AU682675B2 - Steel bar for prestressed concrete excellent in delayed fracture resistance at weld zone - Google Patents

Description

'p -1- PRegulAo 3211 Regulation 32

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT o 0 0 *0 o 0 0 0 *00

ORIGINAL

00 0 0 00*0 0 *000 0* 0 0 000 0 ~*0*00 0 Name of Applicants: Actual Inventors: Address for service in Australia: NKK CORPORATION and NETUREN CO., LTD.

Tetsuo SHIRAGA, Eiji YAMASHITA, Hajime NITTA, Shigeru MIZOGUCHI and Moriyuki ISHIGURO CARTER SMITH BEADLE 2 Railway Parade Camberwell Victoria 3124 Australia Invention Title: STEEL BAR FOR PRESTRESSED EXCELLENT IN DELAYED FRACTURE AT WELD ZONE

CONCRETE

RESISTANCE

The following statement is a full description of this invention, including the best method of performing it kliot'n to us la- REFERENCE TO PATENTS, APPLICATIONS AND PUBLICATIONS PERTINENT TO THE INVENTION As far as we know, there are available the following prior art documents pertinent to the present invention: Japanese Patent Publication No. 55-11,726 published on March 27, 1980; and Japanese Patent Provisional Publication No. 2-240,238 10 published on September 25, 1990.

The contents of the prior art disclosed in the above-mentioned prior art documents will be discussed hereafter under the heading of the "BACKGROUND OF THE

INVENTION."

15 BACKGROUND OF THE INVENTION (FIELD OF THE INVENTION) The present invention relates to a steel bar for prestressed concrete excellent in delayed fracture resistance at a weld zone in a spot welding.

(RELATED ART STATEMENT) While having a high compression resistance, concrete is low in tension resistance. For the purpose of overcoming this drawback, a steel bar for prestressed concrete is used as a material having a high strength for prestressed concrete. The Japanese Industrial Standard (JIS G-3109) specifies lower limit values for proof stress tensile strength (TS) and elongation (El) (a tensile strength of at least 1,200 N/mm2 for example), and an upper limit value for a relaxation value as important mechanical properties of a steel bar for prestressed concrete.

The steel bar for prestressed concrete is usually manufactured by subjecting a killed steel material to a hot rolling followed by any one of such methods as a 15 stretching, a drawing and a heat treatment, or a combination thereof. For a high-carbon steel containing at least 0.8 wt.% carbon, in general, the above-me.itioned drawing method is applied, and for a low-carbon or mediumcarbon steel, the above-mentioned heat treatment method o 20 comprising a quenching and a tempering is applied.

Since it is difficult to apply a spot welding to the steel bar for prestressed concrete manufactured from the above-mentioned high-carbon steel by means of the drawing method, the operational efficiency is seriously reduced when forming a cage. The spot welding is, on the other hand, applicable to the steel bar for prestressed 2 concrete manufactured from the above-mentioned low-carbon or medium-carbon steel by means of the heat treatment method comprising a quenching and a tempering so that a cage can be efficiently formed by spot-welding a portion between a main steel bar and an auxiliary steel bar.

Use of the steel bar for prestressed concrete in concrete poses however the following problem: When placing concrete, if concrete contains salt or low-pH water is employed, there occurs a delay fracture, a phenomenon in which a sudden fracture is caused in a material having a high strength under a load stress within a strength limit thereof after the lapse of a certain period of time, in the steel bar for prestressed concrete.

