Plated steel sheet excellent in strength, formability, and surface quality, and method for producing the same

Abstract

The present invention relates to an ultra-low carbon plated steel sheet having excellent strength, formability and surface quality that can realize weight reduction of automobiles, and a manufacturing method thereof. More specifically, the present invention relates to a zinc or zinc alloy plated steel sheet having high strength and high formability that can be suitably used as a raw material for automobile exterior panels, and a manufacturing method thereof.

Description

Technical Field

[0001] The present invention relates to an extra-low carbon steel plated sheet excellent in strength, formability, and surface quality capable of realizing weight reduction of an automobile, and more particularly to a zinc-based plated sheet having high strength and high formability suitable as a material for an outer panel of an automobile and a method for manufacturing the same.

Background Art

[0002] A cold-rolled steel sheet processed by press working or the like is used as an outer panel of an automobile, and generally high formability is required. In recent years, from the viewpoint of preventing global warming, as a measure for regulating carbon dioxide emissions, a new target for improving the fuel efficiency of automobiles has been set, and a preferential tax system for low-fuel consumption automobiles has been introduced. Therefore, an improvement in the fuel efficiency of automobiles is required. To improve the fuel efficiency of an automobile, weight reduction of the automobile body is an effective means, and from such a weight reduction viewpoint, slimming of the steel sheet for the automobile body has been required. On the other hand, from the viewpoint of ensuring the stability of the automobile body, high strength of the steel sheet for the automobile body has been required. To meet the requirements of such slimming and high strength of the steel sheet, a zinc-based plated high-tensile steel sheet excellent in surface appearance and press formability is required as a steel sheet for an automobile body pressed into a complex shape.

[0003] In order to improve the formability of a steel sheet for an automobile, there is so-called IF steel (Interstitial Free Steel) in which Ti or Nb is added alone or in combination to an extra-low carbon cold-rolled steel sheet, and solid solution elements such as C, N, and S are precipitated in the form of carbides and nitrides to increase the elongation and the plastic deformation ratio, thereby improving the formability. Therefore, conventionally, high purification has been achieved at the steelmaking stage, and the aging phenomenon due to solid solution elements has been limited by a method of adding carbonitride-forming elements such as Ti and precipitating solid solution elements. In addition, in the case of a high-tensile steel sheet, a method of containing solid solution strengthening elements such as Si, Mn, and P in the steel to improve the strength of the steel sheet has been carried out.

[0004] On one hand, in order to manufacture a high-strength zinc-based plated steel sheet, annealing treatment is performed in a mixed atmosphere of hydrogen and nitrogen to ensure the material quality. In such an annealing atmosphere, reduction of the base iron (Fe) occurs. In the case of elements that are prone to oxidation, such as Si, Mn, Al, etc., they react with trace amounts of O2 or H2O contained in the annealing atmosphere to form oxides. When oxides are formed on the surface of the base iron, problems such as unplated areas occurring during subsequent plating processes or the formation of non-uniform plating layers will occur.

[0005] Therefore, a technology at a level that can meet the demand for zinc-plated steel sheets with excellent surface quality, high strength, and high formability has not yet been developed.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] One aspect of the present invention aims to provide a plated steel sheet with excellent surface quality, high strength, and high formability, and a manufacturing method thereof.

[0008] The problems of the present invention are not limited to the above content. Anyone with ordinary knowledge in the technical field to which the present invention belongs will have no difficulty in understanding further problems of the present invention from the content throughout the specification of the present invention.

Means for Solving the Problems

[0009] One aspect of the present invention is base iron, and a plating layer formed on the base iron, and includes The above-described cast iron contains, by weight%, C: 0.003 to 0.009%, Si: 0.05% or less (excluding 0%), Mn: 0.4 to 1.0% (excluding 0%), P: 0.04 to 0.09%, S: 0.01% or less (excluding 0%), N: 0.005% or less (excluding 0%), Sol.Al: 0.1% or less (excluding 0%), Mo: 0.03 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.045%, Cu: 0.04 to 0.15%, B: 0.0015% or less (excluding 0%), and the balance Fe and other inevitable impurities, To provide a plated steel sheet that satisfies the following relational expressions 1 and 2.

[0010] [Relational expression 1] 0 < 10×[Si] / [Mn] ≦ 1.3 (In the above relational expression 1, the above [Si] represents the content of the average weight% of Si in the base iron, and the above [Mn] represents the content of the average weight% of Mn in the base iron.)

[0011] [Relational expression 2] 0 ≦ [Ao] / [At] ≦ 0.15 (In the above relational expression 2, the above [At] represents the area of the region between the interface line and the separated line when, with respect to the cross-section of the above plated steel sheet, a line is drawn 0.3 μm apart in the thickness direction of the plating layer side from the interface line having a length of 500 nm or more between the base iron and the plating layer, and the above [Ao] represents the area occupied by the Mn-Si-O-based composite oxide in the region between the interface line and the separated line.)

[0012] Also, still another aspect of the present invention is By weight, C: 0.003 to 0.009%, Si: 0.05% or less (excluding 0%), Mn: 0.4 to 1.0% (excluding 0%), P: 0.04 to 0.09%, S: 0.01% or less (excluding 0%), N: 0.005% or less (excluding 0%), Sol.Al: 0.1% or less (excluding 0%), Mo: 0.03 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.045%, Cu: 0.04 to 0.15%, B: 0.0015% or less (excluding 0%), the balance being Fe and other inevitable impurities, after continuously casting the steel satisfying the above-mentioned relational expression 1, a step of machining and removing 2 to 5 mm in the thickness direction from the surface of the steel, After reheating the steel slab obtained after the above-mentioned machining and removing at 1180 to 1230°C, hot rolling at Ar3 or higher to provide a hot-rolled steel sheet, A step of winding up the above-mentioned hot-rolled steel sheet at 600 to 650°C, A step of cold rolling the wound hot-rolled steel sheet at a reduction ratio of 70 to 83% to provide a cold-rolled steel sheet, A step of annealing the above-mentioned cold-rolled steel sheet at 740 to 830°C, Performing hot-dip galvanized coating on the annealed cold-rolled steel sheet, and performing alloying heat treatment on the steel sheet having a zinc-based coating layer formed on the surface at 500 to 560°C, A step of temper rolling at a reduction ratio of 0.6 to 1.2% using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm, Providing a method for manufacturing a coated steel sheet, including the above.

