A yield 460mpa grade low-density medium plate and a preparation method thereof

Abstract

The application discloses a yield 460MPa grade low-density medium plate and a preparation method thereof, and belongs to the technical field of low-density steel. The mass percentage of the chemical components is as follows: C 0.30-0.46%, Al 4.8-5.7%, Mn 18.1-22.3%, Nb 0.015-0.06%, O 0.0005-0.0010%, Si 0.05-0.20%, S≤0.005%, P≤0.005%, N≤0.0005%, and the balance is Fe and inevitable impurities. The preparation method of the yield 460MPa grade low-density medium plate comprises the following steps: smelting and casting an ingot, controlled temperature rolling, and normalizing heat treatment. The low-density medium plate has an equiaxed austenite structure, a yield strength of 460MPa or more, an elongation after fracture of more than 40%, a Charpy impact energy at-40 DEG C of greater than 100J, and a density of less than 7.36g / cm 3 .

Description

Technical Field

[0001] This invention belongs to the field of low-density steel technology, specifically relating to a low-density normalized medium plate with a yield strength of 460MPa and its preparation method. Background Technology

[0002] Lightweight transportation equipment is a crucial measure for achieving energy conservation and emission reduction. Various transportation vehicles, such as automobiles and high-speed trains, must ensure excellent safety during lightweight design. This requires the steel plates used to possess not only low density but also excellent impact resistance. To this end, by adding the lightweight element Al to steel to reduce its density, and then adding elements such as Mn and C to stabilize austenite, Fe-Mn-Al-C austenitic low-density steel can be obtained. Single-phase austenitic steel has high low-temperature impact toughness and also exhibits low or no magnetic properties, which can enhance the stability of electronic equipment. It possesses both structural and functional properties, making it a high-performance steel with broad application prospects. However, the low yield strength of this type of steel is a common technical challenge. How to achieve high strength, high elongation, and high toughness in austenitic low-density steel requires further exploration of key material processes.

[0003] Domestic patents concerning austenitic low-density medium plates involve low-density high-strength steels with increased carbon content. While this improves the steel's strength, it also deteriorates weldability. For example, CN 115323278 A, "A low-density steel with a yield strength of 700MPa and its heat treatment method," has a carbon content of 0.85-1.15%; CN 114892084 A, "A high-strength austenitic lightweight steel with high impact toughness and its manufacturing method," has a carbon content of 0.83-0.92%, and the post-rolling heat treatment of the steel plate is quenching, making the steel more prone to cracking. CN 112281074 A, "A high-manganese medium-thick plate for low-density LNG storage tanks and its preparation method," involves rapidly cooling the steel plate to room temperature at a rate greater than 19℃ / s after hot rolling. This rapid cooling process generates significant internal stress in the steel's microstructure, resulting in a high tendency to crack and insufficient performance stability, which is incompatible with the high safety requirements of transportation equipment.

[0004] Currently, in order to meet the requirements of practical applications, there is still an urgent need to develop a low-density medium plate that has excellent comprehensive mechanical properties such as high strength, high impact toughness and high elongation, as well as excellent weldability and performance stability. Summary of the Invention

[0005] The purpose of this invention is to provide a low-density medium plate with a yield strength of 460 MPa and its preparation method. Through the combined design of matrix components such as Fe, Mn, Al, and C with the strengthening element Nb, the yield strength of the steel can reach 460 MPa while reducing the weight of the steel, and at the same time maintaining good elongation and low-temperature impact toughness. It is mainly used in transportation equipment that requires welding.

[0006] This invention discloses a low-density medium-density fiberboard with a yield strength of 460 MPa, comprising the following chemical composition by mass percentage: C 0.30-0.46%, Al 4.8-5.7%, Mn 18.1-22.3%, Nb 0.015-0.06%, O 0.0005-0.0010%, Si 0.05-0.20%, S≤0.005%, P≤0.005%, N≤0.0005%, with the balance being Fe and unavoidable impurities.

[0007] Preferably, in order to make the microstructure of the steel a single-phase equiaxed austenitic grain, the contents of C, Al and Mn elements satisfy: 3.5≤(Mn+C) / (Al+C)≤4.

[0008] The design principle of the chemical composition of a low-density medium-density fiberboard with a yield strength of 460 MPa described in this invention is as follows:

[0009] C: Carbon plays a role in stabilizing austenite and producing solid solution strengthening, thereby improving the strength of steel; however, excessive C content can lead to the formation of carbides in the steel, and too many carbides will deteriorate toughness and be detrimental to welding. Therefore, the C content is set at 0.30-0.46%.

