CN112226701B

CN112226701B - High-aluminum-content fine-grain low-density full-high-temperature ferrite steel and preparation method thereof

Info

Publication number: CN112226701B
Application number: CN202010956557.8A
Authority: CN
Inventors: 延泽鹏; 王学敏; 徐翔宇; 尚成嘉
Original assignee: University of Science and Technology Beijing USTB
Current assignee: University of Science and Technology Beijing USTB
Priority date: 2020-09-11
Filing date: 2020-09-11
Publication date: 2021-12-31
Anticipated expiration: 2040-09-11
Also published as: CN112226701A

Abstract

The invention provides a high-aluminum-content fine-grain low-density full-high-temperature ferrite medium plate for an ocean platform and a preparation method thereof. The steel comprises the following chemical components in percentage by mass: 0.000 to 0.039% of C, 4.01 to 6.00% of Al, Mn: 0.00-1.40%, Cr: 0.01-2.00%, B: 0.002-0.003%, Ti: 0.02-0.06%, Ni: 0.01-0.39%, Nb is less than or equal to 0.007%, P is less than or equal to 0.015%, and S is less than or equal to 0.005%. The others are Fe and other inevitable impurities. By controlling the components and the structure, the corrosion resistance of the alloy is more than 70 percent better than that of the currently and commonly used weathering steel Corten-A under the marine atmospheric environment, and the average grain size is thinned to be less than 50 mu m.

Description

High-aluminum-content fine-grain low-density full-high-temperature ferrite steel and preparation method thereof

Technical Field

The invention belongs to the field of metal materials, and particularly relates to a preparation method of high-aluminum-content fine-grain low-density full-high-temperature ferrite steel for an ocean platform.

Background

With the continuous development of ocean economy, the demand of steel for ocean platforms is on a gradually increasing trend. Because the marine environment is extremely complex and is subject to erosion and damage by natural forces such as sea waves, sea earthquakes, low temperature and the like, marine engineering equipment is in service in a severe marine environment and is subject to severe loads and complex corrosive environments. From the perspective of energy conservation and emission reduction and the safety of an ocean platform structure, the steel for ocean engineering has the weight reduction requirement. The density of the steel is reduced after Al is added, and meanwhile, the corrosion resistance of the steel plate can be improved, so that the low-density weathering steel can meet the requirements of ships and ocean engineering on low-density weathering steel by adding Al into low-carbon steel. However, increasing Al content expands the δ -ferrite phase region, so that the γ -phase region is rapidly reduced or even disappears, the steel with all- δ -ferrite cannot be subjected to structure control by a conventional process, mechanical properties are poor, Al has the effect of stabilizing κ carbide relative to other elements, and toughness of the steel material is seriously reduced. In addition, because the microstructure of the all-delta ferrite steel is single and the components are uniformly distributed, the all-delta ferrite is the microstructure with the best corrosion resistance.

The patent No. CN110066969B entitled "preparation method of high-corrosion-resistance high-aluminum-content low-density steel" discloses a rolling and heat treatment process of high-corrosion-resistance high-aluminum-content low-density steel. The patent is the first patent of the manufacturing method of the high-aluminum all-delta ferrite medium plate in China. The alloy component provided in this patent CN110066969B has high contents of Ni, Mo, and Nb elements in the component system, and this patent mainly utilizes Mo as a solid solution in δ ferrite to improve pitting corrosion resistance and intergranular corrosion resistance, and utilizes Ni as a solid solution in δ ferrite to improve corrosion resistance, but Ni and Mo elements are expensive alloy elements and are high in cost. In the patent, Nb is used for inhibiting grain growth and refining delta ferrite grains, but Nb also has the function of inhibiting recrystallization, so that the refining function of Nb on the delta ferrite grains is expected to be limited, and the cost of Nb is higher. The patent uses rare earth elements for refining delta ferrite grains and improving corrosion resistance, but the rare earth elements have the same higher cost and the smelting process is extremely difficult to control. In addition, the rolling process provided by the patent is complex, the rolling direction needs to be changed in the rolling process, the requirement on equipment is high, and the original size of the plate blank and the final size of the plate are also limited. Therefore, the preparation process of the patent is complex, has a plurality of technical problems which are not beneficial to industrial production and application, and has higher cost.

