EP2612945B1 - High-strength steel plate and method for producing same - Google Patents

High-strength steel plate and method for producing same Download PDF

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Publication number
EP2612945B1
EP2612945B1 EP11838055.9A EP11838055A EP2612945B1 EP 2612945 B1 EP2612945 B1 EP 2612945B1 EP 11838055 A EP11838055 A EP 11838055A EP 2612945 B1 EP2612945 B1 EP 2612945B1
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less
equal
steel
steel plate
strength
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German (de)
English (en)
French (fr)
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EP2612945A4 (en
EP2612945A1 (en
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Tatsuya Kumagai
Michinori Gotoh
Norimasa Kawabata
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • 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
    • C22C38/18—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
    • C22C38/26—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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—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
    • 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

  • the present invention relates to a high-strength steel plate having excellent weldability and a method of manufacturing the same.
  • the present invention relates to a high-strength steel plate which is used as a structural member of a construction machine or an industrial machine, has a yield strength of equal to or more than 885 MPa, a tensile strength of equal to or more than 950 MPa and equal to or less than 1130 MPa, and generally has a thickness of equal to or more than 6 mm and equal to or less than 25 mm, and a method of manufacturing the same.
  • weld crack sensitivity is very largely influenced by diffusible hydrogen
  • the diffusible hydrogen content in weld metal be suppressed to be low.
  • various management including a management of lubricant oil and a cleaning of a groove surface of a welding wire and the like, as well as a selection and a management of welding material, is necessary so that hydrogen is not mixed in at the time of performing the welding operation, and thereby a load in the operation is increased.
  • steel even when approximately 3.0 to 5.0 ml/100g of the diffusible hydrogen content in the carbon dioxide arc welding, which is thought to be mixed in when welding operation management is slightly insufficient, is contained in the steel, it is preferable for steel to have a sufficiently low crack sensitivity in which cracking is not generated when welding is performed without preheating.
  • tensile strength is, for example, in a range of equal to or more than 950 MPa and equal to or less than 1130 MPa and an upper limit of tensile strength is present.
  • a steel plate used for a construction machine or the like is usually bent and when the tensile strength of the steel plate exceeds a specified upper limit, a load which is necessary for bending work is increased. For this reason, it is necessary that the tensile strength of a steel plate not be excessively increased in consideration of a case where work is limited due to facility capacity.
  • high tensile strength steel plates having a tensile strength of 950 MPa-class are disclosed in Patent Documents 1 and 2.
  • these steel plates are relatively thick and used for a penstock. Due to this, a large amount ofNi is added to these steel plates as an essential element in order to secure toughness without particularly considering bending workability and thereby, the steel plates are not economical for use in a construction machine.
  • Patent Document 3 high tensile strength steel having excellent weldability and economic efficiency is disclosed.
  • a weld crack sensitivity index Pcm is suppressed to be equal to or less than 0.29 so that weldability is secured.
  • the lowest crack stopping preheating temperature is 100°C in a y-groove weld cracking test, and it is thought that weldability cannot be secured in welding without preheating.
  • Patent Document 4 a technology relating to high tensile strength steel having excellent weldability and arrestability is disclosed.
  • the technology it is necessary to add Ni in order to secure toughness, and the steel plate is not economical for use in a construction.
  • the diffusible hydrogen content is 1.2 ml/100g under the test conditions. Due to this, in this case, it is estimated that a load is increased at the time of performing a welding operation to manage the diffusible hydrogen content of weld metal.
  • Patent Document 5 a technology relating to high tensile strength steel having excellent weldability and HIC resistance is disclosed.
  • the technology it is necessary to add Ni and 0.6% or more of Mo in order to secure toughness, and the steel plate is not economical for use in a construction machine.
  • the diffusible hydrogen content is limited to 1.5 ml/100g under the test conditions. Due to this, in this case, it is estimated that a load is increased at the time of performing a welding operation to manage the diffusible hydrogen content in weld metal.
