JPH0565540A - Manufacture of high strength bolt - Google Patents

Abstract

PURPOSE:To manufacture a high strength bolt having excellent delayed fracture resistance and fatigue strength as well as excellent in the process of manufacturing and heat profitability with tempering treatment after hardening obviated. CONSTITUTION:Steel stock having a compsn. constituted of, by weight, one or two kinds among 0.04 to 0.15% C, 0.05 to 1.5% Si, 0.5 to 1.5% Mn, 0.1 to 1.5% Cr, 0.0003 to 0.005% B, 0.01 to 0.04% Ti and 0.01 to 0.06% sol.Al and the balance Fe with impurities is subjected to cold forming into the shape of a bolt. Then, this bolt is heated to the temp. range of the Ac3 point or above, is htereafter cooled at least to <=400 deg.C at the cooling rate of >=1 deg.C/sec and is hardened to form its structure into a one essentially consisting of martensite or bainite or a mixed one of martensite and bainite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bolt manufacturing method used for manufacturing bolts used as fastening parts for various mechanical structures, and more particularly to 100 Kgf / m.
The present invention relates to a method for producing a high-strength bolt having a large tensile strength exceeding m ² , excellent delayed fracture resistance, and good fatigue strength.

[0002]

^2. Description of the Related Art Conventionally, JIS standard S45 has been used to manufacture high strength bolts having a tensile strength of 90 kgf / mm ² or more.
Carbon steel such as C, or MnB steel (Fe-0.35% C
(-0.8% Mn-0.002% B) or the like, it is usually manufactured by forming into a bolt shape and then subjecting it to heat treatment such as quenching and tempering.

In the case of high-strength bolts having a tensile strength of 120 Kgf / mm ² or more, such as engine head bolts, it is necessary to lower the tempering temperature after quenching in order to obtain high strength with the above carbon steel and MnB steel. Since this is a generally known temperature range that causes temper brittleness, there are problems such as a decrease in toughness and an increase in delayed fracture susceptibility. Therefore, JIS standard SCM440, etc., which can take a high tempering temperature It is common practice to use alloy steel.

[0004]

However, when an alloy steel such as SCM440 is used in the manufacturing process of such a high-strength bolt, the alloy steel contains Mo, so that the cost is high and the steel cost is high. Since the hardness of the material is high and the deformation resistance is large, it is necessary to perform spheroidizing annealing for a long time before cold working, and in order to obtain high strength and high toughness, be sure to perform tempering treatment after quenching. Must
There have been various problems that increase the cost such as consuming a lot of energy.

Therefore, in order to reduce such problems, for example, in order to improve cold workability or omit tempering treatment after quenching and obtain high toughness,
Although it suffices to reduce the carbon content, in that case, there is a problem that the hardenability of the steel material remarkably deteriorates, and it has been a problem to solve these problems.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and is to reduce the composition of the steel material used for the production of bolts by adding B to supplement the hardenability. , While ensuring good cold workability of the steel material and omitting the tempering treatment after quenching,
An object of the present invention is to provide a method for manufacturing a high-strength bolt which has excellent delayed fracture resistance and fatigue strength, consumes less energy in the manufacturing process, and is economical.

[0007]

The method for producing a high strength bolt according to the present invention is, in weight%, C: 0.04 to 0.15.
%, Si: 0.05 to 1.5%, Mn: 0.5 to 1.5
%, Cr: 0.1 to 1.5%, B: 0.0003 to 0.
005%, Ti: 0.01 to 0.04% and Sol.
Al: one or two of 0.01 to 0.06%,
And cold working shaped bolt shape by using a steel material having a composition the balance being Fe and impurities, and then after heating to the austenite temperature region of more than _three points A _C, at least 400 ° in 1 ° C / sec or more cooling rate It is characterized in that cooling is performed to a temperature of C or lower and quenching is performed so that the main structure is martensite or bainite or a mixed structure of martensite and bainite and the tempering process after quenching is omitted. The structure of the invention relating to the method for manufacturing a high-strength bolt having excellent delayed fracture resistance and fatigue strength is used as means for solving the above-mentioned conventional problems.

Next, the reasons for limiting the content of alloying elements in the steel material used in the method for manufacturing a high-strength bolt according to the present invention will be described.

C: 0.04 to 0.15% C is an element necessary to secure the strength, and it is necessary to contain at least 0.04% in order to maintain sufficient strength especially in the as-quenched state. There is. But 0.15
If added in excess of%, cold workability is impaired and
In the as-quenched state, the strength becomes too high, the toughness decreases significantly, and the delayed fracture resistance deteriorates. Therefore, tempering is necessary to avoid these, so the C content should be 0. It was set to 0.04 to 0.15%.

