JPH09241795A - Pc steel bar excellent in delayed fracture characteristic in weld zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a PC steel bar having spot weldability, mechanical properties, and relaxation characteristic equal to those of the conventional steel, improved in delayed fracture characteristic in a spot weld zone, and reduced in costs. SOLUTION: This steel bar has a composition containing, by weight, 0.20-0.40% C, 0.1-0.3% Si, 0.2-0.5% Mn, <=0.02% P, <=0.02% S, and 0.03-0.2% V or further containing 0.02-0.05% Nb, as essential components, and also containing two or more kinds among 0.01-0.05% Ti, 0.01-0.05% Al, and 0.001-0.005% B, and having the balance iron with inevitable impurities.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PC steel rod having a base material having characteristics equivalent to those of conventional steel and having excellent delayed fracture characteristics of welded portions.

[0002]

2. Description of the Related Art Steel bars for prestressed concrete (P
C steel bar) is specified by JIS G3109 “PC steel bar” having a tensile strength of 1200 N / mm ² or more, and the standard characteristic values are mainly mechanical characteristics. In particular, tensile strength, yield point, elongation, and relaxation value are the most important properties for PC steel bar applications.

PC steel rods are those which are heat-treated after hot rolling to control the tensile strength to 1200 N / mm ² or more, and have a carbon content of 0.4% or less and can be welded. Therefore,
The hoop streak can be spot-welded to a PC steel rod to braid a basket, which has good workability.

One of the main applications of such PC steel rods is concrete poles and concrete piles. In such applications, PC steel rods are spot-welded with hoop reinforcements to form rebar cages, concrete is added to the steel cages, and the steel bars are cured by steam or an autoclave with stress applied near the yield point of the steel rods. Used in tension on steel bars for a period of time. In order to maintain this tension force stably, it is preferable that the relaxation property is small.

PC steel bars reinforced in concrete have p
Corrosion is unlikely to occur due to the presence of a very high H of 12, but water containing a low salt content or low pH due to the use of a small amount of concrete fogging and imperfections in concrete construction. May intrude through cracks in concrete, which may cause delayed fracture of the PC steel rod.

In particular, the welded portion obtained by spot-welding the hoop muscle has a high Vickers hardness of 700 or more and has a high sensitivity to delayed fracture, so that it often becomes the starting point of delayed fracture. For this reason, it has been required to improve the delayed fracture characteristics of spot welds of PC steel rods.

The delayed fracture characteristics of such PC steel rods are described in JP-A-48-51816, which is a PC steel material having an increased Si content, and in JP-A-5-295481, which is a composite material containing Si and Ca. Method for controlling morphology, JP-A-2-240
236, JP-A-2-240237, JP-A-2-2402
JP-A-44 discloses a PC steel rod improved in corrosion resistance and delayed fracture characteristics by adding Ni, Cu and W.

However, although the conventional method has an effect of improving the delayed fracture property of the base metal portion of the PC steel rod, the effect of improving the delayed fracture property of the spot weld is not sufficient, and more expensive alloy is added. In this case, the manufacturing cost increases and the economy is poor.

Further, as a PC steel rod having improved delayed fracture characteristics of spot welds, as disclosed in JP-A-6-212346 and JP-A-6-212357, Ni is disclosed.
Has been proposed for a component type steel type in which the steel content is reduced to 0.25%. Although the target properties are almost satisfied with this component system, it is necessary to increase the Ni content in order to achieve high delayed fracture properties, and there is room for improvement in terms of alloy cost, and oxide scale peeling due to Ni addition There is a problem that processing work becomes complicated due to deterioration of sex.

[0010]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems of the prior art, the present invention aims to suppress the alloy addition as much as possible without adding expensive Ni and to thereby improve the mechanical properties without impairing the spot weldability. The purpose of the present invention is to provide a low-cost PC steel rod having the relaxation characteristics equivalent to those of conventional steels and improved delayed fracture characteristics of spot welds.

[0011]

As a result of intensive studies to solve the above technical problems, the present inventor has found that delayed fracture characteristics, particularly in spot welds, without deteriorating mechanical characteristics and relaxation characteristics. It was found that it is effective to satisfy the following conditions in order to improve delayed fracture characteristics.

