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WO2008062984A1 - Steel excellent in resistance to corrosion by sulfuric acid and method for manufacturing the same - Google Patents

Steel excellent in resistance to corrosion by sulfuric acid and method for manufacturing the same Download PDF

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Publication number
WO2008062984A1
WO2008062984A1 PCT/KR2007/005824 KR2007005824W WO2008062984A1 WO 2008062984 A1 WO2008062984 A1 WO 2008062984A1 KR 2007005824 W KR2007005824 W KR 2007005824W WO 2008062984 A1 WO2008062984 A1 WO 2008062984A1
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WO
WIPO (PCT)
Prior art keywords
steel
less
weight
sulfuric acid
content
Prior art date
Application number
PCT/KR2007/005824
Other languages
French (fr)
Inventor
Soo Hee Lee
Kee Jo Jeong
Gyo Sung Kim
Jai Ik Kim
Sung Hwan Kim
Original Assignee
Posco
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Posco filed Critical Posco
Priority to JP2009538322A priority Critical patent/JP5239089B2/en
Publication of WO2008062984A1 publication Critical patent/WO2008062984A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a steel having corrosion resistance to sulfuric acid, which is used as materials of a duct, an air preheater, a boiler pipe and their component parts in a power plant boiler, and a method for manufacturing the same, and more particularly, to a steel having corrosion resistance to sulfuric acid capable of extending life span of equipments by improving corrosion resistance to sulfuric acid in a low temperature-sulfuric acid concentration zone, and a method for manufacturing the same.
  • Korean Patent Laid-open Publication No. 2001-010931 discloses a technique for improving corrosion resistance to sulfuric acid through the addition of a Cu-Co complex.
  • Korean Patent Laid-open Publication No. 2003-0047470 discloses a technique for improving corrosion resistance and surface defects by adding Cr and Ni to a Cu-Co steel, respectively, and
  • Korean Patent Laid-open Publication No. 2003-0047469 discloses a technique for securing strength by adding Nb to a Cu-Co steel.
  • the Cu-Co steel systems have improved corrosion resistance to sulfuric acid, but require corrosion resistance to sulfuric acid under low temperature-low concentration conditions, and therefore the physical properties of the Cu-Co steel systems remain to be improved further.
  • the mechanical properties of the components in the Cu-Co steel systems are not sufficient to apply to the high temperature environment, which makes it difficult to be installed in boiler ducts, air preheaters, or the like.
  • Japanese Patent Laid-open Publication No. 2002-327236 discloses a steel showing mechanical properties that are suitable for use at a high temperature by adding alloy components such as Cu-Cr-(Ti, Nb, V, Mo, W, etc.) to steel, but the steel has a problem that it has deteriorated corrosion resistance characteristics at low temperature - sulfuric acid concentration ranges. Disclosure of Invention Technical Problem
  • An aspect of the present invention provides a steel having a steel having a more improved corrosion resistance to sulfuric acid and a superior strength at low temperature-low concentration ranges.
  • Another aspect of the present invention provides a high-temperature and high- strength steel having excellent mechanical properties at a temperature range from a room temperature to 500 0 C, and a method for manufacturing the same.
  • a steel having excellent corrosion resistance to sulfuric acid at low temperature-low concentration ranges including, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities.
  • a high- temperature and high-strength steel having corrosion resistance to sulfuric acid including, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities, wherein the steel has at least one selected from the group consisting of low temperature structures of accicular ferrite, bainitic ferrite and bainite.
  • a method for manufacturing a high-temperature and high-strength steel having corrosion resistance to sulfuric acid including: hot-rolling a steel including, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities; cooling the steel acceleratedly; and coiling the steel at a temperature of 500 to 66O 0 C so that the steel can have a low temperature structure.
  • the content of Mn may be in a range from 1.51 to 2.0% by weight in the steel according to the present invention.
  • the high-temperature and high-strength steel having corrosion resistance to sulfuric acid according to the present invention may further include vanadium (V).
  • the high-temperature and high-strength steel having corrosion resistance to sulfuric acid according to the present invention may include 20% by weight or more of at least one low temperature structure from the group consisting of accicular ferrite, bainitic ferrite and bainite.
  • the components such as Mn, Cr, Ni and Nb preferably satisfy the following equation.
  • the steel according to the present invention may be useful to improve corrosion resistance at low temperature-sulfuric acid concentration ranges, and also to improve strengths at a room temperature and a high temperature.
