JPH10245656A - Martensitic stainless steel excellent in cold forgeability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make better the cold forgeability and corrosion-resistance after quenching and tempering of a steel and to obtain high strength therein by specifying the contents of C, Si, Mn, N, Cr, Mo, W, Cu and N and the relation of each content in a stainless steel. SOLUTION: The compsn. of this steel is composed of, by weight,>0.03 to 0.15% C, <=0.1% Si, <0.5% Mn, <=0.5% (including zero) Ni, 11.0 to 14.0% Cr, <=2.0% (including zero) Mo or Mo and W by Mo+1/2W, >0.2 to <1.0% Cu, 0.02 to 0.15% N, and the balance substantial Fe. Also, the components are regulated in such a manner that A value shown by the formula I satisfies <=13 and B value shown by the formula II satisfies >=15. Furthermore, it is preferable that, by weight, Ni/Cu<=0.3 is satisfied. Moreover, for improving its hardness and ductility, one or two or more kinds among V, Ti and Nb may be incorporated therein by <=0.15% in total.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a martensitic stainless steel which is easy to form by cold forging and has excellent cold forgeability.

[0002]

2. Description of the Related Art Conventionally, precipitation hardening stainless steels such as SUS630 have been used for high-strength corrosion-resistant parts in which austenitic stainless steel such as SUS304 has insufficient strength. When these stainless steels are machined into parts, they are often machined into parts. Recently, for the purpose of cost reduction, what has been processed by machining is now being processed by cold forging. However, these stainless steels do not always have sufficient cold forgeability, and when they are formed by cold forging, they have problems such as cracking and large deformation resistance, making it difficult to form.

[0003]

Accordingly, there has been a demand for a martensitic stainless steel which is easy to cold forge and has good corrosion resistance and strength comparable to SUS630 after quenching and tempering. An object of the present invention is to provide an inexpensive martensitic stainless steel having good cold forgeability, good corrosion resistance after quenching and tempering, and capable of obtaining high strength.

[0004]

Means for Solving the Problems The inventor of the present invention has conducted intensive studies on 13% Cr-based martensitic stainless steel in order to achieve both good cold forgeability, corrosion resistance and high strength. As a result, in order to enhance cold forgeability, Ni
In order to keep the amount low and increase the corrosion resistance, Cu,
It has been found that it is effective to make N an essential addition.
Further, to suppress the delta ferrite which impairs the corrosion resistance, the alloy is adjusted so that the A value corresponding to the Cr equivalent shown in the following formula (1) is kept low and the B value shown in the following formula (2) is made high. The inventors have found that it is necessary to optimize the balance of elements, and have reached the present invention.

[0005] That is, the present invention provides C 0.
More than 03% and 0.15% or less, Si 1.0% or less, Mn
Less than 0.5%, Ni 0.5% or less (including 0%),
Cr 11.0-14.0%, Mo or Mo and W 2
The species is 2.0% or less (including 0%) at Mo + 1 / 2W,
Cu more than 0.2% and less than 1.0%, N 0.02-
0.15%, balance substantially consisting of Fe; and (1)
This is a martensitic stainless steel excellent in cold forgeability, having an A value represented by the formula of 13 or less and a B value represented by the formula (2) of 15 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb ………………………………… (1) = Cr + 3.3Mo + 1.65W + Cu + 30N ………… (2)

In order to further improve the cold forgeability in the present invention, it is desirable that the composition by weight% satisfies the expression (3). Ni / Cu ≦ 0.3 (3) In the present invention, in order to improve hardness and ductility, one or more of V, Ti, and Nb by weight% is 0.15 in total. % Or less. Further, in order to improve the hot workability in the present invention, it is desirable that S present as an unavoidable impurity element is restricted to 0.005% or less by weight%.

In order to improve the corrosion resistance of the present invention, JI
It is desirable that the amount of (B + C) -based inclusions specified in S be 0.04% or less. Further, in the present invention, one or more of B, Mg, Ca, and Al can be contained in a total of 0.10% or less in order to further improve hot workability.

