High-strength low-density hot-formed steel and method of making

Abstract

This invention discloses a method for preparing high-strength, low-density hot-formed steel, belonging to the field of advanced high-strength steel preparation technology. The method includes: providing an initial steel billet; hot-rolling the initial steel billet to obtain a hot-rolled plate; performing multiple warm rolling operations on the hot-rolled plate, followed by quenching to obtain a warm-rolled plate; heating the warm-rolled plate to a set temperature and holding it thereafter, followed by quenching to obtain high-strength, low-density hot-formed steel. The hot-rolled plate is cooled to the initial rolling temperature of the warm rolling process at a cooling rate between 10°C / s and 30°C / s, the set temperature being within the two-phase region. The initial steel billet composition includes: carbon 0.29–0.35 wt%; manganese 2.6–3.5 wt%; aluminum 3.0–3.5 wt%; silicon 0.2–0.6 wt%; chromium 0.5–1.0 wt%; vanadium 0.20–0.30 wt%; lanthanum 0.001–0.01 wt%; the balance being Fe. The composition of the initial steel billet satisfies the following relationship: (W Mn +W Cr ) / W Al ≥1, where W Mn Indicates the manganese content; W Cr Indicates the chromium content; W Al This indicates the aluminum content. The present invention also discloses a high-strength, low-density hot-formed steel.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength steel preparation technology, specifically to a high-strength, low-density hot-formed steel and its preparation method. Background Technology

[0002] Low-density steel typically achieves its density by adding the alloying element Al. However, adding a large amount of Al leads to the formation of the δ-ferrite phase. δ-ferrite is relatively soft and has coarse grains, resulting in high stress-strain unevenness between it and other matrix phases, leading to poor overall strength and ductility.

[0003] The high-strength, high-toughness lightweight steel disclosed in Chinese invention patent CN 106498307 A, the high-strength medium-carbon chromium-containing low-manganese lightweight steel disclosed in Chinese invention patent CN106086658A, and the high-ductility low-density steel disclosed in Chinese invention patent CN 104928569A have all failed to achieve a tensile strength exceeding 1 GPa, and their manufacturing processes are complex and cumbersome.

[0004] Therefore, while ensuring sufficient ductility, low-density steel still has significant room for strength improvement. In particular, strengthening it to the point of replacing equivalent hot-formed steel and duplex steel will further advance lightweighting. However, simply increasing the austenitizing temperature and room-temperature martensite content will exacerbate the phase incompatibility between martensite and the soft δ-ferrite phase, leading to premature cracking. Therefore, active improvements are needed in process control, microstructure composition, and strain coordination. Simultaneously, simpler processing routes need to be explored to reduce costs and energy consumption. Summary of the Invention

[0005] The present invention aims to solve one of the technical problems in related technologies to a certain extent, and provides a high-strength, low-density hot-formed steel and its preparation method.

[0006] As a first aspect of the present invention, a method for preparing high-strength, low-density hot-formed steel is provided, comprising:

[0007] Initial steel billet provided:

[0008] The initial steel billet is hot-rolled to obtain a hot-rolled plate;

[0009] After the hot-rolled plate is subjected to multiple warm rolling processes, it is then quenched to obtain a warm-rolled plate.

[0010] After heating the warm-rolled plate to a set temperature and holding it thereafter, it is quenched to obtain high-strength, low-density hot-formed steel. The hot-rolled plate is cooled to the initial rolling temperature of the warm roll at a cooling rate between 10°C / s and 30°C / s. The set temperature is within the two-phase region. The initial billet composition includes:

[0011]

[0012] The balance is Fe, and the composition of the initial steel billet satisfies the following relationship:

[0013] (W Mn +W Cr ) / W Al ≥1, where,

[0014] W Mn Indicates the manganese content;

[0015] W Cr Indicates the chromium content;

[0016] W Al This indicates the aluminum content.

[0017] Furthermore, in the quenching step after multiple warm rolling processes on the hot-rolled plate, the total reduction rate of the warm rolling is between 65% and 75%.

