JP3204080B2 - Method for producing precipitation-hardened martensitic stainless steel with excellent cold forgeability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precipitation hardening martensitic stainless steel suitable for use in fastening bolts, nuts, washers and other high-strength bolts of stainless steel structural materials, and for obtaining excellent cold forgeability. The present invention relates to a method for producing steel.

[0002]

[Prior Art] Precipitation hardening stainless steel such as SUS630
It is a steel type that can obtain high strength by cold working in the solution heat treatment state and then aging treatment.Since it is a steel type that can obtain the highest strength among stainless steel, it is used for mechanical structural materials, building It is used for parts that require both high strength and excellent corrosion resistance in structural materials and the like.

[0003] As described above, precipitation hardening stainless steel such as SUS630 is cold-worked into a predetermined shape in a solution heat treatment state. Therefore, in order to obtain excellent cold workability, a solution heat treatment is required. It is necessary to reduce the hardness as much as possible. On the other hand, when actually used as a part, high strength is required, so that it is required that high hardness can be obtained by aging treatment.

[0004] In response to the above requirements, SUS630, which has been widely used in the past, does not decrease in hardness to the extent that excellent cold workability can be obtained even if the hardness is reduced by solution heat treatment. When manufacturing bolts by forging, there was a problem in workability. In order to solve the above problems, Japanese Patent Laid-Open No. 6-129929
Discloses an invention disclosed in Japanese Patent Application Laid-Open Publication No. H11-209,873. The present invention
It is characterized in that the addition amounts of Ni, Cr and Cu are adjusted to an appropriate range, and final annealing is performed at a temperature of 900 to 1100 ° C during hot rolling.

[0005]

[Problems to be solved by the invention]
Even when SUS630 was subjected to the solution heat treatment usually performed, the hardness was as high as about Hv340 to 350, and the cold forgeability was poor. In addition, the invention described in Japanese Patent Application Laid-Open No. 6-129929 makes it possible to obtain a material having a Hv of about 290 to 300 and a lower hardness than the solution heat treated material of SUS630. It cannot be said that the hardness has been lowered sufficiently to easily perform hot forging, and it has been strongly desired to establish a production method capable of obtaining lower hardness and excellent cold forging property.

[0006] The present invention has been made to solve such problems, and an object of the present invention is to provide a precipitation hardening type martensitic stainless steel capable of obtaining excellent cold forgeability before aging treatment. It is to provide a method for producing steel.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies on a novel heat treatment method capable of lowering the hardness of SUS630, and have obtained the present invention by obtaining the following findings. In order to keep the strength before aging treatment low, it is necessary to reduce as much as possible elements C and N, which have the effect of increasing the strength by solid solution strengthening,
As described in the above-mentioned publication, it is well known in the art. The present inventors further added an appropriate amount of Nb according to the amount of C + N, and heated at 980 to 1080 ° C. (first stage heating)
To form a solid solution by heating (solution heat treatment).
By holding at 0 to 880 ° C for a certain time (hereinafter referred to as second stage heating or intermediate annealing), a small amount is contained.
It has been found that C and N are precipitated as Nb carbonitride, and as a result, a large hardness reduction effect is obtained.

Intermediate annealing performed after heating to 980 to 1080 ° C. includes heating to 980 to 1080 ° C., cooling once, and then reheating, or heating to 980 to 1080 ° C., and then cooling to an intermediate annealing temperature. In both cases where the temperature is maintained, Nb carbonitride can be sufficiently precipitated. When heat treatment is performed by the latter method, Nb carbonitride not only precipitates during this heat treatment but also grows into a large carbonitride.
A sufficiently low hardness can be obtained in a state where it is cooled to room temperature after the heating of the stage.

On the other hand, when the intermediate annealing is carried out by reheating, the low hardness cannot be obtained as it is because the precipitated Nb carbonitride does not grow sufficiently with the intermediate annealing. By performing the softening annealing by heating to 600 to 680 ° C, the precipitation and growth of the ε-Cu phase and the generation of retained austenite are promoted, and a large hardness reduction effect is obtained. Has been found that excellent ductility can be obtained as compared with a case where cooling is performed once and heating is performed continuously from the first stage without reheating.

