Large heat input welding resistant steel for deep sea jacket and preparation method thereof

Abstract

The invention relates to high heat input welding resistant steel for a deep sea jacket and a preparation method thereof, and belongs to the technical field of iron-based alloys. The steel plate comprises, by mass, 0.08%-0.10% of C, 0.15%-0.25% of Si, 1.55%-1.70% of Mn, smaller than or equal to 0.0070% of P, smaller than or equal to 0.0030% of S, 0.008%-0.020% of Ti, 0.0020%-0.0040% of N, 0.20%-0.30% of Cu, 0.45%-0.60% of Ni, 0.010%-0.025% of V, 0.0010%-0.0020% of B and the balance Fe and inevitable impurity elements, the yield ratio of the steel plate is 0.84%-0.88, the yield strength is 430 MPa to 480 MPa, the tensile strength is 520 MPa to 580 MPa, and the ballistic work at the temperature of 40 DEG C below zero and 50 DEG C below zero is larger than or equal to 200 J. On the basis that the steel grade component range of the European standard En10025-4 S420ML is met, the carbon content is further reduced, Mn, Ni, Cu-, V and B elements beneficial to the toughness of the steel plate are added, in the TMCP rolling process, the pass reduction is increased, the total pass number is controlled, and the yield of the steel plate is increased. And the steel plate for high heat input welding, which has a mixed structure mainly consisting of acicular ferrite and a small amount of blocky ferrite, is obtained.

Description

Technical Field

[0001] The invention belongs to the field of iron and steel metallurgy, and specifically relates to high-energy-line welding-resistant S420ML (implementation standard: En10025-4) deep-sea jacket steel and a preparation method thereof. Background Art

[0002] The steel plates used for deep-sea jackets must be able to withstand strong natural factors such as earthquakes, typhoons, waves, tides, currents and ice floes in addition to considering the corrosion of seawater conditions and fouling of marine organisms due to the complex changes in the marine environment. The structural materials must not only have high strength and good plasticity, but also have high impact toughness at extremely low temperatures (-40℃, -50℃). Usually, the jacket steel commonly used to support offshore platforms is 355MPa grade steel, that is, high strength with yield strength ≥355MPa and tensile strength ≥490MPa, and excellent low-temperature impact toughness at -40℃. With the expansion of deep-sea resources and the increase in the weight of offshore platforms, the strength of the jacket steel supporting offshore platforms needs to be further improved. Now S420ML and S420NL are also continuously incorporated into the jacket design and successfully applied in deep-sea operations. The service inspection temperature is also further reduced to -50℃.

[0003] In addition, the offshore engineering market is in a severe situation. In order to reduce costs, offshore engineering enterprises continue to upgrade their technology and improve welding efficiency, shorten construction period and improve economic benefits by improving high-energy-line welding technology. In order to meet the common requirements of high-energy-line steel for the offshore engineering industry and other infrastructure industries such as boilers, pressure vessels, bridges, and buildings, steel companies also develop high-energy-line welding steel accordingly. Usually, high-energy-line welding steel is obtained through two technical approaches. One is to use oxide metallurgy, such as Chinese patent publications CN104411849A, CN105102650A, CN101918607A, etc., which can increase the welding line energy of steel plates to 400-600KJ / cm. However, oxide metallurgy technology is difficult to smelt because of the fluctuations in alloy addition, molten steel temperature, and oxygen potential during the smelting process, and the narrow process window makes it difficult to control. If there is a slight deviation, large inclusions are easily formed, affecting the metallurgical quality of the steel billet. Therefore, using this method, under the current equipment and personnel operation level of Chinese enterprises, is highly risky and difficult to be suitable for the stable production of large-volume steel plates that can be welded with high heat input. Another method is to improve the steel plate's resistance to high heat input by adding Ti, such as the technical solution of Chinese patent publication No. CN103114241A, which obtains a large number of dispersed TiN particles through the composition design of high N and low Ti / N ratio, and the composite addition of Ni and Cu, thereby increasing the welding heat input of the steel plate to 600KJ / cm. The prior art does not involve S420ML steel that meets the requirements of high heat input welding.

