High-ctod-value 9ni steel plate and method for manufacturing the same

Abstract

The present application relates to the field of steel material preparation, in particular to a high CTOD value 9Ni steel plate and a manufacturing method thereof. The steel plate is composed of the following chemical components with weight percentage: C: 0.04%~0.06%, Si: 0.05%~0.10%, Mn: 0.60%~0.80%, Ni: 8.90%~9.50%, Mo: 0.25%~0.35%, Nb: 0.020%~0.030%, Ti: 0.010%~0.020%, P≤0.008%, S≤0.001%, and the balance of Fe and inevitable impurities. The manufacturing method comprises: converter double smelting→LF+RH refining→slab continuous casting→slab slow cooling→slab heating→controlled rolling→laminar cooling→QLT heat treatment. The microstructure of the steel plate is tempered sorbite+5%~10% reverted austenite, the CTOD value at-163℃ is greater than or equal to 0.45mm, the yield strength is greater than or equal to 585MPa, the tensile strength is 680~820MPa, the elongation is greater than or equal to 20%, and the impact energy at-196℃ is greater than or equal to 150J. The present application realizes the unity of excellent strength and toughness matching and high CTOD value.

Description

Technical Field

[0001] This invention relates to the field of steel material preparation, specifically to a high CTOD value 9Ni steel plate and its manufacturing method. Background Technology

[0002] Natural gas, as one of the most widely used clean energy sources, continues to see increasing demand. It is predicted that global LNG demand will grow by more than 50% by 2040, reaching 625 million to 685 million tons annually. LNG operates at a temperature of -163°C, requiring materials used to construct LNG storage containers to possess sufficient strength and excellent cryogenic toughness. Furthermore, large onshore LNG storage tanks are required to operate for 50 years without maintenance, placing extremely stringent demands on the low-temperature fatigue performance and fracture toughness of the materials.

[0003] 9Ni steel, with its stable strength and excellent low-temperature toughness, is widely used in the construction of LNG storage tanks. By adding approximately 9% Ni and employing appropriate heat treatment processes, this steel can achieve a microstructure dominated by tempered martensite with a certain amount of reverse-transformed austenite, thus maintaining a good balance of strength and toughness under ultra-low temperature conditions. However, as LNG storage tanks become larger, higher demands are placed on the fracture toughness of 9Ni steel. Crack tip opening displacement (CTOD), as an important indicator for evaluating a material's resistance to brittle fracture, is directly related to the structural integrity and safety of the storage tank during service. Currently, literature reports that the CTOD value of 9Ni steel plates can reach above 0.38 mm at -163℃, but further improving the CTOD value to enhance the safety margin of the storage tank remains a key area of ​​ongoing research in this field.

[0004] Numerous patent applications have been filed regarding the composition optimization and preparation process of 9Ni steel. Chinese patent document CN115786812A discloses "9Ni steel plate for large-thickness, low-yield-strength-ratio LNG storage tanks and its production method," which reduces the yield-strength ratio by controlling rolling and heat treatment to obtain a tempered martensite + ferrite + reverse-transformed austenite microstructure. Chinese patent CN108220802B discloses "An improved 9Ni material suitable for large-section components and its preparation method," which improves fracture toughness and hardenability by optimizing alloy element content. Chinese patent CN119800230B discloses "9Ni steel plate and its production method," which addresses the problem of excessively high yield-strength ratio by controlling heating, rolling, and tempering processes. All of these prior technologies aim to optimize strength, low-temperature impact toughness, or reduce the yield-strength ratio, but none address improving the -163℃ ultra-low temperature CTOD value of 9Ni steel. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention provides a high CTOD value 9Ni steel plate and its manufacturing method. Under the premise of ensuring good strength and toughness matching of 9Ni steel, the CTOD value of the steel plate at ultra-low temperature is significantly improved, and the CTOD value of the steel plate at -163℃ is ≥0.45mm.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A high CTOD value 9Ni steel plate is composed of the following chemical composition by weight percentage: C: 0.04%~0.06%, Si: 0.05%~0.10%, Mn: 0.60%~0.80%, Ni: 8.90%~9.50%, Mo: 0.25%~0.35%, Nb: 0.020%~0.030%, Ti: 0.010%~0.020%, P≤0.008%, S≤0.001%, with the balance being Fe and unavoidable impurities.

