JP4132178B2 - PC steel wire or bar with good delayed fracture resistance and manufacturing method thereof - Google Patents

Description

【００２８】
【表２】

【００２９】
【表３】

【００３０】
【表４】

【００３１】
【発明の効果】
表３および表４から明らかなように、本発明の鋼線は、いずれもマルテンサイトまたは下部ベイナイトが８０％以上の組織を有し、母相中に、焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織が５０％以上を占める帯状の組織が平均で厚み１μｍ以上３０μｍ以下でかつ間隔が平均で１０μｍ以上２００μｍ以内で鋼線の長さ方向と平行に層状に配列している。このような組織を有する鋼線は十分な強度と伸びを有し、さらに遅れ破壊試験の破断時間も顕著に長い。表２から、このような組織を得るための製造方法は、８５０℃以下の低温域で０．８以上の真歪みで圧延され、さらに圧延終了後に１０℃／ｓ以上の冷却速度でＭｓ点以下まで冷却され、その後に加熱速度１０℃／ｓ以上でＡｃ₁ 点以上Ａｃ₁ 点＋８０℃以下に加熱し、焼き入れされたものであることがわかる。また、熱間圧延後の直接冷却と熱処理の間に付加的な伸線工程、表面加工、形状調整工程等を入れても、本発明鋼の特性に何ら悪影響がないこともわかる。
【図面の簡単な説明】
【図１】本発明鋼の金属組織写真の例を示す図である。
【図２】帯状組織内部での焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織の面積率、帯状組織の厚みと間隔の測定法を示す模式図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a PC steel wire or steel rod having excellent delayed fracture resistance and a method for producing the same.
[0002]
[Prior art]
Prestressed concrete is a composite material that combines the advantages of concrete and steel, and is used in many applications such as poles and piles. With the increase in strength of steel materials for prestressed concrete (hereinafter referred to as PC steel materials), improvement in delayed fracture resistance is strongly demanded. Up to now, PC steel materials having high strength and good delayed fracture resistance have been reported in JP-A-48-88018, JP-A-58-157921, JP-A-3-260012, and the like. . These are mainly made by adding alloy elements such as Si, Ni, Ti, B, etc. to improve the balance between strength and delayed fracture resistance. is there. Furthermore, there is a concern that excessive alloy addition may reduce the toughness of the spot weld.
[0003]
On the other hand, a method of imparting strain at a high temperature for the purpose of improving delayed fracture resistance without adding an alloy has been reported in Japanese Patent Publication No. 5-59967, but there is a difficulty in the production method. In addition, as a technique for improving the delayed fracture resistance by controlling the metal structure, there are reports of JP-A-5-287450, JP-A-7-300652 and the like, but the former is an increase in the surface hardness of the spot welded portion. The latter is limited to a technique that suppresses the effect, and the latter has a limited effect because of the martensite single-phase structure.
[0004]
[Problems to be solved by the invention]
The present invention provides a PC steel wire or steel rod having high strength and good delayed fracture resistance and a method for producing the same by building a metal structure unprecedented without relying on excessive alloy addition. It is for the purpose.
[0005]
[Means for Solving the Problems]
The present invention relates to a PC steel wire or steel rod having high strength and good delayed fracture resistance, and a method for producing the same, which can advantageously eliminate the disadvantages of the conventional PC steel materials as described above, and its summary. The place is as follows.
(1) By weight%
C: 0.20 to 0.40%,
Si: 0.05 to 1.5%,
Mn: 0.2 to 1.5%
Al: 0.002 to 0.10%,
Nb: 0.01 to 0.07%
In the parent phase consisting of martensite or lower bainite with a balance of 4 to 12 mm in diameter and comprising a balance of Fe and unavoidable impurities and having a diameter of 4 mm to 12 mm. A band-like structure in which sites or residual austenite or a mixed structure thereof account for 50% or more has an average thickness of 1 μm to 30 μm and an interval of 10 μm to 200 μm on average, and is layered in parallel to the length direction of the steel wire or steel rod PC steel wire or steel rod with good delayed fracture resistance, characterized in that
[0006]
(2) By weight%
Ti: 0.002-0.10%
The PC steel wire or steel bar having good delayed fracture resistance according to (1) above.
(3) By weight%
V: 0.005 to 0.1%
The PC steel wire or steel bar having good delayed fracture resistance according to the above (1) or (2).
[0007]
(4) By weight%
Cu: 0.05 to 0.5%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 0.5%,
Mo: 0.05-0.5%
Co: 0.05-0.5%
W: 0.05-0.5%
The PC steel wire or the steel rod having good delayed fracture resistance according to any one of the above (1) to (3), comprising one or more of the following.
[0008]
(5)% by weight
B: 0.0002 to 0.0025%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of the above (1) to (4), wherein
(6)% by weight
