JPS59211553A - High cr steel with superior toughness and superior strength at high temperature - Google Patents

Abstract

PURPOSE:To improve the toughness of the resulting martensitic high-Cr steel, the strength at high temp. and the oxidation resistance at high temp. and to make the steel suitable for use as the material of a thick member for high temp. use by preparing a low carbon steel having a specified composition contg. Si, Mn, Cr, Mo, Nb, N and P. CONSTITUTION:This martensitic high-Cr steel has a composition consisting of, by weight, <=0.09% C, 0.02-0.15% Si, <=3.0% Mn, 7.0-15.0% Cr, <=3.0% Mo, <=0.10% Nb, 0.05-0.15% N, <=0.015% P and the balance essentially Fe. The composition may further contain <=0.2% V and/or one or more among <=1.0% Ni, <=1.0% Cu, <=0.01% Ca, <=0.01% Mg and <=0.01% rare earth element.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a martensitic high Cr steel having excellent toughness and high high temperature strength and suitable for use as a thick-walled member for high temperature use.

In general, various parts such as heat exchange tubes, piping, joints, and pulp used in high-temperature and high-pressure environments such as nuclear power facilities such as fast breeder reactors, thermal power generation facilities, or general boiler facilities, etc. It goes without saying that it has high high-temperature strength (creep strength) and excellent high-temperature oxidation resistance, but at the same time, it also has excellent weldability and prevents accidents caused by thermal stress. Another important requirement was a sufficiently small coefficient of thermal expansion.

By the way, high-CT steel containing 7 to 45% Cr (hereinafter referred to as weight percentage) has been known as a high-temperature material used on land at such high temperatures of 500 to 600°C. It is widely used in various fields for the following reasons: first, it has excellent high-temperature oxidation properties, the high-temperature strength can be further improved by adding alloying elements such as MO, the coefficient of thermal expansion is small, and the price is relatively low. It's starting to be used. Furthermore, as can be seen in JP-A No. 54-81116, many materials have been proposed that are based on high Cr steel and have specific alloying elements added to it by cooling to improve desired properties. The performance and reliability of the various high-temperature equipment mentioned above have further improved.

On the other hand, based on this technical background, high-temperature equipment is rapidly becoming more efficient and larger in order to deal with the remarkable increase in energy demand. There has also been a strong demand for thinner joints and thicker joint members.

However, as conventional high-Cr-based high-temperature steels containing Nb or V, for example, there is a 9Cr-1Mo steel that exhibits a martensitic structure after heat treatment by increasing the C content, but has poor toughness and weldability.

Furthermore, after a predetermined heat treatment, 9Cr-2Mo steel exhibits a structure containing a large amount of ferrite in martensite. Although the latter shows good bending workability when used for thin-walled pipes, it cannot be used for thick-walled pipes.

When used as thick-walled materials such as forged steel products or cast products,
As the amount of ferrite increases, it becomes difficult to secure the desired strength, and especially in the case of thick-walled steel, long stress relief annealing is required, and further improvement in toughness is strongly required. There is. In particular, in the case of parts manufactured by hot working, 1. There have been frequent reports that the toughness in the direction perpendicular to the processing direction is significantly degraded.

From the above-mentioned viewpoint, the present inventors first conducted numerous experiments and studies to investigate the causes of the above-mentioned problems observed in high-Cr-based high-temperature thick-walled steel materials, and found that (a) Conventional high-Cr high-temperature steels contain about 5 to 30% by volume of ferrite in martensite after heat treatment, but when it comes to thick-walled materials, the cooling rate after standard treatment is slow. Moreover, since the processing rate during hot working cannot be increased sufficiently, the amount of ferrite in the steel increases further, which inevitably leads to a decrease in strength. It is essential for materials to be subjected to stress relief annealing after welding, but as the amount of ferrite increases, the toughness and strength will deteriorate after stress relief annealing, resulting in embrittlement due to prolonged use at high temperatures. (C) In particular, when thick-walled materials are manufactured by hot working, the soft large ferrite phase existing in the hard martensitic phase is stretched thin, and the hot working process A new fact has been discovered that a structure in which coarse ferrite extends in the direction is formed, resulting in anisotropy in the material, which causes the difference in toughness between the processing direction and the direction perpendicular to it to become relatively large. I found it.

