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

Abstract

This invention provides a high-strength cold-rolled steel sheet with excellent surface quality and minimal material deviation, as well as a method for manufacturing the same. [Solution] The high-strength cold-rolled steel sheet contains, by weight %, 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 microstructure contains, by area %, ferrite: 50% or more, with the remainder being 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 It is less than.

Description

Technical Field

[0001] The present invention relates to a high-strength cold-rolled steel sheet used for structural parts with a large amount of molding, 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 with excellent surface quality, few material deviations, and can be suitably used for automobile parts, and a manufacturing method thereof.

Background Art

Background Art

[0002] Recently, the automotive industry has been strengthening regulations on safety and the environment. When manufacturing vehicle bodies to improve fuel efficiency and protect passengers, the use of high-strength steel with a tensile strength of 780 MPa or more is increasing. 客の保護のために車体を製造する際に、引張強度780MPa級以上の高強度鋼の使用が 増加している。

[0003] Conventional high-strength steels used for automobile bodies include DP (Dual Phase) steel composed of a soft ferrite base and a hard martensite two-phase structure, 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. イト2相から構成されたDP(Dual Phase)鋼、残留オーステナイトの変態誘 起塑性を利用したTRIP(Transformation Induced Plas ticity)鋼またはフェライトと硬質のベイナイトまたはマルテンサイトの複合組織 から構成されるCP(Complexed Phase)鋼などがあった。

[0004] <了 However, in high-strength steels, 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, surface defects due to in-furnace dent are caused by the adsorption and accumulation of metal oxides on the steel sheet surface to the annealing furnace rolls, and when passing the steel sheet 性が劣化し、焼鈍時の炉内デントによる鋼板の表面欠陥が発生する問題がある。また、M / n、Cr、Moなどの硬化能元素が多量添加された場合には、熱延コイルの材質偏差が発 生して冷間圧延時に厚さ品質が悪化するという問題がある。このとき、炉内デントによる 生して冷間圧延時に厚さ品質が悪化するという問題がある。このとき、炉内デントによる 表面欠陥は、鋼板表面の金属系酸化物が焼鈍炉ロールに吸着及び集積されて、通板時に鋼 It should be noted that there seems to be an error in the tag "<了 " in the original text which is likely a misspelling. It is retained as is in the translation according to the requirements. This refers to surface defects in steel sheets formed by contact between the sheet and the roll.

[0005] Manufacturing technology for high-strength cold-rolled steel sheets and hot-dip galvanized steel sheets to solve the above-mentioned problems A brief explanation of the prior art related to this is as follows:

[0006] Among the prior art, Patent Document 1 describes a hot-rolled steel sheet containing a low-temperature transformation phase of 60% or more by volume. The process involves cold rolling the steel sheet to a cold reduction ratio of over 60% but less than 80%, and then ferrite the cold-rolled steel sheet. and a process of continuous annealing in the austenite two-phase region to produce high-strength cold-rolled steel sheets and a method for producing the same. This is presented. However, the cold-rolled steel sheet obtained from Patent Document 1 has a strength of 370-590 MPa. It is at level A, making it difficult to apply to automotive impact-resistant components, and is used in interior and exterior paneling. There was a problem with its use being limited to a specific purpose.

[0007] Furthermore, Patent Document 2 describes tempered martensite. By utilizing the ite phase, high strength and high ductility are obtained simultaneously, resulting in excellent sheet shape after continuous annealing. The invention discloses a method for manufacturing steel plates. However, the technology in Patent Document 2 concerns the carbon content in the steel. The high Si content of over 0.2% leads to problems such as degraded weldability and furnace dents due to the high Si content. There was a problem with surface defects occurring due to this. [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 Initiative]

Problems to be Solved by the Invention

[0009] According to one aspect of the present invention, there is provided a high-strength cold-rolled steel sheet having excellent surface quality and less material deviation, and a manufacturing method thereof.

[0010] The problems of the present invention are not limited to the above description. Those with ordinary knowledge in the technical field to which the present invention belongs can easily understand further problems of the present invention from the entire content described in the specification of the present invention.

Means for Solving the Problems

[0011] One aspect of the present invention is by weight%, C: 0.05 to 0.3%, Si: 0.01 to 2.0%, Mn: 1.5 to 3. 0%, Al: 0.01 to 0.1%, P: 0.001 to 0.015%, S: 0.001 to 0 .01%, N: 0.001 to 0.01%, the balance being Fe and other unavoidable impurities, the value defined by the following relational expression 1 satisfies 0.6 or more and less than 0.9, as the microstructure, in area%, ferrite: 50% or more, the balance: including bainite and martensite sites, the average number of surface defects satisfying one or more of the conditions that the depth is 100 μm or more and the short side length is 1 mm or more is less than 10 pieces / m

[0012] , , , , 2 , , , , to provide a high-strength cold-rolled steel sheet. [[ID=4 2]][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. When each of the above elements is not added, 0 is substituted.)

[0012] Another aspect of the present invention is In 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 0.01%, N: 0.001~0.01%, the remainder being Fe and other unavoidable impurities, below Steel slabs that satisfy the value defined by equation 1 to be between 0.6 and 0.9 are 1100 to 135. The step of reheating to 0°C; The next step involves hot-rolling the reheated steel slab at 850-1150°C; The hot-rolled steel sheet described above is cooled to 450-700°C at an average cooling rate of 10-70°C / s. The stage of doing so; 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, The above winding step is based on the overall width of the steel plate, and the surface temperature (Te) at both ends in the width direction is The temperature range is 601-700°C, and the central surface temperature (Tc) is 450-600°C. The present invention provides a method for manufacturing high-strength cold-rolled steel sheets that allows for precise control. [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 are relative to each element) This indicates the average content by weight. If any of the above elements are not added, substitute 0. [Effects of the Invention]

[0013] According to one aspect of the present invention, a high-strength cold-rolled steel sheet with excellent surface quality and low material deviation, and We can provide a manufacturing method.

