KR20200061642A - Steel sheet and method of manufacturing the same - Google Patents

Abstract

According to an embodiment of the present invention, a steel sheet comprises: 0.01 to 0.03 wt% of carbon (C); greater than 0 to 0.2 or less wt% of silicon (Si); 1.3 to 1.6 wt% of manganese (Mn); 0.2 to 0.4 wt% of aluminum (AI); 0.4 to 0.6 wt% of chromium (Cr); greater than 0 to 0.01 or less wt% of sulfur (S); remaining iron (Fe); and other inevitable impurities. The yield strength of the steel sheet is 210 to 260 MPa, the tensile strength is 390 MPa or more, the elongation is 36% or more, a work hardening index is 0.2 or more, and an r-bar value is 1.2 or more. According to the present invention, the steel sheet applicable to a vehicle exterior panel material having excellent formability is provided.

Description

Steel sheet and its manufacturing method{STEEL SHEET AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a steel sheet and a method of manufacturing the same, and more particularly, to a steel sheet and a method of manufacturing the same, which can be applied to an automobile outer plate material having excellent moldability.

The sintered hardened steel, which is widely used as an automotive exterior material, retains solid carbon to form a Cottrell atmosphere during the coating process (170℃, 20min). In this process, the yield strength of the final product (more than 30㎫) is increased, which has the advantage of improving the dent resistance. However, solid carbon in the steel has an activity even at room temperature in addition to the conditions of the coating treatment, and causes a aging phenomenon and elongation of the yield point over a period of time (more than 6 months). In order to overcome this aging phenomenon and use high-strength measures, dual phase steel is being used as an automotive exterior material, but its carbon content is higher than that of hardened hardened steel, resulting in lower elongation and plastic strain ratio (r value). Due to these problems, composite steel is being applied only to some parts with relatively low molding difficulty among automotive exterior materials.

Related prior art is Republic of Korea Application No. 10-2016-0083407, the name of the invention: a method for manufacturing an outer plate material for a sandwich steel plate and a sandwich steel plate using an outer plate material prepared therefrom).

The problem to be solved by the present invention is to provide a steel sheet that can be applied to an automobile outer plate material having excellent moldability and a manufacturing method thereof.

Steel sheet according to an embodiment of the present invention for achieving the above object is carbon (C): 0.01 ~ 0.03% by weight, silicon (Si): more than 0 0.2% by weight or less, manganese (Mn): 1.3 ~ 1.6% by weight, Aluminum (Al): 0.2 to 0.4% by weight, chromium (Cr): 0.4 to 0.6% by weight, sulfur (S): more than 0 to 0.01% by weight or less, and the rest of iron (Fe) and other unavoidable impurities. The yield strength of is 210 to 260 MPa, the tensile strength is 390 MPa or more, the elongation is 36% or more, the work hardening index is 0.2 or more, and the r-bar value is 1.2 or more.

The final microstructure of the steel sheet may include ferrite and martensite.

In the steel sheet, the fraction of martensite is 3 to 7%, and the fraction of ferrite may be other than the fraction of martensite.

Method for producing a steel sheet according to an embodiment of the present invention for achieving the above object is (a) carbon (C): 0.01 ~ 0.03% by weight, silicon (Si): more than 0 0.2% by weight or less, manganese (Mn): 1.3 to 1.6% by weight, aluminum (Al): 0.2 to 0.4% by weight, chromium (Cr): 0.4 to 0.6% by weight, sulfur (S): more than 0 to 0.01% by weight or less, and the remaining iron (Fe) and other unavoidable impurities Reheating the made steel to 1140 to 1170°C; (b) hot rolling the steel material to a rolling end temperature of 880 to 920°C; (c) winding the hot rolled steel at 640 to 680°C; (d) cold rolling the steel material at a reduction ratio of 60 to 80%; (e) annealing step of first annealing the cold-rolled steel material at 600 to 700° C. for 50 to 60 hours, and continuously annealing at 800 to 840° C. for a second time; And (f) temper rolling the annealed steel at a rolling reduction ratio of 0.3 to 1.0%. It includes.

In the method of manufacturing the steel sheet, the microstructure of the steel material after step (e) may include ferrite and austenite, and the microstructure of the steel material after step (f) may include ferrite and martensite.

In the method of manufacturing the steel sheet, the fraction of the martensite of the steel material after the step (f) is 3 to 7%, and the fraction of the ferrite may be other than the fraction of the martensite.

