Aluminum-plated steel sheet, hot forming part, and manufacturing method

Abstract

The present invention discloses an aluminum-plated steel sheet including a substrate and a plating layer on the substrate. The microstructure of the plating layer includes an Mg2Si phase and an AlMgSiFe phase, and the average crystal grain size of the Mg2Si phase is 0.001 to 5 μm. The present invention can reduce the problems of roller adhesion and the risk of hydrogen embrittlement due to melting in the thermoforming process of the aluminum-plated steel sheet. The present invention further provides a method for manufacturing the aluminum-plated steel sheet, and a thermoformed part and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to the field of metal-plated steel sheets, particularly aluminum-plated steel sheets, hot-formed parts, and manufacturing methods.

Background Art

[0002] Aluminum-plated steel sheets are widely used in various fields such as automobiles, household electrical appliances, ovens, and furnaces due to their good heat resistance and corrosion resistance. The aluminum plating layer can prevent oxidation and decarburization of the steel sheet during heat treatment due to its high-temperature oxidation resistance. Therefore, aluminum coatings are widely used in the field of hot-formed steel (especially hot-stamped steel). The global demand for hot-stamped steel with aluminum plating is approximately 2 million tons. However, the hot forming of aluminum-plated steel sheets also faces several challenges, such as the problem of roller adhesion due to aluminum melting during heat treatment and the risk of hydrogen embrittlement.

[0003] The problem of roller adhesion due to aluminum melting reduces the production efficiency and quality of hot-formed parts. To improve this problem, researchers generally aim to control the heating rate to avoid aluminum melting caused by rapid heating. For example, Patent CN101583486B explicitly proposes that the heating rate of an aluminum-plated steel sheet between 20 and 700 °C should not exceed 12 °C / second. In addition, Patent CN109518114A discloses a stepped heating method for preventing aluminum from adhering to the roller due to melting and reducing the heating rate.

[0004] The risk of hydrogen embrittlement will affect properties such as the delayed fracture resistance of hot-formed parts. In order to reduce the risk of hydrogen embrittlement of aluminum-silicon hot-stamped steel, Patent CN100471595C discloses a hot-stamping method in which the risk of hydrogen embrittlement of hot-stamped parts is reduced by controlling the atmosphere of the hot pressing process. Patent CN104160050B discloses a hot-stamped steel in which the risk of hydrogen embrittlement of the steel sheet is reduced by increasing the concentration of Mn-containing inclusions and Mn oxides in the steel.

[0005] The present invention provides a galvanized steel sheet, a hot-formed part, and a manufacturing method that address the drawbacks of existing products and technologies.

Summary of the Invention

Problems to be Solved by the Invention

[0006] The object of the present invention is to solve the problems of roller adhesion due to melting and the risk of hydrogen embrittlement that occur during the heat treatment process when manufacturing hot-formed parts from galvanized steel sheets. The present invention provides a galvanized steel sheet, a hot-formed part, and a manufacturing method that can reduce the problems of roller adhesion due to melting and the risk of hydrogen embrittlement during the hot forming process of galvanized steel sheets.

Means for Solving the Problems

[0007] The present invention provides a galvanized steel sheet including a substrate and a plating layer on the surface of the substrate, wherein the microstructure of the plating layer includes an Mg2Si phase and an AlMgSiFe phase, and the Mg2Si phase has an average crystal grain size of 0.001 to 5 μm.

[0008] By adopting the above technical solution, the problems of roller adhesion due to melting and the risk of hydrogen embrittlement during heat treatment when manufacturing hot-formed parts from galvanized steel sheets can be reduced, and the red rust resistance of hot-formed parts manufactured from galvanized steel sheets can be improved.

[0009] Preferably, the plating layer includes a surface layer and a barrier layer, and the surface layer includes a Mg2Si phase and an AlMgSiFe phase.

[0010] Preferably, the plating layer further includes a barrier layer, the barrier layer includes an Fe-Al alloy and an Fe-Al-Si alloy, and the barrier layer has a thickness of 5 μm or less.

[0011] Preferably, the plating layer of the aluminum-plated steel sheet has a thickness of 5 to 50 μm.

[0012] Preferably, the composition of the substrate of the aluminum-plated steel sheet contains, by mass percentage, 0.05 to 0.5% of C, 0.01 to 2.0% of Si, 0.3 to 3.0% of Mn, 0.005 to 0.3% of Al, 0.01% ≤ Ti < 0.1%, 0.0005% ≤ B < 0.1%, 0.05% ≤ Cr < 0.5%, 0.0005% ≤ Nb < 0.1%, and Fe.

