KR101650258B1 - Austenitic stainless and manufacturing method thereof - Google Patents

Abstract

The austenitic stainless steel according to the present invention is characterized by containing 0.08% or less of C (excluding 0), 0.5 to 1.5% of Si, 2.5 to 3.5% of Mn, 18 to 20% of Cr, , Ni: 0.3 to 1.5%, N: 0.2 to 0.4%, Cu: 0.3 to 1.5%, Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%, V: 0.01 to 0.1%, and the balance Fe and other unavoidable impurities , Wherein the ratio of the fired organomartensite in the tensile fractured section is 10 to 40%, the amount of residual ferrite in the volume fraction is 5% or less, the elongation is 50% or more, and the yield strength is 450 MPa or more.

Description

[0001] AUSTENITIC STAINLESS AND MANUFACTURING METHOD THEREOF [0002]

The present invention relates to austenitic stainless steel and a method of manufacturing the same, and more particularly, to austenitic stainless steel using work hardening of fired organic martensite and a method of manufacturing the same.

Generally, an austenitic stainless steel contains iron (Fe) as a base metal and Cr and Ni as main raw materials. Mo, Cu and other elements are added to the steel to develop various steel types for various applications. In particular, steel is classified as a steel having excellent processability and corrosion resistance.

These austenitic stainless steels are excellent in corrosion resistance and pitting resistance, and contain a low carbon and Ni content of 8% or more by weight. Therefore, there is a problem in that the price fluctuation due to the rise in Ni price is large, and the price is unstable, thereby deteriorating the competitiveness. Therefore, in order to compensate for this, it is necessary to develop a new steel type capable of securing a corrosion resistance equal to or higher than that of general austenitic stainless steel while lowering the Ni content. In addition, most austenitic stainless steels are difficult to use as structural materials because of their low yield strength.

Therefore, development of an austenitic stainless steel having high strength and high ductility, which can be used as a structural material and has excellent moldability, is required.

SUMMARY OF THE INVENTION The present invention has been made to solve such problems, and an object of the present invention is to provide an austenitic stainless steel having excellent formability and strength and corrosion resistance, and a method for producing the same.

In order to achieve the above object, an austenitic stainless steel according to an embodiment of the present invention comprises 0.08% or less of C (excluding 0), 0.5-1.5% of Si, 2.5-3.5% of Mn, , Ni: 0.3 to 1.5%, N: 0.2 to 0.4%, Cu: 0.3 to 1.5%, Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%, V: 0.01 to 0.1% Fe, and other unavoidable impurities, wherein the ratio of the fired organomartensite in the tensile fractured section is 10 to 40%, the amount of residual ferrite in the volume fraction is 5% or less, the elongation is 50% or more, the yield strength is 450 MPa Or more.

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The method of manufacturing an austenitic stainless steel according to one embodiment of the present invention is a method for producing austenitic stainless steel which comprises 0.08% or less of C (excluding 0), 0.5-1.5% of Si, 2.5-3.5% of Mn, 0.3 to 1.5% of Ni, 0.2 to 0.4% of N, 0.3 to 1.5% of Cu, 0.01 to 0.1% of Ti, 0.01 to 0.1% of Nb, 0.01 to 0.1% of V, balance Fe and other unavoidable A cold-rolled steel sheet containing impurities is subjected to cold-rolling annealing at a temperature of 1050 to 1100 占 폚 for 45 to 180 seconds to produce a steel having an amount of residual ferrite of 5% or less in volume fraction, an elongation of 50% .

The austenitic stainless steel according to the present invention and its manufacturing method have the following effects.

First, the cost of raw materials can be reduced by reducing the contents of high-priced Ni, Si, Cu and Mo alloy components.

Second, it can be used as a structural material by realizing high strength and high ductility.

Third, it has excellent workability through high elongation.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the amount of fired organic martensite and elongation according to the cold-rolling annealing temperature.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms as used herein include plural forms as long as the phrases do not expressly express the opposite meaning thereto. Means that a particular feature, region, integer, step, operation, element and / or component is specified, and that other specific features, regions, integers, steps, operations, elements, components, and / And the like.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

Hereinafter, an austenitic 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.

Generally, austenitic stainless steels, such as 304 steel, contain 6% or more of expensive Ni, resulting in a very high raw material cost in production and a large amount of Ni, which is a valuable resource.

In the case of the newly developed 200 series, Ni is replaced by Mn. In order to secure austenite integrity, a large amount of Mn is secured and the cold workability is improved. However, the corrosion resistance is known to be lower than 304. There is a problem that inclusions such as MnS which are detrimental to the corrosion resistance are easily formed and the corrosion resistance is deteriorated. And, when operating the electric furnace, it causes environmental problems due to occurrence of Mn dust and the like. Accordingly, austenitic stainless steels having high strength and capable of being formed, which have improved moldability and corrosion resistance similar to those of the austenitic system while reducing Ni and Mn, and have improved strength, have been developed.

