Steel pipe with excellent expandability and method for producing the same

Abstract

A steel pipe with excellent expandability, comprises, by mass %, C: 0.1 to 0.45%, Si: 0.3 to 3.5%, Mn: 0.5 to 5%, P: less than or equal to 0.03%, S: less than or equal to 0.01%, soluble Al: 0.01 to 0.8% (more than or equal to 0.1% in case Si content is less than 1.5%), N: less than or equal to 0.05%, O: less than or equal to 0.01%, and balance being Fe and impurities, having a mixed microstructure comprising ferrite and one or more selected from fine pearlite, bainite and martensite, and has a tensile strength of more than or equal to 600 MPa and a uniform elongation satisfying the following formula u-el≧28−0.0075 TS, wherein u-el is uniform elongation (%) and TS is tensile strength (MPa).

1 . A steel pipe with excellent expandability, which has a steel composition comprising, by mass %, C: 0.1 to 0.45%, Si: 0.3 to 3.5%, Mn: 0.5 to 5%, P: less than or equal to 0.03%, S: less than or equal to 0.01%, soluble Al: 0.01 to 0.8% (more than or equal to 0.1% in case Si content is less than 1.5%), N: less than or equal to 0.05%, O: less than or equal to 0.01%, and optionally at least one element selected from at least one of Groups (A) to (E) specified below, and balance being Fe and impurities, wherein the steel has a mixed microstructure comprising ferrite and one or more selected from fine pearlite, bainite and martensite, and has a tensile strength of 600 MPa or more and a uniform elongation satisfying the following formula (1).
   u-el≧28−0.0075 TS  (1),   
       wherein u-el means uniform elongation (%), and TS means tensile strength (MPa): 
       wherein Group (A) of elements is; Cr: less than or equal to 1.5% and Cu: less than or equal to 3.0%; 
       wherein Group (B) of elements is; Mo: less than or equal to 1%; 
       wherein Group (C) of elements is; Ni: less than or equal to 2%; 
       wherein Group (D) of elements is; Ti: less than or equal to 0.3%, Nb: less than or equal to 0.3%, V: less than or equal to 0.3%, Zr: less than or equal to 0.3%, and B: less than or equal to 0.01%; 
       wherein Group (E) of elements is: Ca: less than or equal to 0.01%, Mg: less than or equal to 0.01%, and REM: less than or equal to 1.0%. 
     
     
         2 . The steel pipe with excellent expandability according to  claim 1 , wherein the steel pipe has a uniform elongation satisfying the following formula (2).
   u-el≧29.5−0.0075 TS  (2),   
       wherein u-el means uniform elongation (%), and TS means tensile strength (MPa). 
     
     
         3 . The steel pipe with excellent expandability according to  claim 1 , wherein the mixed microstructure further comprises residual austenite. 
     
     
         4 . The steel pipe with excellent expandability according to  claim 2 , wherein the mixed microstructure further comprises residual austenite. 
     
     
         5 . A method for producing a steel pipe with excellent expandability, comprising the steps of:
 (a) heating the steel pipe which has a steel composition comprising, by mass %, C: 0.1 to 0.45%, Si: 0.3 to 3.5%, Mn: 0.5 to 5%, P: less than or equal to 0.03%, S: less than or equal to 0.01%, soluble Al: 0.01 to 0.8% (more than or equal to 0.1% in case Si content is less than 1.5%), N: less than or equal to 0.05%, O: less than or equal to 0.01%, and optionally at least one element selected from at least one of Groups (A) to (E) specified below, and balance being Fe and impurities,   wherein Group (A) of elements is; Cr: less than or equal to 1.5% and Cu: less than or equal to 3.0%;   wherein Group (B) of elements is; Mo: less than or equal to 1%;   wherein Group (C) of elements is; Ni: less than or equal to 2%;   wherein Group (D) of elements is; Ti: less than or equal to 0.3%, Nb: less than or equal to 0.3%, V: less than or equal to 0.3%, Zr: less than or equal to 0.3%, and B: less than or equal to 0.01%;   wherein Group (E) of elements is: Ca: less than or equal to 0.01%, Mg: less than or equal to 0.01%, and REM: less than or equal to 1.0%, to a temperature from 700 to 790° C., and   (b) forced-cooling the steel pipe down to a temperature lower than or equal to 100° C., wherein the steel pipe is forced-cooled with a cooling rate greater than or equal to 100° C./min at a temperature ranging from 700 to 500° C.   
     
