JPH03180448A - High strength matrensitic stainless rolled steel sheet having excellent fatigue resistance in corrosive and erosive environment - Google Patents

Abstract

PURPOSE:To obtain the high strength martensitic stainless steel sheet having excellent fatigue resistance by specifying a compsn. constituted of C, Si, Mn, Cr, Ni, Mo, V, Nb, Al, N and Fe and regulating non-metallic inclusions therein. CONSTITUTION:The high strength martensitic stainless rolled steel sheet has a chemical compsn. contg., by weight, <=0.05% C, <= 1.0% Si, <= 2.0% Mn, 12 to 17% Cr, 1.5 to 6.5% Ni and 0.2 to 2.0% Mo, contg. one or 2 kinds of 0.01 to 0.50% V and 0.01 to 0.50% Nb, furthermore contg. total <=0.05% of 0.005 to 0.025% Al and <=0.01% N, moreover contg., at need, 0.2 to 5.0% Cu and the balance Fe with inevitable impurities and has a structure of which non-metallic inclusions are dispersed into a steel at <=0.01% areal occupying rate. The steel sheet has excellent fatigue resistance in a corrosive and erosive environment and is suitable for structural members used in a high speed water current such as sea water or the like.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention is used in high-speed water currents such as hydrofoils of high-speed boats and high-speed rotating equipment, especially in environments with strong corrosive or erosive properties. The present invention relates to a high-strength martensitic stainless steel rolled steel sheet that is suitable as a structural member and has excellent fatigue resistance in corrosive or erosive environments.

For water turbine runners and boat screws that operate in high-speed water currents, corrosion resistance and high strength are generally required, so 13Cr stainless steel containing Ni is generally used, and its representative steel type is developed as cast steel or forged steel. It was done^
There is STM CA6 NM (13Cr4Ni) steel, and the yield strength of that steel is at most 60-70 kg f
/ mm”.

However, in recent years, with the speed amplifiers of high-speed boats and the remarkable increase in the speed of rotating equipment, it has become possible to maintain corrosion resistance, have a higher strength than before, with a yield strength of 80 kgf/mm2 or more, and have excellent fatigue resistance. There was a demand for new steel.

(Prior Art) JP-A-1427620 discloses a method of manufacturing a martensitic stainless steel plate containing Ni by hot rolling, and the steel plate is manufactured by the above-mentioned ASTM CA6NM.
Compared to wXI cast steel products and forged steel products, since it is a hot-rolled hollow material, even if material deterioration caused by defects due to casting or forging is reduced, it still lacks strength.

Furthermore, Japanese Patent Application Laid-open No. 62wt24218 describes martensitic stainless steel containing Ni.
A method for manufacturing a high-strength stainless steel material is disclosed, in which q is regulated and heat treated in a temperature range of 550°C to 675°C. However, there is no mention of fatigue resistance in corrosive or erosive environments.

(Problems to be Solved by the Invention) An object of the present invention is to provide a high-strength martensitic stainless steel that has high strength and excellent fatigue resistance in a corrosive or erosive environment.

(Means for Solving the Problems) The present invention was developed as a result of various experimental studies on the i'iit fatigue characteristics of high-strength martensitic stainless steel containing Ni in a corrosive or erosive environment. We obtained the knowledge that fatigue resistance in corrosive or erosive environments is determined by the chemical composition range of steel and the area occupation rate of nonmetallic inclusions in steel.

That is, this invention has the following characteristics: C: 0.05wt% or less, Si: 1.0wt% or less, n: 2.0wt% or less, r: 12wt% or more and 17-t% or less, Ni:
1.5 wt% or more and 6.5 wt% or less Mo:
0.2 hL% or more and 2.0wt% or less, and V: o, o1wt% or more and 0.50wt% or less, Nb
: One or two types selected from 0.01 wt% or more and 0.50 wt% or less, Al: 0.005 wt% or more and 0.025 wt%
t% or less N: Contains a total of 0.01 wt% or less with C, 05 wt% or less, and the remainder is iron and unavoidable impurities, and non-metallic inclusions in the steel are o, oi% A high-strength martensitic stainless steel rolled steel sheet having excellent fatigue resistance in a corrosive or erosive environment, characterized by a dispersed structure with an area occupancy of 0.05 wt% or less, and C: 0.05 wt% or less; Si: 1.0wt% or less, n: 2.0wt% or less, r: 12wt% or more and 17wt% or less, Ni:
1.5 wt% or more and 6.5 ivt% or less and Mo:
0.2 wt% or more and 2.0 wt% or less, and V: 0.01 wt% or more and 0.50 wt% or less, Nb
Cu: 0.2 wt% t% 5.0 wt% t%, Al
: 0.005 wt%t% 0.025 wt%t%
and N: 0.0 in total with C at o, otwt% or less.
Corrosion or erosion characterized by a chemical composition in which the non-metallic inclusions in the steel are dispersed with an area occupation rate of 0.01% or less, with the remainder being iron and unavoidable impurities. This is a high-strength martensitic stainless steel rolled steel sheet with excellent fatigue resistance in the environment.

