Austenite-type stainless steel hot-rolling steel material with excellent corrosion resistance, proof-stress, and low-temperature toughness and production method thereof

Abstract

An austenitic stainless steel hot-rolled steel material can be provided which has sea-water resistance and strength superior to conventional steel. Low-temperature toughness can be maintained, which is preferable in a structural member of speedy craft. The steel material can include an austenitic stainless steel hot-rolled steel material which excels in the properties of corrosion resistance, proof stress, and low-temperature toughness. In such austenitic stainless steel hot-rolling steel material, e.g., PI[=Cr+3.3(Mo+0.5W)+16N] ranges from 35 to 40, δ cal [=2.9 (Cr+0.3Si+Mo+0.5W)−2.6 (Ni+0.3Mn+0.25Cu+35C+20N)−18] ranges from −6 to +2, and a 0.2% proof stress at room temperature is not less than 550 MPa, Charpy impact value measured using a V-notch test piece at −40° C. is not less than 100 J/cm2, and the pitting potential measured in a deaerated aqueous solution of 10% NaCl at 50° C. (Vc′100) is not less than 500 mV (as it relates to saturated Ag/AgCl).

1 - 10 . (canceled)  
   
   
       11 . An austenitic stainless hot-rolled steel material having a superior corrosion resistance, a proof stress, and a low-temperature toughness, comprising, 
 C: about 0.001 to 0.03 mass %,    Si: about 0.1 to 1.5 mass %,    Mn: about 0.1 to 3.0 mass %,    P: about 0.005 to 0.05 mass %,    S: about 0.0001 to 0.003 mass %,    Ni: about 15.0 to 21.0 mass %;    Cr: about 22.0 to 28.0 mass %,    Mo: about 1.5 to 3.5 mass %,    N: about 0.15 to 0.35 mass %, and    O: about 0.0005 to 0.007 mass %,    wherein:    a Pi value expressed by the following formula ranges from about 35 to 40:      PI=Cr+3.3(Mo+0.5W)+16N, and    a δ cal value expressed by the following formula ranges from about −6 to +2:      δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+35C+20N)−18    a remnant of the steel material comprising Fe and inevitable impurities,    a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %,    about 0.2% proof stress at a room temperature is at least about 550MPa,    a Charpy impact value measured using a V-notch test piece at about −40° C. is at least about 100 J/cm 2 , and    a pitting potential measured in a deaerated aqueous solution of about 10% NaCl at about 50° C. (Vc′100) is at least about 500 mV as compared to a saturated solution of Ag/AgCl.    
   
   
       12 . The steel material according to  claim 11 , further comprising at least one of: 
 W: about 0.3 to 3.0 mass %, or    Al: about 0.005 to 0.1 mass %.    
   
   
       13 . The steel material according to  claim 11 , further comprising at least one of: 
 W: about 0.3 to 3.0 mass %,    Al: about 0.005 to 0.1 mass %,    Cu: about 0.3 to 2.0 mass %, or    Sn: at most about 0.1 mass %.    
   
   
       14 . The steel material according to  claim 11 , further comprising at least one of: 
 W: about 0.3 to 3.0 mass %,    Al: about 0.005 to 0.1 mass %,    Cu: about 0.3 to 2.0 mass %,    Sn: about at most about 0.1 mass %,    Ca: about 0.0005 to 0.0050 mass %,    Mg: about 0.0005 to 0.0050 mass %, or    REM: about 0.005 to 0.10 mass %.    
   
   
       15 . The steel material according to  claim 11 , further comprising at least one of: 
 W: about 0.3 to 3.0 mass %,    Al: about 0.005 to 0.1 mass %,    Cu: about 0.3 to 2.0 mass %,    Sn: about at most about 0.1 mass %,    Ca: about 0.0005 to 0.0050 mass %,    Mg: about 0.0005 to 0.0050 mass %,    REM: about 0.005 to 0.10 mass %, or    B: about 0.0003 to 0.0060 mass %.    
   
