CN110066969B - A kind of high corrosion resistance high aluminum content low density steel and preparation method thereof - Google Patents

Abstract

The invention provides a preparation method of a high-corrosion-resistance high-aluminum-content low-density medium plate for an ocean platform. The steel comprises, by mass, 0.010-0.035% of C, 4.01-6.00% of Al, 0.010-0.20% of Mn, 1.00-3.00% of Ni, 0.010-0.30% of Si, 0.008-0.020% of Nb, 0.10-0.80% of Mo, 0.00-0.050% of Ce, less than or equal to 0.015% of P, and less than or equal to 0.005% of S. The others are Fe and inevitable impurities. By controlling the components and the structure, the corrosion resistance of the alloy steel is improved by more than 50 percent and the density is reduced by more than 6 percent compared with the corrosion resistance of the alloy steel Corten-A which is generally used at present.

Description

A kind of high corrosion resistance high aluminum content low density steel and preparation method thereof

technical field

The invention belongs to the field of metal materials, and in particular relates to a preparation method of high corrosion-resistant, high-aluminum and low-density steel for offshore platforms.

Background technique

With the continuous development of my country's marine development, the demand for steel for offshore platforms continues to expand, and the current total annual demand for steel for offshore platforms is more than 1 million tons. According to the "Medium and Long-Term Development Plan of Offshore Engineering Equipment Manufacturing Industry", it is expected that the annual average renewal and new increment will be about 50, and the steel consumption of offshore platforms is expected to reach more than 2 million tons. Due to the long-term exposure of steel for offshore platforms to high (low) temperature, high pressure, high humidity, chloride corrosion and marine biological erosion, severe electrochemical corrosion will occur, which will seriously affect the mechanical properties of the material and shorten its service life. Therefore, in addition to high strength and high and low temperature toughness, steel for offshore platforms must also have good corrosion resistance. The addition of Al can significantly improve the corrosion resistance of steel for offshore platforms. But the addition of Al will reduce the toughness of the steel material.

A number of patents have been formed on steel plates with high aluminum content at home and abroad. The patent number is CN103484771B, which is entitled "A high-aluminum and low-density medium-thick steel plate for offshore platforms and its preparation method", which discloses a high-aluminum and low-density offshore platform. Plate and its rolling and heat treatment process, and achieve low density and high strength, but impact toughness is not disclosed. The composition disclosed in the patent CN103484771B must have an austenite phase region. For example, according to the alloy composition of Example 4 in the patent CN103484771B, the phase composition calculated by Thermo-Calc software shows that the maximum austenite content reaches more than 10%. According to the manufacturing process adopted by the patent CN103484771B, the microstructure finally obtained must be a dual-phase structure. Due to the different alloy content of the two phases, galvanic corrosion will occur during the corrosion process, and the corrosion resistance is lower than that of the single-phase microstructure. In addition, the structure transformed from austenite is the hard phase in the two phases, which will reduce the toughness of the material, and the precipitation of Cu during the tempering process will also reduce the toughness of the material. Therefore, the high-aluminum and low-density medium and thick plates disclosed in the patent CN103484771B can predictably have poor impact toughness.

Patent Publication No. CN106498278 discloses a method for preparing a medium and thick plate with high strength, high elongation and low density, which realizes low density and high strength and solves the problem of low temperature impact toughness, but obtains a multiphase structure, which also exists Corrosion performance is lower than the problem of single-phase structure.

The patent publication number CN101033520A discloses the composition and preparation method of an AlSi type economical weathering steel. By adding Al, Si, P and RE elements to Q235 steel, the patent has good corrosion performance compared to Q235 steel, but It does not disclose its impact properties. According to the composition of the patent CN101033520A, it can be inferred that the obtained microstructure is similar to the microstructure of the low-density medium-thick plate described in the patent CN103484771B. It is foreseeable that the atmospheric corrosion-resistant steel described in the patent CN101033520A also has both impact toughness and corrosion resistance. The problem.

Invention content:

The purpose of the present invention is to provide high corrosion resistance, high aluminum content and low density steel and a preparation method thereof. By controlling the composition and structure, the corrosion resistance in the marine atmosphere environment is higher than that of the currently commonly used alloy steel Corten-A. The performance is improved by more than 50%, and the density is reduced by more than 6%.

