Steel plate API 2Y-50 for large-thickness high-standard ocean structure and preparation method of steel plate API 2Y-50

Abstract

The invention relates to a steel plate API2Y-50 for a large-thickness high-standard ocean structure, and belongs to the technical field of iron-based alloys. The chemical components of the steel plate are based on API Spec 2Y standard requirements and comprise, by weight, 0.07%-0.12% of C, 0.10%-0.40% of Si, 1.00%-1.50% of Mn, smaller than or equal to 0.010% of P, smaller than or equal to 0.008% of S, 0.010%-0.030% of Nb, 0.020%-0.050% of Al, 0.010%-0.020% of Ti, 0.10%-0.25% of Cr, 0.20%-0.60% of Ni and the balance Fe and inevitable impurity elements. The carbon equivalent CEV is less than or equal to 0.43%, and the cold crack sensitivity coefficient Pcm is less than or equal to 0.24%. On the basis of an API Spec 2Y standard component system, the steel plate which is matched in high strength and toughness, easy to weld and excellent in lamellar tearing resistance and corrosion resistance is produced through the working procedures of KR, BOF smelting, LF, RH, slab continuous casting, slab slow cooling, heating, rolling, steel plate slow cooling, heat treatment and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a steel plate API 2Y-50 for marine structures and a preparation method thereof. Background Art

[0003] Different from the existing GB 712 and EH36 steel for offshore platforms under the classification society standards, Middle Eastern countries require steel to meet the requirements of the American Petroleum Institute standard API Spec 2Y. The API Spec 2Y standard not only has more stringent requirements on carbon equivalent, crack sensitivity coefficient, addition of alloy elements, and conventional mechanical properties, but also has additional requirements for S1-12.

[0004] After searching, the document with patent publication number CN105803175BA discloses a low compression bit thick EH36 ship plate steel and its preparation method, which adopts high Mn component design and adds a large amount of precious elements such as Nb, V, Ni, etc. The alloy composition, carbon equivalent and crack sensitivity coefficient do not meet the API Spec 2Y standard requirements, and the structure is tempered bainite. The document with patent publication number CN115216680A discloses a low carbon equivalent and low crack sensitivity coefficient EH36 for offshore wind power and its production method, which adopts low carbon component design and TMCP process production, and the structure is acicular ferrite, with a maximum thickness of only 100mm. At present, there is no report on the production of 150mm thick high-standard marine structure steel plate API 2Y-50 with excellent comprehensive performance in the existing technology. Summary of the invention

[0005] The present invention adopts continuous casting billets to produce high-standard marine structure steel plates API 2Y-50 steel plates with a maximum thickness of 150 mm. Based on the API Spec 2Y standard component system, the steel plates are subjected to the processes of KR molten iron pretreatment → BOF smelting → LF furnace refining → RH vacuum degassing → slab continuous casting → slab slow cooling → heating → rolling → steel plate slow cooling → heat treatment, etc., so as to produce steel plates with high strength and toughness matching, easy welding, excellent lamellar tear resistance and corrosion resistance.

[0006] The technical solution adopted by the present invention is: a thick high-standard steel plate API 2Y-50 for marine structures and a preparation method thereof. The chemical composition of the steel plate is based on the requirements of API Spec 2Y standard, and the content of harmful elements such as P and S in the steel grade is controlled to improve the purity of the steel. The good welding performance of the steel plate is ensured by the low C and low Mn composition design. The Al, Nb, and Ti grain refining elements are fully utilized to improve the strength and toughness of the steel plate. By adding hardenability elements such as Ni and Cr, the uniformity of the performance in the thickness direction of the steel plate is improved. The chemical composition by weight percentage is: C: 0.07~0.12%, Si: 0.10~0.40%, Mn: 1.00~1.50%, P: ≤0.010%, S: ≤0.008%, Nb: 0.010~0.030%, Al: 0.020~0.050%, Ti: 0.010~0.020%, Cr: 0.10~0.25%, Ni: 0.20~0.60%, and the balance is Fe and unavoidable impurity elements.

[0007] The technical solution adopted by the present invention to solve the above problem is a thick high-standard marine structure steel plate API2Y-50, with carbon equivalent CEV≤0.43% and cold crack sensitivity coefficient Pcm≤0.24%. The carbon equivalent CEV and cold crack sensitivity coefficient Pcm are calculated by the smelting analysis composition using the following formula, where the element symbol represents the mass percentage of the corresponding element.

