WO2023214634A1 - Cold-rolled steel sheet and method for manufacturing same - Google Patents

Abstract

Provided in the present invention is a cold-rolled steel sheet comprising: carbon (C): 0.15-0.20% by weight, silicon (Si): 1.0-2.0% by weight, manganese (Mn): 1.5-3.0% by weight, phosphorus (P): greater than 0 and 0.02% by weight or less, sulfur (S): greater than 0 and 0.003% by weight or less, aluminum (Al): 0.01-0.3% by weight, nitrogen (N): greater than 0 and 0.01% by weight or less, titanium (Ti): 48/14·[N] to 0.1% by weight (where [N] is the weight % value of nitrogen), and the remaining being iron (Fe) and other unavoidable impurities, wherein a final microstructure consists of ferrite, acicular retained austenite, a martensite/austenite composite structure, and massive martensite, wherein the area fraction of the ferrite is 30-60%, the area fraction of the acicular retained austenite is 5-12%, the area fraction of the martensite/austenite composite structure is 25-50%, the area fraction of the massive martensite is 5-12%, and the carbon concentration in the retained austenite is 1.1% by weight or more.

Description

Cold rolled steel sheet and manufacturing method thereof

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, and more specifically, to a cold-rolled ultra-high-strength low-carbon steel sheet with excellent formability and a manufacturing method thereof.

Ultra-high-strength steel for automobile steel plates is being developed to meet the two factors of reducing vehicle weight in response to environmental regulatory issues and strengthening crash safety standards due to strengthening safety regulations. However, since strength and elongation have a trade-off relationship, the problem of deterioration of formability as strength increases has emerged, and several studies have been conducted to ensure formability of high-strength steel.

TRIP-aided steel, which utilizes the TRIP (TRansformation Induced Plasticity) phenomenon that transforms residual austenite into martensite during the transformation of retained austenite in the microstructure, is being developed as a 3rd generation steel sheet that can secure both high strength and high elongation. The physical properties of these TRIP-aided steels are determined by the phase stability and fraction of retained austenite that causes the TRIP phenomenon, so securing stable retained austenite in the microstructure is important in manufacturing the steel.

Related prior art includes Korean Patent Application No. 2018-0033119.

The technical problem to be achieved by the present invention is to provide a cold rolled ultra-high strength low carbon steel sheet with excellent formability and a manufacturing method thereof.

The cold-rolled steel sheet according to an embodiment of the present invention for solving the above problems includes carbon (C): 0.15 to 0.20% by weight, silicon (Si): 1.0 to 2.0% by weight, manganese (Mn): 1.5 to 3.0% by weight, Phosphorus (P): more than 0 and not more than 0.02% by weight, sulfur (S): more than 0 and not more than 0.003% by weight, aluminum (Al): 0.01 to 0.3% by weight, nitrogen (N): more than 0 and not more than 0.01% by weight, titanium (Ti ): 48/14·[N] to 0.1% by weight ([N] is the weight percent value of nitrogen) and the remaining iron (Fe) and other inevitable impurities, and the final microstructure is ferrite and needle-like retained austenite. , composed of a composite structure of martensite/austenite and bulky martensite, wherein the area fraction of the ferrite is 30 to 60%, the area fraction of the acicular retained austenite is 5 to 12%, and the martensite/austenite The area fraction of the composite structure is 25 to 50%, the area fraction of the blocky martensite is 5 to 12%, and the carbon enrichment amount in the retained austenite is 1.1% by weight or more.

In the cold rolled steel sheet, the ferrite is composed of polygonal ferrite and acicular ferrite, and the area fraction of the acicular ferrite among the ferrite may be 40% or more.

The cold rolled steel sheet may have a tensile strength (TS) of 980 to 1180 MPa and an elongation (El) of 23 to 25%.

The method of manufacturing a cold rolled steel sheet according to an embodiment of the present invention to solve the above problem is (a) carbon (C): 0.15 to 0.20% by weight, silicon (Si): 1.0 to 2.0% by weight, manganese (Mn): 1.5 to 3.0% by weight, phosphorus (P): more than 0 and less than 0.02% by weight, sulfur (S): more than 0 and less than 0.003% by weight, aluminum (Al): 0.01 to 0.3% by weight, nitrogen (N): more than 0 and 0.01% by weight % or less, titanium (Ti): 48/14·[N] to 0.1% by weight (the [N] is a weight percent value of nitrogen) and the remaining iron (Fe) and other inevitable impurities. (b) hot rolling the reheated steel; (c) cold rolling the hot rolled steel; (d) First annealing including maintaining the cold rolled steel at a first annealing temperature of (Ac1 + 30°C) or more (Ac3 - 30°C) and then cooling to a cooling end point temperature of 340°C or less. heat treatment step; and (e) After maintaining the steel material at a second annealing temperature of Ac1 or higher (Ac3 - 30°C) or lower, the martensite transformation start temperature (Ms) or higher (bainite transformation start temperature (Bs) - 15°C) or lower. A second annealing heat treatment step including a process of over-aging after cooling to the cooling end point temperature, wherein the second annealing temperature is lower than the first annealing temperature.

In the method of manufacturing the cold rolled steel sheet, step (a) includes reheating the steel at 1180 to 1300°C, and step (b) includes a finish rolling temperature of 850 to 950°C and a coiling temperature of 450 to 650°C. It includes the step of hot rolling under conditions of ℃, and step (c) may include cold rolling at a reduction ratio of 40 to 70%.

In the method of manufacturing the cold rolled steel sheet, step (d) is performed by maintaining the cold rolled steel at the first annealing temperature for 30 to 120 seconds and then cooling the cold rolled steel at a cooling rate of 15°C/s or more to a cooling end point temperature of 340°C or less. It may include a cooling process.

In the method of manufacturing the cold rolled steel sheet, after performing step (d), the area fraction of ferrite in the microstructure of the steel may be 30 to 50%.

In the method of manufacturing the cold rolled steel sheet, step (e) is performed by maintaining the steel material at the second annealing temperature for 30 to 120 seconds and then cooling the steel material at a cooling rate of 15°C/s or more to the martensite transformation onset temperature (Ms). It may include a process of over-aging for 30 to 300 seconds after cooling to a cooling end point temperature of (bainite transformation start temperature (Bs) - 15°C) or less.

In the method of manufacturing the cold rolled steel sheet, after performing step (e), the microstructure of the steel is composed of ferrite, acicular retained austenite, a composite structure of martensite/austenite, and bulky martensite, wherein the ferrite The area fraction is 30 to 60%, the area fraction of the acicular retained austenite is 5 to 12%, the area fraction of the martensite/austenite composite structure is 25 to 50%, and the area fraction of the bulky martensite may be 5 to 12%.

