JP7550587B2 - Ferritic stainless steel sheet forming method and ferritic stainless steel sheet - Google Patents

Description

The present invention relates to a method for forming a ferritic stainless steel sheet and a ferritic stainless steel sheet.

Ferritic stainless steel sheets are used in a wide range of fields, including home appliances, kitchen appliances, and electronic devices, but their formability is inferior to that of austenitic stainless steels, and their applications have sometimes been limited. However, in recent years, improvements in refining technology have made it possible to produce ferritic stainless steels with extremely low carbon and nitrogen, and furthermore, attempts have been made to improve the formability and corrosion resistance of ferritic stainless steels by adding elements such as Ti and Nb. In addition, there have been attempts to improve the formability of ferritic stainless steels by controlling the component composition and manufacturing method, as in Patent Documents 1 to 3, etc.

These conventional improvement technologies have improved formability, and ferritic stainless steels are now being used in a wide range of applications, but in recent years, there has been an increasing demand for lighter end products, which has led to demands for further improvements. In other words, to reduce the weight of end products, there is a demand for ferritic stainless steels that offer higher formability at thinner plate thicknesses than before. In particular, there is a demand for improvements in formability, such as stretchability, which makes it easier to obtain flat sections and product depth, and thus easier to create designs.

Stretching is a processing method in which the material is not allowed to flow into the die, but is instead shaped by plastic deformation, primarily elongation of the part in contact with the punch. The deformation area is the region from the die shoulder to the punch head. In the case of rectangular tube forming using a die, the contact load between the material and punch is generally greatest around the punch shoulder, restricting the movement of the material, so deformation is concentrated between the punch shoulder and die shoulder, resulting in the greatest reduction in plate thickness. When necking occurs in this area, the material breaks.

One application in which components formed by this stretch forming are particularly needed is the exterior panels of home appliances and kitchens. Traditionally, such exterior panels were made of painted ordinary steel, but this had the problem of rust forming where the paint had peeled off or at the edges. On the other hand, due to the improved formability of stainless steel and the improved design of clear coatings that give a luxurious appearance, stainless steel has come to be used more frequently in recent years as the material for exterior panels. In particular, because exterior panels form the appearance of a product, improved dimensional accuracy is required. For this reason, processing that uses stretch forming is being used more and more.

As mentioned above, there is a demand for lighter weight in products such as home appliances and kitchen equipment, and there is also a strong demand for lighter weight exterior panels. It is believed that this weight reduction can be achieved by applying stainless steel sheets with a thickness of 0.4 mm to less than 0.8 mm, which is thinner than conventional sheets. However, there have been no ferritic stainless steel sheets that can meet the required stretching workability at such a thickness, including those in the above-mentioned Patent Documents 1 to 3.

Patent Document 4 describes a ferritic stainless steel sheet for exterior panels that has a specified chemical composition, a thickness of 0.4 to 0.8 mm, a forming speed of 3 to 10 mm/min, and an overhang height of 10 mm or more when subjected to an Erichsen test. However, in Patent Document 4, the forming speed is limited to 10 mm/min or less, and there is a certain limit to the time required for forming the formed product. Therefore, there is a demand for further improvement in the productivity of formed products.

Japanese Patent Application Laid-Open No. 57-198248 Japanese Unexamined Patent Publication No. 58-61258 JP 2004-217996 A Patent No. 6050701

The present invention was made in consideration of the above circumstances, and aims to provide a method for forming ferritic stainless steel sheets that is excellent for producing rectangular tube products, and a ferritic stainless steel sheet that is suitable for forming rectangular tubes.

