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WO2021046938A1 - 一种超净稀土钢及夹杂物改性控制方法 - Google Patents

一种超净稀土钢及夹杂物改性控制方法 Download PDF

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WO2021046938A1
WO2021046938A1 PCT/CN2019/108858 CN2019108858W WO2021046938A1 WO 2021046938 A1 WO2021046938 A1 WO 2021046938A1 CN 2019108858 W CN2019108858 W CN 2019108858W WO 2021046938 A1 WO2021046938 A1 WO 2021046938A1
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steel
rare earth
inclusions
content
ultra
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PCT/CN2019/108858
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English (en)
French (fr)
Inventor
李殿中
栾义坤
刘宏伟
傅排先
胡小强
王培�
夏立军
杨超云
刘航航
刘洋
刘朋
李依依
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中国科学院金属研究所
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Priority to JP2021571312A priority Critical patent/JP7384935B2/ja
Priority to KR1020217031568A priority patent/KR102470648B1/ko
Priority to US17/611,061 priority patent/US20220259707A1/en
Priority to EP19944733.5A priority patent/EP3943631A4/en
Publication of WO2021046938A1 publication Critical patent/WO2021046938A1/zh
Priority to US18/142,152 priority patent/US12091733B2/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0006—Adding metallic additives
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/06—Deoxidising, e.g. killing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/068—Decarburising
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/076—Use of slags or fluxes as treating agents
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10—Handling in a vacuum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/04—Making ferrous alloys by melting
    • C22C33/06—Making ferrous alloys by melting using master alloys
    • 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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%

Definitions

  • the application belongs to the field of alloys, and relates to a method for controlling ultra-clean rare earth steel and the modification of inclusions.
  • the performance method by simultaneously controlling the T[O]s ⁇ 20ppm of the molten steel before the addition of rare earths and the T[O]r ⁇ 60ppm of the rare earth metal itself, solves the problem of nozzle clogging, refines the grains of inclusions, and improves steel Impact toughness, but the effect of rare earth addition on the modification of steel inclusions is not clear; CN201710059980.6 relates to a high-purity rare earth steel treatment method.
  • the amount of rare earth added is based on dissolved oxygen O dissolved oxygen in molten steel, total oxygen TO,
  • the sulfur content S and the refining slag basicity R CaO/SiO 2 , FeO+MnO total content are added, but the total content of dissolved oxygen O dissolved oxygen, total oxygen TO, sulfur content S and refining slag basicity R are not studied.
  • the relationship and influence with the amount of rare earth additions lack clear guidance on the production practice of different varieties of high-purity rare earth steel.
  • Nippon Steel CN1759199A relates to bearing steel with fine inclusions.
  • the oxide inclusions in the steel are converted into REM oxides.
  • the addition amount of REM satisfies this formula to prevent Al 2 O 3 from not reacting and convert alumina inclusions in steel into REM oxides. .
  • the purpose of controlling the amount of REM addition is to target the formation of REM oxide inclusions, and it does not take into account the influence of the change in the O content in steel caused by the addition of REM on the inclusions and impurities.
  • the effect of element S on inclusions, the rolling fatigue life of the obtained pure bearing steel containing rare earth inclusions is 3.2-9.2 times that of no REM.
  • the main types of inclusions in the bearing steel are Ce 2 O 3 and individual TiN inclusions.
  • the rare earth increases to 0.007% (Ie 70ppm) the main types of inclusions in bearing steel are also Ce 2 O 3 and TiN alone.
  • Ce 2 O 3 stably exists in the steel, although it is reduced
  • the content of TiN inclusions in the steel but the lattice matching between Ce 2 O 3 and TiN inclusions is not good, and the formation of a large amount of rare earth oxides degrades the mechanical properties of the steel.
  • the embodiments of the present application provide an ultra-clean rare earth steel containing 10-200 ppm, preferably 10-100 ppm, more preferably 10-50 ppm, most preferably 15-40 ppm of rare earth elements, and more than 50% of the total number of inclusions in the steel
  • 80% or more, more preferably 95% or more of the part is RE-oxygen-sulfide (RE 2 O 2 S) with an average equivalent diameter D mean of 1-5 ⁇ m, spherical or nearly spherical or granular, and dispersed.
  • RE 2 O 2 S RE-oxygen-sulfide
  • the boundary between the RE-oxygen-sulfide and the Fe matrix is gentle, and the compatibility with the Fe matrix is good.
  • the equivalent diameter refers to the inclusions measured by (maximum particle diameter+minimum particle diameter)/2.
  • the rare earth content in the ultra-clean rare earth steel satisfies the following formula (1):
  • REM is the content of rare earth elements in steel, in ppm;
  • T[O]m is the total oxygen content in the steel, in ppm;
  • T[O]r is the total oxygen content in the rare earth metal or alloy added to the steel, in ppm;
  • T[S]m is the total sulfur content in steel, in ppm
  • m is the correction factor 1, and the value is 2-4.5, preferably 3-4.5;
  • n is the second correction factor, with a value of 0.5-2.5, preferably 1-2.2;
  • k is the correction coefficient three, and the value is 0.5-2.5, preferably 1-2.2.
  • the inventor’s team research shows that by specifying the rare earth content REM in ultra-clean rare earth steel, the total oxygen content of molten steel, total sulfur content, and the total oxygen content of rare earth metals or alloys added to the steel meet the above formula (1), the total number of inclusions can be obtained More than 50%, better more than 80%, more than 95% of fine and dispersed RE-oxygen-sulfide (RE 2 O 2 S) instead of rare earth oxides (RE 2 O 3 ), while ensuring the formation RE 2 O 2 S has an average equivalent diameter of 1-5 ⁇ m, spherical, nearly spherical or granular, diffusely distributed RE-oxygen-sulfide.
  • the above correction coefficients are empirical coefficients to ensure the formation of RE 2 O 2 S.
  • the tension and compression fatigue life of the REM-modified high-purity bearing steel increased to 4.1* 108 times, which is more than 40 times that of the existing high-purity bearing steel, and the rolling contact fatigue life reached 3.08*10 7 , which is higher than
  • the existing high-purity bearing steel has a high rolling contact fatigue life of 9.1 million times, and its fatigue life has been significantly improved.
  • the r value of RE-IF steel is significantly increased by 25% without changing its strength.
  • the growth rate and strong plastic product have been significantly improved; compared with high-strength steel without RE, the low-temperature transverse and longitudinal impact energy of ultra-high-strength steel in the range of 0°C to -40°C is comprehensively improved after adding ultra-low RE.
  • the steel is high-end bearing steel, gear steel, die steel, stainless steel, steel for nuclear power, IF/DP/TRIP steel for automobiles, or ultra-high-strength steel.
  • this application also provides an ultra-clean rare earth steel containing 10-200ppm, preferably 10-100ppm, more preferably 10-50ppm rare earth elements, and the inclusions in the steel include rare earth-oxygen-sulfide in the amount ⁇ 50% (RE 2 O 2 S), ⁇ 50% rare earth-sulfide and 0-10% Al 2 O 3 inclusions.
