EP4640898A1

EP4640898A1 - Cold rolled steel sheet and method for manufacturing same

Info

Publication number: EP4640898A1
Application number: EP23907598.9A
Authority: EP
Inventors: Young-Roc Im; Chang-Hyo Seo; Sang-Hyun Kim; Ki-Taek Jung
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-20
Filing date: 2023-12-14
Publication date: 2025-10-29
Also published as: CN120344699A; WO2024136312A1; MX2025006712A; EP4640898A4; KR20240098200A; JP2025539892A

Abstract

The present invention relates to a cold rolled steel sheet and a method for manufacturing same. One aspect of the present invention is to provide a cold rolled steel sheet having excellent strength and bending formability and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a cold rolled steel sheet and method for manufacturing the same.

Background Art

In order to make automobiles lighter and safer, the development of a high-strength steel sheet has been continuously promoted, and recently, the importance of ultra-high strength steels having a tensile strength of 1500 MPa or higher has increased to improve a driving range of electric vehicles and protect batteries. However, since existing MART steel has shortcomings in terms of formability, if ultra-high strength steel sheets for cold forming with formability are developed, an economic value thereof is expected to be significantly high. In order to improve the formability of steel, a method of introducing retained austenite and utilizing a TRansformation Induced Plasticity (TRIP) phenomenon is widely used to increase an elongation. However, in the case of such TRIP steel sheets, the addition of Si and Al is necessary to introduce retained austenite, and a larger amount of retained austenite may be obtained when accompanied by bainite transformation. However, since bainite is transformed at relatively high temperatures, a tensile strength (TS) is low and a yield strength (YS) is also low for use as ultra-high strength steel.
Accordingly, a recent trend is to adopt a Quenching and Partitioning process to increase the strength of a steel sheet while utilizing the TRIP phenomenon. In the case of so-called Q&P steel, the main structure of the matrix is tempered martensite, which has excellent yield strength and hole expansion ratio (HER), and if retained austenite is actively formed, an appropriate level of elongation may also be obtained.
Meanwhile, the importance of automobile safety from the perspective of passenger protection is increasing on a day by day basis. It is thought that the risk of automobile collisions may be fundamentally resolved by improving autonomous driving technology, but during the transition period until technological maturity, there is a risk that the severity of accidents will increase due to passengers' ignorance of the accident situation, and accordingly, automobile crash test regulations have been strengthened recently. It is known that the bending properties of steel are important in reducing the risk of cracks in automobile structural members during automobile collisions. When the bending properties of steel are excellent, the material may be folded to absorb more collision energy instead of the occurrence of cracks, and the remaining portions may withstand the collision and induce stable structural collapse.
However, in order to obtain excellent bending formability, applying a Q&P process alone may be insufficient, and additional ideas are required to secure excellent bending formability along with high strength of 1500 MPa or more.

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a cold rolled steel sheet and method for manufacturing the same.
A preferred aspect of the present disclosure is to provide a cold rolled steel sheet having excellent strength and bending formability and a method for manufacturing the same.

Solution to Problem

An embodiment of the present disclosure provides a cold rolled steel sheet, including: by wt%, C: 0.15% or more and less than 0.30%, Si: 0.8 to 2.5%, Al: 0.005 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.2%, B: 0.0005 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), and a balance of Fe and other inevitable impurities, wherein a microstructure includes, in area%, ferrite: 15% or less (excluding 0%), fresh martensite: 5% or less (excluding 0%), tempered martensite: 75% or more and less than 95%, retained austenite: 12% or less (excluding 0%), and a balance of bainite, and the following relational expression 1 is satisfied. (RA_unstable+RA_stable)/RA_stable ≤ 2.0
(However, in the relational expression 1, RA_unstable refers to a fraction of unstable retained austenite, and RA_stable refers to the fraction of stable retained austenite.)
The cold rolled steel sheet may further include at least one of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).
The cold rolled steel sheet may further include V: 0.05% or less (excluding 0%).
The retained austenite may have an average grain size of 0.5µm or less.
The cold rolled steel sheet satisfies the following relational expression 2. $4 \times P . El - E . El \geq 13.0$
(However, in the relational expression 2, P.El refers to a post-elongation, and U.El refers to uniform elongation) .
The cold rolled steel sheet has a tensile strength of 1470 MPa or more, a yield strength of 1000 MPa or more, a total elongation (T.El) of 8.5% or more, and bending formability (R_min/t) of 2.5 or less.
An embodiment of the present disclosure provides a method for manufacturing a cold rolled steel sheet including: heating a slab including, by wt%, C: 0.15% or more and less than 0.30%, Si: 0.8 to 2.5%, Al: 0.005 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.2%, B: 0.0005 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), and a balance of Fe and other inevitable impurities; finishing hot rolling the heated slab at a temperature of 830 to 950°C to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a temperature of 400 to 650°C; cold rolling the coiled hot rolled steel sheet to obtain a cold rolled steel sheet; continuously annealing the cold rolled steel sheet at a temperature of 800 to 950°C; primarily cooling the continuously annealed cold rolled steel sheet to a primary cooling end temperature of 500 to 700°C at an average cooling rate of 1°C/s or more and less than 10°C/s; secondarily cooling the primarily cooled cold rolled steel sheet to a secondary cooling end temperature of 25 to 300°C at an average cooling rate of 10°C/s or more; and reheating the secondarily cooled cold rolled steel sheet at 150 to 450°C and then holding the reheated steel sheet for more than 500 seconds and 1,500 seconds or less.
The slab may further include at least one of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).
The slab may further include V: 0.05% or less (excluding 0%).
A heating temperature of the slab may be 1150 to 1250°C.
A cold reduction ratio may be 30 to 60% during the cold rolling.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a cold rolled steel sheet and a method for manufacturing the same could be provided.
According to a preferred aspect of the present disclosure, a cold rolled steel sheet having excellent strength and bending formability and a method for manufacturing the same could be provided.

