KR101770073B1 - Method of manufacturing high strength steel deforemed bar - Google Patents

Abstract

The present invention provides a method to manufacture a high strength rebar, capable of effectively manufacturing a high strength rebar through alloy composition control and process control. According to one embodiment of the present invention, the method comprises: a step of re-heating a slab within a temperature range of 1,000-1,250C; a step of performing finishing hot rolling of the re-heated slap at a temperature of an Arc3 transformation point or more; a step of performing a first accelerated cooling of the rebar formed by the hot rolling at a temperature of 600-700C; a step of air-cooling the rebar for a time of greater than 0 and less than 10 sec after completing the first accelerated cooling; a step of performing a second accelerated cooling of the air-cooled rebar at a temperature of 450-600C and then naturally cooling the cooled rebar at a temperature lower than 250C; and a step of annealing the naturally cooled rebar at a temperature of 670C or more to an Ac1 transformation point or less. The slab includes: 0.35-1.2 wt% of carbon (C); greater than 0 and equal to or less than 1.5 wt% of silicon (Si); 0.5-4.0 wt% of manganese (Mn); greater than 0 and equal to or less than 0.03 wt% of phosphorus (P); greater than 0 and equal to or less than 0.03 wt% of sulfur (S); 0.1-1.0 wt% of chromium (Cr); greater than 0 and equal to or less than 0.4 wt% of copper (Cu); greater than 0 and equal to or less than 0.25 wt% of nickel (Ni); greater than 0 and equal to or less than 0.45 wt% of molybdenum (Mo); greater than 0 and equal to or less than 0.20 wt% of vanadium (V); greater than 0 and equal to or less than 0.20 wt% of niobium (Nb); greater than 0 and equal to or less than 0.050 wt% of titanium (Ti); greater than 0 and equal to or less than 0.030 wt% of nitrogen (N); and the balance of iron (Fe) and inevitable impurities.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a high-

The present invention relates to a method for producing a high strength steel bar.

Generally, carbon steel has an advantage that it is cheap and the material control by heat treatment is easy. In addition, carbon steel is applied throughout the industry, including automobiles and machinery parts. In addition, these carbon steels are being applied to structures for securing space for human activities. As an example, steel for structures has been widely applied to skyscrapers, long bridges, giant marine structures, underground structures, and the like. As the structures in the civil engineering construction field become larger and larger, the weight reduction and strengthening of the steel for the structure can be an indispensable requirement.

As a related prior art, Korean Registered Patent Publication No. 10-1095486 (published on Dec. 19, 2011, titled invention: a method for manufacturing an anti-vibration reinforcement and an anti-vibration reinforcement manufactured thereby).

It is an object of the present invention to provide a method for effectively manufacturing high-strength reinforcing bars through alloy composition control and process control.

A method of manufacturing a high strength steel bar according to one aspect of the present invention is disclosed. The method of manufacturing a high strength steel bar according to claim 1, wherein the steel contains 0.35% to 1.2% carbon (C), 0.5% to 4.0% manganese (Mn) (Cu): more than 0 and not more than 0.4%, nickel (Ni): not more than 0.25%, molybdenum (Mo): not more than 0.03%, sulfur (S): more than 0 and not more than 0.03% ): More than 0 and not more than 0.45%, vanadium (V): more than 0 and not more than 0.20%, niobium (Nb): more than 0 and not more than 0.20%, titanium (Ti) And the remainder is reheating the cast steel containing iron (Fe) and other inevitably contained impurities in a temperature range of 1000 ° C to 1250 ° C; Subjecting the reheated cast steel to finish hot rolling at a temperature equal to or higher than the Arc3 transformation point; Firstly accelerating the reinforcing bar formed by the hot rolling at a temperature of 600 to 700 ° C; Cooling the reinforcing bar for a period of time greater than 0 and less than 10 sec after the first accelerated cooling is completed; Cooling the air-cooled reinforcing bars to a temperature of 450 to 600 ° C for a second time, followed by spontaneous cooling to less than 250 ° C; And annealing the naturally cooled reinforcing bar at a temperature of 670 ° C or higher and an Ac1 transformation point or lower.

