KR20140032061A - Method of manufacturing duplex stainless steel using post heat treatment - Google Patents

Abstract

Disclosed is a method of manufacturing duplex stainless steel using a post heat treatment process. According to the present invention, a method of manufacturing duplex stainless steel comprises: a step for manufacturing half-finished product steel including in wt%, 0.05% or less of carbon (C), 0.3-0.5% of nitrogen (N), 17.5-18.5% of chrome (Cr), 9.5-10.5% of manganese (Mn), 0.01-0.1% of silicon (Si), and residues comprising iron (Fe) and other unavoidable impurities; a step for rolling the half-finished product steel; and a step for forming a duplex structure including 40-75 vol% of austenite and 25-60 vol% of ferrite by heat-treating the rolled steel at 1200-1350°C. [Reference numerals] (AA) Start; (BB) End; (S110) Vacuum melting; (S120) Rolling; (S130) Heat treating

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a two-phase stainless steel using post heat treatment,

The present invention relates to a method of manufacturing stainless steel, and more particularly, to a method of manufacturing a two-phase stainless steel using post heat treatment.

Stainless steels are classified into austenitic stainless steels, ferritic stainless steels, and two-phase stainless steels depending on the microstructure.

Of these, the two-phase stainless steel is a stainless steel which contains both ferrite and austenite at about 50% or more, thereby securing resistance against stress corrosion cracking and mechanical strength at the same time. These two-phase stainless steels have superior corrosion resistance and mechanical characteristics compared to conventional carbon steels and austenitic stainless steels, and thus have advantages in reducing the maintenance cost when applying structural materials, and are used in many fields.

The two-phase stainless steel generally contains a large amount of elements for improving corrosion resistance such as nickel (Ni), chrome (Cr), and molybdenum (Mo). In particular, the two-phase stainless steel contains low nickel compared to the austenitic stainless steel but contains a large amount of nickel as compared with carbon steel, which may increase the manufacturing cost and cause environmental pollution.

Background art related to the present invention is a super duplex stainless steel excellent in corrosion resistance, corrosion resistance, castability and hot workability, which is inhibited from formation of an intermetallic phase disclosed in Korean Patent Laid-Open Publication No. 10-2003-0077239 (published on October 1, 2003) There is a river.

It is an object of the present invention to provide a method for manufacturing a two-phase stainless steel excellent in mechanical properties by lowering the content of an expensive alloy component such as nickel and forming a ferrite and austenite by post heat treatment.

According to another aspect of the present invention, there is provided a method of manufacturing a two-phase stainless steel including the steps of: preparing a semi-finished steel having a stainless steel alloy composition; Rolling the semi-finished steel; And heat treating the rolled steel to adjust the austenite and ferrite content.

At this time, the semi-finished steel in weight%, carbon (C): 0.05% or less, nitrogen (N): 0.3 ~ 0.5%, chromium (Cr): 17.5 ~ 18.5%, manganese (Mn): 9.5 ~ 10.5%, silicon (Si): 0.01 to 0.1% and may be composed of the remaining iron (Fe) and inevitable impurities. In this case, it is preferable that the heat treatment includes a temperature elevating step, an annealing step and a cooling step, and the annealing step is performed at 1200 to 1350 ° C.

Two-phase stainless steel manufacturing method according to another embodiment of the present invention for achieving the above object by weight, carbon (C): 0.05% or less, nitrogen (N): 0.3 ~ 0.5%, chromium (Cr): 17.5 Preparing a semi-finished steel comprising 18.5%, manganese (Mn): 9.5-10.5%, silicon (Si): 0.01-0.1% and the remaining iron (Fe) and unavoidable impurities; Rolling the semi-finished steel; And heat treating the rolled steel to form a two-phase structure containing 40 to 75 vol% of austenite and 25 to 60 vol% of ferrite.

In this case, the annealing may include a temperature elevation step, an annealing step, and a cooling step, and the annealing step may be performed at 1200 to 1350 ° C.

The method of manufacturing a two-phase stainless steel according to the present invention excludes the addition of nickel (Ni), which is an expensive element, and greatly reduces the content of chromium (Cr), and the problems of austenite stabilization and strength decrease, (Mn) and nitrogen (N) and post heat treatment.

Further, according to the method for producing a two-phase stainless steel according to the present invention, the ferrite and the austenite fractions can be freely changed according to the annealing temperature, and thus a two-phase stainless steel having various physical properties can be manufactured. Accordingly, the two-phase stainless steel manufactured by the method according to the present invention can be applied to various fields such as chemical plants and ships.