  • an ultra-clean rare earth steel containing ppm-level rare earth elements in which ⁇ 70%, preferably ⁇ 80%, and more preferably ⁇ 95% of the total number of inclusions in the steel are spherical or nearly spherical, dispersedly distributed O-Al-S -RE and/or RE-OS inclusions, the total content of TiN and MnS inclusions is ⁇ 5%, and the equivalent average diameter of the inclusions is 1-2 ⁇ m; further, the content of rare earth elements in the steel is 10-200ppm, preferably 10 -100ppm, more preferably 10-50ppm.
  • the method for denaturing inclusions of ultra-clean rare earth steel in the present application is to denature at least 80%, preferably at least 90%, and more preferably at least 95% of Al 2 O 3 inclusions that already exist in the steel to RE-oxygen-sulfide, where high When pure rare earth metal or alloy is added, the total oxygen content of molten steel T[O]m ⁇ 25ppm, the total sulfur content of molten steel T[S]m ⁇ 90ppm, the total oxygen content of high-purity rare earth metals or alloys is controlled by T[O]r At 60-200ppm, after adding high-purity rare earth, the RH deep vacuum cycle time satisfies the following formula (2):
  • C RE is the rare earth element content in the steel
  • T 0 is the correction constant, and the value is 3-10 min
  • C RE is the content of rare earth elements in the steel
  • t 0 is the correction constant, and the value is 5-10 min.
  • the VD deep vacuum time refers to the total time for the degassing of molten steel after the vacuum of the VD furnace reaches a certain vacuum (usually below 67 Pa);
  • the RH deep vacuum time refers to the total time for molten steel degassing after the vacuum degree of the RH furnace reaches a certain vacuum degree (generally less than 200 Pa).
  • the casting superheat is 5-15°C higher than steel grades with the same composition but no rare earths; the increase of N in the whole continuous casting process is controlled within 8ppm.
  • This application also provides an inclusion control process for ultra-clean rare earth steel, including:
  • the high-purity rare earth metal or alloy is added before the LF refining station or after RH vacuum treatment for at least 3 minutes, and the total oxygen content T[O]r in the high-purity rare earth metal or alloy is 60-200ppm;
  • the processing time that satisfies the above formula is very conducive to the formation and floating of rare earth-oxygen-sulfide, thereby reducing the number of inclusions;
  • the tightness between the large ladle-tundish-mold and the thickness of the liquid surface covering agent in the tundish are strengthened, and the argon purge on the liquid surface of the tundish is strengthened to avoid air inhalation during the continuous casting process and increase the amount of N throughout the continuous casting process. It is controlled within 8ppm to inhibit the formation of metal nitride inclusions; compared with steel grades with the same composition but no rare earths, the casting superheat is increased by 5-15°C, in order to prevent flocculation.
  • the amount of high-purity rare earth added in the step 3) satisfies W RE ⁇ T[O]m+T[S]m, where ⁇ is the correction coefficient, and the value is 6-30, preferably 8-20, T [O]m is the total oxygen content in the steel, T[S]m is the total sulfur content in the steel, and W RE is the added amount of high-purity rare earth metals or alloys;
  • the T[O]r of high-purity rare earth metals is controlled at 60-200ppm, because when T[O]r is controlled below 60ppm, rare earth metal oxides are mainly formed, and the equivalent diameter is less than 2 ⁇ m, but When T[O]r increases to 200ppm, its size will exceed 10 ⁇ m, it is difficult to float, and will remain in the melt after solidification, resulting in deterioration of the performance of the steel.
  • This application also provides an inclusion control process for ultra-low RE bearing steel.
  • the process route includes electric arc furnace smelting ⁇ LF refining ⁇ RH refining ⁇ continuous casting ⁇ heating ⁇ rolling. The steps are as follows:
  • T[O]m is the total oxygen content in the steel
  • T[S]m is the total sulfur content in the steel
  • the casting superheat is 5-15°C higher than that of bearing steel of the same composition without RE, and the Al content at the end of RH refining is controlled to 0.015-0.030%; during continuous casting, the MgO content in the working layer of the tundish is greater than 85 %, the SiO 2 content of the large package nozzle, the middle package stopper and the immersion nozzle is less than 5%.
  • this application also provides an ultra-low RE IF/DP/TRIP steel inclusion control method, which includes the following steps:
  • RH refining add high-purity rare earth metals after RH deep vacuum for at least 2 minutes, T[O]m in molten steel is less than 25ppm, T[S]m less than 50ppm before adding high-purity rare earths, after adding high-purity rare earths, RH deep vacuum
  • the converter ladle top slag is upgraded, and the tundish molten steel T[O]m content is controlled below 25ppm; the RH refined ladle top slag is upgraded, and the RH incoming molten steel S content is controlled below 0.005%; the continuous casting tundish top Slag modification, through three modification processes, improve the fluidity of slag, improve the removal ability of inclusions, and ensure the cleanliness of steel.
  • this application provides an inclusion control process for ultra-low RE and ultra-high-strength steel, and the production process flow: converter smelting-LF refining-RH refining-continuous casting-rolling-tempering, including the following steps:
  • LF refining ensures that the white slag time is more than 20 minutes, the total oxygen content of molten steel T[O]m is less than 20ppm, and T[S]m is less than 0.005%;
  • Rare earths are added before LF is refined out of the station or added after 3 minutes of RH net circulation;
  • RE has a strong affinity for oxygen and sulfur, and it is easy to quickly form RE-oxygen-sulfide/RE-sulfide, and most of the The existing Al 2 O 3 inclusions are deformed into RE-oxygen-sulfide; the second is that during the process of molten steel refining, the rare earth-oxygen-sulfide/rare-earth-sulfide part formed by argon soft blowing rises, thereby reducing the inclusions.
  • the third reason is that because of the low oxygen content in the melt, the rare earth-oxygen-sulfide is not easy to grow up, and it has good wettability with the steel melt and is not easy to gather together.
  • Figure 1 The effect of rare earth addition on the tensile-compression fatigue and rolling contact fatigue properties of bearing steel GCr15; among them,
  • Figure 2 The effect of rare earth on the deterioration of inclusions, including the mechanical properties of inclusions, the morphology and distribution of inclusions, and the morphology and distribution of inclusions after fatigue failure; among them,
  • Figures (f)-(g) show the inclusions and dislocations around them after fatigue failure of the RE-GCr15 clean steel of the application.
  • This embodiment is a method for denaturing inclusions of RE-GCr15 bearing steel.
  • the production process route is electric arc furnace ⁇ LF refining ⁇ RH refining ⁇ continuous casting ⁇ heating ⁇ rolling, including the following steps:
  • ⁇ is the correction coefficient, the value is 6-30, preferably 8-20, T[O]m is the total oxygen content (ppm) in the steel, and T[S]m is the total sulfur content (ppm) in the steel;
  • the T[O]r of high-purity rare earth metals is controlled at 60-200ppm.
  • the RH deep vacuum cycle time is guaranteed to be more than 10min
  • the Ar gas soft blowing time is guaranteed to be more than 10min, so that the formed rare earth-oxygen-sulfide can float up, thereby reducing inclusions
  • the quantity of substances, the Al content at the end of RH refining is controlled to be 0.015-0.030%, and the content of rare earth elements in the molten steel is 15-30ppm;
  • the tightness between the large ladle-tundish-mold and the thickness of the liquid surface covering agent in the tundish are strengthened, and the argon purge on the liquid surface of the tundish is strengthened to avoid air inhalation during the continuous casting process and increase the amount of N throughout the continuous casting process.
