EP1825014A1

EP1825014A1 - Steel with excellent weather resistance at the seaside atmosphere, and manufacturing method therefor

Info

Publication number: EP1825014A1
Application number: EP05856389A
Authority: EP
Inventors: Hwan-Gyo POSCO JUNG; Jang-Yong POSCO YOO; Kyung-Keun POSCO UM; Eui-Gyeong POSCO JUNG; Jong-Kyo POSCO CHOI; Jae-Sang POSCO LEE
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2004-11-12
Filing date: 2005-09-28
Publication date: 2007-08-29
Anticipated expiration: 2025-09-28
Also published as: EP1825014A4; EP1825014B1; WO2006080658A1; ATE553225T1

Abstract

The invention relates to steel with excellent weather resistance, and a manufacturing method therefor. According to the invention, the steel consists of up to 0.15wt% C, up to 1.0wt% Si, up to 2.0wt% Mn, 0.2 to 1.0wt% Cu, 0.2 to 5.0wt% Ni, 0.001 to 0.1 wt% Al, up to 0.03wt% P, 0.002 to 0.03wt%S, 0.001 to 0.01wt% Ca. The balance contains Fe and unavoidable impurities, and Ca, S, Al, Si contents are determined by an Equation 1 Ca(%)/S(%) > 1.5Al(%)+2Si(%). Also, the steel has at least 30wt% water-soluble CaS inclusion out of Ca-based non-metallic inclusions. The invention provides steel with excellent weather resistance in an atmosphere having a high concentration of salinity.

Description

STEEL WITH EXCELLENT WEATHER RESISTANCE AT THE SEASIDE ATMOSPHERE, AND MANUFACTURING METHOD

THEREFOR

Technical Field

[1] The present invention relates to steel with excellent corrosion resistance against atmospheric exposure, and a manufacturing method therefor. More particularly, the present invention relates to steel with excellent corrosion resistance at the seaside where salinity is high and in an atmosphere with a high concentration of chlorine where calcium chloride is used as anti-freezing agent.

[2]

Background Art

[3] A conventional weather resistant steel contains a very small amount of Cu, Cr and

P, and has atmospheric corrosion resistance four to eight times higher than general steel. The weather resistant steel has corrosion resistance superior to general steel for the following reasons. The weather resistant steel gathers rust, which falls off or becomes detached in a manner similar to general steel during an early period of atmospheric exposure. But with time passing, some of the rust slowly adheres to a base metal, forming a layer of dense stable rust, which subsequently serves as protective layers against corrosive environment. Stable rust that can function as protections against corrosive environment is FeOOH or α-FeOOH having an amorphous structure.

[4]

[5] The conventional weather resistant steel demonstrates superior corrosion resistance after a long period of at least 3 years in a general atmospheric corrosion environment such as a rural area or a factory area containing SO However, the conventional weather resistant steel indicates little improvement from general steel even though time passes at the seashore having a relative high concentration of chlorine or in a region where anti-freezing agent is used. This is because chlorine ion adhered to the steel surface facilitates oxidation of a water film formed thereon, increasing overall corrosion rate of the steel, obstructing formation of stable rust and generating defective corrosive product such as β-FeOOH on the steel surface. Therefore, to enhance weather resistance at the seashore having a high concentration of chlorine or in a region where snow-melting salt is used, β-FeOOH formation should be inhibited but α- FeOOH with a dense structure should be formed to generate a complete passive film.

[6]

[7] A structure of rust layer formed when steel is exposed to atmosphere is affected by corrosive environment. With higher pH of corrosive environment or the water film formed on the steel surface, β-FeOOH formation is inhibited and α-FeOOH formation is expedited. However, pH is hardly controllable via control of atmosphere, and thus attempts have been made to elevate pH of the water film formed on the steel surface by adding chlorine elements to the steel.

[8] Japanese Laid-Open Patent Application Nos. 2-125839 and 5-51668 disclose conventional technologies in which weather resistance is heightened by adding chlorine elements such as Ca and Mg, and increasing pH of the steel surface. The Patent Application No.2- 125839 teaches a method of improving weather resistance by adding Ca to steel to produce complex oxide of (Al, Ca)O. The Patent Application No. 5-51668 teaches a method of increasing pH of the steel surface by adding pretreated powder that mixes CaO particles of up to ID average diameter and iron of up to 200D average diameter.

[9]

[10] In the conventional technology according to the Patent Application No. 2-125839, inclusion produced by addition of Ca is not CaO single oxide but Ca-based complex oxide. Also, as in the Patent Application No. 5-51668, even though CaO is directly injected, CaO injected reacts easily with other oxides present in molten steel such as Al O and SiO and exists as liquid inclusion such as (Al, Si, Ca)O, CaO- 2Al O , CaO- 6Al O compounds. As a result of investigation into the conventional technology conducted by the inventors, Ca-based composite oxide is not dissolved in water, not elevating pH of the material surface significantly and accordingly having little effect on improving weather resistance.

[H]

[12] Further, in connection therewith, the inventors suggest Korean Patent Application

No. 2002-25571 which teaches a technology for enhancing weather resistance by adding Ca-Si wire to molten steel, forming Ca-based composite sulfide and Ca-based composite oxide on steel, and increasing pH of the steel surface. According to the conventional technology, addition of Ca-Si wire enables formation of steel inclusion containing Ca-based composite oxide and Ca-based composite sulfide. However, by weight fraction of the inclusions, 95% thereof exist as Ca-based composite oxide, not contributing to pH increase of the steel surface and thus not enhancing weather resistance substantially.

