JP3378346B2 - Precipitation-hardened martensitic stainless steel with excellent strength and toughness - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is a precipitation hardening type that exhibits high strength and maintains high toughness after aging treatment in a continuous aging furnace, a batch aging furnace, etc. and exhibits excellent toughness in the welded portion. Regarding martensitic stainless steel.

[0002]

2. Description of the Related Art Precipitation hardening type martensitic stainless steel has low hardness before aging treatment and is excellent in punching workability and forming workability. After the aging treatment, it exhibits high strength due to precipitation hardening and is excellent in welding softening resistance. Utilizing this property, it is widely used as a structural material such as a steel belt that requires welding and various spring materials. The present applicant also introduced, as a precipitation hardening type martensitic stainless steel of this type, a steel belt material having high strength and excellent toughness as Japanese Patent Publication No. 59-49303.
In this precipitation hardening type martensitic stainless steel, C, Ti, Mn, Ni, so that a large amount of austenite phase does not remain in the solution treated state or the heat affected zone after welding.
The composition is adjusted among Cr, Cu and Al. As a result, the martensite formation of the welded portion is particularly promoted, and the strength is improved by the aging treatment. A martensitic stainless steel whose toughness has been improved by adding Mo was introduced in JP-A-60-36649.

[0003]

[Problems to be Solved by the Invention] Japanese Patent Publication No. 59-4930
The No. 3 stainless steel satisfies the required properties in terms of strength, but may not satisfy the required toughness depending on the application. In this regard, in the stainless steel disclosed in JP-A-60-36649, the toughness is improved by adding Mo. Precipitation-hardening martensitic stainless steels are beginning to be used in a wide variety of applications. With the development of applications, insufficient strength or insufficient toughness may become a problem depending on the usage conditions. When high toughness is required, the required characteristics cannot be satisfied simply by adding Mo. Precipitation hardening type martensitic stainless steel is aged in a continuous aging furnace, a batch type aging furnace, etc., but the properties such as toughness and strength vary depending on the temperature rising and cooling rates. Especially in the batch type aging furnace where a long time is spent for raising and lowering the temperature,
Compared with the case of using a continuous aging furnace in which the temperature rising / falling time is relatively short, the toughness is greatly reduced depending on the aging temperature. Therefore, in either case of the continuous aging furnace or the batch aging furnace, it is required to develop a steel material having higher strength and higher toughness depending on the application. The present invention has been devised to meet such a demand, and by balancing the components of Ni, C, etc. under Cu reducing conditions,
It is an object of the present invention to provide a precipitation hardening type martensitic stainless steel which has excellent toughness in both the base material and the welded portion, which has not been obtained by conventional steel materials in the high strength region.

[0004]

The precipitation hardening type martensitic stainless steel of the present invention has a C content of 0.03% by weight or less as a material suitable for aging treatment by a continuous aging furnace in order to achieve the object. , Si: 0.5 to 2.0 wt%, Mn: 3.0 wt% or less, P: 0.02 to 0.0
6% by weight, S: 0.005% by weight or less, Ni: 7.5
9.5% by weight, Cr: 12.0 to 15.0% by weight, C
u: 0.3% by weight or less, Mo: 1.0 to 3.0% by weight,
Ti: 0.15 to 0.60% by weight, N: 0.015% by weight or less and Al: 0.30% by weight or less, formula (1)
The E value defined by is regulated to 0.075 or less. E = [(Si%) × (Ti%)] / (Ni%) (1) Suitable for aging treatment by a batch type aging furnace,
The C content is regulated to 0.02% by weight or less. When used in a welded state, the F value defined by equation (2) is 4
The components are adjusted so as to be 0.0 or less. F = 0.7x (Mn%) + (Ni%) + 0.6x (Cr%) + 0.76x (Cu%) + 0.35x (Si%)-0.36x (Al%) + 0. 6 × (Mo%) + 21.5 ・ ・ ・ ・ (2)

[0005]

