JPH09202921A - Production of wire for cold forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively produce a wire for cold forging suitable as the stock for high strength small and long-sized parts having 1,000 to 1,250MPa tensile strength at high efficiency. SOLUTION: A steel having a chemical compsn. contg., by weight, 0.15 to 0.30% C, 0.05 to 0.30% Si, 1.0 to 2.0% Mn, 0.5 to 1.0% Cr, 0.005 to 0.10% Al, 0 to 0.5% Cu, 0 to 0.5% Ni, 0 to 0.5% Mo, 0 to 0.5% V, 0 to 0.2% Nb, 0 to 0.1% Ti, 0 to 0.01% B, <=0.03% P, <=0.03% S, <=0.02% N, and the balance Fe with inevitable impurities is heated to the temp. range of 900 to 1,250 deg.C, is subjected to hot rolling, is finished at >=750 deg.C, is thereafter quenched at a cooling rate of >=2 deg.C/sec, is next tempered in the temp. range of 400 to 600 deg.C and is moreover subjected to cold drawing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold forging suitable for use in long and small parts, such as bolts and tie rods, and capable of simplifying and omitting processing steps such as a heat treatment step and a straightening step. More specifically, the present invention relates to a method for producing a cold forging wire suitable as a raw material for producing a high-strength small-sized long component having a tensile strength of 1000 to 1250 MPa.

[0002]

2. Description of the Related Art High-strength small long components have been manufactured by using carbon steel or alloy steel of JIS for machine structure. For these materials, as shown in FIG. 1 as a conventional method, “spheroidizing annealing” is performed between hot rolling and cold drawing, and “hardening” and “tempering” heat treatment is performed after cold forging. However, the work of "bending straightening" has been performed after quenching and tempering.

Therefore, the production efficiency is low and the manufacturing cost is high.

For this reason, for example, in Japanese Patent Publication No. 60-406, C: 0.15 to 0.30% by weight, Mn:
After patenting the steel wire containing 1-2%, 15
A "method for producing a high-strength bolt" in which cold drawing is performed at -40% and bolt forming is disclosed. However, in the method proposed in this publication, "quenching / tempering" and "bending correction" can be omitted, but "patenting" must be performed instead of the above "spheroidizing annealing". Therefore, the hot-rolled wire has to be heated again to the austenite temperature range, and there are still problems in energy and cost. Further, as is clear from the example, the strength (tensile strength) of the finished bolt product is at most 90.5 kgf / mm ² (about 8
It was as low as 88MPa), and it was not always possible to meet the demand from industry for "high strength".

In Japanese Patent Laid-Open No. 7-54041, the weight% is
C: 0.05 to 0.25%, Mn: 0.50 to 2.
A "method for producing steel for cold forging" has been proposed in which a steel slab having a specific chemical composition such as 0% is hot-rolled and cooled under specific conditions and then cold-drawn. However, since the technique disclosed in this publication is also intended to obtain the material strength after cold drawing of 735 to 830 MPa, the strength of the actual part is at most 940 MPa as apparent from the examples. This is a low value, and it was not always possible to meet the demand for "higher strength" from the industrial world.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and it is possible to simplify or omit processing steps such as a heat treatment step and a straightening step in the production of small-sized long parts. An object of the present invention is to provide a highly efficient and inexpensive manufacturing method of a wire for forging.

Specifically, an object of the present invention is to provide a highly efficient and inexpensive manufacturing method of a wire for cold forging, which is suitable as a raw material for a high-strength small-sized long component having a tensile strength of 1000 to 1250 MPa. That is.

[0008]

Means for Solving the Problems As a result of various studies to achieve the above object, the present inventor has adjusted the composition of the steel material and, after hot rolling, so-called "direct quenching" to perform the entire length. By forming a uniform quenched structure, then tempering, and then adjusting the strength by cold drawing, a highly ductile cold forging wire suitable as a material for small long parts having a desired high strength is obtained. The present invention has been completed by finding that it can be obtained.

