A carbon-free high modulus low density lightweight structural steel plate and its preparation method

Abstract

The present invention provides a carbon-free, high-modulus, low-density lightweight structural steel plate and a preparation method thereof. The lightweight structural steel plate comprises the following components by weight: Mn: 10.0-16.0%, Al: 1.5-4.0%, Cr: 0.0-3.0%, Ti: 1.8-5.0%, B: 1.0-2.1%, Nb: 0.0-0.2%, with the remainder being Fe and impurities; wherein the Ti and B elements are added via an intermediate alloy, with the Ti weight content of Fe-Ti being 40% and the B weight content of Fe-B being 19%. The lightweight structural steel plate, a new type of low-density steel, achieves a physical property of 6.88-7.45 g / cm 3 The new steel achieves a low density while maintaining a high Young's modulus of 220-250 GPa, resulting in a high specific modulus. In terms of mechanical properties, the new steel boasts a yield strength increase of nearly 30-100% compared to conventional Fe-TiB2 steel while maintaining excellent ductility, meeting the demands of lightweight steel material development.

Description

Technical Field

[0001] The present invention relates to the metallurgy and technical fields of metal materials, and in particular to a carbon-free high-modulus low-density lightweight structural steel plate and a preparation method thereof. Background Art

[0002] The development of automotive steels designed with the concept of improving strength and plasticity has become the mainstream solution for component thinning. However, the continuous thinning of steel plates will significantly reduce the component stiffness, that is, the ability to resist elastic deformation, leading to risks such as instability. There is an urgent need to further increase the material's Young's modulus E or reduce the density ρ (increase the specific modulus E / ρ). Although alloying with lightweight elements such as aluminum and magnesium can reduce density, it also sacrifices Young's modulus, resulting in a deterioration in component stiffness and collision safety. Therefore, the development of automotive steel has excessively pursued high strength and plasticity and ignored the significance of specific modulus, limiting its lightweighting potential in structural parts such as chassis and suspension. How to achieve both low density and high Young's modulus while ensuring high strength and plasticity is a hot and difficult issue in the development of lightweight steel materials.

[0003] At present, adding sufficient lightweight high modulus ceramic particles to steel is an effective way to improve the specific modulus. A new type of Fe-TiB2 high modulus steel based on in-situ self-generation (hereinafter referred to as in-situ high modulus steel) is proposed: by utilizing the eutectic reaction of the Fe-Ti-B system, a ferrite matrix and TiB2 particles are generated in situ during the cooling process of the molten steel. The latter has an extremely high Young's modulus (565GPa) and a lower density (4.5 g / cm 3 ), significantly improving the steel's specific modulus. However, the plasticity and formability of this new in-situ high-modulus steel decrease dramatically with increasing ceramic content. Therefore, maintaining high plasticity while maintaining sufficient ceramic content is a key prerequisite for the practical application of in-situ high-modulus steel. Summary of the Invention

[0004] A technical problem to be solved by the present invention is: how to maintain high plasticity under the premise of sufficient ceramic content.

[0005] To solve the above technical problems, the present disclosure provides a carbon-free, high-modulus, low-density lightweight structural steel plate. The components of the lightweight structural steel plate are as follows by weight: Mn: 10.0-16.0%, Al: 1.5-4.0%, Cr: 0.0-3.0%, Ti: 1.8-5.0%, B: 1.0-2.1%, Nb: 0.0-0.2%, and the balance is Fe and impurities.

[0006] Among them, Ti and B elements are added in the form of intermediate alloys, and the weight content of Ti in Fe-Ti is 40%, and the weight content of B in Fe-B is 19%.

[0007] In some embodiments, the aforementioned carbon-free high modulus low density light structural steel plate has a Young's modulus of 220 to 250 GPa and a density of 6.88 to 7.45 g / cm 3 , the yield strength is 300~700 MPa, the tensile strength is 600~865 MPa, and the total elongation is 10~32%.

