JP3435742B2 - Forging tool and its manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a forging tool used for cold or warm forging with a dramatically extended life.

[0002]

2. Description of the Related Art For example, a plus-shaped groove forming a head of a stainless steel screw is formed by forging. Specifically, a method of forming a positive groove by punching a plus screw punch, which is a male type of the positive groove of the screw, on the head of the screw and engraving the groove. In the forging process such as the screw head machining, cracks occur in the surface layer of the Phillips screw punch shortly after starting the engraving when the engraving speed is increased, which causes the forging tool, the male die, to crack. Defects such as chips or breaks occur early and reach the end of their life.

In order to prevent such defects, it has been attempted to subject the working surface of the forging tool to a surface coating treatment such as CVD or PVD. Even with such a latest surface coating treatment, cracks precede wear and wear. Therefore, it is not an effective measure. In addition, attempts have been made to improve the materials that compose forging tools, and Co-based high-speed tool steels containing expensive Co that improve heat resistance and hardness have also been proposed, and in order to further improve toughness, heat treatment hardness It has also been proposed to keep the Vickers hardness as low as 880 or less.

[0004]

DISCLOSURE OF THE INVENTION The present inventors have found that the cause of cracks is the generation of heat and interfacial frictional heat associated with the plastic deformation of the work material during the forging process. Improvements have been made to extend the life by changing the selection and heat treatment conditions. However, the current situation is that a satisfactory tool life has not been obtained even if the alloy composition or the heat treatment condition is changed. By the way, a method of carburizing a high-speed tool steel used for cutting such as a drill and an end mill to improve the service life is disclosed in Japanese Patent Publication No. 58-26430.
JP-A-60-177167, JP-A-5-163563
No., etc. However, in the applications of drills and end mills disclosed in these documents, the toughness of the core is not required as much as the forging tool, so that a material having a relatively high carbon content of 0.4% to 0.9% is used. If such a high carbon content material is used as it is as a forging tool,
If the toughness of the core part is insufficient, the risk of breakage or breakage of the forging tool increases, which is not preferable.

The object of the present invention is to combine hardness and toughness required for a forging tool, and to prevent cracks from developing easily.
It is an object of the present invention to provide a forging tool having an extremely long tool life and a manufacturing method thereof.

[0006]

The present invention has a different idea from the conventional one, and uses a toughness-rich alloy having a low carbon content with respect to the composition of a high-speed tool steel as a raw material. We have developed a forging tool with a long service life that has both toughness and hardness by suppressing the progress of cracks by increasing the carbon content of the surface layer relative to the core to increase hardness and leaving compressive stress on the surface. Is. That is, the present invention is
By weight ratio, C 0.1 to 0.27 %, Si 1.5% or less, Mn 1.0%
Hereinafter, carburizing having a composition of Cr 2.0 to 7.0%, one or two of W or Mo as W equivalent (W + 2Mo) of 3.0 to 24.0%, V 5.0% or less, and the balance Fe and inevitable impurities. With respect to the core portion other than the portion, the carburized surface layer portion has a composition having a higher carbon content than the above composition, and the core portion has a fracture toughness value of 100 kg / (mm 2/3). As described above, the Vickers hardness is 600 or less, the machined surface of the tool is 700 or more in Vickers hardness, and the compressive stress remains in the carburized surface layer portion.

In the present invention, in order to secure the strength of the core portion, the core portion preferably has a Vickers hardness of 400 to 600. Further, in order to maintain abrasion resistance, the depth of the carburized layer is Is preferably 500 μm or more.

The above-mentioned present invention can be obtained by the following manufacturing method of the present invention. That is, the manufacturing method of the forging tool of the present invention is C 0.1 to 0.27 % by weight and Si 1.5% by weight.
Below, Mn 1.0% or less, Cr 2.0 to 7.0%, and one or two of W or Mo as W equivalent (W + 2Mo) 3.0 to 24.0
%, V 5.0% or less, the balance consisting of Fe and inevitably contained impurities, or a part of Fe of the above composition is Co 10.0% or less, Ni 4.0% or less, and Al 0.5% or less In the material with the composition replaced above,
After carrying out carburizing treatment, quenching, tempering treatment, the surface layer of the material is a composition having a carbon content higher than the above composition, the core other than the carburized portion has a fracture toughness value of 100 kg / (mm 2 minutes of
The third feature is that the Vickers hardness is 600 or less and the machined surface of the tool is 700 or more in Vickers hardness, and compressive stress remains in the carburized surface layer.
The forging tool referred to in the present invention is a punch or die for ordinary punching, a punch used for forming a plus-shaped groove or the like in the screw head by plastic working, and a groove for a pulley or the like. It is a tool for plastic working to which high stress is repeatedly applied, such as a rolling die used for.

