Pickled steel strip for coiled tubing with yield strength of 90 ksi grade and method of making

Abstract

The application discloses a pickling steel strip for coiled tubing with a yield strength of 90ksi and a preparation method, and chemical components include C: 0.04-0.08%, Si: 0.30-0.50%, Mn: 0.50-0.90%, P: ≤0.015%, S: ≤0.0030%, Cr: 0.40-0.80%, Cu: 0.15-0.30%, Ni: 0.05-0.10%, Nb: 0.010-0.025%, Ti: 0.060-0.080%, O: ≤30ppm, and N: ≤50ppm in percentage by weight. The structure is F+P and a small amount of B, the yield strength Rt0.5 is 530-580MPa, the tensile strength Rm is 670-720MPa, A50 is 25-30%, the yield strength ratio is 0.76-0.80, the grain size is greater than or equal to 12 levels, and the banded structure is less than or equal to 1.0 levels.

Description

Technical Field

[0001] This invention belongs to the technical field of steel production process for coiled tubing, specifically relating to a pickled steel strip for coiled tubing with a yield strength of 90 ksi and its preparation method. Background Technology

[0002] Coiled tubing is a type of high-strength, high-ductility oil and gas tubing with a single length reaching thousands or even tens of thousands of meters. It is transported and used while wound on a reel. In conjunction with coiled tubing installation machines, it can be used for dozens of operations in oil and gas field workover, logging, drilling, completion, and oil and gas transportation. It offers advantages such as small footprint, safe and efficient operation, low environmental pollution, and convenient transportation and installation. Currently, the steel used in coiled tubing primarily uses low to medium carbon content supplemented with higher amounts of precious elements such as Cr, Ni, Cu, and Mo, achieving high strength, ductility, and corrosion resistance.

[0003] Chinese invention patent (application number 201611162208.9) discloses a CT90 grade continuous tubing steel and its manufacturing method. The steel contains a large amount of Ni and Mo elements, resulting in a high alloy cost. At the same time, it contains a high amount of V, which is detrimental to the toughness and weldability of the product.

[0004] Chinese invention patent (application number 201711325095.4) discloses a hot-rolled steel strip for CT100 grade continuous tubes and its production method, which uses C: 0.10-0.14%, Mn: 1.10-1.50% and Ni: 0.15-0.35%. The high C and Mn content is detrimental to the material's resistance to H2S corrosion. Summary of the Invention

[0005] The purpose of this invention is to solve the above-mentioned technical problems and provide a pickled steel strip for continuous tubing with a yield strength of 90 ksi and a method for its preparation.

[0006] To achieve the above objectives, the present invention provides a pickled steel strip for continuous tubing with a yield strength of 90 ksi, the chemical composition of which, by weight percentage, includes C: 0.04-0.08%, Si: 0.30-0.50%, Mn: 0.50-0.90%, P: ≤0.015%, S: ≤0.0030%, Cr: 0.40-0.80%, Cu: 0.15-0.30%, Ni: 0.05-0.10%, Nb: 0.010-0.025%, Ti: 0.060-0.080%, O: ≤30ppm, N: ≤50ppm; the remainder being Fe and unavoidable inclusions.

[0007] Furthermore, the C content is 0.06% to 0.08%.

[0008] Furthermore, the Si content is 0.40–0.50%.

[0009] Furthermore, the Mn content is 0.80–0.90%.

[0010] Furthermore, the Cr content is 0.60% to 0.80%.

[0011] Furthermore, the Cu content is 0.25% to 0.30%.

[0012] Furthermore, the Ni content is 0.06% to 0.08%.

[0013] Furthermore, the Ti content is 0.075% to 0.080%.

[0014] Lower C, Mn, P, and S contents are used to ensure good H2S resistance of the final product. Mo is not added and Ni is added in small amounts to reduce the cost of alloying elements. Ti is used to compensate for the strength.

