JP2001049399A - High hardness martensitic stainless steel excellent in pitting corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a stainless steel excellent in pitting corrosion resistance by providing a stainless steel with a composition consisting of specific percentages of C, Si, Mn, Cr, Mo, W, N, Ni, and Cu and the balance Fe. SOLUTION: The high hardness martensitic stainless steel has a composition which consists of, by weight, 0.40-0.60% C, <=2.0% Si, <=2.0% Mn, 11.0-18.0% Cr, Mo or Mo and W in amounts within the range satisfying Mo+1/2W=1.0 to 3.0%, 0.04-0.25% N, 0.1-2.5% Ni, 0.1-3.0% Cu, and the balance Fe and in which Ni and Cu satisfy inequality III and the value of A represented by equation I and the value of B represented by equation II are regulated to <=10 and >=20, respectively. It is also preferable to add one or more elements among B, Mg, Ca, and Al to improve hot workability. By carrying out hardening at >=1,000 deg.C and then low temperature tempering at <=300 deg.C or high temperature tempering at 400-500 deg.C, the high hardness martensitic stainless steel having >=58 HRC hardness can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a screw which is used in the atmosphere and may be exposed to tap water, rain water, dew, etc.
Used in applications requiring both excellent corrosion resistance and high hardness, such as nails, bolts, blades, springs, or bearings, pressure-resistant parts, wear-resistant parts, various cold dies, plastic injection molding machine parts, etc. The present invention relates to a high-hardness martensitic stainless steel excellent in pitting corrosion resistance suitable for steel.

[0002]

2. Description of the Related Art Conventionally, screws, nails,
For bolts, blades, springs, bearings, pressure-resistant parts, various cold dies, and the like, carbon steel and low alloy steel containing relatively large amount of carbon are generally widely used. However, since these have a small amount of alloys such as Cr that contribute to corrosion resistance, tap water, rainwater,
Even when exposed to relatively less corrosive water such as dew condensation, there is a problem that rust easily occurs and the appearance and strength deteriorate. On the other hand, stainless steel is used for applications requiring corrosion resistance. However, SUS30
4. Austenitic stainless steel represented by SUS316 etc. has good corrosion resistance, but its hardness rises only up to about 43HRC even after considerable hardening, so it is suitable for applications requiring high hardness. Is inappropriate.
Further, ferritic stainless steel represented by SUS430 or the like has extremely low hardness, and is not suitable for applications requiring high hardness.

On the other hand, as stainless steel having high hardness,
Although martensitic stainless steel is mentioned, SUS which is a representative material widely used for automobiles and industrial use is exemplified.
Even at 410, the corrosion resistance is insufficient and the hardness is at most about 42 HRC, so that both the corrosion resistance and the hardness are not sufficient. SUS440C is a martensitic stainless steel having a very high hardness. Although the C content is as high as about 1%, a high hardness of 58 HRC or more can be obtained, but the corrosion resistance is not necessarily good for stainless steel. I can't say. In addition, stainless steel has relatively high resistance to rust, but even with little rust, it may cause local pitting corrosion called pitting corrosion, and this is likely to be a starting point of destruction in high-strength materials. There was a problem.
In addition, JP-A-57-70265 discloses a high-strength martensitic stainless steel and JP-A-6-2641.
No. 94 discloses martensitic stainless steel and drilling tapping screws having excellent rust resistance.
No. 04354 proposes a high hardness martensitic stainless steel having excellent pitting resistance.

[0004]

SUMMARY OF THE INVENTION The above-mentioned JP-A-57-7
The martensitic stainless steel proposed in No. 0265 contains 1.0 to 3.0% of Cu and 0.2% or less of Ni, and optionally adds 0.5 to 3.0% of Mo. However, this steel has a problem that the content of Cu is large but the content of Ni is small, so that hot workability is not always satisfactory. Further, depending on the combination of the compositions, delta ferrite is easily formed, and in this case, there is a problem that the pitting corrosion resistance is reduced. The martensitic stainless steel proposed in JP-A-6-264194 does not contain Cu but contains a relatively large amount of Mo. However, this steel has a low C of 0.13 to 0.20%,
There was a problem that high hardness of 8 HRC or more could not be obtained. Thus, it was very difficult to achieve both high hardness of 58 HRC or more and good corrosion resistance.