Patent nos. CN103484771B and CN106498278 both disclose methods for producing non-all- δ ferritic low-density steels, the final structure is a multi-phase structure, the multi-phase structure has different alloy contents of each phase, galvanic corrosion occurs during corrosion, and the corrosion resistance is much lower than that of a single-phase microstructure.

Compared with the CN110066969B alloy, the invention provides the high-aluminum content ultra-fine delta ferrite grain low-density steel which has lower component and simpler and more controllable preparation process and the preparation method thereof.

The invention content is as follows:

the invention provides a preparation method of high-aluminum content fine-grain low-density full-high-temperature ferrite steel for an ocean platform.

The invention relates to high-aluminum-content fine-grain low-density steel which is characterized by comprising the following chemical components in percentage by weight: 0.000 to 0.039% of C, 4.01 to 6.00% of Al, Mn: 0.00-1.40%, Cr: 0.01-2.00%, B: 0.002-0.003%, Ti: 0.02-0.06%, Ni: 0.01-0.39%, Nb is less than or equal to 0.007%, P is less than or equal to 0.015%, and S is less than or equal to 0.005%. The others are Fe and other inevitable impurities.

The preferable chemical components by mass percentage are as follows: c:0.000 to 0.012%, Al: 4.01-5.00%, Mn: 0.21-0.29%, Cr: 1.00-2.00%, B: 0.002-0.003%, Ti: 0.02-0.05%, Ni: 0.01-0.39%, Nb is less than or equal to 0.007%, P is less than or equal to 0.010%, and S is less than or equal to 0.005%. The others are Fe and other inevitable impurities. Thereby leading the content of high-temperature ferrite below the solidification temperature to reach 99.95 percent.

The design content and the range of the alloy components used by the steel for the high-aluminum content fine-grain low-density full-high-temperature ferrite ocean platform are as follows:

c: c is a trace amount of solid-soluble element in δ ferrite and plays a role of solid-solution strengthening, and C is an austenite forming element, and the present invention is a pure ferrite steel, and therefore, the carbon content needs to be controlled. Comprehensively considering the properties of the steel such as welding performance, corrosion resistance, impact toughness and the like, and keeping the carbon content at a lower level as much as possible. Therefore, the carbon content in the present invention is controlled to 0.039% or less, preferably 0.012% or less.

Al: al is an element which is good for realizing the light weight of the metal material, is a ferrite forming element, and increases the Al content to enlarge a delta ferrite phase region, so that a gamma phase region is sharply reduced or even disappears. The affinity of aluminum and oxygen is very strong, aluminum oxide is easily generated in steel, so that a rust layer is more compact, the resistance and the reaction resistance of the rust layer are improved, the permeation of chloride ions is favorably hindered, and the corrosion resistance of the steel is improved. Meanwhile, aluminum and nitrogen also have strong affinity, and AlN particles are easy to form. The stabilized AlN particles, i.e., the AlN particles, may serve as crystal nuclei for solidification, and may serve to refine the grains in either the as-cast or reheated solid state of the alloy. The crystal grain growth is avoided by pinning the crystal grain boundary, so that the crystal grain becomes thin and the strength and the toughness of the steel are improved. Therefore, the content of the Al element in the invention is controlled to be 4.01-6.00%, preferably 4.01-5.00%.

Mn: mn is a good deoxidizer and desulfurizer, can be dissolved in delta ferrite in a solid mode to play a role in solid solution strengthening, and simultaneously forms MnS with S to play a role in fixing S. The addition of Mn element makes the steel material not only have enough toughness, but also have higher strength and hardness, and improves the hot workability of steel. However, Mn is an austenite stabilizing element, is easy to segregate in a high-temperature ferrite grain boundary, weakens the corrosion resistance of steel, reduces the welding performance and reduces the low-temperature impact toughness of the material. Therefore, the Mn content in the present invention is controlled to 0.00 to 1.40%, preferably 0.21 to 0.29%.

