High-strength cold-rolled steel sheet with excellent surface quality and minimal material deviation, and method for manufacturing the same.

Abstract

The present invention relates to a high-strength cold-rolled steel sheet having excellent surface quality and little material deviation, and a manufacturing method thereof, and more particularly to a high-strength cold-rolled steel sheet having few surface defects and little material deviation while ensuring high strength and elongation, and being suitable for use in automotive parts, and a manufacturing method thereof.

Description

Technical Field

[0001] The present invention relates to a high-strength cold-rolled steel sheet used for structural members with a large amount of forming, such as fillers, seat rails, and members of an automobile body, and a manufacturing method thereof. More specifically, it relates to a high-strength cold-rolled steel sheet having excellent surface quality, few material deviations, and being suitable for use in automobile parts, and a manufacturing method thereof.

Background Art

[0002] Recently, the automobile industry has been subject to strengthened regulations on safety and the environment. In order to improve the fuel efficiency of vehicles and protect passengers, the use of high-strength steel with a tensile strength of 780 MPa or more has been increasing when manufacturing vehicle bodies.

[0003] Conventional high-strength steel used for automobile bodies includes DP (Dual Phase) steel composed of a soft ferrite base and a hard martensite two-phase, TRIP (Transformation Induced Plasticity) steel that utilizes the transformation-induced plasticity of retained austenite, or CP (Complexed Phase) steel composed of a composite structure of ferrite and hard bainite or martensite.

[0004] However, in high-strength steel, when a large amount of Si, Al, Mn, etc. is added, the weldability deteriorates, and there is a problem of surface defects of the steel sheet due to in-furnace dent during annealing. Also, when a large amount of hardening elements such as Mn, Cr, Mo, etc. is added, there is a problem that material deviation of the hot-rolled coil occurs and the thickness quality deteriorates during cold rolling. At this time, the surface defects due to in-furnace dent mean surface defects of the steel sheet formed by the adsorption and accumulation of metal oxides on the steel sheet surface to the annealing furnace roll and the contact between the steel sheet and the roll during sheet passing.

[0005] The content of the prior art related to the manufacturing technology of high-strength cold-rolled steel sheets and hot-dip galvanized steel sheets for solving the above problems is briefly described as follows. [[ID=!]]

[0006] Among the prior art, Patent Document 1 presents a high-strength cold-rolled steel sheet and a method for manufacturing the same, which involves a process of cold-rolling a hot-rolled steel sheet containing 60% or more of a low-temperature transformation phase by volume to a cold reduction ratio of more than 60% but less than 80%, and a process of continuously annealing the cold-rolled steel sheet in the ferrite and austenite two-phase region. However, the cold-rolled steel sheet obtained from Patent Document 1 has a low strength of 370-590 MPa, making it difficult to apply to automotive impact-resistant components, and thus limiting its use to only interior and exterior panel applications.

[0007] Furthermore, Patent Document 2 discloses a method for producing cold-rolled steel sheets with excellent plate shape after continuous annealing, by utilizing a tempered martensite phase to simultaneously obtain high strength and high ductility. However, the technology in Patent Document 2 has problems with degraded weldability due to a high carbon content of 0.2% or more in the steel, and surface defects caused by furnace dents due to a large amount of Si content. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Korean Published Patent No. 2004-0066935 [Patent Document 2] Japanese Patent Publication No. 2010-090432 [Overview of the project] [Problems that the invention aims to solve]

[0009] According to one aspect of the present invention, the aim is to provide a high-strength cold-rolled steel sheet with excellent surface quality and minimal material deviation, as well as a method for manufacturing the same.

[0010] The problems addressed by the present invention are not limited to those described above. Anyone with ordinary skill in the art to which the present invention pertains will have no difficulty understanding the further problems addressed by the present invention from the overall content described in the specification. [Means for solving the problem]

[0011] One aspect of the present invention is, In weight percent, it contains C: 0.05-0.3%, Si: 0.01-2.0%, Mn: 1.5-3.0%, Al: 0.01-0.1%, P: 0.001-0.015%, S: 0.001-0.01%, N: 0.001-0.01%, with the remainder being Fe and other unavoidable impurities. The value defined by the following relational expression 1 satisfies the condition that it is 0.6 or greater and less than 0.9. In terms of microstructure, by area percentage, ferrite: 50% or more, the remainder: bainite and martensite. The average number of surface defects that satisfy one or more of the following conditions: depth of 100 μm or more and short side length of 1 mm or more, is 10 defects / m². 2 We provide high-strength cold-rolled steel sheets that are less than [amount missing]. [Relationship 1] C+(1.3×Si+Mn) / 6+(Cr+1.2×Mo) / 5+100×B (In the above relational equation 1, C, Si, Mn, Cr, Mo, and B represent the weight percent average content of each element. Substitute 0 if none of the above elements are added.)

