Heavy-wall pipeline steel plate having excellent HIC resistance and manufacturing method therefor

Abstract

The present invention relates to a heavy-wall pipeline steel plate having excellent hydrogen-induced cracking (HIC) resistance and a manufacturing method therefor. The present invention solves the technical problem of poor HIC resistance in high-strength, high-toughness, heavy-wall pipeline steels by means of design of low-segregation chemical composition and slab thickness, modification of sulfide inclusions, and processes of controlled rolling and controlled cooling. The present invention achieves reliable quality of high strength, high toughness, and excellent HIC resistance in the heavy-wall pipeline steel, thereby providing a high-quality and safe pipeline steel for transportation of high-H2S sour oil and gas.

Description

A large wall thickness pipeline steel plate with excellent HIC resistance and a manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to a large wall thickness pipeline steel plate with excellent HIC resistance and a manufacturing method thereof, and belongs to the technical field of steel material manufacturing. BACKGROUND

[0002] Hydrogen Induced Cracking (HIC) is a common corrosion and cracking problem of steel materials in hydrogen-containing (such as high-concentration hydrogen sulfide H2S) medium environments. These problems can cause cracks in the interior of the steel material, severely reducing the safety of storage and transportation structures. Anti-HIC pipeline steel is widely used in hydrogen sulfide-containing oil and gas pipelines due to its excellent hydrogen-induced cracking resistance. Currently, the anti-HIC pipeline steel that has been mass-produced is mainly X52MS to X65MS grade. With the development of the oil and gas industry, there is an increasing demand for long-distance high-pressure hydrogen-containing oil and gas pipelines, and the demand for higher strength, higher toughness, and larger wall thickness anti-HIC pipeline steel is also rising.

[0003] In order to improve the crack resistance of steel materials in wet acidic media, the key is to eliminate factors that promote hydrogen-induced crack nucleation and propagation. This includes improving the cleanliness of the steel liquid during the smelting stage, reducing the content of harmful impurities such as phosphorus (P) and sulfur (S), and optimizing the composition, size, and morphology of non-metallic inclusions. During the casting stage, the homogenization of the slab should be improved, especially the degree of center segregation should be reduced, and the banded microstructure caused by segregation should be eliminated as much as possible during the rolling stage. However, improving the performance of these anti-acid pipeline steels faces technical challenges, such as the need to add more alloying elements, use thicker slabs, and use stronger post-rolling cooling intensity, which can lead to more severe center segregation, thereby affecting the key HIC resistance performance.

[0004] For example, for the development of X70MS level pipeline steel, patent CN113846268A discloses an X70MS acid-resistant pipeline steel plate and a preparation method thereof, the thickness specification of the pipeline steel plate ranges from 10 to 20 mm, the yield ratio is less than 0.88, and the shear area is greater than 90% in the Drop Weight Tear Test (DWTT) at -30℃. Although certain progress has been made in the production of HIC-resistant pipeline steel, there are still some limitations, especially the thickness specification of the steel produced by this technology is limited, and the toughness performance at low temperature is insufficient, which may affect the application performance of the material under extreme climate conditions. Patent CN102330032A focuses on an acid-resistant X70MS grade spiral buried arc welding pipe, which has high strength and excellent acid resistance, and is suitable for transporting acidic oil and gas media. However, due to the high cost, its wide application in long-distance pipeline transportation is limited, especially in cost-sensitive projects. Patent CN108998746B discloses an X70 grade HIC-resistant pipeline steel and a controlled rolling and controlled cooling method thereof. Although the technology is innovative, the complexity of the rolling process and the narrow process window may result in a low performance qualification rate during production, which not only affects the production efficiency but also increases the overall production cost.

[0005] Therefore, in order to address the technical challenges faced by the upgrading of HIC-resistant pipeline steel, more precise optimization and adjustment must be made in the alloy composition, inclusion control, microstructure homogenization design and control method. SUMMARY

[0006] The present application provides a large wall thickness pipeline steel plate with excellent HIC resistance and a manufacturing method thereof, which achieves reliable quality of high strength, high toughness and excellent HIC resistance of large wall thickness pipeline steel, and provides high-quality and safe pipeline steel for high H2S acidic oil and gas transportation.

[0007] The technical scheme adopted by the present application to solve its technical problems is:

[0008] A large wall thickness pipeline steel plate with excellent HIC resistance, for a steel plate with a wall thickness of 30 mm or more, the chemical composition includes the following components by weight percentage: C: 0.04-0.08%, Si: 0.10-0.40%, Mn: 0.60-1.20%, Ni: 0.15-0.50%, Cr: 0.15-0.50%, Mo: 0.05-0.30%, Cu: 0.15-0.50%, Nb: 0.020-0.080%, V: 0.005-0.04%, Ti: 0.008-0.020%, Ca: 0.003-0.006%, Mg: 0.002-0.008%, Al: ≤0.005%, P: ≤0.01%, S: ≤0.002%, the rest being Fe and unavoidable impurities;

[0009] Meanwhile, Ca / S is designed to be ≥2.0, Mg / Al is designed to be ≥1.0, and the segregation sensitivity index satisfies (C+Mn / 6)×t slab ×e ΔT / Tliq ≤65, wherein, t slab is the slab thickness of the steel plate, ΔT is the superheat of the molten steel during casting, and T liq is the liquidus temperature during casting;

[0010] Further, for a steel plate with a wall thickness of 30 mm or more, the following steps are specifically included:

[0011] Step S1: The molten iron is subjected to pre-desulfurization treatment, and then subjected to converter blowing, LF refining treatment, and RH vacuum refining treatment, and then fed into a Ca-Si composite cored wire for calcium treatment. A steel block containing magnesium alloy is added to the molten steel at the beginning of the LF refining treatment, so that the Mg / Al in the molten steel after calcium treatment is ≥1.0, and the Ca / S is ≥2.0, and the modification of oxide and sulfide inclusions is completed.

