STEEL SHEETS AND METHOD FOR MANUFACTURING SAME

Description

TECHNICAL FIELD

The present disclosure relates to a material applied to vehicle members and, more specifically, to a cold-rolled steel sheet (and a plated steel sheet) having highly excellent bending properties, and a method for manufacturing the same.

BACKGROUND ART

Recently, there has been a demand for improved fuel efficiency of vehicles to protect the global environment, and in particular, for vehicle steel sheets, a higher level of high strength is required to reduce the weight of a vehicle body and ensure safety.

Various types of steel sheets having strength and processability have been designed and put into practical use. For example, a composite structure steel sheet in which a ferrite phase and a low-temperature transformation phase such as martensite or bainite are both present may be used as a high-strength steel sheet with excellent processability. A composite steel sheet is formed by dispersing a hard, low-temperature transformation phase in soft ferrite to simultaneously improve strength and processability.

Meanwhile, in the case of a steel material used as a collision member, the steel material may be manufactured using a cold forming method, and there may be demand for the development of ultra-high strength steel with higher processing characteristics, in particular, excellent bending properties, such that research is actively being conducted on a method for manufacturing ultra-high strength steel having a tensile strength of 1500 MPa or more using a single martensite phase.

A Hot Press Forming (HPF) method of forming a material at high temperature, which may be easy to form, and then securing the required strength through water cooling between a die and the material, is being developed. This is because the HPF method is widely used in manufacturing parts because it can secure high strength as compared to the same thickness, but due to a problem in application caused by excessive facility investment and increased process costs, the development of a material for cold stamping is required. Therefore, the development of a cold-rolled steel sheet, which is suitable for use as the material for cold stamping, has a high strength and high yield ratio to secure collision performance, and has excellent bending properties, is required.

A representative Prior Art of this method is Patent Document 1. Patent Document 1 discloses steel having a steel composition including: 0.25 to 0.4% of C, 1.0% or less of Si, 1.5 to 2.5% of Mn, 0.02% or less of P, 0.003% or less of S, 0.01 to 0.1% of Al, 0.005% or less of N, 0.0005 to 0.005% of B, and also including 0.005 to 0.1% of Ti, 0.005 to 0.1% of Nb, a total of 0.005 to 0.1%, wherein a metal structure is a martensite single-phase structure. Using such a steel, a steel sheet is manufactured by heating and storing the steel and maintaining the same in a temperature range from an Ae3 transformation point to 900° C. or lower, then performing rapid cooling of the steel to 200° C. or lower at an average cooling rate of 300° C./s or more, and then tempering the steel at 250° C. or lower, but there is a defect in that the shape (flatness) thereof, due to water cooling, is inferior, resulting in defects during forming.

Patent Document 2 discloses a technology for manufacturing a plate steel sheet having a steel structure including, 0.05% or more and 0.35% or less of C, 0.01% or more and 2.0% or less of Si, 0.8% or more and 3.0% or less of Mn, 0.05% or less of P, 0.005% or less of S, 0.005% or more and 0.10% or less of Al, and 0.0060% or less of N, wherein a ferrite area ratio is 0% or more and 90% or less, a bainite area ratio is 5% or less (including 0%), a martensite and tempered martensite area ratio is 10% or more (including 100%), and a retained austenite area ratio is 2.0% or less (including 0%), and a standard deviation of a yield strength of the plate steel sheet in a width direction is 30 MPa or less, and a maximum deflection of the plate steel sheet when sheared at a length of 1 m is 10 mm or less. However, this also has the problem of shape defects due to rapid cooling after annealing, and thus has limitations which are disadvantageous in practical aspects.

• • (Patent Document 1) Japanese Application No. 2009-098534
  • (Patent Document 2) Japanese Application No. 2018-143806

SUMMARY OF INVENTION

Technical Problem

An aspect of the present disclosure is to overcome the limitations of the prior art described above, and to provide a steel sheet having ultra-high strength having a tensile strength of 1500 MPa or more, while having excellent shape and bending properties by optimizing a steel composition and manufacturing process.

An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.

Solution to Problem

According to an aspect of the present disclosure, provided is a steel sheet,

• • the steel sheet including by weight %: 0.1 to 0.3% of carbon (C), 0.5% or less (excluding 0%) of silicon (Si), 1.3 to 2.5% of manganese (Mn), 0.2% or less (excluding 0%) of chromium (Cr), 0.01 to 0.1% of molybdenum (Mo), 0.0005 to 0.003% of boron (B), 0.1% or less (excluding 0%) of phosphorous (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), 0.01 to 0.1% of aluminum (Al), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.05% of titanium (Ti), and a balance of Fe and other unavoidable impurities,
  • wherein a ratio (a/b×100) of a total content (a) of C and Mn, in a region within 20 μm from a surface thereof, and a total content (b) of C and Mn at a point (¼)×t, where t is a total thickness of the steel sheet, from the surface is 75% or more (excluding 100%).

The steel sheet may include in area %, 10% or less (excluding 0%) of one or more phases selected from the group consisting of ferrite and bainite, as a microstructure, in the region within 20 μm from the surface.

The steel sheet may have a remainder of martensite, as a microstructure, in the region within 20 μm from the surface.

The steel sheet may include in area %, 90 to 99% of martensite, as a microstructure, in the region within 20 μm from the surface.

The total thickness, t may be 0.6 to 2.5 mm.

The steel sheet may include a zinc-based plating layer on the surface of the steel sheet.

The steel sheet may have a tensile strength (TS) of 1500 MPa or more and bendability (R/t) of 3.7 or less.

