Nb-containing 5.5Ni steel plate based on cooling hot rolling process and preparation method thereof

Abstract

The present application relates to a kind of Nb-containing 5.5Ni steel plate based on cooling hot rolling process and its preparation method, belong to alloy steel manufacturing technical field.The present application provides the chemical composition of the Nb-containing 5.5Ni steel plate according to percentage by weight: C:0.03~0.06%, Si:0.05~0.12%, Mn:0.52~1.10%, Ni:5.20~5.70%, Mo:0.10~0.30%, Nb:0.01~0.02%, P:≤0.006%, S:≤0.005%, the balance is Fe and inevitable impurities.The preparation method of the present application is based on the process flow heat treatment process of cooling rolling, two-phase zone quenching plus tempering on the basis of saving Ni alloy, obtains Nb-containing 5.5Ni steel plate with excellent low temperature toughness and high strength plasticity, comprehensive mechanical property reaches 9Ni steel level, realizes the low cost of material.

Description

Technical Field

[0001] This invention belongs to the field of alloy steel manufacturing technology, specifically relating to a niobium-containing 5.5Ni steel plate based on a cooling hot rolling process and its preparation method. Background Technology

[0002] LNG is a liquid formed by liquefying natural gas at -162°C and 0.1 MPa, with a volume approximately 1 / 625th that of gaseous natural gas. Given the unique physicochemical properties of LNG, from a safety perspective, the structural materials for cryogenic storage and transportation facilities require a combination of high strength, excellent cryogenic toughness, good weldability, and ease of processing. Currently, commonly used materials for cryogenic environments include austenitic stainless steel, Ni-based alloys, aluminum alloys, and Ni-based 9Ni steel for cryogenic applications. Compared to austenitic stainless steel and Ni-based alloys, 9Ni steel has lower alloy costs and higher strength; compared to aluminum alloys, 9Ni steel has higher strength and better weldability. Therefore, 9Ni steel is widely used internationally as the inner structural material for cryogenic storage and transportation facilities.

[0003] In recent years, to reduce the production cost of 9Ni steel and its dependence on high-priced Ni, the development of Ni-saving steel plates has become an important direction for Ni-based ultra-low temperature steels. However, the reduction in Ni content in 7Ni steel is limited, so it is necessary to continue reducing Ni content.

[0004] Patent CN201410600875.5 discloses a technology for producing 5Ni steel plates for cryogenic pressure vessels, employing a low-carbon equivalent composition design and a controlled rolling and cooling + heat treatment production process to produce 8-50mm thick continuously cast 5Ni steel plates that meet the requirements of cryogenic pressure vessels. The product features high purity, uniform composition, excellent mechanical properties, good low-temperature impact toughness, excellent weldability, low production cost, and excellent transverse impact strength at -125℃, enabling mass production. However, the applicable temperature range of this patent is -45℃ to -120℃, which cannot meet the service requirements of LNG storage tanks. Furthermore, the heat treatment process, involving high-temperature quenching + sub-temperature quenching + tempering, is complex and energy-intensive.

[0005] Patent CN202310562946.6 discloses a 5.5Ni steel plate for ultra-low temperature environments and its manufacturing method. It improves the hardenability of the steel plate and ensures the core performance by adding Cr and Mo elements. The corresponding mechanical properties are achieved. The tensile strength Rm of the steel plate in the transverse and longitudinal directions is 700-830 MPa, the yield strength Rel≥590 MPa, and the elongation A≥21%. The samples were subjected to room temperature bending at 180° with D=3a and b=2a, and no cracks were observed. Furthermore, under ultra-low temperature conditions of -196℃, the impact energy absorbed by the impact sample KV2≥100J. However, the heat treatment process of this patent, which involves high-temperature quenching + sub-temperature quenching + tempering, is relatively complex, energy-intensive, and has a low impact energy at -196℃. Summary of the Invention

[0006] The purpose of this invention is to produce a niobium-containing 5.5Ni steel plate with strength comparable to 9Ni steel plate and a low-temperature impact energy of ≥150J at -196℃, based on a cooled hot rolling process. The technical problem this invention aims to solve is to overcome the high energy consumption of existing technologies and provide a method for preparing a niobium-containing 5.5Ni steel plate based on a cooled hot rolling process. This manufacturing method, while saving Ni alloy, employs a heat treatment process of cooled rolling, two-phase quenching, and tempering to obtain a niobium-containing 5.5Ni steel plate with excellent low-temperature toughness and high strength-plasticity. Its comprehensive mechanical properties reach the level of 9Ni steel, allowing it to replace 9Ni steel in the construction of LNG storage tanks and other applications, thus achieving low-cost material production.