In order to solve the above problems, the present invention has the following configuration.
[1] A forming method for square cup drawing of a ferritic stainless steel sheet to a target forming depth by press forming using a punch and a die, comprising:
The ferritic stainless steel sheet contains, by mass%, C: 0.0200% or less, Si: 0.70% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Cr: 11.0 to 19.5%, N: 0.020% or less, Al: 0.005 to 0.100%, O: 0.0050% or less, Ti: 0.03 to 0.20%, Nb: 0.010 to 0.300%, Sn: 0.001 to 0.300%, Zr: 0.001 to 0.080%, with the balance being iron and impurities, and has a component composition that satisfies the following formula (1), and has a sheet thickness of 1.0 mm or less,
The average relative speed _V1 between the punch and the die from the forming start position to a 1/4 position of the target forming depth is set to 150 mm/min or less;
The average relative speed _V2 between the punch and the die from the 1/4 position to the 4/5 position of the target forming depth is set to be greater than the average relative speed _V1 and equal to or less than 250 mm/min;
a maximum relative speed _V3 of the punch and the die from a 4/5 position of the target forming depth to the target forming depth being 120 mm/min or less.
(Al×0.4+Zr×0.5+Ti×0.1)/O≧12…(1)
Here, each element symbol in formula (1) represents the content (mass %) of each element in the ferritic stainless steel sheet.
[2] The method for forming a ferritic stainless steel sheet according to [1], wherein the average relative speed _V1 is 50 to 150 mm/min.
[3] The method for forming a ferritic stainless steel sheet according to [1] or [2], characterized in that the ferritic stainless steel sheet contains, in mass%, one or more of Mo: 0.05 to 0.50%, Ni: 0.05 to 0.50%, and Cu: 0.01 to 1.00% in place of a portion of Fe.
[4] The method for forming a ferritic stainless steel sheet according to any one of [1] to [3], characterized in that the ferritic stainless steel sheet contains, in mass%, one or more of B: 0.0003 to 0.0050%, Ga: 0.0001 to 0.2%, and W: 0.001 to 0.300% in place of a portion of Fe.
[5] A method for forming a ferritic stainless steel sheet according to any one of [1] to [4], characterized in that the average Lankford value of the ferritic stainless steel sheet is 1.7 or more, and the in-plane anisotropy (Δr) of the Lankford value is 0.7 or less.
[6] A ferritic stainless steel sheet comprising, by mass%, C: 0.0200% or less, Si: 0.70% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Cr: 11.0 to 19.5%, N: 0.020% or less, Al: 0.005 to 0.100%, O: 0.0050% or less, Ti: 0.03 to 0.20%, Nb: 0.010 to 0.300%, Sn: 0.001 to 0.300%, Zr: 0.001 to 0.080%, with the balance being iron and impurities, and having a component composition that satisfies the following formula (2):
(Al×0.4+Zr×0.5+Ti×0.1)/O≧12…(2)
Here, each element symbol in formula (2) represents the content (mass %) of each element in the ferritic stainless steel sheet.
[7] The ferritic stainless steel sheet according to [6], characterized in that, instead of a portion of Fe, it contains, by mass%, one or more of Mo: 0.05 to 0.50%, Ni: 0.05 to 0.50%, and Cu: 0.01 to 1.00%.
[8] The ferritic stainless steel sheet according to [6] or [7], characterized in that, instead of a portion of Fe, one or more of the following elements are contained, by mass%, B: 0.0003 to 0.0050%, Ga: 0.0001 to 0.2%, and W: 0.001 to 0.300%.
[9] A ferritic stainless steel sheet according to any one of [6] to [8], characterized in that the average Lankford value is 1.7 or more and the in-plane anisotropy (Δr) of the Lankford value is 0.7 or less.

According to the present invention, a method for forming ferritic stainless steel sheets that is excellent for producing rectangular tube molded products and a ferritic stainless steel sheet suitable for rectangular tube molding can be provided. In particular, according to the present invention, when manufacturing thin-walled molded products required as parts for reducing the weight of home appliances and kitchen equipment, a ferritic stainless steel sheet with a good appearance can be used as a blank, and since the molding process is highly productive and has good stretchability, a molded product that satisfies dimensional accuracy and design can be obtained.

FIG. 1 is a diagram showing the effect of shortening the molding time according to the present invention. 1A to 1C are process diagrams illustrating a molding method according to an embodiment of the present invention. 1A to 1C are process diagrams illustrating a molding method according to an embodiment of the present invention. 3 is a graph illustrating a molding method according to an embodiment of the present invention.

The inventors of the present invention have conducted extensive research into how to improve the productivity of ferritic stainless steel sheets, which are generally considered to have lower formability than austenitic stainless steel sheets, when forming them into square tubes, and to obtain formed products with excellent design without causing cracks or forming defects.

In general, the lower the yield ratio of the steel sheet to be formed, the better the workability. The yield ratio is the ratio of the yield stress to the tensile strength. The lower the yield ratio, the wider the load range in which the uniform elongation region is obtained, making it easier to perform plastic processing. On the other hand, in the case of forming, such as square tube forming, which combines deep drawing and bulging deformation, the thickness distribution of the steel sheet during forming becomes large, which promotes the reduction in thickness and makes it easier for cracks to occur from the inclusions present. In order to solve the above problem, the inventors investigated the relationship between the type and content of each element contained in ferritic stainless steel and the square tube formability. Square tube forming is a difficult forming method in which drawing deformation and bulging deformation coexist, and suppressing the reduction in thickness at the stress concentration area is effective in improving formability. If the thickness is reduced, the workability decreases due to work hardening. When the fracture surface of a formed product in which cracks occurred during forming was observed in detail, a large amount of Al oxide and TiN was confirmed. Assuming that these Al oxides and TiN promote cracking, the researchers investigated steel compositions that would suppress uneven distribution of Al oxides and growth of TiN. They then discovered that by using steel sheets with specific steel compositions for blanks for square tube forming, it is possible to achieve square tube formability that satisfies dimensional accuracy and design without reducing productivity.

The inventors also used a ferritic stainless steel sheet with a thickness of 0.8 mm and a composition (by mass%) of 0.007C-0.26Si-0.19Mn-16.7Cr-0.26Nb-0.09Ti-0.26Sn-0.004Zr-0.053Al-0.0007O to investigate the relationship between the processing conditions and the stretchability of the sheet, assuming application to various presses. Specifically, the stroke section of the die from the forming start position to the final forming depth position was divided into three sections, and the effect on the stretchability of changing the forming speed for each divided section was investigated. The results are shown in Tables 1A, 1B, and 1C.

