Hot-forming steel having ultrahigh oxidation resistance and preparation method therefor

Abstract

Disclosed is hot-forming steel having ultrahigh oxidation resistance. The hot-forming steel comprises, sequentially from inside to outside, a matrix, a compact and low-porosity Fe2(Si / Cr / Re)O4 layer, a Si and Fe oxide-rich layer, and a Cr and Fe oxide-rich layer, the thickness of the entire oxide layer being less than 0.2-2.0 μm. The distribution ratios of Si, Cr, and Re in the oxide layer of the hot-forming steel are: (Cr)L1 / (Cr)L2≥1.58; (Si)L1 / (Si)L2≤2.58; (Si+Cr)L / (Si+Cr)B≥1.95; and 1.25≤ReL / ReB≤2.5. Further disclosed is a preparation method for the hot-forming steel having ultrahigh oxidation resistance. The hot-forming steel of the present application has excellent mechanical properties, the yield strength thereof being 1070 MPa, the tensile strength thereof being 1650 MPa, the elongation thereof being 8%, the product of strength and elongation thereof being 13 GPa·%, and the cold bending angle thereof being 65-75°C. In addition, the steel has excellent high-temperature oxidation resistance and thermal conductivity; hot-stamped parts prepared therefrom have thin surface oxidation layers that are complete and do not fall off, uniform surface quality, and metallic luster, and can be coated directly without requiring shot blasting.

Description

A kind of hot-formed steel with super strong oxidation resistance and its preparation method

[0001] Priority Declaration

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on September 18, 2023, with application number 202311209529.X and invention name “A hot-formed steel with super strong oxidation resistance and its preparation method”, the entire contents of which are incorporated by reference into this application. Technical Field

[0003] The present invention belongs to the technical field of metal processing, and in particular relates to a hot-formed steel with super strong oxidation resistance and a preparation method thereof. Background Art

[0004] Safety, environmental protection and energy conservation are the themes of the current development of the automobile manufacturing industry. The use of high-strength steel plates to manufacture the car body can not only effectively reduce the weight of the car body and reduce fuel consumption, but also improve the safety and comfort of the car. It is the best way to achieve lightweight body and improve collision safety at the same time.

[0005] To address the difficulty in forming high-strength steel, the hot stamping process has been successfully developed and widely commercialized. Hot stamping involves heating a high-strength steel sheet to 880-950°C and holding it for approximately 5 minutes to fully austenitize it. The sheet is then quickly transferred to a die equipped with an internal circulating cooling system for stamping. After a certain holding period, the formed part is obtained. The cooling rate must be greater than 44°C / s to ensure martensitic transformation of the austenite structure. The resulting part is essentially martensitic, with some remaining ferrite and bainite. The hot stamping process produces high-precision, high-strength parts while minimizing wear on the die. Due to the high heating temperatures involved in the hot stamping process, the surface of the high-strength steel sheet undergoes severe oxidation, resulting in a large amount of iron scale. This hard iron scale can damage the die surface and reduce its service life. At the same time, since the loose iron oxide scale on the surface is not conducive to subsequent painting treatment, shot peening equipment is required to remove the surface iron oxide scale. Shot peening will cause surface deformation of parts, further increasing production costs and limiting its application on large-size thin-walled parts.

[0006] In order to solve the problem of high temperature oxidation resistance of hot stamping steel, there are currently two main solutions: one is to use coating technology, and the other is to use non-coating technology.

[0007] Among them, coating technology can prevent surface oxidation and decarburization during the forming process, and can also improve the corrosion resistance after painting. For example, the commercial hot stamping steel plates with aluminum-silicon alloy coatings can protect the steel plates from oxidation at the forming heating temperature, so they are used commercially on a large scale. However, this coating technology has many production processes, which invisibly increases production costs and is monopolized by foreign companies.

[0008] Non-plating technology primarily improves the high-temperature oxidation resistance of hot-stamped steel by designing and manipulating alloying elements and their composition ratios. Currently, this technology has less commercial application than coating technology. Regarding non-plating technology, patent number CN111926248A discloses a hot-stamped steel and hot-stamping process that incorporates a Ce alloy. The hot stamping steel with a Ce alloy has the following chemical composition, by mass percentage: C 0.22-0.40%, Mn 1.0-2.5%, Si 1.2-2.0%, Al 0.05-0.5%, Cr 1.2-2.5%, Mo <0.6%, Nb <0.1%, V <0.15%, Ti 0-0.1%, B 0-0.01%, P 0.01-0.03%, O less than 0.004%, S <0.002%, Ce 0.03-0.08%, Ca <0.01%, Mg <0.01%, with the balance being Fe and unavoidable inclusions. The hot stamping steel adopts a completely new composition design and is produced and hot stamped using conventional production processes. By changing the thickness and structure of the oxide scale during the heating and stamping process of hot-formed steel, the problem of its shedding during the heating and stamping process is solved, and finally a very thin, stable and dense oxide layer is formed on the surface of the stamped parts. This solves the problem of mold wear caused by the shedding of the oxide layer of hot-stamped steel, and at the same time, it can avoid shot peening, meet the conditions for direct coating application, and does not pollute the environment. However, the composition of the hot-formed steel in this technical solution is complex, the rare earth Ce content is too high, and the cost is high; too much Ce addition will cause clogging of the nozzle in the continuous casting process, and unstable production; Ce alloys are extremely easy to oxidize, and too much Ce addition will deteriorate the oxidation resistance, resulting in a relatively thick oxide layer thickness, reducing the toughness and weldability of hot-formed parts, and cannot be used in mass production.

