JP2861564B2 - Age-hardened steel bars excellent in cold forgeability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel bar utilizing the age hardening phenomenon by adding Ni, Al, Cu and the like in combination, and particularly to an age-hardened steel bar excellent in cold forgeability. And a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for high-strength steel bars from the viewpoint of achieving lighter automobiles and improving fuel efficiency. However, this steel bar is generally formed into a desired shape by cold forging. At this time, when the strength is increased, the cold forgeability is reduced. An important requirement is to take measures to maintain as good a cold forgeability as possible.

As a means for satisfying the above requirements, there is known a method of softening by spheroidizing treatment and then performing quenching and tempering treatment after forging to secure strength. However, in this method, distortion is caused by quenching. There is a significant drawback that occurs. In addition, non-heat treated steels which have been improved in strength without performing such quenching treatment have been developed, but this technique does not provide cold forgeability comparable to that of spheroidized materials.

On the other hand, since the problem of quenching distortion can be avoided by utilizing aging precipitation in the ferrite region, a high-strength steel utilizing the age hardening phenomenon has been developed. Especially N
It is already known that a large age hardening can be expected by adding i, Al and Cu in combination.
The relationship between the amount of addition of i, Al, etc. and the strength is also studied (for example, “Iron and Steel” Oc).
t. 1972, PP. 502-514). However, since the above-mentioned technique is mainly intended to improve strength, workability before aging is hardly studied.

[0005] In the case of steel for cold forging, ensuring workability is one of the most important issues, and it is particularly important to ensure good drawing characteristics. The composite additives reported to date have a high carbon content, a matrix of ferrite-pearlite, or bainite or martensite structure, poor workability prior to aging, and, of course, lateral drawing. Was insufficient, and the cold forgeability had a large anisotropy.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been made to solve such technical problems, and an object of the present invention is to reduce the strength at the time of cold forging to about 50 kgf / mm ² and to reduce the drawing. tensile rolling direction (wale), tensile direction perpendicular to the rolling direction (short grain) of greater than 80% in any further 25 kgf / mm ² or more, such as a significant strength increase is obtained age-hardening type steel bars by age hardening And a method for producing such a steel bar.

[0007]

The present invention, which has achieved the above objects, comprises: C: 0.006% by weight or less, Si: 0.75% or less.
% By weight, Mn: 1% by weight or less, P: 0.1% by weight or less, S: 0.02% by weight or less, and N
i: 2 to 4% by weight, Al: 0.5 to 1.5% by weight, C
u: 0.5-2 wt%, respectively containing, 4 steel balance consisting of Fe and unavoidable impurities, the finish temperature is subjected to hot rolling as a _three or more points A, continued from finish rolling temperature
This is a method for producing an age-hardened steel bar which has a gist in cooling so that the cooling rate to 50 ° C is 5 to 50 ° C / sec. According to this method, an age hardened steel bar having excellent cold forgeability and low anisotropy of cold forgeability can be obtained.

[0008]

The present inventors have studied from various angles to achieve the above object. Attention was first focused on ultra-low carbon steel, and a composite addition of Ni, Al and Cu was performed on the ultra-low carbon steel.
However, it was found that the object of the present invention could not be achieved only by combining ultra-low carbon and adding Ni, Al and Cu composites. Therefore, further intensive studies were carried out to adjust the amounts of C and Ni, and to control the cooling conditions after hot rolling, thereby changing the structure of the hot-rolled material to a soft polygonal ferrite single phase with good workability ( Low strength,
(High aperture) can be secured, and the present invention has been completed. In the present invention, the reasons for limiting the component composition range are as follows.

C: 0.006% by weight or less C has an effect on the amount of pearlite precipitated and on the formation of a supercooled structure, so that the strength is reduced and the anisotropy of the workability is reduced. In order to do so, it is desirable to have as little as possible. In particular, in the case of Ni-added steel, it is necessary to prevent the formation of a massive structure, and for this purpose, the C content must be suppressed to 0.006% by weight or less.

Si: 0.75% by weight or less Si is added as a deoxidizing agent at the time of melting, but if contained in a large amount, the hardness of the material is increased, the workability is reduced, and the pickling property is reduced. Since it deteriorates, the upper limit was set to 0.75% by weight.

