WO2017222122A1 - Reinforcing bar and manufacturing method therefor - Google Patents

Abstract

According to an embodiment, a manufacturing method for a reinforcing bar comprises the steps of: reheating a billet having a predetermined alloy composition at a temperature of 1100°C to 1200°C; hot rolling the reheated billet at a finish rolling temperature of the Ac1 point to 1000°C; cooling the hot rolled steel to a surface temperature of Ms-100 (°C) to Ms (°C) through a Tempcore process; and subjecting the cooled steel to recuperative heating at a temperature of 500°C to 600°C.

Description

Rebar and its manufacturing method

The present invention relates to reinforcing bars and methods for manufacturing the same, and more particularly, to light steel bars having high strength and high ductility and methods for manufacturing the same.

In general, carbon steel has advantages of low cost and easy control of materials by heat treatment. As an example, carbon steel is applied throughout an industry such as automobiles and mechanical parts. In addition, such carbon steel is being applied to the structure for securing the space of human activities. For example, structural steel is widely applied to skyscrapers, long bridges, large marine structures, underground structures, and the like.

On the other hand, the carbon steel can be applied to various industries in the form of rebar. As an example, in the case of rebars applied to a structure, as the structure becomes very high and large, it is required to maintain a relatively light weight while having high strength and high ductility.

Background art related to the present invention is Korean Patent Publication No. 10-2014-0041279.

The present invention provides a lightweight reinforcing bar having high strength and high ductility through alloying components and process control.

One embodiment of the present invention is 0.1% to 1.5% by weight of carbon (C), 0.05% to 1.0% by weight of silicon (Si), 0.5% to 16.0% by weight of manganese (Mn), phosphorus (P) greater than 0 0.03 Weight%, Sulfur (S) greater than 0 0.03 weight%, Aluminum (Al) greater than 0 9.0 weight%, Chromium (Cr) 0.2 to 4.0 weight%, Nickel (Ni) 0.1 to 0.5 weight%, Nitrogen (N ) 0.005% to 0.015% by weight, 0.03% to 0.5% by weight of at least one of titanium (Ti), niobium (Nb) and vanadium (V), and an alloy composition containing the remaining iron (Fe) and unavoidable impurities Reheating the billet having at 1100 ° C. to 1200 ° C .; Hot rolling the reheated billet to an Ac1˜1000 ° C. finish rolling temperature; Cooling the hot rolled steel to a temperature of Ms-100 (° C.) to Ms (° C.) via a temp core; And recuperating the cooled steel at 500 ° C. to 600 ° C .; It relates to a method for producing a reinforcing bar comprising a.

Another embodiment of the invention is 0.1% to 1.5% by weight of carbon (C), 0.05% to 1.0% by weight of silicon (Si), 0.5% to 16.0% by weight of manganese (Mn), greater than 0 0.03 Weight%, Sulfur (S) greater than 0 0.03 weight%, Aluminum (Al) greater than 0 9.0 weight%, Chromium (Cr) 0.2 to 4.0 weight%, Nickel (Ni) 0.1 to 0.5 weight%, Nitrogen (N ) 0.005% to 0.015% by weight, 0.03% to 0.5% by weight of at least one of titanium (Ti), niobium (Nb) and vanadium (V), and an alloy composition containing the remaining iron (Fe) and unavoidable impurities The present invention relates to a reinforcing bar having a tensile strength (TS) of 1100 MPa or more and a tensile strength x elongation (TS x EL) of 20000 MPa ·% or more.

The reinforcing bar may have a specific gravity of 6.5 gm ⁻³ to 7.0 gm ⁻³ .

The rebar may include a Fe ₂ AlC fine grain particles having an average particle diameter of 10nm to 100nm in a dispersed state.

The reinforcing bar may have a complex structure of ferrite, austenite and kappa (κ) -carbide.

According to the present invention, through the dispersion strengthening, by uniformly forming the Fe ₃ AlC fine grain particles in the base metal, it is possible to provide a lightweight reinforcing bar having high strength and high ductility.

1 shows a method for manufacturing rebar of the present invention.

Figure 2 shows the microstructure of the reinforcing bar prepared in Example 1 of the present invention.

Figure 3 shows the microstructure of the reinforcing bar prepared in Example 3 of the present invention.

