KR20210147254A - Cold rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

The present invention provides a 1.5G class cold rolled steel sheet having an excellent yield ratio and excellent bendability and a manufacturing method thereof. According to an embodiment of the present invention, the cold rolled steel sheet comprises: 0.24 to 0.30 wt% of carbon (C); 0.05 to 0.3 wt% of silicon (Si); 1.0 to 2.5 wt% of manganese (Mn); 0.01 to 0.05 wt% of aluminum (Al); more than 0 and equal to or less than 0.02 wt% of phosphorus (P); more than 0 and equal to or less than 0.01 wt% of sulfur (S); 0.2 to 0.5 wt% of chromium (Cr); 0.1 to 0.3 wt% of molybdenum (Mo); 0.01 to 0.045 wt% of titanium (Ti); more than 0 and equal to or less than 0.003 wt% of boron (B); and the remainder consisting of iron (Fe) and other inevitable impurities, wherein the yield strength is 1,200 to 1,450 MPa, the tensile strength is 1,550 to 1,700 MPa, the elongation is 5 to 8%, and the yield ratio may be equal to or greater than 75%.

Description

Cold-rolled steel sheet and its manufacturing method

The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a 1.5G class ultra-high strength cold-rolled steel sheet having excellent yield ratio and bendability and a method for manufacturing the same.

As the use of ultra-high-strength steel is gradually increasing, the use of steel plates of 1.5 GPa or higher for the purpose of improving the stability of collision members such as bumper beam, side sill, and door impact beam and reducing body weight this is increasing One of the most important performances for use as automotive parts is machinability, and in the case of 1.5GPa grade materials, bendability (R/t) is applied as a standard for evaluation. In addition, it is also important to secure high yield strength for excellent impact energy absorption for parts that need to consider collision stability.

Korean Patent Laid-Open Publication No. 10-2004-0111413

The technical problem to be achieved by the technical idea of the present invention is to provide a 1.5G class ultra-high strength cold-rolled steel sheet having excellent yield ratio and bendability and a method for manufacturing the same. However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, there is provided an ultra-high strength cold-rolled steel sheet excellent in yield ratio and bendability.

According to an embodiment of the present invention, the cold-rolled steel sheet is carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al) : 0.01 to 0.05 wt%, phosphorus (P): more than 0, 0.02 wt% or less, sulfur (S): more than 0, 0.01 wt% or less, chromium (Cr): 0.2 to 0.5 wt%, molybdenum (Mo): 0.1 to 0.3 wt%, titanium (Ti): 0.01 to 0.045 wt%, boron (B): more than 0 and 0.003 wt% or less and the remaining iron (Fe) and other unavoidable impurities, the yield strength is 1200 to 1450 MPa, and the tensile strength is 1550 to 1700 MPa, the elongation may be 5 to 8%, and the yield ratio may be 75% or more.

According to an embodiment of the present invention, the bendability (R/t) of the steel sheet may be 3.0 or less.

According to one embodiment of the present invention, the final microstructure of the steel sheet may be made of 70% or more and less than 100% tempered martensite, more than 0 20% bainite, and more than 0 10% ferrite.

According to another aspect of the present invention, there is provided a method for manufacturing an ultra-high strength cold-rolled steel sheet having excellent yield ratio and bendability.

According to an embodiment of the present invention, the method for manufacturing the cold-rolled steel sheet comprises (a) carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt% %, aluminum (Al): 0.01 to 0.05 wt%, phosphorus (P): more than 0, 0.02 wt% or less, sulfur (S): more than 0, 0.01 wt% or less, chromium (Cr): 0.2 to 0.5 wt%, molybdenum ( Mo): 0.1 to 0.3% by weight, titanium (Ti): 0.01 to 0.045% by weight, boron (B): providing a steel material consisting of more than 0 and 0.003% by weight or less and the remainder iron (Fe) and other unavoidable impurities; (b) hot-rolling the steel material under the condition that the reheating temperature (SRT) is 1150 to 1300 °C, and the hot rolling end temperature (FDT) is 800 to 1000 °C; (c) winding the hot-rolled steel at 500 to 650°C; (d) cold rolling the steel; (e) annealing the cold-rolled steel at 800 to 900° C.; And (f) tempering the annealing heat-treated steel at 100 ~ 250 ℃; includes

According to an embodiment of the present invention, the step (e) comprises annealing the cold rolled steel at 800 ~ 900 ℃; and cooling the steel material to 300 to 450° C. at a cooling rate of 3 to 20° C./s.