Advantages of the Invention

[0013] According to one aspect of the present invention, it is possible to provide a coated steel sheet having excellent surface quality, high strength, and high formability, and a method for manufacturing the same.

[0014] The various and beneficial advantages and effects of the present invention are not limited to the above-mentioned content, and can be more easily understood in the process of describing the specific embodiments of the present invention.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Best Mode for Carrying Out the Invention

[0016] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those having average knowledge in the technical field.

[0017] As a result of intensive research to solve the above-mentioned problems of the prior art, the present inventors added titanium (Ti) and / or niobium (Nb), which are strong carbonitride-forming elements, to steel to minimize solid solution elements such as carbon (C), nitrogen (N), and sulfur (S), thereby ensuring formability. At the same time, solid solution strengthening elements such as Si, Mn, and P were added to ensure a high strength of 390 MPa or more in tensile strength. By controlling the components and the degree of alloying, it was confirmed that excellent plating properties could be ensured, and the present invention was thus completed. Hereinafter, a high-strength alloyed hot-dip galvanized steel sheet with excellent surface quality and a manufacturing method thereof, which are preferably applicable as materials for outer automotive panels of the present invention, will be specifically described.

[0018] As an automotive steel sheet, with the increase in tensile strength, it must satisfy press formability such as deep drawability. The descaled rolled steel sheet serving as the base material of the alloyed hot-dip galvanized steel sheet according to this embodiment uses a high-tensile steel sheet with an extra-low carbon steel as the basic component and added solid solution strengthening elements such as Si, Mn, and P to improve workability.

[0019] That is, the plated steel sheet according to one aspect of the present invention includes a base iron and a plating layer formed on the base iron. At this time, the base iron contains, by weight%, C: 0.003 to 0.009%, Si: 0.05% or less (excluding 0%), Mn: 0.4 to 1.0% (excluding 0%), P: 0.04 to 0.09%, S: 0.01% or less (excluding 0%), N: 0.005% or less (excluding 0%), Sol.Al: 0.1% or less (excluding 0%), Mo: 0.03 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.045%, Cu: 0.04 to 0.15%, B: 0.0015% or less (excluding 0%), and the balance Fe and other inevitable impurities. Hereinafter, the reasons for adding the basic components of the base iron and the reasons for limiting them will be described.

[0020] Carbon (C): 0.003 - 0.009% C is an interstitial solid solution element and has a great influence on the formation of the microstructure of the steel sheet during cold rolling and annealing processes. When the amount of carbon dissolved in the steel increases, the growth of crystal grains having a {111} gamma (γ)-fiber texture, which is advantageous for drawing, is suppressed, the growth of crystal grains having a {110} and {100} texture is promoted, and the drawability of the annealed sheet decreases. Further, when the content of C exceeds 0.009%, the contents of Ti and Nb required to precipitate this as carbides increase, which is not only disadvantageous in terms of economy, but also pearlite or the like may be generated, resulting in a decrease in formability. Therefore, it is preferable to limit the C content to 0.009% or less. Also, when the C content is less than 0.003%, sufficient strength may not be ensured, so it is preferable to limit the C content to 0.003% or more. However, more preferably, the lower limit of the C content may be 0.0038%, and the upper limit of the C content may be 0.0080%.

[0021] Silicon (Si): 0.05% or less (excluding 0%) Si is an element that contributes to the increase in strength by solid solution strengthening. In order to exert the effect of increasing strength by such solid solution strengthening, it is necessary to add Si essentially, so the Si content in the above base iron is limited to exceed 0%. On the other hand, when the above Si content exceeds 0.05%, there is a problem that surface scale defects are induced and the plating surface characteristics deteriorate. Therefore, in the present invention, the above Si content is controlled to be 0.05% or less. However, more preferably, the lower limit of the above Si content may be 0.01%, and the upper limit of the above Si content may be 0.042%.

[0022] Manganese (Mn): 0.4 - 1.0% Mn is a solid solution strengthening element, which not only contributes to the increase in strength but also plays a role in precipitating S in the steel as MnS. When the content of the above Mn is less than 0.4%, a decrease in strength is a concern, while when the content of the above Mn exceeds 1.0%, there is a possibility of surface problems due to oxides. Therefore, it is preferable to limit the content of the above Mn to 0.4 to 1.0%. However, more preferably, the lower limit of the above Mn content may be 0.48%, and the upper limit of the above Mn content may be 0.80%.

[0023] Phosphorus (P): 0.04 - 0.09% P has the best solid solution effect and is the most effective element for ensuring the strength of steel without significantly impairing the drawability. When the content of the above P is less than 0.04%, it is impossible to ensure the target strength, while when the content of the above P exceeds 0.09%, secondary brittleness and surface streak pattern defects due to P segregation may occur. Therefore, it is preferable to limit the content of the above P to 0.04 to 0.09%. However, more preferably, the lower limit of the above P content may be 0.048%, and the upper limit of the above P content may be 0.089%.

[0024] Molybdenum (Mo): 0.03 - 0.08% Mo is an element with a high affinity for P (phosphorus) and plays a role in suppressing P segregation. In order to ensure high strength in ultra-low carbon steel, P must be inevitably utilized, but appropriately adding Mo can partly contribute to improving surface defects caused by P segregation. When the Mo content is less than 0.03%, there is no significant effect on the intended surface improvement. Also, when the Mo content exceeds 0.08%, the price becomes high and the cost competitiveness decreases. Therefore, it is preferable to limit the content of Mo to 0.03 - 0.08%. However, more preferably, the lower limit of the Mo content may be 0.05%, and the upper limit of the Mo content may be 0.078%.

