KR102470747B1 - 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

In 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 soluble 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%, the rest iron (Fe) and unavoidable A high-strength cold-rolled steel sheet with excellent yield ratio and formability, composed 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 relational expression 1 It is about.
[Relationship 1]
70 ≤ TM / (FM + TM) ≤ 85
(In the above relational expression 1, TM is the area fraction (%) of tempered martensite in 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, more preferably having a yield strength of 900 MPa or more, a tensile strength of 1,180 MPa, an elongation of 8.0%, and A method for manufacturing a high-strength cold-rolled steel sheet simultaneously having a yield ratio of 0.7 or more and a high-strength cold-rolled steel sheet manufactured using the same.

Due to the recently strengthened carbon dioxide emission regulations, the need to improve fuel efficiency has increased, and automobile manufacturers are focusing on weight reduction of car bodies. In addition, the application of high-strength steel to automobile structural members is continuously increasing as safety regulations for impact resistance of automobiles are expanded as well as environmental problems.

As one method for producing the above-described high-strength steel, a method of enhancing strength by adding a rapid cooling section (RCS) after annealing in the heat treatment process has been 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 it can promote not only high yield strength but also improvement in formability due to the decrease in the hardness difference between phases.

Due to these advantages, research on steel sheet manufacturing methods including annealing and rapid cooling is being continuously conducted.

For example, in Japanese Patent Laid-open Publication No. 1992-289120, a steel material having 0.18% or more of carbon is continuously annealed, then water-cooled to room temperature, and overaged at 120 to 300 ° C. for 1 to 15 minutes to produce a steel material with martensite of 80 to 97 area% or more suggest a technique to However, since the above manufacturing method is a method of reinforcing strength due to tempering after water cooling, the yield ratio is very high, but cracks occur during molding due to deterioration of the shape quality of the coil and material deviation due to temperature deviation in the width / length direction during water cooling. There is a problem with doing it.

As another example, Korean Patent Laid-open Publication No. 10-2020-0036759 discloses a steel sheet manufacturing method comprising the step of annealing a cold-rolled steel sheet in a temperature range of (Ae3+30°C to Ae3+80°C), but the above method 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 steelmaking 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 Laid-open Patent Publication No. 2010-090432, it is possible to manufacture a high-strength, high-ductility cold-rolled steel sheet using tempered martensite by increasing the alloy component. The above-described manufacturing method not only has a good plate shape after continuous annealing, but also can induce improved formability by reducing the hardness difference between phases due to tempered martensite, but has poor weldability and Si content because it contains more than 0.2% of carbon (C) There is a disadvantage that dents in the furnace can easily occur because the degree is high.

Considering the required physical properties required to solve the above-mentioned problems and secure high yield ratio and formability at the same time, the realizable tensile strength is judged to be about 1180 MPa, and there is a need to develop high-strength cold-rolled steel sheet as soon as possible based on this. have.

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

Therefore, the present invention is to solve the above problems, by controlling the content of the additive elements and manufacturing conditions, the yield strength is 900MPa or more, the tensile strength is 1,180MPa, the elongation is 8.0%, and the yield ratio is 0.7 or more. It is an object of the present invention to provide a method for producing an 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 having excellent formability and yield ratio 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 It is intended to provide

On the other hand, the subject of the present invention is not limited to the above. The subject of the present invention will be understood from the entire contents of this specification, and those skilled in the art will have no difficulty in understanding the additional subject of the present invention.

One aspect of the present invention for achieving the above object is, by weight, 0.14 to 0.2% of carbon (C), 2.5 to 3.0% of manganese (Mn), 0.3 to 0.6% of silicon (Si), acid soluble 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 rest iron (Fe) and unavoidable impurities It is made of, and the structure is composed of ferrite, bainite, fresh martensite, and tempered martensite, and the area fraction of the tempered martensite satisfies the following relational expression 1, in a high-strength cold-rolled steel sheet with excellent yield ratio and formability it's about

[Relationship 1]

70 ≤ TM / (FM + TM) ≤ 85

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

In the above aspect, the area fraction of ferrite and bainite in 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.

