Thin steel sheet for automobile excellent in notch fatigue strength and method for production thereof

Abstract

The present invention provides a thin steel sheet, for automobile use, excellent in notch-fatigue strength, and a method for producing said steel sheet. Specifically, the present invention is a thin steel sheet for automobile use excellent in notch-fatigue strength, said steel sheet containing, in mass, 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, 0.1% or less P, 0.01% or less S and 0.005 to 1% Al, with the balance consisting of Fe and unavoidable impurities, characterized in that, on a plane at an arbitrary depth within 0.5 mm from the surface of said steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of {100}<011> to {223}<110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of {554}<225>, {111}<112> and {111}<110> to random X-ray diffraction strength is 4 or less and that the thickness of said steel sheet is in the range from 0.5 to 12 mm, and a method for producing said steel sheet by subjecting a steel slab containing aforementioned chemical components to rolling at a total reduction ratio of 25% or more in a temperature range of the Ar 3 transformation temperature+100° C. or lower.

1 . A thin steel sheet, for automobile use, excellent in notch-fatigue strength, characterized in: that, on a plane at an arbitrary depth within 0.5 mm from the surface of the steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of {100}<011> to {223}<110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of {554}<225>, {111}<112> and {111}<110> to random X-ray diffraction strength is 4 or less; and that the thickness of the steel sheet is in the range from 0.5 to 12 mm.  
     
     
         2 . A thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 1 , characterized in that the microstructure of the steel sheet is a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage.  
     
     
         3 . A thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 1 , characterized in that the microstructure of the steel sheet is a compound structure containing retained austenite at 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite.  
     
     
         4 . A thin steel sheet for automobile use excellent in notch-fatigue strength according to  claim 1 , characterized in that the microstructure of the steel sheet is a compound structure containing ferrite as the phase accounting for the largest volume percentage and martensite as the second phase.  
     
     
         5 . A thin steel sheet for automobile use excellent in notch-fatigue strength, the steel sheet containing, in mass, 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, 0.1% or less P, 0.01% or less S and 0.005 to 1% Al, with the balance consisting of Fe and unavoidable impurities, characterized in that, on a plane at an arbitrary depth within 0.5 mm from the surface of the steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of {100}<011> to {223}<110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of {554}<225>, {111}<112> and {111}<110> to random X-ray diffraction strength is 4 or less and that the thickness of the steel sheet is in the range from 0.5 to 12 mm.  
     
     
         6 . A thin steel sheet for automobile use excellent in notch-fatigue strength according to  claim 5 , characterized by further containing, in mass, one or more of 0.2 to 2% Cu, 0.0002 to 0.002% B, 0.1 to 1% Ni, 0.0005 to 0.002% Ca, 0.0005 to 0.02% REM, 0.05 to 0.5% Ti, 0.01 to 0.5% Nb, 0.05 to 1% Mo, 0.02 to 0.2% V, 0.01 to 1% Cr and 0.02 to 0.2% Zr.  
     
     
         7 . A thin steel sheet for automobile use excellent in notch-fatigue strength according to  claim 5  or  6 , characterized in that the microstructure of the steel sheet is any one of 1) a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage, 2) a compound structure containing retained austenite at 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite, and 3) a compound structure containing ferrite as the phase accounting for the largest volume percentage and martensite as the second phase.  
     
     
         8 . A thin steel sheet for automobile use excellent in notch-fatigue strength, characterized in that the steel sheet is produced by applying galvanizing to a thin steel sheet for automobile use according to any one of  claims 1  to  7 .  
     
     
         9 . A method for producing a thin steel sheet for automobile use excellent in notch-fatigue strength characterized in that a steel slab containing, in mass, 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, 0.1% or less P, 0.01% or less S and 0.005 to 1% Al, with the balance consisting of Fe and unavoidable impurities, is subjected, in a hot rolling process, to rough rolling and then to finish rolling at a total reduction ratio of 25% or more in terms of steel sheet thickness in the temperature range of the Ar 3  transformation temperature +100° C. or lower, that, on a plane at an arbitrary depth within 0.5 mm from the surface of the steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of {100}<011> to {223}<110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of {554}<225>, {111}<112> and {111}<110> to random X-ray diffraction strength is 4 or less and that the thickness of the steel sheet is in the range from 0.5 to 12 mm.  
     
     
         10 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 9 , characterized by cooling the steel sheet at a cooling rate of 20° C./sec. or higher after the finish rolling and then coiling it at a coiling temperature of 450° C. or higher.  
     
