KR20220086202A - A method of preparing utlra high strength cold-rolled steel sheet having excellent yield ratio and ductility and utlra high strength cold -rolled steel sheet using the same - Google Patents

Abstract

The present invention is, by weight%, carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid value aluminum (sol.Al) 0.02 to 0.05%, molybdenum (Mo) ), boron (B) 0.0001 to 0.002%, 0.1 to 0.3%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, remaining iron (Fe) and inevitable High-strength cold-rolled steel sheet with excellent yield ratio and formability, which is made of impurities, the structure is composed of ferrite, bainite, fresh martensite, and tempered martensite, and the area fraction of the tempered martensite satisfies the following relation 1 is about
[Relational Expression 1]
70 ≤ TM / (FM + TM) ≤ 85
(In Equation 1, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, and FM is the area fraction (%) of fresh martensite)

Description

Manufacturing method of high-strength cold-rolled steel sheet with excellent yield ratio and formability and high-strength cold-rolled steel sheet manufactured using the same USING THE SAME}

The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet having excellent yield ratio and formability, and a high-strength cold-rolled steel sheet manufactured using the same, and more preferably, a yield strength of 900 MPa or more, a tensile strength of 1,180 MPa, an elongation of 8.0% and A method of manufacturing a high-strength cold-rolled steel sheet having a yield ratio of 0.7 or more at the same time, and a high-strength cold-rolled steel sheet manufactured using the same.

Due to the recent tightening of carbon dioxide emission regulations, the need to improve fuel efficiency is increasing, so automakers are focusing on reducing the weight of car bodies. In addition, the application of high-strength steel to structural members of automobiles is continuously increasing as the safety regulations on the impact resistance of automobiles as well as environmental problems are expanded.

As a method for manufacturing the above-described high-strength steel, a method of strengthening the strength by adding a rapid cooling section (RCS) after annealing in the heat treatment process is being studied. Specifically, it is a method of realizing high yield strength and tensile strength by tempering some martensite in the steel sheet by the rapid cooling and transforming it into tempered martensite. This has the advantage that not only high yield strength, but also improvement of formability due to reduction of the difference in hardness between phases can be achieved.

Due to these advantages, research on a steel sheet manufacturing method including annealing and rapid cooling is continuously being made.

As an example, in Japanese Patent Application Laid-Open No. 1992-289120, steel materials with carbon 0.18% or more are continuously annealed, cooled with water to room temperature, and over-aged at 120-300 ° C. suggest techniques to However, since the above manufacturing method is a method of strengthening strength due to tempering after water cooling, the yield ratio is very high, but cracks occur during molding due to deterioration of shape quality of coil due to temperature deviation in width/length direction during water cooling and material deviation. There is a problem that

As another example, Korean Patent Application Laid-Open No. 10-2020-0036759 discloses a steel sheet manufacturing method comprising annealing a cold-rolled steel sheet in a temperature range of (Ae3+30°C to Ae3+80°C). Due to the large amount of Si and Al added to make retained austenite in the silver steel sheet, it is difficult to secure the surface quality during steel making and casting, and it is difficult to obtain the plating surface quality at the level of the outer plate even in the final product.

In addition, as in Japanese Patent Application Laid-Open No. 2010-090432, it is possible to manufacture a high-strength, high-ductility cold-rolled steel sheet using tempered martensite by increasing the alloy composition. The above-described manufacturing method not only has a good plate shape after continuous annealing, but also can induce improvement in formability by reducing the interphase hardness difference due to tempered martensite, but contains 0.2% or more of carbon (C), resulting in poor weldability and poor Si content There is a disadvantage that dents in the furnace can easily occur due to the high temperature.

Considering the physical properties required to solve the above problems and secure high yield ratio and formability at the same time, the achievable tensile strength is judged to be about 1180 MPa level, and it is necessary to develop high-strength cold-rolled steel sheets based on this as soon as possible. have.

Japanese Laid-Open Patent Publication No. 1992-289120 (October 14, 1992) Republic of Korea Patent Publication No. 10-2020-0036759 (2020.04.07) Japanese Laid-Open Patent Publication No. 2010-090432 (2010.04.22)

Therefore, the present invention is to solve the above problems, and by controlling the content and manufacturing conditions of the added elements, yield strength and formability of 900 MPa or more, tensile strength of 1,180 MPa, elongation of 8.0%, and yield ratio of 0.7 or more An object of the present invention is to provide a method for manufacturing this excellent high-strength cold-rolled steel sheet and a high-strength cold-rolled steel sheet manufactured using the same.

In addition, a method for manufacturing a high-strength cold-rolled steel sheet excellent in yield ratio and formability having a product of yield strength and elongation of 7000 MPax% or more (YSxEl) and hole expandability (HER) of 25.0% or more, and a high-strength cold-rolled steel sheet manufactured using the same intended to provide.

In addition, the subject of this invention is not limited to the above-mentioned content. The subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.

An aspect of the present invention for achieving the above object is in weight %, carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid value aluminum (sol.Al) ) 0.02 to 0.05%, molybdenum (Mo) 0.1 to 0.3%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, remaining iron (Fe) and unavoidable impurities The structure is composed of ferrite, bainite, fresh martensite and tempered martensite, and the area fraction of the tempered martensite satisfies the following relation 1, it's about

[Relational Expression 1]

70 ≤ TM / (FM + TM) ≤ 85

(In Equation 1, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, and FM is the area fraction (%) of fresh martensite)

In one aspect, the area fraction of ferrite and bainite of the high-strength cold-rolled steel sheet may be 5.0 to 15.0%.

In the above aspect, the fine fraction of the high-strength cold-rolled steel sheet may satisfy the following relational expression (2).

[Relational Expression 2]

8.0 ≤ (TM + FM) / (F + B) ≤ 16.0

(In Relation 2, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, FM is the area fraction (%) of fresh martensite, F is the area fraction of ferrite (%), and B is the bay It is the area fraction (%) of the knight)

In one aspect, the high-strength cold-rolled steel sheet may have a yield ratio of 0.7 or more, and an elongation of 8.0% or more.

In one aspect, the high-strength cold-rolled steel sheet may have a yield strength of 900 MPa or more, and a tensile strength of 1,180 MPa or more.