[0015] Ti exhibits excellent grain refinement and precipitation strengthening effects, significantly enhancing the strength and toughness of steel. Ti precipitates as fine TiN or Ti(C,N) particles, preventing austenite grain growth during slab heating and inhibiting austenite recrystallization during rolling. This results in high-density dislocations and deformation bands within the deformed austenite, which is beneficial for microstructure refinement. Simultaneously, a suitable TMCP process during cooling yields a typical microstructure consisting of polygonal ferrite with nano-precipitates of the second phase. Using Ti as the primary alloying strengthening element achieves a good balance between high strength and corrosion resistance, while significantly reducing the use of expensive alloys such as Mo and Ni.

[0016] A method for preparing pickled steel strip as described above is also provided, comprising the following steps in sequence: hot metal pretreatment, converter smelting, LF furnace refining, RH furnace refining, continuous casting, slow cooling of billet, heating, rolling, cooling, coiling and pickling.

[0017] Hot metal pretreatment: Perform pre-slag removal and post-slag removal operations to control the [S] content of the desulfurized hot metal to be ≤0.0050%;

[0018] Converter smelting: [P]≤0.015%, [S]≤0.0050%; most of the alloys are added during the tapping process to make the steel composition reach or approach the internal control lower limit; slag blocking operation is carried out to prevent slag from returning to phosphorus;

[0019] LF furnace refining: The top slag of the ladle is fully reduced and the composition is adjusted to the target value or close to the target value;

[0020] RH furnace refining: Adjust all components to target values; vacuum treatment for 12-36 minutes to make the molten steel [H] ≤ 1.2 ppm, and control the [Ca] content at the end point to 15-30 ppm.

[0021] Continuous casting: The tundish is baked for 24-36 hours; the target temperature of the tundish is controlled at 10-25°C above the liquidus temperature; finally, a billet with a thickness of 230mm is cast.

[0022] Slow cooling of billets: Slow cooling of billets for more than 48 hours;

[0023] Heating: The billet enters the heating furnace for heating. The holding time in the soaking section is 60-120 minutes, and the billet exiting the furnace is 1230-1270℃.

[0024] Rolling: The first stage of rough rolling is rolled in the austenite recrystallization zone at a rolling temperature of 1000–1060℃; the second stage of rough rolling is rolled in the non-recrystallization zone at a rolling temperature of 900–1000℃; the finishing rolling stage uses a 7-stand four-high CVC mill for continuous rolling, which increases the deformation band and dislocation density in the deformed austenite through cumulative large deformation, increases the phase deformation nuclei and refines the grains, and the final rolling temperature is 840–860℃;

[0025] After the rolled steel plate is cooled by laminar flow, it is coiled. The cooling method is front-end laminar flow cooling, and the cooling rate is controlled at 20-30℃ / s. The coiling temperature of the cooled steel plate is controlled at 550-600℃.

[0026] After the coiled steel is cooled, it enters the pickling process. The pickling process mainly improves the surface quality and dimensional accuracy of the product.

[0027] Furthermore, the thickness of the pickled steel strip is 2-6 mm, the metallographic structure is F+P+B structure, and the grain size is grade 12.

[0028] The soaking time in the heating zone is no less than 60 minutes, and the billet exit temperature is 1230–1270℃. The main purpose is to ensure complete austenitization of the material, allowing for full solid solution of alloying elements, while simultaneously suppressing excessive austenite grain growth. A two-stage rolling process is employed. The first stage is rolling in the recrystallization region, with the rolling temperature controlled at 1000–1060℃. Through repeated deformation and recrystallization, the austenite grains are significantly refined. The second stage is rolling in the non-recrystallization region, with the rolling temperature controlled at 900–1000℃. This stage involves simultaneous deformation and phase transformation. In this stage, the austenite grains are elongated, and slip bands are generated. The increase in austenite grain boundaries and the appearance of slip bands provide favorable conditions for ferrite nucleation, resulting in fine-grained ferrite.

