JPH02107743A - Ultrahigh tensile strength pc steel wire or steel rod excellent in uniform elongation and its production - Google Patents

Abstract

PURPOSE:To manufacture a PC steel wire or steel rod having high strength level and excellent in uniform elongation characteristics by subjecting a low- alloy steel of relatively low carbon content to hot working under specific conditions and then to cooling control. CONSTITUTION:A steel having a composition containing, by weight, 0.17-0.29% C, <0.5% Si, 0.2-2% Mn, and <=0.2% Al and/or Ti as principal components is subjected to hot working at >=60% total draft at 900-1,000 deg.C for <=1sec. Successively, the above steel is cooled down to <=500 deg.C within 0.5sec at >=50 deg.C/sec cooling rate. By this method, after hot rolling, a structure consisting of 0.5-5% ferrite of <=3mum average grain size and the balance mainly martensite is provided to the above steel, and further, after straightening, the ultrahigh tensile strength PC steel wire or steel rod having >=120kg/mm<2> tensile strength and >=3% uniform elongation can be obtained. Further, <=0.02% B and 2%, in total, of one or more elements among Cr, Cu, and Ni are incorporated to the above steel. This steel stock can be formed into a prestressed concrete structure highly resistant even to impact load and having high stability.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention has a tensile strength of 120 kg/m with excellent uniform elongation.
The present invention relates to a PC steel wire or steel bar having characteristics of 100 m or more, and a method for manufacturing the same.

[Prior Art] In recent years, as steel structures have become larger, the strength of the steel materials used has tended to increase. In particular, the PC steel wire used as concrete reinforcement is JIS G 310
Maximum tensile strength 145 kg/
v++” or more is required.

However, the strength and ductility of steel materials are generally contradictory in their properties, and in particular, the -like elongation (uniform elongation) deteriorates more severely as the steel becomes higher tensile strength.

Conventionally, heat-treated PC steel bars and steel wires used in prestressed concrete structures are made by reheating hot-rolled material to a temperature above the transformation point, then quenching it to form a martensitic structure, and then It has been manufactured using a process that involves a tempering treatment to improve the yield point and restore toughness. However, when trying to obtain a high strength level with such a tempered martensitic structure, the -like elongation is 3 to 5% after quenching and tempering.

Furthermore, deterioration of uniform elongation, such as deterioration of 1 to 1.5% during the straightening → bluing process, is caused by the steel rods serving as reinforcing bars breaking when the concrete structure is subjected to impact loads, etc. 1. It can no longer function as a reinforcing bar, and in some cases, it may lead to an accident that could result in death.

It is known that a PC steel wire or steel bar that is free from the risk of such an accident needs to have a uniform elongation (uniform elongation) of 3% or more during use. Here, the usage state is the state after the steel wire or steel rod, which is generally supplied in the form of a coil, has been straightened and its elongation properties have deteriorated.

The present inventors previously disclosed in Japanese Patent Application No. 56-5929 and Japanese Patent Application No. 56-205709 that C: 0.15 to 0.
．． After hot rolling steel containing 80% and 20.5% Si,
By rapidly cooling to an appropriate temperature range of 500°C or less and maintaining it in that temperature range, steel with a bainite-based structure or a mixed structure of bainite and martensite or retained austenite can be obtained. mm"
It has been disclosed that it has the above-mentioned high strength, excellent uniform elongation, and satisfies the above-mentioned target properties. However, in order to stably generate bainite to obtain these steels, it is necessary to add S to the component.
It is necessary to add a relatively large amount of alloying elements such as i, which is expensive.

Another disadvantage is that rapid cooling and retention can only be carried out in a rolling mill equipped with special direct heat treatment equipment. In addition, if it is necessary to stabilize the properties by making a large amount of retained austenite exist, the C content may be reduced to 0.
．． It is necessary to increase the content to 3% or more, but this has the disadvantage that weldability deteriorates and some restrictions are forced when constructing a frame using PCm wire or steel rods.

[Problem M to be Solved by the Invention] An object of the present invention is to provide a steel material and a method for manufacturing the same that fundamentally solves the problems in the prior art as described above. More specifically, C: 0.17-0.
120 kg after straightening made of 29% low carbon and relatively low alloy steel
Provides a steel wire or steel rod having an ultra-high tensile strength of 3% or more and a uniform elongation of 3% or more, and a method for producing the same that includes hot rolling and cooling conditions without using special heat treatment equipment. This is what I am trying to do.

[Means to solve the problem] The features of the present invention are as follows.

(1) By weight, C: O, 17-0.29%, Si<0.5%.