To overcome the above-mentioned problem, a steel bar for prestressed concrete excellent in delayed fracture resistance is disclosed in Japanese Patent Provisional Publication No. 2-240,238 published on oo• September 25, 1990, which consists essentially of: carbon from 0.2 to 0.4 wt.%, silicon (Si) from 0.2 to 2.0 wt.%, manganese (Mn) from 0.2 to 1.5 wt.%, chromium (Cr) from 0.3 to 2.0 wt.%, molybdenum (Mo) from 0.1 to 0.5 wt.%, -3and the balance being iron (Fe) and incidental impurities, where, the contents of phosphorus and sulfur as said incidental impurities being, respectively: up to 0.020 wt.% for phosphorus, and up to 0.005 wt.% for sulfur (hereinafter referred to as the "prior art The above-mentioned steel bar for prestressed concrete of the prior art 1 has however the following problems: The steel bar for prestressed concrete of the prior art 1 has an improved delayed fracture resistance in its base metal. However, when a spot welding is applied to the steel bar for prestressed concrete of the prior art 1, a weld zone of the steel bar for prestressed concrete 'is hardened under the effect of a welding heat, and the use thereof in concrete causes the occurrence of delayed :20 fracture at the weld zone.

Furthermore, a method for manufacturing a steel bar for prestressed concrete excellent in spot weldability is disclosed in Japanese Patent Publication No. 55-11,726 published on March 27, 1980, which comprises the steps -4of: quenching, starting from an austenitizing temperature region, a steel material consisting essentially of: carbon from 0.2 to 0.3 wt.%, silicon (Si) from 0.2 to 0.6 wt.%, manganese (Mn) from 0.9 to 2.0 wt.%, chromium (Cr) from 0.1 to 0.6 wt.%, and the balance being iron (Fe) and incidental impurities; and then tempering same at a temperature of at least 280 0

C

(hereinafter referred to as the "prior art The steel bar for prestressed concrete manufactured by the method of the prior art 2 has however the *following problem: The steel bar for prestressed concrete manufactured by the method of the prior art 2 has an excellent spot weldability and a high tensile strength after the spot welding. However, when a spot welding is applied to the steel bar for prestressed concrete of the prior art 2, a weld zone of the steel bar for prestressed concrete is hardened under the effect of a welding heat, and the use thereof in concrete also causes the occurrence of delayed -6fracture at the weld zone as in the case of the prior art 1.

Under such circumstances, there is a strong demand for development of a steel bar for prestressed concrete, which has properties such as proof stress, tensile strength and elongation and a relaxation property on the same level as in the conventional steel bar for prestressed concrete, and furthermore, has an excellent delayed fracture resistance at a weld zone thereof in the spot welding, but such a steel bar for prestressed concrete has not as yet been proposed.

SUMMARY OF THE INVENTION An object of the present invention is therefore to provide a steel bar for prestressed concrete, which has properties such as proof stress, tensile strength and elongation and a relaxation property on the same level as in the conventional steel bar for prestressed concrete, and furthermore, has an excellent delayed fracture resistance at a weld zone thereof in the spot welding.

In accordance with one of the features of the present invention, there is 15 provided a steel bar for prestressed concrete excellent in delayed fracture resistance at a weld zone, having tensile strength of at least 1400N/mm 2 and which has a composition comprising: carbon frin 0.2 to 0.6 wt.%, silicon (Si) from 0.2 to 2.0 wt.%, 20 manganese (Mn) from 0.2 to 2.0 wt.%, nickel (Ni) from 0.25 to under 0.8 wt.%, and the balance being iron (Fe) and incidental impurities, where, the contents of phosphorus and sulfur as said incidental impurities being, respectively: up to 0.020 wt.% for phosphorus, and up to 0.015 wt.% for sulfur, and when said steel bar, which is spot-welded, is immersed in an ammonium thiocyanate (NH4SCN) solution having a concentration of 20% at a temperature of TNB:PJD:#15048 17 October 1996 -7and a stress equal to 70% of tensile strength of said steel bar is applied to said spot-welded steel bar, said spot-welded steel bar has a time before fracture at said weld zone of at least 20 hours.

