JPH0813082A - Steel bar or steel wire for prestressed concrete, having high strength and high weldability, and production thereof - Google Patents

Abstract

PURPOSE:To produce a steel bar or a steel wire for prestressed concrete having high strength and high weldability and excellent in mechanical properties by subjecting a steel bar or a steel wire, having a specific composition containing small amounts of Mo, etc., to hardening and tempering under respectively specified temp. conditions. CONSTITUTION:A carbon steel, which has a composition containing, by weight, 0.15>=0.40% C, 0.3-2.0% Si, 0.4-1.6% Mn, and 0.08-0.35% Mo or further containing 0.01-0.05% Ti and 0.0005-0.005% B and satisfying Si+4Mo>=1%, is used. After a pattern for increasing the adhesion of concrete is applied, if necessary, to the surface of a bar or wire of the above carbon steel, this bar or wire is heated, within 20sec, to 850-1050 deg.C and hardened by quenching by means of water quenching. Successively, this bar or wire is heated rapidly within 25sec to a tempering temp. of 400-550 deg.C and subjected, after the application of <=3% bending strain if necessary, to rapid cooling to undergo heat treatment. By this method, the steel bar or the steel wire, having a fine structure of austenite crystalline particle size No. >=9, increased in strength, reduced in high temp. relaxation value, and excellent in weldability, 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 steel rod and a wire used for prestressed concrete, and provides those having excellent mechanical properties, high strength, low high temperature relaxation value and excellent weldability.

[0002]

^2. Description of the Related Art Steel rods and steel wires (hereinafter referred to as PC steel rods) used for prestressed concrete are required to have high tensile strength because they are subjected to high pretension, and JIS standards of 1420 N / mm ² or more are required. It is regulated and various characteristics are required together. In other words, in order to produce concrete piles and concrete columns with high efficiency, autoclave curing is often carried out to heat concrete to several hundreds of degrees to improve the concrete strength. Causes relaxation that reduces. Therefore, there is a demand for a PC steel rod having a low high temperature relaxation value.
Also, a material having a large uniform elongation, which is one of the indicators of toughness, is required.

Further, in some of the fields of use of PC steel rods, weldability may be required. For example, in the production of concrete piles or concrete columns, a spiral reinforcing bar is wound around a plurality of PC steel bars held in parallel, the intersections with the spiral reinforcing bars are overlapped, and spot welding is performed to manufacture a reinforcing bar basket. Therefore, weldability such as sufficient spot welding strength is required.

By the way, in the method for producing a high strength PC steel bar, an excellent material having a well-balanced strength and toughness can be obtained. Therefore, a method of obtaining a predetermined characteristic by a heat treatment process of quenching and tempering is often used today. . In this method, a steel rod or wire made of a material produced by hot rolling is cold drawn to a predetermined size, and a spiral pattern is formed to improve adhesion of concrete, and a quenching and tempering process is performed. It is a thing. Quenching and tempering are usually performed by continuously feeding a steel rod or wire.

In order to improve relaxation characteristics at a high temperature of 100 ° C. or higher, the amount of Si, which was conventionally 0.2 to 0.3% (wt%, the same applies hereinafter), was used as a component of the material of PC steel rods. It can be achieved by adding 2.0%, and up to about 2.3%.
No. 4 is disclosed. Further, in the present invention, in the step of continuous quenching and tempering described above, by applying a minute bending strain in a state of being heated to the tempering temperature and then rapidly cooling, further high temperature relaxation characteristics are obtained while maintaining uniform elongation. Techniques for improvement are disclosed.

On the other hand, regarding weldability, as described above,
The electric resistance of the PC steel rod to which a large amount of is added becomes extremely high, and there is a problem that sufficient joining strength cannot be obtained unless the set value of the welding voltage in spot welding is increased by about 50%. Manufacturers of rebar cages handle both conventional steel rods with a low Si content and steel rods with a high Si content, and the voltage must be set frequently, so the work efficiency decreases. There was a problem. On the other hand, the present inventors previously disclosed in JP-A-3-151445 and JP-A-3-285045 that the addition of Mo has the effect of improving weldability.

[0007]

The present invention is a further improvement of the above-mentioned invention, in which a high-strength PC steel bar having a tensile strength of 1420 N / mm ² or more has excellent high-temperature relaxation characteristics and high toughness without impairing the weldability. Provide things. That is, Mo is a relatively expensive element, and the present invention exerts an effect not only on the improvement of weldability but also on the improvement of other mechanical properties with a minimum amount of addition of Mo, and has a higher performance than the above invention. The purpose is to manufacture PC steel rods.

