Cold-rolled steel sheet and method for manufacturing the same

Abstract

The present invention provides a steel sheet and a method of manufacturing the same. The present invention can be used to manufacture a cold rolled steel sheet, a hot dip galvanized steel sheet and an alloyed hot dip galvanized steel sheet having a tensile strength of 590 MPa or more, an uniform elongation of 18% or more and a hole expansion ratio (HER) of 40% or more.

Description

Technical Field

[0001] This invention relates to a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet for automobile body parts, and a method for manufacturing the same. Background Technology

[0002] Recently, in an effort to mitigate global warming, the steel industry is accelerating the development of various steelmaking technologies that can reduce greenhouse gas emissions during steel production. Meanwhile, the automotive industry is focusing on lightweighting automotive components to improve fuel efficiency by reducing vehicle weight. Specifically, to reduce carbon dioxide emissions during steel production, steel companies are exploring and commercializing technologies beyond the existing blast furnace (which uses iron ore, coke, limestone, etc. to produce pig iron) – converter (although it emits relatively more greenhouse gases, it can produce large quantities of high-quality products with low impurity content). These technologies include electric arc furnaces (which use electricity to produce molten steel from scrap steel itself or scrap steel plus direct reduced iron (DRI or HBI), producing products with relatively higher impurity content) and hydrogen reduction of iron ore to produce pig iron.

[0003] Furthermore, automakers are exploring the increased use of thin, high-strength steel sheets in certain components to reduce the weight of automotive parts. In particular, when steel sheets or blanks suitable for automotive interior structural components are cold-stamped in a single process, dimensional defects such as springback or forming cracks can sometimes occur. Therefore, steel used for these components requires excellent cold formability (e.g., high ductility, extended flange or expansion capability, 90° bending capability, etc.) or machinability during the component forming process, and high impact energy absorption (high yield strength ratio) or fatigue durability is required when the component is actually installed and running in the vehicle.

[0004] Therefore, various patent documents propose methods to meet the strength-elongation balance or stretch flangeability or hole expansion ratio of high-strength hot-dip galvanized steel sheets by optimizing alloy composition, microstructure, hot rolling, cold rolling and annealing manufacturing conditions.

[0005] For example, Patent Document 1 proposes the following technique: a steel billet containing, by weight percent, C: 0.06-0.09%, Mn: 1.5-2.0%, Si: less than 0.1%, P: less than 0.02%, S: less than 0.0020%, Al: 0.005-0.05%, Cr: 0.05-0.4%, Ti: 0.005-0.02%, Nb: 0.005-0.05%, N: less than 0.0050%, and Ca: 0.0001-0.0020% is hot-rolled at 850-950°C, cooled at an average cooling rate of 5-200°C / second, then coiled, pickled, and cold-rolled at 600°C or below, and then subjected to continuous annealing and hot-dip galvanizing heat treatment. Specifically, the following method for manufacturing coated steel sheets is described: heating to an annealing temperature of 800-900°C, cooling at an average cooling rate of 2-50°C / second, stopping cooling in the temperature range of 450-650°C, and air cooling within a range of 10-50 seconds before hot-dip galvanizing or alloying hot-dip galvanizing heat treatment. However, according to the technology described in Patent Document 1, a 1.4mm thick steel sheet composed of a fine microstructure having 80-90% ferrite, 10-20% fresh martensite, and less than 5% of other phases can obtain an alloying hot-dip galvanized steel sheet with a tensile strength of 595-635MPa (yield ratio of 0.76-0.79), a TS×EL value of 18600-20910MPa·%, and a porosity (HER) value of 74% or higher. In addition, Patent Document 1 limits the sulfur (S) content, which is the main steel component, to less than 0.0020%, but does not mention the influence of aspect ratio or clustering size on the porosity of steel, in addition to the size and number fraction of MnS inclusions.

[0006] Furthermore, Patent Document 2 describes a method in which a steel billet is reheated at a temperature below 1300°C and hot-rolled at a temperature above Ar3. The billet, by weight percent, contains: C: 0.07-0.15%, Mn: 0.65-1.30%, Si: 0.005-0.35%, P: less than 0.03%, S: less than 0.05%, Al: 0.003-0.6%, Cr: 0.6-1.40%, Ti: 0.07-0.2%, B: less than 0.001% (excluding 0%), N: less than 0.01%, and selectively treated with Ca. The hot-rolled steel sheet is then cooled by continuous or multi-stage cooling at a cooling rate of 20°C / second or higher, and coiled at a temperature between Ms and Bs to manufacture hot-rolled steel sheet or hot-rolled coated steel sheet. According to the technology described in Patent Document 2, hot-rolled steel sheets are composed of a microstructure consisting of ferrite and carbide-free bainite totaling 80% or more, 3-5% martensite, retained austenite, and selectively fine carbides with a particle size of 30 nm or less. This results in hot-rolled steel sheets and hot-rolled galvanized steel sheets with tensile strengths of 760-940 MPa (yield-to-tensile ratio of 0.83-0.90), TS×EL values ​​of 12700-13800 MPa·%, and porosity (HER) values ​​of 50% or more. Patent Document 2 limits the sulfur (S) content, a major steel component, to below 0.0500%, but in practice, it is shown to be less than 0.0020%. The high porosity of this multiphase composite steel is attributed to the presence of a relatively high fraction of bainite phase. In addition, the literature does not mention in detail the effect of sulfur (S) content or MnS inclusions on porosity.