[0029] Ti, as a strong carbide and nitride forming element, readily forms TiC, TiN, or Ti(C,N). Among these, TiN precipitated during the solidification and homogenization of the cast billet exhibits good high-temperature thermal stability and can effectively inhibit the growth and coarsening of austenite grains. During rolling, deformation-induced precipitation of TiC or Ti(C,N) can suppress austenite recovery and recrystallization, which is beneficial for controlling the rolling of grains in the non-recrystallized zone. TiC precipitated interphase or by supersaturated ferrite dispersion during post-rolling cooling and holding can reach a size of less than 10 nm and has a significant precipitation strengthening effect.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: The 90ksi grade pickled steel strip for coiled tubing produced by the present invention has a microstructure of F+P and a small amount of B, a yield strength Rt0.5 of 530-580MPa, a tensile strength Rm of 670-720MPa, an A50 of 25-30%, a yield ratio of 0.76-0.80, a grain size ≥12, and a banded microstructure ≤1.0. Furthermore, the pickled steel strip has good resistance to H2S corrosion, and the CSR, CLR, and CTR in the HIC test results are all 0. Under the standard stress condition of 0.8 coefficient, the SSC test showed no cracking of the sample. Attached Figure Description

[0031] Figure 1 Metallographic diagram of an embodiment of the present invention;

[0032] Figure 2 This is a scanning electron microscope image of an embodiment of the present invention. Detailed Implementation

[0033] The present invention will be further described below with reference to specific embodiments.

[0034] According to the chemical composition requirements provided by this invention, the process involves: hot metal pretreatment → converter steelmaking → alloy fine-tuning station → LF ladle refining → RH ladle refining → continuous casting → hot rolling → coiling → pickling. After LF ladle refining, the chemical composition of the steel meets the following weight percentage (wt%): C: 0.04~0.08%, Si: 0.30~0.50%, Mn: 0.50~0.90%, P: ≤0.015%, S: ≤0.0030%, Cr: 0.40~0.80%, Cu: 0.15~0.30%, Ni: 0.05~0.10%, Nb: 0.010~0.025%, Ti: 0.060~0.080%, O: ≤30ppm, N: ≤50ppm; the remainder is Fe and unavoidable inclusions. Protective casting is performed throughout the continuous casting process.

[0035] The chemical compositions of the various embodiments of the present invention are shown in Table 1. Component analysis was performed according to GB / T 4336 "Spark Source Atomic Emission Spectrometry Analysis Method (Conventional Method) for Carbon Steel and Medium-Low Alloy Steel".

[0036] Table 1 Chemical composition of various embodiments of the present invention

[0037]

[0038] The rolling process involves heating the slab to 1230–1270°C, descaling with high-pressure water, rough rolling on two stands, finishing rolling on seven stands, laminar flow cooling, coiling, and pickling. The main parameters of the rolling process are shown in Table 2.

[0039] Table 2 Main process parameters for rolling process

[0040] Sample number Thickness / mm Heating temperature / ℃ Final rolling temperature / ℃ Winding temperature / ℃ Example 1 2.70 1235 830 550 Example 2 4.00 1240 850 580 Example 3 5.00 1260 860 590 Example 4 3.50 1270 855 565 Comparative Example 1 4.50 1299 913 500 Comparative Example 2 6.00 1230 845 520 Comparative Example 3 4.50 1299 913 540

[0041] The microstructure of the experimental steels of this invention is all F+P+a small amount of B, and the mechanical properties are shown in Table 3.

[0042] Table 3 Tensile properties of the test steels of this invention

[0043] serial number Yield strength / MPa Tensile strength / MPa The ratio of yield strength A50mm / % Example 1 530 670 0.79 28.5 Example 2 540 682 0.79 30.5 Example 3 548 682 0.80 28.5 Example 4 553 694 0.80 28 Comparative Example 1 480 620 0.77 30 Comparative Example 2 560 680 0.82 27 Comparative Example 3 565 678 0.83 28

[0044] The experimental steel of this invention has good resistance to H2S corrosion, as shown in Table 4.

