KR100920597B1 - Method for manufacturing steel with resistance to coastal atmospheric corrosion having tensile strength of 50kgf/mm2 grade - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing coastal weather resistant steel having excellent corrosion resistance under corrosive environments with a lot of chlorine ions such as bridges, buildings, and especially coastal zones.

This manufacturing method is in weight%, C: 0.05-0.15%, Mn: 1.6% or less, Ni: 0.5-2.0%, Cu: 0.2-0.6%, P: 0.05% or less, S: 0.003-0.035%, W: 0.1 ~ 0.5% or Mo: 0.1 ~ 0.5% of one or two kinds of Ca, Si, Al in wire form simultaneously added to molten steel composed of remaining Fe and other unavoidable impurities, Ca: 0.002 ~ 0.004% by weight, Si : 0.2 ~ 0.7% by weight and Al: 0.01 ~ 0.06% by weight of the steel slab obtained by controlling and casting the re-heating at 900 ~ 1300 ℃, it finishes rolling over the A _r3 temperature and then air-cooled to room temperature.

This manufacturing method adds W or Mo to Si together with Si to provide sufficient coastal corrosion resistance even if the Ca content is hard to be added at 40 ppm or less in the steelmaking process, thereby providing sufficient coastal corrosion resistance. By providing a method of manufacturing the structural coastal weathering steel of grade ² mm, there is an effect that can contribute to the drastic reduction of maintenance costs against corrosion of the coastal environment of steel bridges and buildings.

Weathering Steel, Si, W, Mo, Ca, Tensile Strength

Description

Method for manufacturing steel with resistance to coastal atmospheric corrosion having tensile strength of 50kgf / mm2 grade}

The present invention relates to a method for producing coastal weather resistant steel having excellent corrosion resistance in corrosive environments with a lot of chlorine ions such as bridges and buildings, especially in coastal areas, and more specifically, to produce structural steel having a tensile strength of 50kgf / mm ² . In addition, W or Mo is added to the steel together with Si, which increases the compactness of the rust layer and can inhibit the penetration of chlorine ions, thereby providing sufficient coastal corrosion resistance even if the Ca content is difficult to be maintained at 40 ppm or less due to the high volatility in the steelmaking process. The present invention relates to a method for producing a structural coastal weather resistant steel having a tensile strength of 50kgf / mm ² capable of mass production without quality deviation.

Conventional weathering steels containing trace amounts of Cu, Cr, P, etc. are known to have 4 to 8 times higher atmospheric corrosion resistance than ordinary steels. The reason for such high corrosion resistance is that the initial period of exposure to the atmosphere Similarly, rust occurs, but as time passes, part of the rust gradually forms a dense stable rust adhered to the base material, and the rust layer acts as a protective film to the environment. It is known that the structure of stable rust that acts as anticorrosive is amorphous iron hydroxide or α-FeOOH. However, in the salty environment such as coastal areas, it is difficult to form stable rust as mentioned above, and therefore, it cannot be applied to coastal areas where the amount of adjuvant salt is more than 0.05mdd (mg / dm ² / day).

In the case of general weathering steel, the following three theories have been suggested for the reason that there is no stable rust formation in the coastal region, so that the layered rust exists and the amount of corrosion increases. First, in the coastal areas, unlike the rural and industrial zones, the mainstream of the rust layer is Fe ₃ O ₄ or β-FeOOH. Second, chlorine ions penetrate the rust layer and concentrate at the interface between the base and the rust layer. It is suggested that further promotion and third, because stable rust is already formed, rust will crystallize when it comes into contact with chlorine ion environment. The above three factors are thought to be due to the fact that the pH near the rust layer is likely to decrease with corrosion of the steel in a salty environment. When corrosion of steel starts, the pH of the steel surface tends to decrease. Once the pH in the rust film or at the interface between the steel and rust decreases, the permeability of chlorine ions in the rust increases, so that the concentration of chlorine ions is near the interface between the steel and rust. As a result, chlorine atmosphere is formed at the interface to promote corrosion.

