Ultralow-temperature high-toughness 7Ni steel plate based on composite microalloying and short-process preparation method

Abstract

The invention discloses an ultralow-temperature high-toughness 7Ni steel plate based on composite microalloying and a short-process preparation method, and relates to the technical field of steel production. The steel plate comprises the following chemical components in percentage by mass: 0.03 to 0.06 percent of C, 0.15 to 0.30 percent of Si, 0.80 to 1.20 percent of Mn, 6.80 to 7.20 percent of Ni, 0.3 to 0.6 percent of Cr, 0.10 to 0.20 percent of Mo, 0.010 to 0.020 percent of Nb, 0.008 to 0.015 percent of Ti, less than or equal to 0.004 percent of P, less than or equal to 0.002 percent of S, less than or equal to 0.0015 percent of O and the balance of Fe and inevitable impurities. The preparation method sequentially comprises the steps of KR molten iron pretreatment, oxygen converter smelting, LF / RH refining, continuous casting (casting blank slow cooling for 48 h), heating to 1180-1280 DEG C in the steel rolling stage, two-stage controlled rolling and gradient controlled cooling, and heat treatment is not needed. The transverse and longitudinal Rm of the steel plate is 690-825 MPa, the Rel is larger than or equal to 600 MPa, the A is larger than or equal to 20%, the impact absorption energy AKV at-196 DEG C is larger than or equal to 120 J, the Ni cost and energy consumption are greatly reduced, and the stability and batch manufacturing advantages of the steel plate are improved.

Description

Technical Field

[0001] This invention relates to the field of steel materials and manufacturing technology, specifically to a composite microalloyed 7Ni steel plate for use in cryogenic environments (such as LNG storage tanks and cryogenic pressure vessels) and its short-process preparation method, which is particularly suitable for cryogenic equipment manufacturing scenarios where there are strict requirements for the material's low-temperature toughness, strength, and manufacturing cost. Background Technology

[0002] LNG storage tanks, cryogenic pressure vessels, and other equipment place extremely high demands on the low-temperature toughness, strength, and reliability of steel. Currently, the industry widely uses Ni-based cryogenic steel plates such as 06Ni9DR and 06Ni7DR (refer to patents: CN106011627B, CN119506724A). However, existing technologies have the following core limitations: High cost and heavy resource dependence: Traditional 06Ni9DR steel plates have a Ni content as high as 8.5-10%, and although 06Ni7DR steel plates reduce the Ni content to 6.5-7.5%, they still rely on Ni alone to ensure low-temperature toughness. Ni resources are scarce in my country, and the high Ni content results in raw material costs accounting for more than 40% of the steel plate cost, significantly increasing equipment manufacturing costs.

[0003] The manufacturing process is lengthy and energy-intensive: Existing Ni-based steel plates generally adopt offline heat treatment processes such as "normalizing + tempering", "quenching + tempering" or "double quenching + tempering" (such as the quenching and tempering process disclosed in CN106011627B). These processes require additional electrical / thermal energy consumption (the energy consumption for heat treatment of a single ton of steel is about 300-500kWh), and the process cycle is extended by 2-3 days, which does not meet the development requirements of green manufacturing.

[0004] Performance improvement faces bottlenecks: Although the SA-841 Gr.G Cl.9 steel plate (Ni 6.5-7.5%) specified by the international standard ASME SA-841 / SA-841M is delivered using TMCP (Thermomechanical Control Process), its 196℃ impact absorption energy is only required to be ≥27J and its yield strength is ≥585MPa, which cannot meet the dual requirements of "ultra-high toughness + high strength" for equipment such as large LNG storage tanks.

[0005] Therefore, how to achieve a synergistic improvement in the toughness and strength of steel plates at ultra-low temperatures through composition optimization and process innovation while reducing Ni content, and shortening the manufacturing process and reducing energy consumption, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a composite microalloyed 7Ni steel plate and a short process preparation method to overcome the defects of existing ultra-low temperature Ni-based steel plates, namely "high Ni cost, long process energy consumption, and low toughness limit".

[0007] The technical solution adopted by this invention to solve the above problems is as follows: an ultra-low temperature high-toughness 7Ni steel plate based on composite microalloying, wherein the chemical composition of the steel plate, by mass percentage, is: C: 0.03-0.06%, Si: 0.15-0.30%, Mn: 0.80-1.20%, Ni: 6.80-7.20%, Cr: 0.30-0.60%, Mo: 0.10-0.20%, Nb: 0.010-0.020%, Ti: 0.008-0.015%; harmful element restrictions: P≤0.004%, S≤0.002%, O≤0.0015%; the balance is Fe and unavoidable impurities.

