High-strength electric resistance welded steel pipe and method for producing the same

Abstract

Provided is a high-strength electric resistance welded steel pipe having excellent bendability and which includes a composition containing, on a mass % basis, C: 0.04% to 0.15%, Si: 0.10% to 0.50%, Mn: 1.0% to 2.2%, P: 0.050% or less, S: 0.005% or less, Cr: 0.2% to 1.0%, Ti: 0.005% to 0.030%, and Al: 0.010% to 0.050%, the balance being Fe and unavoidable impurities, and a microstructure including polygonal ferrite with a volume fraction of 70% or more and retained austenite with a volume fraction of 3% to 20%, the balance being at least one selected from martensite, bainite, and pearlite, wherein the polygonal ferrite has an average grain size of 5 μm or more and an aspect ratio of 1.40 or less.

1 . A high-strength electric resistance welded steel pipe comprising:
 a composition containing, on a mass % basis, C: 0.04% to 0.15%, Si: 0.10% to 0.50%, Mn: 1.0% to 2.2%, P: 0.050% or less, S: 0.005% or less, Cr: 0.2% to 1.0%, Ti: 0.005% to 0.030%, and Al: 0.010% to 0.050%, the balance being Fe and unavoidable impurities; and   a microstructure including polygonal ferrite with a volume fraction of 70% or more and retained austenite with a volume fraction of 3% to 20%, the balance being at least one selected from martensite, bainite, and pearlite, wherein the polygonal ferrite has an average grain size of 5 μm or more and an aspect ratio of 1.40 or less.   
     
     
         2 . The high-strength electric resistance welded steel pipe according to  claim 1 , wherein the composition further contains, on a mass % basis, at least one selected from Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, and Co: 1.0% or less. 
     
     
         3 . The high-strength electric resistance welded steel pipe according to  claim 1 , wherein the composition further contains, on a mass % basis, at least one selected from Nb: 0.10% or less and V: 0.10% or less. 
     
     
         4 . The high-strength electric resistance welded steel pipe according to  claim 1 , wherein the composition further contains Ca: 0.0005% to 0.0050% on a mass % basis. 
     
     
         5 . A method for producing the high-strength electric resistance welded steel pipe according to  claim 1 , the method comprising
 a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip;   a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and   an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner,   wherein the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.,   the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20  after 20 seconds from a time t 0  at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80  after 80 seconds from the time t 0  is lower than 650° C., and   the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.   
     
     
         6 . The method for producing a high-strength electric resistance welded steel pipe according to  claim 5 , wherein in the pipe making step, the hot-rolled steel strip is uncoiled and continuously formed using a plurality of rolls to obtain an open pipe having a substantially circular section, then end surfaces of the open pipe in a width direction are butted against each other and heated to a temperature higher than or equal to the melting point thereof, and pressure welding is performed on the butted and welded end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe. 
     
     
         7 . The high-strength electric resistance welded steel pipe according to  claim 2 , wherein the composition further contains, on a mass % basis, at least one selected from Nb: 0.10% or less and V: 0.10% or less. 
     
     
         8 . The high-strength electric resistance welded steel pipe according to  claim 2 , wherein the composition further contains Ca: 0.0005% to 0.0050% on a mass % basis. 
     
     
         9 . The high-strength electric welded steel pipe according to  claim 3 , wherein the composition further contains Ca: 0.0005% to 0.0050% on a mass % basis. 
     
     
         10 . The high-strength electric resistance welded steel pipe according to  claim 7 , wherein the composition further contains Ca: 0.0005% to 0.0050% on a mass % basis. 
     
     
         11 . A method for producing the high-strength electric resistance welded steel pipe according to  claim 2 , the method comprising
 a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip;   a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and   an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner,   wherein the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.,   the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20  after 20 seconds from a time t 0  at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80  after 80 seconds from the time t 0  is lower than 650° C., and   the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.   
     
     
         12 . A method for producing the high-strength electric resistance welded steel pipe according to  claim 3 , the method comprising
 a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip;   a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and   an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner,   wherein the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.,   the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20  after 20 seconds from a time t 0  at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80  after 80 seconds from the time t 0  is lower than 650° C., and   the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.   
     