  • FIGS. 1 and 2 are photographs illustrating micro structures of an inventive steel according to one exemplary embodiment of present invention, depending on the coiling temperature.
  • the inventive steel is coiled at a high temperature in the case of FIG. 1, and the inventive steel that is coiled at a low temperature in the case of FIG. 2.
  • the present invention was designed during a study for preparing the optimum composition in the Cu-Co component system.
  • Mn is added in an increased content of approximately 2.0% by weight to improve strength characteristics of steel having corrosion resistance to sulfuric acid in the Cu-Co component system.
  • Ni is adjusted to the upper limit of 0.1% by weight or less, and Cr is adjusted to a content of 0.1 to 1.0% by weight while adding Nb.
  • the steel according to the present invention has improved physical properties such as corrosion resistance and strength at the low temperature-sulfuric acid concentration ranges in by statistically controlling contents of the components such as Mn, Nb, Cr and Ni.
  • the components such as Mn, Nb, Cr and Ni.
  • Ni in a content of 0.1% by weight or more, but surface defects does not occur in the Nb-Cr- added steel according to the present invention but corrosion resistance of the Nb-Cr-added steel is improved when Ni is added in a content of 0 to 1.0% by weight.
  • the present invention is also characterized in that the steels that is designed to include the above-mentioned components have further improved strength at high temperature when they are subject to the accelerated cooling and low- temperature coiling processes. That is to say, the precipitation strengthening effect and the formation of a low temperature structure allows the steel according to the present invention to improve strength at high temperature.
  • Carbon (C) is preferably in a content of 0.15% by weight or less.
  • Silicon (Si) is preferably in a content of 1.0% by weight or less.
  • Silicon (Si) is an element that is mainly added to improve strength of steel.
  • the silicon (Si) exceeds 1.0% by weight, corrosion resistance of steel in the low temperature-sulfuric acid concentration ranges may be deteriorated. Therefore, it is preferred to add the silicon (Si) in the content of 1.0% by weight or less.
  • Manganese (Mn) is preferably in a content of 2.0% by weight or less.
  • Manganese (Mn) is generally added to prevent red shortness caused by dissolved sulfur (S) by precipitating the dissolved sulfur (S) in conventional steel in the form of manganese sulfides.
  • the manganese (Mn) is also added to prevent red shortness and improve strength of steel at room temperature and high temperature.
  • the content of the manganese (Mn) exceeds 2.0% by weight, the corrosion resistance to sulfuric acid is deteriorated with respect to the strength- improving effect. Therefore, it is preferred to adjust the content of the manganese (Mn) to the upper limit of 2.0% by weight.
  • the content of the manganese (Mn) is adjusted to a range of 1.51 to 2.0% by weight.
  • the content of the manganese (Mn) may be suitably selected to obtain a desired strength, and the content of Mn may range from 0.5 to 1.50% by weight, if necessary.
  • S is preferably in a content of 0.03% by weight or less.
  • the sulfur (S) is preferably added in a content that is as low as possible.
  • the content of the sulfur (S) exceeds 0.03% by weight, the possibility to generate defects may be increased by the hot brittleness. Therefore, it is preferred to adjust the content of the sulfur (S) to the upper limit of 0.03% by weight.
  • Phosphorus (p) is preferably in a content of 0.02% by weight or less.
  • Aluminum (Al) is preferably in a content of 0.01 to 0.1% by weight.
  • Aluminum (Al) is an element that is added for the purpose of deoxidation in a refining process.
  • the content of the aluminum (Al) is less than 0.01% by weight, the deoxidation effect is low, whereas the possibility to generate defects in a steel surface is high due to the increase in Al oxides when the content of the aluminum (Al) exceeds 0.1%.
  • Copper (Cu) is preferably in a content of 0.2 to 1.0% by weight.
  • Copper (Cu) is an element that is inevitably added in consideration of the corrosion resistance to sulfuric acid.
  • the content of the copper (Cu) should exceed 0.2% by weight to show high corrosion resistance characteristics.
  • the corrosion resistance is poorly improved in respect to the increasing amount of the added copper. Therefore, it is preferred to limit the content of the copper (Cu) to the upper limit of 1.0% by weight.
  • Co Co is preferably in a content of 0.02 to 0.1% by weight.
  • cobalt (Co) is an element that is characteristic to sulfuric acid resistant steel.
  • the cobalt (Co) is added together with the copper (Cu)
  • the content of the cobalt (Co) is less than 0.02% by weight, its addition effect is poor.
