JP4352590B2 - Steel bars for reinforced concrete with excellent fatigue resistance of resistance welds - Google Patents

Description

【００３０】
【表２】

【００３１】
本発明例は、いずれも溶接熱影響部の硬化はほとんどなく耐久比0.3 以上の良好な疲労特性を示した。
一方、本発明の範囲を外れる比較例は、溶接熱影響部が硬化し耐久比が0.3 未満であった。
Ｃ、Si、Mnが本発明の範囲の上限を超えた比較例（棒鋼No.18 〜No.20 ）には、熱影響部が硬化し、耐久比が0.3 未満であった。Ti、Nbが不足している比較例（棒鋼No.21 、No.23 ）では、耐久比が低い。また、Ti、Nbが本発明範囲の上限を超えた場合（棒鋼No.22 、No.24 ）では、熱影響部が硬化し、耐久比が低い。また、Ｏ、Ｎが本発明範囲の上限を超えた場合（棒鋼No.25 、No.26 ）では、耐久比が低い。
【００３２】
なお、従来例（棒鋼No.27 ）はJIS SR295 を満足する鉄筋コンクリート用棒鋼の例であり、熱影響部の硬さが大幅に高く、また耐久比は0.23である。
【００３３】
【発明の効果】
本発明によれば、耐久比が0.3 を越える良好な疲労特性を有する抵抗溶接継手を得ることが可能な抵抗溶接部の疲労特性に優れた鉄筋コンクリート用棒鋼を容易に得ることができ、鉄筋コンクリート用部材の高品質化に寄与でき、産業上の格段の効果を奏する。
【図面の簡単な説明】
【図１】コンクリート構造体に用いられる十字状抵抗溶接継手を模式的に示す説明図である。
【図２】十字状抵抗溶接継手接合部の溶接部硬さ測定位置を示す説明図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel bar suitable as a reinforcing steel material for a concrete structure, and more particularly to improvement of fatigue resistance characteristics of a resistance weld.
[0002]
[Prior art]
Conventionally, steel bars for reinforced concrete defined in JIS G 3112 have been used as reinforcing bars for concrete structures.
These steel bars are used as reinforcing bars for reinforcing concrete structures by being arranged and joined in the longitudinal direction or in a cross shape. For joining these steel bars, welding such as gas pressure welding, arc welding, resistance welding or the like is used. In particular, resistance welding is actively used in the production of formwork for a concrete structure because it can be easily joined by making point contact between steel bars (rebars) and applying current under pressure.
[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 reinforcing bars is melted and joined using resistance heat generation at the contact portion. Therefore, the resistance weld is subjected to a heat cycle that is rapidly heated from the melting point to a high temperature in the austenite region and then rapidly cooled. For this reason, austenite grains are coarsened during rapid heating, and during rapid cooling, the weld is markedly hardened due to martensitic transformation and bainite transformation from the coarse austenite, and the low temperature toughness of the resistance welding heat affected zone is significantly deteriorated. For these reasons, it has been desired to improve the weldability of steel bars for reinforced concrete.
[0004]
For improving the weldability of steel bars for reinforced concrete, for example, JP-A-62-287012 discloses a method for producing a low-temperature steel bar having excellent weldability, although not limited to resistance welding. In the production method described in JP-A-62-287012, C is limited to 0.020 to 0.12%, Ni: 0.2 to 1.5%, Ti: 0.04 to 0.15%, Al: 0.01 to 0.07%, B: It is a manufacturing method of a reinforcing bar which performs controlled rolling and controlled cooling to a steel material containing 0.0005 to 0.0030% to make the structure martensite and bainite structure. Reinforcing steel bars manufactured by such a manufacturing method are said to have no coarsening of crystal grains even if they are affected by heat caused by welding.
[0005]
However, the steel bar manufactured by the technique described in Japanese Patent Application Laid-Open No. 62-287012 still has a problem that it is inevitable that the steel bar is hardened due to the heat effect of resistance welding.
Moreover, when manufacturing a concrete structure formwork, as shown in FIG. 1 (a), in the case of arranging and joining steel bars for reinforced concrete in a cross shape, a cross-sectional view taken along line AA in FIG. 1 (a). As shown in FIG. 1B, the joint portion inevitably has a joint shape with a notch. It is well known that the fatigue strength of joints where notches are present is lower than that when smooth, but the structure of joints where such notches are present is not the same as coarse austenite grains. In the case of a hardened structure, it is presumed that notch sensitivity is increased and the fatigue strength is further reduced. Therefore, in order not to significantly reduce the fatigue strength of the concrete structure, it is desired to improve the fatigue strength of the steel rod resistance welded portion for reinforced concrete.
