.75, and excellent comprehensive performance and high surface quality.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel for oil pipes, and in particular relates to a steel for continuous oil pipes with high surface quality and tensile strength of 1000 MPa and a manufacturing method thereof. Background Art

[0002] As shale gas development conditions deteriorate and well depths increase, coiled tubing (CCT) strength is evolving toward ultra-high strength, and the requirements for CTU weld quality are becoming increasingly stringent. However, if the surface quality of the raw steel used for CTU is poor, such as the presence of scale, this can often lead to inclusions in the CTU welds, causing weld defects. Currently, pickling is the common method for removing surface scale in China. This process improves both the surface quality of the raw steel used for CTU and its shape and dimensional accuracy. Hot-rolled, pickled steel strip used as raw material for CTU must be cleaned of surface scale before pipemaking, ensuring a smooth, scale-free surface to ensure weld quality and enhance market competitiveness.

[0003] Ultra-high-strength coiled tubing steel typically achieves its exceptional strength through compositional design, such as the addition of elements like C, Si, Mn, Nb, V, Ti, Cr, Ni, and Cu, and through hot-rolling. Because the addition of these elements promotes the formation of oxide scale, the thickness of the oxide scale on hot-rolled, pickled steel of varying strength levels often varies. Ultra-high-strength hot-rolled steel will typically have thicker oxide scale. This increased oxide scale directly impacts pickling efficiency and the surface quality of the product after pickling. Summary of the Invention

[0004] The object of the present invention is to provide a high-surface-quality steel for coiled tubing with a tensile strength of 1000 MPa and a method for manufacturing the same, which has excellent comprehensive properties and high surface quality.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] The present invention provides a method for manufacturing steel for coiled oil pipes with high surface quality and tensile strength of 1000 MPa. The manufacturing method comprises the following steps: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting → slab heating → primary high-pressure water descaling → rough rolling → secondary high-pressure water descaling → finishing rolling → cooling → coiling → uncoiling → tensioning and straightening → pickling → rinsing → drying → leveling → edge trimming → oiling → coiling.

[0007] During the refining process, the steel undergoes treatment in both the RH and LF furnaces, along with calcium treatment, to remove hydrogen and oxygen. This controls the number, shape, and size of non-metallic inclusions, minimizing their formation. Dynamic soft reduction and electromagnetic braking are employed during the continuous casting process to reduce internal defects in the ingot and improve its toughness.

[0008] During the slab heating step, the temperature at the end of the first heating section is controlled to be ≤1050°C, the temperature at the end of the second heating section is controlled to be between 1200°C and 1230°C, the temperature in the soaking section is controlled to be between 1200°C and 1230°C, and the combined duration of the second heating section and the soaking section is controlled to be ≤120 minutes. Maintaining uniform heating temperature promotes austenite homogenization and sufficient solid solution of alloying elements in austenite, while shortening the heating time helps reduce scale formation.

[0009] In the one-step high-pressure water descaling, the water pressure is controlled at 16-19 MPa to remove the primary iron oxide scale on the surface of the slab, thereby ensuring the surface quality of the product during subsequent rolling processes.

[0010] In the rough rolling step, multiple rolling passes are performed in the austenite recrystallization zone, and the four groups of descaling water between the rough rolling stands R1 and R2 are fully opened with a descaling water pressure of 15 to 18 MPa to remove the primary iron oxide scale; the thickness of the intermediate billet after rough rolling is controlled at 39 to 41 mm.

[0011] In the secondary high-pressure water descaling step, a total of 35 nozzles are opened in the upper and lower descaling manifolds, the spray angle of the descaling manifolds is 24-26°, the descaling water pressure is 15-18 MPa, and the distance from the nozzle to the upper and lower surfaces of the slab is controlled at 90±5 mm.

