Cold-rolled steel sheet, ZN-plated steel sheet and methods for manufacturing same

Abstract

The present invention relates to steels that are suitable as automobile exterior materials and, more specifically, to a cold-rolled steel sheet, a ZN-plated steel sheet obtained by galvanizing the cold-rolled steel sheet, and methods for manufacturing same.

Description

Cold rolled steel sheets, galvanized steel sheets, and their manufacturing methods

[0001] The present invention relates to steel suitable as an automobile exterior panel material, and more specifically, to cold-rolled steel sheets, galvanized steel sheets obtained by galvanizing the cold-rolled steel sheets, and methods for manufacturing the same.

[0002] Automotive exterior panels, especially those for exterior panels, are required to have a certain level of bake hardening resistance. Bake hardening refers to a phenomenon in which the yield strength increases as activated carbon and nitrogen are fixed to the dislocations created during the steel pressing process during paint baking. Steel with excellent bake hardening resistance has the advantage of being easy to form before paint baking and has the characteristic of improving dent resistance in the final product, making it ideal as a material for automotive exterior panels. In addition, automotive exterior panel materials are required to have a certain level of room temperature aging resistance so that they can be guaranteed against aging for a certain period of time (Patent Document 1).

[0003] Meanwhile, manufacturers of automotive exterior panel components, such as hoods and doors, impose stringent requirements on these components. One of these requirements concerns the paint finish of the parts to be painted. Exterior panels with a reasonably good paint finish are highly valued for their mirror-like surface, reflecting light without distortion, resulting in clear reflections.

[0004] The appearance of a paint is influenced not only by the quality of the paint itself, but also by the surface of the coated substrate, which is composed of in-plane structures of various sizes and scales. Smaller structures are characterized by surface roughness, while larger structures are characterized by so-called surface waviness. It is well known that larger surface structures are transmitted through the paint layers. Consequently, the waviness of the coated substrate surface still exists to some extent on the surface of the outer paint layer.

[0005] Recently, automobile manufacturers are seeking to save energy and reduce costs by omitting intermediate steps during the painting process, so the surface waviness of automobiles is becoming more important.

[0006] That is, since surface waviness can be measured after pressing or forming is applied, the surface waviness of the formed part is important. However, as mentioned above, since the intermediate process is omitted, the surface properties of the substrate before coating, i.e., the base steel sheet, are becoming increasingly important. In other words, a technology is required to manufacture the surface of the base steel sheet, which is the initial substrate for manufacturing parts with a painted appearance, so that it can have a uniform and fine texture.

[0007] (Patent Document 1) Korean Patent Publication No. 10-2017-0012865

[0008] One aspect of the present invention is to provide a galvanized steel sheet having excellent image clarity as well as the generally required properties of hardenability and aging resistance, which are suitable as an automobile exterior panel material, and a method for manufacturing the same.

[0009] Another aspect of the present invention is to provide a cold-rolled steel sheet, which is a base steel sheet for obtaining the galvanized steel sheet, and a method for manufacturing the same.

[0010] Meanwhile, the objectives of the present invention are not limited to the above-described content. The objectives of the present invention can be understood from the overall content of this specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention.

[0011] According to one aspect of the present invention, the cold rolled steel sheet may contain, in wt%, carbon (C): 0.0010 to 0.0050%, silicon (Si): 0.050% or less (excluding 0%), manganese (Mn): 0.10 to 0.25%, phosphorus (P): 0.010 to 0.035%, sulfur (S): 0.0100% or less, nitrogen (N): 0.0050% or less, aluminum (Al): 0.100% or less (excluding 0%), niobium (Nb): 0.0010 to 0.0100%, copper (Cu): 0.0100% or less (excluding 0%), the remainder iron and other unavoidable impurities.

[0012] In one embodiment of the present invention, the cold rolled steel sheet can satisfy relational expression 1.

[0013] [Relationship 1]

[0014] Nb(12 / 93) + S(63 / 32) + Cu(32 / 63) + d / 2 ≤ 11

[0015] (In relational expression 1, d means the average crystal grain size (㎛) of ferrite within the polar surface portion, and the polar surface portion means the ND plane (rolling surface) at a point within 100 ㎛ in the thickness direction from the surface of the cold-rolled steel sheet.)

[0016] In one embodiment of the present invention, the steel sheet may have a microstructure composed of ferrite having an area fraction of 95% or more.

[0017] In one embodiment of the present invention, the average crystal grain size of the ferrite may be 20 μm or less.

[0018] According to the present invention, there is an effect of providing a base steel sheet (cold rolled steel sheet) for providing a galvanized steel sheet with improved bake hardenability and sharpness suitable for use as an automobile exterior panel material by limiting the alloy composition and optimizing the relationship between the alloy composition and the microstructure.

[0019]

[0020] According to another aspect of the present invention, a cold rolled steel sheet can be manufactured by including the steps of preparing a steel slab; heating the steel slab at a temperature range of 1100 to 1300°C; finishing hot rolling the heated steel slab at a temperature of Ar3 or higher to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a temperature range of 600 to 650°C; pickling the coiled hot rolled steel sheet and then cold rolling it at a reduction ratio of 70 to 83% to obtain a cold rolled steel sheet; and annealing the cold rolled steel sheet at a temperature range of 760 to 820°C.

[0021] In one embodiment of the present invention, the steel slab may have the alloy composition described above, and the cold-rolled steel sheet manufactured may satisfy the above-described relational expression 1.