[0009] Summary of the Invention

[0010] The present invention aims to address, at least to a certain extent, one of the technical problems in the related art. To this end, the present invention primarily aims to provide a hot-formed steel with a simple composition, which eliminates the need for shot blasting, ensures the surface quality and strength of formed parts, and exhibits excellent high-temperature oxidation resistance. The present application also aims to provide a method for preparing this hot-formed steel with superior high-temperature oxidation resistance.

[0011] The purpose of the present invention is achieved through the following technical solutions:

[0012] In a first aspect, a hot-formed steel with super strong oxidation resistance comprises, from the inside out, a matrix, a dense, low-porosity Fe2(Si / Cr / RE)O4 layer, a Si-rich, Fe oxide layer, and a Cr-rich, Fe oxide layer; and the distribution ratio of Si, Cr, and RE in the oxide layer of the hot-formed steel is: (Cr) L1 / (Cr) L2 ≥1.58; (Si) L1 / (Si) L2 ≤2.58; (Si+Cr) L / (Si+Cr) B ≥1.95; 1.25≤RE L / RE B ≤2.50;

[0013] Among them (Cr) L1 and (Si) L1 Refers to the Cr and Si contents in the Cr-rich and Fe-rich oxide layers, respectively. (Cr) L2 and (Si) L2 Refers to the Cr and Si contents in Si-rich and Fe-rich oxide layers, respectively. (Si+Cr) L Refers to the total content of Cr and Si in the oxide layer, (Si+Cr) B Refers to the total content of Cr and Si in the matrix, RE L Refers to the average rare earth content in the oxide layer, RE B Refers to the average rare earth content in the matrix.

[0014] A method for preparing hot-formed steel with super strong oxidation resistance includes furnace heating, high-pressure water descaling, rough rolling, finish rolling, coiling, straightening, pickling, cold rolling and annealing. The furnace heating process specifically comprises heating the steel continuous casting billet to 1250-1380°C in a heating furnace and keeping it warm for 1-2.5 hours to obtain hot-rolled insulated steel.

[0015] Furthermore, the hot-rolled insulated steel includes, from the inside to the outside, a matrix, a Fe2(Si / Cr / RE)O4 layer with a loose porous structure, a FeO+Fe3O4 mixed layer, and a Fe2O3 layer; the hot-rolled insulated steel with this structure can remove the oxide layer by high-pressure water before rough rolling, thereby preventing the oxide scale from being pressed into the matrix during the subsequent hot rolling process, so as to obtain hot-formed steel with excellent surface quality.

[0016] In a specific embodiment, the coiling process is specifically as follows: the initial coiling temperature is 480-600° C., the steel strip is coiled at the initial coiling temperature, and then air-cooled to room temperature to obtain coiled steel.

[0017] In a specific embodiment, the coiled steel after the coiling process is further subjected to hood annealing, the purpose of which is to reduce the strength of the steel coil and facilitate subsequent processes. The hood annealing process is specifically as follows: in a full hydrogen or near full hydrogen hood annealing furnace, heating from room temperature to 620-720°C at a heating rate of 40-50°C / h, keeping warm for 10-20h, first cooling to 300-350°C at a slow cooling rate of 8-12°C / h, and then cooling to room temperature at a cooling rate of 30-40°C / h to obtain hood annealed steel.

[0018] Furthermore, the coiled steel includes, from the inside to the outside, a matrix, a thin and dense Fe2(Si / Cr / RE)O4 layer, a FeO+Fe3O4 mixed layer and a Fe2O3 layer; the thin and dense Fe2(Si / Cr / RE)O4 layer can prevent the matrix Fe element from diffusing outward and the external O element from diffusing into the matrix, thereby improving the oxidation resistance and reducing the thickness of the oxide layer; the FeO layer in the FeO+Fe3O4 mixed layer is closer to the Fe2(Si / Cr / RE)O4 layer, the FeO oxide layer is loose and thin, and the Fe3O4 and Fe2O3 oxide layers are thick but have cracks; under the combined action of straightening and pickling, hydrochloric acid can easily enter along the cracks, and react with the Fe2(Si / Cr / RE)O4 oxide layer close to the matrix through the loose FeO, thereby removing the entire oxide layer, so as to subsequently obtain hot-formed steel with excellent surface quality.

[0019] In a specific embodiment, a thermoforming process is further included, wherein the thermoforming process is specifically: in an air atmosphere or a nitrogen protective atmosphere, at a thermoforming temperature of 900-960° C., and keeping the temperature for 1-6 minutes.

[0020] In a specific embodiment, the continuous casting steel billet includes, by mass percentage, the following: C: 0.12-0.4%, Si: 1.0-2.0%, Mn: 0.0-2.0%, Al: 0.01-0.5%, Cr: 0.5-1.6%, B: 0-0.004%, Ti: 0-0.1%, RE: 0.003-0.008%, S and P: less than 0.008%, and the rest is Fe.

[0021] Furthermore, the molar ratio of Si to Cr is (2-4):1, and RE is one of Ce and Y or a mixture thereof.

[0022] Compared with the prior art, the present invention has at least the following advantages:

[0023] 1) The hot-formed steel contains less expensive elements such as Cr, which reduces costs from the source; compared with existing Al-Si coated hot-formed steel, the hot-formed steel has faster heat transfer, can reduce the holding time of hot stamping and hot forming, has high production efficiency, and greatly reduces production costs; the lower Cr content can also reduce cold rolling resistance, improve cold rollability, and thus the yield rate of cold-rolled plates; the lower Cr content can also improve the welding performance of hot-formed parts; therefore, the invention achieves cost reduction and efficiency improvement, significantly improving economic benefits.