  • the content of the cobalt (Co) exceeds 0.1% by weight, the corrosion resistance is poorly improved in respect to the amount of the added cobalt (Co), and the manufacturing cost is very high.
  • Chromium (Cr) is preferably in a content of 0.1 to 1.0% by weight.
  • Chromium (Cr) is added to improve corrosion resistance of steel by forming Cr- based oxides in a surface of the steel at a high temperature.
  • a protective film is hardly formed in the surface of the steel when the content of the added chromium (Cr) is less than 0.1% by weight.
  • the protective film is more easily formed with an increasing amount of the added chromium (Cr), which leads to the improved protective effect.
  • the content of the chromium (Cr) exceeds 1.0% by weight, the excessive chromium (Cr) does not nearly affect the protective effect.
  • Nickel (Ni) is preferably in a content of 0.1% by weight or less.
  • Nickel (Ni) is an element that highly inhibits corrosion resistance to sulfuric acid but was added in the past to prevent surface defects that may be caused during a continuous molding or hot-rolling process of a Cu-containing steel. However, when the nickel (Ni) is added in a content of 0.1% by weight or less, the steel has a smooth surface and an improved corrosion resistance at the same time. Also, the manufacturing cost may be also cut down.
  • Niobium (Nb) is preferably in a content of 0.02 to 0.1% by weight.
  • Niobium (Nb) is an element that contributes highly to improving strength of steel at a room temperature and a high temperature due to the precipitates into fine NbC.
  • the niobium (Nb) should be added in a content of 0.02% by weight or more to secure a desired strength of steel. In this case, the strength of the steel is enhanced with an increasing amount of the niobium (Nb).
  • the amount of the added niobium (Nb) exceeds 0.1% by weight, ductility of the steel is highly degraded, and it is also difficult to expect the precipitates into NbC due to the lack of dissolved carbon (C).
  • the content of the niobium (Nb) is preferred to limit the content of the niobium (Nb) to the upper limit of 0.1% by weight.
  • the niobium (Nb) is most desirable since it functions to effectively improve strength of steel without degrading the corrosion resistance of the steel, compared to the other alloy components.
  • Additional alloy components may be added to the steel that meets the requirements for the above-mentioned components, if necessary.
  • vanadium (V) may be added to increase strength of steel.
  • Vanadium (V) is preferably in a content of 0.02 to 0.1% by weight.
  • the steel that meets the requirements regarding the above-mentioned component systems may have the structure of polygonal ferrite.
  • the corrosion resistance of the steel in the range of low temperature and low concentration of sulfuric acid is improved with favorable strength property.
  • the steel including a low-temperature structure is manufactured through the accelerated cooling process, the steel has further improved high-temperature tensile properties.
  • the low-temperature structure includes at least one selected from the group consisting of accicular ferrite, bainitic ferrite and bainite.
  • the low-temperature structure is most preferably present in a ratio of 20 to 100% in the steel.
  • the steel including the above-mentioned composition may be re-heated according to one of the conventional methods known in the art, and hot-rolled to prepare a hot-rolled steel, or the hot-rolled steel may be then cold-rolled to prepare a cold-rolled steel.
  • the steel according to the present invention may have the structure of polygonal ferrite.
  • the corrosion resistance of the steel in the range of low temperature and low concentration of sulfuric acid is improved with favorable strength property.
  • the steel including a low-temperature structure is manufactured through the accelerated cooling process, the steel has further improved high- temperature tensile properties.
  • the low-temperature structure includes at least one selected from the group consisting of accicular ferrite, bainitic ferrite and bainite.
  • the steel having the structure of polygonal ferrite is not manufactured according to one of special methods but one of the conventional methods known in the art. That is to say, the accelerated cooling and low-temperature coiling processes do not apply to the manufacture of the steel according to the present invention. Therefore, the method for manufacturing a steel including a low-temperature structure will be described in more detail on the basis of the accelerated cooling process according to the present invention.
  • a steel that meets the requirements regarding the component systems according to the present invention is hot-rolled, cooled acceleratedly, and then coiled at a temperature of 500 to 66O 0 C. These manufacturing conditions are set to obtain a low- temperature structure.
  • the preferable accelerated cooling process is carried out at an average cooling rate of 30°C/s or more, and more preferably 30 to 50°C/s.
  • the accelerated cooling condition is sufficient when a low- temperature structure may be obtained at the above-mentioned coiling temperature, and the present invention is not particularly limited to the above-mentioned accelerated cooling rate.