(A) In order to improve the delayed fracture characteristics of the spot weld, the point is that cracks generated in the high hardness portion of the weld metal do not propagate to the inside of the base metal, and the boundary between the weld metal and the base metal It was found that it is important to improve the toughness in steel. That is, the boundary between the deposited metal and the base material is affected by heat during spot welding, is heated to a high temperature, and shows a decrease in hardness, and cracks stop in this region (called the HAZ softening layer). HA
The lower the hardness of the Z softening layer, the more improved the delayed fracture characteristics of the spot weld. That is, it was found that the reduction of Si and Mn acts effectively as an alloy component.

Further, by adding V, the hydrogen acts as a trap site for invading hydrogen, so that the invading hydrogen in the steel material can be made harmless, and grain boundary segregation easily occurs in P, S, M.
The reduction of n suppresses the segregation to the grain boundary and strengthens the grain boundary, and the delayed fracture property can be improved by suppressing the propagation from the crack tip generated in the spot weld to the base metal.

(B) The reduction of Si for improving the delayed fracture property of the spot weld deteriorates the relaxation property which is an important characteristic value of the PC steel bar. Therefore, it was found that the addition of V and Nb, which form fine carbonitrides, is effective in order to obtain good relaxation characteristics even with low Si.

(C) In order to improve the delayed fracture property, B is added in order to suppress the deterioration of the hardenability when the Mn is reduced and the hardenability is secured. Here, in order to make B act effectively, Ti and Al are added to form a nitride and N in the steel.
Must be suppressed from binding with B.

(D) Further, the strength is adjusted by quenching and tempering treatment to secure the target strength. At this time, the rapid heating causes the crystal grains to become finer and the carbonitride precipitates also form martensite lath. Fine deposit inside,
To prevent the decrease in grain boundary strength as a result of suppressing precipitation at grain boundaries. According to the present invention based on the above findings (a) to (d), the above problems can be solved by the following.

In other words, the PC steel rod of the present invention has a weight percentage of C: 0.20 to 0.40%, Si: 0.1 to 0.3%,
Mn: 0.2-0.5%, P: <0.02%, S: <
0.02%, V: 0.03 to 0.2% as a basic component, and Ti: 0.01 to 0.05%, A if necessary.
1: 0.01 to 0.05%, B: 0.001 to 0.00
It is a PC steel rod having excellent delayed fracture characteristics of the weld, which is characterized in that 5%, of which two or more kinds are contained and the balance is composed of iron and inevitable impurities.

In addition, the PC steel rod of the present invention has C:
0.20 to 0.40%, Si: 0.1 to 0.3%, M
n: 0.2 to 0.5%, P: <0.02%, S: <0.
02%, V: 0.03-0.2%, Nb: 0.02-
0.05% as a basic component, and if necessary, further Ti:
0.01-0.05%, Al: 0.01-0.05%,
B: 0.001 to 0.005%, and a PC steel rod having excellent delayed fracture characteristics of the welded portion, characterized by containing two or more kinds and the balance consisting of iron and unavoidable impurities.

[0019]

The reason for limiting the components of the present invention will be described below in terms of weight% (hereinafter simply referred to as%). C is an element necessary to enhance hardenability and strength, and 0.2% or more is necessary to secure a strength level of 1420 N / mm ² as a PC steel rod. Further, as the content of C is increased, the effect of improving the strength is obtained, but one of the main objects of the present invention is to deteriorate the delayed fracture resistance of the spot welded portion and to increase the content of C. Since the weldability is deteriorated and embrittlement of the spot weld is observed, it is particularly necessary to suppress the content to 0.4% or less.

Si is effective for delayed fracture properties and relaxation properties of the base material, and the higher the content, the better the properties. However, as the Si content increases, the softening layer hardness of the heat-affected zone of the spot weld becomes higher, and the delayed fracture property of the spot weld deteriorates. Therefore, in order to improve the delayed fracture property, the content must be at least 0.3% or less. However, when Si is less than 0.1%, the delayed fracture characteristics of the spot welded portion are improved, but the yield ratio (yield strength / tensile strength) is lowered and the stress relaxation during tensioning as a PC steel bar is high, and the relaxation value 0.1% or more is necessary to bring about a large decrease.

