JPH0457744B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a cold-rolled steel sheet with excellent press formability and sharpness after painting, and a method for manufacturing the same. The present invention relates to a cold-rolled steel sheet that is resistant to breakage and cracking even in areas subjected to severe processing while ensuring film resistance, and a method for manufacturing the same. <Prior art and its problems> Cold-rolled steel sheets for drawing used in automobile panels, electrical appliances, kitchen appliances, etc. are required to have excellent deep drawability. In order to improve deep drawability, the mechanical properties of the steel sheet need to be high ductility El and high Rankford value. Furthermore, in actual drawing forming (particularly for automobile panels), composite forming with stretch forming is often performed, so the work hardening index (n value) also becomes important. Furthermore, in the case of steel plates for automobiles, it is necessary to spot weld press parts, so the spot weldability of the steel plate becomes important. Furthermore, in the case of automobile exterior panels, the quality of the finish after painting is one of the important quality items for automobile users, which is directly connected to the overall quality of the vehicle. By the way, research on deep drawing forming has been conducted from both sides of the steel sheet material and forming technology. However, as products become more precise and complex, the characteristics required of steel sheets are becoming more sophisticated and diversified. This tendency is particularly strong in cold-rolled steel sheets for automobiles. For example, the current situation is that a large number of press parts are spot welded when assembling an automobile body, but there is a strong demand to reduce the number of spot welds by increasing the size and integrating these parts. Meanwhile, car designs are becoming more complex in order to meet diversifying needs, and as a result, an increasing number of parts are difficult to form using conventional steel plates. In order to meet these demands,
There is a need for cold-rolled steel sheets that have better press formability than conventional ones. In addition, in recent years, there has been a strong demand among automobile manufacturers to improve the quality of their paint finishes. It is rated as a ``feeling of flesh'', ``a feeling of richness'', and ``a feeling of depth'' that define the sense of mass of a paint layer that can be perceived visually by the way light is reflected on the painted surface. . Among these properties, image clarity, which is a combination of gloss and image clarity, is greatly affected by the painting method and the characteristics of the paint, but it is also affected by the surface roughness of the underlying steel plate. to be influenced. By the way, in actual press forming, the conventionally used mechanical properties (values, El, n values) of the steel sheet alone are insufficient as evaluation criteria. For example, the surface roughness of the steel sheet, lubricating oil, etc. have a large effect on press formability. Several known techniques have been disclosed that demonstrate the influence of steel plate surface roughness on press formability and the like. For example, in "Plasticity and Processing" Vol. 3, No. 14 (1962-3), it is shown that in the case of high-viscosity lubricating oil, the drawability is most improved at a steel plate surface roughness of about a few μm.
On the other hand, in Japanese Patent Publication No. 59-3441, the roll surface roughness
Ra and peak number PPI are respectively Ra=2.8 (μm),
This shows a temper rolling method for cold-rolled steel sheets that achieves excellent post-painting appearance and press workability by temper rolling using dull rolls with a PPI of 226. Although these known techniques are excellent in terms of improving press formability, they do not describe a method for manufacturing a cold rolled steel sheet with good image clarity. Generally, a steel plate is subjected to temper rolling at a reduction rate of 0.5 to 1.0% after annealing for the purpose of shape correction and surface roughness adjustment. It is known that when skin pass rolling is applied, material properties that are advantageous for press formability deteriorate. That is, when a steel plate whose material after annealing causes yield point elongation is subjected to temper rolling at a rolling reduction of 0.8 to 1.0%, the yield point is lowered and yield point elongation no longer occurs. Low carbon Al-killed steel, Ti, Nb,
Non-aging materials containing carbonitride-forming elements such as B, Zr, and V can be used without temper rolling.