The method for producing a precipitation-hardened martensitic stainless steel according to the present invention based on the above-described new findings is as follows: C: 0.050% or less; Si: 0.60% or less;
n: 0.80% or less, S: 0.010% or less, Cu: 2.5 to 4.0%, Ni: 3.5 to
6.0%, Cr: 14.00 to 16.00%, Nb: 0.15 to 0.55%, N: 0.030%
Contains the following, and C + N: 0.070% or less and Nb / (C + N) ≧ 5.00
The balance is steel consisting of Fe and impurity elements, or Mo: 0.3 to 2.0% for improving corrosion resistance, B: 0.0005 to 0.0100% for improving hot workability, Ca: 0.0005 to 0.0100
%, Mg: 0.0005 to 0.0100%, REM: at least one of one or more of 0.0005 to 0.0100%
After hot-rolling a steel further containing the above-mentioned elements and cooling to room temperature, the steel is heated to 980 to 1080 ° C. for 15 minutes to 6 hours.
Hold the temperature for a time, then 800 at a rate of 500 ° C / hr or less
A first invention comprising cooling to 880880 ° C., maintaining the temperature at the above temperature for 1 to 16 hours, and then cooling to room temperature;
The same steel as in the first invention is subjected to a solution heat treatment in which the steel is cooled to room temperature after hot rolling, heated to 980 to 1080C, maintained at a temperature for 15 minutes to 6 hours, and then cooled.
After reheating to 0-880 ° C and maintaining the temperature for 1-16 hours,
An intermediate annealing treatment of cooling to a temperature of not more than the Ms point, and finally a soft annealing treatment of heating to 600 to 680 ° C., maintaining the temperature for 1 to 16 hours, and then cooling to room temperature. It is constituted by the invention.

Next, the range of the added amount of the chemical component and the reason for limiting the heat treatment conditions in the production method of the present invention will be described below.
Although the chemical composition partially overlaps with JIS SUS630, in order to increase the hardness reduction effect after heat treatment, SU
The reason is explained below because it is an appropriate range in the range of S630.

C: 0.050% or less, N: 0.030% or less, C + N: 0.070%
Hereafter, C and N are interstitial elements and have an effect of increasing the hardness by solid solution strengthening, and therefore must be reduced as much as possible in the present invention for improving cold forgeability. In the present invention, the structure in which C and N are precipitated as Nb carbonitride by the addition of Nb is formed. However, if C and N are contained in a large amount, even if the structure is precipitated as Nb carbonitride, the structure is solid. An increase in hardness due to solution strengthening is inevitable. Therefore, in the present invention, the upper limits of C, N and C + N are each 0.050.
%, 0.030%, and 0.070%. More preferably, N, C + N
Should be set to 0.020% and 0.060%.

Si: 0.60% or less Si is an element that increases the hardness of a material by solid solution strengthening and inhibits cold forgeability. However, since Si is an indispensable element as a deoxidizing agent, if it is excessively reduced, production becomes difficult.
It could be added up to 60%. More preferably, the upper limit is set to 0.40%.

Mn: 0.80% or less Mn is an element which causes an increase in material hardness due to solid solution strengthening like Si. Therefore, it is preferable to reduce the hardness as much as possible, but if the upper limit is too strict, the production becomes difficult, so the upper limit is set to 0.80%.

S: 0.010% or less S is an element present as an impurity in the production, but if contained in a large amount, sulfide-based nonmetallic inclusions increase, and cracks are likely to occur during cold forging. , And the upper limit was made 0.010%. More preferably, the upper limit is set to 0.005%.

Cu: 2.50 to 4.00% Cu is an indispensable element for securing necessary precipitation hardening ability and corrosion resistance, and at least 2.50% or more, preferably 3.00%
It is necessary to contain the above. However, when a large amount is contained, the hot workability is reduced and the productivity is deteriorated, and the effect of improving the precipitation hardening ability is saturated. Therefore, the upper limit is set to 4.00%. More preferably, the upper limit is 3.60%
Good to be.

