JPH10265841A - Production of high strength cold forging parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the cold forging part which has good cold forgability represented in term of deformation resistance, high dimensional precision and high strength without conducting a heat treatment of hardening, tempering, etc., causing large strain deformation. SOLUTION: A stock, which has a composition consisting of, by weight, 0.12-0.40% C, 0.01-0.35% Si, 0.1-1.40% Mn, <=0.003% P, <=0.035% S, <=0.015% N, <=0.003% O, further, one kind or two kinds or more among 0.005-0.200% Al, 0.0005-0.0050% B, 0.005-0.05% Ti, 0.005-0.05% Zr and the balance Fe with inevitable impurities and has a ferritic.pearitic structure of a hardness of HV120-HV180, is subjected to a cold forging process of >=50% cold working rate and a heat treatment process having an aging treatment index K value of 400-800 defined in a formula K=T×log(t) (where, T is a heating temp. deg.C, its range is 200-450 deg.C, (t) is a heating time (min)) and then is made to a hardness of HV260.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength cold-forged part having excellent strength characteristics.

[0002]

2. Description of the Related Art Carbon steel such as S45C is used for many mechanical structural parts such as spindles, joints, yokes, shafts, sleeves, and flanges used in automobiles and the like. As a method of manufacturing the steel, a steel material is formed into a general shape by hot forging, and then heat treatment such as quenching and tempering is performed to secure a predetermined strength. Thereafter, a desired part shape is formed by cutting or the like. If necessary, induction hardening may be performed.

[0003] In the manufacturing process of these mechanical structural parts, machining costs such as cutting are extremely high. For example, "HKTonshoff and C. Stanske: Proc. Int. Conf. On H.
ighProductivity Machining, Materials and Machinin
g, ASM, (1985), pp. 207-222, show that cutting costs account for 42-67% of the cost of hot forged parts. Therefore, in recent years, it has been promoted to simplify the cutting work by forming these parts into a shape as close as possible to the final part shape by cold forging instead of hot forging.

In cold forging, since steel is formed at a low temperature of room temperature to about 300 ° C., generation of scale in hot forging can be avoided, and high dimensional accuracy can be achieved. On the other hand, since the deformation resistance of the steel is significantly increased, problems such as shortening of the service life of the mold and breakage of the mold become problems. There is also a problem that cracks occur in forged products due to insufficient deformability of steel.

[0005] Therefore, steel materials used for cold forging are required to have low deformation resistance and high deformability. Especially
Carbon steels with a relatively high C content, such as S45C, have high hardness and poor cold workability when hot rolled, so when cold forging, the hardness is reduced by heat treatment such as spheroidizing annealing. Normally, cold forging was performed after the cooling. 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. Research and development of cold forging steels for the purpose of improvement are underway.

For example, JP-A-49-62318, JP-A-53-12521
No. 6, No. 1-38847, No. 5-57350, No. 5-
No. 76522, Japanese Patent Publication No. 7-45695, Japanese Patent Application Laid-Open No. 1-225750, Japanese Patent Application Laid-Open No. 2-129341, Japanese Patent Application Laid-Open No. 2-145744, Japanese Patent Application Laid-Open No. 2-74836,
JP-A-5-59486, JP-A-7-97656, JP-A-7-242989
The invention described in Japanese Patent Application Laid-Open Publication No. H10-209,837 is disclosed. The steels described in these publications reduce the deformation resistance mainly by reducing Si and Mn, and reduce the S, P, N, O, etc. contained as impurities as much as possible to improve the deformability, thereby reducing the coldness. This is to improve 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]

However, even if a forged product having excellent dimensional accuracy is obtained by cold forging, if a heat treatment such as quenching and tempering is performed to secure the strength, the heat treatment strain is reduced. Due to the occurrence, there is a concern that the dimensional accuracy is greatly reduced. Therefore, in order to effectively utilize the excellent dimensional accuracy of cold forging, it is necessary to obtain a predetermined strength without performing a heat treatment such as quenching and tempering with a large heat treatment strain.

As one of the methods, it is conceivable to use work hardening introduced by cold forging to secure the strength as it is in cold forging without performing a heat treatment. There are many. However, there is a good correlation between the hardness obtained by cold forging and the working load of cold forging, as shown in, for example, "Hideaki Kudo, Hisashige Yamato and others: Cold Forging Handbook (1973), Agne, p.60-" Have been. Therefore, if a high strength is to be obtained by cold forging, the processing load is inevitably increased, and cold forging becomes difficult. In addition, each of the various inventive steels having improved cold forgeability described above is intended to reduce the hardness of the steel by various methods to reduce the deformation resistance, and consequently to obtain cold forged as it is. The resulting strength is low.

