Cold-rolled steel sheet and manufacturing method thereof

Abstract

The present invention relates to a cold rolled steel sheet and a manufacturing method therefor and, more particularly, to a high-strength cold rolled steel sheet having excellent hardness and ductility, and a manufacturing method therefor.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet having excellent hardness and ductility and a method for manufacturing the same.

[0002] Recently, with the goal of reducing global greenhouse gas emissions, automotive powertrains are shifting from internal combustion engines to electric motors. Furthermore, the electric vehicle market is fiercely competing for weight reduction to increase driving range per charge. Consequently, aluminum alloys, which have a lower density than steel, are increasingly being used to minimize the weight of battery pack housing components.

[0003] However, automakers and parts manufacturers are expanding their adoption of steel materials that have relatively low material costs, low carbon emissions in terms of life cycle assessment (LCA), and excellent thermal runaway resistance, thanks to mass production for the popularization of electric vehicles and falling battery prices.

[0004] Patent Document 1 demonstrates the use of steel in the lower plate of a battery housing to protect battery cells. This technology utilizes ultra-high-strength steel with a tensile strength of 980-1800 MPa to protect battery cells from impacts generated from the underbody during operation, ensuring crashworthiness. While this ultra-high-strength steel not only provides crashworthiness but also supports heavy battery packs, it also contains a large amount of ferroalloy to ensure high strength. Furthermore, its complex heat treatment process makes manufacturing costs high and its usability, such as weldability, is challenging.

[0005] Patent Document 2 describes a technology for providing cold-rolled steel sheets and hot-dip galvanized steel sheets having a tensile strength of 490 MPa or more, a product of tensile strength (TS) and elongation (El) of 15,000 MPa% or more, and excellent plating properties for use as structural members or reinforcing materials of an automobile body, and describes as a key technology the formation of a uniform concentrated layer on the extreme surface by adding Sb to suppress the formation of oxidative inclusions on the surface, thereby securing plating properties. However, the Sb content range suggested in the document to form this Sb concentrated layer is insufficient for sufficient solution strengthening and transformation strengthening to realize uniform hardness on the surface to secure crash resistance against external factors such as stone chipping that may occur during automobile operation. Therefore, there is a problem that the addition of a large amount of Sb is inevitable, which increases the manufacturing cost, and excessive Sb addition may actually cause the problem of deteriorating plating adhesion.

[0006] Patent Document 3 describes a technology for manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet with a tensile strength of 590 MPa or higher and excellent fatigue properties and crash resistance. In the invention, in order to suppress softening of the surface layer that can normally occur during manufacturing, temper rolling is performed after hot rolling to increase the number of precipitation formation sites within the surface layer structure, thereby implementing an additional precipitation strengthening effect, thereby securing surface hardness. However, this technology requires the addition of expensive alloying elements such as Nb, Ti, Mo, and V, and there is a problem that plating properties are poor, such as non-plating, if the distribution of the alloying elements in the surface layer is uneven. In addition, if the surface hardness depends solely on the precipitation strengthening effect, it may cause transverse cracks to occur during component processing.

[0007] (Patent Document 1) Korean Patent Publication No. 10-2023-0032302 (published on March 7, 2023)

[0008] (Patent Document 2) Korean Patent Registration No. 10-0711358 (Published on April 27, 2007)

[0009] (Patent Document 3) Korean Patent Publication No. 10-2011-0110370 (published on October 6, 2011)

[0010] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same are provided.

[0011] According to one embodiment of the present invention, it is an object to provide a high-strength cold-rolled steel sheet having excellent hardness and ductility and a method for manufacturing the same.

[0012] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0013] According to one embodiment of the present invention, it contains, in wt%, C: 0.050 to 0.150%, Si: 0.010 to 1.100%, Mn: 1.000 to 3.000%, P: 0.001 to 0.050%, S: 0.001 to 0.020%, Cr: 0.020 to 0.300%, Ni: 0.010 to 0.100%, Sol.Al: 0.01 to 0.10%, Cu: 0.010 to 0.400%, N: 0.0010 to 0.0100%, the remainder being Fe and other unavoidable impurities.

[0014] The R1 value defined in the following relational expression 1 is 2.000 to 4.000,

[0015] The microstructure can provide a cold rolled steel sheet containing 65 to 80% ferrite and residual structure in area %.

[0016] [Relationship 1]

[0017] R1 = 17([Cr]+[Ni]) / [Si]

[0018] (In the formula, [Cr], [Ni] and [Si] are the weight percent of each element.)

[0019] The above cold rolled steel sheet may further include, in weight %, one or more of Mo, Nb, and Sb in a total amount of 1.0% or less.

[0020] The above cold rolled steel sheet may further include B: 0.003% or less in weight %.

