Cold-rolled steel sheet and method for manufacturing same

Abstract

Provided are a steel sheet and a method for manufacturing the same. The present invention is useful for manufacturing 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, a uniform elongation of 18% or more, and a hole expansion ratio (HER) of 40% or more.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to cold-rolled steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets used in automobile body parts, and a method for manufacturing the same.

[0002] Recently, in order to delay global warming, the steel industry is accelerating the development of various crude steel technologies that can reduce greenhouse gas emissions during the steel production process. The automotive industry is also focusing on lightweighting automobile components to improve fuel efficiency by reducing the weight of automobiles. In particular, to reduce carbon dioxide emissions during the steel production process, steel companies are attempting to develop and commercially apply process technologies that produce pig iron using electric furnaces (using electricity to produce molten steel using iron scrap itself or iron scrap + direct reduced iron (DRI or HBI)) or hydrogen to reduce iron ore, in addition to the existing blast furnace-converter method (which produces pig iron by injecting iron ore, coke, limestone, etc.) that produces relatively high greenhouse gas emissions but is capable of mass-producing high-quality products with low impurity contents).

[0003] Meanwhile, automakers are attempting to expand the application of high-strength, thin-walled steel plates to individual automotive components to reduce their weight. In particular, when steel plates or blanks used in automotive interior structural members are cold-formed in one step, dimensional defects such as springback or forming cracks can occur. Therefore, steels used in these components require excellent cold formability (e.g., high ductility, stretch flangeability or hole expandability, 90° bendability, etc.) or workability during the forming process. Furthermore, when these components are actually installed in a vehicle and operated (in-service), high impact energy absorption (high yield ratio) or fatigue durability are required.

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

[0005] For example, Patent Document 1 discloses a technology for hot-rolling a steel slab containing, in wt%, C: 0.06 to 0.09%, Mn: 1.5 to 2.0%, Si: 0.1% or less, P: 0.02% or less, S: 0.0020% or less, Al: 0.005 to 0.05%, Cr: 0.05 to 0.4%, Ti: 0.005 to 0.02%, Nb: 0.005 to 0.05%, N: 0.0050% or less, and Ca: 0.0001 to 0.0020%, at 850 to 950°C, cooling at an average cooling rate of 5 to 200°C / sec, and then coiling, pickling, and cold rolling at 600°C or less, and then continuously performing continuous annealing and hot-dip galvanizing heat treatment. Specifically, a method for manufacturing a galvanized steel sheet is described, in which the steel sheet is heated to an annealing temperature of 800 to 900°C, cooled at an average cooling rate of 2 to 50°C / sec, cooled to a temperature range of 450 to 650°C with a cooling stop and air cooling in the range of 10 to 50 sec, and then hot-dip galvanized or alloyed hot-dip galvanized heat treatment is performed. However, according to the technology described in patent document 1, it is described that an alloyed hot-dip galvanized steel sheet can be obtained, in which a 1.4 mm thick steel sheet is composed of a microstructure having 80 to 90% ferrite, 10 to 20% fresh martensite, and 5% or less of other phases, and has a tensile strength of 595 to 635 MPa (yield ratio of 0.76 to 0.79), a TS x EL value of 18600 to 20910 MPa·%, and a hole expandability (HER) value of 74% or more. Meanwhile, in the above patent document 1, the sulfur (S) content as a major steel component is limited to 0.0020% or less, but there is no mention of the influence of the aspect ratio or clustering size other than the size and number fraction of MnS inclusions on the hole expandability of the steel.

[0006] In addition, Patent Document 2 describes a method of manufacturing a hot-rolled steel slab containing, in wt%, C: 0.07 to 0.15%, Mn: 0.65 to 1.30%, Si: 0.005 to 0.35%, P: 0.03% or less, S: 0.05% or less, Al: 0.003 to 0.6%, Cr: 0.6 to 1.40%, Ti: 0.07 to 0.2%, B: less than 0.001% (excluding 0%), N: 0.01% or less, and optionally Ca-treated, by reheating at less than 1300°C and hot-rolling at a temperature of Ar3 or higher. Subsequently, cooling the hot-rolled steel sheet at a cooling rate of 20°C / sec or more by continuous cooling or multi-stage cooling, and coiling it at a temperature between Ms and Bs to manufacture a hot-rolled steel sheet or a hot-rolled plated steel sheet. According to the technology described in patent document 2, a hot-rolled steel sheet is composed of a microstructure in which the sum of ferrite and carbide-free bainite fractions is 80% or more, 3 to 5% tempered martensite, retained austenite, and fine carbides of less than 30 nm are selectively present, and a tensile strength of 760 to 940 MPa (yield ratio of 0.83 to 0.90), a TS x EL value of 12700 to 13800 MPa·%, and a hole expandability (HER) value of 50% or more are obtained. In the above patent document 2, the sulfur (S) content as a major steel component is limited to 0.0500% or less, but in actual practice, it appears to have a content of less than 0.0020%, and the high hole expandability of the multiphase composite structure steel is thought to be due to the presence of a relatively very high fraction of the bainite phase. Meanwhile, the above document does not provide detailed information on the effect of the sulfur (S) content or MnS inclusions on the hole expandability.

[0007] [Prior Art Literature]

[0008] [Patent Document]

[0009] (Patent Document 1) Japanese Patent No. 5239562

[0010] (Patent Document 2) Korean Patent No. 10-2102005

[0011] The present invention aims to provide a high-strength cold-rolled steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a manufacturing method thereof, which have excellent uniform elongation and hole expandability, by limiting the sum of the Cr+Ni+Cu contents to 0.7% or less, appropriately combining the components of alloy elements, and controlling the manufacturing process.

[0012] In addition, the technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0013] Accordingly, one aspect of the present invention relates to a cold-rolled steel sheet containing, in wt%, C: 0.06 to 0.09%, Mn: 1.2 to 1.6%, Si: 1.3 to 1.7%, P: 0.02% or less, S: 0.007% or less, Al: 0.05% or less, Cr: 0.2% or less (including 0%), Ni: 0.2% or less (including 0%), Cu: 0.3% or less (including 0%), Ti: 0.02% or less (including 0%), Ca: 0.0040% or less (including 0%), N: 0.01% or less, and a sum of Cr+Ni+Cu contents of 0.7% or less, the remainder being Fe and other impurities, and satisfying the following relational expression 1.