[0009]

One of the most significant features of the present invention is that, in order to prolong the life of the forging tool, a material having a specific composition characterized by a low carbon content is subjected to carburizing treatment on the surface layer and compression. There is a residual stress. By the application of this compressive residual stress, the progress of cracks is suppressed, and due to the interaction with the composition having a low carbon content of the core and having a high fracture toughness value, such as breakage or chipping of the forging tool The defects can be significantly suppressed. Further, in the present invention, the surface layer portion has a composition having a higher carbon content than the core portion composition to obtain the surface hardness required for the forging tool, and thus it also has an action of preventing the initiation of cracks. Specifically for securing the wear resistance is required to be 700 or more working surface hardness in Vickers hardness.

Another important feature of the present invention is the fracture toughness value (Kic) to be provided as a core by the method of the present invention.
Is more than 100 kg / (mm 2/3). With a forging tool having a fracture toughness value lower than that of the core portion of the forging tool, cracks grow rapidly and the effect of extending the life of the forging tool cannot be expected. Further, the hardness of the core part is defined as 600 or less in Vickers hardness because in a forging tool having a hardness of more than this, the toughness of the core part becomes low, and the effect of suppressing the progress of cracks decreases. is there. In order to obtain mechanical strength as a forging tool, it is preferable that the Vickers hardness is 400 or more.

The manufacturing method of the present invention is characterized in that a material having a specific composition is carburized, then quenched and tempered. By the quenching and tempering treatments after the carburizing treatment of the present invention, the surface of the forging tool has a higher carbon content than the core portion, and a hard surface is obtained. Further, the carbon that has penetrated into the forging tool causes carbides in the material to grow, leaving a compressive stress effective in suppressing the development of cracks in the surface layer portion, and also significantly improving wear resistance. . The thickness of the carburized layer obtained by carburizing treatment, and the hardness varies carburizing conditions, heat treatment conditions after the carburizing, the chemical composition of the matrix phase. The higher the hardness of the carburized part, the better the wear resistance. Therefore, the hardness of the carburized part must be at least HV700 as a forging tool.

If it is too hard, the crack resistance may decrease, so it is desirable that the Vickers hardness is 1000 or less. The depth of the carburized layer is preferably 500 μm or more in order to maintain wear resistance. Further, if the thickness of the carburized layer exceeds 3 mm, even if the crack extension is suppressed, there is a risk that the material to be processed will enter the crack and accelerate the extension of the crack. Therefore, the thickness is preferably 3 mm or less. The forging tool of the present invention further improves wear resistance by forming a hard coating such as titanium nitride on its surface,
It is also possible to improve the life by adding a secondary method such as forming a porous film by homogenization or a method similar thereto to impart lubricity.

Next, the reasons for limiting the alloy composition which is a raw material for manufacturing the forging tool of the present invention and which constitutes the core portion of the forging tool of the present invention will be described. The composition of the core portion is extremely important for ensuring the basic toughness and strength of the forging tool.

C is an element that secures the strength of the core during quenching and tempering after carburization. If it is less than 0.1%, the hardness becomes low, and the strength required as a forging tool cannot be obtained. Although C depends on the balance with the amount of carbide forming elements, if it is less than 0.1%, δ-ferrite is likely to be generated, which causes uneven hardness and causes a decrease in fracture toughness. Therefore, the lower limit of the present invention is set to 0.1%. Further, when the C content is high , the fracture toughness value of the core portion is lowered, and it becomes impossible to obtain the required toughness. Further, if the C content is high, there is a problem that segregation at the time of casting produces carbides of primary crystals, which causes variations in hardness. Therefore, the upper limit of C in the present invention is 0.27 %.
And Si can be contained as an element that improves hardness and heat resistance. However, if it exceeds 1.5%, the mechanical strength decreases, so the upper limit was made 1.5%. Mn can be added to improve hardenability. However, if it exceeds 1.0%, the transformation temperature is lowered and the machinability is impaired, so the upper limit of addition is set to 1.0%.