Table 1A shows the results of an investigation into the occurrence of cracks when the forming speed was changed in the first of three divided stroke sections of the die.
Table 1B shows the results of an investigation into whether or not cracks occurred when the forming speed was changed in the second divided section, in which the first of the three stroke sections of the mold was set to extend from the forming start position to 1/4 of the final forming depth and the forming speed in this section was set to 150 mm/min.
Furthermore, Table 1C shows the results of an investigation into whether or not cracks occurred when the forming speed was changed in the third (last) divided section, where the first of the three divided sections of the stroke section of the mold was from the starting position of forming to 1/4 of the final forming depth, with a forming speed of 150 mm/min, and the second divided section was from 1/4 to 4/5 of the final forming depth, with a forming speed of 250 mm/min.

As shown in Tables 1A to 1C, there is a clear tendency for defective shapes and cracks to occur when the forming speed is increased. In addition, as shown in Tables 1A to 1C, it was discovered that by optimizing the forming speed for each divided section, it is possible to form at a faster forming speed than conventional forming speeds, and form in a shorter time.

Figure 1 shows the time required for forming a square tube of a ferritic stainless steel sheet having the steel composition of the present invention and a thickness of 0.6 mm with a target forming depth of 50 mm, when the forming speed is constant at 80 mm/min and when the stroke section is divided into three and the forming speed is optimized for each divided section. In the latter case, the time required for forming is halved compared to the former case, making it clear that it is possible to reduce the time while maintaining formability. Note that when the stroke section is divided into three, the forming speed is 150 mm/min in the first process (from the forming start position to 1/4 of the final forming depth), 250 mm/min in the second process (from 1/4 to 4/5 of the final forming depth), and 120 mm/min in the third process (from 4/5 of the final forming depth to the final forming depth position).

The present invention was made based on the above findings. Below, we will explain the method for forming a ferritic stainless steel sheet and the ferritic stainless steel sheet suitable for forming a square tube, which are embodiments of the present invention.

The method for forming a ferritic stainless steel sheet according to the present embodiment is a forming method for square-cup-drawing a ferritic stainless steel sheet to a target forming depth by press forming using a punch and a die, in which the ferritic stainless steel sheet contains, in mass%, C: 0.0200% or less, Si: 0.70% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Cr: 11.0 to 19.5%, N: 0.020% or less, The forming method includes: Al: 0.005 to 0.100%, O: 0.0050% or less, Ti: 0.03 to 0.20%, Nb: 0.010 to 0.300%, Sn: 0.001 to 0.300%, Zr: 0.001 to 0.080%, and the balance being iron and impurities; the composition satisfies the following formula (1); and the plate thickness is 1.0 mm or less; the average relative speed _V1 between the punch and the die from the forming start position to a ¼ position of the target forming depth is 150 mm/min or less; the average relative speed _V2 between the punch and the die from the ¼ position to the 4/5 position of the target forming depth is greater than the average relative speed _V1 and is 250 mm/min or less; and the maximum relative speed _V3 between the punch and the die from the 4/5 position of the target forming depth to the target forming depth is 120 mm/min or less.
(Al×0.4+Zr×0.5+Ti×0.1)/O≧12…(1)
Here, each element symbol in formula (1) represents the content (mass %) of each element in the ferritic stainless steel sheet.
In the method for forming a ferritic stainless steel sheet according to this embodiment, the average relative speed _V1 is preferably set to 50 to 150 mm/min.
In addition, the ferritic stainless steel sheet may contain, in mass percent, one or more of Mo: 0.05 to 0.50%, Ni: 0.05 to 0.50%, and Cu: 0.01 to 1.00%, instead of a portion of Fe.
Furthermore, the ferritic stainless steel sheet may contain, in mass percent, one or more of B: 0.0003 to 0.0050%, Ga: 0.0001 to 0.2%, and W: 0.001 to 0.300%, in place of a portion of Fe.

First, we will explain the ferritic stainless steel sheet that is the blank in the forming method of this embodiment. We will explain the reasons for limiting the chemical composition of the ferritic stainless steel sheet. Note that unless otherwise noted, % which is the unit of component content means % by mass.

C: 0.0200% or less C deteriorates formability and corrosion resistance, so the lower the content, the better, with the upper limit set to 0.0200% or less. However, excessive reduction in the C content leads to increased refining costs, so it is desirable to set the lower limit of the C content to 0.0010% or more. The preferred C content is 0.0030 to 0.0070%.

Si: 0.70% or less Although Si may be contained as a deoxidizing element, it is a solid solution strengthening element, so from the viewpoint of lowering the yield stress, the lower the content, the better, and the upper limit is set to 0.70% or less. However, since an excessive reduction in the Si content leads to an increase in refining costs, it is desirable to set the lower limit to 0.01% or more. The preferred Si content is 0.05 to 0.50%.

Mn: 1.00% or less Like Si, Mn is a solid solution strengthening element, so from the viewpoint of reducing yield stress, the lower the content, the better, and the upper limit is set to 1.00% or less. However, since an excessive reduction in the Mn content leads to an increase in refining costs, it is desirable to set the lower limit to 0.01% or more. The preferred Mn content is 0.05 to 0.50%.

