EP3480332B1 - High strength steel plate having excellent low yield ratio characteristics and low temperature toughness and method for manufacturing same - Google Patents

High strength steel plate having excellent low yield ratio characteristics and low temperature toughness and method for manufacturing same Download PDF

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Publication number
EP3480332B1
EP3480332B1 EP17820573.8A EP17820573A EP3480332B1 EP 3480332 B1 EP3480332 B1 EP 3480332B1 EP 17820573 A EP17820573 A EP 17820573A EP 3480332 B1 EP3480332 B1 EP 3480332B1
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Prior art keywords
steel plate
high strength
strength steel
less
yield ratio
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German (de)
English (en)
French (fr)
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EP3480332A1 (en
EP3480332A4 (en
Inventor
Woo-Gyeom KIM
Ki-Hyun Bang
Kyung-Keun Um
Seng-Ho YU
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Posco Holdings Inc
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Posco Co Ltd
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    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C21D1/28—Normalising
    • 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
    • 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
    • C21D8/0247—Modifying the physical properties 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 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C21D2211/001—Austenite
    • 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
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to a high strength steel plate having low yield ratio characteristics and low temperature toughness, and a method for manufacturing the high strength steel plate.
  • steel materials having a low yield ratio have a great difference between yield strength and tensile strength, the steel materials have high formability, and the start of plastic deformation of the steel materials may be delayed until fracture may occur, such that collapses caused by external force may be prevented, owing to absorption of energy during the delay.
  • the structure may be repaired before collapse so as to prevent damage to property and human life caused by breakage of the structure.
  • a low yield ratio has been realized by forming soft ferrite as a first phase, and martensite, pearlite, or bainite as a second phase.
  • the second phase which is relatively hard decreases impact toughness, and since the content of carbon is increased to form the second phase, the toughness of a weld zone deteriorates. Therefore, brittle fracture may occur in a structure at a low temperature.
  • Patent Document 1 a technique for guaranteeing a low yield ratio and low temperature toughness has been disclosed in Patent Document 1.
  • Patent Document 1 a low yield ratio and high low-temperature toughness are guaranteed by a microstructure including MA (a mixed structure of martensite/austenite) in an amount of 2 vol% to 10 vol% and acicular ferrite in an amount of 90 vol% or greater.
  • MA a mixed structure of martensite/austenite
  • Patent Document 1 a yield ratio of about 0.8 may be guaranteed, but this is not sufficient to ensure seismic resistance.
  • Patent Document 1 Korean Patent Application Laid-open Publication No. 2013-0076577
  • JP H07-278656 A discloses a manufacturing method of a high tensile strength steel having low yield ratio and good in ductility and toughness by reheating a bloom having a specific composition and subjecting to hot rolling at a specific temperature, cooling, air cooling, reheating and recooling.
  • JP 2013-049895 A discloses a high-strength welded steel pipe for an API X65 to X70 grade high strength line pipe, achieving both excellent deformability and weld zone toughness; and a method for manufacturing thereof.
  • the welded steel pipe includes a base material with a specific component composition and a weld metal with a specific component composition.
  • the section of the base material has a microstructure in which a first phase is ferrite and a second phase is island martensite dispersed in the first phase at an area ratio of 5-20% and having an average aspect ratio of 2.0 or less, while 90% or more of the island martensite is present at a ferrite grain boundary.
  • EP 2 940 172 A1 discloses a high strength steel sheet having low yield ratio properties and excellent cryogenic temperature toughness.
  • the high strength steel sheet includes a specific component composition, a microstructure including 70% to 90% of ultrafine ferrite and 10% to 30% of MA (martensite/austenite) structure by area fraction, and the yield ratio (YS/TS) is 0.8 or less.
  • EP 3 012 340 A1 discloses an HT550 steel plate with ultrahigh toughness and excellent weldability and a manufacturing method thereof. Based on a component system with ultralow-C, high-Mn, Nb-microalloying, ultramicro Ti treatment, and by optimizing TMCP process, the steel plate has microstructures of fine ferrite plus self-tempered bainite with an average grain size of less than or equal to 15 ⁇ m, yield strength of 460 MPa or more, tensile strength of 550-700 MPa, yield ratio of 0.85 or less, and -60 °C Charpy impact energy (single value) of 60J or more.
