JPH0459924A - Production of maraging steel excellent in strength, toughness, and ductility - Google Patents

Abstract

PURPOSE:To stably produce a maraging steel securing high strength and having mechanical properties well balanced among tensile ductility, Charpy impact value, and fracture toughness by subjecting a steel containing specific amounts of C, Si, Mn, P, S, Ni, Ti, and B as basic components to hot forming and then to specific recrystallization solution treatment. CONSTITUTION:A hot formed maraging steel containing, by weight, <=0.05% C, <=0.2% Si, <=0.2% Mn, <=0.05% P, <=0.05% S, 16.5-26.0% Ni, 1.0-2.5% Ti, and 0.0005-0.0020% B as basic components 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> The present invention is applied 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 maraging steel containing CO (Fe - 88%).
However, since this steel contains expensive Co, the cost of the steel is extremely high, and its uses are currently quite limited. Therefore, as a maraging steel that does not contain expensive CO2
0% Ni system (Japanese Patent Publication No. 39-19885) and 2
Fe such as 5% Ni type (Special Publication No. 39.19886)
Ni-Ti based maraging steels have been developed.

After forming by hot working, Fe-Ni-Ti maraging steel is subjected to solution treatment in which it is heated to an austenite 1 temperature range of 800 to 950'C and then cooled to form a solid solution of alloying elements and fine austenite grains. At the same time, a martensitic structure was obtained through subsequent cooling. At this time, if necessary, deep cooling treatment is performed, and then 260
Hardening is achieved by precipitating intermetallic compounds such as N, i, and Ti by aging treatment for 0.5 to 24 hours in a temperature range of ~650°C.

In addition, in Fe-Ni-Ti maraging steel, in addition to the basic ingredients, B (boron) and Nb are added in combination to strengthen the steel through non-recrystallization solution treatment (Japanese Unexamined Patent Publication No. 64-4783).
4) is known. The unrecrystallized region solution treatment is a heat treatment method in which the austenite in the unrecrystallized region, that is, the austenite that still has a high dislocation density, is cooled to obtain a martensitic structure in the solution treatment. As a result, the lower fibers of martensite are made finer and stronger.

In ordinary maraging steel, the austenite temperature range in the non-recrystallized region is very narrow and cannot be applied industrially. However, when boron and Nb are added, the recrystallization temperature of austenite increases and the non-recrystallization temperature range is expanded, making industrial application possible.

Furthermore, in order to expand the non-recrystallization temperature range, instead of adding B and Nb in combination, it is necessary to add B alone or add B and MO or Ta.
It is also known that it is possible to make it possible by combined addition with etc. (CAMP, ISIJ Vol.

2 (1989)-1806, (702) l.

However, in Fe-Ni-Ti maraging steel containing boron, depending on the hot rolling conditions, boron compounds may precipitate during hot rolling or during subsequent solution treatment, making it impossible to obtain the desired properties. This has become clear through research conducted by the present inventors.

In Fe-Ni-Ti maraging steel containing boron, the precipitates that are generated during hot working (or become the nucleus of formation during the subsequent heat treatment process) are
A normal solution treatment at 0°C does not completely form a solid solution, resulting in a decrease in fracture toughness. In addition, when slowly cooling after high-temperature solution treatment at 800 to 950°C, precipitates are formed at the austenite grain boundaries during cooling, and this suppresses recrystallization during the subsequent solution treatment at 800 to 950°C. is not refined, resulting in low ductility and Nyarby absorbed energy values.

Therefore, there has been a strong desire to develop a method for stably producing Fe-Ni-Ti maraging steel containing boron and having excellent toughness.

<Invention tries to solve! Is> Therefore, the present invention provides a manufacturing method for obtaining maraging steel that secures conventional strength or higher strength and also has mechanical properties that are well-balanced in tensile ductility, Charpy impact value, and fracture toughness. The purpose is to

<Means for Solving the Problems> The present invention provides, in weight percent, 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: 16.5% or more and 26.0% or less, Ti:
Recrystallization solution treatment, non-recrystallization solution treatment and In a method for manufacturing maraging steel that performs aging heat treatment, it 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 then heated to a temperature range of 800 to 950°C for 1 minute or more. This is a method for producing maraging steel having excellent strength, toughness, and ductility, which is characterized by performing recrystallization solution treatment twice, which consists of heating for at least 1 minute and then cooling.