Mn: 0.2-2%.

It also contains 0.2% or less of Al and/or Ti as a main component, and after hot rolling, 0.5 to 5% of ferrite with an average grain size of 3 μm or less and the remainder are mainly martensite. It is an ultra-high tensile strength PC steel wire or steel bar with excellent uniform elongation, having a tensile strength after straightening of 120 kg/mm" or more and a uniform elongation of 3% or more. (2) By weight, C: 0.17-0.29%, Si:<0.5%.

Contains Mn: 0.2 to 2%, and contains 0.2% or less of either or both of Al and Ti as a main component, and furthermore, B≦
0.02% and one or more of Cr, Cu, and Ni in a total amount of 2%, and the average grain size of 0.5 to 5% after hot rolling is 3μ
It has a structure consisting of ferrite of less than m and the remainder is mainly martensite, and has a tensile strength of 120 kg/ma+ after straightening.
``In summary, the ultra-high tensile strength PC steel wire with an excellent uniform elongation of 3% or more is a steel bar (3) by weight.

C: 0.17-0.29%, Si<0.5%.

Mn: 0.2 to 2%, and 0.2% or less of Al and/or Ti as a main component.
Hot working of 60% or more in total within 1 second in the temperature range from ℃ to 1000℃, followed by 50℃ within 0.5 seconds
A method for producing an ultra-high tensile strength PC steel wire or steel bar with excellent uniform elongation, characterized by cooling to 500°C or less at a cooling rate of /s or more, and (4) by weight, C20.17~ 0.29%, SL<0.5%.

Contains Mn: 0.2 to 2%, and contains 0.2% or less of either or both of Al and Ti as a main component, and furthermore, B≦
Steel containing 0.02% and one or more of Cr, Cu, and Nj in a total amount of 2% is subjected to hot working of a total of 60% or more within 1 second in a temperature range of 900°C to 1000°C, and then 500 at a cooling rate of 50℃ or more within 0.5 seconds
Ultra-high tensile strength PC steel wire formation with excellent uniform elongation characterized by cooling to below 0.degree. C. is a method for producing steel bars.

The present invention will be explained in detail below.

The present inventors previously disclosed in Japanese Patent Application No. 58-43813 a hot-rolled steel material having a tensile strength of 70 kg/mm2 or more, consisting of a small amount of ultra-fine grained ferrite and the balance bainite or martensite, and a method for manufacturing the same. Proposed.

The present invention relates to an improvement of the present invention, and relates to a steel wire or a steel bar having significantly high strength and high uniform elongation after straightening, and a method for manufacturing the same.

The basic idea of the present invention is that when low carbon steel is subjected to strong working at temperatures above the transformation point, fine ferrite is induced and generated by this working, and the fine ferrite is caused by non-recrystallization of austenite and subsequent cooling. This method is based on the ground-breaking knowledge that it is possible to obtain high strength and toughness by refining the bainite and martensite that is produced, but in doing so, cooling is strengthened to almost completely transform all but the fine ferrite into marten. It is based on the new findings that, when hardened to the site state, this martensite has a significantly finer texture, higher strength and better uniform elongation than without any ferrite.

The reasons for limiting the steel components of the present invention are as follows.

C is an element effective in increasing the strength of steel, and 0.1
If the content is less than 7%, it is not possible to stably obtain the target strength of 120 kg/mm2 or more even if complete hardening is performed. On the other hand, when the C content exceeds 0.29%, although the strength becomes sufficiently high, the uniform elongation tends to decrease slightly and the weldability also deteriorates.

Si is an element that is present to some extent in steel and contributes to steel strengthening and martensitic hardenability, but since it is a ferrite-forming element, in the steel of the present invention, if it is added in an amount exceeding 0.5%, it will cause deformation. If the amount of ferrite increases too much, the amount of martensite decreases, resulting in a decrease in the strength of the steel.

Mn is also contained to some extent in steel and is an element useful for improving hardenability, and in the steel of the present invention, at least 0.2% is required to ensure martensitic hardenability. However, 2%
If added in excess of 10%, Ar will lower the transformation point, making it difficult to form deformation-induced ferrite.

Either or both of Al and Ti are usually added to deoxidize steel. Furthermore, both A Q and Tx have the effect of fixing N, but if one of them exceeds 0.2%, the toughness of the steel is reduced, so one or more of them is limited to 0.2% or less.

Cr, Ni, Cu, and B all have the effect of improving the martensitic hardenability of steel, and are useful alloying elements for the steel of the present invention. Further, Cr is effective in improving corrosion resistance, and Ni and Cu are effective in improving low temperature toughness.

