Steel substrate for painted parts

Abstract

A steel strip, sheet or blank used for painted parts, the steel strip, sheet or blank is optionally metallic coated. The steel is an Ultra Low Carbon (ULC) steel type having a composition of (in weight %): C: max 0.007 Mn: max 1.2 Si: max 0.5 Al: max 0.1 P: max 0.15 S: 0.003-0.045 N: max 0.01 Ti, Nb, Mo: if Ti≥0.005 and Nb≥0.005: 0.06≤4Ti+4Nb+2Mo≤0.60 otherwise 0.06≤Ti+2Nb+2Mo≤0.60 and one or more of the optional elements: Cu: max 0.10 Cr: max 0.06 Ni: max 0.08 B: max 0.0015 V: max 0.01 Ca: max 0.01 Co: max 0.01 Sn: max 0.01 resulting in a delta Waviness ΔWsa≤0.12 μm of the surface after the forming of the strip, sheet or blank.

1 . A steel strip, sheet or blank used for painted parts, wherein the steel strip, sheet or blank is optionally metallic coated, wherein the steel is an Ultra Low Carbon (ULC) steel type having a composition of (in weight %):
 C: max 0.007   Mn: max 1.2   Si: max 0.5   Al: max 0.1   P: max 0.15   S: 0.003-0.045   N: max 0.01   Ti, Nb, Mo:
   if Ti≥0.005 and Nb≥0.005:
 
   0.06≤4Ti+4Nb+2Mo≤0.60
 
   otherwise 
   0.06≤Ti+2Nb+2Mo≤0.60
 
   and one or more of the optional elements:   Cu: max 0.10   Cr: max 0.06   Ni: max 0.08   B: max 0.0015   V: max 0.01   Ca: max 0.01   Co: max 0.01   Sn: max 0.01   the remainder being iron and unavoidable impurities, having essentially equi-axed grains with a median grain size smaller than 11.0 micrometer   
       resulting in a delta Waviness ΔWsa≤0.12 μm of the surface due to the forming of the strip, sheet or blank, ΔWsa defined as Wsa(Formed) minus Wsa(Flat), in which Wsa(Formed) is the Wsa value of the optionally metallic coated substrate surface after the forming and Wsa(Flat) is the Wsa value of the optionally metallic coated substrate surface before the forming. 
     
     
         2 . The steel strip, sheet or blank according to  claim 1 , wherein the amounts of Ti, Nb and Mo are as follows (in weight %):
   if Ti≥0.005 and Nb≥0.005:
     0.06≤4Ti+4Nb+2Mo≤0.30
     otherwise     0.06≤Ti+2Nb+2 Mo≤0.10.
   
     
     
         3 . The ultra low carbon steel strip, sheet or blank according to  claim 1 , being bake hardenable, wherein the amount of Ti, Nb and Mo are tuned with respect to the C, N and S levels as follows (all in wt %):
   Ti(free)=Ti−3.43N−1.5S
     if Ti(free)≤0 than Ti( c )=0, else Ti( c )=Ti(used)
     and  Csol =C−0.125Mo−0.129Nb−0.25Ti( c )
     such that 0.0008≤ Csol≤ 0.0033
   and furthermore if Ti and Nb are both >0.005 wt %
   0.06≤4(Ti+Nb)+2Mo≤0.60 wt %
 
   otherwise: 0.06≤Ti+2Nb+2Mo≤0.60 wt %.
 
   
     
     
         4 . (canceled) 
     
     
         5 . The steel strip, sheet or blank according to  claim 1 , wherein the essentially equi-axed grains have a median size smaller than 10.0 micrometer. 
     
     
         6 . The steel strip, sheet or blank according to  claim 1 , wherein the undeformed steel surface of the strip, sheet or blank has a waviness Wsa≤0.35 μm where Wsa is measured in the rolling direction. 
     
     
         7 . The steel strip, sheet or blank according to  claim 1 , wherein the strip, sheet or blank is coated with a zinc based coating, a Zn—Al—Mg based coating, or an aluminium based coating, wherein the
 the zinc based coating consists of 0.1-1.2 wt % aluminium and up to 0.3 wt % of other elements, the remainder being unavoidable impurities and zinc, or 
 the Zn—Al—Mg based coating consists of 0.2-3.0 wt % aluminium and 0.2-3.0 wt % magnesium, up to 0.3 wt % of other elements, the remainder being unavoidable impurities and zinc, or 
 the aluminium based coating consists of 0.2-13 wt % silicon, up to 0.3 wt % of other elements, the remainder being unavoidable impurities and aluminium. 
 