[0014] The diverse yet significant advantages and effects of this invention are not limited to those described above. This can be more easily understood in the process of describing specific embodiments of the invention. [Brief explanation of the drawing]

[0015] [Figure 1] This shows photographs of surface defects in 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] This image shows a surface defect as defined in this invention, captured using a high-magnification scanning electron microscope (SEM). [Modes for carrying out the invention]

[0016] The following describes preferred embodiments of the present invention. However, there are several embodiments of the present invention. It can be modified into other forms, and the scope of the present invention is not limited to the embodiments described below. This is not the case. Furthermore, embodiments of the present invention are intended for those with average knowledge in the art. This is provided to explain the meaning more completely.

[0017] Furthermore, the terms used herein are for the purpose of describing specific examples and do not limit the present invention. This is not intended to be definitive. For example, the singular form used herein is as the relevant definition is clearly defined. This includes multiple forms unless they explicitly express opposition. Furthermore, as used in the specification... The meaning of "includes" in "reru" is to concretize the composition and not to exclude the existence or addition of other components. .

[0018] Conventional technology offers high strength with a tensile strength (TS) of 780 MPa or more, and excellent formability. Applicable to structural components with large shape dimensions, yet boasts excellent surface quality and minimal material deviation. The reality is that technology to meet the high-end demand for rolled steel sheets has not yet been developed.

[0019] Therefore, the present inventors have found a way to solve the problems of the prior art while satisfying all of the above-mentioned characteristics. As a result of diligent research into providing cold-rolled steel sheets, we have optimized the composition and manufacturing conditions of the steel sheets. The above-mentioned objectives can be achieved by controlling the characteristics of the microstructure and surface defects. This discovery led to the completion of the present invention.

[0020] In other words, according to the present invention, while having a high strength of 780 MPa or more, the tensile strength and elongation strength are also high. The product of the saturation rates satisfies 12,000 MPa% or more, and among the components that make up the automobile body, it is stable. Suitable for structural components such as fillers that require a balance of strength-elongation and shock absorption. This can effectively provide high-strength cold-rolled steel sheets that can be applied to various applications.

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

[0022] A high-strength cold-rolled steel sheet according to one aspect of the present invention has a composition of C: 0.05-0.3% by weight, Si: 0.01~2.0%, Mn:1.5~3.0%, Al:0.01~0.1%, P:0.0 01~0.015%, S:0.001~0.01%, N:0.001~0.01%, remainder Contains Fe and other unavoidable impurities.

[0023] The following details the reasons for adding components to the cold-rolled steel sheet according to the present invention and the reasons for limiting the content. This will be explained. In this specification, when indicating the content of each element, unless otherwise specified, Unless otherwise specified, it indicates a percentage by weight.

[0024] C: 0.05~0.3% The above carbon (C) is essential for ensuring a martensitic structure that is effective in strengthening steel. It is a very important component. As the amount of C added increases, the martensite phase and bainite phase As the fraction increases, the tensile strength increases. Therefore, in order to ensure high strength, the above The lower limit of the carbon content is controlled to 0.05%. However, as the carbon content increases, during two-phase annealing... By extending the austenite region, the hard phases of martensite and bainai are formed. The ferrite phase fraction increases, while the ferrite phase fraction, which is a soft phase, decreases, resulting in a deterioration of formability and weldability. It deteriorates. Therefore, the upper limit of the above C content should be controlled to 0.3%. On the other hand, the above effect From the viewpoint of further improvement, more preferably, the lower limit of the C content is 0.06%. This is possible, or the upper limit of the above C content can be 0.12%.

[0025] Si: 0.01~2.0% The above silicon (Si) deoxidizes molten steel, has a solid solution strengthening effect, and prevents the formation of coarse carbides. It is an element that is advantageous for delaying the process and improving moldability. However, the above Si content is 0 If the concentration is less than 0.01%, the aforementioned effects are minimal, making it difficult to improve moldability. On the other hand, if the Si content exceeds 2.0%, during hot rolling, red deposits of Si form on the surface of the steel sheet. A color scale is formed severely. This can cause surface defects or surface damage during the annealing process. The surface becomes concentrated, and unplated areas begin to appear. Furthermore, the formation of surface oxides leads to improved plating adhesion. The properties also deteriorate, and there is a problem that the surface quality becomes very poor. Therefore, in the present invention, S The content of i is controlled to 0.01-2.0%. On the other hand, from the perspective of further improving the effects mentioned above... More preferably, the lower limit of the Si content can be 0.4%, or the above The upper limit for the Si content can be 1.2%.

[0026] Mn: 1.5~3.0% The above-mentioned manganese (Mn), like Si, is an effective element for solid solution strengthening of steel. It is an element that greatly increases hardening ability. However, if the Mn content is less than 1.5%, If the above-mentioned effects are not obtained, and the Mn content exceeds 3.0%, the strengthening effect will be significantly reduced. The amount increases, and the ductility decreases. Also, segregation occurs in the center of the slab thickness during continuous casting. The part develops significantly, and during cooling after hot rolling, the microstructure in the thickness direction becomes non-uniform, forming MnS. As a result, formability such as stretch flange properties deteriorates. Therefore, in the present invention, Mn The content of [the substance] will be controlled to 1.5-3.0%. On the other hand, from the perspective of further improving the effects mentioned above, More preferably, the lower limit of the Mn content can be 1.8%, or the Mn The upper limit for the content can be 2.6%.

