Steel sheet for hot press forming parts with excellent paint adhesion and corrosion resistance after painting and method for manufacturing the same

Abstract

The present invention relates to a steel sheet for hot press forming parts having excellent paint adhesion and corrosion resistance after painting, and a method for manufacturing the same. The steel sheet for hot press forming according to one aspect of the present invention includes a base steel sheet and a plated layer formed on the surface of the base steel sheet, and the proportion of the area occupied by cavities observed in the cross section of the surface layer portion cut in the thickness direction of the plated layer with respect to the area of the entire surface layer portion can be 10% or more.

Description

[0001] This application is a divisional application of Chinese patent application No. 201880036329.X, filed on May 31, 2018, entitled "Steel sheet for hot-pressed parts with excellent coating adhesion and corrosion resistance after coating and manufacturing method thereof", and claims priority to Korean applications No. 10-2017-0068651 and 10-2017-0101563. Technical Field

[0002] This invention relates to a steel sheet for hot-pressed parts with excellent coating adhesion and corrosion resistance after coating, and a method for manufacturing the same. Background Technology

[0003] In recent years, due to the depletion of petroleum energy and high concerns about the environment, regulations on improving the fuel efficiency of automobiles have become increasingly stringent.

[0004] In terms of materials, one way to improve the fuel efficiency of automobiles is to reduce the thickness of the steel plates used. However, reducing the thickness can cause safety issues in automobiles, so it must be accompanied by an increase in the strength of the steel plates.

[0005] For the reasons stated above, there is a continuous demand for high-strength steel sheets, and various types of steel sheets have been developed. However, these steel sheets themselves have high strength, thus presenting a problem with poor workability. That is, the product of strength and elongation tends to be a constant value for each grade of steel sheet, therefore, as the strength of the steel sheet increases, the elongation, as an indicator of workability, decreases.

[0006] To address this problem, hot pressing was proposed. Hot pressing involves rapidly cooling a steel sheet to a suitable processing temperature after processing, thereby forming martensite and other low-temperature structures within the steel sheet, thus increasing the strength of the final product. As described above, this method offers the advantage of minimizing processability issues when manufacturing high-strength components.

[0007] However, when using the hot pressing method, the steel plate needs to be heated to a high temperature, and the surface of the steel plate will be oxidized. Therefore, after the stamping process, it is necessary to add a process to remove the oxides from the surface of the steel plate.

[0008] As a method for solving this problem, U.S. Patent Publication No. 6,296,805 has been proposed. In this invention, an aluminized steel sheet is subjected to a process of hot pressing or room-temperature forming followed by heating and rapid cooling (referred to as "post-heat treatment"). Because the aluminized layer exists on the surface of the steel sheet, the steel sheet will not oxidize during heating.

[0009] However, even if the steel sheet does not oxidize during heating when an aluminum coating is present on the surface, the resulting parts after heating and forming are still exposed to a corrosive environment. In particular, during the heating and coating process of the steel sheet, the base iron diffuses into the aluminum coating, forming a hard Fe-Al coating on the surface of the steel sheet. The Fe-Al coating is hard but brittle, so cracks may form in the coating, thereby exposing the base steel sheet to a corrosive environment.

[0010] To prevent this problem, a coating layer can be formed on the hot-pressed part, which requires excellent coating adhesion. Summary of the Invention

[0011] Technical problems to be solved

[0012] According to one aspect of the present invention, a hot-pressing steel sheet is provided, which has excellent coating adhesion and thus enables the manufacture of hot-pressed parts with excellent corrosion resistance after coating.

[0013] The technical problems addressed by this invention are not limited to those described above. Those skilled in the art can readily solve the additional technical problems of this invention based on the entire specification.

[0014] Technical solution

[0015] A hot-pressed steel sheet according to one aspect of the present invention includes a base steel sheet and a coating formed on the surface of the base steel sheet, wherein the proportion of the area occupied by voids in the cross section of the surface portion observed when the coating is cut along the thickness direction relative to the entire surface portion area can be 10% or more.

[0016] In one specific embodiment of the present invention, the proportion of voids in the cross-section of the surface portion observed when the coating is cut along the thickness direction relative to the entire surface portion area can be 15% or more.

[0017] In one specific embodiment of the present invention, the coating may be an aluminum alloy coating.

[0018] In one specific embodiment of the present invention, the average Fe content of the aluminum alloy coating can be 30% by weight or more.

[0019] In one specific embodiment of the present invention, the average Fe content of the aluminum alloy coating can be 40% by weight or more.

[0020] In one specific embodiment of the present invention, the base steel plate may have a composition comprising, by weight percent: C: 0.04-0.5%, Si: 0.01-2%, Mn: 0.01-10%, Al: 0.001-1.0%, P: less than 0.05%, S: less than 0.02%, N: less than 0.02%, with the balance being Fe and other unavoidable impurities.

[0021] In one specific embodiment of the present invention, the composition of the base steel plate, by weight percent, may further include one or more of the following: the sum of one or more selected from Cr, Mo and W: 0.01 to 4.0%; the sum of one or more selected from Ti, Nb, Zr and V: 0.001 to 0.4%; Cu+Ni: 0.005 to 2.0%; Sb+Sn: 0.001 to 1.0%; and B: 0.0001 to 0.01%.

