Large-thickness 3.5 Ni steel with excellent low-temperature toughness and preparation method thereof

Abstract

The invention discloses large-thickness 3.5 Ni steel with excellent low-temperature toughness and a preparation method of the large-thickness 3.5 Ni steel, and relates to the technical field of production of steel for low-temperature containers, and the large-thickness 3.5 Ni steel comprises the following chemical components in percentage by mass: 0.05-0.08% of C, 0.10-0.30% of Si, 0.60-0.95% of Mn, less than or equal to 0.004% of S, less than or equal to 0.006% of P, 3.65-3.75% of Ni, 0.07-0.12% of Mo, 0.020-0.050% of Alt, less than or equal to 0.0020% of O, less than or equal to 0.0050% of N and the balance of Fe and inevitable impurities. Low-temperature heating, controlled rolling and controlled cooling and off-line heat treatment processes are adopted, a structure with tempered sorbite as a main part and fine ferrite is obtained, the steel plate has excellent low-temperature toughness, the impact value of 1 / 2 thickness-101 DEG C is larger than or equal to 130 J, and the produced large-thickness steel plate can meet the construction requirements of facilities such as a low-temperature washing tower.

Description

Technical Field

[0001] The present invention relates to the technical field of steel production for cryogenic vessels, and particularly to a thick 3.5Ni steel with excellent low-temperature toughness and a preparation method thereof. Background Art

[0002] Low-temperature steel with a Ni content of about 3.5% has excellent low-temperature toughness, relatively high strength, and excellent welding performance in a low-temperature environment. It is a material with good toughness for use in cryogenic environments and is mainly used for constructing container equipment such as oil, ethylene, and air separation oxygen production equipment, including methanol scrubbing towers, H2S concentration towers, and CO towers. Its lowest service temperature is -101°C. In the past, the maximum thickness range of thick plates used to manufacture these cryogenic equipment was 100 - 135 mm. With the development of equipment towards large-scale, design units and customers have put forward requirements for thicker steel plates. However, as the thickness of the steel plate increases, the properties of the core of the steel plate will weaken accordingly, especially the impact properties of the core may fail to meet the requirements.

[0003] Chinese Patent CN103305758B discloses a steel plate for cryogenic pressure vessels and a production method thereof. It is produced using continuous casting billets, and the impact energy at 1 / 2 thickness - 101°C > 150 J. However, 0.010 - 0.020% Nb and 0.010 - 0.020% Ti are added to the composition, and the maximum thickness is 50 mm.

[0004] Chinese Patent CN103898418B discloses thick Ni-based steel plates for cryogenic vessels and a production method thereof. The Ni-Cr-Mo-Nb-Ti alloy system is adopted, and the alloy system includes Cr: 0.18 - 0.25%; Nb: 0.010 - 0.020%; Ti: 0.010 - 0.020%. The thickness covers 100 - 150 mm, and the core impact at -101°C can be ≥80 J. However, its casting uses the ingot casting process. The ingots obtained by ingot casting are thick, and the reduction ratio of rolling thick plates is large, which is easy to meet the requirements of core impact properties. However, the yield of ingot casting production is low, the production cycle is long, and the alloy cost after adding microalloy and Cr elements and the manufacturing costs such as the process and procedures brought by ingot casting are relatively high.

[0005] Chinese Patent CN105177445B discloses a preparation method of a high-toughness 3.5 Ni steel plate, which requires a total reduction ratio (casting billet thickness / finished product thickness) ≥5.0 and adopts online heat treatment. It has excellent low-temperature toughness under pilot-plant conditions, but the maximum thickness of the steel plate in the examples is 20 mm.

[0006] Chinese Patent CN109136768A discloses a preparation method of a 3.5% Ni steel plate applicable to an environment of -135°C. The steel plate has excellent low-temperature toughness at -135°C, but the maximum thickness of the steel plate in the examples is only 20 mm.

[0007] Chinese Patent CN105441798B discloses a manufacturing method of medium - thick Ni steel plates for low - temperature containers, which has excellent low - temperature toughness at - 120°C, but the maximum thickness of the steel plate is 30 mm.

[0008] Chinese Patent CN113151737B discloses an 08Ni3DR steel plate resistant to hydrogen - induced cracking and its manufacturing method. Cr, Mo, and Cu alloys are added to the alloy system, and it is produced using a 370 - mm - thick continuous casting billet, which can meet the service requirements at - 100°C and has good HIC resistance. However, the declared thickness in the disclosed patent is 30 - 80 mm, and the maximum thickness in the examples is 65 mm.