[0012] Step S2: The modified molten steel in step S1 is subjected to a continuous casting process, and the superheat of the molten steel is set to ≤25℃, and the whole process is protected by argon and protective slag, to obtain a continuous casting slab with a thickness of 200-320 mm. The non-metallic inclusions of A-type sulfides, B-type aluminum oxides, C-type silicates, and D-type spherical oxides in the continuous casting slab meet the preset inclusion grade requirements.

[0013] A multi-roll dynamic soft reduction is used at the end of the continuous casting to make the center segregation of the continuous casting slab reach the preset rating standard.

[0014] Step S3: The continuous casting slab is sent to a walking beam furnace, and the continuous casting slab is heated to 1150-1230℃ in the walking beam furnace in sections, and the continuous casting slab is further kept in the temperature homogenization zone for 1-3h, so that the Nb carbonitride precipitates Nb(CN) are fully dissolved in the austenite.

[0015] Step S4: pushing the continuous casting slab out of the walking beam furnace, removing the surface oxide scale by high pressure water, and performing the first stage controlled rolling at a high temperature zone of 1050-950℃, setting the rolling average single pass reduction, fully recrystallizing the deformed austenite, and rolling to an intermediate slab thickness of 3-5 times the target thickness of the finished product;

[0016] Step S5: placing the intermediate slab on a warm roller bed to cool, and performing the second stage controlled finishing rolling when the temperature is within 800-880℃ until the steel plate is rolled to the target thickness of the finished product;

[0017] Step S6: performing high-density header laminar flow accelerated cooling on the steel plate after the controlled rolling of step S5, controlling the cooling rate to be ≥20℃ / s, the final cooling temperature to be 250-350℃, and the re-red temperature to be 350-450℃, and controlling the cooling uniformity of the upper and lower surfaces and the length and width directions of the steel plate by the water quantity and the roller speed, so that the phase change structure of the steel plate is a uniform complex structure of fine crystal acicular ferrite and bainite;

[0018] Step S7: performing dynamic reduction straightening on the steel plate after the cooling of step S6 according to the size and shape of the steel plate, until the flatness of the steel plate reaches the preset value;

[0019] Further, in step S1, the magnesium desulfurizer is fully mixed and reacted with the molten iron to make S≤0.002% in the molten iron, and the temperature of the mixed molten iron is ≥1300℃;

[0020] The pre-desulfurized molten iron is poured into a converter, and Fe-Mn, Fe-Ni, Fe-Cr, and Fe-Cu ferroalloy materials prepared in advance are added, and composite oxygen blowing refining is performed by using a top-bottom oxygen lance, and the treatment time is 25-30min;

[0021] The time for LF refining treatment and RH vacuum refining treatment is set to 20-30min;

[0022] The magnesium alloy steel block added at the initial stage of the LF refining treatment is vaporized during the sinking process of the molten steel and reacts with oxygen in the molten steel to form MgO inclusions, which are absorbed into the protection and removed during the argon blowing and stirring process;

[0023] The length of the Ca-Si composite cored wire fed during the calcium treatment is 150-300m;

[0024] Further, in step S2, the molten steel temperature superheat of the tundish is set to 15-25℃, and the non-metallic inclusions of A type sulfides, B type aluminum oxides, C type silicates, and D type spherical oxides in the continuous casting slab meet the inclusion level requirements of A≤0.5, B≤0.5, C≤0.5, and D≤0.5;

[0025] The multi-roller dynamic light press-down at the end of continuous casting presses the thickness of the continuous casting slab by 20-50mm, so that the slab center segregation reaches the Mannesmann rating secondary standard;

[0026] Further, in step S3, the austenite grain size is controlled to be 30-100um;

[0027] Further, in step S4, when the first stage controlled rolling is performed in the high temperature zone, the rolling average single pass reduction is set to be greater than or equal to 30mm;

[0028] Further, in step S5, when the second stage controlled rolling is performed, the cumulative reduction is set to be 67%-80%;

[0029] Further, in step S7, the multi-roller straightening machine is used to perform the dynamic press-down straightening, so that the flatness of the steel plate is less than or equal to 6mm / 2m;

[0030] Further, in the microstructure of the finished steel plate obtained by the manufacturing method, the volume percentage of acicular and polygonal ferrite is 75-90%, the volume percentage of granular bainite is 5-23%, and the volume percentage of island-shaped martensite-austenite component is 2-5%.

[0031] Through the above technical scheme, compared with the prior art, the present application has the following beneficial effects:

[0032] 1. The large wall thickness pipeline steel plate with excellent HIC resistance provided by the present application, when the chemical composition design for the steel plate with a wall thickness greater than or equal to 30mm is performed, the key alloy addition amount is accurately controlled to achieve the balance of high strength, high toughness and excellent hydrogen-induced cracking resistance; the micro-alloying elements are reasonably added to refine the grains and play the precipitation strengthening effect; the harmful elements P and S are strictly controlled, and appropriate amounts of Mg and Ca are added and the addition sequence is controlled to effectively refine the inclusion size, change the inclusion composition, and spheroidize the morphology, thereby avoiding the formation of long strip-shaped inclusions during rolling;

[0033] 2. The manufacturing method of the large wall thickness pipeline steel plate with excellent HIC resistance provided by the present application, based on the low segregation chemical composition and slab thickness design, combined with a series of technical process methods such as refining process, inclusion control, micro-alloying, low segregation continuous casting, controlled rolling and controlled cooling, the reliable quality of high strength, high toughness and excellent HIC and SSC performance of the large wall thickness pipeline steel is obtained, which provides high-quality and safe pipeline steel for high H2S acidic oil and gas transportation. BRIEF DESCRIPTION OF DRAWINGS

[0034] The present application will be further described below in combination with the drawings and examples.

