JPS61113716A - Manufacture of 18% ni maraging steel - Google Patents

Abstract

PURPOSE:To improve the strength without deteriorating tensile strength, notch strength and high ductility, by performing the soln. treatment after heat working the titled steel contg. trace B by dividing it to 2 times at high and low temp. ranges. CONSTITUTION:To conventional 18% Ni maraging steel, >=0.005wt% B is added and incorporated. Said steel is hot worked usually at 600-1,200 deg.C range in hot working, soln. treating and aging thereof. In this case, said steel is hot worked at temp. range excluding 800-900 deg.C range, and at least >=1 pass of the hot working is carried out at >=900 deg.C by >=20% draft per one pass. In next soln. treatment, at first, said material is soln. treated primarily at 930-1,050 deg.C range, cooled then soln. treated secondarily at 750-850 deg.C range, at last, aged to manufacture the titled steel superior in both toughness and strength.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing 18% Ni-based maraging steel containing boron (8), and particularly to a boron-added 18% Ni-based maraging steel with outstanding toughness and strength. The present invention relates to a method of manufacturing aging steel.

Conventional technology In general, 18% Ni-based maraging steel is one of the various maraging steels that can obtain high strength and good toughness through relatively simple heat treatment, and has traditionally been used in solid fuel rocket motor cases. It is used in deep-sea submarines and the rotating cylinders of uranium centrifuges.

This type of 18% Ni-based maraging steel contains around 18% N1, and has CO and MO added as the main age hardening elements, as well as small amounts of TI, Aji', etc. Usually after hot working, 800 to 9
Solution treatment is performed by heating to a temperature within the range of 50°C and then air cooling to room temperature to fully dissolve the age hardening elements, and then
An aging treatment is performed in which the material is heated to around 00° C. for about 1 to 10 hours and air cooled to room temperature to precipitate intermetallic compounds, and then it is used. 18% after such heat treatment
Ni-based maraging steel has a tensile strength of 175 to 245 kof/- and a fracture toughness of 100 to 450 kof/ll1m94.

However, in 18% Ni-based maraging steel, the fracture toughness value decreases as the tensile strength increases, and at a tensile strength of 175 kgf/-, the fracture toughness value is 1 (KIG).
The fracture toughness value was 380-450 kgf/ml19'&, but with a tensile strength of 200kof/-, the fracture toughness value was 250
If the tensile strength decreases to ~300 kof/am% and further becomes 230 kof/-, the fracture toughness value will be 130 to 17.
It decreases to 0 kof/u+%. Due to the decrease in fracture toughness that accompanies this increase in strength, when using 18% Ni-based maraging steel in the above-mentioned packing equipment, from the viewpoint of reliability and safety against fracture, it is necessary to The reality is that the strength has to be limited to a certain level, and as a result, the greatest feature of 18% Ni maraging steel, which is that high strength can be easily obtained, cannot be fully demonstrated.

Therefore, various attempts have been made to improve the toughness of 18% Ni maraging steel without reducing its strength, and these methods are as follows (1) to (4).
It is broadly divided into

(1) A method in which cold working is applied to the martensitic structure by cooling it after solution treatment, and then reheating to the austenitic region.

(2) A method of processing in the austenite region below the recrystallization temperature and then reheating to the austenite region.

(3) A method of repeating austenitization and martensitization.

(4) A method of heating to an austenite region above the recrystallization temperature, and then reheating to an austenite region below the recrystallization temperature.

Problems to be Solved by the Invention Each of the above-mentioned methods is a
Although this method aims to improve toughness by making the grains finer and the martensite laths finer, there is a limit to the improvement of toughness by making these grains into am. was not obtained.

Incidentally, the formation of coarse precipitates is generally cited as a factor in the deterioration of the toughness of steel. In fact, high-Mo maraging steels, such as "Tetsu to Hagane" Vol. 66, No. 8, 117
10%Ni-18%Co-14% as shown on page 7
It is known that in maraging steels such as Mo-0, 2% 7i, coarse precipitates of Fe2MO8 preferentially precipitate at grain boundaries due to strain during hot working, deteriorating toughness. On the other hand, it is said that such coarse precipitates do not form at an MO content similar to that of 18% Ni maraging steel (approximately 6% at most), and therefore, for 18% Ni maraging steel, The reality is that no particular consideration has been given to suppressing the formation of coarse precipitates in order to improve toughness.

