KR20230004237A - Cold-rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

The present invention provides a cold-rolled steel sheet comprising: 0.15 to 0.20 wt% of carbon (C); 1.0 to 2.0 wt% of silicon (Si); 1.5 to 3.0 wt% of manganese (Mn); more than 0 and equal to or less than 0.02 wt% of phosphorus (P); more than 0 and equal to or less than 0.003 wt% of sulfur (S); 0.01 to 0.3 wt% of aluminum (Al); more than 0 and equal to or less than 0.01 wt% of nitrogen (N); 48/14 * [N] to 0.1 wt% of titanium (Ti) where [N] represents the wt% value of nitrogen; and the remainder consisting of iron (Fe) and other inevitable impurities, wherein the final microstructure is composed of ferrite, acicular retained austenite, martensite/austenite composite structure, and bulky martensite, and wherein 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 %, the area fraction of the bulky martensite is 5 to 12 %, and the carbon concentration in the retained austenite is equal to or more than 1.1 wt%. Therefore, provided are a cold-rolled ultra-high-strength low-carbon steel sheet having excellent moldability and a manufacturing method thereof.

Description

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

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

Ultra-high-strength steel for automotive steel is being developed to meet the two factors of vehicle weight reduction in response to environmental regulatory issues and reinforcement of crash safety standards in accordance with strengthened safety regulations. However, since strength and elongation are in a trade-off relationship, the problem of deterioration in formability due to increase in strength has emerged, and various studies have been conducted to secure formability of high-strength steel.

TRIP-aided steel, which uses the TRansformation Induced Plasticity (TRIP) phenomenon, which transforms 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. Since the physical properties of such TRIP-aided steel are determined by the phase stability and fraction of retained austenite that causes the TRIP phenomenon, it is important to secure stable retained austenite in the microstructure in the manufacture of the steel.

As related prior art, there is Korean Patent Application No. 2018-0033119.

A 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.

Cold-rolled steel sheet according to an embodiment of the present invention for solving the above problems is 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 0.02% by weight or less, sulfur (S): greater than 0 and 0.003% by weight or less, aluminum (Al): 0.01 to 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 rest consists of iron (Fe) and other unavoidable impurities, and the final microstructure is ferrite and needle-like retained austenite , Composed of a composite structure of martensite/austenite and massive martensite, the area fraction of ferrite is 30 to 60%, the area fraction of the acicular retained austenite is 5 to 12%, the martensite/austenite The area fraction of the composite structure is 25 to 50%, the area fraction of the bulky martensite is 5 to 12%, and the carbon concentration in 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 ferrites may be 40% or more.

The cold-rolled steel sheet may have tensile strength (TS): 980 to 1180 MPa, and elongation (El): 23 to 25%.

Method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention for solving the above problems 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): greater than 0 and less than 0.02% by weight, sulfur (S): greater than 0 and less than 0.003% by weight, aluminum (Al): 0.01 to 0.3% by weight, nitrogen (N): greater than 0.01% by weight % or less, titanium (Ti): 48/14*[N] to 0.1% by weight (the [N] is the weight % value of nitrogen) and the remaining iron (Fe) and other unavoidable impurities Reheating the steel; (b) hot rolling the reheated steel; (c) cold-rolling the hot-rolled steel material; (d) First annealing including a step of cooling to a cooling end point temperature of 340 ° C or less after maintaining the cold-rolled steel at a first annealing temperature of (Ac1 + 30 ° C) or more (Ac3 - 30 ° C) or less heat treatment step; And (e) after holding at the second annealing temperature of Ac1 or more (Ac3 - 30 ℃) or less with respect to the steel material, martensite transformation start temperature (Ms) or more (bainite transformation start temperature (Bs) - 15 ℃) or less A second annealing heat treatment step comprising a step of overaging after cooling to the cooling end point temperature; sequentially including, wherein the second annealing temperature is lower than the first annealing temperature.

In the cold-rolled steel sheet manufacturing method, step (a) includes reheating the steel material at 1180 to 1300 ° C, and step (b) has 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 ° C., and the step (c) may include the step of cold rolling at a reduction ratio of 40 to 70%.

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

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

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

In the manufacturing method of the cold-rolled steel sheet, after performing the step (e), the microstructure of the steel material is composed of ferrite, needle-shaped retained austenite, martensite/austenite composite structure, and bulky martensite, and the ferrite The area fraction of 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%, the area fraction of the bulky martensite may be 5 to 12%.

According to an embodiment of the present invention, it is possible to implement a cold-rolled ultra-high-strength low-carbon steel sheet with excellent formability and a manufacturing method thereof. Specifically, it is possible to secure excellent weldability by designing a low-carbon steel grade containing less than 0.2% by weight of carbon, and by concentrating a sufficient amount of carbon and manganese into austenite through several stages of redistribution of alloying elements during the heat treatment process, A balance between strength and elongation can be realized, and cold-rolled ultra-high-strength steel with excellent workability that secures a tensile strength of 980 MPa or more and an elongation of 23% or more can be implemented.

