JP2021134395A - Precipitation-hardening martensitic stainless steel - Google Patents

Abstract

To provide a precipitation-hardening martensitic stainless steel that maintains toughness and a higher strength through the execution of an aging heat treatment.SOLUTION: A precipitation-hardening martensitic stainless steel comprises the following on a mass% basis: C: 0.01-0.10%, Si: 1.0-2.0%, Mn: 0.50-1.50%, P: not more than 0.04%, S: not more than 0.01%, Ni: 6.0-8.0%, Cr: 12.0-15.0%, Mo: 0.50-1.50%, Cu: 0.40-1.20%, Ti: 0.20-0.50%, Nb: 0.05-0.40%, N: 0.001-0.02%, Al: 0.001-0.2%, and O: 0.0001-0.01%, with the balance being Fe and inevitable impurities. A Cu phase and a Ni16(Ti,Nb)6Si7 intermetallic compound phase are distributed in the precipitation-hardening martensitic stainless steel, wherein the Nb in the intermetallic compound phase is 0.2-3.0 (at%).SELECTED DRAWING: Figure 1

Description

The present invention relates to precipitation hardening martensitic stainless steels having high strength and ductility after aging heat treatment.

Precipitation hardening type stainless steel is used for applications such as steel belts and press plates because its strength can be increased by subjecting it to aging heat treatment. Typical examples thereof include SUS630 and SUS631.

The above-mentioned SUS631 is a semi-austenitic stainless steel, and in a solid-dissolved state, it is a ferritic austenitic stainless steel. This steel is cold-rolled to form a work-induced martensite structure, and then NiAl is precipitated by aging heat treatment to increase the strength, but there is a problem that the manufacturability is poor. Further, since it contains Al, there is a problem that the δ ferrite phase is easily precipitated at a high temperature and the hot workability is poor.

The above-mentioned SUS630 is a martensitic stainless steel, which has a martensitic structure after solidification heat treatment, and its strength is increased by precipitation of the ε-Cu phase by aging heat treatment, but the ultimate strength is about 1500 MPa (Vickers hardness). About 400).

Further, in the same martensitic stainless steel as SUS630, by adding Ti or Si, in addition to the ε-Cu phase, the Ni ₁₆ Ti ₆ Si ₇ system intermetallic compound phase (hereinafter, may be abbreviated as G phase). ) Is deposited to increase the strength.

Japanese Unexamined Patent Publication No. 2003-73783 Japanese Unexamined Patent Publication No. 11-256282 JP-A-2017-155317

Precipitation hardening martensitic stainless steels with higher strength than the techniques of the above patent documents are widely used. However, as precipitation hardening martensitic stainless steels are used in a wide variety of applications, there are increasing demands according to the applications, and the characteristics may not be sufficient depending on the conditions of use.

Therefore, an object of the present invention is to provide a precipitation hardening martensitic stainless steel that maintains higher strength and toughness by subjecting it to aging heat treatment.

The inventors have made extensive studies focusing on the alloying elements and the strengthening phase precipitated by aging heat treatment in order to solve the above problems. In order to investigate the influence of each element, laboratory melting was performed with various components, and cold rolled material with a plate thickness of 2 mm was produced by hot forging and cold rolling, and then solidification heat treatment and aging heat treatment were performed. The mechanical properties such as tensile test and Vickers hardness test were evaluated, and the nanoscale precipitation hardening phase was evaluated by observation with a transmission electron microscope (TEM) and a scanning transmission electron microscope (STEM).

In particular, detailed and careful observation was performed using a high-resolution STEM, and the precipitated phase was measured by EDS, and the following findings were obtained. It was clarified that not only Ti but also Fe, Mn, and Nb can be substituted for X of the _{G phase (Ni 16 X 6} Si ₇ ) precipitated by the aging heat treatment.

Among them, it was confirmed that Ti is an element that forms the skeleton of the G phase, and it was clarified that Mn is dissolved in the site of X when Nb is not added. However, in this case, the particle size of the G phase is as large as 4 to 20 nm, the Cu phase is also as large as 4 to 50 nm, and the G phase and the Cu phase tend to be unevenly distributed at the grain boundaries, leading to improvement in precipitation hardening. There wasn't.