However, in 18% Ni-based maraging steel, approximately 0.0005 to 0.01% boron has been added for the purpose of improving strength and toughness after aging treatment, but the present inventors et al. According to the experiment, 18%
It has been found that when Ni-based maraging steel contains boron, coarse precipitates are formed depending on hot working conditions or subsequent solution treatment conditions, reducing toughness. In other words, because this fact was not known in the past, it was necessary to select hot working conditions and heat treatment conditions for boron-added 18% Ni maraging steel from the viewpoint of suppressing the formation of coarse precipitates. Therefore, boron-added 18% Ni-based maraging steel with high toughness could not be stably obtained.

Against the background of the above circumstances, the present inventors have already filed a patent application in 1983.
No. 137014 proposes a method for producing a boron-added 18% Nl maraging steel that is stable and has high toughness without sacrificing its high strength. In other words, the present inventors have found that, as mentioned above, in boron-added 18% Ni-based maraging steel, coarse precipitates may be generated depending on hot working conditions and subsequent solution treatment conditions, which may impair toughness. Based on this knowledge, we conducted extensive experiments and studies to find conditions that could stably improve toughness, and as a result, we found an appropriate reduction schedule in the hot working process, and appropriate temperature conditions in the subsequent solution treatment. It was discovered that by combining these, the formation of coarse precipitates can be suppressed and high toughness can be stably obtained, leading to the development of the method proposed above (Japanese Patent Application No. 137014/1982).

That is, in the method for producing 18% Ni-based maraging steel according to plan i, o,ooos weight% or more, usually 0.0
In a method of hot working 18% Ni-based maraging steel material containing 1% or less boron per weight, solution treatment, and further aging treatment, when hot working within the range of 1200 to 600 ° C. Temperature range excluding 900-800℃ (
(excluding 900℃ and 800℃), and in the hot processing process, at least one pass with a processing rate of 20% or more per pass is performed in a temperature range exceeding 900℃, and further When performing solution treatment within the range of 750°C, the solution treatment is performed by heating to a temperature range excluding 930 to 850°C (however, excluding 930°C and 850°C).

However, as a result of further studies by the present inventors, it was found that even with the method proposed above, depending on the solution treatment conditions after hot working, the tensile strength may slightly decrease or the flammability strength may decrease slightly. It turns out that there are times when it happens.

Therefore, it is an object of this invention to further improve the proposed method and provide a method capable of stably producing boron-added 18% Ni-based maraging steel having even higher strength without impairing high toughness. The purpose is to

Means for Solving the Problems As a result of further study on the method proposed in the above-mentioned Japanese Patent Application No. 137014/1987, the inventor of the present invention has determined that the solution treatment after hot working in that method is carried out in a high temperature range. By dividing the treatment into two processes: the first solution treatment at low temperatures and the second secondary solution treatment at low temperatures, high tensile strength and notch strength are not reduced. It was discovered that a 18% Nl-based maraging steel having high strength can be obtained without impairing toughness, and the present invention was completed.

That is, the present invention provides a method for manufacturing 18% Ni-based maraging steel containing o, ooos weight percent or more of boron by solution treatment after hot working and further aging treatment. When performing hot working within a temperature range excluding the temperature range of 900°C or lower and 800°C or higher, and at least one pass or more of the hot working,
It is carried out at a processing rate of 20% or more per pass in a temperature range exceeding 900°C, and when further solution treatment is performed, it is first heated at 1050°C.
It is characterized in that the first solution treatment is performed in a temperature range exceeding 930°C, and after cooling, the second solution treatment is performed in a temperature range of 850°C to 1.750°C or higher.

DETAILED DESCRIPTION OF THE INVENTION The method of the present invention will be described in more detail below.

