Low-density steel for high-speed railway welding bogie and preparation method and application thereof

Abstract

This invention provides a low-density steel comprising the following chemical composition by mass percentage: C 0.27–0.38%, Al 4.5–5.5%, Mn 9.6–11.9%, Mg 0.0005–0.0015%, O 0.0005–0.0010%, Si ≤ 0.20%, S ≤ 0.005%, P ≤ 0.005%, N ≤ 0.0005%, with the balance being Fe and unavoidable impurities; wherein the mass percentages of C, Al, and Mn satisfy: 0.2 ≤ [(Mn / C)] 0.5 ] / Al 2 ≤0.3. This invention also provides a method for preparing low-density medium-thick steel plates for high-speed rail welded bogies, comprising: casting, heating, rolling, and normalizing treatment. This invention also provides the low-density medium-thick steel plates prepared by this method and their application in high-speed rail welded bogies.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting and heat treatment, and specifically relates to a low-density steel, its preparation method and application. Background Technology

[0002] The bogie is one of the most important components in the structure of a rail vehicle, integrating functions such as load-bearing, traction, buffering, steering, and braking, and plays a vital role in the safe operation of the entire rail vehicle.

[0003] Bogies are classified into traditional welded types and fully assembled, weld-free types based on their manufacturing processes. The former uses steel, while the latter uses lightweight new materials. Currently, welded bogies still dominate the market. Welded bogies mostly use S355J2W and SMA490BW weathering steels from the Cr-Ni-Cu alloy system. Both types of steel require normalizing heat treatment. Normalizing refines the grain size of the steel, which not only increases strength but also significantly improves impact toughness, reduces the tendency of components to crack, and thus improves the weldability of the steel. In addition, normalizing also improves the stability of the steel's properties. Due to the high speed of high-speed trains, higher requirements are specified for the performance of steel used in welded bogies, specifically: yield strength not less than 355MPa, tensile strength not less than 490MPa, elongation after fracture not less than 20%, Charpy impact energy at -40℃ not less than 27J, and good weldability, fatigue resistance, and atmospheric corrosion resistance.

[0004] To further improve speed and reduce energy consumption and emissions, the desire to reduce the weight of high-speed trains ("lightweighting") is growing stronger, and continuous efforts and attempts are being made. Each carriage of a high-speed train is equipped with two bogies. Taking a common short-formation train with eight carriages as an example, the entire high-speed train is equipped with 16 bogies. The car body of high-speed trains already uses lightweight aluminum alloys, and the focus of lightweighting is concentrated on welded bogies. Traditional weathering steel is a low-carbon, low-alloy steel with a density typically of 7.85 g / cm³. 3 How to develop lightweight, high-strength, high-toughness, and weldable bogie steel based on existing S355J2W and SMA490BW weathering steel processes and equipment has become a hot topic in the research on lightweighting of high-speed trains.

[0005] Existing technologies have shown that adding aluminum can reduce the density of steel, thereby achieving the goal of lightweighting. Examples include Chinese invention patent applications CN103741057A (published April 23, 2014) entitled "A low-density steel plate with high resistance to marine environment corrosion and its production process," CN106756478A (published May 31, 2017) entitled "An economical low-density low-alloy steel for seawater corrosion resistance and its preparation method," CN114480984A (published May 13, 2022) entitled "A Ti alloyed low-density high-strength steel and its preparation method," CN115323278A (published November 11, 2022) entitled "A low-density steel with a yield strength of 700MPa and its heat treatment method," and CN114107830A (published March 1, 2022) entitled "A low-density wear-resistant steel for wide temperature range and its preparation method," etc. However, in CN103741057A and CN106756478A, the highest percentage of aluminum by mass in these two low-density steels is only 2.0%, which is not a significant contribution to weight reduction. CN114480984 A, CN115323278 A, and CN114107830 A, involve low-density high-strength steels with excessively high carbon content, with the lowest percentage of carbon by mass being 0.7%, and CN114480984 A even exceeding 1.0%. Excessive carbon content severely reduces the steel's ductility and toughness, making it brittle and reducing its weldability; therefore, none of these meet the performance requirements of bogies. CN114892084 A and CN112281074 A both use a rapid cooling process after controlled rolling and cooling, which is incompatible with the existing equipment and processes for normalized steel plates used in bogies. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this invention provides a low-density steel for high-speed rail welded bogies, replacing S355J2W and SMA490BW weathering steel. This low-density steel plate, while meeting the performance requirements of high-speed train welded bogies, significantly reduces the weight of the bogies, achieving train lightweighting, thereby reducing energy consumption, improving energy efficiency, and facilitating further speed increases for high-speed rail. This invention also provides a method for preparing this low-density medium-thick steel plate for high-speed rail welded bogies.

