JP5691644B2 - Cold-rolled steel sheet for automobile outer plates and automotive outer panel components - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet for an automobile outer plate that can reduce the weight of the automobile body by making the steel sheet material used according to various properties required for the members of the automobile body suitable. Specifically, the present invention relates to a cold-rolled steel sheet used for an automobile outer panel (outer panel) such as an automobile door, a hood, or a roof.

In recent years, the weight reduction of automobile bodies has been actively promoted in consideration of the global environment. Furthermore, in recent years, further reduction in the weight of the vehicle body has been demanded by the global CO ₂ emission regulations and the establishment of fuel efficiency improvement regulations.

  Along with such increasing social demands, a technique for arranging steel plate materials used for automobile bodies in the right place at the right place has been proposed. For example, a so-called “tailored blank material” is known in which metal plates having different materials and plate thicknesses are locally joined to a portion where strength, rigidity, and the like are required (for example, Patent Document 1). In addition, a blank material formed by butt welding a combination of dissimilar metals such as an aluminum alloy and steel and a molded part thereof have been proposed (Patent Document 2).

  The merit of using tailored blanks is that, for relatively large members such as hoods and roofs, for example, at present, a single plate that satisfies the characteristics of all parts is molded for each part. By using different materials according to the required properties such as strength and rigidity, the weight of the material can be reduced and the strength can be increased. Compared to the case of using, the material yield can be improved by reducing the amount of cut off of the window and door.

  However, since many of the conventional tailored blank materials are integrated by welding (Tailored Welded Blank, hereinafter also referred to as “TWB material”), there is a problem of molding defects due to material deterioration of the welded portion. . Therefore, in order to solve this problem, a high-strength steel sheet having a strength of 650 MPa or more and a transformation-induced plastic mold steel sheet are welded together, and the ductility or formability of the entire material is the whole material obtained by joining the transformation-induced plastic mold steel sheets together. Optimum work hardening characteristics of high-strength TWB material (Patent Document 3), which is better than the ductility or formability of steel, and materials in which two or more types of base plates with different tensile strengths are continuously welded A TWB material (Patent Document 4) has been proposed in which the amount of strain applied to the high-strength base plate side is increased before breakage to improve press formability.

  However, at present, the application of the TWB material to automobile parts is limited to the inner plate panel and the skeleton member, and the application to the outer plate panel is small. Moreover, even if it is applied to the outer panel, it is only applied to places that are not directly touched, for example, places that cannot be seen unless the door is opened, or places that are covered with a cover. The reason for this is that the bonding quality of the TWB material and the surface quality (surface accuracy) in the vicinity thereof are reduced during press molding, and this is the biggest obstacle to the application of the TWB material to the outer panel.

  In order to solve this problem of deterioration in surface quality, for example, Patent Document 5 discloses a punch surface that forms the vicinity of a weld line when press-molding a TWB material in which metal plates having different thicknesses or materials are joined. There has been proposed a molding method that suppresses surface quality deterioration by lining hard rubber and press molding. However, this method has not yet completely eliminated the deterioration of the surface quality of the joining line and its vicinity.

  Another problem with the TWB material is that the joint line can be seen even after the coating process if it is left as it is. In order to make this joining line invisible, for example, there is a method of building up with wax or the like along the joining line and then grinding, but this is not a practical method in terms of manufacturing cost. In addition, as in Patent Document 6, there is a method of mechanical joining such as caulking and press fitting instead of welding joining, but naturally, a step is generated in the joining portion or a joining portion trace remains. Therefore, it cannot be applied to the outer plate.

  In addition, as other techniques, several blank materials having different thicknesses by rolling or the like and methods for manufacturing the same are disclosed. For example, Patent Document 7 discloses that in continuous hot rolling finish rolling, a steel strip is rolled using a rolling work roll having a differential thickness and / or a taper, and two or more types of plate thicknesses are formed in the width direction of the steel strip. Patent Document 8 discloses a method of forming a vehicle body panel in which a plate having three different plate thicknesses is formed by a rolling device and formed into a predetermined shape by a press. Patent Document 9 discloses a vehicle body panel including a thick wall portion and a thin wall portion manufactured by rolling or cutting, and a middle wall portion whose thickness changes stepwise or continuously from the thick wall portion to the thin wall portion. In the same manner as in Patent Document 8, the blank material is rolled in the longitudinal direction in the same manner as in Patent Document 8 to produce a differential thickness plate in which the thickness of the plate material is partially changed. An apparatus is disclosed.

