KR20100061601A - High strength ship-building steel with excellent general corrosion and pitting corrosion resistance at low ph chloride solution and excellent haz toughness and manufacturing method for the same - Google Patents

Abstract

PURPOSE: A high strength ship-building steel material with excellent general and local corrosion resistances at low pH chloride solution and excellent HAZ toughness and a manufacturing method thereof are provided to prevent the organization of HAZ(Heat-Affected Zone) from being coarsened during welding by adding Ti, Nb, and V to a steel material and controlling the relation with nitrogen. CONSTITUTION: A high strength ship-building steel material comprises one or more elements selected from the group consisting of C 0.02~0.2 weight%, Si 0.05~1.5 weight%, Mn 0.2~2.0 weight%, P 0.015 weight% or less, S 0.003 weight% or less, Cu 0.05~1.0 weight%, Al 0.001~0.1 weight%, N 0.003~0.015 weight%, Ti 0.005~0.05 weight%, Nb 0.005~0.1 weight%, Ni 0.05~3.0 weight%, Cr 0.02~1.0 weight%, Mo 0.02~0.5 weight%, W 0.02~0.5 weight%, and Ca 0.0005~0.01 weight%, and the rest Fe and an inevitable impurity.

Description

STRENGTH SHIP-BUILDING STEEL WITH EXCELLENT GENERAL CORROSION AND PITTING CORROSION RESISTANCE AT LOW PH CHLORIDE SOLUTION AND EXCELLENT HAZ TOUGHNESS AND MANUFACTURING METHOD FOR THE SAME}

The present invention relates to a high-strength marine steel having excellent front and local corrosion resistance and toughness of weld heat affected zone in a strong acid saline solution, and a method of manufacturing the same. More specifically, the present invention relates to a brine contained in crude oil, such as a bottom of an oil tanker tanker. The present invention relates to a high strength marine steel having excellent front and local corrosion resistance as well as excellent weld heat affected zone toughness in a solution environment in which the basicity of the brine is lowered by corrosion.

Among various steels used in ships, especially steels used in oil tankers for oil tankers, very severe corrosion damage occurs due to the environment inside the oil tanks. Inside the crude oil tank, various types of corrosion progress due to volatile or mixed seawater in crude oil, inert gas sent into the tank for explosion prevention of salt in dielectric salt, condensation due to internal temperature difference, etc. It is much larger than that. In particular, induction bottom plate of the crude oil tanker has a large number of holes (Pit) of less than 50mm in diameter, the growth rate of the cultivation is up to 4mm / y. Unlike the bottom plate of a crude oil tank, the top plate has a corrosion rate of up to 0.3 mm / y, which does not significantly exceed the average wear rate of 0.1 mm / y due to corrosion considered in hull design.

In order to avoid such corrosion damage, some anticorrosive coating of crude oil tank material has been carried out, but there is an initial coating cost and a cost of repainting in the future, and in some paint defects, local corrosion may be further promoted. In addition, in consideration of the thickness loss due to corrosion, if the steel sheet thickness is thicker, not only the steel cost but also the weight of the ship itself becomes heavy, causing various problems such as the increase of fuel consumption. There is a demand for development of excellent marine steels.

Differences in the type of corrosion and the rate of corrosion by parts in the crude oil tanks result from the differences in the corrosion environment and the mechanism of corrosion. In the crude oil tank top, corrosion is caused by the reaction of H ₂ S gas evaporated from crude oil and CO ₂ , SO ₂ , O _2, etc. among inert gases injected for explosion protection with condensation formed on the steel surface due to temperature difference. On the other hand, in the bottom plate, corrosion is started by salt water contained in crude oil or seawater introduced from the surrounding area, but the area to be corroded is limited to the site where the defect of the oil coating layer is generated. As this progresses, the salt water present in the corroded portion is acidified. As the hydrogen ions generated by corrosion deteriorate with chlorine ions in the brine and continue to stagnate at the corrosion sites, the local corrosion sites become more acidic and the corrosion speed becomes faster with acidification. do. In fact, the analysis of the solution collected from the inside of the plant found in the ship in operation showed a very low acidity with a pH close to 1.0. Therefore, the steel used as a crude oil tank preferably exhibits excellent corrosion resistance in a strong acid brine atmosphere.

In addition, steel materials used in ships are exposed to various welding environments, so excellent weldability and weld properties are essential. In particular, currently used crude steel for crude oil tank is less than 20mm mainly because the heat input amount during welding is not high, so the toughness of the weld heat affected zone is not serious, but in recent years the steel tank for crude oil tank is also high strength It is getting angry and materialized. In welding, the heat affected zone, in particular the weld heat affected zone near the melting line, is heated to a temperature close to the melting point. Therefore, since the grain size of the weld heat affected zone grows coarsely and the cooling rate increases, the low temperature structure such as martensite or MA (Martensite-Austenite Constituent), which is vulnerable to toughness, is formed. Serious. In particular, as described above, when the steel material is thickened, the welding productivity decreases when welding with a small amount of heat input. Therefore, the welding heat input tends to increase. As the amount of heat input increases, the temperature around the weld increases gradually. The growth rate of the austenite grains is faster and the cooling rate is further increased, thereby making it easier for low temperature transformation. Therefore, it is preferable that austenite grain growth is suppressed at the welded site and low temperature transformation phase does not occur.

In order to improve corrosion resistance of ship steels, the following techniques have been proposed.

Japanese Patent Application Laid-Open No. 2000-17381 proposes a shipbuilding steel having excellent corrosion resistance in an environment of use such as ship shell, ballast tank, cargo oil tank, photocargo hold, etc. %: C: 0.01 to 0.25%, Si: 0.05 to 0.50%, Mn: 0.05 to 2.0%, P: 0.10% or less, S: 0.001 to 0.10%, Cu: 0.01 to 2.00%, Al: 0.005 to 0.10% , Mg: 0.0002 to 0.0150%, the remainder being Fe and inevitable impurities, shipbuilding corrosion resistant steel, preferably Ni, Cr, Mo, W, Ca, REM, Ti, Nb, V , B, Sb, Sn is a steel containing an appropriate amount of one or more.

However, the oil for transporting crude oil having excellent corrosion resistance described in Japanese Patent Application Laid-Open No. 2001-17381 is effective in reducing the speed of local corrosion occurring in the bottom plate because the content of Si in its composition is limited to 0.5% or less. In addition, since Ni, Cu and Cr are limited to more than 0.5%, it may cause problems such as slab surface cracking in the manufacture of steel, and the amount of alloy added to ensure corrosion resistance may be excellent in corrosion resistance. There is a problem that is difficult. In addition, there is a disadvantage that the weldability is poor because the amount of alloying elements is increased.

Further, Japanese Patent Laid-Open No. 2001-214236 proposes a steel having excellent corrosion resistance when storing liquid fuel and raw fuel such as crude oil and heavy oil in crude oil tankers and oil tanks. : 0.003 to 0.30%, Si: 2.0% or less, Mn: 2.0% or less, Al: 0.10% or less, P: 0.050% or less, S: 0.050% or less, In addition to this, Cu: 0.01 to 2.0%, Ni: 0.01 -7.0%, Cr: 0.01-10.0%, Mo: 0.01-4.0%, Sb: 0.01-0.3%, Sn: 0.01-0.3% of any one or two or more thereof is added, and the balance is Fe and unavoidable impurities. Corrosion resistant steel for crude oil and heavy oil storage is described. However, Sb and Sn are mainly added to improve the tensile strength and have a slight effect on corrosion resistance while reducing elongation and impact value. In addition, since both elements are low melting points and cause hot brittleness, tempering brittleness, low temperature brittleness, the addition of these elements may cause problems in the steel manufacturing process.

Another proposal for corrosion resistant steel is Japanese Patent Application Laid-Open No. 2002-173736, which provides steel and a method of producing corrosion resistance even under the environment of a tank transporting and storing crude oil. Steels provided by C: 0.001 to 0.20%, Si: 0.10 to 0.40%, Mn: 0.50 to 2.0%, P: 0.020% or less, S: 0.010% or less, Al: 0.01 to 0.10%, Cu: 0.5 to 1.5%, Ni: 0.5-3.0%, Cr: 0.5-2.0%, or 1.0≤0.3 Ni + 2.0 Cr-0.5 Cu≤3.8 (where Ni, Cr, Cu: content of each element (mass %)) It is characterized by hot rolling to a steel material having a satisfactory composition, and cooling and producing at a cooling rate of 0.1 to 20 ° C / sec. In addition to the above composition, the steel has a composition which may further include one or two or more selected from Mo, Ti, Nb, V, and B, and one or two selected from Zr and Ca.

However, the present invention, like Japanese Patent Publication No. 2001-17381, has a negative effect on the reduction of the propagation speed of local corrosion caused by the bottom plate because it limits Si to 0.4% or less, which is advantageous for improving the corrosion resistance in the crude oil tanking environment. Since Cr is limited to 0.5% or more, it may cause problems such as surface cracks of slabs in the manufacture of steel, and there is a problem in that corrosion resistance cannot be obtained for cost due to the amount of alloy added.

