High toughness high elongation low density medium gauge sheet and method of making same

Abstract

This invention provides a low-density medium-thick plate with high toughness and high elongation, and its preparation method, belonging to the field of steel material technology. The chemical composition of the low-density medium-thick plate with high toughness and high elongation, by weight percentage, is: C 0.28-0.34%, Al 5.0-5.9%, Mn 16.8-17.8%, Mg 0.0005-0.0015%, O 0.0005-0.0010%, S≤0.005%, P≤0.005%, N≤0.0005%, with the balance being Fe and unavoidable impurities. In the rolling process of this invention, the heated steel ingot is rolled in the recrystallization zone at an initial rolling temperature of 1000-1100℃, a final rolling temperature greater than 800℃, and a total reduction rate of 80-90%, to obtain hot-rolled steel, which is then air-cooled to room temperature after rolling. After hot rolling, the steel is normalized by holding the hot-rolled steel plate at 750-950℃ for 40-60 minutes, followed by air cooling to room temperature. The low-density medium-thick plate of this invention has a yield strength ≥300MPa, tensile strength ≥640MPa, elongation after fracture ≥60%, Charpy impact energy at -40℃ ≥200J, and a density of 7.25~7.40g / cm³. 3 The density of alloy steel is reduced by 6.5% to 7.67%.

Description

Technical Field

[0001] This invention belongs to the field of steel materials technology, specifically relating to a low-density medium-thick plate with high toughness and high elongation and its preparation method. Background Technology

[0002] With the continuous development of society and the economy, the number of various transportation equipment such as automobiles, high-speed trains, and ships is increasing, leading to higher energy consumption and consequently, more prominent emissions problems. Reducing the weight of the transportation equipment itself is one of the important ways to achieve emission reduction targets. Therefore, lightweight design of transportation equipment has become an important measure to solve the problem and achieve energy conservation and environmental protection. While various transportation equipment require lightweight design, they also demand extremely high safety, thus requiring related steels to have low density and excellent impact resistance. Several patents have been filed in China related to low-density medium-thick plates. Some of these patents involve low-density medium-thick steel plates resistant to marine corrosion. CN 106756478A discloses "An economical low-density low-alloy steel for seawater corrosion resistance and its preparation method," which involves low-density medium-thick steel plates resistant to marine corrosion. However, considering its application in a marine environment, its aluminum content is too low, with the highest Al mass percentage being only 2.0%, resulting in a weight reduction of only 2.6% compared to traditional non-low-density steel. This limits its application scope and makes it unsuitable for various types of transportation equipment. CN 114892084A discloses a "high-strength austenitic lightweight steel with high impact toughness and its manufacturing method." While increasing the Al content, it also adds a significant amount of carbon to improve strength, with a carbon mass percentage of 0.83-0.92%. Excessive carbon content deteriorates the steel's weldability, and the final heat treatment, quenching, increases the steel's cracking tendency, resulting in insufficient performance stability. CN 112281074A discloses a "high-manganese medium-thick plate for low-density LNG storage tanks and its preparation method." After controlled rolling and cooling, the steel plate undergoes rapid cooling without other heat treatment processes. Therefore, the steel's performance stability is insufficient, and it has a high cracking tendency, failing to meet the high safety performance requirements of transportation equipment.

[0003] In summary, to meet the application needs of various transportation equipment such as automobiles, high-speed trains, and ships, it is still necessary to develop a medium-thick steel plate with low density, high toughness, high elongation, and good performance stability and weldability. Summary of the Invention

[0004] The purpose of this invention is to provide a low-density medium-thick plate with high toughness and high elongation and its preparation method. By designing the composition of Fe, Mn, Al, C and other components and carrying out appropriate heat treatment processes, the steel can have excellent comprehensive mechanical properties such as certain yield strength, high elongation and high low-temperature impact toughness, as well as low density and excellent corrosion resistance, and can be used in transportation equipment such as automobiles, high-speed trains, and ships.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A low-density, medium-thick plate with high toughness and high elongation comprises the following chemical composition by mass percentage: C 0.28-0.34%, Al 5.0-5.9%, Mn 16.8-17.8%, Mg 0.0005-0.0015%, O 0.0005-0.0010%, S≤0.005%, P≤0.005%, N≤0.0005%, with the balance being Fe and unavoidable impurities.

