KR20140069945A - Super ductile lean duplex stainless steel and manufacturing method thereof - Google Patents

Abstract

A highly ductile lean duplex stainless steel and a method for manufacturing the same are disclosed. In the method for manufacturing the highly ductile lean duplex stainless steel of the present invention for manufacturing a thin film by making molten steel to pass through a pair of casting rolls, nitrogen exceeding the nitrogen solubility limit which is contained in the molten steel is discharged to the outside through the casting rolls when coagulating.

Description

TECHNICAL FIELD [0001] The present invention relates to a high ductility duplex stainless steel and a manufacturing method thereof. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a high ductility duplex stainless steel and a method of manufacturing the same. More particularly, the present invention relates to a high ductility duplex stainless steel and a method of manufacturing the same. More particularly, the present invention relates to a high ductility duplex stainless steel, And a manufacturing method thereof.

In general, austenitic stainless steels having excellent processability and corrosion resistance are made of iron (Fe) as a base metal and contain chromium (Cr) and nickel (Ni) as main raw materials. Molybdenum (Mo) and copper And is being developed into a variety of steel types to meet various applications.

304 and 316 stainless steels, which are excellent in corrosion resistance and workability, contain expensive raw materials such as Ni and Mo, and 200 and 400 stainless steels have been discussed as alternatives. However, 200 and 400 stainless steels The steel is disadvantageous in that the formability and the corrosion resistance are less than the stainless steel of 300 series.

On the other hand, duplex stainless steels in which austenite phase and ferrite phase are mixed have all the advantages of austenitic and ferritic systems, and various types of duplex stainless steels have been developed to date.

United States Patent No. 5624504 (Apr. 29, 1997) discloses "high strength, high ductility duplex stainless steel and its manufacturing method ".

The present invention relates to a duplex stainless steel having a volume fraction of martensite-based structure having a particle average diameter of 10 占 퐉 of 20 to 95% and the remainder being a ferrite-based structure, Si: not more than 2.0%, Mn: not more than 4.0%, P: not more than 0.040%, S: not more than 0.010%, Ni: not more than 4.0%, Cr: not more than 10.0 to 20.0% And a balance of not more than 0.02% of O and not more than 4.0% of Cu and further not more than 0.20% of Al, not more than 3.0% of Mo, not more than 0.20% of REM, not more than 0.20% of Y, And other inevitable impurities.

Such duplex stainless steels have excellent corrosion resistance in a variety of corrosive environments and exhibit better corrosion resistance than austenitic stainless steels such as AISI 304 and 316.

However, since such duplex stainless steel contains high-priced elements such as Ni and Mo, the manufacturing cost is increased, which is disadvantageous from other steel types and price competitiveness.

Recently, in order to compensate for the disadvantages of this price competitiveness, a lean duplex system, which eliminates high-priced alloying elements such as Ni and Mo included in duplex stainless steels, adds low-cost alloying elements, There is growing interest in stainless steel.

However, such a lean duplex stainless steel is disadvantageous in that the hot workability is poor due to the difference in strength between ferritic and austenitic phases, and surface cracks and edge cracks are generated in large quantities.

Meanwhile, various studies and developments have been made to improve the hot workability of the duplex stainless steels and the linseed duplex stainless steels. Hereinafter, related arts related thereto will be introduced.

Japanese Unexamined Patent Publication No. 2005-271307 (Apr. 5, 2007) discloses "two-phase stainless steel excellent in hot workability ".

The present invention relates to a high corrosion resistant duplex stainless steel having excellent hot workability in spite of a large amount of N, and more particularly to a high corrosion resistant duplex stainless steel having a high C content of not more than 0.03%, Si: 0.1 - 2.0% 0.05 to less than 0.05%, S: not more than 0.03%, Cr: 20.0 to 30.0%, Ni: 1 to 11%, Cu: 0.05 to 3.0%, Nd: 0.005 to 0.5% , The content of P is 0.05% or less, and the content of S is 0.03% or less, the balance being Fe and at least one of impurities, As regards excellent two-phase stainless steels, it has been found that P is controlled to a very small amount and Nd is added to improve hot workability.