And B, adding a proper amount of B element into the steel can improve the comprehensive mechanical property of the material. A proper amount of B can improve the high-temperature plasticity of the steel. B and C form MC type carbide which can be used as a grain nucleation core in the cooling process to refine delta ferrite grains. Meanwhile, a trace amount of B can generate an unbalanced segregation behavior, can segregate to delta ferrite grain boundaries, hinders dragging and migration of the grain boundaries, and plays a role in refining the delta ferrite grains. The non-equilibrium segregation of B will occur in the process of solidification and cooling, which can obviously refine the as-cast delta ferrite grains and improve the comprehensive mechanical properties of the material. Above 900 ℃, the inhibition effect of the segregation of the B element on the recrystallization of the high-temperature ferrite is not obvious, and below 900 ℃, the inhibition effect of the segregation of the B element on the recrystallization of the high-temperature ferrite is obvious. However, if the B content exceeds 0.007%, brittleness is easily caused. Therefore, the content of the B element in the invention is controlled to be 0.002-0.003%.

Ni: the addition of Ni element can refine ferrite and improve comprehensive mechanical property. Ni is enriched in the corrosion rust layer, so that the rust layer is more compact, the penetration of chloride ions is effectively inhibited, and the corrosion resistance of the steel is improved. However, Ni is an element that expands the austenite phase region, and is not favorable for the formation of ferrite, and Ni forms an intermetallic compound NiAl with Al, and decreases toughness. Therefore, the content of Ni element in the invention is controlled between 0.01-0.39%.

Nb: the Nb element can be dissolved in the delta ferrite to improve the pitting corrosion resistance. Meanwhile, Nb has the function of refining delta ferrite grains, the comprehensive mechanical property of the material is improved, and Nb and C, N form a stable compound to play a role in precipitation strengthening. Excessive Nb cannot be dissolved in the matrix in a solid solution manner, so that the Nb content is controlled to be 0.000-0.007%.

Cr: the Cr element is a reduced austenite phase region and a strong ferrite forming element. Cr is dissolved in delta ferrite to improve corrosion resistance, but the effect of Cr to improve corrosion resistance is reduced as the carbon content increases, and M is formed by fixed carbon₂₃C₆A type carbide. The Cr content cannot be too high for cost saving. Therefore, the Cr content in the invention is controlled to be 0.00-2.00%, preferably 1.00-2.00%.

Ti: the Ti element is a reduced austenite phase region and a strong ferrite forming element. Ti can form stable TiC with C, and the formation of TiC can stabilize C element and avoid the formation of chromium carbide, thus resulting in chromium-poor zone and intergranular corrosion. Therefore, the Ti content in the invention is controlled to be Ti: 0.02 to 0.06%, preferably 0.02 to 0.05%.

The preparation method of the high-aluminum-content fine-grain low-density full-high-temperature ferritic steel is characterized in that in the casting blank cooling process, after the surface of a casting blank is cooled to 1350 ℃, water mist is adopted to accelerate cooling to below 900 ℃, and the cooling rate is 10-50 ℃/s; the average grain size of the as-cast structure is less than 5 mm.

Further, the preparation method comprises the following rolling processes:

keeping the temperature at the homogenizing temperature of 1000-1100 ℃ for 60 ℃ after castingHomogenizing for 80min, wherein the heat preservation temperature and the B, Ti content meet the formula: t1100-]-2000[Ti]+1000[B][Ti]Then rolling at 1000 ℃, wherein the rolling is divided into 2 stages, and the first stage is controlled rolling by a mechanism of sub-crystal polymerization and growth recrystallization: 3-4 passes, the pass interval is not more than 10s, the reduction rate of each pass is not less than 20% and not more than 25%, and the strain rate is less than 0.2s^-1The final rolling temperature is not lower than 950 ℃; the second stage is controlled rolling by a grain boundary bowing recrystallization mechanism: 3-4 passes, 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 strain rate is more than 1s^-1The final rolling temperature is not lower than 800 ℃; after rolling, carrying out online electromagnetic induction heating annealing for 5-10 s, wherein the annealing temperature and the content of B, Ti are in accordance with the formula: t1000-]-4000[Ti]+2000[B][Ti]And then accelerated cooling to room temperature.