Best Mode for Invention

Hereinafter, a cold rolled steel sheet according to an embodiment of the present disclosure will be described. First, an alloy composition will be described. The content of the alloy composition described below refers to weight % unless otherwise specified.

C: 0.15% or more and less than 0.30%

Carbon (C) is an element that secures the strength of steel through solid solution strengthening and precipitation strengthening. When the content of C is less than 0.15%, it may be difficult to secure a tensile strength (TS) of 1.5 GPa. On the other hand, when the content of C is 0.30% or more, the arc weldability and laser weldability deteriorate, and the risk of cracking due to the formation of coarse carbides increases. Accordingly, the content of C may be preferably in the range of 0.15% or more and less than 0.30%. In terms of securing strength, a lower limit of the C content may more preferably be 0.17%. Additionally, in terms of weldability and the possibility of brittleness due to coarse carbides, an upper limit of the C content may more preferably be 0.28%.

Si: 0.8 to 2.5%

Silicon (Si) is a key element of Transformation Induced Plasticity (TRIP) steel that inhibits the precipitation of cementite and thus increases the retained austenite fraction and elongation. When the content of Si is less than 0.8%, almost no retained austenite remains to cause the elongation to become significantly low. On the other hand, when the content of Si exceeds 2.5%, the deterioration of the weld properties due to the formation of LME cracks may not be prevented, and the surface characteristics and plating properties of the steel deteriorate. Accordingly, the Si content may be preferably in the range of 0.8 to 2.5%. In consideration of the elongation, a lower limit of the Si content may more preferably be 0.9%, even more preferably 1.0%, and may be controlled to be 1.1% depending on the elongation required for the application product. An upper limit of the Si content may more preferably be 2.2%, even more preferably 2.0%, and may be controlled to be 1.7% in consideration of the target of use of or the plating method of the applied product.

Al: 0.005 to 0.3%

Aluminum (Al) is not only an element included for deoxidation of steel, but is also an element that is effective in stabilizing retained austenite by suppressing precipitation of cementite. When the content of Al is less than 0.005%, the deoxidation of the steel is not sufficiently performed, and the cleanliness of the steel is impaired. On the other hand, when the content of Al exceeds 0.3%, the castability of the steel is impaired. Accordingly, the content of Al may preferably have a range of 0.005-0.3%. In terms of the cleanliness of the steel, a lower limit of the Al content may more preferably be 0.01%, even more preferably 0.02%, and may be controlled to be 0.03% in consideration of the used product or the request of the user. In consideration of the castability of the steel described above, an upper limit of the Al content may more preferably be 0.27%, even more preferably 0.22%, and may be controlled to be 0.17% in consideration of the used product or the processing environment of the consumer.

Mn: 2.0 to 3.0%

Manganese (Mn) is an element added to secure strength. When the content of Mn is less than 2.0%, it may be difficult to secure strength. On the other hand, when the content thereof exceeds 3.0%, the phase transformation speed decreases to forming an excessive amount of fresh martensite, which may make it difficult to obtain excellent formability. Additionally, band structure is formed due to the segregation of Mn, which may impair the material uniformity and formability of the material. Accordingly, the content of Mn may be preferably in the range of 2.0 to 3.0%. In consideration of the target strength of the steel, a lower limit of the Mn content may more preferably be 2.2%. In consideration of the formability, an upper limit of the Mn content may more preferably be 2.8%.

Cr: 0.001 to 0.5%

Chromium (Cr) is an element added to secure strength and hardenability. When Mn is added alone, a significantly large amount of Mn should be added to exceed a Mn content range of the present disclosure, but this problem may be solved by adding Cr in an amount of 0.001% or more. When the content of Cr exceeds 0.5%, local corrosion deteriorates and oxides are formed on a surface, which may impair the phosphate treatment property. Accordingly, the content of Cr may be preferably in the range of 0.001 to 0.5%. Depending on the characteristics of the used product, an upper limit of the Cr content may be controlled to be 0.3% in consideration of corrosion and phosphate treatment properties.

Mo: 0.001 to 0.2%

Molybdenum (Mo) is added to secure strength and hardenability. In order to obtain such strength and hardenability improvement effects, the content of Mo should be added in an amount of 0.001% or more. However, since molybdenum (Mo) is a relatively expensive element, the economic feasibility of the steel sheet may be poor and the phase transformation may be significantly delayed, which may cause the formation of fresh martensite, and thus, it is preferable that the content of Mo does not exceed 0.2%. In terms of the costs described above, an upper limit of the Mo content may more preferably be 0.15%, and may be controlled to be 0.12%.

B: 0.0005 to 0.0050%

Boron (B) is an element added to secure hardenability. When Mn is added alone, a significantly large amount of Mn should be added to exceed the Mn content range of the present disclosure, but this problem may be solved by adding B in an amount of 0.0005% or more. However, when the content of B exceeds 0.0050%, boron carbides may be formed in the grain boundaries, which rather impairs the hardenability. Accordingly, the content of B may be preferably in the range of 0.0005 to 0.0050%. In terms of hardenability, an upper limit of the content of B may more preferably be 0.0040%, even more preferably 0.0030%, and may be controlled to be 0025%.

