Patent Application
20240011118
The present disclosure relates to a steel sheet that may be used for automobile parts and the like, and to a steel sheet having high strength characteristics and excellent workability and a method for manufacturing the same.
In recent years, the automobile industry has been paying attention to ways to reduce material weight in an effort to protect the global environment and secure occupant safety. In order to meet these requirements for safety and weight reduction, the use of a high strength steel sheet is rapidly increasing. In general, it is commonly known that as the strength of the steel sheet increases, the workability of the steel sheet is lowered. Therefore, in a steel sheet for automobile parts, a steel sheet having excellent workability represented by ductility, a hole expansion ratio, and the like, while having high strength characteristics is required.
Since transformation induced plasticity (TRIP) steel, using transformation-induced plasticity of retained austenite, has a complex microstructure consisting of ferrite, bainite, martensite, retained austenite, and the like, it is known as having a certain level or more of workability, as well as high strength characteristics.
As a technique for further improving the workability of a steel sheet, a method of utilizing tempered martensite is disclosed in Patent Documents 1 and 2. Since tempered martensite made by tempering hard martensite is softened martensite, there is a difference in strength between tempered martensite and existing untempered martensite (fresh martensite). Therefore, when fresh martensite is suppressed and tempered martensite is formed, the workability may increase.
However, by the techniques disclosed in Patent Documents 1 and 2, a balance (TS2*EL1/2) of tensile strength and elongation does not satisfy the range of 3.0*106 to 6.2*106 (Mpa2%1/2), meaning that it is difficult to secure a steel sheet having both of excellent strength and excellent ductility.
Meanwhile, as another technique for improving workability of a steel sheet, Patent Document 3 discloses a method for inducing generation of bainite by means of adding boron (B). In the case of adding boron (B), a ferrite-pearlite transformation is suppressed, while generation of bainite is induced, whereby coexistence of strength and workability can be achieved.
However, by the technique disclosed in Patent Document 3, a balance (BTE) of tensile strength and elongation of 3.0*106 to 6.2*106 (mpa2%1/2), a balance (BTH) of tensile strength and a hole expansion ratio of 6.0*106 to 11.5*106 (mpa2%1/2), and a yield ratio evaluation index (IYR) of 0.15 to 0.42 cannot be secured at the same time, thereby meaning it is difficult to secure a steel sheet having all of excellent strength, an excellent hole expansion ratio, excellent ductility, and an excellent yield ratio.
That is, a demand for a steel sheet having all of an excellent balance (BTE) of tensile strength and elongation, an excellent balance (BTH) of tensile strength and a hole expansion ratio, and an excellent yield ratio evaluation index (IYR) is not satisfied.
According to an aspect of the present disclosure, a steel sheet having all of an excellent balance of tensile strength and elongation, an excellent balance of tensile strength and a hole expansion ratio, and an excellent yield ratio evaluation index and a method manufacturing the same can be provided, by optimizing a composition and microstructures of the steel sheet.
An object of the present disclosure is not limited to the abovementioned contents. Additional objects of the present disclosure are described in the overall content of the specification, and those of ordinary skill in the art to which the present disclosure pertains will have no difficulty in understanding the additional objects of the present disclosure from the contents described in the specification of the present disclosure.
In an aspect of the present disclosure, a high strength steel sheet having excellent workability may comprise: by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; and as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures, wherein the high strength steel sheet may satisfy the following relational expression 1:
0.03≤[B]FM/[B]TM≤0.55 [Relational Expression 1]
The steel sheet may further comprise, by wt %, one or more of the following (1) to (8):
The microstructure of the steel sheets may include, by volume fraction, 10 to 30% of bainite, 50 to 70% of tempered martensite, 10 to 30% of fresh martensite, 2 to 10% of retained austenite, and 5% or less (including 0%) of ferrite.
In the steel sheet, a balance (BTE) of tensile strength and elongation expressed by the following relational expression 2 satisfies 3.0*106 to 6.2*106 (MPa2%1/2), a balance (BTH) of tensile strength and a hole expansion ratio expressed by the following relational 3 expression satisfies 6.0*106 to 11.5*106 (mpa2%1/2), and a yield ratio evaluation index (IYR) expressed by the following relational expression 4 satisfies 0.15 to 0.42:
B
TE=[Tensile Strength (TS, MPa)]2*[Elongation (El, %)]1/2; [Relational Expression 2]
B
TH=[Tensile Strength (TS, MPa)]2*[Hole Expansion Ratio (HER, %)]1/2; and [Relational Expression 3]
I
YR=1−[Yield Ratio (YR)]. [Relational Expression 4]
In an aspect of the present disclosure, a method for manufacturing a high strength steel sheet having excellent workability may comprise: providing a cold-rolled steel sheet including, by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; heating (primarily heating) the cold-rolled steel sheet to a temperature of 700° C. at an average heating rate of 5° C./s or more, heating (secondarily heating) the primarily heated steel sheet to a temperature within a range of Ac3 to 920° C. at an average heating rate of 5° C./s or less, and then maintaining (primarily maintaining) the secondarily heated steel sheet for 50 to 1200 seconds; cooling (primarily cooling) the primarily maintained steel sheet to a temperature within a range of 200 to 400° C. at an average cooling rate of 1° C./s or more; heating (tertiarily heating) the primarily cooled steel sheet to a temperature within a range of 350 to 550° C. at an average heating rate of 5° C./s or more, and then maintaining (secondarily maintaining) the tertiarily heated steel sheet for 50 seconds or more; and cooling (secondarily cooling) the secondarily maintained steel sheet to room temperature at an average cooling rate of 1° C./s or more.