Of course, 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.
Figure 2 is a view illustrating the outline of (a) a first annealing heat treatment process and (b) a second annealing heat treatment process in the manufacturing method of a cold-rolled steel sheet according to an embodiment of the present invention.
3 is a photograph of the microstructure after the first annealing heat treatment in Example 1 among experimental examples.
4 is a photograph of the microstructure after the second annealing heat treatment in Example 1 among experimental examples.
Figure 5 is a photograph of the microstructure after the first annealing heat treatment in Comparative Example 6 of Experimental Examples, Figure 6 is a photograph of the final microstructure in Comparative Example 7 of Experimental Examples, and Figure 7 is a comparison of Experimental Examples. A photograph of the final microstructure in Example 8, Figure 8 is a photograph of the final microstructure in Comparative Example 9 among experimental examples, and Figure 9 is a photograph of the final microstructure after overaging after the second annealing heat treatment (a) needle-like shape ( b) It is a photograph of a lumpy-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. Terms to be described later are terms appropriately selected in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout this specification. Hereinafter, specific details of a cold-rolled ultra-high-strength low-carbon steel sheet with excellent formation and a manufacturing method thereof will be provided.

Korean Patent Application No. 2018-0033119 suggests a method for manufacturing steel (Quenching and Partitioning, Q&P) containing tempered martensite and retained austenite through rapid cooling and partitioning heat treatment after annealing of the steel. Q&P steel has the advantage of being able to obtain physical properties of 980 MPa or more in tensile strength and 21% or more in elongation even at 0.2% by weight of carbon steel. difficult to obtain

Korean Patent Publication No. 2017-0113858 suggests a two-time annealing heat treatment process as a method of securing a microstructure (whole structure) before final annealing to increase ductility of steel by securing lath-shaped ferrite and retained austenite. . However, single-phase annealing is performed to secure a low-temperature structure with a volume fraction of 90% or more after the first annealing, so it is not possible to stably secure a tensile strength of 980 MPa or more in steel with a low carbon content. .

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

steel plate

In 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): 0 More than 0.02% by weight or less, 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 less than 0.01% by weight, titanium (Ti): 48/14* It consists of [N] to 0.1% by weight (the [N] is the weight% value of nitrogen) and the rest iron (Fe) and other unavoidable impurities.

Hereinafter, 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 the steel, and the strength increases as the carbon content increases in the martensitic 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 of the cold-rolled steel sheet according to an embodiment of the present invention. When the content of carbon is less than 0.15% by weight of the total weight, the above-mentioned effect cannot be implemented and there is a problem of not securing sufficient strength. Conversely, when the carbon content exceeds 0.20% by weight of the total weight, problems such as deterioration in weldability and workability appear.

Silicon (Si)

Silicon (Si) is an element added to increase the strength and suppress the formation of carbides due to the ferrite solid solution strengthening effect. In addition, since silicon is well known as a ferrite stabilizing element, ductility can be increased by increasing the ferrite fraction during cooling. In addition, it is known as an element capable of securing strength by promoting the formation of martensite by enriching austenite carbon. On the other hand, silicon is added as a deoxidizer for removing oxygen in steel in the steelmaking process together with aluminum, and may have a solid solution strengthening effect. The silicon may be added in a content ratio of 1.0 to 2.0% by weight of the total weight of the cold-rolled steel sheet according to an embodiment of the present invention. When the content of silicon is less than 1.0% by weight of the total weight, ductility cannot be secured and the above-mentioned effect of adding silicon cannot be properly exhibited. Conversely, when the content of silicon 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, which impairs plating properties, and increasing the carbon equivalent may reduce weldability, and reheating and hot By generating a red scale during rolling, a problem may be given to the surface quality, and there is a problem in that toughness and plastic workability are 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 an 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 of the cold-rolled steel sheet according to an embodiment of the present invention. When the content of manganese is less than 1.5% by weight, the above-described effect of securing strength cannot be sufficiently exerted. In addition, when the manganese content exceeds 3.0% by weight, workability and delayed fracture resistance decrease due to the formation or segregation of inclusions such as MnS, and the carbon equivalent may be increased to decrease weldability.

Phosphorus (P)

Phosphorus (P) can increase the strength by solid solution strengthening and suppress 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 of the cold-rolled steel sheet according to an embodiment of the present invention. When the content of phosphorus exceeds 0.02% by weight, the welded portion is embrittled, low-temperature brittleness is induced, press formability is lowered, and impact resistance is lowered.

