JPH0881740A - Steel for carburizing and carburized member - Google Patents

Abstract

PURPOSE: To produce a steel for carburizing capable of obtaining excellent toughness and produce a carburized member using the same. CONSTITUTION: This is a steel for carburizing having a compsn. contg., by weight, <=0.4% C, 2 to 7% Cr, one or two kinds of W and Mo by 3 to 20% as W equivalent (W+2Mo), 0.5 to <1.1% V, and the balance substantial Fe with inevitable impurities. Furthermore, this carburized member is the one in which a part or the whole part of the surface of the steel for carburizing is carburized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a carburizing steel to be used after carburizing, and a carburizing member using the same, for application under severe conditions such as cold forging dies and sliding members. To do.

[0002]

2. Description of the Related Art In a tool or a sliding member, when a material having a high hardness is used in order to enhance the wear resistance of the surface, when a crack generated on the surface progresses and the life is extended due to chipping or the like, There are many. In order to suppress the development of the above-mentioned cracks, it suffices to increase the toughness of the material, but the toughness and hardness have a contradictory relationship, and increasing the toughness decreases the hardness and shortens the life due to wear. There will be a problem of becoming. Conventionally, for the purpose of imparting wear resistance and heat resistance and increasing toughness to forging tools such as cold forging dies and sliding members, change of heat treatment and change of alloy composition Has been dealt with by.

Recently, a new technique for suppressing the above-mentioned crack development by nitriding or carburizing the surface, rather than the conventional simple heat treatment and addition of alloying elements, has been developed and attracted attention. For example, as described in JP-A No. 61-236923, the surface is carburized and heat-treated to give residual compressive stress to the surface, and the surface hardness and high temperature hardness are increased, so that the interior not carburized has a low Since carbon is carbon, a technique of leaving a composition having low hardness and excellent toughness is disclosed. In particular, the technology of carburizing this low-carbon carburizing steel can suppress the development of cracks generated on the surface of high hardness due to residual compressive stress on the surface, By making it exist, it is an excellent technique that can suppress the development of cracks in the core.

[0004]

The above-mentioned material for carburizing and use, that is, carburizing steel, is present in the material with the same composition after carburization.
The material forming the interior of the material must have sufficient strength to withstand the load applied to the carburized surface, and for that purpose it is necessary to secure the internal hardness to some extent.
As the carburizing steel, it is necessary to secure toughness, which is the most important characteristic, and to obtain sufficient hardness on the carburized surface.

According to the above-mentioned Japanese Patent Laid-Open No. 61-236923, the bearing steel used after carburizing is Cr-Mo-V.
C0.11-, which has a lower C content than M50.
0.15, Mo3.0-4.5, Cr4.0-4.2
5, V1.1-1.3, Ni3.2-3.6, balance Fe
A steel for carburizing is disclosed. When the present inventor examined this material, it was necessary to secure further toughness as a material constituting the inside of the material after carburization. An object of the present invention is to provide a carburizing steel having extremely high toughness and a carburizing member using the same, which can suppress the development of cracks even if the internal hardness is increased to the extent necessary for carburizing steel. Is.

[0006]

Means for Solving the Problems The present inventor focused on carburizing steel to be used after carburization, and studied the relationship among its composition, structure, toughness, and hardness, and set V in a specific range lower than before. It has been found that the toughness can be dramatically improved without decreasing the hardness when the content is restricted to the present invention, and the present invention has been accomplished.

That is, in the present invention, C0.4% by weight is used.
Hereinafter, it is a Fe-based martensitic steel containing Cr 2 to 7%, one or two of W or Mo as W equivalent (W + 2Mo) of 3 to 20%, V 0.5% or more and less than 1.1%. The steel for carburizing is characterized by being used after carburizing.

Also, preferably, in% by weight, C0.1 to 0.
4%, Si 2% or less, Mn 2% or less, Cr 2 to 7%, W
Alternatively, one or two kinds of Mo are contained in a W equivalent (W + 2Mo) of 3 to 18%, V of 0.5% or more and less than 1.1%, and the balance is substantially Fe and inevitable impurities. Also, it is desirable that a part of Fe be 5% or less of Ni.
Replace with. Further, part of Fe may be replaced with 10% or less of Co.

