KR20140091097A

KR20140091097A - Duplex stainless steel

Info

Publication number: KR20140091097A
Application number: KR1020120152577A
Authority: KR
Inventors: 최국선; 하태권
Original assignee: 주식회사 포스코; 재단법인 포항산업과학연구원
Priority date: 2012-12-24
Filing date: 2012-12-24
Publication date: 2014-07-21

Abstract

Disclosed is duplex stainless steel which comprises 0.2-0.5 wt% of C, 0.5-1.5 wt% of acid soluble Si, 0.5-1.5 wt% of Mn, 20-28 wt% of Cr, 8-10 wt% of Ni, 0.1-0.4 wt% of N, 0.1-0.5 wt% Y, and the remainder consisting of Fe and inevitable impurities.

Description

DUPLEX STAINLESS STEEL}

The present invention relates to a duplex stainless steel, and more particularly, to a duplex stainless steel having a yttrium and a high nitrogen content.

Duplex stainless steel is a steel material superior in corrosion resistance and mechanical properties to conventional austenitic stainless steel due to the coexistence of two phases of austenite and ferrite and having a relatively low cost of nickel and a low production cost .

These duplex stainless steels are applied to parts that simultaneously require high temperature strength and corrosion resistance, such as heat reduction reaction tubes for magnesium smelting, pipelines for chemical plants, parts for desalination and desulfurization facilities of thermal power plants and petrochemical industry, and seawater facilities do.

In particular, duplex 2205 stainless steel has higher strength than austenitic stainless steel and has corrosion resistance higher than that of austenitic stainless steel. Therefore, the material of dechlorination and desulfurization equipment exposed to strong corrosion, internal screw conveyor of paper industry, paper industry Which is required to have corrosion resistance and strength at the same time.

Recently, according to the protection policy of the atmospheric environment, it is obligatory to install dechlorination and desulfurization facilities in power plants and petrochemical industry facilities. In addition, it is becoming an indispensable material for air purification facilities of industrial incinerators.

In general, duplex stainless steel is an alloy in which ferrite structure for improving strength and austenite structure for improving corrosion resistance coexist, and also contains a large amount of chromium (Cr) It is well known as a material.

Further, molybdenum (Mo) is an element which further improves the corrosion resistance of the duplex stainless steel. Typical duplex steels include Fe- (21-23)% Cr- (4.5-6.5)% Ni- (2.5-3.5)% Mo- (0.08-0.20)% Mn and C content of 2.0% N alloy (UNS31803 or SAF 2205).

In addition, 2205 stainless steel is designed to have better corrosion resistance by increasing Cr and Mo contents. Fe- (24 ~ 26)% Cr- (6-8)% Ni- (3 ~ 5)% Mo- (0.24 ~ 0.32)% N alloy (SAF 2507).

However, the biggest problem of the Mo-based duplex stainless steel is that it frequently causes defects such as edge cracking and surface rupture due to low formability in a hot working process, which is a process required for manufacturing plate, wire and pipe products.

The feature of duplex stainless steels is also characterized by adding a large amount of N to stabilize austenite to reduce expensive Ni together with Mn.

And N, like W and Mo, increases corrosion resistance significantly by increasing formal resistance. Normally, N contains about 0.02 as an impurity in the stainless steel. However, at least 0.08% of N content is required to attain the above object. When it exceeds 0.345%, corrosion resistance is increased. However, in the case of ingot casting or continuous casting, Limit the casting defects to cause badness of the material.

Further, in duplex stainless steel added with Mo alone, if N exceeds 0.345%, high temperature ductility is deteriorated.

As described above, the duplex stainless steel has excellent corrosion resistance with austenitic stainless steel and has a high strength of about 50%, which is attracting attention as a next-generation corrosion resistant stainless steel. However, the duplex stainless steel has a drawback that it is difficult to produce a hot rolled plate because of low ductility. A common method of improving high temperature ductility in duplex stainless steels is by addition of Ce (J. L. Komi et al., Proc. Of Int. Conf. On Stainless Steel, ISIJ Tokyo, 1991, p807 or U.S. Patent No. 4,765,953). In this method, S is controlled to 30ppm or less, and when Ce is added, it is known that segregation of S is suppressed and high temperature ductility is improved.