The steel slab may further comprise, by wt %, one or more of the following (1) to (8):
The cold-rolled steel sheet may be provided by: heating a steel slab to 1000 to 1350° C.; performing finishing hot rolling at a temperature within a range of 800 to 1000° C.; coiling the hot-rolled steel sheet at a temperature within a range of 350 to 600° C.; pickling the coiled steel sheet; and cold rolling the pickled steel sheet at a reduction ratio of 30 to 90%.
According to an aspect of the present disclosure, it is possible to provide a steel sheet that may be used for automobile parts and the like, and to a steel sheet having an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and hole expansion ratio, and an excellent yield ratio evaluation index, and a method for manufacturing the same.
The present disclosure relates to a high strength steel sheet having excellent workability and a method for manufacturing the same, and exemplary embodiments of the present disclosure will hereinafter be described. Exemplary embodiments of the present disclosure may be modified into various forms, and it is not to be interpreted that the scope of the present disclosure is limited to exemplary embodiments described below. The present exemplary embodiments are provided in order to further describe the present disclosure in detail to those skilled in the art to which the present disclosure pertains.
The inventors of the present disclosure recognized that in a boron (B)-added transformation-induced plasticity (TRIP) steel comprising bainite, tempered martensite, fresh martensite and retained austenite, when the fractions of the tempered martensite, the fresh martensite, and the retained austenite are controlled to be within certain ranges, the contents of the boron (B) contained in the tempered martensite and the fresh martensite are controlled to be within certain ranges, and a shape and a size of the retained austenite are controlled to be within certain ranges, it is possible to simultaneously secure an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and a hole expansion ratio, and an excellent yield ratio evaluation index. Based thereon, the present inventors have conceived of the present disclosure by devising a method such that excellent strength, excellent yield ratio, excellent ductility, and an excellent hole expansion ratio may be simultaneously provided.
Hereinafter, a high strength steel sheet having excellent workability according to an aspect of the present disclosure will be described in more detail.
In an aspect of the present disclosure, a high strength steel sheet having excellent workability may comprise: by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; and as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures, wherein the high strength steel sheet may satisfy the following relational expression 1:
0.03≤[B]FM/[B]TM≤0.55 [Relational Expression 1]
Hereinafter, compositions of steel according to the present disclosure will be described in more detail. Hereinafter, unless otherwise indicated, % indicating a content of each element is based on weight.
The high strength steel sheet having excellent workability according to an aspect of the present disclosure includes: by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities. In addition, the high strength steel sheet may further include one or more of Ti: or less (including 0%), Nb: 0.5% or less (including 0%), V: 0.5% or less (including 0%), Cr: 3.0% or less (including 0%), Mo: 3.0% or less (including 0%), Cu: 4.0% or less (including 0%), Ni: 4.0% or less (including 0%), Ca: 0.05% or less (including 0%), REM: 0.05% or less (including 0%) excluding Y, Mg: 0.05% or less (including 0%), W: 0.5% or less (including 0%), Zr: 0.5% or less (including 0%), Sb: or less (including 0%), Sn: 0.5% or less (including 0%), Y: 0.2% or less (including 0%), Hf: 0.2% or less (including 0%), and Co: 1.5% or less (including 0%).
Carbon (C): 0.1 to 0.25%
Carbon (C) is an unavoidable element for securing strength of a steel sheet, and is also an element for stabilizing retained austenite that contributes to the improvement in ductility of the steel sheet. Accordingly, in the present disclosure, 0.1% or more of carbon (C) may be added in order to achieve such an effect. A preferable content of carbon (C) may be greater than 0.1%, may be 0.11% or more, and may be 0.12% or more. On the other hand, when the content of carbon (C) exceeds a certain level, ductility may be lowered and weldability may be degraded due to an excessive increase in strength. Therefore, an upper limit of the content of carbon (C) of the present disclosure may be limited to 0.25%. The content of carbon (C) may be 0.24% or less, and a more preferable content of carbon (C) may be or less.
Silicon (Si): 0.01 to 1.5% or Less
Silicon (Si) is an element that contributes to improvement in strength by solid solution strengthening, and is also an element improving workability by homogenizing a structure. In addition, silicon (Si) is an element contributing to generation of retained austenite by suppressing precipitation of cementite. Therefore, in the present disclosure, silicon (Si) of 0.01% or more may be added in order to achieve such an effect. A preferable content of silicon (Si) may be 0.02% or more, and a more preferable content of silicon (Si) may be 0.04% or more. However, when the content of silicon (Si) exceeds a certain level, a problem of plating defects, such as non-plating, may be induced during plating, and weldability of a steel sheet may be lowered, so the present disclosure may limit an upper limit of the content of silicon (Si) to 1.5%. A preferable upper limit of the content of silicon (Si) may be 1.48%, and a more preferable upper limit of the content of silicon (Si) may be 1.46%.