[0007] To achieve the above-mentioned technical effects, the present invention adopts the following technical solution:

[0008] A low-density steel for high-speed rail welded bogies comprises the following chemical composition by weight percentage:

[0009] C 0.27~0.38%, Al 4.5~5.5%, Mn 9.6~11.9%, Mg 0.0005~0.0015%, O 0.0005~0.0010%, Si≤0.20%, S≤0.005%, P≤0.005%, N≤0.0005%, balance Fe and unavoidable impurities;

[0010] To ensure that the austenite content in the steel is greater than the δ-ferrite content after normalizing, the mass percentages of C, Al, and Mn must satisfy: 0.2 ≤ [(Mn / C)]. 0.5 ] / Al 2 ≤0.3.

[0011] Preferably, the low-density steel comprises the following chemical composition by mass percentage:

[0012] The composition is as follows: C 0.28~0.33%, Al 4.7~5.2%, Mn 10.4~11.6%, Mg 0.0006~0.0010%, O 0.0005~0.0010%, Si≤0.2%, S≤0.004%, P≤0.0035%, N≤0.00035%, with the balance being Fe and unavoidable impurities. The mass percentages of C, Al, and Mn satisfy: 0.23≤[(Mn / C)] 0.5 ] / Al 2 ≤0.3.

[0013] Another object of the present invention is to provide a method for preparing low-density medium-thick steel plates for welded bogies of high-speed railways, comprising the following steps:

[0014] (1) Casting:

[0015] The low-density steel is smelted according to the composition ratio of the present invention, and the obtained molten steel is cast into steel ingots.

[0016] (2) Heating:

[0017] The steel ingot is heated to 1100~1200℃ and held for 2~3 hours for homogenization.

[0018] (3) Rolling:

[0019] A two-stage rolling process is adopted. The first stage of rolling is carried out in the austenite recrystallization region, with no control over the initial rolling temperature. The final rolling temperature is greater than 950℃, and the reduction rate is 52.5%~63.7%. Then, after the steel plate temperature drops to the two-phase region, the second stage of rolling is carried out, with an initial rolling temperature of 900~950℃ and a final rolling temperature of 700~800℃. The total reduction rate of the two-stage rolling is 80~90%, resulting in hot-rolled steel, which is then air-cooled to room temperature.

[0020] (4) Normalizing treatment:

[0021] The hot-rolled steel sheet cooled to room temperature is held at 750~1200℃ for 1 hour, and then air-cooled to room temperature to obtain the product.

[0022] Preferably, in step (1), the thickness of the steel ingot is 50~150mm.

[0023] Preferably, in step (2), the heat preservation temperature of the steel ingot satisfies:

[0024] T h = 1233.3+16.67×[Al]×100-16.67×[Mn]×100;

[0025] Wherein [Al] and [Mn] are the mass percentages of Al and Mn in the low-density steel, respectively.

[0026] Preferably, in step (3), the thickness of the hot-rolled steel plate is 10~25mm.

[0027] Preferably, in step (3), the initial rolling temperature during the second stage of rolling satisfies:

[0028] Ts = 966.67+8.33×[Al]×100-8.33×[Mn]×100,

[0029] Final rolling temperature meets

[0030] Te = 833.33+16.67×[Al]×100-16.67×[Mn]×100;

[0031] Wherein [Al] and [Mn] are the mass percentages of Al and Mn in the low-density steel, respectively.

[0032] Preferably, in step (4), the holding temperature (T), i.e. the normalizing temperature, satisfies:

[0033] T = 93 + 138.89 × [C] × [Mn] × 10 4 +370.25×[Al]×100-125.76×[Mn]×100;

[0034] Wherein, [Al], [Mn] and [C] are the mass percentages of Al, Mn and C in the low-density steel, respectively.

[0035] Al is used to shrink the austenite phase region, and Mn is used to expand the austenite phase region. Therefore, the T h Ts, Te, and T all increase with increasing Al content and decrease with increasing Mn content.

[0036] A third objective of this invention is to provide a low-density, medium-thick steel plate prepared by the above-described preparation method.