Chinese Patent No. 101613839 (December 30, 2009) discloses "high-nitrogen low-nickel duplex steel and its manufacturing method ".

The steel sheet according to claim 1, wherein the steel sheet contains 0.01 to 0.10% of C, 0.2 to 1.0% of Si, 4 to 12% of Mn, 18 to 23% of Cr, 0.05% or less of P, %, N: 0.2 to 0.4, 1.0% or less of any one of Mo, W and Cu, and the balance of Fe and inevitable impurities. The present invention relates to a high nitrogen low-nickel duplex steel having an increased nitrogen content, Thereby improving economic efficiency.

The present invention relates to a high manganese duplex stainless steel having improved hot workability and a method for producing the same, and more particularly, to a stainless steel duplex stainless steel having improved C: 0.1% or less, Si: 0.05-2.2% And the balance being Fe and unavoidable impurities, wherein the content of Mn is 2.1 to 7.8%, the content of Cr is 20 to 29%, the content of Ni is 3.0 to 9.5%, the content of N is 0.08 to 0.5, the content of Mo is 5.0% or less and the content of W is 1.2 to 8% Limiting the Cu content, and increasing the Mn content.

 Japanese Patent Laid-Open No. 1998-257018 (Oct. 10, 1998) discloses "a method for producing 22% Cr 2-phase stainless steel hot rolled steel strip ".

This is characterized in that when the Cr 2 phase stainless steel hot-rolled steel sheet containing 21 to 23% Cr by weight is produced, the slab is heated at 1050 to 1150 ° C and subjected to rough rolling and finish rolling to 900 ° C or higher.

However, the above-mentioned prior arts still contain a large amount of expensive Ni, Mo and the like, which causes a problem that the product cost increases.

In addition, with respect to the hot workability, if alloying elements are processed according to the conventional general continuous casting method, porosity in the slab due to the difference in nitrogen solubility occurs when solidified from a liquid phase to a solid phase (see FIG. 1) A large amount of defects are generated on the surface of the product in the reheating process and the hot rolling process as a follow-up process, and a separate grinding process is required to remove a large amount of defects. Thus, there is a disadvantage that an unnecessary load is applied to the entire process do.

Further, as shown in FIG. 2, there is a disadvantage in that the striped ferrite structure is formed in the transverse direction to increase the plastic anisotropy, so that the elongation per rolling direction does not satisfy the reference value.

It should be understood that the foregoing description of the background art is merely for the purpose of promoting an understanding of the background of the present invention and is not to be construed as adhering to the prior art already known to those skilled in the art.

United States Patent No. 5624504 (April 29, 1997) Japanese Patent Application Laid-Open No. 2005-271307 (April 5, 2007) Chinese Patent No. 101613839 (December 30, 2009) U.S. Published Patent Application No. 2003-398128 (March 18, 2004) Japanese Patent Laid-Open No. 1998-257018 (1998.9.10.)

In order to solve such a conventional problem, the present invention can control the content of N, Ni, Si and Cu to reduce cost and ensure corrosion resistance and ductility equal to or higher than that of austenitic stainless steel. The present invention is directed to a high ductility duplex stainless steel which solves problems of product defects due to internal pores and surface pores of a steel to be produced, and a manufacturing method thereof.

In order to accomplish the above object, the present invention provides a method of manufacturing a high ductility lineless duplex stainless steel by passing molten steel through a pair of casting rolls to produce a thin plate, Nitrogen exceeding nitrogen solubility limit is discharged to the outside through the casting roll at the time of solidification.

And at least one of the pair of casting rolls is formed with a nitrogen discharge channel in the circumferential direction.

Wherein the molten steel contains, by weight%, 0.08% or less of C, 0.2-3.0% of Si, 2.0-4.0% of Mn, 19.0-23.0% of Cr, 0.3-2.5% of Ni, %, Cu: 0.5 to 2.5%, and the balance being Fe and other unavoidable impurities.