The present inventors have conducted various investigations to obtain even higher toughness in the martensitic stainless steel introduced in JP-A-60-36649. As a result, it is effective to keep the Cu content low. I found a thing. However, if Cu, which is a precipitation hardening element, is reduced, insufficient strength after aging treatment becomes a problem. Therefore, in order to ensure the prescribed strength for steel with reduced Cu content, further investigation and
Repeated research. As a result, by maintaining the Ni content at a high level, suppressing the C content to be low, and adjusting the balance of other alloy elements, the conventional precipitation hardening type martensite can be used even after aging treatment and during welding. It has been clarified that the strength is equal to or higher than that of site-based stainless steel. Also, N
By maintaining a balance of i, Ti, Si, etc., and by containing a certain amount or more of P, high toughness is maintained even in the high strength region, and a martensitic stainless steel having an excellent strength-toughness balance can be obtained. I understood. Furthermore,
For martensitic stainless steel that is aged in a batch aging furnace, it has been clarified that a steel material exhibiting higher toughness than conventional steel can be obtained by keeping the upper limit of C content lower than when using a continuous aging furnace. did.

The alloy components and their contents contained in the precipitation hardening type martensitic stainless steel of the present invention will be described below. C: 0.03% by weight or less, 0.02% by weight or less when aged in a batch system. It is an element effective for improving the strength of steel and suppressing the δ ferrite phase generated at high temperature. However, as the C content increases, a large amount of austenite remains after quenching, and the martensite single phase cannot be obtained even by subsequent temper rolling. for that reason,
It becomes difficult to obtain high strength during aging treatment, and Ti
As a result of promoting the generation of C, the toughness decreases. Further, a large amount of austenite remains during welding due to the re-dissolution of TiC, and high strength cannot be obtained. Therefore, in the present invention, the upper limit of the C content is specified to be 0.03% by weight. When aging treatment is performed in a batch type aging furnace, a longer time is spent to raise and lower the temperature as compared with a continuous aging furnace. Therefore, if the C content exceeds 0.02% by weight, the toughness may decrease due to the precipitation of carbides. Therefore, in this case, the C content is restricted to 0.02% by weight or less.

Si: 0.5 to 2.0 wt% Si has a large solid solution strengthening ability and exhibits a function of strengthening the matrix. In addition, the composite addition with Ti and Ni causes the fine matching precipitation of the intermetallic compound consisting of elements such as Si, Ti, and Ni during the aging treatment to improve the strength of the steel. The intermetallic compound precipitated at this time is Ni
It is a G phase represented by ₁₆ Ti ₆ Si ₇ . Such an effect is remarkable when the Si content is 0.5% by weight or more.
However, when a large amount of Si exceeding 2.0% by weight is contained, the formation of the δ ferrite phase is promoted, and the strength and toughness decrease. Mn: 3.0 wt% or less Mn has an action of suppressing the formation of the δ ferrite phase in a high temperature range. However, addition of a large amount of Mn tends to cause deterioration of the toughness of the welded part and deterioration of welding workability. Therefore,
In the present invention, the upper limit of the Mn content is specified to be 3.0% by weight.

P: 0.02-0.06% by weight In the alloy system in which the C content is reduced as in the present invention, TiC
The distribution amount of Fe decreases, the pinning effect of TiC becomes smaller, and coarsening of crystal grains, which causes deterioration of toughness, easily occurs. P is an alloying element effective in suppressing coarsening of crystal grains during solution treatment or annealing in this low C stainless steel and obtaining high toughness after aging treatment. In order to obtain high toughness after aging treatment, 0.02% by weight or more of P
Is required to be contained. However, even if a large amount of P exceeding 0.06% by weight is contained, the effect of grain refinement commensurate with the increase in the P content is small, which rather causes deterioration of toughness. Moreover, it is not preferable to contain a large amount of P in order to secure the corrosion resistance.

