KR101895695B1 - Lean duplex stainless steel and method of manufacturing the same - Google Patents

Abstract

And a method for producing the same.
The method of manufacturing lean duplex stainless steel according to the present invention comprises the steps of: (a) adding 0.08% or less of C, 0.2-3.0% of Si, 2-4% of Mn, 19-23% of Cr, 0.3-2.5% , 0.2 to 0.3% of N, 0.5 to 2.5% of Cu, and the balance of Fe and other unavoidable impurities, followed by hot rolling and pickling; (b) cold-rolling the hot-rolled annealed and pickled steel; And (c) subjecting the cold-rolled steel to an atmosphere heat treatment at a temperature of 1,000 to 1,200 ° C for at least 10 seconds in a hydrogen gas atmosphere.

Description

TECHNICAL FIELD [0001] The present invention relates to a lean duplex stainless steel and a manufacturing method thereof. BACKGROUND OF THE INVENTION 1. Field of the Invention [0002]

The present invention relates to a lean duplex stainless steel and a manufacturing method thereof, and more particularly, to a lean duplex stainless steel capable of improving corrosion resistance and a manufacturing method thereof.

Stainless steel is used for various purposes such as kitchen appliances, household appliances, interior materials of buildings and exterior materials because of its excellent design property. Therefore, the use environment of stainless steel is diversified, and the quality demanded by customers is also increasing.

The surface of stainless steel is polished such that its surface has a roughness after cold annealing and pickling, and there are No3, No4 and HL. The polishing method of the stainless steel is carried out after the usual hot-rolling annealing and pickling, and is performed by wet or dry using a belt-type abrasive. The stainless steel cold-rolled sheet passes through the rotating abrasive belt in the opposite direction, and the surface is polished in one side or both sides in the longitudinal direction to give a certain degree of roughness to the surface.

STS 304, which mainly contains Ni of 8 wt%, has been widely used in interior and exterior materials of buildings, because it has little change in surface quality and corrosion resistance according to the polishing condition. However, in case of STS 304 steel, high price of Ni content was high and price competitiveness was not high. Therefore, the use of ferritic grades which are close to the characteristics of STS 304 steel and have good price competitiveness is increasing. In the case of ferritic stainless steel, since the surface hardness is lower than that of austenitic steel, when the SUS 304 is polished under the polishing condition, there arises a problem that the quality of the surface polishing is deteriorated and the corrosion resistance is deteriorated.

A number of techniques have been proposed to solve these problems and to increase the corrosion resistance of abrasive products. For example, JP1990-179818 discloses an improvement of polishing oil used in a polishing finishing process. In the case of KR 10-2012-0074802, a technique of managing the surface roughness and the like after surface polishing has been proposed. However, they all have a problem that the polishing oil is limited, which makes it difficult to use the polishing pad in general and increases the cost.

Further, there is a disadvantage that the corrosion resistance is poor unless the S component contained in the polishing oil or the like is sufficiently removed after polishing. Further, if the surface roughness and the like are limited, it becomes impossible to realize various polishing surfaces such as No3 No4, HL and the like as STS 304.

Recently, in the case of lean duplex stainless steel composed of a mixture of austenite phase and ferrite phase for the purpose of replacing STS 304 steel which is generally used, there is no application example such as surface grinding because it has higher strength than STS 304.

The present invention provides a lean duplex stainless steel capable of ensuring excellent corrosion resistance through surface polishing or atmospheric heat treatment and a method for manufacturing the same.

A method for manufacturing a lean duplex stainless steel according to a first embodiment of the present invention comprises the steps of: (a) a step of preparing a stainless steel having a composition of C: more than 0% to less than 0.08%, Si: 0.2 to 3.0% The hot-rolled steel is hot-rolled by a hot-rolling process, and the hot-rolled steel is subjected to hot-rolling and then hot-rolled to obtain a steel comprising 19 to 23% of Cr, 0.3 to 2.5% of Ni, 0.2 to 0.3% of N and 0.5 to 2.5% of Cu and other unavoidable impurities. ; (b) cold-rolling the hot-rolled annealed and pickled steel; And (c) subjecting the cold-rolled steel to an atmosphere heat treatment at a temperature of 1,000 to 1,200 ° C for at least 10 seconds in a hydrogen gas atmosphere.

