JPS59211552A - Martensitic high cr steel with high toughness - Google Patents

Abstract

PURPOSE:To provide high toughness, superior strength at high temp. and superior oxidation resistance at high temp. to the resulting martensitic high Cr steel and to reduce the cost by preparing a low carbon steel having a specified composition contg. Si, Mn, Cr, Mo, N and P. CONSTITUTION:This martensitic high Cr steel with high toughness has a composition consisting of, by weight, <=0.09% C, 0.01-0.15% Si, <=3.0% Mn, 7.0-15.0% Cr, <=3.0% Mo, 0.05-0.15% N, <=0.015% P and the balance essentially Fe. The composition may further contain one or more among <=1% 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 high toughness 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 valves used in high-temperature and high-pressure environments such as nuclear power facilities such as fast breeder reactors, thermal power generation facilities, and general/doiler facilities, etc. Of course, high temperature strength (creep strength) and excellent high temperature oxidation resistance are required, but at the same time, it must also have excellent weldability and not cause accidents due to thermal stress. Another important factor was that it had a sufficiently small coefficient of thermal expansion.

By the way, as a high-temperature material used at such high temperatures of 50Q to 600°C, high CI steel containing 7 to 3% Cr (hereinafter % is a weight percentage) is known. It has been widely used in various fields due to its excellent high-temperature oxidation resistance, the ability to further improve high-temperature strength by adding alloying elements such as MO, low coefficient of thermal expansion, and relatively low price. has become widely used. For example, as seen in JP-A-54-81116, many materials have been proposed that are based on high Cr steel and have specific alloying elements added to it to improve desired properties. The performance and reliability of the various high-temperature equipment mentioned above have become even more advanced.

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

However, conventional high-Cr-based high-temperature steels, for example (d
Although the 9Cr-I Mo steel exhibits a martensitic structure after a predetermined heat treatment, its strength at room temperature is low, and the 9Cr-2MO steel, which has considerably high high-temperature strength, exhibits a structure containing ferrite in martensite.

Therefore, although the latter exhibited good bending workability when used in thin-walled pipes, when used as thick-walled materials such as thick-walled pipes, forged steel products, or cast products, the increase in ferrite content In addition to making it difficult to secure the desired strength, especially in the case of thick-walled materials, stress relief annealing may be performed between the length O1, and further improvement of toughness 1-V is strongly required. Particularly, in the case of a member manufactured by thermal 1uJ processing, the difference in toughness between the processing direction and the direction perpendicular to this becomes relatively large by as much as 13A.

From the above-mentioned viewpoint, the present inventors first conducted numerous experiments and research to find out the cause of the above-mentioned problems observed in high-Cr-based high-temperature thick-walled steel materials. (1) Conventional high-Cr-based high-temperature steels with high high-temperature strength contain ferrite in martensite in the state after heat treatment, but when it becomes thick-walled, As the cooling rate after standard treatment becomes slower and the processing rate during hot working cannot be increased sufficiently, the amount of ferrite in the steel increases further, inevitably leading to a decrease in strength. (b) For thick-walled materials, stress relief annealing is essential after welding, but if the amount of ferrite increases, the toughness will deteriorate after stress relief annealing, and the subsequent high temperature (C) In particular, when thick-walled materials are manufactured by hot working, the soft large ferrite phase existing in the hard martensitic phase is pushed thinly. This results in the formation of a structure in which coarse ferrite extends in the hot working direction, resulting in anisotropy in the material, which causes a significant decrease in toughness and strength in the direction perpendicular to the working direction. Therefore, the present inventors have discovered a new fact that high CR steel has sufficient strength at room temperature or high temperature, and also prevents the decrease in toughness after stress relief annealing or after long-term use. In order to minimize the formation of ferrite, we have reached the conclusion that it is best to choose a single-phase martensite component system that does not generate ferrite even after standard treatment, and that it has excellent high-temperature strength and high-temperature oxidation resistance. Of course,
As a result of further research to create a martensitic steel that has a low coefficient of thermal expansion, is inexpensive, and also has good toughness, we found that (d) 7 to 15% of CF was added to improve high-temperature oxidation resistance. By keeping the Sl content of the obtained steel extremely low and adding a predetermined amount of N, the formation of ferrite is extremely reduced and a steel material with a substantially martensitic single phase can be obtained.
By adjusting the Si content to a specific low range, carbides are finely dispersed, preventing toughness deterioration due to carbide precipitation, and as a result, the toughness of the steel material is significantly improved; (e) N By adding it, the amount of ferrite itself can be reduced as mentioned above, and
(f) Further reducing the C content suppresses the amount of carbide precipitation during stress relief annealing and use at high temperatures, further improving toughness; The decrease in strength due to the reduction in the amount of Ni can be fully compensated for by becoming a martensitic single phase steel. (1)) Adding a predetermined amount of Ni to such high Cr steel
Cu, Ca.

