Hot-formed steel, preparation method and application thereof, preparation method of parts for automobile and train lower doors, etc.

Abstract

The present invention relates to the technical field of metal material processing, and discloses a hot-formed steel and its preparation method and application, and a preparation method for parts for automobile and train lower doors, etc. The hot-formed steel includes a matrix and a dense composite oxide layer from the inside to the outside; wherein, in the dense composite oxide layer, the content of ferric oxide is less than 2wt%. By controlling the microstructure of the dense composite oxide layer and the content of ferric oxide, the hot-formed steel has excellent high-temperature oxidation resistance, and the structure of the matrix is ​​ferrite and pearlite, so that the hot-formed steel has suitable yield strength, tensile strength and elongation; by regulating the heat treatment temperature, hot rolling temperature and coiling temperature, pickling, shot blasting and annealing can be avoided after coiling; after coiling, the hot-formed steel can be directly used to prepare automotive parts, and has excellent yield strength, tensile strength, elongation, surface hardness, oxidation resistance and paint film adhesion, and can achieve a coating-free and shot-free effect.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material processing, and in particular to a hot-formed steel and a preparation method and application thereof, and a preparation method for parts used for lower doors of automobiles and trains. Background Art

[0002] Lightweighting of commercial vehicle cabins is an important trend in the current automotive industry. With the improvement of environmental awareness, reducing the overall weight of vehicles is an effective way to reduce fuel consumption and exhaust emissions, which is in line with the global trend of carbon emission reduction. Lightweighting can improve the fuel economy of commercial vehicles and reduce the pressure on the environment. Lightweighting of commercial vehicles helps to improve carrying capacity and freight efficiency and reduce transportation costs. By reducing the weight of the vehicle itself, the effective load can be increased and higher cargo transportation efficiency can be achieved. Lightweighting can extend the service life of commercial vehicles and reduce maintenance costs. Reducing the weight of the vehicle can reduce the wear and fatigue of parts, improve the reliability and durability of the vehicle, and reduce the frequency and cost of maintenance. Therefore, the demand for lightweighting of commercial vehicles not only stems from the concern for environmental protection and energy efficiency, but also reflects the all-round pursuit of transportation efficiency, economy and safety.

[0003] The steel used in commercial vehicle carriages has changed from low-strength Q235 and Q345 to high-strength T700 and NM450. As the strength continues to increase, the requirements for molds during cold forming of high-strength steel have become more stringent, resulting in more serious damage. Not only is the springback problem after forming more significant, but the parts themselves are prone to cracking during the forming process, which in turn leads to a decrease in the yield rate. At present, commercial vehicles still use ferrite-pearlite 590MPa series and bainite 780MPa series steel plates for cold forming. Cold forming of hot-rolled 2-6mm thick steel plates with a strength of 780MPa is close to the limit of the mold. Microcracks and large internal stresses often occur after forming, which deteriorates fatigue performance and brings safety hazards to the service process.

[0004] Hot forming technology is a process that heats metal materials to above the austenitizing temperature and then performs stamping forming in this state. It not only solves the problems of stamping cracking, severe springback and poor dimensional accuracy of complex parts, but also its hot stamping parts have the advantages of high strength, high dimensional accuracy, good forming quality, good weldability, and can avoid the springback problem of cold forming of high-strength steel. Therefore, hot formed parts are increasingly valued in lightweight automotive materials.

[0005] At present, commercial hot-formed steel is mainly divided into Al-Si coated steel and bare plate (uncoated steel). The main function of Al-Si coating is to prevent decarburization and oxidation of the steel plate surface during austenitization, and the coating can isolate the steel plate substrate from the external environment, so it has a certain anti-corrosion effect. Although Al-Si coating can prevent oxidation and decarburization during hot forming of steel plates, it is easy to stick to the roller, and the production process of Al-Si coated plates is monopolized by foreign companies, resulting in high prices. The bare plate is prone to form a loose oxide layer on the surface during hot stamping, and the oxide layer is very easy to fall off during the hot forming process. Not only does it need to temporarily produce and clean the iron oxide scale in the mold, but the hard iron oxide scale is easy to cause mold scratches, seriously reducing the service life of the mold; on the other hand, the parts of the bare plate need to be shot blasted after hot forming, which pollutes the environment and increases the process and working hours. At the same time, due to the problem of hardenability, the hot-formed steel used in commercial vehicles can only manufacture parts ≤6.0mm to ensure that the core of the part obtains martensite. These all limit the application of hot forming technology on large thick plate parts.

