JPH1096045A - Structural steel excellent in cold forgeability and strength, and production of forged member using the steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a structural steel excellent in cold workability as well as in cold forgeability and strength, and a method for producing forged members by using the steel. SOLUTION: The cold forging steel excellent in strength has a composition containing, as contents of alloying elements, 0.05-0.15%, by weight, C, 0.05-0.15% Si, 0.05-0.35% Mn, 1.00-2.00% Cu, 0.75-1.50% Ni, 0.15-0.60% Cr, and further one or >=2 kinds selected from 0.05-0.15% V, 0.030-0.080% S, 0.005-0.040% Te, 0.03-0.30% Pb, 0.03-0.20% Bi, and 0.0005-0.0050% Ca and having the balance essentially Fe and also has <=180HV hardness in hot rolled state. The cold forged member excellent in strength can be produced by subjecting a stock of this steel to cold forging at >=40% average draft and then to heat treatment consisting of holding at a temp. of 350-550 deg.C for 120-240min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention is excellent in cold workability and does not cause heat treatment distortion or surface oxidation, that is, without impairing the advantages of cold forging.
The present invention relates to a cold forging steel capable of obtaining a high strength equivalent to that of a tempered material of a medium-carbon tough steel, and a method of manufacturing a high-strength forged member using the steel.

[0002]

2. Description of the Related Art A member obtained by quenching and tempering a medium-carbon tough steel such as JIS-SCM440 after hot forging is excellent in mechanical properties and strength, and is therefore often used as a mechanical structural member. However, parts requiring high precision in weight, dimensions and shape require machining after hot forging or quenching and tempering, so that the manufacturing cost of the parts is high. On the other hand, the members formed by cold forging have weight accuracy,
Although it has the features of good dimensional and shape accuracy and good surface texture, it is difficult to cold work medium-carbon tough steel even with annealed material.

[0003]

The cold forging contributes to the improvement of the mechanical performance and the simplification of the machining process by improving the weight and dimensional accuracy, and the environment of the forging factory is better than that of the hot forging. It is desired to replace the member manufacturing process of hot forging, quenching and tempering and machining, which is a field that has not been applied conventionally. However, from the viewpoint of the increase in deformation resistance during cold forging and the occurrence of forging cracks,
There is a limit to the strength that can be obtained with cold forging,
With the strengthening by work hardening, the fatigue strength does not follow the increase in hardness, and a sufficient fatigue strength as a member for machine structure cannot be obtained. Therefore, it is necessary to perform heat treatment accompanied by transformation such as quenching after cold forging, and the shape accuracy and surface properties, which are one of the features of the cold forged material, deteriorate. In other words, it was difficult to achieve both the characteristics of cold forged materials and the strength of medium carbon tough steel.

[0004]

Means for Solving the Problems The inventors of the present invention have investigated the effects of chemical components, particularly aging precipitation elements, on the strength of low carbon steel after cold forging and found the following.

[0005] A low carbon steel containing 1% or more of Cu exhibits precipitation hardening in a relatively short time by aging at a temperature below the transformation point after cold forging, and can obtain strength equivalent to that of a medium carbon tough steel. Can be Further, since the precipitation of Cu is not remarkable in the cooling process of hot rolling, it is not necessary to perform expensive heat treatment such as solution treatment or annealing before cold forging. Therefore,
It is possible to reduce the heat treatment cost while achieving both the cold forgeability and the strength equivalent to that of tough steel. However, in order to secure sufficient cold forgeability while omitting annealing, the hardness in the hot-rolled state is set to 180 HV or less by reducing C, which forms carbide, and Si and Mn, which are ferrite strengthening elements. It is necessary to improve the toughness of pearlite by adding Cr and Cr.
In addition, the hot workability deteriorates due to the addition of Cu.
It is necessary to add at least 1/2 of the u amount of Ni.

There are few studies on the age hardening phenomenon of Cu-added steel, and there is no report on the effect of aging conditions in combination with cold forging. Therefore, the inventor
As a result of examining the effects of cold forging conditions and aging treatment conditions on the strength of Cu-added low carbon steel, the following was found.

The hardness after aging of a low carbon steel containing 1% or more of Cu changes with the aging temperature, the aging time, and the degree of cold working, but the aging temperature at which the influence of the aging time and the degree of cold working becomes small. Area exists. The small influence of the aging time and the degree of cold working is advantageous in terms of the homogeneity of strength when considering the variation in furnace time in heat treatment in a continuous furnace and the strain distribution of cold forgings. However, even in this temperature range, there is an appropriate holding time, so it is important not to deviate from this range. Further, in order to manufacture a forged member having more uniform strength, an average working ratio of 40% or more is required. This makes it possible to manufacture a forged member having high strength while having the features of cold forging.