[0008] Ingredient design principles: C (0.03-0.06%): Low C content can reduce cementite precipitation, avoid low-temperature toughness deterioration, and at the same time ensure a certain solid solution strengthening effect; preferably 0.04-0.05%, to balance strength and toughness.

[0009] Ni (6.80-7.20%): Ni can lower the martensitic transformation temperature (Ms) of steel and promote the formation of reverse austenite, making it a core element for improving low-temperature toughness. Compared with traditional 9Ni steel, this invention reduces the amount of Ni by 1.5-3% by replacing some of the Ni function through composite microalloying.

[0010] Mo-Nb-Ti composite microalloying: Mo (0.10-0.20%): Improves the hardenability of steel, refines the lath bainite structure, inhibits tempering softening, and synergistically enhances strength and toughness; Nb (0.010-0.020%): In the heating stage, it inhibits the growth of austenite grains; in the cooling stage, it strengthens the grains through precipitation of Nb (C,N) and delays recrystallization. Ti (0.008-0.015%) preferentially combines with N to form TiN, avoiding Nb combination leading to Nb failure, and reducing the risk of aging embrittlement.

[0011] Low levels of harmful elements: P≤0.004% can avoid low-temperature embrittlement caused by grain boundary segregation; S≤0.002% reduces MnS inclusions (which can cut the matrix and reduce toughness); O≤0.0015% controls the content of oxide inclusions and ensures the purity of the matrix.

[0012] The present invention provides a short-process preparation method for ultra-low temperature high-toughness 7Ni steel plates based on composite microalloying, comprising the following steps: Step 1: Steelmaking (Purity Control) The raw materials are sequentially subjected to KR hot metal pretreatment (desulfurization to S≤0.002%), oxygen converter smelting (decarburization to target C content), LF ladle refining (adjusting composition, adding Cr, Mo, Nb, Ti alloy, temperature controlled at 1550-1580℃), RH vacuum refining (vacuum degree ≤1Pa, holding time 15-20min, dehydrogenation to H≤2ppm), and continuous casting. Continuous casting parameters: casting speed 0.6-1.3m / min (preferably 0.8-1.1m / min), superheat 15-25℃ (preferably 18-22℃), using electromagnetic stirring (stirring intensity 300-400A) and dynamic light reduction at the end of solidification (reduction amount 2-4mm) to obtain a continuous casting billet with a thickness of 150-300mm; After the continuously cast billet exits the machine, it is placed in an insulation pit for slow cooling for 48 hours to reduce the H content to ≤1.5ppm and avoid hydrogen-induced cracking.

[0013] Step 2: Steel rolling (organizational control) Heating: The continuously cast billet is placed in a heating furnace and heated to 1180-1280℃ (preferably 1200-1250℃), with a total holding time of 2-3 hours (holding at 1200-1250℃ in the high-temperature section for ≥10 minutes to ensure that the Nb and Ti alloys are fully dissolved). Descaling: After exiting the furnace, use ultra-high pressure water to descaling (pressure 25-30MPa, repeated 2 times) to remove iron oxide scale and prevent it from being pressed into the base material; Two-stage controlled rolling: Rough rolling stage (1000-1100℃): ≥6 rolling passes, cumulative reduction ≥60% (preferably 65-75%), breaking the original austenite grains (refining to 50-80μm); Finishing rolling stage (800-850℃, austenite non-recrystallization zone): full longitudinal rolling is adopted, with a pass reduction rate of 8-15%, and the plate shape is controlled (flatness ≤2mm / m), and finally rolled into steel plates with a thickness of 10-50mm; Gradient cooling (a key tissue regulation step): First stage: After rolling, water cool to 500-600℃ at a rate of 30-50℃ / s to suppress the precipitation of proeutectoid ferrite and retain a large amount of supercooled austenite; Second stage: Switch to air cooling + mist cooling hybrid mode, with a cooling rate of 5-15℃ / s (preferably 8-12℃ / s), so that some of the supercooled austenite is converted into stable reverse-transformed austenite (volume fraction 5-8%), and finally a two-phase structure of lath bainite + reverse-transformed austenite is formed (lath bainite grain size ≤5μm, reverse-transformed austenite is uniformly distributed between laths).

[0014] Step 3: Follow-up processing After the steel plate is cooled to room temperature, it is straightened (straightening force 1000-1500kN) and non-destructive tested (UT flaw detection level I qualified). No offline heat treatment is required, and it can be delivered directly.

