JPH0978191A - High strength pc steel bar excellent in delayed fracture characteristic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength PC steel bar excellent in delayed fracture characteristic. SOLUTION: This steel has a composition containing 0.15-0.4% C, 0.05-2% Si, 0.2-2% Mn, 0.005-0.1% Al, 0.005-0.05% Ti, and 0.0003-0.005% B and further containing, if necessary, one or >=2 elements among Cr, Mo, Ni, Cu, V, Nb, Ta, and W. Moreover, this steel is composed of tempered martensitic structure, and the length-to-width ratio of old austenite grains is regulated to >=1.2 at least in the region between the surface layer and a position at a depth of 0.1R (where R is the radius of the PC steel bar) from the surface layer, and further, this steel has >=1,300MPa tensile strength.

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 prestressed concrete structures such as poles, piles and constructions, bridges, etc., and particularly has a strength of 1300 MPa or more. The present invention relates to a high-strength PC steel rod having excellent fracture characteristics and a method for manufacturing the same.

[0002]

2. Description of the Related Art PC steel widely used as a reinforcing material for prestressed concrete structures such as poles, piles and buildings, bridges is generally a PC steel wire and a PC steel stranded wire 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 treatment and then drawing.

On the other hand, PC steel bars are, for example,
No. 684, the C content is 0.25
Manufactured by quenching and tempering treatment using 0.35% medium carbon steel. The strength of PC steel wire is P
Although it is higher than the C steel bar, it has a drawback that spot welding cannot be performed because the C content is high.

[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 on pages 43 to 45 of Maruzen), a high strength PC steel bar having a strength of more than 1275 MPa (130 kgf / mm ² ) is inferior in delayed fracture property to 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 property of PC steel rods, for example, Japanese Patent Publication No. 5-59967 proposes that it is effective to reduce the P and S contents. Certainly, lowering P and lowering S is effective for delayed fracture, but the P and S contents of the current PC steel bar are already around 0.01%, and JIS G 3109
The actual situation is that the amount is lower than that specified in. Although it is possible to further reduce the P and S contents, the manufacturing cost increases.

In Japanese Patent Publication No. 5-41684,
It is proposed that the Si and Mn contents be regulated and that after the quenching treatment, bending or drawing is performed in the tempering process. Further, Japanese Patent Laid-Open No. 5-7963 proposes to provide a resin coating layer around a spot weld portion between a PC steel rod and an iron wire to reduce the susceptibility to delayed fracture. However, none of the proposals has significantly improved delayed fracture characteristics in the tests conducted by the present inventors. As described above, the conventional technology has a limit in producing a high-strength PC steel rod having a radically improved delayed fracture property.

[0007]

The present invention has been made in view of the above situation, and realizes a high-strength PC steel bar having a good delayed fracture property of 1300 MPa or more and a manufacturing method thereof. 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 is caused by hydrogen in steel. Therefore, the delayed fracture characteristics were evaluated by determining the "limit diffusible hydrogen content" at which delayed fracture does not occur.

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

FIG. 1 shows an example of analysis of the relationship between the amount of diffusible hydrogen and the fracture time until delayed fracture. As the amount of diffusible hydrogen contained in the sample decreases, the time until delayed fracture becomes longer, and delayed fracture does not occur when the amount of diffusible hydrogen is below a certain value. This amount of hydrogen is defined as the “limit 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 peculiar 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 the sample can be easily measured with a gas chromatograph.

Therefore, in order to increase the limit diffusible hydrogen content of the high strength PC steel bar, that is, to improve the delayed fracture property,
We have repeatedly examined the effects of austenite grain size, steel composition, heat treatment conditions, and so on. As a result, it was found that the delayed fracture characteristics could not be significantly improved even if any of the above factors was changed significantly.

Since the delayed fracture is a grain boundary crack along the former austenite grain boundary, it is important to prevent the occurrence of the grain boundary crack in order to achieve a great improvement in the delayed fracture property. Reached the conclusion.

Then, as a result of further various studies on means for preventing austenite grain boundary cracking, as a result, austenite grains were formed in a region extending from the surface layer of the PC steel bar containing B to the axial center direction at least 10% of the radius. If a structure with an aspect ratio (major axis / minor axis of austenite grains) of 1.2 or more, which is the ratio of the length to the width, is formed,
It has been discovered that austenite grain boundary cracks can be prevented even in a high strength region exceeding 00 MPa.

That is, in a steel having a tempered martensite structure in which austenite grains are elongated in the rolling direction of a PC steel rod and the aspect ratio is 1.2 or more, the fracture form is intragranular cracking, so the limit diffusible hydrogen content is It was found that there was a completely new finding that the delayed fracture resistance was significantly improved.

