JPH0459923A - Production of maraging steel excellent in strength, toughness, and ductility - Google Patents

Abstract

PURPOSE:To stably produce a maraging steel excellent in strength, toughness, and ductility by subjecting a steel having a specific composition containing C, Si, Mn, P, S, Ni, Co, Mo, Ti, Al, and B to specific recrystallization solution treatment. CONSTITUTION:A hot forged maraging steel having a composition containing, by weight, <=0.05% C, <=0.2% Si, <=0.2% Mn, <=0.05% P, <=0.05% S, 10.0-21.0% Ni, 5.0-<9.5% Co, 3.0-12.0% Mo, 0.2-1.6% Ti, <=0.30% Al, and 0.0005-0.0020% B is subjected to recrystallization solution treatment, unrecrystallization solution treatment, and aging heat treatment. At this time, a two-times recrystallization solution treatment consisting of heating up to 1000-1300 deg.C for >=1min, cooling at >=20 deg.C/min cooling rate, further heating up to 800-950 deg.C for >=1min, and cooling is carried out. By this method, the maraging steel excellent in strength, toughness, and ductility can be obtained.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention is applicable to structural members such as rocket motor cases, pressure vessels, rollers, shafts, etc. that require high strength and excellent toughness and ductility. The present invention relates to a method for producing maraging steel.

<Conventional technology> The main type of maraging steel currently in practical use is CO-containing maraging steel (Fe-18%Ni).
-Co-Mo-Ti-Al). After forming by hot working, maraging steel is subjected to solution treatment in which it is heated to an austenitic temperature range of 800 to 950°C and then cooled to dissolve the alloying elements and refine the austenite grains. A martensitic structure is obtained. and 0.5 in the temperature range of 260-650℃
By aging for ~24 hours, Nf, Mo
, hardening is attempted by precipitating intermetallic compounds such as Ti.

Also, “Japan Institute of Metals Bulletin Volume 25 No. 6 (1986)
On pages 550-552, there is a report on a technology in which boron is added to maraging steel in addition to its basic components, and a non-recrystallized solution treatment is applied to strengthen the maraging steel.What is non-recrystallized solution treatment? , is a heat treatment method in which a martensite structure is obtained by cooling the austenite in the unrecrystallized region, that is, austenite that still has a high dislocation density, during solution treatment to obtain a martensite structure. In maraging steel, the austenite temperature M in such a non-recrystallized region is very narrow (and cannot be applied industrially, but when boron is added, the austenite recrystallization temperature increases and the non-recrystallized temperature range expands. This makes it possible to apply it industrially.As described above, the addition of boron exhibits excellent effects, but it has also been reported that excessive addition of boron causes a decrease in toughness and ductility.

However, even if an appropriate amount of boron is added, it may cause embrittlement depending on the rolling and heat treatment conditions, and as a means to avoid this, a manufacturing method using an appropriate combination of rolling and heat treatment conditions has been proposed. It is reported in the No.

However, in reality, it is often difficult to ensure appropriate rolling and heat treatment conditions depending on the shape, plate thickness, etc. of the steel material.

In addition, in boron-containing maraging steel, precipitates that form during hot working (or become the nucleus of formation during the subsequent heat treatment process) are not completely solidified during the subsequent normal solution treatment at 800 to 950'C. Because it does not dissolve, it reduces fracture toughness. In addition, when slowly cooling after high-temperature solution treatment at 1000°C or higher, precipitates are generated at austenite grain boundaries during cooling, which suppresses recrystallization during subsequent solution treatment at 800 to 950°C, resulting in finer grains. ductility and absorbed energy values are low.

Therefore, in maraging steel containing boron, the strength
There was a strong desire to develop a method that could stably produce steel with excellent toughness and ductility.

<Problems to be Solved by the Invention> The present invention has been made in view of the above points, and does not limit the hot working conditions (by devising only the heat treatment method) The object of the present invention is to provide a method for producing maraging steel that has excellent strength, toughness, and ductility.

<Means for Solving the Problems> The present invention is a heavy! %, C: 0.05% or less, Si: 0.2% or less, M
n: 0.2% or less, P: 0.05% or less, s: o
, os% or less, Ni: 10°0% or more and 21.0% or less, Co:
5.0% or more and less than 9.5%, Mo: 3.0% or more 1
2.0% or less, ri: 0.2% or more and 1.6% or less,
A7: 0.30% or less, B: Maraging that performs recrystallization solution treatment, non-recrystallization solution treatment, and aging heat treatment on heat-opened maraging steel containing 0.0005% or more and 0.0020% or less. In the method of manufacturing steel, the steel is heated to a temperature range of 1000 to 1300°C for 1 minute or more, then cooled at a cooling rate of 20°C/min or more, and further heated to a temperature range of 800 to 950'C for 1 minute or more and then cooled. This is a method for producing maraging steel with excellent strength, toughness, and ductility, which is characterized by performing recrystallization solution treatment twice.

<For production> First, the reason for limiting the components of the present invention will be explained.

C, Si, Mn, P, and S must be kept as low as possible in maraging steel to reduce toughness.
Each upper limit is 0.05.0.2.0.2.0.05
, O,OS%.

Ni is an element necessary for forming a martensitic matrix having excellent toughness in maraging steel, and for this purpose, Ni is required to be present in an amount of 10% or more. However, if it exceeds 21%, residual austenite will be generated and it will be difficult to ensure strength, so the upper limit was set at 21%.