[0007] A niobium-containing 5.5Ni steel plate based on a cooling hot rolling process, wherein the chemical composition of the niobium-containing 5.5Ni steel plate is as follows by weight percentage: C: 0.03-0.06%, Si: 0.05-0.12%, Mn: 0.52-1.10%, Ni: 5.20-5.70%, Mo: 0.10-0.30%, Nb: 0.01-0.02%, P: ≤0.006%, S: ≤0.005%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, the mechanical properties of the niobium-containing 5.5Ni steel plate are as follows: room temperature yield strength of 585-620 MPa, room temperature tensile strength of 700-750 MPa, room temperature elongation after fracture of 22-26%, and impact energy at -196℃ of 150-200 J.

[0009] A method for preparing Nb5.5Ni steel plate based on a cooling hot rolling process includes the following steps:

[0010] (1) According to the chemical composition of the 5.5Ni steel plate containing niobium, steel ingots are produced by vacuum induction melting furnace, the risers are removed, and the ingots are heated to 1200°C and held for 2 hours before being forged into steel billets.

[0011] (2) The steel billet is kept at 1200℃ for 1.5 to 2.5 hours and then cooled to 990 to 1020℃ for multiple passes of rough rolling. The rough rolling stage is in the static recrystallization zone of austenite to obtain rough rolled steel plate.

[0012] (3) The rough-rolled steel plate is subjected to multiple passes of finishing rolling. During the finishing rolling stage, the austenite is in the non-recrystallized region to obtain the rolled steel plate.

[0013] (4) The rolled steel plate is subjected to two-phase quenching heat treatment;

[0014] (5) The steel plate quenched in the two-phase region is tempered at a temperature lower than Ac1 and then air-cooled to room temperature to obtain a niobium-containing 5.5Ni low-temperature steel plate with excellent room temperature strength and plasticity and impact toughness of -196℃.

[0015] Furthermore, in step (1), the thickness of the steel billet is 100-110 mm, and the steel billet is heated in a box-type resistance furnace.

[0016] Furthermore, in step (2), the roughing stage consists of four passes, with a single pass reduction rate of 20-25%, an intermediate billet thickness of 45-50 mm, a cumulative reduction rate of 45-50%, and a roughing final rolling temperature of 890-930℃.

[0017] Furthermore, in step (2), the thickness of the rough-rolled steel plate is 45-50 mm.

[0018] Furthermore, in step (3), the finishing rolling stage consists of three passes, with a single pass reduction rate of 20-25%, a rolled steel plate thickness of 6-20 mm, a total reduction rate of ≥65%, a final rolling temperature of 810-830℃, and water cooling to room temperature after rolling.

[0019] Furthermore, the thickness of the rolled steel plate in step (3) is 6 to 20 mm.

[0020] Furthermore, the sub-temperature quenching temperature in step (4) is between Ac1 and Ac3, the sub-temperature quenching temperature in the two-phase region is 670-710℃, and the holding time is 30-40min.

[0021] Furthermore, the tempering temperature in step (5) is 570–610°C, and the holding time is 18–30 min.

[0022] The following details the function and dosage selection of the components contained in this invention:

[0023] Carbon (C) is the most important strengthening element in steel and also has the greatest impact on toughness, especially low-temperature toughness. For 5.5Ni steel plates, considering that the Ni content is significantly lower than that in 9Ni, the C content is lower than that in conventional 9Ni steel. Based on this, the carbon content is controlled at 0.03–0.06% in this invention.

[0024] Si (Si) is a weak deoxidizing element in steel and is inexpensive. Si strengthens steel through solid solution; however, its deoxidizing effect is poor when the Si content is below 0.05%, while higher Si content leads to decreased toughness and weldability. In this invention, the Si content is controlled at 0.05–0.12%.