Further, as a method for elongating the austenite grains, it is possible to make the aspect ratio 1.2 or more by selecting the optimum hot rolling conditions of the hot rolling temperature and the rolling reduction. Revealed.

Further, in the tempering process of the PC steel bar having a martensite structure by water cooling after hot rolling, increasing the heating rate to the tempering temperature improves the critical diffusible hydrogen content even with the same austenite intragranular cracking. However, they have found that the delayed fracture characteristics are significantly improved.

Based on the above examination results, if the steel material composition, microstructure, hot rolling conditions and heat treatment conditions are optimally selected,
The present invention has been achieved by the conclusion that a high-strength PC steel bar excellent in delayed fracture characteristics can be realized.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) C: 0.15 to 0.40% by weight, Si:
0.05-2.0%, Mn: 0.2-2.0%, Al:
0.005-0.1%, Ti: 0.005-0.05
%, B: 0.0003 to 0.0050%, or
Alternatively, Cr: 0.05 to 2.0%, Mo: 0.0
5 to 1.0%, Ni: 0.05 to 5.0%, Cu: 0.
05-1.0%, V: 0.05-0.3%, Nb: 0.
005-0.1%, Ta: 0.005-0.5%, W:
In steel containing 0.05 to 0.5% of one kind or two or more kinds and the balance of Fe and unavoidable impurities,
It consists of a tempered martensite structure, and the ratio of the length to the width of the former austenite grains is 1.2 or more and the tensile strength is at least 0.1R (R: radius of PC steel bar) from the surface layer. A high-strength PC steel rod with excellent delayed fracture characteristics, characterized by having a pressure of 1300 MPa or more.

(2) When hot rolling the steel having the above chemical composition, it is water-cooled after a step of performing hot rolling with a total reduction of 20% or more in a temperature range of at least 700 to 900 ° C. A method for producing a high-strength PC steel bar having excellent delayed fracture characteristics, which comprises forming a martensite structure by the above, and subsequently heating it to a temperature range of 250 to 550 ° C at a heating rate of 10 ° C / sec or more and tempering.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the components of the steel targeted by the present invention will be described. C: C is an essential element for ensuring the strength of the steel rod of PC, but if it is less than 0.15%, the required strength cannot be obtained in tempered martensite, while if it exceeds 0.40%, spot welding is performed. Property deteriorates, so 0.15 to 0.40%
Limited to the range.

Si: Si has the effect of improving the relaxation characteristics and also of increasing the strength by the solid solution hardening effect. If it is less than 0.05%, the above-mentioned action cannot be exhibited, while 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 effective not only for deoxidation and desulfurization but also for enhancing the hardenability for obtaining a martensitic structure. The effect cannot be obtained, and if it exceeds 2.0%, the spot weldability deteriorates, so the range was limited to 0.2 to 2.0%.

Al: Al is an effect of preventing coarsening of austenite grains by forming AlN during deoxidation and heat treatment, and fixes N to secure solid solution B effective for improving hardenability and delayed fracture characteristics. However, if it is less than 0.005%, these effects are not exhibited, and if it exceeds 0.1%, the effect is saturated.
The range was limited to 0.1%.

Ti: Ti, like Al, has the effect of preventing coarsening of austenite grains by forming TiN during deoxidation and heat treatment, and at the same time, fixing N to improve the hardenability and delayed fracture characteristics. Although it also has the effect of ensuring the melt B, if less than 0.005%, these effects are not exhibited, and if it exceeds 0.05%, the effect is saturated, so the range of 0.005-0.05%. Limited to.

B: B has an effect of improving delayed fracture characteristics in a steel having a tempered martensite structure in which austenite grains are elongated. Further, B segregates at the austenite grain boundaries to remarkably enhance the hardenability, and
It also has the effect of shifting the non-recrystallization temperature range to the high temperature side, and it becomes easy to obtain elongated austenite grains. If B is less than 0.0003%, the above effect is not exhibited and
The effect is saturated even if it exceeds 0050%, so 0.0003
Limited to ~ 0.0050%.

The above are the basic components of the steel targeted by the present invention. In the present invention, however, Cr: 0.
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
One or two or more of a: 0.005-0.5% and W: 0.05-0.5% can be contained.

Cr: Cr is an element effective for improving the hardenability and increasing the softening resistance during the tempering treatment, but if it is less than 0.05%, its effect cannot be sufficiently 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 is an element that has a strong temper softening resistance similar to Cr and is effective for increasing the tensile strength after heat treatment, and also has the effect of improving relaxation characteristics and raising the non-recrystallization temperature. Have, but 0.05%
If it is less than 1.0%, the effect is small, while if it exceeds 1.0%, the spot weldability deteriorates, so the content is limited to 0.05 to 1.0%.