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

[0028] P: 0.001~0.015% The above-mentioned phosphorus (P) is an alloying element with a very large solid solution strengthening effect, and even in small amounts, it is very effective. It has the characteristic of being able to obtain a significant strengthening effect. However, if P is added excessively, grains Brittleness occurs due to boundary segregation, making it prone to the formation of fine cracks during molding, and significantly reducing ductility and impact resistance. It worsens the condition. Also, it can cause defects on the surface during plating. Therefore, P The upper limit of the content is controlled to 0.015%. On the other hand, the above P content is less than 0.001%. And to satisfy this, the manufacturing costs are excessively high, which is not only economically disadvantageous, Since the strength achieved would be insufficient, 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 above P content to 0.001 to 0.015%. It is desirable. On the other hand, from the viewpoint of further improving the above-mentioned effects, more preferably, the P content is as follows. The limit can be 0.003%, or the upper limit of the above P content is 0.012%. It is possible.

[0029] S: 0.001~0.01% The above sulfur (S) is an impurity present in steel, and if the S content exceeds 0.01%, It combines with Mn and other elements to form non-metallic inclusions, which cause microscopic cracks to occur during the cutting process of steel. This is prone to damage, and there is a problem in that the stretch flange and impact resistance are greatly deteriorated. Also, the above S To manufacture the product with a content of less than 0.001%, the steelmaking operation requires a significant amount of time. This leads to a problem of decreased productivity. Therefore, in this invention, the S content is set to 0. It is preferable to control it to 0.01-0.01%. On the other hand, from the viewpoint of further improving the above-mentioned effects More preferably, the lower limit of the S content can be 0.002%, or The upper limit for the above S content can be 0.007%.

[0030] N: 0.001~0.01% Nitrogen (N), along with carbon (C), is a typical solid solution strengthening element, and together with Ti and Al, etc. It contributes to the formation of coarse precipitates. Generally, the solid solution strengthening effect of N is superior to that of carbon. However, there is a problem in that the toughness decreases significantly as the amount of N in the steel increases. To manufacture the product with a content of less than 0.001%, the steelmaking operation requires a significant amount of time. This leads to a problem of decreased productivity. Therefore, in this invention, the N content is set to 0. It is preferable to control it to 0.01-0.01%. On the other hand, from the viewpoint of further improving the above-mentioned effects Therefore, more preferably, the lower limit of the above N content can be 0.002%, and the above N content The upper limit for the quantity can be 0.006%.

[0031] On the other hand, according to one aspect of the present invention, although not particularly limited, selectively, the cold-rolled steel The plate has the following composition in weight percent: Cr: 1.0% or less (including 0%), Mo: 0.2% or less (including 0%), and B: May include one or more items selected from 0.005% or less (including 0%). The following explains the reasons for adding the above-mentioned selective additive elements and the reasons for limiting their content.

[0032] Cr: 1.0% or less (including 0%) Chromium (Cr) is an ingredient added to steel to improve its hardening ability and ensure high strength. It is an element that plays a very important role in martensite formation, and is effective in increasing strength. This minimizes the reduction in ductility and is also advantageous for the production of composite structure steel with high ductility. Therefore, the above-mentioned Cr can be selectively added for the effects described above. However, the above-mentioned Cr-containing If the amount exceeds 1.0%, not only will the effects described above saturate, but an excessive increase in hot-rolled strength will also occur. This leads to a problem where the cold-rollability deteriorates. Also, the martensite fraction after annealing is large. Because a significant increase in chromium can lead to a decrease in the growth rate, the upper limit for chromium content is set to 1.0% or less. To control. On the other hand, from the viewpoint of further improving the above-mentioned effects, more preferably the above Cr content The lower limit can be 0.01%, or the upper limit of the above Cr content can be 0.8%. It is possible.

[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, the present invention allows for the selective addition of Mo. Yes, it is possible. However, if the Mo content exceeds 0.2%, the effect of improving strength will not increase significantly. On the other hand, ductility may deteriorate, which can be economically disadvantageous. Therefore, the above Mo content It is preferable to control the amount to 0.2% or less. On the other hand, from the perspective of further improving the above-mentioned effects More preferably, the lower limit of the Mo content can be 0.01%, or The upper limit for the above Mo content can be 0.1%.

[0034] B: 0.005% or less (including 0%) Boron (B), when present in a solid solution state in steel, stabilizes grain boundaries and contributes to the steel's ability to function at low temperatures. It has the effect of improving the brittleness and greatly increases the hardening ability of steel. Therefore, the above effect For the sake of effect, the above B can be selectively added. However, the upper limit of the above B is 0.005%. If it exceeds this limit, it will delay recrystallization during annealing, forming oxides on the surface and degrading the plating properties. Therefore, it is preferable to control the content of B above to 0.005% or less. On the other hand, from the viewpoint of further improving the above-mentioned effects, more preferably the lower limit of the B content is 0 It can be 0.0003%, or the upper limit of the above B content is 0.0025%. It is possible.

[0035] The remaining component of this invention is iron (Fe). However, in the normal manufacturing process, the raw materials and surrounding rings Because boundary variables can inevitably introduce unintended impurities, these must be eliminated. It's not possible. These impurities are easily recognizable to any engineer involved in the normal steel manufacturing process. Therefore, not all of its contents will be specifically mentioned in this specification.

[0036] According to one aspect of the present invention, the value of the high-strength cold-rolled steel sheet is 0, as defined by the following relational expression 1. The value can be between 6 and 0.9, and satisfying this condition minimizes the material deviation of cold-rolled steel sheets. This process suppresses the occurrence of surface defects and ensures the desired material is achieved. [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 are relative to each element) This indicates the average content by weight. If any of the above elements are not added, substitute 0.

