WO2019124945A1 - Matériau en acier à haute résistance pour environnement de région polaire ayant d'excellentes caractéristiques anti-rupture à basses températures et son procédé de fabrication - Google Patents

Matériau en acier à haute résistance pour environnement de région polaire ayant d'excellentes caractéristiques anti-rupture à basses températures et son procédé de fabrication Download PDF

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WO2019124945A1
WO2019124945A1 PCT/KR2018/016155 KR2018016155W WO2019124945A1 WO 2019124945 A1 WO2019124945 A1 WO 2019124945A1 KR 2018016155 W KR2018016155 W KR 2018016155W WO 2019124945 A1 WO2019124945 A1 WO 2019124945A1
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steel
ferrite
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엄경근
이학철
김우겸
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주식회사 포스코
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Priority to EP18893271.9A priority Critical patent/EP3730659A4/fr
Priority to JP2020533253A priority patent/JP7045459B2/ja
Priority to CN201880081799.8A priority patent/CN111492085B/zh
Publication of WO2019124945A1 publication Critical patent/WO2019124945A1/fr

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    • C—CHEMISTRY; METALLURGY
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • 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
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    • 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
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    • 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
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    • 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
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    • 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
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    • 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
    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
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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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a high-strength steel material for polar environment having excellent resistance to breakdown at low temperatures, which can be preferably applied to steel for shipbuilding and marine structure, and a manufacturing method thereof.
  • the reason why high-strength steel with high thickness used for large ships or oil mining platforms is vulnerable to destruction at low temperatures is as follows.
  • the addition amount of alloying elements such as Mn and Mo is inevitably large, and due to the low rolling reduction rate and the slow accelerated cooling rate during the production of the superfine steel material, And a hard phase texture such as MA is likely to be generated.
  • the steel Due to such microstructure, the steel has a characteristic that the resistance to fracture is very weak at low temperature. Therefore, it is necessary to miniaturize the structure and to reduce the hard tissues such as granulabainite and M-A extremely in order to have the high strength of the extreme post material and the fracture property at the excellent low temperature.
  • the structure is finely refined by controlling rolling at a low temperature by reducing the slab reheating temperature to an extreme level; (2) the strength is improved by fine Cu precipitates by tempering at low temperature by adding at least 1% In order to improve low-temperature toughness such as granular bainite, which is a hard phase, a large amount of Ni is added, and (4) minimization of promoting elements such as C is used in order to extremely reduce the M-A structure.
  • low-temperature toughness such as granular bainite, which is a hard phase
  • Ni granular bainite
  • minimization of promoting elements such as C is used in order to extremely reduce the M-A structure.
  • Patent Document 1 Korean Patent Publication No. 2002-0028203
  • the microstructure includes 70% by area or more of polygonal ferrite and needle-like ferrite in total, and has a high strength with excellent fracture toughness at low temperature including 3.5% by area or less of MA phase (martensite-austenite composite phase) Steel. .
  • each element is a value expressed in weight%).
  • Example 1 is a graph showing the Kca value of a steel material in Inventive Example 1 measured in this embodiment.
  • Example 2 is a microstructure photograph of the steel material of Inventive Example 3 in this embodiment.
  • the present inventors have found that the addition amount of alloying elements such as C, Mo, Cr, and Nb, which are alloying elements that generate carbide, It is necessary to precisely control the addition amount of the alloying element, which has the effect of simultaneously improving the strength and toughness of the ferrite base, in the direction of maximizing the increase.
  • the microstructure of the steel material can contain 70% or more of polygonal ferrite and acicular ferrite in total, and can contain 3.5% or less of the MA phase (martensite-austenite composite phase) , Thereby enabling breakthrough initiation and propagation resistance at low temperatures to be dramatically improved.
  • the steel material having excellent resistance against fracture at low temperature is 0.005 to 0.07% of C, 0.005 to 0.3% of Si, 1.7 to 3.0% of Mn, 0.001 to 0.035% of Sol.Al, , Ni: not more than 0.02% (excluding 0%), V: not more than 0.01% (excluding 0%), Ti: 0.001 to 0.02%, Cu: 0.01 to 1.0% 0.001 to 0.5% of Mo, 0.0002 to 0.005% of Ca, 0.001 to 0.008% of N, 0.02% or less of P (excluding 0%), S of 0.003% or less 0.003% or less (excluding 0%), the remainder Fe and unavoidable impurities, and satisfies the above relational expression 1-2.
