JP4267376B2 - High strength PC steel wire with excellent delayed fracture characteristics and method for producing the same - Google Patents

Description

【００３６】
【表２】

【００３７】
【発明の効果】
本発明は伸線パーライト鋼よりなるＰＣ鋼線において、表層の板状セメンタイトのアスペクト比を制御することによって、引張強さが１６５０ＭＰａ以上の高強度ＰＣ鋼線の遅れ破壊特性を大幅に向上させることを可能にするとともに、鋼の化学成分、伸線減面率、ブルーイング条件を最適に選択することによって、その製造方法を確立したものであり、産業上の効果は極めて顕著なものがある。
【図面の簡単な説明】
【図１】急速加熱温度がFIP試験破断時間に及ぼす影響を示す図である。
【図２】３５０℃、５００℃ブルーイング材のセメンタイト形状を示す図である。
【図３】表層セメンタイトのアスペクト比がFIP破断時間に及ぼす影響を示す図である。
【図４】急速加熱温度がセメンタイトのアスペクト比に及ぼす影響を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a PC steel wire widely used as a reinforcing material for prestressed concrete structures such as poles, piles, buildings, bridges, and the like, and in particular, high strength excellent in delayed fracture characteristics having a strength of 1650 MPa or more. The present invention relates to a PC steel wire and a manufacturing method thereof.
[0002]
[Prior art]
PC steel materials widely used as reinforcing materials for prestressed concrete structures such as poles, piles, buildings, bridges, etc. are usually specified in PC steel wires and PC steel strands specified in JIS G 3536, JIS G 3109. PC steel bars are used. The material used for the PC steel wire is a piano wire material conforming to JIS G 3502, and is manufactured by performing a drawing process after a patenting treatment.
[0003]
On the other hand, PC steel bars are manufactured by quenching and tempering using medium carbon steel having a C content of 0.25 to 0.35%, as described in Patent Document 1, for example. As described in pages 43 to 45 of “Prestressed Concrete Design and Construction Standards / Commentary” (edited by the Architectural Institute of Japan, Maruzen) The strength PC steel bar is inferior in delayed fracture characteristics to the PC steel wire. Therefore, many proposals have been made in the past to improve the delayed fracture characteristics of PC steel bars. For example, Patent Document 2 proposes that reducing the P and S contents is effective. Patent Document 1 proposes that the Si and Mn contents are regulated and that bending or drawing is performed in the tempering process after quenching. On the other hand, regarding the PC steel wire, since the delayed fracture resistance was originally superior to that of the PC steel rod, there has been almost no invention in terms of improving the delayed fracture resistance. However, in recent years, PC steel wires are also required to have high strength or external cables, and at the present time, those having high strength of 1650 MPa class are required. As is well known (for example, “Delayed Fracture” by Keisaku Matsuyama / Nikkan Kogyo Shimbun) Steel materials become more resistant to hydrogen embrittlement as they become stronger or as the usage environment becomes more severe. There is a need for high strength PC steel wires with improved delayed fracture properties. For example, Patent Document 3 proposes to improve delayed fracture characteristics by maintaining a predetermined temperature and a predetermined time after wire drawing. However, this is intended to remove hydrogen in the steel simply by heating without changing the steel structure and mechanical properties, and it cannot suppress the destruction caused by hydrogen that has entered during use. This is not a fundamental measure to improve the characteristics. The same applies to Patent Document 4 and Patent Document 5.
[0004]
[Patent Document 1]
Japanese Patent Publication No. 5-41684 [Patent Document 2]
Japanese Patent Publication No. 5-59967 [Patent Document 3]
Japanese Patent Laid-Open No. 10-259425 [Patent Document 4]
JP-A-8-337844 [Patent Document 5]
Japanese Patent Laid-Open No. 8-337845
[Problems to be solved by the invention]
The present invention has been made in view of the actual situation as described above, and realizes a high strength PC steel wire having a good strength of delayed fracture property of 1650 MPa or more and a method for producing this PC steel wire. It is intended.
[0006]
[Means for Solving the Problems]
The present inventors first analyzed delayed fracture behavior in detail using PC steel wires of various strength levels manufactured by patenting, wire drawing, and brewing. The delayed fracture characteristics of PC steel wires were evaluated by a widely used FIP test. This is a method of measuring the breaking time by applying a load of 70% of the breaking load in the atmosphere to a PC steel wire in a solution obtained by heating a 20 wt% ammonium thiocyanate aqueous solution to 50 ° C. At this time, the amount of the test solution per 1 cm ² of the surface area in contact with the test solution of the PC steel wire (hereinafter referred to as the specific liquid amount) was 55 cc / cm ² . Moreover, the hydrogen storage behavior of the PC steel wire in the FIP test was measured with a temperature rising gas chromatograph.
[0007]
By performing the above test, in order to increase the FIP fracture time of the high strength PC steel wire, that is, to increase the delayed fracture characteristics, the effects of steel material components, austenite heating temperature, wire drawing conditions, bluing conditions, heat treatment conditions, etc. Repeated examination. As a result, in the final step of manufacturing the PC steel wire, after heating to 450 ° C. or higher, preferably 500 ° C. or higher, after performing blueing so as to hold for 5 seconds or longer, at least the surface layer of the steel wire is cooled. 1 / 10d (d is the radius of the steel wire) Desirably, in a region having a depth of 1 / 5d, a plate-like cementite shape and a structure having an average length / thickness ratio (hereinafter referred to as aspect ratio) of 30 or less. It was found that the fracture time of the FIP test does not break for 50 hours or more even in a high strength region exceeding 1650 MPa. There are no restrictions on the method of rapid heating and rapid cooling, and water cooling after immersion in a lead furnace, water cooling after high-frequency heating, and the like are possible.