The martensitic stainless steels proposed in Japanese Patent Publication No. 10-504354 are Mo, C
Although u, Ni, N, etc. are added and have good pitting corrosion resistance, C is lower than 0.15% and 0.40% or less, preferably 0.20 to 0.35%. And 5
There is a problem that it is difficult to obtain a high hardness of 8 HRC or more. Therefore, recently, a martensitic stainless steel that is easy to hot work and has good pitting resistance and high hardness after quenching and tempering has been desired. An object of the present invention is to provide a martensitic stainless steel having good hot workability, good pitting corrosion resistance after quenching and tempering, and capable of obtaining a high hardness of 58 HRC or more.

[0006]

Means for Solving the Problems The present inventor has proposed that Cr is 1
So-called 13-containing in the range of 1.0-18.0%
A 17% Cr-based martensitic stainless steel has been extensively studied to achieve both high hardness of 58 HRC or more and good pitting corrosion resistance. As a result, in order to obtain high hardness, it is necessary to increase C and N. In order to increase pitting resistance, Mo and N are essential additions, and addition of Cu is very effective. In addition, when Mo is added, delta ferrite is easily formed, and pitting corrosion resistance and hot workability are reduced. Therefore, a small amount of Ni is added in the range of Ni / Cu> 0.2 for the purpose of suppressing formation of harmful delta ferrite. It was found that it was necessary to add while maintaining the above, and to add a large amount of N. Further, for suppressing delta ferrite, the A value corresponding to the Cr equivalent shown in the following equation (1) is kept low, and in order to increase the pitting corrosion resistance, the B value shown in the following equation (2) is used. It is one of the features of the present invention to optimize the balance of the alloying elements so as to increase the value. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N …… (2) (However, the non-added elements among the selected elements are calculated as zero)

[0007] Of the above, the addition of Cu is desirable to be contained as much as possible in order to improve the pitting corrosion resistance. However, when the added amount of Cu is increased, the problem that the hot workability is reduced occurs. In addition, in 13 to 17% Cr-based high-hardness martensitic stainless steel containing Mo, N, or the like, which is an element that reduces hot workability, Ni and Cu are co-added within a specific range, respectively, and Ni is added. And Cu
It is one of the features of the present invention that good corrosion resistance can be obtained by setting the amount ratio to Ni / Cu> 0.2, and that hot workability is not significantly impaired. Further, to obtain a high hardness of 58HRC or more without greatly impairing the pitting corrosion resistance,
Another feature of the present invention is to add a large amount of N after adjusting the C amount to a slightly higher appropriate amount.

[0008] That is, the first invention of the present invention is that, in terms of% by weight, C: more than 0.40% and 0.60% or less;
0% or less, Mn: 2.0% or less, Cr: 11.0 to 1
8.0%, Mo or two kinds of Mo and W are Mo + ／
W exceeds 1.0% and 3.0% or less, N: 0.04 to 0.4%.
25%, Ni: 0.1-2.5% and Cu:
0.1 to 3.0%, and the relationship between Ni and Cu is within a range satisfying the expression (3), and the balance is substantially Fe
And the A value represented by the equation (1) is 10 or less;
A high hardness martensitic stainless steel excellent in pitting corrosion resistance, wherein the B value represented by the formula (2) is 20 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N (2) (However, the elements that are not added among the selected elements are calculated as zero) Ni / Cu> 0.2 (3)

