JPS58199846A - Superhigh tensile steel - Google Patents

Abstract

PURPOSE:To obtain a superhigh tensile steel with superior toughness and strength, by especially reducing the content of C and by adding prescribed percentages of Ni, Mo, Ti, Al and Cu. CONSTITUTION:This high tensile steel consists of, by weight, <=0.05% C, 10- 25% Ni, 0.2-4.0% Mo, 0.1-5.0% Ti, 0.01-3.0% Al, 0.01-6.0% Cu and the balance essentially Fe. The steel is provided with high strength and toughness like 18% Ni maraging steel by utilizing hardening by the precipitation of an intermetallic compound. A martensite matrix phase with superior toughness is formed by adding 10-25% Ni, and Ti, Al and Cu are added as precipitation hardening elements. An intermetallic compound is precipitated in the matrix phase by aging to harden the steel. To ensure toughness at a superhigh strength level, the C content is restricted to a minimum, and the amounts of P and S as impurities are reduced to the utmost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to ultra-high tensile strength steel, and more particularly to aircraft parts, rocket and missile chambers, mandrels,
The present invention relates to ultra-high tensile strength steel suitable for use in tools such as stems, guicasts, and plastic molds, springs, bolts, and fasteners for pressure vessels, plates, and wires, and the like.

In general, typical types of ultra-high tensile strength steels that have been conventionally used or proposed and their problems will be explained.

(1) Quenched and tempered steel (for example, SNCM439.5KD
61) Due to the high C content, this steel has ultra-high strength, low toughness, problems with weldability, large mass effect, poor quality of extremely thick products, large heat treatment distortion, and cold opening. There is a problem of poor workability.

(2) Precipitation hardening stainless steel (e.g. 17-4Pl
(Stainless steel) This steel lacks the strength and toughness of an ultra-high tensile steel, requires complicated heat treatment, and has problems with weldability.

(3) 18% Ni maraging steel This steel has excellent toughness at an ultra-high strength level, (1)
Although the problems of steel explained in (2) have been resolved, Co.
Since it contains a large amount of No, it becomes costly and there are also problems in terms of resource conservation.

(4) 20% Ni, 25% Ni maraging steel This steel has excellent strength, but coarse N15 (T
i, A1) precipitates at grain boundaries, resulting in a problem in that toughness is significantly degraded.

(5) Super strong Kamal A song steel (Japanese Unexamined Patent Application Publication No. 1983-15) No is added to the 20% Ni, 25% Ni maraging steel to increase the strength of the Ni3 (Ti, A1) grain boundaries. This is a maraging steel that prevents precipitation and improves toughness, but the ductility and toughness of this steel vary greatly depending on the aging treatment conditions, and in particular, the problem of significant embrittlement when subjected to strain aging treatment has been demonstrated in experiments. The result is clear.

Therefore, strict temperature control is required during heat treatment.

The present invention has been made in view of the various problems of conventional ultra-high tensile strength steels described above, and provides an ultra-high tensile strength steel having excellent toughness and strength.

The present invention (1)
C50.05%, Ni10-25%, Mo0.2-4.
0%, TiO, 1-5, 0%, AI 0.01-3.
0%, Cu 0.01-6.0%, and the remainder is substantially Fe, and (2) C50.05%, Ni 10-25 %, Mo
0.2-4.0%, Ti0.1%-5.0%, A10
．． 01-3.0%, Cub, 01-6.0%, Co2%
This invention consists of two inventions, the second invention being an ultra-high-strength steel characterized by containing the following, with the remainder being substantially Fe.

The ultra-high tensile strength steel according to the present invention having such a configuration is based on the following.

(1) Similar to 18% Ni maraging steel, strengthening by precipitation of intermetallic compounds is used to strengthen the steel, and the presence of 10 to 25% Ni forms a martensitic matrix with excellent toughness. , Ti, A as precipitation strengthening elements
I, Cu is contained, and intermetallic compounds are precipitated in the martensite matrix through aging treatment to strengthen it. Furthermore, in order to ensure toughness at an ultra-high strength level, the C content is kept as low as possible, and P, S is also kept as low as possible as an impurity element.

(2) By reducing the Co and Mo content and compensating for the accompanying strength loss by precipitation strengthening with Ti, AI, and Cu, Co7 to 14% and Mo of normal 18% Ni maraging steel
The content of Co can be as low as 4-6%, and the content can be as low as 2% or less of Co and 0.2-4.0% of Mo. T to secure
i, A I, and Cu are added in combination. In this way, the content of Co and Mo is reduced for resource saving and economical reasons.

(3) As explained in (1) and (2), Ti and AI are strong precipitation-strengthening elements and are effective for strengthening, but T
This is because the growth rate of Ti and AI precipitates is low and the susceptibility to delayed fracture is high over a wide range of strain-aging treatments, Alternatively, this is because a coherent precipitated phase exists. Therefore, in order to prevent embrittlement due to delayed fracture during sub-aging treatment and ensure stable toughness under a wide range of aging treatment conditions, it is effective to include Cu.
This is because the precipitation of C1l occurs at a lower temperature and in a shorter time than that of T i and A I, and forms precipitates with low delayed fracture susceptibility even during strain aging treatment. Therefore, thermal handling during aging treatment, processing of large products, etc. are facilitated.