The thin plate produced by passing through the pair of casting rolls is rolled by using an inline roller continuously arranged with the pair of casting rolls and subjected to hot rolling annealing, cold rolling and cold rolling annealing successively, and the cold rolling annealing temperature is And is characterized in that it proceeds in the range of 1000 to 1100 占 폚, but proceeds for at least 60 seconds at 1000 to 1050 占 폚 and at least 30 seconds at 1050 to 1100 占 폚.

And the surface of the casting roll is formed with irregularities of 15 to 25 탆.

A plurality of nitrogen discharge channels having a width of 50 to 500 탆 and a depth of 50 to 300 탆 are formed, and a distance between adjacent nitrogen discharge channels is 100 to 1000 탆.

In order to achieve the above object, the present invention provides a high ductility duplex stainless steel comprising, by weight%, 0.08% or less of C (excluding 0%), 0.2-3.0% of Si, 2.0-4.0% of Mn, Molybdenum containing about 23.0% of Ni, 0.3 to 2.5% of Ni, 0.2 to 0.3% of N, 0.5 to 2.5% of Cu and the balance of Fe and other unavoidable impurities is passed through a pair of casting rolls, And nitrogen exceeding the solubility limit of nitrogen contained in the molten steel at the time of solidifying the molten steel is discharged through the casting roll.

The austenite phase of 40 to 75% in volume fraction and the wasteite phase structure of 25 to 60% are formed in a non-directional manner so that plastic anisotropy can be minimized.

And has an elongation of 55% or more with respect to all directions.

The present invention can realize the following effects with the above-described technical configuration.

First, by controlling the alloy component and the N component of the high-priced elements Ni, Si, Cu, and Mo, it is possible to save resources and minimize the manufacturing cost.

Second, since the hot-rolling process can be omitted by utilizing the strip casting process, there is an advantage that the edge cracking and the surface cracking problem occurring in the hot rolling can be solved.

Third, the problem of edge cracking and surface cracking due to nitrogen over nitrogen solubility limit can be solved because the problem of internal pore and surface pore of the thin plate can be solved by discharging nitrogen above the nitrogen solubility limit during solidification in the casting process.

Fourth, there is an advantage that the rate of material failure is improved and the surface grinding process, which is essential for hot rolling, can be omitted.

Fifth, by optimizing annealing heat treatment conditions, it has an advantage of securing a corrosion resistance equal to or higher than that of 304 steel and an elongation of 55% or more.

FIG. 1 is a view showing inner pores generated in a thin plate manufactured using a conventional continuous casting method and a general strip casting process,
FIG. 2 is a view showing the internal microstructure of a thin sheet produced by a conventional continuous casting method and a hot rolling step, FIG.
3 is a schematic illustration of a strip casting process of the present invention,
4 is a schematic view of the nitrogen discharge channel formed in the casting roll of the present invention,
5A is a view showing a dent defect formed on a thin plate by a large amount of pores,
FIG. 5B is a view showing a surface defect state of the high ductility duplex stainless steel of the present invention,
Fig. 6 is a graph comparing the hot workability of the duplex steel and the lean duplex steel,
7 is a view showing a state in which a lean duplex steel is hot rolled,
8 is a view showing the surface of a donut-softened duplex stainless steel manufactured by the present invention,
9 is a graph showing changes in elongation according to the cold-annealing temperature of the present invention,
10 is a graph showing a change in austenite phase fraction according to the cold-rolling annealing temperature of the present invention,
11 is a view showing a change in elongation according to a change in austenite phase fraction of the present invention,
12 is a graph showing the change in elongation according to the annealing temperature and time of the present invention,
13A is a view showing the internal structure of a highly ductile linseed duplex stainless steel manufactured by the present invention,
And Fig. 13B is a diagram showing elongation ratios according to directions.

Hereinafter, a high ductility duplex stainless steel according to a preferred embodiment of the present invention and a method of manufacturing the same will be described with reference to the accompanying drawings.

