JP3251648B2 - Precipitation hardening type martensitic stainless steel and method for producing the same - 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 type martensitic stainless steel which exhibits high strength after aging treatment and has excellent toughness even in a high strength region, and a method for producing the same.

[0002]

2. Description of the Related Art Precipitation hardening martensitic stainless steel has low hardness before aging treatment and is excellent in punching workability and forming workability. On the other hand, after the aging treatment, high strength is exhibited by precipitation hardening. Utilizing this feature, precipitation-hardening martensitic stainless steel is used as various springs, steel belts and the like.

The present applicant has also introduced a steel belt material having high strength and excellent toughness as this kind of precipitation hardening martensitic stainless steel as Japanese Patent Publication No. 59-49303. In the precipitation hardening type martensitic stainless steel introduced here, C, Ti, Mn, Ni, Cr, Cu and Al are added so that a large amount of austenite phase does not remain in the solution-treated state or the heat-affected zone after welding. The components are adjusted between them. This promotes the formation of martensite particularly in the welded portion, and improves the strength by aging treatment.

[0004]

[Problems to be Solved by the Invention] Japanese Patent Publication No. 59-4930
The precipitation-hardened martensitic stainless steel introduced in No. 3 satisfies the required characteristics in terms of strength, but may have insufficient toughness when exposed to a severe use atmosphere as a steel belt or the like. The toughness of precipitation hardening martensitic stainless steel can be improved by adding Mo. However, as the fields of use have been diversified, higher toughness than before has been required. When such high toughness is required, the required characteristics cannot be satisfied only by adding Mo.

The present invention has been devised in order to solve such a problem, and has a structure in which the crystal grains are fine and the number of precipitates is small. It is an object of the present invention to provide a precipitation hardening type martensitic stainless steel that does not cause the precipitation hardening.

[0006]

The precipitation hardening type martensitic stainless steel of the present invention has a C content of 0.08% by weight or less, a Si content of 0.7 to 2.5% by weight, Mn: 3.0% by weight or less, Ni: 6.0 to 10.
0% by weight, Cr: 10.0 to 17.0% by weight, Cu:
0.5 to 2.0% by weight, Mo: 0.5 to 3.0% by weight,
Ti: 0.15 to 0.45% by weight, N: 0.015% by weight or less and S: 0.003% by weight or less, with the balance being Fe and unavoidable impurities. It is characterized by having a structure in which the crystal grain size is 25 μm or less and the precipitates having a grain size of 5 × 10 ⁻² μm or more deposited on the matrix are suppressed to 6 × 10 ⁶ particles / mm ² or less.

The precipitation hardening type martensitic stainless steel has a C content of 0.08% by weight or less and a Si content of 0.7-2.
5% by weight, Mn: 3.0% by weight or less, Ni: 6.0 to 1
0.0% by weight, Cr: 10.0 to 17.0% by weight, C
u: 0.5 to 2.0% by weight, Mo: 0.5 to 3.0% by weight, Ti: 0.15 to 0.45% by weight, N: 0.015
% Of S and 0.003% by weight of S, the remainder having a composition comprising Fe and unavoidable impurities is subjected to a solution treatment, and then subjected to cold rolling of 35% or more, and then 980 to 1130. It is manufactured by performing annealing for heating at a temperature of 5 ° C. for 5 minutes or less and further performing aging treatment.

[0008]

[Operation] In order to further improve the toughness of the martensitic stainless steel introduced in Japanese Patent Application Laid-Open No. 59-493030, in order to further improve the toughness, the crystal grains are refined by recrystallization at the time of annealing, and the factors of the toughness decrease It was considered necessary to re-dissolve precipitates such as TiC in the matrix. Under this assumption, the average grain size of the annealed crystal grains is 25%.
μm or less, and the distribution amount of precipitates is 6 × 10 ⁶ particles / mm ²
It has been experimentally confirmed that when the content is suppressed to below, improvement in toughness is achieved while maintaining high strength.