[0012] In the finishing rolling step, the finishing mill is composed of F1 to F7 frames, and an upper spray header and a lower spray header are provided between the F2-F3 frames. The descaling water pressure is 8 to 10 MPa, which is used to remove the third generation of iron oxide scale in the finishing mill and improve the surface quality of the strip. The finishing rolling temperature is controlled at 800 to 850° C. to avoid excessive mill load caused by rolling in the low-temperature dual-phase region, while avoiding mixed crystals and coarsening of austenite grains caused by excessive finishing temperature. The rolling speed is not less than 6.5 m / s.

[0013] In the cooling and coiling steps, front-stage laminar cooling is adopted, the cooling rate is controlled at 41-50°C / s, and the coiling temperature of the cooled steel plate is controlled at 450-550°C. Rapid cooling and low-temperature coiling are beneficial to reducing the generation of iron oxide scale, making it easier to remove the iron oxide scale on the strip surface during pickling.

[0014] The thickness of the surface iron oxide scale of the high surface quality tensile strength 1000 MPa grade coiled tubing steel before pickling is 3.5 to 8.5 μm.

[0015] In the straightening step, the straightening elongation is controlled at 0.7% to 0.8%. The diameter of the working roll used in the straightening is less than φ80 mm. The bending on the small-diameter working roll causes cracks in the iron oxide scale on the surface of the hot-rolled strip. During pickling, the acid can penetrate better, thereby improving the pickling efficiency.

[0016] In the pickling step, HCl solution is used for pickling, the concentration of the 1# pickling tank is 55-75 g / l, the concentration of the 2# pickling tank is 120-145 g / l, and the concentration of the 3# pickling tank is 155-185 g / l, the nozzle injection pressure in the pickling tank is controlled at 30 kPa-48 kPa, and the pickling solution temperature is 70-80°C.

[0017] In the rinsing step, the steel strip enters a rinsing tank, which is a five-stage overflow tank, and the temperature of the rinsing water is controlled at 40-55°C.

[0018] In the drying step, horizontal circulating steam heating is adopted, with 5 hot gas manifolds opened at the upper and lower parts, and the steam temperature is controlled at 90-100°C.

[0019] In the leveling step, the leveling elongation is 0.3-0.6%.

[0020] The high surface quality and 1000 MPa tensile strength steel for coiled tubing includes the following chemical composition in weight percentage: C: 0.10-0.13%, Si: 0.25-0.29%, Mn: 1.7-1.9%, P≤0.015%, S≤0.002%, Nb+V+Ti≤0.15%, Cr+Ni+Cu≤1.6%, and the remainder is Fe and unavoidable inclusions.

[0021] The metallographic structure of the high surface quality tensile strength 1000 MPa grade coiled tubing steel is ferrite + bainite, and the grain grade is 13.

[0022] The thickness of the high surface quality tensile strength 1000 MPa grade coiled oil pipe steel is 3.0 to 6.0 mm.

[0023] The high-surface-quality 1000MPa-grade tensile-strength continuous oil pipe steel has a yield strength of 786-818MPa, a tensile strength of 1054-1097MPa, an elongation of 12%-17%, a yield-to-tensile ratio of 0.74-0.75, and high surface quality.

[0024] The high surface quality and tensile strength 1000 MPa grade coiled tubing steel provided by the present invention has high surface quality and ultra-high strength. The design principles of the alloying elements and their mass percentages in the steel are as follows:

[0025] 1) Carbon is the most fundamental strengthening element. It dissolves in steel to form interstitial solid solutions, providing solid solution strengthening. It also forms carbides with strong carbide-forming elements, providing precipitation strengthening. However, carbon promotes scale formation, hindering the removal of scale from the strip surface during the pickling process. Therefore, the carbon content is controlled within a range of 0.10-0.13%.

[0026] 2) Si plays a role in solid solution strengthening in steel, thereby improving the strength of steel. It is also a deoxidizing element in steel. However, Si promotes the formation of iron oxide scale. If its content exceeds 0.30%, it will deteriorate the surface quality of hot-rolled strip and further affect the surface quality of pickled steel. Therefore, Si is controlled at 0.25-0.29%.