[0007] [Existing technical documents] [Patent Literature] (Patent Document 1) Japanese Patent No. 5239562 (Patent Document 2) Korean Patent No. 10-2102005 Summary of the Invention

[0008] (a) Technical problems to be solved The purpose of this invention is to provide a high-strength cold-rolled steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and manufacturing method thereof, which achieve excellent uniform elongation and hole expansion properties by limiting the sum of the contents of Cr+Ni+Cu to less than 0.7%, appropriately combining the composition of alloying elements, and controlling the manufacturing process.

[0009] Furthermore, the technical problems that this invention aims to solve are not limited to those mentioned above, and other technical problems not mentioned can be clearly understood by those skilled in the art from the following description.

[0010] (II) Technical Solution Therefore, one aspect of the present invention relates to a cold-rolled steel sheet, which, by weight percent, comprises: C: 0.06-0.09%, Mn: 1.2-1.6%, Si: 1.3-1.7%, P: less than 0.02%, S: less than 0.007%, Al: less than 0.05%, Cr: less than 0.2% (inclusive), Ni: less than 0.2% (inclusive), Cu: less than 0.3% (inclusive), Ti: less than 0.02% (inclusive), Ca: less than 0.0040% (inclusive), N: less than 0.01%, the sum of Cr+Ni+Cu content: less than 0.7%, and the balance being Fe and other impurities, and the cold-rolled steel sheet satisfies the following relationship 1.

[0011] [Relation 1] 0.08≤(Cr+Ni+Cu) / (C+Mn)≤0.7 (weight ratio) The cold-rolled steel sheet can further satisfy the following relationship 2.

[0012] [Relationship 2] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio) The difference between the (Ni+Cu) content at a position 0.1 μm directly below the surface thickness of the cold-rolled steel sheet and the (Ni+Cu) content at a position 50 μm above the surface can satisfy the following relationship 3.

[0013] [Relationship 3] (Ni+Cu) 0.1μm -(Ni+Cu) 50μm ≥0.1% (by weight) One side of the cold-rolled steel sheet may have a hot-dip galvanized layer.

[0014] One side of the cold-rolled steel sheet may have an alloyed hot-dip galvanized layer formed thereon.

[0015] The cold-rolled steel sheet may have a tensile strength of 590 MPa or more, a uniform elongation of 18% or more, and a porosity (HER) of 40% or more.

[0016] Furthermore, the present invention relates to a method for manufacturing a cold-rolled steel sheet, comprising the following steps: heating a steel billet satisfying the composition and relation 1 to a temperature range of 1150-1300°C; hot finishing rolling the heated steel billet at a temperature range of 800-1000°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet and coiling it at a temperature of 510-620°C; cold rolling the coiled steel sheet at a cold rolling reduction rate of 30-70%; and heating the cold-rolled steel sheet to a temperature of 780-820°C and then slowly cooling it to 600-700°C, followed by a second rapid cooling at an average cooling rate of 10°C / second or higher to a temperature range of 350-450°C, and then holding it for 100-700 seconds.

[0017] The steel billet can satisfy the following relationship 2.

[0018] [Relationship 2] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio) The difference between the (Ni+Cu) content at a position 0.1 μm directly below the surface thickness of the manufactured cold-rolled steel sheet and the (Ni+Cu) content at a position 50 μm above the surface can satisfy the following relationship 3.

[0019] [Relationship 3] (Ni+Cu) 0.1μm -(Ni+Cu) 50μm ≥0.1% (by weight) It may further include the step of heating the cold-rolled steel sheet that has been held to a temperature range of 400-470°C and then hot-dip galvanizing it to manufacture hot-dip galvanized steel sheet.

[0020] It may further include the step of heating the hot-dip galvanized steel sheet to a temperature of 500-560°C and then performing an alloying heat treatment to manufacture an alloyed hot-dip galvanized steel sheet.

[0021] The cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet can also be leveled and rolled with an elongation of 0.1-1.5%.

[0022] (III) Beneficial Effects According to the present invention, the steel sheet manufactured with the steel of the stated composition satisfies the correlation HER(%) = -51 + 0.209[TS(MPa)] - 143[S(%)] - 42.6%[Cr+Ni+Cu(%)] + 8.22[MnS longitudinal / transverse ratio] - 27.5[MnS area fraction] - 43.7[MnS size], has a tensile strength of 590 MPa or more, a uniform elongation of 18% or more, and a porosity (HER) of 40% or more. Furthermore, the difference between the (Ni+Cu) content at a position 0.1 μm directly below the surface thickness and the (Ni+Cu) content at a position 50 μm above the surface is 0.1% or more. This high-strength thin steel sheet can be economically manufactured using conventional annealing equipment and can be used as an automotive body panel and structural component. Attached Figure Description

[0023] Figure 1 This is a photograph showing the distribution of MnS inclusions in the cross-section of a steel plate according to Example 3 of the present invention.

[0024] Figure 2 This is a photograph showing the distribution of MnS inclusions in the cross-section of a steel plate of Comparative Example 4 in an embodiment of the present invention.