Sulfur (S)

Sulfur (S) can combine with manganese, titanium, etc. to improve the machinability of steel and form precipitates of fine MnS to improve workability, but is an element that generally 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 of the cold-rolled steel sheet according to an embodiment of the present invention. When the sulfur content exceeds 0.003% by weight, the number of Fes inclusions or MnS inclusions increases, resulting in deterioration in toughness and weldability, poor workability, and problems of segregation during solidification of continuous casting and occurrence of high-temperature cracks.

Aluminum (Al)

Aluminum (Al) is an element mainly used as a deoxidizer, promotes ferrite formation, improves elongation, suppresses carbide formation, and promotes carbon concentration in austenite to stabilize austenite. The aluminum (Al) is preferably added in a content ratio 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 content of aluminum (Al) is less than 0.01% by weight, the above-described effect of adding aluminum may be properly exhibited. Conversely, when the content of aluminum (Al) exceeds 0.3% by weight and is excessively added, aluminum inclusions increase, degrading playability, concentrating on the surface of the steel sheet, degrading plating properties, and forming AlN in the slab to cause hot-rolled cracks. There are problems that cause it.

Nitrogen (N)

Nitrogen (N) is a solid solution strengthening element capable of increasing the strength of a steel sheet, and is an element generally mixed from the atmosphere. Its content must be controlled in the degassing process of the steelmaking process. When the content of nitrogen exceeds 0.01% by weight, problems such as brittleness of the welded part, low-temperature brittleness, deterioration of press formability, and deterioration of impact resistance may occur.

Titanium (Ti)

Titanium (Ti) is a precipitate-forming element, and has the effect of precipitating TiN and refining crystal grains. In particular, the nitrogen content in the steel can be lowered through the precipitation of TiN. Titanium is preferably added in an amount of 48/14*[N] to 0.1% by weight, and when it is less than 48/14*[N] by weight, the amount of TiC precipitated is small, so the effect of adding Ti is insufficient, and the amount of titanium is added in excess of 0.1% by weight. If it is, 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 one embodiment of the present invention having an alloy element composition may be a cold-rolled super-high strength steel sheet having a tensile strength of 980 MPa or more and an elongation of 23% or more and excellent elongation. For example, the cold-rolled steel sheet may have tensile strength (TS) of 980 to 1180 MPa and elongation (El) of 23 to 25%.

The final microstructure of the cold-rolled steel sheet is composed of ferrite, acicular retained austenite, martensite/austenite composite structure, and bulky martensite, and the area fraction of the ferrite is 30 to 60%, the acicular The area fraction of 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 retained austenite It is 1.1 weight% or more of carbon enrichment. The ferrite is composed of polygonal ferrite and acicular ferrite, and the area fraction of the acicular ferrite among the ferrites 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 above-described composition and microstructure will be described.

Steel plate manufacturing method

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

Referring to FIG. 1, a method for manufacturing a steel sheet according to an embodiment of the present invention includes (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 0.02% by weight or less, sulfur (S): greater than 0 and 0.003% by weight or less, aluminum (Al): 0.01 to 0.3% by weight, nitrogen (N): greater than 0 0.01% by weight Hereinafter, titanium (Ti): 48/14 * [N] to 0.1% by weight (the [N] is the weight% value of nitrogen) and the remaining iron (Fe) and other unavoidable impurities in the steel material consisting of reheating step (S100) ; (b) hot rolling the reheated steel material (S200); (c) cold-rolling the hot-rolled steel material (S300); (d) First annealing including a step of cooling to a cooling end point temperature of 340 ° C or less after maintaining the cold-rolled steel at a first annealing temperature of (Ac1 + 30 ° C) or more (Ac3 - 30 ° C) or less Heat treatment step (S400); And (e) after holding at the second annealing temperature of Ac1 or more (Ac3 - 30 ℃) or less with respect to the steel material, martensite transformation start temperature (Ms) or more (bainite transformation start temperature (Bs) - 15 ℃) or less A second annealing heat treatment step (S500) including a process of overaging after cooling to the cooling end point temperature;

The (a) step (S100) may include reheating the slab steel material having the above composition at 1180 to 1300 ° C. The slab is manufactured in the form of a semi-finished product by continuously casting molten steel obtained through the steelmaking process, and through a reheating process, component segregation generated in the casting process is homogenized and made into a state that can be hot rolled. If the slab reheating temperature (Slab Reheating Temperature, SRT) is less than 1180 ° C, there is a problem that the segregation of the slab is not sufficiently re-dissolved, and if it exceeds 1300 ° C, the size of austenite crystal grains increases and the process cost may increase. Reheating of the slabs can take 1 to 4 hours. If the reheating time is less than 1 hour, the segregation zone reduction is not sufficient, and if it exceeds 4 hours, the grain size may increase and the process cost may increase.

