Press-forming mold and method for manufacturing protective film for press-forming mold

Abstract

A press-forming mold has a protective film for preventing seizing during press-forming formed on at least a forming surface that comes into contact with a formed body. The protective film is formed by PVD. An arbitrary selection section extracted from the surface of the protective film is divided into a plurality of individual sections; and, when the gradient of the surface at the n.sup.th division point is represented by (dZn/dXn), taking N to represent the number of divisions, the root-mean-square Rq calculated by the following numerical expression is no greater than 0.032. $R q = \sqrt{\frac{1}{N} {.Math.}_{n = 1}^{N} {(\frac{d Z_{n}}{d X_{n}})}^{2}}$ It is thereby possible to improve the seizing resistance of a press-forming mold having a protective film formed by PVD.

Claims

1. A press-forming mold comprising: a protective film for preventing seizing during press-forming, which is formed on at least a forming surface that comes into contact with a formed body, wherein: said protective film is formed by PVD; an arbitrary selection section extracted from the surface of the protective film is divided into a plurality of individual sections; and, when the n.sup.th division point from an end of said selected section is defined as being located a distance of dXn in the direction of extension of said selected section and dZn in the height direction from the (n1).sup.th division point, and the gradient of the surface at said n.sup.th division point is represented by (dZn/dXn), taking N to represent the number of divisions, the root-mean-square Rq calculated by the following expression is no greater than 0.032: $R q = \sqrt{\frac{1}{N} {.Math.}_{n = 1}^{N} {(\frac{d Z_{n}}{d X_{n}})}^{2}} .$

2. The press-forming mold according to claim 1, wherein said protective film is one that is formed by PVD in which a metal material containing at least 50 atom % of Al is used as a target.

3. The press-forming mold according to claim 2, wherein said protective film is one in which a first thin film comprising a TiAlN-based material is formed on a side that comes into contact with said formed body.

4. The press-forming mold according to claim 3, wherein said protective film is one in which a second thin film comprising a CrN-based material is formed on said forming surface, and said first thin film formed on the second thin film.

5. A method for manufacturing a protective film for a press-forming mold in which a protective film for preventing seizing during press-forming is formed on at least a forming surface of the press-forming mold that comes into contact with a formed body, the method comprising: forming a protective film by PVD on said forming surface in a reaction gas atmosphere using, as a target, a metallic material to become said protective film; and polishing the surface of the protective film; wherein in said polishing the surface of the protective film, the polishing is performed so that: an arbitrary selection section extracted from the surface of said protective film is divided into a plurality of individual sections; and when the n.sup.th division point from an end of said selected section is defined as being located a distance of dXn in the direction of extension of said selected section and dZn in the height direction from the (n1).sup.th division point, and the gradient of the surface at said n.sup.th division point is represented by (dZn/dXn), the root-mean-square Rq calculated by the following expression is no greater than 0.032: $R q = \sqrt{\frac{1}{N} {.Math.}_{n = 1}^{N} {(\frac{d Z_{n}}{d X_{n}})}^{2}} .$

6. The press-forming mold according to claim 1, wherein the protective film comprises: a first film formed on the forming surface and comprising a CrN-based material and having a thickness in range from 2 to 5; and a second film formed on the first film and comprising a TiAlN-based material and having a thickness in range from 1 to 5.

7. The press-forming mold according to claim 1, wherein the root-mean-square Rq is equal to or less than 0.030.

8. The press-forming mold according to claim 1, wherein the PVD comprises arc ion plating.

9. A method for manufacturing a protective film for a press-forming mold, the method comprising: forming a thin film on the mold comprising: placing the mold on a rotary table connected to a bias power source in a chamber; supplying a process gas into the chamber; placing a target comprising a metal material and connected to an arc power source in the chamber; supplying power from the bias and arc power sources, to generate an arc discharge between the mold and the target, and form an arc spot on a surface of the target, such that electrical energy concentrating on the arc spot causes a component of the metal material to instantaneously vaporize and ionize, a product of a reaction between the ionized metal material and the process gas being formed on a surface of the mold to form the thin film; and after the thin film reaches a predetermined thickness, removing the mold from the chamber; and polishing a surface of the thin film until the surface has a root mean square surface roughness (Rq) which is not greater than 0.032.

