JPH10317100A - Reinforcing bar with high strength and high toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a reinforcing bar having the specific desired characteristic values, yield stress, tensile strength, yield ratio, and elongation of a reinforcing bar having high strength and high toughness, while obviating the necessity of complicated manufacturing conditions, by specifying a chemical composition and regulating the ratio between martensitic and bainitic structures to a value in a specific range. SOLUTION: This reinforcing bar has a chemical composition consisting of, by weight, 0.15-0.35% C, 0.10-2.0% Si, 1.8-3.0% Mn, <=0.04% P, <=0.04% S, and the balance iron and containing, if necessary, 0.1-1.5% Cr and/or 0. 0005-0.0030% B, 0.010-0.050% Al, and 0.005-0.050% Ti and also has a structure which is composed of martensitic and/or bainitic structure and in which the proportion between them is regulated to 30-80%. This reinforcing bar has >=600 MPa yield stress, >=700 MPa tensile strength, <=80% yield ratio, >=5% elongation, etc. The above structure can be secured by subjecting a billet with the above composition to hot rolling and then to cooling down to 800-900 deg.C and applying air blast quenching to the resultant wire rod while performing ring spreading on a conveyer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-strength rebar for reinforcing concrete.

[0002]

2. Description of the Related Art In order to cope with the recent rapid increase in size and height of concrete structures, and to improve seismic resistance, reinforcing steel used for concrete reinforcement is required to have high strength and large diameter. And its usage is increasing.
To cope with this, many methods for producing high-strength low-yield ratio rebar have been proposed.

Japanese Patent Publication No. 56-1937 discloses that a rolling structure and a cooling condition are controlled to adjust a composite structure in which the periphery of a bar is made of martensite and bainite having a thickness of about 2 mm and the deep part is made of fine pearlite. A method for producing a high-strength rebar characterized by the following has been proposed.

In Japanese Patent Publication No. 62-86125, C: 0.20 to 0.60%, Si: 0.10 to 0.35
%, Mn: 0.30 to 1.80% by controlling the cooling conditions after hot rolling, tempering martensite in the surface layer, tempering martensite in the center, bainite or supercooled ferrite, pearlite structure. A method for producing a high-strength reinforcing bar characterized by the following has been proposed.

Further, Japanese Patent Publication No. 2-213415 discloses C: 0.20 to 0.60%, Si: 0.10 to 1.0%.
0%, Mn: 0.50 to 1.80% High strength rebar characterized by controlling the cooling conditions after hot rolling of steel to precipitate fine ferrite, granular carbide, and layered carbide in the surface layer. Has been proposed.

Further, Japanese Patent Publication No. 3-19764 discloses that C: 0.30 to 0.50%, Si: 0.60 to 1.
00%, Mn: 1.50 to 3.00%, Al: 0.01
0 to 0.100%, Ti: 0.005 to 0.050%,
V: 0.10 to 0.30%, Nb: 0.010 to 0.1
A method for manufacturing a high-strength rebar has been proposed in which the cooling conditions after hot rolling of a steel consisting of 00% are controlled, the ferrite has a pearlite structure, and the average pearlite grain size is less than 10 μm. .

[0007]

The above conventional manufacturing method requires extremely low-temperature rolling conditions, complicated controlled cooling after rolling, and requires the addition of expensive alloying elements. . It is an object of the present invention to provide high strength, high toughness rebar characteristic target values, yield stress ≧ 600 MPa, tensile strength ≧ 700 MPa, yield ratio ≦ 80 without requiring complicated manufacturing conditions.
%, Elongation ≧ 5%.

[0008]

In order to solve the above-mentioned problems, the present invention relates to a method for preparing C: 0.15 to 0.35% by weight.
Si: 0.10 to 2.0%, Mn: 1.8 to 3.0%,
P: 0.04% or less, S: 0.04% or less, the balance being iron and unavoidable impurities, at least one of martensite and bainite structures, and the ratio is 30 to 80% To provide a high-strength, high-toughness rebar having a structure in the range of: Depending on such components and structure, the yield strength of the rebar is ≧ 600 MPa, tensile strength ≧ 700 MPa, elongation ≧ 5.
% And other excellent mechanical properties. In the above-mentioned components, Mn is set in the range of 0.10 to 3.0%.
r: 0.1-1.5% and / or B: 0.0005-
0.0030%, Al: 0.010 to 0.050%, T
i: 0.005 to 0.050% may be contained.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted research to obtain a high-strength, high-toughness reinforcing bar having the above-mentioned target characteristics.
It has been found that this can be achieved by specifying the component range and the structure as described below. First, the reasons for limiting the components of the present invention will be described.

C: 0.15 to 0.35% C is an element necessary for enhancing hardenability and increasing strength. To secure high strength and high toughness of a reinforcing steel bar, C is required to be 0.1 to 0.35%. 15% or more is necessary. But 0.4%
Exceeds 0.35, the spot weldability deteriorates.
% As the upper limit.

Si: 0.10 to 2.0% Si is added in order to utilize the action as a deoxidizing element and a solid solution strengthening element, but 0.10% or more is necessary.
If added in excess of 2.0%, the ductility of the rebar deteriorates, so 2.0% was made the upper limit.

Mn: 1.8 to 3.0% or 0.10 to 0.10%
3.0% Mn is a deoxidizing element, and is an element necessary for enhancing hardenability and increasing strength. In order to secure strength as a high-strength rebar, Mn alone has a yield stress of ≧ 600 MPa and a tensile strength of 3.0%. To secure ≧ 700 MPa, 1.8% or more is required. Even if added in excess of 3.0%, the effect is saturated and the ductility of the reinforcing steel is also deteriorated. Therefore, the upper limit is set to 3.0%. When Cr and B are added within the range of the present invention to improve the hardenability, the lower limit of the amount of Mn added is set to 0.1.
It can be reduced to 10%. 3.0% even in that case
If the addition exceeds 3, the effect is saturated, and the ductility of the rebar is also deteriorated. Therefore, the upper limit is set to 3.0%.