According to another aspect of the present disclosure, provided is a method for manufacturing a steel sheet,

• • the method including: reheating a steel slab including by weight %, 0.1 to 0.3% of carbon (C), 0.5% or less (excluding 0%) of silicon (Si), 1.3 to 2.5% of manganese (Mn), 0.2% or less (excluding 0%) of chromium (Cr), 0.01 to 0.1% of molybdenum (Mo), 0.0005 to 0.003% of boron (B), 0.1% or less (excluding 0%) of phosphorous (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), 0.01 to 0.1% of aluminum (Al), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.05% of titanium (Ti), and a balance of Fe and other unavoidable impurities, at a temperature within a range of 1100 to 1300° C.;
  • hot rolling the reheated slab;
  • coiling the hot-rolled steel sheet at a temperature within a range of 400 to 600° C.;
  • cold rolling the coiled steel sheet at a reduction ratio of 30 to 80%;
  • annealing the cold-rolled steel sheet by heat treatment to a temperature within a range of Ac3+10° C. to Ac3+80° C.;
  • primarily cooling the annealed steel sheet to a temperature within a range of 680 to 749° C., a primary cooling end temperature range, at an average cooling rate of 10° C./s or less; and
  • secondarily cooling the primarily-cooled steel sheet to a temperature within a range of 100° C. to Mf at an average cooling rate of 60 to 160° C./s.

The annealing may be performed by heat treatment at a temperature within a range of Ac3+10° C. to Ac3+80° C. for 30 seconds or more.

The method may further include an over-aging heat treatment which is reheating the secondarily-cooled steel sheet to a temperature within a range of 150 to 240° C.

The over-aging heat treatment may be performed for 400 to 1000 seconds.

Advantageous Effects of Invention

According to an aspect of the present disclosure, it is possible to overcome the limitations of the prior art and provide a steel sheet having ultra-high strength having a tensile strength of 1500 MPa or more, while having excellent shape and bending properties.

The various and beneficial advantages and effects of the present disclosure are not limited to the above-described contents, and may be more easily understood through descriptions of specific embodiments of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph of a cross-section of a steel sheet in a thickness direction according to Inventive Example 1 of the present disclosure, captured using a scanning electron microscope (SEM).

BEST MODE FOR INVENTION

The terms used herein are for the purpose of describing the present invention and are not intended to limit the present invention. In addition, the singular forms used herein include the plural forms unless the relevant definition clearly indicates a meaning contrary thereto.

The term “comprising” as used in the specification means specifying a configuration, and does not exclude the presence or addition of other configurations.

Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as would be commonly understood by a person of ordinary skill in the art to which the present invention belongs. Terms defined in the dictionary are interpreted to have a meaning consistent with the relevant technical literature and the present disclosure.

Research on a steel sheet having high strength and high yield ratio has been continuously conducted to secure conventional collision performance, but the prior arts have had technical limitations, such as causing defects during forming or shape defects due to rapid cooling after annealing during the process.

In order to solve the above-described problems of the prior art, it is necessary to develop ultra-high strength cold-rolled and plated steel sheets having a tensile strength of 1500 MPa or more and having excellent shape and bending properties, but no technology has been developed until now to satisfy this demand.

Accordingly, the present inventors have conducted extensive research to solve the problems of the above-described prior art, and as a result thereof, have confirmed through experiments that the target properties can be secured when the components and operating conditions satisfy specific relationships, thereby completing the present invention. Accordingly, according to the present invention, in order to overcome the limitations of the prior art, it is possible to provide a steel sheet having ultra-high strength having a tensile strength of 1500 MPa or more, while having excellent shape and bending properties by optimizing the steel composition and manufacturing process.

Hereinafter, the present disclosure will be described in detail. First, an embodiment of a steel sheet according to the present disclosure will be described in detail.

An alloy composition of the steel sheet according to the present disclosure includes, by weight %, 0.1 to 0.3% of carbon (C), 0.5% or less (excluding 0%) of silicon (Si), 1.3 to 2.5% of manganese (Mn), 0.2% or less (excluding 0%) of chromium (Cr), 0.01 to 0.1% of molybdenum (Mo), 0.0005 to 0.003% of boron (B), 0.1% or less (excluding 0%) of phosphorous (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), 0.01 to 0.1% of aluminum (Al), 0.01 to 0.05% of niobium (Nb), 0.01 to 0.05% of titanium (Ti), and a balance of Fe and other unavoidable impurities. Unless specifically stated otherwise in the present invention, the content of each element is based on weight %.

Carbon (C): 0.1 to 0.3 wt %

Carbon (C) is an interstitial solid-solution element, the most effective and important element for improving strength of steel, and is an element which should be added to secure strength of martensite steel. In order to obtain ultra-high strength steel satisfying a yield ratio and tensile strength targeted in the present disclosure, it is preferable that carbon is added in an amount of 0.1% or more, and more preferably, carbon may be added in an amount of 0.15% or more. However, when a content of C exceeds 0.3%, martensite may be excessively formed during cooling due to increased hardenability, which may rapidly increase the strength and deteriorate elongation. In addition, since an increase in the content of C has the problem of impeding weldability, it is preferable to limit an upper limit of the content of C to 0.3%, and more preferably, the upper limit of the content of C may be 0.28% or less. Meanwhile, a lower limit of the content of C may be 0.2%, or the upper limit of the content of C may be 0.25%.