[0007] The effect of selecting the above alloying elements and their contents: 1. C: Carbon is the most effective chemical element for increasing the strength of steel, but at the same time, carbon significantly reduces the toughness of steel and impairs its weldability and fatigue performance. Therefore, this invention precisely controls the C content to 0.04%~0.06%.

[0008] 2. Si: Silicon is detrimental to the toughness and CTOD value of steel. Therefore, the Si content is precisely controlled at 0.05%~0.10% in this invention.

[0009] 3. Mn: Manganese can improve the hardenability, strength, and toughness of steel. However, excessive manganese content can easily lead to the formation of MnS inclusions, causing cracks. Therefore, this invention precisely controls the Mn content to be between 0.60% and 0.80%.

[0010] 4. Ni: Nickel can improve the strength of steel and also give it excellent low-temperature toughness. Nickel is one of the elements that infinitely expands the austenite region. Therefore, after QLT heat treatment, 9Ni steel can obtain a fully refined tempered sorbite structure and 5-10% reverse-transformed austenite, which blunts and bifurcates crack tips, relieves stress concentration, and significantly improves the CTOD value. Therefore, this invention precisely controls the Ni content at 8.90%~9.50%.

[0011] 5. Mo: Molybdenum strongly inhibits temper brittleness, adsorbs and fixes harmful impurities, prevents their segregation to grain boundaries, pins grain boundaries, inhibits austenite grain growth, refines grains, and promotes the formation of reverse-transformed austenite. However, excessive Mo easily forms coarse intermetallic compounds, causing a decrease in the CTOD value. Therefore, this invention precisely controls the Mo content at 0.25%~0.35%.

[0012] 6. Nb: Niobium can form fine compounds with carbon and nitrogen during the two-stage controlled rolling process, inhibiting austenite recrystallization and refining austenite grains. Therefore, the present invention precisely controls the Nb content to 0.020%~0.030%.

[0013] 7. Ti: TiN exhibits extremely high-temperature stability and is not easily completely dissolved during billet heating, effectively preventing abnormal austenite grain growth during slab heating. Furthermore, similar to Nb, titanium can also form fine compounds with carbon and nitrogen during the two-stage controlled rolling process, inhibiting austenite recrystallization and refining austenite grains. Therefore, this invention precisely controls the Ti content to 0.010%~0.020%.

[0014] 8. S: Sulfur easily forms FeS and MnS inclusions in steel, causing hot brittleness and significantly reducing the toughness and fatigue performance of steel. Therefore, the sulfur content in steel should be reduced as much as possible. In this invention, the S content is controlled to be ≤0.001%.

[0015] 9. P: Phosphorus often segregates at grain boundaries in steel, disrupting the continuity of the matrix, significantly reducing the toughness and fatigue performance of steel, and worsening its weldability. Therefore, the phosphorus content in steel should be reduced as much as possible. In this invention, the P content is controlled to be ≤0.008%.

[0016] The microstructure of the aforementioned high CTOD value 9Ni steel plate is tempered sorbite + reverse-transformed austenite, with the volume percentage of reverse-transformed austenite being 5%~10%. The CTOD value of the steel plate at -163℃ is ≥0.45mm. The yield strength of the steel plate is ≥585MPa, the tensile strength is 680~820MPa, the elongation is ≥20%, the V-notch impact energy at -196℃ is ≥150J, and the lateral expansion value is ≥1.0mm. The gas content in the steel plate is controlled as follows: [N]≤0.0040%, [H]≤0.00010%, [O]≤0.0010%. The thickness of the steel plate is 20~50mm.

[0017] The manufacturing method of the above-mentioned high CTOD value 9Ni steel plate includes the following steps: 1) Smelting: The process involves a converter double smelting process, first dephosphorizing and then decarburizing; followed by LF refining and RH vacuum refining.

[0018] 2) Continuous casting and slow cooling: The molten steel obtained in step 1) is continuously cast to obtain a slab, which is then heated and slowly cooled.

[0019] 3) Slab heating: The slab obtained in step 2) is heated to a temperature of 1200~1250℃ and heated in the furnace for 5~6 hours.

[0020] 4) Controlled rolling: The slab obtained in step 3) is subjected to two-stage controlled rolling. The first stage rolling temperature is ≥1050℃; the second stage rolling temperature is 850~890℃, and the final rolling temperature is 800~830℃.

[0021] 5) Laminar flow cooling: The steel plate obtained in step 4) is subjected to laminar flow cooling; the inlet water temperature for laminar flow cooling is 800~830℃, the final cooling temperature is 400~450℃, and it is air-cooled after cooling.