In order to accomplish the above object, a lean duplex stainless steel according to a first embodiment of the present invention comprises C: more than 0% to less than 0.08%, Si: 0.2 to 3.0%, Mn: 2 to 4% To 23% 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, followed by hot rolling and pickling, An oxide film is formed on the surface of the steel by subjecting the steel to an atmosphere heat treatment in a hydrogen gas atmosphere at a dew point of -40 to -70 占 폚 and at 1,000 to 1,200 占 폚 for 10 seconds or longer and the oxide film has a surface thickness of 0.02 占 퐉 And the Si component is distributed in an amount of 10 wt% or more.

A method of manufacturing a lean duplex stainless steel according to a second embodiment of the present invention includes the steps of: (a) a steel sheet having a composition of C: more than 0% to less than 0.08%, Si: 0.2 to 3.0%, Mn: The hot-rolled steel is hot-rolled by a hot-rolling process, and the hot-rolled steel is subjected to hot-rolling and then hot-rolled to obtain a steel comprising 19 to 23% of Cr, 0.3 to 2.5% of Ni, 0.2 to 0.3% of N and 0.5 to 2.5% of Cu and other unavoidable impurities. ; (b) cold-rolling the hot-rolled annealed and pickled steel; (c) cold-annealing and pickling the cold-rolled steel; And (d) surface polishing the cold annealed and pickled steel to a HL or No.4 state.

The lean duplex stainless steel and its manufacturing method according to the present invention can remarkably improve surface gloss and corrosion resistance by controlling the dew point temperature, the heat treatment temperature and the heat treatment holding time in a hydrogen gas atmosphere after cold rolling and performing atmosphere heat treatment.

In addition, the lean duplex stainless steel according to the present invention and its manufacturing method are capable of ensuring excellent polishing surface quality that does not cause change in corrosion resistance due to surface roughness change by uniformly polishing the surface of the steel after cold rolling annealing and pickling have.

As a result, the stainless steel produced by the method of manufacturing the linseed duplex stainless steel according to the present invention is suitable for use as building interior / exterior materials, elevator interior and exterior materials, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow diagram illustrating a method of manufacturing a lean duplex stainless steel according to a first embodiment of the present invention; FIG.
2 is a graph showing oxidation reaction curves of oxides according to heat treatment temperature and dew point in a hydrogen gas atmosphere.
3 is a process flow diagram illustrating a method of manufacturing a lean-duplex stainless steel according to a second embodiment of the present invention.
FIG. 4 is a schematic view showing a surface polishing apparatus used in the surface polishing process of FIG. 3;
FIG. 5 is a photograph showing the corrosion resistance evaluation of the specimen according to Comparative Example 1 and Example 1; FIG.
FIG. 6 is a photograph showing a specimen taken after evaluation of corrosion resistance according to Example 2 and Example 3. FIG.
7 is a graph showing the results of measurement of the oxide element distribution of a specimen according to Comparative Example 1 by GDS analysis.
8 is a graph showing the results of measurement of the oxide element distribution of a specimen according to Comparative Example 3 by GDS analysis.
9 is a graph showing the results of measurement of oxide element distribution of a specimen according to Comparative Example 5 by GDS analysis.
10 is a graph showing the results of measurement of the oxide element distribution of a specimen according to Example 4 by GDS analysis.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and how to accomplish them, will become apparent by reference to the embodiments described in detail below with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a lean duplex stainless steel according to preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The lean duplex stainless steel according to the first embodiment of the present invention is characterized in that it contains C: over 0% to 0.08%, Si: 0.2-3.0%, Mn: 2-4%, Cr: 19-23% 0.3 to 2.5% of N, 0.2 to 0.3% of N, 0.5 to 2.5% of Cu, and the balance of Fe and other unavoidable impurities are subjected to hot rolling, hot rolling and hot rolling, cold- An oxide film is formed on the surface of the steel by holding at a temperature of 1,000 to 1,200 DEG C for 10 seconds or longer under the conditions of a dew point of -40 to -70 DEG C so that the oxide film has an Si content of 10 wt% Or more.

At this time, the lean duplex stainless steel contains 45 to 75 vol% of the austenite phase and 25 to 55 vol% of the ferrite phase per unit area.