[Relationship 2]

8.0 ≤ (TM + FM) / (F + B) ≤ 16.0

(In the above relational expression 2, TM is the area fraction (%) of tempered martensite in 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 knight)

In the above 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 the above 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 above aspect, in 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 selected from among more than one selected from the group consisting of can do.

In another aspect of the present invention, in weight percent, 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%, phosphorus (P) 0.0001 to 0.05%, sulfur (S) 0.0001 to 0.01%, nitrogen (N) 0.0001 to 0.01%, the rest 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, firstly cooling the annealed steel sheet to 620 to 700°C, and firstly cooling the steel sheet to 280 to 580°C. Method for producing a high-strength cold-rolled steel sheet with excellent yield ratio and formability, characterized in that the step of secondary cooling includes the step of overaging the secondary cooled steel sheet to 400 to 500 ° C., and satisfies the following relational expression 1 It is about.

[Relationship 1]

70 ≤ TM / (FM + TM) ≤ 85

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

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

In the above aspect, the secondary cooling may be performed to 280 to 320 °C.

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

In the above aspect, in the secondary cooling, the primary 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 automotive steel sheets for cold forming.

Effects of the present invention are not limited to the above, and may be interpreted as including effects that can be inferred from the description described below by those skilled in the art.

1 is a flowchart for explaining 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.
2 is a scanning electron microscope (SEM) photograph for observing the microstructure of the steel sheet manufactured in Example 1.

Characteristics of the embodiments of the present invention, and methods for achieving them will become clear 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 make the disclosure of the present invention complete, and common knowledge in the art to which the present invention belongs. It is provided to completely inform the person who has the scope of the invention, and the present invention is only defined by the scope of the claims. Like reference numbers designate like elements throughout the specification.

In describing the embodiments of the present invention, if it is determined that a detailed description of a known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted. In addition, terms to be described later are terms defined in consideration of functions in the embodiment of the present invention, which may vary according to the intention or custom of a user or operator. Therefore, the definition should be made based on the contents throughout this specification. Hereinafter, embodiments of the present invention will be described in detail.

In the present invention, by controlling the heat treatment conditions in conjunction with the composition of the steel, the yield ratio is 0.7 and the elongation is 8.0% while maintaining the yield strength (YS) of 900 MPa and the tensile strength (TS) of 1,180 MPa or more. It relates to a high-strength cold-rolled steel sheet having the above yield ratio and excellent formability. 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, formability can be improved while strengthening the collision stability of automobile bodies and structural members, for example, seat rails, pillars, and fenders, where the use of high-strength cold-rolled steel sheets is increasing. have.

For 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, by weight, contains 0.14 to 0.2% of carbon (C), 2.5 to 3.0% of manganese (Mn), 0.3 to 0.6% of silicon (Si), and soluble acid. 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 rest may be made of iron (Fe) and unavoidable impurities.

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

At this time, the martensite may be classified as fresh martensite or tempered martensite. The fresh martensite refers to a structure formed by non-diffusion transformation of austenite formed in an annealing process through a rapid cooling process, and refers to normal martensite.

The tempered martensite means a phase obtained by subjecting the fresh martensite to overaging at 400 to 500°C. The tempered martensite is implemented by forming a Cottrell atmosphere by fixing carbon (C) released from the fresh martensite to dislocations.

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

According to an embodiment, the area fraction of the fresh martensite and the tempered martensite may be formed in a range that satisfies the following relational expression 1.

[Relationship 1]

70 ≤ TM / (FM + TM) ≤ 85

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

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

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

In particular, when the fraction of tempered martensite to the fraction of martensite satisfies 70 to 85, it was found that both the hole expandability (HER) of 25.0% or more and the product of yield strength and elongation (YSxEl) of 7000MPax% or more can be satisfied. did For this reason, the fraction of tempered martensite to the fraction of martensite (TM / (FM + TM) x 100) is preferably 70 to 85, 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 in a 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, high-strength characteristics can be implemented more effectively than other conventionally provided steel plates.