Further, in the present invention, 650 to 950
700N tensile strength after one annealing at ℃
/ Mm ² or less.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation of each element of the present invention will be described below. Ni is an element that increases the tensile strength in the annealed state and deteriorates the cold forgeability. In the present invention, which emphasizes the cold forgeability, it is essential to control the content to be low. It is. If the addition exceeds 0.5%, the cold forgeability becomes insufficient, so the content was made 0.5% or less. For cold forgeability, it is desirably 0.3% or less, and more desirably, no addition. Cu is a very effective element for significantly increasing the corrosion resistance by adding a small amount, and is an important addition element in the present invention. If Cu is added in an amount of 1.0% or more, hot workability tends to deteriorate. On the other hand, if it is 0.2% or less, corrosion resistance becomes insufficient.
u is more than 0.2% and less than 1.0%.

[0010] N is an effective element that forms a solid solution in the martensite matrix to increase the hardness after quenching and also enhances the corrosion resistance and plays a fundamental role in the present invention. 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 it is less than 0.02%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 0.15%, the soundness of the steel ingot is impaired and the productivity is deteriorated. 15%. A desirable range of N is 0.05 to 0.12%.

Mo is an element effective for enhancing the corrosion resistance by strengthening the passive film, and it is desirable to add Mo when importance is attached to the corrosion resistance. W is the same as Mo
It is effective in increasing the corrosion resistance, but the effect is small when W is used alone, and when W is added, a part of Mo is converted to an equivalent amount of W.
(1 / 2W is equivalent to the equivalent Mo). If added in excess of 2.0%, delta ferrite is formed, conversely not only deteriorating corrosion resistance, but also deteriorating hot workability and cold forgeability.
% Or less. In particular, when importance is placed on the cold forgeability, it is desirable that no addition be made.

[0013] The 13Cr of the present invention containing an element such as N or further Mo which reduces hot workability in addition to Cu.
Ni / C in martensitic stainless steels
By setting the u ratio to 0.3 or less, that is, by reducing Ni to Cu, the tensile strength in the annealed state can be reduced, and the cold forgeability can be further improved. When the Ni / Cu ratio is 0.3 or less, the hot workability tends to deteriorate. Therefore, in order to improve the hot workability, the amount of S, which is an impurity element, is particularly suppressed to 0.005% or less. It is effective.

[0013] In the component range specified in the present invention, S is set to 0.1, regardless of the value of the Ni / Cu ratio described above.
If the content exceeds 005%, a sufficient effect of improving hot workability cannot be obtained.
The amount is desirably 0.005% or less. In addition, sulfides and oxides can be mentioned as inclusions that are the starting points of pitting corrosion. Not only the reduction of sulfide-based inclusions by reducing the amount of S, but also the reduction of the amount of oxide-based inclusions further increases corrosion resistance. Can be improved.

C is necessary for obtaining a martensitic structure after quenching 13% Cr stainless steel. C also combines with the carbide-forming elements to form carbides,
Further, a part is an element effective for increasing the hardness by forming a solid solution in the martensite base, but if added in excess of 0.15%, the solid solution forms in the base and the hardness becomes too high,
Too much Cr carbide is formed, which reduces the amount of Cr in the matrix and causes deterioration of corrosion resistance. On the other hand, 0.03%
In the following, not only sufficient hardness cannot be obtained, but also delta ferrite is formed to lower pitting corrosion resistance, hardness, and hot workability. .15% or less. A desirable range of C is 0.
03-0.12%.

Although a small amount of Si is added for deoxidation,
Even if added in excess of 1.0%, no further improvement effect is seen, so the content was set to 1.0% or less. Mn is added in a small amount for deoxidation. However, if added at 0.5% or more, sulfides are formed particularly when S is large, thereby deteriorating corrosion resistance.
It was less than 0.5%. Cr is an important element having an effect of increasing 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 exceeds 14.0%, delta ferrite is formed and the corrosion resistance and hot workability are deteriorated. %.

