A thick-walled steel plate for pipelines with excellent HIC resistance and a method for manufacturing the same

Abstract

The present invention relates to a thick-walled pipeline steel plate with excellent HIC resistance and a manufacturing method thereof, which solves the technical problem of poor hydrogen-induced cracking resistance of high-strength, high-toughness, thick-walled pipeline steel through low-segregation chemical composition and slab thickness design, sulfide inclusion modification, controlled rolling and controlled cooling process methods. The present invention obtains the reliable quality of thick-walled pipeline steel with high strength, high toughness and excellent HIC resistance, and provides high-quality and safe pipeline steel for high-H2S acidic oil and gas transportation.

Description

Technical Field

[0001] The invention relates to a thick-walled pipeline steel plate with excellent HIC resistance and a manufacturing method thereof, belonging to the technical field of steel material manufacturing. Background Art

[0002] Hydrogen Induced Cracking (HIC) is a common corrosion and cracking problem faced by steel in hydrogen-containing media environments (such as high-concentration hydrogen sulfide H2S). These problems can cause cracks inside the steel, seriously reducing the safety of storage and transportation structures. HIC-resistant pipeline steel is widely used in oil and gas pipelines containing hydrogen sulfide due to its excellent resistance to hydrogen-induced cracking. At present, the HIC-resistant pipeline steel that has been applied in batches is mainly X52MS to X65MS grades. With the development of the oil and gas industry, the demand for long-distance, high-pressure hydrogen-containing oil and gas pipelines is increasing, and the demand for HIC-resistant pipeline steel with higher strength, higher toughness and larger wall thickness is also rising.

[0003] In order to improve the crack resistance of steel in moist acidic media, the key is to eliminate factors that promote the nucleation and propagation of hydrogen-induced cracks. This includes improving the cleanliness of molten steel, reducing the content of harmful impurities such as phosphorus (P) and sulfur (S) during the smelting stage, and optimizing the composition, size and morphology of non-metallic inclusions. In the casting stage, the homogenization of the slab should be improved, especially the degree of central segregation of the slab 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 acid-resistant pipeline steels faces technical challenges, such as the need to add more alloying elements, use thicker slabs, and use stronger cooling intensity after rolling, which may lead to more severe central segregation, thereby affecting the key HIC resistance performance.

[0004] For example, for the development of X70MS grade pipeline steel, patent CN113846268A discloses an X70MS acid-resistant pipeline steel plate and its preparation method, the thickness specification range of the pipeline steel plate is 10-20mm, the yield strength ratio is less than 0.88, and the shear area is greater than 90% in the -30 ℃ temperature drop weight test (DropWeightTearTest, DWTT). Although some 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 under low temperature conditions is insufficient, which may affect the application performance of the material under extreme climatic conditions. Patent CN102330032A focuses on an acid-resistant X70MS grade spiral submerged arc welded pipe, which has high strength and excellent acid resistance and is suitable for conveying acidic oil and gas media, but due to its high cost, it is limited to its wide application in long-distance pipelines, especially in cost-sensitive projects. Patent CN108998746B discloses an X70-grade HIC-resistant pipeline steel and a method for controlled rolling and controlled cooling thereof. Although technically innovative, the complexity of the rolling process and the narrow process window may lead to a low performance qualification rate in the production process, which not only affects production efficiency but may also increase overall production costs.

[0005] Therefore, in order to meet the technical challenges faced by the upgrading of HIC-resistant pipeline steel, more refined optimization and adjustment must be made in alloy composition, inclusion control, microstructure homogenization design and control methods. Summary of the invention

[0006] The present invention provides a thick-walled pipeline steel plate with excellent HIC resistance and a manufacturing method thereof, so as to obtain the reliable quality of thick-walled pipeline steel with high strength, high toughness and excellent HIC resistance, and provide high-quality and safe pipeline steel for high-H2S acidic oil and gas transportation.

[0007] The technical solution adopted by the present invention to solve its technical problem is:

[0008] A thick-walled pipeline steel plate with excellent HIC resistance, wherein the chemical composition of the steel plate with a wall thickness of ≥30 mm 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%, and the rest is Fe and unavoidable impurities;

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

[0010] Furthermore, for steel plates with a wall thickness of ≥30 mm, the following steps are specifically included:

[0011] Step S1: pre-desulfurizing the molten iron, and then feeding the Ca-Si composite cored wire for calcium treatment after converter blowing, LF refining and RH vacuum refining. In the early stage of LF refining, a steel block containing magnesium alloy is added to the molten steel, so that Mg / Al≥1.0 and Ca / S≥2.0 in the molten steel after calcium treatment are achieved, thereby completing the modification of oxide and sulfide inclusions.