[0024] 2) The hot-formed steel with super strong oxidation resistance provided by the present invention is obtained by collaboratively designing the ratio of Si and Cr content and introducing a specific proportion of rare earth element RE, so that the final hot-formed steel includes, from the inside to the outside, a matrix, a dense, low-porosity Fe2(Si / Cr / RE)O4 layer, a Si-rich Fe oxide layer and a Cr-rich Fe oxide layer; because Si and Cr have higher oxidation resistance than Fe and are easy to form Si and Cr oxides, an outermost layer of Cr-rich Fe oxide and an intermediate layer of Si and Fe oxide are formed; in this structure, Cr is enriched in the outermost layer, and the Cr2O3 oxide layer is flat and dense, which improves the high-temperature oxidation resistance of the matrix. properties; Si is enriched in the middle layer to form SiO2, which further prevents O from diffusing into the matrix; the innermost dense, low-porosity Fe2(Si / Cr / RE)O4 layer not only improves the bonding strength between the oxide layer and the matrix, but also further prevents O from diffusing into the matrix, thereby making the RE content in the entire oxide layer higher than that in the matrix, optimizing its high-temperature oxidation resistance; and the formed three-layer Si and Cr-rich oxide layer significantly prevents O atoms from diffusing into the internal matrix, improving its high-temperature oxidation resistance and thermal conductivity, so that the hot stamping parts of this steel do not need to be shot peened, thereby avoiding the risk of shot peening deformation of thin-walled parts, while reducing production costs and environmental pollution.

[0025] 3) The hot-formed steel with super strong oxidation resistance provided in this application also has excellent mechanical properties, with a yield strength of 1070MPa, a tensile strength of 1650MPa, an elongation of 8%, a strength-ductility product of 13GPa·% and a cold bending angle of 65-75°C; its mechanical properties are significantly better than the mechanical properties of 22MnB5, with a yield strength of 1055MP, a tensile strength of 1500MPa, an elongation of 6%, a strength-ductility product of 9GPa·% and a cold bending angle of 55-65°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the specific embodiments of the present invention, the following briefly introduces the drawings required for the specific embodiments or the description of the prior art.

[0027] FIG1 is a process flow chart of annealing steel provided in the present invention;

[0028] FIG2 is a graph showing the changes in oxidation weight gain over temperature and time for annealed steels provided in Examples 5, 6, and 7, 22MnB5, and Comparative Example 7 under the same heating environment;

[0029] FIG3 is a graph showing temperature variations over time for annealed steel and 22MnB5 provided in Example 5 of the present invention under the same heating environment and the same size;

[0030] FIG4 is a scanning electron microscope image of the annealed steel provided in Example 5 of the present invention after being hot-formed in an air environment at 950° C. for 5 minutes;

[0031] FIG5 is a microscopic image of the oxide layer at line 5 in FIG4 and its corresponding line scan element distribution map;

[0032] FIG6 is an electron microscope scanning image of the hot-rolled insulated steel provided in Example 5 of the present invention;

[0033] FIG7 is a scanning electron microscope image of the coiled steel provided in Example 5 of the present invention;

[0034] FIG8 is an electron microscope scanning image of the hot-rolled heat-insulated steel in the hot-formed steel provided in Comparative Example 1 of the present invention;

[0035] FIG9 is a surface energy spectrum scan of the oxide layer corresponding to FIG8;

[0036] FIG10 is a physical image of the surface of the ultra-strong oxidation-resistant hot-formed steel and 22MnB5 after quenching provided in Example 5 of the present invention;

[0037] FIG11 is a diagram of a part made of ultra-strong anti-oxidation hot-formed steel after hot forming at 950° C. provided in Example 5 of the present invention;

[0038] FIG12 is a real picture of the hot-formed steel with super strong oxidation resistance after direct coating without shot blasting provided by Example 5 of the present invention. DETAILED DESCRIPTION

[0039] The present invention will be further described in detail below with reference to the accompanying drawings and examples. The following examples are merely illustrative and non-restrictive, and should not be used to limit the scope of protection of the present invention.

[0040] When expressing a certain amount, concentration or other value or parameter in the form of a range, preferred range, or preferred upper and lower numerical limits, it should be understood that it is equivalent to specifically disclosing any range by combining any pair of upper range limits or preferred numerical values ​​with any lower range limit or preferred numerical value, without considering whether the range is specifically disclosed. Unless otherwise indicated, the numerical range values ​​listed herein include the endpoints of the range and all integers and fractions within the range.

[0041] Unless otherwise indicated, all percentages, parts, ratios, etc. herein are by weight.

[0042] The materials, methods, and examples herein are illustrative and, unless otherwise indicated, are not to be construed as limiting.

[0043] To address the oxidation problem of hot-formed steel, shot blasting after forming can largely remove the oxides produced on the surface of the hot-formed steel. However, the parts are severely deformed after shot blasting, making it impossible to manufacture large, thin-walled parts. Due to the current trend of lightweight automobiles, its market space is shrinking. On the other hand, the shot blasting process adds additional production process links, which increases production costs. The process also generates huge noise and dust, which is not in line with environmental protection concepts. Moreover, this method cannot improve the mold loss and reduced production efficiency caused by oxides. In addition to preventing decarburization and oxidation of the steel plate surface during the austenitization process, the protective layer can also isolate the steel plate substrate from the external environment and has a certain anti-corrosion effect. However, the bending fracture strain and delayed cracking resistance of the hot-formed steel plate are significantly lower than those of the bare plate. At the same time, the cost of the coated steel plate is significantly increased compared to the uncoated plate.