According to an embodiment of the present invention, the step (f) may include tempering the annealed heat-treated steel at 100 to 250° C. for 3 to 12 hours.

According to an embodiment of the present invention, the step (b) includes the step of hot rolling at a reduction ratio of 90% or more, and the step (d) includes the step of cold rolling at a reduction ratio of 40 to 60%. can

According to an embodiment of the present invention, the steel sheet after step (f) has a yield strength of 1200 to 1450 MPa, a tensile strength of 1550 to 1700 MPa, an elongation of 5 to 8%, a yield ratio of 75% or more, and bending The workability (R/t) may be 3.0 or less.

According to the technical idea of the present invention, it is possible to implement a 1.5G class ultra-high strength cold-rolled steel sheet having excellent yield ratio and bendability and a manufacturing method thereof. The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a flowchart schematically illustrating a method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention.
2 is a photograph of a hot-rolled microstructure in a method for manufacturing a steel sheet according to an embodiment of the present invention.
3 is a graph showing the material change according to the tempering temperature in the manufacturing method of the cold-rolled steel sheet of the present invention.
4 is a photograph of a cold-rolled microstructure in a method for manufacturing a steel sheet according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Therefore, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

cold rolled steel

A cold-rolled steel sheet according to an embodiment of the present invention is carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al): 0.01 ~ 0.05 wt%, phosphorus (P): more than 0, 0.02 wt% or less, sulfur (S): more than 0, 0.01 wt% or less, chromium (Cr): 0.2 to 0.5 wt%, molybdenum (Mo): 0.1 to 0.3 wt% , titanium (Ti): 0.01 to 0.045% by weight, boron (B): more than 0 0.003% 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 improves the strength of the steel sheet and plays a major role in forming precipitates and bainite phases. In addition, it is an element that has the greatest influence on weldability. Carbon (C) may be added in a content ratio of 0.24 to 0.30 wt% based on the total weight of the cold-rolled steel sheet according to an embodiment of the present invention. When the carbon content is less than 0.24% by weight of the total weight, it is difficult to secure strength because the formation of precipitates and bainite phase for strength are not sufficient. Conversely, when the content of carbon exceeds 0.30% by weight of the total weight, cracks in the cast steel in the steel making and casting process occur and workability and weldability are inferior, so the content is limited.

Silicon (Si)

Silicon (Si) is well known as a ferrite stabilizing element and is well known as an element that increases ductility by increasing the ferrite fraction during cooling. On the other hand, silicon is added together with aluminum as a deoxidizer for removing oxygen in steel in the steelmaking process, and may also have a solid solution strengthening effect. The silicon may be added in a content ratio of 0.05 to 0.3% by weight based on the total weight of the steel sheet according to an embodiment of the present invention. If the content of silicon is less than 0.05% by weight of the total weight, it is not easy to secure ductility and the effect of solid solution strengthening cannot be sufficiently realized. By creating (red scale), the plating property of the steel sheet is lowered, and it may 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 1.0 to 2.5% by weight based on the total weight of the steel sheet according to an embodiment of the present invention. When the manganese content is less than 1.0% by weight, it is difficult to implement the above-described effect and it is not easy to secure strength. In addition, when the content of manganese exceeds 2.5% by weight, the elongation is lowered, weldability is lowered, MnS inclusions and center segregation occur, so that the ductility of the steel sheet is lowered and corrosion resistance can be lowered, It causes cracks to occur and propagate, resulting in reduced workability.