[0035] Fig. 1 is a flowchart of the manufacturing method provided by the present application;

[0036] Figure 2 is a schematic diagram of the process of controlled rolling and controlled cooling in the manufacturing method provided by the present application;

[0037] Figure 3 is a schematic diagram of the band-shaped structure in the core of the steel plate in the comparative example provided by the present application;

[0038] Figure 4 is a schematic diagram of the long strip-shaped inclusions in the steel in the comparative example provided by the present application;

[0039] Figure 5 is a schematic diagram of the microstructure in the core of the steel plate in Example 1 provided by the present application;

[0040] Figure 6 is a schematic diagram of the microstructure in the core of the steel plate in Example 2 provided by the present application;

[0041] Figure 7 is a schematic diagram of the microstructure in the core of the steel plate in Example 3 provided by the present application;

[0042] Figure 8 is a schematic diagram of the microstructure in the core of the steel plate in Example 4 provided by the present application.

[0043] In the figure: 1 is a walking beam furnace, 2 is a continuous casting slab, 3 is high-pressure water for removing surface scale, 4 is a second stage controlled finishing rolling, 5 is laminar water accelerated cooling, 6 is straightening, and 7 is a finished steel plate. DETAILED DESCRIPTION

[0044] The present application will now be further described in detail with reference to the accompanying drawings. The specific dimensions used in the examples are only for the purpose of illustrating the technical solutions and do not limit the scope of protection of the present application.

[0045] As set forth in the background, steel materials generally face corrosion and cracking problems in hydrogen-containing medium environments. In addition to hydrogen-induced cracking, there is also sulfide stress corrosion cracking (SSCC). The so-called sulfide stress corrosion cracking is a brittle cracking phenomenon of metal materials under the combined action of tensile stress and a specific corrosion environment (mainly a sulfide-containing environment). In essence, it is also because sulfides form an acidic environment in water, which promotes corrosion of steel materials. At the same time, the tensile stress breaks the protective film on the metal surface, accelerating the corrosion process. Hydrogen atoms in the steel material penetrate into the material, causing increased brittleness, and thus cracking. Obviously, it is imperative to improve the acid resistance of pipeline steel.

[0046] At present, in the technology of improving the performance of acid-resistant pipeline steel, the thickness specification of steel, production cost and performance qualified rate cannot be guaranteed at the same time, resulting in the problem that the hydrogen-induced cracking resistance of high-strength, high-toughness and large-wall-thickness pipeline steel is still poor. In order to solve the above problems, the present application provides a large-wall-thickness pipeline steel plate with excellent HIC resistance and a manufacturing method thereof. The present application mainly aims at the large-wall-thickness pipeline steel plate with a wall thickness of 30 mm or more, and meets low segregation in chemical composition design, and the slab thickness is also designed. In the matching manufacturing method, multiple designs of sulfide inclusion modification, controlled rolling and controlled cooling process method are carried out, and finally the reliable quality of high strength, high toughness and excellent HIC resistance (and also excellent SSC resistance) of the large-wall-thickness pipeline steel is obtained, which provides high-quality and safe pipeline steel for high-H2S acidic oil and gas transportation.

[0047] The large-wall-thickness pipeline steel plate with excellent HIC resistance provided by the present application has the following components in terms of weight percentage: C: 0.04-0.08%, Si: 0.10-0.40%, Mn: 0.60-1.20%, Ni: 0.15-0.50%, Cr: 0.15-0.50%, Mo: 0.05-0.30%, Cu: 0.15-0.50%, Nb: 0.020-0.080%, V: 0.005-0.04%, Ti: 0.008-0.020%, Ca: 0.003-0.006%, Mg: 0.002-0.008%, Al: ≤0.005%, P: ≤0.01%, S: ≤0.002%, and the rest is Fe and unavoidable impurities. In the above components, the key alloy addition amount is accurately controlled, such as C, Mn, Ni, Cr, Mo and Cu, to achieve the balance of high strength, high toughness and excellent hydrogen-induced cracking resistance; the micro-alloying elements Nb, V and Ti are reasonably added to refine the grain and play the role of precipitation strengthening; the harmful elements P and S are strictly controlled, and appropriate amounts of Mg and Ca are added and the addition sequence is controlled to effectively refine the inclusion size, change the inclusion composition and spheroidize the morphology, and avoid the formation of long strip-shaped inclusions during rolling.

[0048] The design principle is to limit the weight of each element based on its influence in the smelting process. For example, C is a necessary element for controlling phase transformation and ensuring strength, and has a significant effect on improving the strength of the steel through solid solution strengthening and precipitation strengthening. However, too high C content is not conducive to the inhibition of banded structure, plasticity, toughness and weldability of the steel. Therefore, the C content is preferably controlled at 0.04-0.08% from the perspective of comprehensive performance.

[0049] Si is one of the elements for deoxidization and strength improvement during smelting, and can also inhibit the coarsening of bainite, the precipitation and coarsening of cementite, and improve toughness. Therefore, the Si content is preferably 0.10-0.40%.

[0050] Mn: is the main element to ensure the strength, toughness and weldability of steel, can expand the austenite temperature interval, inhibit the austenite to ferrite phase change, is beneficial to refine the final organization, and is also the key element affecting the HIC resistance and sulfide stress cracking performance. But too high content increases the segregation and banded structure degree, and forms MnS inclusions with S in steel, which is not conducive to plasticity, toughness and HIC resistance, therefore the Mn content range is 0.60-1.20%.

