KR102166598B1 - Cold rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

The steel sheet according to an embodiment of the present invention includes carbon (C): 0.06 to 0.10 wt%, silicon (Si): 0.1 to 0.2 wt%, manganese (Mn): 1.5 to 1.7 wt%, aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb): 0.03 to 0.06% by weight, phosphorus (P): more than 0 0.01% by weight, sulfur (S): more than 0 0.005% by weight, nitrogen (N): more than 0 0.006% by weight and It consists of the remaining iron (Fe) and other inevitable impurities, the yield strength is 590 MPa or more, the tensile strength is 650 MPa or more, the elongation is 14% or more, and the yield ratio is 80% or more.

Description

Cold rolled steel sheet and its manufacturing method {COLD ROLLED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a steel sheet and a manufacturing method thereof, and more particularly, to a precipitation hardening type high strength cold rolled steel sheet and a manufacturing method thereof.

Precipitation hardened steel, widely used as a structural material for automobiles, is a steel that realizes the target strength by adding a trace amount of alloying elements that can form precipitates to form precipitates, and strengthening through precipitation reinforcement and grain refinement using these precipitates. . The precipitation hardening type cold-rolled steel sheet is characterized by having a high yield strength (YP) compared to tensile strength (TS), that is, a high yield ratio (YP/TS), and thus has high impact resistance. It is difficult to secure a strength of 500 MPa or more of a yield strength and a tensile strength of 600 MPa using only the reinforcement through precipitate reinforcement and grain refinement in the ferrite matrix structure.

Related prior art is Korean Application No. 10-2004-0111413, Title of Invention: Precipitation-reinforced cold-rolled steel sheet having excellent yield strength and yield ratio and its manufacturing method).

The problem to be solved by the present invention is to provide a steel plate that can be applied to an exterior plate material for automobiles having excellent formability and a manufacturing method thereof.

The cold-rolled steel sheet according to an embodiment of the present invention for achieving the above object is carbon (C): 0.06 to 0.10 wt%, silicon (Si): 0.1 to 0.2 wt%, manganese (Mn): 1.5 to 1.7 wt%, Aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb): 0.03 to 0.06% by weight, phosphorus (P): more than 0 0.01% by weight, sulfur (S): more than 0 0.005% by weight, nitrogen (N): It is made of more than 0 0.006% by weight and the rest of iron (Fe) and other inevitable impurities, the yield strength is 590 MPa or more, the tensile strength is 650 MPa or more, the elongation is 14% or more, and the yield ratio is 80% or more.

The final microstructure of the cold-rolled steel sheet may be made of ferrite having an area fraction of 90 to 95% and bainite having an area fraction of 5 to 10%.

In the cold-rolled steel sheet, the ferrite may have an average grain size of 5 μm or less and a recrystallization ratio of 95% or more.

A method of manufacturing a cold-rolled steel sheet according to an embodiment of the present invention for achieving the above object is (a) carbon (C): 0.06 to 0.10 wt%, silicon (Si): 0.1 to 0.2 wt%, manganese (Mn): 1.5 to 1.7% by weight, aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb): 0.03 to 0.06% by weight, phosphorus (P): more than 0 0.01% by weight or less, sulfur (S): more than 0 0.005% by weight , Nitrogen (N): reheating a steel material consisting of more than 0 0.006% by weight and the remaining iron (Fe) and other inevitable impurities to 1150 to 1250 ℃; (b) hot rolling the steel material so that the hot rolling end temperature (FDT) becomes higher than or equal to A3 temperature; (c) winding the hot-rolled steel at 540 to 600°C; (d) cold rolling the steel material at a reduction ratio of 55% or more; (e) annealing the cold-rolled steel at 770 to 810°C; (f) cooling the annealed steel to 400 ~ 500 ℃; Includes.

In the method of manufacturing the cold-rolled steel sheet, the microstructure of the steel material after step (f) may be made of ferrite having an area fraction of 90 to 95% and bainite having an area fraction of 5 to 10%.

In the method of manufacturing the cold-rolled steel sheet, the ferrite may have an average grain size of 5 μm or less and a recrystallization ratio of 95% or more.

In the method of manufacturing the cold-rolled steel sheet, the yield strength of the steel material after step (f) is 590 MPa or more, tensile strength is 650 MPa or more, elongation is 14% or more, and yield ratio may be 80% or more.

