AU709908B2 - Steel bar for prestressed concrete and method for producing the same - Google Patents

Description

la- TITLE OF THE INVENTION Steel Bar for Prestressed Concrete and Method for Producing the Same" BACKGROUND OF THE INVENTION The present invention relates to a steel bar for prestressed concrete (PC steel bar) used for PC structures such as PC poles and piles and excellent in high-temperature relaxation characteristics and having highly uniform elongation and a method of manufacturing PC steel bar excellent in highly uniform elongation, high-temperature relaxation characteristics and a low yield ratio.

PC steel bars are high-strength members used for prestressed concrete. The PC steel bars are standardized as JIS G 3109 (JIS G 3137) Class B (tensile strength 2 1,080 N/mm 2 yield strength 930 N/mm 2 elongation 2 and relaxation value 5 at room temperature, loaded stress 0.7x tensile strength), and Class C (tensile strength 2 1,230 N/mm 2 yield strength 2 1,080 N/mm 2 elongation and relaxation value at room temperature, loaded stress 0.7x tensile strength), and JIS G 3137 Class D (tensile strength 2 1,420 N/mm 2 yield strength 2 1,275 N/mm 2 elongation 2 and relaxation value 5 at room 2 temperature, loaded stress 0.7x tensile strength).

However, the uniform elongation, yield ratio, and hightemperature relaxation value of PC steel bars are not standardized yet.

A prestressed concrete structure may fail in bending in the event of an earthquake. To prevent this, it is effective to use PC steel bars having a highly uniform elongation to improve bending toughness.

Recent PC steel bars must have a low yield ratio and excellent uniform elongation from the viewpoint of earthquake resistance. Assume that a PC steel bar has excellent uniform elongation but has a high yield ratio.

In this case, when concrete cracks, PC steel bars at the crack locally deform to result in fracture.

Lateral binding wires for reinforcements are often used for these PC steel bars. The lateral binding reinforcements are spot-welded with the PC steel bars.

For this reason, PC steel bars excellent in uniform elongation characteristics before and after spot welding are required.

To shorten the curing period, autoclave curing in an atmosphere having a temperature of 180 to 200 0 C and atm is popular as concrete curing in manufacturing prestressed concrete piles or poles. The autoclave curing, however, has the disadvantage of an increase in relaxation of the PC steel bars in concrete. Therefore, demand has arisen for a PC steel bar having a small

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3 high-temperature relaxation value.

Conventional techniques for improving the individual characteristics of uniform elongation and high-temperature relaxation are separately established.

Several PC steel bars excellent in both uniform elongation and high-temperature relaxation characteristics have been proposed, but are yet to come.

A method disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 3-79410 applies a small bending distortion of 2% or less in tempering. Jpn. Pat. Appln.

KOKAI Publication No. 3-28351 discloses a method of working steel at 900 to 400 0 C or increasing the temperature while performing plastic working on steel to reversely transform the structure into an ultrafine structure. Jpn. Pat. Appln. KOKAI Publication No. 58-120738 discloses a method of cooling steel in straightening using a wet straightening machine.

Jpn. Pat. Appln. KOKAI Publication No. 8-158010 discloses a method of adding Si and Al in a total amount of them as large as 2% to The methods disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 3-79410 and Jpn. Pat. Appln. KOKAI Publication Nos. 3-28351 and 58-120738 require a step of applying distortion, resulting in poor productivity. The method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-158010 is excellent in productivity, uniform elongation, and high-temperature relaxation 4 characteristics, but has a uniform elongation of about in the 1,450 N/mm 2 strength level. It is not yet a remarkable improvement as compared with the state-ofthe-art elongation of The method has a yield ratio as high as 0.95 or more; uniform elongation characteristics may not be actually effected in concrete structures. These references do not describe techniques for realizing a low yield ratio.

Antiearthquake design for forming a crossbeam into a plastic hinge without destructing a pillar to absorb a large earthquake energy at a pillar-crossbeam joint has received a great deal of attention. This design requires a PC steel bar having a strength level equal to or higher than that of JIS G 3109 (JIS G 3137) Class B and a uniform elongation of Most of the conventional techniques require steel bars of 1,420 N/mm 2 from the viewpoint of strength level.

The proposals associated with PC steel bars having a tensile strength of 1,080 N/mm 2 or more and less than 1,420 N/mm 2 corresponding to Class B or C are disclosed in Jpn. Pat. Appln. KOKOKU Publication Nos. 55-8577 and 56-21811. Ductility achieved by the techniques disclosed in Jpn. Pat. Appln. KOKOKU Publication Nos. 55-8577 and 56-21811 is about 8 to 9% in total elongation for the former case and about 3.3 to 4.6% in uniform elongation for the latter case. This strength level does not satisfy the actually required uniform elongation, 8%.

BRIEF SUMMARY OF THE INVENTION It is an object of the present invention to provide a PC steel bar excellent in uniform elongation and high-temperature relaxation characteristics, having a tensile strength of 1,080 N/mm 2 or more and less than 1,420 N/mm 2 and manufactured by a normal step without requiring a distortion applying step, and a method of manufacturing the PC steel bar.

It is an other object of the present invention to provide a method of manufacturing a PC steel bar excellent in uniform elongation, high-temperature relaxation characteristics and a low yield ratio, having a tensile strength of 1,080 N/mm 2 or more, and manufactured by a normal step without requiring a distortion applying step, and a method of manufacturing the PC steel bar.

It is another object of the present invention to provide a PC steel bar excellent in uniform elongation characteristics after spot welding, and a method of manufacturing the same.

In order to achieve the above objects, the present inventors have made extensive studies. According to the findings of the present inventors, to make both uniform elongation and high-temperature relaxation characteristics excellent, it is effective to set the total content of "Si Al" high.

-6- Further, to make both uniform elongation, high-temperature relaxation characteristics excellent and a low yield ratio, it is effective to set the content of C high and quench steel having this composition at a high temperature.

The present invention has been achieved based on the above findings.

First, there is provided a PC steel bar, according to the present invention, with excellent uniform elongation and high-temperature relaxation characteristics comprising steel containing 0.2 to 0.7 wt% of C, 0.2 to 3.0 wt% of Mn, and 2.0 to wt% of a combination of Si and Al, the balance being Fe.