Mn: 1% by weight or less Mn is an element necessary for fixing S. However, when Mn is added in an amount of 1% by weight or more, the hardenability of the material is improved, and the softening of the material is prevented. Therefore, the upper limit of Mn is set to 1% by weight.

P: 0.1% by weight or less Since P is an element causing grain boundary embrittlement, the upper limit is set to 0.1% by weight. S: 0.02% by weight or less S becomes MnS and increases the inclusion density. Therefore, the upper limit is 0.02% by weight in order to ensure the workability of the material.
And

By the way, Ni, Al and Cu are elements related to the essence of the present invention, and the range of components is limited in order to cause age hardening by addition of a composite and to ensure the workability of the material.

Ni: 2 to 4% by weight Ni is an element essential for age hardening and needs to be added in an amount of 2% by weight or more. However, if added in excess of 4% by weight, the hardenability of the material is increased and the workability of the material is degraded, so the upper limit was 4% by weight.

Al: 0.5 to 1.5% by weight Al is an element essential for hardening, and it is necessary to add 0.5% by weight or more. Further, if it is added in excess of 1.5% by weight, the density of inclusions increases and the workability is impaired, and the nozzle is blocked during forging, which lowers the productivity and increases the cost. Therefore, the upper limit of the amount of Al added is set to 1.5% by weight.

Cu: 0.5 to 2% by weight Cu has its own curing ability, but contributes more to composite curing and shortens the curing time. For that purpose, the addition of 0.5% by weight or more is necessary. However, even if it is added in an excessively large amount, the effect is not only saturated, but also hot brittleness is generated and a hot-rolled slab is cracked. Therefore, the upper limit of the addition amount is set to 2% by weight so that the composite effect is sufficiently exhibited and hot brittleness is not caused. In the present invention, each of the above elements is used as a basic component. However, other unavoidable impurity elements may be present, and there is no particular problem as long as it is within a normal range.

Next, the reasons for limiting the manufacturing conditions in the present invention are as follows. Hot rolling finishing temperature is not less than _three points A, terminates the rolling at austenite region. Since this material is liable to cause precipitation of Ni, Al, Cu, etc., Ni, C
In order to ensure age hardening ability while keeping u in a solid solution state, it is necessary to cool from the rolling temperature to 450 ° C. at a rate of 5 ° C./s or more. However, if the cooling is too rapid, the structure is supercooled and becomes a structure having a high transition density, and a structure mainly composed of a desired ferrite phase cannot be obtained. In order to prevent the formation of a supercooled structure, it is necessary to control the balance among the cooling rate, the amount of carbon, and the amount of Ni. The above composition range (C:
(0.006% by weight or less, Ni: 2 to 4% by weight), it is possible to prevent the appearance of a supercooled structure by slow cooling at a cooling rate of 50 ° C / s or less. Thus, according to the present invention, it is possible to easily manufacture a steel bar having a low strength, a large drawing, and a large increase in strength by aging. Here, the cooling rate means a value obtained by dividing a temperature change from a hot rolling finishing temperature to 450 ° C. by a required time.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and the design may be changed in accordance with the above and subsequent points, for example, the composition of the components and the manufacturing conditions. Are all included in the technical scope of the present invention.

[0019]

【Example】

Example 1 A steel having the composition shown in Table 1 below was prepared at a finishing temperature of 950 ° C.
And cooled to 450 ° C. at 30 ° C./sec.

[0020]

[Table 1]

Tensile test specimens were taken from the above materials and subjected to a tensile test in the rolling direction. A tensile test was also performed on a sample obtained by aging this material at 550 ° C. for 3 hours and then air-cooling. The result is shown in FIG. In FIG. 1, the horizontal axis represents Ni.
The vertical axis indicates tensile properties (tensile strength: TS). As is clear from FIG. 1, the strength before aging is 50 kgf / mm ² or less up to 4% by weight of Ni, but the strength sharply increases when the amount of Ni is higher than 4 kg. Also, the drawing (RA), which is an index of the cold forgeability, shows a good value up to 4% by weight, but if it is higher than 4% by weight, it is significantly deteriorated. In addition, the strength increase due to aging shows a large value at 2% by weight or more of Ni, but at 1% by weight, only the strength increase of the conventional Cu-added steel is observed. Therefore, Ni is 2-4% by weight.
It is understood that it is necessary to be within the range.