Figure 4 shows the microstructure of the rebar produced in Comparative Example 1 of the present invention.

One embodiment of the present invention is 0.1% to 1.5% by weight of carbon (C), 0.05% to 1.0% by weight of silicon (Si), 0.5% to 16.0% by weight of manganese (Mn), phosphorus (P) greater than 0 0.03 Weight%, Sulfur (S) greater than 0 0.03 weight%, Aluminum (Al) greater than 0 9.0 weight%, Chromium (Cr) 0.2 to 4.0 weight%, Nickel (Ni) 0.1 to 0.5 weight%, Nitrogen (N ) 0.005% to 0.015% by weight, 0.03% to 0.5% by weight of at least one of titanium (Ti), niobium (Nb) and vanadium (V), and an alloy composition containing the remaining iron (Fe) and unavoidable impurities Reheating the billet having at 1100 ° C. to 1200 ° C .; Hot rolling the reheated billet to an Ac1˜1000 ° C. finish rolling temperature; Cooling the hot rolled steel to a temperature of Ms-100 to Ms (° C.) through a temp core; And recuperating the cooled steel at 500 ° C. to 600 ° C .; It relates to a method for producing a reinforcing bar comprising a. Through this, it is possible to provide a lightweight reinforcing bar having high strength and high ductility by a method of dispersion strengthening to uniformly form Fe 3 AlC fine grain particles in a base metal.

Another embodiment of the invention is 0.1% to 1.5% by weight of carbon (C), 0.05% to 1.0% by weight of silicon (Si), 0.5% to 16.0% by weight of manganese (Mn), greater than 0 0.03 Weight%, Sulfur (S) greater than 0 0.03 weight%, Aluminum (Al) greater than 0 9.0 weight%, Chromium (Cr) 0.2 to 4.0 weight%, Nickel (Ni) 0.1 to 0.5 weight%, Nitrogen (N ) 0.005% to 0.015% by weight, 0.03% to 0.5% by weight of at least one of titanium (Ti), niobium (Nb) and vanadium (V), and an alloy composition containing the remaining iron (Fe) and unavoidable impurities The present invention relates to a reinforcing bar having a tensile strength (TS) of 1100 MPa or more and a tensile strength x elongation (TS x EL) of 20000 MPa ·% or more. This reinforcing bar is achieved by the above-described method for producing rebar.

The rebar may include a Fe ₃ AlC fine grain particles having an average particle diameter of 10nm to 100nm in a dispersed state.

The rebar may have a specific gravity of steel of 6.5gm ⁻³ to 7.0gm ⁻³ .

The reinforcing bar may have a complex structure of ferrite, austenite and kappa (κ) -carbide.

Hereinafter, the role and content of each component included in the rebar according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added to secure the strength and hardness of the steel. Carbon (C) is dissolved in austenite to form martensite structure when quenched. In addition, as the amount of carbon increases, the hardening hardness may be improved, and the deformation potential may be increased during the hardening.

The carbon (C) is added in an amount of 0.1% by weight to 1.5% by weight of the total weight. If the carbon content is less than 0.1% by weight, it is difficult to secure sufficient strength. On the contrary, when the content of carbon (C) exceeds 1.5% by weight, the strength of the steel is increased, but the core hardness is reduced, it is difficult to secure sufficient elongation, and the fraction of the pearlite phase in the microstructure is difficult to secure the desired workability.

silicon( Si )

In the present invention, silicon (Si) is added as a deoxidizer for removing oxygen in the steel in the steelmaking process. In addition, silicon (Si) is a ferrite stabilizing element having a solid solution strengthening effect, which is effective in inducing ferrite formation and improving toughness and ductility of steel. In addition, silicon (Si) also has the effect of increasing the softening resistance phase during tempering.

The silicon (Si) is added in an amount of 0.05 wt% to 1.0 wt% of the total weight. When the content of silicon (Si) is less than 0.05% by weight, the silicon addition effect may not be properly exhibited. On the contrary, when the content of silicon (Si) exceeds 1.0% by weight, there is a problem in that oxides are formed on the surface of the steel, thereby reducing ductility, tube structure, and the like.