[0037] In the present invention, the above relational formula 1 represents the hardening ability of the steel material according to the composition of the present invention (Hardena This is an equation that shows the hardening ability, and the coefficient before each element represents the measure of how much that element contributes to the hardening ability. This is expressed quantitatively. When the hardening ability of steel is high, the bainite phase and martensite phase This is advantageous for securing hard, low-temperature transformation phases, contributes to improved strength, and the lower the hardening ability, the more effective it is. Light transformation is accelerated, which is detrimental to ensuring structural integrity.

[0038] In particular, the present invention aims to ensure high strength with a target tensile strength (TS) of 780 MPa or more. In order for this to happen, the value defined by the above relational equation 1 must be 0.6 or greater. On the other hand, If the value defined by relation 1 is 0.9 or higher, the intensity becomes too high, and the growth rate deteriorates. There is a problem in that... Furthermore, if the value defined from the above relational expression 1 is 0.9 or greater, Immediately after hot rolling, the hot-rolled steel sheet is cooled to 450-700°C at an average cooling rate of 10-70°C / s. The phase transformation of ferrite is significantly delayed at this stage. As a result, the hot-rolled steel sheet is wound up during the subsequent winding stage. Within the bainite phase, the lower bainite phase and martensite phase, which have high hardness, are excessively abundant. As it forms, the material deviation due to position in the width direction becomes severe, and the shape deteriorates. Therefore In this invention, the value defined by the above relational formula 1 satisfies 0.6 or more and less than 0.9. It is preferable to control it. On the other hand, from the viewpoint of maximizing the above-mentioned effect, the above relationship The lower limit of the value defined by Equation 1 can be 0.62, or it can be defined by the above relation Equation 1. The upper limit of the value being defined can be 0.84.

[0039] On the other hand, according to one aspect of the present invention, the above-mentioned high-strength cold-rolled steel sheet has a microstructure that, in terms of area %, Ferrite: 50% or more, remainder: containing bainite and martensite. Furthermore, if the ferrite content is less than 50%, there is a problem of insufficient elongation and deterioration of moldability. Yes. Also, the remainder can be 50% or less as bainite and martensite. If the sum of bainite and martensite exceeds 50%, the strength becomes too high and the elongation rate... There is a problem in that it is insufficient.

[0040] Alternatively, according to one aspect of the present invention, although not particularly limited, elongation and moldability From the perspective of improvement, the microstructure of the above high-strength cold-rolled steel sheet is such that, in area %, ferrite: 50-8 It can contain 5% and a total of 15-50% bainite and martensite.

[0041] If the ferrite content in the above-mentioned high-strength cold-rolled steel sheet exceeds 85%, the target strength will not be achieved. This can lead to a problem where the total of bainite and martensite is less than 15%. If present, it can lead to the problem of not meeting the target intensity. On the other hand, the effects mentioned above... From the viewpoint of further improvement, the microstructure of the above-mentioned high-strength cold-rolled steel sheet is more preferably, in terms of area %, Ferrite: May contain 66-75%.

[0042] Furthermore, according to one aspect of the present invention, although not particularly limited, the above-mentioned high-strength cold-rolled steel sheet The microstructure may contain bainite: 3-7% by area percentage, and / or marten. Site: May contain 19-31%. In the above high-strength cold-rolled steel sheet, bainite is 3% If it is less than 7%, the problem arises that it will not meet the target intensity, and if it exceeds 7%, While it has high strength, it can sometimes have the problem of low elongation. Alternatively, the above high-strength cold The question is whether a rolled steel sheet will not meet the target strength if it contains less than 19% martensite. Problems can arise, and when it exceeds 31%, the strength is high, but the elongation rate is low. It can happen.

[0043] According to one aspect of the present invention, the high-strength cold-rolled steel sheet has a depth of 100 μm or more and a short side length The average number of surface defects that satisfy one or more of the following conditions (1 mm or larger) is 10 defects / m². 2 Less than (0 pieces / m 2(including) In measuring the average number of surface defects, "the depth is 1 Conditions such as "00 μm or larger" and "short side length of 1 mm or larger" are used to measure the average number of surface defects. It is merely a criterion that is sufficient to satisfy in order to do so. Therefore, in this specification, The upper limits for depth and short side length described above are not particularly limited.

[0044] In this invention, a surface defect means a defect having a groove shape, specifically a groove shape in the thickness direction. A defect in the steel plate is a defect that can be confirmed by visually inspecting the surface. The depth of the depression is measured in the thickness direction of the cold-rolled steel sheet (i.e., perpendicular to the rolling direction, relative to the cross-section). (meaning the direction) as a reference, it means the "maximum depth" in the thickness direction for groove-shaped defects. This is possible. Furthermore, the short side length of the above surface defect is measured relative to the surface of the cold-rolled steel sheet. This can mean the shortest length passing through the point of maximum depth mentioned above. On the other hand, the steel plate mentioned above Observe the groove-like surface defects present on the surface and confirm the depth and short-side length of each surface defect. Therefore, images taken using a high-magnification scanning electron microscope (SEM) are shown in Figure 2.

[0045] The inventors have solved the problems of the prior art and ensured the desired level of strength and moldability. To provide a cold-rolled steel sheet that minimizes surface defects and material deviations. We conducted extensive research.

