KR20230093652A - High corrosion resistant austenitic stainless steel with reduced sigma phase and surface defects, and the manufacturing method thereof - Google Patents

Abstract

An object of the present invention is to provide a method for producing a highly corrosion-resistant austenitic stainless steel in which sigma phase and cracking surface defects are suppressed. To this end, C 0.05% or less, Si 1.0% or less, Mn 2.0% or less, Cr 16-20%, Ni 10-15%, Cu 1.0% or less, Mo 2-4%, N 0.1% or less, and the rest by weight% For highly corrosion-resistant austenitic stainless steels containing Fe and unavoidable, after heating slabs, ingots, blooms, and billets produced by continuous casting or ingots in the range of 1140 to 1245 ° C for 1 hour or more, 20 to 40% of After hot rolling at a reduction ratio, stainless steel can be produced by a method of final rolling to a target size after heating for 1 hour or more in the range of 1200 to 1245 ° C. When the hot-rolled stainless steel is heat treated at 1150 to 1200 ° C, the residual sigma phase fraction can be controlled to 0.2% or less, and the crack surface defect can be controlled to 1.0 or less per 10 m of steel.

Description

Highly corrosion-resistant austenitic stainless steel with suppressed sigma phase and surface defects and method for manufacturing the same

The present invention relates to an austenitic stainless steel with suppressed sigma phase and cracking surface defects and excellent corrosion resistance, and a method for manufacturing the same. The austenitic stainless steel presented in the present invention is produced as a plate material through a continuous casting process and can be applied to applications such as chemical plants, pipes and industrial members, and is produced as a billet or bloom and manufactured into wire rods, steel bars, and shaped steel to be used. can

Austenitic stainless steel is a steel material that is used in various fields from kitchen utensils to industrial facilities based on its excellent corrosion resistance. Corrosion resistance in austenitic stainless steel is greatly influenced by the content of alloying elements, and is particularly affected by the content of Cr and Mo. A typical example of austenitic stainless steel with high corrosion resistance containing Cr and Mo is STS316. That is, in order to improve the corrosion resistance of steel materials, it is possible by increasing the amount of Cr and Mo added. However, when the content of Cr and Mo increases, precipitation of the sigma phase, which is an intermetallic compound, is promoted in the microstructure, and when it remains without being completely decomposed, it acts as a cause of lowering corrosion resistance and impact toughness. it is desirable

As a method for suppressing the sigma phase, Patent JP2015-175017 discloses a method of reducing the fraction at which the sigma phase is formed by controlling the contents of Cr, Mo, Ni, and N. For this purpose, in terms of mass%, C: 0.005 to 0.030%, Ni: 10.00 to 30.00%, Cr: 17.00 to 25.00%, Mo: 2.00 to 6.00, N: 0.01 to 0.30%, P: 0.005 to 0.030%, S: 0.0005 to Although it is disclosed that the austenitic stainless steel of 0.0015% and REM≤0.050% satisfies the following formula (1), the process of decomposing the sigma phase is not mentioned. Equation (1): 3ХCr mass%+5ХMo mass%-Ni mass%-3ХN mass%≤60

In addition, in patent KR10-2014-0014280, C: 0.05% or less, Si: 0.01 to 1.0%, Mn: 1.75 to 2.50%, P: 0.05% or less, S: 0.01% or less, Cr: 23.0 to 27.0%, Ni: 20.0 to 24.0%, Mo: 1.8 to 3.2%, N: 0.11% to 0.18% of austenitic stainless steel materials that satisfy 0.1% or less of the sigma phase are provided, but a method by component control and 1080 to Only a general method of removal through solution heat treatment at 1120 ° C is disclosed.

The decomposition of the sigma phase should be performed by heat treatment at high temperature for a long time to diffuse Cr and Mo elements to the surrounding area to be decomposed. In the steelworks process, a process capable of high-temperature, long-term heat treatment is a reheating process in which the casting material is heated before hot rolling, and the more times the reheating process is performed, the more sigma phase can be decomposed. However, due to the growth of austenite grain size during the reheating process, the hot workability of the material is lowered, resulting in increased occurrence of cracked surface defects and an increase in cost. Therefore, it is important to design a process within the range of simultaneously suppressing sigma phase and cracking defects even though reheating is performed twice or more.