[13]

Disclosure of Invention Technical Problem

[14] The present invention has been made to solve the foregoing problems of the prior art and it is therefore an object of the present invention to provide steel with excellent weather resistance at the seaside and in a region having a high concentration of salinity where anti-freezing agent is used, and a method for manufacturing the same. This is made possible by increasing pH of the steel surface at the early stage of corrosion and generating stable rust in a short period without conducting painting or surface treatment by optimizing steel elements and properly controlling weight fraction of water-soluble CaS inclusion out of Ca-based non-metallic inclusions.

[15]

[16] It is another object of the invention to provide steel with excellent weather resistance having at least 300MPa yield strength and at least 480MPa tensile strength in an environment where salinity is high and where anti-freezing agent is used, and a method for manufacturing the same.

[17]

Technical Solution

[18] According to an aspect of the invention for realizing the object, there is provided steel with excellent weather resistance consisting of:

[19] up to 0.15wt% C, up to 1.0wt% Si, up to 2.0wt%, 0.2 to 1.0wt% Cu, 0.2 to

5.0%wt% Ni, 0.001 to 0.1 wt% Al, up to 0.03wt% P, 0.002 to 0.03wt% S, 0.001 to 0.01wt% Ca, the balance being Fe and unavoidable impurities, wherein Ca, S, Al and Si contents are determined by an Equation below:

[20]

[21] Ca(%)/S(%) > 1.5Al(%)+2Si(%) Equation 1,

[22] the steel having 30wt% of water-soluble CaS inclusion out of Ca-based non- metallic inclusions.

[23]

[24] Further, according to the invention, the steel with excellent weather resistance may further comprise at least one selected from a group consisting of 0.005 to 0. lwt% Ti, 0.01 to 1.0wt% Mo and 0.01 to 1.0wt% W.

[25]

[26] If the steel with excellent weather resistance further comprises at least one selected from a group consisting of 0.02 to 0.07wt% Nb, up to 0.1wt% V, 0.003 wt% B, the steel may have its strength improved considerably.

[27]

[28] According to the invention, the steel has 30 to 80wt% water-soluble CaS inclusion out of Ca non-metallic inclusions.

[29]

[30] The steel preferably has Ca-based non-metallic inclusion containing CaS distributed therein by an area fraction of at least 0.04%, [31] wherein the area fraction is determined by dividing inclusion area by observation area. [32] [33] Further, the steel has Ca-based non-metallic inclusion containing CaS sized up to 3D distributed therein by an area fraction of at least 0.02%. [34] [35] A method for manufacturing steel with excellent weather resistance comprising steps of:

[36] preparing molten steel consisting of:

[37] up to 0.15wt% C, up to 1.0wt% Si, up to 2.0wt% Mn, 0.2 to 1.0wt% Cu, 0.2 to

5.0wt% Ni, 0.001 to 0.1wt% Al, up to 0.03wt% P, 0.002 to 0.03wt% S, the balance being Fe and unavoidable impurities; [38] injecting Ca into the molten steel, carrying out bubbling to remove floating inclusion from the molten steel, and then injecting Ca again into the molten steel at least once to constitute 0.001 to 0.01% Ca content, wherein the Ca content is determined by Equation 1 below: [39]

[40] Ca(%)/S(%) > 1.5Al(%)+2Si(%) Equation 1; and

[41] continuously casting the molten steel to fabricate a steel slab and hot-rolling the steel slab under a normal condition to manufacture steel having water-soluble CaS inclusion out of Ca-based non-metallic inclusions by an area fraction of at least 30%. [42] [43] At this time, the molten steel further consists of at least one selected from a group consisting of 0.005 to 0.1wt% Ti, 0.01 to 1.0wt% Mo, 0.01 to 1.0wt% W. [44] The molten steel further consists of at least one selected from a group consisting of

0.02 to 0.07wt% Nb, up to 0. lwt% V, up to 0.003wt% B. [45] The steel preferably has 30 to 80 wt% CaS inclusion out of Ca-based non-metallic inclusions. [46] At this time, the steel has Ca-based non-metallic inclusion containing CaS distributed therein by an area fraction of at least 0.04%, wherein the area fraction is determined by dividing inclusion area by observation area. [47] The steel preferably has Ca-based non-metallic inclusion containing CaS sized up to 3D distributed therein by an area fraction of 0.02%. [48]

Advantageous Effects

[49] The present invention provides steel with excellent weather resistance at the seaside and in a region where salinity is high and where anti-freezing agent is used by optimizing steel compositions, properly controlling weight fraction of water-soluble

CaS inclusion out of Ca-based non-metallic inclusions, increasing pH of the steel surface in the early stage of corrosion, and forming stable rust in a short period without conducting painting or surface treatment. [50] [51] Also, according to one embodiment of the invention, there is provided steel with excellent weather resistance having at least 300MPa yield strength and at least 480MPa tensile strength in an environment having a high concentration of salinity where anti- freezing agent is used. [52]

Brief Description of the Drawings [53] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [54] [55] Figure Ia is a view illustrating the shape of an inclusion and a composition analysis result of comparative steel C; [56] Figure Ib is a view illustrating the shape of an inclusion and a composition analysis result of inventive steel C; [57] Figure 2a shows composition analysis made by randomly selecting 10 out of inclusions distributed in comparative steel C; [58] Figure 2b shows a composition analysis result produced by randomly selecting 10 out of inclusions distributed in inventive steel C; [59] Figure 3a shows thermodynamic calculation of inclusion formation behavior in molten steel of comparative steel; [60] Figure 3b shows thermodynamic calculation of inclusion formation behavior in molten steel of inventive steel;

[61] Figure 4 shows the distribution of inclusions in inventive steel A;

[62] Figure 5 is a graph illustrating temporal pH changes on the surface of comparative and inventive steels; [63] Figure 6 is a graph illustrating temporal pH changes in Ca-based composite oxides observed in comparative steels. [64] Figure 7a is a view illustrating the shape of an inclusion and a composition analysis result of comparative steel D; [65] Figure 7b is a view showing the shape of an inclusion and a composition analysis result of inventive steel B; [66] Figure 8a shows a composition analysis result produced by randomly selecting 10 out of inclusions distributed in comparative steel D; [67] Figure 8b shows a composition analysis result produced by randomly selecting 10 out of inclusions distributed in inventive steel B;

[68] Figure 9 shows the distribution of inclusions in inventive steel A;

[69] Figure 10 is a graph illustrating pH changes on a surface of inventive and comparative steels; and [70] Figure 11 is a graph illustrating pH changes in Ca-based composite oxide observed in comparative steels. [71]

Best Mode for Carrying Out the Invention [72] The present invention will be explained hereunder.

