JP4246117B2 - Deep drawing products using ferritic stainless steel sheet - Google Patents

Description

  The present invention relates to a deep drawn product of ferritic stainless steel that does not break or buckle even if it is subjected to multi-stage forming to obtain a final product.

  Ferritic stainless steel represented by SUS430 exhibits good corrosion resistance and is inexpensive compared to austenitic stainless steel because it does not contain expensive Ni, and is used in a wide range of applications centering on durable consumer materials. . With the development of applications of ferritic stainless steels, the press working conditions for molding into product shapes become more severe, and ferritic stainless steels that exhibit even better deep drawing workability are desired.

  Many studies aiming at improvement of deep drawing workability have been reported, and among these, combined addition of Ti and Nb is typical. The combined addition of Ti and Nb precipitates C and N dissolved in the matrix as carbonitrides, and improves the deep drawability by reducing the C and N concentration of the matrix. Furthermore, improvement of ridging characteristics by Mg inclusions (for example, see Patent Document 1), improvement of Rankford value by combining Ti and Nb combined addition and hot rolling conditions (see Patent Document 2), etc. are also known. Yes.

JP 2000-192199 A Japanese Patent Publication No. 8-26436

  When ferritic stainless steel is used for various applications, multistage formability when forming into the final product shape is also an important required characteristic. Ferritic stainless steel sheets are generally inferior in workability than austenitic stainless steel sheets, but are excellent in the effect of sheet thickness reduction expressed in the Rankford value in deep drawing. However, processing in multistage molding tends to employ severe processing conditions with a high deep drawing ratio, and vertical cracking is likely to occur due to multistage molding close to the fracture limit of the material. Even if molding is possible by multistage molding, galling or wrinkling at the time of molding occurs in the processed product, and measures against these are necessary.

  In deep drawing near the limit, it is difficult to predict the occurrence rate of cracks during processing, and cracks may suddenly occur during operation. The lack of sufficient countermeasures against cracking during multistage molding is due to the fact that the relationship between mechanical properties and molding conditions during multistage molding has not been elucidated.

  An object of the present invention is to provide a deep-drawn product that can be molded into a highly accurate final product shape without causing vertical cracks during multi-stage molding.

  Therefore, the present inventors investigated the relationship between the mechanical properties of steel sheets and press conditions in multi-stage forming. As a result, it was found that when the surface roughness is within a certain range in a steel sheet with guaranteed ductility, the lubrication state between the steel sheet and the mold is stable, and the occurrence of vertical cracks can be suppressed even when multistage forming is performed.

  In order to achieve the object, the deep-drawn product of the present invention has C: 0.015 mass% or less, Si: 0.05-1.0 mass%, Mn: 0.05-0.5 mass%, P : 0.04% by mass or less, S: 0.005% by mass or less, Cr: 10 to 12% by mass, Ti: 0.10 to 0.50% by mass, the balance having the composition of Fe and inevitable impurities, A ferritic stainless steel sheet having a surface roughness Rt of more than 2 μm and less than 6 μm is formed into a shape with a ratio of maximum depth / minimum cylindrical minimum diameter of 2.5 to less than 4 by multi-stage deep drawing three or more times. It is a featured deep drawn product of ferritic stainless steel.

  This processed product is not only a lubrication effect during press molding, but also with this surface roughness, it is a ferritic stainless steel sheet that makes it easy to obtain a good coated surface even if the surface is not ground-treated. It is a processed product molded from Moreover, the ferritic stainless steel sheet of this processed product can be manufactured by hot rolling and then cold rolling with a tandem cold rolling mill and annealing pickling.

  In the present invention, a deep-drawn product excellent in longitudinal crack resistance during multi-stage molding can be obtained by appropriately managing the components and surface roughness.

  As a result of detailed investigation of longitudinal cracks that occur in deep-drawn products formed by multi-stage forming of ferritic stainless steel, the present inventors are prone to cracking during the third and subsequent moldings in a low temperature environment. Investigated that it is caused by brittle cleavage fracture or grain boundary fracture which is hardly seen in the second molding. In addition, it was found that vertical cracks are less likely to occur according to the surface roughness of the steel sheet surface when the lubricating oil thickness is equal. The present invention has been completed on the basis of these findings. By appropriately managing the components and surface roughness, a deep-drawn product with excellent longitudinal crack resistance during multi-stage molding can be obtained.