In the method of manufacturing the steel sheet, the yield strength of the steel sheet is 210 to 260 MPa, the tensile strength is 390 MPa or more, the elongation is 36% or more, the work hardening index is 0.2 or more, and the r-bar value may be 1.2 or more.

According to an embodiment of the present invention, it is possible to implement a steel sheet and a method of manufacturing the same, which can be applied to a vehicle exterior material having excellent moldability. 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 steel sheet according to an embodiment of the present invention.
Figure 2 shows the pole figure (Pole Figure) of the EBSD analysis of the material according to the annealing process after cold rolling in the steel sheet according to the comparative example of the present invention.
Figure 3 shows the {111} fraction of the EBSD analysis of the material according to the annealing process after cold rolling in the steel sheet according to the comparative example of the present invention.
Figure 4 shows the pole figure (Pole Figure) of the EBSD analysis of the material according to the annealing process after cold rolling in the steel sheet according to the embodiment of the present invention.
Figure 5 shows the {111} fraction (fraction) of the EBSD analysis of the material according to the annealing process after cold rolling in the 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 contents throughout the present specification.

The dual phase steel, which is widely used as a material for automobiles, consists of soft ferrite and hard martensite, and exhibits continuous yield behavior during tensile test due to dislocations around martensite and shows the same tensile strength. Also, the yield strength is relatively low, so it is easy to process and has the advantages of excellent baking hardening and aging resistance. However, the reason that it is difficult to be widely used for automotive exterior is due to the disadvantage of low plastic strain ratio (r value). In order to expand the use of composite-structured steel as a steel plate for automobile exterior panels, it is necessary to increase the r value by appropriately adjusting the alloy component or process conditions.

The present invention discloses a method of securing plasticity by improving the plastic strain ratio in a composite structure steel used as an automotive exterior material due to excellent stiffening and aging resistance. Securing moldability in the present invention is made in a hot rolling process, a cold rolling process, and an annealing process. More specifically, the second phase carbide (pearlite and cementite) is controlled in the hot rolling process, and the annealing process is a primary annealing process and a secondary annealing process. It is conducted twice. In the first annealing process (Batch Annealing), heat treatment is performed under conditions of time and temperature to sufficiently develop {111} aggregates in the steel. In the second annealing process (Continuous Annealing), ferrite and austenite fraction formation and carbon distribution And heat treatment to control the microstructure.

The present invention has been made on the basis of the above, and provides the following steel sheet and a method for manufacturing the same.

Grater

Steel sheet according to an embodiment of the present invention carbon (C): 0.01 ~ 0.03% by weight, silicon (Si): more than 0 0.2% by weight or less, manganese (Mn): 1.3 ~ 1.6% by weight, aluminum (Al): 0.2 ~ 0.4% by weight, chromium (Cr): 0.4 to 0.6% by weight, sulfur (S): more than 0 and less than 0.01% by weight, and the rest of iron (Fe) and other inevitable 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 important element for improving the strength of the steel sheet and securing martensite. In addition, it is an element that has the greatest influence on weldability. Carbon (C) may be added in a content ratio of 0.01 to 0.03% by weight of the total weight of the steel sheet according to an embodiment of the present invention. When the carbon content is less than 0.01% by weight of the total weight, it is difficult to secure the martensite structure, and it is difficult to maintain the resistance clothing and aging resistance properties. Conversely, when the carbon content exceeds 0.03% by weight of the total weight, the impact toughness of the base material may be reduced, and there may be a problem that deteriorates formability and weldability.

Silicon (Si)

Silicon (Si) is well known as a ferrite stabilizing element and is known as an element that increases ductility by increasing the ferrite fraction during cooling. On the other hand, silicon is added as a deoxidizer for removing oxygen in the steel in the steelmaking process together with aluminum, and may also have a solid solution strengthening effect. The silicone may be added in a content ratio of more than 0 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 exceeds 0.2% by weight of the total weight, the weldability of the steel decreases when a large amount is added, and the surface quality may be affected by generating a red scale during reheating and hot rolling.

Manganese (Mn)

Manganese (Mn) is an austenite stabilizing element that is used as an element that increases the fraction of the low-temperature phase and increases the strength of the steel with a solid solution strengthening effect. That is, manganese is effective in strengthening the solid solution, and can increase the hardenability of steel. In addition, manganese is an element that stabilizes the austenite structure to form martensite. Manganese may be added in a content ratio of 1.3 to 1.6% by weight of the total weight of the steel sheet according to an embodiment of the present invention. When the content of manganese is less than 1.3% by weight, it is difficult to secure martensite, and it is difficult to secure target strength and properties. In addition, when the content of manganese exceeds 1.6% by weight, elongation decreases, weldability decreases, MnS inclusions and center segregation occur, and thus ductility of the steel sheet may decrease and corrosion resistance may deteriorate.

