A hot rolling process method of a large-wall-thickness acid-resistant pipeline steel X65MS

Abstract

A kind of thick-wall acid-resistant pipeline steel X65MS hot rolling process method, the chemical composition of steel is as follows: C=0.03%~0.04%, Si=0.2%~0.4%, Mn=1.0%~1.2%, Cr=0.2%~0.4%, Cu=0.2%~0.3%, Ni=0.1%~0.4%, Mo=0.10%~0.25%, Nb=0.03%~0.04%, Ti=0.01%~0.015%, Ce=0.004%~0.006%, S≤0.004%, P≤0.008%, the balance is Fe and inevitable impurities;Production is carried out using continuous casting billet with thickness of 360mm and above, and the finished product thickness is 40~55mm;Three-stage rolling process and the matched intermediate billet cooling and post-rolling ultra-fast cooling process are used, the obtained pipeline steel has CLR≤10%, CSR≤1%, CTR≤1% in hydrogen-induced cracking (HIC) experiment under the condition of NACE solution immersion for 96 hours;In sulfide stress corrosion cracking (SSCC) experiment, no crack is found under the condition of four-point bending constant load nominal yield of 90% for 720 hours.

Description

Technical Field

[0001] This invention belongs to the field of steel material processing and relates to a hot rolling process for thick-walled acid-resistant pipeline steel X65MS. Background Technology

[0002] The development of oil and gas resources in extreme environments such as polar regions and deep seas faces numerous challenges, including complex topography and sea areas, extremely cold climates, and corrosion from highly humid acidic media. This necessitates long-distance pipelines with comprehensive technical properties such as larger diameters, thicker walls, higher pressure resistance, excellent low-temperature drop hammer toughness, and acid resistance. Submarine pipeline steel operates under more complex conditions than conventional onshore pipeline steel. Influenced by operating conditions and environmental loads, submarine pipelines require a sufficiently large wall thickness-to-diameter ratio. Therefore, high-pressure, thick-walled submarine pipeline steel has become a development requirement. However, as pipeline steel wall thickness gradually increases, its low-temperature drop hammer performance, low-temperature toughness, and the uniformity of its microstructure become major limiting factors. During the production of thick-gauge pipeline steel, the total compression ratio decreases during rolling, and deformation is difficult to penetrate to the core, resulting in uneven microstructure along the thickness direction of the steel plate and difficulty in eliminating central segregation. This adversely affects the acid resistance of the pipeline steel, ultimately impacting its service performance. Therefore, long-distance pipelines need to have comprehensive technical properties such as larger pipe diameter and wall thickness, higher pressure resistance, excellent low-temperature drop hammer toughness, and acid resistance.

[0003] Controlled rolling and controlled cooling (TMCP) technology can achieve a fine and uniform microstructure by rationally controlling the metal deformation regime and cooling temperature regime during hot rolling. In the finish rolling stage, high-temperature, high-reduction deformation promotes austenite recrystallization, achieving grain refinement through recrystallization. In the finish rolling stage, austenite does not recrystallize; the rolling deformation flattens and elongates the austenite grains, forming strain-accumulated austenite in the form of dislocations, deformation bands, and cellular structures. Strain accumulation increases the nucleation sites for ferrite phase transformation, promoting ferrite nucleation and growth on defects within the deformed austenite grains, thereby refining the phase transformation microstructure. Controlled cooling, by regulating the post-rolling cooling rate, the start temperature, and the end temperature, controls the phase transformation behavior of the high-temperature austenite microstructure during the cooling process, ultimately controlling the microstructure type, morphology, and distribution of the steel, improving its microstructure and mechanical properties. TMCP technology has advantages such as energy saving, simplified production processes, and improved comprehensive mechanical properties of steel plates, making it an important process for pipeline steel production in both domestic and international enterprises.

[0004] CN202410921456.5 discloses a "Thickness Submarine Pipeline Steel Plate and Its Production Method," which mentions a "special controlled rolling process" of "low-temperature high-pressure rolling in the recrystallization zone + instant cooling of the intermediate billet + high-temperature high-pressure rolling in the non-recrystallization zone" and a matching ultra-fast cooling process. CN202511042912.X discloses an "Economic Thick-Walled X70 Pipeline Steel Production Method," which uses a microalloying system of medium carbon + medium manganese + Nb + Cr and a traditional two-stage rolling method to solve the problem of low-temperature drop hammer brittle fracture in medium-carbon pipeline steel. Both of these patents use a traditional two-stage rolling process, and the thickness of the products developed is less than 40mm. Developing a rolling process for thick-walled pipeline steel with a thickness of 40mm or more is a major challenge and bottleneck in production, especially since thick-walled pipeline steel requires both -55℃ low-temperature drop hammer performance and excellent acid resistance. The aforementioned patents do not mention these aspects.

