KR101289154B1 - Hot rolled steel sheet having excellent corrosion resistance and impcat toughness and manufacturing method thereof - Google Patents

Abstract

The present invention is in weight%, carbon (C): 0.03-0.1%, silicon (Si): 0.1-0.5%, manganese (Mn): 0.1-1.0%, aluminum (Al): 0.01-0.7%, chromium (Cr ): 0.5-1.5%, phosphorus (P): 0.03% or less, sulfur (S): 0.01% or less, contains residual iron (Fe) and other unavoidable impurities, and the microstructure is 15% or less perlite, 5% bainite Providing a hot-rolled steel sheet having excellent corrosion resistance and impact toughness including residual ferrite and the following, and reheating the slab satisfying the above-described component system and finishing rolling at 800 ° C. or higher; And it provides a method for producing a hot rolled steel sheet excellent in corrosion resistance and impact toughness comprising the step of cooling the rolled steel sheet to a temperature of 560 ℃ or more.

Seawater, Corrosion Resistance, Impact Toughness, Hot Rolled Steel Sheet

Description

HOT ROLLED STEEL SHEET HAVING EXCELLENT CORROSION RESISTANCE AND IMPCAT TOUGHNESS AND MANUFACTURING METHOD THEREOF}

The present invention relates to a hot rolled steel sheet used in a structure installed in seawater, and a method for manufacturing the same, and more particularly, to a hot rolled steel sheet excellent in corrosion resistance and low temperature impact toughness, and a method for manufacturing the sheet, without a method. will be.

Recently, marine development is rapidly increasing due to the locational and economic limitations of land development. In the case of installing structures in seawater, corrosion of structures is the biggest problem, and a method for preventing such corrosion is required.

Accordingly, a technique of suppressing corrosion of a structure by various kinds of anticorrosive treatments has been proposed. However, when corrosion resistance of the material itself is not secured, the anticorrosive age is only about 30 to 40 years. That is, in order to increase the durability of the structure for a long time more than 50 years, and to reduce the cost of various anticorrosion during the operation of the structure, it is required to strengthen the corrosion resistance of the material itself.

Copper and nickel are the most effective elements which improve the seawater resistance of steel materials. Conventionally, copper and nickel are used to improve the corrosion resistance of steel itself, and nickel is generally added in the range of 50% of the amount of copper to prevent casting cracks generated during casting. However, nickel is an expensive alloying element, and as the content of nickel increases, the price of steel itself increases.

However, in recent years, continuous casting technology has been developed, and the minimum amount of nickel added for preventing casting defects in copper-added steel is decreasing, thereby making it possible to reduce the cost of the product by reducing the amount of expensive nickel added.

Further, alloys effective for seawater corrosion in addition to copper and nickel include chromium, phosphorus, and aluminum, and a technique for improving corrosion resistance in seawater environments by adding such alloying elements has been proposed.

Japanese Patent Laid-Open Publication No. Hei 2-138441 includes such a technique that a large amount of aluminum is added to improve corrosion resistance, but the intermetallic compound is formed by a large amount of aluminum, resulting in a significant decrease in hot workability. There are disadvantages.

Another technique is Japanese Patent Laid-Open No. 6-264176, which improves the corrosion resistance by adding a large amount of chromium and manganese (3.0 to 6.0% by weight), but continuous in case of high manganese content in steel. Segregation easily occurs in the casting process, which has the disadvantage of embrittlement of steel.

The present invention is to control the microstructure of the steel sheet by controlling the component system and manufacturing conditions, to improve the impact toughness characteristics of the steel sheet, to minimize the corrosion rate of the steel sheet to improve the corrosion resistance of the steel sheet itself, and to suppress the occurrence of casting defects The present invention provides a hot rolled steel sheet having excellent impact toughness and a method of manufacturing the same.

In one embodiment, the present invention provides, by weight%, carbon (C): 0.03-0.1%, silicon (Si): 0.1-0.5%, manganese (Mn): 0.1-1.0%, aluminum (Al): 0.01-0.7 %, Chromium (Cr): 0.5-1.5%, Phosphorus (P): 0.03% or less, Sulfur (S): 0.01% or less, balance iron (Fe) and other unavoidable impurities, and the microstructure is 15% or less perlite It provides a hot rolled steel sheet excellent in corrosion resistance and impact toughness, including 5% or less of bainite and residual ferrite.

The steel sheet may further include, by weight percent, copper (Cu): 0.1 to 0.5% and nickel (Ni): 0.05% or less, and the copper and nickel satisfy 6 ≦ copper / nickel ≦ 10. .