[0022]

[0023] According to another aspect of the present invention, a galvanized steel sheet comprising a base steel sheet and a zinc-based plating layer provided on at least one surface of the base steel sheet can be provided.

[0024] In one embodiment of the present invention, the steel sheet may contain, in wt%, carbon (C): 0.0010 to 0.0050%, silicon (Si): 0.050% or less (excluding 0%), manganese (Mn): 0.10 to 0.25%, phosphorus (P): 0.010 to 0.035%, sulfur (S): 0.0100% or less, nitrogen (N): 0.0050% or less, aluminum (Al): 0.100% or less (excluding 0%), niobium (Nb): 0.0010 to 0.0100%, copper (Cu): 0.0100% or less (excluding 0%), the remainder being iron and other unavoidable impurities.

[0025] In one embodiment of the present invention, the steel plate can satisfy relational expression 1.

[0026] [Relationship 1]

[0027] Nb(12 / 93) + S(63 / 32) + Cu(32 / 63) + d / 2 ≤ 11

[0028] (In relational expression 1, d means the average crystal grain size (㎛) of the polar surface, and the polar surface means the ND plane (rolled surface) at a point within 100 ㎛ in the thickness direction of the polar surface from the interface between the base steel sheet and the zinc-based plating layer.)

[0029] According to the present invention, there is an effect of providing a galvanized steel sheet having improved clarity suitable for use as an automobile exterior material by optimizing the alloy composition system. According to one embodiment of the present invention, the galvanized steel sheet may have a ΔWsa defined by the following equation 2 of 0.15 µm or less.

[0030] [Relationship 2]

[0031] ΔWsa = Surface waviness of the steel plate after 5% deformation - Surface waviness of the steel plate before deformation

[0032] In one embodiment of the present invention, the galvanized steel sheet may have a small hardening amount of 30 MPa or more in a tensile test after heat treatment at 170°C for 20 minutes.

[0033] In one embodiment of the present invention, the steel sheet may be the cold-rolled steel sheet described above.

[0034] In one embodiment of the present invention, the galvanized steel sheet may have properties such as a yield strength of 180 MPa or more, a tensile strength of 300 MPa or more, and an elongation of 33% or more.

[0035]

[0036] According to another aspect of the present invention, a method for manufacturing a galvanized steel sheet may include the steps of preparing a steel slab; heating the steel slab at a temperature range of 1100 to 1300°C; finishing hot-rolling the heated steel slab at a temperature of Ar3 or higher to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature range of 600 to 650°C; cold-rolling the coiled hot-rolled steel sheet at a reduction ratio of 70 to 83% after pickling to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature range of 760 to 820°C; hot-dip galvanizing the annealed cold-rolled steel sheet to obtain a galvanized steel sheet; and temper rolling the galvanized steel sheet.

[0037] In one embodiment of the present invention, the steel slab may have the same alloy composition as the steel plate, and may also satisfy the above-described relational expression 1.

[0038] In one embodiment of the present invention, the temper rolling treatment can be performed using a roll having a roughness (Ra) of 1.0 to 2.2 ㎛ and a pressing force of 0.6 to 1.8%.

[0039] In one embodiment of the present invention, the hot-dip galvanizing can be performed in a zinc-based plating bath at 440 to 480°C.

[0040] In one embodiment of the present invention, the temper rolling treatment may be performed by one or more methods selected from electrical discharge machining (EDT), shot blasting (SBT), and topocrom texturing (TCT).

[0041] According to the present invention, it is possible to provide a galvanized steel sheet having excellent hardenability and aging resistance, which are properties required as an automobile exterior panel material, and a cold-rolled steel sheet suitable for obtaining such a galvanized steel sheet.

[0042] Moreover, according to the present invention, the clarity of the galvanized steel sheet is improved, so that it can be suitably applied to parts of automobile exterior panels having a painted appearance.

[0043] The inventors of the present invention have studied in depth a method for improving the hardenability and aging resistance of steel used as an automobile exterior material, and, as the uniformity of the surface of the substrate (base steel plate) is becoming increasingly important in manufacturing parts with a painted appearance using such steel, they have conducted in-depth research on a method for achieving this.

[0044] As a result, it was recognized that it is important to secure uniform and fine crystal grains on the surface of the substrate (base steel plate), and so an attempt was made to optimize the contents of specific elements in the alloy composition. In particular, according to one aspect of the present invention, by using ultra-low carbon steel as a basic component while controlling precipitate-forming elements to secure bake hardenability and aging resistance, it was confirmed that steel with improved image clarity suitable for automobile exterior panel material can be provided, and the present invention was completed.

[0045] Hereinafter, the present invention will be described in detail.

[0046] A cold rolled steel sheet according to one aspect of the present invention may include, in wt%, carbon (C): 0.0010 to 0.0050%, silicon (Si): 0.050% or less (excluding 0%), manganese (Mn): 0.10 to 0.25%, phosphorus (P): 0.010 to 0.035%, sulfur (S): 0.0100% or less, nitrogen (N): 0.0050% or less, aluminum (Al): 0.100% or less (excluding 0%), niobium (Nb): 0.0010 to 0.0100%, and copper (Cu): 0.0100% or less (excluding 0%).

[0047] Below, the reasons for limiting the alloy composition of a cold-rolled steel sheet according to one embodiment of the present invention will be described in detail. Unless otherwise specified, the content of each element is based on weight, and the ratio of the structure is based on area.