In the carburized member of the present invention, a carburized layer is formed on a part or all of the surface of the above-mentioned carburizing steel, the hardness of the surface on which the carburized layer is formed is HRC 60 or more, and the inside other than the carburized portion is A carburized member having a HRC of less than 60.

[0010]

One of the most important features of the present invention is that the V content of the carburizing steel used for carburizing by lowering C from the composition of the high speed tool steel is 0.5% or more and less than 1.1%. It is regulated by. Due to the regulation of V, the carburizing steel of the present invention can obtain extremely high toughness with almost no decrease in hardness.

The reason why the hardness is not lowered and the toughness is remarkably improved by the definition of V in the present invention is unknown.
It is presumed that this is due to the type and morphology of fine secondary hardening precipitates in the matrix after heat treatment (quenching and tempering). In the present invention, when V is 1.1% or more, the toughness is remarkably reduced, so it was defined as less than 1.1%. Preferably 1.
It is less than 0%. Further, if it is less than 0.5%, the toughness is improved, but sufficient secondary hardening in high temperature tempering due to addition of V in the carburized layer cannot be obtained, which results in deterioration of wear resistance and heat resistance. As a result, the life characteristics are deteriorated, which is not preferable. Therefore, the lower limit of V in the present invention is specified to be 0.5%.

The reasons for defining each element other than V will be described below. C is an important element after V and is an element that determines the basic toughness and hardness of the steel of the present invention used after carburizing. C enhances the hardness after quench hardening due to martensitic transformation. It is also an austenite stabilizing element,
It has the effect of suppressing the formation of δ ferrite at the austenitizing temperature. On the other hand, if the C content is too high, the hardness becomes too high and the toughness deteriorates. In the present invention,
Since it is used after being carburized, it is most important to secure the toughness of the core that is not carburized, and the upper limit of C is set to 0.4% in order to prevent the deterioration of toughness due to excessive formation of residual carbide. Further, in order to suppress the formation of δ ferrite, the content is preferably 0.1% or more.

Cr is a solid solution in both the matrix and the carbide, and is an element effective for ensuring the hardenability of the matrix, promoting the solid solution in the matrix during the quenching of the carbide, and increasing the hardness of the carburized surface. If the content of Cr is less than 2%, the hardness after quenching and tempering cannot reach HRC of 40 or more, so the lower limit value was made 2%. Further, if added in an amount of 7% or more, δ ferrite will be stable and the toughness will be significantly deteriorated.

W and Mo are elements having the same action in that they increase the hardness of the carburized portion of the carburizing steel and impart heat resistance. Mo1% by weight is equivalent to W2% and is defined as W equivalent (W + 2Mo). W or M
When o is carburized, it forms fine M6C type carbides and forms a hard carburized layer. Further, since the fine carbides prevent the austenite crystal grains from coarsening, they are also effective in ensuring the toughness. However, if the W equivalent is increased, the hardness of the carburized portion increases, but the hardness of the core portion that is not carburized also becomes harder, which accelerates the crack propagation speed. Also, W
If the equivalent weight is too high, δ ferrite will be stabilized and the toughness will be significantly deteriorated. In the present invention, the W equivalent is specified to be 3% or more in order to obtain a sufficient surface hardness as steel for carburizing, and the upper limit of the W equivalent is set to 20% in order to ensure sufficient toughness. The preferable upper limit of W equivalent is 18%.

Si is known as a deoxidizing element or an element for improving hardness and heat resistance, and it is preferable to add Si. When it is added, if it exceeds 2%, the toughness deteriorates, so the upper limit was made 2%. Mn has the effect of suppressing the harmfulness of S contained as an impurity by precipitating it as a deoxidizing element or as MnS. When added, if over 2%, the toughness deteriorates, so 2% is made the upper limit.

