Hot press molded article and method for manufacturing same

Abstract

Provided are: a hot press-molded article provided with a Zn-based plating layer and an oxidized Zn layer on the surface of the Zn-based plating layer on the surface of a steel material, the upper layer of the Zn-based plating layer having a two-phase structure of a gamma-phase and an Fe-Zn solid solution, and the lower layer other than the upper layer having a single-phase structure of an Fe-Zn solid solution; the root mean square slope R [delta] q of the roughness curve of the surface of the hot press-molded article is 0.25 or less, and the area ratio of the Zn oxide layer on the surface of the hot press-molded article is 30% or less.

Description

Technical Field

[0001] The present invention relates to a hot stamped product, more specifically to a hot stamped product using a Zn-plated steel sheet and a method for producing the same, and particularly to a hot stamped product having excellent coating film adhesion and a method for producing the same. Background Art

[0002] In recent years, in the field of automotive components, the demand for high strength has increased in order to improve fuel efficiency and collision safety. As a solution to this problem, the application of hot stamping technology is expanding. Hot stamping refers to a technology that produces high-shape-freezing and high-strength stamped products by stamping a blank heated to a temperature above the austenite single-phase region (Ac3 point), for example, around 900°C, and simultaneously utilizing rapid cooling (quenching) from the die during hot forming. In addition, due to the increasing demand for high rust prevention, hot stamped products formed from Zn-plated steel sheets are being developed.

[0003] In hot stamped products formed from Zn-plated steel sheets, as shown in Patent Document 1, the formation of an iron oxide film is suppressed, and Zn components remain on the surface of the steel sheet after hot stamping. Therefore, compared with hot stamped products formed from non-plated steel sheets, the corrosion resistance after forming is excellent due to the sacrificial corrosion protection effect.

[0004] However, if Figure 1 As shown, during the hot stamping process, the Zn coating becomes a Γ phase 1, and a portion of the Zn in the coating diffuses into the matrix to become a Fe-Zn solid solution 2. On the other hand, the remaining Zn changes into the Zn oxide layer 3, and a gap 4 is generated between the Γ phase 1 and the Zn oxide layer 3. Therefore, salt water can easily penetrate, resulting in a decrease in the adhesion with the coating 5, which becomes a problem.

[0005] To address this issue, Patent Document 2, for example, discloses that the Zn-plated steel sheet surface can be irradiated with a laser while appropriately adjusting output conditions to remove the Zn oxide layer 3, thereby improving coating adhesion. Furthermore, Patent Documents 3 and 4 disclose that removing the Zn oxide layer 3 by shot peening with steel balls can also improve coating adhesion.

[0006] However, for the hot stamped products described above, when using a Zn-plated steel sheet containing, in addition to the Fe-Zn solid solution 2, a Zn-Fe intermetallic compound (Γ phase 1), which has a high Zn concentration and high sacrificial corrosion protection, the corrosion resistance after coating is superior to that of a Zn-plated steel sheet containing only the Fe-Zn solid solution 2. For example, as described in Patent Documents 5 and 6, the Γ phase contains 10 to 30 mass% of Fe. Generally, the Γ phase 1 comprises approximately 70 to 85 mass% Zn and 15 to 30 mass% Fe, while the Fe-Zn solid solution comprises 10 to 40 mass% Zn and 60 to 90 mass% Fe.

[0007] In addition, a hot stamped product formed from a Zn-plated steel sheet having an Fe-Zn solid solution 2 and a Γ phase 1 can be obtained by heating to 782°C, where the Fe phase is formed in the solid phase, maintaining the temperature for a certain period of time in the mixed temperature range of solid Fe and liquid phase (780-900°C), and then cooling to 780°C before starting hot stamping to precipitate the remaining liquid phase as Γ phase 1, thereby distributing the spherical Fe-Zn solid solution 2 in the matrix. Examples of heating methods used in the manufacturing process of hot stamped products include electric heating and furnace heating.

[0008] In the case of electric heating, the steel plate can be heated rapidly. On the other hand, uniform heating of the steel plate surface and control of the current value are difficult, and there are many manufacturing constraints. Therefore, furnace heating is preferred. Even for hot stamped products formed from Zn-plated steel plates containing Γ phase 1, excellent coating film adhesion is required when it is envisioned to be used in components with severe corrosive environments. By using Zn-plated steel plates containing Γ phase 1, the corrosion resistance after coating is improved. However, as with Fe-Zn solid solution Zn-plated steel plates, during the hot stamping process, a Zn oxide layer 3 with large unevenness is formed on the surface of the Zn-plated layer 9. Therefore, gaps 4 are generated between the Zn-plated layer 9 and the Zn oxide layer 3, making it easier for salt water to penetrate. Furthermore, the chemical conversion treatability is also reduced, so there is a problem with coating film adhesion.

[0009] As a method for improving the coating adhesion of hot stamped products formed from Zn-plated steel sheets containing the Γ phase 1, grinding of the Zn oxide layer 3 by laser ablation or blasting is conceivable, similar to Zn-plated steel sheets containing only the Fe-Zn solid solution 2. However, laser ablation equipment is not universally available, and while shot blasting using steel balls and the like has been studied in detail in the prior art, other blasting treatments have not necessarily been fully studied.

