JPH10298648A - High uniform elongation Low yield ratio - Google Patents

Abstract

(57) Abstract: The present invention provides a method for producing a PC steel excellent in spot weldability. SOLUTION: In weight%, C: 0.1-0.4%, S
i: 0.5 to 2.5%, Mn: 0.3 to 2.0%, P:
A wire rod containing 0.03% or less, S: 0.02% or less, and the balance consisting of Fe and unavoidable impurities is heated to an austenite region, and from this temperature region to 300 to 400 ° C.
Rapidly cooled to a temperature range of 5 to 150 ° C., and kept at a constant temperature for 5 to 150 seconds.
A high uniform elongation and low yield ratio having tempered bainite or tempered martensite or a composite structure containing retained austenite in the structure, characterized by immediately quenching or cooling after tempering for 150 seconds. A method for manufacturing high-tensile steel.

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 (prestressed concrete) steel material having excellent spot weldability and having high ductility at a tensile strength of 1000 MPa or more.

[0002]

2. Description of the Related Art Among concrete piles, those which compress and strengthen concrete for the purpose of improving rigidity and bending strength and preventing cracking of concrete are called PC piles and are manufactured by the following method. First, after a mild steel wire is spirally wound around a PC steel material arranged in parallel on a circumference (hereinafter referred to as a spiral), an intersection of the PC steel material and the spiral is fixed, and a cylindrical cage piece type reinforcing body ( (Hereinafter abbreviated as reinforcement). Next, this reinforcing body was introduced into a mold, and both ends of the PC steel material constituting the reinforcing body were fixed, and 70% of the tensile strength was obtained.
Tension due to front and rear stress. After the concrete poured into the form is solidified, the tension of the PC steel material is removed, and at the same time, a compressive force is applied to the concrete to produce a PC pile. During this manufacturing process, in order to automate the assembling of the reinforcing member, a heat-treated reinforced PC steel material of low-medium carbon steel having good weldability is used, and fixing of the helical muscle to the PC steel material is performed by spot welding.

[0003] In recent years, with the increase in the size of steel structures, the steel materials used for the members tend to have higher strength. For example, a PC steel rod used as a main reinforcing bar in a concrete pile has a tensile strength of 1420 MPa or more as specified in JIS G3109. Generally, the strength and ductility of a wire are opposite to each other, and the ductility represented by elongation decreases as the strength of a high-strength steel increases. In particular, the value of uniform elongation drops extremely. At present, PC steel rods, which are widely used, are given predetermined strength and ductility by quenching and tempering a hot-rolled material. However, when the tempered martensite structure is used to increase the strength, the uniform elongation is about 3%. When the uniform elongation is low, when a large impact load such as an earthquake is applied to the concrete structure, the PC steel bar breaks and no longer functions as a reinforcing bar.

Japanese Patent Application Laid-Open No. 8-158010 discloses a high S
High uniform elongation is achieved with a ferrite-containing structure in the i-Al system. However, in the case of a high Si-Al system, a large amount of scale that reduces the electrical conductivity during heat treatment is generated, and spot weldability is reduced. Also, Japanese Patent Application Laid-Open No. 57-1206
In Japanese Patent Publication No. 22, bainite is produced to achieve high uniform elongation of high-tensile steel. However, in recent years, in order to secure the earthquake resistance of concrete structures, a low yield ratio (low YR) as well as a high uniform elongation tends to be required, but Japanese Patent Application Laid-Open No. 57-120622 discloses a low yield ratio. No study has been conducted.

[0005] Therefore, a high-strength steel material having excellent spot weldability, high uniform elongation, and low yield ratio is required from the viewpoint of ensuring the earthquake resistance of concrete structures.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a PC steel material having excellent spot weldability used for concrete structures such as concrete poles and piles.

[0007]

SUMMARY OF THE INVENTION In the case of PC steel used for concrete structures such as concrete poles and piles, a strength of about 1300 MPa or more is generally required. In order to satisfy both characteristics of high uniform elongation and spot weldability in such a high-tensile steel, the present inventors conducted detailed examination of the effects of manufacturing conditions on uniform elongation characteristics and yield ratio. . As a result, high tensile strength can be achieved by generating fine martensite or bainite by maintaining fine austenite at a constant temperature of 300 to 400 ° C. in high-tensile steel, and high ductility can be achieved by forming bainite. Was found. Further, as shown in FIG. 1, it has been found that a lower yield ratio can be achieved by shortening the isothermal transformation time. This is because if the isothermal transformation time is shortened, the martensite fraction increases, and yielding easily occurs.

