Hot rolled steel sheet for cold rolled steel sheet, hot rolled steel sheet for galvanized steel sheet, and method for producing the same

Abstract

A hot rolled steel sheet having a chemical composition containing, by mass %, C: 0.04% or more and 0.20% or less, Si: 0.7% or more and 2.3% or less, Mn: 0.8% or more and 2.8% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the balance being Fe and inevitable impurities. The microstructure of the hot rolled steel sheet includes ferrite and pearlites, in which the area ratio of the ferrite is 75% or more and less than 95%, the mean grain size of the ferrite is 5 μm or more and 25 μm or less, the area ratio of pearlite is 5% or more and less than 25%, the mean grain size of pearlite is 2.0 μm or more, and the mean free path of pearlite is 5 μm or more.

1 . A hot rolled steel sheet for a cold rolled steel sheet, the hot rolled steel sheet having a chemical composition containing, by mass %, C: 0.04% or more and 0.20% or less, Si: 0.7% or more and 2.3% or less, Mn: 0.8% or more and 2.8% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the balance being Fe and inevitable impurities, and a microstructure of the hot rolled steel sheet including ferrite and pearlite, wherein the area ratio of the ferrite is 75% or more and less than 95%, the mean grain size of the ferrite is 5 μm or more and 25 μm or less, the area ratio of the pearlite is 5% or more and less than 25%, the mean grain size of the pearlite is 2.0 μm or more, and the mean free path of the pearlite is 5 μm or more. 
     
     
         2 . The hot rolled steel sheet for a cold rolled steel sheet according to  claim 1 , wherein a difference in tensile strength between a portion at the center of the width of the hot rolled steel sheet and a position located at ⅛ of the sheet width from the edge in the width direction of the hot rolled steel sheet is 30 MPa or less. 
     
     
         3 . The hot rolled steel sheet for a cold rolled steel sheet according to  claim 1 , further containing, by mass %, at least one element selected from the group consisting of Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less. 
     
     
         4 . The hot rolled steel sheet for a cold rolled steel sheet according to  claim 1 , further containing, by mass %, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. 
     
     
         5 . The hot rolled steel sheet for a cold rolled steel sheet according to  claim 1 , further containing, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         6 . The hot rolled steel sheet for a cold rolled steel sheet according to  claim 4 , further containing, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         7 . A hot rolled steel sheet for a galvanized steel sheet, the hot rolled steel sheet having a chemical composition containing, by mass %, C: 0.04% or more and 0.20% or less, Si: 0.7% or more and 2.3% or less, Mn: 0.8% or more and 2.8% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the balance being Fe and inevitable impurities, and a microstructure of the hot rolled steel sheet including ferrite and pearlite, wherein the area ratio of the ferrite is 75% or more and less than 95%, the mean grain size of the ferrite is 5 μm or more and 25 μm or less, the area ratio of the pearlite is 5% or more and less than 25%, the mean grain size of the pearlite is 2.0 μm or more, and the mean free path of the pearlite is 5 μm or more. 
     
     
         8 . The hot rolled steel sheet for a galvanized steel sheet according to  claim 7 , wherein a difference in tensile strength between a portion at the center of the width of the hot rolled steel sheet and a position located at ⅛ of the sheet width from the edge in the width direction of the hot rolled steel sheet is 30 MPa or less. 
     
     
         9 . The hot rolled steel sheet for a galvanized steel sheet according to  claim 7 , further containing, by mass %, at least one element selected from the group consisting of Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less. 
     
     
         10 . The hot rolled steel sheet for a galvanized steel sheet according to  claim 7 , further containing, by mass %, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. 
     