  • the pouring superheat is controlled between 25-40°C, the superheat control is 5-20°C higher than the conventional superheat control, and the purpose is to prevent flocculation ;
  • the MgO content of the working layer of the tundish is controlled to be greater than 85%; the SiO 2 content of the long nozzle, the tundish stopper and the immersion nozzle of the large bag is less than 5% to ensure the compactness and corrosion resistance of the tundish and the three major components Erosion and erosion resistance; continuous casting and constant casting speed.
  • the tension and compression fatigue life RE-GCr15 steel increased to 4.1 * 10 8
  • high purity GCr15 steel known in the literature of the tension and compression fatigue life of about 10 * 10 6) 40 times, the addition of Ce It reduces the number of inclusions by more than 50% [Figure 2(a)], and reduces inclusions above 5 ⁇ m by at least 35%.
  • the rolling contact fatigue life of RE-GCr15 steel in Figure 1b has also been greatly improved.
  • This embodiment is a method for denaturing Al 2 O 3 inclusions in IF steel.
  • the production process flow is: molten iron inversion station-hot metal pretreatment-converter smelting-RH refining-continuous casting-hot rolling-pickling-cold rolling-annealing , Including the following steps:
  • the top slag of the ladle in the converter process is upgraded.
  • the pre-deoxidation and alloying of manganese are not carried out in the converter process and the RH decarburization process.
  • the oxygen content of the tundish steel is strictly controlled below 25ppm to improve the cleanliness of the IF steel; strictly control the tapping Temperature, hanging tank temperature and amount of slag;
  • RH process ladle top slag is upgraded, the S content of molten steel in the RH station is controlled below 0.003%;
  • the top slag of the tundish is used for upgrading, and the airtightness between the large package-the middle package-the crystallizer is ensured, and the air suction during the continuous casting process is avoided, and the total N absorption of the continuous casting is controlled to be less than 8ppm; the pouring superheat is controlled more than the conventional superheat Control the increase of 5-15°C to prevent the risk of flocculation; continuous casting constant drawing speed control;
  • Table 3 Typical size and quantity distribution of IF steel inclusions (Continued Table 2)
  • adding an appropriate amount of high-purity rare earth metals to IF steel can significantly increase the number of 1-2 ⁇ m-level fine inclusions in the steel by 8% (that is, from 86.67% to 94.67%), and the number and proportion of 5-10 ⁇ m are obvious. Decrease, the maximum diameter of inclusions (1.464 ⁇ m ⁇ 1.431 ⁇ m) is slightly reduced, and the number of inclusions (area ratio 0.146 ⁇ 0.139) is significantly reduced compared to IF steel without rare earth addition; on the other hand, adding a proper amount to IF steel RE can play the purpose of obvious metamorphic inclusions.
  • RE can metamorphize large-size rod-like/cluster-like Al 2 O 3 inclusions into nearly spherical O-Al-S-RE/RE-OS compounds , The size is smaller and dispersed; at the same time, TiN and MnS inclusions lose the Al 2 O 3 nucleation base and it is difficult to nucleate and grow, reducing the fragmentation and anisotropy of such inclusions on the matrix.
  • the characteristics of the distribution of inclusions in the steel of Example 2-1 are: in 22 fields of view, the total number of inclusions is less than 250, and the proportion of inclusions between the equivalent diameter of 1-2 ⁇ m is ⁇ 94.5%, and the equivalent diameter is 2 The proportion of inclusions between -5 ⁇ m is less than 5%, and the proportion of inclusions between 5-10 ⁇ m in equivalent diameter is less than 0.5%.
  • the RE-IF steel can significantly increase the r value by at least 25% (1.820 ⁇ 2.267) compared with the traditional IF steel without changing its strength.
  • the growth rate and strong plastic volume have been significantly improved.
  • This embodiment is a method for denaturing ultra-high-strength grade F marine steel inclusions.
  • the production process is as follows: hot metal pretreatment-converter smelting-LF refining-RH refining-continuous casting-rolling-quenching and tempering.
  • the control process is as follows:
  • the airtightness is ensured between the large ladle-tundish-mold to avoid air suction during the continuous casting process, and the N absorption amount in the whole continuous casting process is controlled to be less than 5ppm; the pouring superheat and the continuous casting constant drawing speed are controlled, The superheat degree is 5-15°C higher than the conventional superheat degree control;
  • Table 5 Composition and content of ultra-high-strength steel
  • Table 7 Typical Inclusion Size Distribution of Ultra High Strength Steel (Continued Table 6)
  • the maximum diameter Dmax of inclusions gradually decreases (from 34 ⁇ 31 ⁇ 19), and the number of inclusions with a diameter of less than 2 ⁇ m increases by at least 4%, and the total amount of inclusions decreases by an average of 18% (0.45 ⁇ 0.37 ⁇ );
  • the average equivalent diameter of inclusions Dmean is reduced by 8% (4.37-4.02), the maximum inclusion diameter/minimum inclusion diameter is significantly reduced, and the area ratio of inclusions also varies to different degrees decline.
  • the typical distribution of inclusions in the steel of Examples 3-1 and 3-2 is: in 20 fields of view, the total number of inclusions is less than 500, and the proportion of inclusions between the equivalent diameter of 1-2 ⁇ m is> 10.5%, The proportion of inclusions with an equivalent diameter of 2-5 ⁇ m is 60-80%, the proportion of inclusions with an equivalent diameter of 5-10 ⁇ m is less than 22.5%, and the proportion of inclusions with an equivalent diameter of> 10 ⁇ m is less than 5%.
  • Example 3 Combined with the SEM+EDS analysis, there are large-size Al 2 O 3 cluster inclusions in the field of view of the sample without RE addition, among which the large-size inclusions are broken and accompanied by elongated MnS-type inclusions, and REM is added in Example 3-
  • the inclusions in the samples of 1 and Example 3-2 are mostly spherical or granular RE-OS compounds, with smaller sizes and dispersed distribution.
  • the modification effect of adding an appropriate amount of high-purity rare earth metal to the inclusions can make the F grade ultra high
  • the low-temperature transverse and longitudinal impact energy of high strength marine steel has been comprehensively improved: the transverse impact energy at 0°C is increased by at least 30J, and the transverse impact energy is increased by at least 60J; the transverse impact energy at -20°C is increased by at least 13J, and the longitudinal impact energy is increased by at least 35J; At -40°C, the transverse impact energy is increased by at least 5J, and the longitudinal impact energy is increased by at least 9J; especially the improvement effect is particularly prominent at the position of 1/2 plate thickness.

