JPH08225845A - Production of high strength bolt excellent in delayed fracture resistance - Google Patents

Abstract

PURPOSE: To produce a bolt excellent in tensile strength and delayed fracture resistance. CONSTITUTION: A bolt worked by using a steel having a compsn. contg., by weight, 0.35 to 0.50% C, <=0.15% Si, <=0.4% Mn, <=0.'015% P, <=0.010% S, 0.8 to 2.4% Ni, 0.8 to 1.5% Cr, 0.8 to 1.4$ Mo, 0.15 to 0.40% V, 0.01 to 0.06%. Al and 0.005 to 0.03% N and contg. <=0.05% Nb if necessary, and the balance substantially Fe with inevitable impurities is hardened from >=175 deg.K and is tempered at >=850 deg.K. Thus, the bolt having >=1500MPa tensile strength and excellent in delayed fracture resistance can be produced.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention has a tensile strength of 1500MP.
The present invention relates to a method for manufacturing a bolt having a delayed fracture resistance of a or higher.

[0002]

2. Description of the Related Art Steel bolts have a tensile strength of 1200 MPa.
It is known that if it exceeds the range, the susceptibility to delayed fracture becomes high, and sudden fracture occurs during use. For this reason, bolts that have been subjected to delayed fracture measures such as reducing impurities in general-purpose toughness steel, for example, JIS-SCM440 steel, or lightly decarburizing the surface of the bolt during heat treatment, are used with 1200 MPa class strength. Has been done. In addition, bolts whose alloy elements and addition amounts are regulated have been put to practical use with a strength of 1400 MPa class.

By the way, in order to reduce the size and weight of structural parts such as machines, automobiles, bridges and buildings, it is necessary to reduce the diameter of bolts used in these industries. Therefore, it is desired to further increase the strength of the bolt so that the fastening force can be maintained even if the diameter is reduced.

Conventionally, the tensile strength is 1400 to 1550M.
A technique has been proposed for increasing the delayed fracture resistance of Pa bolt steel to the same level as that of 1200 MPa class tough steel. However, with these techniques, the tensile strength can be reduced to 1
It is difficult to maintain the delayed fracture resistance of bolts set to 500 MPa to 1700 MPa at the same level as that of 1200 MPa level tough steel.

[0005]

SUMMARY OF THE INVENTION The present invention has a tensile strength of 1
It is an object of the present invention to provide a bolt having a delayed fracture resistance of 500 MPa or more and excellent in resistance to delayed fracture.

[0006]

The present inventors investigated the relationship between delayed fracture resistance and alloying elements and heat treatment conditions,
Medium carbon steel containing Ni, Cr, Mo, V is 1175
It was found that when high temperature tempering at 850 ° K or higher is performed after high temperature quenching at or above ° K, delayed fracture resistance equivalent to that of 1200 MPa level tough steel is exhibited. The present invention
The effect of the content of alloying elements was investigated in detail, and it was clarified that the materials in the following composition ranges were suitable as the steel for bolts.

% By weight, C: 0.35 to 0.50%, S
i: 0.15% or less, Mn: 0.4% or less, P: 0.0
15% or less, S: 0.010% or less, Ni: 0.8 to
2.4%, Cr: 0.8 to 1.5%, Mo: 0.8 to
1.4%, V: 0.15 to 0.40%, Al: 0.01
.About.0.06%, N: 0.005 to 0.03% and, if necessary, Nb: 0.05% or less, with the balance being substantially Fe.
And steel consisting of inevitable impurities.

[0008]

The composition of the high strength bolt steel of the present invention and the reasons for limiting the heat treatment will be described below. C: C is an element effective for obtaining a required strength by heat treatment, and in order to obtain such an effect, it is necessary to contain C by 0.35% or more. However, if the content exceeds 0.5%, the delayed fracture resistance deteriorates, so that
It should be 50% or less. Further, the hardness of the material before bolt forming may be increased, the mold life during bolt forming may be shortened, and the cost may be increased, so 0.50% or less is preferable.

Si: Si is an element that promotes grain boundary oxidation due to high temperature heating during austenitization, and can be a starting point of delayed fracture, and therefore deteriorates delayed fracture resistance. Further, it has an action of promoting segregation of impurity elements such as P and S to the grain boundaries, which also causes deterioration of delayed fracture resistance. Further, the hardness of the material before bolt forming may increase. From these things, it is desirable that the amount of Si is low, and it is limited to 0.15% or less.

Mn: Mn is an element that is effective as a deoxidizer during melting and contributes to the improvement of hardenability.
n promotes grain boundary oxidation during quenching together with Si and deteriorates delayed fracture resistance. Further, similar to Si, it has the effect of promoting segregation of impurity elements such as P and S to the grain boundaries, which also causes a deterioration in delayed fracture resistance. Further, the hardness of the material before bolt forming may increase. For these reasons, the Mn content is preferably low, and is limited to 0.40% or less.

P: P segregates at the austenite grain boundaries due to high-temperature heating for austenitization, embrittles the grain boundaries and deteriorates delayed fracture resistance, and is therefore limited to 0.015% or less.