P: 0.030% or less P is an element that is inevitably mixed in from the raw materials, and is also a solid solution strengthening element like Si and Mn, so its content should be small, and from the viewpoint of elongation, the upper limit is set to 0.030% or less. However, since an excessive reduction in the P content leads to an increase in refining costs, the lower limit may be set to 0.010% or more.

S: 0.005% _or less In the case of Ti-added steel, _{S forms Ti4C2S2 together} with Ti and C, and has the effect of fixing C. _{Ti4C2S2 is} a coarse precipitate _that precipitates at high temperatures, so it has little effect on recrystallization and grain growth behavior, but if this precipitate precipitates in large quantities, it becomes the starting point of rusting and deteriorates corrosion resistance. Therefore, the upper limit of S is set to 0.005% or less. However, since excessive reduction of the S content leads to an increase in refining costs, the lower limit of the S content may be set to 0.0001% or more.

Cr: 11.0-19.5%
A Cr content of 11.0% or more is necessary to improve corrosion resistance, but excessive content deteriorates toughness, worsens manufacturability, and increases yield stress. Therefore, the upper limit of Cr content is 19. The Cr content is preferably 13.0 to 17.5%.

N: 0.020% or less Like C, N deteriorates formability and corrosion resistance, so its content should be small, with the upper limit set to 0.020% or less. However, from the viewpoint of reducing production costs, the lower limit may be set to 0.003% or more. The preferred N content is 0.005 to 0.015%.

Al: 0.005-0.100%
Al may be contained in an amount of 0.005% or more as a deoxidizing element. On the other hand, an excessive content of Al reduces formability and weldability, and may also cause deterioration of surface quality, so the upper limit is set to 0.005%. The Al content is preferably 0.010 to 0.080%.

O: 0.0050% or less O reduces corrosion resistance and workability. Therefore, it is necessary to keep the O content low, and the upper limit is set to 0.0050% or less. However, excessive reduction of the O content increases refining costs, so the lower limit of the O content may be set to 0.0001% or more. The preferred range of the O content is 0.0005 to 0.0030%.

Ti: 0.03~0.20%
Ti combines with C, N, and S to form inclusions, and has the effect of improving corrosion resistance, intergranular corrosion resistance, and deep drawability, so the content is set to 0.03% or more. Since Ti is a solution strengthening element, excessive Ti content leads to an increase in solute Ti, which leads to a decrease in elongation, which is an index of stretchability. Therefore, the upper limit of Ti is set to 0.20%. The preferred Ti content is , 0.08 to 0.12%.

Nb: 0.010-0.300%
Nb is an element that improves formability and corrosion resistance, and its effect is manifested by including 0.010% or more. However, an excessive content leads to a decrease in ductility due to solid solution strengthening, so 0.010% or more is preferable. The Nb content is preferably 0.100 to 0.200%.

Sn: 0.001-0.300%
The inclusion of Sn has the effect of lowering the yield ratio and improving the stretch formability. In order to obtain this effect, the Sn content is set to 0.001% or more. On the other hand, excessive Sn content reduces manufacturability. The upper limit of the Sn content is set to 0.300%. The preferred Sn content is 0.020 to 0.200%.

Zr: 0.001-0.080%
Zr is contained in an amount of 0.001% or more as a deoxidizing element. On the other hand, an excessive content of Zr deteriorates formability, weldability and surface quality, so the upper limit is set to 0.080% or less. The amount is 0.002 to 0.020%.

(Al×0.4+Zr×0.5+Ti×0.1)/O≧12
When the fracture surface of the square molded product where the cracks occurred was observed in detail, aluminum oxides present at the bottom of many dimples and lumpy TiN present unrelated to the dimples were observed, which were the starting points of the cracks. In order to suppress the uneven distribution of Al oxides and the growth of TiN, it is effective to modify the composition of the inclusions, and the use of Zr, a strong deoxidizer, is effective. As a result of closely examining the relationship between the contents of Al, Ti, O and Zr and the occurrence of cracks, it was found that excellent properties were obtained when (Al×0.4+Zr×0.5+Ti×0.1)/O was 12 or more. Therefore, it is preferable that the ferritic stainless steel sheet according to the present embodiment satisfies (Al×0.4+Zr×0.5+Ti×0.1)/O≧12. .

Furthermore, the ferritic stainless steel sheet according to this embodiment may contain, in mass %, one or more of the following elements in place of a portion of Fe: Mo: 0.05-0.50%, Ni: 0.05-0.50%, and Cu: 0.01-1.00%.

Mo, Ni, and Cu are elements that improve corrosion resistance, and in applications requiring corrosion resistance, one or more of them may be included as necessary. Corrosion resistance is improved by including 0.05% or more of Mo and Ni. Excessive inclusion of Mo and Ni leads to hardening and deterioration of formability, so the upper limit of each is set to 0.50% or less. The preferred Mo and Ni amounts are 0.10 to 0.30%. Cu exerts its effect by including 0.01% or more, but excessive Cu content leads to deterioration of formability, especially ductility, so the upper limit of Cu content is set to 1.00% or less. The preferred Cu content is 0.30 to 0.80%.