[0012] Step S2: performing a continuous casting process on the molten steel modified in step S1, setting the superheat of the molten steel to ≤25°C, and casting with argon gas and protective slag throughout the casting process to obtain a continuous casting slab with a thickness of 200-320 mm, wherein the non-metallic inclusions of Class A sulfide, Class B alumina, Class C silicate, and Class D spherical oxide in the continuous casting slab meet the preset inclusion grade requirements;

[0013] At the end of continuous casting, multi-roller dynamic soft pressure is used to make the center segregation of the continuous casting slab reach the preset rating standard;

[0014] Step S3: feeding the continuously cast slab into a walking beam furnace, heating the continuously cast slab to 1150-1230° C. in stages in the walking beam furnace, and keeping the continuously cast slab in a temperature homogenization zone for 1-3 hours, so that Nb carbonitride precipitated particles Nb(CN) are fully dissolved in austenite;

[0015] Step S4: the continuous casting slab is pushed out from the walking furnace, the surface oxide scale is removed by high-pressure water, and the first stage of controlled rolling is carried out in a high temperature zone of 1050-950°C, the average single-pass rolling reduction is set, the deformed austenite is fully recrystallized, and the intermediate slab is rolled to a thickness of 3-5 times the target thickness of the finished product;

[0016] Step S5: placing the intermediate billet on a warming roller table to cool down the temperature, and performing the second stage controlled finishing rolling when the temperature reaches 800-880°C, until the steel plate is rolled to the target thickness of the finished product;

[0017] Step S6: the steel plate after the controlled rolling in step S5 is subjected to accelerated cooling by high-density manifold laminar water, the cooling rate is controlled to be ≥20°C / s, the final cooling temperature is 250-350°C, the red-returning temperature is 350-450°C, and the phase transformation structure in the steel plate is controlled to be a uniform multiphase structure of fine-grained acicular ferrite and bainite by coordinating the water volume and the roller speed, and the cooling uniformity of the upper and lower surfaces and the length and width directions of the steel plate is controlled at the same time;

[0018] Step S7: according to the size and shape of the steel plate after cooling in step S6, a multi-roller straightening machine is used to perform dynamic pressing straightening until the straightness of the steel plate reaches a preset value;

[0019] Furthermore, in step S1, when the molten iron is pre-desulfurized, the magnesium desulfurizer is fully mixed with the molten iron to react, so that the sulfur in the molten iron is ≤0.002%, and the temperature of the molten iron after the mixed reaction is ≥1300°C;

[0020] Pour the pre-desulfurized molten iron into the converter, add pre-configured Fe-Mn, Fe-Ni, Fe-Cr, Fe-Cu ferroalloy materials, and use top-bottom oxygen lances for composite oxygen blowing refining, the processing time is 25-30 minutes;

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

[0022] The magnesium alloy steel blocks added in the early stage of LF refining treatment vaporize during the sinking of the molten steel and react with oxygen in the molten steel to form MgO inclusions, which are removed by suction protection during the argon blowing and stirring process.

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

[0024] Furthermore, in step S2, the superheat degree of the molten steel in the continuous casting is set to 15-25°C, and the non-metallic inclusions of type A sulfide, type B alumina, type C silicate, and type D spherical oxide in the continuous casting slab meet the inclusion grade requirements of: A≤0.5, B≤0.5, C≤0.5, D≤0.5;

[0025] At the end of continuous casting, the multi-roller dynamic soft pressing is used to reduce the thickness of the continuous casting slab by 20-50mm, so that the center segregation of the slab reaches the Mannesmann rating level 2 standard;

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

[0027] Furthermore, in step S4, when the first stage of controlled rolling is performed in the high temperature zone, the average single-pass rolling reduction is set to be ≥30 mm;

[0028] Further, in step S5, during the second stage of finishing rolling, the cumulative reduction is set to 67%-80%;

[0029] Furthermore, in step S7, a multi-roll straightening machine is used to perform dynamic press-down straightening to make the straightness of the steel plate ≤6 mm / 2 m;

[0030] Furthermore, 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 martensite-austenite components is 2-5%.

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

[0032] 1. The thick-walled pipeline steel plate with excellent HIC resistance provided by the present invention, when designing the chemical composition of the steel plate with a wall thickness of ≥30 mm, accurately controls the addition amount of key alloys to achieve a balance between high strength, high toughness and excellent hydrogen-induced cracking resistance; reasonably adds microalloying elements to refine the grains and exert precipitation strengthening effects; strictly controls harmful elements P and S, adds appropriate amounts of Mg and Ca and the order of addition to effectively refine the size of inclusions, change the composition of inclusions, and spheroidize their morphology to avoid the formation of long strip inclusions during rolling;

[0033] 2. The manufacturing method of the thick-walled pipeline steel plate with excellent HIC resistance provided by the present invention is based on low-segregation chemical composition and slab thickness design, combined with a series of technical process methods such as refining process, inclusion control, microalloying, low-segregation continuous casting, controlled rolling, and controlled cooling, to obtain the reliable quality of thick-walled pipeline steel with high strength, high toughness, and excellent HIC and SSC resistance, providing high-quality and safe pipeline steel for high-H2S acidic oil and gas transportation. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0035] Figure 1 is a flow chart of the manufacturing method provided by the present invention;

[0036] Figure 2 It is a schematic diagram of the controlled rolling and controlled cooling processes involved in the manufacturing method provided by the present invention;

[0037] Figure 3 It is a schematic diagram of the banded structure in the core of the steel plate in the comparative example provided by the present invention;

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

[0039] Figure 5 Schematic diagram of the microstructure of the core of the steel plate in Example 1 provided by the present invention;

[0040] Figure 6 Schematic diagram of the microstructure of the core of the steel plate in Example 2 provided by the present invention;

[0041] Figure 7 Schematic diagram of the microstructure of the core of the steel plate in Example 3 provided by the present invention;

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

[0043] In the figure: 1 is a walking furnace, 2 is a continuous casting slab, 3 is high-pressure water to remove surface oxide scale, 4 is the 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 invention will now be described in further detail with reference to the accompanying drawings. The specific dimensions used in this embodiment are only for illustrating the technical solution and do not limit the protection scope of the present invention.

