JP3048238B2 - How to improve delayed fracture resistance of high tension bolts - Google Patents

Description

The present invention relates to a method for manufacturing a high-strength member such as a bolt for an automobile or a high-strength bolt for a long-sized bridge, and more particularly, to a method for forming a high-strength member on a surface of the high-strength member. The present invention relates to a heat treatment method for improving the delayed fracture resistance of a high-strength bolt by preventing phosphorus immersion during quenching by a phosphate film.

[Background Art] There are many bolts that require high strength, such as various types of tightening bolts, cylinder head bolts, connecting rod bolts, and the like, as automotive bolts. In manufacturing these bolts, after rolling and annealing the rolled material, lubrication is applied, then subjected to wire drawing and cold / warm forging processes to form bolts, and finally quenching and tempering. It is common to do

By the way, in high-strength parts represented by the above-mentioned bolts, it is known that when the tensile strength exceeds 120 kgf / mm ² , the delayed fracture resistance rapidly deteriorates. Research has been done. For example, in JP-B-51-21374, JP-A-51-108621, JP-A-60-96714, etc., attention is paid to the fact that the starting point of delayed fracture is at or near the bolt surface, and the delayed fracture sensitivity on the bolt surface is reduced. For this purpose, a method of softening the surface by means such as applying high-frequency heating after quenching is disclosed. However, according to the study of the present inventors, it has been found that the surface hardness is reduced when the method is followed, which may lead to deterioration of fatigue characteristics.

It is thought that the main cause of the delayed fracture is embrittlement due to the action of hydrogen, and countermeasures for hydrogen embrittlement-induced fracture have been studied. However, as a result of re-examination of the effects of component elements on delayed fracture, the inventors of the present invention have found that phosphorus plays an important role, and have found that a lubricant that does not contain phosphorus is used as a lubricant during cold forging. A manufacturing method for improving the delayed fracture resistance by using the same was completed, and the application was filed earlier (Japanese Patent Application No. 1-69031).
That is, the phosphorus content is sufficiently low at the current level of refinement technology, and it is considered that the effect on delayed fracture is small. However, in the production of high-strength components, a phosphate film forming agent is used for the purpose of lubrication during cold forging, and the phosphorus contained in this lubricant remains in the quenching / tempering process while adhering to the material. It has been confirmed by the present inventors that they are brought in and mainly penetrate into the grain boundary portion of the material surface layer (hereinafter referred to as phosphorus immersion) to promote delayed fracture.

As another means of preventing phosphorus immersion, there is a method described in a paper published by the Society of Materials Science of Japan (Summary of the 37th Annual Meeting of the Society, No. 217, p. 104). This prevents phosphorus immersion and improves delayed fracture resistance. However, this method is not preferable because hydrogen generated by the reaction between the acid and the steel material is absorbed into the steel and increases the susceptibility of the hydrogen embrittlement-induced fracture described above.

Furthermore, even in the above-mentioned method using a lubricant containing no phosphorus, the life of the cold forging tool is shortened and the frequency of tool change is increased due to lower lubricating ability as compared with the case where a phosphate film is formed. Have the cost problem of doing so.

[Problems to be Solved by the Invention] The present invention has been made in view of such circumstances, and prevents phosphorus immersion even when a phosphate film is formed by optimizing heat treatment conditions. It is an object of the present invention to provide a method for manufacturing a high-tensile bolt that can be used.

[Means for Solving the Problems] The present invention, which has achieved the above-mentioned object, refers to an intermediate heat treatment under a condition satisfying the following relational expression under an oxidizing atmosphere when heat-treating a bolt formed by cold-forming a phosphate-coated steel material. And then quenching and tempering.

600 ≦ T ≦ 750 (° C.) t ≧ 0.002T ² −3.2T + 1290 (min) (where T is the heating temperature t is the holding time at the temperature T) [Action] The present inventors have conducted intensive studies and found that a phosphate film was formed. Even in the case of using a treated steel material, it has been found that if the intermediate treatment is performed under a specific heat treatment condition, the phosphate film can be removed and the phosphorus impregnation can be prevented. The present invention has been completed by expressing the conditions by a relational expression between heating temperature and heating holding time.