[0014] [Relationship 1]

[0015] 0.08≤(Cr+Ni+Cu) / (C+Mn)≤ 0.7 (weight ratio)

[0016] The above cold rolled steel sheet can additionally satisfy the following relationship 2.

[0017] [Relationship 2]

[0018] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio)

[0019] The difference between the (Ni+Cu) content at a position 0.1 ㎛ directly below the surface thickness of the cold-rolled steel sheet and the (Ni+Cu) content at a position 50 ㎛ above the surface can satisfy the following relationship 3.

[0020] [Relationship 3]

[0021] (Ni+Cu) 0.1㎛ - (Ni+Cu) 50㎛ ≥0.1% (by weight)

[0022] A hot-dip galvanized layer can be formed on one side of the above cold-rolled steel sheet.

[0023] An alloyed hot-dip galvanized layer can be formed on one side of the above cold-rolled steel sheet.

[0024] The above cold-rolled steel sheet can have a tensile strength of 590 MPa or more, a uniform elongation of 18% or more, and a hole penetration rate (HER) of 40% or more.

[0025] In addition, the present invention,

[0026] A step of heating a steel slab satisfying the above composition and relational formula 1 to a temperature range of 1150 to 1300°C;

[0027] A step of obtaining a hot-rolled steel sheet by finishing hot rolling the above-mentioned heated slab in a temperature range of 800 to 1000°C;

[0028] A step of cooling the hot-rolled steel sheet and coiling it at a temperature of 510 to 620°C;

[0029] A step of cold rolling the above-mentioned rolled steel plate at a cold rolling reduction ratio of 30 to 70%;

[0030] The present invention relates to a method for manufacturing a cold-rolled steel sheet, comprising the steps of heating the cold-rolled steel sheet to a temperature of 780 to 820°C, performing a first slow cooling to 600 to 700°C, and then performing a second rapid cooling at an average cooling rate of 10°C / sec or more to a temperature range of 350 to 450°C, and then maintaining the temperature for 100 to 700 seconds.

[0031] The above steel slab can satisfy the following relationship 2.

[0032] [Relationship 2]

[0033] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio)

[0034] The cold rolled steel sheet manufactured above can satisfy the following relationship 3 in terms of the difference between the (Ni+Cu) content at a position 0.1 ㎛ directly below the surface layer thickness of the steel sheet and the (Ni+Cu) content at a position 50 ㎛ above the surface layer.

[0035] [Relationship 3]

[0036] (Ni+Cu) 0.1㎛ - (Ni+Cu) 50㎛ ≥0.1% (by weight)

[0037] The method may further include a step of manufacturing a hot-dip galvanized steel sheet by heating the above-mentioned cold-rolled steel sheet to a temperature range of 400 to 470°C and then hot-dip galvanizing it.

[0038] An additional step of manufacturing an alloyed hot-dip galvanized steel sheet by heating the above-mentioned hot-dip galvanized steel sheet to a temperature of 500 to 560°C and then performing an alloying heat treatment may be included.

[0039] Skin pass rolling can also be performed on the above cold rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet at an elongation of 0.1 to 1.5%.

[0040] According to the present invention, a steel sheet manufactured with the above-described component steel satisfies the relationship of HER(%) = - 51 + 0.209[TS(MPa)] - 143[S(%)] - 42.6%[Cr+Ni+Cu (%)] + 8.22[MnS longitudinal / transverse axis ratio] - 27.5[MnS fraction] - 43.7[MnS size], and has a tensile strength of 590 MPa or more, a uniform elongation of 18% or more, and a hole expandability (HER) of 40% or more, and a high-strength steel sheet in which the difference between the (Ni+Cu) content at a position 0.1㎛ directly below the surface layer thickness and the (Ni+Cu) content at a position 50㎛ of the surface layer is 0.1% or more can be manufactured economically using a conventional annealing facility, and can be used as an automobile body inner panel and a structural component.

[0041] Figure 1 is a photograph showing the distribution of MnS inclusions in a cross-section of a steel plate of Invention Example 3 in an embodiment of the present invention.

[0042] Figure 2 is a photograph showing the distribution of MnS inclusions in a cross-section of a steel plate of Comparative Example 4 in an embodiment of the present invention.

[0043] Figure 3 is a drawing showing the concentration of (Ni+Cu) on the surface of the steel plate of Invention Example 2 and Comparative Example 4 in the embodiment of the present invention.

[0044] Hereinafter, the present invention will be described.

[0045] The present invention is a steel sheet applicable to body panels and structural parts by a cold forming method, characterized in that it provides a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet having a uniform elongation of 18% or more and a hole expandability of 40% or more, even in a steel sheet having a composition in which the difference between the (Ni+Cu) content at a position 0.1㎛ directly below the thickness and the (Ni+Cu) content at a position 50㎛ in the surface layer is 0.1% or more, and a sulfur (S) content is 0.07% or less.

[0046] The cold rolled steel sheet of the present invention contains, in wt%, C: 0.06 to 0.09%, Mn: 1.2 to 1.6%, Si: 1.3 to 1.7%, P: 0.02% or less, S: 0.007% or less, Al: 0.05% or less, Cr: 0.2% or less (including 0%), Ni: 0.2% or less (including 0%), Cu: 0.3% or less (including 0%), Ti: 0.02% or less (including 0%), Ca: 0.0040% or less (including 0%), N: 0.01% or less, and the sum of Cr+Ni+Cu contents is 0.7% or less, the remainder being Fe and other impurities, and can satisfy the relational expression 1.

[0047] Hereinafter, the composition of the steel component of the present invention will be described, and below, “%” means weight% unless otherwise specified.