The steel targeted by this invention is an 18% Ni-based maraging steel containing boron (B) as described above. Specifically, N117-19%, Gd2.0-10.0%, M
o3.0~6.0%, Ti0.1~1.0%, AiFo
, 05 to 0.20% as the main component, and B as o, oo
It contains os% or more. 18%Ni like this
After melting, maraging steel is subjected to hot rolling or hot forging, followed by solution treatment, and further aging treatment before use. The method of the present invention particularly focuses on the process conditions of hot rolling or hot forging (these are collectively referred to as hot working) among the manufacturing processes of boron-added 18% Ni-based maraging steel as described above, and the process conditions thereof. This specifies the conditions for solution treatment of corrosion. In other words, hot working of this type of maraging steel is usually carried out at temperatures of 1200 to 600°C.
However, in this invention, the temperature range for hot working is from 1200 to 600 °C [range to 900 to 900 °C].
Temperature range excluding the range of 800℃, therefore 1200~
Processing shall be performed at a temperature range of 900°C and 800 to 600°C. Furthermore, in hot working, processing is usually performed in multiple passes until the required thickness is achieved, and at least one of the multiple passes is a pass with a reduction rate of 20% or more in a temperature range exceeding 900°C. On the other hand, solution treatment after hot working is usually heated to a temperature range of 1050 to 750°C and then cooled, but in this invention, the temperature range is 1050 to 750°C.
The solution treatment shall be carried out in a temperature range excluding the range of 930 to 850 °C from the temperature range of
It is divided into two stages, and the first solution treatment is carried out at 105 on the high temperature side.
The second solution treatment is carried out in the temperature range of 0 to 930°C, and the second solution treatment is carried out in the lower temperature range of 850 to 750°C. By regulating both the hot working process conditions and the solution treatment conditions in this way, we are able to achieve fracture @ toughness with few coarse precipitates and relatively small precipitates evenly distributed, and with fine crystal grains. Boron-added 18%Ni with high value and high strength
As a result, a maraging steel based on this type can be obtained.

The above-mentioned hot working conditions and solution treatment conditions were discovered through detailed experiments by the present inventors, and therefore, first, the experiments that served as the basis for finding the hot working conditions will be described next.

Steel whose composition is shown in Table 1, that is, 0.003% B
Material steel ingot of 18% Ni-based maraging steel containing 10
After melting 0 kg in a small vacuum melting furnace, it is heated to 1200°C, hot worked in only one pass at various processing temperatures and various reduction rates, and then heated to 95°C and air cooled to room temperature. After that, an aging treatment was carried out by holding at 480° C. for 3 hours. During this process, the structural state after solution treatment was investigated, and the plane strain fracture toughness value after aging treatment was also investigated. Note that the coarse state after solution treatment is the mixed grain size determined by the distribution state of crystal grains, that is, the ratio of coarse grain size to fine grain size (coarse grain size/fine grain size), and the precipitate. The amount of precipitation (area ratio) and the distribution shape of the precipitates ff
! (uniform/heterogeneous) was investigated. (The results are shown in Fig. 1 (A) to (D) in correspondence with the hot processing conditions, that is, the processing temperature l and processing rate. Here, Fig. 1 (A) shows the mixed grain size of the crystal grains, Figure 1 (8) shows the precipitation amount of precipitates, Figure 1 (C) shows the distribution form of precipitates, and Figure 1 (D) shows the fracture toughness value (K Ic ) depending on the hot working conditions. Correspondingly shown.

From Figure 1 (D), the fracture toughness value after aging treatment is 800
The value becomes significantly lower when subjected to processing in the temperature range of ~900℃, and high values are obtained when subjected to severe processing of 20% or more in the temperature range exceeding 900℃ or below 800℃. I understand.

