Ferritic stainless steel and method for manufacturing same

Abstract

A ferritic stainless steel in accordance with an aspect of the present invention contains not more than 0.030% of C, 0.01% to 1.5% of Si, 0.01% to 1.00% of Mn, not more than 0.050% of P, not more than 0.005% of S, 15.0% to 25.0% of Cr, 2.0% to 4.0% of Al, not more than 1.00% of Ni, 0.01% to 0.70% of Nb, not more than 0.030% of N, 0.0003% to 0.01% of B, and 0.01% to 0.20% of REM, in percent by mass, and the other part composed of Fe and an inevitable impurity, the ferritic stainless steel having a dislocation density p of not less than 0.91×10 14 [m −2 ] as derived by the Williamson and Hall method.

1 . A ferritic stainless steel, comprising not more than 0.030% of C, 0.01% to 1.5% of Si, 0.01% to 1.00% of Mn, not more than 0.050% of P, not more than 0.005% of S, 15.0% to 25.0% of Cr, 2.0% to 4.0% of Al, not more than 1.00% of Ni, 0.01% to 0.70% of Nb, not more than 0.030% of N, 0.0003% to 0.01% of B, and 0.01% to 0.20% of REM, in percent by mass, and the other part composed of Fe and an inevitable impurity, the ferritic stainless steel having a dislocation density ρ of not less than 0.91×10 14  [m −2 ] as derived by the Williamson and Hall method,
 wherein in scanning electron microscope observation of random three 30 μm×30 μm portions in a cross section obtained by cutting the ferritic stainless steel along a plane perpendicular to a rolling direction, an average number of carbides each having (i) a Nb concentration of not less than 5 wt % as measured by energy dispersive X-ray spectroscopy and (ii) a particle diameter of not less than 0.1 μm is 2 to 15. 
 
     
     
         2 . The ferritic stainless steel as set forth in  claim 1 , wherein: in a case where the ferritic stainless steel is heated at 1050° C. for 50 hours, the ferritic stainless steel forms an alumina layer mainly composed of alumina; and
 in a cross section of the alumina layer obtained by cutting the alumina layer in a thickness direction, a total length of grain boundaries included in a given region having an area of 2.25 μm 2  is not more than 5.5 km. 
 
     
     
         3 . The ferritic stainless steel as set forth in  claim 2 , wherein a total concentration of a Nb oxide, a Cr oxide, and a REM-based oxide which are present in grain boundaries in the alumina layer is not less than 3.5 wt %. 
     
     
         4 . The ferritic stainless steel as set forth in a  claim 1 , wherein the ferritic stainless steel further comprises at least one selected from the group consisting of not more than 0.50% of Zr, not more than 0.50% of V, not more than 1.0% of Cu, not more than 2.0% of Mo, not more than 2.0% of W, not more than 0.50% of Hf, not more than 0.50% of Sn, not more than 0.5% of Ta, not more than 0.20% of Ti, not more than 0.015% of Mg, and not more than 0.015% of Ca, in percent by mass. 
     
     
         5 . The ferritic stainless steel as set forth in  claim 1 , wherein the ferric stain steel satisfies 100×[C]/[Nb]≤35 where [C] is a percent by mass of C and [Nb] is a percent by mass of Nb. 
     
     
         6 . A method for producing a ferritic stainless steel, the ferritic stainless steel containing not more than 0.030% of C, 0.01% to 1.5% of Si, 0.01% to 1.00% of Mn, not more than 0.050% of P, not more than 0.005% of S, 15.0% to 25.0% of Cr, 2.0% to 4.0% of Al, not more than 1.00% of Ni, 0.01% to 0.70% of Nb, not more than 0.030% of N, 0.0003% to 0.01% of B, and 0.01% to 0.20% of REM, in percent by mass, and the other part composed of Fe and an inevitable impurity, the method comprising:
 an annealing step of annealing a steel strip which has been hot-rolled, such that cooling time taken to cool the steel strip from an annealing temperature to 400 degrees after annealing is not less than 30 seconds; and   a cold rolling step of carrying out, after a final annealing step, cold rolling until a dislocation density ρ derived by the Williamson and Hall method is not less than 0.91×10 14  [m −2 ].   
     
     
         7 . The method as set forth in  claim 6 , wherein in the cold rolling step, a rolling reduction ratio is not less than 65%. 
     
     
         8 . The method as set forth in  claim 6 , wherein in scanning electron microscope observation of random three 30 μm×30 μm portions in a cross section obtained by cutting, along a plane perpendicular to a rolling direction, of the ferritic stainless steel obtained through the cold rolling step, an average number of carbides each having (i) a Nb concentration of not less than 5 wt % as measured by energy dispersive X-ray spectroscopy and (ii) a particle diameter of not less than 0.1 μm is 2 to 15. 
     
     
         9 . The method as set forth in  claim 6 , wherein: the ferritic stainless steel forms an alumina layer in a case where the ferritic stainless steel is heated at 1050° C. for 50 hours; and
 in a cross section of the alumina layer obtained by cutting the alumina layer in a thickness direction, a total length of grain boundaries included in a given region having an area of 2.25 μm 2  is not more than 5.5 μm. 
 
     
     
         10 . The method as set forth in  claim 9 , wherein a total concentration of a Nb oxide, a Cr oxide, and a REM-based oxide which are present in grain boundaries in the alumina layer is not less than 3.5 wt %. 
     
     
         11 . The method as set forth in  claim 6 , wherein the ferritic stainless steel further contains at least one selected from the group consisting of not more than 0.50% of Zr, not more than 0.50% of V, not more than 1.0% of Cu, not more than 2.0% of Mo, not more than 2.0% of W, not more than 0.50% of Hf, not more than 0.50% of Sn, not more than 0.5% of Ta, not more than 0.20% of Ti, not more than 0.015% of Mg, and not more than 0.015% of Ca, in percent by mass. 
     
     
         12 . The method as set forth in  claim 6 , wherein the ferritic stainless steel satisfies 100×[C]/[Nb]≤35 where [C] is a percent by mass of C and [Nb] is a percent by mass of Nb.