[0046] As a result, one or more of the above-mentioned depths of 100 μm or more and short side lengths of 1 mm or more The average number of surface defects that meet the criteria is 10 per square meter. 2 By controlling it to less than the above effect We found that the result can be secured. That is, in the present invention, the average number of surface defects Quantity: 10 pieces / m2 If the above conditions are met, surface dents may occur. From the viewpoint of further improving the effects described above, preferably the average number of surface defects described above is 8. / m 2 It can be the following:

[0047] On the other hand, according to one aspect of the present invention, even if surface defects exist on the surface of the steel plate, the material A cold-rolled steel sheet that can simultaneously ensure the desired level of strength and formability without affecting deviations, etc. Further research was conducted to provide this. As a result, in this invention, even if surface defects are present, the material We have additionally discovered surface defect characteristics that do not affect quality deviations, etc. Specifically, While not particularly limited in this regard, the maximum depth of the above surface defects must be 500 μm or less. This can be achieved. In this case, the maximum depth of the above surface defect is the surface present on the surface of the steel sheet. This can represent the maximum depth of the defect.

[0048] On the other hand, according to one aspect of the present invention, the yield strength of the cold-rolled steel sheet in the width direction is determined at both ends and in the center. The difference from (YS) can be 100 MPa or less. Yield strength of both ends and the center above. By ensuring that the difference is 100 MPa or less, the steel plate is provided with reduced material deviation in the width direction. This allows for a uniform material distribution in the width direction. In this case, the "both ends" refers to the cold Using the total width (100%) as the reference point in the width direction of the rolled steel plate, the 30% section from both ends (total: This refers to the portion corresponding to 60%, and the "central part" mentioned above is defined as the width of the cold-rolled steel sheet, with the overall width as the reference point. This can mean the remaining 40% section after excluding the two ends mentioned above.

[0049] On the other hand, according to one aspect of the present invention, the cold-rolled steel sheet has a tensile strength (TS) of 780 MPa or less. It can be above, preferably 780 MPa or more and less than 1180 MPa, more preferably Or it can be 800 MPa or more and 1100 MPa or less. The tensile strength of the above cold-rolled steel sheet If the pressure is less than 780 MPa, the target strength required for the applicable component cannot be met. Problems may occur, and if the pressure exceeds 1100 MPa, cracks may occur during part molding. This can lead to a significant decrease in the shock absorption capacity of the components.

[0050] Furthermore, according to one aspect of the present invention, the cold-rolled steel sheet has a yield strength (YS) of 380 MPa or less. It can be above, and more preferably between 390 MPa and 650 MPa. If the yield strength of the above cold-rolled steel sheet is less than 380 MPa, the impact resistance of the part will deteriorate. This can lead to problems such as poor moldability when the pressure exceeds 650 MPa. It can happen.

[0051] Furthermore, according to one aspect of the present invention, the cold-rolled steel sheet has a product of tensile strength and elongation of 12.00 0 MPa% or higher (more preferably 12,000 MPa% to 16,500 MPa%) Most preferably, the concentration is between 12,000 MPa% and 16,200 MPa%. It is possible. By satisfying the above-mentioned physical properties, it becomes possible to create a stable and strong component among the parts that make up the automobile body. Suitable for structural components such as fillers that require a balance of degree of elongation and shock absorption. It is possible to ensure usable effects.

[0052] While not particularly limited, selectively, the above cold-rolled steel sheet has a plating layer formed on its surface. It may further include the above-mentioned plating layer formed by a plating process described later. It can be done. Furthermore, the composition of the above plating layer can be applied differently depending on the purpose. Therefore, this specification is not particularly limited, and 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. The manufacturing method for cold-rolled steel sheets according to the Meiji government does not necessarily have to be carried out by the following manufacturing method. That is not what it means.

[0054] Steel slab reheating stage The steel slab satisfying the above composition is reheated to 1100-1350°C. The composition is the same as that of the cold-rolled steel sheet described above, and in this case, the additives of each component in the steel slab The reasons for adding and limiting the content are the same as the explanation given above for cold-rolled steel sheets. On the other hand, if the reheating temperature of the steel slab is less than 1100°C, segregation occurs in the center of the slab. The problem is that residual alloying elements remain, and the starting temperature for hot rolling is too low, resulting in excessive rolling stress. A problem arises. On the other hand, if the reheating temperature of the steel slab exceeds 1350°C, austenite The coarsening of the crystal grains leads to a decrease in strength, which is a problem in this invention. Therefore, 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. When the temperature exceeds 1150℃, the temperature of the hot-rolled steel sheet increases, causing the crystal grain size to become coarser, and heat The surface quality of the rolled steel sheet deteriorates. If the hot rolling temperature is below 850°C, excessive recombination occurs. Due to the development of elongated crystal grains caused by crystallization delay, the load during rolling increases, and the temperature at both ends increases. The material properties decrease, and a non-uniform microstructure is formed during cooling, increasing material deviation and improving moldability. It will also get worse.

[0056] After hot rolling, cooling stage The hot-rolled steel sheet described above is cooled to 450-700°C at an average cooling rate of 10-70°C / s ( Preferably, it is cooled to 20-50°C / s. The cooling temperature of the hot-rolled steel sheet is 45 Below 0°C, the material deviation worsens, and above 700°C, the material deviation deteriorates. Not only does this occur, but internal oxidation of the hot-rolled metal also occurs, leading to problems with surface defect formation. If the average cooling rate is less than 10°C / s, the crystal grains of the matrix structure become coarse, and the microstructure becomes This can lead to uneven cooling. Furthermore, if the average cooling rate exceeds 70°C / s, bainite formation occurs. This leads to the formation of a phase and a martensite phase, which increases the load during cold rolling.

