Cold-rolled steel sheet, plated steel sheet, and method for manufacturing the same

Abstract

The present invention relates to a steel sheet suitable for various uses including automobile parts, and more particularly, to a cold-rolled steel sheet, a plated steel sheet containing residual elements, and a manufacturing method thereof, which are excellent in hole expandability and plating adhesion.

Description

Technical Field

[0001] This invention relates to a steel sheet suitable for various applications, including automotive parts, and more specifically, to a cold-rolled steel sheet containing residual elements and exhibiting excellent hole expansion and coating adhesion, a coated steel sheet, and a method for manufacturing the same. Background Technology

[0002] In response to the global climate crisis and with the goal of achieving carbon neutrality by 2050, automakers are increasingly demanding that steelmakers supply automotive steel sheets with reduced CO2 emissions. Therefore, steelmakers are currently utilizing existing blast furnace-converter or electric arc furnace steelmaking methods to develop automotive steel sheets that reduce CO2 emissions through scrap recycling.

[0003] When scrap steel is used as a raw material, it is difficult to remove residual elements such as Cu and Ni (tramp elements) from the scrap steel during refining, so these elements remain in the steel after refining. These residual elements can reduce the physical properties of the steel or deteriorate its surface quality. Therefore, to date, high-grade thin sheet products, such as automotive steel sheets, are manufactured using molten iron as the main raw material, significantly reducing the C and N content in the steel through a general blast furnace-converter process, while strictly controlling the content of residual elements.

[0004] In addition, the following technology was proposed: using electric arc furnace steelmaking to manufacture automotive steel sheets with excellent workability from electric arc furnace steel containing residual elements.

[0005] Patent Documents 1 and 2 disclose techniques for manufacturing cold-rolled steel sheets with excellent flanging properties. Specifically, they disclose cold-rolled steel sheets with excellent flanging properties even when containing a large amount of residual elements. However, these techniques completely fail to consider the purpose of improving the poor adhesion of plating to the steel sheet and the means to achieve this purpose.

[0006] As mentioned above, although techniques for manufacturing automotive steel sheets containing residual elements have been proposed, automotive steel sheets that improve both hole expansion and plating adhesion even when containing residual elements have not yet been developed.

[0007] (Patent Document 1) Japanese Patent No. 6179698 (Patent Document 2) Japanese Patent Publication No. 2004-250749 Summary of the Invention

[0008] (a) Technical problems to be solved One aspect of the present invention is to provide a cold-rolled steel sheet, a coated steel sheet, and a method for manufacturing the same, which improve hole expansion and coating adhesion even when containing residual elements in the production of automotive steel sheets using recycled scrap steel.

[0009] The technical problems of this invention are not limited to those described above. Those skilled in the art can readily understand the additional technical problems of this invention from the entirety of this specification.

[0010] (II) Technical Solution One aspect of the present invention provides a cold-rolled steel sheet, comprising, by weight percent: carbon (C): greater than 0% to 0.2500%; silicon (Si): greater than 0% to 0.700%; manganese (Mn): greater than 0% to 1.800%; aluminum (Al): greater than 0% to 0.700%; phosphorus (P): less than 0.080%; sulfur (S): less than 0.050%; nitrogen (N): less than 0.0300%; copper (Cu): less than 1.000%; nickel (Ni): less than 1.000%; chromium (Cr): less than 1.000%; magnesium (Mg): less than 0.050%; calcium (Ca): less than 0.050%; and yttrium (Y) excluding... The following elements are selected from the following: rare earth elements (REM): less than 0.050%, tungsten (W): less than 0.50%, zirconium (Zr): less than 0.50%, antimony (Sb): less than 0.500%, tin (Sn): less than 0.500%, cobalt (Co): less than 0.500%, yttrium (Y): less than 0.200%, hafnium (Hf): less than 0.200%; titanium (Ti): less than 0.220%, niobium (Nb): less than 0.220%, vanadium (V): less than 0.220%, molybdenum (Mo): less than 1.000%, and boron (B): less than 0.0200%; the balance being Fe; and unavoidable impurities.

[0011] According to one embodiment of the present invention, a cold-rolled steel sheet is provided, wherein when the average (Cu+Cr+Ni+Sn) content (by weight%) in the surface layer of the cold-rolled steel sheet is set as A, and the average (Cu+Cr+Ni+Sn) content (by weight%) in the center layer is set as B, A / B satisfies 0.15 to 30.00.

[0012] The cold-rolled steel sheet of this invention has excellent hole expansion properties, and after subsequent plating processes, it also has excellent plating adhesion.

[0013] In one embodiment of the present invention, when one or more of titanium (Ti), niobium (Nb) and vanadium (V) are added to the cold-rolled steel sheet, the sum of their contents (Ti+Nb+V) can be less than 0.22%.

[0014] In one embodiment of the present invention, the relationship between the tensile strength (TS) and elongation (E1) of the cold-rolled steel sheet can satisfy the following relationship 1.

[0015] [Relation 1] 0.6×10 6 ≤TS 2 ×√El≤2.3×10 6 (The unit of relation 1 is (MPa)) 2 % 0.5 。 ) In one embodiment of the present invention, the relationship between the tensile strength (TS) and the hole expansion property (HER) of the cold-rolled steel sheet can satisfy the following relationship 2.

[0016] [Relationship 2] 0.9×10 6 ≤TS 2 ×√HER≤3.8×10 6 (The unit of relation 2 is (MPa)) 2 % 0.5 。 ) In one embodiment of the present invention, the yield strength ratio of the cold-rolled steel sheet can be 0.50-0.95.