When 0.18% or less of Ce is added, high hot ductility is increased to obtain better hot workability, but when it exceeds 0.18%, high temperature ductility is deteriorated due to the segregation of coarse oxide inclusions.

Another example is U.S. Patent No. 5,298,093 to Sumitomo, which proposes a duplex stainless steel containing 2 to 4% Mo and 1.5 to 5% W in 23 to 27% Cr with an Mn content of 1.5% or less.

This alloy has been reported to have high strength and excellent corrosion resistance. However, cracks are easily generated during hot rolling and the phase stability is low due to the high alloy, so that a sigma phase which deteriorates the corrosion resistance and impact characteristics may remain.

Like the Mo-based duplex stainless steels described above, the W + Mo-based duplex stainless steel alloy has a disadvantage in that it has a poor defective rate due to poor hot forming property in hot plate forming, wire / rod and pipe manufacturing.

In particular, heat-resistant alloys for heat-reduction reaction tubes for magnesium smelting, which have recently attracted attention, are required to maintain high-temperature strength at 1,200 ^° C and at the same time have high temperature moldability. It is a fact that I can not.

The present invention has been conceived to solve the above problems, including nitrogen (N) in a high content and increase high-temperature strength in a hot forming and 1,200 ^o C by a yttrium (Y) was added at the same time that duplex stainless steel alloys The purpose is to provide material.

In order to achieve the above object, the duplex stainless steel according to an embodiment of the present invention comprises 0.2 to 0.5% of C, 0.5 to 1.5% of acid-soluble Si, 0.5 to 1.5% of Mn, 20 to 28%, Ni: 8 to 10%, N: 0.1 to 0.4%, Y: 0.1 to 0.5%, balance Fe and other unavoidable impurities.

The duplex stainless steel may have a high temperature strength at 1,200 DEG C of 15 MPa or more.

The duplex stainless steel may have an elongation at 1,200 DEG C of 19% or more.

The duplex stainless steel may comprise at least one selected from the group consisting of a thermal reduction reaction tube for magnesium smelting, a pipeline for chemical plants, a component for dechlorination and desulfurization plant of a thermal power plant, a component for a dechlorination and desulfurization plant of the petrochemical industry, Can be used.

According to the present invention, it is possible to provide an alloy having a high temperature strength at 1,200 ^° C of 20 MPa or more and excellent hot moldability in this temperature range by containing nitrogen at a high concentration and adding yttrium at the same time.

Fig. 1 is a view showing a result of a hot rolling test performed at 1,200 ^o C to evaluate the hot-formability of a duplex stainless steel to which nitrogen and yttrium are not added.
Fig. 2 is a graph showing the results of a hot rolling test performed at 1200 ^o C in order to evaluate the hot moldability of a 0.3 wt% duplex stainless steel containing no yttrium and having only a high nitrogen content.
FIG. 3 is a graph showing the results of a hot rolling test performed at 1,200 ^o C for evaluating the hot-formability of the duplex stainless steel according to the present invention containing a high nitrogen content (0.3 wt%) and containing yttrium (0.3 wt%).

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims. Like reference numerals refer to like elements throughout the specification.

Hereinafter, a duplex stainless steel according to a preferred embodiment of the present invention will be described in detail.

The duplex stainless steel according to a preferred embodiment of the present invention may contain 0.2 to 0.5% of C by weight, 0.5 to 1.5% of acid-soluble Si, 0.5 to 1.5% of Mn, 20 to 28% of Cr, Ni : 8 to 10%, N: 0.1 to 0.4%, Y: 0.1 to 0.5%, balance Fe and other unavoidable impurities.

The duplex stainless steel has a high temperature strength at 1,200 캜 of 15 MPa or more.

In addition, the duplex stainless steel has an elongation at 1,200 ° C of 19% or more so that the hot-moldability can be secured.