  • Patent Document 6 a method in which a steel plate having a tensile strength exceeding 980 MPa is manufactured in a non-thermal refining manner is disclosed.
  • the method it is necessary to add a large amount of alloy elements such as 1.5% or more of Mn in the steel in order to secure the tensile strength exceeding 980 MPa with a very small amount of C which is equal to or less than 0.025%, and particularly, when the amount of Mn is large, there is concern that the cracking sensitivity of a segregation portion is degraded.
  • Patent Document 7 a hot rolled steel sheet having a tensile strength of 950 MPa or more in which bending workability and weldability are considered is disclosed. Since it is necessary to add a large amount of Ti in the hot rolled steel sheet, it is thought that weldability is degraded. In addition, since it is necessary to add Ni in order to compensate for a decrease in toughness due to the addition of the large amount of Ti, there is a problem in economic efficiency.
  • Patent Document 8 a method of manufacturing a steel plate which is mainly used for a line pipe and has a tensile strength of 950 MPa or more and excellent toughness and weldability is disclosed. Since it is necessary that the amount of Mn is equal to or more than 1.8%, there is concern that the cracking sensitivity of a segregation portion is degraded, and since low temperature rolling is necessary in a ferrite-austenite two-phase region, productivity is low.
  • JP 11229079 relates to a high strength steel plate for line pipe having a mixed martensite and bainite structure.
  • An object of the present invention is to economically provide a high-strength steel plate having excellent weldability which is used as a structural member of a construction machine or an industrial machine, has a yield strength of equal to or more than 885 MPa, a tensile strength of equal to or more than 950 MPa and equal to or less than 1130 MPa, and generally has a thickness of equal to or more than 6 mm and equal to or less than 25 mm, and a method of manufacturing the same.
  • a high-strength steel plate having excellent weldability which is used as a structural member of a construction machine or an industrial machine, has a yield strength of equal to or more than 885 MPa, and generally has a thickness of equal to or more than 6 mm and equal to or less than 25 mm.
  • weld crack sensitivity index Pcm In order to decrease weld crack sensitivity, it is known that it is effective to decrease a weld crack sensitivity index Pcm.
  • a y-groove weld cracking test (a weld heat input of 1.7 kJ/mm) specified by JIS Z3158 (1993) was performed on steel materials having various chemical compositions with temperature and humidity being adjusted. All testing materials had a thickness of 25 mm, and the test was necessarily performed on two testing materials under the same condition.
  • One of the two testing materials was used as a testing material for analyzing hydrogen content, and immediately after the weld cracking test, a sample was obtained from the testing material to measure a diffusible hydrogen content using gas chromatography.
  • a cracking presence evaluation test was performed on the other testing material only when the diffusible hydrogen content exceeded 5.0 ml/100g as a result of the analysis.
  • the relationship between Pcm and a cracking prevention preheating temperature of steel shown FIG. 1 was obtained from the obtained result. That is, influences of the Pcm and preheating temperature of the steel on the presence of cracking are shown in FIG. 1 . From FIG.
  • a 100-kg/mm 2 steel class steel plate is manufactured by a quenching and tempering process, and generally contains tempered martensite as a main structure.
  • tempered martensite in a case where a main structure is tempered martensite in a component composition (chemical composition) that satisfies a low Pcm of equal to or less than 0.22%.
  • a simple method for obtaining high strength with such low Pcm is that a martensite structure is not subjected to tempering, that is, a martensite structure in an as-quenching state is used.
  • yield strength/tensile strength is low, and when there is an attempt to secure the yield strength specified by the standard, the tensile strength is forced to increase by all means.
  • the yield strength is equal to or more than 885 MPa and the tensile strength is equal to or more than 950 MPa and equal to or less than 1130 MPa.