Ni: 3.50 to 6.00% Ni is a basic element for stainless steel for improving corrosion resistance, and is also an element necessary for suppressing a decrease in hot workability due to formation of δ ferrite at a high temperature. Further, since it is a strong γ-phase forming element and is an element necessary to generate an appropriate amount of retained austenite after the softening annealing in the above-mentioned second invention and sufficiently obtain the effect of reducing hardness, at least 3.50 %, Preferably 4.00% or more, more preferably 4.50% or more.

However, when a large amount is contained, the Ms point is lowered, and residual austenite is formed after solution heat treatment to increase work hardening, so that deformation resistance is increased and precipitation hardening ability is increased due to increased residual austenite. Since the strength decreases and the required strength cannot be obtained, the upper limit is set to 6.00%. Preferably, the upper limit is set to 5.50%.

Cr: 14.00 to 16.00% Cr is a basic element for ensuring the necessary corrosion resistance of stainless steel, and is at least 14.00%, preferably at least 14.00%.
15.00% or more is required. However, when contained in a large amount, the amount of δ ferrite increases and hot workability and toughness deteriorate, so the upper limit was set to 16.00%.

Nb: 0.15 to 0.55%, Nb / (C + N) ≧ 5.00 Nb is an element that easily binds to C and N, and C and N are precipitated as carbonitrides to influence solid solution strengthening. , Is an element that reduces the amount of N and is effective in reducing the hardness after heat treatment. Also,
Nb also has the function of preventing precipitation of Cr carbide by a so-called stabilizing effect and improving corrosion resistance. Therefore, it is necessary to add C and N in an amount sufficient to stabilize it, and it is necessary to contain at least 0.15% or more.

However, when contained in a large amount, the formation of δ ferrite is promoted similarly to Cr and the hot workability and toughness are deteriorated. Therefore, the upper limit is set to 0.55%. In order to sufficiently generate carbonitrides, it is necessary to contain a sufficient amount as compared with the amounts of C and N contained, so that Nb / (C + N) ≧ 5.00 is satisfied. It is necessary to let And
In order to obtain a greater hardness reduction effect, Nb / (C + N) ≧ 7.
It is desirable to adjust the contents of C, N, and Nb so as to satisfy 00.

Mo: 0.3 to 2.0% Mo is an element that can be added as necessary according to the level of corrosion resistance required by the user. In order to obtain superior corrosion resistance compared to steel containing no Mo, ,at least
It must be contained at 0.30% or more. However, when contained in a large amount, the amount of δ ferrite increases and the hot workability decreases, and the toughness also decreases. Therefore, the upper limit is set to 2.0%.

B: 0.0005-0.0100%, Ca: 0.0005-0.0100
%, Mg: 0.0005 to 0.0100%, and REM: 0.0005 to 0.0100% B, Ca, Mg, and REM are elements that can be added as necessary to improve hot workability. In order to sufficiently obtain the above-mentioned effects, it is necessary to contain at least one of the above-mentioned elements in an amount of 0.0005% or more. However, the effect is saturated even if it is contained in large amounts, so the upper limit is set to 0.0100% for each element.
And

Next, the reasons for limiting the heat treatment conditions of the present invention will be described below. The first stage heating temperature is 980 ~ 1080 ℃
The reason for this is that the temperature is appropriate for sufficiently solid-solubilizing carbides as in the case of ordinary solution heat treatment. That is, if the temperature is lower than 980 ° C., the carbide may not sufficiently form a solid solution.
Corrosion resistance may be degraded. The lower limit of the heating and holding temperature is desirably 1020 ° C. in order to ensure the solid solution of the carbide, and the upper limit of the heating and holding temperature is preferably 1060 ° C. in consideration of the formation of δ ferrite. In addition, the heating holding time needs to be a time sufficient for solid solution of the carbide, and is set to at least 15 minutes or more, and the upper limit is set to 6 hours due to productivity problems. Preferably, it is set to 2 hours or less. Further, in the second invention, cooling is performed immediately after the first stage of heating and holding, but this cooling is performed at a cooling rate at which carbides do not precipitate during cooling (for example, water cooling, air cooling, or the like) as in ordinary solution heat treatment. Should not be deposited.) This is because the second-stage heat treatment needs to be performed on steel in which carbides are sufficiently dissolved.