In one embodiment, the cast slab may further include aluminum (Al): more than 0 to 0.040%, antimony (Sb): more than 0 and less than 0.1%, and tin (Sn): more than 0 and less than 0.1%.

In another embodiment, the finish rolling temperature during the hot rolling may be 840 캜 to 1000 캜.

In still another embodiment, the cooling rate of the primary accelerated cooling may be 40 to 50 占 폚 / sec, and the cooling rate of the secondary accelerated cooling may be 70 to 80 占 폚 / sec.

In another embodiment, after the natural cooling, the reinforcing bar has a composite structure including bainite and pearlite, and after the annealing step, the reinforcing bar may have an nodular structure including pearlite and ferrite.

According to another aspect of the present invention, there is provided a high-strength reinforcing steel comprising 0.35% to 1.2% carbon (C), 0.5% to 4.0% manganese (Mn) : More than 0 and not more than 0.03%, sulfur (S): more than 0 and not more than 0.03%, chromium (Cr): 0.1 to 1.0%, copper: more than 0 and not more than 0.4%, nickel (Ni) Molybdenum (Mo): more than 0 and not more than 0.45%, vanadium (V): more than 0 and not more than 0.20%, niobium (Nb): more than 0 and not more than 0.20%, titanium (Ti) And the remainder contains iron (Fe) and other inevitably contained impurities, and has a spheroidizing structure including microperlite and ferrite.

In one embodiment, the reinforcing bars may further include Al (Al): more than 0 to 0.040%, antimony (Sb): more than 0 to 0.1%, and tin (Sn): more than 0 to 0.1%.

According to the present invention, there is provided a method for effectively manufacturing a high-strength steel bar with a high carbon steel material. In general, high carbon steels having the same content ranges as those set forth in the present invention may undergo pearlite transformation during the cooling process after hot rolling, thereby causing an extreme exothermic reaction. As a result, the structure of the high carbon steel material is coarsened and it is difficult to secure high strength.

In the embodiment of the present invention, the cooling step is subdivided and the cooling rate is controlled step by step, so that the transformation heat, which is the coarsening cause of the above-described carbon steel structure, can be minimized. As a result, fine pearlite and ferrite structure can be secured after hot rolling and cooling. Further, there is an advantage that high strength can be realized by securing fine spheroidizing structure through annealing treatment.

1 is a process flow diagram illustrating a method of manufacturing a high strength steel bar according to an embodiment of the present invention.
FIG. 2 and FIG. 3 are photographs showing microstructure of a reinforcing bar according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention may be embodied in many different forms and is not limited to the embodiments described herein. Throughout this specification, the same or similar components are denoted by the same reference numerals. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

The embodiments of the present invention, which will be described below, present reinforced bars manufactured through appropriate component design and process control.

High strength steel

The high strength steel according to an embodiment of the present invention may contain 0.35% to 1.2% by weight of carbon (C), 0.5% to 4.0% of manganese (Mn) : More than 0 and not more than 0.03%, sulfur (S): more than 0 and not more than 0.03%, chromium (Cr): 0.1 to 1.0%, copper: more than 0 and not more than 0.4%, nickel (Ni) Molybdenum (Mo): more than 0 and not more than 0.45%, vanadium (V): more than 0 and not more than 0.20%, niobium (Nb): more than 0 and not more than 0.20%, titanium (Ti) And the remainder contains iron (Fe) and other inevitably contained impurities. The high strength reinforcing bars may further include aluminum (Al): more than 0 to 0.040%, antimony (Sb): more than 0 and less than 0.1%, and tin (Sn): more than 0 and less than 0.1%. The high strength reinforcing bar may have a spheroidizing structure including pearlite and ferrite.

Hereinafter, the role and content of each component included in the essential alloy composition of the high strength steel according to the present invention will be described more specifically.