[0045] As explained in the background technology, steel generally faces 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 refers to a brittle cracking phenomenon of metal materials under the combined action of tensile stress and a specific corrosive environment (mainly a sulfide-containing environment). In essence, it is because sulfides will form an acidic environment in water, which promotes steel corrosion. At the same time, tensile stress breaks the protective film on the metal surface, accelerating the corrosion process. The hydrogen atoms in the steel material penetrate into the material, resulting in increased brittleness and cracking. Obviously, it is imperative to improve the acid resistance of pipeline steel.

[0046] At present, in the technology for improving the performance of acid-resistant pipeline steel, the steel thickness specifications, production costs and performance qualification rates cannot be guaranteed at the same time, resulting in the problem that high-strength, high-toughness, and thick-walled pipeline steels have poor hydrogen-induced cracking resistance and cannot be solved. In order to solve the above problems, the present application provides a thick-walled pipeline steel plate with excellent HIC resistance and a manufacturing method thereof, which is mainly aimed at thick-walled pipeline steel plates with a wall thickness of ≥30 mm, and meets low segregation in chemical composition design, and at the same time, relevant designs are made for the thickness of the slab; in the matching manufacturing method, multiple designs of sulfide inclusion modification, controlled rolling and controlled cooling process methods are carried out, and finally the reliable quality of high strength, high toughness and excellent HIC resistance (and excellent SSC resistance) of thick-walled pipeline steel is obtained, providing high-quality and safe pipeline steel for high H2S acidic oil and gas transportation.

[0047] The heavy-walled pipeline steel plate with excellent HIC resistance provided in the present application comprises the following components in chemical composition 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%, and the rest are Fe and unavoidable impurities. Among the above components, the addition amounts of key alloys, such as C, Mn, Ni, Cr, Mo, and Cu, are precisely controlled to achieve a balance between high strength, high toughness, and excellent resistance to hydrogen-induced cracking. Microalloying elements Nb, V, and Ti are reasonably added to refine the grains and exert precipitation strengthening effects. Harmful elements P and S are strictly controlled, and appropriate amounts of Mg and Ca are added in the order of addition to effectively refine the size of inclusions, change the composition of inclusions, and spheroidize their morphology to avoid the formation of long strip 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. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, too high C content is not conducive to the suppression of banded structure, plasticity, toughness and weldability of steel. From the perspective of comprehensive performance, it is preferred that the C content be controlled at 0.04-0.08%.

[0049] Si: It is one of the elements for smelting deoxidation and improving strength. At the same time, Si can inhibit the coarsening of bainite, the precipitation and coarsening of cementite, and improve toughness. Therefore, the preferred Si content is 0.10-0.40%.

[0050] Mn: It is the main element to ensure the strength, toughness and weldability of steel. It can expand the austenite temperature range, inhibit the phase transformation from austenite to ferrite, and is conducive to the refinement of the final structure. It is also a key element affecting the HIC resistance and sulfide stress cracking performance. However, too high a content increases the degree of segregation and banded structure, and forms MnS inclusions with S in the 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 has an adverse effect on low-temperature toughness and ductility. It may segregate in the center of the slab and aggregate at the grain boundaries, thereby damaging the low-temperature toughness. In the present invention, P is controlled to be no higher than 0.01%.

[0052] S: It is the primary element that is unfavorable to the HIC character. It forms MnS inclusions with Mn, which are easily elongated during the rolling process and become the source of cracks. The S content of the material of the present invention is controlled to be no higher than 0.002%.

[0053] Ni: It has the effects of solid solution strengthening and grain refinement. It is the main alloying element for stabilizing austenite, reduces the phase transition point from austenite to ferrite, promotes ferrite grain refinement, improves hardenability, and can simultaneously improve strength and toughness. However, excessive Ni content increases the sensitivity of steel to hydrogen embrittlement and sulfide stress cracking in an acidic environment. Therefore, the Ni content of the present invention is not higher than 0.50%, and the preferred range is 0.15-0.50%.

[0054] Mo: has the effects of solid solution strengthening, precipitation strengthening, improving hardenability and improving HIC resistance, can effectively improve the uniformity of strength and toughness in the thickness direction of the steel plate, and together with C forms nano-sized carbide MoC to have a precipitation strengthening effect, while improving the hydrogen embrittlement and HIC resistance of steel in an acidic environment. Therefore, the Mo content of the present invention is in the range of 0.05-0.30%.

[0055] Cr: It is the main element that improves the overall and local pitting corrosion resistance in an acidic environment containing H2S. It can capture diffused hydrogen atoms and form carbides to provide precipitation strengthening. It is also a stabilizing element and hardenability element in the strong ferrite region, which can improve the uniformity of the structure of thick pipeline steel plates. Therefore, the Cr content of the present invention is in the range of 0.15-0.50%.