  • the inventors concluded that it is effective that, in order to obtain a high yield ratio in the as-quenching state, the structure in the as-quenching state is controlled to a structure which is mainly composed of lower bainite and the structural fraction of the martensite is lowered.
  • the inventors specifically investigated to the relationship among a structural fraction, strength and yield ratio of steel having various component compositions in which C content is equal to or more than 0.05% and less than 0.10%, and Pcm is equal to or less than 0.22%.
  • C content is equal to or more than 0.05% and less than 0.10%
  • Pcm is equal to or less than 0.22%.
  • the steel plate structure is a structure which is mainly composed of lower bainite (lower bainite single phase structure or a mixed structure of lower bainite and martensite) in order to satisfy a yield ratio of equal to or more than 83%, specifically, the structural fraction of the lower bainite included in the steel plate structure is equal to or more than 70% ( FIG. 2 ).
  • a steel plate having a thickness of 6 to 25 mm in which the sum of the fraction of lower bainite and the fraction of martensite is equal to or more than 90% is used and the structure is controlled by stopping water cooling at 300 to 450°C in the steel plate.
  • the inventors examined a method of stably controlling the structure of the steel plate in a structure which is mainly composed of lower bainite. For example, although a cooling rate at the time of quenching is controlled to be in a predetermined range and lower bainite can be obtained, a range of cooling rates to obtain lower bainite is generally narrow and such controlling of the cooling rate is industrially inadvisable. As a manufacturing process of stably and simply obtaining a structure which is mainly composed of lower bainite, it is effective to stop water cooling at an appropriate temperature in the middle of cooling, and slow down the cooling rate by using air cooling thereafter, instead of accelerated cooling to room temperature during quenching.
  • a water cooling stopping temperature (steel plate temperature to transition from water cooling to air cooling) is lower than 300°C, the martensite fraction is excessively high. Contrarily, when the water cooling stopping temperature is higher than 450°C, upper bainite is easily formed. Therefore, it is desirable that the water cooling stopping temperature be equal to or more than 300°C and equal to or less than 450°C.
  • the inventors investigated in detail the relationship between the structural fraction and strength of the steel in which the sum of the structural fraction of martensite and the structural fraction of lower bainite is equal to or greater than 90% by manufacturing the steel plate having a thickness of 6 to 25 mm under the condition in which the water cooling stopping temperature is equal to or more than 300°C and equal to or less than 450°C with respect to steel grades of various component compositions in which C content is equal to or more than 0.05% and less than 0.10% and Pcm is equal to or less than 0.22%.
  • Mn and Ni have an effect of suppressing the lower bainite transformation, particularly in a process of stopping the water cooling in the middle of the cooling, it become apparent that Mn and Ni have a strong tendency to decrease the structural fraction of the lower bainite, to increase the structural fraction of the martensite when the water cooling stopping temperature is low, and to increase the structural fraction of the upper bainite (upper bainite fraction) when the water cooling stopping temperature is high.
  • Mo and V have a strong tendency to suppress the formation of ferrite and upper bainite and to increase the structural fraction of the lower bainite.
  • a (A value) defined by the following (Formula 6) is adjusted to be equal to or less than 2.0 in addition to the component composition conditions in which the C content is equal to or more than 0.05% and less than 0.10%, and Pcm defined by the following (Formula 5) is equal to or less than 0.22%, and the sum of the structural fraction of the martensite and the structural fraction of the lower bainite is equal to or more than 90%, it is found that the structure having lower bainite fraction of equal to or more than 70% is reliably obtained ( FIG. 3 ).
  • [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are respectively % by mass of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B in the chemical composition.
  • % represents “% by mass”.
  • C is an important element that has a significant effect on the strength of steel of the present embodiment which has a structure which is mainly composed of lower bainite.
  • the C content In order to obtain a yield strength of equal to or more than 885 MPa, it is necessary for the C content to be equal to or more than 0.05%, and preferably equal to or more than 0.055% or equal to or more than 0.060%.