The reason why the composition of the steel according to the present invention is limited to the above range will be described below.

Carbon (C) combines with carbide forming elements such as chromium (Cr), molybdenum (Mo), tungsten (W), niobium (Nb) and vanadium (V) to form carbides of high hardness, Although effective for strength, the excess C content reduces the corrosion resistance by forming excess carbides in the ferrite-austenitic system.

In the steel according to the present invention, when C is added in an amount of 0.5% or more, coarse chromium carbide easily precipitates in the grain boundaries to lower the chromium content around the grain boundaries, thereby reducing the corrosion resistance. Therefore, in the alloy of the present invention, the carbon content should be suppressed to 0.5% or less.

Silicon (Si) is mainly added to the raw material as a residual element, or at least 0.5% is added to increase the fluidity or deoxidization effect in the production of castings, but if it exceeds 1.5%, the mechanical properties related to toughness are greatly reduced Should be suppressed within this range.

Manganese (Mn) is mainly added to improve high temperature ductility in the present invention. Generally, it is known that when Mn is added in a large amount of Mn in stainless steel or steel material, ductility is deteriorated and impact toughness is also decreased. Therefore, the usual amount of addition should be added in the range of 0.5-1.5% do.

Manganese (Mn) can also be added for the reduction of nickel (Ni) addition. Reducing Ni, the most expensive additive element in duplex stainless steels, is economically advantageous. It is generally known that Mn exerts 50% of the ability of Ni to stabilize austenite.

The Mn content in the above range is suitable for improving the fluidity at the time of casting in the present invention so as to produce a thin and complicated shape by casting.

Nickel (Ni) is known as an element that forms austenite. It has been determined that nitrogen (N) and manganese (Mn) play a role in the stabilization of the austenite phase in the alloy of the present invention and is limited to 8.0-10.0% by balance with other ferrite stabilizing elements.

Chromium (Cr) is known as a ferrite stabilizing element and is an essential element added to improve corrosion resistance and to design a duplex structure of austenite and ferrite. The alloy system in high is less than the 20% Cr content because it is the main purpose to induce to have the corrosion resistance is high maintenance of the corrosion resistance becomes difficult when it exceeds 28% the formation of the sigma tends to increase brittleness and 475 ^o in the near C Causing low-temperature embrittlement.

Duplex stainless steel is characterized by adding a large amount of nitrogen (N) to stabilize austenite to reduce expensive Ni as well as manganese (Mn). And N, like W and Mo, increases corrosion resistance significantly by increasing formal resistance.

Normally, N contains about 0.02 as an impurity in stainless steel. However, in order to achieve the above object, N content is required to be at least 0.1% or more. When it exceeds 0.5%, the corrosion resistance is increased. However, in the case of ingot casting or continuous casting, Casting defects are caused to deteriorate the soundness of the material, so it is limited to 0.4% or less.

Further, in the present invention, when yttrium (Y) is added in an amount of 0.1% or more to the base alloy component, high temperature ductility can be increased and excellent hot workability can be obtained. However, when it exceeds 0.5%, the high temperature ductility is deteriorated due to the localization of coarse oxide inclusions.

By using the alloy material of the present invention, it is possible to manufacture a thermal reduction reactor for magnesium smelting, which is used at a temperature of 1,200 ^o C, through casting and hot working, and can perform reliably at a temperature of 1,200 ^° C .

Hereinafter, a duplex stainless steel according to the present invention will be described in detail with reference to embodiments.

[Example]

A steel having the composition shown in Table 1 below was prepared as an ingot having a width of 400 m, a height of 400 mm and a thickness of 150 mm, and subjected to a high temperature tensile and hot rolling test at 1,200 ^° C. The ingots were homogenized and annealed in a heat treatment furnace for 1 hour before the test.