In addition, Japanese Patent Application Laid-Open No. 2003-82435 proposes a steel material for a low cost cargo oil tank having excellent corrosion resistance. C: 0.01 to 0.3% Si: 0.02 to 1% Mn: 0.05 to 2% P: 0.05% or less S : 0.01% or less Ni: 0.03 to 3%, consisting of balance Fe and impurities, and additionally Mo, Cu, Cr, W, Ca, Ti, Nb, V, B, Sb, Sn and Al as needed. We propose steel having composition to include. In another embodiment, C: 0.01 to 0.3% Si: 0.02 to 1% Mn: 0.05 to 2% P: 0.05% or less S: 0.01% or less Ni: 0.01 to 3% Cu: 0.01 to 2% Cr: 0.05% It is less than 30 inclusions per cm < ² > containing Al: 0.07% or less, remainder Fe and an impurity, and exceeding 30 micrometers of particle diameters, It is the mass% of Ap and carbon which are the percentage unit of pearlite in a structure. A steel is proposed in which the relationship of C satisfies Ap / C≤130.

However, the above publication said that the addition of Cr is not preferable because it is harmful to the corrosion resistance in the crude oil tank environment, and not added to 0.05% or less, which is not applicable under strong acid brine conditions, and sufficient corrosion resistance by not adding Cr There is a problem that strength improvement cannot be obtained.

In addition, Korean Patent Laid-Open Publication No. 2005-0008832 shows excellent front and local corrosion resistance against crude oil corrosion generated in steel tanks, and also produces a corrosion product (sludge) containing solid S. To provide crude oil tank steel for welded structure, crude oil tank steel production method, crude oil tank oil and crude oil tank method, which can suppress the pressure of the welded structure. %: C: 0.001-0.2%, Si: 0.01-2.5%, Mn: 0.1-2%, P: 0.03% or less, S: 0.007% or less, Cu: 0.01-1.5%, Al: 0.001-0.3%, N : Cr: 0.001 to 0.01%, Mo: 0.01% to 0.2%, W: 0.01% to 0.5% or more, and more preferably Crude Oil by satisfying the solid solution Mo + solid solution W = 0.005% Of corrosion products (sludge) containing full S and local corrosion resistance in oil tank environment It can be seen that steel is provided to suppress the formation.

Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

However, the Japanese Unexamined Patent Publication No. 2003-82435 also suggests that the addition of Cr is limited to 0.05% or less because it is harmful to corrosion resistance, but the addition of Cr is a crude oil tank environment. Since it is not harmful to the corrosion resistance at all, it is advantageous to improve the corrosion resistance and the strength, there is a problem that can not be obtained through the addition of Cr to limit the corrosion resistance.

In addition, Japanese Patent Application Laid-Open No. 2005-171332 contributes to the extension of the repair repainting life of ship ballast tanks and to the reduction of repair repainting work, and has excellent corrosion resistance to avoid deterioration of weldability, weld part toughness, and rising manufacturing cost. According to the above publication which provides a steel material for ship ballast tanks, a zinc rich primer coated steel which is coated with a zinc rich primer on the surface of the descaled steel, wherein the steel is, in weight%, C: 0.03-0.2%, Si: 0.5% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.005 to 0.06%, Ni: 0.1 to 1.0%, N: 0.0020 to 0.0065%, Ti: 0.005 to 0.024 It is described that it can have corrosion resistance by having the composition which consists of%, and which consists of remaining thing Fe and an unavoidable impurity.

However, the steel provided in the present invention is not only used in the sea water atmosphere, but also to apply a zinc rich primer on the surface of the steel, there is a distance from the steel used in the strong acid salt water atmosphere without coating as in the present invention. In addition, the steel of the present invention also has a low Si content and does not add Cr, so there is a problem that sufficient corrosion resistance cannot be obtained in a strong acid brine atmosphere.

Japanese Patent Laid-Open Publication No. 2005-290479 proposes a steel material for a crude oil tank bottom plate having excellent local corrosion resistance even when used without application of an antirust paint, and as a chemical component, in mass%, C: 0.001 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.01% or less, Cu: 0.1 to 1%, Ni: 0.01 to 2%, Cr: Crude oil comprising 0.1 to 4% of Mo: 0.001 to 1% and having a composition comprising residual Fe and unavoidable impurities, wherein the value of Pcm represented by the following formula (1) is 0.22 or less. Excellent for use in the bottom plates of transport tanks or storage tanks of crude oil is steel for crude oil tank bottom plates with local corrosion resistance. However, the steel materials described in the above publications contain Cu, which is an element having the greatest influence on the corrosion resistance, and thus there is a problem in that sufficient corrosion resistance cannot be secured when Cu is not added.

Korean Patent Publication No. 2006-0069937 is an example of steel having sufficient corrosion resistance even without coating. The above publication is a marine steel material having excellent corrosion resistance that can be put into practical use even without coating or electric corrosion protection, especially in a wet atmospheric atmosphere such as an upper part in a ballast tank or a crude oil tank upper deck in which electrocorrosion prevention does not work. The marine steels provided in the above publication are to provide a marine steel material exhibiting excellent durability against C 0.01 to 0.20% (hereinafter, "%" means mass%), Si 0.01 to 0.50%, Mn 0.01 to 2.0% , 0.05 to 0.50% of Al, 0.01 to 5.0% of Cu and 0.01 to 5.0% of Cr, respectively, except that P 0.020% or less (including 0%) and S 0.010% (including 0%) are respectively suppressed. It is characterized by consisting of Fe and unavoidable impurities.

However, the steel disclosed in the above-mentioned Korean Patent Publication No. 2006-0069937 is a steel material that can be put into practical use even without coating or electrical corrosion prevention, and is used in the upper deck of the ballast or crude oil tank, so that the object of the present invention is As described above, the steel is not used while being immersed in a solution, but is used in a wet atmospheric atmosphere, and thus is different from the steel of the present invention. In addition, since Si is limited to 0.5% or less and Mo and W, which are helpful in improving strength and corrosion resistance, are not added, there is a problem in that sufficient corrosion resistance cannot be obtained.

In addition, the technologies presented so far focus on improving the corrosion resistance as described above, but hardly provide any means for improving the toughness of steel, in particular, the weld heat affected zone toughness.

As a technique for securing corrosion resistance and at the same time securing the weld heat affected zone toughness, Japanese Patent Application Laid-Open No. 2007-177286 discloses a steel material having excellent corrosion resistance and HAZ toughness at high heat input welding. The publication describes that it is possible to secure excellent corrosion resistance by adding Co and Mg to steel materials. According to the results of the present inventors, Co has been found to reduce corrosion resistance in a crude oil tank environment, and also forms TiN to provide excellent corrosion resistance. Although it is described that the welded toughness can be secured, TiN alone could not secure sufficient HAZ toughness, and it was difficult to measure the number of TiN inclusions and to find a correlation between the number of TiN inclusions and the HAZ toughness.

Therefore, marine steel materials that satisfy the corrosion resistance of the steel and the toughness of the weld heat affected zone at the same time are rarely presented until now.

The present invention is to solve the problems of the prior art, according to one aspect of the present invention is a high-strength steel that can not only have excellent resistance to front and local corrosion in strong acid saline solution, but also have excellent weld heat affected zone toughness Is provided.

According to another aspect of the present invention there is provided an advantageous manufacturing method for producing the above-described steel.

The steel for ships of the present invention for solving the problems of the present invention is C: 0.02-0.2% by weight, Si: 0.05-1.5% by weight, Mn: 0.2-2.0% by weight, P: 0.015% by weight or less, S: 0.003% by weight % Or less, Cu: 0.05-1.0 wt%, Al: 0.001-0.1 wt%, N: 0.003-0.015 wt%, Ti: 0.005-0.05 wt%, Nb: 0.005-0.1 wt%, Ni: 0.05-3.0 wt% %, Cr: 0.02 to 1.0% by weight, Mo: 0.02 to 0.5% by weight, W: 0.02 to 0.5% by weight, and Ca: 0.0005 to 0.01% by weight of one or more selected from the group consisting of residual Fe and inevitable impurities , 1.5> C + 2.5Si + 0.3Mn + 4Ni + 3W-3Cu-4Mo-6Cr, Ti / N: 1.5 to 2.5, Nb / N: 1.0 to 10 characterized by having a composition satisfying the relationship.

(In this case, C, Si, Mn, Ni, W, Cu, Mo, Cr, Ti, Nb, N respectively means the content (% by weight) of the corresponding element.)

At this time, V: 0.005 ~ 0.2% and B: 0.0005 ~ 0.005% It is preferable to further include one or both selected from V / N: 0.5 ~ 5, N / B: 10 ~ 40 is satisfied.

(In this case, V, B, and N respectively refer to the content (% by weight) of the corresponding element.)

In addition, it is advantageous that the microstructure is composed of ferrite and perlite and the area fraction of the perlite is 10-40%.

At this time, the average size of the ferrite grains is effective to 20㎛ or less.

In addition, in order to further improve the weld heat affected zone toughness, the average particle diameter and the total number of TiN, Ti-Nb, and Ti-V composite precipitates are preferably 0.3 µm or less and 1 × 10 ⁷ particles / mm ² or more, respectively.

In addition, when the maximum length in the rolling direction of the inclusions present in the steel is 70 μm or less, and the ratio (shape ratio) of the maximum length in the rolling direction and the maximum width in the direction perpendicular to the rolling direction is 30 or less, the corrosion resistance of the steel is greatly increased. Can be improved.

In addition, it is desirable to improve the corrosion resistance of steel materials by satisfying the relationship of Ca / O> 0.25, where Ca and O each represent the content (% by weight) of the corresponding element contained in the steel.

Moreover, it is effective that the area fraction of CaS inclusions among all inclusions is 20% or more.