[0022] 6) QLT heat treatment: The steel plate obtained in step 5) is subjected to QLT heat treatment, including first quenching, second quenching and tempering; the first quenching temperature is 800~830℃, the second quenching temperature is 690~720℃, and the tempering temperature is 580~620℃.

[0023] Further, in step 1), the temperature of the dephosphorization furnace in the converter duplex smelting is 1350~1400℃, with an endpoint P≤0.005%; the temperature of the decarburization furnace is 1650~1700℃; the LF refining deep desulfurization is carried out to S≤0.001%, and calcium treatment is performed to modify inclusions; the RH vacuum refining dehydrogenates H to ≤1.0ppm and N to ≤40ppm.

[0024] Further, in step 2), the casting speed of the continuous casting is 0.8~1.0m / min, the superheat of the tundish is 15~20℃, the entire process is protected during casting and electromagnetic stirring is used; the heating and slow cooling is: heating to 600℃ at a heating rate of ≤50℃ / h, holding at ≥24h, and then slowly cooling to <300℃ with the furnace before air cooling.

[0025] Furthermore, in step 4), the compression ratio of the rolling process is controlled to be ≥4.

[0026] Furthermore, in step 4), the single-pass reduction rate in the first stage is ≥15%.

[0027] Furthermore, in step 6), the first quenching holding time is 3~4 min / mm, the second quenching holding time is 3~4 min / mm, and the tempering holding time is 5~6 min / mm.

[0028] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention employs a low-Si design, avoiding the adverse effects of high Si content on CTOD values ​​in conventional 9Ni steel. Simultaneously, it utilizes Mo+Nb+Ti synergistic microalloying. Mo suppresses temper brittleness, pins grain boundaries, and promotes the formation of reverse austenite. Nb and Ti, during controlled rolling, form fine carbonitrides with carbon and nitrogen to inhibit austenite recrystallization and refine grains. The synergistic effect of these three elements achieves multi-stage effective control over austenite grain growth behavior: TiN pins grain boundaries during heating, Nb (CN) suppresses recrystallization during rolling, and Mo promotes the formation of reverse austenite during tempering, laying the microstructural foundation for achieving a high CTOD value. Furthermore, strict control of P, S, and gas content effectively avoids the damage to low-temperature toughness caused by harmful elements.

[0029] 2. This invention employs a converter double-stage smelting combined with LF+RH double-stage refining to achieve high purity of the molten steel, ensuring the low-temperature toughness of the steel plate from the source. Slow cooling of the slab effectively reduces core segregation and improves microstructure uniformity. In the two-stage controlled rolling process, the first stage achieves fine austenite microstructure through static recrystallization; the second stage ensures sufficient core deformation, promoting austenite flattening and increasing intragranular deformation bands. Post-rolling laminar cooling rapidly crosses the austenite grain growth sensitive temperature range, inhibiting the precipitation of coarse carbides at grain boundaries, refining the original austenite grains and martensite laths, and weakening banded microstructure. Finally, QLT heat treatment yields a homogeneous microstructure of fine-grained tempered sorbite + 5%~10% reverse-transformed austenite.

[0030] 3. First, this invention utilizes a low-Si design combined with Mo+Nb+Ti microalloying to minimize the damage of Si to the CTOD value while ensuring sufficient deoxidation of the molten steel. Simultaneously, the grain refinement and precipitation strengthening effects of Mo, Nb, and Ti compensate for the potential strength loss caused by low Si. Second, the carbonitride precipitation behavior of Nb and Ti is precisely matched with the temperature regime controlling the rolling process. Heating the slab allows for the full dissolution of Nb and Ti, and during rolling, fine Nb(CN) and TiN particles precipitate at grain boundaries and subgrain boundaries, effectively pinning grain boundaries, inhibiting recrystallization and grain growth. Combined with rolling in the non-recrystallized zone, this significantly increases the effective grain boundary area of ​​the γ-α phase deformation nuclei, significantly refining the post-phase transformation microstructure. Third, the Mo element is matched with the tempering temperature window of QLT heat treatment. Within this temperature range, Mo effectively suppresses temper brittleness, promotes the full precipitation of reverse-transformed austenite during tempering, and ensures its stable retention after cooling to room temperature, resulting in 5%~10% reverse-transformed austenite.

