JPH10121193A - Cold forging steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce deformation resistance, to improve deformability, to accelerate the entering into solid solution of a carbide in austenite at the time of induction hardening, and to improve the cold formability and induction hardenability of a steel by providing a composition containing specific amounts of C and Al and also containing Si, Mn, etc., by respectively specified amounts or less. SOLUTION: This steel has a composition consisting of, by weight ratio, >0.30-0.65% C, <0.15% Si, <=0.50% Mn, <=0.030% P, <=0.035% S, >0.10-0.30% Al, <=0.015% N, <=0.003% O, and the balance Fe with impurity elements. Further, one or more kinds among <=0.50% Cr, <=0.13% Mo, and <=0.0100% B are incorporated in order to improve induction hardenability. Moreover, either or both of <=0.05% Ti and <=0.05% Zr are incorporated in order to improve cold forgeability, and also one or more kinds among <=0.15% Pb, <=0.15% Bi, and <=0.01% Ca are incorporated in order to improve machinability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for cold forging which is superior in cold forgeability to carbon steel for machine structure such as S45C and is suitable for use in parts formed by cold forging.

[0002]

2. Description of the Related Art Many mechanical structural parts such as spindles, joints, yokes, shafts, sleeves, and flanges used in automobiles and the like are formed into a predetermined shape by hot forging or cold forging. Parts are manufactured by machining. Among them, cold forging has a higher material yield and higher dimensional accuracy than hot forging, so that material costs and machining costs can be reduced. Therefore, in recent years, the use of cold forging has been increasing in order to reduce the cost of parts.

[0003] Of these mechanical structural parts, those requiring particularly excellent toughness and wear resistance are required to be subjected to quenching and tempering after cold forging a mechanical structural steel such as S45C. Strength is secured. If further strengthening is required, induction hardening is performed.

[0004] Since carbon steel having a relatively high C content, such as S45C, has high hardness and poor cold workability when hot-rolled as it is, when cold forging is performed, heat treatment such as spheroidizing annealing is performed. In general, cold forging is performed after the hardness of the steel is reduced. However, even if a material whose hardness has been reduced by heat treatment is used, sufficient cold forgeability may not be ensured depending on the subsequent cold forging conditions, and cracks will occur in the forged product during cold forging. And the mold life is short. Further, when the working ratio in the cold forging is large, these problems become more remarkable, so that the intermediate heat treatment is usually performed between the cold forging processes. There are also problems such as an increase.

If a carbon steel having a relatively low C content such as S15C is used, the cold forgeability is improved and these problems can be prevented, but it is difficult to secure the strength required as a part. In addition, it is difficult to secure quenching hardness when induction hardening is required. Therefore, there has been a strong demand for the development of a cold forging steel that can secure necessary component strength and induction hardening hardness and has excellent cold forgeability.

[0006] In order to solve these problems, much research and development has been performed so far. For example, JP-A-49-6
No. 2318, JP-A-53-125216, JP-B1-38847, JP-B5-57350, JP-B5-76522, JP-B7-45695,
JP-A 1-225750, JP-A 2- JP 2-293341, JP-A
The inventions described in JP-A-2-145744, JP-A-2-74836, JP-A-5-59486, JP-A-7-97656, and JP-A-7-242989 are disclosed. The steels described in these publications mainly contain Si and
The Mn is reduced to reduce deformation resistance, and S, P, N, O, etc. contained as impurities are reduced as much as possible to improve deformability, thereby improving cold forgeability.

Further, hardenability may be insufficient due to reduction of Si and Mn.
In JP-A 1-225750, Cr, JP-A-53-125216, JP-B-1-38847, JP-A-2-74836, JP-B7-45695 in Cr, B, JP-A-2-145744 Then, Mo is disclosed in
No. 41 describes Mo, B, and JP, 5-59486, Cr, Mo, B
In JP-A-49-62318 and JP-A-7-242989, Ni, Cr, M
o, B, JP-B 5-76522, JP-A 7-97656, Cu,
Ni, Cr, Mo and B are added to secure the necessary hardenability.

[0008]

The invention described in the above publication has excellent cold forgeability as compared with medium carbon steel such as S45C, but the upsetting ratio exceeds 85%, for example. Considering such large deformation cold forging, the improvement of deformability cannot be said to be sufficient, and there is a concern that forging cracks may occur. Although reduction of Si and Mn is effective in reducing deformation resistance, any of these elements is useful as a deoxidizing aid during steelmaking. Therefore, a mere reduction of Si and Mn may lead to poor deoxidation during steel making, and the inclusions may increase, thereby reducing the deformability.