[0004] The existing technology still cannot fully meet the comprehensive requirements of steel for deep - sea jacket, which need to have good impact toughness at extremely low temperatures, extremely low temperature strain aging performance, ultra - high strength and good weldability with large heat input. Therefore, there is still a technical gap in the S420ML - grade steel for deep - sea jacket with good weldability with large heat input. Summary of the Invention

[0005] The object of the present invention is to provide a steel plate for deep - sea jacket of S420ML with good weldability with large heat input and its preparation method. The steel plate meets the mechanical property requirements of En10025 - 4 S420ML. After strain aging, it has a high impact energy at - 50 °C and excellent welding performance when welding with a maximum heat input of 300 KJ / cm.

[0006] The technical solution adopted by the present invention to solve the above problems is as follows: A steel plate for deep - sea jacket of S420ML with good weldability with large heat input and its preparation method. The elemental composition meets the standard requirements of En10025 - 4 S420ML grade. By mass percentage: C: 0.08 - 0.10%, Si: 0.15 - 0.25%, Mn: 1.55 - 1.70%, P: ≤0.0070%, S: ≤0.0030%, Ti: 0.008 - 0.020%, N: 0.0020 - 0.0040%, Cu: 0.20 - 0.30%, Ni: 0.45 - 0.60%, V: 0.010 - 0.025%, B: 0.0010 - 0.0020%, and the balance is Fe and inevitable impurity elements.

[0007] The reasons for the limitation of the composition of the S420ML steel of the present invention are described as follows: C: Reducing the carbon content in the steel plate can improve the extremely low - temperature impact performance and low - temperature strain aging performance of the steel, and improve the sensitivity of the steel plate to welding cold cracks. The lower the carbon content, the lower the carbon equivalent, which can effectively reduce the formation of M - A components in the heat - affected zone during welding with a high heat input (≥100 KJ / cm), and achieve the purpose of improving the extremely low - temperature impact toughness of the heat - affected zone (HAZ) of the weld. However, if the carbon content is too low, the hardenability of the steel will be weakened. Considering that the present invention is a high - strength steel for deep - sea jacket. Therefore, the carbon content is controlled at 0.08 - 0.10%.

[0008] Si: It is mainly used for deoxidation. In the present invention, in order to prevent the weakening of the metallurgical effect of Ti oxides caused by Al deoxidation, Al is not added, and a sufficient amount of Si is required for deoxidation. At the same time, the content of Si should not be too high. If it is too high, the free O in the molten steel will be excessive, forming large inclusions, which will affect the toughness and plasticity of the steel. Therefore, the silicon content is controlled at 0.15 - 0.25%.

[0009] Mn: It has the effect of delaying the transformation of austenite to ferrite in the steel, which is beneficial to refining ferrite and improving strength and toughness. When the content of manganese is low, the above effects are not significant, and the strength and toughness of the steel plate are relatively low, etc. If it is too high, it will cause segregation of continuous casting billets, poor toughness and reduced weldability, etc. Therefore, considering the comprehensive addition of alloys in the present invention, the addition amount of manganese content is specified to be in the range of 1.55 - 1.70%.

[0010] P: Although it can improve corrosion resistance, it will reduce low-temperature toughness and hinder weldability, which is not suitable for structural steel. In the present invention, it is specified to be controlled below 0.0070%.

[0011] S: It forms MnS inclusions, which will also cause center segregation and have an adverse effect on corrosion resistance. In the present invention, it is specified to be controlled below 0.0030%.

[0012] Ti: By forming Ti2O3 particles, it can promote the formation of intragranular ferrite. At the same time, it is also used to fix nitrogen in the steel. Under appropriate conditions, titanium and nitrogen form titanium nitride, which prevents the coarsening of austenite grains during the heating, rolling, especially welding processes at a high temperature of 1350°C, improves the extremely low-temperature toughness of the base metal and the heat-affected zone of welding, and improves welding performance. When titanium is less than 0.008%, the effect is poor. When it exceeds 0.020%, the excess titanium will precipitate in combination with other elements, deteriorating the toughness of the steel.

[0013] N: It is an important element in the present invention. Different from the previous control of harmful elements, it is necessary to ensure a certain nitrogen content in the steel plate. When the ratio of Ti and N atoms in the steel is 1:1, at this time, it is equivalent to the weight ratio of Ti and N being 3.42, and the TiN particles are the smallest and most diffusely distributed, with the strongest refining effect on high-temperature austenite grains. Not only can excellent toughness be obtained, but according to the addition amount of Ti, the oxygen content in the steel, and other nitrogen-fixing elements, the N content in the present invention is controlled to be 0.0020 - 0.0040%.