The high ductility duplex stainless steel of the present invention is a stainless steel having a two-phase structure of austenite phase and ferrite phase and is made of a high alloyed element such as nickel (Ni), molybdenum (Mo), silicon (Si) and copper By increasing the nitrogen content while lowering the content, it is possible to maintain the corrosion resistance equal to or higher than that of the austenitic stainless steel 304, and to achieve a certain level of ductility by applying the strip casting process.

In the method of manufacturing a high ductility duplex stainless steel according to the present invention, a strip casting process is applied to give a rapid cooling rate, so that a liquid steel is directly manufactured into a plate having a thickness of 2 to 5 mm. Unlike ordinary continuous casting or strip casting processes, it is possible to solve the internal pore and surface pore problems caused by the difference in the solubility limit of nitrogen by discharging nitrogen exceeding the nitrogen solubility limit in the molten steel during solidification in the casting process It is a technical feature.

As shown in FIG. 3, the highly ductile linseed duplex steel of the present invention is manufactured by using a strip casting process rather than a general continuous casting process.

The molten steel supplied from the ladle 1 to the tundish 2 passes between the pair of casting rolls 6 through the injection nozzle 3 so that the molten steel is rapidly cooled, The thin plate 8 is manufactured. This thin plate 8 is rolled in an inline roller (IRM) 9 which is disposed continuously with the casting roll 6 and wound around a coil winding facility 10.

On the other hand, on the upper side of the casting roll 6, a manifold shield 4 is mounted so as to prevent the surface of the molten metal from being oxidized by being in contact with air. The inside of the manifold shield 4 is filled with an appropriate gas An oxidation preventing atmosphere is formed.

As described above, the molten steel is rolled through the inline roller 9 while exiting the roll nip 7 where the pair of casting rolls 6 meet, and then is subjected to a heat treatment process and a cold rolling process to manufacture a thin plate having a thickness of 10 mm or less .

One of the most important technical elements in the twin-roll type strip casters for directly manufacturing the above-mentioned thin plates of 10 mm or less is the inner water-cooled twin-drum rolls (for example, 6) and the side dam (5), and the molten steel is rapidly cooled by discharging a large amount of heat through the surface of the water-cooled casting roll (6) so that the thin plate having a desired thickness can be manufactured .

In the method for producing a high ductility lean duplex steel of the present invention, the nitrogen problem above the solubility limit contained in the molten steel, which is the cause of edge cracking and surface cracking, and the problem of lowering hot workability due to the nitrogen content are solved.

That is, the rapid casting is completed while nitrogen exceeding the solid solubility limit is discharged through the casting roll 6, and the rapid casting is completed by using the inline rollers 9 continuously running after the casting, The above-described problems are solved by manufacturing a thin thin plate.

The high ductility duplex stainless steel according to the present invention comprises, by weight%, 0.08% or less of C, 0.2 to 3.0% of Si, 4.0 to 2.0% of Mn, 19.0 to 23.0% of Cr, To 2.5%, N: 0.2 to 0.3%, Cu: 0.5 to 2.5%, and the balance of Fe and other unavoidable impurities. Hereinafter, the above-described numerical reason for limitation will be described.

C is an element for forming an austenite phase, and is an effective element for increasing the strength of a material by solid solution strengthening.

However, in the case of excessive addition, it is easily bonded to a carbide forming element such as Cr effective for corrosion resistance at the ferrite-austenite phase boundary to reduce the corrosion resistance by lowering the Cr content around the grain boundary. Therefore, in order to maximize the corrosion resistance, .

Si is partially added for the deoxidizing effect, and is an element which is concentrated into ferrite upon annealing with an element for forming a ferrite phase.

Therefore, 0.2% or more is added in order to ensure proper ferrite phase fraction.

However, when 3.0% or more is added, the hardness of the ferrite phase is rapidly increased to lower the elongation, which makes it difficult to secure the austenite phase, which affects the elongation.

In addition, when the steel is over-added, the slag fluidity is lowered in the steelmaking process, and the steel is combined with oxygen to form inclusions and reduce corrosion resistance.