[0010] Furthermore, Patent Document 7 discloses that coating adhesion can be improved by forming a chemical conversion coating comprising one or more of titanium oxide, nickel oxide, and tin oxide and a resin on the surface of a Zn-plated steel sheet for hot pressing. Thus, various solutions have been proposed in the prior art to address the issue of coating adhesion in hot-stamped products using Zn-plated steel sheets. However, the demand for high rust resistance has also increased, and there remains a high demand for hot-stamped products that can address this issue.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent No. 3582504

[0014] Patent Document 2: Japanese Patent No. 6211908

[0015] Patent Document 3: Japanese Patent No. 4085876

[0016] Patent Document 4: Japanese Patent No. 5880321

[0017] Patent Document 5: Japanese Patent No. 4072129

[0018] Patent Document 6: Japanese Patent No. 4695459

[0019] Patent Document 7: Japanese Patent No. 6631623 Summary of the Invention

[0020] Problems to be solved by the invention

[0021] Therefore, an object of the present invention is to provide a hot stamped product using a Zn-based plated steel sheet having a novel structure and excellent coating film adhesion, and a method for producing the same.

[0022] Means used to solve problems

[0023] To improve the coating film adhesion of hot-stamped products formed from Zn-plated steel sheets containing the aforementioned Γ phase 1, the inventors investigated various conditions for blasting treatment. As a result, they successfully removed the Zn oxide layer 3 and smoothed the surface, achieving excellent coating film adhesion. The present invention, which achieves the aforementioned objectives, is as follows.

[0024] (1) A hot stamped product comprising a Zn-based plating layer and a Zn oxide layer on the surface of a steel material, wherein an upper layer of the Zn-based plating layer has a two-phase structure of a Γ phase and an Fe-Zn solid solution, and a lower layer other than the upper layer has a single-phase structure of an Fe-Zn solid solution.

[0025] The root mean square slope RΔq of the roughness curve of the surface of the hot stamped product is 0.25 or less, and the area ratio of the Zn oxide layer on the surface of the hot stamped product is 30% or less.

[0026] (2) A method for manufacturing a hot stamped product according to (1) above, characterized in that it includes wet blasting the hot stamped product using a sand-like medium.

[0027] Effects of the Invention

[0028] According to the present invention, a hot stamped product using a Zn-based plated steel sheet having a novel structure and excellent coating film adhesion and a method for producing the same can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The diagram shows a situation in which the Zn-based plating layer of a hot stamped product has a lower layer and an upper layer, and voids are formed between the Fe-Zn solid solution and the Γ phase, and between the Γ phase and the Zn oxide layer in the Zn plating layer, resulting in reduced coating film adhesion.

[0030] Figure 2 This is a graph summarizing the correlation between RΔq and coating film adhesion evaluation results.

[0031] Figure 3 This is a graph summarizing RΔq and the area ratio of the Zn oxide layer, which are conditions required for improving coating film adhesion.

[0032] Figure 4 This figure shows the mechanism of removing the Zn oxide layer assumed when dry blasting is performed using alumina sand as a medium.

[0033] Figure 5 This figure shows the mechanism of removing the Zn oxide layer assumed when dry blasting is performed using steel balls with an average particle size of 300 μm.

[0034] Figure 6 The figures show SEM images (with grids) of the surface of (left) an untreated material and (right) a surface subjected to wet blasting treatment using steel grit as a medium. DETAILED DESCRIPTION

[0035] The inventors studied various conditions for blasting to improve the coating adhesion of hot stamped products formed from Zn-plated steel sheets containing the above-mentioned Γ phase. As a result, they found that surface smoothing and appropriate removal of the Zn oxide layer 3 improve the coating adhesion.

[0036] The inventors first focused on the root mean square slope RΔq of the roughness curve as a factor affecting the coating film adhesion of hot stamped products formed from Zn-based plated steel sheets containing the Γ phase. Figure 2 As shown in FIG. 1 , when blasting treatment is performed under various conditions, in most cases, if RΔq is 0.25 or less, the coating film adhesion is acceptable (A or AA). This is presumably because Figure 1 The Zn oxide layer 3 with large surface irregularities mentioned above is appropriately removed by the blasting treatment, and the voids 4 that are the cause of the easy intrusion of salt water and the reduction of the coating film adhesion disappear. Figure 2 As shown in the circled portion, even when RΔq is 0.25 or less, the coating film adhesion is at an unacceptable level, and therefore it is difficult to adjust the coating film adhesion only by RΔq.

[0037] Therefore, the inventors conducted intensive research and found that, in addition to the above-mentioned RΔq, the area ratio of the Zn oxide layer in the entire surface of the Zn-plated hot stamped product affects the coating film adhesion. Figure 3 The appropriate range of the ratio of RΔq to the area of ​​the Zn oxide layer will be described. Figure 3 The symbols ◎, ○, △, and × in the table represent the evaluation of coating adhesion. A coating peeling area ratio of 30% or more is indicated by an ×, a coating peeling area ratio of 20% or more but less than 30% is indicated by a △, a coating peeling area ratio of 10% or more but less than 20% is indicated by an ○, and a coating peeling area ratio of less than 10% is indicated by a ◎.

[0038] First, if Figure 3 As shown in region I, when RΔq exceeds 0.25, the coating film adhesion is poor regardless of the area ratio of the Zn oxide layer. This is presumably because, as described above, Figure 1The Zn oxide layer 3, with its large surface irregularities, cannot be completely removed, allowing salt water to penetrate through the gaps 4, resulting in reduced coating adhesion. Furthermore, in Region II, RΔq is below 0.25, while the area ratio of the Zn oxide layer exceeds 30%, resulting in poor coating adhesion. This is presumably because, as described later, while the surface is smoothed, the Zn oxide layer is not fully removed and is crushed on the plated surface, causing the coating to peel off starting at the interface between the crushed Zn oxide layer and the plated coating. Furthermore, in Region III, RΔq is below 0.25, and the area ratio of the Zn oxide layer is below 30%, resulting in good coating adhesion.