[0008]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems, in which C: 0.15 to 0.40%, S
i: 0.3 to 2.0%, Mn: 0.4 to 1.6%,
Mo: 0.08 to 0.35%, Ti: 0.01 to 0.05% and B: 0.0 if necessary.
005 to 0.005% is contained, and Si + 4Mo satisfies the relation of 1.0% or more with respect to Si amount and Mo amount,
The balance is Fe and inevitable impurities, and a high strength and high weldability PC steel bar characterized by having a fine structure with an austenite grain size number of 9.0 or more.

As a manufacturing method of this, C: 0.15
To 0.40%, Si: 0.3 to 2.0%, Mn:
0.4 to 1.6%, Mo: 0.08 to 0.35%, Ti: 0.01 to 0.0 if necessary
5% and B: 0.0005 to 0.005%, and Si + 4Mo is 1.0 in terms of Si amount and Mo amount.
% Or more, the balance is Fe and unavoidable impurities of steel rod or wire within 850 to 10 within 20 seconds.
By heating to a quenching temperature of 50 ° C and then rapidly cooling, and further heating to a predetermined tempering temperature within 25 seconds and then rapidly cooling,
A method for producing a PC steel rod, which has a fine structure with an austenite grain size number of 9.0 or more. Further, it is also characterized in that if necessary, a bending strain of 3% or less is applied in the state of being heated to the above-mentioned tempering temperature, followed by quenching.

[0010]

In the present invention, the addition of Mo to the PC steel bar reduces the damage to the weldability of Si added for the purpose of improving the high temperature relaxation characteristics, and is satisfactory even without increasing the welding voltage. It is designed to be weldable. Furthermore, by finding the optimum composition range in the relationship between Mo and Si, and maximizing the grain refining effect of Mo, which is a carbide forming element, and making austenite fine grains with a grain size number of 9.0 or more. A PC steel bar having high strength and excellent toughness can be obtained with a minimum amount of added elements.

The inventors of the present invention investigated the influence of the additive components on the PC steel bar manufactured by subjecting the material after cold drawing to the generally performed continuous quenching and tempering process. Fig. 1 shows various changes in the amounts of Si and Mo.
It is a graph of the high temperature relaxation value in 180 degreeC with and without addition of B, A, B, C, D
Have no Mo additions, 0.1%, 0.2%,
0.3% is shown. In addition, circles etc. are filled with Ti, B is not added, white ones are Ti,
B is added. From this, it is clear that Si has an effect of improving the high temperature relaxation characteristics, but the effect tends to be saturated when it exceeds 1.5%. on the other hand,
It can be seen that the addition of Mo lowers the high temperature relaxation value and has the effect of improving it. On the other hand, T
It can be seen that the presence or absence of addition of i and B does not affect the high temperature relaxation value.

Further, the present inventors conducted a spot welding test on each of the above PC steel rods and investigated the joint strength of the welded portion. That is, a PC steel rod having a diameter of 9.2 mm and a diameter of 3.
The intersection with the 2mm spiral muscle is a normal P with a small amount of Si.
Welded at the voltage of 1.45V used for welding C steel rod,
Welding current is measured and the joint strength of the welded part is determined by JIS
It was tested according to the welding point shear strength test method of G3551 "Welded Wire Mesh".

FIG. 2 shows the result, and Mo
When the Si content exceeds 1% in the additive-free PC steel rod, the welding current gradually becomes difficult to flow in the welded portion, and the joint strength also decreases accordingly. On the other hand, when Mo is added, even if the Si amount exceeds 1.5%, the welding current does not decrease so much and the joint strength does not decrease. It is also understood that the effect of adding Ti and B is also effective.

The present invention has been made with further improvements based on the above experimental facts. First, the chemical components will be described below. C is 0.15% to 0.40%
And If C is less than 0.15%, the required strength as a PC steel rod cannot be obtained, and if it exceeds 0.40%, the toughness decreases and the weldability also deteriorates.

Si is set to 0.3% to 2.0%. Si
If less than 0.3%, even if Mo is added, the high-temperature relaxation characteristics targeted by the present invention cannot be obtained. S
If the amount of i exceeds 2.0%, the effect of improving relaxation characteristics is saturated, and toughness and weldability deteriorate.

Mn is 0.4% to 1.6%. Mn
Improves the hardenability and contributes to the improvement of toughness such as strength and uniform elongation property. At 0.4%, the effect is insufficient,
If it exceeds 1.6%, the amount of retained austenite will increase during quenching and the strength will decrease. Therefore, the above range is set.

