JP2001164337A - High tensile strength steel excellent in delayed fracture characteristic and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce PC steel excellent in delayed fracture characteristics and to provide a method for producing the same. SOLUTION: In this high tensile strength steel rod or steel wire excellent in delayed fracture characteristics, the central part of the steel is provided with tempered martensite in which the fractional ratio of intergranular ferrite or pearlite is <=5%, the difference between the hardness of the surface layer and the hardness of the central part is <=100 Hv, and also, the maximum hardness of the surface layer is lower than the value obtained by adding 50 Hv to the average value of the cross-sectional hardness, and in method for producing the same, a wire rod with a wire diameter of <=9 mm obtained by subjecting a billet containing, by mass, 0.1 to 0.40% C, 0,10 to 0.50% Si, 0.3 to 1.0% Mn, <=0.03% P, <=0.02% S, <=0.005% Al, and the balance iron with inevitable impurities and cast by a billet continuous casting method to hot rolling and thereafter executing cold drawing is quenched in the temperature range of 850 to 1050 deg.C and is tempered in a state of being heated to the temperature range of 350 to 550 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PC (prestressed concrete) steel excellent in spot weldability and a method for producing the same. In particular, the present invention relates to a high-strength steel rod or a steel wire having high ductility at a tensile strength of 1200 MPa or more and excellent in delayed fracture resistance, and a method for producing the same.

[0002]

2. Description of the Related Art Among concrete piles, concrete piles which are reinforced by compressing concrete for the purpose of improving rigidity and bending strength and preventing cracking of concrete are called PC piles and are manufactured by a method described below. 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 muscle), an intersection of the PC steel material and the spiral muscle is fixed, and a cylindrical cage piece type reinforcing body ( (Hereinafter abbreviated as reinforcement)
To manufacture. Next, this reinforcement was introduced into a mold, and both ends of a PC steel material constituting the reinforcement were fixed to 70% of the tensile strength.
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 reinforcement in a concrete pile has a tensile strength of 1420 MPa or more as specified in JIS G3109. After the refining of the converter is completed, the steel produced as wire rod steel is mainly cast into a billet by a continuous casting method. Free oxygen contained in molten steel at the completion of refining is removed as an oxide by adding a deoxidizing material prior to casting. A typical example of the deoxidizing material is deoxidizing using Al. Al ₂ O ₃ as a deoxidation product generated as a result of deoxidation is mostly floated and separated in molten steel, but a part thereof remains in the steel, and remains in the steel after continuous casting. The remaining coarse inclusions increase the hydrogen sensitivity after heat treatment. For this reason, a steel material having excellent delayed fracture resistance is required as a PC steel material by billet casting.

[0004]

In general, in the continuous billet casting method, inclusions tend to be finely dispersed, and the reheated γ grains tend to be fine, and the delayed fracture resistance is improved. However, in the case of a large-diameter PC steel rod having a wire diameter of 9 mm or more, the cooling rate at the center is slowed down, hardenability is reduced, and ferrite or pearlite structure is easily generated, resulting in a non-uniform structure.

[0005] In the case of PC steel materials used for concrete structures such as concrete poles and piles, as described above, a strength of about 1200 MPa or more is required. In order to satisfy the delayed fracture characteristics with such a high-strength steel, the present inventors have conducted detailed studies on the relationship between the structure and hardness and the delayed fracture. As a result, the following facts became clear. A uniform structure has better delayed fracture resistance.

[0006] The finer the structure of tempered martensite, the better the delayed fracture resistance. That is, in the billet casting method, inclusions are finely dispersed, the structure is finely divided by the pinning effect, and the delayed fracture characteristics are improved. But,
When quenching by rapid heating and rapid cooling with a large diameter, the γ grain size is reduced by the billet casting method, so that ferrite and pearlite are likely to be formed in the central portion where the cooling rate is slow. For this reason, a non-uniform structure softens at the center and hardens at the surface layer. As a result, it is necessary to increase the cooling rate during quenching in order to increase the strength of the entire steel, and the delayed fracture susceptibility in the surface layer further increases.