[0006] CN113832407A discloses a method for preparing thick-gauge hot-formed steel, a hot-rolled steel plate and hot-formed steel, which can achieve the effect of no coating and shot blasting. However, the yield strength after hot rolling is too high (greater than 900MPa), which exceeds the performance requirements of the shearing machine in the actual production process and cannot be used for blanking processing; on the other hand, due to the high yield strength after hot rolling, the plate is prone to cracks during cold rolling at room temperature, and cannot be used for indirect hot forming, which limits its application field.

[0007] Therefore, it is necessary to further optimize the preparation process of hot-forming steel alloy composition, which can not only reduce the strength after hot rolling, but also achieve coating-free and shot-blasting-free, and can be used in both direct hot forming and indirect hot forming fields. Summary of the invention

[0008] The purpose of the present invention is to overcome the problems in the prior art that hot-formed steel needs to be treated with Al-Si coating, which has high material cost and complicated process, and may also cause hydrogen embrittlement, roller sticking and coating cracking, while the bare plate of the existing hot-formed steel without coating has insufficient resistance to high-temperature oxidation during the hot-forming process and needs to be shot blasted, and the hot-formed steel needs pickling and annealing processes to produce automotive parts. Provided are hot-formed steel and its preparation method and application, and a preparation method for parts for automobile and train lower doors.

[0009] In order to achieve the above-mentioned object, the first aspect of the present invention provides a hot-formed steel, wherein the hot-formed steel comprises a matrix and a dense composite oxide layer from the inside to the outside;

[0010] Wherein, the content of ferric oxide in the dense composite oxide layer is less than 2wt%.

[0011] A second aspect of the present invention provides a method for preparing hot-formed steel, wherein the method comprises: sequentially subjecting a continuous casting billet to heat treatment, hot rolling and coiling to obtain hot-formed steel;

[0012] The conditions of heat treatment, hot rolling and coiling are such that the content of ferric oxide in the layer formed on the surface of the hot-formed steel is less than 2 wt %;

[0013] The heat treatment temperature is 1260-1340°C; the hot rolling start temperature is 1160-1200°C and the final rolling temperature is 850-890°C; the coiling temperature is 610-680°C.

[0014] A third aspect of the present invention provides a hot-formed steel produced by the above-mentioned production method.

[0015] A fourth aspect of the present invention provides the use of the above-mentioned hot-formed steel in the preparation of parts for automobile and train lower doors, etc.

[0016] A fifth aspect of the present invention provides a method for preparing parts for automobile and train lower doors, etc., the method comprising: subjecting the hot-formed steel to hot-forming treatment.

[0017] Through the above technical solution, the technical solution provided by the present invention achieves the following beneficial effects:

[0018] (1) The hot-formed steel provided by the present invention has a dense composite oxide layer with a content of ferric oxide of less than 2 wt% through a high-temperature coiling process, so that the hot-formed steel has excellent anti-oxidation performance; on the other hand, the matrix structure after high-temperature coiling is ferrite and pearlite, the volume content of ferrite is 55-75%, and the volume content of pearlite is 25-45%, so that the hot-formed steel has suitable yield strength, tensile strength and elongation, which is convenient for shearing and blanking;

[0019] (2) The preparation method of the hot-formed steel provided by the present invention, through the control of the composition design (Si-Cr-RE) and the process parameters such as the heat treatment temperature, the hot rolling temperature and the coiling temperature, makes the content of ferric oxide formed on the surface of the hot-formed steel less than 2wt%, and the pickling, shot blasting and annealing treatment can be avoided after the coiling treatment. By controlling the content of Cr, Si and RE, not only the oxidation resistance of the steel can be improved, but also the dense oxide Cr can be formed before Fe reacts with oxygen. 2 O 3 and SiO 2 In the oxide layer, Si and Cr contribute to the formation of a composite dense oxide layer Fe 2 (Si x Cr y RE z ) 4, and RE elements have good stability for the oxide layer and can improve the bonding strength between the dense composite layer and the matrix. 2 (Si x Cr y RE z ) 4 The layer helps prevent oxygen from diffusing into the matrix and Fe from diffusing into the outside, improves its high temperature oxidation resistance, and promotes dense Fe 3 O 4 generate;