In the one aspect, by weight%, chromium (Cr) 0.5% or less, niobium (Nb) 0.1% or less, titanium (Ti) 0.1% or less, and boron (B) 0.002% or less It further comprises at least one selected from can do.

In another aspect of the present invention, by weight%, carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid value aluminum (sol.Al) 0.02 to 0.05%, molybdenum (Mo) 0.1 to 0.3%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, the remainder consisting of iron (Fe) and unavoidable impurities Preparing a cold-rolled steel sheet, continuously annealing the cold-rolled steel sheet at 700 to 820°C, primary cooling the annealed steel sheet to 620 to 700°C, and heating the primary cooled steel sheet to 280 to 580°C Secondary cooling A method of manufacturing a high-strength cold-rolled steel sheet excellent in yield ratio and formability, comprising the step of over-aging the secondary cooled steel sheet to 400 to 500° C., and satisfying the following relational expression 1 is about

[Relational Expression 1]

70 ≤ TM / (FM + TM) ≤ 85

(In Equation 1, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, and FM is the area fraction (%) of fresh martensite)

In one aspect, the continuously annealing may be performed at 810 to 820 °C.

In the one aspect, the secondary cooling step may be performed to 280 to 320 ℃.

In the one aspect, in the step of primary cooling, the annealed steel sheet may be cooled at a rate of 1 to 10° C./s.

In the one aspect, in the second cooling step, the first cooled steel sheet may be cooled at a rate of 5 to 20 °C / s.

According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet satisfying the yield ratio and formability required for an automotive steel sheet for cold forming.

The effect of the present invention is not limited to the above, and it may be construed as including the effects that those skilled in the art can infer from the description described below.

1 is a flowchart for explaining a method of manufacturing a high-strength cold-rolled steel sheet having excellent yield ratio and formability according to an embodiment of the present invention.
2 is a scanning electron microscope (SEM) photograph for observing the microstructure of the steel sheet prepared in Example 1.

Features of the embodiments of the present invention, and methods of achieving them, will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and only these embodiments allow the disclosure of the present invention to be complete, and common knowledge in the art to which the present invention pertains It is provided to fully inform those who have the scope of the invention, and the present invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout.

In describing the embodiments of the present invention, if it is determined that a detailed description of a well-known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. In addition, the terms to be described later are terms defined in consideration of functions in an embodiment of the present invention, which may vary according to intentions or customs of users and operators. Therefore, the definition should be made based on the content throughout this specification. Hereinafter, embodiments of the present invention will be described in detail.

The present invention maintains a yield strength (YS) of 900 MPa and a tensile strength (TS) of 1,180 MPa or more by controlling the heat treatment conditions in connection with the component composition of the steel, the yield ratio is 0.7, and the elongation is 8.0% It relates to a high-strength cold-rolled steel sheet excellent in yield ratio and formability having the above. In addition, the high-strength cold-rolled steel sheet having excellent yield ratio and formability may satisfy the product of yield strength and elongation (YSxEl) of 7000 MPax% or more and hole expandability (HER) of 25.0% or more. Through this, the formability can be improved while strengthening the collision stability of automobile bodies and structural members, such as seat rails, pillars, and fenders, where the use of high-strength cold-rolled steel sheets is increasing. have.

As an example, the present invention optimizes the temperature at which annealing occurs during annealing and the temperature of the rapid cooling section (RCS) after annealing, so that the yield strength (YS) is 900 MPa or more, and the tensile strength is (TS) of 1,180 MPa or more, yield ratio of 0.7 or more, and elongation of 8.0% or more can be realized.

According to an embodiment, the high-strength cold-rolled steel sheet having excellent yield ratio and formability is, by weight%, carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid value Aluminum (sol.Al) 0.02 to 0.05%, molybdenum (Mo) 0.1 to 0.3%, boron (B) 0.0001 to 0.002%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N ) 0.0001 to 0.01%, and the remaining iron (Fe) and unavoidable impurities.

In addition, the high-strength cold-rolled steel sheet having excellent yield ratio and formability may have a ferrite, bainite and martensitic structure.

In this case, the martensite may be classified into fresh martensite or tempered martensite. The fresh martensite refers to a structure formed by diffusion-free transformation of austenite formed in an annealing process through a rapid cooling process, and refers to conventional martensite.

The tempered martensite refers to a phase changed by overaging the fresh martensite at 400 to 500°C. The tempered martensite is implemented by forming a Cottrell atmosphere by fixing carbon (C) escaping from the fresh martensite to an electric potential.

This has the effect of greatly improving the yield strength instead of slightly reducing the tensile strength of the steel sheet. That is, by appropriately adjusting the area fraction of the fresh martensite in the martensite and the area fraction of the tempered martensite, the strength of the steel sheet can be controlled. In addition, the elongation can be improved due to the austenite remaining at room temperature after final cooling.

According to an embodiment, the area fractions of the fresh martensite and the tempered martensite may be formed in a range satisfying the following Relational Equation 1.

[Relational Expression 1]

70 ≤ TM / (FM + TM) ≤ 85

(In Equation 1, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, and FM is the area fraction (%) of fresh martensite)

That is, the relation 1 means the fraction of tempered martensite with respect to the martensite fraction, and the fraction of tempered martensite with respect to the martensite fraction is formed in 70 to 85, so that the yield strength (YS) is 900 MPa or more, the tensile strength Yield ratio and formability can be optimized when the strength (TS) is 1,180 MPa or more.

When the fraction of tempered martensite with respect to the martensite fraction (TM / (FM + TM) x 100) is less than 70, the fraction of fresh martensite in martensite is improved to 900 MPa in a state where yield ratio and formability are satisfied It is difficult to satisfy the above yield strength and tensile strength above 1,180 MPa. Conversely, when the fraction of tempered martensite with respect to the martensite fraction (TM / (FM + TM) x 100) exceeds 85, the tempered martensite structure in martensite is excessively formed, and the strength is more than 0.7 in a state where the strength is satisfied. It is difficult to realize a yield ratio and an elongation of 8.0% or more.

In particular, it was found that if the fraction of tempered martensite with respect to the martensite fraction satisfies 70 to 85, both the hole expandability (HER) of 25.0% or more and the product of yield strength and elongation (YSxEl) of 7000 MPax% or more can be satisfied. did For this reason, the fraction of tempered martensite relative to the martensite fraction (TM / (FM + TM) x 100) is preferably 70 to 85, and more preferably 75 to 85.