[73] In general, steel with excellent weather resistance has stable rust layers such as γ-

FeOOH or Fe O formed in the early stage. The rust layers degrade weather resistance

3 4 and aesthetic appearance in the early stage due to their fall-off and detachment. Especially, at the seaside having a high concentration of salt, β-FeOOHF, which is worse for weather resistance than rust layers, is also generated, deteriorating weather resistance, and aesthetic appearance at the early stage. However, higher pH of the steel surface inhibits formation of rust layers such as β-FeOOH, γ-FeOOH, Fe O but stimulates formation of stable rust or α-FeOOH which enhances weather resistance.

[74]

[75] For this reason, in the conventional method, Ca was added to steel to increase pH of the surface thereof in the early stage. But if Ca is added to steel, Ca added does not form water-soluble CaO or CaS capable of increasing pH of an aqueous solution but reacts with Si, Al, O present in the steel to form complex oxide (at least 90%) such as (Al, Si, Ca)O. The inclusions are sized at least 3D and distributed unevenly. Moreover, the complex oxide formed in this manner is not dissolved in water, thus not leading to higher pH of the steel surface.

[76]

[77] Therefore the inventors have repeatedly carried out studies and experiments to overcome problems of the prior art. Consequently the inventors were able to manufacture steel with excellent weather resistance having at least 30% water-soluble CaS out of Ca-based non-metallic inclusions in the final product by optimizing Ca, S, Al, Si composition that affects formation of inclusions in the steel and also separating floating Ca-based complex oxide from inclusions in a steel-making process.

[78]

[79] The steel with excellent weather resistance is explained hereunder. [80] Up to 0.15 wt% C

[81] C is added to boost strength, and greater contents thereof enhance hardenability and subsequently strength. However, C content should be preferably lmited to 0.15 wt% or less because C at the amount exceeding this value impairs weldability.

[82]

[83] Up to 1.0 wt% Si

[84] Si serves as a deoxidant and also increases strength. Si content should be preferably limited to 1.0 wt% or less since Si content exceeding this value degrades toughness and weldability, and also reacts with Ca, thereby producing insoluble complex oxide and hindering improvement in weather resistance.

[85]

[86] Up to 2.0 wt% Mn

[87] Mn is effective for boosting strength without degrading toughness. Higher Mn content leads to improved hardneablity and then increased strength. But the Min content should be preferably limited to 2.0 wt% or less since Min at the amount exceeding this value deteriorates weldability.

[88]

[89] 0.2 to 1.0 wt% Cu

[90] Cu is effective for enhancing weather resistance of steel by inducing fineness and density of rust layer particles. Cu content less than 0.2 wt% does not allow improvement in weather resistance, while the content in excess of 1.0 wt% rarely improves weather resistance and also causes Cu with a low melting point to infiltrate into steel grain in case of reheating slab for hot-rolling, causing cracks in a heat process. Therefore the Cu content should be preferably limited to 0.2 to 1.0 wt%.

[91]

[92] 0.2 to 5.0 wt% Ni

[93] Ni is one of important elements conducive to increasing weather resistance.

Addition of Ni allows fineness and density of amorphous rust layer or α-FeOOH rust layer so that permeation into materials through rust layer is inhibited, thus enhancing weather resistance. More particularly, Ni is effective for improving weather resistance at the seaside having a high concentration of salinity. The Ni content less than 0.2 wt% does not ensure the aforesaid effects. But the Ni content in excess of 5.0 wt% rarely improves weather resistance, and also increases manufacturing costs resulting from a great quantity of addition of high-priced Ni. Therefore, the Ni content should be preferably limited to 0.2 to 5.0 wt %.

[94]

[95] 0.001 to 0.1 wt% Al

[96] Al is essentially added for deoxidation in a steel-making process. Al improves shock absorption energy, but like Si, reacts with Ca to form insoluble complex oxide, thus hindering improvement in weather resistance. Al content less than 0.001 wt% does not allow sufficient deoxidation but the content in excess of 0.1 wt% degrades impact toughness. Therefore the Al content should be preferably limited to 0.001 to 0.1 wt%.

[97]

[98] Up to 0.03 wt% P

[99] P is effective for improving weather resistance since it produces PO ion in an

4 aqueous solution when present in steel to boost cation selective permeability of rust layer, and inhibits chlorine ion from permeating rust layer. However, the P content in excess of 0.03 wt% considerably degrades weldability and strength. Accordingly the P content should be preferably limited to 0.03 wt%.

[100]

[101] 0.002 to 0.03 wt% S

[102] S serves as a starting point of corrosion by reacting with Mn to form MnS.

However, in the case where Ca is added as in the present invention, it reacts with Ca to be dissolved in an aqueous solution and forms CaS capable of increasing pH, thereby improving weather resistance. To achieve pH increase by CaS requires addition of S with at least 0.002 wt% in content. But the S content in excess of 0.03 wt% deteriorates impact toughness and weldability. Therefore the S content should be preferably limited to 0.002 to 0.03 wt%.