[0005] CN202410203398.2 discloses a method for producing thick-walled pipeline steel with improved carbon dioxide corrosion resistance and toughness. It mentions methods to improve the strength, low-temperature toughness, HIC resistance and carbon dioxide corrosion resistance of thick-walled pipeline steel, but it focuses more on smelting methods than rolling methods, and does not mention the addition of rare earth element Ce.

[0006] CN202511395276.9 discloses a "Production Method of Thick-Walled Polar Pipeline Steel X65MS," which proposes a method of "ultra-low carbon high Ni + secondary billet opening and extreme cooling + online ultra-rapid cooling of steel plates" to improve the low-temperature drop hammer test, CTOD, HIC resistance, and SSC resistance of thick pipeline steel. This production method uses a "high Ni" composition system, and the "secondary billet opening" is a "two-heat forming" method involving two high-temperature heating processes, which is relatively cumbersome and results in high alloy and process costs.

[0007] Therefore, how to more efficiently solve the low-temperature drop hammer test problem of X65MS acid-resistant pipeline steel with a wall thickness of more than 40mm at -55℃ is a technical challenge. Summary of the Invention

[0008] This invention aims to provide a hot rolling process for thick-walled acid-resistant pipeline steel X65MS with excellent low-temperature drop hammer performance. The produced thick-walled acid-resistant pipeline steel X65MS has excellent low-temperature drop hammer performance, good low-temperature plastic deformation and crack arrest performance.

[0009] The technical solution of the present invention: A hot-rolling process for X65MS, a thick-walled acid-resistant pipeline steel, wherein the chemical composition of the steel, by mass percentage, is: C=0.03%~0.04%, Si=0.2%~0.4%, Mn=1.0%~1.2%, Cr=0.2%~0.4%, Cu=0.2%~0.3%, Ni=0.1%~0.4%, Mo=0.10%~0.25%, Nb=0.03%~0.04%, Ti=0.01%~0.015%, Ce=0. 0.004%~0.006%, S≤0.004%, P≤0.008%, balance being Fe and unavoidable impurities; produced using continuously cast billets with a thickness of 360mm or more, with a finished product thickness of 40~55mm; pipeline steel with a yield strength greater than 470MPa, tensile strength 540~680MPa, Charpy impact strength greater than 200J at -60℃, shear area ratio ≥85% in the -55℃ drop hammer test, resistance to hydrogen-induced cracking (HIC) test with CLR≤10%, CSR≤1%, CTR≤1% after immersion in NACE solution for 96 hours, and resistance to sulfide stress corrosion cracking (SSCC) test with no cracks after immersion in 720 hours under a four-point bending constant load with a nominal yield of 90%; including the following process steps: (1) The continuously cast billet is heated to above the recrystallization temperature of austenite for the first time and held at that temperature. After holding at that temperature, the first stage of rolling is carried out. (2) After the first stage of rolling, the intermediate billet is cooled to the non-recrystallized austenite region and then rolled in the second stage. (3) After the second stage of rolling, the steel plate is rapidly cooled to below 400°C, then heated and kept warm for the second time, and then rolled in the third stage.

[0010] Furthermore, in step (1), the first heating temperature of the continuous casting billet is 1100~1150 ℃, the holding time is 3~5 h, and the first stage of rolling after the continuous casting billet is heated by high temperature and large reduction, the reduction is 35%~50%, and the final rolling temperature is 950~980℃.

[0011] Furthermore, after the first stage of rolling in step (2) is completed, the surface slab is cooled to 750~800 ℃ by intermediate slab cooling, and the second stage of austenitic low temperature rolling is carried out in this temperature range with a rolling reduction of 40%~50%, followed by ultra-rapid cooling to below 400 ℃.

[0012] Furthermore, after the second stage of rolling in step (3) is completed, the steel plate is heated to 780~800℃ for the second time and held for 40~60 min; Furthermore, after the steel plate is heated and kept warm for the second time in step (3), it is rolled in the third stage. After rolling to the finished thickness, the surface of the steel plate is cooled to 350~450℃ using an ultra-fast cooling device, and finally air-cooled to room temperature.

[0013] Furthermore, the microstructure of the steel plate consists of quasi-polygonal ferrite, acicular ferrite, and granular bainite.

[0014] The beneficial effects of this invention are as follows: The pipeline steel produced by the above method has a microstructure mainly composed of quasi-polygonal ferrite, acicular ferrite and granular bainite, and has good low-temperature plastic deformation and crack arrest performance; the yield strength is greater than 470 MPa, the tensile strength is 540~680 MPa, the Charpy impact at -60℃ is greater than 200J, and the shear area ratio in the drop hammer test at -55℃ is ≥85%; in the hydrogen-induced cracking (HIC) test, under the condition of immersion in NACE solution for 96 hours, the CLR≤10%, CSR≤1%, and CTR≤1%; in the sulfide stress corrosion cracking (SSCC) test, under the condition of immersion in constant load at four points with a nominal yield of 90%, no cracks are found after 720 hours. Attached Figure Description

[0015] Figure 1 The image shows the metallographic structure of the acid-resistant pipeline steel X65MS using Example 1 of this invention. Detailed Implementation

[0016] The present invention is illustrated by specific implementation examples 1 to 4 listed below.