10. The method claim 9, wherein the target comprises 52 to 55 atom % of Al.

11. The method of claim 10, wherein the target further comprises 20 to 22 atom % of Ti, 20 to 22 atom % of Cr, and Si.

12. The method of claim 9, further comprising: before the forming of the thin film, forming a base film comprising a CrN-based material on the surface of the mold by arc ion plating using a target comprising metal Cr and 50 atom % or less of Group 4, Group 5, or Group 6A metal elements.

13. The method of claim 9, wherein the polishing comprises: tracing the surface of the protective film using a stylus displacement pickup provided to a surface roughness tester; analyzing a result of the tracing of the surface of the protective film using an analysis software; and calculating the root mean square surface roughness (Rq) based on the analyzed result.

14. The method of claim 13, wherein the calculating of the root mean square surface roughness (Rq) comprises: extracting an arbitrary selection section extracted from the surface of the protective film and dividing the selected section into a plurality of individual sections; and when the n.sup.th division point from an end of said selected section is defined as being located a distance of dXn in the direction of extension of said selected section and dZn in the height direction from the (n1).sup.th division point, and the gradient of the surface at said n.sup.th division point is represented by (dZn/dXn), taking N to represent the number of divisions, calculating the root-mean-square Rq by the following expression: $R q = \sqrt{\frac{1}{N} {.Math.}_{n = 1}^{N} {(\frac{d Z_{n}}{d X_{n}})}^{2}} .$

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an SEM photograph (magnification: 500) showing the surface of a protective film in a press-forming mold according to the present invention;

(2) FIG. 2 is an SEM photograph (magnification: 500) showing the surface of a post-lapping protective film having a large surface roughness;

(3) FIG. 3 is an SEM photograph (magnification: 500) showing the surface of a conventional post-lapping protective film;

(4) FIGS. 4(a) through (c) show the method for adjusting the surface roughness of the protective film in the press-forming mold according to the present invention in comparison with a conventional method for adjusting the surface roughness;

(5) FIG. 5 is a graph showing the change in the number of possible shots that can be performed by the press-forming die relative to the parameter Rq;

(6) FIG. 6 is a schematic diagram showing press-forming of a plate material;

(7) FIGS. 7(a) through (d) show the results of measuring the surface roughness of each test piece in the examples of the present invention;

(8) FIGS. 8(a) and (b) show an example of the surface of the protective film before and after lapping in an instance in which the surface roughness is defined by maximum height; and

(9) FIGS. 9(a) and (b) are schematic diagrams showing the surface states in which the surface roughness is defined using a conventional reference.

BEST MODE FOR CARRYING OUT THE INVENTION

(10) A press-forming mold according to an embodiment of the present invention will now be described. The press-forming mold according to the present invention is a die 11, a punch 12, and other elements in a press-forming device 10 such as that shown, e.g., in FIG. 6, and is used as follows. Specifically, a formed body, e.g., a plate material 2, is placed on the punch 12, and an elastic force is then applied across a pad 13 or a similar element using, e.g., an elastic member, whereby the plate material 2 is fixed on the punch 12. In this state, the die 11 is lowered from above the plate material 2, and the plate material 2 is sandwiched between the die 11 and the punch 12, whereby the plate material 2 is press-formed to a predetermined shape.

(11) In this example shown in FIG. 6, since shearing work and bending work are repeatedly applied to the plate material 2, in portions of the die 11 and the punch 12 that come into contact with the plate material 2, friction occurs and frictional heat is generated, whereby seizing occurs on the surface of the die 11 and the punch 12, and the die 11 and the punch 12 is rendered unusable.

(12) In order to prevent this seizing, in the press-forming mold of the present invention, a protective film for preventing seizing is formed on at least the forming surface that comes into contact with the formed body. In the present invention, the protective film is formed by a PVD technique such as ion plating.