Ni: 7.5-9.5 wt% Ni precipitates as a G phase that contributes to precipitation hardening, and suppresses the formation of a δ ferrite phase. In the alloy system of the present invention, the lower limit of the Ni content is specified to be 7.5 wt% in order to suppress the formation of the δ ferrite phase after annealing and to secure high strength and high toughness. However, when a large amount of Ni exceeding 9.5 wt% is contained, a large amount of austenite remains after annealing or welding, and it becomes difficult to obtain high strength. Cr: 12.0 to 15.0 wt% To obtain corrosion resistance as stainless steel, at least 1
It is necessary to contain 2.0% by weight or more of Cr. However, when a large amount of Cr exceeding 15.0% by weight is contained, a large amount of δ ferrite phase and retained austenite phase are generated, which causes a decrease in strength of the welded portion.

Cu: 0.3% by weight or less The alloy system of the present invention provides high strength regardless of the precipitation strengthening action of Cu. Therefore, when the strength is increased, it is possible to suppress the content of Cu to be low, which tends to cause deterioration of toughness particularly at low temperature aging. Further, addition of a large amount of Cu deteriorates hot workability and causes surface cracking. Therefore, in the present invention, the upper limit of the Cu content is specified to be 0.3% by weight. Mo: 1.0 to 3.0 wt% Mo is an alloying element effective in improving strength and toughness due to precipitation hardening. In order to develop effective age hardening ability,
It is necessary to contain 1.0% by weight or more of Mo. However, even if Mo is contained in excess of 3.0% by weight, the improvement in strength and toughness commensurate with the increase in Mo content cannot be obtained. Moreover, when a large amount of Mo is contained, the formation of the δ ferrite phase is promoted, and the strength and toughness of the welded portion are likely to decrease.

Ti: 0.15 to 0.60 wt% Ti is an alloying element that contributes to precipitation hardening, and it is necessary to contain 0.15 wt% or more of Ti in order to obtain high strength. However, when a large amount of Ti is contained in excess of 0.60% by weight, the strength is improved, but the toughness is lowered due to an excessive precipitation hardening reaction. N: 0.015 wt% or less N has a large affinity with Ti and consumes effective Ti acting as a precipitation hardening element by the formation of TiN. Also, N
The TiN inclusions increase as the content increases, which causes a decrease in fatigue strength and toughness. Therefore, the lower the N content is, the more preferable. In the present invention, the upper limit of the N content is specified to be 0.015% by weight.

S: 0.005 wt% or less S is present in the steel as a non-metallic inclusion such as MnS and adversely affects fatigue strength, toughness, corrosion resistance and the like. In this respect,
The lower the S content, the more preferable, and the upper limit is 0.005% by weight.
Stipulated in. Al: 0.30 wt% or less Al is an element used as a deoxidizing agent, and T
Like i, it also works effectively for precipitation hardening. However, if the Al content exceeds 0.30% by weight, the toughness tends to decrease. Therefore, in the present invention, the upper limit of the Al content is specified to be 0.30% by weight.

E value: In a component system containing an alloying element of 0.075 or less, the E value defined by the formula (1) was further restricted to 0.075 or less. E value is
It is a relationship between alloy components derived from many experiments by the present inventors. E = [(Si%) × (Ti%)] / (Ni%) (1) The E value is an index showing the balance between Si, Ti and Ni necessary for maintaining high toughness. is there. E value is 0.075
If the component is adjusted to exceed, the toughness decreases when the strength is increased by aging treatment.

F value: 40.0 or less When the steel whose composition is designed as above is used for welding, the F value defined by the formula (2) is further restricted to 40.0 or less. Formula (2) is also a relationship between alloy components derived from many experiments by the present inventors. F = 0.7x (Mn%) + (Ni%) + 0.6x (Cr%) + 0.76x (Cu%) + 0.35x (Si%)-0.36x (Al%) + 0. 6 × (Mo%) + 21.5 (2) F value is an index showing the balance between the alloy components necessary for ensuring the annealed state and the high strength of the welded portion. When the composition is adjusted so that the F value exceeds 40.0, a large amount of austenite remains in the heat-affected zone and the bond zone after annealing or after welding. The retained austenite in the annealed state is easily transformed into martensite by some cold rolling. However, since it is not easy to convert retained austenite to martensite in a welded portion where post-treatment such as cold working is difficult, the strength does not sufficiently increase even after the aging treatment. From this point, the upper limit of the F value is defined as 40.0. The F value is 25 for the retained austenite after annealing.
% Or less, the range is preferably set to 39.0 to 40.0.

