EP3492610B1 - High-strength steel sheet - Google Patents

High-strength steel sheet Download PDF

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Publication number
EP3492610B1
EP3492610B1 EP16910563.2A EP16910563A EP3492610B1 EP 3492610 B1 EP3492610 B1 EP 3492610B1 EP 16910563 A EP16910563 A EP 16910563A EP 3492610 B1 EP3492610 B1 EP 3492610B1
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steel plate
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high strength
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German (de)
English (en)
French (fr)
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EP3492610A4 (en
EP3492610A1 (en
Inventor
Masahide Yoshimura
Masanori Minagawa
Norimasa Kawabata
Takeshi Tsuzuki
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Nippon Steel Corp
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Nippon Steel Corp
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    • 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
    • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
    • 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
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • 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
    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • 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
    • 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
    • C21D2211/003—Cementite
    • 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 invention relates to a high strength steel plate.
  • the total elongation is reduced in a case where the plate thickness of a steel plate is limited while increasing the strength of the steel plate to suppress the increase in the weight of members.
  • the plate thickness is limited to 25 mm or less
  • the plate thickness is limited to 8 mm or less
  • the steel plate is required to have not only a strength, but also ductility such as a total elongation.
  • low temperature toughness is also required to prevent brittle fracture.
  • a high strength steel plate having a tensile strength of 780 MPa or greater, or further 950 MPa, and a method of manufacturing the high strength steel plate are proposed.
  • Patent Document 1 proposes a high strength steel plate having excellent toughness which is obtained by hot-rolling and rapidly cooling a steel containing an alloy added thererto and reducing the C content and to obtain appropriate hardenability, and a method of manufacturing the steel plate.
  • Patent Document 1 does not consider the workability of the steel plate.
  • Patent Documents 2 to 4 propose a high strength hot rolled steel sheet which is manufactured by coiling a steel in a coil after hot rolling as a steel sheet which is used for construction machines or the like, and a method of manufacturing the hot rolled steel sheet.
  • Patent Documents 2 to 4 disclose a method of manufacturing a hot rolled steel sheet having a martensitic phase or a tempered martensitic phase as a primary phase by performing hot rolling, rapid cooling to near a martensitic transformation start temperature (Ms), holding for a predetermined period of time, and coniling in a coil.
  • Ms martensitic transformation start temperature
  • tempering heat treatment
  • tempering is preferably performed after the accelerated cooling to transform the microstructure to tempered martensite.
  • the tempering is omitted from the viewpoint of shortening the construction period or suppressing an increase in the manufacturing cost, the microstructure is transformed to martensite, and thus ductility or toughness is reduced although a high strength is obtained.
  • Patent Document 5 proposes a high strength steel plate in which a Mn content and a Ni content are suppressed and a Mo content and a V content are increased to suppress the formation of martensite and to provide a microstructure consisting mainly of lower bainite, and a method of manufacturing the high strength steel plate.
  • Patent document 5 since the technology described in Patent document 5 is based on the premise that the microstructure is obtained by setting a cooling stop temperature to 300°C to 450°C, a sufficient total elongation is not obtained.
  • the inventors produced a steel plate in accordance with the disclosure of Patent Document 5 and performed a test, but a total elongation of 12% or greater was not obtained.
  • Patent Document 6 discloses certain wear-resistant steel plates.
  • a tensile strength (joint strength) thereof is required to be not less than a value required for a base metal in view of reliability of the structure.
  • a welding joint may have a lower tensile strength (joint strength) than a base metal due to softening of a heat-affected zone, and a required value may not be satisfied.
  • the present invention is contrived in view of the circumstances, and an object thereof is to provide a high strength steel plate which is preferably used for construction machines or industrial machines and a method of manufacturing the high strength steel plate.
  • an object of the invention is to provide a high strength steel plate which has a plate thickness of 4.5 to 20 mm, a yield strength of 885 MPa or greater, a tensile strength of 950 MPa or greater, a Charpy absorbed energy of 59 J/cm 2 or greater at -20°C, and a total elongation of 12% or greater, and in which the microstructure consists mainly of martensite, and a tensile strength of a welding joint after welding can be sufficiently secured, and a method of manufacturing the high strength steel plate.
  • the inventors examined the relationship between the ductility of a steel plate and the accelerated cooling stop temperature. As a result, the inventors found that the ductility is reduced in a case where the accelerated cooling stop temperature is 300°C or higher or higher than a martensitic transformation completion temperature (Mf). The inventors further proceeded the examination, and found that in a case where the accelerated cooling is stopped at a temperature of 300°C or higher or a temperature higher than Mf, untransformed austenite transforms to bainite in a microstructure, voids starting from a coarse carbide (cementite) formed caused by the bainite are excessively generated, and thus the ductility is reduced.
  • Mf martensitic transformation completion temperature
  • the inventors studied ways to suppress such a reduction in the ductility.
  • the inventors designed a component capable of increasing hardenability to suppress the above-described bainitic transformation, and found new knowledge that in a case where accelerated cooling to a temperature which is lower than 300°C and not higher than Mf is performed after hot rolling, the microstructure can be allowed to consist mainly of martensite, and thus the ductility of a high strength steel plate can be secured.
