JP3110726B2 - Maraging steel sheet excellent in fatigue characteristics 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 maraging steel sheet having excellent fatigue properties and a method for producing the same.

[0002]

2. Description of the Related Art Maraging steel is strengthened by precipitating an intermetallic compound such as Ti or Mo into a highly tough martensitic matrix composed of very low carbon-Ni or very low carbon-Ni-Co. The intended steel has high toughness and high strength. Moreover, it has various features that have not been seen before, such as good weldability and small dimensional change due to heat treatment. For this reason, it can be used in a wide range of applications from a wide range of fields from structural members in advanced technology fields such as space development, ocean development, nuclear application, aircraft-related and automobile-related, to pressure vessels, tools, extrusion rams, dies, etc. Application is being attempted.

[0003] However, maraging steel has the following problems due to its high strength and strengthening mechanism. In other words, when the strength becomes high, it becomes sensitive to non-metallic inclusions in the material, the fatigue strength decreases due to the stress concentration,
Pulling tends to deteriorate durability.

Therefore, in order to solve such a problem, the cleanliness of nonmetallic inclusions is improved by restricting N and O to be reduced, thereby reducing the amount of nonmetallic inclusions, which are the starting points of fatigue fracture, to reduce fatigue. The characteristics are improved.

[0005]

Although the above technology has improved durability to some extent, in recent years, the use conditions of machines and structures have become severe, and the demands on the strength characteristics of materials have become strict. Therefore, there is a demand for the development of maraging steel having excellent fatigue characteristics in order to further improve durability in order to guarantee long-term stability of mechanical devices and structures.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a thin maraging steel sheet having excellent fatigue characteristics as compared with the prior art, and a method for producing the same.

[0007]

Means for Solving the Problems The present inventor has focused on the fact that Ti and Mo are easily segregated in the chemical composition of maraging steel, and component segregation generated during casting is caused by hot working or heat treatment. If it is not removed, a band structure will occur and the strength will be different inside and outside the band structure after aging treatment.
As a result of intensive research based on the idea that component segregation may hinder improvement in fatigue characteristics, the present invention has been completed.

That is, the maraging steel sheet of the present invention has a chemical composition by weight%, C: 0.01% or less, Ni: 8 to 19%, Co: 8 to 20%, Mo: 2 -9%, Ti: 0.1-2%, Al: 0.15% or less, N: 0.003% or less, O: 0.0015% or less, with the balance being Fe and impurities
In this case, the Ti component segregation ratio and the Mo component segregation ratio in the thickness direction are each 1.3 or less.

The reasons for limiting the components of the maraging steel sheet of the present invention are as follows. C: 0.01% or less C is preferable to form a carbide and reduce the amount of precipitation of intermetallic compounds to lower the fatigue strength, so that the content is preferably as small as possible. In the present invention, 0.01% or less, preferably 0.005% or less Stop at

Ni: 8 to 19% Ni is an indispensable element for forming a matrix structure having high toughness, and if it is less than 8%, the toughness is deteriorated due to an insufficient amount. On the other hand, if it is added excessively, austenite other than martensite is generated in the matrix, and the strength is reduced. Therefore, the lower limit of the Ni content range is set to 8%, preferably 12%, more preferably 16%, and the upper limit is set to 19%.

Co: 8 to 20% Co promotes precipitation of an intermetallic compound containing Mo and improves strength. If it is less than 8%, the strength decreases, while 20%
The toughness is reduced and the addition of ultra forte. For this reason, the lower limit of the Co content range is set to 8%, and the upper limit is set to 20%, preferably 15%.

Mo: 2 to 9% Mo precipitates Fe ₂ Mo and Ni ₃ Mo by aging treatment.
It is an effective element for strengthening steel. Its content is reinforced becomes insufficient at less than 2%, while 9% microsegregation ultra El and in the steel increases, the toughness is reduced. Therefore, the lower limit of the Mo content range is set to 2%, preferably 3%, and the upper limit is set to 9%, preferably 6%.