   
       16 . The steel material according to  claim 11 , further comprising at least one of: 
 W: about 0.3 to 3.0 mass %,    Al: about 0.005 to 0.1 mass %,    Cu: about 0.3 to 2.0 mass %,    Sn: at most about 0.1 mass %,    Ca: about 0.0005 to 0.0050 mass %,    Mg: about 0.0005 to 0.0050 mass %,    REM: about 0.005 to 0.10 mass %,    B: about 0.0003 to 0.0060 mass %,    Ti: about 0.003 to 0.03 mass %,    Nb: about 0.02 to 0.20 mass %,    Zr: about 0.003 to 0.03 mass %,    V: about 0.05 to 0.5 mass %, or    Ta: about 0.01 to 0.1 mass %.    
   
   
       17 . A process for producing an austenitic stainless hot-rolled steel material having a superior corrosion resistance, a proof stress, and a low-temperature toughness, wherein a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, about 0.2% proof stress at a room temperature is at least about 550 MPa, a Charpy impact value measured using a V-notch test piece at about −40° C. is at least about 100 J/cm 2 , and a pitting potential measured in a deaerated aqueous solution of about 10% NaCl at about 50° C. (Vc′100) is at least about 500 mV as compared to a saturated solution of Ag/AgCl, the process comprising: 
 performing, at a temperature of about 1200 to 1300° C. for at least about one hour, a homogenizing-heat treatment on a particular material which is at least one of a cast steel or a semi-finished product of the steel material which comprises: 
 C: about 0.001 to 0.03 mass %,  
 Si: about 0.1 to 1.5 mass %,  
 Mn: about 0.1 to 3.0 mass %,  
 P: about 0.005 to 0.05 mass %,  
 S: about 0.0001 to 0.003 mass %,  
 Ni: about 15.0 to 21.0 mass %;  
 Cr: about 22.0 to 28.0 mass %,  
 Mo: about 1.5 to 3.5 mass %,  
 N: about 0.15 to 0.35 mass %, and  
 O: about 0.0005 to 0.007 mass %,  
 wherein a PI value expressed by the following formula ranges from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N, and  
 a δ cal value expressed by the following formula ranges from about −6 to +2:  
   δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+35C+20N)−18  
 a remnant of the steel material comprising Fe and inevitable impurities,  
   reheating the particular treated material at a temperature of about 1100 to 1300° C.;    while rolling the particular reheated material, maintaining a temperature of at least about 850° C., and by a first draft of at least about 50% at a temperature of at least about 1050° C. and a second draft of at least about 10% at a temperature of about 1050 to 850° C.; and    cooling the rolled particular material at an average cooling rate of about 800 to 500° C. after the rolling procedure is performed for at least about 150° C./min, without a solution treatment.    
   
   
       18 . An austenitic stainless hot-rolled steel material having a superior corrosion resistance and a low-temperature toughness, comprising, 
 C: at most about 0.03 mass %,    Si: about 0.1 to 1.5 mass %,    Mn: about 0.1 to 3.0 mass %,    P: at most about 0.05 mass %,    S: at most about 0.003 mass %,    Ni: about 15.0 to 21.0 mass %,    Cr: about 22.0 to 28.0 mass %,    Mo: about 1.5 to 3.5 mass %,    N: about 0.15 to 0.35 mass %,    Al: about 0.005 to 0.1 mass %, and    O: at most about 0.007 mass %,    wherein    a PI value expressed by the following formula ranges from about 35 to 40:      PI=Cr+3.3(Mo+0.5W)+16N, and    a δ cal value expressed by the following formula ranges from about −6 to +4:      δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+35C+20N)−18    a remnant of the steel material comprising of Fe and substantially inevitable impurities, and a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, and    a value by each element represents the content of the element expressed in terms of mass %.    
   