[0008] CEV (%)=C+Mn / 6+(Cr+Mo+V)5+(Ni+Cu) / 15 Pcm(%)=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B Preferably, the chemical composition of the steel plate is C: 0.11%, Si: 0.30%, Mn: 1.25%, P: ≤0.006%, S: ≤0.001%, Nb: 0.025%, Al: 0.035%, Ti: 0.015%, Cr: 0.20%, Ni: 0.30%, CEV: <0.41%, Pcm: <0.22%, and the remainder is Fe and unavoidable impurity elements.

[0009] The chemical composition design principle of the steel of the present invention: C: The strength of the steel plate can be significantly improved through solid solution strengthening and precipitation strengthening. For thick quenched and tempered steel plates, the hardenability can be improved. However, too high a C content will not only affect the welding performance, but will also cause severe center segregation, thereby affecting the low-temperature impact toughness at 1 / 2 of the thickness of the steel plate. Therefore, the carbon content in the present invention is controlled within the range of 0.07 to 0.12%.

[0010] Si: It is used for deoxidation and improves the strength and corrosion resistance of steel in the form of solid solution strengthening. For thick quenched and tempered steel plates, it can reduce the critical cooling rate of steel and improve the hardenability of steel. However, if its content is too high, it will damage the toughness and welding performance of steel. After rolling, it is easy to form iron oxide scale on the surface, which affects the surface quality of steel. Therefore, its range is controlled at 0.10~0.40%.

[0011] Mn: It has the effect of delaying the transformation of austenite to ferrite, helps to refine ferrite, and improves strength and low-temperature toughness. However, Mn has a high tendency to segregate, and is easy to segregate in the core of the steel plate, reducing the impact toughness of the core of the steel plate and affecting the welding performance of the steel. The present invention stipulates that the manganese content is within the range of 1.00-1.50%.

[0012] Nb: Refining grains. During the rolling process, strain-induced precipitation hinders the recovery and recrystallization of deformed austenite, thereby refining grains and improving the toughness of steel. Too high Nb content is not conducive to welding performance and has a high cost. Therefore, the present invention stipulates that the niobium content is within the range of 0.010-0.030%.

[0013] The addition of Ti alloying elements forms strong carbides, which prevent the coarsening of austenite grains during heating, rolling, and especially welding, and improve the low-temperature toughness of the base material and the welding heat-affected zone. The present invention stipulates that the V content is between 0.010 and 0.020%.

[0014] Al mainly acts as a deoxidizer, and can also reduce the inclusion content in steel and refine grains. However, too much Al will affect the surface quality of the ingot and cause surface cracks. Therefore, the total Al content should be controlled at 0.020~0.050%.

[0015] Ni is mainly used to improve the low temperature toughness of steel, and can also improve the hardenability of steel, and at the same time improve the strength of steel through solid solution strengthening. In addition, the compounding effect with residual Cr and P in steel will help improve the corrosion resistance of steel. However, due to the high price of Ni alloy, in the present invention, the nickel content is specified to be between 0.20% and 0.60%.

[0016] Cr: significantly improves the hardenability of steel and increases tempering stability. For thick quenched and tempered steel plates, it can improve the uniformity of performance in the thickness direction, but excessive Cr will form low-melting-point Cr-Mn composite oxides, causing surface cracks during welding. Therefore, in the present invention, the Cr content is between 0.10-0.20%.

[0017] P, S: P and S are harmful elements in steel, which reduce the physical and mechanical properties of steel. Therefore, the lower the P and S content, the better.

[0018] The present invention further provides a method for preparing the API 2Y-50 steel plate with large thickness and high standard for marine structures, and the specific steps are as follows: 1. Smelting process Select high-quality scrap steel, control LF refining time to more than 50min; RH refining ≥40min, high vacuum (≤133Pa) time >30min; after breaking the vacuum, determine hydrogen [H] ≤1.5ppm, and analyze the N content of the finished product sample, and the [N] content is controlled below 70ppm; then feed wire (silicon calcium wire), and blow argon after feeding, and the soft blowing time is controlled at 15-30min.

[0019] 2. Continuous casting process The tundish preheating time before casting is ≥5H. To ensure that the center segregation of the continuous casting billet is ≤C0.5, low superheat protection pouring is adopted in the whole casting process, the superheat is controlled at 10-25℃, and the pouring speed is strictly controlled to be ≤0.5m / min.