[0008] In some embodiments, the aforementioned carbon-free high modulus low density lightweight structural steel plate, wherein the in-situ generated reinforcement phase of the lightweight structural steel plate is TiB2 particles or M2B particles and TiB2 particles, and the volume fraction of the in-situ generated reinforcement phase is 3.3-17%.

[0009] In some embodiments, in the aforementioned carbon-free high modulus low density lightweight structural steel plate, the in-situ generated reinforcement phase is TiB2 particles, and the volume fraction of the TiB2 particles is 1.2 to 13%; or

[0010] The in-situ generated reinforcing phases are TiB2 particles and M2B particles, and the volume fraction of TiB2 particles and M2B particles is 5-17%.

[0011] In some embodiments, in the aforementioned carbon-free, high-modulus, low-density lightweight structural steel plate, the in-situ generated reinforcing phase is TiB2 particles, and the chemical composition of the lightweight structural steel plate is as follows in terms of weight percentage: Mn: 15.0-16.0%, Al: 3.9-4.0%, Ti: 4.8-5.0%, B: 1.9-2.1%, and the remainder is Fe and impurities.

[0012] In some embodiments, in the aforementioned carbon-free high modulus, low-density lightweight structural steel plate, when the in-situ generated reinforcing phase is TiB2 and M2B particles, the composition of the high modulus lightweight structural steel plate is as follows in terms of weight percentage: Mn: 15.0-16.0%, Al: 1.5-3.7%, Cr: 0.0-3%, Ti: 1.8-3.5%, B: 1.0-2.1%, Nb: 0.0-0.2%, and the remainder is Fe and some unavoidable impurity elements; wherein, the M2B particles are (FeMn)2B or (FeMnCr)2B.

[0013] The second aspect of the present application provides a method for preparing a carbon-free high modulus low density lightweight structural steel plate, comprising the following steps:

[0014] S1. The components of the light structural steel plate are proportioned and smelted and cast to obtain a steel ingot, the steel ingot is heated to 1080-1180° C., kept warm for 2 hours, and the steel ingot is forged into a steel billet with a thickness of 20-30 mm;

[0015] S2, heating the steel billet in S1 to 1080-1180°C, keeping the temperature for 2 hours, hot rolling it into a thin plate with a thickness of 3-5 mm through 6 passes, and cooling it to room temperature;

[0016] The initial rolling temperature is 1050-1130°C, the final rolling temperature is not less than 800°C, and then air-cooled to room temperature;

[0017] S3, pre-annealing the thin plate in S2, keeping the thin plate at 650°C to 950°C for 0.05h to 4h, and air-cooling it to room temperature to obtain experimental steel;

[0018] S4, pickling the experimental steel in S3 with a hydrochloric acid and alcohol solution with a volume fraction of 1:1;

[0019] S51, cold-rolling the experimental steel after pickling in S4 with liquid nitrogen to a thickness of 1 to 1.5 mm to obtain a first cold-rolled steel;

[0020] S6, heat treating the first cold-rolled steel in S51 at 1000° C. for 3 minutes, and air-cooling to room temperature to obtain a light structural steel plate;

[0021] S52, cold rolling the experimental steel after pickling in S4 to a thickness of 1 to 1.5 mm to obtain a second cold-rolled steel;

[0022] S7. Heat-treat the second cold-rolled steel in S52, firstly subjecting it to full austenitization heat treatment at 950°C for 0.8 to 5 minutes, then keeping it within the two-phase temperature range of 600 to 630°C for 1 to 4 hours, and air-cooling it to room temperature to obtain a light structural steel plate.

[0023] In some embodiments, in the aforementioned method for preparing a carbon-free high modulus low density lightweight structural steel plate, the volume fraction of austenite in the steel in S2, S51 and S7 is 65-80%.