According to an embodiment of the present invention, it is possible to implement a cold-rolled steel sheet capable of increasing the strength of an automobile structural material requiring impact resistance and a method of manufacturing the same. Of course, the scope of the present invention is not limited by these effects.

1 is a flowchart schematically showing a method of manufacturing a cold rolled steel sheet according to an embodiment of the present invention.

Hereinafter, a steel sheet according to an embodiment of the present invention and a method of manufacturing the same will be described in detail. Terms to be described later are terms appropriately selected in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout the present specification.

Precipitation hardened steel, widely used as a structural material for automobiles, is a steel that realizes the target strength by adding a trace amount of alloying elements that can form precipitates to form precipitates, and strengthening through precipitation reinforcement and grain refinement using these precipitates. . The precipitation hardening type cold-rolled steel sheet is characterized by having a high yield strength (YP) compared to tensile strength (TS), that is, a high yield ratio (YP/TS), and thus has high impact resistance. It is difficult to secure a strength of 500 MPa or more of a yield strength and a tensile strength of 600 MPa using only the reinforcement through precipitate reinforcement and grain refinement in the ferrite matrix structure. To this end, an attempt was made to secure high strength by adding a component such as B and Mo to delay recrystallization during continuous annealing for recrystallization after cold rolling to introduce a certain fraction of unrecrystallized grains. However, due to these non-recrystallized grains, the elongation is low, and in terms of productiveness, it is difficult to control the mechanical properties of the entire coil in order to secure strength through the non-recrystallized fraction. In addition, when a phase other than ferrite is introduced to increase strength, the yield ratio is generally lowered, and appropriate components and process conditions are required to obtain a high yield ratio. In other trials, it was found that a high yield ratio could be obtained even when some pearlite phases were introduced. However, in this case, the introduction of the pearlite phase as a heterogeneous phase did not solve the disadvantage that it is not sufficient to obtain high strength with a yield strength of 590 MPa or more.

In the present invention, the bainite phase is formed in an area fraction of 5 to 10%, so that the yield strength is 590 MPa or more, the tensile strength is 650 MPa or more, the elongation is 14% or more, and the yield ratio is 80% or more. Provides a steel plate and a manufacturing method thereof.

Cold rolled steel sheet

The cold rolled steel sheet according to an embodiment of the present invention includes carbon (C): 0.06 to 0.10 wt%, silicon (Si): 0.1 to 0.2 wt%, manganese (Mn): 1.5 to 1.7 wt%, aluminum (Al): 0.015 ~ 0.05% by weight, niobium (Nb): 0.03 ~ 0.06% by weight, phosphorus (P): more than 0 0.01% by weight, sulfur (S): more than 0 0.005% by weight, nitrogen (N): more than 0 0.006% by weight or less And the remaining iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the steel sheet according to an embodiment of the present invention will be described.

Carbon (C)

Carbon (C) is an element that plays a major role in improving the strength of the steel sheet and forming a precipitate and bainite phase. In addition, it is an element that has the greatest influence on weldability. Carbon (C) may be added in a content ratio of 0.06 to 0.10% by weight of the total weight of the cold rolled steel sheet according to an embodiment of the present invention. When the carbon content is less than 0.06% by weight of the total weight, formation of precipitates and bainite phase for strength are not sufficient. Conversely, when the content of carbon exceeds 0.10% by weight of the total weight, the content is limited because the cast iron cracks occur and weldability is inferior in the steelmaking process.

Silicon (Si)

Silicon (Si) is well known as a ferrite stabilizing element, and is well known as an element that increases the ductility by increasing the ferrite fraction during cooling. On the other hand, silicon is added together with aluminum as a deoxidizing agent to remove oxygen from the steel in the steelmaking process, and may have a solid solution strengthening effect. The silicon may be added in a content ratio of 0.1 to 0.2% by weight of the total weight of the steel sheet according to an embodiment of the present invention. When the content of silicon is less than 0.1% by weight of the total weight, the solid solution strengthening effect cannot be sufficiently realized, and when a large amount is added exceeding 0.2% by weight, the weldability of the steel is lowered, and red scale is generated during reheating and hot rolling. By doing so, it can give a problem to the surface quality.