In a uniform structure for improving delayed fracture resistance, it is necessary to suppress the difference between the hardness of the surface layer and the hardness of the center portion because the additional load during the delayed fracture test is uniformly applied. Further, the fracture origin of delayed fracture is near the surface layer, and it is necessary to lower the hardness of the surface layer in order to reduce the susceptibility to cracking. For this reason, development of a steel material having excellent delayed fracture resistance as a PC steel material by billet casting has been desired.

[0008]

SUMMARY OF THE INVENTION The present invention has been proposed to solve the above problems, and has a tensile strength of 120.
A steel material including a high-strength steel bar or a steel wire having high ductility at a strength level of 0 MPa or more and excellent in delayed fracture resistance and a method for producing the same, and the gist thereof are as follows. In addition, the mass% defined in the present invention means weight%.

(1) In mass%, C: 0.1 to 0.4%,
Si: 0.1-0.5%, Mn: 0.3-1.0%,
P: 0.03% or less, S: 0.02% or less, Al: 0.
005% or less, the balance being iron and unavoidable impurities, having a wire diameter of 9 mm or more, and having a tempered martensite structure with a grain boundary ferrite or pearlite fraction of 5% or less at the center of the steel, High tensile strength with excellent delayed fracture characteristics characterized in that the difference between the hardness of the surface layer and the hardness at the center is 100 Hv or less, and the maximum hardness of the surface layer is lower than the value obtained by adding 50 Hv to the average value of the cross-sectional hardness. Steel.

(2) Further, as a steel material component, in mass%,
Nb: 0.005 to 0.05%, Ti: 0.005 to
The high-tensile steel material having excellent delayed fracture properties according to claim 1, wherein one or more of 0.05% and V: 0.002 to 0.020% are contained.

(3) Further, as a steel material component, by mass%,
Cu: 0.02-0.1%, Ni: 0.02-0.1
%, Cr: 0.02 to 0.1%, Mo: 0.02 to 0.
The high-tensile steel material having excellent delayed fracture characteristics according to claim 1 or 2, comprising one or more of 10% and B: 0.0005 to 0.005%.

(4) Further, as a steel material component, in mass%,
Ca: 0.0005-0.005%, REM: 0.00
0.05 to 0.005%, Mg: 0.0005 to 0.007
The high-tensile steel material having excellent delayed fracture characteristics according to any one of claims 1 to 3, further comprising one or more of

A slab having a steel material composition according to any one of claims 1 to 4, which is cast by a billet continuous casting method, is hot-rolled and then cold-drawn to obtain a steel material having a wire diameter of 9 mm or more. A method for producing a high-tensile steel material having excellent delayed fracture characteristics, characterized in that the material is quenched after heating to a temperature range of from 10 to 1050 ° C, then rapidly heated to a temperature range of 350 to 550 ° C, and then tempered.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, chemical components used in a high-tensile steel having excellent delayed fracture characteristics according to the present invention are defined as follows. C: C is added together with increasing the strength of martensite, but less than 0.1% has little effect. On the other hand, if C is added in an excessive amount, the hardness of the spot-welded portion increases and the susceptibility to weld cracking increases, so the upper limit is made 0.4%.

Si: Si is an element necessary for securing strength and relaxation properties, and if less than 0.1, there is no effect. On the other hand, if the Si content exceeds 0.5%, the hot workability decreases, so the upper limit was made 0.5%. Mn: Mn is necessary for improving uniform elongation and hardenability. If less than 0.3%, the effect is not obtained. Even if added over 1.0%, the strength improving effect is saturated. In addition, micro-martensite is generated in the center segregation part, and the stretchability is reduced. M
The n amount is in the range of 0.3 to 1.0.

P: P must be made 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 0.02% or less because S also segregates 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. Al: In the billet continuous casting method, it is necessary to reduce Al as much as possible. If the content of Al exceeds 0.005%, the composition of the generated inclusions is mainly composed of Al ₂ O ₃ and is likely to be coarse.
In addition, it is likely to cause nozzle blockage. Therefore, the upper limit is made 0.005%.

In the present invention, in addition to the above elements, the balance is Fe and inevitable impurities. Further, in order to improve the characteristics, Nb, Ti, V, Cu, Ni,
One or more of Cr, Mo, B, Ca, REM, and Mg are contained as necessary. Nb: Nb refines austenite grains and improves ductility by the pinning effect of Nb precipitates. 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 is set to 0.05%. V: V precipitates carbonitrides, refines γ grains, and improves strength and ductility. For that purpose, 0.002% or more must be added, and the lower limit is made 0.002%. However, the effect is saturated with a large amount of addition, so the upper limit was made 0.020%.