[0024] Through the above technical solution, the present invention provides a carbon-free, high-modulus, low-density lightweight structural steel plate with an Al content of less than 4 wt.%, while using a traditional, cost-effective steel manufacturing route. Furthermore, the lightweight structural steel plate has a hot-rolled, heterogeneous microstructure characterized by a fully equiaxed austenite matrix embedded with elongated delta-ferrite bands and micron-sized TiB2 particles. In terms of physical properties, the lightweight structural steel plate achieves a new low-density steel weight of 6.88 g / cm 3 The new steel achieves a low density while maintaining a high Young's modulus of 225 GPa, resulting in a high specific modulus. Mechanically, the new steel exhibits a nearly 100% increase in yield strength compared to conventional Fe-TiB2 steels while maintaining good ductility, meeting the demand for lightweight steel material development.

[0025] At the same time, by optimizing the composition, reducing the Ti content from 5 wt.% to 3.5 wt.%, or adding a certain amount of Cr, the present invention generates two B-containing ceramic reinforcement phases (TiB2 + M2B) in situ in the lightweight structural steel plate. During solidification, the two phases compete for the B element, achieving particle refinement of the ceramic reinforcement phase. Furthermore, during processing, the M2B effectively suppresses the floating and agglomeration of the low-density TiB2 as the molten steel solidifies. This significantly improves the mechanical properties of this lightweight structural steel plate, while maintaining the same specific modulus as high-modulus, low-density steel containing 5 wt.% Ti.

[0026] Therefore, this type of lightweight structural steel plate can maintain high plasticity while having sufficient ceramic content and a higher specific modulus, which can meet the needs of different usage scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a stress-strain curve diagram of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 1 of the present disclosure;

[0028] Figure 2 This is a SEM image of the microstructure of a hot-rolled slab of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 1 of the present disclosure;

[0029] Figure 3 is a stress-strain curve diagram of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 2 of the present disclosure;

[0030] Figure 4 This is a SEM image of the as-cast microstructure of a carbon-free, high modulus, low-density lightweight structural steel plate disclosed in Example 2 of the present disclosure;

[0031] Figure 5 This is a SEM image of the microstructure of a hot-rolled slab of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 2 of the present disclosure;

[0032] Figure 6 is a stress-strain curve diagram of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 3 of the present disclosure;

[0033] Figure 7 is a stress-strain curve diagram of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 4 of the present disclosure;

[0034] Figure 8 is a stress-strain curve diagram of a carbon-free high modulus low density lightweight structural steel plate disclosed in Example 5 of the present disclosure;

[0035] Figure 9 is a stress-strain curve diagram of a carbon-free high modulus low density lightweight structural steel plate disclosed in Example 6 of the present disclosure;

[0036] Figure 10 This is a SEM image of the as-cast microstructure of a carbon-free, high modulus, low-density lightweight structural steel plate disclosed in Example 6 of the present disclosure;

[0037] Figure 11 This is a SEM image of the microstructure of a hot-rolled slab of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 6 of the present disclosure;

[0038] Figure 12 This is a heat treatment process curve diagram of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 1 of the present disclosure;

[0039] Figure 13 This is a heat treatment process curve diagram of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 5 of the present disclosure;

[0040] Figure 14 This is a heat treatment process curve diagram of a carbon-free, high-modulus, low-density lightweight structural steel plate disclosed in Example 4 of the present disclosure;

[0041] Figure 15 This is a heat treatment process curve diagram of a carbon-free high modulus low density lightweight structural steel plate disclosed in the present invention. DETAILED DESCRIPTION

[0042] Example 1:

[0043] The composition and weight percentage of the light structural steel plate in this embodiment are as follows: Mn: 15.50%, Al: 3.95%, Ti: 5.00%, B: 2.15%, and the balance is Fe and impurities. The specific preparation method is as follows:

[0044] S1 is melted at 1650℃ according to the alloy composition ratio of light structural steel plate and forged into a steel billet with a thickness of 30mm;

[0045] S2 heats the steel billet to 1180℃ and keeps it warm for 2h; the initial rolling temperature is 1130℃, the final rolling temperature is not less than 950℃, and the steel billet is hot-rolled into a 5mm thick sheet through 6 passes. The hot-rolled sheet is air-cooled to room temperature.