  • An aspect of the present disclosure may provide a high strength steel plate having low yield ratio characteristics and low temperature toughness, and a method for manufacturing the high strength steel plate.
  • a high strength steel plate having low yield ratio characteristics and low temperature toughness as defined herein comprising, by wt%, C: 0.03% to 0.08%, Si: 0.05% to 0.3%, Mn: 1.0% to 2.0%, Al: 0.005% to 0.04%, Nb: 0.005% to 0.04%, Ti: 0.001% to 0.02%, Cu: 0.05% to 0.4%, Ni: 0.6% to 2.0%, Mo: 0.08% to 0.3%, N: 0.002% to 0.006%, P: 0.01% or less, S: 0.003% or less, and the balance being Fe and inevitable impurities,
  • a method for manufacturing a high strength steel plate having low yield ratio characteristics and low temperature toughness as defined herein comprising:
  • the high strength steel plate of the present disclosure may have low yield ratio characteristics and low temperature toughness. Specifically, the high strength steel plate may have a low yield ratio within the range of 0.65 or less, high formability, and high seismic resistance.
  • the high strength steel plate may be applied not only to shipbuilding and marine structures but also to industrial fields requiring formability and seismic resistance.
  • the inventors have recognized that although formability is somewhat ensured by a yield ratio of about 0.8 in the related art, the yield ratio level obtainable in the related art is not sufficiently low to guarantee seismic resistance. Thus, the inventors have conducted deep research to address this problem.
  • the inventors have found the followings: as the hardness difference between a base material and a second phase increases, a low yield ratio is realized; a uniform distribution of MA (a mixed structure of martensite/austenite) is favored; and in the case of Patent Document 1, the hardness difference between a base material, that is, acicular ferrite, and MA is insufficient, and the MA is formed along grain boundaries and is coarse, thereby failing to obtain a sufficiently low yield ratio.
  • MA a mixed structure of martensite/austenite
  • the inventors have found that a yield ratio of 0.65 or less can be obtained by forming ferrite as the microstructure of a base material and uniformly distributing a fine MA phase along ferrite grain boundaries and in ferrite grains, and this structure can be obtained by adjusting a microstructure to have bainite before a normalizing heat treatment. Then, based on this knowledge, the inventors have invented the present invention.
  • the high strength steel plate having low yield ratio characteristics and low temperature toughness includes, by wt%, C: 0.03% to 0.08%, Si: 0.05% to 0.3%, Mn: 1.0% to 2.0%, Al: 0.005% to 0.04%, Nb: 0.005% to 0.04%, Ti: 0.001% to 0.02%, Cu: 0.05% to 0.4%, Ni: 0.6% to 2.0%, Mo: 0.08% to 0.3%, N: 0.002% to 0.006%, P: 0.01% or less, S: 0.003% or less, and the balance of Fe and inevitable impurities, wherein the microstructure of the high strength steel plate includes, by area%, ferrite in an amount of 80% to 92% and MA (a mixed structure of martensite/austenite) in an amount of 8% to 20%, and the MA has an average size of 3 ⁇ m or less measured in equivalent circular diameter.
  • the alloy composition of the high strength steel plate having low yield ratio characteristics and low temperature toughness will be described in detail according to the aspect of the present disclosure.
  • the content of each element is given in wt%.
  • C is an element leading to solid solution strengthening and being present as a carbonitride of Nb or the like, and thus C guarantees tensile strength.
  • Si is added to assist Al in deoxidizing molten steel and guaranteeing yield strength and tensile strength.
  • Mn is an element that contributes greatly to the strength enhancement effect by solid solution strengthening and helps the formation of bainite.
  • Al is a major deoxidizer of steel and may be added in an amount of 0.005% or greater. However, if the content of Al is greater than 0.04%, this effect is saturated, and the fraction and size of Al 2 O 3 inclusions increase to cause a decrease in low temperature toughness.
  • Nb is an element staying in a solid solution state or precipitating carbonitrides during rolling or cooling, thereby suppressing recrystallization, promoting the formation of a fine microstructure, and increasing strength.
  • the content of Nb is less than 0.005%, the above-mentioned effects are insufficient.
