JPH079038B2 - Low relaxation PC steel manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a method for producing a low relaxation PC steel material. Here, the PC steel material includes PC steel rods, PC wires, and PC strands.

[Conventional technology]

PC steel used for prestressed concrete has a problem of so-called relaxation, in which the introduced prestress decreases with time. When relaxation is large, it is necessary to secure a high safety factor of design stress, and PC steel with low relaxation is desirable.

Also, in the manufacturing process of PC piles, PC steel materials with less relaxation loss are required because they are subjected to steam curing and high temperature / high pressure curing after tensioning the PC steel materials.

Regarding the method for producing low relaxation PC steel, cold drawing of steel containing 0.65 to 0.90% by weight of carbon in Japanese Patent Publication No. 47-13297 is carried out, and 10 to 30% in a temperature range of 150 to 350 ° C.
A method for performing warm wire drawing is disclosed.

Further, JP-A-48-27956 discloses that capstans are provided on the inlet side and the outlet side of a tempering furnace to temper while applying a stress of 60% or more of a tensile load.

Japanese Patent Publication No. 1-24848 discloses that it is effective to perform warm working such as plastic working or straightening with a strain amount of 3 to 90% in the tempering process.

However, these methods have problems that relaxation properties are still insufficient and uniform elongation and straightness are poor.

[Problems to be Solved by the Invention]

The present inventors suspected that the above-mentioned conventional technique has a problem in uniform elongation and straightness because it pays attention only to strain processing, and that improvement in high temperature relaxation may be insufficient. , We conducted many experiments on the relationship between thermal history and relaxation of PC steel. As a result, they have found that it is decisive to apply thermal history by the primary temper and the secondary temper, and have completed the present invention.

The present invention provides such a method.

[Means for Solving the Problems]

The present invention, C: 0.4 wt% or less Si: 1.0 to 2.0 The steel containing by weight% rapidly cooled from A ₃ transformation point or above the temperature at the critical cooling rate or cooling rate to below M _S point, then, 350 ° C.
Heat the steel to 430 ° C or lower to perform primary tempering and impart strain to this steel material, then increase the temperature to 430 ° C or higher and 480 ° C or lower and 20 ° C or higher from the primary temper temperature to perform secondary tempering. It is a characteristic method for producing low relaxation PC steel.

[Action]

The present invention has developed a relationship between relaxation characteristics of PC steel due to temper history and uniform elongation and straightness.

The PC steel used in the present invention has a carbon content of 0.4% by weight or less. If it exceeds 0.4% by weight, the weldability is poor and it cannot be used as an appropriate PC steel material. Although the lower limit is not specified, 0.2% by weight or more is preferable for the commonly used PC steel from the viewpoint of tensile strength.

If the Si content is less than 1.0% by weight, relaxation loss is large, and if it exceeds 2.0% by weight, the material deteriorates.
~ 2.0 wt%, more preferably 1.4-1.7 wt%
Is.

Although quenching treatment is usual practice for this kind steel, wherein the temperature after cooling is not necessary to cool to room temperature, cooled to below M _S point of martensite, and held at this temperature, then the primary Heating to the temper temperature has the advantages of heat energy economy, simplification of the treatment process, etc.
It does not affect the results of the subsequent primary tempering and secondary tempering. Of course, cooling to normal temperature is not excluded.

The primary temper temperature is set to 350 ° C. or higher and 430 ° C. or lower, and the higher the content of C and Si, the higher temperature in this range. This temper temperature is lower than the conventional normal temper temperature. If this temperature is less than 350 ° C., the abrasion of the rollers on the processing line becomes large, which is not suitable. If the temperature exceeds 430 ° C, the effect of the secondary temper after that is diminished, so the upper limit is set to 430 ° C or less.

At this primary temper temperature, known strain imparting is performed.

The secondary temper should be 430 ℃ to 480 ℃ and higher than the primary temper temperature by 20 ℃ or more. The secondary temper temperature is also C,
The higher the Si content, the higher. If the temperature is lower than 430 ° C, the tensile strength is too high, which is not suitable. If the temperature is higher than 480 ° C, the tensile strength is lowered, and desired mechanical properties cannot be obtained.
In addition, it is 20 ℃ to raise the temperature more than 20 ℃ from the primary temper temperature.
If it is less than the above value, movable conversion remains, and it is impossible to sufficiently immobilize dislocations.

〔Example〕

C: 0.29 wt%, Si: 1.55 wt% PC steel material with a diameter of 9.2 mmφ is drawn and heated to 920 ° C, water-cooled and quenched to 200
The temperature was lowered to 0 ° C., and then tempered by the method of the present invention. The primary temper temperature was changed from 330 ℃ to 430 ℃, and the secondary temper was set in the range of 430 to 480 ℃ with a temperature difference of 30 to 130 ℃.

Table 1 shows the tempering conditions of Examples and Comparative Examples and the resulting tensile strength, uniform elongation, elongation after breaking, relaxation value, and straightness data.

The uniform elongation is the limit value of the permanent elongation at which the parallel portion of the test piece is deformed substantially uniformly in the tensile test. Elongation after break refers to elongation at break and is the total elongation of uniform elongation and local elongation.

Relaxation was evaluated by high temperature relaxation. The high temperature relaxation condition is such that the temperature is raised to 180 ° C in 4 hours, kept for 3 hours, and naturally cooled (about 16 hours), and the initial load of high temperature relaxation was 70% of O _b (tensile strength). . The straightness is the height of the arc of PC steel with a length of 1.5 m. This straightness is particularly specified as a quality item because when a user uses PC steel, it will hinder work unless the steel after cutting to a fixed length rolls on the workbench. There is no problem if the straightness is 1 mm or less.

From Table 1, processed under the tempering conditions of the present invention PC steel has a low relaxation value and excellent uniform elongation,
It is known that the elongation after breaking is high and the straightness is excellent, which is preferable.

[Advantages of the Invention] According to the present invention, a low relaxation PC steel material can be obtained, and it becomes possible to manufacture a PC steel material having uniform elongation and excellent straightness.

Claims (1)

[Claims] 1. A steel material containing C: 0.4 wt% or less and Si: 1.0 to 2.0 wt% is rapidly cooled from a temperature of A ₃ transformation point or higher to a M _S point or less at a cooling rate of a critical cooling rate or higher, and 350 ° C. or higher. 430 ° C
It is characterized by performing the primary tempering by heating below and imparting strain to the steel material, and then performing the secondary tempering at 430 ° C. or higher and 480 ° C. or lower, and raising the temperature from the primary temper temperature to 20 ° C. or higher. Manufacturing method of low relaxation PC steel.