Aluminum (Al)

Aluminum (Al) is an element that inhibits ferrite stabilization and carbide formation, such as silicon. That is, aluminum is an element effective to improve the elongation by inducing the growth of ferrite present in the early stage by distributing the carbon in the ferrite as austenite to improve the martensitic curing ability, and enabling high-temperature heat treatment during annealing. In addition, aluminum is added to the steelmaking process as a deoxidizer for removing oxygen in the steel, and can be precipitated in the steel with AlN to contribute to grain refinement. The aluminum may be added in a content ratio of 0.2 to 0.4% 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 0.2% by weight, the above-mentioned effect of adding aluminum is insufficient, and if it exceeds 0.4% by weight, process loads such as increase in steelmaking and annealing temperature occur and difficulty in playing decreases productivity, and alumina, a non-metallic inclusion ( Al ₂ O ₃ ) to form, there may be a problem that the ductility and toughness is lowered.

Chrome (Cr)

Chromium (Cr) is an element that improves quenching and forms a downward effect of yield strength. In addition, chromium is an element having a high effect of inhibiting high temperature phase transformation of ferrite and pearlite. In addition, chromium is a ferrite stabilizing element, which, when added to C-Mn steel, delays the diffusion of carbon due to the solute interference effect, thereby affecting the particle size refinement. The chromium may be added in a content ratio of 0.4 to 0.6% by weight of the total weight of the steel sheet according to an embodiment of the present invention. When the chromium content is less than 0.4% by weight of the total weight, the above-described effect of adding chromium cannot be properly exhibited. Conversely, when the content of chromium exceeds 0.6% by weight of the total weight, when added in large quantities, it may give a problem that the properties of steel are deteriorated in terms of toughness and curability.

Sulfur (S)

Sulfur (S) may form a precipitate of fine MnS to improve processability. The sulfur may be added in a content ratio of more than 0 to 0.01% by weight of the total weight of the steel sheet according to an embodiment of the present invention. When the sulfur content exceeds 0.01% by weight, it may cause surface defects and processing cracks, inhibit toughness and weldability, and lower the low-temperature impact value.

As described above, the steel sheet according to an embodiment of the present invention having an alloy element composition has a yield strength of 210 to 260 MPa, a tensile strength of 390 MPa or more, an elongation of 36% or more, and a work hardening index of 0.2 or more, r The -bar value may be 1.2 or more.

In addition, the final microstructure of the steel sheet according to an embodiment of the present invention having an alloy element composition as described above includes ferrite and martensite, the fraction of martensite is 3 to 7%, and the fraction of ferrite It may be the remainder excluding the fraction of martensite.

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

Steel plate manufacturing method

1 is a flowchart schematically showing a method of manufacturing a steel sheet according to an embodiment of the present invention. The method of manufacturing a steel sheet according to an embodiment of the present invention is characterized by improving elongation and plastic deformation ratio (r value), and the manufacturing process is composed of a hot rolling process, a cold rolling process, and an annealing process.

Referring to Figure 1, the method of manufacturing a steel sheet according to an embodiment of the present invention ((a) carbon (C): 0.01 ~ 0.03% by weight, silicon (Si): more than 0 0.2% by weight or less, manganese (Mn) : 1.3 to 1.6% by weight, aluminum (Al): 0.2 to 0.4% by weight, chromium (Cr): 0.4 to 0.6% by weight, sulfur (S): more than 0 and less than 0.01% by weight, and the remaining iron (Fe) and other unavoidable impurities Reheating the steel consisting of 1140 to 1170 ℃ (S100); (b) hot rolling the steel to a rolling end temperature of 880 ~ 920 ℃ (S200); (c) the hot-rolled steel 640 ~ Winding at 680° C. (S300); (d) Cold rolling the steel material at a reduction rate of 60 to 80% (S400); (e) 50 to 60 hours at 600 to 700° C. During the first annealing (Batch Annealing), and annealing treatment step (S500) of the second continuous annealing (Continuous Annealing) at 800 ~ 840 ℃; and (f) the annealing steel to a rolling reduction of 0.3 ~ 1.0% And temper rolling (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 steelmaking process.

