JP2014051739A - Method of manufacturing two-phase stainless steel using post heat treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing two-phase stainless steel excellent in mechanical physical properties by reducing contents of alloy components such as nickel and forming ferrite and austenite with using a post heat treatment at the same time.SOLUTION: A method of manufacturing a two-phase stainless steel includes melting a mother alloy containing, by wt.%, C:0.05% or less, N:0.3 to 0.5%, Cr:17.5 to 18.5%, Mn:9.5 to 10.5%, Si:0.01 to 0.1% and the balance iron with inevitable impurities in vacuum with a vacuum melting furnace to control a nitrogen content and manufacture a semifinished steel, and rolling the semifinished steel, heating the rolled steel with a velocity of 300 to 450°C/sec., appling it to annealing treatment at 1200 to 1350°C, and cooling with a velocity of 30 to 75°C/sec. to form a two-phase structure containing austenite of 40 to 75 vol.% and ferrite of 25 to 60 vol.%.

Description

  The present invention relates to a method for producing stainless steel, and more particularly to a method for producing duplex stainless steel using post-heat treatment.

  Stainless steel is roughly classified into austenitic stainless steel, ferritic stainless steel, and duplex stainless steel depending on the microstructure.

  Among these, the duplex stainless steel is a stainless steel that simultaneously secures resistance to stress corrosion cracking and mechanical strength by including about 50% of ferrite and austenite. Such a duplex stainless steel has advantages such as a reduction in maintenance costs when structural materials are applied because it has superior corrosion resistance and mechanical properties compared to existing carbon steel and austenitic stainless steel. It is used in many fields.

  The duplex stainless steel usually contains a large amount of corrosion resistance improving elements such as nickel (Ni), chromium (Cr), and molybdenum (Mo). In particular, duplex stainless steel contains nickel that is lower than austenitic stainless steel, but it contains a large amount of nickel compared to carbon steel, which raises manufacturing costs and environmental pollution. It can be awakened.

  As background art related to the present invention, there is a super duplex stainless steel excellent in corrosion resistance, embrittlement resistance, castability and hot workability, which is disclosed in Patent Document 1, in which the formation of intermetallic phases is suppressed. .

Korean Published Patent Publication No. 10-2003-0077239 (2003. 10.01. Published)

  An object of the present invention is to provide a method for producing a duplex stainless steel having excellent mechanical properties by lowering the content of expensive alloy components such as nickel and simultaneously forming ferrite and austenite using post heat treatment. That is.

  In order to achieve the above object, a method for producing a duplex stainless steel according to an embodiment of the present invention comprises: producing a semi-finished steel having a stainless-based alloy composition; rolling the semi-finished steel; Heat treating the formed steel to adjust the austenite and ferrite fraction.

  At this time, the semi-finished steel is, by weight, carbon (C): 0.05% or less, nitrogen (N): 0.3% to 0.5%, chromium (Cr): 17.5% to 18. 5%, manganese (Mn): 9.5% to 10.5%, silicon (Si): 0.01% to 0.1%, and the remainder may be composed of iron (Fe) and inevitable impurities. In this case, the heat treatment includes a temperature raising step, an annealing step, and a cooling step, and the annealing step is preferably performed at 1200 ° C. to 1350 ° C.

  In order to achieve the above object, a method for producing a duplex stainless steel according to another embodiment of the present invention includes carbon (C): 0.05% or less, nitrogen (N): 0.3% to 0.3% by weight. 0.5%, chromium (Cr): 17.5% to 18.5%, manganese (Mn): 9.5% to 10.5%, silicon (Si): 0.01% to 0.1% and The remainder is to produce a semi-finished steel consisting of iron (Fe) and inevitable impurities; rolling the semi-finished steel; and heat treating the rolled steel to austenite 40 vol% to 75 vol% and ferrite 25 vol% Or forming a two-phase structure containing 60 vol%.

  At this time, the heat treatment includes a temperature raising step, an annealing step, and a cooling step, and the annealing step is preferably performed at 1200 ° C. to 1350 ° C.

  The method for producing a duplex stainless steel according to the present invention eliminates the addition of nickel (Ni), which is an expensive element, greatly reduces the content of chromium (Cr), and this can cause austenite stabilization problems and reduced strength. The problem can be solved through the inclusion of manganese (Mn) and nitrogen (N) and post heat treatment.

  In addition, according to the method for producing a duplex stainless steel according to the present invention, the ferrite and austenite fractions can be freely changed depending on the annealing temperature, and thereby a duplex stainless steel having various physical properties can be produced. Therefore, the duplex stainless steel produced by the method according to the present invention can be applied to various fields such as chemical plants and ships.