[0022] A method for manufacturing a steel sheet for hot-pressed components according to one aspect of the present invention includes the following steps: aluminizing the surface of a base steel sheet and coiling it to obtain an aluminized steel sheet; annealing the aluminized steel sheet to obtain an aluminized alloy steel sheet; and cooling the aluminized alloy steel sheet, wherein, taking one side of the steel sheet as a reference, the aluminizing coating weight can be 30-200 g / m². 2 The winding tension during winding can be set to 0.5–5 kg / mm. 2 The annealing can be carried out in a bell-type annealing furnace at a heating temperature range of 550-750°C for 30 minutes to 50 hours. During the annealing, the average heating rate from room temperature to the heating temperature can be set to 20-100°C / hour, but the average heating rate in the 400-500°C range can be set to 1-15°C / hour. The heating rate from -50°C to the heating temperature range can be set to 1-15°C / hour. The temperature difference between the atmosphere temperature and the steel plate temperature in the bell-type annealing furnace can be set to 5-80°C. In the step of cooling the aluminized alloy steel plate, it can be cooled to 500°C at a rate of less than 50°C / hour.

[0023] In one specific embodiment of the present invention, the base steel plate may have a composition comprising, by weight percent: C: 0.04-0.5%, Si: 0.01-2%, Mn: 0.01-10%, Al: 0.001-1.0%, P: less than 0.05%, S: less than 0.02%, N: less than 0.02%, with the balance being Fe and other unavoidable impurities.

[0024] In one specific embodiment of the present invention, the composition of the base steel plate, by weight percent, may further include one or more of the following: the sum of one or more selected from Cr, Mo and W: 0.01 to 4.0%; the sum of one or more selected from Ti, Nb, Zr and V: 0.001 to 0.4%; Cu+Ni: 0.005 to 2.0%; Sb+Sn: 0.001 to 1.0%; and B: 0.0001 to 0.01%.

[0025] Beneficial effects

[0026] According to one aspect of the invention, the surface portion of the steel sheet for hot pressing contains cavities, thereby significantly improving the surface roughness of the part obtained after hot pressing and enabling it to have excellent coating adhesion, and as a result, excellent corrosion resistance after coating is also obtained. Attached Figure Description

[0027] Figure 1 A cross-sectional photograph is shown of a steel plate with a coated cut surface, illustrating an embodiment of the present invention.

[0028] Figure 2 The compositional distribution of the coating on the steel sheet manufactured in Example 1 of the invention is shown by analyzing it using a GDS analyzer.

[0029] Figure 3 A scanning electron microscope image (backscattered electron image) showing a cross-section of the coating on the steel plate manufactured in Example 1 of the invention is shown.

[0030] Figure 4 The compositional distribution of the coating on the steel sheet manufactured in Example 2 of the invention is shown by analyzing it using a GDS analyzer.

[0031] Figure 5 A scanning electron microscope image (backscattered electron image) showing the cross-section of the coating on the steel plate manufactured in Example 2 of the invention is shown.

[0032] Figure 6 The compositional distribution of the coating on the steel sheet manufactured in Comparative Example 1 is shown by analyzing it using a GDS analyzer.

[0033] Figure 7 A scanning electron microscope image of the cross-section of the coating on the steel sheet manufactured in Comparative Example 1 is shown.

[0034] Figure 8 The compositional distribution of the coating on the steel sheet manufactured in Comparative Example 2 is shown by analyzing it using a GDS analyzer.

[0035] Figure 9 A scanning electron microscope image of the cross-section of the coating on the steel sheet manufactured in Comparative Example 2 is shown.

[0036] Figure 10The compositional distribution of the coating on the steel sheet manufactured in Comparative Example 3 is shown by analyzing it using a GDS analyzer.

[0037] Figure 11 A scanning electron microscope image of the cross-section of the coating on the steel sheet manufactured in Comparative Example 3 is shown.

[0038] Best practice

[0039] The present invention will now be described in detail.

[0040] In this invention, "component" refers to a part or material for a part manufactured by hot pressing. Additionally, "steel plate" refers to a steel plate before hot pressing; there are cases where such steel plates are rolled up during the manufacturing process to obtain a coil shape, which is also referred to as a coil.

[0041] Figure 1 A cross-sectional photograph of a steel sheet with a coated surface according to a specific embodiment of the present invention is shown. As shown, the steel sheet of the present invention consists of a base steel sheet and a coating formed on the surface of the base steel sheet. The surface portion of the coating has a large number of pores. This is a phenomenon not observed in conventional hot-pressed aluminized steel sheets, where pores are almost non-existent in the surface portion due to hot-dip aluminizing. However, the main characteristic of the steel sheet of this specific embodiment is the presence of a large number of pores in the surface portion of the coating. In this specific embodiment, the surface portion refers to the area from the surface to a depth of 10 μm or less (the depth is measured from various locations on the rough surface when the surface is rough).

