WO2016105094A1 - Super duplex stainless steel having excellent yield strength and impact toughness and manufacturing method therefor - Google Patents

Super duplex stainless steel having excellent yield strength and impact toughness and manufacturing method therefor Download PDF

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WO2016105094A1
WO2016105094A1 PCT/KR2015/014114 KR2015014114W WO2016105094A1 WO 2016105094 A1 WO2016105094 A1 WO 2016105094A1 KR 2015014114 W KR2015014114 W KR 2015014114W WO 2016105094 A1 WO2016105094 A1 WO 2016105094A1
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phase
yield strength
stainless steel
minutes
duplex stainless
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PCT/KR2015/014114
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French (fr)
Korean (ko)
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전종진
신동익
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(주)포스코
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Priority to CN201580071309.2A priority Critical patent/CN107109603B/en
Priority to JP2017528546A priority patent/JP2018501403A/en
Priority to US15/536,356 priority patent/US20170327923A1/en
Priority to EP15873621.5A priority patent/EP3239340A4/en
Publication of WO2016105094A1 publication Critical patent/WO2016105094A1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling

Definitions

  • the present invention relates to a super duplex stainless steel and a method of manufacturing the same, and more particularly, to a super duplex stainless steel having excellent yield strength and impact toughness by adjusting a reduction ratio and heat treatment temperature.
  • super duplex stainless steel (UNS S32750) containing 24 to 26% chromium (Cr), 6.0 to 8.0% nickel (Ni), 3.0 to 5.0% molybdenum (Mo) and 0.24 to 0.32% nitrogen (N)
  • Cr chromium
  • Ni nickel
  • Mo molybdenum
  • N nitrogen
  • the matrix structure of the super duplex stainless steel has a structure characteristic in which the ferrite phase and the austenite phase are formed in equal proportions.
  • the super duplex stainless steel has a great strength compared to the austenitic stainless steel, and has a great advantage of pitting corrosion and stress corrosion cracking resistance to chlorine ions.
  • the super duplex stainless steel contains a large amount of chromium (Cr) and molybdenum (Mo) to secure the corrosion resistance, when maintained in the 750 °C to 850 °C section, sigma phase is easily generated, the brittleness is strong, corrosion resistance is significantly reduced This causes problems such as deterioration of quality.
  • the method of avoiding a temperature section in which sigma phase generation is easy by increasing the temperature from 600 ° C. to an annealing temperature at a temperature increase rate of 10 ° C./s or more and maintaining it at 1,060 to 1,080 ° C. It is specifically known in the "continuous annealing method of super duplex stainless steel excellent in coil shape" (Patent Publication 10-2013-0034350).
  • This heat treatment method is equally applicable to not only hot rolled coils of 8 mm or less but also thick plates of 10 mm or more.
  • the above annealing method is mainly applicable to the hot rolled coil having a thickness of 8 mm or less, but the same heat treatment method is applicable to a thick plate having a thickness of 10 mm or more, but is 550 MPa or more over a plate of various thicknesses ranging from 5 mm to 50 mm.
  • the problem of not satisfying the 0.2% Off-Set yield strength frequently occurred.
  • the present invention has been made to solve the conventional problems as described above, super-duplex stainless steel excellent in yield strength and impact toughness improved mechanical properties by controlling the reduction rate and annealing conditions in the production of thick material super duplex stainless steel and It provides a manufacturing method.
  • the super duplex stainless steel having excellent yield strength and impact toughness relates to a thick super duplex stainless steel having a thickness of 30 mm or more, in weight percent of Cr: 24 to 26% and Ni: 6.0. ⁇ 8.0%, Mo: 3.5 ⁇ 5.0%, N: 0.24 ⁇ 0.32%, containing the remaining Fe and inevitable impurities, the microstructure consists of a ferrite phase, austenite phase and secondary austenite phase, Its size is 25 ⁇ m or less.
  • the super duplex stainless steel may be characterized by a yield strength of 550 MPa or more.
  • the super duplex stainless steel may have a sum of yield strength and impact toughness of 750 or more.
  • a method of manufacturing super duplex stainless steel having excellent yield strength and impact toughness is weight percent, Cr: 24 to 26%, Ni: 6.0 to 8.0%, Mo: 3.5 to 5.0%, and N:
  • a casting step of producing a slab comprising 0.24 to 0.32% and comprising remaining Fe and inevitable impurities Hot rolling the slab to produce a thick plate having a thickness of 30 mm or more; Heating the thick plate to an annealing temperature to precipitate a CrN phase inside a ferrite phase, and to precipitate a sigma phase and a secondary austenite phase around the CrN phase; And an annealing step of remaining the secondary austenite phase inside the ferrite phase while solidifying the sigma phase to the ferrite phase.
  • the temperature raising step may be characterized in that to increase the temperature at a rate of 0.11 ⁇ 0.17 °C / s from 700 °C to the annealing temperature.
  • the annealing step may be characterized in that the annealing for 20 to 60 minutes at a temperature of 1020 ⁇ 1060 °C.
  • the hot rolling step may be characterized by rolling at a rolling reduction rate of 80% or more so that the grain size of the microstructure is 25 ⁇ m or less.
  • Figure 2 is a photograph showing the microstructure at 800 °C, 1000 °C, 1040 °C temperature according to the temperature increase rate
  • 3 is a view showing the behavior of the precipitate according to the annealing temperature and the annealing time and its microstructure
  • 5 is a graph showing the relationship between the thick plate thickness (rolling down ratio) and the grain size
  • Figure 6 is a photograph comparing the microstructure of the super duplex stainless steel and the comparative material excellent in yield strength and impact toughness prepared according to an embodiment of the present invention.
  • the super duplex stainless steel having excellent yield strength and impact toughness is, by weight, Cr: 24 to 26%, Ni: 6.0 to 8.0%, Mo: 3.5 to 5.0%, and N: 0.24 to 0.32% and the remaining Fe and inevitable impurities.
  • Chromium (Cr) is a ferrite stabilizing element that not only plays a major role in securing the ferrite phase, but is also an essential element for securing corrosion resistance.
  • Cr chromium
  • the corrosion resistance is increased, but it is added in excess of 26%.
  • austenite-forming elements such as expensive nickel (Ni) is increased to maintain the phase ratio, the manufacturing cost increases.
  • the content of chromium (Cr) is preferably limited to the range of 24 ⁇ 26wt%.
  • Molybdenum (Mo) is a very effective element to improve the corrosion resistance while stabilizing the ferrite together with chromium (Cr), but the disadvantage is very expensive. Therefore, the content of molybdenum (Mo) is preferably limited to 3.5 ⁇ 5.0wt%.
  • Nitrogen (N) is one of the elements in which thickening occurs in the austenite phase during annealing. Increasing may result in increased corrosion resistance and higher strength. However, when the content of nitrogen is excessive, nitrogen may be caused to cause surface defects due to the generation of nitrogen pores during casting due to the excess of N (N) solubility.
  • the content of (N) is preferably limited to the range 0.24 ⁇ 0.32wt%.
  • the super duplex stainless steel having excellent yield strength and impact toughness according to an embodiment of the present invention preferably has a grain size of 25 ⁇ m or less in a microstructure composed of a ferrite phase, an austenite phase, and a secondary austenite phase. .
  • the yield strength may be 550 MPa or more, and the sum of the yield strength and the impact toughness may be 750 or more.
  • the super-duplex stainless steel manufacturing method excellent in yield strength and impact toughness is a casting step for producing a slab by playing the molten steel having the composition and rolling to produce a thick plate by hot rolling the slab It includes a step of heating and an annealing step of heating the thick plate material.
  • the super duplex stainless steel annealing heat treatment having both an austenitic phase and a ferrite phase simultaneously controls the temperature increase rate, the annealing temperature and time, and the reduction rate.
  • Figure 1 is a graph showing the sigma phase and CrN phase formation behavior in the temperature increase rate during annealing
  • Figure 2 is a photograph showing the microstructure at 800 °C, 1000 °C, 1400 °C temperature according to the temperature increase rate.
