JP2909089B2 - Maraging steel and manufacturing method thereof - Google Patents

Description

The present invention relates to an 18% Ni-based maraging steel containing B and a method for producing the same, and more particularly to a method for refining the grain size of a maraging steel having excellent toughness. It is.

[Conventional technology]

Maraging steel is known as one of the super-strength steels, and because of its high strength and excellent toughness, it has many uses for special springs, bolts, containers, etc. in the fields of space, aircraft, high-speed rotating equipment, among others. 18% Ni-based maraging steel is widely used in various fields because good toughness can be easily obtained by solution treatment and aging treatment as heat treatment.

However, in the maraging steel tensile strength at equal has a 200 kgf / mm ² or more high strength, ductility with increasing strength, there is a problem that the toughness is degraded, in particular small parts thick, crystal grains Roughness also increases variations in properties such as ductility and toughness, so it is more important to refine crystal grains. One method for solving this problem is to use a method of refining austenite crystal grains. For example, cold working is performed on a plate, a rod, a pipe, etc. that can be cold worked and has a relatively small size. And a solution treatment is further performed.

On the other hand, a method of adding an alloy element effective for improving toughness has also been attempted. For example, Japanese Patent Publication No. 59-34226 contains one or more of B, Zr, Ca, and Mg. Japanese Patent Application Laid-Open No. Sho 61-210156 discloses a maraging steel containing B and a method for producing the same.
No. 23520 describes a manufacturing method in which a maraging steel to which B, Zr, Ca, and V are added simultaneously and a thermomechanical treatment are combined.

[Problems to be solved by the invention]

In the above-described method of performing cold working before the solution treatment, in order to obtain fine crystal grains having a crystal grain size number of 10 or more, it is necessary to suppress the solution treatment temperature to an extent that the solution is substantially insufficient. is there. However, if the solution treatment temperature becomes too low,
Although the crystal grains are refined, the maraging steel containing a relatively large amount of Mo, on the other hand, has a problem that undissolved coarse intermetallic compounds composed of Fe, Mo, and the like remain, reducing ductility and toughness. .

In addition, in Japanese Patent Publication No. 59-34226, a maraging steel containing one or more of B, Zr, Ca, and Mg, when B is added in an amount of 0.0025% or less, deoxidation is strengthened in the same manner as Zr and Ca. Denitrification and M on the grain boundaries
o, it is described that it prevents precipitation of Cr and the like and imparts ductility and toughness, and in JP-A-61-210156, B is 0.0005.
Addition of up to 0.0020% increases the temperature range of non-recrystallized solution treatment and makes it easier to industrially perform non-recrystallized solution treatment, resulting in the production of steel with excellent tensile strength and fracture toughness. It is shown that you can.

However, in the above-mentioned maraging steel and the manufacturing method, the grain size of fine wires or thin plates can be refined, but the grain size of the bar material or plate material of maraging steel used as a normal size is ASTM No. 10 or more. It is difficult to obtain ultrafine and stable toughness or other properties.

Japanese Patent Application Laid-Open No. 52-23520 discloses that a maraging steel to which B, Zr, Ca, and V are simultaneously added at a content of 1% or less is subjected to a solution treatment, rapidly cooled to 50 ° C or less, and then 100 to 350 ° C.
This is a method in which plastic working with a working ratio of 10 to 95% is performed at a temperature of, followed by heating and holding at a temperature from the austenitizing end temperature (Af point) to 1200 ° C, then cooling to room temperature, and then aging treatment. However, there is a drawback that, for example, if a wide plate is continuously subjected to plastic working while being heated to a temperature range of 100 to 350 ° C., expensive equipment is required.

In view of the above, the present invention provides a maraging steel having ultrafine crystal grains and a method for producing the maraging steel.

[Means for solving the problem]

The inventor conducted various studies on alloying elements effective for grain refinement of maraging steel, and found that only a specific solution treatment and a cold working condition were combined with maraging steel to which a certain amount of B was added. It has been found that ultrafine crystal grains can be obtained. Specifically, after hot working of an 18% Ni-based maraging steel in which the amount of B added was changed and an 18% Ni-based maraging steel in which B was not added, as shown in Table 1, the first time Perform a solution treatment,
Thereafter, cold working was performed, and then the second solution treatment was performed. The result is shown in FIG.