(Patent Document 1) Japanese Unexamined Patent Publication No. 2015-175017 (published date: 2015.10.05.)

(Patent Document 2) Korean Patent Laid-Open Publication No. 10-2014-0014280 (published date: 2014.02.05.)

The present invention suppresses the occurrence of sigma phase and cracking surface defects by improving the process without changing the components of the steel material for the highly corrosion resistant austenitic stainless steel, thereby suppressing the sigma phase and surface defects. It is an object of the present invention to provide steel and a manufacturing method thereof.

In a typical austenitic stainless steel manufacturing process, a cast material is extracted by heating it at a temperature in the range of 1200 to 1300 ° C for more than 1 hour, and then hot-rolled to a desired size, followed by solution heat treatment and pickling to produce a hot-rolled product. or made into cold-rolled products through additional cold-rolling and cold-rolling annealing and pickling processes. In the present invention, the cold rolling and cold rolling annealing and pickling processes are not covered, and the casting material is heated and hot rolled, and then the solution heat treatment process is targeted.

According to an embodiment of the present invention, the highly corrosion-resistant austenitic stainless steel in which sigma phase and surface defects are suppressed contains, by weight, C 0.05% or less, Si 1.0% or less, Mn 2.0% or less, Cr 16-20%, Ni 10-15%, Cu 1.0% or less, Mo 2-4%, N 0.1% or less, and the rest includes Fe and unavoidable impurities, and the area ratio of the sigma phase in the microstructure observed in the 5mm ² area of the center of the thickness of the steel is 0.2 % or less is satisfied, and the number of cracked surface defects on the surface of the steel is 1.0 or less per 10 m.

In addition, according to another embodiment of the present invention, a method for producing a highly corrosion-resistant austenitic stainless steel with suppressed sigma phase and surface defects contains, by weight, C 0.05% or less, Si 1.0% or less, Mn 2.0% or less, Cr 16 to 20%, Ni 10 to 15%, Cu 1.0% or less, Mo 2 to 4%, N 0.1% or less, and the rest preparing a slab containing Fe and unavoidable impurities; Extracting the prepared slab after heating at a temperature of 1140 to 1245 ° C. for 1 hour or more and hot rolling at a reduction rate of 20 to 40%; And a step of extracting and hot rolling after heating at a temperature of 1200 ~ 1245 ℃ for 1 hour or more.

According to the present invention, in highly corrosion-resistant austenitic stainless steel used in industrial facilities, electronic devices, etc., austenitic stainless steel capable of securing a sound surface with suppressed surface defects while reducing sigma phase, and a method for manufacturing the same By providing, it is possible to reduce the quality cost of producers and prevent problems of corrosion and impact toughness degradation that may occur during the use of steel materials.

1 is a graph showing the degree of occurrence of cracked surface defects according to the reheating temperature of slabs and intermediate materials.

Hereinafter, the present invention will be described in detail. In general, Mo is added to austenitic stainless steel having corrosion resistance of STS316 or higher to improve corrosion resistance. Austenitic stainless steels widely used for high corrosion resistance include STS316 and STS317, and there are also austenitic stainless steels that exhibit higher corrosion resistance than these, but the amount used is relatively small. Therefore, in the present invention, 0.03% or less of C, 1.0% or less of Si, 2.0% or less of Mn, 16 to 20% of Cr, 10 to 15% of Ni, 2 to 4 The target was limited to austenitic stainless steels containing % Mo, 0.1% or less N, the rest Fe and unavoidable impurities.

