JPH0759738B2 - Ultra-high-strength PC steel wire or steel bar excellent in uniform elongation and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a PC steel wire or steel rod having excellent uniform elongation and tensile strength of 120 kg / mm ² or more, and a method for producing the same. is there.

[Prior Art] Recently, as steel structures become huge, the steel materials used tend to have higher strength. In particular, the PC steel wire used as a concrete reinforcing bar is required to have a maximum tensile strength of 145 kg / mm ² or more as specified in JIS G 3109.

In general, the strength and ductility of steel materials are incompatible with each other, and the uniform elongation (uniform elongation) is extremely deteriorated as the strength of the steel increases.

Conventionally, heat treatment PC steel rod used in the pre-stressed concrete structure, steel wire, after the hot-rolled and reheated to A ₃ transformation point or above the temperature, without a martensitic structure by quenching treatment Then, it has been manufactured by a process of performing tempering treatment for improving the yield point and recovering the toughness. However, when trying to obtain a high strength level by such a tempered martensite structure, the uniform elongation is 3 to 5% after quenching → tempering, and further,
1 to 1.5% deterioration in the process of straightening → bluing.

Such deterioration of uniform elongation may also cause a steel rod, which is a reinforcing bar, to break when the concrete structure is subjected to an impact load, etc., and can no longer function as a reinforcing bar, possibly leading to a life-threatening accident. Become.

It is known that a PC steel wire or steel bar that does not have the risk of such an accident requires a high uniform elongation (uniform elongation) in a used state. Here, the usage state is a state after the steel wire or the steel bar generally supplied in a coil state is straightened and the elongation property is deteriorated.

The inventors of the present invention previously disclosed Japanese Patent Application No. 56-5929 and Japanese Patent Application No.
In 205709, C: 0.15 to 0.80% steel containing Si ≧ 0.5% is hot-rolled, then rapidly cooled to an appropriate temperature range of 500 ° C. or less, and held in that temperature range to obtain bainite-based or bainite-based. And steels having a mixed structure of martensite or retained austenite are obtained.
It has been disclosed that it has a high strength of 100 kg / mm ² or more, is excellent in uniform elongation, and satisfies the above target properties. However, in order to stably generate bainite to obtain these steels, it is necessary to add a relatively large amount of an alloying element such as Si to the composition, which is expensive.

In addition, there is a disadvantage that only the rolling equipment having a special direct heat treatment equipment can be used for the rapid cooling and the retention. Further, when it is necessary to stabilize the characteristics by allowing a large amount of retained austenite to exist, the C content is set to 0.
It is necessary to make it 3% or more, but then there is a drawback that weldability deteriorates and some restrictions are imposed when making a frame with PC steel wire or steel rod.

[Problems to be Solved by the Invention] An object of the present invention is to provide a steel material that fundamentally solves the problems in the prior art as described above and a manufacturing method thereof. More specifically, C: 0.17 to 0.29% low carbon and relatively low alloy steel with ultrahigh tensile strength of 120 kg / mm ² or more immediately after
It is an object of the present invention to provide a steel wire or a steel bar having a uniform elongation of more than 3.5% and a method for producing the same, which comprises hot rolling and cooling conditions without using special heat treatment equipment.