  • the temperature increase step according to an embodiment of the present invention is preferably heated to an annealing temperature having a temperature range of 700 °C to 1030 ⁇ 1050 °C at a rate of 0.11 ⁇ 0.17 °C / s Do.
  • the CrN phases are finely formed inside the ferrite phase near 800 ° C.
  • the formed CrN phase acts as a nucleation site, so that not only the austenite / ferrite phase interface but also the CrN phase is around.
  • Low sigma phase and secondary austenite phases can be formed to refine the tissue.
  • Figure 4 is a graph showing the yield strength and impact toughness according to the annealing conditions.
  • the annealing step according to an embodiment of the present invention is carried out 20 to 40 minutes at a temperature of 1020 ⁇ 1060 °C, more preferably the annealing step of the present invention annealing according to the annealing temperature It is desirable to apply the time differently.
  • the annealing time is carried out 20 ⁇ 40 minutes, when the annealing temperature is 1020 ⁇ 1030 °C, the annealing time is performed 40 ⁇ 60 minutes, when the annealing temperature is 1050 ⁇ 1060 °C The time is 5 to 20 minutes.
  • the secondary austenite phase can remain in the ferrite phase while solidifying the sigma phase inside the ferrite phase, thereby miniaturizing the tissue, and as the annealing temperature increases, the sigma phase and the secondary Although the austenite phase tends to be dissolved, by shortening the annealing time, the secondary austenite phase remains inside the ferrite phase, thereby making it possible to refine the structure.
  • Figure 5 is a graph showing the relationship between the thickness of the thick plate (rolling down rate) and grain size during thick plate production by rolling a slab of 150 mm
  • Figure 6 is excellent in yield strength and impact toughness manufactured according to an embodiment of the present invention This is a picture comparing the microstructure of super duplex stainless steel and comparative material.
  • the reduction ratio of the slab is 80% or more.
  • the yield strength is lowered to 550 MPa or less, which does not satisfy the ASTM standard.
  • This can be improved through the microstructure control method, but by applying a rolling reduction of 82.5%, the grain size of the microstructure can be formed to 25 ⁇ m or less and the yield strength can be improved.
  • the thickness of the super duplex stainless steel excellent in yield strength and impact toughness according to an embodiment of the present invention may be 30 mm or more. That is, the present invention can be usefully applied to thick materials.
  • the upper limit of the thickness is not particularly limited, and may be, for example, 100 mm, 70 mm or 50 mm.
  • the inventors of the present invention while excellent in the properties of the super duplex steel, while forming a CrN phase during the heat treatment by controlling the temperature rise rate to 0.11 ⁇ 0.17 °C / s or less during annealing to secure yield strength and excellent impact toughness of 580 MPa or more at the same time , Sigma phase and secondary austenite phase were finely precipitated inside the ferrite phase.
  • the annealing was performed for 20 to 60 minutes in the temperature range of 1020 to 1060 ° C., and the second austenite phase was left in the ferrite phase while all the sigma phase was dissolved, thereby simultaneously providing the yield strength and impact characteristics of the thick plate having a thickness of 30 mm or more. Improved.
  • Table 1 shows the thickness (rolling down amount), the temperature increase rate, the annealing temperature and the annealing time of the slab for various examples and comparative examples.
  • Examples and Comparative Examples A-Y steels were heated at a rate of 5 ° C./s to 700 ° C. and heated at a temperature increase rate of 1.3 / s, 0.66 ° C./s, 0.33 ° C./s, and 0.17 ° C./s from 700 ° C. to the annealing temperature.
  • the annealing temperatures were 1000 ° C., 1020 ° C., 1040 ° C., 1060 ° C., and 1080 ° C., and the annealing time was 20 minutes, 40 minutes, and 60 minutes, respectively, followed by water cooling.
  • Table 2 shows the change in the microstructure during the temperature increase process when hot rolling and heat treatment under the conditions described in Table 1.
  • the CrN phase was finely formed inside the ferrite phase in the temperature range of 700 to 800 ° C during the temperature rising process, and the secondary phase was carried out at a temperature range of 1020 to 1060 ° C. It can be seen that the austenite phase remains inside the ferrite phase.
  • P-U steels tend to be similar to K-O steels at a temperature increase rate of 0.17 ° C / s, but as the amount of precipitation of CrN phases increases, the remaining secondary austenite phases also tend to increase.
  • the A-U steel has a reduction ratio of 77% and the grains of the final microstructure are coarsened, the size of which exceeds 25 ⁇ m, which is outside the scope of the present invention.
  • the reduction ratio is 82.5%
  • the temperature increase rate is 0.17 °C / s
  • the annealing temperature is 1020 ⁇ 1060 °C V ⁇ X steel satisfying the embodiment of the present invention is a part of the V steel, X steel ( It can be seen that the CrN phase is properly precipitated in all the V3, X1) and W steels, and the second austenite phase is left inside the ferrite phase in the temperature range of 1020 to 1060 ° C to secure the finest structure.
  • the annealing temperature is 1080 ° C as in the case of T steel secondary austenite phase is found to be out of the scope of the present invention.
  • Table 3 shows the characteristics for the representative steel grades (T, R, W) of Table 2.
  • yield strength was taken from the tensile test specimen of JIS No. 5 in the 90 ° direction of the rolling direction and subjected to a tensile test at a crosshead speed of 20 mm / min at room temperature.
  • R steel has 77% reduction in grain size, and its size is larger than the reference value of 25 ⁇ m.
  • R steel has a yield strength of 536MPa, which is less than the standard value of 550MPa, and the yield strength and impact toughness are also 708MPa. It was found that the yield strength and impact toughness were not improved because the reference value was not reached 750 MPa.
  • the sum of yield strength, yield strength and impact toughness satisfies the reference value, but the rolling reduction is 77%, and the grain size exceeds 25 ⁇ m, which is the reference value.
  • the rolling reduction is 82.5%
  • the annealing temperature, the annealing time and the temperature increase rate satisfy the scope of the present invention, and the grain size is fine to 25 ⁇ m or less
  • the yield strength is 585 MPa
  • the yield strength and impact toughness The sum is 778 MPa, yield strength and impact toughness is improved, it can be confirmed that the mechanical properties compared to the comparative material.

Abstract

The present invention relates to a super duplex stainless steel having excellent yield strength and impact toughness, wherein the reduction ratio and the heat treatment temperature are controlled so as to improve mechanical properties. The super duplex stainless steel having excellent yield strength and impact toughness, according to one embodiment of the present invention, is a thick super duplex stainless steel having a thickness of 300 mm or more, the steel comprising 24-26 wt% of Cr, 6.0-8.0 wt% of Ni, 3.5-5.0 wt% of Mo, and 0.24-0.32 wt% of N, and containing the remainder being Fe and inevitable impurities, and a microstructure comprises a ferrite phase, an austenite phase and a secondary austenite phase, wherein the grain size thereof is 25 µm or less.

Description

항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강 및 그 제조방법Super duplex stainless steel with excellent yield strength and impact toughness and its manufacturing method
본 발명은 슈퍼 듀플렉스 스테인리스강 및 그 제조방법에 관한 것으로서, 보다 상세하게는 압하율 및 열처리 온도를 조절하여 기계적 특성을 향상시킨 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강에 관한 것이다.The present invention relates to a super duplex stainless steel and a method of manufacturing the same, and more particularly, to a super duplex stainless steel having excellent yield strength and impact toughness by adjusting a reduction ratio and heat treatment temperature.