The second invention is characterized in that, by weight%, C: 0.4
0% to 0.50% or less, Si: 2.0% or less, M
n: 2.0% or less, Cr: 12.0% to 17.0%, M
o or two kinds of Mo and W are 1.0% at Mo + 1 / 2W
More than 3.0%, N: 0.05 to 0.20%, Ni: more than 0.2, 1.5% or less, and Cu:
0.1 to 2.0%, and the relationship between Ni and Cu is within a range satisfying the expression (3), and the balance is substantially Fe
And the A value represented by the equation (1) is 10 or less;
A high hardness martensitic stainless steel excellent in pitting corrosion resistance, wherein the B value represented by the formula (2) is 20 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N (2) (However, the elements that are not added among the selected elements are calculated as zero) Ni / Cu> 0.2 (3)

In the third invention, C: 0.40% by weight%
Over 0.60% or less, Si: 2.0% or less, Mn:
2.0% or less, Cr: 11.0 to 18.0%, Mo or two kinds of Mo and W are more than 1.0% and 3.0% or less at Mo + 1 / 2W, N: 0.04 to 0% .25%, plus V,
One or more of Ti and Nb are used in total of 0.25
% Or less, Ni: 0.1-2.5% and Cu: 0.1-
3.0%, the relationship between Ni and Cu is within a range satisfying the expression (3), the balance is substantially made of Fe, and the A value shown by the expression (1) is 10 or less, 2) A high hardness martensitic stainless steel excellent in pitting corrosion resistance, wherein the B value represented by the formula is 20 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N (2) (However, the elements that are not added among the selected elements are calculated as zero) Ni / Cu> 0.2 (3)

The steel composition of the stainless steels of the first to third inventions may include B, Mg, Ca, Al
0.10% or less in total of one or more of
Further, for the purpose of increasing the strength after quenching and tempering, 5% or less of Co can be contained. The steel of the present invention having the above composition has a hardness of at least 58 HRC after quenching and tempering, and has a pitting potential Vc ′ in degassed 3.5% salt water at 30 ° C.
Preferably, 100 is equal to or greater than 50 mV (vs SCE), and this property can be achieved by the above-described novel composition range of the present invention.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The action of each element of the steel of the present invention will be described below. C is necessary for obtaining a martensite structure after quenching 13 to 17% Cr stainless steel. C is an element effective for increasing hardness by forming a carbide by combining with a carbide-forming element and further forming a solid solution in a martensitic matrix, but 0.60%
If added in excess of Cr, too much Cr carbide is formed, which causes a decrease in the amount of Cr in the matrix and causes deterioration of corrosion resistance. On the other hand, if it is less than 0.40%, it is difficult to obtain sufficient hardness, so that the content of C exceeds 0.40% to 0.60%
It was as follows. A desirable range of C is 0.40 to 0.4.
50%.

[0013] Si and Mn are added in small amounts for deoxidation, but even if added in excess of 2.0%, no further improvement effect is seen, so that both are set to 2.0% or less. . Further, Si is an element that easily forms ferrite, while Mn is an element that easily forms austenite, and even a small amount slightly affects the structure of the matrix. Is good.

Ni suppresses the formation of delta ferrite and enhances pitting corrosion resistance. However, if it is less than 0.1%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 2.5%, the martensitic transformation point is too low, and it becomes difficult to obtain a complete martensite structure after quenching. 0.1 ~
2.5%. A desirable range of Ni is more than 0.2% and 1.5% or less. Further, Ni is an element effective for preventing a decrease in hot workability due to the addition of Cu, and thus Ni is added in accordance with the added amount of Cu. 1 of the present invention containing an element such as Mo or N which lowers hot workability in addition to Cu.
In a 3-17% Cr-based high hardness martensitic stainless steel, the value of Ni / Cu exceeds 0.2,
Desirably, it is desirable to regulate it to 0.3 or more.