[0012] Another aspect of the present invention is, The step of reheating a steel slab to 1100-1350°C, which contains, by weight percent, C: 0.05-0.3%, Si: 0.01-2.0%, Mn: 1.5-3.0%, Al: 0.01-0.1%, P: 0.001-0.015%, S: 0.001-0.01%, N: 0.001-0.01%, with the remainder being Fe and other unavoidable impurities, and satisfying a value of 0.6 or more and less than 0.9 as defined by the following relational formula 1; The next step involves hot-rolling the reheated steel slab at 850-1150°C; The above-mentioned hot-rolled steel sheet is cooled to 450-700°C at an average cooling rate of 10-70°C / s; The above-mentioned cooled steel plate is rolled up at 450-700°C; The step of cold-rolling the above-mentioned rolled steel sheet with a reduction ratio of 40-70%; and The above cold-rolled steel sheet is continuously annealed at 740-900°C; Includes, In the above winding step, based on the overall width of the steel sheet, a manufacturing method of a high-strength cold-rolled steel sheet is provided, which is controlled such that the surface temperatures (Te) at both ends in the width direction satisfy 601 to 700 °C and the surface temperature (Tc) at the central portion satisfies 450 to 600 °C. [Relational Expression 1] C+(1.3×Si+Mn) / 6+(Cr+1.2×Mo) / 5+100×B (In the above Relational Expression 1, the above C, Si, Mn, Cr, Mo, and B represent the weight % average content for each element. At this time, when each of the above elements is not added, 0 is substituted.) [Advantages of the Invention]

[0013] According to one aspect of the present invention, it is possible to provide a high-strength cold-rolled steel sheet having excellent surface quality and few material deviations and a manufacturing method thereof.

[0014] The diverse and meaningful advantages and effects of the present invention are not limited to the above-described content and can be more easily understood in the process of explaining the specific embodiments of the present invention. [Brief Description of the Drawings]

[0015] [Figure 1] It shows photographs of surface defects of each cold-rolled steel sheet obtained from Invention Example 1 and Comparative Example 1 of the present invention taken with a general low-magnification camera. [Figure 2] It shows photographs of surface defects defined in the present invention taken using a high-magnification scanning electron microscope (SEM). [Modes for Carrying Out the Invention]

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

[0017] Furthermore, the terms used herein are for the purpose of describing specific embodiments and are not intended to limit the present invention. For example, the singular form used herein includes plural forms unless the related definition expresses a clearly opposite meaning. Also, the meaning of "includes" as used in this specification is to specify a configuration and does not exclude the existence or addition of other configurations.

[0018] Conventional technologies have not yet developed a method to meet the high-grade demand for cold-rolled steel sheets that offer high strength (tensile strength of 780 MPa or more), excellent formability, and applicability to structural components in large quantities, while also possessing superior surface quality and minimal material deviation.

[0019] Therefore, the inventors diligently conducted research to provide a cold-rolled steel sheet that satisfies all of the above-mentioned characteristics while solving the problems of the prior art. As a result, they found that the above-mentioned objectives could be achieved by optimizing the composition and manufacturing conditions of the steel sheet and controlling the characteristics of the microstructure and surface defects, thus completing the present invention.

[0020] In other words, according to the present invention, it is possible to effectively provide a high-strength cold-rolled steel sheet that has a high strength of 780 MPa or more, while satisfying a product of tensile strength and elongation of 12,000 MPa% or more, and can be applied to structural member components such as fillers that make up automobile bodies and require a stable balance of strength-elongation and shock absorption.

[0021] The following describes in detail a high-strength steel sheet with excellent surface quality and minimal material deviation according to one aspect of the present invention.

[0022] A high-strength cold-rolled steel sheet according to one aspect of the present invention contains, by weight percent, C: 0.05-0.3%, Si: 0.01-2.0%, Mn: 1.5-3.0%, Al: 0.01-0.1%, P: 0.001-0.015%, S: 0.001-0.01%, N: 0.001-0.01%, with the remainder being Fe and other unavoidable impurities.

[0023] The following will specifically explain the reasons for adding components to the cold-rolled steel sheet according to the present invention and the reasons for limiting their content. In this specification, when indicating the content of each element, the amount will be given in weight percent unless otherwise specified.

[0024] C: 0.05~0.3% The above-mentioned carbon (C) is a very important component in ensuring a martensitic structure that is effective in strengthening steel. As the amount of C added increases, the fractions of the martensite and bainite phases increase, leading to an increase in tensile strength. Therefore, in order to ensure high strength, the lower limit of the C content is controlled to 0.05%. However, as the C content increases, the austenite region expands during two-phase annealing, increasing the fractions of the hard phases, the martensite and bainite phases, while decreasing the fraction of the soft phase, the ferrite phase, thus degrading formability and weldability. Therefore, the upper limit of the C content is controlled to 0.3%. On the other hand, from the viewpoint of further improving the above-mentioned effects, more preferably, the lower limit of the C content can be 0.06%, or the upper limit of the C content can be 0.12%.

[0025] Si: 0.01~2.0% The silicon (Si) mentioned above is an advantageous element for deoxidizing molten steel, providing solid solution strengthening, and delaying the formation of coarse carbides, thereby improving formability. However, if the Si content is less than 0.01%, the above-mentioned effects are minimal, making it difficult to improve formability. On the other hand, if the Si content exceeds 2.0%, a large red scale due to Si forms on the surface of the steel sheet during hot rolling. This can lead to surface defects and surface concentration during the annealing process, resulting in unplated areas. Furthermore, the formation of surface oxides deteriorates plating adhesion, resulting in very poor surface quality. Therefore, in this invention, the Si content is controlled to 0.01-2.0%. On the other hand, from the viewpoint of further improving the above-mentioned effects, more preferably, the lower limit of the Si content can be 0.4%, or the upper limit of the Si content can be 1.2%.

