JPS5871362A - Ferritic stainless steel product with superior corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a ferritic stainless steel product with superior corrosion resistance by working a steel alloy prepared by adding specified amounts of C, Si, Mn, Cr and Cu and/or Ni to Fe and by passivating the surface of the worked alloy. CONSTITUTION:The composition of ferritic stainless steel is composed of, by weight, <0.1% C, 0.1-2% Si, 0.1-2% Mn, 16-18% Cr, 0.1-2% cu and/or 0.1- 2% Ni, and the balance Fe. For example, in the manufacture of a spoke of a car or a motorcycle, the refined ferritic stainless steel is hot worked, annealed, cold worked, especially cold drawn, subjected to necessary mechanical working and passivated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improving the corrosion resistance of ferritic stainless steel products.

For example, products such as spokes for bicycles and motorcycles have generally been made of galvanized hard steel wire, but in recent years there has been a growing trend to make various parts stainless steel. The use of stainless steel has also been attempted for spokes and rims.

Stainless steel materials suitable for such applications include 5
Although LIS 304 is satisfactory in terms of corrosion resistance, it is relatively expensive, so it is only put into practical use in some cases.

In order to promote the use of stainless steel, it is desirable to use low-cost materials. However, since a relatively low-profile material such as 5US430 has insufficient corrosion resistance, especially the salt water resistance required for the above-mentioned applications, there is no practical benefit in making it stainless steel.

The present invention successfully overcomes this situation by improving the corrosion resistance of existing low-cost stainless steel products.

The ferritic stainless steel product of the present invention with excellent corrosion resistance has c: 0.10% or less and Si: 0.10 to 20%.
, Mn: O, ]O~2.0% and Cr: 16~1
8%, as well as ('u: 0.1-2.0% and N10
The steel alloy is characterized in that after processing a steel alloy containing 1 to 20% of one or two of 7 and the remainder substantially 1.e, the surface of the steel alloy is subjected to passivation treatment.

-1- The alloy composition excluding Cu and N1 is the composition of S[JS 130. The role of each alloying element and the reasons for limiting their contents may be considered to be similar to those known for 5LI8430 in the present invention.

Various attempts have been made to passivate stainless steel, but in many cases the corrosion resistance of the passive coating is not as good as expected. It is difficult to succeed in trying to strengthen the film by adding other elements, as shown in the examples using Si, \l, or Mo, which will be shown later.
The fact that corrosion resistance is significantly increased when u and/or N1 are added to 5TJS 430 and combined with passivation treatment is a unique example discovered for the first time by the present inventors.

Both Cu and Ni are effective when present in an amount of 0.1% or more, but Cu is more effective.

Furthermore, at the same (total) amount added, the effect is more pronounced when both are used in combination than either one alone. The upper limit of 2.0% was set because exceeding this value would not only significantly reduce productivity but also be economically disadvantageous.

In the process of manufacturing the ferritic stainless steel product of the present invention, the means for shaping into the product shape is arbitrary. The most typical procedure, taking spokes as an example, is to hot-work the molten material, annealing it, then cold-working it, especially cold drawing, and performing the necessary machining operations such as heading and thread rolling. Later, it will be subjected to passivation treatment.

Regarding the mechanical properties in the annealed state, the material according to the present invention has higher strength and no inferior workability than 5USj30. This tendency is maintained even after work hardening occurs due to cold working. (Irrespective of the deviation, the conditions for the molding process can be determined based on what is conventionally known regarding 5tJS430, with reference to the experimental data shown later.

The conditions for the passivation treatment may also be in accordance with techniques commonly used in the field. For example, this method involves immersion in a heated concentrated nitric acid bath for an appropriate amount of time, adding dichromate to the bath,
It is known that electricity can be applied to increase the effect.

The stainless steel products of the present invention have been explained above, focusing on examples of spokes, and specific data thereof are listed below. SUS 304 grade corrosion resistance is required at a low price, especially for products that must have the corrosion resistance to pass the salt spray test, which represents severe usage conditions such as exposure to wind and rain.
The present invention is applicable.

EXAMPLES Steel having the composition shown in Table 1 was melted in an induction furnace and cast into 25 to 9 ingots. Sample material N [11” is SUS 4
30 and N(L 2-6 are C
'u and/or N1, and N(L
7' to 9'' are those with Si, AI or Mo added. (≠ mark indicates a comparative example.) The ingot was forged into a round bar with a diameter of 20 IIJ, and 75
Annealing was performed by heating at 0°C for 2 hours and then cooling in a furnace.

The mechanical properties of annealed materials at room temperature were investigated.

The results are listed in Table 2. From the data in Table 2,
The alloy having the composition adopted in the present invention is SUS 430
It can be seen that when compared to

Next, the annealed material was cut and polished to a diameter of 16 wJ.
It was finished into a round bar and subjected to passivation treatment by immersing it in a bath with a composition of 20% nitric acid + 2% sodium dichromate at 50°C for 1 hour. Then, a 24 hour x 6 cycle salt water gX fog test was conducted on each of the untreated and treated samples.

The results are shown in Table 3. N (Ll-1N[Ll“
and No. 8" samples already showed rusting after 2 cycles. Nα2 and Nα4, which did not rust at all, were C
It is thought that the effect of the combined use of u and Ni was enhanced.

When the annealed materials were observed under a microscope, they all had the microstructure of normal ferritic stainless steel in which carbides were dispersed in a ferrite base, except for Nα8'.

Table 2 Tensile test piece: JIS No. 4 (d-8(I), GL=30)
Table 3 Next, the above-mentioned annealed material round bar was prepared with a diameter of 16. By cutting the rum and performing cold drawing at various processing rates,
We investigated changes in hardness depending on the processing rate.

It is clear from the table that the hardness increases significantly. The hardness before processing is sufficient to allow processing, while the hardness after processing has increased to a level that is desirable for the product. Although there are some variations, it is generally recognized that the hardness in the annealed state and the hardness after working are in a parallel relationship for each material tested.

In order to examine the mechanical properties of the drawn material, the processing rate was 84.
A wire rod with a diameter of 2.511 JI obtained by drawing % was tested. The results obtained are shown in Table 5.

Strength, as well as hardness, was higher in the annealed state for materials.
The value remains high even after drawing.

In comparison with the basic 5LIS/130, the trend observed in the annealed state, ie, the strength of the material according to the present invention is higher, yet the workability is not inferior, can be confirmed here as well.

Table 5 Patent applicant: Daido Steel Co., Ltd.

Claims (3)

[Claims] (1) C: 0.10% or less, Si: 0.10-2.
0%, to4n: 0.10-2.0% and Cr:1
6-18%, and Cu:Q, ] ~2.0% and Ni
: Corrosion resistance characterized by processing a steel alloy containing one or two of 0.1 to 2.0%, with the remainder essentially consisting of Fe, and then passivating the surface. Excellent ferritic stainless steel products. (2) The stainless steel product according to claim 1, wherein the processing is cold working T, especially cold drawing. (3) The stainless steel product according to claim 2, which is a spoke of a bicycle or a motorcycle.