[0048] C: 0.06 ~ 0.09%

[0049] The above carbon (C) is an element necessary for increasing the strength of steel and securing the fraction of the retained austenite phase. In particular, it is dissolved in the austenite phase during continuous annealing and subsequent cooling, and when the steel sheet is isothermally heat treated at a temperature where the bainite phase is formed, it is dissolved in the austenite phase and reduces the transformation temperature of the martensite phase below room temperature. However, when the content is less than 0.06%, it is difficult to secure the tensile strength of the steel to 590 MPa or more or secure the fraction of the retained austenite phase to less than 5%. On the other hand, when the carbon (C) content exceeds 0.09%, the fraction of the martensite phase increases excessively, making it difficult to secure a uniform elongation of 18% or more.

[0050] Therefore, in the present invention, it is preferable to limit the carbon (C) content to 0.06 to 0.09%. More preferably, it is limited to 0.07 to 0.08%.

[0051] Mn: 1.2 ~ 1.6%

[0052] The manganese (Mn) above is an element that suppresses the formation of a ferrite phase and increases the stability of the austenite phase. As the manganese (Mn) content increases, the transformation temperature of the bainite and martensite phases decreases. In particular, the austenite phase may transform into a martensite phase during the subsequent cooling heat treatment process after the isothermal transformation of the steel, which may reduce the fraction of the retained austenite phase in the final microstructure, thereby reducing the ductility of the steel. If the content is less than 1.2%, it is difficult to secure sufficient strength of the steel, whereas if the content exceeds 1.6%, it is difficult to secure cold formability including the targeted uniform elongation or hole expandability. Therefore, in the present invention, the manganese (Mn) content is preferably limited to 1.2 to 1.6%. More preferably, it is limited to 1.3 to 1.5%.

[0053] Si: 1.3 ~ 1.7%

[0054] The silicon (Si) above is an element that increases the strength of steel through solid solution strengthening or suppresses the formation of iron carbides during the transformation process of the composition. During the isothermal heat treatment of steel, when a portion of the austenite phase transforms into bainite, excess carbon (C) can diffuse into the austenite phase or form iron carbides. Therefore, when the silicon (Si) content is appropriately controlled, the formation of iron carbides can be suppressed, thereby increasing the stability of the austenite phase. If the silicon content is less than 1.3%, iron carbides can be formed during the isothermal holding or isothermal transformation heat treatment or the reheating and overaging heat treatment, which may reduce the fraction of retained austenite in the final microstructure. On the other hand, if the silicon content exceeds 1.7%, excessive formation of silicon (Si)-manganese (Mn) composite oxides in the steel sheet may result in poor plating properties. Therefore, in the present invention, it is preferable to limit the silicon (Si) content to 1.3 to 1.7%. More preferably, it is limited to 1.4 to 1.6%.

[0055] P: 0.02% or less (excluding O%)

[0056] Phosphorus (P) is an element that increases the strength of steel. If its content exceeds 0.02%, the weldability of the steel may deteriorate. Furthermore, if the P content is controlled excessively low, the addition of calcium oxide (CaO) for dephosphorization may be necessary, which can increase the cost of the steelmaking process. Therefore, it is desirable to limit the phosphorus (P) content to 0.02% or less, and more preferably, to 0.01% or less.

[0057] S: 0.007% or less (excluding 0%)

[0058] The above sulfur (S) can cause high-temperature cracks by segregating into MnS non-metallic inclusions or during continuous solidification within the steel. In addition, if there are excessive MnS inclusions within the steel, it can reduce the hole expansion ratio (HER) of the steel, resulting in a sharp decline in cold formability. On the other hand, if the sulfur content is excessively low, it can lead to an increase in steelmaking costs. Therefore, in the present invention, the sulfur (S) content is preferably limited to 0.007% or less. More preferably, it is limited to 0.003% or less.

[0059] Al: 0.05% or less (excluding 0%)

[0060] The above aluminum (Al) is an element added as a deoxidizer. Aluminum (Al) can react with nitrogen (N) in steel to form AlN precipitates, which can cause cracks in the cast steel during the continuous casting process. In addition, aluminum (Al) reacts with oxygen in steel to exist as oxidizing inclusions, and in this case, it can act as a nucleation site for MnS inclusions to make the MnS size fine or relatively uniformly distributed. If the content is added excessively, it can form a large amount of hard oxides such as Al2O3, which can lower the toughness of the steel. Therefore, the content of aluminum (Al) is limited to 0.05% or less (excluding 0%). More preferably, it is limited to 0.03%.

[0061] Cr: 0.2% or less (including 0%)

[0062] The above chromium (Cr) is an element that increases the strength of steel by suppressing ferrite and bainite transformation. In addition, chromium forms a manganese-chromium composite oxide on the surface of the steel, thereby increasing wettability and plating properties during hot-dip galvanization after annealing heat treatment. However, if the content exceeds 0.2%, it is difficult to increase the strength of the steel of the present invention and secure the target uniform elongation. In addition, chromium may be concentrated at specific locations such as ferrite grain boundaries, forming a chromium depletion zone adjacent thereto, which may cause pitting corrosion. Therefore, it is preferable to limit the chromium (Cr) content to 0.2% or less (including 0%).

[0063] Ni: 0.2% or less (including 0%)

[0064] The above nickel (Ni) is an element that increases the stability of the austenite phase. While nickel (Ni) is effective in suppressing high-temperature embrittlement, there is a problem in that the manufacturing cost of steel increases when its content exceeds 0.2%. Therefore, in the present invention, it is preferable to limit the nickel content to 0.2% or less.

[0065] Cu: 0.3% or less (including 0%)

[0066] The above copper (Cu) is an element that increases the stability of the austenite phase. Although the copper (Cu) element is effective in increasing corrosion resistance, it can concentrate on the surface of the cast iron and cause surface cracks, so it is preferable to use it together with the nickel (Ni) element rather than using it alone. In addition, in the present invention, it is understood that it reacts with the sulfur (S) element in the steel to form CuS fine precipitates, thereby having the effect of refining or uniformly dispersing relatively coarse MnS inclusions. In addition, it is understood that it reduces the fraction of the austenite phase in the final microstructure, but increases the amount of carbon (C) in solid solution. However, if the content exceeds 0.3%, it may cause an increase in strength and a decrease in ductility due to solid solution strengthening. Therefore, in the present invention, it is preferable to limit the content of the copper to 0.3% or less.

