GB2290800A

GB2290800A - High strength high weldability steel bars and wires for pretressed concrete

Info

Publication number: GB2290800A
Application number: GB9510785A
Authority: GB
Inventors: Eiji Yamashita; Hajime Nitta; Shigeru Mizoguchi
Original assignee: Neturen Co Ltd
Current assignee: Neturen Co Ltd
Priority date: 1994-06-27
Filing date: 1995-05-26
Publication date: 1996-01-10
Anticipated expiration: 2015-05-26
Also published as: GB9510785D0; CN1121535A; GB2290800B; JP2864348B2; CN1044139C; JPH0813082A; HK1001348A1

Abstract

A high strength steel bar or wire having better weld-ability for prestressed concrete is improved in the relaxation at high temperature thus increasing the toughness. It contains 0.3 % to 2.0 % of silicon, 0.08 % to 0.35 % of molybdenum and predetermined amounts of carbon and manganese. If desired, it may contain titanium and boron. In particular, the silicon and the molybdenum are not less 1 % in the Si+4Mo combination. At heat treatment, the steel bar or wire is rapidly heated within 20 seconds so that a fine austenite microstructure of the grain size number 9.0 or more is formed. More preferably, while the steel bar or wire is heated to a tempering temperature, it is loaded with a bending strain of not more than 3 % before cooled down rapidly.

Description

HIGH STRENGTH, HIGH WELDABILITY STEEL BARS AND WIRES FOR PRESTRESSED CONCRETE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to steel bars and wires for prestressed concrete, which are improved in mechanical properties, increased in strength and weldability, and lowered in high-temperature relaxation.

Description of the Prior Art Steel bars and wires for prestressed concrete (referred to as PC steel bars hereinafter) have been requested to have a tensile strength high enough to ensure effective pretensioning. In the requirements of JIS, high-grade PC steel bars retain as high a tensile strength as 1420 N/mm 2 Commonly, for fabrication of concrete piles and columns at high efficiency, material concrete with PC steel bars embedded is heated to more than 100 t by an autoclave curing process for increasing its physical strength. However, the higher the heating temperature, the more the relaxation is accelerated which lessens the stressing action of the PC steel bars to the concrete. It is thus required for the PC steel bars to be minimum in the relaxation of stress under a high-temperature condition.In addition, the PC steel bars need to have a high uniform elongation which is one of substantial factors for giving the toughness.

In some applications, the weldability is also essential for the PC steel bars. For example, a number of parallel arranged PC steel bars are tied together by and spot welded to a spiral steel wire to form a steel cage structure for reinforcement of a concrete pile or column. The weldability should be high enough to allow the steel bars to be securely welded to spiral steel wire by a common spot welding technique.

It is widely known that the PC steel bars are fabricated by appropriate heat treatment including quenching and tempering for providing desired properties or more particularly, a good combination of hardness and toughness. In the heat treatment, hot-rolled material steel bars or wires are cold drawn to a length, embossed on the surface for ease of bonding with concrete, and subjected to quenching and tempering. Commonly, while the steel bars or wires running continuously, they are heated up and cooled down repeatedly as desired.

For reducing the relaxation at a high temperature of more than 100 t, conventional PC steel bars are increased in the content of silicon from a range of 0.2 to 0.3 % (by weight, as same hereinafter) to a range of 0. 5 to 2. 0 % or to 2.3 % at maximum, as disclosed in Japanese Patent Laid open Publication 62-49334 (1987). Also, the disclosure has attempted to reduce the high-temperature relaxation without losing a uniform elongation by heating PC steel bars to a tempering temperature and simultaneously, applying minimal strains to the same before cooling rapidly.

However, such high-silicon PC steel bars become high in the electrical resistance and thus produce a drawback that the bonding strength of spot welding is declined unless welding voltage is increased by 50 %. As various PC steel bars containing different amounts of silicon are used at a site for fabricating steel cage, the welding voltage has to be varied frequently and the welding process will consume a considerable length of time. To this end, the applicants of the present invention have developed and introduced an improvement of the weldability with application of molybdenum, as depicted in Japanese Patent Laid-open Publications 3-151445 and 3-285045 (1991). As understood, the molybdenum is available in the market only at a high price.It is thus an object of the present invention to provide a high strength PC steel bar which contains less amount of molybdenun but enhancing the weldability and improving the mechanical properties.