[0039] As described above, conditions for improving coating film adhesion require an RΔq value of 0.25 or less and an area ratio of the Zn oxide layer of 0% to 30% or less. Furthermore, from the perspective of improving coating film adhesion, a smaller RΔq value is preferred, and may be, for example, 0.24 or less, 0.22 or less, 0.20 or less, or 0.18 or less. Similarly, from the perspective of further improving coating film adhesion, a smaller area ratio of the Zn oxide layer is preferred, and may be, for example, 28% or less, 25% or less, 22% or less, less than 20%, 18% or less, or 15% or less.

[0040] Next, a method for producing a Zn-plated hot stamped product having an RΔq value of 0.25 or less and an area ratio of the Zn oxide layer of 0% to 30% that satisfies the requirements of the present invention will be described.

[0041] In the prior art of blasting using sand-like media, it has been considered difficult to select the treatment conditions. However, if the Zn oxide layer can be sufficiently removed, the surface will become smooth and the coating adhesion will be steadily improved. Therefore, for example, when aluminum oxide sand is used as the medium for blasting, Figure 4 As shown, compared with the case of using ordinary steel, the removability of the Zn oxide layer is excellent, the surface becomes smoother, and the coating adhesion is steadily improved. However, in the case of general dry blasting, the aluminum oxide sand of the medium remains on a part of the plated surface after the blasting, and the coating adhesion and corrosion resistance after coating are mostly poorer than those of the wet blasting materials described later. It should be noted that sand refers to an angular shape that does not include a sphere. Therefore, the sand-like medium is not particularly limited in material as long as it is a sand-shaped medium, that is, a medium with an angular shape. For example, the sand-like medium can be at least one of aluminum oxide, steel, silicon carbide, ceramics and silicon dioxide. The conditions for the blasting treatment are not particularly limited and can be the conditions commonly used in this technical field.

[0042] (Wet Blasting)

[0043] When the blasting treatment is performed using a sandy medium, the blasting treatment is performed by wet blasting. Wet blasting is also known as wet blasting or liquid honing, and generally uses a liquid (mainly water, and a rust inhibitor may also be added) containing sandy steel with a particle size of 50 to 300 μm as a medium. When the blasting treatment method is implemented under appropriate conditions, the surface smoothing of the Zn-coated hot stamped product and the removal of the Zn oxide layer are fully carried out. For example, the projection pressure of the wet blasting treatment, more specifically the pressure of the compressed air used to spray the liquid containing the medium from the spray nozzle, can be sufficient to remove the Zn oxide layer to the desired level, and is generally 0.10 to 8.00 MPa. The projection pressure can be greater than 0.20 MPa or greater than 0.30 MPa. Similarly, the projection pressure can be less than 6.00 MPa, less than 5.00 MPa, less than 3.00 MPa, less than 1.00 MPa, less than 0.80 MPa or less than 0.50 MPa. However, when the projection pressure is too high, for example, when the projection pressure exceeds 8.00 MPa, although the Zn oxide layer can be sufficiently removed, the surface properties of the hot stamped product deteriorate, and sometimes it is impossible to achieve surface smoothness and thus the desired RΔq. In addition, the projection time of the sand-like medium is not particularly limited, and can be, for example, 1.5 to 80.0 seconds, 2.0 to 60.0 seconds, or 2.0 to 30.0 seconds. Furthermore, in dry spraying, in which the medium is projected without using a liquid, the projected medium may remain on the surface of the Zn-coated hot stamped product and adversely affect the coating adhesion and corrosion resistance after painting. In contrast, in wet spraying, the projected medium flows due to the liquid, and no medium remains on the surface of the Zn-coated hot stamped product, so it does not adversely affect the coating adhesion and corrosion resistance after painting. When the projected medium remains on the surface of the Zn-based plated hot stamped product, the RΔq value tends to increase, and as a result, it is considered that the coating film adhesion decreases.

[0044] (Comparative case of steel ball shot peening)

[0045] As a result of dry shot blasting using steel balls with an average particle size of 300 μm as a common shot blasting method, it was found that Figure 5 As shown in FIG, although it is effective for surface smoothing, depending on the treatment conditions, the Zn oxide layer is not fully removed and is crushed on the plated surface, and the coating film peels off starting from the interface between the crushed Zn oxide layer and the plated surface, thereby reducing the adhesion of the coating film. Here, shot blasting refers to a blasting process using spherical media such as steel balls. Therefore, in the dry blasting process using steel balls, Figure 3 In the region II, although RΔq is small, excellent coating film adhesion cannot be obtained because the area ratio of the Zn oxide layer is large.

[0046] As can be seen from the above description, in order to achieve the desired coating adhesion and, consequently, the desired post-paint corrosion resistance, blasting alone after hot stamping is insufficient. It is extremely important to perform wet blasting using a sand-like medium. By performing wet blasting using a sand-like medium, unlike dry shot blasting using steel balls, the Zn oxide layer is not crushed on the plated surface, and this Zn oxide layer can be appropriately removed. Furthermore, by performing wet blasting rather than dry blasting, the medium does not remain on the surface of the Zn-plated hot stamped part and adversely affect values ​​such as RΔq. Therefore, by performing wet blasting using a sand-like medium, RΔq can be reliably controlled to below 0.25 and the area ratio of the Zn oxide layer to below 30%. As a result, coating adhesion and, consequently, post-paint corrosion resistance can be significantly improved.

[0047] Next, the measuring method and the analyzing method of the requirements of the present invention are described.

[0048] (1) Determination of the root mean square slope RΔq of the roughness curve

[0049] RΔq is measured based on ISO4287-1997. The measurement is performed using a stylus roughness meter manufactured by Tokyo Seimitsu Co., Ltd. with a tip diameter of 5 μm at a scanning speed of 0.25 mm / s. The surface roughness measurement conditions are set to a reference length of 0.8 mm and an evaluation length of 35 mm. It should be noted that, among the various surface roughness indices (arithmetic mean roughness Ra, maximum height roughness Rz, root mean square slope RΔq of the roughness curve) of the surface of the hot stamped product of the present invention, there is no correlation between Ra and Rz, which are general roughness indices, and the coating adhesion, and only a correlation is observed between RΔq and the coating adhesion. It is speculated that this is because Ra and Rz are values ​​indicating the roughness in the direction perpendicular to the surface of the steel plate, while RΔq is a value indicating the inclination of the outermost layer relative to the surface of the steel plate, so only RΔq reflects the "crushed morphology of the Zn oxide layer".