Furthermore, when medium carbon steel is used for cold forging, heat treatment such as spheroidizing annealing is generally performed to improve cold forgeability. However, these spheroidized annealed steels are induction hardened. Short-time rapid heating during quenching makes uniform austenitization insufficient, and ferrite or carbide tends to remain in the induction hardened structure. Therefore Cu, N
Even if the hardenability is improved by adding i, Cr, Mo, B, etc.,
If these effects are not sufficiently exhibited, there is a possibility that a predetermined induction hardening hardness or hardening depth cannot be obtained.

The present invention greatly improves cold forgeability represented by deformation resistance and deformability as compared with conventional structural carbon steels such as S45C, for example, so that the upsetting ratio exceeds 85%. It is an object of the present invention to provide a steel for cold forging capable of performing even large deformation cold forging and ensuring good induction hardenability.

[0011]

Means for Solving the Problems Under the above-mentioned object, the present inventors have conducted intensive studies in order to develop a steel having excellent cold forgeability and induction hardenability. Thus, the present invention has been completed.

N strengthens steel as an interstitial element, as described, for example, in "FB Pickering and T. Gladman: Iron and
Steel Inst. Spec. Rep., 81 (1963), p. 10 ”is a well-known fact, and it is effective to reduce N 2 to improve cold forgeability. However, there is a limit to the reduction in the ordinary steel making method, and it is conceivable to reduce the solute N 2 by fixing N 2 in the steel as nitride. Although elements such as Ti and Zr have a high nitride-forming ability, these elements also have a high carbide-forming ability, which may strengthen the steel by precipitation strengthening due to the formation of TiC or ZrC. It is not desirable as an element. On the other hand, Al has a nitride generation capacity of Ti
Alternatively, it is inferior to Zr or the like, but has a feature of not generating carbide. As a result of a detailed investigation of this nitride generation capability, it was found that the
The solid solution N is sufficiently fixed by the content of Al exceeding 10%,
It has been found that the deformation resistance is reduced.

The addition of Al in excess of 0.10% not only compensates for the lack of deoxidation due to the reduction of Si and Mn, but also makes it possible to reduce the amount of oxygen and harmful inclusions in comparison with ordinary steel containing Si and Mn. Can be reduced. As a result, the deformability is greatly improved, and for example, large deformation cold forging in which the upsetting ratio exceeds 85% becomes possible.

Further, by containing Al in excess of 0.10%,
It was found that uniform austenitization of the spheroidized annealed steel during induction hardening for a short time was promoted. Although the detailed mechanism of this is unknown, it is presumed that Al has an effect of reducing the stability of the carbide, so that it becomes easy for the spherical carbide to form a solid solution in the austenite structure.

[0015] The steel of the present invention completed by obtaining the above-described new findings has a weight ratio of C: more than 0.30% to 0.65%.
% Or less, Si: less than 0.15%, Mn: 0.50% or less, P: 0.030% or less, S: 0.035% or less, Al: more than 0.10% and 0.30% or less, N: 0.01
5% or less, O: contains 0.003% or less, is a steel for cold forging characterized by the balance consisting of Fe and impurity elements, in order to improve induction hardenability, further Cr: 0.50% or less, M
o: One or more of 0.13% or less and B: 0.0100% or less are contained. In addition, in order to improve cold forgeability, Ti: 0.0
One or two types of 5% or less and Zr: 0.05% or less are contained.
Further, in order to improve machinability, further Pb: 0.15% or less, B
One or more of i: 0.15% or less and Ca: 0.01% or less are contained.

The reasons for limiting the component composition in the steel for cold forging of the present invention will be described below. C: More than 0.30% and 0.65% or less C is an element necessary to secure necessary strength and induction hardening hardness, and it is necessary to contain more than 0.30%.
An increase in the C content may impair the cold forgeability, but when higher induction hardening hardness is required, the content is preferably 0.35% or more. However, when the content exceeds 0.65%, the deformation resistance is excessively increased, and it becomes difficult to perform cold forging even if the above-mentioned measures for improving cold forgeability are taken. Therefore, the upper limit is set to 0.65%. If the need for induction hardening hardness is small, the content is preferably 0.60% or less.

Si: less than 0.15% Since the reduction of Si improves the cold forgeability by reducing the deformation resistance, it is necessary to reduce it as much as possible.
%. Conventionally, Si had to be contained to some extent as a deoxidizing auxiliary element, but in the present invention, as described above, since it contains a large amount of Al, there is no particular lower limit.