[0031] In summary, this invention achieves a synergistic match between composition and process through a low-Si+Mo+Nb+Ti synergistic microalloying design, combined with a complete process of high-purity smelting, controlled rolling, laminar flow cooling, and QLT heat treatment. This results in a refined microstructure of tempered sorbite + 5%~10% reverse-transformed austenite. The reverse-transformed austenite, acting as a toughening phase, undergoes plastic deformation under stress at the crack tip, blunting and bifurcating the crack tip, effectively alleviating stress concentration, significantly delaying crack propagation, and thus greatly improving the ultra-low temperature CTOD value of the steel plate. The prepared 9Ni steel plate exhibits a yield strength ≥585MPa, tensile strength of 680~820MPa, elongation ≥20%, V-notch impact energy ≥150J at -196℃, lateral expansion ≥1.0mm, and a CTOD value ≥0.45mm at -163℃, achieving a balance between excellent strength and toughness and a high CTOD value. Attached Figure Description

[0032] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention. Detailed Implementation

[0033] This invention discloses a high CTOD value 9Ni steel plate and its manufacturing method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0034] The production process of this invention is as follows: converter duplex smelting → LF+RH refining → slab continuous casting → slab slow cooling → slab heating → controlled rolling → laminar flow cooling → QLT heat treatment.

[0035] The following is a detailed description of each process: 1) Smelting The process employs a dual-phase converter smelting method, first dephosphorizing and then decarburizing. The dephosphorization furnace temperature is 1350~1400℃, with bottom blowing and strong stirring, and the final P value is ≤0.005%. The decarburization furnace temperature is 1650~1700℃, with rapid decarburization and temperature increase, and the slag can be returned to the dephosphorization furnace.

[0036] Subsequently, the steel undergoes LF refining and RH vacuum refining: LF deep desulfurization to S≤0.001%, and calcium treatment to modify inclusions (MnS→spherical), and fine adjustment of Ni and Ti composition; RH vacuum desulfurization to H≤1.0ppm and N≤40ppm, removing ≤5μm fine inclusions and purifying the molten steel.

[0037] 2) Continuous casting and slow cooling The molten steel obtained in step 1) is continuously cast at a casting speed of 0.8~1.0 m / min and a tundish superheat of 15~20℃. The casting process is protected and electromagnetic stirring is used to reduce secondary oxidation during continuous casting, reduce the inclusion content in the steel, improve the purity of the steel, and obtain a slab.

[0038] After the slabs are cast and removed from the continuous casting line, they are placed in a slow cooling pit for heating and slow cooling. The initial heating temperature is ≥400℃, and the temperature is increased to 600℃ at a heating rate of ≤50℃ / h. After holding at this temperature for ≥24 hours, the slabs are slowly cooled in the furnace to <300℃ and then air-cooled to room temperature. The purpose of heating and slow cooling is to reduce the degree of core segregation through heating.

[0039] 3) Slab heating The slab obtained in step 2) is heated to 1200~1250℃ and heated in the furnace for 5~6 hours to ensure that Mo, Ni, Nb and Ti are completely dissolved into the matrix and diffused evenly.

[0040] 4) Controlled rolling The slab obtained in step 3) is subjected to two-stage controlled rolling, with the compression ratio (slab thickness / finished slab thickness) controlled to be ≥4, in order to ensure deep core deformation.

[0041] The initial rolling temperature is ≥1050℃, and the single-pass reduction rate is ≥15%. In this stage, the original coarse austenite structure is transformed into a fine and uniform austenite structure through static recrystallization, giving full play to the grain refinement effect of controlling rolling.

[0042] The two-stage initial rolling temperature is 850~890℃, and the final rolling temperature is 800~830℃. During rolling in the non-recrystallization zone at 850~890℃, the flattening of austenite and the increase in the number of intragranular deformation bands after rolling deformation increase the effective grain boundary area of ​​the γ-α phase deformation nuclei and improve the phase deformation nucleation rate, thereby obtaining a refined microstructure and fully utilizing the microalloying effect of Nb and Ti.

[0043] 5) Laminar flow cooling The steel plate obtained in step 4) was subjected to laminar flow cooling with an initial water temperature of 800~830℃ and a final cooling temperature of 400~450℃. After cooling, it was air-cooled to obtain a bainite + a small amount of martensite microstructure. Laminar flow cooling can quickly cross the sensitive temperature range for austenite grain growth, inhibit the precipitation of coarse carbides at grain boundaries, refine the original austenite grains and martensite laths, weaken the banded structure, improve the uniformity of the microstructure, and significantly improve the CTOD value of 9Ni steel at -163℃.