The structure in which the crystal grains are fine and the precipitation of TiC or the like is suppressed is obtained by subjecting a martensitic stainless steel having a specified component to cold rolling at a high rolling rate,
It is obtained by annealing in a temperature range of 80 to 1130 ° C. The toughness is an index J _M represented by the maximum tensile stress in a room-temperature tensile test of a specimen with a fatigue pre-crack.
Can be accurately evaluated based on the J _M value of 14
When it is not less than 00 N / mm ^2, a structure is obtained in which the crystal grains after annealing have an average particle size of 25 μm or less, and the distribution of precipitates is suppressed to 6 × 10 ⁶ / mm ² or less.

Hereinafter, the alloy components contained in the precipitation hardening martensitic stainless steel of the present invention and the contents thereof will be described. C: An element effective in improving the strength of steel and suppressing the δ ferrite phase generated at high temperatures. But C
As the content increases, the hardness of the martensite phase generated by quenching increases, and the cold work deformability decreases. As a result, the moldability becomes insufficient, and it becomes difficult to obtain a martensite single phase structure by cooling after the solution treatment. Further, it promotes the generation of TiC in the annealed state and lowers the toughness. Therefore, in the present invention, the upper limit of the C content is specified as 0.08% by weight.

Si: It has a large solid solution strengthening ability and has an effect of strengthening the matrix. Moreover, by adding Ti and Ni in combination, Si, Ti, Ni
Fine consistent precipitation of intermetallic compounds consisting of elements such as
Improve the strength of steel. Such an effect appears remarkably when the Si content is 0.7% by weight or more. However, when a large amount of Si exceeding 2.5% by weight is contained, formation of a δ ferrite phase is promoted, and strength and toughness are reduced. Therefore, the Si content was set in the range of 0.7 to 2.5% by weight. Mn: exhibits an action of suppressing the formation of a δ ferrite phase in a high temperature range. However, the addition of a large amount of Mn tends to cause a decrease in toughness of a welded portion and a decrease in welding workability. Therefore, in the present invention, the upper limit of the Mn content is set to 3.0% by weight.

Ni: contributes to precipitation hardening and suppresses the formation of a δ ferrite phase. In the alloy system of the present invention, in order to maintain high strength and high toughness without lowering the age hardening ability, it is necessary to contain at least 6.0% by weight of Ni. However, when a large amount of Ni exceeding 10.0% by weight is contained, the amount of the retained austenite phase after quenching increases,
The required strength cannot be obtained. Therefore, the Ni content is
It was set in the range of 6.0 to 10.0% by weight. Cr: In order to obtain corrosion resistance as stainless steel, it is necessary to contain at least 10.0% by weight or more of Cr. However, a large amount of Cr exceeding 17.0% by weight
, A δ ferrite phase and a residual austenite phase are formed, which causes a reduction in the strength of the welded portion.
Therefore, the Cr content was set in the range of 10.0 to 17.0% by weight.

Cu: In the alloy system of the present invention, Cu
A high strength can be obtained without particularly emphasizing the precipitation strengthening effect of. However, in order to ensure corrosion resistance in a corrosive environment of sulfur dioxide, it is not sufficient to contain Cr alone, and in this regard, the addition of Cu is effective. When the Cu content is 0.5% by weight or more, the improvement in corrosion resistance becomes remarkable. However, when a large amount of Cu is contained in excess of 2.0% by weight, hot workability is degraded, and defects such as cracks may be generated on the processed material surface. In addition, with the addition of a large amount of Cu, there is a tendency that toughness is reduced when the strength is increased. Therefore, the Cu content was set in the range of 0.5 to 2.0% by weight.