[0015] Steel plate performance indicators Tensile properties: transverse / longitudinal yield strength Rel≥600MPa (preferably ≥650MPa), tensile strength Rm690-825MPa, elongation after fracture A≥20% (preferably ≥22%), yield strength ratio≤0.99; Low-temperature toughness: Impact absorption energy AKV ≥ 120J at -196℃ (actual measurement ≥ 150J), fiber content of cross section 100%; Welding performance: The heat-affected zone (HAZ) at -196℃ has an AKV ≥ 100J, meeting the welding requirements for cryogenic equipment.

[0016] Compared with the prior art, the present invention has the following beneficial effects: Cost advantage: With a 1.5-3% reduction in Ni content and the elimination of heat treatment, the manufacturing cost per ton of steel is reduced by 2,000-3,000 yuan, resulting in significant economic benefits for mass production; Performance advantages: The impact toughness at -196℃ far exceeds that of ASME SA-841 standard (27J) and traditional 9Ni steel (approximately 100J), and the strength index meets the high strength requirements of cryogenic pressure vessels; Green advantages: The short process reduces heat treatment steps, reduces energy consumption per ton of steel by 300-500 kWh, and reduces carbon emissions by 20-30%, which is in line with the trend of green manufacturing. Process stability: Through precise control of composition and gradient cooling, the performance fluctuation of steel plates is ≤5%, and the batch production qualification rate is ≥98%. Detailed Implementation

[0017] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention. Example 1

[0018] Example 1 involves a steel plate with a thickness of 12mm.

[0019] The production process for 12mm thick steel plates is as follows: According to the above chemical composition of the steel plate, the smelting raw materials are configured and carried out in sequence as follows: KR hot metal pretreatment - converter smelting - LF refining - RH refining - continuous casting - continuous casting billet cleaning - continuous casting billet heating - heat preservation treatment - high pressure water descaling - rolling - controlled cooling - straightening - warehousing.

[0020] After the steel refining process, samples were taken from the ladle for analysis. The main components were found to be 0.045% C, 7.10% Ni, 0.45% Cr, 0.12% Mo, 0.016% Nb, and 0.012% Ti.

[0021] Furthermore, a two-stage controlled rolling process is employed: the roughing stage involves a cumulative reduction of 76% to break down the original austenite grains; the finishing stage involves finishing at 820℃ using full longitudinal rolling to control the plate shape. Gradient cooling control is implemented: the first stage involves water cooling to 585℃ at a rate of 30-50℃ / s after rolling; the second stage switches to a mixed mode of air cooling and mist cooling, with an average cooling rate of 10℃ / s, to obtain a dual-phase microstructure of lath bainite + reverse-transformed austenite, giving the steel plate both high deformation capacity and resistance to low-temperature brittle fracture.

[0022] The finished steel plates produced by the above manufacturing process have excellent comprehensive performance, as detailed in Tables 1-2. Example 2

[0023] Example 2 involves a steel plate with a thickness of 25mm.

[0024] The production process for 25mm thick steel plates is as follows: The steelmaking process for steel slab billets is the same as in Example 1.

[0025] To elaborate further: a two-stage controlled rolling process is employed: a cumulative reduction of 67% in the roughing stage; and a finishing stage at 813℃, using full longitudinal rolling to control the plate shape. Gradient cooling control is implemented: in the first stage, the plate is water-cooled to 573℃ at a rate of 30-50℃ / s after rolling; in the second stage, the process switches to a mixed mode of air cooling and mist cooling, with an average cooling rate of 10℃ / s, resulting in a double-phase microstructure of lath bainite + reverse-transformed austenite.

[0026] The finished steel plates produced by the above manufacturing process have excellent comprehensive performance, as detailed in Tables 1-2. Example 3

[0027] Example 3 involves a steel plate with a thickness of 35mm.

[0028] The production process for 35mm thick steel plates is as follows: The steelmaking process for steel slab billets is the same as in Example 1.

[0029] To elaborate further: a two-stage controlled rolling process is employed: a cumulative reduction of 53% in the roughing stage; and a finishing stage at 810℃, using full longitudinal rolling to control the plate shape. Gradient cooling control is implemented: in the first stage, the plate is water-cooled to 570℃ at a rate of 30-50℃ / s after rolling; in the second stage, the process switches to a mixed mode of air cooling and mist cooling, with an average cooling rate of 10℃ / s, to obtain a dual-phase microstructure of lath bainite + reverse-transformed austenite.