[0021] The above cold rolled steel sheet may have an R2 value of 0.150 to 0.500 as defined in the following relational expression 2.

[0022] [Relationship 2]

[0023] R2 = ([Cr 0.5μm ]+[Ni 0.5μm ])-([Cr 1 / 4 ]-[Ni 1 / 4 ])

[0024] (In the formula, [Cr 0.5μm ] and [Ni 0.5μm ] is the maximum weight % of each element in the area from the surface of the steel plate to the point of 0.5 μm in the thickness direction, and [Cr 1 / 4 ] and [Ni 1 / 4 ] is the average weight% of each element at 1 / 4 point in the direction of the thickness of the steel plate.)

[0025] The above residual structure may include at least one of martensite and bainite.

[0026] The above residual structure may contain, in area %, 10 to 35% martensite and bainite.

[0027] The above cold rolled steel sheet may have a tensile strength (TS) of 590 MPa or more, and a product of tensile strength (TS) and elongation (El) of 145000 MPa% or more.

[0028] The above cold rolled steel sheet may have a minimum surface hardness of 170 Hv or more.

[0029] The above cold rolled steel sheet may further include a zinc plating layer on one surface.

[0030] According to one embodiment of the present invention, there is provided a step of reheating a steel slab containing, in wt%, C: 0.050 to 0.150%, Si: 0.010 to 1.100%, Mn: 1.000 to 3.000%, P: 0.001 to 0.050%, S: 0.001 to 0.020%, Cr: 0.020 to 0.300%, Ni: 0.010 to 0.100%, Sol.Al: 0.01 to 0.10%, Cu: 0.010 to 0.400%, N: 0.0010 to 0.0100%, the remainder Fe and other unavoidable impurities, and having an R1 value of 2.000 to 4.000 defined in the following relational expression 1;

[0031] A step of hot rolling the above reheated steel slab;

[0032] A step of coiling the hot-rolled steel sheet;

[0033] A step of cold rolling the above-mentioned rolled steel plate; and

[0034] A method for manufacturing a cold-rolled steel sheet can be provided, including an annealing step of heating and maintaining the cold-rolled steel sheet in a temperature range of Ac1 to Ac3+100°C, and then cooling it to 350°C or lower at a sheet speed of 70 to 200 mpm.

[0035] [Relationship 1]

[0036] R1 = 17([Cr]+[Ni]) / [Si]

[0037] (In the formula, [Cr], [Ni] and [Si] are the weight percent of each element.)

[0038] The above steel slab may further contain, in weight %, one or more of Mo, Nb, and Sb in a total amount of 1.0% or less.

[0039] The above steel slab may further contain B: 0.003% or less in weight %.

[0040] The above reheating step is performed at a temperature range of 1000 to 1350°C,

[0041] The above hot rolling step is performed at a finishing hot rolling temperature of 800 to 1000°C.

[0042] The above winding step is performed at a temperature range of 550 to 650°C,

[0043] The above cold rolling step can be performed at a reduction ratio of 30% or more.

[0044] The step of zinc plating the annealed steel plate may further be included.

[0045] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0046] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet having excellent hardness and ductility and a method for manufacturing the same can be provided.

[0047] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet having excellent hardness and ductility and usable as a structural member or reinforcing material for constituting a battery pack housing and a body of an automobile, and a method for manufacturing the same can be provided.

[0048] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet and a method for manufacturing the same can be provided, which are easy to form parts by enhancing resistance to continuous impacts applied to a vehicle from the outside, such as stone chipping that may occur while the vehicle is being driven, and at the same time ensuring superior ductility compared to ultra-high-strength steel.

[0049] The various advantageous and beneficial effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0050] FIG. 1 is a graph showing the sum of Cr and Ni contents according to the depth in the thickness direction from the surface of steel plates of Invention Example 5, Comparative Example 1, and Comparative Example 2 according to one embodiment of the present invention.

[0051] Figure 2 shows the minimum surface hardness distribution for the product of tensile strength and elongation of an inventive example and a comparative example according to one embodiment of the present invention.

[0052] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0053] Hereinafter, the present invention will be described in detail.

[0054] Below, the steel composition of the present invention is described in detail.

[0055] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0056] A cold rolled steel sheet according to an embodiment of the present invention may include, in weight %, C: 0.050 to 0.150%, Si: 0.010 to 1.100%, Mn: 1.000 to 3.000%, P: 0.001 to 0.050%, S: 0.001 to 0.020%, Cr: 0.020 to 0.300%, Ni: 0.010 to 0.100%, Sol.Al: 0.01 to 0.10%, Cu: 0.010 to 0.400%, N: 0.0010 to 0.0100%.

