JPH0867942A - High strength steel member excellent in delayed breakdown resistance - Google Patents

Abstract

PURPOSE: To produce a practical high strength steel member excellent in delayed breakdown resistance by regulating the content of Ni to a certain one or above and furthermore concentrating the concn. of Ni in the suface compared to that on the inside in a specified range. CONSTITUTION: This high strength steel member is the one contg., by weight, >=0.25%, preferably 0.25 to 7.0% Ni, in which the concn. of Ni in the surface is regulated to double that on the inside or above and having excellent delayed breakdown resistance. This steel member preferably contains 0.2 to 0.6% C, 0.2 to 2.0% Si, 0.2 to 2.0% Mn, <=0.020% P, <=0.015% S and 0.25 to 7.0% Ni. This steel moreover contains one or more kinds among 0.2 to 2.0% Cr, 0.1 to 0.5% Mo, 0.05 to 1.0% Cu, 0.0003 to 0.0050% B, 0.01 to 0.04% Ti and 0.03 to 0.50% W according to necessary. Furthermore, the content of Ni is preferably regulated to 0.25 to 3.0% from the viewpoint of profitability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel member such as a PC steel rod and a high-tensile bolt having excellent delayed fracture resistance.

[0002]

^2. Description of the Related Art It is said that high strength steel members such as PC steel rods and high tensile bolts tend to cause delayed fracture when the tensile strength exceeds 125 kg / mm ² , and in the case of bolts, JIS B1186 or JIS B1186 or F105 for B1051
T-class and 12.9-class are defined as the upper limit strength. In addition, since PC steel rods are generally used in concrete, once they are reinforced in concrete, they usually have a pH value of 12
Since it exists in the environment of PC, delayed fracture cannot occur, but if the concrete contains salt or water with a low pH is constantly supplied due to imperfections in use or construction, PC steel can be used. Delayed rod breakage may occur.

In the case of a high-tensile bolt, delayed fracture is likely to occur, but there is a strong need for higher strength due to weight reduction and construction problems. To meet this, improved delayed fracture characteristics are required. Is essential.

Also, in the case of PC steel rod, 1420 N / m
There is a strong need for improved delayed fracture characteristics in the m ² class. On the other hand, steels for high-strength bolts having improved delayed fracture characteristics are disclosed in JP-A-5-9653 and JP-A-4-329849.
JP-A-2-240244 discloses a method of improving the delayed fracture characteristics of a PC steel rod by adding Ni.
No. 6,086,045.

[0005]

However, in the techniques disclosed in JP-A-5-9653 and JP-A-4-329849, after the material is heat-treated, a test piece is cut out to perform a delayed fracture test. Therefore, the correspondence with the actual part (bolt) is not necessarily taken, and the value itself is not necessarily high in the actual part. Further, in JP-A-2-240244, since Cr and Mo are essential, the cost is high. The present invention has been made in view of the above circumstances, and an object thereof is to provide a practical high-strength steel member having excellent delayed fracture characteristics.

[0006]

Means and Actions for Solving the Problems The inventors of the present application have conducted the research to solve the above problems, and as a result, found the following points. (1) The delayed fracture is caused by hydrogen in the steel, and Ni in the steel
Suppresses the penetration of hydrogen into the steel and improves delayed fracture resistance.

(2) In order to effectively exert such an action of Ni, the Ni concentration on the surface of the steel member may be set to be twice the Ni concentration inside the steel member (entire), which results in delayed fracture resistance. The characteristics are significantly improved.

(3) The effect becomes clearer depending on the final product. Therefore, it is necessary to perform a delayed fracture test on the final product. The present invention has been made based on such findings, and is characterized in that it contains 0.25 wt% or more of Ni, and the Ni concentration on the surface is twice or more the internal concentration. Provide a steel member having excellent delayed fracture characteristics.

The present invention will be specifically described below. As described above, the steel member of the present invention has a Ni content of 0.
In addition to containing 25 wt% or more, the Ni concentration on the surface is twice or more the internal concentration.

The reason why the Ni content of the steel member is 0.25 wt% or more is that Ni is necessary for suppressing the penetration of hydrogen into the steel, but the effect is exhibited at 0.25 wt% or more. This is because if less than 0.25 wt%, the effect is not exhibited.

