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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
  • B21C47/02—Winding-up or coiling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
  • C21D9/46—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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES, PROFILES OR LIKE SEMI-MANUFACTURED PRODUCTS OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
  • B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
  • B21C47/02—Winding-up or coiling
  • B21C47/04—Winding-up or coiling on or in reels or drums, without using a moving guide
• • 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
• • 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
  • C21D2211/009—Pearlite

Definitions

• the present disclosure relates to a steel plate and a method for manufacturing the same, and more particularly, to a high-strength steel plate having excellent corrosion resistance and a method for manufacturing the same.
• a steel material used for a port structure or a pier of offshore bridge may need to use a material having excellent corrosion resistance in a seawater environment to ensure durability.
• a method of increasing Cr and Cu contents in steel may be used.
• a steel type having high Cr and Cu contents may form a Cr oxide film and a Cu concentrated layer on a surface of a steel material, and both methods may have the effect of suppressing corrosion of the steel material in a seawater environment.
• a high-strength steel material may need to be used to increase construction stability of a structure.
• it may be effective to increase a C or Mn content in steel. This may be because a fraction of a carbon-concentrated region having high hardness may be increased in steel having a composite structure of a structure formed in ferrite and a carbon-concentrated region.
• the carbon-concentrated region may be pearlite or bainite.
• the method of increasing the fraction of pearlite or bainite may have the problem of increasing corrosion occurrence sites and inhibiting corrosion resistance.
• An aspect of the present disclosure is to provide a steel plate and a method for manufacturing the same.
• An aspect of the present disclosure is to provide a high-strength steel plate having excellent corrosion resistance and a method for manufacturing the same.
• An aspect of the present disclosure provides a steel plate including, by weight%, C: 0.030-0.070%, Si: 0.50-1.30%, Mn: 0.30-0.70%, Cr: 0.50-1.50%, Al: 0.05% or less, Cu: 0.25-0.50%, Ni: 0.05-0.50%, S: 0.0100% or less, Ti: 0.020-0.050%, Nb: 0.050-0.090%, and a balance of Fe and inevitable impurities,
• the residual structure includes one or more of pearlite and bainite.
• the steel plate has tensile strength of 600 MPa or more and impact toughness of 100 J or more at -5°C.
• Cr in an interfacial surface of a corrosion product of the steel plate, generated after a corrosion test in accordance with KS D ISO 14993, is 0.50% or more by weight%.
• a relative corrosion rate of the steel plate as compared to KS-SS275 is 60% or less.
• An aspect of the present disclosure provides a method of manufacturing a steel plate including reheating a steel slab including, by weight%, C: 0.030-0.070%, Si: 0.50-1.30%, Mn: 0.30-0.70%, Cr: 0.50-1.50%, Al: 0.05% or less, Cu: 0.25-0.50%, Ni: 0.05-0.50%, S: 0.0100% or less, Ti: 0.020-0.050%, Nb: 0.050-0.090%, and a balance of Fe and inevitable impurities, and having R value of 0.75 or more, the R value defined in relational expression 1 as below;
• the reheating step is performed in a temperature range of 1100-1300°C.
• a steel plate and a method for manufacturing the same may be provided.
• a high-strength steel plate having excellent corrosion resistance and a method for manufacturing the same may be provided.
• a high-strength steel plate having excellent corrosion resistance in an environment in which the steel plate is in contact with seawater such as a port structure, an estuary embankment sluice gate, or the like, and a method for manufacturing the same may be provided.
• FIG. 1 is an image indicating results of measuring a Cr content at an interfacial surface of a corrosion product of inventive example 17.
• % indicating a content of each element may be based on weight.
• a steel plate according to an embodiment of the present disclosure may include, by weight %, C: 0.030-0.070%, Si: 0.50-1.30%, Mn: 0.30-0.70%, Cr: 0.50-1.50%, Al: 0.05% or less, Cu: 0.25-0.50%, Ni: 0.05-0.50%, S: 0.0100% or less, Ti: 0.020-0.050%, Nb: 0.050-0.090%, a balance of Fe and inevitable impurities.
• Carbon (C) may be the most economical and effective element for strengthening steel, and may be added at 0.030% or more in the present disclosure.
• the carbon (C) content is less than 0.030%, other alloying elements may need to be added to ensure sufficient strength, which may not be economical.
• 0.035% or more of carbon (C) may be included.