According to an embodiment of the present invention, the highly corrosion-resistant austenitic stainless steel in which sigma phase and surface defects are suppressed contains, by weight, C 0.05% or less, Si 1.0% or less, Mn 2.0% or less, Cr 16-20%, Ni 10-15%, Cu 1.0% or less, Mo 2-4%, N 0.1% or less, and the rest includes Fe and unavoidable impurities, and the area ratio of the sigma phase in the microstructure observed in the 5mm ² area of the center of the thickness of the steel is 0.2 % or less is satisfied, and the number of cracked surface defects on the surface of the steel is 1.0 or less per 10 m.

[Ingredient system]

Hereinafter, the content range of the alloy component will be described. The content of each component means % by weight.

C: 0.05% or less

C (carbon) is a strong austenite phase stabilizing element and is an effective element for increasing material strength by solid solution strengthening. However, when the content is excessive, the content of C is limited to 0.05% or less because carbide-forming elements such as Cr, which are effective for corrosion resistance, are easily combined at the grain boundaries to lower the Cr content around the grain boundaries to reduce corrosion resistance. In order to minimize the risk of carbide precipitation that may impair corrosion resistance, it is preferable to set the C content to 0.03% or less.

Si: 1.0% or less

Si, which also acts as a ferrite phase stabilizing element, is effective in improving corrosion resistance, but when excessive, it promotes the precipitation of intermetallic compounds such as sigma phase and deteriorates mechanical properties and corrosion resistance related to impact toughness, so it is limited to 1.0% or less.

Mn: 2.0% or less

Mn is an austenite phase stabilizing element such as C and Ni, and can improve N solid solubility. However, since an increase in Mn content is involved in the formation of inclusions such as MnS, which is undesirable when corrosion resistance is required, it is preferable to limit the Mn content to 2.0% or less in order to secure corrosion resistance.

Cr: 16-20%

Cr is a basic element that is contained the most among the corrosion resistance improving elements of stainless steel, and must be included at least 16% or more to express corrosion resistance. However, Cr is a ferrite stabilizing element, and as the Cr content increases, the ferrite fraction increases and promotes the formation of a sigma phase, causing deterioration in mechanical properties and corrosion resistance. Therefore, it is preferable to limit the Cr content to 20% or less.

Ni: 10~15%

Ni is the most powerful element among the austenite phase stabilizing elements, and must be contained in an amount of 10% or more to obtain an austenite structure. However, since an increase in Ni content is directly related to an increase in raw material price, it is preferable to limit it to 15% or less.

Mo: 2-4%

Mo is a useful element for improving corrosion resistance, and when added in an amount of 2.0% or more, it can improve corrosion resistance. However, Mo is a ferrite stabilizing element, and when a large amount is added, the ferrite fraction is increased, and the formation of the sigma phase is promoted to cause a decrease in mechanical properties and corrosion resistance, so it is preferable to limit it to 4% or less.

Cu: 1.0% or less

Cu has an advantage of improving corrosion resistance in a sulfuric acid atmosphere. However, in a chlorine atmosphere, pitting resistance is reduced and hot workability is reduced, limiting the Cu content to 1.0% or less.

N: 0.1% or less

N is an element useful for stabilizing the austenite phase as well as improving corrosion resistance in a chlorine atmosphere. However, when added in large amounts, hot workability is reduced to reduce the real yield of steel, so it is preferable to limit it to 0.1% or less.

According to another embodiment of the present invention, a method for producing a highly corrosion-resistant austenitic stainless steel with suppressed sigma phase and surface defects contains, by weight, C 0.05% or less, Si 1.0% or less, Mn 2.0% or less, Cr 16- 20%, Ni 10-15%, Cu 1.0% or less, Mo 2-4%, N 0.1% or less, and the rest preparing a slab containing Fe and unavoidable impurities; Extracting the prepared slab after heating at a temperature of 1140 to 1245 ° C. for 1 hour or more and hot rolling at a reduction rate of 20 to 40%; And a step of extracting and hot rolling after heating at a temperature of 1200 ~ 1245 ℃ for 1 hour or more.