One of the factors that lower the toughness is the presence of δ ferrite. When δ ferrite is observed in the structure after annealing, a brittle fracture surface due to δ ferrite is observed on the fracture surface of the toughness evaluation test piece that has been subjected to a tensile test after aging treatment, and the toughness may decrease. It is confirmed. In order to prevent such an influence of δ ferrite, it is preferable to adjust each alloy component so that the D value defined by the following equation (3) is 2.35 or less. D = [Cr% + 3.5 × (Ti% + Al%) + 1.5 × Si% + Mo%] / [Ni% + 0.3 × Cu% + 0.65 × Mn% + 10 × C%] ... ( 3) D value is Cr equivalent / N in the component system specified in the present invention.
It is a limited formula of i equivalent. When the components are adjusted so that the D value exceeds 2.35, a large amount of δ ferrite is generated when the steel material is heated to the soaking temperature, and the hot workability is deteriorated.
Furthermore, the δ-ferrite phase remains after annealing or welding, which causes a decrease in the strength and toughness of the matrix and the weld.

The balance of the alloy system according to the invention is essentially Fe. However, Ca for the purpose of desulfurization, rare earth metal, and 0.01 added to improve hot workability.
It is also possible to include B or the like in an amount of not more than wt%. In addition, since high strength can be obtained while maintaining excellent toughness by the combined addition with Mo, 3.0 to 6.0% by weight of Co can be contained. In the alloy system containing Co,
A correction term of 0.2 × (Co%) is added to the equation (3). In the above alloy system, the components are designed so that the F value is 40.0 or less. When the F value is 39.5 or more, a certain amount of austenite remains after annealing, and it may not be possible to obtain sufficient age hardening ability even by aging treatment, and it may be difficult to obtain high strength. In such a case, it is preferable to perform temper rolling to convert the retained austenite into martensite and to correct the shape. It is effective to set the rolling ratio of the temper rolling to 3% or more in order to improve the shape characteristics and obtain the high strength during the aging treatment by converting the retained austenite into martensite. However, even if the rolling rate is increased too much, the effect of improving the shape is small, and rather causes deterioration of the toughness. Therefore, when temper rolling, the rolling rate is 3 to
Set in the range of 50%.

In the present invention, the toughness is evaluated by the maximum stress J _M in the notch tensile test of the fatigue pre-cracked test piece. The J _M value enables a detailed investigation of the influence of various factors such as alloying elements and thermomechanical treatment on toughness, as compared with the conventional notch tensile test. This J _M value is 1400N
When it is / mm ² or more, a material excellent in both strength and toughness can be obtained. The annealed stainless steel is subjected to an appropriate temper rolling to be aged. As the aging treatment in the continuous aging furnace, 425 to 5 which is generally performed on precipitation hardening steels.
A heat treatment of heating at 50 ° C. for 10 minutes or more is adopted. For the aging treatment in the batch type aging furnace, heat treatment conditions are employed in which the temperature is raised to 425 to 550 ° C. at a temperature raising rate of 1 to 10 ° C./min, soaking is performed for 110 minutes or more, and then cooling is performed. The cooling rate is not particularly limited,
It is usually set in the range of 0.1 to 5 ° C./minute. High strength is developed by aging treatment, and J _M value is 1400 N / mm
A material having a toughness of ² or more can be obtained. J _M value is 140
Since it 0N / mm ² or more, tensile strength is also increased at least 1400 N / mm ² or more. For example,
Even if the tensile strength is about 1650 N / mm ² , J _M
When the value is 1400 N / mm ² or more, considerably excellent toughness is obtained. That is, in order to obtain high toughness in the high strength region, a J _M value of 1400 N / mm ² or more is required.