• the content exceeds 0.070%, pearlite or bainite fraction in the structure may excessive increase such that corrosion resistance may decreases, and impact toughness may also decrease.
• an upper limit of carbon (C) content may be 0.065%.
• Silicon (Si) may be generally added to deoxidize molten steel, and may also be effective in solid-solution strengthening. Also, silicon (Si) may form a protective film (H 4 SiO 4 ) on the surface and may reduce the fraction of the carbon-concentrated region ultimately differentiated into pearlite or bainite, thereby improving corrosion resistance. When the content of silicon (Si) is less than 0.50%, the above-described effect and corrosion resistance improvement effect may be insufficient. In an embodiment of the present disclosure, a lower limit of silicon (Si) may be 0.55%. When the content exceeds 1.30%, weldability may be reduced, which may be problematic. In an embodiment of the present disclosure, an upper limit may be 1.25%.
• Manganese (Mn) may be effective in solid-solution strengthening of steel, but manganese (Mn) may cause embrittlement of steel and may reduce corrosion resistance by forming MnS.
• Mn manganese
• a lower limit may be 0.35%.
• an upper limit may be 0.65%.
• Chromium (Cr) may be a basic alloying element improving seawater resistance.
• the seawater corrosion resistance was improved by action of chromium (Cr) being concentrated on an interfacial surface of the corrosion product and stabilizing the corrosion layer.
• a lower limit may be 0.55%.
• the content exceeds 1.50%, the seawater corrosion resistance effect may tend to converge.
• an upper limit may be 1.45%.
• Aluminum (Al) may be added to deoxidize molten steel and may improve corrosion resistance. However, when aluminum (Al) is added excessively, a great deal of oxide inclusions may be formed in steel, such that embrittlement of steel may deteriorate. Accordingly, in the present disclosure, the content of aluminum (Al) may be limited to 0.05% or less. In an embodiment of the present disclosure, an upper limit may be 0.045%. In an embodiment of the present disclosure, aluminum (Al) may be included in an amount of 0.001% or more.
• Copper (Cu) may improve seawater resistance.
• copper (Cu) may be added to improve corrosion resistance, but when copper (Cu) is added excessively, economic efficiency may degrade and surface defects known as Cu shortness may be caused, and thus, the content of copper (Cu) may be limited to 0.50% or less in the present disclosure. In an embodiment of the present disclosure, the content may be 0.45% or less. When the content is less than 0.25%, the corrosion resistance improvement effect may be insufficient. In an embodiment of the present disclosure, a lower limit may be 0.27%.
• Nickel (Ni) may be essentially added to prevent surface defects known as Cu hot-shortness in general Cu-added steel. Nickel (Ni) may also be effective for improving seawater resistance together with Cu. However, since nickel (Ni) is an expensive element, excessive addition may not be preferable in terms of economic efficiency. When the content of nickel (Ni) is less than 0.05%, the effect of suppressing Cu shortness and improving corrosion resistance may be insufficient, whereas when the content exceeds 0.50%, the above effects may tend to converge. In an embodiment of the present disclosure, an upper limit may be 0.45%. In an embodiment of the present disclosure, a lower limit of nickel (Ni) may be 0.07%.
• Sulfur (S) may be detrimental to impact toughness and corrosion resistance.
• Sulfur (S) may be combined with manganese (Mn) in steel and may form non-metallic inclusions, MnS, and since the inclusion may act as a corrosion initiation site, it may be preferable to reduce the content as much as possible. Accordingly, the content of sulfur (S) may be limited to 0.0100% or less in the present disclosure. Meanwhile, 0% may be excluded in consideration of the case in which sulfur (S) is unavoidably included during the manufacturing process.
• Titanium (Ti) may be precipitated as TiC, or the like, in steel and may strengthen the steel. Meanwhile, when TiC is precipitated, C forming Cr carbide may be consumed, which ultimately increases solid-solution Cr.
• a lower limit may be 0.025%.
• the content exceeds 0.050%, it may form coarse carbonitride, which may reduce impact strength.
• an upper limit may be 0.045%.
• Niobium (Nb) may have a precipitation strengthening effect similarly to Ti, and may also induce non-recrystallization rolling such that a grain size may become fine, and consequently, niobium (Nb) may increase strength and impact toughness.
• NbC is precipitated, C forming Cr carbide may be consumed, and consequently, solid-solution Cr may be increased.