   
       19 . The steel material according to  claim 18 , further comprising at least one of: 
 Cu: about 0.1 to 2.0 mass %,    Ti: about 0.003 to 0.03 mass %,    Nb: about 0.02 to 0.20 mass %,    V: about 0.05 to 0.5 mass %,    W: about 0.3 to 3.0 mass %,    B: about 0.0003 to 0.0060 mass %,    Ca: about 0.0005 to 0.0050 mass %,    Mg: about 0.0005 to 0.0050 mass %, or    REM: about 0.005 to 0.10.    
   
   
       20 . A process for producing an austenitic stainless hot-rolled steel material having a superior corrosion resistance and a low-temperature toughness, wherein a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, the process comprising: 
 performing, at a temperature of about 1200 to 1300° C. for at least about one hour, a homogenizing-heat treatment on a particular material which is at least one of a cast steel or a semi-finished product of the steel material which comprises: 
 C: at most about 0.03 mass %,  
 Si: about 0.1 to 1.5 mass %,  
 Mn: about 0.1 to 3.0 mass %,  
 P: at most about 0.05 mass %,  
 S: at most about 0.003 mass %,  
 Ni: about 15.0 to 21.0 mass %,  
 Cr: about 22.0 to 28.0 mass %,  
 Mo: about 1.5 to 3.5 mass %,  
 N: about 0.15 to 0.35 mass %,  
 Al: about 0.005 to 0.1 mass %, and  
 O: at most about 0.007 mass %,  
   wherein    a PI value expressed by the following formula ranges from about 35 to 40:      PI=Cr+3.3(Mo+0.5W)+16N, and    a δ cal value expressed by the following formula ranges from about −6 to +4:      δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+35C+20N)−18    a remnant of the steel material comprising of Fe and substantially inevitable impurities,    a value by each element represents the content of the element expressed in terms of mass %, and    the heat treatment is performed so as to reduce the content of the intermetallic compound in the steel material.    
   
   
       21 . A process for producing an austenitic stainless hot-rolled steel material having a superior corrosion resistance, a proof stress, and a low-temperature toughness, wherein a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, about 0.2% proof stress at a room temperature is at least about 550 MPa, a Charpy impact value measured using a V-notch test piece at about −40° C. is at least about 100 J/cm 2 , and a pitting potential measured in a deaerated aqueous solution of about 10% NaCl at about 50° C. (Vc′100) is at least about 500 mV as compared to a saturated solution of Ag/AgCl, the process comprising: 
 performing, at a temperature of about 1200 to 1300° C. for at least about one hour, a homogenizing-heat treatment on a particular material which is at least one of a cast steel or a semi-finished product of the steel material which comprises: 
 C: about 0.001 to 0.03 mass %,  
 Si: about 0.1 to 1.5 mass %,  
 Mn: about 0.1 to 3.0 mass %,  
 P: about 0.005 to 0.05 mass %,  
 S: about 0.0001 to 0.003 mass %,  
 Ni: about 15.0 to 21.0 mass %;  
 Cr: about 22.0 to 28.0 mass %,  
 Mo: about 1.5 to 3.5 mass %,  
 N: about 0.15 to 0.35 mass %,  
 O: about 0.0005 to 0.007 mass %, and  
 at least one of: 
 W: about 0.3 to 3.0 mass %, or  
 Al: about 0.005 to 0.1 mass %.  
 
 wherein a PI value expressed by the following formula ranges from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N,  
 a δ cal value expressed by the following formula ranges from about −6 to +2:  
   δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+35C+20N)−18, and  
 a remnant of the steel material comprising Fe and inevitable impurities;  
   reheating the particular treated material at a temperature of about 1100 to 1300° C.;    while rolling the particular reheated material at a temperature of at least about 850° C., and by a first draft of at least about 1050% at a temperature of at least about 1 050° C. and a second draft of at least about 10% at a temperature of about 1050 to 850° C.; and    cooling the rolled particular material at an average cooling rate of about 800 to 500° C. after the rolling procedure is performed for at least about 150° C./min, without a solution treatment.    
   
