Steel sheet and manufacturing method thereof

Abstract

A steel sheet according to the present invention comprises, in weight%: 0.008-0.015% of C, 0.200% or less of Si; 1.30-2.00% of Mn; 0.5-1.0% of Cr; 0.030% or less of P; 0.010% or less of S; 0.0020-0.0080% of N; and 0.010-0.060% of Al, the remainder being Fe and inevitable impurities, wherein the value of K defined in relational expression 1 is 0 to 15.000, and the microstructure may include, by area%, 0.30-0.80% of a transformed structure, the remainder being ferrite. [Relational expression 1] K = - 651[C] - 2.42[Mn] + 25.7[Cr] - 220[N] (In the formula, [C], [Mn], [Cr] and [N] are the weight% of the respective elements.)

Description

Technical Field

[0001] The present disclosure relates to a steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet having excellent low-temperature bake-hardenability and room-temperature aging resistance, and a method for manufacturing the same.Background Art

[0002] In recent years, a reduction in the thickness of steel sheets through an increase in strength has been continuously demanded for the purpose of weight reduction to improve the fuel efficiency of automobiles. Bake hardenable steel is known as the steel material most suitable for such properties as an exterior panel material. The bake hardening phenomenon refers to a phenomenon in which, during paint baking, activated solid solution carbon and nitrogen are fixed to dislocations generated during pressing, thereby increasing yield strength. Steel having excellent bake hardenability is easily formable before paint baking, and exhibits improved dent resistance in a final product, thereby achieving both excellent formability and high strength.

[0003] However, since bake hardenable steel may undergo aging deterioration, such as the occurrence of yield point elongation, when maintained at room temperature for a long period of time due to solid solution elements contained in the steel, a certain level of room-temperature aging resistance is required so that the steel may withstand aging for a predetermined period of time.

[0004] In general, as a method for manufacturing a cold-rolled steel sheet having bake hardenability, a low-carbon P-added Al-killed steel has mainly been used, in which the steel is simply coiled at a low temperature, that is, by low-temperature coiling at a hot-rolling coiling temperature in a range of 400 to 500°C, and then subjected to a batch annealing method, thereby exhibiting a bake-hardening amount of about 40 to 50 MPa. This is because the coexistence of formability and bake hardenability may be more easily achieved by batch annealing. In the case of a P-added Al-killed steel obtained by a continuous annealing method, bake hardenability may be easily ensured because a relatively high cooling rate is employed; however, there may be a problem in that formability may deteriorate due to rapid heating and short-time annealing, and thus use thereof is limited to automobile exterior panels where workability is not required.

[0005] In accordance with the remarkable advances in steelmaking technology in recent years, it has become possible to control an appropriate amount of solid solution elements in steel, and bake hardenable cold-rolled steel sheets having excellent formability have been manufactured by using Al-killed steel sheets to which strong carbonitride-forming elements such as Ti or Nb are added, resulting in an increasing use of such bake hardenable cold-rolled steel sheets as automobile exterior panel materials requiring dent resistance.

[0006] In recent years, some automobile manufacturers have been considering lowering the bake-hardening temperature after painting as a measure for cost reduction and CO 2 emission reduction. In particular, the number of automobile manufacturers using non-ferrous lightweight materials such as Al or plastic (or CFRP) instead of steel for automobile exterior panels has gradually increased in order to achieve weight reduction of automobiles. To date, steel and non-ferrous materials have been separately press-formed, painted, and baked, and the parts made of the two materials have been assembled in a final stage; however, attempts to improve the efficiency of production processes, reduce energy costs, and help to protect the environment by assembling steel and non-ferrous materials after press forming and performing baking at the same temperature have been increasing, and in such cases, there has been an increasing demand to lower the baking temperature to 120°C or lower, or even to 100°C, in consideration of the curing temperature of non-ferrous materials such as plastic. Such low-temperature baking causes a drastic decrease in the bake-hardening value (BH value) that is conventionally obtained at 170°C, resulting in a problem in that appropriate dent resistance, the purpose of adopting bake hardenable steel, cannot be ensured.

[0007] Conventionally, when a bake-hardening temperature of a steel sheet is lowered, the amount of fixed solid solution carbon and nitrogen decreases, and the time required for fixation is relatively prolonged, resulting in a decrease in bake hardenability. In general, in the bake hardening phenomenon, a pre-strain of several percent is applied, and then heat treatment is performed at 170°C for 20 minutes after painting. As a result, the hardening amount is required to be at least 30 MPa. Accordingly, in order to lower the bake-hardening temperature while ensuring bake hardenability at an appropriate level or higher, it is necessary to maximize bake hardenability at a high temperature; however, when the bake hardenability of a steel sheet increases beyond a certain level, there is a problem in that the aging resistance of the steel sheet deteriorates, thereby increasing the possibility of surface defects occurring during part forming. Therefore, the most desirable condition is to control the difference between the bake-hardening amount obtained in heat treatment at 170°C, which corresponds to a conventional baking, and the BH value obtained in baking at a low temperature, that is, at 100°C, to be small. However, considering that solid solution elements in steel exponentially depend on temperature, it is significantly difficult to reduce the difference in BH values between a low temperature of 100°C and 170°C.

[0008] In order to obtain appropriate bake hardenable steel at a low temperature and to simultaneously ensure bake hardenability superior to that under the conventional condition of 170°C and corresponding aging resistance, various methods have been proposed.

[0009] For example, Patent Documents 1 to 3 propose a method of suppressing room-temperature aging deterioration by increasing the reduction ratio in temper rolling.