On the other hand, in the material to which Nb was added, it was clarified that Nb, not Mn, was dissolved in the site of X. In addition, it promotes precipitation hardening phases such as G phase and Cu phase, and higher strength can be obtained by aging heat treatment for a short time compared to the case where Nb is not added, and precipitation hardening phases such as G phase and Cu phase are 1 to 1. It was found that the particle size became 20 nm and the particle size became finer. Furthermore, it was also shown that these G phase and Cu phase have an action of finely dispersing in crystal grains without being unevenly distributed at the grain boundaries. It was clarified that the effect of these fine precipitations significantly improved precipitation hardening.

That is, the present invention is as follows.
In the following mass%, C: 0.01 to 0.10%, Si: 1.0 to 2.0%, Mn: 0.50 to 1.50%, P: 0.04% or less, S: 0 0.01% or less, Ni: 6.0-8.0%, Cr: 12.0-15.0%, Mo: 5.00-1.50%, Cu: 0.40-1.20%, Ti : 0.25 to 0.50%, Nb: 0.05 to 0.40%, N: 0.001 to 0.02%, Al: 0.001 to 0.2%, O: 0.0001 to 0 .01%, the balance being Fe and inevitable impurities, Cu phase and _{Ni 16 (Ti, Nb) 6} Si 7 intermetallic compound phase distribution, Nb of the intermetallic compound in the phase is 0.2 to 3 It is a precipitation-hardening type martensite-based stainless steel characterized by having a temperature of 0.0 (at%).

Further, in precipitation hardening, the distribution state and size of the precipitation hardening phase have a great influence on the strength. Therefore, the precipitation hardening martensitic stainless steel of the present invention has a Cu phase and Ni ₁₆ (Ti, Nb) _6. It is characterized in that 50% or more of the Si _{7 martensitic compound phase is distributed in the crystal grains.}

Further, the precipitation hardenable martensitic stainless steel of the present invention, Cu phase and _{Ni 16 (Ti, Nb) 6} Si 7 intermetallic compound phase is characterized by an average particle size of 1 to 20 nm.

Further provided are precipitation hardening stainless steels characterized by mechanical properties of 2 to 15% elongation and 400 to 600 Hv hardness.

It is a schematic diagram which shows the precipitation state of the Cu phase and the G phase on the stainless steel of this invention, and shows the vicinity of the grain boundary of three crystal grains.

The reason for limiting the composition of the stainless steel of the present invention will be described. Hereinafter, "%" is "mass%" unless otherwise specified.
C: 0.01 to 0.10%
C is an austenite-forming element and suppresses the formation of a δ-ferrite phase at high temperatures. Further, although the strength is increased by solid solution in the martensite phase, the residual austenite phase after the solid solution heat treatment tends to increase, and sufficient strength cannot be obtained after the aging heat treatment. Further, when the amount of C is high, Ti and Nb, which are constituents of the G phase that contribute to precipitation hardening, are easily consumed by the formation of TiC and NbC. Therefore, the content of C is set to 0.01 to 0.10% in order to reduce the precipitation hardening ability by the aging heat treatment. Further, it is preferably 0.03 to 0.05%.

Si: 1.0-2.0%
Si is 1.0% or more because G phase is generated by aging heat treatment and the strength is greatly increased by precipitation hardening. On the other hand, since it is a ferrite-forming element, if it is contained in a large amount, a δ-ferrite phase is likely to be formed, which promotes hot workability and a decrease in strength at the welded portion. Therefore, the content is set to 2.0% or less. Further, it is preferably 1.30 to 1.90%.

Mn: 0.50 to 1.50%
Mn is an austenite-forming element and suppresses the formation of a δ-ferrite phase at high temperatures. Further, the retained austenite phase after the solid solution heat treatment tends to increase, and the toughness is improved, while the strength is lowered after the aging heat treatment. Further, MnO and MnS are formed to reduce the corrosion resistance. Therefore, the range of Mn is set to 0.50 to 1.50%. Further, it is preferably 0.70 to 1.20%.