[0026] Mn: 1.5~3.0% The manganese (Mn) mentioned above, like Si, is an effective element for solid solution strengthening of steel and greatly increases its hardening ability. However, if the Mn content is less than 1.5%, the above-mentioned effects of the addition are not obtained, and if the Mn content exceeds 3.0%, the strengthening effect increases greatly, but the ductility decreases. In addition, in the continuous casting process, segregation develops significantly in the center of the thickness during slab casting, and during cooling after hot rolling, the microstructure in the thickness direction becomes uneven, forming MnS and resulting in poor formability such as stretch flangeability. Therefore, in the present invention, the Mn content is controlled to 1.5 to 3.0%. On the other hand, from the viewpoint of further improving the above-mentioned effects, more preferably, the lower limit of the Mn content can be 1.8%, or the upper limit of the Mn content can be 2.6%.

[0027] Al: 0.01~0.1% The above-mentioned aluminum (Al) is an ingredient added primarily for deoxidation. If the Al content is less than 0.01%, the effect of the addition is insufficient. On the other hand, if the Al content exceeds 0.1%, it combines with nitrogen to form AlN, which makes corner cracks more likely to occur in the slab during continuous casting and prone to defects due to inclusion formation. Therefore, in the present invention, the Al content is controlled to 0.01 to 0.1%. On the other hand, from the viewpoint of further improving the above-mentioned effects, more preferably, the lower limit of the Al content can be 0.015%, or the upper limit of the Al content can be 0.06%.

[0028] P: 0.001~0.015% The phosphorus (P) mentioned above is an alloying element with a very large solid solution strengthening effect, and is characterized by the fact that a large strengthening effect can be obtained even with a small content. However, if P is added excessively, brittleness occurs due to grain boundary segregation, fine cracks are likely to occur during molding, and ductility and impact resistance are greatly deteriorated. There is also the problem of causing defects on the surface during plating. Therefore, the upper limit of the P content is controlled to 0.015%. On the other hand, if the P content is less than 0.001%, the manufacturing cost required to meet this is excessively high, which is not only economically disadvantageous, but the strength that is secured is also insufficient, so the lower limit of the P content is controlled to 0.001% or higher. Therefore, in the present invention, it is preferable to control the P content between 0.001% and 0.015%. On the other hand, from the viewpoint of further improving the above-mentioned effects, it is more preferable that the lower limit of the P content be 0.003%, or the upper limit of the P content be 0.012%.

[0029] S: 0.001~0.01% The above-mentioned sulfur (S) is an impurity present in steel. If the S content exceeds 0.01%, it combines with Mn and other elements to form nonmetallic inclusions, which makes the steel prone to developing fine cracks during cutting and significantly worsens its tensile flange properties and impact resistance. Furthermore, manufacturing the steel to have an S content of less than 0.001% requires a significant amount of time during steelmaking operations, resulting in reduced productivity. Therefore, in the present invention, it is preferable to control the S content to 0.001 to 0.01%. On the other hand, from the viewpoint of further improving the above-mentioned effects, it is more preferable that the lower limit of the S content be 0.002%, or the upper limit of the S content be 0.007%.

[0030] N: 0.001~0.01% Nitrogen (N), along with carbon (C), is a typical solid solution strengthening element and contributes to the formation of coarse precipitates along with Ti and Al. Generally, the solid solution strengthening effect of N is superior to that of carbon, but there is a problem that the toughness decreases significantly as the amount of N in the steel increases. Furthermore, producing steel with an N content of less than 0.001% requires a lot of time during steelmaking operations, resulting in decreased productivity. Therefore, in the present invention, it is preferable to control the N content to 0.001 to 0.01%. On the other hand, from the viewpoint of further improving the above-mentioned effects, it is more preferable that the lower limit of the N content be 0.002% and the upper limit of the N content be 0.006%.

[0031] On the other hand, according to one aspect of the present invention, although not particularly limited, the cold-rolled steel sheet may further selectively contain one or more elements selected from among Cr: 1.0% or less (including 0%), Mo: 0.2% or less (including 0%), and B: 0.005% or less (including 0%) by weight. The reasons for adding the above selective additive elements and the reasons for limiting their content will be explained below.

[0032] Cr: 1.0% or less (including 0%) Chromium (Cr) is an element added to steel to improve its hardening ability and ensure high strength. It plays a very important role in martensite formation, minimizing the decrease in elongation relative to the increase in strength, and is also advantageous in the production of composite steel with high ductility. Therefore, Cr can be selectively added for the effects described above. However, if the Cr content exceeds 1.0%, not only will the effects described above saturate, but there is also the problem of deterioration of cold rolling properties due to an excessive increase in hot roll strength. Furthermore, there is a problem of a large increase in the martensite fraction after annealing, leading to a decrease in elongation, so the upper limit of the Cr content should be controlled to 1.0% or less. On the other hand, from the viewpoint of further improving the effects described above, more preferably, the lower limit of the Cr content can be 0.01%, or the upper limit of the Cr content can be 0.8%.

[0033] Mo: 0.2% or less (including 0%) Molybdenum (Mo) is an element that suppresses pearlite formation and increases hardening ability. Therefore, in order to ensure the effects described above, Mo can be selectively added in the present invention. However, if the Mo content exceeds 0.2%, the effect of improving strength does not increase significantly, while ductility deteriorates, which can be economically disadvantageous. Therefore, it is preferable to control the Mo content to 0.2% or less. On the other hand, from the viewpoint of further improving the effects described above, it is more preferable that the lower limit of the Mo content be 0.01%, or the upper limit of the Mo content be 0.1%.