[0046] like Figure 1 and Figure 2 As shown in the figure, the carbon-free, high-modulus, low-density lightweight structural steel plate produced in this embodiment, produced using only conventional steel manufacturing routes, has a hot-rolled heterogeneous microstructure in which a fully equiaxed austenite matrix is embedded with elongated delta-ferrite bands and micron-sized TiB2 particles, resulting in a matrix structure of equiaxed austenite + high-temperature ferrite. Specifically, the Young's modulus is 225 GPa and the density is 6.88 g / cm 3The yield strength is 503MPa, the tensile strength is 671MPa, and the total elongation is 15%.

[0047] Example 2:

[0048] The composition and weight percentage of the light structural steel plate in this embodiment are as follows: Mn: 15.50%, Al: 3.7%, Ti: 3.50%, B: 2.15%, Nb: 0.2%, and the balance is Fe and impurities. The specific preparation method is as follows:

[0049] The difference compared with Example 1 is:

[0050] The thickness of the steel billet in S1 is 20 mm;

[0051] The heating temperature of the steel billet in S2 is 1100°C, wherein the initial rolling temperature is 1080°C, the final rolling temperature is not less than 900°C, and it is hot-rolled into a thin plate with a thickness of 4 mm through 5 passes, and the hot-rolled steel plate is air-cooled to room temperature.

[0052] The carbon-free, high-modulus, low-density lightweight structural steel plate prepared in this embodiment has a Ti content reduced by 1.5wt.% compared to the composition of Example 1. Figure 4 As shown in Figure 2, M2B in the as-cast structure presents a fishbone-like eutectic structure; Figure 5 As shown in Figure 2, a hot rolled heterogeneous microstructure (high temperature ferrite + austenite + TiB2 + M2B) with a Young's modulus of 224 GPa and a density of 6.99 g / cm3 was produced via a conventional cost-effective steel manufacturing route. Figure 3 As shown, according to the engineering stress-strain curve of the slab, the yield strength is 442 MPa, the tensile strength is 695 MPa, and the total elongation is 18.3%.

[0053] Example 3:

[0054] The composition and weight percentage of the light structural steel plate in this embodiment are as follows: Mn: 15.50%, Al: 3.7%, Ti: 3.50%, B: 2.15%, Nb: 0.2%, and the balance is Fe and impurities. The specific preparation method is as follows:

[0055] The difference compared with Example 1 is:

[0056] The thickness of the steel billet in S1 is 20 mm;

[0057] The heating temperature of the steel slab in S2 is 1100°C, wherein the initial rolling temperature is 1080°C, the final rolling temperature is not less than 900°C, and the steel slab is hot-rolled into a 4mm thick sheet through 5 passes, and the hot-rolled steel sheet is air-cooled to room temperature;

[0058] S3 puts the hot-rolled plate in S2 into a tubular resistance furnace at a temperature of 950°C for heat treatment, keeps the temperature for 5 minutes, and then air-cools it to room temperature, finally obtaining a carbon-free high modulus lightweight hot-rolled annealed steel plate.

[0059] like Figure 6 As shown, the carbon-free high modulus low density lightweight structural steel plate prepared in this embodiment has a yield strength of 389 MPa, a tensile strength of 691 MPa, and a total elongation of 26.6% according to the engineering stress-strain curve of the slab.

[0060] Example 4:

[0061] The composition and weight percentage of the light structural steel plate in this embodiment are as follows: Mn: 15.50%, Al: 3.7%, Ti: 3.50%, B: 2.15%, Nb: 0.2%, and the balance is Fe and impurities. The specific preparation method is as follows:

[0062] The difference compared with Example 1 is:

[0063] The thickness of the steel billet in S1 is 20 mm;

[0064] The heating temperature of the steel slab in S2 is 1100°C, wherein the initial rolling temperature is 1080°C, the final rolling temperature is not less than 900°C, and the steel slab is hot-rolled into a 4mm thick sheet through 5 passes, and the hot-rolled steel sheet is air-cooled to room temperature;

[0065] S3: The hot-rolled plate in S2 is kept at 650°C for 4 hours, and then air-cooled to room temperature to obtain experimental steel;

[0066] S4: pickling the experimental steel in S3 with a hydrochloric acid and alcohol solution with a volume fraction of 1:1;

[0067] In S5, the pickled experimental steel in S4 was repeatedly rolled into a cold-rolled steel plate with a thickness of 1.5 mm through multiple passes using a cold rolling mill. The reduction in each pass was adjusted according to the rolling force of the rolling mill. During the rolling process, the rolling force on both sides of DS and OS was ensured not to exceed 400 kN, and the rolling speed was 3 mm / s.