The reason why the Ni concentration on the surface is set to be at least twice the Ni concentration inside the steel member is that it is effective to increase the Ni concentration on the surface in order to suppress the penetration of hydrogen into the steel. This is because if the Ni concentration on the surface is at least twice the Ni concentration inside the member, the penetration of hydrogen into the steel is remarkably suppressed, and thus the delayed fracture property is remarkably improved. FIG. 1 shows the results of a delayed fracture test performed by changing the surface Ni concentration of the member. The horizontal axis in FIG. 1 indicates the ratio of the Ni concentration on the surface to the Ni concentration of the material (corresponding to the Ni concentration inside) (hereinafter, the surface N
i / material Ni), and the vertical axis represents the breaking time.
As is clear from FIG. 1, when the surface Ni / material Ni is 2 or more, the delayed fracture characteristics are significantly improved, and when the surface Ni / material Ni is less than 2, it is confirmed that the effect of improving the delayed fracture characteristics is small.

In order to increase the Ni concentration on the surface, (i) increase the rolling temperature during hot rolling, and (i
i) Making an oxidizing atmosphere at the time of heat treatment, (iii) Making a clad, etc. are mentioned. Further, in the present invention, the “surface” is 10 μm from the surface of the base metal excluding scale.
It means a range up to a degree.

It is not necessary to specify the upper limit of Ni in the member in consideration of the above mechanism, but it is preferably 7.0 wt% or less in consideration of the use. Further, considering economic efficiency, the content is preferably 3.0 wt% or less.

The present invention, as described above, uses Ni in the member.
It is not necessary to specify the components other than Ni, since the amount is not less than a certain amount and the Ni concentration on the member surface is increased to obtain a high strength steel member excellent in delayed fracture resistance. However, the use of this kind of high strength material is PC
There are steel rods, bolts, PC steel wires, etc., and there are preferable compositions depending on the application.

First, as the PC steel rod and the bolt, Ni:
0.25-7.0 wt%, C: 0.2-0.6w
t%, Si: 0.2 to 2.0 wt%, Mn: 0.2 to
2.0 wt%, P: 0.020% or less, S: 0.015
Those containing less than wt% are preferable. In addition, C
r: 0.2 to 2.0 wt%, Mo: 0.1 to 0.5 wt
%, Cu: 0.05 to 1.0 wt%, B: 0.0003
~ 0.0050wt%, Ti: 0.01-0.04wt
%, W: at least 1 out of 0.03 to 0.50 wt%
Those containing seeds are preferred.

More specifically, regarding these selective components, P
In the case of a C steel bar, Mo: 0.1-0.5 wt%, C
u: 0.05-1.0 wt%, B: 0.0003-0.
0050 wt%, Ti: 0.01 to 0.04 wt%,
W: 0.03 to 0.50 wt% It is preferable to contain at least one kind, and in the case of bolts, Cr: 0.
2 to 2.0 wt%, Mo: 0.1 to 0.5 wt%, C
u: 0.05 to 1.0 wt%, B: 0.0003 to 0.
A material containing at least one of 0050 wt% and Ti: 0.01 to 0.04 wt% is preferable.

Next, as the PC steel wire, Ni: 0.25
~ 7.0 wt%, C: 0.6-0.9 wt%, S
i: 0.2 to 2.0 wt%, Mn: 0.2 to 2.0 wt
%, P: 0.020% or less, S: 0.015 wt% or less, and Ni: 0.25 to 7.0 wt% are preferable. Further, Cr: 0.2 to 2.0 wt%, C
u: A material containing at least one of 0.05 to 1.0 wt% is preferable.

The reasons for these limitations are as follows. C
Is an element for enhancing hardenability and strength, and P
In order to secure the strength level as a C steel rod and a bolt, it is preferably 0.2 wt% or more. on the other hand,
If it exceeds 0.6 wt%, the spot weldability will decrease, so 0.6
Wt% or less is preferable. When used as a PC steel wire, 0.6 wt% is used to secure the strength after wire drawing.
It is preferable that the content is not less than 0.9%, but if it exceeds 0.9%, wire drawability is deteriorated, so 0.9% by weight or less is preferable.

Si is used as a deoxidizing agent, and is effective for delayed fracture characteristics and relaxation characteristics. Therefore, it is preferable that Si is 0.2 wt% or more so that the effect is exhibited. On the other hand, if it exceeds 2.0 wt%, the toughness of the steel decreases, so 2.0 wt% or less is preferable.