  • the invention is contrived based on such knowledge, and the gist thereof is as follows.
  • a high strength steel plate which has a yield strength of 885 MPa or greater, a tensile strength of 950 MPa or greater, and a total elongation of 12% or greater without containing a large amount of expensive alloying elements.
  • This steel plate exhibits excellent toughness such that a Charpy absorbed energy at -20°C is 59 J/cm 2 or greater.
  • a tensile strength of 950 MPa or greater can be secured at a welding joint where a high strength steel plate according to the invention is a base metal with a predetermined heat input or less in welding.
  • the invention can provide a high strength steel plate which is preferably used for a structural member in construction machines or industrial machines to contribute to an increase in the size or a reduction in the weight of the construction machines or industrial machines without a significant increase in manufacturing cost, and thus very significantly contributes to the industry.
  • a high strength steel plate according to an embodiment of the invention (hereinafter, may be referred to as a high strength steel plate according to this embodiment) will be described in detail.
  • the C content is a useful element for increasing a strength of steel, and is a very important element for determining a total elongation of steel having a martensite structure.
  • the C content is required to be 0.050% or greater to obtain a sufficient strength.
  • the C content is preferably 0.060% or greater, 0.065% or greater, or 0.070% or greater.
  • the C content is required to be 0.100% or less to obtain a good total elongation and good toughness.
  • the C content is preferably adjusted to be 0.095% or less, 0.090% or less, or 0.085% or less.
  • the Si content is limited to 0.50% or less. It is not necessary to particularly determine the lower limit of the Si content, and the lower limit of the Si content is 0%. However, in a case where Si is used for deoxidation, the Si content is preferably adjusted to be 0.03% or greater to obtain a sufficient effect. In addition, Si is also an element which suppresses the generation of a carbide, and in order to obtain this effect, the Si content is preferably adjusted to be 0.10% or greater, and more preferably 0.20% or greater. In a case where it is not necessary to obtain the effects, the upper limit of the Si content may be adjusted to be 0.45%, 0.40%, or 0.35%.
  • Mn is an important element for improving the hardenability of steel.
  • the Mn content is adjusted to be 1.20% or greater to obtain a high strength by increasing a martensite area fraction in a microstructure.
  • the Mn content is preferably adjusted to be greater than 1.20%, 1.25% or greater, or 1.30% or greater, and more preferably 1.35% or greater or 1.39% or greater. In a case where the Mn content is too high, ductility and toughness may be reduced. Accordingly, the Mn content is adjusted to be 1.70% or less. More preferably, the Mn content is adjusted to be 1.60% or less, 1.55% or less, or 1.50% or less.
  • P and S are elements inevitably contained as impurities in steel, and deteriorate the toughness of steel.
  • P and S are elements which deteriorate the toughness of a heat-affected zone in a case where welding is performed. Therefore, the P content is limited to 0.020% or less, and the S content is limited to 0.0050% or less.
  • the P content may be adjusted to be 0.015% or less, and the S content may be adjusted to be 0.0030% or less. Since the P content and the S content are preferably low, and thus preferably reduced as much as possible. Accordingly, it is not necessary to particularly determine the lower limits of the P content and the S content, and the lower limits of the P content and the S content are 0%.
  • the P content may be adjusted to be 0.001 % or greater, and the S content may be adjusted to be 0.0001 % or greater.
  • the B is an element which is segregated in the grain boundary to increase the hardenability of steel, and is a useful element for exhibiting the effect even in a case where the amount thereof is very small.
  • the B content is adjusted to be 0.0003% or greater to increase martensite in a microstructure.
  • the B content is adjusted to be 0.0005% or greater.
  • the B content is adjusted to be 0.0030% or less.
  • the B content is preferably adjusted to be 0.0020% or less or 0.0015% or less.
  • Ti is an element which forms a nitride, and is an element which fixes N in steel as TiN and suppresses the generation of BN.
  • B is an element which increases hardenability, the effect of B is not obtained in a case where BN is formed.
  • the Ti content is required to be 0.003% or greater to secure hardenability by suppressing the formation of BN.
  • the Ti content is preferably adjusted to be 0.005% or greater, and more preferably 0.010% or greater. In a case where the Ti content is too high, TiN becomes coarse, and thus ductility or toughness may be reduced. Accordingly, the Ti content is adjusted to be 0.030% or less.
  • the Ti content is preferably adjusted to be 0.020% or less.
  • Nb is an element which significantly improves the hardenability of steel by being contained together with B.
  • the Nb content is adjusted to be 0.003% or greater to increase a martensite area fraction in a microstructure.
  • Nb is also an element which contributes to grain refining and increases toughness by forming a fine nitride.
  • the Nb content is preferably adjusted to be 0.005% or greater. More preferably, the Nb content is adjusted to be 0.010% or greater or 0.015% or greater. In a case where the Nb content is too high, the nitride becomes coarse, and thus ductility or toughness may be reduced. Accordingly, the Nb content is adjusted to be 0.050% or less.