Therefore, the present inventors have determined that in order to ensure sufficient strength at room temperature or high temperature in high Cr steel and to minimize the decrease in toughness after stress relief annealing or after long-term use, it is necessary to In order to improve weldability, we have concluded that it is best to select a composition system that does not generate ferrite and has a single phase of martensite, and also has a low content of C etc. in order to improve weldability. Not only does it have excellent properties, but it also has a low coefficient of thermal expansion, is inexpensive,
As a result of further research to create a martensitic steel that also has good toughness, (d) the Si content of the steel was kept extremely low by adding 7 to 15% Cr to ensure high-temperature oxidation resistance. When a predetermined amount of N is added to the steel, the formation of ferrite is extremely reduced and a substantially single-phase martensitic steel material is obtained, and by adjusting the Si content to a specific low range, carbides are finely dispersed. As a result, toughness deterioration due to carbide precipitation is prevented, and the toughness of the steel material is greatly improved.(,,) By adding N, the amount of ferrite itself can be reduced as mentioned above. (e) In addition, by adding a predetermined amount of Nl, the toughness of the steel material is further improved and the high temperature strength is further improved. Reducing the content suppresses the amount of carbide precipitation during stress relief annealing and use at high temperatures, further improving toughness. (I)) Adding a predetermined amount of Ni,
Cu, Ca.

(i) The decrease in strength due to the reduction in C content is due to the fact that the steel becomes a martensitic single phase steel, and We have come to the knowledge shown in (d) to (i) above that the strength can be sufficiently covered by addition, and further improvement in strength can be achieved by adding (2).

In order to improve the toughness of r-high Cr steel, it is necessary to obtain a structure consisting only of martensite without forming ferrite after standard treatment, and for this purpose, C and Mn, which are austenite forming elements, are required.

It is desirable to increase the content of Ni, Cu, and N within a range that does not cause embrittlement, and to reduce the content of Si, Cr, and Mo, which are heptagonalite-forming elements. Considering the embrittlement and high temperature strength,
The upper limits of C and N contents and the lower limits of Cr and Mo will be determined by themselves. However, especially C, Cr
The reduction in high-temperature strength caused by reducing Mo and can be sufficiently prevented by adding Nb and N.
Addition of these elements also leads to further improvement of toughness.”
This was done based on the above knowledge based on the fact that
High cr steel, C: 0.09% or less, Si: 0.02-015%.

Mn: 3.0% or less, Cr: 7. O~1
5.0%.

Mo:3. Q related, lower, Nb: 0.10% or less.

N: 0.05-0.15%, P: 0.015% or less.

and, if necessary, Ni: 1.0
% or less, Cu: 1.0% or less.

(Shea: Q, 01% or less, Mg: 0.01
%below.

Rare earth elements (REM): 0.01% or less.

V: Section 02 and below.

Including the first type of carp, )j’e and unavoidable impurities: remainder.

By having a composition consisting of the following, the steel is characterized by a martensitic single-phase steel with excellent toughness and high-temperature strength, and excellent high-temperature oxidation resistance.

Note that the high Cr steel of the present invention exhibits the desired sufficient effect when it is made into a thick-walled product by hot working or used as a cast product, but the conventional high Cr steel It goes without saying that good results can also be obtained by hot-working thin-walled products.

Next, the reason why the composition ratio of chemical components in the steel of the present invention is numerically limited as described above will be explained.

■ CC Does C have the effect of reducing the amount of ferrite and increasing the strength at room temperature and high temperature?
%, the amount of carbide precipitation increases during stress relief annealing or use at high temperatures, significantly reducing toughness.
Because it extremely increases the susceptibility to hot cracking during welding,
The C content is set at 0.009% or less.

5I Si is known as an effective element for deoxidizing steel, but if its content exceeds 0.15%, the amount of ferrite increases, leading to deterioration of toughness. And the Si content is 0
．． It has been found that within a range of 15% or less, carbides in the steel are not finely dispersed, and deterioration in toughness due to carbide precipitation is also suppressed. The lower the 5 content, the better the results, and in the recent melting of steel using A, OD or VOD, sufficient deoxidation is possible even with a low S1 content. Considering economic efficiency, the lower limit was set at 002%. For this reason, the Si content is reduced to 0.
02~0]5%.

Note that the S1 content is preferably 0.10% or less, and within this range, toughness deterioration due to carbide precipitation is significantly suppressed. The effect becomes even more noticeable.