On the other hand, as is clear from Figures 1 (B) and (C), the precipitation amount and distribution form (uniform/heterogeneous) of precipitates after solution treatment largely depend on the processing temperature and processing rate of hot working. Ori,
The processing conditions under which a high fracture toughness value is obtained after the aging treatment as described above corresponds well to the processing conditions under which the amount of precipitates after the solution treatment is small and uniformly distributed. From this, in boron-added 18% Ni maraging steel, in order to obtain a high fracture toughness value, the processing temperature and processing rate must be set so that the precipitates after solution treatment are less solid and uniformly dispersed. It turns out that it is necessary to select

Furthermore, when the experimental results shown in FIGS. 1(A) to (D) are examined in detail, the precipitates after solution treatment in the w4 species targeted in this invention are mainly Fe-M O-
The distribution state of such precipitates is the distribution state of crystal grains l! shown in FIG. 1(A). ! (closely related to mixed particle size). In other words, as shown in Figure 1 (A), when strong working of 20% or more is performed in a temperature range exceeding 900°C, the crystal grains become regular, and in that case, the first
As shown in Figure (C), the precipitates are uniformly dispersed. The reason may be as follows. In other words, it exceeds 900℃! When processing is applied by 20% or more, the entire area is evenly recrystallized by processing of 2096 or more because the processing is in the recrystallization region, and as a result, the crystal structure after solution treatment also changes. It becomes a regular grain. On the other hand, when processed at a low temperature below 900℃, the critical processing rate required for recrystallization is large, resulting in relatively low strain processing, and even when the temperature exceeds 900℃,
If it is less than 0%, the processing rate will be insufficient and grain refinement will occur partially due to recrystallization, and coarse grains larger than the initial grains will partially be generated due to strain-induced grain boundary movement. It is thought that the crystal grains after solution treatment also exhibit a mixed grain structure. Since precipitates preferentially precipitate mainly on prior austenite grain boundaries or recrystallized grain boundaries, it is thought that precipitates are uniformly distributed in a regular-grained structure, whereas they are non-uniformly distributed in a mixed-grained structure. Therefore, uniform dispersion of precipitates can be obtained by processing the material by 20% or more in a temperature range exceeding 900° C. to obtain a regular grain structure.

Further, the amount of precipitates after solution treatment increases when processed in a temperature range of 900 to 800°C.

The reason for this is that in the case of the steel targeted by this invention, the temperature range in which precipitates are most likely to precipitate under dynamic conditions such as during processing is 80°C.
This is thought to be due to the temperature range of 0 to 900°C. Therefore, in order to suppress the amount of precipitates, it is necessary to perform hot working in a temperature range other than the temperature range of 900 to 800°C. Note that even if a sized grain structure is obtained by processing 20% or more in a high temperature range exceeding 900°C, the
If the material is processed in a temperature range of 00°C, the amount of precipitates will increase during processing in the temperature range of 900 to 800°C for the reason mentioned above, so when hot working in multiple passes. , it is necessary to avoid a temperature range of 900 to 800°C in any pass. On the other hand, from the point of view of the above-mentioned grain structure, if 20% or more processing is applied in a temperature range exceeding 900°C even in one pass, other passes may be performed in a temperature range below 800°C or with a processing rate of less than 20%. Even if there is, it is possible to obtain uniform dispersion of the precipitates by controlling the grain size IIJ.

Therefore, if we put the above together, at least one pass is 900.
By processing 20% or more at temperatures exceeding ℃ and avoiding processing in the temperature range of 900 to 800 ℃ throughout the pass, a structure with few precipitates and uniform distribution of precipitates can be obtained. This can bring about a significant improvement in toughness.

According to FIG. 1 (D), the temperature □-cI below 800°C
) 20%, Yo, T, Work, 6□I, □□8 :□1“
However, in the low temperature range below 800℃
% or more is actually not preferable because of the increased load on the rolling mill, etc. Therefore, in the present invention, the strong working of 20% or more may be carried out at a temperature exceeding 900°C.

Furthermore, when hot working at a high temperature exceeding 1200°C, the scale 0 scale increases due to oxidation due to the high temperature, while when hot working at a temperature below 600°C, the deformation resistance increases and processing becomes difficult. . Therefore, the upper limit of the hot working temperature was set to 1200°C, and the lower limit was set to 600°C.

Next, the inventor's experimental results regarding the solution treatment conditions after hot working as described above will be explained.

Regarding the test material (Table 1) with the same composition as that used in the above experiment, 1050
Hot worked at 30% processing rate at ℃.

Next, solution treatment was performed by heating to various temperatures for 1 hour, and then aging treatment was performed at 480° C. for 3 hours, and the fracture toughness value (K rc ) was measured. FIG. 2 shows the relationship between the solution treatment temperature and the fracture toughness value after aging treatment.