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Abstract

一种超净稀土钢及夹杂物控制方法，该钢中含有10-200ppm的稀土元素，钢中夹杂物50％以上的部分为平均等效直径D mean为1-5μm、球状或近球状或粒状、弥散分布的RE-氧-硫化物；该方法是将钢中Al 2O 3夹杂物至少80％、优选至少90％变性为RE-氧-硫化物，与相同成分但不含稀土的钢种相比，钢中夹杂物总量减少18％以上，降低了传统高纯钢中Al 2O 3夹杂物等引起的断裂可能性，显著提高了钢的疲劳寿命等机械力学性能，实现对钢内夹杂物变性的类型、分布和尺寸的精准控制，适用更多品种高性能钢的研发生产。

Description

一种超净稀土钢及夹杂物改性控制方法
本申请要求于2019年9月10日提交中国专利局、申请号为201910855025.2、发明名称为“一种超净稀土钢及夹杂物改性控制方法”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本申请属于合金领域，涉及一种超净稀土钢及夹杂物改性控制方法。
背景技术
近十年来，双低氧技术，即同时控制稀土金属本身的初始含氧量和钢熔体总含氧量被应用后，稀土的作用变得异常稳定和突出，发明人的诸多在先申请均呈现了相关技术，包括：CN201610265575.5，涉及高纯稀土金属的制备方法，可以避免产生夹杂物粗大、材料性能波动、生产过程中堵塞水口等，但对于高纯稀土金属对于钢中夹杂物的影响并没有进行研究；CN201611144005.7，涉及一种超低氧稀土合金和用途，将高纯稀土合金用于处理钢，并给出了处理后的夹杂物对比图及夹杂物评级，但对于高纯稀土合金的加入量与夹杂物尺寸、数量及类型的影响并不明确，使得高性能稀土钢的研发创新一直推进缓慢甚至几乎停滞；CN201410141552.4，涉及一种超低氧纯净钢的冶炼方法，采用两次真空碳脱氧与添加稀土进一步脱氧结合降低金属液中的氧含量，减少合金中的夹杂物数量，改善夹杂物分布，减轻通道偏析，使得产品具有夹杂物少，金属液纯净，高质量将得到超低氧含量、高纯净的金属液，但对于如何通过稀土加入控制钢中夹杂物的形态、数量、类型及分布并不明确；CN201610631046.2，涉及一种钢中添加稀土金属提高性能的方法，通过同时控制添加稀土前钢液的 T[O]s<20ppm和稀土金属本身的T[O]r<60ppm，解决了水口堵塞的问题，细化夹杂物晶粒，提高钢的冲击韧性，但对于稀土添加对钢夹杂物改性的影响尚不明确；CN201710059980.6，涉及一种高纯净稀土钢处理方法，稀土加入量则根据钢水中溶解氧O 溶解氧、全氧T.O、硫含量S以及精炼渣碱度R＝CaO/SiO 2、FeO+MnO总含量进行添加，但并未研究钢水中溶解氧O溶解氧、全氧T.O、硫含量S以及精炼渣碱度R总含量与稀土添加量的关系及影响，对不同品种高纯净稀土钢的生产实践缺乏明确的指导作用。
新日铁CN1759199A涉及含有细内含物的轴承钢，通过将轴承钢REM的添加量控制在-30＜REM－(T.O.×280/48)＜50，将钢中氧化物夹杂转化为REM氧化物夹杂，其中280/48是根据REM 2O 3中REM与O的化学计量比例得出的，REM添加量满足该式，防止Al 2O 3不反应，将钢中氧化铝夹杂转化为REM氧化物。然而，尽管该文献提到了REM氧硫化物，但控制REM添加量的目的是针对REM氧化物夹杂的形成，且根本没有考虑添加REM后引起的钢中O含量变化对夹杂物的影响，以及杂质元素S等对夹杂物的影响，所得的含稀土夹杂物的纯净轴承钢的压延疲劳寿命是不添加REM的3.2-9.2倍。
北京科技大学成国光等人的发明申请201811319185.7中指出稀土Ce对MgAl 2O 4具有很好的改质效果，但仅在控制轴承钢中稀土含量为0.002％(即20ppm)时，获得了CeAlO 3外包裹TiN的复合夹杂物，当轴承钢中稀土的含量达到0.004％(即40ppm)时，轴承钢中主要夹杂物的类型为Ce 2O 3和单独的TiN夹杂物，当稀土增至0.007％(即70ppm)时，轴承钢中主要夹杂物的类型同样以单独的Ce 2O 3和TiN为主，随着钢中稀土含量的进一步增加，Ce 2O 3稳定存在于钢中，尽管降低了钢中夹杂物TiN含量，但Ce 2O 3与TiN夹杂物的晶格匹配性不好，大量稀土氧化物的形成反而劣化钢的机械力学性能。
目前稀土添加对于钢中夹杂物改性的影响尚不明确，稀土加入后控制性差、缺乏系统深入研究，导致夹杂物变性的生产控制工艺难度大、稳定性差，极大地制约了低成本稀土在制备高性能钢，如轴承、齿轮、模具、不锈钢、核电用钢、汽车用钢等以及各种关键零部件上的推广应用。
发明内容
为了实现对稀土添加后对钢内夹杂物变性的类型、分布和尺寸的精准控制，适用更多品种高性能钢的研发生产，发明人团队通过持续的研发创新并与工程实践紧密结合，提出了一种含有ppm级稀土元素的超净稀土钢及其改性控制方法。
为达到上述目的，本申请主要提供如下技术方案：
一方面，本申请的实施例提供一种超净稀土钢，含有10-200ppm，优选10-100ppm，更优选10-50ppm、最优选15-40ppm的稀土元素，钢中夹杂物总数中50％以上，优选80％以上、更优选95％以上的部分为平均等效直径D mean为1-5μm、球状或近球状或粒状、弥散分布的RE-氧-硫化物(RE 2O 2S)。
其中，RE-氧-硫化物与Fe基体的边界平缓，与Fe基体相容性良好。
其中，所述等效直径是指对夹杂物按(最大粒径+最小粒径)/2测量得到。
优选的，所述超净稀土钢中稀土含量满足下式(1)：
-500＜REM-(m*T[O]m)+n*T[O]r+k*T[S]m)＜-30......(1)；
其中REM为钢中稀土元素含量，单位ppm；
T[O]m为钢中全氧含量，单位ppm；
T[O]r为钢中加入的稀土金属或合金中全氧含量，单位ppm；
T[S]m为钢中全硫含量，单位ppm；
m为修正系数一，取值为2-4.5,优选3-4.5；
n为修正系数二，取值为0.5-2.5，优选1-2.2；
k为修正系数三，取值为0.5-2.5，优选1-2.2。
发明人团队研究表明：通过规定超净稀土钢中稀土含量REM与钢水全氧含量、全硫含量以及钢中加入稀土金属或合金中全氧含量满足上式(1)，可得到占夹杂物总数50％以上、更好80％以上、95％以上的细小且弥散的RE-氧-硫化物(RE 2O 2S)，而不是以稀土氧化物(RE 2O 3)为主，同时确保形成 RE 2O 2S平均等效直径为1-5μm、球形、近球形或粒状、弥散分布的RE-氧-硫化物，上述各修正系数为确保形成RE 2O 2S的经验系数。