[103]

[104] 0.001 to 0.01 wt% Ca

[105] Ca reacts with Al, Si, O out of molten steel to form complex oxide such as (Al, Si,

Ca)O and then reacts with S to form CaS. The gross complex oxide does not result in pH increase, having no effect on improving weather resistance but deteriorating impact toughness. However fine CaS inclusion is dissolved in water, increasing pH of the steel surface so that stable rust formation is facilitated to enhance weather resistance. For Ca to enable improvement in weather resistance by CaS formation, at least 0.001 wt% should be added but the Ca content in excess of 0.01 wt% causes wear of refractory material in a steel-making process. Therefore the Ca content should be preferably limited to 0.001 to 0.01 wt%.

[106]

[107] Except for the aforesaid elements, the balance contains Fe and unavoidable impurities.

[108]

[109] Further, according to the invention, better resistance is obtainable by adding to the above elements at least one selected from a group consisting of Ti, Mo and W. [HO]

[111] 0.005 to 0.1 wt% Ti

[112] Ti generates fine oxide and nitride in steel, which due to high melting points, are not dissolved even over a reheating temperature in the case of hot-rolling. Thereby Ti inhibits growth of austenite crystal grain by grain boundary pinning effects and enhances impact toughness. Also, fine Ti oxide serves as a starting point to react with Ca and thus induces fine distribution of inclusions. Ti content less than 0.005 wt% does not ensure the aforesaid effects while Ti content in excess of 0.1 wt% boosts Ti amount of solid solution, deteriorating toughness of the base metal and weld. Therefore the Ti content should be preferably limited to 0.005 to 0.1 wt%.

[113]

[114] 0.01 to 1.0 wt% Mo

[115] Mo generates MoO ion in an aqueous solution to restrain permeation of chlorine

4 ion, thereby improving weather resistance at the seaside, and strength as well. Mo content less than 0.01 wt% does not yield the aforesaid effects, while the content in excess of 1.0 wt% leads to insignificant improvement. As a result, the Mo content should be preferably limited to 0.01 to 1.0 wt%.

[116]

[117] 0.01 to 1.0 wt% W

[118] W is effective for improving weather resistance at the seaside by forming WO ion in an aqueous solution to restrain permeation of chlorine ion that passes rust layer. W content less than 0.01 wt% does not yield the aforesaid effects while the content in excess of 1.0 wt% rarely improves weather resistance. Thereby the W content should be preferably limited to 0.01 to 1.0 wt%.

[119]

[120] Also, higher-strength steel with excellent weather resistance is obtainable by adding at least one selected from a group consisting of Nb, V and B in a content defined later.

[121]

[122] 0.02 to 0.07 wt% Nb

[123] Nb refines crystal grain effectively, and also improves strength significantly. Nb content less than 0.02 wt% does not yield the aforesaid effects, while the Nb content in excess of 0.07 wt% generates gross Nb precipitates, potentially causing cracks in steel and degrading impact properties. Therefore, the Nb content should be preferably limited to 0.02 to 0.07 wt%.

[124]

[125] Up to 0.1 wt% V

[126] V forms carbide- nitride like Nb and thus is effective for boosting strength. V content in excess of 0.1 wt% has little effect on improving strength, also impairing impact properties of steel. Therefore, the V content should be preferably limited to 0.1 wt%.

[127]

[128] 0.003 wt% B

[129] Even a small quantity of B added substantially enhances hardenablity and increases strength of steel. B content in excess of 0.003 wt% forms Fe B, causing red brittleness and impairing weldability. Therefore the B content should be preferably limited to 0.003 wt%.

[130]

[131] As stated above, Ca forms CaS that ensures improved weather resistance while forming complex oxide not contributing thereto. Accordingly, Ca, S, Al, and Si contents should be adequately controlled to inhibit complex oxide and boost CaS inclusion formation. An Equation 1 below was set for restraining complex oxide and enhancing CaS inclusion formation, and derived from repeated experiments of the inventors.

[132] Ca(%)/S(%) > 1.5Al(%)+2Si(%) Equation 1

[133]

[134] Even though elements of the invention are contained in a scope defined, in case where the Equation 1 above is not fulfilled, weight fraction of CaS inclusion out of Ca- based non-metallic inclusions cannot be controlled as desired in the final product. That is, if Ca, S, Al, and Si contents are controlled to satisfy an Equation 1 above, the weight fraction of CaS inclusion out of Ca-based non-metallic inclusions can be properly controlled so as to enhance weather resistance as is the object of the invention.

[135]

[136] Next, steel with excellent weather resistance according to the invention is explained hereunder.

[137] First, according to the invention, prepared is molten steel consisting of up to

0.15wt% C, up to 1.0wt% Si, up to 2.0wt% Mn, 0.2 to 1.0wt% Cu, 0.2 to 5.0wt% Ni, 0.001 to 0.1 wt% Al, up to 0.03wt% P, 0.002 to 0.03wt% S, the balance being Fe and unavoidable impurities.

[138] Then Ca is injected into the molten steel, bubbling is carried out to remove floating inclusion from the molten steel, and then Ca is injected again into the molten steel at least once to constitute 0.001 to 0.01wt% Ca contents to satisfy an Equation 1 below:

[139]

[140] Ca(%)/S(%) > 1.5Al(%)+2Si(%) Equation 1

[141]

[142] According to the invention, such split injection of Ca and composition optimization allows at least 30% weight fraction of CaS inclusion out of Ca-based non-metallic inclusions in the final steel.

[143]

[144] Steel with excellent weather resistance at the seaside is composed as set forth above. The steel, as explained earlier with regard to grounds for limited composition of the steel with excellent weather resistance, may further contain at least one selected from a group consisting of Ti, Mo and W or further contain at least one selected from a group consisting of Nb, V and B.

[145]

[146] The aforesaid manufacturing method according to the invention is explained in greater detail hereunder.

