JP6201534B2 - Steel for carburized parts - Google Patents

Description

This invention relates to carburized parts for steel used in the carburized part article formed are joined by pressure contact to the mating member.

Conventionally, as a method for joining machine structural parts to metal counterparts, welding is performed, especially the part to be joined of the base metal is heated, and only the base material or the base material and filler metal (welding rod, etc.) Has been widely used for melting without solidification of the molten metal in a fused state, and joining without applying mechanical pressure (generally speaking, welding is often referred to as this fusion welding). .
The so-called TIG welding, MIG welding, laser welding, and electron beam welding belong to this fusion welding.

  By the way, for machine parts that require high surface hardness such as gears and bearing parts for automobiles, JIS steel grades such as SCR420 are generally processed into part shapes, then carburized and hardened, and surface hardened for use. This type of carburized component is a member that is difficult to weld.

The carburized component is a high carbon concentration member, and therefore, when this is melted, blown holes are generated by CO ₂ produced by oxidation of C, or cracking occurs due to solidification → martensitic transformation of the molten part, Due to these problems, joining by fusion welding is difficult.
Therefore, conventionally, bolting and riveting have been used exclusively as means for fixing and attaching the carburized parts to the mating member.
However, fixing by bolting and riveting is troublesome, requires high cost, and has a low degree of freedom in shape.

In recent years, joining methods by pressure welding such as friction welding and flash welding have become widespread, and it is conceivable to join the above-mentioned carburized parts to a mating member using such a joining method.
Here, friction welding is a method in which the base metal is abutted against each other, pressure is applied, and frictional heat is applied when the relative rotational movement is applied, and flash welding is a discharge when the base materials to which voltage is applied are butted together. In this method, the joint is heated to a high temperature using the heat generated by the pressure and the pressure is applied in that state, and in each case, the joint is heated and softened, and then the joint is plastically deformed by an external force to perform the joining. Method (pressure welding belongs to a class of welding).

  In these pressure welding, the joint surfaces are joined in a solid phase mainly by the action of external pressure. However, in order to facilitate plastic deformation, auxiliary heating may be performed, which may involve local melting.

FIG. 1 schematically shows an example of a joining method by friction welding.
As shown in the figure, in this friction welding, while one member 10A is rotated, the frictional heat is generated by pressing the member 10B in the axial direction with a low load (friction pressure) once against the other non-rotating member 10B. (See (a), (b), (c)).
Thereafter, a larger pressure (upset pressure) is applied, and at the same time, the rotation-side bonded member 10A is stopped by the brake.
At that time, a large plastic deformation (upset shift margin) is caused in the vicinity of the joint softened by the frictional heat, and the pair of members to be joined 10A and 10B are joined.

  Although it is conceivable to apply this type of pressure welding as a method of joining carburized parts, the problem with carburized parts is that the carburized parts have high hardness (carburized parts are originally carburized to increase their hardness. ) And plastic deformation is difficult, so it is difficult to apply a joining method by pressure welding.

For example, when a member made of steel or cast iron having a high C content with a C content of 1% (mass%) or more is pressed as a mating member, for example, a cast iron member such as FCD400 having a high C content is mated. When pressed as a member, there is a large difference in hardness between the carburized part with high hardness and the mating member, so when applying external force to join the carburized component and the mating member by pressure welding, only the soft mating member is plastically deformed. As a result, the carburized member is not plastically deformed and the joining cannot be performed well.
Similarly, when a carburized part is used as a mating member, it is difficult to perform pressure welding, and any member is difficult to cause plastic deformation necessary for joining, and thus cannot be joined well.
For these reasons, it is generally difficult for carburized parts to be welded instead of fusion welding, and until now, joining with bolts or billets has been performed.

As prior art to the present invention, Patent Document 1 below discloses an invention related to “carburized parts”, and Patent Document 2 discloses an invention related to “carburized parts subjected to surface strengthening treatment by shot peening”. The invention of “low strain vacuum carburizing gas quenching steel and low strain carburized parts produced therefrom” is disclosed in Patent Document 4, “rolling member steel, rolling member, and rolling member manufacturing method” Patent Document 5 discloses an invention relating to “high-strength carburized induction-hardened parts”, and Patent Document 6 discloses an invention relating to “manufacturing method and steel parts of carburized parts”. Steels having compositions similar to those of these steels are disclosed.
However, in these documents, there is no reference to the point where the carburized parts are joined to the mating member by pressure welding, and further, the carburized parts related to the pressure welding and the thermal conductivity of the steel and the like. Are different.

JP 2007-291486 A JP 2008-156673 A JP 2008-195997 A JP 2009-114488 A JP 2008-280610 A JP 2010-90437 A

The present invention is the background of the above circumstances, it is well pressed bonded to the other member, for the purpose of providing a strong bonding force exerts used carburized part article carburized parts steel with the mating member It was made.

