Pickling steel strip with yield strength of 90ksi grade for coiled tubing and preparation method of pickling steel strip

Abstract

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

Description

Technical Field

[0001] The invention belongs to the technical field of production technology of steel for continuous oil pipes, and specifically relates to a pickled steel strip for continuous oil pipes with a yield strength of 90 ksi and a preparation method thereof. Background Art

[0002] Coiled tubing is a high-strength, high-plasticity, and corrosion-resistant oil and gas pipe with a single length of several thousand meters or even tens of thousands of meters. It is wound on a reel for transportation and use. In conjunction with a coiled tubing operation machine, it can be used for dozens of operations in the fields of oil and gas field well repair, logging, drilling, completion, oil and gas transportation, etc. It has the advantages of small footprint, safe and efficient operation, low environmental pollution, and convenient transportation and installation. The existing steel composition for coiled tubing basically adopts medium and low C supplemented with more precious elements such as Cr, Ni, Cu and Mo, which has achieved high strength, plasticity and corrosion resistance.

[0003] A Chinese invention patent (application number 201611162208.9) discloses a CT90 grade continuous oil pipe steel and a manufacturing method thereof, which contains a large amount of Ni and Mo elements and has a high alloy cost. At the same time, it contains a high amount of V, which is not conducive to the toughness and welding performance of the product.

[0004] A Chinese invention patent (application number 201711325095.4) discloses a hot-rolled steel strip for CT100-grade continuous tube and a production method, which uses C: 0.10-0.14%, Mn: 1.10-1.50% and Ni: 0.15-0.35%. Higher C and Mn contents are not good for the material's resistance to H2S corrosion. Summary of the invention

[0005] The purpose of the present invention is to solve the above technical problems and to provide a pickled steel strip for continuous oil pipe with a yield strength of 90 ksi and a preparation method thereof.

[0006] To achieve the above object, the present invention provides a pickled steel strip for coiled tubing with a yield strength of 90 ksi, the chemical composition of which includes, by weight percentage, 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 rest is Fe and unavoidable inclusions.

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

[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 0.60-0.80%.

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

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

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

[0014] Lower C, Mn, P and S contents are used to ensure good H2S resistance of the final product. No Mo and less Ni are added to the product to reduce the alloy element cost of the product, and Ti is used to compensate for the strength.

[0015] Ti element has excellent fine grain strengthening and precipitation strengthening effects, which can significantly improve the strength and toughness of steel. Ti element precipitates in the form of fine TiN or Ti(C, N), preventing the growth of austenite grains during the heating process of the slab, inhibiting the recrystallization of austenite during the rolling process, and causing high-density dislocations and deformation bands in the deformed austenite, which is conducive to the refinement of the structure. At the same time, through the appropriate TMCP process during the cooling process, a typical structure of polygonal ferrite + second phase nano-precipitation is obtained. With Ti as the main alloy strengthening element, a good match between high strength and corrosion resistance is achieved, while the use of precious alloys Mo and Ni is greatly reduced.

[0016] Also provided is a method for preparing the pickled steel strip as described above, which comprises the steps of molten iron pretreatment, converter smelting, LF furnace refining, RH furnace refining, continuous casting, slow cooling of the ingot, heating, rolling, cooling, coiling and pickling in sequence;

[0017] Hot metal pretreatment: perform pre-slag and post-slag operations to control the hot metal [S] after desulfurization to ≤ 0.0050%;

[0018] Converter smelting: [P]≤0.015%, [S]≤0.0050%; most alloys are added during the steel-making process to make the molten steel composition reach or approach the internal control lower limit; do a good job of slag blocking to prevent slag from returning to phosphorus;

[0019] LF furnace refining: The ladle top slag 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 the target values; vacuum treatment for 12 to 36 minutes to make the molten steel [H] ≤ 1.2ppm, and the end point [Ca] content is controlled at 15 to 30ppm.

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

[0022] Slow cooling of ingots: Slow cooling of ingot stacking for more than 48 hours;

[0023] Heating: The billet is heated in the heating furnace. The soaking time is 60 to 120 minutes. The billet out of the furnace temperature is 1230 to 1270°C.

[0024] Rolling: The first stage of rough rolling is carried out in the austenite recrystallization zone, and the rolling temperature is 1000-1060℃; the second stage of rough rolling is carried out in the non-recrystallization zone, and the rolling temperature is 900-1000℃; the finishing rolling stage adopts 7 four-roll CVC rolling mills for continuous rolling, and the deformation band and dislocation density in the deformed austenite are increased by accumulating large deformation, and the phase deformation nucleus points are increased to refine the grains. The final rolling temperature is 840-860℃;

[0025] The rolled steel plate is coiled after laminar cooling. The cooling method is front-stage laminar cooling. The cooling rate is controlled at 20-30°C / s. The coiling temperature of the cooled steel plate is controlled at 550-600°C.

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

[0027] Furthermore, the pickled steel strip has a thickness of 2 to 6 mm, a metallographic structure of F+P+B structure, and a grain size of 12 levels.