We have come to the knowledge shown in (d) to (h) above that the addition of one or more of Mg and rare earth elements further improves the toughness.

This invention was made based on the above findings,
High Cr steel, C: 0.09% or less, Si: 0.01 to 0.15%.

Mn: 3.0% or less, Cr: 7. O～l
5.0%.

Mo: 3.0% or less, N: 0.05-0.1
5%.

P: 0.015 or less.

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

Ca: 0.01% or less, Mg: O, 01% or less.

Rare earth elements (REV): 0.01 or less.

Also includes one or more of the following: 1?0 and unavoidable impurities: remainder.

This steel is characterized by having a martensitic single-phase structure steel with extremely good toughness and satisfactory high-temperature strength and 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 is a matter of course that good results can be obtained even by hot working thin-walled products as shown in (4).

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

■To reduce the amount of ferrite and to reduce the amount of ferrite,
However, if its content exceeds 0.09%, the amount of carbide precipitation increases during stress relief annealing or high temperature use, resulting in a significant decrease in toughness. In addition, the C content was set at 0.09% or less because it also extremely increases the susceptibility to hot cracking during welding.

■S] S is known as an effective element for deoxidizing steel, but if its content exceeds 15% of O, the amount of ferrite will increase and the toughness will deteriorate. It has been found that when the C and Si content is within the range of 0.15% or less, carbides in the steel are finely dispersed and deterioration in toughness due to carbide precipitation is suppressed. In addition, the lower the S1 content, the better the results, and the recent A
Although it has become possible to achieve sufficient deoxidation even if the S1 content is lowered in steel melting by VOD or VOD, the lower limit has been set at 0.01% in consideration of economic efficiency. For this reason, S
1 content is set at 0.01 to 0.15%.

Note that the S1 content is preferably 0.10% or less, and within this range, toughness deterioration due to carbide precipitation is significantly suppressed; however, in the 81 content range of 0.05% or less, The effect is even more pronounced.

@Mn The Mn component is effective in reducing the amount of ferrite as an austenite-forming element, but if it is contained in an amount exceeding 3.0 mm, the toughness of the martensite portion will deteriorate, so the Mn content must be reduced. The number of employees is set at 30 or less.

■ Cr The Cr component has the effect of improving copper oxidation properties and high-temperature strength of steel, but if its content is less than 7.0%, the above effects,
In particular, the desired effect on surface oxidation improvement cannot be obtained, and on the other hand, if the content exceeds 15.0%, the amount of ferrite increases rapidly, and it is no longer possible to eliminate ferrite no matter what means are taken. The Cr content should be set at 70 to 15.
It was set as 0%.

■ J＼4Q l・Ao is an effective element for improving the strength and toughness of acid component steel, but like Cr, 1. Since ferrite is generated and causes a decrease in toughness, the Mo content is reduced to 3 mL.
．． It was set as 0% or less.

■N The N component has the same properties as C, which suppresses the formation of ferrite.
Moreover, it has the effect of improving room temperature and high temperature strength, but if its content is less than 0.05%, the desired effect cannot be obtained. 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 will form. This results in deterioration of toughness. Therefore, the N content was determined to be 0.05 to 0.15%.

@P ■] is an element that exists as an impurity that is unavoidably 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.
It is set at 5% 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, C1, 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, it is necessary to further improve the toughness of the steel. In some cases, one or more types of these are added, but the reason for limiting the amount added will be explained in more detail below, including the accompanying effects.

