US20220259707A1 - Ultra-clean rare earth steel and occluded foreign substance modification control method - Google Patents

Ultra-clean rare earth steel and occluded foreign substance modification control method Download PDF

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US20220259707A1
US20220259707A1 US17/611,061 US201917611061A US2022259707A1 US 20220259707 A1 US20220259707 A1 US 20220259707A1 US 201917611061 A US201917611061 A US 201917611061A US 2022259707 A1 US2022259707 A1 US 2022259707A1
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steel
rare earth
ppm
inclusions
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Dianzhong Li
Yikun Luan
Hongwei Liu
Paixian Fu
Xiaoqiang Hu
Pei Wang
Lijun Xia
Chaoyun YANG
Hanghang LIU
Yang Liu
Peng Liu
Yiyi Li
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Institute of Metal Research of CAS
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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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • 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

Definitions

  • the present application belongs to the field of alloys, and relates to ultra-clean rare earth steel and an occluded foreign substance modification control method.
  • CN201610265575.5 relating to a method for preparing a high-purity rare earth metal, which can avoid the generation of coarse inclusions, property fluctuations of material, nozzle blockage during production, etc., but does not study the influence of the high-purity rare earth metal on inclusions in the steel
  • CN201611144005.7 relating to an extra-low oxygen rare earth alloy and use, wherein a high-purity rare earth alloy is used for treating steel, and a comparison diagram of treated inclusions and rating of the inclusions are given, but the influence of addition amount of the high-purity rare earth alloy on the size, number, and type of the inclusions fail to be figured out, so that the research, development and innovation of high-performance rare earth steel have been advanced slowly and even almost
  • Nippon Steel CN1759199A relates to bearing steel containing fine inclusions, wherein oxide inclusions in the steel are converted into REM oxide inclusions by controlling an addition amount of REM to the bearing steel at ⁇ 30 ⁇ REM ⁇ (T.O. ⁇ 280/48) ⁇ 50, where 280/48 is obtained according to a stoichiometric ratio of REM to O in REM 2 O 3 , the addition amount of REM satisfies this formula, to prevent Al 2 O 3 from not reacting, and convert the alumina inclusions in the steel into REM oxide.
  • the purpose of controlling the addition amount of REM is to address the formation of the REM oxide inclusions, fundamentally without consideration of the influence of the change of O content in the steel on the inclusions caused by the addition of REM, or the influence of the impurity element S or the like on the inclusions, and the resulting rolling contact fatigue life of pure bearing steel containing rare earth inclusions is 3.2-9.2 times that without the addition of REM.
  • the inventor team proposes ultra-clean rare earth steel containing a ppm-level rare earth element and a modification control method thereof through continuous research, development, innovations and close combination with engineering practice.
  • the present application mainly provides the following technical solution:
  • an example of the present application provides 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 50% or more, preferably 80% or more, and more preferably 95% or more, of total number of inclusions in the steel being RE-oxygen-sulfide (RE 2 O 2 S) with a mean equivalent diameter D mean of 1-5 ⁇ m, in a spherical shape or a spheroidal shape or a granular shape, and in dispersed distribution.
  • RE 2 O 2 S RE-oxygen-sulfide
  • the RE-oxygen-sulfide has a gentle boundary with Fe matrix, and good compatibility with the Fe matrix.
  • the equivalent diameter is obtained from (maximum particle size+minimum particle size)/2 by measuring the inclusions.
  • a rare earth content in the ultra-clean rare earth steel satisfies the following formula (1):
  • REM is the content of rare earth elements in the steel, in ppm;
  • T[O]m is a total oxygen content in the steel, in ppm;
  • T[O]r is a total oxygen content in a rare earth metal or alloy added to the steel, in ppm;
  • T[S]m is a total sulfur content in the steel, in ppm;
  • m is the first correction coefficient, with a value of 2-4.5, preferably 3-4.5;
  • n is the second correction coefficient, with a value of 0.5-2.5, preferably 1-2.2;
  • k is the third correction coefficient, with a value of 0.5-2.5, preferably 1-2.2.
  • the research of the inventor team shows that: by specifying that the rare earth content REM in the ultra-clean rare earth steel, the total oxygen content and the total sulfur content in the molten steel, and the total oxygen content in the rare earth metal or alloy added to the steel satisfy the above formula (1), it can be obtained that fine and dispersed RE-oxygen-sulfide (RE2O2S) accounts for 50% or more, preferably 80% or more, 95% or more of the total number of inclusions, rather than rare earth oxide (RE 2 O 3 ) predominating, and it is simultaneously ensured that the formed RE-oxygen-sulfide (RE 2 O 2 S) has a mean equivalent diameter of 1-5 ⁇ m, in a spherical shape, a spheroidal shape or a granular shape, and in dispersed distribution.
