JPH10121200A - Steel material for high strength shear reinforcement and method for producing the same - Google Patents

Abstract

(57) [Problem] To provide a steel material for high-strength shear reinforcement having excellent bending performance and weldability, and a method of manufacturing the steel material at low cost. The method has a specific chemical composition and a maximum particle size of 20 μm.
A steel material for high-strength shear reinforcement having a ferrite-pearlite structure or a ferrite-pearlite bainite structure containing 20 to 60% by area of ferrite having an average particle size of 15 µm or less with a mean particle size of 15 m or less. The manufacturing method is 1050-1
After heating to a temperature range of 250 ° C. and rough rolling, 700-1
In the temperature range of 000 ° C., rolling is performed so that the total rolling reduction of the intermediate rolling and the finish rolling is 10 to 97% in terms of a reduction in area.
Cool at a cooling rate of ° C / min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for high-strength shear reinforcement and a method for producing the same, and more particularly, to a steel for high-strength shear reinforcement having excellent bending performance and weldability, and the steel at low cost. It relates to a method of manufacturing.

[0002]

2. Description of the Related Art As the demand for high-rise buildings made of reinforced concrete excellent in earthquake resistance increases, shear reinforcing bars have been frequently used to secure the strength of the buildings. That is, "shear reinforcement" is used for the purpose of preventing and suppressing the destruction of a beam or a column of a reinforced concrete building due to shearing.

[0003] If the shear reinforcement has a much higher yield strength than the usual so-called "main reinforcement" as "bar for reinforced concrete" standardized by JIS G 3112,
It is possible to increase the allowable load on the building. Furthermore, if a shear reinforcement having a high strength, especially high yield strength is used, overcrowding of the shear reinforcement at the time of construction can be eliminated, and since the weight can be reduced, the workability can be greatly improved. .

[0004] Further, if a shear reinforcement having a large diameter is used,
The allowable load on the building can be further increased,
It is even more effective in eliminating overcrowding of the shear reinforcement.

[0005] However, if the strength of the shear reinforcement is only high, it is difficult to bend and fix it to the main reinforcement. That is, usually, the shear reinforcement is hot-rolled into a bar, and then given strength after being wound into a coil,
After that, it is straightened (the straightening to this straight line,
Hereinafter, this is referred to as “straightening”), and is bent into a spiral shape, cut into required dimensions, and provided for construction on site. Alternatively, it may be cut into a required length after the straightening and then subjected to bending in a ring shape or the like, and then welded to a so-called “closed streak”.

[0006] Under such circumstances, the shear reinforcing bars having excellent strength, bending workability, and weldability, particularly 1 mm in diameter, are required.
There is an increasing demand for large diameter shear reinforcing bars exceeding 3 mm. Specifically, the mechanical properties after straightening described above have a yield strength of 785 MPa or more and a tensile strength of 9 MPa.
A shear reinforcement having 30 MPa or more and elongation at break of 8% or more and excellent bending performance and welding performance is required.

Conventionally, as a method of producing such a steel material for high-strength shear reinforcement, it has been conventionally performed that after hot rolling, the steel is once cooled to room temperature, then reheated, quenched, and further tempered. . However, the above methods have problems that the heat treatment cost is increased and large-scale heat treatment equipment is required.

For this reason, a technique utilizing the sensible heat of a steel material during rolling is disclosed in, for example, Japanese Patent Application Laid-Open No. Sho 59-136428. The technology proposed in this publication is
After hot rolling steel having a specific chemical composition under specific conditions, perform in-line controlled cooling using blast or hot water to obtain a martensite-based structure and obtain a high-strength wire rod. It is assumed that.

However, when the structure of the wire is mainly composed of martensite just by cooling after adjustment, high strength can be obtained but ductility and toughness may be inferior. Neither did the performance required as a muscle be satisfied. Further, a structure mainly composed of martensite can be obtained by controlled cooling with blast or hot water in the case of a thin wire having a diameter of at most about 13 mm. That is, in the case of a wire having a large size such that the diameter exceeds 13 mm, a martensite-based structure is not necessarily obtained when cooled by the above-described means because the retained heat after hot rolling is large. , Strength, ductility and toughness sometimes varied greatly.

