JPH1060597A - High strength dual phase stainless steel excellent in toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide dual-phase stainless steel having >=60% ferrite content in terms of area ratio and combining high strength of >=400N/mm<2> 0.2% proof stress with excellent toughness of >=100J/cm<2> impact value. SOLUTION: This steel is a ferrite-austenite dual-phase stainless steel having 60-90% ferrite content by area ratio. In this steel, the Ni balance value as an index to the stability of ferrite-austenite, represented by equation (Ni balance value)=Ni+0.5Mn+30(C+N)-1.1(Cr+1.5Si+Mo+0.5Nb)+8.2, is regulated to -15 to -10 and also the product of Al content and N content in the steel satisfies inequality Al.N<=0.0023.(Ni balance value)+0.0357, where the symbol of element in the equation and inequality represents its content (wt.%).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength ferrite-austenite duplex stainless steel excellent in toughness, and more particularly to a duplex stainless steel suitable for applications requiring high strength such as a centrifuge.

[0002]

2. Description of the Related Art Ferrite-austenite duplex stainless steel represented by SUS 329 is excellent in general corrosion resistance, stress corrosion cracking resistance, intergranular corrosion resistance and weldability. Another great feature is that the yield strength is twice that of austenitic stainless steel. Duplex stainless steel having such excellent characteristics is widely used for heat exchange tubes, line pipes, steel pipes such as oil country tubular goods, and petrochemical equipment.

The disadvantages of duplex stainless steels are sigma brittleness,
475 is liable to cause brittleness and is inferior in hot workability. Further, it is known that the impact properties are inferior when the ferrite content of the structure increases.

As measures against sigma and 475 brittleness,
There is a method of making a heat history so as to avoid a brittle zone during heating and processing. Further, as a method of improving hot workability, a method of reducing the amount of impurities S and O in the refining process of steel, Al,
A method of adding a strong deoxidizing element such as TiCa and a rare earth element and an S fixing element has been widely adopted.

[0005] N is added to duplex stainless steel to increase strength, improve pitting resistance, and stabilize austenite at low cost.

Al and Ti added as the above-mentioned strong deoxidizing element and S-fixing element also have a very strong affinity for N and a high N content.
If added excessively to steel, AlN and TiN precipitate and the mechanical properties deteriorate.

[0007] These precipitates impair the cleanliness of the steel and, in the case of a high-strength duplex stainless steel having a high ferrite content (60% or more in area ratio), rapidly increase the ductile-brittle transition temperature. Therefore, the impact characteristic becomes a problem. Further, these precipitates also serve as starting points of pitting corrosion and deteriorate corrosion resistance.

[0008] With respect to the deterioration of toughness due to these precipitates,
Measures have been taken to lower the end temperature of hot working as much as possible, to make the crystal recrystallize statically by the subsequent heat treatment, to control the microstructure, and to lower the ductile / brittle transition temperature. However, such a method has various problems such as surface flaws, working load, and σ embrittlement during hot rolling, and cannot be said to be a sufficient measure.

[0009]

According to the present invention, a ferrite content is 60% or more in area ratio and a 0.2% proof stress is 400%.
High strength of N / mm ² or more and impact value of 100J
The present invention provides a duplex stainless steel having excellent toughness of at least / cm ² .

[0010]

In order to improve the toughness of the high-strength duplex stainless steel, the present inventors have studied the solidification of molten steel, the phase transformation in the cooling process, the precipitation behavior of AlN, and the austenitic / ferrite ratio. As a result of various experiments and investigations on the relationship, the following findings were obtained.

(1) Even if the duplex stainless steels have the same Al and N contents, the precipitation amount of AlN increases as the duplex stainless steel has a higher ferrite ratio.

(2) Even in a duplex stainless steel having the same precipitation amount of AlN, the impact value deteriorates as the amount of ferrite increases.

(3) The product of the Al content and the N content is 0.00
233 · Ni balance value +0.0357 or less provides excellent toughness.

The present invention has been made based on such knowledge, and the gist of the invention is that “the amount of ferrite is 6% in area ratio.
A ferrite-austenite duplex stainless steel having a Ni balance value of 0 to 90%, which is an index of ferrite-austenite stability represented by the following equation (1): -1
A high-strength duplex stainless steel excellent in toughness, characterized in that the product of the Al content and the N content in the steel satisfies the following formula (2): 5 to -10.

Ni balance value = Ni + 0.5Mn + 30 (C + N) −1.1 (Cr + 1.5Si + Mo + 0.5Nb) +8.2 (1) Al · N ≦ 0.0023 · Ni balance value + 0.0357 ... (2) However, the symbol of the element in the formula indicates its content, and it is assumed to be% by weight. "It is in.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the limiting conditions for the duplex stainless steel of the present invention will be described.

(1) Ni balance value as an index of austenite-ferrite stability: -15 to -10 The Ni balance value is obtained from the contents of austenite former and ferrite former contained in steel by the following equation. Ni balance value = Ni + 0.5Mn + 30 (C + N)-1.1 (Cr + 1.5S
i + Mo + 0.5Nb) +8.2 When the Ni balance value is less than -15, the amount of ferrite after solution treatment (about 1050 ° C.) becomes 90% or more, and the strength increases remarkably, but when it becomes 90% or more, AlN is almost observed. Even if not performed, the lower limit of the Ni balance value was set to -15 because the toughness was significantly deteriorated and the commercial value was lost.

