JP2008256081A - Ferritic stainless steel screw - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel screw which has strength equivalent to that of the conventional austenite-based stainless steel screw (SUSXM7), extends the lifetime of a die assembly for forming the screw and has corrosion resistance excellent than a temper colored austenitic stainless steel screw after being colored for color matching with a member, the screw using low-cost ferritic stainless steel. <P>SOLUTION: The ferritic stainless steel screw is produced through making the tensile strength of a wire drawing material made of ferritic stainless steel 570 to 700 MPa and further by making the tensile strength after screw form rolling 580 to 730 MPa, the ferritic stainless steel having a chrome volume of 16 to 20% by weight, a carbon volume of 0.005 to 0.02% by weight, and a nitrogen volume of 0.005 to 0.02% by weight. Then, an oxidizing color development process for forming a film of 0.5 μm or less in thickness is applied to the screw. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a ferritic stainless steel screw used for fastening.

  Conventionally, an austenitic stainless steel fastening screw centered on SUSXM7, which has both corrosion resistance and strength, has been used for fastening architectural sashes and the like. This type of fastening screw contains a considerable amount of nickel mainly for imparting corrosion resistance and mainly copper for improving the moldability and die life of the screw. Since these elements are expensive, they increase the price of the material and its product screws, and sometimes induce price fluctuations, which make the supply of the screw material unstable.

  Like austenitic stainless steel screws, ferritic stainless steel screws that contain almost no expensive nickel or copper are not subjected to quench hardening after thread forming, but are processed by the tensile strength of the wire drawing material before threading and screw rolling. Curing determines the tensile strength of the screw that plays an important role in various characteristics such as the axial force of the screw. However, since ferritic stainless steel has a lower work hardening rate than austenitic stainless steel, it is necessary to increase the strength of the wire drawing material (to increase the tensile strength) in order to compensate for this and obtain a high tensile strength of the screw. Necessary. Therefore, Patent Document 1 proposes to increase the strength of a material in order to increase the strength of a plate material that is not a three-dimensional processing such as a screw but is subjected to a two-dimensional processing such as bending. Has been.

  In order to make high-strength, high-corrosion-resistant screws that can replace austenitic stainless steel screws, the problem of internal cracking during forging of the screw head must be solved. In other words, this phenomenon is related to the workability (limit upsetting ratio) characteristics until shear cracking occurs on the side surface when the material is compressed and deformed. In this characteristic, austenitic stainless steel is generally superior to other stainless steels. Because. In order to avoid this internal crack, a high-strength screw cannot be obtained if the hardness of the wire drawing material is lowered by annealing. Patent Document 2 proposes a method of using a stainless steel two-phase region to solve this problem of internal cracks in the head, and making a high-strength, high-corrosion-resistant screw, and processing the screw by complex heat treatment. Has been.

    Conventionally, temper coloring which can be variously colored has been mainly performed for coloring austenitic stainless steel screws, but ferrite stainless steel screws have not been used because corrosion resistance is lowered. In addition, austenitic stainless steel is subjected to black dyeing using sodium hydroxide, which has also been applied to screws. Although coloring is limited to black, Patent Document 3 proposes a black dyeing method using nitric acid in order to perform the same treatment on ferritic stainless steel.

JP 2005-330580 A JP-A-9-314276 Japanese Patent Laid-Open No. 2001-20084

    Using low-cost ferritic stainless steel with less than 0.5% by weight of expensive nickel and copper or impurity levels, the tensile strength of screws is improved to the same level as austenitic stainless steel screws (SUSXM7). In the process, the life of the forging die for screw forming is extended and the cost of screw forming is also reduced. That is, an object is to provide a ferritic stainless steel screw that can be used in place of an austenitic stainless steel screw at low cost.

    Furthermore, in the coloring process of the screw required for color matching with the member, various types of coloring are possible in the same manner as the temper coloring of austenitic stainless steel, and in the corrosion resistance, this austenitic stainless temper colored product It is an object of the present invention to provide a colored screw of the ferritic stainless steel that can be improved to the same level or higher.

  The ferritic stainless steel screw of the present invention is a ferritic stainless steel having a chromium content of 16 to 20% by weight, a carbon content of 0.005 to 0.02% by weight and a nitrogen content of 0.005 to 0.02% by weight. The tensile strength of the wire drawing material before processing the screw is 570 to 700 MPa, and the tensile strength of the screw after thread rolling is 580 to 730 MPa.

  Further, the ferritic stainless steel screw is characterized in that an oxidation coloring layer generation treatment having a thickness of 0.50 μm or less is performed.

  According to the present invention, it is possible to manufacture a ferritic stainless steel screw having the same strength as an austenitic stainless steel screw, and reduce not only the material cost but also the manufacturing cost by extending the die life in the forging, Total cost can be greatly reduced. In addition, various types of coloring are possible in the coloring process required for color matching with members, and it is possible to make ferritic stainless steel screws for fastening that are equal to or better than tempered colored austenitic stainless steel in terms of corrosion resistance. It becomes.