I: wherein the steel has a substantially tempered martensititic, microstructure in o which the carbide in the tempered martensite comprises ultrafine particles having an average diameter of not more than 0.1 pm and a tensile strength of not less than 1,080 N/mm 2 and less than 1,420 N/mm 2 Second, there is provided a PC steel bar with excellent uniform elongation and l high-temperature relaxation characteristics, further containing, as a steel component, at least one element selected from the group consisting of 0.01 to 0.1 wt% of Nb, 0.01 to 0.05 wt% of Ti, 0.0003 to 0.005 wt% of B, 0.2 to 1.0 wt% of Ni, 0.1 to 1.0 wt% of Cr, 0.05 to 0.5 wt% of Mo, and 0.2 to 1.0 wt% of Cu.

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-7- Third, there is provided a PC steel bar with excellent uniform elongation and high-temperature relaxation characteristics, comprising, as steel components, 0.2 to 0.7 wt% of C, 0.2 to 3.0 wt% of Mn, 2.0 to 5.0 wt% of a combination of Si Al, and further containing at least one element selected from the group of 0.01 to 0.1 wt% of Nb, 0.01 to 0.05 wt% of Ti, 0.0003 to 0.005 wt% of B, 0.2 to 1.0 wt% of Ni, 0.1 to 1.0 wt% of Cr, 0.05 to 0.5 wt% of Mo, and 0.2 to 1.0 wt% of Cu, and still further containing Fe and an inevitable impurity for the remaining portion, wherein the steel has a substantially tempered martensitic microstructure in which the carbide in the tempered martensite comprises ultrafine particles having an average diameter of not more than 0.1 im and a tensile strength of not less than 1,080 N/mm 2 and less than 1,420 N/mm 2 Fourth, there is provided a PC steel bar wherein the PC steel bar has a tensile strength of not less than 1,080 N/mm 2 and a yield strength of not less than 930 N/mm 2 15 Fifth, there is provided a PC steel bar wherein the PC steel bar has a tensile strength of not less than 1,230 N/mm 2 and a yield strength of not less than 1,080 N/mm 2 Sixth, there is provided a PC steel bar wherein the PC steel bar contains 0.2 to wt% of C and is used for spot welding and then tempering energization -8- (post welding heat treatment).

First, there is provided a method of manufacturing a PC steel bar, according to the present invention, having a tensile strength of not less than 1,080 N/mm 2 and with excellent highly uniform elongation and high-temperature characteristics at a low yield ratio, comprising hardening the steel at 1,1600C to 1,3000C and then tempering the steel, so as to form a microstructure of ultrafine spherodised cementite in a matrix ferrite the steel containing 0.5 to 0.7 wt% of C, 0.2 to 2.0 wt% of Mn, and a combination of Si and Al within a range of 2.0 wt% Si Al 5. 0 wt%, the balance being Fe.

10 Second, there is provided a method of manufacturing a PC steel bar having a tensile strength of not less than 1,080 N/mm 2 and with excellent highly uniform elongation and high-temperature characteristics at a low yield ratio, comprising hardening the steel at 1,1600C to 1,3000 and then tempering the steel, so as to form a microstructure of ultrafine spherodised cementite in a matrix ferrite 15 the steel containing 0.5 to 0.7 wt% of C, 0.2 to 2.0 wt% of Mn, and a combination of Si and Al within a range of 2.0 wt% Si Al 5.0 wt%, and at least one element selected from the group consisting of 0.01 to 0.1 wt% of Nb, 0.01 to a a 0.05 wt% of Ti, 0.0003 to 0.005 wt% of B, 0.2 to 1.0 wt% of Ni, 0.1 to 1.0 wt% of Cr, 0.05 to 0.5 wt% of Mo, and 0.2 to 1.0 wt% of Cu, the balance being Fe.

Third, there is provided a method of manufacturing a PC steel bar having a tensile strength of not less than 1,080 N/mm 2 and with excellent highly uniform -9elongation and high-temperature characteristics at a low yield ratio, comprising hardening the steel at 1,160 C to 1,300C and then tempering the steel, the steel containing 0.5 to 0.7 wt% of C, 0.2 to 2.0 wt% of Mn, 2.0 to 5.0 wt% of a combination of Si Al, and at least one of the elements selected from the group consisting of 0.01 to 0.1 wt% of Nb, 0.01 to 0.05 wt% of Ti, 0.0003 to 0.005 wt% of B, 0.2 to 1.0 wt% of Ni, 0.1 to 1.0 wt% of Cr, 0.05 to 0.5 wt% of Mo, and 0.2 to 1.0 wt% of Cu, and Fe and an inevitable impurity for the remaining portion.

Fourth, there is provided a method of manufacturing a PC steel bar having 10 highly uniform elongation and high-temperature relaxation characteristics at a low yield ratio, wherein the PC steel bar has a strength level defined by a tensile strength 1,230 N/mm 2 and a yield strength 1,080 N/mm 2 a uniform elongation of not less than 10.0%, a yield ratio of not more than 0.87, and a high-temperature relaxation ratio of not more than 8%.

15 Fifth, there is provided a method of manufacturing a PC steel bar having highly i; uniform elongation and high-temperature relaxation characteristics at a low yield ratio, wherein the PC steel bar has a strength level defined by a tensile strength >1,420 N/mm 2 and a yield strength 1,275 N/mm 2 a uniform elongation of not

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less than a yield ratio of not more than 0.90, and a high-temperature relaxation ratio of not more than 8%.

Sixth, there is provided a method of manufacturing a PC steel bar having highly uniform elongation and high-temperature relaxation characteristics at a low yield ratio, wherein the PC steel bar has a strength level defined by a tensile strength 1,080 N/mm 2 and a yield strength 930 N/mm 2 a uniform elongation of not less than 11.0%, a yield ratio of not more than 0.85, and a high-temperature relaxation ratio of not more than 8%.

Seventh, there is provided a method of manufacturing a PC steel bar with excellent highly uniform elongation and high-temperature relaxation characteristics at a low yield ratio, wherein the tempering temperature is not less than 600 0

C.

The component restriction reasons and manufacturing conditions of the present invention will be described below.

<C>

C is an element necessary for improving the hardening properties and :increasing strength. If the C content is less than 0.2 wt%, the strength level as the PC steel bar cannot be assured. If the C content exceeds 0.7 wt%, ductility degrades. Therefore, the C content falls within the range of 0.2 to 0.7 wt%.