Next, the second-stage heat treatment is performed at a temperature in the range of 800 to 880 ° C. because the first-stage heating is in a state in which the carbides have sufficiently dissolved into solid solution, and the temperature is maintained in this temperature range. By doing so, Nb carbonitride is sufficiently precipitated.
The temperature was set at 800 to 880 ° C because it was in a temperature range suitable for precipitating Nb carbonitride, and even if the temperature was higher or lower than this temperature range, carbonitride did not easily precipitate. It takes time for the treatment, and the hardness does not sufficiently decrease. More preferably, the upper limit of the heating holding temperature is set to 860 ° C. Further, the heating and holding time needs to be long enough to precipitate Nb carbonitride, and is set to at least 1 hour or more, and the upper limit is set to 16 hours from the viewpoint of productivity. In order to sufficiently obtain the effect of reducing the hardness, it is desirable to hold the heating for at least 4 hours.

In the first invention, since the heating from the first stage to the second stage is performed continuously without cooling and reheating once to room temperature or near room temperature, during this heat treatment, Nb carbonitride precipitates and grows and grows. As a result, it is possible to obtain steel of low hardness that is easily cold forged. The reason why the cooling rate is set to 500 ° C / hr or less is that if the cooling rate is more than 500 ° C / hr, the growth of carbonitride becomes insufficient and the hardness does not decrease sufficiently. is there. In order to obtain a particularly low hardness, the temperature is preferably 150 ° C./hr or less, more preferably 50 ° C./hr or less.
In the first invention, the heat treatment is completed by cooling after the second stage of heating, and the desired hardness can be obtained regardless of the cooling condition. However, when the martensitic transformation progresses slowly at about 200 ° C or less (the martensitic transformation temperature slightly changes depending on the component), retained austenite is formed, and although a further hardness reduction effect is obtained, Since high hardness cannot be obtained due to the aging treatment, and the solution heat treatment needs to be performed again as in the case of the second invention, it is desirable to cool the temperature region at a speed higher than air cooling.

Further, in the second invention, the cooling end temperature after the second stage heating and holding is set to a temperature not higher than the Ms point because the martensitic transformation is not performed, and the ε-Cu phase is precipitated by the subsequent softening and annealing treatment. And it cannot be grown. If the cooling ends at a temperature above the Ms point,
When the softening annealing treatment described below is performed, the ε-Cu phase does not precipitate and grow, and an appropriate amount of retained austenite cannot be generated, resulting in insufficient reduction in hardness.

In a second aspect of the present invention, the first stage heat treatment is followed by cooling and reheating to perform the second stage heat treatment.
The Nb carbonitride precipitates due to the second stage of heating and holding, but is cooled before the carbonitride grows sufficiently as in the first invention, so that the carbonitride is in a finely dispersed state. Even if the second-stage heat treatment is completed, the hardness does not decrease to the target hardness (however, the pinning effect of Nb carbonitride has the effect of making the structure finer and improving the ductility).

Then, as the third stage heating, 60
A soft annealing treatment of heating and holding at a temperature of 0 to 680 ° C. for 1 to 16 hours and cooling to room temperature is performed. By this heat treatment ε
The -Cu phase precipitates and grows, and an appropriate amount of retained austenite is formed, and a desired low hardness is obtained. 600 temperature range
~ 680 ° C is the reason for the precipitation of ε-Cu
Since the phase does not grow, there is a problem of hardening instead.If the temperature exceeds 680 ° C, the martensitic structure is transformed to austenite beyond the transformation temperature, and austenite transforms to martensite again during subsequent cooling. This is because there is a problem that the effect of reducing the hardness cannot be obtained. In order to further increase the hardness reduction effect, it is desirable to set the lower limit of the heating holding temperature to 610 ° C. in order to surely grow the ε-Cu phase, and to surely prevent the reverse transformation to austenite. For this purpose, it is desirable to set the upper limit of the heating holding temperature to 660 ° C.