[0056] Cu: It has the function of improving the HIC resistance in an acidic environment containing H2S, and inhibits the entry of hydrogen by forming a protective layer on the steel surface, thereby improving the hydrogen-induced cracking resistance. At the same time, Cu precipitates in the steel during aging, playing a precipitation strengthening role. Therefore, the Cu content of the present invention is in the range of 0.15-0.50%.

[0057] Nb: It is an important element for improving HIC resistance in acidic environment, and is also a microalloying element that plays an effective role in grain refinement and precipitation strengthening. It can regulate the shape and distribution of non-metallic inclusions in steel, inhibit the formation of banded structure, thereby reducing the sensitivity to H2S stress cracking, and significantly improve the high-temperature austenite non-recrystallization zone, increase the austenite dislocation density during the rolling process, effectively refine the final product structure, and form high-density nano-sized NbC and Nb (C, N) precipitation particles with C and N in the steel, which play a precipitation strengthening role. Therefore, the Nb content of the present invention is in the range of 0.02-0.08%.

[0058] V: It has the effects of precipitation strengthening, grain refinement, and improving resistance to HIC and sulfide stress cracking. It forms nano-sized VC and V (C, N) particles with C and N in steel to improve strength, and hinder the phase transformation of austenite to ferrite, achieving fine grain strengthening and toughening effects. At the same time, it acts as a hydrogen trap to improve resistance to hydrogen-induced cracking and sulfide stress cracking in an acid environment. Therefore, the V content of the present invention is in the range of 0.005-0.04%.

[0059] Ti: It has the function of refining the austenite grain size and improving the welding performance. It forms TiN particles with N during the steelmaking process, inhibits the growth and recrystallization of austenite grains, thereby refining the finished ferrite grains, coarsening the grains in the heat-affected zone during the welding process, improving the toughness of the welding part, and can also effectively remove O, N, and S in the steel, forming small and stable Ti-based inclusions, and improving the cleanliness of the molten steel. Therefore, the Ti content of the present invention is in the range of 0.008-0.020%.

[0060] Ca: It is the main element for improving the composition and morphology of non-metallic inclusions, and promotes the formation of calcium-containing inclusions such as calcium aluminate and calcium sulfide, which plays an important role in the corrosion resistance of steel in acidic medium containing H2S and the inhibition of hydrogen-induced cracking. At the same time, calcium-containing inclusions can inhibit the coarsening of the heat-affected zone during welding, and play a role in improving welding toughness. Therefore, the Ca content of the present invention is in the range of 0.003-0.006%.

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

[0062] Al: has a strong deoxidizing effect and is the main element forming oxide inclusions. It competes with Mg and Ti for O in the steelmaking process. In the present invention, it is not conducive to the formation of composite inclusions that promote the acicular ferrite phase transformation. Therefore, the Al content in the present invention should not exceed ≤0.005%.

[0063] In addition to optimizing the design of each chemical composition, it is also necessary to meet the relevant restrictions on low segregation and slab thickness. In this application, Ca / S ≥ 2.0, Mg / Al ≥ 1.0 are designed, and the segregation sensitivity index satisfies (C+Mn / 6)× t slab × e ΔT / Tliq ≤65wt%·mm, where:t slab is the thickness of the steel plate, in mm, ΔT is the superheat of the molten steel during casting, in °C, T liq It is the liquidus temperature during casting, in °C, and the contents of C and Mn are both expressed in mass percentage.

[0064] Next, the present application provides a method for manufacturing a thick-walled pipeline steel plate with excellent HIC resistance. The overall process flow is as follows: Figure 1 As shown, the specific steps include:

[0065] Step S1: pre-desulfurize the molten iron, fully mix the magnesium desulfurizer with the molten iron at a temperature of ≥1300°C to react, so that the S in the molten iron is ≤0.002%, and the sulfur content in the molten iron is reduced by the magnesium desulfurizer, thereby reducing the formation of sulfide inclusions; pour the pre-desulfurized molten iron into a converter, and add pre-configured Fe-Mn, Fe-Ni, Fe-Cr, and Fe-Cu ferroalloy materials, and use a top-bottom oxygen gun for composite oxygen blowing refining, and the treatment time is 25-30 minutes; at the beginning of the LF refining treatment, a steel block containing a magnesium alloy is added, and during the sinking process of the molten steel, the magnesium alloy vaporizes and slowly releases into the molten steel, reacts with oxygen in the molten steel to form MgO inclusions, and the MgO inclusions are relatively stable; then, argon blowing and stirring are used to evenly distribute fine inclusions, and coarse inclusions are caused to float up and be sucked in for protection and removal, thereby further improving the state of inclusions in the molten steel. Then RH vacuum refining treatment is carried out, and 150-300m long Ca-Si composite cored wire is fed after vacuum treatment. The time of the entire LF refining treatment and RH vacuum refining treatment is set to 20-30min. Elements such as magnesium and calcium in the molten steel react with possible oxides and sulfides. After the RH treatment, Mg / Al≥1.0 and Ca / S≥2.0 in the molten steel are modified to complete the modification of oxide and sulfide inclusions. This step is a more prominent innovation of the present application, which can modify the sulfide inclusions in the molten steel into a more stable inclusion form, avoiding the cracking of the steel caused by the internal pressure generated by hydrogen molecules and the cracking caused by the brittleness caused by the penetration of hydrogen atoms into the metal.