Silicon (Si): 0.5 wt % or Less (Excluding 0 wt %)

Silicon is known as a key element of Transformation Induced Plasticity (TRIP) steel, which acts to increase a fraction and elongation of retained austenite. In addition, in the present disclosure, the addition of Si is a factor of suppressing the occurrence of cracks during bending processing by suppressing the precipitation of cementite. Therefore, in order to obtain the above-described effect, an Si content of 0 wt % is excluded. However, if the content of Si exceeds 0.5%, not only will the weldability deteriorated, but the surface properties and plating properties of the steel sheet will also deteriorate, so the content of Si is included in an amount of 0.5 wt %. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Si may be 0.01%, or an upper limit of the content of Si may be 0.3%.

Manganese (Mn): 1.3 to 2.5 wt %

Manganese (Mn) is an element added to secure strength. If the content of Mn is less than 1.3%, hardenability is low, so when a cooling speed is not fast enough during cooling after annealing, martensite is not formed, making it difficult to secure a level of strength required in the present disclosure. On the other hand, when the content of Mn exceeds 2.5%, a Ms temperature is lowered during cooling after annealing, which lowers a final cooling temperature and thus the shape of the steel sheet becomes poor, and it also becomes difficult to secure an initial martensite structure. In addition, during steelmaking/continuous casting operations, a segregation zone occurs in a length direction of a Mn-based slab, which acts as a factor deteriorating bendability, and thus sets the upper limit thereof. That is, since manganese is segregated in a thickness direction, so that manganese bands within the slab is easily formed, which may cause a problem such as increased occurrence of defects during a rolling process along with cracks during continuous casting, it is preferable that the content of manganese (Mn) be 1.3 to 2.5%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Mn may be 1.5%, or an upper limit of the content of Mn may be 2.1%.

Chromium (Cr): 0.2 wt % or Less (Excluding 0 wt %)

Chromium (Cr) is an alloying element facilitating securing a low-temperature transformation structure by suppressing ferrite transformation, and when using a continuous annealing process with slow cooling as in the present disclosure, there is an advantage of suppressing ferrite formation. To obtain the above-described effect, a Cr content of 0 wt % is excluded. However, when the Cr content exceeds 0.2%, delayed fracture resistance may deteriorate. In addition, carbides such as CrC, or the like may be formed, to impair hole expandability and bending processability, and costs may increase due to excessive alloy input. Therefore, the Cr content is preferably in a range of 0.2% or less. Meanwhile, in terms of further improving the above-described effect, an upper limit of the Cr content may be 0.15% or 0.1%. However, there is no need to set a lower limit of the Cr content if it can be manufactured by optimizing the components and operating conditions, but as an example, it can be 0.01%.

Molybdenum (Mo): 0.01 to 0.1 wt %

Molybdenum (Mo) has an effect of improving quenchability of steel, an effect of generating fine carbides containing Mo, serving as a hydrogen trap site, and an effect of improving delayed fracture resistance by refining martensite. However, when a content of Mo exceeds 0.1%, However, when the content of Mo exceeds 0.1%, the effect is not significant compared to an increase in costs due to the addition of high-value alloying elements, so it is preferable to set an upper limit of the content of Mo to 0.1% or less. On the other hand, when the content of Mo is less than 0.01%, the basic characteristics of Mo are not expressed at all, so it was confirmed through experiments that there is no improvement effect in delayed fracture, and a lower limit of the content of Mo was set to 0.01% or more. Meanwhile, in terms of further improving the above-described effect, the lower limit of the content of Mo may be 0.012%, or it is more preferable that the upper limit of the content of Mo is 0.08%.

Boron (B): 0.0005 to 0.003 wt %

Boron (B) is an element that suppresses formation of ferrite, and accordingly, in the present disclosure, B has an advantage of suppressing the formation of ferrite during cooling after annealing. However, if the content of B exceeds 0.003%, ductility may be significantly decreased. On the other hand, when the content of B is less than 0.0005%, there is no hardening effect at all, so not only is the target strength not secured, but ferrite is formed on a surface layer, which tends to result in poor bending properties, thus limiting a lower limit of the B content. Meanwhile, in terms of further improving the above-described effect, the lower limit of the content of B may be 0.0008%, or an upper limit of the content of B may be 0.0022%.

Phosphorus (P): 0.1 wt % or Less (Excluding 0%)

Phosphorus (P) is an impurity element contained in steel, and the lower an amount of phosphorous (P) added in steel, the better, but considering the cases in which it is unavoidably included during the manufacturing process, a Cr content of 0 wt % is excluded. However, if a P content exceeds 0.1%, weldability deteriorates and there is a risk of steel brittleness, so an upper limit of the P content is limited to 0.1%. Meanwhile, in terms of further improving the above-described effect, a lower limit of the P content may be 0.0001%, or the upper limit of the P content may be 0.03%.

Sulfur (S): 0.01 wt % or Less (Excluding 0%)

Sulfur (S), like P, is an impurity inevitably contained in steel, which impairs ductility and weldability of a steel sheet. Thus, it is preferable to manage an S content to be as low as possible, and therefore, it is preferable to limit the S content to 0.01 wt % or less in the present disclosure. However, 0% is excluded considering cases in which it is inevitably included during the manufacturing process. Meanwhile, in terms of further improving the above-described effect, a lower limit of the S content may be. In addition, in order to further contribute to improving bendability by minimizing MnS precipitations in the steel, an upper limit of the S content may be 0.008% or 0.005%.

