JP2002003999A - Steel bars for reinforced concrete with excellent fatigue resistance of resistance welds - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To propose a steel bar for reinforced concrete with less hardening of a resistance welded part and excellent fatigue resistance characteristics. SOLUTION: In mass%, C: 0.02% or less, Si: 1.0%
Mn: 0.30 to 2.5%, P: 0.030% or less, S: 0.03
0% or less, Al: 0.10% or less, Ti: 0.001 to 0.30%, Nb:
0.001 to 0.20%, B: 0.0003 to 0.0050%, O: 0.0100%
Hereinafter, a composition containing N: 0.0150% or less, with the balance being Fe and inevitable impurities. Further, one or more of Cr, Mo, Ni, and Cu, and one or more of V, W, and Co may be contained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel bar suitable as a material for reinforcing bars of concrete structures, and more particularly to improvement of fatigue resistance of a resistance welded portion.

[0002]

2. Description of the Related Art Conventionally, steel bars for reinforced concrete specified in JIS G 3112 have been used as reinforcing bars for concrete structures. These steel bars are arranged and joined in a longitudinal direction or a cross, and are used as reinforcing steel bars for concrete structures. For joining these steel bars, welding such as gas pressure welding, arc welding, or resistance welding is used. Among them, resistance welding is used for steel bars (rebar)
Since they can be easily joined by bringing them into point contact with each other and applying a pressurized current, they are actively used in the production of concrete formwork.

[0003] Resistance welding is a method in which steel bars (rebars) are brought into point contact with each other, a large current exceeding 10,000 A is applied, and a part of the rebars is melted and joined by utilizing the resistance heating of the contact portions. Therefore, the resistance welded part undergoes a heat cycle in which it is rapidly heated from its melting point to a high temperature in the austenite region and then rapidly cooled. Therefore, at the time of rapid heating, austenite grains are coarsened, and at the time of rapid cooling, martensitic transformation and bainite transformation from the coarse austenite cause remarkable hardening of the welded portion, and the low-temperature toughness of the heat-affected zone by resistance welding is significantly deteriorated. For these reasons, improvement in weldability of steel bars for reinforced concrete has been desired.

For improving the weldability of steel bars for reinforced concrete, for example, Japanese Patent Application Laid-Open No. 62-287012 discloses a method for producing a low-temperature steel bar excellent in weldability, which is not limited to resistance welding. I have. The production method described in JP-A-62-287012 limits C to 0.020 to 0.12%,
Further, Ni: 0.2 to 1.5%, Ti: 0.04 to 0.15%, Al: 0.01
This is a method for producing a steel rod containing -0.07% and B: 0.0005 -0.0030%, by subjecting a steel material to controlled rolling and controlled cooling to change the structure to a martensite and bainite structure. It is stated that the steel bars for reinforcing bars manufactured by such a manufacturing method do not have crystal grains coarsened even if they are affected by heat due to welding.

[0005] However, even with the steel bars manufactured by the technique described in Japanese Patent Application Laid-Open No. 62-287012, there remains a problem that it is inevitable that the steel bars harden significantly due to the thermal influence of resistance welding. In addition, when manufacturing a formwork of a concrete structure, when steel bars for reinforced concrete are arranged in a cross shape as shown in FIG.
As shown in FIG. 1B, which is a cross-sectional view taken along the line A-A, the joint has a joint shape in which a notch is inevitably present. It is well known that the fatigue strength of joints with notches is lower than that of smooth joints.However, the structure of the joints with such notches is also reduced by coarse austenite grains. In the case of a hardened structure, it is presumed that the notch sensitivity increases, and the reduction in fatigue strength becomes more remarkable. Therefore, in order not to significantly reduce the fatigue strength of the concrete structure, there is a demand for improvement of the fatigue strength of the steel-bar resistance welded portion for reinforced concrete.

Several proposals have been made for improving the fatigue characteristics of an arc welded portion for high-strength steel sheets, not for steel bars. For example, JP-A-7-62489 discloses that
A high-strength steel that improves the fatigue characteristics of an arc welded part by strengthening the structure of a weld heat affected zone as it is and preventing or suppressing the occurrence of fatigue cracks has been disclosed. JP
The technology described in Japanese Patent Application Laid-Open No. 7-62489 has a C value of 0.03 to 0.20.
%, Si: 0.1 to 2.0%, Mn: 0.6 to 2.0%, Al: 0.01 to
0.08%, further contains P: 0.02 to 0.20%, or further contains B, Ni, Cr, V, etc., and preferably has a bainite structure in the weld heat affected zone to improve fatigue properties. Things.