Carbon (C)

Carbon (C) is added to secure the strength of the rebar. Carbon is solidified in austenite to improve strength by forming a structure like martensite during quenching. In addition, strength and hardness can be improved by forming a carbide by bonding with elements such as iron, chromium, molybdenum, and vanadium.

The carbon (C) is added in an amount of 0.35 to 1.2% by weight based on the total weight of the reinforcing bars. When the content of carbon (C) is less than 0.35% by weight, it may be difficult to secure strength. On the other hand, when the carbon content exceeds 1.2% by weight, the strength is increased but the core hardness and weldability are deteriorated.

silicon( Si )

Silicon (Si) can act as a deoxidizer to remove oxygen in steel during the steelmaking process. Silicon may also perform the function of solid solution strengthening.

The silicon is added in an amount of 0 to 1.5 wt% of the total reinforcing steel weight. If the content of silicon is more than 1.5% by weight, an oxide may be formed on the surface of the steel to lower the weldability of the steel.

Manganese (Mn)

Manganese (Mn) is an element that increases the strength and toughness of steel and increases the ingotability of steel. The manganese is added in an amount of 0.5 to 4.0% by weight based on the total weight of the reinforcing bars. When the content of manganese is less than 0.5% by weight, it may be difficult to secure strength. On the other hand, when the content of manganese exceeds 4.0% by weight, the strength is increased but the amount of MnS-based nonmetallic inclusions is increased, which may cause defects such as cracks during welding.

In (P)

Phosphorus (P) can inhibit cementite formation and increase strength. However, when the content of phosphorus is more than 0.03% by weight, secondary workability may be deteriorated. Therefore, phosphorus (P) is controlled to be not less than 0 and not more than 0.03% by weight of the total reinforcing bar weight.

Sulfur (S)

Sulfur (S) can combine with manganese, molybdenum, etc. to improve machinability of steel. However, if the precipitation occurs in the form of MnS, FeS, and the like, and the amount of such precipitates increases, cracking may occur during hot and cold working. Therefore, the sulfur (S) is controlled to be more than 0 to 0.03% by weight of the total reinforcing steel weight.

Chromium (Cr)

Chromium (Cr) improves the hardenability of the steel and improves the hardenability.

The chromium is added in an amount of 0.1 to 1.0% by weight based on the total weight of the reinforcing bars. When the content of chromium is less than 0.1% by weight, it is difficult to sufficiently secure the above-mentioned effects. On the other hand, when the content of chromium is added in an amount exceeding 1.0% by weight, there is a disadvantage in that the weldability and toughness of the heat-affected zone can be lowered.

Copper (Cu)

Copper (Cu) can serve to improve the hardenability of the steel and the impact resistance at low temperatures. However, if the content of copper is more than 0.4% by weight, it may induce red hot brittleness. Therefore, copper (Cu) is controlled to be 0 to 0.4 wt% or less of the total weight of the reinforcing bars.

nickel( Ni )

Nickel (Ni) increases the strength of the material and ensures a low temperature impact value. However, when the content of nickel exceeds 0.25% by weight of the total weight, the strength at room temperature becomes excessively high, and the weldability and toughness may be deteriorated. Therefore, nickel (Ni) is controlled to be not less than 0 and not more than 0.25% by weight of the total rebar weight.

Molybdenum (Mo)

Molybdenum (Mo) improves strength and toughness and contributes to ensuring stable strength at room or high temperatures. However, when the molybdenum content exceeds 0.45 wt%, the weldability may be deteriorated. Therefore, molybdenum (Mo) is controlled to be 0 to 0.45 wt% or less of the total reinforcing steel weight.

Vanadium (V)

Vanadium (V) acts as a pinning to the grain boundaries and contributes to the improvement of strength. However, when the content of vanadium (V) exceeds 0.20% by weight, the production cost of steel is increased. Therefore, it is preferable that the steel is added in an amount of 0 to 0.20% by weight of the total reinforcing steel weight.

