Hot-stamping shaped article and manufacturing method for same

Abstract

Provided are a hot stamped component comprising a steel material provided on its surface with a Zn-based plating layer comprising a top layer and a bottom layer besides the top layer wherein the top layer is a dual phase structure of a Γ phase and Fe-Zn solid solution, and the bottom layer is a single phase structure of a Fe-Zn solid solution, and a Zn oxide layer on a surface of the Zn-based plating layer, wherein a root mean square slope RΔq of a roughness curve at a surface of the hot stamped component is 0.25 or less, and an area ratio of the Zn oxide layer at a surface of the hot stamped component is 30% or less, and a method of production of the same.

Description

FIELD

[0001] The present invention relates to a hot stamped component, more particularly a hot stamped component using a Zn-based plated steel sheet and a method of production of the same, in particular a hot stamped component excellent in coating adhesion and a method of production of the same.BACKGROUND

[0002] In recent years, in the field of members for automobile use, the need has been rising for higher strength with the aim of improvement of fuel economy and collision safety. As a solution to this, hot stamping technology is being applied more broadly. "Hot stamping" is the technique of press-forming a blank heated to a temperature where it becomes a single austenite phase region (Ac3 point) or more, for example, up to 900°C or so, to hot shape and simultaneously rapidly cool (harden) the blank utilizing robbing of heat by the dies to thereby produce a high strength press-formed component high in shape freezability. Further, due to the rise in need for higher rustproofing, hot stamped components made of Zn-based plated steel sheet are being developed further.

[0003] In a hot stamped component made of Zn-based plated steel sheet, for example, as shown in PTL 1, formation of an iron oxide coating is suppressed. Furthermore, after hot stamping, the Zn constituent remains in the steel sheet surface layer, therefore compared with a hot stamped component made from nonplated steel sheet, due to the sacrificial anticorrosive effect, the corrosion resistance after being shaped is excellent.

[0004] However, as shown in FIG. 1, in the hot stamping process, the Zn plating layer becomes a Γ phase 1. Part of the Zn in the plating diffuses into the base material to form an Fe-Zn solid solution 2, while the remaining Zn changes to a Zn oxide layer 3. Gaps 4 form between the Γ phase 1 and the Zn oxide layer 3, therefore the problem arises that salt water easily penetrates the material and a drop in the adhesion with a coating film 5 is caused.

[0005] To deal with this issue, for example, PTL 2 discloses firing a laser at the surface of the Zn-based plated steel sheet while suitably adjusting the output conditions to thereby remove the Zn oxide layer 3 and enable improvement of the coating adhesion. Further, PTL 3 and PTL 4 disclose that the coating adhesion can be improved if shot blasting steel balls to remove the Zn oxide layer 3.

[0006] In this regard, if using for the hot stamped component a Zn-plated steel sheet provided with a Zn-Fe intermetallic compound with a high a Zn concentration and high sacrificial anticorrosive performance, i.e., a Γ phase 1, in addition to an Fe-Zn solid solution 2, the corrosion resistance after coating becomes more excellent compared with the case of Zn-based plated steel sheet provided with only an Fe-Zn solid solution 2. For example, PTL 5 and PTL 6 describe that the Γ phase contains Fe in 10 to 30 mass%. In this way, in general, the Γ phase 1 is comprised of Zn: 70 to 85 mass% and Fe: 15 to 30 mass% or so, while the Fe-Zn solid solution is comprised of Zn: 10 to 40 mass% and Fe: 60 to 90 mass%.

[0007] Further, a hot stamped component comprised of a Zn-based plated steel sheet provided with an Fe-Zn solid solution 2 and a Γ phase 1 is obtained by raising the temperature up to 782°C where the Fe phase is formed by a solid phase, then holding for a certain time in the mixed temperature region of the solid phase Fe and liquid phase (780 to 900°C), then cooling down to 780°C before the start of hot stamping to thereby make the remaining liquid phase precipitate as the Γ phase 1 and make the spherical Fe-Zn solid solution 2 spread in the matrix. As the heating method in the process of production of a hot stamped component, ohmic heating and furnace heating may be mentioned.

[0008] In the case of ohmic heating, it is possible to rapidly heat the steel sheet. On the other hand, uniform control of the heating and current value at the steel sheet surface is difficult. There are many restrictions in production. Therefore, furnace heating is preferable. Even with a hot stamped component comprised of Zn-based plated steel sheet containing a Γ phase 1, if envisioning application to members used in tough corrosive environments, excellent coating adhesion is sought. By using Zn-based plated steel sheet containing a Γ phase 1, the corrosion resistance after coating is improved, but in the same way as a Fe-Zn solid solution type Zn-based plated steel sheet, in the hot stamping process, the surface of the Zn-based plating layer 9 is formed with a Zn oxide layer 3 with large unevenness, so gaps 4 are formed between the Zn-based plating layer 9 and the Zn oxide layer 3, saltwater easily penetrates the material, and furthermore the chemical convertibility also falls, therefore there is a problem in coating adhesion.

[0009] As a technique for improving the coating adhesion of a hot stamped component comprised of Zn-based plated steel sheet containing a Γ phase 1, in the same way as with Zn-based plated steel sheet provided with only an Fe-Zn solid solution 2, laser peeling or blasting to grind away the Zn oxide layer 3 may be considered. However, laser peeling facilities are not generally available. Further, in the prior art, while shot blasting using steel balls etc. is being studied in detail, other blasting has not necessarily been sufficiently studied.