Mn is a deoxidizing agent similar to Si, and is an element necessary to enhance hardenability and strength like C, so that Mn is preferably 0.2 wt% or more. On the other hand, if it exceeds 2.0 wt%, the ductility decreases, so 2.0
% Or less is preferable.

P is an element which is inevitably contained as an impurity element, but since it deteriorates the delayed fracture characteristics,
It is preferably 0.020 wt% or less. S is also an element that is inevitably contained as an impurity element, but is preferably 0.015 wt% or less because it deteriorates delayed fracture characteristics.

Mo is an element that enhances the hardenability and improves the delayed fracture property. However, if its content is less than 0.1 wt%, its effect is small, so 0.1 wt% or more is preferable. On the other hand, 0.
It is economically disadvantageous to add more than 5 wt%, so 0.5 wt% or less is preferable.

Cu and W are elements for improving the delayed fracture property, but if added in a large amount, it is economically disadvantageous. Therefore, Cu is considered to be 0.05 to 1.0 wt%,
W is preferably 0.03 to 0.5 wt%.

B is an element that improves the hardenability as well as the delayed fracture property. If it is less than 0.0003 wt%, its effect is not clear, while 0.0050 wt%.
%, The hardenability is rather deteriorated, so 0.0003 to 0.0050 wt% is preferable.

Ti is an element added to fix N in the steel and secure the effect of improving the hardenability of B. 0.01
It is preferably 0.04 wt%. Either or both of Zr and Nb having the same action as Ti may be added.

Cr is added for the purpose of improving the hardenability in the case of bolts and for improving the workability in the case of wire drawing, but exhibits its respective effects in any application. Therefore, the content is preferably 0.2 wt% or more, and in any case, if it exceeds 2.0 wt%, the workability is greatly deteriorated, so 2.0 wt% or less is preferable.

[0027]

【Example】

(Example 1) Materials having the composition shown in Table 1 (invention steel N
o. 1-3, 6-10, comparative steel Nos. 4, 5) was used as the test steel and was hot-rolled into a round bar with a diameter of 8 mm.
It was drawn into a round rod having a different diameter of m and then quenched and tempered by high frequency heating. At this time, the surface Ni of each steel
A material having a concentration of twice or more the material Ni concentration (B: the present invention example) and less than twice the concentration (A: comparative example) were manufactured. A was hot-rolled at a heating temperature of 1050 ° C. and B was 1120 ° C. Quenching and tempering treatment is performed at a quenching heating temperature of 92.
The tempering was performed at 0 ° C. at an optimum temperature with a target of a tensile strength of 1420 N / mm ² or more.

Table 1 also shows the surface Ni concentration and the surface Ni / material Ni value of these test materials. The surface Ni concentration was measured by XMA after cutting and polishing. An example of the measurement result is shown in FIG. FIG. 2 shows the steel No. of the present invention. 2 shows a distribution of Ni concentration in the depth direction of No. 2, and in the present invention example, Ni is concentrated in the surface portion, which is 2.9 times that of the entire member, which is more than twice that of the entire member. The degree of thickening is low.

[0029]

[Table 1]

In the delayed fracture test, a deformed round bar after heat treatment was used, spot welding was performed under the following welding conditions, stress of 70% of tensile strength was applied, and heating was performed at 50 ° C.
% NH ₄ SCN solution, and the breaking time was evaluated.

Welding conditions Welding current 3000A Energizing cycle number 2 Pressurizing force 410N Spiral muscle 3.2mm round bar of SWRM8 Surface Ni concentration was measured by XMA after cutting and polishing. Table 2 shows the delayed fracture characteristics of these test materials.

[0032]

[Table 2]

As shown in Table 2, in the examples of the present invention in which Ni was concentrated on the surface more than twice as much as that of the whole member, the delayed fracture characteristics were remarkably better than those of the comparative examples. (Example 2) A material having the composition shown in Table 3 (inventive steel N
o. 1-4, comparative steel No. 5) as test material, diameter 2
It was rolled into a 2 mm round bar and molded into an M22 bolt. Then, non-oxidizing atmosphere A (comparative example) and oxidizing atmosphere B
The quenching and tempering treatment was performed in two atmospheres (Example). Here, quenching was performed at 880 ° C., and tempering was performed at an optimum temperature with a target tensile strength of 1450 N / mm ² .