[0015] Cr is an important element having an effect of improving corrosion resistance, particularly pitting corrosion resistance, by forming a passive film. If it is less than 11.0%, sufficient corrosion resistance cannot be obtained,
On the other hand, if it is added in excess of 18.0%, delta ferrite is formed, and pitting corrosion resistance and hot workability are deteriorated. A desirable range of Cr is 12.0 to 17.0%. Mo is a very effective element for enhancing the pitting corrosion resistance by strengthening the passivation film, and is essentially added to the steel of the present invention. W is also Mo
Similarly to the above, it is effective to enhance the pitting corrosion resistance, but the effect is small when W is used alone, and when W is added, a part of Mo is equivalent to W (１ ／ W corresponds to equivalent Mo). It is desirable to add in the form of substitution. Mo alone or Mo and W
Both are Mo + 1 / 2W and 1.0% or less deteriorates pitting corrosion resistance. On the other hand, if more than 3.0% is added, delta ferrite is formed, and conversely, not only pitting corrosion resistance is deteriorated but also heat 2. Since the workability is also deteriorated, it exceeds 1.0%.
0% or less. Desirably, it is 1.5 to 2.5%.

When Cu is added to steel containing Cr, Mo and N, it is a very effective element for greatly improving the pitting resistance. However, if it is less than 0.1%, a sufficient effect cannot be obtained. On the other hand, if added in excess of 3.0%, the hot workability is impaired, so the content was set to 0.1 to 3.0%. A desirable range of Cu is 0.1 to 2.0%. In order to obtain more stable hot workability, the relationship between Ni and Cu should be limited to Ni / Cu> 0.2, preferably 0.3 or more, as described in the reason for limiting Ni. Is more desirable.

N is a very effective element for forming a solid solution in the martensite matrix to increase the hardness after quenching and to enhance the pitting corrosion resistance. In addition, the effect of suppressing the generation of delta ferrite is great, so that expensive alloy elements such as Ni are saved, and the addition of N instead of Ni suppresses the generation of delta ferrite, thereby producing a material at low cost. It is also effective. If the amount is less than 0.04%, a sufficient effect cannot be obtained. On the other hand, if the amount exceeds 0.25%, the soundness of the steel ingot is impaired and the productivity is deteriorated.
4% to 0.25%. A desirable range of N is 0.0
5 to 0.20%.

V, Ti, and Nb are effective elements for forming primary carbides to refine crystal grains and improve hardness and ductility, and one or more kinds are added as necessary. . If one or more of them add more than 0.25% in total, coarse primary carbides are formed and the cold workability is impaired. It is better to be 25% or less.

B, Mg, Ca and Al need not always be added, but by forming oxides and sulfides, S and O segregating at crystal grain boundaries are reduced, and hot workability is improved. And one or more of them are added as needed. Even if one or more of B, Mg, Ca, and Al are added in a total amount of more than 0.10%, no further improvement effect is obtained, and conversely, the cleanliness is reduced and the heat is reduced. B, Mg, Ca, A
It is preferable that one or two or more of 1 is 0.10% or less in total.

Further, the above-mentioned alloy elements must not only satisfy the individual component ranges but also satisfy the formula specified in the steel of the present invention in order to obtain good pitting corrosion resistance. The A value shown in the equation (1) indicates the Cr equivalent of the steel of the present invention, and the magnitude of the A value in this equation is an important index that affects the ease of forming delta ferrite. A value is Cr, Si, Mo, W, which is an element that easily produces ferrite.
From the values obtained by adding respective coefficients obtained from experiments according to the effects of each element to the weight percentages of V, Ti, and Nb, C, Mn, Ni, Cu, and N, which are elements that easily form austenite, are obtained.
Is a value obtained by subtracting a value obtained by adding a coefficient to the weight% of each element according to the effect of each element. As a result of the experiment, in the steel of the present invention, when the A value exceeds 10, not only delta ferrite is formed, and not only the pitting corrosion resistance is greatly reduced, but also the hot workability and the hardness after quenching are slightly lowered. A shown in equation (1)
The value was set to 10 or less.

The B value shown in the equation (2) is an important index that determines the pitting resistance of the steel of the present invention, and is an element that directly improves pitting resistance, such as Cr, Mo, W, Cu, and N. % By weight
The sum of values obtained by adding coefficients obtained experimentally to the degree of contribution of the effect of each element is shown. In the steel of the present invention, the B value is 2
If it is smaller than 0, good pitting corrosion resistance cannot be obtained.
The B value shown in the equation (2) was set to 20 or more. In addition to the above elements,
Up to 5% by weight of Co may be added to the steel of the present invention.
Co has the effect of increasing the strength after quenching and tempering by forming a solid solution in the matrix, but does not need to be added in a large amount because Co is an expensive element. Further, P and S, which are impurity elements, are not particularly defined as long as they do not cause any problem as long as they are mixed in a usual dissolving step, but a lower pitting corrosion resistance is preferable.