[0010] However, since cold-rolled steel sheets manufactured by such a technique are intended to improve aging resistance by introducing mobile dislocations into the steel sheet, the amount of deformation required for achieving high BH significantly increases. In order to increase the amount of deformation, it is necessary to increase the reduction ratio in temper rolling; however, in high-strength steel sheets, there is a limit to the temper rolling reduction ratio that can be applied, and it becomes practically difficult to manufacture such steel sheets in a continuous line.

[0011] On the other hand, Patent Document 4 discloses a method of controlling the distribution of iron carbide precipitates as a technique for improving low-temperature bake-hardenability. This technique is a hardening technique utilizing precipitation strengthening at a low temperature, and describes a concept different from a technique in which an increase in yield strength, which is necessary for ensuring dent resistance, is obtained by the fixation of solid solution carbon and nitrogen to dislocations.[Related Art Document][Patent Documents]

[0012] (Patent Document 1) Japanese Patent Laid-Open Publication No. H7-75803 (Patent Document 2) Japanese Patent Laid-Open Publication No. 2001-140038 (Patent Document 3) Japanese Patent Laid-Open Publication No. 2001-200337 (Patent Document 4) Japanese Patent Laid-Open Publication No. H6-73498 Summary of InventionTechnical Problem

[0013] An aspect of the present disclosure is to provide a steel sheet and a method for manufacturing the same.

[0014] Another aspect of the present disclosure is to provide a steel sheet having excellent low-temperature bake-hardenability and room-temperature aging resistance, and a method for manufacturing the same.

[0015] An object of the present disclosure is not limited to the above description. Those skilled in the art to which the present disclosure pertains may have no difficulty in understanding an additional object of the present disclosure from the entire contents of the present specification.Solution to Problem

[0016] According to an aspect of the present disclosure, a steel sheet includes, by wt%, 0.008 to 0.015% of C, 0.200% or less of Si, 1.30 to 2.00% of Mn, 0.5 to 1.0% of Cr, 0.030% or less of P, 0.010% or less of S, 0.0020 to 0.0080% of N, 0.010 to 0.060% of Al, and the balance of Fe and inevitable impurities, wherein a K value defined by the following Relational Expression 1 is 0 to 15.000, and a microstructure includes, by area%, 0.30 to 0.80% of a transformed structure and a balance of ferrite. K=−651C−2.42Mn+25.7Cr−220N

[0017] (In the expression, [C], [Mn], [Cr], and [N] represent wt% of the respective elements.)

[0018] The transformed structure may include martensite, bainite, and ferritic bainite.

[0019] The steel sheet may have a tensile strength of 340 MPa or more, a yield strength of 180 to 250 MPa, and an elongation of 34.0% or more.

[0020] The steel sheet may have a bake-hardening amount of 30.0 MPa or more after heat treatment at 100°C for 20 minutes, and may have a difference of 20.0 MPa or less between the bake-hardening amount after heat treatment at 100°C for 20 minutes and the bake-hardening amount after heat treatment at 170°C for 20 minutes.

[0021] The steel sheet may have a yield point elongation (AI) of 0.20% or less after heat treatment at 100°C for 1 hour.

[0022] According to another aspect of the present disclosure, a method for manufacturing a steel sheet includes: reheating a steel slab including, by wt%, 0.008 to 0.015% of C, 0.200% or less of Si, 1.30 to 2.00% of Mn, 0.5 to 1.0% of Cr, 0.030% or less of P, 0.010% or less of S, 0.0020 to 0.0080% of N, 0.010 to 0.060% of Al, and the balance of Fe and inevitable impurities, the steel slab having a K value of 0 to 15.000 defined by the following Relational Expression 1; performing finish hot rolling on the reheated steel slab; cooling the finish hot-rolled steel sheet to a temperature range of 550 to 700°C and then coiling the cooled steel sheet; cold rolling the coiled steel sheet; and continuously annealing the cold-rolled steel sheet in a temperature range of 760 to 830°C. K = − 651 C − 2.42 Mn + 25.7 Cr − 220 N

[0023] (In the expression, [C], [Mn], [Cr], and [N] represent wt% of the respective elements.)

[0024] The reheating may be performed in a temperature range of 1,100 to 1,250°C, the finish hot rolling may be performed in a temperature range of 880°C or higher, and the cold rolling may be performed at a reduction ratio of 60 to 90%.

[0025] The method may further include performing pickling before the cold rolling.

[0026] The method may further include, after the continuous annealing, immersing the continuously annealed steel sheet in a zinc plating bath having a temperature range of 440 to 500°C to perform plating.

[0027] The method may further include subjecting the plated steel sheet to an alloying treatment in a temperature range of 450 to 540°C.

[0028] The method may further include performing temper rolling on the plated steel sheet using a skin-pass roll having a surface roughness (Ra) of 1.0 to 1.6 µm at a temper rolling reduction ratio of 0.5 to 1.5%.Advantageous Effects of Invention

[0029] According to an aspect of the present disclosure, a steel sheet and a method for manufacturing the same may be provided.

[0030] According to another aspect of the present disclosure, a steel sheet having excellent low-temperature bake-hardenability and room-temperature aging resistance and a method for manufacturing the same may be provided.

[0031] According to still another aspect of the present disclosure, it is possible to provide a cold-rolled steel sheet and a plated steel sheet, which have excellent low-temperature bake-hardenability and room-temperature aging resistance, may be used as materials for automobile exterior panels, and exhibit excellent properties in press forming, drawing, and the like, and methods for manufacturing the same.