[0020] (3) The application of the hot-formed steel provided by the present invention in the preparation of parts for automobiles and train lower doors, etc., wherein the matrix of the automobile parts prepared contains martensite accounting for more than 95% by volume, and may or may not contain a small amount of ferrite, bainite or residual austenite, and has excellent mechanical properties, a yield strength of 1030-1180MPa, a tensile strength of 1580-1610MPa, an elongation of 10.2%-10.5% and a surface hardness of >480HV, and can achieve a coating-free and shot-free effect;

[0021] (4) The present invention provides a method for preparing parts for automobile and train lower doors, etc., in which during the hot forming process, the dense composite oxide layer can be deformed together with the substrate, and no oxide layer falls off the surface after hot forming, thereby achieving a coating-free and shot-blasting-free effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a SEM image of the dense composite oxide layer of the hot-formed steel prepared in Example 1 of the present invention;

[0023] Figure 2 is a matrix structure diagram of the hot-formed steel prepared in Example 1 of the present invention;

[0024] Figure 3 is a matrix structure diagram of the hot-formed steel part in Example 1 of the present invention;

[0025] Figure 4 This is a SEM image of the dense composite oxide layer of the automotive part prepared in Example 1 of the present invention;

[0026] Figure 5 This is the XRD diagram of the dense composite oxide layer for automobiles prepared in Example 1 of the present invention;

[0027] Figure 6 This is a SEM image of the composite oxide layer of the hot-formed steel prepared in Comparative Example 7 of the present invention;

[0028] Figure 7 This is an XRD diagram of the composite oxide layer of the automotive part prepared in Comparative Example 7 of the present invention;

[0029] Figure 8 This is a SEM image of the dense composite oxide layer of the automotive part prepared in Comparative Example 7 of the present invention;

[0030] Fig. 9 This is the matrix organization diagram of the automobile parts prepared in Comparative Example 7 of the present invention. DETAILED DESCRIPTION

[0031] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0032] A first aspect of the present invention provides a hot-formed steel, wherein the hot-formed steel comprises a matrix and a dense composite oxide layer from the inside to the outside;

[0033] Wherein, the content of ferric oxide in the dense composite oxide layer is less than 2wt%.

[0034] In the present invention, when the content of ferric oxide in the dense composite oxide layer does not meet the above range, the anti-oxidation effect of the hot-formed steel is weakened, the surface oxide layer will fall off after hot forming, and the effect of direct coating without shot blasting cannot be achieved.

[0035] In the present invention, the term “from inside to outside” means that the structure of the hot-formed steel is a matrix and a dense composite oxide layer.

[0036] According to the present invention, the dense composite oxide layer includes Fe 2 (Si x Cr y RE z ) 4 Layer and Fe 3 O 4 layer.

[0037] According to the present invention, the structure and phase composition of the dense composite oxide layer can be determined by scanning electron microscopy and X-ray diffraction analysis. 2 (Si x Cr y RE z ) 4 The total weight of the layer, the Fe 2 (Si x Cr y RE z ) 4The layer contains 28-32 wt% Fe, 16-20 wt% Si, 34-38 wt% Cr and 0.01-0.02 wt% RE.

[0038] In the present invention, the Fe 2 (Si x Cr y RE z ) 4 In the layer, 0.95≤x+y+z≤1.05. Specifically, there is no special requirement for the values ​​of x, y and z, as long as the value of x+y+z meets the definition of the present invention.

[0039] According to the present invention, based on the Fe 3 O 4 Total weight of the layer, 72-73wt% Fe.

[0040] According to the present invention, the porosity of the dense composite oxide layer is 0.2-2%, preferably 0.2-1%.

[0041] In the present invention, when the porosity content of the dense composite oxide layer does not meet the above range, it is greater than 2%, which weakens the bonding force between the hot-formed steel matrix and the composite oxide layer, and the surface oxide layer will fall off after hot forming, and the effect of direct coating without shot blasting cannot be achieved.

[0042] According to the present invention, the thickness of the dense composite oxide layer is less than 10 μm, preferably less than 8 μm.

[0043] In the present invention, when the thickness of the dense composite oxide layer does not meet the above range, the welding performance of the hot-formed steel is weakened and the requirements of the main engine manufacturer on the welding performance cannot be met.

[0044] According to the present invention, the Fe 2 (Si x Cr y RE z ) 4 The thickness of the layer is 0.05-0.2 μm, preferably 0.1-0.15 μm.