[0066] Step S2: Continuously cast the modified molten steel in step S1, set the superheat of the molten steel to ≤25°C, preferably, set the superheat of the molten steel temperature in the continuous casting to 15-25°C; use argon and protective slag to protect the casting throughout the process, so that the non-metallic inclusions of type A sulfide, type B alumina, type C silicate, and type D spherical oxide in the continuous casting slab 2 meet the inclusion grade requirements of: A≤0.5, B≤0.5, C≤0.5, and D≤0.5. At the end of the continuous casting, multi-roller dynamic light pressing is used to press down the thickness of the continuous casting slab by 20-50mm, so that the center segregation of the slab reaches the Mannesmann rating level 2 standard, and a continuous casting slab with a thickness of 200-320mm is obtained.

[0067] Figure 2 As shown, a schematic diagram of the flow of each device in the controlled rolling and controlled cooling process is provided. Step S3: The continuous casting slab is sent to the stepping furnace 1, and the continuous casting slab is heated to 1150-1230°C in stages in the stepping furnace. This temperature can cause certain changes in the internal structure of the metal. The continuous casting slab is kept in the temperature homogenization area for 1-3 hours, and the continuous casting slab is kept in a relatively stable high temperature environment for a period of time to fully transfer heat and eliminate internal temperature non-uniformity. The Nb carbonitride precipitation particles Nb (CN) are fully dissolved in the austenite, and the austenite grain size is controlled to be 30-100μm.

[0068] Step S4: Push the continuous casting slab out of the stepping furnace, use high-pressure water to remove the surface oxide scale 3, and carry out the first stage of controlled rolling in the high temperature zone of 1050-950℃, setting the average single-pass rolling reduction to ≥30mm, fully recrystallize the deformed austenite, and roll until the intermediate billet thickness is 3-5 times the target thickness of the finished product.

[0069] Step S5: placing the intermediate billet on a warming roller table for cooling, and performing the second stage controlled finishing rolling 4 when the temperature reaches 800-880°C, setting the cumulative reduction to 67%-80%, until 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, and the austenite non-recrystallization temperature is used as the boundary, and the continuous casting slab is subjected to high temperature and low temperature controlled rolling in two stages. The purpose is to optimize the high temperature rolling pass to allow the austenite to be fully recrystallized and refined, and to provide sufficient cumulative rolling pressure in the low temperature zone to provide sufficient particle density for the ferrite phase transformation, and finally refine the finished steel plate 7 structure.

[0071] After the steel plate is rolled, it is cooled online quickly, i.e., step S6: the steel plate after the controlled rolling in step S5 is subjected to high-density manifold laminar water accelerated cooling 5, the cooling rate is controlled to be ≥20°C / s, the final cooling temperature is 250-350°C, and the red-returning temperature is 350-450°C. By coordinating the water volume and the roller speed, the phase transformation structure in the steel plate is controlled to be a uniform multiphase structure of fine-grained acicular ferrite and bainite, the phase transformation is accurately controlled to obtain the volume ratio of fine-grained acicular ferrite, granular bainite and MA islands, and the banded pearlite structure that is not conducive to the HIC resistance is suppressed by the rapid phase transformation process to avoid the formation of the banded structure that is not conducive to the HIC resistance; at the same time, the cooling uniformity of the upper and lower surfaces and the length and width directions 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-roll straightening machine is used to perform dynamic pressing and straightening 6 until the straightness of the steel plate is ≤6mm / 2m.

[0073] As described above, the whole process is controlled from aspects such as alloy element content, steel cleanliness control, process optimization and parameter selection, and microstructure control. In the final microstructure of the finished steel plate, 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 martensite-austenite components is 2-5%; the pipeline steel has high strength, high toughness, and excellent resistance to HIC and SSCC.

[0074] In order to verify the feasibility and superiority of the chemical composition design and manufacturing process of the steel plate for thick-walled pipelines provided in this application, the following 1 comparative example and 4 embodiments are used as comparative examples.

[0075] Comparative Example:

[0076] The conventional X70M pipeline steel is selected, and the chemical composition includes the following components by 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 are Fe and unavoidable impurities, Ca / S=0.16, Mg / Al=0.025, Ceq=0.48%, (C+Mn / 6)× t slab × e ΔT / Tliq =78.6.

[0077] After being treated in the LF furnace and RH furnace for 21min and 23min respectively, the superheat degree during casting in the tundish was 34℃, and the slab was continuously cast into a 230mm thick slab. The core macro segregation level was Class B level 1, and the sulfide inclusion level was 0.5. The slab was heated at 1210℃ and kept warm for 120min. After being removed from the furnace, it was descaled by high-pressure water. The starting rolling temperature was 1080℃, and the rough rolling was to a 120mm thick intermediate slab. The average single-pass reduction was 22mm. The second stage of finishing rolling was carried out when the temperature reached 882℃, and the cumulative reduction was 82mm. The final rolling temperature was 850℃, and it was cooled to 420℃ at a rate of 23℃ / s through ACC accelerated cooling, and then transported to the cooling bed after straightening.

