JP6380910B2 - Steel for high-strength bolts and high-strength bolts - Google Patents

Description

  The present invention relates to steel for high-strength bolts and high-strength bolts. More specifically, the present invention relates to a steel for high-strength bolts excellent in delayed fracture resistance and a high-strength bolt made using the same.

As general bolt steel, alloy steel (for example, SCM435, SCM440, etc.) is used, but heat treatment such as quenching and tempering is required when used in applications requiring high strength. .
However, the high-strength bolt obtained by heat treatment has a problem that delayed fracture is likely to occur. And the higher the strength, the higher the sensitivity of delayed fracture.
For this reason, the bolts used for members such as automobiles are generally set to have a tensile strength of 1200 MPa or less.

  On the other hand, recent reciprocating engines for automobiles have high combustion pressure for high performance, so high axial force is required for bolts that fasten components such as connecting rod mechanisms and multi-link mechanisms. In order to reduce the weight and size of the engine, it is required to reduce the diameter of the bolt.

  On the other hand, a steel component is specified, and a high strength bolt excellent in delayed fracture resistance and having a tensile strength of 1200 MPa or more has been proposed (see Patent Document 1).

  This high-strength bolt excellent in delayed fracture resistance is obtained from high-strength steel having a tensile strength of 1200 MPa or more, and C: 0.3 to 0.5% (mass%: as far as the element amount is concerned, The same), Cr: 0.5 to 5.0%, Mn: 0.2 to 1.0%, and one selected from the group consisting of Mg, Ca, Sr, Ba, Li, Na and K The above is included so as to satisfy predetermined requirements.

JP 2006-219709 A

  However, the high-strength bolt described in Patent Document 1 has a problem that the tensile strength, which is one of the required performances when applied to a high-power engine that requires further higher output, is not sufficient. It was.

  The present invention has been made in view of such problems of the prior art. Another object of the present invention is to provide a steel for high-strength bolts having a required strength and excellent delayed fracture resistance, and a high-strength bolt formed using the same.

  The present inventor has intensively studied to achieve the above object. As a result, it has been found that the above object can be achieved by setting the average crystal grain size of the steel for high-strength bolts within a predetermined range, and the present invention has been completed.

That is, the steel for high-strength bolts of the present invention has an average grain size of all crystals including an austenite crystal and a ferrite crystal and / or a martensite crystal, which have a tensile strength of 1500 MPa or more and are observed in the metal structure observation. Ri is the average grain size of der below 10 [mu] m, carbon 0.50 wt% 0.65 wt% or less, silicon 1.5 wt% to 2.5 wt% or less, chromium 1.0 wt% or more 1.4% by mass or less, molybdenum by 1.6% to 1.8% by mass, niobium and titanium in total of 0.01% by mass to 0.02% by mass with the balance being iron and inevitable impurities It consists of

  The high-strength bolt of the present invention is made using the steel for high-strength bolts of the present invention.

According to the present invention, the average grain size of the high-strength bolt steel is such that the tensile strength is 1500 MPa or more and all crystals including austenite crystals and ferrite crystals and / or martensite crystals observed in the metal structure observation. Ri average crystal grain size der less 10μm is, carbon 0.50 wt% 0.65 wt% or less, silicon 1.5 wt% to 2.5 wt% or less, chromium 1.0 wt% 1.4% by mass or less, 1.6% by mass to 1.8% by mass of molybdenum, 0.01% by mass or more and 0.02% by mass or less of niobium and titanium in total, the balance being iron and inevitable The structure is made of impurities .
Therefore, it is possible to provide a high-strength bolt steel excellent in delayed fracture resistance and a high-strength bolt formed using the same.

FIG. 1 is a partial cross-sectional view showing an example of a reciprocating engine having a multi-link mechanism. FIG. 2 is a cross-sectional view showing an outline of an example of the lower link shown in FIG.

  Hereinafter, steel for high-strength bolts according to an embodiment of the present invention and high-strength bolts using the same will be described in detail.

(First embodiment)
First, the steel for high strength bolts according to the first embodiment of the present invention will be described in detail.
The steel for high-strength bolts of this embodiment has a tensile strength of 1500 MPa or more and an average crystal grain size of 10 μm or less.
By adopting such a configuration, a bolt having excellent delayed fracture resistance and high strength can be realized.

Here, “average crystal grain size” in the present invention means the average grain size of all crystals observed in the observation of the metal structure after heat treatment. For example, examples of the crystal observed in the metal structure observation include an austenite crystal, a ferrite crystal, and a martensite crystal.
The average crystal grain size can be measured and calculated by a conventionally known method from, for example, a metal structure photograph.