[0057] Carbon (C): 0.050~0.150%

[0058] The above carbon (C) is the most economical and effective element for strengthening steel. As the amount added increases, the martensite and bainite fractions increase, which can increase the tensile strength. If the carbon (C) content is less than 0.050%, it may be difficult to sufficiently secure the low-temperature transformation phase. According to one embodiment of the present invention, it may be 0.060% or more. On the other hand, if the content exceeds 0.150%, coarse carbides or pearlite may easily form, which may result in poor formability and poor weldability. According to one embodiment of the present invention, the carbon (C) content may be 0.140% or less.

[0059] Silicon (Si): 0.010~1.100%

[0060] The above silicon (Si) deoxidizes molten steel and has a solid solution strengthening effect. However, if the content is less than 0.010%, the solid solution strengthening effect may be small. According to one embodiment of the present invention, it may be 0.030% or more. On the other hand, if the content exceeds 1.100%, there may be a problem of forming oxides on the steel surface, which may hinder the plating property. According to one embodiment of the present invention, it may be 1.000% or less.

[0061] Manganese (Mn): 1.000~3.000%

[0062] The above manganese (Mn), like Si, has a solid solution strengthening effect and can also increase the hardenability of steel, facilitating the formation of martensite and bainite. However, if the content is less than 1.000%, it may be difficult to sufficiently secure the above-mentioned effect due to addition. According to one embodiment of the present invention, it may be 1.200% or more. On the other hand, if the content exceeds 3.000%, the hardenability increases significantly, and as a large amount of martensite structure is formed, it may be difficult to secure the intended ductility. According to one embodiment of the present invention, it may be 2.800% or less.

[0063] Phosphorus (P): 0.001~0.050%

[0064] The above phosphorus (P) is an element that can be included as an impurity in steel, and its content can be limited to 0.050% or less. If the phosphorus (P) content exceeds 0.050%, there is a risk of reduced weldability and increased brittleness of the steel. Meanwhile, if the P content is controlled to less than 0.001%, there is the problem of a sharp increase in refining costs.

[0065] Sulfur (S): 0.001~0.020%

[0066] The above sulfur (S) is an element that can be included as an impurity in steel, and its content can be limited to 0.020% or less. If the sulfur (S) content exceeds 0.020%, there is a high possibility that weldability will be impaired, and the ductility of the steel plate may also be deteriorated. According to one embodiment of the present invention, the sulfur (S) content can be 0.010% or less. On the other hand, if the sulfur (S) content is controlled to less than 0.001%, there is a problem that the refining cost increases rapidly.

[0067] Chromium (Cr): 0.020~0.300%

[0068] The above chromium (Cr) delays the formation of ferrite during cooling and aids the formation of martensite and bainite. However, if the content is less than 0.020%, the effect of addition cannot be obtained. According to one embodiment of the present invention, it may be 0.030% or more. On the other hand, if the content exceeds 0.300%, martensite structure may be excessively formed, resulting in poor elongation. According to one embodiment of the present invention, it may be 0.250% or less.

[0069] Nickel (Ni): 0.010~0.100%

[0070] The above nickel (Ni) delays the formation of ferrite during cooling and helps the formation of martensite and bainite. However, if the content is less than 0.010%, the effect of addition cannot be obtained. According to one embodiment of the present invention, nickel (Ni) may be 0.020% or more. On the other hand, if the nickel (Ni) content exceeds 0.100%, martensite structure may be excessively formed, resulting in poor ductility. According to one embodiment of the present invention, it may be 0.098% or less.

[0071] Aluminum (Sol.Al): 0.01~0.10%

[0072] The above aluminum (Sol.Al) is a component added primarily for deoxidation. If the content is less than 0.01%, there is a concern that the effect of addition is insufficient. According to one embodiment of the present invention, it may be 0.02% or more. On the other hand, if the content exceeds 0.10%, it combines with nitrogen to form AlN, which is likely to cause corner cracks in the slab during continuous casting, and there is a concern that defects due to the formation of inclusions may occur. According to one embodiment of the present invention, it may be 0.08% or less.

[0073] Copper (Cu): 0.010~0.400%

[0074] The above copper (Cu) is a useful element for ensuring weather resistance and improving hydrogen embrittlement. If the copper (Cu) content is less than 0.010%, the above-mentioned effects may be insufficient. According to one embodiment of the present invention, the content may be 0.012% or more. On the other hand, if the content exceeds 0.400%, there is a risk of a large number of scab defects occurring due to liquefaction of grain boundaries on the slab surface. According to one embodiment of the present invention, the content may be 0.380% or less.

[0075] Nitrogen (N): 0.0010~0.0100%

[0076] The above nitrogen (N) is an element that can be included as an impurity in steel, and its content can be limited to 0.0100% or less. If the content exceeds 0.0100%, the risk of steel becoming brittle increases, and there may be a problem of reduced performance quality due to excessive precipitation of AlN by reacting with Al. On the other hand, if the content is controlled to less than 0.0010%, there may be a problem of a sharp increase in refining costs.