Niobium ( Nb )

Niobium (Nb) improves the strength of steel by forming precipitates in the form of Nb (C, N) or strengthening solids in Fe. However, if the content of niobium (Nb) is more than 0.20 wt%, the precipitation strengthening effect is excessive, and the strength is greatly increased, so that the ductility is deteriorated. Therefore, it is preferable that the steel is added in an amount of 0 to 0.20% by weight of the total reinforcing steel weight.

titanium( Ti )

Titanium (Ti) is an alloying element that forms TiN precipitates during the solidification of steel, inhibits the growth of austenite grains during reheating of the intermediate steel and inhibits the growth of the austenite recrystallized grains during the hot rolling process. However, if the content of titanium (Ti) exceeds 0.050% by weight, the TiN is crystallized in the liquid phase during continuous casting, and the effect of suppressing grain growth is reduced. Therefore, it is preferable that the steel is added in an amount of more than 0 to 0.050% by weight of the total reinforcing steel weight.

Nitrogen (N)

Nitrogen can be combined with other alloying elements such as titanium, vanadium, niobium, aluminum, etc. to form a nitride to function as a fine grain. However, when it is added in a large amount exceeding 0.030%, there is a problem that the amount of nitrogen increases and the elongation and formability of steel are lowered. Therefore, it is preferable that the steel is added in an amount of more than 0 to 0.030% by weight of the total reinforcing steel weight.

Aluminum (Al)

Aluminum (Al) can function as a deoxidizer. However, when the content of aluminum is more than 0.040 wt%, the amount of nonmetal inclusions such as aluminum oxide (Al 2 O 3) can be increased. Therefore, aluminum (Al) is controlled to be not less than 0 to 0.040% by weight of the total reinforcing steel weight.

antimony( Sb )

0 to less than 0.1%

Antimony (Sb) has an effect of suppressing the generation of oxides as a result of inhibiting diffusion of component elements in the steel surface due to the concentration of these elements at the surface and grain boundaries, though these elements themselves do not form an oxide film at high temperatures. In addition, antimony (Sb) effectively suppresses the coarsening of the surface oxide layer when Mn and B are added in combination. However, when the content of antimony (Sb) is more than 0.1% by weight, antimony (Sb) is not more than 0 By weight or less.

Tin (Sn)

Tin (Sn) may be added to ensure corrosion resistance. However, if the content of tin exceeds 0.1%, the elongation can be drastically reduced. Therefore, the tin (Sn) is controlled to be 0 to 0.1 wt% or less of the total reinforcing steel weight.

How to make high strength rebar

Hereinafter, a method of manufacturing a high-strength steel bar according to an embodiment of the present invention will be described.

1 is a process flow diagram illustrating a method of manufacturing a high strength steel bar according to an embodiment of the present invention. Referring to FIG. 1, a high strength steel bar manufacturing method includes a reheating step S110, a hot rolling step S120, a cooling step S130, and an annealing step S140. At this time, the reheating step (S110) may be carried out in order to derive effects such as reuse of precipitates. At this time, the cast steel having the above composition can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process.

Reheat step

In the reheating step of the cast steel, the cast steel having the above composition is reheated in the temperature range of 1000 ° C to 1250 ° C. This reheating can result in re-use of the segregated components and re-use of precipitates during casting. At this time, the cast steel may be a bloom or billet produced by a continuous casting process performed before the reheating step (S110).

If the reheating temperature of the cast steel is less than 1000 ° C, the heating temperature is not sufficient and the segregation component and the precipitate may not be sufficiently reused. Further, there is a problem that the rolling load becomes large. On the other hand, if the reheating temperature exceeds 1250 占 폚, the austenite grains may be coarsened and the strength may be deteriorated.

