JP2990963B2 - High strength bolt steel with excellent delayed fracture resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength bolt steel having excellent delayed fracture resistance.

[0002]

2. Description of the Related Art High tension bolts are used in bridges, buildings, automobiles,
Widely used in machinery, etc., but has a tensile strength of 1300
If N / mm ² is exceeded, delayed fracture is likely to occur. Therefore, in JISB1186 and JISB1051, F10T class and 12.9 class are specified as upper limit strengths, respectively. Low carbon boron steel is mainly used for F10T, and SCM435 is mainly used for 12.9.

[0003]

[SUMMARY OF THE INVENTION Incidentally, although further strengthening the high tensile bolts is also desired in any of the aforementioned areas, even by performing a high-strength delayed fracture properties using these materials Not enough, so 1400N
/ Mm ² class has not yet been standardized. Conventionally, as steel having high strength and excellent delayed fracture characteristics, for example,
1-64815, JP-A-62-86149, JP-A-3-243745 and the like have been proposed. However, it is difficult to say that these steels have sufficiently improved delayed fracture. The present invention has been made in view of the above-described conventional problems, and
It is an object of the present invention to provide a steel for bolts having a high strength of 00 N / mm ² or more and having excellent delayed fracture resistance.

[0004]

Means for Solving the Problems The present inventors have intensively studied to improve the delayed fracture characteristics of high-strength steel for bolts, and as a result, added a large amount of Ni under a specific composition, It has been found that delayed addition characteristics are greatly improved by adding Ti or Zr in combination. FIG.
Those Ni addition amount were investigated on delayed fracture property in a given component 1500 N / mm ² grade steel, the Ni at 1500 N / mm ² class steel as shown in FIG. 2
%, It was found that the rupture time was significantly improved. FIG. 2 shows the effect of the addition of Ni and Ti in relation to delayed fracture characteristics and tensile strength.
As shown in the figure, it was found that the delayed fracture characteristics (K _ISCC ) were further improved by adding 0.04% Ti in addition to 3% Ni.

The present invention has been made based on these findings.
Of the effects of other constituent elements
Therefore, the composition is as follows: C: 0.25 to 0.45 wt%, S
i: 0.25 to 1.0 wt%, Mn: 0.35 to 1.0
wt%, P: 0.005 wt% or less, S: 0.020 w
t% or less, Ni: 2.0 to 6.0 wt%, Cr: 0.5
wt% or more and less than 2.0 wt%, Mo: 1.5 to 3.0 w
high-strength bolt steel containing 0.01% to 0.1% by weight in total of one or two of Ti and Zr, and the balance of Fe and unavoidable impurities is excellent in delayed fracture resistance. It is.

[0006]

[0007]

The reasons for limiting the composition of the high-strength bolt steel of the present invention will be described below. C: An element effective for improving the hardenability and increasing the tempering temperature. If less than 0.25 wt%, the hardenability deteriorates and the tempering temperature decreases. On the other hand, 0.45wt%
If it exceeds, the toughness deteriorates. Therefore, the C content is 0.25
０．４0.45 wt%.

[0008] Si: Si is an element necessary as a deoxidizing agent and is effective for improving delayed fracture characteristics.
A content of not less than wt% is necessary, but if it exceeds 1.0 wt%, toughness is deteriorated. For this reason, the amount of Si is 0.25 to
1.0 wt%. Mn: an element necessary for securing deoxidation and hardenability, and 0.3
A content of 5 wt% or more is required, but if it exceeds 1.0 wt%, toughness and delayed fracture characteristics are deteriorated. For this reason,
The Mn content is 0.35 to 1.0 wt%.

P: Particularly, the higher the strength of the steel, the greater the adverse effect on the delayed fracture characteristics, so the upper limit is made 0.005 wt%. S: Since it exists as inclusions in the form of MnS and adversely affects delayed fracture characteristics, the upper limit is made 0.020 wt%. Ni: an element having a great effect on improving delayed fracture characteristics, it is necessary to add 2.0 wt% or more.
If the content exceeds t%, the effect is saturated and the cost is increased. Therefore, the content is set to 2.0 to 6.0 wt%.

[0010] Cr: hardenability improving an element effective for increasing the tempering temperature, no effect thereof is less than 0.5 wt%, whereas, the effect is saturated at 2.0 wt% or more 0. 5 wt% or more and less than 2.0 wt% . Mo: an element effective for improving hardenability and increasing tempering temperature, and also has an effect for improving delayed fracture characteristics. Therefore, addition of 1.5 wt% or more is required, but 3.0 w
If the content exceeds t%, the effect is saturated and the cost is increased. Therefore, the content is set to 1.5 to 3.0 wt%.

In order to improve the delayed fracture characteristics by fixing Ti, Zr: N and fixing S, and further adding compound with Ni, it is necessary to add 0.01 wt% or more in total. Even if added in excess, the effect is saturated, so the content is 0.01 to 0.1 wt%.

[0012]

EXAMPLES Steels having the component compositions shown in Table 1 were produced and subjected to tests. In the table, No. Nos. 1 to 3 are steels of the present invention; 4 to 7
Is a comparative steel. In addition, No. 4 is 1200N / m at present
a SCM435 mainly used in m ² grade.

Table 2 shows the heat treatment conditions, mechanical properties, and results of the delayed fracture test of each of the above steels. Heat treatment is 25m
Oil quenching after holding the mφ round bar at the predetermined temperature for 40 minutes,
Next, the mixture was kept at a predetermined temperature for 60 minutes, cooled with water, and tempered. Tensile test specimen is JIS4 (parallel part 14mmφ),
The delayed fracture test piece was a square bar type of 10 × 15 × 150 mm, and a 1.5 mm saw notch was inserted at the center.
One having a fatigue notch of 5 mm was used. The delayed fracture test was performed using a cantilever type testing machine by immersing the notch portion of the test piece in 3.5% saline (maintained at 20 ° C.). The results of the analysis using the stress intensity factor K _I represented by the following formula.

[0014]

(Equation 1)

[0015] The KI value at which cracking did not occur even after a 500-hour test was used as the _KISCC for evaluation of delayed fracture characteristics. According to Table 2, the steel of the present invention was 1400 N /
Although it shows high delayed fracture characteristics at a strength of ² mm2 or more,
SCM435 (No. 4 ), comparative steel not containing Ni and Ti (No. 5 ), comparative steel not containing Ti (No. 6 ), and comparative steel not containing Cr and Mo (N.
o. 7 ) shows that the delayed fracture characteristics are inferior.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

The steel for high-strength bolts according to the present invention described above has excellent delayed fracture characteristics that cannot be obtained with conventional steels, despite its high strength of 1400 N / mm ² or more.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the amount of Ni on delayed fracture characteristics in a 1500 N / mm ² grade steel.

FIG. 2 is a graph showing the effect of adding Ni and Ti in relation to delayed fracture characteristics and strength.

Claims (1)

(57) [Claims] 1. C: 0.25 to 0.45 wt%, Si:
0.25 to 1.0 wt%, Mn: 0.35 to 1.0 wt%
%, P: 0.005 wt% or less, S: 0.020 wt%
Hereinafter, Ni: 2.0 to 6.0 wt%, Cr: 0.5 wt%
% To less than 2.0 wt%, Mo: 1.5 to 3.0 wt%
And a high-strength bolt steel containing one or two of Ti and Zr in a total amount of 0.01 to 0.1 wt%, and being excellent in delayed fracture resistance comprising a balance of Fe and unavoidable impurities.