Mn is also an element effective in increasing the strength after quenching and tempering, and the higher the content, the higher the strength obtained. If the content is too small, it is necessary to lower the tempering temperature and sufficient toughness cannot be obtained. As a result, the delayed fracture characteristic also tends to deteriorate, so that at least 0.2% or more is necessary. In the spot-welded portion, the higher the Mn content, the higher the hardness of the softened layer in the heat-affected zone, the higher the susceptibility to delayed fracture, and the worse the delayed fracture characteristics, so the upper limit was made 0.5%.

P is an element which is unavoidable as an impurity element, but is set to 0.02% or less in order to segregate at the grain boundaries and deteriorate the delayed fracture characteristics.

Similar to P, S is an element that cannot be avoided as an impurity element, and is set to 0.02% or less in order to segregate at grain boundaries and deteriorate the delayed fracture characteristics.

V is an important element in the present invention. V
Carbonitride precipitates, makes γ grains finer, and also acts as a trap site for hydrogen that has penetrated into the steel, and improves the delayed fracture property by making hydrogen harmless. Furthermore, the precipitates suppress the movement of dislocations by the pinning effect and improve the relaxation characteristics. In order to exert this effect, a content of 0.003% or more is necessary, and 0.003% was made the lower limit. However, the addition of a large amount saturates the effect and is economically disadvantageous, so the upper limit was made 0.2%.

Nb also precipitates carbonitrides to make γ grains finer,
Improves delayed fracture characteristics. Further, like V, the movement of dislocations is suppressed by the pinning action, and relaxation characteristics are improved. To achieve this effect, at least 0.0
A content of 2% or more is necessary, and 0.02% is the lower limit. However, even if a large amount is blended, the effect is saturated and it is economically disadvantageous, so the upper limit was made 0.05%.

Ti fixes N in the steel and forms a nitride to suppress the reduction of the hardenability improving effect of B due to the precipitation of BN, the improvement of the delayed fracture property due to the refinement of γ grains, and the relaxation due to the precipitate. There is an improvement in characteristics. To obtain this effect, at least 0.01% or more is required. However, if the amount of addition is too large, a large amount of coarse TiN is deposited and the characteristics are deteriorated, which is economically disadvantageous, so the upper limit was made 0.05%.

B is an element that enhances the hardenability by a slight addition, but when N is precipitated as BN, the effect of improving the hardenability of B cannot be obtained. Therefore, Al, Ti
It is necessary to preliminarily fix N in the steel by compounding with the above-mentioned nitride forming element. Further, B preferentially segregates at the grain boundaries, and also has a grain boundary cleaning effect of suppressing grain boundary segregation of P, S, Mn and the like. For this reason, at least 0.001% or more is required to exert the effect of B.
Further, when B is contained in a large amount, Fe23B is precipitated, the delayed fracture characteristics are deteriorated, and the effect of B is saturated, so 0.005% is made the upper limit.

[0028] PC steel bar of the present invention is heat-treated by a tensile strength of 1420N / mm ² or more, the yield strength 1325N / mm ²
As described above, the elongation is adjusted so as to have mechanical properties of 5% or more. In the heat treatment, the steel material is heated to the Ac ₃ transformation point or higher, solution-quenched and rapidly cooled from the austenite state to the Ms point or lower, and quenched to completely obtain a martensite structure, and then tempered to secure toughness. The tempering temperature varies depending on the composition and the target strength, but in order to secure sufficient toughness and to maintain the high yield strength, the tempering temperature is adjusted by heating to 350 ° C. or higher.

At this time, by rapidly heating at the time of quenching, the γ grains become finer, high delayed fracture characteristics can be obtained, re-dissolution of precipitates can be suppressed, and precipitates can be finely distributed, which is effective. Can be used for. Cementite is finely divided by performing rapid heating even during tempering,
A large amount is precipitated in martensite lath to suppress grain boundary embrittlement and improve delayed fracture characteristics.

As described above, in the present invention, Si and Mn are reduced without adding expensive Ni, the softening layer hardness of the welded portion is kept low, the relaxation characteristics are improved by the precipitation of V and Nb, and the hydrogen PC steel with relaxation characteristics equivalent to or better than conventional steel by acting as a trap site to detoxify hydrogen and finely disperse cementite in martensite lath, and excellent delayed fracture characteristics of spot welds. You get a stick.