Hereinafter, the composition range of the present invention will be described in detail. Hereinafter, unless otherwise specified, units are % 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 enhancing the strength of the steel sheet. In addition, the carbon (C) can promote the formation of martensite in the steel sheet to strengthen the steel sheet. In addition, 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 the formation of a composite structure to improve the strength of the steel sheet, but it is difficult to control the elongation rate when the carbon (C) content is included more than necessary. Needs to be.

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

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

Manganese (Mn) is an element that improves the hardenability of steel, and may contribute to the formation of martensite in the steel sheet in particular. 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 inevitably added to the steel sheet, as MnS, it is possible to suppress plate breakage and high-temperature embrittlement due to sulfur (S) during hot rolling. To make this action effective, the manganese (Mn) may be included in an amount of 2.5% by weight and more preferably 2.6% by weight.

However, when the manganese (Mn) content exceeds 3.0% by weight, not only the weldability of the steel sheet is deteriorated, martensite is excessively formed, making the material unstable, and a band-shaped oxide band is formed, resulting in machining cracks. And there is a problem that the risk of plate breakage increases. In addition, when the manganese (Mn) content exceeds 3.0% by weight, manganese (Mn) oxide is eluted from the surface of the steel sheet during annealing, resulting in a problem of impairing plating properties. For this reason, the manganese (Mn) may be included in 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% by weight.

The silicon (Si) may promote the formation of ferrite and promote the formation of martensite by promoting the concentration of carbon (C) into untransformed austenite. That is, silicon (Si) promotes the formation of ferrite with relatively excellent ductility and at the same time promotes the formation of martensite with 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) is preferably included in an amount of 0.3% by weight or more, more preferably 0.35% by weight or more.

However, when the amount of silicon (Si) exceeds 0.6% by weight, plating properties may be impaired by forming silicon (Si) oxide on the surface of the steel sheet, and hydrogen embrittlement and poor weldability may be caused. For this reason, the silicon (Si) is preferably included in an amount of 0.3 to 0.6% by weight, more preferably 0.35 to 0.45% by weight.

Acid-soluble aluminum (sol.Al) may be included in an amount of 0.02 to 0.05% by weight.

The acid-soluble aluminum (sol.Al) is an element added for grain size refinement and deoxidation of the steel sheet, and is preferably included in an amount of 0.02% by weight or more, more preferably 0.03% by weight or more.

However, if the content of acid soluble aluminum (sol.Al) is excessive, surface defects of the steel sheet may occur during plating due to excessive formation of inclusions during steel casting operation. For this reason, the acid-soluble aluminum (sol.Al) may be included in an amount of 0.02 to 0.05% by weight, more preferably 0.03 to 0.04% by weight.

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

The molybdenum (Mo) can improve the hardenability of the steel sheet. Specifically, the molybdenum (Mo) can implement an effect of suppressing the formation of carbides in martensite or bainite, and through this, the amount of carbon (C) in martensite can be controlled. That is, the fraction of tempered martensite and fresh martensite in the steel sheet can be controlled through the molybdenum (Mo). In order to make the above-described action effective, the molybdenum (Mo) is preferably included in an amount of 0.1% by weight or more. However, when the amount of molybdenum (Mo) exceeds 0.3% by weight, the hardness of the welded portion is excessively increased, and 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% by weight, more preferably 0.15 to 0.25% by weight.

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, if the amount of boron (B) is added in excess of 0.002% by weight, the concentration of boron (B) on the surface of the steel sheet may cause deterioration of coating adhesion. More preferably, it may be included in an amount of 0.0015 to 0.002% by weight.

