JPH04228540A - Ferritic stainless steel excellent in high temperature strength and small in rise of strength at room temperature - Google Patents

Abstract

PURPOSE:To offer a ferritic stainless steel excellent in high temp. strength and small in the rise of strength at a room temp. CONSTITUTION:This is a ferritic stainless steel having a feature of contg., by weight, <=0.02% C, <=1.0% Si, <=1.0% Mn, 0.05 to 2.5% Mo, 14 to 20% Cr, <=0.015% N, 0.4 to 1.0% Nb, 0.26 to 1.5% Co and the balance Fe with inevitable impurities. The steel is excellent in high temp. strength and small in the rise of strength at a room temp. The steel may moreover be incorporated with <=0.5% Ti.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel that has excellent high-temperature strength but exhibits little increase in strength at room temperature.

0002

2. Description of the Related Art Conventionally, ferritic stainless steel has often been used as a heat-resistant and oxidation-resistant material. This is because, compared to austenitic stainless steel, ferritic stainless steel (1) has a lower coefficient of thermal expansion, that is, it has better properties (thermal fatigue) in environments where it is subjected to repeated heating. (2) Easy to join to other parts (steel or cast metal). (3) Excellent oxidation resistance in environments subject to repeated heating. (4) It is inexpensive. This is because there are advantages such as.

0003

[Problem to be solved by the invention] However, since ferritic stainless steel is inferior to austenitic stainless steel in terms of high temperature strength, its range of use is naturally limited and it cannot be used in applications where high temperature strength is an issue. Difficult to adapt. Furthermore, as the high temperature strength increases, the strength also increases at room temperature, which causes various troubles. For example, when press forming at room temperature, conventional low-strength materials that had no problems are replaced with high-strength materials, which often causes damage to the equipment, or the equipment cannot be processed into the set mold and the equipment has to be replaced. , and are faced with the need to improve their abilities.

[0004] Normally, elongation, n value, R
Erichsen value, Erichsen value, etc. are used as criteria for whether the material will break or whether it can withstand processing, but in general processing, it is rare to process it to a point close to breaking. These values are not very important. Rather, it is of greater significance whether the strength at room temperature is appropriate.

[0005] An object of the present invention is to provide a ferritic stainless steel that has increased high-temperature strength and exhibits a small increase in strength at room temperature.

[0006]

[Means for Solving the Problems] As a result of extensive review of component elements that affect high-temperature strength, the present inventors have developed an Nb-Mo addition system of ferritic stainless steel or an Nb-Mo addition system of ferritic stainless steel.
In the b-Ti-Mo addition system, Co improves high temperature strength by solid solution strengthening at 0.26% or more, but at 1.5w
The present invention was completed by discovering that no increase in strength occurs at room temperature up to t%. This is because there is a difference in the mechanism of strength improvement at high temperature and room temperature.
This is thought to be because the amount of O differs depending on the temperature.

That is, the present invention provides C: 0.02 wt% or less, Si: 1.0 wt% or less, Mn: 1.0 wt% or less, Mo: 0.05 wt% or more and 2.5 wt% or less, Cr: 1
4 wt% or more and 20 wt% or less, N: 0.015 wt% or less, Nb: 0.4 wt% or more and 1.0 wt% or less, Co: 0.
The object of the present invention is to provide a ferritic stainless steel containing 26 wt% or more and 1.5 wt% or less, with the balance consisting of Fe and unavoidable impurities, which has excellent high-temperature strength and has little increase in strength at room temperature.

[0008] Furthermore, the above steel components may contain Ti in an amount of 0.5 wt% or less.

[0009]

[Operation] The present invention will be explained in more detail below. Next, the reason for limiting the composition range of each component will be explained.

C: 0.02 wt % or less C increases the strength at room temperature and is harmful to high temperature strength, so a lower value is preferable. Therefore, it is limited to 0.02 wt% or less.

Si: 1.0 wt% or less Si is effective in improving oxidation resistance, but if it exceeds 1.0 wt%, workability deteriorates, so the upper limit was set at 1.0 wt%.

Mn: 1.0 wt % or less The lower the Mn content, the better from the viewpoint of processability, but the upper limit was limited to 1.0 wt % in consideration of economic efficiency during manufacturing.

