A 550mpa grade offshore steel with excellent hydrogen-induced cracking resistance and a manufacturing method thereof

Abstract

The application discloses a 550MPa-grade offshore steel with excellent hydrogen-induced cracking resistance and a manufacturing method thereof, and belongs to the technical field of steel material preparation.The offshore steel has the following chemical components and mass percentages: C: 0.06%-0.10%; Si: 0.10%-0.15%; Mn: 0.60%-1.00%; P: ≤0.012%; S: ≤0.002%; Cu: 0.15%-0.40%; Ni: 0.50%-0.90%; Cr: 0.10%-0.20%; Nb: 0.015-0.03%; V: 0.04-0.07%; Mo: 0.25%-0.45%; Ti: 0.007%-0.02%; B: 0.0007%-0.0015%; Als: 0.01%-0.025%; O: ≤0.001%; N: ≤0.005%, and the balance of Fe and inevitable impurities.The produced steel plate has a yield strength of ≥550MPa, a tensile strength of 640-820MPa, an elongation of ≥18%, a Z-direction reduction of area of ≥60%, a Charpy impact energy of ≥100J at the core of the steel plate and 1 / 4 thereof at-40 DEG C, and hydrogen-induced cracking resistance performance of CSR ≤2%, CLR ≤15% and CTR ≤5%, and solves the problem that the strength, plasticity, low-temperature toughness (-40 DEG C) and hydrogen-induced cracking resistance performance of the 550MPa-grade steel plate are difficult to match and harmonize.

Description

Technical Field

[0001] This invention belongs to the field of steel material preparation technology, and specifically relates to a 550MPa grade marine steel with excellent resistance to hydrogen-induced cracking and its manufacturing method. Background Technology

[0002] Steel, as a key structural material for marine engineering equipment, is widely used in offshore wind power, production platforms, and subsea pipelines. Marine engineering equipment typically has a service life of 30 years, 50% longer than traditional ships. The operating environment of marine engineering equipment is extremely harsh, subject not only to its own weight but also to the effects of service conditions. This necessitates that marine engineering steel designs and material selections consider specific sea conditions. How to simultaneously achieve ultra-high strength, high toughness, and high resistance to hydrogen-induced cracking has become a core issue and technical challenge in the development of deep-sea engineering steel. To ensure the safe operation of marine engineering platforms in complex environments, there is an urgent need to develop ultra-high strength marine engineering steel with excellent resistance to hydrogen-induced cracking.

[0003] The patent with publication number CN104946993B, entitled "A Quenched and Tempered Thick Plate Resistant to HIC and SSC and Its Preparation Method", proposes a quenched and tempered thick plate that can resist the occurrence of HIC cracks. However, the low C composition design results in the steel plate yield strength not reaching 550MPa level. The ultra-low Ni, Cr, Cu and other elements make it impossible for this composition process to produce steel plates with a thickness greater than 25mm. The invention patent CN105803327B, entitled "An Economical HIC-Resistant X90 Pipeline Steel Plate and Its Manufacturing Method", proposes an X90-grade pipeline steel plate with a maximum thickness of 20mm and an HIC-resistant steel manufacturing method. However, its thickness is relatively small and cannot meet the large thickness requirements of ultra-high strength marine engineering steel used in marine engineering platform equipment. The invention patent CN106521332A, entitled "A Steel Plate for Stress-Directed Hydrogen-Induced Cracking Resistance and Its Production Method", proposes a steel plate for stress-directed hydrogen-induced cracking resistance. It has the characteristics of high purity, excellent impact toughness at -20℃, excellent resistance to lamellar tearing, and excellent resistance to stress-directed hydrogen-induced cracking. However, it cannot guarantee low-temperature toughness at -40℃.

