Super formable high strength steel sheet and method of manufacturing thereof

Abstract

Disclosed herein are a super formable high strength thin steel sheet suitable for use in various applications, e.g., automobiles, and a method for manufacturing the thin steel sheet. The thin steel sheet has a composition which comprises 0.010 wt % or less of C, 0.02 wt % or less of Si, 1.5 wt % or less of Mn, 0.030.15 wt % or less of P, 0.02 wt % or less of S, 0.030.40 wt % of Sol. Al, 0.004 wt % or less of N, 0.0050.040 wt % of Ti, 0.0020.020 wt % of Nb, one or both of 0.0001 0.02 wt % of B and 0.0050.02 wt % of Mo, and the balance of Fe and inevitable impurities, wherein the components P. Mn, Ti, Nb and B satisfy the relationship represented by the following Formulae 1-1 and 1-2, depending on a desired tensile strength: [Formula 1-1]—tensile strength: 35 kg and 40 kg grades 29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.SNb(%)+0.18[B(ppm) or Mo(%)]15=3544.9 [Formula 1-2]—tensile strength: 45 kg grade 29.1+98.3P(%)+4.6Mn(%)−86.STi(%)+62.SNb(%)+0.21 [B(ppm) or Mo(%)]4550, the components Ti, N, C and Nb satisfy the relationship represented by the 2 0 following Formulae 2 and 3 [Formula 2] 0.6 <(1/0.65)(Ti−3.43N)/4C<3.5 [Formnula 3]0.4

1 . A super formable high strength thin steel sheet having a composition which comprises 0.010 wt % or less of C, 0.02 wt % or less of Si, 1.5 wt % or less of Mn, 0.03˜0.15 wt % or less of P, 0.02 wt % or less of S, 0.03˜0.40 wt % of Sol. Al, 0.004 wt % or less of N, 0.005˜0.040 wt % of Ti, 0.002˜0.020 wt % of Nb, one or both of 0.0001˜0.02 wt % of B and 0.005˜0.02 wt % of Mo, and the balance of Fe and inevitable impurities, 
 wherein the components P, Mn, Ti, Nb and B satisfy the relationship represented by the following Formulae 1-1 and 1-2, depending on a desired tensile strength: at tensile strength of 35 kg and 40 kg grades  
 29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) or Mo(%)]=35˜44.9   Formula 1-1  
 and at tensile strength of 45 kg grade  
 29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50   Formula 1-2,  
 wherein the components Ti, N, C and Nb satisfy the relationship represented by the following Formulae 2 and 3:  
 0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   Formula 2 0.4≦(1/0.35)(Nb/7.75C)≦2.2   Formula 3,  
 and Ti-based and Nb-based precipitates are distributed in an average size ranging from 30˜60 nm.  
 
     
     
         2 . A super formable high strength thin steel sheet with excellent powdering resistance, the thin steel sheet having a composition which comprises 0.010 wt % or less of C, 0.02 wt % or less of Si, 1.5 wt % or less of Mn, 0.03˜0.15 wt % or less of P, 0.02 wt % or less of S, 0.03˜0.40 wt % of Sol. Al, 0.004 wt % or less of N, 0.005˜0.040 wt % of Ti, 0.002˜0.020 wt % of Nb, one or both of 0.0001˜0.02 wt % of B and 0.005˜0.02 wt % of Mo, and the balance of Fe and inevitable impurities, 
 wherein the components P, Mn, Ti, Nb and B satisfy the relationship represented by the following Formulae 1-1 and 1-2, depending on a desired tensile strength: at tensile strength of 35 kg and 40 kg grades  
 29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) or Mo(%)]=35˜44.9   Formula 1-1,  
 and at tensile strength of 45 kg grade  
 29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50   Formula 1-2,  
 and  
 wherein the components Ti, N, C and Nb satisfy the relationship presented by the following Formulae 2 and 3:  
 0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   Formula 2, and 0.4≦(1/0.35)(Nb/7.75C)≦2.2   Formula 3,  
 and  
 wherein Ti-based and Nb-based precipitates are distributed in an average size ranging from 30˜60 nm, the steel sheet has a galvanized layer formed on its surface, and the content of Al in the steel sheet is not less than that calculated from the following formula: weight loss in the plated layer=−0.0642Ln (content of Sol. Al (%) in the steel)−0.0534.  
 
     
     
         3 . The super formable high strength thin steel sheet according to  claim 1 , wherein the Al content is 0.151˜0.4%.  
     
     
         4 . The super formable high strength thin steel sheet according to  claim 1 , wherein the Al content is 0.21˜0.4%.  
     
     
         5 . A method for manufacturing a super formable high strength thin steel sheet, comprising the steps of: 
 finish hot rolling a steel slab having a composition of 0.010 wt % or less of C, 0.02 wt % or less of Si, 1.5 wt % or less of Mn, 0.03˜0.15 wt % or less of P, 0.02 wt % or less of S, 0.03˜0.40 wt % of Sol. Al, 0.004 wt % or less of N, 0.005˜0.040 wt % of Ti, 0.002˜0.020 wt % of Nb, one or both of 0.0001˜0.02 wt % of B and 0.005˜0.02 wt % of Mo, and the balance Fe and inevitable impurities at the austenite monophase zone,    wherein the components P, Mn, Ti, Nb and B satisfy the relationship represented by the following Formulae 1-1 and 1-2, depending on a desired tensile strength: at tensile strength of 35 kg and 40 kg grades    29.1+89.4P(%)+3.9Mn(%)−133.8Ti(%)+157.5Nb(%)+0.18[B(ppm) or Mo(%)]=35˜44.9   Formula 1-1,    and at tensile strength of 45 kg grade    29.1+98.3P(%)+4.6Mn(%)−86.5Ti(%)+62.5Nb(%)+0.21[B(ppm) or Mo(%)]=45˜50   Formula 1-2,    and    wherein the components Ti, N, C and Nb satisfy the relationship represented by the following Formulae 2 and 3:    0.6≦(1/0.65)(Ti−3.43N)/4C≦3.5   Formula 2, and 0.4≦(1/0.35)(Nb/7.75C)≦2.2   Formula 3, and    coiling the resulting steel slab at a temperature meeting the following condition:    730 {square root}(1−(Ti*0.027) 2 )±15° C. [in which Ti*=Ti(%)−3.43N(%)];    cold rolling the coil; and    continuously annealing the cold rolled coil at 780˜860° C.    
     
     
         6 . The method for manufacturing a super formable high strength thin steel sheet according to  claim 5 , wherein the Al content is 0.151˜0.4%.  
     
     
         7 . The method for manufacturing a super formable high strength thin steel sheet according to  claim 5 , wherein the Al content is 0.21˜0.4%.  
     
     
         8 . The method for manufacturing a super formable high strength thin steel sheet according to  claim 1 , wherein the continuous annealing is carried out at 780˜830° C.  
     
     
         9 . The method for manufacturing a super formable high strength thin steel sheet according to  claim 2 , wherein the continuous annealing is carried out at 780˜830° C.  
     
     
         10 . The super formable high strength thin steel sheet according to  claim 2 , wherein the Al content is 0.151˜0.4%.  
     
     
         11 . The super formable high strength thin steel sheet according to  claim 2 , wherein the Al content is 0.21˜0.4%.