SUMMARY OF THE INVENTION The present invention is intended to have a high strength PC steel bar, which retains as high a tensile strength as 1420 N/mm2, improved in the relaxation at high temperature and increased in the toughness without impairing the weldability. More specifically, a high strength, high weldability steel bar or wire for use with prestressed concrete according to the present invention is characterized by containing 0.15 X to 0.40 X of carbon, 0.3 X to 2.0 X of silicon, 0.4 X to 1.6 X of manganese and 0.08 X to 0.35 % of molybdenum each by weight, said silicon and molybdenum being not less than 1.0 % by weight in a Sl+4Mo combination, wherein, the remainder is iron and inherent impurities and having a fine microstructure of the austenitic grain size number 9.0 or more.Preferably the PC steel bar further contains 0.01 X to 0.05 X of titanium and 0.0005 % to 0.005 X of boron each by weight.

A method of producing the foregoing PC steel bars according to the present invention comprises: heating up to 850 t to 1050 t within 20 seconds a material steel bar or wire which contains 0.15 X to 0.40 X of carbon, 0.3 X to 2.0 X of silicon, 0.4 % to 1.6 % of manganese and 0.08 X to 0.35 % of molybdenun each by weight, said silicon and molybdenum being not less than 1.0 % by weight in a SI+4Mo combination, wherein the remainder is iron and inherent impurities, and quenching the same rapidly; and heating the same to a tempering temperature within 25 seconds and cooling down the same rapidly so that a fine microstructure of the austenitic grain size number 9.0 or more is formed.Preferably, the PC steel bar may be loaded with a bending strain of not more than 3 % after heated to the tempering temperature and then, cooled down rapidly.

BRIEF DESCRIPTION OF THE DRAWINGS Fig 1. is a diagram showing the relatlon of contents of silicon and molybdenum to high-tenperature relaxation; Fig. 2 is a diagram showing the relation of the contents of silicon and molybdenum to welding current and welded strength.

DESCRIPTION OF THE PREFERRED EMBODIMENT Detailed Description of the Invention According to the present invention, a PC steel bar contains a proper amount of molybdenum to reduce the unfavorable effect of silicon content upon weldability while the silicon content is essential to decrease the high-temperature relaxation, such that they can securely be welded without increasing the welding voltage. The ratio between molybdenum and silicon in the content is carefully determined for opti mum effect and also, a fine microstructure of the austenitic grain size number 9.0 or more is developed by the effect of molybdenum which makes carbide precipitates. As the result, the PC steel bar of the present invention containing a mini mum amount of costly molybdenum will be increased in both the strength and the toughness.

The inventors of the present invention perforned and examined the addition of major elements to PC steel bars fabricated by a common sequence of cold drawing, quenching and tempering through a series of experiments. Fig. 1 is a representation showing the result of high-temperature relaxation at 180 t on some PC steel bars which are varied in the contents of silicon and molybdenum with or without the presence of titanium and boron, in which A, B ,C and D represent the PC steel bars containing zero, 0. 1 %, 0.2 % and 0.3 % molybdenum respectively and the blank and dark symbols indicate with and without titanium and boron respectively.As apparent, although the presence of silicon contributes to reduction of the high-temperature relaxation, it will saturate the effect when exceeding 1.5 X. It is also clear that the molybdenum is desirable for reducing the hightemperature relaxation. However, the presence of titanium and boron is hardly effective to control the high-temperature relaxation.

The inventors of the present invention conducted a test for examining the bonding strength at spot welded joints between the PC steel bars and spiral steel wire. More spe cifical ly, each test PC steel bar of 9.2 mm in diameter was welded to a 3.2 mm-diameter spiral steel wire by a common electrical spot welding process with a welding voltage of 1.45V and the conductivity between the bar and the wire was measured In currents. Then, the physical strength of the welded joints on the PC steel bars was examined by a shear strength test conforming to the requirements of eluded steel net' of JIS G3551.

The results are shown in Fig. 2. When a PC steel bar contains no molybdenum but more than 1 % silicon, its welded joint is declined in the electrical conductivity and thus the physical strength. The conductivity and strength of the PC steel bar containing molybdenum will remain unchanged or less reduced when the silicon content exceeds 1.5 %. Also, in this case, the presence of titanium and boron will exhibit some effectiveness.

The present invention is directed towards a desired PC steel bar in view of the foregoing results of the experiments.

The chemical composition of the PC steel bar will now be explained.

Preferably, the PC steel bar contains 0.15 to 0.40 % carbon because its strength is reduced to an unfavorable level if the carbon is less than 0. 15 %. If the carbon exceeds 0.40 %, both the toughness and the weldability will be declined.