[0037] In addition, the present invention also provides the application of the above-mentioned low-density medium-thick steel plate in the preparation of welded bogies for railcars.

[0038] Preferably, the rail train refers to a high-speed railway train.

[0039] The design basis and reasons for limiting the effective chemical composition range of the low-density steel for welded bogies of high-speed railways provided by this invention are as follows:

[0040] C: Carbon can stabilize austenite and produce solid solution strengthening, thereby improving the strength of steel; however, excessive C will lead to the formation of carbides in the steel, and too many carbides will deteriorate the toughness, which is not conducive to welding. Therefore, the C content is set at 0.27~0.38%.

[0041] 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 9.6~11.9%.

[0042] Al: Adding 1% Al reduces the density of steel by 1.3%. Simultaneously, the addition of Al can regulate stacking fault energy. Too low an Al content leads 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 results in the formation of δ-ferrite in the microstructure, and excessive δ-ferrite phases also severely impair low-temperature toughness. Therefore, the Al content is set at 4.5~5.5%. The Al content should satisfy the following relationship with C and Mn: 0.2 ≤ [(Mn / C)] 0.5 ] / Al 2 ≤0.3.

[0043] Mg: Trace amounts of Mg can form nanoscale magnesium aluminum spinel with Al, promoting solidification and nucleation, and refining the weld structure. However, excessive Mg content will deteriorate the performance. Therefore, the Mg content is set at 0.0005~0.0015%.

[0044] 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%.

[0045] 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 below 0.20%.

[0046] 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%.

[0047] P: Phosphorus is a ferrite phase-forming element. It can be partially dissolved in α-Fe, shrinking 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 is easy for phosphorus to segregate at the grain boundaries in the form of phosphides, thereby increasing the cold brittleness of steel and deteriorating the plasticity, toughness and weldability of the material. Therefore, the phosphorus content should not be too high. Taking all factors into consideration, the phosphorus content should be controlled below 0.005%.

[0048] 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%.

[0049] The low-density steel provided by this invention has a density of 7.30~7.41 g / cm³. 3 Compared to traditional high-speed rail bogies, this method reduces steel usage by 5.8% to 7.2%. Furthermore, the low-density, medium-thick steel plate obtained through this invention combines lightweight with high strength and high toughness, exhibiting a yield strength of 355-550 MPa, a tensile strength of 640-850 MPa, an elongation after fracture of 20-40%, and a Charpy impact energy of 28-55 J at -40℃. Therefore, the steel plate provided by this invention can be applied to welded high-speed rail bogies, achieving the goal of lightweighting high-speed trains.

[0050] The beneficial effects of this invention are as follows: This invention adopts a Fe-Mn-Al-C composition design and, based on existing weathering steel S355J2W and SMA490BW production equipment, produces high-performance medium-thick steel plates with significantly reduced density. The production cost is low, and the strength, ductility, toughness, and weldability are well matched. The corrosion rate of this steel in natural environment is two-thirds that of traditional weathering steel S355J2W and SMA490BW, and it can replace the above-mentioned weathering steel plates for use in high-speed rail welded bogies. Detailed Implementation

[0051] The present invention will be described below with reference to specific embodiments. Those skilled in the art will understand that these embodiments are for illustrative purposes only and do not limit the scope of the invention in any way.

[0052] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, all raw materials and reagents used in the following examples are commercially available products.

[0053] Example 1: Preparation of a low-density normalized medium-thick steel plate and the resulting steel plate

[0054] Casting: Various components are added to a furnace to melt and cast into steel ingots, obtaining square steel ingots with a thickness of 80 mm. Samples were taken for chemical composition analysis, and the results are shown in Table 1.

[0055] Heating: Heat the steel ingot to 1135℃ and hold for 2 hours.

[0056] Rolling: The heated steel ingot was subjected to the first stage of recrystallization rolling in the recrystallization zone without controlling the initial rolling temperature. The final rolling temperature was 970℃, and the reduction rate was 61.3%. After the steel ingot was slightly cooled, it was subjected to the second stage of rolling in the two-phase zone. The initial rolling temperature was 917℃, the final rolling temperature was 735℃, and the total reduction rate was 86.25%, resulting in a hot-rolled steel plate with a thickness of 11mm. The plate was then air-cooled to room temperature. The tensile properties in the hot-rolled state were measured and are shown in Table 2.

[0057] Normalizing treatment: The steel plate is held at 1050℃ for 1 hour and then air-cooled to room temperature to obtain the steel plate of this embodiment.