[0018] Furthermore, in the quenching step after multiple warm rolling processes on the hot-rolled plate, the reduction rate of the last two warm rolling passes is controlled between 25% and 35%.

[0019] Furthermore, in the quenching step after multiple warm rolling processes of the hot-rolled plate, the initial rolling temperature of the warm rolling is between 800°C and 820°C, the final rolling temperature of the warm rolling is between 700°C and 720°C, and the warm-rolled plate is cooled to room temperature at a cooling rate greater than or equal to 30°C / s.

[0020] Furthermore, in the quenching step after heating the warm-rolled plate to a set temperature and holding it thereafter, the set temperature is between 950°C and 980°C, and the holding time is between 5 minutes and 10 minutes.

[0021] Furthermore, in the quenching step after heating the warm-rolled plate to a set temperature and holding it thereafter, the heating rate is between 25°C / s and 50°C / s, and the cooling rate is greater than or equal to 30°C / s.

[0022] Furthermore, in the hot rolling step of the initial steel billet, the final rolling temperature is between 960°C and 980°C, and the total reduction rate of hot rolling is ≥90%.

[0023] Furthermore, prior to hot rolling the initial steel billet, the preparation method further includes:

[0024] Steel is smelted and cast into ingots to obtain steel ingots;

[0025] The steel ingot is heated to between 1150°C and 1250°C and held at that temperature to obtain a first steel ingot, wherein the heating rate is between 150°C / h and 200°C / h and the holding time is between 2.6h and 3.6h.

[0026] The first steel ingot is forged to obtain a forging billet, wherein the final forging temperature is greater than or equal to 960°C;

[0027] The forging billet is heated to between 1150°C and 1250°C and held at that temperature to obtain the initial steel billet, wherein the heating rate is between 200°C / h and 300°C / h and the holding time is between 1.6h and 2.6h.

[0028] As a second aspect of the present invention, a high-strength, low-density hot-formed steel is also provided, wherein the high-strength, low-density hot-formed steel is prepared by the above-described preparation method.

[0029] Furthermore, the mechanical properties of the high-strength, low-density hot-formed steel are: tensile strength greater than 1500 MPa, yield strength greater than 700 MPa, and elongation greater than or equal to 10.0%; the room temperature microstructure of the high-strength, low-density hot-formed steel includes: martensite, α-ferrite, lath δ-ferrite, and carbides, wherein the volume fraction of the martensite is between 65% and 75%.

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] (1) The hot rolling and warm rolling processes are combined. The hot rolling is carried out in the temperature of the austenite recrystallization zone, which fully refines the original austenite grain size. The hot-rolled plate is cooled at a corresponding cooling rate and then warm-rolled. The faster cooling rate between hot rolling and warm rolling will inhibit the formation of α ferrite, thereby controlling the increase of α ferrite content in the hot-rolled plate. On the other hand, in the temperature range where warm rolling is carried out, a large amount of V carbide will be precipitated by strain induction. It can be used to further refine the grain and precipitate and strengthen δ ferrite. In addition, the lower warm rolling temperature and the larger reduction rate ensure more dislocation accumulation, creating a precursor of fine grain, precipitation and high-density dislocation structure, which is inherited by the subsequent hot forming process.

[0032] (2) The initial billet and hot forming process are designed with appropriate phase content ratios to ensure that the content is 65% ≤ martensite ≤ 75%, with the remainder being (δ+α) ferrite and a small amount of carbides. (W) Mn +W Cr ) / W Al In component designs with ≥1, W Mn and W Cr Compared to W Al High temperature is beneficial for lowering the complete austenitization temperature, forming more austenite, and then further transforming more austenite into more martensite through subsequent quenching, but WMn and W Cr The temperature should not be too high; the martensite volume fraction after quenching should be controlled between 65% and 75%. Al High-performance alloys increase the full austenitization temperature and expand the process window, but excessive W... Al This leads to an increase in the δ-ferrite phase. Lower martensite content results in lower strength, while higher martensite content exacerbates the stress incompatibility between martensite and large-sized δ-ferrite, causing premature failure. Adding appropriate amounts of Mn, Cr, and Al helps control the martensite content and reduce the δ-ferrite content. Furthermore, precise control of hot forming process parameters for steels with suitable compositions, including holding in the two-phase region followed by quenching, controls the martensite content while ensuring the α-ferrite content remains within a suitable range. The α-ferrite existing between the martensite and δ-ferrite phases avoids the incompatibility of deformation stresses between the two phases, forming a strain gradient fit, thereby increasing strain coordination and achieving higher strength and plasticity. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0034] Figure 1 This is a flowchart of one embodiment of the method for preparing high-strength, low-density hot-formed steel provided by the present invention;

