JPS5819743B2 - heat resistant steel - Google Patents

Description

Detailed Description of the Invention 5US403 is a basic steel developed as a steel for turbines.It is inexpensive because it contains only Cu, and has excellent strength at room and high temperatures.
It has excellent ductility, is easy to manufacture, has good fatigue properties, and is widely praised.

However, the biggest drawback of this steel is that its impact value is unstable near room temperature (there is an impact transition temperature near room temperature).

Therefore, the steel of the present invention is a steel having the basic composition of 5US403, which has been improved to improve the impact value without reducing the strength.

The present inventors added small amounts of some elements to SUS 403 as the basic steel to create a material that does not produce any δ ferrite, is easy to hot work, requires only simple quenching and tempering, and has high impact resistance. He invented a heat-resistant steel with excellent properties.

The gist of the present invention is as follows: C0.08-0.25%, Si≦1.5%, Mn≦1.
5%, Ni≦1.0%, Cr10.0-14.0%, N
b +Z ro, 02-0.5%, however, Nb2O, 01
%, Zr2O, 0.1% balance Fe and unavoidable impurities.It is a heat-resistant steel with excellent room temperature toughness, and is used in high-temperature steam, turbines, rotor blades of plates, stationary vane disks, etc.
It is a rust-resistant and heat-resistant steel that can be used in a wide range of applications, including stainless steel for mechanical structures such as pump shafts related to the US403.431 series.

Next, the reason for limiting the composition range of the steel of the present invention will be described.

c: o, os ~ 0.25% C is the most fundamentally important element necessary to impart strength and hardness at room temperature and high temperature.

In order to increase the strength of heat-resistant steel, the first step is to generate carbides made of carbon and carbide-forming elements (Cr, Nb, etc.) through appropriate tempering, and the second step is to make these carbides uniform and fine. It is necessary to distribute C, and for this purpose, the presence of an appropriate amount of C is essential.

However, if the C content is less than 0.08%, not only will the strength be insufficient, but this steel will inevitably generate and remain δ ferrite, which deteriorates the fatigue properties and toughness of the steel as a structural material for turbines. If it exceeds .25%, carbides increase, leading to deterioration of high-temperature strength due to agglomeration of carbides at high temperatures for long periods of time, and promoting embrittlement.

Cr: 10.0-14.0% Cr, like C, is an essential element for wood texture, and is important because it brings about corrosion resistance, heat resistance, and an increase in tempering temperature (shifting of softening to high temperature side due to secondary hardening). Although it is a carbide-forming element, 1
If it is less than 0.0%, it cannot be guaranteed as a heat-resistant steel in terms of heat resistance and corrosion resistance.

Moreover, if it exceeds 14.0%, it becomes difficult to completely eliminate δ ferrite due to the balance between austenite-forming elements such as C, N, and Ni and ferrite-forming elements such as Nb.

Nb Zr: 0.02 to 0.5%, but Nb amount 0.0
1%, Zr2O, 01% co-added Nb is a strong carbide-forming element, which improves high-temperature strength, tempering resistance, and prevents coarsening of crystal grains, and significantly improves toughness due to finely dispersed precipitation of NbC. play a role.

Furthermore, Zr contributes to grain refinement, and therefore Nb and Zr
The effect of refining crystal grains is particularly remarkable when the compound is added in combination.

As a result, it is effective in improving toughness by strengthening grain boundaries.

However, both Nb and Zr are strong ferrite-forming elements, and when added in large amounts, they not only produce δ ferrite and large carbonitrides, impairing cleanliness, but also have favorable effects on other properties, especially fatigue properties. Since there is no Nb + Z r
0.02 to 0.5%, preferably Nb+Z r
It is 0.02-0.2%.

Note that since both elements are expensive, it is uneconomical to add large amounts.

Si: 1.5% or less Si is added as a deoxidizing agent, but if added in large amounts, it tends to promote the formation of δ ferrite and coarsen the crystal grains.
．． It was set to 5% or less.

Mn: 1.5% or less Mn is also added as a deoxidizing agent like Si, but if it exceeds 1.5%, it promotes the growth of retained austenite, so it should be set at 1.5%.
% or less.

Ni: 1.0 or less Although Ni contributes to improving the impact resistance, it significantly lowers the heating transformation point, so the tempering temperature cannot be raised, and the tempering temperature becomes low.

Adding a large amount will generate retained austenite, so 1
．． The upper limit is 0%.

Next, the characteristics of the heat-resistant steel of the present invention will be explained in detail using examples.