Mn: 0.50% or less Reduction of Mn is required to reduce deformation resistance and thereby improve cold forgeability, so that it is necessary to reduce it as much as possible. Since adjustment is required, the upper limit was set to 0.50%. Conventionally, Mn had to be contained to some extent as a deoxidizing auxiliary element like Si, but in the present invention, as described above, because it contains a large amount of Al,
There is no particular lower limit.

P: 0.030% or less P is an element inevitably contained as an impurity, but an element which increases ferrite hardness, increases spheroidizing annealing hardness, and adversely affects cold forgeability by containing a small amount of P. It is. Therefore, it is preferable to reduce as much as possible if only the cold forgeability is taken into consideration. However, since an extreme reduction leads to an increase in steelmaking cost, the upper limit is set to 0.030% in consideration of the process capability. Preferably, it is set to 0.015% or less.

S: 0.035% or less S is an element inevitably contained as an impurity.
By generating inclusions of S, which become the starting point of cold forging cracks, it adversely affects cold forgeability. Therefore, it is preferable to reduce as much as possible if only the cold forgeability is considered.
However, S is an effective element for improving machinability, and an extreme reduction may cause deterioration of machinability. Therefore, the upper limit is set to 0.035%. If not, the content is preferably 0.015% or less.

Al: more than 0.10% and not more than 0.30% As described above, Al reduces the deformation resistance by fixing N, improves the deformability by reducing O and harmful inclusions, and removes carbide during induction hardening. It is an element effective in promoting solid solution in austenite. In order to obtain the sufficient effect, the content must be more than 0.10%, but excessive content
The upper limit is set to 0.30% in order to prevent the solid solution strengthening by Al from being negligible and to increase the deformation resistance.

N: 0.015% or less When N is present as solid solution N, the spheroidizing annealing hardness increases,
A smaller one is preferable because the deformation resistance increases. In the present invention, since the solid solution N 2 is fixed as a nitride by containing a predetermined amount of Al, it is not necessary to extremely reduce N 2.
If the content is too high, solid solution N will not be fixed as nitride
Therefore, the upper limit was set to 0.015% because there is a possibility that the amount of the compound may increase.

0: 0.003% or less O is an element inevitably contained as an impurity, but is an element that generates oxide-based inclusions when contained in a trace amount and adversely affects cold forgeability. Therefore, it is necessary to reduce as much as possible in order to ensure cold forgeability, but there is a possibility that steelmaking costs will increase, so the upper limit was made 0.003%. If only the cold forgeability is taken into account, the reduction is further preferred, and the content is preferably 0.002% or less. In the present invention, since a predetermined amount of Al is contained, reduction of O 2 is relatively easy as compared with ordinary steel.

Cr: 0.50% or less Cr is an element effective for improving hardenability. However, Cr is not added as much as possible because it increases deformation resistance and inhibits solid solution of carbide during induction hardening. Is preferred. However, the hardenability may need to be improved depending on the component shape or the induction hardening conditions, and the content can be added within the range of 0.50% or less as necessary.

Mo: 0.13% or less Mo is an element effective in improving hardenability. However, Mo is not added as much as possible because it increases deformation resistance and inhibits solid solution of carbide during induction hardening. Is preferred. However, depending on the component shape or induction hardening conditions, the hardenability may need to be improved, and if necessary, it can be added within the range of 0.13% or less.

B: 0.0100% or less B is an element that is effective in improving hardenability, and an excessive B content is an element that is also effective in fixing solid solution N 2. Therefore, when improvement of hardenability or improvement of cold forgeability is necessary, 0.
It could be added within the range of 0100% or less. Note that 0.
If the content of B exceeds 0100%, a B carbon compound is excessively generated, and the hardenability and the cold forgeability may be deteriorated. Therefore, the upper limit is set to 0.0100%.

Ti: 0.05% or less Ti is an element effective in fixing N. However, it is preferable that Ti is not added as much as possible, because it inhibits cold forgeability due to precipitation strengthening of carbides. However, in order to stabilize the N-fixing effect, it can be added as needed within the range of 0.05% or less.

Zr: 0.05% or less Zr is an element effective in fixing N. However, it is preferable that Zr is not added as much as possible, since cold forging is impaired by precipitation strengthening of carbides. However, in order to stabilize the N-fixing effect, it can be added as needed within the range of 0.05% or less.

Pb: 0.15% or less Pb is an element effective in improving machinability.
Excessive addition may impair the cold forgeability, so it is preferable not to add as much as possible. However, in many cases, it is important to ensure machinability.
% It can be added within the following range.

