An ultra-high-strength martensitic steel plate and a method for manufacturing the same

Abstract

The application relates to the field of ultra-high-strength steel and processing technology, and discloses an ultra-high-strength martensite steel plate and a preparation method thereof, which comprises the following steps: proportioning alloys according to predetermined mass percentage contents, melting the proportioned alloys and casting ingots, uniformly treating the ingots, high-temperature forging the uniformly treated ingots to obtain slabs, high-temperature hot-rolling the slabs, water-cooling the hot-rolled slabs to room temperature to obtain first semi-finished plate materials, continuously and symmetrically warm-rolling the first semi-finished plate materials through multi-pass large deformation to obtain second semi-finished plate materials, continuously and asymmetrically warm-rolling the second semi-finished plate materials through multi-pass large deformation to obtain third semi-finished plate materials, and quenching the third semi-finished plate materials to room temperature to obtain the ultra-high-strength martensite steel plate. The prepared martensite steel plate has high hardness, high strength and high plasticity, and has the advantages of low cost, resource saving and excellent performance.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength steel and its processing technology, and in particular to an ultra-high strength martensitic steel plate and its preparation method. Background Technology

[0002] Ultra-high strength martensitic steel, due to its extremely high strength and excellent plasticity, has always been an important structural material widely used in the industrial field, especially in the transportation industry such as heavy machinery, high-speed rail, and aerospace, as well as the major defense equipment industry, where the demand for high-strength martensitic steel is increasing rapidly. Moreover, due to its relatively low price and simple processing and manufacturing process, high-strength martensitic steel has a broader application prospect in today's environment of relatively scarce resources and increasing environmental pressure.

[0003] Improving strength while retaining good plasticity has always been the direction for optimizing the performance of structural materials such as high-strength martensitic steel. In addition, the cost of alloying elements added to the selected steel materials and the cost of large-scale industrial production of the preparation method are also important factors to be considered.

[0004] In existing technologies, materials that have undergone various physical treatments often struggle to achieve both high plasticity and high strength simultaneously. Currently, while adding expensive alloying elements or employing complex production processes can partially preserve the good strength and plasticity of martensitic steel, this is not advisable from an industrial application and environmental perspective. Fine-grain strengthening, on the other hand, is an effective means of simultaneously improving material strength and plasticity. Research has found that medium-temperature high-strength deformation treatment can, on the one hand, introduce high-density dislocations into metastable austenite, providing sufficient nucleation sites for carbide precipitation, thereby directly inheriting dislocations and carbides to the post-transformation martensitic structure; on the other hand, it can also sufficiently refine the metastable austenite grains, further refining the post-transformation martensitic structure.

[0005] However, the current method for preparing ultra-high strength martensitic steel using large deformation medium-temperature rolling technology mainly employs large deformation medium-temperature rolling followed by tempering. This method relies on grain refinement and precipitation strengthening to improve the strength of martensitic steel, while the effect of dislocation strengthening is greatly weakened during tempering. Alternatively, a large amount of alloying elements can be added to the martensitic steel to promote the formation of precipitates during medium-temperature rolling. Although this largely preserves the effects of precipitation strengthening and dislocation strengthening, the addition of alloying elements leads to a significant increase in cost, which is not conducive to large-scale industrial production. Summary of the Invention

[0006] This invention provides an ultra-high strength martensitic steel plate and its preparation method to solve the above-mentioned technical problems in the prior art.

[0007] To provide a basic understanding of some aspects of the disclosed embodiments, a brief summary is given below. This summary is not intended as a general commentary, nor is it intended to identify key / important components or to describe the scope of protection of these embodiments. Its sole purpose is to present some concepts in a simple form as a prelude to the detailed description that follows.

[0008] According to a first aspect of the present invention, an ultra-high strength martensitic steel plate is provided.

[0009] In one embodiment, the chemical composition and mass percentage of the alloying elements in the ultra-high strength martensitic steel plate are as follows:

[0010] C≤0.35%, Si:0.3~0.4%, Mn:1.0~1.4%, P:0.005~0.01%, S:0.02~0.03%, Cr:0.1~0.2%, Ni:0.05~0.1%, Mo:0.01~0.03%, Cu:0.01~0.02%, Al:0.01~0.02%, V:0.01~0.03%, Ti:0.01~0.02%, B≤0.0005%, with the remainder being Fe.

[0011] According to a second aspect of the present invention, a method for preparing ultra-high strength martensitic steel plates is provided.

[0012] In one embodiment, the method for preparing the ultra-high strength martensitic steel plate includes:

[0013] The alloy is formulated according to a predetermined mass percentage content, and the formulated alloy is melted and cast into ingots; the obtained ingots are homogenized, and the homogenized ingots are forged at high temperature to obtain slabs.

[0014] The obtained slab is subjected to high-temperature hot rolling and then water-cooled to room temperature to obtain the first semi-finished plate. The first semi-finished plate is subjected to multi-pass large deformation continuous symmetrical warm rolling to obtain the second semi-finished plate. The second semi-finished plate is subjected to multi-pass large deformation continuous asymmetrical warm rolling to obtain the third semi-finished plate.

[0015] After the third semi-finished plate is quenched to room temperature, ultra-high strength martensitic steel plate is obtained.

[0016] Optionally, when melting the proportioned alloy, the percentage content of alloying elements other than C is less than or equal to 2.5 wt.%.

