JP3233830B2 - High-strength PC steel rod excellent in delayed fracture characteristics of spot welds and method of manufacturing the same - Google Patents

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 widely used as a reinforcing material for a prestressed concrete structure such as a pole, a pile, a building, a bridge, etc., and particularly to a delay steel having a strength of 1300 MPa or more. The present invention relates to a high-strength PC steel bar having excellent fracture characteristics and a method for producing the same.

[0002]

2. Description of the Related Art PC steel materials widely used as reinforcements for prestressed concrete structures such as poles, piles, buildings, bridges, etc. are generally PC steel wires and PC steel wires specified in JIS G 3536. JIS G
The PC steel bar specified in 3109 is used.
The material used for the PC steel wire is a piano wire material conforming to JIS G 3502, and is manufactured by performing a patenting process and then drawing.

On the other hand, PC steel bars are, for example,
No. 684, the C content is 0.25
It is manufactured by quenching and tempering using ~ 0.35% medium carbon steel. The strength of PC steel wire is P
Although it is higher than the C steel rod, it has a disadvantage that spot welding cannot be performed due to its high C content.

[0004] On the other hand, although the spot weldability of a PC steel rod is better than that of a PC steel wire, it is referred to in “Prestressed Concrete Design and Construction Standards and Explanation” (edited by the Architectural Institute of Japan,
As described in Maruzen, pp. 43-45, a high-strength PC steel rod having a strength exceeding 1275 MPa (130 kgf / mm ² ) has inferior delayed fracture characteristics as compared with a PC steel wire. Further, as described in Japanese Patent Publication No. 5-59967, the spot weld is rapidly cooled to have a structure mainly composed of martensite, and there is a problem that delayed fracture easily occurs in the spot weld. .

As a conventional finding for improving the delayed fracture characteristics of PC steel bars, for example, Japanese Patent Publication No. 5-59967 proposes that it is effective to reduce the contents of P and S. Certainly, low P and low S are effective for delayed fracture, but the P and S contents of the current PC steel rod are already around 0.01%, and JIS G 3109
In fact, it is at a level lower than the amount specified in. Although it is possible to further reduce the contents of P and S, the production cost increases.

In Japanese Patent Publication No. 5-41684,
It has been proposed to regulate the contents of Si and Mn and to perform bending or drawing in a tempering step after quenching. Further, Japanese Patent Application Laid-Open No. 5-7963 proposes that a resin coating layer is provided around a spot weld between a PC steel rod and an iron wire to reduce the susceptibility to delayed fracture. However, none of the proposals has led to a significant improvement in delayed fracture characteristics in the tests of the present inventors. As described above, in the conventional technique, there is a limit in manufacturing a high-strength PC steel rod with drastically improved delayed fracture characteristics.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and realizes a high-strength PC steel rod having a high strength of 1300 MPa or more with good delayed fracture characteristics and a method of manufacturing the same. It is intended to provide.

[0008]

Means for Solving the Problems The present inventors first made various strength levels of P produced by quenching and tempering.
The delayed fracture behavior was analyzed in detail using a C steel rod. It is already clear that delayed fracture has occurred due to hydrogen in steel. Therefore, the delayed fracture characteristics were evaluated by determining the "critical diffusible hydrogen amount" at which delayed fracture did not occur.

In this method, various levels of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is performed to prevent hydrogen from leaking from the sample into the atmosphere during a delayed fracture test, and thereafter, A predetermined load is applied in the atmosphere to evaluate the amount of diffusible hydrogen at which delayed fracture does not occur.

FIG. 1 shows an example in which the relationship between the amount of diffusible hydrogen and the rupture time until delayed fracture is analyzed. As the amount of diffusible hydrogen contained in the sample decreases, the time until delayed fracture increases, and when the amount of diffusible hydrogen is less than a certain value, delayed fracture does not occur. This amount of hydrogen is defined as “critical diffusible hydrogen amount”.

The higher the critical diffusible hydrogen content is, the better the delayed fracture resistance of the steel material is, which is a value inherent to the steel material determined by the composition of the steel material and the manufacturing conditions such as heat treatment. The amount of diffusible hydrogen in a sample can be easily measured by gas chromatography.