(13) The protective film is formed, e.g., by a PVD technique in which a metal material containing at least 50 atom % of Al is used as the negative electrode (target). In other words, the metal material used as the target contains, e.g., 52 to 55 atom % of Al. The metal material contains, other than Al, e.g., 20 to 22 atom % of Ti, 20 to 22 atom % of Cr, and about 5% of Si.

(14) In the protective film in the present invention, e.g., Nitrogen is used as the process gas, and the metal material is vapor-deposited onto the surface of the mold, whereby a thin film made from a TiAlN-based material (first thin film) is formed at a thickness of, e.g., 1 to 5 m. The TiAlN-based protective film has been conventionally used as a protective film, and is formed by PVD, whereby droplets, resulting from scattering and adhesion of metal that could not be ionized when a part of the target metal of the evaporation source evaporated, are formed on the surface of the protective film. Performing press-forming while the droplets remain adhered to the protective film causes seizing of the press-forming mold, and the droplets are therefore removed by polishing in the present invention. In the present invention, instead of using the conventional reference for the surface roughness, the root-mean-square Rq as stipulated in JIS B0601 and JIS B0031 is used, and this parameter Rq is managed so as to be no greater than 0.032. Specifically, where an arbitrary section of the surface of the protective film is extracted and divided into a plurality of individual sections, with N representing the number of divisions, and the n.sup.th division point from the end of the selected section is defined as being located at a distance of dXn in the distance of extension of the selected section and dZn in the height direction from the (n1).sup.th division point, and the gradient of the surface at the n.sup.th division point is represented by (dZn/dXn), the parameter Rq is calculated by the following expression 3 as the root-mean-square of the gradient at each division point. For example, the section from 1 mm to 4 mm is divided into 1668 to 6667 sections, and the parameter Rq is calculated.

(15) $\begin{matrix} R q = \sqrt{\frac{1}{N} {.Math.}_{n = 1}^{N} {(\frac{d Z_{n}}{d X_{n}})}^{2}} & [Expression 3] \end{matrix}$

(16) FIG. 1 is an SEM photograph showing the surface of the protective film in a press-forming mold according to the present invention; FIG. 2 is an SEM photograph showing the surface of a post-lapping protective film having a high surface roughness; and FIG. 3 is an SEM photograph showing the surface of a conventional post-lapping protective film. The magnification in each of FIGS. 1 to 3 is 500. Conventionally, when the droplets are lapped, the surface roughness of the lapped protective film is managed by maximum height Ry, the ten-point average roughness Rz and the arithmetic average roughness Ra stipulated in JIS B0601; therefore, as shown in FIG. 4(b), only the tips of the convex and concave parts on the surface are removed by lapping, and sharp corners remain at the tips of the convex parts and the concave parts on the surface of the protective film as shown in FIG. 3. Then, a problem is presented in that the protective film is more prone to impact failure or fatigue failure or the adhesion of polishing residue will result in a shorter mold lifespan, or that there will be a variation in the seizing resistance of the mold between an instance in which the slope between the convex and concave parts on the surface of the protective film has a steep gradient and an instance in which the slope has a gentle gradient. However, in the present invention, as described above, the surface roughness is managed by the gradient at each division point using the parameter Rq, and polishing is performed so that the parameter Rq is no greater than 0.032. The convex and concave structures on the surface are thereby polished so as to be uniformly smooth as shown in FIG. 4(c), and no sharp portions remain on the surface of the protective film as shown in FIG. 1. It is thereby possible in the present invention to avoid the abovementioned problems, significantly reduce the friction between the protective film and the press-formed item such as a metal plate, prevent seizing of the mold, and thereby dramatically improve the lifespan of the press-forming mold. Even if the surface roughness of the protective film is managed by the parameter Rq, if the Rq exceeds 0.032, concave parts remain on the surface of the protective film as shown in FIG. 2, and the effect of the present invention cannot be sufficiently obtained.