  • the C content is less than 0.10%, and desirably equal to or less than 0.095% and equal to or less than 0.090%.
  • the Si content is equal to or more than 0.20% and desirably equal to or more than 0.25% or equal to or more than 0.30%.
  • the upper limit of the Si content is 0.50%, and desirably 0.45% or 0.40%.
  • Mn is an element which is effective in improving strength by improving hardenability. For this reason, Mn content is equal to or more than 0.20%, desirably equal to or more than 0.30% or equal to or more than 0.50%.
  • Mn has an effect of suppressing lower bainite transformation, particularly in the process of stopping the water cooling in the middle of the cooling, it becomes apparent that Mn has a strong tendency to decrease the structural fraction of the lower bainite, to increase the structural fraction of the martensite when the water cooling stopping temperature is low, and to increase the upper bainite fraction when the water cooling stopping temperature is high.
  • the Mn content is equal to or more than 1.20%, it is difficult to obtain a yield ratio of equal to or more than 83%, and therefore the Mn content is less than 1.20% and desirably 1.00% or equal to or less than 0.90%.
  • Cr content is equal to or more than 0.20% and desirably equal to or more than 0.25%, or equal to or more than 0.30%.
  • the Cr content is equal to or less than 1.20%, and desirably equal to or less than 1.10% or equal to or less than 1.00%.
  • Mo is effective for stably forming lower bainite in the process of stopping the water cooling in the middle of the cooling described later by suppressing the ferrite formation. For this reason, it is necessary that the Mo content be equal to or more than 0.20% and it is preferable to be equal to or more than 0.25% or equal to or more than 0.30%. However, when a large amount of Mo is added to steel, weldability is deteriorated and also, Mo is an expensive element. Therefore, the Mo content is equal to or less than 0.60%, and desirably equal to or less than 0.58% or equal to or less than 0.55%.
  • Ni has an effect of suppressing lower bainite transformation similar to Mn, particularly in the process of stopping the water cooling in the middle of the cooling, Ni has a strong tendency to decrease the structural fraction of the lower bainite and to increase the structural fraction of the martensite when the water cooling stopping temperature is low, and to increase the upper bainite fraction when the water cooling stopping temperature is high. For this reason, when Ni is added to steel, it is difficult to obtain a yield strength of equal to or more than 83%. Therefore, Ni is not intentionally added to steel, and Ni content is suppressed to be in a range in which Ni is inevitably contained in the steel. Specifically, the upper limit of the Ni content is 0.1%, and desirably 0.05% or 0.04%.
  • the lower limit of the Ni content does not need to be particularly limited and is 0%.
  • Ni whose content is 0.5 times or more of Cu content may be added to steel while limiting the Ni content to less than the above-described Ni content.
  • Nb forms fine carbide during rolling and widens a non-recrystallization temperature region to enhance a controlled rolling effect so that Nb improves toughness by grain refining. Therefore, Nb content is equal to or more than 0.010% and desirably equal to or more than 0.015% or equal to or more than 0.020%. However, when Nb is excessively added to steel, weldability is deteriorate and thereby, the Nb content is equal to or less than 0.050%, and desirably equal to or less than 0.045% or equal to or less than 0.040%.
  • B is used in order to secure suitable hardenability to obtain the lower bainite structure.
  • suitable hardenability it is necessary to secure free B at the time of direct quenching. Since N forms BN to decrease the free B, a suitable amount of Ti is added to steel to not form BN and N is fixed as TiN.
  • Ti is contained in steel to fix N as TiN. That is, Ti content in the steel is equal to or more than 0.005% and desirably 0.010% or equal to or more than 0.012%. However, since an excessive addition of Ti degrades weldability in some cases, the upper limit of the Ti content is 0.030%, and desirably 0.025% or 0.020%.
