JP3460332B2 - Manufacturing method of precipitation hardening stainless steel with excellent cold workability and toughness - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention requires fasteners, various shafts, etc., which are required to have corrosion resistance and strength, have excellent cold workability in the solution heat treatment (ST) state, and have excellent toughness and ductility during precipitation hardening. And a method for producing precipitation hardening stainless steel.

[0002]

2. Description of the Related Art SUS63 has been known as a precipitation hardening type stainless steel which utilizes precipitation hardening in order to increase strength without impairing corrosion resistance.
0 (17Cr-4Ni-4Cu-Nb) is widely used as a typical one, but in the case of SUS630, the hardness in the solution treatment state (hereinafter referred to as ST state) which is the softest state is HRC. It was as high as about 35 and there was a problem that cold workability was insufficient.

Therefore, by reducing the C and N in the steel, ST
There has been proposed a method of lowering the hardness in the state and thereby improving the cold workability (JP-A-56-127754, JP-A-6-76754).
1-147855). Certainly, if C and N in the steel are reduced, the hardness in the ST state can be reduced and the hardness at the time of precipitation hardening can be maintained as before. However, simply reducing C and N in this way In this case, it was found by the study of the present inventors that there is a problem that the toughness and ductility during precipitation hardening becomes extremely low.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a low hardness in the ST state, a high hardness during precipitation hardening, and a high toughness and ductility. An object of the present invention is to provide a method for producing type stainless steel. Thus, the invention of the present application, after performing solution treatment, when aging treatment to produce precipitation hardening stainless steel,
Steel composition in% by weight, C: ≤ 0.030%, Si: ≤ 1.
00%, Mn: ≤ 2.00%, P: ≤ 0.040%,
S: ≤0.030%, Cu: 1.50 to 4.50%, N
i: 3.50 to 6.50%, Cr: 13.0 to 19.0
%, Mo: ≤ 2.50%, N: ≤ 0.030%, Nb:
0.10 to 0.45%, C + N: ≦ 0.040%, the balance being substantially composed of Fe, and the solution treatment was performed at a solution treatment temperature of α ° C. and C + N of β%. It is characterized in that it is sometimes performed under the condition that the following formula (1) is satisfied. α / 910-0.6√β ≦ 1.04 (1)

The present invention has the essence of restricting the individual and total amounts of C and N to small amounts, optimizing the balance of each component in steel, and then performing solution treatment under predetermined conditions. To do. More specifically, after reducing C and N, the solution treatment temperature is also lowered so as to maintain a constant relationship with the amounts of C and N, and according to the present invention. For example, the hardness in the ST state can be lowered to improve the cold workability, and the toughness and ductility during precipitation hardening can be maintained high.

Here, in addition to the above-mentioned components, one or more of Ca, B, Mg, and REM are contained in the steel as Ca: 0.0005 to 0.010% and B: 0.0005, respectively.
~ 0.010%, Mg: 0.0005-0.010%,
REM: It is desirable to contain it in the range of 0.0005 to 0.010% (claim 2). With this, hot workability can be improved.

Next, the reasons for limiting each chemical component and solution treatment condition in the above invention will be described in detail below. C: ≤0.030% The smaller the amount of C, the lower the hardness in the ST state. Therefore, in the present invention, the upper limit is set to 0.030%.

Si: ≦ 1.00% Si is effective as a deoxidizing agent. However, if the amount increases, the amount of δ-ferrite increases and the hardness after aging decreases, so it is necessary to balance with other elements, and in order to increase the hardness, 1.00% or less And need to.

Mn: ≤2.00% Mn is effective as a deoxidizing agent, and it is desirable to increase the content in order to control the amount of δ ferrite. However, adding too much lowers the Ms point too much.
It should be 2.00% or less.

P: ≤0.040% P is a component that causes grain boundary embrittlement, and it is necessary to set it to 0.040% or less.

S: ≤0.030% S is required to be 0.030% or less in order to reduce corrosion resistance and workability.

Cu: 1.50 to 4.50% Cu is an essential component as a precipitation hardening component.
% Or more is required. However, the addition of a large amount lowers the hot workability, so the upper limit must be 4.50%.

Ni: 3.50 to 6.50% Ni has the function of controlling the amount of δ ferrite, and is required to be 3.50% or more in order to suppress the occurrence of flaws during hot working. However, since excessive addition lowers the Ms point and also becomes expensive in terms of cost, it is necessary to control the content to 6.50% or less.

Cr: 13.0 to 19.0% Cr is required to be 13.0% or more to ensure corrosion resistance. However, addition of a large amount increases the amount of δ-ferrite, so in the present invention, it is made 19.0% or less.

Mo: ≦ 2.50% Mo is an effective additive component for further improving the corrosion resistance. However, if added in a large amount, the amount of δ-ferrite increases, so it is necessary to regulate it to 2.50% or less.

N: ≤0.030% The smaller the amount of N, the lower the hardness in the ST state. Therefore, in the present invention, N is 0.03
It is regulated to 0% or less.