According to embodiments of the present invention, a cold rolled ultra-high strength low carbon steel sheet with excellent formability and a manufacturing method thereof can be implemented. Specifically, it can secure excellent weldability by designing it as a low-carbon steel containing less than 0.2% by weight of carbon. During the heat treatment process, sufficient amounts of carbon and manganese are concentrated into austenite through several stages of alloy element redistribution, resulting in excellent weldability. It is possible to achieve a balance between strength and elongation, and to create cold-rolled ultra-high strength steel with excellent processability that secures a tensile strength of more than 980 MPa and an elongation of more than 23%.

Of course, the scope of the present invention is not limited by this effect.

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

Figure 2 is a diagram illustrating the outline of (a) a first annealing heat treatment process and (b) a second annealing heat treatment process in the method of manufacturing a cold rolled steel sheet according to an embodiment of the present invention.

Figure 3 is a photograph taken of the microstructure after the first annealing heat treatment in Example 1 of the experimental examples.

Figure 4 is a photograph taken of the microstructure after the second annealing heat treatment in Example 1 of the experimental examples.

Figure 5 is a photograph taken of the microstructure after the first annealing heat treatment in Comparative Example 6 among the experimental examples, Figure 6 is a photograph taken of the final microstructure in Comparative Example 7 among the experimental examples, and Figure 7 is a photograph of the final microstructure in Comparative Example 7 among the experimental examples. Figure 8 is a picture taken of the final microstructure in Example 8, Figure 8 is a picture taken of the final microstructure in Comparative Example 9 among the experimental examples, and Figure 9 shows (a) a needle-like shape and (a) after overaging after the second annealing heat treatment. b) This is a photograph of a block-shaped tissue.

A cold rolled steel sheet and a manufacturing method thereof according to an embodiment of the present invention will be described in detail. The terms described below are terms appropriately selected in consideration of their functions in the present invention, and definitions of these terms should be made based on the content throughout the present specification. Below, we will provide specific details of the cold-rolled ultra-high-strength low-carbon steel sheet with excellent formability and its manufacturing method.

Korean Patent Application No. 2018-0033119 proposes a method of manufacturing steel (Quenching and Partitioning, Q&P) containing tempered martensite and retained austenite through rapid cooling and partitioning heat treatment after annealing the steel. Q&P steel has the advantage of being able to obtain physical properties of more than 980 MPa in tensile strength and more than 21% in elongation even with 0.2% by weight carbon steel, but the window for process temperature is narrow and the ductility variation is large, so the high elongation targeted by the present invention cannot be achieved stably. difficult to secure.

Korean Patent Publication No. 2017-0113858 proposes a two-time annealing heat treatment process as a method of securing the microstructure (pre-structure) before final annealing to increase the ductility of steel by securing lath-shaped ferrite and retained austenite. . However, since single-phase annealing is performed to secure a low-temperature structure with a volume fraction of more than 90% after the first annealing, a tensile strength of more than 980MPa cannot be stably secured in steel with a low carbon content, and there are concerns that the lifespan of the furnace will be shortened because high-temperature annealing is involved. .

The present invention discloses a cold-rolled ultra-high-strength steel sheet with excellent elongation, having a tensile strength of 980 MPa or more and an elongation of 23% or more, applicable to automobile parts, and a method of manufacturing the same. The microstructure of cold-rolled steel sheets is polygonal ferrite with an area fraction of 20% to 50%, acicular ferrite with an area fraction of 40% or more, acicular retained austenite with an area fraction of 5% or more and 12% or less, and martensite/austenite composite structure with an area fraction of 20% or more and 12% or less. and the remaining bainite, and the alloy amount and appropriate heat treatment conditions to secure the target yield strength, tensile strength, and elongation are disclosed.

steel plate

The cold-rolled steel sheet according to an embodiment of the present invention contains carbon (C): 0.15 to 0.20% by weight, silicon (Si): 1.0 to 2.0% by weight, manganese (Mn): 1.5 to 3.0% by weight, and phosphorus (P): 0. Exceeding 0.02% by weight or less, Sulfur (S): exceeding 0 and not exceeding 0.003% by weight, Aluminum (Al): 0.01 to 0.3% by weight, Nitrogen (N): exceeding 0 and not exceeding 0.01% by weight, Titanium (Ti): 48/14 It consists of [N] to 0.1% by weight (where [N] is the weight% value of nitrogen) and the remaining iron (Fe) and other inevitable impurities.

Below, the role and content of each component included in the cold rolled steel sheet will be described.

Carbon (C)

Carbon (C) is added to secure the strength of steel, and strength increases as the carbon content increases in the martensite structure. Furthermore, it combines with elements such as iron to form carbides to improve strength and hardness. Carbon (C) may be added in a content ratio of 0.15 to 0.20% by weight of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. If the carbon content is less than 0.15% by weight of the total weight, the above-mentioned effect cannot be realized and there is a problem of not securing sufficient strength. On the other hand, when the carbon content exceeds 0.20% by weight of the total weight, weldability and processability are deteriorated.

Silicon (Si)

Silicon (Si) is an element added to increase strength and suppress carbide formation through the solid solution strengthening effect of ferrite. Additionally, silicon is well known as a ferrite stabilizing element, so ductility can be increased by increasing the ferrite fraction during cooling. In addition, it is known as an element that can secure strength by promoting martensite formation through austenite carbon enrichment. Meanwhile, silicon is added along with aluminum as a deoxidizer to remove oxygen in steel during the steelmaking process, and can also have a solid solution strengthening effect. The silicon may be added in an amount of 1.0 to 2.0% by weight of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. If the silicon content is less than 1.0% by weight of the total weight, ductility cannot be secured and the above-mentioned silicon addition effect cannot be properly achieved. On the other hand, when the silicon content exceeds 2.0% by weight of the total weight and a large amount is added, oxides such as Mn ₂ SiO ₄ are formed during the manufacturing process, impairing plating properties, increasing the carbon equivalent, which may reduce weldability, and reheating and hot By generating red scale during rolling, surface quality may be affected, and toughness and plastic workability may be deteriorated.

Manganese (Mn)

Manganese (Mn) is an element that contributes to strength improvement by increasing hardenability, facilitates the formation of a low-temperature transformation phase, and provides the effect of increasing strength through solid solution strengthening. Manganese may be added in a content ratio of 1.5 to 3.0% by weight of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. If the manganese content is less than 1.5% by weight, the above-mentioned effect of securing strength cannot be sufficiently achieved. In addition, when the manganese content exceeds 3.0% by weight, processability and delayed fracture resistance are reduced due to the formation or segregation of inclusions such as MnS, and the carbon equivalent may be increased, thereby reducing weldability.

Phosphorus (P)

Phosphorus (P) increases strength through solid solution strengthening and can perform the function of suppressing the formation of carbides. The phosphorus may be added in a content ratio of more than 0 and less than 0.02% by weight of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. If the phosphorus content exceeds 0.02% by weight, the weld zone may become embrittled, low-temperature brittleness may occur, press formability may deteriorate, and impact resistance may decrease.