[0017] Optionally, when melting the proportioned alloy, uniform stirring and melting can be carried out in a vacuum electric arc furnace.

[0018] Preferably, when homogenizing the obtained ingot, the homogenization temperature is 1200°C, which can fully homogenize the alloy composition of the ingot.

[0019] In one embodiment, when the homogenized ingot is forged at high temperature to obtain a slab, the homogenized ingot is heated to 1150-1250°C in a vacuum or argon-protected atmosphere furnace and held for heat treatment. After heat treatment, it is air-cooled or furnace-cooled to room temperature, wherein the heat treatment time is greater than or equal to 2 hours. After the temperature drops to room temperature, the ingot is reheated to 1150°C and then continuously forged in multiple directions at high temperature to forge the ingot into a slab.

[0020] Optionally, the slab thickness is less than or equal to 50 mm, and during continuous multi-directional high-temperature forging, the forging ratio is ≥5, the initial forging temperature is greater than or equal to 1100℃, and the final forging temperature is greater than or equal to 1050℃. High-temperature forging can eliminate columnar crystals in the ingot and initially refine the grain size of the ingot.

[0021] Preferably, the final forging temperature is 1050℃ and the slab thickness is 35mm.

[0022] In one embodiment, when the obtained slab is subjected to high-temperature hot rolling and then water-cooled to room temperature after hot rolling to obtain the first semi-finished sheet, the obtained slab is heated to 1000-1050°C in a vacuum or argon-protected atmosphere furnace and held for 2-4 hours, followed by high-temperature hot rolling; after hot rolling, it is water-cooled to room temperature to obtain the first semi-finished sheet.

[0023] In one embodiment, the initial rolling temperature of the high-temperature hot rolling process is greater than or equal to 1000°C, the final rolling temperature is greater than or equal to 900°C, the cumulative deformation is greater than 50%, and the thickness of the first semi-finished plate is 20-30 mm.

[0024] Preferably, the furnace is an argon-protected atmosphere furnace, with a heating temperature of 1050℃, a holding time of 3 hours, a cumulative deformation of 60% in multiple hot rolling passes above 1000℃, a final rolling temperature of 900℃, and a final thickness of 25mm for the hot-rolled plate. Through the dynamic recovery and recrystallization that occur during hot rolling, not only can residual stress within the material be eliminated, but the microstructure can also be further homogenized and the grains further refined.

[0025] In one embodiment, the first semi-finished sheet is subjected to multiple passes of large deformation continuous symmetrical warm rolling to obtain the second semi-finished sheet. The first semi-finished sheet is reheated to 900–930°C and held for 1–2 hours, then air-cooled to 700–800°C and subjected to 3–5 passes of large deformation continuous symmetrical warm rolling to obtain the second semi-finished sheet. In this embodiment, medium-temperature rolling prevents the deformed metastable austenite grains from recrystallizing.

[0026] In one embodiment, the rolling time of the large deformation continuous symmetrical warm rolling process is 30s, the cumulative deformation is 70%-80%, the final rolling temperature is greater than or equal to 650℃, and during the large deformation continuous symmetrical rolling process, the diameter and rotation speed of the rolls are the same, and the single pass reduction of symmetrical rolling is greater than or equal to 15%.

[0027] Preferably, during the multi-pass large deformation continuous symmetrical warm rolling process, the plate is reheated at 920°C and held for 1.5 hours, then air-cooled to 750°C to begin rolling, with a final rolling temperature of 680°C.

[0028] Preferably, the large deformation continuous symmetrical warm rolling process is performed in 3 passes, and the deformation rates after each pass are 40%, 60%, and 75%, respectively, with a rolling time of less than or equal to 25 seconds. Using a large deformation, fewer-pass warm rolling process can reduce the warm rolling time and prevent metastable austenite from undergoing phase transformations such as ferrite or pearlite.

[0029] In one embodiment, the second semi-finished sheet is subjected to multi-pass large deformation continuous asymmetric warm rolling to obtain the third semi-finished sheet. The second semi-finished sheet is then subjected to 2-3 passes of large deformation continuous asymmetric warm rolling. The initial rolling temperature is 630-680°C, the linear speed ratio of the rolls on the upper and lower surfaces of the sheet is 1.2-2.5:1, the total reduction in asymmetric rolling is greater than or equal to 15%, the final rolling temperature is greater than or equal to 600°C, and the rolling time is less than or equal to 20 seconds. After rolling, the third semi-finished sheet is obtained. Similarly, in this embodiment, medium-temperature rolling can prevent the deformed metastable austenite grains from recrystallizing.

[0030] Preferably, the initial rolling temperature is 650℃ and the final rolling temperature is 600℃.

[0031] In one embodiment, the large deformation continuous asymmetric warm rolling process is performed in two passes, with the first pass having a reduction of 15% and the second pass having a reduction of 8%, and the rolling time being 10 seconds. This can prevent the metastable austenite from undergoing phase transformations such as ferrite or pearlite due to excessively long warm rolling time.

[0032] In one embodiment, the ultra-high strength martensitic steel plate has a carbon content of less than or equal to 0.35 wt.% and an alloy content of less than or equal to 2.5 wt.%.

[0033] In one embodiment, the ultra-high strength martensitic steel plate has a yield strength greater than or equal to 2100 MPa, a tensile strength greater than or equal to 2600 MPa, a hardness greater than or equal to 700 HV, and an elongation at break greater than or equal to 5%.