Therefore, in order to increase the critical diffusible hydrogen content of the high-strength PC steel rod, that is, to improve delayed fracture characteristics,
The effects of the austenite grain size, steel composition, heat treatment conditions, etc. were repeated. As a result, it was found that the delayed fracture characteristics could not be significantly improved even if any of the above factors was greatly changed.

Since delayed fracture is grain boundary cracking along the former austenite grain boundary, it is important to prevent the occurrence of grain boundary cracking in order to achieve a significant improvement in delayed fracture characteristics. The conclusion has been reached.

As a result of various studies on means for preventing austenite grain boundary cracking, it has been found that if a structure consisting of tempered martensite and grain boundary ferrite is formed, the critical hydrogen content is high, and furthermore, the shaft from the surface layer of the PC steel rod is removed. In a region covering at least 10% of the radius in the center direction, if a structure having an aspect ratio (long / short diameter of austenite grains) of 1.2 or more, which is the ratio of the length and width of austenite grains, is formed, 1300 MPa It has been discovered that austenite grain boundary cracking can be prevented even in a high-strength region exceeding.

That is, a steel having a structure composed of tempered martensite and grain boundary ferrite in which austenite grains are elongated in the rolling direction of a PC steel rod and the aspect ratio is 1.2 or more,
They have found a completely new finding that, since the fracture mode is intragranular cracking, the critical diffusible hydrogen content is greatly increased, and the delayed fracture resistance is significantly improved.

As a method of elongating the austenite grains, it is apparent that the aspect ratio can be increased to 1.2 or more by selecting the optimum hot rolling conditions of the hot rolling temperature and the rolling reduction. did. Furthermore, the relationship between the thickness of grain boundary ferrite and delayed fracture characteristics was analyzed in detail,
The optimum conditions were clarified and the manufacturing conditions were established.

In the tempering process of a PC steel rod having a martensitic structure by water cooling after hot rolling, increasing the heating rate to the tempering temperature increases the critical diffusible hydrogen content even in the same austenite intragranular cracks. And
It has been found that the delayed fracture characteristics are significantly improved.

On the basis of the above examination results, if the steel material composition, microstructure, hot rolling conditions and heat treatment conditions are optimally selected,
The inventors have concluded that a high-strength PC steel rod having excellent delayed fracture characteristics can be realized, and have made the present invention.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) By weight%, C: 0.2-0.6%, Si: 0.0
5 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.0
0.5 to 0.1%, or Cr: 0. 1%.
05-2.0%, Mo: 0.05-1.0%, Ni:
0.05-5.0%, Cu: 0.05-1.0%, V:
0.05-0.3%, Nb: 0.005-0.1%, T
a: 0.005 to 0.5%, W: 0.05 to 0.5%,
Ti: 0.005 to 0.05%, B: 0.0003 to
A steel containing 0.0050% or more of one or more kinds and the balance being Fe and unavoidable impurities, comprising tempered martensite and a grain boundary ferrite structure having a thickness of 0.2 to 10 μm, and at least a surface layer. From 0.1R
(R: radius of the PC steel rod), the ratio of the length and width of the prior austenite grains is 1.2 or more, and the critical diffusion water
Elemental amount is 1.08 ppm or more and tensile strength is 1300
A high-strength PC steel rod excellent in delayed fracture characteristics of a spot welded part, which is not less than MPa.

(2) When hot rolling a steel having the above-mentioned chemical components, after passing through a step of performing hot rolling with a total draft of 20% or more in a temperature range of at least 700 to 900 ° C., Ar ₃ To Ar ₁ for 2 to 100 seconds to precipitate ferrite at the austenite grain boundaries, and then water-cooling to convert the remainder to martensite.
A method for producing a high-strength PC steel rod excellent in delayed fracture characteristics of a spot welded part, wherein the rod is heated and tempered at a heating rate of 0 ° C / sec or more to a temperature range of 250 to 550 ° C.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the components of steel to which the present invention is applied will be described. C: C is an essential element for securing the strength of the steel bar of PC, but if it is less than 0.2%, the required strength cannot be obtained, while if it exceeds 0.6%, the spot weldability is significantly deteriorated. Therefore, the range is limited to the range of 0.2 to 0.6%.