[0013] Preferably, the composition of the substrate of the aluminum-plated steel sheet contains, by mass percentage, 0.05 to 0.5% of C, 0.01 to 2.0% of Si, 0.3 to 3.0% of Mn, 0.005 to 0.3% of Al, 0.01% ≤ Ti < 0.1%, 0.0005% ≤ B < 0.1%, 0.05% ≤ Cr < 0.5%, 0.0005% ≤ Nb < 0.1%, and the balance is Fe and inevitable impurities.

[0014] Preferably, among the inevitable impurities, by mass percentage, P < 0.3%, S < 0.1%, and V < 0.1%.

[0015] Further, the present invention is a method for manufacturing the above aluminum-plated steel sheet, A smelting step; A rolling step; and A process of performing continuous annealing and hot plating, wherein the annealing temperature is 710 - 780 °C, the temperature of the plating solution is 600 - 660 °C, the temperature difference between the temperature of the plating solution and the temperature of the steel sheet entering the plating pot is 5 °C or less, the steel sheet is cooled after exiting the plating pot, the average cooling rate from the temperature of the steel sheet exiting the plating pot to the solidification temperature of the plating layer is greater than 15 °C / second, and the average cooling rate from the temperature of the steel sheet exiting the plating pot to 200 °C is 10 - 30 °C / second. Provided is a method for manufacturing the above aluminum-plated steel sheet, which includes

[0016] Preferably, the chemical composition of the plating solution contains 5 - 11% by mass of Si and 0.5 - 20% by mass of Mg.

[0017] Preferably, the plating solution further contains 1 - 10% by mass of Zn.

[0018] Preferably, the balance of the plating solution is Al and inevitable impurities.

[0019] Preferably, the rolling process includes hot rolling, and the coiling temperature of the hot rolling is 630 °C or lower.

[0020] Preferably, the rolling process includes cold rolling, and the deformation during cold rolling is 10 - 70%.

[0021] Also, the present invention provides a hot-formed part manufactured from the above aluminum-plated steel sheet.

[0022] Preferably, the hot-formed part includes a surface layer and an inner layer, the ratio of the mass percentage of Mg in the surface layer to the mass percentage of Mg in the inner layer is 5 or more, and the hot-formed part has a core hardness HV1 of 300 or more.

[0023] Furthermore, the present invention is a method for manufacturing the above hot-formed part, which includes a step of processing the aluminum-plated steel sheet into a billet; A step of performing heat treatment on a billet, wherein the heating method of the heat treatment is one-stage heating or multi-stage heating; when the heating method of the heat treatment is one-stage heating, the heating stop temperature is a certain temperature within 900 to 1000 °C, and the total heating time is 10 to 600 seconds; when the heating method of the heat treatment is multi-stage heating, the heating stop temperature includes a plurality of temperatures within 700 to 1000 °C, the total heating time is 1 to 15 minutes, the highest temperature among the plurality of temperatures is a certain temperature within 900 to 1000 °C, and the holding time of the billet at 900 to 1000 °C is 10 to 600 seconds; and A step of transferring the billet to a mold for hot forming, wherein the temperature of the billet when transferred to the mold is 650 °C or higher, and the cooling rate of the mold is 30 °C / second or higher Provided is a method for manufacturing the above hot-formed part, including

[0024] Preferably, the hot forming process is hot stamping or hot rolling.

[0025] Preferably, before the step of processing an aluminum-plated steel sheet into a billet, a thickening rolling step is further performed.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Embodiments for Carrying out the Invention

[0027] Hereinafter, embodiments of the present invention will be described with reference to specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention based on the content disclosed in this specification. Although the description of the present invention will be introduced together with preferred embodiments, this does not mean that the features of the present invention are limited only to those embodiments. On the contrary, the description of the present invention together with the embodiments is intended to include other alternative forms or modified forms that can be derived from the claims of the present invention. To provide a complete understanding of the present invention, the following description will include many specific details. However, the present invention can also be implemented without using these specific details. Also, in order to avoid confusion or obscuring the focus of the present invention, some specific details will be omitted from the description. It should be noted that the embodiments in the present invention and the features in the embodiments can be combined with each other as long as there is no contradiction.

[0028] It should be noted that in this specification, like reference numerals and letters refer to like items in the following drawings. Therefore, once an item is defined in a drawing, no further definition and explanation are required in subsequent drawings.

[0029] In the description of the embodiments, terms such as "inside" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings or the normal orientation or positional relationship when the present invention is used, and are for the convenience of explaining and simplifying the description of the present invention. It should be noted that it is not to be construed as limiting the present invention because it does not indicate or imply that the referenced device or element must have a specific orientation and be configured and operated in a specific orientation.