[0034] In one embodiment, the microstructure of the prepared martensitic steel plate is: ultrafine martensitic laths transformed from nanoscale original austenitic layers plus a small amount of residual austenite distributed between the original austenitic layers plus nano-precipitates. The nano-precipitates are carbides of Fe, Cr, Mo or V, and the size of the precipitates is no greater than 10 nm, and they are uniformly and diffusely distributed within the martensitic lamellars and at the lamellar interfaces.

[0035] The technical solutions provided by the embodiments of the present invention may include the following beneficial effects:

[0036] 1. Compared with the prior art, the high-strength martensitic steel plate prepared by the method of the present invention has a fine grain size and a structure of ultrafine martensitic laths transformed from nanoscale original austenitic layers. There is a residual austenitic phase between the original austenitic layers, and high-density nano carbides are uniformly dispersed in the martensitic matrix and at the interface. Therefore, it has excellent comprehensive mechanical properties, with a yield strength of not less than 2100MPa, a tensile strength of not less than 2600MPa, a hardness of not less than 700HV, and a fracture elongation of not less than 5%.

[0037] 2. The material used in this invention is conventional ordinary medium-carbon low-alloy steel, with a carbon content of no more than 0.35% by mass and the content of other alloying elements strictly controlled within 2.5% by mass. Moreover, there are no expensive alloying elements, which greatly reduces the raw material cost of high-strength martensitic steel and makes it very suitable for preparing industrial ultra-high-strength martensitic steel plates.

[0038] 3. The preparation method of this invention mainly employs medium-temperature large deformation rolling, including medium-temperature symmetrical rolling and medium-temperature asymmetrical rolling. Firstly, medium-temperature rolling can suppress the recrystallization and growth of metastable austenite grains after deformation, thereby fully refining the martensite structure after phase transformation and improving the strength and toughness of the material through fine-grain strengthening. Secondly, this invention reduces the subsequent tempering heat treatment process, fully preserving the contribution of dislocation strengthening to yield strength during rolling deformation. Thirdly, the large deformation warm rolling process introduces high-density nano-carbides into the metastable austenite and directly inherits them to the transformed martensite. The carbides are uniformly dispersed in the martensite matrix and at the interface, fully utilizing the precipitation strengthening effect. Finally, the entire preparation process is simple, efficient, and easy to operate, requiring no special mold design or equipment modification. It utilizes conventional industrial production equipment, significantly reducing energy consumption and saving costs, making it very suitable for large-scale preparation of high-strength industrial martensitic steel plates.

[0039] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the invention. Attached Figure Description

[0040] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0041] Figure 1 This is an SEM image of a coarse equiaxed protoauric transformation martensitic steel that has not undergone strong deformation symmetric / asymmetric intermediate-temperature continuous rolling, according to an exemplary embodiment.

[0042] Figure 2 This is a transmission electron microscope (TEM) image of a high-strength, high-toughness, medium-carbon, low-alloy martensitic steel with an ultrafine lamellar structure, as shown in an exemplary embodiment.

[0043] Figure 3 This is a transmission electron microscope (TEM) image of the precipitated phase microstructure in a high-strength and tough medium-carbon low-alloy martensitic steel with an ultrafine lamellar structure, as shown in an exemplary embodiment.

[0044] Figure 4 The tensile mechanical property curves of a high-strength and tough medium-carbon low-alloy martensitic steel sheet with an ultra-fine lamellar structure, as shown in an exemplary embodiment. Detailed Implementation

[0045] The following description and accompanying drawings fully illustrate specific embodiments described herein to enable those skilled in the art to practice them. Some embodiments may include or substitute parts and features of other embodiments. The scope of the embodiments herein encompasses the entire scope of the claims and all available equivalents thereof. Throughout this document, the terms “first,” “second,” etc., are used only to distinguish one element from another without requiring or implying any actual relationship or order between the elements. Indeed, a first element can also be referred to as a second element, and vice versa. Furthermore, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a structure, apparatus, or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a structure, apparatus, or device. Without further limitation, an element defined by the phrase “comprising one…” does not exclude the presence of other identical elements in the structure, apparatus, or device that includes said element. The various embodiments described herein are presented in a progressive manner, with each embodiment focusing on its differences from other embodiments; similar or identical parts between embodiments can be referred to interchangeably.

[0046] The terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer" used in this document to indicate orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings. They are used solely for the convenience of describing the document and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In the description herein, unless otherwise specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to mechanical or electrical connections, or internal connections between two elements; they can be direct connections or indirect connections through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0047] In this document, unless otherwise stated, the term "multiple" means two or more.

[0048] In this article, the character " / " indicates that the objects before and after it are in an "or" relationship. For example, A / B means: A or B.

[0049] In this article, the term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0050] It should be understood that although the steps in the flowchart are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order constraint on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the diagram may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the sub-steps or stages of other steps.

