JPH051321A - Production of high strength bolt excellent in delayed fracture resistance and fatigue characteristic - Google Patents

Abstract

PURPOSE:To improve delayed fracture resistance and fatigue strength by using a boron compound as a lubricant at the time of wiredrawing and also adopting atmospheric conditions of slight-degree decarburizing-extremely-slight-degree carburizing at the time of heat treatment after working. CONSTITUTION:A lubricating film composed essentially of boron compound is formed on the surface of a wire rod of steel for bolt containing 0.20-0.45% carbon, which is wiredrawn, formed into a bolt, and subjected to quench-and- temper treatment. By controlling carbon potential in a hardening atmosphere, carbon concentration in the vicinity of the surface of the bolt after heat treatment is controlled to a fluctuations in the range of -0.15 to +0.02% based on the carbon content in the steel, by which tensile strength can be regulated to 1200-1600N/mm<2>. By this method, a risk of phosphorizing due to conventional phosphate film can be removed, and an effect of improvement in the intergranular binding power of boron compound and also a hardening- supplementing effect can be used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a bolt having excellent delayed fracture resistance and fatigue characteristics.

[0002]

2. Description of the Related Art Due to the trend toward smaller size, lighter weight and higher output of automobiles and various industrial machines, the bolts for fastening parts are also subject to a large load and need to be strengthened. Therefore, as a material for bolts, SCM43, which generally belongs to JIS tough steel category, is generally used.
5, SCM440, SCR440, etc. are used.

However, in these steel bolts,
If the tensile strength is increased to 1200 N / mm ² or more, there is a risk of delayed fracture, so high-strength bolts cannot be used in important parts for safety.

In the production of wire rods, which are the materials for bolts, in many cases, wire drawing using a phosphate coating as a lubricant is carried out. If the steel with the phosphate adhered is kept at the quenching temperature, the phosphorus invasion phenomenon that P invades from the surface of the steel occurs, and the invading P weakens the binding force of the grain boundary near the surface of the bolt, so during use There is a high possibility that delayed destruction will occur. Therefore, the phosphate coating must be removed before quenching.

On the other hand, attempts have been made to reduce the susceptibility to delayed fracture by decarburizing the surface of the bolt to soften it. However, if decarburization is extremely slight, delayed fracture due to the above phosphorus cannot be prevented, and if it becomes excessive, fatigue cracks easily occur in the softened area of the surface and it propagates, resulting in reduced fatigue strength. Another problem of doing it comes out.

In high-strength bolts, carburizing should be avoided. It is common knowledge that even bolts that have not been subjected to phosphorus infiltration are inferior in delayed fracture resistance due to relatively light carburization.

[0007] The applicant has proposed, as one measure for improving the delayed fracture resistance of bolts, to select a specific alloy component and combine it with a high temperature tempering of 500 ° C or more (Japanese Patent Laid-Open No. Sho 6-96). 61-130456).

As a result of subsequent research, it was found that the selection of the lubricating coating during wire drawing and the atmosphere during quenching are important for improving delayed fracture resistance. Further, it has been found that it is advantageous to form the threaded portion, which is performed by the rolling technique in many cases, separately in the roughing process before the heat treatment and the finishing process after the heat treatment.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a high strength bolt, which makes use of the above-mentioned findings and in principle eliminates the risk of phosphorus invasion and improves delayed fracture resistance and fatigue strength. To provide.

[0010]

A method of manufacturing a high strength bolt having excellent delayed fracture resistance and fatigue strength according to the present invention is 0.2
After forming a lubricating coating containing a boron compound as the main component on the surface of a steel wire for bolts containing 0 to 0.45% carbon, wire drawing is performed, and then the bolts are directly processed into quenching.
When tempering is applied, the carbon concentration in the vicinity of the bolt surface after heat treatment varies with the carbon content in the steel.
The heat treatment is performed by controlling the atmosphere so that the tensile strength is 12 to 12 in the range of 0.15 to + 0.02%.
It consists of adjusting to 0 to 1600 N / mm ² .

The thread portion of this type of bolt is usually formed by a rolling technique, and the rolling can be advantageously performed in the present invention. If it is insufficient to obtain the desired threaded portion only by rolling before quenching, or if it is desired to form a particularly precise threaded portion, rolling for finishing may be performed after quenching and tempering.