[0045] Table 4. HIC and SSC properties of the test steel of this invention.

[0046]

[0047] From the above embodiments and comparative examples, it can be seen that Comparative Example 1 adopts a low Ti design, and the tensile properties of the final product cannot reach the level of the embodiments, resulting in insufficient strength; Comparative Example 2 adopts a high Ni composition design, which undoubtedly increases the cost; Comparative Example 3 adopts a medium C design concept, which ensures strength, but the control of P and S content is poor, resulting in insufficient HIC and SSC performance of the final product.

[0048] The above embodiments are merely best examples and are not intended to limit the implementation of the present invention.

[0049] Depend on Figure 1 and Figure 2 It can be seen that the pickled steel strip for 90ksi grade coiled tubing produced by the present invention has a microstructure of F+P and a small amount of B, and a grain size grade ≥12.0.

Claims

1. A pickled steel strip for continuous tubing with a yield strength of 90 ksi, characterized in that: The chemical composition by weight percentage includes C: 0.04–0.08%, Si: 0.30–0.50%, Mn: 0.50–0.90%, P: ≤0.015%, S: ≤0.0030%, Cr: 0.40–0.80%, Cu: 0.15–0.30%, Ni: 0.05–0.10%, Nb: 0.010–0.025%, Ti: 0.060–0.080%, O: ≤30ppm, N: ≤50ppm; the remainder is Fe and unavoidable inclusions.

2. The pickled steel strip for continuous tubing with a yield strength of 90 kSi as described in claim 1, characterized in that: The C content is 0.06% to 0.08%.

3. The pickled steel strip for continuous tubing with a yield strength of 90 ksi as described in claim 1, characterized in that: The Si content is 0.40–0.50%.

4. The pickled steel strip for coiled tubing with a yield strength of 90 kSi as described in claim 1, characterized in that: The Mn content is 0.80–0.90%.

5. The pickled steel strip for continuous tubing with a yield strength of 90 ksi as described in claim 1, characterized in that: The Cr content is 0.60% to 0.80%.

6. The pickled steel strip for coiled tubing with a yield strength of 90 ksi as described in claim 1, characterized in that: The Cu content is 0.25% to 0.30%.

7. The pickled steel strip for coiled tubing with a yield strength of 90 ksi as described in claim 1, characterized in that: The Ni content is 0.06% to 0.08%.

8. The pickled steel strip for continuous tubing with a yield strength of 90 ksi as described in claim 1, characterized in that: The Ti content is 0.075% to 0.080%.

9. A method for preparing the pickled steel strip according to claim 1, characterized in that: The preparation method sequentially includes the following steps: hot metal pretreatment, converter smelting, LF furnace refining, RH furnace refining, continuous casting, slow cooling of the billet, heating, rolling, cooling, coiling, and pickling; characterized in that: The heating process involves the billet entering a heating furnace for heating, with the soaking time in the heat treatment zone being 60–120 min, and the billet exiting the furnace at a temperature of 1230–1270 °C. The rolling process is as follows: the first stage of rough rolling is rolled in the austenite recrystallization zone at a rolling temperature of 1000-1060℃; the second stage of rough rolling is rolled in the non-recrystallization zone at a rolling temperature of 900-1000℃; and the finishing rolling stage is carried out by continuous rolling using a 7-stand four-high CVC mill at a final rolling temperature of 840-860℃. After rolling, the material undergoes laminar flow cooling and is then wound up at 550–600°C.

10. The preparation method according to claim 9, characterized in that: The pickled steel strip has a thickness of 2-6 mm, a metallographic structure of F+P+B, and a grain size of 12.

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