[0067] Ti: 0.02% or less (including 0%)

[0068] The titanium (Ti) is an element that forms precipitates including carbides or nitrides. If its content exceeds 0.02%, coarse TiN precipitates may remain in the steel, which may form microcracks at the interface between the steel and the substrate, thereby reducing the porosity of the steel. On the other hand, in the present invention, the TiN nitride formed in the steel reacts with sulfur (S) to form TiS sulfides, which may uniformly disperse the MnS inclusions, thereby positively affecting the porosity of the steel. However, if the content exceeds 0.02%, the grain size of the steel may become fine, which may increase the yield strength or cause a decrease in ductility. Therefore, in the present invention, it is preferable to limit the titanium content to 0.02% or less.

[0069] Ca: 0.0040% or less (including 0%)

[0070] The above calcium (Ca) is an element that controls the shape of MnS inclusions. However, if its content exceeds 0.0040%, there is a limit to improving the hole opening properties of the steel sheet by changing the shape of the inclusions from a stringy shape to a spherical shape. Therefore, in the present invention, it is preferable to limit the calcium content to 0.0040% or less.

[0071] N: 0.01% or less

[0072] The nitrogen (N) is an element that forms nitrides or increases the stability of the austenite phase. As the nitrogen content in steel increases, the number density of AlN and TiN nitrides increases, thereby reducing the cold formability of the steel. Therefore, in the present invention, it is preferable to limit the nitrogen content to 0.02% or less.

[0073] Meanwhile, in addition to the components described above, the steel plate of the present invention may include one or more types of boron (B) or antimony (Sb) elements to secure the strength of the steel plate or improve the surface hardness.

[0074] B: 0.005% or less

[0075] The above-mentioned boron (B) is an element that increases the hardenability of steel. When added in an appropriate amount, it inhibits the formation of a ferrite phase, thereby increasing hardenability. However, excessive addition can increase the austenite recrystallization temperature or increase brittleness through a decrease in elongation. Therefore, in the present invention, it is preferable to limit the boron content to 0.005% or less.

[0076] Sb: 0.03% or less

[0077] The above-mentioned antimony (Sb) is an element that suppresses surface decarburization of steel. When added in an appropriate amount, it can concentrate on the surface of the steel sheet, suppressing decarburization or inhibiting the formation of oxides of manganese, silicon, and aluminum elements on the surface of the steel sheet during annealing heat treatment. If the content exceeds 0.03%, the above-mentioned effect becomes saturated or manufacturing costs increase. Therefore, in the present invention, it is preferable to limit the content of the above-mentioned antimony to 0.03% or less.

[0078] In the present invention, in addition to the components described above, the remainder is composed of Fe and other impurities.

[0079] Relationship 1

[0080] In the present invention, the chromium (Cr), nickel (Ni), and copper (Cu) contents must satisfy the following relationship 1.

[0081] [Relationship 1]

[0082] 0.08≤(Cr+Ni+Cu) / (C+Mn)≤ 0.7 (weight ratio)

[0083] The above (Cr+Ni+Cu) / (C+Mn) ratio is a condition necessary to secure the cold formability of steel. In particular, it is necessary to secure sufficient ductility including a tensile strength of 590 MPa or more and a uniform elongation of 18% or more. When the contents of chromium (Cr), nickel (Ni), and copper (Cu) are high, it delays the isothermal transformation during the annealing heat treatment of the steel sheet or the bainite transformation during heating, restricts the diffusion of carbon (C) into austenite, or reduces the chemical stability of the retained austenite, thereby reducing the fraction of the retained austenite phase in the final microstructure and increasing the fraction of the martensite phase. As a result, the tensile strength of the steel increases or the elongation decreases.

[0084] Meanwhile, carbon (C) or manganese (Mn) elements are understood to increase the fraction of retained austenite phase, thereby increasing elongation. Therefore, in order to secure a tensile strength of 590 MPa or more and a uniform elongation of 18% or more, the present invention limits the (Cr+Ni+Cu) / (C+Mn) ratio to the range of 0.08 to 0.7.

[0085] Relationship 2

[0086] [Relationship 2]

[0087] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio)

[0088] In the present invention, the above (Al+Cu+Ti+Ca) / (Mn+S) ratio is a desirable condition for securing the porosity of steel by changing the size or distribution pattern of MnS inclusions in the steel plate.

[0089] Specifically, it is a desirable condition for securing a hole expandability of 40% or more. In the present invention, it was confirmed that the distribution and area fraction of MnS inclusions change when the sulfur (S) content increases. It was experimentally confirmed that when the content exceeds 0.5%, MnS inclusions existing in the steel plate are clustered together or the area occupied by MnS inclusions based on the entire area of ​​the steel plate increases rapidly, thereby reducing the hole expandability of the steel plate to half. On the other hand, when the aluminum (Al), copper (Cu), titanium (Ti), and calcium (Ca) contents are appropriately controlled, it was confirmed that MnS inclusions are partially dispersed by Al2O3, TiN, etc., or CuS sulfides are finely precipitated (not confirmed under a scanning electron microscope magnification, but exist in the base steel under a transmission electron microscope magnification), so that the area fraction relatively decreases. As a result, a hole expandability of 40% or more of the steel could be secured. Therefore, as a method for improving the porosity of the steel, it is desirable to limit the (Al+Cu+Ti+Ca) / (Mn+S) ratio to the range of 0.06 to 0.5.

[0090] Relationship 3

[0091] [Relationship 3]

[0092] (Ni+Cu) 0.1㎛ - (Ni+Cu) 50㎛ ≥0.1% (by weight)

[0093] In the present invention, the difference between the (Ni+Cu) content at a position 0.1 ㎛ directly below the surface thickness of the steel plate and the (Ni+Cu) content at a position 50 ㎛ of the surface layer can satisfy the above relationship 3. That is, the above (Ni+Cu) 0.1㎛ - (Ni+Cu) 50㎛The value is a condition necessary to limit the size of the surface layer enrichment to an appropriate level. If the above relationship 3 is not satisfied, it means that nickel (Ni) or copper (Cu) elements are uniformly distributed in the steel. On the other hand, if the enrichment of the above elements is in the surface layer of the steel plate, decarburization in the surface layer of the steel plate can be suppressed, thereby compensating for the decrease in surface hardness. Therefore, as a method to minimize the decrease in surface hardness, (Ni+Cu) 0.1㎛ - (Ni+Cu) 50㎛ It is desirable to control the value to 0.1% or more.