[147] By prior art, in injecting Ca into molten steel, Ca-Si wire was injected into the molten steel one time. The inventors added Ca in this conventional method to evaluate corrosion resistance of steel but found no improvement therein. It was confirmed that in a process of identifying a reason for no effects of the conventional Ca injection method, a complex inclusion such as (Al, Si, Ca)O formed by the conventional method is not dissolved in an aqueous solution, not increasing pH and thus not enhancing corrosion resistance. Therefore, the inventors identified that Ca single inclusion such as CaO, CaS, not complex inclusion is effective for increasing pH of a water film, and discussed a method for forming the Ca single inclusion in steel.

[148] However, in general, Ca has great reactivity and thus easily reacts with Al, Si oxides present in steel when injected into the molten steel to form complex inclusions such as (Al, Si, Ca)O, without forming Ca single inclusion. Consequently, to form single inclusion such as CaO or CaS in steel, other oxides such as Al O or SiO should

° 2 3 2 be removed from steel to the greatest extent. But it was found that if other oxides were removed from steel as described above, dissolved oxides in steel also diminished, rendering CaO generation difficult compared to CaS generation.

[149]

[150] Therefore, the inventors conducted studies and experiments to improve a steel- making process in an attempt to stimulate formation of CaS inclusion in steel. As a result, Ca-Si wire was split-injected at least twice instead of conventional one-time injection, and found a considerable increase in CaS inclusion in steel. That is, when Ca-Si wire is split- injected at least twice as just described, Ca injected initially reacts with oxides present in steel to form complex inclusion, and then is mixed into a slag layer via floatation in steel while waiting for the next Ca injection, thereby diminishing complex inclusion in steel. Therefore, Ca injected after first injcetion reacts with S in steel to effectively form CaS.

[151] [152] In addition, as stated above, in injecting Ca, the Ca, S, Al and Si contents are optimized via an Equation 1 to restrain formation of complex oxide and boost formation of CaS inclusion.

[153]

[154] Further, according to the invention, steel with excellent weather resistance having at least 30% of water-soluble CaS inclusion out of Ca-based non-metallic inclusions in the final product can be manufactured by carrying out continuous casting of molten steel having compositions as described above to fabricate a steel slab, and then hot- rolling the steel slab under a normal condition.

[155]

[156] According to the invention, the steel has at least 30% CaS inclusion out of Ca-based non-metallic inclusions because weight fraction of less than 30% does not lead to improvement in pH of the steel surface. This will be explained in more detail hereunder.

[157]

[158] Ca injected into molten steel reacts with oxygen, aluminum, silicon and sulfur present in the steel to form inclusions such as (Al, Si, Ca)O and CaS. Of these, complex inclusions other than CaS are not dissolved in an aqueous solution, not increasing pH of the steel surface, but only CaS inclusion is dissolved in the aqueous solution to increase pH. Consequently, weight fraction of CaS out of total inclusions serves as a decisive factor for pH increase of a water film. When pH of water film is at least 7.0, a neutral level, it can contribute to enhancing weather resistance. To gain pH of at least 7.0, as described above, the weight fraction of CaS inclusion out of total inclusions should be at least 30%.

[159]

[160] More preferably, water-soluble CaS inclusion out of Ca-based non-metallic inclusions in steel is limited to 30 to 80% for the following reasons. Addition of Ca produces oxide inclusion and sulfide inclusion at the same time. Sulfide inclusion, i.e., CaS may cause nozzle clogging in a continuous casting process, while oxide inclusion may inflict wear on refractory in a continuous casting process. Therefore to enhance weather resistance and secure stability of the continuous casting process, the fraction of CaS inclusion out of total inclusions should be preferably limited to up to 80%.

[161]

[162] According to the invention, Ca-based non-metallic inclusions are Ca-based inclusions formed by injecting Ca into molten steel, and are complex inclusions such as (Al, Si, Ca)O except for CaS. Herein, complex inclusions such as (Al, Si, Ca)O are oxide consisting of at least 2 of Al, Si, Ca, and contain CaAl O , CaAl O , Ca SiO , and Ca Al SiO The inclusions are variously sized and distributed, and impact

2 2 7 properties of steel are altered by the inclusion size. [163] That is, predominance of large-sized inclusions deteriorates impact properties. Thus according to the invention, in view of improved weather resistance, steel preferably should have CaS and Ca-based non-metallic inclusion including complex inclusion such as Al, Si, Ca)O distributed therein by an area fraction of at least 0.04% .

[164] Also, to prevent degradation of impact properties, steel preferably should have Ca- based non-metallic inclusion containing CaS sized up to 3D distributed therein by an area fraction of 0.02%.

[165] [166] As set forth above, the invention provides steel with excellent weather resistance by optimizing steel compositions and adequately controlling weight fraction of CaS inclusion out of Ca-based non-metallic inclusions in the product.

[167]

Mode for the Invention [168] The present invention will be explained in greater detail with the following examples. The examples below are given for the purpose of illustrating the present invention. However, the examples below are not given for any purpose of setting limits or bounds to the definition of the present invention as set forth in the specification and the claims.

[169] [170] First, Examples 1 to 4 have been made to confirm effects of steel with excellent weather resistance and a method for manufacturing the same. Examples 5 to 8 have been made to confirm whether the steel has excellent weather resistance and improved strength if at least one selected from a group consisting of Nb, V and B is additionally added thereto to enhance strength.

[171] [172] Example 1 [173] Molten steel having composition as in Table 1 has been prepared to fabricate a steel slab via continuous casting. Then the steel slab was hot-rolled under a normal condition to manufacture steel. At this time, in the case of inventive steels A to J and comparative steels A to F having Ca added, Ca was injected into molten steel as in Table 2 below. Meanwhile, conventional steel A and conventional steel B were prepared for comparison with inventive steels. Conventional steel A in Table 1 was a general weather resistant steel, and conventional steel B was a general structural steel.