That is, according to the present invention, C: 0.0
5 to 0.15%, Si: 0.05 to 0.15%, Mn:
0.05-0.35%, Cu: 1.00-2.00, N
i: 0.75 to 1.50%, Cr: 0.15 to 0.60
%, V: 0.05 to 0.15%, S:
0.030 to 0.080%, Te: 0.005 to 0.0
40%, Pb: 0.03 to 0.30%, Bi: 0.03
-0.20%, Ca: 0.0005-0.0050%, one or more selected from the group consisting essentially of F
e, and the steel for cold forging having excellent strength characterized in that the hardness in the hot-rolled state is 180 HV or less is defined as the first invention, and the steel corresponding to the first invention has an average working ratio of 40
% Of cold forging, followed by heat treatment at a temperature of 350 ° C. or more and 550 ° C. or less for 120 to 240 minutes to produce a cold forged member having excellent strength. It consists of two inventions.

The reasons for limiting the scope of the present invention will be described below.

C: 0.05 to 0.15% C is an element for improving the strength of steel, and has a content of 0.05%.
%, The strength after cold forging is insufficient. On the other hand, C is an element that forms pearlite and deteriorates cold workability.
If it exceeds 0.15%, the effect becomes significant. Therefore,
The content of C is 0.05 to 0.15%.

Si: 0.05-0.15% Si is an element that strengthens the ferrite phase by solid solution strengthening, and has the effect of improving the strength of steel after cold forging.
If the content is less than 0.05%, the effect is small.
If it exceeds 15%, the ductility of the ferrite decreases, and the cold workability deteriorates. Therefore, the content of Si is 0.05 to 0.1.
15%.

Mn: 0.05-0.35% Mn is an element that strengthens the ferrite phase by solid solution strengthening and improves the forgeability of cold by improving the toughness of pearlite. If it is less than 05%, the effect is small, and if it exceeds 0.35%, work hardening of ferrite is promoted, so that cold workability is deteriorated. Therefore,
The content of Mn is set to 0.05 to 0.35%.

Cu: 1.00 to 2.00% Cu is an element that precipitates by aging treatment following cold forging and improves the strength of the cold forged material.
If it is less than 2,000%, the effect is small, and if it exceeds 2.00%, the hot workability is significantly deteriorated. Therefore, the content of Cu is set to 1.00 to 2.00%.

Ni: 0.75 to 1.50% Ni is an element for preventing deterioration of hot workability due to Cu, but if the content is less than 0.75%, the effect is small, and 1.50% If it exceeds, the machinability deteriorates. Therefore,
The Ni content is 0.75 to 1.50%, and preferably 0.7 to 0.8 times the Cu content.

Cr: 0.15 to 0.60% Cr is an element that improves the deformability in cold forging by improving the toughness of pearlite.
If it is less than 15%, the effect is small, and if it exceeds 0.60%, the amount of pearlite increases and the deformation resistance in cold working increases. Therefore, the content of Cr is 0.15 to 0.5.
60%.

V: 0.05 to 0.15% V is an element which improves the strength of the steel after cold forging by improving the strength of the ferrite phase by precipitation hardening, and is added as necessary. If the content is less than 0.05%, the effect is not remarkable, and if it exceeds 0.15%, the increase in hardness due to aging becomes small. Therefore, the content of V is set to 0.05 to 0.15%.

S: 0.030% to 0.080% S is an element for improving machinability, and is added as needed. However, if it is less than 0.030%, the effect is small.
If it exceeds 0%, the cold workability and strength deteriorate. Therefore, the content of S is set to 0.030 to 0.080%.

Te: 0.005 to 0.040% Te is an element that improves machinability and cold workability by spheroidizing sulfide, and is added as necessary. %, The effect is small, 0.040%
Exceeding the value impairs hot workability. Therefore, the content of Te is set to 0.005 to 0.040%.

Pb: 0.03 to 0.30% Pb is an element for improving the machinability of steel, and is added as needed. However, if it is less than 0.03%, the effect is small.
If it exceeds 0.30%, the strength deteriorates. Therefore, the content of Pb is set to 0.03 to 0.30%.

Bi: 0.03% to 0.20% Bi is an element that improves the chip crushability during cutting.
It is added as needed, but if it is less than 0.03%, the effect is small, and if it exceeds 0.20%, the hot workability deteriorates. Therefore, the content of Bi is set to 0.03 to 0.20%.

Ca: 0.0005 to 0.0050% Ca is an element that improves machinability by controlling the composition of the oxide, and is added as necessary.
If it is less than 05%, the effect is small, and if it exceeds 0.0050%, hard CaS is generated and the machinability deteriorates. Therefore, the content of Ca is set to 0.0005 to 0.0050%.