  Ferritic stainless steel is hardened and has low elongation because it has a higher chromium content than ordinary steel. High stretchability cannot be expected with overhanging deformation, which is greatly affected by the ductility of the material, so we focused on the lubrication status due to ductility, which is a typical numerical value of mechanical properties, and the surface properties between the material and the mold during multi-stage molding. The presence or absence of vertical cracks in multi-stage molding to the final product shape was investigated. The surface roughness of the steel sheet was measured in accordance with JISB0601, using a two-dimensional roughness meter manufactured by Tokyo Seimitsu, and the average value was measured by measuring the rolling direction and the direction perpendicular to the rolling direction by 90 °.

  Hereinafter, the alloy components, contents, and the like of the ferritic stainless steel used in the present invention will be described.

Cr: 10-12 mass%
It is an alloy component essential for improving the corrosion resistance required for stainless steel, and the effect of Cr addition is seen at 10% by mass or more. However, since the hardness of the base increases as the Cr content increases, the upper limit is regulated to 12% by mass.

C: 0.015 mass% or less When the C content is increased, the strength of the cold-rolled annealed plate is increased and the ductility is lowered. Therefore, the upper limit is regulated to 0.015 mass%.

Si: 0.05-1.0 mass%
Si is added in an amount of 0.05% by mass or more for deoxidation. However, since the toughness deteriorates when added over 1.0 mass%, the upper limit is limited to 1.0 mass%.

Mn: 0.05 to 0.5% by mass
Mn is added in an amount of 0.05% by mass or more for deoxidation. However, if the addition exceeds 0.5% by mass, the corrosion resistance deteriorates, so the upper limit is limited to 0.5% by mass.

P: 0.04 mass% or less P is limited to 0.04 mass% or less in order to deteriorate toughness.

S: 0.005 mass% or less S is limited to 0.005 mass% or less in order to deteriorate toughness or corrosion resistance.

Ti: 0.1 to 0.5% by mass
Ti is an alloy component that improves impact resistance and secondary work cracking resistance by fixing C and N and refining ferrite crystal grains, and the effect of adding Ti becomes clear at 0.1 mass% or more. However, when an excessive amount of Ti exceeding 0.5% by mass is added, not only the steel material cost increases, but also surface defects due to Ti inclusions are likely to occur.

Material surface roughness Rt: more than 2 μm and less than 6 μm It is important to stabilize the lubrication between the mold and the steel plate surface in order to perform multi-stage molding. By limiting the surface roughness Rt to more than 2 μm and less than 6 μm Lubrication is stable. Lubrication has the effect of preventing seizure between metals when the material comes into contact with the mold, and usually lubricating oil is used. The amount of this lubricating oil brought into contact with the mold varies depending on the surface roughness. Since the surface of the mold is usually very smooth, bringing the lubricating oil is greatly affected by the surface roughness of the steel sheet. The effect of lubrication becomes clear when the surface roughness exceeds 2 μm. However, when the surface roughness is increased, convex portions begin to exist on the surface. Therefore, there is no lubrication effect in this portion, and seizure occurs.

The surface roughness after molding is stabilized by limiting the surface roughness by multi-stage molding of 3 or more stages and limiting the surface roughness, but depending on the processing, minute wrinkles are formed. Such wrinkles are unsuitable for the final product and are used after polishing. In multistage molding with high deep drawing, cracks are likely to occur starting from such wrinkles, and the upper limit of the drawing ratio is set to less than 4. In addition, when the drawing ratio is less than 2.5, molding can be performed without performing multi-stage molding three or more times, so that the invention was excluded.

The paint ridge has a local gap structure and is likely to be a starting point. In an environment such as a countryside, it is unlikely to cause fire, but it is sometimes used in salt damage areas because it is made of stainless steel. In this environment, it is necessary to completely detoxify the soot, and painting is effective. In addition, the surface of a processed product to which multi-stage molding is applied has a portion smoothened by contact with the mold and a recess in the base material, and has good paintability. Therefore, it is possible to easily obtain a good surface property without performing a ground treatment before coating.