[0025] Figure 3 This is a graph showing the enrichment of Ni+Cu on the surface of the steel plate in Invention Example 2 and Comparative Example 4 of the present invention. Best practice

[0026] The present invention will now be described.

[0027] The present invention is characterized in that, as a steel sheet that can be applied to the body panels and structural components of a vehicle through a cold forming process, even in a steel sheet composed of a (Ni+Cu) content at a position 0.1 μm directly below the surface thickness and a (Ni+Cu) content at a position 50 μm above the surface thickness of the steel sheet, the steel sheet has a uniform elongation of more than 18% and a porosity of more than 40%.

[0028] The cold-rolled steel sheet of this invention, by weight percent, comprises: C: 0.06-0.09%, Mn: 1.2-1.6%, Si: 1.3-1.7%, P: less than 0.02%, S: less than 0.007%, Al: less than 0.05%, Cr: less than 0.2% (inclusive), Ni: less than 0.2% (inclusive), Cu: less than 0.3% (inclusive), Ti: less than 0.02% (inclusive), Ca: less than 0.0040% (inclusive), N: less than 0.01%, the sum of Cr+Ni+Cu content: less than 0.7%, the balance being Fe and other impurities, and can satisfy Equation 1.

[0029] The steel composition of the present invention will be described below. Unless otherwise specified, "%" refers to weight.

[0030] C: 0.06-0.09% Carbon (C) is an essential element for increasing the strength of steel and ensuring the fraction of retained austenite phase. Specifically, it dissolves in the austenite phase during continuous annealing and subsequent cooling, and when the steel sheet is subjected to isothermal heat treatment at the temperature where the bainite phase forms, it dissolves within the austenite phase, lowering the phase transformation temperature of the martensitic phase to below room temperature. However, when the C content is less than 0.06%, it is difficult to ensure a tensile strength of 590 MPa or more, or a retained austenite phase fraction of less than 5%. On the other hand, when the carbon (C) content exceeds 0.09%, the martensitic phase fraction increases excessively, making it difficult to ensure a uniform elongation of more than 18%.

[0031] Therefore, the carbon (C) content in this invention is preferably limited to 0.06-0.09%. More preferably, it is limited to 0.07-0.08%.

[0032] Mn: 1.2-1.6% Manganese (Mn) is an element that inhibits the formation of the ferrite phase and increases the stability of the austenite phase. Increasing the Mn content lowers the phase transformation temperature of the bainite and martensite phases. Specifically, during subsequent cooling heat treatment after the isothermal phase transformation of steel, the austenite phase may transform into martensite, thus reducing the fraction of residual austenite in the final fine microstructure and potentially decreasing the ductility of the steel. When its content is less than 1.2%, it is difficult to ensure sufficient strength of the steel; on the other hand, when the Mn content exceeds 1.6%, it is difficult to ensure cold formability, including the target uniform elongation or porosity. Therefore, the Mn content in this invention is preferably limited to 1.2-1.6%, more preferably 1.3-1.5%.

[0033] Si: 1.3-1.7% The silicon (Si) is an element used to increase the strength of steel through solid solution strengthening or to suppress the formation of iron-carbide during phase transformation of the constituent phase. During isothermal heat treatment of steel, as a portion of the austenite phase transforms into bainite, the remaining carbon (C) can diffuse into the austenite phase or form iron-carbide. Therefore, when the silicon (Si) content is properly controlled, the formation of iron-carbide can be suppressed, thereby increasing the stability of the austenite phase. When the silicon content is less than 1.3%, iron-carbide may form during isothermal holding or isothermal phase transformation heat treatment or reheating and over-aging heat treatment, thus potentially reducing the residual austenite fraction in the final fine microstructure. On the other hand, when the Si content exceeds 1.7%, excessive formation of silicon (Si)-manganese (Mn) composite oxides in the steel sheet leads to poor plating properties. Therefore, the silicon (Si) content in this invention is preferably limited to 1.3-1.7%, more preferably 1.4-1.6%.

[0034] P: Below 0.02% (excluding 0%) Phosphorus (P) is an element that increases the strength of steel. When the P content exceeds 0.02%, the weldability of the steel may decrease. Furthermore, if the P content is controlled too low, quicklime (CaO) needs to be added for dephosphorization, thus increasing the cost of the steelmaking process. Therefore, the phosphorus (P) content is preferably limited to 0.02% or less. More preferably, it is limited to 0.01% or less.

[0035] S: Below 0.007% (excluding 0%) The sulfur (S) can form MnS nonmetallic inclusions within the steel or segregate during continuous casting solidification, potentially causing high-temperature cracking. Furthermore, excessive MnS inclusions within the steel reduce its Hole Expansion Ratio (HER), leading to a sharp decline in cold formability. Additionally, excessively low sulfur content increases steelmaking costs. Therefore, the sulfur (S) content in this invention is preferably limited to 0.007% or less. More preferably, it is limited to 0.003% or less.

[0036] Al: Below 0.05% (excluding 0%) The aluminum (Al) is added as a deoxidizer. Aluminum (Al) can react with nitrogen (N) in steel to form AlN precipitates, which may cause billet cracking during continuous casting. Furthermore, aluminum (Al) reacts with oxygen in steel as an oxidizing inclusion, which can act as a nucleation site for MnS inclusions, leading to finer MnS size or a more uniform distribution of MnS. Excessive addition of aluminum (Al) can result in the formation of large amounts of hard oxides such as Al₂O₃, potentially reducing the toughness of the steel. Therefore, the aluminum (Al) content is limited to 0.05% or less (except 0%). More preferably, it is limited to 0.03%.