[0017] Another aspect of the present invention provides a method for manufacturing a cold-rolled steel sheet, comprising the following steps: preparing a steel billet; heating the steel billet in a temperature range of 900-1300°C; finishing the heated steel billet in an austenite region above the Ar3 phase transformation point or a ferrite region below the Ar3 phase transformation point to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet in a temperature range of 330-750°C; cooling the coiled hot-rolled steel sheet to a temperature range of 250-300°C at a rate of 0.2-10°C / min; cold-rolling the cooled hot-rolled steel sheet at a cold-rolling reduction rate of 30-90% to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet at a temperature above 600°C for more than 10 seconds.

[0018] In one embodiment of the present invention, the steel billet may have the above-described alloy composition.

[0019] In one embodiment of the present invention, the annealing process can be performed by heating the cold-rolled steel sheet obtained by cold rolling to the annealing temperature at a heating rate of 2-60°C / second.

[0020] Furthermore, another aspect of the present invention provides a coated steel sheet comprising a zinc-based coating formed on at least one side of a cold-rolled steel sheet, and a method for manufacturing the coated steel sheet by coating the cold-rolled steel sheet.

[0021] In one embodiment of the present invention, the cold-rolled steel sheet may be the aforementioned cold-rolled steel sheet.

[0022] In one embodiment of the present invention, the plating process can be carried out by hot-dip plating or electroplating.

[0023] In one embodiment of the present invention, hot-dip galvanizing may include a step of hot-dip galvanizing at a temperature range of 440-520°C, and optionally, may further include a step of alloying heat treatment after the hot-dip galvanizing.

[0024] (III) Beneficial Effects According to the present invention, an automotive steel sheet (cold-rolled steel sheet, galvanized steel sheet) with improved porosity and coating adhesion even when containing residual elements can be provided.

[0025] The steel sheet of the present invention is manufactured using recycled scrap steel, and therefore has the advantage of excellent carbon dioxide reduction when applied to materials such as automobile bodies.

[0026] The various and beneficial advantages and effects of the present invention are not limited to those described above, and can be more easily understood in the process of describing specific embodiments of the present invention. Best practice

[0027] The technical terms used herein are for reference only to specific embodiments and are not intended to limit the invention. The singular form used herein includes the plural form unless the statement expressly indicates otherwise.

[0028] The use of "including" or "comprises" in this specification means to specify a particular characteristic, domain, integer, step, action, element, and / or component, and does not exclude the presence or addition of other particular characteristics, domains, integers, steps, actions, elements, components, and / or groups.

[0029] Unless otherwise defined, all terms used herein, including technical and scientific terms, shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms as defined in commonly used dictionaries shall be interpreted as having the meaning consistent with relevant technical literature and current disclosure, and shall not be interpreted as having an ideal or overly formal meaning unless otherwise defined.

[0030] The following is a detailed description of a cold-rolled steel sheet according to one aspect of the present invention, particularly a cold-rolled steel sheet with excellent hole expansion and coating adhesion. It should be noted that, in this invention, the content of each element is expressed as a percentage by weight unless otherwise specified. Furthermore, the proportion of crystals or structures is based on area unless otherwise specified.

[0031] According to one aspect of the present invention, the cold-rolled steel sheet may, by weight percent, comprise: carbon (C): greater than 0% to 0.2500%; silicon (Si): greater than 0% to 0.700%; manganese (Mn): greater than 0% to 1.800%; aluminum (Al): greater than 0% to 0.700%; phosphorus (P): less than 0.080%; sulfur (S): less than 0.050%; nitrogen (N): less than 0.0300%; copper (Cu): less than 1.000%; nickel (Ni): less than 1.000%; chromium (Cr): less than 1.000%; magnesium (Mg): less than 0.050%; calcium (Ca): less than 0.050%. One or more of the following rare earth elements (REM) other than yttrium (Y): less than 0.050%, tungsten (W): less than 0.50%, zirconium (Zr): less than 0.50%, antimony (Sb): less than 0.500%, tin (Sn): less than 0.500%, cobalt (Co): less than 0.500%, yttrium (Y): less than 0.200%, hafnium (Hf): less than 0.200%; and one or more of the following titanium (Ti): less than 0.220%, niobium (Nb): less than 0.220%, vanadium (V): less than 0.220%, molybdenum (Mo): less than 1.000%, and boron (B): less than 0.0200%.

[0032] The reasons for limiting the alloy composition of the cold-rolled steel sheet provided in this invention to the above-described manner will be explained in detail below.

[0033] Carbon (C): Greater than 0% to 0.2500% Carbon (C) is an effective element for ensuring the strength of steel. When Ti, Nb, V, etc. are present in steel, they combine with these elements to form precipitates, which helps to impart strength to the steel plate.

[0034] In one embodiment of the present invention, when the C content exceeds 0.2500%, it is difficult to ensure the weld strength of the welded part during welding. Therefore, the C content can be 0.2500% or less. In another embodiment of the present invention, the C content can be 0.2300% or less or 0.2000% or less.

[0035] Furthermore, carbon (C) is an essential element in steelmaking, with a content greater than 0%. However, in one embodiment of the present invention, when the C content is less than 0.0010%, the effect of reducing C is almost negligible, and it actually leads to increased steelmaking costs, which is therefore undesirable. Therefore, the C content can be 0.0010% or more. In another embodiment of the present invention, the C content can be 0.0015% or more, or 0.0020% or more.

[0036] Silicon (Si): Greater than 0% to 0.700% Silicon (Si) is an element that enhances strength through solid solution strengthening, strengthens ferrite, homogenizes the microstructure, and improves workability. Furthermore, Si is an element required for deoxidation during steelmaking.

[0037] In one embodiment of the present invention, when the Si content exceeds 0.700%, plating defects such as incomplete plating may occur during plating, and the weldability of the steel sheet may be impaired. Therefore, the Si content can be 0.700% or less. In another embodiment of the present invention, the Si content can be 0.600% or less.