Furthermore, the corrosion resistance of the steel grade in the marine atmospheric environment is more than 70% better than that of the currently commonly used weathering steel Corten-A, the average grain size is thinned to be less than 50 mu m, and the Charpy impact power at 0 ℃ is more than 80J. The steel has the advantages that the as-cast structure is a fine delta ferrite structure, the average grain size of the as-cast structure is less than 5mm, the rolling process is simple, the cost is low, the structure in the rolled steel is fine delta ferrite, and the average grain size is thinned to be less than 50 mu m.

The invention relates to a high-aluminum content fine-grain low-density full-high-temperature ferrite steel, which has the structure of full-delta ferrite and does not have phase change in the whole solidification process, so delta ferrite grains can not be refined by a traditional phase change method. Above 900 ℃, the inhibition effect of the segregation of the B element on the recrystallization of the high-temperature ferrite is not obvious, and below 900 ℃, the inhibition effect of the segregation of the B element on the recrystallization of the high-temperature ferrite is obvious. The invention adopts a Mo-free low-Ni trace Nb component system, utilizes Nb solid solution in delta ferrite to improve the pitting corrosion resistance and intergranular corrosion resistance, and has lower cost compared with the traditional Mo-added alloy component system for pitting corrosion resistance and intergranular corrosion resistance. A component system with proper amount of Ti and Cr elements is adopted, Ti has the function of stabilizing the C element, the occurrence of kappa carbide is avoided, and meanwhile, enough Ti and Cr are dissolved in ferrite, so that the corrosion resistance of single-phase delta ferrite can be improved.

The invention not only has innovative component system, but also provides a rolling process aiming at the steel grade of the invention, the rolling process of the first stage is the subgrain polymerization, growth and recrystallization controlled rolling, the inhibition effect on the recrystallization of high-temperature ferrite is not obvious in the stage B, the recrystallization behavior of the high-temperature ferrite is promoted, and the subgrain crystal grains have polymerization behavior, thereby avoiding the occurrence of a large amount of mixed crystals, damaging the uniformity of the structure and reducing the comprehensive mechanical property and the corrosion property of the material. The second stage is controlled rolling by a grain boundary bowing recrystallization mechanism, and the inhibition effect on the recrystallization of the high-temperature ferrite is obvious in the stage B. In the on-line electromagnetic induction heating annealing stage after rolling, B has no obvious inhibition effect on the recrystallization of the high-temperature ferrite, and the high-temperature ferrite is recrystallized in the annealing stage to refine the grains of the high-temperature ferrite. The invention realizes simple rolling process, lower homogenization heat preservation temperature, shorter heat preservation time, lower post-rolling annealing heat preservation temperature, shorter annealing time and lower cost while the rolling process is theoretically innovative, and is beneficial to factory production and application.

The invention is characterized in that the segregation of B is used for refining the as-cast structure of delta ferrite creatively, and a simple and easily controlled rolling process is used for obtaining a thinner single-phase structure of delta ferrite, so that the all-delta ferrite steel has good mechanical properties. The strength of the steel of the invention is improved by utilizing the effect of solid solution strengthening of Mn in delta ferrite. The pitting corrosion resistance and the intergranular corrosion resistance are improved by using a trace amount of Nb which is dissolved in the delta ferrite in a solid manner, and the corrosion resistance is improved by using Cr and Ti which are dissolved in the delta ferrite in a solid manner. The marine atmospheric corrosion is simulated through dry-wet circulation, the weight gain of a corrosion product is measured, and the marine atmospheric corrosion resistance in the patent is found to be improved by more than 70% compared with the current commonly used weathering steel Corten-A.