As mentioned above, general weathering steels are severely limited in their application because stable rust is not formed at the coast. Since most of the social infrastructure to be built now and in the future is built in the coastal zone, the development of economical infrastructure is urgently required to develop coastal weathering steel with corrosion resistance that can be used as a coating in coastal areas. Development has been active.

The most developed and marketed coastal steel is 3.0% Ni-0.4% Cu steel. In the 3.0% Ni-0.4% Cu steel, it is known that the concentrated rust layer of Ni has the effect of blocking salts, so it is known to exhibit corrosion resistance in coastal areas. However, since 3.0% Ni-0.4% Cu steel is expensive due to the large amount of Ni contained, it is necessary to develop low Ni-based coastal weather resistant steel with a significant reduction in the content of expensive elements.

In order to develop coastal weather resistant steel having breakthrough corrosion resistance of Ni content reduction type, a technique using water-soluble basic oxides such as CaO has been proposed. Japanese Laid-Open Patent Publication No. 58-25458 introduces a fine basic oxide powder in a steelmaking step, so that basic oxides such as Ca, Mg, Sr, and Be are dissolved in an aqueous solution at the same time as the corrosion reaction, thereby raising the pH of the steel surface, thereby forming a rust film. It is to prevent the permeation of chlorine ions and to suppress the elution of iron ions to improve the weather resistance of the coast. However, in the current steelmaking technology, it is difficult to uniformly disperse pure oxides in steel, and according to Japanese Patent Laid-Open No. 5-51667, Ca, Mg, Sr, and Be oxide particles having an average particle diameter of 1 μm or less and an average particle diameter are disclosed. A method of uniformly dispersing iron powder of 200 µm or less in a steel mixture is used. However, these inventions have been pointed out as a problem that the manufacturing method of steel materials is uneconomical because expensive basic oxide powders are mixed and added with expensive iron powders.

Since the mixed addition method of basic oxide and iron powder is uneconomical, the attempt to use basic oxide more economically has been actively made in recent years. Such trials include Japanese Patent Application Laid-Open Nos. 11-172370, 2000-54066, 2000-54067, 2000-63981, 2000-17383, and the like. These prior arts improve corrosion resistance by adding Mg in the form of metal to steelmaking, or by adding Ca and other rare earth elements together to produce basic and rare earth metal oxides. However, even in the manufacturing method of these patents, the corrosion resistance is mainly controlled by the Ni content, and when the average splash salt is 0.05 to 0.2mdd (mg / dm ² / day), 1.0 to 2.5% of Ni is required, and the splash salt is 0.2 to 0.5. In the case of mdd, it is known that 2.5 to 5.5% of Ni is required in a severe environment of 2.5 to 3.5% Ni and a droplet salt content of 0.5 to 0.8mdd.

Accordingly, Korean Patent Application No. 2000-70749 uses water-soluble CaS inclusions formed by adding a thin wire-type Ca-Fe that is easy to add steelmaking phase into steel instead of using a basic oxide that is inherently difficult to add steelmaking phase. this tensile strength of 40, a method for preparing a weather-resistant steel alloy elements saving of 50kgf / mm ² class have been proposed in which the coast weatherability severe corrosion environment as 0.2~0.8mdd level. However, in the case of the invention of Korea Patent Application No. 2000-70749, even when the content of Ca is more than 40ppm, it has a certain corrosion resistance. When the content of Ca is more than 40ppm, Ca tends to be unevenly distributed in the steel slab and thus mechanical properties. And there exists a problem that a quality deviation exists in corrosion resistance and the like.

The present invention is to solve the above problems of the prior art, by adding W or Mo to the steel together with Si which can suppress the penetration of chlorine ions to increase the compactness of the rust layer in the steel making it hard to add Ca By providing sufficient coastal corrosion resistance even if the content is maintained at 40ppm or less, to provide a method for producing a structural coastal weathering steel of 50kgf / mm ² grade tensile strength capable of mass production without quality deviation, an object thereof.