Manganese (Mn)

Manganese (Mn) is an element that plays a major role in forming an appropriate bainite phase fraction in the present invention. That is, as an austenite stabilizing element, it is used as an element that increases the fraction of the low-temperature phase and increases the strength of steel with a solid solution strengthening effect. Manganese is effective in solid solution strengthening and can increase the hardenability of steel. Manganese may be added in a content ratio of 1.5 to 1.7% by weight of the total weight of the steel sheet according to an embodiment of the present invention. When the manganese content is less than 1.5% by weight, it is difficult to implement the above-described effect. In addition, when the content of manganese exceeds 1.7% by weight, elongation decreases, weldability decreases, MnS inclusions and center segregation occur, resulting in a decrease in ductility and corrosion resistance of the steel sheet.

Aluminum (Al)

Aluminum (Al), like silicon, is an element that stabilizes ferrite and suppresses the formation of carbides. That is, aluminum is an effective element in increasing the elongation by inducing the growth of ferrite initially present by enabling high-temperature heat treatment during annealing. In addition, aluminum is added to the steelmaking process as a deoxidizing agent for removing oxygen in the steel, and may contribute to grain refinement by depositing in the steel as AlN. The aluminum may be added in a content ratio of 0.015 to 0.05% by weight of the weight of the steel sheet according to an embodiment of the present invention. If the content of aluminum is less than 0.015% by weight, the effect of adding aluminum is insufficient, and if it exceeds 0.05% by weight, the quality of the slab and hot rolled material deteriorates, and there is a process load such as an increase in steel making and annealing temperature, and it is difficult to play. There may be a problem in that productivity is reduced, and ductility and toughness are deteriorated by forming alumina (Al ₂ O ₃ ), which is a non-metallic inclusion.

Niobium (Nb)

Niobium (Nb) plays a major role in forming precipitates for precipitation strengthening together with carbon. For example, it is an element that improves the strength of the base metal and the weld by depositing in the form of NbC or Nb(C,N). The niobium may be added in a content ratio of 0.03 to 0.06% by weight of the weight of the steel sheet according to an embodiment of the present invention. If the content of niobium is less than 0.03% by weight, it is difficult to obtain a sufficient precipitation strengthening effect, and if it exceeds 0.06% by weight, brittle cracking occurs, and there is a concern that non-recrystallized grains may be formed during annealing by increasing the recrystallization temperature, so the content is limited.

Phosphorus (P)

Phosphorus (P) increases the strength of the strength by solid solution strengthening and can function to suppress the formation of carbides. The phosphorus may be added in a content ratio of more than 0 to 0.01% by weight or less of the total weight of the cold-rolled steel sheet according to an embodiment of the present invention. When the content of phosphorus exceeds 0.01% by weight, hot brittleness and weldability may be impaired, and there is a problem that the low-temperature impact value is lowered by the precipitation behavior.

Sulfur (S)

Sulfur (S) can improve processability by forming precipitates of fine MnS. The sulfur may be added in a content ratio of more than 0 to 0.005% by weight or less of the total weight of the steel sheet according to an embodiment of the present invention. When the sulfur content exceeds 0.005% by weight, it may cause surface defects and processing cracks, impair toughness and weldability, and lower low-temperature impact values.

Nitrogen (N)

Nitrogen (N) is an element that increases the strength by depositing vanadium and nitride or carbonitride. The nitrogen may be added in a content ratio of more than 0 to 0.006% by weight or less of the total weight of the steel sheet according to an embodiment of the present invention. Nitrogen content exceeds 0.006% by weight of the total weight, and when a large amount is added, aluminum and nitride are formed, thereby inhibiting the quality of the slab and may act as an element that inhibits toughness.

As described above, the cold-rolled steel sheet according to an embodiment of the present invention having an alloying element composition may have a yield strength of 590 MPa or more, a tensile strength of 650 MPa or more, an elongation of 14% or more, and a yield ratio of 80% or more.

In addition, the final microstructure of the steel sheet according to an embodiment of the present invention having the alloying element composition as described above is made of ferrite with an area fraction of 90 to 95% and bainite with an area fraction of 5 to 10%, the Ferrite may have an average grain size of 5 μm or less and a recrystallization ratio of 95% or more.

Hereinafter, a method of manufacturing a cold-rolled steel sheet according to an embodiment of the present invention having the above-described alloy element composition will be described.

Cold rolled steel sheet manufacturing method

1 is a flowchart schematically showing a method of manufacturing a cold rolled steel sheet according to an embodiment of the present invention.