[0010] Mn: Mn can expand the austenite phase region. Too high a Mn content will worsen low-temperature toughness and reduce weldability; too low a Mn content will affect the austenite phase content and reduce the strength and hardness of the steel. Therefore, the Mn content is set at 18.1-22.3%.

[0011] Al: Adding 1% Al will reduce the density of steel by 1.3%. Simultaneously, the addition of Al can regulate stacking fault energy. Too low an Al content will lead to a decrease in stacking fault energy, making it easier for martensite phases to form during deformation, thus worsening low-temperature toughness. Conversely, too high an Al content will produce δ-ferrite in the microstructure, and excessive δ-ferrite phases also severely impair low-temperature toughness. Therefore, the Al content is set at 4.8-5.7%. The Al content should satisfy the following condition with C and Mn: 3.5 ≤ (Mn+C) / (Al+C) ≤ 4.

[0012] Nitrogen (Nb): The addition of Nb enhances the strength and plasticity of steel through precipitation strengthening and grain refinement; Nb dissolving into austenite increases austenite hardness; however, excessive Nb can cause intergranular inclusions during heat treatment, leading to the formation of inorganic crystal nuclei and affecting the surface quality of the steel, such as causing defects like spots, cracks, and pits. Therefore, the Nb content is set at 0.015-0.06%.

[0013] O: During steelmaking, a certain amount of oxygen is an effective means of removing harmful gases, non-metallic inclusions, and other impurities. However, if excessive oxygen exists in the steel after smelting, it will cause significant harm. Oxygen in steel mainly exists in the form of oxides within non-metallic inclusions, reducing the steel's mechanical strength and toughness significantly, and also promoting aging and increasing hot brittleness. Therefore, the oxygen content is set at 0.0005~0.0010%.

[0014] Si: Si can dissolve in ferrite and austenite to increase the hardness and strength of steel. However, excessive Si content will significantly reduce the plasticity and toughness of steel; Si can also reduce the weldability of steel. Therefore, the Si content should be controlled between 0.05% and 0.20%.

[0015] S: Sulfur is usually a harmful element in steel, which not only affects the strength and weldability of the material, but also easily forms sulfide inclusions, which deteriorates the plasticity and toughness of the material. Therefore, the lower the sulfur content, the better. However, sulfur can improve the machinability of steel. Taking all factors into consideration, the sulfur content should be controlled below 0.005%.

[0016] P: Phosphorus is a ferrite phase-forming element. It can be partially dissolved in α-Fe, reducing the austenite phase region and increasing the content of ferrite in steel, which is relatively beneficial to corrosion resistance. However, if the phosphorus content is too high, it tends to agglomerate at the grain boundaries in the form of phosphides, thereby increasing the cold brittleness of the steel and deteriorating the plasticity, toughness and weldability of the material. Therefore, the phosphorus content should not be too high. Thus, the phosphorus content should be controlled below 0.005%.

[0017] Nitrogen (N) can increase the strength of steel, but it significantly reduces its ductility and toughness, worsens its weldability, and exacerbates cold brittleness. Therefore, the nitrogen content should be controlled below 0.0005%.

[0018] The preparation method of the above-mentioned low-density medium-density fiberboard with a yield strength of 460 MPa includes the following steps:

[0019] (1) Casting:

[0020] The molten steel is smelted according to the composition ratio of a low-density medium plate with a yield strength of 460 MPa, and then cast into steel ingots.

[0021] (2) Heating:

[0022] The steel ingot is held at 1100~1200℃ for 2~3 hours for homogenization treatment;

[0023] (3) Rolling:

[0024] The homogenized steel ingot is rolled in the recrystallization zone to ensure equiaxed grains. The initial rolling temperature is 1000~1100℃, the final rolling temperature is greater than 850℃, and the reduction rate is 80~90% to obtain hot-rolled steel. After rolling, the steel is air-cooled to room temperature.

[0025] (4) Poisoning:

[0026] Hot-rolled steel plates are held at 660~740℃ for 20~35 minutes and then air-cooled to room temperature to obtain low-density medium plates with a yield strength of 460MPa.

[0027] In step (1), the thickness of the steel ingot is 70~120mm.

[0028] In step (3), the thickness of the hot-rolled steel plate is 11~13mm.

[0029] Since Mn is an element that expands the austenite phase region, and Al is an element that expands the ferrite phase region.

[0030] Preferably, in step (3), the initial rolling temperature T of the hot-rolled steel plate is... r =55.44Al-12Mn+1000.

[0031] Preferably, in step (4), the normalizing temperature T of the hot-rolled steel plate n =45.4Al-9.4Mn+650.

[0032] The yield strength of the low-density normalized medium plate with a yield strength of 460MPa is 460~550MPa, the tensile strength is 660~830MPa, the elongation after fracture is 40~60%, the Charpy impact energy at -40℃ is 100~150J, and the density is 7.27~7.36g / cm³. 3 Its density is 6.24-7.41% lower than that of traditional low alloy steel.