[0004] The patent comparison above shows that the marine steels currently available with excellent resistance to hydrogen-induced cracking still have the following shortcomings: 1. The product thickness is relatively small, the strength level is relatively low, and the application range is narrow; 2. The product has a low ultra-low temperature toughness level, which cannot meet the requirements of marine steel to take into account both ultra-high strength and high toughness. Summary of the Invention

[0005] Therefore, the purpose of this invention is to provide a 550MPa grade marine engineering steel with excellent resistance to hydrogen-induced cracking and its manufacturing method. This invention optimizes the alloy content design and adopts a coupled design of smelting-controlled rolling-controlled cooling-quenching and tempering heat treatment processes to finally obtain a 550MPa grade marine engineering steel with excellent resistance to hydrogen-induced cracking with a thickness of up to 80mm. The steel plate of this invention has ultra-high strength (yield strength ≥550MPa, tensile strength 640~820MPa, elongation after fracture ≥18%), excellent low-temperature toughness (core impact energy ≥100J at -40℃), excellent Z-direction properties (Z-direction reduction of area ≥60%), excellent resistance to hydrogen-induced cracking (CSR≤2%, CLR≤15%, CTR≤5%), and uniform microstructure and properties.

[0006] The objective of this invention is achieved through the following means:

[0007] This invention provides a 550MPa grade marine engineering steel with excellent resistance to hydrogen-induced cracking. The chemical composition and mass percentage of the marine engineering steel are as follows: C: 0.06%~0.10%; Si: 0.10%~0.15%; Mn: 0.60%~1.00%; P: ≤0.012%; S≤0.002%; Cu: 0.15%~0.40%; Ni: 0.50%~0.90%. Cr: 0.10%–0.20%; Nb: 0.015%–0.03%; V: 0.04%–0.07%; Mo: 0.25%–0.45%; Ti: 0.007%–0.02%; B: 0.0007%–0.0015%; Als: 0.01%–0.025%; O ≤ 0.001%; ​​N ≤ 0.005%, with the balance being Fe and unavoidable impurities.

[0008] The mechanisms of action of each alloy component in the marine engineering steel of this invention are as follows:

[0009] Carbon (C) is an essential element for ensuring strength and hardenability. It significantly improves steel strength through solid solution strengthening and precipitation strengthening. As C content increases, steel strength increases, but weldability and low-temperature toughness decrease. Excessive C content leads to carbide segregation, which easily causes the formation of a carbon-rich microstructure in the steel plate's core. This carbon-rich microstructure is typically bainite / coarse MA islands. Stress concentration at the boundary between ferrite and the carbon-rich phases becomes the primary source of hydrogen-induced cracking. Therefore, it is preferable to control the C content between 0.06% and 0.10%.

[0010] Si: As a solid solution strengthening element, although Si is beneficial to improving the strength of steel plates and their oxidation resistance at high temperatures, Si promotes packet size coarsening, which seriously impairs the low-temperature toughness, elongation and weldability of ultra-high strength steel plates. Considering the economics and operability of steelmaking, the preferred Si content is 0.10% to 0.15%.

[0011] Mn: As the most important alloying element, in addition to improving the strength of steel plates, it also has the functions of expanding the austenite phase region, lowering the Ar3 point temperature, refining ferrite grains, and improving the low-temperature toughness of steel plates. However, when the quality of Mn is too high, Mn segregation and strip-shaped MnS are easily formed, resulting in poor low-temperature toughness and resistance to hydrogen-induced cracking in the core of thick plates, and a decrease in the performance of the weld heat-affected zone. Therefore, the preferred Mn content range is 0.60% to 1.00%.

[0012] P: is an element that has an adverse effect on the impact value. It can impair low-temperature toughness by segregating in the center of the slab and agglomerating at grain boundaries. The material content of this invention is controlled to be no higher than 0.012%.

[0013] S: is an element that has an adverse effect on the impact value and can form sulfide inclusions, which can become crack initiation sites. The material in this invention is controlled to be no higher than 0.002%.

[0014] Cu: ε-Cu precipitation strengthening can significantly improve the strength of steel. Appropriate amounts of Cu can increase strength without compromising impact toughness. Cu is also an austenite-forming element, which expands the austenite phase region and promotes the formation and stability of austenite during tempering. Cu precipitates can increase the density of effective hydrogen traps, making hydrogen-induced cracks smaller. When used with Ni, it can not only significantly reduce Ar3 but also avoid hot brittleness. However, excessive Cu content can cause hot brittleness, which is detrimental to the toughness of the base material and the heat-affected zone. Therefore, the Cu content in this invention ranges from 0.15% to 0.40%.