[174] [175] Table 1

Contents (wt%) ® ®

C Si Mn Cu Ni Al P S Ca Ti Mo W Cr

[176] * A -J : Inventive Steels A-J

1 1 [177] A -F : Comparative Steels A-F [178] A and B : Conventional Steels A and B

3 3 [179] [180] Table 2

[181] [182] For the steel products manufactured as above, weight fraction of complex inclusion and CaS was measured, respectively. Specifically, as shown in Figure 2(a-b), 10 were randomly selected out of inclusions distributed in comparative steel C and inventive steel C, respectively, to examine compositions thereof, and the results are shown in the CaO-Al O -SiO ternary phase diagram. Herein, for comparative steel C, 9 of the 10 inclusions existed as complex oxide such as (Al, Si, Ca)O, while for inventive steel C, only 3 of the 10 inclusions existed as complex oxide such as (Al, Si, Ca)O. Out of inclusions in each steel type observed in the aforesaid method, weight fractions of complex inclusion and CaS inclusion were measured, respectively, and the results are shown in Table 3.

[183] [184] Table 3

[185] [186] As shown in Tables 1 to 3, in the case of inventive examples 1 to 10 in which compositions are optimized and Ca is split-injected at least twice in a steel-making process, the fraction of water-soluble CaS inclusion out of Ca-based non-metallic inclusions in the final product is at least 30%.

[187] [188] In contrast, in the case of comparative examples 1 and 2 in which compositions are within the scope of the invention but Ca is injected once in a steel-making process, the weight fraction of CaS inclusion was less than 30%.

[189] Also, in the case of comparative examples 3 to 10 which do not fulfill an Equation 1 (Ca(%)/S(%)> 1.5Al(%)+2Si(%)), compared to the invention, the fraction of CaS inclusion out of Ca-based non-metallic inclusions in all products was less than 30% regardless of split injection of Ca.

[190] Figure l(a) and (b) illustrate the shape of inclusion and a composition analysis result of comparative steel C and comparative steel C. As shown in Figure 1 , inclusions in comparative steel C are variously sized and composed, and at least 95% of inclusions have Ca-based complex oxide with the size of at least 3D. But in inventive steel C, 60% of inclusions have fine Cas inclusion with the size of about ID.

[191] [192] Further, in order to compare differences between inclusions produced in inventive and comparative steels according to Ca injection method, inclusion formation behavior in molten steel was calculated thermodynamically, and the results are shown in Figure 3. As shown in Figure 3, in comparative steel in which Ca was injected once, most inclusions out of molten steel exist as complex oxide such as CaAl O , CaAl O , Ca

4 7 12 9 2

SiO 4 , and Ca 2 Al 2 SiO 7 with little CaO formed, but CaS beg Oins to be J p-roduced after addition of at least 0.01% Ca. Meanwhile, in the case of inventive steel in which Ca is split-injected twice, CaS begins to be produced after addition of lOppm Ca and with increase in Ca injection, CaS production is dramatically boosted and thereby most inclusions exist as CaS. This thermodynamic calculation matches observation results of actual inclusions in Figure 1.

[193] [194] Example 2 [195] Size of inclusions and area fraction (inclusion area/observation area) with respect to inclusions present in each steel prepared in example 1 were measured. Then, by analyzing size and distribution of inclusions in steel as in Figure 4, measurement was conducted on average inclusion size, and area fraction of inclusion and fine inclusion, and the results are shown in Table 4. In Table 4, fine inclusion means an inclusion sized up to 3D.

[196] [197] Table 4

[198] [199] As shown in Table 4, inventive examples 1 to 10 in which compositions are optimized and Ca is split-injected in a steel-making process show results superior to co mparative examples 1 to 10 in terms of average inclusion size, and area fraction of inclusion and fine inclusion.

[200] [201] Example 3 [202] To examine effects of inclusion produced by Ca addition on pH of the steel surface, steels of example 1 (inventive steels A,C,D and H, comparative steels A,C and E, conventional steel A) were thinly coated with a water film on the surface to measure pH thereof. The results are shown in FIG.5.

[203] As shown in Figure 5, in inventive steels A, C, D and H in which C was split- injected at least twice, pH of the steel surface was elevated up to 7.8. This pH increase is closely related to weight fraction of CaS inclusion observed in inventive steel. That is, pH increase was the greatest in inventive steel H having the biggest weight fraction of CaS inclusion, whereas pH increase shrank with smaller weight fraction of CaS inclusion.

[204] [205] However in comparative steels A,C and E having small weight fraction of CaS, only comparative steel E having about 20% CaS showed a moderate increase in pH. But in comparative steels A and C having a small weight fraction of CaS showed little pH change like conventional steel having no Ca added (AL: general steel with excellent weather resistance).

[206] When pH changes of inventive steels A and H and comparative steel E having a similar amount of Ca added are compared based on the aforesaid results, split-injection of Ca rather than total amount of Ca added becomes a decisive factor for pH of the water film, and pH is determined by the weight fraction of CaS.

[207] Also, even if comparison of inventive steels A, H and comparative steel E shows a similar amount of Ca addition, pH changes thereof differ because of difference in weight fraction of CaS in inclusions.

[208] Therefore, pH changes little despite a great amount of Ca addition because complex inclusion produced by Ca addition rarely increases pH. This is well identified in Figure 6.

[209] [210] In Figure 6, inclusion was manufactured artificially according to inclusion composition observed in comparative steel C as shown in Figure Ia, and then the inclusion was coated with a water film. Figure 6 shows results of measuring pH changes twice. As in Figure 6, such tests show little pH change because Ca-based complex oxides are not dissolved in water, not contributing to pH increase.