[0044] In the following examples, metallographic structure reliability was tested in accordance with GB / T 13298-1991 Metal Microstructure Test Methods; mechanical property reliability was tested in accordance with GB / T 228.1-2010 Metallic Materials Tensile Test Part 1: Room Temperature Test Methods; oxidation resistance reliability was tested in accordance with GB / T 13303-91 Test Method for Oxidation Resistance of Steel. To better demonstrate the high-temperature oxidation resistance of the hot stamping steel of the present invention, the weight gain method was used for testing; cold bending performance reliability was tested in accordance with T / CSAE 154-2020 Test Method for Ultimate Tip Cold Bending Performance of Ultra-High Strength Automotive Steel Plate.

[0045] The hot-formed steel with super strong oxidation resistance provided by the present application has a chemical composition of the continuous casting steel billet, calculated in mass percentage, including: C: 0.12-0.4%, Si: 1.0-2.0%, Mn: 0-2.0%, Al: 0.01-0.5%, Cr: 0.5-1.6%, B: 0-0.004%, Ti: 0-0.1%, RE: 0.003-0.008%, S and P: less than 0.008%, and the rest is Fe; further, the addition amount of Si and Cr in the hot-formed steel satisfies (2-4):1, wherein the RE is one of Ce and Y or a mixture thereof.

[0046] In this application, the ratio of Si and Cr content and the specific proportion of rare earth elements RE are designed in coordination, and the process parameters such as the furnace temperature and the coiling temperature are controlled, so that the oxide layer formed on the surface of the hot-rolled insulated steel and the coiled steel can be completely removed by high-pressure water descaling and pickling, respectively, to avoid the pressure of iron oxide scale, thereby improving the surface quality and rollability of the subsequent cold-rolled plate; after the annealed steel undergoes the hot forming process, the hot-formed steel finally obtained includes, from the inside to the outside, a matrix, a dense, low-porosity Fe2(Si / Cr / RE)O4 layer, a Si-rich Fe oxide layer and a Cr-rich Fe oxide layer; because Si and Cr have higher oxidation resistance than Fe, they are easy to form Si and Cr oxides, thus forming an outermost layer rich in Cr and Fe oxide and an intermediate layer rich in Si and Fe. Oxide; in this structure, Cr is enriched in the outermost layer, and the Cr2O3 oxide layer is flat and dense, which improves the high-temperature oxidation resistance of the matrix; Si is enriched in the middle layer to form SiO2, which further prevents O from diffusing into the matrix; the innermost dense, low-porosity Fe2(Si / Cr / RE)O4 layer not only improves the bonding strength between the oxide layer and the matrix, but also further prevents O from diffusing into the matrix, thereby making the RE content in the entire oxide layer higher than that in the matrix, optimizing its high-temperature oxidation resistance; and the three-layer Si and Cr-rich oxide layer formed significantly prevents O atoms from diffusing into the internal matrix, improving its high-temperature oxidation resistance and thermal conductivity, so that the hot stamping parts of this steel do not need to be shot peened, thereby avoiding the risk of shot peening deformation of thin-walled parts, while reducing production costs and reducing environmental pollution.

[0047] Other key elements contribute to the following: Carbon (C) is a performance-enhancing element. Combined with microalloying elements, C can act as a precipitation strengthening agent, but excessive C content impairs weldability. Therefore, a carbon content of 0.12-0.4% by weight is used. Manganese (Mn) significantly improves hardenability, but high manganese content increases the carbon equivalent, impairs weldability, and reduces high-temperature oxidation resistance during hot stamping. Therefore, a manganese content of 0-2% by weight is used. Boron (B) enhances hardenability and produces a martensitic structure. Excessive B content can lead to persistent hot brittleness, so the B content is limited to 0-0.004%. Titanium (Ti) can bind nitrogen atoms to form precipitates, reducing B-N bonding and ensuring B's hardenability-enhancing effect. Therefore, a titanium content of 0-0.1% by weight is used. Al (Al) primarily serves to deoxidize and refine grains, improving the uniformity of the steel's microstructure and properties to a certain extent. Therefore, a content of 0.01-0.5% by weight is used. S and P are impurity elements in the test steel and should be controlled to less than 0.008%.

[0048] The present invention provides a method for preparing hot-formed steel with super strong oxidation resistance. The specific process flow is shown in FIG1 . Raw materials are selected according to the final phase composition of the hot-formed steel to prepare the hot-formed steel. The preparation method includes the following steps:

[0049] 1) Heating and high-pressure water descaling: The continuous casting steel billet is heated to 1250-1380°C in a heating furnace and kept warm for 2 hours to obtain hot-rolled insulated steel; the hot-rolled insulated steel is then descaled with high-pressure water at a temperature ≥1200°C and a water pressure ≥20 MPa to remove the furnace-generated iron oxide scale;

[0050] 2) Rough rolling and finish rolling: The hot-rolled insulated steel treated in step 1) is subjected to rough rolling (each pass has a deformation of about 25%) and finish rolling: the rough rolling start temperature is 1150°C; the finish rolling start temperature and the finish rolling end temperature are 1000°C and 860°C, respectively, to obtain hot-rolled steel;

[0051] 3) Coiling: The hot-rolled steel obtained after finishing rolling is water-cooled at a cooling rate of 20°C / s to a coiling initial temperature of 480-600°C. The steel strip is coiled at the coiling initial temperature and then air-cooled to room temperature to obtain coiled steel.