[0038] In addition, in one embodiment of the present invention, the Si content can be greater than 0%, and considering the manufacturing cost, its content can be greater than 0.001%.

[0039] Manganese (Mn): greater than 0% to 1.800% Manganese (Mn) is an element that is useful for improving both the strength and ductility of steel.

[0040] In one embodiment of the invention, when the Mn content exceeds 1.800%, it promotes the phase transformation from austenite to low-temperature phases such as martensite or bainite, resulting in a decrease in the yield strength ratio of the steel plate. Therefore, the Mn content can be below 1.800%. In another embodiment of the invention, the Mn content can be below 1.790% or 1.780%.

[0041] In addition, in one embodiment of the present invention, the Mn content can be greater than 0%, and considering the manufacturing cost, its content can be greater than 0.001%.

[0042] Aluminum (Al): Greater than 0% to 0.700% Aluminum (Al) is an element that combines with oxygen (O) in steel to act as a deoxidizer. Furthermore, similar to Si, it strengthens ferrite, homogenizes the microstructure, and improves machinability.

[0043] In one embodiment of the invention, when the Al content exceeds 0.700%, plating defects such as incomplete plating may occur during the plating process, and the weldability of the steel sheet may decrease. Therefore, the Al content can be 0.700% or less. In another embodiment of the invention, the Al content can be 0.600% or less.

[0044] In addition, in one embodiment of the present invention, the Al content can be greater than 0%, and considering the manufacturing cost, the content can be greater than 0.001%.

[0045] Phosphorus (P): below 0.080% Phosphorus (P) is an element added to improve the strength of steel. In one embodiment of the invention, when the P content exceeds 0.080%, the impact toughness of the steel is deteriorated. According to another embodiment of the invention, the P content may be less than 0.070%.

[0046] In addition, in one embodiment of the present invention, the P content can be greater than 0%, and considering manufacturing costs, it can be more than 0.001%.

[0047] Sulfur (S): below 0.050% Sulfur (S) is an element that is unavoidably added during steelmaking. It combines with Mn in the steel to form MnS inclusions, which impair the steel's ductility. Therefore, in one embodiment of the present invention, the S content can be limited to 0.050% or less. In another embodiment of the present invention, the S content can be 0.040% or less.

[0048] Furthermore, sulfur (S) may inevitably be present in the steel, and therefore its content can be greater than 0%. In one embodiment of the invention, controlling the S content to less than 0.001% would result in a significant increase in manufacturing costs; therefore, the S content can be greater than 0.001%.

[0049] Nitrogen (N): below 0.0300% Nitrogen (N) is an element that is unavoidably added during steelmaking and is responsible for the formation of nitrides during continuous casting, which can cause cracks in the steel billet. With this in mind, according to one embodiment of the invention, N may be contained in amounts of 0.0300% or less.

[0050] In addition, if the N content is controlled to be less than 0.0010%, the manufacturing cost will increase significantly. Therefore, the N content can be limited to more than 0.0010%.

[0051] Copper (Cu): below 1.000% and Nickel (Ni): below 1.000% Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Furthermore, Cu and Ni accumulate on the surface of steel plates, preventing hydrogen from penetrating into the steel interior and thus helping to suppress hydrogen-induced delayed fracture.

[0052] In one embodiment of the invention, excessive amounts of Cu and Ni can impair processability; therefore, their contents can be limited to 1.000% or less. In another embodiment of the invention, the Cu and Ni contents can each be 0.900% or less.

[0053] In addition, in one embodiment of the present invention, the Cu and Ni content can be greater than 0%, and in order to fully obtain the effects brought by these elements, the Cu and Ni content can be greater than 0.001%.

[0054] Chromium (Cr): below 1.000% Chromium (Cr) is an element that inhibits the decomposition of austenite and stabilizes austenite during the alloying process of manufactured steel plates.

[0055] In one embodiment of the invention, processability decreases when the Cr content is too high; therefore, it can be limited to 1.000% or less. In another embodiment of the invention, the Cr content can be 0.900% or less.

[0056] In addition, in one embodiment of the present invention, the Cr content may be greater than 0%, but in order to fully obtain the effect brought by the Cr, the content may be greater than 0.001%.

[0057] In addition to the alloy composition described above, the steel plate of the present invention may also contain one or more of the following residual elements.

[0058] First, it may contain one or more of the following: magnesium (Mg): less than 0.050%, calcium (Ca): less than 0.050%, rare earth elements (REM) other than yttrium (Y): less than 0.050%, tungsten (W): less than 0.50%, zirconium (Zr): less than 0.50%, antimony (Sb): less than 0.500%, tin (Sn): less than 0.500%, cobalt (Co): less than 0.500%, yttrium (Y): less than 0.200%, and hafnium (Hf): less than 0.200%.

[0059] Magnesium (Mg): below 0.050%, Calcium (Ca): below 0.050%, and Rare Earth Elements (REM) other than Yttrium (Y): below 0.050%. Magnesium (Mg), calcium (Ca), and rare earth elements (REMs) other than yttrium (Y) improve the ductility of steel by spheroidizing sulfides in the steel. In one embodiment of the invention, when the content of each element exceeds 0.050%, not only will the above-mentioned effects saturate, but the manufacturing cost will also increase; therefore, their contents can be limited to 0.050% or less.

[0060] Generally, rare earth elements (REM) refer to 17 metallic elements, including scandium (Sc), yttrium (Y), lanthanum (La), and cerium (Ce). According to one embodiment of the present invention, the content of yttrium (Y) will be limited separately below. Therefore, in one embodiment of the present invention, REM refers to 16 elements other than Y.

[0061] Tungsten (W): less than 0.50% and Zirconium (Zr): less than 0.50% Tungsten (W) and zirconium (Zr) are elements that improve the hardenability of steel, thereby increasing its strength. According to one embodiment of the invention, when W and Zr are present in excess, not only will the above-mentioned effects be saturated, but manufacturing costs will also increase; therefore, their contents can be limited to 0.50% or less respectively.