In accordance with another feature of the present invention, there is also provided a steel bar for prestressed concrete excellent in delayed fracture resistance at a weld zone, having tensile strength of at least 1400N/mm 2 and which has a composition comprising: carbon (C) silicon (Si) manganese (Mn) nickel (Ni) from 0.2 to 0.6 wt.%, from 0.2 to 2.0 wt.%, from 0.2 to 2.0 wt.%, from 0.25 to under 0.8 wt.%, at least one element selected from the group consisting of: *e S s .5 molybdenum (Mo) copper (Cu) boron (B) from 0.1 to 0.5 wt.%, from 0.05 to 1.0 wt.%, from 0.0003 to 0.0050 wt.%, and o o *o tungsten (W) fror. 0.03 to 0.50 wt.%, and the balance being iron (Fe) and incidental impurities, where, the contents of phosphorus and sulfur as said incidental impurities being, respectively: up to 0.020 wt.% for phosphorus, and up to 0.015 wt.% for sulfur, and when said steel bar, which is spot-welded, is immersed into an ammonium thiocyanate (NH4SCN) solution having a concentration of 20% at a temperature of 50'C, and a stress equal to 70% of tensile strength of said steel bar is applied to said spot-welded steel bar, said spot-welded steel bar has a time before fracture at said weld zone of at least 20 hours.

C2- (7.

TNB:PJD:# 15048 17 October 1996 -8- DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS From the above-mentioned point of view, extensive studies were carried out to develop a steel bar for prestressed concrete, which has properties such as proof stress, tensile strength and elongation and a relaxation property on the same level as in the conventional steel bar for prestressed concrete, and furthermore, has an excellent delayed fracture resistance at a weld zone thereof in the spot welding.

As a result, the following findings were obtained A steel bar for prestressed concrete has an excellent delayed fracture resistance at a weld zone in a spot welding if the steel bar has a time before fracture of at least 20 hours, when the steel bar not spot-welded is r c o e r o a a r o o r a ~o L' TNBPID:41S048 17 October 1996

ILI

immersed into an ammonium thiocyanate (NH 4 SCN) solution having a concentration of 20% at a temperature of 50°C and a stress equal to 70% of tensile strength of the steel bar is applied to the steel bar to measure the time before fracture thereof.

More specifically, by bringing delayed fracture resistance at a weld zone in a spot-welded steel bar for prestressed concrete to the same level as that of a steel bar for prestressed concrete not spot-welded, it is possible to achieve an excellent delayed fracture resistance even at the weld zone in the spot-welded steel bar. In other words, it can be said that a spot-welded steel bar for prestressed concrete has an excellent delayed fracture resistance at a weld zone thereof on the 15 same level as that in a base metal of a steel bar for prestressed concrete not spot-welded if the spot-welded steel bar has a time before fracture of at least hours, when the spot-welded steel bar is immersed into an ammonium thiocyanate (NH 4 SCN) solution having a a. .0 20 concentration of 20% at a temperature of 50°C and a *a.

stress equal to 70% of tensile strength of the steel bar not spot-welded is applied to the spot-welded steel bar to measure the time before fracture thereof.

In addition, the above-mentioned delayed fracture resistance at the weld zone was found to be determined by the chemical composition of the steel bar for prestressed 9 ril concrete.

The present invention was made on the basis of the above-mentioned findings, and the steel bar for prestressed concrete of the present invention consists essentially of: carbon from 0.2 to 0.6 wt.%, silicon (Si) from 0.2 to 2.0 wt.%, manganese (Mn) from 0.2 to 2.0 wt.%, nickel (Ni) from 0.25 to under 0.8 wt.%, and the balance being iron (Fe) and incidental too impurities,.

where, the contents of phosphorus and sulfur as said incidental impurities being, respectively: up to 0.020 wt.% for phosphorus, *o and 4 up to 0.015 wt.% for sulfur.

oes The steel bar for prestressed concrete of the 20 present invention may additionally contain at least one element selected from the group consisting of: molybdenum (Mo) from 0.1 to 0.5 wt.%, copper (Cu) from 0.05 to 1.0 wt.%, boron from 0.0003 to 0.0050 wt.%, 1 0 I- and tungsten from 0.03 to 0,50 wt.%.

The chemical composition of the steel bar for prestressed concrete of the present invention is limited within a range as described above for the following reasons.