[0052] 4) The coiled steel plate is subjected to tension leveling and pickling, and then subjected to multiple cold rolling passes by cold rolling rollers to obtain a cold-rolled steel plate with a thickness of 0.6-2 mm;

[0053] The pickling conditions are as follows: the hydrochloric acid concentration in the first acid tank is 60g / mL±20g / L, the acid solution temperature is 80±10℃, and the pickling time is 1min; the hydrochloric acid concentration in the second acid tank is 90g / L±20g / L, the acid solution temperature is 80±10℃, the pickling time is 1min, and the iron ion concentration is required to be no more than 90g / L; the hydrochloric acid concentration in the third acid tank is 120g / L±20g / L, the acid solution temperature is 80±10℃, the pickling time is 1min, and the iron ion concentration is required to be no more than 60g / L.

[0054] 5) continuously annealing the cold-rolled steel in a nitrogen and hydrogen mixed environment at 700° C. for 30 minutes to obtain annealed steel;

[0055] The processes of high-pressure water descaling, rough rolling, finishing rolling, pickling and straightening in the preparation method are all conventional processes in this field, and those skilled in the art can make adaptive adjustments based on existing technologies; or adopt conventional process methods in this field.

[0056] The preparation method of the present application also includes hood annealing, which is used to reduce the strength of the steel coil to facilitate subsequent processes. Specifically, the coiled steel plate is heated from room temperature to 620-720°C at a heating rate of 40-50°C / h in a full hydrogen or near full hydrogen hood annealing furnace, and kept warm for 10-20h. It is first cooled to 300-350°C at a slow cooling rate of 8-12°C / h, and then cooled to room temperature at a cooling rate of 30-40°C / h to obtain hood annealed steel, and then other processes are carried out.

[0057] 6) The annealed steel is placed in air or nitrogen atmosphere at a temperature of 900-960°C for 1-6 minutes, and then the steel plate is transferred to a hot stamping press in air. The transfer process is 4-10 seconds, and the temperature of the steel plate after transfer is 780-880°C; the steel plate is hot stamped in a mold with an internal cooling system and is in a pressure holding state with a pressure of 5-30 MPa. It is then quenched and cooled to below 200°C (martensite completion temperature) at a mold cooling rate of 15-200°C / s to obtain a 1600MPa grade super-hard steel. Hot-formed steel products with strong oxidation resistance; the microstructure of the hot-formed steel after die quenching is martensite, with a small amount of ferrite and bainite, a yield strength of 1000-1100MPa, a tensile strength of 1550-1650MPa, an elongation of 7.5-10%, a bending angle of 65-75°, and a surface oxide scale thickness of 0.2-2μm (under the same conditions, when the atmosphere is pure nitrogen, the surface oxide scale thickness is 0.2-1μm; when the atmosphere is air, the surface oxide scale thickness is 0.5-2.0μm).

[0058] 1. Implementation and surface quality

[0059] Examples 1-7

[0060] Examples 1-7 are respectively a kind of hot-formed steel with super strong oxidation resistance. The element composition and proportion of each example, as well as certain process parameters in the preparation method and the surface quality after hot forming are listed in Table 1 in terms of weight percentage:

[0061] Comparative Examples 1-7

[0062] Comparative Examples 1-7 are each a type of hot-formed steel with super strong oxidation resistance. The elemental composition and proportion, certain process parameters in the preparation method, and the surface quality after hot forming of each comparative example are listed in Table 1, calculated by weight percentage. (The preparation methods of Comparative Examples 1-7 are the same as those of Example 5, and the differences are shown in Table 1):

[0063] Table 1 Chemical composition and process parameters of hot-formed steel with super strong oxidation resistance in various embodiments and comparative examples (mass percentage, %)

[0064] The S and P contents in the Examples and Comparative Examples in Table 1 are no greater than 0.008%, with the remainder being Fe. The total mass percentage of all components is 100%. Mark "0" if no oxide layer is detached; "△" if there is slight oxide layer detachment; and "×" if there is a large amount of oxide layer detachment.

[0065] As can be seen from Table 1, the hot-formed steel with super strong oxidation resistance in the present application has no oxide layer shedding during the hot forming process, and can be directly hot-formed in an air environment, reducing production costs. It can be seen from Example 5 and Comparative Examples 1-2 that when the heating temperature and coiling temperature of the continuous casting billet in the preparation method of the present application are not within the process parameter range of the preparation method of the present application, the surface quality of the hot-formed steel obtained cannot meet the requirements; at the same time, it can be seen from Example 5 and Comparative Examples 3-5 that when no rare earth elements are added in the present application or the added rare earth elements are not within the scope of the present application, the hot-formed steel obtained therefrom has a slight shedding of the oxide layer, indicating that the addition of rare earth elements greatly improves the bonding strength between the oxide layer and the matrix, while excessive addition of rare earth elements will deteriorate the oxidation resistance, thereby causing a large amount of oxide layer shedding; at the same time, when excessive rare earth elements are used, the water nozzle is blocked in the continuous casting process, the production is unstable, and thus it is impossible to continuously cast. Continuous stable production; It can be seen from Example 5 and Comparative Examples 6-7 that when the Si / Cr elements in the present application are not within the molar ratio range of the preparation method of the present application, the surface of the prepared hot-formed steel has a slight oxide layer falling off, which cannot meet the relevant technical requirements; In summary, the present application adds Si, Cr and rare earth elements of a specific mass fraction and molar ratio, and combines parameters such as the heating temperature in the furnace and the coiling temperature to achieve effective removal of furnace-born iron oxide scale and the oxide layer after coiling. The synergistic effect of its composition and process makes the oxide layer formed in the hot-forming process of the hot-forming steel in the present application thin, complete and non-falling, the surface quality is uniform and consistent, and it has a metallic luster, thereby achieving the purpose of eliminating the shot blasting process.