[0062] Antimony (Sb): below 0.500%, Tin (Sn): below 0.500%, and Cobalt (Co): below 0.500%. Antimony (Sb), tin (Sn), and cobalt (Co) are elements that improve the wettability and adhesion of steel plating. In one embodiment of the invention, when the content of each element exceeds 0.500%, the brittleness of the steel increases, causing cracking problems during hot or cold working; therefore, the content of each element can be limited to 0.500% or less. According to another embodiment of the invention, Sb, Sn, and Co can each be contained at 0.400% or less. Furthermore, to fully achieve the desired effect by containing these elements, they can each be contained at 0.0005% or more.

[0063] Yttrium (Y): below 0.200% and Hafnium (Hf): below 0.200% Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel. In one embodiment of the invention, the ductility of the steel may decrease when the content of each element exceeds 0.200%, therefore the content of each element can be limited to below 0.200%.

[0064] In addition to the above-mentioned composition, the steel plate of the present invention may further contain one or more of the following: titanium (Ti): 0.220% or less, niobium (Nb): 0.220% or less, vanadium (V): 0.220% or less, molybdenum (Mo): 1.000% or less, and boron (B): 0.0200% or less.

[0065] Titanium (Ti): less than 0.220%, Niobium (Nb): less than 0.220%, and Vanadium (V): less than 0.220%. Titanium (Ti), niobium (Nb), and vanadium (V) are elements that form precipitates in steel, thereby improving the steel's strength and impact toughness. In particular, Ti, Nb, and V can prevent processability degradation caused by dissolved C and N by precipitating dissolved C and N in steel as carbides, nitrides, etc.

[0066] In one embodiment of the present invention, the elements can be added at a concentration of 0.001% or more to achieve the aforementioned effects. However, when the concentration of each element exceeds 0.220%, the effect of addition becomes saturated, and there is an increase in manufacturing costs. Therefore, the upper limit of each element can be limited to 0.220%. According to another embodiment of the present invention, the elements can be contained at a concentration of 0.200% or less.

[0067] Furthermore, according to one embodiment of the present invention, when one or more of Ti, Nb, and V are added, the sum of the contents of these elements (Ti+Nb+V) can be 0.220% or less. According to another embodiment, the sum of the contents of the elements can be 0.200% or less.

[0068] Molybdenum (Mo): below 1.000% Similar to Cr mentioned earlier, molybdenum (Mo) is an element that inhibits the decomposition of austenite and stabilizes austenite during the alloying process of manufactured steel plates.

[0069] In one embodiment of the invention, excessive Mo content reduces processability; therefore, considering this, the content can be limited to 1.000% or less. According to another embodiment of the invention, the Mo content can be 0.900% or less. Furthermore, in one embodiment of the invention, to achieve the desired Mo addition effect, the content can be 0.001% or more; therefore, its lower limit can be limited to 0.001%.

[0070] Boron (B): below 0.0200% Boron (B) is an element that improves the hardenability of steel, thereby increasing its strength, and inhibits the nucleation of grain boundaries.

[0071] In one embodiment of the invention, the deep-drawing properties of the steel may deteriorate when the content of B exceeds 0.0200%. Therefore, the content of B can be limited to below 0.0200%. According to another embodiment, the content of B can be below 0.0100%. Furthermore, in one embodiment of the invention, to achieve the desired effect of B addition, the content of B can be 0.0001% or more; according to another embodiment, the content of B can be 0.0005% or more, or 0.0010% or more.

[0072] In one embodiment of the invention, the cold-rolled steel sheet may contain additional Fe and other unavoidable impurities besides the components described above. However, during the normal manufacturing process, unintended impurities inevitably mix in from the raw materials or the surrounding environment, and therefore these impurities cannot be completely eliminated. These impurities are known to those skilled in the art, and therefore not specifically mentioned in this specification. Furthermore, the addition of further active ingredients beyond the above-described components is not entirely excluded.

[0073] According to one embodiment of the present invention, based on the thickness direction of the cold-rolled steel sheet, the content of certain residual elements at specific locations can be controlled as follows, thereby improving the porosity, pulverization, etc. of the steel sheet.

[0074] Specifically, in one embodiment of the present invention, when the average (Cu+Cr+Ni+Sn) content (by weight%) in the surface layer of the cold-rolled steel sheet is set as A, and the average (Cu+Cr+Ni+Sn) content (by weight%) in the center layer of the cold-rolled steel sheet is set as B, A / B can satisfy 0.15 to 30.00.

[0075] In one embodiment of the invention, it is meaningful to use A / B as an indicator of the porosity and pulverization properties of the steel sheet. When the value of A / B is less than 0.15 or greater than 30.00, certain residual elements contained in the steel cannot be uniformly distributed.

[0076] The central portion refers to the point t / 2 in the thickness direction. Furthermore, the surface portion refers to the point where the concentration of Cu, Cr, Ni, and Sn changes (or changes drastically) when measured along the thickness direction from the surface. As an example, when the (Cu+Cr+Ni+Sn) content on the surface is greater than that at the center, it refers to the region from the measured concentration up to 99% of the (Cu+Cr+Ni+Sn) content at the center; when the (Cu+Cr+Ni+Sn) content on the surface is less than that at the center, it refers to the region from the measured concentration up to 1% of the (Cu+Cr+Ni+Sn) content at the center. As a non-limiting example, the concentrations of the above elements can be measured using FE-SEM or glow discharge spectroscopy (GDS).

[0077] In one embodiment of the present invention, the microstructure of the cold-rolled steel sheet can be dominated by ferrite, and as an example, it can be composed of ferrite with an area fraction of 75-98% and the balance cementite.