■ 1・AJ" 1 to 4n components are effective in reducing the amount of ferrite as austenite-forming elements, but if they are contained in excess of 30%, the toughness of the martensite part will deteriorate, so The Mn content is set at 30 or less.

■ Cr Does the Cr component have the effect of improving the oxidation resistance and high-temperature strength of steel?If its content is less than 70%, the desired effect cannot be obtained, especially the effect of improving oxidation resistance. 15
If the content exceeds 0%, the amount of ferrite increases rapidly, and no matter what measures are taken, it is no longer possible to eliminate ferrite, and the strength and toughness at room temperature are significantly deteriorated. The Cr content was determined to be between 7.0 and 15.0.

■　M.

The Mo component is an effective element for improving the strength and toughness of steel, but like Cr, MO is also a ferrite-forming element, and 3
If the MO content exceeds 0, a large amount of ferrite will be generated, resulting in a decrease in toughness, so the MO content was set at 3.0 or less.

■ Nb The Nb component has the remarkable effect of improving toughness by refining crystal grains and increasing strength at both room temperature and high temperature. However, if it is contained in excess of 0.10%, the susceptibility to hot cracking during welding becomes significant. The Nb content is set at 0.0610% or less because it produces high-ib and ferrite, which reduces both strength and toughness. In addition, N
b exhibits a sufficient effect even when added in an amount of 0.05 or less.

@N Like C, the N component has the effect of suppressing the formation of ferrite and improving the room temperature and high temperature strength, but if its content is less than 0.05%, the desired effect cannot be obtained. I can't. On the other hand, compared to C, the N component causes less deterioration in toughness after stress relief annealing or when used at high temperatures, so it is permissible to add a relatively large amount, but if it is added in excess of 0.15%, nitrides may be formed. This will lead to deterioration of toughness.

Therefore, the N content was determined to be 0.05 to 0.15%.

■ P P exists as an impurity that is inevitably accompanied in steel, and is an element that accelerates the decrease in toughness of martensitic steel containing 7 to 15 Cr due to long-term use at high temperatures. When the P content is 0.015% or less, the toughness deterioration effect is significantly alleviated, so the P content is reduced to o, o1.
It was set as 5m or less.

It was also confirmed that when the P content is suppressed to 0.010% or less, the effect of improving toughness becomes even more remarkable.

■Ni, Cu, Ca, Mg, and rare earth elements These elements have the effect of improving the toughness of the high Cr steel of this invention by adding them, so they can be added when it is necessary to further improve the toughness of the steel. Although one or more types of these are added, the reason for limiting the amount added will be explained in more detail below, including the accompanying effects.

■ Ni and Cu Ni and Cu components are austenite-forming elements and have the effect of suppressing the formation of ferrite, so they are especially added to thick-walled materials where the cooling rate in standard processing is slow and there is a risk of forming ferrite. When contained, it is effective in suppressing ferrite formation and ensuring toughness and strength.

However, even if each content exceeds 1.0%, not only will it not be possible to obtain further improvement effects, but
In particular, since Cu deteriorates hot workability, the content of each was determined to be 1.0% or less.

■ Ca, Mg, and rare earth elements In particular, these elements also have deoxidizing (more effective in the case of low Si) and desulfurizing effects, so they improve the hot workability of steel materials and improve thick-walled materials. It also enables strong hot working and further improves toughness, but
Since hot workability deteriorates if the content exceeds 0%, the content of each is determined to be 0% or less.

Regarding rare earth elements, it is practically preferable to add them in the form of metals in consideration of economic efficiency.

■ V ■ component does not have the crystal grain fine cracking effect like Nb, but like Nb, it has the effect of improving the strength at room temperature and high temperature, and also has the effect of increasing the hot cracking susceptibility of the weld zone. Nb
Since it is smaller, it is extremely good as an element for increasing the strength of high Cr steel. The content is 0.
If it exceeds 2%, the susceptibility to hot cracking during welding increases and the amount of ferrite increases, so the content is set at 0.2% or less.

In addition, the impurity element sb, like P, has a negative effect on toughness, so it is desirable to suppress its content to 0.01% or less, and CO also has the effect of facilitating ferrite formation. In addition to this, when used for nuclear power generation, induced radioactivity is generated, so it should be suppressed as much as possible (0
．． 1% or less) is desirable.

Next, the present invention will be explained by examples and in comparison with comparative examples.