[0066] 2. Mechanical properties test

[0067] This application takes the ultra-strong oxidation-resistant hot-formed steel prepared in Example 5 as an example, and uses the traditional 22MnB5 hot-formed steel material as a comparative example to conduct performance tests. The mechanical properties are tested according to GB / T228.1-2010 and T / CSAE 154-2020 Ultra-High Strength Automotive Steel Sheet Ultimate Tip Cold Bending Performance Test Method. The results are shown in Table 2:

[0068] Table 2 shows the main mechanical properties of Example 5 and 22MnB5 steel

[0069] As can be seen from Table 2, the hot-formed steel with super strong oxidation resistance prepared in this application has excellent mechanical properties. The yield strength, tensile strength, elongation and cold bending angle are comparable to those of 22MnB5, and the strength-ductility product of this application is increased by 44.4% compared with 22MnB5.

[0070] 3. High temperature antioxidant performance

[0071] The present application tests the high-temperature oxidation resistance of the prepared hot-formed steel and 22Mn5 with super strong oxidation resistance by weight gain method:

[0072] The high-temperature oxidation test conditions are: 30 mm × 10 mm × 3 mm specimen, holding temperature 950° C., holding time in air environment 100 h. Specifically, after high-temperature oxidation performance testing of the annealed steels of the above-mentioned embodiments, 22MnB5, and various comparative examples, the thickness of the oxide layer formed, the oxidation rate, the surface state, and the oxidation resistance level (the oxidation resistance level refers to the evaluation criteria of "GB / T 13303-91 Test Method for Oxidation Resistance of Steel") are shown in Table 3:

[0073] Table 3 Surface state and antioxidant level of 22MnB5 and various examples and comparative examples of this patent

[0074] From the high temperature oxidation resistance data given in Table 3, it can be seen from the data of Examples 1-7 that the hot-formed steel with super strong oxidation resistance in this application has excellent high temperature oxidation resistance compared with 22MnB5. After being kept at 950℃ for 100h, the cyclic oxidation rate is <0.4g / m 2 h, the surface is uniform and smooth without any oxide layer falling off, and all of them have reached the antioxidant level; and from Examples 5 to 7, it can be seen that the addition of composite trace RE elements (Ce / Y) can enhance the antioxidant performance of Si / Cr, and when the mass ratio of Ce / Y is 1:1, there is a synergistic enhancement effect, and the obtained ultra-strong antioxidant hot-formed steel has better high-temperature antioxidant performance and reaches the complete antioxidant level. From the data of Comparative Examples 1-7, it can be seen that the process parameters or RE elements or Si / Cr content and ratio are not within the scope of this patent. After being kept at 950°C for 100h, the cyclic oxidation rate is 1-2g / m 2 h, the surface is uneven and a small amount of oxide layer falls off, only reaching the secondary anti-oxidation level.

[0075] A quantitative weight gain experiment was conducted on a high-precision thermogravimetric analyzer (TGA). The high-temperature oxidation experimental conditions were: a 4 mm × 3 mm × 2 mm sample, a heating rate of 20°C / min, a holding temperature of 950°C, a holding time of 30 min, and a cooling rate of 20°C / min in an air atmosphere. Specifically, the oxidation weight gain of the hot-formed steels of Examples 5, 6, and 7, 22MnB5, and Comparative Example 7 over time is shown in FIG2 . As can be seen from FIG2 , the oxidation weight gain of 22Mn5 during the entire process is 17.2 mg / cm 2 The oxidation weight gain of Examples 5, 7 and 6 during the whole process was 2.2 mg / cm 2 , 1.8mg / cm 2 and 1.2 mg / cm2 The weight gain of the whole oxidation process of comparative example 7 is 5.4 mg / cm 2 As shown in Examples 5, 7, and 6, compared with 22MnB5, the synergistic effect of Si, Cr, and RE in the hot-formed steel of the present application results in minimal oxidation weight gain throughout the entire process, significantly improving the high-temperature oxidation resistance of the hot-formed steel. However, as shown in Example 5 and Comparative Example 7, when the Si and Cr contents and ratios added to the hot-formed steel of the present application are outside the range of the present application, the oxidation weight gain increases throughout the entire process, weakening the high-temperature oxidation resistance of the hot-formed steel.

[0076] In summary, when the Si / Cr ratio and the rare earth element ratio in this application are not within the scope of this application, the distribution ratio of Si, Cr and RE in the oxide layer of the hot-formed steel will be affected, so that the final oxide structure layer and element distribution on the surface of the hot-formed steel will change, thereby weakening its high-temperature oxidation resistance.