(17) The reason for the numerical limitation on the parameter Rq in the present invention will now be described with reference to FIG. 5. FIG. 5 is a graph showing the change in the number of possible shots that can be performed by a press-forming mold in relation to the parameter Rq. Each of the plots on the graph represent the result of tests in the examples described further below, White circle plots represent instances in which a TiAlN-based protective film is formed by arc ion plating; black circle plots represent instances in which a TiC-based protective film is formed by arc ion plating; the triangular plot represents an instance in which a VC-based protective film is formed by molten salt immersion; and the square plot represents an instance in which a TiN-based protective film is formed by arc ion plating. The solid line in FIG. 5 represents the relationship between the number of possible shots that can be performed by the press-forming mold in relation to Rq, calculated on the basis of endurance test results for the TiAlN-based protective film. As shown in FIG. 5, in the region in which the parameter Rq exceeds 0.032, the number of possible shots that can be performed by the press-forming mold is less than 300, representing a short mold lifespan. However, once the parameter Rq is equal to or less than 0.032, the number of possible shots dramatically increase to over 1000, and reaches 6000 at a parameter Rq of 0.030. Thus using parameter Rq to manage the surface roughness of the protective film makes it possible to dramatically improve the durability lifespan of the mold. As shown in FIG. 5, the parameter Rq is required to be equal to or less than 0.032, and is preferably equal to or less than 0.030. However, regardless of these values, a smaller Rq is always preferable. As shown in FIG. 5, in an instance in which a VC-based protective film is formed by molten salt immersion, the number of possible shots that can be performed by the press-forming mold is about 2000, and the effect of improving the lifespan is smaller than that in the instance of PVD such as arc ion plating. Accordingly, in the present invention, the protective film is preferably formed by a PVD technique such as arc ion plating.

(18) A protective film representing a base (second thin film) made from, e.g., a CrN-based material may be formed on the surface of the mold at a thickness of, e.g., 2 to 5 m in order to improve the adhesion between the first thin film and the mold, which is the substrate, as well as the pressure resistance of the protective film as a whole.

(19) A method for manufacturing a protective film for a press-forming mold according to the present invention will now be described. First, a mold for protective film formation is introduced into a chamber into which a processing gas, e.g., nitrogen gas, is supplied. The mold is then placed on, e.g., a rotary table connected to, e.g., a bias power source. A target having, e.g., a flat plate shape connected to an arc power source is provided to the side wall of the vacuum chamber. The target is, e.g., a metal plate containing 52 to 55 atom % of Al, and further containing 20 to 22 atom % of Ti, 20 to 22 atom % of Cr, and about 5% of Si. When power is supplied from both of the power sources, an arc discharge is generated between the mold and the target, and an arc spot is formed on the surface of the target. Electrical energy concentrating on the arc spot causes components such as Al, Ti, and Cr in the metal material to instantaneously vaporize and ionize, and scatter in the vacuum chamber. The ionized metal particles react with the process gas, e.g., N, adhere as a thin film on the surface of the mold, and a protective film is formed. When the protective film is formed on the surface of the mold at the predetermined thickness, the mold having the protective film is removed from the vacuum chamber.

(20) Next, the protective film formed on the surface of the mold is polished. Lapping is conventionally performed by hand lapping using, e.g., a rotary tool, or performed using a sponge-type polishing material having a small surface roughness; e.g., no higher than #600. In other words, a soft polishing material is conventionally used to avoid the risk of overpolishing the surface of the protective film. However, in the present invention, lapping is performed through applying a diamond paste on a relatively hard polishing material obtained, e.g., by solidifying felt, so that the surface roughness is, e.g., about #3000. It is thereby possible to resolve any insufficient droplet polishing that results from conventional polishing methods, and to sufficiently polish the surface of the protective film.

(21) When polishing the protective film, the parameter Rq is used to manage the surface roughness of the protective film. Specifically, an arbitrary section of the surface of the protective film is extracted and divided into a plurality of individual sections; and, when the n.sup.th division point from the end of the selected section is defined as being located at a distance of dXn in the direction of extension of the selected section and dZn in the height direction from the (n1).sup.th division point, and the gradient of the surface at the n.sup.th division point is represented by (dZn/dXn), taking N to represent the number of divisions, Rq is calculated by the following expression 4 as the root-mean-square of the gradient at each division point. For example, the section from 1 mm to 4 mm is divided into 1668 to 6667 sections, and the parameter Rq is calculated.