[0020] 3. Slow cooling of continuous casting billet After the continuous casting billet is rolled off the production line in sections, slow cooling and hydrogen expansion are carried out, and the billet is required to be stacked into a pit or covered. The slow cooling temperature is ≥600℃ and the time is ≥72 hours. Then, the surface defects are cleaned at above 200℃.

[0021] 4. Rolling and cooling Heat the billet to 1150-1250℃, control the high temperature section + soaking section time ≥200min, and fully dissolve the alloy elements in the steel. After the billet is removed from the furnace, it is descaled with 22MPa high-pressure water to avoid the surface quality of the steel plate being affected by the pressure of oxides during rolling. Then hot rolling is carried out, and the rolling is carried out using a large reduction process. The rolling speed is ≤1.0m / s. Water spraying is required to descale the surface of the steel plate during the first three rolling processes, so that a thin hardened layer is produced on the surface of the steel plate, so that the deformation penetrates into the core of the continuous casting billet, and the internal cracks, looseness, pores and other defects are bridged. After rolling, the steel plate is taken off the line for slow cooling in the hydrogen expansion pile, and slow cooling is carried out at a high temperature of ≥400℃ for more than 36 hours.

[0022] 5. Heat treatment: Produced using quenching and tempering heat treatment technology.

[0023] Quenching process: Quenching temperature is 860-920°C, and the time in the furnace is 1.4-1.8 min / mm.

[0024] Tempering process: The tempering temperature is 560-640℃, and the time in the furnace is 2.0~4.0min / mm.

[0025] The matrix structure of the steel plate is a fine and uniform mixed structure of ferrite + pearlite, and the average grain size is 8-15μm.

[0026] The technical features of the present invention are as follows: 1) The key to this invention is the low composition design of the strong segregation element "C+Mn", which effectively controls the center segregation of the continuous casting billet, and controls the carbon equivalent CEV to ≤0.43%, and the cold crack sensitivity coefficient Pcm to ≤0.24%. It ensures that the steel plate has good welding performance and provides great convenience for offshore engineering operations. At the same time, by adding "Ni+Cr" strong hardenability elements, matching the excellent heat treatment process, replacing a large amount of alloy accumulation, the uniformity of strength in the thickness direction of the steel plate is ensured.

[0027] 2) API 2Y-50 with a maximum thickness of 150mm is produced using continuous casting billets. Under the unfavorable conditions of low compression ratio (≤3), the deformation is penetrated into the center of the continuous casting billet through the rolling technology of low speed and large reduction + interpass cooling, thereby bridging the defects in the core of the steel plate and ensuring the stability of the impact toughness of the core of the steel plate.

[0028] 3) Use high purity steel smelting technology to strictly control harmful elements such as P, S, O, and H in steel. Inclusions meet the requirements of A≤0.5, B≤0.5, C≤0.5, D≤0.5, and Ds≤0.5. During the continuous casting process, control the low-multiple mass of the continuous casting billet, center segregation ≤C0.5, and center porosity ≤0.5. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The metallographic structure at 1 / 4 thickness of the steel plate of Example 1 of the present invention is uniform and fine ferrite + pearlite.

[0030] Figure 2 This is the metallographic structure at 1 / 2 thickness of the steel plate of Example 1 of the present invention, which is also a mixed structure of uniform and fine ferrite + pearlite. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0032] The chemical composition of the API 2Y-50 steel plate corresponding to each embodiment is shown in Table 1. The data in the table are the mass percentage content of each element, and the rest are Fe and unavoidable impurity elements.

[0033] Table 1 Chemical composition of API 2Y-50 steel plate for large thickness marine structure (wt%) Example C Si Mn P S Al Nb Ti Cr Ni CEV Pc API Spec 2Y(max) 0.18 0.80 1.70 0.020 0.010 ≥0.018 0.06 0.05 1.5 2.00 0.43 0.24 1 0.112 0.287 1.24 0.005 0.0018 0.034 0.026 0.016 0.205 0.293 0.40 0.21 The production process is as follows: (1) Smelting: The molten iron is treated with KR for S removal and then smelted in a 150-ton converter. It is then refined in an LF furnace and subjected to RH vacuum degassing treatment. After degassing, it is treated with Ca.