[0068] S6 high-temperature heat treatment for full austenitization: the cold-rolled plate is placed in a tubular resistance furnace at 950°C for heat treatment, kept warm for 5 minutes, and then air-cooled to room temperature; followed by two-phase zone annealing treatment: the steel plate is placed in a tubular resistance furnace at 600°C for heat treatment, kept warm for 1 hour, and then air-cooled to room temperature, finally obtaining a carbon-free, high-modulus, low-density lightweight structural steel plate.

[0069] like Figure 7 and Figure 14As shown, the carbon-free high modulus lightweight cold-rolled steel plate prepared in this embodiment has a yield strength of 440 MPa, a tensile strength of 720 MPa, and a total elongation of 24% according to the engineering stress-strain curve of the slab. The specific process includes pre-annealing before cold rolling → cold rolling → short-time austenitization → intercritical region annealing, wherein the pre-annealing temperature before cold rolling is between Ac1 and Ac3, the purpose of annealing is to achieve the first pass of Mn element partitioning, and the matrix structure is a three-phase structure of high-temperature ferrite + room-temperature ferrite + austenite to reduce the difficulty of rolling; wherein the short-time austenitization temperature is higher than Ac3, and after austenitization, it is air-cooled to room temperature, and the Mn element is partially recovered during the austenitization process; then the intercritical region annealing temperature is between Ac1 and Ac3, and air-cooled to room temperature. In this stage, the second pass of Mn element partitioning is achieved, and it is enriched in the austenite, and finally a matrix is obtained with a two-phase structure of equiaxed austenite + high-temperature ferrite.

[0070] It can be understood that the cold rolling preparation method is more convenient to operate, and the required reaction conditions are easy to achieve, which is conducive to the efficient production of lightweight structural steel plates.

[0071] Example 5:

[0072] The composition and weight percentage of the light structural steel plate in this embodiment are as follows: Mn: 15.50%, Al: 3.7%, Ti: 3.50%, B: 2.15%, Nb: 0.2%, and the balance is Fe and impurities. The specific preparation method is as follows:

[0073] The difference compared with Example 1 is:

[0074] The thickness of the steel billet in S1 is 20 mm;

[0075] The heating temperature of the steel slab in S2 is 1100°C, wherein the initial rolling temperature is 1080°C, the final rolling temperature is not less than 900°C, and the steel slab is hot-rolled in 5 passes into a sheet with a thickness of 4 mm; and the hot-rolled steel sheet is air-cooled to room temperature;

[0076] S3: The hot-rolled plate in S2 is kept at 950°C for 10 minutes, and then air-cooled to room temperature to obtain experimental steel;

[0077] S4: pickling the experimental steel in S3 with a hydrochloric acid and alcohol solution with a volume fraction of 1:1;

[0078] In S5, the pickled experimental steel in S4 was repeatedly rolled into a 1.5 mm thick cold-rolled steel plate using liquid nitrogen cold rolling. The reduction in each pass was adjusted according to the rolling force of the rolling mill. During the rolling process, the rolling force on both sides of DS and OS did not exceed 400 kN, and the rolling speed was 3 mm / s.

[0079] S6 high temperature heat treatment for full austenitization: The cold rolled plate is placed in a tubular resistance furnace at 1000°C for heat treatment, kept warm for 3 minutes, and then air-cooled to room temperature; finally, a carbon-free, high modulus, low-density, lightweight structural steel plate is obtained.