  • the Nb content is preferably adjusted to be 0.040% or less, 0.035% or less, or 0.030% or less.
  • Total Content of One or Both of Cr and Mo is 0.20% or Greater, and In Case Where Mo Content is Greater Than 0.50%, Cr Content is 0.80% or Less
  • the total content of Cr and Mn is adjusted to be 0.20% or greater to increase a martensite area fraction in a microstructure.
  • the total content of Cr and Mn is preferably adjusted to be 0.30% or greater, and more preferably 0.40% or greater.
  • the lower limits of the Cr content and the Mo content are 0%. If necessary, the lower limit of the Cr content may be adjusted to be 0.20% or 0.30%, and similarly, the lower limit of the Mo content may be adjusted to be 0.20% or 0.30%.
  • the Cr content and the Mo content are adjusted to be 2.00% or less and 0.90% or less, respectively.
  • the Cr content is preferably adjusted to be 1.50% or less or 1.00% or less, and more preferably 0.90% or less or 0.80%.
  • the Mo content is preferably adjusted to be 0.70% or less, and more preferably 0.60% or less or 0.50%.
  • the Cr content is required to be 0.80% or less.
  • the Cr content may be adjusted to be 0.70% or less.
  • the Mo content may be adjusted to be 0.50% or less, and in a case where the Cr content is greater than 1.20%, the Mo content may be adjusted to be 0.40% or less.
  • the total content of Cr and Mo may be adjusted to be 2.50% or less, 2.00% or less, 1.50% or less, 1.30% or less, or 1.10% or less.
  • N is inevitably contained as impurities. N forms BN and inhibits the hardenability improving effect of B. Accordingly, the N content is limited to 0.0080% or less. The N content is preferably limited to 0.0060% or less, and more preferably 0.0050% or less. The N content is preferably reduced as much as possible, and the lower limit thereof is 0%. However, from the viewpoint of cost of denitrification, the N content may be adjusted to be 0.0001% or greater. The N content may be adjusted to be 0.0020% or greater for refining of a microstructure by a nitride.
  • the above elements are contained as essential elements and impurities of the high strength steel plate according to this embodiment, and basically, the high strength steel plate according to this embodiment has components including the above-described essential elements and a remainder consisting of Fe and impurities (the above-described impurity elements and optional impurity elements other than the above-described impurity elements).
  • the high strength steel plate according to this embodiment may further contain, other than the above-described components, one or more of 0.100% or less of Al, 0.50% or less of Cu, 0.50% or less of Ni, 0.100% or less of V, 0.50% or less of W, 0.0030% or less of Ca, 0.0030% or less of Mg, and 0.0030% or less of REM instead of a part of Fe. Since these elements are not essential elements, the contents of the elements may be 0%.
  • Al is a deoxidizing element.
  • the Al content is preferably adjusted to be 0.010% or greater to obtain a sufficient effect.
  • the Al content is limited to 0.100% or less even in a case where Al is contained.
  • the Al content is preferably limited to 0.080% or less, more preferably 0.050% or less, and even more preferably 0.030% or less.
  • Cu and Ni are elements which improve the hardenability of steel.
  • the Cu content and the Ni content are preferably adjusted to be 0.10% or greater, respectively. Since Cu and Ni are expensive elements, the Cu content and the Ni content are adjusted to be 0.50% or less, respectively, even in a case where Cu and Ni are contained.
  • the Cu content and the Ni content are more preferably adjusted to be 0.40% or less, and even more preferably 0.30% or less, respectively.
  • V is an element which forms a carbide or a nitride.
  • the V content is preferably adjusted to be 0.005% or greater in a case where toughness is increased by grain refining by a carbide or a nitride. In a case where the V content is too high, ductility or toughness is reduced. However, since V has less adverse effects than Nb or Ti, the upper limit of the V content is limited to 0.100% in a case where V is contained.
  • the V content is preferably adjusted to be 0.050% or less.
  • W is an element which improves the hardenability of steel.
  • the W content is preferably adjusted to be 0.05% or greater.
  • the W content is adjusted to be 0.50% or less or 0.30% or less even in a case where W is contained. If necessary, the W content may be adjusted to be 0.02% or less or 0.01% or less.
  • Ca is an element which controls the form of an oxide or a sulfide.
  • the Ca content is preferably adjusted to be 0.0001 % or greater.
  • the Ca content is more preferably adjusted to be 0.0005% or greater, and even more preferably 0.0010% or greater.
  • the Ca content is adjusted to be 0.0030% or less even in a case where Ca is contained.
  • Mg is an element which acts to increase the toughness of steel by refining the microstructure.
  • the Mg content is preferably adjusted to be 0.0005% or greater. In a case where the Mg content is too high, the effect is saturated, and ductility or toughness may be reduced due to the formation of inclusions. Therefore, the Mg content is adjusted to be 0.0030% or less even in a case where Mg is contained.