Next, the reasons for limiting the characteristic values of the steel of the present invention will be described. By performing appropriate quenching and tempering, the steel of the present invention can obtain higher hardness than the cold-worked material of SUS304 and the quenched and tempered material of SUS410. In particular, when the steel of the present invention is used for screws, nails, bolts, blades, springs, bearings, pressure-resistant parts, dies, and the like, 58HRC or more is necessary in order to sufficiently exhibit its performance. For steel, after quenching from about 1000 ° C or more, about 3
Low temperature tempering below 00 ° C, or about 400-500 ° C
By performing high temperature tempering, 58HRC or more can be obtained.

When importance is attached to corrosion resistance, it is effective not to leave undissolved carbides as much as possible. Therefore, it is desirable to increase the quenching temperature to reduce undissolved carbides according to the composition. In addition, when the quenching temperature is increased, retained austenite tends to remain, so that it is desirable to perform a sub-zero treatment after quenching to reduce the retained austenite. However, screws,
When delayed fracture resistance is important for nails and bolts, the hardness can be reduced by selecting an appropriate tempering temperature.

The steel of the present invention can obtain good pitting corrosion resistance while maintaining high hardness by performing appropriate quenching and tempering. The pitting potential is one index that indicates the level of pitting resistance, but a member that is used in the air and may be exposed to a relatively mild corrosive environment,
In order to exhibit good pitting resistance even when used for parts, tools, etc., the pitting potential Vc'100 in degassed 3.5% saline at 30 ° C.
Must be 50 mV (vs. SCE) or more. Here, pitting corrosion is a form of corrosion in which small holes are formed in places on the surface of steel in small spots, and is a type of corrosion often seen in stainless steel. When this pitting occurs, not only the appearance deteriorates, but also the hole may be a starting point to cause destruction. The pitting corrosion potential is measured by an electrochemical corrosion evaluation test method according to a measurement method specified in JIS G0577, and is obtained as a potential Vc'100 at a current density of 100 μA / cm 2.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments.
Steel having the chemical components shown in Table 1 was melted by vacuum melting to obtain a steel ingot of 10 kg. Here, steel No. 1-13
Is a steel of the present invention in which the composition, A value, B value, and Ni / Cu ratio are all within the limits of the present invention. 21-26
Is a comparative steel in which any or some of the composition, the A value, the B value, and the Ni / Cu ratio are out of the limited range of the present invention.
These steels are made into 30mm square bars by hot working,
After heating to 860 ° C., furnace-cooled annealing was performed. Further, after heating to 1050 to 1100 ° C and holding for 1 hour, oil-cooling quenching was performed, followed by sub-zero treatment at -75 ° C for 2 hours, and tempering at 150 ° C for 2 hours.

The hardness was measured by a Vickers hardness tester after annealing and by a Rockwell hardness tester after quenching and tempering. For pitting corrosion resistance, see JIS
It was measured in a 3.5% saline solution at 30 ° C. degassed according to G0577, and the potential Vc′100 at which the current density reached 100 μA / cm was determined as the pitting potential. In addition, the hot workability was evaluated as x when the surface and corners were frequently damaged during hot working.
The mark was evaluated by marking the mark with slight scratches, and the mark Δ was added, and the mark without scratches was evaluated with the mark ○. The results are shown in Table 2.