     
         6 . A method for producing a steel pipe with excellent expandability, comprising steps of:
 (a) heating the steel pipe which has a steel composition comprising, by mass %, C: 0.1 to 0.45%, Si: 0.3 to 3.5%, Mn: 0.5 to 5%, P: less than or equal to 0.03%, S: less than or equal to 0.01%, soluble Al: 0.01 to 0.8% (more than or equal to 0.1% in case Si content is less than 1.5%), N: less than or equal to 0.05%, O: less than or equal to 0.01%, and optionally at least one element selected from at least one of Groups (A) to (E) specified below, and balance being Fe and impurities,   wherein Group (A) of elements is; Cr: less than or equal to 1.5% and Cu: less than or equal to 3.0%;   wherein Group (B) of elements is; Mo: less than or equal to 1%;   wherein Group (C) of elements is; Ni: less than or equal to 2%;   wherein Group (D) of elements is; Ti: less than or equal to 0.3%, Nb: less than or equal to 0.3%, V: less than or equal to 0.3%, Zr: less than or equal to 0.3%, and B: less than or equal to 0.01%;   wherein Group (E) of elements is: Ca: less than or equal to 0.01%, Mg: less than or equal to 0.01%, and REM: less than or equal to 1.0%, to a temperature from 700 to 790° C.,   (b) forced-cooling the steel pipe down to a temperature from 250 to 450° C., wherein the steel pipe is forced-cooled with a cooling rate greater than or equal to 100° C./rain at a temperature ranging from 700 to 500° C.,   (c) soaking the steel pipe at a temperature from 250 to 450° C. for 10 min. or more, and then   (d) cooling the steel pipe down to room temperature.   
     
     
         7 . A method for producing a steel pipe with excellent expandability, comprising steps of:
 (a) heating the steel pipe which has a steel composition comprising, by mass %, C: 0.1 to 0.45%, Si: 0.3 to 3.5%, Mn: 0.5 to 5%, P: less than or equal to 0.03%, S: less than or equal to 0.01%, soluble Al: 0.01 to 0.8% (more than or equal to 0.1% in case Si content is less than 1.5%), N: less than or equal to 0.05%, O: less than or equal to 0.01%, and optionally at least one element selected from at least one of Groups (A) to (E) specified below, and balance being Fe and impurities,   wherein Group (A) of elements is; Cr: less than or equal to 1.5% and Cu: less than or equal to 3.0%;   wherein Group (B) of elements is; Mo: less than or equal to 1%;   wherein Group (C) of elements is; Ni: less than or equal to 2%;   wherein Group (D) of elements is; Ti: less than or equal to 0.3%, Nb: less than or equal to 0.3%, V: less than or equal to 0.3%, Zr: less than or equal to 0.3%, and B less than or equal to 0.01%;   wherein Group (E) of elements is: Ca: less than or equal to 0.01%, Mg: less than or equal to 0.01%, and REM: less than or equal to 1.0%, to a temperature from 700 to 790° C.,   (b) forced-cooling the steel pipe down to a temperature from above 250 to 450° C., wherein the steel pipe is forced-cooled with a cooling rate greater than or equal to 100° C./rain at a temperature ranging from 700 to 500° C.,   (c) control-cooling the steel pipe from the finish temperature of the forced-cooling to 250° C. at a cooling rate lower than or equal to 10° C./min, and then   (d) cooling the steel pipe down to room temperature.