[0008]
Based on the above examination results, the present inventors have reached the conclusion that a high-strength PC steel bar with excellent delayed fracture characteristics can be realized by optimally selecting steel components, wire drawing conditions, and heat treatment conditions. .
[0009]
The present invention has been made on the basis of the above findings, and the gist thereof is as follows.
(1) By mass%, C: 0.6-1.1%, Si: 0.12-2.0%, Mn: 0.3-1.0%, P: 0.025% or less, S: 0.0025% or less, and further Al: 0.005-0.1%, Cr: 0.05-2.0%, Mo: 0.05-1.0%, Ni: 0.05-3 0.0%, Cu: 0.05-1.0%, V: 0.05-0.3%, Nb: 0.005-0.1%, W: 0.05-0.5%, Ti: 0.005 to 0.05% B: include one or more of 0.0003 to 0.005 percent, the balance being from iron and unavoidable impurities, the pearlite which is ferrite and cementite lamellar structure Shin The average asperity of plate-like cementite in pearlite in the region of a depth of at least 1 / 10d (d is the radius of the steel wire) of the steel wire surface layer. High strength PC steel wire tensile strength, characterized with excellent delayed fracture resistance of more than 1650MPa that Ratio is 30 or less.
( 2 ) By mass % , C: 0.6-1.1%, Si: 0.12-2.0%, Mn: 0.3-1.0%, P: 0.025 % or less, S: 0.0025 % or less, further Al: 0.005-0.1%, Cr: 0.05-2.0%, Mo: 0.05-1.0%, Ni: 0.05-3 0.0%, Cu: 0.05-1.0%, V: 0.05-0.3%, Nb: 0.005-0.1%, W: 0.05-0.5%, Ti: 0.005 to 0.05% B: include one or more of 0.0003 to 0.005 percent, the balance being made patenting steel consisting of iron and unavoidable impurities and mainly pearlite Then, after cold wire drawing, the tensile strength is excellent at delayed fracture resistance of 1650 MPa or more, characterized by holding at 450 ° C. or more for 5 seconds or more in the final process. Method of manufacturing strength PC steel wire.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
First, the reasons for limiting the components of steel to be the subject of the present invention will be described. Hereinafter, the unit is mass%.
[0011]
C: C is an essential element for securing the strength of the PC steel wire. However, if it is less than 0.6%, the amount of pro-eutectoid ferrite increases during patenting, and the required strength cannot be obtained. If it exceeds 1%, the amount of pro-eutectoid cementite increases and the wire drawing characteristics are remarkably deteriorated.
[0012]
Si: Si has the effect of improving relaxation properties and increasing strength by solid solution hardening. If the content is less than 0.12%, the above-mentioned action cannot be exhibited. On the other hand, if the content exceeds 2%, the effect is saturated, so the content is limited to the range of 0.12 to 2.0%.
[0013]
Mn: Mn is not only necessary for deoxidation and desulfurization, but also has the effect of increasing the strength of the patenting material. However, if the content is less than 0.3%, the above effect cannot be obtained. Segregation at the time became prominent, and micromartensite that deteriorates the wire drawing characteristics at the time of patenting was generated, so the content was limited to a range of 0.3 to 1.0%.
[0014]
P: P co-segregates with Mn, and remarkably enhances the hardenability. Therefore, in order to promote the formation of micromartensite at the time of patenting, the content was made 0.025% or less.
[0015]
S: S is precipitated as MnS and deteriorates the wire drawing characteristics, so the content was made 0.025% or less.
[0016]
The above are the basic components of the steel that is the subject of the present invention. In the present invention, the steel is further divided by mass: Al: 0.005-0.1%, Cr: 0.05-2.0. %, Mo: 0.05 to 1.0%, Ni: 0.05 to 3.0%, Cu: 0.05 to 1.0%, V: 0.05 to 0.3%, Nb: 0. Include one or more of 005 to 0.1%, W: 0.05 to 0.5%, Ti: 0.005 to 0.05%, and B: 0.0003 to 0.0050%. Can do. The reason for adding these elements will be described below.
[0017]
Al: Al has the effect of preventing the coarsening of austenite grains and suppressing the deterioration of toughness by forming AlN during deoxidation and heat treatment, and securing solid solution B effective for fixing delayed N and improving delayed fracture characteristics However, if less than 0.005%, these effects are not exhibited, and even if it exceeds 0.1%, the effect is saturated, so the content is limited to a range of 0.005 to 0.1%.
[0018]
Cr: Cr is an effective element for reducing the spacing of the pearlite lamella and increasing the strength of the patenting material. However, if it is less than 0.05%, the effect cannot be fully exerted, whereas it exceeds 2.0%. In order to saturate the effect, it was limited to 0.05 to 2.0%.
[0019]
Mo: Mo is an effective element for increasing the strength of the patenting material in order to make the pearlite lamella spacing as fine as Cr. However, if it is less than 0.05%, the effect cannot be fully exerted. If it exceeds 0.0%, the progress of pearlite transformation is delayed, so the content was limited to 0.05 to 1.0%.
[0020]
Ni: Ni has an effect of suppressing the intrusion of hydrogen. However, if it is less than 0.05%, the effect cannot be exhibited. On the other hand, if it exceeds 3.0%, the effect corresponding to the added amount cannot be exhibited. Limited to -3.0% range.
[0021]
Cu: Cu also has an effect of suppressing the penetration of hydrogen, but if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 1.0%, the hot workability deteriorates, so 0.05 to 1.0 %.