[0030] The finished steel plates produced by the above manufacturing process have excellent comprehensive performance, as detailed in Tables 1-2.

[0031] Table 1. Main chemical composition (wt%) of the finished product from the example.

[0032] Table 2-1 Mechanical properties of steel plates produced in the examples

[0033] Table 2-2 Mechanical properties of steel plates produced in the examples

[0034] Example Analysis: As the thickness of the steel plate increases, the cumulative reduction rate of rough rolling decreases slightly (from 76% to 53%), and the rate of the first stage of controlled cooling decreases slightly (from 40℃ / s to 35℃). However, through precise control of composition and process adaptation, high strength and ultra-high and low temperature toughness are still achieved, proving the adaptability of this process to steel plates of different thicknesses.

[0035] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A super low temperature high toughness 7Ni steel plate based on complex microalloying, characterized in that, The chemical composition, by mass percentage, is: C: 0.03-0.06%, Si: 0.15-0.30%, Mn: 0.80-1.20%, Ni: 6.80-7.20%, Cr: 0.30-0.60%, Mo: 0.10-0.20%, Nb: 0.010-0.020%, Ti: 0.008-0.015%, P≤0.004%, S≤0.002%, O≤0.0015%, and the balance being Fe and inevitable impurities.

2. The 7Ni steel plate according to claim 1, characterized in that, The mechanical properties of the steel plate satisfy: transverse and longitudinal yield strength Rel≥600MPa, tensile strength Rm=690-825MPa, elongation A≥20%, impact energy AKV≥120J at -196℃, and fiber ratio 100%.

3. The 7Ni steel plate according to claim 1, characterized in that, The chemical composition, by mass percentage, is preferably: C: 0.04-0.05%, Mn: 0.90-1.10%, Ni: 6.90-7.10%, Cr: 0.40-0.50%, Mo: 0.12-0.18%, Nb: 0.014-0.018%, Ti: 0.010-0.013%, P≤0.003%, S≤0.0015%.

4. A short process for the production of a steel sheet according to any one of claims 1 to 3, characterised in that, The method comprises the following steps: S1. Steelmaking: the raw materials are sequentially subjected to KR molten iron pretreatment, oxygen converter smelting, LF external refining, RH vacuum refining, and continuous casting, and the continuously cast slab is slowly cooled in a holding pit for 48h after continuous casting; S2. Rolling: the continuously cast slab is heated to 1180-1280℃ and held for 2-3h, and after discharge, the slab is descaled by superhigh-pressure water, and then subjected to two-stage controlled rolling, and finally gradient controlled cooling to room temperature; the steel plate does not need subsequent offline heat treatment.

5. The preparation method according to claim 4, characterized in that, In step S1, the continuous casting parameters are: casting speed 0.6-1.3m / min, superheat 15-25℃, and electromagnetic stirring and dynamic soft reduction at the end of solidification are used; the vacuum degree of RH vacuum refining is ≤1Pa, the pressure holding time is 15-20min, and dehydrogenation is performed to H≤2ppm.

6. The method of claim 4, wherein, In step S2, the parameters of two-stage controlled rolling are: Coarse rolling stage: temperature 1000-1100℃, rolling pass≥6 passes, and cumulative reduction≥60%; Finish rolling stage: temperature 800-850℃, full longitudinal rolling is used, and pass reduction rate is 8-15%.

7. The preparation method according to claim 4, characterized in that, In step S2, the parameters of gradient controlled cooling are: First stage: water cooling to 500-600℃ at a rate of 30-50℃ / s; Second stage: air cooling + fog cooling to room temperature at a rate of 5-15℃ / s, to obtain a lath bainite + reverted austenite dual-phase structure.

8. The method of claim 4, wherein, In step S2, the heating temperature of the continuously cast slab is preferably 1200-1250℃, and the holding time in the high-temperature section is ≥10min; the pressure of superhigh-pressure water descaling is 25-30MPa, and the descaling is repeated twice.

9. The method of claim 5, wherein, The continuous casting casting speed is preferably 0.8-1.1m / min, and the superheat is preferably 18-22℃; the electromagnetic stirring intensity is 300-400A, and the dynamic soft reduction amount at the end of solidification is 2-4mm.

10. The method of claim 7, wherein, The first stage rate of the gradient controlled cooling is preferably 35-45℃ / s, the second stage rate is preferably 8-12℃ / s; the volume fraction of the reverse austenite is 5-8%, and the size of the lath bainite grain is ≤5μm.

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