JPH0726177B2 - High strength fireproof bolt with excellent delayed fracture resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention has a room temperature strength of 100 kgf / mm.
^It relates to ^two or more fireproof bolts, and since the fireproof bolt of the present invention has excellent delayed fracture resistance, it can be used in civil engineering buildings and bridges where there is a concern that it will be in a high temperature state for several hours due to fire etc. It is useful.

[0002]

2. Description of the Related Art Structural steel plates and fastening bolts have sufficient strength at room temperature, but when exposed to high temperatures such as a fire, they show a significant decrease in strength at around 350 ° C. Has been. Therefore, when building buildings such as buildings and houses, the Building Standard Law requires that a fireproof coating be applied so that the temperature of the steel itself does not exceed 350 ° C.
The cost required for the refractory coating causes a significant increase in construction cost.

However, in March 1987, the new fire protection design method developed in the Ministry of Construction Comprehensive Technology Development Project made it possible to design a method according to each building. Therefore, the use of so-called refractory steel with high high-temperature strength enables the reduction of coating or uncoated design, and a significant reduction in construction cost can be expected. Along with this, various refractory steel materials have been reported. In particular, there are many reports of fire-resistant steel plates, for example, JP-A-2-9647.

However, when a building is built, bolts and welding materials for fastening the plates are also required, and these are naturally required to have fire resistance. Such room temperature at 100 kgf / mm ² or more in the refractory bolt, high temperature (eg 600 ° C.) even 30 kgf / mm ² or more strength is required. 10 at room temperature
In the case of high strength bolts showing 0 kgf / mm ² or more, there is a risk of delayed fracture that suddenly breaks during use, and it is necessary to pay close attention to delayed fracture resistance, but at the present time, both strength and delayed fracture resistance are required. There is no fireproof bolt that satisfies

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a high-strength refractory bolt having excellent delayed fracture resistance and excellent room temperature and high temperature strength. It is a thing.

[0006]

The first invention of the present invention which has been able to solve the above problems is 0.15% ≤C≤0.30% 0.5% ≤Mn≤1.2% 0.7% ≤ Cr ≤ 1.5% 0.15% ≤ Mo ≤ 0.6% P ≤ 0.01% S ≤ 0.008% Si ≤ 0.2% at room temperature strength consisting of balance Fe and unavoidable impurities ( TS) is a refractory bolt of 100 kgf / mm ² or more, P, S and Mo satisfy the formula 1590% P + 386% S-34% Mo ≦ 13, and C, Mn, Cr and Mo are the formula 30 ≦ 19. 2.8% C + 3.9% Mn + 18.5% Cr
It is a high-strength refractory bolt that satisfies + 28.7% Mo-1.9 and has excellent delayed fracture resistance. The second invention is 0.15% ≤ C ≤ 0.30% 0.5% ≤ Mn ≤ 1. 2% 0.7% ≤ Cr ≤ 1.5% 0.15% ≤ Mo ≤ 0.6% P ≤ 0.01% S ≤ 0.008% Si ≤ 0.2% and V ≤ 0.15% Ti ≦ 0.1% Nb ≦ 0.1% of one kind or two kinds or more, and room temperature strength (TS) consisting of balance Fe and unavoidable impurities is 100 kgf / mm.
² or more refractory bolts, P, S and Mo satisfy the formula 1590% P + 386% S-34% Mo ≦ 13, and C, Mn, Cr, Mo, V, Ti and Nb are the formula 30 ≦ 19. 2.8% C + 3.9% Mn + 18.5% Cr +
28.7% Mo + 84.9 (% V +% Ti +% Nb)-
It is a high-strength fireproof bolt satisfying 1.9 and having excellent delayed fracture resistance.

[0007]

The present inventors have conducted a detailed study to obtain a high-strength refractory bolt having excellent delayed fracture resistance and excellent high-temperature strength. As a result, the types and contents of the elements contained are strictly limited. It has been found that all properties can be satisfied by the above. The contained element and the content ratio will be described.

0.15% ≤ C ≤ 0.30% C is an element necessary to secure the strength at room temperature and high temperature, and when it is insufficient, it has sufficient strength (100 kgf / mm at room temperature).
^In order to obtain ( ² or more), it is necessary to lower the tempering temperature, and the treatment temperature tends to be in the brittle temperature range. However, if the amount is too large, the toughness is lowered and the delayed fracture resistance is significantly lowered.

0.5% ≦ Mn ≦ 1.2% Mn has the effect of improving deoxidation and hardenability. In order to exert these effects, it is necessary to contain 0.5% or more, but if it is too large, it segregates to the grain boundaries and embrittles the grain boundaries, resulting in deterioration of delayed fracture resistance.