[0010] Further, PTL 7 discloses forming on the surface of Zn-based plated steel sheet for hot pressing use a chemically converted coating containing one or more of titanium oxide, nickel oxide, or stannous oxide and a resin to thereby enable improvement of the coating adhesion. In this way, in the prior art, various proposals have been made for solving the issue of coating adhesion in a hot stamped component using Zn-based plated steel sheet, but demand for higher rustproofing has also been rising. There is still a high need for a hot stamped component able to solve this problem.[CITATION LIST][PATENT LITERATURE]

[0011] [PTL 1] Japanese Patent No. 3582504 [PTL 2] Japanese Patent No. 6211908 [PTL 3] Japanese Patent No. 4085876 [PTL 4] Japanese Patent No. 5880321 [PTL 5] Japanese Patent No. 4072129 [PTL 6] Japanese Patent No. 4695459 [PTL 7] Japanese Patent No. 6631623 SUMMARY[TECHNICAL PROBLEM]

[0012] Therefore, the present invention has as its object the provision of a hot stamped component using a Zn-based plated steel sheet by a novel constitution, which is excellent in coating adhesion, and a method of production of the same.[SOLUTION TO PROBLEM]

[0013] The inventors studied various conditions in blasting so as to improve the coating adhesion of a hot stamped component comprised of a Zn-based plated steel sheet containing a Γ phase 1 and as a result succeeded in achieving removal of the Zn oxide layer 3 and smoothening of the surface and obtaining excellent coating adhesion. That is, the present invention able to achieve the above object is as follows: (1) A hot stamped component comprising a steel material provided on its surface with a Zn-based plating layer comprising a top layer and a bottom layer besides the top layer wherein the top layer is a dual phase structure of a Γ phase and Fe-Zn solid solution, and the bottom layer is a single phase structure of a Fe-Zn solid solution, and a Zn oxide layer on a surface of the Zn-based plating layer, wherein a root mean square slope RΔq of a roughness curve at a surface of the hot stamped component is 0.25 or less, and an area ratio of the Zn oxide layer at a surface of the hot stamped component is 30% or less. (2) A method of production of the hot stamped component according to the above (1), comprising wet blasting the hot stamped component using a grit media. [ADVANTAGEOUS EFFECTS OF THE INVENTION]

[0014] According to the present invention, it is possible to provide a hot stamped component using a Zn-based plated steel sheet which is excellent in coating adhesion, and a method of production of the same.BRIEF DESCRIPTION OF DRAWINGS

[0015] FIG. 1 is a view showing a state where a Zn-based plating layer of a hot stamped component is provided with a bottom layer and a top layer, a Fe-Zn solid solution and a Γ phase are formed in the Zn plating layer, gaps are formed between the Γ phase and a Zn oxide layer, and coating adhesion falls. FIG. 2 is a view summarizing the correlation between an RΔq and the results of evaluation of coating adhesion. FIG. 3 is a view summarizing the RΔq and area ratio of the Zn oxide layer as conditions sought for improvement of coating adhesion. FIG. 4 is a view showing a mechanism of removal of a Zn oxide layer envisioned in the case of dry blasting using alumina grit as the media. FIG. 5 is a view showing a mechanism of removal of a Zn oxide layer envisioned in the case of dry blasting by steel balls with an average particle size of 300 µm. FIG. 6 is a view showing SEM images (with mesh) of a surface in a case of (left) nontreated material and (right) wet blasting using steel grit as a media. DESCRIPTION OF EMBODIMENTS

[0016] The inventors studied various conditions in blasting so as to improve the coating adhesion of a hot stamped component comprised of a Zn-based plated steel sheet containing a Γ phase and as a result discovered that smoothening of the surface and suitable removal of the Zn oxide layer 3 improve coating adhesion.

[0017] The inventors first focused on a root mean square slope RΔq of a roughness curve as a factor affecting the coating adhesion of a hot stamped component comprised of a Zn-based plated steel sheet including a Γ phase. As shown in FIG. 2, if blasting by various conditions, in many cases, if the RΔq is 0.25 or less, the coating adhesion becomes a passing level (A or AA). This is presumed because the Zn oxide layer 3 with the large surface unevenness referred to in FIG. 1 is suitably removed by blasting and the gaps 4 through which saltwater easily penetrates and becoming the cause of a drop in coating adhesion are eliminated. On the other hand, as shown by the part surrounded by the circle in FIG. 2, even if the RΔq becomes 0.25 or less, there is a point where the coating adhesion becomes a failing level, and therefore it was difficult to assess the coating adhesion by only the RΔq.

[0018] Therefore, the inventors engaged in intensive studies and as a result discovered that in addition to the RΔq, the area ratio of the Zn oxide layer at the surface of the Zn-based plated hot stamped component as a whole affects the coating adhesion. FIG. 3 will be used to explain the suitable ranges of RΔq and the area ratio of the Zn oxide layer. In FIG. 3, ⊚, ○, Δ, and × are symbols indicating the evaluation of the coating adhesion. The state where the delaminated area ratio of the coating film is 30% or more is indicated by the symbol ×, the state where the delaminated area ratio of the coating film is 20% or more and less than 30% is indicated by Δ, the state where the delaminated area ratio of the coating film is 10% or more and less than 20% is indicated by ○, and the state where the delaminated area ratio of the coating film is less than 10% is indicated by ⊚.

[0019] First, as shown by the region I in FIG. 3, if the RΔq is more than 0.25, regardless of the value of the area ratio of the Zn oxide layer, the coating adhesion becomes poor. This is presumed to be because, as explained above, the Zn oxide layer 3 with a large surface unevenness as referred to in FIG. 1 is not completely removed and saltwater penetrating from the gaps 4 causes a drop in the coating adhesion. Further, in the region II, the RΔq is 0.25 or less, but the area ratio of the Zn oxide layer is more than 30% and the coating adhesion becomes poor. This is presumed to be because, as explained later, the surface is smoothened, but the Zn oxide layer is not sufficiently removed and is crushed at the plating surface and the coating film peels off starting from the interface of the crushed Zn oxide layer and plating. Furthermore, in the region III, the RΔq is 0.25 or less and the area ratio of the Zn oxide layer is 30% or less and therefore the coating adhesion is good.

[0020] From the above, as conditions for improvement of the coating adhesion, it is necessary that the value of the RΔq be 0.25 or less and the area ratio of the Zn oxide layer be 0% or more and 30% or less. Furthermore, from the viewpoint of improvement of the coating adhesion, the smaller the value of the RΔq, the more preferable. For example, it may be 0.24 or less, 0.22 or less, 0.20 or less, or 0.18 or less. Similarly, from the viewpoint of improvement of the coating adhesion, the smaller than area ratio of the Zn oxide layer, the more preferable. For example, it may be 28% or less, 25% or less, 22% or less, less than 20%, 18% or less, or 15% or less.

[0021] Next, the method of production of a Zn-based plated hot stamped component with a requirement of the present invention of a value of the RΔq of 0.25 or less and with an area ratio of an Zn oxide layer of 0% or more and 30% or less will be explained.