When a spot-welded PC steel bar is used, the spot-welding properties suffer if 20 the C content exceeds 0.5 wt%.

11 Therefore, when a PC steel bar is spot-welded, the C content is defined to fall within the range of 0.2 to 0.5 wt%. C is also an element for increasing the tempering temperature. Under the condition that Si Al are added in a large amount, C greatly contributes to decreasing the yield ratio and improving the uniform elongation. If the C content is less than wt%, a remarkable improvement in ductility cannot be expected.

<Si, Al> Si and Al are used as deoxidizers and effective for relaxation characteristics and also greatly contribute to improvement of uniform elongation.

However, when the Si Al content is less than 2.0 wt%, the effect is small. When the Si Al content exceeds wt%, the relaxation characteristics and spotwelding properties degrade. Therefore, the Si Al content falls within the range of 2.0 wt% to 5.0 wt%.

<Mn> Mn is a deoxidizer like Si and is an element necessary for improving the hardening properties and increasing strength like C. If the Mn content is less than 0.2 wt%, the effect is small. However, if the Mn content exceeds 3.0 wt%, workability degrades.

Therefore, the Mn content falls within the range of 0.2 to 3.0 wt%. It is preferable that the Mn content is not more than 2.0 wt% in order to improve workability 12 remarkably.

<Nb, Ti, B, Ni, Cr, Mo, Cu> Nb, Ti, B, Ni, Cr, Mo, and Cu are elements contributing to improvement of hardening properties.

At least one of these elements is added depending on desired strength and toughness of a steel bar.

When the content of these elements is too low, no satisfactory effect can be obtained. However, when the content of these elements is too high, the effect is saturated. Therefore, the contents of Nb, Ti, B, Ni, Cr, Mo, and Cu are preferably 0.01 to 0.1 wt%, 0.01 to 0.05 wt%, 0.0003 to 0.005 wt%, 0.2 to 1.0 wt%, 0.1 to wt%, 0.05 to 0.5 wt%, and 0.2 to 1.0 wt%, respectively.

<Metal Structure> The PC steel bar of the present invention is made of a substantially tempered martensite, and the carbide in the tempered martensite is made of fine particles having an average diameter of 0.1 im or less.

The substantially tempered martensite of the present invention includes other structures such as bainite and/or ferrite unless the tempered martensite properties are not interfered.

Changes in the form of carbide in the tempered martensite greatly influence uniform elongation due to easy formation of voids and influence on the hardening behavior during working. When the form of carbide is 13 a bar or plate, the carbide itself tends to crack.

The stress concentrates at the distal end of the carbide to easily form voids, thereby degrading the uniform elongation. The form of carbide must therefore be a particle. Even if the form of carbide is a particle, it is difficult to work-harden at an average particle diameter of the carbide larger than 0.1 pn.

Therefore, the size of the carbide particle is 0.1 gm or less.

<Manufacturing Method> According to the method of manufacturing a PC steel bar according to the present invention, steel having the above composition is obtained as ingot iron by the normal process, and the resultant round steel is hot-rolled or cool-rolled drawn) to obtain a steel bar element. This steel bar element is subjected to a series of heat treatment operations hardening and tempering). The method of manufacturing the PC steel bar according to the present invention does not require a distortion applying step.

The hardening temperature greatly influences the yield ratio through a change in particle size. This greatly improves the uniform elongation accordingly.

When the hardening temperature is less than 1,160 0

C,

the particle size is small, and the yield ratio cannot greatly lower. The hardening temperature is set to 1,160 0 C or more. When the hardening temperature exceeds 14 1,300 0 C, decarbonation typically occurs. Therefore, the hardening temperature is set to 1,300 0 C or less.

The tempering temperature is preferably set to 600 0 C or more in order to greatly improve the uniform elongation.

<Characteristics of PC Steel Bar> A PC steel bar obtained by the manufacturing process of the present invention can attain a uniform elongation of 8.0% or more on the Class B standard strength level (1,080 N/mm 2 or more), a uniform elongation of 6% or more and a high-temperature relaxation ratio of 8% or less on the Class C standard strength level (1,230 N/mm 2 or more). A PC steel bar having a high content of carbon obtained by hardening the steel at a high temperature can attain a uniform elongation of 11.0% or more, a yield ratio of 0.85 or less and a high-temperature relaxation of 8% or less on the Class B standard strength level, a uniform elongation of 10.0% or more, a yield ratio of 0.87 or less and a high-temperature relaxation ratio of 8% or less on the Class C standard strength level (1,230 N/mm 2 or more) and a uniform elongation of or more and a yield ratio of 0.90 or less on the Class D standard strength level (tensile strength 1,420 N/mm 2 and yield strength 1,275 N/mm 2 When a PC steel bar is to be spot-welded, tempering energization is performed upon spot welding, 15 thereby assuring excellent uniform elongation and high-temperature relaxation characteristics.

According to the method of the present invention, there is provided a PC steel bar which can be manufactured by the normal process without requiring an addition step such as a distortion applying step and has a tensile strength of 1,080 N/mm 2 or more, highly uniform elongation, and excellent high-temperature relaxation characteristics at a low yield ratio.

Therefore, the productivity can be greatly improved, and at the same time the reliability of prestressed concrete structures can be greatly improved, thus providing industrial advantages.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the 16 preferred embodiments given below, serve to explain the principles of the invention.

FIGURE is a view showing temperature cycles in a high-temperature relaxation test.

DETAILED DESCRIPTION OF THE INVENTION (Example 1) Steel samples (Nos. 1 to 13) of the present invention and steel samples (Nos. 14 to 19) of control, which had the component compositions shown in Table 1 were used as test samples. Each test sample was rolled into a round steel having a diameter of 24 mm or drawn into a round steel having a diameter of 23 mm, and the rolled or drawn test sample was hardened and then tempered by high-frequency heating. Note that the heat treatment is not limited to high-frequency heating.

The hardening temperature was set to fall within the range of 930 0 C to 1,100 0 C, and tempering was performed on two levels for each test sample so as to obtain Class B and C standard strength levels in JIS G 3109 (JIS G 3137). The resultant samples were then subjected to a tensile test. A uniform elongation test was conducted such that marks were formed on each sample every 2d (d is the diameter), and the elongation was measured at a position 5d or more from the fracture portion by using 10d as a gauge point distance.