Cu: If the Cu content is less than 0.05%, the hardenability is not sufficiently improved, so 0.02% was made the lower limit value. However, if it exceeds 0.1%, hot cracking will occur, so the upper limit is made 0.1%. Ni: If Ni is less than 0.05%, the hardenability is not sufficiently improved, so 0.02% was made the lower limit. However, 0.1
%, The effect is saturated, so the upper limit is set to 0.1%.

Cr: Cr increases the strength of steel for solid solution strengthening and hardenability, but the effect is insufficient if it is 0.02% or less. However, when the content exceeds 0.1%, the effect is saturated, so the upper limit is set to 0.1%. Mo: Mo is an element effective for improving relaxation characteristics. However, at least 0.02
%, The effect is not recognized. Also,
If added in excess of 0.10%, the crack susceptibility of the spot weld increases. Therefore, the component range of Mo is set to 0.02 to 0.1.
It was set to 0%.

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%. Next, the reasons for limiting the heat treatment conditions will be described. Ca: Ca is an element effective for refining the structure. If it is less than 0.0005, there is no effect.
% Was defined as the lower limit. However, when the content exceeds 0.005%, the cleanliness decreases and the inclusions become coarse, so the upper limit value is set to 0.005%.

REM: REM, like Ca, is also an effective element for refining the structure. If it is less than 0.0005, there is no effect, so 0.0005% is set as the lower limit. However, when the content exceeds 0.005%, the cleanliness decreases and the inclusions become coarse, so the upper limit value is set to 0.005%. Mg: Mg is added in an amount of 0.0005% or more because steel forms fine oxides in the steel and makes austenite fine. However, if Mg is added in excess of 0.007%, the oxide becomes coarse and the wire drawing workability is reduced. Therefore, the upper limit is made 0.007%.

Next, a method for producing a high-tensile steel material according to the present invention will be described. Generally, inclusions can be refined by casting a slab by a continuous billet casting method. Hot rolled or cold drawn slabs of 850 to 1050
Quenching is performed by rapid heating and cooling to a temperature range of ° C. Unless heated to 850 ° C. or higher, an untransformed structure remains and a predetermined strength cannot be obtained. On the other hand, when heating is performed at a temperature exceeding 1050 ° C., γ grains are coarsened and the delayed fracture resistance is reduced. The rapid heating is performed so as not to coarsen the γ grains, and the rapid cooling is performed so as to generate martensite.

The tempering temperature is in the range of 350 to 550 ° C. If the temperature is lower than 350 ° C., the strength becomes too high, and delayed fracture easily occurs. On the other hand, if the temperature is 550 ° C. or higher, the strength decreases, and it is not possible to secure predetermined material properties as PC steel.
State the reason for defining the texture and hardness. In order to secure the strength during the delayed fracture test, the fraction of grain boundary ferrite or pearlite at the center of the steel must be 5% or less. It is necessary to increase the hardness of the surface layer when the fraction of grain boundary ferrite or pearlite in the center of the steel is 5% or more, and as a result, the delayed fracture resistance deteriorates. Desirably, it is a tempered martensite single layer.

The difference between the hardness of the surface layer and the hardness of the central part is 100 Hv
If it becomes above, the intensity | strength in a center part will fall and the additional load to a surface layer part will increase apparently. When the maximum hardness of the surface layer is higher than a value obtained by adding 50 Hv to the average value of the cross-sectional hardness, the delayed fracture sensitivity increases. The present invention does not refer to a technique for making the hardness uniform. For example, a small amount of an alloy element may be added to improve hardenability. However, in the present invention, it is added only for the purpose of suppressing the formation of ferrite and pearlite in the central portion, and it is not necessary to add excessively.

FIG. 1 shows the hardness distribution of the C section of the present invention and the conventional steel. As described above, in the conventional steel, the distribution of hardness is obviously not uniform, and especially the height distribution of the surface layer is high, and the hydrogen cracking sensitivity is increased. On the other hand, the steel of the present invention has a substantially constant hardness.