Ti: 0.1 to 2% Ti is an element effective for strengthening steel like Mo by precipitating Ni ₃ Ti and NiTi by aging treatment. If the content is less than 0.1%, the reinforcement becomes insufficient, so the lower limit of the Ti content range is set to 0.1%, preferably 0.3%. On the other hand, if it exceeds 2%, the micro segregation in the steel increases remarkably, and the toughness and the fatigue strength decrease. Moreover, Ti (C,
N) -based nonmetallic inclusions increase and deteriorate durability. Therefore, the upper limit of the Ti content range is 2%, preferably 1.
2%.

Al: 0.15% or less Al is effective for deoxidation, but if it exceeds 0.15%, the amount of alumina-based oxide increases and the durability is reduced. Therefore, the upper limit is made 0.15%. .

N: 0.003% or less N is a harmful element that has an adverse effect on fatigue strength.
% Is important. If it exceeds 0.003%, mainly the TiN sharply increases, and moreover, it becomes a dotted line, so that the fatigue strength is significantly reduced. It is more advantageous for the fatigue strength to have a smaller N, preferably 0.0%.
By setting the content to 02% or less, more preferably 0.001% or less, the durability is further improved.

O: 0.0015% or less O forms oxide-based nonmetallic inclusions, and it is important to reduce the content to 0.0015% or less. If it exceeds 0.0015%, the fatigue strength is significantly reduced. O for fatigue strength
Is more advantageous as the content is smaller, and the durability is further improved by setting the content to preferably 0.0010% or less.

The impurities Si and Mn both form nonmetallic inclusions such as SiO ₂ , MnO and MnS.
Since the fatigue strength is reduced, it is preferable that the amount is smaller, and it is better to keep the respective contents to 0.05% or less, preferably 0.02% or less. Also, as for P and S, since the fatigue strength is lowered due to grain boundary embrittlement and the formation of nonmetallic inclusions, the smaller the content, the better, the better.
It is better to keep it below 002%.

The maraging steel sheet of the present invention has the above-mentioned chemical composition, and its parent phase is substantially composed of a martensite single phase. Further, the segregation ratio of Ti component and Mo component in the structure is Each is set to 1.3 or less. Of the chemical compositions, Ti and Mo, particularly Ti, are apt to segregate, and if the component segregation generated during casting is not removed by hot working or heat treatment, a band structure occurs, and the strength inside and outside the band structure after aging treatment is increased. It fluctuates greatly. Especially when the product thickness is 0 . 5mm
In the case of the following thin plates, the band structure becomes remarkable, and the adverse effect becomes remarkable. For this reason, the boundary of the band structure becomes a starting point of the fatigue fracture, and the fatigue strength is reduced. In this case, as it is clear from examples described later, Ti, rapidly fatigue strength component segregation ratio each 1.3 and Exceeding of Mo decreases. Therefore, in the present invention, the upper limit of each of the component segregation ratios of Ti and Mo is set to 1.3, preferably 1.2. The smaller the segregation ratio, the higher the fatigue strength. The component segregation ratio of Ti and Mo in the present invention means the ratio of the maximum concentration to the minimum concentration of Ti and Mo in the thickness direction of the maraging steel sheet (maximum concentration / minimum concentration) . Although components other than Ti and Mo also segregate, remarkable component segregation occurs.
By keeping the component segregation ratio to a predetermined value, other components such as Co can be kept within a range without any problem.
Only the component segregation ratio of Mo is specified.

The method for producing a maraging steel sheet according to the present invention.
Performs the cast piece of steel having the chemical components were hot-forged at the forging ratio of 4 or more, then the soaking treatment of holding at a temperature in the range of 1,100 to 1,280 ° C. 1 or more times, the total time of the soaking treatment 10 to 100 hours ,
After rolling .

As is clear from the examples described later, the forging ratio of hot forging (cross-sectional area before forging / cross-sectional area after forging) is set to 4 or more because the forging ratio is 4 even under appropriate soaking conditions. If less than, the distance between the segregation peaks of Ti and Mo is large,
Since the diffusion makes it impossible to sufficiently smooth, T
This is because it becomes difficult to reduce the component segregation ratio of i and Mo to 1.3 or less. When the soaking temperature is less than 1100 ° C. or the total soaking time is less than 10 hours, a predetermined segregation ratio of Ti and Mo cannot be obtained even under an appropriate forging ratio. On the other hand, when the soaking temperature is 1280
If the temperature exceeds 100 ° C. or the total soaking time exceeds 100 hours, the crystal becomes extremely coarse, the crystal grain size number becomes less than 8, and the fatigue strength is remarkably reduced. Thus, the lower limit of the soaking temperature is 1100 ° C., preferably 1
180 ° C., and the upper limit is 1280 ° C., preferably 12 ° C.
50 ° C. The lower limit of the total time of the soaking process is set to 10 hours, preferably 20 hours, and the upper limit is set to 10 hours.
0 hr, preferably 72 hr.