Nitrogen (N): 0.01 wt % or Less (Excluding 0%)

Nitrogen (N) is an impurity element, and if an N content exceeds 0.01%, a risk of cracks occurring during continuous casting due to AlN formation, or the like, is greatly increased, so it is preferable to limit an upper limit of the N content to 0.01%. However, 0% is excluded, considering cases in which it is unavoidably included during the manufacturing process. Meanwhile, in terms of further improving the above-described effect, a lower limit of the N content may be 0.0001%, or, more preferably, the upper limit of the N content is 0.008%, and even more preferably, 0.006%.

Aluminum (Al): 0.01 to 0.1 wt %

Aluminum (Al) may be added to remove oxygen in molten steel, and is an element effective in stabilizing residual austenite by suppressing precipitation of cementite during reheating and over-aging operations, as Si. If the Al content is less than 0.01%, deoxidation of a steel material may be insufficient and cleanliness of the steel material is impaired. On the other hand, if the content of Al exceeds 0.1%, not only will castability of a slab deteriorate, but a temperature required for heating a single-phase during annealing will also increase, which may cause production and facility problems. Meanwhile, in terms of further improving the above-described effect, a lower limit of the content of Al may be 0.02%, or an upper limit of the content of Al may be 0.05%.

Niobium (Nb): 0.01 to 0.05 wt %

Niobium (Nb) is an element that segregates at austenite grain boundaries and contributes to increased strength through precipitation strengthening by suppressing growth of austenite grains during annealing heat treatment. However, when the Nb content exceeds 0.05%, precipitation of carbides and nitrides which increases, reduces processability of a base material, and the cost increases as an alloy input amount becomes excessive. On the other hand, when the Nb content is less than 0.01%, it does not contribute to increasing the strength thereof at all, so a lower limit of the Nb content is limited to 0.01%. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Nb content may be 0.02%, or an upper limit of the Nb content may be 0.04%.

Titanium (Ti): 0.01 to 0.05 wt %

Titanium (Ti) is a nitride forming element, and is an element scavenging by precipitating N in steel into TiN. When Ti is not added, there is a possibility that cracks may occur during continuous casting due to AlN formation. However, if the Ti content exceeds 0.05%, the strength of martensite may be reduced by additional carbide precipitation in addition to removal of dissolved N, and hole expandability and bending processability may be impaired by the formation of carbides and nitrides such as TiC and TiN. On the other hand, when the Ti content is less than 0.01%, it does not contribute to increasing strength at all, similar to the Nb element, so a lower limit of the Ti content is set. Meanwhile, in terms of further improving the above-described effect, the lower limit of the Ti content may be 0.02%, or an upper limit of the Ti content may be 0.04%.

The remaining component of the present disclosure is iron (Fe). However, since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.

Hereinafter, the characteristics of the steel sheet according to the present disclosure are described.

A ratio (a/b×100) of a total content (a) of C and Mn in a region within 20 μm from a surface of the steel sheet, and a total content (b) of C and Mn at a point (¼)×t, where t is a total thickness of the steel sheet, from the surface is 75% or more (excluding 100%).

The inventors have repeatedly conducted extensive research, and as a result thereof, have discovered the tendency for the bending properties to be improved when the total content of C and Mn in a region within 20 μm from the surface of the steel sheet satisfies a specific ratio relative to a point (¼)×t, where t is a total thickness of the steel sheet, from the surface of the steel sheet (in a thickness direction of the steel sheet), thereby completing the present disclosure. Therefore, according to the present disclosure, if the ratio of the total content of C and Mn in the region within 20 μm from the surface of the steel sheet to a point of (¼)×t, where t is a total thickness of the steel sheet, from the surface is less than 75%, ferrite or bainite may be excessively formed in the form of clusters on the surface layer portion, and cracks may occur at a boundary between the phases of ferrite and martensite, resulting in reduced bendability.

In order to further improve the above-described effect, a lower limit of the ratio (a/b) may be 78%, or an upper limit of the ratio (a/b) may be 87%.

In addition, according to an embodiment of the present disclosure, the steel sheet may include in area %, 10% or less (excluding 0%) of one or more phases selected from the group consisting of ferrite and bainite, as a microstructure. In the region within 20 μm from the surface, if a ratio (A) of one or more phases selected from the group consisting of ferrite and bainite exceeds 10%, ferrite or bainite, which are soft phases, may be excessively formed around martensite, a hard phase, causing cracks to occur when bending. In this case, a lower limit of the ratio (A) is not particularly limited, but it is advantageous if it can be managed as low as possible within a practically manufacturable range.

In this case, according to an embodiment of the present disclosure, the steel sheet may have a balance of martensite, other than ferrite and bainite, described above, as a microstructure, in the region within 20 μm from the surface.

According to an embodiment of the present disclosure, the steel sheet may include, in area %, 90 to 99% of martensite, as a microstructure, in a region within 20 μm from the surface. Alternatively, as a more preferred range, in a region within 20 μm from the surface, a lower limit of a fraction of martensite may be, in area %, 95%, or in a region within 20 μm from the surface, an upper limit of the fraction of martensite in area %, may be 98%.

According to an embodiment of the present disclosure, the total thickness, t may be 0.6 to 2.5 mm.

Meanwhile, the steel sheet of the present disclosure may further include a plating layer. The plating layer is not particularly limited, so not only the type of plating such as zinc-based plating, aluminum-based plating, or the like, but also the method of plating such as hot-dip plating, electroplating, or the like, are not limited. That is, it is sufficient if it can be used in the technical field to which the present disclosure belongs. However, as a preferred example of the present disclosure, the plating layer may be a zinc-based plating layer.