Nb: 0.001 to 0.05%

Niobium (Nb) is an element added to secure the strength of the steel sheet and refine a structure. When the content of Nb is less than 0.001%, it may be difficult to obtain the effects of strength enhancement and structure refinement, and when the content of Nb exceeds 0.05%, recrystallization is delayed due to local grain fixation, which impairs the uniformity of the structure. Accordingly, the content of Nb may be preferably in the range of 0.001 to 0.05%. In terms of uniformity, an upper limit of the Nb content may more preferably be 0.03%.

Ti: 0.001 to 0.05%

Titanium (Ti) is an element added to secure the strength of the steel sheet and refine the structure. When Ti is added in the content of less than 0.001%, it may be difficult to obtain the effects of strength improvement and structure refinement, and when the content of the Ti exceeds 0.05%, the castability is impaired due to excessive TiN formation and the impact properties of the steel are impaired due to excessive carbide formation. Accordingly, the content of the Ti may be preferably in the range of 0.001 to 0.05%. In terms of improving the impact properties, an upper limit of the Ti content may more preferably be 0.03%.

P: 0.04% or less (excluding 0%)

Phosphorus (P) exists as impurities in steel, and it is advantageous to control a content thereof as low as possible, but phosphorus (P) is also intentionally added to increase the strength of steel. However, when P is added excessively, the toughness of the steel deteriorates, and thus, in the present disclosure, an upper limit thereof may be preferably limited to 0.04% to prevent this problem. In terms of improving toughness, the content of P may more preferably be 0.015% or less, and even more preferably 0.010% or less. When the toughness of the steel is required to be equal to or higher than the standard, phosphorus (P) may be controlled to be 0.007% or less.

S: 0.01% or less (excluding 0%)

Sulfur (S), similarly to phosphorus (P), exists as impurities in steel and it is advantageous to control a content thereof as low as possible. Additionally, since Sulfur (S) deteriorates the ductility and impact characteristics of the steel, an upper limit thereof may be preferably limited to 0.01%. The content of S may more preferably be 0.005% or less, even more preferably 0.003%, and less, and most preferably 0.0015% or less.

N: 0.01% or less (excluding 0%)

