JPH10158780A - Cold tool steel for plasma carburizing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold worked tool combining wear resistance and toughness, particularly, and the one low in crack generating sensitivity of a hardened layer in particular, in the one which is a cold worked tool and having hardness in which matrix hardness is regulated to >=50 HRC. SOLUTION: A steel having a compsn. contg. 0.40 to 0.80% C, 0.05 to 1.50% Si, 0.05 to 1.50% Mn and 1.8 to 6.0% V, furthermore contg. one or >= two kinds among 0.10 to 2.50% Ni, 0.1 to 2.0% Cr and <=3.0% Mo, and the balance Fe with inevitable impurities is subjected to plasma carburizing. This alloy may contain either or both of one or two kinds of <=3.0% Nb and <=3.0% Ta (by <=3.0% in total in the case of two kinds) as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel for producing a tool for cold working by plasma carburization, and a tool produced from the steel.

[0002]

2. Description of the Related Art Conventionally, as a material for a cold working tool requiring severe wear conditions and a high degree of wear resistance, a high-hardness cold die steel or a high-speed tool steel to which primary carbide has been added has conventionally been used. Has been used. The disadvantage of this type of material is that primary carbides are present not only on the surface where wear resistance is required but also on the inside, and if cracks occur on the surface, it easily progresses to the inside, This was the cause of shortening the life.

[0003] As a countermeasure to such a problem, attempts have been made to improve the abrasion resistance by subjecting the material whose hardness has been reduced to a surface treatment, but this method generally increases the cracking sensitivity of the surface treatment layer. Therefore, it has been difficult to obtain a cold tool having both wear resistance and high toughness.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel which overcomes the above-mentioned difficulties and realizes a cold tool having both excellent wear resistance and high toughness, and a steel for the same. An object of the present invention is to provide a method for manufacturing a tool for cold working by using steel and performing a surface treatment by plasma carburization.

[0005]

The cold work tool steel of the present invention comprises:
C: 0.40 to 0.80%, Si: 0.05 to 1.50
%, Mn: 0.05-1.50% and V: 1.8-
6.0%, and Ni: 0.10 to 2.50
%, Cr: 0.1 to 2.0% and Mo: 3.0% or less, and the balance is Fe and unavoidable impurities, with the balance being a cold tool steel for plasma carburization.
This cold tool steel, in addition to the above alloy components,
One or two of Nb: 3.0% or less and Ta: 3.0% or less (in the case of two, a total amount of 3.0% or less) may be contained, and instead or together therewith. , N:
0.02 to 0.15% may be contained.

The method of the present invention for producing a cold tool from these materials comprises forming a cold tool steel for plasma carburizing having any of the above alloy compositions into a substantially tool shape and subjecting the tool to plasma carburizing. VC or V (CN) in carburized layer
Is a cold tool occupying 50% of the total carbide or carbonitride in the area ratio.

[0007]

The present inventors have attempted to overcome the above-mentioned difficulties, and as a means, have chosen to provide a hardened surface layer having excellent wear resistance to a substrate having high toughness. However, cold dies and cold rolls, which are the main applications, are considered to require a base hardness of HRC50 or more from the viewpoint of applied stress, and steel with such hardness is dominant in toughness. Is the occurrence of cracks, and in a mold where cracks occur from the surface hardened part,
Even if the toughness of the matrix is increased, the surface hardened portion is highly sensitive to crack generation, so that cracking proceeds as described above, which shortens the life of the mold and roll.

Therefore, it is necessary to form a surface-hardened layer having a low cracking sensitivity, and in the present invention, plasma carburization was selected in the present invention. VC was found to be optimal as the carbide to be formed in the plasma carburized layer. The VC formed in the carburized layer is hard, and the wear resistance is improved with a small amount of carbide. In addition, VC has cracks in the carburized layer because the individual particles are finer than other carbides. Decrease sensitivity.

A feature of the plasma carburizing method is that quenching and tempering can be continuously performed in the same furnace after carburizing. Taking advantage of this feature, in order to minimize the deformation after carburizing and heat treatment by suppressing the distortion due to heat treatment or to make it possible to omit it, hardenability must be high enough to ensure the strength of the base even for large products. Is done. In order to satisfy this requirement, at least one of Ni, Cr and Mo is added as a hardenability improving element, and one of Nb and Ta or one of Nb and Ta is added to control the form of the primary carbide inevitably formed thereby. Add two.

The reason why the alloy composition is determined as described above in the cold tool steel for plasma carburizing of the present invention is as follows.

C: 0.40 to 0.80% Solid solution in the matrix and determines the hardness of martensite during martensite transformation during quenching. If the amount is too small, the martensite hardness is low, and if the amount is too large, a primary carbide is formed with the carbide forming element before carburizing, and the toughness is reduced. From these points, the above 0.40-0.
An 80% range was defined.

Si: 0.05-1.50% Si is an element necessary for increasing the high-temperature tempering hardness and improving the hardenability. In order to obtain this effect, use 0.
It is necessary that the content is not less than 05%, but if it is too large, the toughness is impaired.

Mn: 0.05-1.50% Element required for enhancing hardenability without forming carbides. For this purpose, at least 0.05% should be present, but if it is present in a large amount, after tempering, Therefore, the upper limit of 1.50% was set because the amount of retained austenite was increased.

V: 1.8 to 6.0% V is an essential component for the steel of the present invention, and is added in a range of 1.8 to 6.0% in order to form an appropriate amount of carbide by carburization.

Ni: 0.10 to 2.50% Works similarly to Mn. If the amount is less than the lower limit of 0.10 to 2.50%, there is no effect, and if the amount exceeds the upper limit, a large amount of austenite remains after quenching.

