Patent Application
20230295780
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.
In recent decade, after the double low oxygen technology, i.e. simultaneously controlling an initial oxygen content of rare earth metal itself and a total oxygen content of a steel melt, is applied, the effect of rare earth becomes abnormally stable and prominent, and many earlier applications of the inventors present related technology, including: 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 stagnated; CN201410141552.4, relating to a smelting method of ultra-low oxygen pure steel, wherein two times of vacuum carbon deoxygenation and further deoxygenation by adding rare earth are combined to reduce an oxygen content in a molten metal, reduce the number of inclusions in the alloy, improve the distribution of the inclusions, and relieve channel segregation, so that the product has fewer inclusions, the molten metal is pure, and the high-purity molten metal with an ultra-low oxygen content is obtained with high quality, but how to control the morphology, number, type and distribution of inclusions in the steel by adding the rare earth is not figured out; CN201610631046.2, relating to a method for adding rare earth metal into steel to improve performance, wherein by simultaneously controlling a molten steel to have T[O]s<20 ppm before adding the rare earth and the rare earth metal itself to have T[O]r<60 ppm, the problem of nozzle blockage is solved, and inclusion grains are refined, then the impact toughness of the steel is improved, but the influence of the rare earth addition on modification of the inclusions in the steel is not clear yet; CN201710059980.6, relating to a high-purity rare earth steel treatment method, wherein an addition amount of rare earth is based on dissolved oxygen Odissolved oxygen, total oxygen T.O, a sulfur content S, refining slag alkalinity R=CaO/SiO2, and a total content of FeO+MnO in a molten steel, but the relationship between the dissolved oxygen Odissolved oxygen, the total oxygen T.O, the sulfur content S, the refining slag alkalinity R, the total content of FeO+MnO and the addition amount of rare earth and also the influence of the addition amount of rare earth on Odissolved oxygen, T.O, S, R and the total content of FeO+MnO are not studied, then clear guidance on the production practice of different varieties of high-purity rare earth steel is lacked.
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 REM2O3, the addition amount of REM satisfies this formula, to prevent Al2O3 from not reacting, and convert the alumina inclusions in the steel into REM oxide. However, although the REM-oxygen-sulfide is mentioned in this document, 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 invention application 201811319185.7 of Cheng Guoguang et al. of University of Science and Technology Beijing pointed out that the rare earth Ce has a good modification effect on MgAl2O4, but only when the content of rare earth in the bearing steel is controlled to be 0.002% (i.e., 20 ppm), composite inclusions with CeAlO3 coated by TiN are obtained, when the content of rare earth in the bearing steel reaches 0.004% (i.e., 40 ppm), types of main inclusions in the bearing steel are Ce2O3 and separate TiN inclusions, when the content of rare earth is increased to 0.007% (i.e., 70 ppm), the types of main inclusions in the bearing steel are also separate Ce2O3 and TiN. As the content of rare earth in the steel is further increased, Ce2O3 stably exists in the steel. Although the content of the inclusions TiN in the steel is reduced, lattice matching property between Ce2O3 and TiN inclusions is unfavorable, then the formation of a large amount of rare earth oxides instead deteriorates the mechanical properties of the steel.
Currently, the influence of the rare earth addition on the modification of the inclusions in the steel is not clear yet, the controllability after the rare earth addition is poor, and systematic in-depth research is lacked, then the production control process of the modification of inclusions is difficult and poor in stability, greatly restricting the promotion and application of the low-cost rare earth on the preparation of high-performance steel, such as bearing, gear, mold, stainless steel, steel for nuclear power, steel for automotive, and various key components.
In order to realize precise control over the type, distribution and size of inclusions (occluded foreign substance) modification in steel after rare earth addition, and be adapted to research, development and production of more varieties of high-performance steel, 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.
In order to achieve the above objective, the present application mainly provides the following technical solution:
In one aspect, 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 (RE2O2S) with a mean equivalent diameter Dmean of 1-5 μm, in a spherical shape or a spheroidal shape or a granular shape, and in dispersed distribution.
In the above, the RE-oxygen-sulfide has a gentle boundary with Fe matrix, and good compatibility with the Fe matrix.
In the above, the equivalent diameter is obtained from (maximum particle size+minimum particle size)/2 by measuring the inclusions.
Preferably, a rare earth content in the ultra-clean rare earth steel satisfies the following formula (1):
−500<REM−(m*T[O]m)+n*T[O]r+k*T[S]m)<−30 (1).
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 (RE2O3) predominating, and it is simultaneously ensured that the formed RE-oxygen-sulfide (RE2O2S) has a mean equivalent diameter of 1-5 μm, 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 RE2O2S.
After test, it was found that the tension-compression fatigue life of the REM-modified high-purity bearing steel is improved 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 of the REM-modified high-purity bearing steel reaches 3.08*107 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; compared with the conventional IF steel, 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; compared with the high-strength steel without addition of RE, 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.
Preferably, 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.
On the other hand, 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 (RE2O2S), 50% or less of rare earth-sulfide, and 0-10% of Al2O3 inclusions.
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 μm; 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 Al2O3 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. After the high-purity rare earth metal or alloy is added, RH deep vacuum circulation time satisfies the following formula (2):
T=(0.1-2.0)CRE+T0 (2)
In the above, CRE is the content of rare earth elements in the steel, and T0 is a correction constant, with a value of 3-10 min; and Ar gas soft blowing time satisfies the following formula (3):
t=(0.05-3.0)CRE+t0 (3)
In the above, CRE is the content of rare earth elements in the steel, and to 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).