일반적으로, 크롬(Cr) 24 ~ 26%, 니켈(Ni) 6.0 ~ 8.0%, 몰리브덴(Mo) 3.0 ~ 5.0% 및 질소(N) 0.24 ~ 0.32%를 함유한 슈퍼 듀플렉스 스테인리스강(UNS S32750)은 오스테나이트와 페라이트 2상 조직으로 구성되는 2상 스테인리스강으로서 내식성 및 기계적 특성이 매우 우수하여 탈황 설비 및 해수 파이프 등의 소재로 사용되고 있다.Generally, super duplex stainless steel (UNS S32750) containing 24 to 26% chromium (Cr), 6.0 to 8.0% nickel (Ni), 3.0 to 5.0% molybdenum (Mo) and 0.24 to 0.32% nitrogen (N) As a two-phase stainless steel composed of austenite and ferrite two-phase structure, it has excellent corrosion resistance and mechanical properties and is used as a material for desulfurization facilities and seawater pipes.
이러한, 슈퍼 듀플렉스 스테인리스강의 기지조직은 페라이트상과 오스테나이트상이 동등한 비율로 구성된 조직특성을 가진다. 아울러 슈퍼 듀플렉스 스테인리스강은 오스테나이트계 스테인리스강에 비해 강도가 높고, 염소 이온에 대한 공식(Pitting Corrosion) 및 응력부식균열 저항성이 우수하다는 큰 장점을 가지고 있다.The matrix structure of the super duplex stainless steel has a structure characteristic in which the ferrite phase and the austenite phase are formed in equal proportions. In addition, the super duplex stainless steel has a great strength compared to the austenitic stainless steel, and has a great advantage of pitting corrosion and stress corrosion cracking resistance to chlorine ions.
하지만, 슈퍼 듀플렉스 스테인리스강은 내식성 확보를 위해 크롬(Cr)과 몰리브덴(Mo)을 다량 함유하기 때문에 750℃ 내지 850℃ 구간에서 유지 시, 시그마상이 쉽게 생성되어 취성이 강해지며, 내식성이 현저히 감소되는 등 품질을 저하시키는 문제점을 유발한다.However, since the super duplex stainless steel contains a large amount of chromium (Cr) and molybdenum (Mo) to secure the corrosion resistance, when maintained in the 750 ℃ to 850 ℃ section, sigma phase is easily generated, the brittleness is strong, corrosion resistance is significantly reduced This causes problems such as deterioration of quality.
이러한 시그마상은 특정 온도구간(750 ~ 850℃)에서 매우 빠르게 생성되기 때문에 슈퍼 듀플렉스 스테인리스강 소둔시 승온 속도를 제어하여 시그마상 생성이 용이한 특정 온도구간에 정체되는 것을 피해야한다.Since the sigma phase is generated very quickly in a certain temperature range (750 ~ 850 ℃), it is necessary to control the temperature rise rate during annealing super duplex stainless steel to avoid stagnation in a specific temperature range that is easy to generate sigma phase.
종래 이러한 문제점을 해결하기 위해 10℃/s 이상의 승온 속도로 600℃부터 소둔 온도까지 승온시켜, 1,060 ~1,080℃로 유지함으로써, 시그마상 생성이 용이한 온도구간을 회피하는 방법에 대해서는 "충격인성 및 코일 형상이 우수한 슈퍼 듀플렉스 스테인리스강의 연속소둔방법(공개특허 10-2013-0034350)" 등에서 구체적으로 공지되어 있다.In order to solve such a problem in the related art, the method of avoiding a temperature section in which sigma phase generation is easy by increasing the temperature from 600 ° C. to an annealing temperature at a temperature increase rate of 10 ° C./s or more and maintaining it at 1,060 to 1,080 ° C. It is specifically known in the "continuous annealing method of super duplex stainless steel excellent in coil shape" (Patent Publication 10-2013-0034350).
이러한 열처리 방법은 8mm 이하의 열연 코일 뿐만 아니라 10 mm 이상의 후판에도 동일하게 적용될 수 있으나, This heat treatment method is equally applicable to not only hot rolled coils of 8 mm or less but also thick plates of 10 mm or more.
상기의 소둔방법은 두께 8㎜ 이하의 열연코일의 경우에 주로 해당되나, 두께 10㎜ 이상의 후판에서도 동일한 열처리 방법이 적용가능하나, 5 mm에서 50 mm에 이르기까지 다양한 두께의 판재에 걸쳐 550 MPa 이상의 0.2% 오프셋(Off-Set) 항복강도를 만족시지 못하는 현상이 빈번하게 발생되는 문제점을 가지고 있었다.The above annealing method is mainly applicable to the hot rolled coil having a thickness of 8 mm or less, but the same heat treatment method is applicable to a thick plate having a thickness of 10 mm or more, but is 550 MPa or more over a plate of various thicknesses ranging from 5 mm to 50 mm. The problem of not satisfying the 0.2% Off-Set yield strength frequently occurred.
본 발명은 상기와 같은 종래의 문제점을 해결하기 위해 안출된 것으로, 후물재 슈퍼 듀플렉스 스테인리스강 제조시, 압하율 및 소둔 조건을 제어하여 기계적 특성이 향상된 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강 및 그 제조방법을 제공한다.The present invention has been made to solve the conventional problems as described above, super-duplex stainless steel excellent in yield strength and impact toughness improved mechanical properties by controlling the reduction rate and annealing conditions in the production of thick material super duplex stainless steel and It provides a manufacturing method.
본 발명의 일 실시예에 따른, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강은 30㎜ 이상의 두께를 갖는 후물 슈퍼 듀플렉스 스테인리스강에 관한 것으로서, 중량%로, Cr: 24 ~ 26%, Ni: 6.0 ~ 8.0%, Mo: 3.5 ~ 5.0%, N: 0.24 ~ 0.32% 를 포함하고, 나머지 Fe 및 불가피한 불순물을 포함하며, 미세조직이 페라이트상, 오스테나이트상 및 2차 오스테나이트상으로 구성되되, 결정립 크기가 25㎛ 이하인 것을 특징으로 한다.According to an embodiment of the present invention, the super duplex stainless steel having excellent yield strength and impact toughness relates to a thick super duplex stainless steel having a thickness of 30 mm or more, in weight percent of Cr: 24 to 26% and Ni: 6.0. ~ 8.0%, Mo: 3.5 ~ 5.0%, N: 0.24 ~ 0.32%, containing the remaining Fe and inevitable impurities, the microstructure consists of a ferrite phase, austenite phase and secondary austenite phase, Its size is 25 µm or less.
상기 슈퍼 듀플렉스 스테인리스강은, 항복강도가 550 MPa 이상인 것을 특징으로 할 수 있다.The super duplex stainless steel may be characterized by a yield strength of 550 MPa or more.
상기 슈퍼 듀플렉스 스테인리스강은, 항복강도와 충격인성의 합이 750 이상인 것을 특징으로 할 수 있다.The super duplex stainless steel may have a sum of yield strength and impact toughness of 750 or more.
본 발명의 일 실시예에 따른, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강 제조방법은 중량%로, Cr: 24 ~ 26%, Ni: 6.0 ~ 8.0%, Mo: 3.5 ~ 5.0%, N: 0.24 ~ 0.32% 를 포함하고, 나머지 Fe 및 불가피한 불순물을 포함하는 슬라브를 제조하는 주조 단계; 상기 슬라브를 열간에서 압연하여 30㎜ 이상의 두께를 갖는 후판재를 생산하는 열간압연 단계; 상기 후판재를 소둔 온도까지 승온하여 페라이트상 내부에 CrN상을 석출시키고, 상기 CrN상 주위에 시그마상 및 2차 오스테나이트상을 석출시키는 승온 단계; 및 상기 시그마상을 상기 페라이트상에 고용시키면서, 상기 2차 오스테나이트상을 상기 페라이트상 내부에 잔류시키는 소둔 단계;를 포함한다.According to an embodiment of the present invention, a method of manufacturing super duplex stainless steel having excellent yield strength and impact toughness is weight percent, Cr: 24 to 26%, Ni: 6.0 to 8.0%, Mo: 3.5 to 5.0%, and N: A casting step of producing a slab comprising 0.24 to 0.32% and comprising remaining Fe and inevitable impurities; Hot rolling the slab to produce a thick plate having a thickness of 30 mm or more; Heating the thick plate to an annealing temperature to precipitate a CrN phase inside a ferrite phase, and to precipitate a sigma phase and a secondary austenite phase around the CrN phase; And an annealing step of remaining the secondary austenite phase inside the ferrite phase while solidifying the sigma phase to the ferrite phase.