Further, in order to clarify the influence of the manufacturing conditions, a material of 3 wt% Ni
Cooling was performed by changing the cooling rate to 50 ° C. from 0.5 to 200 ° C./sec, and the drawing (RA) and tensile strength (T
S) was investigated. The results are shown in FIG. 2, and it is necessary to set the cooling rate in the range of 5 to 50 ° C./sec in order to secure the drawing (RA) which is an index of the cold forgeability before aging. I understand.

Example 2 A steel having the composition shown in Table 2 below was prepared at a finishing temperature of 950 ° C.
, And cooled to 450 ° C. at a cooling rate of 30 ° C./sec.

[0024]

[Table 2]

Tensile test specimens were taken from the above materials and subjected to a tensile test in the rolling direction. A tensile test was also performed on a sample obtained by aging this material at 550 ° C. for 3 hours and then air-cooling. The result is shown in FIG. In FIG. 3, the horizontal axis represents A
The l-axis shows the tensile properties on the vertical axis. As is evident from FIG. 3, the strength before aging does not depend on the Al content, but the strength after aging clearly depends on the Al content. It turns out that Al is required. Example 3 A steel having the composition shown in Table 3 below was prepared at a finishing temperature of 9
It was hot rolled at 50 ° C. and cooled to 450 ° C. at a cooling rate of 30 ° C./sec.

[0026]

[Table 3]

Tensile test specimens were taken from the above materials and subjected to a tensile test in the rolling direction. A tensile test was also performed on a sample obtained by aging this material at 550 ° C. for 3 hours and then air-cooling. The result is shown in FIG. In FIG. 4, the horizontal axis represents Cu.
The amount and the tensile properties are shown on the vertical axis. As is clear from FIG. 4, the strength before aging does not depend on the amount of Cu, but the strength after aging clearly depends on the amount of Cu. It turns out that Cu is necessary. Example 4 A steel having a chemical composition shown in Table 4 below was prepared at a finishing temperature of 950 ° C.
At 450 ° C. at a cooling rate of 30 ° C./sec.

[0028]

[Table 4]

Tensile test specimens were taken from the above-mentioned materials and subjected to tensile tests in the rolling direction and the direction perpendicular thereto. Table 4 also shows the tensile properties. However, No. 3 has a high carbon content and therefore has a high strength and a reduced drawing. No. 4 is M
Since the number of n is large, the strength is similarly increased and the aperture is reduced. No. 5 has a high strength due to a large amount of Si, and as a result, the aperture is reduced. The reduction in the direction perpendicular to the rolling
In Nos. 1 and 2, the value is almost the same as the value in the rolling direction, showing a good value. In No. 3, the reduction in the direction perpendicular to the rolling direction is worse than the value in the rolling direction. That is, No.
It can be seen that in Nos. 1 and 2, the cold workability was excellent and the anisotropy was small.

[0030]

According to the present invention, an age-hardened steel bar having excellent cold forgeability, small anisotropy, and significantly increased strength by heat treatment can be easily obtained. . In particular, when used for automobile gears, bolts, and other main structural members, the steel bar according to the present invention contributes to a significant reduction in vehicle body weight, and is expected to have great industrial significance and profit.

[Brief description of the drawings]

Fig. 1 Strength of steel bar, strength after drawing and aging, and N
It is the graph which showed the relationship of i quantity.

FIG. 2 is a graph showing the relationship between the strength of a steel bar material, the strength after drawing and aging, and the cooling rate after hot rolling.

FIG. 3 is a graph showing the relationship between the strength of a steel bar material, the strength after aging, and the amount of Al.

FIG. 4 is a graph showing the relationship between the strength of a steel bar material, the strength after aging, and the amount of Cu.

Claims (2)

(57) [Claims] 1. C: 0.006% by weight or less, Si: 0.
75% by weight or less, Mn: 1% by weight or less, P: 0.1% by weight or less, S: 0.02% by weight or less, Ni: 2 to 4% by weight, Al: 0.5 to 1% .5 wt%, Cu: 0.5 to 2% by weight respectively and contained, the steel and the balance Fe and unavoidable impurities, the finish temperature is subjected to hot rolling as a _three or more points a, continue rolling finishing temperature A method for producing an age-hardened steel bar excellent in cold forgeability, characterized in that cooling is performed so that the cooling rate from to 50 ° C / sec is 5 to 50 ° C / sec. 2. An age-hardened steel bar manufactured by the method according to claim 1, which is excellent in cold forgeability.