Carbon: Carbon has a function of increasing hardenability and strength of steel. With a carbon content of under 0.2 however, a desired effect as described above is unavailable, leading to a lower hardenability and a lower strength of steel. With a carbon content of over 0.6 on the other hand, spot weldability of steel is e .degraded. The carbon content should therefore be limited within a range of from 0.2 to 0.6 wt.%.

o* Silicon: Silicon has a function of increasing delayed fracture resistance and a relaxation property of steel.

With a silicon content of under 0.2 however, a a.

:20 desired effect as described above is unavailable, leading to a lower delayed fracture resistance and a lower relaxation property of steel. With a silicon content of over 2.0 on the other hand, toughness of steel is degraded. The silicon content should therefore be limited within a range of from 0.2 to 2.0 wt.%.

1 1 l Manganese: Manganese has a function of increasing hardenability and strength of steel. With a manganese content of under 0.2 however, a desired effect as described above is unavailable, leading to the degradation of hardenability and strength of steel. With a manganese content of over 2.0 on the other hand, ductility of steel is degraded. The manganese content should therefore be limited within a range of from 0.2 to wt.%.

Nickel: Nickel has a function of improving delayed fracture resistance at a weld zone. Particularly, it is c possible to achieve a time before fracture of at least hours at a weld zone of a spot-welded steel bar for prestressed concrete, when the spot-welded steel bar is immersed into an ammonium thiocyanate (NH 4 SCN) solution g* having a concentration of 20% at a temperature of 50 °C 0* *a and a stress equal to 70% of tensile strength of the steel bar not spot-welded is applied to the spot-welded bar to measure the time before fracture thereof. With a nickel oa o content of under 0.25 however, a desired effect as described above is unavailable, leading to a poor delayed fracture resistance at the weld zone. A nickel content of at least 0.8 wt.% leads, on the other hand, to a higher cost. The nickel corntent should therefore be limited -12-

D~"~I

within a range of from 0.25 to under 0.8 wt.%.

Phosphorus: Phosphorus is one of impurities inevitably entrapped into the steel bar for prestressed concrete.

While the phosphorus content should preferably be the lowest possible, it is difficult to largely reduce the phosphorus content in an industrial scale from the economic point of view. A phosphorus content of over 0.020 wt.% leads, however, to a lower delayed fracture resistance of steel. The phosphorus content should therefore be limited to up to 0.020 wt.%.

Sulfur: 9 Sulfur is one of impurities inevitably entrapped into the steel bar for prestressed concrete. While the sulfur content should preferably be the lowest possible, it is difficult to largely reduce the sulfur content in an industrial scale from the economic point of view. A sulfur content of over 9.015 wt.% leads, however, to a o e. lower delayed fracture resistance of steel. The sulfur

*N

content should therefore be limited to up to 0.015 wt.%.

Molybdenum: Molybdenum has a function of increasing hardenability and delayed fracture resistance of steel. In the steel bar for prestressed concrete of the present invention, therefore, molybdenum is additionally added as -13required. With a molybdenum content of under 0.1 wt.%, however, a desired effect as described above concerning delayed fracture resistance is unavailable. A molybdenum content of over 0.5 wt.% leads, on the other hand, to a higher cost. The molybdenum content should therefore be limited within a range of from 0.1 to 0.5 wt.%.

Copper: Copper has a function of increasing delayed fracture resistance. In the steel bar for prestressed concrete of the present invention, therefore, copper is additionally added as required. With a copper content of under 0.05 however, a desired effect as described above is unavailable. A copper content of over 1.0 wt.% 0 leads, on the other hand, to a higher cost. The copper content should therefore be limited within a range of from 0.05 to 1.0 wt,%.

Boron: *0 Boron has a function of increasing hardenability and delayed fracture resistance of steel. In the steel bar for prestressed concrete of the present invention, therefore, boron is additionally added as required. With a boron content of under 0.0003 however, a desired effect as described above is unavailable. With a boron content of over 0.0050 on the other hand, ;lelayed fracture resistance of steel is degraded. The boron content should therefore be limited within a range of from 1 4- 0.0003 to 0.0050 wt.%.

