Ultrahigh-strength high-toughness low-cost 9Ni steel and preparation method thereof

Abstract

The invention discloses ultrahigh-strength high-toughness low-cost 9Ni steel and a preparation method thereof, and the 9Ni steel comprises the following chemical components in percentage by mass: 0.03-0.05% of C, 0.15-0.30% of Si, 0.40-0.70% of Mn, less than or equal to 0.005% of P, less than or equal to 0.002% of S, 8.50-9.50% of Ni, 0.30-0.60% of Cu, 0.02-0.04% of Nb, 0.020-0.050% of Alt, less than or equal to 0.004% of N, less than or equal to 0.002% of O and the balance of Fe and inevitable impurities. The preparation method comprises the steps of smelting and continuous casting, slab heating, controlled rolling, controlled cooling and heat treatment. The obtained steel plate has ultrahigh strength and high toughness, and is suitable for ultralow-temperature containers such as liquefied natural gas (LNG) storage tanks and transport ships.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy, specifically relating to an ultra-high strength, high toughness, and low cost 9Ni steel and its preparation method. Background Technology

[0002] As a core structural material in cryogenic (-196℃) environments, the strength level of 9Ni steel directly determines the lightweighting, load-bearing capacity, and service life of LNG containers. With the global LNG industry moving towards larger scale and higher parameters, the strength requirements for materials in container design are continuously upgrading—the diameter and height of large LNG storage tanks are constantly increasing, and the insufficient strength of traditional 9Ni steel has become a key bottleneck restricting equipment lightweighting, cost reduction, and efficiency improvement.

[0003] In existing technologies, improving the strength of 9Ni steel faces multiple challenges: On the one hand, the yield strength of 9Ni steel produced by the traditional QT (quenching and tempering) process is generally only 650-750 MPa, which is far from meeting the "ultra-high strength" requirements of large containers. If the strength is forcibly increased by increasing the carbon content, the low-temperature toughness will drop sharply, losing its value for ultra-low temperature applications. On the other hand, some technologies strengthen the steel by adding precious metal elements such as Mo and Cr (e.g., CN118222921A), which can slightly improve the strength, but the alloy cost increases significantly, and the strength improvement is limited (the yield strength is difficult to exceed 800 MPa), failing to reach the "ultra-high strength" level.

[0004] Furthermore, existing methods for achieving high-strength 9Ni steel largely rely on complex QLT (quenching + two-phase quenching + tempering) processes (such as CN113106351A). This process requires multiple heating and cooling cycles, resulting in high energy consumption, long production cycles, and significant challenges in process control, leading to fluctuations in product strength and hindering large-scale stable production. More importantly, even with the QLT process, the yield strength of traditional 9Ni steel mostly remains in the 750–800 MPa range, failing to achieve a true breakthrough in "ultra-high strength." This cannot meet the lightweight design requirements of large LNG storage tanks, which prioritize strength over thickness. Insufficient strength leads to increased container wall thickness, increasing manufacturing costs and installation difficulty, while also reducing structural flexibility and safety.

[0005] Meanwhile, the contradiction between "high strength" and "high toughness" in existing technologies remains difficult to reconcile: most technologies, in pursuing increased strength, cause the impact energy at -196℃ to drop below 120J, and the yield strength ratio to exceed 0.92, resulting in insufficient material plasticity reserve and susceptibility to brittle fracture. Therefore, developing a 9Ni steel that achieves a breakthrough in ultra-high strength (yield strength ≥850MPa) without relying on expensive alloying elements and through simplified processes, while also possessing excellent low-temperature toughness and a low yield strength ratio, has become a core requirement for addressing industry pain points and is of great significance for promoting the technological upgrading of the cryogenic container industry. Summary of the Invention

[0006] The purpose of this invention is to provide an ultra-high strength, high toughness, and low-cost 9Ni steel and its preparation method. The steel plate obtained by this invention has ultra-high strength and high toughness, and is suitable for cryogenic containers such as liquefied natural gas (LNG) storage tanks and transport ships.

[0007] To achieve the above-mentioned objectives, the technical solution of this invention is as follows: an ultra-high strength, high toughness, and low-cost 9Ni steel, wherein the chemical composition of the 9Ni steel, by mass percentage, is: C: 0.03-0.05%, Si: 0.15-0.30%, Mn: 0.40-0.70%, P≤0.005%, S≤0.002%, Ni: 8.50-9.50%, Cu: 0.30-0.60%, Nb: 0.02-0.04%, Alt: 0.020-0.050%, N≤0.004%, O≤0.002%, with the remainder being Fe and unavoidable impurities.

[0008] The quality properties of the 9Ni steel described in this invention meet the following requirements: yield strength (Rp0.2) ≥ 850 MPa, tensile strength (Rm) ≥ 940 MPa, and elongation after fracture (A) ≥ 20%.

[0009] The quality properties of the 9Ni steel described in this invention meet the following requirements: average Charpy V-notch impact energy (KV2) at 196℃ ≥150J, single value ≥120J, and yield strength ratio ≤0.91.

[0010] The microstructure of the 9Ni steel described in this invention consists of a fine tempered lath martensite matrix with 5-10% reversed austenite distributed on it.

