JPH0741900A - High strength pc rod wire excellent in delayed fracture resistance - Google Patents

Abstract

PURPOSE:To prevent the infiltration of diffusing hydrogen into a PC rod wire and to improve its delayed fracture resistance, at the time of producing the PC rod wire, by finely dispersing MnS into the structure. CONSTITUTION:At the time of producing a construction PC rod wire, as the stock, steel contg., by weight, 0.10 to 0.39% C, 0.10 to 2.0% Si, 0.20 to 2.0% Mn, 0.005 to 0.10% Zr, <=0.015% P, 0.003 to 0.015 S and <=0.010% N or furthermore contg., as material strengthening elements, one or >=two kinds among 0.01 to 1.0% V, 0.01 to 0.5% Ti, 0.01 to 0.5% Nb and 0.0002 to 0.005% B is used. Since the oxides of Zr finely disperse into the steel to work as the effective precipitation site of MnS, MnS infiltrating into the PC rod wire to form the trapping site of diffusing hydrogen causing delayed fracture is finely dispersed to improve its delayed fracture caused by hydrogen, by which the PC rod wire having >=1500Mpa class tensile strength and excellent in delayed fracture resistance can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength PC rod wire having excellent delayed fracture resistance, which is used for construction and the like.

[0002]

2. Description of the Related Art In recent years, tensile strength has increased with the increase in size of structures.
Development of PC steel wire of 500 MPa or more has been required. However, it is well known that when the tensile strength of steel exceeds 1200 MPa, the risk of delayed fracture increases.
Particularly in the case of PC rods, auxiliary reinforcing bars are spot-welded in order to construct a structure, but delayed fracture progresses from the heat-affected zone. As for the upper limit of the working strength, for example, for bolt steel, JIS defines that the limit is 1100 MPa.

As measures against delayed fracture, various researches and developments have been conventionally made on steel for high strength bolts. For example, JP-A-3-243744 and JP-A-3-243 have been published.
No. 745, C = 0.30-wt%, respectively.
0.50, Si = 0.05 to 0.50, Mn <0.5
0, Cr = 0.1-5.0, Al = 0.005-0.1
0, Nb = 0.005 to 0.20, Cu = 0.01 to
0.60, Mo = 0.01 to 0.80, N = 0.005
.About.0.30, and C = 0.35 to 0.50, Si <0.
20, Mn <0.35, Cr <0.25, Al = 0.0
05 to 0.10, V = 0.05 to 0.50, Nb = 0.
005 to 0.20, Ti <0.10, Zr <0.15,
Cu = 0.05-0.60, Ni = 1.0-3.0, M
A high-strength steel containing a chemical composition of o = 0.4 to 1.5 and a manufacturing method thereof are described. These inventions pay attention to the fact that delayed fracture cracks occur with the austenite grain boundary as the starting point and the propagation path, and reduce the grain boundary segregation element, aim at grain boundary strengthening by grain refinement, etc., and provide delayed fracture resistance. It will improve.

[0004]

[Patent Document 1] Japanese Patent Application Laid-Open No. 3-24374
The inventions described in Japanese Patent Application Laid-Open No. 4 and Japanese Patent Application Laid-Open No. 3-243745 disclose the nitrogen content and alloying elements (Al,
By defining the content of Ti, Zr), coarsening of austenite grains during reheating is suppressed. However, since there is a limit to the degree of refinement of austenite obtained by this technique, when applied to PC rods, there is also a limit to the improvement of delayed fracture characteristics.

In "Iron and Steel" 72 (1986) S1518, it is described that delayed fracture is induced by an extremely small amount of diffusible hydrogen (0.3 ppm or less) in a material of 140 kgf / mm ² or more. If not only the grain refining and grain boundary strengthening but also the hydrogen permeating and diffusing into the steel material is not suppressed, the fear of delayed fracture cannot be completely dispelled.

Although a method of coating the surface can be considered as a method of suppressing hydrogen intrusion into the PC rod, it cannot be said to be an effective method because the durability of the coating material such as organic paint is low. In order to reduce diffusible hydrogen, a method of giving a compressive residual stress to the surface layer of the steel material is also effective. Therefore, for example, a method of performing shot peening treatment on a PC bar wire is also expected to be effective. In the case of manufacturing, since the auxiliary reinforcing bar is spot-welded, the compressive residual stress of the welded portion is released by the thermal effect at that time, so a large improvement effect cannot be expected.

[0007]

The gist of the present invention is as follows.