[0007] Also, JP-A-6-235044 and JP-A-6-33
No. 0232 proposes a high-strength steel for a welded structure in which the structure of a heat-affected zone of a weld is controlled as it is and the fatigue strength of the weld is improved. The technology described in Japanese Patent Application Laid-Open No. Hei 6-235044 discloses that C: 0.04 to 0.12%, Si: 0.3 to 0.6%, Mn: 0.
8 to 2.0%, Al: 0.02 to 0.06%, Cu: 0.5 to 1.2%,
B: 0.0005 to 0.002%, Ti: 0.005 to 0.04%, or further contains Ni or the like to make the weld heat affected zone an upper bainite structure to improve fatigue properties. The technology described in JP-A-6-330232 discloses C:
0.03 to 0.20%, Si: 0.6 to 2.0%, Mn: 0.6 to 2.0%,
Al: 0.02 to 0.05%, B: 0.0005 to 0.0020%, or further contains Ni, Cr, V, etc. to reduce the grain boundary pro-eutectoid ferrite in the weld heat affected zone and improve the fatigue properties of the weld zone It is to try to make it.

[0008]

SUMMARY OF THE INVENTION
7-62489, JP-A-6-235044, JP-A-6-3302
The technology described in No. 32 is an improvement in the fatigue resistance of a welded portion mainly by arc welding, and when steel materials manufactured by the technology described in these publications are joined by resistance welding, In addition, the structure of the resistance welded portion tends to be a hardened structure, and no remarkable improvement in fatigue resistance characteristics is not seen. Generally, the heat cycle of the welding portion of resistance welding is rapid heating and quenching as compared with arc welding.

The present invention has been made in view of the above-mentioned problems of the prior art, and proposes a steel bar for reinforced concrete in which the heat affected zone of the weld is hardened even when resistance welding is performed, and the fatigue resistance of the resistance weld is excellent. Aim.

[0010]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have intensively studied the relationship between the composition of a steel bar and the structure of a resistance weld. As a result, the present inventors consider that in order to improve the fatigue characteristics of the welded portion, it is preferable to reduce the cooling rate dependence of the resistance welded structure and prevent the hardening of the weld heat affected zone. I thought. The present inventors have found that in order to reduce the cooling rate dependence of the resistance welded structure, it is most effective to limit the composition of the steel bar, particularly to reduce the C content to 0.02 mass% or less. Was.
By setting C: 0.02 mass% or less, the weld hardening property is reduced, and the notch sensitivity in fatigue is reduced.

The present invention has been completed by further study based on the above findings. That is, the present invention
mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 0.30
~ 2.5%, P: 0.030% or less, S: 0.030% or less, Al:
0.10% or less, Ti: 0.001 to 0.30%, Nb: 0.001 to 0.20
%, B: 0.0003 to 0.0050%, O: 0.0100% or less, N: 0.
0150% or less, is a steel bar for reinforced concrete excellent in fatigue resistance characteristics of the resistance welded portion, characterized by having a composition consisting of the balance Fe and unavoidable impurities, and in the present invention, in addition to the composition, Furthermore, in mass%, Cr: 1.0
%, Mo: 1.0% or less, Ni: 1.0% or less, Cu: 0.8%
It is preferable to contain one or more selected from the following, and in the present invention, in addition to the above-mentioned respective compositions, further, in mass%, V: 0.5% or less, W: 0.5% or less, Co: It is preferable to contain one or more selected from 0.5% or less.

[0012]

First, the reasons for limiting the composition of the steel bar of the present invention will be described. Hereinafter, mass% is simply described as%. C: 0.02% or less C is an element that increases the strength of the base material and increases the hardness of the welded portion. In order to suppress the hardening of the weld heat affected zone, C is limited to 0.02% or less in the present invention. From the viewpoint of securing the strength, the content is preferably 0.005% or more.

Si: 1.0% or less Si is a useful element that acts as a deoxidizing agent and increases the strength by solid solution strengthening. Such an effect
It becomes significant when the content is 0.05% or more. However, if it is contained excessively, the heat affected zone of the weld hardens, so the Si content is limited to 1.0% or less. In addition, it is preferably 0.10 to 0.50%.

Mn: 0.30-2.5% Mn is a useful element that acts as a deoxidizing agent and increases the strength by solid solution strengthening. Further, it is an element necessary for reducing the cooling rate dependence of the steel bar structure. Such an effect is observed at a content of 0.30% or more.
%, The ductility decreases. For this reason, Mn
Was limited to the range of 0.30-2.5%. In addition, preferably 0.8
~ 1.8%.

P: 0.030% or less P increases the strength by forming a solid solution, but segregates at the grain boundaries or concentrates in the final solidified portion of the resistance welded portion, thereby lowering the fatigue strength of the welded portion. For this reason, it is preferable to reduce P as much as possible, but up to 0.030% is acceptable. S: 0.030% or less S is present as sulfide in steel and lowers fatigue strength like P. For this reason, S is preferably reduced as much as possible, but up to 0.030% is acceptable.