The step (b) (S200) is a step of hot rolling the reheated slab. Hot rolling is hot rolling at a finish delivery temperature (FDT) of 850 ~ 950 ℃. When the finish rolling temperature is lower than 850 ° C., the rolling load increases rapidly and productivity decreases, and when the temperature exceeds 950 ° C., the size of grains increases and strength may decrease. After hot rolling, it is cooled to a temperature of 450 ~ 650 ℃ and then wound up. If the coiling temperature is less than 450℃, the shape of the hot-rolled coil becomes non-uniform and the strength increases, so the rolling load increases during cold rolling. If the coiling temperature exceeds 650℃, defects may occur in the post-process due to surface oxidation, etc. A non-uniform microstructure is caused by a difference in cooling rate between the edge portion and the edge portion, and oxidation of the inside of the grain boundary 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 ratio, the higher the formability due to the microstructure effect. In cold rolling, when the reduction is less than 40%, it is difficult to obtain a uniform microstructure, and when the design is greater than 70%, the roll force increases and the process load increases.

After cold rolling, a first annealing heat treatment process and a second annealing heat treatment process are sequentially performed. That is, the cold-rolled cold-rolled steel sheet is subjected to a total of two annealing cycles of primary annealing and secondary annealing. The temperature increase rate for heating from room temperature to the first or second annealing temperature range is not limited and may follow the temperature increase rate of a normal heating furnace facility.

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

The step (d) is performed by reverse annealing heat treatment for 30 to 120 seconds in the first annealing temperature range of (Ac1 + 30 ℃) or more (Ac3 - 30 ℃) or less to secure a dual-phase structure of ferrite and low-temperature phase It is a step. The first annealing heat treatment process is a process of forming a preferred overall structure for securing a lath-shaped acicular ferrite and austenite structure during the second annealing heat treatment process. In this specification, the term 'whole structure' refers to the microstructure of the steel material manufactured by the first annealing heat treatment (S400). During the second annealing heat treatment process, the low-temperature phase structure reversely transforms into austenite, and a lath-type ferrite and austenite microstructure is formed. Here, the low-temperature phase structure refers to a martensite or bainite phase. This lath-type organization is characterized by being able to secure both high strength and high ductility. In the case of annealing in the ideal temperature range, a primary redistribution of carbon and manganese occurs, carbon and manganese are concentrated in the austenite region, and the phase stability of austenite increases.

In order to satisfy both tensile strength of 980 MPa or more and elongation of 23% or more in the steel grade of 0.2% by weight or less proposed in the present invention, carbon and manganese are redistributed into austenite to increase the strength of martensite to increase the strength of martensite. Since it is necessary to secure strength and increase the phase stability of retained austenite to secure sufficient ductility, the first annealing is preferably performed in an ideal range temperature range. When the first annealing temperature exceeds Ac3, austenite crystals become coarse due to high temperature annealing, and a large amount of austenite having a low carbon and manganese content is generated, making it difficult to secure tensile properties of the final steel. On the other hand, if the first annealing temperature is less than (Ac1 + 30 ℃) even if it is Ac1 or higher, the ferrite fraction in the microstructure exceeds 50% after the first annealing heat treatment process, and the soft coarse polygonal ferrite in the final microstructure increases. It is difficult to secure the tensile properties of steel.

Therefore, preferably, after the first annealing heat treatment process, a DP (dual phase) structure composed of ferrite and low-temperature phase should appear in the microstructure, more preferably, the fraction of ferrite is 30% or more in area fraction for strength and ductility balance 50 % or less. In cooling the steel sheet subjected to the primary annealing heat treatment described above to room temperature, it is 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 martensitic structure. Cooling is possible at 25° C./s or higher.

On the other hand, 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 low temperature phase, and the fraction of ferrite is limited to 30% or more and 50% or less in terms of area fraction , The heat treatment temperature of the primary annealing may be limited to (Ac1 + 30 ° C) or more (Ac3 - 30 ° C) or less.

In the step (e), after maintaining the steel material at the second annealing temperature of AAc1 or more (Ac3 - 30 ℃) or less, the martensite transformation start temperature (Ms) or more (bainite transformation start temperature (Bs) - 15 ℃) This is the second annealing heat treatment step including a step of overaging after cooling to the following cooling end point temperature.

The step (e) is a step in which the martensite structure generated in the first annealing heat treatment process is reverse transformed to form lath-type ferrite and austenite. During annealing, the reverse transformation of the primary low-temperature phase and the redistribution of carbon and manganese to austenite occur, so a longer annealing time is desirable for sufficient reverse transformation and redistribution of alloy elements. Time is limited from 30 to 120 seconds.