• the niobium (Nb) content is less than 0.050%, the above effect may not be significant. In an embodiment of the present disclosure, a lower limit may be 0.045%. When the content exceeds 0.090%, the above effect may tend to converge.
• niobium (Nb) may be included by 0.085% or less.
• a remainder of the present disclosure is iron (Fe).
• Fe iron
• inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded.
• a person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.
• an R value defined in relational expression 1 below may be 0.75 or more.
• R Si + Ni / Cu (In the formula, [Si], [Ni] and [Cu] may be the weight% of each element)
• the inventors of the present invention studied to simultaneously ensure mechanical properties and corrosion resistance, and confirmed that corrosion resistance may be deteriorated depending on a relationship between Si, Ni, and Cu content.
• the R value defined in the relational expression 1 is less than 0.75, surface quality may be degraded, and in particular, surface deterioration due to Cu-hot shortness may cause a decrease in impact strength due to a phenomenon in which cracks of tens to hundreds of ⁇ m are left on the surface.
• An upper limit of the R value may not need to be specifically limited, but considering costs of alloy addition, the upper limit of the R value may be 3.00 in the present disclosure.
• % indicating the fraction of the microstructure may be based on an area.
• a microstructure at 1/4 point in the thickness direction from the surface of the steel plate may include 85% or more of ferrite and residual structure by area%, and the residual structure may include one or more of pearlite and bainite.
• the microstructure fraction may be observed using an optical microscope at 1/4 point in the thickness direction from the surface of the steel plate.
• Ferrite may ensure corrosion resistance and impact toughness, and in the present disclosure, ferrite may include 85% or more. When the ferrite fraction is less than 85%, the desired level of corrosion resistance or impact toughness may not be ensured.
• the average grain size of ferrite may be 30 ⁇ m or less.
• the average grain size may be represented as an average value of five points by selecting five points arbitrarily, and measuring the ferrite grain size using a circular cross-section line method described in KS D 0205. Also, the average grain size of ferrite in the present disclosure may be measured at 1/4 points in the thickness direction from the surface, similarly to the microstructure fraction
• a Q value defined in relational expression 2 as below may be 0.3-8.5.
• Q Ti + Nb / Cr (In the formula, [Ti], [Nb] and [Cr] may be weight% of each element present in the form of particles such as precipitates or inclusions in the steel)
• the Q value defined in relational expression 2 is less than 0.3, the amount of Cr precipitation may increase, and the solid-solution Cr content may decrease such that corrosion resistance may not be ensured.
• the value exceeds 8.5 the Ti and Nb precipitates may become coarse and may act as corrosion sites, which may hinder corrosion resistance or may act as crack initiation points, such that impact strength may be deteriorated.
• the solid-solution Cr content may indicate the value obtained by subtracting the solid-solution Cr content from the total Cr content of the steel by measuring the content of Cr present in the form of particles such as precipitates or inclusions in steel. More specifically, in the present disclosure, a method of extracting precipitates and inclusions in the form of particles in steel using electrolytic extraction and measuring the Cr content of the particles using induced defect plasma optical emission spectroscopy was used. The solid-solution Cr content was calculated as the value obtained by subtracting the Cr content present in the form of particles from the steel Cr content.
• the seawater corrosion resistance was determined depending on the Cr content of the interfacial surface of the corrosion product, and that the seawater corrosion resistance was excellent when the Cr content of the interfacial surface of the corrosion product was 0.50% or more. Also, it was confirmed that in order for Cr to be concentrated at a specific level or more in the interfacial surface of the corrosion product, Cr may need to be sufficiently solid-solute in the steel material before the corrosion reaction.
• the solid-solution Cr content is 90% or more of the total Cr content, the Cr content of the interfacial surface of the corrosion product was 0.50% or more, and the seawater corrosion resistance was excellent. Accordingly, in the present disclosure, the solid-solution Cr content may be limited to 90% or more of the total Cr content.
• the interfacial surface of the corrosion product may refer to a region present in the form of an interfacial surface within the corrosion layer formed on the surface of the steel plate after a corrosion test, and the Cr content may be represented as an average value of the measured values through EDS analysis in three positions.
• tensile strength of the steel plate may be 600 MPa or more
• impact toughness may be 100 J or more at -5°C
• the interfacial surface of the corrosion product Cr content generated on the surface may be 0.50% or more
• the relative corrosion rate as compared to KS-SS275 may be 60% or less, such that excellent corrosion resistance characteristics may be ensured.