  • REM rare earth metal
  • MnS a sulfide
  • the REM content is preferably adjusted to be 0.0001 % or greater. In a case where the REM content is too high, inclusions including REM may become coarse, and thus ductility or toughness may be reduced. Therefore, the REM content is adjusted to be 0.0030% even in a case where REM is contained.
  • DI and Pcm which are determined by a chemical composition, are required to satisfy the following ranges, respectively.
  • DI is a hardenability index, and is obtained by (Formula 1).
  • each of [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] in the formula represents a content (mass%) of each element, and 0 is substituted in a case where the element is not contained.
  • DI 7.8 or less.
  • DI is more preferably 7.0 or less, and even more preferably 6.5 or less.
  • DI C 0.5 ⁇ 0.34 ⁇ 1 + 0.64 ⁇ Si ⁇ 1 + 4.1 ⁇ Mn ⁇ 1 + 0.27 ⁇ Cu ⁇ 1 + 0.52 ⁇ Ni ⁇ 1 + 2.33 ⁇ Cr ⁇ 1 + 3.14 ⁇ Mo ⁇ 1.2
  • a tensile strength (joint strength) of a welding joint is required to be not less than a required tensile strength for a base metal provided in welding.
  • a tensile strength (joint strength) of a welding joint may be less than a tensile strength of a base metal due to softening of a heat-affected zone. Accordingly, the inventors manufactured welding joints using various high strength steel plates by changing weld heat input, and performed a test. As a result, the inventors found that by increasing the hardenability of a steel plate, specifically, by adjusting Pcm.
  • Each of [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] represents a content (mass%) of each element, and 0 is substituted in a case where the element is not contained.
  • the inventors further conducted studies on the weld heat input and the strength of the welding joint, and found that the strength of the welding joint can be evaluated by JS which is calculated by (Formula a) using Pcm obtained by (Formula 2) and weld heat input Hi [kJ/cm] from the component composition of the high strength steel plate used in welding, and a joint strength of 950 MPa or greater can be secured at an actual welding joint in a case where JS is 950 MPa or greater.
  • JS 4.3 / Hi + 3.4 ⁇ 1680.7 ⁇ Pcm ⁇ 81.5
  • the weld heat input is preferably reduced as much as possible in order to secure the strength of the welding joint.
  • Pcm necessary for adjusting JS to be 950 MPa or greater is 0.189% from the above formula. That is, by adjusting Pcm to be 0.189% or greater, a joint strength of 950 MPa or greater can be secured.
  • a joint strength of 950 MPa or greater can be secured even in a case where the welding is performed with 10.0 kJ/cm weld heat input with which no special management is required. That is, by adjusting Pcm to be 0.196% or greater, a welding joint strength of 950 MPa or greater can be secured without special welding management.
  • Pcm may be adjusted to be 0.200% or greater, 0.205% or greater, 0.210% or greater, or 0.215% or greater to secure the strength of the welding joint even with higher weld heat input. It is preferable that the weld heat input be high since the number of welding paths can be reduced, and thus the productivity can be improved. It is not necessary to particularly determine the upper limit of Pcm, and the upper limit may be 0.250% or less or 0.240% or less to prevent weld cracking or the like.
  • the inventors further proceeded the examination and studies of the influence of Cr and Mo as hardenability increasing elements on toughness.
  • a ratio of Mo to Cr has an influence on toughness.
  • a ratio ([Mo]/[Cr]) of the Mo content [Mo] to the Cr content [Cr] by mass% is high, a substructure (packet or block) of the martensite is made fine, and as a result, toughness is improved.
  • the ratio may be 0.40 or higher, 0.80 or higher, or 1.00 or higher.
  • FIG. 2 is a diagram showing the relationship between [Mo]/[Cr] and a Charpy absorbed energy at -40°C.
  • the symbol “ ⁇ " represents an actual measurement value
  • the symbol “ ⁇ ” represents an average of the actual measurement values.
  • the Charpy absorbed energy is measured by a Charpy test performed based on JIS Z 2242.
  • the inventors conducted studies on the relationship between the hardenability and the microstructure of a high strength steel plate, and a total elongation. As a result, the inventors found that in a case where the hardenability is poor, the total elongation is reduced, and a reduction in the total elongation, that is, a reduction in the ductility occurs due to the generation of voids starting from a coarse carbide generated caused by the bainite as shown in FIGS. 4A and 4B . In addition, the inventors obtained knowledge that it is necessary to suppress the formation of bainite causing the formation of coarse cementite in order to increase the ductility of a high strength steel plate.
  • a structure mainly including martensite in which 90% or more of a microstructure is martensite is preferably provided.
  • the martensite area fraction in the microstructure is preferably adjusted to be 90% or greater, more preferably 92% or greater, and even more preferably 94% or greater.
  • both the martensite and the bainite are continuous cooling transformation structures, and it may be difficult to accurately distinguish the martensite and the bainite by observing the microstructures. In such a case, it can be judged that the formation of bainite causing the formation of coarse cementite is suppressed in a case where the total area fraction of martensite and bainite is 99% or greater and the total elongation is 12% or greater.
  • the total area fraction of one or both of martensite and bainite is adjusted to be 99% or greater, and the total elongation as a structure index is adjusted to be 12% or greater.