[0077] 4. High thermal conductivity

[0078] The test method is as follows: two groups of identical annealed steels prepared in step 5) are heated from room temperature to 930°C under the same conditions, and the heating rate is measured when the steel reaches the hot forming temperature;

[0079] This test takes Example 5 as an example and introduces 22MnB5 as a comparative example. Under the same test environment, the thermal conductivity of the annealed steel in this application is tested. The results are shown in Figure 3. As can be seen from Figure 3, the annealed steel prepared in Example 5 takes 90 seconds to heat from room temperature to 930°C; the 22MnB5 takes 120 seconds to heat from room temperature to 930°C. That is, the thermal conductivity of the annealed steel in this application saves 30 seconds compared with 22MnB5, and its thermal conductivity is improved by 25% compared with 22MnB5. This will greatly improve the production efficiency of the annealed steel in the subsequent hot forming process and reduce the forming processing cost of the hot-formed steel.

[0080] The present application also introduces the thermal conductivity test of the annealed steel obtained by the preparation method in Comparative Examples 1-7. The annealed steel prepared thereby takes 105-115 seconds to be heated from room temperature to 930°C.

[0081] 5. Morphology and structure test

[0082] 5.1. Morphology and structure of the hot-formed steel of this application

[0083] Taking the hot-formed steel with super strong oxidation resistance prepared in Example 5 as an example, a scanning electron microscope was used for the hot-formed steel. The results are shown in FIG4 . As can be seen from the figure, the hot-formed steel includes, from the inside to the outside, 41-matrix, 42-dense, low-porosity Fe2(Si / Cr / RE)O4 layer, 43-Si-rich, Fe oxide layer, and 44-Cr-rich, Fe oxide layer. At the same time, through the microscopic scanning of the oxide layer at the cold line 5 position in the final hot-formed steel surface oxide layer and its corresponding scanning element distribution, the results are shown in FIG5 . As can be seen from FIG5 , the distribution ratio of Si, Cr and RE in the oxide layer of the hot-formed steel is: (Cr) L1 / (Cr) L2 ≥1.58;

[0084] (Si) L1 / (Si) L2 ≤2.58; (Si+Cr) L / (Si+Cr) B ≥1.95; 1.25≤RE L / RE B ≤2.50; at the same time, it can be clearly seen that Cr is enriched in the outermost layer, and the Cr2O3 oxide layer is flat and dense, which improves the high-temperature oxidation resistance of the matrix; Si is enriched in the middle layer, forming SiO2 to further prevent O from diffusing into the matrix; the innermost layer is Fe2(Si / Cr / RE)O4, which can improve the bonding strength between the oxide layer and the matrix; the RE content in the entire oxide layer is higher than that in the matrix, which further optimizes its high-temperature oxidation resistance. Therefore, the three-layer Si- and Cr-rich oxide layer formed significantly prevents O atoms from diffusing into the internal matrix, thereby improving the high-temperature oxidation performance of the hot-formed steel.

[0085] At the same time, the hot-rolled insulated steel in step 1) of the preparation method was subjected to electron microscope scanning, and the results are shown in FIG6 . As can be seen from the figure, the hot-rolled insulated steel includes, from the inside to the outside, 64-matrix, 63-loose porous structure Fe2(Si / Cr / RE)O4 layer, 62-FeO+Fe3O4 mixed layer and 61-Fe2O3 layer. This structural layer allows the oxide layer to be removed by high-pressure water before rough rolling, thereby preventing the oxide scale from being pressed into the matrix during the subsequent hot rolling process.

[0086] At the same time, the coiled steel of step 2) in the preparation method was scanned by an electron microscope, and the results are shown in Figure 7. It can be seen from the figure that the coiled steel is arranged to include 74-matrix, 73-thin and dense Fe2(Si / Cr / RE)O4 layer, 72-FeO+Fe3O4 mixed layer and 71-Fe2O3 layer from the inside to the outside. Since the FeO layer in the FeO+Fe3O4 mixed layer is closer to the Fe2(Si / Cr / RE)O4 layer, the FeO oxide layer is loose and thin, and the Fe3O4 and Fe2O3 oxide layers are thick but cracked. Under the combined action of straightening and pickling, hydrochloric acid can easily enter along the cracks and react with the Fe2(Si / Cr / RE)O4 oxide layer close to the matrix through the loose FeO, thereby removing the entire oxide layer.

[0087] 5.2. Morphology and structure of hot-formed steel in the control group

[0088] This test takes the hot-formed steel prepared in control group 1 as an example, and performs electron microscope scanning and surface energy spectrum scanning on the hot-rolled insulated steel prepared in step 1); the electron microscope scanning results of the hot-rolled insulated steel are shown in Figure 8, and the surface energy spectrum scanning results of the hot-rolled insulated steel are shown in Figure 9: It can be seen from the figure that the hot-rolled insulated steel includes 84-matrix, 83-thin and dense, low-porosity Fe2(Si / Cr / RE)O4 layer, 82-FeO+Fe3O4 mixed layer and 81-Fe2O3 layer from the inside to the outside. The high-pressure water before rough rolling is not easy to clean up, and it is easy to be pressed into the matrix during the hot rolling process, causing defects, thereby affecting the high-temperature oxidation resistance of the hot-formed steel. This is because the steel is kept at 1200℃ before hot rolling, and the temperature will drop further after it comes out of the furnace, making its temperature lower than the phase transition temperature of fayalite (1173℃), which easily forms a dense, low-porosity Fe2(Cr\Si\RE)O4 layer. This layer is not easy to be cleaned by high-pressure water before rough rolling, and is easily pressed into the matrix during the hot rolling process, causing defects, thereby affecting the apparent quality of the hot-formed steel and increasing the subsequent process and cost of removing the oxide layer.