[0032] According to still another aspect of the present disclosure, it is possible to provide a steel sheet that enables cost reduction and CO 2 reduction due to a low bake-hardening temperature and allows simultaneous baking after painting with non-ferrous lightweight materials such as plastic having a low curing temperature due to a low baking temperature, thereby improving efficiency in automobile body manufacturing processes, and a method for manufacturing the same.Best Mode for Invention

[0033] Hereinafter, preferred example embodiments of the present disclosure will be described. The example embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the example embodiments described below. These example embodiments are provided to enable those skilled in the art to more fully understand the present disclosure.

[0034] The present inventors have made efforts to develop a steel material capable of ensuring appropriate bake hardenability even at a low temperature for simultaneous application with non-ferrous lightweight materials as automobile exterior panel materials, and have developed a steel material that may ensure bake hardenability of 30 MPa or more even when the paint baking temperature is lowered to 100°C in consideration of the curing temperature of materials such as plastic, thereby completing the present disclosure.

[0035] Hereinafter, the present disclosure will be described in detail.

[0036] A steel sheet according to an example embodiment of the present disclosure may include, by wt%, 0.008 to 0.015% of C, 0.2% or less of Si, 1.3 to 2.0% of Mn, 0.5 to 1.0% of Cr, 0.03% or less of P, 0.01% or less of S, 0.002 to 0.008% of N, 0.01 to 0.06% of Al, and the balance of Fe and inevitable impurities.

[0037] Hereinafter, the steel composition of the present disclosure will be described in detail.

[0038] In the present disclosure, unless otherwise particularly specified, "%" represents the content of each element on a weight basis.Carbon (C): 0.008 to 0.015%

[0039] Carbon (C), which is an interstitial solid solution element, effectively contributes to ensuring the strength of steel. In addition, since carbon increases the hardenability of steel and is an important element for ensuring a fraction of martensite, the amount of the carbon (C) added needs to be above a certain level in order to ensure a certain amount of the transformed structure targeted in the present disclosure. To this end, the lower limit of the amount of carbon (C) added may be limited to 0.008%. However, when carbon (C) is excessively added, a transformed structure exceeding a certain fraction targeted for the present inventive steel may be formed, resulting in increased strength and decreased elongation, which may cause wrinkling defects on the product surface during part processing by customers. Accordingly, in the present disclosure, the upper limit of the carbon (C) content may be limited to 0.015%. According to an example embodiment of the present disclosure, the upper limit may be 0.014%.Silicon (Si): 0.200% or less

[0040] Silicon (Si) is an element that contributes to increasing the strength of steel through solid solution strengthening. In the present disclosure, since the desired properties may be obtained even without adding silicon (Si), silicon is not intentionally added. Meanwhile, when the silicon (Si) content exceeds a certain level, there may be a problem in that the plated surface properties are deteriorated. Accordingly, in the present disclosure, the upper limit of the silicon (Si) content may be limited to 0.200%. According to an example embodiment of the present disclosure, the upper limit may be 0.100%.Manganese (Mn): 1.30 to 2.00%

[0041] Manganese (Mn), which is a solid solution strengthening element, not only contributes to increasing the strength of steel but also serves to precipitate S in the steel as MnS. In the present disclosure, manganese, together C and Cr, may contribute to increasing the hardenability of steel and ensuring the fraction of the transformed structure targeted for the present inventive steel. In the present disclosure, it is preferable to add manganese (Mn) in an amount of 1.30% or more in order to ensure a minimum fraction of the transformed structure and to thereby ensure low-temperature bake-hardenability and room-temperature aging resistance. According to an example embodiment of the present disclosure, the lower limit of the manganese (Mn) content may be 1.40%. However, when manganese (Mn) is added in an amount exceeding 2.00%, the fraction of the transformed structure in the steel may not satisfy the conditions defined in the present disclosure, and it may become difficult to satisfy a BH100 value (a bake-hardening amount after heat treatment at 100°C for 20 minutes (lower BH value)) of 30 MPa or more. In addition, excessive addition of manganese (Mn) may cause the formation of annealing oxides, which may lead to surface problems in plated products, and the elongation may decrease, resulting in deteriorated workability. According to an example embodiment of the present disclosure, the upper limit may be 1.80%.Chromium (Cr): 0.5 to 1.0%

[0042] Chromium (Cr), which is a solid solution strengthening element, is one of the most important elements in the present inventive steel, along with C, Mn, and N. Chromium (Cr) effectively contributes to the formation of martensite by increasing the hardenability of steel. In addition, when chromium (Cr) is added to steel, coarse Cr-based carbides such as Cr 23 C 6 are formed during hot rolling, thereby controlling the amount of solid solution C in steel to an appropriate level or lower and suppressing the occurrence of yield point elongation (YPel), such that a complex-phase steel having a low yield ratio may be manufactured. In addition, chromium (Cr) also effectively contributes to ensuring the elongation of complex-phase steel by minimizing the decrease in elongation relative to the increase in strength. Accordingly, in the present disclosure, chromium (Cr) may be essentially added to achieve these effects. When chromium (Cr) is added in an amount of less than 0.5%, the hardenability of the steel may decrease, making it difficult to ensure a sufficient fraction of the transformed structure desired in the present disclosure. This leads to a decrease in the BH100 value, and the BH170-BH100 value (the difference between the bake-hardening amount after heat treatment at 100°C for 20 minutes and the bake-hardening amount after heat treatment at 170°C for 20 minutes) may not satisfy 20 MPa or less as defined in the present disclosure. On the other hand, when the chromium (Cr) content exceeds 1.0%, the formation ratio of martensite may excessively increase, thereby exceeding the conditions of the transformed structure fraction defined in the present disclosure, resulting in deterioration of the BH100 value, and the elongation may decrease due to the excessive addition of chromium (Cr). According to an example embodiment of the present disclosure, the upper limit may be 0.9%.Phosphorus (P): 0.030% or less