[0025] Mn plays a role in solid solution strengthening and deoxidation, but its deoxidation ability is relatively weak. Compared with 9Ni steel, due to its lower C and Ni content, a higher Mn content is designed to improve the strength of the steel plate. In this invention, the Mn content is controlled at 0.52% to 0.98%.

[0026] Ni is a beneficial element that can significantly improve low-temperature toughness, especially affecting the impact toughness and ductile-brittle transition process of steel plates under ultra-low temperature conditions. However, excessively high Ni content will significantly increase the manufacturing cost of steel plates. Therefore, this invention controls its content at 5.20%–5.80%.

[0027] Mo can improve hardenability, thereby increasing strength; improve the tempering stability of steel; and when coexisting with chromium or manganese, it can reduce or suppress temper brittleness caused by other elements.

[0028] Nb: Nb plays all the important roles of microalloying elements in steel. Its effects differ depending on its form; some Nb can be dissolved in the Fe matrix, while others can form stable carbides, nitrides, or carbonitrides with C and N. Nb dissolved in austenite can inhibit the recrystallization of deformed austenite, increasing the austenite recrystallization temperature and thus refining the ferrite grains after rolling. Simultaneously, the cooling hot rolling temperature coincides with the precipitation temperature of Nb in recrystallized austenite, which can promote the precipitation of fine Nb(C,N), one of the innovations of this invention.

[0029] Phosphorus (P) is a harmful impurity element in steel. It combines with iron to form iron phosphide, which easily segregates at grain boundaries, weakening them and causing cold brittleness. This invention controls its content to ≤0.006%.

[0030] S (sulfide) is a harmful impurity element in steel, easily forming defects such as segregation and inclusions. During slab solidification, Mn (magnesium) readily segregates with S at the center of the slab, forming lamellar MnS inclusions, reducing the formation of low-melting-point FeS at austenite grain boundaries. However, MnS inclusions can lead to unstable impact toughness in the steel core. In the later stages of refining, Ca treatment technology is used to spheroidize and uniformly distribute the inclusions, reduce the viscosity of the molten steel, and promote the flotation of inclusions. This invention controls its content to ≤0.005%.

[0031] Elements such as Mn, Mo, and Ni in steel are all elements that improve the hardenability of steel plates. After hot rolling and water cooling, lath-shaped martensite structure is obtained. During the sub-temperature quenching process in the two-phase region, some of the primary martensite is re-austenitized, and new martensite is obtained in the subsequent water cooling process, which refines the grains. During this process, the stability of the primary martensite decreases. During the high-temperature tempering process, C and Ni have certain diffusion and migration capabilities. Some unstable primary martensite phases decompose and break through the potential energy barrier under the action of carbon and nickel elements, and inversely transform into austenite.

[0032] Nitrogen (Nb) in steel can lower the austenite-to-BCC phase transformation temperature. Under the same conditions, especially at high cooling rates, Nb can promote the formation of low-temperature phase transformation products such as acicular ferrite and bainite. During hot rolling, Nb can form stable carbonitrides, preventing and controlling the growth of austenite grains before rolling, refining the original austenite grains, and improving their stability. Furthermore, the dispersed and fine Nb(C,N) particles precipitated during and after rolling can produce a strong precipitation strengthening effect, increasing the strength of the steel.

[0033] In step (2), the billet is cooled to 990–1020°C before undergoing multiple rough rolling passes. This temperature coincides with the precipitation nose temperature of Nb(C,N) in austenite, and when the temperature is ≥810°C, the hot rolling deformation resistance does not change significantly and does not increase the hot rolling strength. Therefore, the cooling hot rolling process can not only refine the original austenite grains but also promote the precipitation of Nb(C,N), achieving a coupled effect of grain refinement strengthening and precipitation strengthening.

[0034] The advantages and beneficial effects of the niobium-containing 5.5Ni steel plate and its preparation method based on a cooling hot rolling process of the present invention compared with the prior art are as follows:

[0035] 1. A method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process was proposed, and the corresponding mechanical properties were achieved. The room temperature yield strength is 585–620 MPa, the room temperature tensile strength is 700–750 MPa, the room temperature elongation after fracture is 22–26%, and the impact energy at -196℃ is 150 J–200 J.