When boron is contained in an amount within the above-mentioned range, the steel bar for prestressed concrete should preferably contain titanium (Ti) within a range of from 0.01 to 0.05 wt.% in order to fix nitrogen in steel. Since zirconium (Zr) and niobium (Nb) have the same function as that of titanium at least one of titanium, zirconium and niobium may be contained in a total amount within a range of from 0.01 to 0.05 wt.%.

(10) Tungsten: Tungsten has a function of increasing delayed fracture resistance of steel. In the steel bar for S prestressed concrete of the present invention, therefore, tungsten is additionally added as required. With a tungsten content of under 0.03 however, a desired effect as described above is unavailable. A tungsten content of over 0.50 wt.% leads, on the other hand, to a higher cost. The tungsten content should therefore be S limited within a range of from 0.03 to 0.50 wt.%.

S

0*

I

S. OsSS 20 Now, the steel bar for prestressed concrete of the present invention is described further in detail by means of examples.

EXAMPLES

B. B I Each of steel materials of the present invention Nos. 1 to 23 having the chemical compositions within the scope of the present invention as shown in Table 1 was hot-rolled into a round bar having a diameter of 8 mm, and then formed into a deformed round bar having a diameter of 7.4 mm by means of the drawing method. Then, the thus formed deformed round bar was quenched at a temperature within a range of from 920 to 1,020°C by the use of the high-frequency heating, and then tempered at such a temperature as to impart a tensile strength of at least 1,420 N/mm 2 The above-mentioned heat treatment may be accomplished by a means other than the highfrequency heating.

Subsequently, the thus heat-treated deformed round 15 bars of the present invention were spot-welded under the following conditions to prepare samples within the scope of the present invention (hereinafter referred to as the "samples of the invention") Nos. 1 to 23: Welding current 3,000 A, Number of energizing cycles: 2, Applied pressure 410 N, and Spiral hoop reinforcement round bar having a diameter of 3.2 mm of of SWRM 8.

Then, samples outside the scope of the present 1 6 r n ~~rr c invention (hereinafter referred to as the "samples for comparison") Nos. 24 to 35 were prepared in the same manner as described above from steel materials Nos. 24 to having the chemical compositions outside the scope of the present invention as shown in Table 1.

0:00 0 Gao 00000 G* 0 SO 0 17 Table 1 (1) (wt. coo aoo p o D 0o 01, *°o0S*

S

too• .o0.

*e O00.0 a, 6096* *.90.

se 0 Chemical composition No.

C Si Mn P S Ni Mo Cu B W Ti 1 0.32' 0.30 0.83 0.0060.009 0.26 2 0.31 0.29 0.50 ).007 0.008 0'.156 3 0.30 0.35 0.61 3.007 0.008 0.-33 0.38 4 0.31 0.30 0.76 3.006 0.003 0._27 0.26 0.32 0.31 0.78 0.006 0.009 0.26 0-0020 0.02 6 0.33 0.45 0.66 3.007 0.008 0.68 0.10 7 0.30 0.25 1.41 P.016 0.002 0.66 0.31 8 0.45 0.28 0.62 0.006 0.009 0.31 0.33 0.30 9 0.28 0.36 0.91 .015 0.00 0.58 0. 24 0.16 0

-P

10 0.32 0.33 0.67 0.00710.00 0.28 0.30 0.0020 0.02 11 0.30 1.42 0.72 1.015 0.002 0.53 0.32 0.0020 0.02 12 0.41 0.42 0.58 0.007 0.00- 0.26 0.31 0.22 H 13 0.32 0.29 0.77 0.006 0.002 0.25 0.25 0.0020 0.02 0 w 14 0.33 0.31 1.58 0.009).009 0.62 0.13 0.0018 0.02 0.32 0.28 0.87 0.0063..009 0.52 0.53 0.0018 0.02 16 0.34 1.02 0.86 0.006 3..009. 0.51 0.27 0.0018 0.02 17 0.33 0.56 0.64 0.006 .008 0.31 0.32 0.18 18 0.34 0.32 0.51 00060.009 0.29 0.0020 0.34 0.02 19 0.32 0.28 0.69 0.007 .010 0.35 0.15 0.13 0.0020 0.02 0.27 0.29 0.79 0.0070.009 0.30 0.20 0.12 0.11 21 0.31 0.45 1.46 0.0080.009 0.27 0.11 0.0020 0.20 0.02 22 0.32 0.38 0.72 0.0070.009 0.32 0.18 0o.0020 0.13 0.02 0.32 0.35 0.60 0.0070.008 0.31 0.141 0.13 0.0020 0_10._2 18 ChTal 1omosi2o (wt. Table 1 (wt. .Chemical composition No.