Manganese (Mn): 1.0 to 4.0%
Manganese (Mn) is a useful element for increasing both strength and ductility. Therefore, in the present disclosure, manganese (Mn) of 1.0% or more may added in order to achieve such an effect. A preferable lower limit of the content of manganese (Mn) may be 1.2%, and a more preferable lower limit of the content of manganese (Mn) may be 1.4%. On the other hand, when manganese (Mn) is excessively added, a bainite transformation time increases and concentration of carbon (C) in austenite becomes insufficient, so there exists a problem in which a desired austenite fraction may not be secured. Therefore, in the present disclosure, an upper limit of the content of manganese (Mn) of the present disclosure may be limited to 4.0%. A preferable upper limit of the content of manganese (Mn) may be 3.9%.
Aluminum (Al): 0.01 to 1.5%
Aluminum (Al) is an element performing deoxidation by combining with oxygen in steel. In addition, aluminum (Al) is also an element for stabilizing retained austenite by suppressing precipitation of cementite like silicon (Si). Therefore, in the present disclosure, aluminum (Al) of 0.01% or more may be added in order to achieve such an effect. A preferable content of aluminum (Al) may be 0.03% or more, and a more preferable content of aluminum (Al) may be 0.05% or more. On the other hand, when aluminum (Al) is excessively added, inclusions in a steel sheet increase, and workability of the steel sheet may be lowered, so the present disclosure may limit an upper limit of the content of aluminum (Al) to 1.5%. A preferable upper limit of the content of aluminum (Al) may be 1.48%.
Phosphorus (P): 0.15% or Less (Including 0%)
Phosphorus (P) is an element which is contained as an impurity and deteriorates impact toughness. Therefore, it is preferable to manage the content of phosphorus (P) to 0.15% or less.
Sulfur (S): 0.03% or Less (Including 0%)
Sulfur (S) is an element which is contained as an impurity to form MnS in a steel sheet and deteriorate ductility. Therefore, it is preferable that the content of sulfur (S) is 0.03% or less.
Nitrogen (N): 0.03% or Less (Including 0%)
Nitrogen (N) is an element which is contained as an impurity and forms nitride during continuous casting to cause cracks in a slab. Therefore, it is preferable that the content of nitrogen (N) is 0.03% or less.
Boron (B): 0 to 0.005%
Boron (B) is an element improving hardenability to increase strength, and is also an element suppressing nucleation of grain boundaries. In addition, in the present disclosure, it is intended to simultaneously secure an excellent balance of tensile strength and elongation, an excellent balance of tensile strength and a hole expansion ratio, and an excellent yield ratio evaluation index, and therefore, boron (B) is to be necessarily added in the present disclosure. Therefore, in the present disclosure, 0.0005% or more of boron (B) may be added in order to achieve such an effect. However, when the content of boron (B) is added beyond a certain level, not only excessive characteristic effects, but also an increase in manufacturing costs is incurred, so the present disclosure may limit an upper limit of the content of boron (B) to 0.005%.
Meanwhile, the steel sheet of the present disclosure has an alloy composition that may be additionally included in addition to the above-described alloy components, which will be described in detail below.
One or More of Titanium (Ti): 0 to 0.5%, Niobium (Nb): 0 to 0.5%, and Vanadium (V): 0 to 0.5%
Titanium (Ti), niobium (Nb), and vanadium (V) are elements that make precipitates and refine crystal grains, and are elements that also contribute to the improvement in strength and impact toughness of a steel sheet, and therefore, in the present disclosure, one or more of titanium (Ti), niobium (Nb), and vanadium (V) may be added in order to achieve such an effect. However, when each of the contents of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates are formed to lower impact toughness and increase manufacturing costs, so the present disclosure may limit the contents of titanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less, respectively.
One or More of Chromium (Cr): 0 to 3.0% and Molybdenum (Mo): 0 to 3.0%
Since chromium (Cr) and molybdenum (Mo) are elements that not only suppress austenite decomposition during alloying treatment, but also stabilize austenite like manganese (Mn), in the present disclosure, one or more of chromium (Cr) and molybdenum (Mo) may be added in order to achieve such an effect. However, when the contents of chromium (Cr) and molybdenum (Mo) exceed certain levels, the bainite transformation time increases and the concentration of carbon (C) in austenite becomes insufficient, so the desired retained austenite fraction may not be secured. Therefore, the present disclosure may limit the contents of chromium (Cr) and molybdenum (Mo) to 3.0% or less, respectively.
One or More of Cu: 0 to 4.0% and Ni: 0 to 4.0%
Copper (Cu) and nickel (Ni) are elements that stabilize austenite and suppress corrosion. In addition, copper (Cu) and nickel (Ni) are also elements that are concentrated on a surface of a steel sheet to prevent hydrogen from intruding into the steel sheet, thereby suppressing hydrogen delayed destruction. Therefore, in the present disclosure, one or more of copper (Cu) and nickel (Ni) may be added in order to achieve such an effect. However, when the contents of copper (Cu) and nickel (Ni) exceed certain levels, not only excessive characteristic effects, but also an increase in manufacturing costs is incurred, so the present disclosure may limit the contents of copper (Cu) and nickel (Ni) to 4.0% or less, respectively.