  • the various correction coefficients above are empirical coefficients for ensuring the formation of RE 2 O 2 S.
  • the tension-compression fatigue life of the REM-modified high-purity bearing steel is improved to 4.1*10 8 times, which is more than 40 times that of the existing high-purity bearing steel, and the rolling contact fatigue life of the REM-modified high-purity bearing steel reaches 3.08*10 7 times, which is 910 ten thousand times higher than that of the existing high-purity bearing steel, thus the fatigue life of the REM-modified high-purity bearing steel is significantly improved;
  • the RE-IF steel has the r value significantly improved by 25%, and meanwhile the elongation and the product of strength and ductility of RE-IF steel obviously get improved without changing the strength thereof;
  • the ultra-high-strength steel is comprehensively improved in low-temperature transverse and longitudinal impact energies in the range of 0° C. to ⁇ 40° C. after the addition of ultra-low content RE.
  • the steel is high-level bearing steel, gear steel, mold steel, stainless steel, steel for nuclear power, IF/DP/TRIP steel for automobile, or ultra-high-strength steel.
  • the present application further provides ultra-clean rare earth steel containing 10-200 ppm, preferably 10-100 ppm, more preferably 10-50 ppm, of rare earth elements, and inclusions in the steel include, in number, 50% or more of rare earth-oxygen-sulfide (RE 2 O 2 S), 50% or less of rare earth-sulfide, and 0-10% of Al 2 O 3 inclusions.
  • RE 2 O 2 S rare earth-oxygen-sulfide
  • Al 2 O 3 inclusions Al 2 O 3
  • the present application further provides ultra-clean rare earth steel containing ppm-level rare earth elements, wherein 70% or more, preferably 80% or more, more preferably 95% or more, of the total number of inclusions in the steel are O—Al—S—RE and/or RE—O—S inclusions in a spherical shape or a spheroidal shape and in dispersed distribution, a sum of contents of TiN and MnS inclusions is 5% or less, and the inclusions have a mean equivalent diameter of 1-2 ⁇ m; further, the rare earth element content in the steel is 10-200 ppm, preferably 10-100 ppm, and more preferably 10-50 ppm.
  • a method of modifying inclusions in ultra-clean rare earth steel of the present application is modifying at least 80%, preferably at least 90%, more preferably at least 95%, of Al 2 O 3 inclusions already existing in the steel into RE-oxygen-sulfide, wherein when a high-purity rare earth metal or alloy is added, a total oxygen content T[O]m of the molten steel is 25 ppm or less, a total sulfur content T[S]m of the molten steel is 90 ppm or less, and a total oxygen content T[O]r of the high-purity rare earth metal or alloy is controlled at 60-200 ppm.
  • RH deep vacuum circulation time satisfies the following formula (2):
  • C RE is the content of rare earth elements in the steel
  • T 0 is a correction constant, with a value of 3-10 min
  • Ar gas soft blowing time satisfies the following formula (3):
  • C RE is the content of rare earth elements in the steel
  • t 0 is a correction constant, with a value of 5-10 min.
  • the VD deep vacuum time refers to total time for degassing the molten steel after a certain vacuum degree in VD furnace is reached (usually 67 Pa or below);
  • the RH deep vacuum time refers to total time for degassing the molten steel after a certain vacuum degree in RH furnace is reached (usually 200 Pa or below).
  • a superheat of casting is increased by 5-15° C. compared with the steel containing the same components but without rare earth; and an N addition in the whole continuous casting is controlled within 8 ppm.
  • the present application further provides a control process of inclusions in ultra-clean rare earth steel, including:
  • the addition amount of high-purity rare earth in the step 3) satisfies W RE > ⁇ T[O]m+T[S]m, where a is a correction coefficient, with a value of 6-30, preferably 8-20, T[O]m is a total oxygen content in the steel, T[S]m is a total sulfur content in the steel, and W RE is an addition amount of the high-purity rare earth metal or alloy;
  • the T[O]r of the high-purity rare earth metal is controlled at 60-200 ppm, because when T[O]r is controlled at less than 60 ppm, the rare earth metal oxide is mainly formed, with an equivalent diameter of less than 2 ⁇ m, but when the T[O]r is increased to 200 ppm, the diameter of the rare earth metal oxide will be more than 10 ⁇ m, wherein the rare earth metal oxide can hardly float up, and will remain in the melt after solidification, thus deteriorating performance of the steel.