[0051] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0052] Example 1

[0053] A preparation process for ultra-high strength martensitic steel plate (nanoprecipitation-reinforced ultrafine lamellar structure high-strength and tough medium-carbon low-alloy martensitic steel plate) is disclosed. The overall technical solution is as follows: First, an alloy is proportioned according to a predetermined mass percentage. The proportioned alloy is then melted and cast into an ingot. The ingot is homogenized and then forged at high temperature to obtain a slab. The slab is hot-rolled at high temperature and then water-cooled to room temperature to obtain a first semi-finished plate. The first semi-finished plate undergoes multiple passes of large deformation continuous symmetrical hot rolling to obtain a second semi-finished plate. The second semi-finished plate undergoes multiple passes of large deformation continuous asymmetrical hot rolling to obtain a third semi-finished plate. The third semi-finished plate is quenched to room temperature to obtain the ultra-high strength martensitic steel plate. The specific steps are as follows:

[0054] (1) Alloy smelting: The chemical composition of the alloy by mass percentage is as follows: C≤0.35%, Si:0.3~0.4%, Mn:1.0~1.4%, P:0.005~0.01%, S:0.02~0.03%, Cr:0.1~0.2%, Ni:0.05~0.1%, Mo:0.01~0.03%, Cu:0.01~0.02%, Al:0.01~0.02%, V:0.01~0.03%, Ti:0.01~0.02% (B content controlled below 0.0005%), with the remainder being Fe element; the percentage content of alloying elements other than carbon is controlled below 2.5wt.%, and the alloy is batched according to the above composition and smelted uniformly in a vacuum induction furnace, then cast into ingots;

[0055] (2) Homogenization treatment and high temperature forging: The ingot obtained by step (1) is homogenized to eliminate compositional segregation. The ingot is heated to 1150-1250°C in a vacuum or argon protective atmosphere furnace and held for no less than 2 hours. Then it is air-cooled or furnace-cooled to room temperature. The ingot is then reheated to 1150°C and continuously multi-directional high temperature forging is carried out. The initial forging temperature is no less than 1100°C and the final forging temperature is no less than 1050°C. The ingot is forged into a slab with a thickness of no more than 50 mm.

[0056] (3) High-temperature hot rolling treatment: The slab obtained after step (2) is heated to 1000-1050℃ in a vacuum or argon protective atmosphere furnace and held for 2-4 hours. Then, it is subjected to high-temperature hot rolling treatment. The initial rolling temperature is not lower than 1000℃, the final rolling temperature is not lower than 900℃, and the cumulative deformation is greater than 50%. The slab is rolled into a plate with a thickness of 20-30mm. After rolling, it is water-cooled to room temperature to obtain the first semi-finished plate.

[0057] (4) Strong deformation symmetrical medium-temperature continuous rolling: The first semi-finished plate obtained after step (3) is reheated to 900-930℃ and held for 1-2 hours, then air-cooled to 700-800℃, and subjected to 3-5 passes of large deformation continuous symmetrical warm rolling. The cumulative deformation is between 70% and 80%, and the final rolling temperature is not lower than 650℃. The diameter and speed of the rolls are the same, the single pass of symmetrical rolling is not less than 15%, and the rolling time is controlled within 30 seconds to obtain the second semi-finished plate;

[0058] (5) Strong deformation asymmetric medium temperature continuous rolling: The second semi-finished plate obtained after step (4) is subjected to 2 to 3 passes of large deformation continuous asymmetric temperature rolling. The initial temperature of the rolls is the actual temperature of the plate after the symmetric medium temperature continuous rolling in step (4), controlled between 630 and 680℃. The linear speed ratio of the rolls on the upper and lower surfaces of the plate is 1.2 to 2.5:1. The total reduction of asymmetric rolling is not less than 15%, the final rolling temperature is not less than 600℃, and the rolling time is controlled within 20s. After rolling, the third semi-finished plate is obtained.

[0059] (6) Water cooling treatment after deformation: After the third semi-finished plate after step (5) is quenched to room temperature, a high-strength and tough medium-carbon low-alloy martensitic steel plate with nano-precipitation reinforced ultrafine lamellar structure is obtained.

[0060] Transmission electron micrographs of high-strength martensitic steel after large deformation warm rolling are attached. Figure 2 Coarse equiaxed proto-austenite grains are transformed into ultrafine proto-austenite lamellars after large deformation warm rolling. Water cooling then yields an ultrafine lamellar martensite structure with nano-retained austenite streaks distributed between the martensite lamellars. Numerous nano-carbides are distributed within the martensite matrix and at the interfaces (see attached figure). Figure 3Typical engineering stress-strain curves of the prepared high-strength martensitic plates are attached. Figure 4 As shown, its yield strength exceeds 2100MPa, tensile strength exceeds 2600MPa, and elongation at break is greater than 5%.

[0061] Example 2

[0062] A preparation process for an ultra-high strength martensitic steel plate (nanoprecipitation-reinforced ultrafine lamellar structure high-strength and tough medium-carbon low-alloy martensitic steel plate) is disclosed. The overall technical solution is as follows: First, an alloy is proportioned according to a predetermined mass percentage. The proportioned alloy is then melted and cast into an ingot. The ingot is homogenized and then forged at high temperature to obtain a slab. The slab is hot-rolled at high temperature and then water-cooled to room temperature to obtain a first semi-finished plate. The first semi-finished plate undergoes multiple passes of large deformation continuous symmetrical warm rolling to obtain a second semi-finished plate. The second semi-finished plate undergoes multiple passes of large deformation continuous asymmetrical warm rolling to obtain a third semi-finished plate. The third semi-finished plate is quenched to room temperature. Unlike Example 1, this example finally performs a low-temperature tempering treatment on the quenched plate to obtain the ultra-high strength martensitic steel plate. The specific steps are as follows:

[0063] (1) Alloy smelting is the same as step (1) in Example 1;

[0064] (2) Homogenization and blanking are the same as step (2) in Example 1;

[0065] (3) High-temperature hot rolling treatment is the same as step (3) in Example 1;

[0066] (4) The strong deformation symmetrical medium temperature continuous rolling treatment is the same as step (4) in Example 1;

[0067] (5) Strong deformation asymmetric medium temperature continuous rolling is the same as step (5) in Example 1;

[0068] (6) The water cooling treatment after deformation is the same as step (6) in Example 1;

[0069] (7) The high-strength and tough medium-carbon low-alloy martensitic steel plate prepared in step (6) is subjected to low-temperature aging treatment. The heating temperature is 150℃ and the holding time is 1h.