According to the present invention, "% by weight, C: 0.
15 to 0.30%, Si: 0.05 to 0.30%, M
n: 1.0 to 2.0%, Cr: 0.5 to 1.0%, A
1: 0.005 to 0.10%, Cu: 0 to 0.5%, N
i: 0-0.5%, Mo: 0-0.5%, V: 0-0.
5%, Nb: 0-0.2%, Ti: 0-0.1%, B:
0-0.01%, P: 0.03% or less, S: 0.03%
Hereinafter, N: 0.02% or less, the balance is steel having a chemical composition consisting of Fe and unavoidable impurities, heated to a temperature range of 900 to 1250 ° C., hot-rolled, and finished at a temperature of 750 ° C. or higher, Quench at a cooling rate of 2 ° C / sec or more, then 4
The gist is a "method for producing a wire for cold forging, characterized by performing tempering in a temperature range of 00 to 600 ° C and further performing cold drawing."

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Each requirement of the present invention will be described in detail below. In addition, “%” of the component content is “% by weight”.
Means

(1) Chemical composition of steel C: C is an element effective for increasing the strength, but a lower value is desirable from the viewpoint of ensuring the ductility and toughness of the final product, small-sized long parts, and the upper limit is preferable. Was 0.30%. Regarding the lower limit, the desired strength (tensile strength is 10
It was set to 0.15% in order to give a pressure of 0 to 1250 MPa).

Si: Si has the functions of stabilizing the deoxidation of steel, strengthening the matrix, and further enhancing the work hardening characteristics. However, if its content is less than 0.05%, the effect of addition is poor, and if it exceeds 0.30%, the deformability of the steel decreases, so the Si content is set to 0.05 to 0.3.
0%.

Mn: Mn is an element necessary for deoxidation and desulfurization of steel, and has the function of enhancing the hardenability of steel and improving the strength. In particular, in the present invention, a desired quenching structure is obtained by "direct quenching" after hot rolling, and a desired strength (tensile strength of 1000 to 1000) is obtained for a small-sized long component as a final product.
It is an essential element for imparting 1250 MPa). To obtain the above effect, the Mn content must be at least 1.0% or more.

On the other hand, if the content exceeds 2.0%, the ductility deteriorates. Therefore, the Mn content is set to 1.0 to 2.0%.

Cr: Cr has the function of enhancing the hardenability of steel and improving the strength. Particularly, in the present invention, a desired quenching structure is obtained by "direct quenching" after hot rolling to obtain a desired strength (tensile strength of 10 or less) for a small-sized long component as a final product.
It is an indispensable element for imparting 0 to 1250 MPa). To obtain the above effect, the content of Cr must be at least 0.5%. On the other hand, if the content exceeds 1.0%, the ductility is deteriorated, the drawability in the cold after tempering is deteriorated, and the ductility of the final product is deteriorated. Therefore, the content of Cr is 0.5
.About.1.0%.

Al: Al has the function of enhancing the cold forgeability of steel. That is, Al fixes N to suppress age hardening during cold forging, and improves the cold forgeability by making the lubricating coating uniform. Further, Al is effective for deoxidizing steel and also has an effect of preventing coarsening of austenite crystal grains during heating of steel before hot rolling. However, if its content is less than 0.005%, the desired effect cannot be obtained, while if it exceeds 0.10%, Al ₂ O ₃ -based inclusions increase and the deformability becomes insufficient, rather causing cold working. It easily cracks during forging. Therefore, the content of Al is set to 0.005 to 0.10%.

Cu: Cu may not be added. If added, it has the effect of enhancing cold forgeability and hardenability. In order to surely obtain such effects, the content of Cu is preferably 0.05% or more. However, the content is 0.5
If it exceeds%, the hot workability is deteriorated and the ductility is lowered, and the cold workability is rather deteriorated.
Therefore, the Cu content is set to 0 to 0.5%.

Ni: Ni may not be added. Addition has the effect of increasing ductility and toughness. In order to reliably obtain such effects, it is preferable that the content of Ni be 0.05% or more. However, if the content exceeds 0.5%, the above effect is saturated, and the cost only increases. Therefore, the Ni content is set to 0 to 0.5%.

Mo: Mo may not be added. If added, it has the effect of strengthening the steel. In order to surely obtain such effects, it is preferable that the content of Mo is 0.02% or more.

However, if its content exceeds 0.5%, ductility deteriorates. Therefore, the Mo content is 0 to 0.5.
%.

V: V may not be added. If added, it has the effect of strengthening the steel. In order to surely obtain such effects, it is preferable that the content of V be 0.01% or more. However, if the content exceeds 0.5%, ductility is deteriorated. Therefore, the V content is set to 0 to 0.5%.