[Means for Solving the Problems] (1) By weight, C: 0.17 to 0.29%, Si <0.5%, Mn: 0.2 to 2%, and 0.2% A structure containing one or both of the following Al and Ti, and B ≦ 0.02% as a main component, and 0.5 to 5% of ferrite having an average grain size of 3 μm or less after hot rolling and the balance mainly composed of martensite. Has a tensile strength of 120 kg / mm ² or more immediately after grading and a uniform elongation of 3.5%
It is a super-high-strength PC steel wire or steel bar with excellent uniform elongation that is super (2) By weight, it contains C: 0.17-0.29%, Si <0.5%, Mn: 0.2-2%, and 0.2 % Or less of Al and / or Ti, and B ≦ 0.02% as a main component, and further contains one or more of Cr, Cu, and Ni in a total amount of 2% or less, and after hot rolling. It has a structure of 0.5 to 5% ferrite with an average grain size of 3 μm or less and the balance mainly of martensite, and has a tensile strength of 120 kg / mm ² or more immediately after shaving and a uniform elongation of 3.5%.
It is an ultra-high-strength PC steel wire or steel rod with excellent uniform elongation that is super (3) By weight, it contains C: 0.17-0.29%, Si <0.5%, Mn: 0.2-2%, and 0.2 % Or less Al and / or Ti, and steel containing B ≦ 0.02% as the main component, a total of 6 within 1 second in the temperature range of 900 ° C to 1000 ° C.
Add 0% or more of hot working and continue to 50 ℃ within 0.5 seconds
It is a manufacturing method of super-high-strength PC steel wire or steel bar with excellent uniform elongation, which is characterized by cooling to 500 ℃ or less at a cooling rate of / s or more. (4) By weight, C: 0.17 to 0.29% , Si <0.5%, Mn: 0.2 to 2%, at least one or both of Al and Ti of 0.2% or less, and B ≦ 0.02% as main components, and Cr, Cu, Steel containing 1% or more of Ni in a total amount of 2% or less is subjected to hot working in a temperature range of 900 ° C to 1000 ° C within 1 second in a total amount of 60% or more, and then within 50 seconds at 50 ° C / s. A method for producing an ultra-high-strength PC steel wire or steel bar with excellent uniform elongation, which is characterized by cooling to 500 ° C or less at the above cooling rate.

The present invention will be described in detail below.

The inventors of the present invention previously disclosed that in Japanese Patent Application No. 58-43813, the tensile strength was 7
We proposed a hot-rolled steel material containing 0 kg / mm ² or more, consisting of a small amount of ultrafine-grained ferrite and the balance bainite or martensite, and a method for producing the hot-rolled steel material.

The present invention relates to an improvement of the present invention, and relates to a steel wire or a steel bar having a remarkably high strength and a high uniform elongation immediately after being pressed and a method for producing the same.

The basic idea of the present invention is that when low-carbon steel is strongly worked at a temperature above the transformation point and above, fine ferrite is induced by this working, and the fine ferrite is generated by unrecrystallization of austenite and subsequent cooling. Starting from the epoch-making finding that the generated bainite and martensite are refined, and thereby high strength and toughness can be obtained, at that time, cooling is strengthened to almost completely eliminate martensite except fine ferrite. It is based on the new finding that when quenched to the site state, this martensite has a significantly finer microstructure, higher strength and better uniform elongation than would be the case without any ferrite.

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

C is an element effective in increasing the strength of steel, and 0.17%
If the content is less than 120k, the target is 120k even after complete quenching.
A strength of g / mm ² or more cannot be stably obtained. On the other hand, when the content of C exceeds 0.29%, the strength becomes sufficiently high, but the uniform elongation tends to slightly decrease with it, and the weldability also deteriorates.

Si is an element that is contained in the steel to some extent and contributes to the strengthening of the steel and the hardenability of martensite, 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%, the amount of work-induced ferrite is increased. Is too much, the amount of martensite decreases, and the strength of the steel decreases.

Mn is also contained in the steel to some extent and is an element useful for improving the hardenability. In the steel of the present invention, at least 0.2% is necessary to secure the martensite hardenability. However, if added in excess of 2%, the Ar ₃ transformation point is lowered and it becomes difficult to form work-induced ferrite.

One or both of Al and Ti are usually added for deoxidizing the steel. Both Al and Ti have the effect of fixing N, but if one of them exceeds 0.2%, the toughness of the steel will decrease, so one or more of them are limited to 0.2% or less.

Cr, Ni, Cu, and B each have an effect of improving the martensite 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, Cr, Ni, and Cu are rather harmful if added in a large amount in excess of 2% for the same reason as Mn.

If B is added in an amount of more than 0.02%, coarse precipitates may be formed and toughness may be deteriorated.