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

The phosphorus (P) can improve the strength of steel by solid solution strengthening. However, if the amount of phosphorus (P) exceeds 0.05% by weight, the amount of segregation of the phosphorus (P) at grain boundaries of the steel sheet excessively increases, and thus the possibility of occurrence of temper brittleness may greatly increase. This is a major cause of plate breakage of the slab during hot rolling. In addition, phosphorus (P) may deteriorate plating surface properties. Theoretically, it is advantageous to control the content of phosphorus (P) 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 phosphorus (P) Since the process is difficult and the production cost increases due to the additional process, it is desirable to set and manage the upper limit. Therefore, it is preferable to manage the phosphorus (P) to include 0.0001 to 0.05% by weight.

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

Sulfur (S) is an impurity element unavoidably contained in a steel sheet, like phosphorus (P), and is an element that impairs 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 low to be close to 0% by weight, similar to the phosphorus (P), but considering the cost and time consumed for this, it is preferable to set and manage the upper limit . Therefore, it is preferable to manage the sulfur (S) to include 0.0001 to 0.01% by weight.

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

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

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 further included in addition to the above-described alloy components, which will be described in detail below.

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

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

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

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

When niobium (Nb) is added to the steel sheet, it is segregated at austenite grain boundaries in the steel sheet, and thus, coarsening of austenite grains may be suppressed during annealing heat treatment. In addition, it may contribute to strength improvement by forming fine carbides in the steel sheet. However, when the amount of niobium (Nb) exceeds 0.1% by weight, the amount of carbon (C) bonded to the niobium (Nb) increases excessively, and the amount of carbon in the steel sheet may decrease. In addition, when the niobium (Nb) is included in an amount of 0.1% by weight or more, manufacturing cost may increase during the manufacture of the steel sheet, resulting in inferior economic feasibility. For this reason, the niobium (Nb) is preferably added in an amount of 0.1% by weight or less, more preferably 0.05% by weight or less.

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

The titanium (Ti) may form fine carbides in the steel sheet to improve yield strength and tensile strength. In addition, titanium (Ti) can precipitate nitrogen (N) in the steel sheet as TiN, thereby suppressing the precipitation of AlN, which is an alloy of nitrogen (N) and aluminum (Al). 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 titanium (Ti) exceeds 0.1% by weight, carbides precipitated in the steel sheet become coarse, and the amount of carbon (C) in the steel sheet can be reduced similarly to the niobium (Nb). 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 titanium (Ti) content is preferably limited to 0.1% by weight or less, more preferably 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 a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

The composition, which is one feature of the present invention, has been described above. Hereinafter, another feature of the present invention, tissue, will be described. Hereinafter, unless otherwise indicated, % representing the ratio of tissues is based on 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 Martensite may be configured in a range that satisfies the above-described relational expression 1.

In addition, in the high-strength steel sheet according to an embodiment of the present invention, the sum of the area fractions of ferrite and bainite is preferably 5.0 to 15.0%. When 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 reduced, making it difficult to secure sufficient strength.

Specifically, the ferrite and bainite may be formed within a range that satisfies the following relational expression 2.

[Relationship 2]

8.0 ≤ (TM + FM) / (F + B) ≤ 16.0

(In the above relational expression 2, TM is the area fraction (%) of tempered martensite in 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 knight)

In other words, it is preferable that the ratio between the martensite to the ferrite and bainite in the steel sheet is 8.0 to 16.0. If the ratio is less than 8.0, sufficient amounts of the tempered martensite and the fresh martensite are 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, the area fractions of ferrite and bainite are relatively reduced, making it difficult to achieve a yield ratio of 0.7 or more and an elongation of 8.0% or more. On the other hand, when the ratio is 8.0 to 16.0, the above-described strength and elongation can be satisfied, and the product of yield strength and elongation (YSxEl) of 7000 MPax% or more and hole expandability (HER) of 25.0% or more can be satisfied.