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 for improving elongation by inducing the growth of ferrite present in the initial stage by enabling high-temperature heat treatment during annealing. In addition, aluminum is added to the steel making process as a deoxidizer for removing oxygen from the steel, and may be precipitated as AlN in the steel to contribute to grain refinement. The aluminum may be added in a content ratio of 0.01 to 0.05% by weight based on the weight of the steel sheet according to an embodiment of the present invention. If the content of aluminum is less than 0.01% by weight, the effect of adding aluminum is insufficient. If it exceeds 0.05% by weight, the quality of the slab and hot-rolled material deteriorates, process loads such as increased steelmaking and annealing temperature occur, and it is difficult to play. Productivity is lowered, and alumina (Al ₂ O ₃ ), which is a non-metallic inclusion, is formed and concentrated on the surface of the steel sheet, so there may be problems in that plating properties are lowered and ductility and toughness are lowered.

Phosphorus (P)

Phosphorus (P) may perform a function of increasing the strength of strength by solid solution strengthening and suppressing the formation of carbides. The phosphorus may be added in a content ratio of greater than 0 and 0.02 wt% or less of the total weight of the cold-rolled steel sheet according to an embodiment of the present invention. If the phosphorus content exceeds 0.02% by weight, crystal grain boundaries and interphase grain boundaries segregate, so that the weld is brittle, the press formability is poor, hot brittleness and weldability may be impaired, and there is a problem in that the low-temperature impact value is lowered by the precipitation behavior.

Sulfur (S)

Sulfur (S) can improve processability by forming fine MnS precipitates. The sulfur may be added in a content ratio of greater than 0 to 0.01% by weight or less based on the total weight of the steel sheet according to an embodiment of the present invention. When the sulfur content exceeds 0.01% by weight, MnS non-metal inclusions in the steel segregate during casting and solidification, causing high-temperature cracks, causing surface defects and processing cracks, impairing toughness and weldability, and lowering the low-temperature impact value. .

Chromium (Cr)

Chromium (Cr) is an element that plays a major role in improving hardenability and increasing strength in the present invention. Chromium may be added in a content ratio of 0.2 to 0.5% by weight based on the total weight of the steel sheet according to an embodiment of the present invention. When the content of chromium is less than 0.2% by weight, it is difficult to implement the above-described effect. In addition, when the content of chromium exceeds 0.5% by weight, there is a problem in that a saturation effect occurs and ductility is lowered.

Molybdenum (Mo)

Molybdenum (Mo) is an element that plays a major role in improving hardenability and increasing strength in the present invention. Molybdenum may be added in a content ratio of 0.1 to 0.3% by weight based on the total weight of the steel sheet according to an embodiment of the present invention. When the content of molybdenum is less than 0.1% by weight, it is difficult to implement the above-described effect and it is difficult to secure strength. In addition, when the content of molybdenum exceeds 0.3% by weight, there is a problem in that the production cost increases because it is more expensive than chromium.

Titanium (Ti)

Titanium (Ti) is an element that serves to refine crystal grains and suppress the formation of boron nitride in the present invention. Titanium may be added in a content ratio of 0.01 to 0.045% by weight based on the total weight of the steel sheet according to an embodiment of the present invention. When the content of titanium is less than 0.01% by weight, there is a problem in that the quality of the slab is deteriorated due to the decrease in ductility of the cast slab due to excessive precipitation of AIN and BN precipitates. In addition, when the content of titanium exceeds 0.045% by weight, it is difficult to refine grains due to the formation of coarse TiN and TiC precipitates, and there is a problem in that the recrystallization temperature is excessively increased to cause a non-uniform structure.

boron (B)

Boron (B) is an element that increases the hardenability of steel and may be an effective element for increasing the hardenability by suppressing the formation of ferrite. The phosphorus may be added in a content ratio of greater than 0 to 0.003% by weight or less based on the total weight of the cold-rolled steel sheet according to an embodiment of the present invention. When the content of boron exceeds 0.003% by weight, there is a problem in that weldability is deteriorated.