Nb: Nb may not be added. If added, it has the effect of refining the steel structure and improving ductility and toughness. In order to reliably obtain such effects, Nb is 0.0
Preferably, the content is at least 05%. However, if its content exceeds 0.2%, the hardenability deteriorates, so that the desired hardened structure cannot be obtained by "direct hardening" after hot rolling. For this reason, it becomes impossible to impart desired strength (tensile strength of 1000 to 1250 MPa) to the small-sized long components as the final product. Therefore, the Nb content is set to 0 to 0.2%.

Ti: Ti may not be added. If added, it has the effect of refining the steel structure and improving ductility and toughness. In order to surely obtain such an effect, Ti is 0.0
Preferably, the content is at least 05%. However, if the content exceeds 0.1%, the Ti carbonitrides become coarse and the hot workability deteriorates, which causes cracking during cold forging. Therefore, the Ti content is 0 to 0.1%.
And

B: B may not be added. If added, it has the effects of enhancing hardenability and improving cold workability. In order to surely obtain such an effect, B is 0.000.
The content is preferably 3% or more. However, if the content exceeds 0.01%, the hot workability is deteriorated. Therefore, the B content is set to 0 to 0.01%.

In the present invention, P as an impurity element,
The contents of S and N are limited as follows.

P: P deteriorates the deformability and ductility of steel,
In particular, if the content exceeds 0.03%, the deformability and ductility are significantly deteriorated. Therefore, the upper limit of the content of P as an impurity element is set to 0.03%.

S: S deteriorates the cold forgeability of steel, and particularly when the content thereof exceeds 0.03%, the cold forgeability is significantly deteriorated. Therefore, the upper limit of the content of S as an impurity element is set to 0.03%.

N: N causes blue-heat embrittlement and deteriorates cold forgeability, and particularly when the content thereof exceeds 0.02%, the cold forgeability deteriorates remarkably. Therefore, the upper limit of the content of N as an impurity element is set to 0.02%. The upper limit of the N content is preferably 0.01%, and further, N
The upper limit value of is more preferably 0.006%.

(2) Manufacturing conditions FIG. 1 shows a comparison between the processing steps of the present invention and the conventional method. In the present invention, hot rolling → direct quenching →
A wire for cold forging is manufactured through each process of tempering → cold drawing. That is, in the present invention, in the cooling process after hot rolling, for example, it is expanded in a ring shape on a Stelmore conveyor, directly quenched by a means such as air cooling, and then wound into a coil and tempered, and then further. A cold forging wire is manufactured by performing cold drawing to adjust the strength.

The wire of which the strength is adjusted according to the present invention is then cold-forged into a small product such as a bolt, which is a final product, and thereby a desired high strength (tensile strength of 1000 to 1250 MPa) is imparted. To be done.

It should be noted that high strength small-sized long components manufactured by tempering treatment such as quenching and tempering are often used after being plated, and in this case, 1 for dehydrogenation.
A heat treatment called “baking” is performed at 80 to 200 ° C. (FIG. 1 (A)). In the case of a small long component cold-forged with the wire according to the present invention, if the above heat treatment is performed after the plating treatment, dehydrogenation is performed and internal stress is also removed, which is more excellent. The characteristics are obtained. In the examples described later, the heat treatment in the above temperature range in the method of the present invention is particularly referred to as "bluing treatment".

According to the present invention, as shown in FIG. 1B, it is possible to omit the spheroidizing annealing before cold drawing in the conventional process. Further, in the present invention, the heat treatment of quenching and tempering which has been conventionally performed on the cold forged product (product shape product) is utilized by the heat retained in the state of the hot rolled steel material (mainly the hot rolled wire rod). And directly quenching, and then tempering. For this reason, it goes without saying that a heating step at the time of quenching is not necessary, but tempering can be performed extremely efficiently because, for example, the coiled steel material itself can be used. In addition, since the desired strength and dimensional accuracy can be obtained by cold forging the wire, the bend straightening process, which is essential in the conventional process, is also unnecessary.

Here, the heating temperature of steel during hot rolling is
In order to prevent coarsening of the austenite crystal grains at the time of rolling heating, suppress the deformation resistance to a low level, and further to sufficiently dissolve precipitates such as carbonitrides in the matrix, 900 to 1250
° C. If the heating temperature is lower than 900 ° C., not only the deformation resistance becomes high, but also the solid solution of the precipitate in the matrix is insufficient, so that the properties of the final product deteriorate. On the other hand, if the heating temperature exceeds 1250 ° C., the austenite crystal grains become coarse and the workability deteriorates during cold drawing or forging. Therefore, the heating temperature range during hot rolling is set to 900 to 1250 ° C.