If the austenite phase fraction is less than 45 vol%, the austenite forming element is excessively concentrated inside the austenite phase, and the sufficient amount of tensile strength of the steel due to the increase in the austenite strength is suppressed However, the ductility degradation phenomenon occurs and the elongation at the SUS 304 level can not be obtained at least 50%. On the other hand, when the austenite phase fraction exceeds 75 vol%, surface cracks occur during hot rolling, resulting in deterioration of hot workability and loss of characteristics of the two-phase stainless steel, so that the austenite phase fraction is preferably less than 75 vol% .

Hereinafter, the role and contents of each component included in the lean duplex stainless steel according to the first embodiment of the present invention will be described.

Carbon (C)

Carbon (C) is an austenite forming element and is an effective element for increasing strength by solid solution strengthening.

However, when a large amount of carbon (C) is added in an amount exceeding 0.08% by weight of the total weight of the lean duplex stainless steel, it easily bonds with a carbide forming element such as Cr, which is effective for corrosion resistance at the ferrite- austenite phase boundary, There is a problem in that the Cr content is lowered to reduce the corrosion resistance. Therefore, in order to maximize the corrosion resistance, it is preferable to add the carbon (C) content in a content ratio of more than 0% to 0.08% of the total weight of the lean duplex stainless steel.

Silicon (Si)

Silicon (Si) is added in part for the deoxidation effect. Silicon (Si) is a ferrite-forming element and is an element that is concentrated in ferrite during annealing. Therefore, silicon (Si) should be added in an amount of 0.2 wt% or more in order to ensure a proper ferrite phase fraction. However, when a large amount of silicon (Si) is added in an amount exceeding 3.0% by weight, the hardness of the ferrite phase is drastically increased, which affects the lowering of the elongation, making it difficult to secure the austenite phase for securing sufficient elongation. Further, when the addition of silicon (Si) is excessive, the slag fluidity is deteriorated at the time of steel making and the inclusion is formed by bonding with oxygen, which adversely affects the corrosion resistance. Therefore, silicon (Si) is preferably added in a content ratio of 0.2 to 3.0 wt% of the total weight of the lean duplex stainless steel.

Manganese (Mn)

Manganese (Mn) is an element that increases nitrogen solubility. Further, when manganese (Mn) is used as a substitute for nickel (Ni) which is expensive as an austenite forming element, when added in an amount exceeding 4 wt%, it becomes difficult to secure the corrosion resistance of SUS 304 level. Therefore, when a large amount of manganese (Mn) is added, it has an effect on nitrogen solubility but forms MnS by binding with S, thereby lowering corrosion resistance. However, when the addition amount of manganese (Mn) is less than 2% by weight, it is difficult to secure a proper austenite phase fraction even if the amount of nickel (Ni), copper (Cu), nitrogen (N) , The solubility of nitrogen (N) added is low, and sufficient employment of nitrogen (N) at normal pressure can not be obtained. Therefore, manganese (Mn) is preferably added at a content ratio of 2 to 4% by weight of the total weight of the lean duplex stainless steel.

Chromium (Cr)

In addition to silicon (Si), chromium (Cr) plays a major role in securing the ferrite phase of a two-phase stainless steel as a ferrite stabilizing element, and is an essential element for ensuring corrosion resistance. However, increasing the amount of chromium (Cr) increases the corrosion resistance. However, since it is necessary to increase the content of nickel and other austenite forming elements in order to maintain the phase fraction, the STS 304 It is preferable that chromium (Cr) is added in a content ratio of 19 to 23% by weight of the total weight of the lean duplex stainless steel.

Nickel (Ni)

Nickel (Ni), together with manganese (Mn), copper (Cu) and nitrogen (N), plays a major role in securing the austenite phase of the two-phase stainless steel as an austenite stabilizing element. (Mn) and nitrogen (N), which are other austenite phase forming elements, are added in place of reducing the amount of expensive nickel (Ni) Balance can be maintained. However, in order to secure sufficient austenite stability in order to suppress the formation of fired organic martensite which occurs during cold working, 0.3 wt% or more should be added. However, if too much nickel (Ni) is added, it is difficult to secure a proper austenite fraction due to an increase in austenite fraction, and it is difficult to secure competitiveness compared to SUS 304 due to increased production cost of nickel (Ni). Therefore, it is preferable that nickel (Ni) is added at a content ratio of 0.3 to 2.5 wt% of the total weight of the lean duplex stainless steel.