[0050] (2) Determination of the area ratio of the Zn oxide layer

[0051] The surface of the hot stamped product after the spray treatment is observed using a BSE image of an SEM at an accelerating voltage of 20 kV within a specified area (1.3 mm × 1.9 mm) magnified 100 times (for example, using a JSM-6610A manufactured by JEOL Ltd.). In the SEM-BSE image, elements with large atomic weights are observed with bright contrast (whiter), so the Zn-based plating layer and the Zn oxide layer can be distinguished based on the difference in contrast. Specifically, the Zn-based plating layer containing a large amount of Zn is observed as white, and the Zn oxide layer is observed as black.

[0052] Then, in order to determine the area ratio of the Zn oxide layer, the SEM image observed at an acceleration voltage of 20 kV and 100 times was as follows: Figure 6 As shown, the grid is divided into two types: black part (the area ratio of black is 50% or more) and white part (the area ratio of white is 50% or more), and the number of each grid is counted. The area ratio of the black part can be calculated based on the ratio of the grid. It should be noted that for grids with the same area ratio of the black part and the white part, the grid with a spacing of 50μm×50μm can be further subdivided to more accurately measure its area fraction, but 0.5 grids can also be counted in the black part and the white part respectively. In the case where the black part and the white part cannot be judged by visual inspection, elemental analysis can also be performed by SEM-EDS in the area of ​​the grid to determine. The results of the elemental analysis are calculated by mass %. When the O content is 40.0% or more, it is counted as a black part, and when it is less than 40.0%, it is counted as a white part.

[0053] Next, the steel plate composition of the Zn-plated steel plate of the present invention is described. As described above, the object of the present invention is to provide a hot stamped product, which is a hot stamped product using a Zn-plated steel plate, and has excellent coating film adhesion. This object is achieved by controlling the root mean square slope RΔq of the roughness curve of the surface of the hot stamped product to be less than 0.25, and controlling the area ratio of the Zn oxide layer on the surface of the hot stamped product to be less than 30%. Therefore, it is clear that the chemical composition of the Zn-plated steel plate or steel material is not a necessary technical feature in achieving the purpose of the present invention. Therefore, the following description refers to a simple example of the preferred chemical composition of steel materials used in automotive components, etc., and is not intended to limit the present invention to the use of steel materials having such a specific chemical composition. In the hot stamping formed product of the present invention, the steel material preferably has, by mass%, C: 0.18% or more and 0.50% or less, Si: 0.10% or more and 1.50% or less, Mn: 0.50% or more and 2.50% or less, P: 0.000% or more and 0.100% or less, S: 0.0000% or more and 0.0100% or less, Al: 0.001% or more and 0.100% or less, N: 0.0000% or more and 0.0100% or less, Nb: 0.00% or more and 0.15% or less, Ti: 0.00% or more and 0.15% or less, The chemical composition includes V: 0.00% to 0.50%, Cr: 0.00% to 0.50%, Mo: 0.00% to 0.50%, Cu: 0.00% to 1.00%, Ni: 0.00% to 2.00%, W: 0.00% to 1.00%, Zr: 0.00% to 1.00%, B: 0.0000% to 0.0100%, the total of REM, Ca, Co, and Mg: 0.0000% to 0.0300%, and the balance: Fe and impurities. Each element is described in more detail below.

[0054] C: 0.18% or more and 0.50% or less

[0055] C is an element that increases the strength of Zn-plated hot-stamped products after hot stamping. If the C content in the steel is too low, the above effect cannot be achieved. Therefore, the lower limit of the C content in the steel is preferably set to 0.18%. On the other hand, if the C content in the steel is too high, the toughness of the steel plate decreases, so the upper limit of the C content is preferably set to 0.50%.

[0056] Si: 0.10% or more and 1.50% or less

[0057] Si is an element that improves the fatigue properties of hot stamped products. Furthermore, Si improves hot-dip zinc plating properties, particularly plating wettability, by forming a stable oxide film during recrystallization annealing in a continuous hot-dip zinc plating line. To achieve these effects, the Si content is 0.10% or greater. Preferably, it exceeds 0.14%, 0.15%, 0.18%, or 0.20%.

[0058] However, if the Si content in the steel is too high, the Si in the steel diffuses during hot stamping heating, forming oxides on the steel surface. These oxides reduce phosphate treatability. Si also increases the Ac3 point of the steel. If the Ac3 point increases, the heating temperature during hot stamping may exceed the evaporation temperature of Zn. Therefore, the upper limit of the Si content is preferably set to 1.50%. Preferably, it is 1.40% or less, 1.20% or less, or 1.00% or less.

[0059] Mn: 0.50% or more and 2.50% or less

[0060] Mn is an element that improves the hardenability of steel and increases the strength of Zn-coated hot stamped products. If the Mn content is too low, this effect cannot be achieved. In order to achieve this effect, the lower limit of the Mn content in the steel is preferably set to 0.50%. The preferred lower limit of the Mn content in the steel is 0.60% or 0.80%. On the other hand, if the Mn content is too high, the effect is saturated. Therefore, the upper limit of the Mn content in the steel is preferably set to 2.50%. The preferred upper limit of the Mn content in the steel is 2.30% or 2.00%.