Mo: In order to improve strength and toughness,
It is an effective alloying element. In order to exhibit effective curing ability, it is necessary to contain 0.5% by weight or more of Mo. However, even if Mo exceeds 3.0% by weight, improvement in strength and toughness commensurate with the increase in Mo content cannot be obtained. In addition, when a large amount of Mo is contained, the formation of the δ ferrite phase is promoted, and the strength of the welded portion is easily reduced. Therefore, the Mo content is 0.5 to 3.0% by weight.
Was set in the range. Ti: An alloy element that contributes to precipitation hardening, and it is necessary to contain 0.15% by weight or more of Ti in order to obtain high strength. However, if a large amount of Ti is contained in excess of 0.45% by weight, the strength is improved by an excessive precipitation hardening reaction, but the toughness is reduced. Therefore,
The Ti content was set in the range of 0.15 to 0.45% by weight.

N: Has a high affinity for Ti, and consumes effective Ti acting as a precipitation hardening element by producing TiN. In addition, TiN inclusions increase with an increase in the N content, which causes a reduction in fatigue strength and toughness. Therefore, the N content is preferably as low as possible. In the present invention, the upper limit of the N content is set to 0.015% by weight. S: Exists in steel as non-metallic inclusions such as MnS, and adversely affects fatigue strength, toughness, corrosion resistance, and the like. In this regard,
The lower the S content, the better, and the upper limit is 0.003% by weight.
Stipulated.

The steel used in the present invention is adjusted in composition so that a martensite single phase structure is substantially formed after solution treatment in the above range of chemical components. The balance of the steel is basically Fe, but Al is added for the purpose of deoxidation, Ca added for the purpose of desulfurization, rare earth metal, hot workability B or the like added in an amount of 0.01% by weight or less in order to improve the content. Since Al exhibits the same action as Ti, a part of Ti may be substituted and contained at a ratio of 0.45% by weight or less. The stainless steel containing the alloy element specified in the present invention is subjected to a solution treatment, subjected to cold rolling, and then annealed. At this time, if cold rolling is performed at a rolling reduction of less than 35%, coarse crystal grains are generated partially or entirely during annealing. When aging treatment is applied to stainless steel having a structure grown to coarse crystal grains, toughness decreases with increasing strength.

In order to ensure high toughness while maintaining high strength, it is necessary to generate uniform and fine recrystallized grains during annealing. When the recrystallized grains average particle size 25μm or less fine particles produced during annealing, 1400 in J _M value
High toughness of N / mm ² or more can be obtained. In order to obtain a fine structure, it is important to introduce a large number of working strains by cold working performed before annealing. In this regard,
Cold working with a rolling reduction of 35% or more was employed. The cold-rolled stainless steel is used for recrystallization and precipitation solubilization.
Anneal in the temperature range of 80 to 1130 ° C. When annealing at a high temperature exceeding 1130 ° C., the growth of recrystallization is promoted, and coarse recrystallized grains are easily generated. Conversely, if the annealing temperature is lower than 980 ° C., precipitates such as TiC present in the steel do not sufficiently dissolve in the matrix, causing a decrease in toughness. In this regard, it is preferable to perform annealing in a temperature range of 1000 to 1100 ° C. In addition, when annealing is performed for a long time, recrystallized grains are likely to become coarse. Therefore, the upper limit of the annealing time is set to 5 minutes.

Although most of the precipitated TiC and the like form a solid solution in the matrix by annealing, it is necessary to raise the annealing temperature or lengthen the annealing time in order to completely dissolve the precipitate. . As a result, the recrystallized grains become coarse, causing a decrease in strength. According to the study by the present inventors, among the precipitates remaining without solid solution after annealing, precipitates having a particle size of 5 × 10 ⁻² μm or more were found to be 6 × 10 ⁶ / mm ^2.
It has been found that both the strength and the toughness after the aging treatment are improved by suppressing the distribution ratio to the following range and maintaining fine crystal grains having an average particle size of 25 μm or less. The annealed stainless steel is formed into a good shape by temper rolling if necessary. 3% to improve shape characteristics
It is preferable to perform temper rolling at the above rolling ratio. However, even if the rolling reduction of the temper rolling is too high, the effect of improving the shape is small, and on the contrary, the toughness is reduced. Therefore, the temper rolling is preferably performed at a rolling rate of 3 to 50%.