As is clear from FIG. 2, solution treatment in the temperature range of 850 to 930°C brings about a significant decrease in fracture toughness.

According to experiments conducted by the present inventors, it has been found that the cause of this is that the amount of precipitates increases when the solution treatment temperature is 850 to 930°C. That is, in the case of the steel targeted by this invention, the temperature range in which precipitates are most likely to precipitate in a static state such as solution treatment is 850 to 930°C, and precipitation progresses slowly at higher or lower temperatures than that. This is thought to be due to. Therefore, the solution treatment temperature needs to be in a temperature range excluding the temperature range of 850 to 930°C in order to obtain high toughness.

As mentioned above, specific hot working conditions and subsequent 850 ~
Boron-added type 18% that has high toughness by combining solution treatment in a temperature range other than 930℃.
It has been found that a Ni-based maraging steel can be obtained. Based on the results, the patent application that had already been proposed was filed in 1973.
This is the method of No. 9-137014. However, after further study by the present inventors, the solution treatment after hot working was carried out in the higher temperature range of the temperature range excluding 850 to 930 °C, that is, in the temperature range exceeding 930 °C. %, there is a risk of a decrease in tensile strength.On the other hand, if solution treatment is performed in a low temperature range, that is, a temperature range of less than 850℃, a decrease in notch strength may occur. It turns out that there is something.

As a result of further experiments and research, we found that 850-930
Solution treatment in a temperature range excluding ℃ is carried out in two parts,
The first solution treatment is performed in a high temperature range of over 930°C, and after the first solution treatment is cooled, the second solution treatment is performed in a low temperature range of less than 850°C. It has been found that by doing so, it is possible to prevent a decrease in tensile strength and notch strength without impairing high toughness. The experiment is described below.

100 kg of the same composition shown in Table 1 as already mentioned
Using a 1.1 mm block, hot rolling was carried out under conditions that satisfied the hot rolling conditions of the present invention, followed by solution treatment under various conditions as shown in Table 2, and further aging treatment. Here, hot rolling is performed at a temperature of 900°C or higher, with a reduction rate of 25%,
The solution treatment was carried out in three passes at 11% and 17%, and the solution treatment was maintained at each temperature in Table 2 for 1 hour and then air cooled. Also, the aging process is 4
After being maintained at 80°C for 3 hours, it was air cooled.

Table 2 also shows the results of examining fracture toughness, tensile strength, and notch tensile strength for each steel material that was solution-treated under various conditions and further subjected to aging treatment. As shown in Table 2, 93
When solution treatment is performed only in a high temperature range exceeding 0℃ (
In condition number 2), the notch strength was low, but the tensile strength was decreased, and this is thought to be due to the coarsening of crystal grains caused by the solution treatment in the high temperature range. On the other hand, when solution treatment is performed only in a low temperature range below 850°C (condition number 5
．． 6) has high tensile strength, but it causes a decrease in notch strength. This is because the solution treatment temperature was low, so the grains became fine and the tensile strength was high.1) However, it is thought that the notch strength was lowered because some rolled structures remained. On the other hand, if the primary solution treatment is performed at a high temperature exceeding 930°C and then the secondary solution treatment is performed at a low mvi of less than 850"C (condition number 1), the tensile strength The reason for this superiority is that the first solution treatment at a high temperature completely eliminates the rolled structure, and the second solution treatment at a low temperature improves the fineness of the grains. This is thought to be due to the

In Table 2, condition number 3.4 indicates that the solution treatment was carried out in the medium temperature range of 850 to 930°C, where precipitates tend to form as mentioned above, and in this case, the fracture toughness value was extremely low ( On the other hand, when the medium temperature range of 850 to 930°C is avoided and the first solution treatment is performed in the high 21 range and the second solution treatment is performed in the low temperature range (condition number 1), the fracture toughness Of course, it will not be damaged.

Furthermore, if the primary solution treatment temperature exceeds 1050°C, then the upper limit temperature of the primary solution treatment is 1050°C.
And so. Furthermore, the lower limit of the secondary solution treatment temperature is 7
The temperature was 50'C.

