JPH10298713A - Quenched and tempered steel wire rod excellent in delayed fracture resistance in spot-weld zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a PC steel having high strength and excellent in delayed fracture resistance in the spot-weld zone by specifying the compsns. of the inside and surface layer part in a steel, the relation between the content of C in the surface layer and the content of C in the center part and the structure of a base metal in accordance with the thickness from the surface layer. SOLUTION: The inside of the wire rod has a compsn. by weight, 0.25 to 0.5% C, 0.15 to 3.0% Si, 0.3 to 2.0%, Mn, <=0.03% P, <=0.01% S, and the balance Fe or the like, and the surface layer part to 0.2 to 1 mm from the surface layer has a compsn. contg. 0.05 to 0.3% C and the other components by the % same as those in the inside. Then, the C content in the surface layer <=the C content in the center part-0.02 is satisfied. As for the structure of the base metal, the part to a thickness of 0.2 to 1 mm from the surface is composed of mixed phases of martensite and intergranular ferrite or mixed phases of martensite, bainite and intergranular ferrite, the content of ferrite content in the old γ grain boundary is regulated to >=10%, and the center part is composed of martensite single phases or mixed phases of martensite and bainite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a prestressed concrete (PC) steel excellent in spot weldability.

[0002]

2. Description of the Related Art Among concrete poles and piles, concrete poles and piles which are reinforced by compressing concrete for the purpose of improving rigidity and bending strength and preventing cracking of concrete are known as P.
It is called a C pole or a PC pile, and is manufactured by the following method. First, a mild steel wire is spirally wound around a PC steel material arranged in parallel on the circumference (hereinafter referred to as a spiral), and then the intersection of the PC steel and the spiral is fixed to fix the cylindrical cage piece type reinforcing body. (Hereinafter abbreviated as reinforcement). Next, the reinforcing body is introduced into a mold, and both ends of the PC steel material constituting the reinforcing body are fixed, and are tensioned with a stress of about 70% of the tensile strength. After the concrete poured into the mold 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 PC poles and PC piles. 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] Since spot welding has a high cooling rate, the delayed fracture resistance of the heat affected zone (hereinafter referred to as HAZ) tends to decrease due to the formation of a low-temperature transformation structure. In order to improve the delayed fracture resistance of the welded portion, it is necessary to firstly reduce the hardness, and in order to reduce the hardness, it is necessary to reduce the steel composition to low C and low Ceq. It is well known that desirable. However, since the PC steel material is required to have high strength and relaxation properties, it is necessary to make the C steel relatively high.

[0004] For example, as disclosed in Japanese Patent Application Laid-Open No. 62-267420, a method of improving the delayed fracture resistance by lowering the C content on the surface and forming a ferrite structure is known as a method of welding even after spot welding. This is a method that can reduce the hardening of the heat-affected zone. However, it is very difficult to control the morphology of the structure of such a multi-layered steel material having different components in the surface layer and the inside, and a relatively high C is required because high strength and relaxation properties are required for PC steel. Because of the necessity, there is a demand for a steel material that satisfies the combined properties of high strength, relaxation properties and delayed fracture resistance.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a PC steel material which is used for concrete structures such as concrete poles and piles and which has high strength and excellent spot fracture resistance to delayed fracture. It is assumed that.

[0006]

SUMMARY OF THE INVENTION In the case of PC steel used as a pretension material in concrete structures such as concrete poles and piles, generally about 14% is used.
A strength of 00 MPa or more is required. In order to satisfy such tensile strength and suppress the delayed fracture of the spot weld, it is necessary to simultaneously secure the strength at the base material of the steel material and suppress the delayed fracture of the spot weld.

The inventors of the present invention have carried out a detailed study of the amount of C in the surface layer and the structure, which affect the delayed fracture characteristics of the base metal and the spot weld, and have clarified the following. 1) The mixed structure of grain boundary ferrite and martensite improves the delayed fracture resistance of the base material. 2) The delayed fracture resistance of the spot weld is improved by reducing C since the HAZ structure becomes martensite.