As a chemical component, C: 0.
10 to 0.39%, Si: 0.10 to 2.0%, Mn:
0.2-2.0%, Zr: 0.005-0.10% is contained, P ≦ 0.015%, S: 0.003-0.015
%, N ≦ 0.010%, the balance consisting of iron and unavoidable impurities, and MnS being precipitated in the Zr oxide, which has a strength of 1500 MPa or more and is excellent in delayed fracture resistance. Strength PC rod.

As a chemical component, C: 0.
10 to 0.39%, Si: 0.10 to 2.0%, Mn:
0.2-2.0%, Zr: 0.005-0.10% is contained, P ≦ 0.015%, S: 0.003-0.015
%, N ≦ 0.010%, and as other strengthening elements,
V: 0.01 to 1.0%, Ti: 0.01 to 0.5%,
Nb: 0.01 to 0.5%, B: 0.0002 to 0.0
1% or 2 or more kinds of 05% is contained, the balance consists of iron and unavoidable impurities, and MnS is precipitated in Zr oxide, which is excellent in delayed fracture resistance with strength of 1500 MPa or more. High strength PC bar wire.

[0010]

The present inventors have found that the high tensile strength P which is excellent in delayed fracture resistance.
As a result of the development of C rods, in order to reduce the diffusible hydrogen that enters the PC rods and is the main cause of delayed fracture, it is possible to finely disperse MnS, which is a hydrogen trapping site, in the PC rods. It has been found that it is effective to improve the delayed fracture resistance of PC rods and it is effective to use Zr oxide as a precipitation site for finely dispersing MnS.

The reasons for limiting the components will be described below.

[C] C is required to be 0.10% or more in order to obtain the strength level as a PC rod wire by quenching and tempering, but if it is too much, it deteriorates toughness as well as delayed fracture resistance and weldability. Therefore, the content is set to 0.39% or less.

<< Si >> Si is an element necessary for deoxidizing steel and enhancing the strength, and 0.1 to obtain the effect.
% Or more is required. Further, it segregates at the γ grain boundary, lowers the grain boundary strength, and deteriorates the delayed fracture property, so the content was made 2.0% or less. However, Si is also an element that needs to be added as a measure against high temperature relaxation during curing of concrete, and 0.5% is sufficient for low temperature curing at 100 ° C. or less. Therefore, when using PC bar wire under the condition that high temperature curing of concrete is required, 0.1% or more and 2.0
%, But considering the adverse effect of Si as a grain boundary embrittlement element, 0.1% or more and 0% or more when a PC rod is used under the condition of low temperature curing treatment of 100 ° C. or less. 0.5% or less is desirable.

<< Mn >> Mn is an element necessary for ensuring deoxidation and hardenability of steel, and is effective for improving delayed fracture characteristics by precipitating MnS on Zr oxide and trapping hydrogen. Although it is an element, if it is less than 0.2%, its effect is difficult to obtain, so the content was made 0.2% or more. On the other hand, if it exceeds 2.0%, the amount segregated at the grain boundaries during heating in the austenite region becomes too large, which significantly embrittles the grain boundaries and deteriorates the delayed fracture resistance.

<< Zr >> Zr is an element necessary for improving the delayed fracture resistance of PC rods, and its addition is the greatest feature of the present invention. Zr is not only an effective element for deoxidizing steel, but its oxide is heavier than normal oxide, so it is finely dispersed in the steel material without being floated and separated during ladle refining. Is an effective precipitation site for MnS. Figure 1
Includes steel type A (0.30% C-0.24% Si-0.7).
9% Mn-0.008% P-0.006% S-0.03
2% Zr) and steel type B (0.30% C-0.24% Si-
0.81% Mn-0.007% P-0.005% S-
For 0.034% Al), a sample with a shape of φ13 × 30 mm is dipped in 30% HCl for 60 minutes, and then the amount of hydrogen that can be detected in the temperature range from room temperature to 300 ° C. is measured by heating gas extraction mass spectrometry in vacuum. As compared with the amount of hydrogen, Zr deoxidized steel reduces the diffusible hydrogen like the steel type A shown in FIG. 1 because the finely dispersed MnS becomes an effective hydrogen trap site. This gives P
Improves delayed fracture resistance of C steel bar. However, in FIG. 2, 0.30% C-1.0% Si-0.82% Mn-0.0
As shown in the relationship between the Zr addition amount and the rupture time for the 08% P-0.0031% N steel, the amount of Zr oxide is insufficient when Zr is less than 0.005%, so that MnS should be dispersed effectively. And the delayed fracture characteristics are not improved. On the other hand, if Zr exceeds 0.10%, the amount of Zr oxide is too large, and a huge oxide is formed, rather the delayed fracture property deteriorates. Therefore, Zr is set to 0.005% or more and 0.1% or less.