   
       22 . A process for producing an austenitic stainless hot-rolled steel material having a superior corrosion resistance, a proof stress, and a low-temperature toughness, wherein a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, about 0.2% proof stress at a room temperature is at least about 550 MPa, a Charpy impact value measured using a V-notch test piece at about −40° C. is at least about 100 J/cm 2 , and a pitting potential measured in a deaerated aqueous solution of about 10% NaCl at about 50° C. (Vc′100) is at least about 500 mV as compared to a saturated solution of Ag/AgCl, the process comprising: 
 performing, at a temperature of about 1200 to 1300° C. for at least about one hour, a homogenizing-heat treatment on a particular material which is at least one of a cast steel or a semi-finished product of the steel material which comprises: 
 C: about 0.001 to 0.03 mass %,  
 Si: about 0.1 to 1.5 mass %,  
 Mn: about 0.1 to 3.0 mass %,  
 P: about 0.005 to 0.05 mass %,  
 S: about 0.0001 to 0.003 mass %,  
 Ni: about 15.0 to 21.0 mass %;  
 Cr: about 22.0 to 28.0 mass %,  
 Mo: about 1.5 to 3.5 mass %,  
 N: about 0.15 to 0.35 mass %,  
 O: about 0.0005 to 0.007 mass %, and  
 at least one of: 
 W: about 0.3 to 3.0 mass %,  
 Al: about 0.005 to 0.1 mass %,  
 Cu: about 0.3 to 2.0 mass %, or  
 Sn: at most about 0.1 mass %,  
 
 wherein a PI value expressed by the following formula ranges from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N,  
 a δ cal value expressed by the following formula ranges from about −6 to +2:  
   δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+35C+20N)−18, and  
 a remnant of the steel material comprising Fe and inevitable impurities,  
 reheating the particular treated material at a temperature of about 1100 to 1300° C.;  
   while rolling the particular reheated material maintaining a temperature of at least about 850° C., and by a first draft of at least about 50% at a temperature of at least about 1050° C. and a second draft of at least about 10% at a temperature of about 1050 to 850° C.; and    cooling the rolled particular material at an average cooling rate of about 800 to 500° C. after the rolling procedure is performed for at least about 150° C./min, without a solution treatment.    
   
   
       23 . A process for producing an austenitic stainless hot-rolled steel material having a superior corrosion resistance, a proof stress, and a low-temperature toughness, wherein a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, about 0.2% proof stress at a room temperature is at least about 550 MPa, a Charpy impact value measured using a V-notch test piece at about −40° C. is at least about 100 J/cm 2 , and a pitting potential measured in a deaerated aqueous solution of about 10% NaCl at about 50° C. (Vc′100) is at least about 500 mV as compared to a saturated solution of Ag/AgCl, the process comprising: 
 performing, at a temperature of about 1200 to 1300° C. for at least about one hour, a homogenizing-heat treatment on a particular material which is at least one of a cast steel or a semi-finished product of the steel material which comprises: 
 C: about 0.001 to 0.03 mass %,  
 Si: about 0.1 to 1.5 mass %,  
 Mn: about 0.1 to 3.0 mass %,  
 P: about 0.005 to 0.05 mass %,  
 S: about 0.0001 to 0.003 mass %,  
 Ni: about 15.0 to 21.0 mass %;  
 Cr: about 22.0 to 28.0 mass %,  
 Mo: about 1.5 to 3.5 mass %,  
 N: about 0.15 to 0.35 mass %,  
 O: about 0.0005 to 0.007 mass %, and at least one of: 
 W: about 0.3 to 3.0 mass %,  
 Al: about 0.005 to 0.1 mass %,  
 Cu: about 0.3 to 2.0 mass %,  
 Sn: about at most about 0.1 mass %,  
 Ca: about 0.0005 to 0.0050 mass %,  
 Mg: about 0.0005 to 0.0050 mass %, or  
 REM: about 0.005 to 0.10 mass %,  
 
 wherein a PI value expressed by the following formula ranges from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N,  
 a δ cal value expressed by the following formula ranges from about −6 to +2:  
   δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6 (Ni+0.3Mn+0.25Cu+35C+20N)−18, and  
 a remnant of the steel material comprising Fe and inevitable impurities, reheating the particular treated material at a temperature of about 1100 to 1300° C.;  
   while rolling the particular reheated material maintaining a temperature of at least about 850° C., and by a first draft of at least about 50% at a temperature of at least about 1050° C. and a second draft of at least about 10% at a temperature of about 1050 to 850° C.; and    cooling the rolled particular material at an average cooling rate of about 800 to 500° C. after the rolling procedure is performed for at least about 150° C./min, without a solution treatment.    
   