According to an embodiment, in the high-strength cold-rolled steel sheet having excellent yield ratio and formability, 10 ¹² or more fine precipitates having an average diameter of 50 nm or less within the range of 1 m x 1 m may be precipitated. The fine precipitates may be Ti, Nb and Mo metal elements or an alloy containing the metal elements. Through this, it is possible to more effectively implement high-strength properties compared to other conventionally provided steel sheets.

Hereinafter, the composition range of the present invention will be described in detail. Hereinafter, unless otherwise specified, the unit is % by weight.

Carbon (C) is included in an amount of 0.14 to 0.2% by weight.

The carbon (C) is an element that decisively contributes to strengthening the strength of the steel sheet. In addition, the carbon (C) may strengthen the steel sheet by promoting the formation of martensite in the steel sheet. In addition, the carbon (C) plays the most important role in forming retained austenite as an austenite stabilizing element. For example, when the content of carbon (C) increases, the formation of martensite in the steel sheet may increase. This is advantageous for forming a complex structure to improve the strength of the steel sheet, but if the content of carbon (C) is included more than necessary, it is difficult to control the elongation. Needs to be.

For the above reasons, the carbon (C) is preferably contained in an amount of 0.14 to 0.2 wt%. If the carbon (C) is less than 0.14% by weight, the strength may decrease due to insufficient martensite formation in the steel sheet. Conversely, when the carbon (C) exceeds 0.2 wt %, elongation is reduced and moldability is reduced. For this reason, the carbon (C) is preferably contained in an amount of 0.14 to 0.2% by weight, and more preferably 0.14 to 0.18% by weight.

Manganese (Mn) may be included in 2.5 to 3.0% by weight.

The manganese (Mn) is an element that improves the hardenability of steel, and in particular may contribute to forming martensite in the steel sheet. At the same time, the manganese (Mn) may contribute to improving the strength of the steel sheet by promoting solid solution strengthening of the steel sheet. In addition, by precipitating sulfur (S), which is unavoidably added to the steel sheet, as MnS, it is possible to suppress the occurrence of plate breakage and high-temperature embrittlement caused by sulfur (S) during hot rolling. In order to make this action effective, the manganese (Mn) may be contained in an amount of 2.5 wt%, more preferably 2.6 wt%.

However, when the manganese (Mn) exceeds 3.0% by weight, not only the weldability of the steel sheet is inferior, but also martensite is excessively formed to make the material unstable, and an oxide band in the form of a band is formed to form a processing crack And there is a problem in that the risk of plate breakage increases. In addition, when the manganese (Mn) content exceeds 3.0 wt %, manganese (Mn) oxide is eluted on the surface of the steel sheet during annealing, thereby deteriorating plating properties. For this reason, the manganese (Mn) may be included in an amount of 2.5 to 3.0% by weight, more preferably 2.5 to 2.9% by weight.

Silicon (Si) may be included in an amount of 0.3 to 0.6 wt%.

The silicon (Si) may promote the formation of ferrite and promote the carbon (C) concentration of untransformed austenite to promote martensite formation. That is, the silicon (Si) promotes the formation of ferrite having relatively excellent ductility and at the same time promotes the formation of martensite having relatively strong strength, thereby contributing to securing strength within the limit of not reducing the ductility of the steel sheet. In order to make this action effective, the silicon (Si) may be included in an amount of 0.3 wt% or more, and more preferably 0.35 wt% or more.

However, when the silicon (Si) exceeds 0.6 wt%, the plating property may be inhibited by forming a silicon (Si) oxide on the surface of the steel sheet, and hydrogen embrittlement and weldability may be deteriorated. For this reason, the silicon (Si) may be included in an amount of 0.3 to 0.6 wt%, more preferably 0.35 to 0.45 wt%.

Aluminum (sol.Al) for acid value may be included in an amount of 0.02 to 0.05 wt%.

The acid value aluminum (sol.Al) is an element added for particle size refinement and deoxidation of the steel sheet, and is preferably included in 0.02 wt% or more, and more preferably in 0.03 wt% or more.

However, if the content of aluminum (sol.Al) for acid value is excessive, the surface defect of the steel sheet may occur during plating due to excessive formation of inclusions during the steel making operation. For this reason, the acid value aluminum (sol.Al) may be included in an amount of 0.02 to 0.05 wt%, more preferably 0.03 to 0.04 wt%.

Molybdenum (Mo) may be included in an amount of 0.1 to 0.3% by weight.

The molybdenum (Mo) may improve the hardenability of the steel sheet. Specifically, the molybdenum (Mo) can implement the effect of suppressing the formation of carbides in martensite or bainite, through which it is possible to control the amount of carbon (C) in martensite. That is, the fraction of tempered martensite and fresh martensite in the steel sheet can be controlled through the molybdenum (Mo). In order to effectively effect the above-described action, the molybdenum (Mo) is preferably contained in an amount of 0.1 wt% or more. However, when the molybdenum (Mo) exceeds 0.3% by weight, the hardness of the welded portion is excessively increased, so that ductility and hole expandability (HER) may be reduced. For this reason, the molybdenum (Mo) may be included in an amount of 0.1 to 0.3 wt%, more preferably 0.15 to 0.25 wt%.

Boron (B) may be included in an amount of 0.001 to 0.002% by weight.

The boron (B) may be added to delay the transformation of austenite into pearlite during annealing of the steel sheet. For this action, the boron (B) is preferably added in an amount of 0.001% by weight or more.

However, when the boron (B) is added in excess of 0.002% by weight, the boron (B) is concentrated on the surface of the steel sheet and may cause deterioration of plating adhesion, so that the content of boron (B) is 0.001 to 0.002% by weight, More preferably, 0.0015 to 0.002% by weight may be included.

Phosphorus (P) may be included in an amount of 0.0001 to 0.05 wt%.

The phosphorus (P) may improve the strength of steel by solid solution strengthening. However, when the phosphorus (P) exceeds 0.05 wt%, the segregation amount of the phosphorus (P) at the grain boundary of the steel sheet is excessively increased, and the possibility of occurrence of temper brittleness may greatly increase. This is the main cause of plate fracture of the slab during hot rolling. In addition, phosphorus (P) may impair the plating surface characteristics. Theoretically, it is advantageous to control the content of the phosphorus (P) to be low so that it is close to 0% by weight, but phosphorus (P) is inevitably contained in the manufacturing process, and to reduce the content of the phosphorus (P) Since the process is complicated and the production cost is increased due to the additional process, it is desirable to set and manage the upper limit. Accordingly, it is preferable to manage the phosphorus (P) to contain 0.0001 to 0.05 wt%.