The present invention for achieving the above object is by weight, C: 0.05 ~ 0.15%, Mn: 1.6% or less, Ni: 0.5 ~ 2.0%, Cu: 0.2 ~ 0.6%, P: 0.05% or less, S: 0.003 Ca: 0.002 by simultaneously adding Ca-Si and Al in the form of wire to molten steel which is composed of one or two kinds of ~ 0.035%, W: 0.1-0.5% or Mo: 0.1-0.5%, the remaining Fe and other unavoidable impurities Steel slabs obtained by controlling and casting to 0.004% by weight, 0.2: 0.7% by weight and Al: 0.01% to 0.06% by weight are reheated at 900 to 1300 ° C, finished rolling over A _r3 temperature, and then cooled to room temperature It is made to include.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In order to solve the problems caused by the non-uniform distribution of Ca in the high Ca steel, the present inventors have conducted research on the development of steels having the same level of corrosion resistance as the high Ca-containing steel even in a low Ca content. On the basis of the present invention has been proposed. The present invention increases the denseness of the rust layer when Si, W or Mo is added to the steel and at the same time exists in an anion state such as WO ₄ ^2- , MoO ₄ ^2- to inhibit the intrusion of Cl ^- ions into the rust layer. It is used to improve the corrosion resistance in the atmosphere, and it is focused on the fact that the addition of Si and W or Mo compensates for the corrosion resistance decrease due to the reduction of Ca content by such corrosion resistance.

Hereinafter, the reason for limitation of the said steel component range, all manufacturing conditions, etc. are demonstrated.

C: 0.05-0.15 wt%

The C (carbon) is an element added to improve the strength, if the content is less than 0.05% by weight it is difficult to secure the strength due to lack of hardenability, when added in excess of 1.5% by weight can increase the strength by improving the hardenability However, because it impairs the weldability, it is preferable to limit the content to 0.05 ~ 0.15% by weight.

Mn: 1.6 wt% or less                     

The Mn (manganese) may increase the hardenability and increase the strength when the content is increased, but damage the weldability, it is preferable to limit the Mn content to 1.6% by weight or less in consideration of ensuring the appropriate strength and weldability.

Ni: 0.5-2.0 wt%

Ni (nickel) is an element that improves coastal weather resistance. As Ni is added, the grain size of rust becomes fine and the dissolution activation of iron ions decreases, so that low pH of the rust and steel interface is suppressed, and the cation selective permeability of the formed rust layer is increased. As a result, the penetration of chlorine ions into the rust layer is suppressed, thereby greatly improving coastal weatherability. As the amount of Ni is increased, the corrosion resistance, strength and toughness are improved, but it is preferable to limit it to 2.0% by weight or less since it is an expensive element, and if the content is less than 0.5% by weight, it is difficult to secure corrosion resistance. Silver is preferably limited to 0.5 to 2.0% by weight.

Cu: 0.2-0.6 wt%

The Cu (copper) is an element that improves the corrosion resistance of the steel due to the refinement and densification of the rust layer particles, but effective in terms of corrosion resistance as the content is increased, but when the slab is reheated for rolling, Cu having a low melting point penetrates into the grain boundaries of the steel and cracks during hot working. Since this may cause hot shortness (hot shortness) is limited to 0.6% by weight or less, if the addition amount is less than 0.2% by weight corrosion resistance is lowered, it is preferable to limit the content to 0.2 to 0.6% by weight.

P: 0.05 wt% or less

The P (phosphorus) is an element that forms PO ₄ ^3- ions in aqueous solution when present in steel to increase the cation selective permeability of the rust layer to inhibit the permeation of the chlorine ion to improve the corrosion resistance, but the weldability is remarkably increased as the amount added is increased. Since deterioration and toughness deteriorate, it is preferable to limit the addition amount to 0.05% by weight or less in consideration of corrosion resistance, weldability and toughness.