[0078] Figure 3 This is a schematic diagram of the banded structure in the core of the steel plate. Figure 4 This is a schematic diagram of long strip inclusions in 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 martensite-austenite (MA) components in the core banded structure. Example

[0079] Embodiment 1:

[0080] The chemical composition of 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)× t slab × e ΔT / Tliq =61.

[0081] After being treated in the LF furnace and RH furnace for 22min and 24min respectively, the superheat degree during casting in the tundish was 23℃, and the slab was continuously cast into a 230mm thick slab, and the core macro segregation level was Class B 0.5. The slab was heated at 1210℃ and kept warm for 120min. After being removed from the furnace, it was descaled by high-pressure water. The starting rolling temperature was 1080℃, and the rough rolling was to a 120mm thick intermediate slab. The average single-pass reduction was 22mm. The second stage of finishing rolling was carried out when the temperature reached 880℃, and the cumulative reduction was 82mm. The final rolling temperature was 850℃, and it was cooled to 390℃ at a rate of 23℃ / s through ACC accelerated cooling, and then transported to the cooling bed after straightening. Figure 5 The core microstructure of Example 1 is shown, in which the proportion of acicular ferrite reaches 75%, the proportion of granular bainite is 20%, and the proportion of island-shaped MA components is 5%.

[0082] Embodiment 2:

[0083] The chemical composition of pipeline steel includes the following components by weight percentage: 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 is Fe and unavoidable impurities, Ca / S=2.9, Mg / Al=1.0, Ceq=0.32%, (C+Mn / 6)× t slab × e ΔT / Tliq =49.8.

[0084] After being treated in the LF furnace and RH furnace for 21min and 23min respectively, the superheat degree during casting in the tundish was 21℃, and continuous casting was used to form a 230mm thick slab, with a core macro-segregation level of Class B 0.5. The slab was heated at 1210℃ and kept warm for 120min. After being removed from the furnace, it was descaled by high-pressure water. The starting rolling temperature was 1082℃, and the rough rolling was to a 120mm thick intermediate slab. The average single-pass reduction was 25mm. The second stage of finishing rolling was carried out when the temperature reached 876℃, with a cumulative reduction of 82mm and a final rolling temperature of 850℃. It was cooled to 410℃ at a rate of 23℃ / s through ACC accelerated cooling, and then transported to the cooling bed after straightening. Figure 6 The core microstructure of Example 2 is shown, in which the proportion of acicular ferrite + polygonal ferrite reaches 85%, the proportion of granular bainite is 10%, and the proportion of island-shaped MA components is 5%.

[0085] Embodiment 3:

[0086] The chemical composition of pipeline steel includes the following components by weight percentage: C: 0.05%, Si: 0.33%, Mn: 0.65%, Ni: 0.45%, Cr: 0.28%, Mo: 0.23%, Cu: 0.28%, Nb: 0.065%, V: 0.04%, Ti: 0.013%, Ca: 0.004%, Mg: 0.003%, Al: 0.002%, P: 0.007%, S: 0.0012%, and the rest are Fe and unavoidable impurities, Ca / S=3.3, Mg / Al=1.5, Ceq=0.32%, (C+Mn / 6)× t slab × e ΔT / Tliq =47.5.

[0087] After being treated in the LF furnace and RH furnace for 23min and 24min respectively, the superheat degree during casting in the tundish was 18℃, and the slab was continuously cast into a 300mm thick slab, and the core macro segregation level was Class B 0.5. The slab was heated at 1210℃ and kept warm for 160min. After being removed from the furnace, it was descaled by high-pressure water. The starting rolling temperature was 1078℃, and the rough rolling was to a 150mm thick intermediate slab. The average single-pass reduction was 32mm. The second stage of finishing rolling was carried out when the temperature reached 862℃, and the cumulative reduction was 112mm. The final rolling temperature was 850℃, and it was cooled to 360℃ at a rate of 23℃ / s through ACC accelerated cooling, and then transported to the cooling bed after straightening. Figure 7 The core microstructure of Example 3 is shown, in which the proportion of acicular ferrite + polygonal ferrite reaches 90% and the proportion of island-shaped MA components reaches 10%.

[0088] Embodiment 4:

[0089] The chemical composition of 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%, and the rest are Fe and unavoidable impurities, Ca / S=2.1, Mg / Al=1.5, Ceq=0.36%, (C+Mn / 6)× t slab × e ΔT / Tliq =53.3.

[0090] After being treated in the LF furnace and RH furnace for 22min and 23min respectively, the superheat degree during casting in the tundish was 19℃, and the slab was continuously cast into a 300mm thick slab, and the core macro segregation level was Class B 0.5. The slab was heated at 1210℃ and kept warm for 160min. After being removed from the furnace, it was descaled by high-pressure water. The starting rolling temperature was 1075℃, and the rough rolling was to a 150mm thick intermediate slab. The average single-pass reduction was 32mm. The second stage of finishing rolling was carried out when the temperature reached 854℃, and the cumulative reduction was 112mm. The final rolling temperature was 834℃, and it was cooled to 420℃ at a rate of 23℃ / s through ACC accelerated cooling, and then transported to the cooling bed after straightening. Figure 8 The core microstructure of Example 4 is shown, in which the proportion of acicular ferrite + polygonal ferrite reaches 85%, the proportion of granular bainite reaches 12%, and the proportion of island-shaped MA components reaches 3%.