Nitrogen (N)

Nitrogen (N) is an element which contributes to the stabilization of austenite phase together with nickel (Ni) in two-phase stainless steel. It is one of the elements in which the austenite phase is thickened during annealing and the increase of nitrogen (N) The corrosion resistance can be increased and the strength can be increased. However, the solubility of nitrogen (N) varies depending on the content of manganese (Mn) added to the lean duplex stainless steel.

At this time, it is preferable to add 0.2 wt% or more of nitrogen (N) in order to improve the corrosion resistance. However, if the addition amount of nitrogen (N) is too low, less than 0.2% by weight, it becomes difficult to secure an appropriate phase fraction. Therefore, nitrogen (N) is preferably limited to a content ratio of 0.2 to 0.3 wt% of the total weight of lean duplex stainless steel.

Copper (Cu)

Copper (Cu) plays a role similar to nickel (Ni). At this time, it is desirable to reduce the amount of copper (Cu) to a minimum to reduce the cost. However, it should be added in an amount of 0.5 wt% or more in order to ensure the stability of the austenite phase so as to inhibit the formation of excess fired organic martensite which occurs during cold rolling. On the other hand, when a large amount of copper (Cu) is added in an amount exceeding 2.5% by weight, product processing may be difficult due to hot brittleness. Therefore, copper (Cu) is preferably added at a content ratio of 0.5 to 2.5 wt% of the total weight of the lean duplex stainless steel.

(Embodiment 1)

1 is a process flow diagram illustrating a method of manufacturing a lean-duplex stainless steel according to a first embodiment of the present invention.

Referring to FIG. 1, a method of manufacturing a lean duplex stainless steel according to a first embodiment of the present invention includes hot rolling annealing and pickling (S110), cold rolling (S120), and atmosphere annealing (S130).

Hot-rolled annealing and pickling

In the hot-rolled annealing and pickling step (S110), C: more than 0% to 0.08%, Si: 0.2 to 3.0%, Mn: 2 to 4%, Cr: 19 to 23%, Ni: 0.3 to 2.5% 0.2 to 0.3% of N, 0.5 to 2.5% of Cu, and the balance of Fe and other unavoidable impurities are hot-rolled, hot-rolled and annealed.

At this time, the steel having the above composition may be a casting plate formed so as to have a thickness of 5 mm or less by mixing molten steel raw materials so as to have a target low-grade composition at the time of steelmaking, and then melting molten steel in steelmaking.

At this time, it is preferable that the hot annealing temperature is 1,030 to 1,200 ° C for 2 to 40 minutes. When the heat treatment temperature for hot-rolling annealing is less than 1,030 占 폚, ferrite and austenite, which are two phases, are defective during rolling, and there is a tendency to decrease elongation due to lack of recovery and recrystallization of heat-treated ferrite and austenite. On the other hand, when the heat treatment temperature for hot-rolling annealing exceeds 1,200 占 폚, the crystal grains of ferrite and austenite become too coarse and it becomes impossible to secure a tensile strength of 800 MPa or more.

In the case where the heat treatment time of the hot rolling annealing is less than 2 minutes, diffusion in which alloying elements are distributed is not sufficient. As a result, the alloying elements are not sufficiently concentrated in the austenite phase, so that it is difficult to control the rate of transformation induced organic martensite formation during cold working. On the other hand, when the heat treatment time of the hot-rolled annealing is more than 40 minutes, the plate is sagged during the heat treatment by the two-phase structure of ferrite and austenite, resulting in lowering of productivity and loss of heat source unit.

Cold rolling

In the cold rolling step (S120), the hot-rolled annealed and pickled steel is cold-rolled. In this case, the cold rolling may be performed so that the hot rolled coil wound with hot rolled annealed and pickled steel has a thickness of 2 mm or less, but the present invention is not limited thereto.

Atmosphere heat treatment

In the atmosphere heat treatment step (S130), an atmosphere heat treatment is performed in which cold-rolled steel is maintained at a temperature of 1,000 to 1,200 DEG C for 10 seconds or more in a hydrogen gas atmosphere. At this time, it is more preferable to perform the heat treatment of the atmosphere at a dew point of -40 to -75 캜.