The hot rolling process itself is not particularly limited except the finishing temperature. Therefore, for example, it may be performed under normal hot rolling conditions.

The finishing temperature for hot rolling must be 750 ° C. or higher. This is because if the finishing temperature is lower than 750 ° C., the desired structure described later will not be obtained even if the quenching treatment is subsequently performed, and thus the desired strength cannot be imparted to the final product.

After finishing the hot rolling at 750 ° C. or higher,
So-called "direct quenching" is performed. When this quenching is performed at a cooling rate of 2 ° C./sec or more, the area ratio in the microstructure is 50.
% Martensite and the balance bainite,
A uniform structure can be obtained in the longitudinal direction of rolling. With the above-mentioned structure, when this is tempered at 400 to 600 ° C, a tensile strength of about 900 to 1000 MPa can be obtained. Then, cold drawing and cold forging are performed to obtain the desired 1
It is possible to give a tensile strength of 000 to 1250 MPa. In order to make the quenching structure more uniform and to stabilize the strength of the product after cold forging, the cooling rate for quenching is preferably 10 ° C./sec or more. There is no particular limitation on the upper limit of the cooling rate in this quenching, and the cooling rate may be the upper limit from the viewpoint of equipment.

Quenching at the above cooling rate is at least 30
It may be performed up to 0 ° C. This means that if quenching at the above cooling rate is performed up to 300 ° C., in the steel having the chemical composition specified in the present invention, the desired quenching structure (structure having martensite of 50% or more in area ratio and the balance bainite) is obtained. Because it will be. Therefore, the cooling rate in the temperature range below 300 ° C. is not particularly limited.

Subsequent to quenching, in order to ensure a certain strength (tensile strength of approximately 900 to 1000 MPa) and ductility and workability that can withstand cold drawing and cold forging in the subsequent steps, 400 to 600 ° C. It is necessary to temper at. In tempering in the temperature range below 400 ° C, the elongation in the tensile test is less than 10%, so it is difficult to withstand cold drawing and cold forging in the subsequent steps, while in the temperature range above 600 ° C In tempering, the tensile strength is less than 900 MPa, and even if cold drawing with a working amount (area reduction rate) described below is performed, a product having the desired tensile strength of 1000 to 1250 MPa cannot be obtained after cold forging. Is. The tempering time is preferably 0.5 to 5 hours.

Tempering for less than 0.5 hours may cause "unevenness" in tempering, and tempering for more than 5 hours increases cost.

After tempering, cold drawing is performed to obtain a wire for cold forging. This cold drawing is a process for improving the dimensional accuracy, ensuring the tensile strength and enhancing the fatigue resistance of the final product, and may be carried out by an ordinary method.

That is, if cold drawing is performed with a reduction amount of 5 to 50%, it is approximately 980 to 1200 MPa.
A wire for cold forging having a tensile strength of 1000 is obtained, and if this is then cold forged, the desired 1000 to 1250MP is obtained.
A product having a tensile strength of a is obtained. In order not only to stably impart the desired strength of 1000 to 1250 MPa to the final product after cold forging but also to provide good ductility, it is preferable to set the above-mentioned surface reduction rate to 15 to 30%.

[0042]

【Example】

(Example 1) Steels having the chemical compositions shown in Table 1 were melted by a usual method.

In Table 1, steels B, C, F, G, J,
K, N and O are target steels of the present invention (hereinafter referred to as steels of the present invention), and steels A, D, E, H, I, L, M and P have a content specified by the present invention It is a comparative steel out of the range.

Then, these steels were formed into billets by a usual method, heated to 1150 ° C., hot-rolled by a usual method, finished at 800 ° C. and made into a wire having a diameter of 13 mm on a Stelmore conveyor. It was developed into a ring shape, and was forcibly air-cooled to 300 ° C. at a cooling rate of 5 ° C./second, quenched, and wound into a coil. Then, after tempering at 500 ° C. for 3 hours, cold drawing was performed by a usual method to obtain a diameter of 11.
A 95 mm wire (area reduction rate: 15.5%) was produced. After that, heat treatment (blueing treatment) was further performed at 190 ° C. for 4 hours.

