Steel tube, vehicle component, method for producing steel tube, and method for producing vehicle component

Abstract

A steel tube of the present disclosure includes a base metal, and an oxide coating on the base metal. The base metal has a chemical composition described in the present description, and has a microstructure composed of, by area fraction, ferrite in an amount of 20% to 60% and pearlite in an amount of 40% to 80%. The oxide coating is composed of, when the sum of peak intensities in X-ray diffraction of Fe 3 O 4 , Fe 2 O 3 , and FeO is taken as 100%, in terms of the peak intensity proportions in the X-ray diffraction, Fe 3 O 4 in an amount of 70% or more, Fe 2 O 3 in an amount of 20% or more, and FeO in an amount of 10% or less, with the balance being impurities. The thickness of the oxide coating is 0.80 to 2.50 μm, and the standard deviation of the thickness is 0.90 μm or less.

The invention claimed is: 
     
         1 . A steel tube, comprising:
 a base metal that has a chemical composition consisting of, in mass %,   C: 0.23 to 0.50%,   Si: 0.01 to 0.50%,   Mn: 0.50 to 2.50%,   P: 0.050% or less,   S: 0.0100% or less,   N: 0.0100% or less,   O: 0.0100% or less,   sol. Al: 0 to 0.080%,   Cr: 0 to 1.50%,   Mo: 0 to 1.00%,   Ni: 0 to 1.00%,   Cu: 0 to 1.00%,   Ti: 0 to 0.100%,   Nb: 0 to 0.100%,   V: 0 to 0.100%,   B: 0 to 0.0050%, and   Ca: 0 to 0.0050%,   with the balance being Fe and impurities, and   that has a microstructure composed of, by area fraction, ferrite in an amount of 20% to 60% and pearlite in an amount of 40% to 80%; and   an oxide coating on the base metal   wherein when a sum of peak intensities in X-ray diffraction of Fe 3 O 4 , Fe 2 O 3 , and FeO is taken as 100%, the oxide coating is composed of, in terms of the peak intensity proportions in the X-ray diffraction, Fe 3 O 4  in an amount of 70% or more, Fe 2 O 3  in an amount of 20% or more, and FeO in an amount of 10% or less, with the balance being impurities, and   wherein a thickness of the oxide coating is 0.80 to 2.50 μm, and a standard deviation of the thickness is 0.90 μm or less.   
     
     
         2 . The steel tube according to  claim 1 , wherein:
 the chemical composition contains one or more elements selected from a group consisting of, in mass %,   sol. Al: 0.001 to 0.080%,   Cr: 0.01 to 1.50%,   Mo: 0.01 to 1.00%,   Ni: 0.01 to 1.00%,   Cu: 0.01 to 1.00%,   Ti: 0.001 to 0.100%,   Nb: 0.001 to 0.100%,   V: 0.001 to 0.100%,   B: 0.0001 to 0.0050%, and   Ca: 0.0001 to 0.0050%.   
     
     
         3 . A vehicle component, comprising:
 a hollow-shell base metal that has a chemical composition consisting of, in mass %,   C: 0.23 to 0.50%,   Si: 0.01 to 0.50%,   Mn: 0.50 to 2.50%,   P: 0.050% or less,   S: 0.0100% or less,   N: 0.0100% or less,   O: 0.0100% or less,   sol. Al: 0 to 0.080%,   Cr: 0 to 1.50%,   Mo: 0 to 1.00%,   Ni: 0 to 1.00%,   Cu: 0 to 1.00%,   Ti: 0 to 0.100%,   Nb: 0 to 0.100%,   V: 0 to 0.100%,   B: 0 to 0.0050%, and   Ca: 0 to 0.0050%,   with the balance being Fe and impurities, and   that has a microstructure composed of tempered martensite,   wherein the base metal has a Vickers hardness in accordance with JIS Z 2244: 2020 of 400 to 550 HV; and   an oxide coating on the base metal   wherein when a sum of peak intensities in X-ray diffraction of Fe 3 O 4 , FeO, and Fe 2 O 3  is taken as 100%, the oxide coating is composed of, in terms of the peak intensity proportions in the X-ray diffraction, Fe 3 O 4  in an amount of 80% or more, FeO in an amount of 15% or less, and Fe 2 O 3  in an amount of 5% or less, with the balance being impurities, and   wherein a thickness of the oxide coating is 3.50 μm or less.   
     
     
         4 . The vehicle component according to  claim 3 , wherein:
 the chemical composition contains one or more elements selected from a group consisting of, in mass %,   sol. Al: 0.001 to 0.080%,   Cr: 0.01 to 1.50%,   Mo: 0.01 to 1.00%,   Ni: 0.01 to 1.00%,   Cu: 0.01 to 1.00%,   Ti: 0.001 to 0.100%,   Nb: 0.001 to 0.100%,   V: 0.001 to 0.100%,   B: 0.0001 to 0.0050%, and   Ca: 0.0001 to 0.0050%.   
     
     
         5 . A method for producing a steel tube, comprising:
 a step of preparing a steel sheet that has a chemical composition consisting of, in mass %,   C: 0.23 to 0.50%,   Si: 0.01 to 0.50%,   Mn: 0.50 to 2.50%,   P: 0.050% or less,   S: 0.0100% or less,   N: 0.0100% or less,   O: 0.0100% or less,   sol. Al: 0 to 0.080%,   Cr: 0 to 1.50%,   Mo: 0 to 1.00%,   Ni: 0 to 1.00%,   Cu: 0 to 1.00%,   Ti: 0 to 0.100%,   Nb: 0 to 0.100%,   V: 0 to 0.100%,   B: 0 to 0.0050%, and   Ca: 0 to 0.0050%,   with the balance being Fe and impurities, and   that has a microstructure composed of, by area fraction, ferrite in an amount of 20% to 60% and pearlite in an amount of 40% to 80%;   a step of subjecting the steel sheet to a heat treatment at 450 to 600° C. for 0.5 to 3.0 minutes; and   a step of subjecting the steel sheet after the heat treatment to electric resistance welding to produce a steel tube.   
     
     
         6 . A method for producing a vehicle component, comprising:
 a step of preparing the steel tube according to  claim 1 ;   a step of subjecting the steel tube to bending;   a step of holding the steel tube after the bending at a temperature within a range of Ac 3 +50° C. or more to 1150° C. or less for 10 seconds or more, and thereafter rapidly cooling the steel tube; and   a step of tempering the steel tube after the rapid cooling by holding the steel tube after the rapid cooling at a temperature of 150 to 350° C. for 10 minutes or more.