[0027]

[Table 1]

[0028]

[Table 2]

As can be seen from Table 2, the steel No. of the present invention. 1
No. 13 to 13 have a quenching and tempering hardness as high as 58 or more HRC, and also have a pitting potential Vc'100 of 50 mV (vs. SC).
E) It shows a high value as described above, and it is understood that the film has both good pitting corrosion resistance and high hardness. In contrast, the composition,
Any one or more of the A value, the B value, and the Ni / Cu ratio are out of the ranges specified in the present invention. It can be seen that the steels of Nos. 21 to 26 are harder than one of the steels of the present invention in one or both of quenching and tempering hardness and pitting potential. In particular, the comparative steel No. in which either the A value or the B value deviates. Nos. 21, 23 to 26 have low pitting potential, and the pitting resistance is insufficient. In addition, comparative steel No. having a low C content. Comparative steel No. 22 has a low hardness and an A value larger than 10. 23 and 26 also have low hardness.

[0030]

As described above, the martensitic stainless steel of the present invention has good hot workability, excellent pitting corrosion resistance after quenching and tempering, and has particularly high hardness.
The steel of the present invention may combine or combine these three properties. Therefore, if it is used for parts, members, tools, etc., such as screws, nails, bolts, blades, springs, bearings, pressure-resistant parts, various cold dies, etc., which are used in the atmosphere, it will be inexpensive and have high reliability and life. Can be greatly improved,
It has an industrially significant effect.

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[Procedure amendment]

[Submission date] February 3, 2000 (200.2.3)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

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[Claims]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

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The second invention is characterized in that, by mass %, C: 0.4
0% to 0.50% or less, Si: 2.0% or less, M
n: 2.0% or less, Cr: 12.0% to 17.0%, M
o or two kinds of Mo and W are 1.0% at Mo + 1 / 2W
More than 3.0%, N: 0.05 to 0.20%, Ni: more than 0.2, 1.5% or less, and Cu:
0.1 to 2.0%, and the relationship between Ni and Cu is within a range satisfying the expression (3), and the balance is substantially Fe
And the A value represented by the equation (1) is 10 or less;
A high hardness martensitic stainless steel excellent in pitting corrosion resistance, wherein the B value represented by the formula (2) is 20 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N (2) (However, the elements that are not added among the selected elements are calculated as zero) Ni / Cu> 0.2 (3)

[Procedure amendment 3]

[Document name to be amended] Statement

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[Correction method] Change

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In the third invention, C: 0.40% by mass %.
Over 0.60% or less, Si: 2.0% or less, Mn:
2.0% or less, Cr: 11.0 to 18.0%, Mo or two kinds of Mo and W are more than 1.0% and 3.0% or less at Mo + 1 / 2W, N: 0.04 to 0% .25%, plus V,
One or more of Ti and Nb are used in total of 0.25
% Or less, Ni: 0.1-2.5% and Cu: 0.1-
3.0%, the relationship between Ni and Cu is within a range satisfying the expression (3), the balance is substantially made of Fe, and the A value shown by the expression (1) is 10 or less, 2) A high hardness martensitic stainless steel excellent in pitting corrosion resistance, wherein the B value represented by the formula is 20 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N (2) (However, the elements that are not added among the selected elements are calculated as zero) Ni / Cu> 0.2 (3)

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction contents]

Further, the above-mentioned alloy elements must not only satisfy the individual component ranges but also satisfy the formula specified in the steel of the present invention in order to obtain good pitting corrosion resistance. The A value shown in the equation (1) indicates the Cr equivalent of the steel of the present invention, and the magnitude of the A value in this equation is an important index that affects the ease of forming delta ferrite. A value is Cr, Si, Mo, W, which is an element that easily produces ferrite.
From values obtained by adding respective coefficients obtained from experiments according to the effect of each element to the mass % of V, Ti, and Nb, C, Mn, Ni, Cu, and N, which are elements that easily generate austenite, are obtained.
Is a value obtained by subtracting a value obtained by adding a coefficient to the weight% of each element according to the effect of each element. As a result of the experiment, in the steel of the present invention, when the A value exceeds 10, not only delta ferrite is formed, and not only the pitting corrosion resistance is greatly reduced, but also the hot workability and the hardness after quenching are slightly lowered. A shown in equation (1)
The value was set to 10 or less.

