JPH05331545A - Manufacture of pc steel bar - Google Patents

Abstract

PURPOSE:To manufacture the objective PC steel bar of stable quality by subjecting a slab having a specified componental compsn. to hot rolling at a specified temp., subjecting it to water cooling at a rate under a specified condition and tempering it. CONSTITUTION:The componental compsn. of steel is constituted of 0.2 to 0.4% C, 0.5 to 2% Si, 0.6 to 2% Mn and 0.5 to 2% Cr, and the balance Fe. The slab thereof is subjected to hot rolling in such a manner that the rolling finishing temp. lies in the range of the Ar1 transformation point + 30 deg.C to 950 deg.C. Immediately, this rolled stock is water-cooled at >=40 deg.C/sec rate so as to regulate the surface temp. to 200 deg.C. After that, it is tempered at a preliminarily obtd. temp. to a prescribed strength. In this way, the objective PC steel bar excellent in size, strength and selectivity as well as free from hardening cracks can be manufactured without largely adding elements required for improving hardenability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a PC steel rod used for the purpose of improving the strength of concrete products.

[0002]

2. Description of the Related Art In recent years, in order to improve the strength of concrete structures, PC steel rods have been increasingly used in place of or in addition to conventional reinforcing bars. For PC steel rods, the concrete structure is cured and solidified in the hollow state, and then PC steel rods are passed through this hollow portion, both ends are fixed while tension is applied to this, and the residual stress of the tension is used to make the entire concrete product. It tries to reinforce the weakness against the tensile force of concrete by giving compressive force.

The properties required for such a PC steel bar are high tensile strength and high yield point,
It has a high toughness and is less likely to undergo stress relaxation.

Basically, various methods shown in the following (1) to (5) are known as methods for producing a PC steel rod. (1) After rolling, after heating by high frequency or heating furnace,
Hardening and tempering is a method of obtaining specified characteristics. (2) A method in which a fine pearlite structure is formed by cooling after rolling, and then stretching and bluing are performed to obtain predetermined characteristics. (3) A method of obtaining predetermined characteristics as a bainite structure by cooling after rolling. (4) A method in which a steel having a predetermined composition is rolled, and then air-cooled to obtain a martensite structure, and then tempering is performed to obtain predetermined properties. (For example, Japanese Examined Patent Publication No. 62-37694) (5) A method in which after rolling, water cooling is performed at a predetermined cooling rate, and then self-tempering is performed to obtain predetermined characteristics.

[0005]

However, in these manufacturing methods, there are some problems as described below. In the method (1), since tempering is performed, there is little variation in strength, and by changing the tempering temperature, products of various desired strengths can be stably obtained. However, since the rolled material is heated again for quenching, and then the material is heated again for tempering, the process becomes long, which is disadvantageous in terms of energy.

In the method (2), stretching and bluing must be performed, which requires a long process. Also, it is not possible to obtain products of various strengths with the same components. Further, the size selectivity is poor, and it is necessary to change the composition of each steel bar in order to obtain products of various sizes with the same strength.

According to the method (3), since high strength is obtained and the yield point is high, post heat treatment such as tempering is unnecessary,
Although the process is comparatively simplified, since it is a method of forming a bainite structure by cooling after rolling, even if the cooling conditions fluctuate even a little, the product strength greatly varies and the quality stability is lacking.

The method (4) has a small variation in strength and is excellent in size selectivity, but requires a tempering step to reduce the variation in strength, and it is also likely to be cooled by air. From the viewpoint of ensuring sufficient hardenability even at low speed cooling, it is necessary to add a relatively large amount of hardenability improving element.

According to the method (5), high strength is obtained, and subsequent steps such as tempering are not required, and the steps are relatively simplified. However, the structure has a tempered martensite structure and an internal structure It has a fine pearlite bainite structure,
The interior has an incompletely quenched structure. Therefore, the relaxation value is not stable.

The present invention has been made in order to solve the above-mentioned problems. After controlling the rolling of a steel slab whose composition has been adjusted, it is accelerated cooled and then tempered, so that the elements necessary for improving the hardenability can be obtained. An object of the present invention is to provide a method for producing a PC steel rod having excellent size and strength selectivity and stable quality without adding much.

[0011]

Means for Solving the Problems The gist of the present invention is C: 0.20.
~ 0.4%, Si: 0.50 ~ 2.00%, Mn: 0.60 ~ 2.00%, Cr:
A steel slab containing 0.50 to 2.00% and the balance Fe and unavoidable impurities was rolled at a finish temperature Ar ₁ transformation point + 30 ° C
Perform hot rolling in the range of 950 ℃, and immediately water-cool to a temperature of 200 ℃ or less at a cooling rate of 40 ℃ / second or more,
After that, it is a method for manufacturing a PC steel rod, which is tempered to a predetermined strength at a temperature that is obtained in advance.