[0034] B: 0.005% or less (including 0%) Boron (B), when present in a solid solution state in steel, has the effect of stabilizing grain boundaries and improving the brittleness of steel at low temperatures, and significantly increasing the hardening ability of steel. Therefore, B can be selectively added for the above-mentioned effects. However, if the upper limit of B exceeds 0.005%, it delays recrystallization during annealing, causing oxide formation on the surface and degrading the plating properties. Therefore, it is preferable to control the B content to 0.005% or less. On the other hand, from the viewpoint of further improving the above-mentioned effects, it is more preferable that the lower limit of the B content be 0.0003%, or the upper limit of the B content be 0.0025%.

[0035] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be introduced due to the raw materials and surrounding environmental conditions, and these cannot be eliminated. Since these impurities are recognizable to any engineer involved in the normal steel manufacturing process, their details are not specifically mentioned herein.

[0036] According to one aspect of the present invention, the value of the high-strength cold-rolled steel sheet can be 0.6 or more and less than 0.9 as defined by the following relational expression 1. By satisfying this condition, the material deviation of the cold-rolled steel sheet can be minimized, the occurrence of surface defects can be suppressed, and the desired material can be secured. [Relationship 1] C+(1.3×Si+Mn) / 6+(Cr+1.2×Mo) / 5+100×B (In the above relational equation 1, C, Si, Mn, Cr, Mo, and B represent the weight percent average content of each element. In this case, if none of the above elements are added, substitute 0.)

[0037] In the present invention, the above relational formula 1 is an equation that shows the hardenability of the steel material according to the composition of the present invention, and the coefficient before each element quantitatively represents the measure of how much that element contributes to the hardenability. A high hardenability of the steel material is advantageous for securing hard, low-temperature transformation phases such as the bainite phase and the martensite phase, and contributes to improved strength, while a low hardenability promotes ferrite transformation, which is disadvantageous for securing strength.

[0038] In particular, in order to secure the high strength of tensile strength (TS) of 780 MPa or more that is the target of this invention, the value defined by the above relational equation 1 must satisfy 0.6 or more. On the other hand, if the value defined by the above relational equation 1 is 0.9 or more, the strength becomes too high, which leads to the problem of poor elongation. Furthermore, if the value defined by the above relational equation 1 is 0.9 or more, the phase transformation of ferrite is greatly delayed at the stage when the hot-rolled steel sheet is cooled to 450-700°C at an average cooling rate of 10-70°C / s immediately after hot rolling. As a result, in the subsequent winding stage, an excessive amount of the hard lower bainite phase and martensite phase are formed in the bainite phase within the hot-rolled steel sheet, resulting in severe material deviation due to position in the width direction and deterioration of the shape. Therefore, in this invention, it is preferable to control the value defined by the above relational equation 1 to satisfy 0.6 or more and less than 0.9. On the other hand, from the viewpoint of further maximizing the above-mentioned effects, the lower limit of the value defined by the above relational equation 1 can be 0.62, or the upper limit of the value defined by the above relational equation 1 can be 0.84.

[0039] On the other hand, according to one aspect of the present invention, the high-strength cold-rolled steel sheet has a microstructure that, by area percent, contains ferrite: 50% or more, with the remainder being bainite and martensite. If the ferrite content in the microstructure is less than 50%, there is a problem that the elongation is insufficient and the formability deteriorates. The remainder can be bainite and martensite, making up 50% or less. If the total of bainite and martensite exceeds 50%, there is a problem that the strength is too high and the elongation is insufficient.

[0040] Alternatively, according to one aspect of the present invention, but not limited to, from the viewpoint of improving elongation and formability, the microstructure of the high-strength cold-rolled steel sheet may include, in area percent, ferrite: 50-85% and bainite and martensite combined: 15-50%.

[0041] In the above-mentioned high-strength cold-rolled steel sheet, if the ferrite content exceeds 85%, the target strength may not be achieved. Similarly, if the total of bainite and martensite is less than 15%, the target strength may not be achieved. On the other hand, from the viewpoint of further improving the effects described above, the microstructure of the above-mentioned high-strength cold-rolled steel sheet can more preferably contain 66-75% ferrite by area percentage.

[0042] Furthermore, according to one aspect of the present invention, although not particularly limited, the microstructure of the high-strength cold-rolled steel sheet may contain bainite: 3-7% and / or martensite: 19-31% by area. If the bainite content in the high-strength cold-rolled steel sheet is less than 3%, the target strength may not be achieved, and if it exceeds 7%, the strength may be high, but the elongation may be low. Alternatively, if the martensite content in the high-strength cold-rolled steel sheet is less than 19%, the target strength may not be achieved, and if it exceeds 31%, the strength may be high, but the elongation may be low.

[0043] According to one aspect of the present invention, the above-mentioned high-strength cold-rolled steel sheet has an average number of 10 surface defects / m that satisfy one or more of the following conditions: a depth of 100 μm or more and a short side length of 1 mm or more. 2 Less than (0 pieces / m) 2 (including) In measuring the average number of surface defects, the conditions of "depth of 100 μm or more" and "short side length of 1 mm or more" are merely criteria that are sufficient to satisfy in order to measure the average number of surface defects. Therefore, in this specification, the above-mentioned upper limits for depth and short side length are not particularly limited.