As for the delayed fracture resistance of the sample, as shown in FIG. 3, a test piece 1 having a notch on its circumference was prepared, and the notch part of the test piece 1 was immersed in a 20% NH ₄ SCN aqueous solution at 50 ° C. By immersing and applying a constant tensile load of 0.7 σ _B to the test piece 1, the time until breakage was measured and evaluated.

<< V, Ti, Nb >> V, Ti, and Nb contribute to the refinement of crystal grains and have a high affinity with hydrogen, and retardation of diffusion and accumulation of hydrogen in steel delays the delay. Since it is an element effective for improving the fracture characteristics, V: 0.01% or more, Ti: 0.01% or more, and Nb: 0.01% or more are required. However, if the amount is too large, the effect is saturated and rather the toughness is deteriorated and the delayed fracture resistance is deteriorated. Therefore, V: 1.0% or less, Ti: 0.5% or less, Nb:
It was set to 0.5% or less.

<< B >> Since B has the effect of further improving the hardenability of steel, it is added when particularly high strength is required for thick PC rods, but if it is less than 0.0002%, its effect is obtained. I can't do that. On the other hand, if the content exceeds 0.005%, the effect is saturated, the toughness also deteriorates, and the delayed fracture resistance deteriorates.
% And 0.005% or less.

<< P, S, N >> P is 0.015% because it is an element that microsegregates during solidification and segregates at grain boundaries during heating in the austenite region, embrittles the grain boundaries, and deteriorates delayed fracture resistance. Below.

S is an element necessary for forming MnS and trapping hydrogen, but if it is less than 0.03%, the amount of MnS produced is too small. Therefore, even with Zr deoxidation, the effect of improving delayed fracture characteristics is obtained. Disappears. If it is too large, it segregates at the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and deteriorates delayed fracture resistance, so the content was made 0.015% or less.

N is an element that segregates at the grain boundaries during austenite heating, embrittles the grain boundaries, and deteriorates the delayed fracture resistance.

[0022]

[Examples] Table 1 shows the chemical composition of test steels of Examples of the present invention.
It shows in Table 2. Strength 1500 MPa with these compositions
With respect to the PC rod, the delayed fracture resistance was evaluated by the above-mentioned fracture evaluation test. The results are also shown in Tables 1 and 2. In addition, Table 3 shows the results of the delayed fracture property evaluation of the PC rod having a strength of 1500 MPa in the comparative example.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

From Tables 1 to 3, the invention examples 1 to 51 are about 2 to 30 times longer in breaking time than the comparative examples 52 to 66.
It became clear that delayed destruction is difficult.

[0027]

According to the present invention, it is possible to obtain a PC rod wire having a tensile strength of 1500 MPa or more and an excellent delayed fracture resistance. This improves the life and safety of the concrete structure.

[Brief description of drawings]

FIG. 1 is a diagram showing an effect of suppressing diffusible hydrogen by Zr.

FIG. 2 is a diagram showing the relationship between the amount of Zr added and the delayed fracture resistance of PC rods.

FIG. 3 is a diagram showing a test piece used for evaluation of delayed fracture characteristics.

[Explanation of symbols]

 1 test piece

Claims (2)

[Claims] 1. As a chemical component, C: 0.1% by weight,
0 to 0.39%, Si: 0.10 to 2.0%, Mn:
0.2-2.0%, Zr: 0.005-0.10% is contained, P ≦ 0.015%, S: 0.003-0.015
%, N ≦ 0.010%, the balance consisting of iron and unavoidable impurities, and MnS being precipitated in the Zr oxide, which has a strength of 1500 MPa or more and is excellent in delayed fracture resistance. Strength PC rod. 2. The chemical component, in% by weight, is C: 0.1.
0 to 0.39%, Si: 0.10 to 2.0%, Mn:
0.2-2.0%, Zr: 0.005-0.10% is contained, P ≦ 0.015%, S: 0.003-0.015
%, N ≦ 0.010%, and as other strengthening elements,
V: 0.01 to 1.0%, Ti: 0.01 to 0.5%,
Nb: 0.01 to 0.5%, B: 0.0002 to 0.0
1% or 2 or more kinds of 05% is contained, the balance consists of iron and unavoidable impurities, and MnS is precipitated in Zr oxide, which is excellent in delayed fracture resistance with strength of 1500 MPa or more. High strength PC bar wire.