[0037] Cr: 0.2% or less (including 0%) Chromium (Cr) is an element that inhibits the phase transformation of ferrite and bainite, thereby increasing the strength of steel. Furthermore, chromium forms a manganese-chromium composite oxide on the surface of the steel, thereby increasing wettability and coating properties during hot-dip galvanizing after annealing heat treatment. However, when the Cr content exceeds 0.2%, it increases the strength of the steel of the present invention, making it difficult to ensure the target uniform elongation. In addition, chromium can accumulate at specific locations such as ferrite grain boundaries, forming chromium-depleted regions in adjacent areas, which may cause pitting corrosion. Therefore, the chromium (Cr) content is preferably limited to 0.2% or less (including 0%).

[0038] Ni: 0.2% or less (including 0%) Nickel (Ni) is an element that increases the stability of the austenitic phase. Nickel (Ni) is effective in suppressing high-temperature brittleness, but when the Ni content exceeds 0.2%, the manufacturing cost of steel increases. Therefore, the nickel content in this invention is preferably limited to 0.2% or less.

[0039] Cu: 0.3% or less (including 0%) The copper (Cu) is an element that increases the stability of the austenite phase. While effective in increasing corrosion resistance, copper (Cu) can accumulate on the surface of the billet, potentially causing surface cracks. Therefore, it is preferred to use it in conjunction with nickel (Ni) rather than alone. Furthermore, this invention has shown that it reacts with sulfur (S) in steel to form fine CuS precipitates, thereby refining or uniformly dispersing relatively coarse MnS inclusions. Additionally, while reducing the austenite phase fraction in the final microstructure, it has been observed to increase the carbon (C) solid solution content. However, when the Cu content exceeds 0.3%, it leads to an increase in strength and a decrease in ductility due to solid solution strengthening. Therefore, the copper content in this invention is preferably limited to 0.3% or less.

[0040] Ti: 0.02% or less (including 0%) Titanium (Ti) is an element that forms carbide or nitride precipitates. When the Ti content exceeds 0.02%, coarse TiN precipitates remain in the steel, which may form microcracks at the iron-matrix interface, thus reducing the porosity of the steel. Furthermore, this invention has confirmed that the TiN nitrides formed in the steel react with sulfur (S) to form TiS sulfides, which can uniformly disperse MnS inclusions, thus positively affecting the porosity of the steel. However, when the Ti content exceeds 0.02%, it causes grain refinement in the steel, leading to an increase in yield strength or a decrease in ductility. Therefore, the titanium content in this invention is preferably limited to 0.02% or less.

[0041] Ca: 0.0040% or less (including 0%) The calcium (Ca) is an element that controls the shape of MnS inclusions. However, when the Ca content exceeds 0.0040%, there is a limit to the ability to improve the porosity of the steel plate by changing the shape of the inclusions from a stringy to a spherical form. Therefore, the calcium content in this invention is preferably limited to 0.0040% or less.

[0042] N: below 0.01% The nitrogen (N) is an element that forms nitrides or increases the stability of the austenite phase. When the nitrogen content in steel increases, the number density of AlN and TiN nitrides increases, thereby reducing the cold formability of the steel. Therefore, the nitrogen content in this invention is preferably limited to 0.02% or less.

[0043] In addition, in addition to the above-mentioned components, the steel plate of the present invention may contain one or more of boron (B) or antimony (Sb) elements in order to ensure the strength of the steel plate or improve the surface hardness.

[0044] B: Below 0.005% Boron (B) is an element that increases the hardenability of steel. Adding an appropriate amount inhibits the formation of the ferrite phase, thereby increasing hardenability; however, excessive addition can raise the austenite recrystallization temperature or increase brittleness by decreasing elongation. Therefore, its content in this invention is preferably limited to 0.005% or less.

[0045] Sb: below 0.03% Antimony (Sb) is an element that inhibits surface decarburization of steel. When added in appropriate amounts, it accumulates on the surface of the steel plate, thereby inhibiting decarburization or suppressing the formation of oxides of manganese, silicon, and aluminum on the steel plate surface during annealing heat treatment. When the Sb content exceeds 0.03%, the effect becomes saturated or manufacturing costs increase. Therefore, the antimony content in this invention is preferably limited to 0.03% or less.

[0046] In this invention, in addition to the components described above, the balance consists of Fe and other impurities.

[0047] Relation 1 The chromium (Cr), nickel (Ni), and copper (Cu) contents described in this invention must satisfy the following relationship 1.

[0048] [Relation 1] 0.08≤(Cr+Ni+Cu) / (C+Mn)≤0.7 (weight ratio) The (Cr+Ni+Cu) / (C+Mn) ratio is essential for ensuring the cold formability of steel. Specifically, it is necessary to ensure sufficient ductility, including a tensile strength of 590 MPa or higher and a uniform elongation of 18% or higher. With high chromium (Cr), nickel (Ni), and copper (Cu) contents, the isothermal transformation during the annealing heat treatment of the steel sheet is delayed by holding or heating, which restricts carbon (C) diffusion within the austenite or reduces the chemical stability of the retained austenite. This results in a decrease in the fraction of retained austenite and an increase in the fraction of martensite in the final microstructure. Consequently, this increases the tensile strength of the steel or decreases the elongation.