Another aspect of the present invention is a method for producing the advantageous steel is C: 0.02-0.2% by weight, Si: 0.05-1.5% by weight, Mn: 0.2-2.0% by weight, P: 0.015% by weight or less, S: 0.003% by weight % Or less, Cu: 0.05-1.0 wt%, Al: 0.001-0.1 wt%, N: 0.003-0.015 wt%, Ti: 0.005-0.05 wt%, Nb: 0.005-0.1 wt%, Ni: 0.05-3.0 wt% %, Cr: 0.02 to 1.0% by weight, Mo: 0.02 to 0.5% by weight, W: 0.02 to 0.5% by weight, and Ca: 0.0005 to 0.01% by weight of one or more selected from the group consisting of residual Fe and inevitable impurities , 1.5> C + 2.5Si + 0.3Mn + 4Ni + 3W-3Cu-4Mo-6Cr, Ti / N: 1.5 ~ 2.5, Nb / N: 1.0 ~ 10 Reheating to a temperature of; Initiating hot finishing rolling of the reheated slab at a temperature of 950 ° C. or less to form steel; Cooling the hot rolled steel to a temperature of 400-600 ° C. at a cooling rate of 5 ° C./s or more; Characterized in that it comprises a.

(In this case, C, Si, Mn, Ni, W, Cu, Mo, Cr, Ti, Nb, N respectively means the content (% by weight) of the corresponding element.)

In this case, the steel slab further comprises one or both selected from V: 0.005 to 0.2% and B: 0.0005 to 0.005%, and satisfies the composition satisfying V / N: 0.5-5, N / B: 10-40. It is desirable to have.

(In this case, V, B, and N respectively refer to the content (% by weight) of the corresponding element.)

In addition, the composition of the steel slab is effective to satisfy the relationship of Ca / O> 0.25 (where Ca and O respectively mean the content (% by weight) of the corresponding element contained in the steel).

According to the present invention, the high-strength steel and its strength can be significantly reduced when applied to ships, as well as excellent in front corrosion and local corrosion resistance as well as excellent weld heat affected zone compared to conventional steel materials A manufacturing method may be provided.

Hereinafter, the present invention will be described in detail.

In general, low alloyed steels are susceptible to corrosion in the brine atmosphere. The corrosion rate depends on the corrosion environment, ie the concentration of salt, temperature and pH of the solution. The higher the salt concentration, the higher the temperature of the solution, the more acidic the solution, i.e. the lower the pH, the faster the corrosion rate.

In particular, since most of the steel used in ships are corroded by seawater, there is no significant difference in salinity or salt acidity except for a slight temperature difference depending on the environment of the ship site. However, the oil for oil tankers of oil tankers is not corroded by seawater, but by saltwater contained in crude oil, and the brine has a high salt concentration compared to seawater. The analysis revealed. When the crude oil is loaded into the crude oil tank, the bottom surface of the crude oil tank is formed with a thin crude oil coating layer, and when the crude oil loading is completed, the brine included in the crude oil due to the specific gravity difference is formed in the lower layer. Crude coating layer produced during crude oil loading serves to prevent corrosion, so the progress of corrosion is delayed. However, corrosion is first started at the part where the crude oil coating layer is damaged by sludge included in the crude oil flow rate. At the site of corrosion, the hydrogen generated by the corrosion reaction and chlorine ion in the brine are electrically coupled to increase the acidity in the solution, and the corrosion progresses rapidly as the acidity in the solution increases. In comparison, corrosion proceeds at a much faster rate. Sudden corrosion in the areas where the coating layer is damaged appears in the form of local corrosion, but strictly different from local corrosion mechanism caused by galvanic corrosion.

Therefore, the type of corrosion that occurs in bottom oil of crude oil tank is not a local corrosion due to the characteristics of steel, but a simple form of corrosion caused by external factors. It was a general opinion that it could be solved even if raised.

However, the study and experiment for the present invention it was found that even in a low pH saline solution according to the characteristics of the steel local corrosion as a formula, the degree is very serious. Particularly, it was a general recognition that local corrosion such as formula is only a problem in austenitic stainless steels, but is not a big problem in general low alloy steels. According to the research results of the present inventors, local corrosion is not affected even in low alloy steels. Seriously could happen. The characteristics of the steel affecting local corrosion include surface cracks and precipitates that occur depending on the components, and the local corrosion characteristics may be completely changed depending on the size and shape of the inclusions.

Therefore, the present inventors have repeated studies and experiments to overcome the problems not found in the prior art, and as a result, C, Mn, Cu, Ni, Cr, which not only affects corrosion resistance but also contributes to surface cracking and precipitate formation. In addition to optimizing the components such as Mo and W, it has been found to be excellent in front corrosion and local corrosion resistance in a low pH saline solution by limiting the formation of stretched oxide-based inclusions widely distributed in a steel sheet. In addition, if additional elements such as Ti, Nb, and V are added to the steel and the relationship between them and nitrogen is properly controlled, precipitates of these elements are formed inside the steel, resulting in coarse structure of the weld heat affected zone during welding. The present invention has been found with the fact that it can be prevented.

Hereinafter, the component system of the ship steel of this invention is demonstrated first.

C: 0.02 to 0.2 wt%

The C is an element added to increase the strength to increase the hardenability by increasing the content, but the strength is increased as the addition amount increases, inhibiting the corrosion resistance of the front surface, and promotes precipitation of carbides, etc. It also has some effect on resistance. To improve front and local corrosion resistance, C content should be reduced. However, if C is less than 0.02% by weight, it is difficult to secure the strength. If it exceeds 0.2% by weight, the weldability is deteriorated, which is undesirable as a steel for welding structures. The range is limited to%. It is more preferable to make C 0.12 weight% or less from a corrosion resistance viewpoint.

Si: 0.05-1.5 wt%

The Si is required to be 0.05% by weight or more in order not only to act as a deoxidizer but also to increase the strength of the steel. In addition, it is advantageous to increase the content because Si contributes to the improvement of the front corrosion resistance, but if the content of Si exceeds 1.5% by weight, the toughness and weldability are inhibited and the peeling of the scale during rolling is difficult. Since it causes surface defects, it is preferable to limit the content to 0.05 to 1.5% by weight. In order to improve corrosion resistance, it is more preferable to add Si by 0.2% by weight or more.

Mn: 0.2-2.0 wt%

Mn is required at least 0.2% by weight to secure strength. If the content is increased, the hardenability is increased to increase the strength, but when added in excess of 2.0% by weight, there is a problem that the weldability is lowered, it is preferable to limit the content to 0.2 to 2.0% by weight. Mn does not have a significant effect but affects the corrosion resistance of the front face. It is more preferable to add Mn at 1.5 weight% or less from the viewpoint of the front corrosion resistance.

P: 0.015% by weight or less

P is an impurity element, and if the content is added in excess of 0.015% by weight, not only the weldability is significantly lowered but also the toughness is degraded, so the content is preferably limited to 0.015% by weight or less.

S: 0.003 wt% or less

S is easily reacted with Mn to form stretched inclusions such as MnS, and the vacancy present at both ends of the stretched inclusions may be a local corrosion start point, so the content is preferably limited to 0.005% by weight or less. In addition, when S is present in the grain boundary as an impurity element, the brittle fracture is greatly reduced along the grain boundary to significantly reduce the toughness of the steel. Therefore, the content thereof is more preferably 0.003% by weight or less in order to improve the toughness of the steel.

Cu: 0.05-1.0 wt%

When Cu is contained in an amount of 0.05% by weight or more together with Ni and Cr, it is effective for improving the front corrosion and local corrosion resistance by delaying the dissolution of Fe. However, if it exceeds 1.0% by weight, it may cause surface cracking during slab manufacture, thereby lowering the resistance to local corrosion, and when reheating the slab for rolling, Cu having a low melting point may penetrate into grain boundaries of the steel, causing cracks during hot working. It is preferable to limit the content to 0.05 to 1.0% by weight. Since surface cracks generated during slab production interact with each other with C, Ni, and Mn contents, the occurrence frequency of surface cracks may vary depending on the content of each element, but the Cu content is most preferably 0.5% by weight or less.

Al: 0.001-0.1 wt%

Al is an element added for deoxidation to form AlN by reacting with N in the steel to refine the austenite grains to improve toughness. At least 0.001% by weight must be added for deoxidation. However, when excessively contained in excess of 0.1% by weight, the inclusions are formed in the coarse oxide in the steelmaking process. The formation of the stretch inclusions encourages the formation of cavities around the inclusions, which act as a starting point for local corrosion and thus serve to inhibit local corrosion resistance. Therefore, it is preferable to limit Al content to 0.1 weight% or less.

N: 0.003-0.015 wt%

N is an element necessary for forming TiN, AlN, BN, (Ti-Nb) N, (Ti-V) N, etc., and the amount of N increases so that the amount of the precipitates is increased to form austenite grains of the weld heat affected zone. Inhibits growth In particular, since the TiN precipitates have a significant influence on the size and spacing, distribution, and high temperature stability of the precipitates themselves, it is preferable to include at least 0.003% by weight. However, if the nitrogen content is excessive, the effect of the precipitate formation is saturated, rather it is preferable to limit the upper limit to 0.015% by weight because the amount of solid solution nitrogen distributed in the weld heat affected zone may decrease the toughness.