[0044] In this invention, a surface defect refers to a defect having a groove shape, specifically a defect that can be visually observed on the surface of a steel sheet as a concave defect in the thickness direction. The depth of the surface defect can be defined as the "maximum depth" in the thickness direction of the groove-shaped defect, based on the cross-section in the thickness direction of the cold-rolled steel sheet (i.e., the direction perpendicular to the rolling direction, based on the cross-section). The short side length of the surface defect can be defined as the shortest length passing through the point of maximum depth, based on the surface of the cold-rolled steel sheet. On the other hand, Figure 2 shows photographs taken using a high-magnification scanning electron microscope (SEM) to observe the groove-shaped surface defects present on the surface of the steel sheet described above and to confirm the depth and short side length of each surface defect.

[0045] The inventors of this invention have diligently conducted research to solve the problems of the prior art and to provide a cold-rolled steel sheet that can minimize surface defects and material deviations while ensuring the desired level of strength and formability.

[0046] As a result, the average number of surface defects that satisfy one or more of the above conditions—a depth of 100 μm or more and a short side length of 1 mm or more—was 10 defects / m². 2 We found that the above-mentioned effects can be ensured by controlling the number to less than 10. In other words, in the present invention, the average number of surface defects is 10 / m². 2 If the number exceeds the above, surface dents may occur. On the other hand, from the viewpoint of further improving the above-mentioned effects, it is preferable that the average number of surface defects is 8 per square meter.2 It can be the following:

[0047] On the other hand, according to one aspect of the present invention, the inventors conducted further research to provide a cold-rolled steel sheet that can simultaneously ensure the desired level of strength and formability without affecting material deviations, etc., even if surface defects are present on the steel sheet surface. As a result, the present invention has additionally discovered surface defect characteristics that do not affect material deviations, etc., even if surface defects are present. Specifically, although not particularly limited in the present invention, the maximum depth of the above surface defects can be 500 μm or less. In this case, the maximum depth of the above surface defects can mean the maximum depth for each surface defect present on the surface of the steel sheet.

[0048] On the other hand, according to one aspect of the present invention, the difference between the yield strength (YS) of both ends and the center of the cold-rolled steel sheet in the width direction can be 100 MPa or less. By satisfying the condition that the difference between the yield strength of both ends and the center is 100 MPa or less, it is possible to provide a steel sheet with reduced material deviation in the width direction, resulting in a uniform material in the width direction. In this case, the "both ends" refers to the 30% section from both ends (totaling 60%) based on the overall width (100%) of the cold-rolled steel sheet in the width direction, and the "center" refers to the remaining 40% section excluding the above-mentioned ends, based on the overall width of the cold-rolled steel sheet in the width direction.

[0049] On the other hand, according to one aspect of the present invention, the cold-rolled steel sheet can have a tensile strength (TS) of 780 MPa or more, preferably 780 MPa or more and less than 1180 MPa, and more preferably 800 MPa or more and 1100 MPa or less. If the tensile strength of the cold-rolled steel sheet is less than 780 MPa, the problem may arise that it cannot satisfy the target strength required for the applied part, and if it exceeds 1100 MPa, the problem may arise that cracks occur during part forming or the impact absorption capacity of the part is significantly reduced.

[0050] Furthermore, according to one aspect of the present invention, the cold-rolled steel sheet may have a yield strength (YS) of 380 MPa or more, and more preferably 390 MPa or more and 650 MPa or less. If the yield strength of the cold-rolled steel sheet is less than 380 MPa, the impact resistance of the part may deteriorate, and if it exceeds 650 MPa, the formability may deteriorate.

[0051] Furthermore, according to one aspect of the present invention, the cold-rolled steel sheet can have a product of tensile strength and elongation of 12,000 MPa% or more (more preferably 12,000 MPa% to 16,500 MPa%, most preferably 12,000 MPa% to 16,200 MPa%). By satisfying the above-mentioned physical properties, it is possible to ensure that the cold-rolled steel sheet is suitable for use in structural components such as fillers, which are components that make up an automobile body and require a stable balance of strength-elongation and shock absorption.

[0052] While not particularly limited, the cold-rolled steel sheet may selectively further include a plating layer formed on its surface. In this case, the plating layer may be formed by a plating process described later. Furthermore, the composition of the plating layer can be applied differently depending on the purpose, and is not particularly limited in this specification; one example is a zinc-based plating layer.

[0053] The following describes in detail a method for manufacturing high-strength cold-rolled steel sheets according to one aspect of the present invention. However, this does not necessarily mean that the cold-rolled steel sheets according to the present invention must be manufactured by the following method.

[0054] Steel slab reheating stage A steel slab satisfying the above-described composition is reheated to 1100-1350°C. The composition of the steel slab is the same as that of the cold-rolled steel sheet described above, and the reasons for adding each component and limiting their content in the steel slab are the same as those explained for the cold-rolled steel sheet described above. On the other hand, if the reheating temperature of the steel slab is less than 1100°C, segregated alloying elements remain in the center of the slab, resulting in a problem where the starting temperature for hot rolling is too low, leading to excessive rolling load. On the other hand, if the reheating temperature of the steel slab exceeds 1350°C, a problem arises where the strength decreases due to the coarsening of the austenite grains. Therefore, in the present invention, it is preferable to control the reheating temperature of the steel slab to 1100-1350°C.