Si: 0.05 to 1.5% Si is an element necessary as a deoxidizing agent for steel and must be contained at least 0.05%. However, if the Si content exceeds 1.5%, the toughness is significantly deteriorated and the delayed fracture susceptibility is increased, so the Si content is set to 0.05 to 1.5%.

Mn: 0.5-1.5% Mn is an element effective for improving hardenability, but 0.5%
If less than 1.5%, the effect is not sufficient, and if added in excess of 1.5%, cold workability is hindered, and further, grain boundary segregation of P is promoted during heat treatment to increase delayed fracture susceptibility,
The Mn content was 0.5 to 1.5%.

Cr: 0.1 to 1.5% Cr is an element useful for increasing the hardenability and strength of steel, but if it is less than 0.1%, its effect cannot be obtained.
On the other hand, if the content exceeds 1.5%, not only the toughness deteriorates and the susceptibility to quenching cracks increases, but also the economical efficiency is impaired. Therefore, the Cr content is set to 0.1 to 1.5%. ..

B: 0.0003 to 0.005% B has the effect of remarkably improving the hardenability of steel with the addition of a trace amount, but if it is less than 0.0003%, the effect of improving the hardenability is not sufficient, Even if added over 005%, not only the effect of improving the hardenability corresponding to the increase of the added amount cannot be expected, but also the problem of degrading the toughness occurs, so the B content is 0.0003 to 0.005. %.

Ti: 0.01 to 0.04% and So
l. Al: 0.01 to 0.06%, or 1 or 2
Species Ti, Sol. Al is an element effective in fixing N and O in the steel as a compound in order to effectively act the added B, but if the content is less than the lower limit of each, the effect is not sufficient, while the content is If the content is excessive, the cleanliness of the material is deteriorated.
0.04% and Sol. Al: 0.01 to 0.06%
One or two of them.

In addition, P segregates at grain boundaries during heat treatment,
It is an impurity element that reduces toughness and delayed fracture resistance, and is an element that impairs cold workability. From the viewpoint of delayed fracture resistance, it is desirable to limit the content to 0.01% or less.

Next, the reasons for limiting the manufacturing conditions will be described.

Conventionally, the tensile strength is 100 Kgf / mm ^2.
The above high-strength bolts are molded into a bolt shape and then
Heated above _C3 points had been Uyori produced by performing tempering in the following point _C1 A after the quenching.

However, such heat treatment for quenching and tempering promotes segregation of P in the steel to the former austenite grain boundaries during tempering, which causes deterioration of toughness and delayed fracture resistance. It was

Therefore, in the present invention, by cold working into a bolt shape and then completely austenitizing the structure of the bolt steel material, that is, after heating to a temperature range of _{AC 3} points or higher, quenching is performed. By allowing the desired strength and toughness to be obtained without tempering and omitting tempering, segregation of P at grain boundaries can be suppressed and delayed fracture resistance is improved. It was made.

The cooling rate during the quenching is 1 ° C.
/ Sec or more is required. This is because if the cooling rate is low, the structure becomes ferrite / pearlite or a mixed structure of bainite and ferrite / pearlite, and it becomes difficult to secure the desired strength in the as-quenched state.

Further, from the viewpoint of delayed fracture property, it is necessary to cool the temperature range of 400 to 600 ° C. where P segregates at the grain boundaries by quenching to prevent this from cooling to at least 400 ° C. or less.

[0022]

In the method for producing a high-strength bolt according to the present invention, the composition of the steel material used for producing the bolt is made to have a low carbon content, and B is added in order to compensate for the deterioration of the hardenability due to the low carbon content. After ensuring good cold workability of the steel material and cold forming into a bolt shape, it is heated to an austenite temperature range of AC ₃ points or higher, and at least 400 ° C or lower at a cooling rate of 1 ° C / sec or higher. Since the main structure is martensite or bainite or a mixed structure of martensite and bainite, it is excellent in delayed fracture resistance while omitting tempering after quenching. Also, a high-strength bolt having fatigue strength and low energy consumption in the manufacturing process and having good economical efficiency can be manufactured.

[0023]

EXAMPLES Next, examples of the present invention will be described together with comparative examples.

Comparative Examples 1 and 3 in which bolts were manufactured by a conventional method using SAE standard 10B35, which is MnB steel, and JIS standard SCM440, which is alloy steel.
The results of comparison tests of Comparative Examples 2 and 4 in which bolts are manufactured by the conventional method using the above as a raw material and Examples 1 and 2 in which bolts are manufactured by the method of the present invention will be described.

First, after drawing each steel material having the chemical composition shown in Table 1, a bolt size is measured by a bolt header machine:
M10, under-neck length: 70 mm, a flanged hexagonal bolt was cold-worked and formed, and then a thread-rolled bolt having a pitch of 1.25 and a thread length of 30 mm was subjected to an experiment.