P: 0.04% or less P increases the susceptibility to solidification and cracking and lowers the hot workability by segregating at the grain boundaries. Therefore, the lower the P content, the more desirable it is, and the P content is 0.04% or less.

S: 0.01% or less S is a harmful component that forms MnS, lowers corrosion resistance, segregates at grain boundaries, and lowers hot workability. Therefore, the lower the S content, the more desirable it is, and the S content is 0.01% or less.

Ni: 6.0-8.0%
Since Ni is an austenite-forming element, is a constituent element of the G phase, and is an important element for precipitation hardening, it is set to 6.0% or more. However, when the Ni content increases, the residual austenite phase after the solid solution heat treatment tends to increase, which causes a decrease in strength. Therefore, the content was set to 8.0% or less.

Cr: 12.0 to 15.0%
Cr was set to 12.0% or more in order to ensure the corrosion resistance of stainless steel. However, since it is a ferrite-forming element, a δ-ferrite phase is likely to be formed at a high temperature, and the hot workability is lowered. Therefore, the content is set to 15.0% or less.

Mo: 0.50 to 1.50%
Mo is an element effective for improving corrosion resistance, but it was set in the range of 0.50 to 1.50% because the δ ferrite phase promotes the formation. Further, it is preferably 0.50 to 1.00%.

Cu: 0.40 to 1.20%
Cu is an element that produces a Cu phase by aging heat treatment and is effective for precipitation hardening. However, excessive addition causes a decrease in strength due to an increase in the retained austenite phase and a decrease in hot workability, resulting in cracking. Therefore, the Cu content is set to 0.40 to 1.20%. Further, it is preferably 0.50 to 1.00%.

Ti: 0.25 to 0.50%
Ti is an essential element for forming the G phase and is an element effective for increasing the strength due to precipitation hardening. However, the range of Ti is set to 0.20 to 0.50% because oxides and nitrides are easily formed and cause defects.

Nb: 0.05 to 0.40%
Nb is an element that constitutes the G phase and is a very important element. Nb is an effective element because it has the effect of controlling the G phase to the Ni ₁₆ (Ti, Nb) ₆ Si _{7 system and promoting nucleation.} Further, there is also an effect of finely dispersing the Cu phase, and the ability of precipitation hardening by the Cu phase and the G phase is remarkably improved. Further, although not particularly limited, it has the effect of forming Nb carbide having a size of about 0.3 to 1 μm to prevent the coarsening of the crystal grains, and is also effective for the miniaturization of the crystal grains. Therefore, the content of Nb is set to 0.05% or more. However, the addition of excess Nb causes a decrease in the amount of solid solution C due to the formation of excess NbC, which causes a decrease in elongation, and thus the content is 0.40% or less. Further, it is preferably 0.10 to 0.30%.

N: 0.001 to 0.02%
Like C, N is an austenite-forming element that dissolves in the martensite phase to increase its strength, but the formation of TiN and NbN consumes Ti and Nb, which are constituents of the G phase that contribute to precipitation hardening. It becomes easier and reduces the precipitation hardening ability by aging heat treatment. Therefore, the range of N is set to 0.001 to 0.02%.

Al: 0.001 to 0.2%
Al is an element effective as a deoxidizing agent for reducing the amount of O. Further, since Nb is an element that is relatively easily oxidized, Nb can be reliably controlled within the scope of the present invention by deoxidizing with Al to reduce the oxygen concentration. However, if it is contained excessively, the formation of a δ ferrite phase is promoted, resulting in a decrease in hot workability and a decrease in toughness. Therefore, the range of Al is set to 0.001 to 0.2%.

O: 0.0001 to 0.01%,
O forms non-metal inclusions with Si and Ti, which are constituents of the G phase that contribute to precipitation hardening, and thus reduces the strength after aging heat treatment. In addition, oxide-based inclusions reduce the cleanliness of the steel and cause defects. However, excessive deoxidation causes an increase in cost, so the range of O is set to 0.0001 to 0.01%.