The steel sheet is carbon (C): 0.01 to 0.03% by weight, silicon (Si): more than 0 to 0.2% by weight or less, manganese (Mn): 1.3 to 1.6% by weight, aluminum (Al): 0.2 to 0.4% by weight, chromium ( Cr): 0.4 to 0.6% by weight, sulfur (S): more than 0 and less than 0.01% by weight, and may be made of the remaining iron (Fe) and other inevitable impurities.

In one embodiment, the steel material may be reheated at a temperature of 1140 to 1170 ℃. When the steel sheet is reheated at the above-described temperature, components segregated during the continuous casting process may be re-used. When the reheating temperature is lower than 1140°C, the solid solution may not be sufficiently employed, and there may be a problem that segregated components are not evenly distributed during the continuous casting process. When the reheating temperature exceeds 1170°C, very coarse austenite grains may be formed, which may make it difficult to secure strength. In addition, when it exceeds 1170 ℃ heating cost is increased and the process time is added, it may lead to an increase in manufacturing cost and productivity decrease.

In the hot rolling step (S200), the reheated steel is hot rolled. The hot rolling may be controlled such that the rolling end temperature is 880 to 920°C, which is the temperature of Ar3 or higher. When the rolling end temperature is less than 880°C, rolling progresses in the unrecrystallized region, whereby the rolling addition may be increased, and the yield ratio of the steel sheet as a result of rolling may be increased. In addition, when the rolling end temperature exceeds 920°C, it may be difficult to secure target strength and toughness.

The step of winding the hot rolled steel (S300) may be performed at 640 ~ 680 ℃. When the coiling temperature is less than 640°C, second phase carbides are formed, which subsequently degrades the aggregated structure during the cold rolling and continuous annealing process. When it exceeds 680°C, surface thickening of elements such as Si or Mn is promoted to improve surface quality. The winding temperature is preferably carried out at a temperature of 640 ~ 680 ℃ because it can lower the.

Cold rolling the steel material (S400) may be performed at a reduction rate of 60 to 80%. If the cold rolling reduction is less than 60%, it is difficult to secure the accumulated energy required for recrystallization, and when it exceeds 80%, it is difficult to process by cold rolling and cracks may occur at the edge of the steel sheet during rolling, so the cold rolling reduction is 60 to 60%. It is desirable to limit it to 80%.

In the step of annealing the cold-rolled steel (S500), the cold-rolled steel is first annealed at 600 to 700°C for 50 to 60 hours (Batch Annealing), and continuously annealed twice at 800 to 840°C ( Continuous Annealing).

The annealing step is carried out a total of two times, first and second, and in the first step, batch annealing can be performed at a temperature of 600 to 700°C for 50 to 60 hours. If the temperature is less than 600°C or heat treatment is performed for less than 50 hours, {111} aggregates are difficult to develop sufficiently, and if the temperature exceeds 700°C or exceeds 60 hours, surface quality may deteriorate. It is desirable to limit it to 60 hours. Secondly, in continuous annealing, it is for promoting ferrite and austenite fraction formation and carbon distribution at the same time as recrystallization. When the recrystallization temperature is less than 800℃, the amount of austenite decreases. When it exceeds 840℃, the amount of austenite is excessive. It is preferable to limit the recrystallization temperature to 800 to 840°C because the ductility is reduced due to formation.

The step of roughly rolling the annealed steel (S600) may be performed at a rolling reduction of 0.3 to 1.0%. After annealing, temper rolling (SPM; Skin Pass Mill) rolling is performed by 0.3% or more to increase the additional dislocation density, but when it exceeds 1.0%, yield strength is increased so that shape freeze deterioration due to resistive ratio does not occur.

The microstructure of the steel material after the annealing step (S500) may include ferrite and austenite. The microstructure of the steel material after the temper rolling step (S600) may include ferrite and martensite, in which case the fraction of martensite is 3 to 7%, and the fraction of ferrite is the fraction of martensite It can be the rest.

The yield strength of the steel sheet according to an embodiment of the present invention implemented by performing the above steps (S100) to (S600) is 210 to 260 MPa, tensile strength is 390 MPa or more, elongation is 36% or more, and work hardening The index is 0.2 or more, and the r-bar value may be 1.2 or more.

Figure 2 shows the pole figure (Pole Figure) of the EBSD analysis results of the material according to the annealing process after cold rolling in the steel sheet according to the comparative example of the present invention, Figure 3 is after cold rolling in the steel sheet according to the comparative example of the present invention It shows the {111} fraction of EBSD analysis results of materials according to the annealing process. Figure 4 shows the pole figure (Pole Figure) of the EBSD analysis of the material according to the annealing process after cold rolling in the steel sheet according to the embodiment of the present invention, Figure 5 is after cold rolling in the steel sheet according to the embodiment of the present invention It shows the {111} fraction of EBSD analysis results of materials according to the annealing process.