[0011] Another object of the present invention is to provide a method for preparing the above-mentioned ultra-high strength, high toughness, and low cost 9Ni steel, the method comprising the following steps: (1) Smelting and continuous casting: The above composition is smelted in a converter or electric furnace, refined by LF, degassed by VD or RH vacuum, and then calcium treated, and then continuously cast into billets; (2) Slab heating: Heating the continuously cast slab to ensure that the alloying elements are fully dissolved; (3) Controlled rolling and controlled cooling: Two-stage rolling is adopted. The first stage is the recrystallization zone rolling, with an initial rolling temperature ≥1050℃ and a cumulative reduction rate >60%; the second stage is the non-recrystallization zone rolling, with an initial rolling temperature ≤850℃ and a final rolling temperature controlled at 750~810℃, with a cumulative reduction rate >70%; accelerated cooling is carried out immediately after rolling, with a cooling rate of 15~30℃ / s and a final cooling temperature ≤300℃; (4) Heat treatment: The offline two-phase quenching and tempering (LT) process is adopted, followed by water cooling.

[0012] Step (2) of the present invention: heating the slab: heating the continuously cast slab to 1180~1220℃, and holding it for a time of 0.8~1.2 times the slab thickness, wherein the holding time is in min and the slab thickness is in mm.

[0013] Step (3) of the present invention controls rolling and cooling: accelerated cooling is carried out immediately after rolling, with a cooling rate of 15-30℃ / s and a final cooling temperature of ≤300℃.

[0014] The heat treatment in step (4) of this invention is as follows: the quenching heating temperature is 670-700℃, the holding time is 1.5-2.5 times the plate thickness, and then water quenching is performed to room temperature; the holding time is in min and the plate thickness is in mm.

[0015] The heat treatment in step (4) of this invention: the tempering heating temperature is 590~620℃, the holding time is 2.5~4.0 times the plate thickness, the holding time is in min, and the plate thickness is in mm.

[0016] The beneficial effects of adopting the above technical solution are as follows: 1. This invention uses "Cu precipitation strengthening" + "Nb fine grain / precipitation strengthening" to significantly improve the strength of the material and maintain good low-temperature toughness by adding a small amount of Cu and Nb elements without relying on expensive alloying elements, while achieving better cost-effectiveness. 2. This invention provides an extremely fine and uniform original microstructure for subsequent heat treatment through optimized TMCP process (especially large reduction and controlled cooling in the non-recrystallization zone), so that the combination of ultra-high strength and high and low temperature toughness that can only be achieved by traditional QLT process can be obtained by simply using LT heat treatment, eliminating one quenching, lowering energy consumption, shortening the process, and increasing production efficiency. 3. The 9Ni steel obtained by this invention has a yield strength exceeding 850MPa, an impact energy exceeding 150J at -196℃, and a yield strength ratio controlled below 0.91, which is significantly better than 9Ni steel treated by traditional QT process, and has performance comparable to or even better than QLT process products, while also possessing excellent weldability. Attached Figure Description

[0017] Figure 1 This is a 500x magnification micrograph of the steel plate from Example 1. Figure 2 This is a 5000x magnification micrograph of the steel plate from Example 1. Figure 3 This is a transmission electron microscope (TEM) image of the steel plate from Example 1; Figure 4 The image shows the electron backscatter diffraction (EBSD) phase distribution of the steel plate in Example 1, where blue represents bcc ferrite and red represents fcc austenite. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with embodiments.

[0019] Examples 1-6 A method for preparing ultra-high strength, high toughness, and low-cost 9Ni steel includes the following steps: (1) Smelting and continuous casting: The above composition is smelted in a converter or electric furnace, refined by LF, degassed by VD or RH vacuum, and then calcium treated, and then continuously cast into billets; (2) Slab heating: Heat the continuously cast slab to 1180~1220℃ and hold it for (slab thickness mm × (0.8~1.2)) min to ensure that the alloying elements are fully dissolved; (3) Controlled rolling and controlled cooling: Two-stage rolling is adopted. The first stage is the recrystallization zone rolling, with an initial rolling temperature ≥1050℃ and a cumulative reduction rate >60%; the second stage is the non-recrystallization zone rolling, with an initial rolling temperature ≤850℃ and a final rolling temperature controlled at 750~810℃, with a cumulative reduction rate >70%; accelerated cooling is carried out immediately after rolling, with a cooling rate of 15~30℃ / s and a final cooling temperature ≤300℃; (4) Heat treatment: Offline two-phase quenching and tempering (LT) process is adopted. The quenching heating temperature is 670~700℃, the holding time is (plate thickness mm × (1.5~2.5)) min, and then water quenching to room temperature; the tempering heating temperature is 590~620℃, the holding time is (plate thickness mm × (2.5~4.0)) min, and then water cooling.

[0020] The chemical composition of the obtained steel plate is shown in Table 1. The slab thickness is 220 mm. The finished product thickness is 20 mm. The performance test results are shown in Table 2. The control parameters for each process are shown in Table 3.

