Slab for high-strength steel sheet and cooling method thereof, producing method of high-strength hot-rolled steel sheet, producing method of high-strength cold-rolled steel sheet, and producing method of high-strength plated steel sheet

Abstract

A slab for a high-strength steel sheet and a cooling method thereof are disclosed. In addition, producing methods of a high-strength hot-rolled steel sheet, a high-strength cold-rolled steel sheet, and a high-strength plated steel sheet using that slab are provided. The slab for a high-strength steel sheet is a slab continuously cast for a high-strength steel sheet, and is characterized in that an average prior austenite grain size at the position 10 mm from a slab surface layer is 2.0 mm or less, and that the slab has a microstructure in which bainitic ferrite and tempered martensite in total account for 50% or more and 97% or less by area, residual austenite accounts for 3% or more and 30% or less by area, ferrite accounts for 20% or less by area, and pearlite and quenched martensite in total account for 20% or less by area.

1 . A slab for a high-strength steel sheet which has been continuously cast,
 wherein   an average prior austenite grain size at a position 10 mm from a slab surface layer is 2.0 mm or less, and that the slab has a microstructure in which a total area ratio of bainitic ferrite and tempered martensite is 50% or more but 97% or less, an area ratio of residual austenite is 3% or more but 30% or less, an area ratio of ferrite is 20% or less, and a total area ratio of pearlite and quenched martensite is 20% or less.   
     
     
         2 . The slab for a high-strength steel sheet according to  claim 1 , wherein
 the slab for a high-strength steel sheet has an ingredient composition containing, in mass %:   C: 0.10% or more but 0.50% or less,   Si: 0.10% or more but 2.50% or less,   Mn: 1.00% or more but 5.00% or less,   P: 0.100% or less,   S: 0.0200% or less,   Al: 0.005% or more but 2.500% or less,   N: 0.0100% or less, and   O: 0.0100% or less, and   further optionally containing at least one type of element selected from the following:   Ti: 0.200% or less,   Nb: 0.200% or less,   V: 0.200% or less,   Ta: 0.10% or less,   W: 0.10% or less,   B: 0.0100% or less,   Cr: 1.00% or less,   Mo: 1.00% or less,   Co: 1.00% or less,   Ni: 1.00% or less,   Cu: 1.00% or less,   Sn: 0.200% or less,   Sb: 0.200% or less,   Ca: 0.0100% or less,   Mg: 0.0100% or less,   REM: 0.0100% or less,   Zr: 0.100% or less,   Te: 0.100% or less,   Hf: 0.10% or less, and   Bi: 0.200% or less,   with the balance being Fe and unavoidable impurities.   
     
     
         3 . The slab for a high-strength steel sheet according to  claim 1 , wherein
 the slab for a high-strength steel sheet has an ingredient composition containing, in mass %:   C: 0.10% or more but 0.50% or less,   Si: 0.70% or more but 2.50% or less,   Mn: 1.00% or more but 5.00% or less,   P: 0.100% or less,   S: 0.0200% or less,   Al: 0.005% or more but 2.500% or less,   N: 0.0100% or less, and   O: 0.0100% or less, and   further optionally containing at least one type of element selected from the following:   Ti: 0.200% or less,   Nb: 0.200% or less,   V: 0.200% or less,   Ta: 0.10% or less,   W: 0.10% or less,   B: 0.0100% or less,   Cr: 1.00% or less,   Mo: 1.00% or less,   Co: 1.00% or less,   Ni: 1.00% or less,   Cu: 1.00% or less,   Sn: 0.200% or less,   Sb: 0.200% or less,   Ca: 0.0100% or less,   Mg: 0.0100% or less,   REM: 0.0100% or less,   Zr: 0.100% or less,   Te: 0.100% or less,   Hf: 0.10% or less, and   Bi: 0.200% or less,   with the balance being Fe and unavoidable impurities.   
     