In this step, the atmosphere heat treatment can be carried out in a brass annealing furnace. At this time, during the heat treatment of the atmosphere, the oxide in the oxide film formed on the surface of the steel is reduced by the heat treatment in the hydrogen gas atmosphere, and the Si and Cr components in the oxide film are enriched and the corrosion resistance is improved.

At this time, in order to improve the corrosion resistance by constituting the main component of the oxide film formed on the surface of the cold-rolled steel with oxides rich in Si and Cr, a hydrogen gas atmosphere with excellent reducing property is required. Here, the hydrogen gas atmosphere means that the atmosphere of the impurity gas and the atmosphere heat treatment, which are inevitably incorporated in the pre-industrial production process, permits the gas to be mixed inevitably in the process of replacing the inside with hydrogen, .

2 is a graph showing oxidation reaction curves of oxides according to heat treatment temperatures and dew points in a hydrogen gas atmosphere.

As can be seen from FIG. 2, in order to form an oxide having excellent corrosion resistance on the surface of the cold-rolled steel, the Fe and Cr oxides formed after cold rolling must be reduced and the surface should be replaced with oxides rich in Si components. At this time, in order to reduce the Fe and Cr oxides and replace the surface with the Si-rich oxides, the temperature of the atmosphere is preferably controlled at a temperature of 1,000 to 1,200 ° C. at a dew point of -40 ° C. or less, more preferably -40 to -75 ° C. .

In the region where the dew point temperature is lower than -75 캜, the Si oxide is reduced and the corrosion resistance is lowered. Therefore, it is necessary to maintain the dew point at -40 to -75 캜 in the atmosphere heat treatment temperature range of 1,000 to 1,200 ° C.

In general, the higher the atmospheric heat treatment temperature is, the better the reduction is. Therefore, the higher the atmospheric heat treatment temperature, the better the characteristics. In the present invention, however, it is preferable to control the atmospheric heat treatment temperature to 1,000 to 1,200 ° C. At this time, when the annealing temperature is less than 1,000 ° C, the recovery and recrystallization of ferrite and austenite tend to be insufficient during the heat treatment of the atmosphere, and the elongation tends to be lowered. When the atmospheric heat treatment temperature exceeds 1,200 占 폚, it is difficult to secure a tensile strength of 800 MPa or more with the crystal grains of the ferrite and austenite being too large.

The atmospheric heat treatment holding time should be maintained for at least 10 seconds or more, and the preferable atmospheric heat treatment holding time may be 10 to 60 seconds. At this time, it was experimentally found that a retention time of at least 10 seconds is required in order to sufficiently reduce Fe and Cr oxides in the surface oxide film and form Si-rich oxides to improve the corrosion resistance.

The lean duplex stainless steel produced by the above-described method according to the first embodiment of the present invention reduces the Fe and Cr oxides in the surface oxide film to form Si-rich oxides. As a result, The Si component is distributed in an amount of 10 wt% or more.

Therefore, the method of manufacturing the lean-duplex stainless steel according to the first embodiment of the present invention is characterized in that after the cold rolling, the dew point temperature, the heat treatment temperature and the heat treatment holding time are controlled in a hydrogen gas atmosphere, Can be improved.

As a result, the stainless steel produced by the method of manufacturing the linseed duplex stainless steel according to the first embodiment of the present invention is suitable for use as building interior / exterior materials, elevator interior and exterior materials, and the like.

(Second Embodiment)

3 is a process flow diagram illustrating a method of manufacturing a lean-duplex stainless steel according to a second embodiment of the present invention.

Referring to FIG. 3, a method of manufacturing a lean duplex stainless steel according to a second embodiment of the present invention includes the steps of hot rolling annealing and pickling (S210), cold rolling (S220), cold annealing and pickling (S230) (S240).

Hot-rolled annealing and pickling

In the hot-rolled annealing and pickling step S210, C: more than 0 to 0.08%, Si: 0.2 to 3.0%, Mn: 2 to 4%, Cr: 19 to 23%, Ni: 0.3 to 2.5% 0.2 to 0.3% of N, 0.5 to 2.5% of Cu, and the balance of Fe and other unavoidable impurities are hot-rolled, hot-rolled and annealed.

At this time, the steel having the above composition may have substantially the same composition and composition as the lean duplex stainless steel according to the first embodiment of the present invention.