On the other hand, it is conceivable to apply a heat treatment as small as possible to generate strain after cold forging, that is, a heat treatment at a temperature lower than the transformation temperature of steel. Japanese Patent Application Laid-Open No. 5-171275 discloses an age hardening type cold forging steel containing relatively large amounts of Ni, Al and Cu in steel and utilizing precipitation strengthening of these alloys or alloy compounds. Due to its high content, it is a very expensive steel material. JP-A-1-100221 and JP-A-1-100222 disclose that the steel has a fine mixed structure of ferrite and bainite and is subjected to a tempering treatment after cold forging to secure the strength. In these inventions, the proof stress is greatly increased, but the amount of increase in the tensile strength is small, as compared with the case of cold forging by tempering. Further, since a fine mixed structure of ferrite and bainite is used, the deformation resistance of the steel is relatively high, and the cold forgeability is not always good. Japanese Patent Application Laid-Open No. 7-70636 discloses that a steel having a predetermined chemical composition is cold forged and then subjected to an aging treatment to improve the elastic modulus, but no study has been made on improving the strength. .

Therefore, it is difficult to obtain a high-strength cold-forged product having good cold-formability and high-strength heat treatment such as quenching and tempering at the time of cold forging. It is difficult, and there is a demand for technology development that can solve them.

[0012]

Means for Solving the Problems The present inventors have conducted intensive studies with the aim of solving the above-mentioned problems, and as a result, obtained the following findings, and completed the present invention.

The phenomenon of increasing the strength of a cold-worked steel when subjected to a relatively low-temperature heat treatment is known as strain aging, and is described, for example, in “Lecture: Modern Metallurgical Materials, Vol. The Japan Institute of Metals, (1985), p.58 ". The results show that strain aging is caused by the solid solution of C and N in the steel, which is fixed to dislocations introduced by cold working. However, these phenomena have been studied in low carbon steels, and the behavior of medium carbon steels with a large difference in C content or structure is largely unknown.

However, strain aging is one of the factors that increase the deformation resistance in cold forging, and studies on suppressing it as much as possible have hitherto been mainly conducted. The present inventors have conducted intensive studies on the strain aging phenomenon in a medium-carbon mechanical structural steel, and as a result, obtained the following knowledge, thereby completing the present invention.

A medium-carbon steel containing a predetermined amount of one or more of Al, B, Ti, and Zr and having N fixed in the steel is used. These steels are made of ferrite having a hardness of HV180 or less. -By adjusting the pearlite structure, a forged material with low deformation resistance and excellent deformability can be obtained. And by performing cold forging with relatively high workability on these forged materials, at the same time as obtaining work hardening by cold working,
Many processing dislocations are introduced. Normally, solid solution N rapidly accumulates on these processing dislocations, and strain aging already occurs during processing, resulting in an increase in deformation resistance. However, in the present invention, since one or more of Al, B, Ti, and Zr strongly trap solid solution N 2 as a nitride, strain aging during processing is suppressed. As for C, diffusion is slower than that of N, so that strain aging is suppressed, and hardly contributes to strain aging during processing. By subjecting a cold forged product manufactured using these steels to a heat treatment at a limited temperature and for a limited time, in addition to an increase in strength due to cold working, a further large age hardening occurs, and a cold forged material having excellent strength is produced. A forged part can be obtained. The heat treatment conditions that cause age hardening are extremely limited. If insufficient, the strain age hardening is small, and if it is excessive, there is a possibility that overaging strain recovery and recrystallization may cause softening.

The present invention, which has been completed by obtaining the new findings described above, has a C: 0.12 to 0.40% by weight, Si:
0.01 to 0.35%, Mn: 0.10 to 1.40%, P: 0.030% or less, S:
0.035% or less, N: 0.015% or less, O: 0.003% or less, Al: 0.005 to 0.200%, B: 0.0005 to 0.0050%, Ti: 0.005
Hardness HV120 containing one or more of Zr: 0.005 to 0.05%, the balance being Fe and impurity elements.
~ HV180 material having a ferrite-pearlite structure, a cold forging process in which the cold working ratio is 50% or more, and a heat treatment process in which the aging treatment index K value obtained by the following formula is 400 to 800 A method for manufacturing a high-strength cold-forged part, wherein the hardness is increased to HV260 or more by applying. K = T × logt where T indicates a heating temperature (° C.), and the range is 200 to
450 ° C. t indicates a heating time (minute). Also, in order to improve machinability, Pb: 0.25% or less, Bi: 0.15% or less,
Ca: One or more of 0.01% or less can be contained.