Sulfur (S) may be included in 0.0001 to 0.01% by weight.

The sulfur (S) is an impurity element unavoidably contained in the steel sheet like phosphorus (P) described above, and is an element that inhibits the ductility and weldability of the steel sheet. In addition, the sulfur (S) may cause hot shortness in the steel sheet. For this reason, it is advantageous to control the sulfur (S) content to be low to be close to 0 wt% like the phosphorus (P), but considering the cost and time consumed for this, it is preferable to set and manage the upper limit. . Accordingly, it is preferable to manage the sulfur (S) to contain 0.0001 to 0.01 wt%.

Nitrogen (N) may be included in an amount of 0.0001 to 0.01% by weight.

The nitrogen (N) may be combined with the acid value aluminum (sol.Al) to form an alumina-based non-metallic inclusion of AlN. The AlN may decrease the playing quality and increase the risk of fracture defects by increasing the brittleness of the steel sheet. For this reason, it is preferable to manage the nitrogen (N) to contain 0.0001 to 0.01 wt%.

On the other hand, the high-strength cold-rolled steel sheet having excellent yield ratio and formability according to an embodiment of the present invention has an alloy composition that may be additionally included in addition to the above-described alloy components, which will be described in detail below.

According to the embodiment, the high-strength cold-rolled steel sheet having excellent yield ratio and formability is at least one selected from 0.5% or less chromium (Cr) 0.5% or less, niobium (Nb) 0.1% or less and titanium (Ti) 0.1% or less by weight% may further include.

Chromium (Cr) may be included in an amount of 0.5 wt% or less.

The chromium (Cr) can improve hardenability by contributing to the formation of martensite in the steel sheet, thereby improving the strength of the steel sheet. In addition, the chromium (Cr) may contribute to securing ductility of the steel sheet by minimizing a decrease in elongation compared to an increase in strength. However, when the chromium (Cr) in the steel sheet exceeds 0.5 wt%, the formation rate of martensite in the steel sheet is excessively increased, and the fraction of coarse chromium (Cr)-based carbides is increased, so the elongation is excessively reduced there is a problem to be In addition, excessive addition of chromium (Cr) may cause hydrogen embrittlement and poor weldability. For this reason, the chromium (Cr) may be limited to 0.5 wt% or less, and more preferably to 0.3 wt% or less.

Niobium (Nb) may be added in an amount of 0.1 wt% or less.

When the niobium (Nb) is added to the steel sheet, it is segregated at the austenite grain boundary in the steel sheet, thereby suppressing coarsening of austenite grains during annealing heat treatment. In addition, by forming fine carbides in the steel sheet, it can contribute to the improvement of strength. However, when the amount of niobium (Nb) exceeds 0.1 wt%, the amount of carbon (C) combined with the niobium (Nb) may be excessively increased to decrease the amount of carbon in the steel sheet. In addition, when the niobium (Nb) is contained in an amount of 0.1 wt % or more, the manufacturing cost increases when the steel sheet is manufactured, and thus economic efficiency may be inferior. For this reason, the niobium (Nb) is preferably added in an amount of 0.1 wt% or less, and more preferably 0.05 wt% or less.

Titanium (Ti) may be added in an amount of 0.1 wt% or less.

The titanium (Ti) may form fine carbides in the steel sheet to improve yield strength and tensile strength. In addition, the titanium (Ti) may inhibit the precipitation of AlN, which is an alloy of the nitrogen (N) and the aluminum (Al), by precipitating nitrogen (N) in the steel sheet as TiN. Through this, the titanium (Ti) can effectively reduce the risk of cracks occurring in the steel sheet during playing. For this reason, titanium (Ti) may be added to the steel sheet.

However, when the content of the titanium (Ti) exceeds 0.1 wt%, the carbide precipitated in the steel sheet becomes coarse, and like the niobium (Nb), the amount of carbon (C) in the steel sheet can be reduced. In addition, when the titanium (Ti) is added in excess of 0.1% by weight, it causes nozzle clogging during playing. For this reason, the content of the titanium (Ti) is preferably limited to 0.1% by weight or less, and more preferably to 0.05% by weight or less.

The remaining component of the present invention is Fe. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The composition, which is one characteristic of the present invention, has been described above. Hereinafter, another feature of the present invention, the tissue, will be described. Hereinafter, unless otherwise indicated, % indicating the proportion of the tissue is based on the area.

As described above, the high-strength cold-rolled steel sheet having excellent yield ratio and formability according to an embodiment of the present invention may be composed of ferrite, bainite, fresh martensite and tempered martensite, among which, the fresh martensite and tempered The martensite may be configured in a range that satisfies the above-described Relational Equation 1.

In addition, in the high-strength steel sheet according to an embodiment of the present invention, it is preferable that the sum of the area fractions of ferrite and bainite is 5.0 to 15.0%. If the sum of the area fractions of ferrite and bainite is less than 5.0%, the Although the strength of the steel sheet may be improved, it is difficult to secure sufficient formability due to a decrease in elongation. Conversely, when the sum of the area fractions of ferrite and bainite exceeds 15.0%, the area fraction of martensite is relatively decreased, so that it is difficult to secure sufficient strength.

Specifically, the ferrite and bainite may be formed in a range satisfying the following relation (2).

[Relational Expression 2]

8.0 ≤ (TM + FM) / (F + B) ≤ 16.0

(In Relation 2, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, FM is the area fraction (%) of fresh martensite, F is the area fraction of ferrite (%), and B is the bay It is the area fraction (%) of the knight)

In other words, the ratio between the martens to the ferrite and bainite in the steel sheet is preferably 8.0 to 16.0. If the ratio is less than 8.0, sufficient amounts of the tempered martensite and the fresh martensite were not formed. Means that. This means that it is difficult to secure a yield strength of 900 MPa or more and a tensile strength of 1,180 MPa or more. Conversely, when the ratio exceeds 16.0, it is difficult to achieve a yield ratio of 0.7 or more and an elongation of 8.0% or more because the area fractions of the ferrite and the bainite are relatively decreased. On the other hand, when the ratio satisfies 8.0 to 16.0, the above-described strength and elongation may be satisfied, the product of yield strength and elongation of 7000 MPax% or more (YSxEl), and hole expandability (HER) of 25.0% or more may be satisfied.