[0057] Winding stage The cooled steel plate is wound up at 450-700°C. If the winding temperature is below 450°C... When cooled and wound, the bainite and martensite phases in the steel are unnecessarily formed. The shape is non-uniform, and the rolling load during cold rolling increases significantly. When removed, the ferrite crystal grains become larger, and a coarse pearlite phase is more easily formed. There is a problem in that an uneven microstructure is formed during annealing, which degrades the formability of the steel. As a result, the amount of hot-rolled oxide increases and is adsorbed onto the roll during annealing, causing the oxide to accumulate on the roll. As a result, surface defects such as dents occur on the surface of the steel plate due to friction between the steel plate and the roll during the passage of the steel plate. This can lead to problems. Also, if hot-rolled oxides remain on the steel sheet, when the steel sheet is plated... This can lead to deterioration of plating quality and adhesion.

[0058] Normally, after winding, both ends of the wound steel sheet (coil) in the width direction are exposed to the surrounding atmosphere. Cooling proceeds quickly in areas that are exposed, while cooling proceeds more slowly in the central part in the width direction. Furthermore, a cooling deviation occurs in the width direction of the steel sheet from the winding stage, causing the position of the wound steel sheet to change Differences in microstructure occur depending on the location, ultimately resulting in material deviations compared to hot-rolled steel sheets. Hot-rolled steel sheets with large quality deviations not only deepen the material deviations during the cold-rolling process, but also during the hot-rolling process. Groove-like surface defects that were not visible to the naked eye on the steel plate deepen further after cold rolling, This leads to a problem where surface defects occur in large quantities. In other words, hot-rolled steel sheets with large material deviations are Not only does the shape deteriorate during cold rolling, but even in the final annealed material, material deviations are induced depending on the position in the width direction. Therefore, the inventors diligently conducted research to solve the above-mentioned problems, and as a result, winding We have adopted a manufacturing method that controls the temperature of both ends and the center to be different at each stage. .

[0059] Specifically, the present invention reduces material deviation in the width direction of the steel plate and suppresses surface defects. As a method for handling the steel plate, when winding it up as described above, the surface temperature of both ends in the width direction is measured based on the overall width of the steel plate. The temperature (Te) is between 601 and 700°C, and the central surface temperature (Tc) is between 450 and 600°C. Control to satisfy the following. In this case, the above "width direction of the steel plate" refers to the surface of the steel plate. This means the direction perpendicular to the direction of transport of the steel plate. Note that the above ends and the center are above The explanation given above applies similarly.

[0060] In this case, if the above Te is below 601°C, the material deviation due to overcooling at both ends deepens. This presents a problem: when the above Te exceeds 700°C, material deviation and surface deformation occur due to deterioration in the central part. There is a problem in that surface defects deepen. Also, if the above Tc is less than 450℃, the central part and There is a problem in that the temperature difference between the two ends becomes severe, worsening the material deviation, as described above in Tc When the temperature exceeds 600°C, the central part becomes too hot, causing material deviations and surface defects. be.

[0061] Thus, in the winding stage described above, the surface temperature of both ends in the width direction of the steel plate and the middle Various methods can be applied to control the surface temperature of the central part to be different, This is not particularly limited. For example, when winding the steel plate, the temperatures of both ends and the center of the steel plate may be different. To control this, during the cooling stage before winding, the cooling water injected into both ends of the steel It is possible to shut off the cooling water before it reaches the plate, or to control the amount of cooling water injected. Alternatively, two methods can be used in parallel. For example, according to one aspect of the present invention, During the cooling stage before winding, using the overall width of the steel plate as a reference, on both ends in the width direction. The amount of cooling water injected into the central part, excluding the ends mentioned above, is greater than the main amount of cooling water injected. The water volume can be controlled to increase even further.

[0062] Furthermore, according to one aspect of the present invention, although not particularly limited, the material deviation can be further reduced. From the viewpoint of reducing and further improving the effect of suppressing surface defects, the winding step described above is performed The difference between the surface temperature at the edges and the surface temperature in the center (Te-Tc) can be kept below 150°C. At this time, if the Te-Tc value exceeds 150°C, the material deviation in the width direction will worsen. This can sometimes lead to a problem. However, the temperature deviation calculated from the above Te-Tc is small. Since this is preferable, the lower limit does not need to be separately specified, and it is preferably 0°C. This is possible. On the other hand, more preferably, the lower limit of the Te-Tc value can be 50°C. The upper limit of the above Te-Tc value can be 90°C.

[0063] Maintenance stage inside the heat-retaining cover After the winding stage described above, the material is selectively moved into the heat-insulating cover and heated to 400-500°C. It can be maintained within the range for more than 6 hours. After the winding stage, it can be kept in the heat-insulating cover for a long time. By maintaining this, the temperature at both ends and the center of the steel plate in the width direction will be 601-70°C, respectively. When steel plates are maintained at temperatures of 0°C and in the range of 450-600°C for a long period of time, both ends of the coil... A large amount of bainite structure is uniformly formed in the parts and center, resulting in excellent shape quality during cold rolling. It is possible to manufacture cold-rolled steel sheets with low rolling load and uniform thickness.

[0064] During the maintenance phase inside the heat-insulating cover, adjust the surface temperature of the steel plate to 400-500°C. This is possible. At this time, during the maintenance phase inside the heat-insulating cover, the surface temperature of the steel plate is less than 400°C. Therefore, the aforementioned effects cannot be secured, and when the temperature exceeds 500°C, coarse carbon forms locally. The formation of oxides and the increase in hot-rolled oxides can potentially worsen the formability and surface quality of the steel.

[0065] Furthermore, if the maintenance time inside the above-mentioned heat-insulating cover is less than 6 hours, a problem arises where material deviation occurs. This may occur, and there is no particular upper limit on the duration of maintenance inside the above heat-insulating cover, but as an example... It can be 8 hours or less.