[0061] P: 0.000% or more and 0.100% or less

[0062] Phosphorus (P) is an impurity contained in steel. P segregates at the grain boundaries of steel, reducing the steel's toughness and delayed fracture resistance. Therefore, the P content is set to 0.100% or less, preferably 0.050% or less. The lower limit of the P content is 0.000%, but may also be 0.001%.

[0063] S: 0.0000% or more and 0.0100% or less

[0064] S is an impurity contained in steel. S forms sulfides, reducing the toughness of steel and lowering its delayed fracture resistance. Therefore, the upper limit of the S content is 0.0100%. The S content is preferably as low as possible. The lower limit of the S content is 0.0000%, but may also be 0.0001%.

[0065] Al: 0.001% or more and 0.100% or less

[0066] Al is an element effective for deoxidizing steel. To achieve this effect, the lower limit of the Al content is set to 0.001% or higher. On the other hand, if the Al content is too high, the Ac3 point of the steel sheet increases, and the heating temperature required for hot stamping may exceed the evaporation temperature of the Zn-based coating. Therefore, the upper limit of the Al content in the steel is preferably set to 0.100%. It is more preferably 0.070% or less, or 0.050% or less.

[0067] N: 0.0000% or more and 0.0100% or less

[0068] N is an impurity inevitably contained in steel. N is an element that forms nitrides and reduces the toughness of steel. When N contains B, it combines with B to reduce the amount of solid-solution B. The amount of solid-solution B decreases, thereby reducing hardenability. Therefore, the N content of steel is preferably as low as possible. When the N content of steel exceeds 0.0100%, its influence becomes significant, so the upper limit of the N content of steel can be set to 0.0100%. The lower limit of the N content does not need to be specifically specified, and the lower limit of the N content is 0.0000%, or it can be 0.0001%.

[0069] The balance of the chemical composition of the steel material of this embodiment may be Fe and impurities. In this embodiment, impurities refer to impurities mixed from the raw material ore, waste or manufacturing environment and / or impurities allowed within the range that do not adversely affect the hot stamping product of this embodiment.

[0070] The steel material of this embodiment may contain at least one of the following elements as an optional element instead of a part of Fe. When the following elements are not contained, the content of each optional element is 0%.

[0071] Nb: 0.00% or more and 0.15% or less

[0072] Nb contributes to the strength of the steel sheet through carbide precipitation, and therefore can be included as needed. However, excessive Nb content generates a large amount of carbides, reducing the toughness of the steel sheet. Therefore, the Nb content can be set to 0.15% or less, 0.10% or less, or 0.05% or less. The lower limit of the Nb content is 0.00%, but may also be 0.01%.

[0073] Ti: 0.00% or more and 0.15% or less

[0074] Ti also contributes to the strength of the steel sheet through the precipitation of carbides, and therefore can be included as needed. However, excessive Ti content generates a large amount of carbides, reducing the toughness of the steel sheet. Therefore, the Ti content can be set to 0.15% or less, 0.10% or less, or 0.01% or less. The lower limit of the Ti content is 0.00%, but it can also be 0.001% or 0.005%.

[0075] V: 0.00% or more and 0.50% or less

[0076] V also contributes to the strength of the steel sheet through carbide precipitation, and therefore can be included as needed. However, excessive V content generates a large amount of carbides, reducing the toughness of the steel sheet. Therefore, the V content can be set to 0.50% or less. The lower limit of the V content is 0.00%, and can also be 0.01%.

[0077] Cr: 0.00% or more and 0.50% or less

[0078] Cr is an arbitrary element and may not be present. If present, Cr improves the hardenability of steel and affects its strength. To achieve this effect, the preferred lower limit of the Cr content in the steel is 0.01%. However, if the Cr content in the steel is too high, Cr carbides are formed, which become difficult to dissolve during hot stamping heating. Consequently, austenitization of the steel becomes difficult, reducing its hardenability. Therefore, the upper limit of the Cr content in the steel is preferably set to 0.50%.

[0079] Mo: 0.00% or more and 0.50% or less

[0080] Mo is an optional element and may not be present. If present, Mo improves the hardenability of the steel and affects its strength. To achieve this effect, the preferred lower limit of the steel's Mo content is 0.01%. However, if the Mo content is too high, the aforementioned effect saturates. Therefore, the upper limit of the steel's Mo content is preferably set to 0.50%.

[0081] Cu: 0.00% or more and 1.00% or less

[0082] Cu is an arbitrary element and may not be contained. When contained, Cu is an element that dissolves in the steel and can improve the strength without compromising toughness. To achieve this effect, the Cu content is preferably 0.01% or more. If the Cu content of the steel is too high, micro cracks may sometimes be generated on the surface during rolling. Therefore, the preferred upper limit of the Cu content is 1.00% or 0.60%, and more preferably 0.40% or 0.25%.

[0083] Ni: 0.00% or more and 2.00% or less

[0084] Ni is an optional element and may not be present. If present, Ni improves the toughness of the steel and also affects its strength. Furthermore, it suppresses embrittlement caused by liquid Zn during heating during hot stamping. To achieve these effects, the Ni content is preferably 0.01% or greater. If the Ni content of the steel is too high, these effects become saturated. Therefore, the preferred upper limit of the Ni content is 2.00%.

[0085] W: 0.00% or more and 1.00% or less

[0086] W is an optional element and may not be present. Therefore, the lower limit is 0.00%. When present, W affects strength improvement, grain control, crack prevention, and corrosion resistance. To achieve these effects, the W content is preferably 0.01% or higher. However, if the W content of the steel is too high, these effects become saturated. Therefore, the preferred upper limit of the W content is 1.00%.

[0087] Zr: 0.00% or more and 1.00% or less

[0088] Zr is an optional element and may not be present. Therefore, the lower limit is 0.00%. If present, Zr affects strength and corrosion resistance. To achieve these effects, the Zr content is preferably 0.01% or higher. However, if the Zr content of the steel is too high, these effects become saturated. Therefore, the preferred upper limit of the Zr content is 1.00%.