Generally speaking, increasing the strength of a steel material will result in a decrease in corrosion resistance and fatigue strength in a corrosive environment. For the above-mentioned structural members used in corrosive environments, it is desired to develop steel materials that have not only high strength but also excellent fatigue resistance. In order to improve fatigue resistance, it is necessary to have excellent rust resistance and erosion resistance, which greatly affect fatigue strength.

Rust resistance refers to the fact that when rust occurs, fatigue fracture occurs at the rusting point, reducing the fatigue strength.Erosion resistance refers to the damage caused by water or seawater colliding with the steel material at high speed. In this case, there is a high possibility that cavitation and erosion will occur on the surface of the steel material.Erosion will be accelerated especially when corrosive like seawater, and if erosion occurs on the surface of the steel material, the fatigue strength will be significantly affected by the surface properties. lower.

Therefore, in order to improve the fatigue resistance of steel materials used in corrosive or erosive environments, it is necessary to increase their rust resistance and erosion resistance.

(for production) The reason for the limitation of this invention will be explained.

C: The hardness of martensite is set to 0.05 wt% or less in order to ensure weldability without making the hardness more than necessary.

Si: This is an essential component for deoxidizing, but there is no need to worry about it as adding too much will reduce toughness.1.
The upper limit is 0 wt%.

Mn = Mn has the effect of fixing S in steel and widening the high-temperature austenite single phase region to improve hardenability, but if added in large amounts, the toughness decreases, so the upper limit is 2.0 wt%. Prevent deterioration.

Cr: In martensitic stainless steel, the lower limit is 12ht% to ensure corrosion resistance. On the other hand, in order to ensure that the high-temperature austenite region is sufficiently wide in balance with the amounts of other component elements and to prevent the amount of retained austenite from being too large, the upper limit is set at 17-t%.

Ni: 1.5 wt% is required in order to prevent the hot rolling temperature range from becoming narrower due to the narrowing of the high-temperature austenite range due to the reduced content of C and N.

In addition, in order to ensure the toughness of high-strength martensitic stainless steel, it is more preferable to add 3.5 wt%. On the other hand, if it exceeds 6.5 wt%, a large amount of austenite remains even after cooling from the high temperature austenite region,
6.5 wt without any concerns as it will cause insufficient strength.
The upper limit is %.

io: An element that improves corrosion resistance in seawater. It is necessary to add at least 0.2% to impart corrosion resistance, but if it is too large, the high-temperature austenite region becomes narrow and strength cannot be ensured. The upper limit is set at 2.0 i1t%, where there is no need to worry about it becoming difficult.

V, Nb: Elements that suppress the precipitation of Cr carbides at grain boundaries, and their content is 0.01wt.
If it is less than 0.0%, there is no effect, and if it is more than 0.50 wt%, the hot workability is reduced.
The range shall be below wt%.

Cu: Has effects such as expansion of the high temperature austenite region, adjustment of the Ms point, and precipitation hardening, so it is desirable to add 0.2 wt% when such effects are expected.

On the other hand, if the content exceeds 5.0 wt%, the hot workability deteriorates, so the upper limit is set at 5.0 wt%, which does not have such good properties.

AI: A necessary component for deoxidizing, but A1 in steel
*The AI remaining after deoxidation is 0.00 so that there is no concern that it will remain as 0.03 and cause deterioration of fatigue properties.
The range is 511t% or more and 0.025wt%t%.

N: Since it is an element that tends to generate blowholes when electron beam welding is performed, the content should be 0.01 wt% or less.

C+N: Set to 0.05wt% or less since the risk of iM contact cracking increases.

Nonmetallic inclusions: If the area occupation rate of nonmetallic inclusions in the cross section in the rolling direction exceeds 0.01%, sufficient fatigue properties in salt water cannot be obtained, so the area occupation rate is 0.01%.
% or less, resulting in a uniformly dispersed structure.

Next, the relationship between the area occupation rate of nonmetallic inclusions and fatigue properties will be explained based on experimental results.

Figure 1 shows the results of a tensile fatigue test conducted in a 3.5 wt% NaCl aqueous solution at 400 MPa, repetition rate: IHz, and environment: 3.5 wt% NaCl aqueous solution for steels whose chemical compositions are compatible with the present invention and have different area occupancies of nonmetallic inclusions. Shown below. Here, the area occupation rate of the nonmetallic inclusions is determined by measuring the size and number of the nonmetallic inclusions using an optical microscope after polishing a cross section parallel to the rolling direction at a magnification of 800 times and a field of view of 120. This was determined through image analysis. As is clear from FIG. 1, as the area occupation rate of nonmetallic inclusions decreases, the rate at which the number of repetitions until fracture increases increases, and when the area occupation rate is 0.
The number of repetitions (Nf) until breakage at 01% or less is lXl0
' more than once.

(Example) C: 0.03wt%, N: 0.01wt%, Cr
: 13.5wt%, Si: 0.30wt%, M
n: 0.60wt%, P: 0.020wt%
, S: 0.004 wt% as a basic component is melted in a converter, and this is used as a base material to undergo secondary refining in a small ESR furnace. Through this refining, nonmetallic inclusions in the steel are removed. 22 types of steel were produced by adjusting the amount of steel and adding trace elements.