Therefore, the Si content is preferably limited to 0.2 to 3.0%.

N is an element contributing greatly to the stabilization of the austenite phase together with Ni in duplex stainless steel and is one of the elements which is concentrated in the austenite phase during annealing heat treatment.

Therefore, by increasing the N content, the corrosion resistance can be improved and the strength can be increased. However, since the solubility of N may vary depending on the content of Mn added, it is necessary to control the content thereof.

If the N content exceeds 0.3% in the Mn range of the present invention, blowholes and pinholes are generated during casting due to excess nitrogen solubility, thereby causing surface defects of the product have.

In order to secure corrosion resistance at the level of 304, N should be added in an amount of at least 0.2%, and if the N content is too low, it becomes difficult to secure an appropriate phase fraction.

Therefore, it is preferable to limit the N content to 0.20 to 0.30%.

Mn is an element that increases deoxidizing agent and nitrogen solubility and is added in place of expensive Ni as an austenite forming element.

If the Mn content is more than 4%, it becomes difficult to obtain the corrosion resistance at the level of 304. When Mn is added more than the above, it is effective to improve the nitrogen solubility, but MnS is formed by bonding with S in the steel, .

When the content of Mn is less than 2%, it is difficult to secure a proper austenite phase fraction even when Ni, Cu, N, etc. as the austenite forming elements are controlled, and the solubility of N added is low, Can not be obtained.

Therefore, it is preferable to limit the content of Mn to 2% to 4%.

Cr is a ferrite stabilizing element together with Si, which plays a major role in securing ferrite phase of two-phase stainless steel and is an essential element for securing corrosion resistance.

Increasing the Cr content increases the corrosion resistance, but there is a disadvantage that the content of expensive Ni and other austenite forming elements must be increased to maintain the phase fraction.

Therefore, the content of Cr is limited to 19 to 23% in order to maintain the phase fraction of the two-phase stainless steel and ensure corrosion resistance higher than 304 steel.

Ni is an austenite stabilizing element together with Mn, Cu and N, and plays a major role in securing the austenite phase of the duplex stainless steel.

In order to reduce the cost, instead of reducing the Ni content which is high in price, it is possible to increase the Mn and N, which are the other austenite phase forming elements, to sufficiently maintain the phase fraction balance by the reduction of Ni.

However, Ni is one of the major elements for securing the elongation rate during processing or molding by utilizing the fired organic martensite or mechanical twin generated during cold working. When Ni is excessively sintered organic martensite, the Ni is subjected to cold working, It is difficult to ensure the stability of the austenitic phase. In order to ensure the stability of the austenitic phase to such an extent as to inhibit the excessive formation of fired organic martensite, 0.3% or more should be added.

When a large amount of Ni is added, an austenite fraction is increased and it is difficult to obtain a proper austenite fraction. Moreover, austenite phase is too stabilized to suppress the generation of calcined organic martensite during cold working, It is difficult to secure competitiveness compared to the 304 steel.

Therefore, it is preferable to limit the Ni content to 0.3% to 2.5%.

In order to reduce the cost, it is desirable to reduce the Cu content, which has the same role as Ni, to a minimum. In order to secure sufficient austenite phase stability to suppress the formation of excess fired organic martensite during cold working, it should be added at least 0.5%.

On the other hand, if the Cu content is 2.5% or more, the product is difficult to process due to hot brittleness, and the Cu content is preferably adjusted to 0.5 to 2.5%.

On the other hand, the inventors of the present invention produced lean duplex steels with molten steel having the composition shown in the following Table 1, in order to confirm the effect of nitrogen exceeding the solubility limit in the molten steel.