[0049] Furthermore, it has been confirmed that the presence of carbon (C) or manganese (Mn) increases the fraction of the retained austenite phase, thereby increasing elongation. Therefore, as a method to ensure a tensile strength of 590 MPa or more and a uniform elongation of 18% or more, the (Cr+Ni+Cu) / (C+Mn) ratio is limited to the range of 0.08-0.7 in this invention.

[0050] Relation 2 [Relationship 2] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio) The (Al+Cu+Ti+Ca) / (Mn+S) ratio described in this invention is a preferred condition for changing the size or distribution morphology of MnS inclusions in the steel plate, thereby ensuring the porosity of the steel.

[0051] Specifically, the preferred conditions for ensuring a porosity of 40% or more are as follows. This invention confirms that the distribution and area fraction of MnS inclusions change with increasing sulfur (S) content. Experiments have shown that when the sulfur (S) content exceeds 0.5%, the MnS inclusions within the steel plate cluster together, or the area occupied by MnS inclusions relative to the total area of ​​the steel plate increases sharply, thereby reducing the porosity of the steel plate to half. Furthermore, it has been confirmed that when the contents of aluminum (Al), copper (Cu), titanium (Ti), and calcium (Ca) are appropriately controlled, MnS inclusions are partially dispersed by Al2O3, TiN, etc., or CuS sulfides are finely precipitated (although not detectable under scanning electron microscopy, they exist in the matrix iron under transmission electron microscopy), resulting in a relatively reduced area fraction. As a result, a porosity of 40% or more in the steel can be ensured. Therefore, as a method to improve the porosity of steel, it is preferable to limit the (Al+Cu+Ti+Ca) / (Mn+S) ratio to the range of 0.06-0.5.

[0052] Relation 3 [Relationship 3] (Ni+Cu) 0.1μm -(Ni+Cu) 50μm ≥0.1% (by weight) In this invention, the difference between the (Ni+Cu) content at a position 0.1 μm directly below the surface thickness of the steel plate and the (Ni+Cu) content at a position 50 μm below the surface thickness satisfies the aforementioned relationship 3. That is, the (Ni+Cu) content... 0.1μm -(Ni+Cu) 50μm This value represents a necessary condition for limiting the enrichment size of the steel surface layer to an appropriate level. In cases where relationship 3 is not satisfied, it means that nickel (Ni) or copper (Cu) elements are uniformly distributed in the steel. Furthermore, when the enrichment of these elements exists on the surface of the steel sheet, decarburization of the steel surface can be suppressed, thereby compensating for the decrease in surface hardness. Therefore, as a method to minimize the decrease in surface hardness, it is preferable to use (Ni+Cu) 0.1μm -(Ni+Cu) 50μm The value should be controlled above 0.1%.

[0053] Furthermore, the steel sheet of the present invention can have a fine microstructure consisting of ferrite, retained austenite, and a mixture of bainite and martensite, wherein the sum of the fractions of the bainite and martensite phases is preferably 9 area % or less. Additionally, the retained austenite fraction is preferably 5% or less. When the sum of the fractions of the bainite and martensite phases exceeds 9 area %, the addition of Cr, Cu, and Ni increases, thus decreasing the retained austenite fraction and increasing the martensite fraction. This results in increased tensile strength but decreased yield strength, potentially making it difficult to ensure the desired properties. Furthermore, when the retained austenite fraction exceeds 5 area %, the addition of C and Mn increases, thereby increasing the strength of the steel but potentially worsening its porosity.

[0054] As described above, the steel sheet of the present invention, having a steel composition and fine microstructure, can have a tensile strength of 590 MPa or more, a uniform elongation of 18% or more, and a porosity (HER) of 40% or more.

[0055] Next, the steel plate manufacturing method of the present invention will be described.

[0056] This invention manufactures steel billets by melting steel plates that satisfy the stated composition and relational formula 1. The melting method can selectively utilize either blast furnace or electric arc furnace steelmaking.

[0057] The steel plate manufacturing method of the present invention includes the following steps: heating a steel billet that satisfies the above-mentioned composition and relation 1 to a temperature range of 1150-1300°C; hot finishing rolling the heated steel billet at a temperature range of 800-1000°C to obtain a hot-rolled steel plate; cooling the hot-rolled steel plate and coiling it at a temperature of 510-620°C; cold rolling the coiled steel plate at a cold rolling reduction rate of 30-70%; and heating the cold-rolled steel plate to a temperature of 780-820°C and then slowly cooling it once to 600-700°C, followed by a second rapid cooling at an average cooling rate of 10°C / second or higher to a temperature range of 350-450°C, and then holding it for 100-700 seconds.

[0058] First, in this invention, the steel billet satisfying the above-mentioned composition and relational formula 1 is heated to a temperature range of 1150-1300°C. When the heating temperature is below 1150°C, it is difficult to ensure the rolling temperature of the steel plate during hot rolling, resulting in an increase in rolling load. On the other hand, when the reheating temperature exceeds 1300°C, a large amount of oxide scale is generated on the surface of the steel billet or slab. Therefore, in this invention, the reheating temperature of the steel billet is preferably controlled within the range of 1150-1300°C.