[0030] To make the objectives, technical solutions, and advantages of the present invention more clear, the embodiments of the present invention will be described in more detail together with the accompanying drawings.

[0031] The present invention provides an aluminum-plated steel sheet including a substrate and a plating layer on the surface of the substrate, wherein the microstructure of the plating layer contains Mg2Si phase and AlMgSiFe phase, and the Mg2Si phase has an average crystal grain size of 0.001 to 5 μm.

[0032] By forming Mg2Si phase and AlMgSiFe phase in the plating layer, the ratio of Al phase or Al-Si phase in the plating layer mainly composed of Al is reduced, the aggregation of these Al-containing phases on the surface of the plating layer is eliminated, and they can be dispersed as much as possible. Thereby, the melting of aluminum during heat treatment is reduced, and the problem of roller adhesion caused by aluminum melting during heat treatment can be alleviated. As a result, the aluminum-plated steel sheet can withstand a higher heating rate, and the production efficiency can be improved. In addition, the probability that aluminum reacts with H2O in the air to generate H2 during heat treatment is reduced, the content of H2 in the atmosphere during heat treatment is minimized, thereby reducing the risk of hydrogen embrittlement.

[0033] In an embodiment of the present invention, the average crystal grain size of the Mg2Si phase in the high-quality plating layer is 0.001 to 5 μm. The smaller the average crystal grain size of the Mg2Si phase, the easier it is to distribute on the surface of the plating layer, and it contributes more to reducing the risk of hydrogen embrittlement.

[0034] Preferably, the plating layer includes a surface layer containing Mg2Si phase and AlMgSiFe phase.

[0035] The plating layer is mainly composed of Al phase and Si-rich phase, and the Mg2Si phase and AlMgSiFe phase are uniformly distributed in the surface layer in a cluster shape or a network shape. Since the Mg-containing phase tends to accumulate on the surface of the plating layer, the Mg2Si phase and AlMgSiFe phase in the plating layer are preferentially distributed on the surface of the plating layer during heat treatment. This effectively blocks the diffusion or penetration of H2 from the external atmosphere to the substrate, further reducing the risk of hydrogen embrittlement.

[0036] The plating layer further includes a barrier layer containing an Fe-Al alloy and an Fe-Al-Si alloy, and the barrier layer has a thickness of 5 μm or less.

[0037] When a substrate mainly composed of Fe is immersed in a plating solution mainly composed of Al and Si, the molten Al and Si will naturally alloy with Fe on the surface of the substrate to form a barrier layer mainly composed of an Fe-Al alloy and an Fe-Al-Si alloy. The barrier layer is located between the substrate of the steel sheet and the surface layer of the plating layer. In the actual production process, the thickness of the plating barrier layer can be adjusted by controlling the immersion time of the steel sheet in the plating solution and other conditions. In the embodiments of the present invention, the thickness of the barrier layer should be controlled within 5 μm. If the thickness of the barrier layer is too large, it will affect the change of the microstructure of the plating layer during cooling. For example, the formation of the Mg2Si phase and the AlMgSiFe phase will be hindered, the crystal grains will become excessively large, and in some cases, the peeling of the surface layer may occur during subsequent hot forming.

[0038] Preferably, the plating layer of the aluminum-plated steel sheet has a thickness of 5 to 50 μm.

[0039] During actual production, the thickness of the plating layer can be controlled by adjusting the immersion time of the substrate in the plating solution, the air flow intensity of the air knife, etc. The longer the immersion time, the thicker the plating layer. On the other hand, the higher the air flow intensity of the air knife, the thinner the plating layer.

[0040] Preferably, the composition of the substrate of the aluminum-plated steel sheet, by mass percentage, includes 0.05 to 0.5% of C, 0.01 to 2.0% of Si, 0.3 to 3.0% of Mn, 0.005 to 0.3% of Al, 0.01% ≤ Ti < 0.1%, 0.0005% ≤ B < 0.1%, 0.05% ≤ Cr < 0.5%, 0.0005% ≤ Nb < 0.1%, and Fe.

[0041] Preferably, the composition of the substrate of the aluminum-plated steel sheet, in mass percent, is 0.05 to 0.5% C, 0.01 to 2.0% Si, 0.3 to 3.0% Mn, 0.005 to 0.3% Al, 0.01% ≤ Ti < 0.1%, 0.0005% ≤ B < 0.1%, 0.05% ≤ Cr < 0.5%, 0.0005% ≤ Nb < 0.1%, with the balance being Fe and inevitable impurities.