P, S: 0.04% or less P, S is preferably as small as possible in order to ensure bending strength, ductility and the like. If P and S exceed 0.04%, they have an adverse effect on ductility and the like.

Cr: 0.1 to 1.5% Cr is an element that enhances hardenability and increases strength similarly to Mn. In order to exhibit the effect as a high-strength rebar,
0.1% or more is necessary. Even if added in excess of 1.5%, the effect is saturated and the ductility of the rebar also deteriorates, so the upper limit was 1.5%.

B: 0.0005% to 0.0030% B is an element that enhances the hardenability and increases the strength similarly to Cr and Mn. In order to exhibit the effect as a high-strength rebar, 0.0005% or more is required. is necessary. 0.0030%
If the content exceeds 0.003, the effect is saturated.
0% was made the upper limit.

Al: 0.010 to 0.050% Ti: 0.005 to 0.050% Al, Ti is added for the purpose of suppressing the reaction between B and N and effectively exhibiting the effect of improving the hardenability of B. Is what you do. A
If l is less than 0.010% and Ti is less than 0.005%, the effect cannot be sufficiently exhibited.
The effect is saturated even if it is added in excess of 50%.
The upper limit was 50%.

Next, the reason for limiting the organization will be described.
FIG. 1 shows the relationship between the martensite and bainite ratios and the mechanical properties of the reinforcing steel. It is necessary to secure a range of 30 to 80% as a ratio, which is a target value of high strength and high toughness, yield stress ≧ 600 MPa, tensile strength ≧ 700 MPa, yield ratio ≦ 80.
% And elongation ≧ 5% are indispensable. This structure is obtained by hot rolling a billet having the component range of the present invention, cooling the billet to a temperature in the range of 800 to 900 ° C., and then expanding the obtained wire in a ring on a conveyor. It can be ensured by cooling at a cooling rate of s or more, and does not require any complicated equipment, rolling and cooling conditions. Similar target characteristics were obtained even when the structure was martensite alone or bainite alone.

The effects of the present invention will be described below with reference to examples.

[0019]

EXAMPLES Table 1 shows the characteristics of reinforcing bars whose components and structures are adjusted within the scope of the present invention, and Table 2 shows examples outside the scope of the present invention. These rebars are cooled to 850 ° C. after hot rolling, then developed on a conveyor, and subjected to blast cooling to 3 ° C./s.
It is manufactured by cooling at a cooling rate of. Yield stress ≧ 6
Obviously, in order to achieve the target properties of 00 MPa, tensile strength ≧ 700 MPa, yield ratio ≦ 80%, and elongation ≧ 5%, it is necessary to define the components and the structure within the scope of the present invention.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

[Table 5]

[0025]

[Table 6]

[0026]

As explained in detail above, according to the present invention, high strength having the target properties of yield stress ≧ 600 MPa, tensile strength ≧ 700 MPa, yield ratio ≦ 80%, elongation ≧ 5%,
Since it becomes possible to provide high-toughness rebar stably and at low cost, and to dramatically improve the strength and safety of concrete structures, the social and economic value is extremely large.

[Brief description of the drawings]

FIG. 1 shows the effect of the structure on the mechanical properties of a reinforcing bar.

Claims (4)

[Claims] C: 0.15 to 0.35%, Si: 0.10 to 2.0%, Mn: 1.8 to 3.0%, P: 0.04% or less, by weight% S: 0.04% or less, the balance being iron and inevitable impurities, at least one of martensite and bainite structure,
And having a structure in which the ratio is in the range of 30 to 80%, yield stress ≧ 600 MPa, tensile strength ≧ 700 MP
a, High strength, high toughness rebar with elongation ≧ 5%. 2. In% by weight, C: 0.15 to 0.35%, Si: 0.10 to 2.0%, Mn: 0.10 to 3.0%, Cr: 0.1 to 1. 5%, P: 0.04% or less, S: 0.04% or less, the balance being iron and unavoidable impurities, at least one of martensite and bainite structure,
And having a structure in which the ratio is in the range of 30 to 80%, yield stress ≧ 600 MPa, tensile strength ≧ 700 MP
a, High strength, high toughness rebar with elongation ≧ 5%. 3. In% by weight, C: 0.15 to 0.35%, Si: 0.10 to 2.0%, Mn: 0.10 to 3.0%, B: 0.0005 to 0. 0030%, Al: 0.010 to 0.050%, Ti: 0.005 to 0.050%, P: 0.04% or less, S: 0.04% or less, with the balance being iron and unavoidable impurities And at least one of martensite and bainite structures,
And having a structure in which the ratio is in the range of 30 to 80%, yield stress ≧ 600 MPa, tensile strength ≧ 700 MP
a, High strength, high toughness rebar with elongation ≧ 5%. 4. In% by weight, C: 0.15 to 0.35%, Si: 0.10 to 2.0%, Mn: 0.10 to 3.0%, Cr: 0.1 to 1. 5%, B: 0.0005 to 0.0030%, Al: 0.010 to 0.050%, Ti: 0.005 to 0.050%, P: 0.04% or less, S: 0.04% With the balance comprising iron and unavoidable impurities, at least one of martensite and bainite structures,
And having a structure in which the ratio is in the range of 30 to 80%, yield stress ≧ 600 MPa, tensile strength ≧ 700 MP
a, High strength, high toughness rebar with elongation ≧ 5%.