EP3385399A1 - Tôle d'acier non traitée thermiquement ayant une limite élastique élevée dans laquelle la dureté d'une zone affectée par la chaleur de soudage et la dégradation de la ténacité à basse température de la zone affectée par la chaleur de soudage sont supprimées - Google Patents

Tôle d'acier non traitée thermiquement ayant une limite élastique élevée dans laquelle la dureté d'une zone affectée par la chaleur de soudage et la dégradation de la ténacité à basse température de la zone affectée par la chaleur de soudage sont supprimées Download PDF

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EP3385399A1
EP3385399A1 EP16870530.9A EP16870530A EP3385399A1 EP 3385399 A1 EP3385399 A1 EP 3385399A1 EP 16870530 A EP16870530 A EP 16870530A EP 3385399 A1 EP3385399 A1 EP 3385399A1
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Prior art keywords
less
heat
content
yield strength
bainite
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German (de)
English (en)
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EP3385399A4 (fr
Inventor
Koji Kurata
Haruya KAWANO
Kiichiro TASHIRO
Motoki KAKIZAKI
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
    • B21C37/08—Making tubes with welded or soldered seams
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • 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
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    • 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
    • C22C38/46—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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a non-heat-treated steel plate having high yield strength in which hardness of a weld heat-affected zone and degradation of low-temperature toughness of the weld heat-affected zone are suppressed. More specifically, the present invention relates to a non-heat-treated steel plate having a high yield strength of API standard X80 grade to be used in line pipes for transportation of oil, natural gas, and the like.
  • Controlled rolling is exemplified as a manufacturing method to satisfy these requirements.
  • Controlled rolling is a technique which includes refining crystal grains by appropriately controlling the temperature and rolling reduction during hot-rolling and the like, and performing accelerated cooling after the hot-rolling.
  • the controlled rolling does not need any thermal refining, including heating after the accelerated cooling, and the like.
  • the steel plate obtained by such a method is generally called a non-heat-treated steel plate.
  • Patent Documents 1 to 4 disclose a method for manufacturing a steel plate that has a high yield strength of API standard X80 grade in a non-heat-treated state.
  • weld heat-affected zone As line pipes are installed in cold regions in many cases, it is essential for a weld heat-affected zone (HAZ) to have excellent low-temperature toughness. Furthermore, from the viewpoint of weldability, suppression of a hardness of the weld heat-affected zone has been strongly desired in recent years.
  • Patent Document 1 and Patent Document 2 are not controlled to have a low Ceq, which is an index for evaluating the toughness and hardness of the weld heat-affected zone. Consequently, the toughness of the weld heat-affected zone might be degraded, and the weld heat-affected zone might be hardened.
  • the present invention has been made in view of the foregoing circumstances, and it is an object of the present invention to provide a non-heat-treated steel plate having high yield strength in which hardness of a weld heat-affected zone and degradation of low-temperature toughness of the weld heat-affected zone are suppressed.
  • the non-heat-treated steel plate having high yield strength in which hardness of a weld heat-affected zone and degradation of low-temperature toughness of the weld heat-affected zone are suppressed includes, in percent by mass: C: more than 0.04% and 0.10% or less; Si: 0.15 to 0.50%; Mn: 1.20 to 2.50%; P: more than 0% and 0.020% or less; S: more than 0% and 0.0050% or less; Nb: 0.
  • the above-mentioned non-heat-treated steel plate is used for line pipes.
  • a non-heat-treated steel plate having high yield strength can be obtained in which hardness of a weld heat-affected zone and degradation of low-temperature toughness of the weld heat-affected zone are suppressed.
  • the present inventors have considered factors that control a yield strength of a non-heat-treated steel plate.
  • the yield strength of the non-heat-treated steel plate has close correlation with respective area ratios of bainite and martensite-austenite constituent in a metal structure and the maximum hardness of bainite, and by controlling these factors within predetermined ranges, the high yield strength of the API standard X80 grade can be achieved.
  • the present inventors have studied non-heat-treated steel plates from various perspectives in order to achieve a non-heat-treated steel plate having high yield strength in which hardness of a weld heat-affected zone and degradation of low-temperature toughness of the weld heat-affected zone are suppressed.
  • a chemical composition is controlled to satisfy the relationships of the above-mentioned formulas (1) to (3), the degradation of the low-temperature toughness of the weld heat-affected zone can be suppressed, and further the hardness of the weld heat-affected zone can be reduced. In this way, the present invention has been completed.
