JPS63111156A - Cold rolled steel sheet having excellent press formability and vividness after painting and its production - Google Patents

Abstract

PURPOSE:To provide a cold rolled steel sheet which has excellent vividness and the excellent press formability to have the resistance to breakdown and rupture even in the section to be subjected to severe working by specifying the flatness range in the projecting parts on the surface of a steel sheet and the average height of the valleys at the central surface. CONSTITUTION:The titled cold rolled steel sheet which is so formed that the flatness P in the projecting part on the surface of the steel sheet expressed by the equation satisfies 0<=P<=0.20 and the average height Rv of the valleys at the surface center is Rv>=0 is provided. The surface roughness of the steel sheets at the final stand of cold rolling or before the final temper rolling is increased and thereafter the cold rolling or the temper rolling is ended with the rolls of the small surface roughness in order to obtain the cold rolled steel sheet mentioned above. The cold rolled steel sheet having the die galling property at the time of pressing and the vividness after painting in compatibility which heretofore were considered to be in diametrically opposite relations is thereby obtd. while the deterioration in the material quality after the temper rolling is suppressed.

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 ensures excellent image clarity through control and is resistant to breakage and cracking even in areas subjected to severe processing, 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, high ductility (EjZ) and high Lankford value (
lower price) is required.

Furthermore, in actual drawing molding (especially for automobile panels), it is often a composite molding with stretch molding.
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 is also 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 has been carried out 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, in the assembly of automobile bodies, a large number of press parts are currently spot welded, but there is a strong demand to increase 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, a cold-rolled steel sheet is required that has 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. ”, ``feeling of fleshiness'', ``feeling of richness'', and ``feeling of depth'' that define the sense of mass of the paint layer visually perceived by the way light is reflected on the painted surface.
It is evaluated as. Among these properties, image clarity, which is a combination of gloss and image clarity, is greatly affected by the coating method and characteristics of the paint, but is also affected by the surface roughness of the underlying steel plate.

By the way, in actual press forming,

As for the evaluation criteria, the conventionally used mechanical properties of the steel plate (lower value, El, n value) alone are not sufficient.

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.

Tatoeha “Plasticity and Processing JVO1, 3, No. 14 (19
62-3) shows that in the case of high-viscosity lubricating oil, the drawability is improved most when the steel plate surface roughness is about a few degrees. On the other hand, in Japanese Patent Publication No. 59-3441, the roll surface roughness (
Ra) and peak number (PPI) are each Ra=2.8.
(-) shows a temper rolling method for a cold rolled steel sheet that has excellent post-painting appearance and press workability by temper rolling with dull rolls having 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, steel plates are used for shape correction and surface roughness adjustment.
After annealing, skin pass rolling is performed at a reduction rate of 0.5 to 1.0%. 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 exhibits elongation at yield point is subjected to skin pass rolling at a rolling reduction of 0.8 to 1.0%, the yield point is lowered and elongation at yield point no longer occurs.

Low carbon single-killed steel, Ti, and Nb made by box annealing.

Non-aging materials containing carbonitride-forming elements such as B, Zr, and V have a low yield point and no yield point elongation, even without temper rolling. When a non-aging steel plate is subjected to skin pass rolling, the yield point increases and the press load increases, but the elongation decreases, which has the disadvantage of causing necking and rupture in areas that undergo severe processing. appear.

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,
Breakage is more likely to occur.

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 on the final skin-pass rolling surface, cold rolling or skin-pass rolling is completed using rolls with a small surface roughness, resulting in press formability and coating. A cold-rolled steel sheet with good post-sharpness and a method for manufacturing the same have been invented.

<Structure of the Invention> According to the first aspect of the present invention, the flatness P of the convex portions on the surface of the steel plate as expressed by the following formula satisfies 0≦P≦0.20, and the center plane average valley height Rv≧1 Provided is a cold-rolled steel sheet with a diameter of .0 μm and excellent press formability and sharpness after painting.

Here, Pi: peak mountain height from the central plane
According to this embodiment, a hot-rolled steel sheet is cold-rolled or temper-rolled in a conventional manner so that the flatness P of the convex portions on the steel sheet surface is o as expressed by the following formula.
<p≦0.20 and center plane average valley height Rv≧
When producing a cold rolled steel sheet with a surface roughness of 1.0-, first the steel sheet surface roughness Ra is set to Ra≧1.0 μm in the first rolling process, and then in the second rolling process, the surface roughness of the steel plate is made into a tall roll with a roll surface roughness Ra of 51.2 mm. Press formability and sharpness after painting characterized by rolling at a rolling reduction of i (Re) of 0.2 to 1.0% and a roll surface roughness of Ra≦2 (1,2-Re). Provided is a method for producing a cold-rolled steel sheet with excellent properties.