According to an embodiment of the present disclosure, an ultra-high strength steel sheet having a tensile strength (TS) of 1500 MPa or more, and excellent bending properties having bendability (R/t) of 3.7 or less, may be provided.

In addition, according to an embodiment of the present disclosure, a steel sheet having a tensile strength (TS) of 1500 MPa or more, an elongation (El) of 3% or more (or, a more preferable range of elongation is 5% or more, and in particular, an upper limit thereof is not calculated because it is advantageous as the upper limit is higher), and a bendability (R/t) of 3.7 or less, may be provided.

Next, a preferred method for manufacturing a steel sheet according to the present disclosure will be described.

First, a steel slab satisfying the above-described composition is heated to a temperature within a range of 1100 to 1300° C. The present process is performed to smoothly perform a subsequent hot rolling process, and sufficiently obtain target properties of the steel sheet. In this case, if the reheating temperature is lower than 1100° C., a problem in which a hot rolling load increases rapidly occurs. On the other hand, if the reheating temperature exceeds 1300° C., an amount of surface scales increases, and yield of a material decreases, so it is limited.

The reheated slab is hot rolled. In this case, hot rolling may be performed at a temperature within a range of Ar3 to 1000° C. A final hot rolling temperature of the reheated slab is limited to Ar3 (a temperature at which ferrite begins to appear when austenite is cooled) or higher, limited because, below Ar3, rolling in a dual phase region of ferrite and austenite or ferrite region is performed to create a mixed structure, and there may be concerns about malfunctions due to fluctuations in the hot rolling load.

Next, the hot-rolled steel sheet is coiled at a temperature within a range of 400 to 600° C. When a coiling temperature exceeds 600° C., an oxide film may be excessively generated on a surface of the steel sheet, which may cause defects, and surface properties of a plating material may deteriorate, so an upper limit thereof is limited. In addition, it is preferable to maintain a low coiling temperature to secure material uniformity over an entire length and entire width by forming a single-phase structure rather than a composite structure as much as possible in a hot-rolled steel sheet. However, as the coiling temperature decreases, the strength of the hot-rolled steel sheet increases, which increases a rolling load of cold rolling, a subsequent process, and there may be a factor that makes actual production impossible, so a lower limit of the coiling temperature is limited to 400° C. or higher. Meanwhile, more preferably, the lower limit of the coiling temperature may be 420° C., an upper limit of the coiling temperature may be 520° C., and water cooling treatment cooling may be performed after coiling.

Next, an oxide layer formed on a surface of the hot-rolled steel sheet coiled after the hot rolling is removed through a pickling process, and then cold rolling is performed at a reduction ratio of 30 to 80%. If the reduction ratio of the cold rolling is less than 30%, not only is it difficult to secure a target thickness, but there is also a concern that residual hot-rolled grains may affect austenite formation and final properties during annealing heat treatment. In addition, if the reduction ratio of the cold rolling exceeds 80%, there is a problem that material deviation of a final steel sheet may occur due to an uneven amount of reduction rolled in the length and width directions from the work hardening occurring during cold rolling, and it may be difficult to secure the target thickness due to a rolling load.

After cold rolling, heat treatment is performed in an annealing temperature within a range of Ac3+10° C. to Ac3+80° C. for 30 seconds or more. Since an Ac3 temperature varies depending on the component, it is determined by Formula 1 below. When the annealing temperature is lower than Ac3+10° C., a mixed grain structure may be formed by dual-phase annealing than rather single-phase annealing throughout an entire coil length, which has a detrimental effect on the material. Therefore, the lower limit is defined as Ac3+10° C. On the other hand, if the annealing temperature exceeds Ac3+80° C., facility troubles may occur due to overload of an annealing furnace, so the upper limit is set to Ac3+80° C. Meanwhile, more preferably, a lower limit of the annealing temperature may be 823° C., or an upper limit of the annealing temperature may be 916° C.

Ac ⁢ 3 = 910 - 203 ⁢ √ [ C ] - 15.2 [ Ni ] + 44.7 [ Si ] + 104 [ V ] + 31.5 [ Mo ] + 13.1 [ W ] [ Formula ⁢ 1 ]

In Formula 1 above, [C], [Ni], [Si], [V], [Mo] and [W] represent a weight % content of each element in parentheses.

According to an embodiment of the present disclosure, the heat treatment during the annealing may be performed (maintained) for 30 seconds or more in a temperature within a range of Ac3+10° C. to Ac3+80° C. If the heat treatment time at a temperature within a range of Ac3+10° C. to Ac3+80° C. is less than 30 seconds, there may be a problem that the structure is not heat treated sufficiently in a single phase, making it difficult to ultimately secure a martensite structure.

Next, the annealed steel sheet is subjected to primary cooling at an average cooling rate of 10° C./s or less (exceeding 0° C./s) to a temperature within a range of 680 to 749° C., a primary cooling end temperature range. When the primary cooling end temperature is lower than 680° C., according to the experimental results, in a surface layer portion in a region within 20 μm from the surface, a ratio of other phase structures (one or more phases selected from the group consisting of ferrite and bainite) other than martensite structures exceeds 10% in area %, and/or when a total content (a) of C and Mn in a region within 20 μm from the surface is compared with a total content (b) of C and Mn at a point (¼)×t from the surface, where t is a total thickness of the steel sheet, the ratio (a/b) is less than 75%, which may result in a bendability (R/t) evaluation value exceeding 3.7, which may deteriorate the formability. On the other hand, if the primary cooling end temperature exceeds 749° C., not only can it not be reproduced in terms of facility configuration, but the structure may be coarsened and the strength may deteriorate, so the primary cooling end temperature is limited to 680 to 749° C. Meanwhile, more preferably, a lower limit of the primary cooling end temperature may be 700° C., or an upper limit of the primary cooling end temperature may be 730° C.