[0089] B: 0.0000% or more and 0.0100% or less

[0090] B is an optional element and may not be present. If present, B improves the hardenability of steel and the strength of Zn-plated hot-stamped products. To achieve this effect, the B content is preferably 0.0001% or more or 0.0005% or more. If the B content of the steel is too high, the effect becomes saturated. Therefore, the upper limit of the B content in the steel is preferably set to 0.0100%.

[0091] Total of REM, Ca, Co, and Mg: 0.0000% or more and 0.0300% or less

[0092] REM, Ca, Co, and Mg are arbitrary elements and may not be contained. Therefore, the lower limit is 0.00%. When contained, they are elements that control sulfides and oxides into preferred shapes, suppress the formation of coarse inclusions, and thus suppress the occurrence of roughness during spot welding. In order to reliably obtain this effect, the preferred lower limit of the total content of REM, Ca, Co, and Mg is 0.0003%, and the content of any one of REM, Ca, Co, and Mg can be 0.0003% or more. If the total content of REM, Ca, Co, and Mg is too high, the effect is saturated. Therefore, the upper limit of the total content of REM, Ca, Co, and Mg is preferably set to 0.0300%.

[0093] The chemical composition of the steel can be determined using standard analytical methods. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used. It should be noted that C and S are measured using the combustion-infrared absorption method, while N is measured using the inert gas fusion-thermal conductivity method. The surface coating can be removed by mechanical grinding before chemical composition analysis.

[0094] (Plating Structure of the Present Invention)

[0095] Next, the Zn-based plating structure of the present invention is described. First, the plating structure of the Zn-based plated hot stamped product of the present invention is as described above. Figure 1 The steel sheet shown in FIG. 1 includes a lower layer 7 and an upper layer 8. The upper layer 8, which is the surface layer side of the Zn-based plating layer 9, has a two-phase structure in which the Fe-Zn solid solution 2 is distributed in an island shape in the Γ phase 1. The lower layer 7, which is the steel sheet 6 side of the Zn-based plating layer 9, has a single-phase structure of the Fe-Zn solid solution 2.

[0096] (Plating composition of the present invention)

[0097] In addition, regarding the composition of the Zn-based plating in the present embodiment, the Zn content of the Zn-based plating is 30.0% or more by mass%. As needed, the lower limit of the Zn content can also be set to 35.0%, 40.0% or 50.0%. The upper limit of the Zn content is preferably set to 80.0%. As needed, the upper limit of the Zn content can also be set to 78.0% or 75.0%. The content of elements other than Zn does not need to be particularly specified, but the chemical composition of the Zn-based plating (excluding Zn) is, for example, preferably Fe: 20.0-70.0%, Al: 0-1.0%, Si: 0-1.0%, Mg: 0-1.0%, Mn: 0-1.0%, Ni: 0-1.0%, Sb: 0-1.0%, and the balance: impurities.

[0098] In addition, the Zn-based plating in the present invention refers to a range in which the Fe content is below 95.0%, and the position for analyzing the chemical composition of the Zn-based plating is the center of the thickness of the Zn-based plating (the center of the film thickness). The chemical composition analysis method is to measure the Fe content in the thickness direction of the Zn-based hot-stamped product (i.e., from the surface of the Zn-based hot-stamped product toward the center of the plate thickness) by GDS (glow discharge luminescence analysis) from the surface of the Zn-based hot-stamped product, and determine the range from the surface of the Zn-based hot-stamped product to the Fe content exceeding 95.0%. Then, the content of each element in the center of the distance from the position where the Fe content initially becomes 95.0% to the surface (this range is the Zn-based plating) (i.e., the center of the thickness of the Zn-based plating) is analyzed by GDS, and the analyzed value is used as the chemical composition of the Zn-based plating.

[0099] Since there is a Zn oxide layer on the Γ phase, the position where the Zn content reaches 80.0% (if there are multiple positions, the position closest to the surface) is considered to be the surface position of the Zn-based plating layer. However, if there is no region in the outermost layer of the hot stamped product where the Zn content exceeds 80.0%, the center of the Zn-based plating thickness (the center of the film thickness) is determined by considering the outermost layer as the surface position of the Zn-based plating.

[0100] (Plating Conditions of the Zn-Based Plated Steel Sheet of the Present Invention)

[0101] The coating weight is set to 60 g / m 2 Above and 150g / m 2 Below. Preferably 80g / m 2 If the coating weight is 80g / m 2 If the thickness is 150 g / m 2 Below, good appearance can be obtained after plating treatment, so it is preferred (plating adhesion is more than 150g / m 2 When molten Zn-based plating is performed, the plating sags and the appearance deteriorates, and sometimes the appearance of the hot stamped product also deteriorates). It should be noted that the Zn-based coating of the hot stamping steel material is a molten Zn-plated steel sheet (GI) in which the amount of oxides in the molded product during hot stamping is small. The alloyed molten Zn plating (GA) generates a large amount of oxides in the molded product during hot stamping, and sometimes the appearance deteriorates. Therefore, the hot stamping steel material as the raw material of the hot stamping molded product is preferably a molten Zn-plated steel sheet (GI). It should be noted that GA refers to an alloyed molten Zn-plated steel sheet, and GI refers to an unalloyed molten Zn-plated steel sheet.

[0102] Next, the various conditions for hot stamping performed before the blasting process will be described. Hot stamping can be performed under any appropriate conditions known to those skilled in the art, and therefore the various conditions for hot stamping are not particularly limited. Therefore, the following description is intended to simply illustrate preferred conditions for hot stamping and is not intended to limit the manufacturing method of the present invention to a manufacturing method that includes hot stamping performed under these specific conditions.