[0045] In the present invention, when the Fe 2 (Si x Cr y RE z ) 4 When the thickness of the layer does not meet the above range, the oxidation resistance of the hot-formed steel and the adhesion of the composite oxide layer are weakened.

[0046] According to the present invention, the Fe 3 O 4 The thickness of the layer is 9.8 μm or less, preferably 8 μm or less.

[0047] In the present invention, when the Fe 3 O 4 When the thickness of the layer does not satisfy the above range, the weldability of the hot-formed steel is impaired.

[0048] In the present invention, the dense composite oxide layer generally contains ferric oxide in an amount of less than 2 wt %.

[0049] According to the present invention, the structure of the matrix is ​​ferrite and pearlite.

[0050] According to the present invention, based on the total volume of the matrix, the volume content of the ferrite in the matrix is ​​55-75%, and the volume content of the pearlite is 25-45%.

[0051] According to the present invention, the yield strength of the hot-formed steel is 500-600 MPa, the tensile strength is 700-800 MPa and the elongation is 18-24%.

[0052] According to the present invention, based on the total weight of the hot formed steel, the hot formed steel contains 0.12-0.4wt% of C, 1-2wt% of Si, 0.01-0.5wt% of Al, 0.5-1.6wt% of Cr, 0.003-0.008wt% of RE, less than 0.008wt% of S+P, not more than 2wt% of Mn, not more than 0.004wt% of B, not more than 0.1wt% of Ti, and 93.381-96.698wt% of Fe+impurities.

[0053] Further, based on the total weight of the hot-formed steel, the hot-formed steel contains 0.2-0.3wt% of C, 1.5-1.9wt% of Si, 0.2-0.4wt% of Al, 1.0-1.5wt% of Cr, 0.004-0.008wt% of the RE, less than 0.005wt% of S+P, not more than 1.5wt% of Mn, not more than 0.002wt% of B, not more than 0.03wt% of Ti, and 94.455-96.0842wt% of Fe+impurities.

[0054] According to the present invention, the RE is selected from Ce and / or Y. The RE used in the following embodiments is Ce.

[0055] A second aspect of the present invention provides a method for preparing hot-formed steel, wherein the method comprises: sequentially subjecting a continuous casting billet to heat treatment, hot rolling and coiling to obtain hot-formed steel;

[0056] The conditions of heat treatment, hot rolling and coiling are such that the content of ferric oxide in the layer formed on the surface of the hot-formed steel is less than 2 wt %;

[0057] The heat treatment temperature is 1260-1340°C; the hot rolling start temperature is 1160-1200°C and the final rolling temperature is 850-890°C; the coiling temperature is 610-680°C.

[0058] In the present invention, by regulating the heat treatment temperature, hot rolling temperature and coiling temperature, the content of ferric oxide in the layer formed on the surface of the hot-formed steel is less than 2wt%, and pickling, shot blasting and annealing can be avoided after coiling.

[0059] According to the present invention, the continuous casting billet needs to be subjected to a steelmaking process before.

[0060] According to the present invention, the heat treatment conditions include: temperature of 1270-1320° C., preferably 1280-1300° C.; time of 1.3-2.2 h, preferably 1.6-2 h.

[0061] In the present invention, when the heat treatment conditions do not meet the above range, the temperature is greater than 1320°C, Fe 2 (Si x Cr y RE z ) 4 The layer completely turns into liquid and diffuses into the oxide layer along the grain boundaries, forming diffusion channels for oxygen atoms and iron atoms, promoting the oxidation of the matrix and reducing the high-temperature oxidation resistance of the matrix; the temperature is lower than 1260°C, which weakens the subsequent descaling effect, causing the unremoved oxide scale to be pressed into the matrix during the rolling process, reducing the matrix's oxidation resistance and surface quality.

[0062] According to the present invention, the heat treatment process comprises: performing a first full high-pressure water descaling after the heat treatment, and the descaling water pressure is ≥20MPa.

[0063] According to the present invention, the hot rolling process includes: rough rolling and finish rolling.

[0064] According to the present invention, the rough rolling process includes: the starting rolling temperature is 1160-1200°C, preferably 1170-1190°C; the final rolling temperature is 990-1080°C, preferably 1030-1050°C; 3-7 rough rolling passes, preferably 4-6 passes, with a deformation of each pass ≥20%; and then a second high-pressure water descaling is performed.