Hwang (S)

Sulfur (S) improves the machinability of steel by combining with manganese, titanium, etc., and can improve machinability by forming fine MnS precipitates, but is generally an element that inhibits ductility and weldability. The sulfur may be added in a content ratio of more than 0 and less than 0.003% by weight of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. If the sulfur content exceeds 0.003% by weight, the number of Fes inclusions or MnS inclusions increases, which reduces toughness and weldability and machinability, and may cause segregation during solidification during continuous casting, causing high-temperature cracks.

Aluminum (Al)

Aluminum (Al) is an element mainly used as a deoxidizer. It promotes the formation of ferrite, improves elongation, suppresses the formation of carbides, and stabilizes austenite by increasing carbon concentration in austenite. The aluminum (Al) is preferably added in an amount of 0.01 to 0.3% by weight of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. When the aluminum (Al) content is less than 0.01% by weight, the above-described effect of adding aluminum can be properly achieved. On the other hand, if the aluminum (Al) content is added excessively, exceeding 0.3% by weight, aluminum inclusions increase, reducing playability, concentrating on the surface of the steel sheet, deteriorating plating ability, and forming AlN in the slab, causing hot rolling cracks. There is a problem that causes this.

Nitrogen (N)

Nitrogen (N) is a solid solution strengthening element that can increase the strength of steel sheets, and is generally an element mixed from the atmosphere. Its content must be controlled by the degassing process in the steelmaking process. If the nitrogen content exceeds 0.01% by weight, the weld zone may become embrittled, low-temperature brittleness may occur, press formability may deteriorate, and impact resistance may decrease.

Titanium (Ti)

Titanium (Ti) is a precipitate forming element and has the effect of precipitating TiN and refining grains. In particular, the nitrogen content inside the steel can be lowered through precipitation of TiN. Titanium is preferably added in an amount of 48/14·[N] to 0.1% by weight. If it is less than 48/14·[N], the effect of adding Ti is insufficient due to the small amount of TiC precipitated, and if added in excess of 0.1% by weight. If this happens, it is difficult to secure strength by reducing the carbon solubility in the base material.

As described above, the cold-rolled steel sheet according to an embodiment of the present invention having the alloy element composition may be a cold-rolled ultra-high strength steel sheet with excellent elongation, having a tensile strength of 980 MPa or more and an elongation of 23% or more. For example, the cold rolled steel sheet may have a tensile strength (TS) of 980 to 1180 MPa and an elongation (El) of 23 to 25%.

The final microstructure of the cold rolled steel sheet consists of ferrite, needle-shaped retained austenite, a composite structure of martensite/austenite, and bulky martensite, and the area fraction of the ferrite is 30 to 60%, and the needle-shaped residual austenite The area fraction of the retained austenite is 5 to 12%, the area fraction of the martensite/austenite composite structure is 25 to 50%, the area fraction of the bulky martensite is 5 to 12%, and the area fraction of the retained austenite is 5 to 12%. The carbon enrichment amount is more than 1.1% by weight. The ferrite is composed of polygonal ferrite and acicular ferrite, and the area fraction of the acicular ferrite among the ferrite may be 40% or more.

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

Manufacturing method of steel plate

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, the method of manufacturing a steel sheet according to an embodiment of the present invention is (a) carbon (C): 0.15 to 0.20% by weight, silicon (Si): 1.0 to 2.0% by weight, manganese (Mn): 1.5 to 3.0% by weight, phosphorus (P): more than 0 and less than 0.02% by weight, sulfur (S): more than 0 and less than 0.003% by weight, aluminum (Al): 0.01 to 0.3% by weight, nitrogen (N): more than 0 and 0.01% by weight. Hereinafter, titanium (Ti): 48/14·[N] to 0.1% by weight (where [N] is a weight percent value of nitrogen) and the remaining iron (Fe) and other inevitable impurities, a step of reheating (S100). ; (b) hot rolling the reheated steel (S200); (c) cold rolling the hot rolled steel (S300); (d) First annealing including maintaining the cold rolled steel at a first annealing temperature of (Ac1 + 30°C) or more (Ac3 - 30°C) and then cooling to a cooling end point temperature of 340°C or less. Heat treatment step (S400); and (e) After maintaining the steel material at a second annealing temperature of Ac1 or higher (Ac3 - 30°C) or lower, the martensite transformation start temperature (Ms) or higher (bainite transformation start temperature (Bs) - 15°C) or lower. A second annealing heat treatment step (S500) including a process of over-aging after cooling to the cooling end point temperature is sequentially included.

The step (a) (S100) may include reheating the slab steel having the above composition at 1180 to 1300°C. Slabs are manufactured in the form of semi-finished products by continuously casting molten steel obtained through the steelmaking process, and through a reheating process, component segregation occurring in the casting process is homogenized and made ready for hot rolling. If the Slab Reheating Temperature (SRT) is below 1180℃, there is a problem that the segregation of the slab cannot be sufficiently re-employed, and if it exceeds 1300℃, the size of austenite grains increases and process costs may increase. Reheating of the slab can take 1 to 4 hours. If the reheating time is less than 1 hour, the reduction in the segregation zone is not sufficient, and if it exceeds 4 hours, the grain size increases and process costs may increase.

The step (b) (S200) is a step of hot rolling the reheated slab. Hot rolling is performed at a finish delivery temperature (FDT) of 850 to 950°C. If the finish rolling temperature is lower than 850°C, the rolling load increases rapidly, reducing productivity, and if it exceeds 950°C, the size of the grains may increase and strength may decrease. After hot rolling, it is cooled to a temperature of 450 to 650°C and then wound. If the coiling temperature is less than 450℃, the shape of the hot-rolled coil becomes uneven and its strength increases, which increases the rolling load during cold rolling. If it exceeds 650℃, it may cause defects in the post-process due to surface oxidation, etc., and the center of the steel sheet It causes non-uniform microstructure due to the difference in cooling rate between the and edge portions, and problems such as oxidation inside the grain boundaries may occur.

Step (c) is a step of pickling the hot rolled coil to remove the surface scale layer and performing cold rolling. The thickness reduction rate during cold rolling is approximately 40 to 70%. The higher the reduction rate, the higher the formability due to the tissue refinement effect. In cold rolling, if the reduction is less than 40%, it is difficult to obtain a uniform microstructure, and if the design is over 70%, the roll force increases and the process load increases.

After cold rolling, the first annealing heat treatment process and the second annealing heat treatment process are sequentially performed. That is, the cold rolled steel sheet is annealed twice in total, including primary annealing and secondary annealing. The temperature increase rate of heating from room temperature to the first or second annealing temperature range is not limited and can follow the temperature increase rate of normal heating furnace equipment.

The step (d) includes maintaining the cold rolled steel at a first annealing temperature of (Ac1 + 30°C) or more (Ac3 - 30°C) and then cooling to a cooling end point temperature of 340°C or less. This is the first annealing heat treatment step.