[0036] 2. The chemical composition design adds Nb element to improve its stability by expanding the austenite phase region and refining the original austenite grain size, thereby obtaining residual austenite in the final microstructure, and precipitation strengthening can be achieved through Nb(C,N).

[0037] 3. The cooling hot rolling process is adopted to stably coincide with the precipitation nose of Nb(C,N) in austenite, which promotes the precipitation of Nb(C,N) in austenite and enhances the precipitation strengthening effect.

[0038] 4. The hot-rolled water-cooled + LT process is adopted. Compared with the traditional QLT process, it can not only improve the inheritance of hot-rolled microstructure (fine grains and precipitation, etc.), but also eliminate the high-energy-consuming step of high-temperature quenching, making it more green and efficient. Attached Figure Description

[0039] Figure 1 Here is a typical SEM microstructure of #1 steel in its rolled state prepared using a cooling hot rolling process, as shown in Example 1.

[0040] Figure 2 The final heat treatment microstructure of #1 steel prepared in Example 1 based on a cooling hot rolling process is shown in the diagram.

[0041] Figure 3 The stress-strain curve of a typical tensile engineering project of No. 1 steel prepared based on a cooling hot rolling process in Example 1 is shown.

[0042] Figure 4 The final heat treatment microstructure of #2 steel prepared in Example 2 based on a cooling hot rolling process is shown in the figure.

[0043] Figure 5 The final heat treatment microstructure of #3 steel prepared in Example 3 based on a cooling hot rolling process is shown in the diagram.

[0044] Figure 6 The final heat treatment microstructure of #4 steel prepared in Example 4 based on a cooling hot rolling process is shown in the diagram.

[0045] Figure 7 The final heat treatment microstructure of 5# steel prepared in Example 5 based on a cooling hot rolling process is shown in the diagram.

[0046] Figure 8 The final heat treatment microstructure of 6# steel prepared in Example 6 based on a cooling hot rolling process is shown in Figure 6.

[0047] Figure 9 The longitudinal tensile properties curves of 2#-6# steel prepared by cooling hot rolling process in Examples 2-6 are shown. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and do not limit the scope of the invention.

[0049] The hot rolling forming process of the following embodiments was carried out on a 450mm two-roll reversible hot rolling experimental mill.

[0050] Example 1

[0051] A niobium-containing 5.5Ni steel plate based on a cooling hot rolling process, designated as No. 1 steel plate, has the following chemical composition by weight percentage: C: 0.045%, Si: 0.112%, Mn: 0.95%, Ni: 5.58%, Mo: 0.216%, Nb: 0.019%, P: 0.005%, S: 0.003%, with the balance being Fe and unavoidable impurities.

[0052] A method for preparing Nb5.5Ni steel plate based on a cooling hot rolling process includes the following steps:

[0053] (1) According to the chemical composition of the 5.5Ni steel plate containing niobium, steel ingots were produced by vacuum induction melting furnace, the risers were removed, and the ingots were heated to 1200℃ and held for 2 hours to form a steel billet No. 1 with a thickness of 100mm.

[0054] (2) The billet is kept at 1200℃ for 2 hours, and then the billet No. 1 is cooled to 1000℃ for rough rolling in the austenitic static recrystallization zone. The initial rolling temperature is 1004℃, the final rolling temperature is 897℃, the rolling passes are 4, the single pass reduction rate is 20-22%, and the total reduction rate is 53.4%. After rolling, it is water-cooled to obtain the rough rolled steel plate.

[0055] (3) After rough rolling, multiple passes of finishing rolling are carried out. The finishing rolling stage is in the non-recrystallized austenite region. There are three passes in the finishing rolling stage. The initial rolling temperature is 864℃ and the final rolling temperature is 824℃. The single pass reduction rate is 20-22% and the total reduction rate is 68.2%. After rolling, air cooling is carried out to obtain rolled steel plate.

[0056] (4) The rolled steel plate is subjected to two-phase quenching heat treatment. The two-phase sub-temperature quenching temperature is 690℃, the holding time is 36min, and the cooling method is air cooling.