[0042] In the composition of the substrate, the elements P, S, and V are inevitable impurities, and the lower their content in the substrate, the better. Specifically, in the embodiments of the present application, in mass percent, P < 0.3%, S < 0.1%, and V < 0.1%.

[0043] Further, the present invention is a method for manufacturing the above aluminum-plated steel sheet, comprising: A step of smelting; A step of rolling; and A step of performing continuous annealing and molten plating, wherein the annealing temperature is 710 to 780°C, the temperature of the plating solution is 600 to 660°C, the temperature difference between the temperature of the plating solution and the temperature of the steel sheet entering the plating pot is 5°C or less, the steel sheet is cooled after exiting the plating pot, the average cooling rate from the temperature of the steel sheet exiting the plating pot to the solidification temperature of the plating layer is greater than 15°C / second, and the average cooling rate from the temperature of the steel sheet exiting the plating pot to 200°C is 10 to 30°C / second. The present invention provides a method for manufacturing the above aluminum-plated steel sheet, including the above steps.

[0044] If the annealing temperature is less than 710°C, it may affect the plating property of the steel sheet, causing plating leakage or poor plating adhesion of the plating layer. If the annealing temperature exceeds 780°C, it will cause energy waste and may further affect the surface state of the steel sheet, potentially affecting the surface quality of the plating layer, the crystal grain size of the Mg2Si phase in the plating layer, and the formation of the AlMgSiFe phase.

[0045] The temperature of the plating solution affects the alloying reaction of molten Al and Fe, thereby affecting the composition and thickness of the barrier layer. In the present application, the temperature of the plating solution is controlled within the range of 600 to 660 °C, and the temperature of the steel sheet entering the plating pot is controlled slightly lower than the temperature of the plating solution, whereby a barrier layer having an appropriate thickness and microstructure can be obtained, and the desired AlMgSiFe phase and Mg2Si phase are surely formed in the surface layer during subsequent processing, and peeling of the surface layer is prevented.

[0046] Not only whether the temperature of the plating solution is high or low, but also a significant difference between the temperature of the steel sheet entering the plating pot and the temperature of the plating solution may affect the surface quality of the plating layer, the crystal grain size of the Mg2Si phase in the plating layer, and the formation of the AlMgSiFe phase. This may result in the average crystal grain size of the Mg2Si phase being larger than 5 μm and / or the inability to form the AlMgSiFe phase. If the average particle size of the Mg2Si phase in the plating layer is too large, the surface of the plating layer will become significantly rough, affecting the appearance of the steel sheet.

[0047] Both the average cooling rate from the temperature of the steel sheet exiting the plating pot to the solidification temperature of the plating layer and the average cooling rate from the temperature of the steel sheet exiting the plating pot to 200 °C are important. If these two cooling rates are too slow, the growth rate of the Al-Si phase will become too fast, suppressing the formation of the Mg2Si phase and the AlMgSiFe phase. As a result, the ability to overcome the problems of roller adhesion due to melting and the risk of hydrogen embrittlement faced during the hot forming process of the aluminum-plated steel sheet of the present application cannot be realized. Also, if the cooling rate is excessively slow, large crystal grain Mg2Si phase and AlMgSiFe phase will precipitate in the plating layer, and as a result, the surface of the plating layer will become rough, possibly affecting the appearance of the product. Conversely, if these two cooling rates are too fast, the strength of the steel sheet will become excessive, possibly impairing its elongation or causing other secondary damages such as surface scratches.

[0048] The temperature of the steel sheet entering the plating pot can be adjusted according to the thickness and width of the steel sheet. By appropriately increasing the temperature of the steel sheet entering the plating pot and the cooling rate after leaving the plating pot (including the average cooling rate from the temperature of the steel sheet leaving the plating pot to the solidification temperature of the plating layer and the average cooling rate from the temperature of the steel sheet leaving the plating pot to 200°C), the uniform distribution of Mg2Si phase and AlMgSiFe phase in the surface layer and the refinement of crystal grains can be further improved.

[0049] During the processes of continuous annealing and molten plating, the control of the cooling rate can be achieved by adjusting the output of the blower.

[0050] Preferably, the chemical composition of the plating solution contains 5 - 11% Si and 0.5 - 20% Mg by mass percentage.

