Method for producing low yield strength cold rolled steel sheet excellent in uniformity

Abstract

A method produces a high-strength cold-rolled steel sheet includes hot-rolling and cold-rolling steel having a composition which contains, by % by mass, over 0.01% to less than 0.08% of C, 0.2% or less of Si, 0.8% to less than 1.7% of Mn, 0.03% or less of P, 0.02% or less of S, 0.3% or less of sol. Al, 0.01% or less of N, and over 0.4% to 2% of Cr, and which satisfies 1.9<[Mneq]<3 and 0.34≦[% Cr]/[% Mn], the balance being composed of iron and inevitable impurities; heating at an average heating rate of less than 3° C./sec in a temperature range of 680° C. to 740° C.; annealing at an annealing temperature of over 740° C. to less than 820° C.; cooling at an average cooling rate of 2 to 30° C./sec in a temperature range of the annealing temperature to 650° C.; cooling at an average cooling rate of 10° C./sec or more in the temperature range of 650° C. to Tc° C.

1 . A method for producing a high-strength cold-rolled steel sheet comprising:
 hot-rolling and cold-rolling steel having a composition which contains, by % by mass, over 0.01% to less than 0.08% of C, 0.2% or less of Si, 0.8% to less than 1.7% of Mn, 0.03% or less of P, 0.02% or less of S, 0.3% or less of sol. Al, 0.01% or less of N, and over 0.4% to 2% of Cr, and which satisfies 1.9<[Mneq]<3 and 0.34≦[% Cr]/[% Mn], the balance being composed of iron and inevitable impurities;   heating at an average heating rate of less than 3° C./sec in a temperature range of 680° C. to 740° C.;   annealing at an annealing temperature of over 740° C. to less than 820° C.;   cooling at an average cooling rate of 2 to 30° C./sec in a temperature range of the annealing temperature to 650° C.;   cooling at an average cooling rate of 10° C./sec or more in a temperature range of 650° C. to Tc° C. represented by equation (1) below; and   cooling at an average cooling rate of 0.2 to 10° C./sec in a temperature range of Tc° C. to 200° C.,
     Tc= 410−40×[% Mn]−30×[% Cr]  (1)
 
   wherein [Mneq] represents the Mn equivalent shown by [Mneq]=[% Mn]+1.3×[% Cr] and [% Mn] and [% Cr] represent the contents of Mn and Cr, respectively.   
     
     
         2 . The method according to  claim 1 , wherein during annealing, heating is performed at an average heating rate of less than 2° C./sec in a temperature range of 680° C. to 740° C. 
     
     
         3 . The method according to  claim 1 , wherein the steel satisfies 0.55≦[% Cr]/[% Mn]. 
     
     
         4 . The method according to  claim 1 , wherein the steel further comprises, by % by mass, 0.005% or less of B. 
     
     
         5 . The method according to  claim 1 , wherein the steel further comprises, by % by mass, at least one of 0.1% or less of Mo and 0.2% or less of V. 
     
     
         6 . The method according to  claim 1 , wherein the steel further comprises, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu. 
     
     
         7 . The method according to  claim 2 , wherein the steel satisfies 0.55≦[% Cr]/[% Mn]. 
     
     
         8 . The method according to  claim 2 , wherein the steel further comprises, by % by mass, 0.005% or less of B. 
     
     
         9 . The method according to  claim 3 , wherein the steel further comprises, by % by mass, 0.005% or less of B. 
     
     
         10 . The method according to  claim 7 , wherein the steel further comprises, by % by mass, 0.005% or less of B. 
     
     
         11 . The method according to  claim 2 , wherein the steel further comprises, by % by mass, at least one of 0.1% or less of Mo and 0.2% or less of V. 
     
     
         12 . The method according to  claim 3 , wherein the steel further comprises, by % by mass, at least one of 0.1% or less of Mo and 0.2% or less of V. 
     
     
         13 . The method according to  claim 4 , wherein the steel further comprises, by % by mass, at least one of 0.1% or less of Mo and 0.2% or less of V. 
     
     
         14 . The method according to  claim 2 , wherein the steel further comprises, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu. 
     
     
         15 . The method according to  claim 3 , wherein the steel further comprises, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu. 
     
     
         16 . The method according to  claim 4 , wherein the steel further comprises, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu. 
     
     
         17 . The method according to  claim 5 , wherein the steel further comprises, by % by mass, at least one of less than 0.014% of Ti, less than 0.01% of Nb, 0.3% or less of Ni, and 0.3% or less of Cu.