  • such a non-heat-treated steel plate can be preferably manufactured by hot-rolling a steel material satisfying a predetermined composition through heating and then cooling the hot-rolled steel from a cooling start temperature of 730°C or higher to a cooling stop temperature of 370 to 550°C at an average cooling rate of 10 to 50°C/sec.
  • high yield strength of the API standard X80 grade means that the yield strength of a steel plate in the plate width direction is 555 MPa or more and 705 MPa or less.
  • the area ratios of respective structures of bainite and martensite-austenite constituent to the entire metal structure in a 1/4 position of the thickness t of the steel plate satisfy bainite: 80% by area or more; and martensite-austenite constituent: 0% by area or more and 0.26% by area or less, and the maximum hardness of bainite is 270 HV or more.
  • Bainite 80% by area or more
  • Bainite is a structure that contributes to the improvement of yield strength and further is an important structure for securing high yield strength of the API standard X80 grade. If the bainite area ratio is less than 80% by area, the yield strength of steel is reduced. For this reason, the lower limit of the bainite area ratio is 80% by area or more assuming that the area of the entire metal structure is 100%. The lower limit of the bainite area ratio is preferably 82% by area or more, and more preferably 84% by area or more.
  • Fig. 1 is a graph showing a relationship between bainite area ratio and yield strength in non-heat-treated steel plates manufactured using steels of types A to X shown in Table 1 of Examples to be mentioned later, under manufacturing conditions No. 1 to 24 shown in Table 2.
  • the bainite area ratio in the metal structure is 80% or more.
  • it is found effective to increase the bainite area ratio to 80% or more.
  • the yield strength does not satisfy 555 MPa or more despite the bainite area ratio being 80% or more.
  • the bainite hardness to be mentioned later is less than 270HV, or the MA area ratio is more than 0.26%.
  • the maximum hardness of bainite is critical for suppressing variations in yield strength to obtain a stable high yield strength, and needs to be controlled to be 270 HV or more. Thus, the high yield strength of the API standard X80 grade can be stably secured.
  • the lower limit of the maximum hardness of bainite is preferably 275 HV or more.
  • the upper limit of the maximum hardness of bainite is preferably 310 HV or less, and more preferably 300 HV or less.
  • Fig. 2 is a graph showing a relationship between maximum hardness of bainite and yield strength in non-heat-treated steel plates manufactured by using the steels of types A to X shown in Table 1 of Examples to be mentioned later, under manufacturing conditions No. 1 to 24 shown in Table 2.
  • the maximum hardness of bainite in the metal structure is 270 HV or more.
  • the yield strength does not satisfy 555 MPa or more despite the maximum hardness of bainite being 270 HV or more.
  • the bainite area ratio is less than 80%, or the MA area ratio is more than 0.26%.
  • maximum hardness of bainite means an average value of measured values at the top three points obtained when the Vickers hardness of bainite is measured by a method mentioned in a section of Examples mentioned later. The present inventors have found that the high yield strength can be stably obtained by controlling the maximum hardness of bainite.
  • martensite-austenite constituent 0% by area or more and 0.26% by area or less.
  • the martensite-austenite constituent is a structure that reduces the yield strength, in order to secure the desired high yield strength, it is necessary to reduce the MA area ratio. For that reason, the upper limit of the MA area ratio is 0.26% by area or less, assuming that the area of the entire metal structure is set at 100%. The upper limit of the MA area ratio is preferably 0.25% by area or less.
  • Fig. 3 is a graph showing a relationship between martensite-austenite constituent area ratio and yield strength in non-heat-treated steel plates manufactured by using steels of types A to X shown in Table 1 of Examples to be mentioned later, under manufacturing conditions No. 1 to 24 shown in Table 2.
  • the MA area ratio in the metal structure is 0.26% or less.
  • the yield strength does not satisfy 555 MPa or more despite the MA area ratio being 0.26% or less.
  • the bainite area ratio is less than 80%, or the maximum hardness of bainite is less than 270 HV.
  • the structure of the non-heat-treated steel plate according to the present invention is as mentioned above.
  • Other remaining structures, except for those mentioned above, may include ferrite, martensite, and/or pearlite.
  • Ceq defined by the formula (1) mentioned above is an index that is important for determining the low-temperature toughness of the HAZ and the hardness of the HAZ. If Ceq is 0.44 or more, the low-temperature toughness of the HAZ and the hardness property of the HAZ are drastically degraded. If Ceq is less than 0.44, the HAZ can secure satisfactory low-temperature toughness and hardness. For this reason, the upper limit of Ceq is set at less than 0.44.