The tensile test results are shown in Table 2.

A relaxation test was conducted complying with the JIS 17 G 3109 (JIS G 3137) method. The relaxation test was conducted by applying a 70% stress of the standard tensile strength to each sample at room temperature.

A change in load 10 hours after the start of the test was measured. The relaxation was evaluated by the ratio of the changes in loads to the initial load.

Note that 10-hour values were used in Example 1 for the sake of simplicity, although 1,000-hour values are standards since 1994. In addition, a high-temperature relaxation test was conducted by applying a 70% stress of the standard tensile strength to each sample at 180 0

C.

Each sample was heated and cooled in accordance with a pattern shown in FIGURE. A change in load 23 hours after the start of the test was measured. The hightemperature relaxation was evaluated by the ratio of the changes in loads to the initial loads. The forms of carbide and sizes of tempered martensites in the samples were measured using a transmission electron microscope using the extraction residue method.

The diameter of each carbide was observed at 89,000x in five fields, and the average value of the measurement values was calculated. The results are shown in Table 2.

As shown in Table 2, steel sample Nos. 1 to 13 of the present invention attained a uniform elongation of or more and a high-temperature relaxation ratio of 8% or less on the Class B standard strength level 18 (tensile strength 2 1,080 N/mm 2 and yield strength 930 N/mm 2 and a uniform elongation of 6% or more and a high-temperature relaxation ratio of 8% or less on the Class C standardization strength level (tensile strength 1,230 N/mm 2 and yield strength 1,080 N/mm 2 These samples had both characteristics much better than those of conventional steel characteristics.

The carbides in the tempered martensites were in the form of a particle whose diameter was 0.1 pn or less.

To the contrary, although steel sample No. 14 of control had excellent relaxation characteristics, ductility degraded due to the large particle size of the carbide because the C content exceeded 0.7 wt%.

Steel sample No. 15 had poor ductility because the Mn content exceeded 3.0 wt%. Steel sample No. 16 had insufficient uniform elongation and degraded relaxation characteristics because the Si Al content was less than 2 wt%. The carbide of this steel sample had the form of a rod. Steel sample No. 17 had degraded relaxation characteristics because the Si Al content exceeded 5 wt%. Although steel sample Nos. 18 and 19 had appropriate chemical compositions, the form of carbide in sample No. 18 was a rod, and the uniform elongation of steel sample No. 19 lowered because the diameter exceeded 0.1 pim although the form of carbide was a particle.

19 (Example 2) Steel samples containing 0.5 wt% or less of C, of the steel samples (Nos. 1 to 13) of the present invention and the steel samples (Nos. 14 to 19) of control, which had the component compositions shown in Table 1 were used as test samples. Each test sample was rolled into a round steel having a diameter of 8 mm or drawn into a profile round steel having a diameter of 7.1 mm, and the rolled or drawn steel sample was hardened and then tempered by high-frequency heating.

Note that the heat treatment is not limited to highfrequency heating. The hardening temperature was set to fall within the range of 930 0 C to 1,100 0

C,

and tempering was performed for each steel sample on optimal conditions so as to obtain the Class B standard strength level in JIS G 3109 (JIS G 3137). The resultant samples were then subjected to spot welding and tempering energization under the following welding and tempering conditions.

Welding and Tempering Conditions: welding current 2,500 A tempering current 2,700 A number of energization cycles 2 pressure 410 N spiral hoop reinforcement SWRM 8-3.1 mm The uniform elongation and relaxation test methods follow the same procedures as described above.

20 The test results are shown in Table 3. As can be apparent from Table 3, the steel samples of the present invention can assure uniform elongations of 6% or more and have excellent relaxation characteristics by performing tempering energization upon spot welding.

:i Table 1 Steel Samples of Present Invention Steel Samples of Control No. C 1 0.22 Si Mn 3.02 0.72 2 3 4 6 7 8 9 11 12 13 14 0.33 0.46 0.65 0.32 0.31 0.3 0.33 0.32 0.32 0.32 0.30 0.31 0.73* 0.31 3.04 2.98 3.01 3.54 4.01 3.52 3.08 3.05 2.03 2.52 4.08 4.02 2.53 3.56 0.69 0.65 0.68 0.73 0.74 0.73 1.54 2.56 0.73 0.72 2.01

P

0.007 0.006 0.008 0.008 0.007 0.007 0.007 0.007 0.005 0.005 0.007 0.007 0.005 0.006 0.006 S Al 0.008 0.008 0.008 0.009 0.007 0.008 0.006 0.008 0.008 0.008 0.009 0.008 0.006 0.006 n nnf 0.023 0.021 0.022 0.02 0.027 0.022 1.28 0.021 0.024 0.024 0.021 0.022 0.024 0.035 n n, (wt% Si+Al 3.04 3.06 3.00 3.03 3.57 4.03 4.80 3.1 3.07 2.05 2.54 4.1 4.04 2.57 3 n 2.74 0.72 3.18* 0.006 nfl;. 5 o 16 0.32 1.01 0.71 0.007 0.008 0.021 1.03* 17 0.31 4.03 0.72 0.007 0.007 1.21 5.24* 18 0.66 1.15 2.78 0.007 0.008 1.26 2.41 19 0.68 2.26 0.68 0.006 0.008 0.029 2.29 Note) The asterisk represents the range falling outside the present invention (Continued)

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Table 1 No.

Steel Samples of Present Invention 2 3 4 6 7 8 9 11 12 13 Steel Samples of 14 Control 16 17 18 19 Nb Ti

B

0.003 0.003 Ni 0.5 0.5 Cr Mo 'wt% Cu 0.04 0.04 0.02 0.02 0.5 0.8 0.1 0.2 0.3 0.03 0.02 10.002

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Table 2 No.