The austenitic stainless steel according to any one of claims 1 to 3, wherein the content of C is 0.08% or less (excluding 0), 0.5 to 1.5% of Si, 2.5 to 3.5% of Mn, 18 to 20% of Cr, 0.3 to 1.5% The steel sheet contains 0.2 to 0.4% of N, 0.3 to 1.5% of Cu, 0.01 to 0.1% of Ti, 0.01 to 0.1% of Nb, 0.01 to 0.1% of V and balance of Fe and other unavoidable impurities. And the proportion of the fired organic martensite is 10 to 40%.

C is an austenite forming element and is an effective element for increasing the strength of a material by solid solution strengthening. However, the Cr content and the carbide around the grain boundaries are lowered at the boundary between the ferrite phase and the austenite phase when Cr is added. In order to maximize the corrosion resistance, it is preferable to add C in the range of 0.08% or less, since this results in reducing the corrosion resistance.

Si is an element added for the deoxidation effect. Particularly, when the content of Ni is small, 0.5% or more is added to stabilize the austenite phase. When 1.5% or more of Si is added, the fired organic martensite formed on the austenite is insufficient and sufficient work hardening can not be obtained, thereby reducing the ductility. Also, Si is a substitutional element, and its solubility enhancement effect increases as the content of Si increases. On the other hand, if it is added in excess, the slag fluidity is lowered during steelmaking, and SiO ₂ inclusions are formed by binding with oxygen, thereby lowering the corrosion resistance. Therefore, it is preferable to limit the Si content to 0.5 to 1.5% so as to secure softness by forming sintered organic martensite while increasing the strength by solid solution strengthening.

N is an element contributing to the stabilization of typical austenite phase together with Ni in duplex stainless steel. N is an element necessary for supplementing Ni when it is an austenite forming element, and it can be intensified as an interstitial element. N is an element for improving corrosion resistance, and it is possible to compensate for the effect of reducing Mn by addition of Mn. If the N content exceeds 0.4%, blowholes, pinholes, and the like may be generated during casting due to excessive use of the resin, thereby causing surface defects. On the other hand, if the N content is too low, the effect of lowering the corrosion resistance caused by Mn addition becomes large, so that the N content is preferably limited to 0.20 to 0.4%.

Mn is an element that increases the solubility of deoxidizing agent and N, and can be used for Ni substitution as an austenite forming element. However, addition of more than 4% makes it difficult to obtain corrosion resistance at the level of 304 steel. This is because MnS combined with S in the steel is formed as an inclusion in the steel to deteriorate the corrosion resistance. When the content is less than 2.5%, it is difficult to secure the required austenite phase fraction even when the contents of the austenite forming elements Ni, Cu, N and the like are controlled, and the solubility of N to be added is low, . Therefore, the content of Mn is preferably limited to 2.5% to 4%.

Cr is an essential element for ensuring the corrosion resistance of stainless steel. Increasing the content increases the corrosion resistance, but in order to maintain the fraction of austenite phase, it is necessary to further increase the content of Ni and other austenite forming elements. Therefore, it is preferable to limit the Cr content to 18 to 20%.

Ni, together with Mn, Cu and N, plays a major role in securing the austenite phase of stainless steel as an austenite stabilizing element. Instead of reducing the costly Ni content as much as possible to reduce costs, the other austenite phase forming elements, Mn and N, are increased to compensate for the austenite phase reduction due to Ni reduction to maintain the phase fraction balance. When the content of Ni exceeds 1.5%, the austenite is excessively stabilized and the calcined organic martensite is not formed during the cold working. As a result, there is a problem that the elongation rate is lowered and the formability is lowered. As the content of Ni, which is an expensive element, becomes higher, it becomes difficult to secure the competitiveness of the product. When Ni is substituted with Mn and N, the minimum Ni content capable of retaining the austenite single phase and imparting acid resistance when used in an acid container is 0.3% or more. Therefore, the content of Ni is preferably limited to 0.3% to 1.5%.