However, for the same reason as Mn, when Cr, Ni, and Cu are added in large amounts exceeding 2% in total, they become harmful.

If B is added in an amount exceeding 0.02%, coarse precipitates may be formed and toughness may deteriorate.

Next, a desirable manufacturing method for the steel of the present invention will be explained.

The steel of the present invention is usually produced by steelmaking means such as an LD converter, and is sometimes subjected to preliminary rolling to form a steel billet.
There are no restrictions on the history to reach the final processing stage, and any process may be used.

The steel of the present invention should consist essentially only of an austenite phase immediately before the final stage of hot working.

That is, Ar must be above the transformation point, but in order to generate a very small amount of deformation-induced ferrite,
It was found that the processing temperature range should be 900-1000°C. In a temperature range exceeding 1000°C, it is difficult to generate 0.5% or more deformation-induced ferrite, which is effectively effective.On the other hand, in a temperature range below 900°C, the amount of ferrite becomes too large at 5% or more. This causes a decrease in the strength of the steel.

Although deformation-induced ferrite is not produced unless a large reduction is applied, applying a large reduction in one bath is difficult in industrial rolling. However, in continuous hot rolling of wire rods, steel bars, etc., multiple reductions are applied within an extremely short period of time, so the cumulative strain produces the same effect as a large reduction in one pass. In the case of the steel of the present invention, it is only necessary to apply a total reduction of 60% or more within one second, and processing that applies a large reduction in such a short time can be achieved industrially by continuous hot rolling of wire rods, steel bars, etc. be done.

Such deformation-induced ferrite, which is generated by machining that applies a single pass or a large cumulative reduction, is usually a fine particle of about 0.5 to 2 μm immediately after generation, and basic research results indicate that it exists. This suppresses the softening caused by the recovery recrystallization of austenite processed under large pressure and preserves the processing strain of austenite, so that even after it transforms into martensite at low temperatures, this is carried over and produces a toughening effect due to processing heat treatment. It is.

Furthermore, after hot working, deformation-induced ferrite is extremely unstable in the temperature range of 900 to 1000°C above the equilibrium transformation point, and disappears if left in this temperature range for more than 0.5 seconds. Therefore, it is necessary to start rapid cooling within 0.5 seconds after hot working.

In addition, A, below the transformation point, fine ferrite becomes the core,
The pro-eutectoid ferrite starts to grow, but the ferrite is 3 μm thick.
If the steel becomes larger than the
It is necessary to continue rapid cooling to below ℃.

The higher the cooling rate is, the better; however, in order to achieve the above objective, a cooling rate of at least 50°C/s is required, and preferably 100°C/s or more.

For cooling in a temperature range of 500° C. or lower, rapid cooling may be necessary for steels with a low martensitic transformation point. In this case, the lower limit temperature for cooling is approximately 350° C. or higher, at which the bainite transformation rate is extremely slow.

The steel produced in this way has a
It has a structure consisting of 5% fine grains of 3 μm or less and the remainder mainly martensite. Depending on the steel composition and cooling rate, some bainite may be mixed into martensite, but this amount should be as small as possible, preferably 30% at most. However, it is usually difficult to quantitatively distinguish bainite and martensite using easily performed experimental methods such as optical microscopy. Therefore, we do not specify a clear amount.

Due to the presence of the fine ferrite, the martensitic structure becomes fine and dense, as shown in FIG. 1(a).

This effect becomes clear when compared with the structure shown in FIG. 1(b) obtained by quenching steel with the same composition through a normal heat treatment process.

Since the steel of the present invention has such a dense structure, it has high strength and exhibits excellent uniform elongation. These characteristics make the PC steel wire suitable for steel bars, as described above. In particular, P
When Cm wire or steel rods are manufactured in the form of coils, it is necessary to straighten them into a straight shape before use.
At that time, if the maximum plastic strain is about 2% and then heating is performed to a temperature range of 300 to 400°C to remove the strain, generally,
Ductility decreases due to strain aging phenomenon. However, in the steel of the present invention, the deterioration of uniform elongation due to strain aging is small. This is because, as mentioned above, since the structure is dense, localization of strain due to local stress concentration is difficult to occur.

[Example] Four types of steel having the compositions shown in Table 1 were melted in a large top-bottom blowing converter, continuously cast into a bloom with a cross section of 300 mm x 500 mm, and then bloomed into a bloom of 120 mm x 1 by blooming.
It was made into a billet with a cross section of 2 mm. This billet.