[0033] Compared with existing technologies, the scientific composition design and process flow of this invention, which yields low-density, high-strength steel, differ from previously disclosed techniques in the field in the following ways:

[0034] 1. This invention adds the strengthening element Nb to the Fe-Mn-Al-C matrix elements, which refines the austenite grains, significantly improves the yield strength and ductility of the steel, and at the same time reduces the ductile-brittle transition temperature, giving the steel good weldability and formability.

[0035] 2. The present invention uses normalizing heat treatment as the final heat treatment, which can reduce internal stress, improve the toughness and weldability of steel, and improve the performance stability of steel; it can be used in transportation equipment that requires welding, such as train bogies. Attached Figure Description

[0036] Figure 1 This is a Kikuchi strip contrast diagram of the low-density normalized medium plate with a yield strength of 460 MPa prepared in Example 1 of this invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0038] Example

[0039] After being batched according to the chemical composition shown in Table 1, the raw materials were added to the furnace for melting and cast into ingots, obtaining ingots with the thickness shown in Table 2. The ingots were held at 1100℃ for 2 hours to homogenize their internal composition. Subsequently, they were rolled according to the process parameters in Table 2, and then air-cooled to room temperature. Following this, normalizing heat treatment was performed to weaken the internal stress in the steel structure and improve toughness and elongation. The properties and density of the embodiments are shown in Table 2.

[0040] Depend on Figure 1 It can be seen that the microstructure of the medium plate obtained by the above process is a fully austenitic structure. Because it is rolled in the recrystallization zone, the austenitic grains are equiaxed.

[0041] The comparative example uses the embodiment disclosed in CN 112281074 A, where the mass fractions of each element in the steel are: C 0.68%, Al 5.10%, Mn 18.3%, Si 0.18%, P 0.0018%, and S 0.0016%. Its yield strength is 471.3 MPa, tensile strength is 823.7 MPa, elongation at break is 46.5%, and Charpy impact energy at -196℃ is 155.4 J. The strength of the steel plate is improved by adding a high amount of carbon (C) and using a post-rolling water-cooling heat treatment process. However, excessively high C content will deteriorate the weldability of the steel, and the excessively rapid cooling rate of the water-cooling process will increase the internal stress in the steel and reduce its performance stability. Therefore, it is not suitable for transportation equipment requiring welding and with high safety requirements.

[0042] Table 1. Smelting composition (wt%) of steel ingots in embodiments of the present invention

[0043]

[0044] Table 2. Process parameters of the embodiments of the present invention

[0045]

[0046] Table 3. Mechanical properties and density of embodiments of the present invention

[0047]

Claims

1. A method for preparing a low-density medium-density fiberboard with a yield strength of 460 MPa, characterized in that, The chemical composition includes the following percentages by mass: C 0.30-0.46%, Al 4.8-5.7%, Mn 18.1-22.3%, Nb 0.015-0.06%, O 0.0005-0.0010%, Si 0.05-0.20%, S≤0.005%, P≤0.005%, N≤0.0005%, balance is Fe and unavoidable impurities, its chemical composition conforms to 3.5≤(Mn+C) / (Al+C)≤4; smelting, casting, and heating are carried out according to the composition ratio, the molten steel obtained after smelting is cast into steel ingots, the heated steel ingots are rolled in the recrystallization zone, the initial rolling temperature Tr=55.44Al-12Mn+1000, the final rolling temperature is greater than 850℃, the total reduction rate is 80~90%, and hot-rolled steel plate is obtained. After rolling, it is air-cooled to room temperature, and then normalized at a normalizing temperature Tn=45.4Al-9.4Mn+650. The hot-rolled steel plate is held at 660~740℃ for 20~35min, and then air-cooled to room temperature to obtain a low-density medium plate with a yield strength of 460MPa.

2. The preparation method according to claim 1, characterized in that, The medium-strength plate has a yield strength of 460~550MPa, a tensile strength of 660~830MPa, an elongation after fracture of 40~60%, a Charpy impact energy of 100~150J at -40℃, and a density of 7.27~7.36g / cm³. 3 .

3. The preparation method according to claim 1, characterized in that, The ingot is formed by casting molten steel obtained after smelting into a steel ingot with a thickness of 70~120mm.

4. The preparation method according to claim 1, characterized in that, The heating process involves heating the steel ingot to 1100~1200℃ and holding it at that temperature for 2~3 hours for homogenization.

5. The preparation method according to claim 1, characterized in that, The thickness of the hot-rolled steel plate is 11~13mm.

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