  • B has an effect for improving hardenability of steel, and it is necessary that B content is equal to or more than 0.0003% to exhibit the effect, and preferably equal to or more than 0.0005% or equal to or more than 0.0010%.
  • the B content is equal to or less than 0.0030%, and desirably equal to or less than 0.0025% or equal to or less than 0.0020%.
  • N content is suppressed to be equal to or less than 0.0080%, and desirably equal to or less than 0.0060% or equal to or less than 0.0050%.
  • the lower limit of the N content does not need to be particularly limited, and is 0%.
  • Al is added to steel as a deoxidizing material, and Al content in the steel is generally equal to or more than 0.01%. However, since an excessive addition of Al degrades toughness in some cases, the upper limit of the Al content is 0.10%, and desirably 0.08% or 0.05%.
  • P is a harmful element that degrades toughness. Therefore, P content is suppressed to be equal to or less than 0.012%, and desirably equal to or less than 0.010% or equal to or less than 0.008%. In addition, since P is an inevitable impurity, the lower limit of the P content does not need to be particularly limited and is 0%.
  • the S content is suppressed to be equal to or less than 0.005%, and desirably equal to or less than 0.004% or equal to or less than 0.003%.
  • the lower limit of the S content does not need to be particularly limited and is 0%.
  • the elements described above are basic components (basic elements) of the steel according to the present embodiment, and the chemical composition containing the basic elements and composed of a balance Fe and inevitable impurities is a basic composition of the present embodiment.
  • the following elements may be further contained in the present embodiment as needed. In addition, even when these selective elements are inevitably mixed, the effect of the present embodiment is not impaired.
  • one or more kinds selected from V, Cu, and Ca can be added to the steel as the selective element, in addition to the basic components.
  • V enhances a hardenability, has a precipitation strengthening effect of a tempered martensite structure or a tempered bainite structure and is effective in improving strength
  • V may be added as needed.
  • a V content is equal to or less than 0.10%, and desirably equal to or less than 0.090% or equal to or less than 0.080%.
  • the lower limit of the V content is 0%.
  • Cu is an element that improves strength by solid-solution strengthening, and Cu may be added as needed.
  • Cu can be added to steel so that a Cu content is equal to or more than 0.05%.
  • the Cu content is equal to or less than 0.50%, and desirably equal to or less than 0.40% or equal to or less than 0.30%.
  • Cu is an expensive element, it is unnecessary to intentionally add Cu to the steel, and the lower limit of the Cu content is 0% to reduce the alloy cost.
  • Ca has an effect of reducing a decrease in bending workability due to MnS by spheroidizing a sulfide of a steel plate, and Ca may be added to steel as needed.
  • Ca is added to steel to achieve the object, and 0.0001% or more of Ca may be contained in the steel.
  • the upper limit of the Ca content is equal to or less than 0.0030%, and desirably equal to or less than 0.0020% or equal to or less than 0.0010%.
  • the lower limit of the Ca content is 0% to reduce alloy cost.
  • the high-strength steel plate of the present embodiment contains the above-mentioned basic elements and has the chemical composition composed of the balance Fe and inevitable impurities, or contains the above-mentioned basic elements, one or more kinds selected from the above-mentioned selective element and has the chemical composition composed of the balance Fe and inevitable impurities.
  • the component composition is adjusted so that the Pcm defined in the above (Formula 5) is equal to or less than 0.22% in order to secure sufficient weldability as described above.
  • the component composition is adjusted so that A (A value) defined by the above (Formula 6) is equal to or less than 2.0.
  • Pcm and "A" are respectively defined by the following (Formula 7) and (Formula 8).
  • the (Formula 7) and (Formula 8) correspond to the above (Formula 5) and (Formula 6) respectively.
  • Pcm C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 20 + Mo / 15 + 5 ⁇
  • B A Mn + 1.5 ⁇ Ni / Mo
  • a component composition satisfying the respective element content ranges and the conditions of Pcm and A is the component composition of the present embodiment.