[0015] Ni: Nickel dissolves in austenite, inhibiting austenite recrystallization, refining austenite grains, and improving the low-temperature toughness of steel plates; Ni can reduce the diffusion rate of various elements in steel and delay the decomposition transformation of austenite, thereby improving the hardenability of steel; the simultaneous addition of Ni and Mo can improve the toughness of steel while increasing its strength. However, excessive Ni content will increase Ceq and Pcm, affecting welding performance. Therefore, the Ni content in this invention is controlled at 0.50% to 0.90%.

[0016] Cr: It can improve the hardenability and strength of steel plates. The presence of Cr can enhance the tempering stability of steel and ensure that the steel obtains tempered bainitic structure during high-temperature tempering. Cr has a similar solid solution strengthening effect to Mn and is not prone to segregation. However, if the Cr content is too high, it will increase the tendency for temper brittleness and increase the difficulty of welding, while if the content is too low, it cannot effectively exert its strengthening effect. In this invention, the Cr content is controlled at 0.10% to 0.20%.

[0017] Niobium (Nb) is added to promote grain refinement in the rolled steel microstructure, simultaneously improving strength and toughness. During controlled rolling, niobium effectively refines the microstructure by inhibiting austenite recrystallization and strengthens the matrix through precipitation. The presence of Nb precipitates also increases the density of effective hydrogen traps, resulting in smaller hydrogen-induced cracks. The combined addition of Nb and boron effectively inhibits austenite recrystallization and grain refinement during rolling while also suppressing carbon diffusion. This effectively inhibits the precipitation of M23(C,B)6 at austenite grain boundaries, ensuring effective segregation of boron at ferrite nucleation sites, improving hardenability, and giving the steel higher strength and better low-temperature toughness. The addition of Nb and the formation of Nb(C,N) in the steel suppress the formation of coarse M23(C,B)6 microstructure. This reduces carbon diffusion, improves boron utilization efficiency, and promotes microstructure homogenization. However, when the Nb content exceeds a certain range, MA islands will form in the welded HAZ, which is detrimental to toughness. The Nb content of this invention is preferably controlled at 0.015-0.03%.

[0018] V: Adding V to steel can refine the grain structure, improve strength and toughness, and enhance the steel plate's resistance to tempering softening during tempering. The effect is not significant when the addition amount is too low; excessive V content reduces the steel's toughness and weldability. With an appropriate nitrogen content, V can fully precipitate, significantly reducing the particle size and spacing in the steel, resulting in precipitation strengthening and thus improving strength. Therefore, this invention controls the V content to be between 0.04% and 0.07%.

[0019] Mo (Mo) can shift the C-curve of steel, thereby significantly improving its hardenability. It promotes the formation of martensite or bainite with numerous dislocations within the grains over a wider cooling range, resulting in phase transformation strengthening and dislocation strengthening, significantly improving the strength and microstructure uniformity of the steel. In this invention, the combined addition of Mo and Nb not only provides the strengthening effect of adding molybdenum or niobium alone, but also allows molybdenum to segregate at the NbC matrix interface, preventing the coarsening of NbC particles. The combined addition of Mo and B increases the content of effective (solid-dissolved) B in the steel, inhibiting the diffusion of C to grain boundaries, suppressing the precipitation and growth of M23(C,B)6 at grain boundaries, promoting the segregation of B at grain boundaries, reducing the nucleation sites of ferrite, and improving the hardenability of the steel. The combined addition of Mo and Ni can improve the toughness of steel while increasing its strength; however, with the increase of Mo, the amount of Mo-containing carbides also increases. Carbides distributed at grain boundaries disrupt the continuity of the matrix and deteriorate the toughness of the steel. Therefore, the Mo content in this invention is controlled at 0.25%–0.45%.

[0020] Ti: Inhibits excessive austenite grain growth during slab heating, improving steel toughness. Ti also helps to solidify nitrogen (N), ensuring a certain amount of dissolved boron (B) in the steel and improving hardenability. However, excessive Ti content can lead to TiC precipitation as TiC on martensite / bainite laths and grain boundaries, severely degrading the low-temperature toughness of the steel. Therefore, the Ti content in this invention is controlled between 0.007% and 0.02%.

[0021] B: It can improve the hardenability and strength of steel plates, but if the B content is too high, it will form coarse BN particles that are detrimental to hardenability and toughness. At the same time, it will also affect the weldability and surface quality of steel plates. Therefore, the Mo content in this invention is controlled at 0.0007% to 0.002%.