The steel of the present invention is a precipitation hardening martensitic stainless steel having excellent strength by simultaneously precipitating _{Cu phase and G phase Ni 16} X ₆ Si _7. The distribution state of these precipitation-hardened phases and the size of the precipitation-hardened phase itself have a great influence on mechanical properties such as hardness and elongation.

For example, when a large amount of precipitates are present on the grain boundaries and the abundance ratio in the crystal grains is low, the precipitates tend to grow coarsely and become brittle. On the other hand, the strength increases when the precipitates are uniformly distributed regardless of the positions in the crystal grains and on the grain boundaries. Therefore, the precipitates are uniformly dispersed by solution heat treatment and aging heat treatment under appropriate conditions. The appropriate heat treatment conditions here are not particularly limited, but the solution heat treatment is carried out at 1000 to 1150 ° C. for 1 to 5 minutes, and then the aging heat treatment is carried out at 400 to 600 ° C. for 30 min to 10 hr. Point to.

There is an optimum value for the ratio of Nb in the G phase. That is, if Nb is too low, the distribution of the G phase becomes non-uniform, and a large amount is distributed on the grain boundaries, and the hardness of the present invention cannot be satisfied. On the other hand, if Nb in the G phase is too high, the elongation becomes less than 2%, which is the lower limit of the range of the present invention, and the elongation is not sufficient and processing cannot be performed. When _{Nb in Ni 16} (Ti, Nb) ₆ Si ₇ is x, the G phase is represented as Ni ₁₆ (Ti _(1-x) , Nb _x ) ₆ Si ₇ . Therefore, the atomic ratio (at%) of Nb in the G phase can be expressed by x / (16 + 6 + 7). Although not particularly limited, the hardness and elongation can be controlled within the range of the present invention by setting the Nb (at%) in the G phase to the range of 0.2 to 3.0. In order to realize this range, the Nb content may be 0.05 to 0.40%.

Further, in this steel, by adding Nb, nucleation of the precipitated phase can be promoted, and the precipitate can be uniformly dispersed. Therefore, Cu phase and _Ni 16 (Ti, _Nb) and 6 Si ₇ system more than 50% of the intermetallic phase is possible to distribute in the crystal grains by defining the Nb in the scope of the present invention.

Furthermore, the size of these precipitation-hardened phases themselves has a great influence on the strength, which is a very important value in the present application. Even if the dislocations move, the strength will increase if the movement can be stopped by the precipitate.

The action of this precipitate as an obstacle changes depending on the size of the precipitated phase, and there is an optimum size of the precipitated phase. In this steel, when the size of the precipitation phase is 1 nm or more and 20 nm or less, the action as an obstacle to the dislocation of the precipitate is maximized, so that it is necessary to optimize the size of the precipitate. Therefore, solution heat treatment under appropriate conditions, Cu phase and _{Ni 16 (Ti, Nb) 6} Si 7 intermetallic compound phase by defining the aging and Nb in the range of the present invention has an average particle size 1~20nm It is possible to

Precipitation hardening type stainless steel is used for applications such as steel belts and press plates because its strength can be increased by subjecting it to aging heat treatment. These are required to have strength and fatigue characteristics, but in order to increase these characteristics, hardness of HV400 or more is required. On the other hand, if the hardness is very high, the elongation will decrease, so the HV is set to 600 or less. In addition, since toughness is required, the elongation should be 2 to 15% in balance with hardness.

Next, examples will be presented to further clarify the constitution and effects of the present invention, but the present invention is not limited to the following examples.
Table 1 shows the chemical composition of the test material, the presence or absence of a precipitation hardening phase, the amount of Nb in the G phase, the proportion of precipitates in the grains, the Vickers hardness, and the elongation. Numerical values whose chemical composition is outside the scope of the present invention are shown in parentheses.