Mo is set to 0.08% to 0.35. M
o improves the high temperature relaxation characteristics together with Si and prevents the weldability from deteriorating when the amount of Si is large. Further, it improves the hardenability and refines the crystal grains to contribute to the improvement of strength and toughness. If it is less than 0.08%, the effect cannot be expected, and if it is 0.35% or more, the effect is saturated and it is disadvantageous in cost.

Further, regarding the amount of Si and the amount of Mo, Si + 4
It is necessary to satisfy the relationship that Mo is 1.0% or more. Si + 4M to obtain the specified high temperature relaxation characteristics
It is necessary to set the value of o to 1.0% or more, preferably 1.4% or more. By satisfying the individual values of the Si amount and the Mo amount, and further satisfying this condition, the weldability and other The characteristics can also be improved.

Further, if necessary, Ti and B can be added simultaneously to improve weldability and delayed fracture characteristics. If Ti is less than 0.01%, its effect is small, and if it exceeds 0.05%, the cleanliness of the material is impaired, which is not preferable. If B is less than 0.0005%, its effect is small, and if it exceeds 0.005%, the effect is saturated, and if it is more, it causes cracking during hot working in the production of the raw material.

The PC steel rod of the present invention has a fine structure with an austenite grain size number of 9.0 or more. The grain size number is in JIS G 0551, and the number n of crystal grains per mm ² is n = 2 ^{N + 3} where N is the grain size number. This austenite grain size is that of the austenite phase under the quenching and heating conditions in the process of the present invention, and by refining it, an extremely fine martensite structure can be obtained by quenching. A PC steel bar having high strength and high toughness can be obtained with a smaller amount of added elements.

In the method for producing a PC steel rod of the present invention, a predetermined diameter is obtained by a method such as cold drawing, and if necessary, a pattern is formed on the surface for the purpose of improving concrete adhesion, followed by quenching and tempering. Quenching heating is 850-10 within 20 seconds
Heat to a quenching temperature of 50 ° C. Such rapid heating of the material of the present invention increases the number of nucleation sites of recrystallization, and austenite can be made into fine particles having a grain size number of 9.0 or more. As means for rapid heating, high frequency induction heating or direct current heating is used. When the time for heating to the quenching temperature is longer than 20 seconds, a small number of recrystallized nuclei generated during the temperature increase grows, and as a result, fine crystal grains aimed at by the present invention cannot be obtained.

The holding time of the steel bar at the quenching temperature is set to the minimum time for obtaining a uniform austenite structure. In the material of the present invention, since the carbide of Mo is precipitated, the austenite crystal grains do not suddenly increase in size, but the austenite crystal grains are not held for a long time, preferably within 20 seconds. After that, quenching is performed, but rapid cooling of 100 ° C./second or more is performed to obtain a complete martensite structure.

Subsequent to this, tempering is carried out, but rapid tempering within 25 seconds to the tempering temperature is desirable for tempering to obtain a uniform tempered martensite structure. The tempering temperature is a temperature at which a predetermined tensile strength, for example, 1420 N / mm ² or more can be obtained, and is usually in the range of 400 ° C to 550 ° C. Cooling after the tempering is preferably maintained at the tempering temperature for, for example, about 10 seconds, and thereafter cooled relatively quickly so that the strength does not decrease due to the coarsening of the structure, which is preferable.

Further, as a particularly preferred manufacturing method of the present invention, there is a method of applying a bending strain of 3% or less to the tempering temperature during heating and then cooling. This can further improve the high temperature relaxation characteristics. The method of applying bending strain may be such that a device for heating a continuously fed steel rod to a tempering temperature, for example, an induction heating device is followed by a roll row for repeated bending similar to that used in a straightening device. . Although dislocations introduced into the material are entangled with each other by applying such a minute bending strain, movement of dislocations is hindered by precipitates such as carbides and nitrides, and strength and relaxation characteristics are improved. The amount of bending strain is 3
% Or less, preferably 2% or less is appropriate, and if it is too much, the toughness deteriorates and the characteristic values such as uniform elongation deteriorate. Further, if strain is introduced by repeated bending and the tempering temperature is kept for a long time for a long time, the precipitates become coarse and the effect thereof decreases, so that the cooling should be forcibly performed within 10 seconds, for example.