[0022] Al (Al₂O₃): A deoxidizing and grain-refining element. At high temperatures, Al forms fine AlN precipitates, which inhibit austenite grain growth during the austenitization process of slabs / steel plates, thus refining the austenite grains and improving the toughness of the steel at low temperatures. Excessive Al content leads to the formation of larger Al oxides, reducing the low-temperature impact resistance of the steel plate. Furthermore, it makes the slab prone to edge and corner cracks during continuous casting. The preferred Al content is controlled between 0.010% and 0.025%.

[0023] O: An element that has an adverse effect on impact toughness. It combines with other elements in steel to form non-metallic inclusions, which become crack initiation sites. In this invention, the O content is controlled to be no higher than 0.001%.

[0024] N: N element will form BN with B element and form coarse ALN with AL, which precipitates along the original austenite grain boundary, affecting the hardenability and low temperature impact toughness of steel. In this invention, the N content is controlled to be no higher than 0.005%.

[0025] Based on the above technical solution, the marine steel further has a yield strength ≥550MPa, tensile strength 640-820MPa, elongation at section ≥18%, Z-direction reduction of area ≥60%, Charpy impact energy at -40℃ in the core and 1 / 4 section of the steel plate ≥100J, and resistance to hydrogen-induced cracking CSR≤2%, CLR≤15%, and CTR≤5%.

[0026] This invention also provides a method for manufacturing the above-mentioned 550MPa grade marine steel with excellent resistance to hydrogen-induced cracking, which mainly includes the following steps: smelting process → continuous casting and billet slow cooling process → rolling process → rapid cooling process → slow cooling process → quenching process → tempering process.

[0027] Based on the above technical solution, further, the molten iron is desulfurized before the smelting process, and the smelting process includes converter smelting, ladle refining and vacuum treatment.

[0028] Based on the above technical solution, further, in the continuous casting and billet slow cooling process, the superheat of the molten steel in the ladle is 20-25℃, the casting is protected throughout the process, the light reduction is 5-8mm, the thickness of the continuous casting billet is 250-360mm, the continuous casting billet is stacked and slow cooled after it leaves the line, the stacking temperature is ≥650℃, and the stacking slow cooling rate is 5-6℃ / h.

[0029] Based on the above technical solution, furthermore, in the rolling process, the continuously cast billet adopts a 4-stage heating process, namely a preheating stage, heating stage 1, heating stage 2, and soaking stage. The heating temperature of the preheating stage is 750-850℃, the heating temperature of heating stage 1 is 900-1100℃, the heating temperature of heating stage 2 is 1100-1200℃, the heating temperature of the soaking stage is 1050-1150℃, the heating time of the preheating stage is 0.1-0.2 min / mm, the heating time of heating stage 1 is 0.2-0.4 min / mm, the heating time of heating stage 2 is 0.1-0.3 min / mm, and the heating time of the soaking stage is 0.3-0.5 min / mm.

[0030] The rolling process is carried out in two stages. The first stage rolling temperature is 1000-1050℃, and the single-pass reduction rate is ≥15% except for the widening pass. The thickness of the intermediate billet is 1-3 times the thickness of the finished product. The intermediate billet is water-cooled with a cooling rate of ≥2℃ / s. The second stage rolling temperature is 780-840℃, the single-pass reduction rate is ≥10%, and the final rolling temperature is 730-820℃.

[0031] Based on the above technical solution, the billet is further subjected to high-pressure water descaling 1 to 5 times after being heated and tapped out of the furnace, which removes the iron oxide scale on the surface and reduces the temperature of the continuous casting billet.

[0032] Based on the above technical solution, the final rolled steel plate directly enters the rapid cooling process, with an average cooling rate of ≥3℃ / s and a reddening temperature of 350~400℃.

[0033] Based on the above technical solution, the rapid cooling process further adopts the DQ+ACC rapid cooling system.

[0034] Based on the above technical solution, further, in the slow cooling process, the temperature of the tank entering the tank is not lower than 300℃, and the slow cooling time is not less than 24 hours.