In addition, when an average cooling rate of the primary cooling exceeds 10° C./s, problems may occur with the shape of the plate, so an upper limit of the average cooling rate of the primary cooling is set to 10° C./s. In addition, a lower limit of the average cooling rate of primary cooling may not be set separately because it may be as low as possible due to the specific facility configuration, and is therefore set to exceed 0° C./s (or 1° C./s or more). Meanwhile, more preferably, the lower limit of the average cooling rate of the primary cooling may be 4.3° C./s, or the upper limit of the average cooling rate of the primary cooling may be 7.7° C./s.

Next, the primarily-cooled steel sheet is subjected to secondary cooling (rapid cooling) at an average cooling rate of 60 to 160° C./s from 100° C. to Mf. In this case, Mf refers to a martensite transformation finish temperature (Mf) and is measured using a dilatometer.

In order to secure the level of strength required in the present disclosure, it is preferable to maintain rapid cooling conditions during the secondary cooling. Specifically, when an average cooling rate of the secondary cooling is less than 60° C./s, some bainite structures may be formed during cooling, making it difficult to secure the target strength. On the other hand, if the average cooling rate of the secondary cooling exceeds 160° C./s, problems of shape deterioration and material deviation in the width direction of the steel sheet may occur due to a rapid martensite transformation rate at the time of the secondary cooling. Meanwhile, in order to further improve the above-described effect, a lower limit of the average cooling rate of the secondary cooling may be 80° C./s, or an upper limit of the average cooling rate of the secondary cooling may be 153° C./s.

In addition, a cooling end temperature of the secondary cooling is 100° C. to the Mf temperature, and if the cooling end temperature of the secondary cooling exceeds the Mf temperature, martensite transformation may not occur sufficiently, making it difficult to secure the microstructure desired by the present disclosure. On the other hand, when the cooling end temperature of the secondary cooling is less than 100° C., it is disadvantageous in terms of shape because cooling is performed at too low temperatures, and exceeds the manufacturing process range of the facility, so the lower limit of the cooling end temperature of the secondary cooling is limited to 100° C. Meanwhile, in order to further improve the above-described effect, the lower limit of the cooling end temperature of the secondary cooling may be 106° C., or the upper limit of the cooling end temperature of the secondary cooling may be 152° C.

Although not particularly limited, a method for manufacturing a steel sheet according to an embodiment of the present disclosure may satisfy the following Relational Expression 1. The present inventors have repeatedly conducted extensive research, and as a result thereof, the inventors have additionally confirmed that a steel sheet having excellent properties such as strength and bendability may be secured by satisfying a specific relationship, such as the following Relational Expression 1, between cooling end temperatures during primary and secondary cooling.

4.5 ≤ T ⁢ 1 / T ⁢ 2 ≤ 7 [ Relational ⁢ Expression ⁢ 1 ]

In Relational Expression 1 above, T1 is a cooling end temperature during primary cooling (° C.), and T2 is a cooling end temperature during secondary cooling (° C.).

Next, according to an embodiment of the present disclosure, the secondarily-cooled steel sheet is reheated at a temperature within a range of 150 to 240° C. to perform an over-aging heat treatment. Yield strength may be increased by transforming martensite obtained through the secondary cooling process of the above-described rapid cooling into tempered martensite through the reheating and over-aging heat treatment. When the over-temperature heat treatment temperature is lower than 150° C., there is a disadvantage in that tempering is not sufficiently performed, yield strength is low, and sufficient toughness cannot be secured. On the other hand, when the over-treatment temperature exceeds 240° C., there is a disadvantage in that bending processability becomes poor due to the precipitation and coarsening of large amounts of carbides.

Meanwhile, the lower a lower limit of the over-aging heat treatment temperature, the more advantageous the bendability is, but considering the facility characteristics, the lower limit of the over-aging heat treatment temperature is recommended to be 150° C. or higher, and more preferably, it may be 170° C. or higher. In addition, an upper limit of the over-aging heat treatment temperature is preferably 200° C., and more preferably 198° C.

Although not particularly limited, a method for manufacturing a steel sheet according to an embodiment of the present disclosure may satisfy the following Relational Expression 2. The present inventors have repeatedly conducted extensive research, and as a result thereof, the inventors have additionally found that a steel sheet having more excellent properties, such as strength and bendability, can be secured by satisfying a specific relationship, such as the following Relational Expression 2, between a cooling end temperature during secondary cooling and an over-aging heat treatment temperature.

1.2 ≤ T ⁢ 3 / T ⁢ 2 ≤ 1.8 [ Relational ⁢ Expression ⁢ 2 ]

In Relational Expression 2 above, T2 is a cooling end temperature of secondary cooling (° C.), and T3 is a temperature of over-aging heat treatment (° C.).

According to an embodiment of the present disclosure, the over-aging heat treatment may be performed for 400 seconds or longer. Here, the over-aging heat treatment time means a time maintained within an over-aging heat treatment temperature range.

When the over-aging heat treatment time is less than 400 seconds, tempering may not be sufficiently performed, which may result in lower yield strength. On the other hand, there is no specific limitation on an upper limit of the over-aging heat treatment time, but it is difficult to exceed 1000 seconds due to the characteristics of continuous annealing facility. Accordingly, the over-aging heat treatment time may be performed for 400 to 1000 seconds, and in terms of further improving the above-described effect, a lower limit of the over-aging heat treatment time may be 428 seconds, or the upper limit of the over-aging heat treatment time may be 600 seconds.