[0042] When the surface layer of a steel plate contains a large number of cavities, when the steel plate is heated and stamped, some of the cavities in the surface layer are opened due to the stress applied during the stamping process, which increases the roughness of the coating surface.

[0043] Alloying occurs on the surface of hot-pressed parts obtained by hot pressing aluminized steel sheets. The resulting alloy layer is relatively stable compared to the unalloyed aluminized layer, and therefore has weak reactivity with phosphates used to improve coating adhesion. There is little room for improvement in coating adhesion through conventional phosphate treatment alone. Although the increased roughness during hot pressing can improve coating adhesion to some extent, this improvement has limitations.

[0044] Therefore, in this specific embodiment, in order to improve this situation, cavities are formed in the coating of the steel plate as described above, so that the cavities collapse during subsequent stamping and forming, thereby helping to increase the roughness.

[0045] Therefore, relative to the entire surface area, the proportion of voids in the cross-section of the surface portion observed when the coating of the steel sheet is cut along the thickness direction can be 10% or more, or 15% or more. In the case described above, when the steel sheet is hot-pressed, the surface roughness is increased, thereby significantly improving coating adhesion and post-coating corrosion resistance. Regarding coating adhesion or post-coating corrosion resistance, it is not necessary to specifically limit the upper limit of surface roughness, but the void ratio can generally be set to 70% or less, or 60% or less. There are various methods for measuring the void ratio; in one specific embodiment of the present invention, a method using an image analyzer to measure the proportion of the portion containing voids can be used.

[0046] In this invention, to form a coating with a high void ratio in the surface layer of a steel plate as described above, the coating can be an aluminum alloy coating, and in one specific embodiment, the coating can be an Al-Fe alloy coating. According to a specific embodiment of the invention, the Al-Fe alloy coating can be obtained by alloying the Al-plated steel plate under appropriate conditions. That is, the invention utilizes the phenomenon that when the Al-plated steel plate is heated under appropriate conditions, diffusion occurs between the Al in the coating and the Fe in the base steel plate, and Al and Fe alloy together, during which a large number of voids are formed in the surface layer.

[0047] In order to form cavities, the average Fe content of the coating can be 30% by weight or more, more preferably 40% by weight or more, and even more preferably 50% by weight or more. That is, cavities can only be obtained in the surface layer if sufficient alloying occurs, so the average Fe content of the coating can be 30% by weight or more, 40% by weight or more, or 50% by weight or more. Although it is not necessary to specifically limit the upper limit of the average Fe content, when considering the efficiency of alloying, the upper limit of the average Fe content can be set to 80% by weight or less. Here, the average Fe content refers to the average Fe content in the entire coating. Various measurement methods can be used, but in this specific embodiment, the following measurement value can be used: when analyzing from the surface of the coating to the interface of the steel plate by glow discharge emission spectroscopy (GDS), the displayed Fe content curve according to depth (thickness) is integrated, and then divided by the coating thickness. There are various criteria for determining the interface between the coating and the steel plate, but in this specific implementation, the interface between the coating and the steel plate can be set at the position where the Fe content is 92% of the Fe content of the base material, based on the GDS results.

[0048] The steel sheet of the present invention is a hot-press forming steel sheet, and its composition is not particularly limited as long as it is used for hot pressing. However, according to one aspect of the present invention, the steel sheet of the present invention may have a composition comprising the following components, in weight percent (hereinafter, it should be noted that unless otherwise specified, the composition of the steel sheet and the coating of the present invention is based on weight): C: 0.04 to 0.5%, Si: 0.01 to 2%, Mn: 0.01 to 10%, Al: 0.001 to 1.0%, P: less than 0.05%, S: less than 0.02%, and N: less than 0.02%.

[0049] C: 0.04–0.5%

[0050] The carbon (C) is an essential element for improving the strength of heat-treated components and can be added in an appropriate amount. Specifically, to sufficiently ensure the strength of the heat-treated component, more than 0.04% of the C can be added. In one specific embodiment, the lower limit of the C content can be 0.1%. However, if the C content is too high, in the case of producing cold-rolled materials, the strength of the hot-rolled material becomes excessively high during cold rolling, resulting in significantly poor cold-rollability and a substantial reduction in spot weldability. Therefore, to ensure sufficient cold-rollability and spot weldability, less than 0.5% of C can be added. Alternatively, the C content can also be limited to less than 0.45% or less than 0.4%.

[0051] Si: 0.01–2%

[0052] The Si is added not only as a deoxidizer in steelmaking, but also suppresses the formation of carbides, which have the greatest impact on the strength of hot-pressed parts. Furthermore, after martensite is formed during hot pressing, it enriches carbon at the martensitic lath grain boundaries, thereby ensuring retained austenite. Therefore, Si can be added at a content of 0.01% or more. Additionally, when aluminizing the rolled steel sheet, the upper limit of the Si content can be set to 2% to ensure sufficient coating properties. In one specific embodiment of the invention, the Si content can be limited to 1.5% or less.