     
         4 . A cooling method of a slab for a high-strength steel sheet, wherein
 the slab for a high-strength steel sheet with the ingredient composition according to  claim 2  is cooled such that a retention time in a temperature range of 1200° C. or higher but 1450° C. or lower at the position of a widthwise center 10 mm below a surface layer of the slab is 130 seconds or less; then cooled such that an average cooling rate while a surface temperature at a widthwise center of the slab is in a range of 700° C. or higher but 850° C. or lower is 25° C./hr or more; cooled such that an average cooling rate while the surface temperature is in a range of 550° C. or higher but lower than 700° C. is 20° C./hr or more; cooled such that an average cooling rate while the surface temperature is in a range of 400° C. or higher but lower than 550° C. is 15° C./hr or more; cooled such that an average cooling rate until the surface temperature reaches a cooling stop temperature of 250° C. or higher but lower than 400° C. is 10° C./hr or more; heated so that the surface temperature reaches a reheating temperature of higher than the cooling stop temperature but 450° C. or lower; and then cooled such that an average cooling rate while the surface temperature is in a range of 200° C. or higher but the reheating temperature or lower is 30° C./hr or less.   
     
     
         5 . A cooling method of a slab for a high-strength steel sheet, wherein
 the slab for a high-strength steel sheet with the ingredient composition according to claim  3  is cooled such that a retention time in a temperature range of 1200° C. or higher but 1450° C. or lower at the position of a widthwise center 10 mm below a surface layer of the slab is 130 seconds or less; cooled such that an average cooling rate while a surface temperature at a widthwise center of the slab is in a range of 700° C. or higher but 850° C. or lower is 25° C./hr or more; cooled such that an average cooling rate while the surface temperature is in a range of 550° C. or higher but lower than 700° C. is 20° C./hr or more; cooled such that an average cooling rate while the surface temperature is in a range of 400° C. or higher but lower than 550° C. is 10° C./hr or more; and then cooled such that an average cooling rate while the surface temperature is in a range of 200° C. or higher but lower than 400° C. is 30° C./hr or less.   
     
     
         6 . A producing method of a high-strength hot-rolled steel sheet, wherein
 that the slab for a high-strength steel sheet according to  claim 1  is heated such that a slab heating temperature is 1000° C. or higher but 1300° C. or lower, rough rolled, and then finish rolled such that a finish rolling end temperature is 750° C. or higher but 1000° C. or lower, and wound such that a winding temperature is room temperature or higher but 750° C. or lower.   
     
     
         7 . A producing method of a high-strength cold-rolled steel sheet, wherein
 a high-strength hot-rolled steel sheet produced by the producing method according to claim  6  is pickled and then cold-rolled such that a rolling reduction is 30% or more but 80% or less; and optionally, one process selected from the following is further performed:   (a) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, cooled to a cooling stop temperature of 300° C. or higher but 600° C. or lower, and then cooled to 100° C. or lower;   (b) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, cooled to a cooling stop temperature of 130° C. or higher but 400° C. or lower, reheated to a temperature of 200° C. or higher but 450° C. or lower, and then cooled to 100° C. or lower, and   (c) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, followed by water quenching at 500° C. or higher, cooled by water to 100° C. or lower, and then reheated at 100° C. or higher but 300° C. or lower.   
     
     
         8 . A producing method of a high-strength plated steel sheet, wherein
 a high-strength cold-rolled steel sheet obtained by the cold rolling according to claim  7  is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower; the high-strength cold-rolled steel sheet is subjected to a molten-metal plating process and turned into a plated steel sheet; the plated steel sheet is then cooled under a condition of a cooling stop temperature of 150° C. or lower; and the molten-metal plating process optionally adopts one type selected from zinc plating, zinc-based-alloy plating, zinc-Al-alloy plating, and Al plating.   
     
     
         9 . The producing method of a high-strength plated steel sheet according to  claim 8 , wherein
 the plated steel sheet having been subjected to the molten-metal plating process is subjected to an alloying process.   
     
     
         10 . A producing method of a high-strength plated steel sheet, wherein
 using a high-strength cold-rolled steel sheet produced by the producing method according to  claim 7 , an electroplating process is performed on a surface; and optionally, the electroplating process adopts one type selected from zinc plating, zinc-based-alloy plating, zinc-Al-alloy plating, and Al plating.   
     
     
         11 . A producing method of a high-strength hot-rolled steel sheet, wherein
 that the slab for a high-strength steel sheet according to  claim 2  is heated such that a slab heating temperature is 1000° C. or higher but 1300° C. or lower, rough rolled, and then finish rolled such that a finish rolling end temperature is 750° C. or higher but 1000° C. or lower, and wound such that a winding temperature is room temperature or higher but 750° C. or lower.   
     