V, Ti, and Nb are not necessarily added, but are effective elements for forming primary carbides to refine crystal grains and improve hardness and ductility.
One or more kinds are added as needed. If one or more of them add more than 0.15% in total, coarse primary carbides are formed and the cold workability is impaired. 15%
It is better to do the following.

B, Mg, Ca, and Al are not necessarily added, but form oxides and sulfides to reduce S and O segregated at crystal grain boundaries and improve hot workability. 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 can be obtained, and conversely, the cleanliness is reduced and the heat is reduced. B, Mg, C, because it impairs the workability and cold forgeability
a, Al or one or more of Al in total of 0.1
It is better to be 0% or less.

In order to enhance the corrosion resistance, it is effective to reduce inclusions, which are one of the starting points of pitting corrosion. In the present invention, (B +
C) If the inclusions are reduced, the corrosion resistance can be further improved.
If the amount of (B + C) -based inclusions specified in JIS is more than 0.04%, a sufficient effect cannot be obtained, so the content is set to 0.04% or less.

Further, the above-mentioned alloy elements must not only satisfy the respective component ranges but also satisfy the formula specified in the present invention in order to obtain good corrosion resistance. The A value shown in the equation (1) indicates the Cr equivalent 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. The A value is Cr, Si, Mo, W, V, which is an element that easily produces ferrite.
From the values obtained by adding the respective coefficients obtained from experiments according to the effects of each element to the weight percent of Ti and Nb, the weight percent of C, Mn, Ni, Cu, and N, which are elements that easily form austenite, are calculated. The values obtained by subtracting the respective coefficients according to the effect of the above are subtracted. As a result of the experiment, in the steel of the present invention, this A
If the value exceeds 13, delta ferrite is formed and not only the corrosion resistance is greatly reduced, but also the hot workability and the hardness after quenching are slightly lowered.
3 or less.

The B value shown in the equation (2) is an important index that determines the corrosion resistance of the present invention, and is the weight percentage of Cr, Mo, W, Cu, and N, which are elements that directly improve the pitting corrosion resistance. 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, if the B value is less than 15, good pitting corrosion resistance cannot be obtained.
The B value shown in the equation was set to 15 or more. In addition to the above elements, weight%
In addition, 5% or less 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 require a large amount of Co because Co is an expensive element. P, which is an impurity element, is not particularly limited as long as it is at a level which is mixed in a normal melting step, but is not particularly limited, but a lower pitting corrosion resistance is desirable.

The above-mentioned steel of the present invention is also characterized in that it can be reduced to a sufficiently low hardness by a relatively simple single annealing. Particularly when performing cold forming such as cold drawing, cold rolling, cold forging, screw rolling, and cold bending, the tensile strength after annealing may be 700 N / mm ² or less. desirable. In particular, in the present invention, one annealing at 650 to 950 ° C. increases the tensile strength to 700 N / m.
m ² or less.

[0022]

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. 1-16
Is a steel of the present invention in which the composition, the amount of inclusions, the A value and the B value are all within the limits of the present invention. Nos. 31 to 37 are comparative steels in which any or some of the composition, the amount of inclusions, the A value, the B value, and the Ni / Cu ratio are out of the limited range of the present invention. 38 is a comparative steel SUS630.
No. Steels Nos. 1 to 37 were formed into bars of 30 mm square by hot working, heated to 860 ° C., and then annealed by furnace cooling. After heating to 1000 ° C. and holding for 30 minutes, oil-quenched quenching was performed, and then tempering was performed at 300 ° C. for 2 hours. Comparative steel No. No. 38 was aged at 550 ° C. after solution treatment at 1040 ° C.

The tensile strength was measured at JIS at room temperature using a round bar test piece having a parallel part diameter of 6 mm and a gauge length of 30 mm.
A tensile test specified in Z2241 was performed to determine the tensile strength. Regarding the cold forgeability, a round bar test piece having a diameter of 5 mm and a length of 7.5 mm in an annealed state or a solution-treated state was subjected to compression deformation to form SUS63 without cracking.
Those that can be compressed with a deformation resistance lower than 0 were evaluated with ○, and those with cracks or high deformation resistance were evaluated with x. The corrosion resistance is specified in JIS Z2371 in a quenched and tempered state or an aged state.
A salt spray test was conducted for 100 hours using 5% salt water at a temperature of 5 ° C., and the presence or absence of rust was evaluated. The hot workability was evaluated by marking a mark with a mark during hot working with a mark x and a mark without a crack with a mark with o. Table 2 shows the results.