Example 1 First, steels 1 to 36 having the compositions shown in Table 1 were melted in an electric furnace and formed into ingots. Next, each obtained steel ingot was forged into a plate with a thickness of 1009 mm, and then subjected to the standards and tempering treatment shown in Table 2, followed by a tensile test.
Charpy impact test and further 600 after tempering treatment
Charpy impact tests were conducted on samples that had been aged for 1000 hours at .degree. C., and creep strength measurement examples.The results are also shown in Table 2.

All Charpy impact test pieces were taken perpendicular to the forging direction, and the impact absorption energy values after aging treatment were measured based on values that correspond to actual usage conditions subjected to thermal history. This is to find out. The creep strength is expressed as the breaking stress after 1000 hours at a temperature of 650°C.

From the results shown in Table 2, the steel of the present invention has sufficiently high strength not only at room temperature but also in creep strength at 600℃
The Charpy impact energy value after 000 hr of aging treatment is also extremely high, and it is clear that embrittlement is extremely low when compared with the as-tempered phase.

On the other hand, comparative steels 26 to 36 are, for example, steel yuzu 9 to
As shown in 2↓, the strength was low due to the presence of ferrite, or the toughness was significantly deteriorated due to aging treatment, or the creep strength was low, so it is fully satisfactory as a high-temperature steel. It can be seen that it does not have the characteristics.

Example 2 Inventive steels 37 and 38 and comparative steel (conventional steel) 39 having the composition shown in Table 3 were melted in an electric furnace, and cast into sand molds to a length of 500 mm. Width is 200
Finally, a plate-shaped casting with a thickness of 80 mm was created.

Next, these samples were subjected to standard and tempering treatments as shown in Table 4, and then their tensile properties and Charpy impact properties were examined in the same manner as in Example 1.

These results are also shown in Table 4.

From the results shown in Table 4, it is clear that the steel of the present invention has sufficient strength and toughness even when used as a casting material, whereas the steel of the present invention has sufficient strength and toughness even when used as a casting material. It can be seen that steel has low strength and is extremely poor in toughness, especially toughness after aging treatment.

As described above, according to the present invention, it is possible to obtain at a low cost a steel with a low coefficient of thermal expansion that is suitable for room temperature and high temperature strength, has extremely good toughness, and has excellent high temperature oxidation resistance. This brings about extremely useful effects industrially, such as the ability to further improve the performance of equipment components.

Applicant: Mitsubishi Heavy Industries, Ltd. Applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] (1) M content ratio: C: 0.09% or less. Si: 0.02-0.15%. Mn: 3.0918 or less. Cr: 7. o~15.0%. Mo: 3. O under high yield. Jsub: 010% or less. 1 chi: 0.05 to 0.15 chi. P: 0.015% or less. Martensitic high Cr copper with high toughness and high temperature strength, characterized by containing ]I″e and unavoidable impurities: residue. (2) In weight proportion, C2O , C19 or less. Si: O, O'2~015%. Mn: 3.0% or less. Cr: 7.0~15.0%. Mo: 3.0% or less. Nb: 0.10 or less N: 0.05 to 0.15%. P: 0.015% or less. Ni: 1.0% or less. Cu: 1.0% or less. Ca: 0.01% or less. .Mg20.01% or less.Rare earth elements 20.01% or less.Maltene with excellent toughness and high temperature strength, characterized by containing one or more of the following, Fe and unavoidable impurities: balance P. Site-based high Cr steel. (3) In terms of weight percentage, C: 0.09% or less. Si: 0.02 to 0.15%. Cr: 30% or less. Cr: 7.0 to 1. 0%.Mo', 3.0% or less.Nb2, 0.10% or less.N: 0.05 to 0.15%.P2, 0.015% or less.Also contains 120.2%. f, t of toughness and high-temperature strength, characterized by comprising the following: Fe and the remainder of unavoidable impurities.
FTC martenzite high Cr steel. (4) C20.09 or less in terms of weight percentage. Si: 0.02-0.15%. l Acceptable n: 3.0 or less. Cr: 70-15.0%. Mo: 3.0 section or less. Nl): 0.10% or less. 1li: 005-015%. P:0. O15 and below. and furthermore, Ni: 1.0% or less. Cu: 1. O under high yield. Ca: 0.01% or less. Mg: 0.01% or less. Rare earth elements 0.01% or less. Contains one or more of the following, and ■202% or less. Also contains Fe and unavoidable impurities: residual υ. A martensitic high Cr steel with excellent toughness and high temperature strength.