Above, the high-strength cold-rolled steel sheet having excellent yield ratio and formability according to an embodiment of the present invention has been described. 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 for 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 manufacturing method of a high-strength cold-rolled steel sheet having excellent yield ratio and formability includes preparing a cold-rolled steel sheet made of the aforementioned components; continuously annealing the cold-rolled steel sheet at 700 to 820° C.; Primary cooling the annealed steel sheet to 620 to 700 ° C; secondarily cooling the firstly cooled steel sheet to 280 to 580°C; and overaging the secondary cooled steel sheet to 400 to 500°C.

The cold-rolled steel sheet according to an embodiment of the present invention contains 0.14 to 0.2% of carbon (C), 2.5 to 3.0% of manganese (Mn), 0.3 to 0.6% of silicon (Si), 0.02 to 0.05% of acid soluble aluminum (sol.Al), 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 rest iron (Fe) and unavoidable impurities can be used.

According to an embodiment, as one method for manufacturing the cold-rolled steel sheet, providing a hot-rolled steel sheet by reheating a steel slab having a predetermined component and then performing hot-rolling in a temperature range of Ar3 to Ar3 + 50 ° C. It can be manufactured through the steps of winding a hot-rolled steel sheet at a temperature range of 400 to 700° C. and cold-rolling the coiled 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 rest iron (Fe) and unavoidable impurities By reheating at 1,000 ° C to 1,350 ° C, hot rolling to be described later may be prepared.

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

The steel slab heated in the above-described 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 temperature of the Ar3 transformation point to Ar3 + 50 ° C based on the temperature at the exit side of the finishing mill. If the outlet temperature is less than Ar3, the hot deformation resistance is likely to increase rapidly, and if the outlet temperature exceeds Ar3 + 50 ° C, excessively thick oxide scale is generated, resulting in a decrease in productivity and grain This is because it is formed coarsely and may cause deterioration of physical properties of the final steel sheet.

The hot-rolled steel sheet after hot rolling may be cooled and wound at 400 to 700°C. When the coiling temperature is less than 400° C., martensite or bainite is excessively generated in the steel sheet, 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 the increase in surface scale.

Thereafter, the coiled steel sheet may be pickled to remove surface scale, and the pickled steel sheet may be cold-rolled to manufacture a cold-rolled steel sheet. Pickling and cold rolling conditions are not particularly limited, but cold rolling is preferably carried out at an elongation of 40 to 70%. If the elongation of cold rolling is less than 40%, the driving force for recrystallization is weakened, so problems may occur in obtaining good recrystallized grains, and shape correction may be very difficult.

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

Although the cold-rolled steel sheet manufacturing method has been described above, it is not limited thereto, and it is of course possible to manufacture a cold-rolled steel sheet provided within the above-described composition range by any known method.

Annealing may be continuously performed on the prepared cold-rolled steel sheet to prepare the steel sheet to have the above-described structure. The annealing is preferably performed at 700 to 820 ° C., when the annealing is performed at a temperature of less than 700 ° C., it may be difficult to secure an elongation rate because the 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 annealed oxide on the surface of the steel sheet is accelerated and the coating adhesion with the steel sheet is deteriorated during hot-dip galvanization, so plating peeling may occur on the surface during part molding. have. This causes plating peeling to occur on the surface during component 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 that is then transformed from the austenite increases, which is advantageous for 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 the secondary cooling described later. Through this, deformation due to cooling of the steel sheet can be prevented and inferiority can be suppressed.

According to an embodiment, the first cooling may be cooled to a first 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 it is difficult to implement the effect of preventing deformation and suppressing inferior position when the primary cooling rate exceeds 10 ° C / sec. Conversely, if the cooling rate is less than 1° C./sec, too much time is consumed in the process of performing the primary cooling, and productivity may be reduced. For this reason, the primary cooling rate is preferably 1 to 10°C/sec, 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, it is preferable to perform the secondary cooling more rapidly than the primary cooling, because the secondary cooling changes the microstructure of the steel sheet and affects strength and toughness.