For this reason, it is also possible to secure the strength, ductility, and toughness by performing quenching treatment such as water quenching using the sensible heat of the steel material after rolling to form a martensite structure and tempering it off-line. Attempted. But,
In this case, it is necessary to perform the tempering off-line, so that the cost is increased, and it is not always possible to meet the demand of the industry to obtain a high-strength shear reinforcement at low cost.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a steel material for high-strength shear reinforcement having excellent bending performance and weldability, and a method for producing the steel material at low cost. The purpose is to do. Specifically, an object of the present invention is to determine that the mechanical properties after straightening have a yield strength of 785M.
Pa or more, tensile strength of 930 MPa or more, and elongation at break of 8%
As described above, a steel material for a high-strength shear reinforcing bar having excellent bending performance and welding performance, in particular, a steel material for a large-diameter shear reinforcing bar having a diameter exceeding 13 mm and a method for producing the steel at a low cost are provided. Is to do.

[0012]

The gist of the present invention resides in a method for producing a steel material for high-strength shear reinforcement as shown in the following (1) and a method for producing a steel material for high-strength shear reinforcement as shown in the following (2).

(1) In weight%, C: 0.15 to 0.50
%, Si: 0.15 to 1.50%, Mn: 0.30
2.50%, Cr: 0.02 to 2.00%, V: 0.0
1 to 0.50%, Nb: 0.01 to 0.50%, N:
0.005 to 0.02%, Cu: 0 to 0.50%, N
i: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 0
1.00%, Ti: 0 to 0.20%, Zr: 0 to 0.2
0% and Al: 0 to 0.065%, the balance being Fe
A ferrite-pearlite structure or a ferrite-pearlite bainite structure containing 20 to 60% by area of ferrite having a maximum particle size of 20 μm or less and an average particle size of 15 μm or less, which is composed of a chemical composition of unavoidable impurities. A steel material for high-strength shear reinforcing bars, characterized in that:

(2) A method for producing a steel material for a high-strength shear reinforcing bar, wherein the rolling step comprises rough rolling, intermediate rolling and finish rolling, wherein C: 0.15 to 0.50 by weight%
%, Si: 0.15 to 1.50%, Mn: 0.30
2.50%, Cr: 0.02 to 2.00%, V: 0.0
1 to 0.50%, Nb: 0.01 to 0.50%, N:
0.005 to 0.02%, Cu: 0 to 0.50%, N
i: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 0
1.00%, Ti: 0 to 0.20%, Zr: 0 to 0.2
0% and Al: 0 to 0.065%, the balance being steel having a chemical composition of Fe and unavoidable impurities, 1050 to 1
Rough rolling is performed by heating to a temperature range of 250 ° C.
In a temperature range of 0 to 1000 ° C, rolling is performed so that the total reduction amount of the intermediate rolling and the finish rolling is 10 to 97% in terms of a reduction in area,
Immediately thereafter, the temperature in the temperature range of 400 to 600 ° C.
A method for producing a steel material for a high-strength shear reinforcing bar, comprising cooling at a cooling rate of 600 ° C./min.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, “%” of the component content is “% by weight”.
Means

(A) Chemical composition of steel C: 0.15 to 0.50% C is an element effective for increasing the strength. However, if the content is less than 0.15%, a desired high strength cannot be obtained. On the other hand, if it exceeds 0.50%, the pearlite fraction (area ratio) increases, and consequently, the area ratio of ferrite decreases, resulting in deterioration of toughness and bending properties. Further, the content of C exceeding 0.50% causes deterioration of weldability. Therefore, the content of C is set to 0.15 to 0.5.
0%. In addition, the preferable content of C is 0.20 to 0.20.
0.50%.

Si: 0.15 to 1.50% Si is an element effective for stabilizing the deoxidation of steel and improving the strength. However, if the content is less than 0.15%, the desired effect cannot be obtained, and if it exceeds 1.50%, the toughness is reduced. For this reason, the content of Si is set to 0.15.
-1.50%.