The steel of the present invention has a 0.2% proof stress of 400 N / m.
m ² or more but high strength in which the target, the Ni balance value is greater than -10, since the strength ferrite content after solution heat treatment is less than 60% can not be obtained, the upper limit of the Ni balance value Was set to -10.

(2) Al · N ≦ 0.0023 · Ni balance value + 0.0357 This equation is an empirical equation obtained through many experiments.
-N is 0.00233-Ni balance value + 0.0357
(Al and N in the formulas are contents by weight%)
The impact value substantially determined by the amount of ferrite and the amount of AlN precipitation is less than 100 J / cm ² (according to JIS Z 2203 No. 3 test piece), and the commercial value is lost. Therefore, the product of the Al content and the N content is 0.0023 · Ni Balance value +0.
0357 or less.

The duplex stainless steel of the present invention can achieve excellent toughness and high strength if the above conditions are satisfied.
The chemical components other than those represented by the conditional expressions are not particularly limited, but preferred chemical components are shown below.

In weight%, C: 0.03% or less, Si: 2
%, Mn: 2% or less, Ni: 3 to 15%, Cr: 2
0 to 30%, P: 0.04% or less, S: 0.02% or less, Al: 0.02 to 0.15%, N: 0.1 to 0.2
%, And when corrosion resistance needs to be improved, Cu, M
o, W and V are preferably contained.

[0022]

EXAMPLE In a high-frequency vacuum induction furnace, a 100 kg ingot of duplex stainless steel having the chemical components shown in Table 1 was melted.

[0023]

[Table 1]

As shown in Table 1, the Ni balance value was
While variously changing in the range of 9.8 to -15.6, A
The component was designed so that 1 · N also changed within the range of 0.0008 to 0.0149.

After heating these ingots at 1200 ° C., they were subjected to slab rolling and hot rolling to produce a hot-rolled steel sheet having a thickness of 30 mm, and subjected to a solution treatment at 1050 ° C.

From the center in the width direction of each solution-treated hot-rolled steel sheet, a JIS Z 220113B tensile test piece and a JIS
S3 impact test pieces and micro test pieces were collected. Measure tensile strength, 0.2 proof stress, impact value by impact test,
The strength and toughness were evaluated. After polishing the micro test piece, the amount of AlN precipitated was determined by the method of JIS G0555.

Table 2 shows the results of these measurements.

[0028]

[Table 2]

FIG. 1 shows the results of Table 2 with the Ni balance value on the horizontal axis and the amount of Al and N on the vertical axis, and divides the 0.2% proof stress and the impact value into three stages shown in FIG. This is shown in the figure. In the figure, a region surrounded by oblique lines indicates a range defined by the present invention.

As is clear from Table 2 and FIG. 2, the steel having a Ni balance value of −10 or less according to the present invention has a 0.2% proof stress as high as 400 N / mm ² or more. N in Comparative Example
In Nos. 7 to 9, the Ni balance value falls within the range specified in the present invention and the 0.2% proof stress is as high as 400 N / mm ² or more, but the Al · N value is 0.0023 · Ni. Since the value exceeds the balance value +0.0357, the impact value is 86 to 62.
J / cm ² is low. The impact value of No. 10 of the comparative example was 1
Although it is as high as 87 J / cm ² , the 0.2% proof stress is 389 N / m.
m ² and low strength.

In No. 6, the yield strength was 489 because the Ni balance value was slightly outside the range specified in the present invention.
N / mm ^{2, which} is extremely high, and the impact value is low despite the small amount of AlN deposited.

In Nos. 1 to 5 of the present invention, the amount of AlN precipitated was small, the strength was high, and the impact value was superior to the comparative example.

[0033]

According to the present invention, the Ni balance value is -1.
Within the range of 5 to -10, the amount of Al · N is 0.0023 ·
A duplex stainless steel with a high yield strength and impact value can be obtained by designing the components so that the Ni balance value is equal to or less than +0.0357, and high strength and high toughness such as a lid and a body of a centrifuge are required. Demonstrates excellent effects in applications.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship among a Ni balance value, an Al.N amount, strength, and an impact value.

Claims (1)

[Claims] 1. A ferrite-austenite duplex stainless steel having an area ratio of 60 to 90% in ferrite, and having a Ni balance value of -15 as an index of ferrite-austenite stability expressed by the following formula (1). ~ -10
A high-strength duplex stainless steel with excellent toughness, wherein the product of the Al content and the N content in the steel satisfies the following formula (2). Ni balance value = Ni + 0.5Mn + 30 (C + N) -1.1 (Cr + 1.5Si + Mo + 0.5Nb) + 8.2 ... (1) Al N ≤ 0.0023 · Ni balance value + 0.0357 ...・ (2) However, the symbol of the element in the formula indicates its content, and it is assumed to be wt%.