[0006]
Several proposals have been made for improving the fatigue characteristics of arc welds not for bar steel but for high-tensile steel plates. For example, Japanese Patent Application Laid-Open No. 7-62489 discloses a high-strength steel that improves the fatigue characteristics of arc welds by strengthening the structure of the heat-affected zone while welding and preventing or suppressing the occurrence of fatigue cracks. Is disclosed. The technology described in JP-A-7-62489 includes C: 0.03 to 0.20%, Si: 0.1 to 2.0%, Mn: 0.6 to 2.0%, Al: 0.01 to 0.08%, and P: 0.02 to 0.20. %, Or further contains B, Ni, Cr, V, etc., preferably the weld heat-affected zone has a bainite structure to improve the fatigue characteristics.
[0007]
JP-A-62-235044 and JP-A-6-330232 propose high-strength steel for welded structures that improves the fatigue strength of welds by controlling the structure of the weld heat-affected zone while welding. Has been. The technology described in JP-A-62-235044 is as follows: C: 0.04-0.12%, Si: 0.3-0.6%, Mn: 0.8-2.0%, Al: 0.02-0.06%, Cu: 0.5-1.2%, B : 0.0005 to 0.002%, Ti: 0.005 to 0.04%, or further containing Ni or the like, the weld heat-affected zone is made an upper bainite structure to improve fatigue characteristics. The technology described in JP-A-6-330232 contains C: 0.03-0.20%, Si: 0.6-2.0%, Mn: 0.6-2.0%, Al: 0.02-0.05%, B: 0.0005-0.0020%. Alternatively, it further contains Ni, Cr, V and the like to reduce the grain boundary pro-eutectoid ferrite in the weld heat affected zone and to improve the fatigue characteristics of the weld zone.
[0008]
[Problems to be solved by the invention]
However, the techniques described in JP-A-7-62489, JP-A-6-35044, and JP-A-6-330232 are improvements in fatigue resistance characteristics of welds mainly composed of arc welding. When steel materials manufactured by the technique described in the publication are joined by resistance welding, the structure of the resistance welded portion still tends to be a hardened structure, and no significant improvement in fatigue resistance is observed. In general, the thermal cycle of resistance welding is rapid heating and quenching compared to arc welding.
[0009]
The present invention has been made in view of the above-described problems of the prior art, and an object thereof is to propose a steel bar for reinforced concrete that has little hardening of the heat affected zone and is excellent in fatigue resistance characteristics of the resistance welded portion even when resistance welding is performed. .
[0010]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors diligently studied the relationship between the steel bar composition and the structure of the resistance weld. As a result, in order to improve the fatigue characteristics of the welded part, the inventors believe that it is better to reduce the cooling rate dependency of the resistance welded part structure and prevent hardening of the heat affected zone. I came up with it. The present inventors have found that it is most effective to limit the steel bar composition, particularly to reduce the C content to 0.02 mass% or less, in order to reduce the cooling rate dependency of the resistance weld structure. It was. C: By making it 0.02 mass% or less, the weld curability is lowered and the notch sensitivity in fatigue is lowered.
[0011]
The present invention has been completed by further study based on the above findings.
That is, the present invention is 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 -0.30%, Nb: 0.001-0.20%, B: 0.0003-0.0050%, O: 0.0100% or less, N: 0.0150% or less, resistance welding characterized by having a composition consisting of the balance Fe and inevitable impurities In addition to the above composition, in the present invention, in addition to the above-mentioned composition, in addition to mass%, Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, Cu : It is preferable to contain 1 type (s) or 2 or more types selected from 0.8% or less, and in the present invention, in addition to each of the above-mentioned compositions, mass%, V: 0.5% or less, W: 0.5 % Or less, Co: 0.5% or less is preferably included.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
First, the reasons for limiting the composition of the steel bar of the present invention will be described. Hereinafter, mass% is simply referred to 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, and is limited to 0.02% or less in the present invention in order to suppress hardening of the weld heat affected zone. From the viewpoint of securing strength, it is preferable to contain 0.005% or more.
[0013]
Si: 1.0% or less
Si is a useful element that acts as a deoxidizer and increases strength by solid solution strengthening. Such an effect becomes remarkable when the content is 0.05% or more. However, since the weld heat affected zone hardens if contained excessively, Si is limited to 1.0% or less. In addition, Preferably it is 0.10 to 0.50%.
[0014]
Mn: 0.30 to 2.5%
Mn is a useful element that acts as a deoxidizer and increases strength by solid solution strengthening. Moreover, it is an element necessary for reducing the cooling rate dependency of the steel bar structure. Such an effect is recognized at a content of 0.30% or more, but when it exceeds 2.5%, the ductility is lowered. For this reason, Mn was limited to the range of 0.30 to 2.5%. In addition, 0.8 to 1.8% is preferable.