Furthermore, the ferritic stainless steel sheet according to this embodiment may contain, in mass %, one or more of the following elements in place of a portion of Fe: B: 0.0003-0.0050%, Ga: 0.0001-0.2%, and W: 0.001-0.300%.

B is an element that improves secondary workability, and 0.0003% or more of B may be contained as necessary. However, since excessive B content reduces elongation, the upper limit of B content is set to 0.0050% or less. The preferred B content is 0.0010 to 0.0020%.

Ga is an element that forms GaS and improves corrosion resistance. It is a very effective element because it can suppress the precipitation of MnS and eliminate the starting point of rust. Since no effect is observed at a content of less than 0.0001%, if Ga is to be contained, it is recommended that the content be 0.0001% or more. On the other hand, excessive Ga content leads to solid solution hardening. Therefore, the upper limit of the Ga content is set to 0.2% or less.

Like Nb and Ti, W is an element that fixes C and N, prevents sensitization caused by Cr carbonitrides, and improves corrosion resistance. To achieve this effect, the W content should be 0.001% or more. On the other hand, if the W content exceeds 0.300%, the stainless steel plate becomes hard and its workability decreases. Therefore, it is preferable to keep the W content at 0.300% or less.

The ferritic stainless steel sheet according to this embodiment is composed of Fe and impurities (including unavoidable impurities) in addition to the elements described above. In addition to the elements described above, other elements may be contained within a range that does not impair the effects of the present invention. In this embodiment, for example, Bi, Pb, Se, H, etc. may be contained, but in this case, it is preferable to reduce the amount as much as possible. On the other hand, the content ratio of these elements is controlled to the extent that the problem of the present invention is solved, and Bi may be contained at 0.01% or less, Pb at 0.01% or less, Se at 0.01% or less, and H at 0.01% or less, as necessary.

In the forming method of this embodiment, a ferritic stainless steel sheet having a thickness of 1.0 mm or less is to be formed. The preferred thickness is 0.4 to 0.8 mm. The ferritic stainless steel sheet according to this embodiment has excellent extrusion properties due to the adjustment of its chemical composition, so it is particularly suitable for use in applications where forming processing is required for a thin sheet thickness, such as home appliances and kitchen equipment.

In addition, the ferritic stainless steel sheet according to this embodiment preferably has an average Lankford value (hereinafter referred to as the average r value, also referred to as the average plastic strain ratio) of 1.7 or more and an in-plane anisotropy (Δr) of the Lankford value of 0.7 or less. By making the average r value 1.7 or more, reduction in sheet thickness at the punch shoulder is less likely to occur, making it possible to suppress the occurrence of cracks during forming. Furthermore, by making the in-plane anisotropy (Δr) 0.7 or less, the in-plane anisotropy of the steel sheet during forming is reduced, and formability can be further improved.

The average r value and Δr can be measured by the plastic strain ratio test method of JIS Z 2254:2008. The average r value can be calculated by the following formula (A) in accordance with JIS Z 2254:2008. The in-plane anisotropy (Δr) can be calculated by the following formula (B) in accordance with JIS Z 2254:2008.

Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 (A)

Δr=(r ₀ -2r ₄₅ +r ₉₀ )/2...(B)

In the formulas (A) and (B), _r0 represents the r value in the rolling direction, _r90 represents the r value in the direction perpendicular to the rolling direction, and _r45 represents the r value in the 45 degree direction to the rolling direction.

The ferritic stainless steel sheet of this embodiment can be manufactured by a general method and is not particularly limited. That is, it may be manufactured by casting a slab having the desired chemical composition by steelmaking and continuous casting, hot rolling, annealing and pickling after hot rolling, cold rolling, and final annealing after cold rolling. However, in order to set the average r value and in-plane anisotropy (Δr) in the preferred range, it is preferable that the cold rolling reduction is in the range of 86 to 94%, the heating rate of the final annealing after cold rolling is in the range of 5 to 20°C/sec, the soaking rate and soaking time of the final annealing are in the ranges of 850 to 920°C and 0 to 2 minutes, respectively, and the cooling rate from the end of soaking to 500°C is in the range of 15 to 50°C/sec.

Next, a forming method using the above-mentioned ferritic stainless steel sheet will be described. The forming method of this embodiment is a forming method in which the ferritic stainless steel sheet is pressed using a punch and a die to form a square cylinder to the target forming depth.

FIG. 2 shows an example of a molded product that can be molded by the molding method of this embodiment.
The formed product 2 shown in Fig. 2 is manufactured by drawing a ferritic stainless steel sheet 1 (hereinafter sometimes referred to as steel sheet 1) into a square cylinder. The formed product 2 has a rectangular bottom surface portion 21, a flange portion 22, and four vertical wall portions 23 connecting the bottom surface portion 21 and the flange portion 22. The distance between the bottom surface portion 21 and the flange portions 22, i.e., the height d of the vertical wall portions, corresponds to the target forming depth D during square cylinder forming.

The molded product 2 shown in FIG. 2 can be used, for example, as an exterior panel for home appliances or kitchen appliances. When the molded product 2 is used as such an exterior panel, it is not limited to the shape shown in FIG. 2, and the bottom surface portion 21, flange portion 22, and vertical wall portion 23 may be appropriately modified.