According to this manufacturing method, ordinary equipment such as forging equipment and annealing furnace can be used without using special equipment.
By performing hot forging and soaking under a predetermined forging ratio and soaking conditions, a forged piece of maraging steel having a component segregation ratio of Ti or Mo of 1.3 or less can be obtained.
Can be. Thereafter , hot rolling or cold rolling is appropriately performed to process the product to a thickness.

[0022]

EXAMPLE Steel having the chemical composition shown in Table 1 below was melted, and a cast piece (1000 kgf) was hot forged in accordance with the manufacturing conditions shown in Tables 2 and 3, and further soaked if necessary.
Hot rolling and cold rolling were performed to form a thin plate having a thickness of 0.3 mm. 100m length from this thin plate along the rolling direction
A test piece having a width of 10 mm and a width of 10 mm was collected, subjected to a solution treatment at 820 ° C. × 1 hr, subjected to an aging treatment at 480 ° C. × 4 hr, and then subjected to a NH ₃ gas nitriding treatment at 450 ° C. × 6 hr.

Using the thus obtained sample, Ti, M
The component segregation ratio of o was examined. As for the component segregation ratio, the maximum value and the minimum value of the Ti and Mo concentrations were measured by performing line analysis with EPMA in the thickness direction of each sample, and the ratio (maximum value / minimum value) was calculated. Since a nitride layer is present on the surface layer having a thickness of 30 μm, X-rays were scanned except for that portion.

A cross section of each sample along the rolling direction (length direction) was observed with an optical microscope (400 times).
The grain size number was measured according to the austenitic grain size test method for ISG0511 steel.

Further, fatigue characteristics were evaluated using each sample. The fatigue characteristics were evaluated by performing a pulsating test under a constant cyclic stress of 30 kgf / mm ^{2, and} calculating the number of repetitions (N) until the test piece was broken. The results of these investigations are shown in Tables 2 and 3. FIGS. 7 and 8 show examples of EPMA analysis results of the sample used for calculating the Ti component segregation ratio. FIG. 7 shows an example of the invention (sample N
o. 27), and FIG. 8 shows a comparative example (sample No. 21).

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

From Tables 2 and 3, it can be seen that the invention examples all have a repetition number of 1 × 10 ⁹ or more and have excellent fatigue properties. FIG. 1 shows a graph in which the relationship between the Ti component segregation ratio and the number of repetitions of the fatigue test is arranged for Sample Nos. 21 to 27. From this graph, when the Ti component segregation ratio is 1.3 or less, the fatigue characteristics rapidly increase. It turns out that it improves. A similar tendency is observed for Mo.

Further, for Sample Nos. 1 to 5 which were subjected to soaking at 1100 ° C. for 10 hours after hot forging using steel type A, which is an inventive component, a graph summarizing the relationship between the forging ratio and the Ti component segregation ratio. As shown in FIG. From this, it can be seen that the Ti component segregation ratio decreases as the forging ratio increases, and that the Ti component segregation ratio becomes 1.3 or less by setting the forging ratio to 4 or more. The same applies to Mo.

Sample Nos. 11 to 11 were prepared by using steel type C, which is an inventive component, and performing soaking under various soaking temperature conditions with a holding time of 20 hours after hot forging at a forging ratio of 4.
FIG. 3 shows a graph in which the relationship between the soaking temperature and the Ti component segregation ratio for No. 18 was arranged. From this, it can be seen that the Ti component segregation ratio decreases as the soaking temperature increases, and that the Ti component segregation ratio becomes 1.3 or less by setting the soaking temperature to 1100 ° C. or higher. The same applies to Mo.