Cu is an element stabilizing austenite together with Mn, Ni and N, and plays a main role in securing the austenite phase of the duplex stainless steel. It is possible to maintain the balance by canceling the reduction of the austenite phase fraction due to the reduction of Ni by increasing Cu, Mn, and N, which are other austenite phase forming elements, instead of reducing the high Ni content as much as possible. However, when Cu is excessively added, it is difficult to secure a sufficient elongation rate because the formation of fired organic martensite is suppressed during cold working. In addition, if a large amount of Cu is added, it may precipitate in the form of inclusions exceeding the solubility of Cu, which may cause a welding defect problem in the subsequent process. Therefore, copper should be added in an amount of 1.5% or less. In order to exert the function of securing the austenite phase with Cu solved, a content of at least 0.3% or more is required. Therefore, it is preferable to limit the Cu content to 0.3% to 1.5%.

Ti, Nb, and V are precipitation hardening elements that bond with carbon or nitrogen. The addition of these elements can suppress the formation of Cr precipitates which are generated during cooling during cold annealing. Further, when cooling the welded portion, the Ti, Nb and V precipitates are formed earlier than the Cr precipitates, and the formation of Cr precipitates in the welded portions is suppressed, so that deterioration in corrosion resistance can be prevented. At least 0.01% or more of these alloying elements should be added to suppress the formation of Cr precipitates. However, when these elements are added in an amount exceeding 0.1%, they are pitted out of the molten steel during casting, causing clogging of the casting nozzle, and precipitating as inclusions, which causes surface defects during hot rolling and cold rolling. These inclusions of Ti, Nb and V precipitate on the grain boundaries during hot rolling annealing and cold rolling annealing to interfere with the growth of the crystal grains and inhibit the formation of fired organic martensite during cold working, resulting in a problem of lowering the elongation. Therefore, it is preferable to limit the content of Ti, Nb and V to 0.01% to 0.1%.

The fired organic martensite formed on the austenite is a structure that affects the work hardening. In order to improve the workability, the ratio of the fired organic martensite should be 10% or more. That is, when the ratio of the fired organic martensite at the fracture site is less than 10%, the localized necking occurring during processing can not be suppressed, so that the fracture occurs and the formability is lowered. However, when the amount of the fired organic martensite exceeds 40%, the strength rapidly increases at the beginning of the processing, and processing can not be performed to the desired molding range. This is because the fired organic martensite is formed rapidly. Therefore, in order to maximize the workability, it is preferable to limit the amount of fired organic martensite formed at the rupture portion to 10 to 40%.

The volume fraction of the residual ferrite is preferably 5% or less.

When the proportion of ferrite in the structure of the austenitic stainless steel is controlled to 5% or less, the corrosion resistance can be secured. If the content of ferrite exceeds 5%, defects may occur on the surface due to heterogeneous oxidation due to the ferrite phase upon annealing. Further, the excessive ferrite phase content causes a decrease in corrosion resistance due to nonuniform distribution of alloying elements. Therefore, the volume fraction of ferrite is preferably 5% or less.

It is preferable that the elongation is 50% or more and the yield strength is 450 MPa or more.

By limiting the amount of fired organic martensite as described above, the elongation and strength increase simultaneously, but the elongation and strength can be achieved by limiting the elongation to 50% or more and the yield strength to 450 MPa or more.

C Cr Mn Ni Si Cu N Ti Nb V Comparative Example 1 0.04 18 1.3 7.8 0.4 - 0.04 - - - Comparative Example 2 0.025 21.84 4.5 1.9. 0.45 0.2 0.19 0.03 - - Comparative Example 3 0.05 19.8 2.98 1.02 0.34 2.07 0.238 - 0.05 - Comparative Example 4 0.025 18.5 3.5 0.3 1.7 0.7 0.27 0.06 0.03 0.07 Example 1 0.047 18.26 3.7 1.41 0.619 1.41 0.26 0.04 0.06 0.05 Example 2 0.047 18.42 3.84 1.42 0.96 1.43 0.286 0.07 0.02 0.09 Example 3 0.042 19.5 2.6 1.0 0.55 1.4 0.35 0.04 0.06 0.05 Example 4 0.047 18.26 3.94 1.42 0.619 1.38 0.256 0.06 0.04 0.02 Example 5 0.06 18.48 3.05 0.4 1.4 1.4 0.26 0.04 0.03 0.05 Example 6 0.047 18.26 3.94 1.42 0.619 0.6 0.256 0.09 0.06 0.05

Yield strength
(MPa) Elongation
(%) ferrite
(%) Fired organic martensite (%) Annealing condition Comparative Example 1 278 61 1.7 43 1080 ° C
80 seconds Comparative Example 2 530 44 25 10 Comparative Example 3 510 46 35 30 Comparative Example 4 535 43 31 27 Example 1 480 57 2.1 38 Example 2 459 51.9 3.4 20 Example 3 530 53.2 4.3 22 Example 4 570 52 4.0 15 Example 5 470 59 1.5 31 Example 6 542 56.5 2.6 33

Specimens of austenitic stainless steels according to the composition range of the present invention and specimens used as comparative examples were prepared by using vacuum melting. The prepared alloy was subjected to hot rolling, hot rolling annealing, cold rolling and cold rolling annealing to measure the phase fraction of the material and the physical properties of the material. Table 1 shows the alloy composition (% by weight) for the experimental steel types, and Table 2 shows the yield strength, the elongation, the amount of ferrite and the amount of fired organic martensite at the time of fracture.