(22) $\begin{matrix} R q = \sqrt{\frac{1}{N} {.Math.}_{n = 1}^{N} {(\frac{d Z_{n}}{d X_{n}})}^{2}} & [Expression 4] \end{matrix}$

(23) In the present invention, polishing is performed so that the parameter Rq of the surface roughness is no greater than 0.032, whereby the convex and concave structures on the surface are polished so as to be uniformly smooth, and as shown in FIG. 1, no sharp portions remain on the surface of the protective film. It is thereby possible to avoid the problem of the protective film being more prone to impact failure or fatigue failure or the adhesion of polishing residue resulting in a shorter mold lifespan, or of there being a variation in the seizing resistance of the mold. In addition, it is possible to: significantly reduce the friction between the protective film and the press-formed article, e.g., the metal plate; prevent seizing of the mold; and thereby dramatically improve the lifespan of the press-forming mold.

(24) In an instance in which a protective film representing a base (second thin film) made from, e.g., a CrN-based material is formed on the surface of the mold, the second thin film may be formed by arc ion plating, prior to the formation of the first thin film, using a target containing metal Cr and 50 atom % or less of Group 4, Group 5, or Group 6A metal elements.

EXAMPLES

(25) The effect of the configuration of the present invention will now be described through a comparison of examples thereof with comparative examples. First, a protective film was formed on the surface of a press-forming mold. The mold in the present examples is a die and a punch made from steel equivalent to SKD11 in which the hardness has been adjusted to 60 HRC. The protective film was formed by molten salt immersion (TD) or arc ion plating. In molten salt immersion, the mold was immersed in a VC-based salt bath and a VC-based protective film was formed on the surface, quenching and tempering processes were then applied, and lapping was then performed, whereby a protective film was formed at a thickness of 8 m. In arc ion plating, using a target (negative electrode) material containing 50 atom % of Ti and 50 atom % of Al (with each of the components also containing unavoidable impurities), and using nitrogen gas or a gas mixture of nitrogen and a hydrocarbon such as methane as the process gas to be introduced into the chamber, a TiC-based, TiN-based, or a TiAlN-based protective film was formed at a variety of thicknesses on the surface of the mold by arc ion plating in each of the process gas atmospheres.

(26) Then, each of the protective films was polished. The polishing was managed by tracing the surface of each of the protective films using a stylus displacement pickup (tip shape: conical; tip diameter: 5 m) provided to a surface roughness tester (Tokyo Seimitsu; HANDYSURF E-35B; height-direction resolution: 0.01 m), analyzing the tracing results using an analysis software (Tokyo Seimitsu; TiMS Light), and calculating the Rq in compliance with JIS B0601 and JIS B0031. The cutoff value c of the measurement results was set to 0.8 mm, and the measurement length l was set to 4.0 mm. At the same time, the arithmetic average roughness Ra and the maximum height Ry stipulated in JIS B0601 were calculated for each of the examples and comparative examples. For each of the examples and comparative examples, the die 11 and the punch 12 having a protective film formed thereon were placed on a press-forming device 10 as shown in FIG. 6, and a metal plate 2 was placed on the punch 12. An elastic force was then applied across a pad 13 or a similar element using, e.g., an elastic member, whereby the plate material 2 was fixed on the punch 12. In this state, the die 11 was lowered from above the plate material 2, and the plate material 2 was sandwiched between the die 11 and the punch 12, whereby the plate material 2 was press-formed. For the metal plate 2, a hot-rolled soft steel plate (SPH590) having a plate thickness of 3.2 mm was used, and was press-formed without a lubricant oil (i.e., with only the anti-rust material for the work material). The processing speed was 40 shots per minute (spm), and the ironing ratio (amount of reduction in plate thickness/original plate thickness) was 7%. The results of measuring the number of possible shots in each of the examples and comparative examples are shown in Table 1. For the third example and the fourth, seventh, and eighth comparative examples, tracing was performed using the abovementioned surface roughness tester, and the tracing results were analyzed using the abovementioned analysis software. The analyzed surface roughness curves of the protective films are shown in FIGS. 7(a) to (d).