[0014] Cu: It is an element that improves the hardenability of the steel. In addition, it can effectively reduce the δ phase region, which is beneficial to steel smelting and reduces the thermal shrinkage during continuous casting. During the welding process, it also reduces the solid solution of TiN in the δ phase region, increases the high-temperature pinning effect of TiN, and reduces the coarsening of austenite grains. The combined addition of Cu + Ni is beneficial to the improvement of impact toughness. When the content of Cu is less than 0.10%, the effect is not obvious. However, if the content is too high, it is easy to cause Cu segregation, resulting in poor surface quality of the steel plate. Considering economy at the same time, the Cu content in the present invention is controlled to be 0.20 - 0.30%.

[0015] Ni: Like Cu, it is an element that increases the hardenability of steel and reduces the δ-phase region. It is also the most commonly used element to effectively improve the low-temperature toughness of steel. In addition, when combined with Cu and P in steel, it will help improve the corrosion resistance of steel. However, if the addition amount is too high, it will significantly increase the cost of steel. Therefore, in this invention, the nickel content is specified to be between 0.45% and 0.60%.

[0016] V: A strong carbide and nitride forming element. It forms second-phase particles such as VC and V(CN) in steel, which can refine the grains, improve the strength and low-temperature toughness of steel. However, if the content of V is too high, it will reduce the weldability of steel. Therefore, its content is controlled between 0.010% and 0.025%.

[0017] B: It can form compounds similar to carbides in steel, and these compounds are more stable and finer than carbides. They can increase the intergranular strength and intragranular strength of steel, and at the same time improve the plastic deformation performance of steel, thus greatly improving the strength and toughness of steel. The addition of boron element in steel can cause the refinement of carbides, thereby reducing the precipitates generated due to supersaturation, damping the movement of dislocations, making the deformation behavior of steel more uniform, and further reducing the brittleness of steel. However, if the B content in the steel for large heat input welding is too high, it is easy to cause segregation. Therefore, in this invention, the boron content is specified to be in the range of 0.0010% to 0.0020%.

[0018] The preparation method of the S420ML deep-sea jacket steel that can be welded with large heat input includes the following processes: Smelting and continuous casting process: Using hot metal pretreatment to desulfurize the hot metal so that the sulfur content is lower than 0.0020%, converter smelting, secondary refining, vacuum degassing, slab continuous casting and other processes. Among them, the C content at the end of converter smelting is ≤0.02%. During the tapping process of the converter, bottom blowing argon and nitrogen are carried out for 15 - 20 minutes. When performing secondary refining, Si alloy is added for deoxidation. The alloy addition sequence is Si, Mn, Ni, V, Cu, Ti, Ca, B. Finally, feeding Ti, Ca and B wires is to improve the recovery rate of Ti, Ca and B, and prevent Ti from forming precipitates at too high temperatures, which affects the large heat input welding effect; the molten steel is cast into slabs by continuous casting process.

[0019] Rolling process: The compression ratio of the continuous casting slab to the finished product thickness is ≥3.5. The thermo-mechanical control process (TMCP) is adopted. The reheating temperature of the continuous casting slab is 1180℃ - 1200℃, and the heating time is 1.0 - 1.5 mm / min. Two-stage controlled rolling of rough rolling and finish rolling is adopted. The reduction ratio per pass of rough rolling is 12 - 20%. The starting temperature of rough rolling is 1100 - 1150℃, and the thickness of the intermediate slab obtained after rough rolling is ≥2.0 times the thickness of the finished product; the starting rolling temperature of finish rolling is 800 - 830℃. After rolling, accelerated cooling is adopted. The final cooling temperature of the steel plate is 550 - 590℃, and the cooling rate is 5 - 8℃ / s. Then it is air-cooled.