  Embodiments of the present invention will be described below with reference to the drawings.

  In the chemical composition of the wire drawing material 1 shown in FIG. 1 (a), in order to produce a uniform and stable passive film on the screw surface to obtain high corrosion resistance, the chromium amount is set to 16% by weight or more, and at the time of screw forming In order to prevent embrittlement (in this case, sigma brittleness) that is harmful to the product characteristics of the screw, the upper limit is 20% by weight.

  The amount of carbon is 0.05% by weight or less in order to prevent martensite generation during cooling due to the formation of an austenite phase at a high temperature of 800 ° C. or higher and to avoid the risk of embrittlement that is harmful to screws. The Furthermore, in order to increase the limit upsetting rate and improve the forging workability, each including nitrogen was limited to 0.02% by weight or less. However, the lower limit is set to 0.005% by weight or more in order to ensure the strength by work hardening. Thus, the strength of the screw can be ensured while avoiding embrittlement.

  On the other hand, when the carbon content and nitrogen content are within the above ranges, precipitation of chromium carbide and chromium nitride is suppressed, so that the passive film itself is stabilized by leaving sufficient solid solution chromium on the surface, and chromium at the grain boundary is also present. Minimize the occurrence of deficient phases and suppress intergranular corrosion. Limiting the amount of carbon and the amount of nitrogen to the above ranges greatly contributes to the improvement of the corrosion resistance of the screw.

  In general, the tensile strength of a screw is the same as that of the wire drawing material 1 for screw shown in FIG. 1 (a) and that for forming a thread valley 6 in the screw 5 after rolling shown in FIG. 1 (c). It depends on the work hardening by construction. The ferritic stainless steel wire drawing material 1 is rolled and drawn, subjected to predetermined annealing treatment, and then subjected to wire drawing (skin pass) at a low processing rate. However, the tensile strength of the wire drawing material 1 is determined by these processing histories. Considering that it is difficult to increase the strength by refining the crystal grains of ferritic stainless steel, we will reduce the heat treatment parameters (annealing temperature x common logarithm of heating time) slightly lower than the standard annealing conditions. By optimal adjustment, the tensile strength of the wire drawing material 1 is set to be higher than that of a normal ferritic stainless steel screw drawing material, and the range of the screw 2 after screw head processing shown in FIG. In the above, it was confirmed that no cracks or defects were generated in the screw head 3 or the screw groove 4, and knowledge about the optimum annealing conditions and the range of the tensile strength corresponding thereto was obtained. That is, by setting the annealing temperature and the heating time in a certain range, the tensile strength of the wire drawing material 1 is set to 570 to 700 MPa. In this range, the inside and outside of the screw head 3 and the screw groove 4 are formed during screw forging. No cracks occur in the peripheral part, and the tensile strength of the screw 5 after the rolling process is increased to 580 to 730 MPa by work hardening when the thread valley part 6 in FIG. 1 (c) is rolled. I found it.

  Table 1 shows that there is no cracking in the inner and outer periphery of the screw head 3 and the screw groove 4 during the screw forging shown in FIG. 1 (b), and the life of the mold used for forging is guaranteed to be 20000 times or more. Regarding the range of tensile strength of the drawn wire 1 and the tensile strength of the thread 5 after the rolling process, a typical austenitic stainless steel SUSXM7 screw (hereinafter referred to as Comparative Example 1) and a general ferritic stainless steel A comparison of a SUS430 screw (referred to as Comparative Example 2) and a product of the present invention (Example 1 and Example 2) is shown. If the tensile strength of the wire drawing material 1 before each threading is increased, the tensile strength of the screw also increases, but a load is applied to the mold, and the life is shortened due to wear or fatigue. The tensile strength of the wire drawing material of Comparative Example 1 is limited to 500 to 540 MPa, and if it exceeds this, mass production becomes impossible with an early life of the mold. In Comparative Example 2, since the limit upsetting rate is small, the tensile strength of the wire drawing material 1 is as shown in Table 1 in order to prevent internal and external defects of the screw head 3 and the screw groove 4 during the above-described forging. Limited to range.

  The coloring of various screws required for color matching with the member can be processed at a low temperature of 120 ° C. or less so as not to deteriorate the corrosion resistance, and is high chromium, low carbon and low nitrogen. Utilizing the characteristics of chemical components, a highly corrosion-resistant oxidation coloring film is formed. First, as a pretreatment, the screw is sufficiently degreased and washed, and then immersed in 10 to 20% nitric acid to passivate the surface. For dipping, it is preferable to rotate and agitate a barrel case in which a certain number of screws are charged in order to generate a uniform passive film on the surface of the screw having a complicated shape. In this way, the passive film uniformly forms a passive film to every corner of the screw head 3, the screw groove portion 4 and the screw thread valley portion 5 to ensure the corrosion resistance, and the thickness of the oxide layer in the subsequent oxidation color treatment is uniform. It plays a role in minimizing color unevenness. Thereafter, it is immersed in an oxidizing agent solution based on sulfuric acid at 120 ° C. or lower to form an oxidized coloring layer having a thickness corresponding to a desired color, and a colored screw 7 shown in FIG. 1 (d) is obtained. .

