Steel plate excellent in machineability and in toughness and weldability and method of production of the same

Abstract

The present invention provides steel plate excellent in machineability and in toughness and weldability and a method of production of the same. Steel with C, Si, Mn, P, S, Al, and N limited to predetermined ranges and further containing, as necessary, Mo, Cr, Nb, Ti, V, Cu, Ni, B, REM, Ca, Zr, or Mg is further strictly defined in the balance of steel ingredients and strictly controlled in the conditions of rolling, water cooling, etc. in the method of production to obtain steel plate where, when the plate thickness is 4 mm to less than 10 mm, the ferrite fraction of locations exactly ¼ and ¾ of the plate thickness inside from a top surface of the steel plate is 30% to 90% and a ferrite fraction of a location exactly ½ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90% and, when the plate thickness is 10 mm to 100 mm, a ferrite fraction of a location 2 mm inside from a front and rear surface of the steel plate is 30% to 90% and a ferrite fraction of locations exactly ¼, ½, and ¾ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%.

1 . Steel plate excellent in machineability and in toughness and weldability characterized in that the steel comprises, by mass %, a steel composition comprising: 
 C: 0.005 to 0.2%,    Si: 0.01 to 1%,    Mn: 0.01 to 2%,    P: 0.02% or less,    S: 0.035% or less    Al: 0.001 to 0.1%    N, 0.01% or less, and    the balance of iron and unavoidable impurities,    X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,    when the plate thickness is 4 mm to less than 10 mm, a ferrite fraction of locations exactly ¼ and ¾ of the plate thickness inside from a top surface of the steel plate is 30% to 90% and a ferrite fraction of a location exactly ½ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%, and    when the plate thickness is 10 mm to less than 100 mm, a ferrite fraction of a location 2 mm inside from a top and rear surface of the steel plate is 30% to 90% and a ferrite fraction of locations exactly ¼, ½, and ¾ of the plate thickness inside from a top surface of the steel plate surface is 20% to 90%.    
   
   
       2 . Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in  claim 1  wherein: 
 Mn: 0.01 to 1.4%,    S: 0.01% or less,    X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,    X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0,    the structure forming the steel is a structure having a ferrite fraction of 30% to 90% and the balance of a hard structure mainly comprised of bainite and martensite or a structure having a ratio with a micro Vickers hardness of 190 HV or less of 20% or more, and    the steel has a Vickers hardness of 165 HV to 300 HV.    
   
   
       3 . Steel plate excellent in machineability and in toughness and weldability, characterized in that the steel comprises, by mass %, a steel composition as set forth in  claim 1  wherein: 
 Mn: 0.01 to 1.4%,    S: over 0.01% to 0.035%,    X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is 0.24 or less,    X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0,    the structure forming the steel is a structure having a ferrite fraction of 30% to 90% and the balance of a hard structure mainly comprised of bainite and martensite or a structure having a ratio with a micro Vickers hardness of 190 HV or less of 20% or more, and    the steel has a Vickers hardness of 165 HV to 300 HV.    
   
   
       4 . Steel plate excellent in machineability and in toughness and weldability as set forth in  claim 1 , characterized in that said steel further comprises, by mass %, one or more of: 
 Mo: 0.01 to 1%,    Cr: 0.01 to 1%,    Nb: 0.001 to 0.1%,    Ti: 0.001 to 0.1%,    V: 0.001 to 0.1%,    Cu: 0.005 to 1%,    Ni: 0.01 to 2%,    B: 0.0002 to 0.005%,    REM: 0.0005 to 0.1%,    Ca: 0.0005 to 0.02%,    Zr: 0.0005 to 0.02%, and    Mg: 0.0005 to 0.02%    
   
   
       5 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 1  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T1(° C.) expressed by T1=35 ln(X2/2)-25√t+1070, X2=(Si/5+Mo+Cr/2)/Mn, to 720° C. and a total reduction rate of 30% to 95%, starting water cooling after the end of the rolling at a flow rate of 0.2 m 3 /m 2 ·min to 5.0 m 3 /m 2 ·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.  
   
   
       6 . A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in  claim 5 , characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.  
   