[0021] Figure 1 This is a 500x magnification micrograph of the steel plate from Example 1. Figure 2 This is a 5000x magnification micrograph of the steel plate from Example 1. Figure 3 This is a transmission electron microscope (TEM) image of the steel plate from Example 1; Figure 4The diagram shows the electron backscatter diffraction (EBSD) phase distribution of the steel plate in Example 1, where blue represents bcc ferrite and red represents fcc austenite. The corresponding figures for the other examples are similar, so they are omitted.

[0022] Figure 1 (Example 1) and Figure 2 The tempered lath martensite matrix and the dispersed reverse austenite are clearly presented; Figure 3 (Example 1) Morphological characteristics of reversed austenite were observed by TEM; Figure 4 (Example 1) The content and uniform distribution of reversed austenite (red area) were visually verified by the EBSD phase distribution, which is consistent with the data in Table 4.

[0023] Table 1 Chemical composition (wt.%) of steel plates from Examples 1-6

[0024] The balance in Table 1 is Fe and unavoidable impurities.

[0025] Table 2 Control parameters for slab heating, controlled rolling, and controlled cooling processes in Examples 1-6

[0026] Table 3. Control parameters for heat treatment processes in Examples 1-6

[0027] Table 4 Properties of steel plates from Examples 1-6

[0028] The results of the above embodiments show that the present invention significantly improves the strength of 9Ni steel by adding Cu and Nb in combination and optimizing the process, while simplifying the heat treatment, and maintains excellent low-temperature toughness, which fully meets the requirements for high-performance LNG storage tanks.

[0029] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A high-strength, high-toughness, low-cost 9Ni steel, characterized in that, The chemical composition of the 9Ni steel, by mass percentage, is as follows: C: 0.03–0.05%, Si: 0.15–0.30%, Mn: 0.40–0.70%, P≤0.005%, S≤0.002%, Ni: 8.50–9.50%, Cu: 0.30–0.60%, Nb: 0.02–0.04%, Alt: 0.020–0.050%, N≤0.004%, O≤0.002%, with the remainder being Fe and unavoidable impurities.

2. The ultra-high strength, high toughness, and low-cost 9Ni steel according to claim 1, characterized in that, The quality properties of the 9Ni steel meet the following requirements: yield strength (Rp0.2) ≥ 850 MPa, tensile strength (Rm) ≥ 940 MPa, and elongation after fracture (A) ≥ 20%.

3. The ultra-high strength, high toughness, and low-cost 9Ni steel according to claim 1 or 2, characterized in that, The quality properties of the 9Ni steel meet the following requirements: average Charpy V-notch impact energy (KV2) at 196℃ ≥150J, single value ≥120J, and yield strength ratio ≤0.

91.

4. The ultra-high strength, high toughness, low cost 9Ni steel according to claim 1 or 2, characterized in that, The microstructure of the 9Ni steel consists of a fine tempered lath martensitic matrix with 5-10% reversed austenite distributed on it.

5. A method for preparing ultra-high strength, high toughness, and low-cost 9Ni steel according to any one of claims 1-4, characterized in that, The preparation method includes the following steps: (1) Smelting and continuous casting: The above composition is smelted in a converter or electric furnace, refined by LF, degassed by VD or RH vacuum, and then calcium treated, and then continuously cast into billets; (2) Slab heating: Heating the continuously cast slab to ensure that the alloying elements are fully dissolved; (3) Controlled rolling and controlled cooling: Two-stage rolling is adopted. The first stage is the recrystallization zone rolling, with an initial rolling temperature ≥1050℃ and a cumulative reduction rate >60%; the second stage is the non-recrystallization zone rolling, with an initial rolling temperature ≤850℃ and a final rolling temperature controlled at 750~810℃, and a cumulative reduction rate >70%; accelerated cooling is carried out immediately after rolling. (4) Heat treatment: offline two-phase quenching + tempering process is adopted, followed by water cooling.

6. The method for preparing ultra-high strength, high toughness, and low-cost 9Ni steel according to claim 5, characterized in that, Step (2) Slab heating: The continuously cast slab is heated to 1180~1220℃ and the holding time is 0.8~1.2 times the slab thickness. The holding time is in min and the slab thickness is in mm.

7. The method for preparing ultra-high strength, high toughness, and low-cost 9Ni steel according to claim 5, characterized in that, Step (3) Control rolling and control cooling: Accelerated cooling is carried out immediately after rolling, with a cooling rate of 15-30℃ / s and a final cooling temperature of ≤300℃.

8. The method for preparing ultra-high strength, high toughness, and low-cost 9Ni steel according to any one of claims 5-7, characterized in that, The heat treatment in step (4) is as follows: the quenching heating temperature is 670-700℃, the holding time is 1.5-2.5 times the plate thickness, and then the plate is water quenched to room temperature; the holding time is in min and the plate thickness is in mm.

9. The method for preparing ultra-high strength, high toughness, and low-cost 9Ni steel according to any one of claims 5-7, characterized in that, The heat treatment in step (4) is as follows: the tempering heating temperature is 590~620℃, and the holding time is 2.5~4.0 times the plate thickness. The holding time is in min and the plate thickness is in mm.

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