[0051] P: is an element that adversely affects low temperature toughness and ductility, which can segregate in the center of slab and gather at grain boundaries, etc. to damage low temperature toughness, the present application controls P not higher than 0.01%.

[0052] S: is the primary element that is not conducive to HIC resistance, forms MnS inclusions with Mn, which is easily elongated during rolling and becomes a crack source, the present application controls S content not higher than 0.002%.

[0053] Ni: has solid solution strengthening and fine grain effect, is the main alloying element to stabilize austenite, reduces the austenite to ferrite phase transition point, promotes ferrite grain refinement, improves hardenability, and can improve strength and toughness at the same time. But too high Ni content increases the hydrogen brittleness and sulfide stress cracking sensitivity of steel in acidic environment. Therefore, the Ni content of the present application is not higher than 0.50%, and the preferred range is 0.15-0.50%.

[0054] Mo: has solid solution strengthening, precipitation strengthening, improves hardenability and improves HIC resistance, can effectively improve the uniformity of strength and toughness in the thickness direction of steel plate, forms nanometer-sized carbide MoC with C to provide precipitation strengthening effect, and improves the hydrogen brittleness and HIC performance of steel in acidic environment. Therefore, the Mo content range of the present application is 0.05-0.30%.

[0055] Cr: is the main element to improve the overall and local pitting corrosion resistance in H2S-containing acidic environment, can capture diffused hydrogen atoms, and at the same time forms carbides to provide precipitation strengthening effect, is also a strong ferrite zone stabilizing element and hardenability element, and can improve the organization uniformity of large thickness pipeline steel plate. Therefore, the Cr content range of the present application is 0.15-0.50%.

[0056] Cu: has the effect of improving HIC resistance in H2S-containing acidic environment, forms a protective layer on the surface of the steel to inhibit hydrogen from entering, to improve the hydrogen-induced cracking resistance effect. At the same time, Cu is aged and precipitated in steel, which has precipitation strengthening effect. Therefore, the Cu content range of the present application is 0.15-0.50%.

[0057] Nb: is an important element to improve the resistance to HIC in acid environment, and is a micro-alloying element to play effective grain refinement and precipitation strengthening. It can control the shape and distribution of non-metallic inclusions in steel, inhibit the formation of banded structure, thereby reducing the sensitivity of H2S stress cracking, and significantly improving the high temperature austenite non-recrystallization zone, increasing the austenite dislocation density during rolling process, effectively refining the final product structure, and forming high-density nanometer-sized NbC and Nb(C, N) precipitated particles with C and N in steel, thereby playing a precipitation strengthening effect. Therefore, the Nb content range of the present application is 0.02-0.08%.

[0058] V: has the effects of precipitation strengthening, grain refinement and improving the resistance to HIC and sulfide stress cracking, forms nanometer-sized VC and V(C, N) particles with C and N in steel to improve strength, and hinders the phase transition of austenite to ferrite, thereby achieving the effects of fine-grain strengthening and toughening, and improving the resistance to hydrogen-induced cracking and sulfide stress cracking in acid environment as a hydrogen trap. Therefore, the V content range of the present application is 0.005-0.04%.

[0059] Ti: has the effects of refining austenite grain size and improving welding performance, forms TiN particles with N during steelmaking process to inhibit austenite grain growth and recrystallization, thereby refining the ferrite grain of the finished product, improving the toughness of the welded part, and effectively removing O, N and S in steel to form fine and stable Ti-based inclusions and improve the cleanliness of the molten steel. Therefore, the Ti content range of the present application is 0.008-0.020%.

[0060] Ca: is a main element to improve the composition and morphology of non-metallic inclusions, promotes the formation of calcium aluminate and calcium sulfide inclusions, which plays an important role in the corrosion resistance and inhibition of hydrogen-induced cracking of steel in H2S-containing acid medium, and the calcium-containing inclusions can inhibit the grain coarsening in the heat-affected zone during welding process, thereby improving the welding toughness. Therefore, the Ca content range of the present application is 0.003-0.006%.

[0061] Mg: plays a role in desulfurization and deoxidization during steelmaking process, is a main element to improve the cleanliness, composition and shape of inclusions in molten steel, forms (Mg, Ca)O-(Ca, Mn)S composite spherical inclusions, and promotes the refinement and homogenization of microstructure during the phase transition process after rolling as high-density acicular ferrite nucleation sites, thereby avoiding the formation of banded pearlite structure and having an important role in improving the resistance to HIC. At the same time, the Mg-containing composite inclusions can promote the formation of acicular ferrite in the heat-affected zone during welding, which is beneficial to improve the welding toughness. Therefore, the Mg content range of the present application is 0.002-0.008%.

[0062] Al: has strong deoxidation effect, is the main forming element of oxide inclusions, competes with Mg and Ti for O in the steelmaking process, is not conducive to the formation of composite inclusions promoting acicular ferrite phase transition in the application. Therefore, the content of Al in the application should not exceed ≤0.005%.

[0063] In addition to the optimization design of each chemical component, it is also necessary to meet the related limitations of low segregation and slab thickness. In this application, Ca / S≥2.0, Mg / Al≥1.0, and the segregation sensitivity index satisfies (C+Mn / 6)×t slab ×e ΔT / Tliq ≤65wt%·mm, wherein t slab is the slab thickness of the steel plate, unit: mm, ΔT is the superheat of the molten steel during casting, unit: ℃, T liq is the liquidus temperature during casting, unit: ℃, the content of C element and Mn element is in mass percent.