As described above, the cold rolled steel sheet according to an embodiment of the present invention having the alloy element composition has a yield strength of 1200 to 1450 MPa, a tensile strength of 1550 to 1700 MPa, an elongation of 5 to 8%, and a yield ratio of 75% or more. have. Furthermore, the bendability (R/t) of the cold-rolled steel sheet may be 3.0 or less. That is, the cold-rolled steel sheet may have a minimum bending radius (R) of less than three times the thickness (t) of the steel sheet when bending at 90°.

In addition, the final microstructure of the steel sheet according to an embodiment of the present invention having the alloy element composition as described above is 70% or more and less than 100% tempered martensite, more than 0 20% or less bainite, and more than 0 10% It may be made of the following ferrite. The volume fraction is based on a result of analyzing a 1/4 point in the thickness direction in a direction perpendicular to the rolling direction with a scanning electron microscope (SEM).

If the fraction of tempered martensite is less than 70%, the target strength cannot be obtained. Bainite is a microstructure that is inevitably generated due to an insufficient cooling rate, and is a major factor that reduces strength. The smaller the fraction, the better. If the fraction of bainite exceeds 20%, the target strength cannot be obtained. Ferrite is a microstructure that is inevitably generated during production, and the smaller the fraction, the more advantageous. When the fraction of ferrite exceeds 10%, strength and formability are reduced.

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

Manufacturing method of cold rolled steel sheet

1 is a flowchart schematically showing a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention, FIG. 2 is a photograph of a hot-rolled microstructure in a method for manufacturing a steel sheet according to an embodiment of the present invention, and FIG. 3 is a graph showing the material change according to the tempering temperature in the manufacturing method of the cold-rolled steel sheet of the present invention. 4 is a photograph of a cold-rolled microstructure in a method for manufacturing a steel sheet according to an embodiment of the present invention.

Referring to FIG. 1 , the method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention includes (a) carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, Aluminum (Al): 0.01 to 0.05 wt%, Phosphorus (P): more than 0, 0.02 wt% or less, Sulfur (S): more than 0, 0.01 wt% or less, Chromium (Cr): 0.2 to 0.5 wt% %, molybdenum (Mo): 0.1 to 0.3% by weight, titanium (Ti): 0.01 to 0.045% by weight, boron (B): more than 0 and 0.003% by weight or less, and the remaining iron (Fe) and other unavoidable impurities. to (S100); (b) hot-rolling the steel material under the condition that the reheating temperature (SRT) is 1150 to 1300 °C, and the hot rolling end temperature (FDT) is 800 to 1000 °C (S200); (c) winding the hot-rolled steel at 500 to 650° C. (S300); (d) cold rolling the steel material (S400); (e) annealing the cold-rolled steel at 800 to 900° C. (S500); (f) tempering the annealed heat-treated steel at 100 to 250° C. (S600); includes

The step of hot rolling (S200) in a broad sense includes reheating the steel sheet having the above-described predetermined composition. The steel sheet may be manufactured through a continuous casting process after obtaining molten steel of a desired composition through a steelmaking process.

In one embodiment, the steel may be reheated at a temperature of 1150 ~ 1300 ℃. When the steel sheet is reheated at the above-mentioned temperature, segregated components are re-dissolved during the continuous casting process, which affects the uniformity of the structure and the dissolution of precipitates in the slab. When 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 that the segregated components are not sufficiently evenly distributed during the continuous casting process. When the reheating temperature exceeds 1300°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 1300 ℃ heating cost is increased and process time is added, which may result in an increase in manufacturing cost and a decrease in productivity.

The reheated steel material is hot rolled. The hot rolling is performed so that the hot rolling end temperature (FDT) is in the range of 800 ~ 1000 ℃. When the hot-rolling end temperature is less than 800° C., the rolling proceeds in the non-recrystallized region, thereby increasing the rolling addition. The hot rolling may have a rolling reduction of 90% or more. The hot-rolled microstructure is made of martensite, bainite, and ferrite (FIG. 2).