[0091] Table 1 summarizes the chemical composition of the steels in the comparative examples and the examples.

[0092] Table 1 Chemical composition of the embodiments of the present invention and the comparative steel (weight percentage)

[0093]

[0094] The carbon equivalent Ceq calculation formula is as follows:

[0095]

[0096] Taking conventional X70M pipeline steel as a comparative steel, C and Mn exceed the upper limit of the present invention, conventional Al deoxidation is adopted in the steelmaking process, P and S content are not specially removed, and no Ca wire is fed after refining. However, all components of the embodiment of the present invention are within the scope of the invention components. In addition, the refining process does not adopt the Al deoxidation method, but adopts the Mg alloy slow release process to deoxidize in the LF furnace, and Ca treatment is performed after the RH refining treatment, ensuring that the Mg / Al value is above 1.0, the Ca / S value is above 2.0, and the macrosegregation index (C+Mn / 6)× t slab × e ΔT / Tliq Not higher than 65.

[0097] Table 2 summarizes the smelting and refining methods and continuous casting slab defect ratings of the comparative examples and the embodiments.

[0098] Table 2 Refining method and slab defect rating of the embodiment of the present invention

[0099]

[0100] Table 3 summarizes the controlled rolling and controlled cooling methods of the comparative examples and the embodiments.

[0101] Table 3 Controlled rolling and controlled cooling methods of embodiments of the present invention

[0102]

[0103] Table 4 summarizes the mechanical properties and HIC resistance of the comparative examples and embodiments.

[0104] Table 4 Mechanical properties and HIC resistance of the embodiments of the present invention

[0105]

[0106] From the comparative data in Tables 1 to 4, combined with Figure 3-Figure 8 It is further confirmed that the present application obtains the reliable quality of high strength, high toughness and excellent HIC and SSC resistance of thick-walled pipeline steel through a series of technical process methods of low segregation sensitivity index chemical composition, refining process, inclusion control, microalloying, low segregation continuous casting, controlled rolling and controlled cooling, providing high-quality and safe pipeline steel for high H2S acidic oil and gas transportation.

[0107] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art to which this application belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless defined as herein.

[0108] The meaning of "and / or" described in this application means that the situations where each exists alone or both exist at the same time are included.

[0109] The term “connection” as used in this application may mean a direct connection between components or an indirect connection between components via other components.

[0110] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A thick-walled pipeline steel plate with excellent HIC resistance, characterized in that: Its 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%, and the rest is Fe and unavoidable impurities; At the same time, the design Ca / S ≥ 2.0, Mg / Al ≥ 1.0, and the segregation sensitivity index satisfies (C+Mn / 6) × t slab × e ΔT / Tliq ≤ 65, where t slab is the thickness of the steel plate, ΔT is the superheat of the molten steel during casting, T liq is the liquidus temperature during casting; The wall thickness of the steel plate is ≥30mm; The manufacturing method specifically comprises the following steps: Step S1: pre-desulfurizing the molten iron, and then after converter blowing, LF refining and RH vacuum refining, feeding the Ca-Si composite cored wire for calcium treatment, adding a steel block containing a magnesium alloy to the molten steel at the beginning of the LF refining treatment, so that Mg / Al ≥ 1.0 and Ca / S ≥ 2.0 in the molten steel after calcium treatment, and completing the modification of oxide and sulfide inclusions; Step S2: performing a continuous casting process on the molten steel modified in step S1, setting the superheat of the molten steel to ≤25°C, and casting with argon gas and protective slag throughout the casting process to obtain a continuous casting slab with a thickness of 200-320 mm, wherein the non-metallic inclusions of type A sulfide, type B alumina, type C silicate, and type D spherical oxide in the continuous casting slab meet the inclusion grade requirements of: A≤0.5, B≤0.5, C≤0.5, and D≤0.5; At the end of continuous casting, multi-roll dynamic soft pressure is used to make the center segregation of the slab reach the Mannesmann grade II standard; Step S3: feeding the continuously cast slab into a walking beam furnace, heating the continuously cast slab in sections to 1150-1230° C. in the walking beam furnace, and keeping the continuously cast slab in a temperature homogenization area for 1-3 hours, so that the Nb carbonitride precipitated particles Nb(CN) are fully dissolved in the austenite; Step S4: the continuous casting slab is pushed out from the walking beam furnace, the surface oxide scale is removed by high-pressure water, and the first stage of controlled rolling is carried out in a high temperature zone of 1050-950°C, the average single-pass rolling reduction is set to be ≥30mm, the deformed austenite is fully recrystallized, and the intermediate slab is rolled to a thickness of 3-5 times the target thickness of the finished product; Step S5: placing the intermediate billet on a warming roller table to cool it down, and performing the second stage controlled finishing rolling when the temperature reaches 800-880°C, until 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 accelerated cooling by high-density manifold laminar water, the cooling rate is controlled to be ≥20°C / s, the final cooling temperature is 250-350°C, the red-returning temperature is 350-450°C, and the cooling uniformity of the upper and lower surfaces and the length and width directions of the steel plate is controlled at the same time; Step S7: according to the size and shape of the steel plate after cooling in step S6, a multi-roll straightening machine is used to perform dynamic pressing straightening until the straightness of the steel plate is ≤ 6 mm / 2 m; In the microstructure of the finished steel plate, 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 martensite-austenite components is 2-5%.