Si: Si has the effect of improving relaxation properties and increasing the strength by solid solution hardening. If it is less than 0.05%, the above effect cannot be exerted,
On the other hand, even if it exceeds 2%, an effect commensurate with the added amount cannot be expected, so the range is limited to 0.05 to 2.0%.

Mn: Mn is an element not only necessary for deoxidation and desulfurization but also effective for enhancing the hardenability for obtaining a martensitic structure. No effect can be obtained. On the other hand, if the content exceeds 2.0%, the spot weldability deteriorates. Therefore, the content is limited to the range of 0.2 to 2.0%.

Al: Al forms AlN at the time of deoxidation and heat treatment, thereby preventing the austenite grains from becoming coarse and fixing N to secure solid solution B effective for improving hardenability and delayed fracture characteristics. However, if less than 0.005%, these effects are not exhibited, and if it exceeds 0.1%, the effect is saturated, so that 0.005% or less.
Limited to the range of ~ 0.1%.

The above are the basic components of the steel which is the subject of the present invention. In the present invention, the steel further contains Cr: 0.1.
05-2.0%, Mo: 0.05-1.0%, Ni:
0.05-5.0%, Cu: 0.05-1.0%, V:
0.05-0.3%, Nb: 0.005-0.1%, T
a: 0.005 to 0.5%, W: 0.05 to 0.5%,
Ti: 0.005 to 0.05%, B: 0.0003 to
One or more of 0.0050% can be contained.

Cr: Cr is an element effective for improving hardenability and increasing softening resistance during tempering, but if it is less than 0.05%, its effect cannot be fully exhibited.
On the other hand, if it exceeds 2.0%, the spot weldability deteriorates, so the content is limited to 0.05 to 2.0%.

Mo: Mo, like Cr, has a strong tempering softening resistance and is an effective element for increasing the tensile strength after heat treatment, and further has the effect of improving relaxation properties and increasing the non-recrystallization temperature. Have, but 0.05%
If it is less than 1.0%, the effect is small. On the other hand, if it exceeds 1.0%, the spot weldability deteriorates.

Ni: Ni is added in order to improve ductility, which deteriorates with increasing strength, to improve hardenability during heat treatment, and to increase tensile strength.
If it is less than 5.0%, the effect is small. On the other hand, if it exceeds 5.0%, the effect corresponding to the added amount cannot be exhibited.
Restricted to 0% range.

Cu: Cu is an element effective for increasing the tempering softening resistance. However, if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 1.0%, hot workability is deteriorated. 0.05% to 1.0%.

V: V has the effect of reducing the austenite grains and increasing the relaxation value by forming carbonitrides during the quenching treatment.
If it is less than 0.05%, the effect of the above-mentioned effect cannot be obtained. On the other hand, if it exceeds 0.3%, the effect is saturated.
Limited to 3%.

Nb: Like V, Nb is also an effective element for forming carbonitrides to refine austenite grains. In addition, Nb has the effect of significantly increasing the non-recrystallization temperature, and has the advantage that a steel with austenite grains elongated can be easily produced even at a high hot rolling finish temperature. If it is less than 0.005%, the above effect is insufficient, while if it exceeds 0.1%, this effect is saturated, so that it is 0.00%.
Limited to 5-0.1%.

Ta: Ta also has the effect of increasing the non-recrystallization temperature similarly to Nb. However, if it is less than 0.005%, the above effect is not exhibited. Since the effect is saturated, the content is limited to 0.005 to 0.5%.

W: W is an element effective for improving the delayed fracture characteristics of a high-strength PC steel rod, but if the content is less than 0.05%, the above-mentioned effect is not exhibited. Even if added in excess, the effect saturates, so 0.05-0.5%
Limited to the range.

Ti: Ti has an effect of preventing austenite grains from being coarsened by forming deoxidation and forming TiN, and has an effect of securing N and securing solid solution B effective for improving delayed fracture characteristics. However, if the content is less than 0.005%, these effects are not exhibited, and if the content exceeds 0.05%, the effect is saturated, so that the content is limited to the range of 0.005 to 0.05%.

B: B has the effect of improving delayed fracture characteristics, and further has the effect of segregating at austenite grain boundaries to significantly enhance hardenability.
If it is less than 03%, the above effect is not exhibited, and
%, The effect is saturated, so that 0.0003 to 0.0
Limited to 050%.