[0035] Figure 2 These are microscopic tissue photographs of Embodiment 1 of the present invention;

[0036] Figure 3 This is the room temperature stretching curve of Embodiment 1 of the present invention. Detailed Implementation

[0037] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described are intended to explain the present invention and should not be construed as limiting the invention.

[0038] The terms "an embodiment," "example," or "trademark" used in this specification refer to a particular feature, structure, or characteristic described in connection with the embodiment itself that may be included in at least one embodiment disclosed in this invention. The phrase "in an embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0039] like Figure 1 As shown in the figure, this invention provides a method for preparing high-strength, low-density hot-formed steel, wherein the preparation method includes:

[0040] In step S110, an initial steel billet is provided;

[0041] In step S120, the initial steel billet is hot-rolled to obtain a hot-rolled plate;

[0042] In step S130, the hot-rolled plate is subjected to multiple warm rolling processes and then quenched to obtain a warm-rolled plate.

[0043] In step S140, the warm-rolled plate is heated to a set temperature and held thereafter, then quenched to obtain high-strength, low-density hot-formed steel. The hot-rolled plate is cooled to the initial rolling temperature of the warm roll at a cooling rate between 10°C / s and 30°C / s, with the set temperature falling within the two-phase region. The initial billet composition includes:

[0044]

[0045] The balance is Fe, and the initial billet composition satisfies the following relationship:

[0046] (W Mn +W Cr ) / W Al ≥1, where,

[0047] W Mn Indicates the manganese content;

[0048] W Cr Indicates the chromium content;

[0049] W Al This indicates the aluminum content.

[0050] In step S110, the initial steel billet and hot forming process, based on component design, are used to ensure a suitable phase content ratio, satisfying 65% ≤ martensite content ≤ 75%, with the remainder being (δ+α) ferrite and a small amount of carbides. (W) Mn +W Cr ) / W Al In component designs with ≥1, W Mn and W Cr Compared to W Al A higher temperature is beneficial for lowering the complete austenitization temperature, forming more austenite, which can then be further transformed into more martensite through subsequent quenching. However, W... Mn and W Cr The temperature should not be too high; the martensite volume fraction after quenching should be controlled between 65% and 75%. AlMartensite content increases the full austenitization temperature of steel, leading to an increase in the δ-ferrite phase. Lower martensite content results in lower strength, while higher martensite content exacerbates the stress incompatibility between martensite and large-sized δ-ferrite, causing premature failure. Adding appropriate amounts of Mn, Cr, and Al can control the martensite content and reduce the δ-ferrite content.

[0051] In step S120, the initial steel billet is hot rolled. As an optional implementation, the final rolling temperature of the hot rolling is controlled between 960°C and 980°C, and the total reduction rate of the hot rolling is ≥90%. This allows the initial steel billet to be hot rolled within the austenite recrystallization region temperature, thereby fully refining the original austenite grain size in the steel billet.

[0052] In step S130, after the hot-rolled plate is cooled at a corresponding cooling rate, it is further subjected to warm rolling. The faster cooling rate between hot rolling and warm rolling will inhibit the formation of α-ferrite, thereby controlling the increase of α-ferrite content in the hot-rolled plate.