The high-strength cold-rolled steel sheet excellent in yield ratio and formability according to an embodiment of the present invention has been described above. Hereinafter, a method for manufacturing a high-strength cold-rolled steel sheet having excellent yield ratio and formability according to an embodiment of the present invention will be described.

1 is a flowchart for explaining a method of manufacturing a high-strength cold-rolled steel sheet having excellent yield ratio and formability according to an embodiment of the present invention.

Referring to FIG. 1 , the method for manufacturing a high-strength cold-rolled steel sheet having excellent yield ratio and formability includes the steps of preparing a cold-rolled steel sheet composed of the above-described compositional components; continuously annealing the cold-rolled steel sheet at 700 to 820°C; First cooling the annealed steel sheet to 620 to 700 ℃; Secondary cooling of the primary cooled steel sheet to 280 to 580 °C; and overaging the secondary cooled steel sheet to 400 to 500°C.

A cold-rolled steel sheet according to an embodiment of the present invention is carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid value aluminum (sol.Al) 0.02 to 0.05%, Molybdenum (Mo) 0.1 to 0.3%, boron (B) 0.0001 to 0.002%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, the remainder iron (Fe) Cold-rolled steel sheet composed of and unavoidable impurities can be used.

According to an embodiment, as one method for manufacturing the cold-rolled steel sheet, a steel slab having a predetermined component is reheated and then finish hot-rolled in a temperature range of Ar3 to Ar3+50° C. to provide a hot-rolled steel sheet; It may be manufactured through the steps of winding a hot-rolled steel sheet in a temperature range of 400 to 700° C. and cold rolling the wound hot-rolled steel sheet at an elongation of 40 to 70%.

Specifically, carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid soluble aluminum (sol.Al) 0.02 to 0.05%, molybdenum (Mo) 0.1 to 0.3 %, boron (B) 0.0001 to 0.002%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, the remaining iron (Fe) and unavoidable impurities. By reheating to 1,000° C. to 1,350° C., hot rolling to be described later can be prepared.

If the reheating temperature of the steel slab is less than 1,000 ℃, there is a possibility that the target physical properties may not be realized by hot rolling at a low temperature during the finish hot rolling. Conversely, if the reheating temperature of the steel slab exceeds 1,350℃, it may reach the melting point of the steel and melt. For this reason, the reheating temperature is preferably 1,000 °C to 1,350 °C.

The steel slab heated in the above temperature range may be hot-rolled and provided as a hot-rolled steel sheet. At this time, the temperature at which the hot rolling is performed is preferably from the Ar3 transformation point to Ar3 + 50°C based on the exit temperature of the finishing mill. When the outlet temperature is less than Ar3, the hot deformation resistance is highly likely to increase rapidly, and when the outlet temperature exceeds Ar3 + 50℃, excessively thick oxide scale occurs and productivity decreases, and the crystal grains of the hot-rolled steel sheet This is because it is formed coarsely and may cause deterioration of the physical properties of the final steel sheet.

The hot-rolled steel sheet after the hot rolling has been completed may be cooled and wound at 400 to 700°C. When the coiling temperature is less than 400° C., martensite or bainite in the steel sheet is excessively generated, which may cause shape defects during cold rolling. On the other hand, if the coiling temperature exceeds 700 ℃, the pickling property may be deteriorated due to an increase in the surface scale.

Thereafter, the wound steel sheet may be pickled to remove scale on the surface, and the pickled steel sheet may be cold rolled to manufacture a cold rolled steel sheet. Although pickling and cold rolling conditions are not particularly limited, cold rolling is preferably performed at an elongation of 40 to 70%. When the elongation of the cold rolling is less than 40%, the recrystallization driving force is weakened, and there is a high possibility that a problem may occur in obtaining good recrystallized grains, and shape correction may be very difficult.

Conversely, when the elongation exceeds 70%, cracks in the edge portion of the steel sheet are highly likely to occur, and the rolling load may rapidly increase.

Although the cold-rolled steel sheet manufacturing method has been described above, the present invention is not limited thereto, and any known method can be used as long as the cold-rolled steel sheet is provided within the composition range described above.

Annealing may be performed continuously on the prepared cold-rolled steel sheet to prepare the steel sheet to have the structure described above. The annealing is preferably performed at 700 to 820° C., but when the annealing is performed at a temperature of less than 700° C., it may be difficult to secure elongation because recrystallization of the ferrite is not sufficiently performed. On the other hand, when the annealing is performed at a temperature exceeding 820°C, the formation of annealing oxide on the surface of the steel sheet is accelerated, and plating adhesion with the steel sheet during hot-dip galvanizing is reduced, so plating peeling may occur on the surface during part molding. have. This causes plating peeling on the surface during part molding.

According to an embodiment, the annealing may be performed at 810°C to 820°C. When the annealing is performed at 810° C. or higher, the fraction of ferrite decreases and the fraction of austenite increases during the annealing process. This means that the fraction of martensite transformed from the austenite thereafter increases, which is advantageous in securing a high-strength steel sheet. For this reason, the annealing may be performed at 810°C to 820°C.

The primary cooling is a step of cooling at a relatively gentle rate than the secondary cooling before secondary cooling to be described later. Through this, it is possible to prevent deformation due to cooling of the steel sheet and suppress inferiority.

According to an embodiment, the primary cooling may be cooled to a primary cooling stop temperature of 620 to 700° C., and may be cooled at a cooling rate of 1 to 10° C./sec.

As described above, the primary cooling is preferably performed at 1 to 10 °C / sec, because when the primary cooling rate exceeds 10 °C / sec, it is difficult to implement the effect of deformation prevention and inferior position suppression. Conversely, if the cooling rate is less than 1° C./sec, excessive time is consumed in the process of performing primary cooling, thereby reducing productivity. For this reason, the primary cooling rate is preferably 1 to 10° C./sec, and more preferably 2 to 5° C./sec.

Thereafter, the primary cooled steel sheet may be secondary cooled to 280 to 580 °C.