In the present disclosure, nitrogen (N) is inevitably included in the steel as an impurity element during the process, and an upper limit thereof may be preferably limited to 0.01%. The content of N may more preferably be 0.007% or less, even more preferably 0.005% or less, and most preferably 0.003% or less.
In addition to the steel composition described above, a balance may include Fe and inevitable impurities. The inevitable impurities may be unintentionally mixed during a typical steel manufacturing process, and may not be completely excluded, and a person skilled in the typical steel manufacturing field may easily understand a meaning thereof. Additionally, the present disclosure does not completely exclude the addition of other compositions other than the aforementioned steel composition.
Meanwhile, the cold rolled steel sheet according to the present disclosure may further include at least one of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%) according to an embodiment. Copper (Cu) and nickel (Ni) are elements increasing the strength of steel. However, copper (Cu) and nickel (Ni) are elements increasing the strength and hardenability of steel, but when Copper (Cu) and nickel (Ni) are added in excessive amounts, a target strength grade may be exceeded, and since copper (Cu) and nickel (Ni) are expensive elements, each of upper limits thereof may be preferably to 0.1% from an economic perspective. Meanwhile, since copper (Cu) and nickel (Ni) act as solid solution strengthening elements, when copper (Cu) and nickel (Ni) are added in amounts of less than 0.03%, the solid solution strengthening effect may be minimal, and thus, copper (Cu) and nickel (Ni) may be added in amounts of 0.03% or more, respectively.
Additionally, the cold rolled steel sheet according to the present disclosure may further include V: 0.05% or less (excluding 0%) according to an embodiment. Vanadium (V) may increase the strength of steel even with a small amount of addition, but since vanadium (V) does not have a significant effect on improving elongation, a content thereof may be preferably controlled to be 0.05% or less. The content of V may more preferably be 0.04% or less, and even more preferably 0.03% or less.
The cold rolled steel sheet according to the present disclosure may have a microstructure including, in area %, ferrite: 15% or less (excluding 0%), fresh martensite 5% or less (excluding 0%), tempered martensite 75% or more and less than 95%, retained austenite: 12% or less (excluding 0%), and a balance of bainite.
The cold rolled steel sheet according to the present disclosure is intended to secure excellent bending formability having a tensile strength of 1470 MPa or more, and specifically, in order to obtain high local formability, a hardness difference between microstructure phases forming the steel sheet should be reduced. When a fraction of the softest ferrite exceeds 15%, the yield strength decreases and the hole expandability and bending formability deteriorate. Additionally, when the fraction of fresh martensite, which is brittle and has high strength due to non-tempering, exceeds 5%, the formability also deteriorates. Tempered martensite is a structure obtained by tempering martensite formed by cooling the steel sheet below the Ms temperature, through a reheating or temperature maintenance process, and has high strength, and when a fraction thereof is 75% or more, high strength of 1470 MPa or more and excellent bending formability may be obtained. However, when the fraction of the tempered martensite is 95% or more, an entire microstructure becomes a full martensite structure that has high strength but insufficient ductility, and thus, the necessary formability may not be obtained. Retained austenite refers to austenite remaining at room temperature after a process of manufacturing a steel sheet is completed, and in the case in which the retained austenite has sufficient stability, when additional deformation is applied to the steel, the retained austenite does not immediately transform into martensite, but gradually transforms into martensite depending on the deformation level, thereby contributing to the elongation and bending characteristics. When the retained austenite is sufficiently stable, it is better to have a high fraction thereof, but when the fraction exceeds 12%, the stability thereof decreases and the bending characteristics thereof may be significantly reduced.
The retained austenite may have an average grain size of 0.5 µm or less. When an average grain size of the retained austenite exceeds 0.5 µm, it may be difficult to secure excellent bending characteristics. Meanwhile, the retained austenite may be measured through EBSD, and a phase classified as FCC when analyzed through the EBSD may be regarded as retained austenite.
The cold rolled steel sheet according to the present disclosure may preferably satisfy the following relational expression 1. (RA_unstable+RA_stable)/RA_stable ≤ 2.0
(However, in the relational expression 1, RA_unstable refers to a fraction of unstable retained austenite, and RA_stable refers to the fraction of stable retained austenite.)
The unstable retained austenite may be defined as a difference between a fraction of retained austenite before a tensile test and a fraction of retained austenite after the tensile test after measuring a fraction of retained austenite in a grip portion of a tensile specimen before and after the tensile test, and the stable retained austenite may be defined as a fraction of retained austenite after the tensile test. The pressure applied to the grip portion during the tensile test may be 250 N. The retained austenite fraction may be measured through XRD peak analysis. In order to have good bending characteristics, a ratio of stable retained austenite that remains as austenite even when micro-deformation is applied should be higher than that of unstable retained austenite that easily transforms into martensite when the micro-deformation is applied.
As described above, the cold rolled steel sheet according to the present disclosure may satisfy the following relational expression 2. $4 \times P . El - U . El \geq 13.0$
(However, in the relational expression 2, P.El refers to a post-elongation, and U.El refers to a uniform elongation.)
During a tensile test of a steel sheet, a material is deformed relatively uniformly from the start of the tensile test until a flow stress reaches the tensile strength (TS), but after exceeding the tensile strength (TS), which corresponds to a maximum flow stress that may be supported by the steel sheet, deformation heterogeneity between microstructures within the material increases, and eventually, it may be known that the steel sheet undergoes a process of void generation, coalescence, and macroscopic fracture in upper boundaries. A final fracture process of the material during the tensile test may be represented by a post-elongation value, and an extreme surface area in which fracture occurs during a bending test is also a process in which the material fractures due to high strain, which was found to be highly related to the post-elongation. Accordingly, the post-elongation and bending characteristics in the tensile test are closely related, and accordingly, the inventors of the present disclosure were able to determine that the post-elongation increases when a hardness difference between phases is reduced by controlling a phase fraction of the microstructure and an alloy composition range is controlled by suppressing excessive alloy addition. On the other hand, when the fraction of retained austenite is high, a uniform elongation of the material increases, but when the steel plate is deformed to the extent that the tensile strength reaches a limit thereof, some of the retained austenite transforms into high-carbon martensite, which deteriorates the bending characteristics. From these results, it was found that the bending characteristics of the steel sheet may be improved by limiting the post-elongation to a high level and the uniform elongation to a relatively low level, as in the relational expression 2. When a value of the relational expression 2 is less than 13.0, the bending characteristics may be inferior, and it may be difficult to obtain a bending formability (R_min/t) of 2.5 or less. Meanwhile, a total elongation (T.El) may be measured through the tensile test, and a total nominal strain until the specimen breaks is known as a total elongation (T.El). The total elongation (T.El) may be divided into the uniform elongation (U.El) until reaching the tensile strength and the post-elongation (P.El) until a final break thereafter.
The cold rolled steel sheet may have a tensile strength of 1470 MPa or more, a yield strength of 1000 MPa or more, a total elongation (T.El) of 8.5% or more, and a bending formability (R_min/t) of 2.5 or less. In the present disclosure, since the tensile strength, yield strength, and total elongation are preferably higher, there is no limitation on an upper limit thereof, but the tensile strength is unlikely to exceed 1650 MPa, the yield strength is unlikely to exceed 1300 MPa, and the total elongation is unlikely to exceed 16%. Additionally, in the present disclosure, since the bending formability (R_min/t) is preferably lower, there is no limitation on a lower limit thereof, but as an example, the lower limit of the bending formability (R_min/t) may be 0.8. Meanwhile, in the bending formability (R_min/t), R_min refers to a minimum punch curvature radius at which cracks do not occur when a 90-degree V-bending bending test is performed using punches having various curvature radii (R), and t refers to a thickness (mm) of the steel sheet. When bending is performed using a 90-degree V-bending die, if the bending is possible with an acute punch with a small R value, the bending characteristics of the material may be considered to be better. Accordingly, the smaller the minimum R value (R_min) at which cracks do not occur, the better the bending characteristics of the material. On the other hand, the R_min value is affected by a thickness of the tested material, and since it is difficult to have a small R_min value as a thickness thereof increases, the bending characteristics are usually represented by a ratio of the R_min value and the thickness t.