The chemical composition mass percentage of the high corrosion resistance, high aluminum content and low density steel of the present invention is: C: 0.010-0.035%, Al: 4.01-6.00%, Mn: 0.010-0.20%, Ni: 1.00-3.00%, Si: 0.010- 0.30%, Nb: 0.008-0.020%, P≤0.015%, S≤0.005%, Mo: 0.10-0.80%, Ce: 0.00-0.050%. Others are Fe and other inevitable impurities. In order to make the minimum content of ferrite reach more than 99%, the content of C, Al, Mn and Ni should conform to the formula 190[C]-0.4286[Al]+[Mn]+1.3796[Ni]-0.1576[Al][Ni] < 5.338.

The preferred chemical composition mass percentages are: C: 0.010-0.024%, Al: 4.01-5.00%, Mn: 0.010-0.20%, Ni: 1.00-1.50%, and the contents of C, Al, Mn, and Ni should conform to formula 190[ C]-0.14[Al][Ni]+[Mn]+1.196[Ni]<4.6, so that the ferrite content below 1000°C reaches 99.9%.

The microstructure in the above-mentioned high corrosion resistance, high aluminum content, and low density steel is a single delta ferrite.

The alloy composition design content and the scope of the steel used for the high corrosion resistance, high aluminum and low density offshore platform of the present invention are based on the following:

C: C can dissolve in austenite and expand the austenite phase region. However, the present invention is a pure ferritic steel, so the carbon content needs to be controlled. Moreover, excessive carbon content will be detrimental to the atmospheric corrosion resistance of the steel, and will reduce the weldability, cold brittleness, and stamping properties of the steel. Therefore, the carbon content of the present invention is controlled to be 0.035% or less, preferably 0.024% or less.

Al: Aluminum contributes to the formation of ferrite. As a lightweight element added to steel, it can effectively reduce the density of steel and at the same time play a role in solid solution strengthening. Furthermore, in order to ensure the content of ferrite during the homogenization treatment, it is necessary to increase the content of aluminum element. Studies have shown that during the corrosion process, aluminum will be enriched in the inner rust layer, which is helpful for the formation of nano-scale fine-grained compounds in the inner rust layer, thereby enhancing the protection of the rust layer and improving the corrosion resistance of steel. , especially when the aluminum content is 4.00%, the corrosion resistance of steel is significantly improved. Al can reduce the ferrite stacking fault energy. When the Al content reaches 4.00%, the stacking fault energy is 100mJ/m2. The reduction of the stacking fault energy is beneficial to the occurrence of static recrystallization during hot deformation. However, if the aluminum content is too high, Fe ₃ Al intermetallic compounds will be formed, which will further deteriorate the impact toughness of the steel. Therefore, the content of aluminum in the present invention is controlled between 4.01-6.00%, preferably 4.01-5.00%.

Mn: The addition of manganese can effectively desulfurize, and the manganese element can improve the toughness and strength of the steel and improve the hot workability of the steel, but the manganese element will stabilize the austenite, which is not conducive to the formation of ferrite and reduces the corrosion resistance of the steel. Therefore, the content of manganese element in the present invention is controlled between 0.01% and 0.20%.

Ni: Ni can make the self-corrosion potential of the steel positive, which increases the stability of the steel. The enrichment of Ni in the corrosion rust layer can effectively inhibit the intrusion of the corrosive anion Cl ^- , promote the rapid formation of a protective rust layer, and improve the corrosion resistance of the steel. Ni is an oxidant-type corrosion inhibitor for steel, making it self-passivating. However, Ni will expand the austenite phase region, which is not conducive to the formation of ferrite, and Ni will form an intermetallic compound NiAl with Al, which deteriorates the impact toughness. Therefore, the content of Ni element in the present invention is controlled between 1.00% and 3.00%.

Si: Silicon has a good deoxidation effect on molten steel. At the same time, silicon prevents the formation of acid in the rust layer and can prevent ^Cl- from invading the inner rust layer. Si mainly exists in the spinel oxide as a divalent oxide. The inner rust layer is made dense, the intrusion of Cl ^- is hindered, the corrosion resistance is improved, and the silicon element can improve the strength and hardness of the steel. Therefore, the content of silicon in the present invention is controlled between 0.01% and 0.30%.

Nb: Nb can play the role of grain refinement strengthening and precipitation strengthening, which can effectively improve the strength of steel. However, the present invention is a ferritic non-transformation steel, the solid solubility of Nb is lower than that of traditional low alloy steel at 800-1300°C, and excess Nb cannot be solid-dissolved into the matrix, so the niobium content in the present invention is controlled at 0.008～1300°C between 0.02%.

