JPH1112687A - Bar steel for age hardening, excellent in cold forgeability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bar steel for age hardening, excellent in cold forgeability and having high strength equal to that of a heat treated medium carbon steel material. SOLUTION: As to the contents of alloying elements, this bar steel has a composition consisting of, by weight, 0.05-0.15% C, >0.10-0.45% Si, 0.20-0.65% Mn, 0.015-0.100% P, 1.00-2.00% Cu, 0.50-1.50% Ni, and the balance essentially of Fe and containing, if necessary, one or >=2 kinds selected from 0.005-0.080% S, 0.003-0.040% Te, 0.005-0.300% Pb, 0.005-0.200% Bi, and 0.0005-0.0050% Ca.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention is excellent in cold forgeability 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 high strength equivalent to that of a tempered material of medium carbon steel.

[0002]

2. Description of the Related Art A member obtained by hot forging non-heat treated steel or a member subjected to quenching and tempering after hot forging medium carbon steel is excellent in mechanical properties and strength. I have. However, for parts where weight accuracy, size and shape accuracy are problematic, machining is required after quenching and tempering or after hot forging, so the manufacturing cost of the parts is high. On the other hand, the members formed by cold forging have weight accuracy,
It has the features of good dimensional and shape accuracy and good surface texture, so machining can be omitted or simplified. However, cold forging costs are low for medium carbon steel and non-heat treated steel due to poor cold workability. It is expensive, and it cannot be expected that the manufacturing cost of parts will be reduced even if the cost of machining is reduced.

[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 working environment is better than that of the hot forging. It is desired to replace the manufacturing process of a hot forged member, which is a field that has not been applied conventionally. However, from the viewpoint of increasing the 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 as it is, and when strengthening by work hardening, the fatigue strength does not follow the increase in hardness. In addition, sufficient fatigue strength cannot be obtained as a member for machine structure. Therefore, it is necessary to perform heat treatment accompanied by transformation such as quenching after cold forging, which impairs the shape accuracy and surface properties which are one of the features of the cold forged material. In other words, it was difficult to achieve both the characteristics of cold forging and the strength of tempered steel.

[0004]

The inventors of the present invention have investigated the effects of chemical components, particularly aging precipitated elements, on the strength and cold forgeability of low carbon steel after cold forging. I found that.

[0005] 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 has a strength equivalent to that of a medium carbon steel tempered material. Is obtained. Since this heat treatment is performed at a temperature lower than the transformation point, the shape accuracy and surface properties of the cold forged product are not deteriorated. Further, since precipitation of Cu is not remarkable in the cooling process after hot working, 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 as well as to achieve both the cold forgeability and the strength equivalent to that of the tempered material of medium carbon steel.

When producing such Cu-added steel, C
Deterioration of hot workability due to the presence of u becomes a problem. In particular, in steel bar rolling, circumferential surface cracks generated in a temperature range around 1000 ° C. significantly degrade the surface quality of a steel material, and often induce cracks in cold forging. Then, the inventor found out the following as a result of investigating the effect of alloying elements on the phenomenon peculiar to the rolling of such Cu-added steel bars in more detail.

Circumferential surface cracks occurring at around 1000 ° C. are caused by the formation of a molten layer enriched with Cu immediately under the scale during hot rolling and penetrating into grain boundaries to which tensile stress has been applied. is there.

[0008] Ni is an element which is concentrated just below the scale similarly to Cu and raises the melting point of the Cu-enriched layer, thereby having the effect of suppressing circumferential surface cracks caused by Cu. However, it is necessary to add Ni in the same amount or more as that of Cu, so that the cold forgeability is degraded due to the increase in the material hardness and the cost of the steelmaking raw material is increased.

P is an element that tends to segregate at the grain boundaries, and is an element that reduces the ductility in hot working at 1200 ° C. or more such as in the initial rolling of continuous cast slabs and ingots.
It is effective in suppressing circumferential surface cracks generated at around 1000 ° C. in the u-added steel. However, in order to prevent this by P alone, a large amount of P needs to be added, and it becomes difficult to perform slab rolling at 1200 ° C or higher. Cu 1
By adding P together with about / 2 of Ni, hot workability and surface quality in a wide temperature range can be ensured.

That is, according to the present invention, C: 0.0
5 to 0.15%, Si: more than 0.10 to 0.45%, M
n: 0.20 to 0.65%, P: 0.015 to 0.10
0%, Cu: 1.00-2.00%, Ni: 0.50
It is an age hardening steel bar containing 1.50% and the balance substantially consisting of Fe and having excellent cold forgeability (Claim 1). Further, if necessary, S: 0.005 to 0.080% , T
e: 0.003 to 0.040%, Pb: 0.005 to
0.300%, Bi: 0.005 to 0.200%, C
a: It is an age-hardened steel bar excellent in cold forgeability containing one or more selected from 0.0005 to 0.0050%. (Claim 2)

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

C: 0.05 to 0.15% C is an element that improves the strength of steel materials. If C is less than 0.05%, its effect is small, and if it exceeds 0.15%, cracking during cold forging occurs. The probability of occurrence increases. Therefore, the content of C is set to 0.05 to 0.15%.

Si: more than 0.10 to 0.45% Si is an element which improves the strength of steel by solid solution strengthening.
If it exceeds, the ductility of ferrite decreases, and the probability of cracking during cold forging increases. Therefore, the content of Si is 0.1
More than 0 to 0.45%.