[0094] Meanwhile, the steel sheet of the present invention may have a mixed microstructure of ferrite, retained austenite, and bainite+martensite, and it is preferable that the sum of the bainite and martensite phase fractions is 9 area% or less. And the retained austenite fraction is preferably 5% or less. If the sum of the bainite and martensite phase fractions exceeds 9 area%, the addition amounts of Cr, Cu, and Ni increase, and accordingly, the fraction of the retained austenite structure decreases and the martensite fraction increases, so that the tensile strength increases, but the yield strength decreases, making it difficult to secure desired properties. And if the retained austenite fraction exceeds 5 area%, the addition amounts of C and Mn increase, so that the strength of the steel increases, but the hole expandability may deteriorate.

[0095] The steel plate of the present invention having the steel composition and microstructure as described above can have a tensile strength of 590 MPa or more, a uniform elongation of 18% or more, and a hole expandability (HER) of 40% or more.

[0096] Next, the method for manufacturing a steel plate of the present invention is described.

[0097] The present invention manufactures a slab by melting a steel plate satisfying the above composition components and the composition relation equation 1. As the melting method, a blast furnace or an electric furnace steelmaking method can be selectively utilized.

[0098] The method for manufacturing a steel plate of the present invention comprises the steps of: heating a steel slab having the above-described composition and satisfying the relational expression 1 to a temperature range of 1150 to 1300°C; finishing hot-rolling the heated steel slab at a temperature range of 800 to 1000°C to obtain a hot-rolled steel plate; cooling the hot-rolled steel plate and coiling it at a temperature of 510 to 620°C; cold-rolling the coiled steel plate at a cold reduction ratio of 30 to 70%; heating the cold-rolled steel plate to a temperature of 780 to 820°C, performing a first slow cooling to 600 to 700°C, and then performing a second rapid cooling to a temperature range of 350 to 450°C at an average cooling rate of 10°C / sec or more, and then maintaining the temperature for 100 to 700 seconds.

[0099] First, in the present invention, a steel slab satisfying the above-described composition and relational formula 1 is heated to a temperature range of 1150 to 1300°C. If the heating temperature is lower than 1150°C, it is difficult to secure the rolling temperature of the steel plate during the hot rolling process, and there is a problem that the rolling load increases. On the other hand, if the reheating temperature exceeds 1300°C, there is a problem that a large amount of scale is generated on the surface of the slab or bar plate. Therefore, in the present invention, it is preferable to control the slab reheating temperature to be in the range of 1150 to 1300°C.

[0100] Additionally, in the present invention, the steel slab may satisfy the above-described relational expression 2.

[0101] Next, in the present invention, a hot-rolled steel sheet is manufactured by subjecting the reheated steel slab to a finish hot rolling at a temperature range of 800 to 1000°C. At this time, the hot finish rolling is performed at a temperature higher than the Ar3 temperature, which is a normal condition, and specifically, it is preferably performed at a temperature range of 800 to 1000°C. If the hot finish rolling temperature is less than 800°C, the rolling load increases significantly, and if it exceeds 1000°C, there is a problem that the thermal fatigue of the rolling rolls increases significantly. Therefore, in the present invention, the hot finish rolling temperature is performed at 800 to 1000°C.

[0102] And in the present invention, the manufactured hot-rolled steel sheet is cooled on a run-out table and coiled at a temperature of 510 to 620°C. If the coiling temperature is lower than 510°C, martensite and bainite may be excessively formed in the hot-rolled steel sheet, which may increase the rolling load during subsequent cold rolling. On the other hand, if the coiling temperature exceeds 620°C, decarburization may occur on the surface layer or excessive scale may be generated, so that iron scale may exist despite descaling operation, which may lower the plating properties of the final plated steel sheet.

[0103] Next, the coiled hot-rolled steel sheet may be pickled using a conventional method to remove the surface oxide layer.

[0104] Next, in the present invention, the coiled steel plate is cold rolled at a cold reduction ratio of 30 to 70% to manufacture a cold rolled steel plate having a target thickness. The cold reduction ratio during cold rolling can be performed in the range of 30 to 70%. If the cold reduction ratio is less than 30%, the accumulated reduction amount is insufficient, which may result in insufficient austenite recrystallization during continuous annealing. On the other hand, if the cold reduction ratio exceeds 70%, austenite recrystallization may be excessive, which may result in the formation of coarse austenite or ferrite grains during continuous annealing. Therefore, in the present invention, it is preferable to control the cold reduction ratio in the range of 30 to 70%.

[0105] Subsequently, in the present invention, the cold-rolled steel sheet is subjected to continuous annealing heat treatment. This continuous annealing heat treatment process is a very important process in providing a steel sheet having excellent cold formability, with a tensile strength of 590 MPa or more and a uniform elongation of 18% or more, by securing a steel sheet microstructure composed of an appropriate fraction of ferrite, bainite, martensite, and retained austenite phases.

[0106] Specifically, in the present invention, the cold-rolled steel sheet is heated to a temperature of 780 to 820°C, then first slowly cooled to 600 to 700°C, then second rapidly cooled to a temperature range of 350 to 450°C at an average cooling rate of 10°C / sec or more, and then isothermal transformation is maintained for 100 to 700 seconds.

[0107] First, in the present invention, the cold-rolled steel sheet is heated to an annealing temperature of 780 to 820°C.

[0108] If the annealing temperature is lower than 780°C, the austenite fraction is insufficient, making it difficult to secure the target strength. On the other hand, if it exceeds 820°C, the austenite fraction is sufficient, but coarse grains are formed, which may reduce the stability of austenite and the retained austenite fraction during the subsequent cooling process, thereby reducing the elongation. Therefore, in the present invention, it is preferable to control the annealing temperature to 780 to 820°C.