Specifically, the secondary cooling may generate a fresh martensite (FM) phase in the steel sheet. Specifically, by quenching the steel sheet to a low temperature below the martensite formation start temperature (Ms) by the secondary cooling, the movement of carbon in the steel material may be hindered so that the austenite phase may be non-diffusion-transformed into the martensite phase.

According to an embodiment, the section in which the secondary cooling is performed may be named 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 the 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., deformation may occur in the steel sheet due to a cooling deviation in the width direction or the length direction of the steel sheet. On the other hand, if the secondary cooling stop temperature exceeds 580 ° C., there is a possibility that the desired tissue may not 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 °C. When the secondary cooling stop temperature is 320° C. or less, the movement of carbon (C) of austenite formed in the annealing process may be hindered to further increase the fraction of martensite. This means that the strength of the steel sheet can be improved. Conversely, if the secondary cooling stop temperature exceeds 320 ° C., the fraction of fresh martensite generated after secondary cooling decreases, and thus affects 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, it is more preferable that the secondary cooling stop temperature is 280 to 320°C. Even more preferably, it is more preferable to select in consideration of the width and thickness of the steel sheet to be manufactured from 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 transforming into tempered martensite by inducing carbon (C) movement on the fresh martensite by holding the secondary cooled steel sheet at 400 to 500° C. for 200 to 400 seconds. Through the overaging treatment, it is possible to adjust the fraction of the tempered martensite and the fresh martensite phase among the martensite.

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

On the other hand, if the overaging treatment temperature exceeds 500° C., the carbide becomes excessively coarse, so there is a risk of strength deterioration. For this reason, the overaging treatment is preferably performed at 400 to 500°C, more preferably at 430 to 480°C.

In addition, the overaging treatment is preferably performed for 100 to 600 seconds. If the overaging treatment time is less than 100 seconds, it is insufficient to realize a sufficient overaging effect. On the other hand, if the overaging treatment time exceeds 600 seconds, the change in effect is insignificant and productivity may be lowered. For this reason, the overaging treatment is preferably performed for 100 to 600 seconds, and more preferably for 200 to 400 seconds.

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

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

Finally, a step of thirdly cooling the steel sheet on which the hot-dip galvanization is completed or the steel sheet on which the second cooling has been performed to 20 to 100° C. may be performed.

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

According to the embodiment, after the tertiary cooling, 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 during the temper rolling is less than 0.1%, the effect of increasing the yield strength is insignificant, and it is difficult to mold into a desired shape. Conversely, when the elongation of the temper rolling exceeds 2.0%, workability may be greatly deteriorated due to high elongation. 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 should be noted that the following examples are only for illustrating 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 the matters reasonably inferred therefrom.

[Production Example]

The slabs composed of the components disclosed in Tables 1 and 2 below and the balance of Fe 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, pickling was performed to remove surface scale, followed by cold rolling at a cold elongation rate 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 steel 1 0.15 2.6 0.4 0.2 - 0.03 0.02 0.0018 0.03 0.01 0.002 0.003 invention steel 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 steel 3 0.15 2.9 0.4 0.2 - 0.03 0.02 0.0018 0.03 0.01 0.002 0.003 Invention Steel 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 steel 5 0.15 2.9 0.4 0.2 0.2 - 0.02 0.0018 0.03 0.01 0.002 0.003 invention steel 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 the preparation example was subjected to annealing 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 secondarily cooled steel sheet and thirdly cooled to 25°C at a cooling rate of 10°C/s.