[0059] Furthermore, the steel billet described in this invention can also satisfy the above-mentioned relation 2.

[0060] Next, in this invention, the reheated steel billet is hot-rolled within a temperature range of 800-1000°C to produce hot-rolled steel sheet. At this time, the hot finishing rolling is performed under normal conditions, i.e., above Ar3 temperature, specifically, preferably within a temperature range of 800-1000°C. When the hot finishing rolling temperature is below 800°C, the rolling load increases significantly; when it exceeds 1000°C, there is a problem of a significant increase in thermal fatigue of the rolling rolls. Therefore, in this invention, the hot finishing rolling is performed within a temperature range of 800-1000°C.

[0061] Furthermore, in this invention, the manufactured hot-rolled steel sheet is cooled on the output roller table and coiled at a temperature of 510-620°C. When the coiling temperature is below 510°C, excessive martensite and bainite will form within the hot-rolled steel sheet, potentially increasing the rolling load during subsequent cold rolling. On the other hand, when the coiling temperature exceeds 620°C, decarburization or excessive oxide scale will occur on the surface, and iron-oxide scale may remain even after descaling, potentially reducing the plating properties of the final coated steel sheet.

[0062] Next, the hot-rolled steel sheet that has been coiled can be pickled using conventional methods to remove the surface oxide layer.

[0063] Next, in this invention, the coiled steel sheet is cold-rolled at a reduction rate of 30-70% to produce a cold-rolled steel sheet with the target thickness. The cold rolling reduction rate can be within the range of 30-70%. When the cold rolling reduction rate is less than 30%, the cumulative reduction is insufficient, and austenite recrystallization during continuous annealing may be inadequate. On the other hand, when the cold rolling reduction rate exceeds 70%, austenite recrystallization is excessive, and coarse austenite or ferrite grains may form during continuous annealing. Therefore, in this invention, the cold rolling reduction rate is preferably controlled within the range of 30-70%.

[0064] Subsequently, the cold-rolled steel sheet is subjected to continuous annealing heat treatment in this invention. This continuous annealing heat treatment process is very important for ensuring the fine microstructure of the steel sheet composed of appropriate fractions of ferrite, bainite, martensite and retained austenite phases, thereby providing a steel sheet with tensile strength of more than 590 MPa and uniform elongation of more than 18% and excellent cold formability.

[0065] Specifically, in this invention, the cold-rolled steel plate is heated to an annealing temperature of 780-820°C and then slowly cooled to 600-700°C. Then, it is rapidly cooled a second time at an average cooling rate of 10°C / second or higher to a temperature range of 350-450°C. Finally, it is held for 100-700 seconds for an isothermal phase transformation.

[0066] First, in this invention, the cold-rolled steel sheet is heated to an annealing temperature of 780-820°C.

[0067] When the annealing temperature is below 780°C, the austenite fraction is insufficient, making it difficult to ensure the target strength. On the other hand, when the annealing temperature exceeds 820°C, although the austenite fraction is sufficient, coarse grains will form, leading to decreased austenite stability and a reduction in the residual austenite fraction during subsequent cooling, which may result in a decrease in elongation. Therefore, the annealing temperature in this invention is preferably controlled at 780-820°C.

[0068] Next, the heated cold-rolled steel sheet is subjected to a slow cooling process to 600-700°C. While the present invention does not impose a particular limitation on the initial slow cooling temperature, considering the characteristics of the actual annealing line, the slow cooling temperature is controlled within the range of 600-700°C. Furthermore, the slow cooling rate is not limited at this stage; for example, a slow cooling rate of 2°C / second can be used.

[0069] Furthermore, in this invention, the steel plate that has undergone slow initial cooling is subjected to a second rapid cooling at an average cooling rate of 10°C / second or higher, cooling it to a temperature range of 350-450°C.

[0070] In this invention, the secondary rapid cooling termination temperature is preferably controlled within the range of 350-450°C. When the secondary rapid cooling termination temperature is below 350°C, it is almost close to the Ms temperature of the steel plate, making it difficult to promote bainitic phase transformation without further reheating and ensuring a sufficient residual austenite fraction. On the other hand, when the rapid cooling termination temperature exceeds 450°C, the possibility of carbide formation increases, which may reduce the carbon (C) content diffused into the austenite phase or the residual austenite fraction. Therefore, in this invention, the secondary rapid cooling termination temperature is preferably controlled within the range of 350-450°C.

[0071] Furthermore, the average cooling rate during this secondary cooling is preferably controlled to be 8°C / second or higher. When the cooling rate is less than 8°C / second, excessive formation of new ferrite or pearlite phases may occur during the cooling process, thus potentially reducing the austenite fraction participating in the bainitic phase transformation.

[0072] Subsequently, in this invention, the cold-rolled steel sheet after the secondary rapid cooling is held for 100-700 seconds. When the holding time is less than 100 seconds, carbon diffusion, which helps improve the stability of retained austenite, may be insufficient. On the other hand, when the holding time exceeds 700 seconds, there is a problem of reduced steel sheet manufacturing productivity. Therefore, the holding time in this invention is preferably controlled within the range of 100-700 seconds.