[0043] Phosphorus (P) is an element that is inevitably included as an impurity in steel. In addition, phosphorus is also the most effective element for ensuring the strength of steel through solid solution strengthening without significantly impairing drawability. However, when phosphorus (P) is excessively added, the possibility of brittle fracture increases, which may cause slab breakage during hot rolling and may also significantly deteriorate the surface properties of the plated steel sheet. Accordingly, in the present disclosure, the upper limit of the phosphorus (P) content may be limited to 0.030%.Sulfur (S): 0.010% or less

[0044] Sulfur (S) is also an element that is inevitably included as an impurity in steel, and it is preferable to control a content thereof to be as low as possible in order to ensure excellent weldability. In particular, since sulfur (S) in steel may cause hot shortness, in the present disclosure, the upper limit of the sulfur (S) content may be limited to 0.010%.Nitrogen (N): 0.0020 to 0.0080%

[0045] Nitrogen (N) is also an element that is inevitably introduced into steel as an impurity. Therefore, it is preferable to control a content thereof to be as low as possible; however, in the present disclosure, nitrogen is utilized as a very important element for ensuring low-temperature bake-hardenability and room-temperature aging resistance. Nitrogen (N) has a very high diffusion rate and may simultaneously cause bake hardenability and aging deterioration. In order to suppress aging deterioration caused by nitrogen (N), it is necessary to ensure a certain fraction of the transformed structure through an appropriate combination of C, Mn, and Cr. In steel containing such a transformed structure, in order to obtain low-temperature bake-hardenability at 100°C, a certain amount or more of nitrogen (N) may be required. When the nitrogen (N) content is less than 0.0020%, it may be difficult to obtain bake hardenability having a BH100 value of 30 MPa or more. According to an example embodiment of the present disclosure, the lower limit may be 0.0050%. According to an example embodiment of the present disclosure, the lower limit may be 0.0060%. On the other hand, when the content exceeds 0.0080%, it becomes difficult to simultaneously ensure both low-temperature bake-hardenability and room-temperature aging resistance.Aluminum (Al) : 0.010 to 0.060%

[0046] Aluminum (Al) is an element added for grain refinement and deoxidation of steel. In the present disclosure, in order to manufacture a stable Al-killed steel, the lower limit of the aluminum (Al) content may be limited to 0.010%. According to an example embodiment of the present disclosure, the lower limit may be 0.020%. However, when aluminum (Al) is excessively added, the strength of steel increases due to grain refinement, but during steelmaking and continuous casting operations, an excessive amount of inclusions may be formed, thereby deteriorating the surface quality of the steel sheet and increasing manufacturing costs. Accordingly, in the present disclosure, the upper limit of the aluminum (Al) content may be limited to 0.060%. According to an example embodiment of the present disclosure, the upper limit may be 0.050%.

[0047] The steel of the present disclosure may include, in addition to the above-described composition, the balance of iron (Fe) and inevitable impurities. The inevitable impurities may be unintentionally introduced during ordinary manufacturing processes and therefore cannot be completely excluded. Since such impurities are well known to those skilled in the art of steel manufacturing, a detailed description thereof is omitted herein.

[0048] The steel sheet according to an example embodiment of the present disclosure may have a K value defined by the following Relational Expression 1 of 0 to 15.000.

[0049] K = - 651 C - 2.42 Mn + 25.7 Cr - 220 N

[0050] (In the expression, [C], [Mn], [Cr], and [N] represent wt% of the respective elements.)

[0051] The present inventors have found that the interaction among C, Mn, Cr, and N is significantly important for ensuring a bake-hardening amount of 30 MPa or more at a low temperature of 100°C. In the present disclosure, in order to simultaneously ensure low-temperature bake-hardenability and room-temperature aging resistance, the microstructure is strictly limited. This is because C, Mn, and Cr are identified as essential elements for obtaining the transformed structure targeted in the present disclosure, and N is an element that enables the steel to exhibit a certain level of bake hardenability even at a low temperature.

[0052] In the present disclosure, in order to ensure appropriate bake hardenability at a low temperature, solid solution elements having high diffusion coefficients are required. N is presumed to be a significantly advantageous element for obtaining bake hardenability at a low temperature. That is, since the diffusion rate of N is more than 100 times faster than that of C, N is advantageous for obtaining bake hardenability even at a low temperature compared to C. Meanwhile, solid solution nitrogen also affects room-temperature aging resistance. That is, solid solution nitrogen may easily cause not only bake hardenability but also yield point elongation, which represents aging resistance. In the present disclosure, in order to solve the problem of deterioration of aging resistance caused by N, the deterioration may be controlled by utilizing the transformed structure generated in the steel. C, Mn, and Cr are advantageous for forming the transformed structure, and since a large number of mobile dislocations exist around the transformed structure, the movement of N may be hindered. That is, by appropriately utilizing N together with C, Mn, and Cr, aging does not occur at room temperature, while bake hardenability (BH property) may be obtained even at a low baking temperature. Accordingly, in the present disclosure, Relational Expression 1 is proposed by utilizing C, Mn, Cr, and N.