Next, if necessary, temper rolling or tension levelling may be passed to improve the plate shape.

In addition, if necessary, the method for manufacturing a steel sheet may further include an operation of forming a plating layer on a surface of the steel sheet. The plating may be performed by a hot-dip plating method in which a plating bath is installed and the steel sheet is dipped in a molten plating solution, or by an electroplating method in an electrolyte after annealing is completed. The plating conditions are not particularly limited as long as the conditions are generally known in the technical field to which the present disclosure belongs.

According to the manufacturing method described above, it is possible to effectively obtain an ultra-high strength steel sheet having an ultra-high strength having a tensile strength of 1500 MPa or more and excellent shape and bendability with bendability (R/t) of 3.7 or less.

Mode for Invention

Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following examples are only for describing the present disclosure by illustration, and not intended to limit the right scope of the present disclosure. The reason is that the right scope of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.

EXAMPLE

Molten steel having the alloy composition shown in Table 1 below was cast into an ingot and then sized and rolled to prepare a steel slab. The steel slab was heated to a temperature of 1200° C., maintained for 1 hour, and then was subjected to finish hot rolling at a temperature of 900° C., charged into a heated furnace, which set with various conditions, maintained for 1 hour, and then furnace-cooled to simulate hot rolling. After pickling the hot-rolled steel sheet prepared as described above, cold rolling was performed at a cold rolling reduction rate of 50%, and then was subjected to annealing heat treatment, primary cooling (slow cooling), secondary cooling (rapid cooling), reheating, and overaging heat treatment under the conditions shown in Table 2 below to form a cold-rolled steel sheet and then electrogalvanized under normal conditions.

For the cold-rolled steel sheet prepared as described above, a microstructure thereof was evaluated through an optical microscope and SEM structure observation. In particular, the structure observation of a surface layer portion corresponding to a region within 20 μm of the surface was performed using a 3000×SEM structure, and a fraction for each structure phase was obtained by analyzing an area ratio through image analysis of each phase, and an average value through three analyses was set to be a representative value.

In addition, quantitative analysis was performed using a microanalyzer (FE-EPMA) with a TEM device to measure a total content ratio of each C and Mn at a point (¼)×t from the surface and in a region within 20 μm from the surface.

That is, local concentrations of C and Mn was obtained by quantitatively analyzing a concentration ratio (%) of C and Mn for each of 5 points within a diameter of approximately 20 μm, and an arithmetic mean thereof was used as a representative value. In addition, in order to compare the overall concentration ratio of C and Mn, an average total content ratio of C and Mn in the area within 20 μm of the surface relative to the point (¼)×t from the surface was accurately evaluated using a line profile technique for relative comparison.

A tensile strength (TS) and yield strength (YS) were measured by collecting a tensile test sample of a size of JIS No. 5 in a direction perpendicular to a rolling direction and then performing a tensile test at a strain rate of 0.01/s.

For R/t (bending properties, a cold-rolled steel sheet was processed into a sample having a width of 100 mm*a length of 30 mm, then was subjected to a 90° bending test at a test speed of 100 mm/min, and then checking for cracks in a bent portion using a microscope to improve the reliability of the results.

Flatness was measured by scanning the shape using a 3D scanner technique over the entire width after cutting the steel sheet in 200 mm in a longitudinal direction and then measuring the flatness by section. In general, in the present disclosure, a value for the flatness of 3 mm or less is determined to be satisfactory.

Table 1 below shows a range of components used to manufacture Inventive Steel and Comparative Steel, and Table 2 summarizes each operating condition of Inventive Steel and Comparative Steel. In Inventive Steel, operating conditions, which are outside the scope of the present disclosure, are marked with *, and in Comparative Example, cases which are outside the scope of the present disclosure are also marked with *.

TABLE 1
Division	C	Si	Mn	Cr	Mo	B	P	S	N	S•Al	Nb	Ti	Ac3

Inventive	0.15	0.1	1.7	0.1	0.015	0.002	0.008	0.001	0.003	0.03	0.02	0.03	836
Steel 1
Inventive	0.18	0.1	1.8	0.1	0.012	0.0018	0.007	0.0021	0.004	0.03	0.03	0.023	829
Steel 2
Inventive	0.21	0.2	1.5	0.1	0.03	0.0022	0.006	0.0023	0.002	0.025	0.025	0.03	827
Steel 3
Inventive	0.24	0.1	1.9	0.08	0.02	0.001	0.008	0.0022	0.005	0.04	0.03	0.04	816
Steel 4
Inventive	0.28	0.2	2.1	0.15	0.04	0.0008	0.005	0.0012	0.006	0.02	0.04	0.02	813
Steel 5
Inventive	0.22	0.2	1.9	0.12	0.08	0.0021	0.006	0.008	0.006	0.03	0.03	0.023	826
Steel 6
Comparative	0.31	0.2	0.8	0.2	0.2	0.0008	0.007	0.007	0.004	0.04	—	0.03	812
Steel 1
Comparative	0.26	0.3	2.7	0.1	0.18	0.0021	0.008	0.006	0.005	0.03	0.003	0.025	826
Steel 2