Thus, claim 1 is a steel used for carburized parts that are vacuum carburized and joined to the mating member by pressure welding, and in terms of mass%, C: 0.10 to 0.30%, Si: 0.40 to 3.00%, Mn : 0.30 to 3.00%, P: 0.030% or less, S: 0.030% or less, Cu: 0.01 to 1.00%, Ni: 0.01 to 3.00%, Cr: 0.30 to 3.00%, balance Fe and inevitable impurities In addition, the following formulas (1) and (2) are satisfied.
[Si%] + [Ni%] + [Cu%] − [Cr%]> 0.3 (1)
24 x [Si%] + 3.2 x [Mn%] + 6.8 x [Cr%]> 28 ... Formula (2)

Effects and effects of the invention

In order to join carburized parts to the mating member by pressure welding,
1) There should be as little oxide as possible in the planned joining part of the carburized part to the mating member. This is because if an oxide is present at the bonding interface to the mating member, the oxide is hard, does not melt, does not stick, and easily peels off.
2) When the carburized component is favorably joined to the mating member by pressure welding, the presence of carbide in the joining portion of the carburized component is also a problem. This is because the carbide is also hard, does not melt, does not stick, and is easy to peel off.
3) Ensuring the hardness of the carburized part, so that the carburized part must be easy to join to the mating member even under conditions where plastic deformation of the carburized part is difficult.

The first problem, a solution by the vacuum carburizing parts the carburizing components may FIG Rukoto.
In the case of conventional gas carburizing performed in an atmosphere containing oxygen, an oxide film is formed on the surface of the carburized component, and therefore the carburized component cannot be joined to the mating member by pressure welding.
With where immersion vacuum carburizing parts charcoal component, and the absence of only oxides possible predetermined joining portions of the carburized parts to solve the problem it is FIG Rukoto.

For example, in friction welding of carburized parts, even if there is an oxide in the part to be joined, if it is very small, it can be discharged as a burr, and the new surfaces can be pressed and adhered to each other for joining. Is a factor that inhibits good bonding.
However, the presence of such oxides can be reduced or eliminated as much as possible in vacuum carburized parts, and the problems caused by oxides can be solved.

Second problem, i.e., generation of carbides predetermined joining portions of the carburized parts, which for the problem of inhibiting the bonding, the steel used for carburizing parts, component so as to inhibit the generation of carbide, specifically the solved by a composition satisfying the formula (1) may FIG Rukoto in.

To third problem, a solution by lowering the thermal conductivity of steel, i.e. the thermal conductivity of the carburized part can Figure Rukoto.
Specifically, the content of Si, which is the most effective in reducing thermal conductivity, is increased to a certain level or more, and other components Mn and Cr that also contribute to lowering thermal conductivity are contributed to lowering thermal conductivity. it contained, specifically, the composition of the steel, i.e. carburized parts to solved by to satisfy the equation (2) may FIG isosamples according to.

By reducing the thermal conductivity, heat is concentrated on the joint surface as much as possible while keeping the hardness of the carburized component high, so that the vicinity of the joint surface is effectively partially softened by heat, The problem of difficulty in joining due to high hardness can be solved .

By doing in the above way , even carburized parts that have been conventionally difficult to be welded can be joined to the mating member with good pressure welding.
In addition, the carburized component can be well bonded to the mating member that has been particularly difficult to press.
Thus, the bonded carburized parts generate a strong bonding force with the mating member, and constitute a pressure-bonded joint with the mating member having a high bonding strength.

In the vacuum carburizing, various hydrocarbon gases such as acetylene, ethylene and propane can be used as the carburizing gas. The carburization pattern is also arbitrary.
Moreover, although the carburizing process can be performed at various vacuum degrees, it is desirable to set the pressure to 2 kPa or less in order to eliminate the influence of oxygen.
Oxide content of the bonding scheduled portions in carburizing components it is desirable that not more than 30% by area ratio. More preferably, the area ratio is set to zero.
Moreover, it is desirable that the amount of carbide in the planned joining portion is 15% or less in terms of area ratio, and more desirably, the area ratio is zero.

The present invention relates to steel for carburized parts, and manufactures carburized parts by vacuum carburizing using the steel for carburized parts, and joins the mating member by pressure welding so that the joint strength is high and the strength quality of the joint. High pressure carburized parts and a mating member can be formed.