Steel 1 (Class B) Samples 1 (Class C) of 2 (Class B) Present 2 (Class C) Invention 3 (Class B) 3 (Class C) 4 (Class B) 4 (Class C) (Class B) (Class C) 6 (Class B) 6 (Class C) 7 (Class B) 7 (Class C) 8 (Class B) 8 (Class C) 9 (Class B) 9 (Class C) (Class B) (Class C) 11 (Class B) 11 (Class C) 12 (Class B) 12 (Class C) 13 (Class B) 13 (Class C) Hardening Temperature (Oc) 1040 1040 1020 1020 990 990 960 960 1080 1080 1090 1090 1100 1100 1000 1000 980 980 940 940 960 960 1070 1070 1040 1040 Tempering Temperature (c) 630 560 670 600 690 620 710 630 695 630 730 665 670 600 670 600 680 610 680 600 710 610 740 670 740 675 0.2% Yielding Strength (N/mmZ) 1056 1215 1090 1223 1068 1231 1084 1211 1041 1170 1060 1183 1051 1177 1058 1207 1078 1219 1136 1256 1123 1270 1066 1210 1054 1208 Tensile Strengtl (N/mm 1132 1267 1168 1275 1145 1284 1162 1263 1143 1250 1150 1250 1154 1257 1134 1259 1155 1271 1169 1286 1155 1300 1156 1279 1143 1277 Elongation 17.8 15.6 17.5 15.8 17.6 15.2 18 15.1 19.3 16.2 20.3 16.6 19.6 16.4 17.4 15.5 18.9 16.2 15.7 12.9 16.6 13.7 21.5 17.2 22.8 18.1 Uniform Elongation 6.8 8.4 6.9 8.6 6.7 8.4 6.9 12.1 8.3 13.2 10.3 12.2 7.9 8.6 7.2 8.2 6.1 8.8 6.4 14.1 11.2 14.8 11.8 (Continued) Tensile Test Ip~ Table 2 Steel Samples o Present Invention No.

1 (Class

B)

1 (Class C) 2 (Class B) 2 (Class C) 3 (Class B) 3 (Class C) 4 (Class B) 4 (Class C) (Class B) (Class C) 6 (Class B) 6 (Class C) 7 (Class B) 7 (Class C) 8 (Class B) 8 (Class C) 9 (Class B) 9 (Class C) (Class B) (Class C) 11 (Class B) 11 (Class C) 12 (Class B) 12 (Class C) 13 (Class B) 13 (Class

C)

Relaxation Test Room 80"C Temperature 0.54 6.5 0.53 6.6 0.41 6.4 0.43 6.2 0.39 6.0 0.38 6.0 0.39 6.1 0.40 6.3 0.41 6.3 0.44 6.5 0.38 5.9 0.38 6.3 0.48 6.5 0.49 6.4 0.53 6.5 0.52 6.4 0.54 6.8 0.53 6.7 0.55 6.3 0.57 6.5 0.58 6.5 0.59 6.8 0.50 6.2 0.54 6.5 0.55 6.5 0.53 6.2 Form of Carbide Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Particle Diameter of Carbide 01 m) 0.02 0.01 0.03 0.02 0.05 0.03 0.09 0.06 0.03 0.02 0.02 0.01 0.03 0.02 0.04 0.02 0.05 0.03 0.04 0.03 0.04 0.02 0.02 0.01 0.03 0.02 (Continued) Table 2 Tensile Test 0.2% Hardening Tempering Yielding Tensile Uniform Temperature Temperature Strength Strength Elongation Elongation No. (Clas B) 7N/m N/ mmZ) Steel 14 (Class B) 930 730 1119 1151 11.2 5.6* Samples of 14 (Class C) 930 630 1235 1264 10.7 5.2* Control 15 (Class B) 1030 695 1045 1147 13.1 6.3* (Class C) 1030 630 1193 1275 12.2 5.4* 16 (Class B) 930 620 1128 1161 11.8 5.8* 16 (Class C) 930 530 1256 1286 11.6 5.8* 17 (Class B) 1100 710 1060 1150 20.2 12.9 17 (Class C) 1100 615 1203 1272 16.4 10.1 18 (Class B) 940 730 1062 1156 17.3 7.3* 18 (Class C) 940 650 1220 1275 15.5 5.7* 19 (Class B) 930 710 1115 1149 11.5 5.9* 19 (Class C) 930 600 1224 1267 10.6 5.6* Note) The a tenr is

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T e nL une range falling outside the present invention (Continued) Table 2 Steel Samples of Control No.

14 (Class B) 14 (Class C) 15 (Class B) (Class C) 16 (Class B) 16 (Class C) 17 (Class B) 17 (Class C) 18 (Class B) 18 (Class C) 19 (Class B) 19 (Class C) Relaxation Test Room 80°C Temperature 0.45 5.8 0.47 6.0 0.44 6.8 0.47 6.8 0.73 10.2* 0.78 11.0* 0.87 11.5* 0.89 11.3* 0.68 9.7* 0.67 9.5* 0.46 5.8 0.47 6.3 Form of Carbide Particle Particle Particle Particle Rod Rod Particle Particle Rod Rod Particle Particle Diameter of Carbide m) 0.17* 0.14* 0.07 0.05 0.15(length) 0.12(length) 0.03 0.02 0.2 (length) 0.17(length) 0.14* 0.11* I l- mi noue) nTe asterisk represents the range falling outside the present invention

~B~PV:

Table 3 Tensile Test Relaxatio 0.2% Hardening Tempering Yielding Tensile Uniform Room Tempera- Tempera- Strength Strength Elonga- Elonga- Tempera- No. ture ture (C N/mmZ i N/mm tion tion ture Samples of1 040 630 1048 1128 13.2 6.5 0.55 Present 2 1020 670 1078 1165 135 6.3 0.46 Invention 5 6.3 046 3 990 690 1045 1123 14.2 6.5 0.41 4 1080 695 1037 1142 13.9 6.7 0.39 6 1090 730 1055 1148 12.8 6.9 0.40 7 1100 670 1045 1150 13.1 6.8 0.47 8 1000 670 1039 1130 14.5 6.3 0.52 9 980 680 1068 1149 13.7 6.5 0.57 940 680 1032 1166 13.7 6.4 0.60 11 960 710 1021 1152 14.1 6.7 0.56 12 1070 740 1056 1151 12.9 6.3 0.47 13 1040 740 1050 1139 13.0 6.3 0.51 Steel 14 Samples of Control 15 1030 695 1034 1142 12.9 5.9* 0.41 16 930 620 1115 1154 11.5 5.7* 0.71 17 1100 710 1047 1145 6.5 3.2* 0.87 Note) The asterisk represents the range falling outside the present invention n Test 180°C

L--

6.6 6.6 6.1 6.7 6.2 6.3 6.8 6.6 7.2 10.8* 12.1* 28 (Example 3) Steel samples having component compositions shown in Table 4 were used. Each sample was rolled into a round steel having a diameter of 8 mm and drawn into a profile round steel having a diameter of 7.1 mm.