  • the upper limit of Ceq is preferably 0.43 or less, and more preferably 0.42 or less. Meanwhile, in consideration of the lower limit of the content of each element and the like, the lower limit of Ceq is preferably 0.37 or more, and more preferably 0.38 or more.
  • a value: 2.50 or more A value 1.15 ⁇ Mn + 2.20 ⁇ Mo + 6.50 ⁇ Nb
  • Mn, Mo, and Nb represent the contents of Mn, Mo, and Nb in percent by mass, respectively.
  • the A value is first found by the present inventors and is a parameter that controls the respective contents of Mn and Mo, which are effective for suppressing the ferrite transformation among the elements constituting Ceq mentioned above, and further controls the content of Nb so as to satisfy the above formula (2).
  • the bainite area ratio which is important for achieving the high yield strength, can be secured while suppressing an increase in Ceq.
  • the higher the A value, the better, and hence the lower limit of the A value is set at 2.50 or more in order to secure the high yield strength of the API standard X80 grade.
  • the lower limit of the A value is preferably 2.52 or more, and more preferably 2.54 or more.
  • the upper limit of the A value is preferably 3.00 or less, and more preferably 2.95 or less.
  • Mn, Ni, and Nb represent the contents of Mn, Ni, and Nb in percent by mass, respectively.
  • the B value is first found by the present inventors and is a parameter that controls the respective contents of Mn, Ni, and Nb, which are effective for introducing dislocations at high densities, to satisfy the above formula (3) by decreasing the bainite transformation temperature.
  • the B value is set to 2.37 or more, the maximum hardness of bainite can be secured while suppressing an increase in Ceq.
  • the higher the B value, the better, and hence the lower limit of the B value is set at 2.37 or more in order to secure the high yield strength of the API standard X80 grade.
  • the lower limit of the B value is preferably 2.39 or more.
  • the upper limit of the B value is preferably 2.70 or less, and more preferably 2.68 or less.
  • the lower limit of C content needs to be set at more than 0.04%.
  • the lower limit of the C content is preferably 0.05% or more, and more preferably 0.06% or more.
  • the upper limit of the C content needs to be set at 0.10% or less.
  • the upper limit of C content is preferably 0.09% or less, and more preferably 0.08% or less.
  • the lower limit of Si content is 0.15% or more.
  • the lower limit of the Si content is preferably 0.18% or more, and more preferably 0.20% or more.
  • the upper limit of the Si content needs to be set at 0.50% or less.
  • the upper limit of the Si content is preferably 0.45% or less, and more preferably 0.40% or less.
  • Mn is an element effective in improving the yield strength of the base metal.
  • the lower limit of Mn content needs to be 1.20% or more.
  • the lower limit of the Mn content is preferably 1.50% or more, and more preferably 1.70% or more.
  • the upper limit of the Mn content is set at 2.50% or less.
  • the upper limit of the Mn content is preferably 2.20% or less, and more preferably 2.00% or less.
  • P is an element inevitably contained in the steel material. If the P content exceeds 0.020%, the low-temperature toughness of the HAZ is remarkably degraded. For this reason, the upper limit of the P content is set at 0.020% or less.
  • the upper limit of the P content is preferably 0.015% or less, and more preferably 0.010% or less. It should be noted that P is an impurity inevitably contained in the steel, and hence the P content is difficult to set at 0% in terms of industrial production.
  • S is an element that affects the low-temperature toughness of the HAZ, like P mentioned above. If the S content exceeds 0.0050%, coarse sulfides are formed, degrading the low-temperature toughness of the HAZ. Thus, the upper limit of the S content is set at 0.0050% or less. The upper limit of the S content is preferably 0.0030% or less, and more preferably 0.0020% or less. It should be noted that S is an impurity inevitably contained in the steel, and hence the S content is difficult to set at 0% in terms of industrial production.
  • Nb is an element effective in enhancing the yield strength of the steel and the low-temperature toughness of the base metal without degrading the weldability.
  • the lower limit of the Nb content needs to be 0.020% or more.
  • the lower limit of the Nb content is preferably 0.030% or more, and more preferably 0.040% or more.
  • the upper limit of Nb content is set at 0.100% or less.
  • the upper limit of the Nb content is preferably 0.070% or less, and more preferably 0.060% or less.