[0009] Chinese Patent CN115341152A discloses a nickel - saving - type low - temperature steel for - 100°C and its manufacturing method, with Ni: 2.8% - 3.2%, Mo: 0.10% - 0.30%, Nb: 0.02% - 0.06%, Ti: 0.02% - 0.06%; the V - notch impact energy at - 100°C ≥ 150 J. Reducing the content of Ni element is beneficial to controlling the alloy cost, and micro - alloying treatment will increase the alloy cost. The maximum thickness of the steel plate in the examples is 50 mm.

[0010] Chinese Patent CN116145014A discloses an 08Ni3DR steel plate with low compression ratio and ensuring the properties of the core part. The maximum thickness produced using a 370 - mm - thick continuous casting billet is 120 mm, and the heat treatment is carried out in sequence of high - temperature quenching + sub - critical quenching + high - temperature tempering, which has excellent low - temperature toughness. The number of thermal cycles in the heat treatment is large, and the heat treatment cycle is long, which will affect the heat treatment production capacity and increase the process cost.

[0011] Chinese Patent CN118854164A discloses a 3.5Ni steel and its production method and application, with a thickness range of 6 - 100 mm, and the lowest impact test temperature is only - 80°C, which cannot meet the standard requirements.

[0012] In summary, there is an urgent need for a manufacturing technology that uses continuous casting billets to produce large - thickness steel plates with a thickness ≥ 120 mm and the properties of the core part meeting the standard requirements. Summary of the Invention

[0013] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a large - thickness 3.5Ni steel with excellent low - temperature toughness and its preparation method.

[0014] To solve the above - mentioned technical problems, the technical solution of the present invention is as follows: A thick 3.5Ni steel with excellent low-temperature toughness, the chemical composition and mass percentage of which are as follows: C: 0.05-0.08%, Si: 0.10-0.30%, Mn: 0.60-0.95%, S≤0.004%, P≤0.006%, Ni: 3.65-3.75%, Mo: 0.07-0.12%, Alt: 0.020-0.050%, O≤0.0020%, N≤0.0050%, and the balance is Fe and inevitable impurities.

[0015] The present invention also provides a preparation method for a thick 3.5Ni steel with excellent low-temperature toughness, including: Steelmaking: Using a converter for smelting, and then refining through an LF furnace and an RH furnace in sequence; Continuous casting: Controlling the casting speed to be 0.65±0.10 m / min, the superheat of the molten steel to be 10-25°C, and subjecting the cast slab to bank cooling treatment; Slab heating: Controlling the heating temperature to be 1100-1150°C, and controlling the temperature uniformity of the slab to be less than or equal to 15°C; Hot rolling: Adopting two-stage controlled rolling, and using an ultra-fast cooling system and a laminar cooling system to cool the rolled piece to below 180°C after rolling; Heat treatment: Sequentially performing offline quenching and tempering for heat treatment.

[0016] As a preferred scheme of the preparation method for the thick 3.5Ni steel with excellent low-temperature toughness according to the present invention, wherein: in the steelmaking process, the vacuum degree of the RH furnace is less than or equal to 40 Pa, and the vacuum treatment time is greater than or equal to 25 min.

[0017] As a preferred scheme of the preparation method for the thick 3.5Ni steel with excellent low-temperature toughness according to the present invention, wherein: in the continuous casting process, controlling the thickness of the continuous casting slab to be 460 mm.

[0018] As a preferred scheme of the preparation method for the thick 3.5Ni steel with excellent low-temperature toughness according to the present invention, wherein: the two-stage controlled rolling includes: Rough rolling in the austenite recrystallization zone, controlling the starting rolling temperature to be 980-1130°C, the total reduction of rough rolling to be 50-60%, and the reduction in the first three passes to be ≥35 mm; Finish rolling in the austenite non-recrystallization zone, controlling the starting rolling temperature to be lower than 820°C, and the finishing rolling temperature to be 780-820°C.

[0019] As a preferred scheme of the preparation method for the thick 3.5Ni steel with excellent low-temperature toughness according to the present invention, wherein: the offline quenching process includes: heating the steel plate to 820°C-840°C and holding for 20-50 minutes, and then performing water quenching on a roll press quenching machine.

[0020] As a preferred solution of the method for preparing thick 3.5Ni steel with excellent low temperature toughness of the present invention, the tempering process comprises heating the steel plate to 590-620°C, tempering for 40-80 min, and then air cooling after taking out of the furnace.