In addition, according to another embodiment of the present invention, a method for manufacturing a highly corrosion-resistant austenitic stainless steel with suppressed sigma phase and surface defects is hot-rolled austenitic stainless steel twice after hot rolling at 1100 to 1180 Annealing heat treatment in the range of ℃, it may include the step of controlling the area ratio of the sigma phase in the final microstructure observed in the central 5mm ² region of the thickness of the steel to 0.2% or less.

In addition, according to another embodiment of the present invention, in the method for manufacturing a highly corrosion-resistant austenitic stainless steel with suppressed sigma phase and surface defects, the number of cracked surface defects on the surface of the steel material may be 1.0 or less per 10 m. .

[Limitation of reheating temperature of cast material and intermediate material]

As described above, in order to decompose the sigma phase, it is necessary to perform heat treatment at high temperature for a long time to diffuse Cr and Mo elements contained in the sigma phase to the surroundings. The only process that can be used is the heating furnace process that heats the cast material before hot rolling. However, in such high-temperature and long-term heat treatment, it is unavoidable that the microstructure of the cast material grows and becomes larger, and when the microstructure of the cast material grows excessively, crack defects occur easily during hot rolling along fragile grain boundaries. Therefore, while decomposing the sigma phase, it is necessary to set an appropriate temperature range in which the growth of the microstructure of the cast material does not occur excessively. In the present invention, through experiments under various conditions, the appropriate reheating temperature range was limited to 1140 to 1245 ° C. for the cast material and 1200 to 1245 ° C. for the intermediate material.

Since the columnar crystals formed in the process of casting steel are exposed to the surface as they are during hot rolling, in order to prevent cracking surface defects, the crystal grain growth must be further suppressed by reheating at a lower temperature than the intermediate material. However, when reheated below 1140 ° C., the decomposition rate of the sigma phase slows down and the sigma phase suppression effect decreases, and the temperature of the material during hot rolling is not sufficient, resulting in a decrease in hot workability and an increase in the rolling load. In addition, when reheating at a temperature of 1245 ° C or higher, the crystal grain growth of the casting material is excessive and cracking surface defects increase.

In the case of the intermediate material, the reheating temperature range was limited to 1200 ~ 1245 ℃ for the same reason as above, but the cumulative reduction in the intermediate material is greater than the process of making the cast material into the intermediate material, so the reheating temperature should be higher than that of the cast material to hot work to the desired size. It becomes easier.

[Limited to casting material rolling reduction range]

After reheating the casting material in the temperature range of 1140 ~ 1245 ℃ for more than 1 hour, hot rolling in the range of 20 ~ 40% reduction ratio helps to suppress cracking defects. When hot rolling is performed after reheating, shear stress is applied to the material, which can play a role in refining the surface texture of the cast material. However, when the reduction ratio is less than 20%, the shear stress transmitted to the surface is weak, so that the recrystallization action of refining crystal grains during rolling is weakened, so that it does not play a large role in suppressing cracking defects. In addition, although the cast material can be hot-rolled at a reduction ratio of 40% or more, since there is a subsequent process of hot-rolling the intermediate material thereafter, it is preferable to increase productivity by hot-rolling only to the extent that cracking surface defects can be prevented.

[limited range of annealing heat treatment temperature]

In general, annealing heat treatment after hot rolling of austenitic stainless steel is performed at a temperature of 1050 ° C. or higher to recrystallize the microstructure of the material and re-dissolve precipitates such as carbide that may be formed during hot rolling. However, for austenitic stainless steels in which sigma phase precipitation is a concern, heat treatment should be performed by raising the heat treatment temperature to 1100° C. or higher to decompose the sigma phase. As described above, the decomposition of the sigma phase is facilitated by heat treatment at a high temperature, but when the heat treatment of the final product is performed at a high temperature, crystal grains grow excessively, causing deterioration of materials such as hardness and strength. In addition, when performing heat treatment at a high temperature of 1200 ° C. or higher in a normal continuous process, the life of the heat treatment equipment may be shortened due to strain on the equipment.

In the present invention, since the sigma phase can be reduced in the process of reheating the slab and the intermediate material, a sigma phase fraction of less than 0.2% can be obtained by heat treatment in the range of 1100 to 1180 ° C.