[0015]
P: 0.030% or less P increases in strength by solid solution, but segregates at the grain boundary or concentrates in the final solidified portion of the resistance welded portion to lower the fatigue strength of the welded portion. For this reason, it is preferable to reduce P as much as possible, but it is acceptable up to 0.030%.
S: 0.030% or less S is present as a sulfide in steel and, like P, reduces fatigue strength. For this reason, it is preferable to reduce S as much as possible, but it is acceptable up to 0.030%.
[0016]
Al: 0.10% or less
Al acts effectively as a deoxidizer, as does Si, but if the content exceeds 0.10%, alumina inclusions increase and fatigue strength is reduced. For this reason, Al was limited to 0.10% or less. In addition, Preferably, it is 0.01 to 0.05%.
Ti: 0.001 to 0.30%
Ti is a useful element that combines with N to form TiN, contributes to refinement of the structure, and reduces notch sensitivity. Ti also acts as a deoxidizer. Such an effect is recognized when the content is 0.001% or more. However, if the content exceeds 0.30%, the hardenability is improved, the heat-affected zone is hardened during resistance welding, and the fatigue strength is lowered. For this reason, Ti was limited to 0.001 to 0.30%. In addition, from a viewpoint of ensuring the solid solution B, Preferably it is 0.05 to 0.10%.
[0017]
Nb: 0.001 to 0.20%
Nb precipitates as carbonitride, contributes to the refinement of the structure by the pinning effect, and improves hardenability as solute Nb. In order to obtain the pinning effect, it is necessary to contain at least 0.001% or more. However, if it exceeds 0.20%, the hardenability is excessively improved and the heat-affected zone during resistance welding is hardened, resulting in fatigue. Reduce strength. For this reason, Nb was limited to 0.001 to 0.20%. In addition, Preferably, it is 0.01 to 0.10%.
[0018]
B: 0.0003-0.0050%
B is an element that improves hardenability and is an element that is useful for reducing the dependency of strength on the cooling rate. In order to exhibit such an effect, the content of 0.0003% or more is necessary. On the other hand, even if it contains excessively exceeding 0.0050%, the effect will be saturated and the effect corresponding to content cannot be expected. For this reason, B was limited to the range of 0.0003 to 0.0050%. In addition, Preferably, it is 0.0010 to 0.0030%.
[0019]
O: 0.0100% or less O is present as an inclusion in steel, lowering cleanliness and reducing fatigue strength. For this reason, it is preferable to reduce O as much as possible, but it is acceptable up to 0.0100%.
N: 0.0010 to 0.0150%
N combines with Ti and Nb and precipitates as nitride or carbonitride, and has the effect of improving the fatigue strength by refining the structure as a pinning site that suppresses crystal grain growth during heating or resistance welding. ing. In order to fully exhibit the effect of refining the structure due to precipitation of nitrides such as TiN and carbonitrides, a content of 0.0010% or more is required. On the other hand, if the content exceeds 0.0150%, the effect is saturated, and the toughness of the steel bar rather decreases due to an increase in solute N, and the notch sensitivity increases in the heat-affected zone of resistance welding, and the fatigue strength decreases. For this reason, N was limited to 0.0010 to 0.0150%.
[0020]
The basic composition has been described above. In the present invention, in addition to the basic composition described above, one or two of Cr, Mo, Ni, and Cu as elements that further improve the hardenability and thus improve the strength. More than one species can be selected and contained.
One or more selected from Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, Cu: 0.8% or less
Cr is an element useful for reducing the strength cooling rate dependency, but if it is contained in a large amount exceeding 1.0%, the toughness is lowered. For this reason, Cr is preferably limited to 1.0% or less. In addition, More preferably, it is 0.1 to 0.8%.
[0021]
Mo, like Cr, is an element useful for reducing the dependence of strength on the cooling rate, but if contained in a large amount exceeding 1.0%, the toughness is reduced. For this reason, Mo is preferably limited to 1.0% or less. In addition, More preferably, it 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%, the effect is saturated 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, it is 0.1 to 0.8%.
[0022]
Cu is an element effective for improving the strength, but like P and S, it concentrates in the final solidified portion of the resistance welded portion and lowers the fatigue strength. For this reason, Cu is preferably limited to 0.8% or less. In addition, More preferably, it is 0.1 to 0.6%.
In the present invention, in addition to the basic composition described above, one or more of V, W and Co can be contained as an element for increasing the strength. Alternatively, one or more of Cr, Mo, Ni, and Cu may be added, and one or more of V, W, and Co may further be contained.