[0018]

【Example】

—Continuous Aging Treatment— For each stainless steel having the components shown in Table 1 and Table 2, a hot rolled sheet having a thickness of 6 mm was manufactured from a steel ingot of 100 kg through hot rolling. After cutting the hot rolled sheet, it is solution heat treated, then cold rolled at a rolling rate of 40% and annealed by heating at 1030 ° C. for 60 seconds. Molded. In addition, A in Table 1
The group is the invention steel. On the other hand, Group B in Table 2 is a comparative steel, and the content of alloying elements such as C, Ni, P, Cu and Mo, or E value and F value are out of the ranges specified in the present invention. Group C indicates conventional steel.

[0019]

[Table 1]

[0020]

[Table 2]

For each stainless steel, temper rolling condition,
The mechanical properties such as the aging state of the base material and the tensile strength and J _M value of the aged welded part during the aging treatment in the continuous aging furnace were investigated. Use continuous aging furnace for aging treatment 4
A heat treatment condition was employed in which the temperature was raised to 525 ° C. at a temperature increase rate of 0 ° C./min, and soaking was continued for 60 minutes. Hereinafter, this heat treatment is referred to as continuous aging treatment. Invention Steels A1 to 9 and Comparative Steel B
About 1 to 10 and conventional steels C1,2, shown in Table 3 and Table 4 combined hardness at the base metal and the weld at the time of continuous aging treatment at 525 ° C., the J _M value or the like, the characteristics of tempered material . J _M
The test piece 1 shown in FIG. 1 was used for the measurement of the value. The test piece 1 is formed into a rectangular shape having a length of 160 mm and a width of 45 mm, and has a diameter of 16 mm at a position 28 mm from each end.
The circular holes 2 and 3 were opened. A center hole 4 having a diameter of 4 mm was opened in the center of the test piece 1, and notches 5 and 6 having a length of 2.5 mm and a width of 0.3 mm extending in the width direction from the center hole 4 were cut by electric discharge machining. Then, fatigue pre-cracks 7 and 8 having a length of 3.5 mm were introduced by a fatigue tester. The tensile test using this test piece 1 is different from the conventional notch tensile test in which the occurrence of cracks and the resistance to progress are simultaneously evaluated, and only the progress of cracks can be evaluated. Further, since the stress concentration on the fatigue pre-cracks 7 and 8 is higher than that of the conventional notched tensile test piece, the toughness of the material at the crack bottom is evaluated more strictly.

[0022]

[Table 3]

[0023]

[Table 4]

FIG. 2 shows J _{M of the} base metal and the welded part after continuous aging treatment at 525 ° C. shown in Tables 3 and 4 for some steels.
It is the graph which arranged the value by the E value which made the content of Si, Ti, and Ni a factor. As is clear from FIG. 2, a high J _M value of 1400 N / mm ² or more is obtained in the region where the E value is 0.075 or less. J _M value is E value is 0.0
When it exceeds 75, it drops sharply. In both the invention steels of group A and the comparative steels of group B, the intermetallic compound mainly represented by Ni ₁₆ Ti ₆ Si ₇ is precipitated during the aging treatment, and the precipitation is hardened. However, in order to maintain high toughness, it is understood from FIG. 2 that these constituent alloy elements must be contained in a well-balanced manner and the E value should be 0.075 or less. In Comparative Steel B7 and Conventional Steel C1, although the E value was higher than the range specified in the present invention, the J _M value was high and good toughness was shown. This is because in steels having a C content higher than the range specified in the present invention, such as Comparative Steel B7 and Conventional Steel C1, a part of Ti was fixed as TiC during cooling after hot rolling, and after aging. It is presumed that this is because the amount of Ti, which affects the strength and toughness, is smaller than that of steel having a low C content.