Here, Pi: the height of the peak from the central plane.
According to the embodiment, a hot-rolled steel sheet is cold-rolled or temper-rolled by a conventional method, and the convex flatness P of the steel sheet surface is determined by the following formula.
When manufacturing a cold-rolled steel sheet that satisfies = 0 and has a center surface average valley height Rv≧1.0, first the steel plate surface roughness Ra≧1.0 μm is achieved in the first rolling, and then the second rolling In rolling,
The reduction rate (Re) is 0.2 to 1.0% with bright roll.
Provided is a method for producing a cold-rolled copper plate having excellent press formability and sharpness after painting, which is characterized by flattening the convex portions on the surface of the steel plate by rolling the steel plate.

Here, Pi is the height of the peak from the central plane.In the embodiments of 'fJ2 and 3 of the present invention, it is preferable that both the first rolling and the second rolling be performed at 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: 0.0
08%, Mn: 0.24%, P: 0.006%
, S: 0.005%, 5ofl, Aj2.
: Q, 026%, N: (1,00
25%, Ti: 0.031%, Nb: 0.008
%, B: 0.0001%, Zr: 0.0010
A hot-rolled copper plate with a thickness of 3.2 mm was produced in a laboratory, and the remaining portion was Fe and unavoidable impurities. After pickling, the work roll surface roughness of the final pass during cold rolling was By changing the steel plate surface roughness (Ra) of 0.2 to 2.0-1
Cold rolling (first rolling) was performed so that the plate thickness was 0.8+nm.

Next, the mixture was heated to 850° C. at a heating rate of 20° C./S, held soaked for 20 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 2.01 m or less, temper rolling (second rolling) was performed at a reduction rate of 0.5%, and the frictional resistance of the steel plate surface at that time and the sharpness after painting were measured. When we investigated the properties, we obtained the results shown in Figure 1.

Here, the method for measuring frictional resistance is as shown in Figure 3.
The steel plate was pressed down with a compressive force (P) of 50 kg, the steel plate was pulled out as it was, and the pulling force (2F) at that time was determined.

Note that the coefficient of friction (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 clarity after painting, first, as the painting conditions, a phosphate film treatment was applied as a base treatment, and then the painting processes of cationic electrodeposition painting, intermediate coating, and top coating were applied.

As a method for evaluating sharpness, DOI (Distinction
ctness of Reflected Image
)It was adopted. As shown in Figure 4, the 001 value is the intensity of reflected light when light incident at an incident angle of 30° is reflected at a reflection angle of 30° as Rs, and the reflection angle is 30° ± 0.3°.
When the intensity of the reflected light reflected at is R63, it is expressed as DOI = 100 (Rs Ro, 3)/RS.

When the DOI value is 85 or more, the image clarity after painting is excellent.

Next, we investigated the material properties and the temper rolling reduction ratio that affects the frictional resistance.

(Experiment 2) After pickling the same hot rolled steel sheet as in Experiment 1, cold rolling (first rolling) was performed, and the steel sheet surface roughness (Ra) after cold rolling was 1.5-1.
The plate thickness was set to (L8 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, skin pass rolling (second rolling) with a reduction ratio of 0.2 to 1.3% was performed using a work roll with a work roll surface roughness (Ra) of 1.8 pm or less, and the ductility (E2) and the complete degreasing were evaluated. Frictional resistance and image clarity after painting were also investigated. 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 surface roughness (Ra) of the steel plate after cold rolling (first rolling) is 1.0, and the surface roughness of the skin pass rolling roll (Ra) is 1.0 during the second rolling. It was found that the surface friction resistance of steel under the condition of .24 was significantly lower than the friction resistance of steel outside the above range.

Also, from Figure 2, the skin pass rolling reduction ratio (Re) is 0.2
~1.0%, temper rolling roll surface roughness (Ra)≦2(1
, 2-Re) and under the condition that the surface roughness (Ra) of the temper rolling roll is ≦1.2-, it is found that the steel has low frictional resistance and also has good image clarity after painting. It was done.