[0022] In the prior art of blasting by a grit media, selection of the treatment conditions was considered to be difficult, but if the Zn oxide layer can be sufficiently removed, the surface also becomes smooth and the coating adhesion is stably improved. Therefore, for example, if using an alumina grit as the blasting media, as shown in FIG. 4, compared with the case of using usual steel, the removability of the Zn oxide layer is more excellent, the surface also becomes smoother, and the coating adhesion is stably improved. However, in the case of general dry blasting, after the blasting, the alumina grit media partially remains at the plating surface. Compared with the later explained wet blasting material, the coating adhesion and corrosion resistance after coating were often inferior. "Grit" indicates a material, not including spherical shape, of an angular shape. Therefore, a grit media need only be a media with a grit shape, i.e., angular shape. The material is not particularly limited. For example, the grit media may be at least one of alumina, steel, silicon carbide, ceramic, and silica. The conditions of the blasting are not particularly limited and may be conditions generally applied in this technical field.(Wet Blasting)

[0023] At the time of blasting using a grit media, the blasting is performed by wet blasting. Wet blasting is also called "wet type blasting" or "liquid honing" and usually uses a liquid containing steel grit with a particle size of 50 to 300 µm as a media (mainly comprised of water, corrosion inhibitor also possibly added). If performing this blasting under suitable conditions, the Zn-based plated hot stamped component surface is sufficiently smoothened and the Zn oxide layer is also sufficiently removed. For example, the blasting pressure of the wet blasting, more specifically the pressure of the compressed air for spraying the liquid containing the media from the spray nozzles need only be a pressure sufficient for removing the Zn oxide layer down to a desired level and in general is 0.10 to 8.00 MPa. The blasting pressure may also be 0.20 MPa or more or 0.30 MPa or more. Similarly, the blasting pressure may also be 6.00 MPa or less, 5.00 MPa or less, 3.00 MPa or less, 1.00 MPa or less, 0.80 MPa or less, or 0.50 MPa or less. However, if the blasting pressure is too high, for example, if the blasting pressure becomes more than 8.00 MPa, the Zn oxide layer is sufficiently removed, but the surface properties of the hot stamped component deteriorate and sometimes the smoothening of the surface and in turn the desired RΔq cannot be achieved. Further, the blasting time of the grit media is not particularly limited, but, for example, may be 1.5 to 80.0 seconds, 2.0 to 60.0 seconds, or 2.0 to 30.0 seconds. Furthermore, in dry blasting which blasts a media without using a liquid, the blasted media remains at the surface of the Zn-based plated hot stamped component and can have a detrimental effect on the coating adhesion and corrosion resistance after coating, while with wet blasting, the blasted media is flushed away by the liquid, the media does not remain at the surface of the Zn-based plated hot stamped component, and therefore there is no possibility of the coating adhesion and corrosion resistance after coating being detrimentally affected. If the blasted media remains at the surface of the Zn-based plated hot stamped component, a tendency is seen for the value of the RΔq to become higher and as a result it is believed the coating adhesion falls.(Case of Shot Blasting Steel Balls as Comparison)

[0024] The usual shot blasting of dry shot blasting using steel balls of an average particle size of 300 µm was performed. As a result, as shown in FIG. 5, it was learned that while this was effective for smoothening the surface, depending on the treatment conditions, the Zn oxide layer is not sufficiently removed and is crushed at the plating surface, the coating film peels off starting from the interface of the crushed Zn oxide layer and the plating, and therefore the coating adhesion falls. Here, the "shot blasting" means blasting by steel balls or other spherical media. Therefore, with the dry blasting method using steel balls, as shown in the region II in FIG. 3, the RΔq becomes small, but the area ratio of the Zn oxide layer is large, therefore excellent coating adhesion cannot be obtained.

[0025] From the above explanation, to achieve the desired coating adhesion and furthermore the desired corrosion resistance after coating, it is learned that it is not sufficient to just perform blasting after hot stamping and that performing wet blasting using a grit media is extremely important. By wet blasting using a grit media, unlike the case of dry shot blasting by steel balls etc., the Zn oxide layer is not crushed at the plating surface and the Zn oxide layer can be suitably removed. Furthermore, by performing not dry, but wet blasting, the media never remains at the surface of the Zn-based plated hot stamped component and disadvantageously acts on the value of the RΔq etc. Therefore, by wet blasting using a grit media, it is possible to reliably control the RΔq to 0.25 or less and control the area ratio of the Zn oxide layer to 30% or less and as a result it becomes possible to remarkably improve the coating adhesion and furthermore the corrosion resistance after coating.

[0026] Next, the methods of measurement and methods of analysis according to the requirements of the present invention will be explained.(1) Measurement of Root Mean Square Slope RΔq of Roughness Curve

[0027] The RΔq is measured based on ISO4287-1997. A stylus type roughness meter with a tip diameter of 5 µm made by Accretech was used to measure this by a scan speed of 0.25 mm / s. The measurement conditions of the surface roughness are a reference length of 0.8 mm and an evaluation length of 35 mm. At the surface of the hot stamped component in the present invention, among the various surface roughness indicators (arithmetic average roughness Ra, maximum height roughness Rz, and root mean square slope RΔq of the roughness curve), there is no correlation between the general roughness indicators Ra and Rz and the coating adhesion. Correlation is only seen between the RΔq and coating adhesion. This is because Ra and Rz are values showing the roughness in the vertical direction to the steel sheet surface, while RΔq is a value showing a gradient of the surface-most layer to the steels sheet surface, therefore it is presumed that only RΔq reflects the "crushed form of the Zn oxide layer".(2) Measurement of Area Ratio of Zn Oxide Layer

[0028] The surface of the hot stamped component after blasting is examined by an SEM by a BSE image enlarged 100X in a predetermined area range (1.3 mm×1.9 mm) using an acceleration voltage of 20 kV (for example, using JSM-6610A made by JEOL). In the SEM-BSE image, large atomic weight elements are observed bright in contrast (white), therefore this difference in contrast can be used to differentiate the Zn-based plating layer and Zn oxide layer. Specifically, the Zn-based plating layer containing a large amount of Zn is observed as white while the Zn oxide layer is observed as black.