测试后发现：经REM改性的高纯轴承钢的拉压疲劳寿命提高到4.1*10 8次，是现有高纯轴承钢的40多倍，且滚动接触疲劳寿命达到3.08*10 7，比现有高纯轴承钢的滚动接触疲劳寿命高910万次，其疲劳寿命得到显著提高；RE-IF钢较传统IF钢在基本不改变其强度的前提下，r值显著提高25％，同时伸长率和强塑积得到明显提升；相较于不加RE的高强度钢，加入超低RE后超高强度钢在0℃至-40℃范围内的低温横向及纵向冲击功得到全面提升。
优选的，所述钢为高端轴承钢、齿轮钢、模具钢、不锈钢、核电用钢、汽车用IF/DP/TRIP钢、或超高强度钢。
另一方面，本申请还提供了一种超净稀土钢，含有10-200ppm，优选10-100ppm，更优选10-50ppm的稀土元素，钢中夹杂物包括数量≥50％的稀土-氧-硫化物(RE 2O 2S)、≤50％的稀土-硫化物和0-10％的Al 2O 3夹杂物。
以及一种含有ppm级的稀土元素的超净稀土钢，钢中夹杂物总数中≥70％、优选≥80％、更优选≥95％为球状或近球状、呈弥散分布的O-Al-S-RE和/或RE-O-S夹杂物，TiN和MnS类夹杂物含量之和≤5％，夹杂物等效平均直径为1-2μm；进一步的，钢中稀土元素含量为10-200ppm，优选10-100ppm，更优选10-50ppm。
本申请超净稀土钢的夹杂物变性方法是将钢中已存在的至少80％、优选至少90％、更优选至少95％的Al 2O 3夹杂物变性为RE-氧-硫化物，其中高纯稀土金属或合金加入时钢水的全氧含量T[O]m≤25ppm，钢水的全硫含量T[S]m≤90ppm，高纯稀土金属或合金的全氧含量的T[O]r控制在60-200ppm，加入高纯稀土后，RH深真空循环时间满足下式(2)：
T＝(0.1-2.0)C RE+T 0…(2)
其中：C RE为钢中稀土元素含量，T 0为修正常数，取值为3-10min；
以及，Ar气软吹时间满足下式(3)：
t＝(0.05-3.0)C RE+t 0…(3)
其中C RE为钢中稀土元素含量，t 0为修正常数，取值为5-10min。
所述VD深真空时间是指VD炉真空度达到一定真空度后(通常为67Pa以下)进行钢水脱气的总时间；
所述RH深真空时间是指RH炉真空度达到一定真空度后(一般为200Pa以下)进行钢水脱气的总时间。
并且，加入高纯稀土后，浇注过热度与相同成分但不含稀土的钢种相比，提高5-15℃；连铸全程增N量控制在8ppm以内。
本申请还提供了一种超净稀土钢的夹杂物控制工艺，包括：
1)在LF精炼时保证白渣时间20min以上，稳定渣碱度>5，全硫含量T[S]m≤90ppm，全氧含量T[O]m≤25ppm；
2)高纯稀土金属或合金在LF精炼出站前加入或在RH真空处理至少3min后加入，高纯稀土金属或合金中全氧含量T[O]r为60-200ppm；
3)稀土加入后RH深真空循环时间满足下式：T＝(0.1-2.0)C RE+T 0，其中C RE为钢中稀土元素含量，T 0为修正常数，取值为3-10min；
以及，Ar气软吹时间满足下式：t＝(0.05-3.0)C RE+t 0，其中C RE为钢中稀土元素含量(ppm)，t 0为修正常数，取值为5-10min，满足上述公式的处理时间，非常有利于稀土-氧-硫化物形成并上浮，从而减少夹杂物的数量；
4)连铸中强化大包-中包-结晶器之间密闭性和中包液面覆盖剂厚度，强化中包液面氩气吹扫，避免连铸过程吸气，连铸全程增N量控制在8ppm以内，抑制金属氮化物夹杂物的形成；浇注过热度与相同成分但不含稀土的钢种相比，提高5-15℃，目的是防止絮瘤。
优选的，所述步骤3)中高纯稀土加入量满足W RE≥α×T[O]m+T[S]m，其中α为修正系数，取值为6-30，优选8-20，T[O]m为钢中全氧含量，T[S]m为钢中全硫含量，W RE为高纯稀土金属或合金的加入量；
其中，将高纯稀土金属的T[O]r控制在60-200ppm，原因是当T[O]r被控制在低于60ppm时，主要形成稀土金属氧化物，其等效直径小于2μm， 但当T[O]r增大至200ppm时，它的尺寸将超过10μm，难以上浮，凝固后会留在熔体中，导致钢的性能恶化。
本申请还提供了一种超低RE轴承钢的夹杂物控制工艺，工艺路线包括电弧炉冶炼→LF精炼→RH精炼→连铸→加热→轧制，步骤如下：
1)电弧炉冶炼；
2)LF精炼：调整精炼渣系的渣碱度>5，控制钢水中T[O]m≤25ppm和全S含量T[S]m低于90ppm；
3)RH精炼：
RH真空处理至少5min后加入高纯稀土金属或合金，高纯稀土加入量满足W RE≥α×T[O]m+T[S]m，其中α为修正系数，取值为6-30，优选8-20，T[O]m为钢中全氧含量，T[S]m为钢中全硫含量；
高纯稀土加入后RH深真空循环时间满足T＝(0.1-2.0)C RE+T 0，其中C RE为钢中稀土元素含量，T 0为修正常数，取值为3-10min；Ar气软吹时间满足下式：t＝(0.05-3.0)C RE+t 0，其中C RE为钢中稀土元素含量，t 0为修正常数，取值为5-10min；
4)连铸：连铸全程增N量控制在8ppm以内，防止增氧，并抑制金属氮化物夹杂物的形成；
5)加热后轧制及热处理。
其中，连铸时，浇注过热度较不含RE的相同成分的轴承钢提高5-15℃，RH精炼终点的Al含量控制为0.015-0.030％；连铸时，中间包工作层MgO含量大于85％，大包长水口、中包塞棒与浸入式水口的SiO 2含量小于5％。
再一方面，本申请还提供了一种超低RE的IF/DP/TRIP钢的夹杂物控制方法，包括如下步骤：
1)转炉冶炼；
2)RH精炼：RH深真空至少2min后添加高纯稀土金属，高纯稀土加入前钢水中T[O]m小于25ppm，T[S]m低于50ppm，高纯稀土加入后，RH深真空循环时间满足T＝(0.1-2.0)C RE+T 0，其中C RE为钢中稀土元素含量，T 0 为修正常数，取值为3-10min；破空后Ar气软吹时间满足下式：t＝(0.05-3.0)C RE+t 0，其中C RE为钢中稀土元素含量，t 0为修正常数，取值为5-10min；
3)连铸：保证大包-中包-结晶器之间保证密闭性，避免连铸过程中吸气，连铸全程吸N量小于8ppm；浇注过热度与相同成分但不含稀土的钢种相比提高5-15℃；
4)轧钢及热处理。
优选的，转炉钢包顶渣改质，控制中间包钢水T[O]m含量在25ppm以下；RH精炼钢包顶渣改质，RH进站钢水S含量控制在0.005％以下；连铸中间包顶渣改质，通过三次改质工艺，提高炉渣的流动性、提高夹杂物的去除能力，保证钢的洁净度。