[0025] Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.005% or less (excluding 0%) S and N are impurities present in steel and are inevitably added. Therefore, the S and N contents in the base iron each independently exceed 0%. However, in order to ensure excellent welding characteristics, it is preferable to control their contents as low as possible. Thus, in the present invention, the S content is controlled to be 0.01% or less, and the N content is controlled to be 0.005% or less. However, more preferably, the lower limit of the S content may be 0.0015%, and the upper limit of the S content may be 0.0034%. Also, more preferably, the lower limit of the N content may be 0.0008%, and the upper limit of the N content may be 0.004%.

[0026] Aluminum (Al): 0.1% or less (excluding 0%) Al precipitates AlN and contributes to improving the drawability and ductility of steel. In order to exert such an effect of improving drawability and ductility, the Al content in the base iron exceeds 0%. However, when the Al content exceeds 0.1%, there is a problem that internal defects of the steel sheet occur due to excessive formation of Al inclusions during steelmaking operations. Therefore, it is preferable to control the Al content to 0.1% or less. However, more preferably, the lower limit of the Al content may be 0.025%, and the upper limit of the Al content may be 0.08%.

[0027] Titanium (Ti): 0.005 - 0.03% Ti is an element that greatly contributes to improving the drawability of steel sheets by reacting with dissolved carbon and dissolved nitrogen during hot rolling to precipitate Ti-based carbonitrides. When the above Ti content is less than 0.005%, carbonitrides cannot be sufficiently precipitated, and the drawability decreases. On the other hand, when the above Ti content exceeds 0.03%, it is difficult to manage inclusions during steelmaking operations, and there is a possibility of inclusion property defects. Therefore, it is preferable to limit the content of the above Ti to 0.005 - 0.03%. However, more preferably, the lower limit of the above Ti content may be 0.007%, and the upper limit of the above Ti content may be 0.012%.

[0028] Niobium (Nb): 0.02 - 0.045% Nb is the most effective element that can produce very fine grains through the rolling and cooling processes when the unrecrystallized region in the austenite range becomes wider at high temperatures due to the hot rolling solute drag and precipitate pinning effects. When the above Nb content is less than 0.02%, the range of the austenite unrecrystallized temperature region in the steel becomes narrow, and the effect of grain size refinement is slight. On the other hand, when the above Nb content exceeds 0.045%, there is a problem that the high temperature strength increases and it brings difficulties to hot rolling. Therefore, it is preferable to limit the content of the above Nb to 0.02 - 0.045%. However, more preferably, the lower limit of the above Nb content may be 0.028%, and the upper limit of the above Nb content may be 0.044%.

[0029] Boron (B): 0.0015% or less (excluding 0%) B is an element added to prevent secondary processing brittleness caused by P addition in steel. In order to exhibit the above-mentioned effect of preventing secondary processing brittleness, the B content in the base iron exceeds 0%. However, when the B content exceeds 0.0015%, it is accompanied by a decrease in the ductility of the steel sheet. Therefore, it is preferable to limit the content of the above B to 0.0015% or less. On the other hand, more preferably, the lower limit of the above B content may be 0.0004%, and the upper limit of the above B content may be 0.001%.

[0030] Copper (Cu): 0.04 - 0.15% Cu is an element added to ensure strength and is an element that is difficult to remove when adjusting the steel composition by steelmaking. Therefore, it is preferable to add 0.04% or more of Cu to ensure strength. However, if the Cu content exceeds 0.15%, it will lead to grain boundary embrittlement and cost increase. Therefore, the Cu content is limited to 0.15% or less. On the other hand, more preferably, the lower limit of the Cu content may be 0.06%, and the upper limit of the Cu content may be 0.10%.

[0031] In addition to this, the remaining Fe and inevitable impurities are included. The addition of effective components other than the above composition is not excluded. On the other hand, all the inevitable impurities can be included as long as they can be unintentionally mixed in the normal manufacturing process of the plated steel sheet. Since those skilled in the art can easily understand its meaning, it is not particularly limited.

[0032] In addition, the above plated steel sheet can satisfy the following relational expression 1.

[0033] [Relational Expression 1] 0 < 10×[Si] / [Mn] ≤ 1.3 (In the above relational expression 1, the above [Si] represents the content of the average weight% of Si in the base iron, and the above [Mn] represents the content of the average weight% of Mn in the base iron.)

[0034] That is, when the value of 10×[Si] / [Mn] defined by the above relational expression 2 exceeds 1.3, surface Si oxides occur frequently during annealing and the plating wettability deteriorates. Therefore, there is a possibility of surface defects due to unplated or uneven plating in the final product. This is judged to result in the formation of Si alone and composite oxides easily during annealing because Si is more likely to undergo an oxidation reaction than Mn, leading to the occurrence of surface defects. On the other hand, from the perspective of further improving the above-described effects, more preferably, the lower limit of the value of 10×[Si] / [Mn] may be 0.48, and the upper limit of the value of 10×[Si] / [Mn] may be 0.68.

[0035] By satisfying the above-described component system, the present invention can effectively provide a high-strength, extra-low-carbon alloyed hot-dip galvanized steel sheet for automobile outer panels with excellent formability.

[0036] That is, the present invention relates to an alloyed hot-dip zinc-based galvanized steel sheet having a high-tensile steel sheet containing extra-low-carbon steel as a basic component and strengthening elements such as Si, Mn, and P as a base material (base iron) in order to improve formability. The present invention can effectively provide an alloyed hot-dip zinc-based galvanized steel sheet for automobile outer panels having an excellent surface appearance and a method for producing the same by appropriately adjusting the composition and degree of alloying in order to prevent ungalvanized areas and the formation of non-uniform plating layers due to oxides on the surface of the base iron.