  • Ti is an element effective in improving the yield strength of the base metal. Further, Ti is the element necessary for improving the low-temperature toughness of the HAZ because Ti precipitates as TiN in steel, thereby suppressing austenite grains in the HAZ from being coarsened during welding. To effectively exhibit these effects, the lower limit of the Ti content needs to be 0.003% or more. The lower limit of the Ti content is preferably 0.005% or more, and more preferably 0.007% or more. However, if the Ti content is excessive, the low-temperature toughness of the HAZ is degraded due to an increase in the amount of solid-solution Ti or TiC precipitates. Thus, the upper limit of the Ti content is set at 0.020% or less. The upper limit of the Ti content is preferably 0.018% or less, and more preferably 0.016% or less.
  • the lower limit of the N content needs to be 0.0010% or more.
  • the lower limit of the N content is preferably 0.0020% or more, and more preferably 0.0030% or more.
  • the upper limit of the N content is set at 0.0075% or less.
  • the upper limit of the N content is preferably 0.0070% or less, and more preferably 0.0065% or less.
  • the Zr is an element that forms an oxide and disperses it in the steel, thereby contributing to improvement of the low-temperature toughness of the HAZ.
  • the lower limit of the Zr content needs to be 0.0001% or more.
  • the lower limit of the Zr content is preferably 0.0003% or more, and more preferably 0.0005% or more.
  • the upper limit of the Zr content needs to be 0.0100% or less.
  • the upper limit of the Zr content is preferably 0.0050% or less, and more preferably 0.0030% or less.
  • Ca is an element that has the function of controlling the form of a sulfide and forms CaS to suppress the formation of MnS, thereby improving the low-temperature toughness of the HAZ.
  • the lower limit of the Ca content needs to be 0.0005% or more.
  • the lower limit of the Ca content is preferably 0.0006% or more.
  • the upper limit of the Ca content is 0.0030% or less.
  • the upper limit of the Ca content is preferably 0.0028% or less, and more preferably 0.0026% or less.
  • a rare-earth metal (REM) as rare-earth element is an element effective in controlling the form of a sulfide and suppresses the formation of MnS which adversely affects the low-temperature toughness of the HAZ.
  • the lower limit of the REM content is set at 0.0001% or more.
  • the REM content is preferably 0.0003% or more, and more preferably 0.0005% or more.
  • the upper limit of the REM content is set at 0.0050% or less.
  • the upper limit of the REM content is preferably 0.0040% or less, and more preferably 0.0030% or less.
  • REM means lanthanoid elements (15 elements from La to Lu), scandium (Sc), and yttrium (Y).
  • the steel preferably contains at least one element selected from the group consisting of Ce, La, and Nd, and more preferably contains at least one of the Ce and La.
  • the lower limit of the Al content needs to be 0.010% or more.
  • the lower limit of the Al content is preferably 0.015% or more, and more preferably 0.018% or more.
  • the upper limit of the Al content needs to be 0.050% or less.
  • the upper limit of the Al content is preferably 0.045% or less, and more preferably 0.042% or less.
  • the upper limit of the B content is set at 0.0003% or less.
  • the upper limit of the B content is preferably 0.0002% or less, and more preferably 0.0001% or less. It should be noted that if B is added in an amount exceeding 0.0003%, the combined addition of Mo and B induces an excessive increase in the yield strength of the base metal.
  • Mo, Cu, Ni, Cr, and V are elements effective in improving the yield strength of the steel. These elements may be added alone or in combination. The reasons for setting the content ranges of these elements are as follows.
  • the lower limit of the Mo content is preferably 0.01% or more.
  • the lower limit of the Mo content is more preferably 0.05% or more, and even more preferably 0.10% or more.
  • the upper limit of the Mo content is 0.30% or less.
  • the upper limit of the Mo content is preferably 0.25% or less, and more preferably 0.20% or less.
  • the lower limit of the Cu content is preferably 0.01% or more.
  • the lower limit of the Cu content is more preferably 0.05% or more, and even more preferably 0.10% or more.
  • the upper limit of the Cu content is set at 0.30% or less.
  • the upper limit of Cu content is preferably 0.27% or less, and more preferably 0.25% or less.
  • Ni more than 0% to 0.30% or less
  • Ni is an element effective in improving the yield strength of a base metal.
  • the lower limit of the Ni content is preferably 0.01% or more.
  • the lower limit of the Ni content is more preferably 0.05% or more, and even more preferably 0.10% or more.