  • the steel microstructure contains 70% by area or more of polygonal ferrite and acicular ferrite in total, and contains 3.5% by area or less of MA phase (martensite-austenite composite phase).
  • the alloy composition of the steel of the present invention and the reason for limiting the content thereof will be described in detail.
  • the unit of each element content is% by weight.
  • C is an element that plays an important role in promoting the formation of needle-like ferrite or lath bainite and securing strength by generating cementite or pearlite. If the C content is less than 0.01%, the diffusion of C is hardly occurred and the transformation occurs relatively quickly, so that the steel is transformed into a coarse ferrite structure and the strength and toughness of the steel may be deteriorated. On the other hand, when the C content is more than 0.07%, cementite or MA phase is not only excessively formed, but also has a problem in that it is formed to a great extent and can significantly deteriorate the fracture initiation resistance at low temperature. Therefore, it is preferable that the content of C is in the range of 0.01 to 0.07%. The content of C is more preferably 0.01 to 0.06%, and still more preferably 0.01 to 0.05%.
  • Si is an element that is generally added for the purpose of strengthening employment together with deoxidation and desulfurization effect.
  • the effect of increasing the yield and tensile strength is negligible, while the stability of the austenite in the weld heat affected zone is greatly increased and the fraction of the MA phase is increased.
  • the lower limit of the Si content is preferably 0.005%. Therefore, it is preferable that the content of Si is in the range of 0.005 to 0.3%.
  • the Si content is more preferably 0.005 to 0.25%, and still more preferably 0.005 to 0.2%.
  • Mn has a large effect of increasing the strength by solid solution strengthening, and toughness reduction at low temperature is not large. Therefore, Mn is added in an amount of 1.7% or more to ensure a sufficient strength. However, if Mn is added excessively, segregation becomes serious in the thickness direction center of the steel sheet, and at the same time, it promotes the formation of MnS, which is a non-metallic inclusion together with segregated S. The MnS inclusions generated in the center portion are stretched by the subsequent rolling, and the segregation site has a high hardening ability, so that the low-temperature structure of high hardness is easily generated, and as a result, the breakdown initiation and propagation resistance at low temperatures are greatly lowered, Is preferably 3.0%. Therefore, the content of Mn is preferably 1.7 to 3.0%. The content of Mn is more preferably 1.7 to 2.8%.
  • Sol.Al is used as a strong deoxidizer in the steelmaking process together with Si and Mn, and at least 0.005% should be added at the time of single or multiple deoxidation to obtain sufficient effect.
  • the content of Sol.Al is more than 0.035%, the above-mentioned effect is saturated and the fraction of Al 2 O 3 in the oxidative inclusions produced as a result of deoxidation increases more than necessary, the size of the inclusions becomes large, There is a problem that the low temperature toughness of the steel material is greatly reduced.
  • the generation of the MA phase in the weld heat affected zone is promoted, and the breakdown initiation and propagation resistance at low temperatures can be greatly reduced. Therefore, the content of Sol.Al is preferably 0.005 to 0.035%.
  • the content of Sol.Al is more preferably 0.005 to 0.03%, still more preferably 0.005 to 0.02%.
  • Nb 0.02% or less (excluding 0%)
  • Nb is dissolved in the austenite during the reheating of the slab to increase the hardenability of the austenite and precipitates into fine carbonitrides (Nb, Ti) (C, N) during hot rolling to inhibit recrystallization during rolling and cooling, Is a very large element.
  • Nb is added in an excessively large amount, the hardenability of the weld heat affected zone is excessively increased to promote the generation of the MA phase, which significantly lowers destruction initiation and propagation resistance at low temperatures. Therefore, the Nb content in the present invention is 0.02% Excluding 0%).
  • the content of Nb is more preferably 0.015% or less, and still more preferably 0.012% or less.
  • V 0.01% or less (excluding 0%)
  • V is almost completely re-heated at the time of reheating of the slab, and it is mostly precipitated during cooling after rolling to improve strength. In the heat affected zone of welding, it dissolves at high temperature to greatly increase hardenability, thereby promoting the formation of MA phase. Therefore, in the present invention, the content of V is limited to 0.01% or less (excluding 0%). The content of V is more preferably 0.008% or less, and still more preferably 0.005% or less.