JP 2006-095596 A Japanese Patent Laid-Open No. 08-309445 Japanese Patent No. 4044434 JP 2000-309843 A JP 2010-036222 A JP 2004-130344 A JP-A-11-192502 JP 2002-326589 A JP 2002-331317 A JP 2008-264850 A

  However, the technology of Patent Document 7 described above is that the product is a hot-rolled steel sheet, and because the shape is given to the rolling roll with unevenness, there is no plate thickness inclined portion, and a clear plate thickness step Therefore, it is unsuitable for automotive skin panel applications. Further, the technique of Patent Document 8 is mainly intended to improve the dimensional accuracy of the plate thickness step portion, and since there is a plate thickness step, it is not suitable for the outer panel. Further, in the technique of Patent Document 9, the step portion cross section when the plate thickness changes stepwise is an arc, and the plate thickness changes rapidly. There is also a description that the thickness is continuously changed, but there is no specific description about how to change the thickness, and no contrivance for obtaining the outer panel is found. In addition, although it is possible to give a plate thickness change by grinding, the quality requirement as an outer plate panel cannot be satisfied with grinding. Further, the technique of Patent Document 10 relates to a rolling apparatus that places importance on plate thickness dimensional accuracy, and furthermore, since the cross-sectional shape of the plate thickness step portion is an arc shape, it is also not suitable for an outer plate panel. As described above, none of the techniques of Patent Documents 7 to 10 assume an outer panel, and no consideration is given to surface quality.

  The present invention has been made in view of the above-described problems of the prior art, and the purpose thereof can be directly seen from the outside even for a tailored blank material having a plate thickness difference or a plate thickness inclined portion. An object of the present invention is to provide a cold-rolled steel sheet for an automobile outer plate that can be applied to an automobile outer plate.

  In general, the surface quality of automobile exterior panels is evaluated by sensory tests such as visual judgment that is judged by visual feeling and tactile judgment that is judged by touch when touched by hands, and so-called “look” is particularly important. Often done. Therefore, in order to solve the above-mentioned problems, the inventors have earnestly considered a cold-rolled steel sheet that satisfies the surface quality as an automobile outer plate by visual inspection, on the premise of a tailored blank material that has no welded portion and has a plate thickness inclined portion. Repeated examination. As a result, people mainly recognize the degree of angle change rather than plate thickness difference, and even if there is a plate thickness difference or plate thickness inclined part, the change cannot be visually recognized. The present invention was developed by finding that there is a range that satisfies the surface quality.

The present invention based on the above knowledge has a plate thickness equal thickness portion with a constant plate thickness, and a plate thickness inclined portion where the plate thickness changes at least on one side, the plate thickness equal thickness portion and the plate thickness inclined portion, When the angle θ formed by the surface of the equal thickness portion and the surface of the inclined thickness portion is curved to the curvature of the automotive outer panel component using the steel plate, the boundary portion cannot be visually recognized. The following formula (1);
Record
θc (°) = 0.589 × (1 / R) ^0.187 (when R <8000)
= 1.175 × (1 / R) ^0.274 (when 8000 ≦ R <20000)
= 0.342 × (1 / R) ^0.151 (when R ≧ 20000)
... (1)
However, in the formula, R (mm) represents a radius of curvature, (1 / R) represents a curvature, and when R is ∞ (when it can be regarded as flat), θc is set to 0.06 °.
It is set as the angle below the critical inclination | tilt angle (theta) c defined by (4), It is a cold-rolled steel plate for motor vehicle outer plates.

In addition, the cold rolled steel sheet for an automobile outer sheet of the present invention is characterized in that an angle θ formed by the surface of the equal thickness portion and the surface of the inclined thickness portion is 0.20 ° or less.

In addition, the cold rolled steel sheet for an automobile outer sheet according to the present invention is characterized in that an angle θ formed by the surface of the equal thickness portion and the surface of the inclined thickness portion is 0.10 ° or less.

  Moreover, the cold rolled steel sheet for an automobile outer sheet according to the present invention is characterized in that a difference in sheet thickness between both ends of the sheet thickness inclined portion is 0.2 mm or more.