  • the content of Nb exceeds 0.04%, the toughness of a base material and the toughness of the steel plate after welding may be lowered.
  • Ti forms precipitates by combining with oxygen or nitrogen, thereby suppressing coarsening of a microstructure, contributing to refinement, and improving toughness.
  • Cu is an element that does not markedly deteriorate impact characteristics and improves strength by solid solution strengthening and precipitation strengthening. For sufficient strength improvements, Cu is required to be added in an amount of 0.05% or greater. However, if the content of Cu exceeds 0.4%, surface cracks may be formed in the steel plate due to Cu thermal shock.
  • Ni improves strength and toughness at the same time and promotes the formation of bainite by lowering the Ar3 temperature.
  • Mo serving as an austenite stabilizing element, has an effect on increasing the amount of MA and plays a large role in improving strength.
  • Mo prevents a decrease in strength during a heat treatment process and promotes the formation of bainite.
  • N is an element useful in improving strength and toughness because N forms precipitates together with elements such as Ti, Nb, or Al and thus leads to the formation of fine austenite when a slab is heated.
  • Phosphorus (P) 0.01% or less
  • the upper limit of the content of P is adjusted.
  • the upper limit of the content of P is adjusted to be 0.01% or less.
  • the upper limit of the content of S is adjusted.
  • the upper limit of the content of S is adjusted to be 0.003% or less.
  • the other component of the steel plate of the present disclosure is iron (Fe).
  • Fe iron
  • impurities of raw materials or manufacturing environments may be inevitably included in the steel plate, and such impurities may not be removed from the steel plate.
  • Such impurities are well-known to those of ordinary skill in manufacturing industries, and thus specific descriptions of the impurities will not be given in the present disclosure.
  • the microstructure of the high strength steel plate having low yield ratio characteristics and low temperature toughness includes: by area%, ferrite in an amount of 80% to 92% and MA in an amount of 8% to 20%, and the MA has an average size of 3 ⁇ m or less measured in equivalent circular diameter.
  • the fraction of a microstructure refers to an area fraction (area%) unless otherwise specified.
  • the fraction of ferrite basically guarantees toughness and strength, and it is preferable that the fraction of ferrite be 80% or greater. In addition, it is preferable that the upper limit of the fraction of ferrite be 92% for guaranteeing the formation of sufficient MA. Furthermore, the ferrite of the steel plate may not include acicular ferrite. Since the difference in hardness between acicular ferrite and MA is small, acicular ferrite does not guarantee a sufficient low yield ratio.
  • the fraction of MA When the fraction of MA is less than 8%, it is difficult to obtain a low yield ratio with the range of 0.65 or less, and when the fraction of MA is greater than 20%, impact toughness may decrease, and elongation may decrease.
  • the average size of MA measured in equivalent circular equivalent is greater than 3 ⁇ m, MA is mainly formed along grain boundaries, thereby making it difficult to guarantee a uniform distribution of MA and a low yield ratio.
  • Inevitable phases other than the ferrite and MA may be included in the steel plate, and these phases may not be excluded.
  • pearlite may be included in the steel plate in an area% of 1 or less.
  • MA be present on five to thirteen sites on the straight line.
  • MA may be present at five to thirteen sites on each straight line on average.
  • MA formed along grain boundaries mainly leads to the start of fracture, and when the above-described condition is satisfied, a low yield ratio may be obtained because the distribution of MA is uniform along grain boundaries and in grains.
  • the ratio of MA existing in ferrite grains to MA existing along ferrite grain boundaries may be 1:3 to 1:10.
  • the ratio refers to the ratio of the numbers of MA sites, and when the ratio is within the above-mentioned range, MA may uniformly exist in ferrite grains in an amount of 0.5 area% to 5 area%.
  • the ferrite of the steel plate may have an average size measured in equivalent circular diameter within the range of 20 ⁇ m or less. If the average size of ferrite exceeds 20 ⁇ m, it may be difficult to ensure sufficient toughness and strength.
  • the steel plate of the present disclosure is a steel plate treated by a normalizing heat treatment, and the microstructure of the steel plate before the normalizing heat treatment may have bainite in an amount of 50 area% to 90 area%.