From Fig. 3, the maraging steel with B added has finer crystal grains than the maraging steel without B addition, and in particular, the maraging steel containing 0.0003% or more of B has a grain size number of 10 or more. It is extremely fine. However, even if the same maraging steel to which B is added is not subjected to cold working after the first solution treatment, it can be seen that the crystal grains hardly become fine even after the second solution treatment.

The inventors conducted similar tests on each element such as Ca, Zr, Y, and Mg in addition to B. As a result, it was newly found that elements effective for grain refinement are effective only when B is added. And led to the present invention. That is, in the first invention, C is 0.03% or less, Si is 0.1% or less, and Mn 0.1
%, P 0.01% or less, S 0.01% or less, Ni 16-20%, C
o 7-14%, Mo 3.0-5.5%, Al 0.2% or less, Ti 0.3-2.
Ultra-fine crystals containing 0%, N 0.01% or less, B 0.0003-0.01%, the balance being substantially Fe, and having a grain size of 10 or more in ASTM No. A second invention is a maraging steel having grains, wherein the maraging steel having the composition described in the first invention is obtained by hot working.
An ultrafine crystal characterized by performing a solution treatment at a temperature of 800 to 950 ° C., then performing a cold working at a working rate of 10% or more, and further performing a solution treatment at a temperature equal to or higher than a recrystallization temperature. This is a method for producing a maraging steel having grains.

In the present invention, the first solution treatment is performed in order not to leave undissolved intermetallic compounds containing Fe and Mo as main components, and 800 ° C. or higher is required to achieve the above object. If the temperature exceeds 950 ° C, the crystal grains become coarse.
The temperature range of the first solution treatment is limited to 800 to 950 ° C.
The cold working performed after the first solution treatment is performed in order to add processing strain to the material and finely recrystallize by the second solution treatment performed thereafter. For example, when the results of Step C and Steps E to I shown in Table 1 are summarized, as shown in FIG. It can be seen that cold working has no effect.

Therefore, the rate of cold working performed after the first solution treatment is limited to 10% or more. Desirably a cold working rate of 20%
That is all. The subsequent solution treatment needs to be performed at least at a temperature higher than the recrystallization temperature in order to apply a strain to the material by cold working in the previous step and recrystallize the crystal grains. FIG. 1 shows that after the hot working in steps A to D shown in Table 1, a first solution treatment was performed at 840 ° C. for 1 hour, followed by a 30% cold working, and then a second solution working. FIG. 4 is a diagram showing an example of examining the relationship between the solution treatment temperature and the crystal grain size of the sample. From FIG. 1, it can be seen that in order to make the crystal grain size fine, the second solution treatment temperature is preferably higher than the recrystallization temperature and at a lower temperature. However, since the appropriate heating holding time, recrystallization temperature, and the like change depending on the dimensions of the material, the rate of cold working before the second solution treatment, and the like, it is desirable to appropriately select the conditions for the second solution treatment.

[Action]

 The reasons for limiting the composition of the present invention are described below.

Ni must be at least 16 to form low C martensite, the matrix of the maraging steel.
% Is a necessary element, but if it exceeds 20%, austenite is stabilized and it becomes difficult to form a martensite structure. Therefore, Ni is set to 16 to 20%.

Co strengthens the precipitation by reducing the solid solubility of Mo by promoting the formation of fine intermetallic compounds by reducing the solid solubility of Mo without significantly affecting the stability of the martensite structure, which is the matrix. However, if the content is less than 7%, the effect is not always sufficiently obtained,
If it exceeds 14%, it tends to become brittle,
The content of Co was limited to 7-14%.

Mo forms a fine intermetallic compound by aging treatment,
It is an element that contributes to strengthening by precipitating in the matrix, but its effect is small when its content is less than 3.0%, and when it exceeds 5.5%, the main elements are Fe and Mo, which deteriorate ductility and toughness. In order to easily form coarse precipitates, the content of Mo was set to 3 to 5.5%.