  • the upper limit of the Ni content is set at 0.30% or less.
  • the upper limit of the Ni content is preferably 0.27% or less, and more preferably 0.25% or less.
  • the lower limit of the Cr content is preferably 0.01% or more.
  • the lower limit of the Cr content is more preferably 0.05% or more, and even more preferably 0.10% or more.
  • the upper limit of the Cr content is 0.30% or less.
  • the upper limit of the Cr content is preferably 0.27% or less, and more preferably 0.25% or less.
  • V more than 0% and 0.050% or less
  • V is an element effective in improving the yield strength.
  • the lower limit of the V content is preferably 0.001% or more.
  • the lower limit of the V content is more preferably 0.002% or more, and even more preferably 0.003% or more.
  • the upper limit of the V content is set at 0.050% or less.
  • the upper limit of the V content is preferably 0.030% or less, and more preferably 0.010% or less.
  • the elements of the steel used in the present invention have been mentioned above, with the balance substantially being iron. It should be noted that inevitable impurities are allowed to be brought and contained in steel, depending on the situations, including raw materials, other materials, facilities, and the like.
  • the above-mentioned inevitable impurities can include, for example, As, Sb, Sn, O, H, and the like.
  • the steel plate of the present invention can be manufactured, for example, by producing a cast strip, such as a slab, heating and hot-rolling the obtained cast strip, followed by accelerated cooling.
  • REM and Ca are added to steel material after deoxidation by adding Al and Zr to steel material and forming Al 2 O 3 and ZrO.
  • Ca is an element which easily forms oxides, and Ca forms an oxide (CaO) more easily than a sulfide (CaS).
  • the time up to completion of the casting is preferably controlled. For that reason, when Al, Zr, REM, and Ca are added in this order in a molten steel treatment step, solidification of the molten steel is preferably completed within 200 minutes after the addition of Ca.
  • the time from the addition of REM to the addition of Ca is preferably secured for four minutes or more.
  • Ca and REM tend to be present in the form of sulfides without forming any oxide.
  • the cast strip is heated and hot-rolled.
  • the heating temperature at which the cast strip is heated is preferably set in a range of 1,000 to 1, 200°C. If the heating temperature is extremely low, Nb in the steel cannot be solid-soluted sufficiently, failing to secure the high yield strength. Thus, the lower limit of the heating temperature is more preferably 1,100°C or higher, and even more preferably 1,120°C or higher. However, if the heating temperature is extremely high, austenite grains are coarsened, degrading the low-temperature toughness of the base metal. Thus, the upper limit of the heating temperature is more preferably 1,180°C or lower.
  • a hot-rolling start temperature is preferably 900 to 1,100°C. If the hot-rolling start temperature is extremely low, rolling in a austenite recrystallization region cannot be secured, thereby making austenite grains coarse, which might degrade the low-temperature toughness of the base metal. Thus, the lower limit of the hot-rolling start temperature is more preferably 930°C or higher, and even more preferably 950°C or higher. Meanwhile, if the hot-rolling start temperature is extremely high, austenite grains become coarse after recrystallization, which might degrade the low-temperature toughness of the base metal. Thus, the upper limit of the hot-rolling start temperature is more preferably 1090°C or lower, and even more preferably 1,080°C or lower.
  • the rolling reduction performed at temperatures from 950°C to a hot-rolling end temperature is preferably 40 to 80%. If the rolling reduction is extremely low at temperatures from 950°C to the hot-rolling end temperature, strain to be introduced into austenite grains cannot be secured, so that the grains after the bainite transformation become coarse, which might degrade the low-temperature toughness of the base metal. Thus, the lower limit of the rolling reduction is more preferably 50% or more, and even more preferably 60% or more. Meanwhile, if the rolling reduction performed at temperatures from 950 °C to the hot-rolling end temperature is extremely high, excessive strain is introduced into austenite grains, thus degrading the hardenability of the steel. Thus, the upper limit of the rolling reduction is more preferably 77% or less, and even more preferably 75% or less.
  • the hot-rolling end temperature is preferably 770 to 880°C. If the hot-rolling end temperature is extremely low, excessive strain is introduced into austenite grains, thus degrading the hardenability of the steel. Thus, the lower limit of the hot-rolling end temperature is more preferably 790°C or higher, and even more preferably 800°C or higher. Meanwhile, if the hot-rolling end temperature is extremely high, strain to be introduced into austenite grains cannot be secured, making the grains after the bainite transformation coarse, which might degrade the low-temperature toughness of the base metal. Thus, the upper limit of the hot-rolling end temperature is more preferably 860°C or lower, and even more preferably 850°C or lower.