Hot rolling

In the hot rolling step (S120), the reheated cast steel is hot-rolled into a bar shape. In the hot rolling step (S120), the finish hot rolling may proceed at temperatures above the Arc3 transformation point. As an example, the finish rolling temperature may be 840 ° C to 1000 ° C. If the finish rolling temperature exceeds 1000 캜, the austenite grains are coarsened, and after the transformation, the ferrite grain refinement can not be sufficiently performed, which may make it difficult to secure the strength. On the contrary, when the finish rolling temperature is lower than 840 占 폚, a problem such as generation of blast-furnace structure due to abnormal reverse rolling may occur.

Cooling

In the cooling step (S130), the steps of the first accelerated cooling step, the air cooling step, the second accelerated cooling step, and the natural cooling step may be performed in order to ensure sufficient strength and toughness.

In the first accelerated cooling step, the reinforcing bars formed by the hot rolling are cooled to a temperature of 600 to 700 캜. At this time, the cooling rate of the first-order accelerated cooling may be 40 to 50 ° C / sec. By accelerating and cooling in this cooling rate range, it is possible to effectively suppress the formation of pro-eutectoid ferrite in the grain boundary and effectively increase the austenite high- .

In the air cooling step, after the first accelerated cooling is completed, the reinforcing bars are air-cooled for a period of time exceeding 0 and 10 sec. At this time, a minute pearlite may be partially formed in the steel bar.

In the second accelerated cooling step, the air-cooled reinforcing bars are secondarily accelerated to a temperature of 450 to 600 ° C. At this time, the cooling rate of the secondary accelerated cooling may be 70 to 80 占 폚 / sec. By accelerating and cooling the cooling rate in this cooling speed range, bainite can be formed as a final targeted hot deformation structure.

In the natural cooling step, after the secondary accelerated cooling is completed, the reinforcing bars are naturally cooled to less than 250 캜.

As described above, when the cooling step is performed after the hot rolling, the reinforcing bars to be manufactured may have a composite structure including micro pearlite and bainite.

Annealing

In the annealing step S140, the naturally cooled reinforcing bars are annealed at a temperature of 670 DEG C or higher and Ac1 transformation point or lower. The annealing heat treatment may be performed for about 15 hours as an example. After the annealing, the reinforcing bars may have a spheroidizing structure including fine perlite and ferrite.

In the manufacturing method according to the embodiment of the present invention, the high-strength reinforcing bars can be manufactured by controlling the process steps from the hot rolling step to the subsequent annealing heat treatment. Particularly, by controlling the primary accelerated cooling and the secondary accelerated cooling steps and conducting the air cooling step between the primary cooling step and the secondary cooling step, it is possible to suppress the transformation heating phenomenon, which is the cause of texture coarsening of carbon steel, To form a complex structure of pearlite and bainite. Then, through the annealing heat treatment, a spheroidizing structure including micro pearlite and ferrite can be realized.

Example

Hereinafter, the structure and function of the present invention will be described in more detail with reference to preferred embodiments and comparative examples of the present invention. It should be understood, however, that this is provided as illustrative of the present invention and is not to be construed as limiting the invention in any way.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Preparation of specimens

The billets having the alloy compositions shown in Table 1 were processed under the hot rolling and cooling conditions shown in Table 2 to prepare Comparative Examples and Examples 1 and 2 specimens.

Chemical composition (% by weight) C Si Mn P S Cr Cu Ni Mo Nb Ti V N Comparative Example 0.34 0.16 1.25 0.015 0.015 - 0.25 0.1 0.1 0.01 0.01 0.05 0.008 Example 1 0.35 0.18 2.10 0.015 0.015 0.15 0.25 0.20 0.25 0.10 0.05 0.05 0.008 Example 2 0.35 0.17 2.40 0.015 0.015 0.15 0.25 0.20 0.25 0.10 0.05 0.05 0.008

Rolling conditions Annealing condition Reheating temperature (℃) Finishing rolling temperature (캜) Primary accelerated cooling rate (° C / sec) Primary cooling temperature (℃) Air cooling time (sec) Secondary accelerated cooling rate (° C / sec) Secondary cooling temperature (℃) Temperature (℃) Hour (hour) Comparative Example 1140 840 50 650 - - - 680 15 Example 1 1140 840 50 650 5 80 450 680 15 Example 2 1140 860 50 650 5 80 450 680 15