[0077] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0078] According to one embodiment of the present invention, one or more of Mo, Nb, and Sb may be further included in a total amount of 1.0% or less.

[0079] The above molybdenum (Mo) and niobium (Nb) are elements that increase hardenability and contribute to the formation of martensite and bainite, and antimony (Sb) is an element that is effective in securing plating properties by forming a uniformly concentrated layer on the surface. However, if the sum of the contents of these elements exceeds 1.0%, there is a problem that the manufacturing cost excessively increases, and in particular, if excessive surface concentration is caused, a problem of poor plating adhesion may occur, so the content can be controlled so as not to exceed the above content. According to one embodiment of the present invention, it may be 0.9% or less.

[0080] According to one embodiment of the present invention, B may further include 0.003% or less.

[0081] The above boron (B) is an element that increases hardenability and contributes to the formation of martensite and bainite. However, if its content exceeds 0.003%, manufacturing costs may increase excessively, and especially, if excessive surface thickening occurs, plating adhesion may be deteriorated. According to one embodiment of the present invention, the content may be 0.002% or less.

[0082] A cold rolled steel sheet according to one embodiment of the present invention may have an R1 value defined in the following relational expression 1 of 2.000 to 4.000.

[0083] [Relationship 1]

[0084] R1 = 17([Cr]+[Ni]) / [Si]

[0085] (In the formula, [Cr], [Ni] and [Si] are the weight percent of each element.)

[0086] According to one embodiment of the present invention, hardness and ductility can be secured by controlling the relationship between the contents of Cr and Ni, which are hardenable elements, through relational expressions 1 and 2.

[0087] Specifically, according to one embodiment of the present invention, a hardenable alloy element is utilized to introduce a solid solution and transformation strengthening effect within a certain depth from directly below the surface scale layer, thereby preventing a decrease in surface hardness.

[0088] If the R1 value defined in relational expression 1 is less than 2.000, there may be a problem in that sufficient transformation strengthening effect cannot be secured in the surface layer. According to one embodiment of the present invention, the R1 value may be 2.200 or more. On the other hand, if the value exceeds 4.000, there may be a problem in that excessive transformation strengthening effect and ductility decrease due to carbide precipitation. According to one embodiment of the present invention, the R1 value may be 3.800 or less.

[0089] A cold rolled steel sheet according to one embodiment of the present invention may have an R2 value of 0.150 to 0.500 as defined in the following relational expression 2.

[0090] [Relationship 2]

[0091] R2 = ([Cr 0.5μm ]+[Ni 0.5μm ])-([Cr 1 / 4 ]-[Ni 1 / 4 ])

[0092] (In the formula, [Cr 0.5μm ] and [Ni 0.5μm ] is the maximum weight % of each element in the area from the surface of the steel plate to the point of 0.5 μm in the thickness direction, and [Cr 1 / 4 ] and [Ni 1 / 4 ] is the average weight% of each element at 1 / 4 point in the direction of the thickness of the steel plate.)

[0093] The surface layer of the steel plate may react with oxygen and, when undergoing phase transformation, delay secondary phase transformation. This may result in a localized decrease in hardness in the surface layer, which may result in reduced formability. According to one embodiment of the present invention, the surface layer may refer to a region extending from the surface of the steel plate to a point 0.5 μm in the thickness direction.

[0094] According to one embodiment of the present invention, by appropriately concentrating Cr and Ni in the surface layer of the steel sheet through relational expression 2, the phenomenon of local hardness decline can be prevented. That is, according to one embodiment of the present invention, by controlling the maximum value of the Cr and Ni contents in the surface layer of the steel sheet and controlling the average value of the Cr and Ni contents at the 1 / 4 point in the direction of the thickness of the steel sheet, the phenomenon of secondary phase transformation being delayed can be prevented by securing hardenable elements capable of forming a secondary phase in the surface layer. Therefore, by controlling relational expression 2, local hardness decline in the surface layer can be prevented.

[0095] If the R2 value defined in relational expression 2 is less than 0.150, it may be difficult to secure the hardness proposed in the present invention due to the formation of coarse ferrite that occurs locally in an area from the surface of the steel plate to a point of 0.5 μm in the direction of the center of the thickness. According to one embodiment of the present invention, it may be 0.170 or more. On the other hand, if the value exceeds 0.500, there may be a problem of causing a decrease in the ductility of the steel plate due to excessive thickening in the surface layer. According to one embodiment of the present invention, it may be 0.450 or less.