[0044] 6) QLT heat treatment The steel plate obtained in step 5) is subjected to QLT heat treatment, including a first quench, a second quench, and tempering. The first quench temperature is 800–830℃, and the holding time is 3–4 min / mm; the second quench temperature is 690–720℃, and the holding time is 3–4 min / mm; the tempering temperature is 580–620℃, and the holding time is 5–6 min / mm. The purpose of QLT heat treatment is to obtain tempered sorbite with uniform composition and microstructure + 5%–10% reverse-transformed austenite, ensuring that 9Ni steel obtains good strength, low-temperature toughness, and CTOD performance.

[0045] The chemical composition of the embodiments of the present invention is shown in Table 1, the smelting and continuous casting process is shown in Table 2, the slow cooling process of the continuous casting billet is shown in Table 3, the rolling and heating process is shown in Table 4, the QLT heat treatment process is shown in Table 5, and the mechanical properties are shown in Table 6.

[0046] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0047] Table 2 Smelting and continuous casting processes of embodiments of the present invention

[0048] Table 3 Slow Cooling Process of Continuous Casting Billet in Embodiments of the Invention

[0049] Table 4 Rolling and heating processes of embodiments of the present invention

[0050] Table 5. QLT heat treatment process of the present invention embodiments

[0051] Table 6 Mechanical properties of embodiments of the present invention

[0052] As shown in Table 6, the 9Ni steel plates prepared in the various embodiments of the present invention have excellent mechanical properties: yield strength of 650~675MPa, tensile strength of 755~780MPa, elongation of 23%~25%, V-notch impact energy at -196℃ of 175~183J, lateral expansion value of 1.81~1.93mm, CTOD value at -163℃ of 0.50~0.55mm, and reverse austenite content of 7%~10%.

[0053] Comparative Example 1 The same process steps as in Example 1 were used, the only difference being that the Si content in the chemical composition was 0.22% (exceeding the 0.05%~0.10% range defined in this invention), while the remaining components were the same as in Example 1. Testing showed that the CTOD value at -163℃ was 0.28 mm, and the V-notch impact energy at -196℃ was 142 J, significantly lower than that of the embodiment of this invention (CTOD value at -163℃ was 0.50~0.55 mm, and impact energy at -196℃ was 175~183 J). This indicates that when the Si content increases to a conventional level, both the CTOD value and low-temperature impact toughness decrease significantly, verifying the necessity of the low-Si design of this invention.

[0054] Comparative Example 2 The same chemical composition and process steps as Example 1 were used, the only difference being the absence of Mo (i.e., Mo content was 0%). All other components were identical to Example 1. Testing revealed that the reverse austenite content of this comparative steel plate was only 2.8%, and the CTOD value at -163℃ was only 0.31 mm, significantly lower than that of the present invention (reverse austenite content of 7%~10%, CTOD value at -163℃ 0.50~0.55 mm). This indicates that the addition of Mo plays a crucial role in promoting the formation of reverse austenite and improving the CTOD value; even with the same QLT heat treatment process, sufficient reverse austenite cannot be obtained without Mo.

[0055] Comparative Example 3 Using the same chemical composition as Example 1, but employing conventional quenching and tempering (i.e., one quenching + high-temperature tempering, without a second quenching), the quenching temperature was 830℃ with a holding time of 3 min / mm, and the tempering temperature was 620℃ with a holding time of 6 min / mm. Testing revealed that the reverse-transformed austenite content of this comparative steel plate was only 3.2%, and the CTOD value at -163℃ was only 0.33 mm, significantly lower than that of the embodiment of this invention (reverse-transformed austenite content of 7%~10%, and CTOD value at -163℃ of 0.50~0.55 mm). This indicates that the second quenching (sub-temperature quenching at 690~720℃) in QLT heat treatment plays an irreplaceable role in obtaining sufficient reverse-transformed austenite and achieving a high CTOD value.

[0056] As can be seen from the above comparative examples, the high Si content in Comparative Example 1 severely impaired the ultra-low temperature CTOD value and impact toughness of the steel plate; in Comparative Example 2, the absence of Mo led to a significant decrease in the reverse austenite content and CTOD value; in Comparative Example 3, the use of conventional quenching and tempering treatment instead of the QLT process also resulted in a significant reduction in the reverse austenite content and CTOD value. The embodiments of this invention, through the combined effects of low-Si design, Mo+Nb+Ti synergistic microalloying, and QLT heat treatment, achieved a reverse austenite content of 7%~10% and a CTOD value of 0.50~0.55 mm at -163℃, significantly superior to the comparative examples.