1 is a flowchart schematically showing a method of manufacturing a two-phase stainless steel according to an embodiment of the present invention.
2 is a graph showing a change in austenitic and ferrite content with temperature in an alloy composition applied to the present invention.
3 and 4 are photographs of microstructures of the steel specimens 1 and 2 according to comparative examples.
5 to 7 are microstructural photographs of the steel specimens 3 to 5 corresponding to the examples.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described in detail below. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

Hereinafter, a method for manufacturing a two-phase stainless steel using a post-heat treatment according to the present invention will be described in detail with reference to the accompanying drawings.

1 is a flowchart schematically showing a method of manufacturing a two-phase stainless steel according to an embodiment of the present invention.

Referring to FIG. 1, a method for manufacturing a two-phase stainless steel according to the present invention includes a semi-finished steel making step S110, a rolling step S120, and a post-annealing step S130.

In the semi-finished steel production step (S110), a semi-finished steel having a stainless steel-based alloy composition is produced.

More preferably, the stainless steel alloy composition to be applied in the present invention is a weight%, carbon (C): 0.05% or less, nitrogen (N): 0.3-0.5%, chromium (Cr): 17.5-18.5%, manganese (Mn) ): 9.5 ~ 10.5%, silicon (Si): 0.01 ~ 0.1% and may be composed of the remaining iron (Fe) and inevitable impurities.

In the case of the stainless steel-based alloy composition, the addition of nickel (Ni) and molybdenum (Mo) is excluded so that the price competitiveness is high and the content of chromium (Cr) is 17.5 to 18.5% by weight. Further, in order to prevent the austenite stability from being deteriorated due to a low chromium content or the like, the above alloy composition is characterized in that manganese (Mn): 9.5 to 10.5 wt% and nitrogen (N): 0.3 to 0.5 wt% have.

More specifically, the present invention includes carbon (C), nitrogen (N), chromium (Cr), manganese (Mn), and silicon (Si). The remainder in addition to the above components are iron (Fe) and inevitable impurities.

Herein, unavoidable impurities are not intentionally contained but are inevitably included in the steelmaking process and include nickel (Ni): 0.01 wt% or less, phosphorus (P): 0.04 wt% or less, sulfur (S) Aluminum (Al): 0.05 weight% or less, and the like. Of the elements that can be included as impurities, nickel causes environmental problems. In the case of phosphorus and sulfur, it causes deterioration of mechanical properties. In the case of aluminum, it deteriorates the nitrogen addition effect. Therefore, it is preferable that nickel is removed during steelmaking. Range.

Hereinafter, the alloy composition applied to the manufacturing method of the two-phase stainless steel according to the present invention will be described.

Carbon (C)

Carbon (C) contributes to mechanical properties and high temperature strength of stainless steel. The carbon is preferably contained in 0.05% by weight or less of the total weight of the steel. When the content of carbon exceeds 0.05% by weight, corrosion resistance is deteriorated by precipitation of chromium carbide (Cr ₂₃ C ₆ ) on the grain boundary.

Nitrogen (N)

Nitrogen (N) is an austenite stabilizing element, an element that can replace nickel, and can greatly increase the resistance to formaldehyde. The nitrogen is preferably contained in an amount of 0.3 to 0.5 wt% of the total weight of the steel. When the content of nitrogen is less than 0.3% by weight, the effect of the addition is insufficient. Conversely, when the content of nitrogen exceeds 0.5 wt%, it is difficult to control the nitrogen content at the time of steel making under normal pressure.

Chromium (Cr)

Chromium (Cr) creates a chromium oxide layer on the surface of the steel, contributing to the improvement of corrosion resistance.

The chromium is preferably contained in an amount of 17.5 to 18.5% by weight of the total weight of the steel. If the content of chromium is less than 17.5% by weight, the effect of the addition is insufficient. On the other hand, when the content of chromium exceeds 18.5% by weight, ferrite stability is increased, which makes it difficult to control the two-phase structure.

Manganese (Mn)

Manganese (Mn) is an element that stabilizes austenite and is effective in increasing nitrogen solubility. The manganese is preferably contained in an amount of 9.5 to 10.5% by weight of the total weight of the steel. When the content of manganese is less than 9.5% by weight, it may be difficult to secure the austenite stability in consideration of the fact that nickel is not added in the present invention. On the other hand, when the content of manganese exceeds 10.5% by weight, the austenite stability is increased, thereby making it difficult to control the two-phase structure and reducing the corrosion resistance.