[0053] Mn: 0.01~10%

[0054] The Mn content not only ensures solid solution strengthening, but also, in hot-pressed parts, can be added at a content of 0.01% or more to reduce the critical cooling rate required to ensure martensite formation. Furthermore, to ensure the workability of the hot-pressing process, reduce manufacturing costs, and improve spot weldability by appropriately maintaining the strength of the steel sheet, the Mn content can be set to 10% or less; in one specific embodiment of the invention, the Mn content can be set to 9% or less or 8% or less.

[0055] Al: 0.001~1.0%

[0056] The Al, together with Si, acts as a deoxidizer in steelmaking, thereby improving the cleanliness of the steel. Therefore, Al can be added at a content of 0.001% or more. Furthermore, to prevent the Ac3 temperature from becoming excessively high and to ensure the heating required for hot pressing within an appropriate temperature range, the Al content can be set to 1.0% or less.

[0057] P: below 0.05%

[0058] Phosphorus (P) exists as an impurity in steel, and its content should be as low as possible. Therefore, in one specific embodiment of the invention, the P content can be below 0.05%. In another specific embodiment of the invention, the P content can also be limited to below 0.03%. Since P is an impurity element that is more advantageous in lower amounts, it is not necessary to specifically set an upper limit for the P content. However, excessively reducing the P content may increase manufacturing costs, so taking this into consideration, a lower limit for the P content can be set at 0.001%.

[0059] S: below 0.02%

[0060] S is an impurity in steel, an element that hinders the ductility, impact properties, and weldability of the component. Therefore, the maximum content of S is set to 0.02% (preferably less than 0.01%). Furthermore, when the minimum content of S is less than 0.0001%, manufacturing costs increase. Therefore, in a specific embodiment of the present invention, the lower limit of the S content can be set to 0.0001%.

[0061] N: below 0.02%

[0062] The nitrogen (N) is an element contained in steel as an impurity. To reduce the susceptibility to cracking during slab continuous casting and to ensure impact characteristics, a lower N content is more advantageous; therefore, it can be less than 0.02% N. Although it is not necessary to specifically set a lower limit for the N content, considering factors such as increased manufacturing costs, in a particular embodiment, the N content can be set to 0.001% or higher.

[0063] In addition to the steel composition described above, one or more of the following may be added as needed: a sum of one or more selected from Cr, Mo and W: 0.01 to 4.0%; a sum of one or more selected from Ti, Nb, Zr and V: 0.001 to 0.4%; Cu+Ni: 0.005 to 2.0%; Sb+Sn: 0.001 to 1.0%; and B: 0.0001 to 0.01%.

[0064] The sum of one or more of Cr, Mo, and W: 0.01–4.0%

[0065] The Cr, Mo, and W elements can ensure strength and grain refinement by improving hardenability and precipitation strengthening effects. Therefore, they can be added at a rate of 0.01% or more, based on the sum of the contents of one or more of Cr, Mo, and W. Furthermore, to ensure the weldability of the component, the sum of the contents of one or more of Cr, Mo, and W can be limited to 4.0% or less. Moreover, when the contents of these elements exceed 4.0%, the further improvement in effect is minimal; therefore, limiting the contents to 4.0% or less prevents cost increases resulting from further element addition.

[0066] The sum of one or more of Ti, Nb, Zr, and V: 0.001–0.4%

[0067] The Ti, Nb, and V improve the strength of heat-treated components by forming fine precipitates, and also have the effect of stabilizing retained austenite and improving impact toughness through grain refinement. Therefore, the total content of one or more of Ti, Nb, Zr, and V can be added at 0.001% or more. However, when the above-mentioned addition exceeds 0.4%, not only will its effect be saturated, but the addition of too much alloy iron will also lead to an increase in cost.

[0068] Cu+Ni: 0.005~2.0%

[0069] The Cu and Ni elements enhance strength by forming fine precipitates. To achieve the aforementioned effect, the sum of one or more of the Cu and Ni components can be set to 0.005% or more. However, a Cu+Ni value exceeding 2.0% would increase costs excessively; therefore, the upper limit for Cu+Ni is set at 2.0%.

[0070] Sb+Sn: 0.001~1.0%

[0071] The Sb and Sn are enriched on the surface during the annealing heat treatment used for Al-Si plating, inhibiting the formation of Si or Mn oxides on the surface, thereby improving plating performance. To achieve the above-mentioned effect, more than 0.001% Sb+Sn can be added. However, when the amount of Sb+Sn added exceeds 1.0%, excessive alloy iron cost is required, and Sb and Sn are dissolved in the grain boundaries of the slab, which can induce edge cracks in the coil during hot rolling. Therefore, the upper limit of Sb+Sn is set at 1.0%.

[0072] B: 0.0001~0.01%

[0073] The boron (B) not only improves hardenability with small amounts, but also segregates at the original austenite grain boundaries, thereby suppressing the brittleness of hot-pressed parts caused by the segregation of P and / or S grain boundaries. Therefore, more than 0.0001% of B can be added. However, when the B content exceeds 0.01%, it not only saturates the effect but also leads to brittleness during hot rolling. Therefore, the upper limit of the B content can be set to 0.01%, and in one specific embodiment, the B content can be set to less than 0.005%.