Nb: 0.10 to 0.45% Nb is a component necessary for adjusting the Ms point, and particularly for fixing C and N, and is contained at 0.10% or more. However, excessive addition impairs hot workability, so 0.45
% Or less.

C + N: ≤0.040% The lower the contents of C and N, the lower the hardness in the ST state. Therefore, in the present invention, the total amount is 0.040%
It regulates to the following.

Α / 910−0.6√β ≦ 1.04 (1) (where, α: ST temperature (° C.), β: C + N (%)) By reducing C + N as described above, ST Although the hardness in the state can be reduced, on the other hand, if the amount of C + N is simply reduced, the toughness and ductility will be extremely reduced in a region near peak aging such as H900 (480 ° C.). However, sufficient toughness and ductility can be ensured by performing the solution treatment according to the above formula (1).

Ca: 0.0005 to 0.010% B: 0.0005 to 0.010% Mg: 0.0005 to 0.010% REM: 0.0005 to 0.010% Ca, B, Mg, REM By incorporating any one or more of the above in the above range, the hot workability can be improved.

[0021]

EXAMPLES Examples of the present invention will be described in detail below. In a vacuum induction furnace, 150 kg of steel ingots of various chemical compositions shown in Table 1 were melted, hot forged, and then hot rolled to 17 m.
An mφ coil material was manufactured. Then, the presence or absence of fatal flaws during hot forging and hot rolling was examined to evaluate the hot workability.

Then, solution treatment was performed under the temperature condition (α) shown in Table 2, and then H900 (480 ° C.) / 4 hr.
-Aging treatment was performed under the condition of air cooling. Then, a compression test was performed in the solution heat treatment state, and the deformation resistance when the strain amount was 0.6 was obtained to evaluate the cold workability. Further, the hardness of the sample after the aging treatment was determined on a 5-point average, and the JIS No. 4 tensile test was also performed to evaluate the characteristics. The results are shown in Table 2.

[0023]

[Table 1]

[0024]

[Table 2]

From the above results, in Comparative Examples C1 to C5 in which the chemical components were the same as those of the present invention and the solution treatment conditions were out of the range of the present invention, the hot workability was good and the hardness in the ST state was Although low, as shown in the results of the tensile test after aging treatment, the values of elongation and drawing are inferior to those of the examples of the present invention (tensile strength is almost the same), and the toughness and ductility during precipitation hardening are inferior. I understand.

In Comparative Example C6 in which Si was out of the range of the present invention and Comparative Example C7 in which Cu was out of the range of the present invention, the hot workability was poor and it was not possible to use it for the subsequent test. Further, in the case of Comparative Example C8 to which Nb was not added, the hardness at the time of precipitation hardening was not sufficiently obtained, and therefore the tensile test was also omitted.

On the other hand, in the case of the comparative example steel C9 (SUS630), the hardness in the ST state is high, which makes cold working difficult. In addition, in all of the examples of the present invention, the characteristics at the time of aging treatment are equal to or higher than those of SUS630.

The embodiment of the present invention has been described in detail above, but this is merely an example, and the present invention can be implemented in various modifications without departing from the spirit of the present invention.

[0029]

As described above in detail, according to the present invention, when the precipitation hardening stainless steel is manufactured, the hardness in the ST state can be lowered to enhance the cold workability, and the precipitation The toughness and ductility at the time of hardening can be maintained high. Further, according to the second aspect, it is possible to further improve the hot workability.

Claims (2)

(57) [Claims] 1. When a precipitation hardening stainless steel is produced by subjecting to solution treatment and then aging treatment, the steel composition is expressed in weight% and C: ≤ 0.030% Si: ≤ 1.00% Mn: ≦ 2.00% P: ≦ 0.040% S: ≦ 0.030% Cu: 1.50 to 4.50% Ni: 3.50 to 6.50% Cr: 13.0 to 19.0% Mo : ≤ 2.50% N: ≤ 0.030% Nb: 0.10 to 0.45% C + N: ≤ 0.040% The balance is composed essentially of Fe, and the solution treatment is carried out by solution treatment. A method for producing a precipitation hardening stainless steel excellent in cold workability and toughness and ductility, which is performed under the condition that the following formula (1) is satisfied when the treatment temperature is α ° C. and C + N is β%. α / 910-0.6√β ≦ 1.04 (1) 2. The manufacturing method according to claim 1, wherein the steel composition is represented by weight%, C: ≦ 0.030% Si: ≦ 1.00% Mn: ≦ 2.00% P: ≦ 0.040% S : ≤ 0.030% Cu: 1.50 to 4.50% Ni: 3.50 to 6.50% Cr: 13.0 to 19.0% Mo: ≤ 2.50% N: ≤ 0.030% Nb: 0.10 to 0.45% C + N: ≦ 0.040%, and further contains 1 of Ca, B, Mg and REM.
Each contains at least Ca: 0.0005 to 0.010% B: 0.0005 to 0.010% Mg: 0.0005 to 0.010% REM: 0.0005 to 0.010%, A method for producing a precipitation hardening stainless steel which is excellent in cold workability and toughness and ductility, characterized in that the balance is composed essentially of Fe.