In Comparative Example 1, molten steel having a specific composition was cast using a general continuous casting method. In Comparative Example 2, molten steel having a specific composition was cast using a general strip casting (rapid casting) method. In Examples 1 to 5 Is cast in a strip casting process while discharging nitrogen above the solid solubility limit in molten steel.

division C Si Mn Cr Ni Cu N Casting method Nitrogen emission Internal porosity Comparative Example 1 0.05 1.35 2.8 20.3 1.06 1.0 0.23 Continuous casting X ○ Comparative Example 2 0.05 1.35 2.8 20.3 1.06 1.0 0.23 Rapid casting X ○ Example 1 0.045 1.08 3.02 19.63 0.98 0.98 0.272 Rapid casting ○ X Example 2 0.071 1.3 3.81 19.69 1.14 0.5 0.24 Rapid casting ○ X Example 3 0.051 1.28 3.07 20.02 1.0 0.503 0.24 Rapid casting ○ X Example 4 0.051 1.27 3.09 20.41 1.03 0.5 0.25 Rapid casting ○ X Example 5 0.02 1.21 2.63 20.53 0.85 0.793 0.22 Rapid casting ○ X

As shown in FIG. 5A, in the case of Comparative Example 1 and Comparative Example 2, it was confirmed that a large amount of pores inside the thin plate were formed, and dent defects were present.

This is due to the difference in the solubility of nitrogen generated when molten steel passes through the casting roll and solidifies.

The nitrogen composition of the highly ductile duplex stainless steel of the present invention is in the range of 2000 to 3000 ppm.

The process of solidification of molten steel from liquid to solid phase proceeds in the order of liquid-> liquid + delta-> delta-> delta + austenite. When the liquid phase changes to delta phase, the nitrogen solubility is about 1164ppm, There is a difference in employment of about 1836ppm.

Some of the supersaturated nitrogen in the liquid phase is gasified during solidification to form various pores in the solidified material and also to form many pores in the solidification cell formed on the surface of the material.

As described above, there are many pores in the actual solidified material, and some of these pores may be squeezed during hot rolling. However, uncombined pores proceed to internal defects, Surface defects.

On the other hand, as shown in FIG. 5B, it was confirmed that the thin sheets cast according to Examples 1 to 5 had no internal pores due to the difference in nitrogen solubility.

Various means may be proposed to remove nitrogen above the solubility limit contained in the molten steel during the strip casting process. For example, in the method of manufacturing a highly ductile duplex stainless steel according to the present invention, a nitrogen discharge channel is formed on the surface of a casting roll And the amount of nitrogen exceeding the solid solubility limit was discharged.

As shown in FIG. 4, the problem of internal porosity due to nitrogen is generated in a process in which molten steel passes through a pair of casting rolls and rapidly cooled.

Therefore, the nitrogen exceeding the solid solubility limit in the molten steel must proceed simultaneously with the molten steel passing through the casting roll. For this purpose, it is preferable that a nitrogen discharge channel 6a is formed on the surface of the casting roll so that nitrogen can be discharged during casting.

The nitrogen discharge channel 6a is a fine channel to which only nitrogen gas can be discharged while molten steel can not pass through. The nitrogen discharge channel 6a may be formed in various ways in the casting roll, and may be formed in the circumferential direction on the surface of the casting roll to guide and discharge the nitrogen gas toward the outside of the casting roll in accordance with the rotation of the casting roll.

The nitrogen discharge channel 6a corresponds to a fine channel having a width of 50 to 500 mu m and a depth of 50 to 300 mu m and a plurality of holes are formed in the circumferential direction of the casting roll, Is preferably about 100 to 1000 mu m.

The shape, structure, and application position of the nitrogen discharge channel 6a may be variously modified as long as the functions thereof can be accomplished.

When a plurality of such nitrogen discharge channels 6a are formed, the contact area between the casting roll and the molten steel passing through the casting roll can be reduced. To prevent this, the surface of the casting roll is provided with projections and depressions .

These irregularities have an average size of 15 to 25 mu m.