Referring to Figure 1, a method of manufacturing a cold-rolled steel sheet according to an embodiment of the present invention is (a) carbon (C): 0.06 to 0.10% by weight, silicon (Si): 0.1 to 0.2% by weight, manganese (Mn): 1.5 to 1.7% by weight, aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb): 0.03 to 0.06% by weight, phosphorus (P): more than 0 0.01% by weight or less, sulfur (S): more than 0 0.005% by weight , Nitrogen (N): reheating a steel material consisting of more than 0 0.006% by weight and the remaining iron (Fe) and other inevitable impurities to 1150 to 1250 ℃ (S100); (b) hot rolling the steel material so that the hot rolling end temperature (FDT) is equal to or higher than the A3 temperature (S200); (c) winding the hot-rolled steel at 540 ~ 600°C (S300); (d) cold rolling the steel material at a reduction ratio of 55% or more (S400); (e) annealing the cold-rolled steel at 770 ~ 810 °C (S500); And (f) cooling the annealed steel material to 400 ~ 500°C (S600).

First, in the reheating step (S100), the steel sheet having the above-described predetermined composition is reheated. The steel sheet may be manufactured through a continuous casting process after obtaining molten steel having a desired composition through a steel making process.

The steel sheet is carbon (C): 0.06 to 0.10% by weight, silicon (Si): 0.1 to 0.2% by weight, manganese (Mn): 1.5 to 1.7% by weight, aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb ): 0.03 to 0.06% by weight, phosphorus (P): more than 0 and not more than 0.01% by weight, sulfur (S): more than 0, 0.005% by weight, nitrogen (N): more than 0 and not more than 0.006% by weight, and the rest of iron (Fe) and others It can be made of inevitable impurities.

In one embodiment, the steel material may be reheated at a temperature of 1150 to 1250 ℃. When the steel sheet is reheated at the above-described temperature, segregated components are re-used during the continuous casting process, and it affects the uniformity of the structure in the slab and the dissolution of precipitates. If the reheating temperature is lower than 1150°C, there may be difficulties in completely re-dissolving the precipitates, the solid solution of various carbides may not be sufficient, and there may be a problem in that segregated components are not sufficiently evenly distributed during the continuous casting process. When the reheating temperature exceeds 1250°C, the austenite grain size becomes coarse, making it difficult to obtain a fine hot-rolled structure, and it may be difficult to secure strength. In addition, if it exceeds 1250 ℃ heating cost increases and process time is added, it may bring about an increase in manufacturing cost and productivity decrease.

In the hot rolling step (S200), the reheated steel is hot-rolled. In the hot rolling, the hot rolling end temperature (FDT) is set at A3 or higher. When the hot rolling end temperature is less than the A3 temperature, rolling is performed in the non-recrystallized region, so that the rolling addition may increase.

Subsequently, a step (S300) of winding the hot-rolled steel is performed. Winding temperature (CT) may be performed at 540 ~ 600 ℃. When winding at a temperature of less than 540°C, it is difficult to obtain a sufficient precipitation strengthening effect due to the difficulty of forming a precipitate, and when the temperature exceeds 600°C, the precipitation strengthening effect is difficult to obtain sufficiently.

The step of cold rolling the steel material (S400) may be performed at a reduction ratio of 55% or more. If the cold reduction ratio is less than 55%, recrystallization does not occur sufficiently, and there is a concern about occurrence of non-recrystallization during annealing. Therefore, the cold reduction ratio is preferably limited to 55% or more.

Annealing the cold-rolled steel at 770 to 810°C (S500) is performed. The annealing temperature is a factor that affects recrystallization and coarsening of precipitates. If it is less than 770°C, there is a concern about occurrence of non-recrystallization, and if it exceeds 810°C, the precipitates become coarse, reducing the effect of precipitation strengthening, which makes it difficult to obtain target strength.

A step (S600) of cooling the annealed steel is performed. After annealing, the cooling temperature is 400 ~ 500℃. The temperature after cooling affects the phase change of the austenite formed during annealing. If it is less than 400℃, there is a concern about achieving the target yield ratio due to the formation of martensite, and if it exceeds 500℃, pearlite instead of the target bainite is formed, making it difficult to obtain the target strength.

The yield strength of the steel sheet according to an embodiment of the present invention implemented by performing the above-described steps (S100) to (S600) is 590 MPa or more, tensile strength is 650 MPa or more, elongation is 14% or more, and yield ratio is 80 May be greater than or equal to %.

On the other hand, the final microstructure of the steel sheet according to an embodiment of the present invention implemented by performing the above-described steps (S100) to (S600) is a ferrite with an area fraction of 90 to 95% and an area fraction of 5 to 10%. It can be made of bainite. The ferrite may have an average grain size of 5 μm or less and a recrystallization ratio of 95% or more.