[0008] In order to reduce the hardness of martensite in the spot weld, the surface layer is made low in C to reduce the delayed fracture susceptibility. The base metal structure is preferably a ferrite single phase from the viewpoint of suppressing the starting point of delayed fracture, but the strength of the material as a whole is reduced, so that the fracture time cannot be lengthened. For this reason, it is necessary to generate a mixed structure of grain boundary ferrite and martensite in the surface layer in order to reduce the C in the entire area of the HAZ of the spot weld, and to generate martensite in the center to secure the strength. It was found that it was effective in suppressing delayed fracture of materials. In addition, by suppressing the amount of reduction in the C content of the entire material, the relaxation value was improved.

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) The weight of the inside of the wire is C: 0.25 to 0.5
%, Si: 0.15 to 3.0%, Mn: 0.3 to 2.0
%, P: 0.03% or less, S: 0.01% or less, the balance being Fe and unavoidable impurities, and the surface layer portion of the steel wire 0.2 to 1 mm from the surface layer is expressed by weight%. ,
C: 0.05 to 0.3%, Si: 0.15 to 3.0%,
Mn: 0.3 to 2.0%, P: 0.03% or less, S:
0.01% or less, the balance being Fe and unavoidable impurities, and the relationship between the C content of the surface layer and the C content of the central portion is as follows: C content of the surface layer ≦ C content of the central portion−0.02. The matrix structure is a mixed phase of martensite and grain boundary ferrite having a thickness of 0.2 to 1 mm from the surface or a mixed phase of martensite, bainite and grain boundary ferrite, and the former γ grain boundary has a ferrite occupancy of 10%. As described above, a quenched and tempered steel wire excellent in delayed fracture resistance of a spot weld, wherein a central portion is a single phase of martensite or a mixed phase of martensite and bainite.

(2) Further, as a chemical component of the wire rod, Nb: 0.005 to 0.05% and Ti: 0.0% by weight.
05 to 0.15%, Al: 0.10% or less, V: 0.0
0.05 to 0.060%, Cu: 0.05 to 1.0%, N
i: 0.05 to 1.0%, Cr: 0.05 to 1.0%,
Mo: 0.05-0.35%, B: 0.0005-0.
The quenched and tempered steel wire according to the above (1), which comprises one or more kinds of 005% of the steel.

[0011]

Hereinafter, the present invention will be described in detail. The present invention relates to a steel wire rod that has been subjected to quenching and tempering treatment. C: C is added to obtain good mechanical properties and hardenability. If the C content is less than 0.25%, it is difficult to secure the strength by quenching. On the other hand, if the C content exceeds 0.5%, it becomes difficult to secure predetermined strength and toughness by quenching and tempering. Therefore, the C content was set to 0.25 to 0.5%.

Si: Si is added in an amount of 0.15% or more from the viewpoint of ensuring high-temperature relaxation. However, if it exceeds 3.0%, a satisfactory spot welding state cannot be obtained, and welding workability is reduced. Therefore, the amount of Si is 0.15 to 3.0%.
And Mn: Mn is required for uniform elongation and hardenability improvement, but if less than 0.3%, there is no effect. On the other hand, 2.0%
If the amount exceeds the range, the strength improving effect is saturated, and a micro-martensite phase is generated in the central segregation part to lower the stretchability. Therefore, the Mn content is in the range of 0.3 to 2.0%.