Ti is an element that contributes to strengthening by forming and precipitating a fine intermetallic compound by aging treatment like Mo, but its effect is small when its content is less than 0.3%, and exceeds 2.0% If added, the ductility and toughness deteriorate, so the content of Ti is set to 0.3 to 2.0%.

Al not only contributes to aging-precipitated strengthening, but also has a deoxidizing effect. However, if contained in excess of 0.2%, toughness deteriorates, so its content was made 0.2% or less.

N is an impurity element that, when its content exceeds 0.01%, forms a carbonitride with Ti and C and remains as an inclusion, and also reduces the effective Ti amount contributing to the effect. Was set to 0.01% or less.

B is an element necessary and effective for refining crystal grains. However, if the content is less than 0.0003%, sufficient effect cannot be obtained, and if the content exceeds 0.01%, toughness is reduced. Due to deterioration, the content was made 0.0003% to 0.01%.

The larger the grain size number, the higher the strength and toughness.
If the value is smaller than 10, the effect is insufficient, and according to the method of the present invention, 10 or more can be achieved.

〔Example〕

 Hereinafter, the present invention will be described with reference to examples.

18% Ni maraging steel having the composition of Sample Nos. 1 to 11 shown in Table 2 was melted by vacuum induction melting, cast into a 10 kg ingot, homogenized and annealed at 1200 ° C.
Hot rolling was performed to make a 4.5 mm thick flat material.

After performing the heat treatment and the cold working of steps C, D, E and I in the steps shown in Table 1 using this flat material,
Further, aging treatment was performed in the range of 480 ° C. to 520 ° C., and the grain size of the longitudinal section was measured. The results are shown in Table 2.

As shown in Table 2, 1 to 10 of the alloys of the present invention containing B are fine grains having a grain size of No. 10 or more.
It can be seen that the comparative alloy 11 with no addition has a small fine-grain effect even after cold working. Step I of the comparative method, in which the second solution treatment was carried out without performing the cold working after the first solution treatment, was carried out with respect to 1 to 10 of the alloy of the present invention containing B. It turns out that it does not granulate.

〔The invention's effect〕

ADVANTAGE OF THE INVENTION According to this invention, the crystal grain of the maraging steel containing B can be easily refined | reduced. , Excellent toughness, and especially for parts with small thickness, it is expected that the variation in ductility, toughness, etc. is small due to the ultra-fine crystal grains, and if used for various tool materials, structural members, etc. It is expected to have industrially remarkable effects, such as showing tool life and mechanical properties.

[Brief description of the drawings]

FIG. 1 shows that a maraging steel containing B and a maraging steel not containing B are subjected to cold working 2
FIG. 2 is a diagram showing a relationship between a second solution treatment temperature and a crystal grain size number, and FIG. 2 shows cold working performed between two solution treatments of a maraging steel containing B and a maraging steel not containing B; FIG. 3 is a diagram showing the relationship between the working ratio and the grain size number, and FIG. 3 is a diagram showing the relationship between the B content of the maraging steel and the grain size number.

Claims (2)

(57) [Claims] (1) In weight%, C is 0.03% or less, Si is 0.1% or less,
Mn 0.1% or less, P 0.01% or less, S 0.01% or less, Ni 16 ~ 2
0%, Co 7-14%, Mo 3.0-5.5%, Al 0.2% or less, Ti
0.3 to 2.0%, N 0.01% or less, B 0.0003 to 0.01%, the balance is substantially composed of Fe, and the grain size is ASTM N
A maraging steel having ultra-fine crystal grains, characterized by having fine grains of 10 or more in o. 2. A maraging steel having a composition according to claim 1 is subjected to a solution treatment at a temperature of 800 to 950 ° C. after hot working, and then to a cold working at a working ratio of 10% or more. Thereafter, a solution treatment is performed at a temperature higher than a recrystallization temperature to produce a maraging steel having ultrafine crystal grains.