[0027]

Embodiments of the present invention will be described below. Table 1
Table 3 shows the chemical composition, production conditions and material results of the test steel. 12
A 5 mm square billet was cast, and the hot rolled wire was quenched and tempered. In the conventional method using comparative steel, a bloom cast material was machined into a 125 mm square and then heat-treated.
For the hardness distribution, the Vickers hardness of the C section was measured.

Since steel 10 is not an appropriate steel component, mechanical properties or weldability could not be ensured. Steel 10 is Al
The inclusion amount was too large to control inclusions, and the delayed fracture resistance was reduced. Steels 11 to 12 do not have proper production conditions, and cannot obtain material properties. In Steel 11, since the billet casting method was not performed, the gamma grains were coarsened and the delayed fracture resistance was reduced. In Steel 12, the tempering temperature was low, so the structure was hardened and the delayed fracture resistance was reduced. Steel 13-15
In this case, material properties cannot be obtained because proper structure and hardness are not secured. In steel 13, since the ferrite fraction in the center is high, in steel 14, the difference between the hardness of the surface layer and the hardness of the center is 1
Since steel H is at least 00 Hv, the difference between the maximum hardness of the surface layer and the average value of the cross-sectional hardness of steel 15 is 50 Hv or more, and the delayed fracture resistance deteriorated.

[0029]

[Table 1]

[0030]

According to the present invention, a high-strength steel material for a PC steel wire excellent in delayed fracture resistance can be obtained, which is industrially very useful.

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[Procedure amendment]

[Submission Date] January 20, 2000 (2000.1.2
0)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Added

[Correction contents]

[Brief description of the drawings]

FIG. 1 is a diagram showing the hardness distribution of the C section of the present invention steel and conventional steel.

Continued on the front page (72) Inventor Satoshi Sugimaru 1 Kimitsu, Kimitsu City, Chiba Prefecture Inside the Nippon Steel Corporation Kimitsu Works (72) Inventor Hiroshi Oba 1 Kimitsu, Kimitsu City, Chiba Prefecture Kimitsu Corporation F term in the steelworks (reference) 4K043 AA02 AB00 AB01 AB02 AB04 AB07 AB10 AB13 AB15 AB18 AB21 AB22 AB25 AB26 AB27 AB29 AB30 AB33 BB01 BB02 BB06 BB08 DA01 DA04 EA04 FA03 FA12 FA13

Claims (5)

[Claims] 1. Mass%, C: 0.1 to 0.4%, Si: 0.1 to 0.5%, Mn: 0.3 to 1.0%, P: 0.03% or less, S: 0.02% or less, Al: 0.005% or less, the balance being iron and unavoidable impurities, a wire diameter of 9 mm or more, and a fraction of grain boundary ferrite or pearlite at the center of the steel of 5%. % Or less, the difference between the hardness of the surface layer and the hardness of the central part is 100 Hv or less,
A high tensile strength steel excellent in delayed fracture characteristics, characterized in that the maximum hardness of the surface layer is lower than a value obtained by adding 50 Hv to the average value of the cross-sectional hardness. 2. As a steel component, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, V: 0.002 to 0.020% by mass%. The high-tensile steel material having excellent delayed fracture characteristics according to claim 1, wherein the steel material contains two or more kinds. 3. Further, as a steel material component, in mass%, Cu: 0.02 to 0.1%, Ni: 0.02 to 0.1%, Cr: 0.02 to 0.1%, Mo: 0 3. A high-tensile steel material having excellent delayed fracture characteristics according to claim 1 or 2, wherein the steel material contains one or more of 0.02 to 0.10% and B: 0.0005 to 0.005%. . 4. As a steel material component, one of the following mass%: Ca: 0.0005 to 0.005%, REM: 0.0005 to 0.005%, Mg: 0.0005 to 0.007% 4. A high-tensile steel material having excellent delayed fracture characteristics according to claim 1, wherein the steel material contains two or more kinds. 5. A steel piece having a wire diameter of 9 mm or more obtained by subjecting a slab having the steel material component according to any one of claims 1 to 4 to cold drawing after hot rolling after casting by a billet continuous casting method. And quenching after heating to a temperature range of 850 to 1050 ° C., then rapidly heating to a temperature range of 350 to 550 ° C., and then tempering the material.