[Procedure amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0021

[Correction method] Change

[Correction contents]

The B value shown in the equation (2) is an important index that determines the pitting resistance of the steel of the present invention, and is an element that directly improves pitting resistance, such as Cr, Mo, W, Cu, and N. % By mass
The sum of values obtained by adding coefficients obtained experimentally to the degree of contribution of the effect of each element is shown. In the steel of the present invention, the B value is 2
If it is smaller than 0, good pitting corrosion resistance cannot be obtained.
The B value shown in the equation (2) was set to 20 or more. In addition to the above elements,
Up to 5% by weight of Co may be added to the steel of the present invention.
Co has the effect of increasing the strength after quenching and tempering by forming a solid solution in the matrix, but does not need to be added in a large amount because Co is an expensive element. Further, P and S, which are impurity elements, are not particularly defined as long as they do not cause any problem as long as they are mixed in a usual dissolving step, but a lower pitting corrosion resistance is preferable.

[Procedure amendment 6]

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[0027]

[Table 1]

Claims (6)

[Claims] (1) In terms of% by weight, C: more than 0.40%;
60% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr: 11.0 to 18.0%, Mo or Mo and W
Contains Mo more than 1.0% and 3.0% or less at Mo + 1 / 2W, N: 0.04 to 0.25%, and Ni: 0.1
-2.5% and Cu: 0.1-3.0%, and the relationship between Ni and Cu satisfies the expression (3),
High hardness excellent in pitting corrosion resistance, characterized in that the balance is substantially composed of Fe, and the A value represented by the formula (1) is 10 or less and the B value represented by the formula (2) is 20 or more. Martensitic stainless steel. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N (2) (However, the elements that are not added among the selected elements are calculated as zero) Ni / Cu> 0.2 (3) 2. In% by weight, C: more than 0.40%.
50% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr: 12.0% to 17.0%, Mo or two kinds of Mo and W are 1.0% by Mo + 1 / 2W. Over 3.0%
Hereinafter, N: 0.05 to 0.20% is contained.
More than 2 and 1.5% and Cu: 0.1 to 2.0%, and the relationship between Ni and Cu is within a range satisfying the expression (3), and the balance substantially consists of Fe; And the A value represented by the formula (1) is 10 or less, and the B value represented by the formula (2) is 2 or less.
A high-hardness martensitic stainless steel excellent in pitting corrosion resistance, which is 0 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N (2) (However, the elements that are not added among the selected elements are calculated as zero) Ni / Cu> 0.2 (3) 3. C: Exceeding 0.40% by weight%.
60% or less, Si: 2.0% or less, Mn: 2.0% or less, Cr: 11.0 to 18.0%, Mo or Mo and W
Are Mo + ／ W of more than 1.0% to 3.0% or less, N: 0.04 to 0.25%, V, Ti, Nb
At least 0.25% or less in total, N
i: 0.1 to 2.5% and Cu: 0.1 to 3.0%, and the relationship between Ni and Cu is within a range satisfying the expression (3), and the balance is substantially from Fe A high hardness martensitic stainless steel excellent in pitting corrosion resistance, wherein A value represented by the formula (1) is 10 or less and B value represented by the formula (2) is 20 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb …… (1) (However, the non-added elements among the selected elements are calculated as zero) B = Cr + 3.3Mo + 1.65W + Cu + 30N (2) (However, the elements that are not added among the selected elements are calculated as zero) Ni / Cu> 0.2 (3) 4. The steel composition according to claim 1, wherein one or more of B, Mg, Ca, and Al are contained in a total of 0.10% or less. High hardness martensitic stainless steel. 5. A high-hardness martensite made of the martensitic stainless steel according to claim 1 and having a hardness after quenching and tempering of 58 HRC or more, having excellent pitting corrosion resistance. Series stainless steel. 6. A deaerated 3.5 of 30 ° C. made of the martensitic stainless steel according to claim 1.
% Pitting potential Vc'100 in 50% salt water (vs.
CE) or higher, a high hardness martensitic stainless steel having excellent pitting resistance.