The addition of Mo: Mo element can improve the resistance to grain boundary corrosion, and Mo can passivate the steel surface in reducing acid and strong oxidizing salt solution, so it can generally improve the corrosion resistance of steel and prevent steel from being exposed to chlorides. Pitting in solution. Therefore, the addition of Mo element can effectively improve the corrosion resistance of steel. However, the price of Mo is too high, so the Mo content in the present invention is controlled between 0.1% and 0.8%.

Ce: After the rare earth Ce is added to the steel, the relative grain boundary energy of the steel is reduced. The rare earth atoms distributed on the grain boundary reduce the surface energy of the grain boundary, making it difficult for new phases to precipitate along the grain boundary, the grain boundary is relatively clean, and most impurities are combined into stable compounds. After the rare earth is added to the steel, the rare earth changes the conventional inclusions in the steel into rare earth inclusions (metamorphism), which reduces the segregation of impurity elements such as phosphorus on the grain boundaries, thereby reducing the number of intergranular and hindering the formation of intergranular cracks and expansion, increasing its toughness. However, the addition of excessive rare earth Ce will lead to the decline of the mechanical properties of the steel. Therefore, the Ce content in the present invention is controlled between 0 and 0.05%.

The preparation method of the high corrosion resistance high aluminum content low density steel of the present invention is as follows:

Rolling process: Hold the forged billet at a homogenization temperature of 1050℃～1150℃ for 80～120min for homogenization treatment, and then start rolling at 980℃-1020℃. The rolling is divided into 3 stages. Rolling in the crystallization zone, 3 to 4 passes, the pass interval is not more than 10s, the reduction rate of each pass is not less than 15% and not higher than 20%, and the final rolling temperature is not less than 950 ° C; the second stage is not Transverse rolling in the recrystallization zone, 60°～80° from the rolling direction of the first stage, 3 to 4 passes, the pass interval is not more than 15s, and the reduction rate of each pass is not less than 20% and not higher than 30%; the third-stage non-recrystallization zone is longitudinally rolled in the same direction as the first-stage rolling, 3 to 4 passes, the pass interval is not more than 20s, and the reduction rate per pass is not low At 25% and not higher than 35%, the finish rolling temperature is not lower than 700℃. After rolling, heat preservation and cooling are performed to obtain a steel sheet with a final thickness of 4 to 20 mm.

The post-rolling heat preservation is on-line heat preservation for 5-15 minutes, and the heat preservation temperature and the Nb content should conform to the formula: T=950-5000 [Nb].

The cooling is accelerated cooling to below 400°C at a cooling rate greater than 5°C/s after heat preservation, and then air-cooled to room temperature.

After rolling, keep on-line for 5 to 15 minutes, and the temperature and Nb content should conform to the formula: T=950-5000[Nb]. After heat preservation, accelerated cooling to below 400°C at a cooling rate of more than 5°C/s, and then air-cooled to room temperature, the final thickness of the steel plate is 4-20mm.

The present invention is characterized in that a single-phase structure of delta ferrite is obtained, thereby realizing the characteristics of good marine atmospheric corrosion resistance. By simulating marine atmospheric corrosion through a dry-wetting cycle, and measuring the weight gain of corrosion products, it is found that the steel in the present invention has an ability to resist marine atmospheric corrosion by more than 50% and a density reduction of more than 6% compared with the currently commonly used weathering steel Corten-A. , 0 ℃ Charpy impact energy greater than 47J. Through thermodynamic derivation, Thermo-Calc software calculation and experimental verification, the contents of C, Al, Mn and Ni satisfy the formula: 190[C]-0.4286[Al]+[Mn]+1.3796[Ni]-0.1576[Al][Ni ]<5.338, realizing the single-phase structure of ferrite.