Subsequently, a step (S300) of winding the hot-rolled steel is performed. Coiling temperature (CT) may be performed at 500 ~ 650 ℃. When winding at a temperature of less than 500℃, it makes the shape of the hot-rolled coil non-uniform, causes a load during cold rolling, and it is difficult to form precipitates, making it difficult to obtain a sufficient precipitation strengthening effect. It causes a non-uniform microstructure by

The cold rolling of the steel material (S400) may be performed at a reduction ratio of 40 to 60%. If the cold reduction ratio is less than 40%, recrystallization does not occur sufficiently, so there is a concern that non-recrystallization occurs during annealing, and it is difficult to obtain a uniform microstructure.

Annealing the cold-rolled steel at 800 to 900° C. (S500) is performed. The annealing temperature is a factor that affects recrystallization and coarsening of precipitates. When it is less than 800°C, there is concern about non-recrystallization, and when it exceeds 900°C, the precipitate becomes coarse and the precipitation strengthening effect is reduced, which makes it difficult to obtain the target strength.

As a specific example, in the annealing heat treatment process, the temperature may be increased to a temperature of Ac3 or higher at a temperature increase rate of 1 to 10°C. Preferably, it can be maintained at a temperature between 800 and 900° C. for 60 to 600 seconds. After that, it can be cooled to 300-450°C, which is the end temperature (RCS) during secondary cooling, at an average cooling rate of 3-20°C/s and maintained for 10-100 seconds.

The step (S600) of tempering the annealed heat-treated steel at 100 ~ 250 ℃ is performed. For a specific example, tempering at a temperature of 100 ~ 250 ℃ for 3 ~ 12 hours. When the temperature is 100° C. or less, it is difficult to obtain the target yield strength, and when it is 250° C. or more, it is difficult to obtain the target bendability.

3 is a graph showing the material change according to the tempering temperature in the manufacturing method of the cold rolled steel sheet of the present invention. 3 , the horizontal axis means tempering temperature, the left vertical axis means the strength value, and the right vertical axis means bending workability (R/t).

Referring to Figure 3, when the tempering temperature is increased from 100 ℃ to 280 ℃, the yield strength is gradually increased, it can be seen that the tensile strength is gradually decreased, it can be confirmed that when the tempering temperature is too high, the bending properties are lowered can

During tempering in an appropriate temperature range, carbon is fixed to dislocation and lath boundaries, which increases yield strength and reduces dislocation inside martensite, improving bending properties. However, carbides are formed depending on the tempering temperature and time, and the size increases proportionally as the temperature and time increase. If the tempering temperature is too high, the carbide is coarsened to deteriorate the bending properties.

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 1200 to 1450 MPa, the tensile strength is 1550 to 1700 MPa, and the elongation is 5 to 8%, The yield ratio may be 75% or more, and the bendability (R/t) may be 3.0 or less.

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 70% or more and less than 100% tempered martensite, 0 to 20% or less It may consist of bainite and greater than 0 and less than or equal to 10% ferrite (see FIG. 4 ).

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping 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 %) of the compositional component system in this experimental example, and Table 2 is Comparative Examples 1-9 and Examples 1-9 implemented with the compositional component system and process conditions described in Table 1 3 shows the mechanical properties of the specimen.

ingredient system C Si Mn Al P S Cr Mo Ti B A 0.27 0.1 2.0 0.03 0.01 0.005 0.4 0.2 0.03 0.0025 B 0.23 0.2 2.2 0.03 0.01 0.005 0.3 0.2 0.06 0.0025 C 0.25 0.02 2.6 0.03 0.01 0.005 0.4 0.1 0.025 0.0025

Referring to Table 1, the compositional component system A is carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al): 0.01 to 0.05 wt%, phosphorus (P): more than 0, 0.02 wt% or less, sulfur (S): more than 0, 0.01 wt% or less, chromium (Cr): 0.2 to 0.5 wt%, molybdenum (Mo): 0.1 to 0.3 wt%, Titanium (Ti): 0.01 to 0.045 wt%, boron (B): more than 0 0.003 wt% or less, and the remaining iron (Fe) satisfies the composition of an embodiment of the present invention.