Ti: 0.005-0.05 wt%

Ti is an indispensable element in the present invention because it not only forms TiN precipitates as carbide or nitride forming elements, but also forms complex precipitates such as Ti-Nb and Ti-V to affect grain refinement of the austenite phase. In order to exhibit the effect of fine precipitates, it should be added at least 0.005% by weight. However, when the added amount is increased, coarse precipitates are formed, which act as corrosion initiation sites to promote local corrosion and have no effect on the refinement of austenite grains in the weld heat affected zone, so the upper limit thereof is limited to 0.05% by weight or less. It is preferable.

Nb: 0.005 to 0.1 wt%

Nb is finely re-precipitated in the form of Nb (C, N) at a temperature around 900 ℃ to increase the strength and to suppress the recrystallization of austenite generated during the secondary hot rolling to play a role to refine the ferrite particles. In addition, Ti-Nb complex precipitates are combined with Ti to promote ferrite transformation in the weld heat affected zone. In order to show the effect of the addition of Nb, it should be added at least 0.005% by weight. However, the coarse Nb precipitates produced as the Nb content increases can act as a corrosion initiation site to promote local corrosion as well as increase the low temperature transformation structure in the weld heat affected zone, so the upper limit is limited to 0.1 wt% or less. It is preferable.

In addition to the above-mentioned advantageous composition, at least one of Ni, Cr, Mo, W and Ca to be described below needs to be included.

Ni: 0.05-3.0 wt%

When Ni is contained in an amount of 0.05% by weight or more like Cu, it is effective for improving the front corrosion and local corrosion resistance. In addition, when added together with Cu, there is an effect of suppressing the problem of cracking during hot working by suppressing the formation of a low melting point Cu phase by reacting with Cu. Ni is also an effective element for improving the toughness of the base material. However, since it is an expensive element, the addition of more than 3.0% by weight is disadvantageous in terms of economics or weldability, so it is preferable to limit the content to 0.05 to 3.0% by weight. Since the effect of Ni on corrosion resistance is not higher than that of Cu, it is more preferable to contain 1.5 times or less of Cu content to suppress surface cracking due to Cu addition rather than adding a large amount to improve corrosion resistance.

 Cr: 0.02-1.0 wt%

The effect of Cr is not great but not only improves the corrosion resistance of the front surface but also contributes to the improvement of strength. In order to show the effect of the addition of Cr, it should be contained 0.02% by weight or more. However, if excessively contained in excess of 1.0% by weight, rather than encourage local corrosion, such as pitting (pitting) and adversely affect the toughness and weldability, it is preferable to limit the content to 0.02 ~ 1.0% by weight.

Mo: 0.02-0.5 wt%

Mo is an element contributing to the improvement of corrosion resistance and strength and should be added at least 0.02% by weight in order to exhibit the effect. However, Mo must be employed in steel to improve corrosion resistance. Mo contained more than the solid solution can contribute to the increase of strength by forming precipitates, but these precipitates can form a ferrite and galvanic rather increase the corrosion rate, and if the precipitate is coarse, the pores formed in the precipitate and steel interface It acts as a starting point for local corrosion and reduces local corrosion resistance. Therefore, the upper limit is preferably 0.5% by weight or less. Therefore, Mo content is limited to 0.02 to 0.5% by weight. Although the high capacity of Mo can be increased by controlling the steelmaking or rolling process, since the amount of Mo that can be dissolved in a solid solution without particular limitation on the process is 0.1 wt%, Mo is more preferably added at 0.1 wt% or less.

W: 0.02-0.5 wt%

W is an element that plays the same role as Mo and contributes to the improvement of corrosion resistance and strength and should be added in an amount of 0.02% by weight or more. However, like Mo, W must be dissolved in steel to improve corrosion resistance. W contained above the solid solution limit may form precipitates and contribute to the improvement of strength, but these precipitates may form ferrite and galvanic to increase the corrosion rate, and if the precipitates are coarse, the pores formed in the precipitate and steel interface It acts as a starting point for local corrosion and reduces local corrosion resistance. Therefore, the upper limit is preferably 0.5% by weight or less. Therefore, the W content is limited to 0.02 to 0.5% by weight. The high capacity of W can be increased by controlling the steelmaking or rolling process, but the high capacity of W is smaller than that of Mo, and thus the amount of W that can be dissolved without particular restriction on the process is 0.05% by weight, more preferably W Is controlled to 0.05% by weight or less.

Ca: 0.0005 ~ 0.01 wt%

The Ca is effective in controlling the inclusions, and thus inhibits the formation of the stretch inclusions, and CaO, CaS, produced by the addition of these elements is easily dissolved in the solution, which delays the acidification of the solution. In order to exhibit these characteristics, it should contain 0.0005% by weight or more. On the other hand, the upper limit is limited to 0.01% by weight, in which the inclusions are excessively coarse to be in a range that spoils local corrosion resistance, ductility and toughness. Therefore, the Ca is preferably limited to 0.0005 to 0.01 wt%.

Therefore, the steel of the present invention is C: 0.02-0.2% by weight, Si: 0.05-1.5% by weight, Mn: 0.2-2.0% by weight, P: 0.015% by weight or less, S: 0.003% by weight or less, Cu: 0.05-1.0% by weight %, Al: 0.001-0.1 wt%, N: 0.003-0.015 wt%, Ti: 0.005-0.05 wt%, Nb: 0.005-0.1 wt%, Ni: 0.05-3.0 wt%, Cr: 0.02-1.0 wt% , Mo: 0.02 to 0.5% by weight, W: 0.02 to 0.5% by weight, and Ca: 0.0005 to 0.01% by weight of at least one selected from the group consisting of, a balance consisting of Fe and inevitable impurities.

When the steel composition of the present invention has the above-described composition, not only the resistance to front corrosion or local corrosion is greatly improved, but also excellent weld heat affected zone toughness can be secured. In addition, in addition to the above component system, when controlling one or both of the elements of V and B in the following range, the weld heat affected zone toughness of the steel of the present invention may be further improved, and thus additionally include the elements as necessary. can do. Hereinafter, the composition range of each element and its advantageous advantages are demonstrated.

V: 0.005 to 0.2 wt% or less

V forms VN when N is present in the steel in a sufficient amount, but in general, V precipitates in the ferrite region in the form of VC to increase strength. In addition, Ti-V complex precipitates (carbide, nitride or carbonitride) are combined with Ti to promote ferrite transformation in austenite grains. In addition, V lowers the vacancy carbon concentration upon transformation to austenite-ferrite and VC provides a nucleation site for cementite formation. To achieve this effect, 0.005% by weight or more must be added. However, the strength is increased as the cementite fraction is increased, but as the cementite fraction is increased, the ferrite and galvanic corrosion of the surroundings are encouraged, so that the weight loss due to corrosion may be increased, and the upper limit is preferably limited to 0.2% by weight.

B: 0.0005 to 0.005 wt%

B is a component effective for improving the strength of the steel by remarkably increasing the hardenability of the steel even with a small amount of addition. In addition, BN precipitates are formed to inhibit austenite grain growth and to promote needle and polygonal ferrite transformation in grain boundaries and in the mouth. In order to show the effect of the addition of B, it should be added at least 0.0005% by weight. On the contrary, when the content of B exceeds 0.005% by weight, Fe ₃ B is formed, causing red brittleness, and the hardenability is excessive, so that the hardening and low temperature cracking of the weld heat affected zone may occur, so the content is 0.0005 to 0.005% by weight. It is desirable to limit to%.

On the other hand, as described above, Cu, Ni, Cr, Mo, W and the like are elements that contribute to the improvement of the front corrosion and local corrosion resistance, but may also promote local corrosion depending on the content of each element. This is because surface cracks and coarse precipitates, which may be the starting point of local corrosion, depend on the content of each alloying element.

Surface cracking is a phenomenon that occurs when the aposperm section is excessive during continuous casting. The range of the aposphere section changes depending on the content of the added alloying element. C, Mn, Cu, Ni, etc. are elements that promote surface crack formation by extending the reaction zone with austenite stabilizing elements, while Si, Mo, W, and Cr reduce the reaction zone with ferrite stabilization elements, forming surface cracks. Becomes an element that suppresses However, among these, the influence on the surface cracking is slightly different. Among these elements, C also affects the range of the trapping reaction zone, but it is important to determine whether or not the passage of the apolytic reaction zone is ultimately determined by the amount of C. It is known that surface cracks are significantly affected by Cu and Ni contents, which are austenite stabilizing elements, rather than stabilizing elements.

Unlike the effect of each alloying element on the surface cracking, precipitate formation is related to the solid solution limit of ferrite stabilizing elements. Mo, W, Cr, etc. are elements that can form carbides by reacting with C. Especially, W, Mo has a very high solid solution limit in steel, and if it is excessively contained over a certain range, it forms coarse carbides. Can act as a point of view. On the other hand, Cr has a higher solubility limit than Mo and W, so it is difficult to form carbide unless it contains a large amount.

As described above, it is difficult to theoretically quantify the effects of not only different surface cracks or precipitate formation on each alloy element, but also there is interaction between alloy elements on these properties. Accordingly, the present inventors have derived an empirical formula as shown in Equation 1 in consideration of the influence of each element on the surface crack and precipitate formation.

1.5> C + 2.5 Si + 0.3Mn + 4Ni + 3W-3Cu-4Mo-6Cr

However, here, C, Si, Mn, Ni, W, Cu, Mo, Cr means the content (wt%) of each component.