[0027] 3) Mn is a cheap element that can improve the strength of steel through solid solution strengthening. However, too much Mn can easily cause severe center segregation, so the Mn content is controlled at 1.7-1.9%.

[0028] 4) P and S are harmful residual elements that easily cause segregation and aggregation. The P and S contents in the steel should be reduced as much as possible. Therefore, the P and S contents are controlled to P≤0.015% and S≤0.002%.

[0029] 5) Cr, Cu and Ni can effectively improve the strength of steel, but if too much Cr and Mn are added to the steel at the same time, it will lead to the formation of low-melting-point Cr-Mn composite oxides, forming surface cracks during hot working and seriously deteriorating the welding performance. Excessive Cu and Ni will promote the formation of iron oxide scale, which is not conducive to the pickling process to remove the iron oxide scale on the surface of the strip. Therefore, Cr, Cu and Ni are controlled within Cr+Ni+Cu≤1.6%.

[0030] 6) Nb, V, and Ti are essential microalloying elements in pipeline steel, improving both the strength and low-temperature toughness of the material. However, excessive Nb, V, and Ti content not only hinders their full effectiveness but also may lead to coarse precipitation particles, negatively impacting the toughness of the steel. Therefore, the Nb, V, and Ti content should be controlled within the range of Nb+V+Ti≤0.15%.

[0031] The manufacturing method of the steel for coiled tubing with high surface quality and 1000 MPa tensile strength provided by the present invention has the following process design ideas:

[0032] 1) Surface quality control: The thickness of the iron oxide scale on the surface of the strip is controlled by rationally designing and matching the chemical composition, hot rolling process and pickling process. First, the present invention belongs to hot-rolled pickled ultra-high strength continuous oil pipe steel with a tensile strength greater than 1000MPa. The composition design adds relatively high C, Si, Mn, Cr, Ni, Cu, Nb, V, and Ti elements. However, elements such as C, Si, Ni, and Cu promote the formation of iron oxide scale, which affects the surface quality of the final product. Therefore, the influence of the promotion of iron oxide scale formation is reduced by reducing the addition amount of C, Si, Ni, and Cu elements. Secondly, the heating system is optimized. Under the condition of ensuring that the alloy elements are fully dissolved in austenite, the heating temperature and heating time are reduced as much as possible to reduce the formation of iron oxide scale, thereby realizing a fast and low-temperature heating process system. Then, the formation of iron oxide scale is reduced by matching and controlling the rolling speed, high-pressure water dephosphorization process, coiling temperature, and cooling speed during the hot rolling process. Finally, the iron oxide scale on the surface of the steel strip is removed by controlling each process in the pickling process, thereby obtaining a hot-rolled pickled steel strip for continuous oil pipe with excellent surface quality.

[0033] 2) Strength performance control: Through composition and hot rolling process design, the type and size of Nb, V, and Ti precipitates are controlled to improve grain refinement and precipitation strengthening effects; through deformation, temperature, and cooling rate control, ultra-high strength performance with a tensile strength greater than 1000 MPa is achieved.

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] (1) The method for manufacturing high-surface-quality steel with a tensile strength of 1000 MPa for continuous oil pipes provided by the present invention can control the thickness of the iron oxide scale of the hot-rolled pickled steel strip base material to within 9 μm, thereby avoiding the influence of the subsequent pickling efficiency and the surface quality of the pickled product due to the excessive thickness of the iron oxide scale, and avoiding the pressure-in of the iron oxide scale during the hot rolling process, which causes product degradation.

[0036] (2) In the manufacturing method of high surface quality steel for continuous oil pipe with tensile strength of 1000 MPa provided by the present invention, the pickling process is used to remove the thickness of iron oxide scale. After pickling, the strip has excellent surface quality, the surface of the pickled strip is smooth, and no iron oxide scale remains.