[0057] (5) The steel plate quenched in the two-phase region was tempered at a temperature lower than Ac1 and then air-cooled to room temperature. The tempering temperature was 590℃ and the holding time was 18min to obtain steel plate No. 1.

[0058] Typical SEM microstructure of #1 steel in rolled state as follows Figure 1 As shown, the final heat-treated microstructure is as follows Figure 2As shown, the stress-strain curve of a typical tensile test for steel #1 is as follows: Figure 3 As shown in Table 1, tensile and impact specimens were taken from the niobium-containing 5.5Ni steel plate prepared in this embodiment, which has excellent room temperature strength and plasticity and low temperature impact toughness, along the rolling direction and transverse direction. The room temperature tensile strength and the impact energy at -196℃ are shown in Table 1.

[0059] Example 2

[0060] A niobium-containing 5.5Ni steel plate based on a cooling hot rolling process, designated as No. 1 steel plate, has the following chemical composition by weight percentage: C: 0.045%, Si: 0.112%, Mn: 0.95%, Ni: 5.58%, Mo: 0.216%, Nb: 0.019%, P: 0.005%, S: 0.003%, with the balance being Fe and unavoidable impurities.

[0061] A method for preparing Nb5.5Ni steel plate based on a cooling hot rolling process includes the following steps:

[0062] (1) According to the chemical composition of the 5.5Ni steel plate containing niobium, steel ingots were produced by vacuum induction melting furnace, the risers were removed, and the ingots were heated to 1200℃ and held for 2 hours before being forged into No. 2 steel billets with a thickness of 100mm.

[0063] (2) The billet was kept at 1200℃ for 2 hours, and then the No. 2 billet was directly subjected to rough rolling in the austenitic dynamic recrystallization zone. The initial rolling temperature was 1091℃, the final rolling temperature was 942℃, the number of rolling passes was 4, the single pass reduction rate was 20-22%, and the total reduction rate was 52.0%. After rolling, it was water-cooled to obtain the rough rolled steel plate.

[0064] (3) After rough rolling, multiple passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone. There are three passes in the finishing rolling stage. The initial rolling temperature is 870℃ and the final rolling temperature is 844℃. The single pass reduction rate is 21-26% and the total reduction rate is 66.7%. After rolling, air cooling is carried out to obtain rolled steel plate.

[0065] (4) The rolled steel plate is subjected to two-phase quenching heat treatment. The two-phase sub-temperature quenching temperature is 690℃, the holding time is 36min, and the cooling method is air cooling.

[0066] (5) The steel plate quenched in the two-phase region was tempered at a temperature lower than Ac1 and then air-cooled to room temperature. The tempering temperature was 590℃ and the holding time was 18min to obtain steel plate #2.

[0067] Tensile and impact specimens were taken from the niobium-containing 5.5Ni steel plate prepared in this embodiment, which has excellent room temperature strength and plasticity and low temperature impact toughness, along the rolling direction and transverse direction. The room temperature tensile strength and the impact energy at -196℃ are shown in Table 1.

[0068] Example 3

[0069] A niobium-containing 5.5Ni steel plate based on a cooling hot rolling process, designated as No. 1 steel plate, has the following chemical composition by weight percentage: C: 0.043%, Si: 0.107%, Mn: 1.01%, Ni: 5.47%, Mo: 0.192%, Nb: 0.01%, P: 0.005%, S: 0.003%, with the balance being Fe and unavoidable impurities.

[0070] A method for preparing Nb5.5Ni steel plate based on a cooling hot rolling process includes the following steps:

[0071] (1) According to the chemical composition of the 5.5Ni steel plate containing niobium, steel ingots were produced by vacuum induction melting furnace, the riser was removed and heated to 1200℃, and after holding for 2 hours, they were forged into No. 3 steel billets with a thickness of 100mm.

[0072] (2) The billet was kept at 1200℃ for 2 hours, and then the No. 3 billet was directly subjected to rough rolling in the austenitic dynamic recrystallization zone. The initial rolling temperature was 1103℃, the final rolling temperature was 950℃, the rolling passes were 4, the single pass reduction rate was 20-22%, and the total reduction rate was 52.6%. After rolling, the billet was water-cooled to obtain the rough rolled steel plate.