[0037] It is preferable that the above galvanized steel sheet satisfies the following relational expression 2. By satisfying the following relational expression 2, the galvanized steel sheet according to the present invention can stably control the ratio of interfacial oxides in the vicinity of the interface between the base iron and the plating layer, thereby ensuring excellent surface quality.

[0038] [Relational Expression 2] 0 ≦ [Ao] / [At] ≦ 0.15 (In the above relational expression 2, [At] indicates the area of the region between the interface line and the separated line when a line is drawn 0.3 μm apart in the thickness direction of the plating layer side from an interface line having a length of 500 nm or more between the base iron and the plating layer with respect to the cross section of the above galvanized steel sheet. [Ao] indicates the area occupied by the Mn-Si-O-based composite oxide in the region between the interface line and the separated line.)

[0039] When the value of [Ao] / [At] exceeds 0.15, the plating wettability deteriorates due to the surface oxide, and surface defects may occur due to unplated or uneven plating. On the other hand, since the present invention includes the case where the Mn—Si—O-based composite oxide does not exist near the interface between the base iron and the plating layer, the lower limit of the [Ao] / [At] value defined by the above relational expression 2 may be 0 (that is, the value of [Ao] / [At] includes 0, and the fact that the value of [Ao] / [At] is 0 can mean the case where the Mn—Si—O-based composite oxide is not included near the interface between the base iron and the plating layer).

[0040] On the other hand, the lower limit of the value of [Ao] / [At] may be 0%, and the upper limit of the value of [Ao] / [At] may be 0.08. Alternatively, in the case where the Mn—Si—O-based composite oxide exists near the interface between the base iron and the plating layer, the lower limit of the value of [Ao] / [At] can be more preferably 0.001%.

[0041] In this specification, the thickness direction can mean a direction perpendicular to the rolling direction. Further, the length of the interface line for calculating [Ao] and [At] in the above relational expression 2 may be 500 nm or more. Here, it can mean the value obtained by measuring the total length of the interface line drawn along the boundary between the base iron and the plating layer with respect to the cross section of the plated steel sheet as a reference. Therefore, when measuring the [Ao] and [At] values defined by the above relational expression 2, it can be measured on the basis that the total length of the interface line is 500 nm or more.

[0042] On the one hand, the measuring methods of [Ao] and [At] defined in the above relational expression 2 are schematically shown in FIG. 4. That is, with reference to the cross-section in the thickness direction of the plated steel sheet, after drawing an interface line 10 with a total interface length of 500 nm or more between the base iron 1 and the plating layer 2, a line 20 is drawn parallel to the thickness direction and separated from the interface line 10 by 0.3 μm. Thereby, the area At of the region between the interface line 10 and the separated line 20 can be obtained, and further, the area Ao of the Mn-Si-O-based composite oxide 100 existing in the region between the interface line 10 and the separated line 20 can be obtained. At this time, the cross-section of the plated steel sheet can be observed using a measuring device such as EDS so that the vicinity of the boundary between the base iron 1 and the plating layer 2 can be clearly seen. For example, the value of the above Ao can be confirmed by obtaining the area of the region corresponding to the hatched portion in FIG. 4.

[0043] In addition, the unit of [Ao] / [At] defined in the above relational expression 2 only needs to have the units of [Ao] and [At] unified. For example, if the unit of the above [Ao] is μm 2 then the unit of [At] is also μm 2 and unified, and if the unit of [Ao] is nm 2 then the unit of [At] is also nm 2 After unifying, the value of [Ao] / [At] can be obtained.

[0044] As a result of repeated research, the inventors have found that by suppressing the generation of the Mn-Si-O-based composite oxide to a specific amount or less in the vicinity of the interface between the base iron and the plating layer, a plated steel sheet having excellent surface quality and high strength suitable for use as an outer panel material for automobiles can be obtained. Therefore, by satisfying the above relational expression 2, the application range of the high-strength cold-rolled steel sheet containing P to an automobile body can be expanded to a range that has never been applied before (for example, also for side outer, etc.), and as a result, the weight of the automobile body can be further reduced.

[0045] At this time, the above Mn-Si-O-based composite oxide is Mn a SiO 2+awhere a can be a real number satisfying 0 < a ≤ 2. For example, as the Mn—Si—O composite oxide, there can be MnSiO3, Mn2SiO4, Mn 0.9 SiO 2.9 (that is, 0.9MnO·SiO2), and the like.

[0046] According to one aspect of the present invention, the average diameter of the Mn—Si—O composite oxide may be 200 nm or less. When the average diameter of the Mn—Si—O composite oxide exceeds 200 nm, there may occur a problem that dot-shaped unplated defects are generated. However, since the smaller the Mn—Si—O composite oxide is, the more advantageous it is for preventing surface defects, the lower limit of its average diameter may not be particularly limited. On the other hand, from the viewpoint of further improving the above-described effects, the upper limit of the average diameter of the Mn—Si—O composite oxide may be 100 nm, and the lower limit of the average diameter of the Mn—Si—O composite oxide may be 0 nm (that is, it means the case where the number of Mg—Si—O composite oxides is 0).

[0047] At this time, the average diameter of the Mn—Si—O composite oxide can mean the average value of the values obtained by measuring the circle-equivalent diameter with respect to the above-described Mn—Si—O composite oxide, based on the cut surface in the thickness direction (that is, the direction perpendicular to the rolling direction) of the plated steel sheet.

[0048] Further, according to one aspect of the present invention, when a line is drawn by separating the interface line between the base iron and the plating layer by 0.3 μm in the thickness direction on the plating layer side on the cut surface of the plated steel sheet, the area occupied by the Mn—Si—O composite oxide in the region between the interface line and the separated line can be larger than the area occupied by the Mn—Si—O composite oxide in the region between the separated line and the surface line of the plating layer. This is judged to be because the above-described Mn—Si—O composite oxide is mainly formed on the surface of the base iron during annealing in the manufacturing process and exists in the vicinity of the interface between the base iron and the plating layer while performing the hot-dip zinc plating.