The boron compound used as the lubricating coating is
Boric acid, sodium borate, potassium borate or the like may be used. Borax, that is, sodium borate decahydrate, is preferable in that it can be obtained at a low cost and vitrifies at a processing temperature to exhibit good lubricating performance.

In order to keep the carbon concentration near the surface of the bolt after heat treatment within a range of −0.15 to + 0.02% with respect to the carbon content in steel, the carbon potential of the quenching atmosphere may be controlled. .

Steel for bolts contains C: 0.20 to 0.45%
Various steel types can be used as long as they contain C, but a typical one is a steel having a C content in the above range among SCM and SCR.

[0015]

As described above, the use of phosphate as a lubricant during wire drawing is not preferable in principle because of the risk of phosphorus penetration. Therefore, in the present invention, a boron compound is used instead. . Lubrication by a boron compound not only eliminates phosphorus, but also leaves a lubricating coating during molding and heat treatment after wire drawing, so that a trace amount of B penetrates into the bolt surface layer to improve the bond strength of grain boundaries. It has the function of increasing the hardenability of the surface layer as well as increasing the hardness. It is considered that the mechanism by which the invaded B enhances the bond strength of the grain boundary is to suppress the precipitation of impurities such as P remaining in the steel.

When the bolt is heat treated and exposed to an oxidizing atmosphere, Si, Mn, Cr,
Alloying elements such as Al are oxidized. Since these oxides mainly form at grain boundaries, they provide a starting point for cracks due to fatigue, and there is a risk of promoting initial propagation of cracks. It is considered that B segregates at the grain boundaries to suppress the precipitation of these oxides at the grain boundaries. Oxidation of alloying elements reduces the amount of alloying components in the matrix, and as a result, the hardenability deteriorates and an incompletely hardened structure is formed, so that the bolt surface layer becomes soft. The invasion of B complements this decrease in hardenability.

As described above, the carbon concentration in the surface layer of the bolt after quenching varies by -0.15% with respect to the carbon content of steel.
The meaning of staying in the range of + 0.02% means not to deviate from the region from mild decarburization to extremely mild carburization. If decarburization stronger than this limit is carried out, the hardenability deteriorates remarkably, and it becomes impossible to deal with it by supplementing the hardenability due to the penetration of B as described above. On the other hand, carburization that exceeds the above-mentioned limit has no effect on the improvement of the grain boundary bonding force by the invading B. Such control of decarburization to carburization can be performed by selecting the carbon potential of the atmosphere according to the carbon content of the steel for bolts used as a material, and may be performed according to what is known in the art of gas carburization.

The strength after heat treatment is adjusted to 1200 N / mm ² or more because this is the level required for high strength bolts, and the upper limit of 1600 N / mm ² is that when strength is higher than this. This is because it is difficult to improve the delayed fracture resistance only by controlling the surface properties of the bolt.

[0019]

Example A steel wire rod having the alloy composition shown in Table 1 (weight%, balance impurities and Fe) was prepared. Some of them are formed into bolts by cold forging and rolling after forming a lubricating coating, and another is formed into a lubricating coating after machining into tensile test pieces and delayed fracture test pieces, Both were heat-treated.

Table 1 Steel C Si Mn Cr Other A 0.260 0.31 0.81 1.12 Mo0.17 B 0.421 0.30 0.77 1.16-C 0.332 0.05 0. 34 1.02 Mo0.61, V0.14 D 0.379 0.11 0.29 1.25 Mo0.31, V0.31 Nb0.06 lubricating coating, sodium borate _{Na 2 B 4 O 7 · 5H} 2 It was formed by immersing in an aqueous solution of O at a concentration of 200 g / l at a temperature of 90 ° C. for 10 minutes, pulling it up, and air-drying (this is represented by “B system”). For comparison, a product having a phosphate-based lubricating coating formed thereon was also prepared (this is referred to as "P-based").

The above test pieces and bolts were heat-treated at various quenching and tempering temperatures and under an atmosphere ranging from decarburization to carburization. Table 2 shows the heat treatment conditions, the carbon concentration in the surface layer of the test piece (delayed fracture test piece) after the heat treatment, and how much it changed with respect to the carbon concentration in the steel.