The reasons for limiting the method of manufacturing a cold forged part according to the present invention will be described below. C: 0.12% to 0.40% or less C is an element necessary for securing necessary strength and obtaining age hardening ability, and it is necessary to contain 0.12% or more. However, if it exceeds 0.40%, the deformation resistance increases due to the increase in hardness of the forged material, and the cold forgeability may be impaired. Therefore, the upper limit was set to 0.40%.

Si: 0.01 to 0.35% Since Si is an effective element as an auxiliary deoxidizer in the steel making process, the lower limit is set to 0.01%. However, an excessive content increases the deformation resistance, so the upper limit is set to 0.35%.

Mn: 0.10 to 1.40% Mn is an element effective as an auxiliary deoxidizing material similar to Si, and is an element effective for imparting necessary strength or hardenability to steel. Is required.
However, an excessive content may increase the deformation resistance and impair the cold forgeability, so the upper limit is 1.10%.

P: 0.030% or less P is an element inevitably contained as an impurity, but is an element which increases the ferrite hardness by a small amount and adversely affects the cold forgeability. 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 steel making 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 0.010% or less.

N: 0.015% or less When N is present as solid solution N, the hardness of the forged material increases and, during cold forging, strain aging occurs to deteriorate the cold forgeability. It is desirable. In the present invention, since solid solution N 2 is fixed as nitride by the inclusion of Al, B 2, Ti or Zr, no extreme reduction is necessary, but if the N content is excessively large, the solid solution N is not fixed as nitride. Since the amount of dissolved N may increase, the upper limit is set to 0.015%.

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, it is further reduced and preferably 0.001% or less.

Al: 0.005 to 0.200% Al is an effective element as a deoxidizing material in steel making. However, by further fixing solid solution N, deformation resistance is reduced and strain aging generated during cold forging is reduced. Has the effect of doing In order to exert these effects, the content of 0.005% or more is necessary.However, excessive addition causes aggregation and coarsening of oxide-based inclusions, which may lower the deformability, so that the upper limit is 0.20.
0%.

B: 0.0005% to 0.0050% B is an element which has a strong affinity for N and has an effect of fixing solid solution N like Al. 0.0005 to get those effects
% Or more is required. However, the excessive content may excessively generate the B carbon compound and may lower the cold forgeability, so the upper limit is set to 0.0100%. Since B has the function of improving the hardenability, it is necessary to consider the required hardenability when using B.

Ti: 0.005% to 0.05% Ti is an element having an effect of fixing N 2, like Al. To obtain the effect, the content of 0.005% or more is required. However, excessive addition may impair the cold forgeability due to precipitation strengthening of Ti carbonitride, so the upper limit was made 0.05%.

Zr: 0.005 to 0.05% Zr is an element that is effective in fixing N 2, like Ti, and it is necessary to contain 0.005% or more to obtain the effect. However, an excessive addition may impair cold forgeability due to precipitation strengthening of Zr carbonitride, so the upper limit was made 0.05%.

Pb: 0.25% 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, so 0.25%
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, it is often important to ensure machinability, so 0.15%
It can be added within the following range.

Ca: 0.01% or less Ca 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, so 0.01%
It can be added within the following range.

The reason for setting the hardness of steel satisfying the above composition to HV120 to HV180 as a method of manufacturing a forged material is as follows. From the viewpoint of cold forgeability, the lower the hardness of the forging material, the better. However, if it is less than HV120, it becomes difficult to obtain a hardness of HV260 or more even by subsequent cold forging and heat treatment. In addition, if it exceeds HV180, the cold forgeability deteriorates, so the upper limit was set to HV180.

Regarding the microstructure, the bainite or martensite microstructure is a hard microstructure, and its coexistence may cause deterioration of cold forgeability. In addition, the spheroidized annealed structure, which is usually used for cold forging materials, is very good from the viewpoint of cold forgeability, but has relatively small work hardening ability and can obtain the necessary component strength. Is excluded from the scope of the present invention because it is not possible and spheroidizing annealing is a very expensive heat treatment. The ferrite-pearlite structure of the present invention is hot-rolled or hot-forged,
Alternatively, it can be obtained by performing controlled rolling, controlled cooling, or the like as necessary. This structure can provide good cold forgeability and improved hardness after cold forging. Further, in order to further adjust the hardness to a predetermined range, by using normalizing or annealing treatment, it is possible to obtain further good cold forgeability and improvement in hardness after cold forging. .
The shape of the forged material can take various shapes such as a rolled steel material, a cut product, or a forged product, depending on the shape of the cold forged part.