It is a flowchart showing roughly the manufacturing method of the duplex stainless steel concerning the example of the present invention. It is a graph showing the austenite and ferrite fraction change by temperature in the alloy composition applied to this invention. It is a microstructure picture of the steel specimen 1 applicable to a comparative example. It is a microstructure photograph of the steel specimen 2 applicable to a comparative example. It is a microstructure photograph of the steel specimen 3 applicable to an Example. It is a microstructure photograph of the steel specimen 4 applicable to an Example. It is a microstructure photograph of the steel specimen 5 applicable to an Example.

Hereinafter, a method for producing a duplex stainless steel using post-heat treatment according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a flow chart schematically illustrating a method for producing a duplex stainless steel according to an embodiment of the present invention.

Referring to FIG. 1, the method for producing a duplex stainless steel according to the present invention includes a semi-finished steel production step (S110), a rolling step (S120), and a post heat treatment step (S130).
In the semi-finished steel production stage (S110), semi-finished steel having a stainless alloy composition is produced.

  More preferably, the composition of the stainless steel alloy applied to the present invention is, by weight, carbon (C): 0.05% or less, nitrogen (N): 0.3% to 0.5%, chromium (Cr). : 17.5% to 18.5%, Manganese (Mn): 9.5% to 10.5%, Silicon (Si): 0.01% to 0.1%, and the rest is inevitable with iron (Fe) It can consist of impurities.

  In the case of the stainless steel alloy composition, the addition of nickel (Ni) and molybdenum (Mo) is eliminated and the price competitiveness is high, and the chromium (Cr) content is 17.5 wt% to 18.5 wt%. It is characterized by being relatively lower. Further, in the case of the alloy composition, manganese (Mn): 9.5 wt% to 10.5 wt% and nitrogen (N): 0 to prevent the austenite stability from being lowered due to a low chromium content or the like. .3 wt% to 0.5 wt% is included.

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

  Here, the inevitable impurities are components inevitably included in the steel making process and the like that are not intentionally included. Nickel (Ni): 0.01 wt% or less, phosphorus (P): 0.04 It may be less than or equal to wt%, sulfur (S): 0.01% or less, aluminum (Al): 0.05 wt% or less. Of the elements that can be included as impurities, nickel causes environmental problems, phosphorus and sulfur cause mechanical properties to deteriorate, and aluminum reduces the effect of nitrogen addition. Therefore, it is preferably removed during steelmaking, and remains as an impurity. Even so, it is preferable to be limited to the above range.

  Hereinafter, the alloy composition in which the duplex stainless steel according to the present invention is applied to the production method will be described.

Carbon (C)
Carbon (C) helps improve the mechanical properties and high temperature strength of stainless steel. The carbon is preferably contained at 0.05% by weight or less of the total weight of the steel. When the carbon content exceeds 0.05% by weight, corrosion resistance is reduced by precipitation of chromium carbide (Cr ₂₃ C ₆ ) at the grain boundaries.

Nitrogen (N)
Nitrogen (N) is an austenite stabilizing element that can replace nickel, and can greatly increase pitting resistance. The nitrogen is preferably included at 0.3 wt% to 0.5 wt% of the total weight of the steel. If the nitrogen content is less than 0.3% by weight, the effect of addition is not sufficient. Conversely, if the nitrogen content exceeds 0.5% by weight, it is difficult to control the nitrogen content during steelmaking under normal pressure.

Chrome (Cr)
Chromium (Cr) helps to improve corrosion resistance by forming a chromium oxide layer on the steel surface.
The chromium is preferably included at 17.5 wt% to 18.5 wt% of the total weight of the steel. If the chromium content is less than 17.5% by weight, the effect of addition is not sufficient. Conversely, if the chromium content exceeds 18.5% by weight, there is a difficulty in controlling the two-phase structure due to increased ferrite stability.

Manganese (Mn)
Manganese (Mn) is an effective element for stabilizing austenite and increasing nitrogen solid solubility. The manganese is preferably contained in an amount of 9.5 to 10.5% by weight based on the total weight of the steel. When the manganese content is less than 9.5% by weight, it is difficult to ensure austenite stability, considering that nickel is not added in the present invention. On the other hand, if the manganese content exceeds 10.5% by weight, the austenite stability is increased, thereby causing difficulty in controlling the two-phase structure and reducing the corrosion resistance.

Silicon (Si)
Silicon (Si) is a ferrite forming element and acts as a deoxidizer. The silicon is preferably contained in an amount of 0.01% to 0.1% by weight of the total steel weight. If the silicon content is less than 0.01% by weight, the effect of addition is not sufficient. On the other hand, when silicon is excessively added exceeding 0.1% by weight, there is a problem that the toughness of the steel is lowered.

  The semi-finished steel made of the alloy component can be in the form of a slab, ingot, billet or the like.

  Such semi-finished steel can be manufactured by various methods, and a vacuum melting method can be presented as an example.

  The vacuum melting can be performed in a series of processes including a master alloy charging stage, a vacuum maintaining stage, a master alloy melting stage, a nitrogen content adjusting stage, a molten alloy stirring stage, and a semi-finished steel forming stage.