(4) The heat treatment is performed to austenite at a temperature higher than the Af temperature, and the material is cooled to room temperature (the cooling rate does not matter) to form a martensitic phase (deep cooling treatment is not required). 300°~
Precipitation strengthening is performed by aging at 650°C for 0.5-100 hours.

Next, we will explain in detail the components and component ratios of the ultra-high-strength steel according to the present invention. 9C has the effect of increasing strength, but when C is used for strengthening, the conventional quenched and tempered ultra-high-strength steel In the ultra-high tensile steel according to the present invention, C
Since C is considered to be an impurity element and the precipitation of intermetallic compounds is used for strengthening, the C content is set to 0.05% or less.

Ni is an important element that forms the martensite matrix, and in order to generate a high-strength lath martensite phase with excellent toughness, this effect cannot be obtained if the Ni content is less than 10%. If the content exceeds %, the austenite phase becomes stabilized and does not become 100% martensite phase at chamber fM. Therefore, the Ni content is set at 10 to 25%. Also, N: is T1, l\1. due to aging treatment. cu, M
It has the effect of forming an intermetallic compound with o and increasing the strength.

Mo is an element that prevents grain boundary embrittlement at ultra-high strength levels.
If the content is less than 0.2%, this effect will be small, and the aging treatment will precipitate N13Mo and Fe2Mo and cause strange strengthening, but if the content increases, it is not economical.
．． Do not contain more than 0%. Therefore, Mo content is 0
．． 2 to 4.0%.

T1 is an important element that imparts strength, and N
i5Ti precipitates and strengthens significantly, but the content is 0.1
If it is less than 5.0%, precipitation strengthening cannot be expected much, and if it is contained in a large amount, it will become brittle, so it is not necessary to contain it in excess of 5.0%. Therefore, the Ti content is 0.1 to 5.0%
shall be.

A1 is an element that has a great effect on strengthening, and increases the strength by precipitating N, i, and s^1 during aging treatment, but if the content is less than 0.01%, this effect is small;
．． If it is contained in a large amount exceeding 0%, it becomes brittle. Therefore,
The A1 content is 0.01 to 3.0%.

Cu is an element effective in increasing precipitation strength and preventing embrittlement by aging treatment, and Ti
, When reinforced with AI, delayed fracture susceptibility increases during sub-aging treatment and tends to become brittle, but when the Cu content is 0
．． It is effective to prevent this embrittlement and ensure stable toughness under a wide range of aging treatment conditions by containing more than 0.01%.

Since Cu precipitation occurs at lower temperatures and shorter times than Ti and AI, by combining Cu, Ti, and AI precipitation, it is possible to create a structure with low delayed fracture susceptibility under a wide range of aging treatment conditions from sub-aging to over-aging. This prevents embrittlement and increases strength. Moreover, if the content exceeds 6.0%, it becomes brittle as it is strengthened, and high-temperature oxidation tends to occur. Therefore, the Cu content is set to 0.01 to 6.0%. In addition,
Cu also has the effect of improving corrosion resistance.

□ Co is said to have the effect of reducing the solid solubility of Mo and promoting the precipitation of N13Mo, but in the ultra-high tensile steel of the present invention, the precipitation effect is due to Ti, A1, and Cu rather than Mo. The effect of CO containing gold is small. Further, since Co is expensive, the Co content is set to 2% or less.

Since Si, Mn, p, and S are considered as impurity elements, they must be kept as low as possible, and the content is S1≦0.3%, M≦0.3%, P≦0.05%.
, and it is desirable that S≦0.05%.

Examples of ultra-high tensile strength steel according to the present invention will be described together with comparative examples.

The ingredients and proportions shown in Example Table 1 are as follows:
Each steel type was melted and cast using the usual melting method. Thereafter, the sample was prepared by processing.

Table 2 shows the heat treatment conditions and mechanical properties.

FIG. 1 also shows ultra-high tensile strength steels according to the present invention No. 5, No. 5, N007, and No. 088, and No. 16. The drawings show the reduction of area and tensile strength compared to No. 19 comparative steel.

As is clear from this Table 2 and FIG. It is tension steel.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between aging treatment temperature, reduction of area, and tensile strength.

Claims (2)

[Claims] (1) C50.05%, Ni 10-25%, Mo 0
．． 2-4.0%, TiO, 1-5.0%, AlO,
Contains 01-3,0%, Cu O,01-6,0%,
An ultra-high tensile strength steel characterized in that the remainder is substantially Fe. (2) C50.05%, Ni 10-25%, MoO
,2-4.0%, Ti 0.1-5.0%, A10.0
1-3.0% Cu, 0.01-6.0% Cu, and 2% or less Co, with the remainder being substantially Fe.