P: P must be 0.03% or less 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 0.01% 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 present invention relates to a quenched and tempered steel wire rod. The chemical composition of the surface layer is defined as follows. C: C is added to obtain good mechanical properties and hardenability. If the C content is less than 0.05%, it is difficult to secure the strength by quenching, and the strength of the entire steel wire becomes insufficient.
It is necessary to do above. On the other hand, when the C content exceeds 0.3%, the HAZ hardness of the spot weld increases, which promotes delayed fracture. Therefore, the range of the amount of C is 0.05-0.3%
And

Si: Si is added in an amount of 0.15% or more from the viewpoint of ensuring high-temperature relaxation. However, if it exceeds 3.0%, a satisfactory spot welding state cannot be obtained, and welding workability is reduced. Therefore, the amount of Si is 0.15 to 3.0%.
And Mn: Mn is required for uniform elongation and hardenability improvement, but if less than 0.3%, there is no effect. On the other hand, 2.0%
If the amount exceeds the range, the strength improving effect is saturated, and a micro-martensite phase is generated in the central segregation part to lower the stretchability. Therefore, the Mn content is in the range of 0.3 to 2.0%.

P: P is required to be 0.03% or less because P segregates at the grain boundary to 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 0.01% 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.

In order to secure the material properties of the entire steel wire, not only control of the above components, but also mechanical properties and hardenability by the internal high C structure, and spotting by reducing the surface C In order to suppress the delayed fracture of the welded portion, it is necessary that at least C content of the surface layer ≦ C content of the central portion−0.02.
The steel wire rod of the present invention contains the above-mentioned components, and the balance consists of Fe and unavoidable impurities. However, in order to further improve the properties, the steel wire rod may contain one or more of the following elements. .

Nb: Nb refines the structure by the pinning effect of Nb precipitates and improves the delayed fracture resistance. For that purpose, 0.005% or more must be added. However, if added in excess of 0.05%, a large amount of coarse NbCN precipitates, thus deteriorating the material properties. Therefore, the upper limit of Nb is set to 0.05%. Ti: Ti refines the structure by the pinning effect of Ti precipitates and improves delayed fracture resistance. for that purpose,
It is necessary to add 0.005% or more. However, 0.1
If more than 5% is added, a large amount of coarse TiN precipitates,
Deteriorates material properties. Therefore, the upper limit of Ti is set to 0.1
5%.

Al: If the content of Al exceeds 0.10%, the cleanliness of the steel decreases and the hot workability deteriorates.
10% or less. V: V precipitates carbonitrides, refines γ grains, acts as a trap site for hydrogen in steel, and improves delayed fracture resistance. For that purpose, 0.005% or more must be added, and the lower limit is set to 0.005%.
However, since the effect is saturated when a large amount is added, the upper limit is set to 0.060%.

Cu: hardenability when Cu is less than 0.05%,
Since the corrosion resistance was not sufficiently improved, the lower limit was set to 0.05%. However, if it exceeds 1.0%, hot cracking occurs, so the upper limit was made 1.0%. Ni: If Ni is less than 0.05%, hardenability and corrosion resistance are 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, but the effect is insufficient if less than 0.05%. However, if the content 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, at least 0.05
The effect is not recognized unless% is added. Also, 0.3
If added in excess of 5%, the hardness at the spot weld increases, and the delayed fracture resistance deteriorates. For this reason, Mo
Range of 0.05 to 0.35%.

B: If B is less than 0.0005%, the hardenability is not sufficiently improved, so 0.0005% was made the lower limit. However, if the content exceeds 0.005%, the effect is saturated, so the upper limit is set to 0.005%. When assembling the reinforcement by spot welding, to obtain sufficient joint strength,
Given the required heat input, the HAZ is 0.2-1 mm deep. Therefore, if the thickness of the surface layer is less than 0.2 mm, the central portion of high C is hardened by the influence of heat and the delayed fracture resistance deteriorates. However, if the thickness of the low-C, low-strength surface layer exceeds 1 mm, the strength of the entire steel wire cannot be maintained at a level required for a PC steel material. For this reason, the low C region of the surface layer is 0.2
１1 mm.