  Cold rolling of a stainless steel plate is usually performed using a Sendzimir mill, and a very smooth surface can be formed. On the other hand, in a tandem mill applied to ordinary steel, the lubricating oil is sandwiched between the plate surface and the cold-rolled roll and a recess is easily formed on the plate surface, and the surface roughness is not as smooth as that of the Sendzimir mill. By using a manufacturing method of cold rolling with this tandem mill and finishing by annealing pickling, it becomes possible to make the surface roughness within the scope of the invention.

  By combining the conditions of component identification and surface roughness in this way, deep drawing without vertical cracks even when multistage molding is performed on the final product shape where the ratio of maximum drawing depth / minimum cylindrical diameter is 2.5 or more and less than 4. A processed product is obtained.

  Table 1 component ferritic stainless steel was melted, cast, and hot-rolled to a thickness of 5 mm. After hot-rolled sheet annealing, it was cold-rolled to a plate thickness of 2 mm using a tandem mill or Sendzimir mill, and then annealed, and then pickled.

  A rolling direction (L direction) JIS No. 13B test piece was cut out from the final annealed material of each ferritic stainless steel and subjected to a tensile test, and the hardness of the plate cross section was measured. Further, a blank having a diameter of 50 mm was cut out from the final annealed material, and a molded product having a different drawing ratio was processed by a four-stage drawing in which the punch / die diameter was changed. After painting this processed product, it was exposed to a position about 100m from the coast for one month, and the state of bruising was investigated.

  As can be seen from the results of the investigation in Table 2, the processed products satisfying the conditions specified in the present invention for the component coating did not cause vertical cracks in any of the processes, and did not start in the beach area.

  Even when the conditions specified by the components were satisfied, in Example A2 and Example C3 where the surface roughness Rt was outside the range, vertical cracks occurred during multi-stage molding.

  The comparative steel D containing excess C (carbon) has a high material hardness, a tendency of low ductility, and frequent vertical cracks. In Example E in which Cr exceeded the range, as in Example D, the tendency for low ductility was remarkable and vertical cracking occurred frequently. In Example F where Ti exceeded the range, a large amount of Ti oxide was present, which was the starting point of cracking. Further, in Examples A2 and C3 manufactured by Sendzimir mill, the surface roughness Rt was lower than the range of the invention, and galling occurred during multistage molding.

  From the above results, by making all of the components, surface roughness, and coating within the conditions, it is possible to stably obtain a deep-drawn product that does not generate vertical cracks and does not generate even by multistage molding.

  As described above, when all of the components, elongation, surface roughness, coating, and hardness are appropriately controlled, it can be processed into a final product shape free of vertical cracks by multi-stage molding. Ferritic stainless steel used as a raw material is an inexpensive material compared to austenitic stainless steel. Utilizing such advantages, a processed product suitable for industrial equipment such as a capacitor case which requires a highly accurate and complicated shape is provided.

Claims (3)

  C: 0.015 mass% or less, Si: 0.05 to 1.0 mass%, Mn: 0.05 to 0.5 mass%, P: 0.04 mass% or less, S: 0.005 mass% or less , Cr: 10 to 12% by mass, Ti: 0.10 to 0.50% by mass, the balance being Fe and inevitable impurities, and a surface roughness Rt of more than 2 μm and not more than 6 μm A deep-drawn product of a ferritic stainless steel sheet, wherein the ratio of maximum depth / minimum cylindrical diameter is formed to be 2.5 or more and less than 4 by multi-stage deep drawing three or more times.   A deep-drawn product of salt-resistant ferritic stainless steel sheet, wherein the surface of the processed product according to claim 1 is coated without being subjected to a ground treatment.   Furthermore, the deep drawing processed article of Claim 1 or 2 shape | molded using the ferritic stainless steel plate manufactured by cold rolling with the tandem cold rolling mill after hot rolling, and manufactured by the annealing pickling process.