[0065] In the present invention, when the temperatures of the start rolling and the final rolling of the rough rolling do not meet the above ranges, the start rolling temperature is greater than 1200°C, resulting in grain coarsening, affecting the ultimate strength of the material; the start rolling temperature is less than 1160°C, resulting in uneven organizational structure and slow grain growth, affecting the plasticity and toughness of the material, making the deformation of the material more difficult; the final rolling temperature is greater than 1080°C, resulting in grain coarsening, affecting the ultimate strength of the material; the final rolling temperature is less than 990°C, and the incompletely recrystallized organization leads to higher hardness and strength of the material, but relatively poor toughness.

[0066] According to the present invention, the conditions for finishing rolling include: the starting rolling temperature is 900-980°C, preferably 850-890°C; the final rolling temperature is 850-890°C, preferably 860-880°C; after 4-7 passes of finishing rolling, the target thickness is 2-6mm.

[0067] In the present invention, when the temperatures of the start rolling and the final rolling of the finish rolling do not meet the above-mentioned range, the start rolling temperature is greater than 980°C, which leads to grain coarsening and affects the ultimate strength of the material; the start rolling temperature is less than 900°C, and the low start rolling temperature leads to uneven organizational structure and slow grain growth, which affects the plasticity and toughness of the material; the final rolling temperature is greater than 890°C, which easily produces coarse austenite grains and mixed crystal structures, affecting the ultimate strength of the material; the final rolling temperature is less than 850°C, which leads to uneven organizational structure and increases residual stress.

[0068] According to the present invention, the coiling temperature is 630-650°C.

[0069] In the present invention, when the coiling temperature does not meet the above range, the temperature is greater than 680°C, the thickness of the surface composite oxide layer increases, which is not conducive to the subsequent hot forming process and weakens the welding performance; the temperature is less than 610°C, the Fe in the composite oxide layer increases. 2 O 3 content, reducing its antioxidant properties; on the other hand, the decrease in curling temperature causes the matrix to form bainite or martensite, resulting in an increase in its yield strength, which is not conducive to subsequent blanking and punching processes.

[0070] A third aspect of the present invention provides a hot-formed steel produced by the above-mentioned production method.

[0071] A fourth aspect of the present invention provides the use of the above-mentioned hot-formed steel in the preparation of parts for automobile and train lower doors, etc.

[0072] A fifth aspect of the present invention provides a method for preparing parts for automobile and train lower doors, etc., the method comprising: subjecting the hot-formed steel to hot-forming treatment.

[0073] In the present invention, the hot forming treatment is direct hot forming or indirect hot forming; the direct hot forming comprises: in an air or nitrogen protective atmosphere, keeping the hot forming steel at 900-960°C for 4-8 minutes, and then quickly transferring it to a mold for hot forming to obtain automotive parts;

[0074] The indirect hot forming includes: firstly cold stamping the hot-formed steel for 10-20% pre-forming, and keeping it at 850-930℃ for 4-6min, and then quickly transferring it to the mold for pressure quenching, completing the remaining 80-90% deformation, and obtaining the parts for the train door, wherein the temperature before entering the mold is 810-890℃, the pressure holding pressure is 100-600MPa, and the pressure holding time is 10-40s. The obtained train parts have good forming quality, no warping, deformation, cracking, yield strength greater than 1000MPa, and tensile strength greater than 1500MPa.

[0075] The present invention will be described in detail below by way of examples. In the following examples,

[0076] The content of each element is measured in accordance with GB / T1467-2008 "General Principles and General Provisions for Chemical Analysis Methods of Metallurgical Products";

[0077] The porosity was measured using ImageJ software based on the microstructural photographs of the oxide layer;

[0078] The mechanical properties are measured according to GB / T228.1-2010 Tensile test of metal materials Part 1: Room temperature test method;

[0079] Metallographic structure reliability: tested in accordance with "GB / T13298-1991 Metal Microstructure Test Method";

[0080] The paint film adhesion is measured in accordance with the "ASTM B571 Standard Practice for Qualitative Adhesion Tests of Metal Coatings" and the coating adhesion reliability standard;

[0081] The raw materials used in the examples and comparative examples are all commercially available.