Next, a desirable method for producing the steel of the present invention will be described.

The steel of the present invention is produced by a steel making means such as an LD converter, and is usually preliminarily rolled into a billet, and there is no restriction on the history to reach the final processing stage. , It doesn't matter what process.

The steel of the present invention should consist essentially of the austenite phase immediately before the final stage hot working. That is, it is necessary to have the Ar ₃ transformation point or higher, but it has been found that the working temperature range should be 900 to 1000 ° C. in order to generate a very small amount of the work-induced ferrite. Substantially effective in the temperature range over 1000 ° C
It is difficult to generate processing-induced ferrite of 0.5% or more,
On the other hand, in the temperature range of less than 900 ° C, the amount of ferrite becomes too large at 5% or more, and the strength of the steel decreases.

Work-induced ferrite is not generated unless large reduction is applied, but it is difficult to apply large reduction in one pass in industrial rolling. However, in continuous hot rolling of a wire rod, a steel bar and the like, since a plurality of reductions are applied within an extremely short time, cumulative strain has the same effect as a one-pass large reduction. In the case of the steel of the present invention, a total of 60% or more may be processed within 1 second, and the processing of applying a large reduction in such a short time is industrially achieved by continuous hot rolling of wire rods and bars. To be done.

The work-induced ferrite generated by such a process of applying one pass or cumulative large reduction is usually as fine as 0.5 to 2 μm immediately after the generation, and according to the basic research results, the existence of this Since the softening of the austenite processed by large reduction is suppressed by the recovery recrystallization and the processing strain of the austenite is preserved, it is inherited even after transformation into martensite at a low temperature, and the toughening effect by the thermomechanical treatment is produced.

Furthermore, after hot working, the work-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. It is necessary to start quenching within 0.5 seconds after hot working.

At a transformation temperature of A ₃ or lower, fine ferrite becomes nuclei and pro-eutectoid ferrite starts to grow, but if ferrite exceeds 3 μm and grows and bainite is formed, the strength of steel after quenching decreases. Therefore, it is necessary to continue the rapid cooling down to 500 ° C or less.

The cooling rate at this time is preferably as high as possible, but in order to achieve the above-mentioned object, a cooling rate of at least 50 ° C / s is required, and preferably 100 ° C / s or more.

Cooling in a temperature range of 500 ° C. or lower may also require rapid cooling for a steel having a low martensite transformation point. In this case, the lower limit cooling temperature is about 350 ° C. or higher at which the bainite transformation rate becomes extremely slow.

The steel produced in this way is 0.5 to 5 as described above.
%, Having a fine grain of 3 μm or less and the balance mainly consisting of martensite. Depending on the steel composition and cooling rate, some bainite may be mixed in martensite, but this amount should be as small as possible, at most 30
% Is desirable. However, it is often difficult to quantitatively distinguish bainite and martensite by an easily-executable experimental method such as an optical microscope. Therefore, there is no definite amount.

Due to the presence of the fine ferrite, the martensite structure becomes fine and dense. The effect of this is clear when compared with a structure in which steel of the same composition is quenched in a normal heat treatment process.

Since the steel of the present invention has such a dense structure, it has high strength and excellent uniform elongation. Such characteristics are suitable for the PC steel wire or steel bar as described above. In particular, when it is manufactured in the form of a coil of PC steel wire or steel rod, it is necessary to straighten it before use, but at that time, it receives a plastic strain of up to about 2% and further strain relief. For
When heated in the temperature range of 300 to 400 ° C, the ductility is generally reduced due to the strain aging phenomenon. Therefore, in the steel of the present invention, deterioration of uniform elongation due to strain aging is small. This is because, as described above, the strain is localized due to local stress concentration due to the dense structure.