[0035] Based on the above technical solution, further, in the quenching process, a two-stage heating quenching heat treatment is adopted. The first stage is held at a temperature of 720-750℃, and after the steel plate thickness reaches 1 / 2 of the temperature, it is held for 5-10 minutes. The second stage is held at a temperature of 880-920℃, and after the steel plate thickness reaches 1 / 2 of the temperature, it is held for 10-30 minutes. After quenching, the plate is taken out of the furnace and quenched to room temperature.

[0036] Based on the above technical solution, further, in the tempering process, the tempering temperature is 580~640℃, the furnace time is 3~5min / mm, and after exiting the furnace, it is air-cooled to obtain the final product.

[0037] The advantages of this invention over the prior art are as follows:

[0038] (1) This invention prepares marine engineering steel with good comprehensive mechanical properties through the coupled design of alloy composition design-smelting-controlled rolling-controlled cooling-quenching and tempering heat treatment process. It solves the problem of difficulty in matching and harmonizing the strength, plasticity, low temperature toughness (-40℃) and resistance to hydrogen-induced cracking of 550MPa grade steel plates. The produced steel plates have a yield strength ≥550MPa, tensile strength 640-820MPa, elongation at section ≥18%, Z-direction reduction of area ≥60%, Charpy impact energy at -40℃ in the core and 1 / 4 section of the steel plate ≥100J, and resistance to hydrogen-induced cracking CSR≤2%, CLR≤15%, and CTR≤5%.

[0039] (2) This invention fully leverages the technical equipment advantages of the heavy plate rolling mill and heat treatment unit, and combines 250-300mm thick continuous casting slabs to develop ultra-high strength and high toughness marine engineering steel thick plates with a maximum thickness of 80mm.

[0040] (3) The present invention adopts a reasonable rolling process and offline quenching and tempering process. Compared with online quenching of ultra-high strength steel of the same strength, offline quenching can accurately control the temperature at the start of quenching, and the temperature of the whole plate is uniform, resulting in more stable performance of the produced steel plate.

[0041] (4) The microstructure of the steel plate is tempered bainite. Attached Figure Description

[0042] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0043] Figure 1 This is a schematic diagram of the two-stage heat preservation and quenching process in the embodiment;

[0044] Figure 2 Metallographic image (500x) of the marine steel prepared in Example 1. Detailed Implementation

[0045] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0046] Examples 1-14

[0047] This embodiment provides a method for preparing 550MPa grade marine engineering steel with excellent resistance to hydrogen-induced cracking. The chemical composition and weight percentage of the marine engineering steel in this embodiment are shown in Table 1:

[0048] Table 1 shows the chemical composition and weight percentage (wt%) of the marine steel in the examples.

[0049] Example C Si Mn P S Cu Ni Cr Nb V Mo Ti B Alt O N 1 0.079 0.12 0.89 0.011 0.002 0.19 0.52 0.19 0.024 0.055 0.26 0.008 0.001 0.024 0.0009 0.0031 2 0.089 0.14 0.79 0.009 0.002 0.35 0.64 0.14 0.019 0.061 0.33 0.018 0.0014 0.014 0.001 0.0027 3 0.089 0.13 0.96 0.007 0.002 0.24 0.74 0.18 0.028 0.059 0.38 0.013 0.0011 0.013 0.001 0.0035 4 0.061 0.14 0.86 0.012 0.001 0.45 0.59 0.16 0.016 0.051 0.29 0.011 0.0012 0.016 0.0007 0.0036 5 0.071 0.14 0.69 0.008 0.002 0.41 0.81 0.14 0.023 0.059 0.35 0.012 0.0013 0.021 0.0007 0.0040 6 0.096 0.11 0.65 0.011 0.001 0.36 0.71 0.19 0.019 0.048 0.29 0.016 0.0009 0.011 0.0008 0.0043 7 0.073 0.14 0.78 0.010 0.002 0.39 0.66 0.16 0.021 0.064 0.26 0.014 0.001 0.019 0.0007 0.0035 8 0.069 0.10 0.79 0.007 0.001 0.33 0.89 0.19 0.028 0.068 0.41 0.010 0.0008 0.011 0.0007 0.0027 9 0.081 0.12 0.74 0.007 0.002 0.21 0.76 0.19 0.019 0.059 0.38 0.012 0.0013 0.018 0.0008 0.0036 10 0.064 0.13 0.69 0.008 0.001 0.44 0.71 0.18 0.022 0.066 0.29 0.017 0.0012 0.011 0.0009 0.0035 11 0.065 0.11 0.98 0.01 0.001 0.33 0.77 0.13 0.024 0.046 0.39 0.012 0.0011 0.018 0.0008 0.0038 12 0.075 0.10 0.79 0.006 0.002 0.26 0.51 0.17 0.019 0.062 0.29 0.013 0.0008 0.023 0.0008 0.0041 13 0.076 0.15 0.99 0.009 0.002 0.19 0.64 0.19 0.028 0.045 0.29 0.010 0.0014 0.018 0.0009 0.0040 14 0.079 0.14 0.97 0.008 0.001 0.19 0.79 0.19 0.019 0.043 0.31 0.009 0.0012 0.019 0.0007 0.0034