[0070] In this embodiment, low-temperature aging treatment is used to further increase the number density of nano-precipitates and eliminate residual stress caused by large deformation, thereby preparing high-strength and tough medium-carbon low-alloy martensitic steel plates. The prepared plates have a yield strength of 1900 MPa, a tensile strength of 2500 MPa, and a fracture elongation of nearly 10%.

[0071] Example 3

[0072] A preparation process for an ultra-high strength martensitic steel plate (nanoprecipitation-reinforced ultrafine lamellar structure high strength and toughness medium carbon low alloy martensitic steel plate) is as follows: First, alloy is proportioned according to a predetermined mass percentage content, the proportioned alloy is melted and cast into an ingot; the ingot is homogenized and then forged at high temperature to obtain a slab; the slab is hot-rolled at high temperature and then water-cooled to room temperature after hot rolling to obtain a first semi-finished plate; the first semi-finished plate is subjected to multi-pass large deformation continuous symmetrical warm rolling to obtain a second semi-finished plate; the second semi-finished plate is subjected to multi-pass large deformation continuous asymmetrical warm rolling to obtain a third semi-finished plate; the third semi-finished plate is quenched to room temperature to obtain an ultra-high strength martensitic steel plate. Unlike Example 1, this example adjusts step (4) strong deformation symmetrical medium temperature continuous rolling, increases the rolling deformation to 90%, and extends the total rolling time to 40s. The specific steps are as follows:

[0073] (1) Alloy smelting is the same as step (1) in Example 1;

[0074] (2) Homogenization and blanking are the same as step (2) in Example 1;

[0075] (3) High-temperature hot rolling treatment is the same as step (3) in Example 1;

[0076] (4) Strong deformation symmetrical medium-temperature continuous rolling: The first semi-finished plate obtained after step (3) is reheated to 900-930℃ and held for 1-2 hours, then air-cooled to 700-800℃, and subjected to 5 passes of large deformation continuous symmetrical warm rolling, with a cumulative deformation of more than 90% and a final rolling temperature of not less than 600℃. The diameter and speed of the rolls are the same, and the deformation rates after each pass are 50%, 65%, 78%, 87% and 92%, respectively. The 5 passes of rolling are completed within 40 seconds to obtain the second semi-finished plate;

[0077] (5) Strong deformation asymmetric medium temperature continuous rolling is the same as step (5) in Example 1;

[0078] (6) The water cooling treatment after deformation is the same as step (6) in Example 1;

[0079] The prepared high-strength and tough medium-carbon low-alloy martensitic steel plate has a yield strength of 2000 MPa, a tensile strength of 2500 MPa, and a fracture elongation of about 5%.

[0080] Example 4

[0081] A preparation process for an ultra-high strength martensitic steel plate (nanoprecipitation-reinforced ultrafine lamellar structure high strength and toughness medium carbon low alloy martensitic steel plate) is as follows: First, alloy is proportioned according to a predetermined mass percentage content, the proportioned alloy is melted and cast into an ingot; the ingot is homogenized and then forged at high temperature to obtain a slab; the slab is hot-rolled at high temperature and then water-cooled to room temperature after hot rolling to obtain a first semi-finished plate; the first semi-finished plate is subjected to multi-pass large deformation continuous symmetrical warm rolling to obtain a second semi-finished plate; the second semi-finished plate is subjected to multi-pass large deformation continuous asymmetrical warm rolling to obtain a third semi-finished plate; the third semi-finished plate is quenched to room temperature to obtain an ultra-high strength martensitic steel plate. Unlike Example 1, this example adjusts step (4) strong deformation symmetrical medium temperature continuous rolling, reduces the number of rolling passes, and increases the reduction per pass. The specific steps are as follows:

[0082] (1) Alloy smelting is the same as step (1) in Example 1;

[0083] (2) Homogenization and blanking are the same as step (2) in Example 1;

[0084] (3) High-temperature hot rolling treatment is the same as step (3) in Example 1;

[0085] (4) Strong deformation symmetrical medium-temperature continuous rolling: The first semi-finished plate obtained after step (3) is reheated to 900-930℃ and held for 1-2 hours, then air-cooled to 700-800℃, and subjected to four passes of large deformation continuous symmetrical warm rolling. The cumulative deformation is greater than 90%, and the final rolling temperature is not lower than 600℃. The diameter and speed of the rolls are the same, and the deformation rates after each pass are 60%, 70%, 78%, 87%, and 93%, respectively. The four passes are completed within 40 seconds to obtain the second semi-finished plate.

[0086] (5) Strong deformation asymmetric medium temperature continuous rolling is the same as step (5) in Example 1;

[0087] (6) The water cooling treatment after deformation is the same as step (6) in Example 1;

[0088] The prepared high-strength and tough medium-carbon low-alloy martensitic steel plate has a yield strength of 2100 MPa, a tensile strength of 2300 MPa, and a fracture elongation of about 4.5%.