[0007] Preferably, in order to make the ferrite content in the steel microstructure less than 15%, the chemical composition of C, Al and Mn meets the condition 2.8≤(Mn+C) / Al≤3.2.

[0008] The design basis and reasons for limiting the effective chemical composition range of the high-toughness, high-elongation, low-density medium-thick plate of this invention are as follows:

[0009] C: Carbon plays a role in stabilizing austenite and also produces 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 toughness, which is detrimental to welding. Therefore, the C content is set at 0.28-0.34%.

[0010] Mn: Mn expands 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 16.8-17.8%.

[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 5.0-5.9%. The Al content should satisfy the following condition with C and Mn: 2.8 ≤ (Mn+C) / Al ≤ 3.2.

[0012] Mg: Trace amounts of Mg can form nanoscale magnesium aluminum spinel with Al, promoting solidification and nucleation, and refining the weld microstructure. However, excessive Mg content will deteriorate the performance. Therefore, the Mg content is set at 0.0005–0.0015%.

[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] 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%.

[0015] 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%.

[0016] 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%.

[0017] A method for preparing a low-density, medium-thick plate with high toughness and high elongation involves smelting, casting, and heating the steel ingot according to the specified composition ratio. The ingot thickness is 55-140 mm. The heated ingot is then rolled in the recrystallization zone at an initial rolling temperature of 1000-1100℃ and a final rolling temperature greater than 800℃, with a total reduction rate of 80-90%, yielding a hot-rolled steel plate with a thickness of 10-20 mm. After rolling, the plate is air-cooled to room temperature. The hot-rolled steel plate is then subjected to normalizing heat treatment, i.e., held at 750-950℃ for 40-60 min, followed by air cooling to room temperature, resulting in a low-density, medium-thick plate with high toughness and high elongation.

[0018] Preferably, considering that Mn expands the austenite phase region and Al shrinks it, the recrystallization zone rolling of the heated steel ingot is performed at a starting temperature of T. r =71.4Al-25.7Mn+1100.6.

[0019] Preferably, considering that Mn expands the austenite phase region and Al shrinks the austenite phase region, the normalizing heat treatment temperature T of the hot-rolled steel plate is... n =172.4Al–44.8Mn+685.9.

[0020] Preferably, the thickness of the steel ingot is 70-120 mm.

[0021] Preferably, the heating involves heating the steel ingot to 1100–1200°C and holding it at that temperature for 2–3 hours for homogenization.

[0022] Preferably, the thickness of the hot-rolled steel plate is 11-13 mm.

[0023] The aforementioned high-toughness, high-elongation, low-density, medium-thick plate has a yield strength ≥300MPa, tensile strength ≥640MPa, elongation after fracture ≥60%, Charpy impact energy at -40℃ ≥200J, and a density of 7.25~7.40g / cm³. 3 The density of alloy steel is reduced by 6.5% to 7.67%.

[0024] The microstructure of the high-toughness, high-elongation, low-density medium-thick plate is austenitic matrix and δ-ferrite. After normalizing heat treatment, the ferrite content in the microstructure is 3% to 15%.

[0025] The beneficial effects of this invention are as follows: This invention employs an Fe-Mn-Al-C composition design to produce medium-thick steel plates with significantly reduced density, high toughness, and high elongation, resulting in low production costs. The normalizing process refines the grain structure of the steel, not only improving strength but also significantly enhancing impact toughness and reducing the tendency of components to crack, thus improving the weldability of the steel. Simultaneously, normalizing further improves the stability and safety of the steel's properties. Due to the addition of Al, the steel in this invention exhibits excellent corrosion resistance. It can be used in transportation vehicles such as automobiles, high-speed trains, and ships. Attached Figure Description

[0026] Figure 1 This is a Kikuchi strip contrast diagram of the low-density, medium-thick plate with high toughness and high elongation prepared in Example 1 of this invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0028] Example 1

[0029] Casting: According to the chemical composition requirements of this invention (Table 1), various components are added to the furnace for melting and then cast into steel ingots to obtain square steel ingots with a thickness of 80 mm. Samples are taken for chemical composition analysis, and the chemical composition results are shown in Table 1.