Ni has the effect of remarkably suppressing the formation of δ-ferrite and grading the change in the hardness of the carburized surface, and is an extremely effective additionable element for improving the fatigue life characteristics. However, if it exceeds 5%, the A1 transformation point is lowered, the annealing hardness is increased, and the machinability is deteriorated.

Co is mainly solid-dissolved in the matrix and has the effect of improving hardness and heat resistance. However, as the amount added is increased, the strength and toughness gradually decrease. In particular, it is a desirable element to be added to a bearing member that requires high hardness.
If 10% or more of Co is added, the toughness deteriorates so much that it becomes unsuitable as a bearing member, so the upper limit was made 10%.

Further, in the present invention, Nb is a specific additive element. Nb is an element effective in refining the alloy structure, and is effective in increasing the toughness of carburizing steel. If Nb is added too much, the hardness will be too high and the crack growth rate will be accelerated. Therefore, when Nb is added, it is 0.5% or less.

An example of a preferable specific composition range of the carburizing steel of the present invention containing Ni is as follows:
C 0.1 to 0.4%, Si 2% or less, Mn 2% or less, N
i 5% or less, Cr 2 to 7%, 1 or 2 kinds of W or Mo as W equivalent (W + 2Mo) 3 to 18%, V0.
Steel containing 5% or more and less than 1.1%, and the balance being essentially Fe. Furthermore, an example of a preferred specific composition range of the carburizing steel of the present invention containing Nb is as follows:
% By weight, C 0.1 to 0.4%, Si 2% or less, Mn 2
% Or less, Ni 5% or less, Cr 2 to 7%, and 1 or 2 kinds of W or Mo as W equivalent (W + 2Mo) 3 to 18
%, V0.5% or more, less than 1.1%, Nb0.5
% Or less, the balance being steel consisting essentially of Fe.

According to the present invention, the surface hardness of the carburized layer is HRC6 by carburizing the carburizing steel satisfying the above-mentioned regulations.
The internal hardness of 0 or more and other than the carburized portion can be less than HRC60. Further, the carburizing steel of the present invention is a carburizing treatment,
The subsequent heat treatment allows the compressive stress to remain on the surface. This compressive stress field can suppress the development of cracks on the surface, and the inside having high toughness can also prevent the development of cracks inside, so that the life characteristics can be remarkably improved.

[0021]

【Example】

(Example 1) The carburizing steels and comparative steels of the present invention shown in Table 1 were obtained by melting by a vacuum melting method and then subjected to a soaking treatment. For the obtained carburizing steel,
In order to evaluate the properties of the material constituting the inside of the member after carburization, the product was rolled without being carburized to 15 mm × 15 mm for Charpy impact test pieces, and quenched and tempered to obtain an evaluation sample. The quenching and tempering are performed at 1100 ° C. and at a temperature 10 ° C. higher than the tempering temperature at which peak hardness at which secondary hardening is obtained by high temperature tempering is obtained. The obtained evaluation sample was subjected to hardness measurement and Charpy impact test.

The results are shown in Table 1. Further, in order to evaluate as a carburized member, carburizing steel processed to φ24 mm is subjected to carburizing treatment at 950 ° C. for 48 hours, and after removing 1 mm from the surface, the above quenching and tempering treatment is performed,
A carburized member having a carburized layer of about 2.5 mm was obtained. The obtained carburized member was confirmed by X-ray analysis to have residual compressive stress on the surface. Table 1 shows the results of measuring the surface hardness of the carburized member. In order to evaluate the softening resistance at higher temperatures, the tempering temperature was changed to 650 ° C and the surface hardness was evaluated.

[0023]

[Table 1]

The measurement results are shown in Table 1. Although the carburizing steel of the present invention has hardness equal to or higher than that of the comparative steel having a high V content (Sample No. 18), it is about 2
It has an extremely high impact value of 2 to 4 times, which shows that it is excellent as a carburizing steel. Further, the carburizing steel of the present invention has an extremely high impact value even when the hardness is increased by increasing C (Sample Nos. 2 and 3). It can be seen that the comparative steel sample No. 19 having a higher C than that of the carburizing steel of the present invention is not preferable because the hardness becomes high and the impact value decreases even if the V amount is within the specified range of the present invention. .