In the steel for high-strength bolts of the present embodiment, carbon is 0.50% by mass or more, silicon is 1.5% by mass or more, chromium is 1.0% by mass or more, molybdenum is 1.6% by mass or more, Niobium and titanium are preferably contained in a total amount of 0.1% by mass or more, and the balance is made of iron and inevitable impurities.
By adopting such a configuration, a bolt having excellent delayed fracture resistance and high strength can be realized more reliably.

(Carbon (C): 0.50 mass% or more)
When the carbon content is less than 0.50% by mass, sufficient temper softening resistance cannot be obtained, and high-temperature tempering described later cannot be performed, so that delayed fracture resistance is not excellent.
In addition, it is preferable that content of carbon is 0.65 mass% or less. Since the increase in the amount of cementite that accumulates hydrogen can be suppressed, delayed fracture resistance is excellent.

(Silicon (Si): 1.5% by mass or more)
When the silicon content is less than 1.5% by mass, sufficient temper softening resistance cannot be obtained, and high-temperature tempering described later cannot be performed, so that delayed fracture resistance is not excellent.
Further, by increasing the silicon content, the diffusion coefficient of hydrogen in the steel is reduced, and concentration of hydrogen causing delayed fracture can be suppressed.
In addition, it is preferable that content of silicon is 2.5 mass% or less. It is excellent in forgeability and easy to form a predetermined bolt.

(Chromium (Cr): 1.0% by mass or more)
When the chromium content is less than 1.0% by mass, sufficient temper softening resistance cannot be obtained, and high-temperature tempering described later cannot be performed, so that delayed fracture resistance is not excellent.
In addition, although it does not specifically limit, It is preferable that content of chromium is 1.4 mass% or less.

(Molybdenum (Mo): 1.6% by mass or more)
If the molybdenum content is less than 1.6% by mass, the amount of molybdenum-based carbide that renders hydrogen harmless will not be sufficient, so that the delayed fracture resistance will not be excellent.
In addition, although it does not specifically limit, It is preferable that content of molybdenum is 1.65 mass% or less.

(Total of niobium and titanium (Nb + Ti): 0.01% by mass or more)
When the total content of niobium and titanium is less than 0.01% by mass, the steel grains tend to be coarse. And when a crystal grain becomes coarse, since concentration of hydrogen causing delayed fracture is caused, delayed fracture resistance is not excellent.
In addition, although it does not specifically limit, It is preferable that the total content of niobium and titanium is 0.02 mass% or less.

(Second Embodiment)
Next, the high strength bolt according to the second embodiment of the present invention will be described in detail.
The high-strength bolt of this embodiment is formed using the steel for high-strength bolts according to the first embodiment described above.
By setting it as such a structure, it will be excellent in delayed fracture resistance and will have high intensity | strength.

Here, although not particularly limited, for example, the above-described high-strength bolt steel is first cold forged, then thread rolled, and quenched at 900 ° C. or higher. The above-described high-strength bolt can be obtained by performing a heat treatment that is tempered at a temperature higher than or equal to ° C. Even if the order of the screw rolling and the heat treatment (quenching and tempering) is changed, the above-described high-strength bolt can be obtained.
For example, when tempering at a high temperature such as 580 ° C. or higher, flake cementite on the grain boundary that causes embrittlement is spheroidized, and the grain boundary strength can be improved.

Moreover, in the high intensity | strength volt | bolt of this embodiment, it is preferable to have either one or both of an iron phosphate film and a chromium plating film on the surface.
By forming such a film on the surface, the delayed fracture resistance can be further improved.

  Here, an example of the high-strength bolt of the present embodiment will be described in more detail with reference to the drawings. In addition, the dimension ratio of drawing is exaggerated on account of description, and may differ from an actual ratio. FIG. 1 is a partial cross-sectional view showing an example of a reciprocating engine having a multi-link mechanism. FIG. 2 is a cross-sectional view schematically showing an example of the lower link shown in FIG.

  As shown in FIG. 1, the reciprocating engine 100 includes an upper link 103 connected to the piston pin 102 of the piston 101, a lower link 106 connecting the upper link 103 and the crank pin 105 of the crankshaft 104, and one end thereof. A control link 107 is supported on the engine 100 side so as to be swingable and the other end is connected to the lower link 106. The upper link 103 and the lower link 106 are rotatably connected to each other via an upper pin 108, and the control link 107 and the lower link 106 are rotatably connected to each other via a control pin 109.