One or More of Calcium (Ca): 0 to 0.05%, Magnesium (Mg): 0 to 0.05%, and Rare Earth Element (REM) Excluding Yttrium (Y): 0 to 0.05%
Here, the rare earth element (REM) is scandium (Sc), yttrium (Y), and a lanthanide element. Since calcium (Ca), magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y) are elements that contribute to the improvement in ductility of a steel sheet by spheroidizing sulfides, in the present disclosure, one or more of calcium (Ca), magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y) may be added in order to achieve such an effect. However, when the contents of calcium (Ca), magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y) exceed certain levels, not only excessive characteristic effects, but also an increase in manufacturing costs is incurred, so the present disclosure may limit the contents of calcium (Ca), magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y) to 0.05% or less, respectively.
One or More of Tungsten (W): 0 to 0.5% and Zirconium (Zr): 0 to 0.5%
Since tungsten (W) and zirconium (Zr) are elements that increase strength of a steel sheet by improving hardenability, in the present disclosure, one or more of tungsten (W) and zirconium (Zr) may be added in order to achieve such an effect. However, when the contents of tungsten (W) and zirconium (Zr) exceed certain levels, not only excessive characteristic effects, but also an increase in manufacturing costs is incurred, so the present disclosure may limit the contents of tungsten (W) and zirconium (Zr) to or less, respectively.
One or More of Antimony (Sb): 0 to 0.5% and Tin (Sn): to 0.5%
Since antimony (Sb) and tin (Sn) are elements that improve plating wettability and plating adhesion of a steel sheet, in the present disclosure, one or more of antimony (Sb) and tin (Sn) may be added in order to achieve such an effect. However, when the contents of antimony (Sb) and tin (Sn) exceed certain levels, brittleness of a steel sheet increases, and thus, cracks may occur during hot working or cold working, so the present disclosure may limit the contents of antimony (Sb) and tin (Sn) to 0.5% or less, respectively.
One or More of Yttrium (Y): 0 to 0.2% and Hafnium (Hf): 0 to 0.2%
Since yttrium (Y) and hafnium (Hf) are elements that improve corrosion resistance of a steel sheet, in the present disclosure, one or more of the yttrium (Y) and hafnium (Hf) may be added in order to achieve such an effect. However, when the contents of yttrium (Y) and hafnium (Hf) exceed certain levels, ductility of the steel sheet may deteriorate, so the present disclosure may limit the contents of yttrium (Y) and hafnium (Hf) to 0.2% or less, respectively.
Cobalt (Co): 0 to 1.5%
Since cobalt (Co) is an element that promotes a bainite transformation to increase a TRIP effect, in the present disclosure, cobalt (Co) may be added in order to achieve such an effect. However, when the content of cobalt (Co) exceeds a certain level, since weldability and ductility of a steel sheet may deteriorate, the present disclosure may limit the content of cobalt (Co) to 1.5% or less.
The high strength steel sheet having excellent workability according to an aspect of the present disclosure may include a balance of Fe and other unavoidable impurities in addition to the components described above. However, in a general manufacturing process, unintended impurities may inevitably be mixed from raw materials or the surrounding environment, and thus, these impurities may not be completely excluded. Since these impurities are known to those skilled in the art, all the contents are not specifically mentioned in the present specification. In addition, a further addition of effective components other than the above-described components is not entirely excluded.
The high strength steel sheet having excellent workability according to an aspect of the present disclosure may include, as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures.
Both untempered martensite (fresh martensite, FM) and tempered martensite (TM) are microstructures that improve the strength of a steel sheet. However, compared with tempered martensite, fresh martensite has a characteristic of greatly reducing ductility and burring workability of a steel sheet. In addition, compared with tempered martensite, fresh martensite has a tendency of reducing a yield ratio of a steel sheet. These are because a microstructure of tempered martensite is softened by a tempering heat treatment. Therefore, in the present disclosure, it is preferable to control fractions of tempered martensite and fresh martensite in order to secure a balance (TS2*EL1/2) of tensile strength and elongation, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) targeted by the present disclosure. In order to satisfy a balance (TS2*EL1/2) of tensile strength and elongation of 3.0*106 or more, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio of 6.0*106 or more, and a yield ratio evaluation index (1-YR) of 0.42 or less, it is preferable to limit a fraction of the tempered martensite to 50 vol % or more, and to limit a fraction of the fresh martensite to 10 vol % or more. A more preferable fraction of tempered martensite may be 52 vol % or more, or 54 vol % or more, and a more preferable fraction of fresh martensite may be 12 vol % or more. On the other end, when tempered martensite or fresh martensite is excessively formed, ductility and burring workability are lowered, so that a balance (TS2*EL1/2) of tensile strength and elongation of 3.0*106 or more, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio of 6.0*106 or more, and a yield ratio evaluation index (1-YR) of 0.42 or less cannot be satisfied at the same time. Therefore, the present disclosure may limit a fraction of tempered martensite to 70 vol % or less, and limit a fraction of fresh martensite to 30 vol % or less. A more preferable fraction of tempered martensite may be 68 vol % or less, or 65 vol % or less, and more preferable fraction of fresh martensite may be 25 vol % or less.