상기 승온 단계는, 700℃ 부터 상기 소둔 온도까지 0.11 ~ 0.17 ℃/s의 속도로 승온시키는 것을 특징으로 할 수 있다.The temperature raising step may be characterized in that to increase the temperature at a rate of 0.11 ~ 0.17 ℃ / s from 700 ℃ to the annealing temperature.
상기 소둔 단계는, 1020 ~ 1060℃의 온도로 20 ~ 60 분간 소둔하는 것을 특징으로 할 수 있다.The annealing step may be characterized in that the annealing for 20 to 60 minutes at a temperature of 1020 ~ 1060 ℃.
상기 열간압연 단계는, 미세조직의 결정립 크기가 25㎛이하가 되도록, 80% 이상의 압하율로 압연하는 것을 특징으로 할 수 있다.The hot rolling step may be characterized by rolling at a rolling reduction rate of 80% or more so that the grain size of the microstructure is 25 μm or less.
본 발명의 실시예에 따르면, CrN상의 석출을 유도하여 페라이트상 내부에 2차 오스테나이트상의 형성을 촉진시켜 후물재 슈퍼 듀플렉스 스테인리스강의 항복강도 및 충격인성 등 기계적 특성을 향상시킬 수 있는 효과가 있다.According to the embodiment of the present invention, by inducing the precipitation of the CrN phase to promote the formation of the secondary austenite phase inside the ferrite phase has the effect of improving the mechanical properties such as yield strength and impact toughness of the material super duplex stainless steel.
도 1은 소둔시 승온 속도에 다른 시그마상 및 CrN상의 생성 거동을 보여주는 그래프이고,1 is a graph showing the sigma phase and CrN phase formation behavior in the temperature increase rate during annealing,
도 2는 승온 속도에 따른 800℃, 1000℃, 1040℃ 온도에서 미세조직을 보여주는 사진이며,Figure 2 is a photograph showing the microstructure at 800 ℃, 1000 ℃, 1040 ℃ temperature according to the temperature increase rate,
도 3은 소둔 온도 및 소둔 시간에 따른 석출물의 거동과 그 미세조직을 보여주는 도면이고,3 is a view showing the behavior of the precipitate according to the annealing temperature and the annealing time and its microstructure,
도 4는 소둔 조건에 따른 항복강도 및 충격인성을 나타낸 그래프이며,4 is a graph showing the yield strength and impact toughness according to the annealing conditions,
도 5는 후판 두께(압하율)와 결정립 크기의 관계를 나타낸 그래프이고,5 is a graph showing the relationship between the thick plate thickness (rolling down ratio) and the grain size,
도 6은 본 발명의 일 실시예에 따라 제조된 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강과 비교재의 미세조직을 비교한 사진이다.Figure 6 is a photograph comparing the microstructure of the super duplex stainless steel and the comparative material excellent in yield strength and impact toughness prepared according to an embodiment of the present invention.
이하 첨부된 도면들을 참조하여 본 발명의 바람직한 실시예를 상세하게 설명하지만, 본 발명이 실시예에 의해 제한되거나 한정되는 것은 아니다. 참고로, 본 발명을 설명함에 있어서, 관련된 공지 기술에 대한 구체적인 설명이 본 발명의 요지를 불필요하게 흐릴 수 있다고 판단되거나, 당업자에게 자명하다고 판단되는 내용은 생략될 수 있다.Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, but the present invention is not limited or limited by the embodiments. For reference, in describing the present invention, it may be determined that the detailed description of the related well-known technology may unnecessarily obscure the subject matter of the present invention, or the contents determined to be obvious to those skilled in the art may be omitted.
본 발명의 일 실시예에 따른, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강은 중량%로, Cr: 24 ~ 26%, Ni: 6.0 ~ 8.0%, Mo: 3.5 ~ 5.0%, N: 0.24 ~ 0.32% 를 포함하고, 나머지 Fe 및 불가피한 불순물을 포함한다.According to an embodiment of the present invention, the super duplex stainless steel having excellent yield strength and impact toughness is, by weight, Cr: 24 to 26%, Ni: 6.0 to 8.0%, Mo: 3.5 to 5.0%, and N: 0.24 to 0.32% and the remaining Fe and inevitable impurities.
이하, 본 발명에 실시예에 따른, 성분 함량의 수치 한정 이유에 대하여 설명하기로 한다.Hereinafter, the reason for numerical limitation of the component content according to the embodiment of the present invention will be described.
Cr: 24 ~ 26wt%Cr: 24 ~ 26wt%
크롬(Cr)은 페라이트 안정화 원소로 페라이트상 확보에 주된 역할을 할 뿐만 아니라, 내식성 확보를 위한 필수 원소로서, 크롬(Cr)의 함량이 증가되면 내식성이 증가되나, 26%를 초과하여 과량 첨가되는 경우, 상분율 유지를 위해 고가의 니켈(Ni) 등 오스테나이트 형성원소의 함량을 증가됨에 따라 제조원가가 상승시킨다.Chromium (Cr) is a ferrite stabilizing element that not only plays a major role in securing the ferrite phase, but is also an essential element for securing corrosion resistance. When the content of chromium (Cr) is increased, the corrosion resistance is increased, but it is added in excess of 26%. In this case, as the content of austenite-forming elements such as expensive nickel (Ni) is increased to maintain the phase ratio, the manufacturing cost increases.
따라서, 크롬(Cr)의 함량은 24 ~ 26wt% 범위로 한정하는 것이 바람직하다.Therefore, the content of chromium (Cr) is preferably limited to the range of 24 ~ 26wt%.
Ni: 6.0 ~ 8.0wt% Ni: 6.0-8.0 wt%
니켈(Ni)은 망간(Mn), 구리(Cu) 및 질소(N)와 함께 오스테나이트 안정화 원소로, 오스테나이트상의 안정도 증대에 주된 역할한다. 따라서, 페라이트상과 오스테나이트상의 상분율을 유지하기 위해 그 함량을 6.0 ~ 8.0wt%로 제한한다.Nickel (Ni), together with manganese (Mn), copper (Cu) and nitrogen (N), is an austenite stabilizing element and plays a major role in increasing the stability of the austenite phase. Therefore, in order to maintain the phase fractions of the ferrite phase and the austenite phase, the content is limited to 6.0 to 8.0 wt%.
Mo: 3.5 ~ 5.0wt%Mo: 3.5 ~ 5.0wt%
몰리브덴(Mo)은 크롬(Cr)과 함께 페라이트를 안정화 하면서 내식성 개선에 매우 유효한 원소이나, 가격이 매우 비싼 단점이 있다. 따라서, 몰리브덴(Mo)의 함량은 3.5 ~ 5.0wt%로 한정하는 것이 바람직하다.Molybdenum (Mo) is a very effective element to improve the corrosion resistance while stabilizing the ferrite together with chromium (Cr), but the disadvantage is very expensive. Therefore, the content of molybdenum (Mo) is preferably limited to 3.5 ~ 5.0wt%.
