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

Abstract

Provided are an ultra-clean rare earth steel and an occluded foreign substance modification control method, the steel includes 10-200 ppm of rare earth elements, 50% or more occluded foreign substances in the steel are dispersed into RE-oxygen-sulfide with the average equivalent diameter Dmean ranging from 1-5 μm in a spherical shape or a substantially spherical shape or a granular shape; according to the method, at least 80%, preferably at least 90%, of Al2O3 occluded foreign substances in the steel are modified into RE-oxygen-sulfide, compared with steel with the same components without rare earth, the total amount of the occluded foreign substances in the steel is reduced by 18% or higher, the cracking probability caused by occluded foreign substances such as Al2O3 in traditional pure steel is reduced, the mechanical performance such as the fatigue life of the steel is remarkably improved.

1 - 12  (canceled) 
     
     
         13 . An ultra-clean rare earth steel, containing 10-200 ppm of rare earth elements, wherein 50% or more of total number of inclusions in the steel are RE-oxygen-sulfides (RE 2 O 2 S) with a mean equivalent diameter D mean  of 1-5 μm in a spherical shape or a spheroidal shape or a granular shape, and in dispersed distribution, and a content of rare earth elements REM in the steel satisfies the following formula:
   −500<REM−( m*T[O]m )+ n*T[O]r+k*T[S]m )<−30,
 
 where 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; 
 n is the second correction coefficient, with a value of 0.5-2.5; and 
 k is the third correction coefficient, with a value of 0.5-2.5. 
 
     
     
         14 . The ultra-clean rare earth steel according to  claim 13 , wherein the steel contains 10-100 ppm of rare earth elements. 
     
     
         15 . The ultra-clean rare earth steel according to  claim 14 , wherein the steel contains 10-50 ppm of rare earth elements. 
     
     
         16 . The ultra-clean rare earth steel according to  claim 13 , wherein the content of RE-oxygen-sulfides (RE 2 O 2 S) accounts for 80% or more of total number of inclusions in the steel. 
     
     
         17 . The ultra-clean rare earth steel according to  claim 16 , wherein the content of RE-oxygen-sulfide (RE 2 O 2 S) accounts for 95% or more of total number of inclusions in the steel. 
     
     
         18 . The ultra-clean rare earth steel according to  claim 13 , wherein the mean equivalent diameter D mean  of RE-oxygen-sulfide (RE 2 O 2 S) in the steel is 1-2 μm. 
     
     
         19 . The ultra-clean rare earth steel according to  claim 13 , wherein the steel is high-level bearing steel, gear steel, mold steel, stainless steel, steel for nuclear power, IF/DP/TRIP steels for automobile or ultra-high-strength steel. 
     
     
         20 . The ultra-clean rare earth steel according to  claim 13 , wherein inclusions in the steel comprise 50% or more of rare earth-oxygen-sulfides (RE 2 O 2 S), 50% or less of rare earth-sulfides, and 0-10% of Al 2 O 3  inclusions. 
     
     
         21 . The ultra-clean rare earth steel according to  claim 20 , wherein inclusions in the steel comprise 85% or more of rare earth-oxygen-sulfides (RE 2 O 2 S), 10% or less of rare earth-sulfides, and 5% or less of Al 2 O 3  inclusions. 
     
     
         22 . An ultra-clean rare earth steel, containing ppm-level rare earth elements, wherein 70% or more of the total number of inclusions in the steel are RE-O—S inclusions (RE 2 O 2 S) in a spherical shape or a spheroidal shape or a granular shape and in dispersed distribution, a sum of contents of TiN and MnS inclusions is 5% or less, and the RE-O-S inclusions have a mean equivalent diameter of 1-5 μm, wherein a proportion of RE-O—S inclusions with the equivalent diameter of 1-2 μm is greater than or equal to 94.5%, and the proportion of RE-O—S inclusions with the equivalent diameters of 2-5 μm is less than 5%. 
     
     
         23 . The ultra-clean rare earth steel according to  claim 22 , wherein 80% or more of the total number of inclusions in the steel are RE-O—S inclusions (RE 2 O 2 S). 
     
     
         24 . The ultra-clean rare earth steel according to  claim 23 , wherein 95% or more of the total number of inclusions in the steel are RE-O—S inclusions (RE 2 O 2 S). 
     