Al: 0.10% or less Al acts effectively as a deoxidizing agent like Si, but when the content exceeds 0.10%, alumina inclusions increase, and the fatigue strength decreases. For this reason, Al was limited to 0.10% or less. In addition, Preferably, it is 0.01-0.05%. Ti: 0.001 to 0.30% Ti is a useful element that combines with N to form TiN, contributes to the refinement of the structure, and reduces the notch sensitivity. Ti also acts as a deoxidizer. These effects are
Although it is recognized at a content of 0.001% or more, an excessive content exceeding 0.30% improves hardenability, causes hardening of the heat-affected zone during resistance welding, and lowers fatigue strength. For this reason, Ti
Is limited to 0.001 to 0.30%. In addition, from a viewpoint of ensuring solid solution B, it is preferably 0.05 to 0.10%.

Nb: 0.001 to 0.20% Nb precipitates as carbonitrides, contributes to the refinement of the structure by a pinning effect, and improves the hardenability as solid solution Nb. To obtain the pinning effect, the content must be at least 0.001% or more. However, if the content exceeds 0.20%, the hardenability will be excessively improved, and the heat-affected zone will be hardened during resistance welding, resulting in fatigue. Decrease strength. Therefore, Nb is limited to 0.001 to 0.20%. In addition, Preferably, it is 0.01 to 0.10%.

B: 0.0003% to 0.0050% B is an element that improves the hardenability and is an element useful for reducing the cooling rate dependence of strength. In order to exhibit such an effect, the content needs to be 0.0003% or more. On the other hand, if the content exceeds 0.0050%, the effect saturates and an effect commensurate with the content cannot be expected.
Therefore, B is limited to the range of 0.0003 to 0.0050%. In addition, Preferably, it is 0.0010-0.0030%.

O: 0.0100% or less O is present as an inclusion in steel and reduces the cleanliness and the fatigue strength. For this reason, it is preferable to reduce O as much as possible, but up to 0.0100% is acceptable. N: 0.0010 to 0.0150% N combines with Ti and Nb and precipitates as nitride or carbonitride, refines the structure as a pinning site that suppresses crystal grain growth during heating or resistance welding, and reduces fatigue strength. Has the effect of improving. In order to sufficiently exhibit the effect of refining the structure due to precipitation of nitrides such as TiN and carbonitrides, the content of 0.0010% or more is required. On the other hand, 0.0150%
If the content exceeds 3, the effect is saturated, the toughness of the steel bar is rather lowered due to the increase of the solute N, and the notch sensitivity is increased in the heat-affected zone of resistance welding, and the fatigue strength is reduced. For this reason, N was limited to 0.0010 to 0.0150%.

Although the basic composition has been described above, in the present invention, in addition to the basic composition described above, Cr, as an element for further improving the hardenability and, consequently, the strength, is used.
One, two or more of Mo, Ni and Cu can be selected and contained. Cr: 1.0% or less, Mo: 1.0% or less,
Ni: 1.0% or less, Cu: 0.8% or less 1
Species or two or more types of Cr are useful elements for reducing the dependence of strength on the cooling rate, but when contained in a large amount exceeding 1.0%, toughness is reduced. For this reason, Cr is preferably limited to 1.0% or less. More preferably, the content is 0.1 to 0.8%.

Mo, like Cr, is a useful element for reducing the dependence of strength on the cooling rate, but when contained in a large amount exceeding 1.0%, toughness is reduced. For this reason, Mo is 1.
Preferably, it is limited to 0% or less. More preferably, the content is 0.1 to 0.8%. Ni is an element useful for improving strength and toughness, but is expensive, and even if it is contained in a large amount exceeding 1.0%, its effect saturates and an effect commensurate with the content cannot be expected. For this reason, Ni is preferably limited to 1.0% or less. In addition, more preferably, 0.
1 to 0.8%.

Cu is an element effective for improving the strength, but, like P and S, is concentrated in the final solidified portion of the resistance welded portion to lower the fatigue strength. For this reason, Cu is preferably limited to 0.8% or less. In addition, more preferably, 0.
1 to 0.6%. In the present invention, in addition to the above-described basic composition, V, W, Co
One or more of these may be contained. Or,
One or more of Cr, Mo, Ni, and Cu may be added, and one or more of V, W, and Co may be further contained.

V: 0.5% or less, W: 0.5% or less, Co: 0.
One or more selected from among 5% or less V, W, and Co are all effective elements for improving the strength, but if contained more than 0.5%, the notch in the heat-affected zone Increased sensitivity. Therefore, V, W, and Co
Preferably, it is limited to 0.5% or less. More preferably, V is 0.05 to 0.25%, W is 0.05 to 0.25%, and Co is 0.05 to 0.25%.
~ 0.25%.