[0082] Example 1

[0083] The continuously cast billet is sequentially subjected to heat treatment, hot rolling and coiling to obtain hot-formed steel;

[0084] Among them, the continuous casting billet includes 0.25wt% C, 1.7wt% Si, 0.3wt% Al, 1.2wt% Cr, 0.006wt% RE, 0.003wt% S+P, 1.2wt% Mn, 0.001wt% B, 0.02wt% Ti, the rest is Fe and other inevitable impurities; the conditions of heat treatment include: temperature of 1290℃, time of 1.8h; the first full high-pressure water descaling, the rough rolling process includes: the starting rolling temperature is 1180℃, the final rolling temperature is 1040℃, after 5 passes of rough rolling, it is rolled to an intermediate billet of 18mm, and then the second full high-pressure water descaling is carried out; the conditions of finishing rolling include: the starting rolling temperature is 940℃, the final rolling temperature is 870℃, after 6 passes of finishing rolling, it is rolled to a target thickness of 4mm, the coiling temperature is 640℃, and the hot-formed steel A1 is obtained.

[0085] Figure 1 The SEM image of the dense composite oxide layer of the hot-formed steel prepared in Example 1 of the present invention shows that 1 is Fe 2 (Si x Cr y RE z ) 4 layer, 2 is Fe 3 O 4 layer.

[0086] Figure 2 This is a matrix microstructure diagram of the hot-formed steel prepared in Example 1 of the present invention. It can be seen from the figure that 1 (dark gray area) is pearlite and 2 (white area) is ferrite.

[0087] Figure 3 This is a matrix structure diagram of the hot-formed steel part in Example 1 of the present invention. It can be seen from the figure that the matrix is ​​almost entirely lath-shaped martensite.

[0088] Figure 4 This is a SEM image of the dense composite oxide layer of the automotive part prepared in Example 1 of the present invention. It can be seen from the image that the structure of the dense composite oxide layer on the surface of the part does not change significantly after hot forming, and the thickness only increases slightly.

[0089] Figure 5 The XRD diagram of the dense composite oxide layer for automobiles prepared in Example 1 of the present invention shows that the dense composite oxide layer is composed of Fe 3 O 4 and Fe 2 (Si x Cr y RE z ) 4 Two-phase composition.

[0090] Embodiment 2-5

[0091] Hot-formed steels were prepared according to the method of Example 1, except that the types and amounts of the elements in the continuous casting billet were different from those in Example 1, as shown in Table 1; the heat treatment conditions, the rough rolling process, the finish rolling conditions and the coiling temperature were different from those in Example 1, as shown in Table 1. Hot-formed steels A2-A5 were prepared respectively.

[0092] Comparative Examples 1-7

[0093] Hot-formed steels were prepared according to the method of Example 1, except that the types and amounts of the elements in the continuous casting billet were different from those in Example 1, as shown in Table 1; the heat treatment conditions, the rough rolling process, the finish rolling conditions and the coiling temperature were different from those in Example 1, as shown in Table 1. Hot-formed steels D1-D7 were prepared respectively.

[0094] Figure 6 This is a SEM image of the composite oxide layer of the hot-formed steel prepared in Comparative Example 7 of the present invention. It can be seen from the figure that the surface oxide layer not only has loose Fe 2 O 3 , and no continuous and dense Fe 2 (Si x Cr y RE z ) 4 , reducing the oxidation resistance of hot-formed steel.

[0095] Figure 7 This is the XRD diagram of the composite oxide layer of the automotive parts prepared in Comparative Example 7 of the present invention. It can be seen from the figure that 1 (light gray area) is pearlite, 2 (white area) is ferrite, and 3 (dark gray area) is carbide.

[0096] Figure 8 This is a SEM image of the dense composite oxide layer of the automotive parts prepared in Comparative Example 7 of the present invention. It can be seen from the figure that the composite oxide layer of the hot-formed steel contains Fe 2 O 3 , Fe 3 O 4 and Fe 2 (Si x Cr y RE z ) 4 Three phases.

[0097] Fig. 9 This is a matrix organization diagram of the automotive part prepared in Comparative Example 7 of the present invention. It can be seen from the figure that the matrix is ​​composed of martensite, ferrite and bainite, and has not been fully hardened, thereby reducing the mechanical properties of the part.

[0098] Table 1 Type and amount of each element in the continuous casting billet of each embodiment and comparative example (wt%)

[0099]

[0100] Table 1 (Continued) Preparation process parameters of each embodiment and comparative example

[0101]

[0102] Test Example 1

[0103] The hot-formed steels prepared in the embodiments and comparative examples were tested, and the results are shown in Table 2.

