Patent Publication Number: US-11655520-B2

Title: Steel sheet annealing method and steel sheet annealing furnace

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is the U.S. National Phase application of PCT/JP2019/004131, filed Feb. 6, 2019, which claims priority to Japanese Patent Application No. 2018-042645, filed Mar. 9, 2018, the disclosures of these application being incorporated herein by reference in their entireties for all purposes. 
     FIELD OF THE INVENTION 
     The present invention relates to a steel sheet annealing method and a steel sheet annealing furnace. 
     BACKGROUND OF THE INVENTION 
     Generally, in a production line of steel sheets such as thin steel sheets, a process of annealing a rolled steel sheet is carried out prior to a predetermined process, such as application of surface coating. The surface layer of a hearth roll used in an annealing furnace is required to be made of a material having properties such as a high thermal resistance and a high thermal expansion resistance under a high-temperature condition, as well as a moderate hardness, and, generally, a carbon material of a metallic graphite core body is used. For example, Patent Literatures 1 and 2 disclose that a graphitic carbon material having a carbon purity equal to or higher than 98%, a bulk specific gravity equal to or more than 1.65, a specific resistance equal to or less than 1000 μΩ·cm, and a degree of graphitization equal to or more than 0.60 is suitable as the material of the roll, for a use in a temperature equal to or higher than 800° C. Furthermore, Patent Literature 3 discloses that, if a roll made of the material disclosed in Patent Literatures 1 and 2 is used in a temperature equal to or higher than 900° C., degradation due to oxidization spreads very quickly during the use, because of the low Shore hardness of the material, and result in local formation of recesses on the roll surface, and such recesses become a cause of a pickup. Therefore, Patent Literature 3 discloses that a roll having properties of a hardness equal to or more than 50, a porosity between 5% to 15%, and a degree of graphitization equal to or higher than 0.6% is most suitable. 
     PATENT LITERATURE 
     
         
         Patent Literature 1: U.S. Pat. No. 2,603,578 
         Patent Literature 2: Japanese Examined Patent Application Laid-open No. S44-3694 
         Patent Literature 3: Japanese Patent Application Laid-open No. S57-137419 
       