FIG. 3 shows the relationship between the F value and the tensile strength of the welded portion after continuous aging treatment at 525 ° C. shown in Tables 3 and 4 for the steels with varying F values. When the F value is 40.0 or less, the weld shows a high tensile strength of 1700 N / mm ² or more, but when the F value exceeds 40.0, the strength of the weld decreases sharply. I understand from In addition, the invented steel A9 and the comparative steel B2 have F values exceeding 40.0, and the comparative steel B7 has a C content outside the scope of the present invention. With respect to the present invention steel A3 and the comparative steel B2, the retained austenite amount and hardness of the welded portion were investigated. The amount of retained austenite was about 35% in the heat-affected zone of the invention steel A3, but was 45% or more in the comparative steel B2. Further, the hardness of the welded portion was HV400 or more in the invention steel A3, whereas it was locally reduced to about HV350 in the comparative steel B2. From this comparison, it was confirmed that when the F value exceeds 40.0, the amount of retained austenite in the welded portion becomes large and the strength is reduced. Further, in Steel A9 of the present invention, as is clear from Table 3 above, although the base metal portion shows high strength and toughness, the toughness of the welded portion deteriorates because the F value exceeds 40.0. ing. That is, in order to secure high strength of the welded portion, it is desirable to balance each alloy component so that the F value is 40.0 or less.

On the other hand, in Comparative Steel B7, the F value is 40.0 or less, but the strength of the welded portion is low.
This is because the C content of comparative steel B7 is higher than the range specified in the present invention. That is, it is presumed that TiC exists in a non-solid solution state due to a large amount of C, and TiC is solid-solved in the matrix due to heat input during welding, and as a result, a large amount of austenite remains after welding. Therefore, in order to obtain a high-strength welded portion, it is necessary to reduce the C content to the range specified in the present invention and suppress the generation of TiC. For steels with varying P content, the J _M values of the base metal and welds after continuous aging treatment at 525 ° C. shown in Tables 3 and 4 are arranged in relation to the P content and shown in FIG. . Inventive steel A4 having a P content of about 0.02% by weight has a high J _M value of 1400 N / mm ² or more. On the other hand, in comparative steel B3 having a low P content of 0.005% by weight, the J _M value is significantly reduced.

For the invention steel A4 and the comparative steel B3, the prior austenite grain size after annealing was investigated. As a result, the comparative steel B3 had a thickness of 30 μm, whereas the inventive steel A4 had a thickness of 30 μm.
In the former, the austenite grains were refined to 10 to 15 μm, and the structure was finer than that of Comparative Steel 4. Although the mechanism of obtaining fine particles by the P content is not clear, it is presumed that the high toughness shown in FIG. 4 is obtained by the effect of P. High J _M value gives P content of 0.06% by weight
It is maintained even if the dose is increased. However, when P is contained in an amount of more than 0.06% by weight, the J _M value is 1400 N / mm.
It fell below ² and the toughness deteriorated. From this, in order to obtain high toughness, the P content is 0.02 to 0.06% by weight.
It has been confirmed that it is necessary to set within the range. 425
The J _M value of the steel A4 of the present invention continuously aged at 525 ° C.
It is shown in FIG. 5 in comparison with the J _M value of comparative steel B8. Although the comparative steel B8 has a Cu content higher than the range specified in the present invention, in order to compare the toughness by making the strength of the base metal and the welded portion equal to that of the steel A4 of the present invention, the austenite forming element Ni is used. The content of is lower than that of the steel A4 of the present invention, and the Ti content is adjusted.

It is shown in FIG. 5 that the invention steel A4 and the comparative steel B8 exhibit equivalent J _M values when aged at a high temperature of 500 ° C. or higher. However, the J _M value of comparative steel B8 is 5
It is lower than the J _M value of the invention steel A4 due to low temperature aging of 00 ° C. or less. The decrease in J _M value is especially 450 ° C
And 480 ° C aged material, which is remarkable, 1400N /
less than mm ² . It is speculated that this decrease in toughness is due to the aging precipitation of Cu-based precipitates due to the aging treatment at around 450 ° C. Therefore, in order to obtain high toughness even at low temperature aging, it is necessary to balance the components under the condition where the Cu content is reduced. 52 shown in Table 3 and Table 4
The relationship between the tensile strength and the J _M value after continuous aging treatment at 5 ° C is
This is shown in FIGS. 6 and 7. FIG. 6 shows the base metal portion, and FIG. 7 shows the welded portion. In Comparative Steels B5 and B6, the Ni content is outside the range specified in the present invention, and the strength of the welded portion is low, or the J _M value of the base material and the welded portion is low. The conventional steel C1 shows relatively good toughness in both the base material and the welded portion, but the comparative steel C2 having further improved strength has reduced toughness.