[0109] Next, in the present invention, the heated cold-rolled steel sheet is subjected to a primary slow cooling to 600 to 700°C. The present invention is not particularly limited to the primary slow cooling temperature, but the slow cooling temperature is controlled to a range of 600 to 700°C in consideration of the characteristics of the actual annealing line. In addition, the slow cooling speed at this time is not limited, and a slow cooling rate of 2°C / s, for example, can be used.

[0110] And in the present invention, the first-cooled steel plate is rapidly cooled for a second time at an average cooling rate of 10°C / sec or more to a temperature range of 350 to 450°C.

[0111] In the present invention, the secondary rapid cooling stop temperature is preferably controlled in the range of 350 to 450°C. If the secondary rapid cooling stop temperature is less than 350°C, it is difficult to promote bainite transformation without additional reheating because it is close to the Ms temperature of the steel plate, making it difficult to secure a sufficient fraction of retained austenite. On the other hand, if the secondary rapid cooling stop temperature exceeds 450°C, the possibility of carbide formation increases, which may reduce the carbon (C) content diffused into the austenite phase or reduce the fraction of the retained austenite phase. Therefore, in the present invention, the secondary rapid cooling stop temperature is preferably controlled in the range of 350 to 450°C.

[0112] In addition, it is desirable to control the average cooling rate during this secondary cooling to 8°C / sec or more. If the cooling rate is less than 8°C / sec, an excessive amount of new ferrite or pearlite phase may be formed during the cooling process, thereby reducing the austenite fraction that should be involved in the bainite transformation.

[0113] Subsequently, in the present invention, the secondarily rapidly cooled cold-rolled steel sheet is maintained for 100 to 700 seconds. If the holding time is less than 100 seconds, carbon diffusion, which can increase the stability of retained austenite, may not occur sufficiently. On the other hand, if the holding time exceeds 700 seconds, there is a problem of reduced steel sheet manufacturing productivity. Therefore, in the present invention, the holding time is preferably controlled within the range of 100 to 700 seconds.

[0114] Using the manufacturing process described above, (Ni+Cu) 0.1㎛ - (Ni+Cu) 50㎛ A cold-rolled steel sheet having a value controlled to 0.1% or more can be obtained, and further, a cold-rolled steel sheet having a tensile strength of 590 MPa or more, a uniform elongation of 18% or more, and a hole expandability (HER) of 40% or more can be obtained.

[0115] Meanwhile, in the present invention, if necessary, the maintained cold-rolled steel sheet can be heated to a temperature of 400 to 470°C, and then a hot-dip galvanized steel sheet can be manufactured using a conventional method.

[0116] Furthermore, in the present invention, an alloyed hot-dip galvanized steel sheet can be manufactured by heating the hot-dip galvanized steel sheet to a temperature of 500 to 560°C and then performing an alloying heat treatment.

[0117] In addition, in the present invention, skin pass rolling can be performed for the purpose of shape correction or surface roughness control on the cold-rolled steel sheet, hot-dip galvanized steel sheet, or alloyed hot-dip galvanized steel sheet manufactured above, and since excessive skin pass rolling can reduce the ductility of the heat-treated steel sheet, it is preferable that the skin pass elongation be performed within the range of 0.1 to 1.5%.

[0118] Hereinafter, the present invention will be described in detail through examples.

[0119] (Example)

[0120] Classification CSiMnPSSAlCrNiTiCuNCaRelationship 1Relationship 2Relationship 3Inventive steel 10.0761.461.470.00900.00300.030.0400.02400.0690.00600.00060.090.0681.88Inventive steel 20.0801.471.450.00900.00600.030.0390.0260.0020.0660.00600.00050.090.0680.10Inventive steel 30.0881.431.460.00900.00200.030.1730.1230.0020.3920.00640.00100.440.2910.10Inventive steel 40.0901.481.470.00900.00700.030.1770.1260.0020.3930.00720.00060.450.2880.10Inventive steel 50.0881.461.450.00900.00200.030.1060.0730.0020.2300.00600.00050.270.1810.10Inventive steel 60.0861.471.440.01000.00700.030.1020.0740.0020.2320.00550.00050.270.1830.10Comparison steel 10.0871.461.440.00900.01200.030.0390.0250.0020.0670.00570.00040.090.0680.06Comparison steel 20.0871.461.460.00900.00200.030.3410.2410.0020.7460.00600.00040.860.5320.18Comparison steel 30.0891.501.490.00900.00600.030.3470.2470.0020.7720.00620.00070.870.5380.17Comparison steel 40.0891.491.490.00900.01100.030.3490.2460.0020.7690.00640.00060.860.5340.09Comparison steel 50.0871.441.440.00900.01200.030.1700.1200.0020.3820.00670.00040.440.2850.10ComparisonStrength60.0851.491.440.0800.01100.030.1080.0730.0020.2290.00540.00030.270.1800.10BalGangMyeong70.051.501.470.0090.00100.0120.050.1000.0250.0500.00600.1320.0600.11Inventive steel 80.081.501.220.0090.00100.0150.100.0010.0010.1500.0050.0040.1930.1360.12Inventive steel 90.081.201. 450.0090.00600.030.1750.1220.0100.2450.00690.00050.3540.1960.10Invention steel 100.0631.201.450.0090.00600.030.1420.1220.0100.2430.00690.00050.3350.1950.10.