[0049] On the one hand, according to one aspect of the present invention, since the base iron corresponds to an extra-low carbon steel with a C content of less than 0.01%, the base iron can have a ferrite-based microstructure. At this time, the ferrite-based microstructure can include other tissues that are inevitably generated.

[0050] Specifically, according to one aspect of the present invention, in terms of the area fraction, the microstructure of the base iron has 95% or more ferrite, and a small amount of pearlite or the like can remain (for example, the balance is pearlite). Alternatively, more preferably, the microstructure of the base iron has 99% or more ferrite in terms of area fraction and 1% or less pearlite. Alternatively, most preferably, the microstructure of the base iron may be a single-phase ferrite. By satisfying such microstructural characteristics, excellent formability can be ensured. That is, if the content of the microstructure such as pearlite other than the above-mentioned ferrite in the base iron exceeds 5%, there may be a problem that the formability deteriorates.

[0051] According to one aspect of the present invention, although not particularly limited, in the above base iron, the average grain size of the ferrite may be 5 to 15 μm (that is, 5 μm or more and 15 μm or less). If the average grain size of the ferrite is less than 5 μm, there may be a problem that the strength is too high and the elongation cannot be sufficiently ensured. Further, if the average grain size of the ferrite exceeds 15 μm, there may be a problem that the target strength cannot be ensured. However, from the perspective of further improving the above-mentioned effects, the lower limit of the average grain size of the ferrite may be 6 μm, and the upper limit of the average grain size of the ferrite may be 10 μm.

[0052] Here, the average grain size of the ferrite can mean the average value of the values obtained by measuring the equivalent circle diameter for the grains based on the cross-section in the thickness direction of the plated steel sheet (that is, the direction perpendicular to the rolling direction).

[0053] In this specification, the equivalent circular diameter described above can mean the value obtained by measuring the particle size when assuming spherical particles in which the longest length penetrating the inside of the crystal grains is depicted as the particle size.

[0054] On the other hand, according to one aspect of the present invention, the plating layer may be a hot-dip zinc-based plating layer or a zinc-based alloy plating layer. Although not particularly limited, as an example, the plating layer can have a composition containing Fe: 8 to 13% by weight, with the balance being Zn and other inevitable impurities, and excellent powdering properties can be easily ensured by satisfying the composition of the plating layer described above.

[0055] According to one aspect of the present invention, the tensile strength of the plated steel sheet may be 390 MPa or more (more preferably in the range of 390 to 480 MPa). Thus, by satisfying the range of the tensile strength of the plated steel sheet being 390 to 480 MPa, weight reduction of automobiles using high-tensile steel can be achieved.

[0056] Also, according to one aspect of the present invention, the elongation of the plated steel sheet is 15% or more, more preferably 28 to 43%, and most preferably 28 to 38%. Thus, by satisfying the range of the elongation of the plated steel sheet, excellent formability and workability can be ensured.

[0057] Next, the manufacturing method of the plated steel sheet will be described in detail. However, it does not necessarily mean that the plated steel sheet of the present invention should be manufactured by the following manufacturing method.

[0058] The manufacturing method of the plated steel sheet according to one aspect of the present invention includes a step of continuously casting steel having the composition described above, and the description of the composition of the steel is similarly applicable to the composition of the plated steel sheet described above.

[0059] After continuously casting the steel, it is possible to perform a machining process to remove 2 mm or more and 5 mm or less in the thickness direction (at this time, the thickness direction means the direction perpendicular to the rolling direction) from the surface of the steel. If the thickness to be machined is less than 2 mm, the segregation band of the difficult-to-oxidize element may not be removed, and surface defects may occur. If the thickness to be machined exceeds 5 mm, there may be a problem of a decrease in the actual yield rate.

[0060] In addition, the method for manufacturing the plated steel sheet includes the step of reheating the steel slab obtained by continuous casting to 1180 to 1230 °C and then performing hot rolling above Ar3 to provide a hot-rolled steel sheet. At this time, if the reheating temperature of the slab is less than 1180 °C, production problems may occur due to the rolling load in the FM section. If the reheating temperature of the slab exceeds 1230 °C, surface scale defects may occur. Further, the hot rolling can be performed so that the finish rolling temperature is above Ar3, and more specifically, it can be performed in the range of 880 to 970 °C. If the hot rolling is less than 880 °C, it is cooled in the two-phase region (i.e., below Ar3), and coarse grains are generated in the surface layer, resulting in non-uniform grain sizes in the surface layer and ultimately potential problems with mapability. If it exceeds 970 °C, the grain size may not become sufficiently fine, and there may be a problem of insufficient strength of the final material.

[0061] According to one aspect of the present invention, the coiling of the hot-rolled steel sheet can be performed in the range of 600 to 650 °C. If the coiling temperature is less than 600 °C, precipitates such as (Ti, Nb)C may not be sufficiently generated, which may cause problems such as precipitation during annealing, affecting recrystallization and grain growth, and making it difficult to ensure the desired strength and elongation. If the coiling temperature exceeds 650 °C, there may be a problem of deterioration of surface characteristics due to the formation of secondary hot-rolled scale.

[0062] Further, according to one aspect of the present invention, after coiling the hot-rolled steel sheet, it can be subjected to a pickling process, and then cold-rolled at a reduction ratio of 70 to 83% to obtain a cold-rolled steel sheet. When the reduction ratio during the cold rolling is less than 70%, the {111} texture may not grow sufficiently, resulting in a problem of reduced formability. On the other hand, when the reduction ratio during the cold rolling exceeds 83%, there may be a problem because the load on the rolling roll during on-site production is extremely intense and the shape deteriorates. Therefore, it is preferable to control the reduction ratio to 70 to 83%, and more preferably to 74 to 80%.