Further, bainite effective for uniform elongation and martensite for lowering the yield ratio are stably formed by isothermal transformation at 300 to 400 ° C., and subsequently 400 to 550 ° C.
It has been found that tempering in the temperature range described above can optimize strength and achieve high uniform elongation mainly by tempering martensite that has been excessively hardened in the early stage. The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) C: 0.1 to 0.40% by weight
%, Si: 0.5 to 2.5%, Mn: 0.3 to 2.0
%, P: 0.03% or less, S: 0.02% or less, the balance being Fe and unavoidable impurities.
Heat to austenite range, and from this temperature range 300
Quenched to a temperature range of ~ 400 ° C,
Tempered bainite or tempered martensite or a structure characterized by being quenched or immediately cooled after tempering for 50 seconds at a constant temperature and subsequently for 5 to 150 seconds in a temperature range of 400 to 550 ° C. For producing a high-tensile-strength, low-yield-ratio, high-tensile-strength steel material having a composite structure containing residual austenite.

(2) Further, as chemical components of the wire rod, Nb: 0.005 to 0.5% and Ti: 0.00% by weight.
5 to 0.5%, Al: 0.10% or less, V: 0.005
0.060%, Cu: 0.05 to 1.0%, Ni:
0.05-1.0%, Cr: 0.05-1.0%, M
o: 0.05-0.35%, B: 0.0005-0.0
The method for producing a high-tensile steel material having a high uniform elongation and a low yield ratio according to claim 1, wherein one or more kinds of the steels are contained at a content of 05%.

[0011]

Hereinafter, the present invention will be described in detail. First,
The reasons for limiting the chemical components of the present invention will be described. C: C is added to increase the strength of martensite or bainite, but its effect is small if less than 0.1%. On the other hand, if C is added in an excessive amount, the hardness of the spot weld increases, and the susceptibility to weld cracking increases. Therefore, the upper limit is set to 0.4%.

Si: Si is an element necessary for securing strength and relaxation properties, but if less than 0.5%, the effect is not obtained. On the other hand, if the Si content exceeds 2.5%, a large amount of scale harmful to spot welding is generated.
The upper limit was set to 2.5%. Mn: Mn is required for uniform elongation and hardenability improvement, but if less than 0.3%, there is no effect. On the other hand, even if Mn is added in an amount of more than 2.0%, the strength improving effect is saturated, and micro-martensite is generated in the central segregation portion to lower the stretchability. Therefore, the Mn content is in the range of 0.3 to 2.0.

P: P must be not more than 0.03% in order to segregate at the grain boundaries and easily cause grain boundary embrittlement. It is desirable that P, which is an impurity element, be reduced as much as possible. S: S is also required to be 0.02% or less because S is also segregated at the grain boundary similarly to P and easily causes grain boundary embrittlement. It is desirable that S, which is an impurity element, be reduced as much as possible.

The steel material of the present invention contains the above-mentioned elements, with the balance being Fe and unavoidable impurities. In order to further improve the properties, one or more of the following elements are used.
It can contain more than one species. Nb: Nb refines austenite grains by the pinning effect of Nb precipitates and improves ductility. For that purpose, 0.005% or more must be added. However,
Addition of more than 0.05% increases the hardness of the spot weld and increases the susceptibility to weld cracking. Therefore, the appropriate range of Nb is set to 0.005 to 0.05%.

Ti: Ti refines the structure similarly to Nb due to the pinning effect of Ti precipitates. For that, 0.
005% or more must be added. However, 0.05%
If added excessively, coarse TiN precipitates in large quantities, deteriorating the material properties. Therefore, the upper limit of Ti is set to 0.05%. Al: Al is added to steel as a deoxidizing element, but if added in excessive amounts, coarse alumina-based oxides are formed and ductility is reduced, so the upper limit was made 0.10%.

V: V precipitates carbonitride and refines γ grains to improve strength and ductility. For that, 0.00
It is necessary to add 5% or more, and the lower limit was made 0.005%. However, the effect is saturated with a large amount of addition, so the upper limit was made 0.060%. Cu: If Cu is less than 0.05%, hardenability is not sufficiently improved, so 0.05% was made the lower limit. However, 1.0
%, Hot cracking is caused.
0%.

Ni: If Ni is less than 0.05%, the hardenability is not sufficiently improved, so 0.05% was made the lower limit. However, if the content exceeds 1.0%, the effect is saturated, so the upper limit is set to 1.0%. Cr: Cr increases the strength of the steel by solid solution strengthening and hardenability improvement. However, if the content is less than 0.05%, the effect is insufficient, so the lower limit is made 0.05%. On the other hand, if it exceeds 1.0%, the effect is saturated, so the upper limit is set to 1.0%.

Mo: Mo is an element effective for improving relaxation characteristics. However, the effect is not recognized unless at least 0.05% is added.
Further, when the addition exceeds 0.35%, the crack susceptibility of the spot weld increases. Therefore, the Mo component range is set to 0.05.
-0.35%. B: If B is less than 0.0005%, the hardenability is not sufficiently improved, so 0.0005% was made the lower limit. But,
If the content exceeds 0.005%, the effect is saturated, so the upper limit was made 0.005%.