     
         11 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 9 , characterized by retaining the steel sheet for 1 to 20 sec. in the temperature range from the Ar 1  transformation temperature to the Ar 3  transformation temperature after the finish rolling then cooling it at a cooling rate of 20° C./sec. or higher and then coiling it at a coiling temperature in the range from higher than 350° C. to lower than 450° C.  
     
     
         12 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 11 , characterized by coiling the steel sheet at a coiling temperature of 350° C. or lower after the cooling.  
     
     
         13 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to any one of  claims 9  to  12 , characterized by applying lubrication rolling to the steel sheet in the hot rolling.  
     
     
         14 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to any one of  claims 9  to  13 , characterized by applying descaling to the steel sheet after the completion of the rough rolling in the hot rolling.  
     
     
         15 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength, characterized in that a steel slab containing, in mass, 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, 0.1% or less P, 0.01% or less S and 0.005 to 1% Al, with the balance consisting of Fe and unavoidable impurities, is subjected to rough rolling, then finish rolling at a total reduction ratio of 25% or more in terms of steel sheet thickness in the temperature range of the Ar 3  transformation temperature +100° C. or lower, pickling, cold rolling at a reduction ratio of less than 80% in terms of steel sheet thickness and then annealing for recovery or recrystallization comprising the processes of retaining the cold-rolled steel sheet for 5 to 150 sec. in the temperature range from the recovering temperature to the Ac 3  transformation temperature +100° C. and then cooling it, that, on a plane at an arbitrary depth within 0.5 mm from the surface of the steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of {100}<011> to {223}<110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of {554}<225>, {111}<112> and {111}<110> to random X-ray diffraction strength is 4 or less and that the thickness of the steel sheet is in the range from 0.5 to 12 mm.  
     
     
         16 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 15 , characterized by subjecting the steel sheet after the cold rolling to a heat treatment comprising the processes of retaining the cold-rolled steel sheet for 5 to 150 sec. in the temperature range from the Ac 1  transformation temperature to the Ac 3  transformation temperature +100° C. and then cooling it.  
     
     
         17 . A method for producing a thin steel sheet for automobile use excellent in notch-fatigue strength according to  claim 15 , characterized by subjecting the steel sheet to a heat treatment comprising the processes of, in sequence, retaining the cold-rolled steel sheet for 5 to 150 sec. in said temperature range, cooling it at a cooling rate of 20° C./sec. or higher to the temperature range from higher than 350° C. to lower than 450° C., retaining it for 5 to 600 sec. in said temperature range, and then cooling it at a cooling rate of 5° C./sec. or higher to the temperature range of 200° C. or lower.  
     
     
         18 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 15 , characterized in subjecting the steel sheet to a heat treatment comprising the processes of retaining the cold-rolled steel sheet for 5 to 150 sec. in said temperature range and then cooling it at a cooling rate of 20° C./sec. or higher to the temperature range of 350° C. or lower.  
     
     
         19 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength, characterized in that the steel sheet produced by the method according to any one of  claims 11  to  18  further contains, in mass, one or more of 0.2 to 2% Cu, 0.0002 to 0.002% B, 0.1 to 1% Ni, 0.0005 to 0.002% Ca, 0.0005 to 0.02% REM, 0.05 to 0.5% Ti, 0.01 to 0.5% Nb, 0.05 to 1% Mo, 0.02 to 0.2% V, 0.01 to 1% Cr and 0.02 to 0.2% Zr.  
     
     
         20 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 10  or  16 , characterized in that the microstructure of the steel sheet is a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage.  
     
     
         21 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 11  or  17 , characterized in that the microstructure of the steel sheet is a compound structure containing retained austenite at 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite.  
     
     
         22 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 12  or  18 , characterized in that the microstructure of the steel sheet is a compound structure containing ferrite as the phase accounting for the largest volume percentage and martensite as the second phase.  
     
     
         23 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength characterized by, after producing a hot-rolled steel sheet or a steel sheet annealed for recovery or recrystallization according to any one of  claims 9  to  22 , further applying galvanizing to the surfaces of the steel sheet by dipping the steel sheet in a zinc plating bath.  
     
     
         24 . A method for producing a thin steel sheet, for automobile use, excellent in notch-fatigue strength according to  claim 23 , characterized by further subjecting the steel sheet to an alloying treatment after the galvanizing.