S: S, like P, causes segregation at austenite grain boundaries due to high temperature heating during austenitization, embrittles the grain boundaries, and deteriorates delayed fracture resistance.
Since MnS is formed to deteriorate the delayed fracture resistance,
It is limited to 010% or less.

Ni: Ni is an element that contributes to the improvement of the hardenability, so it is preferable to adjust the addition amount according to the size of the high-strength component, etc., thereby ensuring the hardenability of the bolt. Further, in order to maintain the delayed fracture resistance at a strength level of 1500 MPa or more, it is necessary to enhance not only the prior austenite grain boundaries but also the toughness of the matrix. Ni is effective in improving the toughness of the matrix. Therefore, the Ni content is set to 0.
8% or more. However, excessive addition may increase the hardness of the material before bolt molding, shorten the mold life during bolt molding, and increase the cost. Therefore, 2.
It is better to be 4% or less.

Cr: Cr, like Ni, is an element that contributes to the improvement of hardenability. Therefore, the addition amount should be adjusted according to the size of the high-strength parts. Secure. Therefore, the content of Cr is 0.8
% And above. However, excessive addition promotes intergranular oxidation and deteriorates delayed fracture resistance similarly to Si and Mn, so it is necessary to set an upper limit. In addition, the hardness of the material before bolt forming may increase, so the content is preferably 1.5% or less.

Mo: Mo is an element that contributes not only to improving the hardenability but also to refining the crystal grains and improving the strength of the austenite grain boundaries, and further, it is possible to obtain a sufficient secondary hardening during tempering. In order to make it possible and the tempering temperature for obtaining the tensile strength of 1500 MPa or more, the tempering temperature is set to 0.80% or more in order to exceed 850 ° K. However, when added in a large amount, a huge primary carbide crystallizes out and remains during quenching, resulting in deterioration of not only toughness but also delayed fracture resistance, so the upper limit is made 1.4% or less.

V: V is 0.15% or more because it is an element effective for making it possible to obtain sufficient secondary hardening during high temperature tempering. However, when added in a large amount, huge primary carbides crystallize out and remain during quenching, causing not only toughness but also delayed fracture resistance, so the upper limit is made 0.4% or less.

Al is an element effective in forming AlN together with N to refine the crystal grains and improve the toughness.
In order to obtain such an effect, it is set to 0.01% or more. However, if too much, large inclusions that will cause ground marks are generated, and
Since 1 ₂ O ₃ is the starting point of fatigue, the content was made 0.06% or less.

N is an element effective in forming AlN together with Al to make the crystal grains finer and improve the toughness, so the content is made 0.005% or more to obtain such an effect. The amount of N added is about 1/2 of the amount of Al added.
However, if too much, large inclusions that will cause ground marks are generated, so the content was made 0.03% or less.

Nb: Nb forms fine carbonitrides, has an effect of refining the crystal grains, and is effective in strengthening the toughness of the matrix, so it is added if necessary. However, even if added over 0.05%, the effect is saturated, so 0.05% or less is preferable.

Quenching temperature: The quenching temperature is 117 in order to solidify the carbide before quenching and obtain a sufficient secondary hardening.
It is desirable to set it to 5 ° K or higher. If tempered at a temperature lower than this, 1500 MPa due to tempering
The above tensile strength cannot be obtained. Tempering temperature: The tempering temperature is set to 850 ° K or higher in order to effectively utilize the secondary hardening of Mo and V carbides. When tempering is performed at a temperature lower than this, not only the target strength cannot be obtained, but also the delayed fracture resistance deteriorates significantly.

[0021]

EXAMPLE Steel A according to the present invention having the chemical composition shown in Table 1
-F and Comparative Examples GM were melted and then ingoted, and each steel was rolled into a wire rod having a diameter of 10 mm.

[0022]

[Table 1]

Then, each wire was annealed and processed into a tensile test piece. A reduced JIS No. 4 test piece was used for the tensile test. The results of the tensile test after conditioning are shown in Tables 2 and 3. The quenching was carried out at a predetermined temperature for 30 minutes and then oil cooling. The tempering was carried out by air cooling after holding at a predetermined temperature for 1 hour.

After annealing each wire, it was processed into a delayed fracture test piece. In the test of delayed fracture characteristics, a bending type accelerated test piece (l ₁ = 20 mm, d ₁ shown in FIG. 1 was used.
= 6 mm, d ₂ = 4 mm, R = 0.1 mm),
A cantilever bending load was applied. In addition, the test environment, 0.1N-HCl aqueous solution is dropped into the notch of the test piece,
Bending stress was applied.

The delayed fracture resistance of each test material was expressed by the ratio of the static bending stress (σSB) to the strength (σ30h) after 30 hours of the delayed fracture test, that is, the delayed fracture strength ratio σ30h / σSB. The results are shown in Tables 2 and 3.
Shown in the table. In addition, about the test piece whose tensile strength is less than 1500 MPa, the delayed fracture test was not performed except Y of the comparative example.