Mn: 0.30 to 2.50% Mn is an element necessary for increasing the strength, and for that purpose, the content must be 0.30% or more.
On the other hand, if the content exceeds 2.50%, the hardenability becomes extremely high, and the desired structure and mechanical properties cannot be obtained. Therefore, the content of Mn is reduced to 0.30 to 2.50%.
And

Cr: 0.02 to 2.00% Cr has the effect of increasing the strength similarly to Mn. Furthermore,
It also has the effect of making pearlite colonies smaller and increasing ductility. However, if the content is less than 0.02%, the above effects cannot be obtained. On the other hand, when the content exceeds 2.00%, the hardenability is remarkably increased, and the desired structure and mechanical properties cannot be obtained. Therefore, the content of Cr is set to 0.1.
02 to 2.00%.

V: 0.01 to 0.50% V transforms an austenite phase to a ferrite phase, and its nitride and carbonitride are dispersed and precipitated in the ferrite phase to strengthen ferrite. Further, it has the effect of promoting the refinement of crystal grains and improving the strength while maintaining ductility.
However, if the content is less than 0.01%, the desired effect cannot be obtained. If the content exceeds 0.50%, the effect of improving strength is saturated, and only the production cost is increased. Therefore, the content of V is set to 0.01 to 0.50%. In addition, the preferable content of V is 0.01 to 0.30%.

Nb: 0.01 to 0.50% Nb is a very important element whose nitride or carbonitride contributes to suppressing coarsening of austenite crystal grains and contributing to precipitation strengthening. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, if the content exceeds 0.50%, the effect of improving the strength is saturated and only the production cost is increased. Therefore, the content of Nb is set to 0.01 to
0.50%. The preferable content of Nb is 0.1.
01 to 0.40%.

N: 0.005 to 0.02% N forms a carbonitride in ferrite with V and Nb described above, and has the effect of increasing the strength and refining the crystal grains to strengthen the steel. . However, its content is 0.005
%, The desired effect cannot be obtained, and if it exceeds 0.02%, the toughness is rather lowered. Therefore, the content of N is set to 0.005 to 0.02%.

Cu: 0 to 0.50% Cu need not be added. Addition has the effect of increasing the strength. In order to ensure this effect, Cu should be set to 0.1.
The content is preferably at least 02%. However, when the content exceeds 0.50%, the above effect is saturated.
In addition, the hot workability is deteriorated, which leads to an increase in cost. Therefore, the content of Cu is set to 0 to 0.50%.
And

Ni: 0 to 0.50% Ni may not be added. Addition has the effect of increasing the strength. To ensure this effect, Ni should be 0.02
% Is preferable. However, when the content exceeds 0.50%, the above-mentioned effect is saturated and the cost is increased. Therefore, the content of Ni is set to 0 to 0.50%.

Mo: 0 to 0.50% Mo need not be added. Addition has the effect of increasing the strength. To ensure this effect, Mo should be set to 0.
The content is preferably at least 02%. But,
If the content exceeds 0.50%, not only the above effect is saturated, but also the hardenability is remarkably increased, and the desired structure and mechanical properties cannot be obtained. Therefore, the content of Mo is set to 0
０．５0.50%.

W: 0 to 1.00% W may not be added. Addition has the effect of increasing the strength. To ensure this effect, W should be 0.02
% Is preferable. However, when the content exceeds 1.00%, the above-mentioned effect is saturated, and the cost is increased. Therefore, the content of W is set to 0 to 1.00%.

Ti: 0 to 0.20% Ti need not be added. When added, they form nitrides and carbonitrides, contribute to the refinement of crystal grains and strengthen precipitation, and have the effect of toughening steel. In addition, since the nitride of Ti has a relatively small solubility product, it is stably present in the steel.
The precipitation of coarse nitride of Nb is suppressed, and the fine precipitation of carbonitride of V and Nb is promoted. In order to surely obtain such an effect, the content of Ti is preferably set to 0.01% or more. However, when the content exceeds 0.20%, the toughness is rather deteriorated. Therefore, the content of Ti is reduced to 0.
０．２0.20%.