The steel sheet may further include one or two kinds of titanium (Ti): 0.1% or less and niobium (Ni): 0.07% or less.

As another embodiment of the present invention, in weight%, carbon (C): 0.03 to 0.1%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.1 to 1.0%, aluminum (Al): 0.01 to 0.7 %, Chromium (Cr): 0.5-1.5%, phosphorus (P): 0.03% or less, sulfur (S): 0.01% or less, after reheating the slab containing residual iron (Fe) and other unavoidable impurities Finishing rolling in; And it provides a method for producing a hot rolled steel sheet excellent in corrosion resistance and impact toughness comprising the step of cooling the rolled steel sheet to a temperature of 560 ℃ or more.

The slab is preferably reheated at 1000 ~ 1200 ℃.

According to the present invention, it is possible to provide a hot rolled steel sheet having improved corrosion resistance of the steel sheet itself, no casting defects, and excellent impact toughness.

In order to improve seawater resistance and maximize the effect by chromium, which is an element for improving corrosion resistance, the present invention controls the content of copper, nickel, etc., and optimizes the range of rolling temperature and cooling end temperature, etc. The present invention relates to a hot rolled steel sheet having excellent corrosion resistance and impact toughness.

Hereinafter, the component system of the hot-rolled steel sheet of the present invention will be described.

Carbon (C): 0.03 to 0.1 wt%

Carbon is the most useful element to improve the strength of steel. In order to obtain such an effect in the present invention, it is preferably included 0.03% by weight or more. However, if the content is high, the carbon equivalent is high, the weldability, formability and impact characteristics are deteriorated, so in the present invention, it is preferable to limit the upper limit to 0.1% by weight in consideration of the other element affecting the carbon equivalent. Do.

Silicon (Si): 0.1 to 0.5 wt%

Silicon is generally added to deoxidize molten steel and added as a solid solution hardening element. This effect is insignificant when the content of silicon is less than 0.1% by weight. On the other hand, if the content exceeds 0.5% by weight, the scale is formed on the surface by the excessive silicon, there is a problem in lowering the surface quality.

Manganese (Mn): 0.1-1.0 wt%

Manganese is a very effective element in solidifying steel. This effect is negligible when the manganese content is less than 0.1% by weight. On the other hand, when the content exceeds 1.0% by weight, a large amount of MnS is formed to lower the corrosion resistance and impact characteristics.

Aluminum (Al): 0.01 ~ 0.07 wt%

Aluminum is added to deoxidize molten steel and is a useful element for improving corrosion resistance. This effect is insignificant when the aluminum content is less than 0.01% by weight. On the other hand, when the content is more than 0.07% by weight, aluminum and oxygen are combined by excessive aluminum content to form oxide-based inclusions in the steel, thereby lowering the impact properties of the steel.

Chromium (Cr): 0.5-1.5%

Chromium is a useful element for improving seawater resistance. When included in less than 0.5% by weight of the present invention this effect is insignificant. On the other hand, when the content is more than 1.5% by weight, local corrosion occurs due to the chromium oxide formed by combining chromium and oxygen.

Phosphorus (P): 0.03% by weight or less

Phosphorus is an element that improves the corrosion resistance of steel, but when excessively added, it is contained in steel and causes a problem of causing central segregation during casting. This problem occurs when the phosphorus content exceeds 0.03% by weight, so the upper limit of the phosphorus content is preferably limited to 0.03% by weight.

Sulfur (S): 0.01 wt% or less

Sulfur is an inevitable impurity, and reacts with manganese to form MnS, which acts as a starting point for corrosion. In theory, the sulfur content is advantageously limited to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, the upper limit of the sulfur content in the present invention is preferably limited to 0.01% by weight.

The remaining component of the present invention is iron (Fe). However, in the usual steel manufacturing process, impurities which are not intended from raw materials or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.

In addition, in addition to the advantageous component system described above, when copper (Cu) and nickel (Ni) are additionally added, these elements can further increase the seawater resistance of chromium.

Copper (Cu): 0.5 wt% or less

Copper is an element that improves seawater resistance, but the corrosion resistance may be improved as the content of copper increases, but when it exceeds 0.5% by weight, the effect of improving corrosion resistance is not so great. It is preferable to limit an upper limit to 0.5 weight%.