[0031]

Embodiments of the present invention will be described below. It is needless to say that these examples are examples showing the effect of the present invention and do not limit the technical scope of the present invention.

[0032]

EXAMPLES The inventor of the present invention has steel composition No. 1 having the composition shown in Table 1. 1 to No. 19, comparative steel No. 20-No. 29
Was hot-rolled into a round bar having a diameter of 8 mm as a test steel, descaled, drawn into a deformed round bar having a diameter of 7.4 mm, and then subjected to quenching and tempering treatment by high-frequency heating. Here, quenching is performed at 880 to 920 ° C., and tempering is performed at 350 to 45 so that the tensile strength is 1420 N / mm ² or more.
The temperature was controlled in the range of 0 ° C. It is obvious that the heat treatment means is not limited to high frequency heating.

[Table 1]

The heat-treated steel material was subjected to S with a diameter of 3.2 mm.
Delayed fracture test at the welded part with WRM8 mild steel wire rod crossed at right angles as hoop bars, pressed by a pressure of 410N and applied with a welding current of 2500A for 2 cycles at 0.05S for spot welding. And a relaxation test was performed.

Here, in the delayed fracture test, as outlined in FIG. 1, a constant stress 6 of 70% of the standard tensile strength was added, and 20% NH ₄ SCN heated to 50 ° C. in the container 3 was added. The PC steel rod 1 including the welded portion 4 was dipped in the solution 2 and the breaking time was evaluated. The container 3 is a double jacket, and hot water at 50 ° C. flows from the lower right to the upper right in the figure in the gap between the outer and inner jackets.

In the weldability test, the PC steel bar after welding was pulled, and the case where the strength and the elongation satisfied the standard of JIS G3109 was marked with ◯.

In the tensile test of the base material, the one after heat treatment was subjected to a tensile test at a rating distance of 60 mm.

The relaxation test was carried out at 75 ° C. in consideration of steam curing. At this time, the standard tensile strength of 70
% Stress was added, and the mixture was kept at 75 ° C. for 5 hours, then allowed to cool in the furnace, the load change amount after 23 hours was measured, and the ratio with the initial load was evaluated.

As shown in Table 2, all of the steels of the present invention have mechanical properties and weldability comparable to those of conventional steels, and satisfy the properties as a PC steel bar.
It can be seen that the delayed fracture rupture time of the spot welded portion was 30 hours or more, and the relaxation characteristics were also improved as compared with the conventional steel.

[Table 2]

[0039]

As described above, according to the present invention, 14
It has a tensile strength of 20 N / mm ² or more, excellent delayed fracture properties of the welded part without deterioration of properties even after welding, other relaxation properties and mechanical properties similar to those of conventional steel, and This is an industrially very useful invention because a low-cost PC steel rod can be obtained and can be sufficiently used even in an environment where delayed fracture is likely to occur.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a delayed fracture test method.

[Explanation of symbols]

1 ... PC steel rod for testing 2 ... 20% NH ₄ SCN solution heated to 50 ° C. 3 ... Container for circulating hot water at 50 ° C. 4 ... Spot welding part of PC steel rod 5 ... Packing for preventing leakage of solution 6 ... Loading direction

Claims (2)

[Claims] 1. By weight%, C: 0.20 to 0.40%, Si: 0.05 to 0.3.
%, Mn: 0.2 to 0.5%, P: 0.02% or less S: 0.02% or less V: 0.03 to 0.2%, as a basic component, and further Ti: if necessary. 0.01-0.05%, Al: 0.01-
0.05%, B: 0.001 to 0.005%, and a balance of two or more kinds, and the balance consisting of iron and unavoidable impurities. 2. By weight%, C: 0.20 to 0.40%, Si: 0.1 to 0.3%,
Mn: 0.2 to 0.5%, P: 0.02% or less S: 0.02% or less V: 0.03 to 0.2%, Nb: 0.02 to 0.05
% As a basic component, and if necessary, further Ti: 0.01
.About.0.05%, Al: 0.01 to 0.05%, B: 0.
PC steel rod with excellent delayed fracture characteristics of the weld, characterized by containing two or more of 001 to 0.005% and the balance consisting of iron and inevitable impurities.