JPS6114211B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a molding material for automobiles that has excellent moldability, surface texture, and rust resistance. Automotive molding materials, ie, decorative materials around window frames and bodies, are usually processed into a predetermined shape by roll forming or press molding. Due to its purpose, the above-mentioned molding material is required to have a high degree of aesthetic appearance, and is also required to have rust resistance during use. Furthermore, regarding workability, elongation of the material is often required, such as in strong tensile bending processing. In addition, the surface quality after processing must be free from roping and roughness, and the rust resistance is, for example, against the occurrence of red rust caused by table salt and rock salt, which are sprayed as anti-freezing agents on road surfaces. Alternatively, it is required to have rust resistance against sulfur dioxide gas in industrial areas. SUS-304, SUS-430, and SUS-434 are conventionally used as molding materials for automobiles.
Mainly made of stainless steel such as SUS-304
Although it has significantly better rust resistance and formability than SUS-430, it is very expensive. On the other hand, SUS―
430 has insufficient moldability and rust resistance, and is problematic as a molding material for automobiles. Also, since SUS-434 is made by adding 1% Mo to SUS-430, its rust resistance is superior to SUS-430, but the addition of 1% Mo reduces elongation and deteriorates formability. Roping tends to occur on the surface and the surface quality deteriorates. Furthermore, Mo is extremely expensive and subject to rapid price fluctuations, making it difficult to recommend it as a molding material for automobiles. The present inventors have discovered that Mo is applied to ferritic stainless steel.
With the aim of developing a molding material for automobiles that has at least the same corrosion resistance as SUS-434 without the addition of ferrite, has improved formability and surface properties, and can be supplied stably and inexpensively, we have developed a ferritic stainless steel. By adding a combination of Nb and Cu to steel and reducing C, we have discovered a stainless steel that is soft, has excellent formability and surface texture, and has rust resistance equivalent to SUS-434. The present invention uses this stainless steel as a raw material and subjects it to a special heat treatment described later to make it an optimal material for automotive molding material. That is, the automotive molding material of the present invention contains up to 0.05% C, 0.75%
Si up to 1.0%, Mn up to 1.0%, Cr up to 14-18%, 0.3
~0.8 Cu, up to 1.2% Nb 8 times the amount of C and up to 1.2% Nb, and the balance essentially consists of Fe except for unavoidable impurities. After hot rolling, the stainless steel is heated and cooled to 900 to 1050°C, and then cold rolled. It is a ferritic stainless steel characterized by being brightly annealed at 850-1050℃. The reasons for limiting each component are described below. C is an impurity element that greatly affects the rust resistance of steel, and it is desirable to reduce it as much as possible.
On the other hand, in terms of processability, a low C content is preferable since a large amount of C increases yield strength, hardness, etc., and also reduces elongation and deteriorates overall formability. Moreover, assuming that it is mass-produced at low cost, C is 0.05%.
The following are desirable. Si is an inevitable element as a deoxidizing element in steelmaking reactions, but if it exceeds 0.75%, it will harden the material, so it was limited to 0.75%. Like Si, Mn is an unavoidable deoxidizing element in the steelmaking reaction, but if the content is too large, the rust resistance will deteriorate, so it was limited to 1.0% or less. Cr is the most important element among the components of ferritic stainless steel that imparts rust resistance, but if it is less than 14%, it will not be sufficiently effective. However, if it is contained in a large amount, it is not economical and causes embrittlement of the material, and it is also bad in terms of rust resistance.
The upper limit was limited to 18% because even 18% had a sufficient effect. Nb fixes C as an impurity element and is an important element from both rust resistance and roping resistance. The reason why the lower limit of Nb content was set to 8 times the C content was because
This is because below this amount it is not sufficient to provide good rust resistance and roping resistance.
The reason why the content was limited to 1.2% was because if it exceeds 1.2%, the cleanliness of the steel will be significantly impaired, the surface quality of the molding product will deteriorate, and the cost will be high economically. Cu content is limited to 0.3 to 0.8% because if it is less than 0.3% or more than 0.8%, the yield strength and hardness will increase and elongation will be reduced, and the roping resistance will deteriorate. Next, the reason for limiting the heat treatment conditions will be described. The reason why the heating temperature after hot rolling is limited to 900 to 1050°C is that if it is below 900°C, a sufficient recrystallized structure cannot be obtained and the roping resistance deteriorates. On the other hand, if the temperature exceeds 1050°C, the crystal grains will become extremely coarse, resulting in extremely rough skin after product processing and deterioration in elongation. In addition, the bright annealing temperature after cold rolling is 850 to 1050℃.
This is because below 850℃, it becomes hard and elongation deteriorates.
On the other hand, if the temperature exceeds 1050℃, similar to hot-rolled sheet annealing, roughness will occur due to grain coarsening, and bright annealing is used when a high degree of aesthetic appearance and surface quality are required due to its decorative properties, such as in this molding application. This is because the combination of normal cold rolling, air annealing, and pickling methods cannot provide the appropriate surface quality of the steel sheet, and bright annealing is desirable from the viewpoint of rust resistance, which will be discussed later. be. Next, embodiments of the invention will be described. Steel with each component shown in Table 1 below was made into cold rolled steel sheets under the manufacturing conditions shown in Table 2, and in order to determine whether each steel sheet is suitable as a molding material for automobiles, the mechanical properties and after forming were determined. The evaluation of the surface properties and various types of rust resistance were investigated. In addition,
In Table 1, comparative steel 9 corresponds to SUS-430,
10 has a composition corresponding to SUS-434.