These samples were hardened and then tempered by highfrequency heating. Note that the heat treatment is not limited to high-frequency heating. The heating temperature was set to fall within the range of 1,160 0

C

to 1,300 0 C (examples of the present invention), and tempering was performed to obtain the Class C or D standard strength level in JIS G 3109 (JIS G 3137).

The resultant samples were subjected to a tensile test.

A uniform elongation test was conducted such that marks were formed on each sample every 2d (d is the diameter), and the elongation was measured at a position 5d or more from the fracture portion by using 10d as a gauge point distance. The tensile test results are shown in Table 5. A relaxation test was conducted complying with the JIS G 3109 (JIS G 3137) method at room temperature. Note that as in Example 1 10-hour values were used in Example 3 for the sake of simplicity, although 1,000-hour values are standards since 1994.

In addition, a high-temperature relaxation test was conducted by applying a 70% stress of the standard tensile strength to each sample, at 180 0 C. Each sample was heated and cooled in accordance with a pattern 29 shown in FIG. i. A change in load 23 hours after the start of the test was measured. The high-temperature relaxation was evaluated by the ratio of the changes in loads to the initial loads. The test results are shown in Table As shown in Table 5, steel sample Nos. 1 to 13 of the present invention attained a uniform elongation of 10.0% or more, a yield ratio of 0.87 or less, and a high-temperature relaxation ratio of 8% or less on the Class C standard strength level (tensile strength 1,230 N/mm 2 and yield strength 1,080 N/mm 2 and a uniform elongation of 8% or more, a yield ratio of 0.90 or less, and a high-temperature relaxation ratio of 8% or less on the Class D standardization strength level (tensile strength 1,420 N/mm 2 and yield strength 1,275 N/mm 2 These samples had both characteristics much better than those of conventional steel characteristics. Note that although a tempering temperature of 600 0 C or more is desired to further improve the uniform elongation, the steel samples having the components of the present invention attain the Class D standard strength level and uniform elongations of 8% or more at a tempering temperature of 600'C or more.

To the contrary, steel sample Nos. 14 and 16 of control are short in the C and Si Al contents, respectively, so their tempering temperature is low and the yield ratio cannot greatly lower. The uniform 30 elongation also degrades as low as 4 to 5% on the Class D standard and about 7% on the Class C standard. Steel sample Nos. 15 and 17 have high tempering temperatures and low yield ratios. However, as the C and Mn contents of sample Nos. 15 and 17 exceed optimal amounts, the uniform elongations lower as compared with the samples of the present invention. Sample Nos. and 17 cannot attain, as target values, the uniform elongation of 10% or more on the Class C standard and 8% or more on the Class D standard. Although steel sample No. 18 contains appropriate chemical components, it cannot attain the target uniform elongation value because the yield ratio cannot lower due to a low hardening temperature of 1,100 0

C.

(Example 4) Steel samples having component compositions shown in Table 4 were used. Each sample was rolled into a round steel having a diameter of 24 mm and drawn into a profile round steel having a diameter of 23 mm.

These samples were hardened and then tempered by highfrequency heating. Note that the heat treatment is not limited to high-frequency heating. The heating temperature was set to fall within the range of 1,160'C to 1,300 0 C (samples of the present invention), and tempering was performed to obtain the Class B standard strength level in JIS G 3109 (JIS G 3137). The resultant samples were subjected to a tensile test and 31 a relaxation test, following the same procedures as in Example 3.

The heat treatment conditions and the resultant mechanical characteristics are shown in Table 6. As is apparent from Table 6, steel sample Nos. 1 to 13 of the present invention attained a uniform elongation of 11.0% or more, a yield ratio of 0.85 or less, and a high-temperature relaxation ratio of 8% or less on the Class B standard strength level. These samples had both characteristics much better than those of conventional steel characteristics. To the contrary, steel sample Nos. 14 and 16 of control are short in the C and Si Al contents, respectively, so their tempering temperature is lower than that of the present invention, and the yield ratio cannot greatly lower as in Example 3. The uniform elongation degrades to 8% on the Class B standard. Steel sample Nos. 15 and 17 have high tempering temperatures and low yield ratios.

However, as the C and Mn contents of sample Nos. 15 and 17 exceed optimal amounts, the uniform elongations slightly lower as compared with the samples of the present invention. Sample Nos. 15 and 17 cannot attain, as target values, the uniform elongation of 11% or more on the Class B standard. Although steel sample No. 18 contains appropriate chemical components, it cannot attain the target uniform elongation value because the yield ratio cannot lower due to a low hardening -32temperature of 1,100 0 C as in Example 3.

Additional advantages and modifications will readily occurs to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein.

Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Throughout this specification (including the claims if present), unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

o o• p p po p P Table 4 Chemical Components of Test Samples No, Steel Samples of 2 Present Invention 3 4 6 7 C Si Mn p S Al Si+A 0.52 3.02 0.7 0.007 0.007 0.02 3.04 8 9 11 12 13 14 15 16 17 18 0.6 3.01 0.68 0.008 0.009 0.02 3.03 0.68 3.06 0.68 0.007 0.009 0.02 3.08 0.62 3.55 0.72 0.009 0.008 0.03 3.58 0.61 2.27 0.76 0.009 0.008 0.02 2.29 0.59 2.51 0.72 0.007 0.007 2.13 4.64 0.6 3.01 0.38 0.007 0.009 0.03 3.04 0.62 3.06 1.2 0.007 0.007 0.02 3.08 0.62 3.09 1.85 0.008 0.007 0.020 3.11 0.60 3.02 0.7 0.007 0.008 0.020 3.04 0.60 3.04 0.7 0.009 0.008 0.030 3.07 0.58 3.02 0.72 0.007 0.008 0.020 3.04 0.60 3.04 0.74 0.008 0.008 0.030 3.07 0.32* 3.02 0.72 0.007 0.007 0.02 3.04 0.78* 3.01 0.7 0.007 0.008 0.020 3.03 0.58 1.53 0.8 0.008 0.008 0.260 1.79* 0.6 3.04 3.12* 0.007 0.008 0.02 3.06 0.6 3.01 0.68 0.008 0.009 0.02 3.03 (Continued) Steel Samples of Control

I

Table 4 Chemical Components of Test Samples ~p" Table 5 Mechanical Characteristics of Test Samples Steel Samples of Present Invention No.