Experimental example

Hereinafter, a preferred experimental example is presented to aid the understanding of the present invention. However, the following experimental examples are only to aid understanding of the present invention, and the present invention is not limited by the following experimental examples.

Table 1 shows the specific alloying element composition (unit: wt%) of the composition component system in this experimental example, and Table 2 shows Examples 1 to 4 and Comparative Examples 1 to 4 implemented with the composition component system and process conditions shown in Table 1. Figure 7 shows the mechanical properties of the specimen.

Composition component C Si Mn Al Nb P S N
(ppm) One 0.08 0.2 1.6 0.02 0.04 <0.01 <0.003 60 2 0.08 0.2 1.2 0.02 0.04 <0.01 <0.003 60 3 0.08 0.2 1.9 0.02 0.04 <0.01 <0.003 60

Referring to Table 1, the composition component 1 is carbon (C): 0.06 to 0.10 wt%, silicon (Si): 0.1 to 0.2 wt%, manganese (Mn): 1.5 to 1.7 wt%, aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb): 0.03 to 0.06% by weight, phosphorus (P): more than 0 0.01% by weight, sulfur (S): more than 0 0.005% by weight, nitrogen (N): more than 0 0.006% by weight and The composition of the remaining iron (Fe) is satisfied.

On the other hand, the composition components 2 and 3 of Table 1, respectively, unlike the composition of the steel sheet according to an embodiment of the present invention, the weight ratio of manganese (Mn) is out of the range of 1.5 to 1.7% by weight.

division Steel grade Annealing temperature
(℃) After annealing
Cooling
Temperature
(℃) ferrite
Grain size
(㎛) Bainite
Fraction
(%) ferrite
Recrystallization fraction (%) YP
(MPa) TS
(MPa) EL
(%) YR
(%) Example 1 One 770 460 3.4 7.2 98.1 653 770 15.3 85 Example 2 One 800 460 3.9 8.1 97.8 608 705 16.7 86 Example 3 One 800 420 3.7 7.4 97.7 628 706 16.4 89 Example 4 One 800 500 4.1 7.9 98.5 594 673 17.8 88 Comparative Example 1 One 740 460 2.8 7.4 68.4 691 802 11.4 86 Comparative Example 2 One 820 460 4.2 7.8 98.3 527 648 21.7 81 Comparative Example 3 One 800 380 3.7 4.3 98.6 564 712 17.2 79 Comparative Example 4 2 770 460 3.3 3.9 98.2 572 660 15.9 87 Comparative Example 5 2 800 460 3.8 3.3 99.1 530 622 20.1 85 Comparative Example 6 3 770 460 3.9 15.4 87.4 720 845 11.3 85 Comparative Example 7 3 800 460 3.7 14.8 91.7 659 749 13.7 88

Referring to Table 2, the composition of Examples 1 to 4 is carbon (C), which is a composition of a steel sheet according to an embodiment of the present invention: 0.06 to 0.10% by weight, silicon (Si): 0.1 to 0.2% by weight, Manganese (Mn): 1.5 to 1.7% by weight, aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb): 0.03 to 0.06% by weight, phosphorus (P): more than 0 0.01% by weight or less, sulfur (S): More than 0 0.005% by weight, nitrogen (N): more than 0 and 0.006% by weight or less, and the composition of the remaining iron (Fe) is satisfied. In addition, the process conditions of Examples 1 to 4 are conditions for performing the above-described steps (S100) to (S600), specifically, the step of reheating the steel material to 1150 to 1250°C (S100); (b) hot rolling the steel material so that the hot rolling end temperature (FDT) is equal to or higher than the A3 temperature (S200); (c) winding the hot-rolled steel at 540 ~ 600°C (S300); (d) cold rolling the steel material at a reduction ratio of 55% or more (S400); (e) annealing the cold-rolled steel at 770 ~ 810 °C (S500); And (f) cooling the annealed steel to 400 ~ 500 ℃ step (S600); satisfies the range of conditions to perform.

It can be seen that the steel sheets according to Examples 1 to 4 had a yield strength of 590 MPa or more, a tensile strength of 650 MPa or more, an elongation of 14% or more, and a yield ratio of 80% or more.

Comparative Example 1, unlike the method of manufacturing a steel sheet according to an embodiment of the present invention, the temperature for annealing the cold-rolled steel material is out of the range of 770 ~ 810 ℃ bar, the elongation of the steel sheet is less than 14%, the final fine The recrystallization ratio of ferrite was found to be less than 95% in the structure.