     
         13 . A method for producing the high-strength electric resistance welded steel pipe according to  claim 4 , the method comprising
 a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip;   a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and   an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner,   wherein the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.,   the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20  after 20 seconds from a time t 0  at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80  after 80 seconds from the time t 0  is lower than 650° C., and   the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.   
     
     
         14 . A method for producing the high-strength electric resistance welded steel pipe according to  claim 7 , the method comprising
 a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip;   a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and   an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner,   wherein the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.,   the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20  after 20 seconds from a time t 0  at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80  after 80 seconds from the time t 0  is lower than 650° C., and   the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.   
     
     
         15 . A method for producing the high-strength electric resistance welded steel pipe according to  claim 8 , the method comprising
 a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip;   a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and   an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner,   wherein the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.,   the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20  after 20 seconds from a time t 0  at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80  after 80 seconds from the time t 0  is lower than 650° C., and   the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.   
     
     
         16 . A method for producing the high-strength electric resistance welded steel pipe according to  claim 9 , the method comprising
 a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip;   a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and   an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner,   wherein the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.,   the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20  after 20 seconds from a time t 0  at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80  after 80 seconds from the time t 0  is lower than 650° C., and   the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.   
     
     
         17 . A method for producing the high-strength electric resistance welded steel pipe according to  claim 10 , the method comprising
 a pipe material production step of heating, hot-rolling, and then cooling a steel material to obtain a hot-rolled steel strip and coiling the hot-rolled steel strip;   a pipe making step of forming the hot-rolled steel strip into an open pipe having a substantially circular section, then butting end surfaces of the open pipe in a width direction against each other, heating the end surfaces of the open pipe in the width direction to a temperature higher than or equal to the melting point thereof and performing pressure welding on the butted and heated end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe; and   an in-line heat treatment step of heat-treating an electric resistance welded part of the electric resistance welded steel pipe in an in-line manner,   wherein the heating in the pipe material production step is performed at a heating temperature of 1100° C. to 1250° C.,   the cooling after the hot-rolling in the pipe material production step is continuously performed to a cooling stop temperature of 600° C. to 450° C. while the cooling is controlled so that, at a central position of the steel strip in a thickness direction, a temperature T 20  after 20 seconds from a time t 0  at which a final pass of the hot-rolling is finished is higher than 650° C. and a temperature T 80  after 80 seconds from the time t 0  is lower than 650° C., and   the heat treatment in the in-line heat treatment step includes heating the electric resistance welded part so that a minimum temperature portion of the electric resistance welded part in a thickness direction has a temperature of 800° C. or higher and a maximum heating temperature is 1150° C. or lower and then performing water cooling or allowed to cooling on the electric resistance welded part so that a maximum temperature of the electric resistance welded part in a thickness direction is 500° C. or lower.   
     
     
         18 . The method for producing a high-strength electric resistance welded steel pipe according to  claim 11 , wherein in the pipe making step, the hot-rolled steel strip is uncoiled and continuously formed using a plurality of rolls to obtain an open pipe having a substantially circular section, then end surfaces of the open pipe in a width direction are butted against each other and heated to a temperature higher than or equal to the melting point thereof, and pressure welding is performed on the butted and welded end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe. 
     
     
         19 . The method for producing a high-strength electric resistance welded steel pipe according to  claim 12 , wherein in the pipe making step, the hot-rolled steel strip is uncoiled and continuously formed using a plurality of rolls to obtain an open pipe having a substantially circular section, then end surfaces of the open pipe in a width direction are butted against each other and heated to a temperature higher than or equal to the melting point thereof, and pressure welding is performed on the butted and welded end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe. 
     
     
         20 . The method for producing a high-strength electric resistance welded steel pipe according to  claim 17 , wherein in the pipe making step, the hot-rolled steel strip is uncoiled and continuously formed using a plurality of rolls to obtain an open pipe having a substantially circular section, then end surfaces of the open pipe in a width direction are butted against each other and heated to a temperature higher than or equal to the melting point thereof, and pressure welding is performed on the butted and welded end surfaces of the open pipe in the width direction to obtain an electric resistance welded steel pipe.