JPH0833902A - Manufacture of steel plate with superior press-formability - Google Patents

Abstract

PURPOSE:To demonstrate efficiently neither too much nor too little a function as a lubricant reservoir for a recessed part which is transferred on the surface of a steel plate and to enable smooth press-forming with a low forming load without causing flaws such as necking due to insufficient lubricant. CONSTITUTION:The surface of a rolling work roll with an average roughness Ra of 0.8mum or less in the center line is irradiated by a high density energy; and a projected part is formed, which has a build-up height of 15 to 30mum and a build-up volume of 28000mum<3> or above, on the surface of the roll for 9 pieces or more per 1mm<2>. With the use of such work roll, a temper rolling is performed at a draft of 0.3% or higher, the projected part is transferred to the surface of a steel plate as a recessed part, and a lubricant reservoir is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet having an improved press formability by forming a myriad of concave portions uniformly distributed on the surface thereof. It is extremely useful as an outer panel material or a building material for household electric appliances.

[0002]

2. Description of the Related Art In recent years, steel sheets used as outer panel materials for automobiles, in addition to the social demand for weight reduction for the purpose of improving fuel efficiency and reducing exhaust gas, reduce material costs and reduce the number of parts. Various improvements have been pursued in order to simplify the manufacturing and pretreatment processes. As part of such research, as the demand for weight reduction of car bodies by thinning steel plates and simplification of molding and assembly by reducing the number of parts, etc., the materials are required to have even better workability. How to increase the elongation and Rankford value (r value) in mild steel sheets, and how to achieve both high tension and elongation in high-tensile steel sheets are becoming major issues in the future.

By the way, in press forming, in addition to the formability of the material itself, lubrication of the steel sheet surface during forming is extremely important. The dull finish is processed. This is because innumerable recesses are formed on the surface of the steel sheet and the lubricating oil is stored in the recesses to reduce friction between the die and the steel sheet during press forming and prevent seizure. Generally adopted as this method is a temper rolling process for finally adjusting the surface roughness of the steel sheet,
This is a method of using a rolling roll whose surface has been dull-finished by shot blasting, electric discharge machining, laser treatment, or the like, and transferring the unevenness of the roll surface to the steel plate surface.

Among them, the method of temper rolling using a roll whose surface is dull-finished by irradiation with high-density energy such as a laser beam makes it easy to control the concave-convex pattern transferred to the surface of the steel sheet and obtains stable press formability. It is considered to be an extremely effective method because it is easy to form regular unevenness with good paint clarity, and various improvements have been studied.

For example, JP-A-62-168602 and JP-A-6-168602.
In JP-A 2-230402, when the surface of a temper rolling roll is dull-finished by laser processing, the pattern of surface irregularities given by the finishing treatment is controlled to provide excellent press formability and sharpness after painting. It discloses that a steel sheet obtained from Also, JP-A-63-1216
No. 36 and No. 64-2705, the surface roughness pattern and the surface average roughness are appropriately controlled. Further, in JP-A-63-303011 and JP-A-2-179302, the area of the recess is It has been clarified that a steel sheet excellent in press formability and sharpness can be manufactured by appropriately controlling the average depth and the pattern of the uneven portion.

However, as the processing conditions become more and more severe due to the thinning and integral forming of the steel sheet as described above, satisfactory results cannot be obtained with the conventional dull finishing method as described above. . That is, when deep drawing is performed on a mild steel plate by press forming, a large surface pressure acts between the press die and the steel plate, and the surface pressure causes the unevenness of the steel plate surface to be crushed and the lubricating oil may remain. It is inevitable that the deep drawability will decrease due to the increase in frictional resistance, the occurrence of mold galling due to necking and seizure, etc., and that in the press forming of high-strength steel sheets, the decrease in material elongation due to the increase in tensile strength cannot be avoided. Therefore, improvement in workability cannot be expected.

Therefore, in order to improve the workability, it is necessary to improve the lubrication performance, but in the method of increasing the lubricity by increasing the viscosity of the lubricating oil, the deterioration of the cleaning property in the subsequent process becomes a big problem, In addition, the large amount of lubricant applied not only wastefully consumes the lubricant but also adversely affects the cleaning efficiency in the subsequent process. In press forming of a steel sheet, the amount of lubricant that can be retained on the surface of the steel sheet in the press forming process seems to be deeply involved in the press formability, but as described above, in the steel sheet rolled using the conventional dull finishing roll, There is no idea that the area and depth of the recesses formed on the surface of the steel sheet and the concavo-convex pattern are only specified, and the volume of the recesses, which is considered to be an important requirement for the lubricating oil sump, is also strictly specified. Therefore, the function as an oil sump varies, and in fact, stable press formability is not obtained.