Table 1 shows the composition of the ingot prepared to have the alloy component according to the present invention and the results of the high temperature tensile and hot rolling tests performed at 1,200 ^° C.

alloy
Kinds Chemical composition (wt.%) Elongation Seal
burglar crack
Occur C Si Mn Cr Ni N Y Al P S (%) (MPa) Whether Comparative Example 1 0.38 1.4 0.87 24.9 7.8 - - - - 38 14.1 X Comparative Example 2 0.39 1.3 0.89 24.8 7.9 0.31 - - - 14 15.1 O Comparative Example 3 0.36 1.4 0.98 25.2 8.0 - 0.3 - - 38 14.5 X Inventory 1 0.35 1.2 1.00 25.1 7.9 0.32 0.3 - - 19 19.4 X Comparative Example 4 0.35 1.4 0.99 24.9 20.1 - - - - 29 9.2 X Comparative Example 5 0.37 1.3 0.95 26.0 10.1 - - - - 67 18.2 X Comparative Example 6 0.39 1.4 0.92 25.9 9.9 0.12 - - - 47 22.2 O Comparative Example 7 0.43 1.5 0.94 26.0 10.0 0.21 - - - 46 24.9 O Comparative Example 8 0.42 1.5 0.96 25.9 10.2 0.11 - - - 55 23.5 O Comparative Example 9 0.35 1.5 0.95 20.1 - 0.09 0.1 5.0 - 16 4.01 O Comparative Example 10 0.32 1.4 1.02 26.0 10.2 - 0.3 - - 13 15.7 X Inventory 2 0.36 1.2 1.05 25.8 9.9 0.1 0.29 - - 86 15.8 X Inventory 3 0.36 1.3 0.94 26.4 10.1 0.21 0.31 - - 58 22.2 X Honorable 4 0.35 1.4 0.98 26.2 10.2 0.3 0.32 - - 22 29.9 X Inventory 5 0.32 1.4 0.98 25.7 9.9 0.31 0.1 - - 59 24.9 X Inventory 6 0.36 1.4 0.91 25.7 9.8 0.21 0.19 - - 67 22.2 X

FIGS. 1 to 3 show the results of Comparative Examples 1 and 2 and Inventive Example 1, respectively, of the results of the hot rolling test at 1,200 ^o C for evaluating the hot moldability. As in Comparative Example 2, when nitrogen alone was added, cracks were generated in hot rolling. However, by adding yttrium additionally, a sound hot rolled plate was obtained.

As shown in Comparative Example 2 of FIG. 2, when nitrogen (N) alone is added, the strength is as high as about 15 MPa, but the formability is poor to such an extent that hot rolling is impossible.

3, when yttrium was added together with nitrogen, the high-temperature strength was as high as 19 MPa and the hot-moldability was also excellent.

In Examples 2 to 6 of Table 1, it can be seen that when both nitrogen and yttrium are added, the hot-moldability and the hot-moldability are improved. Especially, as the nitrogen content increased, the high temperature strength was also improved.

Comparative Example 4, which is the most commonly used austenitic stainless steel in an industrial field, has a tensile strength of about 9 MPa at a temperature of 1,200 ^° C. On the other hand, in the case of Comparative Example 9 which is a heat resistant alloy, the tensile strength is only 4 MPa.

On the contrary, Inventive Example 1 shown in FIG. 3, which is a steel material according to the present invention, shows a tensile strength of about 19 MPa, which is twice that of a conventional 300 series alloy.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

Claims

0.5 to 1.5% of Cr, 20 to 28% of Cr, 8 to 10% of Ni and 0.1 to 0.4% of N in weight percent (wt%) of C, , Y: 0.1 to 0.5%, balance Fe and other unavoidable impurities.

The method according to claim 1,
Wherein the high-temperature strength at 1,200 ° C is at least 15 MPa.

The method according to claim 1,
And an elongation at 1,200 DEG C of 19% or more.

Characterized in that it is used in at least one selected from the group consisting of a thermal reduction reaction tube for magnesium smelting, a pipeline for chemical plant, a component for dechlorination and desulfurization facility of thermal power plant, a component for dechlorination and desulfurization facility of petrochemical industry, Wherein the stainless steel is a stainless steel.