[0048] Carbon (C): 0.0010~0.0050%

[0049] Carbon (C) is an interstitial solid-solution element that has a great influence on the formation of the texture of steel sheets during cold rolling and annealing. When the amount of solid-solution carbon in steel increases, the growth of grains having a {111} gamma (γ)-fiber texture, which is advantageous for drawing processing, is suppressed, while the growth of grains having {110} and {100} textures is promoted, thereby reducing the drawability of the annealed cold-rolled steel sheet. Accordingly, the C may be included in an amount of 0.0050% or less, and if the C content exceeds 0.0050%, there is a concern that the aging resistance of the steel sheet may be inferior. Meanwhile, in terms of securing the bake hardenability of the steel sheet, the C may be included in an amount of 0.0010% or more.

[0050] Therefore, according to one embodiment of the present invention, the C may be included in an amount of 0.0010 to 0.0050%. According to another embodiment of the present invention, the C may be included in an amount of 0.0011% or more or 0.0012% or more, and according to another embodiment, the C may be included in an amount of 0.0045% or less.

[0051] Silicon (Si): 0.050% or less

[0052] Silicon (Si) is an element that contributes to increased strength through solidification. However, in the present invention, even if Si is not intentionally added, the physical properties can be secured without any problems. However, considering the level of silicon that is unavoidably added during the steel sheet manufacturing process, 0% can be excluded.

[0053] In one embodiment of the present invention, if the content of Si exceeds 0.050%, there is a problem of causing scale defects on the surface of the steel sheet, thereby deteriorating the plating surface properties, so the content may be limited to 0.050% or less.

[0054] Therefore, in one embodiment of the present invention, the Si may be included in an amount of 0.050% or less, 0% may be excluded, and in another embodiment, the Si may be included in an amount of 0.040% or less.

[0055] Manganese (Mn): 0.10~0.25%

[0056] Manganese (Mn) is a solid solution strengthening element that not only contributes to increasing the strength of steel, but also has the effect of suppressing embrittlement caused by S by precipitating S present in the steel as MnS.

[0057] In one embodiment of the present invention, if the content of Mn is less than 0.10%, the intended strength cannot be secured, whereas if the content exceeds 0.25%, there is a risk of surface defects occurring due to oxides.

[0058] Accordingly, in one embodiment of the present invention, the Mn may be included in an amount of 0.10 to 0.25%, and in another embodiment, the Mn may be included in an amount of 0.11% or more, or 0.13% or more. In another embodiment, the Mn may be included in an amount of 0.23% or less, or 0.21% or less.

[0059] Phosphorus (P): 0.010~0.035%

[0060] Phosphorus (P) is the element with the best employment effect and is effective in securing strength without significantly impairing the drawability of steel.

[0061] In one embodiment of the present invention, if the content of P is less than 0.010%, the aforementioned effect cannot be sufficiently obtained, whereas if the content exceeds 0.035%, there is a concern that the strength may become excessively high or secondary brittleness and surface stripe defects may occur due to P segregation.

[0062] Therefore, in one embodiment of the present invention, the P may be included in an amount of 0.010 to 0.035%. According to another embodiment of the present invention, the P may be included in an amount of 0.015% or more, and according to another embodiment, the P may be included in an amount of 0.030% or less.

[0063] Sulfur (S): 0.0100% or less and nitrogen (N): 0.0050% or less

[0064] Sulfur (S) and nitrogen (N) are impurities present in steel and are inevitably added during the steelmaking process. To ensure the weldability of steel, it is advantageous to keep the content of these elements as low as possible.

[0065] In one embodiment of the present invention, the S may be 0.0100% or less, and the N may be 0.0050% or less. However, considering the level that is inevitably added during the steel manufacturing process, the content of S and N may each exceed 0%.

[0066] Aluminum (Al): 0.100% or less (excluding 0%)

[0067] Aluminum (Al) combines with N present in steel to precipitate as AlN, and this precipitate contributes to improving the drawability and ductility of the steel.

[0068] In one embodiment of the present invention, if the Al content exceeds 0.100%, there is a risk that defects may occur inside the steel sheet due to excessive formation of Al inclusions during steelmaking operations.

[0069] Therefore, in one embodiment of the present invention, Al may be included in an amount of 0.100% or less, and 0% may be excluded. In another embodiment of the present invention, Al may be included in an amount of 0.080% or less. In yet another embodiment of the present invention, Al may be included in an amount of 0.005% or more.

[0070] Niobium (Nb): 0.0010~0.0100%

[0071] Niobium (Nb) is the most effective element for forming very fine grains during the rolling and cooling process when the austenite non-recrystallization temperature range is widened to high temperatures due to the solute drag and precipitate pinning effects during hot rolling.

[0072] In one embodiment of the present invention, if the content of Nb is less than 0.0010%, the range of the austenite non-recrystallization temperature range becomes narrow, and the grain refinement effect cannot be realized. On the other hand, if the content exceeds 0.0100%, there is a problem of reduced bake hardenability due to scavenging of C.

[0073] Therefore, in one embodiment of the present invention, Nb may be included in an amount of 0.0010 to 0.0100%. According to another embodiment of the present invention, Nb may be included in an amount of 0.0020% or more, or 0.0030% or more. According to yet another embodiment of the present invention, Nb may be included in an amount of 0.0090% or less, or 0.0085% or less.

[0074] Copper (Cu): 0.0100% or less (excluding 0%)

[0075] Copper (Cu) is an element that is difficult to control when adjusting the composition of steel in a steelmaking process. In one embodiment of the present invention, if the content of Cu exceeds 0.0100%, Cu2S precipitates are formed, which causes uneven grain size and may cause grain boundary embrittlement.