Manganese (Mn)

Manganese (Mn) is a substituted element having an atomic diameter similar to iron, which increases the strength and toughness of the steel and increases the hardenability of the steel, but decreases the ductility at the time of increasing the strength more than the addition of carbon (C). In addition, manganese (Mn) is a very effective element for solid solution strengthening contributes to the hardenability of the steel, and serves to improve the hardenability of the steel.

The manganese (Mn) is added in an amount of 0.5 wt% to 16.0 wt% of the total weight. If the content of manganese (Mn) is less than 0.5% by weight it may not be sufficient to improve the yield strength by strengthening the ferrite solid solution. On the contrary, when the content of manganese (Mn) exceeds 16% by weight, the amount of MnS-based nonmetallic inclusions increases, which may cause defects such as cracking during piping, and may cause hardening cracks, deformation, or hardening ability. Over-improvement increases the likelihood of cold microstructure being expressed in the final tissue.

Phosphorus (P)

Phosphorus (P) is an element that partially contributes to the strength improvement, but when excessively included, deteriorates the ductility of the steel and causes the final material deviation due to the billet center segregation. P is not a problem if it is uniformly distributed in the steel, but usually forms a harmful compound of Fe ₃ P. The Fe ₃ P is extremely fragile and segregated so that it does not become homogenized even after annealing, but increases in processing such as forging and rolling.

The phosphorus (P) is limited to a content of more than 0 and 0.03% by weight of the total weight. When the content of phosphorus (P) exceeds 0.03% by weight, as well as the center segregation, as well as micro segregation is formed to adversely affect the material, and also may worsen the ductility and tubulation.

Sulfur (S)

Sulfur (S) is an element that contributes in part to the improvement of workability, but when excessively contained, it inhibits the toughness and ductility of the steel and combines with manganese to form MnS non-metallic inclusions, thereby causing cracks during processing of the steel. Sulfur can combine with iron to form FeS if there is not enough manganese in the steel.

The sulfur (S) is limited to a content of more than 0 and 0.03% by weight of the total weight. When the content of sulfur (S) exceeds 0.03% by weight, there is a problem that greatly inhibits the ductility and excessively generates the MnS non-metallic inclusions.

Aluminum (Al)

Aluminum (Al) serves as a deoxidizer to remove oxygen in the steel. Al is generally used as a deoxidizer, but this patent aims to increase the lattice constant and replace Fe atoms by adding up to 9% by weight in order to maximize the effect of reducing the specific gravity of the steel grade itself. Adding about 7.5% wt has a specific gravity reduction of 10%.

The aluminum (Al) is added in an amount of more than 0 9.0 wt% of the total weight. If the content of aluminum (Al) exceeds 9.0% by weight, there is a problem that to form the Al ₂ O ₃ decreases the toughness.

chrome( Cr )

Chromium (Cr) is an effective element added to secure the strength, and can improve the hardenability of the steel and combine with carbon to form carbide to improve the strength. By forming a homogeneous oxide film on the surface of the plate, it is possible to reduce the decarburized portion of the surface layer portion of the plate.

Chromium (Cr) is added in an amount of 0.2% to 4.0% by weight of the total weight. If the content of chromium (Cr) is less than 0.2% by weight, it may be difficult to secure the strength even if the carbon (C) content is high. On the contrary, when the content of chromium (Cr) is more than 4.0% by weight, there is a problem of deteriorating ductility or heat affected zone (HAZ) toughness.

nickel( Ni )

Nickel (Ni) refines grains and is dissolved in austenite and ferrite to strengthen the matrix and improve hardenability. In particular, nickel (Ni) is an element effective in promoting the formation of low-temperature phase microstructure to improve low-temperature impact toughness. When coexisted with chromium or molybdenum, it exhibits excellent hardening ability and facilitates heat treatment of large steels. Since it is an austenite stabilizing element, it is combined with chromium to form austenitic stainless steel and heat resistant steel. It strengthens low temperature toughness of steel and does not harm weldability and malleability. By slowing the diffusion of carbon or nitrogen, it prevents deterioration of heat-resistant steel and improves the expansion rate, stiffness rate, and ceramic rate.

The nickel (Ni) is added in an amount of 0.1 wt% to 0.5 wt% of the total weight of the steel sheet according to the present invention. If the content of nickel (Ni) is less than 0.1% by weight, it may be difficult to properly exhibit the effect of adding nickel (Ni). On the contrary, when a large amount of nickel (Ni) is added in excess of 0.5% by weight, red brittleness may be caused.