■Ni and Cu N1 and Cu components are austenite-forming elements and have the effect of suppressing the formation of ferrite, so they are added to thick-walled materials where the cooling rate in standard processing is slow and there is a risk of forming ferrite. It is definitely effective in suppressing ferrite formation and ensuring toughness and strength.

However, even if the content exceeds 0% of each, no further improvement effect can be obtained, and in particular, Cu deteriorates hot workability, so each content The amount is set at 1.0% or less/ζ0 (jj) Ca, Mg, and rare earth elements, especially these elements are deoxidized (more effective in the case of low S1)
, also has a desulfurization effect, so it improves the heat [u] processability of the preparation, enables strong hot working even in thick-walled materials, and further improves toughness. Since hot workability deteriorates if the content exceeds 0.01% or less, the content of each is determined to be 0.01% or less.

Note that it is practically preferable to add the rare earth element in the form of Mitsushi metal in consideration of economic efficiency.

In addition, the impurity element Sb, like P, has a negative effect on toughness, so it is important to keep its content below 0.01 factor. In addition to having a
12% or less) is desirable.

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

Example 1 Steel having the composition shown in Table 1 was melted in an electric furnace for 1 to 2 days and then formed into an ingot. Next, each of the steel ingots was forged into a 100 mm thick plate, and after being subjected to baking and tempering treatments as shown in Table 2, it was subjected to a tensile test, a Charpy impact test, and a plate with a thickness of 100 mm. Furthermore, Charpy impact tests were conducted on the samples subjected to aging treatment at 600° C. for 1000 hours, and the results of creep strength measurement examples are also shown in Table 2.

The Charpy impact test specimens were taken perpendicular to the forging direction, and the impact absorption energy values after aging were measured using one straight test specimen that was subjected to thermal history and was subjected to actual usage conditions. This is to find out. The creep strength was expressed as the breaking stress after 1000 hours at a temperature of 650°C.

From the results shown in Table 2, it is clear that the steel of the present invention not only has sufficient strength at room temperature, but also has sufficiently high creep strength.
The 7 yalpy impact energy value after aging treatment for 000 hours is also extremely high, and it is clear that embrittlement is extremely low when compared with the post-temperature phase of the untempered steel.

On the other hand, comparative steels 20 to 28 have low strength, as seen in steel type 28, or are aged (r(
Therefore, it can be seen that the steel does not have sufficiently satisfactory characteristics as a high-temperature steel, as shown by the significantly deteriorated toughness (9) or the low creep strength.

Example 2 Inventive steels 29 and 30 and comparative steel (conventional steel) 31 having the composition shown in Table 3 were melted in an electric furnace, and cast into sand molds with a length of 500 mjn and a width. is 2
A plate-shaped casting having a size of 00 mjl and a thickness of 80 m was produced.

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

These results are also shown in Table 4. From the results shown in Table 4, it is clear that this steel has sufficient strength and toughness to be used as a casting material. for,
It can be seen that comparative steels whose compositions are outside the scope of the present invention have low strength and significantly inferior 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 having high strength at room temperature and high temperature, extremely good toughness, and excellent high temperature oxidation resistance and a low coefficient of thermal expansion. 3. Applicant: Mitsubishi Heavy Industries, Ltd. Applicant: Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] (1) In terms of weight ratio, C: 0.09q6 or less. Si: O, O1 to 0.15. Mll: 3.0% or less. Cr: '7.0-15.0%. Mo: 3. O under high yield. N: 0.05 to 0.15 yen. P: 0.015% or less. Contains 1'i'e and unavoidable impurities: residue. Marunzite-based high Cr material with good toughness, characterized by consisting of: (2) In terms of weight ratio, C: 0.09 or less. Si: 0.01-0.15%. Mn: 3. O under high yield. Cr: 7.0-15.0%. Mo: 3.0% or less. N: 0.05 to 0.15 inches. P: 0.01□5% or less. and furthermore, Ni: 1.0 or less. Cu:1. Oq6 or less. Ca: 0.01 or less. Mg: 0.01% or less. Rare earth elements 2001 or less. Contains one or more of the following: Fe and unavoidable impurities: Remaining. A martensitic high Cr steel with good toughness, characterized by comprising: Te