In the present invention, the toughness is evaluated by the maximum stress J _M in a notch tensile test of a test piece with a fatigue pre-crack. The J _M value enables a detailed investigation of the effects of various factors such as alloying elements and thermomechanical treatment on the toughness, as compared to conventional notch tensile tests. This J _M value is 1400
When it is at least N / mm ² , the above-mentioned microstructure with few precipitates is formed, and a material excellent in both strength and toughness can be obtained. The annealed stainless steel is subjected to aging treatment through appropriate temper rolling. As the aging treatment, a heat treatment generally performed on precipitation hardened steel at 425 to 550 ° C. for 10 minutes or more is adopted. By the aging treatment, a high strength material is obtained, and a material having a J _M value of 1400 N / mm ² or more and excellent toughness is obtained.

Since the J _M value is 1400 N / mm ² or more, the tensile strength is at least 1400 N / mm ² or more. For example, the tensile strength is 1650N /
Even if it is about ² mm, considerably excellent toughness can be obtained if the J _M value is 1400 N / mm ² or more. However, 1
At a J _M value of less than 400 N / mm ² , the toughness sharply decreases. That is, in order to obtain high toughness in a high strength region, a J _M value of 1400 N / mm ² or more is required.

[0021]

【Example】

Example 1 For each of the stainless steels having the components shown in Table 1, a 100 kg ingot was subjected to hot rolling to a sheet thickness of 6 mm.
Was manufactured. After the hot-rolled sheet was cut, the sheet was subjected to cold rolling at a rolling ratio of 40%, annealing at 1030 ° C. for 180 seconds, and further temper rolling at 15% to form a steel strip having a thickness of 2 mm. In addition, J1 to J5 in Table 1 are steels to which the present invention is applied. On the other hand, a to c are comparative steels, and the Mo content or the Cu content is out of the component range specified in the present invention.

[Table 1]

Aging treatment at 480 ° C. for 1 hour
After performing the% temper rolling material were examined hardness, tensile strength, notched tensile strength, the mechanical properties of J _M value or the like. The notch tensile strength was measured by a method similar to the conventional method according to ASTM E338-81. The test piece 1 shown in FIG. 1 was used for measuring the J _M value. Test piece 1 is 160 mm long
And formed into a rectangular shape having a width of 45 mm, and circular holes 2 and 3 having a diameter of 16 mm were formed at positions 28 mm from both ends. Further, a center hole 4 having a diameter of 4 mm was formed 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, 3.5 mm long pre-fatigue cracks 7 and 8 were introduced by a fatigue tester. When the test piece 1 is subjected to a notch tensile test, unlike the conventional notch tensile test in which the crack generation and the propagation resistance are simultaneously evaluated, only the crack propagation resistance is evaluated. In addition, since the degree of stress concentration on the fatigue pre-cracks 7 and 8 is higher, the toughness of the material at the crack bottom is more strictly evaluated.

Table 2 shows the measurement results. Also, the ratio of the notch tensile strength NTS to the tensile strength TS, NTS / T
When the ratio J _M / TS of the S and J _M values is arranged by the tensile strength TS, the relationship shown in FIG. 2 is established between the ratios NTS / TS and J _M / TS and the tensile strength TS. I understood. In FIG. 2, a circle indicates the ratio NTS / TS, and a triangle indicates the ratio J _M / TS. In addition, the outline is the target steel of the present invention, and the black is the comparative steel. As shown in Table 2 and FIG. 2, the steel targeted by the present invention does not differ much in strength from the comparative steel. Further, when toughness is evaluated by a conventional notch tensile test, the target steel of the present invention is at the same toughness level as the comparative steel. However, when evaluating the toughness in J _M value, the target steel of the present invention exhibits excellent toughness than the comparative steels, it is found that has a strong development resistance to cracking. The steels J1 to J5 are all at the same strength level as the conventional precipitation hardening type steel, and can be subjected to various processes by the same processing method as the conventional martensitic steel.