另外，本申请提供了一种超低RE的超高强度钢的夹杂物控制工艺，生产工艺流程：转炉冶炼-LF精炼-RH精炼-连铸-轧制-调质，包括如下步骤：
1)转炉冶炼：
2)LF和RH精炼：
稀土加入前，LF精炼保证白渣时间20min以上，钢水全氧含量T[O]m低于20ppm，T[S]m低于0.005％；
稀土在LF精炼出站前加入或在RH净循环3min后加入；
稀土加入后RH深真空循环时间满足T＝(0.1-2.0)C RE+T 0，其中C RE为钢中稀土元素含量，T 0为修正常数，取值为3-10min，RH负压，常规Ca处理后，Ar气软吹时间满足下式：t＝(0.05-3.0)C RE+t 0，其中C RE为钢中稀土元素含量，t 0为修正常数，取值为5-10min；
3)连铸：大包-中包-结晶器之间保证密闭性，控制连铸全程吸N量小于5ppm；控制浇注过热度与相同成分但不含稀土的钢种相比提高5-15℃；
4)轧制与调质工艺。
需要指出的是，本申请中钢中夹杂物发生重要变化的原因有三方面，一是RE与氧和硫的亲和力强，容易快速形成RE-氧-硫化物/RE-硫化物，同 时将大部分已经存在的Al 2O 3夹杂物变形为RE-氧-硫化物；二是在钢水精炼过程中，通过氩气软吹形成的稀土-氧-硫化物/稀土-硫化物部分上浮，从而减少夹杂物的数量；三是由于熔体中氧含量低，稀土-氧-硫化物不易长大，且与钢熔体的润湿性好，不易聚集在一起。
本申请改性的反应式如下：
Figure PCTCN2019108858-appb-000001
本申请具有以下突出的技术效果：
(1)明确了高纯稀土添加对于高纯钢夹杂物改性的机理，提供一种科学系统化的高纯钢夹杂物控制方法，在此基础上，可将高纯稀土对于高纯净钢的改性处理推广应用于更多的高性能钢种的开发上，如高端轴承、齿轮、模具、不锈钢、核电用钢、汽车用钢等以及各种关键零部件上，其效果相当于钢中微结构控制；
(2)采用稀土改性后的钢中夹杂物RE-氧-硫化物的硬度较Al 2O 3夹杂物更低，塑性变形能力较好，导致边界处的微应力/应变集中较低，降低了应变集中引起断裂的可能性；其中：RE改性的高纯轴承钢的疲劳寿命提高到4.1*10 8次，是现有高纯轴承钢的40多倍，且滚动接触疲劳寿命达到3.08*10 7，比现有高纯轴承钢的滚动接触疲劳寿命高910万次，其疲劳寿命得到显著提高；RE-IF钢较传统IF钢在基本不改变其强度的前提下，r值显著提高25％，同时伸长率和强塑积得到明显提升；在0℃至-40℃范围内，相较于不加RE的高强度钢，加入超低RE后超高强度钢低温横向及纵向冲击功得到全面提升；
(3)通过规定超净稀土钢中稀土含量REM与钢水全氧含量、以及钢中加入稀土金属或合金中全氧含量满足上式，控制得到占夹杂物总数50％以上的RE-氧-硫化物(RE 2O 2S)，而不是以稀土氧化物(RE 2O 3)为主，且使得RE- 氧-硫化物尺寸微细化，能够获得等效直径为1-5μm、球形、近球形或粒状、弥散分布的RE-氧-硫化物。
(4)控制稀土加入时钢水的全氧含量T[O]m≤25ppm、全硫含量T[S]m≤90ppm、高纯稀土的含氧量及加入量、加入时机、加入之后的RH精炼时间的控制、Ar气软吹时间、浇注过热度和连铸全程吸N量，使形成的RE-氧-硫化物得到充分上浮，减少夹杂物的数量，这些工艺控制要点的协同作用共同保证了钢中夹杂物发生改性化，最终将钢中已存在的至少80％的Al 2O 3夹杂物变性为RE-氧-硫化物，且获得了小尺寸(1-5μm)、球形、近球形或粒状、弥散分散的RE-氧-硫化物。
附图说明
图1：稀土添加对轴承钢GCr15拉压疲劳与滚动接触疲劳性能的作用；其中，
(a)图为最大应力载荷±800MPa(2KHz)下的拉压接触疲劳寿命；
(b)图为载荷Fa＝8.82KN转速2000r/min下的轴承钢滚动接触疲劳寿命；
图2：稀土对夹杂物变质的作用，包括夹杂物力学性质、夹杂物形貌和分布，以及疲劳失效后夹杂物的形态和分布；其中，
(a)图为传统GCr15与本申请RE-GCr15钢锭中的夹杂物数量统计的比较；
(b)图为稀土氧硫化物与Al 2O 3的纳米压痕测试比较；
(c)图为传统GCr15洁净钢(无REM)中的夹杂物SEM形貌；
(d)图为本申请RE-GCr15洁净钢中的稀土氧硫化物SEM形貌；
(e)图为本申请RE-GCr15洁净钢在TEM下稀土氧硫化物的形貌与衍射图谱；
(f)-(g)图为本申请RE-GCr15洁净钢在疲劳失效后，夹杂物及其周围的位错塞积。
具体实施方式
下面结合具体实施方式对本申请做进一步详细说明，但本申请的保护 范围并不限于此。
实施例1
本实施例为RE-GCr15轴承钢的夹杂物变性方法，生产工艺路线为电弧炉→LF精炼→RH精炼→连铸→加热→轧制，包括如下步骤：
1)电弧炉冶炼；
2)LF精炼：合理调整精炼渣系，稳定渣碱度>5，保证白渣时间20min以上，控制钢水中T[O]m≤10ppm和T[S]m含量不高于0.005％。
3)RH精炼：RH真空处理至少5min后在料仓中加入高纯稀土金属，高纯稀土加入量满足下式：
W RE≥α×T[O]m+T[S]m，
其中：α为修正系数，取值为6-30，优选8-20，T[O]m为钢中全氧含量(ppm)，T[S]m为钢中全硫含量(ppm)；
高纯稀土金属的T[O]r控制在60-200ppm，加入后RH深真空循环时间保证10min以上，Ar气软吹时间保证10min以上，使形成的稀土-氧-硫化物上浮，从而减少夹杂物的数量，控制RH精炼终点的Al含量为0.015-0.030％、钢水成分中稀土元素含量为15-30ppm；
4)连铸中强化大包-中包-结晶器之间密闭性和中包液面覆盖剂厚度，强化中包液面氩气吹扫，避免连铸过程吸气，连铸全程增N量控制在8ppm以内，抑制TiN夹杂物的形成，保证钢的纯净度；浇注过热度控制在25-40℃之间，该过热度控制较常规过热度控制提高5-20℃，目的是防止絮瘤；中间包工作层MgO含量控制为大于85％；大包长水口、中包塞棒与浸入式水口的SiO 2含量小于5％，保证中间包的紧实度和耐蚀度以及三大件的抗冲刷与侵蚀性；连铸恒拉速浇注。
5)常规轧制工艺。
从本实施例得到的轧制成品中提取多个试样，对改性后的GCr15钢的夹杂物进行了分析，结果表明：相较于不添加稀土的高纯GCr15钢，添加高纯稀土对夹杂物的改性导致RE-GCr15钢产生了前所未有的优异疲劳性能， 如图1a所示，稀土元素的加入改变了疲劳寿命的规律，在最大应力±800MPa和20kHz的循环载荷拉伸/压缩实验中，RE-GCr15钢的拉压疲劳寿命提高到4.1*10 8次，是高纯GCr15钢(已有文献报道的拉压疲劳寿命约为10*10 6次)的40多倍，稀土的加入使夹杂物的数量减少了50％以上[图2(a)]，并使得5μm以上的夹杂物降低了至少35％。此外，作为轴承钢的另一项重要指标，图1b中RE-GCr15钢的滚动接触疲劳寿命也得到了大幅度提高，在轴向载荷Fa＝8.82KN，转速为2000r/min的情况下，RE-GCr15钢的滚动接触疲劳寿命为3.08*10 7，比高纯GCr15钢的滚动接触疲劳寿命高910万次。