[0064] Then the application provides a manufacturing method of large wall thickness pipeline steel plate with excellent HIC resistance. The overall process flow is shown in Figure 1, which specifically includes the following steps:

[0065] Step S1: pre-desulfurization treatment of molten iron, fully mix and react magnesium desulfurizer with molten iron at a temperature of ≥1300℃, so that S≤0.002% in the molten iron, reduce the sulfur content in the molten iron by magnesium desulfurizer, reduce the formation of sulfide inclusions; pour the pre-desulfurized molten iron into the converter, and add the pre-configured Fe-Mn, Fe-Ni, Fe-Cr, Fe-Cu ferroalloy, and use top-bottom oxygen lance for composite oxygen blowing refining, the treatment time is 25-30min; add steel blocks containing magnesium alloy at the beginning of LF refining treatment, the magnesium alloy is slowly released into the molten steel during the sinking process of the molten steel, and reacts with the oxygen in the molten steel to form MgO inclusions, which are relatively stable; then through argon blowing stirring, the fine inclusions are uniformly distributed, and the coarse inclusions are promoted to float up and be absorbed into the protection and removed, further improving the state of inclusions in the molten steel. Then RH vacuum refining treatment is carried out, after vacuum treatment, 150-300m long Ca-Si composite cored wire is fed, the time of the whole LF refining treatment and RH vacuum refining treatment is set to 20-30min, magnesium, calcium and other elements in the molten steel react with possible oxides and sulfides, after RH treatment, Mg / Al≥1.0, Ca / S≥2.0 in the molten steel, and the modification of oxide and sulfide inclusions is completed. This step is a prominent innovation point of the application, which can modify the sulfide inclusions in the molten steel into a relatively stable inclusion form, avoiding the cracking of steel caused by the internal pressure of hydrogen molecules and the cracking problem caused by the brittleness of hydrogen atoms penetrating into the metal.

[0066] Step S2: the molten steel modified in step S1 is subjected to a continuous casting process, the superheat of the molten steel is set to be ≤25℃, preferably, the superheat of the molten steel in the tundish is set to be 15-25℃; the continuous casting is carried out under the protection of argon and protective slag, so that the non-metallic inclusions of the A-type sulfide, the B-type alumina, the C-type silicate and the D-type spherical oxide in the continuous casting slab 2 meet the inclusion level requirements: A≤0.5, B≤0.5, C≤0.5, D≤0.5. At the end of the continuous casting, a multi-roll dynamic soft reduction is adopted, and the thickness of the continuous casting slab is reduced by 20-50mm, so that the slab center segregation reaches the Mannesmann rating level two standard, and the continuous casting slab with a thickness of 200-320mm is obtained.

[0067] As shown in FIG. 2, a process flow diagram of each device in the controlled rolling and controlled cooling process is provided. Step S3: the continuous casting slab is sent to the walking beam furnace 1, and the continuous casting slab is heated to 1150-1230℃ in the walking beam furnace. This temperature can change the internal structure of the metal to a certain extent. The continuous casting slab is kept in the temperature homogenization zone for 1-3h, so that the heat is fully transferred, and the internal temperature inhomogeneity is eliminated. The Nb carbonitride precipitated particles Nb(CN) are fully dissolved in the austenite, and the austenite grain size is controlled to be 30-100μm.

[0068] Step S4: the continuous casting slab is pushed out from the walking beam furnace, the surface oxide scale 3 is removed by high-pressure water, and the first stage controlled rolling is carried out in the high-temperature zone of 1050-950℃. The average single pass reduction amount is set to be ≥30mm, the deformed austenite is fully recrystallized, and the intermediate billet is rolled to 3-5 times of the target thickness of the finished product.

[0069] Step S5: the intermediate billet is placed on the temperature waiting roller way for cooling, and the second stage controlled finishing rolling 4 is carried out when the temperature is 800-880℃. The cumulative reduction amount is set to be 67%-80%, and the steel plate is rolled to the target thickness of the finished product.

[0070] In the rolling process stage from step S4 to step S5, four-roll reversible hot rolling is adopted. The continuous casting slab is subjected to high-temperature and low-temperature two-stage controlled rolling with the austenite non-recrystallization temperature as the boundary. The purpose is to optimize the high-temperature rolling pass to make the austenite fully recrystallize and refine, to give sufficient cumulative rolling reduction to the low-temperature zone to provide sufficient particle density for the ferrite phase change, and finally to refine the structure of the finished steel plate 7.

[0071] The steel plate is subjected to on-line rapid cooling after rolling, i.e., step S6: the steel plate after the controlled rolling in step S5 is subjected to high-density header laminar flow water accelerated cooling 5, the cooling rate is controlled to be greater than or equal to 20 ℃ / s, the final cooling temperature is 250-350 ℃, the re-red temperature is 350-450 ℃, the water amount and the roller speed are matched to control the phase change microstructure of the steel plate to be a uniform complex microstructure of fine acicular ferrite and bainite, the volume ratio of fine acicular ferrite, granular bainite and M-A island is precisely controlled, the banded pearlite structure which is not conducive to HIC resistance is inhibited in the rapid phase change process, and the banded structure which is not conducive to HIC resistance is avoided; meanwhile, the cooling uniformity of the upper and lower surfaces and the length-width direction of the steel plate is controlled to ensure the flatness of the steel plate.

[0072] Step S7: according to the size and shape of the steel plate after cooling in step S6, a multi-roller straightening machine is used for dynamic pressure straightening 6 until the flatness of the steel plate is less than or equal to 6 mm / 2 m.

[0073] The above-mentioned full-process process control is performed from the alloy element content, steel cleanliness control, process optimization and parameter selection, microstructure control and the like, and finally the volume percentage of acicular and polygonal ferrite in the microstructure of the finished steel plate is 75-90%, the volume percentage of granular bainite is 5-23%, and the volume percentage of island-shaped martensite-austenite group elements is 2-5%; the pipeline steel has high strength, high toughness, and excellent HIC and SSCC performance.