N: 0.24 ~ 0.32wt%N: 0.24-0.32 wt%
질소(N)는 탄소(C), 니켈(Ni)과 함께 오스테나이트 상의 안정화에 크게 기여하는 원소로, 소둔 열처리 시 오스테나이트 상에 농화가 발생하는 원소 중의 하나로서, 질소(N)의 함량을 증가시키면 부수적으로 내식성 증가 및 고강도화를 꾀할 수 있으나, 질소(N)의 함량이 과도한 경우, 질소(N) 고용도 초과에 의한 주조 시 질소 포어(pore) 발생에 의한 표면 결함을 유발할 수 있기 때문에 질소(N)의 함량은 0.24 ~ 0.32wt% 범위로 한정하는 것이 바람직하다.Nitrogen (N), together with carbon (C) and nickel (Ni), contributes greatly to stabilization of the austenite phase. Nitrogen (N) is one of the elements in which thickening occurs in the austenite phase during annealing. Increasing may result in increased corrosion resistance and higher strength. However, when the content of nitrogen is excessive, nitrogen may be caused to cause surface defects due to the generation of nitrogen pores during casting due to the excess of N (N) solubility. The content of (N) is preferably limited to the range 0.24 ~ 0.32wt%.
본 발명의 일 실시예에 따른 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강은 페라이트상과 오스테나이트상 및 2차 오스테나이트상으로 구성되는 미세조직의 결정립 크기가 25㎛이하로 형성되는 것이 바람직하다.The super duplex stainless steel having excellent yield strength and impact toughness according to an embodiment of the present invention preferably has a grain size of 25 μm or less in a microstructure composed of a ferrite phase, an austenite phase, and a secondary austenite phase. .
또한, 항복강도는 550MPa이상이고, 항복강도와 충격인성의 합은 750이상인 것을 특징으로 할 수 있다.In addition, the yield strength may be 550 MPa or more, and the sum of the yield strength and the impact toughness may be 750 or more.
한편, 본 발명의 일 실시예에 따른 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강 제조방법은 상기의 조성을 갖는 용강을 연주하여 슬라브를 제조하는 주조단계와 슬라브를 열간압연하여 후판재를 생산하는 압연단계와 후판재를 가열하는 승온단계 및 소둔단계를 포함한다.On the other hand, the super-duplex stainless steel manufacturing method excellent in yield strength and impact toughness according to an embodiment of the present invention is a casting step for producing a slab by playing the molten steel having the composition and rolling to produce a thick plate by hot rolling the slab It includes a step of heating and an annealing step of heating the thick plate material.
본 발명에서 미세조직을 제어하기 위해 오스테나이트상 및 페라이트상을 동시에 갖는 슈퍼 듀플렉스 스테인리스강 소둔 열처리시, 승온속도, 소둔 온도 및 시간, 압하율을 제어하게 되며, 보다 구체적으로 승온 단계에서 승온속도를 제어하여 승온 도중 CrN상의 석출을 유도한 다음 CrN상 주위로 시그마상 및 2차 오스테나이트상의 석출을 유도하고, 소둔 단계에서 소둔 온도 및 시간을 제어하여 승온 단계에서 석출된 시그마상을 페라이트상 내부에 고용시키면서,2차 오스테나이트상을 페라이트상 내부에 잔류시킨다.In the present invention, in order to control the microstructure, the super duplex stainless steel annealing heat treatment having both an austenitic phase and a ferrite phase simultaneously controls the temperature increase rate, the annealing temperature and time, and the reduction rate. Control to induce the precipitation of CrN phase during the temperature increase, and then induce precipitation of the sigma phase and the secondary austenite phase around the CrN phase, and control the annealing temperature and time in the annealing step to the inside of the ferrite phase While solid solution, the secondary austenite phase remains inside the ferrite phase.
도 1은 소둔시 승온 속도에 다른 시그마상 및 CrN상의 생성 거동을 보여주는 그래프이고, 도 2는 승온 속도에 따른 800℃, 1000℃, 1400℃ 온도에서 미세조직을 보여주는 사진이다.1 is a graph showing the sigma phase and CrN phase formation behavior in the temperature increase rate during annealing, Figure 2 is a photograph showing the microstructure at 800 ℃, 1000 ℃, 1400 ℃ temperature according to the temperature increase rate.
도 1 내지 도 2에 도시된 바와 같이, 본 발명의 일 실시예에 따른 승온단계는 700℃ 부터 1030 ~ 1050℃의 온도범위를 갖는 소둔 온도까지 0.11 ~ 0.17℃/s의 속도로 승온시키는 것이 바람직하다.1 to 2, the temperature increase step according to an embodiment of the present invention is preferably heated to an annealing temperature having a temperature range of 700 ℃ to 1030 ~ 1050 ℃ at a rate of 0.11 ~ 0.17 ℃ / s Do.
이에, 페라이트상 내부에 CrN상을 미세하게 석출시키면서 동시에 CrN상 주위로 시그마 상을 형성시킬 수 잇기 때문이다. This is because it is possible to form a sigma phase around the CrN phase while simultaneously depositing the CrN phase in the ferrite phase.
즉, 승온속도가 0.17℃/s를 초과하는 경우, 800℃ 근처에서 페라이트상 내부에 CrN상들이 형성되지 못하며, 900 ~ 1000℃로 온도가 상승하더라도 안정성인 시그마상 및 2차 오스테나이트상은 페라이트상 과 오스테나이트상 계면에서 형성되여 조직의 미세화 효과를 얻을 수 없다.In other words, if the temperature increase rate exceeds 0.17 ℃ / s, CrN phases are not formed inside the ferrite phase near 800 ℃, even if the temperature rises to 900 ~ 1000 ℃ stable sigma phase and secondary austenite phase is a ferrite phase It is formed at the austenite phase interface with the micronized effect of the tissue.
반면, 승온속도가 0.17℃/s이하 인 경우, 800℃ 근처에서 페라이트상 내부에 CrN상들이 미세하게 형성되며, 이때 형성된 CrN상이 핵생성 사이트로 작용하여 오스테나이트/페라이트상 계면 뿐만 아니라 CrN상 주위로 시그마상 및 2차 오스테나이트상들이 형성되어, 조직을 미세화할 수 있다.On the other hand, when the temperature increase rate is 0.17 ° C / s or less, the CrN phases are finely formed inside the ferrite phase near 800 ° C. At this time, the formed CrN phase acts as a nucleation site, so that not only the austenite / ferrite phase interface but also the CrN phase is around. Low sigma phase and secondary austenite phases can be formed to refine the tissue.
도 3은 소둔 온도 및 소둔 시간에 따른 석출물의 거동과 그 미세조직을 보여주는 도면이고, 도 4는 소둔 조건에 따른 항복강도 및 충격인성을 나타낸 그래프이다.3 is a view showing the behavior of the precipitate according to the annealing temperature and the annealing time and its microstructure, Figure 4 is a graph showing the yield strength and impact toughness according to the annealing conditions.
도 3 및 도 4에 도시된 바와 같이, 본 발명의 일 실시예에 따른 소둔단계는 1020 ~ 1060℃의 온도로 20 ~ 40분 실시하며, 보다 바람직하게 본 발명의 소둔 단계는 소둔 온도에 따라 소둔 시간을 다르게 적용하는 것이 바람직하다.As shown in Figure 3 and 4, the annealing step according to an embodiment of the present invention is carried out 20 to 40 minutes at a temperature of 1020 ~ 1060 ℃, more preferably the annealing step of the present invention annealing according to the annealing temperature It is desirable to apply the time differently.
소둔 온도가 1030 ~ 1050℃ 인 경우 소둔 시간은 20 ~ 40분을 실시하고, 소둔 온도가 1020 ~ 1030℃ 인 경우 소둔 시간은 40 ~ 60분을 실시하며, 소둔 온도가 1050 ~ 1060℃ 인 경우 소둔 시간은 5~ 20분을 실시한다.When the annealing temperature is 1030 ~ 1050 ℃, the annealing time is carried out 20 ~ 40 minutes, when the annealing temperature is 1020 ~ 1030 ℃, the annealing time is performed 40 ~ 60 minutes, when the annealing temperature is 1050 ~ 1060 ℃ The time is 5 to 20 minutes.