Comparative Example 2, unlike the method of manufacturing a steel sheet according to an embodiment of the present invention, the temperature at which the cold-rolled steel is annealed is out of the range of 770 to 810°C, and the yield strength of the steel sheet is less than 590 MPa, and the tensile strength is It was found to be less than 650 MPa.

Comparative Example 3, unlike the method of manufacturing a steel sheet according to an embodiment of the present invention, the cooling temperature after annealing is out of the range of 400 ~ 500 ℃ bar, the yield strength of the steel sheet is less than 590 MPa, the final microstructure of bainite The area fraction does not satisfy the range of 5 to 10%.

Comparative Example 4, unlike the composition of the steel sheet according to an embodiment of the present invention, the weight ratio of manganese (Mn) is out of the range of 1.5 to 1.7% by weight bar, the yield strength of the steel sheet is less than 590 MPa, the bay in the final microstructure The area fraction of the knight does not satisfy the range of 5 to 10%.

Comparative Example 5, unlike the composition of the steel sheet according to an embodiment of the present invention, the weight ratio of manganese (Mn) is out of the range of 1.5 to 1.7% by weight bar, the yield strength of the steel sheet is less than 590 MPa, the tensile strength is less than 650 MPa And the area fraction of bainite in the final microstructure does not satisfy the range of 5 to 10%.

Comparative Example 6, unlike the composition of the steel sheet according to an embodiment of the present invention, the weight ratio of manganese (Mn) is out of the range of 1.5 to 1.7% by weight bar, the elongation of the steel sheet is less than 14%, and the bay in the final microstructure. The area fraction of the knight does not satisfy the range of 5 to 10%.

Comparative Example 7, unlike the composition of the steel sheet according to the embodiment of the present invention, the weight ratio of manganese (Mn) is out of the range of 1.5 to 1.7% by weight bar, the elongation of the steel sheet is less than 14%, and the bay in the final microstructure. The area fraction of the knight does not satisfy the range of 5 to 10%.

In the above, the embodiments of the present invention have been described mainly, but various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention as long as they do not depart from the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims (7)

Carbon (C): 0.06 to 0.10% by weight, silicon (Si): 0.1 to 0.2% by weight, manganese (Mn): 1.5 to 1.7% by weight, aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb): 0.03 ~ 0.06% by weight, phosphorus (P): more than 0 and not more than 0.01% by weight, sulfur (S): more than 0, 0.005% by weight, nitrogen (N): more than 0 and not more than 0.006% by weight and the rest of iron (Fe) and other inevitable impurities Done,
Yield strength is 590 MPa or more, tensile strength is 650 MPa or more, elongation is 14% or more, and yield ratio is 80% or more,
The final microstructure is composed of ferrite with an area fraction of 90 to 95% and bainite with an area fraction of 5 to 10%, and ferrite has an average grain size of 5 μm or less and a recrystallization ratio of 95% or more.
Cold rolled steel sheet.
delete delete (a) Carbon (C): 0.06 to 0.10% by weight, silicon (Si): 0.1 to 0.2% by weight, manganese (Mn): 1.5 to 1.7% by weight, aluminum (Al): 0.015 to 0.05% by weight, niobium (Nb ): 0.03 to 0.06% by weight, phosphorus (P): more than 0 and not more than 0.01% by weight, sulfur (S): more than 0, 0.005% by weight, nitrogen (N): more than 0 and not more than 0.006% by weight, and the rest of iron (Fe) and others Reheating the steel material made of inevitable impurities to 1150 to 1250°C;
(b) hot rolling the steel material so that the hot rolling end temperature (FDT) becomes higher than or equal to A3 temperature;
(c) winding the hot-rolled steel at 540 to 600°C;
(d) cold rolling the steel material at a reduction ratio of 55% or more;
(e) annealing the cold-rolled steel at 770 to 810°C;
(f) cooling the annealed steel to 400 ~ 500 ℃; Including,
The microstructure of the steel material after the step (f) is composed of ferrite with an area fraction of 90 to 95% and bainite with an area fraction of 5 to 10%,
Ferrite has an average grain size of 5 μm or less, a recrystallization ratio of 95% or more,
The steel after the step (f) has a yield strength of 590 MPa or more, a tensile strength of 650 MPa or more, an elongation of 14% or more, and a yield ratio of 80% or more,
Cold rolled steel sheet manufacturing method.




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