[0024]

[Table 1]

[0025]

[Table 2]

As can be seen from Table 2, the steel No. of the present invention. 1
No.-16 have a tensile strength of 700N in the annealed state.
/ Mm ² or less, indicating that the cold forgeability is sufficient. In addition, the steel of the present invention has a high tensile strength after quenching and tempering, shows no rust due to salt water spray, and has high strength and good corrosion resistance.

On the other hand, the composition, amount of inclusions, A value, B
Comparative steel No. in which one or more of the values are out of the range specified in the present invention. 31 to 38 show that one or more properties of the tensile strength in the annealed state, the tensile strength in the quenched and tempered state, the corrosion resistance by salt spray, and the hot workability are inferior to those of the steel of the present invention. . In particular, the comparative steel No. in which either the A value or the B value deviates. Rust is observed in 31 to 34 and 37,
Insufficient corrosion resistance. In addition, the comparative steel No.
31, comparative steel No. 31 with a large amount of inclusions Rust is also observed in No. 36, and the corrosion resistance is insufficient. In addition, Comparative Steel No. Comparative steel No. 31 having a high Cu content. No. 35, comparative steel No. with a large amount of inclusions. No. 36 has poor hot workability and poor material productivity. In addition, in the case of No. No. 36, where the amount of Mo is large. No. 37 has a high tensile strength in an annealed state and is inferior in cold forgeability.

[0028]

As described above, the martensitic stainless steel of the present invention has good hot workability, low tensile strength in an annealed state, and good cold forgeability.
Excellent corrosion resistance after quenching and tempering, and has high strength.
The steel of the present invention can combine or combine these four properties. Therefore, if it is formed by hot working or cold forging and is used for parts, members, and the like used in the atmosphere, it is inexpensive, and its reliability and life can be greatly improved, which has a remarkable industrial effect.

Claims (7)

[Claims] (1) In% by weight, C exceeds 0.03% and 0.1%.
15% or less, Si 1.0% or less, Mn less than 0.5%,
Ni 0.5% or less (including 0%), Cr 11.0 to 1
4.0%, Mo or two kinds of Mo and W are Mo + ／
W is 2.0% or less (including 0%), Cu is more than 0.2% and less than 1.0%, N is 0.02 to 0.15%, the balance is substantially made of Fe, and (1) A value shown by the formula is 1
A martensitic stainless steel excellent in cold forgeability, wherein the B value represented by the formula (2) is 3 or less and 15 or more. A = -40C + 6Si-2Mn-4Ni + Cr + 4Mo + 2W-2Cu-30N + 11V + 10Ti + 5Nb ………………………………… (1) = Cr + 3.3Mo + 1.65W + Cu + 30N ………… (2) 2. The martensitic stainless steel excellent in cold forgeability according to claim 1, wherein the composition by weight% satisfies the expression (3). Ni / Cu ≦ 0.3 (3) 3. A marten having excellent cold forgeability according to claim 1, wherein one or more of V, Ti and Nb are contained in a total of 0.15% or less by weight%. Sight stainless steel. 4. A martensitic material excellent in cold forgeability according to claim 1, wherein S present as an unavoidable impurity element is restricted to 0.005% by weight or less. Stainless steel. 5. The amount of (B + C) inclusions specified in JIS is 0.
The martensitic stainless steel excellent in cold forgeability according to any one of claims 1 to 4, wherein the content is 04% or less. 6. The cold forgeability according to claim 1, wherein one or more of B, Mg, Ca, and Al are contained in a total amount of 0.10% or less. Excellent martensitic stainless steel. 7. The cold forging according to claim 1, wherein a tensile strength after one annealing at 650 to 950 ° C. is 700 N / mm ² or less. Martensitic stainless steel with excellent properties.