Since the present invention is pure delta ferritic steel, the structure cannot be refined through phase transformation, and the crystal grains can only be refined through deformation and recrystallization. For ferritic steel, which has a body-centered cubic crystal structure, the climbing and cross-slip effects of dislocations during hot deformation are stronger than those of austenite, and the driving force for recrystallization is weaker. The invention utilizes Al to reduce the stacking fault energy of ferrite, compared with other ferrite non-transformation steels (silicon steel, ferritic stainless steel), the recovery is reduced, and the recrystallization driving force is increased. The invention adopts methods such as changing the rolling direction, controlling the reduction amount of the pass, and controlling the interval time of the pass, etc., to increase the deformation accumulation, thereby promoting the recrystallization behavior of the steel of the present invention, and achieving the purpose of refining the structure. The key technology of the present invention is to change the rolling direction, which has two core functions. One is that rolling in different directions will activate different slip systems in the grains, making it easier for dislocations to react during the deformation process. , dislocation crossover, dislocation entanglement and other behaviors, which hinder dislocation climbing and cross-slip during the recovery process, thereby promoting recrystallization nucleation; the second is in ferritic steel (100)[011] It is difficult for the grains to recrystallize. By changing the rolling direction, the formation of the (100)[011] texture is reduced due to the rotation of the grains in different directions during the rolling process. [011] The grains, due to the promotion of dislocation entanglement, improve the deformation energy storage, and promote the phagocytosis of the (100)[011] grains by the recrystallized grains near the (100)[011] grains , which also achieves the purpose of grain refinement.

The advantage of the present invention is that it has the advantages of low density and high corrosion resistance compared with the traditional weathering steel. Through measurement, it is found that the density reduction of the low-density weathering steel of the present invention reaches 6% compared with Corten-A. In terms of marine atmospheric corrosion resistance, the marine atmospheric corrosion resistance of the low-density weathering steel of the present invention is improved by more than 50% compared with Corten-A. The corrosion evaluation method of the present invention refers to GB/T 20853-2007 to obtain the relative corrosion rate compared with Corten-A. The Charpy impact energy at 0°C of the low-density weathering steel of the invention is greater than 47J, which meets the toughness requirements of steel for ships and marine engineering.

Description of drawings

FIG. 1 is the metallographic structure of Example 1. FIG.

Detailed ways

In order to express the content of the present invention more clearly, further description will be given below with reference to the preferred embodiments. Those skilled in the art should understand that the specific content described below is non-limiting but illustrative, and should not limit the protection scope of the present invention.

Example 1

The steel grade composition (mass percentage) of Example 1 of the present invention is shown in Table 1

Table 1: Alloy Composition (wt%)

The steel billet with a thickness of 170mm after forging was held at a homogenization temperature of 1100 ° C for 100 minutes for homogenization treatment, and then rolled at 1000 ° C. The rolling was divided into 3 stages. The first stage was rolled in the recrystallization zone, with 3 passes. , the pass reduction rates are 15.8%, 16.6%, and 17.3%, respectively, the pass intervals are 6s, 8s, and 10s, respectively, and the final rolling temperature is 960 °C; In the first stage, the rolling direction is 60°, with 3 passes, the pass reduction rates are 22.0%, 23.1%, and 23.3%, respectively, and the pass intervals are 10s, 12s, and 15s, respectively; the third stage is non-recrystallization Longitudinal rolling in the zone, the same as the rolling direction of the first stage, 3 passes, the pass reduction rates are 26.1%, 26.5%, and 28%, respectively, and the pass intervals are 15s, 18s, and 20s, respectively. The final rolling temperature is 750°C. After rolling, heat preservation for 5 minutes, the heat preservation temperature is 850 °C, after heat preservation, the cooling rate is greater than 5 °C/s to accelerate cooling to 380 °C, and then air-cooled to room temperature, and the final thickness of the steel plate is 18 mm.

The mechanical properties of Example 1 were finally obtained: the yield strength was 462 MPa, the tensile strength was 576 MPa, the elongation after fracture was 30.2%, the impact energy at 0°C was 80 J, and the relative corrosion rate with Corten-A as a reference was 31%. The metallographic structure of Example 1 is shown in FIG. 1 , and all the constituent phases are delta ferrite.

Example 2

The steel grade composition (mass percentage) of Example 2 of the present invention is shown in Table 2

Table 2: Alloy Composition (wt%)