Step (d) is a biphasic annealing heat treatment for 30 to 120 seconds in the first annealing temperature range of (Ac1 + 30℃) or higher (Ac3 - 30℃) to secure a dual-phase structure of ferrite and low temperature phase. It's a step. The first annealing heat treatment process is a process of forming a desirable overall structure to secure lath-shaped needle-shaped ferrite and austenite structures during the second annealing heat treatment process. In this specification, 'full structure' refers to the microstructure of steel manufactured through the first annealing heat treatment (S400). During the second annealing heat treatment process, the low-temperature phase structure reversely transforms into austenite, and lath-shaped ferrite and austenite microstructure are formed. Here, the low-temperature phase structure refers to the martensite or bainite phase. This lath-type organization has the characteristic of securing both high strength and high ductility. When annealing in an ideal temperature range, primary redistribution of carbon and manganese occurs, which enriches carbon and manganese in the austenite region and increases the phase stability of austenite.

In order to satisfy both the tensile strength of 980 MPa or more and the elongation of 23% or more in the steel grade of 0.2% by weight or less proposed in the present invention, sufficient tensile strength is required by redistributing carbon and manganese to austenite beyond conventional heat treatment to increase the strength of martensite. Since it is necessary to secure strength and secure sufficient ductility by increasing the phase stability of retained austenite, it is preferable that the first annealing is performed in the ideal temperature range. If the first annealing temperature exceeds Ac3, austenite crystals become coarse due to high-temperature annealing, and a large amount of austenite with low carbon and manganese content is generated, making it difficult to secure the tensile properties of the final steel. On the other hand, if the first annealing temperature is above Ac1 but below (Ac1 + 30°C), the ferrite fraction in the microstructure exceeds 50% after the first annealing heat treatment process, resulting in an increase in soft and coarse polygonal ferrite in the final microstructure. There are difficulties in securing the tensile properties of steel.

Therefore, preferably, after the first annealing heat treatment process, the microstructure should exhibit a DP (dual phase) structure composed of ferrite and a low-temperature phase, and more preferably, for strength and ductility balance, the fraction of ferrite should be 30% or more by area fraction. It can be limited to % or less. When cooling the above-described primary annealed heat-treated steel sheet to room temperature, it is preferably cooled at 15°C/s or more to suppress the formation of polygonal ferrite, which adversely affects physical properties during cooling, and to secure a low-temperature martensite structure. It can be cooled to over 25℃/s.

Meanwhile, in a modified embodiment of the present invention, if the microstructure after the first annealing heat treatment process is a DP (dual phase) structure composed of ferrite and a low-temperature phase, and the fraction of ferrite is limited to 30% or more and 50% or less as an area fraction. , the heat treatment temperature for primary annealing may be limited to (Ac1 + 30°C) or more and (Ac3 - 30°C) or less.

In step (e), the steel is maintained at a second annealing temperature of AAc1 or higher (Ac3 - 30°C) and then martensite transformation start temperature (Ms) or more (bainite transformation start temperature (Bs) - 15°C). This is the second annealing heat treatment step including the process of over-aging after cooling to the cooling end point temperature below.

In step (e), the martensite structure generated in the first annealing heat treatment process is reverse transformed to form lath-shaped ferrite and austenite. During annealing, reverse transformation of the primary low-temperature phase and redistribution of carbon and manganese to austenite occur, so a longer annealing time is preferable for sufficient reverse transformation and redistribution of alloy elements. However, if the annealing time is too long, there is a risk of decreased productivity, so annealing is maintained. Time is limited to 30 to 120 seconds.

The secondary annealed heat-treated steel sheet is cooled to a temperature between the martensite transformation start temperature (Ms) and the bainite transformation start temperature (Bs) and held for 30 to 300 seconds to induce redistribution of carbon and manganese alloy elements to form retained austenite. This is the step to increase the stability of the costume. When cooling to the cooling end point temperature above the martensite transformation start temperature (Ms) (bainite transformation start temperature (Bs) - 15°C) or below, if the cooling rate is less than 15°C/s, polygonal ferrite is generated during cooling and the final Because it causes inferiority in the tensile properties of steel, the cooling rate is set to 15°C/s or more, preferably 25°C/s or more. If the cooling end point temperature is higher than (bainite transformation start temperature (Bs) - 15°C), ferrite or pearlite is generated during the holding stage, causing a decrease in strength and elongation, and the cooling end point temperature is higher than the bainite transformation start temperature (Bs). ) At temperatures just below this, bainite transformation and carbon redistribution do not occur in a balanced manner due to the high-temperature bainite formation section. Conversely, if the cooling end temperature is lower than the martensite transformation start temperature (Ms), fresh martensite is generated by cooling, greatly increasing the strength of the steel, while retained austenite is reduced to 23%, which is the target of the present invention. It becomes impossible to secure sufficient elongation. Therefore, it is preferable that the cooling end point temperature is determined to be a temperature higher than the martensite transformation start temperature (Ms) (bainite transformation start temperature (Bs) - 15°C) or lower. After cooling to the cooling end point temperature, it is maintained for 30 to 300 seconds for additional redistribution of carbon and manganese and then cooled to room temperature. At this time, the cooling rate to room temperature is not specifically limited, but is preferably 10℃/s or more for productivity. do.

Furthermore, in the method of manufacturing a cold rolled steel sheet of the present invention, the second annealing temperature is lower than the first annealing temperature. If the second annealing temperature is higher than the first annealing temperature, the austenite fraction generated in the second annealing heat treatment (S500) becomes higher than the low-temperature phase fraction of the structure after the first annealing heat treatment (S400). Austenite reversely transformed at a low temperature appears as a lamellar structure of acicular ferrite and austenite, but the austenite produced in excess due to high annealing temperature develops into a blocky form, and as a result, the fraction of blocky martensite increases in the final microstructure. The tensile strength of the steel increases significantly, while the elongation decreases.

The microstructure of the steel material finally realized through the above-described heat treatment process is composed of ferrite, needle-like retained austenite, a composite structure of martensite/austenite, and bulky martensite, and the area fraction of the ferrite is 30 to 60%. , the area fraction of the acicular retained austenite may be 5 to 12%, the area fraction of the martensite/austenite composite structure may be 25 to 50%, and the area fraction of the blocky martensite may be 5 to 12%. .

The steel grade composed of the above-described heat treatment process within the composition range described in the present invention and the microstructure obtained therefrom is a low-carbon type with excellent formability with a tensile strength (TS): 980 to 1180 MPa and an elongation (El): 23 to 25%. Cold-rolled ultra-high-strength steel sheets can be realized.

Hereinafter, the above-described annealing heat treatment process will be described with reference to the drawings.

Figure 2 is a diagram illustrating the outline of (a) a first annealing heat treatment process (S400) and (b) a second annealing heat treatment process (S500) in the method of manufacturing a cold rolled steel sheet according to an embodiment of the present invention.

First annealing heat treatment (S400)

Referring to (a) of Figure 2, the a-b section corresponds to the step of maintaining the first annealing temperature of (Ac1 + 30°C) or more and (Ac3 - 30°C) or less, and the b-c section is the first half of the cooling section and is a slow cooling process. Corresponds to , the c-d section is the latter half of the cooling section and corresponds to the quenching process, and the d-e section corresponds to the over-aging process. In a modified embodiment of the present invention, the slow cooling process in sections b-c and the over-aging process in sections d-e can be omitted.