Here, the steel having the above composition may be a casting plate formed so as to have a thickness of 5 mm or less by mixing molten steel raw materials so as to have a desired small steel composition at the time of steelmaking, and then melting molten steel in steelmaking.

At this time, it is preferable that the hot annealing temperature is 1,030 to 1,200 ° C for 2 to 40 minutes. When the heat treatment temperature for hot-rolling annealing is less than 1,030 占 폚, ferrite and austenite, which are two phases, are defective during rolling, and there is a tendency to decrease elongation due to lack of recovery and recrystallization of heat-treated ferrite and austenite. On the other hand, when the heat treatment temperature for hot-rolling annealing exceeds 1,200 占 폚, the crystal grains of ferrite and austenite become too coarse and it becomes impossible to secure a tensile strength of 800 MPa or more.

In the case where the heat treatment time of the hot rolling annealing is less than 2 minutes, diffusion in which alloying elements are distributed is not sufficient. As a result, the alloying elements are not sufficiently concentrated in the austenite phase, so that it is difficult to control the rate of transformation induced organic martensite formation during cold working. On the other hand, when the heat treatment time of the hot-rolled annealing is more than 40 minutes, the plate is sagged during the heat treatment by the two-phase structure of ferrite and austenite, resulting in lowering of productivity and loss of heat source unit.

Cold rolling

In the cold rolling step (S220), the hot-rolled annealed and pickled steel is cold-rolled.

In this case, the cold rolling may be performed so that the hot rolled coil wound with hot rolled annealed and pickled steel has a thickness of 2 mm or less, but the present invention is not limited thereto.

Cold annealing and pickling

In the cold annealing and pickling step S230, the cold-rolled steel is cold-annealed and pickled.

At this time, the cold annealing is carried out at 1,050 to 1,100 ° C for 2 to 5 minutes to complete the austenite and ferrite phase recrystallization of the microstructure in the steel and to secure the required room temperature elongation of 40% or more. If the cold annealing temperature is less than 1,050 占 폚, the required recrystallization ratio can not be secured. If the cold annealing temperature is more than 1,100 DEG C, there is a problem that the shape of the steel becomes poor.

Surface polishing

In the surface polishing step (S240), the cold annealed and pickled steel is surface-polished to HL or No.4 state.

By performing such surface polishing, the irregular surface of the steel surface subjected to the cold-rolling annealing and pickling treatment is uniformly polished and the Fe and Cr oxide films are formed on the surface, so that the corrosion resistance can be improved.

4 is a schematic view showing a surface polishing apparatus used in the surface polishing process of FIG.

As shown in Fig. 4, in the surface polishing, the surface of the cold-annealed and pickled steel is physically polished.

At this time, the surface grinding apparatus 1 includes a grinding belt 2, a contact roll 3, an idle roll 4, a blank roll 5, an inlet nozzle 6, an outlet nozzle 7, an inlet breaker roll 8 And an outgoing breaker roll 9 as shown in Fig.

Here, the contact roll 3 and the idle roll 4 are spaced apart from each other. The abrasive belt 2 is installed in a structure in which the contact roll 3 and the idle roll 4 are wound in a closed loop form. The abrasive belt 2 is sandwiched between the contact roll 3 and the nonwoven fabric roll 5.

At this time, the contact roll 3 is rotationally driven by the driving source so that the abrasive belt 2 rotates clockwise in the contact roll 3 and the idle roll 4.

That is, the steel 11 subjected to the cold-rolling annealing and pickling treatment is conveyed in the first direction L by applying a suitable tension to the inlet side and the outlet side breaker rolls 8, 9, The surface is polished when it passes between the polishing pad 2 and the polishing pad 2. The inlet nozzle 6 and the outlet nozzle 7 are arranged before and after the cold rolling annealing and pickling of the steel 11 with the contact roll 3 interposed therebetween to polish and lubricate the polishing oil. Here, the roughness of the abrasive belt 2 is selected and polished according to the specifications of surface finishing of HL or No. 4.

The steel 11 subjected to the cold annealing and pickling treatment is subjected to surface polishing using the surface grinding apparatus 1 so that the surface of the steel 11 is flattened and the surface of the steel 11 is coated with Fe and / A Cr oxide film is formed and corrosion resistance can be improved.