  • accelerated cooling is preferably performed in the following manner. It should be noted that the cooling is not necessarily limited to this condition.
  • a cooling start temperature after the end of the hot-rolling is preferably 730°C or higher. If the cooling start temperature is lower than 730°C, ferrite transformation is promoted to precipitate ferrite, so that the metal structure does not become bainite, thus making it difficult to secure the high yield strength of the base metal in some cases.
  • the lower limit of the cooling start temperature is more preferably 735°C or higher, and even more preferably 740°C or higher.
  • the upper limit of the cooling start temperature is not particularly limited, but is more preferably 860°C or lower, and even more preferably 850°C or lower.
  • the accelerated cooling is quickly performed preferably at an average cooling rate of 10 to 50°C/sec.
  • an average cooling rate of the accelerated cooling is set to 10°C/sec or higher, the untransformed austenite is caused to be transformed into a bainite structure, thereby making it possible to prevent the precipitation of ferrite.
  • the maximum hardness of bainite is enhanced, thereby easily improving the yield strength of the steel.
  • the lower limit of the average cooling rate is more preferably 13°C/sec or higher, and even more preferably 15°C/sec or higher.
  • the upper limit of the average cooling rate is preferably 50°C/sec or lower.
  • the upper limit of the average cooling rate is more preferably 45°C/sec or lower in consideration of the formability into a steel pipe.
  • the cooling stop temperature is preferably set at 370 to 550°C.
  • the cooling stop temperature is set at 370 to 550°C, so that the MA area ratio is reduced, and the high yield strength of 555 MPa or more can be easily obtained.
  • the lower limit of the cooling stop temperature is more preferably 390°C or higher, and even more preferably 400°C or higher.
  • the upper limit of the cooling stop temperature is more preferably 540°C or lower, and even more preferably 530°C or lower.
  • the non-heat-treated steel plate of the present invention can be obtained by cooling the steel plate to a temperature between 370 and 550°C, followed by normal cooling, such as allowing the steel plate to cool, to room temperature.
  • the average cooling rate at this time is preferably approximately 0.1 to 5°C/sec.
  • the plate thickness of the steel plate according to the present invention is not particularly limited, but when using the steel plate as the material for line pipes, the lower limit of the plate thickness is preferably 6 mm or more, and more preferably 10 mm or more. Meanwhile, the upper limit of the plate thickness of the steel plate is preferably 32 mm or less, and more preferably 30 mm or less from the viewpoint of suppressing precipitation of ferrite while securing a necessary cooling rate.
  • the non-heat-treated steel plate obtained in the way mentioned above is suitable for use, particularly, in line pipes.
  • a line pipe obtained by using the non-heat-treated steel plate of the present invention reflects the properties of the non-heat-treated steel plate, so that the line pipe has excellent low-temperature toughness and hardness property of the HAZ and excellent yield strength.
  • 35Fe-30REM-35Si alloys containing 50% of Ce and 20% of La were used as REM.
  • REM and Ca were added in this order, and the time from the addition of REM to the addition of Ca was set at 4 minutes or more. Further, a cast strip was produced so that solidification was completed within 200 minutes after the addition of Ca.
  • the steel plate was allowed to cool to room temperature.
  • the average cooling rate at this time is approximately 1°C/sec.
  • a test piece with dimensions of 20 mm ⁇ 15 mm ⁇ 15 mm was cut out of the above-mentioned steel plate, and its cross section parallel to the rolling direction was polished and subjected to nital corrosion. Then, the structure of the test piece at the 1/4 position of the plate thickness t was observed using an optical microscope at a magnification of 100 times, and a bainite area ratio was measured by image analysis assuming that the entire metal structure was set at 100%. The bainite area ratio was measured for three fields of view in total of each test piece, and an average of the measured values was determined. In Examples, the same observation as in bainite was performed on the remaining structure except for MA.
  • a test piece with dimensions of 20 mm ⁇ 15 mm ⁇ 15 mm was cut out of the above-mentioned steel plate, and its cross section parallel to the rolling direction was polished and subjected to repeller corrosion. Then, the structure of the test piece at the 1/4 position of the plate thickness t was observed using an optical microscope at a magnification of 1,000 times, and a MA area ratio was measured by image analysis assuming that the entire metal structure was 100%. The MA area ratio was measured for three fields of view in total of each test piece, and an average of the measured values was determined.