[0104] Table 2

[0105]

[0106] Table 2 (continued)

[0107] serial number Yield strength / MPa Tensile strength / MPa Elongation / % A1 550 760 22 A2 540 750 20 A3 520 800 18 A4 560 730 21 A5 600 780 24 D1 550 789 13 D2 562 852 12 D3 668 875 11 D4 1056 1354 10 D5 1120 1450 9.8 D6 895 1350 9.9 D7 520 781 15.2

[0108] As can be seen from Table 2, the hot-formed steel prepared by the present invention controls the type and content of each raw material and process parameters such as heat treatment temperature, hot rolling temperature and coiling temperature, so that the content of ferric oxide in the layer formed on the surface of the hot-formed steel is less than 2wt%, the matrix structure is ferrite and pearlite, the volume content of ferrite is 56-68%, the volume content of pearlite is 32-44%, and the porosity of the dense composite oxide layer is 0.2-0.8%, so that the hot-formed steel has suitable yield strength, tensile strength and elongation, which is convenient for shearing and blanking; and the pickling, shot blasting and annealing treatments can be omitted after the coiling treatment in the preparation method. The hot-formed steels prepared in Comparative Examples 1-7 have a surface formed ferric oxide content of more than 2wt%, a ferrite volume content of 42-82%, a pearlite volume content of 15-51%, and the matrix structure produces other structures in addition to ferrite and pearlite, such as Fig. 9 The martensite and bainite structures shown in Figure 3 have a dense composite oxide layer with a porosity of 2.4-6.7%, which results in excessively high yield strength and tensile strength and low elongation, making it difficult to shear and blank.

[0109] Test Example 2

[0110] The hot-formed steels prepared in the examples and comparative examples were used to prepare automobile parts.

[0111] The specific method for preparing automotive parts is as follows:

[0112] Under a nitrogen protective atmosphere, the hot-formed steel is kept at 930°C for 6 minutes and then quickly transferred to a mold for hot forming to obtain automotive parts.

[0113] The results are shown in Table 3.

[0114] Table 3

[0115]

[0116]

[0117] It can be seen from Table 3 that the automobile parts prepared by the present invention, by adopting the hot-formed steel prepared by the present application, after heat treatment, the matrix of the automobile parts prepared contains more than 95% martensite by volume, and contains or does not contain a small amount of ferrite, bainite or residual austenite, so that it has excellent mechanical properties, yield strength, tensile strength, elongation, surface hardness, oxidation resistance (no oxide layer falling off) and paint film adhesion.

[0118] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A hot-formed steel, It is characterized in that The hot-formed steel comprises a matrix and a dense composite oxide layer from the inside to the outside; Wherein, the content of ferric oxide in the dense composite oxide layer is less than 2wt%; The porosity of the dense composite oxide layer is 0.2-2%; The dense composite oxide layer includes Fe 2 (Si x Cr y RE z ) 4 Layer and Fe 3 O 4 layer; Based on the Fe 2 (Si x Cr y RE z ) 4 The total weight of the layer, the Fe 2 (Si x Cr y RE z ) 4 The layer contains 28-32wt% of Fe, 16-20wt% of Si, 34-38wt% of Cr and 0.01-0.02wt% of RE, wherein 0.95≤x+y+z≤1.05, and the RE is selected from Ce and / or Y; Based on the Fe 3 O 4 The total weight of the layer, the Fe 3 O 4 The layer contains 72-73wt% Fe; The method for preparing hot-formed steel comprises: sequentially subjecting a continuous casting billet to heat treatment, hot rolling and coiling to obtain hot-formed steel; The conditions of heat treatment, hot rolling and coiling are such that the content of ferric oxide in the layer formed on the surface of the hot-formed steel is less than 2 wt %; The temperature of the heat treatment is 1260-1340°C; the starting temperature of the hot rolling is 1160-1200°C and the final rolling temperature is 850-890°C; the coiling temperature is 610-680°C.

2. The hot-formed steel according to claim 1, in, The porosity of the dense composite oxide layer is 0.2-1%.

3. The hot-formed steel according to claim 1 or 2, in, The thickness of the dense composite oxide layer is less than 10 μm.