The yield point is low and yield point elongation does not occur, but when such a completely non-aging steel plate is subjected to temper rolling, the yield point increases and the press load increases, but the elongation decreases. However, this has the disadvantage of causing netting and breakage in areas that undergo severe processing. If the rolling reduction rate is increased, the transfer rate of the roll surface roughness to the steel plate will increase, and there is a risk that the image clarity after painting will decrease. Therefore, it is preferable to avoid increasing the rolling reduction ratio as much as possible. On the other hand, with too little reduction rolling, not only the above-mentioned objectives of shape correction and surface roughness adjustment cannot be achieved, but also the processing conditions are severe during press forming, and in areas lacking press oil, the steel plate and press Friction with the mold increases, making it more likely to break. As a result of extensive research, the inventors of the present invention have found that there is no problem with shape correction and surface roughness adjustment of steel sheets by temper rolling, and even with temper rolling, there is no material deterioration or sharpness after painting. We have discovered a method for producing cold-rolled steel sheets with excellent press formability and little deterioration in properties. In other words, after roughening the surface roughness of the steel sheet before the final stand of cold rolling or before final skin-pass rolling, the press formability and coating are manufactured by completing cold-rolling or skin-pass rolling using rolls with a small surface roughness. We have invented a cold-rolled steel sheet with good post-sharpness and a method for manufacturing the same. <Configuration of the Invention> According to the first aspect of the present invention, a cold-rolled steel sheet having chemical components as follows: C≦0.12% Mn≦0.50% P≦0.040% S≦0.045%, the steel plate having the following formula: A cold-rolled steel sheet with excellent press formability and post-painting sharpness, characterized by a surface convex flatness P satisfying 0≦P≦0.20 and a center surface average valley height Rv≧1.0 μm. is provided. =1/n _o 〓Pi ^h=1 P=1/n _o 〓 ⁱ⁼¹ |−Pi|/ Here, Pi: height of the peak from the central plane According to the second aspect of the present invention, the conventional method The hot-rolled steel plate is cold rolled or temper rolled to ensure that the flatness of the convex portions on the steel plate surface, expressed by the following formula, satisfies 0<P≦0.20, and that the center plane average valley height Rv≧1.0μm. When manufacturing a certain cold-rolled steel plate, first the steel plate surface roughness Ra≧1.0μm is achieved in the first rolling, then in the second rolling,
A dull roll with a roll surface roughness of Ra≦1.2μm and a rolling reduction ratio of 0.2 to 1.0%, and a roll surface roughness of Ra
Provided is a method for producing a cold-rolled steel sheet with excellent press formability and post-painting sharpness, which is characterized by rolling at ≦2 (1.2-Re). =1/n _o 〓Pi ⁱ⁼¹ P=1/n _o 〓 ⁱ⁼¹ |−Pi|/ Here, Pi: height of the peak from the central plane According to the third aspect of the present invention, the conventional method The hot-rolled steel sheet is cold-rolled or temper-rolled so that the flatness of the convex portions on the steel sheet surface, expressed by the following formula, satisfies P = 0, and the center surface average valley height Rv≧1.0μm. When manufacturing a rolled steel plate, first the surface roughness of the steel plate is determined in the first rolling process.
Press formability characterized by setting Ra≧1.0 μm, and then rolling with a bright roll at a reduction rate of 0.2 to 1.0% in the second rolling to flatten the convex portions on the surface of the steel sheet. Provided is a method for producing a cold-rolled steel sheet with excellent image clarity after painting. =1/n _o 〓Pi ⁱ⁼¹ P=1/n _o 〓 ⁱ⁼¹ |−Pi|/ Here, Pi: Height of the peak from the center plane Here, in the second and third aspects of the present invention It is preferable that both the first rolling and the second rolling be performed in the cold rolling stage. Moreover, in the second and third aspects of the present invention, it is preferable that the first rolling is performed in the cold rolling stage and the second rolling is performed in the temper rolling stage. Furthermore, in the second and third aspects of the present invention, it is preferable that both the first rolling and the second rolling be performed at the temper rolling stage. The present invention will be explained in detail below. First, the experiments that formed the basis of the present invention will be explained. (Experiment 1) Weight ratio: C: 0.0021%, Si: 0.008%,
Mn: 0.24%, P: 0.006%, S: 0.005%, Sol.