传统的硬脆性Al 2O 3氧化物和长条状MnS夹杂物(＞100μm)在高纯GCr15钢中很普遍[图2(c)]，而对于稀土改性的GCr15钢来说，这些传统夹杂物急剧消失，取而代之的是具有高度典型性和规律性的小尺寸、球形、分散的RE-氧-硫化物和RE-硫化物[2(d)]。TEM进一步观察表明，这些稀土-氧-硫化物夹杂物大多为RE 2O 2S与Fe基体的边界平缓[图2(e)]。
RE 2O 2S夹杂物的弹性、杨氏和剪切模量以及硬度都远低于传统的Al 2O 3夹杂物，目前的纳米压痕实验测量也证实了这些结果[图2(b)]。由于RE 2O 2S夹杂物比传统的硬Al 2O 3夹杂物与Fe基体的相容性更好，内部微应力和应变集中的不均匀程度将远低于传统钢，如图2(f)所示的EDS和/或选区衍射图的结果表明，复合夹杂物由RE-O-S夹杂物(≥85％)和/或O-Al-S-RE夹杂物，稀土-硫化物(≤10％)、极少量(≤5％)Al 2O 3夹杂物组成[图2(f)]，经过拉压加载循环后，稀土-氧-硫化物夹杂物内部出现了许多位错[图2(g)，但稀土-氧-硫化物和稀土-硫化物附近基体中的板条仍然完好，板条之间的边界仍然清晰；相比之下，Al 2O 3颗粒内部几乎没有位错，板条断裂，它们之间的边界消失。这一对比表明，稀土-氧-硫化物的硬度较Al 2O 3夹杂物更低，塑性变形能力较好，导致边界处的微应力/应变集中较低，进一步降低了应变集中引起断裂的可能性。
实施例2
本实施例为一种IF钢中Al 2O 3夹杂物变性方法，生产工艺流程：铁水倒罐站-铁水预处理-转炉冶炼-RH精炼-连铸-热轧-酸洗-冷轧-退火，包括如下步骤：
1)转炉冶炼：
转炉工序钢包顶渣改质，同时在转炉工序及RH脱碳过程不进行锰的预脱氧和合金化，严格控制中间包钢水氧含量在25ppm以下，以提高IF钢洁净度；严格控制出钢温度、挂罐温度和下渣量；
2)RH精炼：
RH工序钢包顶渣改质，RH进站钢水S含量控制在0.003％以下；RH进站定氧，脱氧合金化后加高纯稀土前定氧，高纯稀土加入前钢水中全氧含量T[O]m不大于20ppm，T[S]m不大于30ppm；真空脱碳、脱氧、合金化后，RH深真空至少2min后，在高位料仓添加高纯稀土，高纯稀土中全氧含量60-100ppm，高纯稀土加入后，RH深真空底吹氩时间不低于10min，破空后负压软吹时间不低于15min；
3)连铸环节技术要求：
采用中间包顶渣改质，并保证大包-中包-结晶器之间保证密闭性，避免连铸过程中吸气，控制连铸全程吸N量小于8ppm；浇注过热度控制较常规过热度控制提高5-15℃，防止絮瘤风险；连铸恒拉速控制；
4)常规轧钢及热处理工艺。
从本实施例得到的退火成品中提取多个样品，对改性后的IF钢成分、气体含量、夹杂物形貌及尺寸分布等进行详细分析：
表1：IF钢的成分及含量
Figure PCTCN2019108858-appb-000002
注：除RE为ppm外，其他元素均为wt％，余量为Fe和不可避免的杂质元素；对比例1的组份和制备控制工艺与实施例2-1相同，但不添加REM。
表2：IF钢夹杂物的典型尺寸及数量分布
Figure PCTCN2019108858-appb-000003
表3：IF钢夹杂物典型尺寸及数量分布(续表2)
编号 Dmax/μm Dmin/μm Dmean/μm Dmax/Dmin 面积比例
对比例1 1.890 1.027 1.464 1.841 0.146
实施例2-1 1.817 1.044 1.431 1.741 0.139
本实施例在IF钢中添加适量高纯稀土金属，一方面可使得钢中1-2μm级微细夹杂物数量显著增加8％(即从86.67％增加至94.67％)，5-10μm数量及比例明显降低，夹杂物最大直径(1.464μm→1.431μm)略有减小，而且较不添加稀土的IF钢而言，夹杂物数量(面积比例0.146→0.139)明显减少；另一方面IF钢中添加适量RE可起到明显变质夹杂的目的，结合SEM+EDS分析，发现RE可将大尺寸棒状/团簇状Al 2O 3夹杂物变质为近球状O-Al-S-RE/RE-O-S类化合物，尺寸更加细小且弥散分布；同时使得TiN、MnS类夹杂物失去Al 2O 3形核基底而难以形核长大，减轻此类夹杂对基体的割裂作用和各向异性。
实施例2-1的钢中夹杂物分布特征为：在22个视场中，夹杂物总数量小于250个，其中等效直径1-2μm之间的夹杂物比例≥94.5％，等效直径2-5μm之间的夹杂物比例＜5％、等效直径5-10μm之间的夹杂物比例＜0.5％。
结合JIS-5板材标样拉伸实验检测结果，证实RE-IF钢较传统IF钢而言在基本不改变其强度的前提下使得r值显著提高至少25％(1.820→2.267)，同时使得伸长率和强塑积得到明显提升。
表4：IF钢的典型性能指标
Figure PCTCN2019108858-appb-000004
实施例3
本实施例为一种超高强度F级海工钢夹杂物变性方法，生产工艺流程：铁水预处理-转炉冶炼-LF精炼-RH精炼-连铸-轧制-调质，控制工艺如下：
1)冶炼及稀土添加环节：稀土加入前，LF精炼保证白渣时间20min以上，钢水全氧含量T[O]m不高于10ppm，T[S]m含量不高于0.003％；高纯稀土金属在LF出站前加入或在RH净循环3min后加入，稀土加入时采用与钢水同材质的钢管包覆或采用铝箔包裹的形式，目的是避免稀土金属加入过程中氧化或与钢渣接触，稀土金属中全氧含量80-100ppm，其中实施例3-2的稀土加入量是实施例3-1的2倍，实施例3-2的稀土可分成两次加入；
2)稀土加入后RH深真空时间保证15min以上，RH复压，常规Ca处理后Ar气软吹时间保证15min以上；
3)连铸工艺中，大包-中包-结晶器之间保证密闭性，避免连铸过程中吸气，控制连铸全程吸N量小于5ppm；控制浇注过热度和连铸恒拉速，过热度较常规过热度控制提高5-15℃；
4)常规轧制与调质工艺。
通过上述工艺控制，从本实施例得到的调质成品中提取多个样品，对改性后的超高强度钢成分、气体含量、夹杂物形貌及尺寸分布等进行详细分析：
表5：超高强度钢的成分及含量
Figure PCTCN2019108858-appb-000005
注：除RE为ppm外，其他元素均为wt％，余量为Fe和不可避免的杂质元素；对比例2的组份和制备控制工艺与实施例3-1、实施例3-2相同，但不添加REM。