  • the sum of the martensite fraction and the lower bainite fraction is equal to or more than 90%, and the lower bainite fraction needs to be equal to or more than 70% to satisfy a yield ratio of equal to or more than 83% while weldability required for general welding operation management is secured.
  • the inventors found that specifically, the fact that the number density of the relatively coarse cementite having a diameter (equivalent circle diameter) of equal to or more than 50 nm is equal to or less than 20 pieces/ ⁇ m 3 in the steel plate structure is a preferable condition to contain a large amount of the fine cementite and remarkably improve the yield strength. It is possible to easily achieve the yield ratio of equal to or more than 83% by containing a large amount of the fine cementite in the steel plate structure. In addition, the lower limit of the number density of the cementite is 0 pieces/ ⁇ m 3 .
  • a base material of a steel plate with a predetermined volume is eluted by electrolysis using an extraction replica method to prepare a sample which is obtained by extracting cementite, and the sample is observed by a transmission electron microscope (TEM) to obtain the number (number density) of the cementite having an equivalent circle diameter of equal to or more than 50 nm (cementite of equal to or more than 50 nm) per unit volume.
  • TEM transmission electron microscope
  • an aspect ratio of prior austenite is equal to or more than 2 as described later.
  • the aspect ratio of prior austenite is a ratio (axial ratio) of a long axis length to a short axis length of the prior austenite and an average value of each axial ratio of each prior austenite grain. Therefore, the lower limit of the aspect ratio is 1.
  • the high-strength steel plate was manufactured by the following method using a slab (steel) in which the component composition in the steel is adjusted by addition or the like so as to satisfy the component composition conditions of the present embodiment.
  • FIG. 4 schematically shows an outline the method of manufacturing the high-strength steel plate according to the present embodiment.
  • the slab is heated to a temperature (heating temperature) of equal to or more than 1100°C (S1). While the upper limit of the heating temperature is not particularly limited, it is preferable to be 1300 °C since productivity is decreased or the grain diameter of the austenite at the time of heating is extremely increased.
  • the heated slab is subjected to hot rolling to have a target thickness so that a cumulative rolling reduction ratio in the non-recrystallization temperature region is equal to or more than 60% (S2).
  • the hot-rolled slab that is, steel plate (steel) generally has a thickness of 6 to 25 mm, and the thickness is not necessarily limited thereto.
  • the cumulative rolling reduction ratio in the non-recrystallization temperature region is equal to or more than 60%, it is possible to introduce sufficient working strain and to appropriately control strength properties of the steel plate.
  • the non-recrystallization temperature region is a temperature region of equal to or more than Ar3 and equal to or less than 960°C, in which recrystallization (reduction of working strain) after rolling can be prevented.
  • the Ar3 (Ar3 transformation point) is a temperature in which the ferrite transformation is started at the time of cooling and can be measured by a hot working simulator manufactured by Fuji Electronic Industrial Co., Ltd (THERMECMASTOR-Z).
  • a hot working simulator manufactured by Fuji Electronic Industrial Co., Ltd (THERMECMASTOR-Z).
  • a volume change at the time of cooling is measured to determine Ar3 on the basis of the volume change.
  • the cumulative rolling reduction ratio in the non-recrystallization temperature region is less than 100%.
  • On-line accelerated cooling (water cooling) is performed on the steel plate (steel) obtained by the hot rolling after the hot rolling from the temperature of equal to or more than Ar3 (water cooling starting temperature). Hardenability can be increased by performing the on-line accelerated cooling, which is advantageous to decrease Pcm.
  • the reason that the accelerated cooling starting temperature is set to the temperature of equal to or more than Ar3 is that ferrite or upper bainite is formed and the strength of the steel plate is significantly degraded when the accelerated cooling is started from the temperature of less than Ar3.