[0012]

The reason for limiting the range of chemical components and the manufacturing conditions in the present invention will be described below. C is an element necessary for obtaining quenching strength, and 0.20% or more must be added to obtain the tensile strength required for a PC steel bar, but if too much C is added, quench cracking occurs. Therefore, the amount of C added is in the range of 0.20 to 0.4%.

Si is an element effective in improving hardenability and resistance to temper softening. Addition of less than 0.50% cannot expect such an effect, and addition of more than 2.00% significantly deteriorates toughness. To do. Therefore, the amount of Si added is
The range is 0.50 to 2.00%.

Mn is indispensable as an element for improving hardenability, and in order to exert its effect, it is necessary to add 0.60% or more. Further, Mn has a function of stabilizing the austenite structure, and if it is added in an excessively large amount, retained austenite is generated after quenching and sufficient hardness cannot be obtained.
Therefore, the addition amount of Mn is set to the range of 0.60 to 2.00%.

Cr is an element for improving hardenability, and it is necessary to add 0.50% or more in order to exert its effect.
Even if added too much, the effect becomes saturated, and the upper limit was set to 2.00% in consideration of economic efficiency. Therefore, the addition amount of Cr is set to the range of 0.50 to 2.00%.

The finish temperature of hot rolling needs to be cooled from a stable austenite region in order to perform sufficient quenching, so the lower limit is set to Ar ₁ transformation point + 30 ° C. Although hardenability improves when the finishing temperature becomes high, the toughness deteriorates when the temperature becomes too high, so the upper limit is set to 9
It was set to 50 ° C.

In the cooling method after rolling, it is necessary to cool at a cooling rate of 40 ° C./sec or more in order to obtain a sufficient quenching effect by cooling. Therefore, the cooling means is water cooling. Further, the cooling end temperature was set to 200 ° C. or lower at the surface temperature in order to obtain a uniform martensite structure up to the central part.

After that, the tempering is performed at the tempering temperature for obtaining the predetermined strength which is obtained in advance.

[0019]

EXAMPLES Examples of the present invention will be described below, but the following examples do not limit the present invention. The test material is a steel piece with the chemical composition shown in Table 1, and the diameter under the manufacturing conditions shown in Table 2.
It is a finished PC steel rod of 23 to 32 mm. Direct quenching was performed immediately after hot rolling. The presence or absence of quench cracking was investigated for these PC steel rods, and the results are shown in Table 2. Then, tempering was performed, and a tensile test, a relaxation test, and a structure observation were performed. The results are shown in Table 3 together with the JIS standard. In addition, No. 3 was aimed at self-tempering by raising the cooling end temperature and it was not tempered.

The method of the present invention, as shown in Tables 2 and 3,
All of them satisfy the JIS standard without any quench cracks.

As shown in Table 2, quench cracks were observed in Nos. 4 and 5 using steel types A and B having a C content of more than 0.40%.

No. 3 is intended for self-tempering, but the quenching structure cannot be obtained because the cooling rate is slow and the relaxation value is poor.

No. 5 using steel type C has a low Cr content, so a completely quenched structure cannot be obtained, and the relaxation value is poor.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

According to the present invention, C: 0.20-0.4%, Si: 0.50-
A steel slab containing 2.00%, Mn: 0.60 to 2.00%, Cr: 0.50 to 2.00% and the balance Fe and unavoidable impurities is hot-rolled at a rolling finish temperature Ar ₁ transformation point + 30 ° C to 950 ° C. The method for producing a PC steel rod is to immediately perform water cooling at a cooling rate of 40 ° C / sec or more to a temperature of 200 ° C or less, and then temper it to a predetermined strength at a temperature that is obtained in advance. According to the method of the present invention, it is possible to produce a PC steel bar having excellent quality and stability with excellent selectivity in size and strength without adding many elements necessary for improving hardenability and having stable quality. ..

Claims (1)

[Claims] 1. C: 0.20 to 0.4%, Si: 0.50 to 2.00%, M
n: 0.60 to 2.00%, Cr: 0.50 to 2.00%, with a balance of Fe and inevitable impurities, a steel slab with a rolling finish temperature
Hot-rolling is performed in the range of Ar ₁ transformation point + 30 ° C to 950 ° C, and immediately water-cooled to a temperature of 200 ° C or less at a cooling rate of 40 ° C / sec or more, and then at a predetermined temperature. A method for manufacturing a PC steel rod, which comprises tempering to the strength of.