[0121]

[0122] Classification Rolling temperature (℃) Coiling temperature (℃) Speed ​​(mpm) Annealing (℃) Primary cooling (℃) Rapid cooling (℃) Maintenance / heating (℃) GI plating GI cooling (℃) GA temperature (℃) GA cooling (℃) Remarks Invention Honorary 191052490801639399400 / 440460-510280 Invention steel 1 Invention Honorary 290850890799639400400 / 420460-507280 Invention steel 2 Invention Honorary 390950890787637364364 / 400460-510280 Invention steel 3 Invention Honorary 491055490797639398398 / 400460-510280 Invention steel 4 Invention Honorary 590651390800640399399 / 400460280506280 Invention 5 Invention Example 691852290801640400400 / 400460280510280 Invention 6 Comparative Example 190952790803639399400 / 400460280510280 Comparative Example 1 290951990820640400400 / 400460280510280 Comparative Example 2 390058090800640407407 / 400460280510280 Comparative Example 3 491455390802638405405 / 400460280510280Comparative steel 4Comparative example 590058090803640400399 / 400460280510280Comparative steel 5Comparative example 690058090800640400400 / 400460280510280Comparative steel 6Comparative example 792160090770650324400 / 450460-510280Inventive steel 5Comparative example 8922530120850650324400 / 450460-510280Invention 6 Invention Example 792153090830650324400-150--Invention 7 Invention Example 8903510112799639398400-150--Invention 8 Invention Example 991254395802638399400 / 451460280--Invention 9 Invention Example 10913541102804638399400 / 473460280--Invention 10

[0123] Using steel slabs having the same composition as in Table 1 above, hot rolling, continuous annealing, and hot-dip galvanizing or alloying hot-dip galvanizing heat treatment were performed under the conditions of Table 2 above to manufacture cold-rolled steel sheets and coated steel sheets having a thickness of 1.2 mm. Specifically, the field-manufactured slabs or lap-manufactured ingots before hot rolling were homogenized by heating in the range of 1200±20℃ for 60 minutes. Next, individual slabs or ingots were subjected to rough rolling and finish rolling to be coiled at a temperature of 510 to 580℃ to manufacture hot-rolled steel sheets having a thickness of 2.4 mm, and then a cold-rolled steel sheet was manufactured by applying a cold reduction ratio of 50%, and then cold-rolled steel sheets or coated steel sheets were manufactured through continuous annealing heat treatment or plating heat treatment. In the above Table 1, the invention steel (1-10) satisfies the relation 1-3, whereas the comparative steel 1-6 does not satisfy at least one of the relations 1-3. Table 2 shows the specific hot rolling temperature, coiling temperature, and continuous speed or hot dip galvanizing heat treatment conditions performed on the invention steel and the comparative steel. At this time, the secondary cooling rate was uniformly set to 10℃ / s.

[0124] The results of measurements of the compositional fraction in the microstructure, the carbon content of the retained austenite phase, the MnS inclusion fraction, the tensile strength, and the hole expandability (HER) of the cold-rolled steel sheet and the plated steel sheet manufactured as described above are shown in Table 3 below.

[0125] The microstructure of the steel sheet was observed using a scanning electron microscope at a position of 1 / 4 of the steel sheet thickness, and the results were quantitatively evaluated using an image analyzer, and the results are shown in Table 3 below. In addition, the fraction of the retained austenite phase was measured using an X-ray diffraction analyzer, and the carbon solid solution content in the retained austenite phase was calculated using a commonly known empirical formula. Meanwhile, in Table 3 below, the residual structure of the cold-rolled steel sheet and the plated steel sheet is ferrite.

[0126] In addition, the fraction of MnS inclusions was quantitatively analyzed by taking six or more images of the cross-section of the steel sheet using an optical microscope at X500 magnification and using an image analyzer.

[0127] And the tensile material was evaluated after processing and heat treating the specimen in the direction perpendicular to the rolling direction according to the JIS standard, and the hole expandability (HER) evaluation was measured according to the VDA standard.

[0128] ClassificationAret (%)Aret[C]B + M (Area%)MnS Area Fraction(%)Yield Strength(MPa)Tensile Strength(MPa)Total Elongation(%)Uniform Elongation(%)Hole Expandability(%)RemarksInvention Honorary 13.21.267.60.0539260733.420.967.2Invention Steel 1Invention Honorary 22.51.316.40.0940661030.719.442.1Invention Steel 2Invention Honorary 32.01.408.20.0938765528.918.940.2Invention Steel 3Invention Honorary 42.11.438.50.1039066530.918.344.5Invention Steel 4Invention Honorary 54.21.177.50.0339163235.019.948.3Inventive steel 5Inventive example 63.41.357.80.1039362532.820.551.0Inventive steel 6Comparative example 12.91.327.50.1539764431.819.540.8Comparative steel 1Comparative example 22.41.356.20.1440361733.619.942.3Comparative steel 2Comparative example 32.91.489.30.0232872626.015.938.4Comparative steel 3Comparative example 42.61.5110.20.1132673327.215.437.8Comparative example 4 52.11.4810.70.1333974224.715.437.4Comparative example 5 63.61.267.80.1339361731.219.547.7Comparative example 6 73.41.327.90.0334359532.219.651.1Inventive example 5 84.21.238.60.1137669227.317.539.4Inventive example 6 73.01.297.60.0633852138.723.255.2 Invention 7 Invention example 83.01.307.70.0542168729.518.054.1 Invention 8 Invention example 92.91.348.00.0841067928.217.347.6 Invention 8 Invention example 102.91.348.00.0740764532.719.948.3 Invention 10

[0129] *In Table 3, Aret(%) represents the area fraction of retained austenite, and Aret[C] represents the amount of dissolved carbon in the retained austenite phase.

[0130] As shown in Table 1-3 above, the cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet of the invention example (1-10), which have steel composition components and steel manufacturing process conditions satisfying the scope of the present invention, were measured to have a relatively low carbon content within the retained austenite phase and a sum of bainite and martensite fractions of 9 area% or less. Therefore, according to the above results, it can be confirmed that they exhibit a uniform elongation of 18% or more and a hole expandability (HER) of 40% or more.

[0131] In contrast, Comparative Examples 1-6 did not satisfy Equation 1-3 in terms of steel composition, and the sum of the fractions of bainite and martensite phases exceeded 9 area% relatively, or the uniform elongation value was low at less than 18%, and the hole expandability was also poor. In addition, Comparative Examples 7-8, in which the steel composition was within the scope of the present invention but the steel manufacturing conditions were outside the scope of the present invention, exhibited poor mechanical properties. Specifically, Comparative Example 7, in which the annealing temperature was low, had poor yield strength, whereas Comparative Example 8, in which the annealing temperature was excessively high, had poor uniform elongation and hole expandability.

[0132] Meanwhile, Fig. 1 shows the results of measuring the distribution of MnS inclusions observed across the thickness of the cold-rolled steel sheet of Invention Example 2 in an embodiment of the present invention. And Fig. 2 shows the results of measuring the distribution of MnS observed across the thickness of the cold-rolled steel sheet of Comparative Example 4. In the case of the present invention example, it can be seen that the sulfur (S) content is relatively low, and the area fraction and distribution of the MnS inclusions are relatively uniform.