On the other hand, composition B of Table 1, unlike the composition of the steel sheet according to an embodiment of the present invention, carbon (C): 0.24 to 0.30 wt%, titanium (Ti): out of the range of 0.01 to 0.045 wt%. And the compositional component system C of Table 1, unlike the composition of the steel sheet according to an embodiment of the present invention, silicon (Si): 0.05 to 0.3% by weight, manganese (Mn): out of the range of 1.0 to 2.5% by weight.

ingredient system RCS
(℃) tempering
(℃) YP
(MPa) ts
(MPa) EL
(%) yield ratio
(YP/TS)X100 R/t Example 1 A 350 150 1280 1585 5.9 81 2.8 Example 2 A 450 120 1331 1641 7.5 81 2.6 Example 3 A 350 100 1246 1670 5.9 75 2.8 Comparative Example 1 A 250 150 1304 1415 7.0 92 3.4 Comparative Example 2 B 250 150 1293 1417 6.5 91 3.4 Comparative Example 3 C 250 150 1316 1420 5.9 93 3.6 Comparative Example 4 B 350 150 1222 1493 6.5 82 2.6 Comparative Example 5 C 350 150 1277 1585 5.9 81 3.6 Comparative Example 6 B 450 120 1135 1543 8.7 74 2.6 Comparative Example 7 C 450 120 1313 1629 6.2 81 3.6 Comparative Example 8 A 350 280 1361 1425 8.1 93 3.4 Comparative Example 9 A 350 25 1169 1686 6.5 69 4.8

Referring to Table 2, Examples 1 to 3 of the present invention are carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, Aluminum (Al): 0.01 to 0.05% by weight, phosphorus (P): more than 0 and 0.02% by weight or less, sulfur (S): more than 0 to 0.01% by weight or less, chromium (Cr): 0.2 to 0.5% by weight, molybdenum (Mo) : 0.1 to 0.3% by weight, titanium (Ti): 0.01 to 0.045% by weight, boron (B): more than 0 and 0.003% by weight or less, and the remaining iron (Fe). The end temperature (RCS) satisfies the range of 300 ~ 450 ℃, the tempering temperature satisfies the range of 100 ~ 250 ℃.

The steel sheets of Examples 1 to 3 of the present invention satisfying these composition and process conditions have a yield strength of 1200 to 1450 MPa, a tensile strength of 1550 to 1700 MPa, an elongation of 5 to 8%, and a yield ratio of 75% or more. It can be confirmed that the bending workability (R/t) satisfies the range of 3.0 or less.

On the other hand, Comparative Example 2, Comparative Example 4, Comparative Example 6 of the present invention has a compositional component system B, unlike the composition of the steel sheet according to an embodiment of the invention, carbon (C): 0.24 to 0.30 wt%, titanium ( Ti): does not satisfy the range of 0.01 to 0.045% by weight. Looking at these comparative examples, it can be seen that in the cold-rolled steel sheet having the compositional component B, the tensile strength is low due to an insufficient carbon content, and titanium is excessively added, resulting in a decrease in strength due to the effect of refining the structure and excessive carbide formation.

Comparative Examples 3, 5, and 7 of the present invention have a compositional component system C, and unlike the composition of the steel sheet according to an embodiment of the present invention, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn) : Does not satisfy the range of 1.0 ~ 2.5% by weight. Looking at these comparative examples, it can be seen that the cold-rolled steel sheet having the compositional component C has a problem in that the bendability deteriorates due to the excessive addition of Mn.

In Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention, unlike the method of manufacturing a steel sheet according to an embodiment of the present invention, the end temperature (RCS) during secondary cooling of the annealing heat treatment process is in the range of 300 to 450 ° C. are not satisfied with Looking at these comparative examples, when the secondary cooling temperature is less than 300 °C, it can be confirmed that the bendability is lowered due to excessive tempering of martensite by auto tempering.