  • Ti has an effect of suppressing crystal grain growth of the base material and the weld heat affected zone by forming precipitates of (Ti, Nb) (C, N) precipitates mainly in the form of fine hexagonal TiN type precipitates at high temperatures or by adding them such as Nb .
  • the Ti content is more than 0.02%, coarse carbonitride is produced more than necessary, which acts as a starting point of the fracture crack, which can greatly reduce the impact characteristics of the weld heat affected zone. Therefore, the Ti content is preferably 0.001 to 0.02%.
  • the content of Ti is more preferably 0.001 to 0.017%, and still more preferably 0.001 to 0.015%.
  • Cu is an element capable of significantly improving the strength by solubilization and precipitation without greatly deteriorating breakdown initiation and propagation resistance.
  • the content of Cu is preferably in the range of 0.01 to 1.0%.
  • the content of Cu is more preferably 0.01 to 0.6%, still more preferably 0.01 to 0.4%.
  • Ni has almost no effect of increasing the strength, but is effective in improving fracture initiation and propagation resistance at low temperatures.
  • Cu when added, it has an effect of suppressing surface cracking due to selective oxidation occurring at reheating of the slab.
  • the addition of Ni has the effect of improving the toughness at low temperature even if a coarse hard tissue is produced due to the high temperature and rapid cooling rate of the weld heat affected zone.
  • the content of Ni is in the range of 0.01 to 2.0%.
  • the content of Ni is more preferably 0.2 to 1.8%, still more preferably 0.3 to 1.2%.
  • Cr has a small effect of increasing the yield and tensile strength due to employment, but it has an effect of improving strength and toughness by allowing fine materials to be formed at a slow cooling rate of a post-material because of its high hardenability.
  • the content of Cr is in the range of 0.01 to 0.5%.
  • the content of Cr is more preferably 0.01 to 0.4%, still more preferably 0.01 to 0.25%.
  • Mo has the effect of delaying the phase transformation in the accelerated cooling process and consequently increasing the strength, and is an element having an effect of preventing the deterioration of toughness due to grain boundary segregation of impurities such as P and the like.
  • the Mo content is less than 0.01%, the above-mentioned effect is insufficient.
  • the Mo content exceeds 0.65%, the generation of the MA phase in the weld heat affected zone is accelerated due to the high hardenability, and the breakdown initiation and propagation resistance at low temperature can be greatly reduced. Therefore, it is preferable that the Mo content is in the range of 0.01 to 0.65%.
  • the Mo content is more preferably 0.01 to 0.5%, and still more preferably 0.01 to 0.4%.
  • Ca When Ca is Al-deoxidized and added to molten steel during steelmaking, it is combined with S existing mainly in MnS, thereby suppressing MnS formation and forming spherical CaS, thereby suppressing cracks in the center of the steel. Therefore, Ca should be added in an amount of 0.0002% or more in order to sufficiently form added S in CaS.
  • the upper limit of the Ca content is preferably 0.005%. Therefore, the Ca content is preferably in the range of 0.0002 to 0.005%.
  • the content of Ca is more preferably 0.0005 to 0.003%, still more preferably 0.0005 to 0.0025%.
  • N is an element that forms a precipitate together with added Nb, Ti and Al to improve the strength and toughness of the base material by refining the crystal grains of the steel.
  • Nb titanium, Ti and Al
  • the addition amount of N is limited to the range of 0.001 to 0.006% considering that the Ti content is 0.001 to 0.02%.
  • the content of N is more preferably 0.001 to 0.005%, and still more preferably 0.001 to 0.0045%.
  • P acts to increase the strength, but it is an element that lowers the low temperature toughness. Particularly, there is a problem that low-temperature toughness is largely deviated due to grain boundary segregation in the heat-treated steel. Therefore, it is preferable to control P as low as possible. However, excessively removing P from the steelmaking process is expensive, so it is limited to 0.02% or less.
  • the content of P is more preferably 0.015% or less, and still more preferably 0.012% or less.
  • S is a main cause of MnS inclusions mainly in the thickness direction center of the steel sheet by binding with Mn, thereby lowering the low temperature toughness. Therefore, it is desirable to remove S as much as possible in the steelmaking process in order to secure the deformation aging property at low temperature. However, it may be excessive cost, so it should be limited to less than 0.003%.