  • the present application further provides a control process of inclusions in ultra-low-RE bearing steel, wherein a process flow includes electric arc furnace smelting ⁇ LF refining ⁇ RH refining ⁇ continuous casting ⁇ heating ⁇ rolling, and steps are as follows:
  • a high-purity rare earth metal or alloy after performing RH vacuum treatment for at least 5 min, adding a high-purity rare earth metal or alloy, with an addition amount of the high-purity rare earth satisfying W RE > ⁇ T[O]m+T[S]m, where a is a correction coefficient, with a value of 6-30, preferably 8-20, T[O]m is a total oxygen content in the steel, T[S]m is a total sulfur content in the steel;
  • a superheat of casting is increased by 5-15° C. compared with the bearing steel having the same components but without RE, and an Al content at the end of RH refining is controlled to be 0.015-0.030%; in the continuous casting, a MgO content of an working lining of a tundish is greater than 85%, and SiO 2 contents of a long nozzle of laddle, a stopper rod of tundish, and a submerged nozzle are less than 5%.
  • the present application further provides a control method of inclusions in ultra-low-RE IF/DP/TRIP steel, including the following steps:
  • top slag of a converter ladle is modified, and the content T[O]m of molten steel in the tundish is controlled to be 25 ppm or less; top slag of RH refining ladle is modified, and the S content of the molten steel before RH refining is controlled to be 0.005% or less; and tundish top slag is modified in the continuous casting.
  • the fluidity of the slag is improved, the capability of removing the inclusions is improved, and the cleanness of the steel is ensured.
  • the present application provides a control process of inclusions in ultra-low-RE and ultra-high-strength steel, wherein a process flow is: converter smelting ⁇ LF refining ⁇ RH refining ⁇ continuous casting ⁇ rolling ⁇ quenching and tempering, and includes the following steps:
  • the RE has strong affinity with oxygen and sulfur, then it is easy to rapidly form the RE-oxygen-sulfide/RE-sulfide, and meanwhile, most of the existing Al 2 O 3 inclusions are modified into the RE-oxygen-sulfide;
  • the second one is that in the process of molten steel refining, the rare earth-oxygen-sulfide/rare earth-sulfide formed by argon soft blowing partially float up, thereby reducing the number of inclusions;
  • the third one is that as the oxygen content in the melt is low, the rare earth-oxygen-sulfide is not easy to grow up, and has good wettability with the steel melt, then it is not easy to gather together.
  • a reaction formula of modification of the present application is as follows:
  • the hardness of the RE-oxygen-sulfide inclusions in the rare earth-modified steel is lower than that of the Al 2 O 3 inclusions, and has good plastic deformation capability, which results in low micro-stress/strain concentration at the boundary, and reduces the possibility of crackage caused by strain concentration, wherein the fatigue life of the RE-modified high-purity bearing steel is increased to 4.1*10 8 times, which is more than 40 times that of the existing high-purity bearing steel, and the rolling contact fatigue life reaches 3.08*10 7 , which is 910 ten thousand times higher than that of the existing high-purity bearing steel, thus the fatigue life of the RE-modified steel is significantly improved; compared with the conventional IF steel, the RE-IF steel has the r value significantly improved by 25%, and meanwhile obviously improved the elongation and the product of strength and ductility, without changing the strength thereof; compared with the high-strength steel without addition of RE, the ultra-high-strength steel is comprehensively improved in low-temperature
  • the rare earth content REM in the ultra-clean rare earth steel, the total oxygen content in the molten steel, and the total oxygen content in the rare earth metal or alloy added to the steel satisfy the above formula it is controlled to obtain that the RE-oxygen-sulfide (RE 2 O 2 S) accounts for 50% or more of the total number of inclusions, rather than the rare earth oxide (RE 2 O 3 ) predominating, and the size of the RE-oxygen-sulfide is minimized, and the RE-oxygen-sulfide with an equivalent diameter of 1-5 ⁇ m, in a spherical shape, a spheroidal shape or a granular shape, and in dispersed distribution can be obtained; and
  • FIG. 1 effect of rare earth addition on tension-compression fatigue and rolling contact fatigue properties of bearing steel GCr15, wherein