[0096] According to one embodiment of the present invention, the sum of the maximum contents of Cr and Ni in a region from the surface of the steel sheet to a point 0.5 μm in the thickness direction may be 0.1% or more. According to one embodiment of the present invention, it may be 0.2% or more. According to one embodiment of the present invention, the sum of the maximum contents of Cr and Ni in a region from the surface of the steel sheet to a point 0.5 μm in the thickness direction may be 0.9% or less. According to one embodiment of the present invention, it may be 0.8% or less.

[0097] Below, the steel microstructure of the present invention is described in detail.

[0098] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0099] The microstructure of the cold-rolled steel sheet according to one embodiment of the present invention may include, in terms of area %, 65 to 80% ferrite and a residual structure. According to one embodiment of the present invention, the residual structure may include at least one of martensite and bainite. According to one embodiment of the present invention, the residual structure may include 10 to 35% martensite and the remainder bainite.

[0100] If the ferrite fraction is less than 65%, there may be a problem in securing the product of tensile strength and elongation proposed in the present invention due to a decrease in ductility. According to one embodiment of the present invention, the ferrite fraction may be 70% or more. On the other hand, if the fraction exceeds 80%, there may be a problem in securing sufficient strength due to a decrease in the fractions of bainite and martensite. According to one embodiment of the present invention, the ferrite fraction may be 78% or less.

[0101] If the martensite fraction in the residual structure is less than 10%, there may be problems in securing the desired level of strength. According to one embodiment of the present invention, it may be 11% or more. On the other hand, if the fraction exceeds 35%, there may be a problem of reduced ductility due to excessive secondary phase formation. According to one embodiment of the present invention, tempered martensite is not intentionally included, but a small amount may inevitably be formed, and this may be included as a martensite fraction.

[0102] The remaining structure excluding martensite may include bainite. The bainite fraction is not particularly limited, but may be 20% or less according to one embodiment of the present invention. If the bainite fraction exceeds 20%, there may be a problem in securing ductility due to a decrease in the ferrite fraction. According to one embodiment of the present invention, it may be 19% or less. According to one embodiment of the present invention, it may be 18% or less. Meanwhile, the lower limit of the bainite fraction is not particularly limited, but according to one embodiment of the present invention, it may be 0%. According to one embodiment of the present invention, it may be 1% or more.

[0103] According to one embodiment of the present invention, martensite and bainite can be distinguished based on the degree of lath development of the sub-structure, and the two structures can be distinguished based on the crystal orientation relationship to indicate the fraction.

[0104] Additionally, the microstructure of the steel sheet according to one embodiment of the present invention may include ferrite, martensite, and bainite, but may also include other inevitable structures. The inevitable structures may include retained austenite, pearlite, etc.

[0105] According to one embodiment of the present invention, a cold-rolled steel sheet may have a tensile strength (TS) of 590 MPa or more, a product of tensile strength (TS) and elongation (El) of 145000 MPa% or more, and a minimum surface hardness value of 170 Hv or more.

[0106] A cold-rolled steel sheet according to one embodiment of the present invention may include a plating layer formed on one surface. The plating layer may be one of a zinc plating layer, a zinc alloy plating layer, and an alloyed zinc plating layer.

[0107] Below, the steel manufacturing method of the present invention is described in detail.

[0108] A cold-rolled steel sheet according to one embodiment of the present invention can be manufactured by reheating, hot rolling, coiling, cold rolling, and annealing a steel slab satisfying the above-described alloy composition.

[0109] Reheating

[0110] A steel slab satisfying the alloy composition according to one embodiment of the present invention can be reheated in a temperature range of 1000 to 1350°C.

[0111] The above steel slab can be manufactured by melting steel, and it does not matter whether the melting method is the converter method or the electric furnace method.

[0112] During the above reheating, if the temperature is below 1000°C, it is not easy to form a scale layer by selective oxidation of Fe, and thus a problem may arise in which the content levels of Cr and Ni directly beneath the scale layer proposed in the present invention cannot be sufficiently achieved. On the other hand, if the temperature exceeds 1350°C, there may be a problem in which a large amount of surface scale is generated.

[0113] hot rolling

[0114] The above reheated steel slab can be hot rolled at a finishing hot rolling temperature of 800 to 1000°C.

[0115] If the above finishing hot rolling temperature is below 800℃, there is a risk that the rolling load will increase significantly. On the other hand, if the temperature exceeds 1000℃, there may be a problem of a significant increase in thermal fatigue of the rolling roll.

[0116] Winding

[0117] The above hot-rolled steel plate can be coiled at a temperature range of 550 to 650°C.

[0118] If the coiling temperature is controlled below 550°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 650°C, it may act as a cause of reduced plating properties due to excessive scale formation. According to one embodiment of the present invention, after coiling, an additional pickling treatment may be performed using a conventional method, and the oxide layer may be removed through the pickling treatment.