   
       7 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 1  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it, starting water cooling when the steel plate surface temperature is T2(° C.) expressed by T2=910−310×C−80×Mn−20×Cu−15×Cr−55×Ni−80×Mo+0.0006t2−0.56t−8.6 to 650° C. by a flow rate of 0.2 m 3 /m 2 ·min to 5.0 m 3 /m 2 ·min, and ending the water cooling at 500° C. or less, where t is the plate thickness.  
   
   
       8 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 1  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, air cooling to 500° C. or less, reheating the steel plate to a T3(° C.) expressed by T3=0.0017t 2 +0.17t+730 to 850° C., then starting the water cooling, and ending the water cooling at 500° C. or less, where t is the plate thickness.  
   
   
       9 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 2  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)-25t+1100 to an Ar 3  point by a total reduction rate of 30% to 95%, then speedily starting water cooling after the end of the rolling at a flow rate of 0.2 m 3 /m 2 ·min to 5.0 m 3 /m 2 ·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.  
   
   
       10 . A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in  claim 9 , characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.  
   
   
       11 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 2  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.  
   
   
       12 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 2  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar 3  point to a temperature lower than the Ar 3  point by 150° C., starting water cooling at a flow rate of 0.2 m 3 /m 2 ·min to 5.0 m 3 /m 2 ·min, and ending the water cooling at 500° C. or less.  
   
   
       13 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 2  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to 500° C. or less, reheating the steel plate to 730° C. to less than 900° C., then water cooling it, and ending the water cooling to 500° C. or less.  
   
   
       14 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 3  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, then rough rolling it by a total reduction rate of 30% to 95%, then finish rolling it by a first pass bite temperature of a T4(° C.) expressed by T4=35 ln(X2/2)−25√t+1100 to an Ar 3  point by a total reduction rate of 30% to 95%, then speedily starting water cooling at a flow rate of 0.2 m 3 /m 2 ·min to 5.0 m 3 /m 2 ·min, and ending the water cooling at 600° C. or less, where t is the plate thickness.  
   
   
       15 . A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in  claim 14 , characterized in that in the water cooling started after the end of the rolling, an average cooling rate from a water cooling start temperature to over 650° C. is 1° C./s to 5° C./s and an average cooling rate from 650° C. to a water cooling stop temperature is 10° C./s to 100° C./s.  
   
   
       16 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 3  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rolling it, cooling it to 500° C. or less, reheating the steel plate to 900° C. to 1050° C., water cooling it by an average cooling rate of 1° C./s to 100° C./s, and ending the water cooling at 500° C. or less.  
   
   
       17 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 3  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then air cooling it to an Ar 3  point to a temperature lower than the Ar 3  point by 150° C., starting water cooling at a flow rate of 0.2 m 3 /m 2 ·min to 5.0 m 3 /m 2 ·min, and ending the water cooling at 500° C. or less.  
   
   
       18 . A method of production of steel plate excellent in machineability and in toughness and weldability characterized by heating a steel slab or cast slab having a steel composition as described in  claim 3  and having an X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B of 0.24 or less and an X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn of 0.15 to 10.0, rough rolling it by a total reduction rate of 30% to 95%, finish rolling it by a total reduction rate of 30% to 95%, then cooling it to 500° C. or less, reheating the steel plate to 730° C. to 900° C., water cooling it, and ending the water cooling at 500° C. or less.  
   
   
       19 . A method of production of steel plate excellent in machineability and in toughness and weldability as set forth in  claim 5 , characterized in that said steel slab or cast slab further contains, by mass %, one or more of: 
 Mo: 0.01 to 1%,    Cr: 0.01 to 1%,    Nb: 0.001 to 0.1%,    Ti: 0.001 to 0.1%,    V: 0.001 to 0.1%,    Cu: 0.005 to 1%,    Ni: 0.01 to 2%,    B: 0.0002 to 0.005%,    REM: 0.0005 to 0.1%,    Ca: 0.0005 to 0.02%,    Zr: 0.0005 to 0.02%, and    Mg: 0.0005 to 0.02%