  • the content of S is more preferably 0.002% or less, still more preferably 0.0015% or less.
  • O is made into an oxidative inclusion by adding a deoxidizing agent such as Si, Mn, Al in the steel making process. If the amount of the deoxidizing agent and the process for removing inclusions are insufficient, the amount of the oxidative inclusions remaining in the molten steel increases, and the size of the inclusions increases greatly.
  • the coarse oxidative inclusions which have not been removed in this way are then left in a crushed form or spherical form during the rolling process in the steel making process and serve as a starting point of fracture at low temperature or as propagation paths of cracks. Therefore, in order to secure impact characteristics and CTOD characteristics at low temperatures, it is necessary to suppress coarse oxidative inclusions as much as possible and limit the O content to 0.003% or less.
  • the content of O is more preferably 0.0025% or less, still more preferably 0.0022% or less.
  • the remainder of the present invention is iron (Fe).
  • Fe iron
  • elements or impurities which are not intended from the raw material or the surrounding environment in a conventional manufacturing process, since they may be inevitably incorporated.
  • impurities 5 ppm or less of boron (B) or the like.
  • the alloy composition of the present invention is required to contain Mn, Ni, Cu, Cr, and Nb so as to satisfy not only the above-described respective element content but also the following relational expression 1-2.
  • each element is a value expressed in weight%).
  • Mn, Ni, and Cu constituting the above-mentioned relational expression 1 are typical face-centered cubic metals. These elements are elements that not only increase the strength by solid solution strengthening when added to steel materials, but also do not significantly affect toughness even at low temperatures.
  • the inventors of the present invention designed Equation 1 in consideration of the influence of the elements on the steel strength and toughness. As the value of Equation 1 increases, the solid solution strengthening effect increases and the strength of the steel material and weld heat affected portion increases. Therefore, in order to obtain sufficient strength, it is preferable to control the value of the relational expression 1 to 2.5 or more.
  • the value of the relational expression (2) is a formula designed in consideration of the influence of an element promoting the formation of an MA phase, which is a representative structure that largely affects the toughness of the steel material and the weld heat affected zone.
  • the MA phase fraction increases greatly,
  • the ductile-brittle transition temperature which is a low-temperature impact characteristic, increases. That is, as the value of the relational expression 2 increases, the low temperature toughness tends to decrease. Therefore, it is preferable to control the value of the relational expression (2) to 0.5 or less in order to sufficiently secure the low temperature impact property of the steel material, particularly the CTOD value.
  • the SC-HAZ (Sub-Critically Reheated Heat Affected Zone), which is the most important position for assuring the low temperature CTOD value, has a microstructure almost similar to the microstructure of the base material,
  • the value of the relational expression 2 is more preferably 0.48 or less, and still more preferably 0.45 or less.
  • the microstructure of the steel of the present invention contains 70% by area or more of polygonal ferrite and needle-like ferrite in total, and contains 3.5% by area or less of MA phase (martensite-austenite composite phase).
  • the acicular ferrite is the most important and basic microstructure to not only increase the strength due to the fine grain size effect but also to prevent crack propagation at low temperatures. Since polygonal ferrite is relatively large compared to acicular ferrite, it contributes relatively little to the increase in strength, but it has a low dislocation density and high grain boundaries and is a microstructure that contributes greatly to suppressing propagation at low temperatures.
  • the total of the polygonal ferrite and the needle-shaped ferrite is preferably 70% by area or more, more preferably 85% by area or more, still more preferably 90% by area or more.
  • the polygonal ferrite and the needle-shaped ferrite have a ratio of the grain size of the large diameter angle in which the difference in crystal orientation between the crystal grains is defined as not less than 15 degrees is not less than 40% in the total grain boundaries and the length of the large- / mm < 2 > or more.
  • the MA phase does not accept deformation due to its high hardness, it not only concentrates the deformation of the soft ferrite base around it, but also separates the interface with the surrounding ferrite base or destroys the MA phase, Lt; / RTI > Therefore, it is the most important cause for deteriorating the low-temperature fracture characteristics of the steel, so the MA phase should be controlled as low as possible, and it is preferable to control the MA phase to 3.5% or less.