Ni: Ni is added to improve ductility, which deteriorates with higher strength, and hardenability during heat treatment to increase tensile strength.
If it is less than 5.0%, the effect is small, while if it exceeds 5.0%, the effect corresponding to the added amount cannot be exhibited, so that 0.05 to 5.
The range was limited to 0%.

Cu: Cu is an element effective for increasing the resistance to temper softening, but if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 1.0%, the hot workability deteriorates, so 0 It was limited to 0.05 to 1.0%.

V: V has the effect of refining 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 action cannot be obtained, while if it exceeds 0.3%, the effect is saturated, so that 0.05 to 0.
Limited to 3%.

Nb: Nb is also an element effective for refining austenite grains by forming carbonitrides like V. Further, Nb has an effect of significantly increasing the non-recrystallization temperature, and has an advantage that a steel in which austenite grains are elongated can be easily manufactured even if the hot rolling finishing temperature is high. If it is less than 0.005%, the above effect is insufficient, while if it exceeds 0.1%, the effect is saturated, so 0.00
It was limited to 5 to 0.1%.

Ta: Ta also has the effect of raising the non-recrystallization temperature like Nb, but if it is less than 0.005%, the above effect is not exhibited, and if it exceeds 0.5%, it is added. Since the effect is saturated, it is limited to 0.005 to 0.5%.

W: W is an element effective for improving the delayed fracture property of a high strength PC steel rod, but if it is less than 0.05%, the above effect is not exhibited, while on the other hand, 0.5% is used. The effect is saturated even if added in excess, so 0.05-0.5%
Limited to the range.

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. Further, N has the effect of refining austenite grains by forming nitrides of Ti, Al, V, and Nb.
0.015% is a preferable range.

Next, the reason for limiting the structural morphology of the PC steel rod, which is the most important point for improving the delayed fracture property of the high-strength PC steel rod aimed at by the present invention, will be described. FIG. 2 shows an example of analysis of the influence of the aspect ratio on the critical diffusible hydrogen content and delayed fracture morphology of a PC steel rod having a tempered martensite structure. In the figure, black marks indicate intergranular cracks, white marks indicate intragranular cracks, and half white marks indicate that intergranular cracks and intragranular cracks coexist. Here, a PC with an aspect ratio of 1.0
The steel rod is manufactured by conventional quenching and tempering treatment, and is a steel in which austenite grains are not elongated.

As is clear from the figure, as the austenite grains are elongated and the aspect ratio is increased, the fracture mode shifts from intergranular cracking to intragranular cracking, and when 1.2 or more, intragranular cracking occurs. Correspondingly, the amount of critical diffusible hydrogen increases, and the delayed fracture characteristic is remarkably improved. Here, when the aspect ratio is less than 1.2, the delayed fracture characteristics are not significantly improved, so the lower limit of the aspect ratio is limited to 1.2. In addition,
When the aspect ratio is 1.5 or more, the effect of improving the delayed fracture property becomes remarkable, so 1.5 or more is a preferable range of the aspect ratio.

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

For this reason, the region having an aspect ratio of 1.2 or more is limited to a region extending from the surface layer of the PC steel rod to 0.1R (R: radius of the PC steel rod), at least. As is clear from FIG. 3, a preferable condition is 0.2R or higher because the effect of improving delayed fracture characteristics is high at 0.2R or higher.

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 the austenite grains, and after rolling, the austenite grains are martensitic by stretching with water, followed by tempering. .

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

Hot rolling reduction: If 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, and Since it becomes difficult to set the region having an aspect ratio of 1.2 or more to 0.1R or more, the lower limit of the total rolling reduction is limited to 20%.

In the present invention, the hot rolling is followed by water cooling to obtain a martensitic structure. At this time, ferrite, pearlite, bainite, retained austenite or a mixed structure thereof having a volume fraction of less than 20% is formed. Even if it is generated, the delayed fracture characteristic is not deteriorated, and there is no limitation.

Tempering heating rate: When the heating rate (temperature rising rate) at the time of tempering martensite in which austenite grains are elongated is less than 10 ° C./sec, even if the delayed fracture mode is intragranular cracking, critical diffusible hydrogen The lower limit of the heating rate is limited to 10 ° C./second because the amount is low and the delayed fracture characteristics cannot be significantly improved. A preferable condition for stably producing a PC steel rod excellent in delayed fracture property is 20 ° C./sec or more.

Tempering temperature: If the tempering temperature is less than 250 ° C., the tempering effect is small, while if it exceeds 550 ° C., the strength of tempered martensite is apt to decrease and it becomes difficult to manufacture a high strength PC steel bar. Therefore, the tempering temperature range is limited to 250 to 550 ° C. In the present invention, even if the wire diameter is adjusted after the hot rolling or the tempering treatment and the light wire drawing is performed for other purposes, the delayed fracture property and the mechanical property are not deteriorated, and there is no limitation.