It is heated to 900 to 1100℃ in a heating furnace, and the final release rate is 26% by a 25-stage fully continuous wire mill for fragrances 1 to 3.
It was rolled at 0 m/s to obtain a wire rod with a diameter of 5.5 mm.

The final stage of the rolling is carried out in a 10 series block mill with a rolling process of approximately 0.
．． A cumulative pressure reduction of 87% was applied in 4 seconds.

The rolled wire rod enters the water-cooled tube in about 0.1 seconds, is cooled to about 400°C in about 0.7 seconds, is then wound into a loose coil, and then cooled to 300°C at a cooling rate of 12°C/s. Forced air cooling.

The material for flavoring 4 was rolled into a 16 mm diameter steel bar using the same rolling mill without passing through a block mill. At that time, a reduction of 61% was applied within 0.9 seconds in the final three passes.

Next, water cooling was started 0.4 seconds later, and the temperature was cooled to 430°C at a cooling rate of 60°C/s. Table 2 shows the rolling temperature and cooling conditions of each flavored steel, and the structure and mechanical properties of the obtained steel. The structure of the steel shown in Fragrance 1 is shown in Figure 1 (a), whereas the structure of the same steel heated at 900°C for 5 minutes and then water-cooled is shown in Figure 1 (b). This material and structure are shown as Comparative Material 7 in Table 2.

Comparative material 5 whose rolling temperature is higher than the temperature specified in the present invention and comparative material 7 obtained by reheating this material have poor uniform elongation, and comparative material 6 whose cooling stop temperature is high is insufficient in strength.

In contrast, Examples 1 to 4 of the present invention showed 1 even after strain aging.
It has a tensile strength of 25 to 190 kg/mm'' and a uniform elongation of 3% or more, demonstrating the remarkable effects of the present invention.

[Effects of the Invention] According to the present invention, it is possible to obtain an F'C steel wire or steel bar that has an extremely high strength level and excellent uniform elongation characteristics, and when this is used as a concrete reinforcing bar,
This has the effect of creating a highly safe prestressed concrete structure that is resistant to impact loads.

[Brief explanation of the drawing]

FIG. 1(a) is a photograph of the metal structure showing the martensitic structure of the steel of the present invention. FIG. 1(b) is a photograph of the metal structure obtained by quenching steel with the same composition as in FIG. 1(a) through a normal heat treatment process. It is. Patent applicant: Nippon A-Tetsu Co., Ltd.

Claims (4)

[Claims] (1) By weight, C: 0.17-0.29%, Si<0.5%, Mn: 0
．． 2 to 2%, and 0.2% or less of either or both of Al and Ti as a main component, and 0.5 to 5% of ferrite with an average grain size of 3 μm or less after hot rolling. An ultra-high tensile strength PC steel wire or steel rod having a structure in which the remainder is mainly martensite, has a tensile strength after straightening of 120 kg/mm^2 or more, and has an excellent uniform elongation of 3% or more. (2) By weight, C: 0.17-0.29%, Si: <0.5%, Mn:
0.2 to 2%, and contains 0.2% or less of either or both of Al and Ti as a main component, and furthermore, B≦
0.02% and one or more of Cr, Cu, and Ni in a total amount of 2%, and the average grain size of 0.5 to 5% after hot rolling is 3μ
It has a structure consisting of ferrite of less than m and the remainder is mainly martensite, and the tensile strength after straightening is 120 kg/mm^
Ultra-high tensile strength PC steel wire or steel bar with excellent uniform elongation of 2 or more and uniform elongation of 3% or more. (3) By weight, C: 0.17-0.29%, Si<0.5%, Mn: 0
．． 2% to 2% and 0.2% or less of either or both of Al and Ti as a main component.
Hot working of 60% or more in total within 1 second in the temperature range from ℃ to 1000℃, followed by 50℃ within 0.5 seconds
A method for producing an ultra-high tensile strength PC steel wire or steel bar with excellent uniform elongation, characterized by cooling to 500°C or less at a cooling rate of /s or more. (4) By weight, C: 0.17-0.29%, Si<0.5%, Mn: 0
．． 2 to 2%, and contains 0.2% or less of either or both of Al and Ti as a main component, and furthermore, B≦
Steel containing 0.02% and one or more of Cr, Cu, and Ni in a total amount of 2% is subjected to hot working of a total of 60% or more within 1 second in a temperature range of 900 ° C to 1000 ° C, and then 50 at a cooling rate of 50°C/s or more within 0.5 seconds
A method for producing ultra-high tensile strength PC steel wire or steel bar with excellent uniform elongation, characterized by cooling to below 0°C