  • (b) shows the rolling contact fatigue life of the bearing steel at a load Fa of 8.82 KN and a rotating speed of 2000 r/min;
  • FIG. 2 effects of the rare earth on the modification of the inclusions, including mechanical properties of the inclusions, morphology and distribution of the inclusions, and morphology and distribution of the inclusions after fatigue failure, wherein
  • the present example provides a method of modifying inclusions in RE-GCr15 bearing steel, wherein a process flow is electric arc furnace smelting ⁇ LF refining ⁇ RH refining ⁇ continuous casting ⁇ heating ⁇ rolling, and includes the following steps:
  • LF refining reasonably adjusting a refining slag system, stabilizing slag alkalinity to be greater than 5, ensuring white slag time to be 20 min or more, and controlling molten steel to have T[O]m of 10 ppm or less and content T[S]m to be not higher than 0.005%;
  • RH refining adding a high-purity rare earth metal to an overhead bin after at least 5 min of RH vacuum treatment, with an addition amount of the high-purity rare earth satisfying the following formula:
  • T[O]m is a total oxygen content (ppm) in the steel
  • T[S]m is a total sulfur content (ppm) in the steel
  • T[O]r of the high-purity rare earth metal to be 60-200 ppm, ensuring RH deep vacuum circulation time to be 10 min or more after adding the high-purity rare earth metal, ensuring Ar gas soft blowing time to be 10 min or more, so that formed rare earth-oxygen-sulfide floats up, thereby reducing the number of inclusions, and controlling Al element content at the end of RH refining to be 0.015-0.030%, and rare earth element content in the molten steel to be 15-30 ppm;
  • an MgO content of an working lining of the tundish to be more than 85%, and an SiO 2 contents in a ladle long nozzle, a tundish stopper rod and a submerged nozzle to be less than 5%, to ensure compactness and corrosion resistance of the tundish and also erosion resistance and corrosion resistance of the three main components; and performing constant casting speed in the continuous casting; and 5) a conventional rolling process.
  • results show that: compared with the high-purity GCr15 steel without addition of rare earth, the modification of inclusions by adding the high-purity rare earth enables the RE-GCr15 steel to generate unprecedented excellent fatigue property, as shown in FIG.
  • the addition of rare earth elements changes the law of the fatigue life, in the cyclic load tension/compression experiments of maximum stress of ⁇ 800 MPa and 20 kHz, the tension-compression fatigue life of the RE-GCr15 steel is improved to 4.1*10 8 times and is more than 40 times that of the high-purity GCr15 steel (the tension-compression fatigue life reported on the literature is about 10*10 6 times), and the addition of rare earth reduces the number of inclusions by 50% or more [ FIG. 2( a ) ] and reduces the inclusions of 5 ⁇ m or more by at least 35%.
  • the rolling contact fatigue life of the RE-GCr15 steel in FIG. 1 b is also greatly improved.
  • the rolling contact fatigue life of the RE-GCr15 steel is 3.08 ⁇ 10 7 , which is 910 ten thousand times higher than that of the high-purity GCr15 steel.
  • the RE 2 O 2 S inclusions have much lower elasticity, Young's modulus, shear modulus and hardness than the conventional Al 2 O 3 inclusions, and these results are also confirmed by the current nano-indentation experiment measurements [ FIG. 2( b ) ]. As the RE 2 O 2 S inclusions have better compatibility with the Fe matrix than the conventional hard Al 2 O 3 inclusions, the non-uniform degree of internal micro-stress and strain concentration will be far lower than that of the conventional steel. As shown by the results of EDS and/or selected area diffraction patterns shown in f of FIG.
  • the composite inclusions consist of RE-O-S inclusions ( ⁇ 85%) and/or O-Al-S-RE inclusions, rare earth-sulfide ( ⁇ 10%), and a very small amount ( ⁇ 5%) of Al 2 O 3 inclusions ( FIG. 2( f ) ), and after the tension-compression loading circulation, many dislocations appear inside the rare earth-oxygen-sulfide inclusions ( FIG. 2( g ) , but laths in the matrix in the vicinity of the rare earth-oxygen-sulfide and the rare earth-sulfide are still intact, and boundaries between the laths are still clear; in contrast, Al 2 O 3 particles almost have no dislocation inside, the laths crack, and the boundaries between them disappear.