[0008]

As described above, in the conventional steel sheet for press forming, the volume of the recess formed by the dull finish is not properly controlled, and therefore the function as the oil sump is insufficient. As a result, there is often experienced a problem that excess or deficiency of the lubricating oil occurs, causing increase in frictional resistance due to running out of the lubricating oil, deterioration of deep drawability, and defective molding such as generation of galling due to necking or seizure. The present invention has been made by paying attention to such drawbacks of the prior art, and an object thereof is to provide a lubricating oil existing between a die and a steel plate to be processed in press forming using a lubricating oil. It is intended to establish a manufacturing method of a steel sheet that can stably exhibit excellent press formability without causing excess or deficiency.

[0009]

The constitution of the steel sheet excellent in press formability according to the present invention, which has been able to solve the above-mentioned problems, has unevenness formed on the surface of a work roll for rolling on one side or both sides of the steel sheet. Upon transfer, by irradiating the surface of the work roll for rolling having a center line average roughness Ra of 0.8 μm or less with high-density energy, a swelling height: 15 to 30 μm, a swelling volume: 28,000
Use a work roll for rolling that has 9 or more μm ³ protrusions per 1 mm ² on the roll surface.
The above is the main point of temper rolling.

[0010]

As described above, according to the present invention, by irradiating the surface of the work roll for rolling having a small center line average roughness with high density energy in a pulsed manner, the convex portion having the specified height and volume can be specified. Using a work roll for rolling formed at a density or more, by temper rolling at a specific reduction rate or more, to transfer the convex portion of the surface of the rolling roll to the surface of the steel sheet,
The reason for defining the respective constitutional requirements is to increase the press formability by forming an appropriate concave portion as a lubricating oil reservoir.

First, the center line average roughness R of the rolling roll material
The value of a is set to 0.8 μm or less because when a base roll having a surface roughness exceeding this is used, the convex portions formed by the high-density energy irradiation are hidden by the irregularities of the base, and the uniform rising height is increased. It becomes impossible to form a convex portion of the depth, and as a result the convex portion becomes uneven and the concave portion transferred to the steel plate becomes extremely uneven, and it is not possible to secure a uniform volume as the lubricating oil sump portion, so it is stable. Lubrication performance cannot be exhibited.

Next, the height of the protrusions formed on the roll surface is set to be in the range of 15 to 30 μm in order to make the recesses transferred to the steel plate have a proper depth as the lubricating oil reservoir. When the thickness is less than 15 μm, the recesses transferred to the steel sheet are insufficient in depth and the function as a lubricating oil reservoir cannot be fully exerted, and sufficient press formability cannot be obtained due to insufficient lubrication. On the other hand, when the height of the bulge exceeds 30 μm, the bulge is significantly damaged during temper rolling, and the depth of the recesses transferred to the steel sheet becomes uneven, so that a stable lubricating effect cannot be obtained. . A more preferable rising height is in the range of 20 to 27 μm in the sense that a recess having a uniform and sufficient depth is transferred to the surface of the steel sheet.
The planar shape of the convex portion can be arbitrarily changed according to the irradiation method of high-density energy and irradiation conditions, and may be any shape such as a ring shape, a circular shape, a semicircular shape, or a polygonal shape. .

By the way, according to the present invention, the convex portion is transferred as a concave portion to the surface of the steel sheet by temper rolling using the rolling roll having the convex portion formed on the surface as described above. As a method of forming the convex portion, for example, a method of irradiating the roll surface with high-density energy such as a laser pulse to instantaneously melt the metal on the roll surface to form a raised portion on the peripheral side and cooling and solidification are Adopted. The shape of the convex portion formed by such a method determines the shape of the concave portion transferred to the surface of the steel sheet, but as described above, it is cooled and solidified in the state of melting and rising due to high-density energy irradiation. Since the upper surface of the convex portion formed in this way is not flat and the molten metal does not rise uniformly from the irradiation center in all directions, the height and shape of the convex portion are different from those of the molten metal during high-density energy irradiation. The height changes significantly depending on the rising direction, and a convex portion having a uniform height and shape is not always formed. As a result, the depth and shape of the recess formed on the surface of the steel sheet due to the transfer of the projection also become non-uniform, and the amount of lubricating oil retained in the recess changes accordingly.