Bi: 0.15% or less Bi is an element effective in improving machinability,
Excessive addition may impair the cold forgeability, so it is preferable not to add as much as possible. However, in many cases, it is important to ensure machinability.
% It can be added within the following range.

Ca: 0.01% or less Ca is an element effective for improving machinability,
Excessive addition may impair the cold forgeability, so it is preferable not to add as much as possible. However, in many cases, it is important to ensure machinability.
% It can be added within the following range.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION The steel for cold forging according to the present invention contains a predetermined amount of carbon and, in addition to reducing the contents of Si and Mn, further contains a predetermined amount of Al, so that N is fixed. Cold forgeability and induction hardening superior to conventional steel by reducing deformation resistance, improving deformability by reducing O and harmful inclusions, and promoting solid solution of carbides in austenite during induction hardening. Sex can be obtained. Hereinafter, the characteristics of the steel for cold forging of the present invention are compared with comparative steels and conventional steels, and are clarified by examples.

[0033]

EXAMPLES The characteristics of the present invention will be clarified below with reference to comparative examples and conventional steels. Table 1 shows the chemical components of the test materials used in the examples.

[0034]

[Table 1]

Steel having the components shown in Table 1 was melted in an electric furnace, and a round bar having a diameter of 38 mm was manufactured by hot rolling.
The test material was used. Among the steels shown in Table 1, steels 1 to 13 are steels of the present invention, steels 14 to 16 are comparative steels in which some elements do not satisfy the conditions of the present invention, and steels 17 and 18 are conventional steels. S45C and S55C equivalent steel.

A 38 mm diameter round bar having the components shown in Table 1 was used.
After heating at 740 ° C for 4 hours, 10 ° C / hr up to 640 ° C
Subjected to spheroidizing annealing to gradually cool at a cooling rate of
It was subjected to induction hardening and machinability tests.

In the cold forgeability test, a round bar-shaped specimen W having a diameter of 20 mm and a height of 40 mm was obtained from the spheroidized test material.
1 is machined, and a test piece W1 is applied by an upper mold 10 into an indented lower mold composed of dies 11, 12 in the manner shown in FIGS. 1 (a) and 1 (b). Was performed, and the deformation load and the cracking limit working rate associated with the deformation of the test material W2 were determined. The test piece was subjected to a bond treatment,
An 800T hydraulic forging press was used for cold forging. The deformation load used was the forming load at upsetting rates of 70% and 90%.

In the induction hardening test, a round bar-shaped test piece having a diameter of 18 mm and a height of 30 mm was machined from the spheroidized test material, and a fixed hardening coil was used at a frequency of 100 kHz. Induction hardening was performed, and the induction hardening hardness and the quenching depth were determined. Induction quenching adjusts the heating power,
The test was performed under the two conditions of a heat affected depth of 2.5 mm and 4 mm. In addition, the induction hardening hardness was set to a depth of 0.1 mm from the surface, and the quenching depth was set to the limit depth at which the hardness of HV450 was obtained.

In the machinability test, a bar having a rectangular cross section was machined from the test material subjected to the above-described spheroidizing annealing, and the bars were subjected to thickness measurement.
Drill cutting was performed on a test piece that had been cold forged into a 3 mm flat plate shape, and the tool life was determined. Since the machinability is strongly affected by the hardness, all the specimens are installed by changing the shape of the rectangular cross section according to the specimen so that they fall within the hardness range of HV230-270. The hardness was adjusted by changing the work-in rate. SK for machinability test
Using a 3mm φ straight drill made of H51, a through hole was drilled in a flat plate with a thickness of 3mm, and the number of drillings until the wear amount of the drill bit reached 0.2mm was defined as the tool life. Table 2 shows the performance evaluation results of each test material.

[0040]

[Table 2]

As shown in Table 2, the steels of the present invention, 1, 2
Steel and steels 4 to 13 have a lower deformation load than conventional steel 17 of the same carbon content, or steel 3 of the present invention has lower carbon steel than conventional steel 18 of the same carbon content. In addition, the cracking limit working rate is remarkably improved. In particular, the steel according to the present invention has a characteristic of having a cracking limit working ratio of 92% or more, which indicates that a large deformation cold forging such that the upsetting ratio exceeds 85% is also possible. Furthermore, the N-fixing effect is stabilized by Ti or Zr, and steels 7 to 9 and 13 are effective in reducing the deformation load, especially at a working rate of 90%, as compared with other invention steels. In addition, in conventional steels, cracks were generated due to lack of deformability when measuring the deformation load, but the deformation load was directly used for comparison.