  • the hot-rolled test samples were immersed at 7O 0 C in a 50% sulfuric acid solution for 3 hours, and then measured for corrosion resistance to sulfuric acid under low temperature-low concentration conditions. Also, the hot-rolled test samples were evaluated for mechanical properties by performing a tensile test at room temperature/high temperature (500 0 C). The results are listed in the following Table 2. When the tensile test was carried out at a high temperature, the tensile specimens were heated to a desired temperature and kept for 30 minutes to minimize the difference in temperature. In the steel types of Al -A 12 as listed in Table 1, aluminum (Al) is present in a content of 0.03%.
  • FIG. 1 shows the change in micro structures of an A4 steel when the A4 steel was coiled at a high temperature, and then coiled at a low temperature in an accelerated cooling process. As shown in FIG. IB, it was revealed that a low-temperature structure is distributed over the A4 steel.
  • the A2 steel was a Ti-added steel that has a satisfactory level of a low-temperature structure, but its corrosion resistance to sulfuric acid was poor. Therefore, it was seen that Ti was not the optimum alloy element in consideration of the effect of Ti on the corrosion resistance. Also, it was revealed that the A5 steel was a P-added steel that has a satisfactory effect on the increase in strength, but it was detrimental to the corrosion resistance.

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Abstract

There is provided a steel having superior corrosion resistance to condensed sulfuric acid, and a method for manufacturing the same. The steel having corrosion resistance to sulfuric acid includes, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities. Accordingly, the steel may have a polygonal ferrite structure, or have at least one selected from the group consisting of low temperature structures of accicular ferrite, bainitic ferrite and bainite. Also, the steel including a low temperature structure exhibits excellent mechanical properties in a wide temperature range from a room temperature to 5000C.

Description

Description
STEEL EXCELLENT IN RESISTANCE TO CORROSION BY SULFURIC ACID AND METHOD FOR MANUFACTURING THE
SAME
Technical Field
[1] The present invention relates to a steel having corrosion resistance to sulfuric acid, which is used as materials of a duct, an air preheater, a boiler pipe and their component parts in a power plant boiler, and a method for manufacturing the same, and more particularly, to a steel having corrosion resistance to sulfuric acid capable of extending life span of equipments by improving corrosion resistance to sulfuric acid in a low temperature-sulfuric acid concentration zone, and a method for manufacturing the same.
[2]
Background Art
[3] When a sulfur-containing fuel is burned, SOx is formed in exhaust gas, and then chemically reacts with moisture in the exhaust gas to form sulfuric acid. When the exhaust gas is cooled to a dew point (approximately 16O0C) of sulfuric acid, the sulfuric acid is condensed onto a surface of a steel, and therefore the steel is seriously corroded by the condensed sulfuric acid. Materials, which are used at a low temperature range of 2000C or below, in equipments of a thermoelectric power plant require excellent characteristics such as corrosion resistance to sulfuric acid, but may be used for conventional low alloy steels when mechanical properties of the materials meets the requirement of the conventional low alloy steels. However, the boiler duct or the air preheater, which operates at a temperature of 2000C or above, require excellent high temperature tensile properties in addition to the excellent corrosion resistance to sulfuric acid.
[4] Generally, there have been developed a variety of steels whose corrosion resistance is secured by adding a combination of copper (Cu) and other corrosion resistant alloys to the steels, and representative technologies known in the art includes Korean Patent Laid-open Publication Nos. 2001-010931, 2003-0047470 and 2003-0047469, and Japanese Patent Laid-open Publication No. 2002-327236.
[5] Korean Patent Laid-open Publication No. 2001-010931 discloses a technique for improving corrosion resistance to sulfuric acid through the addition of a Cu-Co complex. Korean Patent Laid-open Publication No. 2003-0047470 discloses a technique for improving corrosion resistance and surface defects by adding Cr and Ni to a Cu-Co steel, respectively, and Korean Patent Laid-open Publication No. 2003-0047469 discloses a technique for securing strength by adding Nb to a Cu-Co steel. In the above publications, the Cu-Co steel systems have improved corrosion resistance to sulfuric acid, but require corrosion resistance to sulfuric acid under low temperature-low concentration conditions, and therefore the physical properties of the Cu-Co steel systems remain to be improved further. Also, the mechanical properties of the components in the Cu-Co steel systems are not sufficient to apply to the high temperature environment, which makes it difficult to be installed in boiler ducts, air preheaters, or the like.