  • the martensite area fraction is preferably 90% or greater.
  • the martensite of the microstructure is as quenched, and is different from tempered martensite obtained after tempering treatment. Tempered martensite is not preferable since cementite grows by tempering for a long period of time.
  • the remainder other than the above microstructures may include one or more of ferrite, pearlite, and residual austenite.
  • the distinguishment of the microstructure and the measurement of the martensite area fraction are performed using an optical microscope. Specifically, near a 1/4 t-portion in a cross-section parallel to a rolling direction (a portion at a depth of 1/4 of a plate thickness t from a steel plate surface in a plate thickness direction) is subjected to nital etching, and two regions within a range of 120 ⁇ m ⁇ 100 ⁇ m are photographed at 500-fold magnification using an optical microscope to measure an area fraction of a microstructure in which an acicular lath structure is developed.
  • the cross-section of the steel plate is subjected to electrolytic polishing, and then a portion near the 1/4 t-portion in the cross-section of the steel plate is observed by a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • the magnification is 5,000 times, and the photographing is performed within a range of 50 ⁇ m ⁇ 40 ⁇ m.
  • the acicular structure is defined as martensite, and an area fraction of the region is obtained.
  • the product of the acicular structure area fraction measured using the optical microscope and the martensite area fraction measured using SEM is an area fraction of the martensite structure of this steel.
  • the orientation of the long axis direction of cementite in two or more directions in the block may not be clearly distinguished.
  • the area fraction of a microstructure in which an acicular lath structure is developed is defined as the total area fraction of martensite and bainite.
  • the inventors found that by controlling an accelerated cooling stop temperature, the number fraction of the coarse carbide (particularly, cementite) having a length of 1.0 ⁇ m or greater in a long axis direction can be reduced, and as a result, the generation of voids can be suppressed and the total elongation can be improved. Specifically, the inventors found that the total elongation can be improved by adjusting the number fraction of cementite having a length of 1.0 ⁇ m or greater in a long axis direction in cementite having a length of 0.1 ⁇ m or greater in a long axis direction to be 5% or less.
  • the invention by stopping accelerated cooling at a temperature which is not higher than Mf and lower than 300°C, it is possible to provide a structure mainly including martensite in which the generation of a coarse carbide is suppressed. That is, the generation of voids starting from coarse cementite having a length of 1.0 ⁇ m in a long axis direction can be suppressed by controlling an accelerated cooling stop temperature.
  • the number density of cementite is measured using a scanning electron microscope (SEM). Specifically, a cross-section of the steel plate is subjected to electrolytic polishing, and then a portion near a 1/4 t-portion in the cross-section of the steel plate is photographed within a range of 50 ⁇ m ⁇ 40 ⁇ m at 5,000-fold magnification by a scanning electron microscope (SEM). Based on the contrast in the obtained image, the number of precipitates as cementite having an aspect ratio of 2.0 or greater and a length of 0.1 ⁇ m or greater in a long axis direction is counted using image analysis software.
  • SEM scanning electron microscope
  • the number of cementite having an aspect ratio of 2.0 or greater and a length of 1.0 ⁇ m or greater in a long axis direction is counted.
  • the obtained number of precipitates of 1.0 ⁇ m or greater is divided by the number of cementite of 0.1 ⁇ m or greater, and thus the number fraction (%) of cementite of 1.0 ⁇ m or greater is obtained.
  • the shape of the carbide is not particularly limited. However, in a case where the carbide has an ellipsoidal shape, the "length in a long axis direction" refers to a major axis.
  • the aspect ratio of prior austenite grains is adjusted to be 2.0 or greater. In a case where the aspect ratio is less than 2.0, toughness may be reduced.
  • the aspect ratio of prior austenite grains can be adjusted to be 2.0 or greater.
  • quenching is performed after rolling, cooling, and subsquent reheating, the structure worked by rolling is not taken over, and the aspect ratio of prior austenite grains becomes less than 2.0.
  • the aspect ratio of prior austenite grains is measured by the following method. That is, near a 1/4 t-portion in a cross-section parallel to a rolling direction, which is positioned at a depth of 1/4 of a plate thickness t from a surface in a plate thickness direction, is etched with nital, and two regions within a range of 120 ⁇ m ⁇ 100 ⁇ m are photographed at 500-fold magnification using an optical microscope. From the obtained image, long axis lengths and short axis lengths of at least 50 prior austenite grains are measured, and the long axis length is divided by the short axis length to obtain an aspect ratio of each grain. An average of the aspect ratios of the prior austenite grains is obtained.
  • a high strength steel plate which is used for cranes or the like has a plate thickness of 4.5 to 20 mm. Therefore, the high strength steel plate according to this embodiment has a plate thickness of 4.5 to 20 mm. However, the thickness is preferably 4.5 to 15 mm in view of contribution to a reduction in the weight.
  • high-strengthening is required to contribute to an increase in the size or a reduction in the weight of construction machines or industrial machines.
  • the upper limit may be 1,100 MPa or less.