Table 2 No. Steel film Quenching temperature Tempering temperature Surface layer C% difference (℃) (℃) C (%) Example 1 AB series 870 450 0.210 -0.050 〃 2 B 〃 850 490 0.273 -0.148 〃 3 C 〃 875 600 0.294 -0.038 〃 4 D 〃 930 580 0.255 -0.077 5 C 890 570 370 0.391 +0.012 6 D 920 620 620 0.364 -0.015 Comparative Example 1 AP type 865 430 0.250 -0.010 〃 2 B 〃 855 470 0.250-0.171 〃 3 C 〃 925 610 0.339 +0.007 〃 4 DB system 950 560 0.412 +0.033 〃 5 B 〃 870 480 0.200 -0.221 〃 6 C P system 880 520 0.299 -0.033 The following tests were performed on each of the above test pieces or bolt products.

(Delayed Fracture Test) Diameter 6 mm × Length 40 mm
V-shaped notch with an angle of 60 ° at the center of the round bar
The test piece was provided with m and a radius of 0.1 mm at the bottom. A bending stress was applied to the notch while dropping a corrosive liquid (0.1N-HCl). For some specimens, 0.1 mmR tool steel (HRC6
0) was pressed at a pressure of 200 kgf / mm ² to simulate rolling processing conditions.

Delayed fracture resistance is defined as the stress that breaks after 30 hours when a bending stress is applied under the above conditions (referred to as "30-hour strength"), when a bending stress is applied without dropping a corrosive liquid. The ratio of the stress (referred to as "bending stress") was taken and evaluated as the delayed fracture strength ratio. (The closer the value is to 1, the better the result.) Delayed fracture strength ratio = 30-hour strength / Bending stress (fatigue test) An average stress of 790 N / mm ² was applied to the above bolts using a physical fatigue tester. The stress amplitude that does not break after 3 × 10 ⁶ cycles was used as a measure of fatigue strength.

The results of the above two types of tests are shown in Table 3.

Table 3 No. Rolling simulation Tensile strength Slow fracture Fatigue strength Configuration (N / mm ²⁾ intensity ratio (N / mm ²⁾ Example 1 None 1200 0.80 96 〃 2 〃 1280 0.81 88 〃 3 〃 1250 0.88 103 〃 4 there 1540 0.78 125 〃 5 〃 1370 0.75 116 〃 6 None 1410 0.84 124 Comparative Example 1 〃 1240 0.59 50 〃 2 〃 1310 0.86 34 34 〃 3 〃 1230 0.38 57 〃 4 〃 5 1 80 ッ 0.31 1300 0.89 41 〃 6 〃 1320 0.62 64

[0027]

EFFECT OF THE INVENTION In the method for producing a high strength bolt of the present invention, since a boron compound is used as a lubricant during wire drawing in place of the conventional phosphate, the risk of phosphorus penetration is eliminated in principle. , B to improve the grain boundary bond strength and complement the hardenability, and by adopting the atmosphere conditions of light decarburization to ultra-light carburization during the heat treatment, the delayed fracture resistance is improved and the fatigue property is improved. Succeeded in improving.

Therefore, this manufacturing method is effective when it is used to manufacture bolts from various tough steels.

Claims (5)

[Claims] 1. A lubricating coating containing a boron compound as a main component is formed on the surface of a steel wire for bolts containing 0.20 to 0.45% carbon, drawn and then formed into bolts as it is. Quenching and tempering treatment, while controlling the atmosphere so that the fluctuation of the carbon concentration in the vicinity of the bolt surface after heat treatment with respect to the carbon content in the steel remains within the range of -0.15 to + 0.02%. A method for producing a high-strength bolt having excellent delayed fracture resistance and fatigue properties, which comprises performing heat treatment to adjust the tensile strength to 1200 to 1600 N / mm ² . 2. The manufacturing method according to claim 1, wherein quenching
A method for manufacturing high-strength steel for bolts, which includes a step of finishing the threaded portion after tempering. 3. The method according to claim 1, wherein the forming process and / or the finishing process of the thread portion of the bolt are performed by rolling.
Manufacturing method of high strength bolts. 4. The method for producing a high strength bolt according to claim 1, wherein a boron compound selected from boric acid, sodium borate and potassium borate is used. 5. The method for producing a high-strength bolt according to claim 1, wherein the bolt steel is SCM or SCR.