[0073] (3) After rough rolling, multiple passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone. There are three passes in the finishing rolling stage. The initial rolling temperature is 881℃ and the final rolling temperature is 847℃. The single pass reduction rate is 21-26% and the total reduction rate is 67.0%. After rolling, air cooling is carried out to obtain rolled steel plate.

[0074] (4) The rolled steel plate is subjected to two-phase quenching heat treatment. The two-phase sub-temperature quenching temperature is 690℃, the holding time is 36min, and the cooling method is air cooling.

[0075] (5) The steel plate quenched in the two-phase region was tempered at a temperature lower than Ac1 and then air-cooled to room temperature. The tempering temperature was 590℃ and the holding time was 18min to obtain steel plate #3.

[0076] Tensile and impact specimens were taken from the niobium-containing 5.5Ni steel plate prepared in this embodiment, which has excellent room temperature strength and plasticity and low temperature impact toughness, along the rolling direction and transverse direction. The room temperature tensile strength and the impact energy at -196℃ are shown in Table 1.

[0077] Example 4

[0078] A niobium-containing 5.5Ni steel plate based on a cooling hot rolling process, designated as No. 1 steel plate, has the following chemical composition by weight percentage: C: 0.043%, Si: 0.107%, Mn: 1.01%, Ni: 5.47%, Mo: 0.192%, Nb: 0.01%, P: 0.005%, S: 0.003%, with the balance being Fe and unavoidable impurities.

[0079] A method for preparing Nb5.5Ni steel plate based on a cooling hot rolling process includes the following steps:

[0080] (1) According to the chemical composition of the 5.5Ni steel plate containing niobium, steel ingots were produced by vacuum induction melting furnace, the risers were removed, and the ingots were heated to 1200℃ and held for 2 hours before being forged into No. 4 steel billets with a thickness of 100mm.

[0081] (2) The billet was kept at 1200℃ for 2 hours, and then the No. 4 billet was cooled to 1000℃ for rough rolling in the austenitic static recrystallization zone. The initial rolling temperature was 1011℃, the final rolling temperature was 903℃, the rolling passes were 4, the single pass reduction rate was 20-22%, and the total reduction rate was 52.9%. After rolling, the billet was water-cooled to obtain the rough rolled steel plate.

[0082] (3) After rough rolling, multiple passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone. There are three passes in the finishing rolling stage. The initial rolling temperature is 851℃ and the final rolling temperature is 817℃. The single pass reduction rate is 20-22% and the total reduction rate is 67.9%. After rolling, air cooling is carried out to obtain rolled steel plate.

[0083] (4) The rolled steel plate is subjected to two-phase quenching heat treatment. The two-phase sub-temperature quenching temperature is 690℃, the holding time is 36min, and the cooling method is air cooling.

[0084] (5) The steel plate quenched in the two-phase region was tempered at a temperature lower than Ac1 and then air-cooled to room temperature. The tempering temperature was 590℃ and the holding time was 18min to obtain steel plate #4.

[0085] Tensile and impact specimens were taken from the niobium-containing 5.5Ni steel plate prepared in this embodiment, which has excellent room temperature strength and plasticity and low temperature impact toughness, along the rolling direction and transverse direction. The room temperature tensile strength and the impact energy at -196℃ are shown in Table 1.

[0086] Example 5

[0087] A niobium-containing 5.5Ni steel plate based on a cooling hot rolling process, designated as No. 1 steel plate, has the following chemical composition by weight percentage: C: 0.045%, Si: 0.112%, Mn: 0.95%, Ni: 5.58%, Mo: 0.216%, Nb: 0.019%, P: 0.005%, S: 0.003%, with the balance being Fe and unavoidable impurities.

[0088] A method for preparing Nb5.5Ni steel plate based on a cooling hot rolling process includes the following steps:

[0089] (1) According to the chemical composition of the 5.5Ni steel plate containing niobium, steel ingots were produced by vacuum induction melting furnace, the risers were removed, and the ingots were heated to 1200℃ and held for 2 hours before being forged into 5# steel billets with a thickness of 100mm.