Mo: Mo: 0.05 wt% or more and 2.5 wt% or less Mo
is added to improve high temperature strength. The effect is 0.05
It appears above wt% and saturates at 2.5wt%. Since Mo is a very expensive element, it is limited to 0.05 to 2.5 wt%.

Cr: 14 wt% or more and 20 wt% or less Cr
is a main element that imparts oxidation resistance, but if it is less than 14 wt%, it does not have sufficient oxidation resistance, and if it exceeds 20 wt%, the toughness and workability deteriorate significantly, so the upper limit was set at 20 wt%.

N: 0.015 wt% or less Like C, it increases the strength at room temperature, but if it is 0.015 wt% or less, there is no problem in practical terms.

Nb: 0.4 wt% or more and 1.0 wt% or less Nb is added to improve high temperature strength. 0.4wt%
If it is less than 1.0 wt%, the high temperature strength will not be sufficient, and if it exceeds 1.0 wt%, the workability will deteriorate significantly, so it is limited to 0.4 to 1.0 wt%.

Co: 0.26 wt% or more and 1.5 wt% or less Co is a very important element for the present invention. Figure 1 shows 850℃;
The relationship between the tensile strength at 25°C and the amount of Co added is shown. High temperature strength is improved at 0.26 wt%. On the other hand 25
The strength at ℃ hardly changes up to 1.5 wt%. Therefore, it is limited to 0.26 to 1.5 wt%.

Ti: 0.5 wt% or less Ti is preferably added to improve workability. 0.5wt
If it exceeds 0.5 wt %, there will be many inclusions and the surface quality will deteriorate, so the upper limit is limited to 0.5 wt %.

[0019]

EXAMPLES The present invention will be specifically explained below based on examples.

(Example 1) Test materials with various compositions shown in Table 1 were hot rolled, annealed, and
Made as a 1.5 mm thick cold rolled plate by cold rolling, 1
After annealing at 000°C, pickling was performed. Thereafter, these test materials were subjected to a high temperature tensile test at 850°C and a tensile test at room temperature (25°C). Usually, high-temperature tensile tests are often measured using hot-rolled plates or cast round bars, but the strength was measured using actual conditions, that is, cold-rolled and annealed plates. The results are also shown in Table 1.

[0021] When Nb and Mo are not added, the strength is the lowest at both 25°C and 850°C, as in comparative steel D. Nb, Mo
Although the addition improves the high-temperature strength as in Comparative Steel A, if Nb is further added to further improve the strength at 850°C based on Comparative Steel A, the strength at 25°C as in Comparative Steel E increases. The strength also increases significantly.

However, invention steel No. When the 850°C strength is improved by adding Co instead of Nb as in Nos. 1, 2, and 3, the strength at 25°C hardly changes. Comparative steel B has an insufficient amount of Co, so the 850°C strength is not improved that much. In addition, in comparative steel C, due to excess Co, 850
The strength increased both at ℃ and 25℃. Therefore, it is clear that the steel within the range of the present invention has excellent high temperature strength and also has a small increase in strength at room temperature.

[0023]

[Table 1]

[0024]

According to the present invention, by adding an appropriate amount of Co to Nb- and Mo-added steel, it is possible to obtain a ferritic stainless steel that further improves high-temperature strength and exhibits a small increase in strength at room temperature. Therefore, for example, when making a mold by press working at room temperature, such as a flange part for high temperature, it is a material that can be processed as well as conventional materials and has high high temperature strength, which is very advantageous in terms of design and industrial use. The value is high.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of Co on strength at 25°C and 850°C.

Claims (2)

[Claims] Claim 1: C: 0.02wt% or less, Si: 1.
0wt% or less, Mn: 1.0wt% or less, Mo: 0.05wt% or more and 2.5wt% or less, Cr: 1
4 wt% or more and 20 wt% or less, N: 0.015 wt% or less, Nb: 0.4 wt% or more and 1.0 wt% or less, Co: 0.
A ferritic stainless steel having excellent high-temperature strength and little increase in strength at room temperature, characterized by containing 26 wt% or more and 1.5 wt% or less, with the remainder consisting of Fe and unavoidable impurities. 2. The ferritic stainless steel according to claim 1, further containing 0.5 wt% or less of Ti.