[0073] Using the manufacturing process described above, (Ni+Cu) can be obtained. 0.1μm -(Ni+Cu) 50μmThe value is controlled to be above 0.1% for cold-rolled steel sheets, thereby obtaining cold-rolled steel sheets with tensile strength of above 590MPa, uniform elongation of above 18% and porosity (HER) of above 40%.

[0074] In addition, as needed, the cold-rolled steel sheet that has been held can be heated to a temperature of 400-470°C and then hot-dip galvanized steel sheet can be manufactured using conventional methods.

[0075] Furthermore, in this invention, the hot-dip galvanized steel sheet can also be heated to a temperature of 500-560°C and then subjected to alloying heat treatment to manufacture alloyed hot-dip galvanized steel sheet.

[0076] Furthermore, in this invention, the manufactured cold-rolled steel sheet, hot-dip galvanized steel sheet, or alloyed hot-dip galvanized steel sheet can be flattened for the purpose of shape correction or surface roughness control. Excessive flattening will reduce the ductility of the heat-treated steel sheet. Therefore, the flattening elongation is preferably in the range of 0.1-1.5%. Detailed Implementation

[0077] The present invention will now be described in detail through examples.

[0078] (Example) [Table 1] [Table 2] Using steel billets with the composition shown in Table 1, hot finishing rolling, continuous annealing, and hot-dip galvanizing or even alloying hot-dip galvanizing heat treatments are performed under the conditions in Table 2 to manufacture cold-rolled steel sheets and coated steel sheets with a thickness of 1.2 mm. Specifically, on-site steel billets or laboratory-made ingots manufactured before hot rolling are homogenized by heating at 1200±20°C for 60 minutes. Then, each steel billet or ingot is rough-rolled and finish-rolled, and after being coiled at a temperature of 510-580°C to manufacture hot-rolled steel sheets with a thickness of 2.4 mm, a cold rolling reduction of 50% is applied to manufacture cold-rolled steel sheets, and then cold-rolled steel sheets or coated steel sheets are manufactured by continuous annealing heat treatment or even coating heat treatment. The inventive steels (1-10) in Table 1 satisfy relations 1 to 3, while comparative steels 1 to 6 do not satisfy at least one of relations 1 to 3. Table 2 shows the specific hot finishing temperature, coiling temperature, and continuous speed or hot-dip galvanizing heat treatment conditions applied to the invented steel and the comparative steel. In this case, the secondary cooling rate is uniformly 10°C / second.

[0079] The results of the measurements of the fraction of constituent phases, carbon solid solution content of retained austenite phase, area fraction of MnS inclusions, tensile properties and porosity (HER) in the microstructure of cold-rolled steel sheets and galvanized steel sheets manufactured as described above are shown in Table 3 below.

[0080] The microstructure of the steel sheet was observed at one-quarter of its thickness using a scanning electron microscope, and the results were quantitatively evaluated using an image analyzer. The results are shown in Table 3 below. Furthermore, the fraction of retained austenite phase was measured using X-ray diffraction analysis, and the carbon solid solution content within the retained austenite phase was calculated using commonly known empirical formulas. Additionally, the residual microstructure of the cold-rolled and clad steel sheets in Table 3 below is ferrite.

[0081] In addition, the area fraction of MnS inclusions is determined by taking more than six images of the billet cross section with an optical microscope at X500 magnification, and the results are quantitatively analyzed using an image analyzer.

[0082] In addition, tensile properties are evaluated by shearing the specimen in a direction perpendicular to the rolling direction, processing and heat treatment according to JIS standards, and the porosity (HER) is evaluated according to VDA standard reference.

[0083] [Table 3] In Table 3, Aret(%) represents the area fraction of the retained austenite, and Aret[C] represents the amount of dissolved carbon in the retained austenite phase.

[0084] As shown in Tables 1 to 3, the cold-rolled steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets of the inventive examples (1-10) whose steel composition and steel manufacturing process conditions meet the scope of this invention, were found to have relatively low carbon solid solution content within the retained austenite phase and relatively low sum of bainite and martensite fractions, wherein the sum of bainite and martensite fractions is less than 9 area %. Therefore, based on the results, it can be confirmed that they exhibit a uniform elongation of more than 18% and a porosity (HER) of more than 40%.

[0085] On the other hand, Comparative Examples 1 to 6 represent steel compositions that do not satisfy Relationships 1 to 3. Relatively speaking, the sum of the fractions of bainite and martensite phases exceeds 9% by area, or the uniform elongation value is less than 18%, and the porosity is also poor. Furthermore, Comparative Examples 7 to 8, whose steel compositions are within the scope of this invention but whose steel manufacturing conditions exceed the scope of this invention, exhibit low mechanical properties. Specifically, Comparative Example 7, with its low annealing temperature, has a lower yield strength; on the other hand, Comparative Example 8, with its excessively high annealing temperature, has poor uniform elongation and porosity.

[0086] in addition, Figure 1This is a measurement result of the MnS inclusion distribution observed in the thickness direction of the cold-rolled steel sheet in Example 2 of the present invention. Furthermore, Figure 2 These are the measurement results of the MnS distribution observed in the thickness direction of the cold-rolled steel sheet of Comparative Example 4. It can be seen that in the case of the present invention, the sulfur (S) content is relatively low, and the area fraction and distribution of MnS inclusions are relatively uniform.