The secondary cooling is a step of cooling the steel sheet cooled to the primary cooling stop temperature through primary cooling to the secondary cooling stop temperature. As described above, the secondary cooling is preferably performed more rapidly than the primary cooling, because the microstructure of the steel sheet is changed due to the secondary cooling, thereby affecting strength and toughness.

Specifically, the secondary cooling may generate a fresh martensite (FM) phase in the steel sheet. Specifically, by rapidly cooling the steel sheet to a low temperature below the martensite formation starting temperature (Ms) by the secondary cooling, it is possible to prevent the movement of carbon in the steel and cause diffusionless transformation of the austenite phase into the martensite phase.

According to an embodiment, the section in which the secondary cooling is performed may be called a rapid cooling section (RCS).

According to an embodiment, the secondary cooling may cool the steel sheet cooled to the primary cooling stop temperature to a secondary cooling stop temperature of 280 to 580 °C, and may be cooled at a rate of 5 to 20 °C/s.

When the secondary cooling stop temperature is less than 280°C, there is a possibility that a cooling deviation occurs in the width direction or the length direction of the steel sheet, thereby causing deformation of the steel sheet. On the other hand, when the secondary cooling stop temperature exceeds 580° C., there is a possibility that the desired tissue cannot be realized. For this reason, the secondary cooling stop temperature is preferably 280 to 580 °C.

More preferably, the secondary cooling stop temperature may be 280 to 320 ℃. When the secondary cooling stop temperature is 320° C. or less, the carbon (C) movement of austenite formed in the annealing process is prevented, and thus the fraction of martensite can be further increased. This means that the strength of the steel sheet can be improved. Conversely, when the secondary cooling stop temperature exceeds 320° C., the fraction of fresh martensite generated after secondary cooling is reduced, thereby affecting the fraction of tempered martensite in the overaging process to be described later. This causes a decrease in strength of the steel sheet. For this reason, the secondary cooling stop temperature is more preferably 280 to 320 ℃. Even more preferably, it is more preferably selected in consideration of the width and thickness of the steel sheet to be manufactured up to 280 to 320 °C.

Thereafter, an overaging treatment step of maintaining the steel sheet at the secondary cooling stop temperature for a predetermined time may be performed.

The overaging treatment refers to a process of maintaining the secondary cooled steel sheet at 400 to 500° C. for 200 to 400 seconds to induce carbon (C) movement on the fresh martensite to transform into tempered martensite. A fraction of the tempered martensite and the fresh martensite phase among the martensite may be adjusted through the over-aging treatment.

The overaging treatment is preferably performed at 400 to 500 ° C. If the over aging temperature is less than 400 ° C., tempered martensite is not sufficiently formed during the over aging treatment, so that the tensile strength is excessively increased, and the carbon (C ) is not sufficiently adhered to the dislocation, so it is difficult to secure the target yield strength.

On the other hand, when the overaging treatment temperature exceeds 500° C., the carbide becomes too coarse, and there is a risk of a decrease in strength. For this reason, the overaging treatment is preferably performed at 400 to 500°C, and more preferably at 430 to 480°C.

In addition, the overaging treatment is preferably performed for 100 to 600 seconds. When the overaging treatment time is less than 100 seconds, it is insufficient to realize a sufficient overaging effect. On the other hand, when the over-aging treatment time exceeds 600 seconds, the change in the effect is insignificant, which may reduce productivity. For this reason, the overaging treatment is preferably performed for 100 to 600 seconds, more preferably 200 to 400 seconds.

According to an embodiment, after secondary cooling, hot-dip galvanizing may be selectively performed.

The hot-dip galvanizing may include alloying hot-dip galvanizing. The composition and plating method of the plating layer during the hot-dip galvanizing are not particularly limited, and a known composition and plating method may be applied.

Finally, the step of tertiary cooling of the steel sheet on which the hot-dip galvanizing has been completed or the steel sheet subjected to the second cooling to 20 to 100°C may be performed.

Specifically, the steel sheet cooled to the secondary cooling stop temperature through the tertiary cooling can be cooled to the tertiary cooling stop temperature of 20 to 100 °C, and cooled at a rate of 5 to 20 °C/s like the secondary cooling. can be

According to an embodiment, after the tertiary cooling, the temper rolling treatment may be selectively performed.

The temper rolling may be performed to improve the yield strength of the steel sheet, and more preferably, may be performed at an elongation of 0.1 to 2.0%. When the elongation is less than 0.1% during the temper rolling, there is a difficulty in forming a desired shape as well as insignificant effect of increasing the yield strength. Conversely, when the elongation of the temper rolling exceeds 2.0%, operability may be greatly deteriorated by the high stretching operation. For this reason, the temper rolling is preferably performed at an elongation of 0.1 to 2.0%.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[Production Example]

The slabs composed of the component compositions and the remainder Fe disclosed in Tables 1 and 2 below were vacuum melted, heated at 1,200° C., and then hot-rolled at 880° C. based on the outlet temperature. The hot-rolled steel sheet was wound in a temperature range of 650°C, and then, after removing the surface scale by pickling, it was cold-rolled at a cold elongation of 50% to prepare a cold-rolled steel sheet.

steel grade Ingredients (wt%) C Mn Si Mo Cr Nb Ti B Al P S N Invention lecture 1 0.15 2.6 0.4 0.2 - 0.03 0.02 0.0018 0.03 0.01 0.002 0.003 Invention lecture 2 0.15 2.6 0.4 0.2 0.2 0.03 0.02 0.0018 0.03 0.01 0.002 0.003 Invention lecture 3 0.15 2.9 0.4 0.2 - 0.03 0.02 0.0018 0.03 0.01 0.002 0.003 Invention lecture 4 0.15 2.9 0.4 0.2 0.2 0.03 0.02 0.0018 0.03 0.01 0.002 0.003 Invention River 5 0.15 2.9 0.4 0.2 0.2 - 0.02 0.0018 0.03 0.01 0.002 0.003 Invention lecture 6 0.15 2.9 0.4 0.2 0.2 0.03 - 0.0018 0.03 0.01 0.002 0.003

[Example]

The cold-rolled steel sheet prepared according to Preparation Example was annealed at T ₁ in Table 3, and the annealed steel sheet was first cooled to 650°C at a cooling rate of 3°C/s. Secondary cooling was performed up to T ₂ at a cooling rate of 11° C./s for the steel sheet on which the primary cooling was performed.