[0025]

[Examples] The materials of the hot-rolled steel bars having the chemical compositions shown in Table 1 were cold drawn to have a diameter of 9.2 mm. This was heated in an induction heating furnace at 950 ° C. for 7 seconds, kept at this temperature for 6 seconds, and then cooled with water. This material was heated to a tempering temperature for 5 seconds by induction heating, held for 10 seconds, and then cooled with water. As another step, after heating to the tempering temperature similarly, 0.9
% Bending strain was applied by the roll row, and after that, after 5 seconds at the tempering temperature, a water-cooled material was also prepared. As a further step, as for heating for quenching, it took 25 seconds to heat up to 950 ° C., and some materials were also used. The tempering temperature was set for each material so as to satisfy the standard value of tensile strength of 1420 N / mm ² or more.

[0026]

[Table 1]

The test results of austenite grain size, mechanical properties, high temperature relaxation test, weld joint strength, etc. are shown in Tables 2 and 3. According to this, it can be seen that the mechanical properties of the present invention are equal to or higher than those of the conventional products. Particularly, the material of the present invention having a grain size number of 9.0 or more has a large uniform elongation and is excellent in each characteristic.

[0028]

[Table 2]

[0029]

[Table 3]

The relaxation test shows that the yield stress is 80% of the standard value 1275 N / mm ² , which is 1020 N / mm ^2.
The stress reduction rate is shown when the stress is applied and the holding interval is maintained as it is. The test was performed at room temperature and a high temperature of 180 ° C. The high temperature relaxation was measured by heating up to 180 ° C. in 4 hours, holding for 3 hours, and cooling in the furnace to measure 23 hours after the load. S as component range
It can be seen that the relaxation property is excellent when i + 4Mo satisfies the condition of the present invention of 1.0% or more.
It can also be seen that the process of applying a slight bending strain at the tempering temperature significantly improves the high temperature relaxation characteristics even with materials having the same components.

The strength test of the welded joint was carried out by the method described above, and in particular, as shown in the graph above, Si was tested.
It can be seen that in a material containing a large amount of Mo, the effect of improving strength by Mo is great. Further, the addition of Ti and B reduces the variation in strength and improves the characteristics as a whole.

[0032]

INDUSTRIAL APPLICABILITY The PC steel rod of the present invention has excellent high-temperature relaxation characteristics, which is a problem in autoclave curing during mass production of prestressed concrete material, and special welding conditions for spot welding performed when producing a reinforcing steel basket. Sufficient joint strength can be obtained without setting. Further, even a material of the highest strength class in the standard value of PC steel bar is excellent in toughness, and it is possible to provide a PC steel bar having extremely excellent characteristics.

[Brief description of drawings]

FIG. 1 is a graph showing an example of the relationship between the contents of Si and Mo and high temperature relaxation.

FIG. 2 is a graph showing an example of the relationship between the Si and Mo contents, the welding current value, and the weld joint strength.

Claims (5)

[Claims] 1. C: 0.15 to 0.40 in% by weight.
%, Si: 0.3 to 2.0%, Mn: 0.4 to 1.
6%, Mo: 0.08 to 0.35%, and S
Regarding the amount of i and the amount of Mo, Si + 4Mo satisfies the relation of 1.0% or more, and the balance is Fe and inevitable impurities,
A steel rod or wire for high-strength and high-weldability prestressed concrete, which has a fine structure with an austenite grain size number of 9.0 or more. 2. The steel rod or wire further comprises Ti: 0.01.
To 0.05% and B: 0.0005 to 0.005
%, The steel rod or steel wire for high-strength and high-weldability prestressed concrete according to claim 1. 3. C: 0.15 to 0.40 in% by weight.
%, Si: 0.3 to 2.0%, Mn: 0.4 to 1.
6%, Mo: 0.08 to 0.35%, and S
Regarding the amount of i and the amount of Mo, Si + 4Mo satisfies the relation of 1.0% or more, the balance is Fe and the steel rod or steel wire which is an unavoidable impurity is heated within 20 seconds to a quenching temperature of 850 to 1050 ° C. and then rapidly cooled. Of the high-strength and high-weldability prestressed concrete steel bar or steel wire characterized by having a fine structure with an austenite grain size number of 9.0 or more by heating to a tempering temperature within 25 seconds and then rapidly cooling. Production method. 4. The steel rod or wire further comprises Ti: 0.01.
To 0.05% and B: 0.0005 to 0.005
% Is contained, The manufacturing method of the steel rod or steel wire for high strength and high weldability prestressed concrete of Claim 3 characterized by the above-mentioned. 5. 3% when heated to the tempering temperature
The method for producing a steel rod or steel wire for high-strength and high-weldability prestressed concrete according to claim 3 or 4, characterized by quenching after applying the following bending strain.