[0119] cold rolling

[0120] The above-mentioned rolled steel plate can be cold rolled at a reduction ratio of 30% or more.

[0121] During cold rolling, the reduction ratio is not particularly limited, but in order to suppress the formation of coarse ferrite during recrystallization in the subsequent annealing process, according to one embodiment of the present invention, it can be performed at a reduction ratio of 30% or more.

[0122] Sodun

[0123] The above cold-rolled steel sheet can be heated and maintained in a temperature range of Ac1 to Ac3+100℃, and then annealed to cool to 350℃ or lower at a sheet speed of 70 to 200mpm.

[0124] During annealing, if the heating temperature is lower than Ac1, a large amount of soft polygonal ferrite may be formed, resulting in reduced strength. On the other hand, if the heating temperature exceeds Ac3+100℃, it may be difficult to secure sufficient ferrite, resulting in reduced ductility.

[0125] When the line speed (L / S) exceeds 200 mpm, the non-uniformity of martensite and bainite formation increases, which can lead to deteriorated steel sheet shape, such as wave phenomena, and sheet fracture due to sheet drift. On the other hand, when the line speed is less than 70 mpm, the problem of reduced productivity can occur.

[0126] If the cooling end temperature exceeds 350℃, there is a risk that the desired level of strength may not be achieved due to insufficient martensite fraction.

[0127] [ceremony]

[0128] Ac1 = 723-10.7[Mn]-16.9[Ni]+29.1[Si]+16.9[Cr]

[0129] (In the formula, [Mn], [Ni], [Si] and [Cr] are the weight percent of each element.)

[0130] Ac3 = 910-203√([C])-15.2[Ni]+44.7[Si]+31.5[Mo]

[0131] (In the formula, [C], [Ni], [Si] and [Mo] are the weight percent of each element.)

[0132] According to one embodiment of the present invention, if necessary, a plating process for plating the annealed steel sheet may be additionally performed. According to one embodiment of the present invention, a zinc plating or zinc alloy plating process may be additionally performed, and the plating method is not particularly limited. According to one embodiment of the present invention, electrogalvanizing may be performed. According to one embodiment of the present invention, hot-dip galvanizing may be performed. In addition, if necessary, an alloying process for alloying the zinc-plated steel sheet may be additionally performed.

[0133] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0134] (Example)

[0135] Steel slabs having the alloy compositions disclosed in Table 1 below were manufactured. Cold-rolled steel sheets were manufactured using each of the steel slabs prepared above using the conditions shown in Table 2 below. At this time, the reheating temperature was applied at the same level of 1200°C, and the reduction ratio during cold rolling was applied at 50%.

[0136] Steel alloy composition (weight %) relationship 1CSiMnPSCrNiSol.AlCuNAdditional elementR1A0.0900.3701.7500.0110.0020.0410.0250.030.0800.0026-3.032B0.0860.3811.7310.0110.0060.0310.0250.030.0600.0034-2.499C0.0701.0002.3000.0100.0060 .0520.0800.050.0900.0030Mo+Sb0.52.244D0.0701.0002.3000.0100.0060.1120.031 0.030.2200.0030-2.431E0.0701.0002.3000.0100.0060.1120.0970.060.3700.0030- 3.553F0.0900.3601.8100.0120.0070.1720.1240.030.3700.0034-13.978G0.0870.3 601.7100.0100.0110.0020.0020.050.0900.0031-0.189H0.0860.3811.7310.0110.00 60.0410.0250.030.0850.0034-2.945I0.0860.3811.7310.0110.0060.0410.0250.060 .1050.0034-2.945J0.0860.3811.7310.0110.0060.0410.0250.050.1100.0034-2.945

[0137] [Relationship 1] R1 = 17([Cr]+[Ni]) / [Si]

[0138] (In the formula, [Cr], [Ni] and [Si] are the weight percent of each element.)

[0139] Specimen number Steel grade Hot rolled Coiling Ac1 Ac3 Annealing Finishing Temperature (℃) Temperature (℃) Temperature (℃) Plate passing speed (mpm) Cooling temperature (℃) 1A8905807158658001302802B9805807168678051202903C9805807279008051202904D9056007299018151503005E90558 07289008151503106F8955607158638051302907G9055907158668151502808H8 90580716867800652909I90057071686798013029010J900570716867700130300

[0140] [Formula] Ac1 = 723-10.7[Mn]-16.9[Ni]+29.1[Si]+16.9[Cr]

[0141] (In the formula, [Mn], [Ni], [Si] and [Cr] are the weight percent of each element.)

[0142] Ac3 = 910-203√([C])-15.2[Ni]+44.7[Si]+31.5[Mo]

[0143] (In the formula, [C], [Ni], [Si] and [Mo] are the weight percent of each element.)