Mn: 0.20 to 0.65% Mn is an element that improves hot workability and suppresses surface cracking due to Cu. However, if it is less than 0.20%, the effect is small, and 0.65% is reduced. If it exceeds, the deformation resistance during cold forging significantly increases. Therefore, the content of Mn is 0.20 to 0.6.
5%.

P: 0.015 to 0.100% P is an element that suppresses the generation of circumferential surface cracks generated at around 1000 ° C. in the Cu-added steel. If it exceeds 0.100%, it will be 1200
The hot workability at ℃ or more is significantly deteriorated. Therefore, the content of P is set to 0.015 to 0.100%. P is preferably at least 0.031%.

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: 0.50 to 1.50% Ni is an element that prevents the deterioration of hot workability due to Cu, but has no effect if the content is less than 0.50%.
If it exceeds 1.50%, the machinability deteriorates remarkably. Therefore, the content of Ni is set to 0.50 to 1.50%.

S: 0.005 to 0.080% S is an element for improving machinability and is added as necessary. However, if it is less than 0.005%, there is no effect.
%, The cold forgeability deteriorates. Therefore, the content of S is set to 0.005 to 0.080%.

Te: 0.003 to 0.040% Te is an element for improving machinability, and is added as necessary. However, if it is less than 0.003%, there is no effect.
If it exceeds 0%, the cold forgeability deteriorates. Therefore, the content of Te is set to 0.003 to 0.040%.

Pb: 0.005 to 0.300% Pb is an element for improving the machinability of steel, and is added as needed. %, The cold forgeability deteriorates. Therefore, the content of Pb is set to 0.005 to 0.300%.

Bi: 0.005 to 0.200% Bi is an element that improves the chip crushability during cutting.
It is added as needed, but if it is less than 0.005%, there is no effect, and if it exceeds 0.200%, the cold forgeability deteriorates. Therefore, the content of Bi is 0.005 to 0.20.
0%.

Ca: 0.0005% to 0.0050% Ca is an element which 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%.

[0023]

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 35 mm was manufactured by hot rolling. Invention steel 1 and invention steel 2 correspond to the first invention of the present invention, and invention steels 3 to 8 correspond to the second invention. Comparative steel A is JI
S carbon steel S45C, and comparative steel B is a comparative steel in which P is not added to the first invention.

[0024]

[Table 1]

In order to evaluate the frequency of occurrence of surface cracks in the hot-rolled state, a round bar having a diameter of 35 mm and a length of 100 mm was sampled from each of ten places, and cut in a longitudinal section passing through the center.
The number of surface cracks having a depth of 0.5 mm or more in the cross section was measured. Also, in order to evaluate the cold forgeability, a length of 50 mm
After being cut into pieces, the surface of the rolled surface is cold-forged at various compression ratios in the axial direction, and the cylindrical surface is observed with a stereoscopic microscope at a magnification of 20 times.
The presence or absence of cracks was determined. Furthermore, for invention steels 1 to 8 and comparative steel B, in order to evaluate the strength of the aged material,
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, is performed without lubrication, and the cold forged material is aged at 500 ° C for 1 hour. After processing, the diameter of the test section is 3 mm from the shaft center.
mm tensile test pieces were collected by machining. Comparative steel A
The test piece was retained at 850 ° C. for 1 hour in a machined material having a diameter of 20 mm and a length of 100 mm, and then water-cooled and hardened at 550 ° C. for 1 hour.

Table 2 shows the number of cracks on the hot-rolled surface, the critical compressibility at which cracking occurs during cold forging, and the tensile strength of the aged material. 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.

[0027]

[Table 2]

In Table 2, invention steels 1 to 8 all have a lower material hardness than comparative steel A, and no surface cracks are observed in the hot-rolled state. Is 70% or more, which is better than Comparative Steel A. Further, the tensile strength of the aged material is higher than that of the tempered material of Comparative Steel A. On the other hand, Comparative Steel B has a high tensile strength of the aged material, but has a large number of surface cracks in a hot-rolled state, so that the critical compressibility in cold forging is extremely low.

That is, the steel material satisfying the claims of the present invention has a low material hardness and has good cold forgeability since no surface cracks are generated in a hot-rolled state. Therefore, it is possible to obtain a higher strength than the tempered material of medium carbon steel.

[0030]

As described above, according to the present invention, a mechanical structural member having the same strength as a medium-carbon steel quenched and tempered material, high shape accuracy by cold forging and high-quality surface skin, which has been difficult in the past. 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: more than 0.10 to 0.45%, Mn: 0.20 to 0.6
5%, P: 0.015 to 0.100%, Cu: 1.00
2.00%, Ni: 0.50 to 1.50%,
Age hardened steel bar with excellent cold forgeability consisting essentially of Fe. 2. C: 0.05 to 0.15% by weight, S
i: more than 0.10 to 0.45%, Mn: 0.20 to 0.6
5%, P: 0.015 to 0.100%, Cu: 1.00
2.00%, Ni: 0.50 to 1.50%,
Further, S: 0.005 to 0.080%, Te: 0.0
03-0.040%, Pb: 0.005-0.300
%, Bi: 0.005 to 0.200%, Ca: 0.00
An age hardening steel bar containing one or more selected from 0.05 to 0.0050%, and the balance being substantially Fe and having excellent cold forgeability.