Hereinafter, a method for manufacturing a cold rolled steel sheet according to an embodiment of the present disclosure will be described.
First, a slab having the above-described alloy composition is heated. The slab heating temperature may be 1150 to 1250°C. When the slab heating temperature is lower than 1150°C, hot rolling, which is a subsequent operation, may not be performed, while if the slab heating temperature exceeds 1250°C, a large amount of energy is unnecessarily consumed to increase a slab temperature. Accordingly, the slab heating temperature may be preferably in the range of 1150 to 1250°C. In consideration of the condition temperature of a hot rolling process, a lower limit of the slab heating temperature may more preferably be 1170°C, and even more preferably 1180°C. In terms of costs and energy consumption reduction, an upper limit of the slab heating temperature may more preferably be 1230°C, and even more preferably 1220°C.
Then, the heated slab is subjected to finishing hot rolling at a temperature of 830 to 950°C to obtain a hot-rolled steel sheet. When the finishing hot rolling temperature (hereinafter referred to as 'FDT') is less than 830°C, a rolling load increases and shape defects increase, which deteriorates productivity. On the other hand, when the finishing hot rolling temperature exceeds 950°C, the surface quality deteriorates due to an increase in oxides caused by excessive high-temperature task. Therefore, the finishing hot rolling temperature may preferably have a range of 830 to 950°C. In terms of the productivity described above, a lower limit of the finishing hot rolling temperature may more preferably be 850°C, and eve more preferably 880°C. In terms of securing surface quality, an upper limit of the finishing hot rolling temperature may more preferably be 930°C, and even more preferably 910°C.
Then, the hot-rolled steel sheet is coiled at a temperature of 400 to 650°C. When the coiling temperature (hereinafter referred to as 'CT') exceeds 650°C, there is a disadvantage that coarse hot-rolled internal oxidation occurs and surface characteristics deteriorate, and when the coiling temperature is less than 400°C, this corresponds to a transition boiling range, and thus, the controllability of the coiling temperature deteriorates and a shape of the steel sheet deteriorates. In order to prevent the above-mentioned problems, a lower limit of the coiling temperature may more preferably be 440°C, and even more preferably 480°C. Additionally, an upper limit of the coiling temperature may more preferably be 610°C, and even more preferably 570°C.
Then, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold rolled steel sheet. During the cold rolling, a cold reduction ratio may be 30 to 60%. When the cold reduction ratio is less than 30%, it is difficult to secure targeted thickness precision and to perform shape correction of the steel sheet. On the other hand, when the cold reduction ratio exceeds 60%, cracks are likely to occur at an edge of the steel sheet, and a cold rolling load may excessively increase. Accordingly, the cold reduction ratio may have a range of 30 to 60%.
Then, the cold rolled steel sheet is continuously annealed at a temperature of 800 to 950°C. When the continuous annealing temperature (hereinafter referred to as 'SS') is less than 800°C, there may be a concern that sufficient austenite reverse transformation may not occur, and thus the ferrite phase may be formed on a level exceeding 15% after annealing. On the other hand, when the continuous annealing temperature exceeds 950°C, the surface quality and productivity may deteriorate, and coarse austenite may be formed, which may deteriorate the material. A lower limit of the continuous annealing temperature may more preferably be 820°C, and even more preferably 840°C. An upper limit of the continuous annealing temperature may more preferably be 930°C, and even more preferably 900°C.
Then, the continuously annealed cold rolled steel sheet is primarily cooled to a primary cooling end temperature (hereinafter referred to as 'SCS') of 500 to 700°C at an average cooling rate of 1°C/s or more and less than 10°C/s. The primary cooling end temperature may be defined as a point in time at which the rapid cooling equipment applied in the primary cooling is additionally applied and secondary cooling (rapid cooling) is initiated. When the cooling process is divided into primary cooling and secondary cooling and performed in stages, the temperature distribution of the steel sheet may be made uniform in a slow cooling operation, thereby reducing a final temperature and material deviation. When the primary cooling end temperature is less than 500°C, there may be a concern that soft bainite transformation may be induced, and it may be difficult to cool to less than 500°C at a cooling rate of less than 10°C/s based on an actual equipment length. When the primary cooling end temperature exceeds 700°C, the cooling amount up to the secondary cooling end temperature increases, resulting in poor steel sheet shape. On the other hand, when the primary cooling rate is less than 1°C/s, a ferrite phase is formed during cooling, making it difficult to obtain high-strength steel, and when the primary cooling rate is 10°C/s or more, the cooling amount in the secondary cooling increases, which may increase final temperature deviation and material deviation. Accordingly, the primary cooling rate may preferably have a range of 1°C/s or more and less than 10°C/s. A lower limit of the primary cooling rate may more preferably be 2°C/s, and even more preferably 3°C/s. An upper limit of the primary cooling rate may more preferably be 7°C/s, and even more preferably 5°C/s.
Then, the primarily-cooled cold rolled steel sheet is secondarily cooled to a secondary cooling end temperature (hereinafter referred to as 'RCS') of 25 to 300°C at an average cooling rate of 10°C/s or more. The secondary cooling end temperature is intended to ensure that a temperature of the steel sheet is lower than Ms so that martensite transformation occurs during cooling, and the martensite ultimately becomes tempered martensite through a reheating operation as a post process. It may be difficult to cool the secondary cooling end temperature below 25°C, which is lower than room temperature. On the other hand, when the secondary cooling end temperature exceeds 300°C, martensite is not sufficiently generated during cooling, making it difficult to obtain sufficient yield strength, tensile strength, and bending formability. When the secondary cooling speed is less than 10°C/s, even when a target secondary cooling end temperature is reached, a high-temperature phase such as upper bainite is mixed during cooling, making it impossible to obtain the target tempered martensite fraction and high strength. Meanwhile, in the present disclosure, as faster the secondary cooling rate is faster, this is more advantageous, and thus, there is no particular limitation on an upper limit thereof, but it is difficult to exceed 100°C/s due to equipment limitations.
Then, the secondarily-cooled cold rolled steel sheet is heated at 150 to 450°C and held for more than 500 seconds and 1,500 seconds or less. Through the process, interphase carbon distribution and additional phase transformation necessary for stabilizing the retained austenite are obtained. In the present disclosure, an end point temperature of a heating section is conveniently referred to as a reheating temperature (hereinafter, also referred to as 'RHS'), and an end point temperature of a holding section is conveniently referred to as an over-aging temperature (hereinafter, also referred to as 'OAS'). When the RHS or OAS temperature is less than 150°C, the strength becomes excessively high and the formability deteriorates. On the other hand, when the RHS or OAS temperature exceeds 450°C, it may be difficult to obtain the high strength desired by the present disclosure. When the holding time in the holding section is equal to or less than 500 seconds, there may be a concern that a total phase transformation amount is insufficient at an end time of the holding operation, the retained austenite fraction increases, and a large amount of fresh martensite is generated, which may deteriorate the bending characteristics. Additionally, a value of the relational expression 1 may exceed 2.0. A lower limit of the holding time may more preferably be 600 seconds, and even more preferably 700 seconds. An upper limit of the holding time may more preferably be 1300 seconds, and even more preferably 1000 seconds. Meanwhile, the holding time in the holding section refers to the time required from an end point of the heating section to an end point of the holding section.