  • the upper limit may be 1,300 MPa or less or 1,250 MPa or less.
  • the total elongation is adjusted to be 12% or greater.
  • the total elongation may be a structure index indicating whether the formation of bainite, which causes the formation of coarse cementite is suppressed.
  • the yield strength, the tensile strength, and the total elongation are measured by performing a tensile test based on JIS Z 2241.
  • the value of the total elongation measured by the tensile test depends on the shape of a test piece.
  • the limit (12% or greater) of the total elongation is a value in a case where a No. 5 test piece of JIS Z2241 (a flat test piece in which original gauge length is 50 mm, a parallel portion has a width of 25 mm, and the thickness of the test piece is equal to that of the steel plate) is used as a tensile test piece.
  • the elongation conversion formula based on a difference in the test piece shape is also specified in ISO2566-1.
  • a No. 5 test piece of JIS Z2241 in a case where a No. 13B test piece of JIS Z2241 (a flat test piece in which original gauge length is 50 mm, a parallel portion has a width of 12.5 mm, and the thickness of the test piece is equal to that of the steel plate) is used as a tensile test piece, the elongation can be converted into 10.4%, and in a case where a No.
  • test piece of JIS Z2241 (a flat test piece in which original gauge length is 80 mm, a parallel portion has a width of 20 mm, and the thickness of the test piece is equal to that of the steel plate) is used as a tensile test piece, the elongation can be converted into 9.5%.
  • the high strength steel plate may be required to have low temperature toughness. Therefore, the Charpy absorbed energy at -20°C is 59 J/cm 2 or greater. More preferably, the Charpy absorbed energy at -40°C is preferably 59 J/cm 2 or greater.
  • the Charpy absorbed energy is measured by collecting a test piece in which a longitudinal direction is a rolling direction from a thickness middle portion, and performing a Charpy test based on JIS Z 2242 at -20°C or -40°C. It may be difficult to collect a full-sized test piece of 10 mm ⁇ 10 mm depending on the plate thickness of the steel plate, and in such a case, a sub-sized test piece is used.
  • the Charpy absorbed energy is a value (J/cm 2 ) obtained by dividing the absorption energy by a cross-sectional area (cm 2 ) of the test piece in a bottom part of a V-notch.
  • the high strength steel plate according to this embodiment can be manufactured as follows: a molten steel having a chemical composition within the above-described range is melted in the usual manner, a slab obtained by casting the molten steel is heated to perform hot rolling, accelerated cooling is performed, and after the accelerated cooling is stopped, air cooling to room temperature is performed.
  • a heat treatment such as tempering is not performed after the accelerated cooling is stopped or after the air cooling to room temperature is performed.
  • martensite changes to tempered martensite. That is, the high strength steel plate according to this embodiment is manufactured through non-heat treated manufacturing steps in which a heat treatment is omitted to shorten the construction period or reduce the manufacturing cost.
  • the high strength steel plate according to this embodiment manufactured through the non-heat treated manufacturing steps may be referred to as a non-heat treated high strength steel plate.
  • the high strength steel plate according to this embodiment is required to contain a predetermined amount of alloying elements to increase hardenability. Therefore, a carbide or a nitride of alloying elements is generated in a slab provided in hot rolling. In heating of the slab, it is necessary to decompose the carbide or the nitride in order to form a solid solution in the steel, and the heating temperature is adjusted to be 1,100°C or higher. In a case where the heating temperature of the slab is too high, the grain size becomes coarse, and thus toughness may be reduced. Accordingly, the heating temperature is adjusted to be 1,250°C or higher.
  • a heated slab is hot-rolled. After the hot rolling, it is necessary to start accelerated cooling at a temperature at which the microstructure is austenite in order to provide a microstructure mainly including martensite through the accelerated cooling. Accordingly, the hot rolling should be ended at a temperature at which the microstructure is austenite.
  • the hot rolling finishing temperature is adjusted to be Ar3 (°C) or higher.
  • Ar3 (°C) is a temperature at which transformation from austenite to ferrite starts during cooling, and can be obtained from thermal expansion behavior.
  • Ar3 (°C) can be simply obtained by (Formula b).
  • Ar 3 868 ⁇ 396 ⁇ C + 24.6 ⁇ Si ⁇ 68.1 ⁇ Mn ⁇ 36.1 ⁇ Ni ⁇ 20.7 ⁇ Cu ⁇ 24.8 ⁇ Cr + 29.6 ⁇ Mo
  • each of [C], [Si], [Mn], [Ni], [Cu], [Cr], and [Mo] represents a content (mass%) of each element, and 0 is substituted in a case where the element is not contained.
  • the hot rolling may be performed in the usual manner, and it is preferable that recrystallization region rolling be performed with a cumulative rolling reduction of 50% to 80% within a temperature range not lower than 1,050°C, or non-recrystallization region rolling be performed with a cumulative rolling reduction of 50% to 90% within a temperature range of Ar3 to 950°C.
  • the cooling rate of the accelerated cooling is required to be 30 °C/s or higher to increase a martensite area fraction. A sufficient martensite area fraction is not obtained when the cooling rate is lower than 30 °C/s.