In the case of Comparative Example 1, the conventional austenitic stainless steel has an elongation as high as about 60%. However, since the yield strength is less than 300 MPa, there is a limit to use as a structural material. Also, since the Ni content is high, the manufacturing cost is high. In the present invention, in order to solve such a problem, Ni is decreased and Mn and N are increased. As in the case of Comparative Examples 2 to 4, when a large amount of ferrite is present, it exhibits relatively good yield strength properties, but is disadvantageous in terms of elongation because it has a low fraction of austenite that is transformed into martensite. In the case of the embodiment, it is possible to secure ductility (at least 50% or more) equivalent to that of Comparative Example 1 and 1.5 times or more the strength (450MPa or more) of Comparative Example 1.

Steel grade Formula potential (mV) Annealing
Condition Comparative River 1 310

1080 ° C
80 seconds Inventive Steel 1 320 Invention river 2 350 Invention steel 3 345 Inventive Steel 4 380 Invention steel 5 322 Invention steel 6 331

Table 3 shows the corrosion resistance of some steels. In the test method, the cold rolled annealed cold rolled steel sheet was surface-polished with # 600 abrasive paper, and the formal potential was measured in a 3.5% NaCl solution. It can be seen that the formula potential of Comparative Example 1 is about 310 mV, and 320 to 380 mV can be secured for Examples 1 to 6. Therefore, it can be seen that the embodiment of the present invention has a corrosion resistance equal to or higher than that of Comparative Example 1.

The method of manufacturing an austenitic stainless steel according to one embodiment of the present invention is a method for producing austenitic stainless steel which comprises 0.08% or less of C (excluding 0), 0.5-1.5% of Si, 2.5-3.5% of Mn, 0.3 to 1.5% of Ni, 0.2 to 0.4% of N, 0.3 to 1.5% of Cu, 0.01 to 0.1% of Ti, 0.01 to 0.1% of Nb, 0.01 to 0.1% of V, balance Fe and other unavoidable The cold-rolled steel sheet containing impurities is preferably subjected to cold-rolling annealing at a temperature of 1050 to 1100 占 폚 for 45 to 180 seconds.

The numerical limitation on the respective properties is replaced with the description of the above-described austenitic stainless steel.

As shown in Fig. 1, when the annealing temperature is less than 1050 占 폚 or the annealing time is less than 45 seconds, the fired organic martensite is formed to less than 10%, so the elongation rate is decreased. When the annealing temperature exceeds 1100 占 폚 If it exceeds 180 seconds, fired organic martensite is produced in excess of 40%. As a result, the elongation percentage becomes less than 50%. Therefore, the annealing temperature and the annealing time are preferably limited as described above.

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

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims (4)

The steel sheet according to any one of claims 1 to 3, wherein the steel contains, by weight%, 0.08% or less of C, 0.5 to 1.5% of Si, 2.5 to 3.5% of Mn, 18 to 20% of Cr, 0.3 to 1.5% of Ni, 0.2 to 0.4% 0.3 to 1.5% of Cu, 0.01 to 0.1% of Ti, 0.01 to 0.1% of Nb, 0.01 to 0.1% of V, balance Fe and other unavoidable impurities,
The ratio of the fired organic martensite in the tensile fractured section is 10 to 40%
The residual ferrite content is 5% or less by volume,
An elongation percentage of 50% or more, and a yield strength of 450 MPa or more.
delete delete The steel sheet according to any one of claims 1 to 3, wherein the steel contains, by weight%, 0.08% or less of C, 0.5 to 1.5% of Si, 2.5 to 3.5% of Mn, 18 to 20% of Cr, 0.3 to 1.5% of Ni, 0.2 to 0.4% A cold rolled steel sheet containing 0.3 to 1.5% of Cu, 0.01 to 0.1% of Ti, 0.01 to 0.1% of Nb, 0.01 to 0.1% of V and balance of Fe and other unavoidable impurities at a temperature of 1050 to 1100 ° C and 45 to 180 Cold annealing for a second,
Wherein a steel having an amount of residual ferrite of 5% or less in volume fraction, an elongation of 50% or more, and a yield strength of 450 MPa or more is produced.

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