The steel sheet subjected to the secondary annealing heat treatment is cooled to a temperature between the martensitic transformation start temperature (Ms) and the bainite transformation start temperature (Bs) and maintained for 30 to 300 seconds to induce redistribution of carbon and manganese alloy elements to induce retained austenite. This step increases the stability of the costume. In cooling to the cooling end temperature below the martensite transformation start temperature (Ms) or higher (bainite transformation start temperature (Bs) - 15 ℃), if the cooling rate is less than 15 ℃ / s, polygonal ferrite is generated during cooling and the final The cooling rate is 15°C/s or more, preferably 25°C/s or more, because it causes inferior tensile properties of the steel. If the cooling end temperature is higher than (bainite transformation start temperature (Bs) - 15 ℃), ferrite or pearlite is generated during the holding step, causing a decrease in strength and elongation, and the cooling end temperature is the bainite transformation start temperature (Bs ), bainite transformation and carbon redistribution do not occur in a balanced manner to the high-temperature bainite formation zone at the temperature directly below. Conversely, if the cooling end point temperature is lower than the martensite transformation start temperature (Ms), fresh martensite is created by cooling, which greatly increases the strength of the steel, while reducing the retained austenite to 23%, which is the target of the present invention. The above sufficient elongation rate cannot be secured. Therefore, the cooling end point temperature is preferably determined at a temperature equal to or higher than the martensite transformation start temperature (Ms) (bainite transformation start temperature (Bs) - 15 ° C) or less. After cooling to the cooling end point temperature, hold for 30 to 300 seconds for additional redistribution of carbon and manganese, and then cool to room temperature. At this time, the cooling rate to room temperature is not particularly limited, but it is preferable to be 10 ℃ / s or more for productivity. Do.

Furthermore, in the method for manufacturing a cold-rolled steel sheet according to the present invention, the second annealing temperature is lower than the first annealing temperature. When the second annealing temperature is higher than the first annealing temperature, the austenite fraction generated in the second annealing heat treatment (S500) is higher than the low temperature phase fraction of the structure after the first annealing heat treatment (S400). Austenite reversely transformed in the low temperature phase appears as a lamellar structure of acicular ferrite and austenite. 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, acicular retained austenite, martensite/austenite composite structure, 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 bulky martensite may be 5 to 12%. .

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

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

2 is a view 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.

1st annealing heat treatment (S400)

Referring to (a) of Figure 2, the section a-b corresponds to the step of maintaining at the first annealing temperature of (Ac1 + 30 ℃) or more (Ac3 - 30 ℃) or less, section b-c is the first half of the cooling section, a slow cooling process Corresponds to, and the c-d section is the second half of the cooling section and corresponds to the rapid cooling process, and the d-e section corresponds to the overaging process. In a modified embodiment of the present invention, the slow cooling process in the b-c section and the overaging process in the d-e section can be omitted.

The first annealing heat treatment process may 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 embodiment is 0.2% by weight or less, in order to secure the tensile strength of the steel, the redistribution of alloy elements must be increased compared to the existing steel type. lead to 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. Since the fraction of austenite with a low element content (lean) increases, it is difficult to achieve the target tensile properties. If the holding time exceeds 120 seconds, the size of crystal grains may become coarse and productivity may decrease. The annealed cold-rolled steel is cooled at a cooling rate of 15 °C/s or more to a temperature of 340 °C or less. At this time, when the cooling end point temperature exceeds 340 ° C, it is difficult to obtain a lath-shaped structure in the second annealing heat treatment process due to carbide precipitation, and when the cooling rate is less than 15 ° C / s, a large amount of polygonal ferrite is generated during cooling Unfavorable for securing tensile strength.

Section b-c is a step of slowly cooling the annealed and heat-treated steel sheet. In cooling the steel sheet subjected to the annealing heat treatment, a slow cooling section may be included according to heat treatment facilities. In the case of including the slow cooling section, the slow cooling end point temperature or cooling rate is not particularly limited, but the slow cooling end point temperature is preferably 740°C or higher and the cooling rate is -5°C/s so that a large amount of polygonal ferrite is not generated during cooling. may be ideal

Section c-d is the second half of the cooling section and corresponds to the rapid cooling process, and is a step of cooling the steel sheet cooled by the slow cooling process to a temperature of 340 ° C or lower. In cooling the primary annealed heat treated steel sheet or the primary cooled steel sheet, cooling at -15 ° C / s or more to suppress the formation of polygonal ferrite, which adversely affects physical properties, and form bainite or martensite, which is a low-temperature phase, , preferably at least 25°C/s. The cooling rate is maintained to a temperature below the martensite transformation start temperature (Ms) represented by Equation (1) below, and then cooled to room temperature through the overaging section of the facility. Alternatively, it is okay to directly cool to room temperature by omitting the overaging section.