[0057] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high CTOD value 9Ni steel plate, characterized in that, It is composed of the following chemical components in weight percentage: C: 0.04%~0.06%, Si: 0.05%~0.10%, Mn: 0.60%~0.80%, Ni: 8.90%~9.50%, Mo: 0.25%~0.35%, Nb: 0.020%~0.030%, Ti: 0.010%~0.020%, P≤0.008%, S≤0.001%, balance being Fe and unavoidable impurities; The microstructure of the steel plate is tempered sorbite + reverse-transformed austenite, wherein the volume percentage of reverse-transformed austenite is 5%~10%. The steel plate has a CTOD value ≥ 0.45 mm at -163℃.

2. The high CTOD value 9Ni steel plate according to claim 1, characterized in that, The steel plate has a yield strength ≥585MPa, a tensile strength of 680~820MPa, an elongation ≥20%, a V-shaped impact energy at -196℃ ≥150J, and a lateral expansion value ≥1.0mm.

3. The high CTOD value 9Ni steel plate according to claim 1, characterized in that, The gas content in the steel plate is controlled as follows: [N]≤0.0040%, [H]≤0.00010%, [O]≤0.0010%.

4. A high CTOD value 9Ni steel plate according to claim 1, characterized in that, The thickness of the steel plate is 20~50mm.

5. A method for manufacturing a high CTOD value 9Ni steel plate as described in any one of claims 1 to 4, characterized in that, Includes the following steps: 1) Smelting: The process involves a double-phase converter smelting process, followed by dephosphorization and decarburization; then, it undergoes LF refining and RH vacuum refining. 2) Continuous casting and slow cooling: The molten steel obtained in step 1) is continuously cast to obtain a slab, and the slab is heated and slowly cooled. 3) Slab heating: The slab obtained in step 2) is heated to a temperature of 1200~1250℃ and the heating time in the furnace is 5~6h. 4) Controlled rolling: The slab obtained in step 3) is subjected to two-stage controlled rolling. The first stage rolling temperature is ≥1050℃; the second stage rolling temperature is 850~890℃, and the final rolling temperature is 800~830℃. 5) Laminar flow cooling: The steel plate obtained in step 4) is subjected to laminar flow cooling; the inlet water temperature for laminar flow cooling is 800~830℃, the final cooling temperature is 400~450℃, and it is then air-cooled. 6) QLT heat treatment: The steel plate obtained in step 5) is subjected to QLT heat treatment, including first quenching, second quenching and tempering; the first quenching temperature is 800~830℃, the second quenching temperature is 690~720℃, and the tempering temperature is 580~620℃.

6. The method for manufacturing a high CTOD value 9Ni steel plate according to claim 5, characterized in that, In step 1), the temperature of the dephosphorization furnace in the converter duplex smelting is 1350~1400℃, and the endpoint P≤0.005%; the temperature of the decarburization furnace is 1650~1700℃. The LF is refined and desulfurized to S≤0.001%, and then calcium treatment is performed to modify inclusions; The RH vacuum refining process removes H to ≤1.0ppm and N to ≤40ppm.

7. A method for manufacturing a high CTOD value 9Ni steel plate according to claim 5, characterized in that, In step 2), the casting speed is 0.8~1.0 m / min, the tundish superheat is 15~20℃, and electromagnetic stirring is used during the entire casting process with full protection. The heating and slow cooling process involves heating the furnace at a rate of ≤50℃ / h to 600℃, holding the temperature for ≥24h, and then slowly cooling it to <300℃ before air cooling.

8. A method for manufacturing a high CTOD value 9Ni steel plate according to claim 5, characterized in that, In step 4), the compression ratio of the rolling process is controlled to be ≥4.

9. A method for manufacturing a high CTOD value 9Ni steel plate according to claim 5, characterized in that, In step 4), the single-pass reduction rate in the first stage is ≥15%.

10. A method for manufacturing a high CTOD value 9Ni steel plate according to claim 5, characterized in that, In step 6), the first quenching holding time is 3~4 min / mm, and the second quenching holding time is 3~4 min / mm. The tempering holding time is 5~6 min / mm.

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