[Examples] Three types of steel having the composition shown in Table 1 were melted in a large-scale upper-bottom blowing converter, continuously cast into a bloom of 300 mm x 500 mm cross section, and then billet of 120 mm x 120 mm cross section was obtained by slab rolling. And This billet was heated to 900 to 1100 ° C. in a heating furnace, and those of trial Nos. 1 and 3 were rolled at a final discharge speed of 60 m / s by a 25-stage fully continuous wire rod mill to form a wire rod having a diameter of 5.5 mm.
The final stage of the rolling is a block mill of 10 stations, about 0.4
A cumulative reduction of 87% was applied during the second.

The rolled wire rod enters the water cooling tube in about 0.1 seconds and
It was cooled to 400 ° C., then wound on a loose coil and then forced air cooled to 300 ° C. at a cooling rate of 12 ° C./s.

The material of trial number 2 was rolled into a 16 mmφ steel rod in the same rolling mill without passing through a block mill. At that time, 0.9 in the last 3 passes
Within a second, a 61% reduction was applied.

Then, after 0.4 seconds, water cooling is started and the cooling rate is 60 ° C / s.
Cooled to 30 ° C. Table 2 shows each trial number The rolling temperature and cooling conditions of steel and the structure and mechanical properties of the obtained steel are shown.

The comparative material 4 having a rolling temperature higher than the temperature defined in the present invention and the comparative material 6 obtained by reheating the same have poor uniform elongation, and the comparative material 5 having a high cooling stop temperature has insufficient strength.

On the contrary, in Examples 1 and 2 of the present invention, even after strain aging, 12
Since it has a tensile strength of 0 kg / mm ² or more and a uniform elongation of more than 3.5%, the effect of the present invention is remarkable.

[Effect of the Invention] According to the present invention, it is possible to obtain a PC steel wire or steel rod having an extremely high strength level and excellent in uniform elongation characteristics. It has the effect of being a prestressed concrete structure that is strong and highly safe.

Claims (4)

[Claims] 1. By weight, C: 0.17 to 0.29%, Si <0.5%, Mn: 0.2 to 2%, and 0.2% or less of one or both of Al and Ti, and B ≦ 0.02% as a main component, 0.5 to 5% after hot rolling, having a structure of ferrite having an average grain size of 3 μm or less and the balance mainly composed of martensite, and having a tensile strength immediately after compression of 120 kg / mm ² or more, Uniform elongation 3.5%
Ultra-high-strength PC steel wire or steel bar with superb uniform elongation. 2. By weight, it contains C: 0.17 to 0.29%, Si <0.5%, Mn: 0.2 to 2%, 0.2% or less of either or both of Al and Ti, and B ≦ It contains 0.02% as a main component, and further contains one or more of Cr, Cu, and Ni in a total amount of 2% or less. After hot rolling, 0.5 to 5% of ferrite having an average grain size of 3 μm or less and the balance are mainly martens. It has a structure consisting of sites, and has a tensile strength of 120 kg / mm ² or more immediately after coring and a uniform elongation of 3.5.
% Ultra-high-strength PC steel wire or steel bar with excellent uniform elongation of over%. 3. By weight, it contains C: 0.17 to 0.29%, Si <0.5%, Mn: 0.2 to 2%, 0.2% or less of either or both of Al and Ti, and B ≦ Steel containing 0.02% as the main component, a total of 6 within 1 second in the temperature range of 900 ℃ to 1000 ℃.
Add 0% or more of hot working and continue to 50 ℃ within 0.5 seconds
A method for producing an ultra-high-strength PC steel wire or steel bar with excellent uniform elongation, which comprises cooling to 500 ° C or less at a cooling rate of / s or more. 4. By weight, C: 0.17 to 0.29%, Si <0.5%, Mn: 0.2 to 2%, and 0.2% or less of either or both of Al and Ti, and B ≦ Steel containing 0.02% as the main component and 1% or more of Cr, Cu, Ni in a total amount of 2% or less in total within 60 seconds within a temperature range of 900 ° C to 1000 ° C. A method for producing ultra-high-strength PC steel wire or steel bar with excellent uniform elongation, characterized by adding cold working and subsequently cooling to 500 ° C or less at a cooling rate of 50 ° C / s or more within 0.5 seconds