The balance other than the above components is Fe and inevitable impurities.

[0025]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter and made into blooms having a cross section of 400 × 560 mm by a continuous casting method. These blooms were hot-rolled into 12 mmφ steel bars for reinforced concrete. . The mechanical properties (tensile properties, cross-sectional hardness) of these steel bars as they were rolled were investigated. (1) Tensile properties A test piece having a length of 300 mm was sampled from each bar and subjected to a tensile test in accordance with the provisions of JIS Z 2204 to determine the yield strength YS, tensile strength TS, and elongation El. (2) Hardness test A test piece for hardness measurement was taken from each steel bar, and measured at 5 points using a Vickers hardness tester (load: 10 kg) at a D / 4 position of a cross section (L cross section) parallel to the rolling direction. And their average value Hv
_mean was the hardness of each bar.

Next, the fatigue properties of the welded portions of these steel bars were examined. (3) Fatigue properties of welded parts A 350 mm long test piece material was sampled from each steel bar, arranged in a cross shape as shown in Fig. 1 and subjected to resistance welding and joined to produce a resistance welded joint. A test piece was used. Resistance welding conditions are: pressure 5000N, welding current 15000A, welding time
0.3 s.

Using the fatigue test piece manufactured as described above, a partial pulsating tensile fatigue test at a frequency of 7 Hz was performed. The minimum tensile stress was 3000N. In addition, the number of repetitions: the strength that satisfies the 10 ⁷ times was the fatigue strength. Also,
The ratio between the fatigue strength and the as-rolled tensile strength was defined as the durability ratio. (4) Hardness test of welded part For a resistance welded joint in which steel bars were joined in a cross shape by resistance welding, the joint was cut and polished, and the hardness distribution was measured with a Vickers hardness meter (load 1 kg) as shown in FIG. Measure
The maximum value was defined as the maximum hardness Hv _max .

Table 2 shows the results.

[0029]

[Table 1]

[0030]

[Table 2]

In each of the examples of the present invention, hardening of the heat affected zone was hardly observed, and good fatigue characteristics with a durability ratio of 0.3 or more were exhibited. On the other hand, in Comparative Examples outside the scope of the present invention, the heat affected zone was hardened and the durability ratio was less than 0.3. Comparative examples in which C, Si and Mn exceeded the upper limit of the range of the present invention (steel bars No. 18 to No. 20).
In (), the heat-affected zone was cured and the durability ratio was less than 0.3. Comparative examples lacking Ti and Nb (bars No.21, No.23
), The durability ratio is low. When Ti and Nb exceed the upper limit of the range of the present invention (steel bars No. 22 and No. 24), the heat-affected zone is hardened and the durability ratio is low. When O and N exceed the upper limit of the range of the present invention (steel bars No. 25 and No. 26), the durability ratio is low.

The conventional example (bar steel No. 27) is JIS SR295
This is an example of steel bars for reinforced concrete satisfying the following conditions. The hardness of the heat-affected zone is significantly higher, and the durability ratio is 0.23.

[0033]

According to the present invention, it is possible to easily obtain a steel bar for reinforced concrete excellent in fatigue characteristics of a resistance welded portion, which can provide a resistance welded joint having a good fatigue characteristic having a durability ratio exceeding 0.3. It can contribute to the improvement of the quality of the reinforced concrete member, and has a remarkable industrial effect.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing a cruciform resistance welding joint used for a concrete structure.

FIG. 2 is an explanatory view showing a weld hardness measurement position of a cruciform resistance weld joint joint.

Continuing on the front page (72) Inventor Kenichi Amano 1-chome, Kawasaki-dori, Mizushima, Kurashiki-shi, Okayama Pref. Kawasaki Steel Corporation Mizushima Steel Works

Claims (3)

[Claims] 1. Mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 0.30 to 2.5%, P: 0.030% or less, S: 0.030% or less, Al: 0.10% or less, Ti: 0.001 to 0.001% 0.30%, Nb: 0.001 to 0.20%, B: 0.0003 to 0.0050%, O: 0.0100% or less, N: 0.0150% or less, and has a composition comprising the balance of Fe and unavoidable impurities. Steel bars for reinforced concrete with excellent fatigue resistance. 2. In addition to the above composition, in mass%, C
r: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, C
2. The steel bar for reinforced concrete according to claim 1, wherein the steel bar contains one or more members selected from u: 0.8% or less. 3. In addition to the composition, in mass%,
The steel bar for reinforced concrete according to claim 1 or 2, wherein the steel bar contains one or more selected from V: 0.5% or less, W: 0.5% or less, and Co: 0.5% or less.