[0037] (2) The mechanical properties of the high surface quality and 1000 MPa tensile strength steel for continuous oil pipes provided by the present invention are as follows: yield strength: 786-818 MPa, tensile strength: 1054-1097 MPa, elongation: 12%-17%, yield strength ratio: 0.74-0.75, and ultra-high strength performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1Schematic diagram of the thickness of the iron oxide scale on the surface of the hot-rolled steel strip before pickling in Example 1;

[0039] Figure 2 Schematic diagram of the thickness of the iron oxide scale on the surface of the hot-rolled steel strip before pickling in Example 2;

[0040] Figure 3 Schematic diagram of the thickness of the iron oxide scale on the surface of the hot-rolled steel strip before pickling in Example 3;

[0041] Figure 4 Schematic diagram of the thickness of the iron oxide scale on the surface of the hot-rolled steel strip before pickling in Example 4;

[0042] Figure 5 This is a picture of the surface quality of the hot-rolled steel strip after pickling in Example 1;

[0043] Figure 6 This is a picture of the surface quality of the hot-rolled steel strip after pickling in Comparative Example 2;

[0044] Figure 7 This is the metallographic structure diagram of the finished coiled tubing steel product in Example 1;

[0045] Figure 8 Figure 1 shows the composite irregular Nb and Ti precipitated particles in the finished coiled tubing steel product (A) and elemental analysis (B). The particles are mainly composed of NbC and TiC, and are relatively small, approximately 60 nm in size.

[0046] Figure 9 Figure 1 shows the VC precipitation particle diagram (A) and elemental analysis diagram (B) in the finished coiled tubing steel product of Example 1. The square particles are VC-based precipitates with a relatively small particle size of approximately 38 nm. DETAILED DESCRIPTION

[0047] The present invention provides a method for manufacturing steel for coiled tubing with high surface quality and tensile strength of 1000 MPa, comprising the following steps: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting → slab heating → primary high-pressure water descaling → rough rolling → secondary high-pressure water descaling → finishing rolling → cooling → coiling → uncoiling → tension leveling → pickling → rinsing → drying → leveling → edge trimming → oiling → coiling;

[0048] In the slab heating step, the temperature at the end of the first heating section is controlled to be ≤1050°C, the temperature at the end of the second heating section is controlled to be between 1200°C and 1230°C, the temperature in the soaking section is controlled to be between 1200°C and 1230°C, and the total time of the second heating section and the soaking section is controlled to be ≤120 minutes.

[0049] In the one high-pressure water descaling step, the water pressure is controlled at 16-19 MPa;

[0050] In the rough rolling step, multiple passes are performed in the austenite recrystallization zone, and the four groups of descaling water between the rough rolling stands R1 and R2 are fully opened, and the descaling water pressure is 15-18 MPa; the thickness of the intermediate billet after rough rolling is controlled to be 39-41 mm;

[0051] In the secondary high-pressure water descaling step, a total of 35 nozzles are opened on the upper and lower descaling manifolds, the spray angle of the descaling manifolds is 24-26 degrees, the descaling water pressure is 15-18 MPa, and the distance from the nozzle to the upper and lower surfaces of the slab is controlled at 90±5 mm;

[0052] In the finishing rolling step, the finishing mill is composed of F1 to F7 frames, and an upper spray header and a lower spray header are provided between the F2-F3 frames. The descaling water pressure is 8 to 10 MPa, which is used to remove the tertiary iron oxide scale generated in the finishing mill and improve the surface quality of the strip; the finishing rolling temperature is controlled at 800 to 850°C; and the rolling speed is not less than 6.5 m / s.