  • the cooling rate is preferably increased, but there is a restriction caused by a plate thickness or facility. Accordingly, the upper limit may be 200 °C/s or lower.
  • the cooling rate is calculated as follows: a temperature variation of a steel plate surface after hot rolling is measured, and a difference between a surface temperature before the water cooling is started and a surface temperature immediately after the water cooling is stopped is divided by a time required for cooling.
  • a martensitic transformation start temperature Ms (°C) is obtained by (Formula 3).
  • a martensitic transformation end temperature Mf (°C) is lower than Ms (°C) by approximately 150°C, and is obtained by (Formula 4).
  • each of [C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], and [Al] represents a content (mass%) of each element, and 0 is substituted in a case where the element is not contained.
  • cooling to at least Ms (°C) or lower is required, and in a case where cooling (rapid cooling) to Mf (°C) or lower is performed, 90% or more of the microstructure transforms to martensite.
  • the cooling stop temperature is 300°C or higher, the cooling may become unstable and a part of the microstructure transforms to not martensite but bainite. Accordingly, the cooling stop temperature is adjusted to be not higher than Mf (°C) and lower than 300°C.
  • the accelerated cooling stop temperature is very important, and a precondition is set that the accelerated cooling is stopped at a temperature lower than the martensitic transformation start temperature Ms (°C).
  • the microstructure transforms to a structure mainly including martensite in which the generation of a carbide is suppressed.
  • the ductility of a high strength steel plate is influenced by hardenability. That is, in a case where the hardenability is increased, the formation of bainite is suppressed, and the generation of a coarse cementite-based carbide is suppressed. Accordingly, the total elongation is improved, and the variation is also reduced.
  • FIG. 1 The relationship between the accelerated cooling stop temperature Tcf and Mf and the total elongation, and the influence of DI and the C content on the total elongation are qualitatively summarized as schematically shown in FIG. 1 .
  • the vertical axis represents a total elongation
  • the horizontal axis represents accelerated cooling stop temperature Tcf
  • DI represents a hardenability index which is obtained by (Formula 1).
  • the total elongation is improved together with a reduction of Tcf.
  • DI is increased, and thus the formation of bainite is suppressed. Accordingly, the generation of a coarse carbide is suppressed, and thus the total elongation is improved.
  • the lower limit of the accelerated cooling stop temperature is not particularly limited, and accelerated cooling may also be performed to room temperature.
  • the accelerated cooling stop temperature is preferably 100°C or higher to increase a yield strength by, for example, the action of locking carbon atoms in the dislocation.
  • a slab obtained by steel making for chemical components (including a remainder consisting of Fe and impurities) shown in Table 1 was formed into a steel plate having a plate thickness of 4.5 to 20 mm under manufacturing conditions shown in Table 2.
  • the "heating temperature” represents a reheating temperature of the slab
  • the “rolling end temperature” represents a temperature at which hot rolling is ended
  • the "water cooling start temperature” represents a surface temperature of the steel plate when accelerated cooling (water cooling) is started
  • the “cooling rate” represents an average cooling rate in a thickness middle portion within a temperature range of Ar3 (°C) to an accelerated cooling stop temperature
  • the “water cooling stop temperature” represents a surface temperature of the steel plate when water cooling is stopped.
  • the surface temperature of the steel plate was measured by a radiation-type thermometer, and the "cooling rate” was calculated by obtaining a temperature of the thickness middle portion from the surface temperature through thermal conductivity calculation. Tempering was not performed on any steel plate.
  • microstructure and the mechanical properties were evaluated.
  • the distinguishment of a microstructure and the measurement of a martensite area fraction and a bainite area fraction were performed by the following method.
  • a cross-section of a steel plate was subjected to mirror polishing, and then near a 1/4 t-portion in the cross-section parallel to a rolling direction was subjected to nital etching.
  • Two regions within a range of 120 ⁇ m ⁇ 100 ⁇ m were photographed at 500-fold magnification using an optical microscope to measure an area fraction of a microstructure in which an acicular lath structure was developed.
  • the cross-section of the steel plate was subjected to electrolytic polishing, and then a portion near the 1/4 t-portion in the cross-section of the steel plate was observed by a scanning electron microscope (SEM).
  • the magnification was 5,000 times, and the photographing was performed within a range of 50 ⁇ m ⁇ 40 ⁇ m.
  • the acicular structure was defined as martensite, and an area fraction of the region was obtained.
  • the product of the acicular structure area fraction measured using the optical microscope and the martensite area fraction measured using SEM was an area fraction of the martensite structure of this steel.
  • An acicular structure other than the martensite was determined as bainite.
  • the area fraction of a microstructure in which an acicular lath structure was developed was defined as the total area fraction of martensite and bainite using an optical microscope.
  • a microstructure (remainder) other than the "martensite and bainite" described in Table 3 includes one or more of ferrite, pearlite, and residual austenite.
  • a cross-section of the steel plate was subjected to electrolytic polishing, and then a portion near a 1/4 t-portion in the cross-section of the steel plate was observed by a scanning electron microscope (SEM) to measure the number density of cementite.