Next, the reasons for limiting the heat treatment conditions will be described. When subjected to isothermal transformation at a temperature higher than 400 ° C., pseudo-pearlite is partially generated, a predetermined strength cannot be obtained, and stable bainite transformation does not proceed. On the other hand, in a temperature range lower than 300 ° C., bainite transformation hardly occurs, and high ductility cannot be achieved. As described above, it is necessary to increase the generation rate of bainite in order to achieve high uniform elongation, and it is necessary to increase the martensite fraction in order to reduce the yield ratio. It is necessary to change the isothermal transformation temperature accordingly. In the present invention, the balance of the fractions of the structure is not mentioned, but when it is used as a normal strength PC steel, the temperature is desirably 350 ° C. or less.

Further, when the bainite transformation is terminated, the martensite fraction at the time of constant temperature transformation is reduced, so that a low yield ratio cannot be achieved. For this reason, the constant temperature transformation time needs to be 150 seconds or less. On the other hand, when the isothermal transformation time is less than 5 seconds, bainite transformation hardly occurs, and uniform elongation decreases. Continuously tempering to a temperature range of 400 to 550 ° C. continuously to mainly adjust the strength. If the tempering temperature exceeds 550 ° C., the strength is significantly reduced. Also, 40
If the temperature is lower than 0 ° C., the effect of tempering cannot be obtained, and the uniform elongation cannot be improved. Therefore, the tempering temperature is 400 to 550.
° C. If the tempering time is less than 5 seconds, there is no effect.
Even if the time exceeds 50 seconds, the effect on the strength is almost saturated.

Next, the reason for defining the organization will be described. It is necessary to form a composite structure of bainite and martensite to ensure strength, and to form a bainite structure to ensure ductility. For this reason, bainite and martensite are generated during the isothermal transformation. In order to secure a desired strength, a composite structure of tempered bainite and tempered martensite is used. That is, from the viewpoint of ensuring high uniform elongation, it is necessary to increase the generation of bainite, but by tempering martensite, a decrease in uniform elongation can be suppressed. In addition, martensite is generated for lowering the yield ratio.

In the present invention, since the amount of Si is high, a small amount of retained austenite is generated, but does not deteriorate the material properties. For this reason, even if it is quenched or left to cool after the tempering process, the material properties do not deteriorate. Therefore, quenching or cooling is selected as needed.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment An embodiment of the present invention will be described below. Using test steels with chemical components shown in Tables 1 and 2 (continued from Table 1),
The heat treatment shown in Table 3 was performed. Table 4 shows the results of the mechanical properties and the spot weldability of the steel material obtained in this manner. Further, the method of evaluating the uniform elongation in the tensile test was such that the GL (evaluation part) was 30d, and the average value of the uniform elongation calculated from the breaking position and the length excluding both sides thereof was shown.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

The steels 1 to 10 of the present invention satisfied the material characteristics. Since steels 11 to 14 are not appropriate steel components, good mechanical properties or weldability could not be secured. Steel 11 had insufficient strength due to low C content. Steel 12 had a high C content and reduced spot weldability. Steel 13 had a low Si content and reduced relaxation characteristics. In the steel 14, the spot weldability was reduced due to excessive addition of Si.

[0029] Steels 15 to 21 were not under appropriate production conditions, and good material properties could not be obtained. Since steel 15 had a long isothermal transformation time, the martensite fraction was reduced, and a low yield ratio could not be achieved. In steel 16, bainite was not generated because the isothermal transformation time was shortened, and uniform elongation was reduced. Steel 17 had a high isothermal transformation temperature and generated pearlite, resulting in a decrease in strength. Steel 18 had a low isothermal transformation temperature, a reduced bainite content, and reduced uniform elongation. Steel 19 had a high tempering temperature after the isothermal transformation, and thus had a reduced strength.

[0030]

According to the present invention, high strength P excellent in weldability is obtained.
A steel material for C steel wire can be obtained, which is industrially very useful.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a constant temperature transformation time and a yield ratio.

Claims (2)

[Claims] C .: 0.1 to 0.40%, Si: 0.5 to 2.5%, Mn: 0.3 to 2.0%, P: 0.03% or less by weight% S: 0.02% or less, and the balance consisting of Fe and unavoidable impurities is heated to an austenite region, rapidly cooled from this temperature region to a temperature region of 300 to 400 ° C. Hold at constant temperature for 150 seconds, then 400-550 ° C
After performing tempering for 5 to 150 seconds in the temperature range of
A method for producing a high-tensile-strength, low-yield-ratio, high-strength steel material having a tempered bainite or tempered martensite or a composite structure containing retained austenite in the structure, which is immediately quenched or left to cool. 2. The composition according to claim 1, further comprising:
Nb: 0.005 to 0.5%, Ti: 0.005 to 0.5%, Al: 0.10% or less, V: 0.005 to 0.060%, Cu: 0.05 to 1 0.0%, Ni: 0.05 to 1.0%, Cr: 0.05 to 1.0%, Mo: 0.05 to 0.35%, B: 0.0005 to 0.005% The method for producing a high-tensile-strength steel material having a high uniform elongation and a low yield ratio according to claim 1 or 2 or more.