[0022]
V: V is an effective element for refining austenite grains by producing carbonitride. Further, it precipitates as carbides during patenting and functions as a hydrogen trap site, so that it has an effect of improving delayed fracture resistance. However, if it is less than 0.05%, the above effect cannot be obtained. Even if it exceeds%, the effect is saturated, so it was limited to 0.05 to 1.0%.
[0023]
Nb: Nb is also an element effective for reducing the austenite grains and improving ductility and toughness by producing carbonitrides in the same manner as V. If it is less than 0.005%, the above effect is insufficient. On the other hand, if it exceeds 0.1%, this effect is saturated, so the content is limited to 0.005 to 0.1%.
[0024]
W: W, like V, is an element effective for improving the delayed fracture characteristics of a high-strength PC steel wire. However, if it is less than 0.05%, the above effect cannot be exhibited, while 0.5% Even if it is added over the range, the effect is saturated, so the content is limited to 0.05 to 0.5%.
[0025]
Ti: Ti has the effect of preventing the coarsening of austenite grains by forming deoxidation and TiN, and has the effect of fixing N and ensuring effective solute B for improving delayed fracture characteristics. If it is less than 0.005%, these effects are not exhibited, and even if it exceeds 0.05%, the effect is saturated, so the content is limited to a range of 0.005 to 0.05%.
[0026]
B: B has an effect of improving delayed fracture characteristics. However, if B is less than 0.0003%, the above effect is not exhibited, and even if it exceeds 0.0050%, the effect is saturated. Limited to 0050%.
[0027]
N: N has an effect of refining austenite grains by forming nitrides of Al, V, Nb, and Ti, and can contribute to improvement of ductility and toughness, so 0.003 to 0.015% is preferable range It is.
[0028]
Next, the reasons for limiting the microstructure of the PC steel wire and the rapid heating temperature, which are the most important points for improving the delayed fracture characteristics of the high strength PC steel wire intended in the present invention, will be described.
[0029]
FIG. 1 shows an example in which the influence of the rapid heating temperature on the average rupture time by the FIP test is analyzed. Here, high speed (frequency is 40 kHz) was used for rapid heating, and the heating rate was 200 ° C./s or more. The FIP test is a result of carrying out 12 pieces for each steel type with a specific liquid amount of 55 cc / cm ² and a load of 1200 MPa. As is clear from the figure, the fracture time in the FIP test is increased as the heating temperature is increased, indicating that the delayed fracture resistance is improved. A remarkable effect is observed when the heating temperature is 450 ° C. or higher, and more preferably 500 ° C. or higher.
[0030]
In the drawn pearlite structure, the cementite elongated in a plate shape is dissolved and divided by bluing at a high temperature as shown in FIG. FIG. 3 shows the result of analyzing the influence of the heating temperature on the ratio (aspect ratio) of the length and thickness of plate-like cementite in the steel wire surface layer. The measurement is performed by cutting the test piece in the longitudinal direction (L cross section), applying resin-embedded polishing, etching with a saturated picral solution, and then scanning with a scanning electron microscope (SEM). The part was photographed at a magnification of 5000, and the analysis was performed by image analysis. The figure shows that cementite is divided and spheroidized as the heating temperature increases.
[0031]
FIG. 4 shows the effect of the aspect ratio of the surface cementite on the FIP fracture time. As the aspect ratio of cementite decreases, the FIP fracture time increases. A remarkable effect is recognized when the aspect ratio is 30 or less, and more preferably 20 or less.
[0032]
【Example】
After rolling the test material having the chemical composition shown in Table 1 under normal hot rolling conditions, patenting and drawing are performed in various temperature ranges, and rapid heating / quenching is carried out at different temperatures. PC steel wire Manufactured. Table 2 shows the results of evaluating the mechanical properties, structure morphology, and delayed fracture characteristics using the above PC steel wires. Delayed fracture characteristics were carried out at a load stress of 1200 MPa in the FIP test. Evaluation was carried out by taking the average value of the rupture times of 12 FIP tests.
[0033]
In Tables 1 and 2, A to T are examples of the present invention, and the others are comparative examples. As can be seen from the table, all of the examples of the present invention have a blueing temperature of 450 ° C. or higher and a tensile strength of 1650 MPa or higher. FIP breaking time is 30 hours or more.
[0034]
U, which is a comparative example, is an example in which 1650 MPa could not be achieved by rapid heating at 450 ° C. or higher because the steel material component and the wire drawing area reduction ratio were not appropriate. V, which is a comparative example, is an example in which the brewing time was less than 5 seconds and the cementite aspect ratio did not become 30 or less up to a predetermined depth. Furthermore, W to Z as comparative examples are all manufactured by a conventional manufacturing method. That is, an example in which blueing is performed at 400 ° C. or less and 1650 MPa or more is achieved, but the cementite aspect ratio is greater than 30, the FIP test has a fracture time of 30 hours or less, and delayed fracture characteristics are poor It is.
[0035]
[Table 1]