Next, the reasons for limiting the chemical components in the present invention and reference examples will be described below.
C: 0.10 to 0.30%
C is an element necessary for obtaining strength as a machine part. If the content is less than 0.10%, ferrite is generated in the core of the part and the strength is lowered, so 0.10% or more is contained. On the other hand, if it exceeds 0.30%, the workability, in particular the machinability, is lowered and disadvantageous for the molding of the parts.

Si: 0.40 to 3.00%
Si, together with Cu and Ni described later, is an element that suppresses the formation of carbides, and is an element that is useful for lowering the thermal conductivity. The reason why the lower limit of Si is set to 0.40% is to reduce thermal conductivity, and too little Si reduces seizure resistance and galling resistance and decreases strength. On the other hand, the upper limit of Si is set to 3.00% because workability, particularly machinability, is deteriorated if there is too much Si.

Mn: 0.30 to 3.00%
Mn is added as a deoxidizer during the melting of steel. Mn has little effect on the formation of carbides, but is useful as an element that lowers the thermal conductivity.
However, in a small amount that does not reach 0.30%, ferrite is generated in the core and the strength is lowered, and the contribution to the decrease in thermal conductivity is insufficient. On the other hand, if the amount exceeds 3.00%, workability, particularly machinability, is lowered.

P: 0.030% or less S: 0.030% or less Since these are impurities and are components that are not preferable for the mechanical properties of the parts, such as embrittlement, the amount is preferably low. Both of the above values are acceptable limits.

Cu: 0.01 to 1.00%
Cu is an element that suppresses the formation of carbides together with Si described above and Ni described later. The reason why the lower limit of Cu is set to 0.01% is that if the amount of Cu is too small, the hardenability decreases and the strength decreases. On the other hand, the reason why the upper limit of Cu is set to 1.00% is that if there is too much Cu, workability, particularly machinability, deteriorates.

Ni: 0.01 to 3.00%
Ni is an element that suppresses the formation of carbides together with the aforementioned Si and Cu. The reason why the lower limit of Ni is set to 0.01% is that if Ni is too small, the hardenability is lowered and the strength is lowered. On the other hand, the upper limit of Ni is set to 3.00% because if Ni is too much, workability, particularly machinability, deteriorates.

Cr: 0.30 to 3.00%
Cr is a component that promotes the formation of carbides. In the present invention, the content is restricted to 3.00% or less from the viewpoint of inhibiting the formation of carbides, and further from the viewpoint of workability, particularly machinability. However, if the content is too small, the hardenability becomes low and the mechanical properties of the product become unsatisfactory, and the content is made 0.30% or more in order to contribute to a decrease in thermal conductivity.

Mo: 2.00% or less It can be added to improve hardenability and increase temper softening resistance. If the amount is too large, the workability of steel deteriorates, so the content should be 2.00% or less. The lower limit of the desirable content is 0.01%.

Al: 0.20% or less Al has a function of suppressing the coarsening of crystal grains, and can be contained if the effect is desired. A desirable amount is 0.005% or more. However, if the content is excessive, alumina is formed in the steel, resulting in a decrease in strength, and the workability is impaired, so the content is made 0.20% or less.

Nb: 0.20% or less
Ti: 0.20% or less These components are effective in suppressing the growth of crystal grains generated during carburizing and maintaining a sized structure. However, excessive addition has an adverse effect on processability, so the content should be 0.20% or less. The lower limit values of the desirable contents of Nb and Ti are 0.001% and 0.001%, respectively.

N: 0.05% or less When N is present, there is an effect of preventing the coarsening of crystal grains. Therefore, it is preferable that at least 0.001% be present. Since this effect is saturated at about 0.05%, the content is made less than this.

B: 0.01% or less B is effective in improving hardenability, so is added as desired. Since the presence of a large amount is harmful to processability, the addition amount is selected within a range of 0.01% or less. The lower limit of the desirable content is 0.001%.

[Si%] + [Ni%] + [Cu%] − [Cr%]> 0.3 (1)
Si, Ni and Cu suppress the formation of carbides, while Cr increases.
In the present invention aiming at preventing the carbide generated in the carburized part from interfering with the pressure contact between the carburized part and the mating member, the amount of addition of Si, Ni, Cu and Cr is balanced, and at the time of vacuum carburizing Prevents carbide formation. For that purpose, Si, Ni, Cu and Cr are contained so as to satisfy the above formula (1).

24 x [Si%] + 3.2 x [Mn%] + 6.8 x [Cr%]> 28 ... Formula (2)
Si, Mn, and Cr have the effect of reducing the thermal conductivity of steel. In formula (2), the coefficients of Si, Mn, and Cr represent the degree of contribution to the decrease in thermal conductivity.
By adjusting the components so that the value of the left side of Equation (2) exceeds 28, diffusion of frictional heat generated on the joint surface of the carburized component during pressure welding can be suppressed, and bonding can be performed satisfactorily.