Equation 1 is a condition in which local corrosion does not occur in consideration of the influence of each component, and when the value of the right side (also called fitting index) of Equation 1 is 1.5 or more, the content of each element is described above. Even if it meets the range, local corrosion occurs rapidly and the area occupied by local erosion after 144 hours immersion in 10% NaCl solution with pH 1 exceeds 10%. However, when the component range of the present invention is satisfied and the condition of Equation 1 is satisfied, the progress of local corrosion can be significantly prevented.

In addition, in order to secure the toughness of the weld heat affected zone when welding steel, it is necessary to control the structure of the weld heat affected zone so that it does not coarsen during welding. In the present invention, it is necessary to form precipitates such as TiN. In addition, in the present invention, TiN precipitates and Ti-Nb and Ti-V composite precipitates which are stable at high temperatures are used together. Ti-Nb and Ti-V composite precipitates are dispersed in the base material to suppress the growth of austenite grains in the weld heat affected zone during welding, and to promote the formation of polygonal ferrite during cooling, which prevents the formation of low temperature transformation phase. It helps to improve the nature of disapproval. To this end, as described below, it is preferable to control so as to satisfy a relationship of less than Ti / N: 1.5 to 2.5, Nb / N: 1.0 to 10, V / N: 0.5 to 5, and N / B: 10 to 40. .

Since TiN is stable at high temperatures, the present invention is also effectively used as an element for preventing coarsening of crystal grains in the weld heat affected zone. In other words, the precipitates such as high nitrogen TiN are more stable at higher temperatures because the solubility level showing the stability of the precipitates at higher temperatures such as the weld heat affected zone becomes smaller. However, due to the phenomenon that the welding heat affected zone near the melting line is heated to a high temperature around 1400 ° C. during welding of ship steel, the TiN precipitates in the weld zone are partially dissolved or grown to coarsen, thereby reducing the number of TiN precipitates in the weld zone. Night grain growth inhibition effect is reduced. This inferior stability of TiN at high temperature is related to the ratio of Ti and N. In high nitrogen environment, the solid solution concentration of Ti decreases and the diffusion rate decreases, and the solid solution stability of TiN precipitate is improved. Generally, it is known that the Ti / N ratio is controlled from 3.4 to 2.5, which is a stoichiometric ratio, for the fine TiN precipitation in the base metal during steel manufacturing. I.e., at intervals of less than Ti / N is from 1.5 to 2.5 HAZ after the employment Ti amount is decreased to improve the high temperature Anseong of the TiN precipitates when they have welded the scope of the TiN precipitates 0.5㎛ 1.0 × 10 ⁷ pieces / It will be distributed more than mm ² . In other words, when the ratio is less than 1.5, solid solution nitrogen increases, which is detrimental to the toughness of the weld heat affected zone. On the other hand, if the amount exceeds 2.5, the remaining amount of Ti does not precipitate as TiN, and the remaining Ti remains in solid solution, which adversely affects the toughness of the weld heat affected zone. Therefore, it is preferable to limit the Ti / N ratio to 1.5 to 2.5.

As described above, Nb forms a Ti-Nb composite precipitate to serve to promote ferrite transformation in the weld heat affected zone. If the Nb / N ratio (where Nb and N represent the weight percent of each element) is less than 1.0, the number of Ti-Nb composite precipitates required for the austenite grain growth inhibition is insufficient, so that it will be effective as a needle-like nucleus site. On the contrary, if the ratio is 10 or more, the austenite grain growth inhibitory effect loses its function as a ferrite nucleation site when saturated, and thus the range of Nb / N is preferably limited to 1.0 to 10.

V is also an element that forms a VN, VC or Ti-V composite precipitate, it can promote the ferrite transformation in the austenite grains by the precipitate. If the V / N ratio is less than 0.5, the number of Ti-V composite precipitates required for the austenite grain growth inhibition is insufficient, and thus it may not be effective as a needle-like nucleus site. When saturation loses its function as a ferrite nucleation site, it is preferable to limit the range to 0.5-5.

In addition, BN plays a role in promoting polygonal ferrite transformation at the austenite grain boundary. To this end, it is necessary to appropriately control the ratio of N / B. If the ratio of N / B is less than 10, if the amount of precipitation of BN that promotes polygonal ferrite transformation at the austenite grain boundary is insufficient during the post-weld cooling process, if the N / B ratio is over 40, the amount of solid solution is increased when the effect is saturated. Since the toughness of a heat affected zone is impaired, it is preferable to limit the range to 10-40.

However, it should be noted that V / N and N / B are established only when V and B are actively added, respectively.

In addition, according to the results of the present inventors, it is more preferable to control them because inclusions of various sizes generated during deoxidation in addition to surface cracks and coarse precipitates caused by alloying elements affect local corrosion.

Oxide inclusions produced during the steelmaking process have various compositions and sizes. These various inclusions are present in the steel in various forms due to the deformations applied during rolling. Small spherical inclusions retain their shape during rolling and are in close contact with the steel so that no voids form around the inclusions. On the other hand, coarse oxide inclusions that are easily broken or coarse inclusion clusters of several inclusions are easily broken during rolling and stretched inclusions that are elongated along the metal movement during rolling. The stretched inclusions that are long and broken due to the large inclusions have an irregular shape and thus are not in close contact with the steels, and a void is created between the inclusions and the steels. When immersed in a corrosion solution, the corrosion progresses rapidly around these public areas, resulting in local corrosion.

Therefore, the size and shape of the inclusions must be controlled in order to produce steel having excellent local corrosion resistance. The present inventors have repeatedly observed the inclusions and stretched inclusions having a length of 70 μm or more along the rolling direction, or the ratio (shape ratio) of the maximum width in the rolling direction and the direction perpendicular to the rolling direction is 30. It was confirmed that local corrosion was initiated around these inclusions if stretched inclusions were present. Therefore, in order to secure excellent local corrosion resistance, the maximum inclusion length in the rolling direction should be limited to 70 μm or less, and the ratio of the maximum length in the rolling direction and the maximum width in the direction perpendicular to the rolling direction does not exceed 30. It is desirable to limit the number of times.

The method commonly used to control the size and shape of the oxide inclusions in the final product is to separate the coarse inclusions as much as possible in the steelmaking process, leaving only the fine spherical inclusions in the molten steel and casting the slab. As mentioned above, the fine spherical inclusions are broken during rolling and are not long and do not form a long line, so they maintain good adhesion with the steel, so that no void is formed between the steel and the inclusions, and thus does not create a place where local corrosion can be started. In order to fully remove and remove coarse inclusions in the steelmaking process, bubbling must be carried out at least 5 minutes before continuous casting after the converter operation.

Another method for controlling the size and shape of the oxide inclusions in the final product is a method of adding Ca to react with Al-based oxides to produce inclusions with low melting points to make them liquid in the molten steel. In general, liquid inclusions are more easily separated than solid phase inclusions, and thus, Ca can be added to remove coarse Al oxide inclusions, and as is well known, Ca plays a role of spheroidizing inclusions, thereby removing local corrosion initiation points. effective. In addition, CaO or CaS inclusions are easily dissolved in the solution to increase the pH of the solution. Therefore, when a large amount of CaO or CaS inclusions plays a role in increasing the pH of the solution, it is also possible to increase the front corrosion resistance of the steel. The effect of this Ca input is considerably different depending on the Ca input method and the present invention will be described with respect to a more preferable Ca input method that can maximize the Ca input effect.

In general, Ca is a highly reactive element, which easily reacts with Al oxide remaining in molten steel when it is injected into molten steel to form various types of complex inclusions, but since the 12Al ₂ O ₃ -7CaO compound has the lowest melting point, liquid inclusion formation is difficult. Easy and easy to separate

However, when the other oxides are removed from the molten steel, the dissolved oxidation amount in the molten steel is also reduced, thereby facilitating the generation of CaS.

Therefore, the present inventors conducted research and experiment to improve the steelmaking process in the direction of promoting the formation of CaS inclusions in the molten steel, and as a result, instead of the conventional method of inserting the conventional Ca-Si wire, the Ca-Si wire was replaced with 2 It has been found that CaS inclusions in the steel are greatly increased by splitting more than once. In more detail, when the Ca-Si wire is divided into two or more times as described above, the initially added Ca reacts with the remaining oxide in the molten steel to form a composite inclusion, and then waits for the subsequent Ca input. Since the floating separation in the molten steel is mixed into the slag layer, the composite inclusion in the molten steel is reduced. Therefore, when Ca is added after two batches, Ca can react with S in molten steel to form CaS effectively.

In addition, as described above, in order to form liquid inclusions that are easily separated by flotation through Ca, and to form spherical CaS inclusions that are easily dissolved in a solution, the amount of Ca must be appropriately controlled according to dissolved oxygen. As shown in Equation 2 below, when the amount of Ca in the molten steel and the dissolved oxygen ratio at the time of Ca exceeds 0.25, the liquid inclusions start to form, and as the ratio increases, the fraction of the liquid inclusions gradually increases. Suppressed. Therefore, in order to obtain a local corrosion resistance effect according to Ca addition, the ratio of Ca in molten steel and dissolved oxygen at the time of Ca injection should be limited to 0.25 or more, more preferably 0.5 or more.

Ca / O> 0.25

Here, Ca and O respectively mean the content (% by weight) of the corresponding element contained in the steel.