  • the microstructure of the steel plate before the heat treatment has bainite in which carbides are contained, MA may be uniformly distributed along grain boundaries and in grains after the heat treatment. Thus, it is preferable that the microstructure of the steel plate before the heat treatment have bainite in an amount of 50 area% to 90 area%.
  • the steel plate of the present disclosure has a yield ratio within the range of 0.5 to 0.65 and a low temperature impact property within the range of 100 J or greater at -40°C. Since the steel plate has a yield ratio within the range of 0.65 or less, that is, has a great difference between yield strength and tensile strength, the steel plate may have high formability, and the start of plastic deformation of the steel plate may be delayed until fracture may occur such that collapse caused by external force may be prevented owing to absorption of energy during the delay.
  • the steel plate may be applied not only to shipbuilding and marine structures but also to industrial fields requiring formability and seismic resistance.
  • the steel plate has a yield strength within the range of 350 MPa to 400 MPa and a tensile strength within the range of 600 MPa or greater.
  • the method for manufacturing a high strength steel plate having low yield ratio characteristics and low temperature toughness includes: heating a slab having the above-described alloy composition to 1050°C to 1200°C; hot rolling the heated slab such that a finish rolling termination temperature ranges from 760°C to 850°C, so as to obtain a hot-rolled steel plate; cooling the hot-rolled steel plate to 450°C or less at a cooling rate of 5°C/s or greater; and performing a normalizing heat treatment by heating the cooled hot-rolled steel plate to a temperature range of 850°C to 960 °C and then maintaining the steel plate for [1.3t + (10 to 30)] minutes.
  • t refers to the thickness of the steel plate measured in millimeters (mm) .
  • a slab having the above-described alloy composition is heated to a temperature of 1050°C to 1200°C.
  • austenite grains may be coarsened to cause a decrease in toughness, and if the heating temperature is lower than 1050 °C, elements such as Ti and Nb may not be sufficiently dissolved to cause a decrease in strength.
  • the heated slab is hot rolled such that a finish rolling termination temperature may range from 760°C to 850°C, so as to obtain a hot-rolled steel plate.
  • a normal rolling process is performed on heat-treated steel materials at a rolling temperature of 850°C to 1000°C.
  • bainite is formed as an initial microstructure.
  • a controlled rolling process is required to finish rolling at a low temperature.
  • Recrystallization rolling is required during the hot rolling process to obtain fine austenite grains, and as the reduction ratio per pass increases, physical properties may be improved.
  • the finish rolling termination temperature may be set to be 760°C to 850°C. If the finish rolling termination temperature exceeds 850°C, it is difficult to suppress ferrite-pearlite transformation. If the finish rolling termination temperature is less than 760°C, a non-uniform microstructure may be formed in a thickness direction, and an intended microstructure may not be formed because of a decrease in rolling force by rolling roll loads.
  • the finish rolling is terminated at a temperature of 760°C to 850°C to suppress ferrite-pearlite transformation and form bainite by cooling.
  • Bainite is formed as an initial microstructure to obtain uniform distribution of MA after a heat treatment.
  • MA is mainly formed along grain boundaries whereas in a bainite microstructure, MA is formed along grain boundaries and in grains as well.
  • the hot-rolled steel plate is cooled to a temperature of 450°C or less at a cooling rate of 5°C/s or greater.
  • Bainite is required to obtain fine and uniform MA, and the termination temperature and rate of the cooling are adjusted to form bainite.
  • the cooling termination temperature is higher than 450 °C, coarse grains may be formed, and coarse MA may be formed after a heat treatment because of coarse carbides, causing a decrease in toughness and making it difficult to form bainite in an amount of 50 area% or greater.
  • acicular ferrite or a ferrite+pearlite microstructure may be formed in large amounts to result in a decrease in strength; the formation of a coarse ferrite+pearlite microstructure instead of the formation of a two-phase microstructure of ferrite+MA or a sharp decrease in the amount of a second phase may be resulted after a heat treatment; and it is difficult to form bainite in an amount of 50 area% or greater.
  • t refers to the thickness of the steel plate measured in millimeters (mm).
  • the normalizing temperature is less than 850°C or the maintaining period of time is less than (1.3t + 10) minutes, cementite and MA may not easily dissolve in pearlite and bainite, and thus the amount of dissolved carbon (C) may decrease. Therefore, it is difficult to guarantee strength, and a finally remaining hardened phase is coarse.