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 influence of the steel plate surface roughness, especially the flatness of convex portions, on its image clarity and mold galling property.
, the center plane average valley height Rv was investigated. The results are shown in FIG.

Note that the flatness P of the convex portion is defined by the equation shown below.

Pi: Peak mountain height from the center plane From FIG. 5, the convex flatness P is 0.20 or less and the sharpness is good, and the center plane average valley height Rv is 1.0 or more. It has been found that the steel plate has a small surface frictional resistance, and it is preferable to define Rv and P within appropriate ranges in order to satisfy both image clarity and frictional resistance.

Next, the reason for limiting the manufacturing conditions of the present invention will be explained below.

First rolling, that is, before the final stand of cold rolling or ri
The reason why the surface roughness (Ra) of the steel sheet before final skin pass rolling is set to 1.0- or more is that if it is less than 1.0-,
This is because frictional resistance increases at the part where the v1 lubricating oil runs out, making it easier for mold galling to occur.

Roll surface roughness of dull roll during second rolling; Ra
) is set to 1.2- or less because if it exceeds 1.2 $1, the surface roughness of the steel plate will become rough, leading to a decrease in image sharpness after painting. 0.2-1.0%
And the roll surface roughness (Ra)≦2(1,2-Re)
The reason for this is that when Re is more than t, o% or less than 0.2%, or in the range of Ra > 2 (1,2-Re), the rolling reduction is too high, which reduces the ductility and at the same time reduces the friction on the steel plate surface. This is because resistance increases and press formability deteriorates.

Here, the steel plate surface roughness (Ra) is 51. 〇- or more rolling process (first rolling process), and then re-rolling under the above-mentioned limited conditions so that the convex flatness P is 0.20 or less and the center plane average valley height Rv) is 1.0- As for the timing of performing the step of rolling so as to achieve the above (second rolling step), the following three bad conditions are preferable.

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 stand of cold rolling, and the second rolling process is performed at the final stand of the cold rolling process. By doing this, it is possible to flatten the convex parts of the steel plate surface roughness, improve the sharpness of the image, and also make it possible to give deep roughness in four parts because it is possible to perform large reduction rolling. effective.

Next, the first rolling process is performed at the final stand of cold rolling, and the second rolling process is performed at the temper rolling stage. By doing this, surface scratches and shape defects that occur during the cold rolling-annealing process can be removed. There is an effect that 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.

Convex portions in the steel plate surface roughness must be flat or the convex 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- or more. The reason is that: Pi: If the peak height P from the central plane exceeds 0.20, the steel plate surface will be too rough, the specular reflectance of light will decrease, and the image clarity will deteriorate. In addition, if the center plane average valley height (Rv) is less than 0.50, the amount of steel plate wear powder flowing into the recess cannot be fully consumed during press working,
This is because it tends to cause galling due to seizure.

Note that the rolls used in cold rolling and temper rolling in the present invention are applicable to the present invention regardless of whether they are manufactured by shot blasting, electric discharge dulling, or laser dulling. be.

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.

<Example> Next, the present invention will be explained in more detail based on examples.

Example L: Plate thickness 3 manufactured under the conventional hot rolling conditions shown in Table 1
．． A 2o+m w4A-G hot-rolled steel sheet was pickled, and the roll surface roughness (Ra) and rolling reduction of the 4th and 5th stands of a 5-stand cold rolling mill or a temper rolling mill were determined as shown in Table 2. This was changed to a cold-rolled plate with a thickness of 0.8 mm.

Meanwhile, before temper rolling, steels A-F were subjected to continuous annealing to
Annealing was performed at 50° C. for 20 seconds, 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 mold galling test, the equipment shown in Figure 3 was used to pull out 100 mm of 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) O: No marking scratches △: Lightly scratched ×: Scratches such as peeling off sliding parts Table 1 shows the mechanical properties of each steel plate. Shown with ingredients. The tensile properties were determined using a JIS No. 5 test piece during skin pass rolling at a reduction rate of 0.8%. r value gives 15% tensile strain, 3
Measured by the point method, the average value r = (r L+ r (+ 2 rD ) / It was found in 4.