[0029] Further, for measurement of the area ratio of the Zn oxide layer, the above-mentioned SEM image observed by an acceleration voltage of 20 kV and 100X power may, as shown in FIG. 6, be divided into 50 µm×50 µm pitch grid areas. These are divided into the two types of areas of black parts (black area ratio of 50% or more) and white parts (white area ratio of 50% or more). The respective numbers of the grid areas are counted and the area ratio of the black parts may be calculated from the ratios of the grid areas. For grid areas with the same extents of area ratios of the black parts and white parts, the 50 µm×50 µm pitch grid areas may be further subdivided and the area ratios measured more accurately, but it is also possible to count the black parts and white parts in 0.5 grid areas. If the black parts and white parts cannot be judged visually, it is also possible to analyze the elements and perform judgment at the grid area regions by SEM-EDS. As a result of the elemental analysis, areas with O contents by mass% of 40.0% or more are counted as black parts, while with less than 40.0%, they are counted as white parts.

[0030] Next, the steel sheet constituents of the Zn-based plated steel sheet according to the present invention will be explained. The present invention, as explained above, has as its object the provision of a hot stamped component using Zn-based plated steel sheet in which the coating adhesion is excellent. The object is achieved by controlling the root mean square slope RΔq of the roughness curve at the surface of the hot stamped component to 0.25 or less and controlling the area ratio of the Zn oxide layer at the surface of the hot stamped component to 30% or less. Therefore, the chemical composition of the Zn-based plated steel sheet or steel material clearly is not an essential technical feature in achieving the object of the present invention. Therefore, the following explanation is only intended as an illustration of the preferable chemical composition of the steel material for application in members for automotive use etc. and is not intended to limit the present invention to a steel material having such a specific chemical composition. In the hot stamped component according to the present invention, the steel material preferably has a chemical composition containing, for example, 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, V: 0.00% or more and 0.50% or less, Cr: 0.00% or more and 0.50% or less, Mo: 0.00% or more and 0.50% or less, Cu: 0.00% or more and 1.00% or less, Ni: 0.00% or more and 2.00% or less, W: 0.00% or more and 1.00% or less, Zr: 0.00% or more and 1.00% or less, B: 0.0000% or more and 0.0100% or less, a total of REM, Ca, Co, and Mg: 0.0000% or more and 0.0300% or less, and a balance: Fe and impurities. Below, the elements will be explained in more detail.C: 0.18% or More and 0.50% or Less

[0031] C is an element raising the strength of a Zn-based plated hot stamped component after hot stamping. If the C content in the steel material is too low, the above effect cannot be obtained. For this reason, the lower limit of the C content in the steel material is preferably 0.18%. On the other hand, if the C content in the steel material is too high, the toughness of the steel sheet falls, therefore the upper limit of the C content is preferably 0.50%.Si: 0.10% or More and 1.50% or Less

[0032] Si is an element improving the fatigue characteristics of a hot stamped component. Further, Si is an element which forms a stable oxide coating during annealing for recrystallization on a continuous hot dip galvanization line and thereby improves the hot dip galvanizing ability, in particular the plating wettability. To obtain these effects, the Si content is 0.10% or more. Preferably, it is more than 0.14%, 0.15% or more, 0.18% or more, or 0.20% or more.

[0033] However, if the Si content in the steel material is too high, the Si in the steel material diffuses during the heating in the hot stamping and forms oxides at the steel material surface. The oxides cause the phosphatization ability to fall. Si, furthermore, has the action of making the Ac3 point of the steel material rise. If the Ac3 point rises, the heating temperature at the hot stamping sometimes exceeds the evaporation temperature of the Zn. For this reason, the upper limit of the Si content is preferably 1.50%. Preferably, it is 1.40% or less, 1.20% or less, or 1.00% or less.Mn: 0.50% or More and 2.50% or Less

[0034] Mn is an element raising the hardenability of the steel material and raising the strength of a Zn-based plated hot stamped component. If the Mn content is too low, that effect cannot be obtained. If obtaining this effect, the lower limit of the Mn content of the steel material is preferably 0.50%. The preferable lower limit of the Mn content of the steel material is 0.60% or 0.80%. On the other hand, if the Mn content is too high, the effect becomes saturated. Therefore, the upper limit of the Mn content of the steel material is preferably 2.50%. The preferable upper limit of the Mn content is 2.30% or 2.00%.P: 0.000% or More and 0.100% or Less

[0035] P is an impurity contained in a steel material. P segregates at the grain boundaries of the steel material to cause the toughness of the steel and the delayed fracture resistance to fall. Therefore, the P content is 0.100% or less, preferably 0.050% or less. The lower limit of the P content is 0.000% and may also be 0.001%.S: 0.0000% or More and 0.0100% or Less

[0036] S is an impurity contained in a steel material. S forms sulfides to lower the toughness of the steel and lower the 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% and may also be 0.0001%.Al: 0.001% or More and 0.100% or Less

[0037] Al is an element effective for deoxidizing steel. To obtain this effect, the lower limit of the Al content is 0.001% or more. On the other hand, if the Al content is too high, the Ac3 point of the steel sheet rises and sometimes the heating temperature required at the time of hot stamping becomes more than the evaporation temperature of the Zn-based plating layer. Therefore, the upper limit of the Al content of the steel material is preferably 0.100%. Preferably it is 0.070% or less or 0.050% or less.N: 0.0000% or More and 0.0100% or Less

[0038] N is an impurity unavoidably contained in a steel material. N is an element forming nitrides to lower the toughness of the steel material. If B is contained, N has the effect of bonding with the B and reducing the amount of solid solution B. By the amount of solid solution B being reduced, the hardenability falls. Therefore, the N content of the steel material is preferably as low as possible. If the N content of the steel material becomes more than 0.0100%, the effect becomes remarkable, therefore the upper limit of the N content of the steel material may be 0.0100%. The lower limit of the N content does not particularly have to be prescribed, but the lower limit of the N content may be 0.0000% or may be 0.0001%.

[0039] The balance of the chemical composition of the steel material according to the present embodiment may be comprised of Fe and impurities. In the present embodiment, the "impurities" mean constituents entering due to the ore, scrap, and other sources from the production environment etc. as raw materials and / or constituents allowed to an extent not detrimentally affecting the hot stamped component according to the present embodiment.