[Table 2]

Example 2 A hot rolled sheet of steel J1 shown in Table 1 was cut and cold rolled at a rolling rate of 20 to 60%. Next, after annealing at 1030 ° C. for 3 minutes, a 2 mm-thick strip was manufactured by temper rolling at a rolling reduction of 15%. Apply aging treatment to 480 ° C for 1 hour on the strip,
The tensile strength and J _M value aging material was measured. When the measurement results were arranged by the cold rolling reduction before annealing, the tensile strength TS
And the relationship shown in FIG. 3 between the J _M value and the cold rolling rate was established. As is clear from FIG. 3, the tensile strength TS
Shows a tendency to increase as the cold rolling reduction increases, but there is no significant change. On the other hand, the J _M value changes greatly at a cold rolling reduction of 35%. Observation of the structure of the test piece having a cold rolling reduction of less than 35% revealed that the crystal grains had grown greatly during annealing. On the other hand, in the test piece having a cold rolling reduction of 35% or more, the growth of crystal grains was suppressed, and the test piece had a fine crystal structure having an average particle size of 25 μm or less. Therefore, it is subjected to cold rolling at a rolling reduction of 35% or more before annealing, J _M
It can be seen that the value is 1400 N / mm ² or more, which is effective in increasing the strength and toughness after aging treatment to high levels.

Example 3 Steel J1 in Table 1 was annealed by heating for 3 minutes while changing the annealing temperature in the range of 950 to 1150 ° C. Other about 480 ° C. aged material was subjected to the same machining heat treatment as in Example 1, was measured J _M value. The measurement results were organized in annealing temperature, the relationship shown in FIG. 4 between the J _M value and the annealing temperature was established. As is clear from FIG. 4, the J _M value is low at a low annealing temperature of less than 980 ° C. This is presumed to be due to the fact that the precipitate was not sufficiently re-dissolved. Also, 1130
At a high annealing temperature exceeding ℃, the crystal grains grow actively, and the J _M value decreases. In contrast, 980-1
In the case where the annealing temperature is maintained in the temperature range of 130 ° C., 14
A high J _M value exceeding 00 N / mm ² is obtained. From this, it can be seen that it is necessary to perform annealing in a temperature range of 980 to 1130 ° C. in order to make the material excellent in both strength and toughness.

Example 4 A hot-rolled sheet of steel J1 shown in Table 1 was sequentially subjected to cutting, cold rolling and annealing, and then a rolling reduction of 15
% To produce a strip having a thickness of 2 mm. 48
An aging treatment of heating to 0 ° C. for 1 hour was applied to the strip, and the tensile strength and J _M value of the aging material were measured. At this time, the average crystal grain size of recrystallized grains and the distribution of precipitates in the structure after annealing were adjusted by variously changing the rolling reduction and the annealing temperature. When the measurement results are arranged by the crystal grain size and the precipitate distribution amount after annealing, the relationship shown in FIGS. 5 and 6 between the tensile strength TS and J _M value and the crystal grain size and the precipitate distribution amount respectively is shown. It was established. The amount of precipitate distribution was determined by dissolving the mother phase several μm using a non-aqueous solvent-based electrolyte and counting the number of precipitates remaining on the mother phase that could be detected in a 20,000-fold field of view. And the average number per unit area (pcs /
mm ² ). Precipitates consist of TiC, Ti ₄ C ₂ S _2, etc., and precipitates having a particle size of 5 × 10 ⁻² μm or more were observed in this visual field.

