JPH02217422A - Production of pc steel bar - Google Patents

Abstract

PURPOSE:To attain superior hardenability without adding large amounts of hardenability-improving elements by subjecting a steel containing specific amounts of C, Si, and Mn to hot rolling at a specific temp., to water cooling under specific conditions, and then to self-tempering. CONSTITUTION:A steel having a composition containing, by weight, 0.2-0.6% C, 0.15-0.45% Si, and 0.3-2% Mn is hot-rolled so that finishing temp. is regulated to a temp. between (Ar1 point + 30 deg.C) and 950 deg.C. Subsequently, water cooling is applied at >=40 deg.C/sec cooling rate until surface temp. reaches a temp. between the Ms point and (Ms point -200 deg.C). Then, the above steel is self-tempered by means of its internally retained heat so as to be formed into a PC steel bar. Further, <=0.4% Cr and/or <=0.005% B is incorporated to the above steel composition, if necessary. By this method, the process can be simplified, and PC steel bars on various strength levels can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an improvement in the method for manufacturing prestressed steel bars used for the purpose of improving the strength of concrete products.

[Prior Art] In recent years, for the purpose of further improving the strength of concrete structures, PCtI4 bars have been increasingly used instead of or in addition to conventional reinforcing bars. After curing and solidifying a concrete structure in a hollow state, PCt1 is placed in the hollow part.
The idea is to apply tension through four rods and fix both ends, and use the residual stress of the tension to apply compressive force to the entire concrete product, thereby reinforcing the concrete's weakness against tensile force. The properties required for such a PC steel bar are (a) high tensile strength, (b)
) have a high yield point, (C) have high toughness, (d)
For example, stress relaxation is difficult to occur.

The manufacturing method for PC steel bars is basically the following (1) ~
Various methods shown in (IV) are known.

(1) A method in which predetermined properties are obtained by heating with a high frequency or a heating furnace, followed by quenching and tempering.

(n) forming a fine pearlite structure by cooling after rolling;
Stretching and bluing are then performed to obtain desired properties.

(Ill) A method of obtaining predetermined properties as a bainite structure by cooling after rolling.

(IV) A method of obtaining a martensitic structure by air-cooling steel with predetermined components after rolling, and then tempering it to obtain predetermined properties (for example, Japanese Patent Publication No. 37694/1983)
.

[Problems to be Solved by the Invention] However, each of these manufacturing methods has the following drawbacks.

In method N), since tempering is performed, there is little variation in strength, and by changing the tempering temperature, products with various desired strengths can be stably obtained. However, since the rolled material is heated again and hardened, and then heated again and then tempered, the process becomes long and is disadvantageous in terms of energy.

In method (II), stretching and bluing must be performed, resulting in a long process. In addition, changing the strength requires changing the component composition, and conversely speaking, it is not possible to obtain products with various strengths using the same ingredients. Furthermore, size selectivity is poor, and in order to obtain products of various sizes with the same strength, it is necessary to change the composition for each steel bar.

In the method (III'), high strength is obtained and the yield point is high, so post-treatments such as tempering are not required, and the process is relatively simple. Therefore, if the cooling conditions change even slightly, the strength of the product will vary greatly, resulting in a lack of stability in quality.

Although the (rV) method has little variation in strength and is excellent in size selectivity, it requires a tempering process to reduce variation in strength, and low-speed cooling such as air cooling is sufficient. From the viewpoint of ensuring good hardenability, it is necessary to add a relatively large amount of hardenability improving elements.

The present invention has been made to improve such conventional technology, and its purpose is to achieve good hardenability without adding many hardenability-improving elements, simplify the process, and select size. The purpose of the present invention is to provide a method for manufacturing a PC steel bar with excellent properties.

[Means for Solving the Problems] The present invention that achieves the above object is as follows: C: 0.2 to 0.6
Weight %, S f : 0.15 to 0.45 weight %, Mn
:0.3 to zTL content% at finishing temperature (Ar
, point +30°C) to 950°C, followed by water cooling at a cooling rate of 40°C/sec or more until the surface temperature reaches Ms point - (Ms point -200°C), and then self-tempering. This is a method for manufacturing a PC steel bar.

The present invention can ensure sufficient hardenability by adopting the above configuration, but from the viewpoint of further improving the hardenability, Cr: 0.4% by weight or less and B
It is also effective to add one or more hardenability improving elements selected from the group consisting of: 0.005% by weight.

[Function] In the present invention, the reason why the component composition range and manufacturing conditions are limited is as follows.

C: 0.2-0. a l amount % C is an inexpensive element necessary to obtain high strength (quenching hardness), and it has the tensile strength (100 kgf) required for a PC steel bar.
/mm" or more), it is necessary to add 0.2% by weight or more. However, if too much C is added, quench cracking will occur, so the upper limit is set at 0.6% by weight.

Si: 0.15 to 0.45% by weight St is an element necessary as a deoxidizer, and if it is less than 0.15% by weight, deoxidation will be insufficient. On the other hand, even if it is added in an amount exceeding 0.45% by weight, the deoxidizing effect becomes saturated and is meaningless. Therefore, in the present invention, the amount of St added is limited to 0.15 to 0.45% by weight.

Mn: 0.3 to 2% by weight Mn is essential as an inexpensive element for improving hardenability, and in order to exhibit its effect, it is necessary to add 0.3% by weight or more. However, Mn has the effect of stabilizing the austenite structure, and if too much Mn is added, it will contain a large amount of residual austenite after quenching, making it impossible to obtain sufficient hardness (therefore, the upper limit was set at 21% by weight).