Rem: 0.002 to 0.10%,
Ca: 0.0003 to 0.0030%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of the above (1) to (5), characterized by containing one or two of the following.
[0009]
(7)% by weight
Mg: 0.0003 to 0.01%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of the above (1) to (6), comprising:
(8) When the steel comprising the chemical component according to any one of the above (1) to (7) is hot-rolled to a steel wire or a steel rod having a diameter of 4 mm or more and 12 mm or less, the truth until the final wire diameter is reached. The subsequent rolling is performed at a temperature of 850 ° C. or less from the wire diameter at which the strain becomes 0.8 or more, and after the rolling is completed, the cooling is performed at a cooling rate of 10 ° C./s or less to the Ms point or less, and then the heating rate is 10 ° C./s or more. The method for producing a PC steel wire or a steel rod with good delayed fracture resistance according to any one of (1) to (7), wherein the steel is heated to Ac ₁ point or more and Ac ₁ point + 80 ° C. or less and quenched. .
[0010]
Here, the true strain ε is defined by ε = ln (d ₀ / d ₁ ). Here, ln is a natural logarithm, and d ₀ and d ₁ are a predetermined wire diameter during rolling and a final rolling wire diameter, respectively.
(9) After direct cooling after hot rolling, after additional processing steps, shape adjustment steps, etc., heat at a heating rate of 10 ° C / s or higher to Ac ₁ point or higher and Ac ₁ point + 80 ° C or lower and quench. The method for producing a PC steel wire or steel bar having good delayed fracture resistance according to (8) above.
[0011]
Hereinafter, the present invention will be described in detail.
The technical idea forming the basis of the present invention is as follows.
Since delayed fracture is caused by hydrogen, it is effective to prevent hydrogen from entering and to reduce defects that become hydrogen trap sites. For the former, a method is known in which 3.5% or more of Ni is added to form a stable film on the surface of the steel material, thereby suppressing hydrogen intrusion. On the other hand, for the latter, a large amount of a solid solution strengthening element such as Si is added to increase the strength at the time of quenching, and the amount corresponding to the strength increase amount is reduced by increasing the tempering temperature. A method of obtaining a steel material having the same strength and few various lattice defects is known.
[0012]
The present invention does not rely on the addition of such alloy elements, but is intended to delay the propagation of delayed fracture cracks by controlling the metallographic structure. In general, delayed fracture is a fracture phenomenon in which a crack progresses in the cross-sectional direction of the steel material and eventually breaks, and often exhibits a so-called grain boundary fracture mode in which the prior austenite grain boundaries before quenching become fracture paths. Therefore, if the structure has so-called anisotropy in which the structure is arranged in a direction perpendicular to the cross section of the steel material, the progress of the delayed fracture crack in the steel material cross section direction is prevented, and the delayed fracture itself can be suppressed. is there. In particular, it has been found that this effect is remarkable when a structure having a hardness higher than that of a normal matrix structure is arranged in a direction perpendicular to the cross section.
[0013]
The reasons for limiting the present invention will be described below.
C is an element effective for strengthening steel, and if it is less than 0.20%, sufficient strength as a PC steel wire or steel rod cannot be obtained. On the other hand, when the content exceeds 0.40%, the spot welded portion is excessively hardened and easily cracks.
Si is effective as a deoxidizing element and as a strengthening element for steel, but if its content is less than 0.05%, it has no effect. On the other hand, if it exceeds 1.5%, the toughness of the spot weld will be impaired.
[0014]
Mn is an element effective for strengthening steel, and if it is less than 0.2%, a sufficient effect cannot be obtained. On the other hand, when the content exceeds 1.5%, the workability of steel is deteriorated.
Al is added as a deoxidizing element. If the content is less than 0.002%, the effect is not obtained. If the content exceeds 0.10%, the surface properties of the steel are impaired.