The method of manufacturing the lean-duplex stainless steel according to the second embodiment of the present invention described above can uniformly polish the surface of a steel after cold rolling annealing and pickling to obtain an excellent polishing surface quality that does not cause a change in corrosion- .

As a result, the stainless steel produced by the method of manufacturing the linseed duplex stainless steel according to the second embodiment of the present invention is suitable for use as building interior / exterior materials, elevator interior and exterior materials, and the like.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. 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.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Specimen Manufacturing

The specimens according to Examples 1 to 5 and Comparative Examples 1 to 5 were produced under the composition of Table 1 and the process conditions of Tables 2 and 3.

In all of the specimens according to Examples 1 to 5 and Comparative Examples 1 to 5, the casting plate having a thickness of 3 mm was hot-rolled in a strip casting process, hot rolled and annealed at a hot rolling annealing temperature of 1080 ° C for 3 minutes, And then cold rolling was performed.

[Table 1]

[Table 2]

[Table 3]

2. Corrosion resistance test

Table 4 shows the corrosion resistance test results of the specimens according to Examples 1 to 3 and Comparative Example 1 measured.

At this time, the corrosion resistance evaluation was performed by spraying 5% NaCl aqueous solution at 30 ° C for 2 hours using a combined cycle corrosion test apparatus, drying at 60 ° C and 20% relative humidity for 4 hours and wet at 90% The cycle was repeated for a total of 60 cycles to evaluate the corrosion resistance. In order to measure the perforation rate, photographing was firstly performed by washing the saline present on the surface of the specimen with running water and photographing the photograph by minimizing the reflection by the light source during photographing. After taking a photo, the blowing rate was measured using an image analyzer. The evaluation of the blowing rate of the test piece was evaluated according to ASTM D610 standard.

[Table 4]

Referring to Tables 1 to 4, in the case of the test piece according to Comparative Example 1, the surface roughness of the surface after 60 cycles was about 35%, and the roughness evaluation rating was Grade 1 in ASTM D610. Which is very low.

On the other hand, as in the specimens according to Examples 1 and 2, in the case of the HL and No. 4 surfaces subjected to cold-rolled annealing and pickling treated steel surfaces, the blowing rate was drastically reduced to about 1% In the case of ASTM D610, the rating of the outbreak rating was Grade 5, which was improved by 4 compared to Comparative Example 1.

Further, as in the case of the specimen according to Example 3, in the case of the surface subjected to the atmosphere heat treatment after cold rolling, the dusting rate was approximately 1% or less, and the ASTM D610 dusting rate evaluation grade was Grade 6, . Grade 6 means that no glowing occurs when observed with naked eyes.

FIG. 5 is a photograph of the specimen after the evaluation of corrosion resistance of the specimen according to Comparative Example 1 and Example 1, FIG. 6 is a photograph showing the specimen after the corrosion resistance evaluation according to Example 2 and Example 3 to be.

As shown in FIG. 5 (a), in the case of the test piece according to the comparative example 1, it can be seen that the test piece is visually confirmed with the test grade Grade 1. On the other hand, as shown in FIGS. 5 (b) and 6 (a) and 6 (b), in the case of the specimens according to Examples 1 to 3, .

On the other hand, Table 5 shows the corrosion resistance test results of the specimens according to Examples 3 to 5 and Comparative Examples 2 to 5 measured.

[Table 5]

As shown in Table 1, Table 3, and Table 5, it can be seen that the specimens according to Examples 3 to 4 exhibited a blowing rate of 1% or less and that the ASTM D610 outbreak rating grade satisfied Grade 6.

On the other hand, in the case of the specimens according to Comparative Examples 2 and 3, since the dew point temperature was higher than -40 ° C., the Fe and Cr oxides were not sufficiently reduced on the surface, The grade was only Grade 3.

In the case of the specimen according to the comparative example 4, the dew point temperature was in a sufficient state at -45 캜, but since the heat treatment holding time was not sufficient for 5 seconds, the Fe and Cr oxides remaining on the surface were not sufficiently reduced, 10.1% and ASTM D610 was rated grade 3 only.

In the case of the specimen according to Comparative Example 5, the Fe and Cr oxides were sufficiently reduced at the dew point temperature of -80 ° C, but the Si oxide was not sufficiently formed. Therefore, the ASTM D610 elongation evaluation grade was only Grade 4.