[0053] In the cooling and coiling steps, front-stage laminar cooling is adopted, the cooling rate is controlled at 41-50°C / s, and the coiling temperature of the cooled steel plate is controlled at 450-550°C;

[0054] The thickness of the iron oxide scale on the surface of the 1000 MPa tensile strength coiled tubing steel before pickling is 3.5-8.5 μm;

[0055] In the drawing and straightening step, the drawing and straightening elongation is controlled at 0.7% to 0.8%, and the diameter of the working roll used in the drawing and straightening is φ80 mm; in the pickling step, HCl solution is used for pickling, the concentration of the 1# pickling tank is 55 to 75 g / l, the concentration of the 2# pickling tank is 120 to 145 g / l, and the concentration of the 3# pickling tank is 155 to 185 g / l, the nozzle injection pressure in the pickling tank is controlled at 30 kPa to 48 kPa, and the pickling solution temperature is 70 to 80°C.

[0056] In the rinsing step, the steel strip enters a rinsing tank, which is a five-stage overflow tank, and the temperature of the rinsing water is controlled at 40-55°C.

[0057] In the drying step, horizontal circulating steam heating is adopted, with 5 hot gas manifolds opened at the upper and lower parts, and the steam temperature is controlled at 90-100°C.

[0058] In the leveling step, the leveling elongation is 0.3-0.6%.

[0059] The high surface quality and 1000 MPa tensile strength steel for coiled tubing includes the following chemical components in weight percentage: C: 0.10-0.13%, Si: 0.25-0.29%, Mn: 1.7-1.9%, P≤0.015%, S≤0.002%, Nb+V+Ti≤0.15%, Cr+Ni+Cu≤1.6%, and the remainder is Fe and unavoidable inclusions.

[0060] The present invention is described in detail below with reference to the embodiments.

[0061] The chemical compositions and weight percentages of the various examples and comparative examples of the present invention are shown in Table 1, with the balance being iron and unavoidable impurities. Composition analysis was performed in accordance with GB / T 4336, "Carbon steel and low- and medium-alloy steels—Determination of multi-element content—Spark discharge atomic emission spectrometry (conventional method)."

[0062] Table 1 Chemical compositions of various embodiments of the present invention

[0063]

[0064]

[0065] The main rolling process parameters of the embodiments of the present invention and the comparative examples are shown in Table 2.

[0066] Table 2 Main parameters of temperature control at each stage of rolling process

[0067]

[0068] The phosphorus removal process parameters of the embodiments of the present invention and the comparative examples are shown in Table 3.

[0069] Table 3 Main parameters of phosphorus removal process at each stage of rolling process

[0070]

[0071] The pickling process parameters of the embodiments of the present invention and the comparative examples are shown in Table 4.

[0072] Table 4 Main parameters of pickling process

[0073]

[0074] The results of the surface oxide scale thickness of the hot-rolled steel strips before pickling in various embodiments of the present invention are shown in Table 5.

[0075] Table 5 Measurement of surface oxide scale thickness of hot-rolled steel strip before pickling

[0076] Example Thickness / mm Iron oxide scale thickness / μm Example 1 3.0 6.51-8.15 Example 2 4.0 6.03-8.26 Example 3 5.0 6.59-7.37 Example 4 6.0 3.79-7.04 Comparative Example 1 4.0 12.1-16.5 Comparative Example 2 5.0 7.6-14.1 Comparative Example 3 6.0 12.1-16.5

[0077] The mechanical properties and surface oxide scale residue of the finished coiled tubing steel products in the embodiments of the present invention and the comparative examples are shown in Table 6.

[0078] Table 6 Mechanical properties

[0079]

[0080]

[0081] As can be seen from the examples in the table above, by selecting raw materials according to the weight percentages of the chemical components given in the present invention and then producing according to the manufacturing method given in the present invention, a 1000 MPa grade hot-rolled pickled steel strip for coiled tubing with excellent surface quality and ultra-high strength can be obtained.