It is necessary to optimize a fraction of bainite in order to secure a balance (TS2*EL1/2) of tensile strength and elongation, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) at levels targeted by the present disclosure. In order to secure a balance (TS2*EL1/2) of tensile strength and elongation of 3.0*106 or more, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio of 6.0*106 or more, and a yield ratio evaluation index (1-YR) of 0.42 or less, it is preferable to control a fraction of bainite to 10 vol % or more. A more preferable fraction of bainite may be 12 vol % or more, or 14 vol % or more. On the other end, when bainite is excessively formed, it causes a fraction reduction of tempered martensite, so that a fraction of bainite may be limited to 30 vol % or less, in order to secure a balance (TS2*EL1/2) of tensile strength and elongation, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) targeted by the present disclosure. A preferable fraction of the bainite may be 12 vol % or more, 14 vol % or more, 28 vol % or less, or 26 vol % or less.
A steel sheet including retained austenite has excellent ductility and bending workability due to transformation-induced plasticity occurring during transformation from austenite to martensite during processing. When a fraction of the retained austenite is lower than a certain level, a balance (TS2*EL1/2) of tensile strength and elongation may be less than 3.0*106 (Mpa2%1/2), and is not preferably. On the other hand, when a fraction of retained austenite exceeds a certain level, local elongation may be lowered, or point weldability may be lowered. Therefore, in the present disclosure, a fraction of retained austenite may be limited to be in a range of 2 to 10 vol % in order to obtain a steel sheet having an excellent balance (TS2*EL1/2) of tensile strength and elongation. A preferable fraction of retained austenite is 3 vol % or more, or 8 vol % or less.
As the unavoidable structure, the steel sheet of the present disclosure may include ferrite, pearlite, martensite austenite constituent (M-A), and the like. When ferrite is excessively formed, strength of the steel sheet may be lowered, so the present disclosure may limit a fraction of ferrite to 5 vol % (including 0%). Moreover, when pearlite is excessively formed, workability of the steel sheet may be lowered or a fraction of retained austenite may be lowered, so the present disclosure intends to limit the formation of pearlite as much as possible.
The high strength steel sheet having excellent workability according to an aspect of the present disclosure may satisfy the following relational expression 1:
0.03≤[B]FM/[B]TM≤0.55 [Relational Expression 1]
The present disclosure not only controls the fractions of the tempered martensite, the fresh martensite, and the retained austenite to be in certain ranges, but also controls the content ratios of the boron (B) contained in the tempered martensite, and the fresh martensite to be in certain ranges, while controlling a ratio of the retained austenite of specific size, shape and type with respect to the entire retained austenite to be in a certain range, in order to secure a balance (TS2*EL1/2) of tensile strength and elongation, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) targeted thereby.
The present disclosure controls a ratio of the content ([B]TM, wt %) of boron (B) contained in the fresh martensite to the content ([B]FM, wt %) of boron (B) contained in the tempered martensite to be in a range of 0.03 to 0.55 as shown in relational expression 1, so as to secure a balance (BTE) of tensile strength and elongation of 3.0*106 to 6.2*106 (mpa2%1/2), a balance (BTH) of tensile strength and a hole expansion ratio of 6.0*106 to 11.5*106 (Mpa2%1/2), and a yield ratio evaluation index (IYR) of 0.15 to 0.42 at the same time.
The inventors of the present disclosure conducted in-depth research on a method for securing physical properties of a boron (B)-added TRIP steal, and as a result, noted that the physical properties targeted by the present disclosure may be secured only when a ratio of a content of boron (B) contained in fresh martensite to a content of boron (B) contained in tempered martensite satisfies a certain range, even though the theoretical basis thereof is not clearly identified. In particular, it was able to identify that a yield ratio of a steel sheet has a constant tendency according to a content ratio of boron (B) contained in tempered martensite and fresh martensite. Therefore, the present disclosure limits the ratio of the content of boron (B) contained in the fresh martensite to the content of boron (B) contained in the tempered martensite to be in a range of 0.03 to 0.55 as shown in relational expression 1, thereby securing a balance (TS2*EL1/2) of tensile strength and elongation, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) targeted thereby.
In the high strength steel sheet having excellent workability according to an aspect of the present disclosure, a balance (BTE) of tensile strength and elongation expressed by the following relational expression 2 may satisfy 3.0*106 to 6.2*106 (Mpa2%1/2), a balance (BTH) of tensile strength and a hole expansion ratio expressed by the following relational expression 3 may satisfy 6.0*106 to 11.5*106 (Mpa2%1/2), and a yield ratio evaluation index (IYR) expressed by the following relational expression 4 may satisfy 0.15 to 0.42:
B
TE=[Tensile Strength (TS, MPa)]2*[Elongation (El, %)]1/2h; [Relational Expression 2]
B
TH=[Tensile Strength (TS, MPa)]2*[Hole Expansion Ratio (HER, %)]1/2; and [Relational Expression 3]
I
YR=1−[Yield Ratio (YR)]. [Relational Expression 4]
Hereinafter, an example of a method for manufacturing a steel sheet of the present disclosure will be described in detail.
A method for manufacturing a high strength steel sheet having excellent workability according to an aspect of the present disclosure may comprises: heating (primarily heating) a cold-rolled steel sheet having a predetermined alloy composition to a temperature of 700° C. at an average heating rate of 5° C./s or more, heating (secondarily heating) the primarily heated steel sheet to a temperature within a range of Ac3 to 920° C. at an average heating rate of 5° C./s or less, and then maintaining (primarily maintaining) the secondarily heated steel sheet for 50 to 1200 seconds; cooling (primarily cooling) the primarily maintained steel sheet to a temperature within a range of 200 to 400° C. at an average cooling rate of 1° C./s or more; heating (tertiarily heating) the primarily cooled steel sheet to a temperature within a range of 350 to 550° C. at an average heating rate of 5° C./s or more, and then maintaining (secondarily maintaining) the tertiarily heated steel sheet for 50 seconds or more; and cooling (secondarily cooling) the secondarily maintained steel sheet to room temperature at an average cooling rate of 1° C./s or more.