note steel grade _T1
(℃) _T2
(℃) Properties Phase fraction (%) YS
(MPa) TS
(MPa) El
(%) yield ratio YSxEl
(MPax%) HER
(%) F B FM TM Example 1 invention steel 1 810 280 952 1183 9.1 0.80 8663 28 8 3 17 72 Example 2 invention steel 2 810 280 1055 1246 9.1 0.85 9601 31 8 3 16 73 Example 3 invention steel 2 810 310 907 1276 8.6 0.71 7800 26 8 3 26 63 Example 4 invention steel 1 820 280 1036 1201 8 0.86 8288 30 3 3 19 75 Example 5 invention steel 2 820 280 1012 1238 8.6 0.82 8703 29 3 3 19 75 Example 6 invention steel 1 820 310 905 1217 8.8 0.74 7964 25 3 3 26 68 Example 7 invention steel 2 820 310 922 1255 8.1 0.73 7468 26 3 3 26 68 Example 8 invention steel 3 810 280 976 1205 8.7 0.81 8491 28 8 3 18 71 Example 9 Invention Steel 4 810 280 1028 1231 9.2 0.84 9458 29 7 3 19 71 Example 10 invention steel 5 810 280 1037 1240 9 0.84 9333 30 7 3 19 71 Example 11 invention steel 6 810 280 1041 1242 8.9 0.84 9265 30 7 3 19 71 Comparative Example 1 invention steel 1 770 310 714 1232 7.1 0.58 5069 20 19 3 26 52 Comparative Example 2 invention steel 1 770 350 726 1259 7 0.58 5155 18 19 9 72 0 Comparative Example 3 invention steel 1 790 310 728 1193 8.5 0.61 6188 21 14 3 30 53 Comparative Example 4 invention steel 1 790 350 710 1247 8.1 0.57 5751 20 13 8 79 0 Comparative Example 5 invention steel 1 800 280 855 1194 9.4 0.72 8037 27 9 3 19 69 Comparative Example 6 invention steel 2 800 280 883 1269 8.4 0.7 7417 29 9 3 18 70 Comparative Example 7 invention steel 1 800 310 787 1214 8.1 0.65 6375 22 10 3 32 55 Comparative Example 8 invention steel 2 800 310 827 1303 7.2 0.63 5954 23 10 3 30 57 Comparative Example 9 invention steel 1 810 350 790 1213 8.4 0.65 6636 24 9 7 84 0 Comparative Example 10 invention steel 1 810 400 699 1234 9.2 0.57 6431 21 9 9 82 0 Comparative Example 11 invention steel 1 810 450 718 1274 9.4 0.56 6749 21 9 9 82 0 Comparative Example 12 invention steel 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, which satisfy all of 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 the above-described relational expressions 1 and 2.

Referring to FIG. 2, which was actually photographed by SEM in 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 the relational expression 1. .

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

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 900 MPa or more with 952 MPa, and the tensile strength is 1,180 MPa or more with 1,183 MPa. At the same time, the elongation is 9.1%, which is more than 8.0%, and the yield ratio is 0.8, which is more than 0.7. In addition, in Example 1, it can be seen that the product of yield strength and elongation (YSxEl) 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 both the above-described relational expressions 1 and 2, so that the yield strength (YS) of 900 MPa or more and the tensile strength of 1,180 MPa or more (which are the physical properties targeted by the present invention) TS), it was confirmed to have an elongation of 8.0% or more and a yield ratio of 0.7 or more, and may additionally have a product of yield strength and elongation (YSxEl) of 7000 MPax% or more and hole expandability (HER) of 25.0% or more.

On the other hand, Comparative Examples 1 to 4 had a yield strength of 770 to 790 MPa, which was 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 generated during annealing is mixed, and finally 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 relatively decreased to 72 to 78. In addition, the fraction of tempered martensite relative to the fraction 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 the relational expression 1 can know that

In addition, it can be confirmed that the yield strength of Comparative Examples 10 to 13 is less than 900 MPa as well. 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., as described above, some austenite formed by the annealing is transformed into bainite, 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 increased to 6 to 9, and tempered martensite was not formed, so the fraction of tempered martensite relative to the fraction of martensite (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 have a yield strength of less than 900 MPa.