Al: 0.026%, N: 0.0025%, Ti: 0.031%, Nb:
A hot-rolled steel plate with a thickness of 3.2 mm containing 0.008%, B: 0.0001%, and Zr: 0.0010%, with the remainder being Fe and unavoidable impurities, was manufactured in a laboratory, and after pickling and cold rolling. The work roll surface roughness of the final bath is changed, and the steel plate surface roughness Ra is 0.2 to 2.0 μm, and the plate thickness is
Cold rolling (first rolling) was performed so that the thickness was 0.8 mm. Then, after heating to 850℃ at a heating rate of 20℃/S,
The mixture was soaked for seconds and cooled to room temperature at a cooling rate of 10°C/S. Subsequently, using a work roll with a roll surface roughness Ra of 2.0 μm or less, temper rolling (second rolling) was performed at a reduction rate of 0.5%, and the steel plate surface friction resistance and image clarity after painting were investigated. , the results shown in FIG. 1 were obtained. Here, as shown in FIG. 3, the frictional resistance was measured by pressing down the steel plate with a compressive force P of 50 kg, pulling out the steel plate as it was, and determining the pulling force 2F at that time. Note that the friction coefficient μ is expressed as μ=F/P. When the frictional resistance is 50 Kgf or less, there is no risk of breakage during press forming, and press formability is excellent. Regarding the sharpness after painting, first, as the painting conditions, a phosphate film treatment was applied as a base treatment, and then a cationic electrodeposition coating, an intermediate coating, and a top coating were applied. As a method of evaluating sharpness, DOI
(Distinctness of Reflected Image) was adopted. As shown in Figure 4, the DOI value is the intensity of the reflected light when light incident at an incident angle of 30° is reflected at a reflection angle of 30°, and Rs is the intensity of the reflected light when the light is reflected at a reflection angle of 30° ± 0.3°. When the intensity of reflected light is R _0.3 , it is expressed as DOI = 100 (Rs - R _0.3 )/Rs. If the DOI value is 85 or higher, the image clarity after painting will be excellent. Next, we investigated the material properties and the temper rolling reduction ratio that affects the frictional resistance. (Experiment 2) The same hot rolled steel sheet as in Experiment 1 was pickled and then cold rolled (first rolling) to determine the surface roughness Ra of the steel sheet after cold rolling.
The plate thickness was 1.5 μm and 0.8 mm. Next, the mixture was heated to 830°C at a heating rate of 20°C/s, held for 30 seconds, and cooled to room temperature at a cooling rate of 15°C/s. Subsequently, temper rolling (second rolling) was performed using a work roll with a work roll surface roughness Ra of 1.8 μm or less at a reduction rate of 0.2 to 1.3%, and the ductility El, frictional resistance during complete degreasing, and sharpness after painting were evaluated. I also looked into it. The results are shown in Figure 2. From these Experiments 1 and 2, the following became clear. First, as is clear from Fig. 1, the steel plate surface roughness after cold rolling (first rolling) is Ra≧1.0μm, and the skin pass rolling roll surface roughness Ra≦1.2μm during second rolling. It was found that the surface frictional resistance of the steel plate was significantly lower than that of steel outside the above range. Also, from Figure 2, the temper rolling reduction ratio Re is 0.2
~1.0%, temper rolling roll surface roughness Ra≦2 (1.2−
It was found that steel that satisfies Re) and has a surface roughness of the temper rolling roll Ra≦1.2 μm has low frictional resistance and good image clarity after painting. Furthermore, in the second rolling using bright rolls, it was expected that the roll surface roughness would be significantly reduced. (Experiment 3) Regarding the steel plate manufactured in (Experiment 2), the effects of the steel plate surface roughness on its image clarity and mold-gathering properties, particularly the flatness P of the convex portions and the center plane average valley height Rv, were investigated. The results are shown in FIG. Note that the flatness P of the convex portion is defined by the equation shown below. =1/n _o 〓Pi ⁱ⁼¹ P=1/n _o 〓 ⁱ⁼¹ ｜−Pi｜/ Pi: Height of the peak from the center plane From Figure 5, it is clear that the convex flatness P is 0.20 or less. The center plane average valley height Rv is
Steel plates with a diameter of 1.0 μm or more have a small surface frictional resistance, and are required to satisfy both image clarity and frictional resistance.