Nb is well known to be effective in suppressing recrystallization of austenite during hot rolling, so 0.01% or more is added. Since the effect of Nb is saturated when it exceeds 0.07%, the addition amount is set to 0.07% or less.
[0015]
Ti functions effectively in terms of crystal grain refinement and precipitation hardening when added in a small amount, but excessive addition causes precipitation embrittlement. For this reason, the upper limit of the addition amount is set to 0.10%. On the other hand, since there is no effect if the addition amount is too small, the lower limit of the addition amount is set to 0.002%.
V causes precipitation strengthening when added in a small amount, but excessive addition causes precipitation embrittlement. For this reason, the upper limit of the addition amount is set to 0.10%. On the other hand, since there is no effect if the addition amount is too small, the lower limit of the addition amount is set to 0.005%.
[0016]
Cu, Ni, Cr, Mo, Co, and W are all elements that strengthen the steel by solid solution strengthening. In the case of the present invention, the strength of the steel can be increased by the addition, but an excessive amount of addition hardens the steel and makes it easy to crack, so Cu ≦ 0.5%, Ni ≦ 1.0%, Cr ≦ 0. .5%, Mo ≦ 0.5%, Co ≦ 0.5%, W ≦ 0.5%. Moreover, since there will be no effect if there is too little addition amount, the minimum of addition amount shall be 0.05% with any element.
[0017]
B is an element that improves the hardenability of steel. In the present invention, the addition can increase the strength of the steel, but excessive addition increases the precipitates of B and impairs the toughness of the steel. Therefore, the upper limit of the addition amount is set to 0.0025%. Moreover, since there will be no effect if there is too little addition amount, the minimum of addition amount shall be 0.0002%.
Rem and Ca are effective for detoxification of S, but if the addition amount is small, S remains harmful, and excessive addition produces a large amount of oxides and impairs toughness. 10%, Ca: Add in the range of 0.0003-0.0030%.
[0018]
Mg becomes a fine oxide and refines the structure of steel to improve toughness. If it is less than 0.0003%, the effect is not obtained, and if it exceeds 0.01%, cracks starting from oxides tend to occur, so the content is made 0.0003 to 0.01%.
Next, the characteristics of the metal structure of the steel wire or steel bar in the present invention will be described.
The strength of PC steel wire or steel bar is classified according to JIS, and TS of 1420 MPa or more is defined for the highest strength. In order to obtain such high strength, 80% or more of the metal structure needs to be martensite or lower bainite.
[0019]
In general, delayed fracture often causes cracks to propagate through so-called prior austenite grain boundaries, leading to fracture. Therefore, when a cross section perpendicular to the axial direction of the bar / line is considered, if the austenite grain boundary faces in parallel to the diameter direction of the cross section, the crack easily penetrates the cross section and causes the final fracture. Conversely, if a hard structure that can suppress the growth of cracks is arranged perpendicular to the diameter direction of the cross-section, that is, parallel to the axial direction of the rod / line, the crack generated from the surface of the bar / line is -It becomes easy to branch in the axial direction of the line, and the probability of causing the final break through the cross section is low.
[0020]
In the case of a PC steel wire or a steel bar, since the matrix phase itself is tempered martensite having a high strength as described above, the structure having a higher hardness than this needs to be martensite as transformed. Moreover, since retained austenite is transformed into martensite by strain-induced transformation when stress is applied in an environment where a PC steel wire or a steel rod is used, the same effect as martensite is obtained as a result. In order to improve the delayed fracture resistance, it is necessary to arrange a band-like structure mainly composed of as-quenched martensite or retained austenite or a mixed structure thereof in the matrix. If the martensite or retained austenite or their mixed structure as quenched inside is less than 50% in terms of area ratio, the crack growth suppressing effect is small. In addition, if the band-like structure has an average thickness of less than 1 μm, the effect of suppressing crack growth is also small. However, when the thickness exceeds 30 μm, the PC steel wire or the steel rod itself is brittle. In addition, if the interval between the strip-like structures is less than 10 μm on average, the hard phase is too much to cause embrittlement of the PC steel wire or the steel rod itself. On the other hand, if the interval exceeds 200 μm on average, the crack growth suppressing effect is small.