[0053] When the K value of Relational Expression 1 is less than 0, the fraction of the transformed structure is outside the range of the present disclosure, resulting in a low BH100 value and a high BH170-BH100 value, that is, a large difference in BH values depending on the baking temperature. On the other hand, when the K value of Relational Expression 1 exceeds 15.000, it may include, in most cases, conditions in which the contents of C, Mn, Cr, and N are outside the ranges the standards defined in the present disclosure. This causes the fraction of the transformed structure to fall outside the range targeted in the present disclosure, and there is a concern that the BH100 value cannot be satisfied. According to an example embodiment of the present disclosure, the lower limit of the K value may be 1. According to an example embodiment of the present disclosure, the lower limit may be 1.500.

[0054] Hereinafter, the microstructure of the steel of the present disclosure will be described in detail.

[0055] In the present disclosure, unless otherwise particularly specified, "%" represents the fraction of the microstructure on an area basis.

[0056] According to an example embodiment of the present disclosure, the microstructure of the steel sheet may include, by area%, 0.30 to 0.80% of a transformed structure and a balance of ferrite. According to an example embodiment of the present disclosure, the transformed structure may include martensite, bainite, and ferritic bainite. According to an example embodiment of the present disclosure, the transformed structure may be martensite.

[0057] When bake hardenability of 30 MPa or more is ensured as the BH100 value, a higher bake hardenability, that is, a higher BH170 value than the BH100 value, may be obtained at a high baking temperature. In this case, a problem of deterioration of room-temperature aging resistance may occur due to a high content of solid solution elements in the steel. Therefore, the smaller the difference between the BH170 and BH100 bake hardenability values, the more advantageous the aging resistance may become. For this reason, in the present disclosure, in order to ensure room-temperature aging resistance, the difference in bake hardenability between the BH170 value and the BH100 value is limited to 20 MPa or less.

[0058] Meanwhile, when a certain fraction of the transformed structure is present in the microstructure, the aging resistance may be improved due to abundant mobile dislocations around the transformed structure. However, an excessive fraction of the transformed structure requires an increase in alloying elements for its formation and may also cause an increase in strength. Furthermore, there is a concern that low-temperature bake-hardenability may decrease. Therefore, in the present disclosure, the fraction of the transformed structure is limited to 0.30 to 0.80%.

[0059] When the fraction of the transformed structure is less than 0.30%, a sufficient number of mobile dislocations cannot be obtained, and thus, the BH100 value cannot satisfy the level proposed in the present disclosure. In addition, the added C affects not only the BH property but also the aging properties, which may cause an increase in the BH170 value, and, consequently, an increase in the BH170-BH100 value, as well as deterioration of room-temperature aging resistance. On the other hand, when the fraction of the transformed structure exceeds 0.80%, the aging properties may be improved due to the excessive transformed structure; however, the BH100 value may decrease.

[0060] The steel sheet according to an example embodiment of the present disclosure may have a tensile strength of 340 MPa or more, a yield strength of 180 to 250 MPa, and an elongation of 34.0% or more, may have a bake-hardening amount of 30.0 MPa or more after heat treatment at 100°C for 20 minutes, may have a difference of 20.0 MPa or less between the bake-hardening amount after heat treatment at 100°C for 20 minutes and the bake-hardening amount after heat treatment at 170°C for 20 minutes, and may have a yield point elongation (AI) of 0.20% or less after heat treatment at 100°C for 1 hour, thereby exhibiting excellent low-temperature bake-hardenability and room-temperature aging resistance. According to an example embodiment of the present disclosure, the tensile strength may be 400 MPa or less.

[0061] The BH value refers to a bake-hardening amount at a specific temperature, which is a value measured as an increase in lower yield strength after baking at the specific temperature for a certain period of time, based on the flow stress after 2% pre-strain.

[0062] Hereinafter, a method for manufacturing steel according to the present disclosure will be described in detail.

[0063] The steel sheet according to an example embodiment of the present disclosure may be manufactured by reheating a steel slab satisfying the above-described alloy composition, performing finish hot rolling, cooling, coiling, cold rolling, and continuous annealing.Reheating

[0064] A steel slab satisfying the alloy composition of the present disclosure may be reheated to a temperature range of 1,100 to 1,250°C.

[0065] The reheating may be performed to smoothly carry out a subsequent hot rolling process and to sufficiently obtain the desired properties of the steel sheet.

[0066] When the reheating temperature is lower than 1,100°C, inclusions in the steel slab may not be sufficiently redissolved, which may cause variations in material properties and surface defects after hot rolling. On the other hand, when the reheating temperature exceeds 1,250°C, the strength may decrease due to excessive growth of austenite grains, and excessive scale formation may occur, thereby deteriorating the surface quality of the steel sheet.Finish hot rolling

[0067] The reheated steel slab may be subjected to finish hot rolling at a temperature of 880°C or higher.

[0068] When the finish hot rolling is performed in an austenite single-phase region, pancake-shaped austenite grains and deformation bands are formed, which may be advantageous in terms of refining the final structure.

[0069] When the finish hot rolling temperature is lower than 880°C, the rolling is performed in the dual-phase region of austenite and ferrite, which may cause heterogeneity in material properties and may result in excessive rolling load. Therefore, in the present disclosure, the finish hot rolling temperature may be limited to 880°C or higher so that the hot rolling is completed in the austenite single-phase region. According to an example embodiment of the present disclosure, the upper limit of the finish hot rolling temperature may be 950°C.Cooling and coiling

[0070] The finish hot-rolled steel sheet may be cooled to a temperature range of 550 to 700°C and then coiled.

