JP2002363640A - Method for producing martensitic heat resistant steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method for more excelling the toughness of the following martensitic heat resistant steel. SOLUTION: Steel having a composition containing, by weight, 0.05 to 0.20% C, <=0.15% Si, <=0.20% Mn, <=0.020% P, <=0.010% S, <=0.20% Cu, <=0.25% Ni, 9.80 to 10.70% Cr, 0.50 to 0.90% Mo, 1.60 to 2.00% W, 2.90 to 3.60% Co, 0.10 to 0.30% V, 0.03 to 0.07% Nb, 0.03 to 0.07% Ta, 0.003 to 0.008% B and 0.010 to 0.035% N, and the balance Fe with inevitable impurities is subjected to soaking at 1,180 to 1,250 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a martensitic heat-resistant steel, and more particularly to a method for producing a martensitic heat-resistant steel having excellent toughness.

[0002]

2. Description of the Related Art Martensitic heat-resistant steel is used for steam turbine shafts and blades for thermal power generation, plants, and the like. Conventionally, martensitic heat-resistant steels used in these applications include C: 0.17%, Ni: 0.5
%, Cr: 11%, Mo: 1%, V: 0.2%, Ta:
0.05%, N: 0.05, balance being Fe and unavoidable impurities, C: 0.2%, Cr: 10.5%, M
o: 1.3%, W: 1.0%, V: 0.2%, Nb:
0.2%, B: 0.04%, N: 0.02, balance Fe
And those composed of unavoidable impurities are known. These martensitic heat-resistant steels are used after being melted, cast and formed into ingots, soaked at about 1150 ° C., and then subjected to hot working, quenching and tempering.

Recently, C: 0.12%, Cr: 1
0.3%, Mo: 0.7%, W: 1.8%, Co: 3.%
3%, V: 0.20%, Nb: 0.05%, Ta: 0.
A martensitic heat-resistant steel containing 0.05%, B: 0.005% and N: 0.023, with the balance being Fe and unavoidable impurities, has been developed. This martensitic heat-resistant steel is used after being melted and cast into an ingot in the same manner as the above-mentioned conventional heat-resistant steel, and then subjected to soaking at about 1150 ° C., followed by hot working, quenching and tempering. . However, this martensitic heat-resistant steel has excellent heat resistance such as creep rupture strength, but has not yet had sufficient toughness.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a manufacturing method for improving the toughness of the recently developed martensitic heat-resistant steel.

[0005]

Means for Solving the Problems In order to solve the above problems, the present inventors have conducted intensive research on the above martensitic heat resistant steels. It has been found that when soaking is performed at a temperature higher than the normal soaking temperature of the heat-resistant steel, the toughness becomes more excellent. The present invention
The present invention has been made based on these findings.

That is, in the method for producing a martensitic heat-resistant steel of the present invention, C: 0.05 to 0.20%;
Si: 0.15% or less, Mn: 0.20% or less, P:
0.020% or less, S: 0.010% or less, Cu: 0.
20% or less, Ni: 0.25% or less, Cr: 9.80-
10.70%, Mo: 0.50 to 0.90%, W: 1.
60-2.00%, Co: 2.90-3.60%, V:
0.10 to 0.30%, Nb: 0.03 to 0.07%,
Ta: 0.03 to 0.07%, B: 0.003 to 0.0
10.8-12% and N: 0.010-0.035%, the balance being Fe and unavoidable impurities.
Soaking at 50 ° C., followed by hot working, quenching and tempering.

[0007]

Next, a method for producing a martensitic heat-resistant steel of the present invention will be described in detail. First, the reason why the component composition of the martensitic heat-resistant steel of the present invention is specified as described above will be described. C: 0.05 to 0.20% C is an element contained for increasing the strength. To obtain the necessary strength, it is necessary to contain 0.05% or more, but if the amount is too large, welding becomes difficult, so the upper limit is made 0.20%.

[0008] Si: 0.15% or less Si is a deoxidizing agent at the time of smelting, but if the amount is large, the toughness and high-temperature strength are reduced.
%. Mn: 0.20% or less

Mn, like Si, is a deoxidizing agent at the time of melting, but if its amount increases, the creep rupture strength deteriorates, so the upper limit is made 0.20%. P: 0.020% or less P is an impurity and reduces the weldability and the toughness and strength. S: 0.010% or less S is an impurity and reduces hot workability,
Since the toughness and strength are reduced, the content is made 0.010% or less.

[0010] Cu: 0.20% or less, Ni: 0.25%
Although Cu and Ni lower the high-temperature strength, they are 0.20% or less or 0.2% or less of the usual impurities mixed from the melting raw material.
If the content is 5% or less, the effect is small, so the content is set to 0.20% or less or 0.25% or less.

Cr: 9.80-10.70% Cr is an element to be contained for improving oxidation resistance and finely as a carbide in a matrix to improve creep strength. . In order to obtain these effects, it is necessary to contain the element in an amount of 9.80 or more. However, if the amount is too large, δ ferrite is likely to be formed, lowering the high-temperature strength and deteriorating the toughness. .70%.