The first annealing heat treatment process can be maintained for 30 to 120 seconds at a first annealing temperature of (Ac1 + 30°C) or more (Ac3 - 30°C) or less. When the carbon content proposed in this example is 0.2% by weight or less, the redistribution of alloy elements must be increased compared to existing steel types to secure the tensile strength of the steel, so the first annealing heat treatment process is performed in the ideal temperature range to remove the primary alloy elements. Induce redistribution. At this time, if the annealing temperature is too low, a large amount of polygonal ferrite is formed in the microstructure after the first annealing heat treatment process, making it difficult to secure sufficient tensile strength, and if it exceeds (Ac3 - 30℃), grain coarsening and alloying may occur due to high temperature annealing. The austenite fraction with a low element content (lean) increases, making it difficult to achieve the target tensile properties. If the holding time exceeds 120 seconds, the size of the grains may become coarse and productivity may decrease. The annealed cold-rolled steel is cooled to a temperature of 340°C or lower and a cooling rate of 15°C/s or higher. At this time, if the cooling end point temperature exceeds 340℃, it is difficult to obtain a lath-shaped structure in the second annealing heat treatment process due to carbide precipitation, and if the cooling rate is less than 15℃/s, a large amount of polygonal ferrite is generated during cooling. It is disadvantageous in securing tensile strength.

Section b-c is a step of slowly cooling the annealed heat-treated steel sheet. When cooling the annealed heat-treated steel sheet, a slow cooling section may be included depending on the heat treatment equipment. When a slow cooling section is included, the slow cooling end point temperature or cooling rate is not specifically limited, but to prevent a large amount of polygonal ferrite from being generated during cooling, the slow cooling end point temperature is preferably 740°C or higher and the cooling rate is -5°C/s. It could be more than that.

The c-d section is the latter part of the cooling section and corresponds to a rapid cooling process, and is a step in which the steel sheet cooled through the slow cooling process is cooled to a temperature of 340°C or lower. When cooling the primary annealed heat-treated steel sheet or the primary cooled steel sheet, the steel sheet is cooled to -15°C/s or higher to suppress the formation of polygonal ferrite, which adversely affects physical properties, and to form bainite or martensite, which are low-temperature phases. , preferably 25°C/s or higher. The cooling rate is maintained up to a temperature below the martensite transformation start temperature (Ms) expressed in the following equation (1), and then cooled to room temperature through the overaging section of the equipment. Alternatively, the over-aging section may be omitted and the product may be cooled directly to room temperature.

Equation (1)

Ms (℃) = 491.1 - 302.6[C] - 14.5[Si] -30.6[Mn] - 16.6[Ni] - 8.9[Cr] + 2.4[Mo] - 11.3[Cu] + 8.58[Co] + 7.4[W ]

Here, [C], [Si], [Mn], [Ni], [Cr], [Mo], [Cu], [Co], and [W] are carbon, silicon, manganese, nickel, This is the mass percent value of chromium, molybdenum, copper, cobalt, and tungsten.

Second annealing heat treatment (S500)

Referring to (b) of Figure 2, the p-q section corresponds to the step of maintaining the second annealing temperature of Ac1 or higher (Ac3 - 30°C), and the q-r section is the first half of the cooling section and corresponds to the slow cooling section, r-s The section is the second half of the cooling section and corresponds to the rapid cooling section, and the s-t section corresponds to the overaging section. Meanwhile, in the second annealing heat treatment process disclosed in (b) of Figure 2, the dotted line profile between the bainite transformation start temperature (Bs) and the martensite transformation start temperature (Ms) corresponds to the case where the plating process is performed in a plating bath. .

The second annealing heat treatment process can be maintained for 30 to 120 seconds at a second annealing temperature of Ac1 or higher (Ac3 - 30°C) or lower. Furthermore, the second annealing temperature is characterized in that it is lower than the first annealing temperature. A biphasic annealing heat treatment step is performed for 30 to 120 seconds at a temperature of Ac1 or higher but lower than the primary annealing temperature. This is the stage in which the low-temperature phase structure created in the first annealing heat treatment (S400) undergoes reverse transformation to form lath-shaped ferrite and austenite. During annealing, reverse transformation of the primary low-temperature phase and redistribution of carbon (C) and manganese (Mn) to austenite occur. Therefore, a longer annealing time is preferable to ensure sufficient reverse transformation and redistribution of alloy elements. However, if the annealing time is too long, productivity is reduced. Because there is concern about deterioration, the annealing holding time is limited to 30 to 120 seconds. If the annealing temperature of the second annealing heat treatment (S500) is higher than the annealing temperature of the first annealing heat treatment (S400), the austenite fraction generated in the secondary annealing is larger than the low-temperature phase fraction in the entire structure. As a result, the development of lath-shaped austenite, which undergoes reverse transformation at low temperatures, is inhibited, and bulky austenite is generated in proportion to the excess fraction. This blocky austenite lowers the phase stability of austenite by reducing carbon (C) and manganese (Mn) that are redistributed into lath-shaped austenite. This impedes the effect of the first annealing heat treatment (S400), which creates a lath-shaped structure required to secure the elongation desired in the present invention. Therefore, the second annealing heat treatment (S500) is preferably performed at a lower temperature than the first annealing heat treatment (S400).

Afterwards, it is cooled to a cooling end temperature below the martensite transformation start temperature (Ms) (bainite transformation start temperature (Bs) - 15°C) at a cooling rate of 15°C/s or above and maintained for 30 to 300 seconds (overaging). This is a step to increase the phase stability of retained austenite by inducing redistribution of carbon (C) and manganese (Mn) alloy elements.

The bainite transformation initiation temperature (Bs) can be expressed by the following equation (2).

Equation (2)

Bs (℃) = 656 - 57.7[C] - 75[Si] - 35[Mn] - 15.3[Ni] - 34[Cr] - 41.2[Mo]

Here, [C], [Si], [Mn], [Ni], [Cr], and [Mo] are the mass percent values of carbon, silicon, manganese, nickel, chromium, and molybdenum in the steel.

When cooling below the martensite transformation start temperature (Ms) or below (bainite transformation start temperature (Bs) - 15°C), if the cooling rate is less than 15°C/s, polygonal ferrite is generated during cooling, resulting in inferior tensile properties of the final steel. Therefore, the cooling rate is 15°C/s or more, preferably 25°C/s or more.