4. The hot-formed steel according to claim 3, in, The thickness of the dense composite oxide layer is less than 8 μm.

5. The hot-formed steel according to claim 3, in, The Fe 2 (Si x Cr y RE z ) 4 The thickness of the layer is 0.05-0.2 μm.

6. The hot-formed steel according to claim 5, in, The Fe 2 (Si x Cr y RE z ) 4 The thickness of the layer is 0.1-0.15 μm.

7. The hot formed steel according to claim 3, in, The Fe 3 O 4 The thickness of the layer is 9.8 μm or less.

8. The hot-formed steel according to claim 7, in, The Fe 3 O 4 The thickness of the layer is 8 μm or less.

9. The hot-formed steel according to any one of claims 4 to 8, in, The matrix has a structure of ferrite and pearlite.

10. The hot formed steel according to claim 9, in, Based on the total volume of the matrix, the volume content of the ferrite in the matrix is ​​55-75%, and the volume content of the pearlite is 25-45%.

11. The hot-formed steel according to any one of claims 4 to 8, in, The hot-formed steel has a yield strength of 500-600 MPa, a tensile strength of 700-800 MPa and an elongation of 18-24%.

12. The hot formed steel according to claim 1 or 2, in, Based on the total weight of the hot-formed steel, the hot-formed steel contains 0.12-0.4wt% of C, 1-2wt% of Si, 0.01-0.5wt% of Al, 0.5-1.6wt% of Cr, 0.003-0.008wt% of the RE, less than 0.008wt% of S+P, not more than 2wt% of Mn, not more than 0.004wt% of B, not more than 0.1wt% of Ti, and 93.381-96.698wt% of Fe+impurities.

13. The hot formed steel according to claim 12, in, Based on the total weight of the hot-formed steel, the hot-formed steel contains 0.2-0.3wt% of C, 1.5-1.9wt% of Si, 0.2-0.4wt% of Al, 1-1.5wt% of Cr, 0.004-0.008wt% of the RE, less than 0.005wt% of S+P, not more than 1.4wt% of Mn, not more than 0.002wt% of B, not more than 0.03wt% of Ti, and 94.455-96.0842wt% of Fe+impurities.

14. A method for preparing the hot-formed steel according to any one of claims 1 to 13, It is characterized in that The preparation method comprises: sequentially heat treating, hot rolling and coiling the continuously cast billet to obtain hot-formed steel; The conditions of heat treatment, hot rolling and coiling are such that the content of ferric oxide in the layer formed on the surface of the hot-formed steel is less than 2 wt %; The temperature of the heat treatment is 1260-1340°C; the starting temperature of the hot rolling is 1160-1200°C and the final rolling temperature is 850-890°C; the coiling temperature is 610-680°C.

15. The preparation method according to claim 14, in, The heat treatment conditions include: temperature of 1270-1320° C. and time of 1.3-2.2 h.

16. The preparation method according to claim 15, in, The heat treatment conditions include: temperature of 1280-1300° C. and time of 1.6-2 h.

17. The preparation method according to any one of claims 14 to 16, in, The heat treatment process includes: performing the first full high-pressure water descaling after the heat treatment.

18. The preparation method according to claim 17, in, The hot rolling process includes: rough rolling and finish rolling.

19. The preparation method according to claim 18, in, The rough rolling process includes: the starting rolling temperature is 1160-1200° C.; the final rolling temperature is 990-1080° C.; 3-7 rough rolling passes; and then a second full high-pressure water descaling is performed.

20. The preparation method according to claim 19, in, The rough rolling process includes: the starting rolling temperature is 1170-1190° C.; the final rolling temperature is 1030-1050° C., 4-6 passes of rough rolling are performed, and then a second full high-pressure water descaling is performed.

21. The preparation method according to any one of claims 18 to 20, in, The conditions of the finishing rolling include: the starting rolling temperature is 900-980° C.; the final rolling temperature is 850-890° C.; and the finishing rolling is performed for 4-7 passes.

22. The preparation method according to claim 21, in, The conditions of the finish rolling include: the starting rolling temperature is 930-950°C; and the final rolling temperature is 860-880°C.

23. The preparation method according to claim 21, in, The coiling temperature is 630-650°C.

24. Use of the hot-formed steel according to any one of claims 1 to 13 in the manufacture of parts for lower doors of automobiles and trains.

25. A method for preparing parts for automobile and train lower doors, etc. include: The hot-formed steel according to any one of claims 1 to 13 is subjected to hot forming treatment.

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