In each steel, the raw materials were melted by a high-frequency induction furnace and cast into a cast iron mold to prepare an ingot having a scale of 20 kg. These were hot forged at 1000 to 1200 ° C. to obtain a forged plate having a thickness of 12 mm. Then, a cold-rolled material having a thickness of 2 mm was produced by cold rolling, and the solution heat treatment and the aging heat treatment were performed on the cold rolled material. The solid solution heat treatment is performed to dissolve the precipitates existing in the steel, and martensitic transformation occurs due to rapid cooling after the heat treatment. The cold-rolled material was subjected to a solution heat treatment for 2 minutes at 1050 ° C.

The aging heat treatment is a treatment in which the precipitation hardening phase is finely dispersed and precipitated after the solidification heat treatment, and in this steel, the Cu phase and the G phase are finely dispersed and precipitated. The above cold-rolled material was subjected to a solution heat treatment for 1 hr at 480 ° C.

These test materials were evaluated for mechanical properties such as tensile test and Vickers hardness test, microstructure evaluation by optical microscope and SEM, and nanoscale precipitation hardening phase evaluation by TEM and STEM observation.

Regarding the presence, distribution, and size of the Cu and G phases, which are precipitation hardening phases, an element mapping image by an energy dispersive X-ray (EDS) analyzer mounted on a STEM using a thin film sample prepared by a focused ion beam (FIB). The measurement was performed using more image analysis.

Hereinafter, the evaluation criteria for each item of the evaluation shown in Table 1 will be described.
(Presence / absence of Cu phase, presence / absence of G phase)
When the Cu phase and the G phase were present, the evaluation was evaluated as ◯. If either phase does not exist, it is marked with x. The criterion for determining the presence or absence of each phase was that there was a case where the amount of ^{precipitates was 0.001 (pieces / nm 2) or more when observed in an arbitrary field of view.}

(Nb (at%) in G phase)
G-phase _{Ni 16 (Ti, Nb) 6} Si 7 in Nb (at%) _{is, Ni 16 (Ti (1-} x), Nb x) when the 6 Si _7, represented by x / (16 + 6 + 7 ). The amount of Nb in the G phase during this aging heat treatment was determined using the thermodynamic calculation software Thermo-Calc. In addition, the amount of Nb in the G phase obtained from this thermodynamic calculation showed good agreement with the results of STEM-EDS analysis. From the mechanical properties such as hardness and elongation, Nb (at%) in the G phase was evaluated as ◯ in the range of 0.2 to 3.0.

(Percentage of precipitates in grains (%))
Evaluation ◯ was given when the ratio of the precipitate in the grain was 50% or more.

(Precipitated phase average particle size (nm))
The case where the average particle size of the precipitate was 1 to 20 nm was evaluated as ◯.

(Hardness Hv (10 kg load))
In the Vickers hardness test, the above cold-rolled material with a plate thickness of 2 mm is heat-treated, the rolled surface is polished with # 800, and then 5 points are measured with a load of 10 kg on the surface, and the average value is calculated. I asked. A hardness of 400 to 600 Hv was evaluated as ◯.

(stretch(%))
In the tensile test, the cold-rolled material having a plate thickness of 2 mm was heat-treated, and a JIS13B flat tensile test piece having the tensile direction as the rolling direction was cut out and measured. From the measurement results, the range of elongation of 2 to 15% was evaluated as ◯.

(comprehensive evaluation)
From the above results, when all the evaluations were ◯, the overall evaluation was ⊚, when there were 1 or 2 x, the overall evaluation was ◯, and when x was 3 or more, the evaluation was x. The overall evaluation of the invention example is ⊚ or ◯, while the overall evaluation of the comparative example is ×.

In Comparative Example 6, the amount of Nb is low and the proportion of precipitates in the grains is low. Further, since the amounts of Ni and Cu are high, the amount of residual γ tends to be large and the hardness is small.

In Comparative Example 7, since the amount of Ti is low and the G phase does not exist, it is out of the range of the present invention and the hardness is low.

In Comparative Example 8, although the G phase is present, the amount of Cu is low, so that the Cu phase is not present and the hardness and elongation are low.