It has been found that it is preferable to define Rv and P within appropriate ranges. Next, the reason for limiting the manufacturing conditions of the present invention will be explained below. The steel plate surface roughness Ra before the first rolling, that is, before the final stand of cold rolling or before the final temper rolling, is 1.0 μm.
The reason for the above reason is that if the thickness is less than 1.0 μm, frictional resistance becomes large in areas where lubricating oil runs out during press molding, and mold galling is likely to occur. The reason why the roll surface roughness Ra of the dull roll during the second rolling is set to 1.2 μm or less is because if it exceeds 1.2 μm, the surface roughness of the steel plate will become rough, leading to a decrease in sharpness after painting. Rolling reduction rate Re 0.2~1.0%
And the reason why we set the roll surface roughness Ra≦2 (1.2−Re) is that when Re is more than 1.0% or less than 0.2%, or in the range of Ra>(1.2−Re), the rolling reduction is too high.
This is because as the ductility decreases, the frictional resistance on the surface of the steel sheet increases, and press formability deteriorates. Here, the steel sheet surface roughness Ra is rolled to 1.0 μm or more (first rolling step), and then re-rolled under the above-mentioned limited conditions to achieve a convex flatness P of 0.20 or less.
The following three embodiments are preferable as the timing for performing the step of rolling (second rolling step) so that the center plane average valley height Rv becomes 1.0 μm or more. First of all, both the first rolling process and the second rolling process are performed at the cold rolling stage, specifically, the first rolling process is performed before the final cold rolling stand, and the second rolling process is performed at the same final stand. By doing this, it is possible to flatten the convex parts of the steel plate surface roughness, improve the sharpness of the image, and because it is possible to perform large reduction rolling, it is possible to impart deep roughness to the concave parts. There is. Next, the first rolling process is performed at the final cold rolling stand.
The second rolling process is performed in the temper rolling stage, and by doing so, there is an effect that surface flaws and shape defects caused in the cold rolling-annealing process can be improved. Finally, the first rolling process is performed before the final stand for skin pass rolling, and the second rolling process is performed at the final stand.By doing this, the steel plate with good image clarity and pressability is produced in one process. It can be done in-house, which has the effect of shortening the process. Furthermore, the surface roughness of the steel plate of the present invention will be described. The reason why the protrusions in the steel plate surface roughness must be flat or the protrusion flatness P as shown in the formula below must be 0.20 or less, and the center plane average valley height Rv must be 1.0 μm or more is as follows: = 1 /n _o 〓Pi ⁱ⁼¹ P=1/n _o 〓 ⁱ⁼¹ ｜−Pi｜/ Pi: Peak mountain height from the center plane If P is more than 0.20, the steel plate surface is too rough and the specular reflectance of light is low. This is because the sharpness of the image deteriorates. In addition, if the average valley height Rv of the center plane is less than 0.50 μm, the amount of steel plate wear powder flowing into the recesses during press working cannot be fully consumed, which tends to cause seizure-type galling. In addition, the rolls used in cold rolling and temper rolling in the present invention are those manufactured by any of the shot blasting method, the electric discharge dulling method, and the laser dulling method, and the rolls used in the present invention do not apply. It is possible. Further, attached FIG. 6a is a diagram showing a three-dimensional profile of the surface of a steel plate according to the present invention, and FIG. 6b is a diagram showing a three-dimensional profile of the surface of a conventional steel plate. As is clear from the figure, the surface of the steel plate according to the present invention had more flat parts than the conventional steel plate and had good image clarity. <Examples> Next, the present invention will be described in more detail based on Examples. [Example] Hot-rolled steel plates of steels A to G with a thickness of 3.2 mm manufactured under the conventional hot-rolling conditions shown in Table 1 were pickled.