[0015] Cr is an element effective in securing both the hardenability of the matrix, accelerating the solid solution in the matrix during the quenching of the carbide, and increasing the hardness during carburization, since both the matrix and the carbide are in solid solution. . If it is 2% or less, hardness at the time of quenching and tempering cannot be secured at HRC 40 or more, so the lower limit value was made 2.0%. If 7.0% or more is added, δ
The upper limit was set to 7.0% because ferrite easily forms and stabilizes, and the hardness distribution in the carburized part becomes too steep, which promotes the occurrence of heat cracks.

W and Mo are essential elements for ensuring heat resistance and wear resistance as a forging tool. W and Mo have a substitution property, and 1% Mo and 2% W in weight ratio can be regarded as almost equivalent, which is shown by W equivalent (W + 2Mo). W
The equivalent is required to be at least 3.0% and there is a balance with C, but the hardness of the carburized portion increases as the W equivalent increases. Further, M ₆ C type carbides are generated in the carburized portion, and the undissolved carbides act to impart wear resistance and prevent coarsening of austenite crystal grains. In the present invention, the W equivalent is such that carbides in the surface layer portion formed by carburization can be made extremely fine, and the upper limit is 24.0%, which is higher than that of ordinary high speed tool steel. If the content exceeds this range, the fracture toughness value required for the core of the forging tool cannot be obtained, which is not preferable.

When V coexists, V forms a hard MC type carbide and is the most effective element for imparting the wear resistance required for a forging tool. 0.7% even when coexisting with carbon
The following will also form a solid solution in the matrix and enhance the secondary curability. If it is 5.0% or more, the amount of MC-type carbides generated is too large, and the fracture toughness value of the core portion decreases, so the upper limit was made 5.0%.

The elements that can be selectively contained in the present invention will be described below. Co mainly forms a solid solution in the matrix and has the effect of improving hardness and heat resistance. On the other hand, the mechanical strength and fracture toughness values gradually decrease as the added amount increases. Therefore,
In the present invention, it is not an indispensable additional element, but it is added in applications where high absolute hardness is required. At this time, if the content is 10.0% or more, the mechanical strength and the fracture toughness value are too low to be used as a forging tool, so the upper limit value in the case of adding is set to 10.0%.

Ni has the effect of suppressing the formation of δ-ferrite and grading the change in hardness of the carburized and hardened part, and it is often effective to contain Ni. However, more than 4.0%, the lower the A ₁ transformation temperature, increasing the annealing hardness, as it reduces the machinability, was the upper limit of 4.0%.

Al is also used as a deoxidizer, but δ
It can be contained because it has the same effect as Ni, that is, suppression of ferrite. However, if it exceeds 0.5%, the inclusions increase and secondary products such as AlN are generated to lower the fracture toughness value, so the upper limit value was made 0.5%.

[0021]

【Example】

Example 1 An example of the present invention will be shown below. Material No. of Table 1 JIS M40F Phillips screw punches were formed by a cold hobbing method on an annealed material of 12 mmφ material having the composition shown in 1-10. This punch is carburized at 780 ° C for 6 hours, then quenched at 1160 ° C and 1.5Hr 3 at 560 ° C.
The tempering process was performed once. With this punch, SUS304
A plus-shaped groove was formed at a rate of 160 times / minute on a workpiece (diameter 3.5 mmφ), and the number of stamps until the punch was broken or reached the wear life was measured. At this time, the life was evaluated when the plus screw punch was worn or broken by 0.3 mm or more.

As a comparative example, No. 1 in Table 1 was used. Phillips screw punches having the compositions shown in 11 to 13 were manufactured in the same manner as in the example of the present invention, and the same life evaluation was performed. Obtained life, Vickers hardness of the surface that is the tool processing surface formed on the Phillips screw punch, surface residual stress value, carburized layer depth, Vickers hardness of the core, and Charpy impact value and fracture toughness of the core Values (Kic) are shown in Tables 2 and 3. The Charpy impact value and the fracture toughness value (Kic) are measured with a test piece from which a carburized layer of a Phillips screw punch manufactured under the same conditions is removed, that is, a core portion in the present invention.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

[0027]

[Table 5]

Further, the material No. of the present invention. CV after carburizing on 1
The same evaluation was made for a plus screw punch in which a Ti (CN) film was formed to a thickness of 5 μm by D treatment and then subjected to quenching and tempering treatment, and for a hardened and tempered finished product in which a Ti (CN) film was formed in a thickness of 3 μm by PVD. I went. This result also shows the sample Nos. Shown as 14 and 15.