[0040] The steel material according to the present embodiment may contain at least one type of element among the following as optional elements in place of part of the Fe. If not containing the following elements, the contents of the respective optional elements are 0%.Nb: 0.00% or More and 0.15% or Less

[0041] Nb is an element contributing to improvement of the strength of the steel sheet by carbide precipitation and having an effect on the strength, therefore may be contained in accordance with need. However, if excessively containing Nb, a large amount of carbides is formed and the toughness of the steel sheet is made to fall. For this reason, the Nb content is 0.15% or less, 0.10% or less, or 0.05% or less. The lower limit of the Nb content is 0.00% and may also be 0.01%.Ti: 0.00% or More and 0.15% or Less

[0042] Ti is an element contributing to improvement of the strength of the steel sheet by carbide precipitation and having an effect on the strength, therefore may be contained in accordance with need. However, if excessively containing Ti, a large amount of carbides is formed and the toughness of the steel sheet is made to fall. For this reason, the Ti content may be 0.15% or less, 0.10% or less, or 0.01% or less. The lower limit of the Ti content is 0.00% and may also be 0.001% or 0.005%.V: 0.00% or More and 0.50% or Less

[0043] V is an element contributing to improvement of the strength of the steel sheet by carbide precipitation and having an effect on the strength, therefore may be contained in accordance with need. However, if excessively containing V, a large amount of carbides is formed and the toughness of the steel sheet is made to fall. For this reason, the V content may be 0.50% or less. The lower limit of the V content is 0.00% and may also be 0.01%.Cr: 0.00% or More and 0.50% or Less

[0044] Cr is an optional element and need not be included. Cr, if contained, is an element which raises the hardenability of the steel material and has an effect on the strength. To obtain this effect, the preferable lower limit of the Cr content of the steel material is 0.01%. However, if the Cr content of the steel material is too high, Cr carbides are formed and the carbides become hard to dissolve at the time of the heating in the hot stamping. For this reason, austenization of steel material becomes harder and the hardenability falls. Therefore, the upper limit of the Cr content of the steel material is preferably 0.50%.Mo: 0.00% or More and 0.50% or Less

[0045] Mo is an optional element and need not be included. Mo, if contained, is an element which raises the hardenability of the steel material and has an effect on the strength. To obtain this effect, the preferable lower limit of the Mo content of the steel material is 0.01%. However, if the Mo content is too high, the above effect become saturated. Therefore, the upper limit of the Mo content of the steel material is preferably 0.50%.Cu: 0.00% or More and 1.00% or Less

[0046] Cu is an optional element and need not be included. Cu, if contained, is an element dissolving in the steel to raise the strength without impairing the toughness. To obtain this effect, the Cu content is preferably 0.01% or more. If the Cu content of the steel material is too high, at the time of rolling etc., sometimes fine cracking is caused to occur at the surface. Therefore, the preferable upper limit of the Cu content is 1.00% or 0.60%, more preferably is 0.40% or 0.25%.Ni: 0.00% or More and 2.00% or Less

[0047] Ni is an optional element and need not be included. Ni, if contained, is an element which raises the toughness of the steel material, has an effect on the strength as well, and suppresses the embrittlement caused by the liquid phase Zn at the time of heating in the hot stamping. To obtain these effects, the Ni content is preferably 0.01% or more. If the Ni content is too high, the above effect become saturated. Therefore, the preferable upper limit of the Ni content is 2.00%.W: 0.00% or More and 1.00% or Less

[0048] W is an optional element and need not be included. Therefore, the lower limit is 0.00%. W, if contained, is an element affecting the improvement of strength or control of the crystal grains and prevention of cracking or corrosion resistance. To obtain these effects, the W content is preferably 0.01% or more. However, if the W content of the steel material is too high, the above effect become saturated. Therefore, the preferable upper limit of the W content is 1.00%.Zr: 0.00% or More and 1.00% or Less

[0049] Zr is an optional element and need not be included. Therefore, the lower limit is 0.00%. Zr, if contained, is an element affecting the improvement of strength and corrosion resistance. To obtain these effects, the Zr content is preferably 0.01% or more. However, if the Zr content of the steel material is too high, the above effect become saturated. Therefore, the preferable upper limit of the Zr content is 1.00%.B: 0.0000% or More and 0.0100% or Less

[0050] B is an optional element and need not be included. B, if contained, is an element raising the hardenability of steel and raising the strength of a Zn-based plated hot stamped component. To obtain this effect, the B content is preferably 0.0001% or more or 0.0005% or more. If the B content of the steel material is too high, the effect becomes saturated. Therefore, the upper limit of the B content of the steel material is preferably 0.0100%.Total of REM, Ca, Co, and Mg: 0.0000% or More and 0.0300% or Less

[0051] REM, Ca, Co, and Mg are optional elements and need not be included. Therefore, the lower limit is 0.00%. They are elements which, if contained, control sulfides and oxides to preferable forms and keep rough inclusions from forming to thereby suppress the formation of roughness at the time of spot welding. To reliably obtain this effect, the preferable lower limit of the total of the contents of REM, Ca, Co, and Mg is 0.0003%. The content of any one of REM, Ca, Co, and Mg may be 0.0003% or more. If the total of the contents of REM, Ca, Co, and Mg is too high, the effect becomes saturated. Therefore, the upper limit of the total of the contents of REM, Ca, Co, and Mg is preferably 0.0300%.