[0087] in addition, Figure 3 The (Ni+Cu) enrichment amount measured on the surface of the steel plate in Invention Example 2 and Comparative Example 4 is shown. It can be seen that the surface enrichment amount is relatively small in steel with low (Ni+Cu) content. Based on the degree of enrichment, the decrease in surface hardness caused by decarburization of the steel plate surface can be compensated.

[0088] As described above, preferred embodiments of the present invention have been presented in the detailed description of the invention. However, those skilled in the art can make various modifications without departing from the scope of the invention. Therefore, the scope of the invention should not be limited to the described embodiments, but should be determined by the following claims and their equivalents.

Claims

1. A cold-rolled steel sheet, by weight percent, comprising: C: 0.06-0.09%, Mn: 1.2-1.6%, Si: 1.3-1.7%, P: less than 0.02%, S: less than 0.007%, Al: less than 0.05%, Cr: less than 0.2% and including 0%, Ni: less than 0.2% and including 0%, Cu: less than 0.3% and including 0%, Ti: less than 0.02% and including 0%, Ca: less than 0.0040% and including 0%, N: less than 0.01%, the sum of Cr+Ni+Cu content: less than 0.7%, and the balance being Fe and other impurities, and the cold-rolled steel sheet satisfies the following relationship 1. [Relation 1] 0.08≤(Cr+Ni+Cu) / (C+Mn)≤0.7 (weight ratio).

2. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet further satisfies the following relationship 2. [Relationship 2] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio).

3. The cold-rolled steel sheet according to claim 1, wherein, The difference between the (Ni+Cu) content at a position 0.1 μm directly below the surface thickness of the steel plate and the (Ni+Cu) content at a position 50 μm below the surface thickness satisfies the following relationship 3. [Relationship 3] (Ni+Cu) 0.1μm -(Ni+Cu) 50μm ≥0.1% (by weight).

4. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet further comprises one or more of boron (B) at less than 0.005% and antimony (Sb) at less than 0.03%.

5. The cold-rolled steel sheet according to claim 1, wherein, One side of the cold-rolled steel sheet has a hot-dip galvanized layer.

6. The cold-rolled steel sheet according to claim 1, wherein, One side of the cold-rolled steel sheet has an alloyed hot-dip galvanized layer.

7. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet has a fine microstructure comprising less than 9% bainite + martensite, less than 5% retained austenite, and the remainder ferrite, by area percentage.

8. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet has a tensile strength of over 590 MPa, a uniform elongation of over 18%, and a porosity (HER) of over 40%.

9. A method for manufacturing cold-rolled steel sheet, comprising the following steps: The steel billet is heated to a temperature range of 1150-1300℃. The steel billet, by weight%, contains: C: 0.06-0.09%, Mn: 1.2-1.6%, Si: 1.3-1.7%, P: less than 0.02%, S: less than 0.007%, Al: less than 0.05%, Cr: less than 0.2% and including 0%, Ni: less than 0.2% and including 0%, Cu: less than 0.3% and including 0%, Ti: less than 0.02% and including 0%, Ca: less than 0.0040% and including 0%, N: less than 0.01%, the sum of Cr+Ni+Cu content: less than 0.7%, and the balance being Fe and other impurities. The steel billet satisfies the following relationship 1. The heated steel billet is hot-rolled in a temperature range of 800-1000℃ to obtain hot-rolled steel plate; The hot-rolled steel sheet is cooled and coiled at a temperature of 510-620°C; The coiled steel sheet is then cold-rolled with a cold rolling reduction rate of 30-70%. as well as The cold-rolled steel sheet is heated to 780-820°C and then slowly cooled to 600-700°C. It is then rapidly cooled a second time at an average cooling rate of 10°C / second or higher to 350-450°C, and held for 100-700 seconds. [Relation 1] 0.08≤(Cr+Ni+Cu) / (C+Mn)≤0.7 (weight ratio).

10. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, The steel billet further satisfies the following relationship 2. [Relationship 2] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio).

11. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, The steel billet further contains one or more of boron (B) at less than 0.005% and antimony (Sb) at less than 0.03%.

12. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, The manufactured cold-rolled steel sheet has a fine microstructure comprising less than 9% bainite + martensite, less than 5% retained austenite, and the balance ferrite, by area percentage.

13. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, The difference between the (Ni+Cu) content at a position 0.1 μm directly below the surface thickness of the cold-rolled steel sheet and the (Ni+Cu) content at a position 50 μm below the surface thickness satisfies the following relationship 3. [Relationship 3] (Ni+Cu) 0.1μm -(Ni+Cu) 50μm ≥0.1% (by weight).

14. The method for manufacturing cold-rolled steel sheet according to claim 9, wherein, The manufacturing method further includes the step of heating the cold-rolled steel sheet that has been held to a temperature range of 400-470°C and then hot-dip galvanizing it to manufacture a hot-dip galvanized steel sheet.

15. The method for manufacturing cold-rolled steel sheet according to claim 14, wherein, The manufacturing method further includes the step of heating the hot-dip galvanized steel sheet to a temperature of 500-560°C and then performing an alloying heat treatment to manufacture an alloyed hot-dip galvanized steel sheet.

16. The method for manufacturing cold-rolled steel sheet according to claim 15, wherein, The alloyed hot-dip galvanized steel sheet is leveled and rolled with an elongation of 0.1-1.5%.

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