[0023]
One or more selected from V: 0.5% or less, W: 0.5% or less, and Co: 0.5% or less are effective elements for improving the strength. If the content exceeds 0.5%, the notch sensitivity of the heat-affected zone increases. For this reason, it is preferable that V, W, and Co are all 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%.
[0024]
The balance other than the above components is Fe and inevitable impurities.
[0025]
【Example】
Steels having the composition shown in Table 1 were melted in a converter and made into blooms having a cross section of 400 × 560 mm by a continuous casting method, and these blooms were hot rolled to form 12 mmφ steel bars for reinforced concrete.
For these steel bars, the mechanical properties (tensile properties, cross-sectional hardness) as rolled were investigated.
(1) Tensile properties A test piece having a length of 300 mm was taken from each steel bar and subjected to a tensile test in accordance with the provisions of JIS Z 2204 to determine yield strength YS, tensile strength TS, and elongation El.
(2) Hardness test Samples for hardness measurement were taken from each steel bar and measured at 5 points using a Vickers hardness meter (load 10 kg) at a position D / 4 of the cross section (L cross section) parallel to the rolling direction. The average value Hv _mean was taken as the hardness of each steel bar.
[0026]
Next, the fatigue properties of the welds were investigated for these bars.
(3) Fatigue characteristics of welded part Sample materials for 350 mm long specimens were taken from each steel bar, placed in a cross shape as shown in Fig. 1 and joined by resistance welding to produce a resistance welded joint. A test piece was obtained. The resistance welding conditions were a pressure of 5000 N, a welding current of 15000 A, and a welding time of 0.3 s.
[0027]
Using the thus produced fatigue test piece, a partial piece swing tensile fatigue test with a frequency of 7 Hz was performed. The minimum tensile stress was 3000N. The strength satisfying the number of repetitions of 10 ⁷ times was defined as fatigue strength. The ratio between the fatigue strength and the tensile strength as rolled was defined as the durability ratio.
(4) Hardness test of welded part For resistance welded joints in which steel bars are joined in a cross shape by resistance welding, the joints are cut and polished, and the hardness distribution is obtained by the Vickers hardness tester (load 1 kg) as shown in FIG. And the maximum value was taken as the maximum hardness Hv _max .
[0028]
The results are shown in Table 2.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
In all of the inventive examples, there was almost no hardening of the weld heat-affected zone, and good fatigue characteristics with a durability ratio of 0.3 or more were exhibited.
On the other hand, in the comparative example outside the scope of the present invention, the weld heat affected zone was cured and the durability ratio was less than 0.3.
In the comparative examples (bars No. 18 to No. 20) in which C, Si, and Mn exceeded the upper limit of the range of the present invention, the heat affected zone was hardened and the durability ratio was less than 0.3. In the comparative examples (bars No. 21 and No. 23) lacking Ti and Nb, the durability ratio is low. Moreover, when Ti and Nb exceed the upper limit of the range of the present invention (bars No. 22 and No. 24), the heat affected zone is hardened and the durability ratio is low. Further, when O and N exceed the upper limit of the range of the present invention (bars No. 25 and No. 26), the durability ratio is low.
[0032]
The conventional example (bar No. 27) is an example of a steel bar for reinforced concrete that satisfies JIS SR295. The heat-affected zone has a significantly higher hardness and a durability ratio of 0.23.
[0033]
【The invention's effect】
According to the present invention, it is possible to easily obtain a steel bar for reinforced concrete having excellent fatigue characteristics of a resistance welded portion capable of obtaining a resistance welded joint having a durability ratio exceeding 0.3, and a member for reinforced concrete. It can contribute to the improvement of quality and has a remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is an explanatory view schematically showing a cross-shaped resistance welded joint used for a concrete structure.
FIG. 2 is an explanatory diagram showing a weld hardness measurement position of a cruciform resistance weld joint joint.

Claims (3)

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 0.30%, Nb: 0.001 to 0.20%,
B: 0.0003 to 0.0050%, O: 0.0100% or less,
N: A steel bar for reinforced concrete excellent in fatigue resistance of a resistance welded portion, characterized by having a composition composed of the balance Fe and unavoidable impurities, including 0.0150% or less. In addition to the above composition, it may further contain at least one selected from mass: Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less, Cu: 0.8% or less. The steel bar for reinforced concrete according to claim 1. In addition to the above composition, the composition further contains at least one selected from the group consisting of mass%, V: 0.5% or less, W: 0.5% or less, and Co: 0.5% or less. The steel bar for reinforced concrete according to 1 or 2.

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