Table 1 shows the steels of the present invention and comparative steels.

81~S6 is a 12% Cr heat-resistant steel with C0.1% and Zr0.
Nb is added to 0.01 to 0.15% based on 0.03%, and S7 to S9 have a C content of 0.2%C to 12%C.
This is a heat-resistant steel with a composite addition of Nb and Zr.

Naori 1. B2 is a 12% Cr heat-resistant steel with a single Cr content, and S3
．． S4. B5 is a comparison steel containing only 0.05% NbO2 and 0.05%.

Figure 1 shows the influence of Nb and Zr on tempering resistance.
．． By adding 0.05%, the tempering temperature can be increased by about 70°C from the conventional tempering temperature of 600°C or higher.

Further, by adding 0.04% Zr in combination, the temperature can be increased by 100°C.

This shows that it is extremely effective despite being added in a small amount.

FIG. 2 shows the mechanical properties at room temperature of materials to which 0.02 to 0.04% of Zr was added and to which the amount of Nb was added in increasing amounts.

As the amount of Nb is increased, the strength and hardness increase without decreasing the toughness.

However, if it is added in an amount of 0.02% or more, no further improvement is observed.

Figure 3 shows 0.1%C with Zr0.02~0104% added.
The influence of Nb on the transition temperature of 2Ttr/L* impact value of -12%Cr heat-resistant steel is shown.

The addition of Nb lowers the shock transition temperature near room temperature to about -25° C. at 0.02%, to -30° C. at 0.05%, and further lowers it by adding Zr.

From this figure, the preferable range of Nb is 0.02 to 0.05
%.

Generally, the brittle fracture rate is 50% when the absorbed energy is 6 to 8 kg, m, so it is 7 to 9. The absorbed energy of m was taken.

FIG. 4 shows the effects of Nb and Zr on the impact curve.

With the addition of 0.02% Nb, the shock transition curve has already decreased to below 0°C, and with the addition of Zr, it has further decreased.
It can be seen that the synergistic effect of b and Zr is remarkable.

FIG. 5 shows the influence of Nb and Zr on the relationship between impact resistance and strength of 0.2%C-2%Cr heat-resistant steel.

As with the 0.1% C class, it can be seen that addition of Nb and Zr improves toughness even at the same strength level.

Table 2 shows the influence of Nb and Zr on the creep-loveture rupture time of 0.1%C-12%Cr heat-resistant steel.

In a 1-point test conducted at a low temperature for a short period of time, when the hardness level is the same, the addition of Nb and Zr extends the rupture life by 6 to 8 times.

From the above results, there is a trace amount of Nb in the 5US403 series.

When Zr is added, the impact transition properties and creep rupture properties are significantly improved, and an excellent and inexpensive heat-resistant steel can be obtained.

[Brief explanation of drawings]

Figure 1 shows the influence of Nb and Zr on the tempering hardness of 0.1%C-12%Cr heat-resistant steel.
This was done under oil cooling at 0°C. Figure 2 shows the influence of Nb on the mechanical properties of 0.1%C-■2%Cr heat-resistant steel at room temperature, and the solid line is 9950.
℃ for 30 minutes and then air-cooled at 680℃ for 1 hour.The dotted line is the same <', after 30 minutes of oil cooling at 950℃, 720℃.
It was air cooled for 1 hour. For the impact value, a 5mm Unotch was used at 21°C, for the curvature test, a test piece with a diameter of 12mmφ was used, and for the elongation, a test piece with a gauge length of 40mm was used. Figure 3 shows the influence of Nb on the shock transition temperature of o,i%C-12%Cr heat-resistant steel, and the condition is 2mm.
Using V-notch Charpy, the hardness of the test piece is HRC20.
~22, Heat treatment conditions are 650-720℃ with oil cooling at 950℃
It is air cooled. Figure 4 is a diagram showing the influence of Nb and Zr on the impact transition curve of 0.1%C-12%Cr heat-resistant steel, and the conditions such as notch pyrolysis and heat treatment of the test piece were the same as in Figure 3. be. FIG. 5 is a diagram showing the influence of Nb and Zr on the hardness and impact value of 0.24C-12%Cr heat-resistant steel.

Claims (1)

[Claims] I C0.08~0.25%, Si≦1.5%, Mn
≦1.5%, Ni≦1.0%, Cr1O, 0-14.0
%, Nb+Zr0.02-0.5% However, Nb2O,01
%, Zr2O, 0.1% balance Fe and unavoidable impurities.Heat-resistant steel with excellent toughness at room temperature.