[0051] Si in the plating solution is essential mainly for suppressing the thickness of the barrier layer. If the Si content in the plating solution is too low, the thickness of the barrier layer will become too thick, resulting in poor workability of the steel sheet. On the other hand, if the Si content in the plating solution is too high, its suppressing effect on the barrier layer will be limited, and at the same time, it will also affect the fluidity of the plating solution, increasing the difficulty of production. Therefore, the Si content in the plating solution is 5 - 11%. The presence of Mg in the plating layer is mainly to improve the corrosion resistance and promote the formation of the Mg2Si phase. Mg in the plating layer is derived from the plating solution. When the Mg content in the plating solution exceeds a certain value, the Mg2Si phase can be formed during cooling. However, the solubility of Mg in the Al - Si plating solution has a limit. If the Mg content in the plating solution is too high, Mg will be extremely easily oxidized to form slag, making production difficult. Therefore, the Mg content in the plating solution is 0.5 - 20%.

[0052] Preferably, the plating solution further contains 1 - 10% by mass of Zn. Zn in the plating layer functions as a sacrificial anode, providing sacrificial protection and enhancing the corrosion resistance of the steel.

[0053] Preferably, the remainder of the plating solution is Al and inevitable impurities.

[0054] Preferably, the rolling process includes hot rolling, and the coiling temperature of the hot rolling is 630°C or lower. If the coiling temperature is too high, excessive oxide scale may be generated on the surface of the steel plate, and the oxide scale cannot be completely removed during pickling after rolling, which may affect the surface quality of the plating layer during subsequent aluminum plating.

[0055] Preferably, the rolling process further includes cold rolling. When the steel plate produced in the above hot rolling process does not meet the requirements of user applications, cold rolling can be further performed on the hot-rolled steel coil. In the embodiment of the present application, the deformation during cold rolling is 10-70%.

[0056] The aluminum-plated steel plate can be directly used for cold stamping forming or hot stamping forming.

[0057] Furthermore, the present invention provides a hot-formed part manufactured from the above aluminum-plated steel plate.

[0058] Preferably, the hot-formed part includes a surface layer and an inner layer, the ratio of the mass percentage of Mg in the surface layer to the mass percentage of Mg in the inner layer is 5 or more, and the hot-formed part has a core hardness HV1 of 300 or more.

[0059] During the hot forming process of the aluminum-plated steel plate, the surface layer and the barrier layer of the previously formed plating layer are transformed into the surface layer and the inner layer of the hot-formed part. The corresponding microstructure also changes. The surface layer was originally composed of an Al-Si alloy, but it will change to an Fe-Al-Si alloy. The barrier layer of the Fe-Al-Si alloy will further undergo alloy diffusion, and the Fe content will increase. The inner layer of the part refers to the substrate of the hot-formed part to the dark-colored Fe-rich layer in the plating layer, and the surface layer extends from the dark-colored Fe-rich layer in the plating layer to the surface of the plating layer.

[0060] The Mg2Si phase and the AlMgSiFe phase are distributed in the surface layer of the plating layer and still preferentially distribute on the surface of the plating layer during heat treatment. After heat treatment, Mg is mainly distributed in the surface layer of the part, and the ratio of the mass percentage of Mg in the surface layer to the mass percentage of Mg in the inner layer of the part is 5 or more, which is determined by the aggregation characteristics of Mg. Since Mg is abundantly present on the surface of the hot-formed part, the rust resistance of the hot-formed part during transportation and storage can be improved.

[0061] The substrate of the aluminum-plated steel sheet will become the core of the hot-formed part after hot forming. The microstructure of the core of the hot-formed part includes one or more of martensite, bainite, and ferrite. The specific composition and content are determined by the composition of the substrate and the cooling rate of the mold during hot forming. The final microstructure of the core will affect the core hardness of the hot-formed part.

[0062] Moreover, the present invention is a method for manufacturing the above-mentioned hot-formed part, comprising: a step of processing the aluminum-plated steel sheet into a billet; a step of performing heat treatment on the billet, wherein the heating method of the heat treatment is one-stage heating or stepwise heating; when the heating method of the heat treatment is one-stage heating, the heating stop temperature is a certain temperature within 900 - 1000 °C, and the total heating time is 10 - 600 seconds; when the heating method of the heat treatment is stepwise heating, the stop temperature of the stepwise heating includes a plurality of temperatures within 700 - 1000 °C, the total heating time is 1 - 15 minutes, the highest temperature among the plurality of temperatures is a certain temperature within 900 - 1000 °C, and the holding time of the billet at 900 - 1000 °C is 10 - 600 seconds; and a step of transferring the billet to a mold for hot forming, wherein the temperature of the billet when transferred to the mold is 650 °C or higher, and the cooling rate of the mold is 30 °C / second or higher The present invention provides a method for manufacturing the above-mentioned hot-formed part. In the embodiments of the present application, the mold for hot forming is water-cooled, and the cooling rate of the mold is controlled by adjusting conditions such as the flow rate, flow velocity, and pressure of the cooling water.