[0066] Furthermore, in order to further improve the effects described above, the rolled-up steel plate is rolled up It can be stored inside the above heat-insulating cover within 90 minutes immediately afterwards, and stored inside the above heat-insulating cover If the time before cooling exceeds 90 minutes, excessive air cooling will cause overcooling in the center in the width direction. The temperature may not meet the 450-600°C range. Alternatively, the temperature inside the above heat-insulating cover may not be maintained. After the initial stage, further cooling to room temperature can be performed using air or water.

[0067] Cold rolling stage The above-mentioned rolled steel sheet is cold-rolled at a cold reduction ratio of 40-70%. If the percentage is less than 40%, it is not only difficult to secure the target thickness, but also the shape of the steel plate is affected. While achieving a positive result is difficult, exceeding 70% will cause cracks to occur at the edge of the steel plate. This is highly likely to occur and presents the problem of causing a load during cold rolling. Therefore, in the present invention It is preferable to limit the above cold reduction ratio to 40-70%.

[0068] Annealing stage The cold-rolled steel sheet described above is continuously annealed at 740-900°C. The annealing temperature described above is 740°C. If the value is less than the target, recrystallization will not occur, resulting in problems such as not meeting the target strength and elongation. This can occur, and if the annealing temperature exceeds 900°C, a problem arises where surface oxides are formed. There are also the above. On the other hand, from the viewpoint of further improving the above-mentioned effects, the above annealing temperature The temperature can be set to 750-850°C.

[0069] Furthermore, although not particularly limited, according to one aspect of the present invention, the continuous annealing step Following this stage, there is a step of selective primary cooling to 650-700°C at a cooling rate of 1-10°C / second. After the primary cooling step described above, the temperature is cooled from Ms-100℃ to Ms+100℃ at a rate of 11-20℃ / second. The step of secondary cooling at the cooling rate may further include. The process can further include a step of selectively overaging while maintaining a constant temperature. The above conditions for the primary cooling stage, secondary cooling stage, and overaging stage are met, thereby increasing strength and elongation. The annealing rate can be further improved. At this time, the above Ms is the amount of malte when the steel sheet is cooled after annealing. This refers to the starting temperature at which ionsite is formed, 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 are the weight percent of each element) This indicates the average content. If any of the above elements are not added, 0 is substituted.

[0070] Furthermore, according to one aspect of the present invention, the cold-rolled steel sheet is selectively plated (preferably with hot-dip zinc plating). The process may further include a step of plating, and by performing the above plating, a plated steel sheet can be obtained. It is possible. [Examples]

[0071] The present invention will be described in more detail below with reference to examples. However, the following examples are illustrative. This is merely to explain the present invention, and not to limit the scope of the rights of the present invention. It is important to note that this is not the case. The scope of the rights of this invention is limited to the matters described in the claims and This is because it will be determined by factors that can be reasonably inferred from the following.

[0072] (Examples) After reheating the steel slabs meeting the composition of Table 1 below at 1200°C and hot-rolling them at 900°C, After cooling to 450-700°C at a cooling rate of 20-50°C / s, it was wound up. When winding as described above, the overall width in the width direction of the steel plate is used as the reference, and the 30% section from both ends The surface temperature of the steel plate at both ends (Te) and the surface temperature of the steel plate in the remaining 40% (Tc) are shown in the table below. Cooling water is injected onto both ends in the width direction of the steel plate to satisfy the hot rolling conditions described in 2. The amount of cooling water injected into the central part, excluding the ends mentioned above, is even larger than the amount of water being injected. It was controlled to do so. Also, the hot-rolled steel sheet that had been rolled up was moved into the heat-insulating cover, and below The thermal insulation cover conditions listed in Table 2 include the average temperature before and after installation inside the cover and the temperature maintained during maintenance. The process was controlled to fill the gap. Next, the hot-rolled steel sheet was cold-rolled with a cold reduction ratio of 50%, and 800 After continuous annealing at °C, primary cooling was performed to 670°C at an average cooling rate of 8°C / s, followed by Ms+ Cold-rolled steel sheets were obtained by secondary cooling at an average cooling rate of 12°C / s up to 100°C.

[0073] For each cold-rolled steel sheet thus obtained, the microstructure, mechanical properties, and Average number of surface defects per unit area observed on the surface (defects / m²) 2 ) Measure the following Table 3~ As shown in 5. In this case, YS, TS, and El are the 0.2% offset yield strengths, respectively. This refers to tensile strength and elongation at fracture, measured by measuring a JIS No. 5 standard test specimen perpendicular to the rolling direction. The results of tests conducted by taking test specimens from the central and both ends are shown. The described microstructure was examined using a scanning electron microscope (FE-SEM) at 3,000 to 5,000x magnification. This is the result of measuring the area percentage from photographs observed at a rate. Furthermore, the average number of surface defects was calculated during manufacturing. Visually inspect the surface of the steel plate and identify any holes with a depth of 100 μm or more and a short side length of 1 mm or more. This measures the average number of surface defects that satisfy one or more conditions. In particular, the above surface defects The maximum depth of the depression was measured in the same manner as described herein. Also, cold-rolled steel sheets... For the test specimens taken from the center and both ends in the width direction, use the same method as described above. The yield strength was measured, and the material deviation in the width direction was measured relative to these values, as shown in Tables 4 and 5 below. did.

[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 1-5 above, the composition and manufacturing conditions of the present invention are met. In the cases of Examples 1-6, a tensile strength (TS) of 780 MPa or more can be secured, Cold-rolled steel sheets with suppressed quality deviations and surface defects were obtained. At this time, from Invention Examples 1 to 6 of this application We confirm that the maximum depth of surface defects measured in the resulting cold-rolled steel sheet is 500 μm or less. did.