[0053] On the other hand, within the temperature range of warm rolling, strain induces the precipitation of a large amount of V carbides, which can be used to further refine the grains and precipitate and strengthen δ ferrite. In addition, the lower warm rolling temperature and the greater reduction rate ensure more dislocation accumulation, creating a precursor of fine grain, precipitation and high-density dislocation structure, which is inherited by subsequent hot forming processes.

[0054] Optionally, the total reduction rate of warm rolling is between 65% and 75%, and the reduction rate of the last two warm rolling passes is controlled between 25% and 35%.

[0055] Optionally, the initial rolling temperature of the warm rolling is controlled between 800°C and 820°C, and the final rolling temperature of the warm rolling is between 700°C and 720°C.

[0056] Optionally, the warm-rolled plate is cooled to room temperature at a cooling rate of 30°C / s or greater.

[0057] In step S140, a hot forming process is employed, preserving the advantages of dislocation density and grain refinement found in warm-rolled steel. Precise control of hot forming process parameters is achieved for steel with a suitable composition design. After holding in the two-phase region and then quenching, the martensite content is controlled while ensuring that the α-ferrite content remains within a suitable range. The α-ferrite, existing between the martensite and δ-ferrite phases, avoids the incompatibility of deformation stresses between the two phases, forming a strain gradient fit, thereby increasing strain coordination and achieving higher strength and plasticity.

[0058] As an optional implementation, the temperature is set between 950°C and 980°C, and the holding time is between 5 minutes and 10 minutes.

[0059] As an optional implementation, in the above-mentioned thermoforming process, the heating rate is between 25°C / s and 50°C / s, and the cooling rate is greater than or equal to 30°C / s.

[0060] In this embodiment of the invention, no special limitation is placed on how the initial steel billet is obtained. For example, the initial steel billet can be obtained by purchasing it externally. Another example is that the initial steel billet can be obtained directly from steel through smelting, calcination, and heat treatment.

[0061] Specifically, the preparation method further includes, prior to hot rolling of the initial steel billet:

[0062] Steel is smelted and cast into ingots to obtain steel ingots;

[0063] The steel ingot is heated to between 1150°C and 1250°C and held at that temperature to obtain the first steel ingot. The heating rate is between 150°C / h and 200°C / h, and the holding time is between 2.6h and 3.6h. This step is a conventional homogenization process.

[0064] The first steel ingot is forged to obtain a forging billet, wherein the final forging temperature is greater than or equal to 960℃;

[0065] The forging billet is heated to between 1150°C and 1250°C and held at that temperature to obtain the initial steel billet. The heating rate is between 200°C / h and 300°C / h, and the holding time is between 1.6h and 2.6h.

[0066] The process of this invention is simple and the heat treatment cycle is greatly reduced, which is conducive to achieving low-cost and high-efficiency production.

[0067] The high-strength, low-density hot-formed steel of this invention is obtained from an initial steel billet designed with the above-mentioned composition and prepared by the above-mentioned method. The mechanical properties of the high-strength, low-density hot-formed steel are: tensile strength greater than 1500 MPa, yield strength greater than 700 MPa, and elongation greater than or equal to 10.0%. The obtained high-strength, low-density hot-formed steel includes martensite, α-ferrite, lath δ-ferrite, and carbides, wherein the volume fraction of martensite is between 65% and 75%.

[0068] The present invention will be further illustrated below through specific embodiments and comparative examples.

[0069] Example

[0070] In the following embodiments of the present invention, the composition of the steel billet used is as follows:

[0071]

[0072] The balance is Fe and unavoidable impurity elements, and the initial billet composition satisfies the following relationship:

[0073] (W Mn +W Cr ) / W Al ≥1, where,

[0074] W Mn Indicates the manganese content;

[0075] W Cr Indicates the chromium content;

[0076] W Al Indicates the aluminum content;

[0077] The density of this steel is 7.55 g / cm³. 3 .

[0078] Example 1

[0079] A method for preparing high-strength, low-density hot-formed steel, comprising:

[0080] Step 1: Smelt the steel billet to produce a cast billet;

[0081] Step 2: Heat the billet to 1200℃ at a heating rate of 200℃ / h, and hold for 3 hours to homogenize it.