[0071] When the coiling temperature is lower than 550°C, the shape of the steel sheet may become poor, and a large amount of low-temperature transformed phases such as martensite or bainite may be formed, which may cause an excessive increase in the strength of the steel sheet. On the other hand, when the coiling temperature exceeds 700°C, coarse ferrite grains may be formed, and coarse carbides and nitrides are likely to form, which may deteriorate the material properties of the steel. In addition, as the coiling temperature increases, oxides of the hot-rolled sheet such as Mn and Si may increase, which may lead to partial remaining of oxides during pickling, or even when the oxides are completely removed, enrichment may occur in the surface layer of the steel sheet, thereby causing surface defects during plating.

[0072] According to an example embodiment of the present disclosure, after coiling and before performing a subsequent cold rolling process, pickling may be additionally performed to remove surface scale. The conditions for the pickling are not particularly limited, and any conditions commonly used in conventional processes may be applied.Cold rolling

[0073] The coiled steel sheet may be cold-rolled at a reduction ratio of 60 to 90%.

[0074] When the reduction ratio during cold rolling is less than 60%, the driving force for recrystallization due to cold rolling is insufficient, such that recrystallization of ferrite may not be completed, causing an unrecrystallized ferrite structure to remain. On the other hand, when the reduction ratio exceeds 90%, the rolling load on the rolls becomes excessively high during operation, which may lead to deterioration of the shape of the steel sheet. In particular, cracks may occur at the edge portion of the steel sheet, thereby increasing the cold rolling load.Continuous annealing

[0075] The cold-rolled steel sheet may be continuously annealed in a temperature range of 760 to 830°C.

[0076] When the continuous annealing temperature is lower than 760°C, recrystallization may not be sufficiently completed, resulting in the occurrence of unrecrystallized structures. In addition, since the annealing is performed in a ferrite single-phase region, the desired microstructure intended in the present disclosure cannot be obtained. On the other hand, when the temperature exceeds 830°C, excessive formation of austenite may occur, which may lead to a decrease in the carbon concentration within the austenite and consequently a reduction in its stability. As a result, during cooling after annealing, reverse transformation into ferrite may easily occur, or even when austenite transforms into martensite, the hardenability may become insufficient due to a lack of carbon concentration, making it difficult to achieve sufficient bake hardenability and room-temperature aging resistance.Plating

[0077] According to an example embodiment of the present disclosure, the continuously annealed steel sheet may be immersed in a zinc plating bath having a temperature range of 440 to 500°C to perform plating.

[0078] In the present disclosure, a hot-dip zinc-based plated steel sheet may be manufactured by immersing the continuously annealed cold-rolled steel sheet in the zinc plating bath.Alloying treatment

[0079] In addition, according to an example embodiment of the present disclosure, the plated steel sheet may be subjected to alloying treatment in a temperature range of 450 to 540°C.

[0080] When the alloying treatment temperature is lower than 450°C, unplated areas may occur across the entire width of the annealed steel sheet. On the other hand, when the temperature exceeds 540°C, excessive alloying may lead to deterioration of powdering properties due to the influence of the brittle Fe-Zn intermetallic compound (Γ).Temper rolling

[0081] According to an example embodiment of the present disclosure, temper rolling may be performed on the steel sheet after continuous annealing or on the zinc-plated steel sheet.

[0082] Specifically, according to an example embodiment of the present disclosure, temper rolling may be performed using a skin-pass roll having a surface roughness (Ra) of 1.0 to 1.6 µm at a temper rolling reduction ratio of 0.5 to 1.5%.

[0083] When the temper rolling reduction ratio is less than 0.5%, sufficient dislocations may not be generated, which is disadvantageous in terms of sheet shape and may cause plating surface defects. In addition, the aging resistance may also deteriorate. On the other hand, when the reduction ratio exceeds 1.5%, an excessive increase in dislocation density at the surface layer may lead to a deterioration of material properties, and side effects such as strip breakage may occur due to the limitation of equipment capability.Mode for Invention

[0084] Hereinafter, the present disclosure will be described more specifically with reference to Examples. However, it should be noted that the following Examples are merely intended to illustrate the present disclosure in more detail, and are not intended to limit the scope of the present disclosure.(Examples)

[0085] After preparing a steel slab having the alloy composition shown in Table 1, reheating, finish hot rolling, coiling, and cold rolling were performed under the manufacturing conditions shown in Table 2. The cold-rolled steel sheet was continuously annealed at the temperature conditions specified in Table 2, and then, furnace cooling was performed. Subsequently, the steel sheet was immersed in a hot-dip galvanizing bath maintained at approximately 460°C to perform hot-dip galvanizing, and thereafter, a temper rolling reduction ratio of 0.8% was applied to manufacture a final hot-dip galvanized steel sheet. [Table 1]Steel typeAlloy composition (wt%)Relational Expression 1CSiMnPSAlCrNA0.0120.1002.000.0120.0050.0250.60.00501.668B0.0120.0501.900.0150.0060.0351.00.005012.190C0.0100.1001.500.0100.0080.0400.70.00606.530D0.0120.0111. 600.0100.0050.0350.60.00502.636E0.0120.0151.500.0150.0020.0400.90.006010.368F0.0100.0101.700.0200.0050.0400.70.00506.266G0.0100.0501.700.0150.0060.0350.90.007010.966H0.0080.0501. 600.0100.0040.0500.80.00709.940I0.0100.1001.400.0120.0050.0440.90.006011.912J0.0100.1001.500.0100.0050.0301.00.007014.020K0.0070.0152.000.0120.0060.0350.40.00350.113L0.0140.1002.500.0120.0070.0400.50.0050-3.414M0.0100.0502.500.0100.0050.0450.40.0030-2.940N0.0160.0501.100.0200.0040.0200.70.00803.152O0.0200.0501.700.0150.0030.0350.50.0100-6.484P0.0060.0501.400.0250.0070.0550.30.0050-0.684Q0.0400.1001.000.0200.0090.0400.50.0040-16.490R0.0350.1001.500.0150.0070.0300.40.0120-18.775S0.0080.1001.350.0200.0080.0331.00.005016.972T0.0030.1000.900.0150.0070.0350.90.005017.899 K=−651C−2.42Mn+25.7Cr−220N