Moreover, after the addition of the high-purity rare earth, 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:
Preferably, the addition amount of high-purity rare earth in the step 3) satisfies WRE≥α ×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 WRE is an addition amount of the high-purity rare earth metal or alloy;
In the above, 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 μm, but when the T[O]r is increased to 200 ppm, the diameter of the rare earth metal oxide will be more than 10 μm, 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:
In the above, in the continuous casting, 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 SiO2 contents of a long nozzle of laddle, a stopper rod of tundish, and a submerged nozzle are less than 5%.
In another aspect, the present application further provides a control method of inclusions in ultra-low-RE IF/DP/TRIP steel, including the following steps:
Preferably, 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. Through three modification processes, the fluidity of the slag is improved, the capability of removing the inclusions is improved, and the cleanness of the steel is ensured.
Besides, 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:
It should be pointed out that there are three reasons for important changes of the inclusions in the steel in the present application. One is that 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 Al2O3 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; and 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 present application has the following prominent technical effects:
The present application is further described below in combination with specific embodiments, but the scope of protection of the present application is not limited thereto.
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:
W
RE
≥α×T[O]m+T[S]m,
where α is a correction coefficient, with a value of 6-30, preferably 8-20, T[O]m is a total oxygen content (ppm) in the steel, and T[S]m is a total sulfur content (ppm) in the steel;
A plurality of samples were extracted from rolled products obtained in the present example, and the inclusions in the modified GCr15 steel were analyzed. 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
Conventional hard brittle Al2O3 oxides and strip-shaped MnS inclusions (>100 μm) are quite common in the high-purity GCr15 steel [
The RE2O2S inclusions have much lower elasticity, Young's modulus, shear modulus and hardness than the conventional Al2O3 inclusions, and these results are also confirmed by the current nano-indentation experiment measurements [
The present example provides a method of modifying Al2O3 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:
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:
In the present example, 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 μm 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 μm level can be obviously decreased, the maximum diameter (1.464 μm→1.431 μm) 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 Al2O3 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 Al2O3 nucleation matrix, thus it is difficult for the nucleation to grow large, thereby reducing the cleavage effect and anisotropy of such inclusions on the matrix.
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 μm is 94.5% or greater, the proportion of the inclusions with an equivalent diameter of 2-5 μm is less than 5%, and the proportion of the inclusions with an equivalent diameter of 5-10 μm is less than 0.5%.
In conjunction with the testing results of tension test of JIS-5 sheet standard samples, it is confirmed that compared with the conventional IF steel, 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.
Example 3
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:
Through the above process control, a plurality of samples were extracted from tempered products obtained in the present example, and the modified ultra-high-strength steel was analyzed in detail in terms of component, gas content, morphology and size distribution of inclusions and so on:
Note: except that RE is in ppm, all of other elements are in wt %, and the balance is Fe and inevitable impurity elements; the components and the preparation and control process of Comparative Example 2 are the same as those of Example 3-1 and Example 3-2, but without REM.
Study results indicate that as the addition amount of RE increases, the maximum diameter Dmax of the inclusions gradually decreases (34→31→19), and that the number of inclusions with a diameter of less than 2 μm increases by at least 4%, the total number of inclusions decreases by a mean of 18% (0.45% c->0.37% o); after the addition of RE, the mean equivalent diameter Dmean of the inclusions is reduced by 8% (4.37-4.02), the maximum/minimum inclusion diameter is obviously reduced, and the area proportion of the inclusions is also reduced to different degrees.
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 μm is greater than 10.5%, the proportion of the inclusions with an equivalent diameter of 2-5 μm is 60-80%, the proportion of the inclusions with an equivalent diameter of 5-10 μm is less than 22.5%, and the proportion of the inclusions with an equivalent diameter of less than 10 μm is less than 5%.
In conjunction with the SEM+EDS analysis, there were large-sized Al2O3 cluster inclusions in the field of samples without adding RE, in which the large-size inclusions were comminuted, accompanied by strip-shaped MnS inclusions, while the inclusions in the samples of Example 3-1 and Example 3-2 with REM were mostly spherical or granular RE-O—S compounds, with a smaller size and in dispersed distribution.
Note: All the samples in Table 9 were taken at positions of ½ plate thickness.
The above analysis results indicate that, in the range of 0° C. to −40° C., compared with the F grade ultra-high-strength marine steel without addition of RE, 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 ½ plate thickness is especially remarkable.
The examples above are merely preferred embodiments of the present application, but should not be construed as limitation on the scope of protection of the present application. It should be indicated that a person ordinarily skilled in the art still could make several modifications, substitutions and improvements without departing from the concept of the present application, all of which fall within the scope of protection of the present application.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201910855025.2 | Sep 2019 | CN | national |
This Application is a divisional of application Ser. No. 17/611,061. The present application claims priorities from application Ser. No. 17/611,061, filed Nov. 12, 2021, from PCT Application No. PCT/CN2019/108858, filed Sep. 29, 2019, and from the Chinese patent application with the filing No. 201910855025.2, filed on Sep. 10, 2019 with the Chinese Patent Office, and entitled “Ultra-clean Rare Earth Steel and Occluded Foreign Substance Modification Control Method”, the contents of which are incorporated herein by reference in entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17611061 | Nov 2021 | US |
| Child | 18142152 | US |