이에, 온도가 낮더라도 소둔시간을 증가시킴으로써, 페라이트상 내부에 시그마상을 고용시키면서 2차 오스테나이트상을 페라이트상 내에 잔류시켜 조직을 미세화시킬 수 있으며, 소둔 온도가 높아짐에 따라 시그마상 및 2차 오스테나이트상이 고용되려는 경향이 강하나 소둔시간을 짧게함으로써, 페라이트상 내부에 2차 오스테나이트상을 잔류시킴으로써 조직을 미세화할 수 있는 효과가 있다.Thus, by increasing the annealing time even at a low temperature, the secondary austenite phase can remain in the ferrite phase while solidifying the sigma phase inside the ferrite phase, thereby miniaturizing the tissue, and as the annealing temperature increases, the sigma phase and the secondary Although the austenite phase tends to be dissolved, by shortening the annealing time, the secondary austenite phase remains inside the ferrite phase, thereby making it possible to refine the structure.
도 5는 150㎜의 슬라브를 압연하여 후판 생산시, 후판 두께(압하율)와 결정립 크기의 관계를 나타낸 그래프이고, 도 6은 본 발명의 일 실시예에 따라 제조된 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강과 비교재의 미세조직을 비교한 사진이다.Figure 5 is a graph showing the relationship between the thickness of the thick plate (rolling down rate) and grain size during thick plate production by rolling a slab of 150 mm, Figure 6 is excellent in yield strength and impact toughness manufactured according to an embodiment of the present invention This is a picture comparing the microstructure of super duplex stainless steel and comparative material.
본 발명의 일 실시예에 따른 열간압연 단계는 슬라브의 압하율이 80% 이상인 것이 바람직하다. In the hot rolling step according to an embodiment of the present invention, it is preferable that the reduction ratio of the slab is 80% or more.
도 5 및 도 6에 도시된 바와 같이, 150㎜ 두께의 슬라브를 10 ~ 35㎜의 두께를 갖는 후판으로 압연하는 경우 후판의 두께가 두꺼워 질수록 결정립 크기가 증가됨을 알 수 있다.As shown in FIG. 5 and FIG. 6, it can be seen that when the slab having a thickness of 150 mm is rolled into a thick plate having a thickness of 10 to 35 mm, the grain size increases as the thickness of the thick plate increases.
이에, 30㎜ 이상의 두께를 갖는 후물재의 경우, 항복강도가 550MPa 이하로 저하되어 ASTM 규격을 만족시키지 못한다. 이는 미세조직 제어 방법을 통해 개선 가능하나 압하량 82.5%를 적용함으로써, 미세조직의 결정립 크기를 25㎛ 이하로 형성함과 동시에 항복강도를 향상시킬 수 있다Accordingly, in the case of a thick material having a thickness of 30 mm or more, the yield strength is lowered to 550 MPa or less, which does not satisfy the ASTM standard. This can be improved through the microstructure control method, but by applying a rolling reduction of 82.5%, the grain size of the microstructure can be formed to 25 µm or less and the yield strength can be improved.
본 발명의 일 실시예에 따른 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강의 두께는 30 mm 이상일 수 있다. 즉, 본 발명은 후물재에 유용하게 적용될 수 있다. 두께의 상한치는 특별히 제한되지 않고, 예를 들어 100 mm, 70 mm 또는 50 mm일 수 있다.The thickness of the super duplex stainless steel excellent in yield strength and impact toughness according to an embodiment of the present invention may be 30 mm or more. That is, the present invention can be usefully applied to thick materials. The upper limit of the thickness is not particularly limited, and may be, for example, 100 mm, 70 mm or 50 mm.
이하, 실시예를 이용하여 본 발명의 일 실시예에 따른 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스강의 조직제어 방법에 관하여 상세히 설명한다.Hereinafter, the structure control method of the super duplex steel excellent in yield strength and impact toughness according to an embodiment of the present invention by using an embodiment will be described in detail.
본 발명자는 슈퍼 듀플렉스 강의 제반 성질이 우수하면서, 580 MPa 이상의 항복강도 및 우수한 충격 인성을 동시에 확보하기 소둔시 승온 속도를 0.11 ~ 0.17℃/s 이하로 제어함에 의하여, 열처리 도중 CrN상을 형성시킨 후, 시그마상 및 2차 오스테나이트상이 페라이트상 내부에 미세하게 석출시켰다.The inventors of the present invention, while excellent in the properties of the super duplex steel, while forming a CrN phase during the heat treatment by controlling the temperature rise rate to 0.11 ~ 0.17 ℃ / s or less during annealing to secure yield strength and excellent impact toughness of 580 MPa or more at the same time , Sigma phase and secondary austenite phase were finely precipitated inside the ferrite phase.
그리고, 1020 ~ 1060℃ 온도범위에서 20 ~ 60 분간 소둔을 실시하여 2차 오스테나이트상을 페라이트상 내부에 잔류시키면서 시그마상은 전량 고용시킴으로서, 30㎜ 이상의 두께를 가는 후판의 항복강도 및 충격특성을 동시에 개선하였다.In addition, the annealing was performed for 20 to 60 minutes in the temperature range of 1020 to 1060 ° C., and the second austenite phase was left in the ferrite phase while all the sigma phase was dissolved, thereby simultaneously providing the yield strength and impact characteristics of the thick plate having a thickness of 30 mm or more. Improved.
구분division 공정변수 Process variables 비고 Remarks
슬라브 두께(압하량) Slab thickness (load reduction) 승온속도 (℃/s)Temperature rise rate (℃ / s) 소둔온도 (℃)Annealing Temperature (℃) 소둔시간 (min)Annealing time (min)
A A 77%77% 1.31.3 10001000 20분/40분/60분20 minutes / 40 minutes / 60 minutes 비교예Comparative example
B B 77%77% 1.31.3 10201020 20분/40분/60분20 minutes / 40 minutes / 60 minutes
C C 77%77% 1.31.3 10401040 20분/40분/60분20 minutes / 40 minutes / 60 minutes
D D 77%77% 1.31.3 10601060 20분/40분/60분20 minutes / 40 minutes / 60 minutes
E E 77%77% 1.31.3 10801080 20분/40분/60분20 minutes / 40 minutes / 60 minutes
F F 77%77% 0.660.66 10001000 20분/40분/60분20 minutes / 40 minutes / 60 minutes
G G 77%77% 0.660.66 10201020 20분/40분/60분20 minutes / 40 minutes / 60 minutes
H H 77%77% 0.660.66 10401040 20분/40분/60분20 minutes / 40 minutes / 60 minutes
I I 77%77% 0.660.66 10601060 20분/40분/60분20 minutes / 40 minutes / 60 minutes
J J 77%77% 0.660.66 10801080 20분/40분/60분20 minutes / 40 minutes / 60 minutes
K K 77%77% 0.330.33 10001000 20분/40분/60분20 minutes / 40 minutes / 60 minutes
L L 77%77% 0.330.33 10201020 20분/40분/60분20 minutes / 40 minutes / 60 minutes
M M 77%77% 0.330.33 10401040 20분/40분/60분20 minutes / 40 minutes / 60 minutes
N N 77%77% 0.330.33 10601060 20분/40분/60분20 minutes / 40 minutes / 60 minutes
O O 77%77% 0.330.33 10801080 20분/40분/60분20 minutes / 40 minutes / 60 minutes
P P 77%77% 0.170.17 10001000 20분/40분/60분20 minutes / 40 minutes / 60 minutes
Q Q 77%77% 0.170.17 10201020 20분/40분/60분20 minutes / 40 minutes / 60 minutes
R R 77%77% 0.170.17 10401040 20분/40분/60분20 minutes / 40 minutes / 60 minutes
S S 77%77% 0.170.17 10601060 20분/40분/60분20 minutes / 40 minutes / 60 minutes
T T 77%77% 0.170.17 10801080 20분/40분/60분20 minutes / 40 minutes / 60 minutes
U U 82.50%82.50% 0.170.17 10001000 20분/40분/60분20 minutes / 40 minutes / 60 minutes
VV V1V1 82.50%82.50% 0.170.17 10201020 20분20 minutes
V2V2 40분40 minutes
V3V3 60분60 minutes 실시예Example
W W 82.50%82.50% 0.170.17 10401040 20분/40분/60분20 minutes / 40 minutes / 60 minutes
XX X1X1 82.50%82.50% 0.170.17 10601060 20분20 minutes
X2X2 40분40 minutes 비교예Comparative example
X3X3 60분60 minutes
Y Y 82.50%82.50% 0.170.17 10801080 20분/40분/60분20 minutes / 40 minutes / 60 minutes
표 1은 다양한 실시예 및 비교예에 대하여 슬라브의 두께(압하량)과 승온 속도, 소둔 온도 및 소둔 시간을 나타내었다.Table 1 shows the thickness (rolling down amount), the temperature increase rate, the annealing temperature and the annealing time of the slab for various examples and comparative examples.