[0133] Meanwhile, Fig. 3 shows the amount of (Ni+Cu) enrichment measured on the surface of steel plates of Invention Example 2 and Comparative Example 4. It can be seen that the amount of enrichment on the surface is relatively small in steels with a low (Ni+Cu) content, and depending on the degree of the enrichment, the decrease in surface hardness due to decarburization on the surface of the steel plate can be compensated for.

[0134] As described above, the detailed description of the present invention has described preferred embodiments of the present invention. However, it will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described below but also by equivalents thereof.

Claims

1. A cold rolled steel sheet containing, in wt%, C: 0.06 to 0.09%, Mn: 1.2 to 1.6%, Si: 1.3 to 1.7%, P: 0.02% or less, S: 0.007% or less, Al: 0.05% or less, Cr: 0.2% or less (including 0%), Ni: 0.2% or less (including 0%), Cu: 0.3% or less (including 0%), Ti: 0.02% or less (including 0%), Ca: 0.0040% or less (including 0%), N: 0.01% or less, and a sum of Cr+Ni+Cu contents of 0.7% or less, the remainder being Fe and other impurities, and satisfying the following Relationship Formula 1. [Relationship 1] 0.08≤(Cr+Ni+Cu) / (C+Mn)≤ 0.7 (weight ratio) 2. A cold rolled steel sheet further satisfying the following relational expression 2 in paragraph 1. [Relationship 2] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio) 3. A cold rolled steel sheet in accordance with claim 1, wherein the difference between the (Ni+Cu) content at a position 0.1 ㎛ directly below the surface thickness of the steel sheet and the (Ni+Cu) content at a position 50 ㎛ above the surface satisfies the following relationship 3. [Relationship 3] (Ni+Cu) 0.1㎛ - (Ni+Cu) 50㎛ ≥0.1% (by weight) 4. A cold rolled steel sheet further comprising one or more of boron (B): 0.005% or less and antimony (Sb): 0.03% or less in accordance with paragraph 1.

5. A cold rolled steel sheet in accordance with paragraph 1, wherein a hot-dip galvanized layer is formed on one side of the cold rolled steel sheet.

6. A cold rolled steel sheet in accordance with paragraph 1, wherein an alloyed hot-dip galvanized layer is formed on one side of the cold rolled steel sheet.

7. In the first paragraph, the cold rolled steel sheet is a cold rolled steel sheet having a microstructure including, in area %, bainite + martensite: 9% or less, residual austenite: 5% or less, and residual ferrite.

8. A cold rolled steel sheet having a tensile strength of 590 MPa or more, a uniform elongation of 18% or more, and a hole expandability (HER) of 40% or more in accordance with paragraph 1.

9. A step of heating a steel slab, which contains, by weight%, C: 0.06 to 0.09%, Mn: 1.2 to 1.6%, Si: 1.3 to 1.7%, P: 0.02% or less, S: 0.007% or less, Al: 0.05% or less, Cr: 0.2% or less (including 0%), Ni: 0.2% or less (including 0%), Cu: 0.3% or less (including 0%), Ti: 0.02% or less (including 0%), Ca: 0.0040% or less (including 0%), N: 0.01% or less and the sum of Cr+Ni+Cu contents is 0.7% or less, the remainder being Fe and other impurities, and satisfying the following relationship 1, at a temperature range of 1150 to 1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated slab in a temperature range of 800 to 1000℃; A step of cooling the hot-rolled steel plate and coiling it at a temperature of 510 to 620°C; A step of cold rolling the above-mentioned rolled steel plate at a cold reduction ratio of 30 to 70%; and A method for manufacturing a cold rolled steel sheet, comprising the steps of heating the cold rolled steel sheet to a temperature of 780 to 820°C, performing a first slow cooling to 600 to 700°C, and then performing a second rapid cooling at an average cooling rate of 10°C / sec or more to a temperature range of 350 to 450°C, and then maintaining the temperature for 100 to 700 seconds. [Relationship 1] 0.08≤(Cr+Ni+Cu) / (C+Mn)≤ 0.7 (weight ratio) 10. A method for manufacturing a cold rolled steel sheet in accordance with claim 9, wherein the steel slab additionally satisfies the following relational expression 2. [Relationship 2] 0.06≤(Al+Cu+Ti+Ca) / (Mn+S)≤0.5 (weight ratio) 11. A method for manufacturing a cold rolled steel sheet in claim 9, wherein the steel slab additionally contains one or more of boron (B): 0.005% or less and antimony (Sb): 0.03% or less.

12. A method for manufacturing a cold rolled steel sheet in claim 9, wherein the manufactured cold rolled steel sheet has a microstructure including, in terms of area %, bainite + martensite: 9% or less, residual austenite: 5% or less, and residual ferrite.

13. In the 9th paragraph, a method for manufacturing a cold rolled steel sheet, wherein the difference between the (Ni+Cu) content at a position 0.1㎛ directly below the surface layer thickness of the steel sheet and the (Ni+Cu) content at a position 50㎛ above the surface layer satisfies the following relationship 3. [Relationship 3] (Ni+Cu) 0.1㎛ - (Ni+Cu) 50㎛ ≥0.1% (by weight) 14. A method for manufacturing a cold-rolled steel sheet, further comprising the step of manufacturing a hot-dip galvanized steel sheet by heating the maintained cold-rolled steel sheet to a temperature range of 400 to 470°C and then hot-dip galvanizing it in accordance with paragraph 9.

15. A method for manufacturing a cold rolled steel sheet, further comprising the step of manufacturing an alloyed hot-dip galvanized steel sheet by heating the hot-dip galvanized steel sheet to a temperature of 500 to 560°C and then performing an alloying heat treatment in accordance with paragraph 14.

16. A method for manufacturing a cold rolled steel sheet, wherein skin pass rolling is performed on the alloyed hot-dip galvanized steel sheet at an elongation of 0.1 to 1.5% in accordance with paragraph 15.

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