S: 0.003-0.035 wt%

The S (sulfur) is present in the steel mainly as CaS in combination with the added Ca. CaS enhances the corrosion resistance of steel by promoting the formation of α-FeOOH, which is stable rust by the action of HS ^- ions when dissolved in aqueous solution. Therefore, the content of S is better, but is limited to 0.035% by weight or less as an element harmful to impact toughness and weldability, and the content is limited to 0.003 to 0.035% by weight since the effect of improving the corrosion resistance is less than 0.003% by weight. It is desirable to.

W: 0.1-0.5 weight% or Mo: 0.1-0.5 weight% 1 type or 2 types

The W (tungsten) is an element that contributes to the refinement of the grain size of the rust layer when present in the steel and forms WO ₄ ^2- ions in aqueous solution to increase the cation selective permeability of the rust layer, thereby inhibiting the penetration of the chlorine ion into the rust layer, thereby improving corrosion resistance. As the added amount is increased, the weldability is considerably lowered and the toughness is deteriorated, so it is limited to 0.5% by weight or less, and when the added amount is less than 0.1% by weight, the effect of improving corrosion resistance cannot be expected. In consideration of corrosion resistance, weldability, and toughness, it is preferable to limit the amount to 0.1 to 0.5% by weight.

Mo (molybdenum) also acts similar to W, contributes to the refinement of the grain size of the rust layer when present in the steel, and form MoO ₄ ^2- ions in the aqueous solution to increase the cation selective permeability of the rust layer to inhibit the rust penetration of the chlorine ion Improves corrosion resistance. As the added amount is increased, the weldability is considerably lowered and the toughness is deteriorated, so it is limited to 0.5% by weight or less, and when the added amount is less than 0.1% by weight, the effect of improving corrosion resistance cannot be expected. In consideration of corrosion resistance, weldability and toughness, the Mo addition amount is preferably limited to 0.1 to 0.5% by weight.

In addition to the above compositions, the remainder is composed of Fe and other unavoidable impurities.

Ca is added simultaneously with Al in the form of Ca-Si in the form of wire in the steelmaking process to make the steel having the above composition. Adding Ca-Si and Al simultaneously adds Ca-Si, adds Al later or Al deoxidation first, then adds Ca-Si to form coarse CaO · Al ₂ O ₃ and mix it into slag This is because the error rates of Ca and Al are greatly reduced.

The Ca (calcium) is present as inclusions such as CaS, Al ₂ O ₃ · CaS, CaO, Al ₂ O ₃ · CaO when added to the steel and dissolved upon contact with the corrosion solution to increase the pH of rust and the surface of the steel or It is an element that promotes formation of α-FeOOH, which is stable rust, and greatly improves coastal weather resistance. As the added amount increases, the effect of improving corrosion resistance increases. However, since Ca is highly volatile, the recovery rate is low, which is uneconomical and causes quality deviations such as the mechanical properties of the product. If it is less than 0.002%, since the effect of improving corrosion resistance is slight, it is preferable to limit the content to 0.002 to 0.004% by weight.

The Si (silicon) is a component added as a deoxidizer, in the present invention serves to increase the corrosion resistance by inhibiting the permeability of Cl ^- ions to contribute to the miniaturization of the corrosion rust layer and to increase the strength. If the content is less than 0.2% by weight, the effect of improving the corrosion resistance is insignificant, and as the Si content is increased, the corrosion resistance increases, but when the content exceeds 0.7% by weight, the weldability is remarkably degraded and it is very difficult to remove the scale during rolling. Therefore, the Si content is preferably limited to 0.20 to 0.7% by weight.

Al improves shock absorption energy as an essential element for deoxidation, but impairs impact toughness when a large amount is added. When the content of Al is less than 0.01% by weight, deoxidation is insufficient, and when the content of Al exceeds 0.06% by weight, impact toughness is impaired. Therefore, the content is preferably limited to 0.01 to 0.06% by weight.