[0078] In one embodiment of the present invention, when the microstructure of cold-rolled steel sheet contains a ferrite phase, the porosity may deteriorate when the area fraction of ferrite is less than 75%; on the other hand, the strength may decrease when it exceeds 98%.

[0079] In one embodiment of the present invention, the ferrite may simultaneously contain recrystallized ferrite and non-recrystallized ferrite, and there is no particular limitation on their fractional range. The alloy composition and manufacturing process according to one embodiment of the present invention will be naturally determined, and this is hereby stated.

[0080] Furthermore, the cold-rolled steel sheet according to one embodiment of the present invention is not intended to exclude trace amounts of impurities inevitably included in the process, which are of course also included within the scope of the present invention. Here, the impurity structure is not particularly limited; as an example, it may be pearlite or bainite or martensite phases formed at low temperatures.

[0081] The cold-rolled steel sheet according to one embodiment of the present invention not only has high strength, but also excellent hole expansion properties.

[0082] Specifically, according to one embodiment of the present invention, the relationship between the tensile strength (TS) and elongation (E1) of the cold-rolled steel sheet can satisfy the following relationship 1, and the relationship between the tensile strength (TS) and the expansion property (HER) can satisfy the following relationship 2.

[0083] [Relation 1] 0.6×10 6 ≤TS 2 ×√El≤2.3×10 6 (The unit of relation 1 is (MPa)) 2 % 0.5 。 ) [Relationship 2] 0.9×10 6 ≤TS 2 ×√HER≤3.8×10 6 (The unit of relation 2 is (MPa)) 2 % 0.5 。 ) Furthermore, according to one embodiment of the invention, the cold-rolled steel sheet may have a yield strength ratio in the range of 0.50-0.95.

[0084] The coated steel sheet according to another aspect of the present invention will now be described.

[0085] According to one embodiment of the present invention, the coated steel sheet includes a zinc-based coating formed on at least one surface of the cold-rolled steel sheet. In one embodiment of the present invention, the zinc-based coating is a coating with zinc (Zn) as the main component, and the composition of coatings generally applicable in this art can also be applied. In this case, the zinc-based coating may also include a zinc-based coating alloyed by an alloying treatment.

[0086] That is, the coated steel sheet according to one embodiment of the present invention is a zinc-coated steel sheet, which may be a hot-dip galvanized steel sheet (GI), an alloyed hot-dip galvanized steel sheet (GA), an electrogalvanized steel sheet (EG), an alloyed electrogalvanized steel sheet, etc.

[0087] In one embodiment of the present invention, the cold-rolled steel sheet having a zinc-based coating formed on at least one surface can be a cold-rolled steel sheet according to an embodiment of the present invention, and therefore the above description can also be applied to the cold-rolled steel sheet, which is hereby stated.

[0088] Furthermore, according to one embodiment of the present invention, the cold-rolled steel sheet may have a concentration gradient of specific elements, such as residual elements Cu, Cr, Ni, and Sn, along the thickness direction from its surface. When a zinc-based coating is formed on at least one surface of such a cold-rolled steel sheet, the residual elements may further diffuse into the coating due to factors such as the temperature during coating or the temperature during alloying treatment. Therefore, in the coated steel sheet, the concentration gradient of residual elements can be achieved from the interface between the base material and the coating as the substrate steel sheet towards the thickness direction of the base material, or it can be achieved starting from the alloy layer formed at the interface between the base material and the coating.

[0089] That is, the residual elements can have a concentration gradient represented by A / B as mentioned above because of the difference in the diffusion rate of residual elements to the surface side of the steel sheet during the manufacturing process of cold-rolled steel sheet. This characteristic of cold-rolled steel sheet will also be exhibited in galvanized steel sheet.

[0090] According to one embodiment of the present invention, the coated steel sheet has the characteristic of excellent powdering properties of the coating, specifically, the powdering peeling width of the coating is less than 7 mm.

[0091] The following describes in detail a method for manufacturing cold-rolled steel sheet and clad steel sheet according to another aspect of the present invention.

[0092] First, according to one embodiment of the present invention, cold-rolled steel sheet can be obtained through a series of processes, such as [billet heating - hot rolling - coiling - cooling - cold rolling - annealing]. The process steps are described in detail below, and the following manufacturing process is an example for manufacturing cold-rolled steel sheet and clad steel sheet according to one embodiment of the present invention.

[0093] According to one embodiment of the present invention, steel billets or ingots can be manufactured by an electric furnace or a new blast furnace-converter process, wherein the raw materials used in the electric furnace or blast furnace-converter process are scrap steel and pig iron. Here, pig iron refers to molten iron obtained in the blast furnace-converter process or its corrugate or hot briquetted iron (HBI), etc.

[0094] In the case of electric arc furnaces, desulfurization can be performed through ladle refining after steel tapping, and further desulfurization and subsequent vacuum degassing can be carried out. Furthermore, by adding alloying elements while degassing the steel obtained in the electric arc furnace, the final desired alloy composition can be achieved. Here, the RH method and DH method are common vacuum degassing methods, but oxygen can be injected into the degassing tank in parallel. Regarding oxygen injection, there is an oxygen injection method using a top-blown lance.

[0095] [Bill Heating] After preparing the steel billet, it can be heated. At this time, the steel billet can also be a steel ingot. In one embodiment of the invention, the steel billet can have the above-described alloy composition; therefore, the alloy composition of the steel billet can be replaced by the above description.

[0096] In one embodiment of the invention, the heating temperature for heating the prepared steel billet or ingot can be in the range of 900-1300°C. When the heating temperature exceeds 1300°C, the steel may reach its melting point and melt; on the other hand, when the temperature is below 900°C, the rolling load during subsequent hot rolling will increase, potentially reducing hot rolling stability. In another embodiment of the invention, the heating can be performed below 1160°C.