  In general, a stainless steel oxide layer is light transmissive, and is recognized as a color by reflecting and emphasizing a specific wavelength due to the optical path difference due to the difference in refractive index and thickness of the layer. If the thickness exceeds 0.50 μm, there is a possibility that a problem such as peeling may occur due to a difference in physical properties between the oxidized portion and the inside. The passivated film is an oxide layer of about several nanometers, and the present invention includes a case where only a passivating process is performed and the metal white screw is used as a transparent oxide film.

  In this way, the passivating process and the oxidative coloring process utilizing the ingredients of the material enable a coloring process having corrosion resistance, which was difficult with conventional ferrite stainless steel screws. In other words, by limiting the amount of chromium, the amount of carbon, and the amount of nitrogen to the above ranges, the surface is almost uniform and sufficiently covered with a solid solution surface of chromium, and a local chromium-deficient layer is not generated, and oxidation coloring Even after treatment, the surface structure becomes highly corrosion-resistant, making it difficult to produce a local battery. Thus, the ferritic stainless steel screw of the present invention is superior in corrosion resistance by being covered with uniform chromium oxide rather than the SUSXM7 of Comparative Example 1 subjected to the temper coloring treatment.

  An embodiment in the case where an M4 truss machine screw is manufactured according to the present invention will be described.

    NSSC160R (Nippon Steel & Sumikin Stainless Steel Co., Ltd.) wire drawing rod with reduced carbon and nitrogen content was used as the material. The main components are chromium content of 16.5% by weight, carbon content of 0.01% by weight and nitrogen content of 0.01% by weight, and nickel and copper are each 0.5% by weight or less. The above-mentioned material is made into a coil wire φ5.5 mm by continuous casting and hot rolling, and after drawing this to φ3.53 mm by hole die drawing, it is annealed and then drawn at a low processing rate ( (Skin pass) was performed to produce a wire drawing material 1 for screw forming with a diameter of 3.45 mm. After this, the screw head 3 and the screw groove portion 4 are processed by a forging machine (header) to produce the screw 2 after the screw head processing, and then the screw thread valley portion 6 is rolled by a flat rolling device to form an M4 truss. This was formed into a screw 5 after the rolling process. The screw 5 after the rolling process was sufficiently degreased and washed, and then subjected to a passivation treatment by immersing it in a 20% nitric acid solution for about 30 minutes while rotating and stirring the barrel case for treatment. Thereafter, a solution containing an oxidizing agent based on sulfuric acid was heated to 90 ° C. to obtain a bronze oxidative coloring 8 and was taken as Example 1. In Example 2, only the passivation treatment was performed to obtain a metal white screw. Moreover, the comparative example 1 shows SUSDXM7 and the comparative example 2 shows the bronze M4 truss machine screw of SUS430. However, the comparative example 1 performed the temper coloring process generally performed to austenitic stainless steel, and the comparative example 2 performed the passivation process equivalent to the present invention product, and the bronze oxidation coloring 8.

  Table 2 shows the test results of the intermediate processed products and products of Examples 1 and 2 and Comparative Examples 1 and 2. The die life is the life of a die for forging the screw head 3 and the screw groove 4. Corrosion resistance was evaluated by a salt spray test (5% saline, JIS) for the colored screw 7 as a product. Examples 1 and 2 are superior to Comparative Example 1 in mold life and corrosion resistance, and are superior to Comparative Example 2 in tensile strength and corrosion resistance of screws.

It is a figure which shows the process of manufacturing the screw concerning this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Wire drawing material 2 Screw after screw head processing 3 Screw head 4 Screw groove part 5 Screw after rolling process 6 Screw thread valley part 7 Screw processed with coloring 8 Oxidation coloring

Claims (2)

  A ferritic stainless steel screw having a chromium content of 16 to 20% by weight, a carbon content of 0.005 to 0.02% by weight, and a nitrogen content of 0.005 to 0.02% by weight. A ferritic stainless steel screw characterized in that the tensile strength of the previous wire drawing material is 570 to 700 MPa, and the tensile strength of the screw after thread rolling is 580 to 730 MPa. 2. The ferritic stainless steel screw according to claim 1, wherein the ferritic stainless steel screw for fastening is subjected to an oxidation coloring layer generation treatment having a thickness of 0.50 μm or less.

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