[0090] (2) The billet was kept at 1200℃ for 2 hours, and then the No. 5 billet was cooled to 1000℃ for rough rolling in the austenitic static recrystallization zone. The initial rolling temperature was 1004℃, the final rolling temperature was 897℃, the rolling passes were 4, the single pass reduction rate was 20-22%, and the total reduction rate was 53.4%. After rolling, the billet was water-cooled to obtain the rough rolled steel plate.

[0091] (3) After rough rolling, multiple passes of finishing rolling are carried out. The finishing rolling stage is in the non-recrystallized austenite region. There are three passes in the finishing rolling stage. The initial rolling temperature is 846℃ and the final rolling temperature is 824℃. The single pass reduction rate is 20-22% and the total reduction rate is 68.2%. After rolling, air cooling is carried out to obtain rolled steel plate.

[0092] (4) The rolled steel plate is subjected to two-phase quenching heat treatment. The sub-temperature quenching temperature of the two-phase region is 650℃, the holding time is 36min, and the cooling method is air cooling.

[0093] (5) The steel plate quenched in the two-phase region was tempered at a temperature lower than Ac1 and then air-cooled to room temperature. The tempering temperature was 590℃ and the holding time was 18min to obtain No. 5 steel plate.

[0094] Tensile and impact specimens were taken from the niobium-containing 5.5Ni steel plate prepared in this embodiment, which has excellent room temperature strength and plasticity and low temperature impact toughness, along the rolling direction and transverse direction. The room temperature tensile strength and the impact energy at -196℃ are shown in Table 1.

[0095] Example 6

[0096] A niobium-containing 5.5Ni steel plate based on a cooling hot rolling process, designated as No. 1 steel plate, has the following chemical composition by weight percentage: C: 0.045%, Si: 0.112%, Mn: 0.95%, Ni: 5.58%, Mo: 0.216%, Nb: 0.019%, P: 0.005%, S: 0.003%, with the balance being Fe and unavoidable impurities.

[0097] A method for preparing Nb5.5Ni steel plate based on a cooling hot rolling process includes the following steps:

[0098] (1) According to the chemical composition of the 5.5Ni steel plate containing niobium, steel ingots were produced by vacuum induction melting furnace, the risers were removed, and the ingots were heated to 1200℃ and held for 2 hours before being forged into 5# steel billets with a thickness of 100mm.

[0099] (2) The billet was kept at 1200℃ for 2 hours, and then the No. 5 billet was cooled to 1000℃ for rough rolling in the austenitic static recrystallization zone. The initial rolling temperature was 1004℃, the final rolling temperature was 897℃, the rolling passes were 4, the single pass reduction rate was 20-22%, and the total reduction rate was 53.4%. After rolling, the billet was water-cooled to obtain the rough rolled steel plate.

[0100] (3) After rough rolling, multiple passes of finishing rolling are carried out. The finishing rolling stage is in the non-recrystallized austenite region. There are three passes in the finishing rolling stage. The initial rolling temperature is 846℃ and the final rolling temperature is 824℃. The single pass reduction rate is 20-22% and the total reduction rate is 68.2%. After rolling, air cooling is carried out to obtain rolled steel plate.

[0101] (4) The rolled steel plate is subjected to two-phase quenching heat treatment. The sub-temperature quenching temperature of the two-phase region is 730℃, the holding time is 36min, and the cooling method is air cooling.

[0102] (5) The steel plate quenched in the two-phase region was tempered at a temperature lower than Ac1 and then air-cooled to room temperature. The tempering temperature was 590℃ and the holding time was 18min to obtain No. 5 steel plate.

[0103] Tensile and impact specimens were taken from the niobium-containing 5.5Ni steel plate prepared in this embodiment, which has excellent room temperature strength and plasticity and low temperature impact toughness, along the rolling direction and transverse direction. The room temperature tensile strength and the impact energy at -196℃ are shown in Table 1.

[0104] The final heat treatment microstructure of #2-6 is as follows Figure 4-8 As shown, the longitudinal tensile property curve is as follows: Figure 9 As shown.