In the present invention, the steel slab is manufactured by continuous casting of molten steel whose composition is controlled as described above, and then hot rolled under normal conditions so that the maximum inclusion length in the final steel product does not exceed 70 μm or the maximum inclusion length and inclusions. Steels for ships with excellent front and local corrosion resistance can be produced in strong acid saline solutions of pH 3 or less, whose maximum width ratio does not exceed 30.

It is more preferable that the ratio of dissolved oxygen (ie, Ca / O) at the time of Ca and Ca injection is more than 0.75 in order to improve not only local corrosion resistance but also front corrosion resistance due to Ca input. If the above criteria are met, spherical fine CaS is produced together with the liquid inclusions.

In the present invention, in order to improve the front corrosion resistance according to the Ca input, the area of the CaS-based non-metallic inclusions is controlled to be 20% or more of the total inclusions. If the fraction is less than 20%, the pH of the steel surface can be effectively obtained. Because you can't.

That is, Ca introduced into the molten steel reacts with oxygen, aluminum, silicon, sulfur, etc. present in the molten steel to form inclusions (Al, Si, Ca) O and CaS. Of these inclusions, except for CaS, the complex inclusions do not dissolve in the aqueous solution and thus do not contribute to the pH increase of the steel surface, and only CaS inclusions are dissolved in the aqueous solution to raise the pH. Therefore, the ratio of CaS in the total inclusions is a major factor in determining the degree of rise of the pH of the solution, and the pH of the water film can be contributed to improving the corrosion resistance only when the corrosion rate is 3.0 or more. In order to obtain a pH of 3.0 or more as described above, the area fraction of CaS inclusions in the total inclusions should be 20% or more.

More preferably, the area fraction of the water-soluble CaS inclusions in the Ca-based nonmetallic inclusions in steel is limited to 20 to 80%. The reason for this is as follows. When Ca is added, oxide inclusions and emulsion inclusions are produced simultaneously. CaS, an emulsion-based inclusion, may cause nozzle clogging in the continuous casting process, and oxide-based inclusions may cause melting of the refractory in the continuous casting process. Therefore, in order to improve both weather resistance and secure the stability of the continuous casting process, it is more preferable to limit the upper limit of the fraction of CaS inclusions in the total inclusions to 80% or less.

Further, in order to secure the strength of the steel in addition to the advantageous composition and inclusion conditions, it is more preferable to limit the microstructure of the steel to the following range. That is, in order to manufacture high strength ship steel having excellent front corrosion or local corrosion resistance at a low pH, for example, 3.0 or less, more preferably 1.0 or less, the microstructure of the steel is ferrite and perlite, but the area of perlite is used. It is preferable to make fraction into 10 to 40% of range, and it is more preferable to make the size of a ferrite grain into 20 micrometers or less. Low-temperature transformation phases such as bainite, martensite, and MA (Martensite-Austenite Constituent) other than ferrite and pearlite are formed, and the toughness decreases. These low-temperature transformation phases are high energy phases and are more corrosion resistant than ferrite phases. As it forms, the overall corrosion rate increases.

That is, the total corrosion rate is increased by the galvanic corrosion between ferrite and perlite in the tissue mainly composed of ferrite and perlite, it is preferable to limit the perlite fraction to 40% or less. In terms of corrosion resistance, the perlite fraction may be limited to 40% or less. However, the higher the ferrite phase in the ferrite + pearlite composite structure, the higher the toughness and elongation of the base material.For improving the toughness of the weld heat affected zone, the perlite fraction is less than 30%. It is more preferable to limit to. When the fraction of the pearlite is increased, the yield strength and the tensile strength of the steel are increased, so it is more preferable that the fraction of the pearlite is 10% or more in order to obtain high strength of 400 MPa or more.

In addition, it is preferable to limit the average grain size of the final product to 20 μm or less in the strong acid brine solution having a pH of 3 or less and excellent in corrosion resistance and toughness at the front and local corrosion. If the ferrite grain size is 20 ㎛ or more, the austenite average grain size of the weld heat affected zone during welding is 80 μm or more. .

In addition, in order to improve the toughness of the weld heat affected zone, the average particle diameter and the total number of TiN, Ti-Nb, and Ti-V composite precipitates are limited to 0.3 µm or less and 1 × 10 ⁷ / mm ² or more, respectively, under the conditions satisfying the above composition. It is desirable to. When it exceeds 0.3 micrometer, the austenite grain growth inhibitory effect becomes small, and when the number is less than 1x10 <^7> piece / mm <^2> , it is difficult to control the size of the weld heat-affected zone austenite grain below 80 micrometers which is a threshold value.

As described above, the present invention can satisfy both excellent front and local corrosion resistance and excellent weld heat affected zone toughness by optimizing steel components and controlling the shape and structure of various precipitates.

The steel as described above can be produced by those who have ordinary knowledge in the technical field to which the present invention is sought without a great difficulty. However, the present inventors have found that the steel of the present invention can be provided more effectively when the steel of the present invention is manufactured by a more special method other than the conventional method, which will be described below.

In the method of manufacturing the steel of the present invention, as described below, after heating the steel slab having the above-described composition to a temperature range of 1050 ~ 1180 ℃, hot rolling to a finish rolling start temperature of 950 ℃ or more 5 ℃ / s It characterized in that the steel is produced by cooling to the temperature range of 400 ~ 600 ℃ at the above cooling rate. Hereinafter, the manufacturing method of the steel of the present invention will be described for each major configuration.

Reheating Temperature: 1050 ~ 1180 ℃

Austenite grows on reheating, so the reheating temperature should be lowered to improve toughness. However, when the reheating temperature is too low, rolling becomes difficult, and when Nb, V and other elements are added, solid solution of these elements does not occur, so it is preferable to limit the lower limit to 1050 ° C. On the other hand, if the reheating temperature is increased, the precipitation element is easy to employ, but the austenite is grown and the final ferrite grain is coarse to deteriorate the toughness, so it is preferable to limit it to 1180 ° C or less. Therefore, in order to improve the toughness, the reheating temperature is limited to 1050 ~ 1180 ℃.

Finish rolling start temperature: Below 950 ℃

If the rolling temperature is high, grains caused by recrystallization after rolling may be regrown, or grain size may be uneven due to partial recrystallization, thereby decreasing toughness. Therefore, in order to obtain fine grains uniformly, rolling must be started at a low temperature. Therefore, in order to improve toughness by grain refinement, it is preferable to limit finish rolling start temperature to 950 degrees C or less. In addition, in order to suppress excessive pearlite formation, the finish rolling end temperature is preferably limited to 800 ° C. or more, which is equal to or higher than the austenite-ferrite transformation temperature.

Cooling end temperature: 400 ~ 600 ℃

In order to control the grain size it is necessary to cool at an appropriate cooling rate after rolling, for example water cooling is preferred. If the cooling end temperature is too high at the time of cooling, the ferrite grains are not terminated and the ferrite grains transformed at the time of air cooling after the end of water cooling are coarse than the ferrite grains transformed at the time of water cooling, so that the overall grain size becomes large and uneven, thereby harming toughness. On the contrary, if the cooling end temperature is too low, low-temperature phases such as MA are generated and thus the toughness is impaired. Therefore, it is desirable to limit the cooling end temperature to 400 ~ 600 ℃.

Cooling rate: 5 ℃ / S or more

Toughness can be controlled through cooling rate control. In other words, when the cooling rate is too low, fine ferrite cannot be obtained during transformation, and the cooling rate is preferably a predetermined level or more. In particular, in order to obtain a high toughness steel, it is preferable to limit the cooling rate to 5 degrees C / s or less.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following examples are intended to illustrate the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

Example 1

To prepare a molten steel having a composition as shown in Table 1 below to prepare a steel slab by using a continuous casting. Bubbling was carried out for more than 5 minutes to separate the inclusions before casting by controlling the maximum size of inclusions affecting local corrosion to less than 70㎛ and the shape ratio to 30 or less.

In Table 1, the fitting index means C + 2.5Si + 0.3Mn + 4Ni + 3W-3Cu-4Mo-6Cr, which is the right side of Equation 1.

As can be seen in Table 1, the invention steel A to invention steel M not only satisfy the component range defined by the present invention, the fitting index is also less than 1.5 is a steel that satisfies all the composition control conditions of the present invention. In addition, by performing bubbling for 5 minutes or more, the maximum inclusion size is all 70 µm or less, and the aspect ratio is also 30 or less, which is a steel material that satisfies all the preferable conditions of the present invention. However, the comparative steel N shows the components of the conventional ship steel of yield strength 320MPa class, not only does the carbon content not meet the conditions defined in the present invention, but Cu, Ti, Nb is not added at all, and Cr, Ni The case where at least one element among Mo, W, and Ca is not added at all. Comparative steel O to Comparative steel S is a case in which the Cu, Ti and Nb is not added to deviate from the component specification prescribed in the present invention, Comparative steel S is not added to Cu, Ti and Nb as well as the case where the Mo content is excessive, Comparative steels T to W represent a case where Ti and Nb are not added.

Steel slabs manufactured under the conditions of Table 1 were hot-rolled under the conditions of Table 2 to prepare steel materials. In addition, the prepared steels by adding HCl to 10% NaCl solution by weight in Table 2 below to adjust the pH of the total solution to 0.85 immersed the invention steel and comparative steel 144 hours in weight loss after 144 hours After the measurement, the corrosion rate was calculated and the front corrosion rate was calculated. The relative value of the corrosion rate of the comparative steel N, which is a general steel for ships, was set to 100 (relative corrosion rate). In addition, the food pore fraction shown in Table 2 below is a value obtained by dividing the area of the food pore by the total area of the entire specimen surface area. The pearlite fraction means the area fraction of the pearlite measured when the surface of the product is observed in a direction parallel to the rolling direction.