[211] As set forth above, simple addition of Ca to molten steel does not ensure higher pH of the steel surface. But pH of the steel surface is elevated by split- injecting Ca to obtain at least a certain amount of water-soluble CaS inclusion. In this case, pH increase is proportional to the weight fraction of CaS out of inclusions. To gain at least 10% increase from the initial pH, the fraction of CaS out of inclusions should be at least 30%.

[212] [213] Example 4 [214] The steel of example 1 was exposed to a salt spray environment of lmdd to measure corrosion depth. The results are shown in Table 5.

[215] [216] Table 5

[217] [218] As shown in Table 5, inventive examples 1 to 10 in which compositions are optimized and Ca is split-injected at least twice in a steel-making process indicate superior weather resistance. Especially in the case of inventive examples 4 to 10 in which at least one selected from a group consisting of Ti, Mo and W is added, inventive examples 4 to 10 showed weather resistance superior to inventive examples having no Ti, Mo and W. Improved weather resistance according to the invention is achieved since CaS inclusion formed by Ca addition is dissolved in a water film condensed on the steel surface, increasing pH of the steel surface and thus forming stable rust in the early stage.

[219] [220] In contrast, overall inferior results are shown in the case of comparative examples 1 to 10 in which compositions and Ca injection method are beyond the scope of the invention. Especially conventional steels A and B having no Ca added indicated worst properties.

[221]

[222] Example 5 [223] Molten steel having composition as in Table 6 below was prepared and steel slab was fabricated via continuous casting. Then the fabricated steel slab was hot-rolled under a normal condition to manufacture steel. At this time, for inventive steels A to K and comparative steels A to I having Ca, Ca was injected into molten steel as in Table 7. Meanwhile, conventional steels A and B were prepared in the examples for comparison with inventive steels. Conventional steel A in Table 6 below was a general weather resistant steel and conventional steel B was a general structural steel.

[224] Table 6

[226] * A -K : Inventive Steels A-K i i [227] A -I : Comparative Steels A-I [228] A and B : Conventional Steels A and B [229] [230] Table 7

[231] [232] For the steel products manufactured as above, the weight fraction of complex inclusion and CaS was measured, respectively. More specifically as shown in Figure 8 (a-b), 10 were randomly selected out of inclusions distributed in comparative steel D and inventive steel B, respectively, to investigate compositions thereof. The results are shown in CaO-Al O -SiO ternary phase diagram. Herein, for comparative steel D, 9 of the 10 inclusions existed as complex oxide such as (Al, Si, Ca)O, while for inventive steel B, only 3 of the 10 inclusions existed as complex oxide such as (Al, Si, Ca)O. Out of inclusions in each steel type observed in the above method, the weight fraction of complex inclusion and CaS inclusion was measured, respectively, as shown in Table 8.

[233] [234] Table 8

[235] [236] As shown in Tables 6 to 8, in the case of inventive examples 1 to 11 in which compositions are optimized and Ca is split-injected at least twice in a steel-making process, the fraction of CaS inclusion out of Ca-based non-metallic inclusions in the final product is at least 30%.

[237] [238] In contrast, for comparative examples 1 to 3 in which compositions are within the scope of the invention but Ca is injected once in a steel-making process, the weight fraction of CaS inclusion was less than 30%.

[239] Also, in the case of comparative examples 1 to 14 which do not fulfill an Equation l(Ca(%)/S(%)> 1.5Al(%)+2Si(%)), the fraction of CaS inclusion out of Ca-based non- metallic inclusions in all products was less than 30% regardless of split injection of Ca.

[240] [241] Figure 7 (a) and (b) show the shape of inclusion and a composition analysis result of comparative steel D and inventive steel B. As shown in Figure 7 (a) and (b), inclusions in comparative steel D are variously sized an composed, and at least 95% of inclusions are sized at least 3D, while 60% of inclusions in inventive steel B are fine CaS inclusions sized about ID.

[242] [243] Example 6 [244] With respect to inclusions present in the steel prepared in example 5, measurement was conducted on the size of inclusions and their area fraction (inclusion area/ observation area). In this case, by analyzing the size distribution of inclusions in the steel as in Figure 9, measurement was conducted on the average inclusion size and the area fraction of inclusion and fine inclusion in inventive and comparative steels. The results are shown in Table 9. Herein fine inclusion in Table 9 means an inclusion sized up to 3D.

[245] [246] Table 9

[247] [248] As shown in Table 9, inventive examples 1 to 11 in which compositions are optimized and Ca is split-injected in a steel-making process indicate results superior to comparative examples 1 to 14 in terms of average inclusion size and the area fraction of inclusion and fine inclusion.

[249] [250] Example 7 [251] To examine effects of inclusion produced by Ca addition on pH of the steel surface, steels of example 1 (inventive steels B,F and H comparative steels B,F and G conventional steel A) were thinly coated with a water film to measure pH changes of the water film. The results are shown in Figure 10.

[252] As shown in Figure 10, in the case of inventive steels B, F and H having Ca split- injected at least twice, pH of the steel surface was elevated up to 7.8. This pH increase is closely related to the weight fraction of CaS inclusion observed in inventive steel. That is, inventive steel H having the biggest weight fraction of CaS inclusion indicated the highest increase in pH, while the pH increase shrank with smaller weight fraction of CaS inclusion.

[253] However, in the case of comparative steels B, F and G having a small weight fraction of CaS, only comparative steel G having about 20% of CaS showed a moderate increase in pH. Meanwhile in comparative steels B and F having a small weight fraction of CaS showed little pH change as in conventional steel (A, a general weather resistant steel) having no Ca added.

[254] [255] When inventive steel F and comparative steel F having a similar amount of Ca added are compared based on the aforesaid results, split-injection of Ca rather than total Ca amount added is a decisive factor for pH of the water film, and pH is determined by the weight fraction of CaS.

[256] Further, when inventive steel F and comparative steel F are compared, pH changes thereof are different despite a similar amount of Ca added. This results from difference in the weight fraction of CaS.