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

2nd annealing heat treatment (S500)

Referring to (b) of Figure 2, the p-q section corresponds to the step of maintaining at the second annealing temperature of Ac1 or more (Ac3 - 30 ℃) or less, the q-r section corresponds to the slow cooling section as the first half of the 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. On the other hand, in the second annealing heat treatment process disclosed in (b) of FIG. 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 may be maintained for 30 to 120 seconds at the second annealing temperature of Ac1 or more (Ac3 - 30 ℃) or less. Furthermore, the second annealing temperature is characterized in that lower than the first annealing temperature. Perform a step of reverse annealing heat treatment for 30 to 120 seconds at a temperature less than the primary annealing temperature above Ac1. This is a step in which the low-temperature phase structure generated in the first annealing heat treatment (S400) is reverse transformed to form lath-shaped ferrite and austenite. During annealing, the reverse transformation of the primary low-temperature phase and the redistribution of carbon (C) and manganese (Mn) to austenite occur, so a longer annealing time is desirable for sufficient reverse transformation and redistribution of alloy elements. Since deterioration is concerned, the annealing holding time is limited to within 30 seconds to 120 seconds. When the annealing temperature of the second annealing heat treatment (S500) is equal to or higher than the annealing temperature of the first annealing heat treatment (S400), the austenite fraction generated in the secondary annealing is greater than the low temperature phase fraction in the entire structure. As a result, the development of lath-type austenite, which undergoes reverse transformation in the low-temperature phase, is inhibited, and bulk-type austenite is produced by an excess fraction. Such blocky austenite reduces the phase stability of austenite by reducing carbon (C) and manganese (Mn) that are redistributed into lath-shaped austenite. This hinders the effect of the first annealing heat treatment (S400) for generating a lath-shaped structure required to secure the elongation to be achieved 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).

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

The bainite transformation start temperature (Bs) can be expressed by Equation (2) below.

Equation (2)

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

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

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

If the cooling end point temperature exceeds the bainite transformation start temperature (Bs) or less (bainite transformation start temperature (Bs) - 15 ℃), austenite transforms into ferrite or pearlite during the holding stage, causing reduction in strength and elongation, If it is directly below the bainite transformation start temperature (Bs), it is difficult to secure the phase stability of retained austenite due to insufficient carbon redistribution. Conversely, if the cooling end point temperature is less than the martensitic transformation start temperature (Ms), fresh martensite is generated and the strength of the steel greatly increases, while the retained austenite decreases, so that sufficient elongation of 23% or more targeted in the present invention can be secured. there will be no In addition, when the holding time is less than 30 seconds, the redistribution time is insufficient and the redistribution effect is reduced, and when the holding time exceeds 300 seconds, productivity may be reduced.

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

weight% acicular blocky carbon (C) 0.6±0.05 0.3±0.05 Manganese (Mn) 3.5±0.5 2.25±0.25

According to the above-mentioned cold-rolled steel sheet and its manufacturing method according to the technical concept of the present invention, it is possible to secure excellent weldability by designing a low-carbon steel grade containing less than 0.2% by weight of carbon, and to redistribute alloy elements in several stages during the heat treatment process. Through this process, a sufficient amount of carbon (C) and manganese (Mn) can be concentrated in austenite to realize an excellent balance of strength and elongation, and cold-rolled super-high-strength steel with excellent workability that secures a tensile strength of 980 MPa or more and an elongation of 23% or more. can provide.

Experimental example

Hereinafter, preferred experimental examples are presented to aid 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.

1. Specimen fabrication

In this experimental example, specimens having the alloy element composition (unit: weight%) of 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 components in Table 2 are 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, which are the composition of the cold-rolled steel sheet according to an embodiment of the present invention. , 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 to 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 ([N] is the weight% value of nitrogen) and the remaining iron (Fe) composition is satisfied.

According to the components of Table 2, the bainite transformation start temperature (Bs) is 437.6 ° C, and the martensite transformation start temperature (Ms) is calculated as 341.6 ° C. The temperature is calculated by the following relational expression.

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 ]

On the other hand, according to the components of Table 2, the Ac1 temperature is 754 °C and the Ac3 temperature is 900 °C.

In the experimental example of the present invention, the steel material having the above composition was reheated at 1250 ° C. for 4 hours, then hot rolled to a thickness of 3.5 mm at a finish rolling temperature (FDT) of 850 ° C., and then wound at a coiling temperature of 600 ° C. Thereafter, surface oxide scale was removed through pickling, and cold rolling was performed to a thickness of 1.2 mm. Thereafter, the cold-rolled steel sheet was subjected to two consecutive heat treatments according to the configuration disclosed in FIG. 2 .