   
       24 . A process for producing an austenitic stainless hot-rolled steel material having a superior corrosion resistance, a proof stress, and a low-temperature toughness, wherein a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, about 0.2% proof stress at a room temperature is at least about 550 MPa, a Charpy impact value measured using a V-notch test piece at about −40° C. is at least about 100 J/cm 2 , and a pitting potential measured in a deaerated aqueous solution of about 10% NaCl at about 50° C. (Vc′100) is at least about 500 mV as compared to a saturated solution of Ag/AgCl, the process comprising: 
 performing, at a temperature of about 1200 to 1300° C. for at least about one hour, a homogenizing-heat treatment on a particular material which is at least one of a cast steel or a semi-finished product of the steel material which comprises: 
 C: about 0.001 to 0.03 mass %,  
 Si: about 0.1 to 1.5 mass %,  
 Mn: about 0.1 to 3.0 mass %,  
 P: about 0.005 to 0.05 mass %,  
 S: about 0.0001 to 0.003 mass %,  
 Ni: about 15.0 to 21.0 mass %;  
 Cr: about 22.0 to 28.0 mass %,  
 Mo: about 1.5 to 3.5 mass %,  
 N: about 0.15 to 0.35 mass %, p 2  O: about 0.0005 to 0.007 mass %, and at least one of: 
 W: about 0.3 to 3.0 mass %,  
 Al: about 0.005 to 0.1 mass %,  
 Cu: about 0.3 to 2.0 mass %,  
 Sn: at most about 0.1 mass %,  
 Ca: about 0.0005 to 0.0050 mass %,  
 Mg: about 0.0005 to 0.0050 mass %,  
 REM: about 0.005 to 0.10 mass %, or  
 B: about 0.0003 to 0.0060 mass %,  
 
 wherein a PI value expressed by the following formula ranges from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N,  
 a δ cal value expressed by the following formula ranges from about −6 to +2:  
   δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+35C+20N)−18, and  
 a remnant of the steel material comprising Fe and inevitable impurities,  
   reheating the particular treated material at a temperature of about 1100 to 1300° C.;    while rolling the particular reheated material maintaining a temperature of at least about 850° C., and by a first draft of at least about 50% at a temperature of at least about 1050° C. and a second draft of at least about 10% at a temperature of about 1050 to 850° C.; and    cooling the rolled particular material at an average cooling rate of about 800 to 500° C. after the rolling procedure is performed for at least about 150° C./min, without a solution treatment.    
   