  Moreover, the cold-rolled steel sheet for automobile outer sheets according to the present invention is characterized in that the steel sheet surface is subjected to surface treatment.

  Moreover, this invention is an automotive outer panel component formed by using any one of the cold-rolled steel sheets described above.

  The automotive outer panel component according to the present invention is characterized in that a plate thickness inclined portion is applied to a portion having a curvature radius of 8000 mm or less.

  According to the present invention, even if there is a plate thickness difference or a plate thickness inclined part in the material steel plate, the change cannot be recognized by visual sensory inspection depending on the angle change, so tailored that satisfies the surface quality as an automobile outer plate A blank material can be provided. Therefore, since the steel plate that satisfies the conditions of the present invention can be suitably used as a cold rolled steel plate for an automobile outer plate, the application of a tailored blank material that has been conventionally applicable only to an automobile inner plate or a strength member that is invisible to the human eye. The range can be greatly expanded. Furthermore, by applying the steel plate of the present invention to an automobile outer panel component, not only can the weight of the automobile body be increased and the strength can be increased, but also it is possible to integrally form automobile parts, or the material yield. Since improvement can also be achieved, the effect produced in the industry is extremely great.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram explaining the cold-rolled steel plate for motor vehicle outer plates of this invention, (a) is a type (type A) from which plate | board thickness changes only to the single side | surface side of a steel plate, (b) is a board on the both surfaces side of a steel plate. The type in which the thickness changes (type B) is shown. It is a schematic diagram explaining the jig | tool which curves a steel plate and visually inspects. It is a graph which shows the relationship between curvature 1 / R when a steel plate is curved, and critical inclination | tilt angle (theta) c from which a boundary part becomes invisible.

FIG. 1 is a schematic diagram for explaining a cold-rolled steel sheet for automobile outer plates according to the present invention, and FIG. 1 (a) shows plate thickness equal thickness portions on both ends in the length direction (or width direction) of the steel plate, In the middle, it has a plate thickness inclined portion in which the plate thickness is changed only on one side of the steel plate surface, and the angle formed by the surface of the plate thickness equal thickness portion and the surface of the plate thickness inclined portion is θ (hereinafter, This angle θ is an example of a steel plate (type A) which is also referred to as “inclination angle” or “plate thickness inclination angle”, and FIG. 1B shows both ends in the length direction (or width direction) of the steel plate. Another steel plate example of the present invention having a plate thickness equality portion on the side, a plate thickness inclined portion with a change in plate thickness on both sides, and a plate thickness inclination angle of each surface being θ (Type B) is shown. Therefore, the steel sheet of type B in FIG. 1B has a plate thickness change rate per unit length (plate thickness difference / plate thickness inclined portion length) even if the plate thickness inclination angle θ is the same. This is twice that of type A steel plate (a). In the steel sheet of the present invention, the plate thickness of the plate thickness inclined portion changes linearly, and therefore the steel plate surface of the plate thickness inclined portion is flat, and the common tangent line is formed at the boundary between the steel plate surface and the plate thickness equal thickness portion. It shall be connected discontinuously without having. In FIG. 1B, the thickness inclination angle θ is the same on the front and back surfaces, but the thickness inclination angle θ may be different on the front and back surfaces.

  First, in order to confirm whether or not the plate thickness inclination angle θ is less than or equal to how many times the boundary between the plate thickness equal thickness portion and the plate thickness inclination portion cannot be recognized by human visual inspection, A test specimen having a different thickness inclination angle θ is manufactured with the type A shown in FIG. 1A and visually inspected in the state of a flat plate, and the critical inclination in which the presence of the thickness inclination portion cannot be visually recognized by human visual inspection. The angle θc was investigated. As a result, it was found that the critical inclination angle θc in the flat plate state was 0.06 °.

  Subsequently, using the jig as shown in FIG. 2, the test piece (type A) is brought to the surface with the thickness inclination side (that is, the surface without the thickness inclination is in contact with the jig). Bend with various curvature radii R and be fixed to a jig with bolts. Visual inspection is performed in the same manner as described above, and the critical inclination angle θc at which the presence of the plate thickness inclined portion cannot be confirmed by human visual inspection is set. confirmed. As a result, as shown in Table 1 and FIG. 3, it was found that the critical inclination angle θc increases as the radius of curvature R to be bent decreases (the curvature 1 / R increases).