[6] Japanese Patent Laid-open Publication No. 2002-327236 discloses a steel showing mechanical properties that are suitable for use at a high temperature by adding alloy components such as Cu-Cr-(Ti, Nb, V, Mo, W, etc.) to steel, but the steel has a problem that it has deteriorated corrosion resistance characteristics at low temperature - sulfuric acid concentration ranges. Disclosure of Invention Technical Problem
[7] An aspect of the present invention provides a steel having a steel having a more improved corrosion resistance to sulfuric acid and a superior strength at low temperature-low concentration ranges.
[8] Another aspect of the present invention provides a high-temperature and high- strength steel having excellent mechanical properties at a temperature range from a room temperature to 5000C, and a method for manufacturing the same.
Technical Solution
[9] According to an aspect of the present invention, there is provided a steel having excellent corrosion resistance to sulfuric acid at low temperature-low concentration ranges, the steel including, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities.
[10] According to another aspect of the present invention, there is provided a high- temperature and high-strength steel having corrosion resistance to sulfuric acid including, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities, wherein the steel has at least one selected from the group consisting of low temperature structures of accicular ferrite, bainitic ferrite and bainite.
[11] According to still another aspect of the present invention, there is provided a method for manufacturing a high-temperature and high-strength steel having corrosion resistance to sulfuric acid, the method including: hot-rolling a steel including, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities; cooling the steel acceleratedly; and coiling the steel at a temperature of 500 to 66O0C so that the steel can have a low temperature structure.
[12] The content of Mn may be in a range from 1.51 to 2.0% by weight in the steel according to the present invention. Also, the high-temperature and high-strength steel having corrosion resistance to sulfuric acid according to the present invention may further include vanadium (V).
[13] The high-temperature and high-strength steel having corrosion resistance to sulfuric acid according to the present invention the present invention may include 20% by weight or more of at least one low temperature structure from the group consisting of accicular ferrite, bainitic ferrite and bainite.
[14] For the steel according to the present invention, the components such as Mn, Cr, Ni and Nb preferably satisfy the following equation.
[15]
(λln+Cr+Xb ) in Ki
Advantageous Effects
[16] The steel according to the present invention may be useful to improve corrosion resistance at low temperature-sulfuric acid concentration ranges, and also to improve strengths at a room temperature and a high temperature.
[17]
Brief Description of the Drawings
[18] FIGS. 1 and 2 are photographs illustrating micro structures of an inventive steel according to one exemplary embodiment of present invention, depending on the coiling temperature. Here, the inventive steel is coiled at a high temperature in the case of FIG. 1, and the inventive steel that is coiled at a low temperature in the case of FIG. 2.
[19]
Best Mode for Carrying Out the Invention
[20] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[21] To improve physical properties such as corrosion resistance and strength at low temperature-sulfuric acid concentration ranges in which steel is apt to corrode, the present invention was designed during a study for preparing the optimum composition in the Cu-Co component system. For the present invention, Mn is added in an increased content of approximately 2.0% by weight to improve strength characteristics of steel having corrosion resistance to sulfuric acid in the Cu-Co component system. In order to prevent the decrease in corrosion resistance through the addition of the Mn, Ni is adjusted to the upper limit of 0.1% by weight or less, and Cr is adjusted to a content of 0.1 to 1.0% by weight while adding Nb. Unlike the steel having corrosion resistance to sulfuric acid in the Cu-Co component system that has been widely known in the art, the steel according to the present invention has improved physical properties such as corrosion resistance and strength at the low temperature-sulfuric acid concentration ranges in by statistically controlling contents of the components such as Mn, Nb, Cr and Ni. In order to solve the problems regarding the surface defects caused by the addition of Cu in the Cu-Co component system, it is necessary to add Ni in a content of 0.1% by weight or more, but surface defects does not occur in the Nb-Cr- added steel according to the present invention but corrosion resistance of the Nb-Cr-added steel is improved when Ni is added in a content of 0 to 1.0% by weight.
[22] Furthermore, the present invention is also characterized in that the steels that is designed to include the above-mentioned components have further improved strength at high temperature when they are subject to the accelerated cooling and low- temperature coiling processes. That is to say, the precipitation strengthening effect and the formation of a low temperature structure allows the steel according to the present invention to improve strength at high temperature.
[23] Carbon (C) is preferably in a content of 0.15% by weight or less.
[24] When the content of the carbon (C) exceeds 0.15% by weight, the corrosion resistance to sulfuric acid and the welding characteristics may be deteriorated, which leads to the shortened life span of equipments used in the present invention, as well as the increased possibility to generate defects. Therefore, it is preferred to add the carbon (C) in the content of 0.15% by weight or less.