Average working ratio: 40% or more The working ratio in cold forging affects the strength after aging treatment.
If the average working ratio is less than 40%, sufficient strength cannot be obtained, and if it is more than 40%, the change in strength is small. Therefore, the average processing rate is set to 40% or more.

Aging temperature: 350 to 550 ° C. Aging time: 120 to 240 minutes The steel material corresponding to claim 1 of the present invention is improved in strength by aging after cold forging. 350
If the temperature is lower than ℃, sufficient strength cannot be obtained in a portion where the processing strain is low, and if it exceeds 550 ° C, sufficient hardness cannot be obtained due to recovery of the processing strain. Further, if the processing time is less than 120 minutes, the hardening due to the aging is not remarkable, and if the processing time exceeds 240 minutes, the hardness is reduced. Therefore, the aging temperature is 350 to 550 ° C and the aging time is 120 to
240 minutes.

[0024]

The present invention will be described below with reference to examples. After melting steel having the chemical composition shown in Table 1 in an arc furnace, a round bar having a diameter of 25 mm was manufactured by hot rolling.

[0025]

[Table 1]

In Table 1, for Invention Steels 1 to 5,
After machining to a diameter of 20 mm and a length of 100 mm, cold forging with a compressibility of 50% in the direction perpendicular to the axis, that is, in the diameter direction, was performed without lubrication. After aging treatment of the cold forged material under the conditions shown in Table 2, a tensile test piece and a tensile compression fatigue test piece having a test part diameter of 3 mm were sampled from the shaft center by machining to evaluate the strength. Comparative steel A is JIS-SCM44
0, and after hot forging to a similar shape at 1200 ° C,
After holding at 850 ° C. for 1 hour, water-cooled quenching and after holding at 550 ° C. for 1 hour, water-cooled tempering were performed, and test specimens were collected from the shaft center. Furthermore, in order to evaluate the cold workability, the diameter 2
A cylindrical machined product having a length of 0 mm and a length of 30 mm was cold-forged at various compression ratios in the axial direction, and the cylindrical surface was observed with a stereoscopic microscope having a magnification of 20 times to determine the presence or absence of cracks.

The tensile strength, Table 2 shows the 10 ⁷ times tensile compressive fatigue strength and crack occurrence limit compression ratio. As the crack occurrence limit compression ratio, a compression ratio at which a crack occurrence probability becomes 5% in cold forging of a rolled material was employed.

[0028]

[Table 2]

In Table 2, Invention Examples 1 to 5 were prepared by subjecting invention steels 1 to 5 corresponding to claim 1 of the present invention to cold forging and aging treatment under the conditions corresponding to claim 2 of the present invention. The test was conducted with a hardness of 300 HV or more and a tensile strength of 96.
0MPa or more, fatigue strength 520MPa or more, SC
Strength equal to or higher than Comparative Example 1 obtained by quenching and tempering M440 was obtained. In addition, the critical compressibility is 70% or more in each case, and good cold forgeability is obtained.

That is, a steel material satisfying claim 1 of the present invention is manufactured, and the steel material is subjected to cold working and aging treatment by a method corresponding to claim 2 of the present invention.
It is possible to achieve both good cold forgeability and strength equivalent to that of a medium-carbon toughened steel quenched and tempered material.

[0031]

As described above, according to the present invention, a mechanical structure having the same strength as that of a medium-carbon toughened steel quenched and tempered material, high shape accuracy by cold forging and high-quality surface skin, which has been difficult in the past. The member can be manufactured at low cost, and the industrial advantage is extremely large.

Claims (2)

[Claims] 1. C: 0.05 to 0.15% by weight, S
i: 0.05 to 0.15%, Mn: 0.05 to 0.35
%, Cu: 1.00-2.00, Ni: 0.75-1.
50%, Cr: 0.15-0.60%, V: 0.05-0.15%, S: 0.030-0.
080%, Te: 0.005 to 0.040%, Pb:
0.03 to 0.30%, Bi: 0.03 to 0.20%,
Ca: one or more selected from 0.0005 to 0.0050%, the balance being substantially Fe, and a hardness in a hot-rolled state of 180 HV or less. Excellent cold forging steel. 2. C: 0.05 to 0.15% by weight, S
i: 0.05 to 0.15%, Mn: 0.05 to 0.35
%, Cu: 1.00-2.00, Ni: 0.75-1.
50%, Cr: 0.15-0.60%, V: 0.05-0.15%, S: 0.030-0.
080%, Te: 0.005 to 0.040%, Pb:
0.03 to 0.30%, Bi: 0.03 to 0.20%,
Ca: 350% after subjecting a steel containing at least one selected from 0.0005% to 0.0050% to at least 40% or more to a steel substantially composed of Fe and having a balance of at least 40%.
A method for producing a cold forged member having excellent strength, comprising performing a heat treatment at a temperature of not less than 550C and not more than 550C for 120 to 240 minutes.