     
         11 . The hot rolled steel sheet for a galvanized steel sheet according to  claim 7 , further containing, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         12 . The hot rolled steel sheet for a galvanized steel sheet according to  claim 11 , further containing, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         13 . A method for producing a hot rolled steel sheet for a cold rolled steel sheet, the method comprising:
 hot rolling a steel slab having a chemical composition containing, by mass %, C: 0.04% or more and 0.20% or less, Si: 0.7% or more and 2.3% or less, Mn: 0.8% or more and 2.8% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the balance being Fe and inevitable impurities under the condition that a finish rolling delivery temperature is 850° C. or higher;   
       cooling the hot rolled steel sheet at a mean cooling rate of 20° C./s to 90° C./s in a temperature range from the finish rolling delivery temperature to 650° C.;
 further cooling the hot rolled steel sheet at a mean cooling rate of 5° C./s to 35° C./s in a temperature range down to a coiling temperature; and 
 
       coiling the cooled steel sheet at a temperature of 470° C. to 640° C. 
     
     
         14 . The method for producing a hot rolled steel sheet for a cold rolled steel sheet according to  claim 13 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less. 
     
     
         15 . The method for producing a hot rolled steel sheet for a cold rolled steel sheet according to  claim 13 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. 
     
     
         16 . The method for producing a hot rolled steel sheet for a cold rolled steel sheet according to  claim 13 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         17 . The method for producing a hot rolled steel sheet for a cold rolled steel sheet according to  claim 15 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         18 . A method for producing a hot rolled steel sheet for a galvanized steel sheet, the method comprising:
 hot rolling a steel slab having a chemical composition containing, by mass %, C: 0.04% or more and 0.20% or less, Si: 0.7% or more and 2.3% or less, Mn: 0.8% or more and 2.8% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the balance being Fe and inevitable impurities under the condition that a finish rolling delivery temperature is 850° C. or higher;   
       cooling the hot rolled steel sheet at a mean cooling rate of 20° C./s to 90° C./s in a temperature range from the finish rolling delivery temperature to 650° C.; 
       further cooling the hot rolled steel sheet at a mean cooling rate of 5° C./s to 35° C./s in a temperature range down to a coiling temperature; and 
       coiling the cooled steel sheet at a temperature of 470° C. to 640° C. 
     
     
         19 . The method for producing a hot rolled steel sheet for a galvanized steel sheet according to  claim 18 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Cr: 0.05% or more and 1.0% or less, V: 0.005% or more and 0.5% or less, Mo: 0.005% or more and 0.5% or less, Ni: 0.05% or more and 1.0% or less, and Cu: 0.05% or more and 1.0% or less. 
     
     
         20 . The method for producing a hot rolled steel sheet for a galvanized steel sheet according to  claim 18 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. 
     
     
         21 . The method for producing a hot rolled steel sheet for a galvanized steel sheet according to  claim 18 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         22 . The method for producing a hot rolled steel sheet for a galvanized steel sheet according to  claim 20 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         23 . The hot rolled steel sheet for a cold rolled steel sheet according to  claim 3 , further containing, by mass %, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. 
     
     
         24 . The hot rolled steel sheet for a cold rolled steel sheet according to  claim 3 , further containing, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         25 . The hot rolled steel sheet for a cold rolled steel sheet according to  claim 23 , further containing, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         26 . The hot rolled steel sheet for a galvanized steel sheet according to  claim 9 , further containing, by mass %, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. 
     
     
         27 . The hot rolled steel sheet for a galvanized steel sheet according to  claim 9 , further containing, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         28 . The hot rolled steel sheet for a galvanized steel sheet according to  claim 27 , further containing, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         29 . The method for producing a hot rolled steel sheet for a cold rolled steel sheet according to  claim 14 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. 
     
     
         30 . The method for producing a hot rolled steel sheet for a cold rolled steel sheet according to  claim 14 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         31 . The method for producing a hot rolled steel sheet for a cold rolled steel sheet according to  claim 29 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         32 . The method for producing a hot rolled steel sheet for a galvanized steel sheet according to  claim 19 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ti: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.1% or less, and B: 0.0003% or more and 0.0050% or less. 
     
     
         33 . The method for producing a hot rolled steel sheet for a galvanized steel sheet according to  claim 19 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less. 
     
     
         34 . The method for producing a hot rolled steel sheet for a galvanized steel sheet according to  claim 32 , wherein the steel slab further contains, by mass %, at least one element selected from the group consisting of Ca: 0.001% or more and 0.005% or less and REM: 0.001% or more and 0.005% or less.