Similarly, the relationship between the soaking temperature and the grain size number was determined for Sample Nos. 21 to 28 which were subjected to soaking at various soaking temperatures using steel grade E, which is an invention component, with a forging ratio of 4 and a soaking time of 72 hours. Is shown in FIG. Thus, the grain size number decreases as the soaking temperature increases (i.e., the crystals become coarser)
However, when the soaking temperature exceeds 1280 ° C., the crystal grain size number becomes smaller than 8. As is clear from sample No. 28, when the crystal grain size number is less than 8, the fatigue strength is significantly reduced. Samples Nos. 21 and 22 have good crystal grain size, but do not have a proper Ti component segregation ratio due to a low soaking temperature.

Sample No. 3 was prepared by using a steel type G, which is an inventive component, and performing soaking for various soaking times at a soaking temperature of 1100 ° C. after hot forging at a forging ratio of 4.
FIG. 5 shows a graph in which the relationship between the soaking time and the Ti component segregation ratio for 1 to 36 is arranged. From this, Ti
It can be seen that the component segregation ratio decreases with an increase in the soaking time, and that the Ti component segregation ratio becomes 1.3 or less by setting the soaking time to 10 hours or more. The same applies to Mo.

Similarly, Sample Nos. 41 to 47 were prepared by using a steel type I, which is an invention component, at a forging ratio of 4 and at a soaking temperature of 1280 ° C. for various soaking times.
FIG. 6 shows a graph in which the relationship between the soaking time and the crystal grain size number was arranged. As a result, the grain size number decreases as the soaking time increases, and the soaking time becomes 1
If it exceeds 00 hr, the grain size number will be less than 8,
As is clear from No. 47, the fatigue strength is significantly reduced.

[0035]

As described above, according to the maraging steel sheet of the present invention, the chemical component is a predetermined component in which N and O are regulated, and the component segregation ratio of Ti and Mo in the thickness direction is 1 respectively. 0.3 or less, so that the cleanliness of non-metallic inclusions is improved and the segregation of components is also suppressed, and the occurrence of non-metallic inclusions and micro-strength differences, which are the starting points of fatigue fracture, is suppressed and prevented. It has the fatigue characteristics. Further, according to the production method of the present invention, the above maraging steel sheet can be easily produced using ordinary equipment, and the productivity is excellent.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a Ti component segregation ratio and fatigue characteristics (number of repetitions) in Examples.

FIG. 2 is a graph showing a relationship between a training ratio and a Ti component segregation ratio in Examples.

FIG. 3 is a graph showing a relationship between a soaking temperature and a Ti component segregation ratio in Examples.

FIG. 4 is a graph showing a relationship between a soaking temperature and a crystal grain size number in Examples.

FIG. 5 is a graph showing a relationship between a soaking time and a Ti component segregation ratio in Examples.

FIG. 6 is a graph showing a relationship between a soaking time and a grain size number in Examples.

FIG. 7 is a graph showing an example of a Ti concentration distribution in a plate thickness direction in the invention example.

FIG. 8 is a graph showing an example of a Ti concentration distribution in a plate thickness direction in a comparative example.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/10 C21D 9/46-9/48

Claims (3)

(57) [Claims] 1. A chemical composition in weight%, C: 0.01% or less, Ni: 8 to 19%, Co: 8 to 20%, Mo: 2 to 9%, Ti: 0.1 to 2%, Al: 0.15% or less, N: 0.003% or less, O: 0.0015% or less, including residual Fe and impurities
A maraging steel sheet excellent in fatigue characteristics, wherein each of the Ti component segregation ratio and the Mo component segregation ratio in the thickness direction is 1.3 or less. 2. The method according to claim 1, wherein the plate thickness is 0.5 mm or less.
Maraging steel sheet loaded. 3. Casting of steel having the composition according to claim 1.
The slab is hot forged at a forging ratio of 4 or more, then 1100-1
Perform a soaking process at a temperature of 280 ° C once.
Or twice or more, and the total time of the soaking process is 10 to
The method according to claim 1, wherein the rolling is performed for 100 hours, and thereafter, the rolling is performed.
A method for producing a maraging steel sheet having excellent fatigue properties.