[0082] Step 3: Forge the billet from Step 2 to obtain a forged billet, wherein the final forging temperature is maintained at or above 960℃.

[0083] Step 4: Heat the forging billet to 1150℃ at a heating rate of 300℃ / h and hold for 2 hours;

[0084] Step 5: The forging billet after step 4 is hot rolled to obtain a hot-rolled plate. The final hot rolling temperature is controlled at 960℃, and the total reduction rate of 5 passes is 90%.

[0085] Step 6: Cool the hot-rolled plate to the warm rolling start temperature at a cooling rate of 20℃ / s. The warm rolling start temperature is controlled at 800℃. Perform four warm rolling passes, and then cool to room temperature at a cooling rate of about 50℃ / s to obtain the warm-rolled plate. The final rolling temperature of the warm rolling is controlled at 700℃, the total reduction rate of the four passes is 65%, and the reduction rate of the last two warm rolling passes is controlled at 25%.

[0086] Step 7: Heat the warm-rolled plate to 950°C at a heating rate of 30°C / s, hold for 5 minutes, and cool to room temperature at a cooling rate of about 50°C / s to obtain high-strength, low-density hot-formed steel.

[0087] Micro-organisms such as Figure 2 As shown, the room temperature microstructure contains approximately 66% martensite by volume, with the remainder consisting of alternating α-ferrite and long lath δ-ferrite to ensure coordinated deformation. The room temperature tensile curve is shown in the figure. Figure 3As shown in Tables 1 and 2, other relevant process parameters and mechanical properties are also shown.

[0088] Example 2

[0089] High-strength, low-density hot-formed steel was prepared using the same method as in Example 1. The difference was that the total reduction rate of the four-pass warm rolling in Example 2 was 75%, and the heating temperature in the seventh step was 980℃. Specific process parameters are shown in Table 1, and the mechanical properties and microstructure of the finished product are shown in Table 2. The increased martensite content further improved the strength.

[0090] Comparative Example 1

[0091] High-strength, low-density hot-formed steel was prepared using the same method as in Example 1, except that the heating temperature in step seven of Comparative Example 1 was 1050℃. Specific process parameters are shown in Table 1, and the mechanical properties and microstructure of the finished product are shown in Table 2. Excessive heating temperature led to a significant increase in martensite and almost complete disappearance of α-ferrite, impairing interphase strain compatibility and causing premature failure.

[0092] Comparative Example 2

[0093] High-strength, low-density hot-formed steel was prepared using the same method as in Example 1. The difference was that Comparative Example 2 underwent a single-pass hot rolling process, with the total thickness reduction being the same as in Example 1. The final rolling temperature was 850°C. Specific process parameters are shown in Table 1, and the mechanical properties and microstructure of the finished product are shown in Table 2. Less dislocation packing and less V carbide precipitation ultimately resulted in lower strength.

[0094] Comparative Example 3

[0095] High-strength, low-density hot-formed steel was prepared using the same method as in Example 1. The difference was that the total reduction rate of the four warm rolling passes in Comparative Example 3 was 40%, and the reduction rates of the last two passes were 18%. Specific process parameters are shown in Table 1, and the mechanical properties and microstructure of the finished product are shown in Table 2. The steel exhibited less dislocation packing and coarser grains, ultimately resulting in slightly lower strength and plasticity.

[0096] Test case

[0097] Microscopic tissue images were obtained using an OLS4100 laser confocal microscope. The final microstructure in Example 1 is shown below. Figure 2 As shown in Table 2, the microstructure statistics of Example 1 include approximately 66% martensite + (δ+α) ferrite, fine grains, high-density dislocations, and V carbide precipitation.

[0098] Example 2 increases the total reduction in warm rolling and increases the heating temperature in the two-phase region. The microstructure consists of approximately 74% martensite + (δ + α) ferrite, with fine grains, high-density dislocations, and V carbide precipitation, the same as in Example 1, but with increased martensite content.