[0034]

[Table 3]

The delayed fracture test was carried out in such a manner that two 20 mm-thick plates were manufactured using the bolts manufactured as described above by the nut rotation angle method.
After tightening at 0 °, it was repeatedly dried and wet in a 3.5% saline solution (normal temperature) every day for 6 months, and evaluated by the number of breaks in 20 pieces. Quenching and tempering atmosphere, surface Ni / material N
Table 4 shows the value of i, the tensile strength, and the number of bolt breakages.

[0036]

[Table 4]

As shown in Table 4, the steel of the present invention No. 1 to
In No. 4, in the non-oxidizing atmosphere A, the surface concentration of Ni did not occur and the fracture of the bolt occurred in the delayed fracture test, but in the oxidizing atmosphere B, Ni was concentrated on the surface and the fracture of the bolt did not occur at all. It was

Comparative steel No. In No. 5, the fracture of the bolt occurred in any atmosphere. Ni enrichment at this time (surface Ni
/ Material Ni) was 1.0 (no thickening) in Comparative Example A and 2.6 in Example B.

(Example 3) A clad steel wire having a composition shown in Table 5 for the core part and the outer layer part was produced. C,
E and F are examples of the present invention, and A, B and D are comparative examples.

These are outer layer steel pipes having an outer diameter of 80 mm and an inner diameter of 70 mm, and a core rod having an outer diameter slightly smaller than the inner diameter. The rod is inserted into the steel pipe and the ends are welded in vacuum. Then, it was hot rolled into a wire having a diameter of 12 mm. This is drawn to 9 mm, then the tensile strength of 80
% NH ₄ SC heated to 50 ° C with a stress applied
The rupture time was evaluated by immersing in N solution. As a result, as shown in Table 5, C, which satisfies the present invention,
It was confirmed that E and F had significantly better delayed fracture characteristics than the comparative example.

[0041]

[Table 5]

[0042]

As described above, according to the present invention,
A practical high-strength steel member having excellent delayed fracture resistance is provided by controlling the Ni content in the member to be a certain amount or more and concentrating the Ni on the member surface in a specific range. Therefore, a high-strength member that can be sufficiently used even in an environment where delayed fracture may occur is obtained, and its industrial value is extremely high.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between surface Ni / material Ni and delayed fracture characteristics.

FIG. 2 is a diagram showing a state in which the surface of Ni of the present invention and the comparative example is concentrated.

Front page continuation (72) Inventor Moriyuki Ishiguro 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd. (72) Inventor Shigeru Mizoguchi 5893 Tamura, Hiratsuka-shi, Kanagawa High-frequency smelting stock company Shonan Works

Claims (7)

[Claims] 1. A steel member having excellent delayed fracture resistance, characterized by containing 0.25 wt% or more of Ni and having a surface Ni concentration of at least twice the internal concentration. 2. The steel member contains Ni of 0.25 to 7.0.
The steel member excellent in delayed fracture resistance according to claim 1, characterized in that the steel member contains wt%. 3. The steel member comprises C: 0.2 to 0.6 wt.
%, Si: 0.2 to 2.0 wt%, Mn: 0.2 to 2.
0 wt%, P: 0.020% or less, S: 0.015 wt
% Or less, Ni: 0.25 to 7.0 wt% is contained, and the steel member excellent in delayed fracture resistance according to claim 1. 4. The steel member further comprises Cr: 0.2 to
2.0 wt%, Mo: 0.1 to 0.5 wt%, Cu:
0.05-1.0 wt%, B: 0.0003-0.00
50 wt%, Ti: 0.01 to 0.04 wt%, W:
The steel member excellent in delayed fracture resistance according to claim 1, containing at least one of 0.03 to 0.50 wt%. 5. The steel member comprises C: 0.6 to 0.9 wt.
%, Si: 0.2 to 2.0 wt%, Mn: 0.2 to 2.
0 wt%, P: 0.020% or less, S: 0.015 wt
% Or less, Ni: 0.25 to 7.0 wt% is contained, and the steel member excellent in delayed fracture resistance according to claim 1. 6. The steel member further comprises Cr: 0.2 to
The steel member excellent in delayed fracture resistance according to claim 5, which contains at least one of 2.0 wt% and Cu: 0.05 to 1.0 wt%. 7. The steel member contains Ni of 0.25 to 3.0.
The steel member excellent in delayed fracture resistance according to any one of claims 1 to 6, wherein the steel member contains wt%.