[0021]
FIG. 1 shows an example of the metal structure of the steel of the present invention. FIG. 2 shows a method of measuring the area ratio of as-quenched martensite or retained austenite or a mixed structure thereof, the thickness (width in the plate thickness direction) and the interval (in the plate thickness direction) of the band structure. An example of
Next, the features of the method for producing a PC steel wire or steel bar in the present invention will be described.
[0022]
The reason why the diameter is set to 4 mm or more and 12 mm or less is that the size of the PC steel wire or the steel rod that is usually used is almost within this range.
Moreover, the subsequent hot rolling temperature was set to 850 ° C. or less from the wire diameter at which the true strain until the final wire diameter reaches 0.8 or more because of the synergistic effect of Nb addition and rolling in the low temperature region. Both the dynamic recrystallization and the static recrystallization are suppressed, and by applying a strain of 0.8 or more, the prior austenite grains before the transformation are sufficiently stretched, and the rolling direction by the transformation in the subsequent direct cooling process is performed. This is to obtain a structure stretched parallel to (the length direction of the steel wire).
[0023]
Although it cools directly after hot rolling, if the cooling rate at that time is less than 10 ° C./s, it is difficult to obtain martensite or lower bainite in an area ratio of 80% or more. Cooling should be below the Ms point to obtain a sufficient amount of martensite or lower bainite.
It is necessary for the improvement of the delayed fracture resistance, which is the object of the present invention, to pass through additional processing steps such as wire drawing, surface processing, and shape adjustment as needed before the heat treatment which is the next step after cooling. It is not an obstacle.
[0024]
In the heat treatment process, heating is performed at a heating rate of 10 ° C./s or more to Ac ₁ point or more and Ac ₁ point + 80 ° C. or less, and quenching is performed, but if the heating rate is less than 10 ° C./s, the structure becomes coarse during heating, Delayed fracture resistance deteriorates. Further, when the maximum heating temperature is less than Ac ₁ point, reverse-transformed austenite cannot be obtained, and as-quenched martensite or residual austenite or a mixed structure thereof cannot be obtained even by quenching after heating. On the other hand, if the heating temperature exceeds Ac ₁ point + 80 ° C., the amount of reverse-transformed austenite is too much and the hardenability is lowered, and as-quenched martensite or residual austenite or a mixed structure thereof cannot be obtained sufficiently. Therefore, the range of the heating temperature is limited to Ac ₁ point or more and Ac ₁ point + 80 ° C. or less.
[0025]
It is preferable that quenching after heating is performed as quickly as possible, and thus a sufficient amount of as-quenched martensite, retained austenite, or a mixed structure thereof can be secured.
By passing through the above process, it becomes possible to obtain the metal structure of the PC steel wire or steel bar of the present invention.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in detail based on examples.
First, the steel having the chemical composition shown in Table 1 was made into a steel wire having the diameter shown in Table 2 under the production conditions shown in Table 2. The metal structure of this steel wire, the thickness of the hard phase, the spacing, the area ratio of the as-quenched martensite or retained austenite inside the band structure or their mixed structure, tensile test results and delayed fracture test results (in the FIP delayed fracture test) f ₅₀ hours) are also shown in Tables 3 and 4 (continued in Table 3).
[0027]
[Table 1]