This is because the steel sheets prepared in Comparative Examples 5 to 8 did not have an appropriate amount of martensite formed. As described above, this is because some ferrite was not transformed into austenite because the annealing temperature (T ₁ ) was less than 810° C., 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 between the ferrite and the martens to bainite ((TM + FM) / (F + B)) in the steel sheet defined in the relational expression 2 was 6.7 to 7.3. It can be cross-validated through being formed.

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

That is, in the high-strength cold-rolled steel sheet according to an embodiment of the present invention, by weight, 0.14 to 0.2% of carbon (C), 2.5 to 3.0% of manganese (Mn), 0.3 to 0.6% of silicon (Si), aluminum for acid availability (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 rest iron (Fe) and unavoidable The cold-rolled steel sheet made of impurities is annealed at 700 to 820°C, first cooled to 620 to 700°C, and secondarily cooled to 280 to 580°C to form ferrite, bainite, fresh martensite, and tempered martensite in the steel sheet. It is characterized in that the phase fraction is formed within a range that satisfies both of the above-described relational expressions 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 900MPa or more, and a tensile strength of 1,180MPa or more, and at the same time, the product of yield strength and elongation (YSxEl) of 7000MPax% or more, hole expandability of 25.0% or more ( HER) was found to be satisfied.

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

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 apparent that branch substitutions, modifications and alterations are possible.

Claims (11)

In weight percent, 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 0.1% 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 rest iron (Fe) and unavoidable impurities ,
The structure is composed of ferrite, bainite, fresh martensite and tempered martensite,
The area fraction of ferrite and bainite of the high-strength cold-rolled steel sheet is 5.0 to 15.0%,
The area fraction of the tempered martensite satisfies the following relational expressions 1 and 2, high-strength cold-rolled steel sheet having excellent yield ratio and formability.
[Relationship 1]
70 ≤ TM / (FM + TM) ≤ 85
[Relationship 2]
8.0 ≤ (TM + FM) / (F + B) ≤ 16.0
(In the above relations 1 and 2, TM is the area fraction (%) of tempered martensite in high-strength cold-rolled steel sheet, FM is the area fraction (%) of fresh martensite, F is the area fraction (%) of ferrite, and B is It is the area fraction (%) of bainite) delete delete According to 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. According to 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. According to claim 1,
Yield, characterized by further comprising at least one member selected from 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, by weight%. High-strength cold-rolled steel sheet with excellent ratio and formability. In weight percent, 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 0.1% 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 rest iron (Fe) and unavoidable impurities Preparing the steel plate;
continuously annealing the cold-rolled steel sheet at 810 to 820° C.;
primary cooling of the annealed steel sheet at 620 to 700° C.;
Secondary cooling of the primary cooled steel sheet at 280 to 320 ° C; and
Including; overaging the secondary cooled steel sheet at 400 to 500 ° C.
The area fraction of ferrite and bainite of the high-strength cold-rolled steel sheet is 5.0 to 15.0%,
The area fraction of the tempered martensite is a method for producing a high-strength cold-rolled steel sheet having excellent yield ratio and formability that satisfies the following relational expressions 1 and 2.
[Relationship 1]
70 ≤ TM / (FM + TM) ≤ 85
[Relationship 2]
8.0 ≤ (TM + FM) / (F + B) ≤ 16.0
(In the above relations 1 and 2, TM is the area fraction (%) of tempered martensite in high-strength cold-rolled steel sheet, FM is the area fraction (%) of fresh martensite, F is the area fraction (%) of ferrite, and B is It is the area fraction (%) of bainite) delete delete According to claim 7,
The primary cooling step is characterized in that the annealed steel sheet is cooled at a rate of 1 to 10 ℃ / s, a method for producing a high-strength cold-rolled steel sheet excellent in yield ratio and formability. According to claim 7,
The secondary cooling step is characterized in that the primary cooled steel sheet is cooled at a rate of 5 to 20 ℃ / s, a method for producing a high-strength cold-rolled steel sheet excellent in yield ratio and formability.

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