On the other hand, the present inventors evaluated the hot workability of ordinary duplex steel and lean duplex steel having the composition shown in Table 2 by using Gleiber.

division C Si Mn Cr Ni Cu N Remarks Comparative Example 3 0.02 0.43 1.52 21.5 2.07 0.81 0.17 Hot rolling Comparative Example 4 0.03 0.57 5 21.2 1.45 0.3 0.21 Hot rolling Comparative Example 5 0.05 1.5 3.0 20.09 1.04 1.0 0.246 Annual rolling Comparative Example 6 0.051 0.84 2.98 19.94 1.02 1.0 0.242 Hot rolling Example 6 0.05 1.5 3.0 20.09 1.04 1.0 0.246 Strip casting

Comparative Example 3 was obtained by hot rolling the ordinary duplex steel 329LA, Comparative Example 4 was obtained by hot-rolling the 200-series stainless steel 2101, and Comparative Example 5 and Comparative Example 6 were obtained by hot rolling the lean duplex steel .

As in the ratio shown in FIG. 6, Comparative Example 3 showed excellent hot workability. In Comparative Example 4, the hot workability was lower than Comparative Example 3, but the minimum value for hot rolling was 50% Respectively.

On the other hand, in the case of Comparative Example 5 and Comparative Example 6 which are lean duplex, it was confirmed that rapid hot workability deteriorates at a high temperature.

This is lower than that of the comparative steel 4 by about 20%. If a reheating or hot rolling process is performed after the slab is manufactured by a conventional method, there is a possibility that a large amount of edge cracks and surface cracks due to insufficient hot workability Show.

As shown in Fig. 7, it was confirmed that edge cracking and surface cracking occurred in Comparative Example 5 and Comparative Example 6 in the case of ordinary hot rolling.

On the other hand, as shown in FIG. 8, the highly ductile duplex stainless steel of the present invention produced by the strip casting process has no surface cracks and edge cracks generated during hot rolling due to the omission of the hot rolling process I could.

As described above, the lean duplex steel produced by a conventional method has various problems such as internal pore caused by high nitrogen generated during casting, large amount of cracks due to hot workability dislocation, and the like, and a strip casting process , It is possible to manufacture thin thin plates of 1 to 5 mm without surface cracks and edge cracks through in-line rolling in succession.

On the other hand, according to the method of manufacturing a high ductility duplex stainless steel of the present invention, a thin plate is manufactured by using a casting roll and an inline roller continuously arranged with the casting roll, and the thin plate is subjected to hot rolling annealing, cold rolling and cold- And the cold-rolled annealing proceeds at 1000 ° C or higher, whereby a thin plate having an elongation of 55% or more can be produced.

The inventors of the present invention conducted cold rolling of hot rolled thin plates produced using a strip casting process and cold rolling and annealing for about 5 minutes in Examples 1 to 5 of Table 1 to measure elongation change according to cold annealing temperature Respectively.

As shown in Fig. 9, when the cold-rolling annealing temperature is 1000 占 폚 or higher, a desired elongation percentage of 55% or more can be secured.

The inventors of the present invention measured the austenite phase fraction according to the cold-rolling annealing temperature in Examples 1 to 5 prepared by the strip casting process, and this is shown in FIG.

As shown in FIG. 10, the maximum austenite phase fraction was confirmed at the annealing temperature of 1100 ° C., and when the annealing temperature was reheated to 1100 ° C. or more, it was found that the austenite phase fraction decreased, It was found that the austenite phase fraction for securing an elongation of about 55% or more after cold rolling and annealing was about 40 to 75%.

As shown in FIG. 11, the elongation of the cold-rolled sheet produced by the strip casting process according to the percentage of austenite phase is in the range of 40 to 75% except for the annealing temperature of 900 ° C. , The elongation of the lean duplex steel manufactured by the strip casting process is secured to about 55% or more.

Further, as shown in Fig. 12, after the hot-rolled sheet produced by the strip casting process was cold-rolled, elongation change was measured at 1000 占 폚 and 1050 占 폚 with annealing time.

When the annealing temperature is 1000 ° C for about 30 seconds, the elongation of about 50% can be obtained, which is less than the target value of 55%. However, when the annealing time is increased to 60 seconds at the same annealing temperature, I could.