Zr: 0 to 0.20% Zr may not be added. If added, it has the effect of forming nitrides and carbonitrides to refine crystal grains and toughening steel. In order to surely obtain this effect, the content of Zr is preferably set to 0.01% or more. However, when the content exceeds 0.20%, the toughness is rather deteriorated. Therefore, the content of Zr is set to 0 to 0.20%.

Al: 0 to 0.065% Al may not be added. When added, it has the effect of stabilizing the deoxidation of steel. Further, it has an effect of forming nitride by reacting with N to make crystal grains fine. In order to surely obtain such an effect, the content of Al is preferably set to 0.005% or more. However, if the content exceeds 0.065%, the formation of nitrides and carbonitrides of V and Nb effective for strengthening is inhibited, and the strength is reduced. Therefore, the content of Al is set to 0 to 0.065%. The upper limit of the Al content is preferably set to 0.05%.

(B) Structure of Steel Material As will be described in detail in Examples, the steel material has a predetermined chemical composition, and the structure is such that ferrite having a maximum grain size of 20 μm or less and an average grain size of 15 μm or less has an area ratio of 20%. In the case of a ferrite-pearlite structure or a ferrite-pearlite bainite structure containing up to 60%, even after straightening with a roller leveler or a wire straightener, for example, a large elongation at break of 8% or more is stable in a steel material. High strength and good bending properties.

That is, in order to impart desired strength, elongation and bending characteristics to the steel material after straightening, the structure of the steel material must first be a ferrite-pearlite structure or a ferrite-pearlite bainite structure. Here, "ferrite-pearlite structure" refers to a mixed structure of ferrite and pearlite, and "ferrite-pearlite bainite structure" refers to a mixed structure of ferrite, pearlite, and bainite.

In the above structure, when the grain size of the ferrite is large, that is, the maximum grain size of the ferrite is 20 μm.
If the average particle diameter exceeds 15 μm or the average particle diameter exceeds 15 μm, ductility and strength are reduced, and desired characteristics cannot be obtained. Therefore, the ferrite is specified to have a maximum particle size of 20 μm or less and an average particle size of 15 μm or less.

When the area ratio of the ferrite having the maximum particle size of 20 μm or less and the average particle size of 15 μm or less is less than 20%, the ductility is reduced and the elongation at break of 8% or more is obtained after straightening. Is no longer available, while the area ratio is 60%
If it exceeds, the strength is lowered and a desired high strength cannot be obtained after straightening. Therefore, the area ratio of the ferrite having the maximum particle diameter of 20 μm or less and the average particle diameter of 15 μm or less is 2
0 to 60%. The area ratio of the ferrite is 30 to 5
It is preferably set to 0%.

When the structure of the steel material is a structure containing bainite (a ferrite-pearlite-bainite structure), ductility and toughness are ensured, and the elongation at break of 8% or more and good bending characteristics after straightening are ensured. In order to achieve this, the area ratio of bainite is preferably set to 20% or less.

(C) Hot Rolling (C-1) Heating In the present invention, high strength is achieved by utilizing precipitation hardening of Nb and V. For this purpose, it is necessary to sufficiently dissolve Nb and V in austenite during heating before rolling (before rough rolling). Therefore, the steel having the above chemical composition is heated to 1050 ° C. or higher. On the other hand, when heating at a high temperature exceeding 1250 ° C., austenite grains are remarkably coarsened, and a desired structure and mechanical properties cannot be obtained. Therefore, the heating of rolling was limited to a temperature range of 1050 to 1250 ° C.

(C-2) Intermediate Rolling and Finish Rolling The intermediate rolling and finish rolling after heating to the above-mentioned temperature range and rough rolling are carried out from fine austenite recrystallized grains to fine ferrite / pearlite structure or fine ferrite / pearlite. In order to obtain a bainite structure and to further finely precipitate nitrides and carbonitrides of Nb and V, the total reduction in the temperature range of 700 to 1000 ° C. is 10 to 97% in terms of reduction in area. There is a need.