[0047]

Example A sample material having the chemical composition shown in Table 1 was rolled under various hot rolling conditions and then water-cooled to obtain a martensite structure. Then, tempering was performed under various tempering conditions to manufacture a PC steel rod having a tempered martensite structure.

Table 2 shows the results of evaluation of the mechanical properties, microstructure, delayed fracture characteristics and spot weldability of the above samples. Spot weldability test is for PC steel rod and JI
Performed using SWM-B of SG 3532. After the cross-welding, a tensile test was performed with 10 test pieces, and when the fracture rate of the spot-welded portion was 50% or less, the spot-weldability was good (indicated by a circle). The delayed fracture characteristic was evaluated by using the sample subjected to spot welding and the critical diffusible hydrogen amount described above, and the load stress was evaluated under the condition of 80% of the tensile strength.

Test No. of Table 2 1 to 11 are examples of the present invention,
Others are comparative examples. As can be seen from the table, in each of the examples of the present invention, the tensile strength of the PC steel bar is 1300 MPa or more, the aspect ratio is 1.2 or more, and the aspect ratio with respect to the radius of the PC steel bar is 1.2. Since the above ratio is 0.1 or more, the fracture mode is intragranular cracking, the critical diffusible hydrogen content is higher than that of the conventional PC steel bar,
PC steel rods with excellent delayed fracture properties have been realized. Also, spot weldability is excellent.

On the other hand, No. which is a comparative example. 12, 1
Nos. 3 and 17 are manufactured by the conventional manufacturing method. That is, it is manufactured by quenching and tempering after hot rolling, and is an example in which austenite grains are not elongated. Therefore, the delayed fracture mode is intergranular cracking, the amount of critical diffusible hydrogen is low, and the delayed fracture characteristics are poor.

Comparative example No. No. 16 is an example in which the delayed fracture characteristics were not improved because the aspect ratio was too small. In addition, in Comparative Example No. No. 14 is an example in which the austenite grains were elongated and the aspect ratio was satisfactory, but the delayed fracture characteristics could not be significantly improved due to the slow heating rate during the tempering treatment.

Further, No. 1 which is a comparative example. 15, 18, 1
No. 9 is an example in which the chemical composition of steel is inappropriate. That is, No. In No. 15, since the C content is too low, the high-strength PC steel rod of 1300 MPa or more, which is the object of the present invention, cannot be realized. In addition, No. No. 18 has a C content that is too high. No. 19 is an example in which the spot weldability was poor because the Mn content was too high.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

INDUSTRIAL APPLICABILITY The present invention changes the delayed fracture mode of PC steel rods from intergranular cracks to intragranular cracks by elongating old austenite grains, and delays high strength PC steel rods having a tensile strength of 1300 MPa or more. It is possible to significantly improve the fracture characteristics and to establish the manufacturing method by optimally selecting the chemical composition of steel, hot rolling conditions, and heat treatment conditions. There is something remarkable.

[Brief description of 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 table showing an example of analysis of the relationship between the critical diffusible hydrogen content and the aspect ratio.

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

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication C22C 38/54 C22C 38/54 38/58 38/58 (72) Inventor Toshihiko Takahashi 20 Shintomi, Futtsu-shi -1 Technology Development Division, Nippon Steel Corporation

Claims (4)

[Claims] 1. By weight%, C: 0.15 to 0.40%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0. 1%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0050%, with the balance being Fe and unavoidable impurities, a steel having a tempered martensite structure and at least Also, in the region of 0.1 R (R: radius of PC steel bar) from the surface layer, the ratio of the length and width of the prior austenite grains (hereinafter referred to as the aspect ratio) is 1.2 or more, and the tensile strength is 1300 MPa or more. A high-strength PC steel bar with excellent delayed fracture characteristics. 2. By weight%, 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%, one kind or The high-strength PC steel rod excellent in delayed fracture characteristics according to claim 1, containing two or more kinds. 3. By weight%, C: 0.15 to 0.40%, Si: 0.05 to 2.0%, Mn: 0.2 to 2.0%, Al: 0.005 to 0. 1%, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0050%, the balance being Fe and inevitable impurities. At the time of hot rolling, at least 700 to After a step of hot rolling with a total reduction of 20% or more in a temperature range of 900 ° C., a martensite structure is obtained by cooling with water, and subsequently 10 ° C.
A method for producing a high-strength PC steel rod having excellent delayed fracture characteristics, which comprises heating and tempering in a temperature range of 250 to 550 ° C. at a heating rate of not less than 1 / second. 4. By weight%, 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%, one kind or The method for producing a high-strength PC steel rod having excellent delayed fracture characteristics according to claim 3, characterized in that it contains two or more kinds.