   
       25 . A process for producing an austenitic stainless hot-rolled steel material having a superior corrosion resistance, a proof stress, and a low-temperature toughness, wherein a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, about 0.2% proof stress at a room temperature is at least about 550 MPa, a Charpy impact value measured using a V-notch test piece at about −40° C. is at least about 100 J/cm 2 , and a pitting potential measured in a deaerated aqueous solution of about 10% NaCl at about 50° C. (Vc′100) is at least about 500 mV as compared to a saturated solution of Ag/AgCl, the process comprising: 
 performing, at a temperature of about 1200 to 1300° C. for at least about one hour, a homogenizing-heat treatment on a particular material which is at least one of a cast steel or a semi-finished product of the steel material which comprises: 
 C: about 0.001 to 0.03 mass %,  
 Si: about 0.1 to 1.5 mass %,  
 Mn: about 0.1 to 3.0 mass %,  
 P: about 0.005 to 0.05 mass %,  
 S: about 0.0001 to 0.003 mass %,  
 Ni: about 15.0 to 21.0 mass %;  
 Cr: about 22.0 to 28.0 mass %,  
 Mo: about 1.5 to 3.5 mass %,  
 N: about 0.15 to 0.35 mass %,  
 O: about 0.0005 to 0.007 mass %, and  
 at least one of: 
 W: about 0.3 to 3.0 mass %,  
 Al: about 0.005 to 0.1 mass %,  
 Cu: about 0.3 to 2.0 mass %,  
 Sn: at most about 0.1 mass %,  
 Ca: about 0.0005 to 0.0050 mass %,  
 Mg: about 0.0005 to 0.0050 mass %,  
 REM: about 0.005 to 0.10 mass %,  
 B: about 0.0003 to 0.0060 mass %,  
 Ti: about 0.003 to 0.03 mass %,  
 Nb: about 0.02 to 0.20 mass %,  
 Zr: about 0.003 to 0.03 mass %,  
 V: about 0.05 to 0.5 mass %, or  
 Ta: about 0.01 to 0.1 mass %,  
 
 wherein a PI value expressed by the following formula ranges from about 35 to 40: PI=Cr+3.3(Mo+0.5W)+16N,  
 a δ cal value expressed by the following formula ranges from about −6 to +2:  
   δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+35C+20N)−18, and  
 a remnant of the steel material comprising Fe and inevitable impurities,  
   reheating the particular treated material at a temperature of about 1100 to 1300° C.;    while rolling the particular reheated material maintaining a temperature of at least about 850° C., and by a first draft of at least about 50% at a temperature of at least about 1050° C. and a second draft of at least about 10% at a temperature of about 1050 to 850° C.; and    cooling the rolled particular material at an average cooling rate of about 800 to 500° C. after the rolling procedure is performed for at least about 150° C./min, without a solution treatment.    
   
   
       26 . A process for producing an austenitic stainless hot-rolled steel material having a superior corrosion resistance and a low-temperature toughness, wherein a content of intermetallic compounds contained in the steel material is at most about 0.5 mass %, the process comprising: 
 performing, at a temperature of about 1200 to 1300° C. for at least about one hour, a homogenizing-heat treatment on a particular material which is at least one of a cast steel or a semi-finished product of the steel material which comprises: 
 C: at most about 0.03 mass %,  
 Si: about 0.1 to 1.5 mass %,  
 Mn: about 0.1 to 3.0 mass %,  
 P: at most about 0.05 mass %,  
 S: at most about 0.003 mass %,  
 Ni: about 15.0 to 21.0 mass %,  
 Cr: about 22.0 to 28.0 mass %,  
 Mo: about 1.5 to 3.5 mass %,  
 N: about 0.15 to 0.35 mass %,  
 Al: about 0.005 to 0.1 mass %, and  
 O: at most about 0.007 mass %, and  
 at least one of: 
 Cu: about 0.1 to 2.0 mass %,  
 Ti: about 0.003 to 0.03 mass %,  
 Nb: about 0.02 to 0.20 mass %,  
 V: about 0.05 to 0.5 mass %,  
 W: about 0.3 to 3.0 mass %,  
 B: about 0.0003 to 0.0060 mass %,  
 Ca: about 0.0005 to 0.0050 mass %,  
 Mg: about 0.0005 to 0.0050 mass %, or  
 REM: about 0.005 to 0.10,  
 
 wherein 
 a PI value expressed by the following formula ranges from about 35 to 40:  
   PI=Cr+3.3(Mo+0.5W)+16N, and  
 a δ cal value expressed by the following formula ranges from about 6 to +4:  
   δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni +0.3Mn+0.25Cu+35C+20N)−18  
 a remnant of the steel material comprising of Fe and substantially inevitable impurities,  
 a value by each element represents the content of the element expressed in terms of mass %, and  
 the heat treatment is performed so as to reduce the content of the intermetallic compound in the steel material.