In the cold forging process, the cold working ratio is increased to 50%.
The reason for the above is as follows. In order to obtain sufficient strength with a hardness of HV260 or more by work hardening by cold forging and age hardening by predetermined heat treatment after cold forging, it is necessary to significantly increase work dislocations in steel. It is necessary to do at least 50%. Depending on the shape of the part, it can be considered that it is difficult to make the cold working rate of the whole part 50% or more. What is necessary is that the working ratio is satisfied.

The reason why the age hardening index K is set to 400 to 800 in the heat treatment step after the cold forging is as follows. Since the strain aging due to this heat treatment is governed by the diffusion, accumulation and precipitation of C atoms in the steel, the heat treatment temperature T (° C.) and the heat treatment time (t) in the heat treatment step are very important. Therefore, the aging treatment index K value indicating the relationship between the heat treatment temperature T and the heat treatment time t is K = T × log
If the aging index K is less than 400,
Diffusion is insufficient and sufficient age hardening cannot be obtained.
If it exceeds 300, the precipitates will be aggregated and coarsened, and sufficient hardening cannot be obtained, and work hardening introduced by cold forging may be softened by strain recovery or recrystallization.

[0035]

Next, embodiments of the present invention will be described in detail. It will be clarified by examples in comparison with steel.

Example 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 produced by hot rolling to obtain a test material. Among the steels shown in Table 1, A to D steels have the chemical composition according to claim 1 of the present invention, and E to G steels have the chemical composition according to claim 2. H to K steels are comparative steels, and H steel has a lower C content than that of the present invention.
In steel, Al, B, Ti, and Zr do not satisfy the claimed content of the present invention, J steel has an N content exceeding the claimed content of the present invention, and K steel has a Mn content of the present invention. Exceeds the claimed content.

[0036]

[Table 1]

From a hot-rolled steel material having a diameter of 38 mm having the components shown in Table 1, a round bar-shaped test piece W having a diameter of 20 mm and a height of 40 mm was prepared.
1 is machined, and a test piece W1 is put into a concave lower die composed of dies 11 and 12 by an upper die 10 in the manner shown in FIGS. 1 (a) and 1 (b). Pressure) to determine the deformation load associated with the deformation of the test material W2 having an upsetting rate of 70%. The test piece was subjected to a bond treatment, and an 800T hydraulic forging press was used for cold forging.

These cold forged products were heated at a heating temperature of 350 ° C.
Heat treatment (age hardening index K value = 517) was performed for 30 minutes, and the hardness was measured. The hardness was also measured on hot-rolled steel and after cold forging. Table 2 shows the performance evaluation results for each sample.
Shown in

[0039]

[Table 2]

As shown in Table 2, the forged materials of the A to G steels of the present invention all have a ferrite-pearlite structure satisfying the hardness range of HV120 to HV180. It showed good forgeability of 100tf or less. Further, it can be seen that the hardness after the heat treatment was as high as HV260 or more. From the difference between the hardness after the heat treatment and the hardness after the cold forging, when the amount of age hardening due to the heat treatment is determined, it can be seen that the A to G steels of the present invention have a large age hardening of ΔHV32 to 44.

On the other hand, H steel having a low C content has a low deformation load and good forgeability because the hardness of the forged material is low.
Hardness after heat treatment is low, and a predetermined strength cannot be obtained. Al,
Steel I, which does not contain predetermined amounts of B, Ti, and Zr, has high deformation resistance and poor forgeability even though the hardness of the forged material is in the range of HV120-180. Also, the amount of age hardening is small. The J steel containing an excessive amount of N also has high deformation resistance and poor forgeability, despite the hardness of the forged material being in the range of HV120 to 180, like the I steel. K steel with an excessive amount of Mn has a high hardness of the forged material and a high deformation resistance.

[0042]

Example 2 The same test piece processing as in Example 1 was performed on steel A shown in Table 1 and upsetting cold forging was performed. The cold working rate was changed to 10-80%. Thereafter, a heat treatment (age hardening index K value = 517) was performed at a heating temperature of 350 ° C. for 30 minutes in the same manner as in Example 1, and the hardness was measured. FIG. 2 shows the evaluation results.