[0052] The above-mentioned chemical composition of the steel material may be measured by a general method of analysis. For example, it may be measured using ICP-AES (inductively coupled plasma-atomic emission spectrometry). C and S may be measured using the combustioninfrared absorption method, while N may be measured using the inert gas melting-thermal conductivity method. The chemical composition may be analyzed after removing the surface plating layer by mechanical grinding.(Plating Structure According to Present Invention)

[0053] The Zn-based plating structure according to the present invention will be explained. First, the plating structure of the Zn-based plated hot stamped component in the present invention, as shown in the above-mentioned FIG. 1, is provided with a bottom layer 7 and a top layer 8. The region at the surface layer side of the Zn-based plating layer 9 forming the top layer 8 is a dual phase structure with a Fe-Zn solid solution 2 distributed in an Γ phase 1. Further, the region at the steel material 6 side of the Zn-based plating layer 9 forming the bottom layer 7 is a single phase structure of the Fe-Zn solid solution 2.(Plating Constituents According to Present Invention)

[0054] Further, regarding the constituents of the Zn-based plating layer in the present embodiment, the Zn-based plating layer comprises by mass% a Zn content of 30.0% or more. In accordance with need, the lower limit of the Zn content may be 35.0%, 40.0%,, or 50.0%. The upper limit of the Zn content is preferably 80.0%. In accordance with need, the upper limit of the Zn content may also be 78.0% or 75.0%. The contents of the elements other than Zn do not have to be particularly prescribed, but the chemical composition of the Zn-based plating layer (however, excluding Zn) is preferably, for example, by mass%, Fe: 20.0 to 70.0%, Al: 0 to 1.0%, Si: 0 to 1.0%, Mg: 0 to 1.0%, Mn: 0 to 1.0%, Ni: 0 to 1.0%, Sb: 0 to 1.0%, and a balance: impurities.

[0055] Further, the "Zn-based plating layer" in the present invention has an Fe content of 95.0% or less in range. The position for analysis of the chemical composition of the Zn-based plating layer is made the center of the thickness of the Zn-based plating layer (center of plating thickness). The method of analysis of the chemical composition comprises measuring the Fe content by GDS (glow discharge spectrometry) in the thickness direction of the Zn-based plated hot stamped component from the surface of the Zn-based plated hot stamped component (i.e., in the direction from the surface of the Zn-based plated hot stamped component toward the center of sheet thickness) and identifying the range from the surface of the Zn-based plated hot stamped component where the Fe content is more than 95.0%. After that, first, the contents of the elements at the center of the distance from the position where the Fe content became 95.0% to the surface (this range is the Zn-based plating layer) (i.e., the center of thickness of the Zn-based plating layer) are analyzed by GDS. The analysis values are made the chemical composition of the Zn-based plating layer.

[0056] Further, the Γ phase has a Zn oxide layer on it, therefore the position where the Zn content becomes 80.0% (if several, the position closest to the surface) is deemed the surface position of the Zn-based plating layer. However, if there is no region with a Zn content of more than 80.0% at the surface-most layer of the hot stamped component, the surface-most layer is deemed the surface position of the Zn-based plating layer in identifying the center of thickness of the Zn-based plating layer (center of plating thickness).(Plating Conditions of Zn-Based Plated Steel Sheet According to Present Invention)

[0057] The amount of plating deposition is 60 g / m 2< or more and 150 g / m 2< or less. Preferably it is 80 g / m 2< or more. If the amount of plating deposition is 80 g / m 2< or more, an effect of improvement of the corrosion resistance of the hot stamped component is obtained. If 150 g / m 2< or less, an excellent appearance is obtained after plating, therefore this is preferable (if making the amount of plating deposition greater than 150 g / m 2< , if performing hot dip Zn-based plating, sometimes the plating will drip down and the appearance will become worse and the appearance of the hot stamped component will also become worse). The Zn-based plating layer of a steel material for hot stamping use is a hot dip galvanized (GI) layer on the steel sheet with little amount of oxides at the time of hot stamping. Hot dip galvannealing (GA) sometimes results in a large amount of oxides formed at the component at the time of hot stamping and poorer appearance. Therefore, as the steel material for hot stamping use used as the material of the hot stamped component, a hot dip galvanized (GI) steel sheet is preferable. "GA" means a hot dip galvanized layer on steel sheet which has been alloyed while "GI" means a hot dip galvanized layer which has not been alloyed.

[0058] Next, the conditions in the hot stamping performed before the blasting will be explained. The hot stamping can be performed under any suitable conditions known to persons skilled in the art. Therefore, the conditions of the hot stamping are not particularly limited. Therefore, the following explanation is only intended as an illustration of the preferable conditions in the hot stamping and is not intended to limit the method of production of the present invention to one including hot stamping performed under such specific conditions.(Hot Stamping Heating Conditions)

[0059] The hot stamping heating temperature is preferably the Ac3 point or more and 950°C or less while the heating time is preferably 240 seconds to 600 seconds. If the heating temperature is less than Ac3, hardening becomes difficult. For this reason, the heating temperature is preferably the Ac3 point or more. The Ac3 point (°C) is found by cutting out a small piece from the steel sheet and measuring the heat expansion of that small piece during heating from room temperature by 10°C / s to 1000°C. If the heating temperature is 950°C or more, sometimes surface oxidation of the Zn-based plated hot stamped component (formation of Zn oxide layer) excessively proceeds. For this reason, the heating temperature is preferably less than 950°C. If the heating time is less than 240 seconds, sometimes hardening is not possible. For this reason, the heating time is preferably 240 seconds or more. If the heating time is more than 600 seconds, sometimes surface oxidation of the Zn-based plated hot stamped component (formation of Zn oxide layer) excessively proceeds. For this reason, the heating time is preferably 600 seconds or less.

[0060] Further, in hot stamping, usually a die inside of which a cooling medium (for example, water) is circulated is used to press the steel material for hot stamping use. When pressing a steel material for hot stamping use, the steel material for hot stamping use is hardened by heat being robbed by the die. The Zn-based plated hot stamped component is produced by such a process.(Cooling Conditions of Hot Stamping)

[0061] To make the top layer of the Zn-based plating layer a dual phase structure of the Γ phase and Fe-Zn solid solution and make the bottom layer a single phase of the Fe-Zn solid solution, the temperature for starting pressing of the steel material for hot stamping use (starting temperature of rapid cooling) is preferably the lower limit of the temperature where the liquid phase Zn contained in the Zn-based plating layer completely solidifies, i.e., the plating solidification point (about 750°C) or less in cooling. For the specific temperature range, for example, advance experiments can be conducted etc. to easily find the temperature range giving the above structure. It is sufficient to start the rapid cooling, i.e., the hot stamping, from a temperature range found in this way.