Next, an example of a press molding machine suitable for use in the molding method of this embodiment will be described. FIG. 3 is a process diagram outlining the molding method of this embodiment. The press molding machine 30 shown in FIG. 3(a) is a so-called servo press machine, and is equipped with a die 31, a punch 32, and a die cushion pad 33. The die cushion pad 33 is also generally called a blank holder or blank holder. The press molding machine 30 improves the formability of the molded product 2 by sandwiching a part 1a of the steel sheet 1 between the peripheral portion of the die 31 and the die cushion pad 33 during press molding.

In this embodiment, a servo motor (not shown) is connected as a drive source to the die 31. By using a servo motor as a drive source for the die 31, it is possible to freely control the sliding motion of the die 31 and punch 32. In the following explanation of the operation of the press molding machine 30, the punch 32 is assumed to be the fixed side, and the die 31 and die cushion pad 33 are assumed to be the movable side.

In this embodiment, the punch 32 is the fixed side, and the die 31 and die cushion pad 33 are the movable side, but the present invention is not limited to this. The punch 32 may be the movable side, and the die 31 and die cushion pad 33 may be the fixed side. In this case, a servo motor may be used as the drive source for the punch 32 and the die cushion pad 33. Furthermore, the punch 32, the die 31, and the die cushion pad 33 may each be configured to be movable. In this case, a servo motor may be used as the drive source for the punch 32, the die 31, and the die cushion pad 33.

In general, when press molding is performed using a servo press, the maximum molding speed (relative speed of the punch to the die) during the molding process is about 200 mm/min. If molding is performed at this molding speed, it will cause softening due to significant processing heat, and the processing formability will decrease. As a result of the inventors' investigation of the molding processing conditions using a servo press, it was found that for ferritic stainless steel sheets in the above-mentioned component composition range, if molding is performed using a general oil-based lubricant at a molding speed of 80 mm/min or less, which is slower than the general conditions, the moldability will be improved and the sheets can be used. However, if the molding speed is set to 80 mm/min or less, the productivity will decrease significantly compared to molding at about 200 mm/min. The inventors have found conditions that prevent cracking and shape defects without decreasing productivity.

In the press forming method of this embodiment, first, as shown in FIG. 3(a), the steel sheet 1 is placed between the die 31 and the punch 32, the steel sheet 1 is placed on the die 31, and a part 1a of the steel sheet is further restrained by the die cushion pad 33. Next, as shown in FIG. 3(b), while the steel sheet 1 is still restrained by the die cushion pad 33, the steel sheet 1 is raised together with the die 31 and the die cushion pad 33 to approach the punch 32, and the steel sheet 1 is brought into contact with the tip of the punch 32. The relative position of the punch 32 to the die 31 at this time is the forming start position.

Next, as shown in FIG. 3(c), the steel sheet 1 is further raised together with the die 31 and die cushion pad 33 until the punch 32 reaches the bottom dead center, i.e., until the punch 32 reaches the position of the target forming depth D shown in FIG. 3(c), to perform square cylinder drawing. Here, the target forming depth D is a depth corresponding to the forming depth d in the formed product 2, and more specifically, is the distance between the forming start position and the position of the target forming depth.

In this embodiment, in order to prevent cracks and defective shapes, the relative speed of the punch 32 with respect to the die 31 is controlled in stages as described below while the relative position of the punch 32 with respect to the die 31 moves from the forming start position to the position of the target system depth D.

Figure 4 shows a schematic graph to explain the relative speed of the punch 32 with respect to the die 31. The horizontal axis of Figure 4 indicates time, and the vertical axis indicates the relative position of the punch with respect to the die. In this embodiment, the section from the forming start position to the position of the target forming depth D is divided into three divided sections, and the relative speed of the punch 32 with respect to the die 31 is controlled for each divided section. The divided sections are the first section from the forming start position to the 1/4 position of the target forming depth, the second section from the 1/4 position of the target forming depth to the 4/5 position of the target forming depth, and the third section from the 4/5 position of the target forming depth to the target forming depth position. The length of the first section is 1/4 of the target forming depth D, and the length of the third section is 1/5 of the target forming depth D. The length of the second section is 11/20 of the target forming depth D. In FIG. 4, for ease of explanation, the relationship between the displacement of the punch 32 and time is depicted as a straight line, but the relationship between the displacement of the punch 32 and time is not limited to a linear relationship.

The relative speed of the punch 32 with respect to the die 31 in the first section (from the forming start position to the 1/4 position of the target forming depth) is an average relative speed _V1 of 150 mm/min or less, more preferably an average relative speed _V1 of 50 to 150 mm/min. The period from the forming start position to the 1/4 position of the target forming depth is an important phase where plastic deformation occurs in the steel sheet 1. By setting the average relative speed _V1 in this range to a relatively low speed of 150 mm/min or less, it is possible to ensure the effect of deformation resistance and lubrication. However, if the average forming speed _V1 is too low, productivity will decrease, so it is preferable to set the lower limit to 50 mm/min or more. The preferred range of the average forming speed _V1 is 50 to 100 mm/min. The average forming speed _V1 is a value obtained by dividing the length of 1/4 of the target forming depth D by the time required from when the punch 32 reaches the forming start position to when it reaches the 1/4 position of the target forming depth.