If the cooling end point temperature is below the bainite transformation start temperature (Bs) (bainite transformation start temperature (Bs) - 15°C), austenite is transformed into ferrite or pearlite during the holding step, causing a decrease in strength and elongation, If it is directly below the bainite transformation onset temperature (Bs), it is difficult to secure the phase stability of the retained austenite due to insufficient carbon redistribution. Conversely, if the cooling end point temperature is below the martensite transformation start temperature (Ms), fresh martensite is generated and the strength of the steel increases significantly, while the retained austenite decreases, making it possible to secure sufficient elongation of 23% or more, which is the target of the present invention. There will be no more. Additionally, if the holding time is less than 30 seconds, the redistribution effect may be reduced due to insufficient redistribution time, and if the holding time is longer than 300 seconds, productivity may decrease.

After cooling to the cooling end point temperature, over-aging for 30 to 300 seconds to redistribute carbon (C) and manganese (Mn) and then cooling to room temperature. The temperature during overaging does not need to be maintained isothermally at the cooling end point temperature, and may be cooled if necessary, but the temperature must be above Ms to prevent the formation of fresh martensite. In addition, the cooling rate to room temperature is not specifically limited, but is preferably 10°C/s or more for productivity. The redistribution effect of carbon (C) and manganese (Mn) during overaging varies depending on the austenite shape, and is greater in needle-shaped than in block-shaped. This is because the diffusion distance of carbon (C) and manganese (Mn) is shorter in the needle-shaped shape, so diffusion occurs more easily during the same time, and as shown in Figure 9 and Table 1, carbon is present in the needle-shaped and block-shaped structures after overaging. (C), As a result of analyzing the manganese (Mn) content, it can be seen that more carbon (C) and manganese (Mn) enrichment occurs in the needle-shaped shape. As a result, in the microstructure after final cooling, acicular austenite remains as a composite structure of martensite/austenite, and blocky austenite remains as a blocky martensite.

weight%	bed type	weird type
Carbon (C)	0.6±0.05	0.3±0.05
Manganese (Mn)	3.5±0.5	2.25±0.25

According to the cold-rolled steel sheet and its manufacturing method according to the technical idea of the present invention described above, excellent weldability can be secured by designing it as a low-carbon steel containing less than 0.2% by weight of carbon, and various stages of alloy element redistribution during the heat treatment process. By concentrating a sufficient amount of carbon (C) and manganese (Mn) into austenite, an excellent balance of strength and elongation can be realized, and it is a cold-rolled ultra-high strength steel with excellent workability that secures a tensile strength of over 980 MPa and an elongation of over 23%. can be provided.

Experiment example

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

1. Specimen production

In this experimental example, specimens having the alloy element composition (unit: weight %) shown in Table 2 are provided.

C	Si	Mn	P	S	Al	Ti	N	Fe
0.18	1.70	2.30	0.01	0.001	0.03	0.015	0.003	Bal.

The component system in Table 2 is the composition of the cold rolled steel sheet according to an embodiment of the present invention: carbon (C): 0.15 to 0.20% by weight, silicon (Si): 1.0 to 2.0% by weight, manganese (Mn): 1.5 to 3.0% by weight. , Phosphorus (P): more than 0 and not more than 0.02% by weight, sulfur (S): more than 0 and not more than 0.003% by weight, aluminum (Al): 0.01 to 0.3% by weight, nitrogen (N): more than 0 and not more than 0.01% by weight, titanium ( Ti): 48/14·[N] to 0.1% by weight ([N] is the weight% value of nitrogen) and the remaining iron (Fe) composition is satisfied. According to the components in Table 2, the bainite transformation start temperature (Bs) is calculated to be 437.6°C, and the martensite transformation start temperature (Ms) is calculated to be 341.6°C. The temperature is calculated by the following relational equation.

Bs(℃) = 656 - 57.7[C] - 75[Si] - 35[Mn] - 15.3[Ni] - 34[Cr] - 41.2[Mo]

Ms (℃) = 491.1 - 302.6[C] - 14.5[Si] - 30.6[Mn] - 16.6[Ni] - 8.9[Cr] + 2.4[Mo] - 11.3[Cu] + 8.58[Co] + 7.4[W ]

Meanwhile, according to the components in Table 2, the Ac1 temperature is 754°C and the Ac3 temperature is 900°C.

In an experimental example of the present invention, the steel having the above composition was reheated at 1250°C for 4 hours, hot rolled to a thickness of 3.5 mm at a finish rolling temperature (FDT) of 850°C, and then coiled at a coiling temperature of 600°C. Afterwards, the surface oxidized scale was removed through pickling and cold rolled to a thickness of 1.2 mm. Thereafter, the cold rolled steel sheet was heat treated twice in succession according to the configuration shown in FIG. 2.

2. Process conditions and physical property evaluation

Table 3 shows the process conditions of the primary annealing heat treatment and secondary annealing heat treatment applied in the experimental examples of the present invention.

	A. Primary annealing temperature	B. First annealing time	C. Slow cooling end temperature	D. Rapid cooling end temperature	E. Overexposure time	F. Secondary annealing temperature	G. Secondary annealing time	H. Rapid cooling end temperature	I. Over-aging end temperature	J. Overexposure time
Example 1	850	60	800	340	180	830	60	400	360	180
Example 2	850	60	800	340	180	840	120	400	360	300
Example 3	860	60	800	340	180	830	60	400	360	180
Example 4	850	60	-	25	-	830	60	400	360	180
Comparative Example 1	850	60	800	340	180	830	60	440	400	180
Comparative example 2	850	60	800	340	180	850	60	440	400	180
Comparative Example 3	850	60	800	340	180	850	60	400	360	25
Comparative example 4	850	60	800	340	180	850	60	320	280	180
Comparative Example 5	890	60	800	340	180	830	60	400	360	180
Comparative Example 6	910	60	800	340	180	830	60	440	400	180
Comparative example 7	910	60	800	340	180	830	60	400	360	180
Comparative example 8	850	60	800	340	180	870	60	400	360	180
Comparative Example 9	-	-	-	-	-	850	60	400	360	180

In Table 3, item A is the annealing temperature of the first annealing heat treatment process (S400) and corresponds to the annealing temperature in the a-b section in (a) of Figure 2, and item B is the annealing time of the first annealing heat treatment process (S400). It corresponds to the process time of the a-b section in (a) of Figure 2, and item C is the slow cooling end temperature of the first annealing heat treatment process (S400), and point c is the end temperature of the slow cooling process in the b-c section in (a) of Figure 2. Corresponds to the temperature of, item D is the rapid cooling end temperature of the first annealing heat treatment process (S400) and corresponds to the temperature at point d, which is the end temperature of the rapid cooling process in the section c-d in (a) of Figure 2, and item E is The over-aging time of the first annealing heat treatment process (S400) corresponds to the process time of the over-aging process in the section d-e in (a) of Figure 2. In addition, item F in Table 3 is the over-aging time of the second annealing heat treatment process (S500). The annealing temperature corresponds to the annealing temperature of the p-q section in (b) of Figure 2, and the G item is the annealing time of the secondary annealing heat treatment process (S500) and corresponds to the process time of the p-q section in (b) of Figure 2, The H item is the rapid cooling end temperature of the secondary annealing heat treatment process (S500) and corresponds to the temperature at point s, which is the end temperature of the rapid cooling process in the r-s section in Figure 2 (b), and the I item is the secondary annealing heat treatment process ( The over-aging end temperature of S500) corresponds to the temperature at point t, which is the end temperature of the over-aging process in the s-t section in Figure 2 (b), and the J item is the over-aging time of the secondary annealing heat treatment process (S500). In (b) of 2, it corresponds to the process time of the over-aging process in the s-t section.