[0103] (Hot stamping heating conditions)

[0104] The hot stamping heating temperature is preferably set to be above the Ac3 point and below 950°C, and the heating time is preferably set to 240 seconds to 600 seconds. When the heating temperature is lower than Ac3, quenching becomes difficult. Therefore, the heating temperature is preferably above the Ac3 point. Regarding the Ac3 point (°C), a small piece is cut out from the steel plate and obtained based on the thermal expansion of the small piece during the process of heating from room temperature to 1000°C at 10°C / second. When the heating temperature is above 950°C, the surface oxidation (formation of Zn oxide layer) of the Zn-plated hot stamped product sometimes proceeds excessively. Therefore, the heating temperature is preferably less than 950°C. When the heating time is less than 240 seconds, quenching is sometimes not possible. Therefore, the heating time is preferably more than 240 seconds. When the heating time exceeds 600 seconds, the surface oxidation (formation of Zn oxide) of the Zn-plated hot stamped product sometimes proceeds excessively. Therefore, the heating time is preferably less than 600 seconds.

[0105] In hot stamping, a die with a cooling medium (e.g., water) circulating through it is typically used to stamp the hot stamping steel. During stamping, the heat removed from the die quenches the hot stamping steel. Through the above process, Zn-plated hot stamped products are manufactured.

[0106] (Hot stamping cooling conditions)

[0107] In order to make the upper layer of the Zn-based plating a two-phase structure of Γ phase and Fe-Zn solid solution and the lower layer a single phase of Fe-Zn solid solution, the temperature at which the hot stamping steel material is started (the rapid cooling start temperature) is preferably cooled below the lower limit of the temperature at which the liquid phase Zn contained in the Zn-based plating layer completely solidifies, that is, the plating solidification point (about 750°C). The specific temperature range can be easily determined by conducting preliminary tests, for example, to achieve the above-mentioned structure. Rapid cooling can be started from the temperature range thus determined, and hot stamping can be started.

[0108] If the average cooling rate from the quenching start temperature to 450°C is less than 20°C / s, sufficient strength may not be achieved. Therefore, the average cooling rate from the quenching start temperature to 450°C is preferably 20°C / s or higher. Furthermore, the average cooling rate from 450°C to 200°C is preferably 15°C / s or higher.

[0109] Example

[0110] Next, examples of the present invention are described. Table 1 shows examples of hot stamping heating conditions, spraying conditions, RΔq, the area ratio of the zinc oxide layer, and evaluations using a hot stamping heating condition, a molten zinc-plated steel sheet (GI) obtained from a steel sheet having a chemical composition of C: 0.19%, Si: 0.20%, Mn: 1.90%, Al: 0.030%, Ti: 0.03%, S: 0.0010%, P: 0.003%, and N: 0.0030% as a sample (base material).

[0111]

[0112] As the test materials, GI-HS material / Γ phase-retained type was used, while as the reference example (Comparative Example 1), GI-HS material / Fe-Zn solid solution type was used. Here, HS stands for hot stamping, Γ phase-retained type refers to Zn-plated steel sheets that contain, in addition to the Fe-Zn solid solution, a Zn-Fe intermetallic compound (Γ phase) with a high Zn concentration and high sacrificial corrosion protection, while Fe-Zn solid solution type refers to molten Zn-plated steel sheets consisting solely of an Fe-Zn solid solution formed by diffusion of a portion of the Zn plating into the substrate during the hot stamping process. More specifically, Sample No. 1 in Table 1 has a Zn-based plating layer composed solely of an Fe-Zn solid solution, while Samples Nos. 2 to 23 have Zn-based plating layers with an upper layer consisting of a dual-phase structure of the Γ phase and the Fe-Zn solid solution, and a lower layer consisting of a single-phase structure of the Fe-Zn solid solution. Furthermore, all the Zn-based plating layers in Samples No. 1 to 23 have a chemical composition consisting of, in mass %, Zn: 34.5 to 69.5%, Fe: 30.0 to 65.0%, Al: 0.1 to 0.9%, and the balance: impurities.

[0113] It should be noted that the blasting conditions for the "steel" "ball" were performed using a direct pressure device (BA-1 model, manufactured by Atsugi Metal Works Co., Ltd.) (air pressure 0.35 MPa), and the "aluminum oxide" "sand" was dry blasted using a suction device (BS-1 model, manufactured by Atsugi Metal Works Co., Ltd.), with an air pressure of 0.20 MPa and a projection distance of 150 mm. For the wet blasting, a W8MN-Q008, manufactured by Moko Co., Ltd., was used, with a projection distance of 150 mm and an air pressure of 0.25 MPa. The blasting time was performed according to the conditions listed in Table 1.

[0114] The coating weight was measured by the following method. A sample (30 mm x 30 mm) was cut from a molten Zn-plated steel sheet before hot stamping. The surface opposite the evaluation surface was covered with masking tape. The sample was then immersed in a 5% aqueous HCl solution containing 0.02% of an inhibitor (ibit 700A, Asahi Chemical Co., Ltd.) to inhibit the dissolution of iron in the base material at room temperature for 10 minutes. The entire coating was dissolved, and the weight change before and after dissolution was calculated. The completion of coating dissolution was determined by the end of foaming caused by hydrogen generation during dissolution.

[0115] (Chemical conversion treatment conditions)

[0116] The Zn-plated hot stamped products (plate-shaped) were phosphate-treated using a zinc phosphate treatment solution (trade name: PALBOND 3020) manufactured by Nippon Parkersei Co., Ltd. (treatment conditions were the standard conditions for this treatment solution).