The invention has the advantages that the delta ferrite cast structure is refined by using the alloy element B instead of the traditional expensive Nb element and rare earth element, and the thinner delta ferrite structure is obtained while the cost is reduced. The thinner delta ferrite structure can be obtained by adopting the traditional hot rolling equipment and additionally arranging electromagnetic induction on-line heat treatment equipment, and sufficient possibility is provided for the production and popularization of high-aluminum all-delta ferrite steel. Compared with the traditional weathering steel, the high-density high-corrosion-resistance weathering steel has the advantages of low density and high corrosion resistance. In the aspect of marine atmospheric corrosion resistance, the marine atmospheric corrosion resistance of the high-aluminum content fine-grain low-density steel is improved by over 70 percent compared with the traditional weathering steel Corten-A. The corrosion evaluation method refers to GB/T20853-2007 to obtain the relative corrosion rate compared with Corten-A. The Charpy impact energy of the high-aluminum content fine-grain low-density steel at 0 ℃ is more than 80J, and the toughness requirement of the steel for ships and ocean engineering is greatly met.

Drawings

Fig. 1 shows the metallographic structure of example 1, and the structure is fine δ ferrite.

Detailed Description

In order to make the present invention more clear, the following description is given with reference to preferred embodiments. It will be understood by those skilled in the art that the details of the following description are not to be interpreted as limiting, but rather as illustrative and not limiting the scope of the invention.

Example 1

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

Table 1: alloy composition (wt%)

C	Al	Mn	Ni	B	Ti	Nb	Cr	Fe
									0.009	4.17	0.25	0.22	0.0022	0.021	0.004	1.21	Balance of

After casting, carrying out homogenization treatment at a homogenization temperature of 1050 ℃ for 60min, then carrying out initial rolling at 1000 ℃, wherein the rolling is divided into 2 stages, and the first stage is controlled rolling by a mechanism of a sub-crystal polymerization and growth recrystallization: 3 passes, pass reduction rates of 21.6%, 22.5% and 24.3%, pass interval of not more than 10s, strain rate of less than 0.2s^-1The final rolling temperature is not lower than 950 ℃; the second stage is controlled rolling by a grain boundary bowing recrystallization mechanism: 3 passes, pass reduction rates of 16.2%, 17.8% and 18.6% respectively, pass interval of not more than 10s, strain rate of more than 1s^-1The final rolling temperature is not lower than 800 ℃; and (3) carrying out online electromagnetic induction heating annealing for 5-10 s after rolling, wherein the annealing temperature is 900 ℃. After which accelerated cooling to room temperature. Most preferably of steel plateThe final thickness was 12 mm.

The mechanical properties of example 1 were finally obtained: the yield strength is 496MPa, the tensile strength is 618MPa, the elongation after fracture is 35 percent, the Charpy impact energy at 0 ℃ is 98J, and the relative corrosion rate by taking Corten-A as a reference is 25 percent. The metallographic structure of example 1 is shown in fig. 1, and the structure is ultra-fine δ ferrite.

Example 2

The steel grade composition (mass fraction) of example 2 of the invention is shown in Table 2

Table 2: alloy composition (wt%)

C	Al	Mn	Ni	B	Ti	Nb	Cr	Fe
									0.008	4.21	0.23	0.19	0.0025	0.025	0.006	1.69	Balance of

After casting, carrying out homogenization treatment at a homogenization temperature of 1050 ℃ for 60min, then carrying out initial rolling at 1000 ℃, wherein the rolling is divided into 2 stages, and the first stage is controlled rolling by a mechanism of a sub-crystal polymerization and growth recrystallization: 3 passes, pass reduction rates of 22.5%, 23.2% and 23.7%, pass intervals of not more than 10s, and strain rate of less than 0.2s^-1The final rolling temperature is not lower than 950 ℃; the second stage is controlled rolling by a grain boundary bowing recrystallization mechanism: 3 passes, the pass reduction rate is respectively 15.9%, 16.8% and 19.6%, the pass interval is not more than 10s, and the strain rate is more than 1s^-1The final rolling temperature is not lower than 800 ℃; and (3) carrying out online electromagnetic induction heating annealing for 5-10 s after rolling, wherein the annealing temperature is 900 ℃. After which accelerated cooling to room temperature. The final thickness of the steel plate was 18 mm.