[0076] Therefore, in one embodiment of the present invention, Cu may be included in an amount of 0.0100% or less, in another embodiment, Cu may be included in an amount of 0.0095% or less, and in another embodiment, Cu may be included in an amount of 0.0090% or less.

[0077] In one embodiment of the present invention, considering that Cu is not easily removed in a steelmaking process, the Cu may be included in an amount exceeding 0%, and as an example, may be 0.0010% or more.

[0078] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0079] In one embodiment of the present invention, the cold-rolled steel sheet may have the above-described alloy composition, and may contain carbon in an amount of 0.0050% or less in the alloy composition. Accordingly, in one embodiment of the present invention, the microstructure of the cold-rolled steel sheet may be mainly composed of a ferrite phase. As one example, the ferrite phase may be included at an area fraction of 95% or more. As another example, the microstructure of the cold-rolled steel sheet may be composed of a ferrite single-phase structure.

[0080] In one embodiment of the present invention, the cold rolled steel sheet may include 5% or less of other inevitably generated structures, and as one example, may include 5% or less of pearlite.

[0081] In one embodiment of the present invention, the average grain size of ferrite may be 20㎛ or less. If the average grain size of ferrite exceeds 20㎛, the surface of the galvanized steel sheet obtained by galvanizing a cold-rolled steel sheet becomes uneven during forming, making it difficult to secure sharpness. Here, the average grain size of ferrite means the average value of the ferrite grain size for the entire thickness cross-section of the cold-rolled steel sheet. In other words, the average grain size of ferrite is distinct from the average grain size (d) of the polar surface in the following relational expression 1.

[0082] A cold-rolled steel sheet according to one embodiment of the present invention can satisfy the following relational expression 1 indicating the relationship between the alloy composition and the microstructure (crystal grain size).

[0083] [Relationship 1]

[0084] Nb(12 / 93) + S(63 / 32) + Cu(32 / 63) + d / 2 ≤ 11

[0085] (In relational expression 1, d means the average crystal grain size (㎛) of ferrite within the polar surface portion, and the polar surface portion means the ND plane (rolling surface) at a point within 100 ㎛ in the thickness direction from the surface of the cold-rolled steel sheet.)

[0086] The inventors of the present invention have discovered that, in order to secure uniform and fine grains on the surface of a cold-rolled steel sheet, it is important to control precipitated elements that affect the structural recrystallization during the annealing process during the process of manufacturing a cold-rolled steel sheet, and have derived the above-mentioned relational expression 1.

[0087] In one embodiment of the present invention, Nb constituting the above relational expression 1 inhibits the bake hardenability of the cold-rolled steel sheet by combining with C when the content is excessive, and S and Cu have a problem of forming Cu2S precipitates during the annealing process during the steel manufacturing process, causing unevenness of the structure. Therefore, according to one embodiment of the present invention, by controlling the relationship between the content of Nb, S, and Cu and the average grain size of ferrite in the polar surface portion as relational expression 1, the bake hardenability of the cold-rolled steel sheet can be improved. Furthermore, the sharpness of the galvanized steel sheet obtained by galvanizing the cold-rolled steel sheet with improved bake hardenability can be improved.

[0088] In one embodiment of the present invention, the value of the above relational expression 1, i.e., the relationship between the contents of Nb, S, and Cu and the average grain size of the extreme surface of the cold-rolled steel sheet exceeds 11, which means that the surface of the cold-rolled steel sheet is formed unevenly. As a result, the bake hardenability of the cold-rolled steel sheet cannot be secured, and further, the sharpness of the galvanized steel sheet cannot be secured.

[0089] In one embodiment of the present invention, the average crystal grain size (d) of the polar surface of the above relational expression 1 is distinct from the average crystal grain size of the ferrite mentioned above, so it is to be noted that the value of d may be outside the range of the average crystal grain size of the ferrite mentioned above.

[0090] The average crystal grain size referred to in the present invention refers to the size of the circle-equivalent diameter of each crystal grain, and means the average value of these diameters.

[0091]

[0092] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method is an example for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention.

[0093] According to one embodiment of the present invention, a cold-rolled steel sheet can be manufactured by subjecting a heated steel slab to a [hot rolling - cold rolling - annealing] process, and each process step is specifically described below.

[0094] [Heating of steel slabs]

[0095] After preparing a steel slab according to one embodiment of the present invention, the steel slab may be heated. The heating process of the steel slab is a process for smoothly performing the hot rolling process described below. As an example, the steel slab may have the same alloy composition as the cold-rolled steel sheet according to one embodiment of the present invention, and therefore, the description of the alloy elements is replaced with the aforementioned matters.

[0096] In one embodiment of the present invention, the heating of the steel slab may be performed at a temperature range of 1100 to 1300°C. If the heating temperature is lower than 1100°C, there is a risk of a rolling load occurring in a subsequent hot rolling process, whereas if the temperature exceeds 1300°C, there is a risk of surface scale defects occurring.

[0097] [Hot rolling]

[0098] The above heated steel slab can be hot rolled to obtain a hot rolled steel sheet.

[0099] In one embodiment of the present invention, the finishing hot rolling can be performed at a temperature of Ar3 or higher during the hot rolling, and if the temperature is lower than Ar3, abnormal rolling may be performed, which may cause tissue unevenness.

[0100] In one embodiment of the present invention, in order to secure fine crystal grains on the surface of the steel sheet during the finishing hot rolling, the finishing hot rolling can be performed at a temperature range of 880 to 950°C.

[0101] [Winding]

[0102] The hot-rolled steel sheet obtained by the above hot rolling can be coiled.