[0063] When the heating method of the heat treatment is one-step heating, the heating stop temperature is a certain temperature within 900 - 1000 °C, and the total heating time is the time from the start to the end of heating the billet. When the heating method of the heat treatment is stepwise heating, the stop temperature includes a plurality of temperatures within the range of 700 - 1000 °C, and the total heating time is the time from the start to the end of heating the billet. To ensure that the steel is completely austenitized and to form the desired structure during cooling, the final heating stop temperature should be 900 °C or higher regardless of whether it is one-step heating or stepwise heating. For energy conservation, the upper limit of the heating stop temperature is set to 1000 °C.

[0064] Preferably, the process of hot forming is hot stamping or hot rolling.

[0065] Preferably, before processing the aluminum-plated steel sheet into a billet, a sizing rolling process is further performed.

Examples

[0066] Examples 1 - 6 and Comparative Examples 1 - 2 Using the following manufacturing method, aluminum-plated steel sheets and hot-formed parts of Examples 1 - 6 and Comparative Examples 1 - 2 were manufactured. Step 1: Smelting was carried out to obtain a substrate having the composition shown in Table 1. Step 2: Rolling was carried out to obtain a rolled steel sheet. After rolling, pickling was performed to remove the oxide layer on the surface of the steel sheet. Step 3: Continuous annealing and electroplating were carried out. In this step, the rolled steel sheet was continuously annealed, and then placed in a plating pot (immersed in the plating solution). After immersion, the steel sheet was cooled to obtain an aluminum-plated steel sheet. The specific process parameters of rolling, continuous annealing, and electroplating are shown in Table 2. Step 4: The aluminum-plated steel sheet was processed into a billet. Step 5: The billet was heat-treated. Step 6: The heat-treated billet was transferred to a mold for hot forming to obtain a hot-formed part. Table 3 shows the specific process parameters of heat treatment and hot forming.

[0067] According to the following test methods, the aluminum-plated steel sheets and hot-formed parts of Examples 1 to 6 and Comparative Examples 1 to 2 were tested. The test results are shown in Tables 2 and 3.

[0068] 1) Average crystal grain size of Mg2Si phase (μm) The crystal grain size was calculated using the sectioning method. Average crystal grain size = length of the section cross-section / number of crystal grains.

[0069] 2) Presence of AlMgSiFe phase Observation was carried out using a Zeiss scanning electron microscope of EVO10 in combination with energy-dispersive X-ray spectrometer (EDS) analysis.

Number

[0070] 3) Roller adhesion phenomenon Judgment was made by visual inspection. "×" indicates the absence of adhesion,

Number

[0071] 4) Hydrogen embrittlement resistance The hydrogen content of the hot-formed parts was evaluated using a G4-PHONEX micro hydrogen concentration analyzer. The maximum heating temperature did not exceed 400 °C. The amount of hydrogen released was recorded. The larger the released amount, the worse the hydrogen embrittlement resistance. The evaluation scale ranges from 1 (worst) to 5 (best).

[0072] 5) Mass percentage of Mg in the surface layer of the part / Mass percentage of Mg in the inner layer of the part It was tested using a GDS850A glow discharge spectrometer. The inner layer of the part refers to the part from the substrate of the thermoformed part to the dark-colored Fe-rich layer in the plating layer, and the surface layer of the part refers to the part from the dark-colored Fe-rich layer in the plating layer to the surface of the plating layer.

[0073] 6) Rust resistance It was evaluated using a neutral salt spray test. The thermoformed parts to be evaluated did not have electrophoretic coating. After 24 hours, the degree of rust coverage is evaluated, and a coverage of less than 5% indicates the best performance. In this experiment, an evaluation scale from 1 (worst) to 5 (best) is used.

[0074] 7) Core hardness HV1 The Vickers hardness of the thermoformed parts was measured according to the GB / T4340.1-2009 standard.

[0075] Figure 1 was obtained by scanning the plating layer of the aluminum-plated steel sheet obtained in Example 2 of the present invention using a Zeiss field emission electron microscope.

[0076] The thermoformed parts obtained in Example 2 of the present invention were tested using a GDS850A glow discharge spectrometer to obtain Figure 2 showing the change in the mass percentage of Mg as a function of the depth of the plating layer.

[0077] Table 1 shows the chemical element compositions of the substrates of Examples 1 to 6 and Comparative Examples 1 to 2.

[0078]

Table 1

[0079] Table 2 shows the rolling, continuous annealing, and molten plating process parameters, the composition of the plating solution, and the microstructure and thickness of the plating layer of the steel sheets of Examples 1 to 6 and Comparative Examples 1 to 2.