[0062] If the average cooling speed from the starting temperature of rapid cooling down to 450°C is less than 20°C / s, sometimes sufficient strength is not obtained. For this reason, the average cooling speed from the starting temperature of rapid cooling down to 450°C is preferably 20°C / s or more. Further, the average cooling speed from 450°C to 200°C is preferably 15°C / s or more.EXAMPLES

[0063] Next, examples according to the present invention will be explained. Examples using as samples (types of base materials) hot dip Zn-based plated steel sheet (GI) obtained from 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% are shown in Table 1 together with the hot stamping heating conditions, blasting conditions, RΔq, area ratio of zinc oxide layer, and evaluation.[Table 1]

[0064] Table 1Sam pleHot stamping heating conditionBlasting conditionsRΔqArea ratio of Zn oxide layer %EvaluationRemarksSample no.Type of base materialPlating deposition amount g / m 2< Zn-based plating layer structureSheet peak temp. °CHeating time sMedia materialShapeDry / wetParticle size µmBlasting time sChemical convertibilityCoating adhesionCorrosion resistance after coating1GI100Fe-Zn solid solution920600No treatment----0.3589.5BCBComp.ex.2GI100Γ phase+Fe-Zn solid solution900405No treatment----0.3390.0BCAAComp.ex.3GI100Γ phase+Fe-Zn solid solution900405SteelBallsDry30023.80.3080.0ACAAComp.ex.4GI100Γ phase+Fe-Zn solid solution900405SteelBallsDry30047.60.2473.9ACAAComp ex.5GI100Γ phase+Fe-Zn solid solution900405SteelBallsDry30071.40.1463.2ACAAComp.ex.6GI100Γ phase+Fe-Zn solid solution900405SteelBallsDry3001190.1152.5ACAAComp.ex.7GI100Γ phase+Fe-Zn solid solution900405SteelBallsDry5023.80.3055.0ACAAComp.ex.8GI100Γ phase+Fe-Zn solid solution900405SteelBallsDry5047.60.2751.0ACAAComp.ex.9GI100Γ phase+Fe-Zn solid solution900405SteelBallsDry5071.40.1640.0ABAAComp.ex.10GI100Γ phase+Fe-Zn solid solution900405SteelBallsDry501190.1235.0ABAAComp.ex.11GI100Γ phase+Fe-Zn solid solution900405AluminaGritDry10023.80.3034.5ABAComp.ex.12GI100Γ phase+Fe-Zn solid solution900405AluminaGritDry10047.60.2721.6ABAComp.ex.13GI100Γ phase+Fe-Zn solid solution900405SteelBallsWet1001.90.3075.0ACAComp.ex.14GI100Γ phase+Fe-Zn solid solution900405SteelGritWet1001.90.2515.7AAAAInv. ex.15GI100Γ phase+Fe-Zn solid solution900405SteelGritWet1007.60.2215.7AAAAAInv. ex.16GI100Γ phase+Fe-Zn solid solution900405SteelGritWet10030.40.2211.8AAAAAInv. ex.17GI100Γ phase+Fe-Zn solid solution900405SteelGritWet10060.80.156.1AAAAAInv. ex.18GI100Γ phase+Fe-Zn solid solution900405AluminaGritWet1007.60.2327.2AAAAInv. ex.19GI100Γ phase+Fe-Zn solid solution900405AluminaGritWet10030.40.1313.2AAAAAInv. ex.20GI100Γ phase+Fe-Zn solid solution900405AluminaGritWet10060.80.117.2AAAAAInv. ex.21GI100Γ phase+Fe-Zn solid solution900600SteelGritWet10060.80.1513.2AAAAAInv. ex.22GI100Γ phase+Fe-Zn solid solution880360SteelGritWet10060.80.207.9AAAAAInv. ex.23GI100Γ phase+Fe-Zn solid solution920480SteelGritWet10060.80.147.9AAAAAInv. ex.Underlines indicate outside scope of present invention.

[0065] As the test material, a GI-HS material / residual Γ phase type is used. As the test material of the reference example (Comparative Example 1), a GI-HS material / Fe-Zn solid solution type is used. Here, "HS" indicates for hot stamping use, a "residual Γ phase type" indicates a Zn-plated steel sheet provided with a Fe-Zn solid solution plus an Zn-Fe intermetallic compound (Γ phase) with a high Zn concentration and a high sacrificial anticorrosive performance, while an "Fe-Zn solid solution type" indicates a hot dip Zn plated steel sheet comprised of only an Fe-Zn solid solution formed by part of the Zn in the plating diffusing into the base material in the hot stamping process. More specifically, Sample No. 1 of Table 1 has a Zn-based plating layer comprised of only a Fe-Zn solid solution, while each of Sample Nos. 2 to 23 has a Zn-based plating layer with a top layer comprised of a dual phase structure of a Γ phase and an Fe-Zn solid solution and with a bottom layer comprised of a single phase structure of an Fe-Zn solid solution. Further, the Zn-based plating layers at Sample Nos. 1 to 23 all had chemical compositions comprised of, by mass%, Zn: 34.5 to 69.5%, Fe: 30.0 to 65.0%, Al: 0.1 to 0.9%, and balance: impurities.

[0066] In the blasting conditions, for the "Steel" "Balls , a direct pressure type device (model BA-1 made by Ascon) (air pressure 0.35 MPa) was used. For the "Alumina" "Grit", a suction type device was used. The dry blasting was performed using model BS-1 made by Ascon. The air pressure as made 0.20 MPa and the blasting distance was standardized to 150 mm in each case. The wet blasting was performed using W8MN-Q008 made by Macoho. The blasting distance was made 150 mm, and the air pressure was made 0.25 MPa. The blasting time of the blasting process was made the condition described in Table 1.