C Al Mn Ni Si Nb Mo Ce Fe 0.02 4.26 0.051 1.08 0.19 0.018 0.41 0.030 margin

The steel billet with a thickness of 170 mm after forging is kept at a homogenization temperature of 1150 ° C for 80 to 120 minutes for homogenization treatment, and then rolled at 1000 ° C. The rolling is divided into 3 stages. The first stage is rolled in the recrystallization zone. Passes, the pass reduction rates are 15.3%, 15.9%, and 17.2%, respectively, the pass intervals are 5s, 8s, and 9s, respectively, and the final rolling temperature is 955 °C; Rolling at 60° to the rolling direction of the first stage, 3 passes, the pass reduction rates are 20.8%, 21.0%, and 21.8%, respectively, and the pass intervals are 11s, 13s, and 15s. The longitudinal rolling in the recrystallization zone is the same as the rolling direction of the first stage, with three passes, the pass reduction rates are 32%, 32.3%, and 34%, respectively, and the pass intervals are 15s, 17s, and 20s, respectively. The rolling temperature was 730°C. After rolling, heat preservation for 5 minutes, the heat preservation temperature is 860 °C, after heat preservation, the cooling rate is greater than 5 °C/s to accelerate cooling to 350 °C, and then air-cooled to room temperature, and the final thickness of the steel plate is 15 mm.

The mechanical properties and corrosion properties of Example 2 are finally obtained as follows: the yield strength is 487MPa, the tensile strength is 594MPa, the elongation after fracture is 32.8%, the impact energy at 0°C is 73J, and the relative corrosion rate with Corten-A as a reference is 33 %.

Example 3

The steel grade composition (mass percentage) of Example 3 of the present invention is shown in Table 3

Table 3: Alloy Composition (wt%)

C Al Mn Ni Si Nb Mo Ce Fe 0.018 4.26 0.042 1.06 0.19 0.020 0.29 0.032 margin

The steel billet with a thickness of 150 mm after forging is kept at a homogenization temperature of 1150 ° C for 80 to 120 minutes for homogenization treatment, and then rolled at 1000 ° C. The rolling is divided into three stages. The first stage is rolled in the recrystallization zone. Passes, the pass reduction ratios are 16.0%, 17.5%, and 19.2%, respectively, the pass intervals are 6s, 9s, and 10s, respectively, and the final rolling temperature is 965 ° C; It was rolled at 75° to the rolling direction of the first stage, with 3 passes, the pass reduction rates were 21.4%, 25.7%, and 26.5%, respectively, and the pass intervals were 11s, 14s, and 14s. The longitudinal rolling in the recrystallization zone is the same as the rolling direction of the first stage, with 3 passes, the pass reduction rates are 27.8%, 30.8%, and 33.3%, respectively, and the pass intervals are 16s, 19s, and 20s, respectively. The rolling temperature was 720°C. After rolling, heat preservation for 5 minutes, the heat preservation temperature is 850 °C, after heat preservation, the cooling rate is greater than 5 °C/s to accelerate cooling to 390 °C, and then air-cooled to room temperature, and the final thickness of the steel plate is 15mm.

The mechanical properties and corrosion properties of Example 3 are finally obtained as follows: the yield strength is 470MPa, the tensile strength is 589MPa, the elongation after fracture is 32.6%, the impact energy at 0°C is 96J, and the relative corrosion rate with Corten-A as a reference is 29 %.

The mechanical properties and corrosion properties of Examples 1, 2, and 3 are shown in Table 4:

Table 4

Comparative Example 1

Comparative Example 1 refers to the alloy composition of Example 4 in Patent CN103484771B. It is smelted in a vacuum smelting furnace, and its alloy composition is shown in Table 5.

table 5

C Si Mn Cu Ni Nb Al Fe 0.067 0.32 0.98 1.03 0.86 0.041 3.95 margin

The alloy compositions shown in the table are added to the smelting furnace for smelting and cast into billets. The slab was heated to 1200°C for 2 hours and then forged into a slab with a thickness of 80 mm, and the slab was heated to 1150°C for 2 hours and rolled. The rolling temperature is 1050°C, the rolling is controlled in two stages, the temperature in the recrystallization zone is ≥980°C, the rolling temperature in the non-recrystallization zone is ≤930°C, the cumulative deformation in the non-recrystallization zone is greater than 50%, and the water is quenched after rolling.

The rolled steel sheet was tempered for 1 h, and the tempering temperature was 700° C. The mechanical properties and corrosion properties of the obtained steel of the comparative example are listed in Table 6.

Table 6

Comparative Example 2

Comparative Example 2 refers to the alloy composition of Example 4 in the patent CN106498278. It is smelted in a vacuum smelting furnace, and the alloy composition (mass fraction) is shown in Table 7

Table 7

According to the components in Table 7, the raw materials were smelted and cast into slabs. Forging at 1200 ℃, the forging temperature is 1200 ℃, the final forging temperature is higher than 1000 ℃, and the water quenching and rapid cooling after the forging is completed.