[0020] The mechanical properties of the steel plate of the present invention meet the performance requirements of the European standard En10025-4 S420ML steel grade, and it has excellent extremely low-temperature impact toughness, resistance to low-temperature strain aging, and the ability to withstand large heat input welding of 300 KJ / cm. Specific properties: Yield strength (transverse tension): 430 - 480 MPa, Tensile strength: 520 - 580 MPa, Yield ratio is 0.84 - 0.88. The yield ratio within this range ensures fine grains, enabling the steel plate to have low-temperature toughness and the ability to resist brittle fracture. The elongation of the steel plate is ≥25%. The longitudinal impact toughness values at -40°C and -50°C are ≥200 J; after strain aging, the impact toughness values at -40°C and -50°C do not decrease significantly, showing excellent resistance to low-temperature aging. After welding with a heat input of 300 KJ / cm, the properties of the weld position and the heat-affected zone of the weld do not deteriorate, and the impact performance at -50°C still meets the requirements of the En10025-4 S420ML steel grade. The strength of the welded joint is 530 - 580 MPa. When a tensile test is carried out on the welded specimen, the base metal fractures while the welded joint does not fracture. The steel plate of the present invention meets the comprehensive performance requirements of high strength, low-temperature toughness, strain aging resistance, and large heat input welding resistance for S420ML steel used in deep-sea jacket platforms. It has a stable production process and is easy for mass production.

[0021] The present invention has the following characteristics: 1. On the basis of meeting the composition range of the European standard En10025-4 S420ML steel grade, the present invention further reduces the carbon content and adds elements beneficial to the strength and toughness of the steel plate, such as Mn, Ni, Cu, V, and B. During the TMCP rolling process, the reduction per pass is increased and the total number of passes is controlled to obtain a steel plate for large heat input welding with a mixed microstructure mainly composed of acicular ferrite + a small amount of blocky ferrite.

[0022] 2. By using the traditional Ti addition technology in combination with low C, the addition of active elements such as Mg, Ca, and even rare earths involved in oxide metallurgy, which are difficult to control, is avoided, the smelting difficulty is reduced, the product stability is improved, and it is easy for mass smelting and continuous casting control.

[0023] 3. By adding microalloying Ti, Ti2O3 particles and a small amount of TiN particles are formed to promote the formation of intragranular ferrite and prevent the coarsening of austenite grains, thereby improving the extremely low-temperature toughness of the base metal and the heat-affected zone of the weld. At the same time, alloying elements such as Cu and Ni are added as auxiliary to reduce the δ phase region and avoid the dissolution of TiN in the δ phase region. During the smelting process, Si, Mn, V, Cu, Ti, Ca, and B are added in sequence to give full play to the role of elements beneficial to large heat input welding.

[0024] 4. By means of a reasonable addition sequence of alloys and microalloys, the present invention increases the recovery rates of microalloying elements such as Ti and B. In combination with the control of N content, the precipitation probability of second-phase particles is increased, which hinders the rapid growth of austenite at high temperatures. At the same time, it effectively prevents the premature precipitation of Ti at high temperatures during smelting and the precipitation of large inclusions.

[0025] 5. By using composition design and TMCP supporting processes, the present invention obtains a mixed structure composed of acicular ferrite and a small amount of blocky ferrite, with a large process window. The present invention can be extended and applied to the preparation methods of steel materials for other offshore facilities in cold regions, such as. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is the typical structure of the steel plate in Example 1 of the present invention, which is a mixed structure composed of acicular ferrite and a small amount of blocky ferrite.

[0027] Figure 2 It is a macrostructure schematic diagram of the welded joint of the steel plate in Example 1 of the present invention welded with a large heat input of 300 KJ / cm. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] The present invention will be further described in detail below in conjunction with the embodiments. The described embodiments are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention.

[0029] The chemical compositions of the steel plates in each embodiment are shown in Table 1. The data in the table are the mass percentage contents of each element, and the rest are Fe and inevitable impurity elements.

[0030] Table 1

[0031] Production process of the steel plate: Smelting and continuous casting process: Hot metal pretreatment is adopted to desulfurize the hot metal so that the sulfur content is lower than 0.0020%. Then, converter smelting, secondary refining and vacuum degassing refining, slab continuous casting and other processes are carried out. The C content at the end of converter smelting is ≤0.02%. During the tapping process of the converter, bottom blowing of argon and nitrogen is carried out for 15 - 20 minutes. During refining, no Al is added and an appropriate amount of Si is added. The addition sequence of alloys is Si, Mn, Ni, V, Cu, Ti, Ca, B. Finally, feeding Ti, Ca and B wires is to improve the recovery rates of Ti, Ca and B and prevent the precipitation of Ti at too high temperatures, which affects the welding effect of large heat input.