[0097] [Hot Rolled] The heated steel billet or ingot can be hot-rolled to obtain hot-rolled steel sheet.

[0098] In one embodiment of the invention, the finishing temperature during hot rolling can be any one of the austenite region above the Ar3 transformation point or the ferrite region below the Ar3 transformation point. However, if the finishing temperature is too low, it will increase the rolling load during hot rolling, so its lower limit temperature can be limited to above 750°C.

[0099] [Collection] The hot-rolled steel sheet obtained by hot rolling can be rolled into a coil shape.

[0100] In one embodiment of the invention, the winding process can be performed within a temperature range of 330-750°C. When the winding temperature is below 330°C, unevenness may occur on the surface of the hot-rolled coil due to the excessively low temperature; on the other hand, when the temperature exceeds 750°C, an excessively thick oxide scale may form on the hot-rolled surface. According to another embodiment of the invention, the winding process can be performed at a temperature above 350°C.

[0101] [cool down] Cooling can be performed simultaneously with uncoiling and recoiling of the hot-rolled steel sheet.

[0102] In one embodiment of the invention, post-winding cooling can be carried out at a cooling rate of 0.2-10.0°C / min to a temperature range of 250-300°C. By cooling at this relatively slow rate, uniform grain size within the microstructure can be ensured.

[0103] In one embodiment of the invention, when the cooling rate is less than 0.2°C / min, the cooling rate must be excessively reduced, which may burden the equipment and prevent uniform grain formation. On the other hand, when the cooling rate exceeds 10.0°C / min, the grains may become coarser.

[0104] In one embodiment of the present invention, when the cooling termination temperature is below 250°C, the cooling time will be too long, which may reduce productivity. On the other hand, when the cooling termination temperature exceeds 300°C, it will cause load problems during subsequent cold rolling.

[0105] [Cold Rolled] The cooled uncoiled hot-rolled steel sheet can be cold-rolled to obtain a cold-rolled steel sheet.

[0106] In one embodiment of the invention, cold rolling can be performed with a cold rolling reduction rate of 30-90%. When the cold rolling reduction rate is less than 30%, it may be difficult to ensure the target thickness and to correct the shape of the steel sheet. In one embodiment of the invention, there is no particular upper limit to the cold rolling reduction rate, but when the cold rolling reduction rate is too high, it may cause cold rolling load; therefore, taking this into consideration, it can be limited to below 90%.

[0107] [annealing] The manufactured cold-rolled steel sheet can be subjected to annealing heat treatment.

[0108] In one embodiment of the present invention, the annealing heat treatment can be carried out in a general continuous annealing furnace, and can be carried out under the condition that the cold-rolled steel sheet is heated to above 600°C and then subjected to heat treatment for more than 10 seconds. When the temperature of the annealing heat treatment is lower than 600°C or the time is less than 10 seconds, the precipitates in the cold-rolled steel sheet after annealing heat treatment may not be finely formed or may not be uniformly distributed.

[0109] According to one embodiment of the present invention, there is no particular upper limit on the temperature and time of the annealing heat treatment. However, considering the possibility of equipment failure and deterioration of pulverization caused by high-temperature annealing, 960°C and 15 minutes can be set as upper limits, respectively. That is, when the temperature of the annealing heat treatment is too high or the heat treatment is performed for a long time, the pulverization of the coated steel sheet obtained by the subsequent plating process may deteriorate.

[0110] Furthermore, according to one embodiment of the invention, when heating the cold-rolled steel sheet to the temperature required for annealing heat treatment, a heating rate of 2-60°C / second can be used. If the heating rate is too slow or too fast, the precipitates may not form finely or may not be evenly distributed.

[0111] Cold-rolled steel sheets manufactured by a series of processes according to one embodiment of the present invention can have the desired strength and hole-expanding properties.

[0112] In particular, according to one embodiment of the present invention, the relationship between tensile strength (TS) and elongation (E1) of the cold-rolled steel sheet can satisfy the following relationship 1, and the relationship between tensile strength (TS) and expansion property (HER) can satisfy the following relationship 2.

[0113] [Relation 1] 0.6×10 6 ≤TS 2 ×√El≤2.3×10 6 (The unit of relation 1 is (MPa)) 2 % 0.5 。 ) [Relationship 2] 0.9×10 6 ≤TS 2 ×√HER≤3.8×10 6 (The unit of relation 2 is (MPa)) 2 % 0.5 。 ) Furthermore, according to one aspect of the invention, a coated steel sheet can be obtained by plating a cold-rolled steel sheet, wherein the cold-rolled steel sheet can be a cold-rolled steel sheet manufactured by a series of processes according to an embodiment of the invention. Thus, the coated steel sheet can have a coating on at least one surface of the cold-rolled steel sheet.

[0114] In one embodiment of the present invention, the plating process can be hot-dip plating or electroplating, and each plating process can be carried out according to the conditions commonly practiced in this technical field.

[0115] As an example, the hot-dip galvanizing can be performed by immersing the cold-rolled steel sheet, which has undergone the annealing heat treatment, in a hot-dip galvanizing bath with zinc as the main component. The conditions for the hot-dip galvanizing process can be typical and are therefore not specifically limited in this specification. However, as a particular embodiment, GI hot-dip galvanizing can be performed under typical conditions within a temperature range of 440-520°C.

[0116] Furthermore, according to one embodiment of the present invention, the coated steel sheet with a zinc-based coating can be selectively subjected to alloying heat treatment to obtain alloyed hot-dip galvanized steel sheet (GA). The alloying heat treatment can also be performed under normal conditions and is therefore not specifically limited. As an example, the alloying heat treatment can be performed in a temperature range of 500-560°C.