[0030] Heating: Heat the steel ingot to 1100℃ and hold for 2 hours;

[0031] Rolling: The heated steel ingot is rolled in the recrystallization zone at an initial rolling temperature of 1060℃ and a final rolling temperature of 900℃, with a total reduction of 85.0%, to obtain a hot-rolled steel plate with a thickness of 12mm. After rolling, the plate is air-cooled to room temperature.

[0032] Normalizing heat treatment: The steel plate was held at 880℃ for 60 minutes, and then air-cooled to room temperature. The mechanical properties and density after normalizing are shown in Table 2.

[0033] Depend on Figure 1 It can be seen that the microstructure of the medium-thick plate obtained by the above process is a dual-phase structure of austenite + δ-ferrite, with δ-ferrite distributed in bands along the rolling direction.

[0034] The ferrite content of the low-density medium-thick plate with high toughness and high elongation prepared in Example 1 is about 8%, and the rest is an austenitic matrix.

[0035] Example 2

[0036] Casting: According to the chemical composition requirements of this invention (Table 1), various components are added to the furnace for melting and then cast into steel ingots to obtain square steel ingots with a thickness of 100 mm. Samples are taken for chemical composition analysis, and the chemical composition results are shown in Table 1.

[0037] Heating: Heat the steel ingot to 1150℃ and hold for 2 hours;

[0038] Rolling: The heated steel ingot is rolled in the recrystallization zone at an initial rolling temperature of 1060℃ and a final rolling temperature of 900℃, with a total reduction of 87%, to obtain a hot-rolled steel plate with a thickness of 13mm. After rolling, the plate is air-cooled to room temperature.

[0039] Normalizing heat treatment: The steel plate was held at 897℃ for 60 minutes, and then air-cooled to room temperature. The mechanical properties and density after normalizing are shown in Table 2.

[0040] The ferrite content of the low-density medium-thick plate with high toughness and high elongation prepared in Example 2 is about 10%, and the rest is an austenitic matrix.

[0041] Example 3

[0042] Casting: According to the chemical composition requirements of this invention (Table 1), various components are added to the furnace for melting and then cast into steel ingots to obtain square steel ingots with a thickness of 90 mm. Samples are taken for chemical composition analysis, and the chemical composition results are shown in Table 1.

[0043] Heating: Heat the steel ingot to 1150℃ and hold for 2 hours;

[0044] Rolling: The heated steel ingot is rolled in the recrystallization zone at an initial rolling temperature of 1050℃ and a final rolling temperature of 900℃, with a total reduction of 87.78%, to obtain a hot-rolled steel plate with a thickness of 11mm. After rolling, the plate is air-cooled to room temperature.

[0045] Normalizing heat treatment: The steel plate was held at 864℃ for 60 minutes, and then air-cooled to room temperature. The mechanical properties and density after normalizing are shown in Table 2.

[0046] The ferrite content of the low-density medium-thick plate with high toughness and high elongation prepared in Example 3 is about 11%, and the rest is an austenitic matrix.

[0047] Example 4

[0048] Casting: According to the chemical composition requirements of this invention (Table 1), various components are added to the furnace for melting and then cast into steel ingots to obtain square steel ingots with a thickness of 110 mm. Samples are taken for chemical composition analysis, and the chemical composition results are shown in Table 1.

[0049] Heating: Heat the steel ingot to 1150℃ and hold for 2 hours;

[0050] Rolling: The heated steel ingot is rolled in the recrystallization zone at an initial rolling temperature of 1060℃ and a final rolling temperature of 900℃, with a total reduction of 88.2%, to obtain a hot-rolled steel plate with a thickness of 13mm. After rolling, the plate is air-cooled to room temperature.