[0055] Hot rolling stage The reheated steel slab described above is hot-rolled at 850-1150°C. If the hot-rolling temperature exceeds 1150°C, the temperature of the hot-rolled steel sheet becomes high, the grain size becomes coarser, and the surface quality of the hot-rolled steel sheet deteriorates. If the hot-rolling temperature is below 850°C, the development of elongated grains due to excessive recrystallization delay increases the load during rolling, the temperature at both ends decreases significantly, and an uneven microstructure is formed during cooling, increasing material deviation and deteriorating formability.

[0056] After hot rolling, cooling stage The hot-rolled steel sheet is cooled to 450-700°C at an average cooling rate of 10-70°C / s (more preferably 20-50°C / s). If the cooling temperature of the hot-rolled steel sheet is below 450°C, the material deviation deteriorates, and if it exceeds 700°C, not only does material deviation occur, but internal oxidation of the hot-rolled material occurs, leading to the problem of surface defects. Furthermore, if the average cooling rate is below 10°C / s, the crystal grains of the matrix structure become coarse, resulting in a non-uniform microstructure. Moreover, if the average cooling rate exceeds 70°C / s, the bainite and martensite phases are more easily formed, leading to an increased load during cold rolling.

[0057] Winding stage The cooled steel sheet is wound at 450-700°C. If the steel is wound at a temperature below 450°C, the bainite and martensite phases in the steel are unnecessarily formed, resulting in an uneven shape and a significant increase in the rolling load during cold rolling. If wound at a temperature exceeding 700°C, the ferrite grains become larger, making it easier for coarse pearlite phases to form. This leads to the formation of an uneven microstructure during annealing, resulting in a deterioration of the steel's formability. Furthermore, the amount of hot-rolled oxides increases, and these oxides are adsorbed onto the rolls during annealing, causing an accumulation of oxides on the rolls. This can lead to surface defects such as dents on the steel sheet due to friction between the steel sheet and the rolls during feeding. Additionally, if hot-rolled oxides remain on the steel sheet, they can lead to a deterioration in plating quality and adhesion during plating.

[0058] Typically, after winding, the ends of the wound steel sheet (coil) cool faster in the width direction due to exposure to the ambient atmosphere, while the central part in the width direction cools more slowly. This causes a cooling deviation in the width direction of the steel sheet from the winding stage, resulting in differences in the microstructure depending on the position of the wound steel sheet, ultimately leading to a material deviation compared to the hot-rolled steel sheet. Hot-rolled steel sheets with such large material deviations not only deepen the material deviation during the cold-rolling process, but groove-like surface defects that were not visually observed in the hot-rolled steel sheet deepen further after cold rolling, resulting in a problem of large surface defects. In other words, hot-rolled steel sheets with large material deviations not only have inferior shape during cold rolling, but also induce material deviations depending on the position in the width direction even in the final annealed material. As a result of diligent research to solve the above problems, the inventors have devised a manufacturing method that controls the temperature of both ends and the central part to be different during the winding stage.

[0059] Specifically, in this invention, as a method to reduce material deviation in the width direction of the steel sheet and suppress surface defects, during winding, the surface temperature (Te) at both ends in the width direction is controlled to be 601 to 700°C, and the surface temperature (Tc) at the center is controlled to be 450 to 600°C, based on the overall width of the steel sheet. In this case, the "width direction of the steel sheet" refers to the direction perpendicular to the transport direction of the steel sheet, based on the surface of the steel sheet. The same explanation as above applies to both ends and the center.

[0060] In this case, if Te is less than 601°C, there is a problem that material deviations deepen due to overcooling at both ends, and if Te exceeds 700°C, there is a problem that material deviations and surface defects deepen due to deterioration in the central part. Furthermore, if Tc is less than 450°C, there is a problem that the temperature difference between the central part and both ends becomes severe, worsening material deviations, and if Tc exceeds 600°C, there is a problem that the temperature in the central part is too high, causing material deviations and surface defects.

[0061] Thus, in the winding stage described above, various methods can be applied to control the surface temperature of the steel plate at both ends and the central part in the width direction to be different, and are not particularly limited. For example, in order to control the temperature of the steel plate at both ends and the central part in the winding stage described above, in the cooling stage before winding, the cooling water injected at both ends can be cut off before it reaches the steel plate, or the amount of cooling water injected can be controlled to be different, or both methods can be used in parallel. As an example, according to one aspect of the present invention, in the cooling stage before winding described above, the amount of cooling water injected into the central part excluding the ends can be controlled to be even larger than the amount of cooling water injected into the ends in the width direction, based on the overall width of the steel plate.

[0062] Furthermore, according to one aspect of the present invention, although not particularly limited, from the viewpoint of further reducing material deviation and improving the effect of suppressing surface defects, the winding step can be performed to make the difference between the surface temperature of both ends and the surface temperature of the center (Te-Tc) 150°C or less. In this case, if the value of Te-Tc exceeds 150°C, a problem may arise in which the material deviation in the width direction deteriorates. However, since a smaller temperature deviation calculated from Te-Tc is preferable, its lower limit does not need to be separately limited and can preferably be 0°C. On the other hand, more preferably, the lower limit of the value of Te-Tc can be 50°C, and the upper limit of the value of Te-Tc can be 90°C.