Emphasis heating
Temperature
(℃) Finish rolling start temperature
(℃) Finish Rolling End Temperature
(℃) Cooling end
Temperature
(℃) Cooling
speed
(℃ / s) YS
(MPa) TS
(MPa) -40 ℃ Welding Heat Impact Impact Energy (J) Relative Corrosion Rate Food
Fraction Invention steel A-1 1150 950 880 550 10 405 497 135 25.4 0.3 B-2 1130 940 870 530 10 407 495 130 31.5 1.5 C-3 1130 940 870 530 10 420 508 140 22.8 0.1 D-4 1100 930 860 480 12 425 514 156 39.4 4.7 E-5 1110 930 860 450 13 446 550 174 34.2 3.1 F-6 1120 920 860 480 11 435 534 205 34.1 0.7 G-7 1130 920 860 450 12 442 535 224 15.4 0 H-8 1150 900 840 550 7 435 520 186 28.9 5.4 I-9 1120 900 840 580 7 420 503 201 20.1 0.5 J-10 1120 900 810 520 10 452 550 223 34.5 3.6 K-11 1100 890 810 480 10 455 555 185 35.9 2.5 L-12 1120 910 820 480 11 437 511 163 38.5 1.5 M-13 1100 930 840 500 11 440 531 197 35.9 3.4 A-14 1130 930 840 510 11 445 550 175 35.1 0 E-15 1120 900 810 500 10 440 532 196 15.5 1.1 Comparative steel B-16 1180 1050 970 650 10 385 470 36 30.6 1.4 D-17 1200 1030 950 500 13 405 502 27 40.2 4.3 H-18 1200 1000 920 720 4 402 494 18 27.0 5.1 N-19 1210 1040 960 700 One 324 407 15 100 0.2 O-20 1200 1000 920 680 2 339 421 17 95.4 24.7 P-21 1040 1020 930 650 9 341 426 25 92.1 15.3 Q-22 1090 980 910 720 6 314 401 17 99.7 0.8 R-23 1120 980 910 700 2 325 399 23 98.2 27.8 S-24 1180 1000 920 700 7 320 405 25 101.5 0 T-25 1180 1040 950 680 11 324 417 18 85.1 32.8 U-26 1220 1020 930 650 One 319 403 29 87.6 30.1 V-27 1090 990 920 600 One 337 417 31 88.7 31.4 W-28 1190 980 920 620 2 361 446 28 91.2 27.9 R-29 1150 950 870 450 9 350 451 32 92.1 15.3 S-30 1130 940 850 550 8 367 458 62 101.2 0.2 T-31 1180 950 850 600 9 355 449 47 84.9 32.8

As can be seen in Table 2, in the case of the comparative steel N, which is not added to Cr, Ni, Cu, Mo, W, etc., the food porosity is not high, but the relative corrosion rate is very high compared to the invention steel. In other words, it is necessary to suppress the front corrosion by the addition of these elements, but the front corrosion is easy to occur in the comparative steel N without addition of the element. A large corrosion rate was observed for the comparative steels O to S without Cu added. However, comparative steel O, comparative steel P, comparative steel R of these steels can be confirmed that the food content ratio is also increased at the same time when the fitting index is greater than 1.5.

The relationship between the fitting index of Table 1 and the food porosity of Table 2 is shown in FIG. As can be seen from the drawing, the food content fraction is less than 5% until the fitting index reaches 1.5, but when the fitting index reaches 1.5, the food content fraction increases abruptly, and the tendency increases as the food index increases. It tends to increase linearly. That is, when the fitting index exceeds 1.5, it can be seen that the local corrosion resistance suddenly decreases. Therefore, in order to obtain good local corrosion resistance, the fitting index should be limited to less than 1.5. In addition, the relationship between the fitting index and the front corrosion rate is shown in FIG. The speed increases rapidly, reaching a level similar to that of ordinary steel.

Table 3 shows the results of observing the average size and number of Ti-based precipitates (including complex precipitates) of the inventive steels and the comparative steels. Since the comparative steel was a component system without adding Ti, almost no Ti-based precipitate could be observed.

Emphasis Ti-based precipitate average size (㎛) Ti-based precipitates (pieces / mm²) Average grain size (㎛) Perlite fraction (%) Invention steel A-1 0.025 1.4 x 10 ⁷ 15 10 B-2 0.018 1.3 x 10 ⁷ 14 11 C-3 0.022 1.7 x 10 ⁷ 15 10 D-4 0.02 2.1 x 10 ⁷ 16 9 E-5 0.025 1.8 x 10 ⁷ 12 15 F-6 0.037 1.7 x 10 ⁷ 11 14 G-7 0.029 2.5 x 10 ⁷ 15 16 H-8 0.031 2.1 x 10 ⁷ 13 22 I-9 0.034 2.7 x 10 ⁷ 12 14 J-10 0.052 1.8 x 10 ⁷ 10 21 K-11 0.031 3.5 x 10 ⁷ 11 14 L-12 0.054 3.3 x 10 ⁷ 13 20 M-13 0.037 3.8 x 10 ⁷ 10 23 A-14 0.025 1.4 x 10 ⁷ 11 16 E-15 0.025 1.7 x 10 ⁷ 13 10 Comparative steel N-16 32 44 O-17 36 41 P-18 32 42 Q-19 26 46 R-20 29 47 S-21 42 41 T-22 39 48 U-23 40 43 V-24 23 41 W-25 29 47 P-26 29 41

As can be seen in Table 3, in the case of the invention steel that satisfies the conditions of the present invention, a large amount of Ti-based (composite) precipitates are formed therein, so that the ferrite grain size can be controlled to 20 µm or less, and the ratio of the pearlite crystal is also It could be controlled in the 10-40% range. However, in the case of comparative steel in Table 3, since Ti precipitates were hardly observed inside, the blanks were marked. In the case of the comparative steel, the size of the ferrite grains also exceeded 20 μm because the grain growth inhibition effect by the precipitate could not be achieved. In addition, the area fraction of the pearlite also exceeds 40% to deviate from the conditions of the preferred welding steel materials defined in the present invention. In this case, the old austenite grain size of the weld heat affected zone at the time of welding becomes 80 µm or more, so the toughness of the weld heat affected zone is extremely deteriorated, which makes it difficult to use it as a steel for welding.

When the invention steels B, D, and H are reheated at a high temperature which is outside the manufacturing range limited in the present invention and rolled at a high temperature, the component satisfies the limited range, but the toughness of the weld heat affected zone decreases rapidly. On the other hand, although comparative steels R, S, and T were manufactured within the limited manufacturing range in the present invention, the toughness of the welded heat affected zone is partially improved, but shows poor toughness characteristics that are less than 100J. That is, it can be seen that it is necessary to limit both the component range and the manufacturing range limited in the present invention in order to obtain high strength and excellent toughness of the weld heat-facing part together with excellent front and local corrosion resistance.

Example 2

After melting the steel having the same composition as the invention steel prepared in Example 1, and then casting a different bubbling time for the separation of the inclusions before casting to manufacture a slab and then hot-rolled the slab under normal conditions Steel sheet was prepared. At this time, the maximum inclusion length and shape ratio (inclusion length / width) were measured by analyzing the size distribution of the inclusions in the steel, and the food porosity was measured after the immersion test.

Steel grade division Bubbling time (minutes) Maximum length of inclusions (㎛) Maximum aspect ratio (length / width) Food fraction (%) Inventive Steel A Inventive Example 1 5 15 3.4 0.3 Inventive Steel A Comparative Example 1 2 47 20.4 11.4 Inventive Steel B Inventive Example 2 6 12 2.5 1.5 Inventive Steel B Comparative Example 2 2 54 25.3 13.5 Invention Steel C Inventive Example 3 7 9 2.1 0.1 Invention Steel C Comparative Example 3 2 49 15.4 14.8 Inventive Steel D Inventive Example 4 6 10 2.2 4.7 Inventive Steel D Comparative Example 4 One 62 30.4 20.7 Inventive Steel E Inventive Example 5 6 11 1.9 3.1 Inventive Steel E Comparative Example 5 One 58 28.6 19.2 Inventive Steel F Inventive Example 6 6 12 2.4 0.7 Inventive Steel F Comparative Example 6 2 42 19.8 14.3 Invention Steel G Inventive Example 7 5 25 3.3 0 Invention Steel G Comparative Example 7 One 71 32.5 20.1 Inventive Steel H Inventive Example 8 7 8 1.4 5.4 Inventive Steel H Comparative Example 8 2 39 17.6 13.8 Inventive Steel I Inventive Example 9 6 15 1.8 0.5 Inventive Steel I Comparative Example 9 2 55 22.9 16.7

As shown in Table 4, the maximum inclusion length and shape ratio are increased, that is, in the form of stretch inclusions, by optimizing its composition and varying the bubbling time for inclusion separation during steelmaking process satisfying Equation 1. It can be seen that the food content is changed and the food fraction increases. Figure 3 shows the aspect of the planting occurred in each steel after the immersion test, it can be seen that even the same invention steel according to the invention and comparative examples completely different form of the planting.