2 to 5, in the comparative example of the present invention, the annealing process performed only the continuous annealing process, and in the embodiment of the present invention, the annealing process was carried out after the annealing process.

In the comparative example of the present invention, the γ-fiber value of the composite tissue steel subjected to continuous annealing is 25 to 30%, but in the embodiment of the present invention, the γ-fiber value of the material subjected to continuous annealing after the normal annealing is 42 to 47. %, up more than 12%. This indicates that the {111} aggregate structure in the material developed during the normal annealing process, and it is possible to improve the moldability of the final product by improving the plastic strain ratio only by changing the process conditions without adding expensive alloying components.

Experimental Example

Hereinafter, a preferred experimental example is presented to help 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 alloy element composition (unit: weight ratio %) of the compositional component system in this Experimental Example, and Table 2 shows Examples 1 to 2 and Comparative Examples 1 to 2 implemented with the compositional component system and process conditions shown in Table 1. 8 shows the mechanical properties of the specimen.

Specifically, the steel composition as shown in Table 1 was reheated at 1160° C. and then hot rolled at 900° C. to prepare a steel sheet having a thickness of 3.0 mm. After cooling, it was wound at 660° C., pickled, and cold rolled to a thickness of 0.8 mm. The cold-rolled steel sheet was subjected to annealing heat treatment at a temperature of 820° C., temper rolling was performed at 0.5%, and a tensile test was performed on specimens taken in the vertical direction in the rolling direction to obtain material performance as shown in Table 2.

Composition ingredient system C Si Mn S Al Cr One 0.031 0.112 1.42 0.003 0.305 0.503 2 0.019 0.108 1.62 0.004 0.299 0.505 3 0.021 0.106 1.43 0.003 0.042 0.497 4 0.020 0.106 1.36 0.006 0.222 0.475 5 0.020 0.100 1.35 0.007 0.254 0.475

Referring to Table 1, composition components 4 and 5 are the composition of the steel sheet according to an embodiment of the present invention, carbon (C): 0.01 to 0.03 wt%, silicon (Si): more than 0 and 0.2 wt% or less, manganese (Mn ): 1.3 to 1.6% by weight, aluminum (Al): 0.2 to 0.4% by weight, chromium (Cr): 0.4 to 0.6% by weight, sulfur (S): more than 0 to 0.01% by weight or less, and a composition composed of the remaining iron (Fe) It is a satisfactory ingredient system. On the other hand, the composition component system 1, 2, 3 of Table 1, respectively, unlike the composition of the steel sheet according to an embodiment of the present invention, the weight percentage of carbon (C) is outside the range of 0.01 to 0.03% by weight, and manganese (Mn ) Is out of the range of 1.3 to 1.6% by weight, and the weight of aluminum (Al) is outside the range of 0.2 to 0.4% by weight.

Referring to Table 2, the composition of Example 1 and Example 2 is the composition of the steel sheet according to an embodiment of the present invention carbon (C): 0.01 ~ 0.03% by weight, silicon (Si): more than 0 0.2% by weight or less , Manganese (Mn): 1.3 to 1.6% by weight, aluminum (Al): 0.2 to 0.4% by weight, chromium (Cr): 0.4 to 0.6% by weight, sulfur (S): more than 0 and less than 0.01% by weight, and the remaining iron (Fe ). In addition, the process conditions of Examples 1 and 2 are conditions for performing the above-described steps (S100) to Step (S600), specifically, reheating the steel to 1140 to 1170°C (S100); Hot rolling the steel material to a rolling end temperature of 880 to 920°C (S200); Winding the hot-rolled steel material at 640 ~ 680 ℃ (S300); Cold rolling the steel material at a reduction rate of 60 to 80% (S400); Annealing step (Batch Annealing) of the cold-rolled steel material at 600 ~ 700 ℃ for 50 ~ 60 hours (Batch Annealing), and continuous annealing (Continuous Annealing) at 800 ~ 840 ℃ for a second time (S500); And it satisfies the condition range to perform the step (S600) of the annealing the steel material to the rolling reduction rate of 0.3 to 1.0%.