     
         12 . A producing method of a high-strength hot-rolled steel sheet, wherein
 that the slab for a high-strength steel sheet according to  claim 3  is heated such that a slab heating temperature is 1000° C. or higher but 1300° C. or lower, rough rolled, and then finish rolled such that a finish rolling end temperature is 750° C. or higher but 1000° C. or lower, and wound such that a winding temperature is room temperature or higher but 750° C. or lower.   
     
     
         13 . A producing method of a high-strength cold-rolled steel sheet, wherein
 a high-strength hot-rolled steel sheet produced by the producing method according to claim  11  is pickled and then cold-rolled such that a rolling reduction is 30% or more but 80% or less; and optionally, one process selected from the following is further performed:   (a) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, cooled to a cooling stop temperature of 300° C. or higher but 600° C. or lower, and then cooled to 100° C. or lower;   (b) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, cooled to a cooling stop temperature of 130° C. or higher but 400° C. or lower, reheated to a temperature of 200° C. or higher but 450° C. or lower, and then cooled to 100° C. or lower, and   (c) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, followed by water quenching at 500° C. or higher, cooled by water to 100° C. or lower, and then reheated at 100° C. or higher but 300° C. or lower.   
     
     
         14 . A producing method of a high-strength cold-rolled steel sheet, wherein
 a high-strength hot-rolled steel sheet produced by the producing method according to claim  12  is pickled and then cold-rolled such that a rolling reduction is 30% or more but 80% or less; and optionally, one process selected from the following is further performed:   (a) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, cooled to a cooling stop temperature of 300° C. or higher but 600° C. or lower, and then cooled to 100° C. or lower;   (b) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, cooled to a cooling stop temperature of 130° C. or higher but 400° C. or lower, reheated to a temperature of 200° C. or higher but 450° C. or lower, and then cooled to 100° C. or lower, and   (c) a process in which the high-strength cold-rolled steel sheet obtained by the cold rolling is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower, followed by water quenching at 500° C. or higher, cooled by water to 100° C. or lower, and then reheated at 100° C. or higher but 300° C. or lower.   
     
     
         15 . A producing method of a high-strength plated steel sheet, wherein
 a high-strength cold-rolled steel sheet obtained by the cold rolling according to claim  13  is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower; the high-strength cold-rolled steel sheet is subjected to a molten-metal plating process and turned into a plated steel sheet; the plated steel sheet is then cooled under a condition of a cooling stop temperature of 150° C. or lower; and the molten-metal plating process optionally adopts one type selected from zinc plating, zinc-based-alloy plating, zinc-Al-alloy plating, and Al plating.   
     
     
         16 . A producing method of a high-strength plated steel sheet, wherein
 a high-strength cold-rolled steel sheet obtained by the cold rolling according to claim  14  is heated such that an annealing temperature is 750° C. or higher but 950° C. or lower; the high-strength cold-rolled steel sheet is subjected to a molten-metal plating process and turned into a plated steel sheet; the plated steel sheet is then cooled under a condition of a cooling stop temperature of 150° C. or lower; and the molten-metal plating process optionally adopts one type selected from zinc plating, zinc-based-alloy plating, zinc-Al-alloy plating, and Al plating.   
     
     
         17 . The producing method of a high-strength plated steel sheet according to  claim 15 , wherein
 the plated steel sheet having been subjected to the molten-metal plating process is subjected to an alloying process.   
     
     
         18 . The producing method of a high-strength plated steel sheet according to  claim 16 , wherein
 the plated steel sheet having been subjected to the molten-metal plating process is subjected to an alloying process.   
     
     
         19 . A producing method of a high-strength plated steel sheet, wherein
 using a high-strength cold-rolled steel sheet produced by the producing method according to  claim 13 , an electroplating process is performed on a surface; and optionally, the electroplating process adopts one type selected from zinc plating, zinc-based-alloy plating, zinc-Al-alloy plating, and Al plating.   
     
     
         20 . A producing method of a high-strength plated steel sheet, wherein
 using a high-strength cold-rolled steel sheet produced by the producing method according to  claim 14 , an electroplating process is performed on a surface; and optionally, the electroplating process adopts one type selected from zinc plating, zinc-based-alloy plating, zinc-Al-alloy plating, and Al plating.