[0074] In order to verify the feasibility and superiority of the chemical composition design and manufacturing process of the thick-walled pipeline steel plate provided in the present application, the following one comparative example and four examples are used for comparison.

[0075] Comparative Example:

[0076] A conventional X70M pipeline steel is selected, and the chemical composition includes the following components in terms of weight percentage: C: 0.10%, Si: 0.25%, Mn: 1.45%, Ni: 0.33%, Cr: 0.30%, Mo: 0.18%, Cu: 0.26%, Nb: 0.035%, Ti: 0.012%, Ca: 0.0008%, Mg: 0.0008%, Al: 0.032%, P: 0.012%, S: 0.005%, and the rest is Fe and unavoidable impurities, Ca / S=0.16, Mg / Al=0.025, Ceq=0.48%, (C+Mn / 6)xt slab ×e ΔT / Tliq = 78.6.

[0077] After 21 min and 23 min treatment in LF and RH furnace respectively, the superheat is 34°C at tundish casting, the 230 mm thickness slab is formed by continuous casting, the macro-segregation level in the core is B class 1 level, and the sulfide inclusion level is 0.5 level. The slab is soaked at 1210°C for 120 min, after descaling by high pressure water, the starting rolling temperature is 1080°C, the rough rolling is to 120 mm thickness intermediate slab, the average single pass reduction is 22 mm, the second stage finishing rolling is performed when the temperature is 882°C, the cumulative reduction is 82 mm, the finish rolling temperature is 850°C, the slab is cooled to 420°C by ACC accelerated cooling at a rate of 23°C / s, and then is transported to the cooling bed after straightening.

[0078] Figure 3 is a schematic diagram of the banded structure in the core of the steel plate, and Figure 4 is a schematic diagram of the long strip inclusions in the steel, the volume percentage of polygonal ferrite is 80%, the volume percentage of banded pearlite is 20%, there is no acicular ferrite, and there are long strip MnS inclusions and large block martensite-austenite (M-A) components in the banded structure in the core.

[0079] Example 1:

[0080] The chemical composition in the pipeline steel includes the following components by weight percentage: C: 0.07%, Si: 0.38%, Mn: 0.86%, Ni: 0.23%, Cr: 0.34%, Mo: 0.12%, Cu: 0.18%, Nb: 0.031%, V: 0.01%, Ti: 0.013%, Ca: 0.003%, Mg: 0.003%, Al: 0.003%, P: 0.008%, S: 0.0015%, the rest is Fe and unavoidable impurities, Ca / S = 2.0, Mg / Al = 1.0, Ceq = 0.33%, (C+Mn / 6) x t slab x e ΔT / Tliq = 61.

[0081] After 22 min and 24 min treatment in LF and RH furnace respectively, the superheat is 23°C at tundish casting, the 230 mm thickness slab is formed by continuous casting, the macro-segregation level in the core is B class 0.5 level. The slab is soaked at 1210°C for 120 min, after descaling by high pressure water, the starting rolling temperature is 1080°C, the rough rolling is to 120 mm thickness intermediate slab, the average single pass reduction is 22 mm, the second stage finishing rolling is performed when the temperature is 880°C, the cumulative reduction is 82 mm, the finish rolling temperature is 850°C, the slab is cooled to 390°C by ACC accelerated cooling at a rate of 23°C / s, and then is transported to the cooling bed after straightening. Figure 5 shows the core microstructure of Example 1, in which the proportion of acicular ferrite reaches 75%, the proportion of granular bainite is 20%, and the proportion of island M-A component is 5%.

[0082] Example 2:

[0083] The chemical composition of the pipeline steel includes the following components in percentage by weight: C: 0.06%, Si: 0.25%, Mn: 0.94%, Ni: 0.18%, Cr: 0.17%, Mo: 0.16%, Cu: 0.37%, Nb: 0.051%, V: 0.02%, Ti: 0.014%, Ca: 0.005%, Mg: 0.003%, Al: 0.003%, P: 0.007%, S: 0.0017%, the rest being Fe and unavoidable impurities, Ca / S = 2.9, Mg / Al = 1.0, Ceq = 0.32%, (C + Mn / 6) x t slab ΔT / Tliq = 49.8.

[0084] After being treated in the LF furnace and RH furnace for 21 min and 23 min respectively, the superheat is 21°C at the time of tundish casting, and a 230 mm thick slab is formed by continuous casting. The macroscopic segregation level of the core is B class 0.5. The slab is soaked at 1210°C for 120 min, and after descaling by high-pressure water, the starting rolling temperature is 1082°C. The intermediate billet is rolled to 120 mm thick, and the average single pass reduction is 25 mm. The second stage finishing rolling is performed when the temperature is 876°C, and the cumulative reduction is 82 mm. The final rolling temperature is 850°C, and the slab is cooled to 410°C at a rate of 23°C / s by ACC accelerated cooling, and then transported to the cooling bed after straightening. Figure 6 shows the core microstructure of Example 2, the ratio of acicular ferrite + polygonal ferrite is 85%, the ratio of granular bainite is 10%, and the ratio of island-shaped M-A constituent is 5%.

[0085] Example 3:

[0086] The chemical composition of the pipeline steel includes the following components in percentage by weight: C: 0.06%, Si: 0.25%, Mn: 0.94%, Ni: 0.18%, Cr: 0.17%, Mo: 0.16%, Cu: 0.37%, Nb: 0.051%, V: 0.02%, Ti: 0.014%, Ca: 0.005%, Mg: 0.003%, Al: 0.003%, P: 0.007%, S: 0.0017%, the rest being Fe and unavoidable impurities, Ca / S = 2.9, Mg / Al = 1.0, Ceq = 0.32%, (C + Mn / 6) x t slab ΔT / Tliq = 47.5.