Mo: 0.50 to 0.90% Mo is an element to be contained for improving the creep rupture strength by forming a solid solution in the matrix. In order to obtain the effect, it is necessary to contain 0.50% or more. However, when the amount is increased, δ ferrite is generated, and on the contrary, the creep rupture strength is deteriorated. Therefore, the upper limit is made 0.90%. . W: 1.60 to 2.00% W is an element to be contained for improving the creep rupture strength by forming a solid solution in the matrix similarly to Mo. In order to obtain the effect, it is necessary to contain 1.60% or more. However, when the content is increased, δ ferrite and a large amount of Laves phase are generated, and conversely, the creep rupture strength is deteriorated. 0.000%.

Co: 2.90% to 3.60% Co forms a solid solution in the matrix to suppress the formation of δ ferrite, so that Cr, W, M which forms δ ferrite
A large amount of strengthening elements such as o can be added, and as a result, the creep rupture strength can be improved and the tempering softening resistance can be increased. In order to obtain these functions and effects, it is necessary to contain 2.90% or more. However, when the content is increased, an intermetallic compound such as a σ phase is easily generated, and when such an intermetallic compound is generated, the creep rupture strength is deteriorated. Therefore, the upper limit is set to 3.60%.

V: 0.10 to 0.30% V can significantly increase the creep rupture strength even if precipitated as a carbonitride or solid solution in a matrix. It is. To obtain the effect, it is necessary to contain 0.10% or more,
If the amount increases, the creep rupture strength decreases, so the upper limit is set to 0.30%.

Nb: 0.03-0.07%, Ta: 0.
03-0.07% Nb and Ta enhance the high-temperature strength by precipitating as carbides or carbonitrides, and also form a solid solution in the matrix to improve the strength of the matrix. It is. In order to obtain those effects, it is necessary to contain 0.03% or more.
If the amount is large, carbides or carbonitrides are coarsened and the toughness is reduced, so the upper limit is made 0.07%. The preferable upper limit of the content is that the sum of Nb and Ta is 0.12%.
It is.

B: 0.003 to 0.008% B is an element to be included for increasing the grain boundary strength and consequently improving the creep rupture strength. In order to obtain the effect, it is necessary to contain the element in an amount of 0.003% or more. However, when the amount is increased, the weldability is deteriorated and the toughness is also reduced.
%.

N: 0.010-0.035% N is an element to be contained for improving the strength even when solid-dissolved in the matrix or precipitated as nitride or carbonitride. . In order to obtain the effect, it is necessary to contain the element in an amount of 0.010% or more. However, when the amount is increased, the amount of BN generated is increased, and the effect of containing B is reduced. %.

Next, the reason why the martensitic heat-resistant steel according to the present invention is soaked at 1180 to 1250 ° C. will be described. Conventional martensitic heat-resistant steel is normally soaked at about 1150 ° C as described above, but the martensitic heat-resistant steel according to the present invention has a temperature of 1180 ° C or higher as shown in Table 3 and FIG. When soaking at a temperature, the tensile strength, elongation, hardness, etc. are almost the same as those soaked at 1150 ° C., but the toughness (impact value) becomes extremely high, so that the soaking is performed at a temperature of 1180 ° C. or more. There is a need. However, if soaking is performed at a temperature higher than 1250 ° C., the toughness is reduced due to the precipitation of ferrite, so the upper limit is set to 1250 ° C.

Hereinafter, embodiments of the present invention will be described.

EXAMPLE A test steel having the composition shown in Table 1 below was melted, cast into an ingot by a usual method, subjected to soaking under the conditions shown in the following Table 2, and then subjected to hot working by a usual method. And used as test materials.

[0020]

[Table 1]

Thereafter, a rod-shaped tensile test piece specified in JIS Z 2201 and an impact test piece specified in JIS Z 2202 were prepared from these test materials, and quenched and tempered under the conditions shown in Table 2 below. Using these heat-treated test pieces, a tensile test and an impact test were performed. The results are shown in Table 3 below and FIG. The impact value (absorbed energy) was an average value of three test pieces.

[0022]

[Table 2]

[0023]

[Table 3]

From these results, it was found that the example of the present invention had a proof stress of 8
39-860 N / mm ² , tensile strength 978-998 N / m
m ^2, elongation of 18 to 20%, diaphragm 53 to 65%, the hardness was HRC 31 to 34 and impact value 64～94J.
On the other hand, in the comparative example, the proof stress, tensile strength, elongation, and drawing were almost the same as those of the present invention, but the impact value was 38 to 4
3J.

[0025]

According to the method for producing a martensitic heat-resistant steel of the present invention, by adopting the above constitution, the toughness of the martensitic heat-resistant steel is produced by a conventional production method without deteriorating other properties. It has an excellent effect that it can be improved by about 50 to 120% as compared with that of the present invention.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the tensile strength and the impact value of the inventive examples and comparative examples.

Claims (1)

[Claims] 1. C: 0.05-% by weight (the same applies hereinafter)
0.20%, Si: 0.15% or less, Mn: 0.20%
Hereinafter, P: 0.020% or less, S: 0.010% or less,
Cu: 0.20% or less, Ni: 0.25% or less, Cr:
9.80 to 10.70%, Mo: 0.50 to 0.90
%, W: 1.60-2.00%, Co: 2.90-3.
60%, V: 0.10 to 0.30%, Nb: 0.03 to
0.07%, Ta: 0.03 to 0.07%, B: 0.0
03-0.008% and N: 0.010-0.035%
Containing steel and the balance consisting of Fe and inevitable impurities
A method for producing a martensitic heat-resistant steel, comprising soaking at 180 to 1250 ° C.