[0080]

Table 2

[0081] Table 3 shows the process parameters of the heat treatment, the presence or absence of the roller adhesion phenomenon, the process parameters of the hot forming of the aluminum-plated steel sheet, and the characteristics of the hot formed parts in Examples 1 to 6 and Comparative Examples 1 and 2.

[0082]

Table 3

[0083] From Tables 1 to 3, it can be seen that the aluminum-plated steel sheets obtained in Examples 1 to 6 show the microstructure of the plating layer containing the Mg2Si phase and the AlMgSiFe phase. The average crystal grain size of the Mg2Si phase is 1 to 5 μm. During the heat treatment, the roller adhesion phenomenon due to melting does not occur. The hot formed parts obtained in Examples 1 to 6 are excellent in hydrogen embrittlement resistance, and the ratio of the mass percentage of Mg in the surface layer to the mass percentage of Mg in the inner layer is 5 or more. Moreover, the hot formed parts show excellent red rust resistance, and the core hardness HV1 is 300 or more.

[0084] FIG. 1 is a scanning spectrum of the plating layer of the aluminum-plated steel sheet of Example 2 of the present invention, and it can be seen that the microstructure of the plating layer contains the Mg2Si phase and the AlMgSiFe phase. FIG. 2 shows the change in the mass percentage of Mg in the hot formed part of Example 2 of the present invention as a function of the depth of the plating layer, and it can be seen that the mass percentage of Mg is higher the closer the measurement is to the surface of the plating layer.

[0085] In contrast, for Comparative Example 1, the average cooling rate from the temperature of the steel sheet exiting the plating pot to the solidification temperature of the plating layer is too slow, only 10 °C / second. The chemical composition of the plating solution does not contain Mg, and the plating layer of the aluminum-plated steel sheet does not contain the Mg2Si phase and the AlMgSiFe phase. The roller adhesion phenomenon due to melting occurs during heat treatment. The temperature of the billet when transferred to the mold is too low, only 600 °C. The cooling rate of the mold is too low, only 25 °C / second. As a result, the hydrogen embrittlement resistance and red rust resistance of the hot-formed parts are inferior, and the core hardness HV1 is only 250.

[0086] For Comparative Example 2, the difference between the temperature of the plating solution and the temperature of the steel sheet entering the plating pot is significant, with a 20 °C difference. The average cooling rate from the temperature of the steel sheet exiting the plating pot to the solidification temperature of the plating layer is too slow, only 5 °C / second. The average cooling rate from the temperature of the steel sheet exiting the plating pot to 200 °C is too slow, only 8 °C / second. The Mg content in the plating solution is only 0.3%, and the Mg2Si phase and the AlMgSiFe phase do not exist. The roller adhesion phenomenon due to melting occurs during heat treatment. The temperature of the billet when transferred to the mold is too low, only 600 °C. The cooling rate of the mold is also low, only 25 °C / second. As a result, the hydrogen embrittlement resistance of the hot-formed parts is inferior, and the ratio of the mass percentage of Mg in the surface layer to the mass percentage of Mg in the inner layer of the part is only 3. Moreover, the red rust resistance is inferior, and the core hardness HV1 is only 250.

[0087] Since the process parameters of the steel sheets in Comparative Examples 1 and 2 are not properly controlled during continuous annealing, molten plating, and hot forming, the resulting hot-formed parts do not have the desired properties of the present application.

[0088] In summary, the present invention provides an aluminum-plated steel sheet, a hot-formed part, and a manufacturing method that can reduce the problem of roller adhesion due to melting during heat treatment of the aluminum-plated steel sheet, reduce the risk of hydrogen embrittlement, and improve the red rust resistance of the hot-formed part.

[0089] Although the present invention has been illustrated and described with reference to some preferred embodiments thereof, those skilled in the art should understand that the above content further describes the present invention in more detail together with specific embodiments, and it should not be considered that the specific implementation of the present invention is limited only to the above description. Those skilled in the art may make various changes in form and details, including performing a certain degree of deduction or substitution without departing from the spirit and scope of the present invention.

Explanation of Reference Numerals

[0090] 1 Mg2Si phase 2 AlMgSiFe phase

Claims

1. An aluminum-plated steel sheet including a substrate and a plating layer on the surface of the substrate, wherein the microstructure of the plating layer contains Mg ２ Si phase and AlMgSiFe phase, and the Mg ２ Si phase has an average crystal grain size of 0.001 to 5 μm. Aluminum-plated steel sheet.

2. The plating layer contains the above Mg ２ The aluminum-plated steel sheet according to claim 1, comprising a surface layer containing an Si phase and the above AlMgSiFe phase.