[0103] In one embodiment of the present invention, the coiling can be performed at a temperature range of 600 to 650°C. If the temperature during the coiling is lower than 600°C, a large amount of dissolved Nb, etc., is present in the steel, which adversely affects the suppression of recrystallization and grain growth during the subsequent annealing process. On the other hand, if the temperature exceeds 650°C, secondary scale is generated, and there is a risk that the surface of the steel sheet may be deteriorated.

[0104] [Cold rolling]

[0105] The above-mentioned hot-rolled steel sheet can be uncoiled and cold-rolled to obtain a cold-rolled steel sheet. At this time, the reduction ratio can be controlled so that the desired thickness is obtained.

[0106] In one embodiment of the present invention, the cold rolling can be controlled to 70 to 83% based on the cumulative reduction ratio. If the reduction ratio during the cold rolling is less than 70%, there is a concern that the {111} texture may not grow sufficiently, resulting in poor formability. On the other hand, if it exceeds 83%, the load on the rolling roll becomes very severe, resulting in poor shape.

[0107] According to another embodiment of the present invention, the cold rolling can be performed at a cumulative reduction ratio of 74% or more, and according to another embodiment, it can be performed at a cumulative reduction ratio of 80% or less.

[0108] In one embodiment of the present invention, prior to performing the cold rolling, a pickling process may be additionally performed for the purpose of removing surface scale from the coiled hot-rolled steel sheet. The pickling process may be performed under normal conditions, and there are no particular limitations on the conditions.

[0109] [Sodun]

[0110] The cold rolled steel sheet obtained by the above cold rolling can be annealed.

[0111] In one embodiment of the present invention, the annealing treatment may be performed at a temperature higher than the recrystallization temperature in order to remove deformation caused by rolling and to improve workability by softening.

[0112] In one embodiment of the present invention, the annealing treatment may be performed at a temperature range of 760 to 820°C. If the temperature during the annealing treatment is lower than 760°C, sufficient recrystallization driving force cannot be secured, whereas if the temperature exceeds 820°C, there is a risk that the crystal grains may become coarser.

[0113]

[0114] Hereinafter, a galvanized steel sheet according to another aspect of the present invention will be described in detail.

[0115] According to one embodiment of the present invention, the galvanized steel sheet may include a base steel sheet; and a plating layer provided on at least one surface of the base steel sheet.

[0116] In one embodiment of the present invention, the plating layer may be a zinc-based plating layer containing zinc as a main component, and a composition of a plating layer commonly applied in the relevant technical field may be applied to the zinc-based plating layer in the same manner.

[0117] In one embodiment of the present invention, the base steel sheet for providing the plating layer may be a cold-rolled steel sheet according to one embodiment of the present invention. As previously mentioned, the cold-rolled steel sheet according to one embodiment of the present invention secures not only bake hardenability but also surface uniformity by controlling the alloy composition and microstructure, as well as optimizing the relationship between the alloy composition and microstructure.

[0118] A galvanized steel sheet according to one embodiment of the present invention having a galvanized layer, for example a zinc-based galvanized layer, on at least one surface of the cold-rolled steel sheet can have excellent sharpness.

[0119] In one embodiment of the present invention, the galvanized steel sheet may have a characteristic in which ΔWsa defined by the following relational expression 2 is 0.15 ㎛ or less. That is, the galvanized steel sheet according to one embodiment of the present invention is obtained by using a cold-rolled steel sheet with improved surface uniformity as a base steel sheet, and thus the amount of surface change after deformation can be minimized. In one embodiment of the present invention, the fact that the ΔWsa value according to relational expression 2 exceeds 0.15 ㎛ means that the amount of change after deformation is relatively large, which is disadvantageous for securing sharpness.

[0120] [Relationship 2]

[0121] ΔWsa = Surface waviness of the steel plate after 5% deformation - Surface waviness of the steel plate before deformation

[0122] In relational expression 2 according to one embodiment of the present invention, ΔWsa is a Wsa measurement value of a plated steel sheet, and represents the difference between the Wsa value before processing (deformation) of the plated steel sheet and the Wsa value after processing (deformation). It is to be noted that the processing (deformation) may be imparted through money-making forming as an example, but is not limited thereto.

[0123] In one embodiment of the present invention, the galvanized steel sheet has excellent bake hardening properties, and specifically, the bake hardening amount (L-BH, tensile test after heat treatment at 170°C for 20 minutes) may be 30 MPa or more.

[0124] In addition, in one embodiment of the present invention, the galvanized steel sheet can have properties of a yield strength of 180 MPa or more, a tensile strength of 300 MPa or more, and an elongation of 33% or more, and thus has an effect of being suitably applicable as an automobile exterior panel material.

[0125]

[0126] Hereinafter, a method for manufacturing a galvanized steel sheet according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method is an example for manufacturing a galvanized steel sheet according to one embodiment of the present invention.

[0127] According to one embodiment of the present invention, a galvanized steel sheet may include the steps of preparing a base steel sheet; hot-dip galvanizing the base steel sheet to obtain a galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the base steel sheet; and subjecting the galvanized steel sheet to a temper rolling process. Each process step is described in detail below.

[0128] [Preparation of the steel plate]

[0129] First, a base steel sheet for performing a galvanizing process according to one embodiment of the present invention is prepared. In one embodiment of the present invention, the base steel sheet may be a cold-rolled steel sheet, and the cold-rolled steel sheet may be a cold-rolled steel sheet according to one embodiment of the present invention. That is, since it may be the aforementioned cold-rolled steel sheet, it is to be noted that the contents of the base steel sheet are replaced with the contents of the aforementioned cold-rolled steel sheet.