[0028]
[Table 2]

[0029]
[Table 3]

[0030]
[Table 4]

[0031]
【The invention's effect】
As is apparent from Tables 3 and 4, the steel wire of the present invention has a structure in which martensite or lower bainite is 80% or more, and in the matrix, as-quenched martensite or residual austenite or The band-like structures in which the mixed structure occupies 50% or more have an average thickness of 1 μm to 30 μm and an interval of 10 μm to 200 μm on average, and are arranged in layers in parallel with the length direction of the steel wire. A steel wire having such a structure has sufficient strength and elongation, and further, the fracture time of the delayed fracture test is remarkably long. From Table 2, the manufacturing method for obtaining such a structure is rolled at a true strain of 0.8 or higher in a low temperature region of 850 ° C. or lower, and further, at the cooling rate of 10 ° C./s or higher after the rolling, the Ms point or lower. It was found that the sample was cooled to Ac ₁ point to Ac ₁ point + 80 ° C. at a heating rate of 10 ° C./s or higher and then quenched. It can also be seen that there is no adverse effect on the properties of the steel of the present invention even if an additional wire drawing step, surface treatment, shape adjustment step, etc. are inserted between the direct cooling after the hot rolling and the heat treatment.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a metallographic photograph of steel of the present invention.
FIG. 2 is a schematic diagram showing a method for measuring the area ratio of as-quenched martensite or retained austenite or a mixed structure thereof, the thickness of the band structure, and the interval inside the band structure.

Claims (9)

% By weight
C: 0.20 to 0.40%,
Si: 0.05 to 1.5%,
Mn: 0.2 to 1.5%
Al: 0.002 to 0.10%,
Nb: 0.01 to 0.07%
In the parent phase consisting of martensite or lower bainite with a balance of 4 to 12 mm in diameter and comprising a balance of Fe and unavoidable impurities and having a diameter of 4 mm to 12 mm. A band-like structure in which sites or residual austenite or a mixed structure thereof account for 50% or more has an average thickness of 1 μm to 30 μm and an interval of 10 μm to 200 μm on average, and is layered in parallel to the length direction of the steel wire or steel rod PC steel wire or steel rod with good delayed fracture resistance, characterized in that % By weight
Ti: 0.002-0.10%
The PC steel wire or steel bar having good delayed fracture resistance according to claim 1. % By weight
V: 0.005 to 0.1%
3. A PC steel wire or a steel bar having good delayed fracture resistance according to claim 1 or 2, characterized by comprising: % By weight
Cu: 0.05 to 0.5%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 0.5%,
Mo: 0.05-0.5%
Co: 0.05-0.5%
W: 0.05-0.5%
The PC steel wire or the steel bar having good delayed fracture resistance according to any one of claims 1 to 3, wherein the PC steel wire or the steel rod has good delayed fracture resistance. % By weight
B: 0.0002 to 0.0025%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of claims 1 to 4. % By weight
Rem: 0.002 to 0.10%,
Ca: 0.0003 to 0.0030%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of claims 1 to 5, wherein the PC steel wire or the steel rod has good delayed fracture resistance. % By weight
Mg: 0.0003 to 0.01%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of claims 1 to 6. When the steel comprising the chemical component according to any one of claims 1 to 7 is hot-rolled to a steel wire or a steel rod having a diameter of 4 mm to 12 mm, the true strain until the final wire diameter is 0. carried from the line diameter of 8 or more subsequent rolling at 850 ° C. or less, rolling end after cooling to Ms point or lower at 10 ° C. / s or more cooling rate, then the heating rate 10 ° C. / s or higher at Ac ₁ or more points The method for producing a PC steel wire or a steel rod with good delayed fracture resistance according to any one of claims 1 to 7, wherein the steel is heated to Ac ₁ point + 80 ° C or lower and quenched.
Here, the true strain ε is defined by ε = ln (d ₀ / d ₁ ). Here, ln is a natural logarithm, and d ₀ and d ₁ are a predetermined wire diameter during rolling and a final rolling wire diameter, respectively. After direct cooling after hot rolling, after additional processing steps, shape adjustment steps, etc., heating is performed at a heating rate of 10 ° C./s to Ac ₁ point to Ac ₁ point + 80 ° C. and quenching. The method for producing a PC steel wire or a steel bar having good delayed fracture resistance according to claim 8.

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