As mentioned above, in the present invention, it is effective to add small amounts of Cr and B as hardenability-improving elements in addition to the above-mentioned additive elements, but adding large amounts of these elements is disadvantageous from a cost perspective. It is. Therefore, in the present invention, the upper limits of Cr and B are set at 0.4% by weight and 0.4% by weight, respectively. Further, the PC steel rod of the present invention may contain elements such as P, S, and other unavoidable elements.

Finishing temperature of hot rolling: (Ar, point +30°C) ~ 950
°C In order to perform sufficient hardening, it is necessary to cool from the stable austenite region. Finishing temperature of hot rolling is Ar
If the temperature is lower than +30°C, there is a risk that pro-eutectoid ferrite will occur during cooling, and the hardness will not be sufficiently increased. On the other hand, if the temperature is too high, the toughness deteriorates even if the hardenability improves, so the upper limit was set at 950°C.

Cooling method after rolling: Water cooling at a cooling rate of 40°C/sec. In order to ensure good hardenability without adding excessive hardenability-enhancing elements, it is necessary to cool at a cooling rate of 40°C/sec or more. Further, for this purpose, it is necessary to use water cooling as the cooling means.

Cooling end temperature Two surface temperatures are from Ms point to (Ms point -200°C
Although the steel bar is water-cooled at a cooling rate of 40° C./second or higher until the temperature reaches 40° C., if the cooling temperature is too low, quench cracking will occur in the steel bar. Therefore, in the present invention, cooling is stopped when the surface of the steel bar reaches a predetermined temperature, and thereafter the bar is self-tempered using internal heat.

Therefore, in the present invention, the lower limit of the cooling end temperature is set as (Ms point -20
0°C). On the other hand, the upper limit of the cooling end temperature was determined to be below the Ms point due to the need to generate a martensitic structure. The shape of the PC steel bar during cooling is not limited in any way, but from the viewpoint of uniform cooling and eliminating variations in material strength, it is preferably a straight bar. Moreover, the PCi rod obtained in this way has a tempered martensitic structure on the surface and fine pearlite inside.

It becomes a suitably cooled structure such as bainite.

Hereinafter, the present invention will be explained in more detail with reference to Examples, but the following Examples do not limit the present invention, and the design may be changed in accordance with the purpose of the above and below, for example, the component composition, manufacturing conditions, etc. may be changed as appropriate. Anything that is included in the technical scope of the present invention.

[Example] Using two types of test steels A and B having the compositions shown in Table 1 below, PCffl bars N011 to 6 were manufactured under the manufacturing conditions shown in Table 2 below.
was manufactured, and the presence or absence of quench cracking was investigated.

The Ms point of this shell was determined from the following equation (1), and was 274°C for sample tlIA and 319°C for sample steel B. In addition, the Ar point is approximately 695°C for specimen @A and approximately 69°C for specimen steel B.
It was 0℃.

M s (tl:)=561-474 C(!k)
-33M n (treasure) -17Cr (t)-1
7N i (%F)-21M O(X)... (1
) As is clear from Table 2, the cooling end temperature is low (40t
-Ms point -234°C) In No. 1 Te, quench cracking occurred.

Next, the mechanical properties of the PC steel bars No. 2 to No. 6 without quench cracking were investigated.

For No. 2.4.5, the mechanical properties when tempered at 450°C were also investigated. The result is the third
As shown in the table, Table 3 also shows the JIS standard values for PC steel bars.

From Table 3, the following can be considered. No. 3, which has a low cooling rate during quenching (34°C/sec), cannot be used as a PC steel bar because it does not have sufficient strength and has a high relaxation value, but all other bars have the characteristics of a PCCFI4 bar. I'm fully satisfied. Moreover, PCl4 rods of various sizes and various strength levels have been obtained even though they have the same composition.

[Effects of the Invention] In the present invention, by employing the above configuration, the following effects can be obtained.

■Since the material is rapidly cooled by water cooling, good hardenability can be ensured without adding too many hardenability-improving elements.

■Even with water cooling, cooling is stopped at an appropriate temperature without being completely cooled, and self-tempering is performed thereafter, so quench cracking does not occur.

■Since self-tempering is performed, there is no need for tempering, which simplifies the process and improves toughness. However, as indicated in the implementation order, it is possible to actually perform tempering for strength adjustment, and this may be done as desired.

■ By changing the cooling rate depending on the amount of water and time, it is possible to obtain various strength ranks for the same 11 types in various sizes.

Claims (2)

[Claims] (1) C: 0.2-0.6% by weight, Si: 0.15-0
．． 45% by weight, Mn: 0.3 to 2% by weight, was hot rolled to a finishing temperature of (Ar_1 point + 30°C) to 950°C, and then the surface temperature reached Ms point to (Ms
A PC characterized by being water-cooled at a cooling rate of 40°C/sec or more until the temperature reaches -200°C, and then self-tempering.
Method of manufacturing steel rods. (2) C: 0.2-0.6% by weight, Si: 0.15-0
．． 4% by weight, Mn: 0.3 to 2% by weight, and Cr:
Steel containing one or more selected from the group consisting of 0.4% by weight or less and B: 0.005% by weight or less is hot rolled at a finishing temperature of (Ar_1 point + 30°C) to 950°C. , the surface temperature continues to increase from Ms point to (Ms point -20
0°C) at a cooling rate of 40°C/sec or more,
A method for producing a PC steel bar, which is characterized in that it is subsequently self-tempered.