[0130] [Hot-dip galvanizing]

[0131] By performing hot-dip galvanization on the prepared cold-rolled steel sheet, i.e., the cold-rolled steel sheet, in a continuous hot-dip galvanization line, a galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the cold-rolled steel sheet can be obtained.

[0132] In one embodiment of the present invention, hot-dip galvanizing may be performed by immersing a steel sheet in a plating bath containing zinc as a main component, and the components in the plating bath are not particularly limited. In other words, it should be noted that the conditions typically applied to manufacturing galvanized steel sheets using the hot-dip galvanizing process may be applied.

[0133] In one embodiment of the present invention, the hot-dip galvanizing can be performed at a temperature range of 440 to 480°C. The temperature range refers to the temperature of the plating bath.

[0134] [Temperature rolling]

[0135] Temper rolling can be performed on the above galvanized steel sheet.

[0136] In one embodiment of the present invention, temper rolling can be performed using a roll having a roughness (Ra) of 1.0 to 2.2 ㎛ at a rolling force of 0.6 to 1.8%.

[0137] In one embodiment of the present invention, if the roughness (Ra) of the roll during the temper rolling is less than 1.0 ㎛, the target surface cannot be secured due to low roughness work, and on the other hand, if it exceeds 2.2 ㎛, the subsequent forming process, for example, the press process, cannot be performed smoothly.

[0138] In one embodiment of the present invention, if the pressure is less than 0.6% during temper rolling using a roll with controlled surface roughness, there is a concern that the hardenability may be poor, whereas if it exceeds 1.8%, there is a concern that the yield strength may be excessively high.

[0139] In one embodiment of the present invention, the temper rolling treatment may be performed by one or more methods selected from electrical discharge machining (EDT), shot blasting (SBT), and topocrom texturing (TCT). As an example, topocrom texturing (TCT) is a method of forming roughness in a chromium (Cr) coating layer itself by controlling the temperature and current density of a coating solution during a chromium (Cr) coating process, without a process of mechanically and physically processing the roughness.

[0140] A galvanized steel sheet manufactured according to one embodiment of the present invention not only has excellent hardenability and aging resistance, but also has excellent sharpness by uniformly securing surface grains.

[0141]

[0142] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended solely to illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0143] (Example)

[0144] A steel slab having an alloy composition shown in Table 1 below and a thickness of 250 mm was prepared, heated at 1300°C or lower, and then subjected to hot rolling, coiling, cold rolling, and annealing treatments under the conditions shown in Table 2 below to produce a cold-rolled steel sheet. Each cold-rolled steel sheet was immersed in a molten zinc-plating bath at 455±5°C to perform plating, thereby obtaining a galvanized steel sheet. Thereafter, each galvanized steel sheet was subjected to temper rolling using rolls according to the conditions shown in Table 2 below.

[0145] The mechanical properties, hardenability, and deep drawing workability of the galvanized steel sheet manufactured as described above were evaluated, and the results are shown in Table 3 below.

[0146] Tensile properties such as yield strength (YS), tensile strength (TS), and elongation (T-El) were measured using a universal tensile testing machine according to ISO6892 after collecting test specimens according to JIS No. 5 standard.

[0147] For the test specimens identical to those used to evaluate the above tensile properties, a 2% pre-strain was applied, and then baked at 170°C for 20 minutes. Then, the L-BH (Lower Yield Stress) was measured to evaluate the bake hardening property. At this time, only those with an L-BH value of 30 MPa or higher were judged to be passed.

[0148] In order to measure the r value, which is an indicator of deep drawing processing, JIS 5 tensile test pieces were collected for each galvanized steel sheet in three directions: parallel to the rolling direction, 45° direction, and perpendicular direction, and then the r value of each test piece was measured. The r value was derived by measuring the change in plate thickness and plate width at the time of tensile deformation of approximately 15% in the previously performed tensile test and calculating the ratio of the change in plate width to plate thickness. At this time, the r value in the direction parallel to the rolling direction was r0, and the r value in the 45° direction was r 45 , the r value in the orthogonal direction is r 90When determining, each value was substituted into the (formula) below to obtain the final r value, and a case where the value was 1.0 or higher was judged as passing.

[0149] (Formula) r value = r0+ (2×r 45 ) + (r 90 / 4)

[0150] Meanwhile, in order to confirm the grain distribution on the extreme surface of the cold-rolled steel sheet, which is the base steel sheet of the galvanized steel sheet, the average grain size was measured using the orientation imaging microscopy (OIM) analysis software of TSL (Te×SEM Laboratories) through electron backscattered diffraction (EBSD) analysis of the annealed cold-rolled steel sheet. In addition, when the microstructure of each cold-rolled steel sheet was confirmed by an optical microscope, ferrite with an area fraction of 95% or more was observed in all steel types (invention steels 1 to 9 and comparative steels 1 to 6). At this time, the remaining structure excluding ferrite was pearlite phase.

[0151] Then, in order to measure the surface waviness of the galvanized steel sheet, a blank measuring 225 mm × 225 mm was manufactured, and then the blank was pressed into a cup using a press having the force of a hollow punch with a diameter of 75 mm and a blank holder so that any material movement between the blank holder and the die was completely suppressed. At this time, the deformation of the cup was performed with a punch pulling depth of approximately 17 to 18 mm so that the thickness deformation rate of the bottom was approximately 5% ± 0.2%. Thereafter, the surface waviness of the blank before deformation and the surface waviness after deformation were measured, and the difference (ΔWsa) between the values ​​was obtained.