According to the above-described method, the hardness can be reduced as compared with the case where the solution heat treatment is generally performed, so that excellent cold forgeability can be obtained. However, in the second invention, the ε-Cu phase precipitates and grows and the residual austenite is generated by the heat treatment of the present invention, so that an increase in hardness cannot be expected even if the aging treatment is performed as it is. Therefore, for stainless steel manufactured by the method of the second invention,
After processing into a predetermined shape by cold forging, it is necessary to perform solution heat treatment again and then perform aging treatment. Thereby, sufficient strength can be obtained.

By combining the above-described ranges of the chemical components and the production conditions with the respective desirable ranges, the hardness HRC 27 which has been almost impossible in the past can be obtained.
The following can be achieved:

That is, as in the invention of the third aspect (third invention), the weight ratio of C: 0.050% or less, Si: 0.40% or less, M:
n: 0.80% or less, S: 0.010% or less, Cu: 3.0 to 3.6%, Ni: 4.5 to
5.5%, Cr: 15.00 to 16.00%, Nb: 0.15 to 0.55%, N: 0.020%
And C + N: 0.060% or less and Nb / (C + N) ≧ 7.00
Is satisfied, the balance is steel consisting of Fe and impurity elements, or Mo: 0.3 to 2.0% for improving corrosion resistance, B: 0.0005 to 0.0100% for improving hot workability, Ca: 0.0005 to 0.0100
%, At least one of one or more of Mg: 0.0005 to 0.0100%, REM: 0.0005 to 0.0100%
After hot rolling the steel further containing the above-mentioned elements to the steel and cooling to room temperature, the steel is heated to 102 to 1060 ° C. for 15 minutes to 6 minutes.
Hold the temperature for a time, then 800 at a rate of 150 ° C / hr or less
After cooling to 860860 ° C., maintaining the temperature at the above temperature for 4 to 16 hours, and then cooling to room temperature, the hardness after heat treatment can be reduced to HRC 27 or less.

Further, as in the invention of the fourth aspect (the fourth invention), the weight ratio of C: 0.05% or less, Si: 0.40% or less, Mn: 0.8
0% or less, S: 0.010% or less, Cu: 3.0 to 3.6%, Ni: 4.5 to 5.5%,
Cr: 15.00 to 16.00%, Nb: 0.15 to 0.55%, N: 0.020% or less, and C + N: 0.060% or less and Nb / (C + N) ≧ 7.00, the balance being Fe and impurities Elemental steel or Mo: 0.3-2.0% for improving corrosion resistance, B: 0.0005-0.0100%, Ca: 0.0005-0.0100%, M for improving hot workability
g: 0.0005 to 0.0100%, REM: 0.0005 to 0.0100%, and at least one element of at least one element of the above steels was further rolled to a room temperature after hot rolling. After that, it is subjected to a solution heat treatment of heating to 102 to 1060 ° C., maintaining the temperature for 15 minutes to 6 hours, and then cooling.
Next, after reheating to 800 to 860 ° C and maintaining the temperature for 4 to 16 hours, an intermediate annealing treatment of cooling to a temperature below the Ms point is performed, and finally heating to 610 to 660 ° C for 4 to 16 hours. Even after cooling to room temperature, the hardness after heat treatment
HRC 27 or less is possible.

[0034]

Next, the characteristics of the steel according to the present invention will be described with reference to examples in comparison with comparative examples. Table 1 shows the chemical components of these test steels. The test steel was prepared by heating a steel ingot melted by a 30 kg vacuum induction melting furnace to 1200 ° C. and forging.

[0035]

[Table 1]

In Table 1, A to F steels are steels within the composition range of the present invention, and G to J steels are comparative steels in which any component is out of the range specified in the invention. In addition. G ~ J
Among the steels, G and J steels are steels within the composition range of SUS630,
H steel is added to steel in the range of SUS630 to improve hot workability.
Is a steel containing a small amount of Nb, and steel I is a steel having a low Nb content relative to SUS630 and containing a small amount of Ca for improving hot workability.