The cold-rolled steel sheet may be provided by: heating steel slab having a predetermined alloy composition to 1000 to 1350° C.; performing finishing hot rolling at a temperature within a range of 800 to 1000° C.; coiling the hot-rolled steel sheet at a temperature within a range of 350 to 600° C.; pickling the coiled steel sheet; and cold rolling the pickled steel sheet at a reduction ratio of 30 to 90%.
Preparation and Heating of Steel Slab
A steel slab having a predetermined alloy composition is prepared. Since the steel slab according to the present disclosure includes an alloy composition corresponding to an alloy composition of the steel sheet described above, the description of the alloy compositions of the slab is replaced by the description of the alloy composition of the steel sheet described above.
The prepared steel slab may be heated to a temperature within a certain range, and the heating temperature of the steel slab at this time may be in the range of 1000 to 1350° C. When the heating temperature of the steel slab is less than 1000° C., the steel slab may be hot rolled at a temperature within a range below a desired finish hot rolling temperature range, and when the heating temperature of the steel slab exceeds 1350° C., the temperature reaches a melting point of steel, and thus, the steel slab may be melted.
Hot Rolling and Coiling
The heated steel slab may be hot rolled, and thus, provided as a hot-rolled steel sheet. During the hot rolling, the finish hot rolling temperature is preferably in the range of 800 to 1000° C. When the finish hot rolling temperature is lower than 800° C., an excessive rolling load may be a problem, and when the finish hot rolling temperature exceeds 1000° C., grains of the hot-rolled steel sheet are coarsely formed, which may cause a deterioration in physical properties of the final steel sheet.
After the hot rolling has been completed, the hot-rolled steel sheet may be cooled at an average cooling rate of 10° C./s or more, and may be coiled at a temperature within a range of 350 to 650° C. When the coiling temperature is lower than 350° C., coiling is not easy, and when the coiling temperature exceeds 650° C., surface scale may be formed into the inside of the hot-rolled steel sheet, which may make pickling difficult.
Pickling and Cold Rolling
After uncoiling the coiled hot-rolled coil, in order to remove the scale generated on the surface of the steel sheet, the pickling may be performed, and the cold rolling may be performed. Although the conditions of the pickling and the cold rolling are not particularly limited in the present disclosure, the cold rolling is preferably performed at a cumulative reduction ratio of 30 to 90%. When the cumulative reduction ratio of the cold rolling exceeds 90%, it may be difficult to perform the cold rolling in a short time due to the high strength of the steel sheet.
The cold-rolled steel sheet may be manufactured as a non-plated cold-rolled steel sheet through the annealing heat treatment process, or may be manufactured as a plated steel sheet through a plating process to impart corrosion resistance. As the plating, plating methods such as hot-dip galvanizing, electro-galvanizing, and hot-dip aluminum plating may be applied, and the method and the type are not particularly limited.
Annealing Heat Treatment
In the present disclosure, in order to simultaneously secure the strength and workability of the steel sheet, the annealing heat treatment process is performed.
The cold-rolled steel sheet is heated (primarily heated) to a temperature of 700° C. at an average heating rate of 5° C./s or more, is heated (secondarily heated) to a temperature within a range of Ac3 to 920° C. at an average heating rate of 5° C./s or less, and then is maintained (primarily maintained) for 50 to 1200 seconds.
When the average heating rate to a temperature of 700° C. of the primary heating is less than 5° C./s, lump austenite is formed from ferrite and cementite generated during heating, and as a result, fine tempered martensite and retained austenite cannot be formed as a final structure. Therefore, a targeted balance (TS2*EL1/2) of tensile strength and elongation, and a targeted balance (TS2*HER1/2) of tensile strength and a hole expansion ratio cannot be implemented. In addition, when the secondary heating rate up to the primary maintaining temperature exceeds 5° C./s, transformation from cementite generated during heating to austenite is accelerated, so that a large amount of lump austenite is formed, the final structure is coarsened, and boron (B) may not be sufficiently concentrated into tempered martensite. As a result, [B]FM/[B]TM exceeds 0.55, and targeted levels of a balance (TS2*EL1/2) of tensile strength and elongation and a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (IYR) cannot be implemented.
When the primary maintaining temperature is lower than Ac3 (two-phase region), 5 vol % or more of ferrite is formed, and therefore, a balance (TS2*EL1/2) of tensile strength and elongation and a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio may be lowered. In addition, when the primary maintaining time is less than 50 seconds, the structure may not be sufficiently homogenized and the physical properties of the steel sheet may be lowered. Upper limits of the primary maintaining temperature and the primary maintaining time are not particularly limited, but it is preferable that the primary maintaining temperature is limited to 920° C. or less, and the primary maintaining time is limited to 1200 seconds or less, in order to prevent toughness reduction due to coarsened grains.