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

C, Si, Mn, P, and S need to be kept as low as possible in maraging steel in order to reduce toughness, and the upper limit of each is set to 0.05.0.2.0.2.0.05.
It was set to 0.05%.

Ni forms a martensitic matrix with excellent toughness in Fe-Ni-Ti system maraging steel, and N1d
It is an element that contributes to strengthening through the precipitation of i.

For this purpose, a content of 16.5% or more is required, but if it exceeds 26%, a complete martensitic structure cannot be obtained even by deep cooling treatment, so the upper limit was set at 26%.

Ti is a precipitation hardening element like Ni, but if it is less than 1.0%, its effect is small, and if it exceeds 2.5%, it becomes brittle, so the upper limit was set to 2.5%.

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 0.020%, it will cause embrittlement, so the upper limit is 0.00%.
It was set at 20%.

Note that elements other than these, such as MO1Nb and AZ, may be added within a range that does not affect the effects of the present invention.

Next, the reasons for limiting the 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 longer is necessary. Furthermore, if the heating temperature exceeds 7300°C, the surface quality of the steel material will deteriorate and the heat treatment cost will increase, so the upper limit is 1300°C.
'C.

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. Grains are not refined and tensile ductility and Charpy absorbed energy decrease. Therefore, after high-temperature solution treatment, a cooling rate of 20° C./min or higher 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 V properties are low. It is necessary to carry out solution treatment for 1 minute or more in a temperature range of ゛C. 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. Moreover, if the temperature exceeds 950°C, the austenite grains will become too coarse, so the upper limit is 950°C.
It was set as C.

By performing recrystallization solution treatment twice at high and low temperatures, fracture toughness, ductility, and
It becomes possible to obtain a Fe-Ni-Ti maraging steel having excellent Charpy impact properties.

<Example> The steel shown in Table 1 was made into a steel plate with a thickness of 20 m by hot working after melting. Steel A is a steel that corresponds to the basic composition of the present invention.

In addition, BD steel is a steel that further contains MOINb and AI. Furthermore, classes E and F each have compositions in which boron is not covered by the present invention. Further, after the heat treatments shown in Table 2 were performed, the tensile properties, Charpy absorbed energy, and plane strain fracture toughness were examined.

Tensile properties and fracture toughness were measured at room temperature. Charpy absorbed energy was measured using a V-notch full-size test piece at 0°C. 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 Nn4 sea bream after high-temperature solution treatment is below the lower limit of the present invention, it has low ductility and toughness.
Although sea bream is a steel to which conventional heat treatment conditions, ie, high-temperature solution treatment, are not applied, it is clear that the ductility, Charpy absorbed energy, and fracture toughness are lower than the steel of the present invention. Although No. 6 steel does not require solution treatment to refine the grain, it has low ductility and Charpy absorbed energy, similar to No. 4 steel. Seeds 7, 8, and 9m are respectively FIo, Nb, and A7.
These steels are obtained by applying the heat treatment of the present invention to the steel to which the above-mentioned is added, and all of them have good tensile properties, Charpy impact properties, and fracture toughness. Nα10 steel is produced by the method of the present invention, but has a strength of 1, which is obtained by omitting the solution treatment in the non-recrystallized region.
Although the toughness is somewhat low, it is significantly superior to conventional methods and comparative methods, and it is clear that the effect of the present method does not depend on the presence or absence of solution treatment in the non-recrystallized region.
Steel No. 2 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> By the heat treatment of the present invention, Fe-Ni-
It becomes possible to provide Ti-based maraging steel, which makes it possible to reduce the weight of structures and improve reliability.

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: 16.5% or more and 26.0% or less, Ti: 1.0% or more and 2.5% or less, B: 0. 0.0005% or more and 0.0020% or less as a basic composition, the hot-formed maraging steel is subjected to recrystallization solution treatment, non-recrystallization solution treatment, and aging heat treatment.
After heating to a temperature range of 300℃ for more than 1 minute, heat to 20℃/
Excellent strength, toughness, and ductility, characterized by performing recrystallization solution treatment twice, which consists of cooling at a cooling rate of 1 minute or more, and then cooling after heating to a temperature range of 800 to 950 °C for 1 minute or more. A method for producing maraging steel.