  • the MA phase may have an average size measured at a circle-equivalent diameter of 2.5 mu m or less.
  • the average size of the MA phase is more than 2.5 ⁇ , the MA is more likely to be broken due to more concentrated stress, and acts as a starting point of crack initiation.
  • the polygonal ferrite and the needle-like ferrite may not be work-hardened by hot rolling. That is, the polygonal ferrite and the needle-like ferrite may not be drawn by hot rolling, and the polygonal ferrite and the needle-shaped ferrite may be produced after hot rolling.
  • the microstructure of the steel of the present invention may include bainitic ferrite, cementite, etc. in addition to the polygonal ferrite, acicular ferrite and MA phase described above.
  • bainitic ferrite is a transformed structure at low temperature and has many internal potentials.
  • bainitic ferrite has a relatively stronger characteristic than ferrites and also contains an MA phase therein, so that its strength is high. However, And therefore should be controlled to a minimum.
  • the steel material of the present invention may contain inclusions having a size of 10 m or more in a range of 11 pieces / cm 2 or less.
  • the size is the size measured in the circle equivalent diameter.
  • the inclusions having a size of 10 ⁇ or more are more than 11 pieces / cm 2 , there arises a problem of acting as a crack initiation point at a low temperature.
  • the steel material of the present invention has a yield strength of 460 MPa or more, an impact energy value at -60 ⁇ of 300 J or more, and a CTOD value at -20 ⁇ of 0.2 mm or more.
  • the steel material of the present invention may have a tensile strength of 570 MPa or more.
  • the steel material of the present invention may have a DBTT (ductile-brittle transition temperature) of -80 ⁇ or lower.
  • a method of manufacturing a steel material according to the present invention includes the steps of: preparing a steel slab satisfying the above-described alloy composition; Heating the steel slab to a temperature of 1000 to 1200 ⁇ ; Finishing hot-rolling the heated slab in a temperature range of 650 ° C or higher; And cooling the finish hot-rolled hot-rolled steel sheet to a cooling end temperature of 200 to 550 ° C at a cooling rate of 2 to 30 ° C / s.
  • a steel slab satisfying the alloy composition as described above is provided.
  • the steel slab is heated to 1000 to 1200 ° C.
  • the heating temperature of the slab is less than 1000 ° C., it is difficult to reuse the carbides generated in the slab during the performance, and the homogenization of the segregated elements becomes insufficient. Therefore, it is preferable to heat to at least 1000 ° C, at which 50% or more of the added Nb can be reused.
  • the austenite grain size may grow excessively large, and further fineness may be insufficient due to subsequent rolling, and the mechanical properties such as tensile strength and low temperature toughness of the steel sheet may be greatly reduced .
  • the heating temperature of the steel slab is more preferably 1000 to 1160 DEG C, and still more preferably 1000 to 1140 DEG C.
  • the heated slab is subjected to finish hot rolling at 650 DEG C or higher, which is the bainite formation initiation temperature, to obtain a hot-rolled steel sheet.
  • the finish hot rolling temperature is lower than 650 ° C, coarse bainite is produced and the work hardens during rolling, and the strength is excessively increased excessively.
  • the impact toughness at low temperature is greatly reduced, . That is, when the hot rolling temperature is low, coarse erosion ferrite is produced before the hot rolling finish, and after that, it is stretched by rolling and work hardening is performed, and the remaining austenite remains in a band form, And the low-temperature toughness is lowered.
  • sufficient strain energy is accumulated in the austenite by performing a total reduction ratio of 30% or more (except for the recrystallization reverse reduction ratio) in the non-recrystallization inverse temperature region, so that polygonal and needle- It is preferable to sufficiently generate ferrite and ensure the ratio and density of the large-diameter grain boundaries.
  • the reduction ratio is more preferably 40% or more, and still more preferably 45% or more.
  • the finished hot-rolled steel sheet is cooled.
  • the hot-rolled steel sheet it is preferable to cool the hot-rolled steel sheet to a cooling end temperature of 200 to 550 ° C at a cooling rate of 2 to 30 ° C / s. If the cooling rate is less than 2 DEG C / s, the cooling rate is too slow to coarse the ferrite, pearlite and bainite transformation sections, and the strength and low-temperature toughness may suffer from thermal degradation. Rabynite or martensite is formed and the strength is increased, but the low-temperature toughness may be extremely dull.