Nitrogen (N)

Nitrogen (N) is an unavoidable impurity, and there is a problem of lowering the internal quality of steel by forming inclusions such as AlN and TiN. Nitrogen is added in an amount of 0.005% to 0.015% by weight.

If the content of nitrogen (N) is less than 0.005% by weight, there is a difficulty in management, with an increase in manufacturing cost due to the very small amount of nitrogen to be controlled. On the contrary, when the content of nitrogen (N) exceeds 0.015% by weight, there is a problem in that solid solution nitrogen increases, which lowers the impact property and elongation of the steel and greatly reduces the toughness of the weld.

titanium( Ti ), Naobium ( Nb ), And vanadium (V)

The alloy composition may further include one or more of titanium (Ti) niobium (Nb) and vanadium (V) in an amount of 0.03% by weight to 0.5% by weight.

In the present invention, titanium (Ti) forms a carbide upon reheating, thereby suppressing austenite grain growth, thereby miniaturizing the structure of the steel and increasing strength. The carbide forming ability of Ti, V, and Nb is stronger than chromium, and is used for improving stainless steel or cutting tool steel because it refines crystal grains. In addition, since the compound is formed with other metal elements and the precipitation hardening effect is remarkable, it is used for precipitation hardening steel or permanent magnets.

The titanium (Ti) may be added in an amount of 0.03% to 0.5% by weight of the total weight. If the content of titanium (Ti) is less than 0.03% by weight, the titanium addition effect may not be properly exhibited. On the contrary, when the content of titanium (Ti) is more than 0.5% by weight, the carbonized precipitates are coarsened, so that the effect of suppressing grain growth is reduced, and it is difficult to secure ductility.

Niobium (Nb) combines with carbon (C) and nitrogen (N) at high temperatures to form carbides or nitrides. Niobium-based carbides or nitrides suppress grain growth during rolling to refine grains, thereby improving strength and low temperature toughness of the steel.

The niobium (Nb) is added in an amount of 0.03% to 0.5% by weight of the total weight. When the content of niobium (Nb) is less than 0.03% by weight, the niobium addition effect cannot be properly exhibited. On the contrary, when the content of niobium (Nb) exceeds 0.5% by weight, the ductility of the steel is lowered, and the strength and low temperature toughness according to the increase in niobium content are not improved any more, and are present in solid solution in the ferrite. There is a risk of deterioration.

Vanadium (V) combines with carbon (C) and nitrogen (N) at high temperatures to form carbides or nitrides. In addition, vanadium (V) serves to improve the strength of the steel through the precipitation strengthening effect by the formation of precipitates.

The vanadium is added in an amount of 0.03% to 0.5% by weight of the total weight. If the content of vanadium is less than 0.03% by weight, it may be difficult to properly exhibit the effect of adding vanadium. On the contrary, when the amount of vanadium is added in excess of 0.5% by weight, the low temperature impact toughness may be reduced.

In addition to the components of the alloy composition described above, the remainder is made of iron (Fe) and impurities that are inevitably included in the steelmaking process.

Manufacturing method of rebar

1 is a flow chart schematically showing a method for manufacturing a rebar according to an embodiment of the present invention. Referring to FIG. 1, the method of manufacturing the rebar includes a reheating step S100, a hot rolling step S200, a cooling step S300, and a reheating step S400. At this time, the reheating step (S100) is not necessarily to be performed, but is carried out in order to derive the effect, such as re-use of the precipitate.

In the rebar manufacturing method according to the invention the steel of the semi-finished state, which is the target of the hot rolling process has the alloy composition described above. The semi-finished steel may for example be a billet.

Reheat

In the reheating step (S100), the billet of the alloy composition described above is reheated to 1100 ° C to 1200 ° C. Steel having the alloy composition may be obtained through a continuous casting process after obtaining a molten steel of a desired composition through a steelmaking process. By reheating these steels, the segregated components can be reclaimed during casting.