  • a test piece with dimensions of 20 mm ⁇ 15 mm ⁇ 15 mm was cut out of the above-mentioned steel plate, and its cross section parallel to the rolling direction was exposed. Then, the bainite hardness of the structure of each test piece at a 1/4 position of the thickness t was measured in 20 points at equal intervals within a range of 100 ⁇ m ⁇ 100 ⁇ m with a Vickers tester under a load of 5 gf (0.049 N). Among these, an average value of measured values at the top three hardnesses refers to the maximum hardness of bainite.
  • test piece was cut out of the above-mentioned steel plate based on the API5L standard such that the direction of the steel plate perpendicular to the rolling direction is oriented along the longitudinal side of the test piece. Then, 0.5% proof stress of each test piece was measured as the yield strength. Test pieces having a yield strength of the API standard X80grade, namely, 555 MPa or more and 705 MPa or less, were rated as pass.
  • a test piece with dimensions of 12 mm ⁇ 32 mm ⁇ 55 mm was cut from each of the above-mentioned steel plates Nos. 1 to 24 shown in Table 2 such that the direction of the steel plate perpendicular to the rolling direction is oriented along the longitudinal side of the test piece.
  • This test piece was regarded as a reproducible heat cycle test piece.
  • a heat cycle was applied to the reproducible heat cycle test piece.
  • the maximum heating temperature was 1,350°C while simulating a coarse-grain heat affected zone in the vicinity of a melting line.
  • the heat cycle includes heating at 1,350°C and holding at this temperature for 5 seconds, and then cooling in a temperature range of 800 to 500°C for 30 seconds.
  • the low-temperature toughness of the HAZ was then evaluated by performing a Charpy impact test in the way specified by the API5L standard.
  • the low-temperature toughness of the HAZ was evaluated by performing the Charpy impact test at -10°C. Test pieces having an absorption energy of 27 J or more were rated as pass.
  • a reproducible heat cycle test piece was taken out of each of the steel plates Nos. 1 to 24 shown in Table 2, in the same manner as the evaluation on the low-temperature toughness of the weld heat-affected zone.
  • the heat cycle was applied to the test pieces.
  • the Vickers hardness test was performed on each test piece to evaluate the hardness property of the HAZ.
  • the hardness of the HAZ refers to the maximum value of the Vickers hardness among the measured values at three points under a load of 98 N.
  • test pieces having a hardness of the HAZ of less than 225 HV were rated as pass.
  • Steel plates Nos. 17 to 24 shown in Table 2 were manufactured using steel types Q to X shown in Table 1, satisfying the composition specified by the present invention, under manufacturing conditions for Nos. 17 to 24 shown in Table 2, satisfying the preferable requirements specified by the present invention. It is found that these steel plates thus obtained had satisfactory low-temperature toughness and hardness property of the HAZ as well as high yield strength of 555 MPa or more.
  • the steel plate No. 1 shown in Table 2 was an example in which individual elements of the composition satisfied the requirements specified by the present invention.
  • this steel plate used the steel type A in Table 1 having a large Ceq. Consequently, the maximum hardness of the HAZ was increased due to the large Ceq, resulting in reduced low-temperature toughness of the HAZ.
  • the steel plate No. 2 shown in Table 2 was an example of using the steel type B shown in Table 1, in which a content of B was large and the A value and B value were small. Consequently, the yield strength was reduced because the bainite area ratio was small and the maximum hardness of bainite was low, and the low-temperature toughness of the HAZ was reduced because the B content was large.
  • the steel plate No. 3 shown in Table 2 was an example of using the steel type C shown in Table 1, in which the contents of B and Ti were large, and the A value and B value were small. Consequently, the low-temperature toughness of the HAZ was reduced because of the large contents of B and Ti. It should be noted that in this steel plate, although the A value and B value were small, the B content exceeded 0.0003%, and Mo was added in a complex manner, whereby the bainite area ratio, the maximum hardness of bainite, and the yield strength were increased.
  • the steel plate No. 4 shown in Table 2 was an example of using the steel type D shown in Table 1, in which the A value and B value were small. Consequently, the bainite area ratio was small, and the maximum hardness of bainite was low, resulting in reduced yield strength.
  • the steel plate No. 5 shown in Table 2 was an example of using the steel type E shown in Table 1, in which the A value was small. Consequently, the bainite area ratio was small, thus resulting in reduced yield strength.