[0144] The content behavior of Cr and Ni at 50 points was analyzed using a glow discharge spectrometer (GDS) from the surface of the manufactured cold-rolled steel sheet to a depth of 1 μm in the thickness direction, and the average value was derived by measuring the sum of the maximum contents of Cr and Ni elements in the area from the surface of the steel sheet to a point of 0.5 μm. In addition, the average value of the Cr and Ni element contents at 1 / 4 points in the thickness direction from the surface of the steel sheet was calculated by measuring the contents of Cr and Ni elements at 50 points using the SEM-EDS technique. The R2 value of Relationship 2 was calculated using the above two values ​​and is shown in Table 3 below.

[0145] The microstructural fraction was derived by simulating the manufacturing conditions applied during the production of cold-rolled steel sheets using a dilatometer, thereby deriving the ferrite and austenite fractions. Furthermore, the results were compared with those obtained by etching the final cold-rolled steel sheets using the Nital etching method and analyzing them using a scanning electron microscope (SEM) and an image analyzer at magnifications of 1,000 to 5,000 times, and then described.

[0146] In addition, the tensile strength and hardness of the cold-rolled steel sheet manufactured above were measured and shown in Table 3 below. Here, TS and El represent tensile strength and fracture elongation, and the results are from testing ASTM standard test specimens taken parallel to the rolling direction. Hardness was measured by measuring the Vickers hardness value on the surface of the steel sheet using a Micro-Vickers tester. At this time, a load of 500 g was applied, the analysis area was 20 mm × 10 mm, and a total of 50 points were measured at 2 mm intervals. Here, the minimum value of the measured hardness value was recorded.

[0147] Specimen number Steel grade Thickness (mm) Relationship 2 Microstructure (area %) Physical property Classification R2FBMTS (MPa) El (%) TS x El (MPa %) Surface hardness (Hv) 1A 1.20.20 475 111 46 8 8 24 165 12 199 Invention example 12 B 1.20.19 27 7 8 15 6 7 9 25 16 9 7 5 178 Invention example 23 C 1.30.20 87 212 16 8 19 21 17 19 9 233 Invention example 34 D 1.70.32 9 6 9 11 20 8 3 7 20 16 7 4 0 250 Invention example 45 E 1.40 .3626718158541916226255Invention Example 56F1.20.7566015257321913908214Comparative Example 17G1.20.036774196322515800154Comparative Example 28H1.20.24688575102814280135Comparative Example 39I1.20.2626017237181913642211Comparative Example 410J1.20.24290555332714391151Comparative Example 5

[0148] * F: ferrite, B: bainite, M: martensite [Relationship 2]

[0149] R2 = ([Cr 0.5μm ]+[Ni 0.5μm ])-([Cr 1 / 4 ]-[Ni 1 / 4 ])

[0150] (In the formula, [Cr 0.5μm ] and [Ni 0.5μm ] is the maximum weight % of each element in the area from the surface of the steel plate to the point of 0.5 μm in the thickness direction, and [Cr 1 / 4 ] and [Ni 1 / 4 ] is the average weight% of each element at 1 / 4 point in the direction of the thickness of the steel plate.)

[0151] As shown in Table 3 above, in the case of an invention example that satisfies the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention were also secured.

[0152] On the other hand, Comparative Example 1 is an example in which the R1 value of Relationship 1 exceeded the scope of the present invention. As a result, the R2 value of Relationship 2 was also excessively high, and martensite and bainite were excessively formed. As a result, the desired product of tensile strength and elongation could not be achieved.

[0153] Comparative Example 2 is an example in which the R1 value of Relationship 1 falls short of the scope of the present invention. As a result, the R2 value of Relationship 2 also falls short, and the desired hardness value cannot be secured.

[0154] Comparative Examples 3 to 5 are examples in which the alloy composition conditions proposed in the present invention are satisfied, but the annealing conditions are not satisfied. In Comparative Example 3, the sheet-feeding speed during annealing was below the suggested range, and sufficient strength could not be secured due to excessive ferrite formation. In addition, the surface hardness value was also inferior. In Comparative Example 4, the annealing temperature exceeded the range of the present invention, and sufficient ferrite was not secured, resulting in excessive formation of martensite and bainite in the final microstructure. It was confirmed that this resulted in a very poor elongation. In Comparative Example 5, the annealing temperature was below the range of the present invention, and insufficient austenite was secured for the formation of martensite and bainite, resulting in excessive ferrite fraction in the final microstructure. As a result, it was confirmed that the strength and surface hardness values ​​also did not satisfy the suggested levels.

[0155] FIG. 1 is a graph showing the sum of Cr and Ni contents according to the depth in the thickness direction from the surface of steel plates of Invention Example 5, Comparative Example 1, and Comparative Example 2 according to one embodiment of the present invention.