If the thickness of the mixed phase of martensite and grain boundary ferrite or the mixed phase of martensite, bainite and grain boundary ferrite in the surface layer is less than 0.2 mm, the delayed fracture resistance cannot be improved, and If this thickness exceeds 1 mm, the desired mechanical properties cannot be achieved only by heat treatment, so the thickness was set to 0.2 to 1.0 mm. Also,
Unless the occupation ratio of the ferrite in the old γ grain boundary becomes 10% or more, delayed fracture cannot be suppressed.

Although the wire diameter of the steel wire rod of the present invention is not particularly limited, a normal diameter of 7 to 12 mmφ can easily secure mechanical properties and delayed fracture characteristics. There is no problem in applying to other wire diameters. The steel wire of the present invention is obtained by quenching and tempering. It is heated above the Ac ₃ transformation point, turned into a solution, cooled from the austenite region to below the MS point, hardened to form a structure mainly composed of martensite, and then toughened by tempering to secure high toughness. .
In this case, the delayed fracture resistance is improved by rapidly cooling during quenching to refine the structure. In the present invention, the quenching and tempering temperature is not described because it differs depending on the components and the target strength. However, it is preferable that the quenching temperature be 850 ° C or higher and the tempering temperature be 250 ° C or higher.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment An embodiment of the present invention will be described below. Tables 1 and 2 (continuation-1 in Table 1), Table 3 (continuation-2 in Table 1), and Table 4 (continuation-3 in Table 1) using surface decarburization and multilayer slabs. Adjusted to chemical composition. Occupancy is 30
The grain boundary ferrite length in the prior γ grain size was determined by SEM observation at × 00. Table 5 shows the material properties of each material.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

[Table 5]

[0031] Steels 1 to 14 of the present invention exhibited good delayed fracture resistance and mechanical properties. Steel 15 promoted delayed fracture due to the small thickness of the surface layer. Steel 16 had insufficient strength due to a low C content in the surface layer. Steel 17 was not able to obtain a predetermined strength due to a low internal C content. Steel 18 had a large thickness of the surface layer, and thus could not obtain a predetermined strength, and the relaxation characteristics were deteriorated. In Steel 19, the internal C was high, the strength was excessive, and the delayed fracture susceptibility of the base metal increased. Steel 20 has surface C
Since the amount was high and the difference from the inside was as low as 0.01%, the delayed fracture resistance deteriorated. Steel 21 had insufficient strength because the surface layer was a ferrite single phase. Steel 22 has a low occupation ratio, so that the delayed fracture resistance cannot be improved.

[0032]

Industrial Applicability According to the present invention, a steel material for PC steel wire having high strength and excellent in delayed fracture resistance of a spot weld can be obtained, which is industrially very useful.

Claims (2)

[Claims] 1. Inside of a wire is C: 0.25 to 0.5%, Si: 0.15 to 3.0%, Mn: 0.3 to 2.0%, P: 0. 0.3% or less, S: 0.01% or less, the balance being Fe and unavoidable impurities,
And the surface portion of the steel wire of 0.2 to 1 mm from the surface layer is represented by weight%
C: 0.05 to 0.3%, Si: 0.15 to 3.0%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.01% or less , The balance consisting of Fe and unavoidable impurities,
The relationship between the C content in the surface layer and the C content in the central portion is as follows: C content in the surface layer ≦ C content in the central portion−0.02, and the base material structure is 0.2 to 1 mm thick from the surface of martensite and grains. A mixed phase of intergranular ferrite or a mixed phase of martensite, bainite and grain boundary ferrite. The ferrite occupancy of the former γ grain boundary is 10% or more, and the center is a single phase of martensite or a mixed phase of martensite and bainite. A quenched and tempered steel wire excellent in delayed fracture resistance of spot welds, characterized in that: 2. The composition according to claim 1, further comprising:
Nb: 0.005 to 0.05%, Ti: 0.005 to 0.15%, 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% 2. The quenched and tempered steel wire according to claim 1, wherein the steel wire comprises two or more types.