(27) TABLE-US-00001 TABLE 1 Film Film No. of formation Film thickness Ra ry possible No. method type m m m Rq shots Examples 1 PVD(AIP) TiAlN 10 0.06 0.80 0.032 No 2 PVD(AIP) TiAlN 10 0.07 0.59 0.031 seizing 3 PVD(AIP) TiAlN 10 0.04 0.50 0.025 at 3000 Comparative 4 Molten salt VC 8 0.04 0.53 0.021 2000 examples immersion (TD) 5 PVD(AIP) TiN 3 0.13 1.37 0.042 300 6 PVD(AIP) TiC 5 0.08 0.85 0.050 945 7 PVD(AIP) TiAlN 10 0.12 1.08 0.054 40 8 PVD(AIP) TiAlN 10 0.08 0.78 0.036 120

(28) As shown in Table 1, in the first through third examples, which satisfy the configuration of the present invention, the number of possible shots dramatically improved in comparison with the fourth comparative example, in which the protective film was formed by molten salt immersion, and with fifth through eighth comparative examples in which the parameter Rq exceeded 0.0352, and no seizing occurred even after 3000 shots.

(29) As shown in Table 1, when a comparison is made between the first, second, and third examples and the seventh and eighth comparative examples, all of which involve the use of a TiAlN-based coating, it can be seen that there is a clear correlation between the number of possible shots and Rq. In addition, in light of the fact that there is a large difference in the number of possible shots between the second example and the eighth comparative example, between which the arithmetic average roughness Ra has a substantially identical value, and between the first example and the eighth comparative example, between which the maximum height Ry is substantially identical, it can be seen that the lifespan of the mold is strongly correlated with Rq. When the third example and the fourth comparative example are compared, the shot lifespan is greater in the third example, even though the fourth comparative example has a smaller Rq; this indicates that the TiAlN coating formed by PVD has a longer lifespan. Also, as shown in FIG. 7(a) (seventh comparative example) and FIG. 7(b) (eighth comparative example), both the convex and concave structures on the surface of the protective film are larger when Rq is exceeding 0.032, and the convex and concave structures decrease in size with the decrease in Rq. Once the parameter Rq falls to 0.032 or less, the convex and concave structures on the surface of the protective film become extremely small as shown in FIG. 7(c) (third example). Therefore, in the present invention, performing polishing while managing the surface roughness using the parameter Rq makes it possible to reliably improve the lifespan of the press-forming mold. In FIG. 7(d) (fourth comparative example), since the protective film is formed by molten salt immersion, adhesion of droplets does not take place, and the effect of the present invention of managing the surface roughness using the parameter Rq is smaller.

INDUSTRIAL APPLICABILITY

(30) The present invention makes it possible to improve the seizing resistance of the press-forming mold having a protective film formed by PVD, and contributes towards improving the wear resistance of the press-forming mold.

KEY

(31) 10 Press-forming device 11 Die 12 Punch 13 Pad 2 Plate material (metal plate)

Press-forming mold and method for manufacturing protective film for press-forming mold

Assignee

Inventors

Cpc classification

Classification Explorer

C23C14/5886

CHEMISTRY; METALLURGY

Classification Explorer

C23C14/0641

CHEMISTRY; METALLURGY

Classification Explorer

B21D37/01

PERFORMING OPERATIONS; TRANSPORTING

Classification Explorer

B29C33/68

PERFORMING OPERATIONS; TRANSPORTING

International classification

Classification Explorer

B29C33/68

PERFORMING OPERATIONS; TRANSPORTING

Classification Explorer

C23C14/06

CHEMISTRY; METALLURGY

Classification Explorer

B21D37/01

PERFORMING OPERATIONS; TRANSPORTING

Classification Explorer

C23C14/58

CHEMISTRY; METALLURGY

Abstract

Claims

Description