JPS63171857A - Manufacture of precipitation hardening-type stainless steel excellent in fatigue characteristic - Google Patents

Abstract

PURPOSE:To manufacture a stainless steel improved in fatigue characteristics, by casting a steel having a specific composition containing precipitation harden ing elements while regulating the Ms point to a value in a specific range to cause central segregation so as to provide compressive residual stress to the surface layer of the finished product. CONSTITUTION:A stock for manufacturing precipitation hardening-type martensitic stainless steel has a composition consisting of, by weight, <=0.1% C, <=2% Si, <=1% Mn, 10-20% Cr, 3-10% Ni, one or more kinds, as precipitation hardening elements, among 0.1-3% Mo, 0.1-3% Ti, 0.1-1% Nb, and 0.5-5% Cu, and the balance Fe with inevitable impurities. Casting is carried out by regulating Ms represented by an equation, determined based on the above chemical components, to 80-200 deg.C so as to cause solidification segregation, particularly central segregation. In this way, compressive residual stress can be provided to the surface layer without shot peening, so that fatigue strength of steels of this kinds can be easily improved.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing martensitic precipitation hardening stainless steel with excellent fatigue properties obtained from rapidly cooled materials such as continuous casting materials. This is a proposal for a method for manufacturing steel that is suitably used in the fields of materials, marine materials, and chemical equipment materials.

This type of martensitic precipitation hardening stainless steel is
A metal that dissolves in solid solution in austenite but has almost no solubility in martensite is precipitated from the martensite after the austenite-martensite transformation, and is characterized by the combination of martensitic transformation and precipitation hardening. It has the following.

(Prior Art) Conventionally, as a method for improving the fatigue strength of the martensitic precipitation hardening stainless steel, there has been a method using shot peening as described in, for example, Metal Surface Technology Handbook (1976), page 1204. Are known. In short, this is a method of generating compressive residual stress in the surface layer by shot peening. In general, fatigue failure of metals is often caused by tensile stress, and in particular, tensile residual stress reduces the strength of the material, whereas compressive residual stress acts to reduce it. In other words, since shot peening is a process in which countless small balls collide with the surface of a workpiece to stretch it, compressive residual stress is generated on the surface of the workpiece, improving the so-called fatigue strength.

(Problems to be Solved by the Invention) Although fatigue properties can be improved by applying compressive residual stress to the surface of the material, in the case of the above-mentioned conventional technology, a surface treatment process called shot peening is essential, which reduces manufacturing efficiency and There was a disadvantage in terms of cost.

The object of the present invention is to propose an advantageous technique that can be applied naturally in the conventional general manufacturing process without relying on the above-mentioned surface treatment process of shot peening.

(Means for Solving the Problems) As a result of repeated development and research in order to achieve the above-mentioned purpose, the present inventors have discovered a martensitic transformation point (hereinafter referred to as rMs) determined based on chemical components. If casting is carried out in such a way that solidification segregation, especially central segregation where precipitation hardening elements are concentrated, is carried out within a specific range (abbreviated as "point"), compressive residual stress can be imparted to the surface layer of the final product, and its It was found that fatigue properties were improved as a result. That is, in the present invention, C: 0.1 wt% or less, St: 2 wt% or less, Mn:
1 wt% or less, Cr: 10-20 wt%,
Contains Ni: 3-10 wt%, Mo as a precipitation hardening element: 0.1-3 wt%, Ti: 0.1-3 wt%
3Ht%Nb: 0.1-1 wt% and Cu:
Contains at least one type of 0.5 to 5 wt%, the remainder consists of Fe and inevitable impurities, and has the following Ms point;
s (“C) = 1229.9-1666, 7 (C+
N) -27,8Si-33,3Mn-41,7Cr-
61,1Ni-44,3Mo+59.2Ti+140.
We have come up with a technology that consists of a patented method for producing precipitation hardening stainless steel based on a patented tensitic system, which is characterized by casting 8Nb-30,6Cu steel at 80 to 200°C in a manner that causes center segregation. .

(Function) The idea of the present invention is based on the fact that the Ms point of precipitation hardening stainless steel of the required composition is 80 to 2.
The alloy is designed to have a temperature of 00°C, and as it is rapidly cooled as in continuous casting, specific components (Ni, Ni,
Cr. The point is that shot peening becomes unnecessary when positive segregation in which Mo, etc.) is concentrated occurs.

In short, by continuously casting a steel having the above-mentioned composition, a part enriched with major alloying elements such as Ni and Cr and precipitation hardening elements such as Cu and Ti can be left as segregation in the center of the continuously cast slab. As a result, the Ms point in the center becomes lower than that in the surface layer, and even if the surface layer undergoes martensitic transformation at room temperature or lower, the center portion remains as austenite. Therefore, there is a difference in structure between the center and the surface layer, and compressive residual stress can be applied to the surface layer.

Therefore, the present inventors first investigated in detail the relationship between chemical components and the Ms point. The results will be explained below.

Casting and rolling of melted materials (5 kg) of various component compositions, 1
The sample was heated to 050° C. for heat treatment, and then the Ms point was measured using a thermal expansion tester. Then C+ Sit Mn.

For each coefficient of Cr and Ni, Bichervan
The coefficients of Mo + Ti. Ta.

Ms('e)-1229. 9-1666. 7
(C+N)-27.8Si-33. 3Mn-41.
7Cr-61. INi-44.3Mo+59.2T
i+140.8Nb-30.6Cu Next, the relationship between the above Ms point and the structure of the product subjected to the continuous casting process was investigated. As a result, when the calculated Ms point was 80°C or less, the center showed an austenitic structure due to an alloying element enriched layer, but the near surface also showed a mixed austenite-martensitic structure. Further, when the calculated Ms point was 200° C. or higher, the entire structure up to the center was martensitic.