[0027] As apparent from FIG. 5, the tensile strength and J _M value both showed a tendency to decrease according to the average grain diameter increases. Although the degree of degradation of the tensile strength is negligible, J _M value was greatly reduced when the average crystal grain size exceeds 25 [mu] m. In addition, as shown in FIG. 6, the tensile strength showed a tendency to slightly decrease as the amount of distribution of the precipitate increased. On the other hand, the J _M value was determined to be 6 × 10 ⁶ precipitates / mm.
It dropped sharply after ² . From this, the J _M value was 1400 N / m while maintaining the tensile strength at a high level.
In order to impart excellent toughness of at least m ² , the average crystal grain size after annealing should be 25 μm or less and the amount of precipitate distribution should be 6 ×
It turns out that it is effective to suppress the number to 10 ⁶ pieces / mm ² or less. That is, even with stainless steel having the same composition, by adjusting the amount of stress strain introduced by cold rolling and the strain released by annealing and the growth rate of recrystallization, high strength and excellent toughness are obtained. Thus, a precipitation-hardened martensitic stainless steel is obtained.

[0028]

As described above, according to the present invention, the composition of C, Si, Mn, Ni, Cr, Cu, Mo, Ti and the like is adjusted, and the average grain size of the crystal structure after annealing is obtained. The diameter and the amount of precipitate distribution are adjusted. As a result, a material having higher toughness than before can be obtained while maintaining high strength. In addition, the properties can be improved at the same manufacturing cost as the conventional one. The obtained precipitation-hardened martensitic stainless steel is required to have the same strength and corrosion resistance as conventional steels, and is used as various structural materials, such as springs, steel belts, and the like, which require higher toughness.

[Brief description of the drawings]

Fig. 1 Specimen used to measure the index J _M value for evaluating toughness

FIG. 2 Notch tensile strength N with respect to tensile strength TS
Graph showing the relationship between TS ratio NTS / TS and J _M value of the ratio J _M / TS tensile strength TS

FIG. 3 is a graph showing the effect of the cold rolling reduction on the tensile strength TS and J _M value.

FIG. 4 is a graph showing the effect of annealing temperature on J _M value.

FIG. 5 shows that the average grain size after annealing is different from the tensile strength and J _M
Graph showing the effect on values

FIG. 6 shows that the distribution of precipitates after annealing shows tensile strength and J _M
Graph showing the effect on values

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Seiichi Ohashi 4976 Nomura Minami-cho, Shinnanyo-shi, Yamaguchi Pref. 60-36623 (JP, A) JP-A-63-171857 (JP, A) JP-A-60-152660 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 302 C21D 8/00 C21D 9/46 C22C 38/50

Claims (2)

(57) [Claims] C: 0.08% by weight or less, Si: 0.7
-2.5 wt%, Mn: 3.0 wt% or less, Ni: 6.
0 to 10.0% by weight, Cr: 10.0 to 17.0% by weight, Cu: 0.5 to 2.0% by weight, Mo: 0.5 to 3.0%.
0% by weight, Ti: 0.15 to 0.45% by weight, N: 0.
015% by weight or less and S: 0.003% by weight or less, with the balance being Fe and unavoidable impurities.
And that the average crystal grain size after annealing is 25 μm or less, and that the precipitates having a grain size of 5 × 10 ⁻² μm or more precipitated in the matrix are suppressed to 6 × 10 ⁶ grains / mm ² or less. A precipitation-hardening martensitic stainless steel with excellent strength and toughness. 2. C: 0.08% by weight or less, Si: 0.7
-2.5 wt%, Mn: 3.0 wt% or less, Ni: 6.
0 to 10.0% by weight, Cr: 10.0 to 17.0% by weight, Cu: 0.5 to 2.0% by weight, Mo: 0.5 to 3.0%.
0% by weight, Ti: 0.15 to 0.45% by weight, N: 0.
015% by weight or less and S: 0.003% by weight or less
And, after the balance has been subjected to solution treatment the <br/> Ru steel to have a composition consisting of Fe and unavoidable impurities, subjected to rolling of 35% or more of cold, then 5 minutes or less at a temperature of 980 to 1,130 ° C. heating A method for producing a precipitation-hardening martensitic stainless steel excellent in strength and toughness, characterized by performing annealing and further aging treatment.