[25] Silicon (Si) is preferably in a content of 1.0% by weight or less.
[26] Silicon (Si) is an element that is mainly added to improve strength of steel.
However, when the content of the silicon (Si) exceeds 1.0% by weight, corrosion resistance of steel in the low temperature-sulfuric acid concentration ranges may be deteriorated. Therefore, it is preferred to add the silicon (Si) in the content of 1.0% by weight or less.
[27] Manganese (Mn) is preferably in a content of 2.0% by weight or less.
[28] Manganese (Mn) is generally added to prevent red shortness caused by dissolved sulfur (S) by precipitating the dissolved sulfur (S) in conventional steel in the form of manganese sulfides. In the case of the present invention, the manganese (Mn) is also added to prevent red shortness and improve strength of steel at room temperature and high temperature. When the content of the manganese (Mn) exceeds 2.0% by weight, the corrosion resistance to sulfuric acid is deteriorated with respect to the strength- improving effect. Therefore, it is preferred to adjust the content of the manganese (Mn) to the upper limit of 2.0% by weight. To improve strength at room temperature and high temperature, it is preferred to adjust the content of the manganese (Mn) to a range of 1.51 to 2.0% by weight. When the content of the manganese (Mn) is within the above range, the strength of steel may be improved without degrading toughness of the steel. Of course, the content of the manganese (Mn) may be suitably selected to obtain a desired strength, and the content of Mn may range from 0.5 to 1.50% by weight, if necessary.
[29] Sulfur (S) is preferably in a content of 0.03% by weight or less.
[30] The sulfur (S) is preferably added in a content that is as low as possible. When the content of the sulfur (S) exceeds 0.03% by weight, the possibility to generate defects may be increased by the hot brittleness. Therefore, it is preferred to adjust the content of the sulfur (S) to the upper limit of 0.03% by weight.
[31] Phosphorus (p) is preferably in a content of 0.02% by weight or less.
[32] When the content of the phosphorus (P) exceeds 0.02% by weight, the increase in strength of steel can be expected, but the corrosion resistance to sulfuric acid may be highly degraded. Therefore, it is preferred to limit the content of the phosphorus (P) to the upper limit of 0.02% by weight.
[33] Aluminum (Al) is preferably in a content of 0.01 to 0.1% by weight.
[34] Aluminum (Al) is an element that is added for the purpose of deoxidation in a refining process. When the content of the aluminum (Al) is less than 0.01% by weight, the deoxidation effect is low, whereas the possibility to generate defects in a steel surface is high due to the increase in Al oxides when the content of the aluminum (Al) exceeds 0.1%.
[35] Copper (Cu) is preferably in a content of 0.2 to 1.0% by weight.
[36] Copper (Cu) is an element that is inevitably added in consideration of the corrosion resistance to sulfuric acid. Here, the content of the copper (Cu) should exceed 0.2% by weight to show high corrosion resistance characteristics. When the content of the copper (Cu) exceeds 1.0% by weight, the corrosion resistance is poorly improved in respect to the increasing amount of the added copper. Therefore, it is preferred to limit the content of the copper (Cu) to the upper limit of 1.0% by weight.
[37] Cobalt (Co) is preferably in a content of 0.02 to 0.1% by weight.
[38] In addition to the copper (Cu), cobalt (Co) is an element that is characteristic to sulfuric acid resistant steel. When the cobalt (Co) is added together with the copper (Cu), it is possible to secure very superior corrosion resistance, compared to when the cobalt (Co) is used alone. When the content of the cobalt (Co) is less than 0.02% by weight, its addition effect is poor. On the contrary, when the content of the cobalt (Co) exceeds 0.1% by weight, the corrosion resistance is poorly improved in respect to the amount of the added cobalt (Co), and the manufacturing cost is very high.
[39] Chromium (Cr) is preferably in a content of 0.1 to 1.0% by weight.
[40] Chromium (Cr) is added to improve corrosion resistance of steel by forming Cr- based oxides in a surface of the steel at a high temperature. Here, a protective film is hardly formed in the surface of the steel when the content of the added chromium (Cr) is less than 0.1% by weight. The protective film is more easily formed with an increasing amount of the added chromium (Cr), which leads to the improved protective effect. However, when the content of the chromium (Cr) exceeds 1.0% by weight, the excessive chromium (Cr) does not nearly affect the protective effect.