Comparative Example 9 has a Cu phase and a G phase, but the amounts of Ti and Nb are high and the elongation is low.

Since the amount of Mn in Comparative Example 10 is high, the amount of γ remaining is very large as in the case of Comparative Example 6, and since the amount of Nb is low, Nb in the G phase is low and the proportion of precipitates in the grains is also high. Since it is low, the hardness is low and the elongation is high.

As described above, the comparative example is inferior to the invention example in the mechanical properties of either or both of hardness and elongation.

That is, the present invention is as follows.
In the following mass%, C: 0.01 to 0.05 %, Si: 1.0 to 2.0%, Mn: 0.70 to 1.50%, P: 0.04% or less, S: 0 0.01% or less, Ni: 6.0-8.0%, Cr: 12.0-15.0%, Mo: 5.00-1.50%, Cu: 0.40-1.20%, Ti : 0.25 to 0.50%, Nb: 0.05 to 0.40%, N: 0.001 to 0.005 %, Al: 0.001 to 0.2%, O: 0.0001 to 0 .01%, the balance being Fe and inevitable impurities, Cu phase and _{Ni 16 (Ti, Nb) 6} Si 7 intermetallic compound phase distribution, Nb of the intermetallic compound in the phase is 0.2 to 3 It is a precipitation-hardening type martensite-based stainless steel characterized by having a temperature of 0.0 (at%).

Further, in precipitation hardening, the distribution state and size of the precipitation hardening phase have a great influence on the strength. Therefore, the precipitation hardening type martensite-based stainless steel of the present invention has a Cu phase and Ni ₁₆ (Ti, Nb) _6. From the _{element mapping image of the Si 7-} based intermetallic compound phase by an energy dispersive X-ray (EDS) analyzer mounted on a scanning transmission electron microscope (STEM) using a thin film sample prepared by a focused ion beam (FIB). The distribution is obtained by observing and evaluating the precipitation-hardened phase on a nanoscale using image analysis, and it is characterized in that 50% or more is distributed in the crystal grains.

Next, examples will be presented to further clarify the constitution and effects of the present invention, but the present invention is not limited to the following examples.
Table 1 shows the chemical composition of the test material, the presence or absence of a precipitation hardening phase, the amount of Nb in the G phase, the proportion of precipitates in the grains, the Vickers hardness, and the elongation. Numerical values whose chemical composition is outside the scope of the present invention are shown in parentheses. In addition, * 2 and * 5 are reference examples.

Claims (5)

In the following mass%, C: 0.01 to 0.10%, Si: 1.0 to 2.0%, Mn: 0.50 to 1.50%, P: 0.04% or less, S: 0 0.01% or less, Ni: 6.0 to 8.0%, Cr: 12.0 to 15.0%, Mo: 0.50 to 1.50%, Cu: 0.40 to 1.20%, Ti : 0.25 to 0.50%, Nb: 0.05 to 0.40%, N: 0.001 to 0.02%, Al: 0.001 to 0.2%, O: 0.0001 to 0 .01%, the balance being Fe and inevitable impurities, Cu phase and _{Ni 16 (Ti, Nb) 6} Si 7 intermetallic compound phase distribution, Nb of the intermetallic compound in the phase is 0.2 to 3 Precipitated and hardened martensite-based stainless steel characterized by being .0 (at%). The precipitation hardening martensitic stainless steel according to claim 1, wherein 50% or more of the Cu phase and the Ni ₁₆ (Ti, Nb) ₆ Si _{7 intermetallic compound phase are distributed in the crystal grains.} steel. The Cu phase and the _{Ni 16 (Ti, Nb) 6} Si 7 intermetallic compound phase has an average particle diameter of claim 1 or 2 precipitation hardenable martensitic stainless steel according to characterized in that it is a 1~20nm .. The precipitation hardening martensitic stainless steel according to any one of claims 1 to 3, wherein the elongation is 2 to 15%. The precipitation hardening martensitic stainless steel according to any one of claims 1 to 4, wherein the hardness is 400 to 600 Hv.

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