In addition to applying the hot working conditions already described above, the solution treatment after hot working was first carried out at 930 to 1050°C.
After cooling, the secondary solution treatment is carried out at
18% boron-added type with excellent strength and toughness by solution treatment at a low temperature range of 50 to 750℃
This made it possible to stably obtain Ni maraging steel.

Note that the aging treatment after the solution treatment may be carried out in accordance with a conventional method, for example, by heating to around 500° C. for about 1 to 10 hours and cooling in air.

Example The practical aspects of this invention are described below along with comparative examples.

Three types of boron-added type 18% shown in W4A, 8.0 in Table 3
After melting Ni-based maraging resin according to a conventional method, it was remelted in vacuum in a concell arc furnace, hot worked and solution treated under various conditions, and then aged at 480°C for 3 hours. . Regarding the steel obtained under each hardening condition,
The tensile strength, notch tensile strength and plane fracture toughness values were investigated. Each process condition and trial I! 5! The results are shown in Table 4. In addition, fracture toughness test (AST Ivl E-3
The test was carried out by a three-point bending test according to 99.

As shown in Table 3, when manufactured within the condition range of this invention (condition number 5.12.13), the tensile strength and notch strength were both 210, which was extremely excellent, exceeding ``/-''. Furthermore, the fracture toughness value (K Ic ) is also 380 k.
A value of gf/av% or higher was obtained.

On the other hand, when solution treatment is performed only in a high temperature range exceeding 930°C (condition number 4.11), the tensile strength is low, and when solution treatment is performed only in a low temperature range below 850°C (condition number 4.11), the tensile strength is low. The notch strength is low in all conditions (condition numbers 7, 15, 16).Furthermore, in a comparative example (conditions No. 1.2.3), a comparative example in which hot working was performed in the temperature range of 900 to 800°C (Condition No. 8.9.10)
), and a comparative example (condition number @2.6.9.14) in which the solution treatment temperature is in the temperature range of 930 to 850°C, both have low fracture toughness values, and even 118B has a low fracture toughness value of 338 kgf/I! 1
It was I11'4.

Effects of the Invention As described above, according to the method of this invention, boron-doped type 1
In producing 8% Ni-based maraging steel, it is possible to produce an inert material that not only has extremely high fracture toughness, but also has stable and extremely high strengths such as tensile strength and notch strength. Therefore, by applying the manufacturing method of the present invention, the reliability and safety of boron-added 18% Ni-based maraging steel can be significantly improved compared to the conventional ones, and the fields of use thereof can be further expanded.

Table 1: Chemical composition of sample materials (wt%)

[Brief explanation of the drawing]

Figures 1 (A) to (D) are diagrams showing the influence of hot working conditions on boron-added 18% Ni maraging steel; (A) shows the effect of heat on the grain size after solution treatment; A diagram showing the influence of hot working conditions, (B) a diagram showing the influence of hot working conditions on the precipitation amount (area ratio) of precipitates after solution treatment,
(C) is a diagram showing the influence of hot working conditions on the distribution form of precipitates after solution treatment, and (D) is a diagram showing the influence of hot working conditions on fracture toughness [(KIC) after aging treatment]. FIG. The M2 diagram is a correlation diagram showing the relationship between the boron-added 18% N+ maraging true solution treatment temperature and the fracture toughness value after aging treatment.

Claims (1)

[Claims] In manufacturing 18% Ni-based maraging steel containing 0.0005% by weight or more of boron, solution treatment is performed after hot working, and further aging treatment is performed at 1200 to 600°C.
When hot working within the range of 900℃ or less, 800℃
Processing is performed in a temperature range excluding the temperature range above ℃, and at least one pass of the hot processing is performed at a processing rate of 20% or more per pass in a temperature range exceeding 900℃, and further solution treatment is performed. Before doing this, first, heat the temperature to 1050℃.
Below, the first solution treatment is performed in a temperature range exceeding 930℃, and after cooling, the second solution treatment is performed in a temperature range of less than 850℃ and 750℃ or higher.
Boron-added type 1 characterized by performing a subsequent solution treatment
Method for producing 8% Ni-based maraging steel.