  • the cooling end temperature is more preferably 200 to 500 ⁇ ⁇ , and still more preferably 200 to 450 ⁇ ⁇ .
  • the cooled hot-rolled steel sheet is heated to 450 to 650 ° C., maintained for (1.3 ⁇ t + 5) to (1.3 ⁇ t + 200) (Where t is the thickness of the hot-rolled steel sheet measured in mm).
  • MA or martensite is excessively produced, MA or martensite is decomposed to remove the high dislocation density therein, and precipitated Nb or the like, which is a small amount, is solidified with carbonitrides to further improve the yield strength or low temperature toughness It is for this reason.
  • the heating temperature is lower than 450 ⁇ ⁇ , the softening of the ferrite base is not sufficient, and the embrittlement phenomenon due to the P segregation or the like appears, which may deteriorate toughness.
  • the heating temperature is higher than 650 ° C, the recovery and growth of the crystal grains occur rapidly, and when the temperature is higher, the steel is partially transformed into austenite and the yield strength is lowered and the low temperature toughness may be deteriorated.
  • the CTOD value (-20 DEG C) of the weld heat affected zone (SCHAZ) was measured after welding the above-prepared steel material, and the results are shown in Table 3 below.
  • the CTOD value (-20 ° C) of the steel is higher than that of the weld heat affected part, so the CTOD value (-20 ° C) for the steel is not separately measured.
  • the microstructure of the steel material was polished to a specular surface after polishing the cross section of the steel material, and etched with Nital or LePera according to the purpose, and a certain area of the specimen was measured with an optical or scanning electron microscope at a magnification of 100 to 5000 times, The fraction of the phase was measured from the measured image using an image analyzer. In order to obtain a statistically significant value, the same specimen was repeatedly measured by changing its position, and the average value thereof was determined.
  • the Nittal etched specimens were subjected to EBSD (Electron Back Scatter Diffraction) measurement with a scanning electron microscope to quantitatively measure the grain boundary characteristics of the produced steel
  • the physical properties of the steel are described by measuring from the nominal strain-nominal stress curve obtained by ordinary tensile tests.
  • the impact energy value (-60 ° C) of the weld heat affected zone was measured by Charpy V-notch impact test.
  • the CTOD value (-20 ° C) shall be determined by machining the specimen in the size of B (thickness) x B (width) x 5B (length) perpendicular to the rolling direction according to BS 7448 standard and to make the fatigue crack length approximately 50% After the fatigue crack was inserted, the CTOD test was performed at -20 ° C. Where B is the thickness of the steel produced.
  • Kca was tested three times by the ESSO test method, and the graph of the propagation stop temperature and the K value of the crack measured in each test was obtained.
  • the K value was obtained from the K value (Kca: crack arrest K) at the temperature of -10 degrees.
  • the crack arrest temperature (CAT) was measured from the NRL-ductility transition temperature (NDTT) and calculated from the equation (1).
  • B represents the thickness of the steel material.
  • the relation 1 is Mn + 0.5 x (Ni + Cu), and the relation 2 is Mo + Cr + 1.5 x Si + 10 x Nb.
  • the ferrite system means the sum of polygonal ferrite and needle-shaped ferrite.
  • Inventive Examples 1 to 4 which satisfy both the alloy composition and the manufacturing conditions proposed in the present invention, are excellent in fracture at low temperature in consideration of yield strength, tensile strength, impact energy value, Kca and CAT It can be confirmed that the toughness resistance is excellent and the CTOD value in the weld heat affected zone is also high. Particularly, as shown in FIG. 1, the value of Kca measured in Inventive Example 1 shows a value greatly exceeding the required value of 8000. These excellent strengths and low temperature toughness characteristics are also obtained from the fine polygonal and acicular ferrite structures sufficiently generated as shown in Fig.
  • Comparative Example 1 when the C content exceeds the range of the present invention, added C is the most powerful element promoting granulobenite and MA. Therefore, the excessively high C content causes a significant reduction in the fraction of ferrite that is favorable to toughness, so that the strength in the base material is high, but the low temperature toughness such as impact energy value is poor.
  • Comparative Example 2 is a case where the added Mn content exceeds the range of the present invention.