After the primary maintaining, the primarily maintained steel sheet may be cooled (primarily cooled) to a primary cooling stop temperature in a range of 200 to 400° C. at an average cooling rate of 1° C./s or more. When the average cooling rate of the primary cooling is less than 1° C./s, a fraction of retained austenite becomes insufficient due to a slow cooling, and therefore, a balance (TS2*EL1/2) of tensile strength and elongation of the steel sheet may be lowered. An upper limit of the average cooling rate of the primary cooling does not need to be particularly specified, but is preferably set to 100° C./s or less. When the primary cooling stop temperature is lower than 200° C., tempered martensite is excessively formed, and retained austenite becomes insufficient, whereby a balance (TS2*EL1/2) of tensile strength and elongation and a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio of the steel sheet may be lowered. On the other hand, when the primary cooling stop temperature exceeds 400° C., bainite is excessively formed, and tempered martensite becomes insufficient, whereby a balance (TS2*EL1/2) of tensile strength and elongation and a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio of the steel sheet may be lowered.
After the secondary cooling, the primarily cooled steel sheet may be heated (tertiarily heated) to a temperature within a range of 350 to 550° C. at an average heating rate of 5° C./s or more, and then may be maintained (secondarily maintaining) for 50 seconds or more. An upper limit of the average heating rate of the tertiary heating does not need to be particularly specified, but is preferably set to 100° C./s or less. When the secondary maintaining temperature is lower than 350° C. or the secondary maintaining time is less than 50 seconds, tampered martensite is excessively formed, and therefore, it is difficult to secure a fraction of retained austenite. As a result, a balance (TS2*EL1/2) of tensile strength and elongation and a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio may be lowered. When the secondary maintaining temperature exceeds 550° C. or the secondary maintaining time exceeds 155,000 seconds, a fraction of retained austenite becomes insufficient, and therefore, a balance (TS2*EL1/2) of tensile strength and elongation of the steel sheet may be lowered.
After the secondary maintaining, the secondarily maintained steel sheet may be cooled (secondarily cooling) to room temperature at an average cooling rate of 1° C./s or more.
The high strength steel sheet having excellent workability manufactured by the aforementioned manufacturing method may comprise, as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures, and as a preferable example, may comprise, by volume fraction, 10 to 30% of bainite, 50 to 70% of tempered martensite, 10 to 30% of fresh martensite, 2 to 10% of retained austenite, and 5% or less (including 0%) of ferrite.
In the high strength steel sheet having excellent workability manufactured by the aforementioned manufacturing method, a balance (BTE) of tensile strength and elongation expressed by the following relational expression 2 may satisfy 3.0*106 to 6.2*106 (MPa2%1/2), a balance (BTH) of tensile strength and a hole expansion ratio expressed by the following relational expression 3 may satisfy 6.0*106 to 11.5*106 (MPa2%1/2), and a yield ratio evaluation index (IYR) expressed by the following relational expression 4 may satisfy 0.15 to 0.42:
B
TE=[Tensile Strength (TS, MPa)]2*[Elongation (El, %)]1/2; [Relational Expression 2]
B
TH=[Tensile Strength (TS, MPa)]2*[Hole Expansion Ratio (HER, %)]1/2; and [Relational Expression 3]
I
YR=1−[Yield Ratio (YR)]. [Relational Expression 4]
Hereinafter, a high strength steel sheet having excellent workability and a method for manufacturing same according to an aspect of the present disclosure will be described in more detail. It should be noted that the following examples are only for the understanding of the present disclosure, and are not intended to specify the scope of the present disclosure. The scope of the present disclosure is determined by matters described in claims and matters reasonably inferred therefrom.
A steel slab having a thickness of 100 mm having alloy compositions (a balance of Fe and unavoidable impurities) shown in Table 1 below was prepared, heated at 1200° C., and then subjected to finish hot rolling at 900° C. Thereafter, the steel slab was cooled at an average cooling rate of 30° C./s, and coiled at a coiling temperature of Tables 2 and 3 to manufacture a hot-rolled steel sheet having a thickness of 3 mm. Thereafter, after removing a surface scale by pickling, cold rolling was performed to a thickness of 1.5 mm.
Thereafter, the heat treatment was performed under the annealing heat treatment conditions shown in Tables 2 to 5 to manufacture the steel sheet. In Tables 2 and 3, the single-phase region means a temperature range of Ac3 to 920° C., and the two-phase region means a temperature range below Ac3° C.
The microstructure of the thus prepared steel sheet was observed, and the results were shown in Tables 6 and 7. Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM) and pearlite (P) were observed through SEM after nital-etching a polished specimen cross section. After nital-etching, a structure having no concave-convex portions on a surface of a specimen was classified as ferrite, and a structure having a lamella structure of cementite and ferrite is classified as pearlite. Since both of bainite (B) and tempered martensite (TM) were observed in a form of lath and block, and were difficult to distinguish each other, the fractions of bainite and tempered martensite were calculated using an expansion curve after evaluating dilatation. That is, a value obtained by subtracting the fraction of tempered martensite calculated using the expansion curve from the fraction of bainite and tempered martensite measured by the SEM observation was determined as the fraction of bainite. Meanwhile, since fresh martensite (FM) and retained austenite (retained γ) are also difficult to distinguish each other, a value obtained by subtracting the fraction of retained austenite calculated by an X-ray diffraction method from the fraction of martensite and retained austenite observed by the SEM was determined as the fraction of the fresh martensite.