The mechanical properties of example 2 were finally obtained: the yield strength was 502MPa, the tensile strength was 624MPa, the elongation after fracture was 34.6%, the Charpy impact energy at 0 ℃ was 89J, and the relative corrosion rate with Corten-A as a reference was 23%.

Example 3

The steel grade composition (mass fraction) of example 3 of the invention is shown in Table 3

Table 3: alloy composition (wt%)

C	Al	Mn	Ni	B	Ti	Nb	Cr	Fe
									0.004	4.35	0.28	0.31	0.0027	0.03	0.003	1.85	Balance of

After casting, carrying out homogenization treatment at a homogenization temperature of 1050 ℃ for 60min, then carrying out initial rolling at 1000 ℃, wherein the rolling is divided into 2 stages, and the first stage is controlled rolling by a mechanism of a sub-crystal polymerization and growth recrystallization: 3 passes, pass reduction rates of 22.9%, 23.7% and 24.5%, pass intervals of not more than 10s, and strain rate of less than 0.2s^-1The final rolling temperature is not lower than 950 ℃; the second stage is controlled rolling by a grain boundary bowing recrystallization mechanism: 3 passes, pass reduction rates of 16.4%, 18.2% and 19.1%, respectively, pass interval of not more than 10s, strain rate of more than 1s^-1The final rolling temperature is not lower than 800 ℃; and (3) carrying out online electromagnetic induction heating annealing for 5-10 s after rolling, wherein the annealing temperature is 900 ℃. After which accelerated cooling to room temperature. The final thickness of the steel plate was 20 mm.

The mechanical properties of example 3 were finally obtained: the yield strength is 488MPa, the tensile strength is 602MPa, the elongation after fracture is 32.8 percent, the Charpy impact energy at 0 ℃ is 86J, and the relative corrosion rate by taking Corten-A as a reference is 25 percent.

The mechanical and corrosion properties of examples 1, 2, 3 are shown in table 4:

TABLE 4

Comparative example 1

Comparative example 1 refers to the alloy composition of example 1 in patent CN 110066969B. The alloy components are shown in Table 5

Table 5: alloy composition (wt%)

C	Al	Mn	Ni	Si	Nb	Mo	Ce	Fe
									0.017	4.20	0.013	1.48	0.20	0.020	0.21	0.040	Balance of

Homogenizing a casting blank with the thickness of 170mm at the homogenization temperature of 1100 ℃ for 80-120 min, then starting rolling at 1000 ℃, wherein the rolling is divided into 3 stages, the first stage is used for rolling in a recrystallization area, 3 passes are carried out, the pass reduction rates are respectively 15.8%, 16.6% and 17.3%, and the final rolling temperature is 960 ℃; the second-stage non-recrystallization zone is transversely rolled, and is rolled at an angle of 60-80 degrees along the rolling direction of the first stage, and the rolling reduction of 3 passes is respectively 22.0%, 23.1% and 23.3%; and (3) longitudinally rolling the non-recrystallization zone in the third stage, wherein the rolling direction is the same as that of the first stage, the rolling directions are 3 passes, the pass reduction rates are respectively 26.1%, 26.5% and 28%, and the final rolling temperature is 750 ℃. And (3) keeping the temperature for 5min after rolling, wherein the heat preservation temperature is 850 ℃, cooling to 380 ℃ at a cooling speed of more than 5 ℃/s after heat preservation, then cooling to room temperature in air, and the final thickness of the steel plate is 18 mm.

The mechanical properties finally obtained are: the yield strength was 462MPa, the tensile strength was 576MPa, the elongation after fracture was 30.2%, the 0 ℃ impact energy was 80J, and the relative corrosion rate with Corten-A as a reference was 31%.