The heat treatment is also performed under the conditions shown in Table 1,
Comparative Examples 1 and 2 and Inventive Example 3 and Comparative Examples 3 and 4 and Inventive Example 2 have the same strength level, that is, tensile strengths of 125 Kgf / mm ² and 140 Kgf, respectively.
/ Mm < ² >.

Table 2 shows the results of the delayed fracture evaluation test by the acid immersion method.

In this delayed fracture resistance evaluation test, the bolt was attached to a jig, stress was applied to the yield point of the bolt by the nut rotation angle method, and then the specimen was immersed in 0.1N HC1 for 2 minutes and washed with water. A method was used in which the dried product was left at room temperature and evaluated for damage after 48 hours.

[0029]

[Table 1]

[0030]

[Table 2]

From the results shown in Tables 1 and 2, the bolts according to the present invention can be obtained by omitting the tempering treatment after quenching.
Tensile strength 125 Kgf / mm ² , 140 Kgf / mm ²
-Grade high strength is obtained, and segregation to the former austenite grain boundaries during P tempering is prevented by omitting the tempering treatment, and delayed fracture susceptibility is improved by lowering C and optimizing the amounts of Si, Mn, and P. Since it can be reduced, it can be seen that compared with the comparative example using the conventional steel, it has extremely good delayed fracture resistance at any strength level.

Next, using the bolts of Comparative Examples 3 and 4 and the example of the present invention described above, a fatigue test was carried out by repeatedly applying tension-tension load with the bolt alone, and the results are shown in FIG.

As is apparent from FIG. 1, when compared at the same tensile strength level, the bolt according to Example 2 of the present invention has superior fatigue strength to the bolts according to Comparative Examples 3 and 4.

In this experiment, the damaged portion is the screw portion, and the root portion of the screw is used as the starting point of fatigue, but the bolt according to the present invention has a significantly reduced sensitivity to notches against repeated input due to the low C. Therefore, it can be seen that the stress concentration portion is effective for the member that causes the fatigue, such as the screw bottom portion of the bolt.

Next, in order to determine the lower limit of the cooling rate at the time of quenching, the bolts using the steel materials shown in Example 1 of the present invention in Table 1 were used as specimens, and various bolts were selected from the austenite temperature range of AC ₃ points or higher. The result of measuring the hardness of the product cooled at a speed is shown in FIG.

As is apparent from FIG. 2, the cooling rate is 1 °.
An inflection point of the relationship between hardness and cooling rate is recognized near C / sec. This is because the cooling rate is almost 1 ° C / sec.
This is because the structure becomes a structure in which ferrite + pearlite is mixed from the bainite simple substance at a later time than that, and an inflection point was recognized in the tensile strength as well as in the decrease in hardness.

Therefore, if the cooling rate is 1 ° C./sec or more, the structure can be martensite or bainite excellent in delayed fracture resistance and fatigue strength or a mixed structure thereof.

[0038]

According to the method for producing a high strength bolt according to the present invention, the composition of the steel member used for producing the bolt does not include expensive Mo and the carbon content is lowered to improve the cold workability. , B are added to ensure hardenability, and C
By selecting steel with optimized amounts of r, Mn, Si, Ti, Al, etc., we ensure excellent cold workability of the steel material when manufacturing bolts and omit tempering treatment after quenching. Therefore, it brings great economic effects such as material cost reduction, process reduction, energy consumption reduction, equipment investment reduction, and also has a quality with excellent delayed fracture resistance and fatigue strength. The great effect that an excellent high-strength bolt can be obtained is brought about.

[Brief description of drawings]

FIG. 1 is a graph showing the results of fatigue tests performed on bolts of Example 2 of the present invention, Comparative Example 3, and Comparative Example 4.

FIG. 2 is a graph showing the results of examining the effect of the cooling rate from the austenite temperature range of AC ₃ points or higher on hardness.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/00 301 A 7217-4K 38/32

Claims (2)

[Claims] 1. C: 0.04 to 0.15% by weight,
Si: 0.05-1.5%, Mn: 0.5-1.5%,
Cr: 0.1-1.5%, B: 0.0003-0.00
5%, Ti: 0.01 to 0.04% and Sol. A
1: Cold-worked into a bolt shape using a steel material having a composition of one or two of 0.01 to 0.06%, the balance Fe and impurities, and then in a temperature range of _{AC 3} points or higher. After heating, quenching is performed by cooling to a temperature of at least 400 ° C or less at a cooling rate of 1 ° C / sec or more,
A method for producing a high-strength bolt excellent in delayed fracture resistance and fatigue strength, characterized in that the main structure is martensite or bainite or a mixed structure of martensite and bainite. 2. The method for producing a high-strength bolt according to claim 1, wherein a steel material containing P: 0.01% or less in impurities is used.