[0117] (Electrodeposition coating conditions)

[0118] After the phosphate treatment, the hot-stamped sheet products of each test number were electroplated with a cationic electrodeposition coating manufactured by Nippon Paint Co., Ltd. using a 160V slope voltage. The coating was then baked at 160°C for 20 minutes. The coating film thickness after the electrodeposition coating was controlled using the pre-hot-stamped plated steel sheets to achieve a thickness of 10 μm.

[0119] (Evaluation of chemical conversion treatability)

[0120] For the samples before electrodeposition coating, the amount of P adhesion was measured using a fluorescent X-ray device. The P adhesion amount was less than 2.0 mg / m 2 Denote it as B, and 2.0 mg / m 2 The above was recorded as A, and the chemical conversion treatability was evaluated.

[0121] (Evaluation of corrosion resistance after painting)

[0122] For hot stamped products after electrodeposition coating, cross-cutting was performed in a manner that reached the steel material of the substrate, and a composite corrosion test (neutral salt water spray cycle test specified in JIS H 8502 (1999)) was performed. Specifically, a corrosion evaluation test was conducted with 5% salt water spray (35°C, 2h), drying (60°C, 25% RH, 4h), and wetting (50°C, 98% RH, 2h) as one cycle. After 180 cycles of evaluation tests, the maximum corrosion depth of each sample was measured. Based on the evaluation results of Fe-Zn solid solution Zn-plated steel sheets (maximum corrosion depth of 0.5mm), the maximum corrosion depth exceeding 0.4mm was evaluated as B, the maximum corrosion depth of 0.2mm or more and 0.4mm or less was evaluated as A, and the maximum corrosion depth of less than 0.2mm was evaluated as AA to evaluate the corrosion resistance after coating.

[0123] (Evaluation of coating film adhesion)

[0124] Evaluation of coating adhesion is carried out by salt water immersion test. Immerse in 5% by mass NaCl aqueous solution at 50°C for 300h, attach tape to the surface of the sample taken out after immersion, measure the area of ​​the coating attached to the tape after peeling, and calculate the coating peeling area rate (the ratio of the coating area to the area of ​​the tape). Regarding the area of ​​the coating, scan the peeled tape and measure it by black and white binarization using image processing software. The coating peeling area rate of 30% or more is set to C, the coating peeling area rate of 20% or more and less than 30% is set to B, the coating peeling area rate of 10% or more and less than 20% is set to A, and the coating peeling area rate of less than 10% is set to AA to evaluate the coating adhesion.

[0125] The cases where the coating film adhesion was evaluated as A or AA were hot stamped products using Zn-based plated steel sheets and were evaluated as hot stamped products with excellent coating film adhesion.

[0126] Referring to Table 1, Comparative Examples 1 and 2, because they did not undergo blasting, could not control the RΔq and the area ratio of the Zn oxide layer within the desired range, resulting in reduced coating adhesion. Comparative Example 3, despite undergoing shot peening with steel balls, similarly failed to control the RΔq and the area ratio of the Zn oxide layer within the desired range, resulting in reduced coating adhesion. Comparative Examples 4-6 were able to control RΔq within the desired range by using steel balls for longer shot peening times than in Comparative Example 3. However, the Zn oxide layer was crushed on the plated surface, failing to be fully removed. Consequently, the area ratio of the Zn oxide layer could not be controlled within the desired range, resulting in reduced coating adhesion. Comparative Examples 7 and 8, by changing the steel ball particle size to 50 μm, were able to reduce the area ratio of the Zn oxide layer compared to Comparative Examples 3 and 4, which had the same shot peening time. However, the RΔq and the area ratio of the Zn oxide layer could not be controlled within the desired range, resulting in reduced coating adhesion. Comparative Examples 9 and 10 were able to control RΔq within the desired range by using steel ball shot blasting for a longer time than in Comparative Examples 7 and 8. However, the Zn oxide layer was crushed on the plated surface, and it could not be fully removed. As a result, the area ratio of the Zn oxide layer could not be controlled within the desired range, and the coating adhesion was reduced. Comparative Examples 11 and 12 used sand-like media for blasting, but because it was a dry blasting process, RΔq and / or the area ratio of the Zn oxide layer could not be controlled within the desired range, and the coating adhesion was reduced. Comparative Example 13 performed a wet blasting process using steel balls, but RΔq and the area ratio of the Zn oxide layer could not be controlled within the desired range, and the coating adhesion was reduced.

[0127] In contrast, Examples 14 to 23 of the present invention, through wet blasting using sand-like media, were able to control RΔq and the area ratio of the Zn oxide layer within the desired range, resulting in excellent coating adhesion. In particular, in Examples 14 to 23 of the present invention, the wet blasting allowed the projected sand-like media to flow due to the liquid, preventing the sand-like media from remaining on the surface of the Zn-plated hot-stamped product. This significantly improved post-coating corrosion resistance in addition to coating adhesion.

[0128] Description of Reference Numerals

[0129] 1Γ phase

[0130] 2Fe-Zn solid solution

[0131] 3 Zn oxide layer

[0132] 4 gaps

[0133] 5 coating

[0134] 6 Steel

[0135] 7 Lower Level

[0136] 8 Upper Floor

[0137] 9Zn coating

Claims

1. A hot stamping product, characterized in that: The hot stamped product comprises a Zn-based plating layer and a Zn oxide layer on the surface of the Zn-based plating layer on the surface of the steel material, wherein the upper layer of the Zn-based plating layer is a two-phase structure of a Γ phase and an Fe-Zn solid solution, and the lower layer excluding the upper layer is a single-phase structure of an Fe-Zn solid solution. The root mean square slope RΔq of the roughness curve of the surface of the hot stamped product is 0.25 or less, and the area ratio of the Zn oxide layer on the surface of the hot stamped product is 30% or less.

2. A method for manufacturing a hot stamped product according to claim 1, characterized in that: It involves wet blasting of hot stamped parts using sand-like media.

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