The comparative steel B7 has the same C content as that of the conventional steel and has improved strength and toughness as compared with the conventional steel by reducing the Cu content and increasing the Ni content. The strength is low. Also, the F value is 4
In the present invention steel A9 exceeding 0.0, the toughness of the welded portion is low. On the other hand, the invention steels A1 to A8 are
Both are 1740 N / mm ² or more at the base metal and 17 at the weld.
It exhibits a high strength of 00 N / mm ² or more, maintains a high J _M value, and is superior in toughness as compared with the comparative steel of group B and the conventional steel of group C.

-Batch-type aging treatment-For the stainless steels shown in Tables 1 and 2, the hardness, tensile strength, and J after aging treatment in a batch-type aging furnace
The mechanical properties such as _M value were investigated. For aging treatment, 1 ℃ /
A heat treatment condition was adopted in which the temperature was raised to 525 ° C. at a heating rate of 1 minute, the soaking treatment was continued at this temperature for 90 minutes, and then the cooling was performed at a cooling rate of 1 ° C./minute. Hereinafter, this aging treatment is referred to as batch type aging treatment. For some test pieces, 52
The toughness of continuous aging treatment at 5 ° C. was compared with that of batch aging treatment. Invention Steel A1, 3, 4, 7,
8, shown in Table 5 by comparing comparative steels B1,7,9,10 and conventional steel C1 batch aged the hardness after the J _M value or the like, characteristics after refining and.

[0031]

[Table 5]

FIG. 8 is a graph in which the J _M values after batch aging treatment shown in Table 5 for some steels are arranged by E values having Si, Ti and Nb contents as factors. As is clear from FIG. 8, it is 1 when the E value is 0.075 or less.
A high J _M value of 400 N / mm ² or more is obtained. However, when the E value exceeds 0.075, the J _M value sharply decreases. Inventive Steels A1, 3, 4, 7, 8 and Comparative Steel B
FIG. 9 shows the relationship between the J _M value and the C content after the batch aging treatment shown in Table 5 for 7, 9, 10 and conventional steel C1. The results of continuous aging treatment are also shown in FIG. As shown in Table 5, the composition of each steel is adjusted by batch type aging treatment so as to obtain almost the same strength, and in reality, 1727 to 1749N.
/ Mm ² The strength is shown. Invention Steel A8 and Comparative Steel B7,
In No. 9, the C content exceeds 0.02% by weight, or the E value is higher than the range specified in the present invention. Comparative steel B10 is M
The o content is lower than the range specified in the present invention.

As shown in FIG. 9, the comparative steel B10 had a J _M value of 1400 N / mm ² regardless of the aging treatment conditions.
It shows low toughness as below. That is, it is confirmed that it is necessary to contain a predetermined amount of Mo in order to obtain high toughness. Steels other than comparative steel B10 that were subjected to continuous aging treatment exhibited a high J _M value of 1400 N / mm ² or more irrespective of the C content, indicating that they have excellent toughness. On the other hand, when the batch aging treatment is performed, in steels having a C content of 0.02% by weight or less, the J _M value is lower than that in the continuous aging treatment, but the difference is small and the J _M value is 1400.
High toughness of N / mm ² or more is maintained. But,
When the C content exceeds 0.02% by weight, the J _M value is 140.
It is 0 N / mm ² or less, and the toughness is reduced. In the conventional steel C1, the strength equivalent to that of the steel of the present invention is secured by lowering the Ni content and utilizing the precipitation hardening of Cu, but the toughness after the batch aging treatment is as low as that of the comparative steel. Has become. Although the reason for the decrease in toughness after the batch type aging treatment with the increase in the C content is not clear, it is presumed that it is due to the precipitation of carbides during the aging treatment. However, in the aging treatment using the batch aging furnace, in order to maintain high toughness as in the case of using the continuous aging furnace, it is necessary to suppress the C content to 0.02% by weight or less.