7 is a graph showing the oxide element distribution of the specimen according to Comparative Example 1 measured by GDS analysis, and FIG. 8 is a graph showing the oxide element distribution of the specimen according to Comparative Example 3, to be. 9 is a graph showing the results of measurement of the oxide element distribution of the specimen according to Comparative Example 5 by GDS analysis, and FIG. 10 is a graph showing the results of measurement of the oxide element distribution of the specimen according to the GDS analysis to be.

As shown in FIG. 7, the oxide element distribution was measured by GDS analysis. As a result, in the case of the specimen according to Comparative Example 1, the major components distributed in the region within 0.01 μm in the thickness direction were Fe and Cr oxide (Fe component> Cr component).

In addition, as shown in FIG. 8, in the case of the specimen according to Comparative Example 3, the dew point temperature was -35 Deg.] C, the Fe oxide is sufficiently reduced, but the Cr oxide is not sufficiently reduced, and the Si oxide-rich oxide is also formed on the surface of the region within 0.01 [micro] m in the thickness direction. 4.

8, in the case of the specimen according to Comparative Example 5, since the dew point temperature is -80 deg. C and the Si oxide is reduced in this atmosphere, the Si oxide is formed on the surface in the region within 0.01 mu m in the thickness direction And the Fe and Cr oxides were also subjected to a sufficient reduction treatment, so that the release rate was measured to be higher than in Example 4.

On the other hand, as shown in Fig. 10, in the case of the specimen according to Example 4, the Fe and Cr oxides were sufficiently reduced under the atmosphere heat treatment conditions, and the surface of the region within 0.01 mu m in the thickness direction was coated with an oxide (Si component> Fe, Cr component).

Based on the above experimental results, it was found from the results of the GDS analysis and the combined cycle corrosion test that the components of the surface oxides of the atmosphere heat treated specimens were at least 10 wt% in the region of 0.01 μm or less in the thickness direction, The above results indicate that it is sufficient to contain sufficient corrosion resistance.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. These changes and modifications may be made without departing from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Hot rolling annealing and pickling step
S120: Cold rolling step
S130: atmosphere heat treatment step

Claims (10)

(a) from 0 to less than 0.08% of C, from 0.2 to 3.0% of Si, from 2 to 4% of Mn, from 19 to 23% of Cr, from 0.3 to 2.5% of Ni, from 0.2 to 0.3% of N, 0.5 to 2.5% of Cu, and the balance of Fe and other unavoidable impurities, followed by hot rolling and pickling;
(b) cold-rolling the hot-rolled annealed and pickled steel; And
(c) subjecting the cold-rolled steel to an atmosphere heat treatment at a temperature of 1,000 to 1,200 ° C for 10 to 60 seconds in a hydrogen gas atmosphere,
In the step (c), the atmosphere heat treatment is performed at a dew point of -40 to -75 ° C,
After step (c), the steel comprises 45 to 75 vol% of austenite phase and 25 to 55 vol% ferrite phase per unit area.
The method according to claim 1,
In the step (a)
The hot-
At a temperature of 1,030 to 1,200 DEG C for 2 to 40 minutes.
delete The method according to claim 1,
After the step (c)
Wherein the steel is reduced in Fe and Cr oxides in the surface oxide film to form an Si oxide-rich oxide film.
5. The method of claim 4,
The oxide film
Wherein the Si component is distributed in an amount of 10 wt% or more within 0.02 mu m in the thickness direction of the surface.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet comprises, by weight, C: more than 0% to 0.08%, Si: 0.2-3.0%, Mn: 2-4%, Cr: 19-23%, Ni: 0.3-2.5% 0.5 to 2.5% and the balance Fe and other unavoidable impurities is hot-rolled, hot-rolled and hot-rolled, and cold-rolled steel is heat-treated at a temperature ranging from -40 to -70 ° C. in a hydrogen gas atmosphere at a temperature of 1,000 to 1,200 ° C. By performing an atmosphere heat treatment for 10 to 60 seconds,
An oxide film is formed on the surface of the steel, and the oxide film has a Si content of 10 wt% or more within 0.02 mu m in the surface thickness direction,
Austenite phase in an amount of 45 to 75 vol% and a ferrite phase in an amount of 25 to 55 vol% per unit area.
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