Although P and S are not particularly limited,
From the viewpoint of improving the delayed fracture characteristics of the PC steel rod, the respective ranges are preferably 0.015% or less. In addition, since N has an effect of reducing austenite grains by generating nitrides of Al, V, Nb, and Ti,
0.015% is a preferable range.

Next, the reason for limiting the structural form of the PC steel rod, which is the most important point for the improvement of the delayed fracture characteristics of the high-strength PC steel rod aimed at in the present invention, will be described. FIG. 2 shows an example in which the influence of the aspect ratio on the critical diffusible hydrogen content of a PC steel rod composed of tempered martensite and a grain boundary ferrite structure is analyzed. Here, the aspect ratio is 1.0
Is a steel in which austenite grains are not elongated.

As is apparent from the figure, as the aspect ratio is increased by elongating the austenite grains, the amount of critical diffusible hydrogen increases, and the delayed fracture characteristics are remarkably improved.
Here, if the aspect ratio is less than 1.2, it is difficult to prevent austenite grain boundary cracking, and the improvement in delayed fracture characteristics is not remarkable. Therefore, the lower limit of the aspect ratio is limited to 1.2. Note that when the aspect ratio is 1.5 or more, the effect of improving delayed fracture characteristics becomes remarkable. Therefore, 1.5 or more is a preferable range of the aspect ratio.

FIG. 3 shows an example of an analysis of the relationship between the critical diffusible hydrogen content and the ratio of the PC steel rod radius to the depth in the axial center direction from the surface of the PC steel rod having an aspect ratio of 1.2 or more. It is. The area from the surface of the PC steel bar having an aspect ratio of 1.2 or more with respect to the radius of the PC steel bar,
If it is less than 0.1, the effect of improving the critical diffusible hydrogen amount is small,
It can be seen that there is no significant effect on the delayed fracture characteristics.

Therefore, the region having an aspect ratio of 1.2 or more is limited to a region extending at least 0.1 R (R: radius of the PC steel bar) from the surface of the PC steel bar. As is apparent from FIG. 3, since the effect of improving delayed fracture characteristics is high at 0.2R or more, the preferable condition is 0.2R or more.

FIG. 4 shows an example in which the influence of the grain boundary ferrite thickness on the critical diffusible hydrogen content of a PC steel rod in which austenite grains are elongated is analyzed. When the grain boundary ferrite thickness is "0", the steel is made of tempered martensite having no grain boundary ferrite. As is apparent from the figure, as the thickness of the ferrite increases, the critical diffusible hydrogen amount increases, and the delayed fracture characteristics are significantly improved.

Here, the thickness of the grain boundary ferrite is 0.2 μm.
If it is less than 10, the delayed fracture characteristics are not remarkable, so the lower limit of the grain boundary ferrite thickness is limited to 0.2 μm. In addition, 0.5μ
Since a remarkable effect is exhibited at m or more, preferable conditions are 0.5 μm or more. On the other hand, if it exceeds 10 μm, the volume fraction of ferrite increases and the strength tends to decrease, so the upper limit was limited to 10 μm.

The volume fraction of ferrite is not particularly limited, but the strength tends to decrease as the volume fraction increases, so that the preferred upper limit of the volume fraction of ferrite including intragranular ferrite is 20%. Also, the grain boundary ferrite does not need to be present over the entire cross section of the PC steel rod, and at least 1 PC of the radius of the PC steel rod must
If it is generated in the region of 5%, there is an effect of improving delayed fracture characteristics.

High strength PC excellent in delayed fracture characteristics of the present invention
In the method of manufacturing a steel rod, hot rolling is performed at a low temperature to elongate austenite grains, and in a cooling process after rolling, a predetermined temperature range is maintained to generate grain boundary ferrite, and the remainder is water-cooled. And then tempered.

The reasons for limiting the manufacturing conditions will be described below. Hot rolling temperature: When the hot rolling finishing temperature exceeds 900 ° C., recrystallization is likely to occur even when an element that raises the non-recrystallization temperature is added, and it is difficult to obtain elongated austenite grains, and the aspect ratio is low. 1.2 or more area
Since it is difficult to increase the temperature to 1R or more, the upper limit temperature is set to 90.
Limited to 0 ° C. On the other hand, if the temperature is lower than 700 ° C., the deformation resistance becomes too large and hot rolling becomes difficult, so the lower limit temperature is limited to 700 ° C.