[0152] Steel grade alloy composition (weight %) CSiMnPSAlNbCuN Inventive steel 10.00160.0250.130.0200.00600.0340.00300.00100.0015 Inventive steel 20.00170.0210.180.0190.00460.0350.00390.00800.0021 Inventive steel 30.00210.0350.190.0220.00800.0330.00300.00500.0018 Inventive steel 40.00180.0320.150.0210.00430.0350.00300.00800.0022 Inventive steel 50.00230.0180.150.0200.00620.0320.00480.00650.0019 Invention steel 60.00180.0220.180.0250.00540.0350.00300.00240.0018 Invention steel 70.00190.0240.150.0200.00870.0310.00500.00210.0025 Invention steel 80.00150.0280.170.0240.0100.0290.00300.00800.0023 Invention steel 90.00170.0200.200.0180.0100.0320.00350.01000.0015Comparison steel 10.00180.0260.180.0220.00880.0350.00320.00500.0013Comparison steel 20.00180.0300.140.0230.00920.0400.00540.13000.0022Comparison steel 30.00180.0240.180.0210.00900.0390.00800.15000.0018Comparison steel 40.00180.0280.150.0200.01100.0310.00110.12000.0016Comparative steel 50.00160.0250.130.0200.00600.0340.00300.00100.0027Comparative steel 60.00170.0210.180.0190.00460.0350.01200.00800.0024 In Table 1, comparative steels 1 and 5 are marked as comparative steels because the alloy composition satisfies the present invention, but the process conditions in Table 2 below deviate from the present invention.

[0153] Steel grade Finishing Hot rolling temperature (℃) Coiling temperature (℃) Cold rolling reduction ratio (%) Annealing temperature (℃) Temper rolling roll surface roughness (Ra, ㎛) Rolling force (%) Inventive steel 1910611787881.81.52 Inventive steel 2915620797902.01.62 Inventive steel 3905613808102.11.3 Inventive steel 4921618788202.11.31 Inventive steel 5899625807912.01.51 Inventive steel 6912610818001.90.8 Inventive steel 7916605797952.11.27 Inventive steel 8925619808162.01.4 Inventive steel 9890624808112.01.31 Comparison Steel 1911621808392.01.54 Comparison Steel 2905613808101.91.3 Comparison Steel 3899625807912.11.51 Comparison Steel 4912610818002.11.6 Comparison Steel 5910620607902.01.52 Comparison Steel 6908615808352.01.4

[0154] Steel grade d (㎛) Relationship 1 YS (MPa) TS (MPa) El (%) r value L-BH (MPa) ΔWsa (㎛) Inventive steel 1 178.5 20 8 3 16 4 2 1.5 6 6 0.07 Inventive steel 2 189.0 19 9 3 11 4 5 1.5 4 0 0.06 Inventive steel 3 20 10 19 4 3 4 6 1.4 3 8 0.08 Inventive steel 4 19 9 2 3 0 8 4 1 1.8 5 0.13 Inventive steel 5 189.0 20 0 3 24 4 3 1.6 3 2 0.05 Inventive steel 6 21 10 5 21 23 17 4 2 1.5 4 5 0.08 Inventive steel 7 157.5 21 13 0 9 4 0 1.5 5 5 0.09 Inventive steel 8209.0193313461.8470.11 Inventive steel 9199.5205327431.6390.10 Comparative steel 12512.5191339481.8350.17 Comparative steel 22311.6201318451.5420.19 Comparative steel 32412.1198308431.6490.16 Comparative steel 42311.6208328421.5480.18 Comparative steel 52512.5232340400.9280.16 Comparative steel 6189.0178301481.7310.07 In Table 3, d is the average grain size of ferrite in the extreme surface layer of the annealed cold-rolled steel sheet. As for the size, it is the average size of crystal grains measured on the ND plane (rolled surface) at a point within 100 ㎛ in the thickness direction from the surface of the cold-rolled steel sheet. Equation 1 represents the value derived from [Nb(12 / 93) + S(63 / 32) + Cu(32 / 63) + d / 2].

[0155]

[0156] As shown in Tables 1 to 3 above, the invention steels 1 to 9, which satisfy both the alloy composition and manufacturing conditions proposed in the present invention, have uniform and fine surface grains formed (satisfying Equation 1) on the base steel sheet (cold rolled steel sheet). As a result, all of the invention steels 1 to 9 have an L-BH, which is an indicator of bake hardenability, of 30 MPa or more, and an r value, which is an indicator of deep drawing processing, of 1.0 or more, so that not only are they excellent in bake hardenability and aging resistance, but also the difference in surface waviness before and after deformation (ΔWsa) is secured as 0.15 ㎛ or less, so that it can be confirmed that they have excellent sharpness.

[0157] On the other hand, comparative steels 1 to 6, which do not satisfy the alloy composition or manufacturing conditions proposed in the present invention, had at least one or more inferior properties.

[0158] Although the alloy composition satisfies the present invention, the comparative steel 1, which had an annealing temperature that was too high among the manufacturing conditions, had poor sharpness because the surface grains of the base steel sheet (cold rolled steel sheet) were non-uniform and did not satisfy equation 1.

[0159] Comparative steels 2 to 4, which had excessive Cu content in the steel, showed a difference in surface waviness (ΔWsa) before and after deformation exceeding 0.15 ㎛ due to the value of relational expression 1 exceeding 11. In other words, the sharpness was inferior.

