Hot-rolled steel sheet and method for manufacturing the same

Abstract

A high-strength hot-rolled steel sheet is provided having a high low-temperature toughness and a low yield ratio that is suitable for a steel pipe material. The hot-rolled steel sheet has a composition that contains C: 0.03% to 0.10%, Si: 0.01% to 0.50%, Mn: 1.4% to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005% to 0.10%, Nb: 0.02% to 0.10%, Ti: 0.001% to 0.030%, Mo: 0.01% to 0.50%, Cr: 0.01% to 0.50%, and Ni: 0.01% to 0.50%. The composition of the hot-rolled steel sheet has an Moeq value in a range of 1.4% to 2.2%. The hot-rolled steel sheet includes an inner layer, which has a microstructure that contains a main phase and a second phase, and an outer layer, which has a microstructure that contains a tempered martensite phase or a tempered martensite phase and a tempered bainite phase.

1 . A hot-rolled steel sheet having a composition comprising, on a mass percent basis:
 C: 0.03% to 0.10%, Si: 0.01% to 0.50%, Mn: 1.4% to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005% to 0.10%, Nb: 0.02% to 0.10%, Ti: 0.001% to 0.030%, Mo: 0.01% to 0.50%, Cr: 0.01% to 0.50%, Ni: 0.01% to 0.50%, and a remainder of Fe and incidental impurities,   the hot rolled steel sheet comprising:
 an inner layer having a microstructure that contains a main phase and a second phase, the main phase being bainitic ferrite having an average grain size of 10 μm or less, the second phase having an area fraction in a range of 1.4% to 15% and containing massive martensite having an aspect ratio of less than 5.0, and 
 an outer layer having a microstructure that contains (i) a tempered martensite phase or (ii) a tempered martensite phase and a tempered bainite phase. 
   
     
     
         2 . The hot-rolled steel sheet according to  claim 1 , wherein the composition of the steel sheet has an Moeq value, which is defined by the following formula (1), in a range of 1.4% to 2.2% by mass:
   Moeq (%)=Mo+0.36Cr+0.77Mn+0.07Ni  (1)
   wherein, Mn, Ni, Cr, and Mo denote the corresponding element contents (% by mass).   
     
     
         3 . The hot-rolled steel sheet according to  claim 1 , wherein the chemical composition of the steel sheet further comprises, on a mass percent basis, at least one of the following: Cu: 0.50% or less, V: 0.10% or less, and B: 0.005% or less. 
     
     
         4 . The hot-rolled steel sheet according to  claim 1 , wherein the chemical composition of the steel sheet further comprises, on a mass percent basis, Ca: 0.0005% to 0.0050%. 
     
     
         5 . The hot-rolled steel sheet according to  claim 1 , wherein the massive martensite has a maximum size of 5.0 μm or less and an average size in the range of 0.5 to 3.0 μm. 
     
     
         6 . The hot-rolled steel sheet according to  claim 1 , wherein a hardness of the hot-rolled steel sheet at a depth of 0.5 mm from a surface thereof in a thickness direction is 95% or less of a maximum hardness in the thickness direction. 
     
     
         7 . A method for manufacturing a hot-rolled steel sheet, comprising:
 subjecting steel to a hot-rolling step, a cooling step, and a coiling step to form the hot-rolled steel sheet, wherein:   the steel contains, on a mass percent basis, C: 0.03% to 0.10%, Si: 0.01% to 0.50%, Mn: 1.4% to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005% to 0.10%, Nb: 0.02% to 0.10%, Ti: 0.001% to 0.030%, Mo: 0.01% to 0.50%, Cr: 0.01% to 0.50%, Ni: 0.01% to 0.50%, and a remainder of Fe and incidental impurities,   the hot-rolling step includes:
 heating the steel to a heating temperature in a range of 1050° C. to 1300° C., 
 rough-rolling the heated steel to form a sheet bar, and 
 finish-rolling the sheet bar such that a cumulative rolling reduction at a temperature of 930° C. or less is 50% or more, thereby forming a hot-rolled steel sheet, 
   the cooling step includes first cooling, second cooling, third cooling, and fourth cooling in this order,
 the first cooling being started immediately after completion of the finish rolling, and including cooling the hot-rolled steel sheet to a martensitic transformation start temperature or less at an average cooling rate of 100° C./s or more with respect to surface temperature, 
   the second cooling including, after completion of the first cooling, holding the hot-rolled steel sheet for 1 s or more at a surface temperature of 600° C. or more,
 the third cooling including, after completion of the second cooling, cooling the hot-rolled steel sheet to a cooling stop temperature in a range of 600° C. to 450° C. at an average cooling rate in a range of 5° C. to 30° C./s with respect to a temperature at half a thickness of the hot-rolled steel sheet, and 
 the fourth cooling including (i) cooling the hot-rolled steel sheet from the cooling stop temperature of the third cooling to a coiling temperature at an average cooling rate of 2° C./s or less with respect to the temperature at half the thickness of the hot-rolled steel sheet or (ii) alternatively holding the hot-rolled steel sheet at a temperature in the range extending from the cooling stop temperature of the third cooling to the coiling temperature for 20 s or more, and 
   the coiling step includes coiling the hot-rolled steel sheet at a surface temperature of 450° C. or more.   
     
     
         8 . The method for manufacturing a hot-rolled steel sheet according to  claim 7 , wherein the composition has an Moeq value, which is defined by the following formula (1), in a range of 1.4% to 2.2% by mass:
   Moeq (%)=Mo+0.36Cr+0.77Mn+0.07Ni  (1)
   wherein, Mn, Ni, Cr, and Mo denote the corresponding element contents (% by mass).   
     
     
         9 . The method for manufacturing a hot-rolled steel sheet according to  claim 7 , wherein the hot-rolled steel sheet further contains, on a mass percent basis, at least one of the following: Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less. 
     
     
         10 . The method for manufacturing a hot-rolled steel sheet according to  claim 7 , wherein the hot-rolled steel sheet further contains Ca: 0.0005% to 0.0050% by mass.