JPH11293407A - Maraging steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a maraging steel strip excellent in fatigue strength with the acceptance of the presence of inclusions to some degree. SOLUTION: In the case the diameter of a circle outwardly connected with Ti series inclusions observed in the cross section of the structure is defined as Dmax and the diameter of a circle inwardly connected with them is defined as Dmin, Dmax is regulated to <=8 μm, and as for the existing Ti series inclusions of 8 to 2 μm Dmax, the ratio of Dmin/Dmax is regulated to >=0.75. Preferably, the content of N is regulated to <=13 ppm, and the content of C is regulated to <=0.01%, and more preferably, the content of N is regulated to <=9 ppm, and the content of C is regulated to <=0.006%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a maraging steel strip used for a belt of a belt type continuously variable transmission of an automobile.

[0002]

2. Description of the Related Art A belt for a continuously variable transmission is known as a so-called steel belt, and is used after being formed into a thin plate having a thickness of 1 mm or less by rolling. Since extremely high fatigue strength and toughness are required for this steel belt, maraging steel has often been used. Incidentally, in the case of high-strength steel such as maraging steel, the presence of inclusions greatly affects the fatigue strength. That is, it is well known that large inclusions become the starting point of fatigue fracture. Since maraging steel contains Ti as an essential element,
Combined with inevitably present C and / or N which is a gas component, relatively large TiC, TiN or Ti
(C, N) Ti-based inclusions are easily formed. Therefore, conventionally, in order to improve the fatigue strength by reducing inclusions,
Measures have been adopted to reduce the gas components and carbon content that are the origin of inclusions.

[0003]

In recent years, the level of fatigue strength required for steel belts has become increasingly higher, and the stability of quality has also been required. On the other hand, in addition to such demands, there is an extremely strict demand for cost reduction. Therefore, high-purity maraging steel obtained at the laboratory level is too expensive for mass production and is not practical. An object of the present invention is to provide a maraging steel strip having excellent fatigue strength characteristics while allowing the presence of inclusions to some extent.

[0004]

Means for Solving the Problems The present inventors have found that Ti-based inclusions in which Ti and C and / or N are contained in maraging steel usually have a cubic shape. It has been found that a member having a notch effect, such as a steel belt for a continuously variable transmission, in which the compressive stress and the tensile stress are constantly changed and cannot be used, is likely to be a starting point of fatigue. As a result of intensive studies, the present inventor has changed the form of a cubic inclusion that adversely affects fatigue strength,
The inventors have found that it is possible to approximate a spherical shape with a small notch effect, and have reached the present invention.

That is, the present invention relates to a method for producing a Ti
Assuming that the diameter of a circle circumscribing the system inclusion is Dmax and the diameter of a circle circumscribing the system inclusion is Dmin, the Ti-based inclusion having a Dmax of 8 μm or less and an existing Dmax of 8 to 2 μm has a Dmin / Dmax of 0.75. This is the maraging steel strip described above. More preferably, the N content is 13 ppm
Hereinafter, the C content is set to 0.01% or less, and more desirably,
The N content is 9 ppm or less and the C content is 0.006% or less.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One of the most significant features of the present invention is that a cubic Ti-based inclusion, which contributes to a reduction in the fatigue strength of a maraging steel strip, is formed into a spherical shape that is unlikely to cause a reduction in the fatigue strength. Is controlled by the inclusions. In the present invention, Dmax is specified to be 8 μm or less, since inclusion of a large inclusion having a Dmax exceeding 8 μm is not preferable as a steel strip even if the absolute value of the fatigue strength is too low.
On the other hand, in a structure having no Ti-based inclusions having a Dmax of 2 μm or more, the inclusions are too fine to make the dependence of the shape of the inclusions on the fatigue strength unclear, and there is little need to control the morphology of the inclusions. In addition, it is out of the scope of the present invention because the control of impurities is very costly.

In the present invention, Dmax is defined as a range in which Ti-based inclusions are spherical and improvement in fatigue strength is recognized.
Dmin / Dmax of the inclusions of 8 to 2 μm was specified to be 0.75 or more. The most major inclusions in the maraging steel strip, TiC, TiN or Ti (C,
Specific means for controlling the size and shape of the Ti-based inclusions in N) include, for example, limiting the amounts of N and C to a certain degree or less, and increasing the solidification rate when producing an ingot by melting and casting. It is effective to do.

[0008] For example, the amount of N and C, but if the amount of N is large, the temperature at which Ti-based inclusions are formed in molten steel (crystallization temperature)
And Ti-based inclusions grow during molten steel or during solidification to form a large cubic shape. To prevent this, it is preferable to suppress the N content to 13 ppm or less, preferably 9 ppm or less. Further, when the amount of C is large, the crystallization temperature does not change so much as when the amount of N is increased, but is slightly increased, so that the Ti-based inclusions are increased and the amount is increased, and the fatigue strength is reduced. Cause Therefore, C needs to be 0.01% or less, preferably 0.006% or less.