As shown in the figure, the roll surface roughness of the 4th to 5th stand of a 5-stand cold rolling mill or a skin pass rolling mill.
A cold-rolled plate with a thickness of 0.8 mm was obtained by changing Ra and rolling reduction. Meanwhile, before temper rolling, Steels A to F were annealed at 850°C for 20 seconds by a continuous annealing method, and Steel G was annealed at 700°C for 3 hours by a box annealing method. Afterwards, a sliding test was conducted to examine the frictional resistance and mold galling properties of the steel plate surface, as well as the image clarity after painting. The results are shown in Table 2. In the die galling test, the equipment shown in Figure 3 is used to pull out 100 mm from a steel plate without applying lubricating oil, examine the state of scratches on the sliding parts due to the pullout, and perform the following evaluations. Ta. (Mold galling test evaluation) ○: No scratches △: Lightly scratched ×: Scratches such as peeling off the sliding part The mechanical properties of each steel plate are shown in Table 1 along with the steel composition. show. Tensile properties are measured at 0.8% skin pass rolling.
It was determined using a JIS No. 5 test piece. The values are measured using a three-point method with 15% tensile prestrain, and are measured in the L (rolling) direction,
The average value in the C (90° to the rolling direction) direction and the D (45° to the rolling direction) direction = (r _L +r _C +2r _D )/4. The limit drawing ratio (LDR) was determined from the maximum blank diameter Do max that can be deep drawn using a die with a punch diameter of 33 mm, and the ratio to the punch diameter dp. That is, LDR=Do max/dp The drawing conditions were all the same, with a drawing speed of 1 mm/s and no lubricating oil. As is clear from Table 2, in steels A to G, the steel plate surface roughness Ra after the first rolling process is 1.0 μm or more, and the second rolling process reduction rate Re is 0.2 to 1.0.
%, by setting the second rolling process roll surface roughness Ra to 1.2 μm or less, the steel plate convex flatness P is 0<P≦0.20
Or flat surface, and center surface average valley height
By setting Rv to 1.0 μm or more, it was possible to produce a cold-rolled steel sheet with good mold resistance and image clarity. On the other hand, the steel plate surface roughness after the first rolling process
A steel plate under the conditions where Ra is less than 1.0 μm, or the second rolling process roll surface roughness Ra is small, and the second rolling process reduction rate is less than 0.2% or more than 1.0%,
Although the image clarity after painting was good, mold galling was likely to occur. In addition, the sharpness after coating of the steel plate rolled with a roll having a surface roughness Ra of over 1.2 μm was relatively poor, although it was related to the rolling reduction.

【table】

[Table] Box annealed materials

[Table] <Effects of the Invention> As detailed above, according to the present invention, after roughening the surface roughness of a steel sheet before the final stand of cold rolling or before final skin-pass rolling, cold rolling is performed using rolls with a small surface roughness. By manufacturing steel sheets after rolling or skin-pass rolling, we can suppress material deterioration after skin-pass rolling, and at the same time improve mold galling properties during pressing and brightness after painting, which were previously thought to have a contradictory relationship. It has now become possible to achieve both image quality and image quality. Furthermore, in addition to drawing steel sheets, there is no deterioration in sharpness as a general cold-rolled steel sheet, and it can be used as a high-definition steel sheet.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the steel plate surface roughness after cold rolling and the temper rolling roll surface roughness on the frictional resistance and DOI value of the steel plate. Figure 2 shows the frictional resistance of the steel plate and
FIG. 3 is a diagram showing the relationship between skin pass rolling roll surface roughness and skin pass rolling reduction ratio on DOI value. FIG. 3 is an explanatory diagram of a steel plate sliding test. FIG. 4 is an explanatory diagram showing a method of measuring a DOI value representing image clarity. FIG. 5 is a diagram showing the relationship between center plane average valley height Rv and convex flatness P on image clarity (DOI value) and steel sheet surface frictional resistance. FIG. 6a is a diagram showing a three-dimensional profile of the surface of a steel plate according to the present invention, and FIG. 6b is a diagram showing a three-dimensional profile of the surface of a conventional steel plate.