    
     SUMMARY OF THE INVENTION 
     Patent Literature 3 discloses that pickups can be suppressed effectively by increasing the Shore hardness of the roll. However, according to some experiments carried out by the inventors of the present invention, when a steel sheet having a high Si content (e.g., a Si content of 3% or so) was annealed in an annealing process in a temperature equal to or higher than 950° C., pickups occurred, even on a carbon roll having properties within the ranges disclosed in Patent Literature 3. In other words, the inventors found out that pickups cannot be suppressed sufficiently merely by increasing the Shore hardness of the roll. 
     Aspects of the present invention have been made in consideration of the issue described above, and an object according to aspects of the present invention is to provide a steel sheet annealing method and a steel sheet annealing furnace capable of suppressing pickups sufficiently even in a temperature equal to or higher than 950° C. 
     To solve the problem and achieve the object, a method of annealing a steel sheet according to aspects of the present invention in an annealing furnace including hearth rolls configured to support and convey a steel sheet. The method includes: using a full-ceramic hearth roll as a hearth roll located in an area where a furnace temperature is equal to or higher than 950° C., wherein a main constituent of the full-ceramic hearth roll is silicon nitride with use of an Al—Y-based sintering aid. 
     Moreover, the method of annealing the steel sheet according to aspects of the present invention further includes: a step of adjusting a torque of each hearth rolls such that a difference between a torque of the full-ceramic hearth roll and a torque of the hearth roll made of another material becomes equal to or smaller than 5%, when the full-ceramic hearth roll is used together with the hearth roll made of another material in the annealing furnace. 
     Moreover, a steel sheet annealing furnace according to aspects of the present invention including hearth rolls configured to support and convey a steel sheet. The annealing furnace includes: a full-ceramic hearth roll located in an area where a furnace temperature is equal to or higher than 950° C., wherein a main constituent of the full-ceramic hearth roll is silicon nitride with use of an Al—Y-based sintering aid. 
     Moreover, the steel sheet annealing furnace according to aspects of the present invention further includes: a torque adjusting means configured to adjust a torque of each hearth rolls such that a difference between a torque of the full-ceramic hearth roll and a torque of the hearth roll made of another material becomes equal to or smaller than 5%, when the full-ceramic hearth roll is used together with the hearth roll made of another material in the annealing furnace. 
     With the steel sheet annealing method and the annealing furnace according to aspects of the present invention, it is possible to suppress pickups sufficiently, even in a temperature equal to or higher than 950° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a general schematic illustrating a configuration of a steel sheet annealing line that uses a steel sheet annealing method according to one embodiment of the present invention. 
         FIG.  2    is a general schematic illustrating how a steel sheet is conveyed in the annealing furnace illustrated in  FIG.  1   . 
         FIG.  3    is a schematic illustrating results of cross sectional analyses with EPMA. 
         FIG.  4    is a schematic illustrating results of roll surface observations. 
         FIG.  5    is a schematic illustrating results of axial vibration measurements. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The inventors of the present invention made efforts to come up with a hearth roll causing an extremely small number of pickups even in a temperature equal to or higher than 950° C., in an annealing process of a steel sheet with a high Si content. Carbon, which is a common roll material, goes through oxidation-reduction reactions with iron powder or the like on the surface of the steel sheet, and, as a result of the reactions, recesses are formed locally on the roll surface. The recesses then become filled with agglomerates, such as those of iron powder, and such agglomerates grow as the agglomerates are rubbed and rotated against the steel sheet. Chunks of the grown agglomerates then protrude from the roll surface and cause pickups. Focusing on this point, the inventors carried out a research on a ceramic, as a material that is less reactive to a steel sheet. As a result of conducting a research on sintering aids, the inventors confirmed that, a ceramic using a Mg-based sintering aid reacts with Al or Si on the steel sheet surface (goes through reactions such as 4Al 2 O 3 +3Mg→3Al 2 MgO 4 +2Al), and the reactions induce pickups, because Mg becomes easily oxidized. However, it was confirmed that a ceramic using an Al—Y-based sintering aid does not react with the steel sheet, has the surface layer that remains clean, and exhibits a high pickup resistance, whereas recesses, which can originate pickups, are formed on the surface layer of the carbon roll. Based on the above-described fact, the ceramic preferably contains 5 weight % to 20 weight % of Al 2 O 3  and Y 2 O 3 , as a sintering aid. 
     The inventors also carried out some experiments on a roll with a ceramic sleeve, having a layer of ceramic laid on the surface layer of a metallic roll, in order to achieve a cost reduction, and confirmed that the axial vibration increases after a long use, and such a roll was found out to be unsuitable for commercialization from the viewpoint of durability. By contrast, it was confirmed that a full-ceramic hearth roll in which the entire shaft portion and roll body are made of ceramic experiences less axial vibration compared with a carbon hearth roll, and has better durability. Based on these results, with a full-ceramic hearth roll, a repair cost can be reduced. Furthermore, although hearth rolls made of carbon or a heat-resistant alloy are generally used as the hearth rolls in an annealing furnace, it was found out that, when such hearth rolls are used together with a full-ceramic hearth roll, and the circumferential velocities of the respective rolls are set constant, the surface of a hearth roll made of materials other than the ceramic and located near the full-ceramic hearth roll become degraded, and pickups are promoted by the degradations. This is due to lost balance in the amounts of the steel sheet transported by the respective rolls because different materials have different friction coefficients. To address this issue, the inventors adjusted the toques of the respective hearth rolls approximately to the same level, and succeeded in reducing the frequency of occurrence of pickups on the carbon hearth roll or the heat-resistant alloy hearth roll that is used near the full-ceramic hearth roll to the level that is the same as those on the other carbon hearth rolls. Based on the above-described fact, it was confirmed that, by making torque adjustments, it is possible to implement an operation using a full-ceramic hearth roll together with hearth rolls made of other materials, and to introduce ceramic hearth rolls incrementally and selectively. 