Cr: 0.1 to 2.0% Cr is added as appropriate because it enhances hardenability and contributes to secondary hardening. The lower limit is the minimum addition amount at which the effect is recognized. A one carbide forming elements, at a large amount M ₃ C,
Since the primary carbide of M ₇ C ₃ is precipitated, the upper limit of 2.0%
Select the addition amount within.

Mo: 3.0% or less Mo improves the hardenability of bainite, and forms a solid solution in the matrix to perform a secondary hardening action.
If the amount is too large, primary carbides are formed to be crystallized and the overall toughness is reduced, so the upper limit is 3.0%.

Nb: 3.0% or less, Ta: 3.0% or less These are carbide forming elements next to V, and bind preferentially to C to suppress crystallization of primary carbides of other elements. When a part is crystallized, it is finely crystallized to prevent a decrease in toughness. If the amount of addition is excessive, a large amount of primary carbides are formed to impair toughness, and the ratio of VC in the total carbides is unfavorably lowered. Therefore, the upper limit is 3.0%. Add an amount within (in total).

N: 0.02 to 0.15% Primary carbide which may inevitably crystallize is refined, and the form of VC formed during carburization is improved.

The amount of VC or V (NC) is at least 50% of the total carbides or carbonitrides in terms of area ratio. By satisfying these conditions, it is possible to achieve both the wear resistance and the toughness of the cold work tool steel. .

Examples and Comparative Examples Cold tool steels for plasma carburizing having the alloy compositions listed in Table 1 were produced.

[0022] Table 1 No. C Si Mn V Ni Cr Mo Nb Ta N Example 1 0.55 0.50 0.50 2.5 0.15 0.50 0.25 - - 0.004 2 0.45 0.90 1.43 3.1 0.20 0.10 0.10 - - 0.008 3 0.65 1.10 0.80 2.0 0.33 1.25 0.85 - -0.026 4 0.73 0.53 0.15 4.4 0.11 2.10 0.02 0.28-0.044 5 0.59 0.18 1.27 3.5 1.75 1.34 1.62 0.10 0.12 0.027 6 0.61 1.07 0.89 2.9 0.51 1.41 0.04 0.15 0.01 0.009 7 0.59 0.28 0.42 3.3 0.12 1.35 1.85--0.035 Comparative example 11 1.51 0.35 0.41 0.29-11.8 1.1---12 0.84 0.33 0.42 1.87-4.1 5.2---13 0.53 0.66 0.34--6.3 0.1---14 0.69 0.52 0.55 0.44-0.2 6.3---15 0.59 0.32 0.22 0.61-0.1 0.1 ---In addition to the above, No. No. 12 shows W: 6.3%, No. 15 contains W: 7.2%.

A test piece having a square cross section of 10 mm on a side and a length of 55 mm was prepared from each steel, and (Ar + H ₂ + C ₃ H)
₈ ) Plasma carburizing was performed in a mixed gas under the treatment conditions of 900 ° C. × 1 hour, followed by quenching (gas cooling) and tempering. Hardness of base, proportion of VC in total carbides,
Abrasion resistance (by Ogoshi type testing machine. Counterpart material SCM41
5, a load of 6 kgf, a sliding speed of 1 m / sec) and a Charpy impact value (10R notched Charpy test piece) were measured. Table 2 shows the results.

Table 2 No. Quenching Tempering Base Hardness VC / Total Carbonization Wear Resistance Charpy (° C) (° C) Length (HRC) Material Area Ratio (%) (mm ² / kg) Impact Value (J / cm ² ) Example 1 1100 500 55 82 8 × 10 ^-7 30 2 1100 500 57 85 7 × 10 ^-7 26 3 1100 500 54 60 9 × 10 ^-7 18 4 1100 500 53 71 4 × 10 ^-7 16 5 1100 500 57 44 ^{9 × 10 -7 26 6 1100 500} 54.0 59 8 × 10 -7 24 7 1100 500 56.2 53 6 × 10 -7 30 Comparative example 11 1050 500 56 9 20 × 10 -7 8 12 1100 540 61 21 18 × 10 - ⁷ 7 13 1100 500 43 0 39 × 10 ^-7 16 14 1100 500 47 17 39 × 10 ^-7 17 15 1100 500 44 31 27 × 10 ^-7 17

[0025]

The cold work tool steel for plasma carburizing of the present invention
When plasma carburizing is selected as the carburizing method, grain boundary oxidation associated with the conventional carburizing method is avoided, a factor that impairs the toughness of the hardened layer is eliminated, and VC as a carbon (nitride) is used.
As a main component, crystallization of carbides in the finest form is realized, so that the combination thereof suppresses the susceptibility of the hardened layer to cracking, and provides both wear resistance and toughness.

If this steel is used as a material, the distortion due to the heat treatment following the plasma carburization is small,
The intended cold tool can be manufactured. The present invention is particularly significant when applied to the production of cold forging dies, cold rolls, and the like.

Claims (3)

[Claims] 1. C: 0.40 to 0.80%, Si: 0.
0.5-1.50%, Mn: 0.05-1.50% and V: 1.8-6.0%, and further Ni: 0.10
-2.50%, Cr: 0.1-2.0% and Mo:
3.0% or less of one or more kinds, the balance being F
e, and cold tool steel for plasma carburizing, consisting of unavoidable impurities. 2. In addition to the alloy component according to claim 1, N
b: 3.0% or less and Ta: 3.0% or less, one or two kinds (in the case of two kinds, the total amount is 3.0% or less), and / or N: 0.02 to 0.15 % Cold tool steel for plasma carburization. 3. The cold-work tool steel for plasma carburization according to claim 1 or 2 is formed into a substantially tool shape and subjected to plasma carburization, and VC or V (CN) in the carburized layer is reduced in area ratio to a total. Cold tools that make up 50% of the carbide or total carbonitride.