[0032] Rolling process: The compression ratio of the continuous casting billet to the finished product thickness is ≥3.5. The thermo-mechanical control process (TMCP) is adopted. The reheating temperature of the continuous casting billet is 1180°C - 1200°C, and the heating time is 1.0 - 1.5 mm / min. Two-stage controlled rolling of rough rolling and finish rolling is adopted. The reduction ratio per pass in rough rolling is 12 - 20%, the starting temperature of rough rolling is 1050 - 1100°C, and the thickness of the intermediate billet obtained after rough rolling is ≥2.0 times the thickness of the finished product. The starting temperature of finish rolling is 800 - 830°C. Accelerated cooling is adopted after rolling. The final cooling temperature of the steel plate is 550 - 590°C, and the cooling rate is 5 - 8°C / s. Then it is air-cooled. The rolling process is shown in Table 2, and the cooling process is shown in Table 3.

[0033] Table 2

[0034] Table 3

[0035] The mechanical properties of the steel plates in each example are shown in Tables 4 and 5. From the tensile properties of the examples, the transverse tensile yield strength of the steel plate is 430 - 480 MPa, the tensile strength is 520 - 580 MPa, the yield ratio is 0.84 - 0.88, and the elongation is ≥25%. The low-temperature longitudinal impact energy at -40°C and -50°C is ≥200 J. After strain aging at -40°C and -50°C (after 5% strain and holding at 250°C for 1 hour), the low-temperature longitudinal impact toughness value is still ≥200 J.

[0036] Table 4

[0037] Table 5

[0038] The steel plate of Example 1 is subjected to a weldability test under the condition of a linear energy of 300 ± 15 KJ / cm. The test steel plate is not preheated before welding and not heat-treated after welding. The welding groove is V-shaped. The impact energy at -50°C in the heat-affected zone of the welded joint is not less than 80 J. The tensile impact properties of the welded joint are shown in Table 6.

[0039] Table 6

[0040] In addition to the above examples, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution shall fall within the protection scope of the claims of the present invention.

Claims

1. A steel plate, characterized in that: The element composition by mass percentage is C: 0.08~0.10%, Si: 0.15~0.25%, Mn: 1.55~1.70%, P: ≤0.0070%, S: ≤0.0030%, Ti: 0.008~0.020%, N: 0.0020~0.0040%, Cu: 0.20~0.30%, Ni: 0.45~0.60%, V: 0.010~0.025%, B: 0.0010~0.0020%, and the balance is Fe and unavoidable impurity elements.

2. The steel plate according to claim 1, characterized in that: The metallographic structure of the steel plate is acicular ferrite and a small amount of blocky ferrite. The yield strength ratio of the steel plate is 0.84-0.88, the yield strength is 430-480 MPa, the tensile strength is 520-580 MPa, and the impact energy at -40℃ and -50℃ is ≥200 J.

3. A method for manufacturing the steel plate according to claim 1, characterized in that: The process includes molten steel smelting and rolling. Molten steel is smelted according to elemental composition, and the molten steel is cast into slabs. The TMCP rolling process is adopted, and the rolling reduction ratio is ≥3.

5. The slab reheating temperature is 1180℃~1200℃, the heating time is 1.0~1.5mm / min, and the rolling is controlled in two stages of rough rolling and finishing rolling. The single-pass reduction rate of rough rolling is 12~20%, the starting temperature of rough rolling is 1100~1150℃, and the thickness of the intermediate slab obtained after rough rolling is greater than 2.0 times the final thickness. The starting temperature of finishing rolling is 800~830℃, and accelerated cooling is adopted after finishing rolling, with a cooling rate of 5~8℃ / s, and the final cooling temperature of the steel plate is 550~590℃, followed by air cooling.

4. The method according to claim 3, characterized in that: Molten steel smelting includes molten iron pretreatment: desulfurization of molten iron to make the sulfur content of molten iron less than 0.0020%; Converter smelting: smelting end point C ≤ 0.02%, converter steelmaking process adopts 15 to 20 minutes of bottom blowing of argon and nitrogen; refining: Si alloy deoxidation is used to adjust the element content, the order of alloy addition is Si, Mn, Ni, V, Cu, Ti, Ca, B, and the molten steel is vacuum degassed to reduce the gas element content.

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