[0058] Corrosion resistance test: The steel plate was subjected to a periodic immersion corrosion test according to the TB / T 2375-1993 standard. The periodic immersion test lasted for 75 hours. Pickling and rust removal were performed according to the GB / T 16545-2015 standard, and the weight loss rate of the steel plate was measured.

[0059] The mechanical properties, density, and weight loss rate of the normalized steel plate obtained in this embodiment are shown in Table 3.

[0060] The data in Table 3 shows that this embodiment has excellent corrosion resistance, with a relative corrosion rate of 58% compared to S355J2W weathering steel plate in natural environment.

[0061] Example 2: Preparation of a low-density normalized medium-thick steel plate and the resulting steel plate

[0062] Casting: Various components are added to the furnace to melt and cast into steel ingots to obtain square steel ingots with a thickness of 130 mm; samples are taken for chemical composition analysis, and the chemical composition results are shown in Table 1.

[0063] Heating: Heat the steel ingot to 1140℃ and hold for 2 hours;

[0064] Rolling: The heated steel ingot was subjected to the first stage of recrystallization rolling, with no control over the initial rolling temperature and a final rolling temperature of 975℃, resulting in a reduction rate of 63.3%. After slight cooling, the steel ingot was subjected to the second stage of two-phase rolling, with an initial rolling temperature of 920℃ and a final rolling temperature of 740℃, resulting in a total reduction rate of 90.2%, to obtain a hot-rolled steel plate with a thickness of 13mm. The plate was then air-cooled to room temperature. The tensile properties in the hot-rolled state were measured and are shown in Table 2.

[0065] Normalizing treatment: The steel plate is held at 950°C for 1 hour and then air-cooled to room temperature to obtain the steel plate of this embodiment.

[0066] Corrosion resistance test: The steel plate was subjected to a periodic immersion corrosion test according to the TB / T 2375-1993 standard. The periodic immersion test lasted for 75 hours. Pickling and rust removal were performed according to the GB / T 16545-2015 standard, and the weight loss rate of the steel plate was measured.

[0067] The mechanical properties, density, and weight loss rate of the normalized steel plate obtained in this embodiment are shown in Table 3.

[0068] The data in Table 3 shows that this embodiment has excellent corrosion resistance, with a relative corrosion rate of 61% compared to S355J2W weathering steel plate in natural environment.

[0069] Example 3: Preparation of a low-density normalized medium-thick steel plate and the resulting steel plate

[0070] Casting: Various components are added to the furnace to melt and cast into steel ingots to obtain square steel ingots with a thickness of 70 mm; samples are taken for chemical composition analysis, and the chemical composition results are shown in Table 1.

[0071] Heating: Heat the steel ingot to 1148℃ and hold for 2 hours;

[0072] Rolling: The heated steel ingot was subjected to the first stage of recrystallization rolling in the recrystallization zone without controlling the initial rolling temperature. The final rolling temperature was 960℃, and the reduction rate was 59.3%. After the steel ingot was slightly cooled, it was subjected to the second stage of rolling in the two-phase zone. The initial rolling temperature was 924℃, the final rolling temperature was 748℃, and the total reduction rate was 82.9%, resulting in a hot-rolled steel plate with a thickness of 12mm. The plate was then air-cooled to room temperature. The tensile properties in the hot-rolled state were measured and are shown in Table 2.

[0073] Normalizing treatment: The steel plate is held at 980°C for 1 hour and then air-cooled to room temperature to obtain the steel plate of this embodiment.

[0074] Corrosion resistance test: The steel plate was subjected to a periodic immersion corrosion test according to the TB / T 2375-1993 standard. The periodic immersion test lasted for 75 hours. Pickling and rust removal were performed according to the GB / T 16545-2015 standard, and the weight loss rate of the steel plate was measured.

[0075] The mechanical properties, density, and weight loss rate of the normalized steel plate obtained in this embodiment are shown in Table 3.

[0076] The data in Table 3 shows that this embodiment has excellent corrosion resistance, with a relative corrosion rate of 62% compared to S355J2W weathering steel plate in natural environment.

[0077] Comparative Example 1: Preparation of a low-density normalized medium-thick steel plate and the resulting steel plate

[0078] Comparative Example 1 has the same composition as Example 1 (see Table 1) and is prepared in accordance with the steps and process of Example 1. However, during the normalizing treatment, the steel plate is held at 650°C for 1 hour.