Although the comparative steel 14 steel having a low Al content has reduced Si, Mn and the like similarly to the steel of the present invention, the cold forgeability is improved as compared with the conventional steel 17 steel. Since the N-fixing effect is insufficient, the deformation load increases and the cracking limit working ratio is lower than that of the steels of the present invention (excluding the three steels having different carbon contents). The comparative steel 15 steel with excessive Al content has a remarkable strengthening of solid solution strengthening of Al, and the deformation load increases and the cracking limit working rate is higher than that of the present invention steel (excluding three steels with different carbon contents). Is low. That is, it can be seen that in order to obtain excellent cold forgeability, the Al content needs to be limited to the scope of the present invention.

The induction hardening hardness of the steel of the present invention was 1, 2
Steels and steels 4 to 13 have equivalent values compared to conventional steel 17 of the same carbon content, or steel 3 of the present invention has the same value as conventional steel 18 of the same carbon content. Under the quenching condition with a heat affected depth of 2.5 mm, an equivalent quenching depth was obtained. Since deeper quenching depth may be required for some parts, evaluation was performed under quenching conditions with a heat affected depth of 4 mm. The present invention steels 4 to 6, 8 steel, 9 steel and
Steels 11 to 13 have improved induction hardenability without impairing cold forgeability and have a quench depth equivalent to that of conventional steel 17 steel.

Since the comparative steel 14 steel has a low Al content, the solid solution of carbides in austenite during induction hardening is insufficient, and the steel has a higher frequency than the steel of the present invention (except for three steels having different carbon contents). Hardening hardness and induction hardening depth are reduced.
Compared steel 16 steel has low C content, so induction hardening hardness is HV
It is 600 or less, and the hardness required for ordinary induction hardened parts cannot be satisfied.

Further, regarding the machinability, it can be confirmed that 10 to 13 steels to which machinability elements are added show excellent machinability without impairing the cold forgeability as compared with 1 to 3 steels. Was.

[0046]

According to the present invention, the cold forgeability represented by deformation resistance and deformability is greatly improved as compared with conventional structural carbon steel such as S45C, and for example, the upsetting rate is 85%. The purpose of this invention is to provide cold forging steel that enables even large deformation cold forging to be performed, and that can further secure good induction hardenability. It greatly contributes to the reduction of the cost of forged parts and the net shaping of cold forged parts by expanding the forming limits of parts, and is of great industrial significance.

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[Procedure amendment]

[Submission date] December 20, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Added

[Correction contents]

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a relationship between a die and a test material in an upsetting cold forging process according to the present invention.

[Description of Signs] W1, W2: Test material, 10: Upper die, 11, 12: Dice

Claims (5)

[Claims] (1) As a weight ratio, C: more than 0.30% and 0.65% or less, Si: less than 0.15%, Mn: 0.50% or less, P: 0.030% or less, S:
0.035% or less, Al: more than 0.10%, 0.3% or less, N: 0.015% or less,
O: A steel for cold forging characterized by containing 0.003% or less, with the balance being Fe and impurity elements. 2. The weight ratio of C: more than 0.30% to 0.65% or less, Si: less than 0.15%, Mn: 0.50% or less, P: 0.030% or less, S:
0.035% or less, Al: more than 0.10%, 0.3% or less, N: 0.015% or less,
O: 0.003% or less, Cr: 0.50% or less, Mo: 0.13% or less, B: 0.01
A steel for cold forging, characterized by containing one or more of the following types, with the balance being Fe and impurity elements. 3. A weight ratio of C: more than 0.30% to 0.65% or less, Si: less than 0.15%, Mn: 0.50% or less, P: 0.030% or less, S:
0.035% or less, Al: more than 0.10%, 0.3% or less, N: 0.015% or less,
A cold forging steel containing one or two of O: 0.003% or less, Ti: 0.05% or less, and Zr: 0.05% or less, with the balance being Fe and impurity elements. 4. A weight ratio of C: more than 0.30% to 0.65% or less, Si: less than 0.15%, Mn: 0.50% or less, P: 0.030% or less, S:
0.035% or less, Al: more than 0.10%, 0.3% or less, N: 0.015% or less,
O: 0.003% or less, Cr: 0.50% or less, Mo: 0.13% or less, B: 0.01
1% or less of at least 00%, and one or more of Ti: 0.05% or less and Zr: 0.05% or less, with the balance being Fe
And a cold forging steel comprising an impurity element. 5. The method according to claim 1, wherein:
By weight ratio, further Pb: 0.15% or less, Bi: 0.15% or less, C
a: contains one or more of 0.01% or less, with the balance being Fe
And a cold forging steel comprising an impurity element.