Meanwhile, [B]FM/[B]TM, a balance (TS2*EL1/2) of tensile strength and elongation, a balance (TS2*HER1/2) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (IYR) of the steel sheet were measured and evaluated, and the results thereof were shown in Tables 8 and 9.
The concentrations of boron (B) in fresh martensite and tempered martensite measured using an electron probe microanalyser (EPMA) were determined as a content ([B]FM) of boron (B) contained in fresh martensite, and a content ([B]TM) of boron (B) contained in tempered martensite.
Tensile strength (TS) and elongation (El) were evaluated through a tensile test, and the tensile strength (TS) and the elongation (El) were measured by evaluating the specimens collected in accordance with JIS No. 5 standard based on a 90° direction with respect to a rolling direction of a rolled sheet. The hole expansion ratio (HER) was evaluated through a hole expansion test, and was calculated by the following relational expression 5, after forming a punching hole (die inner diameter of 10.3 mm, clearance of 12.5%) of 10 mmψ, inserting a conical punch having an apex angle of 60° into a punching hole in a direction in which a burr of the punching hole faces outward, and then compressing and expanding a peripheral portion of the punching hole at a moving speed of 20 mm/min:
Hole Expansion Ratio (HER, %)={(D−D0)/D0}×100 [Relational Expression 5]
In the above relational expression 5, D is a hole diameter (mm) when cracks penetrate through the steel sheet along the thickness direction, and D0 is the initial hole diameter (mm).
As shown in Tables 1 to 9 above, it could be seen that in the case of the specimens complying with the conditions presented in the present disclosure, relational expression 1 is satisfied, a balance (BTE) of tensile strength and elongation satisfies 3.0*106 to 6.2*106 (Mpa2%1/2), a balance (BTH) of tensile strength and a hole expansion ratio satisfies 6.0*106 to 11.5*106 (Mpa2%1/2), and a yield ratio evaluation index (IYR) satisfies 0.15 to 0.42.
In specimen 2, the primary average heating rate was less than 5° C./s, so that tempered martensite and retained austenite were insufficient. As a results, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 2.
In specimen 3, the secondary average heating rate was more than 5° C./s, so that lump austenite was formed, and boron (B) was not concentrated into tempered martensite. As a result, [B]FM/[B]TM was more than 0.55, a yield ratio evaluation index (IYR) was more than 0.42, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 3.
In specimen 4, the primary maintaining temperature was in a two-phase region less than Ac3, so that a fraction of ferrite was excessive. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 4.
In specimen 5, the primary average cooling rate was less than 1° C./s, so that a fraction of retained austenite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, in specimen 5.
In specimen 6, the primary cooling stop temperature was less than 200° C., so that a fraction of tempered martensite was excessive, and a fraction of retained austenite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 6.
In specimen 7, the primary cooling stop temperature was more than 400° C., so that a fraction of bainite was excessive, and a fraction of tempered martensite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 7.
In specimen 8, the secondary maintaining temperature was less than 350° C., so that a fraction of tempered martensite was excessive, and a fraction of retained austenite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 8.
In specimen 9, the secondary maintaining temperature was more than 550° C., so that a fraction of retained austenite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, in specimen 9.
In specimen 10, the secondary maintaining time was less than 50 seconds, so that a fraction of tempered martensite was excessive, and a fraction of retained austenite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 10.
In specimen 11, the secondary maintaining time was more than 155,000 seconds, so that a fraction of retained austenite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, in specimen 11.
In specimen 33, a content of carbon (C) was low, so that a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106.
In specimen 34, a content of carbon (C) was high, so that a fraction of tempered martensite was insufficient, a fraction of fresh martensite was excessive, and a fraction of retained austenite was excessive. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 34.
In specimen 35, a content of silicon (Si) was low, so that a fraction of retained austenite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, in specimen 35.
In specimen 36, a content of silicon (Si) was high, so that a fraction of fresh martensite was excessive. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 36.
In specimen 37, a content of Aluminum (Al) was high, so that a fraction of fresh martensite was excessive. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 37.
In specimen 38, a content of manganese (Mn) was low, so that pearlite was generated, and a fraction of retained austenite was insufficient. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, in specimen 38.
In specimen 39, a content of manganese (Mn) was high, so that a fraction of fresh martensite was excessive. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 39.
In specimen 40, a content of chromium (Cr) was high, so that a fraction of fresh martensite was excessive. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 40.
In specimen 41, a content of molybdenum (Mo) was high, so that a fraction of fresh martensite was excessive. As a result, a balance (BTE) of tensile strength and elongation was less than 3.0*106, and a balance (BTH) of tensile strength and a hole expansion ratio was less than 6.0*106, in specimen 41.
In specimen 42, a content of boron (B) was low, so that boron (B) was not concentrated into tempered martensite. As a result, [B]FM/[B]TM was more than 0.55, and a yield ratio evaluation index (IYR) was more than 0.42, in specimen 42.
In specimen 43, a content of boron (B) was high, so that boron (B) was excessively concentrated into tempered martensite. As a result, [B]FM/[B]TM was less than 0.03, and a yield ratio evaluation index (IYR) was less than 0.15, in specimen 43.
While the present disclosure has been described in detail through exemplary embodiment, other types of exemplary embodiments are also possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to exemplary embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2020-0177474 | Dec 2020 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2021/017989 | 12/1/2021 | WO |