     Examples of the hearth rolls made of the other materials include a carbon hearth roll and a hearth roll made of a heat-resistant steel or a heat-resistant alloy. 
     A steel sheet annealing method according to one embodiment of the present invention will now be explained with reference to some drawings. 
       FIG.  1    is a general schematic illustrating a configuration of a steel sheet annealing line that uses the steel sheet annealing method according to one embodiment of the present invention.  FIG.  2    is a general schematic illustrating how a steel sheet S is conveyed in an annealing furnace  4  illustrated in  FIG.  1   . As illustrated in  FIG.  1   , in this steel sheet annealing line  1  that uses the steel sheet annealing method according to the one embodiment of the present invention, the steel sheet S taken out of a coil  2  on the incoming side is washed with alkali in a cleaning section  3  so that rolling oil, iron powders, and the like attached on the surface are removed. The steel sheet S is then annealed continuously inside of the annealing furnace  4 . As illustrated in  FIG.  2   , the steel sheet S is supported and conveyed horizontally by hearth rolls  11  inside the annealing furnace  4  in such a manner that the steel sheet S does not become distorted. Referring back to  FIG.  1   , the steel sheet S annealed in the annealing furnace  4  is sent to a coater  5 , and the coater  5  applies a coating liquid to the surface of the steel sheet S. The steel sheet S having the surface applied with the coating liquid is conveyed into a baking furnace  6 , and the coating liquid is dried and baked in the baking furnace  6 . The steel sheet S is then wound as a coil  7 , on the outgoing side of the annealing line  1 . 
     A hearth roll according to aspects of the present invention is a full-ceramic hearth roll in which a main constituent of the entire shaft portion and roll body is silicon nitride with the use of an Al—Y-based sintering aid, and is capable of exerting an effect of reducing pickups sufficiently when deployed in an area where the furnace temperature reaches a temperature equal to or higher than 950° C., or preferably equal to or higher than 900° C., in the annealing furnace  4 . Furthermore, when this full-ceramic hearth roll is used together with a carbon hearth roll as the hearth rolls  11 , torque adjustments are needed. Without the torque adjustments, the surface layer of the carbon hearth roll located near the full-ceramic hearth roll becomes degraded, and, as chunks of agglomerates of powders are rubbed and rotated against the steel sheet, and the chunks grow on the surface layer, and the chunks of the grown agglomerates protrude from the roll surface, and cause pickups. The torque herein means a driving force for causing a roll to rotate. The torque can be interpreted as a current level in the driving motor, for example, and an acceptable range of a difference between the current levels for the respective rolls is equal to or less than 5%. The torque can be adjusted by changing the circumferential velocities of the rolls, that is, by changing the forward slips for the respective rolls, for example. 
     Examples 
     In this example, sintering aids for a ceramic hearth roll were examined through experiments. Specifically, ceramic hearth roll pieces (15t×18w×38L) using an Al—Y-based sintering aid and a Mg-based sintering aid, respectively, were placed on powder of a steel sheet containing 3.3 mass % of Si and 0.7 mass % of Al, and a 383-gram weight was placed on the ceramic hearth roll pieces to adjust the surface pressure. The hearth roll pieces were then left in an atmosphere of 20% H 2 —N 2  with a dew point set to −40° C. for one hour with the temperature being kept at 1050° C. As a result, reactant was found only on the surface of the ceramic hearth roll piece with the Mg-based sintering aid. The results of cross sectional analyses of the ceramic hearth roll pieces using an Electron Probe Micro Analyzer (EPMA) is illustrated in  FIG.  3   . As illustrated in  FIG.  3   , the reactant observed on the surface layer of the ceramic hearth roll with the Mg-based sintering aid was found to be an oxide resultant of a reaction with Si or Al in the steel sheet. 
     In addition, in an annealing furnace for annealing a steel sheet the Si content of which was 1 mass % or more, a carbon sleeve roll and a full-ceramic hearth roll were used for seven months, in an area where the furnace temperature was kept equal to or higher than 950° C., and the roll surfaces were observed, and measurements of their axial vibrations were collected. The results of the roll surface observations and the axial vibration measurements are indicated in FIGS.  4 ( a ),  4 ( b ), and  5 , and Table 1. As indicated in  FIGS.  4 ( a ) and  4 ( b ) , scratches and cavity holes, which can originate pickups, were observed on the carbon sleeve roll (No. 4), but the full-ceramic hearth roll (No. 1) with the use of the Al—Y-based sintering aid had no damage at all, and remained in a very good condition. Furthermore, as illustrated in  FIG.  5   , the carbon sleeve roll vibrated very much, and made rattling sound, but the full-ceramic hearth roll with the use of the Al—Y-based sintering aid did not vibrate very much, and made no noise. The ceramic sleeve roll was cracked after 2 months. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Pickup/ 
                   