CO reduces the solid solubility of 10, which contributes to precipitation hardening, and N
It is an effective element that promotes the precipitation of 1Jo and thereby increases strength. For that purpose, 5.0% or more is required. However, if 9.5% or more is added, the strength increases but the toughness decreases significantly, and the cost increases, so the content was set to be less than 9.5%.

Mo is an important element that contributes to age hardening of maraging steel, and for this purpose 3% or more is required, but if added in excess, it may cause strength reduction due to the formation of retained austenite or embrittlement due to coarse precipitates. The upper limit is 12
．． It was set to 0%.

T4 is a precipitation hardening element like FIo, but if it is less than 0.2%, its effect is small, and if it exceeds 1.6%, it becomes brittle, so the upper limit was set at 1.6%.

M is also an element that contributes to age hardening, but if it exceeds 0.30%, it becomes brittle, so the upper limit was set at 0.30%.

B is an effective element for solution treatment in the non-recrystallized region, and for this purpose, 0.0005% or more is required. But 0.0
If it exceeds 02Q%, it will cause embrittlement, so the upper limit is 0.00.
It was set at 20%.

Next, the reasons for limiting the recrystallization solution heat treatment conditions are as follows.

First, in order to completely dissolve the precipitates that precipitate during hot working (or become the nucleus of precipitation during subsequent solution treatment), heating at 1000° C. or higher for 1 minute or more is necessary. Furthermore, if the heating temperature exceeds 1300°C, the surface quality of the steel material will deteriorate and the heat treatment cost will increase, so the upper limit is 1300°C.
And so.

In addition, if the cooling rate after heating is lower than 20°C/min, precipitation will occur on the austenite grain boundaries during cooling, and austenite recrystallization will be significantly suppressed during solution treatment at low temperatures, which will be described later. is not refined, and tensile ductility and Charpy absorbed energy decrease. Therefore? After hot solution treatment, a cooling rate of 20'C/min or more is required.

By the way, after the above-mentioned high-temperature solution treatment, which was carried out to dissolve the precipitates, the crystal grains have become coarse and the tensile ductility and Charpy impact properties are low. It is necessary to perform solution treatment for 1 minute or more at a temperature range of . In order to recrystallize the austenite phase and refine the grains, it is necessary to heat the material at 800° C. or higher for 1 minute or more. In addition, if the temperature exceeds 950'C, the austenite grains will become too coarse, so the upper limit is 950'C.
And so.

By performing recrystallization solution treatment twice at high and low temperatures, fracture toughness, ductility, and
It becomes possible to obtain maraging steel with excellent Charpy impact properties.

<Example> The steel shown in Table 1 was heated to a thickness of 2On+m w4vi after melting. A-E steel is the composition that is the object of the present invention. In addition, F and G steels each have boron compositions that are outside the scope of the present invention. Further, the tensile properties, Charpy absorbed energy, and plane strain fracture toughness were examined after heat treatment, which is not shown in Table 2. Tensile properties and fracture toughness were measured at room temperature. Also, the Charpy absorbed energy is v at O'C
Measurement was performed using a Notti full size test piece. The measurement results are also shown in Table 2.

It is clear that all the materials according to the invention have good tensile properties, Charpy impact properties and fracture toughness.

Although the cooling rate of Nα4111 after high-temperature solution treatment is below the lower limit of the present invention, the ductility and toughness are low.

Although Nα5 steel is a steel to which conventional heat treatment conditions, that is, high temperature solution treatment is not applied, it is clear that the ductility, Charpy absorbed energy, and fracture toughness are lower than the steel of the present invention.隘6 steel does not require solution treatment to refine grains,
The 9th grade 11 steel, which has low ductility and low Charpy absorbed energy like Nα414, is produced by the method of the present invention, but the solution treatment in the non-recrystallized region is omitted. k
Although the strength is slightly lower than that of lotH, it has excellent properties, and it is clear that the effect of the method of the present invention does not depend on the presence or absence of solution treatment in the non-recrystallized region. Seeds 12 and 13
Steel is a steel whose boron content deviates from the present invention, but no effect is observed even if the heat treatment conditions of the present invention are applied.

<Effects of the Invention> The heat treatment of the present invention makes it possible to provide maraging steel with far superior fracture toughness, tensile ductility, and Charpy impact properties than conventional maraging steels, resulting in lighter structures and improved reliability. can do.

Claims (1)

[Claims] In weight %, C: 0.05% or less, Si: 0.2% or less, Mn: 0.
2% or less, P: 0.05% or less, S: 0.05% or less, Ni: 10.0% or more and 21.0% or less, Co: 5.0% or more and less than 9.5%, Mo: 3. Hot-formed maru containing 0% or more and 12.0% or less, Ti: 0.2% or more and 1.6% or less, Al: 0.30% or less, and B: 0.0005% or more and 0.0020% or less. In a method for manufacturing maraging steel in which aging steel is subjected to recrystallization solution treatment, non-recrystallization solution treatment, and aging heat treatment, heating to a temperature range of 1000 to 1300℃ for 1 minute or more and then cooling at 20℃/minute or more Maraging steel with excellent strength, toughness and ductility, characterized by being subjected to recrystallization solution treatment twice, which consists of cooling at a high speed, heating to a temperature range of 800 to 950°C for 1 minute or more, and then cooling. manufacturing method.