[0063] Maintenance stage inside the heat-retaining cover After the winding stage described above, the steel sheet can be selectively moved into a heat-insulating cover and maintained at a temperature of 400-500°C for more than 6 hours. By maintaining the steel sheet in the heat-insulating cover for an extended period after the winding stage, the temperatures at both ends and the center of the steel sheet in the width direction are maintained at temperatures of 601-700°C and 450-600°C, respectively. This allows for the uniform formation of a large amount of bainite structure at both ends and the center of the entire coil, resulting in superior shape quality, reduced rolling load during cold rolling, and the production of cold-rolled steel sheets with uniform thickness.

[0064] During the maintenance phase inside the heat-insulating cover, the surface temperature of the steel plate can be adjusted to 400-500°C. However, if the surface temperature of the steel plate is below 400°C during the maintenance phase inside the heat-insulating cover, the above-mentioned effects cannot be secured, and if it exceeds 500°C, coarse carbides may be formed locally, increasing the amount of hot-rolled oxides and potentially degrading the formability and surface quality of the steel.

[0065] Furthermore, if the maintenance time inside the heat-insulating cover is less than 6 hours, a problem may occur where material deviation occurs. While there is no particular upper limit to the maintenance time inside the heat-insulating cover, it can be 8 hours or less as an example.

[0066] Furthermore, from the viewpoint of further improving the effects described above, the rolled steel plate can be stored in the heat-insulating cover within 90 minutes immediately after rolling. If the time before storage in the heat-insulating cover exceeds 90 minutes, excessive air cooling may occur in the center in the width direction, causing the temperature to fall below the 450-600°C range. Alternatively, after the maintenance stage inside the heat-insulating cover, further air cooling or water cooling to room temperature can be performed.

[0067] Cold rolling stage The rolled steel sheet described above is cold-rolled at a cold reduction ratio of 40-70%. If the cold reduction ratio is less than 40%, it is difficult to secure the target thickness and to correct the shape of the steel sheet. If it exceeds 70%, there is a high possibility of cracks occurring at the edges of the steel sheet, which puts a load on the cold rolling process. Therefore, in this invention, it is preferable to limit the cold reduction ratio to 40-70%.

[0068] Annealing stage The cold-rolled steel sheet described above is continuously annealed at 740 to 900°C. If the annealing temperature is below 740°C, recrystallization may not occur, resulting in problems such as not meeting the target strength and elongation. If the annealing temperature exceeds 900°C, problems such as surface oxide formation may occur. On the other hand, from the viewpoint of further improving the effects described above, the annealing temperature can more preferably be set to 750 to 850°C.

[0069] Furthermore, although not particularly limited, according to one aspect of the present invention, the process may further include: a step of selective primary cooling to 650-700°C at a cooling rate of 1-10°C / second after the continuous annealing step; and a step of secondary cooling to Ms-100°C to Ms+100°C at a cooling rate of 11-20°C / second after the primary cooling step. In addition, a step of selective overaging while maintaining a constant temperature may further be included after the secondary cooling step. By satisfying the conditions of the primary cooling step, the secondary cooling step, and the overaging step, the strength and elongation can be further improved. In this case, Ms represents the starting temperature at which martensite is generated when the annealed steel sheet is cooled, and can be determined from the following relational equation 2. [Relationship 2] Ms=539-423×C-30.4×Mn-12.1×Cr-17.7×Ni-7.5×Mo (In the above relational equation 2, C, Mn, Cr, Ni, and Mo represent the weight percent average content of each element. Substitute 0 if none of the above elements are added.)

[0070] Furthermore, according to one aspect of the present invention, the step of selectively plating (preferably hot-dip galvanizing) the cold-rolled steel sheet can be further included, and a plated steel sheet can be obtained by performing the above plating. [Examples]

[0071] The present invention will be described more specifically below with reference to examples. However, it should be noted that the following examples are merely illustrative and not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0072] (Examples) Steel slabs meeting the composition shown in Table 1 were reheated at 1200°C, hot-rolled at 900°C, cooled to 450-700°C at a cooling rate of 20-50°C / s, and then wound up. During the winding process, the amount of cooling water injected into the central portion (excluding the ends) was controlled to be greater than the amount of cooling water injected into the ends in the width direction of the steel sheet, so that the surface temperature (Te) of the 30% section from both ends and the surface temperature (Tc) of the central portion (the remaining 40%) met the hot-rolling conditions described in Table 2. Furthermore, the wound hot-rolled steel sheet was moved into a heat-insulating cover, and the heat-insulating cover conditions described in Table 2 were controlled to meet the average temperature and maintenance time before and after insertion into the cover. Next, the hot-rolled steel sheet was cold-rolled with a cold reduction ratio of 50%, followed by continuous annealing at 800°C. After primary cooling to 670°C at an average cooling rate of 8°C / s, a secondary cooling was performed at an average cooling rate of 12°C / s up to Ms+100°C to obtain a cold-rolled steel sheet.