As for the steel of the present invention, the welded portion is 525.
The strength of the batch type aging treatment at 0 ° C is slightly inferior to that of the continuous aging treatment at the same temperature, but the difference is small and the strength of 1650 N / mm ² or more is maintained. However, comparative steel B2 having an F value of more than 40.0 and comparative steel B7 having a C content of more than 0.03% by weight.
The strength after the batch type aging treatment of the welded portion is lower than that of the steel of the present invention and is 1600 N / mm ² or less. Therefore, in the case of batch-type aging treatment as well as continuous aging treatment, the C content is set to 0.03 wt% or less or the F value is set to 40.0 or less in order to maintain the strength of the welded portion at a high level. I know I need to In addition,
Each of the group A steels according to the present invention had hardness almost the same as that of the conventional precipitation hardening steel before the aging treatment. This indicates that the steel of the present invention can be subjected to various types of processing by the same processing technique as that used for processing conventional martensitic steel.

[0035]

As described above, in the present invention, C, Si, Mn, P, Ni, Cr, Cu, Mo, T are used.
In addition to adjusting the components such as i, the component balance among Ni, Si, and Ti is optimized so that the E value is 0.075 or less, and the F value is 40 so that a large amount of austenite does not remain in the welded portion. It is regulated to 0.0 or less. As a result, it is possible to obtain a material having a higher strength than the conventional one including the welded portion while maintaining high strength after the aging treatment in the continuous aging furnace or the batch aging furnace. The obtained precipitation hardening type martensitic stainless steel is used as various springs, steel belts and other structural materials that require strength equivalent to that of conventional steels and higher toughness, and welding is particularly required. It has excellent advantages in applications where it is used for parts.

[Brief description of drawings]

FIG. 1 Test piece used to measure an index J _M value for evaluating toughness

FIG. 2 is a graph showing the relationship between the J _M value and the E value for the base metal and welded portion after continuous aging treatment at 525 ° C.

FIG. 3 shows the welded part after continuous aging treatment at 525 ° C.
Graph showing the relationship between tensile strength and F value

FIG. 4 is a graph showing the relationship between the J _M value and the P content in the base metal and welds after continuous aging treatment at 525 ° C.

FIG. 5 is a graph showing the relationship between the J _M value and the aging temperature of inventive steel A4 and comparative steel B8.

[Fig. 6] Regarding the base metal after the continuous aging treatment at 525 ° C, J
Graph showing the relationship between _M value and tensile strength

FIG. 7: Regarding the welded part after continuous aging treatment at 525 ° C.
Graph showing the relationship between J _M value and tensile strength

FIG. 8 is a graph showing the relationship between the J _M value and the E value for the base material after the batch aging treatment at 525 ° C.

FIG. 9 is a graph showing the relationship between the J _M value and the C content after the batch aging treatment at 525 ° C. and after the continuous aging treatment at 525 ° C.

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. C: 0.03 wt% or less, Si: 0.5
~ 2.0 wt%, Mn: 3.0 wt% or less, P: 0.0
2 to 0.06% by weight, S: 0.005% by weight or less, N
i: 7.5 to 9.5% by weight, Cr: 12.0 to 15.0
% By weight, Cu: 0.3% by weight or less, Mo: 1.0-3.
0% by weight, Ti: 0.15 to 0.60% by weight, N: 0.
Precipitation hardening type martensitic stainless steel containing 015% by weight or less and Al: 0.30% by weight or less and having an E value defined by the formula (1) of 0.075 or less and having excellent strength and toughness. E = [(Si%) × (Ti%)] / (Ni%) ... (1) 2. A precipitation hardening martensitic stainless steel for batch aging, wherein the C content according to claim 1 is restricted to 0.02% by weight or less. 3. The precipitation hardening type martensitic stainless steel according to claim 1, which has an F value defined by the formula (2) of 40.0 or less. F = 0.7x (Mn%) + (Ni%) + 0.6x (Cr%) + 0.76x (Cu%) + 0.35x (Si%)-0.36x (Al%) + 0. 6 × (Mo%) + 21.5 ・ ・ ・ ・ (2)