[41] Nickel (Ni) is preferably in a content of 0.1% by weight or less.
[42] Nickel (Ni) is an element that highly inhibits corrosion resistance to sulfuric acid but was added in the past to prevent surface defects that may be caused during a continuous molding or hot-rolling process of a Cu-containing steel. However, when the nickel (Ni) is added in a content of 0.1% by weight or less, the steel has a smooth surface and an improved corrosion resistance at the same time. Also, the manufacturing cost may be also cut down.
[43] Niobium (Nb) is preferably in a content of 0.02 to 0.1% by weight.
[44] Niobium (Nb) is an element that contributes highly to improving strength of steel at a room temperature and a high temperature due to the precipitates into fine NbC. The niobium (Nb) should be added in a content of 0.02% by weight or more to secure a desired strength of steel. In this case, the strength of the steel is enhanced with an increasing amount of the niobium (Nb). However, when the amount of the added niobium (Nb) exceeds 0.1% by weight, ductility of the steel is highly degraded, and it is also difficult to expect the precipitates into NbC due to the lack of dissolved carbon (C). Therefore, it is preferred to limit the content of the niobium (Nb) to the upper limit of 0.1% by weight. The niobium (Nb) is most desirable since it functions to effectively improve strength of steel without degrading the corrosion resistance of the steel, compared to the other alloy components.
[45] Additional alloy components may be added to the steel that meets the requirements for the above-mentioned components, if necessary. As a representative example of the additional alloy components, vanadium (V) may be added to increase strength of steel.
[46] Vanadium (V) is preferably in a content of 0.02 to 0.1% by weight.
[47] When the vanadium (V) is added in a content of 0.02% by weight or less, strength of steel is poorly enhanced, whereas corrosion resistance of the steel may be degraded when the content of the vanadium (V) exceeds 0.1% by weight.
[48] The steel that meets the requirements regarding the above-mentioned component systems may have the structure of polygonal ferrite. In this case, the corrosion resistance of the steel in the range of low temperature and low concentration of sulfuric acid is improved with favorable strength property. Furthermore, when a steel including a low-temperature structure is manufactured through the accelerated cooling process, the steel has further improved high-temperature tensile properties. For the present invention, the low-temperature structure includes at least one selected from the group consisting of accicular ferrite, bainitic ferrite and bainite. The low-temperature structure is most preferably present in a ratio of 20 to 100% in the steel.
[49] For the steel that meets the requirements regarding the component systems according to the present invention, physical properties, such as corrosion resistance and strength, of the steel are optimized when the components such as Mn, Nb, Ni and Cr are statistically controlled according to the following equation, as follows. However, the present invention is not particularly limited thereto.
[50]
(Mn+Cr+Nb ) Xi
[51] Hereinafter, a method for manufacturing a steel according to the present invention will be described in detail.
[52] According to the present invention, the steel including the above-mentioned composition may be re-heated according to one of the conventional methods known in the art, and hot-rolled to prepare a hot-rolled steel, or the hot-rolled steel may be then cold-rolled to prepare a cold-rolled steel.
[53] The steel according to the present invention may have the structure of polygonal ferrite. In this case, the corrosion resistance of the steel in the range of low temperature and low concentration of sulfuric acid is improved with favorable strength property. Furthermore, when a steel including a low-temperature structure is manufactured through the accelerated cooling process, the steel has further improved high- temperature tensile properties. For the present invention, the low-temperature structure includes at least one selected from the group consisting of accicular ferrite, bainitic ferrite and bainite.
[54] The steel having the structure of polygonal ferrite is not manufactured according to one of special methods but one of the conventional methods known in the art. That is to say, the accelerated cooling and low-temperature coiling processes do not apply to the manufacture of the steel according to the present invention. Therefore, the method for manufacturing a steel including a low-temperature structure will be described in more detail on the basis of the accelerated cooling process according to the present invention.
[55] First, a steel that meets the requirements regarding the component systems according to the present invention is hot-rolled, cooled acceleratedly, and then coiled at a temperature of 500 to 66O0C. These manufacturing conditions are set to obtain a low- temperature structure. The preferable accelerated cooling process is carried out at an average cooling rate of 30°C/s or more, and more preferably 30 to 50°C/s. For the present invention, the accelerated cooling condition is sufficient when a low- temperature structure may be obtained at the above-mentioned coiling temperature, and the present invention is not particularly limited to the above-mentioned accelerated cooling rate.
[56]
Mode for the Invention [57] Hereinafter, exemplary embodiments of the present invention will be described in more detail.