That is, in the method of the present invention, a steel material subjected to a phosphate film treatment is formed into a bolt by cold forging, and then, in order to remove the phosphate film adhering to the surface of the bolt, in an oxidizing atmosphere, for 600 hours. T ≧ 0.002T at heating temperature up to 750 ℃
² Heat and hold for a time that satisfies the relationship of -3.2 + 1290.
As a combination satisfying the above relational expression, for example, a method of heating and holding at around 700 ° C. for about 30 minutes is generally used.

If the intermediate heat treatment is performed under the above conditions, the phosphate film on the surface of the steel material is oxidized, so that phosphorous impregnation in the subsequent quenching step can be prevented, and as a result, delayed fracture resistance is improved.

The reason for limiting the intermediate heat treatment heating temperature to the range of 600 ° C or more and 750 ° C or less is that at temperatures higher than 750 ° C, the diffusion reaction of phosphorus into steel becomes more active than the oxidation reaction of the film,
As a result, phosphorus immersion proceeds in the state of the intermediate heat treatment, which is not preferable. On the other hand, when the temperature is lower than 600 ° C., the phosphate film is stabilized due to the low temperature, and is not easily oxidized. Therefore, it takes a very long time for the intermediate heat treatment, so that it is difficult to put to practical use.

[Example] A steel material having the composition shown in Table 1 was hot-rolled into a wire having a diameter of 10.3 mm, subjected to spheroidizing annealing, and then pickled. Next, after treatment with various lubricating film forming agents, MIC bolts were formed by skin pass and cold forging to obtain test pieces.
The test piece was subjected to an intermediate heat treatment under the various conditions shown in Table 2, then held at 860 ° C for 40 minutes and oil-quenched, and then at 430 ° C for 90 minutes.
After heating and holding for 1 minute, water-cooled and tempered, tensile strength 130K
Tempered to gf / mm ² strength. When a delayed fracture test described below was performed on each of the obtained high-strength bolts, the results shown in Table 2 were obtained.

The delayed fracture test is performed by immersing the bolts in a 15% HCl aqueous solution for 30 minutes, washing them with water, and drying them.
The stress was measured by measuring a stress value that does not break even after the elapse of 0 hour, and this stress value was defined as a 100-hour delayed fracture strength.
The bolt surface was analyzed by EPMA to determine the phosphorus content.

Examples 1 to 5 according to the intermediate heat treatment conditions of the present invention
It has a 100-hour delayed fracture strength of 5 kgf / mm ² or more, and has excellent delayed fracture resistance.

In contrast, Comparative Examples 1 to 10 lack any of the conditions in the intermediate heat treatment of the method of the present invention as described below,
The 100-hour delayed fracture strength is as low as 140 kgf / mm ² or less.

Comparative Examples 1 to 3 are examples where the holding time is too short, Comparative Example 4 is an example where the heating temperature is too low, and Comparative Examples 5 and 6 are examples where the heating temperature is too high.

Comparative Example 7 is an example in which intermediate heat treatment was performed in an RX gas atmosphere, and Comparative Examples 8 and 9 did not perform intermediate heat treatment.
Comparative Example 10 is an example in which a heat treatment is performed in a vacuum.

Although most of the test pieces had a zinc phosphate film formed thereon, Example 5, Comparative Examples 9 and 10 each formed an iron phosphate as a lubricating film.

[Effects of the Invention] Since the present invention is configured as described above, it is possible to provide various steel members having high strength and excellent in delayed fracture resistance, and it is possible to further improve the bolts for automobiles and bolts for long bridges. Safety can be improved.

[Brief description of the drawings]

FIG. 1 is a graph showing a range of intermediate heat treatment conditions of the present invention.

Claims (1)

(57) [Claims] The present invention is characterized in that when heat treatment is performed on a bolt formed by cold-forming a steel material treated with a phosphate film, an intermediate heat treatment is performed in an oxidizing atmosphere under conditions satisfying the following relational expression, followed by quenching and tempering. A method for improving the delayed fracture resistance of tension bolts. 600 ≦ T ≦ 750 (℃) t ≧ 0.002T ² −3.2T + 1290 (min) (However, T is heating temperature t is holding time at temperature T)