The steel sheet according to Examples 1 and 2 has a yield strength (YP) of 210 to 260 MPa, a tensile strength (TS) of 390 MPa or more, an elongation (EL) of 36% or more, and a work hardening index (n) of 0.2. Above, it can be confirmed that the r-bar value is 1.2 or more.

In Comparative Examples 1 to 3, unlike the composition of the steel sheet according to an embodiment of the present invention, the weight percent of carbon (C) is outside the range of 0.01 to 0.03 percent by weight, or the weight percent of manganese (Mn) is 1.3. ~ 1.6% by weight out of the range, or the aluminum (Al) weight% has a composition outside the range of 0.2 to 0.4% by weight bar, r-bar value of 1.2 or more was not secured, Comparative Example 1 has an elongation of 36% or more A work hardening index of 0.2 or more was not secured.

Comparative Example 4, unlike the method of manufacturing a steel sheet according to an embodiment of the present invention, the hot-rolled steel was wound in a range that does not satisfy 640 to 680°C, and an r-bar value of 1.2 or more was not secured. .

In Comparative Example 5, unlike the method of manufacturing a steel sheet according to an embodiment of the present invention, cold rolled steel was subjected to a batch annealing process for 50 to 60 hours in a range that does not satisfy 600 to 700°C. Bar, r-bar value of 1.2 or more could not be secured.

In Comparative Example 6, unlike the method of manufacturing a steel sheet according to an embodiment of the present invention, a cold-rolled steel material was subjected to a batch annealing process at 600 to 700° C. for less than 50 hours. The r-bar value was not secured.

In Comparative Example 7, unlike the method of manufacturing a steel sheet according to an embodiment of the present invention, a cold-rolled steel material was subjected to a continuous annealing process in a range that does not satisfy 800 to 840° C. The elongation was not secured.

In the above, although the description has been mainly focused on the embodiment of the present invention, various changes or modifications can be made at the level of those skilled in the art. It can be said that such modifications and variations belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention should be judged by the claims set forth below.

Claims (7)

Carbon (C): 0.01 to 0.03% by weight, silicon (Si): more than 0 to 0.2% by weight, manganese (Mn): 1.3 to 1.6% by weight, aluminum (Al): 0.2 to 0.4% by weight, chromium (Cr): 0.4 ~ 0.6% by weight, sulfur (S): more than 0 and less than 0.01% by weight and the rest of the iron (Fe) and other unavoidable impurities steel sheet,
The steel sheet has a yield strength of 210 to 260 MPa, a tensile strength of 390 MPa or more, an elongation of 36% or more, a work hardening index of 0.2 or more, and an r-bar value of 1.2 or more.
Grater. According to claim 1,
The final microstructure of the steel sheet is characterized in that it comprises ferrite and martensite,
Grater. According to claim 2,
The fraction of the martensite is 3 to 7%, characterized in that the fraction of the ferrite is the remainder excluding the fraction of martensite,
Grater. (a) Carbon (C): 0.01 to 0.03% by weight, silicon (Si): more than 0 to 0.2% by weight or less, manganese (Mn): 1.3 to 1.6% by weight, aluminum (Al): 0.2 to 0.4% by weight, chromium ( Cr): 0.4 to 0.6% by weight, sulfur (S): more than 0 to less than 0.01% by weight and the remaining iron (Fe) and other unavoidable impurities steel reheating step to 1140 to 1170 ℃;
(b) hot rolling the steel material to a rolling end temperature of 880 to 920°C;
(c) winding the hot rolled steel at 640 to 680°C;
(d) cold rolling the steel material at a reduction ratio of 60 to 80%;
(e) annealing step of first annealing the cold-rolled steel material at 600 to 700° C. for 50 to 60 hours, and continuously annealing at 800 to 840° C. for a second time; And
(f) temper rolling the annealed steel material at a rolling reduction ratio of 0.3 to 1.0%; Containing,
Method of manufacturing a steel sheet. The method of claim 4,
The microstructure of the steel material after the step (e) includes ferrite and austenite,
After the step (f), the microstructure of the steel material includes ferrite and martensite,
Method of manufacturing a steel sheet. The method of claim 5,
The fraction of the martensite of the steel material after the step (f) is 3 to 7%, and the fraction of the ferrite is the remainder excluding the fraction of the martensite,
Method of manufacturing a steel sheet. The method of claim 4,
The steel sheet has a yield strength of 210 to 260 MPa, a tensile strength of 390 MPa or more, an elongation of 36% or more, a work hardening index of 0.2 or more, and an r-bar value of 1.2 or more.
Method of manufacturing a steel sheet.

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