  • the steel plate No. 6 shown in Table 2 was an example of using the steel type F shown in Table 1, in which the B value was small. Consequently, although the bainite area ratio was 80% by area or more, the maximum hardness of bainite was low, thus resulting in reduced yield strength.
  • the steel plate No. 7 shown in Table 2 was an example of using the steel type G shown in Table 1, in which Mo, Cu, Ni, Cr, and V were not contained, and the A value and B value were small. Consequently, none of Mo, Cu, Ni, Cr, and V was included, the bainite area ratio was small, and the maximum hardness of bainite was low, thus resulting in reduced yield strength.
  • the steel plate No. 8 shown in Table 2 was an example of using the steel type H shown in Table 1, in which the C content was small and the B value was small. Consequently, the C content was small, and the maximum hardness of bainite was low, thus resulting in reduced yield strength.
  • the steel plate No. 9 shown in Table 2 was an example of using the steel type I shown in Table 1, in which the Si content was small and the A value was small. Consequently, the Si content was small and the bainite area ratio was small, thus resulting in reduced yield strength.
  • the steel plate No. 10 shown in Table 2 was an example of using the steel type J shown in Table 1, in which the Mn content was small and the A value and B value were small. Consequently, the Mn content was small, the bainite area ratio was small, and the maximum hardness of bainite was low, thus resulting in reduced yield strength.
  • the steel plate No. 11 shown in Table 2 was an example of using the steel type K shown in Table 1, in which the Mn content and the Nb content were small, and the A value and B value were small. Consequently, the Mn content and the Nb content were small, the bainite area ratio was small, and the maximum hardness of bainite was low, thus resulting in reduced yield strength.
  • the steel plate No. 12 shown in Table 2 was an example of using the steel type L shown in Table 1, in which the Nb content was small, Mo, Cu, Ni, Cr, and V were not contained, and the A value and B value were small. Consequently, the Nb content was small and none of Mo, Cu, Ni, Cr, and V was contained, the bainite area ratio was small, and the maximum hardness of bainite was low, thus resulting in reduced yield strength.
  • the steel plate No. 13 shown in Table 2 was an example of using the steel type M shown in Table 1, in which the Ni content was large, and the A value and B value were small. Consequently, the amount of MA became large, and the bainite area ratio became small, thus resulting in reduced yield strength.
  • the steel plate No. 14 shown in Table 2 was an example of using the steel type N shown in Table 1, in which the Cr content was large and the A value and B value were small. Consequently, the amount of MA was large, the bainite area ratio was small, and the maximum hardness of bainite became low, thus resulting in reduced yield strength.
  • the steel plate No. 15 shown in Table 2 was an example of using the steel type O shown in Table 1, in which the Mn content was small, the Cr was large, and the A value and B value were small. Consequently, the Mn content was small, the amount of MA was large, the bainite area ratio was small, and the maximum hardness of bainite became low, thus resulting in reduced yield strength.
  • the steel plate No. 16 shown in Table 2 was an example of using the steel type P shown in Table 1, in which the V content was large, and the A value and B value were small. Consequently, the amount of MA was large, the bainite area ratio was small, and the maximum hardness of bainite became low, thus resulting in reduced yield strength.

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JP5704706B2 (ja) * 2011-03-16 2015-04-22 株式会社神戸製鋼所 Ｈａｚ靭性に優れた高強度厚鋼板
JP5910219B2 (ja) * 2012-03-23 2016-04-27 Ｊｆｅスチール株式会社 鋼板内の材質均一性に優れた大入熱溶接用高強度鋼板及びその製造方法
JP6101132B2 (ja) * 2012-04-20 2017-03-22 株式会社神戸製鋼所 耐水素誘起割れ性に優れた鋼材の製造方法
JP6226542B2 (ja) * 2013-03-22 2017-11-08 株式会社神戸製鋼所 溶接熱影響部の靭性に優れた鋼材
CN106133175B (zh) * 2014-03-31 2018-09-07 杰富意钢铁株式会社 耐应变时效特性和耐hic特性优良的高变形能力管线管用钢材及其制造方法以及焊接钢管

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US11401568B2 (en) 2018-01-30 2022-08-02 Jfe Steel Corporation Steel material for line pipes, method for producing the same, and method for producing line pipe

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JP2017106107A (ja) 2017-06-15
EP3385399A4 (fr) 2019-05-22
WO2017094593A1 (fr) 2017-06-08
KR20180085791A (ko) 2018-07-27

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