[0034] (2) Continuous casting: The molten steel is cast into continuous casting billets with a cross section of 450 mm by a straight arc continuous casting machine. The casting temperature is controlled at 10-25°C above the liquidus line. The pulling speed is controlled at ≤0.5m / min to ensure that the center segregation of the continuous casting billet is ≤C0.5 level and the center looseness is ≤0.5 level.

[0035] (3) Slow cooling and hydrogen expansion of billets: The continuous casting billets are stacked after they come off the production line, with a slow cooling temperature of ≥600°C and a time of ≥72 hours, followed by surface defect cleaning at a temperature above 200°C.

[0036] (4) Rolling: The billet enters the heating furnace for secondary heating, and the temperature is controlled at 1150-1250℃. The high temperature section + soaking section time is ≥200min, so as to fully dissolve the alloy elements in the steel and ensure the uniformity of the composition and performance of the finished steel plate. After the billet is removed from the furnace, it is descaled by high-pressure (≥22MPa) water to avoid the surface quality of the steel plate being affected by the pressure of oxides during rolling. Then hot rolling is carried out, and the rolling process is adopted with a large reduction process. The rolling speed is ≤1.0m / s. Water spray cooling is required during the first three rolling processes to produce a thin hardened layer on the surface of the steel plate, so that the deformation can more easily penetrate into the core of the continuous casting billet, and fill the internal cracks, looseness, pores and other defects. After rolling, the offline pile is slowly cooled.

[0037] (5) Slow cooling and hydrogen expansion of steel plates: hydrogen expansion is carried out by stacking and slow cooling at above 400°C for ≥36 hours.

[0038] (6) Quenching and tempering heat treatment: The steel plate after slow cooling is quenched at a quenching temperature of 860-920°C and a time of 1.4-1.8 min / mm from entering the furnace to exiting the furnace. After exiting the furnace, the steel plate enters the quenching machine and is tempered after cooling to room temperature at a tempering temperature of 560-640°C and a time of 2.0-4.0 min / mm from entering the furnace to exiting the furnace. Air cool after exiting the furnace.

[0039] (7) After tempering, the steel plates were subjected to transverse tensile tests and transverse Charpy impact tests at 1 / 4 and 1 / 2 of the thickness and metallographic structure observation according to API Spec 2Y standard requirements.

[0040] The properties of the example steel plates produced according to the above process flow are shown in Table 3. The non-proportional extension strength of the transverse tensile performance of the 150mm thick API 2Y-50 steel plate at 1 / 4 and 1 / 2 of the thickness of the head and tail is between 360-375MPa, the tensile strength is between 520-530MPa, and the elongation is ≥35.5%; the transverse impact energy at -40℃ at 1 / 4 and 1 / 2 of the thickness of the head and tail of the steel plate is between 355-421J; the Z-direction tensile performance at 1 / 2 of the thickness of the head and tail of the steel plate is between 60-77%. After 1h of heat preservation at 250℃, the aging impact energy at -40℃ with 5% strain is between 197-313J. The characteristic value of CTOD at -10℃ is 0.431-1.145mm. All meet the requirements of API Spec 2Y standard.

[0041] The steel plate has good shape, and there are no defects such as cracks, wrinkles, welts, delamination, and scale indentation on the surface. The whole plate flaw detection meets the requirements of ASTM A578 / A578M C level. The steel plate carbon equivalent CEV is less than 0.41%, and the cold crack sensitivity coefficient Pcm is less than 0.22%. It has excellent welding performance and crack resistance, which provides great convenience for offshore engineering operations.

[0042] Figure 1 , 2 Typical microstructure photos of the embodiment are given. The microstructure of the finished steel plate is a uniform and fine mixed structure of ferrite + pearlite, with a grain size of 8-15μm. Under the premise of large thickness and low compression ratio, the low C + low Mn composition design is matched with advanced low-speed large reduction + interpass cooling technology and tempering process to ensure the uniformity of the structure in the thickness direction of the steel plate. While ensuring the high strength of the steel plate, it also takes into account the low-temperature impact toughness of -40℃.

[0043] Example Finished steel plate thickness, mm Blank thickness, mm Billet heating temperature, ℃ Main pass reduction mm Main pass reduction rate % Rolling speed m / s 1 150 450 1230-1250 30+32+35 14+15+16 0.5-1.0 Table 3 Tensile, impact, Z-direction properties, and aging impact of the embodiments of the present invention

[0044] *T is the thickness of the steel plate.