[0074] In addition to the components mentioned above, the balance may include iron and unavoidable impurities, as long as they are components that can be included in hot-formed steel sheets, there are no special restrictions.

[0075] Hereinafter, an example of a method for manufacturing a hot-pressed steel sheet according to one aspect of the present invention will be described. However, it should be noted that the following method for manufacturing a hot-pressed steel sheet is merely an example, and the hot-pressed steel sheet of the present invention does not necessarily have to be manufactured using this method. Any manufacturing method that satisfies the claims of the present invention can be used to implement various specific embodiments of the present invention.

[0076] The steel plate of the present invention can be obtained by the following method: using a hot-rolled or cold-rolled base steel plate, hot-dip aluminizing the surface of the base steel plate, and annealing the plated steel plate.

[0077] [Aluminizing process]

[0078] In one specific embodiment of the present invention, the following steps are performed: a base steel plate is prepared, the surface of the base steel plate is aluminized under appropriate conditions and then coiled to obtain an aluminized steel plate (coil).

[0079] 30-200g / m² per side 2 The amount of coating is used to coat the surface of the base steel plate with aluminum.

[0080] Aluminizing can be applied to the surface of rolled steel sheets. Aluminizing typically uses either Type I AlSi plating (containing more than 80% Al and 5-20% Si, with additional elements added as needed) or Type II plating (containing more than 90% Al, with additional elements added as needed). Hot-dip aluminizing can be performed to form the coating, or the steel sheet can be annealed before plating. During plating, the appropriate coating weight is 30-200 g / m², based on one side. 2 If the coating amount is too large, it will take too much time to alloy the surface; on the other hand, if the coating amount is too small, it will be difficult to obtain sufficient corrosion resistance.

[0081] The winding tension after plating is set to 0.5–5 kg / mm. 2

[0082] When the steel sheet is coiled after plating to obtain a coil, the coiling tension can be adjusted. Depending on the adjustment of the coiling tension, the alloying behavior and surface quality of the coil during subsequent annealing will differ.

[0083] [Annealing treatment]

[0084] Anneal the steel sheet that has been aluminized by the above steps under the following conditions to obtain an aluminized alloy steel sheet.

[0085] In a bell-type annealing furnace, the annealing process is carried out at a temperature of 550–750°C for 30 minutes to 50 hours.

[0086] Aluminized steel sheet (coil) is heated in a batch annealing furnace. When heating the steel sheet, the target heat treatment temperature and holding time are preferably within the range of 550–750°C (in this invention, the highest temperature reached by the material within this temperature range is called the heating temperature) for 30 minutes to 50 hours, based on the steel sheet temperature. The holding time refers to the time from when the coil temperature reaches the target temperature until cooling begins. In one specific embodiment of the invention, if sufficient alloying is not achieved, the coating will peel off during roll straightening; therefore, for sufficient alloying, the heating temperature can be set to 550°C or higher. Furthermore, to prevent excessive oxide formation on the surface and ensure spot weldability, the heating temperature can be set to 750°C or lower. Additionally, to ensure sufficient coating while preventing a decrease in productivity, the holding time can be set to 30 minutes to 50 hours. In one specific embodiment of the invention, the steel sheet temperature can have a heating mode in which the temperature continues to rise until the heating temperature is reached without a cooling process.

[0087] The average heating rate is set to 20–100 °C / hour, and the temperature is heated to the desired temperature.

[0088] When heating the steel sheet at the aforementioned heating temperature, in order to ensure sufficient productivity and uniform alloying of the coating throughout the entire steel sheet (coil), the average heating rate can be set to 20–100 °C / hour, based on the temperature of the steel sheet (coil) across the entire temperature range (from room temperature to the heating temperature). Furthermore, the overall average heating rate can be controlled within the numerical range described above; however, in one specific embodiment of the present invention, the heating rate within a specific temperature range is controlled together as described below to achieve the technical problem of the present invention. In another specific embodiment of the present invention, the average heating rate across the entire temperature range can be set to 70 °C / hour.

[0089] During heating, the average heating rate in the 400-500℃ range is set to 1-15℃ / hour.

[0090] In one specific embodiment of the present invention, in order to prevent the residual rolling oil in the temperature range from being vaporized during rolling and causing surface stains, and to ensure sufficient productivity, the average heating rate in the 400-500°C range can be set to 1-15°C / hour. In one specific embodiment of the present invention, the lower limit of the average heating rate in the 400-500°C range can be set to 4°C / hour; in another specific embodiment, the lower limit of the average heating rate in the 400-500°C range can be set to 5°C / hour.

[0091] During the heating process, the average heating rate within the range of -50℃ to the heating temperature is set to 1–15℃ / hour. heating

[0092] To prevent adhesion during alloying (a surface defect caused by adhesion between coils due to surface alloying) and to ensure sufficient productivity while fully forming cavities, the average heating rate within the range of -50°C to the heating temperature can be set to 1–15°C / hour. In one specific embodiment of the invention, the lower limit of the average heating rate within this range can be set to 4°C / hour; in another specific embodiment, the lower limit of the average heating rate within this range can be set to 5°C / hour.