As shown in Tables 2 and 3, the sample No. which is a comparative example. Nos. 11 to 13 have a lower fracture toughness value of the core than the sample of the present invention, and therefore the tool life is significantly shorter than that of the present invention. It can be seen that the Phillips screw punch of the present invention has a long life of about 10 times or more in absolute value as compared with the comparative example. In addition, the sample Nos. 14
As shown in FIGS. 15 and 15, C is added to the Phillips screw punch of the present invention.
It was found that the life can be further improved by performing the surface coating treatment of Ti (CN) with VD or PVD.

(Example 2) No. 1 shown in Table 1 After finishing the material having the composition of No. 1 into a rolling die shape for groove machining of a pulley having an outer diameter of 200 mmφ, a width of 40 mm, and a thread groove depth of 3 mm, carburizing and quenching and tempering similar to Example 1 are performed. Sample No. of die making I got 16. Also, conventionally used JIS SKH51 and JIS SKD
No. 11 has the same rolling die shape.
17 and 18 were obtained. Using these rolling dies, JI
When I manufactured a 50 mmφ pulley made by S SPCC,
As shown in Table 4 and Table 5, rolling die sample N of the present invention
o. It was found that 16 has a tool life 10 times or more that of a conventional rolling die.

[0031]

As described above, the forging tool of the present invention has both the hardness and the toughness required, and it is possible to suppress the propagation of cracks generated under the use condition in which a high stress is repeatedly applied. As a result, It is possible to obtain a long tool life. Further, since the forging tool of the present invention can suppress the propagation of cracks, the processing speed can be increased and a great effect can be expected in improving productivity.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI C23C 8/22 C23C 8/22 // B21K 1/48 B21K 1/48 B (58) Fields investigated (Int.Cl. ⁷ , DB name) B21J 1/00-13/14 B21J 17/00-19/04 B21K 1/00-31/00 C22C 38/00-38/60

Claims (6)

(57) [Claims] 1. A weight ratio of C 0.1 to 0.27 % and Si 1.5%
Below, Mn 1.0% or less, Cr 2.0 to 7.0%, and one or two of W or Mo as W equivalent (W + 2Mo) 3.0 to 24.0
%, V 5.0% or less, with the balance being Fe and the unavoidably contained impurities other than the carburized portion, the carburized surface layer portion has a higher carbon content than the above composition. The fracture toughness value of the core is 100 kg / (m
m / 3 power) or more, Vickers hardness is 600 or less,
A forging tool characterized in that the machined surface of the tool is 700 or more in Vickers hardness, and compressive stress remains in the carburized surface layer. 2. The composition of the core portion according to claim 1, Fe
A forging tool characterized by replacing a part of Co with 10.0% or less. 3. A forging tool, wherein a part of Fe in the composition of the core portion according to claim 1 or 2 is replaced by Ni 4.0% or less . 4. A forging tool, wherein a part of Fe in the composition of the core portion according to claim 1 is replaced by Al 0.5% or less . 5. The Vickers hardness of the core is 400 to 600.
Heading tool according to any one of claims 1 to 4, characterized in that. 6. A weight ratio of C 0.1 to 0.27 % and Si 1.5%
Below, Mn 1.0% or less, Cr 2.0 to 7.0%, and one or two of W or Mo as W equivalent (W + 2Mo) 3.0 to 24.0
%, V 5.0% or less, the balance consisting of Fe and inevitably contained impurities, or a part of Fe of the above composition is Co 10.0% or less, Ni 4.0% or less, and Al 0.5% or less In the material with the composition replaced above,
After carrying out carburizing treatment, quenching, tempering treatment, the surface layer of the material is a composition having a carbon content higher than the above composition, the core other than the carburized portion has a fracture toughness value of 100 kg / (mm 2 minutes of
(3rd power) or more, the Vickers hardness is 600 or less, the machined surface of the tool is Vickers hardness of 700 or more, and the compressive stress is left on the carburized surface layer portion.