[0086] (In the expression, [C], [Mn], [Cr], and [N] represent wt% of the respective elements.) [Table 2]Specimen No.Steel typeReheatingFinish hot rollingCoiling after coolingCold rollingContinuous annealingTemperature (°C)Temperature (°C)Temperature (°C)Reduction ratio (%)Temperature (°C)1A1200900650707902B1200900650707903B1200900650707404B1200900650708705C1180910650708006D1190910650707907E1180920650708108F1200910650707809G12009006507079010H12009206507080011I11909106507081012J11909106507081013K11809006507080014L11809206507079015M12009206507081016N12009206507080017O12009006507082018P11809006507081019Q11909106507080020R11909106507081021S11909006507080022T120092065070810

[0087] For each of the manufactured hot-dip galvanized steel sheets, the microstructure was analyzed at a quarter thickness of the steel sheet after Le Pera etching by using an optical microscope, and the results are illustrated in Table 3. At this time, the transformed structure included martensite, bainite, and ferritic bainite. Ferrite was observed as another structure. In addition, a tensile test was conducted in the rolling direction in accordance with the JIS-5 standard to measure the yield strength (YP), tensile strength (TS), and elongation (El) of the plated steel sheets, and the results are illustrated in Table 3. In addition, for the same standard, the lower yield stress (L-BH) was measured after applying a 2% pre-strain, and then performing baking at 100°C for 20 minutes and at 170°C for 20 minutes. The results for the BH100 value (the bake-hardening amount after heat treatment at 100°C for 20 minutes) and the BH170-BH100 value (the difference between the bake-hardening amount after heat treatment at 170°C for 20 minutes and the bake-hardening amount after heat treatment at 100°C for 20 minutes) are shown in Table 3. Specifically, the BH value represents the bake-hardening amount at a specific temperature, which is a value measured as an increase in lower yield strength after baking at the specific temperature for a certain period of time, based on the flow stress after 2% pre-strain. The room-temperature aging resistance (AI) was determined, as described above, by maintaining the specimen in a water bath kept at 100°C for 1 hour, and then measuring the yield point elongation (YPel) obtained from the tensile test. [Table 3]Specimen No.Steel typeMicrostructurePropertiesClassificationTransformed structure (area%)FYP (MPa)TS (MPa)El (%)BH100 (MPa)BH170-BH100 (MPa)AI (%)1A0.30Balance24338636.137.211.10Inventive Example 12B0.35Balance23939135.241.79.60Inventive Example 23B0Balance25740233.322.110.50.10Comparative Example 14B0.19Balance27941932.19.742.60.41Comparative Example 25C0.60Balance22039036.340.24.50Inventive Example 36D0.53Balance24738835.340.73.30Inventive Example 47E0.56Balance21341036.539.44.20Inventive Example 58F0.65Balance22840136.143.87.10InventiveExample 69G0.71Balance23539335.140.99.70Inventive Example 710H0.77Balance20942036.530.911.30Inventive Example 811I0.49Balance19939734.337.76.60Inventive Example 912J0.58Balance20740734.944.94.10Inventive Example 1013K0.25Balance20539933.326.837.80.25Comparative Example 314L0.91Balance25146531.219.722.10Comparative Example 415M0.86Balance24444332.39.327.40Comparative Example 516N0.08Balance27945628.316.525.30.35Comparative Example 617O0.22Balance26643230.310.637.10Comparative Example 718P0.03Balance27740134.322.330.10.40Comparative Example 819Q3.60Balance27750125.35.212.30Comparative Example 920R2.80Balance27348922.310.112.10Comparative Example 1021S0.45Balance22541132.327.116.30.10Comparative Example 1122T0.15Balance25938433.19.735.10.30Comparative Example 12* M: martensite, F: ferrite

[0088] As shown in Table 3, in the Inventive Examples satisfying the conditions of the present disclosure, the microstructural characteristics proposed in the present disclosure were achieved, and the properties desired in the present disclosure could also be obtained.

[0089] In contrast, Comparative Example 1 satisfies the alloy composition and Relational Expression 1 of the present disclosure, but the annealing temperature did not reach the range defined in the present disclosure. As a result, annealing was performed in the ferrite single-phase region, and no transformed phase was obtained.

[0090] In Comparative Example 2, the annealing temperature was outside the range of the present disclosure, and due to an insufficient carbon concentration in the austenite, most of the austenite was reverse-transformed into ferrite during cooling, resulting in a low fraction of the transformed structure. In addition, the transformed structure that was formed lacked stability, leading to a significantly low BH100 value and poor aging resistance.

[0091] Comparative Example 3 corresponds to a case in which the Cr content was lower than the range proposed in the present disclosure. Due to the low Cr content, the fraction of the transformed structure was insufficient, and the value of Relational Expression 1 did not satisfy the conditions of the present disclosure, such that the BH100 value was not achieved. The room-temperature aging resistance was also inferior.

[0092] Comparative Example 4 corresponds to a case in which the Mn content exceeded the range proposed in the present disclosure, and Comparative Example 5 corresponds to a case in which the Mn content exceeded the proposed range and the Cr content was lower than the proposed range. As a result, an excessive amount of the transformed structure was formed, and the BH100 value did not satisfy the conditions of the present disclosure.