Therefore, in order to secure a more suitable amount of lubricating oil, it is not enough to define the area and depth of the recess, and in order to ensure the function of the oil reservoir, It is necessary to properly control the volume of each recess. From this point of view, in the present invention, not only the rising height of the convex portion formed on the roll surface but also the volume of each convex portion is defined, and thereby the volume of the concave portion transferred to the steel sheet as a lubricating oil reservoir is appropriately adjusted. However, as a requirement for forming a concave portion having a sufficient volume as the lubricating oil reservoir, it is necessary to set the volume per convex portion formed on the roll surface to 28,000 μm ³ or more. However, if the volume of the convex portion is less than 28,000 μm ³ , the concave portion having a sufficient volume cannot be formed on the surface of the steel sheet, and the problem of defective press molding due to insufficient lubricating oil cannot be avoided.
Although the upper limit of the volume of the convex portion is not particularly specified, if the convex portion becomes too large, the concave portion formed on the surface of the steel sheet due to transfer becomes excessively large, and the lubricating oil retention amount increases more than necessary, and lubrication is increased. In addition to wasteful consumption of oil, consumption of rust preventive oil often applied for rust prevention after press molding also increases, which is disadvantageous in terms of cost. Therefore, the volume of the convex portion is preferable. stops below about 50000 ^3, it is preferable to like can be transferred shaping recess that can secure necessary and sufficient lubricating oil retention amount.

The cross-sectional shape of the convex portion is not particularly limited, and may have any shape such as an inverted V shape, an inverted U shape, a trapezoidal shape, and an arc shape, but the number thereof is not limited. , 1m of roll surface
It is an essential requirement to form at a density of 9 or more per m ² . However, if the number of convex portions is insufficient, the number of concave portions transferred to the steel plate will naturally be small, and the area of the parallel portion that does not have the lubricating oil retaining effect will be relatively large with respect to the concave portion that constitutes the lubricating oil sump. Therefore, it becomes impossible to avoid defective molding due to lack of lubricating oil, such as deterioration of deep drawability due to increase in frictional resistance during press molding, necking, and mold scuffing due to seizure.

As a method for forming the convex portions having the above-mentioned size, shape and number on the roll surface, a high density energy beam such as a laser or an electron beam is regularly irradiated on the roll surface in a pulse shape. By adjusting the irradiation conditions at this time (irradiation energy and degree of convergence, etc.) and pulse intervals, the cooling rate after irradiation, the rotation cycle of the non-processing roll, and the like, the height of the convex portions formed on the roll surface and volume,
The shape, number, etc. can be adjusted arbitrarily.

In order to efficiently transfer the convex portion formed on the rolling roll as described above to one side or both sides of the steel sheet, it is necessary to set the reduction rate during temper rolling to 0.3% or more. . However, if the rolling reduction is less than the above value, the convex portion of the proper height and volume formed on the roll surface at an appropriate angle is not accurately transferred to the steel plate surface, and the depth and volume of the concave portion as the lubricating oil sump are insufficient. This is because a steel sheet having satisfactory press formability cannot be obtained. The upper limit of the rolling reduction is not particularly specified, but if the rolling reduction becomes too large, the steel plate becomes hardened due to work hardening of the steel plate at the time of transfer, and the press formability is adversely affected by the increase of the steel plate height. It is desirable to keep the rolling reduction to about 5% or less. From the meaning of accurately transferring the convex portion of the roll surface to the steel sheet surface while suppressing the work hardening of the steel sheet to a minimum, a more preferable reduction ratio is in the range of 0.5 to 1.2%.

As described above, in the present invention, a work roll for rolling is used in which the surface of a roll having a small center line average roughness is irradiated with high density energy to form protrusions having an appropriate height and volume at an appropriate density. Then, this work roll is used to temper-roll the steel sheet at an appropriate reduction ratio, thereby transferring the above-mentioned convex portions to the steel sheet surface to form concave portions, so that there is no excess or deficiency of lubricating oil retention in the press forming process. A steel sheet with excellent press formability that can guarantee the amount and does not cause forming defects such as increased frictional resistance due to running out of lubricating oil, reduced deep drawability, and die galling due to necking or seizure. Has been successfully obtained.

Further, according to the present invention, the height, volume and number of the convex portions formed on the surface of the work roll are specified as described above, and the steel sheet is temper-rolled at a predetermined rolling reduction using the roll. Since the above-mentioned convex portion is successfully transferred to the surface of the steel plate to form a concave portion having an appropriate depth and volume as a lubricating oil sump, there is no limitation on the steel plate material itself, and the most general The present invention can be applied not only to typical cold-rolled steel sheets, but also to hot-rolled steel sheets or various surface-treated steel sheets obtained by subjecting these to surface treatment such as plating.

[0020]

EXAMPLES Next, examples of the present invention will be shown, but the present invention is not limited by the following examples, and may be carried out with appropriate modifications within a range compatible with the gist of the preceding and following description. Of course, it is possible, and all of them are included in the technical scope of the present invention.

EXAMPLE Rolling rolls having various convex portions formed on the surface were prepared by the laser pulse method, and cold-rolled steel sheets (SPC) were formed by using the rolls.
By subjecting material D) to temper rolling, the convex portions were transferred to the surface of the steel sheet to form concave portions. Using each of the obtained steel plates, the forming load was examined in order to compare the sliding characteristics during press forming, and the presence or absence of necking was also examined.