[0117] Furthermore, as an example, the electroplating can involve placing a base steel sheet (cold-rolled steel sheet) on the cathode of a vertical plating tank type electroplating simulator, and then circulating a plating solution containing zinc (Zn) to form a zinc-based coating on at least one surface of the base steel sheet (cold-rolled steel sheet). Detailed Implementation

[0118] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative and are not intended to limit the scope of the invention. The scope of the invention is determined by the matters set forth in the claims and those reasonably inferred therefrom.

[0119] (Example) After preparing a steel billet with the alloy composition shown in Table 1 below, the billet is subjected to a series of processes according to the conditions shown in Table 2 below to obtain a steel sheet (cold-rolled steel sheet). At this time, the coiled material is cooled to a temperature range of 250-300°C.

[0120] The steel billet is obtained through the following process. First, molten iron from an existing blast furnace and a batch of scrap steel are fed into an electric arc furnace to obtain molten steel. This molten steel is then transferred to a ladle and subjected to vacuum degassing in an RH degassing furnace (0.1 torr), while alloying elements are added to produce molten steel with the target composition. Afterward, the molten steel is continuously cast into steel billets.

[0121] The microstructure and mechanical and physical properties of each cold-rolled steel sheet were measured and evaluated, and all results are shown in Table 3 below.

[0122] First, to measure the microstructure, samples were collected from t / 4 of the thickness direction of each steel plate (where t refers to the thickness (mm) of the cold-rolled steel plate), ground, and then the cross-section of the ground sample was etched with nitric acid and alcohol. The cross-section was then observed using a scanning electron microscope (SEM). The microstructure with a smooth surface after nitric acid and alcohol etching was identified as ferrite, while the microstructure with spherical or layered structures was identified as cementite. Furthermore, for unrecrystallized ferrite containing numerous dislocations, due to the crystal orientation difference within the grains, the crystal orientation of the ferrite was measured using FESEM-EBSD. Then, the kernel average misorientation (KAM) method was used to distinguish unrecrystallized ferrite within the ferrite, and its fraction was measured.

[0123] In addition, tensile tests and hole expansion tests were conducted to evaluate the physical properties of each cold-rolled steel sheet. The tensile tests were performed using the rolling direction of each cold-rolled steel sheet as a reference, with the 0° direction as the reference. Test pieces were collected according to JIS 5 standard, and the relationship between tensile strength (TS) and elongation (E1) (corresponding to Equation 1) and yield strength ratio (YR) were calculated. The hole expansion test involved forming a 10mm diameter hole (Ø) with the punch burr facing outwards using a conical punch with a 60° apex angle at a speed of 20mm / min (die inner diameter 10.3mm, clearance 12.5%). The hole expansion ratio (HER) was then calculated using the following formula.

[0124] (Formula) HER(%) = {(D-D0) / D0} × 100 (D: Hole diameter (mm) when the crack penetrates the plate thickness, D0: Initial hole diameter (mm)) In addition, to confirm the content (wt%) of specific elements (Cu, Cr, Ni, and Sn) at points along the thickness direction of the cold-rolled steel sheet, the cross-sections of each sample were removed using FIB, and compositional analysis was performed by FE-SEM. At this point, according to the definition of the surface layer, the content within the surface layer and the content at the center (t / 2) point along the thickness direction were measured separately. After repeating the measurements 30 times at random points, the maximum and minimum values ​​of each element were removed, and the content was calculated using the average value.

[0125] In addition, in order to evaluate the powdering property, a 45-degree V-bending test method was performed on the plated steel sheets obtained by plating each cold-rolled steel sheet in a hot-dip galvanizing bath at 440 - 480°C. At this time, the evaluation surface was positioned on the inner side of the bend, and after bending at 60 degrees using a die with a tip curvature radius of 1 mm, a tape was adhered to the inner side and then removed, and the powdering property of the coating layer peeled off together with the tape was evaluated with a full score of 5 points. The specific evaluation criteria are stipulated as follows, and a score of 2 or more is judged as qualified.

[0126] Peeling width less than 2 mm: 5 points Peeling width of 2 mm or more and less than 3 mm: 4 points Peeling width of 3 mm or more and less than 5 mm: 3 points Peeling width of 5 mm or more and less than 7 mm: 2 points Peeling width of 7 mm or more: 1 point [Table 1] [Table 2] [Table 3] As shown in Tables 1 to 3, in Invention Examples 1 to Invention Examples 7 that all satisfy the alloy composition and manufacturing conditions according to an embodiment of the present invention, the microstructure is mainly composed of ferrite and cementite is appropriately formed, so both the strength and the hole expansion property are excellent. In addition, the powdering property after plating is also excellent.

[0127] On the other hand, in Comparative Examples 1 to Comparative Examples 24 that deviate from any one or more of the alloy composition and manufacturing conditions according to an embodiment of the present invention, since the microstructure is not formed as expected or the distribution of specific elements is uneven, at least one or more physical properties such as strength, hole expansion property, powdering property (plating adhesion) are poor.

[0128] As described above, according to the present invention, excellent hole expansion property and plating adhesion can be achieved simultaneously, which has technical significance.

Claims

1. A cold-rolled steel sheet, comprising, by weight percent: carbon (C): greater than 0% to 0.2500%; silicon (Si): greater than 0% to 0.700%; manganese (Mn): greater than 0% to 1.800%; aluminum (Al): greater than 0% to 0.700%; phosphorus (P): less than 0.080%; Sulfur (S): less than 0.050%; Nitrogen (N): less than 0.0300%; Copper (Cu): less than 1.000%; Nickel (Ni): less than 1.000%; Chromium (Cr): less than 1.000%; Magnesium (Mg): less than 0.050%; Calcium (Ca): less than 0.050%; Rare Earth Elements (REM) other than Yttrium (Y): less than 0.050%; Tungsten (W): less than 0.50%; Zirconium (Zr): less than 0.50%; Antimony (Sb): less than 0.500%; Tin (Sn): less than 0.500%; Cobalt (Co): less than 0.500%; Yttrium (Y): less than 0.200%; Hafnium (Hf): less than 0.200%; Selected from one or more of the following: Titanium (Ti): less than 0.220%; Niobium (Nb): less than 0.220%; Vanadium (V): less than 0.220%; Molybdenum (Mo): less than 1.000%; and Boron (B): less than 0.0200%; Balance Fe; and unavoidable impurities. When the average (Cu+Cr+Ni+Sn) content (wt%) in the surface layer is defined as A, and the average (Cu+Cr+Ni+Sn) content (wt%) in the central layer is defined as B, the ratio of A to B satisfies 0.15 to 30.