[0080] like Figure 8 and Figure 13 As shown, the carbon-free, high-modulus, lightweight cold-rolled steel plate obtained in this embodiment has a yield strength of 349 MPa, a tensile strength of 682 MPa, and a total elongation of 32.7% according to the engineering stress-strain curve of the slab. The specific process includes pre-annealing before liquid nitrogen cold rolling → deep cold rolling → short-time austenitization, wherein the pre-annealing before liquid nitrogen cold rolling is higher than Ac3 to reduce the difficulty of rolling; after short-time austenitization, air cooling to room temperature occurs, and the Mn element is partially recovered during the austenitization process, and finally a two-phase structure of equiaxed austenite + high-temperature ferrite is obtained as the matrix.

[0081] It is understandable that the light structural steel plates prepared by liquid nitrogen cold rolling have better yield strength and tensile strength. The steel is softer and will not break easily when used in low temperature scenarios, ensuring the normal use of the steel.

[0082] Example 6:

[0083] The composition and weight percentage of the light structural steel plate in this embodiment are as follows: Mn: 15.50%, Al: 3.0%, Cr: 3.0%, Ti: 3.50%, B: 2.15%, Nb: 0.2%, and the balance is Fe and impurities. The specific preparation method is as follows:

[0084] The difference compared with Example 1 is:

[0085] The thickness of the steel billet in S1 is 20 mm;

[0086] The heating temperature of the steel slab in S2 is 1100°C, wherein the initial rolling temperature is 1080°C, the final rolling temperature is not less than 900°C, and the steel slab is hot-rolled into a sheet with a thickness of 4.5 mm through 5 passes, and the hot-rolled steel sheet is air-cooled to room temperature;

[0087] like Figure 9 、 Figure 10 and Figure 11 As shown in the figure, the carbon-free high modulus low density lightweight structural steel plate prepared in this embodiment introduces 3wt.% Cr content and the M2B in the cast structure presents a fishbone eutectic structure. Specifically, the Young's modulus is 234 GPa and the density is 6.92 g / cm 3. The yield strength is 496MPa, the tensile strength is 753MPa, and the total elongation is 17%. Among them, the M2B particles are (FeMn)2B or (FeMnCr)2B. It should be explained that during the solidification process, Ti and Nb, as the strongest boride-forming elements, will preferentially form an extremely stable TiB2 hexagonal structure to consume B atoms, and extremely low levels of Nb are dissolved in the TiB2 ceramic phase. The remaining B atoms react with Fe (atomic radius of about 1.24 Å), Mn (atomic radius of about 1.26 Å) or Fe, Mn, Cr (atomic radius of about 1.28 Å) to form (FeMn)2B or (FeMnCr)2B. The specific reasons are as follows: The atomic radii of Fe, Mn and Cr are very close (the radius difference is about 2-3%). When the atomic radius difference is less than 15%, it is considered to be a favorable condition for the formation of a continuous solid solution. Therefore, when Mn and Cr atoms replace M sites in the M2B (M=Fe) lattice, the resulting lattice distortion is minimal, and the energy cost of solid solution is low, consistent with thermodynamic principles. The similar bonding properties and electronegativity between Fe, Cr, and Mn do not significantly alter the bonding characteristics of the M2B phase (M=Fe). This facilitates the formation of the more stable (FeMn)2B (no Cr added to the alloy) or (FeMnCr)2B phases with an atomic radius of 1.43 Å. Larger Al atoms, while more inclined to solid solution in the matrix, are unable to dissolve in the M2B phase due to their large atomic size and structural incompatibility.

[0088] The contents of the above embodiments should be understood as these embodiments are only used to more clearly illustrate the present invention, and are not used to limit the scope of the present invention. Various equivalent modifications of the present invention by those skilled in the art fall within the scope defined by the claims attached to this application.

Claims

1. A carbon-free, high modulus, low-density, lightweight structural steel plate, characterized by: The components of the light structural steel plate are as follows by weight: Mn: 10.0-16.0%, Al: 1.5-4.0%, Cr: 0.0-3.0%, Ti: 1.8-5.0%, B: 1.0-2.1%, Nb: 0.0-0.2%, and the balance is Fe and impurities; Among them, Ti and B elements are added in the form of intermediate alloys, and the weight content of Ti in Fe-Ti is 40%, and the weight content of B in Fe-B is 19%.