Silicon (Si)

Silicon (Si) is a ferrite-forming element and also acts as a deoxidizer. The silicon is preferably contained in an amount of 0.01 to 0.1% by weight based on the total weight of the steel. If the content of silicon is less than 0.01% by weight, the effect of the addition is insufficient. Conversely, if the silicon content exceeds 0.1% by weight There is a problem that the toughness of the steel is lowered.

The semi-finished steel comprising the alloy components may be in the form of a slab, ingot, billet, or the like.

Such semi-finished steel can be manufactured in various ways, and one example is a vacuum melting method.

Vacuum melting can be performed by a series of processes including a parent alloy charging step, a vacuum holding step, a mother alloy melting step, a nitrogen content adjusting step, a molten alloy stirring step, and a semi-product steel forming step.

In the parent alloy charging step, parent alloys such as electrolytic iron, Fe-50 weight% Mn, Fe-60 weight% Cr, Fe-58.8 weight% Cr-6.6 weight% N and the like were weighed according to the alloy composition ratio of the two- And then charged into a vacuum melting furnace.

Next, in the vacuum holding step, the inside of the vacuum melting furnace is maintained at a vacuum of about 10 ^-3 torr or less for the purpose of solution dissolution.

Next, in the mother alloy melting step, the vacuum melting furnace is heated to about 1600 DEG C or higher through electric resistance heating, induction heating, etc., and the mother alloy is melted to form a molten alloy.

Next, in the nitrogen content adjusting step, the nitrogen content of the molten alloy is adjusted to 0.1 to 0.3 wt% by injecting nitrogen gas. At this time, it is possible to pressurize the inside of the vacuum melting furnace to obtain the target nitrogen content, but this is not necessarily performed.

Next, in the molten alloy stirring step, the molten alloy is stirred to remove the segregation of the alloying element or to suppress the occurrence of the alloying element segregation.

Next, in the semi-finished steel forming step, a molten alloy is spouted from the inside of the vacuum melting furnace to form semi-finished steel in the form of ingots, slabs, billets and the like.

Next, in the rolling step (S210), the semi-finished steel is rolled.

Rolling may include reheating the semi-finished steel to austenite single phase and homogenizing the hot semi-finished steel, and hot rolling the reheated semi-finished steel at approximately 1000 to 900 ° C. After the hot rolling, a process of quenching to room temperature may be performed.

Next, in the post-heat treatment step S130, the rolled steel is heat-treated to adjust the austenite and ferrite content.

At this time, the heat treatment may include a temperature rising step, an annealing step and a cooling step.

In the present invention, the annealing step is performed at a temperature of 1200 to 1350 DEG C within approximately one hour.

Under these annealing conditions, it is possible to form a two phase structure comprising 40 to 75 vol% of austenite and 25 to 60 vol% of ferrite. This can be more clearly understood with reference to FIG.

FIG. 2 is a graph showing a change in austenite and ferrite fraction according to temperature in an alloy composition applied to the present invention, and a Thermo-Calc program (Thermo-Calc Software Inc.) was used.

Referring to FIG. 2, the ferrite fraction may be 25 to 60 vol% at an annealing temperature of 1200 ° C. or higher and 1350 ° C. or lower in the post heat treatment. When the annealing temperature is lower than 1200 占 폚 or exceeds 1350 占 폚, the ferrite fraction is excessively low or high and it is difficult to exhibit the characteristics of the two-phase stainless steel.

On the other hand, it is more preferable that the temperature increase to the annealing temperature during the post-heat treatment is performed at about 100 to 400 ° C at a rate of 300 to 450 ° C / sec. If the heating rate is less than 300 ° C / sec, the strength of the steel produced by crystal grain coarsening may be reduced. On the other hand, if the heating rate exceeds 450 ° C / sec, the manufacturing cost of the steel may be greatly increased due to excessive heating, and the heating rate control may become difficult.

Further, it is more preferable that cooling after the annealing in the post heat treatment is performed at a rate of 30 to 75 ° C / sec. If the cooling rate is less than 30 ° C / sec, unwanted phase transformation may occur. Conversely, when the cooling rate exceeds 75 DEG C / sec, it is difficult to control the cooling rate.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Fabrication of Steel Specimen

Steel specimens 1 to 5 made of the composition and the balance iron shown in Table 1 were prepared.