[Sigma phase fraction]

The sigma phase is an intermetallic compound containing Cr and Mo as main components, and since Cr and Mo, which are corrosion resistance forming elements, are supplied from the surroundings and precipitated, a depletion layer with a lower content of Cr and Mo is formed around the sigma phase than in other regions. , which acts as a starting point for corrosion to occur in this depleted layer. In addition, since the hardness of the sigma phase is higher than that of the parent material, the degree of polishing is different when surface polishing is performed, which acts as a factor in generating irregularities.

Corrosion resistance can be evaluated in various ways, but it is generally evaluated according to the ASTM G48 method. At this time, when the fraction of the sigma phase present on the cross section is less than 0.2%, weight loss due to corrosion when evaluated according to ASTM G28 Method A It was confirmed that there is almost no In addition, in the case of irregularities generated after surface polishing, the defect generation rate was as low as less than 1% when the sigma phase fraction was less than 0.2%. Therefore, in the present invention, the fraction of the sigma phase was limited to be less than 0.2%. At this time, the method of measuring the sigma image is to selectively express only the sigma image by etching with a NaOH solution after mirror polishing the cross section in the thickness direction of the material as the observation surface, and then using an optical microscope to obtain 10 consecutive microscopic images at a magnification of 50 times. Tissues were photographed and analyzed. In this case, the sigma phase is classified as black or dark gray, and the rest of the matrix tissue appears as white. Only the sigma phase fraction is identified using an image analysis tool. In this way, when 10 continuous microstructures are photographed at a magnification of 50 times, the photographed area corresponds to 5 mm ² .

[number of surface defects]

When processing austenitic stainless steel such as rolling, extrusion, forging, etc. in a hot state, cracking defects are likely to occur on the surface of the material. The intensity of cracking defects varies depending on the type of steel depending on the degree to which the material is easy to process in a hot state. Generally, less than 1.0 per 10m occurs when cracking surface defects are good. If more than that occurs, a process of removing cracked surface defects caused by grinding or polishing the surface after hot working is performed, which leads to an increase in production time and cost.

Examples will be described below.

(Example)

C Si Mn Cr Ni Mo Cu N PREN A 0.018 0.41 1.17 16.72 10.14 2.06 0.28 0.019 23.82 B 0.021 0.54 1.32 17.35 14.12 2.61 0.31 0.035 26.52 C 0.024 0.48 1.27 17.57 13.79 3.24 0.76 0.089 29.69

The pitting resistance index (PREN) was calculated as PREN = Cr + 16.0 × N - 0.5 × Mn, and steel grades A to C showed a PREN value of 23.82 or more.

After the steel having the chemical composition shown in Table 1 was melted in a vacuum induction melting furnace, the ingot was reheated and hot rolled to be processed into an intermediate material. The reheating temperature before hot rolling was carried out in the range of 1140 to 1280 ° C, and the holding time was adjusted to be more than 1 hour. When the ingot was hot rolled, the reduction ratio was adjusted in the range of 10 to 30%. After hot rolling the ingot under the above conditions, it was cooled to room temperature to be processed into an intermediate material, and reheating was performed to hot-roll the intermediate material again. When reheating the intermediate material, the reheating temperature was carried out in the range of 1215 ~ 1280 ℃, and the holding time was adjusted to be more than 1 hour. After hot-working the intermediate material to make a plate having a thickness of 6 mm in the above method, after cooling to room temperature, heat treatment was performed in the range of 1120 to 1180 ° C. for 1 minute, and cooled to room temperature using the water cooling method. The sigma phase fraction for each manufacturing condition and the number of surface defects after hot rolling are shown in Table 2 below.