Each of these test steels was evaluated for hardness after heat treatment, critical working ratio, hardness after aging treatment, corrosion resistance, and hot workability. The test method is described below. (1) Heat treatment method The heat treatment conditions are as follows:
The cooling rate during the heat treatment was varied as shown in Tables 2 and 3. However, for the first stage heating corresponding to the solution heat treatment, 1050 ° C x
30 minutes (The results in Table 2 were cooled at the rate shown in Table 2 after heating and holding.
The results in Table 3 were unified under the condition of heating and holding and water cooling). In addition, cooling after the second stage of heating was performed by gradually cooling to 700 ° C (furnace cooling in a heating furnace), and cooling to 700 ° C or less by air cooling to room temperature (excluding test No. 33). .). In addition, in order to compare the difference of the effect with the conventional heat treatment, the measurement was performed in the same manner even when the ordinary solution heat treatment was performed (Test Nos. 20, 44 shown in Tables 2 and 3).

As an embodiment relating to the second invention,
In order to examine the effect of the cooling end temperature after the second stage heating, a test (test No. 33) in which the cooling end temperature was set to 300 ° C. higher than the Ms point was also performed. Using the test piece heat-treated by the method described above, the hardness, critical working rate, hardness after aging treatment, and corrosion resistance were evaluated as described below.

(2) Hardness after heat treatment After the above heat treatment, the hardness of the D / 4 part was measured on a Rockwell C scale based on JISZ2245.

(3) Cold Forgeability The test piece subjected to the heat treatment described above was machined to a diameter of 14%.
mm, height 21mm, depth 0.8mm notched in the longitudinal direction of the test piece (Standard No.2 cold forging subcommittee of the Japan Society for Technology of Plasticity) and compression-processed with a 100ton Amsler testing machine. The compression rate (critical processing rate) when cracks were visually confirmed was measured. In the compression working, a tool with concentric grooves was used in order to make the friction condition hard to vary.

(4) Hardness after aging treatment The test piece subjected to the heat treatment was further subjected to 590 ° C. ×
Aging treatment under the condition of air cooling after heating and holding for 4 hours,
The hardness was measured in the same manner as in (2). In the case of performing the heat treatment corresponding to the second invention, in order to obtain necessary precipitation hardening, after performing a solution heat treatment under the conditions of heating and holding at 1030 ° C. × 1 hr and water cooling before aging treatment, Hardness was measured in the same manner.

(5) Corrosion Resistance After the same heat treatment as the test piece whose hardness after the aging treatment was measured, the test piece was subjected to a method in accordance with “Testing method for ferric chloride corrosion of stainless steel” specified in JIS G0578. After the test at a temperature of 35 ° C., the corrosion resistance was evaluated by measuring the corrosion loss. Tables 2 and 3 show the measured corrosion weight loss (g / m ² hr).
Is indicated by ◎ when less than 25, ○ when 25 or more and less than 35, and × when 35 or more.

(6) Hot workability The hot workability is determined by preparing a test piece having a parallel part diameter of 10 mm from a forged material, heating it to 1100 ° C. for 100 seconds by applying current, and maintaining the temperature for 1 minute. Then, after breaking at a tensile speed of 50 mm / sec, evaluation was performed by measuring the drawing ratio of the fractured surface. Tables 2 and 3 show 場合, 90 when the drawing ratio is 95% or more.
% And less than 95%, and ○ when 80% or more and less than 90%. The test results are shown in Table 2 (Examples of the first and third inventions) and Table 3 (Examples of the second and fourth inventions).

[0044]

[Table 2]

[0045]

[Table 3]

Of the results shown in Tables 2 and 3, Test Nos. 1 to
As is clear from the results of Nos. 15, 21 to 39, even when using the A to F steels whose chemical components are within the scope of the present invention, any one of the conditions such as the heating temperature and the cooling rate is the present invention. If the temperature is out of the range, heat treatment (1050 ° C. × 30
Test No. 2 which is the result of water-cooled solution heat treatment)
It can be seen that the hardness after heat treatment is higher than that of the present invention, but the cold forgeability is lower than that of the present invention.