In addition, if the instantaneous maximum value of the relative speed in the first section is 300 mm/min or more, the plastic deformation of the steel plate 1 cannot keep up, which may cause cracks or defective shapes, so the instantaneous maximum value of the relative speed in the first section may be less than 300 mm/min.

Furthermore, if the relative speed is too high between the time the punch reaches the forming start position and 2 seconds later, the plastic deformation of the steel sheet 1 cannot keep up, which may cause cracks or defects in the shape, so the average relative speed during this period may be set to 5 to 80 mm/min.

Next, the relative speed of the punch 32 to the die 31 in the second section (from the 1/4 position to the 4/5 position of the target forming depth) is set to an average relative speed _V2 that is greater than the average relative speed _V1 and is equal to or less than 250 mm/min. In this section, it is possible to make the relative speed of the punch 32 to the die 31 relatively high. On the other hand, if the average relative speed _V2 exceeds 250 mm/min, the forming speed is too fast, so friction between the die 31 and the punch 32 and the steel sheet 1 increases, and the processing formability decreases. In addition, if the average forming speed _V2 is too low, productivity decreases, so it is preferable to set the lower limit to more than the average relative speed _V1 . Preferably, the average relative speed _V2 is set to 150 to 200 mm/min. The average forming speed _V2 is set to a value obtained by dividing the length of 11/20 of the target forming depth D by the time required for the punch 32 to reach the 1/4 position of the target forming depth and reach the 4/5 position.

In addition, if the instantaneous maximum value of the relative velocity in the second section is 300 mm/min or more, the plastic deformation of the steel plate 1 cannot keep up, which may cause cracks or defective shapes, so the instantaneous maximum value of the relative velocity in the second section may be less than 300 mm/min.

The maximum relative speed _V3 of the punch 32 with respect to the die 31 in the third section (from the 4/5 position of the target forming depth to the target forming depth position) (the instantaneous maximum value of the relative speed in the third section) is set to 120 mm/min or less. This is because the maximum relative speed _V3 of the punch 32 needs to be lowered from the final 4/5 position to the target forming depth position in order to obtain dimensional accuracy of the formed product. At a speed exceeding 120 mm/min, a stable formed shape with high dimensional accuracy cannot be obtained. It is preferably 80 mm/min or less. However, if the maximum relative speed is too low, productivity may decrease, so the maximum relative speed _V3 may be set to 50 mm/min or more.

In addition, the average relative speed from one second before the punch reaches the target forming depth position to the time it reaches the target forming depth position may be set to 30 mm/min or less, preferably 20 mm/min or less. This can further improve the dimensional accuracy of the molded product.

By performing forming processing under the above-mentioned forming processing conditions, even thin ferritic stainless steel sheets can be used as parts for home appliances and kitchen equipment. On the other hand, if the forming processing conditions are not controlled within the above ranges and are changed to commonly used forming processing conditions, undesirable results will occur, such as cracks, defective shapes, and poor aesthetics.

The present invention will now be described in more detail with reference to examples.
Ferritic stainless steel having the composition shown in Table 2 was melted and cast, and hot-rolled to a thickness of 5.0 mm. In the composition shown in Table 2, the balance is Fe and impurities. After that, hot-rolled sheet annealing was performed, pickled, cold-rolled to a thickness of 0.4 to 1.2 mm, annealed, pickled, and then temper rolled to obtain the steel sheet shown in Table 3A. In addition, No. 1 to 7 and No. 11 to 22, the cold rolling ratio was in the range of 86 to 94%, the temperature increase rate of the final annealing after cold rolling was in the range of 5 to 20 ° C / sec, the soaking rate and soaking time of the final annealing were in the range of 850 to 920 ° C and 0 to 2 minutes, respectively, and the cooling rate from the end of soaking to 500 ° C was in the range of 15 to 50 ° C / sec. Using the steel sheet obtained in this way, a square tube forming test was performed. In addition, when a distance block was used, it was indicated as "Yes" in Table 3B.

A DSF-N1-1500 servo press machine manufactured by AIDA ENGINEERING was used for the forming test. The blank size was 175 x 175 mm, and a die with the following shapes was used: Die: 82 mm x 62 mm, corner r9 mm, die r5 mm, punch: 80 mm x 60 mm, corner r8 mm, punch r8 mm. The cushion pressure applied by the die cushion pad was 500 kN. A PVC sheet was attached to the square tube forming surface, and a water-soluble lubricant was lightly applied. These forming conditions were fixed, and forming tests were performed under various conditions in which the forming speed of the punch relative to the die was changed. The target forming depth was set to 50 mm.

The 0.2% yield strength and elongation of the steel plates were measured using JIS No. 13B test pieces taken from a 0° direction parallel to the rolling direction in accordance with the conditions described in JIS Z 2241. Furthermore, the average r value and Δr were measured under conditions where a 16% strain was applied to samples taken from test pieces taken at 0°, 45°, and 90° directions relative to the rolling direction.