     
     
         8 . The method for producing a steel strip according to  claim 1 , wherein the steel strip is hot rolled and cold rolled, and in that the last stand or the only stand of the cold rolling mill contains work rolls having a roughness Ra between 0.5 μm and 7.0 μm. 
     
     
         9 . The method according to  claim 8 , wherein the roughness Ra of the work rolls in the last stand or the only stand is between 0.55 μm and 5.0 μm. 
     
     
         10 . The method according to  claim 8 , wherein the cold rolling mill contains one stand, with work rolls having a roughness Ra between 0.5 μm and 7.0 μm. 
     
     
         11 . The method according to  claim 8 , wherein the cold rolling mill contains two stands, the work rolls of the first stand having a roughness Ra between 0.6 μm and 3.0 μm, and the work rolls of the last stand having a roughness Ra between 0.5 μm and 7.0 μm. 
     
     
         12 . The method according to  claim 8 , wherein the cold rolling mill contains three or more stands, the work rolls of the first stand having a roughness Ra between 0.6 μm and 3.0 μm, the work rolls of the intermediate stands having a roughness Ra between 0.3 μm and 0.8 μm and the work rolls of the last stand having a roughness Ra between 0.5 μm and 7.0 μm. 
     
     
         13 . The method according to  claim 8 , wherein the cold rolled strip is skin passed, optionally after applying a metallic coating, using temper rolls having a roughness between 0.5 and 4.0 μm. 
     
     
         14 . The strip produced with the method according to  claim 8 , wherein the surface of the strip has a roughness Ra lower than 2.0 μm and a waviness Wsa lower than 0.6 μm in rolling direction of the strip for a strip coated with an aluminium based coating having a coating thickness between 4 and 12 μm. 
     
     
         15 . The steel strip, sheet or blank according to  claim 1 , wherein the undeformed steel surface of the strip, sheet or blank has a waviness Wsa≤0.32 μm where Wsa is measured in the rolling direction. 
     
     
         16 . The steel strip, sheet or blank according to  claim 1 , wherein the undeformed steel surface of the strip, sheet or blank has a waviness Wsa≤0.29 μm where Wsa is measured in the rolling direction. 
     
     
         17 . The steel strip, sheet or blank according to  claim 1 , wherein the undeformed steel surface of the strip, sheet or blank has a waviness Wsa≤0.26 μm where Wsa is measured in the rolling direction. 
     
     
         18 . The method according to  claim 8 , wherein the roughness Ra of the work rolls in the last stand or the only stand is between 0.6 μm and 4.0 μm. 
     
     
         19 . The method according to  claim 8 , wherein the roughness Ra of the work rolls in the last stand or the only stand is between 0.6 μm and 2.0 μm. 
     
     
         20 . The method according to  claim 8 , wherein the cold rolling mill contains one stand, with work rolls having a roughness Ra between 0.55 μm and 5.0 μm. 
     
     
         21 . The method according to  claim 8 , wherein the cold rolling mill contains one stand, with work rolls having a roughness Ra between 0.6 μm and 4.0 μm. 
     
     
         22 . The method according to  claim 8 , wherein the cold rolling mill contains one stand, with work rolls having a roughness Ra between 0.6 μm and 2.0 μm. 
     
     
         23 . The method according to  claim 11 , wherein the work rolls of the last stand having a roughness Ra between 0.55 μm and 5.0 μm. 
     
     
         24 . The method according to  claim 11 , wherein the work rolls of the last stand having a roughness Ra between 0.6 μm and 4.0 μm. 
     
     
         25 . The method according to  claim 11 , wherein the last stand having a roughness Ra between 0.6 μm and 2.0 μm. 
     
     
         26 . The method according to  claim 12 , wherein the work rolls of the last stand having a roughness Ra between 0.55 μm and 5.0 μm. 
     
     
         27 . The method according to  claim 12 , wherein the work rolls of the last stand having a roughness Ra between 0.6 μm and 4.0 μm. 
     
     
         28 . The method according to  claim 12 , wherein the work rolls of the last stand having a roughness Ra between 0.6 μm and 2.0 μm. 
     
     
         29 . The method according to  claim 8 , wherein the cold rolled strip is skin passed, optionally after applying a metallic coating, using temper rolls having a roughness greater than 0.5 and ≤2.8 μm.