[0026]

[Table 2]

[0027]

[Table 3]

As shown in Table 2, the tensile strength is 1200MP.
The delayed fracture strength ratio of the a-class comparative example Y steel is 0.70.
Therefore, it was decided to judge the superiority or inferiority of the delayed fracture resistance based on whether or not the delayed fracture strength ratio of 0.70 or more can be obtained.

As shown in Table 2, the steels A to F of the examples of the present invention.
Indicates a tensile strength of 1500 MPa or more due to heat treatment in the quenching temperature and tempering temperature ranges of the claims. Further, it shows a higher delayed fracture strength ratio than Comparative Example Y.

On the other hand, in Comparative Examples G, P and R, since the contents of C, Mo and V were lower than the claimed ranges, respectively, an adequate secondary curing amount could not be obtained and a tensile strength of 1500 MPa or more was obtained. Couldn't get U, W and X are
Since the content of Al, N, and Nb was higher than the claimed range, large inclusions were generated, which became a starting point of fracture and fractured at a low tensile stress, so that 1500 MPa or more could not be obtained.

Comparative examples H, I, J, L, M, N,
It was found that the tensile strengths of O, Q, S, T, and V were 1500 MPa or more, but the delayed fracture strength ratio was smaller than 0.7. In Comparative Example H, since C exceeds the upper limit of the claimed range, the delayed fracture susceptibility is increased. In Comparative Example 1, since Si exceeds the upper limit of the claimed range, the amount of segregation of the impurity element to the austenite grain boundaries increases, and the delayed fracture susceptibility of the old austenite grain boundaries increases.
In Comparative Example J, since Mn exceeds the upper limit of the claimed range, the amount of segregation of the impurity element to the austenite grain boundaries is increased, and the delayed fracture susceptibility of the old austenite grain boundaries is increased. In Comparative Example L, since P exceeds the upper limit of the claimed range, the segregation amount of the impurity element to the austenite grain boundaries increases, and the delayed fracture susceptibility of the old austenite grain boundaries increases. In Comparative Example M, since S exceeded the upper limit of the claim range, the segregation amount of the impurity element to the austenite grain boundary was increased, and the delayed fracture susceptibility of the old austenite grain boundary was increased. In Comparative Example N, since Ni was below the lower limit of the claimed range, the delayed fracture susceptibility of the base was increased. In Comparative Example O, since Cr exceeded the claimed brittle range,
This is because the depth of the grain boundary oxide layer during heat treatment became deeper and the delayed fracture susceptibility near the specimen increased. In Comparative Example Q, M
Because o exceeds the upper limit of the claimed range, a huge primary eutectic carbide is generated, which deteriorates the toughness of the matrix and increases the delayed fracture susceptibility. In Comparative Example S, since V exceeded the upper limit of the claimed range, huge primary eutectic carbide was generated, which deteriorated the toughness of the matrix and increased the delayed fracture susceptibility. In Comparative Example T, since Al was less than the claimed range, the crystal grains were coarsened, which deteriorated the toughness of the matrix and increased the delayed fracture susceptibility. Comparative Example V
, N was below the claimed range, so the crystal grains became coarse,
This is because the toughness of the base deteriorates and the susceptibility to delayed fracture increases.

Table 3 shows steels A to A in the examples of the present invention shown in Table 2.
The tensile strength and delayed fracture strength ratios of F having quenching temperature and tempering temperature range within the present claims and those exceeding the claims are shown.

When the quenching temperature was lower than 1175K, the tensile strength was lower than 1500 MPa for all the steel types. This is due to the fact that the carbide did not form a solid solution at the time of quenching and heating, so that the secondary curing amount became small.

When the tempering temperature was lower than 850 ° K, a tensile strength of 1500 MPa or more was obtained for all steel types. However, it can be seen that the delayed fracture strength ratio is less than 0.70 of Comparative Example Y in Table 2. This is because the carbides that contribute to the secondary hardening were not sufficiently precipitated, so that the amount of trapped hydrogen was reduced, and the diffusion of hydrogen to the delayed fracture starting point portion could not be prevented.

[0035]

As described above, according to the present invention, the tensile strength is 1500 MPa or more and 1200 MPa.
It is possible to manufacture a bolt having a delayed fracture resistance equal to or higher than that of a high-strength steel tempered to a. As a result, the weight and size of the structural component can be reduced, which has a great industrial effect.

[Brief description of drawings]

FIG. 1 shows a delayed fracture test piece shape.

Claims (1)

[Claims] 1. C: 0.35 to 0.50% by weight,
Si: 0.15% or less, Mn: 0.4% or less, P: 0.
015% or less, S: 0.010% or less, Ni: 0.8 to
2.4%, Cr: 0.8 to 1.5%, Mo: 0.8 to
1.4%, V: 0.15 to 0.40%, Al: 0.01
.About.0.06%, N: 0.005 to 0.03% and, if necessary, Nb: 0.05% or less, with the balance being substantially Fe.
And a bolt processed using steel made of unavoidable impurities are characterized by quenching from a temperature of 1175 ° K or higher and tempering at a temperature of 850 ° K or higher, with a tensile strength of 1500 MPa or more and delay resistance. A method for manufacturing a high-strength bolt having excellent destructiveness.