TABLE 2
								Over-	Over-
				Primarily		Secondary	Secondary	aging	aging
				cooling	Primarily	cooling	rapid	heat	heat
	Steel	Coiling	Annealing	end	cooling	end	cooling	treatment	treatment
	type	temperature	temperature	temperature	rate	temperature	rate	temperature	time
Division	No.	(° C.)	(° C.)	(° C.)	(° C./s)	(° C.)	(° C./s)	(° C.)	(sec.)
Inventive	Inventive	450	852	712	4.5	121	80	180	452
Example 1	Steel 1
Inventive	Inventive	452	854	705	5.2	118	85	182	456
Example 2	Steel 1
Comparative	Inventive	451	848	721	4.7	70 *	200 *	170	350
Example 1	Steel 2
Comparative	Inventive	460	853	652 *	2.3	150	120	187	455
Example 2	Steel 2
Inventive	Inventive	520	862	706	7.7	152	121	186	435
Example 3	Steel 3
Inventive	Inventive	478	852	712	4.8	118	108	178	428
Example 4	Steel 3
Inventive	Inventive	430	855	723	4.6	122	105	182	480
Example 5	Steel 4
Comparative	Inventive	452	800 *	705	4.5	108	151	186	501
Example 3	Steel 4
Inventive	Inventive	455	849	712	4.3	106	153	183	450
Example 6	Steel 5
Comparative	Inventive	640 *	862	715	4.8	240 *	32 *	350 *	482
Example 4	Steel 5
Inventive	Inventive	462	846	721	6.1	113	95	198	485
Example 7	Steel 6
Inventive	Inventive	455	862	716	6.8	121	92	186	495
Example 8	Steel 6
Comparative	Comparative	550	845	715	3.2	350 *	55 *	278 *	496
Example 5	Steel 1
Comparative	Comparative	526	861	713	4.3	260 *	86	215	520
Example 6	Steel 1
Comparative	Comparative	530	855	650 *	4.5	112	95	186	512
Example 7	Steel 2
Comparative	Comparative	540	849	713	4.2	115	105	182	532
Example 8	Steel 2

TABLE 3
						Flatness
	YS	TS	Elongation	Marked	Marked	{circle around (3)}	Bendability
Division	(MPa)	(MPa)	(%)	{circle around (1)}	{circle around (2)}	(mm)	(R/t)
Inventive	1180	1502	7.1	8.2	82	2.1	3.5
Example 1
Inventive	1210	1520	7.2	5.3	87	2.2	3.5
Example 2
Comparative	1245	1510	6.5	6.5	72 *	8.7 *	4.7 *
Example 1
Comparative	1246	1520	6.8	12.3 *	70 *	1.8	4.3 *
Example 2
Inventive	1282	1520	6.5	5.6	82	1.9	3.4
Example 3
Inventive	1256	1562	7.2	6.5	86	1.8	3.5
Example 4
Inventive	1235	1582	7.1	7.8	78	1.6	3.5
Example 5
Comparative	1241	1590	7.5	11.5 *	68	1.5	5.5 *
Example 3
Inventive	1231	1543	6.8	4.5	81	2.2	3.5
Example 6
Comparative	1210	1556	6.2	8.7	62	2.3	4.8 *
Example 4
Inventive	1260	1550	6.6	6.5	79	2.4	3.5
Example 7
Inventive	1245	1549	6.3	6.6	81	2.1	3.6
Example 8
Comparative	1021	1352 *	6.5	12.5 *	68 *	2.7	4.5 *
Example 5
Comparative	1032	1401 *	5.6	12.3 *	65 *	2.5	4.6 *
Example 6
Comparative	1265	1596	5.8.	14.1 *	71*	2.9	4.8 *
Example 7
Comparative	1256	1586	6.5	10.7 *	65 *	2.3	4.3 *
Example 8

Meanwhile, in Table 3 above, {circle around (1)} to {circle around (3)} represent the following values.

{circle around (1)} Area ratio (%) of one or more phases selected from the group consisting of ferrite and bainite, in a region within 20 μm of the surface

{circle around (2)} Ratio (a/b×100) of a total content (a) of C and Mn to a total content (b) of C and Mn at a point (¼)×t, where t is a total thickness of the steel sheet from the surface, in a region within 20 μm of the surface

As can be seen from Table 1, in Inventive Examples of the present disclosure, it was confirmed that a tensile strength is 1500 MPa or more, bendability (R/t) is 3.7 or less, and flatness is 3 mm or less, so that the steel sheet has ultra-high strength and excellent shape and bendability.

In particular, FIG. 1 illustrates a photograph of a cross-section of the steel sheet in a thickness direction according to Inventive example 1 of the present disclosure, taken using a scanning electron microscope (SEM).

On the other hand, in the case of Comparative examples of the present disclosure, it was confirmed that one or more of the above-described characteristics such as strength, shape, and bendability were inferior because they did not meet the conditions required by the present disclosure.

Specifically, in Comparative examples 1 and 2, when the first and second cooling end temperatures and cooling rates are outside, the flatness or bendability is outside the range required in the present disclosure, and it can be seen that the bendability is inferior because a ratio of one or more mixed grain structures selected s from the group consisting of ferrite and bainite other than a martensite structure within a surface layer of 20 μm exceeds 10%, or the total content ratio of C and Mn is outside the required range of the present disclosure.

In Comparative examples 3 and 4, it can be seen that the bendability is also inferior under the manufacturing conditions in which a coiling temperature and annealing temperature are outside the range of the present disclosure, and in Comparative examples 5 to 8, it can be seen that the steel component is outside the target range and bendability are also inferior.

That is, by controlling the components and operating conditions required in the present disclosure, a 1500 MPa grade annealed and electrogalvanized steel sheet having the target shape (flatness) and excellent bendability may be prepared as in the case of Inventive Example.