The forged 100mm thick steel billet was heated to 1130°C for 120min for homogenization treatment, and then two-stage rolling was performed. The initial rolling temperature was 1000°C and the final rolling temperature was 850°C. After six passes of deformation, the thickness of the steel plate was 13mm. The total reduction is 87%. After rolling, the steel sheet is water quenched and cooled to room temperature at a cooling rate greater than 10°C.

In the critical heat treatment experiment, the steel plate was heated to 738 °C for isothermal tempering for 30 min and then air-cooled. The obtained mechanical properties and corrosion properties are shown in Table 8.

Table 8

Comparative Example 3

Comparative Example 3 refers to the alloy composition of Example 2 in patent CN101033520A. Using Q235 as the base material, according to the set value of the composition, the amount of Al, ferrosilicon, etc. required for alloying is calculated. After adding the charging, vacuum magnetron arc casting is performed, and it is repeated more than 10 times, so that the alloying is sufficient and uniform. Finally, the smelted steel samples were sampled for composition analysis. The compositions of the obtained alloys are listed in Table 9.

Table 9

C Si Mn S P Al RE Fe 0.18 0.35 0.55 0.008 0.017 4.02 0.13 margin

The final mechanical properties and corrosion properties are shown in Table 10:

Table 10

Claims (5)

1. The high-corrosion-resistance high-aluminum-content low-density steel is characterized by comprising the following chemical components in percentage by mass: 0.010-0.035% of C, 4.01-6.00% of Al, 0.010-0.20% of Mn, 1.00-3.00% of Ni, 0.01-0.30% of Si, 0.008-0.020% of Nb, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.10-0.80% of Mo, 0.00-0.050% of Ce and the balance of Fe and inevitable impurities, wherein the contents of C, Al, Mn and Ni are in accordance with the formula: 190[ C ] -0.4286[ Al ] + [ Mn ] +1.3796[ Ni ] -0.1576[ Al ] [ Ni ] <5.338, the structure in the steel is single ferrite.

2. The high-corrosion-resistance high-aluminum-content low-density steel as claimed in claim 1, wherein the chemical composition comprises, in mass percent: 0.010-0.024% of C, 4.01-5.00% of Al, 0.01-0.20% of Mn, 1.00-1.50% of Ni, 0.010-0.30% of Si, 0.008-0.02% of Nb, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.10-0.80% of Mo, 0.00-0.05% of Ce, and the balance of Fe and inevitable impurities, wherein the contents of C, Al, Mn and Ni are in accordance with a formula 190[ C ] -0.14[ Al ] [ Ni ] + [ Mn ] +1.196[ Ni ] < 4.6.

3. The method for preparing the steel with high corrosion resistance, high aluminum content and low density according to claim 1, wherein the specific rolling process comprises the following steps:

carrying out homogenization treatment on the forged steel billet at a homogenization temperature of 1050-1150 ℃ for 80-120 min, then carrying out initial rolling at 980-1020 ℃, wherein the rolling is divided into 3 stages, the first stage is rolling in a recrystallization zone, 3-4 passes are carried out, the pass interval is not more than 10s, the reduction rate of each pass is not less than 15% and not more than 20%, and the final rolling temperature is not less than 950 ℃; the second-stage non-recrystallization zone is transversely rolled, the rolling is carried out in a 60-80-degree angle with the rolling direction of the first stage, 3-4 passes are carried out, the pass interval is not more than 15s, and the reduction rate of each pass is not lower than 20% and not higher than 30%; and (3) longitudinally rolling the non-recrystallization zone in the third stage in the same direction as the first stage, wherein the rolling direction is 3-4 passes, the pass interval is not more than 20s, the reduction rate of each pass is not less than 25% and not more than 35%, the final rolling temperature is not less than 700 ℃, and the steel plate with the final thickness of 4-20 mm is obtained after heat preservation and cooling after rolling.

4. The method for preparing the high-corrosion-resistance high-aluminum-content low-density steel according to claim 3, wherein the post-rolling heat preservation is an on-line heat preservation for 5-15 min, and the heat preservation temperature and the Nb content meet the formula: t is 950-.

5. The method for preparing a steel with high corrosion resistance, high aluminum content and low density according to claim 3, wherein the cooling is performed by holding the temperature, then performing accelerated cooling to below 400 ℃ at a cooling rate of more than 5 ℃/s, and then performing air cooling to room temperature.

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