• the interfacial surface of the corrosion product may indicate a region present in the form of an interfacial surface within the corrosion layer formed on the surface of the steel plate after a corrosion test, and the Cr content may be represented as an average value of the measured values through EDS analysis in three positions.
• the steel plate according to an embodiment of the present disclosure may be manufactured by reheating, hot-rolling, primary cooling, secondary winding and cooling a steel slab satisfying the above-described alloy composition.
• the steel slab satisfying the alloy composition in the present disclosure may be reheated.
• the reheating temperature may not be limited to any particular example, but a reheating temperature commonly used in the same technical field may be applied. In an embodiment of the present disclosure, reheating may be performed in a temperature range of 1100-1300°C.
• the reheated steel slab may be hot-rolled at a finishing rolling temperature of 750-900°C.
• the finishing rolling temperature may be a factor affecting the grain size. As the finishing rolling temperature decreases, it may be advantageous for grain refinement, and it may be difficult to ensure the target fine grain at a temperature exceeding 900°C. A temperature less than 750°C may be difficult to be applied to the manufacturing process due to excessive rolling load.
• the hot-rolled steel plate may be primary cooled to a temperature range of 500-650°C at an average cooling rate of 5-20°C/s and may be wound.
• the grain size may decrease.
• the average cooling rate may be controlled to be 5°C/s or more and the winding temperature to be 650°C or less to ensure the target grain size.
• the average cooling rate exceeds 20°C/s or the winding temperature is less than 500°C, it may be difficult to implement the embodiment at a thickness of 10 mm or more, and the material may be locally overcooled, which may not be applicable.
• the wound steel plate may be secondary cooled to 400°C at an average cooling rate of 0.40°C/min or more.
• Cooling rate (°C/s) Winding temperature (°C) Cooling rate (°C/min) 1 883 12 561 0.58 2 795 11 642 0.69 3 779 9 639 0.47 4 824 19 548 0.62 5 783 9 570 0.60 6 887 11 637 0.45 7 841 11 501 0.54 8 890 5 554 0.43 9 801 12 610 0.51 10 767 19 528 0.49 11 879 12 549 0.40 12 892 14 576 0.70 13 896 13 513 0.50 14 897 5 558 0.58 15 799 19 546 0.69 16 833 6 549 0.70 17 900 15 638 0.66 18 860 5 623 0.57 19 834 14 565 0.47 20 869 16 630 0.70 21 892 19 544 0.47 22 795 11 636 0.62 23 837 9 565 0.68 24 802 17 638 0.61 25 789 18 610 0.56 26 844 17 569 0.67 27 841
• the ferrite, pearlite and bainite fractions, and the grain size of ferrite were observed and listed. Also, the content of precipitates for Ti, Nb and Cr was measured, and relational expression 2 and the solid-solution Cr content fraction were calculated and listed in Table 3 below. The tensile strength, impact toughness at -5°C, Cr content in the interfacial surface of the corrosion product after corrosion, corrosion rate, and relative corrosion rate as compared to KS-SS275 were measured and listed.
• microstructure fraction was measured using an optical microscope after nital-etching the sample, and the ferrite grain size was measured at five points using the circular cross-section method described in KS D 0205 and the average value was listed. In this case, the microstructure characteristics were observed at 1/4 points in the thickness direction from the surface of the steel plate
• the corrosion test was carried out for 120 cycles (960 hours) as specified in the KS D ISO 14993 standard.
• the interfacial surface of the corrosion product may indicate the region present in the form of the interfacial surface within the corrosion layer formed on the surface of the steel plate after the corrosion test, and the Cr content of the interfacial surface was measured through EDS analysis at the corresponding position.
• the corrosion products was removed by the KS D ISO 8407 method, and the corrosion rate was measured using the equation as below. Specifically, the conditions of 2 hours of 5% NaCl salt spray at 35°C, 4 hours of drying at 60°C, and 2 hours of wetting at 50°C were repeated for 960 hours.
• W is the weight loss after corrosion (mg)
• D is the specific gravity of the metal (g/cm3)
• A is the area exposed to the corrosive environment (cm2)
• T is the corrosion test time (hour).
• 960 was used as a fixed value.
• the manganese content exceeds the range suggested in the present disclosure. Accordingly, pearlite was excessively formed, such that ferrite was insufficiently formed, and accordingly, corrosion occurred excessively and rapidly.

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