The silicon is 0.3 to 2.0 %. If less than 0.3 %, the high-temperature relaxation can hardly be reduced regardless of the presence of molybdenum. If the silicon is more than 2.0 X, the toughness and the weldability will be declined while the decrease of the relaxation being saturated.

The PC steel bar contains 0.4 to 1.6 X manganese. The presence of manganese allows the hardening process to be eased and contributes to improvements in the uniform elongation and the toughness. If less than 0.4 %, such advantages are lost. If more than 1.6 %, an unnegligible amount of austenite will remain after the quenching thus decreasing the physical strength. Hence, a range from 0.4 to 1.6 X is most preferable.

The molybdenum is 0.08 to 0.35 X. The molybdenum suppresses the high-temperature relaxation in combination with the silicon and also, maintains the weldability even if the amount of the silicon is increased. Furthermore, it increases the hardening effect and contributes to the microstructure of fine grains thus enhancing the physical strength and toughness. If the molybdenun is less than 0.08 X, the above advantages are fallen and if more than 0.35 X, the cost rill be increased while the advantageous effects remain saturated.

In particular, it is necessary to maintain the combina- tion of silicon and molybdenum, in Si+4Mo not less than 1.0 %. For restricting the high-temperature relaxation to a given level, the Si+4Mo value is preferably not less than 1.0 % and preferably, 1.4 % or more. It is found from the experiments that an optimum requirements of the PC steel bar is achieved when the above combination of silicon and molybdenum is satisfied with maintaining their respective predetermined anounts.

Also, titanium and boron may be added for enhancing the weldability. The titanium Is less effective when below 0.01 X and will impair the cleanness when exceeding 0.05 X.

The boron is less effective when below 0.0005 X and will remain the same in the effectiveness when exceeding 0.005 X.

A largely excessive amount of the boron will result in crack or breakage during hot rolling of the steel bars.

The PC steel bar of the present invention exhibits an fine prior austenite grain of the grain size number 9.0 or more. The grain size number is defined in JIS G0551, where the count n of grains per square millimeter Is expressed by n=2N+3 , when the grain size number is N. The prior austenite microstructure has been achieved by the heating process for quenching according to the present invention. The austenite phase is then converted by quenching to a martensite phase which consists of more fine grains and consequently, the PC steel bar will be higher in both the strength and the toughness than the conventional steel bars of less than grain size number 9. 0.

The PC steel bars of the present invention are fabri cated by cold drawing a material steel to a desired diameter size, embossing its surface, if necessary, for ease of bonding with concrete, and quenching and tempering. In the quenching, the PC steel bars are rapidly heated up to 850 to 1050 t within 20 seconds. This rapid heating conserves a lot of nucleation sites of austenite grains introduced by cold drawing, thus develops a fine austenite microstructure of grain size number 9.0 or more. The rapid heating may be carried out by high-frequency induct ion heating or direct resistance heating. If the heating up to the austenizing temperature takes more than 20 seconds, a less number of nuclei which generated during the temperature rise grow into large grains, thus the intended fine microstructure cannot be obtained.

The holding time at the temperature for quenching should be minimum as far as developing an uniform austenite structure. Although the material for the present invention contains Mo-carbide precipitates which suppress the growing of austenite grains, the holding time at the temperature is preferably completed within 20 seconds. Quenching after the heating is at a rate of not less than 100 t/sec so that complete martensite structure is developed.

The tempering is then followed in which the heating to a tempering temperature is as fast as within 25 seconds to have a uniform tempered martensite structure. The tempering temperature which is ordinarily within the range of 400 to 550 t is determined so that the prescribed tensile strength, for instance 1420 N/mm2 , is obtained. After heating up to and maintaining at the tempering temperature, for instance about 10 seconds, the steel bars nay be cooled down rather quickly to prevent declination of strength due to coarsening of structure.

As more preferable process in the present invention, a bending strain of less than 3 X is exerted during the heating to the tempering temperature. This will further reduce the high-temperature relaxation. The exertion of the bending strain may be implemented with a row of repetitive bending rolls which are similar to those of a straightener and are disposed after a heating apparatus such as an Induct ion heater for heating the steel bars fed in a succession. Dislocations introduced In the material by bending strain tangle each other and are restricted to move under influence of the tangling precipitates such as carbides and nitrides and so on.

This causes reducing relaxation.