3. The aluminum-plated steel sheet according to claim 2, wherein the plating layer further includes a barrier layer containing an Fe—Al alloy and an Fe—Al—Si alloy, and the barrier layer has a thickness of 5 μm or less.

4. The aluminum-plated steel sheet according to claim 1, wherein the plating layer has a thickness of 5 to 50 μm.

5. The aluminum-plated steel sheet according to claim 1, wherein the composition of the substrate of the aluminum-plated steel sheet contains, by mass percentage, 0.05 to 0.5% of C, 0.01 to 2.0% of Si, 0.3 to 3.0% of Mn, 0.005 to 0.3% of Al, 0.01% ≤ Ti < 0.1%, 0.0005% ≤ B < 0.1%, 0.05% ≤ Cr < 0.5%, 0.0005% ≤ Nb < 0.1%, and Fe.

6. The composition of the substrate of the aluminum-plated steel sheet contains, by mass percentage, 0.05 to 0.5% of C, 0.01 to 2.0% of Si, 0.3 to 3.0% of Mn, 0.005 to 0.3% of Al, 0.01% ≤ Ti < 0.1%, 0.0005% ≤ B < 0.1%, 0.05% ≤ Cr < 0.5%, 0.0005% ≤ Nb < 0.1%, and the balance is Fe and inevitable impurities. Among the inevitable impurities, by mass percentage, P < 0.3%, S < 0.1%, and V < 0.1%. The aluminum-plated steel sheet according to claim 5.

7. A method for manufacturing the aluminum-plated steel sheet according to claim 1, comprising: a step of steelmaking; a step of rolling; and a step of performing continuous annealing and molten plating, wherein the annealing temperature is 710 to 780°C, the temperature of the plating solution is 600 to 660°C, the temperature difference between the temperature of the plating solution and the temperature of the steel sheet entering the plating pot is 5°C or less, the steel sheet is cooled after exiting the plating pot, the average cooling rate from the temperature of the steel sheet exiting the plating pot to the solidification temperature of the plating layer is greater than 15°C / second, and the average cooling rate from the temperature of the steel sheet exiting the plating pot to 200°C is 10 to 30°C / second. A method for manufacturing an aluminum-plated steel sheet, including the above steps.

8. The method for manufacturing an aluminum-plated steel sheet according to claim 7, wherein the chemical composition of the plating solution contains, by mass percentage, 5 to 11% of Si and 0.5 to 20% of Mg.

9. The method for manufacturing an aluminum-plated steel sheet according to claim 8, wherein the plating solution further contains 1 to 10% by mass of Zn.

10. The method for manufacturing an aluminum-plated steel sheet according to claim 8, wherein the remainder of the plating solution is Al and inevitable impurities.

11. The method for manufacturing an aluminum-plated steel sheet according to claim 7, wherein the rolling step includes hot rolling, and the coiling temperature of the hot rolling is 630°C or lower.

12. The method for manufacturing an aluminum-plated steel sheet according to claim 11, wherein the rolling step includes cold rolling, and the deformation during the cold rolling is 10 to 70%.

13. A hot-formed part manufactured using the aluminum-plated steel sheet according to claim 1.

14. The hot-formed part according to claim 13, wherein the hot-formed part includes a surface layer and an inner layer, the ratio of the mass percentage of Mg in the surface layer to the mass percentage of Mg in the inner layer is 5 or more, and the hot-formed part has a core hardness HV1 of 300 or more.

15. A method for manufacturing a hot-formed part according to claim 13, comprising: a step of processing the aluminum-plated steel sheet into a blank; a step of performing heat treatment on the blank, wherein the heating method of the heat treatment is one-step heating or stepwise heating; when the heating method of the heat treatment is one-step heating, the heating stop temperature is a certain temperature within 900 to 1000°C, and the total heating time is 10 to 600 seconds; when the heating method of the heat treatment is stepwise heating, the heating stop temperature includes a plurality of temperatures within 700 to 1000°C, the total heating time is 1 to 15 minutes, the highest temperature among the plurality of temperatures is a certain temperature within 900 to 1000°C, and the holding time of the blank at 900 to 1000°C is 10 to 600 seconds; and a step of transferring the blank to a mold for hot forming, wherein the temperature of the blank when transferred to the mold is 650°C or higher, and the cooling rate of the mold is 30°C / second or higher. A method for manufacturing a hot-formed part.

16. The method for manufacturing a hot-formed part according to claim 15, wherein the hot-forming process is hot stamping or hot rolling.

17. The method for manufacturing a hot-formed part according to claim 15, wherein a step of incremental rolling is further performed before the step of processing the aluminum-plated steel sheet into a blank.

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