  From the viewpoint of securing rigidity, the outer plate of an automobile body is not used in a flat state and is usually given some curvature, and the curvature radius R at that time is 8000 mm or less. Often. Therefore, from FIG. 3, even when the plate thickness inclined portion is provided, if the plate thickness inclined angle θ is 0.10 ° or less, the human visual inspection almost eliminates the boundary between the plate thickness equal thickness portion and the plate thickness inclined portion. Since it cannot recognize, if it is a site | part whose curvature radius R is 8000 mm or less, it turns out that it can fully be used as a cold-rolled steel plate for motor vehicle outer plates. Further, FIG. 3 also shows that a steel plate having a thickness inclination angle θ of 0.20 ° or less can be sufficiently used for an automobile outer plate at a portion having a curvature radius R of 320 mm. That is, the plate thickness inclination angle θ is appropriately set depending on the radius of curvature (curvature) of a portion used as an automobile outer plate. The shape of the automotive outer panel component changes locally to various curvatures depending on the design, but considering the assumed radius of curvature, the radius of curvature R is preferably in the range of about 300 to 8000 mm.

  As a result of further investigation by the inventors, the relationship between the curvature and the critical inclination angle θc shown in Table 1 and FIG. 3 is the type A in which the plate thickness changes only on one side and the type in which the plate thickness changes on both sides. It was also found that B was almost the same.

  Further, the purpose of using the tailored blank material for an automobile outer plate is to reduce the weight of the automobile body by making the thickness of the material to be used appropriate. In order to enjoy such an effect, the thickness inclination angle θ in the cold rolled steel sheet for automobile outer sheets of the present invention is preferably 0.02 ° or more, and more preferably 0.03 ° or more. .

  For the same purpose, it is preferable that the difference in thickness between the both ends of the inclined portion of the cold-rolled steel sheet for automobile outer sheets of the present invention is 0.2 mm or more. If the thickness is less than 0.2 mm, the weight reduction effect is small, and the merit of using the tailored blank material is small. In addition, the upper limit of the plate thickness difference in the steel plate of the present invention is not particularly limited, but in the case of Type A in which the plate thickness is changed only on one side, the maximum is about 1 mm, and the plate thickness is changed on both sides. In the case of B, the maximum thickness is preferably about 2 mm (total thickness difference including the thickness differences on both sides). The reason for this is that, in order to obtain a thickness difference exceeding 1 mm in the case of type A (a thickness difference exceeding 2 mm in the case of type B), the length of the thickness inclination portion is This is because the size of the steel plate becomes too large and the size of the steel plate becomes too large to exceed the practical range.

  Further, the cold rolled steel sheet for an automobile outer plate of the present invention may be a cold rolled steel sheet, but the surface of the steel sheet is subjected to Zn-based and Al-based hot dipping or electroplating such as Zn, Zn-Al, Zn-Sn. Steel plate that has been subjected to various surface treatments, such as a steel plate that has been subjected to chemical treatment, a steel plate that has been subjected to chemical conversion treatment, a steel plate that has been subjected to a chemical conversion treatment, or a steel sheet that has been coated with a solid lubricant or the like. .

  Also, there is no particular limitation on the method of forming the thickness-thinned portion in the cold rolled steel sheet for an automobile outer sheet of the present invention, for example, a method of changing the sheet thickness in the rolling length direction or width direction by a rolling method, press There are a method of changing the plate thickness by processing or forging, a method of changing the plate thickness by polishing, grinding, etching, or the like, and any method may be used. However, in the case where the plate thickness difference is given by polishing or grinding, the surface condition is greatly different between the processed part and the non-processed part, and the difference may be recognized even after painting. It is preferable to make the surface properties uniform by performing a light shot blasting process. In particular, it is preferable to grind the surface of the steel plate so that the grinding marks (grinding irons) disappear after the grinding from the viewpoint of securing the surface quality as an automobile outer plate.