  In the mother alloy charging stage, an attempt is made to produce a mother alloy such as electrolytic iron, Fe-50 wt% Mn, Fe-60 wt% Cr, Fe-58.8 wt% Cr-6.6 wt% N, etc. After weighing according to the alloy composition ratio of the duplex stainless steel to be performed, it is charged into a vacuum melting furnace.

Next, in the vacuum maintenance step, the inside of the vacuum melting furnace is maintained at a vacuum of about 10 ⁻³ torr or less for vacuum melting.

  Next, in the master alloy melting stage, the vacuum melting furnace is heated to about 1600 ° C. or higher through electric resistance heating, induction heating, etc., and the master 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 wt% to 0.3 wt% through nitrogen gas injection. At this time, in order to obtain a target nitrogen content, the inside of the vacuum melting furnace can be pressurized, but this does not necessarily have to be performed.

  Next, in the molten alloy agitation step, the molten alloy is agitated to remove the alloy element schist and to suppress the generation of the alloy element schist.

  Next, in the semi-finished steel forming stage, the molten alloy is discharged from the inside of the vacuum melting furnace to form semi-finished steel in the form of an ingot, slab, billet or the like.

Next, in the rolling stage (S120), the semi-finished steel is rolled.
Rolling may include reheating and heating the semi-finished steel in an austenitic single phase region and hot rolling the reheated semi-finished steel at about 900 ° C to 1000 ° C. After hot rolling, a process of quenching to room temperature can be performed.

Next, in the post heat treatment step (S130), the rolled steel is heat treated to adjust the austenite and ferrite fractions.
At this time, the heat treatment may include a heating step, an annealing step, and a cooling step.

In the present invention, the annealing step is performed at 1200 ° C. to 1350 ° C. within about 1 hour.
Under such annealing conditions, a two-phase structure including austenite 40 vol% to 75 vol% and ferrite 25 vol% to 60 vol% can be formed. This can be more clearly understood with reference to FIG.

  FIG. 2 is a graph showing changes in the austenite and ferrite fraction with temperature in the 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 vol% to 60 vol% when the annealing temperature is 1200 ° C. or higher and 1350 ° C. or lower during the post heat treatment. If the annealing temperature is less than 1200 ° C. or exceeds 1350 ° C., the ferrite fraction becomes too low or too high, making it difficult to exhibit the characteristics of the duplex stainless steel.

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

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

EXAMPLES Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred examples of the present invention. However, this is presented as a preferred illustration of the present invention and should not be construed as limiting the invention in any way. Since the contents not described here can be technically analogized by those skilled in the art, the description thereof will be omitted.

1. Production of Steel Specimens Steel specimens 1 to 5 having the composition shown in Table 1 and the remaining iron were produced.

  Steel specimens 1 and 2 were manufactured by reheating an ingot made of the corresponding alloy composition at 1000 ° C. for 1 hour, hot rolling to 900 ° C., and then quenching to room temperature.

Steel specimens 3 to 5 are the same as steel specimens 1 and 2 until the ingot composed of the corresponding alloy composition is reheated at 1000 ° C. for 1 hour, hot rolled to 900 ° C., and quenched to room temperature. After quenching, post-heat treatment was performed under the conditions described in Table 2.

2. Evaluation of Physical Properties Steel specimens 1 to 5 were subjected to a tensile test using an ASTM specimen, and the results are shown in Table 3.

  Referring to Table 3, the steel specimens 3 to 5 manufactured by the method including post-heat treatment presented in the present invention are not subjected to post-heat treatment even though elements such as nickel and molybdenum are not added. It can be seen that the strength characteristics are relatively superior to those of the steel specimens 1 and 2.

3. Microstructure Evaluation FIGS. 3 and 4 are photographs of the microstructure of steel specimen 1 (FIG. 3) and steel specimen 2 (FIG. 4) corresponding to the comparative example. 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. 7) corresponding to the embodiment.

  3 to 7, it can be seen that all the steel specimens 1 to 5 have a two-phase structure of ferrite and austenite. That is, as in the case of steel specimens 1 and 2, when a large amount of chromium, nickel, and molybdenum is contained, a two-phase structure can be formed. However, when heat treatment is performed after the present invention is presented, such nickel and molybdenum And a two-phase structure can be formed even when the chromium content is low.

  On the other hand, referring to FIGS. 3 to 5, steel specimens 1 to 2 (FIGS. 3 to 4) represent a typical duplex stainless steel structure, whereas steel specimens 3 to 5 (FIGS. 5 to 7). Represents a two-phase stainless steel structure in which ferrite is formed at the austenite grain boundaries.

  The present invention has been described with reference to the embodiments. However, the embodiments are merely illustrative, and various modifications and equivalent other implementations may be performed by those having ordinary knowledge in the art. I think you can understand that an example is possible. Therefore, the true technical protection scope of the present invention must be defined by the claims.

S110: Semi-finished steel manufacturing stage S120: Rolling stage S130: Post heat treatment stage

Claims (12)

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