0.7% ≤ Cr ≤ 1.5% Cr has the effect of improving the hardenability and the strength and toughness in a well-balanced manner, and further contributing to the improvement of the high temperature characteristics. 0.7 for these effects
%, But if too much, the deformation resistance of the material becomes too high, which may cause deterioration of the tool life during bolt forming, so the upper limit was made 1.5%.

0.15% ≤ Mo ≤ 0.6% Mo has the effect of improving the high temperature strength and the effect of improving the toughness at room temperature, and therefore has the effect of significantly improving the delayed fracture resistance. In order to exert these effects
It is necessary to contain 0.15% or more, but even if it exceeds 0.6%, the effects will be saturated, and not only the cost will increase, but also the deformation resistance of the material will become too high if it is too much, and the tool life during bolt forming will increase. Therefore, the upper limit was made 0.60%.

P.ltoreq.0.01% S.ltoreq.0.008% P and S are elements segregated at the crystal grain boundaries, and it is preferable that P and S be as small as possible so as to embrittle the grain boundaries and deteriorate the delayed fracture resistance.

Si ≦ 0.2% Si is an element effective as a deoxidizing element, but since it also increases the deformation resistance and deteriorates the tool life during bolt molding, the upper limit was made 0.2% or less.

V ≦ 0.15% (second invention) Ti ≦ 0.1% (second invention) Nb ≦ 0.1% (second invention) V, Ti and Nb are combined with C and N in steel to form carbon / nitride. Since it has the effect of significantly improving the strength at high temperature, the effect of improving the high-temperature yield strength is large even if added in a small amount.
Moreover, the crystal grains can be made finer, which has the effect of improving delayed fracture resistance. However, all of them are expensive elements, and if the amount exceeds a certain level, a significant increase in the effect cannot be recognized, so the respective upper limits were set.

In order to obtain further excellent delayed fracture resistance, M
It is necessary that the contents of o, P and S satisfy the formula 1590% P + 386% S-34% Mo ≦ 13.

As a result of various investigations by the present inventors, the delayed fracture resistance was determined by the 1
It was found that the content of the above elements and σ _100D (kgf / mm ² ) are approximated by the following regression equation when expressed by the maximum additional stress (σ _100D ) that does not break at 00 hours. σ _100D = 183- (1590% P + 386% S-34% Mo) Also, in practice, it is determined that delayed fracture resistance of σ _100D is required to be 170 kgf / mm ² or more. 1590% P + 386% S-34% Mo ≦ 13 was derived.

Furthermore, in order to obtain excellent high temperature strength, C, Mn, Cr, Mo, V, Ti and Nb are represented by the formula 30 ≦ 19.8% C + 3.9% Mn + 18.5% Cr + 28.7% Mo + 84.9 (% It is necessary to satisfy V +% Ti +% Nb) -1.9 (However, in the first invention, V, T
i and Nb are 0%). That is, high temperature strength of 600 ℃
It was found that the content of the above elements and YP ₆₀₀ are approximated by the following regression equation when expressed by the yield strength (YP ₆₀₀ ) in the tensile test at. YP ₆₀₀ = 19.8% C + 3.9% Mn + 18.5% Cr + 28.7% Mo + 84.9 (% V +% Ti +% Nb) -1.9 Since it is judged that high temperature strength is required to be 30kgf / mm ² or more according to the design specifications, the above By rearranging the formula, the formula 30 ≦ 19.8% C + 3.9% Mn + 18.5% Cr + 28.7% Mo + 84.9 (% V +% Ti +% Nb) -1.9 was derived.

In the case of the bolt of the present invention, A is used as a deoxidizing agent.
1 may be contained in the range of 0.05% or less.

The present invention will be described in more detail with reference to the following examples, but the following examples do not limit the present invention.
All modifications and implementations that do not depart from the spirit of the description below are included in the technical scope of the present invention.

[0020]

EXAMPLE Steels having chemical compositions Nos. 1 to 18 shown in Tables 1 and 2 were hot-rolled and spheroidized into 22 mmφ steel bars, and M22 bolts were produced by a multistage former. These bolts were subjected to quenching and tempering treatment and tempered so that the tensile strength at room temperature was around 115 kgf / mm ² . Using the obtained bolt, a high temperature tensile test piece shown in FIG. 1 was produced by machining, and a high temperature tensile test was carried out at 600 ° C. Further, notched delayed fracture test pieces shown in FIG. 2 were produced for some of them, and an accelerated delayed fracture test in water was carried out. The results are summarized in Table 2.