It is the figure which represented typically an example of the joining method by friction welding. It is the figure which showed the shape of the tension test piece.

Next, examples of the present invention will be described below.
Steels having chemical compositions shown in the examples , reference examples, and comparative examples in Table 1 were melted and processed into rods having a diameter of 12 mm by hot forging, followed by vacuum carburization.
The vacuum carburization was performed under the following conditions.
Here, a drop-type gas carburizing furnace was used, using methanol decomposition gas as the carrier gas and propane as the enriched gas. The inside of the furnace was evacuated to a reduced pressure of 1.5 kPa, and the bar was heated to 950 ° C. and carburized for 60 minutes.
Next, after 60 minutes of diffusion treatment under 1.5 kPa, the temperature was lowered to the quenching temperature of 850 ° C. and held for 30 minutes, followed by oil quenching. Further, tempering was performed at 180 ° C. for 60 minutes.

The oxide area ratio and the carbide area ratio were measured for the test pieces vacuum-carburized as described above, and a pressure welding test was performed using a JIS FCD400 rod of φ12 mm as a mating member. Further, a tensile test piece including a joint surface was cut out from a joint portion by pressure welding and a tensile test was performed.
Here, the oxide area ratio and the carbide area ratio were obtained as follows. The pressure contact test was performed under the following conditions.

<Measurement of oxide area ratio and carbide area ratio>
The surface of the bar material after carburizing was corroded with nital, and SEM observation (1000 times) was performed to measure the area ratio of oxides and carbides.

<Pressure welding test>
Using a brake friction welding machine, friction welding was performed under the conditions of a friction pressure of 40 MPa, an upset pressure of 100 MPa, and a total displacement of 2 mm.

<Tensile test>
A tensile test piece including a bonded surface was cut out from the bonded portion by pressure welding, and a tensile test was performed.
FIG. 2 shows the shape of the tensile test piece. The test piece is based on a JIS Z 2201 No. 14A test piece, and the left half in the figure is a carburized part and the right half is an FCD400 material.
These results are shown in Table 2.

From Tables 1 and 2, the following can be understood.
In Comparative Example 1, the amount of Si added is less than the lower limit of the present invention, and the value of the left side ([Si%] + [Ni%] + [Cu%] − [Cr%]) of the formula (1) is It is below the lower limit of the invention, and carbides are generated in the carburized parts after carburizing. Moreover, the value of the left side (24 × [Si%] + 3.2 × [Mn%] + 6.8 × [Cr%]) of the formula (2) is also below the lower limit of the present invention, and the thermal conductivity of the carburized parts is Not low enough.
As a result, the strength of the joint by pressure welding is low, and the joint is broken in the tensile test.

On the other hand, in Comparative Examples 2 and 3, although the addition amount of Si is within the range of the present invention, the value on the left side of the formula (1) is lower than the lower limit value of the present invention, and carbides are generated after carburizing. . Moreover, the value of the left side of Formula (2) is less than the lower limit of this invention, and the thermal conductivity of a carburized component is not low.
As a result, in the tensile test after pressure welding, fracture occurs at the joint and the joint strength is low.

  In Comparative Example 4, since gas carburizing is performed, oxides are generated in the carburized parts after carburizing, fractures are generated at the joints in the tensile test after pressure welding, and the bonding strength is low.

In Comparative Examples 5 and 6, the value on the left side of Equation (2) is below the lower limit of the present invention, and the thermal conductivity of the carburized component is not low.
As a result, the strength of the joint portion in the pressure welding is low, and the fracture occurs in the joint portion in the tensile test.

On the other hand, Examples and References in which the value of the left side of Equation (1) and the value of the left side of Equation (2) are both vacuum carburized and pressure welded using steel satisfying the range defined in the present invention. In the example , the strength of the joint portion by pressure welding is sufficiently high, and therefore, in the tensile test after pressure welding, the fracture occurs at the FCD400 base material side without breaking at the joint portion.

        10A, 10B Joined member

Claims (1)

Steel used for carburized parts that are vacuum carburized and joined by pressure welding to the mating member,
By mass% C: 0.10 to 0.30%
Si: 0.40 to 3.00%
Mn: 0.30 to 3.00%
P: 0.030% or less S: 0.030% or less
Cu: 0.01 to 1.00%
Ni: 0.01 to 3.00%
Cr: 0.30 to 3.00%
A steel for carburized parts having a composition of remaining Fe and inevitable impurities and satisfying the following formulas (1) and (2).
[Si%] + [Ni%] + [Cu%] − [Cr%]> 0.3 (1)
24 x [Si%] + 3.2 x [Mn%] + 6.8 x [Cr%]> 28 ... Formula (2)

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