[0063] Next, the cold-rolled steel sheet can be annealed at a temperature above the recrystallization temperature in the range of 740 to 830°C. By annealing (annealing) at a temperature above the recrystallization temperature, the deformation generated by rolling can be removed, the steel sheet can be softened, and the workability can be improved. That is, if the annealing temperature is less than 740°C, the recrystallization of the ferrite phase may not be completed, and there may be a problem of insufficient elongation. If the annealing temperature exceeds 830°C, the grain growth may proceed excessively after the recrystallization is completed, and there may be a problem of insufficient strength.

[0064] On the other hand, although not particularly limited, according to one aspect of the present invention, the annealing can be performed by heat-treating at a temperature in the range of 740 to 850°C, then performing primary cooling at an average cooling rate of 2 to 6°C / s, and then performing secondary cooling at an average cooling rate of 6.5 to 15°C / s. By performing annealing so as to satisfy the above-described conditions, the amount of the complex oxide can be controlled within an appropriate range in the vicinity of the interface between the base iron and the plating layer, thereby preventing the non-uniformity of alloying caused by the complex oxide formed during annealing.

[0065] Also, although not particularly limited, according to one aspect of the present invention, the annealing can be performed in a range where the dew point temperature is -60 to -20°C. When the dew point temperature is less than -60°C during the annealing, there may be a problem of inferior economic efficiency in order to maintain the furnace atmosphere temperature. When the dew point temperature exceeds -20°C during the annealing, there may be a problem of frequent occurrence of surface oxides.

[0066] Also, after the annealing, the cold-rolled steel sheet can be directly subjected to hot-dip zinc-based plating in a continuous hot-dip zinc-based plating line. At this time, the zinc-based plating refers to plating performed by immersing in a plating bath containing 60% or more of Zn. As an example, the plating can be performed by immersing in a plating bath containing Al: 0.121 to 0.133%, with the balance being Zn and other inevitable impurities. Then, after the hot-dip zinc-based plating, an alloying heat treatment can be performed in the range of 500 to 560°C. At this time, if the alloying heat treatment temperature is less than 500°C, alloying does not proceed sufficiently, and if it exceeds 560°C, excessive alloying proceeds and the plating layer becomes brittle, which may induce problems such as peeling of the plating due to processing such as pressing.

[0067] Furthermore, the step of temper rolling can be further included for the alloying heat-treated steel sheet using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm at a reduction rate of 0.6 to 1.2% (which can mean the average reduction rate). During the temper rolling, if the roughness (Ra) of the skin pass roll is less than 1.0 μm, not only can surface defects caused by the Mn-Si-O-based composite oxide not be sufficiently suppressed, but there may also be insufficient imageability showing beautiful surface characteristics after painting. On the other hand, during the temper rolling, if the roughness (Ra) of the skin pass roll exceeds 1.6 μm, problems may occur in press formability. Also, if the reduction rate of the temper rolling is less than 0.6%, problems may occur in shape correction, etc., and if it exceeds 1.2%, problems may occur such that the yield strength exceeds the reference value due to the work hardening effect.

[0068] Note that from the perspective of further improving the above-described effects, the temper rolling is more preferably performed using a skin pass roll having a roughness (Ra) of 1.1 to 1.5 μm at a reduction rate of 0.6 to 1.2%.

[0069] Alternatively, according to one aspect of the present invention, although not particularly limited, the above temper rolling is performed by first performing primary temper rolling at a reduction rate of 0.05 to 0.4, and then performing secondary temper rolling at a reduction rate of 0.6 to 1.0%, thereby suppressing surface defects caused by complex oxides existing near the interface between the base iron and the plating layer, and contributing to ensuring excellent surface characteristics. At this time, the average reduction rate for the above primary temper rolling and secondary temper rolling satisfies the reduction rate of 0.6 to 1.2% described above.

Embodiments for Carrying Out the Invention

[0070] Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are for explaining the present invention through exemplification and do not limit the scope of rights of the present invention. The scope of rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0071] (Experimental Example 1) A steel slab with a thickness of 250 mm having the alloy compositions (the balance being Fe and other impurities, unit: wt%) shown in Tables 1 and 2 below was subjected to a machining treatment of 2 to 4 mm, then reheated to 1230°C, and hot rolling, coiling, cold rolling, annealing, plating, and alloying treatment were performed under the conditions shown in Table 3 below to produce a plated steel sheet. At this time, the above annealing was heat-treated at a dew point temperature in the range of -60 to -20°C and a temperature in the range of 740 to 850°C, and then, based on the surface temperature of the steel sheet, primary cooling was performed to 650°C at an average cooling rate of 2 to 6°C / s, and then secondary cooling was performed to 550°C at an average cooling rate of 6.5 to 15°C / s. Further, during the above plating, it was immersed in a zinc plating bath containing Al: 0.121 to 0.133% and the balance being Zn and other inevitable impurities to perform alloying hot-dip zinc plating.

[0072] For the electroplated steel sheet thus obtained, the occupancy ratio of the oxide ([Ao] / [At] value) according to Relational Expression 2 was measured using a TEM (Transmission electron microscopy)-EDS (Energy dispersive spectroscopy) apparatus. Specifically, the vicinity of the interface between the base iron and the hot-dip galvanized layer of the cross-section (cut surface in the thickness direction) of the hot-dip galvanized test piece was measured and elementally analyzed to measure the occupancy ratio of the oxide, and this value was shown in Table 4 below.

[0073] In addition, the yield strength (YS), tensile strength (TS), and elongation at break (El) were measured and shown in Table 4 below. At this time, after measuring the width, length of the parallel part, and thickness of the test piece, the test piece was mounted on a tensile testing machine and waited until the test piece was broken, and then the yield strength, tensile strength, and elongation at break of the test piece were measured. The yield strength is the limit stress when elastic deformation occurs, and usually shows a value with a 0.2% offset. The tensile strength shows the value obtained by dividing the maximum load by the original cross-section. The elongation at break represents the amount of deformation of the test piece after fracture from the tensile test in %. At this time, when the tensile strength was 390 MPa or more and the elongation rate was 15% or more, it was evaluated as qualified.