Hot rolling reduction: When the total reduction in the temperature range of 700 to 900 ° C. is less than 20%, it is difficult to obtain elongated austenite grains having an aspect ratio of 1.2 or more, Since it is difficult to make the area having an aspect ratio of 1.2 or more 0.1R or more, the lower limit of the total reduction is limited to 20%.

Retention temperature and time after hot rolling: The reason for keeping the temperature range of Ar ₃ to Ar ₁ after hot rolling is to generate ferrite at austenite grain boundaries. Here, when the holding temperature is Ar ₃ or higher, no ferrite is formed, and when the holding temperature is lower than Ar ₁ , pearlite is formed. Therefore, the holding temperature range is limited to Ar ₃ to Ar ₁ . The holding time varies depending on the holding temperature and the component, but it is difficult to generate grain boundary ferrite in less than 2 seconds,
On the other hand, if it exceeds 100 seconds, the thickness of the grain boundary ferrite becomes 10
The holding time was limited to 2 to 100 seconds because it easily exceeded μm.

After the holding, the remainder of the grain boundary ferrite is formed into martensite by water cooling, but the martensite structure has a volume fraction of 20%.
If less than 10% of pearlite, bainite, retained austenite or a mixed structure thereof is formed, there is no deterioration in delayed fracture characteristics and there is no limitation.

Tempering heating rate: When the heating rate (temperature rising rate) at the time of tempering a PC steel rod is less than 10 ° C./sec, the critical diffusible hydrogen amount is low even if the microstructures are tempered martensite and grain boundary ferrite. Therefore, the lower limit of the heating rate was limited to 10 ° C./sec.
Preferred conditions for stably producing a PC steel rod having excellent delayed fracture characteristics are 20 ° C./sec or more.

Tempering temperature: If the tempering temperature is less than 250 ° C., the effect of tempering is small, while if it exceeds 550 ° C., the tensile strength tends to decrease, and it becomes difficult to produce a high-strength PC steel rod. Tempering temperature range 250-55
Limited to 0 ° C.

In the present invention, even if the wire diameter is adjusted after the hot rolling or the tempering treatment, and the light drawing is performed for other purposes, the delayed fracture characteristics and the mechanical characteristics are not deteriorated, and there is no limitation. Absent.

[0052]

EXAMPLE After a test material having the chemical composition shown in Table 1 was rolled under ordinary hot rolling conditions, it was maintained at various temperature ranges.
After water cooling, a tempering treatment was performed to produce a PC steel rod. Table 2 shows the results of evaluation of the mechanical properties, microstructure, and delayed fracture characteristics using the above samples. The delayed fracture characteristics
Using the spot-welded sample, evaluation was performed using the critical diffusible hydrogen content described above, and the applied stress was 80% of the tensile strength.
It carried out on condition of.

Test No. 2 in Table 2 1 to 11 are examples of the present invention,
Others are comparative examples. As can be seen from the table, all of the examples of the present invention have a structure composed of tempered martensite and grain boundary ferrite, and have an aspect ratio of 1.2 or more and a radius of PC steel rod of 1.2 or more. since the ratio of the aspect ratio of 0.1 or more, the critical diffusible hydrogen amount is much higher than the conventional PC steel bar, the critical diffusible hydrogen amount
Is 1.08 ppm or more and the tensile strength is 1300MP
A high-strength PC steel rod excellent in delayed fracture characteristics of spot welds of a or more is realized.

On the other hand, the comparative example No. 12, 1
No. 3 was manufactured by a conventional manufacturing method.
That is, this is an example in which the steel sheet is manufactured by quenching and tempering after hot rolling, does not include grain boundary ferrite, and does not elongate austenite grains. For this reason, this is an example in which the fracture mode is austenite grain boundary cracking, the critical diffusible hydrogen content is low, and the delayed fracture characteristic is poor.

The comparative example No. Nos. 15 and 18 are examples in which the total reduction ratio in hot rolling is too low and the aspect ratio is not 1.2 or more, so that the delayed fracture characteristics have not been significantly improved.