The limit drawing ratio (L, D, R,) is punch diameter = 33 mm
The maximum blank diameter Domax that can be deep drawn using the mold was determined from the ratio to the punch diameter ctp. That is, 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 w4A-G, the steel plate surface roughness (Ra) after the first rolling process is 1.0 or more,
Furthermore, the second rolling step reduction rate (Re) is 0.2 to 1.0.
%, the second rolling process roll surface roughness (Ra) is 1.2- or less, so that the steel plate convex flatness P is 0, P≦0620 or a flat surface, and the average valley height Rv below the center plane ) was 1.02 or more, it was possible to produce a cold-rolled steel sheet with good mold galling properties and sharpness.

On the other hand, if the steel plate surface roughness (Ra) after the first rolling process is less than 1.0- or the second rolling process roll surface roughness (
Steel plates with a small Ra) and a second rolling process reduction of less than 0.2% or more than 1.0% had good image clarity after painting, but were susceptible to mold galling. No way.

In addition, the sharpness after painting of a steel plate rolled with a roll having a surface roughness (Ra) of more than 1.2 was relatively poor, although it was related to the rolling reduction.

<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, the surface roughness of the steel sheet can be cold rolled or tempered using rolls with a small surface roughness. By manufacturing steel sheets after finishing temper rolling, we can suppress material deterioration after temper rolling, and at the same time improve mold galling during pressing and image clarity after painting, which were previously thought to have contradictory relationships. It became possible to achieve both.

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 surface roughness of the steel plate after cold rolling and the surface roughness of the temper rolling roll on the frictional resistance and DOI value of the steel plate. FIG. 2 is a diagram showing the relationship between the skin pass rolling roll surface roughness and the skin pass rolling reduction ratio on the frictional resistance and DOT value of a steel plate. FIG. 3 is an explanatory diagram of a sliding test on a steel plate. FIG. 4 is an explanatory diagram showing a method for measuring a DOI value representing image clarity. FIG. 5 is a drawing showing the relationship between center plane average valley height Rv) and convex flatness (P) on the sharpness (DOI value), 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. FIG, 1 4 Oka j anti-concave surface phase degree Ra () 1 m) i station of the trowel, get off at Sansho 05'/. FIG, 2 Pressure reduction + Re ('/-) FIG, 3 Pulling force 2F ↑ FIG, 4 FIG, 5 Convex part 乎 cu 7 degrees P

Claims (9)

[Claims] (1) The flatness P of the convex portion of the steel plate surface is 0≦P≦ as shown in the following formula
0.20 and center plane average valley height Rv≧1.0
A cold-rolled steel sheet with excellent press formability and sharpness after painting. @P@=(1/n)Σ＾n_i_=_1PiP=(1/
n) Σ＾n_i_=_1 [(｜P-Pi｜)/(@P@
)] Here, Pi: Height of the peak from the center plane (2) The hot-rolled steel sheet is cold-rolled or temper-rolled using a conventional method, and the flatness of the convex portions on the surface of the steel sheet, P, as shown by the following formula, is 0<P≦0.
20, and center plane average valley height Rv≧1.0μm
When manufacturing a cold-rolled steel sheet, first, the steel sheet surface roughness Ra≧1.0 μm is set in the first rolling, and then, in the second rolling,
Rolling is performed with a dull roll having a roll surface roughness Ra≦1.2 μm at a rolling reduction ratio (Re) of 0.2 to 1.0% and a roll surface roughness Ra≦2 (1.2-Re). A method for producing cold-rolled steel sheets that is characterized by excellent press formability and sharpness after painting. @P@=(1/n)Σ＾n_i_=_1PiP=(1/
n) Σ＾n_i_=_1 [(｜P-Pi｜)/(@P@
)] Here, Pi: Height of the peak from the center plane (3) 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 cold rolling stage. (4) Manufacturing 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 cold rolling stage and the second rolling is performed in a temper rolling stage. Method. (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 in a conventional manner so that the flatness P of the convex portions on the steel plate surface as shown by the following formula satisfies P=0, and the 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 ratio (Re) is set to 0.0 μm using a bright roll in the second rolling process. A method for producing a cold-rolled steel sheet with excellent press formability and post-painting sharpness, characterized by flattening the convex portions on the surface of the steel sheet by rolling at 2 to 1.0%. @P@=(1/n)Σ＾n_i_=_1PiP=(1/
n) Σ＾n_i_=_1 [(｜P-Pi｜)/(@P@
)] Here, Pi: Height of the peak from the center plane (7) 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 cold rolling stage. (8) Manufacturing 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. Method. (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.