If the above-mentioned rolling is carried out at a temperature exceeding 1000 ° C., recrystallization becomes remarkable, a fine structure cannot be obtained, and desired mechanical properties cannot be achieved after straightening. On the other hand, 7
In the rolling at a temperature lower than 00 ° C., the deformation resistance of the steel material increases, and the load on the rolling mill becomes extremely large. Therefore, the temperature range of the rolling was set to 700 to 1000 ° C.

The total rolling reduction in the above-mentioned rolling is 1 in terms of reduction in area.
If it is less than 0%, fine recrystallized grains cannot be obtained and the desired structure cannot be obtained. Therefore, desired mechanical properties cannot be obtained after straightening. On the other hand, the total reduction amount is 97
%, The cumulative strain due to recrystallization increases and Nb
And V are coarsely precipitated in the steel, and the desired mechanical properties cannot be achieved after straightening. Therefore, the total rolling reduction of the rolling was set to 10 to 97% in terms of a reduction in area. A preferable value of the total rolling reduction in the rolling is 30 to 91% in terms of a reduction in area.

(D) Cooling after Rolling After the hot rolling is completed, it is necessary to control the cooling rate immediately to cool the steel sheet. If the cooling rate of this cooling is less than 5 ° C./min, a desired fine structure cannot be obtained, and desired mechanical properties cannot be achieved after straightening. On the other hand, 600 ° C /
In the case where the amount exceeds minutes, a so-called “low-temperature transformation structure” such as bainite or martensite is formed, and desired mechanical properties cannot be obtained after straightening. Therefore, the cooling rate after rolling must be 5 to 600 ° C./min.

Cooling at the above-mentioned cooling rate is carried out immediately after rolling.
It is necessary to perform the process up to a temperature in the temperature range of 00 to 600 ° C. When the cooling is stopped at a temperature exceeding 600 ° C.,
The desired ferrite / pearlite structure or ferrite / pearlite bainite structure cannot be obtained. Meanwhile, 4
If the cooling is performed at the above-mentioned cooling rate to a temperature in a temperature range lower than 00 ° C., the area ratio of a so-called “low-temperature transformation structure” such as bainite or martensite increases, and a desired structure cannot be obtained. For this reason, in any case, desired mechanical properties cannot be achieved after straightening. The cooling after cooling under the above conditions may be allowed to cool.

(A) having the chemical composition described in (A),
The steel material for high-strength shear reinforcement having the following structure or the steel material for high-strength shear reinforcement having the chemical composition described in (A) and manufactured by the method described in (C) and (D) is The straightening process is performed by a normal method using a roller leveler, a wire straightener, or the like without undergoing an off-line tempering process. This straightened steel material has a yield strength of 7
It is 85 MPa or more, has a tensile strength of 930 MPa or more, and has a breaking elongation of 8% or more, and has excellent bending performance and welding performance. This steel material is then bent into a spiral shape, cut into required dimensions, and finished into a so-called "spiral bar" as a shear reinforcing bar. Further, after being cut to a required length, it is bent into a required shape such as a ring shape and then welded to be finished into a so-called "closed reinforcing bar". Alternatively, after being cut to a required length, it is bent into a required shape and further attached with a hook to be finished into a so-called "reinforcement with a hook".

[0042]

【Example】

(Example 1) Steels having the chemical compositions shown in Tables 1 and 2 were melted by an ordinary method. In Tables 1 and 2, steels A to Q are steels having the chemical composition specified in the present invention, and steels a to h are steels in which one of the components is out of the content range specified in the present invention.

[0043]

[Table 1]

[0044]

[Table 2]

Next, these steels were turned into billets by a usual method, and then formed into a wire rod having a diameter of 16 mm by hot rolling comprising the steps of rough rolling, intermediate rolling and finish rolling, then cooled, and wound into a coil. A steel material for shear reinforcement was manufactured.

Table 3 shows hot rolling conditions and cooling conditions. The cooling after “the temperature at which the cooling was completed” described in Table 3 was allowed to cool.

[0047]

[Table 3]

A microstructure observation test piece was cut out from the above steel material and microstructure observation was performed. The structure was determined by observing a portion 4 mm from the surface of a steel material having a diameter of 16 mm with an optical microscope.