The slab, which is formed as described above, is reheated at 900 to 1300 ° C. If the reheating temperature is less than 900 ℃ high deformation resistance of the steel is not possible to roll with a conventional hot rolling mill, and if it exceeds 1300 ℃ to bring about abnormal grain growth (grain normal) of the grains and deterioration of mechanical properties , The reheating temperature is preferably limited to 900 ~ 1300 ℃.

After the reheating, the rolling is terminated at a temperature higher than A _r3 (starting temperature at which the steel is transformed from austenite to ferrite when cooling at a high temperature), and then cooled to room temperature. When the rolling finish temperature is lower than A _r3 , the microstructure is composed of ferrite + pearlite + processed strained ferrite, and the ferrite processed in the two-phase region has a high stress state, thereby providing a cause of local corrosion, thereby reducing corrosion resistance. There is concern. Therefore, the hot rolling end temperature is preferably limited to more than A _r3 temperature.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

Inventive steels (A to D) and comparative steels (E to S), which are prepared as shown in Table 1 below, were manufactured as slabs. Ca, Si and Al content adjustments in the production of the inventive steels A, B, C, D and comparative steels E and F steel were performed by adding Ca-Si wire and Al simultaneously. Comparative steel G was prepared by administering Al deoxidation followed by administration of Ca-Fe in molten steel. The cast slab was sufficiently heated at 1250 ° C. to form a 13 mm steel sheet by continuous hot rolling with an average reduction of 17%, and then air cooled. The hot-rolled during rolling end temperature was 850 ℃ higher than the Ar ₃ temperature (780 ~ 820 ℃). Inventive material is prepared by the Cyclic Corrosion Test, which simulates the corrosive environment of the coast with a level of 0.8mdd (mg / dm ² / day) by making specimens of 70mm × 150mm × 5mm size from the invention material and the comparative material. The coastal corrosion resistance of and non-comparative materials was investigated. Repeated corrosion test conditions were salt spray (30 ℃, 10min)-wet (30 ℃, 80% RH, 60min)-drying (40 ℃, 50% RH, 50min)-washing (water spray, 10min) Repeat drying (40 ° C, 35% RH, 80 minutes). 480 repeated tests were performed under these conditions, and the results are shown in Table 2 below. The 480 repeated tests in the repeated composite corrosion test are known to coincide with the degree of corrosion after 4 years on the shore with a splash of 0.8 mdd.

The crystal structure of the rust layer formed by the 480 cycle repeated corrosion test was analyzed by X-ray diffraction test and hardness test to identify the corrosion resistance mechanism of the inventive steel, and the results are shown in Table 3 below. In addition, an AC impedance test was conducted in a thin water film to investigate the effect of W and Mo elements on the polarization resistance in the presence of Cl ⁻ ions. 0.01 M and 0.1 M NaCl aqueous solutions were used as the water film, and the thickness of the water film was adjusted to 100 μm. Electrochemical Impedance Spectroscopy (EIS) was used, and the impedance change according to the frequency change was measured by applying an AC voltage of 10 mV in the frequency range of 10 ⁻² to 10 ⁻⁴ Hz. The polarization resistances of the inventive and comparative materials obtained by the AC impedance test are shown in Table 4 below.

Mechanical properties of the inventive and comparative materials were evaluated by tensile and impact tests, and are shown in Table 5 below.

As shown in Tables 1 and 2, the inventive materials (1 to 4) in which the Si and W or Mo contents fall within the scope of the present invention are 38.48 to 42.85 µm in corrosion depth when exposed to 480 times. Corrosion resistance was very good at 1/6 or less. Comparative materials (5) and (6), which are almost similar to the chemical composition range of the present invention but do not contain W or Mo, have excellent corrosion resistance compared to general steel but have a corrosion depth of 53.16 ~ 66.77㎛, which is higher than that of the invention. Fell significantly. Comparative material (7) is a case where the Si content is lower than the chemical composition of the present invention, it shows that the corrosion resistance is better than the comparative materials (5 to 6), but the corrosion resistance is better than the invention material. It can be seen from this that the Si element is a very important element for implementing the corrosion resistance of the present invention.                     