In the comparative example No. Nos. 14 and 17 are examples in which the holding time after hot rolling was too long, the volume fraction of ferrite increased, and the desired high-strength PC steel rod could not be achieved.

In the comparative example No. 16, austenite grains are examples of the grain boundary ferrite was not generated because the holding temperature after hot rolling those lengthening exceeds the Ar _3. In addition, in Comparative Example No. No. 19 is an example in which the critical diffusible hydrogen amount was low due to the low heating rate during the tempering treatment, and the delayed fracture characteristics deteriorated.

[0058]

[Table 1]

[0059]

[Table 2]

[0060]

According to the present invention, the delayed fracture mode of PC steel rods is changed from intergranular cracks to intragranular cracks by forming a structure composed of tempered martensite and grain boundary ferrite in which the former austenite grains are elongated, and the tensile strength is increased. It is possible to greatly improve the delayed fracture characteristics of a high-strength PC steel rod having a strength of 1300 MPa or more, and to optimally select the chemical composition of the steel, hot rolling conditions, and heat treatment conditions, thereby producing the steel. The industrial effects are extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a chart showing an example of the relationship between the amount of diffusible hydrogen and delayed fracture time.

FIG. 2 is a chart showing an example of analyzing a relationship between a critical diffusible hydrogen amount and an aspect ratio.

FIG. 3 is a table showing an example of analyzing a relationship between a critical diffusible hydrogen amount and a ratio of an aspect ratio to a radius of 1.2 or more.

FIG. 4 is a chart showing an example of analyzing a relationship between a critical diffusible hydrogen amount and a thickness of grain boundary ferrite.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Toshihiko Takahashi 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division (56) References JP-A-2-107743 (JP, A) JP-A-2 -236223 (JP, A) JP-A-7-224355 (JP, A) JP-A-5-222450 (JP, A) JP-A-4-358802 (JP, A) (58) Fields investigated (Int. . ^7, DB name) C22C 38/00 - 38/60 C21D 8/00 - 8/10

Claims (4)

(57) [Claims] C .: 0.2 to 0.6%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0.5% by weight. 1%, with the balance being Fe and unavoidable impurities, tempered martensite and a thickness of 0.2 to 10 μm.
m, and the ratio between the length and width of prior austenite grains is at least 1.2 in a region at least 0.1 R (R: radius of a PC steel bar) from the surface layer, and
Spot the critical diffusible hydrogen amount at least 1.08 ppm, and a tensile strength is equal to or not less than 1300MPa
G High strength PC steel rod with excellent delayed fracture characteristics of welds . 2. Cr: 0.05-2.0%, Mo: 0.05-1.0%, Ni: 0.05-5.0%, Cu: 0.05-1. 0%, V: 0.05 to 0.3%, Nb: 0.005 to 0.1%, Ta: 0.005 to 0.5%, W: 0.05 to 0.5%, Ti: 0 2. The spot welded part according to claim 1, further comprising one or more of 0.005 to 0.05% and B: 0.0003 to 0.0050%. Strength PC steel bar. 3. A high-strength PC steel rod according to claim 1.
A that method, in weight%, C: 0.2~0.6%, Si : 0.05~2.0%, Mn: 0.2~2.0%, Al: 0.005~0 When hot rolling steel containing 0.1% and the balance being Fe and unavoidable impurities, a step of performing hot rolling with a total draft of 20% or more in a temperature range of at least 700 to 900 ° C. After that, the ferrite was held at a temperature range of Ar _{3 to} Ar ₁ for 2 to 100 seconds to precipitate ferrite at the austenite grain boundary, and then water-cooled to convert the remainder to martensite. Spot welding characterized by heating and tempering to a temperature range of up to 550 ° C
Method for producing high-strength PC steel rods with excellent delayed fracture characteristics at the part . 4. Cr: 0.05-2.0%, Mo: 0.05-1.0%, Ni: 0.05-5.0%, Cu: 0.05-1. 0%, V: 0.05 to 0.3%, Nb: 0.005 to 0.1%, Ta: 0.005 to 0.5%, W: 0.05 to 0.5%, Ti: 0 4. The spot welding part according to claim 3, further comprising one or more of 0.005 to 0.05% and B: 0.0003 to 0.0050%. Strength PC steel bar.