Thereafter, hot-dip galvanizing was performed on the second cooled steel sheet and the third cooling was performed at a cooling rate of 10°C/s to 25°C.

note steel grade T ₁
(℃) T ₂
(℃) Properties Phase fraction (%) YS
(MPa) ts
(MPa) El
(%) yield ratio YSxEl
(MPax%) HER
(%) F B FM TM Example 1 Invention lecture 1 810 280 952 1183 9.1 0.80 8663 28 8 3 17 72 Example 2 Invention lecture 2 810 280 1055 1246 9.1 0.85 9601 31 8 3 16 73 Example 3 Invention lecture 2 810 310 907 1276 8.6 0.71 7800 26 8 3 26 63 Example 4 Invention lecture 1 820 280 1036 1201 8 0.86 8288 30 3 3 19 75 Example 5 Invention lecture 2 820 280 1012 1238 8.6 0.82 8703 29 3 3 19 75 Example 6 Invention lecture 1 820 310 905 1217 8.8 0.74 7964 25 3 3 26 68 Example 7 Invention lecture 2 820 310 922 1255 8.1 0.73 7468 26 3 3 26 68 Example 8 Invention lecture 3 810 280 976 1205 8.7 0.81 8491 28 8 3 18 71 Example 9 Invention lecture 4 810 280 1028 1231 9.2 0.84 9458 29 7 3 19 71 Example 10 Invention River 5 810 280 1037 1240 9 0.84 9333 30 7 3 19 71 Example 11 Invention lecture 6 810 280 1041 1242 8.9 0.84 9265 30 7 3 19 71 Comparative Example 1 Invention lecture 1 770 310 714 1232 7.1 0.58 5069 20 19 3 26 52 Comparative Example 2 Invention lecture 1 770 350 726 1259 7 0.58 5155 18 19 9 72 0 Comparative Example 3 Invention lecture 1 790 310 728 1193 8.5 0.61 6188 21 14 3 30 53 Comparative Example 4 Invention lecture 1 790 350 710 1247 8.1 0.57 5751 20 13 8 79 0 Comparative Example 5 Invention lecture 1 800 280 855 1194 9.4 0.72 8037 27 9 3 19 69 Comparative Example 6 Invention lecture 2 800 280 883 1269 8.4 0.7 7417 29 9 3 18 70 Comparative Example 7 Invention lecture 1 800 310 787 1214 8.1 0.65 6375 22 10 3 32 55 Comparative Example 8 Invention lecture 2 800 310 827 1303 7.2 0.63 5954 23 10 3 30 57 Comparative Example 9 Invention lecture 1 810 350 790 1213 8.4 0.65 6636 24 9 7 84 0 Comparative Example 10 Invention lecture 1 810 400 699 1234 9.2 0.57 6431 21 9 9 82 0 Comparative Example 11 Invention lecture 1 810 450 718 1274 9.4 0.56 6749 21 9 9 82 0 Comparative Example 12 Invention lecture 1 810 500 811 1354 7.5 0.6 6083 23 8 7 85 0

Referring to Table 2, it can be seen that Examples 1 to 11 satisfying all the heat treatment conditions of the present invention include all of ferrite, bainite, fresh martensite, and tempered martensite. In particular, it can be confirmed that the phase fraction at this time satisfies both Relational Expressions 1 and 2 described above.

In fact, referring to FIG. 2 taken by SEM of Example 1, it can be confirmed that it has a composite structure composed of ferrite, bainite, fresh martensite and tempered martensite as described above, and specifically, the area fraction is ferrite 8 %, bainite 3%, fresh martensite 17% and tempered martensite 72%.

At this time, it can be seen that the ratio of the tempered martensite to the martensite (fresh martensite and tempered martensite) in Example 1 is 80.9, which satisfies 70 to 85 defined in relation 1 above. .

In addition, it can be seen that the ratio of martensite (fresh martensite and tempered martensite) to ferrite and bainite in Example 1 is 7.33, which satisfies 8.0 to 16.0 defined in Relation 2 above.

That is, as a result of the phase fraction of Example 1 satisfying both Examples 1 and 2, it can be seen that the yield strength is 952 MPa, which is 900 MPa or more, and the tensile strength is 1,183 MPa, which is 1,180 MPa or more. At the same time, the elongation is 9.1%, which is 8.0% or more, and the yield ratio is 0.8 or more, which is 0.7 or more. In addition, in Example 1, it can be seen that the product (YSxEl) of the yield strength and the elongation is 8663 MPax%, which is 7000 MPax% or more, and the hole expandability (HER) is 28%, which is 25.0% or more.

That is, in Example 1, the phase fraction in the steel sheet is formed within a range that satisfies all of the above-described Relations 1 and 2, and thus the physical properties of the present invention are: a yield strength (YS) of 900 MPa or more, a tensile strength of 1,180 MPa or more ( TS), it was confirmed that it has an elongation of 8.0% or more and a yield ratio of 0.7 or more, and additionally it may have a product of yield strength and elongation of 7000 MPax% or more (YSxEl) and hole expandability (HER) of 25.0% or more.

On the other hand, in Comparative Examples 1 to 4, it can be seen that the yield strength is 770 to 790 MPa, which is less than 900 MPa. This means that, since the annealing temperature (T ₁ ) is less than 810° C., cooling proceeds in a state in which ferrite produced during annealing is mixed, and finally, the yield strength is reduced.

As a result, it can be seen that the ferrite fraction in Comparative Examples 1 to 4 increased to 13 to 19, and the martensite fraction was relatively decreased to 72 to 78. In addition, the fraction of tempered martensite relative to that of martensite in Comparative Examples 1 to 4 (TM / (FM + TM)) is 0 or 63.9 to 66.7, which does not satisfy 70 to 85 defined in Relation 1 it can be seen that

In addition, it can be confirmed that Comparative Examples 10 to 13 also have a yield strength of less than 900 MPa. This is because, when the secondary cooling temperature (T ₂ ) exceeds 320° C., tempered martensite does not exist and all are transformed into fresh martensite.

That is, when the secondary cooling temperature (T ₂ ) exceeds 320 °C, some austenite formed by the annealing is transformed into bainite as described above, and the fraction of the martensite phase is reduced so that tempered martensite is not formed.