The comparative specimen does not contain chromium among the alloy components proposed in the present invention. Further, the specimen of the comparative example was subjected to finish rolling at 840 캜, followed by accelerated cooling to 650 캜, followed by natural cooling. On the other hand, the first and second specimens were subjected to the first cooling, the air cooling and the second cooling sequentially after the finish rolling. In the comparative example, the first and second specimens were annealed at 680 DEG C for 15 hours.

2. Property evaluation

The mechanical properties of tensile strength (TS), yield strength (YS) and elongation (EL) of the comparative examples and the specimens 1 and 2 prepared under the process conditions shown in Table 1 and Table 2 were measured and shown in Table 3 .

Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Comparative Example 572 712 13 Example 1 671 862 18 Example 2 696 922 17

Referring to Table 3, it can be confirmed that, in the case of Examples 1 and 2, superior material properties are exhibited in terms of yield strength, tensile strength, and elongation.

3. Tissue observation

FIG. 2 and FIG. 3 are photographs showing microstructure of a reinforcing bar according to an embodiment of the present invention. Specifically, FIG. 2 is a photograph of the microstructure of a reinforcing bar after proceeding only to the hot rolling and cooling steps according to the embodiment of the present invention. FIG. 3 is a graph showing the microstructure of the reinforcing bars after the hot rolling, cooling and annealing steps according to the embodiment of the present invention It is a microstructure photograph of reinforcing bars. Referring to FIG. 2, it can be seen that the reinforcing bars have a composite structure including micro pearlite and bainite. Also, referring to FIG. 3, it can be confirmed that the reinforcing bar is a composite structure including microperlite and ferrite, and changed into a spheroidized structure through annealing heat treatment.

S110: Reheating step
S120: Hot rolling step
S130: cooling step
S140: annealing step

Claims (7)

(a) from 0.35% to 1.2% by weight of carbon (C), from 0.5% to 4.0% of manganese (Mn) S: more than 0 and not more than 0.03%, Cr: 0.1 to 1.0%, Cu: more than 0 and not more than 0.4%, Ni: more than 0 and not more than 0.25%, molybdenum (Mo) (N): more than 0 and not more than 0.20%, titanium (Ti): more than 0 and not more than 0.050%, nitrogen (N): more than 0 and not more than 0.030% Reheating a cast slab containing iron (Fe) and other inevitably contained impurities in a temperature range of 1000 ° C to 1250 ° C;
(b) subjecting the reheated cast steel to finish hot rolling at a temperature equal to or higher than the Arc3 transformation point;
(c) first accelerating and cooling the reinforcing bar formed by the hot rolling at a cooling rate of 40 to 50 DEG C / sec to 600 to 700 DEG C;
(d) after the first accelerated cooling is completed, cooling the reinforcing bar for a period of time greater than 0 and less than 10 sec;
(e) secondarily accelerating the air-cooled reinforcing bar at a cooling rate of 70 to 80 캜 / sec to a temperature of 450 to 600 캜, followed by spontaneous cooling to less than 250 캜; And
(f) annealing the naturally-cooled reinforcing bar at a temperature of 670 캜 or more and a Ac1 transformation point or less,
After step (e), the reinforcing bar has a composite structure including micro pearlite and bainite, and after step (f), the reinforcing bar has a microstructure including micro pearlite and ferrite
Method of manufacturing high strength steel bars. The method according to claim 1,
wherein said cast steel of step (a) further contains Al: Al more than 0 to 0.040%, Antimony (Sb): more than 0 and not more than 0.1%, and tin (Sn)
Method of manufacturing high strength steel bars.
The method according to claim 1,
The finish rolling temperature in step (b)
840 ° C to 1000 ° C
Method of manufacturing high strength steel bars.
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