Table 4 shows the results of the organization survey. In the microstructure column of Table 4, F means ferrite, P means pearlite, and B means bainite.

[0050]

[Table 4]

Next, the diameter 16 m obtained as described above was used.
m of the steel for shear reinforcement was straightened straight by a normal method using a straightener (roller leveler or wire straightener) of the type described in Table 3. The elongation in the above correction is about 0.4% on average. Thereafter, the straightened steel material was subjected to an actual tensile test and an actual bending test.

The test results are shown in Table 4 above. The bending characteristics were evaluated based on the presence or absence of cracking after bending by 180 degrees with a bending radius of 48 mm. In the table, "○"
Indicates that no cracks occurred, and “x” indicates that cracks occurred.

From the results shown in Table 4, it can be seen that a steel material having the chemical composition specified by the present invention and subjected to the “hot rolling-cooling” treatment under the conditions specified by the present invention (test numbers 1 to 5). 1
In 3), since it has a specified structure, after straightening, a predetermined yield strength of 785 MPa or more, a tensile strength of 930 MPa or more, and a breaking elongation of 8% or more are obtained, and the bending properties are also excellent. it is obvious.

Next, the straightened steel material of Test Nos. 1 to 13 according to the present invention was cut into a length of 500 mm, and the electrode interval was 45 mm and the welding interval was 25 using a pneumatic butt welding machine.
A welding test was performed under normal welding conditions of mm, a pressure of 5 kgf / mm ² , and a current of 5 kA. As a result, it was confirmed that welding could be performed without any welding cracks or poor joining, and that the weldability was good.

[0055]

The steel material for high-strength shear reinforcement according to the present invention is excellent in strength, bending properties and weldability, so that shear reinforcement that prevents and suppresses destruction of beams and columns of a reinforced concrete building due to shearing. Can be used for business. This steel material for high-strength shear reinforcement can be relatively easily manufactured at low cost by the method of the present invention.

Claims (2)

[Claims] C .: 0.15 to 0.50% by weight, S
i: 0.15 to 1.50%, Mn: 0.30 to 2.50
%, Cr: 0.02 to 2.00%, V: 0.01 to 0.
50%, Nb: 0.01 to 0.50%, N: 0.005
-0.02%, Cu: 0-0.50%, Ni: 0-0.
50%, Mo: 0 to 0.50%, W: 0 to 1.00%,
Ti: 0 to 0.20%, Zr: 0 to 0.20% and A
l: 0 to 0.065%, the balance being the chemical composition of Fe and unavoidable impurities, with a maximum grain size of 20 μm
A ferrite-pearlite structure or a ferrite-pearlite bainite structure having a ferrite pearlite structure or a ferrite pearlite bainite structure containing ferrite having an average particle diameter of 15 μm or less in an area ratio of 20 to 60%. 2. A method for producing a steel material for high-strength shear reinforcing bars, wherein the rolling step comprises the steps of rough rolling, intermediate rolling and finish rolling, wherein C: 0.15 to 0.50% by weight. S
i: 0.15 to 1.50%, Mn: 0.30 to 2.50
%, Cr: 0.02 to 2.00%, V: 0.01 to 0.
50%, Nb: 0.01 to 0.50%, N: 0.005
-0.02%, Cu: 0-0.50%, Ni: 0-0.
50%, Mo: 0 to 0.50%, W: 0 to 1.00%,
Ti: 0 to 0.20%, Zr: 0 to 0.20% and A
l: 0 to 0.065%, the balance being steel having a chemical composition consisting of Fe and unavoidable impurities, heated to a temperature range of 1050 to 1250 ° C to perform rough rolling, and then 700 to 100
In a temperature range of 0 ° C., rolling is performed so that the total reduction amount of the intermediate rolling and the finish rolling is 10 to 97% in terms of a reduction in area, and immediately thereafter, the temperature is reduced to 5 to 600 ° C./400° C./600° C.
A method for producing a steel material for a high-strength shear reinforcing bar, comprising cooling at a cooling rate of 1 minute.