[0089] Example 5

[0090] A preparation process for an ultra-high strength martensitic steel plate (nanoprecipitation-reinforced ultrafine lamellar structure high strength and toughness medium carbon low alloy martensitic steel plate) is as follows: First, alloy is proportioned according to a predetermined mass percentage content, the proportioned alloy is melted and cast into an ingot; the ingot is homogenized and then forged at high temperature to obtain a slab; the slab is hot-rolled at high temperature and then water-cooled to room temperature after hot rolling to obtain a first semi-finished plate; the first semi-finished plate is subjected to multi-pass large deformation continuous symmetrical warm rolling to obtain a second semi-finished plate; the second semi-finished plate is subjected to multi-pass large deformation continuous asymmetrical warm rolling to obtain a third semi-finished plate; the third semi-finished plate is quenched to room temperature to obtain an ultra-high strength martensitic steel plate. Unlike Example 1, this example adjusts step (2) homogenization and billet opening, shortening the homogenization heat treatment time and reducing the size of grain growth to a certain extent. The specific steps are as follows:

[0091] (1) Alloy smelting is the same as step (1) in Example 1;

[0092] (2) Homogenization and billet treatment: The ingot obtained in step (1) is homogenized at 1150-1250℃, the holding time is shortened to 1h, and the cooling method is air cooling to obtain a billet with homogeneous structure. The homogenization time is shortened here, which reduces the size of grain growth to a certain extent. Then, high-temperature forging is carried out. The initial forging temperature is not lower than 1100℃, and multiple upsetting and drawing are carried out. The final forging temperature is not lower than 1050℃, and the final forging thickness is not higher than 50mm.

[0093] (3) High-temperature hot rolling treatment is the same as step (3) in Example 1;

[0094] (4) The strong deformation symmetrical medium temperature continuous rolling treatment is the same as step (4) in Example 1;

[0095] (5) The strong deformation asymmetric medium-temperature continuous rolling treatment is the same as step (5) in Example 1;

[0096] (6) The water cooling treatment after deformation is the same as step (6) in Example 1;

[0097] The prepared high-strength and tough medium-carbon low-alloy martensitic steel plate has a yield strength of 2050 MPa, a tensile strength of 2450 MPa, and a fracture elongation of about 6%.

[0098] Example 6

[0099] A preparation process for an ultra-high strength martensitic steel plate (nanoprecipitation-reinforced ultrafine lamellar structure high strength and toughness medium carbon low alloy martensitic steel plate) is as follows: First, the alloy is proportioned according to a predetermined mass percentage content, the proportioned alloy is melted and cast into an ingot; the ingot is homogenized and then forged at high temperature to obtain a slab; the slab is hot-rolled at high temperature and then water-cooled to room temperature after hot rolling to obtain a first semi-finished plate; the first semi-finished plate is subjected to multi-pass large deformation continuous symmetrical hot rolling to obtain a second semi-finished plate; the second semi-finished plate is subjected to multi-pass large deformation continuous asymmetrical hot rolling to obtain a third semi-finished plate; the third semi-finished plate is quenched to room temperature to obtain an ultra-high strength martensitic steel plate. Unlike Example 1, this example adjusts step (1) alloy melting, increasing the carbon mass fraction to 0.4%. The specific steps are as follows:

[0100] (1) Alloy smelting: The chemical composition of the alloy by mass percentage is as follows: C: 0.4%, Si: 0.3-0.4%, Mn: 1.0-1.4%, P: 0.005-0.01%, S: 0.02-0.03%, Cr: 0.1-0.2%, Ni: 0.05-0.1%, Mo: 0.01-0.03%, Cu: 0.01-0.02%, Al: 0.01-0.02%, V: 0.01-0.03%, Ti: 0.01-0.02% (B content controlled below 0.0005%), with the remainder being Fe. After the alloy is batched according to the above composition, it is loaded into the furnace and smelted by uniform stirring in a vacuum induction furnace, and then cast into ingots. Here, the mass fraction of carbon is increased to 0.4%.

[0101] (2) Homogenization and blanking are the same as step (2) in Example 1;

[0102] (3) High-temperature hot rolling treatment is the same as step (3) in Example 1;

[0103] (4) The strong deformation symmetrical medium temperature continuous rolling treatment is the same as step (4) in Example 1;

[0104] (5) The strong deformation asymmetric medium-temperature continuous rolling treatment is the same as step (5) in Example 1;

[0105] (6) The water cooling treatment after deformation is the same as step (6) in Example 1;

[0106] The prepared high-strength and tough medium-carbon low-alloy martensitic steel plate has a yield strength of 2200 MPa, a tensile strength of 2800 MPa, and a fracture elongation of about 4%.