[0093] Comparative Example 6 corresponds to a case in which the Mn content was lower than the conditions defined in the present disclosure. The low Mn content resulted in insufficient hardenability, such that a sufficient amount of the transformed structure was not obtained. Because the transformed structure was hardly formed, a sufficient amount of mobile dislocations was not ensured, resulting in high yield strength and low elongation. In addition, the BH property did not satisfy the conditions of the present disclosure.

[0094] Comparative Example 7 corresponds to a case in which the N content was excessively high, and the BH100 value targeted in the present disclosure was not satisfied. In addition, due to the high N content, an increase in yield strength was observed.

[0095] Comparative Example 8 corresponds to a case in which the C content was significantly low, and Comparative Example 9 corresponds to a case in which the C content exceeded the proposed range. In Comparative Example 8, sufficient hardenability could not be ensured due to the low C content, and thus, the fraction of the transformed structure did not reach the targeted level. The K value did not satisfy the range proposed in the present disclosure, resulting in an unsatisfactory BH100 value. In Comparative Example 9, the excessively high C content resulted in significantly increased hardenability, and an excessive amount of the transformed structure was formed. As a result, the tensile strength increased, and the BH property was suppressed.

[0096] Comparative Example 10 corresponds to a case in which the amounts of C, Cr, and N added were outside the conditions defined in the present disclosure. In particular, because the C content was high, the hardenability increased, resulting in low elongation. In addition, since the K value of Relational Expression 1 was outside the range defined in the present disclosure, the conditions for the BH value proposed in the present disclosure could not be satisfied.

[0097] Comparative Example 11 corresponds to a case in which the alloy composition satisfied the ranges defined in the present disclosure but the conditions of Relational Expression 1 were not satisfied. As a result, the BH100 value at the level targeted in the present disclosure could not be ensured.

[0098] Comparative Example 12 corresponds to a case in which the C content was low and the K value of Relational Expression 1 was outside the range proposed in the present disclosure. As a result, the fraction of the transformed structure was insufficient, and the BH100 value also did not reach the proposed standard.

[0099] Although the present disclosure has been described in detail with reference to the example embodiments, other example embodiments may also be implemented. Therefore, the technical idea and the scope of the present disclosure defined by the appended claims should not be limited to the example embodiments.

Claims

1. A steel sheet comprising, by wt%, 0.008 to 0.015% of C, 0.200% or less of Si, 1.30 to 2.00% of Mn, 0.5 to 1.0% of Cr, 0.030% or less of P, 0.010% or less of S, 0.0020 to 0.0080% of N, 0.010 to 0.060% of Al, and the balance of Fe and inevitable impurities, wherein a K value defined by the following Relational Expression 1 is 0 to 15.000, and a microstructure includes, by area%, 0.30 to 0.80% of a transformed structure and a balance of ferrite: K = - 651 C - 2.42 Mn + 25.7 Cr - 220 N wherein [C], [Mn], [Cr], and [N] represent wt% of the respective elements.

2. The steel sheet of claim 1, wherein the transformed structure includes martensite, bainite, and ferritic bainite.

3. The steel sheet of claim 1, wherein the steel sheet has a tensile strength of 340 MPa or more, a yield strength of 180 to 250 MPa, and an elongation of 34.0% or more.

4. The steel sheet of claim 1, wherein the steel sheet has a bake-hardening amount of 30.0 MPa or more after heat treatment at 100°C for 20 minutes, and has a difference of 20.0 MPa or less between the bake-hardening amount after heat treatment at 100°C for 20 minutes and the bake-hardening amount after heat treatment at 170°C for 20 minutes.

5. The steel sheet of claim 1, wherein the steel sheet has a yield point elongation (AI) of 0.20% or less after heat treatment at 100°C for 1 hour.

6. A method for manufacturing a steel sheet, the method comprising: reheating a steel slab including, by wt%, 0.008 to 0.015% of C, 0.200% or less of Si, 1.30 to 2.00% of Mn, 0.5 to 1.0% of Cr, 0.030% or less of P, 0.010% or less of S, 0.0020 to 0.0080% of N, 0.010 to 0.060% of Al, and the balance of Fe and inevitable impurities, the steel slab having a K value of 0 to 15.000 defined by the following Relational Expression 1; performing finish hot rolling on the reheated steel slab; cooling the finish hot-rolled steel sheet to a temperature range of 550 to 700°C and then coiling the cooled steel sheet; cold rolling the coiled steel sheet; and continuously annealing the cold-rolled steel sheet in a temperature range of 760 to 830°C: K = - 651 C - 2.42 Mn + 25.7 Cr - 220 N wherein [C], [Mn], [Cr], and [N] represent wt% of the respective elements.

7. The method of claim 6, wherein the reheating is performed in a temperature range of 1,100 to 1,250°C, the finish hot rolling is performed in a temperature range of 880°C or higher, and the cold rolling is performed at a reduction ratio of 60 to 90%.

8. The method of claim 6, further comprising performing pickling before the cold rolling.

9. The method of claim 6, further comprising, after the continuous annealing, immersing the continuously annealed steel sheet in a zinc plating bath having a temperature range of 440 to 500°C to perform plating.

10. The method of claim 9, further comprising subjecting the plated steel sheet to an alloying treatment in a temperature range of 450 to 540°C.

11. The method of claim 9, further comprising performing temper rolling on the plated steel sheet using a skin-pass roll having a surface roughness (Ra) of 1.0 to 1.6 µm at a temper rolling reduction ratio of 0.5 to 1.5%.

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