Mode for Invention

Hereinafter, the present disclosure will be described more specifically through examples. However, it should be noted that the following examples are only intended to illustrate and concretize the present disclosure and are not intended to limit the scope of the rights of the present disclosure. This is because the scope of the rights of the present disclosure is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

(Example)

A cold rolled steel sheet was manufactured by performing slab heating, hot rolling, coiling, cold rolling, annealing, primary cooling, secondary cooling, reheating, and holding processes on a slab having an alloy composition described in Tables 1 and 2 below under the conditions described in Tables 3 and 4 below. Meanwhile, the conditions described in Tables 3 and 4 below were based on a surface temperature of the steel sheet.
The microstructure and mechanical properties of the cold rolled steel sheet manufactured in this manner were measured, and the results are shown in Tables 5 and 6 below.
The microstructure was measured using a Point Counting method from images observed using a scanning electron microscope (SEM), and specifically, the fraction of retained austenite was measured using XRD. An average grain size of retained austenite was measured using EBSD analysis.
The tensile strength (TS), yield strength (YS), and total elongation (T.El) were measured by taking specimens, perpendicular to a rolling direction for the cold rolled steel sheet and performing a tensile test on the specimens. Additionally, the total elongation (T.El) was divided into the uniform elongation (U.El) until reaching the tensile strength and the post-elongation (P.El) until the final fracture. The No. 5 tensile test specimen of the KS B0801 standard was used for the tensile test, and a gauge length was 50 mm, and a width of a tensile test section was 25 mm. In the tensile test, the elongation was measured by the so-called nominal strain, which is calculated by dividing the amount of elongation of the specimen by an initial gauge length (50 mm), and the total nominal strain until the specimen breaks is known as a total elongation (T.El). The strength was measured by the so-called nominal stress, which is calculated by dividing the load measured during the tensile test by an initial cross-section of the specimen, and a stress value when this nominal stress reaches the maximum value is known as a tensile strength (TS). Here, the uniform elongation (U.El) refers to a nominal strain value when the nominal stress reaches the tensile strength (TS), and the post-elongation (P.El) refers to a nominal strain amount from the time at which the tensile strength is reached until the final fracture occurs. In other words, the total elongation is a sum of the uniform elongation and the post-elongation.

The bending characteristics were obtained by performing a 90-degree V-bending test on the cold rolled steel sheet and observing a surface of the steel sheet with a magnifying glass of 1000x magnification to obtain a minimum R_min/t value in which no cracks occurred. In this case, a 90-degree V-bending test method was used in which 90-degree punches having different radii (R) were prepared, and then a steel sheet having a constant thickness (t) was placed between 90-degree V-shaped dies and the punch and the die were brought into close contact.

Table 1:

Steel Type	Alloy Composition (wt%)
Steel Type	C	Si	Mn	Al	Mo	Ti	B	Cr
A	0.237	1.93	2.54	0.0382	0.057	0.023	0.0024	0.03
B	0.279	1.94	2.63	0.0473	0.103	0.022	0.0023	0.02
C	0.170	1.48	2.62	0.0192	0.007	0.020	0.0013	0.28
D	0.246	1.96	2.50	0.2040	0.113	0.025	0.0024	0.01
E	0.270	2.32	2.55	0.0460	0.103	0.024	0.0016	0.02
F	0.197	0.95	2.61	0.0206	0.108	0.019	0.0022	0.01
G	0.240	0.94	2.22	0.0308	0.101	0.022	0.0022	0.02
H	0.240	1.91	2.47	0.0230	0.106	0.021	0.0020	0.01
I	0.335	1.97	2.52	0.0440	0.108	0.022	0.0020	0.01
J	0.410	2.00	2.47	0.0390	0.002	0.019	0.0019	0.02
K	0.176	1.51	2.58	0.0440	0.050	0.021	0.0022	0.47
L	0.191	1.90	2.58	0.0430	0.103	0.021	0.0023	0.02
M	0.391	1.98	2.46	0.0468	0.110	0.024	0.0022	0.01