     
         25 . A method of modifying inclusions in the ultra-clean rare earth steel according to  claim 13 , wherein the content of rare earth elements REM in the ultra-clean rare earth steel, the total oxygen content T[O]m in the steel, the total sulfur content T[S]m in the steel, and the total oxygen content T[O]r in the rare earth metal or alloy added to the steel are controlled to satisfy the following formula:
   −500<REM−( m *T [O] m )+ n *T [O] r+k *T [S] m )<−30,
   where 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;   n is the second correction coefficient, with a value of 0.5-2.5;   k is the third correction coefficient, with a value of 0.5-2.5; and   at least 80% of Al 2 O 3  inclusions in the steel are modified into RE-oxygen-sulfides (RE 2 O 2 S).   
     
     
         26 . The method according to  claim 25 , wherein at least 90% of Al 2 O 3  inclusions in the steel are modified into RE-oxygen-sulfides (RE 2 O 2 S). 
     
     
         27 . The method according to  claim 26 , wherein at least 95% of Al 2 O 3  inclusions in the steel are modified into RE-oxygen-sulfides (RE 2 O 2 S). 
     
     
         28 . The method according to  claim 25 , wherein when a high-purity rare earth metal or alloy is added, the total oxygen content T[O]m of molten steel is 25 ppm or less, the total sulfur content T[S]m of the molten steel is 90 ppm or less, and the total oxygen content T[O]r of the high-purity rare earth metal or alloy is controlled at 60-200 ppm. 
     
     
         29 . The method according to  claim 25 , wherein after the high-purity rare earth is added, RH or VD deep vacuum circulation time satisfies the following formula: T=(0.1-2.0)C RE +T 0 , where C RE  is a content (in ppm) of rare earth elements in the steel, and T 0  is a correction constant, with a value of 3-10 min; and Ar gas soft blowing time satisfies the following formula: t=(0.05-3.0)C RE +t 0 , where C RE  is a content (in ppm) of rare earth elements in the steel, and t 0  is a correction constant, with a value of 5-10 min. 
     
     
         30 . The method according to  claim 25 , wherein after the addition of the high-purity rare earth, a superheat of casting is increased by 5-15° C. compared with a steel containing the same composition but without rare earth elements; and a N addition in the whole continuous casting is controlled within 8 ppm. 
     
     
         31 . A method of modifying inclusions in the ultra-clean rare earth steel according to  claim 22 , wherein the content of rare earth elements REM in the ultra-clean rare earth steel, the total oxygen content T[O]m in the steel, the total sulfur content T[S]m in the steel, and the total oxygen content T[O]r in the rare earth metal or alloy added to the steel are controlled to satisfy the following formula:
   −500<REM−( m *T [O] m )+ n *T [O] r+k* T [S] m )<−30,
   where 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;   n is the second correction coefficient, with a value of 0.5-2.5;   k is the third correction coefficient, with a value of 0.5-2.5; and   at least 80% of Al 2 O 3  inclusions in the steel are modified into RE-oxygen-sulfides (RE 2 O 2 S).   
     
     
         32 . A control process of inclusions in ultra-clean rare earth steel, wherein the content of rare earth elements REM in the ultra-clean rare earth steel, the total oxygen content T[O]m, the total sulfur content T[S]m in the steel, and the total oxygen content T[O]r in a rare earth metal or alloy added to the steel are controlled to satisfy the following formula:
   −500<REM−( m *T [O] m )+ n *T[O] r+k *T[S] m )<−30,
   where 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;   n is the second correction coefficient, with a value of 0.5-2.5;   k is the third correction coefficient, with a value of 0.5-2.5; and   the process comprises:   a) guaranteeing white slag time to be 20 min or more, stabling slag alkalinity to be greater than 5, a total sulfur content T[S]m to be 90 ppm or less, and a total oxygen content T[O]m to be 25 ppm or less during LF refining;   b) adding a high-purity rare earth metal or alloy before the LF refining departure or after at least 3 min of RH vacuum treatment, wherein a total oxygen content T[O]r in the high-purity rare earth metal or alloy is 60-200 ppm;   c) after adding the rare earth, making RH or VD deep vacuum circulation time satisfy the following formula: T=(0.1-2.0)C RE +T 0 , where C RE  is a content (in ppm) of rare earth elements in the steel, and T 0  is a correction constant, with a value of 3-10 min; and making Ar gas soft blowing time satisfy the following formula: t =(0.05-3.0)C RE +t 0 , where C RE  is a content (in ppm) of rare earth elements in the steel, and t 0  is a correction constant, with a value of 5-10 min; and   d) strengthening gas tightness between a ladle, a tundish and a crystallizer and thickness of a covering agent on a liquid surface of the tundish in the continuous casting, strengthening argon purging on the liquid surface of the tundish and controlling a N addition in the whole continuous casting to be within 8 ppm, wherein compared with a steel having the same composition but without rare earth elements, a superheat of casting is increased by 5-15° C.