[0043]

FIG. 2

FIG. 2 shows the relationship between the hardness in the cold-worked state and the cold-working rate by black circles, and the relationship between the hardness in the heat-treated state after the cold-working and the cold-working rate by white circles. From this, as the cold working rate increases, the difference ΔH between the hardness after cold working and the hardness after cold working increases, and at a cold working rate of 50% or more, ΔHv becomes 30 Hv or more. You can see that there is. From this, it can be seen that sufficient age hardening can be obtained by performing an appropriate heat treatment at a cold working ratio of 50% or more. Also in this example, it can be seen that high strength of HV260 or more can be stably obtained if the cold working ratio is 50% or more.

[0045]

Example 3 The same test piece processing as in Example 1 was performed on steel A shown in Table 1 and cold forging was performed at an upsetting rate of 70%.
Thereafter, heat treatment was performed in which the age hardening index K was changed by changing the heating temperature and time in various ways, and the hardness was measured. FIG. 3 shows the evaluation results.

[0046]

FIG. 3

In FIG. 3, black circles indicate hardness after 70% cold working, and white circles indicate various age hardening indexes K after cold working.
The value indicates the hardness when heat-treated. As shown in FIG. 3, when the aging index K is in the range of 400 to 800, the age hardening amount is ΔHv.
It is about 30 to 40 Hv, and it can be seen that the hardness is improved by sufficient age hardening. If the aging treatment index K value is smaller than 400 or larger than 800, ΔHv becomes 30 Hv or less, and a sufficient improvement in hardness cannot be obtained. In this example, if the aging index K is in the range of 400 to 800, the HV260
It can also be seen that the above high strength can be obtained stably.

[0048]

According to the present invention, the cold forgeability represented by deformation resistance is good, and by cold work hardening and age hardening, heat treatment such as quenching and tempering with large strain deformation can be performed.
It is intended to provide a high-strength cold forged part having excellent dimensional accuracy. In addition, it greatly contributes to cost reduction of cold forged parts by improving mold life, miniaturization of in-process press, or elimination of quenching and tempering processing, and to net formation of cold forged parts by improving part dimensional accuracy. It is of great industrial significance.

[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.

FIG. 2 is an explanatory view showing the effect of the cold working rate on the hardness of the cold-processed and heat-treated steel after cold working in the present invention.

FIG. 3 is an explanatory diagram showing the effect of an age hardening index K value on the hardness of a material subjected to heat treatment after cold working in the present invention.

[Explanation of symbols]

 W1, W2: test material, 10: upper die, 11, 12: die

Claims (2)

[Claims] 1. A weight ratio of C: 0.12-0.40%, Si: 0.01
~ 0.35%, Mn: 0.10 ~ 1.40%, P: 0.030% or less, S: 0.035%
Below, N: 0.015% or less, O: 0.003% or less, further A
l: 0.005 ~ 0.200%, B: 0.0005 ~ 0.0050%, Ti: 0.005 ~ 0.0
Hardness HV120-HV containing one or more of 5%, Zr: 0.005-0.05%, with the balance being Fe and impurity elements
A material having a ferrite-pearlite structure of 180 is sequentially subjected to a cold forging process in which the cold working ratio is 50% or more, and a heat treatment process in which the aging treatment index K value obtained by the following equation is 400 to 800. A method for producing a high-strength cold-forged part, wherein the hardness is HV260 or more. K = T × logt where T indicates a heating temperature (° C.), and the range is 200 to
450 ° C. t indicates a heating time (minute). 2. C: 0.12 to 0.40% by weight, Si: 0.01
~ 0.35%, Mn: 0.10 ~ 1.40%, P: 0.030% or less, S: 0.035%
Below, N: 0.015% or less, O: 0.003% or less, further A
l: 0.005 ~ 0.200%, B: 0.0005 ~ 0.0050%, Ti: 0.005 ~ 0.0
5%, Zr: 0.005 to 0.05% of one or more kinds and Pb:
0.25% or less, Bi: 0.15% or less, Ca: 0.01% or less containing one or more kinds, and the balance is made of a material having a ferrite-pearlite structure with a hardness of HV120 to HV180 consisting of Fe and impurity elements, A cold forging process in which the working ratio is 50% or more, and an aging treatment index K value obtained by the following equation is 400 to
By applying the heat treatment process in order of 800, the hardness is HV2
A method for producing a high-strength cold-forged part, wherein the method is 60 or more. K = T × logt where T indicates a heating temperature (° C.), and the range is 200 to
450 ° C. t indicates a heating time (minute).