[0050] Includes the following steps:

[0051] 1) Smelting, continuous casting and slow cooling of billet: The production process is carried out by deep desulfurization of molten iron, converter smelting, ladle refining, vacuum treatment and continuous casting. The superheat of molten steel in the tundish is shown in Table 2. The casting process is protected throughout, and electromagnetic stirring is turned on and combined with a light reduction process. The amount of light reduction is shown in Table 2. After continuous casting, a continuous casting billet is obtained with a thickness of 250-300 mm. After the continuous casting billet is removed from the line, it is stacked and slow cooled. The stacking temperature and stacking slow cooling rate are shown in Table 2.

[0052] Table 2 shows the continuous casting and billet slow cooling stacking process parameters for marine engineering steel in the embodiments.

[0053]

[0054] 2) Rolling process: The continuously cast billet is heated in four stages: preheating stage, heating stage 1, heating stage 2, and soaking stage. The heating temperatures and times for the preheating stage, heating stage 1, heating stage 2, and soaking stage are shown in Table 3. After the billet exits the furnace, it is descaled with high-pressure water 2-3 times to remove surface iron oxide scale and reduce the temperature of the continuously cast billet. Two-stage rolling is adopted. The starting rolling temperature and minimum single-pass reduction rate (excluding the widening pass) of the first stage are shown in Table 4 to improve the as-cast microstructure of the slab, reduce the billet thickness before heating, and shorten the heating time of the steel plate. The thickness of the intermediate billet is twice the thickness of the finished product. The intermediate billet is water-cooled, and the cooling rate is shown in Table 4. The starting rolling temperature, minimum single-pass reduction rate, and final rolling temperature of the second stage are shown in Table 4. The two-stage rolling process controls the uniform and refined phase transformation microstructure, preparing the initial microstructure for tempering heat treatment.

[0055] 3) Cooling process: The steel plate is directly cooled after final rolling using the DQ+ACC rapid cooling system. The average cooling rate and reddening temperature are shown in Table 4.

[0056] Table 3 shows the slab heating process parameters for marine engineering steel in the examples.

[0057]

[0058] Table 4 shows the rolling and cooling process parameters for marine engineering steel plates in the embodiments.

[0059]

[0060] 4) Slow cooling process: After cooling is completed, immediately place the product into a slow cooling tank. The temperature of the product entering the tank should not be lower than 300℃, and the slow cooling time should not be less than 24 hours.

[0061] 5) Quenching process: The steel plate is subjected to two-stage heating and quenching heat treatment. The steel plate thickness, the first stage holding temperature and the holding time after the steel plate thickness reaches 1 / 2 of the temperature are shown in Table 5. Then, it enters the second stage. The second stage holding temperature and the holding time after the steel plate thickness reaches 1 / 2 of the temperature are shown in Table 5. After quenching, the plate is taken out of the furnace and quenched to room temperature.

[0062] 6) Tempering process: The steel plate is tempered. The tempering temperature and furnace time are shown in Table 5. After being taken out of the furnace, it is air-cooled to obtain the finished steel plate. The mechanical properties and hydrogen-induced cracking resistance of the finished steel plate are shown in Tables 6-7.

[0063] Table 5 shows the heat treatment process parameters for marine steel in the embodiments.

[0064]

[0065]

[0066] Table 6 shows the mechanical properties of marine steel in the examples.