[0036]
[Table 2]

[0037]
【The invention's effect】
The present invention significantly improves the delayed fracture characteristics of a high strength PC steel wire having a tensile strength of 1650 MPa or more by controlling the aspect ratio of the surface layered cementite in the PC steel wire made of drawn pearlite steel. In addition, the manufacturing method has been established by optimally selecting the chemical composition of steel, the wire drawing area reduction ratio, and the bluing conditions, and the industrial effects are extremely remarkable.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of rapid heating temperature on FIP test break time.
FIG. 2 is a view showing a cementite shape of a 350 ° C. and 500 ° C. bluing material.
FIG. 3 is a diagram showing the influence of the aspect ratio of surface cementite on the FIP fracture time.
FIG. 4 is a diagram showing the influence of rapid heating temperature on the aspect ratio of cementite.

Claims (2)

In mass%, C: 0.6-1.1%, Si: 0.12-2.0%, Mn: 0.3-1.0%, P: 0.025% or less, S: 0.0025 In addition, Al: 0.005-0.1%, Cr: 0.05-2.0%, Mo: 0.05-1.0%, Ni: 0.05-3.0% Cu: 0.05-1.0%, V: 0.05-0.3%, Nb: 0.005-0.1%, W: 0.05-0.5%, Ti: 0.005 -0.05%, B: 0.0003-0.005 % of one type or two or more types, the balance is made of iron and inevitable impurities, and a structure obtained by drawing pearlite, which is a layered structure of ferrite and cementite The average aspect ratio of plate-like cementite in pearlite in the region of depth of at least 1 / 10d (d is the radius of the steel wire) of the steel wire surface layer High strength PC steel wire tensile strength, characterized with excellent delayed fracture resistance of more than 1650MPa to be 30 or less. mass % C: 0.6-1.1%, Si: 0.12-2.0%, Mn: 0.3-1.0%, P: 0.025 % Hereinafter, S: 0.0025 % In addition, Al: 0.005-0.1%, Cr: 0.05-2.0%, Mo: 0.05-1.0%, Ni: 0.05-3.0%, Cu: 0.05-1.0%, V: 0.05-0.3%, Nb: 0.005-0.1%, W: 0.05-0.5%, Ti: 0.005- 0.05%, B: 0.0003 to 0.005 % After the steel is composed of iron and inevitable impurities and the remainder is patented to make a pearlite structure as a main component and then cold-drawn, it is processed at 450 ° C. or higher in the final step. A method for producing a high-strength PC steel wire excellent in delayed fracture resistance with a tensile strength of 1650 MPa or more, characterized by holding for at least 2 seconds.

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