  A steel sheet having a width of 120 to 500 mm and a length of 800 to 900 mm is sampled from a cold rolled steel sheet for drawing (JIS G 3141 (2009)) with a thickness of 1.0 to 1.6 mm, and the steel sheet is ground. Thus, a test piece having a plate thickness equal thickness portion (thick wall portion, thin wall portion) shown in Table 2 and a plate thickness inclined portion was produced. In addition, the “one side” of the differential thickness type shown in Table 2 is a type A having a plate thickness inclined portion in which the plate thickness is changed only on either one of the front and back surfaces of the steel plate as shown in FIG. In addition, “both sides” means a type B type having a plate thickness inclined portion in which the plate thickness is changed on both sides of the steel plate as shown in FIG. Moreover, in order to remove the grinding wrinkles on the ground steel plate surface and make the surface state equivalent to the non-ground surface, first, the grinding wrinkles (machine eye) are dropped with alumina cloth paper # 80, and finally, The entire surface was polished with # 200 cloth paper until the grinding wrinkle disappeared. In the polishing, first, only the flat portion was polished while covering the inclined portion, and then the inclined portion was polished so as to cover the flat portion so as not to obscure the boundary portion.

  Next, the surface of the test piece was subjected to baking coating consisting of three layers of base coating, color coating, and clear coating equivalent to those of an automobile body (color coating is a product number of Nippon Paint Industry: NTK2009-023 and NTK2009) -1190 paint is used, the total film thickness is 100-120 μm), the surface on which the thickness changes is the top surface, and remains flat (curvature radius ∞) or outside the car body Assuming plate panel parts, when using the jig shown in FIG. 2 to change the curvature in various ways (bend holes in the panel and fix them to the jig with bolts) In addition, it was evaluated whether or not the presence of the plate thickness inclined portion could be confirmed by visual observation.

  The results of the evaluation test are also shown in Table 2. Thus, the steel sheet of the invention example in which the thickness inclined portion is formed under the conditions that conform to the conditions of the present invention should be used by being curved with an appropriate curvature radius even if the thickness inclination angle θ is 0.06 ° or more. Thus, it can be seen that it has sufficient surface quality as an outer plate material of an automobile body. Further, Table 2 shows the weight reduction rate when the plate thickness inclined portion is provided with respect to the case where all the test pieces have the plate thickness of the thick portion. It can be seen that it can be expected to contribute to weight reduction.

  The technology of the present invention is not limited to automobile outer panel components, and can be used in the field of home appliances, for example.

1: Bending jig 2: Evaluation test piece 3: Fixing bolt

Claims (7)

A plate thickness equal thickness portion having a constant plate thickness and a plate thickness inclined portion where the plate thickness changes at least on one side, and the plate thickness equal thickness at the boundary between the plate thickness equal thickness portion and the plate thickness inclined portion. The angle θ formed by the surface of the part and the surface of the sloped part of the plate thickness is defined by the following equation (1) where the boundary portion cannot be visually recognized when the steel plate is curved to the curvature of the automotive outer panel component used. A cold-rolled steel sheet for an automobile outer sheet, characterized in that it is set to an angle equal to or less than the critical inclination angle θc.
Record
θc (°) = 0.589 × (1 / R) ^0.187 (when R <8000)
= 1.175 × (1 / R) ^0.274 (when 8000 ≦ R <20000)
= 0.342 × (1 / R) ^0.151 (when R ≧ 20000)
... (1)
However, in the formula, R (mm) represents a radius of curvature, (1 / R) represents a curvature, and when R is ∞ (when it can be regarded as flat), θc is set to 0.06 °. The cold rolled steel sheet for an automobile outer sheet according to claim 1 or 2, wherein an angle θ formed by the surface of the equal thickness part and the surface of the inclined part of the sheet thickness is 0.20 ° or less. The cold rolled steel sheet for an automobile outer sheet according to claim 1 or 2 , wherein an angle θ formed by the surface of the equal thickness part and the surface of the inclined part of the sheet thickness is 0.10 ° or less. The cold-rolled steel sheet for automobile outer sheets according to any one of claims 1 to 3 , wherein a difference in sheet thickness at both ends of the sheet thickness inclined part is 0.2 mm or more. The cold-rolled steel sheet for automobile outer sheets according to any one of claims 1 to 4 , wherein the steel sheet surface is subjected to surface treatment. An automotive outer panel component formed by using the cold-rolled steel sheet according to any one of claims 1 to 5 . The automotive outer panel component according to claim 6 , wherein the plate thickness inclined portion is applied to a portion having a radius of curvature of 8000 mm or less.

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