2. The carbon-free high modulus low density lightweight structural steel plate according to claim 1, characterized in that: The Young's modulus of the lightweight structural steel plate is 220-250 GPa, and the density is 6.88-7.45 g / cm 3 , the yield strength is 300~700 MPa, the tensile strength is 600~865 MPa, and the total elongation is 10~32%.

3. The carbon-free high modulus low density lightweight structural steel plate according to claim 1, characterized in that: The in-situ generated reinforcement phase of the light structural steel plate is TiB2 particles or M2B particles and TiB2 particles, and the volume fraction of the in-situ generated reinforcement phase is 3.3-17%.

4. The carbon-free high modulus low density lightweight structural steel plate according to claim 3, characterized in that: The in-situ generated reinforcing phase is TiB2 particles, and the volume fraction of the TiB2 particles is 1.2 to 13%; or The in-situ generated reinforcing phase is TiB2 particles and M2B particles, and the volume fraction of the TiB2 particles and M2B particles is 5-17%.

5. The carbon-free high modulus low density lightweight structural steel plate according to claim 3, characterized in that: When the in-situ generated reinforcing phase is TiB2 particles, the chemical composition of the lightweight structural steel plate is as follows in terms of weight percentage: Mn: 15.0-16.0%, Al: 3.9-4.0%, Ti: 4.8-5.0%, B: 1.9-2.1%, and the remainder is Fe and impurities.

6. The carbon-free high modulus low density lightweight structural steel plate according to claim 3, characterized in that: When the in-situ generated reinforcing phase is TiB2 and M2B particles, the composition of the lightweight structural steel plate is as follows in terms of weight percentage: Mn: 15.0-16.0%, Al: 1.5-3.7%, Cr: 0.0-3%, Ti: 1.8-3.5%, B: 1.0-2.1%, Nb: 0.0-0.2%, and the balance is Fe and some unavoidable impurity elements; Wherein, the M2B particles are (FeMn)2B or (FeMnCr)2B.

7. A method for preparing the carbon-free high modulus low density lightweight structural steel plate according to any one of claims 1 to 6, characterized in that: The steps include: S1. The components of the light structural steel plate are proportioned and smelted and cast to obtain a steel ingot, the steel ingot is heated to 1080-1180° C., kept warm for 2 hours, and the steel ingot is forged into a steel billet with a thickness of 20-30 mm; S2, heating the steel billet in S1 to 1080-1180°C, keeping the temperature for 2 hours, hot rolling it into a thin plate with a thickness of 3-5 mm through 6 passes, and cooling it to room temperature; The initial rolling temperature is 1050-1130°C, the final rolling temperature is not less than 800°C, and then air-cooled to room temperature; S3, pre-annealing the thin plate in S2, keeping the thin plate at 650°C to 950°C for 0.05h to 4h, and air-cooling to room temperature to obtain experimental steel; S4, pickling the experimental steel in S3 with a hydrochloric acid and alcohol solution with a volume fraction of 1:1; S51, cold-rolling the experimental steel after pickling in S4 with liquid nitrogen to a thickness of 1 to 1.5 mm to obtain a first cold-rolled steel; S6, heat treating the first cold-rolled steel in S51 at 1000° C. for 3 minutes, and air-cooling to room temperature to obtain the light structural steel plate; S52, cold rolling the experimental steel after pickling in S4 to a thickness of 1 to 1.5 mm to obtain a second cold-rolled steel; S7. Heat-treating the second cold-rolled steel in S52, first performing a full austenitizing heat treatment at 950°C for 0.8 to 5 minutes, then keeping the steel in the two-phase region at 600 to 630°C for 1 to 4 hours, and air-cooling the steel to room temperature to obtain the light structural steel plate.

8. The method according to claim 7, characterized in that The volume fraction of austenite in steels S2, S51 and S7 is 65-80%.

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