[0082] The detailed description of a high-surface-quality 1000 MPa tensile strength steel for coiled tubing and its manufacturing method with reference to the above-mentioned embodiments is illustrative rather than restrictive. Several embodiments may be enumerated within the limited scope. Therefore, changes and modifications that do not depart from the overall concept of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for manufacturing high surface quality and 1000 MPa tensile strength steel for coiled tubing, characterized in that: The manufacturing method comprises the following steps: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting → slab heating → primary high-pressure water descaling → rough rolling → secondary high-pressure water descaling → finishing rolling → cooling → coiling → uncoiling → tensioning and leveling → pickling → rinsing → drying → leveling → trimming → oiling → coiling; In the one high-pressure water descaling step, the water pressure is controlled at 16-19 MPa; In the rough rolling step, multiple passes are rolled in the austenite recrystallization zone, and the four groups of descaling water between the rough rolling stands R1 and R2 are fully opened, and the descaling water pressure is 15-18 MPa; In the secondary high-pressure water descaling step, the descaling manifold spray angle is 24-26°, the descaling water pressure is 15-18 MPa, and the distance from the nozzle to the upper and lower surfaces of the slab is controlled at 90±5 mm; In the finishing rolling step, the finishing mill is composed of F1 to F7 frames, an upper spray header and a lower spray header are provided between the F2-F3 frames, and the descaling water pressure is 8 to 10 MPa; the rolling speed is not less than 6.5 m / s; In the cooling and coiling steps, the cooling rate of the front-stage laminar cooling is controlled at 41-50°C / s, and the coiling temperature of the cooled steel plate is controlled at 450-550°C; The high surface quality and 1000 MPa tensile strength steel for coiled tubing includes the following chemical composition in weight percentage: C: 0.10-0.13%, Si: 0.25-0.29%, Mn: 1.7-1.9%, P≤0.015%, S≤0.002%, Nb+V+Ti≤0.15%, Cr+Ni+Cu≤1.6%, and the remainder is Fe and unavoidable inclusions.

2. The manufacturing method according to claim 1, characterized in that In the rough rolling step, the thickness of the intermediate billet after rough rolling is controlled to be 39 to 41 mm.

3. The manufacturing method according to claim 1, characterized in that In the slab heating step, the temperature at the end of the first heating section is controlled at ≤1050°C, the temperature at the end of the second heating section is controlled at 1200-1230°C, the temperature of the soaking section is controlled at 1200-1230°C, and the sum of the time of the second heating section and the soaking section is controlled at ≤120min.

4. The manufacturing method according to claim 1, characterized in that In the finishing rolling step, the finishing rolling temperature is controlled at 800-850°C.

5. The manufacturing method according to claim 1, characterized in that The thickness of the iron oxide scale on the surface of the 1000 MPa tensile strength coiled tubing steel before pickling is 3.5 to 8.5 μm.

6. The manufacturing method according to claim 1, characterized in that In the straightening step, the straightening elongation is controlled at 0.8%, and the diameter of the working roll used in the straightening is φ80 mm or less.

7. The manufacturing method according to claim 1, characterized in that In the pickling step, HCl solution is used for pickling, the concentration of the 1# pickling tank is 55-75 g / l, the concentration of the 2# pickling tank is 120-145 g / l, and the concentration of the 3# pickling tank is 155-185 g / l, the nozzle injection pressure in the pickling tank is controlled at 30 kPa-48 kPa, and the pickling solution temperature is 70-80°C.

8. The manufacturing method according to claim 1, characterized in that The temperature of the rinsing water is controlled at 40-55° C.; and the flattening elongation is 0.3-0.6%.

9. The manufacturing method according to claim 1, characterized in that The metallographic structure of the 1000 MPa tensile strength steel for coiled tubing is ferrite + bainite, and the grain grade is 13.

10. The manufacturing method according to claim 1, characterized in that The high-surface-quality 1000MPa-grade tensile-strength continuous oil pipe steel has a yield strength of 786-818MPa, a tensile strength of 1054-1097MPa, an elongation of 12%-17%, a yield-to-tensile ratio of 0.74-0.75, and high surface quality.

11. Steel for coiled tubing with high surface quality and tensile strength of 1000 MPa, manufactured by the manufacturing method according to any one of claims 1 to 10.

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