[0079] Corrosion resistance test: The steel plate was subjected to a periodic immersion corrosion test according to the TB / T 2375-1993 standard. The periodic immersion test lasted for 75 hours. Pickling and rust removal were performed according to the GB / T 16545-2015 standard, and the weight loss rate of the steel plate was measured.

[0080] The mechanical properties, density, and weight loss of the normalized steel plates obtained in this comparative example are shown in Table 3.

[0081] The data in Table 3 shows that the mechanical properties of the steel plate in this comparative example cannot meet the requirements of high-speed rail bogies.

[0082] Table 1. Smelting composition (wt%) of steel ingots from Examples 1-3 and Comparative Example 1

[0083] Example C Al Mn Mg O Si S P N Example 1 0.32 5.12 11.0 0.0010 0.0006 0.13 0.001 0.001 0.0001 Example 2 0.27 4.8 10.4 0.0008 0.0009 0.15 0.002 0.002 0.0001 Example 3 0.30 4.6 9.7 0.0007 0.0005 0.11 0.001 0.003 0.0002 Comparative Example 1 0.32 5.12 11.0 0.0010 0.0006 0.13 0.001 0.001 0.0001

[0084] Table 2 Mechanical properties of hot-rolled steel plates from Examples 1-3 before normalizing

[0085] Example <![CDATA[R p0.2 / MPa]]> <![CDATA[R m / MPa]]> A / % -40℃ AKV / J Example 1 304.6 1107.0 9.3 7 Example 2 352.3 1045.1 12.3 13 Example 3 361.2 998.2 15.0 14

[0086] Table 3 Mechanical properties, density, and weight loss rate of the steel plates from Examples 1-3 and Comparative Example 1

[0087] Example Normalizing temperature / °C <![CDATA[R p0.2 / MPa]]> <![CDATA[R m / MPa]]> A / % -40℃ AKV / J <![CDATA[ρ / g / cm 3 ]]> <![CDATA[Weight loss rate / g*(cm 2 ·h) -1 > Example 1 1050 413.5 814.5 23.1 35 7.35 1.68 Example 2 950 395.4 768.1 25% 47 7.37 1.77 Example 3 980 400.2 776.4 26% 50 7.38 1.80 Comparative Example 1 650 351.1 865.2 15.1 21 7.35 1.68

[0088] Comparative Example 2: Cu-Cr-Ni alloy system weathering steel plate S355J2W

[0089] The steel plate composition for this comparative example is shown in Table 4. A cyclic immersion corrosion test was conducted on the steel plate according to TB / T 2375-1993 standard, with each cyclic immersion test lasting 75 hours. Pickling and rust removal were performed according to GB / T 16545-2015 standard. The mechanical properties, density, and weight loss rate of the steel plate for this comparative example are shown in Table 5.

[0090] Table 5 shows that the mechanical properties of weathering steel plate S355J2W meet the requirements for use in high-speed railway bogies; however, its density is 7.85 g / cm³.3 The steel plate is larger than that of the present invention; the weight loss rate in the cyclic immersion corrosion test is 2.90 g / cm³. 2 The density h is much larger than that of the steel plate of the present invention, and its corrosion resistance is not as good as that of the low-density steel plate of the present invention.

[0091] Table 4. Composition (wt%) of S355J2W weathering steel plate in Comparative Example 2

[0092] C Si Mn P S N Cr Cu S355J2W 0.19 0.55 0.98 0.03 0.02 0.005 0.74 0.41

[0093] Table 5. Properties, density, and weight loss (wt%) of S355J2W weathering steel plate from Comparative Example 2

[0094] <![CDATA[R p0.2 / MPa]]> <![CDATA[R m / MPa]]> A / % -40℃ AKV / J <![CDATA[ρ / g / cm 3 ]]> <![CDATA[Weight loss rate / g*(cm 2 ·h) -1 > S355J2W 365 523 22 31 7.85 2.90

[0095] In summary, this invention provides a low-density, high-strength, weather-resistant steel plate that can meet the requirements for steel in high-speed rail welded bogies and helps to achieve lightweighting of high-speed trains.