As a result, the fraction of bainite in Comparative Examples 10 to 13 was increased to 6 to 9, and tempered martensite was not formed, so the fraction of tempered martensite relative to the martensite fraction (TM / (FM + TM) )) is 0.

On the other hand, it can be seen that the steel sheets prepared in Comparative Examples 5 to 8 also had a yield strength of less than 900 MPa.

The reason for this is that in the steel sheets prepared in Comparative Examples 5 to 8, an appropriate amount of martensite was not formed. This is because, as described above, the annealing temperature (T ₁ ) was less than 810° C., so that some ferrite could not be transformed into austenite, and the ferrite was mixed in the cooling process. As a result, in the steel sheets prepared in Comparative Examples 5 to 8, the ratio ((TM + FM) / (F + B)) between the martens to the ferrite and bainite in the steel sheet defined in Relation 2 was 6.7 to 7.3. It can be cross-validated by forming.

This can also be confirmed through the fact that the steel sheets prepared in Examples 1 to 11 satisfy all of the ratios ((TM + FM) / (F + B)) between ferrite and bainite to martens of 8.0 to 16.0.

That is, the high-strength cold-rolled steel sheet according to an embodiment of the present invention is, by weight%, carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid value aluminum (sol. Al) 0.02 to 0.05%, molybdenum (Mo) 0.1 to 0.3%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, the remainder iron (Fe) and inevitable Cold-rolled steel sheet made of impurities is annealed at 700 to 820°C, first cooled to 620 to 700°C, and secondary cooled to 280 to 580°C, into ferrite, bainite, fresh martensite and tempered martensite in the steel sheet. There is a characteristic that the constituting phase fraction is formed in a range that satisfies both the above-described Relations 1 and 2.

Due to these characteristics, it can have a yield ratio of 0.7 or more, an elongation of 8.0% or more, a yield strength of 900 MPa or more, and a tensile strength of 1,180 MPa or more, and at the same time, the product of yield strength and elongation of 7000 MPax% or more (YSxEl), hole expandability of 25.0% or more ( HER) is satisfied.

In addition, referring to Table 2, it can be confirmed that the yield strength, yield ratio, product of yield strength and elongation (YSxEl) and hole expandability are improved as each annealing temperature is higher or the quenching temperature is lower.

In the above description, various embodiments of the present invention have been presented and described, but the present invention is not necessarily limited thereto. It will be readily appreciated that branch substitutions, transformations and alterations are possible.

Claims (11)

By weight%, carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid soluble aluminum (sol.Al) 0.02 to 0.05%, molybdenum (Mo) 0.1 to 0.3%, boron (B) 0.0001 to 0.002%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, the remaining iron (Fe) and inevitable impurities. ,
The structure is composed of ferrite, bainite, fresh martensite and tempered martensite,
The high-strength cold-rolled steel sheet having excellent yield ratio and formability, in which the area fraction of the tempered martensite satisfies the following relation (1).
[Relational Expression 1]
70 ≤ TM / (FM + TM) ≤ 85
(In Equation 1, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, and FM is the area fraction (%) of fresh martensite) The method of claim 1,
A high-strength cold-rolled steel sheet having excellent yield ratio and formability, characterized in that the area fraction of ferrite and bainite of the high-strength cold-rolled steel sheet is 5.0 to 15.0%. 3. The method of claim 2,
The high-strength cold-rolled steel sheet having excellent yield ratio and formability, characterized in that the area fraction of the high-strength cold-rolled steel sheet satisfies the following relational expression (2).
[Relational Expression 2]
8.0 ≤ (TM + FM) / (F + B) ≤ 16.0
(In Relation 2, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, FM is the area fraction of fresh martensite (%), F is the area fraction of ferrite (%), and B is the bay It is the area fraction (%) of the knight) The method of claim 1,
The high-strength cold-rolled steel sheet has a yield ratio of 0.7 or more and an elongation of 8.0% or more. 5. The method of claim 4,
The high-strength cold-rolled steel sheet has a yield strength of 900 MPa or more and a tensile strength of 1,180 MPa or more. The method of claim 1,
By weight%, chromium (Cr) 0.5% or less, niobium (Nb) 0.1% or less, titanium (Ti) 0.1% or less and boron (B) 0.002% or less, characterized in that it further comprises at least one selected from the group consisting of, yield, High-strength cold-rolled steel sheet with excellent ratio and formability. By weight%, carbon (C) 0.14 to 0.2%, manganese (Mn) 2.5 to 3.0%, silicon (Si) 0.3 to 0.6%, acid soluble aluminum (sol.Al) 0.02 to 0.05%, molybdenum (Mo) 0.1 to Cold rolling consisting of 0.3%, boron (B) 0.0001 to 0.002%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, the remainder iron (Fe) and unavoidable impurities preparing a grater;
continuously annealing the cold-rolled steel sheet at 700 to 820°C;
First cooling the annealed steel sheet to 620 to 700 ℃;
Secondary cooling of the primary cooled steel sheet to 280 to 580 °C; and
Including; overaging the secondary cooled steel sheet to 400 to 500 ℃;
A method of manufacturing a high-strength cold-rolled steel sheet excellent in yield ratio and formability, characterized in that it satisfies the following Relational Expression 1.
[Relational Expression 1]
70 ≤ TM / (FM + TM) ≤ 85
(In Equation 1, TM is the area fraction (%) of tempered martensite in the high-strength cold rolled steel sheet, and FM is the area fraction (%) of fresh martensite) 8. The method of claim 7,
The continuous annealing is a method of manufacturing a high-strength cold-rolled steel sheet having excellent yield ratio and formability, characterized in that it is performed at 810 to 820 °C. 9. The method of claim 8,
The method of manufacturing a high-strength cold-rolled steel sheet excellent in yield ratio and formability, characterized in that the secondary cooling step is performed to 280 to 320 ℃. 8. The method of claim 7,
The primary cooling step is a method of manufacturing a high-strength cold-rolled steel sheet having excellent yield ratio and formability, characterized in that the annealed steel sheet is cooled at a rate of 1 to 10°C/s. 8. The method of claim 7,
The secondary cooling step is a method of manufacturing a high-strength cold-rolled steel sheet excellent in yield ratio and formability, characterized in that the primary cooled steel sheet is cooled at a rate of 5 to 20 °C/s.

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