실시예 및 비교예인 A~Y 강은 700℃까지 5℃/s 속도로 가열하고 700℃부터 소둔 온도까지 1.3/s, 0.66℃/s, 0.33℃/s, 0.17℃/s의 승온속도로 가열시켰으며, 소둔 온도는 1000℃, 1020℃, 1040℃, 1060℃, 1080℃이고, 소둔 시간은 각각 20분, 40분, 60분으로 열처리 후 수냉을 실시하였다.Examples and Comparative Examples A-Y steels were heated at a rate of 5 ° C./s to 700 ° C. and heated at a temperature increase rate of 1.3 / s, 0.66 ° C./s, 0.33 ° C./s, and 0.17 ° C./s from 700 ° C. to the annealing temperature. The annealing temperatures were 1000 ° C., 1020 ° C., 1040 ° C., 1060 ° C., and 1080 ° C., and the annealing time was 20 minutes, 40 minutes, and 60 minutes, respectively, followed by water cooling.
구분division 압하량(%) Rolling amount (%) 승온속도(℃/s) Temperature increase rate (℃ / s) 소둔온도(℃) Annealing Temperature (℃) 소둔시간(min) Annealing time (min) CrN상CrN phase 2차 오스테나이트상Second austenite phase 결정립 평균 크기Grain Average Size 비고Remarks
A~JA ~ J 7777 1.3~0.66 1.3 ~ 0.66 1000~1080 1000-1080 20~60 20-60 X X X X 4141 비교예Comparative example
K~N K ~ N 7777 0.330.33 1000~1060 1000-1060 20~60 20-60 O O O O 3333
O O 7777 0.330.33 10801080 20~60 20-60 O O X X 3838
P~S P ~ S 7777 0.170.17 1000~1060 1000-1060 20~60 20-60 O O O O 3030
T~U T ~ U 7777 0.170.17 10801080 20~60 20-60 O O X X 3636
V~X V to X 82.582.5 0.170.17 1000~1060 1000-1060 20~60 20-60 O O O O 2222 실시예 Example
YY 82.582.5 0.170.17 10801080 20~60 20-60 O O X X 2626 비교예 Comparative example
표 2는 표 1에 기재된 조건으로 열간 압연 및 열처리를 실시한 경우 승온 과정에서 나타난 미세조직의 변화를 나타내었다.Table 2 shows the change in the microstructure during the temperature increase process when hot rolling and heat treatment under the conditions described in Table 1.
표 2에 나타난 바와 같이, 승온 속도가 0.66 ~ 1.3℃/s인 A~J 강의 경우 승온과정에서 CrN상이 형성되지 않아, 2차 오스테나이트상 또한 페라이트상 내부에 형성되지 않아, 결정립의 크기가 조대해져 본 발명의 범위를 벗어남을 확인할 수 있다.As shown in Table 2, in the case of A-J steel having a temperature rising rate of 0.66 to 1.3 ° C / s, no CrN phase is formed during the temperature raising process, and thus, the secondary austenite phase is not formed inside the ferrite phase, so that the grain size is coarse. It can be confirmed that outside the scope of the present invention.
한편 K~N 강의 경우, 승온 속도가 0.33℃/s로 느려짐에 따라 승온과정에서 700 ~ 800℃의 온도 범위에서 페라이트상 내부에 CrN상이 미세하게 형성되었으며, 1020~1060℃의 온도 범위에서 2차 오스테나이트상이 페라이트상 내부에 잔류함을 알 수 있다.Meanwhile, in the case of K ~ N steel, as the temperature increase rate is slowed down to 0.33 ° C / s, the CrN phase was finely formed inside the ferrite phase in the temperature range of 700 to 800 ° C during the temperature rising process, and the secondary phase was carried out at a temperature range of 1020 to 1060 ° C. It can be seen that the austenite phase remains inside the ferrite phase.
O 강은 K~N 강과 유사하게 CrN상이 형성되나 소둔 온도가 1080℃를 초과함에 따라 2차 오스테나이트상이 고용되어 석출되지 않았다.In the O steel, CrN phase was formed similarly to K ~ N steel, but the secondary austenite phase was dissolved due to the annealing temperature exceeding 1080 ° C.
P~U 강은 0.17℃/s의 승온 속도로, K ~ O 강과 유사한 경향을 보이나, CrN상의 석출량이 증가함에 따라, 잔류하는 2차 오스테나이트 상도 증가되는 경향을 보인다.P-U steels tend to be similar to K-O steels at a temperature increase rate of 0.17 ° C / s, but as the amount of precipitation of CrN phases increases, the remaining secondary austenite phases also tend to increase.
또한, A ~ U강은 압하율이 77%로 최종 미세조직의 결정립이 조대해져 그 크기가 25㎛를 초과하여 본 발명의 범위를 벗어남을 알 수 있다.In addition, it can be seen that the A-U steel has a reduction ratio of 77% and the grains of the final microstructure are coarsened, the size of which exceeds 25 μm, which is outside the scope of the present invention.
한편, 압하율이 82.5% 이고, 승온 속도가 0.17℃/s이며, 소둔 온도가 1020 ~ 1060℃로 본 발명의 실시예를 만족하는 V~X강은 소둔 시간에 따라 V강, X강의 일부(V3, X1) 및 W강 전부에서 승온 과정에서 CrN상을 적절히 석출시킴과 동시에 1020 ~ 1060℃의 온도 영역에서 페라이트상 내부에 2차 오스테나이트상을 잔존시켜 가장 미세한 조직을 확보함을 알수 있다.On the other hand, the reduction ratio is 82.5%, the temperature increase rate is 0.17 ℃ / s, the annealing temperature is 1020 ~ 1060 ℃ V ~ X steel satisfying the embodiment of the present invention is a part of the V steel, X steel ( It can be seen that the CrN phase is properly precipitated in all the V3, X1) and W steels, and the second austenite phase is left inside the ferrite phase in the temperature range of 1020 to 1060 ° C to secure the finest structure.
반면, Y강의 경우 소둔 온도가 1080℃로 T강과 마찬가지로 2차 오스테나이트상이 고용되어 본 발명의 범위를 벗어남을 알 수 있다.On the other hand, in the case of Y steel, the annealing temperature is 1080 ° C as in the case of T steel secondary austenite phase is found to be out of the scope of the present invention.
구분division 압하량(%) Rolling amount (%) 승온속도(℃/s) Temperature increase rate (℃ / s) 소둔온도(℃) Annealing Temperature (℃) 소둔시간(min) Annealing time (min) 결정립크기(㎛)Crystal grain size (㎛) 항복강도 (A)Yield strength (A) 충격인성 (B)Impact Toughness (B) (A+B)(A + B) 비고Remarks
TT 7777 0.38~0.17 0.38 ~ 0.17 10801080 4040 36~43 36-43 536536 172172 708708 비교예 Comparative example
R R 7777 0.33~0.17 0.33 ~ 0.17 10401040 4040 28~33 28-33 569569 187187 756756
W W 82.582.5 0.33~0.17 0.33 ~ 0.17 10401040 4040 21~24 21-24 585585 193193 778778 실시예 Example
표 3은 표 2의 대표 강종 (T, R, W)에 대한 특성을 나타내었다.Table 3 shows the characteristics for the representative steel grades (T, R, W) of Table 2.
