Low-yield-ratio high-strength hot-rolled steel plate with excellent low-temperature toughness and process for producing same

Abstract

A low yield ratio and high-strength hot rolled steel sheet having a composition containing, on a mass percent basis, 0.03% to 0.10% C, 0.10% to 0.50% Si, 1.4% to 2.2% Mn, 0.005 % to 0.10% Al, 0.02% to 0.10% Nb, 0.001% to 0.030% Ti, 0.05% to 0.50% Mo, 0.05% to 0.50% Cr, and 0.01% to 0.50% Ni, in which Moeq preferably satisfies the range of 1.4% to 2.2%; and a microstructure including a main phase that contains bainitic ferrite having an average grain size of 10 μm or less and a secondary phase that contains massive martensite having an aspect ratio of less than 5.0 in an area ratio of 1.4% to 15%.

1 . A hot rolled steel sheet comprising a composition containing, on a mass percent basis:
 0.03% to 0.10% C; 0.10% to 0.50% Si;   1.4% to 2.2% Mn; 0.025% or less P;   0.005% or less S; 0.005% to 0.10% Al;   0.02% to 0.10% Nb; 0.001% to 0.030% Ti;   0.05% to 0.50% Mo; 0.05% to 0.50% Cr;   0.01% to 0.50% Ni; and   the balance comprising Fe and incidental impurities; and a microstructure including a main phase that contains bainitic ferrite and a secondary phase that contains massive martensite having an aspect ratio of less than 5.0 in an area ratio of 1.4% to 15%, wherein the average grain size of the bainitic ferrite is 10 μm or less.   
     
     
         2 . The hot rolled steel sheet according to  claim 1 , wherein the composition is such that Moeq defined by expression (1) described below satisfies the range of 1.4% to 2.2% by mass,
   M oeq  (%)=Mo+0.36Cr+0.77Mn+0.07Ni  (1)
   
       where Mn, Ni, Cr, and Mo represent the respective contents (% by mass). 
     
     
         3 . The hot rolled steel sheet according to  claim 1 , further comprising, on a mass percent basis, one or more elements selected from 0.50% or less Cu, 0.10% or less V, and 0.0005% or less B, in addition to the composition. 
     
     
         4 . The hot rolled steel sheet according to  claim 1 , further comprising, on a mass percent basis, 0.0005% to 0.0050% Ca, in addition to the composition. 
     
     
         5 . The hot rolled steel sheet according to  claim 1 , wherein the massive martensite has a maximum size of 5 μm or less and an average size of 0.5 to 3.0 μm. 
     
     
         6 . A method for producing a hot rolled steel sheet, comprising:
 subjecting a steel to a hot rolling step, a cooling step, and a coiling step to provide a hot rolled steel sheet, wherein the steel has a composition containing, on a mass percent basis:   0.03% to 0.10% C; 0.10% to 0.50% Si;   1.4% to 2.2% Mn; 0.025% or less P;   0.005% or less S; 0.005% to 0.10% Al;   0.02% to 0.10% Nb; 0.001% to 0.030% Ti;   0.05% to 0.50% Mo; 0.05% to 0.50% Cr;   0.01% to 0.50% Ni; and   the balance comprising Fe and incidental impurities,   the hot rolling step includes heating the steel to a heating temperature of 1050° C. to 1300° C., subjecting the heated steel to rough rolling to form a sheet bar, and subjecting the sheet bar to finishing rolling in the temperature range of 930° C. or lower at a cumulative rolling reduction of 50% or more to provide a hot rolled steel sheet,   the cooling step includes primary cooling in which cooling is started immediately after the completion of the finishing rolling, and the steel sheet is cooled to a cooling stop temperature in the temperature range of 600° C. to 450° C. at an average cooling rate of 5 to 30° C./s, the average cooling rate being measured at the middle portion of the sheet in the thickness direction; and secondary cooling in which the steel sheet is cooled from the cooling stop temperature to a coiling temperature at an average cooling rate of 2° C./s or lower or in which the steel sheet is held in the temperature range of the cooling stop temperature to the coiling temperature for a holding time of 20 s or more, and the coiling step includes coiling the steel sheet at a coiling temperature of 450° C. or higher, the coiling temperature being based on a surface temperature.   
     
     
         7 . The method for producing a hot rolled steel sheet according to  claim 6 , wherein the composition is such that Moeq defined by expression (1) described below satisfies the range of 1.4% to 2.2% by mass,
   M oeq  (%)=Mo+0.36Cr+0.77Mn+0.07Ni  (1)
   
       where Mn, Ni, Cr, and Mo represent the respective contents (% by mass). 
     
     
         8 . The method for producing a hot rolled steel sheet according to  claim 6 , further comprising, on a mass percent basis, one or more elements selected from 0.50% or less Cu, 0.10% or less V, and 0.0005% or less B, in addition to the composition. 
     
     
         9 . The method for producing a hot rolled steel sheet according to  claim 6 , further comprising, on a mass percent basis, 0.0005% to 0.0050% Ca, in addition to the composition. 
     
     
         10 . The hot rolled steel sheet according to  claim 2 , further comprising, on a mass percent basis, one or more elements selected from 0.50% or less Cu, 0.10% or less V, and 0.0005% or less B, in addition to the composition. 
     
     
         11 . The hot rolled steel sheet according to  claim 2 , further comprising, on a mass percent basis, 0.0005% to 0.0050% Ca, in addition to the composition. 
     
     
         12 . The hot rolled steel sheet according to  claim 3 , further comprising, on a mass percent basis, 0.0005% to 0.0050% Ca, in addition to the composition. 
     
     
         13 . The hot rolled steel sheet according to  claim 10 , further comprising, on a mass percent basis, 0.0005% to 0.0050% Ca, in addition to the composition. 
     
     
         14 . The hot rolled steel sheet according to  claim 2 , wherein the massive martensite has a maximum size of 5 μm or less and an average size of 0.5 to 3.0 μm. 
     
     
         15 . The hot rolled steel sheet according to  claim 3 , wherein the massive martensite has a maximum size of 5 μm or less and an average size of 0.5 to 3.0 μM. 
     
     
         16 . The hot rolled steel sheet according to  claim 10 , wherein the massive martensite has a maximum size of 5 μm or less and an average size of 0.5 to 3.0 μM. 
     
     
         17 . The hot rolled steel sheet according to  claim 4 , wherein the massive martensite has a maximum size of 5 μm or less and an average size of 0.5 to 3.0 μm. 
     
     
         18 . The hot rolled steel sheet according to  claim 11 , wherein the massive martensite has a maximum size of 5 μm or less and an average size of 0.5 to 3.0 μm. 
     
     
         19 . The hot rolled steel sheet according to  claim 12 , wherein the massive martensite has a maximum size of 5 μm or less and an average size of 0.5 to 3.0 μm. 
     
     
         20 . The hot rolled steel sheet according to  claim 13 , wherein the massive martensite has a maximum size of 5 μm or less and an average size of 0.5 to 3.0 μm.