[0087] ​​After 23 min and 24 min treatment in LF and RH furnace respectively, the slab was continuously cast into 300 mm thickness with 18 °C superheat at tundish pouring, the core macro-segregation level was B class 0.5. The slab was soaked at 1210 °C for 160 min, after descaling by high pressure water, the starting rolling temperature was 1078 °C, rough rolling to 150 mm thickness, the average single pass reduction was 32 mm, the second stage finishing rolling was performed when the temperature was up to 862 °C, the cumulative reduction was 112 mm, the finishing temperature was 850 °C, after ACC accelerated cooling with 23 °C / s rate to 360 °C, then after straightening, it was transported to the cooling bed. Figure 7 shows the core microstructure of Example 3, the ratio of acicular ferrite + polygonal ferrite reached 90%, the ratio of island M-A constituent reached 10%.

[0088] Example 4:

[0089] The chemical composition of the pipeline steel includes the following components by weight percentage: C: 0.04%, Si: 0.26%, Mn: 1.15%, Ni: 0.35%, Cr: 0.33%, Mo: 0.08%, Cu: 0.27%, Nb: 0.076%, V: 0.04%, Ti: 0.013%, Ca: 0.003%, Mg: 0.005%, Al: 0.002%, P: 0.009%, S: 0.0014%, the rest is Fe and unavoidable impurities, Ca / S = 2.1, Mg / Al = 1.5, Ceq = 0.36%, (C+Mn / 6) x t slab ×e ΔT / Tliq = 53.3.

[0090] After 22 min and 23 min treatment in LF and RH furnace respectively, the slab was continuously cast into 300 mm thickness with 19 °C superheat at tundish pouring, the core macro-segregation level was B class 0.5. The slab was soaked at 1210 °C for 160 min, after descaling by high pressure water, the starting rolling temperature was 1075 °C, rough rolling to 150 mm thickness, the average single pass reduction was 32 mm, the second stage finishing rolling was performed when the temperature was up to 854 °C, the cumulative reduction was 112 mm, the finishing temperature was 834 °C, after ACC accelerated cooling with 23 °C / s rate to 420 °C, then after straightening, it was transported to the cooling bed. Figure 8 shows the core microstructure of Example 4, the ratio of acicular ferrite + polygonal ferrite reached 85%, the ratio of granular bainite reached 12%, the ratio of island M-A constituent reached 3%.

[0091] Table 1 summarizes the steel chemical composition of the comparative examples and examples,

[0092] Table 1 summarizes the steel chemical composition of the comparative examples and examples,

[0093] Wherein the carbon equivalent Ceq is calculated according to the following formula:

[0094] Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15

[0095] With the conventional X70M pipeline steel as the comparative steel, C and Mn exceed the upper limit of the present application, the conventional Al deoxidization is used in the steelmaking process, the P and S contents are not specially removed, and Ca wire is not fed after refining. All the components of the present application are within the component range of the present application, in addition, the Al deoxidization method is not used in the refining process, but the Mg alloy slow-release process is used for deoxidization in the LF furnace, and Ca treatment is carried out after RH refining treatment, so that the Mg / Al value is above 1.0, the Ca / S value is above 2.0, and the macrosegregation index (C + Mn / 6) x t slab x e ΔT / Tliq is not higher than 65.

[0096] Table 2 summarizes the smelting and refining methods and continuous casting slab defect ratings of the comparative examples and the examples,

[0097] Table 2 Smelting and refining methods and slab defect ratings of the examples of the present application

[0098] Table 3 summarizes the controlled rolling and controlled cooling methods of the comparative examples and the examples,

[0099] Table 3 Controlled rolling and controlled cooling methods of the examples of the present application

[0100] Table 4 summarizes the mechanical properties and HIC resistance of the comparative examples and the examples,

[0101] Table 4 Mechanical properties and HIC resistance of the examples of the present application

[0102] From the comparative data in Tables 1-4, combined with Figures 3-8, it is further confirmed that the present application obtains reliable quality of high strength, high toughness, and excellent HIC and SSC resistance of large wall thickness pipeline steel through the series of technical process methods of low segregation sensitivity chemical composition, refining process, inclusion control, microalloying, low segregation continuous casting, controlled rolling, and controlled cooling, which provides high-quality and safe pipeline steel for high-H2S acidic oil and gas transportation.

[0103] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the meaning consistent with their meaning in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0104] The meaning of "and / or" as used in this application includes both situations where each exists alone or both exist simultaneously.

[0105] The term "connection" as used in this application can mean a direct connection between components or an indirect connection between components through other components.

[0106] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A large wall thickness steel plate for a pipeline having excellent HIC resistance, characterized by: For the steel plate with wall thickness ≥ 30mm, the chemical composition includes the following components by weight percentage: C: 0.04-0.08%, Si: 0.10-0.40%, Mn: 0.60-1.20%, Ni: 0.15-0.50%, Cr: 0.15-0.50%, Mo: 0.05-0.30%, Cu: 0.15-0.50%, Nb: 0.020-0.080%, V: 0.005-0.04%, Ti: 0.008-0.020%, Ca: 0.003-0.006%, Mg: 0.002-0.008%, Al: ≤0.005%, P: ≤0.01%, S: ≤0.002%, the rest is Fe and inevitable impurities; Meanwhile, Ca / S is designed to be ≥2.0, Mg / Al is designed to be ≥1.0, and a segregation sensitivity index satisfies (C+Mn / 6)×t slab ×e ΔT / Tliq ≤65, wherein t slab is a slab thickness of the steel plate, ΔT is a superheat of the molten steel at the time of casting, and T liq is a liquidus temperature at the time of casting.