이때, 항복강도는 압연방향의 90°방향으로 JIS 5호 인장시편을 채취하여 상온에서 20㎜/min의 변형속도(Crosshead Speed)로 인장시험을 실시하였다.At this time, yield strength was taken from the tensile test specimen of JIS No. 5 in the 90 ° direction of the rolling direction and subjected to a tensile test at a crosshead speed of 20 mm / min at room temperature.
R강은 압하율이 77%로 결정립이 조대해져 그 크기가 기준값인 25㎛을 초과하며, 특히 R강의 경우 항복강도가 536MPa로 기준값인 550MPa에 미달되고, 항복강도와 충격인성의 합 또한 708MPa로 기준값인 750MPa에 미달하여 항복강도 및 충격인성 특성이 향상되지 못하였음을 알 수 있다.R steel has 77% reduction in grain size, and its size is larger than the reference value of 25㎛. In particular, R steel has a yield strength of 536MPa, which is less than the standard value of 550MPa, and the yield strength and impact toughness are also 708MPa. It was found that the yield strength and impact toughness were not improved because the reference value was not reached 750 MPa.
또한, T강의 경우, 항복강도 및 항복강도와 충격인성의 합은 기준값을 만족하지만, 압하량이 77%로 결정립의 크기가 기준값인 25㎛를 초과한다.In addition, in the case of T steel, the sum of yield strength, yield strength and impact toughness satisfies the reference value, but the rolling reduction is 77%, and the grain size exceeds 25 μm, which is the reference value.
반면, W강은, 압하량이 82.5%이며, 소둔 온도, 소둔시간 및 승온 속도가 본원 발명의 범위를 만족하여 결정립의 크기는 25㎛ 이하로 미세하며, 항복강도는 585MPa이고, 항복강도와 충격인성의 합은 778MPa로 항복강도 및 충격인성이 향상되어 기계적 특성이 비교재에 비하여 향상되었음을 확인할 수 있다.On the other hand, in the W steel, the rolling reduction is 82.5%, the annealing temperature, the annealing time and the temperature increase rate satisfy the scope of the present invention, and the grain size is fine to 25 µm or less, the yield strength is 585 MPa, the yield strength and impact toughness The sum is 778 MPa, yield strength and impact toughness is improved, it can be confirmed that the mechanical properties compared to the comparative material.
상술한 바와 같이, 본 발명의 바람직한 실시예를 참조하여 설명하였지만 해당 기술분야의 숙련된 당업자라면 하기의 청구번위에 기재된 본 발명의 사상 및 영역으로부터 벗어나지 않는 범위 내에서 본 발명을 다양하게 수정 및 변경시킬 수 있음을 이해할 수 있을 것이다.As described above, the present invention has been described with reference to the preferred embodiments, but those skilled in the art can variously modify and change the present invention without departing from the spirit and scope of the present invention as set forth in the following claims. I can understand that you can.

Claims (7)

  1. 30㎜ 이상의 두께를 갖는 후물 슈퍼 듀플렉스 스테인리스강에 관한 것으로서,A thick super duplex stainless steel having a thickness of 30 mm or more,
    중량%로, Cr: 24 ~ 26%, Ni: 6.0 ~ 8.0%, Mo: 3.5 ~ 5.0%, N: 0.24 ~ 0.32% 를 포함하고, 나머지 Fe 및 불가피한 불순물을 포함하며,By weight, including Cr: 24 to 26%, Ni: 6.0 to 8.0%, Mo: 3.5 to 5.0%, N: 0.24 to 0.32%, including the remaining Fe and inevitable impurities,
    미세조직이 페라이트상, 오스테나이트상 및 2차 오스테나이트상으로 구성되되, 결정립 크기가 25㎛ 이하인 것을 특징으로 하는, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강.A super duplex stainless steel having excellent yield strength and impact toughness, wherein the microstructure is composed of a ferrite phase, an austenite phase, and a secondary austenite phase, and has a grain size of 25 μm or less.
  2. 청구항 1에 있어서,The method according to claim 1,
    상기 슈퍼 듀플렉스 스테인리스강은,The super duplex stainless steel,
    항복강도가 550 MPa 이상인 것을 특징으로 하는, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강.A super duplex stainless steel excellent in yield strength and impact toughness, characterized by a yield strength of at least 550 MPa.
  3. 청구항 2에 있어서,The method according to claim 2,
    상기 슈퍼 듀플렉스 스테인리스강은,The super duplex stainless steel,
    항복강도와 충격인성의 합이 750 이상인 것을 특징으로 하는, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강.A super duplex stainless steel excellent in yield strength and impact toughness, characterized in that the sum of yield strength and impact toughness is 750 or more.
  4. 중량%로, Cr: 24 ~ 26%, Ni: 6.0 ~ 8.0%, Mo: 3.5 ~ 5.0%, N: 0.24 ~ 0.32% 를 포함하고, 나머지 Fe 및 불가피한 불순물을 포함하는 슬라브를 제조하는 주조 단계;A casting step of producing a slab comprising Cr: 24 to 26%, Ni: 6.0 to 8.0%, Mo: 3.5 to 5.0%, N: 0.24 to 0.32% by weight, and containing the remaining Fe and inevitable impurities;
    상기 슬라브를 열간에서 압연하여 30㎜ 이상의 두께를 갖는 후판재를 생산하는 열간압연 단계;Hot rolling the slab to produce a thick plate having a thickness of 30 mm or more;
    상기 후판재를 소둔 온도까지 승온하여 페라이트상 내부에 CrN상을 석출시키고, 상기 CrN상 주위에 시그마상 및 2차 오스테나이트상을 석출시키는 승온 단계; 및Heating the thick plate to an annealing temperature to precipitate a CrN phase inside a ferrite phase, and to precipitate a sigma phase and a secondary austenite phase around the CrN phase; And
    상기 시그마상을 상기 페라이트상에 고용시키면서, 상기 2차 오스테나이트상을 상기 페라이트상 내부에 잔류시키는 소둔 단계;를 포함하는, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강 제조방법.And annealing the second austenite phase to remain in the ferrite phase while dissolving the sigma phase in the ferrite phase. The method according to claim 1, wherein the yield strength and impact toughness are excellent.
  5. 청구항 4에 있어서,The method according to claim 4,
    상기 승온 단계는,The temperature raising step,
    700℃ 부터 상기 소둔 온도까지 0.11 ~ 0.17 ℃/s의 속도로 승온시키는 것을 특징으로 하는, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강 제조방법.The method of manufacturing a super duplex stainless steel excellent in yield strength and impact toughness, characterized in that to increase the temperature from 700 ℃ to the annealing temperature at a rate of 0.11 ~ 0.17 ℃ / s.
  6. 청구항 5에 있어서,The method according to claim 5,
    상기 소둔 단계는,The annealing step,
    1020 ~ 1060℃의 온도로 20 ~ 60 분간 소둔하는 것을 특징으로 하는, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강 제조방법.A super-duplex stainless steel manufacturing method excellent in yield strength and impact toughness, characterized in that the annealing for 20 to 60 minutes at a temperature of 1020 ~ 1060 ℃.
  7. 청구항 4에 있어서,The method according to claim 4,
    상기 열간압연 단계는,The hot rolling step,
    미세조직의 결정립 크기가 25㎛이하가 되도록, 80% 이상의 압하율로 압연하는 것을 특징으로 하는, 항복강도 및 충격인성이 우수한 슈퍼 듀플렉스 스테인리스강 제조방법.A method for producing a super duplex stainless steel excellent in yield strength and impact toughness, characterized by rolling at a reduction ratio of 80% or more so that the grain size of the microstructure is 25 µm or less.
PCT/KR2015/014114 2014-12-26 2015-12-22 Super duplex stainless steel having excellent yield strength and impact toughness and manufacturing method therefor WO2016105094A1 (en)

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