[0028] The holding time of the soaking section is not less than 60 minutes, and the temperature of the cast billet out of the furnace is 1230-1270℃. Its main purpose is to ensure that the material is completely austenitized, so that the alloy elements are fully dissolved, and at the same time, the excessive growth of austenite grains can be suppressed. Two-stage rolling is adopted. The first stage is rolling in the recrystallization zone. The rolling temperature is controlled at 1000-1060℃. Through repeated deformation and recrystallization, the austenite grains are significantly refined. The second stage is rolling in the non-recrystallization zone. The rolling temperature is controlled at 900-1000℃. It is a stage where deformation and phase transformation are carried out simultaneously. In this stage, the austenite grains are elongated and slip bands are generated. The increase of austenite grain boundaries and the appearance of slip bands provide favorable conditions for ferrite nucleation, thereby obtaining fine-grained ferrite.

[0029] As a strong carbon and nitride forming element, Ti easily forms TiC, TiN or Ti(C, N) with it. Among them, TiN precipitated during the solidification and soaking process of the ingot has good high-temperature thermal stability and can effectively inhibit the growth and coarsening of austenite grains; TiC or Ti(C, N) induced by deformation during rolling can inhibit the recovery and recrystallization of austenite, which is conducive to the controlled rolling and grain refinement of the non-recrystallized area; TiC precipitated interphase or dispersed in ferrite supersaturation during the cooling and heat preservation process after rolling can reach a size of less than 10nm, which has a significant precipitation strengthening effect.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: the pickled steel strip for continuous oil pipe of 90ksi grade produced by the present invention has a structure of F+P and a small amount of B, a yield strength Rt0.5: 530-580MPa, a tensile strength Rm: 670-720MPa, A50: 25-30%, a yield strength ratio: 0.76-0.80, a grain size ≥12, and a banded structure ≤1.0; and the pickled steel strip has good resistance to H2S corrosion, and CSR, CLR and CTR in the HIC test results are all 0; and the sample has no cracking under the SSC test condition of a standard stress of 0.8 coefficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The metallographic structure diagram of the embodiment of the present invention;

[0032] Figure 2 The following is a scanning electron microscope photograph of an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The present invention will be further described below in conjunction with specific embodiments.

[0034] According to the chemical composition requirements provided by the present invention, hot metal pretreatment → converter steelmaking → alloy fine-tuning station → LF refining → RH refining → continuous casting → hot rolling → coiling → pickling are adopted. After LF refining, the chemical composition of the steel meets the 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 rest is Fe and unavoidable inclusions. Protective casting is performed during the whole continuous casting process.

[0035] The chemical compositions of the embodiments of the present invention are shown in Table 1. The composition detection is carried out according to GB / T 4336 "Spark Source Atomic Emission Spectrometric Analysis Method for Carbon Steel and Medium and Low Alloy Steel (Conventional Method)".

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

[0037]

[0038] The rolling process includes heating the slab to 1230-1270℃ → high-pressure water descaling → 2-stand rough rolling → 7-stand finishing rolling → laminar cooling → coiling → pickling. The main parameters of the rolling process are shown in Table 2.

[0039] Table 2 Main process parameters of rolling process

[0040] Sample No. Thickness / mm Heating temperature / ℃ Final rolling temperature / ℃ Coiling 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 test steel of the present invention is F+P+a small amount of B, and the mechanical properties are shown in Table 3.

[0042] Table 3 Tensile properties of the test steel of the present invention

[0043] serial number Yield strength / MPa Tensile strength / MPa Yield-to-strength ratio 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 test steel of the present invention has good H2S corrosion resistance, as shown in Table 5.

[0045] Table 5 HIC and SSC properties of the test steel of the present invention

[0046]

[0047] From the above embodiments and comparative examples, comparative example 1 adopts low Ti design, and the tensile properties of the final product cannot reach the level of the embodiment, and there is a phenomenon of insufficient strength; comparative example 2 adopts high Ni component design, and its cost is undoubtedly increased; comparative example 3 adopts the design concept of medium C, which ensures the strength, but the control of P and S content is poor, and the HIC and SSC properties of the final product are insufficient.

[0048] The above embodiments are only the 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 obtained that the pickled steel strip for 90 ksi grade continuous oil pipe produced by the present invention has a structure of F+P and a small amount of B, and a grain size grade of ≥12.0.

Claims

1. A pickled steel strip for coiled 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 rest is Fe and unavoidable inclusions.

2. The pickled steel strip for coiled tubing with a yield strength of 90 ksi according to claim 1, characterized in that: The C content is 0.06-0.08%.

3. The pickled steel strip for coiled tubing with a yield strength of 90 ksi according to 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 according to claim 1, characterized in that: The Mn content is 0.80-0.90%.

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

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

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

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

9. A method for preparing the pickled steel strip according to claim 1, characterized in that: The preparation method comprises the steps of molten iron pretreatment, converter smelting, LF furnace refining, RH furnace refining, continuous casting, slow cooling of ingot, heating, rolling, cooling, coiling and pickling in sequence; and is characterized in that: The heating: the billet is heated in a heating furnace, the soaking section is kept warm for 60 to 120 minutes, and the billet exits the furnace at a temperature of 1230 to 1270°C; The rolling process includes: the first stage of rough rolling is carried out in the austenite recrystallization zone, and the rolling temperature is 1000-1060°C; the second stage of rough rolling is carried out in the non-recrystallization zone, and the rolling temperature is 900-1000°C; the finishing rolling stage adopts 7 four-roll CVC rolling mills for continuous rolling, and the final rolling temperature is 840-860°C; After rolling, the steel is cooled by laminar flow and coiled at 550-600°C.

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

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