00. Wherein, the central part refers to the point t / 2 in the thickness direction, and the surface part is the point where the concentration of Cu, Cr, Ni and Sn changes when the concentrations of Cu, Cr, Ni and Sn are measured along the thickness direction on the surface. When the (Cu+Cr+Ni+Sn) content on the surface is greater than that in the central part, it refers to the region up to 99% of the (Cu+Cr+Ni+Sn) content in the central part in the measured concentration value. When the (Cu+Cr+Ni+Sn) content on the surface is less than that in the central part, it refers to the region up to 1% of the (Cu+Cr+Ni+Sn) content in the central part in the measured concentration value.

2. The cold-rolled steel sheet according to claim 1, wherein, When one or more of the aforementioned titanium (Ti), niobium (Nb), and vanadium (V) are added, the sum of their contents (Ti+Nb+V) is less than 0.220%.

3. The cold-rolled steel sheet according to claim 1, wherein, The microstructure of the cold-rolled steel sheet comprises 75-98% ferrite and the balance cementite.

4. The cold-rolled steel sheet according to claim 1, wherein, The tensile strength (TS) and elongation (E1) of the cold-rolled steel sheet satisfy the following relationship: Equation 1. [Relation 1] 0.6×10 6 ≤TS 2 ×√El≤2.3×10 6 The unit of expression 1 is (MPa). 2 % 0.5 .

5. The cold-rolled steel sheet according to claim 1, wherein, The tensile strength (TS) and hole expansion property (HER) of the cold-rolled steel sheet satisfy the following relationship 2. [Relationship 2] 0.9×10 6 ≤TS 2 ×√HER≤3.8×10 6 The unit of expression 2 is (MPa). 2 % 0.5 .

6. The cold-rolled steel sheet according to claim 1, wherein, The yield strength ratio of the cold-rolled steel sheet is 0.50 to 0.

95.

7. A method for manufacturing cold-rolled steel sheet, comprising the following steps: Prepare a steel billet, which, by weight percent, comprises: carbon (C): greater than 0% to 0.2500%; silicon (Si): greater than 0% to 0.700%; manganese (Mn): greater than 0% to 1.800%; aluminum (Al): greater than 0% to 0.700%; phosphorus (P): less than 0.080%. Sulfur (S): less than 0.050%; Nitrogen (N): less than 0.0300%; Copper (Cu): less than 1.000%; Nickel (Ni): less than 1.000%; Chromium (Cr): less than 1.000%; Magnesium (Mg): less than 0.050%; Calcium (Ca): less than 0.050%; Rare Earth Elements (REM) other than Yttrium (Y): less than 0.050%; Tungsten (W): less than 0.50%; Zirconium (Zr): less than 0.50%; Antimony (Sb): less than 0.500%; Tin (Sn): less than 0.500%; Cobalt (Co): less than 0.500%; Yttrium (Y): less than 0.200%; Hafnium (Hf): less than 0.200%; Selected from one or more of the following: Titanium (Ti): less than 0.220%; Niobium (Nb): less than 0.220%; Vanadium (V): less than 0.220%; Molybdenum (Mo): less than 1.000%; and Boron (B): less than 0.0200%; Balance Fe; and unavoidable impurities. The steel billet is heated in a temperature range of 900-1300℃; The heated steel billet is precision rolled in the austenite region above the Ar3 phase transformation point or the ferrite region below the Ar3 phase transformation point to obtain hot-rolled steel sheet. The hot-rolled steel sheet is coiled within a temperature range of 330-750℃; The hot-rolled steel sheet that has been coiled is cooled to a temperature range of 250-300°C at a rate of 0.2-10°C / minute. The cooled hot-rolled steel sheet is cold-rolled at a reduction rate of 30-90% to obtain a cold-rolled steel sheet; and The cold-rolled steel sheet is annealed at a temperature above 600°C for more than 10 seconds.

8. The method for manufacturing cold-rolled steel sheet according to claim 7, wherein, The annealing process involves heating the cold-rolled steel sheet to the annealing temperature at a heating rate of 2-60℃ / second.

9. The method for manufacturing cold-rolled steel sheet according to claim 7, wherein, When the steel billet contains one or more of titanium (Ti), niobium (Nb), and vanadium (V), the sum of their contents (Ti+Nb+V) is less than 0.220%.

10. A galvanized steel sheet, comprising: Cold-rolled steel sheet according to any one of claims 1 to 6; as well as A zinc-based coating formed on at least one side of the cold-rolled steel sheet.

11. The galvanized steel sheet according to claim 10, wherein, The width of the powdering and peeling of the coating is less than 7 mm.

12. A method for manufacturing a galvanized steel sheet, wherein, A coated steel sheet is obtained by coating the cold-rolled steel sheet according to any one of claims 1 to 6, wherein the coating process is hot-dip coating or electroplating.

13. The method for manufacturing galvanized steel sheet according to claim 12, wherein, The hot-dip galvanizing process includes a step of hot-dip galvanizing at a temperature range of 440-520°C, and optionally, further includes a step of alloying heat treatment after the hot-dip galvanizing process.

Images