From sample Nos. 1, 4, and 5 which are carburizing steels of the present invention in which the addition amount of V is changed, it is understood that the impact value tends to decrease when the V amount exceeds 1%. Further, the sample No. of the comparative steel having a V content smaller than the specified range of V of the carburizing steel of the present invention. Although 20 has a high impact value, when the surface is carburized, sufficient secondary curing is not obtained, and the softening resistance at high temperatures is also unfavorable.
Sample No. 6 of the present invention in which the amounts of W, Cr and Co added were changed,
It can be seen that also in 7, 8, 9, 10, 11, 12, 16 and 17, V has a significantly better impact value than Comparative Steel No. 18 in which V is higher than the range of the present invention.

Sample No. 1 in which the amount of Ni added was changed
From 3, 14 and 15, it can be seen that the impact value increases with the increase of Ni, and the addition of Ni is effective for securing the toughness. Further, the surface hardness of the carburized member of the present invention in Table 1 is HRC 60 or more, and a carburized member composed of a highly hard surface capable of ensuring abrasion resistance and an extremely high toughness inside can be constructed. I understand that When the hardness (softening resistance) when tempered at 650 ° C. was measured, the comparative material No. 20 having V of 0.34% was remarkably softened with respect to the steel of the present invention. For the steel of the present invention, the higher the softening resistance is, the better for heat resistance, so V is 0.5%.
If less than 60%, the maximum hardness does not reach 60 HRC, and the softening resistance is low, which is not preferable.

Example 2 Samples were similarly prepared with the components shown in invention steel Nos. 1 and 2 and comparative steel Nos. 18 and 20 in Table 1. The quenching temperature was set to 1100 ° C without carburizing,
Tempering was performed at a predetermined temperature between 300 ° C. and 600 ° C., and the relationship between the tempering temperature and toughness was investigated. The toughness was evaluated by the impact value obtained by the Charpy impact test as in Example 1.
The result is shown in FIG. The carburizing steel No1 of the present invention having a low V has excellent toughness as compared with the comparative steel No18 having a high V. Further, the carburized steel No. 2 of the present invention is one in which C is adjusted to be high in the present invention, and the internal hardness is increased.
Since C is increased, the impact value is slightly lower. But 53
When tempered at 0 ° C., a high hardness of HRC52 and a higher impact value than Comparative Steel No. 18 having a high V were obtained,
It can be seen that the carburizing steel of the present invention having a reduced V can secure high toughness even if the hardness is increased by C.

[0028]

According to the carburizing steel of the present invention, when carburized into a carburized member, it is possible to maintain extremely excellent toughness characteristics and sufficient strength. It is possible to dramatically improve. Therefore, it is extremely useful as a carburizing steel for cold forging dies, sliding members, tools such as punches and end mills, which are used under severe conditions where cracks are expected to occur.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a tempering temperature and a toughness of a steel of the present invention and a comparative steel.

Claims (5)

[Claims] 1. By weight%, C 0.4% or less, Cr2 to 7
%, One or two of W or Mo is equivalent to W (W + 2M
o) Fe-based martensitic steel containing 3 to 20%, V 0.5% or more and less than 1.1%, which is used after carburizing. 2. C0.1-0.4% by weight%, Si2
% Or less, Mn 2% or less, Cr 2 to 7%, and 1 or 2 of W or Mo as W equivalent (W + 2Mo) 3 to 18
%, V 0.5% or more and less than 1.1%, and the balance consisting essentially of Fe and unavoidable impurities, and used for carburizing before use. 3. F according to claim 1 or 2.
Steel for carburizing, characterized in that a part of e is replaced by 5% or less of Ni. 4. F according to any one of claims 1 to 3.
Steel for carburizing, characterized in that a part of e is replaced by 10% or less of Co. 5. A carburized layer is formed on a part or all of the surface of the carburizing steel according to claim 1, and the hardness of the surface on which the carburized layer is formed is HRC 60 or more, other than the carburized part. The carburized member has a hardness of less than HRC60.