[0067] Further, the amount of plating deposition was measured by the following method. A sample (30 mm×30 mm) cut out from the hot dip Zn-based plated steel sheet before hot stamping heating was covered at the evaluation surface and opposite surface by masking tape, then was dipped in a 5% HCl aqueous solution containing 0.02% of an inhibitor suppressing dissolution of the iron in the base material (ibit 700A, Asahi Chemical Co., Ltd.) at ordinary temperature for 10 minutes to dissolve the entire plating layer. The change of weight before and after dissolution was used for calculation. Whether the plating layer has finished being dissolved was judged based on the end of foaming due to generation of hydrogen at the time of dissolution.(Chemical Conversion Conditions)

[0068] The Zn-based plated hot stamped component (sheet shape) is phosphated using a zinc phosphate treatment solution made by Nihon Parkerizing Co., Ltd. (product name: Palbond 3020) (treatment conditions are standard conditions with the treatment solution).(Electrodeposition Coating Conditions)

[0069] After the above-mentioned phosphatization was performed, the sheet-shaped hot stamped components of the different test numbers were coated with cationic electrodeposition paints made by Nippon Paint Co., Ltd. by electrodeposition by a slope current of a voltage 160V. These were baked on at a temperature of 160°C for 20 minutes. The coating thickness of the paints after electrodeposition was controlled under conditions giving electrodeposition coating of 10 µm at the plated steel sheet before hot stamping.(Evaluation of Chemical Convertibility)

[0070] A sample before electrodeposition coating was measured for the amount of deposition of P by a fluorescent X-ray apparatus. The chemical convertibility was evaluated as "B" in the case of an amount of deposition of P of less than 2.0 mg / m 2< and was evaluated as "A" in the case of an amount of deposition of P of 2.0 mg / m 2< or more.(Evaluation of Corrosion Resistance After Coating)

[0071] The electrodeposition coated hot stamped component was cross cut so as to reach the base steel material and subjected to a combined corrosion test (neutral salt spray cyclic test prescribed in JIS H 8502 (1999)). Specifically, a test for evaluation of corrosion was performed by 5% saltwater spraying (35°C , 2h), drying (60°C , 25%RH, 4h), and wetting (50°C , 98%RH, 2h) as one cycle. After 180 cycles of the evaluation test, the maximum corrosion length of each sample was measured. Based on the result of evaluation at an Fe-Zn solid solution type Zn-based plated steel sheet (maximum corrosion length of 0.5 mm), the corrosion resistance after coating was evaluated as "B" in the case of a maximum corrosion depth of more than 0.4 mm, as "A" in the case of a maximum corrosion depth of 0.2 mm or more and 0.4 mm or less, and as "AA" in the case of a maximum corrosion depth of less than 0.2 mm.(Evaluation of Coating Adhesion)

[0072] The coating adhesion was evaluated by a saltwater immersion test. A sample was immersed in a 50°C 5 mass% NaCl aqueous solution for 300 hours. After immersion, the sample was taken out and taped at its surface. The tape was peeled off, then the area of the coating film deposited on the tape was measured and the delaminated area ratio of the coating film (ratio of coating film area to area of tape) was calculated. The area of the coating film was measured by scanning the peeled off tape and using image processing software to binarize it to white and black. The coating adhesion was evaluated as "C" in the case of a delaminated area ratio of the coating film of 30% or more, as "B" in the case of a delaminated area ratio of the coating film of 20% or more and less than 30%, as "A" in the case of a delaminated area ratio of the coating film of 10% or more and less than 20%, and as "AA" in the case of a delaminated area ratio of the coating film of less than 10%.

[0073] Cases where the coating adhesion was evaluated as A and AA were evaluated as hot stamped components using Zn-based plated steel sheets and as hot stamped components excellent in coating adhesion. The results are shown in Table 1.

[0074] Referring to Table 1, in each of Comparative Examples 1 and 2, blasting was not performed, therefore the RΔq and area ratio of the Zn oxide layer could not be controlled to within the desired ranges and the coating adhesion fell. In Comparative Example 3, shot blasting using steel balls was performed, but in the same way as Comparative Examples 1 and 2, the RΔq and area ratio of the Zn oxide layer could not be controlled to within the desired ranges and the coating adhesion fell. In each of Comparative Examples 4 to 6, the blasting time of the shot blasting by the steel balls was made longer than Comparative Example 3 whereby it was possible to control the RΔq to within the desired range. However, the Zn oxide layer was crushed at the plating surface and the Zn oxide layer could not be sufficiently removed. As a result, the area ratio of the Zn oxide layer could not be controlled to within the desired range and the coating adhesion fell. In each of Comparative Examples 7 and 8, the particle size of the steel balls was changed to 50 µm whereby it was possible to reduce the area ratio of the Zn oxide layer compared with Comparative Examples 3 and 4 of the same blasting time. However, the RΔq and area ratio of the Zn oxide layer could not be controlled to within the desired ranges and the coating adhesion fell. In each of Comparative Examples 9 and 10, the blasting time of the shot blasting by the steel balls was made longer than Comparative Examples 7 and 8 whereby it was possible to control the RΔq to within the desired range. However, the Zn oxide layer was crushed at the plating surface and the Zn oxide layer could not be sufficiently removed. As a result, the area ratio of the Zn oxide layer could not be controlled to within the desired range and the coating adhesion fell. In each of Comparative Examples 11 and 12, a grit media was used for blasting, but it was dry blasting, therefore the RΔq and / or area ratio of the Zn oxide layer could not be controlled to within the desired ranges and the coating adhesion fell. In Comparative Example 13, wet blasting was performed by steel balls, but the RΔq and area ratio of the Zn oxide layer could not be controlled to within the desired ranges and the coating adhesion fell.

[0075] In contrast to this, in each of Invention Examples 14 to 23, a grit media was used for wet blasting and the RΔq and area ratio of the Zn oxide layer could be controlled to within the desired ranges. As a result, excellent coating adhesion could be achieved. In particular, in each of Invention Examples 14 to 23, by wet blasting being performed, the blasted grit media was flushed away by the liquid and no grit media remained at the surface of the Zn-based plated hot stamped component, therefore it was possible to remarkably improve the corrosion resistance after coating in addition to the coating adhesion.REFERENCE SIGNS LIST

[0076] 1Γ phase 2Fe-Zn solid solution 3Zn oxide layer 4gap 5coating film 6steel material 7bottom layer 8top layer 9Zn-based plating layer

Claims

1. A hot stamped component comprising a steel material provided on its surface with a Zn-based plating layer comprising a top layer and a bottom layer besides the top layer wherein the top layer is a dual phase structure of a Γ phase and Fe-Zn solid solution, and the bottom layer is a single phase structure of a Fe-Zn solid solution, and a Zn oxide layer on a surface of the Zn-based plating layer, wherein a root mean square slope RΔq of a roughness curve at a surface of the hot stamped component is 0.25 or less, and an area ratio of the Zn oxide layer at a surface of the hot stamped component is 30% or less.

2. A method of production of the hot stamped component according to claim 1, comprising wet blasting the hot stamped component using a grit media.

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