In the reheating step (S100), when the reheating temperature is less than 1100 ° C, the segregated components during casting may not be sufficiently reusable, and there is a problem that a rolling load is caused during hot rolling. In addition, titanium is not sufficiently reusable in the alloy component, coarsening of precipitates occurs, and it is difficult to secure sufficient strength. On the contrary, when the reheating temperature is higher than 1200 ° C., the austenite grain size may be increased, thereby making it difficult to secure the strength. Due to the excessive heating process, only the steel manufacturing cost may increase.

Hot rolled

In the hot rolling step (S200), the reheated steel is hot rolled to a finish rolling temperature of Ac1 to 1000 ℃. In such a case, grains may be refined due to repetition of recovery and recrystallization during rolling.

If the finish rolling temperature (FRT) is less than Ac1, the mixed structure is generated by the abnormal region rolling, which makes it difficult to secure the workability of the steel sheet and may cause a load in the rolling process. On the contrary, when the finish rolling temperature (FRT) is higher than 1000 ° C., the quality of the steel sheet due to the generation of surface scale of the steel sheet is deteriorated, and the size of grains due to the high temperature rolling is increased, thereby making it impossible to secure the strength.

Finish rolling temperature (FRT) in the hot rolling step (S200), for example, may be 850 ~ 1000 ℃.

Cooling

In the cooling step (S300), the hot rolled steel is cooled to a temperature of Ms-100 (° C) to Ms (° C) via a temp core. The effect of suppressing coarse grain growth to the maximum in the cooling temperature range is excellent.

Recurrent

In the recuperation step (S400), the cooled rolled material is regenerated to 500 ° C to 600 ° C. It is possible to secure a microstructure having ferrite, austenite and kappa (κ) -carbide in the recuperation temperature range. If the recuperation temperature is less than 550 ° C., it is difficult to secure workability due to an increase in strength and a decrease in ductility due to grain refinement. On the contrary, when the coiling temperature exceeds 600 ℃ ductility is secured but the strength is lowered.

By proceeding the process including the steps S100 to S400 described above, it is possible to manufacture a rebar according to an embodiment of the present invention. According to an embodiment of the present invention, through the dispersion strengthening to uniformly form the Fe ₃ AlC fine grain particles in the base metal, it is possible to provide a lightweight reinforcing bar having high strength and high ductility. As an example, the rebar according to the embodiment of the present invention may have a tensile strength (TS) of 1100 MPa or more, and a tensile strength x elongation (TS × EL) value of 20000 MPa · or more.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. However, this is presented as a preferred example of the present invention and in no sense can be construed as limiting the present invention.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Preparation of Specimen

To prepare a billet consisting of the alloy composition shown in Table 1 and the remaining iron (Fe) and inevitable impurities.

The billets were hot-rolled under the conditions shown in Table 2 to prepare specimens of Examples 1 to 3 and Comparative Examples 1 and 2.

division	Chemical composition (% by weight)													importance
	C	Si	Mn	S	P	Al	Cr	Ni	Nb	Ti	V	Cu	N
Example 1	0.79	0.25	13.06	0.005	0.02	6.28	2.29	0.1	-	0.02	0.03	-	0.0120	6.98
Example 2	1.18	0.24	13.25	0.004	0.02	7.97	2.24	0.11	0.02	-	0.04	-	0.0111	6.93
Example 3	1.19	0.24	13.19	0.005	0.02	7.88	4.59	0.11	0.02	-	0.04	-	0.0130	6.91
Comparative Example 1	0.35	0.25	1.4	0.005	0.03	0.01	0.2	0.1	-	-	0.05	0.2	0.0080	7.89
Comparative Example 2	0.25	0.20	20.0	0.005	0.02	10.0	-	0.10	-	-	-	-	-	6.94

division	Rolling Condition (℃)
	Reheat	final	Recurrent
Example 1	1200	970	520
Example 2	1200	973	516
Example 3	1200	965	526
Comparative Example 1	1030	930	570
Comparative Example 2	1200	900	570

2. Property evaluation

Table 3 shows the mechanical property evaluation results for the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 and 2. The physical property evaluation was shown by measuring the tensile strength (TS), yield strength (YS) and elongation (EL).

division	YS (MPa)	TS (MPa)	EL (%)	TS * EL (MPa%)	Microstructure
Example 1	992	1,133	24.8	28098.4	α + γ + κ
Example 2	1,185	1,285	29	37265	α + γ + κ
Example 3	1,094	1,179	19.8	23344.2	α + γ + κ
Comparative Example 1	651	832	14.3	11897.6	α + Fe 3 C
Comparative Example 2	600	770	10.1	7,777	γ + κ