Claims

1. A low-density steel for high-speed rail welded bogies, having the following chemical composition by weight percentage: C 0.27~0.38%, Al 4.5~5.5%, Mn 9.6~11.9%, Mg 0.0005~0.0015%, O 0.0005~0.0010%, Si≤0.20%, S≤0.005%, P≤0.005%, N≤0.0005%, balance Fe and unavoidable impurities; in, The mass percentages of C, Al, and Mn satisfy the following condition: 0.2 ≤ [(Mn / C)] 0.5 ] / Al 2 ≤0.3; The low-density steel is prepared by the following method: (1) Casting: The low-density steel is smelted according to the composition ratio described above, and the molten steel is cast into steel ingots. (2) Heating: The steel ingot is heated to 1100~1200℃ and held for 2~3 hours for homogenization. (3) Rolling: A two-stage rolling process is adopted. The first stage of rolling is carried out in the austenite recrystallization region, with no control over the initial rolling temperature. The final rolling temperature is greater than 950℃, and the reduction rate is 52.5%~63.7%. Then, after the steel plate temperature drops to the two-phase region, the second stage of rolling is carried out, with an initial rolling temperature of 900~950℃ and a final rolling temperature of 700~800℃. The total reduction rate of the two-stage rolling is 80~90%, resulting in hot-rolled steel, which is then air-cooled to room temperature. (4) Normalizing treatment: The hot-rolled steel sheet cooled to room temperature is held at 750~1200℃ for 1 hour, and then air-cooled to room temperature to obtain the product.

2. The low-density steel according to claim 1, characterized in that, The low-density steel has the following chemical composition by mass percentage: C 0.28~0.33%, Al 4.7~5.2%, Mn 10.4~11.6%, Mg 0.0006~0.0010%, O 0.0005~0.0010%, Si≤0.2%, S≤0.004%, P≤0.0035%, N≤0.00035%, with the balance being Fe and unavoidable impurities; the mass percentages of C, Al, and Mn satisfy: 0.23≤[(Mn / C)] 0.5 ] / Al 2 ≤0.

3.

3. A method for preparing low-density medium-thick steel plates for welded bogies of high-speed railways, comprising the following steps: (1) Casting: The low-density steel is smelted according to the composition ratio of claim 1 or 2, and the resulting molten steel is cast into steel ingots. (2) Heating: The steel ingot is heated to 1100~1200℃ and held for 2~3 hours for homogenization. (3) Rolling: A two-stage rolling process is adopted. The first stage of rolling is carried out in the austenite recrystallization region, with no control over the initial rolling temperature. The final rolling temperature is greater than 950℃, and the reduction rate is 52.5%~63.7%. Then, after the steel plate temperature drops to the two-phase region, the second stage of rolling is carried out, with an initial rolling temperature of 900~950℃ and a final rolling temperature of 700~800℃. The total reduction rate of the two-stage rolling is 80~90%, resulting in hot-rolled steel, which is then air-cooled to room temperature. (4) Normalizing treatment: The hot-rolled steel sheet cooled to room temperature is held at 750~1200℃ for 1 hour, and then air-cooled to room temperature to obtain the product.

4. The preparation method according to claim 3, characterized in that, In step (1), the thickness of the steel ingot is 50~150mm.

5. The preparation method according to claim 3, characterized in that, In step (2), the holding temperature of the steel ingot satisfies: T h = 1233.3 + 16.67 × [Al] × 100 - 16.67 × [Mn] × 100; Wherein [Al] and [Mn] are the mass percentages of Al and Mn in the low-density steel, respectively.

6. The preparation method according to claim 3, characterized in that, In step (3), the thickness of the hot-rolled steel plate is 10~25mm.

7. The preparation method according to claim 3 or 6, characterized in that, In step (3), the initial rolling temperature during the second stage of rolling satisfies: Ts = 966.67+8.33×[Al]×100-8.33×[Mn]×100, The final rolling temperature must meet the following requirements: Te = 833.33+16.67×[Al]×100-16.67×[Mn]×100; Wherein [Al] and [Mn] are the mass percentages of Al and Mn in the low-density steel, respectively.

8. The preparation method according to claim 3, characterized in that, In step (4), the heat preservation temperature T satisfies: T = 93+138.89×[C]×[Mn]×10 4 +370.25×[Al]×100-125.76×[Mn]×100; Wherein, [Al], [Mn] and [C] are the mass percentages of Al, Mn and C in the low-density steel, respectively.

9. A low-density medium-thickness steel plate, prepared by the preparation method described in any one of claims 3 to 8.

10. The application of the low-density medium-thick steel plate of claim 9 in the manufacture of welded bogies for railcars.

11. The application according to claim 10, characterized in that, The rail train in question refers to a high-speed railway train.