【table】

[Table] Table 3 below shows the mechanical properties and surface properties of the above test materials after processing. Invention steel 3 with Nb/C of 8 or more has annealing conditions of 900 to 1050 after hot rolling and cold rolling.
℃ and between 850 and 1050℃, as shown in the conditions, surface roping, roughness, etc. will not occur after processing and molding, and
0.2% A material with low yield strength and hardness, high elongation value, and excellent workability can be obtained. On the other hand, methods that deviate from the method of the present invention even if they have the same component composition,
In other words, if the annealing temperature after hot rolling or cold rolling is 850°C or lower, a sufficient recrystallized structure cannot be obtained, and the yield strength and
The hardness increases and the elongation deteriorates, resulting in insufficient formability, and moreover, roping tends to occur after processing and forming. Furthermore, if the annealing temperature after cold rolling exceeds 1050°C, the elongation will be significantly reduced and the crystal grains will become coarser, resulting in roughness on the surface after processing and forming, which will significantly impair the aesthetic appearance. Further, even if proper annealing according to the present invention is performed, the corrosion resistance of Sample 6, in which the amount of Cu is outside the range of the present invention, is lower than that of the steel according to the present invention, as shown in Table 5-A. Furthermore, as shown in Table 5-A, Sample 7, in which the Nb/C ratio is outside the range of the present invention, is inferior in corrosion resistance compared to the steel of the present invention. Sample 9 of SUS-430 equivalent steel shown as comparison steel and SUS-
Sample 10 of steel equivalent to 434 is hard compared to the material properties of steel under the conditions of Invention Steel 3,
Moreover, roping occurs on the surface after processing.

[Table] "Roping" in the surface texture after processing and molding in the table above refers to visually checking the surface of the test piece after applying 20% tensile processing using a JIS No. 5 tensile test piece. The roughness of the machined surface was visually confirmed using the Erichsen test. From the above explanation, it is clear that appropriate Nb/C
By selecting the annealing conditions after hot rolling and cold rolling according to the method of the present invention between 900 and 1050°C for the former and 850 and 1050°C for the latter, the steel of the present invention has good softness and formability. Moreover, it has become possible to manufacture a steel sheet that is ideal for automotive moldings and does not cause roping or roughness on the surface after processing. Next, we will discuss the evaluation results of rust resistance as a molding material for automobiles. As shown in Table 4 below,
A test was conducted to measure the pitting corrosion potential of each test material containing the steel of the present invention, with respect to one whose entire surface was polished to #600 and one whose surface was brightly annealed. As a result, Sample 3 of the steel of the present invention was compared with SUS-
Compared with Sample 9 of 430-equivalent steel and Sample 10 of SUS-434-equivalent steel, both the polished skin and the bright annealed skin showed a high pitting corrosion potential, and the pitting corrosion resistance was improved. In addition, the pitting corrosion resistance of the steel sample 3 of the present invention is further enhanced by bright annealing, and compared with the bright annealed materials of sample 9 of SUS-430 equivalent steel and sample 10 of SUS-434 equivalent steel, A pitting corrosion potential nearly three times higher can be obtained. As is clear from the above description, the steel of the present invention exhibits remarkable effects in terms of resistance to pitting corrosion caused by chlorine ions when combined with bright annealing, which is commonly applied as a molding material for automobiles.

[Table] Table 5-A below shows sample 3 of the invention steel, samples 6 and 7 as comparison steel, sample 9 of SUS-430 equivalent steel, and sample 10 of SUS-434 equivalent steel, respectively, regarding bright annealed materials. Table 5-B shows the results of the salt spray test and the dry/wet cyclic test, and the conditions for the dry/wet cyclic test. Among these test methods, the dry-wet cyclic test in particular serves as an important guideline for practical evaluation of automotive molding materials. Inventive steel sample 3 was tested in both the salt spray test and the dry-wet cyclic test, compared to other comparison steel sample 6.
7 and SUS-430 equivalent steel sample 9, SUS-434 steel
Compared to No. 10, the time and number of times until rusting occurs is longer, and the rust resistance is extremely excellent. As mentioned above, the steel of the present invention is effective against the occurrence of rust in environments containing chlorine ions.
It is clearly superior to 434.

【table】

[Table] As is clear from the various evaluation examples above, the present invention is softer and has better formability than SUS-434 and SUS-430, and has no roping or roping on the surface after processing.
A material with excellent surface properties without roughness can be obtained. SUS has rust resistance in environments containing chlorine ions.
A material with comparable rust resistance to 434, but even better when combined with bright annealing, can be obtained. Among the elements that make up stainless steel, it does not contain Mo or Ni, which are expensive and are easily affected by supply and demand due to fluctuations in the economic environment, making it a relatively economical and stable material. . Stainless steel, which is used as molding material for automobiles, undergoes severe forming processing, and the beauty of the surface after processing is important, and mass production is required because it is required to be rust resistant in the environment containing chlorine ions when it is mounted on a car and is running. The premise is more stable material supply and economic efficiency,
However, the present invention has made it possible to obtain an optimal material that satisfies the above conditions.

Claims (1)

[Claims] 1 C up to 0.05%, Si up to 0.75%, Mn up to 1.0%, 14-18% Cr, 0.3-0.8 Cu, C amount of 8
Stainless steel consisting of Nb up to 1.2% and the remainder substantially Fe except for unavoidable impurities is heated and cooled to 900 to 1050°C after hot rolling, and then cold rolled.
A method for producing an automotive molding material, further comprising bright annealing at 850 to 1050°C.