  • the accelerated cooling is stopped at a temperature of equal to or more than 300°C and equal to or less than 450°C (water cooling stopping temperature), and air cooling is performed (S3).
  • the accelerated cooling is cooling in which an average cooling rate in 1/4t parts of the steel plate is equal to or more than 10°C/s in a temperature region which is equal to or more than the cooling stopping temperature and equal to or less than Ar3, and the upper limit of the average cooling rate of the accelerated cooling is not particularly limited.
  • the air cooling (retained in the atmosphere) is cooling in which an average cooling rate in 1/4t parts of the steel plate is equal to or less than 1°C/s in a temperature region which is equal to or more than the room temperature and less than the cooling stopping temperature, and the lower limit of the average cooling rate of the air cooling is not particularly limited.
  • the 1/4t parts of the steel plate are a portion which is distant from a surface of the steel plate in a thickness center (depth) direction by a distance of 1/4 of the thickness, and the cooling rate of the 1/4t parts is obtained from temperature change obtained by performing a thermal analysis.
  • 70% or more of lower bainite can be obtained and sufficiently fine cementite can be secured.
  • the number density of relatively coarse cementite of equal to or more than 50 nm is equal to or less than 20 pieces/ ⁇ m 3 with respect to the most of the obtained steel plates.
  • the sum of the lower bainite fraction and the martensite fraction is equal to or more than 90%
  • the lower bainite fraction is equal to or more than 70%
  • the aspect ratio of the prior austenite is equal to or more than 2 as a property the structure of the steel plate manufactured by the on-line accelerated cooling.
  • the aspect ratio of the prior austenite in the steel plate is less than 2.0.
  • tempering is necessary to secure the yield ratio, the number of processes and process time are increased and industrially, cost is increased.
  • the cooling rate in the time of air cooling is significantly decreased, and the number density of relatively coarse cementite of equal to or more than 50 nm exceeds 20 pieces/ ⁇ m 3 .
  • the coil-shaped steel plate be subjected to air cooling after the accelerated cooling and it is desirable that the steel plates be left to be air-cooled without overlapping each other until the temperature of the steel plate is equal to or less than 250°C. That is, until the temperature of the steel plate is equal to or less than 250°C, it is desirable that the steel plates be not overlapped over each other (for example, so that the surfaces of the steel plates can be in contact with air) and be air-cooled. After the temperature of the steel plate reaches equal to or less than 250°C, the steel plates may be air-cooled in an overlapped manner.
  • Steel composition Nos. A to AP having component compositions shown in Tables 1 and 2 were smelted to obtain slabs and using the slabs, steel plates with numbers 1 to 55 having thickness of 6 to 25 mm were manufactured according to manufacturing conditions in Tables 3 and 4.
  • Tables 1 and 2 when Cu, Ni, V and Ca are not intentionally added to the steel, the amounts of these chemical components are provided with parentheses.
  • Tables 3 and 4 after the accelerated cooling (water cooling) was stopped, the steel plates were not wound and were air-cooled one by one, until the temperature of the steel plate is 250°C.
  • Example 1 A 1180 25 64 804 325 42 2 A 1170 6 70 761 420 105 3 B 1150 25 66 819 360 40 4 B 1175 9 62 750 425 71 5 C 1200 25 64 794 330 38 6 D 1165 25 63 820 370 40 7 D 1130 12 61 767 410 66 8 E 1150 25 64 809 335 41 9 E 1135 12 66 764 395 67 10 F 1160 25 66 798 405 40 11 G 1155 25 64 802 375 44 12 H 11235 25 70 799 360 37 13 I 1125 25 64 779 375 38 14 J 1180 25 65 794 410 37 15 K 1150 25 67 804 350 38 16 L 1160 25 66 787 320 41 17
  • the long axis length and the short axis length of the prior austenite were measured, and an aspect ratio was obtained by dividing the long axis length by the short axis length from an image obtained by observing a cross-section which is parallel to a rolling direction (longitudinal direction) of the steel plate in the vicinity of the 1/4t parts (L-shaped cross-section, a cross-section perpendicular to a thickness center direction).