Table 2:

Steel Type	Alloy Composition (wt%)
Steel Type	Nb	P	S	N	Cu	Ni	V
A	0.001	0.0120	0.0017	0.0035	0.032	0.011	0.003
B	0.019	0.0103	0.0022	0.0037	0.007	0.005	0.001
C	0.005	0.0082	0.0018	0.0043	0.025	0.007	0.005
D	0.019	0.0107	0.0024	0.0044	0.041	0.014	0.004
E	0.018	0.0095	0.0020	0.0032	0.023	0.021	0.003
F	0.019	0.0085	0.0019	0.0049	0.017	0.005	0.002
G	0.018	0.0095	0.0019	0.0029	0.033	0.015	0.001
H	0.018	0.0085	0.0028	0.0030	0.027	0.004	0.003
I	0.003	0.0097	0.0026	0.0034	0.021	0.003	0.004
J	0.006	0.0090	0.0029	0.0044	0.034	0.008	0.005
K	0.003	0.0080	0.0009	0.0033	0.016	0.013	0.006
L	0.020	0.0089	0.0021	0.0049	0.012	0.011	0.011
M	0.019	0.0103	0.0022	0.0047	0.037	0.022	0.007

Table 3:

Division	Steel Type	Slab Heating Tempera ture (°C)	Finishing Hot Roll Temperatu re (FDT) (°C)	Thickness of Hot Rolled Steel Sheet (mm)	Coiling Tempera ture (CT) (°C)	Cold reductio n ratio (%)	Thicknes s of Cold Rolled Steel Sheet (mm)	Continuous Annealing Temperature (SS) (°C)
Inventive Example 1	A	1178	912	3.2	624	56	1.4	867
Inventive Example 2	B	1211	927	2.8	605	50	1.4	823
Inventive Example 3	C	1223	913	2.5	557	36	1.6	850
Inventive Example 4	D	1193	882	2.3	540	39	1.0	878
Inventive Example 5	E	1197	870	2.2	492	36	1.4	865
Inventive Example 6	F	1218	846	2.0	447	40	1.2	873
Inventive Example 7	G	1182	851	2.1	633	33	1.4	882
Inventive Example 7	H	1201	904	2.3	582	39	1.4	871
Comparative Example 1	I	1211	899	2.5	577	44	1.4	892
Comparative Example 2	J	1193	912	2.4	630	50	1.2	867
Comparative Example 3	K	1231	881	2.1	442	52	1.0	838
Comparative Example 4	L	1242	842	2.1	438	33	1.4	821
Comparative Example 5	M	1187	895	2.6	644	46	1.4	888

Table 4:

Division	Steel Type	Primary Cooling End Tempera ture (SCS) (°C)	Primary Average Cooling Rate (°C/s)	Secondary Cooling End Temperatu re (RCS) (°C)	Secondary Average Cooling rate (°C/s)	Heating Section End Temperat ure (RHS) (°C)	Holding Section End Temperat ure (OAS) (°C)	Holding Time (Seconds)
Inventive Example 1	A	602	3.8	155	35	403	402	695
Inventive Example 2	B	588	4.4	147	38	411	40	735
Inventive Example 3	C	592	4.4	35	48	212	202	596
Inventive Example 4	D	578	4.6	196	33	318	262	625
Inventive Example 5	E	647	5.1	140	47	305	311	1092
Inventive Example 6	F	628	4.1	212	39	247	252	811
Inventive Example 7	G	606	3.2	245	41	298	261	754
Inventive Example 8	H	620	2.8	248	32	296	292	621
Comparati ve Example 1	I	597	3.7	221	38	412	372	622
Comparati ve Example 2	J	592	3.4	198	42	411	366	592
Comparati ve Example 3	K	605	4.7	321	27	462	452	545
Comparati ve Example 4	L	602	5.2	246	28	398	403	425
Comparati ve Example 5	M	577	3.5	197	35	388	372	550

Table 5:

Division	Microstructure Fraction (area%)					RA Characteristics
Division	F	TM	FM	B	RA	Average Grain Size (µm)	Formula 1
Inventive Example 1	1.2	92.3	0.3	1.0	5.2	0.32	1.2
Inventive Example 2	8.4	78.0	2.2	0.2	11.2	0.31	1.4
Inventive Example 3	11.1	87.6	0.2	0.2	0.9	0.27	0.8
Inventive Example 4	5.6	88.9	0.5	0.2	4.8	0.45	1.5
Inventive Example 5	3.1	88.5	0.3	1.8	6.3	0.33	1.1
Inventive Example 6	0.3	93.6	1.5	2.5	2.1	0.41	0.9
Inventive Example 7	1.1	91.1	3.5	0.5	3.8	0.44	1.6
Inventive Example 8	2.9	80.1	3.8	1.7	11.5	0.48	1.4
Comparative Example 1	1.5	72.9	2.5	5.6	17.5	0.66	2.1
Comparative Example 2	0.4	69.1	4.3	6.7	19.5	0.72	3.1
Comparative Example 3	10.8	49.7	9.5	23.5	6.5	0.54	2.4
Comparative Example 4	22.3	62.4	8.7	5.4	1.2	0.65	2.3
Comparative Example 5	0.3	67.2	3.8	10.2	18.5	0.77	2.9
F: Ferrite, RM: Tempered Martensite, FM: Fresh Martensite, B: Bainite, and RA: Retained Austenite
[Formula 1] (RA_unstable+RA_stable)/RA_stable
(In the Formula 1, RA_unstable refers to a fraction of retained austenite after the tensile test, and RA_stable refers to a fraction of retained austenite before tensile test.)