1 2 3 4 6 7 8 9 11 12 13 Hardening Tempering Yielding Temperature Temperature Strength Class C) N/mm 2 D 1200 630 1310 C 1200 720 1101 D 1200 670 1306 C 1200 745 1087 D 1200 700 1281 C 1200 760 1085 D 1200 690 1283 C 1200 760 1081 D 1200 640 1310 C 1200 720 1091 D 1250 650 1278 C 1250 730 1082 D 1200 670 1300 C 1200 745 1122 D 1200 670 1286 C 1200 745 1083 D 1250 680 1288 C 1250 755 1089 D 1200 670 1310 C 1200 745 1095 D 1200 670 1312 C 1200 745 1088 D 1200 670 1302 C 1200 745 1092 D 1200 670 1304 C 1200 745 1084 Tensile Strength N/mm 2 1456 1265 1489 1295 1490 1292 1475 1303 1472 1283 1452 1288 1444 1290 1478 1274 1481 1297 1489 1288 1474 1280 1480 1285 1465 1290 Total Elongation 13.2 15.6 13.4 16 14 16.8 13.8 16.7 13 15.4 13 15.6 13.1 15.8 13.8 16.5 14.2 16.6 13.5 16.2 13.4 16.1 13.8 16.5 13.7 16.2

I

(Continued)

BQ~$:

Table 5 Mechanical Characteristics of Test Samples No.

Steel 1 Samples of Present 2 Invention 3 4 6 7 8 9 11 12 13 Relaxation Ratio at Uniform Room Elongation Yield Temperature Ratio 8.2 0.90 0.62 10.2 0.87 0.61 8.5 0.88 0.68 10.3 0.84 0.72 8.7 0.86 0.62 10.5 0.84 0.65 8.7 0.87 0.66 10.6 0.83 0.66 8.2 0.89 0.68 10.1 0.85 0.72 8.1 0.88 0.63 0.84 0.69 8.4 0.90 0.70 10.1 0.87 0.72 8.6 0.87 0.69 10.7 0.85 0.65 8.6 0.87 0.62 10.7 0.84 0.64 8.5 0.88 0.70 10.2 0.85 0.72 8.4 0.89 0.63 10.4 0.85 0.63 8.4 0.88 0.65 10.4 0.85 0.66 8.3 0.89 0.62 10.5 0.84 0.70 Relaxation Ratio at 180°C 6.3 6.2 6.1 6.7 6.3 6.1 5.9 6.1 6.3 6.2 7.2 6.2 5.8 6.3 6.2 6.6 6.4 7.1 6.1 6.8 6.1 5.6 6.2 (Continued)

"I

~e~PP" Table 5 Mechanical Characteristics of Test Samples Hardening Tempering Yielding Tensile Total Temperature Temperature Strength Strength Elongatio o. Class "C N/mm 2 N/mm 2 %o 4 D 1200 520 1410 1469 10.3 C 1200 600 1184 1260 15.8 D 1200 720 1285 1460 12.6 C 1200 780 1065 1268 14.8 6 D 1200 590 1384 1457 7.9 C 1200 685 1199 1275 10.2 7 D 1200 680 1278 1469 12.5 C 1200 760 1088 1295 14.3 3 D 1100* 670 1368 1471 13.6 C 1100* 745 1147 1289 15.9 (Continued) Ai W Ii Table 5 Mechanical Characteristics of Test Samples Relaxation Ratio at Uniform Room Relaxation Elongation Yield Temperature Ratio at No. Ratio 180°C el 14 4.9 0.96 0.68 6.8 tples of 6.9 0.94 0.47 6.9 trol 15 7.3 0.88 0.65 6.7 9.4 0.84 0.62 6.1 16 4.2 0.95 0.78 6.8 0.94 0.76 17 7.2 0.87 0.63 ii 111 Table 6 Mechanical Characteristics of Test Samples Steel Samples of Present Invention Steel Samples of Control No.

1 2 3 4 6 7 8 9 11 12 13 14 15 16 17 18 Hardening Temperature Class 0

_C)

B 1200 B 1200 B 1200 B 1200 B 1200 B 1250 B 1200 B 1200 B 1250 B 1200 B 1200 B 1200 B 1200 B 1200 B 1200 B 1200 B 1200 B 1100* Tempering Temperature oc) 740 780 810 800 750 760 780 780 790 780 780 780 780 670 790 700 790 780 Yielding Tensile Total Strength Strength Elongatio N/mm 2 N/mm 2 1005 1182 17.6 968 1180 18 964 1175 18.9 940 1160 18.6 984 1186 17.4 937 1143 17.6 1000 1176 17.6 958 1154 18.5 961 1172 18.4 967 1165 18.3 944 1137 18.2 955 1150 18.5 972 1185 18.2 1080 1174 17.8 980 1195 16.9 1042 1133 12.4 973 1186 16.2 1018 1170 17.9 (Continued) izd Table 6 Mechanical Characteristics of Test Samples Steel Samples of Present Invention Steel Samples of Control Uniform Elongation No. 1 11.6 2 11.8 3 12.2 4 12.1 11.8 6 11.7 7 12.0 8 12.2 9 12.2 11.9 11 12.0 12 11.9 13 12.1 14 8.4 15 10.6 16 8.3 17 10.8 18 9.5 Relaxation Yield Ratio at Room Ratio Temperature 0.85 0.62 0.82 0.70 0.82 0.65 0.81 0.64 0.83 0.70 0.82 0.66 0.85 0.72 0.83 0.67 0.82 0.63 0.83 0.70 0.83 0.64 0.83 0.66 0.82 0.66 0.92 0.51 0.82 0.65 0.92 0.76 0.82 0.64 0.87 0.71 Relaxation Ratio at 180°C 6.3 6.2 6.4 6.6 6.2 6.8 6.1 6.2 6.6 6.6 5.9 6.4 6.4 7.8 6.2 -41 The Claims defining the invention are as follows: 1. A PC steel bar with excellent uniform elongation and high-temperature relaxation characteristics comprising steel containing 0.2 to 0.7 wt% of C, 0.2 to wt% of Mn, and 2.0 to 5.0 wt% of a combination of Si and Al, the balance bieng Fe, wherein the steel has a substantially tempered martensitic microstructure in which the carbide in the tempered martensite comprises ultrafine particles having an average diameter of not more than 0.1 tm and a tensile strength of not less than 1,080 N/mm 2 and less than 1,420 N/mm 2 10 2. A bar according to claim 1, further containing, as a steel component, at least one element selected from the group consisting of 0.01 to 0.1 wt% of Nb, 0.01 to 0.05 wt% of Ti, 0.0003 to 0.005 wt% of B, 0.2 to 1.0 wt% of Ni, 0.1 to 1.0 wt% of Cr, 0.05 to 0.5 wt% of Mo, and 0.2 to 1.0 wt% of Cu.