Nickel (Ni): 0.05 wt% or less

Nickel is generally added to prevent casting cracks generated during casting in copper-added steels, and is useful for improving seawater resistance with copper, but it is a costly element that causes the price to rise and prevents cracking due to the development of casting technology. It is preferable to limit the minimum amount for the upper limit to 0.05% by weight.

6≤copper / nickel≤10

In the present invention, the content ratio (each weight%) of copper and nickel is preferably limited to 6-10. When the ratio of copper / nickel is less than 6, the content of expensive nickel is high and economic efficiency is remarkably decreased. On the other hand, when the copper / nickel content ratio exceeds 10, there is a problem that the defect of the casting occurs because the content of copper is too high compared to nickel.

In addition, the steel of the present invention can further improve the effects of the present invention when additionally adding one or more elements of titanium (Ti) and niobium (Nb) described below.

Titanium (Ti): 0.1 wt% or less

Titanium is an element that plays a role of improving the strength by the precipitation strengthening effect by bonding Ti with carbon in the steel, when the content exceeds 0.1% by weight, the strength improvement effect is not large compared to the increase in the content of the present invention In the upper limit of the content is preferably limited to 0.1% by weight.

Niobium (Nb): 0.07 wt% or less

Niobium is an element that enhances the strength by precipitation strengthening effect by forming NbC by combining with carbon in steel like titanium. When the content exceeds 0.07% by weight, the strength improvement effect is not increased. Since the upper limit of the content in the present invention is not large, it is preferable to limit it to 0.07% by weight.

As the steel sheet having the above-described component system, it is necessary to limit the microstructure of the steel sheet to preferable conditions for becoming a steel sheet excellent in corrosion resistance and impact characteristics in a seawater environment. The microstructure of the steel sheet of the present invention is 15% or less in the pearlite area fraction, 5% or less in the bainite area fraction, and the balance includes ferrite. If the area fraction of bainite is more than 5% or the area fraction of pearlite is more than 15%, it is difficult to secure the impact toughness energy value intended by the present invention.

The most preferred method elicited by the present inventors for producing the steel which satisfies the object of the present invention as described above is described below.

In the manufacturing method of the present invention, the slab having the steel composition of the present invention is schematically heated at 1000 to 1200 ° C., and then the heated slab is subjected to finish rolling at a rolling end temperature of 800 ° C. or higher, cooled, and cooled at 560 ° C. or higher. Terminate and wind up.

Hereinafter, detailed conditions of each step will be described.

Slab reheating temperature: 1000 ~ 1200 ℃

The reheating temperature of the slab of the present invention is preferably 1000 ° C. or higher, in order to solidify the carbonitride formed during casting. However, when reheating excessively high temperature, austenite may coarsen, so the reheating temperature is preferably 1200 ° C or lower.

Rolling end temperature: 800 ℃ or more

The reheated slab is preferably rolled to finish rolling at a temperature of 800 ° C. or higher. When rolling is complete | finished below 800 degreeC, the area fraction of bainite exceeds 5%, and impact toughness falls.

Cooling end temperature: 560 ℃ or higher

It is preferable to perform cooling at the temperature of 560 degreeC or more by performing cooling of the hot rolled sheet steel by which the said rolling was complete | finished. When cooling is finished below 560 ° C, the area fraction of bainite exceeds 5%, and the microstructure intended by the present invention cannot be secured.

Hereinafter, the present invention will be described in detail through examples.

(Example)

After casting the ingot (ingot) satisfying the component system shown in Table 1, and then heated at 1200 ℃ for 1 hour, starting rolling to finish rolling at 820 ℃ to prepare a 12 mm thick hot rolled steel sheet to 580 ℃ After cooling for 1 hour at the temperature to give a cooling effect and winding effect, the furnace was cooled. The seawater resistance test was done using the hot-rolled steel sheet obtained by such a process. Corrosion test was repeated for 2 hours of 35 ℃ brine spray, 60 ℃ 4 hours drying, 50 ℃ 2 hours wet conditions in accordance with ISO 14993, was performed for a total of 480 hours, the corrosion rate was measured in Table 1 Indicated. In addition, using the actual continuous casting machine for each specimen to observe the occurrence of surface casting defects during continuous casting is shown in Table 2 below. In addition, Charpy impact toughness test was performed on the inventive steel 1 and the comparative steel 3, and the results are shown in FIG.