[0093] The temperature difference between the atmosphere and the steel plate in the bell-type annealing furnace is set to 5–80℃.

[0094] In typical bell-type annealing furnaces, heating is achieved by raising the temperature of the atmosphere within the furnace, rather than directly heating the steel sheet (coil). In this case, a temperature difference between the atmosphere and the steel sheet is unavoidable. However, to minimize material and coating quality variations at different locations within the steel sheet, and based on the time required to reach the target heat treatment temperature, the temperature difference between the atmosphere and the coil can be set below 80°C. Ideally, the temperature difference should be as small as possible, but this would slow down the heating rate, making it difficult to meet the overall average heating rate requirement. Therefore, considering this, the temperature difference between the atmosphere and the steel sheet can be set above 5°C. Here, the steel sheet temperature refers to the temperature measured at the bottom (lowest part) of the loaded steel sheet (coil), while the atmosphere temperature refers to the temperature measured at the center of the furnace's interior space.

[0095] [Cooling Process]

[0096] After annealing, cool to 500°C at a rate of less than 50°C / hour.

[0097] After holding at the target temperature for a certain period, the aluminized alloy steel sheet (coil) is cooled. Various cooling methods can be used, such as furnace cooling, air cooling, and water cooling. There is no particular limitation on the average cooling rate throughout the cooling range; rapid cooling can be employed to improve productivity. However, to prevent adhesion defects and ensure material uniformity while fully forming cavities, the cooling rate in the temperature range from heating to 500°C can be set below 50°C / hour. There is no specific limitation on the lower limit of the cooling rate, but considering productivity, the lower limit can be set above 1°C / hour. Detailed Implementation

[0098] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the contents of the claims and the contents reasonably deduced therefrom.

[0099] (Example)

[0100] Steel plate manufacturing

[0101] Invention Example 1

[0102] Prepare cold-rolled steel sheets for hot pressing with the compositions shown in Table 1 below. Coat the surface of the steel sheets using a Type I coating bath with a composition of Al-9% Si-2.5% Fe. The coating amount is 70 g / m² per side. 2 To adjust, the winding tension after plating is adjusted to 2.2 kg / mm. 2 And then roll it up into a sheet.

[0103] [Table 1]

[0104] element C Si Mn Al P S N Additional elements content(%) 0.21 0.2 1.3 0.03 0.01 0.003 0.005 Ti: 0.03, B: 0.002, Cr: 0.2

[0105] In a bell-type annealing furnace, the plated steel sheet is heated to 650°C under the following conditions.

[0106] Overall average heating rate to 650℃: 20℃ / hour

[0107] Average heating rate in the 400–500℃ temperature range: 10℃ / hour

[0108] Average heating rate in the 600–650℃ temperature range: 10℃ / hour

[0109] The temperature difference between the atmosphere and the coil at the heating temperature: 30℃

[0110] After heating, the steel sheet is held at the same temperature for 10 hours. Then, the steel sheet is cooled to 500°C at an average cooling rate of 40°C / hour, and then cooled to 100°C at an average cooling rate of 55°C / hour to obtain a hot-pressed steel sheet.

[0111] The results of analyzing the coating of steel plates using a GDS analyzer can yield results such as... Figure 2 The compositional distribution shown is such that the average Fe content calculated is 51.5% by weight. The cross-sectional shape of the steel plate is as follows. Figure 3 As shown, it can be confirmed that a coating is formed on the outside of the base steel plate, and the area ratio of the cavities formed in the surface layer from the surface of the coating to a position of 10 μm in the thickness direction is 22.8%.

[0112] Invention Example 2

[0113] The surface of a steel sheet having the composition described in Table 1 was coated using a Type I plating bath with a composition of Al-9%Si-2.5%Fe. The coating amount during plating was 80 g / m² per side. 2 Adjust the winding tension after plating to 2 kg / mm. 2 And then roll it up into a sheet.

[0114] Then, in a bell-type annealing furnace, the plated steel sheet is heated to 700°C under the following conditions.

[0115] Overall average heating rate to 700℃: 20℃ / hour

[0116] Average heating rate in the 400–500℃ temperature range: 12℃ / hour

[0117] Average heating rate in the 650–700℃ temperature range: 8℃ / hour

[0118] The temperature difference between the atmosphere and the steel plate at the heating temperature: 40℃

[0119] After heating, the steel plate is held at the same temperature for 1 hour. Then, the steel plate is cooled to 500°C at an average cooling rate of 30°C / hour, and then cooled to 100°C at an average cooling rate of 57°C / hour to obtain a hot-pressed steel plate.

[0120] The results of analyzing the coating of steel plates using a GDS analyzer can yield results such as... Figure 4 The compositional distribution shown is such that the average Fe content calculated is 53.7% by weight. The cross-sectional shape of the steel plate is as follows. Figure 5 As shown, it can be confirmed that a coating is formed on the outside of the base steel plate, and the area ratio of the cavities formed in the surface layer from the surface of the coating to a position of 10 μm in the thickness direction is 28.5%.