[0160] In addition, even in the case of comparative steel 5, which satisfies the alloy composition of the present invention but has too low a reduction ratio during cold rolling, it was not possible to uniformly secure the crystal grains on the surface of the base steel sheet (cold rolled steel sheet) so that relational expression 1 was satisfied, and as a result, the sharpness was inferior, and the workability and hardenability were also inferior.

[0161] Meanwhile, in the case of comparative steel 6, relational expression 1 was satisfied, but the yield strength was inferior due to the excessive Nb content in the steel and the annealing being performed at a high temperature.

[0162] Although not shown in the above Tables 1 to 3, the invention steels 1 to 9 and the comparative steels 1 to 6 contained carbon contents in the range of 0.001 to 0.005%, so that the ferrite phase was formed in the microstructure at 95 area% or more.

Claims

1. Contains, in weight%, carbon (C): 0.0010 to 0.0050%, silicon (Si): 0.050% or less (excluding 0%), manganese (Mn): 0.10 to 0.25%, phosphorus (P): 0.010 to 0.035%, sulfur (S): 0.0100% or less, nitrogen (N): 0.0050% or less, aluminum (Al): 0.100% or less (excluding 0%), niobium (Nb): 0.0010 to 0.0100%, copper (Cu): 0.0100% or less (excluding 0%), the remainder iron and other inevitable impurities. Cold rolled steel sheet satisfying the following relational expression 1. [Relationship 1] Nb(12 / 93) + S(63 / 32) + Cu(32 / 63) + d / 2 ≤ 11 (In relational expression 1, d means the average grain size (㎛) of ferrite within the polar surface portion, and the polar surface portion means the ND plane (rolling surface) within 100 ㎛ in the thickness direction from the surface of the cold rolled steel sheet.) 2. In paragraph 1, The above cold rolled steel sheet is a cold rolled steel sheet whose microstructure is composed of ferrite with an area fraction of 95% or more.

3. In paragraph 2, Cold rolled steel sheet having an average grain size of the above ferrite of 20㎛ or less.

4. A step for preparing a steel slab containing, in wt%, carbon (C): 0.0010 to 0.0050%, silicon (Si): 0.050% or less (excluding 0%), manganese (Mn): 0.10 to 0.25%, phosphorus (P): 0.010 to 0.035%, sulfur (S): 0.0100% or less, nitrogen (N): 0.0050% or less, aluminum (Al): 0.100% or less (excluding 0%), niobium (Nb): 0.0010 to 0.0100%, copper (Cu): 0.0100% or less (excluding 0%), the remainder iron and other unavoidable impurities; A step of heating the above steel slab in a temperature range of 1100 to 1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab at a temperature of Ar3 or higher; A step of coiling the above hot-rolled steel plate at a temperature range of 600 to 650°C; A step of obtaining a cold rolled steel sheet by cold rolling the above-mentioned hot rolled steel sheet after pickling it with a reduction ratio of 70 to 83%; and A method for manufacturing a cold rolled steel sheet, comprising a step of annealing the cold rolled steel sheet at a temperature range of 760 to 820°C.

5. In paragraph 4, A method for manufacturing a cold rolled steel sheet, wherein the cold rolled steel sheet subjected to the above annealing treatment satisfies the following relational expression 1. [Relationship 1] Nb(12 / 93) + S(63 / 32) + Cu(32 / 63) + d / 2 ≤ 11 (In relational expression 1, d means the average grain size (㎛) of ferrite within the polar surface portion, and the polar surface portion means the ND plane (rolling surface) within 100 ㎛ in the thickness direction from the surface of the cold rolled steel sheet.) 6. A galvanized steel sheet comprising a zinc-based plating layer provided on at least one surface of the steel sheet, Galvanized steel sheet having ΔWsa of 0.15㎛ or less, as defined by the following relational expression 2. [Relationship 2] ΔWsa = Surface waviness of the steel plate after 5% deformation - Surface waviness of the steel plate before deformation 7. In paragraph 6, The above galvanized steel sheet is a galvanized steel sheet with a baking hardening amount (L-BH, tensile test after heat treatment at 170℃ for 20 minutes) of 30 MPa or more.

8. In paragraph 6, The above-mentioned steel sheet is a galvanized steel sheet which is a cold-rolled steel sheet according to any one of claims 1 to 3.

9. In paragraph 6, The above galvanized steel sheet is a galvanized steel sheet having a yield strength of 180 MPa or more, a tensile strength of 300 MPa or more, and an elongation of 33% or more.

10. Step for preparing the steel plate; A step of hot-dip galvanizing the above-mentioned base steel sheet to obtain a galvanized steel sheet having a zinc-based plating layer formed on at least one surface of the above-mentioned base steel sheet; and It includes a step of subjecting the above galvanized steel sheet to temper rolling treatment, A method for manufacturing a galvanized steel sheet, wherein the above-mentioned temper rolling treatment is performed using a roll having a roughness (Ra) of 1.0 to 2.2 ㎛ and a pressing force of 0.6 to 1.8%.

11. In paragraph 10, A method for manufacturing a galvanized steel sheet, wherein the above-mentioned hot-dip galvanizing is performed in a zinc-based plating bath at 440 to 480°C.

12. In paragraph 10, A method for manufacturing a galvanized steel sheet, wherein the above-mentioned temper rolling treatment is performed by at least one method selected from electrical discharge machining (EDT), shot blasting (SBT), and topocrom texturing (TCT).

13. In paragraph 10, The above-mentioned steel sheet is a method for manufacturing a galvanized steel sheet, which is a cold-rolled steel sheet according to any one of claims 1 to 3.