division C Si Mn P S Cu Cr Ni Al Nb Ti Corrosion rate
(mm / yr) Casting defect Inventive Steel 1 0.05 0.3 0.5 0.01 0.005 - 0.5 - 0.03 - - 0.53 none Invention river 2 0.05 0.3 0.5 0.01 0.005 - 1.5 - 0.03 - - 0.21 none Invention steel 3 0.05 0.3 0.5 0.01 0.005 - One - 0.03 - - 0.78 none Inventive Steel 4 0.05 0.3 0.5 0.01 0.005 - One - 0.07 - - 0.72 none Invention steel 5 0.05 0.3 0.5 0.03 0.005 - One - 0.03 - - 0.53 none Invention steel 6 0.05 0.3 0.5 0.01 0.005 0.5 One 0.05 0.03 - - 0.07 none Invention steel 7 0.05 0.5 0.5 0.01 0.005 - One - 0.03 - - 0.78 none Inventive Steel 8 0.05 0.3 1.0 0.01 0.005 - One - 0.03 - - 0.54 none Invention river 9 0.05 0.3 0.5 0.01 0.005 - One - 0.03 - 0.1 0.81 none Invented Steel 10 0.05 0.3 0.5 0.01 0.005 - One - 0.03 0.05 0.1 0.90 none Comparative River 1 0.05 0.3 0.5 0.01 0.005 - 0.4 - 0.03 - - 1.53 none Comparative River 2 0.05 0.3 0.5 0.01 0.005 - - - 0.03 - - 2.13 none Comparative Steel 3 0.05 0.3 0.7 0.11 0.005 0.55 0.45 0.45 0.03 - - 1.40 none Comparative Steel 4 0.05 0.3 0.5 0.01 0.005 0.4 One 0.036 0.03 - - 0.13 Occur

In Table 1, the content unit of each element is weight percent.

Comparative steel 1, the chromium content is less than the limit of the present invention, the corrosion rate was measured to 1.53mm / year, comparative steel 2 was chromium-free steel was measured as the corrosion rate 2.13mm / year, comparison Steel 3 contains copper and nickel but the content of chromium is less than the limit of the present invention, the corrosion rate is measured to 1.40mm / year it can be confirmed that the corrosion resistance is not excellent. On the contrary, it can be confirmed that the corrosion rates of the inventive steels 1 to 10 were significantly reduced to 0.90 mm / year or less.

In addition, Comparative Steel 4 contained copper and nickel, but the ratio of copper / nickel exceeded 10, and the corrosion rate was measured to be 0.13 mm / year, but the corrosion resistance was excellent, but it was confirmed that defects occurred during casting.

And, as shown in Figure 1, Comparative steel 3 is a steel grade to ensure corrosion resistance by the addition of P, the maximum absorption energy (Upper shelf energy) is about 60J, whereas the maximum absorption energy (Upper shelf energy) of the invention steel 1 ) Is about 320J, which shows that the impact characteristics are about 5 times higher than that of the comparative steel.

1 is a graph showing Charpy impact energy values according to temperatures of inventive steel 1 and comparative steel 3;

Claims (6)

By weight%, carbon (C): 0.03 to 0.1%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.1 to 1.0%, aluminum (Al): 0.01 to 0.07%, chromium (Cr): 0.5 ~ 1.5%, phosphorus (P): 0.03% or less, sulfur (S): 0.01% or less, contains residual iron (Fe) and other unavoidable impurities, and the microstructure is 15% or less perlite, 5% or less bainite and balance Hot rolled steel sheet with excellent corrosion resistance and impact toughness, including ferrite. The method of claim 1, wherein the steel sheet is a weight percent, copper (Cu): 0.1 to 0.5% and nickel (Ni): 0.05% or less, the copper and nickel satisfy 6≤ copper / nickel≤10 Hot rolled steel sheet excellent in corrosion resistance and impact toughness. delete The hot rolled steel sheet according to claim 1, wherein the steel sheet includes, by weight%, one or two of titanium (Ti): 0.1% or less and niobium (Ni): 0.07% or less. By weight%, carbon (C): 0.03 to 0.1%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.1 to 1.0%, aluminum (Al): 0.01 to 0.07%, chromium (Cr): 0.5 ˜1.5%, phosphorus (P): 0.03% or less, sulfur (S): 0.01% or less, reheating the slab containing residual iron (Fe) and other unavoidable impurities and finishing rolling at 800 ° C. or higher; And The method of manufacturing a hot rolled steel sheet excellent in corrosion resistance and impact toughness comprising the step of cooling the rolled steel sheet to a temperature of 560 ℃ or more. The method of claim 5, wherein the slab reheating is performed at 1000 ~ 1200 ℃ excellent hot corrosion steel and impact toughness manufacturing method.

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