In addition, from the X-ray diffraction test results for the green layer formed at 480 cycles of the repeated composite corrosion test shown in Table 3, inventions (1-4) containing W or Mo together with Si were known to be effective for beach corrosion resistance. It is noted that the relative fraction of -FeOOH is at least 41.01%, significantly higher than 17.19% in general structural steel. In addition, the comparative material (5 to 7) having a lower content of Si, W or Mo than the inventive material was found to have a lower fraction of α-FeOOH than the inventive material. From this, it can be seen that the invention material is significantly better in coastal corrosion resistance than the comparative material because the addition of Si, W or Mo promotes the formation of α-FeOOH, which is stable rust even at the coast. Table 3 also shows the hardness test results for the rust layer, showing that the hardness of the invention material rust layer is significantly higher than the comparative material. In view of the fact that the hardness of the rust layer is due to the fineness and compactness of the rust layer, in the present invention, the improvement of the corrosion resistance by addition of Si, W or Mo is because these elements contribute greatly to the refinement and densification of the rust layer. Able to know.

Table 4 shows the corrosion polarization resistance under the thin NaCl of the invention and the comparative material, it shows that the corrosion polarization resistance under NaCl atmosphere more than two times increased as added W or Mo.

From the results of the electrochemical behavior under NaCl atmosphere of Table 3 and Table 4, and the structural analysis of the rust layer, it is well understood that addition of Si, W, and Mo increases the coastal corrosion resistance of the steel through the refinement of the rust layer, the crystal structure change, and the ionic property change. Can be. Therefore, even if the Ca content in the present invention is 20 to 40ppm, which is the quality deviation management range, the corrosion resistance equivalent to or higher than that of the high Ca steel having a Ca content of 40ppm or more is ensured due to the significant corrosion resistance improvement effect of Si, W, and Mo. You can see well.

Table 5 shows the mechanical properties of the invention and the comparative material. In Table 5, the yield strength of the invention material is 34.4 ~ 42.9kgf / mm ² , the tensile strength is 55.4∼61.1kgf / mm ² , and the impact toughness is 55.9∼91.9J. The tensile strength is 50kgf / mm ² grade. 33 kgf / mm < ^2> or more, tensile strength 50-62kgf / mm <^2> , 0 degreeC impact toughness 28J). From this, the invention material is not only excellent in corrosion resistance but also has mechanical properties as a structural strength of 50kgf / mm ² grade tensile strength, it can be seen that it can be sufficiently applied to economic bridge building construction.

As described above, the present invention is capable of mass production without quality deviation by adding W or Mo to Si in steel to provide sufficient coastal corrosion resistance even if the Ca content is hard to be added at 40 ppm or less in the steelmaking process. By providing a method of manufacturing structural coastal weather resistant steel with a tensile strength of 50kgf / mm ² , it can contribute to the drastic reduction of maintenance costs against corrosion of steel bridges and buildings.

Claims (1)

By weight%, C: 0.05-0.15%, Mn: 1.6% or less, Ni: 0.5-2.0%, Cu: 0.2-0.6%, P: 0.05% or less, S: 0.003-0.035%, W: 0.1-0.5% Or Mo: 0.1-0.5% of one or two kinds of steel, which is composed of the remaining Fe and other unavoidable impurities, simultaneously adding Ca-Si and Al in the form of wire, Ca: 0.002-0.004% by weight, Si: 0.2-0.7 Weight% and Al: tensile strength 50kgf / mm, including reheating the steel slab obtained by controlling to 0.01 ~ 0.06% by weight and casting the steel slab at 900 ~ 1300 ° C, finishing rolling above the temperature of A _r3 and then cooling it to room temperature Method for manufacturing grade ² coastal weather resistant steel.