[0089] Comparing Figures 6 and 8, the hot-rolled insulated steel in step 1) of the present application, when the heating temperature is increased to 1280°C, which is higher than the melting point of the Fe2(Si / Cr / RE)O4 phase, so that it becomes liquid, the hot rolling process temperature is lower than its melting point and the time is short, and the Fe2(Cr\Si\RE)O4 layer generated is discontinuous granular, that is, a loose and porous structure of the Fe2(Si / Cr / RE)O4 layer, which can remove the oxide layer by high-pressure water, avoiding the oxide scale from being pressed into the matrix during the subsequent hot rolling process, and providing technical support for obtaining a thin, complete and non-falling oxide layer, a uniform surface quality, and a hot-formed steel with a metallic luster.

[0090] 6. Thermoformed appearance

[0091] The hot-formed steel prepared in Example 5 of the present application and the traditional 22MnB5 hot-formed workpiece were press-quenched at a holding temperature of 930°C and a holding time of 5 minutes. The surface quality comparison after forming is shown in Figure 10. It can be seen from the figure that the hot-formed steel of the present application has a complete and non-shedding oxide layer during the stamping process, and the surface quality is uniform and has a metallic luster; while the 22MnB5 material has severe surface oxidation, an uneven distribution of the oxide layer, obvious shedding, and no metallic luster. At the same time, the annealed steel prepared in Example 5 was hot-stamped at 930°C and a nitrogen protective atmosphere for a holding time of 5 minutes. The front and back of the part are shown in Figure 11. No oxide layer has fallen off the surface of the part. It is bright and has a clear metallic luster and can be directly painted without shot blasting. The photo of the part after electrophoretic coating is shown in Figure 12. The entire surface of the part is covered with a paint film, which is flat and smooth, and has a good coating effect.

[0092] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and description of the present invention.

Claims

1. A hot-formed steel with super strong oxidation resistance, characterized in that: The hot-formed steel includes, from the inside to the outside, a matrix, a dense, low-porosity Fe2(Si / Cr / RE)O4 layer, a Si-rich, Fe oxide layer, and a Cr-rich, Fe oxide layer, and the entire oxide layer has a thickness of less than 2 μm; and the distribution ratio of Si, Cr and RE in the oxide layer of the hot-formed steel is: (Cr) L1 / (Cr) L2 ≥1.58; (Si) L1 / (Si) L2 ≤2.58; (Si+Cr) L / (Si+Cr) B ≥1.95; 1.25≤RE L / RE B ≤2.50; The oxide layer thickness refers to the average thickness of the entire oxide layer from the substrate, including the Fe2(Si / Cr / RE)O4 layer, the Si-rich, Fe oxide layer and the Cr-rich, Fe oxide layer; and (Cr) L1 and (Si) L1 Refers to the Cr and Si contents in the Cr-rich and Fe-rich oxide layers, respectively. (Cr) L2 and (Si) L2 Refers to the Cr and Si contents in Si-rich and Fe-rich oxide layers, respectively. (Si+Cr) L Refers to the total content of Cr and Si in the oxide layer, (Si+Cr) B Refers to the total content of Cr and Si in the matrix, RE L Refers to the average rare earth content in the oxide layer, RE B Refers to the average rare earth content in the matrix.

2. A method for preparing hot-formed steel with super strong oxidation resistance according to claim 1, comprising furnace heating, high-pressure water descaling, rough rolling, finish rolling, coiling, straightening, pickling, cold rolling and annealing, characterized in that: The furnace heating process is specifically as follows: heating the steel continuous casting billet to 1250-1380° C. in a heating furnace and keeping the temperature for 1-2.5 hours to obtain hot-rolled heat-insulated steel.

3. The method for preparing hot-formed steel with super strong oxidation resistance according to claim 2, characterized in that: The coiling process is specifically as follows: the initial coiling temperature is 480-600° C., the steel strip is coiled at the initial coiling temperature, and then air-cooled to room temperature to obtain coiled steel.

4. The method for preparing hot-formed steel with super strong oxidation resistance according to claim 2, characterized in that: The method also includes annealing the coiled steel after the coiling process, wherein the annealing process is specifically as follows: in a full hydrogen or near full hydrogen annealing furnace, heating from room temperature to 620-720°C at a heating rate of 40-50°C / h, keeping warm for 10-20h, first cooling to 300-350°C at a slow cooling rate of 8-12°C / h, and then cooling to room temperature at a cooling rate of 30-40°C / h to obtain annealed steel.

5. The method for preparing hot-formed steel with super strong oxidation resistance according to claim 1, characterized in that: The method further includes a thermoforming process, which specifically comprises: in an air or nitrogen protective atmosphere, at a thermoforming temperature of 900-960° C., and keeping the temperature for 1-6 minutes.

6. The method for preparing hot-formed steel with super strong oxidation resistance according to claim 2, characterized in that: The continuous casting steel billet comprises, by mass percentage, C: 0.12-0.4%, Si: 1.0-2.0%, Mn: 0-2.0%, Al: 0.01-0.5%, Cr: 0.5-1.6%, B: 0-0.004%, Ti: 0-0.1%, RE: 0.003-0.008%, S and P: less than 0.008%, and the rest is Fe.

7. The method for preparing hot-formed steel with super strong oxidation resistance according to claim 6, characterized in that: The molar ratio of Si to Cr is Si / Cr (2-4): 1, and RE is one of Ce and Y or a mixture thereof.

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