2. Process conditions and property evaluation

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

A.
1st annealing temperature B. 1st annealing time C.
slow cooling end temperature D.
quench end temperature E.
overaging time F.
2nd annealing temperature G.
2nd annealing time H.
Quick cooling end temperature I.
Overaging end temperature J.
overaging 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 primary annealing heat treatment process (S400) and corresponds to the annealing temperature of the a-b section in FIG. Corresponds to the process time of the section a-b in (a) of 2, and item C is the end temperature of the slow cooling of the first annealing heat treatment process (S400), point c, which is the end temperature of the slow cooling process of the section b-c in (a) of FIG. Corresponds to the temperature of, item D is the end temperature of the rapid cooling of the primary annealing heat treatment process (S400) and corresponds to the temperature of point d, which is the end temperature of the rapid cooling process in the section c-d in FIG. As the overaging time of the primary annealing heat treatment process (S400), it corresponds to the process time of the overaging process of the d-e section in FIG. 2 (a).

In addition, item F in Table 3 is the annealing temperature of the secondary annealing heat treatment process (S500) and corresponds to the annealing temperature of the p-q section in FIG. Corresponds to the process time of the p-q section in FIG. 2 (b), and H item is the end temperature of the rapid cooling process of the r-s section in FIG. 2 (b) as the end temperature of the rapid cooling of the secondary annealing heat treatment process (S500) Corresponds to the temperature at point s, and item I is the overaging end temperature of the secondary annealing heat treatment process (S500) and corresponds to the temperature at point t, which is the end temperature of the overaging process in the s-t section in FIG. 2 (b), Item J is the overaging time of the secondary annealing heat treatment process (S500) and corresponds to the process time of the overaging process of the s-t section in FIG. 2 (b).

A.
ferrite B.
cold phase C.
ferrite D.
PF E.
Lath F F.
RA G.
M/A H.
blocky 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 (unit: %) of the microstructure and the amount of carbon enrichment (unit: wt%) in retained austenite in the experimental example 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 the carbon content in the retained austenite.

In Table 4, item A is the area fraction of the ferrite phase realized after the primary annealing heat treatment, item B is the area fraction of the low temperature phase realized after the primary annealing heat treatment, item C is the area fraction of the ferrite phase implemented after the secondary annealing heat treatment, and , item D is the area fraction of the polygonal ferrite phase among the ferrites realized after the secondary annealing heat treatment, item E is the area fraction of the acicular ferrite phase among the ferrites realized after the secondary annealing heat treatment, and item F is the area fraction of the ferrite phase implemented after the secondary annealing heat treatment is the area fraction of the needle-shaped retained austenite phase, item G is the area fraction of the composite martensite/austenite phase realized after the secondary annealing heat treatment, and item H is the area fraction of the massive martensite phase realized after the secondary annealing heat treatment fraction, and item I is the amount of carbon enrichment in retained austenite realized after the secondary annealing heat treatment.

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

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

TS T.El TS X T.El 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 together, Example 1, Example 2, Example 3, and Example 4 are performed by appropriately performing the first annealing heat treatment (S400) and the second annealing heat treatment (S500) proposed in the present invention. Tensile strength of 980 MPa or more (eg, 980 to 1180 MPa), elongation of 23% or more (eg, 23 to 25%), and TS x El of 22,000 MPa% or more to be achieved in the present invention are satisfied.

Referring to FIG. 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 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 the same as in FIG. It can be seen that it consists of

In Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4, the first annealing heat treatment (S400) is performed at an ideal temperature of 850 ° C. to secure a sufficient amount of 45% ferrite in the microstructure after the first annealing heat treatment. However, since the redistribution of alloying elements was not smoothly performed in the second annealing heat treatment (S500), the retained austenite fraction and phase stability were not sufficiently secured, so the tensile strength was sufficiently high, such as 1000 MPa or more, but the elongation was 23% to be achieved in the present invention did not reach much.

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

In Comparative Example 3, like Examples 1, 2, and Examples, the cooling was terminated at an appropriate temperature equal to or lower than the martensite transformation start temperature (Ms) (bainite transformation start temperature (Bs) - 15 ° C.), but the holding time (shown The elongation rate fell short of the target value (more than 23%) because the effective time) was short, less than 30 seconds, and a sufficient amount of redistribution was not achieved. In Example 2, the holding time was increased in the second annealing heat treatment (S500) than in Comparative Example 3, and the redistribution of carbon (C) and manganese (Mn) was sufficiently made to greatly increase the elongation, so that sufficient maintenance 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 is less than the martensite transformation start temperature (Ms), and martensite is formed at the end of cooling to reduce the austenite fraction, and due to the low temperature, carbon (C), The redistribution of manganese (Mn) was not effective, so the elongation fell short of the target value (more than 23%).