Comparative Examples 8 and 9 of the present invention, unlike the method for manufacturing a steel sheet according to an embodiment of the present invention, the tempering temperature does not satisfy the range of 100 ~ 250 ℃. Looking at these comparative examples, when the tempering temperature exceeds 250 ℃, the amount of dissolved carbon is greatly reduced to lower the tensile strength, and when the tempering temperature is less than 100 ℃, it can be confirmed that the bending properties are greatly reduced due to excessive dislocation in martensite.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (8)

Carbon (C): 0.24 to 0.30 wt%, Silicon (Si): 0.05 to 0.3 wt%, Manganese (Mn): 1.0 to 2.5 wt%, Aluminum (Al): 0.01 to 0.05 wt%, Phosphorus (P): 0 More than 0.02 wt%, sulfur (S): more than 0 and less than 0.01 wt%, Chromium (Cr): 0.2 to 0.5 wt%, Molybdenum (Mo): 0.1 to 0.3 wt%, Titanium (Ti): 0.01 to 0.045 wt% , boron (B): more than 0 0.003% by weight or less and the remainder consisting of iron (Fe) and other unavoidable impurities,
The yield strength is 1200 ~ 1450 MPa, the tensile strength is 1550 ~ 1700 MPa, the elongation is 5 ~ 8%, characterized in that the yield ratio is 75% or more,
cold rolled steel sheet. The method of claim 1,
The bending workability (R / t) of the steel sheet is characterized in that 3.0 or less,
cold rolled steel sheet. The method of claim 1,
The final microstructure of the steel sheet is characterized in that it consists of 70% or more and less than 100% tempered martensite, more than 0 and less than 20% bainite, and more than 0% and less than 10% of ferrite,
cold rolled steel sheet. (a) carbon (C): 0.24 to 0.30 wt%, silicon (Si): 0.05 to 0.3 wt%, manganese (Mn): 1.0 to 2.5 wt%, aluminum (Al): 0.01 to 0.05 wt%, phosphorus (P) ): more than 0 and less than 0.02 wt%, sulfur (S): more than 0 and less than or equal to 0.01 wt%, chromium (Cr): 0.2 to 0.5 wt%, molybdenum (Mo): 0.1 to 0.3 wt%, titanium (Ti): 0.01 to 0.045% by weight, boron (B): more than 0 and 0.003% by weight or less and the remaining iron (Fe) and other unavoidable impurities to provide a steel material;
(b) hot-rolling the steel material under the condition that the reheating temperature (SRT) is 1150 to 1300 °C, and the hot rolling end temperature (FDT) is 800 to 1000 °C;
(c) winding the hot-rolled steel at 500 to 650°C;
(d) cold rolling the steel;
(e) annealing the cold-rolled steel material at 800 ~ 900 ℃; and
(f) tempering the annealed heat-treated steel material at 100 ~ 250 ℃; containing,
A method for manufacturing a cold rolled steel sheet. 5. The method of claim 4,
The step (e) is annealing the cold-rolled steel at 800 ~ 900 ℃; and cooling the steel material to 300 to 450° C. at a cooling rate of 3 to 20° C./s.
A method for manufacturing a cold rolled steel sheet. 5. The method of claim 4,
The step (f) comprises the step of tempering the annealed heat-treated steel at 100 ~ 250 ℃ for 3 ~ 12 hours,
A method for manufacturing a cold rolled steel sheet. 5. The method of claim 4,
The step (b) includes the step of hot rolling at a reduction ratio of 90% or more,
The step (d) comprises the step of cold rolling at a reduction ratio of 40 to 60%,
A method for manufacturing a cold rolled steel sheet. 5. The method of claim 4,
The steel sheet after step (f) has a yield strength of 1200 to 1450 MPa, a tensile strength of 1550 to 1700 MPa, an elongation of 5 to 8%, a yield ratio of 75% or more, and a bendability (R/t) of 3.0 or less characterized in that
A method for manufacturing a cold rolled steel sheet.

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