[0080] On the other hand, in the case of Comparative Examples 1 to 16, which do not satisfy one or more of the composition and manufacturing conditions of the present invention, Material deviations may deteriorate, surface defects may occur, and / or the physical properties intended in this invention may not be achieved. It was difficult to secure.

[0081] In particular, comparative steel 1 had a Si content exceeding 2.0%, and therefore could not satisfy relational equation 1. Therefore, in the case of Comparative Examples 13 and 14 using the above comparative steel 1, the manufacturing strip presented in the present invention Even if the requirements are met and the material deviation is good, there is a dent problem due to the accumulation of Si oxide in the annealing furnace. This resulted in a problem where the average number of surface defects on the product exceeded the target value.

[0082] Furthermore, the comparative steel 2 mentioned above had a small amount of alloy additive and could not satisfy relational equation 1. Therefore, In the case of Comparative Examples 15 and 16 using the above comparative steel 2, the manufacturing conditions presented in the present invention are met. Even if surface defects and material deviations are good, the tensile strength is less than 780 MPa and does not meet the target material requirements. I couldn't add it.

[0083] Furthermore, in the case of Comparative Examples 1, 5, and 9, the temperatures at both ends and the center in the width direction are as presented in the present invention. Examples of manufacturing at a temperature higher than the specified temperature are shown, and in the case of Comparative Examples 4, 8, and 12 above, the temperature of the heat-insulating cover is An example where the reference temperature was exceeded is shown. As a result, in the comparative example above, excessive hot-rolled oxide was generated. As a result, a large number of surface defects occurred in the final steel sheet due to the oxide in question.

[0084] Furthermore, in the case of Comparative Examples 2, 6, and 10, the temperatures at both ends and the center in the width direction are presented in the present invention. The temperature is lower than the target temperature, and the difference between the surface temperature of the relevant ends and the surface temperature of the center (Te-Tc) is 1 Examples exceeding 50°C are shown, and in the cases of Comparative Examples 3, 7, and 11 above, examples where a heat-insulating cover is not applied are shown. This demonstrates that, in the above comparative example, the target material properties of the annealed steel sheet can be secured. Although the average number of surface defects was good, the deviation of the yield strength in the width direction of the annealed steel sheet was not at the target value. There was a problem where the pressure exceeded 100 MPa.

Claims

1. In 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 0.01%, N: 0.001-0.01%, 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, remainder: bainite and marten. Including the site, Surface defects satisfying one or more of the following conditions: depth of 100 μm or more and short side length of 1 mm or more. The average number of pits is 10 per meter. ２ High-strength cold-rolled steel sheet that is less than [amount missing]. [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 are relative to each element) This indicates the average content by weight percent. If none of the elements are added, 0 is substituted.

2. The aforementioned microstructure consists of, by area percentage, ferrite: 50-85% and bainite and martensitic sugar. The high-strength cold-rolled steel sheet according to claim 1, comprising a total of 15-50% of the following.

3. The microstructure comprises 66-75% ferrite by area percentage, according to claim 1, for high strength. degree cold rolled steel plate.

4. The microstructure comprises 3-7% bainite by area percentage, as described in claim 3 for high-strength cold Rolled steel plate.

5. The microstructure comprises, by area percentage, 19-31% martensite, as described in claim 3. High strength cold rolled steel plate.

6. In weight percent, Cr: 1.0% or less (including 0%), Mo: 0.2% or less (including 0%), and B: Further comprising one or more selected from 0.005% or less (including 0%), claim 1 High-strength cold-rolled steel sheet as described above.

7. Claim 1, wherein the tensile strength is 780 MPa or more and the yield strength is 380 MPa or more. The high-strength cold-rolled steel sheet described.

8. The high-strength cold-filled material according to claim 1, wherein the product of tensile strength and elongation is 12,000 MPa% or more. Rolled steel plate.

9. In the width direction of the cold-rolled steel sheet, the difference in yield strength between both ends and the center is 100 MPa or less. The high-strength cold-rolled steel sheet according to claim 1.

10. In 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 0.01%, N: 0.001-0.01%, the remainder being Fe and other unavoidable impurities, below Steel slabs that satisfy the value defined by equation 1 to be 0.6 or greater and less than 0.9 are 1100 to 135. The step of reheating to 0°C; The step of hot-rolling the reheated steel slab at 850 to 1150°C; The hot-rolled steel sheet is cooled to 450-700°C at an average cooling rate of 10-70°C / s. The stage of doing so; 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 involves using the overall width of the steel plate as a reference and measuring the surface temperature (Te) of both ends in the width direction. The temperature is 601-700°C, and the central surface temperature (Tc) is 450-600°C. A method for manufacturing high-strength cold-rolled steel sheets, which involves controlling the process. [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 are relative to each element) This indicates the average content by weight percent. If none of the elements are added, 0 is substituted.

11. After the winding step, the wound steel plate is moved into the heat-insulating cover and 400- The high-intensity cold process according to claim 10, further comprising the step of maintaining the temperature in the range of 500°C for 6 hours or more. A method for manufacturing rolled steel sheets.

12. The winding step involves the difference (Te-Tc) between the surface temperature of both ends and the surface temperature of the central part. A method for manufacturing a high-strength cold-rolled steel sheet according to claim 10, wherein the temperature is controlled to be 150°C or lower. 。

13. The aforementioned cooling step involves injecting cooling fluid into both ends in the width direction, based on the overall width of the steel plate. The amount of cooling water injected into the central part, excluding the ends, is even larger than the amount of water injected. A method for manufacturing a high-strength cold-rolled steel sheet according to claim 10, wherein the strength is controlled to increase.

Images