2. The method of manufacturing a large-walled pipe line steel plate having excellent HIC resistance according to claim 1, characterized in that, for a steel plate having a wall thickness of 30 mm or more: Specifically includes the following steps: Step S1: the molten iron is pre-desulfurization treated, then after converter blowing, LF refining treatment and RH vacuum refining treatment, Ca-Si composite cored wire is fed for calcium treatment, magnesium alloy containing steel block is added to the molten steel in the early stage of LF refining treatment, so that Mg / Al≥1.0 and Ca / S≥2.0 in the molten steel after calcium treatment, and the modification of oxide and sulfide inclusions is completed; Step S2: the molten steel modified in step S1 is subjected to continuous casting process, the superheat of the molten steel is set to be ≤25℃, argon and protective slag are used for protection during casting, and a continuous casting slab with a thickness of 200-320mm is obtained, the non-metallic inclusions of A type sulfide, B type aluminum oxide, C type silicate and D type spherical oxide in the continuous casting slab meet the preset inclusion grade requirements; Multi-roll dynamic light pressing is used at the end of continuous casting to make the center segregation of the continuous casting slab reach the preset evaluation standard; Step S3: the continuous casting slab is sent to a walking beam furnace, the continuous casting slab is heated to 1150-1230℃ in the walking beam furnace in sections, and the continuous casting slab is kept in the temperature homogenization zone for 1-3h, so that the Nb carbon nitride precipitates are fully dissolved in the austenite; Step S4: the continuous casting slab is pushed out from the walking beam furnace, high-pressure water is used to remove the surface iron oxide scale, and the first stage controlled rolling is carried out in the high temperature zone of 1050-950℃, the average single pass reduction amount is set, the deformed austenite is fully recrystallized, and the rolling is carried out until the intermediate billet thickness is 3-5 times of the target thickness of the finished product; Step S5: the intermediate billet is placed on the waiting temperature roller way for cooling, the second stage controlled finishing rolling is carried out when the temperature is in the range of 800-880℃, and the steel plate is rolled to the target thickness of the finished product; Step S6: the steel plate after the controlled rolling in step S5 is subjected to high-density header laminar flow accelerated cooling, the cooling rate is controlled to be ≥20℃ / s, the final cooling temperature is 250-350℃, the re-red temperature is 350-450℃, the water amount and the roller speed are matched to control the phase change structure in the steel plate to be uniform complex structure of fine crystal needle-shaped ferrite and bainite, and the cooling uniformity of the upper and lower surfaces and the length and width directions of the steel plate is controlled; Step S7: according to the size and shape of the steel plate after the cooling in step S6, the multi-roll straightening machine is used for dynamic reduction straightening until the flatness of the steel plate reaches the preset value.

3. The method of producing a large-walled pipe line steel plate having excellent HIC resistance according to claim 2, characterized by: In step S1, the molten iron is pre-desulfurized by mixing the magnesium desulfurizer with the molten iron to make S≤0.002% in the molten iron, and the temperature of the mixed molten iron is ≥1300℃; The pre-desulfurized molten iron is poured into a converter, and Fe-Mn, Fe-Ni, Fe-Cr, Fe-Cu ferroalloy materials are added, and the top-bottom oxygen lance is used for composite blowing oxygen refining, and the treatment time is 25-30 min; The time of LF refining treatment and RH vacuum refining treatment is set to 20-30 min; The magnesium alloy steel block added in the initial stage of LF refining treatment vaporizes in the process of sinking in the molten steel, reacts with oxygen in the molten steel to form MgO inclusions, and the MgO inclusions are absorbed into the protection and removed in the process of argon stirring; The length of the Ca-Si composite cored wire fed during calcium treatment is 150-300 m.

4. The method of producing a large-walled pipe line steel plate having excellent HIC resistance according to claim 2, characterized by: In step S2, the molten steel temperature superheat in the tundish during continuous casting is set to 15-25℃, and the non-metallic inclusions of A type sulfides, B type alumina, C type silicates, and D type spherical oxides in the continuous casting slab meet the inclusion level requirements: A≤0.5, B≤0.5, C≤0.5, and D≤0.5; The multi-roll dynamic light pressing is used at the end of continuous casting, and the thickness of the continuous casting slab is pressed by 20-50 mm, so that the slab center segregation reaches the Mannesmann rating level two standard.

5. The method of producing a large-walled pipe line steel plate having excellent HIC resistance according to claim 2, characterized by: In step S3, the austenite grain size is controlled to be 30-100 μm.

6. The method of producing a large-walled pipe line steel plate having excellent HIC resistance according to claim 2, characterized by: In step S4, when the first stage controlled rolling is performed in the high temperature zone, the rolling average single pass reduction is set to be ≥30 mm.

7. The method of producing a large-walled pipe line steel plate having excellent HIC resistance according to claim 2, characterized by: In step S5, when the second stage controlled finishing rolling is performed, the cumulative reduction is set to be 67%-80%.

8. The method of producing a large-walled pipe line steel plate having excellent HIC resistance according to claim 2, characterized by: In step S7, the multi-roll straightening machine is used for dynamic pressing straightening, so that the flatness of the steel plate is ≤6 mm / 2 m.

9. The method of producing a large-walled pipe line steel plate having excellent HIC resistance according to claim 2, characterized by: The finished steel plate obtained by the manufacturing method of any one of claims 2-8 has a microstructure in which the volume percentage of acicular and polygonal ferrite is 75-90%, the volume percentage of granular bainite is 5-23%, and the volume percentage of island-shaped martensite-austenite component is 2-5%.

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