[0021]

[Table 1]

[0022]

[Table 2]

In the case of Example 1, the high temperature yield strength at 600 ° C. was 30 kgf / mm ² or more, and the 100-hour delayed fracture strength was 170 kgf / mm ² or more, and all the characteristics were excellent. Examples 2 to 5 are cases in which V, Ti, and Nb are added individually or in combination, but the material strength does not change much in either case, but the high temperature yield strength and 100 hour delayed fracture strength are significantly increased. It was well understood that the element has a remarkable improving effect even in a small amount.

On the other hand, Comparative Example 6 having a low Mo content is low in high temperature yield strength, and Comparative Example 7 having a high Mo content is not preferable because the hardness of the bolt material becomes too high and the deformation resistance increases. Comparative Example 8 had a value of (1590% P + 386% S-34% Mo) of 13
The fracture strength after 100 hours is significantly reduced. Comparative Example 9 having low Mn is preferable in terms of low material strength and deformation resistance, but deoxidation and the like become insufficient, inclusions increase, notch sensitivity increases, and delayed fracture characteristics deteriorate. Comparative Example 10 in which Mn is high has a high material strength, but because of segregation of Mn grain boundaries, delayed fracture characteristics are deteriorated, which is not preferable.

Further, Comparative Example 11 in which the amount of Cr is too small cannot obtain a sufficient high temperature yield strength, and Comparative Example 12 in which the amount of Cr is high is significantly increased in the material strength, which is expected to adversely affect the tool life. Comparative Example 13 in which P is high (1590% P +
386% S-34% Mo) exceeds 13 and 1
00 hour delay Fracture strength is significantly reduced. Comparative Example 14 in which S is too high is (1590% P + 386% S-34%
Mo) value satisfies 13 or less, but still 100
The delayed fracture strength is decreasing. Comparative Example 15 in which Si is too high is not preferable because the hardness of the material becomes too high.

Further, Comparative Example 16 shows the result when V is added out of the claimed range, taking V among the trace elements as an example.
This is not preferable because the material strength is significantly increased and the tool life may deteriorate. In Comparative Example 17 in which C is too low, when the strength is secured, the tempering occurs in the tempering brittleness temperature range of less than 400 ° C., and therefore the 100-hour delay strength is significantly low. On the other hand, in Comparative Example 18 in which C is too high, the hardness of the material is increased, the tool is adversely affected, the toughness of the heat treated bolt is reduced, and the 100-hour delayed fracture strength is low.

[0027]

EFFECTS OF THE INVENTION Since the present invention is constituted as described above, it has excellent delayed fracture resistance and high strength at high temperature even though the tensile strength is 100 kgf / mm ² or more. It has become possible to provide high strength fireproof bolts. Therefore, by combining the high-strength fire-resistant bolt of the present invention with a fire-resistant steel plate or the like, it is possible to further improve the safety and cost of a building or structure.

[Brief description of drawings]

FIG. 1 is a diagram showing the shape of a test piece used in a high temperature tensile test in Examples.

FIG. 2 is a diagram showing a shape of a test piece used for an accelerated delayed fracture test in Examples.

Claims (2)

[Claims] Claims: 0.15% ≤ C ≤ 0.30% (% by weight, the same unless otherwise specified) 0.5% ≤ Mn ≤ 1.2% 0.7% ≤ Cr ≤ 1.5% 0.15% ≤ Mo ≤ 0.6% P ≤ 0.01% S ≤ A refractory bolt containing 0.008% Si ≤ 0.2% and consisting of balance Fe and unavoidable impurities and having a room temperature strength (TS) of 100 kgf / mm ² or more, wherein P, S and Mo are expressed by the formula 1590% P + 386% S-34%. Excellent delayed fracture resistance characterized by satisfying Mo ≦ 13, and C, Mn, Cr and Mo satisfying the formula 30 ≦ 19.8% C + 3.9% Mn + 18.5% Cr + 28.7% Mo-1.9 High strength fireproof bolt. 2. 0.15% ≦ C ≦ 0.30% 0.5% ≦ Mn ≦ 1.2% 0.7% ≦ Cr ≦ 1.5% 0.15% ≦ Mo ≦ 0.6% P ≦ 0.01% S ≦ 0.008% Si ≦ 0.2% and V ≦ 0.15% Ti ≦ 0.1% Nb ≦ 0.1% One or more kinds are contained, and the balance Fe and Room temperature strength (TS) consisting of inevitable impurities is 100 kgf / mm
² or more refractory bolts, P, S and Mo satisfy the formula 1590% P + 386% S-34% Mo ≦ 13, and C, Mn, Cr, Mo, V, Ti and Nb are the formula 30 ≦ 19. 2.8% C + 3.9% Mn + 18.5% Cr +
28.7% Mo + 84.9 (% V +% Ti +% Nb)-
A high-strength fire-resistant bolt having excellent delayed fracture resistance, which satisfies 1.9.