Table 6:

Division	YS (MPa)	TS (MPa)	T.El (%)	U.El (%)	P.El (%)	R_min/t	Formula 2
Inventive Example 1	1361	1506	8.9	2.4	6.5	1.8	23.6
Inventive Example 2	1435	1546	12.4	5.6	6.8	2.5	21.6
Inventive Example 3	1178	1496	10.2	4.7	5.5	2.5	17.3
Inventive Example 4	1102	1572	12.7	7.0	5.7	2.5	15.8
Inventive Example 5	1370	1707	10.3	5.3	5.0	2.5	14.7
Inventive Example 6	1057	1502	10.2	5.4	4.8	2.0	13.8
Inventive Example 7	1102	1479	10.0	5.4	4.6	2.4	13.0
Inventive Example 8	1042	1493	11.7	6.6	5.1	2.1	13.8
Comparative Example 1	1158	1524	16.7	12.6	4.1	2.9	3.8
Comparative Example 2	1121	1540	18.9	14.7	4.2	3.2	2.1
Comparative Example 3	774	1509	6.2	4.7	1.5	2.8	1.3
Comparative Example 4	722	1495	8.4	6.8	1.6	4.3	-0.4
Comparative Example 5	1006	1543	17.9	14.5	3.4	5.7	-0.9
[Formula 2] 4×P.El - U.El

As may be seen from Tables 1 to 6, in Inventive Examples 1 to 8 satisfying the alloy composition and manufacturing conditions proposed by the present disclosure, excellent mechanical properties are ensured by ensuring the microstructure obtained by the present disclosure.
In the case of Comparative Examples 1, 2 and 5, the manufacturing conditions proposed by the present disclosure are satisfied, but the alloy composition is not satisfied, from which it may be seen that the microstructure desired by the present disclosure is not secured, and for this reason, mechanical properties are inferior.
In the case of Comparative Examples 3 and 4, the alloy composition proposed by the present disclosure is satisfied, but the manufacturing conditions are not satisfied, from which the microstructure desired by the present disclosure is not secured, and thus mechanical properties are inferior.

Claims

A cold rolled steel sheet, comprising: by wt%, C: 0.15% or more and less than 0.30%, Si: 0.8 to 2.5%, Al: 0.005 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.2%, B: 0.0005 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), and a balance of Fe and other inevitable impurities,
wherein a microstructure includes, in area%, ferrite: 15% or less (excluding 0%), fresh martensite 5%: or less (excluding 0%), tempered martensite: 75% or more and less than 95%, retained austenite: 12% or less (excluding 0%), and a balance of bainite, and

the following relational expression 1 is satisfied, (RA_unstable+RA_stable)/RA_stable ≤ 2.0

where RA_unstable refers to a fraction of unstable retained austenite, and RA_stable refers to the fraction of stable retained austenite.
The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet further includes at least one of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).
The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet further includes V: 0.05% or less (excluding 0%).
The cold rolled steel sheet of claim 1, wherein the retained austenite has an average grain size of 0.5µm or less.
The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet satisfies the following relational expression 2, $4 \times P . El - U . El \geq 13.0$ where P.El refers to a post-elongation, and U.El refers to uniform elongation.
The cold rolled steel sheet of claim 1, wherein the cold rolled steel sheet has a tensile strength of 1470 MPa or more, a yield strength of 1000 MPa or more, a total elongation (T.El) of 8.5% or more, and bending formability (R_min/t) of 2.5 or less.
A method for manufacturing a cold rolled steel sheet, comprising:
heating a slab including, by wt%, C: 0.15% or more and less than 0.30%, Si: 0.8 to 2.5%, Al: 0.005 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.2%, B: 0.0005 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), and a balance of Fe and other inevitable impurities;

finishing hot rolling the heated slab at a temperature of 830 to 950°C to obtain a hot rolled steel sheet;

coiling the hot rolled steel sheet at a temperature of 400 to 650°C;

cold rolling the coiled hot rolled steel sheet to obtain a cold rolled steel sheet;

continuously annealing the cold rolled steel sheet at a temperature of 800 to 950°C;

primarily cooling the continuously annealed cold rolled steel sheet to a primary cooling end temperature of 500 to 700°C at an average cooling rate of 1°C/s or more and less than 10°C/s;

secondarily cooling the primarily cooled cold rolled steel sheet to a secondary cooling end temperature of 25 to 300°C at an average cooling rate of 10°C/s or more; and

reheating the secondarily cooled cold rolled steel sheet at 150 to 450°C and then holding the reheated steel sheet for more than 500 seconds and 1,500 seconds or less.
The method for manufacturing a cold rolled steel sheet of claim 7, wherein the slab further includes at least one of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).
The method for manufacturing a cold rolled steel sheet of claim 7, wherein the slab further includes V: 0.05% or less (excluding 0%).
The method for manufacturing a cold rolled steel sheet of claim 7, wherein a heating temperature of the slab is 1150 to 1250°C.
The method for manufacturing a cold rolled steel sheet of claim 7, wherein a cold reduction ratio is 30 to 60% during the cold rolling.