In addition, when annealing is performed at an annealing temperature of 1050 占 폚 for about 30 seconds, an elongation of 55% or more can be secured, so that an effect of shortening the annealing time can be obtained.

Therefore, it is possible to obtain a steel having a desired plane elongation of 55% by securing an annealing time of 60 seconds or more at 1000 ° C and 30 seconds or more at 1050 ° C.

FIG. 13A is a view showing the microstructure of a thin plate when annealing is performed under a constant annealing time at 1000 ° C or 1050 ° C after cold-rolling a hot-rolled sheet produced by the above-described strip casting process.

Conventionally, the band-shaped ferrite (see FIG. 2) has disappeared, and the microstructure is randomly oriented with non-directionality. As a result, it has been confirmed that plastic anisotropy is minimized by such microstructure.

FIG. 13B is a diagram showing elongation in the rolling direction (0 DEG), elongation in the direction of 45 DEG from the rolling direction, and elongation in the 90 DEG direction from the rolling direction.

In the case of the thin plate 1 and the thin plate 2 produced through the conventional continuous casting method and hot rolling, the elongation at 90 ° from the rolling direction is less than 50%, but the thin plate 3 and the thin plate 4 produced by the strip casting process It was confirmed that the elongation in all directions was 55% or more.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims It will be apparent to those of ordinary skill in the art.

1: Ladle 2: Tundish
3: injection nozzle 4: manifold shield
5: side dam 6: casting roll
6a: Nitrogen discharge channel
7: Roll nip 8: Lamination
9: Inline roller 10: Coil winding device

Claims (9)

A method of manufacturing a high ductility duplex stainless steel in which a thin plate is produced by passing molten steel through a pair of casting rolls,
Wherein the nitrogen exceeding the nitrogen solubility limit contained in the molten steel is discharged to the outside through the casting roll at the time of solidification.
The method according to claim 1,
Wherein a nitrogen discharge channel is formed in at least one of the pair of casting rolls in the circumferential direction.
The method of claim 2,
Wherein the molten steel contains, by weight%, 0.08% or less of C, 0.2-3.0% of Si, 2.0-4.0% of Mn, 19.0-23.0% of Cr, 0.3-2.5% of Ni, %, Cu: 0.5 to 2.5%, and the balance being Fe and other unavoidable impurities.
The method according to any one of claims 1 to 3,
The thin plate produced by passing through the pair of casting rolls is rolled by using an inline roller continuously arranged with the pair of casting rolls and subjected to hot rolling annealing, cold rolling and cold rolling annealing successively, and the cold rolling annealing temperature is Wherein the process is carried out in the range of 1000 to 1100 占 폚, but is progressed for at least 60 seconds in the range of 1000 to 1050 占 폚 and at least 30 seconds in the range of 1050 to 1100 占 폚.
The method according to claim 1,
And the surface of the casting roll is provided with a concavity and convexity of 15 to 25 占 퐉.
The method according to claim 1,
Characterized in that a plurality of the nitrogen discharge channels are formed with a width of 50 to 500 탆 and a depth of 50 to 300 탆 and an interval between adjacent nitrogen discharge channels is 100 to 1000 탆. Way.
The steel sheet according to any one of claims 1 to 3, wherein the steel contains 0.08% or less of C (excluding 0%), 0.2 to 3.0% of Si, 2.0 to 4.0% of Mn, 19.0 to 23.0% of Cr, 0.3 to 2.5% of Ni, 0.5 to 2.5% of Cu, and the balance of Fe and other unavoidable impurities is passed through a pair of casting rolls during rapid casting, and nitrogen above the nitrogen solubility limit contained in the molten steel upon solidification of the molten steel, Wherein the stainless steel is produced by discharging through a stainless steel pipe.
The method of claim 7,
Characterized in that the austenite phase in an amount of 40 to 75% and the waste light phase structure in 25 to 60% are formed in a non-directional manner so that plastic anisotropy can be minimized.
The method of claim 8,
And has an elongation of 55% or more with respect to all directions.

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