The bending strain is 3 X or less and more preferably, 2 % or less. If the strain is high, the toughness and the uniform elongation will be declined. When the steel bars are maintained at the tempering temperature for long after the exertion of the bending strain, the dislocation introduced therein will annihilate and lose their favorable prop erties. Thus, the forced cooling should be followed before long, for instance, within about 10 seconds.

Examples Groups of hot-rolled steel naterials having different chemical compositions listed in Table 1 were cold drawn to steel bars of 9. 2 mn in diameter. The steel bars were heated up to 950 t by an induction heating within 7 seconds and naintained at the temperature for 6 seconds before quenched with water. One group of the steel bars were then heated to a tempering temperature in 5 seconds and maintained at the temperature for 10 seconds before cooled down vith water.

Another group was also heated to the same tempering temper- ature, stressed by a bending strain of 0.9 X, after that maintained at the temperature for 5 seconds before cooled down with vater. A further group of the steel bars were heated in 25 seconds to the same temperature 950 t as the previous groups. The tempering temperature was varied depending on the conpositions so that the resultant PC steel bars retain a tensile strength of more than 1420 N/mm2 The results of a series of tests examining the austenitic grain size, the mechanical properties, the stress relaxation, and the welded strength are explained in Tables 2 and 3.As apparent, the PC steel bars of the present invenlion are equivalent to or higher in the mechanical properties Table 1

ChemiCal COmPOSitiOn (X) No. Mn r T B C Si Mn P S Mo Ti B Si+4Mo 1 1 0.31 0.51 0.90 0.011 0.010 0.27 0.02 0.0016 1.59 2 2 0.28 0.82 0.62 0.015 0.008 0.24 0.02 0.0019 1.78 0 3 3 0.32 1.01 0.82 0D09 0.008 0.20 0.02 0.0018 1.81 C 4 0.29 1.53 0.98 0.012 0.011 0.12 0.02 0.0018 2.01 .

5 5 0.33 1.82 0.70 0.011 0.009 0.11 0.02 0.0020 2.26 6 6 0.30 0.35 1.20 0.016 0.010 0.31 0.02 0.0017 1.59 - 7 0.34 0.82 1.10 0.010 0.008 0.25 - - 1.82 8 8 0.30 1.00 0.80 0.008 0.012 0.22 - - 1.88 9 9 0.29 1.72 0.85 0.009 0.009 0.18 - - 2.44 0. 10 0.32 0.25 0.92 0.010 0.011 0.18 0.02 0.0022 0.97 o 11 11 0.28 0.50 0.80 0.012 0.008 0.12 0.02 0.0020 0.98 12 0.30 0.83 0.76 0.009 0.012 - 0.02 0.0018 0.83 a E 13 0.27 1.05 0.84 0.014 0.011 - 0.02 0.0017 1.05 0 Q 14 0.31 2.03 0.82 0.010 0.009 - 0.02 0.0021 2.03 Table 2

conditions * grain yield tensile uniform elongation No. raped bending size stress strength elongation after late- rapid bending number fracture rial heating strain (N/ D2) (N/XE2) (%) (%) 1A Y Y 1 10.1 1451 1479 3.6 10.9 1B Y N 10.1 1452 1478 3.6 11.1 2A 2 Y Y 10.3 1442 1475 3.5 10.9 c 3A Y Y 10.4 1439 1466 3.7 10.8 o - 3 - 3B Y N 10.4 1453 1474 3.5 10.6 4A 4 Y Y 10.6 1452 1476 3.8 10.8 c 5A Y Y 10.7 1449 1463 3.5 10.8 - 5 5B Y N 10.7 1452 0 1482 3.6 11.0 m 6A 6 Y Y 10.2 1440 1473 3.7 11.2 7A 7 Y Y 10.5 1445 1479 3.5 10.7 8A 8 Y Y 10.3 1450 1481 3.6 10.7 9A 9 Y Y 10.4 1444 1478 3.5 10.8 1C 1 N Y 8.6 1450 1468 3.1 9.9 3C 3 N Y 8.5 1453 1464 2.9 10.2 5C 5 N Y 8.7 1455 1470 3.0 10.1 C: : 10A 10 Y Y 10.4 1448 1482 3.4 10.8 m - 11A Y Y 10.4 1450 1478 3.5 10.6 o 11B 11 Y N 10.4 1439 1473 3.4 10.9 a O 11C N Y 8.6 1457 1475 2.9 10.0 12A 12 Y Y 9.9 1445 1472 3.3 10.3 13A 13 Y Y 10.0 1447 1470 3.4 10.7 14A 14 Y Y 9.7 1445 1468 3.3 10.4 * Y : yes. N : no Table 3