[0099] Comparative Example 1 further increased the heating temperature. The excessively high heating temperature caused the grains to coarsen, the V carbides to disappear and solidify, and the microstructure to become almost completely austenitic, resulting in a large increase in martensite and the near disappearance of α ferrite. The microstructure consisted of about 82% martensite + δ ferrite, and the grain size was relatively large.

[0100] Comparative Example 2 uses a one-time hot rolling process, with the same total deformation as Example 1, but lacks multiple hot rolling passes, resulting in a reduction in dislocation density, V carbide precipitation, and reduced grain refinement and strengthening effects. The microstructure consists of approximately 66% martensite + (δ + α) ferrite, low-density dislocations, and less V carbide precipitation.

[0101] Comparative Example 3 reduced the total reduction rate of warm rolling, resulting in insufficient deformation and no strain-induced V carbides. This reduced internal dislocation accumulation and insufficient grain refinement. The microstructure consisted of approximately 66% martensite + (δ + α) ferrite, with low-density dislocations and relatively large grain size.

[0102] The mechanical properties were tested using a universal testing machine (MTS E45.305). The final mechanical properties of Example 1 are as follows: Figure 3 As shown in the figure, the horizontal axis represents strain %, and the vertical axis represents stress, with the unit being MPa. In the figure, "B" represents the tensile initiation point, "M" represents the elastic modulus, "Rp" represents the yield strength, and "P" represents the maximum tensile strength. The mechanical properties of the remaining embodiments and comparative examples are shown in Table 2.

[0103] Example 1 contains approximately 66% martensite by volume, with the remainder consisting of alternating α-ferrite and long lath δ-ferrite to ensure coordinated deformation. Its yield strength is 783 MPa, tensile strength is 1574 MPa, and elongation is 14%.

[0104] Example 2 improved the total reduction rate during warm rolling, further refined the grains, and increased the heating temperature in the two-phase region, resulting in increased martensite content and further enhanced strength. Yield strength was 825 MPa, tensile strength was 1612 MPa, and elongation was 10%.

[0105] Comparative Example 1 further increased the heating temperature. The excessive heating temperature caused the grains to coarsen, the V carbides to disappear and solidify, and the microstructure to become almost completely austenitic, resulting in a large increase in martensite and the near disappearance of α ferrite. This damaged the interphase strain coordination and caused premature failure. The yield strength of Comparative Example 1 was 743 MPa, the tensile strength was 1329 MPa, and the elongation was 4%.

[0106] Comparative Example 2 uses a one-step hot rolling process, resulting in less dislocation accumulation and less V carbide precipitation, ultimately leading to lower strength. Comparative Example 2 has a yield strength of 661 MPa, a tensile strength of 1350 MPa, and an elongation of 7%.

[0107] Comparative Example 3 had a lower total reduction during warm rolling, resulting in insufficient deformation, less dislocation accumulation, and coarser grains, ultimately leading to slightly lower strength and plasticity. Comparative Example 3 had a yield strength of 722 MPa, a tensile strength of 1404 MPa, and an elongation of 13%.

[0108] Table 1. Process parameters of embodiments and comparative examples of the present invention.

[0109]

[0110] Table 2 Mechanical properties and microstructure of embodiments and comparative examples of the present invention

[0111]

[0112] As can be seen from Tables 1 and 2, the total reduction rate of warm rolling in Example 1 is smaller than that in Example 2. The heating temperature in the two-phase region of Example 1 is lower than that in Example 2. The martensite content in Example 1 is lower than that in Example 2, resulting in insufficient strengthening. The tensile strength of Example 1 is lower than that of Example 2, but the elongation is higher. In Comparative Example 1, the excessively high heating temperature leads to grain coarsening, the disappearance of V carbides and solid solution, and near-complete austenitization of the microstructure, resulting in a large increase in martensite and the near disappearance of α-ferrite, which damages the interphase strain coordination and leads to premature failure. Comparative Example 2 uses a one-time hot rolling process, resulting in less dislocation accumulation and less precipitation of V carbides, ultimately leading to lower strength. Comparative Example 3 has a lower total reduction rate of warm rolling, insufficient deformation, less dislocation accumulation, and relatively coarse grains, ultimately resulting in slightly lower strength and plasticity.