[0073] For each cold-rolled steel sheet thus obtained, the microstructure, mechanical properties, and average number of surface defects per unit area (units / m²) observed on the surface were determined for the inventive example and the comparative example. 2 The following measurements were taken and are shown in Tables 3-5 below. In this case, YS, TS, and El represent the 0.2% offset yield strength, tensile strength, and fracture elongation, respectively, and the results are shown for tests conducted by taking test specimens from the center and both ends of a JIS No. 5 standard test specimen perpendicular to the rolling direction. Furthermore, the microstructure described above was obtained by using a scanning electron microscope (FE-SEM) and measuring the area % from photographs observed at 3,000 to 5,000 magnification. The average number of surface defects was measured by visually observing the surface of the manufactured steel sheet and finding the average number of surface defects that met one or more of the following conditions: depth of 100 μm or more and short side length of 1 mm or more. In particular, the maximum depth for the above surface defects was measured in the same manner as described herein. Furthermore, the yield strength was measured in the same manner as described above for test specimens taken from the center and both ends in the width direction of the cold-rolled steel sheet, and the material deviation in the width direction was measured and is shown in Tables 4 and 5 below.

[0074] [Table 1]

[0075] [Table 2]

[0076] [Table 3]

[0077] [Table 4]

[0078] [Table 5]

[0079] As can be seen from the experimental results in Tables 1 to 5 above, in the case of Invention Examples 1 to 6, which satisfy the composition and manufacturing conditions of the present invention, cold-rolled steel sheets were obtained that could secure a tensile strength (TS) of 780 MPa or more while suppressing material deviation and surface defects. At this time, it was confirmed that the maximum depth of surface defects measured in the cold-rolled steel sheets obtained from Invention Examples 1 to 6 of this application was 500 μm or less.

[0080] On the other hand, in the case of Comparative Examples 1 to 16, which did not satisfy one or more of the composition and manufacturing conditions of the present invention, material deviations deteriorated, surface defects occurred, and / or it was difficult to secure the physical properties targeted by the present invention.

[0081] In particular, comparative steel 1 had a Si content exceeding 2.0%, and therefore could not satisfy relational equation 1. Consequently, in the case of comparative examples 13 and 14 using comparative steel 1, even though the manufacturing conditions presented in the present invention were met and the material deviation was good, a dent problem occurred due to the accumulation of Si oxide in the annealing furnace, resulting in the average number of surface defects in the product exceeding the target value.

[0082] Furthermore, the comparative steel 2 described above had a low amount of alloy additive and could not satisfy relational equation 1. Therefore, in the case of comparative examples 15 and 16 using the comparative steel 2 described above, even if the manufacturing conditions presented in the present invention were met and the surface defects and material deviations were good, the tensile strength was less than 780 MPa and the target material could not be satisfied.

[0083] Furthermore, Comparative Examples 1, 5, and 9 show examples where the temperatures at both ends and the center in the width direction were higher than the temperatures presented in the present invention, and Comparative Examples 4, 8, and 12 show examples where the temperature of the heat-insulating cover exceeded the reference temperature. As a result, in the above comparative examples, excessive hot-rolled oxides were generated, and a large number of surface defects occurred in the final steel sheet due to these oxides.

[0084] Furthermore, in Comparative Examples 2, 6, and 10, the temperatures at both ends and the center in the width direction were lower than the temperatures presented in the present invention, and the difference between the surface temperature at the ends and the surface temperature of the center (Te-Tc) exceeded 150°C. In Comparative Examples 3, 7, and 11, examples were shown where no heat-insulating cover was applied. As a result, in the above comparative examples, the target material properties of the annealed steel sheet could be secured, and the average number of surface defects was good, but there was a problem in that the deviation of the yield strength in the width direction of the annealed steel sheet exceeded the target value of 100 MPa.

Claims

1. The step of reheating a steel slab to 1100-1350°C, which contains, by weight percent, C: 0.05-0.3%, Si: 0.01-2.0%, Mn: 1.5-3.0%, Al: 0.01-0.1%, P: 0.001-0.015%, S: 0.001-0.01%, and N: 0.001-0.01%, with the remainder being Fe and other unavoidable impurities, and satisfying a value of 0.6 or more and less than 0.9 as defined by the following relational formula 1; The step of hot-rolling the reheated steel slab at 850 to 1150°C; A step of cooling the hot-rolled steel sheet to 450-700°C at an average cooling rate of 10-70°C / s; The step of winding the cooled steel plate at 450 to 700°C; The step of cold-rolling the wound steel sheet at a reduction ratio of 40 to 70%; and The process includes the step of continuously annealing the cold-rolled steel sheet at 740 to 900°C; The winding step is a method for manufacturing high-strength cold-rolled steel sheets, in which the winding step is controlled so that, based on the overall width of the steel sheet, the surface temperature (Te) at both ends in the width direction is 601 to 700°C and the surface temperature (Tc) at the center is 450 to 600°C. [Relationship 1] C+(1.3×Si+Mn) / 6+(Cr+1.2×Mo) / 5+100×B (In the above relational formula 1, C, Si, Mn, Cr, Mo, and B represent the weight percent average content of each element. If none of the elements are added, substitute 0.)

2. A method for manufacturing a high-strength cold-rolled steel sheet according to claim 1, further comprising the step of moving the wound steel sheet into a heat-insulating cover after the winding step and maintaining it at a temperature in the range of 400 to 500°C for 6 hours or more.

3. The method for manufacturing a high-strength cold-rolled steel sheet according to claim 1, wherein the winding step is controlled so that the difference between the surface temperature of both ends and the surface temperature of the central part (Te-Tc) is 150°C or less.

4. The method for manufacturing a high-strength cold-rolled steel sheet according to claim 1, wherein the cooling step is controlled such that, based on the overall width of the steel sheet, the amount of cooling water injected into the central part excluding the ends is greater than the amount of cooling water injected into the ends in the width direction.

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