[0017] A hot rolling process for thick-walled acid-resistant pipeline steel X65MS, the main process of which adopts a three-stage rolling process and supporting intermediate billet cooling and post-rolling ultra-rapid cooling processes, including the following process steps: (1) The continuously cast billet is heated to above the recrystallization temperature of austenite for the first time and held at that temperature. After holding at that temperature, the first stage of rolling is carried out. (2) After the first stage of rolling, the intermediate billet is cooled to the non-recrystallized austenite region and then rolled in the second stage. (3) After the second stage of rolling, the steel plate is rapidly cooled to below 400°C, then heated and kept warm for the second time, and then rolled in the third stage.

[0018] The composition of the acid-resistant pipeline steel X65MS in Examples 1-4 is shown in Table 1. Table 2 shows the main process parameters for Examples 1-4. The performance test results for Examples 1-4 are shown in Table 3.

[0019] Table 1 Chemical composition (wt.%, Fe balance) of the steel produced in the examples .

[0020] Table 2 Process Parameters for Examples .

[0021] Table 3 Performance test results of the steel in the examples .

[0022] Figure 1 The microstructure of the acid-resistant pipeline steel X65MS produced in Example 1 using a three-stage rolling process mainly consists of quasi-polygonal ferrite, acicular ferrite, and granular bainite. The refined microstructure exhibits good low-temperature plastic deformation and crack arrest properties, meeting the requirements for drop hammer testing at -55℃.

Claims

1. A hot rolling process for thick-walled acid-resistant pipeline steel X65MS, characterized in that: The chemical composition of the steel, by mass percentage, is: C=0.03%~0.04%, Si=0.2%~0.4%, Mn=1.0%~1.2%, Cr=0.2%~0.4%, Cu=0.2%~0.3%, Ni=0.1%~0.4%, Mo=0.10%~0.25%, Nb=0.03%~0.04%, Ti=0.01%~0.015%, Ce=0.004%~0.006%, S≤0.004%, P≤0.008%, with the balance being Fe and unavoidable impurities; The pipeline steel is produced using continuously cast billets with a thickness of 360mm or more, and the finished product thickness is 40~55mm. The yield strength of the pipeline steel is greater than 470MPa, the tensile strength is 540~680MPa, the Charpy impact strength at -60℃ is greater than 200J, the shear area ratio in the drop hammer test at -55℃ is ≥85%, the resistance to hydrogen-induced cracking (HIC) test is CLR≤10%, CSR≤1%, and CTR≤1% under the condition of immersion in NACE solution for 96 hours, and the resistance to sulfide stress corrosion cracking (SSCC) test is no crack after immersion in NACE solution for 720 hours under the condition of 90% nominal yield under constant load at four points. The process includes the following steps: (1) The continuously cast billet is heated to above the recrystallization temperature of austenite for the first time and held at that temperature. After holding at that temperature, the first stage of rolling is carried out. (2) After the first stage of rolling, the intermediate billet is cooled to the non-recrystallized austenite region and then rolled in the second stage. (3) After the second stage of rolling, the steel plate is rapidly cooled to below 400°C, then heated and kept warm for the second time, and then rolled in the third stage.

2. The hot rolling process for thick-walled acid-resistant pipeline steel X65MS according to claim 1, characterized in that: Step (1) The first heating temperature of the continuous casting billet is 1100~1150℃, and the holding time is 3~5 h. After the continuous casting billet is held, the first stage of rolling adopts the high temperature and large reduction method, with a reduction of 35%~50% and a final rolling temperature of 950~980℃.

3. The hot rolling process for thick-walled acid-resistant pipeline steel X65MS according to claim 1, characterized in that: Step (2) After the first stage of rolling is completed, the surface slab is cooled to 750~800 ℃ by intermediate slab cooling, and the second stage of austenitic low temperature rolling is carried out in this temperature range with a rolling reduction of 40%~50%, followed by ultra-rapid cooling to below 400 ℃.

4. The hot rolling process method for thick-walled acid-resistant pipeline steel X65MS according to claim 1, characterized in that... Step (3) After the second stage of rolling is completed, the steel plate is heated to 780~800 ℃ for the second time and held for 40~60 min.

5. The hot rolling process for thick-walled acid-resistant pipeline steel X65MS according to claim 1, characterized in that: Step (3) After the second heating and heat preservation, the steel plate is rolled in the third stage. After rolling to the finished thickness, the surface of the steel plate is cooled to 350~450℃ using an ultra-fast cooling device, and finally air-cooled to room temperature.

6. The hot rolling process method for thick-walled acid-resistant pipeline steel X65MS with excellent low-temperature drop hammer performance according to claims 1-5, characterized in that... The microstructure of the steel plate consists of quasi-polygonal ferrite, acicular ferrite, and granular bainite.

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