Further, a test was conducted using a test material in which a part of the components specified in the present invention is out of the range.
As is clear from the results of Nos. 16 to 19 and 40 to 43, even if the steel is within the range of SUS630 standardized by JIS or has a component very similar to SUS630, the effect of the new heat treatment is It can be seen that there is a case where the hardness is not sufficiently obtained and the hardness is not sufficiently reduced. In Test Nos. 20 and 44, in which SUS630 was merely heat-treated by a conventional method, the hardness after heat treatment was significantly higher than that of the above-mentioned comparative example.

In contrast to the comparative example and the conventional example described above, the examples corresponding to the present invention all have sufficiently low hardness (HRC 28 or less, and HRC 27 or less when performed under desirable conditions). It can be seen that the rate is also superior to the conventional example. In addition, it was confirmed that the hardness after precipitation hardening was equal to or higher than that of the conventional example, and the corrosion resistance and hot workability were equal to or higher than those of the conventional example and the comparative example. In this example, since the case of aging treatment at 590 ° C. is shown, the hardness is about HRC35,
If the temperature is lower (for example, 480 ° C. [equivalent to H900]), higher hardness can be naturally obtained. Table 2,
Although the results are not shown in FIG. 3, the same test materials were used and the same heat-treated materials were subjected to tensile tests.
The heat-treated material of the invention has an elongation of 12 to 15%, and the second and fourth inventions have an elongation of 22 to 25%. Thus, it has been found that the second and fourth inventions are superior in ductility.

In the experiment described above, in order to test a number of conditions in a short time, an experiment was conducted using a melting material in a 30 kg VIM melting furnace, and the results were shown. It was confirmed that similar results were obtained.

In the above embodiment, only the case where the first stage heating condition is performed at a constant temperature of 1050 ° C. is shown. However, the purpose of this heating is the same solid solution of carbide as in the conventional solution heat treatment.
If the temperature is in the range of 980 to 1080 ° C., which is within the range of the present invention, even if the first-stage heating is performed at a temperature other than 1050 ° C., the carbides are sufficiently dissolved to obtain exactly the same effects as those of the above-described embodiment. be able to. The cooling conditions after the second stage of heating and holding are shown only for slow cooling up to 700 ° C and below for air cooling, but the same experiment was conducted for air cooling or water cooling immediately after heating and holding. Was carried out, and it was confirmed that a similarly excellent hardness reduction effect was obtained.

[0051]

As described above, according to the present invention, by optimizing the component range and performing heat treatment under specific conditions,
Conventionally, cold forging was regarded as a very difficult steel type
The hardness of SUS630 (or steel obtained by additionally adding a corrosion resistance improving element (Mo) and a hot workability improving element (B, Ca, Mg, REM) to SUS630) can be suppressed. Therefore, the load on the die and punch used for cold forging is reduced, and the life of the die and punch can be greatly improved to enable cold forging to process parts. Both corrosion resistance and strength are required, and it is possible to increase the efficiency of manufacturing components suitable for application of precipitation hardening stainless steel.

Claims (4)