The molding load was measured with a width of 40 mm and a length of 2
Approximately 5 g of lubricating oil for press (“Knoxlast 530” manufactured by Parker Co.) on a cold rolled steel plate 1 having a thickness of 50 mm and a thickness of 0.8 mm
m ² is applied, and this is set using a beaded die 2 as shown in FIG. 1 and a wrinkle presser 3, and a wrinkle presser pressure of 400 k
The punch 4 is pushed up under gf / cm ² and the height is 65 mm.
It was squeezed until it became, and the molding load and the presence or absence of necking at this time were examined.

Table 1 shows in detail the dimension and shape of the convex portion formed on the surface of each temper rolling roll by using a laser microscope, and the temper rolling is performed at a predetermined reduction ratio using each roll. It shows the results of examining the forming load and the presence or absence of necking when the steel sheet thus obtained was subjected to the press forming test. 2 (A), (B) and (C) are the same as those shown in Table 1. 3, 10,
13 is a schematic enlarged view showing the cross-sectional shape of the convex portion formed on the surface of the rolling roll used in 12. Furthermore, FIG.
6 is a graph showing the effects of the height and volume of the convex portions on the press formability (presence / absence of necking) of the steel sheet, arranged from the results of Table 1.

[0024]

[Table 1]

From Table 1 and FIGS. 2 and 3, the following can be considered. No. Examples 1 to 7 are Nos. 1 to 7 satisfying all the requirements of the invention. Nos. 8 to 17 are comparative examples lacking any of the specified requirements. In each of the comparative examples, the molding load during press molding exceeds 1750 kgf, and necking is recognized in all cases, whereas in the examples, comparison is made. Compared to the examples, the molding load during press molding was small and necking did not occur.

2 (A) (roll No. 3) and FIG. 2 (B) (roll No. 10), the volumes V _a and V _b of both are almost the same, but the former is While the height h _{a of the} raised portion (convex portion) 5 is high and the skirt is narrow, the latter has a shape in which the height h _b of the raised portion 5 is low and the skirt is wide. (C) (No.
2 (A), the height h _c of the raised portion 5 is almost the same as that of FIG. 2 (A), but its volume V _c is small. As is clear from the comparison of the forming load and the presence or absence of necking of the steel sheet temper-rolled using these rolls in Table 1,
It can be seen that, in order to achieve the object of the present invention, the raised portion 5 formed on the roll surface must have an appropriate height h and volume V.

Further, FIG. 3 shows from the results of Table 1 that the height h and the volume V of the raised portion 5 formed on the surface of the temper rolling roll are V.
Is a graph showing the effects on press formability (presence or absence of necking) of the obtained rolled steel sheet, where the height h of the raised portion 5 is 15 μm or more and the volume V is 2800.
Steel sheets that have been temper-rolled using rolling rolls of 0 μm ³ or more (all have a rolling reduction of 0.3% or more) have no necking and have excellent press formability. I understand.

[0028]

EFFECTS OF THE INVENTION The present invention is configured as described above, and by forming a raised portion having an appropriate height and volume on the surface of a temper rolling roll and performing temper rolling at an appropriate reduction ratio,
It is possible to effectively exert the function of the recessed part transferred to the steel sheet surface as a lubricating oil reservoir part without excess or deficiency, and to smoothly perform press forming with a low forming load without causing defects such as necking due to insufficient lubrication. It is now possible to provide a steel material that can be manufactured.

[Brief description of drawings]

FIG. 1 is an explanatory cross-sectional view of a main part showing a method for measuring a molding load, which was performed to examine press moldability.

FIG. 2 is an enlarged schematic view showing the shape of a raised portion (convex portion) on the surface of a temper rolling roll used in an experiment.

FIG. 3 is a graph summarizing the effect of the height h and volume V of the raised portion 5 formed on the surface of the temper rolling roll on the press formability (presence or absence of necking) of the obtained rolled steel sheet. .

[Explanation of symbols]

 1 Specimen steel plate 2 Die 3 Wrinkle retainer 4 Punch 5 Raised part (convex part)

Claims (1)

[Claims] 1. When transferring unevenness formed on the surface of a work roll for rolling to one side or both sides of a steel plate, high density energy is applied to the surface of the work roll for rolling having a center line average roughness Ra of 0.8 μm or less. By irradiating the surface of the roll with a projection height of 15 to ³⁰ μm and a projection volume of 28000 μm ³ or more, 1 mm ² of the roll surface
Use the work rolls for rolling formed more than 9 per
A method for producing a steel sheet excellent in press formability, which comprises temper rolling at a rolling reduction of 0.3% or more.