Mn 24 0.,31 0,05 0 .92 37015

S

3.003 INi Cu

B

Ti 4- 4 0 00000 00 o 00 0 0.30 0.27 0.78 0.0100.008 0.0020 0..02 26 0.31 0.03 0.96 .0153.0030.62 0.,25 0 27 0.18 0.51 0.89 k,008D.002 0.35 28 0.62 0.34 0.76 0.010).003 0.68 0 o 29 0.33 0.11 0.82 0.015).003 0.44 O 30 0.35 2.23 .0.51 P. 0090.0100.37 31 0.31 0.46 0.10 .0110.00 80.51 32 0.36 0.33 2.09 .0110. 0010..30 33 0.34 0.30 0.88 0.0250.000.48 0 2 0 34 0.34 0.42 0.67 0.010 .0180.56 35 0.31 0.55 0.86 0.009 .oo 0.20 The tempering temperature and time (second) in the Sample Nos. 1 to 35 of the examples of the present specification are as follows: Sample TempTm l ep. Tim Sampl Temp. Time] 1 380 3 13 380 3 25 380 3 2 380 3 14 380 3 J26 410 3 3 450 3 15 380 3 27 390 3 4 380 3 16 440 3 28 380 3 390 3 17 400 3 29 380 3 6 390 3 18 380 3 30 560 3 7 440 3 19 400 3 31 390 3 8 440 3 20 410 3 32 380 3 9 420 3 21 400 3 33 380 3 440 3 22 410 3 34 390 3 11 520 3 23 410 3 35 390 3 12 450 3 24 370 3 _j -19nb:dmw.# I5O4 8 .r2 26 Febwary] 1997 gIMM For each of the thus prepared samples of the invention Nos. 1 to 23 and samples for comparison Nos. 24 to 35, a tensile test, a relaxation test and a weldability test of the base metal, and a test of delayed fracture resistance of the weld zone, were carried out in the following procedures: Test of delayed fracture resistance of weld zone: Each of the samples was immersed into an ammonium thiocyanate (NH 4 SCN) solution having a concentration of 20% at a temperature of 50°C and a stress equal to 70% of tensile strength of the base metal of the sample was applied to the sample to measure a time before fracture at the weld zone in the above-mentioned spot-welding of the sample.

Tensile test: o A tensile test was carried out with a gauge length of 60 mm, for each of the samples after the abovementioned heat treatment in conformity to the specification of the Japanese Industrial Standard JIS Z- 2241.

Weldability test: A tensile test was carried out for each of the samples after the spot-welding, and those satisfying the specifications of the Japanese Industrial Standard JIS G- 3109 in terms of tensile strength (TS) and elongation (El) were evaluated by the mark "0 and those not satisfying same, by the mark Relaxation test: A relaxation test was carried out at temperatures of 20 °C (room temperature) and 75°C for each of the samples. At the room temperature, the test was in compliance with the specifications of the Japanese Industrial Standard JIS G-3109. At the temperature of the test comprised applying a stress equal to 70% of "tensile strength as specified in the above-mentioned JIS standard to the sample for five hours, then, after naturally cooling the sample in the furnace for 23 hours, measuring an amount of load change, and determining a percentage of the amount of load change to an initial load as a relaxation value.

The results of the above-mentioned test are shown in Table 2.