  • the Mn content is high, the probability of segregation at the center of the steel greatly increases, impact energy at the center of the thickness direction of steel is largely damped, and the hardened structure, And the CTOD value dropped significantly due to a pop-in phenomenon.
  • Comparative Example 3 is a case where the content of Nb, which is widely used for strength improvement and texture refinement, generally exceeds the range of the present invention.
  • Nb which is widely used for strength improvement and texture refinement
  • the addition of Nb is advantageous in simultaneously increasing the strength and toughness by refining the structure.
  • the Nb is added more than necessary, the formation of polygonal and needle-like ferrite favorable to toughness is suppressed and the structure such as granular bainite is promoted . Therefore, it is possible to relatively easily propagate cracks by greatly reducing the density and the ratio of the large-diameter grain boundaries of 15 DEG or more, which is advantageous for suppressing propagation of cracks.
  • the strength was sufficiently high, but the impact energy value in the base material and the CTOD value in the weld heat affected zone were inferior.
  • Comparative Example 6 is a case where the equations (1) and (2) are outside the scope of the invention. That is, the component favorable to low-temperature toughness is insufficient, and the component disadvantageous to low-temperature toughness is exceeded, and all the low-temperature toughness properties are heated.
  • Comparative Example 7 is a case in which the components of the steel meet all of the inventions but do not fall within the range of the invention of the non-recrystallized reverse rolling total reduction in the manufacturing process of the steel. That is, the amount of ferrite which interferes with the propagation of cracks in the microstructure of the steel is low due to insufficient amount of reduction in the non-recrystallized zone, and the ratio and density of the large-angle grain boundaries are drastically decreased, .
  • Comparative Example 8 the steel composition satisfies all the ranges of the invention. However, in the case where the steel is manufactured by air cooling without applying accelerated cooling after controlled rolling in the manufacturing process of steel, ferrite Is sufficiently generated, but the strength of the coarsening is greatly lowered.

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Abstract

Cette invention concerne un matériau d'acier à haute résistance pour un environnement de région polaire ayant d'excellentes caractéristiques anti-rupture à basses températures et son procédé de fabrication. Un matériau d'acier à haute résistance ayant d'excellentes caractéristiques anti-rupture à basses températures selon l'invention comprend, en termes de % en poids : 0,005 à 0,07 % de C ; 0,005 à 0,3 % de Si ; 1,7 à 3,0 % de Mn ; 0,001 à 0,035 % d'Al sol. ; 0,02 % ou moins (0 % exclus) de Nb ; 0,01 % ou moins (0 % exclus) de V ; 0,001 à 0,02 % de Ti ; 0,01 à 1,0 % de Cu ; 0,01 à 2,0 % de Ni ; 0,01 à 0,5 % de Cr ; 0,001 à 0,5 % de Mo ; 0,0002 à 0,005 % de Ca ; 0,001 à 0,008 % de N ; 0,02 % ou moins (0 % exclus) de P ; 0,003 % ou moins (0 % exclus) de S ; 0,003 % ou moins (0 % exclus) de O ; le reste du Fe et les inévitables impuretés. Le matériau d'acier à haute résistance ayant d'excellentes caractéristiques anti-rupture à basses températures selon l'invention satisfait la relation 1 et la relation 2, et la microstructure de celui-ci comprend au moins 70 % en surface d'un total de ferrite polygonale et de ferrite aciculaire et comprend 3,5 % en surface ou moins de phase MA (phase composite martensitique-austénitique).
PCT/KR2018/016155 2017-12-22 2018-12-18 Matériau en acier à haute résistance pour environnement de région polaire ayant d'excellentes caractéristiques anti-rupture à basses températures et son procédé de fabrication WO2019124945A1 (fr)

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JP2020533253A JP7045459B2 (ja) 2017-12-22 2018-12-18 低温での耐破壊特性に優れた極地環境用高強度鋼材及びその製造方法
CN201880081799.8A CN111492085B (zh) 2017-12-22 2018-12-18 低温下抗断裂性优异的极地环境用高强度钢材及其制造方法

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EP4039844A4 (fr) * 2019-10-01 2023-09-13 Posco Acier ultra-épais à haute résistance à excellente ténacité aux chocs après vieillissement sous contrainte cryogénique au coeur de celui-ci et son procédé de fabrication

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