Regarding the solidification rate, the present inventors have studied that when the solidification rate is extremely high, no large Ti-based inclusions are formed even if the N content is as high as about 20 ppm. This is presumably because even if Ti-based inclusions are formed at a high temperature, the solidification rate is high and the Ti-based inclusions do not grow significantly. Since reducing the amount of N increases costs, it is preferable to control the solidification rate to control the form of inclusions by setting the amount of N to 5 ppm or more. Under actual manufacturing conditions, the structure changes due to the interaction with the amounts of N and C and the amounts of other elements or the difference in the local solidification rate caused by the mass effect at the time of manufacturing the ingot. No numerical limitation is imposed on the manufacturing conditions. A belt for a continuously variable transmission, which is one of the uses of the present invention, is often used by applying a surface treatment such as nitriding treatment or Ni plating treatment to maraging steel. The maraging steel strip of the present invention naturally includes a belt for performing post-treatment such as nitriding.

[0010]

EXAMPLES Steel having the composition shown in Table 1 was melted and cast in vacuum to produce a 10 kg ingot. Here, in order to see the effect of solidification speed, a product cast in a mold (solidification speed is high)
And those cast in a lost wax mold (having a low solidification rate) and those heated and wound with a mold and a heat insulating material (the solidification rate is intermediate between the former two). These are 1280 ° C x
After soaking for 20 hours and then forming into a 20 mm × 70 mm plate by hot forging and hot rolling, 820
A solution treatment at 1 ° C. × 1 h was performed. Thereafter, the sheet was finished to a sheet thickness of 0.5 mm by cold rolling, and then subjected to a solution treatment at 820 ° C. × 1 h and an aging treatment at 480 ° C. × 5 h.

[0011]

[Table 1]

From the obtained maraging steel strip, in order to determine the fatigue strength assuming repeated bending on the belt, 10 samples were cut out in a 100 mm × 9.6 mm strip shape, the bending angle was 10 °, the distance between fulcrums was 50 mm, and the rotation was performed. The fatigue strength of 10 7 times when a leaf spring fatigue test was performed at several thousand cpm was determined. Table 2 shows the results. Also,
The metal structure of the obtained maraging steel strip was observed at 800 times. 1 to 4 show typical examples of relatively large inclusions among the inclusions observed in the maraging steel strip obtained with the sample No. corresponding to Table 1. What is illustrated is Ti-based inclusions. Table 2 shows the maximum value of Dmax,
And Dmin / D of inclusions having a Dmax of 2 to 8 μm
Indicates the minimum value of max.

[0013]

[Table 2]

Sample 1 which is the maraging steel strip of the present invention
The maximum value of the size Dmax of the inclusions of Nos. 3 to 3 is 5 μm or less, and the fatigue strength of 10 7 times in the leaf spring fatigue test is 10% or more than that of Comparative Example 4 having the inclusion of a large cubic shape. high. Among the samples of the present invention, sample No. having relatively large inclusions was used. In Comparative Example N, the inclusions are not rectangular, but have rectangular inclusions of similar size.
o. 5 shows that the fatigue strength was increased. On the other hand, in the sample No. 4 shows that the fatigue strength in the leaf spring fatigue test is clearly deteriorated as compared with Samples 1 to 3 of the present invention.

[0015]

According to the present invention, the fatigue strength of maraging steel can be greatly improved, and this is an indispensable technique for practical use of a belt for a continuously variable transmission with higher performance.

[Brief description of the drawings]

FIG. 1 is a photograph showing inclusions in a metal microstructure of a maraging steel strip of the present invention.

FIG. 2 is a photograph showing another example of inclusions in the metal microstructure of the maraging steel strip of the present invention.

FIG. 3 is a photograph showing another example of inclusions in the metal microstructure of the maraging steel strip of the present invention.

FIG. 4 is a photograph showing inclusions in a metal microstructure of a maraging steel strip of a comparative example.

FIG. 5 is a photograph showing inclusions in a metal microstructure of a maraging steel strip of a comparative example.

Claims (3)

[Claims] 1. The diameter of a circle circumscribing a Ti-based inclusion observed in a tissue section is Dmax, and the diameter of a circle circumscribed is Dmi.
n, Dmax is 8 μm or less, and the existing Ti-based inclusions having Dmax of 8 to 2 μm are Dmin /
A maraging steel strip, wherein Dmax is 0.75 or more. 2. The amount of N is 13 ppm or less, and the amount of C is 0.01 or less.
% Or less, the maraging steel strip according to claim 1. 3. The amount of N is 9 ppm or less and the amount of C is 0.006.
% Or less, the maraging steel strip according to claim 1 or 2.