Referring to Table 3, in the case of Examples 1 to 3, which are specimens according to the embodiment of the present invention, all of the tensile strength (TS) 1100 MPa or more were satisfied. In addition, in Examples 1 to 3, all of tensile strength x elongation (TS x EL) of 20000 MPa ·% or more were satisfied. In particular, in the case of the specimen of Example 2, the tensile strength (TS) is 1200 MPa or more, and the tensile strength x elongation (TS x EL) is 35000 MPa ·% or more.

The specimens of Comparative Examples 1 and 2 had a tensile strength (TS) of less than 1000 MPa and a tensile strength x elongation (TS × EL) of less than 20000 MPa ·%.

2 to 4 show the microstructure cross sections of Example 2, Example 3 and Comparative Example 1 specimens, respectively.

In the case of the specimens of Example 2 and Example 3, the composite structure of ferrite, austenite and kappa (κ) -carbide was shown, and in the comparative example 1 specimen, the composite structure of ferrite and cementite was shown.

In the above description, the embodiment of the present invention has been described, but various changes and modifications can be made at the level of those skilled in the art. Such changes and modifications may belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention will be determined by the claims described below.

Claims (8)

1. 0.1 wt% to 1.5 wt% of carbon (C), 0.05 wt% to 1.0 wt% of silicon (Si), 0.5 wt% to 16.0 wt% of manganese (Mn), 0.03 wt% of phosphorus (P) above 0, sulfur (S) More than 0, 0.03% by weight, aluminum (Al), more than 0, 9.0% by weight, chromium (Cr) 0.2% to 4.0%, nickel (Ni) 0.1% to 0.5%, nitrogen (N) 0.005% to 0.015% A billet having an alloy composition containing 0.03% to 0.5% by weight of one or more of%, titanium (Ti), niobium (Nb) and vanadium (V), and the remaining iron (Fe) and unavoidable impurities; Reheating at ° C;

   Hot rolling the reheated billet to an Ac1˜1000 ° C. finish rolling temperature;

   Cooling the hot rolled steel to a temperature of Ms-100 (° C.) to Ms (° C.) via a temp core; And

   Reheating the cooled steel at 500 ° C. to 600 ° C .; Rebar manufacturing method comprising a.
2. The method of claim 1,

   Reinforcing bar production method that satisfies the tensile strength (TS) of 1100 MPa or more, tensile strength x elongation (TS × EL) of 20000 MPa ·% or more of the manufactured rebar.
3. The method of claim 1,

   The prepared rebar is

   Rebar manufacturing method comprising a Fe ₃ AlC fine grain particles having an average particle diameter of 10nm to 100nm in a dispersed state.
4. The method of claim 1,

   The prepared reinforcing bar has a composite structure of ferrite, austenite and kappa (κ) -carbide.
5. 0.1 wt% to 1.5 wt% of carbon (C), 0.05 wt% to 1.0 wt% of silicon (Si), 0.5 wt% to 16.0 wt% of manganese (Mn), 0.03 wt% of phosphorus (P) above 0, sulfur (S) More than 0, 0.03% by weight, aluminum (Al), more than 0, 9.0% by weight, chromium (Cr) 0.2% to 4.0%, nickel (Ni) 0.1% to 0.5%, nitrogen (N) 0.005% to 0.015% Tensile strength (TS) having an alloy composition comprising%, 0.03% to 0.5% by weight of at least one of titanium (Ti), niobium (Nb) and vanadium (V), and the remaining iron (Fe) and unavoidable impurities Reinforcing bar satisfying 1100 MPa or more and tensile strength x elongation (TS x EL) 20000 MPa ·% or more.
6. The method of claim 5,

   The reinforcing bar is to include the Fe ₃ AlC fine grains having an average particle diameter of 10nm to 100nm in a dispersed state.
7. The method of claim 5,

   The rebar has a specific gravity of 6.5gm ^-3 to 7.0gm ^-3

   rebar.
8. The method of claim 5,

   The reinforcing bar has a complex structure of ferrite, austenite and kappa (κ) -carbide.

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