In the comparative example of the inventive steel, the reason why the area of cultivation is increased is due to the stretch inclusions as shown in Table 4 above. 4 shows a process in which local corrosion generated in a stretch inclusion of steel of Comparative Example 1 grows into a large plant. In the drawings, the photograph of the steel hole after the immersion test is a photograph observing the presence of the inclusion 3 times before the immersion test.

Comparative Example 1 Before the immersion test, when the inclusions in the steel were observed, the inclusions observed at position 3 had the shape of an elongated inclusion having a length of 110 μm while the inclusions observed at other locations had a relatively fine spherical inclusion shape. As a result of observing the shape after the immersion test of the same steel, it can be seen that the hole was generated at the position 3 with the extension inclusions. Therefore, it can be seen that the presence of the stretch inclusion promotes the formation of food balls, thereby deteriorating local corrosion resistance. Considering the maximum inclusion size and the shape ratio shown in Table 4, by limiting the maximum inclusion length to 70㎛ or less or 30 or less to form ratio can be obtained excellent local corrosion resistance.

Example 3

In Example 2, it was confirmed that the stretch inclusions generated inside the steel were a factor that inhibited the local corrosion resistance. As a method for realizing this, when bubbling before casting in the molten steel was sufficiently performed, the large inclusions were separated and removed to remove the internal inclusions. No stretch inclusions are produced at. As a method of flotation and separation of large inclusions, there is a method to facilitate flotation by generating inclusions having low melting point in molten steel by adding Ca. Low melting point inclusions exist in the liquid phase in the molten steel and facilitate flotation. The formation of such liquid inclusions is related to the amount of Ca and the amount of dissolved oxygen in molten steel when Ca is added. 5 is a graph predicting the type of inclusions formed according to Ca input amount and dissolved oxygen content in molten steel through thermodynamic calculation.

The molten aluminate (molten aluminate) shown in Figure 5 is a liquid inclusion with a low melting point, the liquid inclusions start to generate when [Ca] / [O] exceeds 0.25, the amount of liquid inclusions increases as the value increases Therefore, [Ca] / [O] should exceed 0.25 to suppress the formation of stretch inclusions. When the value of [Ca] / [O] exceeds 1, spherical CaS inclusions are formed. CaS is easily dissolved in the solution and increases the pH of the solution. Table 5 shows the Ca input amount, dissolved oxygen amount, inclusion maximum length and shape ratio, CaS inclusion fraction, front corrosion rate, and food pore fraction of Ca added invention steel and comparative steel.

Steel grade Ca amount
(weight%) Dissolved oxygen
(weight%) [Ca] / [O] Inclusions max
Length (㎛) Maximum aspect ratio of inclusions
(Length / width) CaS fraction
(%) Food fraction
(%) Invention Steel J 0.0018 0.0025 0.72 28 2.4 49 3.6 Inventive Steel K 0.0011 0.0022 0.50 11 2.0 41 2.5 Inventive Steel L 0.0013 0.0027 0.48 30 4.8 38 1.5 Inventive Steel M 0.0010 0.0028 0.35 14 3.4 31 3.4 Comparative Steel V 0.0005 0.0030 0.16 9 1.4 8 31.4 Comparative Steel W 0.0006 0.0032 0.18 10 1.9 13 27.9

As shown in Table 5, small inclusions close to the spherical shape were observed in the inventive steel exceeding [Ca] / [O] by 0.25 or more, while stretch inclusions were observed in the comparative steel having [Ca] / [O] of less than 0.25. Due to the effect of inhibiting the formation of stretched inclusions due to the addition of Ca, the food content ratio between the inventive steel and the comparative steel shows a big difference. That is, since Ca is added so that [Ca] / [O] exceeds 0.25, excellent local corrosion resistance can be obtained. In addition, when the [Ca] / [O] ratio is high, it was also confirmed that the CaS ratio also increases to contribute to increase the pH of the solution.

1 is a graph showing the correlation between the fitting index and the degree of food hole generation,

2 is a graph showing the correlation between the fitting index and the relative corrosion rate;

Figure 3 is a photograph showing the difference in the tendency to appear when the steel is manufactured when the steel is manufactured by varying the bubbling time for the same molten steel,

Figure 4 is a micrograph observing the phenomenon that the food is generated in the elongated inclusions during the immersion test, and

5 is a graph showing the relationship between the appropriate Ca and O content for forming molten aluminate in molten steel.

Claims (11)

C: 0.02-0.2% by weight, Si: 0.05-1.5% by weight, Mn: 0.2-2.0% by weight, P: 0.015% by weight or less, S: 0.003% by weight or less, Cu: 0.05-1.0% by weight, Al: 0.001-- 0.1 weight%, N: 0.003-0.015 weight%, Ti: 0.005-0.05 weight%, Nb: 0.005-0.1 weight% Ni: 0.05-3.0 weight%, Cr: 0.02-1.0 weight%, Mo: 0.02-0.5 Wt%, W: 0.02 to 0.5% by weight and Ca: 0.0005 to 0.01% by weight, at least one selected from the group consisting of Fe and unavoidable impurities, 1.5> C + 2.5Si + 0.3Mn + 4Ni + 3W- 3Cu-4Mo-6Cr, Ti / N: 1.5 ~ 2.5, Nb / N: 1.0 ~ 10 Front corrosion and local corrosion resistance and weld heat affected zone toughness in strong acid saline solution characterized by the composition Excellent high strength marine steels. However, here, C, Si, Mn, Ni, W, Cu, Mo, Cr, Ti, Nb, N means the content (% by weight) of the corresponding element, respectively. The method of claim 1, further comprising one or both selected from V: 0.005 to 0.2% and B: 0.0005 to 0.005%, and satisfies V / N: 0.5 to 5 and N / B to 10 to 40. High strength ship steel with excellent corrosion resistance and local corrosion resistance and toughness of welding heat affected zone in strong acid salt solution. Here, V, B, N means the content (% by weight) of the corresponding element, respectively. The surface corrosion and local corrosion resistance and welding heat in the strong acid saline solution according to claim 1 or 2, characterized in that the microstructure is composed of ferrite and perlite, and the area fraction of the perlite is 10-40%. High strength ship steel with excellent impact toughness. [4] The high strength ship steel of claim 3, wherein the ferrite grains have an average size of 20 µm or less, which is excellent in front corrosion and local corrosion resistance and weld heat affected zone toughness in a strong acid saline solution. The strong acid salt according to claim 1 or 2, wherein the average particle diameter and the total number of TiN, Ti-Nb, and Ti-V composite precipitates are 0.3 µm or less and 1x10 ⁷ particles / mm ² or more, respectively. High strength marine steel with excellent corrosion resistance and local corrosion resistance and toughness in welding heat affected zone in aqueous solution. 2. The maximum length in the rolling direction of the inclusions present in the steel is 70 µm or less, and the ratio (shape ratio) of the maximum width in the rolling direction and the maximum width in the direction perpendicular to the rolling direction is 30 or less. Steel for ships with excellent resistance to front and local corrosion in strong acid brine solution. 2. The front surface of a strong acid saline solution according to claim 1, wherein a content of Ca / O> 0.25 is satisfied, wherein Ca and O each represent a content (% by weight) of a corresponding element contained in the steel. Marine steel with excellent corrosion and local corrosion resistance. The ship steel having excellent front corrosion and local corrosion resistance in a strong acid saline solution according to claim 1, wherein the area fraction of CaS inclusions in the total inclusions is 20% or more. C: 0.02-0.2% by weight, Si: 0.05-1.5% by weight, Mn: 0.2-2.0% by weight, P: 0.015% by weight or less, S: 0.003% by weight or less, Cu: 0.05-1.0% by weight, Al: 0.001-- 0.1 weight%, N: 0.003-0.015 weight%, Ti: 0.005-0.05 weight%, Nb: 0.005-0.1 weight% Ni: 0.05-3.0 weight%, Cr: 0.02-1.0 weight%, Mo: 0.02-0.5 Wt%, W: 0.02 to 0.5% by weight and Ca: 0.0005 to 0.01% by weight, at least one selected from the group consisting of Fe and unavoidable impurities, 1.5> C + 2.5Si + 0.3Mn + 4Ni + 3W- Reheating the steel slab having a composition satisfying the relationship of 3Cu-4Mo-6Cr, Ti / N: 1.5 to 2.5, and Nb / N: 1.0 to 10 at a temperature of 1050 to 1180 ° C; Initiating hot finishing rolling of the reheated slab at a temperature of 950 ° C. or less to form steel; Cooling the hot rolled steel to a temperature of 400-600 ° C. at a cooling rate of 5 ° C./s or more; Method of producing a ship steel material excellent resistance to front and local corrosion in a strong acid saline solution comprising a. However, here, C, Si, Mn, Ni, W, Cu, Mo, Cr, Ti, Nb, N means the content (% by weight) of the corresponding element, respectively. The steel slab of claim 9, wherein the steel slab further comprises one or both selected from V: 0.005 to 0.2% and B: 0.0005 to 0.005%, and V / N: 0.5 to 5 and N / B to 10 to 40. The method for producing marine steels having excellent front and local corrosion resistance in a strong acid saline solution, characterized in that it has a composition that satisfies. Here, V, B, N means the content (% by weight) of the corresponding element, respectively. The method of claim 9, wherein the composition of the steel slab is characterized by satisfying the relationship of Ca / O> 0.25, where Ca and O respectively represent the content (% by weight) of the corresponding element contained in the steel A method for producing marine steels with excellent resistance to front and local corrosion in strong acid saline solution.

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