On the other hand, in the case where the Ms point was 80 to 200°C, an austenite structure was observed only in the center. From these results, it was confirmed that the organization can be determined by the Ms point.

Furthermore, when bending fatigue tests were conducted on the three cases mentioned above, it was found that steel having an austenitic structure only in the center exhibited the highest fatigue strength. This can be understood as follows. That is, during the solidification process, solidification segregation usually occurs.

In the case of square steel ingots, because they go through a pressing process, this segregation is considerably reduced and the inside is homogenized. However, in the case of rapidly cooled slabs such as slab-type steel ingots and continuous casting, the solidification segregation due to the layer enriched with alloying elements that occurs in the center does not disappear until the final product (remains as it is. Ni.

If the Ms point of the part enriched with alloying elements such as Cr is above room temperature, everything from the surface layer to the center becomes martensite, whereas when the Ms point of the surface layer is above room temperature, the M of the center becomes martensite.
When the s point is below room temperature, the surface layer and the center become martensite and austenite, respectively, resulting in a so-called structural difference.

As a result, volumetric expansion occurs in the surface layer due to martensitic transformation, which introduces strain between it and the center portion, which remains retained austenite, generating compressive residual stress in the surface layer, resulting in a decrease in fatigue strength. It will improve.

Next, the reason for limiting the components of the material for producing precipitation hardening stainless steel of the present invention will be described.

C: An element necessary to ensure strength, 0.1%
If it exceeds 1 t% (hereinafter simply abbreviated as "%"), the steel plate becomes hard, so the upper limit was set at 0.1%.

Si: Like C, it is effective as a deoxidizing agent while ensuring strength, but if it exceeds 2%, toughness deteriorates, so the upper limit should be set at 2%.
%.

Mn: Effective for improving strength and toughness, but if it exceeds 1%, the mechanical properties of the steel sheet deteriorate, so the upper limit was set at 1%.

Cr: This is the main element for martensitic precipitation hardening stainless steel, and it is essential to contain 10% or more in order to obtain the necessary corrosion resistance.On the other hand, if it is contained in 20% or more, the amount of δ ferrite increases rapidly and heat Since it deteriorates machinability, it was limited to 10 to 20%.

Ni: An element that suppresses the formation of δ ferrite phase, and C
It depends to some extent on the amount of r, but if it is too low, it will reduce the precipitation hardening phenomenon which is a characteristic of steel that is limited by the method of the present invention, so it is set to at least 3%. On the other hand, if it is too high, retained austenite phase tends to form, so the upper limit was set at 10%.

N: N improves the strength, but if it exceeds 0.2%, the steel plate becomes hard, so IMo is set to 0.2% or less.

Ti, Nb, and Cu are elements for precipitation hardening, and are used singly or in combination; however, below a limited amount, their effectiveness is small and, above that, embrittlement or reduction in hot workability occur. Therefore, Mo: 0.1-3%,
Ti: 0.1-3%, Nb: 0.1-1%, Cu
: 0.5 to 5%.

(Example) 18 tons of martensitic precipitation-hardening stainless steel having the composition shown in Table 1 was melted and cast using continuous casting, and a slab that was pressed after normal ingot making was the same. After hot rolling, heat treatment, and cold rolling,
A final 1 m long strip was manufactured, and then subjected to solution treatment at 1050°C and precipitation hardening treatment A and C at 480°C for 1 hr to obtain test materials 1 to 12.

For this sample material, the Ms point was calculated, the presence or absence of retained austenite in the center was determined by microstructural observation, and a bending fatigue test was performed.

For the bending fatigue test, a Nishihara type double swing plate bending fatigue tester was used. The results are shown in Table 2. As is clear from Table 2, those with a calculated Ms point of 80 to 200°C have undergone a continuous casting process compared to those that have undergone a normal ingot-pressing process. It has better fatigue strength. 1st
The figure shows the quality of each microstructure of the inventive material and comparative material.
Retained austenite (white layered material) was confirmed in the center of the steel obtained by the method of the present invention.

In addition, although the level of fatigue strength exhibited by the steel produced by the method of the present invention is the highest level that can be obtained with Fe-based materials, it can be manufactured with a relatively small amount of alloying elements added and by a simple manufacturing process.

(Effects of the Invention) As explained above, according to the method for manufacturing martensitic precipitation hardening stainless steel according to the present invention, compressive residual stress is generated in the surface layer without performing any special treatment (shot peening). Therefore, the fatigue strength of this type of steel can be easily and inexpensively improved.

[Brief explanation of the drawing]

FIG. 1 is a photograph of the metallographic structure in the surface layer and center of each martensitic precipitation hardening stainless steel obtained by the method of the present invention and the comparative method. Patent applicant Nippon Yakin Kogyo Co., Ltd. Representative patent attorney Hidetoshi Sugimura Akira Sugimura Patent attorney No. 1
figure

Claims (1)

[Claims] 1. C: 0.1 wt% or less, Si: 2 wt% or less, Mn
: 1wt% or less, Cr: 10~20wt%, Ni: 3~
Mo containing 10 wt% and as a precipitation hardening element:
0.1 to 3 wt%, Ti: 0.1 to 3 wt%, Nb: 0.
A steel containing at least one type of 1 to 1 wt% and 0.5 to 5 wt% of Cu, the remainder consisting of Fe and unavoidable impurities, and having the following Ms point of 80 to 200 °C is cast in such a way that center segregation occurs. A method for manufacturing a martensitic precipitation hardening stainless steel steel having excellent fatigue properties. Ms(℃)=1229.9-1666.7(C+N)-
27.8Si-33.3Mn-41, Cr-61.1N
i-44.3Mo+59.2Ti+140.8Nb-3
0.6Cu