Comparative example 2

Comparative example 2 refers to the alloy composition of example 4 in patent CN 103484771B. The alloy components are shown in Table 6

Table 6: alloy composition (wt%)

C	Si	Mn	Cu	Ni	Nb	Al	Fe
								0.068	0.32	1.06	1.04	0.84	0.040	3.93	Balance of

Heating the steel ingot to 1200 ℃, preserving heat for 2 hours, forging the steel ingot into a plate blank with the thickness of 80mm, heating the plate blank to 1150 ℃, preserving heat for 2 hours, carrying out two-stage controlled rolling on the plate blank at the initial rolling temperature of 1050 ℃, controlling the rolling temperature of a recrystallization zone to be more than or equal to 980 ℃, controlling the rolling temperature of a non-recrystallization zone to be less than or equal to 930 ℃, controlling the accumulated deformation of the non-recrystallization zone to be more than 50%, and carrying out water quenching after rolling. The rolled steel sheets were tempered for 1 hour at a tempering temperature of 700 ℃, and the mechanical properties and corrosion properties of the steels of comparative examples obtained are shown in table 7.

TABLE 7

Comparative example 3

Comparative example 3 refers to the alloy composition of example 3 in patent CN 106498278B. Smelting in a vacuum smelting furnace, wherein the alloy components (mass fraction) are shown in Table 8

Table 8: alloy composition (wt%)

C	Al	Mn	Ni	Nb	Cu	V	Si	S	P	N	Fe
												0.25	3.95	2.41	2.97	0.015	0.5	0.03	0.19	0.001	0.001	0.001	Balance of

The raw materials were smelted and cast into ingots according to the compositions in table 8. Forging at 1200 deg.C, the forging temperature is 1200 deg.C, the final forging temperature is higher than 1000 deg.C, and water quenching and quick cooling are carried out after forging.

The forged billet with the thickness of 100mm is heated to 1150 ℃ and is kept warm for 60min for homogenization treatment, and then two-stage rolling is carried out. The initial rolling temperature is 1000 ℃, the thickness of the steel plate is 13mm after six-pass deformation, the total reduction is 87%, and the final rolling temperature is 830 ℃. And after rolling, quenching the steel plate with water at a cooling speed of more than 10 ℃ and cooling to room temperature. The critical heat treatment experiment was then performed: heating the steel plate to 730 ℃, isothermally tempering for 30min, and then air cooling. The mechanical properties and corrosion properties obtained are shown in table 9.

TABLE 9

Claims

1. A high-aluminum content fine-grain full-high-temperature ferrite low-density steel is characterized by comprising the following chemical components: 0.001 to 0.039% of C, 4.01 to 6.00% of Al, Mn: 0.01-1.40%, Cr: 0.01-2.00%, B: 0.002-0.003%, Ti: 0.02-0.06%, Ni: 0.01-0.39%, Nb is less than or equal to 0.007%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, and the balance of Fe and other inevitable impurities; the microstructure is full high temperature ferrite and is suitable for an ocean platform;

in the casting blank cooling process of the high-aluminum-content fine-grain low-density full-high-temperature ferritic steel, after the surface of the casting blank is cooled to 1350 ℃, water mist is adopted to accelerate cooling to below 900 ℃, and the cooling rate is 10-50 ℃/s; the average grain size of the as-cast structure is less than 5 mm;

the cast structure of the steel is a fine delta ferrite structure, the rolling process is simple, the cost is low, the structure in the rolled steel is ultrafine delta ferrite, the average grain size is thinned to be below 50 mu m, and the Charpy impact power at 0 ℃ is more than 80J; the corrosion resistance of the steel grade in the marine atmospheric environment is more than 70 percent better than that of the currently commonly used weathering steel Corten-A.

2. The high aluminum content fine-grained low density all high temperature ferritic steel according to claim 1 is characterized by the chemical composition in mass percent: c: 0.001-0.012%, Al: 4.01-5.00%, Mn: 0.21-0.29%, Cr: 1.00-2.00%, B: 0.002-0.003%, Ti: 0.02-0.05%, Ni: 0.01-0.39%, Nb is less than or equal to 0.007%, P is less than or equal to 0.01%, S is less than or equal to 0.005%, and the balance of Fe and other inevitable impurities.

3. The method for preparing a high aluminum content fine-grained low density full high temperature ferritic steel according to claim 1 is characterized by the following rolling process:

the first stage rolling process is the crystallization controlled rolling of the growing of the sub-crystal polymerization;

the second stage rolling process is controlled rolling by a grain boundary bowing recrystallization mechanism.