[0074] Note that for the evaluation of the r value, which is an index of deep drawing process, JIS No. 5 tensile test pieces were taken in three directions: parallel to the rolling direction, 45° direction, and perpendicular direction from the alloyed hot-dip galvanized steel sheet, and the r values of each test piece were measured and shown in Table 4 below. That is, for the measurement of the r value, the change value of the plate thickness and the change value of the plate width at the time when about 15% of tensile deformation was performed in the above tensile test were measured, and the ratio of the change value of the plate width to the plate thickness was obtained. And when the r value parallel to the rolling direction is r0, the r value in the 45° direction is r 45 and the r value in the perpendicular direction is r 90 when, the r values in the following respective directions were obtained from the following Relational Expression A, and when the r value was 1.2 or more, it was regarded as qualified.

[0075] [Relational Expression A] r = r0 + 2 × r 45 + r 90 / 4

[0076] The surface quality (presence or absence of white defects) of the above galvannealed steel sheet was evaluated, and the results are shown together in Table 4 below. At this time, the method and criteria for evaluating the surface quality were set as follows.

[0077] Surface quality: It is the degree to which the galvannealed layer coats the base steel sheet well, and the appearance of the plated steel sheet was observed visually. After performing annealing heat treatment on a cold-rolled steel sheet (F / H) under normal continuous annealing line conditions, if oxides are excessively or non-uniformly generated on the surface of the base iron, a non-uniform plating layer due to the difference in alloying speed is formed, and a defect where the region where the plating layer is relatively thickly formed appears white and conspicuous. When such a defect appears, it cannot be used for automotive outer panels, so the presence or absence of white defects was set as a measure for judging surface quality.

[0078] [Table 1]

[0079] [Table 2] Ar3 = 910 - 310×[C] - 80×[Mn] - 20×[Cu] - 15×[Cr] - 55×[Ni] - 80×[Mo] - 0.35×(thickness - 8)

[0080] [Table 3] FDT * : Finishing rolling CT * : Coiling Ra * : Skin pass roll roughness

[0081] [Table 4]

[0082] As can be seen from the experimental results in Tables 1 to 4 above, by satisfying the composition and manufacturing conditions of the substrate iron of the present invention, in the case of Invention Examples 1 to 8 that satisfy Relational Expressions 1 and 2, it was confirmed that the microstructure of the substrate iron with respect to the plated steel sheet is a ferrite single phase in terms of area fraction, the tensile strength is 390 MPa or more, the yield strength is in the range of 230 to 330 MPa, and while having high strength, the elongation rate is 15% or more and it is not only excellent in formability but also has good surface characteristics. Among these, a photograph of the cross-section in the thickness direction of the plated steel sheet obtained from Invention Example 4 of the present application is shown in FIG. 2. Specifically, FIG. 2 is a photograph taken using a transmission electron microscope (TEM) at a magnification of 40,000 near the interface between the substrate iron and the plating layer.

[0083] On the other hand, in Comparative Examples 1 to 4, the value of 10×[Si] / [Mn] in Relational Expression 1 exceeded 1.3, and further, the occupancy ratio [Ao] / [At] of the complex oxide according to Relational Expression 2 did not satisfy the range of the present invention, and it was confirmed that white defects occurred due to the non-uniform plating layer. Among these, a photograph of the surface of the plated steel sheet obtained from Comparative Example 4 taken with an optical camera at a magnification of 1 is shown in FIG. 1, and it was visually confirmed that white defects occurred. Also, a photograph of the cross-section in the thickness direction of the plated steel sheet obtained from Comparative Example 2 taken using EDS at a magnification of 100,000 is shown in FIG. 3, and as shown in FIG. 3, it was confirmed that an Mn-Si-O-based complex oxide exists near the interface between the substrate iron and the plating layer.

[0084] (Experimental Example 2) During temper rolling, except that primary and secondary temper rolling were performed under the conditions described in Table 5 below, finish rolling, coiling, cold rolling, annealing, plating, and alloying treatment were carried out under the conditions of Table 5 below in the same manner as in Experimental Example 1 described above to produce a plated steel sheet.

[0085]

Table 5

[0086] For each of the electroplated steel sheets obtained by the above method, the average diameter of the Mn—Si—O composite oxide was measured, and the occupancy ratio, yield strength, tensile strength, elongation at break, r value, and presence or absence of white defects on the surface of the composite oxide were evaluated in the same manner as in Experimental Example 1 of ferrite, and are shown in Table 6 below. Also, in the same manner as the method described in the present specification, the average diameter of the Mn—Si—O composite oxide and the average crystal grain size of ferrite were measured, and are shown in Table 6 below.

[0087]

Table 6

[0088] As shown in Table 6 above, in the case of Comparative Example 5 that does not satisfy the composition and production conditions of the base iron of the present invention, the relational expressions 1 and 2 are not satisfied, whereby the tensile strength is less than 390 MPa, not only is the strength insufficient, but also white defects due to a non-uniform plating layer occurred.

[0089] On the other hand, in the case of Invention Examples 9 to 13 that satisfy the composition and production conditions of the base iron of the present invention, the relational expressions 1 and 2 are satisfied, and compared with Comparative Example 5, not only are the strength characteristics excellent, but also the surface characteristics are good.

[0090] Further, with reference to a cross-section in the thickness direction of the electroplated steel sheet manufactured from Invention Example 13 described above, when a line is drawn such that an interface line having a length of 500 nm between the base iron and the plating layer is separated by 0.3 μm in the thickness direction on the plating layer side, it was confirmed that the area occupied by the Mn—Si—O composite oxide in the region between the interface line and the separated line is larger than the area occupied by the Mn—Si—O composite oxide in the region between the separated line and the surface line of the plating layer.