[Claims]

1 Chemical components are C≦0.003wt%, Si≦0.05wt%, Mn≦0.5wt%,
P≦0.02wt%, S≦0.02wt%, Al≦0.1wt%, N
≦0.003wt%, 48/12C+48/14+48/32S (wt%)<Ti≦0.1wt%, 0.002wt%≦Nb≦0.02wt%, 0.0002wt%≦B≦0.002wt%, and the remainder unavoidable impurities and Fe , and in a thin steel sheet for press working with an average grain size of 20 μm or more, the regularity parameter S representing the regularity of the steel sheet surface roughness expressed by the following formula is S≦0.25 in at least one direction, and the average surface roughness A cold press for press processing that is characterized by a geometric surface structure that satisfies the relationship Ra≧1.2μm and is less likely to cause roughness during processing.

Claims (1)

Then, the flatness P of the protrusions on the steel plate surface shown in the following formula is 0.
≦P≦0.20 and center plane average valley height Rv
When producing a cold rolled steel sheet with a surface roughness of Ra≧1.0μm, first the steel plate surface roughness is Ra≧1.0μm in the first rolling, and then rolled with a dull roll with a roll surface roughness Ra≦1.2μm in the second rolling. A method for producing a cold-rolled steel sheet with excellent press formability and sharpness after coating, characterized by rolling with a rolling ratio Re of 0.2 to 1.0% and a roll surface roughness Ra≦2 (1.2-Re). =1/n _o 〓Pi ⁱ⁼¹ P=1/n _o 〓 ⁱ⁼¹ |−Pi|/ Here, Pi: Peak height from the center plane 3 Both the first rolling and the second rolling are in the cold rolling stage. A method for producing a cold-rolled steel sheet having excellent press formability and image clarity after painting according to claim 2. 4. The method for producing a cold rolled steel sheet with excellent press formability and post-painting sharpness according to claim 2, wherein the first rolling is performed in a cooling stage and the second rolling is performed in a temper rolling stage. 5. The method for producing a cold-rolled steel sheet with excellent press formability and post-painting sharpness according to claim 2, wherein both the first rolling and the second rolling are performed in the temper rolling stage. 6 Cold-roll or temper-roll the hot-rolled steel plate using a conventional method to obtain a convex flatness P on the steel plate surface expressed by the following formula.
=0 and center plane average valley height Rv≧
When manufacturing a cold-rolled steel sheet with a thickness of 1.0 μm, the steel sheet surface roughness Ra≧1.0 μm is first rolled in the first rolling process, and then the reduction rate Re is set using a bright roll in the second rolling process.
A method for producing a cold-rolled steel sheet with excellent press formability and sharpness after painting, characterized by flattening the convex portions on the surface of the steel sheet by rolling with 0.2 to 1.0%. =1/n _o 〓Pi ⁱ⁼¹ P=1/n _o 〓 ⁱ⁼¹ |−Pi|/ Here, Pi: Peak height from the center plane 7 Both the first rolling and the second rolling are in the cold rolling stage. A method for producing a cold-rolled steel sheet having excellent press formability and image clarity after painting according to claim 6. 8. The method for producing a cold rolled steel sheet with excellent press formability and post-painting sharpness according to claim 6, wherein the first rolling is performed in a cold rolling stage and the second rolling is performed in a temper rolling stage. 9. The method for producing a cold-rolled steel sheet with excellent press formability and post-painting sharpness according to claim 6, wherein both the first rolling and the second rolling are performed in the temper rolling stage.