ESR (, - The cast ingots were heated at a temperature of 1200°C for 4 hours and then bloomed into slabs with a thickness of 100 mm. These slabs were heated at a temperature of 1200°C for 2 hours and then finished at a finishing temperature. These steel plates were hot rolled to a temperature of 900''C to produce steel plates with a thickness of 30 mm.These steel plates were further subjected to a standard treatment of 930''C and then tempered to 600''C to produce products.

These steel plates were evaluated for fatigue properties in seawater using a tensile fatigue test, rust resistance using a salt spray test, and erosion resistance in salt water.

These evaluation results are shown in Table 1 along with the chemical composition of the steel sheet.

The above test conditions and the symbols shown in Table 1 are described below.

Fatigue properties in seawater: A tensile fatigue test is conducted in a 3.5 wt% NaCl aqueous solution under conditions of a stress of 400 MPa and a repetition rate of 111z. In Table 1, samples with the number of repetitions until breakage (Nf)≧lXl0' are marked with ◎, and (Nf)
< I

Rust resistance: A 16-hour salt spray test was conducted using a 3.5 wt% NaCl aqueous solution. In Table 1, samples with the number of rust points per unit area of 0.1 pieces/cm or less are marked with ◎, samples with more than 0.1 to 1 piece/cmZ are marked with ○, and 1.
For samples with super ~10 pieces/mark 2, mark Δ, 10 pieces/cm”
In the case of super, an x mark was added to evaluate the rust resistance.

Erosion resistance: Tested in a 3.5 wt% NaCl aqueous solution using an opposed magnetostrictive vibration type cavitation erosion tester under the following conditions.

Frequency: 20kllz Amplitude: max 25μll Test surface - horn tip: 0.5n+m Test time: 24h In Table 1, samples with weight loss due to erosion of 15g/% or less are marked with ○, and samples with weight loss exceeding this are marked with △. The erosion resistance was evaluated.

As is clear from Table 1, the compliant steel of the present invention has an area occupation rate of nonmetallic inclusions of C, Ni, Mo, and V.

Nb, AI, N, etc. are superior in fatigue resistance in seawater, rust resistance in salt spray tests, erosion resistance, etc. compared to comparative steels that are outside the limited range of this invention, and yield strength is 80 kgf/in. “It shows a value of more than ”.

(Effect of the invention) The area occupancy rate of nonmetallic inclusions in 13Cr stainless steel containing Ni is regulated to a certain value or less, and in the component composition,
In addition to improving seawater rust resistance by regulating C, N, etc. and adding Mo appropriately, adding one or more of Nb and V suppresses the precipitation of C carbides at grain boundaries. It is possible to obtain a high-strength martensitic stainless steel that is high in strength and has excellent fatigue properties, rust resistance, erosion resistance, etc. in seawater.

The high-strength martensitic stainless steel obtained by this invention can be advantageously used in structural materials used in environments under fluctuating loads and high-speed water currents in seawater, such as hydrofoils of high-speed ships and high-speed rotating equipment. be able to.

[Brief explanation of drawings]

FIG. 1 shows a relationship diagram between the area occupation rate of nonmetallic inclusions and the number of repetitions until breakage in a tensile fatigue test. Figure 1

Claims (1)

[Claims] 1. C: 0.05wt% or less, Si: 1.0wt% or less, Mn: 2.0wt% or less, Cr: 12wt% or more and 17wt% or less, Ni: 1.5wt% or more6. 5wt% or less and Mo: 0
．． Contains 2wt% or more and 2.0wt% or less, and V: 0.01wt% or more and 0.50wt% or less, Nb: 0
．． One or two types selected from 0.01 wt% or more and 0.50 wt% or less, Al: 0.005 wt% or more and 0.025 wt% or less, and N: 0.01 wt% or less, with a total of 0.05 wt% with C.
% or less, with the remainder being iron and unavoidable impurities, and a corrosive or erosive environment characterized by a structure in which nonmetallic inclusions are dispersed in steel with an area occupation rate of 0.01% or less. High-strength martensitic stainless steel sheet with excellent fatigue resistance. 2. C: 0.05 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, Cr: 12 wt% or more and 17 wt% or less, Ni: 1.5 wt% or more and 6.5 wt% or less, and Mo: 0
．． Contains 2wt% or more and 2.0wt% or less, and V: 0.01wt% or more and 0.50wt% or less, Nb: 0
．． Cu: 0.2 wt% or more and 5.0 wt% or less, Al: 0.01 wt% or more and 0.50 wt% or less.
005wt% or more and 0.025wt% or less and N: 0.0
It contains C at a total of 0.05 wt% or less at 1 wt% or less, and the remainder is iron and unavoidable impurities, and nonmetallic inclusions are dispersed in the steel with an area occupation rate of 0.01% or less. A high-strength martensitic stainless steel rolled steel sheet with excellent fatigue resistance in corrosive or erosive environments.