[Table 1]

Steel specimens 1 and 2 were prepared by reheating ingots of the alloy composition at 1000 ° C for 1 hour, hot rolling to 900 ° C, and quenching to room temperature.

The steel specimens 3 to 5 were prepared by reheating the ingot having the alloy composition at 1000 캜 for 1 hour and hot rolling to 900 캜 and quenching to room temperature until the quenching was performed. Lt; RTI ID = 0.0 > 0 < / RTI >

[Table 2]

2. Property evaluation

Tensile tests based on ASTM test specimens were performed on the steel specimens 1 to 5, and the results are shown in Table 3.

 [Table 3]

Table 3 shows that in the case of the steel specimens 3 to 5 prepared by the method including the heat treatment as described in the present invention, even though no element such as nickel or molybdenum was added, The strength characteristics are relatively superior to those of Examples 1 and 2.

3. Microstructure assessment

Figs. 3 and 4 are microstructural photographs of the steel specimen 1 (Fig. 3) and the steel specimen 2 (Fig. 4) which are comparative examples. Figs. 5 to 7 are microstructural photographs of the steel specimen 3 (Fig. 5), the steel specimen 4 (Fig. 6) and the steel specimen 5 (Fig.

3 to 7, it can be seen that both of the steel specimens 1 to 5 have a ferrite and austenite two phase structure. That is, two-phase structure may be formed when chromium, nickel and molybdenum are contained in a large amount as in the steel specimens 1 and 2. However, when the heat treatment is performed after the present invention, nickel and molybdenum are excluded, It is possible to form a two-phase structure even when the content of the two-phase structure is lowered.

3 to 5, a typical two-phase stainless steel structure is shown for the steel specimens 1 to 2 (Figs. 3 to 4), while for steel specimens 3 to 5 (Figs. 5 to 7), austenite Phase stainless steel structure in which ferrite is formed in grain boundaries.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Accordingly, the true scope of protection of the present invention should be defined by the claims.

S110: Semi-finished steel making step
S120: rolling step
S130: Post-heat treatment step

Claims (12)

Preparing a semifinished steel having a stainless-based alloy composition;
Rolling the semi-finished steel; And
And heat treating the rolled steel to adjust the austenite and ferrite content.
The method of claim 1,
The semi-finished steel
By weight%, carbon (C): 0.05% or less, nitrogen (N): 0.3-0.5%, chromium (Cr): 17.5-18.5%, manganese (Mn): 9.5-10.5%, silicon (Si): 0.01- A method for producing a two-phase stainless steel, characterized by consisting of 0.1% and the remaining iron (Fe) and inevitable impurities.
3. The method of claim 2,
The heat treatment includes a temperature elevating step, an annealing step and a cooling step,
Wherein the annealing step is performed at 1200 to 1350 < 0 > C.
The method of claim 3,
The heating step
Wherein the annealing is performed at a rate of 300 to 450 ° C / sec.
The method of claim 3,
The cooling step
Wherein the annealing is performed at a rate of 30 to 75 ° C / sec.
By weight%, carbon (C): 0.05% or less, nitrogen (N): 0.3-0.5%, chromium (Cr): 17.5-18.5%, manganese (Mn): 9.5-10.5%, silicon (Si): 0.01- Preparing a semi-finished steel consisting of 0.1% and the remaining iron (Fe) and inevitable impurities;
Rolling the semi-finished steel; And
And heat treating the rolled steel to form a two-phase structure including 40 to 75 vol% of austenite and 25 to 60 vol% of ferrite.
The method according to claim 6,
The heat treatment includes a temperature elevating step, an annealing step and a cooling step,
Wherein the annealing step is performed at 1200 to 1350 < 0 > C.
8. The method of claim 7,
The heating step
Wherein the annealing is performed at a rate of 300 to 450 ° C / sec.
8. The method of claim 7,
The cooling step
Wherein the annealing is performed at a rate of 30 to 75 ° C / sec.
10. The method according to any one of claims 1 to 9,
The semi-finished steel
Wherein the stainless steel is produced by a vacuum dissolving method.
11. The method of claim 10,
The vacuum melting
Charging the parent alloy into a vacuum furnace,
Maintaining the inside of the vacuum melting furnace in vacuum;
Melting the parent alloy to form a molten alloy;
Adjusting the nitrogen content of the molten alloy;
Stirring the molten alloy;
And forming a semi-finished steel from the molten alloy.
10. The method according to any one of claims 1 to 9,
The rolling
Reheating the semi-finished steel;
And hot-rolling the reheated semi-finished steel.

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