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

Referring to FIG. 5 showing 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) is composed of a low-temperature phase by single-phase annealing. 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 polygonal ferrite is reduced in the final microstructure after the second annealing heat treatment (S500), and acicular ferrite and retained austenite are increased. The fraction of nitrite 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 the bulky martensite contributing to the increase in strength is Very small elongation in Comparative Example 5 and Comparative Example 7 satisfies the target value (23% or more) of the present invention, but the tensile strength is not satisfied.

In Comparative Example 6, the cooling end temperature in the second annealing heat treatment (S500) is set to 440 ° C. exceeding the bainite transformation start temperature (Bs) to reduce redistribution of carbon (C) and manganese (Mn) after cooling to increase tensile strength Although sufficient over 1022MPa was secured, the elongation fell short of the target value (over 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 contrary to the heat treatment method proposed in the present invention. When the second annealing temperature is higher than the first annealing temperature, the austenite fraction generated in the second annealing heat treatment (S500) is higher than the low temperature phase fraction of the structure after the first annealing heat treatment (S400). Austenite reversely transformed in the low temperature phase appears as a lamellar structure of needle-like ferrite and austenite, but austenite formed in excess due to high annealing temperature develops a conglomerate form, and as a result, the fraction of conglomerate martensite increases in the final microstructure. The tensile strength of sea steel increases significantly, while the elongation decreases (see Fig. 7).

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

According to the experimental examples described so far, if the ideal structure composed of ferrite and low-temperature phase is not secured in the first annealing heat treatment (S400) or 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 in which the target tensile strength and elongation were balanced.

Although the above has been described based on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. As long as these changes and modifications do not depart from the scope of the present invention, it can be said to belong to the present invention. Therefore, the scope of the present invention will be determined by the claims described below.

Claims (9)

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 0.02% by weight or less, 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 (the above [N ] is composed of the weight % value of nitrogen) and the rest of iron (Fe) and other unavoidable impurities,
The final microstructure consists 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 complex structure is 25 to 50%, the bulky martensite The area fraction is 5 to 12%,
Characterized in that the carbon concentration in retained austenite is 1.1% by weight or more,
cold rolled steel. According to claim 1,
The ferrite is composed of polygonal ferrite and acicular ferrite,
The area fraction of the acicular ferrite in the ferrite is 40% or more,
cold rolled steel. According to claim 1,
Tensile strength (TS): 980 to 1180 MPa, elongation (El): characterized in that 23 to 25%,
cold rolled steel. (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 0.02% by weight or less, sulfur ( S): greater than 0 and 0.003% by weight or less, aluminum (Al): 0.01 to 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 ( Reheating a steel material consisting of the [N] is the weight percent value of nitrogen) and the remaining iron (Fe) and other unavoidable impurities;
(b) hot rolling the reheated steel;
(c) cold-rolling the hot-rolled steel material;
(d) First annealing including a step of cooling to a cooling end point temperature of 340 ° C or less after maintaining the cold-rolled steel at a first annealing temperature of (Ac1 + 30 ° C) or more (Ac3 - 30 ° C) or less heat treatment step; and
(e) After holding at the second annealing temperature of Ac1 or more (Ac3 - 30 ℃) or less with respect to the steel material, martensite transformation start temperature (Ms) or more (bainite transformation start temperature (Bs) - 15 ℃) or less cooling A second annealing heat treatment step including a process of overaging after cooling to the end point temperature; sequentially including,
The second annealing temperature is characterized in that lower than the first annealing temperature,
Manufacturing method of cold-rolled steel sheet. According to claim 4,
Step (a) includes reheating the steel at 1180 to 1300 ° C,
The step (b) includes hot rolling under the condition that the finish rolling temperature is 850 to 950 ° C and the coiling temperature is 450 to 650 ° C,
Step (c) comprises cold rolling at a reduction ratio of 40 to 70%,
Manufacturing method of cold-rolled steel sheet. According to claim 4,
The step (d) includes a step of holding the cold-rolled steel at the first annealing temperature for 30 to 120 seconds and then cooling to a cooling end temperature of 340 ° C or less at a cooling rate of 15 ° C / s or more,
Manufacturing method of cold-rolled steel sheet. According to claim 6,
Characterized in that the area fraction of ferrite in the microstructure of the steel after performing step (d) is 30 to 50%,
Manufacturing method of cold-rolled steel sheet. According to claim 4,
In the step (e), the steel material is maintained at the second annealing temperature for 30 to 120 seconds, and then martensite transformation start temperature (Ms) or higher (bainite transformation start temperature (Bs) at a cooling rate of 15 ° C / s or more) ) - including a process of overaging for 30 to 300 seconds after cooling to the cooling end temperature below 15 ℃),
Manufacturing method of cold-rolled steel sheet. According to claim 8,
After performing the 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 complex structure is 25 to 50%, the bulky martensite Characterized in that the area fraction is 5 to 12%,
Manufacturing method of cold-rolled steel sheet.

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