  • This comparison indicates that the rare earth-oxygen-sulfide has lower hardness than the Al 2 O 3 inclusions and better plastic deformability, resulting in lower micro-stress/strain concentration at the boundaries, further reducing the cracking probability caused by strain concentration.
  • the present example provides a method of modifying Al 2 O 3 inclusions in IF steel, wherein a process flow is: molten iron reladling station—molten iron pretreatment—converter smelting—RH refining—continuous casting—hot rolling—acid pickling—cold rolling—annealing, and includes the following steps:
  • modifying the ladle top slag in an RH refining process and controlling an S content of the molten steel to be 0.003% or less when entering the RH refining; controlling the oxygen content before entering the RH refining and also before adding the high pure rare earth but after deoxygenation and alloying, wherein the total oxygen content T[O]m was not more than 20 ppm, and T[S]m was not more than 30 ppm in the molten steel before adding the high-purity rare earth; after vacuum decarburization, deoxygenation and alloying, and after at least 2 min of RH deep vacuum, adding the high-purity rare earth with a total oxygen content of 60-100 ppm to an overhead bin, after adding the high-purity rare earth, making RH deep vacuum bottom argon blowing time not less than 10 min, and negative pressure soft blowing time not less than 15 min after the vacuum being broken;
  • a plurality of samples were extracted from annealed products obtained in the present example, and the modified IF steel was analyzed in detail in terms of components, gas content, morphology and size distribution of inclusions, and so on:
  • an appropriate amount of high-purity rare earth metal is added to the IF steel, then on the one hand, the number of fine inclusions of 1-2 ⁇ m level in the steel can be significantly increased by 8% (namely, from 86.67% to 94.67%), the number and proportion of fine inclusions of 5-10 ⁇ m level can be obviously decreased, the maximum diameter (1.464 ⁇ m ⁇ 1.431 ⁇ m) of the inclusions can be slightly decreased, and compared with the IF steel without addition of rare earth, the number of inclusions (area proportion 0.146 ⁇ >0.139) is obviously decreased; on the other hand, adding an appropriate amount of RE to the IF steel can achieve the purpose of obviously modifying the inclusions, and in conjunction with SEM+EDS analysis, it is found that RE can modify large-size rod-like/clustered Al 2 O 3 inclusions into O—Al—S-RE/RE-O—S compounds in a spheroidal shape, with finer size and in dispersed distribution; meanwhile, TiN and MnS inclusions lose the
  • the distribution of the inclusions in the steel of Example 2-1 are characterized in that, in 22 fields, the total number of inclusions is less than 250, wherein the proportion of the inclusions with an equivalent diameter of 1-2 ⁇ m is 94.5% or greater, the proportion of the inclusions with an equivalent diameter of 2-5 ⁇ m is less than 5%, and the proportion of the inclusions with an equivalent diameter of 5-10 ⁇ m is less than 0.5%.
  • the RE-IF steel has the r value significantly increased by at least 25% (1.820 ⁇ 2.267), and meanwhile obviously improved the elongation and the product of strength and ductility without substantially changing the strength thereof.
  • the present example provides a method of modifying inclusions in ultra-high-strength F grade marine steel, wherein a process flow is: molten iron pretreatment—converter smelting—LF refining—RH refining—continuous casting—rolling—quenching and tempering, and a control process is as follows:
  • Typical distribution of the inclusions in the steel of Example 3-1 and Example 3-2 is as follows: in 20 fields, the total number of inclusions is less than 500, wherein the proportion of the inclusions with an equivalent diameter of 1-2 ⁇ m is greater than 10.5%, the proportion of the inclusions with an equivalent diameter of 2-5 ⁇ m is 60-80%, the proportion of the inclusions with an equivalent diameter of 5-10 ⁇ m is less than 22.5%, and the proportion of the inclusions with an equivalent diameter of less than 10 ⁇ m is less than 5%.
  • the modification effect of the addition of an appropriate amount of high-purity rare earth metal on the inclusions can allow the low-temperature transverse and longitudinal impact energies of the F grade ultra-high-strength marine steel to be fully improved: at 0° C., the transverse impact energy is increased by at least 30 J, and the transverse impact energy is increased by at least 60 J; at ⁇ 20° C., the transverse impact energy is increased by at least 13 J, and the longitudinal impact energy is increased by at least 35 J; at ⁇ 40° C., the transverse impact energy is increased by at least 5 J, and the longitudinal impact energy is increased by at least 9 J; in particular, the improvement effect at the positions of 1 ⁇ 2 plate thickness is especially remarkable.

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