division steel grade Slab* reheating temperature (℃) Primary
Reduction rate (%) intermediate goods
Reheat temperature (℃) Hot-rolled steel heat treatment temperature (℃) Sigma phase fraction (%) Number of surface defects (ea/10m) Comparative Example 1 A 1280 30 1245 1180 0.00 3.19 Comparative Example 2 A 1265 32 1220 1175 0.05 2.56 Example 1 A 1221 30 1220 1175 0.06 0.89 Comparative Example 3 A 1160 30 1280 1180 0.08 2.35 Example 2 A 1140 30 1220 1180 0.10 0.24 Example 3 A 1140 30 1220 1100 0.14 0.17 Comparative Example 4 B 1268 30 1220 1180 0.04 5.16 Comparative Example 5 B 1265 31 1218 1150 0.05 4.82 Example 4 B 1220 30 1215 1180 0.07 0.65 Example 5 B 1140 30 1220 1180 0.07 0.32 Example 6 B 1140 20 1218 1178 0.08 0.31 Example 7 B 1160 20 1220 1152 0.06 0.28 Comparative Example 6 B 1150 10 1220 1160 0.07 2.78 Example 8 B 1220 20 1225 1120 0.05 0.71 Comparative Example 7 C 1280 30 1240 1180 0.09 3.24 Comparative Example 8 C 1260 30 1245 1150 0.09 2.82 Example 9 C 1160 30 1220 1180 0.14 0.56 Example 10 C 1240 20 1245 1150 0.10 0.84

The slab* may include a slab, an ingot, a billet, a bloom, and the like. In the case of Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, Comparative Example 5, Comparative Example 7, and Comparative Example 8, regardless of the component system, when the temperature for reheating the ingot or intermediate material is as high as 1245 ° C or higher, Sigma Although the phase fraction was good, it was confirmed that the number of surface defects exceeded 1.0 per 10 m.

In addition, in the case of Comparative Example 6, when the reduction ratio of the ingot was as low as 10%, the shear stress formed on the surface was not sufficient, so the coarse columnar structure formed during ingot casting remained on the surface, and as a result, the reheating temperature was low, but the number of cracked surface defects occurred more than 1.0 per 10 m after hot rolling of the intermediate material.

In addition, in Examples 1 to 10 in which the reheating temperature was as low as 1245 ° C. or less and the reduction ratio of the ingot was maintained at 20 to 40%, the sigma phase fraction was less than 0.2% and the number of cracked surface defects was less than 1.0 per 10 m.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those skilled in the art are within the scope not departing from the concept and scope of the claims described below. It will be appreciated that various changes and variations are possible in

Claims (4)

0.05% or less of C, 1.0% or less of Si, 2.0% or less of Mn, 16-20% Cr, 10-15% Ni, 1.0% or less Cu, 2-4% Mo, 0.1% or less N, and the balance being Fe and Sigma phase and surface containing unavoidable impurities, the area ratio of the sigma phase in the microstructure observed in the 5 mm ² area of the center of the thickness of the steel material satisfies 0.2% or less, and the number of cracking surface defects on the surface of the steel material is 1.0 or less per 10 m. High corrosion resistance austenitic stainless steel with suppressed defects. 0.05% or less of C, 1.0% or less of Si, 2.0% or less of Mn, 16-20% Cr, 10-15% Ni, 1.0% or less Cu, 2-4% Mo, 0.1% or less N, and the balance being Fe and Preparing a slab containing unavoidable impurities;
Extracting the prepared slab after heating at a temperature of 1140 to 1245 ° C. for 1 hour or more and hot rolling at a reduction rate of 20 to 40%; and
A method for producing highly corrosion-resistant austenitic stainless steel with suppressed sigma phase and surface defects, comprising the step of extracting and hot rolling after heating at a temperature of 1200 to 1245 ° C. for 1 hour or more. The method of claim 2,
Hot-rolled austenitic stainless steel is hot-rolled twice and then subjected to annealing heat treatment in the range of 1100 ^to 1180 ° C. A method for producing a highly corrosion-resistant austenitic stainless steel in which sigma phase and surface defects are suppressed, comprising the steps of: The method of claim 3,
A method for producing a highly corrosion-resistant austenitic stainless steel in which the number of cracked surface defects on the surface of the steel material is 1.0 or less per 10 m, and sigma phase and surface defects are suppressed.

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