(57) [Claims] Claims: C: 0.050% or less by weight, Si: 0.60% or less, Mn: 0.80% or less, S: 0.010% or less, Cu: 2.5 to 4.0%, Ni:
3.5-6.0%, Cr: 14.00-16.00%, Nb: 0.15-0.55%, N:
Contains 0.030% or less, and C + N: 0.070% or less and Nb / (C + N)
Satisfies ≥ 5.00, the balance is steel consisting of Fe and impurity elements, or Mo: 0.3 to 2.0% for improving corrosion resistance, B: 0.0005 to 0.0100% for improving hot workability, Ca: 0.0005
~ 0.0100%, Mg: 0.0005 ~ 0.0100%, REM: 0.0005 ~ 0.010
After hot rolling a steel further containing at least one element of at least one of 0% or two or more thereof, the steel is cooled to room temperature, and then heated to 980 to 1080C.
Maintaining the temperature for 15 minutes to 6 hours, then cooling at a rate of 500 ° C./hr or less to 800 to 880 ° C., maintaining the temperature at the above temperature for 1 to 16 hours, and then cooling to room temperature. Method for producing precipitation-hardened martensitic stainless steel with excellent cold forgeability. 2. The weight ratio of C: 0.05% or less, Si: 0.60% or less, Mn: 0.80% or less, S: 0.010% or less, Cu: 2.5 to 4.0%, Ni:
3.5-6.0%, Cr: 14.00-16.00%, Nb: 0.15-0.55%, N:
Contains 0.030% or less, and C + N: 0.070% or less and Nb / (C + N)
Satisfies ≥ 5.00, the balance is steel consisting of Fe and impurity elements, or Mo: 0.3 to 2.0% for improving corrosion resistance, B: 0.0005 to 0.0100% for improving hot workability, Ca: 0.0005
~ 0.0100%, Mg: 0.0005 ~ 0.0100%, REM: 0.0005 ~ 0.010
After hot rolling a steel further containing at least one element of at least one of 0% or two or more thereof, the steel is cooled to room temperature, and then heated to 980 to 1080C.
A solution heat treatment of cooling after maintaining the temperature for 15 minutes to 6 hours, then reheating to 800 to 880 ° C., maintaining the temperature for 1 to 16 hours, and then cooling to a temperature below the Ms point Annealed, and finally heated to 600-680 ° C
A method for producing a precipitation hardening martensitic stainless steel excellent in cold forgeability, characterized by performing a soft annealing treatment in which the temperature is maintained for up to 16 hours and then cooled to room temperature. 3. The weight ratio of C: 0.050% or less, Si: 0.40% or less, Mn: 0.80% or less, S: 0.010% or less, Cu: 3.0 to 3.6%, Ni:
4.5-5.5%, Cr: 15.00-16.00%, Nb: 0.15-0.55%, N:
Contains 0.020% or less, and C + N: 0.060% or less and Nb / (C + N)
Satisfies ≧ 7.00, the balance is steel consisting of Fe and impurity elements, or Mo: 0.3 to 2.0% for improving corrosion resistance, B: 0.0005 to 0.0100% for improving hot workability, Ca: 0.0005
~ 0.0100%, Mg: 0.0005 ~ 0.0100%, REM: 0.0005 ~ 0.010
0% or more of at least one element of at least one of the above-mentioned steels is further rolled to room temperature after hot rolling, and then heated to 102 to 1060 ° C.
The temperature is maintained for 15 minutes to 6 hours, then cooled to 800 to 860 ° C. at a rate of 150 ° C./hr or less, the temperature is maintained for 4 to 16 hours, and then cooled to room temperature. To
A method for producing a precipitation-hardening martensitic stainless steel excellent in cold forgeability, characterized by having an HRC of 27 or less. 4. C: 0.05% or less by weight, Si: 0.40% or less, Mn: 0.80% or less, S: 0.010% or less, Cu: 3.0 to 3.6%, Ni:
4.5-5.5%, Cr: 15.00-16.00%, Nb: 0.15-0.55%, N:
Contains 0.020% or less, and C + N: 0.060% or less and Nb / (C + N)
Satisfies ≧ 7.00, the balance being steel consisting of Fe and impurity elements, or Mo: 0.3 to 2.0% for improving corrosion resistance, and B: 0.0005 to 0.0100% for improving hot workability, Ca: 0.0005
~ 0.0100%, Mg: 0.0005 ~ 0.0100%, REM: 0.0005 ~ 0.010
0% or more of at least one element of at least one of the above-mentioned steels is further rolled to room temperature after hot rolling, and then heated to 102 to 1060 ° C.
A solution heat treatment of cooling after maintaining the temperature for 15 minutes to 6 hours, then reheating to 800 to 860 ° C, maintaining the temperature for 4 to 16 hours, and then cooling to a temperature below the Ms point Annealed, and finally heated to 610-660 ° C
A method for producing a precipitation-hardening martensitic stainless steel excellent in cold forgeability, characterized in that after maintaining the temperature for 16 hours, it is cooled to room temperature and the hardness after heat treatment is set to HRC 27 or less.