  The lower link 106 receives the combustion pressure received by the piston 101 from the upper pin 108 via the upper link 103, and transmits the force to the crank pin 105 by the operation using the control pin 109 as a fulcrum. Accordingly, a large combustion pressure or inertial load received by the piston 101 is input to the lower link 106 from the upper pin bearing portion 108a via the piston pin 102, the upper link 103, and the upper pin 108. At the same time, loads are also generated in the crank pin bearing portion 105a and the control pin bearing portion 109a so as to balance this load. Therefore, the surface pressure of each bearing portion (108a, 105a, 109a) is higher than that of a reciprocating engine having a general single link mechanism.

  Therefore, the required strength for the lower link is increased. Further, from the viewpoint of improving fuel efficiency, it is desired to make the lower link compact and lightweight.

  As shown in FIG. 2, the lower link 106 usually has a structure in which the divided lower link parts 106 </ b> A and 106 </ b> B are fastened with the high-strength bolt 1. In order to obtain a lower link that satisfies the required performance as described above, a high-strength bolt excellent in delayed fracture resistance is required. The high-strength bolt 1 of the present embodiment is not particularly limited, but is particularly suitable for fastening such a lower link component. In addition, 108a, 105a, 109a is each bearing part.

  Hereinafter, the present invention will be described in more detail with reference to examples.

(Example 1 to Example 5, Comparative Example 1 to Comparative Example 5)
The steel for high-strength bolts having the composition shown in Table 1 is subjected to cold forging, then thread rolling, and then heat-treated at 900 ° C. or higher and tempered at a predetermined tempering temperature. Thus, high strength bolts of each example were obtained. Here, the tempering temperature is 580 ° C. or higher for the examples in Table 1, but the comparative example in Table 1 is not necessarily 580 ° C. or higher because of the low temper softening resistance. The average crystal grain size in Table 1 is measured after heat treatment.

[Performance evaluation]
(Tensile strength evaluation)
The tensile strength of each example bolt was evaluated by a tensile test. The obtained results are shown in Table 1 as the results of Test 1. In the item of Test 1, “OK” indicates that the tensile strength of the bolt of each example is 1500 MPa or more.

(Evaluation of delayed fracture resistance)
The bolts of each example were soaked in hydrochloric acid having a specified concentration for a specified time, and it was observed whether or not the bolts were broken. The obtained results are shown in Table 1 as the results of Test 2. In the item of Test 2, “OK” indicates that the bolt was not damaged, and “NG” indicates that the bolt was damaged.

  Table 1 shows that Examples 1 to 5 belonging to the scope of the present invention are high strength bolts having a tensile strength of 1500 MPa or more and excellent delayed fracture resistance. On the other hand, it can be seen that Comparative Examples 1 to 5 outside the present invention are not excellent in delayed fracture resistance.

  As mentioned above, although this invention was demonstrated with some embodiment and an Example, this invention is not limited to these, A various deformation | transformation is possible within the range of the summary of this invention.

  For example, the configuration described in each of the above-described embodiments and examples is not limited to each embodiment or each example. For example, the composition of each embodiment, detailed conditions for manufacturing, the film The presence / absence can be changed, or the configuration of each embodiment or each example can be changed to a combination other than each embodiment or each example described above.

  Further, for example, in the above-described embodiment, the lower link in the engine having the multi-link mechanism is exemplified as the application site of the high-strength bolt. Needless to say.

1 High Strength Bolt 100 Reciprocating Engine 101 Piston 102 Piston Pin 103 Upper Link 104 Crankshaft 105 Crankpin 105a Crankpin Bearing 106 Lower Link 106A, 106B Lower Link Component 107 Control Link 108 Upper Pin 108a Upper Pin Bearing 109 Control Pin 109a Control Pin Bearing part

Claims (4)

Tensile strength of not less than 1500 MPa, and Ri average crystal grain size der less 10μm an average particle size of the total crystals and a is the austenite crystal and ferrite crystals and / or martensite crystals observed in the metal structure observation,
0.50% to 0.65% by mass of carbon, 1.5% to 2.5% by mass of silicon, 1.0% to 1.4% by mass of chromium, and 1.6% to molybdenum A high content characterized by containing at least 0.01 mass% and at most 1.8 mass%, niobium and titanium in a total content of 0.01 mass% to 0.02 mass%, with the balance being iron and inevitable impurities. Steel for strength bolts. The steel for high-strength bolts according to claim 1, characterized in that it contains 1.9% by mass or less of silicon and 1.2% by mass or less of chromium.   A high-strength bolt comprising the high-strength bolt steel according to claim 1.   The high-strength bolt according to claim 3, wherein the surface has an iron phosphate film and / or a chromium plating film.

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