[0091] In particular, in the case of Invention Examples 10 and 11 in which primary temper rolling with a rolling reduction rate of 0.05 to 0.4% and secondary temper rolling with a rolling reduction rate of 0.6 to 1.0% were performed during temper rolling, compared with Invention Examples 9 and 12 that do not satisfy the conditions of the above-described primary temper rolling and secondary temper rolling, it was confirmed that the deep drawability and surface characteristics are further improved.

[0092] On the one hand, in the case of a test piece with good white defects on the surface, visually, the defects shown in FIG. 1 are not confirmed. In the case of a very good test piece, not only are no defects observed visually, but when observing the cross-section of the plating layer by utilizing a scanning electron microscope (SEM), it can be confirmed that the deviation of the minimum thickness and the maximum thickness in the plating layer of the cross-section [(maximum thickness - minimum thickness) / (maximum thickness)] does not exceed 0.1.

Explanation of symbols

[0093] 1: Substrate iron 2: Plating layer 10: Interface line where the total interface length between the substrate iron and the plating layer is 500 nm or more 20: Line separated 0.3 μm parallel to the thickness direction of the interface line At: Area of the region between the interface line 10 and the line 20 separated therefrom 100: Mn-Si-O-based composite oxide Ao: Area of the Mn-Si-O-based composite oxide existing in the region between the interface line 10 and the line 20 separated therefrom

Claims

1. A base iron, and a plating layer formed on the base iron, wherein the base iron contains, by weight %, C: 0.003 to 0.009%, Si: 0.05% or less (excluding 0%), Mn: 0.4 to 1.0%, P: 0.04 to 0.09%, S: 0.01% or less (excluding 0%), N: 0.005% or less (excluding 0%), Sol. Al: 0.1% or less (excluding 0%), Mo: 0.03 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.045%, Cu: 0.04 to 0.15%, B: 0.0015% or less (excluding 0%), and the balance consists of Fe and other inevitable impurities, and satisfies the following relational expressions 1 and 2, a plated steel sheet. 【Relational Expression 1】 0 < 10 × [Si] / [Mn] ≤ 1.3 (In the relational expression 1, [Si] represents the content of the average weight % of Si in the base iron, and [Mn] represents the content of the average weight % of Mn in the base iron.) 【Relational Expression 2】 0 ≤ [Ao] / [At] ≤ 0.15 (In the relational expression 2, [At] represents the area of the region between the interface line between the base iron and the plating layer and a line drawn 0.3 μm apart in the thickness direction on the plating layer side with respect to the cross section of the plated steel sheet, and [Ao] represents the area occupied by the Mn—Si—O based composite oxide in the region between the interface line and the line drawn 0.3 μm apart.)

2. The plated steel sheet according to claim 1, wherein the average diameter of the Mn—Si—O based composite oxide is 200 nm or less.

3. When a line is drawn 0.3 μm apart in the thickness direction on the plating layer side with respect to the cross section in the thickness direction of the plated steel sheet, the area occupied by the Mn—Si—O based composite oxide in the region between the interface line and the line drawn 0.3 μm apart is larger than the area occupied by the Mn—Si—O based composite oxide in the region between the line drawn 0.3 μm apart and the surface line of the plating layer, the plated steel sheet according to claim 1.

4. The plated steel sheet according to claim 1, wherein the tensile strength is 390 MPa or more and the elongation is 15% or more.

5. The plated steel sheet according to claim 1, wherein the microstructure of the base iron is, in area fraction, 99% or more ferrite and the balance is pearlite.

6. The plated steel sheet according to claim 5, wherein the average crystal grain size of the ferrite is in the range of 5 to 15 μm.

7. By weight, C: 0.003 to 0.009%, Si: 0.05% or less (excluding 0%), Mn: 0.4 to 1.0%, P: 0.04 to 0.09%, S: 0.01% or less (excluding 0%), N: 0.005% or less (excluding 0%), Sol. Al: 0.1% or less (excluding 0%), Mo: 0.03 to 0.08%, Ti: 0.005 to 0.03%, Nb: 0.02 to 0.045%, Cu: 0.04 to 0.15%, B: 0.0015% or less (excluding 0%), consisting of the balance Fe and other inevitable impurities, after continuously casting a steel that satisfies relational expression 1, a step of machining and removing 2 to 5 mm in the thickness direction from the surface of the steel, After reheating the steel slab obtained after the machining and removing step at 1180 to 1230°C, hot rolling at a temperature above Ar3 to provide a hot-rolled steel sheet, A step of coiling the hot-rolled steel sheet at 600 to 650°C, A step of cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 70 to 83% to provide a cold-rolled steel sheet, A step of annealing the cold-rolled steel sheet at 740 to 850°C, Performing hot-dip galvanized coating on the annealed cold-rolled steel sheet, and performing alloying heat treatment on the steel sheet with a zinc-based coating layer formed on the surface at 500 to 560°C, A step of temper rolling at a reduction ratio of 0.6 to 1.2% using a skin pass roll having a roughness (Ra) of 1.0 to 1.6 μm, including, The annealing step is performed in a range where the dew point temperature is -60 to -20°C. After heat treatment at a temperature in the range of 740 to 850°C, primary cooling is performed at an average cooling rate of 2 to 6°C / s, and then secondary cooling is performed at an average cooling rate of 6.5 to 15°C / s. The method for manufacturing a coated steel sheet according to claim 1. [Relational expression 1] 0 < 10 × [Si] / [Mn] ≤ 1.3 (In the relational expression 1, [Si] represents the content of the average weight% of Si in the steel, and [Mn] represents the content of the average weight% of Mn in the steel.)

8. The temper rolling step performs primary temper rolling at a reduction ratio of 0.05 to 0.4, and then performs secondary temper rolling at a reduction ratio of 0.6 to 1.0%. The method for manufacturing a coated steel sheet according to claim 7.

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