[0107] Example 7

[0108] A preparation process for an ultra-high strength martensitic steel plate (nanoprecipitation-reinforced ultrafine lamellar structure high strength and toughness medium carbon low alloy martensitic steel plate) is as follows: First, alloy is proportioned according to a predetermined mass percentage content, the proportioned alloy is melted and cast into an ingot; the ingot is homogenized and then forged at high temperature to obtain a slab; the slab is hot-rolled at high temperature and then water-cooled to room temperature after hot rolling to obtain a first semi-finished plate; the first semi-finished plate is subjected to multi-pass large deformation continuous symmetrical warm rolling to obtain a second semi-finished plate; the second semi-finished plate is subjected to multi-pass large deformation continuous asymmetric warm rolling to obtain a third semi-finished plate; the third semi-finished plate is quenched to room temperature to obtain an ultra-high strength martensitic steel plate. Unlike Example 1, this example adjusts step (5) strong deformation asymmetric medium temperature continuous rolling, where the linear speed ratio of the rolls on the upper and lower surfaces of the plate is increased to 3:1. The specific steps are as follows:

[0109] (1) Alloy smelting is the same as step (1) in Example 1;

[0110] (2) Homogenization and blanking are the same as step (2) in Example 1;

[0111] (3) High-temperature hot rolling treatment is the same as step (3) in Example 1;

[0112] (4) The strong deformation symmetrical medium temperature continuous rolling treatment is the same as step (4) in Example 1;

[0113] (5) Strong deformation asymmetric medium temperature continuous rolling: The second semi-finished plate obtained after step (4) is subjected to 2 to 3 passes of large deformation continuous asymmetric temperature rolling. The initial temperature of the rolls is the actual temperature of the plate after the symmetric medium temperature continuous rolling in step (4), controlled between 630 and 680℃. The linear speed ratio of the rolls on the upper and lower surfaces of the plate is 3:1. The total reduction of the asymmetric rolling is not less than 15%, the final rolling temperature is not less than 600℃, and the rolling time is controlled within 20s. After the rolling is completed, the third semi-finished plate is obtained.

[0114] (6) The water cooling treatment after deformation is the same as step (6) in Example 1;

[0115] The prepared high-strength and tough medium-carbon low-alloy martensitic steel plate has a yield strength of 2100 MPa, a tensile strength of 2600 MPa, and a fracture elongation of about 4%.

[0116] Example 8

[0117] A preparation process for an ultra-high strength martensitic steel plate (nanoprecipitation-reinforced ultrafine lamellar structure high strength and toughness medium carbon low alloy martensitic steel plate) is as follows: First, alloy is proportioned according to a predetermined mass percentage content, the proportioned alloy is melted and cast into an ingot; the ingot is homogenized and then forged at high temperature to obtain a slab; the slab is hot-rolled at high temperature and then water-cooled to room temperature after hot rolling to obtain a first semi-finished plate; the first semi-finished plate is subjected to multi-pass large deformation continuous symmetrical warm rolling to obtain a second semi-finished plate; the second semi-finished plate is subjected to multi-pass large deformation continuous asymmetrical warm rolling to obtain a third semi-finished plate; the third semi-finished plate is quenched to room temperature to obtain an ultra-high strength martensitic steel plate. Unlike Example 1, this example adjusts the air cooling treatment after deformation in step (6), changing the water cooling treatment to air cooling treatment. The specific steps are as follows:

[0118] (1) Alloy smelting is the same as step (1) in Example 1;

[0119] (2) Homogenization and blanking are the same as step (2) in Example 1;

[0120] (3) High-temperature hot rolling treatment is the same as step (3) in Example 1;

[0121] (4) The strong deformation symmetrical medium temperature continuous rolling treatment is the same as step (4) in Example 1;

[0122] (5) Strong deformation asymmetric medium temperature continuous rolling is the same as step (5) in Example 1;

[0123] (6) Air cooling treatment after deformation: After the third semi-finished plate after step (5) is air cooled to room temperature, a high-strength and tough medium carbon (0.35wt.%) low alloy (≤2.5wt.%) martensitic steel plate with nano-precipitation reinforced ultrafine lamellar structure is obtained. The high-strength and tough medium carbon low alloy martensitic steel plate prepared has a yield strength of 2000MPa, a tensile strength of 2500MPa, and a fracture elongation of about 7%.

[0124] This invention uses ordinary industrial medium-carbon low-alloy steel as raw material, with a carbon content of no more than 0.35 wt% and the mass percentage of other alloying elements controlled below 2.5%. It employs conventional processes such as alloy smelting, homogenization treatment, high-temperature forging, high-temperature hot rolling, medium-temperature symmetrical continuous rolling, and asymmetric rolling. Utilizing the shear force generated during over 90% of the warm rolling process, the metastable austenite structure is fully refined, transforming coarse equiaxed austenite grains into nanoscale austenite lamellae. Subsequent water cooling yields a multiphase structure of ultrafine martensite laths and retained austenite. Back stress strengthening is generated by the difference in mechanical properties between the two phases. The high-strength martensitic steel prepared by this process, in addition to possessing the advantages of fine-grain strengthening and dislocation strengthening inherent in general large deformation rolling, also benefits from the high-density nanoparticles uniformly dispersed within the martensite matrix and at the martensite lath interfaces introduced during the large deformation warm rolling process. The resulting precipitation strengthening plays a crucial role in improving its yield strength. Simultaneously, the presence of retained austenite significantly enhances the plasticity of the warm-rolled martensitic steel, resulting in excellent overall mechanical properties.

[0125] This invention is not limited to the structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this invention is limited only by the appended claims.