  • SEM scanning electron microscope
  • a cross-section of the steel plate was subjected to electrolytic polishing, and then a portion near a 1/4 t-portion in the cross-section of the steel plate was photographed within a range of 50 ⁇ m ⁇ 40 ⁇ m at 5,000-fold magnification by a scanning electron microscope (SEM).
  • the number of precipitates as cementite having an aspect ratio of 2.0 or greater and a length of 0.1 ⁇ m or greater in a long axis direction was counted using image analysis software.
  • the number of cementite having an aspect ratio of 2.0 or greater and a length of 1.0 ⁇ m or greater in a long axis direction was counted.
  • the obtained number of precipitates of 1.0 ⁇ m or greater was divided by the number of cementite of 0.1 ⁇ m or greater, and thus the number fraction (%) of cementite of 1.0 ⁇ m or greater was obtained. In a case where the number fraction of cementite of 1.0 ⁇ m or greater was 5% or less, this was judged as a good result.
  • the aspect ratio of prior austenite grains was measured. Specifically, near a 1/4 t-portion in a cross-section parallel to a rolling direction was etched with nital, and two regions within a range of 120 ⁇ m ⁇ 100 ⁇ m were photographed at 500-fold magnification using an optical microscope. From the obtained image, long axis lengths and short axis lengths of at least 50 prior austenite grains were measured, and the long axis length was divided by the short axis length to obtain an aspect ratio of each grain. An average of the aspect ratios of the prior austenite grains was obtained and defined as an aspect ratio of the prior austenite grains. In a case where the aspect ratio of the prior austenite grains was 2.0 or greater, this was judged as a good result.
  • a test piece (overall thickness) was collected from the steel plate, and a tensile strength, a yield strength, and a total elongation were measured based on JIS Z2241. In addition, a Charpy absorbed energy at -20°C and a Charpy absorbed energy at -40 °C were measured based on JIS Z 2242.
  • the tensile test piece is a No. 5 test piece (overall thickness) collected such that a longitudinal direction is perpendicular to the rolling direction, and the yield strength is 0.2% proof stress.
  • the Charpy test piece is a sub-sized test piece of 10 mm ⁇ 5 mm collected from a thickness middle portion such that a longitudinal direction is the rolling direction.
  • the mechanical properties were evaluated to be good in a case where the yield strength was 885 MPa or greater, the tensile strength was 950 MPa or greater, the total elongation was 12% or greater, and the absorbed energy value at -20°C (vE -20 ) was 59 J/cm 2 or greater as a result of the tests.
  • Welding joints were produced using steel plates having good mechanical properties (steel plate Nos. 1 to 16) and a steel plate No. 32 in which Pcm was less than 0.189%.
  • the welding method was MAG welding, and the weld heat input was 7.0 kJ/cm or 10.0 kJ/cm.
  • the welding conditions were set as follows: a current of 280 A, a voltage of 27 V, and a welding rate of 65 cm/min.
  • the welding conditions were set as follows: a current of 305 A, a voltage of 29 V, and a welding rate of 53 cm/min.
  • the tensile strength (joint strength) of the welding joint was evaluated by a tensile test specified in JIS Z 3121, and evaluated to be good in a case where the tensile strength was 950 MPa or greater.
  • Steel plate Nos. 1 to 16 are invention examples, and an excellent strength, excellent ductility, and excellent toughness are obtained.
  • the joint strength is 950 MPa or greater.
  • Mo/Cr is 0.20 or higher, excellent toughness is obtained even at a test temperature of -40°C.
  • Steel plate Nos. 17 to 35 are comparative examples, and one or more of the yield strength, the tensile strength, the total elongation, and vE -20 does not satisfy a target value.
  • a steel plate No. 20 had a low Mn content and had low hardenability. Accordingly, ferrite and bainite were formed other than martensite, and thus the amount of martensite formed did not satisfy the range of the invention. As a result, the strength was significantly reduced.
  • a steel plate No. 31 had a high Cr content and a high Mo content, and DI was too high. Accordingly, the toughness and the total elongation were low.
  • the invention it is possible to provide a high strength steel plate which has a yield strength of 885 MPa or greater, a tensile strength of 950 MPa or greater, and a total elongation of 12% or greater without containing a large amount of expensive alloying elements.
  • this steel plate exhibits excellent toughness such that a Charpy absorbed energy at -20°C is 59 J/cm 2 or greater. Accordingly, the invention is useful for industry.

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BR112018071948A2 (pt) 2019-02-05
CN109154041B (zh) 2020-07-31
KR102142472B1 (ko) 2020-08-07
CN109154041A (zh) 2019-01-04
KR20180126591A (ko) 2018-11-27
JP6501042B2 (ja) 2019-04-17
EP3492610A4 (en) 2020-03-11
BR112018071948B1 (pt) 2022-03-03
WO2018020660A1 (ja) 2018-02-01
EP3492610A1 (en) 2019-06-05
JPWO2018020660A1 (ja) 2019-02-28

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