[0051] Normalizing heat treatment: The steel plate was held at 883℃ for 60 minutes, and then air-cooled to room temperature. The mechanical properties and density after normalizing are shown in Table 2.

[0052] The ferrite content of the low-density medium-thick plate with high toughness and high elongation prepared in Example 4 is about 9%, and the rest is an austenitic matrix.

[0053] Comparative Example

[0054] This comparative example is an embodiment published in CN 114892084 A, whose main chemical composition by mass percentage is C 0.92%, Al 8.11%, Mn 25%, Si 0.1%, Cr 0.07%, Cu 0.1%, Nb 0.04%, S 0.005%, and P 0.002%. The steel, after quenching and solution treatment, has a yield strength of 561 MPa, a tensile strength of 908 MPa, a fracture elongation of 50%, and a Charpy impact energy of 261 J at -40°C. The high C content and the presence of up to 25% Mn, as well as elements such as Cr and Cu, significantly increase the steel's hardening tendency, deteriorate its weldability, and make it unsuitable for use in transport equipment requiring welding.

[0055] Table 1. Smelting composition (wt%) of steel ingots in embodiments of the present invention

[0056] Example C Al Mn Mg O S P N 1 0.3 5.52 16.9 0.0006 0.0007 0.002 0.0015 0.0002 2 0.33 5.8 17.6 0.008 0.0006 0.002 0.003 0.0004 3 0.29 5.4 16.8 0.0010 0.0008 0.004 0.003 0.0003 4 0.32 5.64 17.3 0.0012 0.0005 0.003 0.002 0.0003

[0057] Table 2. Mechanical properties and density of embodiments of the present invention

[0058] Example Normalizing temperature / °C Rp0.2 / MPa Rm / MPa A / ％ -40℃AKV / J <![CDATA[ρ / g·cm -3 ]]> 1 880 304.6 647.5 66.8 208.7 7.32 2 897 322.3 651.3 65.1 206.4 7.29 3 864 310.1 641.3 68.0 212.1 7.33 4 883 314.4 647.3 67.3 203.8 7.31

Claims

1. A method for preparing a low-density, medium-thick plate with high toughness and high elongation, characterized in that, The chemical composition includes the following percentages by mass: C 0.28-0.34%, Al 5.0-5.9%, Mn 16.8-17.8%, Mg 0.0005-0.0015%, O 0.0005-0.0010%, S≤0.005%, P≤0.005%, N≤0.0005%, with the balance being Fe and unavoidable impurities. The C, Al, and Mn chemical compositions satisfy 2.8≤(Mn+C) / Al≤3.

2. The ingot is smelted, cast, and heated according to the composition ratio. The heated ingot is then rolled in the recrystallization zone. The initial rolling temperature is Tr=71.4Al-25.7Mn+1100.6, and the final rolling temperature is greater than 800℃. The total reduction rate is 80-90%, resulting in a thickness of 10-20 mm. The hot-rolled steel sheet is rolled to mm and then air-cooled to room temperature. The hot-rolled steel sheet is then subjected to normalizing heat treatment at a temperature of Tn = 172.4Al - 44.8Mn + 685.9 for 40-60 minutes, followed by air cooling to room temperature to obtain a low-density medium-thick plate with high toughness and high elongation. The microstructure of the medium-thick plate is austenitic matrix and δ-ferrite. After normalizing heat treatment, the proportion of δ-ferrite in the microstructure is 3% to 15%.

2. The preparation method according to claim 1, characterized in that, Medium-thick plates have a yield strength ≥300MPa, tensile strength ≥640MPa, elongation after fracture ≥60%, Charpy impact energy at -40℃ ≥200J, and density of 7.25~7.40g / cm³. 3 .

3. The preparation method according to claim 1, characterized in that, The thickness of the ingot is 55-140 mm.

4. The preparation method according to claim 1, characterized in that, The thickness of the ingot is 70-120 mm.

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

6. 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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