-21- Table 2 (1) Mechanical property Delayed Relaxation 4 fracture value Proof Tensile Elan- resist- M% stress strength atia No.m a)ncem) hr 20 0 C 75 0

C

1390 1470 0.60

S

211380 1460 10 45 0.60 6.0 Q 3 1370 1470 9 35 0.59 6 .3 Q 4 1380 1470 10 30 -0.62 7.0 0 1380 1470 10 20 0.59 6.6 0 6 1390 1480 9 45 0.56 6.8 0 7 1380 1470 9 80 0.50 5.8 Q 8 1410 1490 9 4,0 0.60 5.8 0 9 1400 1480 10 70 0.53 5.6 Q 1380 1480 9 35 0.62 5. 0 11 1400 14990 10 80 0.48 5.3 0i 12 1400 1480 9 35 0.58 5.6 0 13 1380 1460 10 25 0.54 7.0 C0 14 141 0 1460 10 6 0 0.54 5.7 0 1425 1470 10 1 60 0.59 4.2 (9 16 141 5 1460 10 50 0.49 5.1 17 1370 1470 10 30 0.50 6.4 0 18 1380 1470 9 30 0.59 6.2 0 19 1390 1490 9 40 0.60 5.6 (0 1340 1480I 10 35 0.61 5.5 (9 21 1380 1480 10 35 0.58 6.3 (9 [22 1390 1480 9 45 0.59 5.7 (9 0S~~ 1370 1470 0.60 5.3 0 i J. 22- Table 2 (2) >1 Mechanical property Delayed Relaxation 4 fracture value H o. Proof Tensile Elon rsit stress strength' ance (Nmm) (Nm (hr) 20 0 C 75 0 C q 2 140 1490 9 2 1.02 8.6 0 1430 1470 10 4 0.50 6.0 0Q 26 1400 1480 9 15 0.80 8.5 0 0 In 27 1100 1390. 28 1410 1480 10 1 0.52 6.0 X 0 a 29 1380 1460 9 7 0.80 8.6 0- Lo 30 1400 1480 5 10 0.48 L)31 1390 1480 9 10 0.56 6.1 (0 32 1400 1470 5 10 0.55 7.0 Q 33 1 390 1460 9 15 0.54 5.3 0J 34 1380 1460 10 10 0.56 5.7 0 j 1390 1480 11 17 0.52 5.6 1Q 0000.0 23 0 0 0D 0 0.0 0* .00 As is clear from Table 2, all the samples of the invention Nos. 1 to 23 had properties such as proof stress tensile strength (TS) and elongation (El) and a relaxation property on the same level as in the conventional steel bar for prestressed concrete, and furthermore, had such an excellent delayed fracture resistance as typically represented by a time before fracture of at least 20 hours at the weld zone in the spot welding.

In contrast, the samples for comparison Nos. 24 and 25, which contained no nickel, were poor in delayed fracture resistance at the weld zone. The samples for comparison Nos. 26 and 29, which had a low silicon content outside the scope of the present invention, showed a poor delayed fracture resistance at the weld zone. The sample for comparison No. 27, which had a low carbon content outside the scope of the present invention, showed a poor proof stress thus making it impossible to use same as a steel bar for prestressed concrete. The sample for comparison No. 28, which had a high carbon content outside the scope of the present invention, showed a poor weldability. The sample for comparison No.

which had a high silicon content outside the scope of the present invention, showed a degraded toughness of steel, thus resulting in a poor weldability and a poor delayed fracture resistance at the weld zone. The samples for comparison Nos. 31 and 32, which had a low or a high 0S CC Cs 2 4 P"sgss~"ps manganese content outside the scope of the present invention, were poor in strength or in ductility, respective4 and showed a poor delayed fracture resistance at the weld zone. The samples for comparison No. 33 and 34, which had a high content of phosphorus or sulfur outside the scope of the present invention, or the sample for comparison No. 35, which had a low nickel content outside the scope of the present invention, showed a poor delayed fracture resistance of the weld zone.