	A. ferrite	B. low temperature	C. ferrite	D. PF	E. Lath F	F. R.A.	G. M/A	H. Weird M	I. C in RA
Example 1	43	57	53	18	35	8.9	33	5.1	1.12
Example 2	45	55	43	13	30	9.0	38	10.0	1.15
Example 3	30	70	55	17	38	9.4	30	5.6	1.21
Example 4	37	63	56	19	37	8.3	26	9.7	1.18
Comparative Example 1	45	55	55	18	37	4.0	29	12.0	1.03
Comparative example 2	45	55	41	10	31	4.3	41	13.7	1.07
Comparative example 3	45	55	44	11	33	4.0	39	13.0	0.94
Comparative Example 4	45	55	45	11	34	3.1	35	16.9	1.15
Comparative Example 5	6	94	44	7	37	12.5	42	1.5	1.00
Comparative Example 6	0	100	42	4	38	13.7	35	9.3	1.00
Comparative Example 7	0	100	42	4	38	15.6	40	2.4	1.07
Comparative example 8	45	55	40	5	35	4.0	36	20.0	1.02
Comparative Example 9	-	-	55	39	16	3.0	24	18.0	1.17

Table 4 shows the area fraction of microstructure (unit: %) and the amount of carbon enrichment (unit: weight %) in retained austenite in the experimental examples of the present invention. The microstructure was analyzed using a scanning electron microscope (SEM), and XRD analysis was used to analyze the retained austenite fraction and carbon content in retained austenite. In Table 4, item A is the area fraction of the ferrite phase realized after the first annealing heat treatment, item B is the area fraction of the low-temperature phase realized after the first annealing heat treatment, and item C is the area fraction of the ferrite phase realized after the second annealing heat treatment. , D item is the area fraction of polygonal ferrite phase among ferrites realized after secondary annealing heat treatment, E item is the area fraction of acicular ferrite phase among ferrites realized after secondary annealing heat treatment, and F item is materialized after secondary annealing heat treatment. This is the area fraction of the acicular retained austenite phase, G is the area fraction of the martensite/austenite composite structure realized after the secondary annealing heat treatment, and H is the area of the blocky martensite phase realized after the secondary annealing heat treatment. It is a fraction, and item I is the amount of carbon enrichment in the retained austenite realized after the secondary annealing heat treatment.

Table 5 shows tensile properties in experimental examples of the present invention. Tensile properties were evaluated by performing a tensile test according to KS No. 5 standard using Zwick/Roell Corp Z100.

In Table 5, the TS item represents the tensile strength (unit: MPa), the T.El item represents the elongation (unit: %), and the TS It is shown.

	TS	T.El	TS
Example 1	985	24.3	23,936
Example 2	1011	23.4	23,657
Example 3	990	23.7	23,463
Example 4	1029	23.8	24,490
Comparative Example 1	1053	18.6	19,586
Comparative example 2	1090	18.1	19,729
Comparative example 3	1106	17.7	19,576
Comparative example 4	1158	14.6	16,907
Comparative Example 5	968	23.8	23,038
Comparative Example 6	1022	20.8	21,258
Comparative example 7	946	25.1	23,745
Comparative example 8	1142	16.7	19,071
Comparative Example 9	1164	15.8	18,391

Referring to Tables 2 to 5, Examples 1, 2, 3, and 4 are obtained by appropriately performing the first annealing heat treatment (S400) and the second annealing heat treatment (S500) proposed in the present invention. The present invention satisfies the tensile strength of 980 MPa or more (e.g., 980 to 1,180 MPa), elongation of 23% or more (e.g., 23 to 25%), and TS x El of 22,000 MPa or more. Referring to Figure 3, the structure after the first annealing heat treatment (S400) of Example 1, that is, the entire structure, is composed of 43% ferrite and 57% low-temperature phase as an area fraction, and is satisfied under the conditions of the present invention (area fraction of ferrite : 30 to 50%) is satisfied. The microstructure after the second annealing heat treatment (S500) of Example 1 is shown in Figure 4, and the target fraction of ferrite, needle-like retained austenite, martensite/austenite composite structure, and bulky martensite is the target fraction of the present invention. You can see that it is composed of .

Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 were subjected to the first annealing heat treatment (S400) at an ideal range temperature of 850°C to secure a sufficient amount of ferrite of 45% in the microstructure after the first annealing heat treatment. However, in the second annealing heat treatment (S500), the redistribution of alloy elements was not performed smoothly, and the retained austenite fraction and phase stability were not sufficiently secured, so the tensile strength was sufficiently high at 1000 MPa or more, but the elongation was 23%, which is the goal of the present invention. It did not fall far short of .

Specifically, in Comparative Examples 1 and 2, in the second annealing heat treatment (S500), the cooling end point temperature was higher than the bainite transformation start temperature (Bs), and the redistribution of carbon (C) and manganese (Mn) occurred during the holding time after the cooling end point. Because it was not effective, the elongation rate was below the target value (more than 23%).

In Comparative Example 3, like Examples 1, 2, and Examples, cooling was completed at an appropriate temperature above the martensite transformation start temperature (Ms) (bainite transformation start temperature (Bs) - 15°C) or below, but the holding time (shown) As the effective time was short (less than 30 seconds), a sufficient amount of redistribution was not achieved, and the elongation rate fell short of the target value (more than 23%). In Example 2, the holding time was increased in the second annealing heat treatment (S500) compared to Comparative Example 3, and the redistribution of carbon (C) and manganese (Mn) was sufficiently achieved to greatly increase the elongation, so that sufficient retention was required to increase the elongation. You can see that it takes time.

In Comparative Example 4, in the second annealing heat treatment (S500), the cooling end temperature was less than the martensite transformation start temperature (Ms), martensite was formed at the end of cooling, the austenite fraction was reduced, and due to the low temperature, carbon (C), The redistribution of manganese (Mn) was not effective, so the elongation was below the target value (more than 23%).

In Comparative Examples 5, 6, and 7, the annealing temperature was high in the first annealing heat treatment (S400), so the ferrite fraction in the microstructure after the first annealing heat treatment was 6% and 0%, respectively, within the range proposed by the present invention (30%). to 50%).

Referring to Figure 5, which shows the entire structure after the first annealing heat treatment (S400) of Comparative Example 6, it can be confirmed that the entire structure after the first annealing heat treatment (S400) by single-phase annealing is composed of a low temperature phase. In this way, as the annealing temperature of the first annealing heat treatment (S400) increases, the ferrite fraction in the entire structure decreases and the low-temperature phase fraction increases, and the polygonal ferrite decreases in the final microstructure after the second annealing heat treatment (S500), and acicular ferrite and residual auste The fraction of nite increases, and the fraction of blocky martensite decreases.