[0121] Comparative Example 1

[0122] Aluminized steel sheet, identical to the one described in Invention Example 1 but only subjected to plating without heating or cooling, is used as Comparative Example 1.

[0123] The results of analyzing the coating of steel plates using a GDS analyzer can yield results such as... Figure 6 The compositional distribution shown is such that the average Fe content calculated is 23.6% by weight. The cross-sectional shape of the steel plate is as follows. Figure 7 As shown, it can be confirmed that a coating is formed on the outside of the base steel plate, and almost no voids are formed in the surface layer from the surface of the coating to a position of 10 μm in the thickness direction. The area ratio of the formed voids is 0%.

[0124] Comparative Example 2

[0125] Aluminized steel sheet, identical to the one described in Invention Example 2 but only subjected to plating without heating or cooling, is used as Comparative Example 2.

[0126] The results of analyzing the coating of steel plates using a GDS analyzer can yield results such as... Figure 8 The compositional distribution shown is such that the average Fe content calculated is 21% by weight. The cross-sectional shape of the steel plate is as follows: Figure 9 As shown, it can be confirmed that a coating is formed on the outside of the base steel plate, and almost no voids are formed in the surface layer from the surface of the coating to a position of 10 μm in the thickness direction. The area ratio of the formed voids is 0%.

[0127] Comparative Example 3

[0128] The surface of a steel sheet having the composition described in Table 1 was coated using a Type I plating bath with a composition of Al-9%Si-2.5%Fe. The coating amount during plating was 90 g / m² per side. 2 Adjust the winding tension after plating to 2 kg / mm. 2 And then roll it up into a sheet.

[0129] Then, in a bell-type annealing furnace, the plated steel sheet is heated to 650°C under the following conditions.

[0130] Overall average heating rate to 650℃: 50℃ / hour

[0131] Average heating rate in the 400–500℃ temperature range: 10℃ / hour

[0132] Average heating rate in the 600–650℃ temperature range: 70℃ / hour

[0133] The temperature difference between the atmosphere and the steel plate at the heating temperature: 30℃

[0134] After heating, the steel sheet is held at the same temperature for 10 hours. Then, the steel sheet is cooled to 500°C at an average cooling rate of 45°C / hour, and then cooled to 100°C at an average cooling rate of 60°C / hour to obtain a hot-pressed steel sheet.

[0135] The results of analyzing the coating of steel plates using a GDS analyzer can yield results such as... Figure 10 The compositional distribution shown is such that the average Fe content calculated is 48.4% by weight. The cross-sectional shape of the steel plate is as follows. Figure 11 As shown, it can be confirmed that a coating is formed on the outside of the base steel plate, and the area ratio of the cavities formed in the surface layer at a position of 10 μm in the thickness direction from the surface of the coating is 3.5%.

[0136] Hot pressing

[0137] The steel plates of the Invention Examples 1 and 2 and Comparative Examples 1 to 3 were heated to 950°C, held at the temperature for 5 minutes, and then hot-pressed by stamping and rapid cooling to obtain hot-pressed parts.

[0138] The surface roughness (Ra) was observed by examining the cross-section of the obtained component, and the results are shown in Table 2 below.

[0139] [Table 2]

[0140] distinguish Surface roughness (Ra) Invention Example 1 2.01 Invention Example 2 2.23 Comparative Example 1 1.12 Comparative Example 2 1.27 Comparative Example 3 1.48

[0141] As can be seen from Table 2, the surface roughness (Ra) of Invention Example 1 and Invention Example 2 is 2.01 μm and 2.23 μm, respectively, but the surface roughness (Ra) of Comparative Example 1, Comparative Example 2 and Comparative Example 3 is only 1.12 μm, 1.27 μm and 1.48 μm, respectively.

[0142] The components obtained in the various inventive examples and comparative examples were subjected to phosphate treatment and electrodeposition coating to form cross-shaped scratches on the surface of the steel plate. A cyclic corrosion test was then conducted to observe the degree of blistering at the cross-shaped scratches. In the cyclic corrosion test, one cycle consisted of 24 hours of exposure: 2 hours in a humid atmosphere - 2 hours in a salt spray - 1 hour drying - 6 hours in a humid atmosphere - 2 hours drying - 6 hours in a humid atmosphere - 2 hours drying - 3 hours cooling, for a total of 50 cycles. It was confirmed that the maximum width of all blistering in Inventive Examples 1 and 2 was less than 1 mm, but the maximum width of blistering in Comparative Examples 1, 2, and 3 was 3.2 mm, 2.9 mm, and 2.4 mm, respectively, indicating poorer corrosion resistance after coating compared to the inventive examples.

[0143] Therefore, the advantageous effects of the present invention can be confirmed.