[0067]

[0068] Table 7 shows the hydrogen-induced cracking resistance of marine steel in the examples.

[0069]

[0070] Note: The solution used was NACEA, which, according to relevant standards, consisted of 5% NaCl + 0.5% CH3COOH + saturated H2S aqueous solution and was continuously soaked for 96 hours.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A 550MPa grade marine steel with excellent resistance to hydrogen-induced cracking, characterized in that, The chemical composition and mass percentage of the marine engineering steel are as follows: C: 0.06%–0.10%; Si: 0.10%–0.14%; Mn: 0.60%–0.89%; P: ≤0.012%; S≤0.002%; Cu: 0.15%–0.40%; Ni: 0.50%–0.90%; Cr: 0.10%–0.20%; Nb: 0.015%–0.03%; V：0.04～0.07％； Mo: 0.25%–0.45%; Ti: 0.007%–0.02%; B: 0.0007%–0.0015%; Als: 0.01%–0.025%; O ≤ 0.001%; ​​N ≤ 0.005%, with the balance being Fe and unavoidable impurities; The marine steel described above has a yield strength ≥550MPa, tensile strength 640-820MPa, elongation at section ≥18%, Z-direction reduction of area ≥60%, Charpy impact energy at -40℃ in the core and 1 / 4 section of the steel plate ≥100J, and resistance to hydrogen-induced cracking CSR≤2%, CLR≤15%, and CTR≤5%. The manufacturing method of the 550MPa grade marine steel with excellent resistance to hydrogen-induced cracking mainly includes the following steps: smelting process → continuous casting and billet slow cooling process → rolling process → rapid cooling process → slow cooling process → quenching process → tempering process. In the rolling process, the continuously cast billet is heated in four stages: preheating stage, heating stage 1, heating stage 2, and soaking stage. The heating temperature of the preheating stage is 750-850℃, the heating temperature of heating stage 1 is 900-1100℃, the heating temperature of heating stage 2 is 1100-1200℃, and the heating temperature of the soaking stage is 1050-1150℃. The heating time of the preheating stage is 0.1-0.2 min / mm, the heating time of heating stage 1 is 0.2-0.4 min / mm, the heating time of heating stage 2 is 0.1-0.3 min / mm, and the heating time of the soaking stage is 0.3-0.5 min / mm. Two-stage rolling is adopted. The first stage rolling temperature is 1000-1050℃, and the single-pass reduction rate is ≥15% except for the widening pass. The thickness of the intermediate billet is 1-3 times the thickness of the finished product. The intermediate billet is water-cooled with a cooling rate ≥2℃ / s. The second stage rolling temperature is 780-840℃, the single-pass reduction rate is ≥10%, and the final rolling temperature is 730-820℃. The molten iron undergoes deep desulfurization before the smelting process, which includes converter smelting, ladle refining, and vacuum treatment. In the continuous casting and billet slow cooling processes, the superheat of the molten steel in the tundish is 20-25°C, the casting is carried out under full protection, the light reduction is 5-8mm, the thickness of the continuous casting billet is 250-360mm, and the billet is stacked and slow cooled after leaving the line, with a stacking temperature ≥650°C and a stacking slow cooling rate of 5-6°C / h. The steel plate after final rolling directly enters the rapid cooling process, with an average cooling rate of ≥3℃ / s and a reddening temperature of 350~400℃. In the slow cooling process, the temperature of the tank entering the tank shall not be lower than 300℃, and the slow cooling time shall not be less than 24 hours; In the quenching process, a two-stage heating and quenching heat treatment is adopted. The first stage is held at a temperature of 720-750℃, and after the steel plate thickness reaches 1 / 2 of the temperature, it is held for 5-10 minutes. The second stage is held at a temperature of 880-920℃, and after the steel plate thickness reaches 1 / 2 of the temperature, it is held for 10-30 minutes. After quenching, the plate is taken out of the furnace and then quenched to room temperature. In the tempering process, the tempering temperature is 580-640℃, the furnace time is 3-5 min / mm, and after taking it out of the furnace, it is air-cooled to obtain the final product.

2. The 550MPa grade marine steel with excellent resistance to hydrogen-induced cracking as described in claim 1, characterized in that, After being heated and removed from the furnace, the billet is descaled with high-pressure water for 1 to 5 times.

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