[0156] In comparison with Comparative Examples 1 and 2, it can be confirmed that the maximum value of the sum of the Cr and Ni contents in the area up to 0.5 μm from the steel surface is distributed within an appropriate range in Inventive Example 5. On the other hand, Comparative Example 1 did not satisfy Relationship 2 because its maximum value exceeded 1%. In Comparative Example 2, the maximum value was at the level of 0.04%, confirming that Cr and Ni were not sufficiently concentrated in the surface layer.

[0157] Figure 2 illustrates the minimum surface hardness distribution for the product of tensile strength and elongation of an inventive example and a comparative example according to an embodiment of the present invention. It can be confirmed that the inventive example according to an embodiment of the present invention satisfies both the product of tensile strength and elongation and the surface hardness value proposed in the present invention.

[0158] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, in wt%, C: 0.050~0.150%, Si: 0.010~1.100%, Mn: 1.000~3.000%, P: 0.001~0.050%, S: 0.001~0.020%, Cr: 0.020~0.300%, Ni: 0.010~0.100%, Sol.Al: 0.01~0.10%, Cu: 0.010~0.400%, N: 0.0010~0.0100%, the remainder being Fe and other unavoidable impurities. The R1 value defined in the following relational expression 1 is 2.000 to 4.000, Cold rolled steel sheet containing 65-80% ferrite and residual structure by area %. [Relationship 1] R1 = 17([Cr]+[Ni]) / [Si] (In the formula, [Cr], [Ni] and [Si] are the weight % of each element.) 2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further containing, in weight %, one or more of Mo, Nb, and Sb in a total amount of 1.0% or less.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further comprising B: 0.003% or less in weight %.

4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having an R2 value of 0.150 to 0.500 as defined in the following relational expression 2. [Relationship 2] R2 = ([Cr 0.5μm ]+[Ni 0.5μm ])-([Cr 1 / 4 ]-[Ni 1 / 4 ]) (In the formula, [Cr 0.5μm ] and [Ni 0.5μm ] is the maximum weight % of each element in the area from the surface of the steel plate to a point of 0.5 μm in the thickness direction, and [Cr 1 / 4 ] and [Ni 1 / 4 ] is the average weight % of each element at 1 / 4 point in the direction of the thickness of the steel plate.) 5. In claim 1, The above residual structure is a cold rolled steel sheet including at least one of martensite and bainite.

6. In claim 1, The above residual structure is a cold rolled steel sheet containing 10 to 35% martensite by area % and the remainder bainite.

7. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a tensile strength (TS) of 590 MPa or more and a product of tensile strength (TS) and elongation (El) of 14500 MPa% or more.

8. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet with a minimum surface hardness of 170 Hv or higher.

9. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further including a zinc plating layer on one surface.

10. A step of reheating a steel slab containing, by weight%, C: 0.050 to 0.150%, Si: 0.010 to 1.100%, Mn: 1.000 to 3.000%, P: 0.001 to 0.050%, S: 0.001 to 0.020%, Cr: 0.020 to 0.300%, Ni: 0.010 to 0.100%, Sol.Al: 0.01 to 0.10%, Cu: 0.010 to 0.400%, N: 0.0010 to 0.0100%, the remainder being Fe and other unavoidable impurities, and having an R1 value of 2.000 to 4.000 defined in the following Relationship 1; A step of hot rolling the above reheated steel slab; A step of coiling the hot-rolled steel plate; A step of cold rolling the above-mentioned rolled steel plate; and A method for manufacturing a cold rolled steel sheet, comprising an annealing step of heating and maintaining the cold rolled steel sheet in a temperature range of Ac1 to Ac3+100°C, and then cooling it to 350°C or lower at a sheet passing speed of 70 to 200 mpm. [Relationship 1] R1 = 17([Cr]+[Ni]) / [Si] (In the formula, [Cr], [Ni] and [Si] are the weight % of each element.) 11. In claim 10, A method for manufacturing a cold rolled steel sheet, wherein the steel slab further contains, in weight %, one or more of Mo, Nb, and Sb, in a total amount of 1.0% or less.

12. In claim 10, A method for manufacturing a cold rolled steel sheet, wherein the steel slab further comprises B: 0.003% or less in weight%.

13. In claim 10, The above reheating step is performed at a temperature range of 1000 to 1350℃. The above hot rolling step is performed at a finishing hot rolling temperature of 800 to 1000°C. The above winding step is performed at a temperature range of 550 to 650°C. A method for manufacturing a cold rolled steel sheet, wherein the above cold rolling step is performed at a reduction ratio of 30% or more.

14. In claim 10, A method for manufacturing a cold rolled steel sheet further comprising the step of zinc plating the annealed steel sheet.

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