[257] [258] Therefore, comparative steel shows little pH change despite a great amount of Ca added because complex inclusion produced by Ca addition is not significantly conducive to increasing pH. This is well confirmed in Figure 11.

[259] Inclusions were produced artificially according to inclusion compositions observed in comparative steel D as shown in Figure 7a. Then the inclusions were coated with a water film to measure pH changes twice. The results are shown in Figure 11. As shown in Figure 11, the experiments show little pH change because Ca-based complex oxide is not dissolved in water, not contributing to pH increase.

[260] As stated above, simple addition of Ca does not ensure pH increase of the steel surface. The pH of the steel surface is elevated only by split-injection of Ca to obtain at least a certain amount of CaS inclusion. In this case, increase in pH is proportional to the weight fraction of CaS out of inclusions. To gain at least 10% increase from the initial pH, the fraction of CaS out of inclusions should be at least 30%.

[261] [262] Example 8 [263] The steel of example 5 was exposed to a salt spray environment of lmdd for 300 days of to measure corrosion depth thereof. The results are shown in Table 10.

[264] Also, for the steel of example 1, yield strength and tensile strength were measured through tensile tests, and the results are shown in Table 11.

[265] [266] Table 10

[267] [268] As shown in Table 10, inventive examples 1 to 11 in which compositions are optimized and Ca is split-injected at least twice in a steel-making process indicate superior weather resistance. Especially, inventive examples 3 to 11 in which at least one selected from Ti, Mo and W is added, show results superior to inventive examples having no Ti, Mo and W added. According to the invention, weather resistance is enhanced because CaS inclusion produced by Ca addition is dissolved in a water film condensed on the steel surface so that pH of the steel surface is elevated to form stable rust.

[269] In contrast, comparative examples 1 to 14 in which compositions and Ca injection method are beyond the scope of the invention indicate overall inferior results. Especially conventional steels A,B having no Ca added showed poor properties.

[270] [271] Table 11

[272]

[273] As shown in Table 11, inventive examples according to the invention can produce high strength steel having at least 307MPa of yield strength and at least 483MPa of tensile strength.

Claims

[1] Steel with excellent weather resistance consisting of: up to 0.15wt% C, up to 1.0wt% Si, up to 2.0wt% Mn, 0.2 to 1.0wt% Cu, 0.2 to

5.0%wt% Ni, 0.001 to 0.1wt% Al, up to 0.03wt% P, 0.002 to 0.03wt% S, 0.001 to 0.01wt% Ca, the balance being Fe and unavoidable impurities, wherein Ca, S,

Al and Si contents are determined by an Equation below:

Ca(%)/S(%) > 1.5Al(%)+2Si(%) Equation 1, the steel having 30wt% of water-soluble CaS inclusion out of Ca-based non- metallic inclusions. [2] The steel with excellent weather resistance according to claim 1, wherein the steel further comprises at least one selected from a group consisting of 0.005 to

0.1 wt% Ti, 0.01 to 1.0wt% Mo and 0.01 to 1.0wt% W. [3] The steel with excellent weather resistance according to claim 1, wherein the steel further comprises at least one selected from a group consisting of 0.02 to

0.07wt% Nb, up to 0.1 wt% V, 0.003 wt% B. [4] The steel with excellent weather resistance according to claim 1, wherein the steel has 30 to 80wt% water-soluble CaS inclusion out of Ca non-metallic inclusions. [5] The steel with excellent weather resistance according to claim 1, wherein the steel has Ca-based non-metallic inclusion containing CaS distributed therein by an area fraction of at least 0.04%, wherein the area fraction is determined by dividing inclusion area by observation area. [6] The steel with excellent weather resistance according to claim 1, wherein the steel has Ca-based non-metallic inclusion containing CaS sized up to 3D distributed therein by an area fraction of at least 0.02%, wherein the area fraction is determined by dividing inclusion area by observation area. [7] A method for manufacturing steel with excellent weather resistance comprising steps of: preparing molten steel consisting of: up to 0.15wt% C, up to 1.0wt% Si, up to 2.0wt% Mn, 0.2 to 1.0wt% Cu, 0.2 to

5.0wt% Ni, 0.001 to 0.1wt% Al, up to 0.03wt% P, 0.002 to 0.03wt% S, the balance being Fe and unavoidable impurities; injecting Ca into the molten steel, carrying out bubbling to remove floating inclusion from the molten steel, and then injecting Ca again into the molten steel at least once to constitute 0.001 to 0.01wt% Ca content, wherein the Ca content is determined by Equation 1 below: Ca(%)/S(%) > 1.5Al(%)+2Si(%) Equation 1; and continuously casting the molten steel to fabricate a steel slab and hot-rolling the steel slab under a normal condition to manufacture steel having water-soluble CaS inclusion out of Ca-based non-metallic inclusions by an area fraction of at least 30%.

[8] The method according to claim 7, wherein the steel further consists of at least one selected from a group consisting of 0.005 to 0. lwt% Ti, 0.01 to 1.0wt% Mo, 0.01 to 1.0wt% W.

[9] The method according to claim 7, wherein the steel further consists of at least one selected from a group consisting of 0.02 to 0.07wt% Nb, up to 0. lwt% V, up to 0.003 wt% B.

[10] The method according to claim 7, wherein the steel has 30 to 80 wt% CaS inclusion out of Ca-based non-metallic inclusions.

[11] The method according to claim 7, wherein the steel has Ca-based non-metallic inclusion containing CaS distributed therein by an area fraction of at least 0.04%, wherein the area fraction is determined by dividing inclusion area by observation area.

[12] The method according to claim 7, wherein the steel has Ca-based non-metallic inclusion containing CaS sized up to 3D distributed therein by an area fraction of 0.02%, wherein the area fraction is determined by dividing inclusion area by observation area.