I stress relaxation (%) weldabllity No. shearlng neo. Irw teip. bigh temp. strength (keg) 1A 0.34 7.3 151 IB 0.33 12.3 150 2A 0.41 7.4 153 a 3A 0.38 6.5 150 o - 3B 0.33 11.0 148 c 4A 0.39 6.4 142 a) c S A 0.30 6.9 141 5B 0.35 11.8 143 - 6A 0.31 8.0 155 c 7 A 0.38 7.6 150 8A 0.37 6.4 155 9A 0.34 7.0 148 1C 0.35 8.5 147 3C 0.36 8.1 ( 142 5C 0.36 8.2 140 C: 0.41 9.5 145 0 10.1 ~ 11 A 0.39 10.1 151 X 11 11B 0.40 15.5 148 a 11C 0.38 13.0 139 12A 0.33 10.3 138 9.4 13A 0.35 9.4 4 115 14A 0.34 7.7 88 than the conventional PC steel bars. Particularly, the PC steel bars of the present invention which has grain size number of 9.0 or more are high in the uniform elongation and also excellent in another properties.

The relaxation test was conducted at two different temperatures, a room temperature and 180 t to examine a decrease in the stress at an a constant grip distance after loaded with the initial stress of 1020 N/mm2 , 80 X of the prescribed yielding stress 1275 N/mm2 . For examining the hightemperature relaxation, the test bars were heated to 180 t within 4 hours and maintained at the high temperature for 3 hours before cooled down gradually. The measurement was made 23 hours after the loading with the initial stress. As shown, the best result is achieved on the PC steel bar which contains more than 1.0 % in the Sis4Uo combination.It is also aPParent that the reduction of the high-temperature relaxation is encouraged by applying a snall bending strain during the tempering in comparison of the sane contents material.

The welded strength test was conducted by the same manner as described previously. As aPParent, the PC steel bars containing a higher amount of silicon exhibits more advantage of molybdenum in increasing the tensile strength.

It is also clear that the presence of titanium and boron reduces inconsistency of the strength thus increasing the mechanical properties.

As set forth above1 the PC steel bars of the present invention are desirable for reducing the high-temperature relaxation which is unfavorably accelerated during the autoclave curing for mass production of prestressed concrete.

They are also desirable for providing a weldability high enough to spot weld to form a steel cage reinforcement without specific welding conditions. Also, the PC steel bars remain high in the toughness when its strength is as high as satisfying the top-grade requirements of relevant standards, thus providing better mechanical properties.

Claims

What is claimed is:

1. A high strength, high weldability steel bar or wire for use with prestressed concrete, containing 0.15 % to 0.40 % of carbon, 0.3 X to 2.0 X of silicon, 0.4 % to 1.6 % of manganese and 0.08 X to 0.35 X of molybdenum each by weight, said silicon and molybdenum being not less than 1.0 X by weight in a Si+4Mo combination, wherein the remainder is iron and inherent impurities and having a fine microstructure of the austenitic grain size number 9.0 or more.

2. A high strength, high weldability steel bar or wire for use with prestressed concrete according to claim 1, further containing 0.01 % to 0.05 X of titanium and 0.0005 X to 0.005 X of boron each by weight.

3. A method of producing a high strength, high weldability steel bar or wire for use with prestressed concrete, comprising the steps of: heating up to 850 t to 1050 t within 20 seconds a material steel bar or wire which contains 0.15 X to 0.40 X of carbon, 0.3 % to 2.0 X of silicon, 0.4 X to 1.6 X of manganese and 0.08 % to 0.35 % of molybdenum each by weight, said silicon and molybdenum being not less than 1.0 X by weight in a Si+4Mo combination, wherein the remainder is iron and inherent impurities, and quenching the same rapidly; and heating the same to a tempering temperature within 25 seconds and cooling down the same rapidly so that a fine microstructure of the austeni tic grain size number 9.0 or more is formed.

4. A method of producing a high strength, high weldability steel bar or wire for use with prestressed concrete according to claim 3, wherein the steel bar or wire further contains 0.01 % to 0.05 % of titanium and 0.0005 X to 0.005 X of boron each by weight.

5. A method of producing a high strength, high welded ability steel bar or wire for use with prestressed concrete according to claim 3 or 4, wherein after heated to the tempering temperature, the steel bar or wire is loaded with a bending strain of not more than 3 % and then, cooled down rapidly.