2. The method for manufacturing a thick-walled pipeline steel plate having excellent HIC resistance according to claim 1, characterized in that: The steel plate wall thickness is ≥30mm, and specifically comprises the following steps: Step S1: pre-desulfurizing the molten iron, and then after converter blowing, LF refining and RH vacuum refining, feeding the Ca-Si composite cored wire for calcium treatment, adding a steel block containing a magnesium alloy to the molten steel at the beginning of the LF refining treatment, so that Mg / Al ≥ 1.0 and Ca / S ≥ 2.0 in the molten steel after calcium treatment, and completing the modification of oxide and sulfide inclusions; Step S2: performing a continuous casting process on the molten steel modified in step S1, setting the superheat of the molten steel to ≤25°C, and casting with argon gas and protective slag throughout the casting process to obtain a continuous casting slab with a thickness of 200-320 mm, wherein the non-metallic inclusions of type A sulfide, type B alumina, type C silicate, and type D spherical oxide in the continuous casting slab meet the inclusion grade requirements of: A≤0.5, B≤0.5, C≤0.5, and D≤0.5; At the end of continuous casting, multi-roll dynamic soft pressure is used to make the center segregation of the slab reach the Mannesmann grade II standard; Step S3: feeding the continuously cast slab into a walking beam furnace, heating the continuously cast slab in sections to 1150-1230° C. in the walking beam furnace, and keeping the continuously cast slab in a temperature homogenization area for 1-3 hours, so that the Nb carbonitride precipitated particles Nb(CN) are fully dissolved in the austenite; Step S4: the continuous casting slab is pushed out from the walking beam furnace, the surface oxide scale is removed by high-pressure water, and the first stage of controlled rolling is carried out in a high temperature zone of 1050-950°C, the average single-pass rolling reduction is set to be ≥30mm, the deformed austenite is fully recrystallized, and the intermediate slab is rolled to a thickness of 3-5 times the target thickness of the finished product; Step S5: placing the intermediate billet on a warming roller table to cool it down, and performing the second stage controlled finishing rolling when the temperature reaches 800-880°C, until 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 accelerated cooling by high-density manifold laminar water, the cooling rate is controlled to be ≥20°C / s, the final cooling temperature is 250-350°C, the red-returning temperature is 350-450°C, and the cooling uniformity of the upper and lower surfaces and the length and width directions of the steel plate is controlled at the same time; Step S7: according to the size and shape of the steel plate after cooling in step S6, a multi-roll straightening machine is used to perform dynamic pressing straightening until the straightness of the steel plate is ≤ 6 mm / 2 m; In the microstructure of the finished steel plate, 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 martensite-austenite components is 2-5%.

3. The method for manufacturing a thick-walled pipeline steel plate having excellent HIC resistance according to claim 2, characterized in that: In step S1, when the molten iron is subjected to pre-desulfurization treatment, the magnesium desulfurizer is fully mixed with the molten iron to react, so that the sulfur in the molten iron is ≤ 0.002%, and the temperature of the molten iron after the mixed reaction is ≥ 1300°C; Pour the pre-desulfurized molten iron into the converter, add pre-configured Fe-Mn, Fe-Ni, Fe-Cr, Fe-Cu ferroalloy materials, and use top-bottom oxygen lances for composite oxygen blowing refining, with a treatment time of 25-30 minutes; The time of LF refining treatment and RH vacuum refining treatment was set to 20-30 min; The magnesium alloy steel blocks added in the early stage of LF refining treatment vaporize during the sinking of the molten steel and react with oxygen in the molten steel to form MgO inclusions, which are removed during the argon blowing and stirring process. The length of the Ca-Si composite cored wire fed during calcium treatment is 150-300m.

4. The method for manufacturing a thick-walled pipeline steel plate having excellent HIC resistance according to claim 2, characterized in that: In step S2, the superheat degree of the molten steel temperature in the continuous casting is set to 15-25°C, and the non-metallic inclusions of type A sulfide, type B alumina, type C silicate, and type D spherical oxide in the continuous casting slab meet the inclusion grade requirements of: A≤0.5, B≤0.5, C≤0.5, D≤0.5; At the end of continuous casting, the multi-roller dynamic soft pressing is used to reduce the thickness of the continuous casting slab by 20-50 mm, so that the center segregation of the slab reaches the Mannesmann rating level 2 standard.

5. The method for manufacturing a thick-walled pipeline steel plate having excellent HIC resistance according to claim 2, characterized in that: In step S3, the austenite grain size is controlled to be 30-100 μm.

6. The method for manufacturing a thick-walled pipeline steel plate having excellent HIC resistance according to claim 2, characterized in that: In step S5, during the second stage of finishing rolling, the cumulative reduction is set to 67%-80%.

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