表6：超高强度钢的典型夹杂物尺寸分布
Figure PCTCN2019108858-appb-000006
表7：超高强度钢的典型夹杂物尺寸分布(续表6)
编号 Dmax/μm Dmin/μm Dmean/μm Dmax/Dmin 面积比例/‰
对比例2 34.40 1.78 4.37 19.33 0.45
实施例3-1 31.46 1.78 4.02 17.67 0.36
实施例3-2 19.53 1.78 4.21 10.97 0.37
研究结果表明：随RE加入量增加，夹杂物的最大直径Dmax逐渐减小(从34→31→19)，且＜2μm直径的夹杂物数量增加了至少4％，夹杂物总量平均减少18％(0.45‰→0.37‰)；添加RE后夹杂物平均等效直径Dmean降低了8％(4.37-4.02)，最大夹杂物直径/最小夹杂物直径明显减小，而夹杂物面积比例也有不同程度的下降。
实施例3-1、3-2的钢中夹杂物典型分布是：在20个视场中，夹杂物总数量小于500个，其中等效直径1-2μm之间的夹杂物比例＞10.5％，等效直径2-5μm之间的夹杂物比例60-80％、等效直径5-10μm之间的夹杂物比例＜22.5％、等效直径＞10μm的夹杂物比例＜5％。
结合SEM+EDS分析，无RE添加样品视场中存在大尺寸Al 2O 3团簇夹杂，其中大尺寸夹杂物碎化，并伴有长条状MnS类夹杂，而添加REM的实施例3-1和实施例3-2的样品中夹杂物多为球状或粒状的RE-O-S化合物，尺寸更加细小且弥散分布。
表8：超高强度钢的典型微观组织比较
Figure PCTCN2019108858-appb-000007
表9：超高强度钢的典型低温横向及纵向冲击性能
Figure PCTCN2019108858-appb-000008
注：表9中取样均为1/2板厚位置。
上述分析结果表明，在0℃至-40℃范围内，相较于不加RE的F级超高强度海工钢，添加适量高纯稀土金属对夹杂物的改性作用可使得F级超高强度海工钢低温横向及纵向冲击功得到全面提升：0℃下的横向冲击功至少提高30J，横向冲击功至少提高60J；-20℃下横向冲击功至少提高13J，纵向冲击功至少提高35J；-40℃下横向冲击功至少提高5J，纵向冲击功至少提高9J；尤其是1/2板厚位置处改善效果尤为突出。
以上实施例仅是本申请的优选实施方式，并不能理解为本申请保护范围的限制。应当指出的是，对于本领域的普通技术人员，在不脱离本申请构思的前提下，还可以做出若干变形、替代及改进，这些均属于本申请的保护范围。

Claims (10)

  1. 一种超净稀土钢，其特征在于：含有10-200ppm，优选10-100ppm，更优选10-50ppm的稀土元素，钢中夹杂物总数中50％以上，优选80％以上、更优选95％以上的部分为平均等效直径D mean为1-5μm、优选1-2μm的球状或近球状或粒状、且呈弥散分布的RE-氧-硫化物(RE 2O 2S)。
  2. 根据权利要求1所述的超净稀土钢，其特征在于：钢中稀土元素REM含量满足下式：
    -500＜REM-(m*T[O]m)+n*T[O]r+k*T[S]m)＜-30；
    其中REM为钢中稀土元素含量，单位ppm；
    T[O]m为钢中全氧含量，单位ppm；
    T[O]r为钢中加入的稀土金属或合金中全氧含量，单位ppm；
    T[S]m为钢中全硫含量，单位ppm；
    m为修正系数一，取值为2-4.5；
    n为修正系数二，取值为0.5-2.5；
    k为修正系数三，取值为0.5-2.5。
  3. 根据前述任一权利要求所述的超净稀土钢，其特征在于：所述钢为高端轴承钢、齿轮钢、模具钢、不锈钢、核电用钢、汽车用IF/DP/TRIP钢、或超高强度钢。
  4. 一种超净稀土钢，其特征在于：含有10-200ppm，优选10-100ppm，更优选10-50ppm的稀土元素，钢中夹杂物包括数量≥50％的稀土-氧-硫化物(RE 2O 2S)、≤50％的稀土-硫化物和0-10％的Al 2O 3夹杂物。
  5. 一种超净稀土钢，其特征在于：含有ppm级的稀土元素的超净稀土钢，钢中夹杂物总数中≥70％，优选≥80％、更优选≥95％为球状或近球状或粒状的呈弥散分布的RE-O-S夹杂物(RE 2O 2S)，TiN和MnS类夹杂物含量之和≤5％，RE-O-S夹杂物平均等效直径为1-5μm，优选1-2μm。
  6. 一种前述权利要求任一所述的超净稀土钢的夹杂物变性方法，其特征在 于：将钢中Al 2O 3夹杂物至少80％、优选至少90％、更优选至少95％变性为RE-氧-硫化物(RE 2O 2S)。
  7. 根据权利要求6所述的方法，其特征在于：高纯稀土金属或合金加入时钢水的全氧含量T[O]m≤25ppm，钢水的全硫含量T[S]m≤90ppm，高纯稀土金属或合金的全氧含量T[O]r控制在60-200ppm。
  8. 根据权利要求7所述的方法，其特征在于：加入高纯稀土后，RH或VD深真空循环时间满足下式：T＝(0.1-2.0)C RE+T 0，其中C RE为钢中稀土元素含量ppm，T 0为修正常数，取值为3-10min；以及Ar气软吹时间满足下式：t＝(0.05-3.0)C RE+t 0，其中C RE为钢中稀土元素含量(ppm)，t 0为修正常数，取值为5-10min。
  9. 根据权利要求7或8所述的方法，其特征在于：加入高纯稀土后，浇注过热度与相同成分但不含稀土的钢种相比，提高5-15℃；连铸全程增N量控制在8ppm以内。
  10. 一种超净稀土钢的夹杂物控制工艺，包括：
    a)在LF精炼时保证白渣时间20min以上，稳定渣碱度＞5，全硫含量T[S]m≤90ppm，全氧含量T[O]m≤25ppm；
    b)高纯稀土金属或合金在LF精炼出站前加入或在RH真空处理至少3min后加入，高纯稀土金属或合金中全氧含量60-200ppm；
    c)稀土加入后RH或VD深真空循环时间满足下式：T＝(0.1-2.0)C RE+T 0，其中C RE为钢中稀土元素含量ppm，T 0为修正常数，取值为3-10min；Ar气软吹时间满足下式：t＝(0.05-3.0)C RE+t 0，其中C RE为钢中稀土元素含量ppm，t 0为修正常数，取值为5-10min；
    d)连铸中强化大包-中包-结晶器之间密闭性和中包液面覆盖剂厚度，强化中包液面氩气吹扫，连铸全程增N量控制在8ppm以内；浇注过热度与相同成分但不含稀土的钢种相比，提高5-15℃。
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