Observation of the microstructure of a wire rod having a diameter of 13 mm as-quenched (with forced air cooling) revealed that all the steels except the comparative steels I and M had a mixed structure of martensite and bainite. It was confirmed that the area ratio of martensite exceeded 50%.

From the wire thus obtained, JIS 9B
JIS No. 1 rotary bending fatigue test piece having a diameter of 6 mm in parallel with the No. 10 tensile test piece was subjected to room temperature tensile test and room temperature air,
An Ono-type rotary bending fatigue test was conducted under the condition of 3600 rpm.

Table 2 shows the test results. From this Table 2, it is clear that the present invention provides a wire having an appropriate strength level, good ductility and high fatigue strength.

[0048]

[Table 1]

[0049]

[Table 2]

(Example 2) Using steel G which is the steel of the present invention shown in Table 1, a diameter of 13 mm was obtained under the same conditions as in Example 1.
After rolling and quenching, it is wound into a coil, 350-65
A wire having a diameter of 11.95 mm (area reduction ratio: 15.
5%). After that, heat treatment (blueing treatment) was further performed at 190 ° C. for 4 hours.

In the same manner as in Example 1, the wire thus obtained was subjected to a room temperature tensile test and a room temperature atmosphere of 3600 rp.
An Ono-type rotary bending fatigue test was conducted under the condition of m.

Table 3 shows the test results. It is clear from Table 3 that even with the steel of the present invention, a wire having an appropriate strength level, good ductility and large fatigue strength can be obtained only when tempered under the conditions specified in the present invention. is there.

[0053]

[Table 3]

(Example 3) Using Steel G which is the steel of the present invention shown in Table 1, a diameter of 13 mm was obtained under the same conditions as in Example 1.
Rolled into a coil, rolled up into a coil, and wound into a coil at 500 ° C for 3
Tempering was performed for a period of time, and cold drawing was performed by an ordinary method to prepare a wire having a diameter of 11.95 mm (area reduction rate: 15.5%). Then, M12 bolts were cold forged by a usual method, electrogalvanized, and heat-treated (bluing) at 190 ° C. for 4 hours. Using the bolts thus obtained, a substantial tensile test without a wedge and a substantial fatigue test were performed at room temperature. In the actual fatigue test, an electrohydraulic tension / compression fatigue tester is used to apply a unilateral stress at a frequency of 10 Hz, and the maximum stress when failure is not reached after 10 ⁷ repetitions is the actual “fatigue strength”. And

Table 4 shows the test results. For comparison, Table 4 shows steel G manufactured by the conventional conventional method of "hot rolling → spheroidizing annealing → cold drawing → cold forging → quenching → tempering → straightening → plating → baking treatment". The results of the actual tensile test at room temperature and the actual fatigue test performed using M12 bolts as the material are also shown.

According to Table 4, in the case of the bolt forged by cold forging using the wire according to the present invention, it is possible to obtain excellent strength and ductility by performing the above heat treatment (blueing) after plating. Balance and fatigue resistance are obtained,
It is clear that it is comparable to the characteristics of the bolt manufactured by the conventional method.

[0057]

[Table 4]

[0058]

EFFECTS OF THE INVENTION According to the present invention, processing steps such as a heat treatment step and a straightening step can be simplified or omitted. Therefore, the present invention is suitable as a material for a high-strength small-sized long component having a tensile strength of 1000 to 1250 MPa. The cold forging wire can be provided with high efficiency and at low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a comparison between a conventional method and a method of the present invention after hot rolling. (A) shows the conventional method and (B) shows the method of the present invention.

Claims (1)

[Claims] 1. C: 0.15 to 0.30% by weight, S
i: 0.05 to 0.30%, Mn: 1.0 to 2.0%,
Cr: 0.5-1.0%, Al: 0.005-0.10
%, Cu: 0 to 0.5%, Ni: 0 to 0.5%, Mo:
0 to 0.5%, V: 0 to 0.5%, Nb: 0 to 0.2
%, Ti: 0 to 0.1%, B: 0 to 0.01%, P:
0.03% or less, S: 0.03% or less, N: 0.02%
Hereinafter, the balance is steel having a chemical composition consisting of Fe and unavoidable impurities, heated to a temperature range of 900 to 1250 ° C., hot-rolled, finished at a temperature of 750 ° C. or higher, and then cooled at a rate of 2 ° C./sec or higher. A method for manufacturing a wire for cold forging, characterized in that the wire is tempered in a temperature range of 400 to 600 ° C., and then cold drawn.