[58] [Embodiments] [59] An ingot was prepared by dissolving the components that meets the requirements of the composition as listed in the following Table 1, re -heated at 125O0C for 1 hour in a heating furnace, and then hot-rolled. The hot-rolling finish temperature was set to a temperature range of 870 to 89O0C, and the coiling temperature of the composition was set to two temperature ranges of 56O0C and 66O0C, and the final thickness of the hot- rolled steel plate was set to a thickness range of 6.0 mm in consideration of the major customer s desired thickness. The hot-rolled test samples were immersed at 7O0C in a 50% sulfuric acid solution for 3 hours, and then measured for corrosion resistance to sulfuric acid under low temperature-low concentration conditions. Also, the hot-rolled test samples were evaluated for mechanical properties by performing a tensile test at room temperature/high temperature (5000C). The results are listed in the following Table 2. When the tensile test was carried out at a high temperature, the tensile specimens were heated to a desired temperature and kept for 30 minutes to minimize the difference in temperature. In the steel types of Al -A 12 as listed in Table 1, aluminum (Al) is present in a content of 0.03%.
[60] [61] Table 1
Figure imgf000009_0001
Figure imgf000010_0001
[62] [63] Table 2
Figure imgf000010_0002
Figure imgf000011_0001
Figure imgf000012_0001
[64] [65] As listed in Table 2, it was revealed that the steel types such as A4, A7, A8, A9, AlO, Al 1 and A 12, which meet the requirements regarding the component systems according to the present invention, have a desired structure although they were coiled at a low temperature in the accelerated cooling process. In particular, the strengths of the steels at the room and high temperatures were highly enhanced when they were coiled at the low temperature in the accelerated cooling process. FIG. 1 shows the change in micro structures of an A4 steel when the A4 steel was coiled at a high temperature, and then coiled at a low temperature in an accelerated cooling process. As shown in FIG. IB, it was revealed that a low-temperature structure is distributed over the A4 steel. [66] The A2 steel was a Ti-added steel that has a satisfactory level of a low-temperature structure, but its corrosion resistance to sulfuric acid was poor. Therefore, it was seen that Ti was not the optimum alloy element in consideration of the effect of Ti on the corrosion resistance. Also, it was revealed that the A5 steel was a P-added steel that has a satisfactory effect on the increase in strength, but it was detrimental to the corrosion resistance.
[67]
[68]

Claims

Claims[1] A steel having excellent corrosion resistance to sulfuric acid, comprising, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities.[2] The steel of claim 1, wherein the components Mn, Cr, Ni and Nb (% by weight) satisfy the following equation. [3] A steel having excellent corrosion resistance to sulfuric acid, comprising, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities, wherein the steel includes at least one low temperature structure selected from the group consisting of accicular ferrite, bainitic ferrite and bainite.[4] The steel of claims 1 and 3, wherein the content of Mn is in a range from 1.51 to2.0% by weight.[5] The steel of claims 1 and 3, wherein the content of Mn is in a range from 0.5 to
1.50% by weight.
[6] The steel of claims 1 and 3, wherein vanadium (V) is present in a content of 0.02 to 0.1% by weight in the steel having corrosion resistance to sulfuric acid.
[7] The steel of claim 3, wherein the at least one low temperature structure selected from the group consisting of accicular ferrite, bainitic ferrite and bainite is present in a content of 20% by weight or more.
[8] A method for manufacturing a steel having excellent corrosion resistance to sulfuric acid, the method comprising: hot-rolling a steel comprising, by weight: C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.03% or less, P: 0.02% or less, Al: 0.01 to 0.1%, Cu: 0.2 to 1.0%, Co: 0.02 to 0.1%, Cr: 0.1 to 1.0%, Ni: 0.1% or less and Nb: 0.02 to 0.1%, and the balance of Fe and other inevitable impurities; cooling the steel acceleratedly; and coiling the steel at a temperature of 500 to 66O0C so that the steel has a low temperature structure.
[9] The method of claim 8, wherein the accelerated cooling operation is carried out at an average cooling rate of 30 to 50°C/s. [10] The method of claim 8, wherein the content of Mn is in a range from 1.51 to
2.0% by weight. [11] The method of claim 8, wherein the content of Mn is in a range from 0.5 to
1.50% by weight. [12] The method of claim 8, wherein vanadium (V) is present in a content of 0.02 to
0.1% in the steel having corrosion resistance to sulfuric acid.
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