:o".The steel bar for prestressed concrete of the present invention has, as described above in detail, properties such as proof stress, tensile strength and elongation and a relaxation property on the same level as in the conventional steel bar for prestressed concrete, and furthermore has an excellent delayed fracture o resistance at a weld zone in the spot welding, and is usable in an environment tending to cause a delayed 0* 00 o fracture, thus providing many industrially useful effects.

oooa• 11~

Claims (3)

1. A steel bar for prestressed concrete excellent in delayed fracture resistance at a weld zone, having tensile strength of at least 1400N/rmm, and which has a composition comprising: carbon (C) silicon (Si) manganese (Mn) nickel (Ni) from 0.2 to 0.6 wt.%, from 0.2 to 2.0 wt.%, from 0.2 to 2.0 wt.%, from 0.25 to under 0.8 wt.%, o 0 S* 4 5 S 55555 S. and the balance being iron (Fe) and incidental impu'ities, where, the contents of phosphorus and sulfur as said incidental impurities being, respectively: up to 0.020 wt.% for phosphorus, and up to 0.015 wt.% for sulfur, and when said steel bar which is spot-welded, is immersed in an ammonium thiocyanate (NH 4 SCN) solution having a concentration of 20% at a temperature of 50 0 C, and a stress equal to 70% of tensile strength of said steel bar is applied to said spot-welded steel bar, said spot-welded steel bar has a time before fracture at said weld zone of at least 20 hours.

2. A steel bar for prestressed concrete excellent in delayed fracture resistance at a weld zone, having tensile strength of at least 1400N/mm 2 and which has a composition comprising: carbon (C) silicon (Si) manganese (Mn) nickel (Ni) from 0.2 to 0.6 wt.%, from 0.2 to 2.0 wt.%, from 0.2 to 2.0 wt.%, from 0.25 to under 0.8 wt.%, at least one element selected from the group consisting of: TNB:PJD:f#5048 17 October 19 Lll~ -27- molybdenurm (Mo) from 0.1 to 0.5 wt.%, copper (Cu) from 0.05 to 1.0 wt.%, boron from 0.0003 to 0.0050 wt.%, and tungsten from 0.03 to 0.50 wt.%, and the balance being iron (Fe) and incidental impurities, where, the contents of phosphorus and sulfur as said incidental impurities being, respectivelyp to 0.020 wt.% for phosphorus, and up to 0.015 wt.% for sulfur, and when said steel bar which is spot-welded, is immersed into an ammonium thiocyanate (NH 4 SCN) solution having a concentration of 20% at a temperature of 0 C, and a stress equal to 70% of tensile strength of said steel bar is applied to said spot-welded steel bar, said spot-welded steel bar has a time before fracture at said weld zone of at least 20 hours.

3. A steel bar for prestressed concrete as claimed in claim 1 or 2, wherein said p steel has a composition substantially as hereinbefore described in any one of p. Examples 1 to 23. DATED: 17 October 1996 o p CARTER SMITH BEADLE Patent Attorneys for the Applicant: NKK CORPORATION and NETUREN CO., LTD a 17 October 1996 ~ra~sl~s -L n~Llll~-6-U ABSTRACT OF THE DISCLOSURE A steel bar for prestressed concrete excellent in delayed fracture resistance at a weld zone, which consists essentially of: carbon(C) from 0.2 to 0.6 wt.%, silicon (Si) from 0.2 to 2.0 wt.%, manganese (Mn) from 0.2 to 2.0 wt.%, nickel (Ni) from 0.25 to under 0.8 wt.%, and the balance being iron (Fe) and incidental impurities, where, the contents of phosphorus and sulfur as the incidental impurities being, respectively: up to 0.020 wt.% for phosphorus, and up to 0.015 wt.% for sulfur. The above-mentioned steel bar may additionally contain at 0oo least one element selected from the group consisting of: molybdenum (Mo) from 0.1 to 0.5 wt.%, copper (Cu) from 0.05 to 1.0 wt.%, boron from 0.0003 to 0.0050 wt.%, and tungsten from 0.03 to 0.50 wt.%. -28- 91~ IC-~IPIIII~RYSII~