3. A PC steel bar with excellent uniform elongation and high-temperature 15 relaxation characteristics, comprising as steel components, 0.2 to 0.7 wt% of C, 0.2 to 3.0 wt% of Mn, 2.0 to 5.0 wt% of a combination of Si Al, and further containing at least one element selected from the group of 0.01 to 0.1 wt% of SNb, 0.01 to 0.05 wt% of Ti, 0.0003 to 0.005 wt% of B, 0.2 to 1.0 wt% of Ni, 0.1 to wt% of Cr, 0.05 to 0.5 wt% of Mo, and 0.2 to 1.0 wt% of Cu, and still further containing Fe and an inevitable impurity for the remaining portion

Claims (8)

1. 4. A bar according to any one of claims 1 to 3, wherein the PC steel bar has a tensile strength of not less than 1,080 N/mm 2 and a yield strength of not less than 930 N/mm 2 A bar according to any one of claims 1 to 3, wherein the PC steel bar has a tensile strength of not less than 1,230 N/mm 2 and a yield strength of not less than 1,080 N/mm 2
2. 6. A bar according to any one of claims 1 to 5, wherein the PC steel bar contains 0.2 to 0.5 wt% of C and is used for spot welding and then tempering energization.
3. 7. A method of manufacturing a PC steel bar having a tensile strength of not less than 1,080 N/mm 2 and with excellent highly uniform elongation and high- temperature characteristics at a low yield ratio, comprising hardening the steel at 1,160 0 C to 1,300 0 C and then tempering the steel, the steel containing 0.5 to 0.7 wt% of C, 0.2 to 2.0 wt% of Mn, and a combination of Si and Al within a range of 2.0 wt% O 9lt -43- Si Al 5.0 wt%, the balance being Fe.
4. 8. A method of manufacturing a PC steel bar having a tensile strength of not less than 1,080 N/mm 2 and with excellent highly uniform elongation and high- temperature characteristics at a low yield ratio, comprising hardening the steel at 1,160 0 C to 1,300 0 C and then tempering the steel, so as to form a microstructure of ultrafine spherodised cementite in a matrix ferrite the steel containing 0.5 to 0.7 wt% of C, 0.2 to 2.0 wt% of Mn, and a combination of Si and Al within a range of 2.0 wt% Si Al 5.0 wt%, and at least one element selected from the group consisting of 0.01 to 0.1 wt% of Nb, 10 0.01 to 0.05 wt% of Ti, 0.0003 to 0.005 wt% of B, 0.2 to 1.0 wt% of Ni, 0.1 to wt% of Cr, 0.05 to 0.5 wt% of Mo, and 0.2 to 1.0 wt% of Cu, the balance being Fe.
5. 9. A method of manufacturing a PC steel bar having a tensile strength of not less than 1,080 N/mm 2 and with excellent highly uniform elongation and high- o S 15 temperature characteristics at a low yield ratio, comprising hardening the steel w at 1,160 0 C to 1,300 0 C and then tempering the steel, the steel containing 0.5 to 0.7 wt% of C, 0.2 to 2.0 wt% of Mn, 2.0 to 5.0 wt% of a combination of Si Al, and at least one of the elements selected from the group consisting of 0.01 to 0.1 wt% of Nb, 0.01 to 0.05 wt% of Ti, 0.0003 to 0.005 wt% of B, 0.2 to 1.0 wt% of Ni, 0.1 to 1.0 wt% of Cr, 0.05 to 0.5 wt% of Mo, and 0.2 to 1.0 wt% of Cu, and Fe and an inevitable impurity for the remaining portion -44- A method according to any one of claims 7 to 9, wherein the PC steel bar has a strength level defined by a tensile strength 1,230 N/mm 2 and a yield strength >1,080 N/mm 2 a uniform elongation of not less than 10.0%, a yield ratio of not more than 0.87, and a high-temperature relaxation ratio of not more than 8%.
6. 11. A method according to any one of claims 7 to 9, wherein the PC steel bar has a strength level defined by a tensile strength 1,420 N/mm 2 and a yield strength 1,275 N/mm 2 a uniform elongation of not less than a yield ratio of not more than 0.90, and a high-temperature relaxation ratio of not more than 8%.
7. 12. A method according to any one of claims 7 to 9, wherein the PC steel bar has a strength level defined by a tensile strength 1,080 N/mm 2 and a yield strength 930 N/mm 2 a uniform elongation of not less than 11.0%, a yield ratio aof not more than 0.85, and a high-temperature relaxation ratio of not more than 15 8%. e.
8. 13. A method according to any one of claims 7 to 12, wherein the tempering temperature is not less than 600 0 C. .14. A PC steel bar, substantially as hereinbefore described with reference to the Examples but excluding the prior art examples. 20 15. A manufacturing method of manufacturing PC steel bar substantially as hereinbefore described with reference to the Examples but excluding the prior art examples. Dated this TWELFTH day of MARCH 1998 NKK CORPORATION and NETUREN CO., LTD. Applicants Wray Associates Perth, Western Australia Patent Attorneys for the Applicant _M i ABSTRACT OF THE DISCLOSURE A method of manufacturing a PC steel bar having a tensile strength of 1,080 N/mm 2 or more and excellent in highly uniform elongation and high-temperature characteristics at a low yield ratio, includes hardening steel at 1,160 0 C to 1,300 0 C and then tempering the steel. The steel contains 0.5 to 0.7 wt% of C, 0.2 to 2.0 wt% of Mn, and Si and Al within the range of wt% Si Al 5.0 wt%. o 0 0 0