[0021] As a preferred embodiment of the method for preparing thick 3.5Ni steel with excellent low-temperature toughness according to the present invention, the yield strength of the prepared steel plate near the surface and at 1 / 4 thickness is greater than or equal to 400 MPa, the tensile strength is 520-580 MPa, and the KV2 at -101°C is greater than or equal to 180 J; the yield strength at 1 / 2 thickness is greater than or equal to 380 MPa, the tensile strength is 510-550 MPa, and the KV2 at -101°C is greater than or equal to 130 J.

[0022] As a preferred solution of the method for preparing thick 3.5Ni steel with excellent low-temperature toughness of the present invention, the near-surface non-plastic transition temperature of the prepared steel plate is less than or equal to -110°C.

[0023] The beneficial effects of the present invention are: The thick 3.5Ni steel with excellent low-temperature toughness provided by the present invention has a small number of alloy types in its components, and a narrow controlled composition range, which is beneficial to controlling the stability of organizational performance; and the steel has excellent low-temperature toughness, especially a large margin of 1 / 2 thickness low-temperature toughness (relative to 60 J required by the standard), and NDT≤-110°C, which can meet the service requirements of the lowest working condition of -101°C; at the same time, quenching at a temperature above the A3 line is adopted, which is beneficial to the production of organizational structure and the control of plate shape; in addition, continuous casting billets are used to produce thick steel plates, with low process costs and high yield rates, and the steel has obvious cost advantages and delivery cycle advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0025] Figure 1 It is a schematic diagram of the tempered structure at 1 / 4 thickness of a 150 mm steel plate prepared by the present invention after being corroded by 4% nitric acid alcohol solution; Figure 2 The figure is a schematic diagram of the tempered structure at 1 / 2 thickness of a 150 mm steel plate produced by the present invention corroded by 4% nitric acid alcohol solution. DETAILED DESCRIPTION

[0026] To make the content of the present invention more clearly understood, the following further detailed description of the present invention will be given according to specific embodiments in conjunction with the accompanying drawings.

[0027] This application provides a large-thickness 3.5Ni steel with excellent low-temperature toughness, and its chemical composition and mass percentage are as follows: C: 0.05 - 0.08%, Si: 0.10 - 0.30%, Mn: 0.60 - 0.95%, S ≤ 0.004%, P ≤ 0.006%, Ni: 3.65 - 3.75%, Mo: 0.07 - 0.12%, Alt: 0.020 - 0.050%, O ≤ 0.0020%, N ≤ 0.0050%, and the balance is Fe and unavoidable impurities.

[0028] The design principle of the above composition is as follows: C: The solid solution strengthening element of steel. As the C content increases, the yield strength and tensile strength increase, but too high C content will lead to an increase in the ductile-brittle transition temperature and affect the welding performance. In order to ensure the low-temperature impact toughness and weldability of the steel plate, it is desirable that the C content in the steel is relatively low, and it can also weaken the segregation at the center of the continuous casting billet; but too low C content leads to a relatively high austenite grain boundary migration rate, which is likely to cause poor austenite grain uniformity during hot rolling and austenitization, and even form a mixed grain structure. At the same time, too low C content will also result in insufficient strength margin, especially it is difficult to ensure the strength at the center of the large-thickness steel plate. During design, considering comprehensively, the C content should be as low as possible on the premise of ensuring strength. The preferred range of this application is 0.05% - 0.08%.

[0029] Si: It is a deoxidizing element, which can inhibit the segregation of P and Mn at grain boundaries. The Si element can increase the yield strength, but too high Si content will lead to the coarsening of cementite on the original austenite grain boundaries. Such grain boundary positions are prone to form crack sources and are not conducive to low-temperature toughness. The content of Si in this application is controlled within 0.10 - 0.30%.

[0030] Mn: It is a solid solution strengthening element, which significantly improves the hardenability of the material. It can increase the yield strength through solid solution strengthening and precipitation strengthening. However, too high Mn content is likely to cause segregation at the original austenite boundaries and form MnS inclusions during the cooling process of the continuous casting billet, which is not conducive to low-temperature toughness. The Mn content range of this application is 0.60 - 0.95%.

[0031] Ni: It is the most important element to ensure low-temperature toughness. It can form α and γ phase solid solutions with Fe, can be infinitely soluble in the γ phase, and increases the strength through solid solution strengthening; it can expand the γ phase region, is an austenite formation and stabilization element, reduces the γ→α phase transformation temperature, and refines the α phase. In order to ensure the low-temperature toughness of the large-thickness steel plate, especially the low-temperature toughness at the 1 / 2 thickness, it is added according to the upper limit within the standard range. The range of this application is 3.65 - 3.75%.