[0045] Table 4 CTOD test of steel plate welding in the embodiment of the present invention

[0046] In addition to the above embodiments, the present invention also includes other implementation modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A thick and high-standard steel plate API2Y-50 for marine structures, characterized by: The chemical composition of the steel plate is based on the requirements of API Spec 2Y standard, and is calculated by weight as follows: C: 0.07-0.12%, Si: 0.10-0.40%, Mn: 1.00-1.50%, P: ≤0.010%, S: ≤0.008%, Nb: 0.010-0.030%, Al: 0.020-0.050%, Ti: 0.010-0.020%, Cr: 0.10-0.25%, Ni: 0.20-0.60%, and the balance is Fe and unavoidable impurity elements.

2. The steel plate according to claim 1, characterized in that: Carbon equivalent CEV≤0.43%, cold crack sensitivity coefficient Pcm≤0.24%.

3. The steel plate according to claim 1, characterized in that: The chemical composition by weight percentage is C: 0.11%, Si: 0.30%, Mn: 1.25%, P: ≤ 0.006%, S: ≤ 0.001%, Nb: 0.025%, Al: 0.035%, Ti: 0.015%, Cr: 0.20%, Ni: 0.30%, CEV: <0.41%, Pcm: <0.22%, the balance is Fe and inevitable impurity elements.

4. A method for preparing the steel plate according to claim 1, characterized in that: include, Step 1: Molten Steel Smelting LF refining time ≥50min; RH refining time ≥40min, ≤133Pa high vacuum treatment time>30min; after breaking the vacuum, determine hydrogen [H] ≤1.5ppm, and analyze the N content of the finished product sample, and the [N] content is controlled below 70ppm; then feed the silicon calcium wire, and blow argon after the wire feeding, and the soft blowing time is controlled at 15-30min; Step 2: Continuous Casting The tundish preheating time before casting is ≥5H. To ensure that the center segregation of the continuous casting billet is ≤C0.5, low superheat protection pouring is adopted in the whole casting process. The superheat is controlled at 10-25℃ and the pouring speed is ≤0.5m / min. Step 3: Slow cooling of continuous casting billet After the continuous casting billet is rolled off the production line in sections, slow cooling and hydrogen expansion are carried out; Step 4: Rolling and cooling The billet is heated to 1150-1250℃, and the heating time of the high temperature section + soaking section is controlled to be ≥200min, so as to fully dissolve the alloy elements in the steel; after the billet is taken out of the furnace, it is descaled by high-pressure water to remove the oxides on the surface of the billet, and then hot rolled, and rolled by a large reduction process, with a rolling speed of ≤1.0m / s. During the first three rolling passes, water is sprayed to descale, so that a thin hardened layer is produced on the surface of the steel plate, so that the deformation penetrates into the core of the continuous casting billet, and after rolling, the steel plate is taken off the line for slow cooling in the hydrogen expansion pile, and slow cooling is carried out at a high temperature of ≥400℃ for more than 36 hours; Step 5: Heat treatment The steel plate is subjected to tempering heat treatment.

5. The method according to claim 4, characterized in that: Step 3: The continuous casting ingots are stacked and placed in a pit or covered for slow cooling. The slow cooling temperature is ≥ 600°C and the time is ≥ 72 hours. Then, surface defects are cleaned at above 200°C.

6. The method according to claim 4, characterized in that: Step five, the tempering heat treatment includes quenching and tempering. The quenching process: the quenching temperature is 860-920°C, and the time in the furnace is 1.4-1.8 min / mm; the tempering process: the tempering temperature is 560-640°C, and the time in the furnace is 2.0-4.0 min / mm.

7. The method according to claim 4, characterized in that: Step 5: After heat treatment, the matrix structure of the steel plate is a fine and uniform mixed structure of ferrite + pearlite, and the average grain size is 8-15μm.

8. The method according to claim 4, characterized in that: The non-proportional extension strength of the transverse tensile performance of the steel plate head and tail at 1 / 4 and 1 / 2 of the thickness is: 360-375MPa, tensile strength: 520-530MPa, elongation ≥35.5%; The transverse impact energy at -40℃ at 1 / 4 and 1 / 2 of the thickness of the head and tail of the steel plate is 355-421J; the Z-direction tensile performance at 1 / 2 of the thickness of the head and tail of the steel plate is 60-77%; the aging impact energy at -40℃ with 5% strain after insulation at 250℃ for 1h is between 197-313J; the CTOD characteristic value at -10℃ is 0.431-1.145mm.

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