[0113] Therefore, by using the component design and preparation method of the present invention, low-density steel with better comprehensive performance in terms of tensile strength and elongation can be obtained. The product of Example 2 has a yield strength of up to 852 MPa, a tensile strength of up to 1612 MPa or more, and an elongation of 10%.

[0114] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing high-strength, low-density hot-formed steel, characterized in that, include: Provide initial steel billets; The initial steel billet is hot-rolled to obtain a hot-rolled plate; After the hot-rolled plate is subjected to multiple warm rolling processes, it is quenched to obtain a warm-rolled plate. The total reduction rate of the warm rolling is between 65% and 75%, and the reduction rate of the last two warm rolling passes is between 25% and 35%. After heating the warm-rolled plate to a set temperature and holding it thereafter, it is quenched to obtain high-strength, low-density hot-formed steel. The hot-rolled plate is cooled to the initial rolling temperature of the warm roll at a cooling rate between 10°C / s and 30°C / s. The set temperature is within the two-phase region. The initial billet composition includes: Carbon 0.29~0.35wt%; Manganese 2.6~3.5 wt%; Aluminum 3.0~3.5wt%; Silicon 0.2~0.6wt%; Chromium 0.5~1.0 wt%; Vanadium 0.20~0.30wt%; Lanthanum 0.001~0.01wt%; The balance is Fe, and the composition of the initial steel billet satisfies the following relationship: (WMn+WCr) / WA1≥1, where, WMn indicates the manganese content; WCr indicates the chromium content; WAl indicates the aluminum content.

2. The preparation method according to claim 1, characterized in that, In the quenching step after multiple warm rolling processes of the hot-rolled plate, the initial rolling temperature is between 800°C and 820°C, the final rolling temperature is between 700°C and 720°C, and the warm-rolled plate is cooled to room temperature at a cooling rate of greater than or equal to 30°C / s.

3. The preparation method according to claim 1, characterized in that, In the quenching step after heating the warm-rolled plate to a set temperature and holding it at that temperature, the set temperature is between 950°C and 980°C, and the holding time is between 5 minutes and 10 minutes.

4. The preparation method according to claim 1, characterized in that, In the quenching step after heating the warm-rolled plate to a set temperature and holding it thereafter, the heating rate is between 25°C / s and 50°C / s, and the cooling rate is greater than or equal to 30°C / s.

5. The preparation method according to claim 1, characterized in that, In the hot rolling step of the initial steel billet, the final rolling temperature is between 960°C and 980°C, and the total reduction rate of hot rolling is ≥90%.

6. The preparation method according to any one of claims 1 to 5, characterized in that, Before hot rolling the initial steel billet, the preparation method further includes: Steel is smelted and cast into ingots to obtain steel ingots; The steel ingot is heated to between 1150°C and 1250°C and held at that temperature to obtain a first steel ingot, wherein the heating rate is between 150°C / h and 200°C / h and the holding time is between 2.6h and 3.6h. The first steel ingot is forged to obtain a forging billet, wherein the final forging temperature is greater than or equal to 960°C; The forging billet is heated to between 1150°C and 1250°C and held at that temperature to obtain the initial steel billet, wherein the heating rate is between 200°C / h and 300°C / h and the holding time is between 1.6h and 2.6h.

7. A high-strength, low-density hot-formed steel, characterized in that, The high-strength, low-density hot-formed steel is obtained by the preparation method described in any one of claims 1 to 6.

8. The high-strength, low-density hot-formed steel according to claim 7, characterized in that, The mechanical properties of the high-strength, low-density hot-formed steel are: tensile strength greater than 1500 MPa, yield strength greater than 700 MPa, and elongation greater than or equal to 10.0%. The room temperature microstructure of the high-strength, low-density hot-formed steel includes martensite, α-ferrite, lath δ-ferrite, and carbides, wherein the volume fraction of the martensite is between 65% and 75%.

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