  • a base material of a steel plate with a predetermined volume from the steel plates Nos.
  • 1 to 55 was eluted by electrolysis using the extraction replica method to prepare a sample which was obtained by extracting cementite, and the sample was observed by a transmission electron microscope (TEM) to obtain the number density of the cementite having an equivalent circle diameter of equal to or more than 50 nm.
  • TEM transmission electron microscope
  • Ar3 Ar3 transformation point
  • FAMECMASTOR-Z Fuji Electronic Industrial Co., Ltd
  • the yield strength and the tensile strength were measured by acquiring lA-type specimens for a tensile test specified in JIS Z 2201 (1998) from the steel plates Nos. 1 to 55 according to a tensile test specified in JIS Z 2241 (1998).
  • the yield strength is equal to or more than 885 MPa
  • the tensile strength is equal to or more than 950 MPa and equal to or less than 1130 MPa
  • the yield strength and the tensile strength of the steel plate were respectively evaluated as "Pass".
  • a y-groove weld cracking test specified by JIS Z 3158 (1993) was performed on the steel plates Nos. 1 to 55 to evaluate weldability.
  • temperature and humidity were adjusted to perform carbon dioxide arc welding at a heat input of 15 kJ/cm, and the steel plate provided for the evaluation had a thickness of 25 mm.
  • the weldability of the steel plate was evaluated as "Pass".
  • each of two testing materials was subjected to welding in which the same conditions such as temperature, humidity and a heat input were set, and one of the two testing materials was sampled immediately after the welding so that the diffusible hydrogen content of the weld metal was measured using the gas chromatography method specified by JIS Z 3118 (2007).
  • the diffusible hydrogen content exceeded 5.0 ml/100g, the other testing material was subjected to the evaluation test of weldability (presence of cracking).
  • the sum of the lower bainite fraction and the martensite fraction (lower bainite fraction + martensite fraction) is equal to or more than 90%, the lower bainite fraction is equal to or more than 70%, and the yield strength, tensile strength, yield ratio, weldability and toughness satisfied the target value.
  • the y-groove weld cracking test which was performed to evaluate the weldability, since the diffusible hydrogen content in the weld metal was in a range of 5.1 to 6.0 ml/100g, it was confirmed that weld cracking was not generated in the range.
  • the diffusible hydrogen content is 3.0 to 5.0 ml/100g which is thought to be mixed when the welding operation management was slightly insufficient, the diffusible hydrogen content is lower than the diffusible hydrogen content in the range so that it can be considered that the weld cracking is not generated.
  • the number density of the relatively coarse cementite of equal to or more than 50 nm was increased and the weld strength was degraded in comparison with a case where tempering was not performed.
  • each chemical component amount and values of Pcm and A satisfied the conditions of the present invention.
  • any one of manufacturing conditions did not satisfy the conditions of the present invention.
  • the structure condition of the steel plate one or more of lower bainite + martensite fractions and the lower bainite fractions did not satisfy the conditions of the present invention and one or more of the yield strength, tensile strength and toughness were also failed.
  • the steel plate No. 54 after the slab was rolled to manufacture a steel plate and air cooling was performed on the steel plate, the steel plate was reheated to 930°C, and cooled in a temperature region which is from 810°C to 350°C at a cooling rate of 40°C/s. Therefore, for example, manufacturing cost was increased in the steel plate No. 54 in comparison with the steel plate No. 52.

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EP2612945A4 (en) 2013-07-24
TWI418641B (zh) 2013-12-11
BR112013010765B1 (pt) 2018-12-18
CN103189537B (zh) 2016-01-20
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JP5037744B2 (ja) 2012-10-03
CN103189537A (zh) 2013-07-03
KR101374422B1 (ko) 2014-03-17

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