[0105] Table 1 Summary of Mechanical Properties

[0106]

[0107] The results of the examples show that the preparation method of niobium-containing 5.5Ni steel plate based on the cooled hot rolling process of the present invention can achieve excellent room temperature strength and plasticity, and -196℃ impact energy while maintaining simplicity, efficiency, and low cost. Furthermore, the comparison of the results of Examples 1 and 2 shows that the cooled hot rolling process can effectively improve room temperature strength and plasticity, and -196℃ impact energy. The comparison of the results of Examples 1 and 4 shows that within an appropriate range, increasing the Nb content can effectively improve room temperature strength without reducing its room temperature plasticity and -196℃ impact energy. The comparison of the results of Examples 1 and Examples 5 / 6 shows that excessively high or low critical zone annealing temperatures will result in a lower ferrite content in the critical zone. Although this can effectively improve room temperature strength, it significantly reduces room temperature plasticity and -196℃ impact energy.

Claims

1. A niobium-containing 5.5Ni steel plate based on a cooled hot rolling process, characterized in that, The chemical composition of the niobium-containing 5.5Ni steel plate, by weight percentage, is as follows: C: 0.03~0.06%, Si: 0.05~0.12%, Mn: 0.52~1.10%, Ni: 5.20~5.70%, Mo: 0.10~0.30%, Nb: 0.01~0.02%, P: ≤0.006%, S: ≤0.005%, with the balance being Fe and unavoidable impurities; The mechanical properties of the niobium-containing 5.5Ni steel plate are as follows: room temperature yield strength of 585~620 MPa, room temperature tensile strength of 700~750 MPa, room temperature elongation after fracture of 22~26%, and impact energy at -196℃ of 150J~200J.

2. A method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process as described in claim 1, characterized in that, Includes the following steps: (1) According to the chemical composition of the 5.5Ni steel plate containing niobium, steel ingots were produced by vacuum induction melting furnace, the risers were removed, and the ingots were heated to 1200°C and held for 2 hours before being forged into steel billets. (2) The billet is kept at 1200℃ for 1.5~2.5h, and then cooled to 990~1020℃ for multiple passes of rough rolling. The rough rolling stage is in the static recrystallization zone of austenite to obtain rough rolled steel plate. (3) The rough-rolled steel plate is subjected to multiple passes of finishing rolling. During the finishing rolling stage, the austenite is in the non-recrystallized region to obtain the rolled steel plate. (4) The rolled steel plate is subjected to two-phase quenching heat treatment; (5) The steel plate quenched in the two-phase region is tempered at a temperature lower than Ac1 and then air-cooled to room temperature to obtain a niobium-containing 5.5Ni low-temperature steel plate with excellent room temperature strength and plasticity and impact toughness of -196℃.

3. The method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process according to claim 2, characterized in that, In step (1), the thickness of the steel billet is 100~110mm, and the steel billet is heated in a box-type resistance furnace.

4. The method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process according to claim 2, characterized in that, In step (2), the roughing stage consists of four passes, with a single pass reduction rate of 20-25%, an intermediate billet thickness of 45-50 mm, a cumulative reduction rate of 45-50%, and a roughing final rolling temperature of 890-930℃.

5. The method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process according to claim 2, characterized in that, In step (2), the thickness of the rough-rolled steel plate is 45~50mm.

6. The method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process according to claim 2, characterized in that, In step (3), the finishing rolling stage consists of three passes, with a single pass reduction rate of 20-25%, a rolled steel plate thickness of 6-20 mm, a total reduction rate of ≥65%, a final rolling temperature of 810-830℃, and water cooling to room temperature after rolling.

7. The method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process according to claim 2, characterized in that, The thickness of the rolled steel plate in step (3) is 6~20mm.

8. The method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process according to claim 2, characterized in that, The sub-temperature quenching temperature in step (4) is between Ac1 and Ac3, and the sub-temperature quenching temperature in the two-phase region is 670~710℃, and the holding time is 30~40min.

9. The method for preparing niobium-containing 5.5Ni steel plate based on a cooling hot rolling process according to claim 2, characterized in that, The tempering temperature in step (5) is 570~610℃, and the holding time is 18~30min.

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