                   
               
               
                   
                 Roll 
                   
                 Damage 
               
               
                 No. 
                 Type 
                 Aid 
                 Condition 
                 Vibrations 
                 Remarks 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 
                 Full 
                 Al—Y 
                 A 
                 A 
                 Example 
               
               
                 2 
                 Ceramic 
                 Mg 
                 C 
                 A 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                 Example 
               
               
                 3 
                 Ceramic 
                 Al—Y 
                 A 
                 C 
                 Comparative 
               
               
                   
                 Sleeve 
                   
                   
                   
                 Example 
               
               
                 4 
                 Carbon 
                 — 
                 C 
                 C 
                 Comparative 
               
               
                   
                 Sleeve 
                   
                   
                   
                 Example 
               
               
                   
               
            
           
         
       
     
     The full-ceramic hearth roll with the use of Al—Y-based sintering aid was used, in an annealing furnace for annealing a steel sheet with a Si content of 1 mass % or more, in an area where the furnace temperature was equal to or higher than 950° C., and the carbon sleeve roll was used in an area where the furnace temperature was lower than 950° C. The torque of the ceramic roll and that of the carbon sleeve roll were then changed variously, and the surfaces of the respective rolls were observed after the seven-month operation. The results of the roll surface observations are indicated in Table 2 below. As indicated in Table 2, when the torque difference was equal to or smaller than 1%, no formation of pickups was observed (evaluation: A). When the torque difference was 3% or 5%, some cavity holes, which could originate pickups, were formed (evaluation: B). When the torque difference exceeded 5%, obvious pickups such as dents or scratches were formed (evaluation: C). As a result, it was confirmed that, by setting the torque difference between the ceramic roll and the carbon sleeve roll equal to or smaller than 5%, pickups can be suppressed effectively even when a ceramic roll is used in a high-temperature area, together with a carbon sleeve roll used in the low-temperature area of the annealing furnace. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Conditions 
               
               
                   
                 Torque of 
                 Torque of 
                 Torque 
                 of Pickup 
               
               
                 No. 
                 Ceramic Roll 
                 Carbon Roll 
                 Difference 
                 Formation 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 −4%  
                 5% 
                 9% 
                 C 
               
               
                 2 
                 −3%  
                 4% 
                 7% 
                 C 
               
               
                 3 
                 0% 
                 5% 
                 5% 
                 B 
               
               
                 4 
                 1% 
                 4% 
                 3% 
                 B 
               
               
                 5 
                 3% 
                 4% 
                 1% 
                 A 
               
               
                   
               
            
           
         
       
     
     Although some embodiments that are applications of the invention made by the inventors are explained above, the scope of the present invention is not limited to the embodiments including the descriptions and the drawings making up a part of disclosure of the present invention, in any way. In other words, other embodiments, examples, operation technologies, and the like made by a person skilled in the art on the basis of the embodiment all fall within the scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     According to aspects of the present invention, it is possible to provide a steel sheet annealing method and a steel sheet annealing furnace capable of suppressing pickups sufficiently even in a temperature equal to or higher than 950° C. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  steel sheet annealing line 
               2 ,  7  coil 
               3  cleaning section 
               4  annealing furnace 
               5  coater 
               6  baking furnace 
               11  hearth roll 
             S steel sheet