[0032] Mo: The yield strength can be increased by solution strengthening, and the hardenability can be improved during the quenching process. It can also enhance the tempering stability of the microstructure and effectively inhibit grain boundary embrittlement. However, the Mo element is expensive, so it should be as low as possible while ensuring the performance. The Mo content range in this application is 0.07 - 0.12%.

[0033] Al: It is the main deoxidizing element in steel. Generally, a certain amount of Al is contained in steel to ensure the purity of molten steel. A certain amount of Al is also beneficial for refining grains. However, excessive Al is likely to form large-size inclusions, which is not conducive to low-temperature toughness. The range in this application is 0.020 - 0.050%: S and P: S is prone to form precipitates MnS with Mn, which is not conducive to low-temperature toughness. P is likely to segregate at grain boundaries, reducing the ability of grain boundaries to resist crack propagation and deteriorating low-temperature toughness. Therefore, in order to improve low-temperature toughness, the lower the content of these two inclusion elements, the better. However, considering production efficiency and cost, on the basis of ensuring material performance, the S content is determined to be ≤0.004%, and the P content is determined to be 0.006%.

[0034] O and N: Steel purity is the premise to ensure low-temperature toughness. Excessive O content will lead to too many oxide inclusions in steel. O and N are prone to form high-melting-point precipitates Al2O3 and AlN with Al, and the diameter of the precipitates is relatively large, reaching several micrometers. Stress concentration is likely to occur near the precipitates, becoming the crack source and seriously affecting the low-temperature toughness of the matrix. Therefore, in this application, the O content is controlled at ≤0.0020%, and the N content is controlled at ≤0.0050%.

[0035] This application also provides a preparation method of large-thickness 3.5Ni steel with excellent low-temperature toughness, including: Steelmaking: Converter smelting is adopted, and refining is carried out successively through the LF furnace and the RH furnace. Among them, the vacuum degree of the RH furnace is less than or equal to 40 Pa, and the vacuum treatment time is greater than or equal to 25 min.

[0036] Continuous casting: Low-speed casting withdrawal, low superheat pouring, and weak secondary cooling water cooling scheme are adopted for production. The casting speed is controlled at 0.65 ± 0.10 m / min, the superheat of molten steel is 10 - 25°C, the thickness of the continuous casting billet is 460 mm, and the casting billet is subjected to stacking and cooling treatment.

[0037] Slab heating: In order to control the original austenite grain size before hot rolling, low-temperature heating is adopted, and the heating temperature is controlled at 1100 - 1150°C, and the temperature uniformity of the slab is controlled at less than or equal to 15°C; Hot rolling: Two-stage controlled rolling is adopted, and after rolling, an ultra-fast cooling system and a laminar cooling system are used to cool the rolled piece to below 180°C. Among them, the two-stage controlled rolling includes: rough rolling in the austenite recrystallization zone, controlling the starting rolling temperature at 980 - 1130°C, the total reduction ratio of rough rolling at 50 - 60%, and the reduction in the first three passes ≥ 35 mm; finish rolling in the austenite non-recrystallization zone, controlling the starting rolling temperature below 820°C, and the finishing rolling temperature at 780 - 820°C.

[0038] Heat treatment: Offline quenching and tempering are adopted for heat treatment in sequence. Among them, the hot-rolled plate is subjected to offline quenching and tempering heat treatment. The steel plate is heated to 820°C - 840°C and held for 20 - 50 minutes, quenched in water on a roll press quenching machine, and then heated to 590 - 620°C for tempering for 40 - 80 min, and then air-cooled after leaving the furnace.

[0039] The above scheme is further described below through specific embodiments.

[0040] The compositions of 3.5Ni steel in Examples 1 - 3 of the present invention are specifically shown in Table 1.

[0041]

[0042] Table 1 In Examples 1 - 3, converter smelting is used for C and P removal, partial alloying is carried out, and an LF furnace is used for temperature control, desulfurization, and precise alloying; in order to reduce the influence of inclusion gas elements such as H on low-temperature toughness, RH vacuum degassing treatment is adopted, the vacuum degree is maintained at ≤ 40 Pa for ≥ 25 min, and the content of H element is controlled below 2 ppm. A 460 mm thick slab is produced by continuous casting process, and gas shielded casting is carried out throughout the continuous casting production process. The superheat of the molten steel is controlled in the range of 10 - 25°C to ensure the quality of the slab and weaken the central segregation. In order to control the austenite grain size, low-temperature heating is adopted, the heating temperature is 1100 - 1150°C, and after leaving the furnace, two-stage controlled rolling is adopted. After rolling, it is directly cooled to below 180°C by an ultra-fast cooling plus laminar flow cooling system. The steel plate is subjected to offline heat treatment to obtain a structure mainly composed of tempered sorbite plus small-sized ferrite, so as to ensure excellent low-temperature toughness, especially the low-temperature toughness at 1 / 2 thickness. The 3.5Ni steel thick plate produced can fully meet the service requirements under the condition of -101°C.