Referring to FIG. 6 showing the final microstructure of Comparative Example 7, the microstructure is generally composed of acicular ferrite, a composite structure of martensite/austenite, and retained austenite, and bulky martensite contributes to the increase in strength. It is very small, so the elongation in Comparative Examples 5 and 7 satisfies the target value of the present invention (23% or more), but the tensile strength does not satisfy.

In Comparative Example 6, in the second annealing heat treatment (S500), the cooling end temperature was set to 440°C, which exceeds the bainite transformation start temperature (Bs), and the redistribution of carbon (C) and manganese (Mn) was reduced after the end of cooling to increase the tensile strength. Although it was sufficiently secured above 1022MPa, the elongation rate fell short of the target value (more than 23%) due to insufficient redistribution.

In Comparative Example 8, the annealing temperature (second annealing temperature) of the second annealing heat treatment (S500) is higher than the annealing temperature (first annealing temperature) of the first annealing heat treatment (S400), which is against the heat treatment method proposed in the present invention. If the second annealing temperature is higher than the first annealing temperature, the austenite fraction generated in the second annealing heat treatment (S500) becomes higher than the low-temperature phase fraction of the structure after the first annealing heat treatment (S400). Austenite reversely transformed at a low temperature appears as a lamellar structure of needle-shaped ferrite and austenite, but the austenite produced in excess due to high annealing temperature develops into a blocky form, and as a result, the fraction of blocky martensite increases in the final microstructure. While the tensile strength of the steel increases significantly, the elongation decreases (see Figure 7).

In Comparative Example 9, only the conventional one-time annealing heat treatment was performed, and a microstructure consisting of blocky bainite, martensite, and ferrite appeared, as shown in FIG. 8. It exhibits high tensile strength and low elongation due to its high blocky martensite fraction, low martensite/austenite composite structure, and retained austenite fraction.

According to the experimental example described so far, if an abnormal structure consisting of ferrite and low-temperature phase is not secured in the first annealing heat treatment (S400) or if acicular ferrite and austenite are not properly secured in the second annealing heat treatment (S500), in the present invention It was confirmed that it was difficult to secure physical properties that balanced the target tensile strength and elongation.

Although the above description focuses on the embodiments of the present invention, various changes and modifications can be made at the level of those skilled in the art. These 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 rights of the present invention should be determined by the claims described below.

Claims (9)

1. Carbon (C): 0.15 to 0.20% by weight, Silicon (Si): 1.0 to 2.0% by weight, Manganese (Mn): 1.5 to 3.0% by weight, Phosphorus (P): greater than 0 and less than or equal to 0.02% by weight, Sulfur (S): 0.003% by weight or less, aluminum (Al): 0.01 to 0.3% by weight, nitrogen (N): 0.01% by weight or less, titanium (Ti): 48/14·[N] to 0.1% by weight (the [N) ] is the weight percent value of nitrogen) and the remainder consists of iron (Fe) and other inevitable impurities,

   The final microstructure consists of ferrite, acicular retained austenite, martensite/austenite composite structure, and blocky martensite.

   The area fraction of the ferrite is 30 to 60%, the area fraction of the acicular retained austenite is 5 to 12%, the area fraction of the martensite/austenite composite structure is 25 to 50%, and the bulky martensite The area fraction is 5 to 12%,

   Characterized in that the carbon enrichment amount in the retained austenite is 1.1% by weight or more,

   Cold rolled steel plate.
2. According to claim 1,

   The ferrite is composed of polygonal ferrite and acicular ferrite,

   Among the ferrites, the area fraction of the needle-shaped ferrite is 40% or more,

   Cold rolled steel plate.
3. According to claim 1,

   Tensile strength (TS): 980 to 1180 MPa, and elongation (El): 23 to 25%.

   Cold rolled steel plate.
4. (a) Carbon (C): 0.15 to 0.20% by weight, silicon (Si): 1.0 to 2.0% by weight, manganese (Mn): 1.5 to 3.0% by weight, phosphorus (P): greater than 0 and less than 0.02% by weight, sulfur ( S): more than 0 and 0.003% by weight or less, aluminum (Al): 0.01 to 0.3% by weight, nitrogen (N): more than 0 and 0.01% by weight or less, titanium (Ti): 48/14·[N] to 0.1% by weight ( [N] is a weight percent value of nitrogen) and reheating the steel consisting of the remaining iron (Fe) and other inevitable impurities;

   (b) hot rolling the reheated steel;

   (c) cold rolling the hot rolled steel;

   (d) First annealing including maintaining the cold rolled steel at a first annealing temperature of (Ac1 + 30°C) or more (Ac3 - 30°C) and then cooling to a cooling end point temperature of 340°C or less. heat treatment step; and

   (e) The steel material is maintained at a second annealing temperature of not less than Ac1 (Ac3 - 30°C) and then cooled to not less than the martensite transformation start temperature (Ms) (bainite transformation start temperature (Bs) - 15°C). A second annealing heat treatment step including a process of over-aging after cooling to the end point temperature; sequentially including,

   Characterized in that the second annealing temperature is lower than the first annealing temperature,

   Manufacturing method of cold rolled steel sheet.
5. According to claim 4,

   Step (a) includes reheating the steel at 1180 to 1300°C,

   Step (b) includes hot rolling under conditions where the finish rolling temperature is 850 to 950°C and the coiling temperature is 450 to 650°C,

   Step (c) includes cold rolling at a reduction ratio of 40 to 70%,

   Manufacturing method of cold rolled steel sheet.
6. According to claim 4,

   The step (d) includes maintaining the cold rolled steel at the first annealing temperature for 30 to 120 seconds and then cooling it to a cooling end point temperature of 340°C or less at a cooling rate of 15°C/s or more.

   Manufacturing method of cold rolled steel sheet.
7. According to claim 6,

   After performing step (d), the area fraction of ferrite in the microstructure of the steel is 30 to 50%,

   Manufacturing method of cold rolled steel sheet.
8. According to claim 4,

   In step (e), the steel is maintained at the second annealing temperature for 30 to 120 seconds, and then cooled at a cooling rate of 15°C/s or more to the martensite transformation start temperature (Ms) or more (bainite transformation start temperature (Bs). ) - Including the process of cooling to a cooling end point temperature of 15℃ or lower and then over-aging for 30 to 300 seconds,

   Manufacturing method of cold rolled steel sheet.
9. According to claim 8,

   After performing step (e), the microstructure of the steel is composed of ferrite, acicular retained austenite, martensite/austenite composite structure, and bulky martensite,

   The area fraction of the ferrite is 30 to 60%, the area fraction of the acicular retained austenite is 5 to 12%, the area fraction of the martensite/austenite composite structure is 25 to 50%, and the bulky martensite Characterized in that the area fraction is 5 to 12%,

   Manufacturing method of cold rolled steel sheet.

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