Claims

1. A method for manufacturing a steel plate for hot-pressed parts, comprising the following steps: The surface of the base steel plate is aluminized and then coiled to obtain an aluminized steel plate; The aluminized steel sheet is annealed to obtain an aluminized alloy steel sheet; and The aluminum-coated steel sheet is cooled. Specifically, taking one side of the base steel plate as a reference, the aluminum coating amount is 30~200g / m². 2 , The winding tension during winding should be set to 0.5~5 kg / mm. 2 , The annealing is carried out in a bell-type annealing furnace at a heating temperature range of 550~750℃ for 30 minutes to 50 hours. During the annealing process, the average heating rate from room temperature to the specified heating temperature is set to 20~100℃ / hour. The temperature difference between the atmosphere and the steel plate in the bell-type annealing furnace is set to 5~80℃. In the step of cooling the aluminized alloy steel sheet, the temperature is cooled to 500°C at a rate of less than 50°C / hour.

2. The method for manufacturing steel plates for hot-pressed components according to claim 1, wherein, The base steel plate, by weight percent, has a composition comprising the following components: C: 0.04-0.5%, Si: 0.01-2%, Mn: 0.01-10%, Al: 0.001-1.0%, P: less than 0.05%, S: less than 0.02%, N: less than 0.02%, with the balance being Fe and other unavoidable impurities.

3. The method for manufacturing steel plates for hot-pressed components according to claim 2, wherein, The composition of the base steel plate, by weight percent, further comprises one or more of the following: 0.01 to 4.0% of one or more selected from Cr, Mo and W; 0.001 to 0.4% of one or more selected from Ti, Nb, Zr and V; 0.005 to 2.0% of Cu+Ni; 0.001 to 1.0% of Sb+Sn; and 0.0001 to 0.01% of B.

4. A hot-pressed component comprising a base steel plate and an aluminum alloy coating formed on the surface of the base steel plate, wherein the base steel plate has a composition comprising, by weight percent: C： 0.04~0.5%, Si: 0.01~2%, Mn: 0.01~10%, Al: 0.001~1.0%, P: less than 0.05%, S: less than 0.02%, N: less than 0.02%, balance Fe and other unavoidable impurities. Relative to the total area of ​​the surface layer, the proportion of voids in the cross-section of the surface layer observed when the aluminum alloy coating is cut along its thickness direction is 10% or more, wherein the surface layer is the area extending from the surface of the aluminum alloy coating to a depth of 10 μm or less. The average Fe content of the aluminum alloy coating is 30% by weight or more.

5. The hot-pressed forming component according to claim 4, wherein, The proportion of voids in the cross-section of the surface portion observed when the aluminum alloy coating is cut along the thickness direction relative to the total area of ​​the surface portion is 15% or more.

6. The hot-pressed component according to claim 5, wherein, The proportion of voids in the cross-section of the surface portion observed when the aluminum alloy coating is cut along the thickness direction relative to the total area of ​​the surface portion is less than 70%.

7. The hot-pressed forming component according to claim 6, wherein, The average Fe content of the aluminum alloy coating is 40% by weight or more.

8. The hot-pressed component according to claim 7, wherein, The average Fe content of the aluminum alloy coating is below 80% by weight.

9. The hot-pressed forming component according to claim 4, wherein, The composition of the base steel plate, by weight percent, further comprises one or more of the following: 0.01 to 4.0% of one or more selected from Cr, Mo and W; 0.001 to 0.4% of one or more selected from Ti, Nb, Zr and V; 0.005 to 2.0% of Cu+Ni; 0.001 to 1.0% of Sb+Sn; and 0.0001 to 0.01% of B.

10. A hot-pressed component comprising a base steel plate containing martensite and an aluminum alloy coating formed on the surface of the base steel plate. Relative to the total area of ​​the surface layer, the proportion of voids in the cross-section of the surface layer observed when the aluminum alloy coating is cut along its thickness direction is more than 10%, wherein... The surface layer is the area extending from the surface of the aluminum alloy coating to a depth of less than 10 μm. The average Fe content of the aluminum alloy coating is 30% by weight or more.

11. The hot-pressed forming component according to claim 10, wherein, The proportion of voids in the cross-section of the surface portion observed when the aluminum alloy coating is cut along the thickness direction relative to the total area of ​​the surface portion is 15% or more.

12. The hot-pressed forming component according to claim 11, wherein, The proportion of voids in the cross-section of the surface portion observed when the aluminum alloy coating is cut along the thickness direction relative to the total area of ​​the surface portion is less than 70%.

13. The hot-pressed component according to claim 12, wherein, The average Fe content of the aluminum alloy coating is 40% by weight or more.

14. The hot-pressed component according to claim 13, wherein, The average Fe content of the aluminum alloy coating is below 80% by weight.

15. The hot-pressed component according to claim 10, wherein, The base steel plate, by weight percent, has a composition comprising the following components: C: 0.04-0.5%, Si: 0.01-2%, Mn: 0.01-10%, Al: 0.001-1.0%, P: less than 0.05%, S: less than 0.02%, N: less than 0.02%, with the balance being Fe and other unavoidable impurities.

16. The hot-pressed component according to claim 15, wherein, The composition of the base steel plate, by weight percent, further comprises one or more of the following: 0.01 to 4.0% of one or more selected from Cr, Mo and W; 0.001 to 0.4% of one or more selected from Ti, Nb, Zr and V; 0.005 to 2.0% of Cu+Ni; 0.001 to 1.0% of Sb+Sn; and 0.0001 to 0.01% of B.

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