Claims

1. A method for preparing ultra-high strength martensitic steel plates, characterized in that, The chemical composition and mass percentage of alloying elements in ultra-high strength martensitic steel plates are as follows: C ≤ 0.35%, Si: 0.3~0.4%, Mn: 1.0~1.4%, P: 0.005~0.01%, S: 0.02~0.03%, Cr: 0.1~0.2%, Ni: 0.05~0.1%, Mo: 0.01~0.03%, Cu: 0.01~0.02%, Al: 0.01~0.02%, V: 0.01~0.03%, Ti: 0.01~0.02%, B ≤ 0.0005%, with the remainder being Fe. The method includes: The alloy is formulated according to a predetermined mass percentage content, and the formulated alloy is melted and cast into ingots; the obtained ingots are homogenized, and the homogenized ingots are forged at high temperature to obtain slabs. The obtained slab is subjected to high-temperature hot rolling and then water-cooled to room temperature to obtain the first semi-finished plate. The first semi-finished plate is subjected to multi-pass large deformation continuous symmetrical warm rolling to obtain the second semi-finished plate. The second semi-finished plate is subjected to multi-pass large deformation continuous asymmetrical warm rolling to obtain the third semi-finished plate. After the third semi-finished plate is quenched to room temperature, ultra-high strength martensitic steel plate is obtained. The process of obtaining the second semi-finished sheet by performing multi-pass large deformation continuous asymmetric warm rolling to obtain the third semi-finished sheet includes: performing 2-3 passes of large deformation continuous asymmetric warm rolling on the second semi-finished sheet, with an initial rolling temperature of 630-680℃, a linear speed ratio of 1.2-2.5:1 between the rolls on the upper and lower surfaces of the sheet, a total reduction of ≥15% in asymmetric rolling, a final rolling temperature of ≥600℃, and a rolling time of ≤20 s, thereby obtaining the third semi-finished sheet after rolling.

2. The method for preparing ultra-high strength martensitic steel plate according to claim 1, characterized in that, When melting the proportioned alloy, it is done by uniform stirring in a vacuum electric arc furnace.

3. The method for preparing ultra-high strength martensitic steel plate according to claim 1, characterized in that, The homogenization temperature for the obtained ingots was 1200 ℃.

4. The method for preparing ultra-high strength martensitic steel plate according to claim 1, characterized in that, The homogenized ingot is then subjected to high-temperature forging to obtain a slab comprising: The homogenized ingot is heated to 1150~1250 ℃ in a vacuum or argon-protected atmosphere hot furnace and held for heat treatment. After heat treatment, it is air-cooled or furnace-cooled to room temperature. The heat treatment time is greater than or equal to 2 hours. After the temperature drops to room temperature, the ingot is reheated to 1150 ℃ and then subjected to continuous multi-directional high-temperature forging to forge the ingot into a slab.

5. The method for preparing ultra-high strength martensitic steel plate according to claim 4, characterized in that, The slab thickness is less than or equal to 50 mm, and during continuous multi-directional high-temperature forging, the forging ratio is ≥5, the initial forging temperature is greater than or equal to 1100 ℃, and the final forging temperature is greater than or equal to 1050 ℃.

6. The method for preparing ultra-high strength martensitic steel plate according to claim 1, characterized in that, The obtained slab is subjected to high-temperature hot rolling and then water-cooled to room temperature to obtain the first semi-finished sheet material, which includes: The obtained slab is heated to 1000~1050 ℃ in a vacuum or argon-protected atmosphere furnace and held for 2~4 h, followed by high-temperature hot rolling. After hot rolling, the material is cooled to room temperature to obtain the first semi-finished sheet.

7. The method for preparing ultra-high strength martensitic steel plate according to claim 6, characterized in that, The high-temperature hot rolling process has an initial rolling temperature greater than or equal to 1000 ℃, a final rolling temperature greater than or equal to 900 ℃, a cumulative deformation of greater than 50%, and a thickness of 20~30 mm for the first semi-finished plate.

8. The method for preparing ultra-high strength martensitic steel plate according to claim 1, characterized in that, The first semi-finished sheet is subjected to multi-pass large deformation continuous symmetrical warm rolling to obtain the second semi-finished sheet, which includes: The first semi-finished sheet is reheated to 900~930 ℃ and kept warm for 1~2 hours, then air-cooled to 700~800 ℃, and subjected to 3~5 passes of large deformation continuous symmetrical warm rolling to obtain the second semi-finished sheet.

9. The method for preparing ultra-high strength martensitic steel plate according to claim 8, characterized in that, The rolling time for the large deformation continuous symmetrical warm rolling process is 30 s, the cumulative deformation is 70%-80%, the final rolling temperature is greater than or equal to 650 ℃, and the diameter and speed of the rolls are the same during the large deformation continuous symmetrical warm rolling process, and the single pass reduction of symmetrical rolling is greater than or equal to 15%.

10. The method for preparing ultra-high strength martensitic steel plate according to claim 9, characterized in that, The large deformation continuous symmetrical warm rolling process consists of 3 passes, and the deformation rates after each pass are 40%, 60%, and 75%, respectively, with a rolling time of less than or equal to 25 seconds.

11. The method for preparing ultra-high strength martensitic steel plate according to claim 1, characterized in that, The large deformation continuous asymmetric warm rolling process consists of two passes, with a reduction of 15% in the first pass and 8% in the second pass, and a rolling time of 10 seconds.

12. The method for preparing ultra-high strength martensitic steel plate according to claim 1, characterized in that, The ultra-high strength martensitic steel plate has a yield strength greater than or equal to 2100 MPa, a tensile strength greater than or equal to 2600 MPa, a hardness greater than or equal to 700 HV, and an elongation at break greater than or equal to 5%.

Images