After molding, if the molded product had cracks or a defective shape, it was rated as NG, and if no cracks or a defective shape occurred, it was rated as OK. The results are shown in Table 3B.

As shown in Tables 2, 3A, and 3B, when molded under conditions within the scope of the present invention, the molded product did not crack or have a defective shape, and the design of the molded product was also good.

On the other hand, when molding was performed under conditions outside the range of the present invention, the molded product had cracks and was defective in shape.
In Nos. 2, 16, and 21, the average relative speed _V1 from the forming start position to the 1/4 position of the target forming depth exceeded 150 mm/min, so cracks occurred in the early stage of forming. Therefore, forming was stopped when forming was performed to the 1/4 position of the target forming depth.

In Nos. 7, 13, 17 and 20, the maximum relative speed _V3 from the 4/5 position of the target forming depth to the position of the target forming depth exceeded 120 mm/min, so the dimensional accuracy of the molded product was reduced and a molded product of the desired shape was not obtained.

In No. 10, the thickness of the steel plate exceeded 1.0 mm, and cracks occurred in the molded product.

In Nos. 6, 9, 19, and 21, the average relative speed _V2 from the 1/4 position to the 4/5 position of the target forming depth exceeded 250 mm/min, so the workability was reduced and a formed product having the desired shape could not be obtained.

Nos. 23 to 40 had cracks and poor shape because the chemical composition of the blank ferritic stainless steel plate was outside the range of the present invention.

Claims (9)

A forming method for square cup drawing of a ferritic stainless steel sheet to a target forming depth by press forming using a punch and a die, comprising the steps of:
The ferritic stainless steel plate contains, by mass%, C: 0.0200% or less, Si: 0.70% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Cr: 11.0 to 19.5%, N: 0.020% or less, Al: 0.005 to 0.100%, O: 0.0050% or less, Ti: 0.03 to 0.20%, Nb: 0.010 to 0.300%, Sn: 0.001 to 0.300%, Zr: 0.001 to 0.080%, with the balance being iron and impurities, and has a component composition that satisfies the following formula (1), and has a plate thickness of 1.0 mm or less,
The average relative speed _V1 between the punch and the die from the forming start position to a 1/4 position of the target forming depth is set to 150 mm/min or less;
The average relative speed _V2 between the punch and the die from the 1/4 position to the 4/5 position of the target forming depth is set to be greater than the average relative speed _V1 and equal to or less than 250 mm/min;
a maximum relative speed _V3 of the punch and the die from a 4/5 position of the target forming depth to the target forming depth being 120 mm/min or less.
(Al×0.4+Zr×0.5+Ti×0.1)/O≧12…(1)
Here, each element symbol in formula (1) represents the content (mass %) of each element in the ferritic stainless steel sheet. The method for forming a ferritic stainless steel sheet according to claim 1, wherein the average relative velocity _V1 is set to 50 to 150 mm/min. The method for forming a ferritic stainless steel sheet according to claim 1 or 2, characterized in that the ferritic stainless steel sheet contains, in mass%, one or more of Mo: 0.05-0.50%, Ni: 0.05-0.50%, and Cu: 0.01-1.00% in place of a portion of Fe. The method for forming a ferritic stainless steel sheet according to any one of claims 1 to 3, characterized in that the ferritic stainless steel sheet contains, in mass%, one or more of B: 0.0003 to 0.0050%, Ga: 0.0001 to 0.2%, and W: 0.001 to 0.300% in place of a portion of Fe. The method for forming a ferritic stainless steel sheet according to any one of claims 1 to 4, characterized in that the average Lankford value of the ferritic stainless steel sheet is 1.7 or more, and the in-plane anisotropy (Δr) of the Lankford value is 0.7 or less. A ferritic stainless steel plate characterized by containing, by mass%, C: 0.0200% or less, Si: 0.70% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Cr: 11.0 to 19.5%, N: 0.020% or less, Al: 0.005 to 0.100%, O: 0.0050% or less, Ti: 0.03 to 0.20%, Nb: 0.010 to 0.300%, Sn: 0.001 to 0.300%, Zr: 0.001 to 0.080%, with the balance being iron and impurities, and having a component composition that satisfies the following formula (2):
(Al×0.4+Zr×0.5+Ti×0.1)/O≧12…(2)
Here, each element symbol in formula (2) represents the content (mass %) of each element in the ferritic stainless steel sheet. The ferritic stainless steel sheet according to claim 6, characterized in that it contains, in mass%, one or more of the following elements in place of a portion of Fe: Mo: 0.05-0.50%, Ni: 0.05-0.50%, and Cu: 0.01-1.00%. The ferritic stainless steel sheet according to claim 6 or 7, characterized in that it contains, in mass%, one or more of the following elements in place of a portion of Fe: B: 0.0003-0.0050%, Ga: 0.0001-0.2%, and W: 0.001-0.300%. A ferritic stainless steel sheet according to any one of claims 6 to 8, characterized in that the average Lankford value is 1.7 or more, and the in-plane anisotropy (Δr) of the Lankford value is 0.7 or less.

Images