  • the normalizing temperature exceeds 960°C or the maintaining period of time is greater than (1.3t + 30) minutes, all the carbides existing in bainite grains may move to grain boundaries or carbide coarsening may occur.
  • final MA may not have an intended size and may not be uniformly distributed.
  • grain growth may occur to result in a decrease in strength and deterioration of impact characteristics.
  • Slabs were prepared by performing a continuous casting process on molten steel having the compositions shown in Table 1 below.
  • Steel plates were manufactured by performing rolling, cooling, and normalizing heat treatment processes on the slabs under the manufacturing conditions shown in Table 2 below.
  • Table 3 shows the bainite fraction and mechanical properties of each of the steel plates measured before the normalizing heat treatment process.
  • the MA fraction, the average MA size, the number of MA sites on a 100- ⁇ m long line, and the mechanical properties of each of the steel plates were measured as shown in Table 4.
  • ferrite was formed in addition to MA, and the average grain size of the ferrite was 20 ⁇ m or less (not recorded).
  • the average MA size refers to an equivalent circular diameter, and the number of MA sites on a 100- ⁇ m long line was measured by vertically or horizontal drawing ten straight lines on a microstructure photograph having a size of 100 ⁇ m ⁇ 100 ⁇ m, counting the number of MA sites on each of the straight lines, and averaging the numbers.
  • inventive examples satisfying all of the alloy composition and the manufacturing conditions proposed in the present disclosure have a yield ratio of 0.65 or less and an impact toughness of 100 J or greater at -40°C.
  • Test Nos. 6, 7, 9, and 10 satisfied the alloy composition proposed in the present disclosure, but did not satisfy the manufacturing conditions proposed in the present disclosure. Thus, test Nos. 6, 7, 9, 10 failed to guarantee a sufficiently low yield ratio and had poor impact toughness at -40°C within the range of less than 100 J.
  • Test Nos. 11 to 14 satisfied the manufacturing conditions proposed in the present disclosure, but did not satisfy the alloy composition proposed in the present disclosure. Thus, test Nos. 11 to 14 failed to guarantee a sufficiently low yield ratio, and the impact toughness of test Nos. 11 and 14 was poor within the range of less than 100 J at -40°C.
  • the MA fractions in the inventive examples are greater than those in the comparative examples.
  • the reason for this is that: since the fraction of bainite was high before the normalizing heat treatment process as shown in Table 3 above, carbides existing in grains and along grain boundaries of the initial bainite were transformed into fine MA.
  • FIGS. 1 and 2 showing images of the microstructure of test No 1 (inventive example), the formation of fine and uniform MA can be seen.

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  • Engineering & Computer Science (AREA)
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  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
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  • Heat Treatment Of Steel (AREA)
EP17820573.8A 2016-07-01 2017-06-30 High strength steel plate having excellent low yield ratio characteristics and low temperature toughness and method for manufacturing same Active EP3480332B1 (en)

Applications Claiming Priority (2)

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KR1020160083588A KR101799202B1 (ko) 2016-07-01 2016-07-01 저항복비 특성 및 저온인성이 우수한 고강도 강판 및 그 제조방법
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KR102236852B1 (ko) 2018-11-30 2021-04-06 주식회사 포스코 우수한 저항복비 및 저온인성 특성을 가지는 구조용강 및 그 제조방법
CN110373524B (zh) * 2019-07-16 2021-05-04 河钢股份有限公司承德分公司 一种90公斤级焊丝用钢热处理软化工艺
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JP7338811B1 (ja) 2022-04-20 2023-09-05 Ｊｆｅスチール株式会社 鋼板およびその製造方法
WO2023203815A1 (ja) * 2022-04-20 2023-10-26 Ｊｆｅスチール株式会社 鋼板およびその製造方法

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JP6771047B2 (ja) 2020-10-21
EP3480332A1 (en) 2019-05-08
EP3480332A4 (en) 2019-06-26
WO2018004297A1 (ko) 2018-01-04
CN109328240A (zh) 2019-02-12
KR101799202B1 (ko) 2017-11-20
JP2019524987A (ja) 2019-09-05

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