[0043] The manufacturing process parameters such as steel heating, rolling, and heat treatment in Examples 1 - 3 of the present invention are specifically shown in Table 2.

[0044]

[0045] Table 2 The heat treatment manufacturing process parameters of the steel in Examples 1 - 3 of the present invention are specifically shown in Table 3.

[0046]

[0047] Table 3 The mechanical properties of Examples 1-3 of the present invention are specifically shown in Table 4.

[0048]

[0049] Table 4 It can be seen from this that the present invention adopts a low-temperature heating, controlled rolling and controlled cooling + offline heat treatment process to obtain a structure mainly composed of tempered sorbite plus fine ferrite. The thickness range of the produced finished product is 120-150 mm. The steel plate has excellent low-temperature toughness, and the impact value at 1 / 2 thickness -101 °C is ≥130 J. The produced thick steel plate can meet the construction requirements of facilities such as low-temperature scrubbers.

[0050] In addition to the above embodiments, the present invention may have other embodiments; all technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A thick 3.5Ni steel with excellent low temperature toughness, characterized in that: Its chemical composition and mass percentage are: C: 0.05-0.08%, Si: 0.10-0.30%, Mn: 0.60-0.95%, S≤0.004%, P≤0.006%, Ni: 3.65-3.75%, Mo: 0.07-0.12%, Alt: 0.020-0.050%, O≤0.0020%, N≤0.0050%, and the balance is Fe and unavoidable impurities.

2. A method for preparing a thick 3.5Ni steel with excellent low temperature toughness according to claim 1, characterized in that: include: Steelmaking: smelting is done in a converter, and then refined in LF furnace and RH furnace in turn; Continuous casting: Control the casting speed to 0.65±0.10 m / min, the superheat of molten steel to 10~25℃, and perform pile cooling treatment on the ingot; Slab heating: control the heating temperature to 1100-1150℃, and control the slab temperature uniformity to be less than or equal to 15℃; Hot rolling: two-stage controlled rolling is adopted, and after rolling, the ultra-fast cooling system and layer cooling system are used to cool the rolled piece to below 180°C; Heat treatment: Offline quenching and tempering are used for heat treatment in sequence.

3. The method for preparing a thick 3.5Ni steel having excellent low temperature toughness according to claim 2, characterized in that: In the steelmaking process, the vacuum degree of the RH furnace is less than or equal to 40 Pa, and the vacuum treatment time is greater than or equal to 25 minutes.

4. The method for preparing a thick 3.5Ni steel having excellent low temperature toughness according to claim 2, characterized in that: In the continuous casting process, the thickness of the continuous casting billet is controlled to be 460 mm.

5. The method for preparing thick 3.5Ni steel with excellent low temperature toughness according to claim 2, characterized in that: The two-stage controlled rolling includes: Rough rolling is carried out in the austenite recrystallization zone, and the starting rolling temperature is controlled at 980-1130℃, the total rough rolling reduction is 50-60%, and the first three passes have a reduction of ≥35 mm; Finish rolling is carried out in the austenite non-recrystallization zone, and the start rolling temperature is controlled below 820℃, and the final rolling temperature is 780-820℃.

6. The method for preparing thick 3.5Ni steel with excellent low temperature toughness according to claim 2, characterized in that: The offline quenching process includes: heating the steel plate to 820° C.-840° C. and keeping the temperature for 20-50 minutes, and then water quenching on a roller quenching machine.

7. The method for preparing thick 3.5Ni steel with excellent low temperature toughness according to claim 2, characterized in that: The tempering process includes heating the steel plate to 590-620° C., tempering for 40-80 min, and then taking the steel plate out of the furnace and air cooling.

8. The method for preparing thick 3.5Ni steel with excellent low temperature toughness according to claim 2, characterized in that: The yield strength of the steel plate near the surface and at 1 / 4 thickness is greater than or equal to 400 MPa, the tensile strength is 520-580 MPa, and the KV2 at -101°C is greater than or equal to 180 J. The yield strength at 1 / 2 thickness is greater than or equal to 380 MPa, the tensile strength is 510-550 MPa, and the KV2 at -101°C is greater than or equal to 130 J.

9. The method for preparing thick 3.5Ni steel with excellent low temperature toughness according to claim 2, characterized in that: The near-surface non-plastic transition temperature of the prepared steel plate is less than or equal to -110°C.

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