Steel substrate for painted parts

Abstract

The invention relates to a steel strip, sheet or blank used for painted parts, wherein 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 method for producing a bake hardenable steel strip, sheet or blank used for painted parts, comprising
 hot rolling and cold rolling the steel strip, wherein the steel strip is cold rolled in a cold rolling mill and 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,   
       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 
 
 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 
 
       wherein C, Mn and Si are present, the remainder being iron and unavoidable impurities, having essentially equi-axed grains with a median grain size smaller than 11.0 micrometer,
 resulting in the optionally metallic coated steel strip, sheet or blank as an optionally metallic coated substrate such that a surface of a 145 mm×145 mm sample of the optionally metallic coated substrate, after forming of the sample into a cup, has a delta Waviness ΔWsa≤0.12 μm, according to a forming test wherein the cup is produced by pressing the 145 mm×145 mm sample of the optionally metallic coated steel strip, sheet or blank in a press with a hollow punch with diameter 75 mm and a blankholder force such that any material movement of the (coated) substrate between the blankholder and then die is completely suppressed, wherein deformation of the cup is such that thickness strain in a bottom of the cup is 9%+0.3%, 
 wherein ΔWsa is 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, 
 wherein Wsa is measured according to standard SEP 1941, 
 wherein Wsa (Flat) is lower than 0.6 μm in rolling direction of the strip. 
 
     
     
         2 . The method 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,   wherein Wsa (Flat) is ≤0.35 μm in rolling direction.   
     
     
         3 . The method according to  claim 1 , 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≤(Ti+Nb)+2Mo≤0.60 wt % 
 otherwise: 0.06≤Ti+2Nb+2Mo≤0.60 wt. 
 
     
     
         4 . The method according to  claim 3 , wherein the optionally metallic coated substrate surface of the strip before the forming has a roughness Ra between 0.7 and 1.6 μm and wherein Wsa (Flat) is between 0.15 and 0.35 μm, wherein Wsa (Formed) is ≤0.47 μm. 
     
     
         5 . The method according to  claim 1 , wherein the essentially equi-axed grains have a median size smaller than 10.0 micrometer. 
     
     
         6 . The method 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 method 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 according to  claim 1 , 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. 
     
     
         9 . The method according to  claim 1 , 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. 
     
     
         10 . The method according to  claim 1 , 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 1 , 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 1 , 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 1 , 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 method according to  claim 1 , 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, wherein the strip is coated with an aluminium based coating having a coating thickness between 4 and 12 μm. 
     
     
         15 . The method according to  claim 1 , wherein the optionally metallic coated strip, sheet or blank before the forming has a waviness Wsa≤0.32. μm where Wsa is measured in the rolling direction. 
     
     
         16 . The method according to  claim 1 , wherein the optionally metallic coated strip, sheet or blank before the forming has a waviness Wsa≤0.29 μm where Wsa is measured in the rolling direction. 
     
     
         17 . The method 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 1 , 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. 
     
     
         19 . The method according to  claim 1 , wherein the cold rolling mill contains one stand, with work rolls having a roughness Ra between 0.55 μm and 5.0 μm. 
     
     
         20 . The method according to  claim 1 , wherein the cold rolling mill contains one stand, with work rolls having a roughness Ra between 0.6 μm and 4.0 μm. 
     
     
         21 . The method according to  claim 1 , wherein the cold rolling mill contains one stand, with work rolls having a roughness Ra between 0.6 μm and 2.0 μm. 
     
     
         22 . The method according to  claim 1 , wherein the work rolls of the last stand having a roughness Ra between 0.55 μm and 5.0 μm. 
     
     
         23 . The method according to  claim 1 , wherein the work rolls of the last stand having a roughness Ra between 0.6 μm and 4.0 μm. 
     
     
         24 . The method according to  claim 1 , wherein the last stand having a roughness Ra between 0.6 μm and 2.0 μm. 
     
     
         25 . The method according to  claim 1 , wherein the work rolls of the last stand having a roughness Ra between 0.55 μm and 5.0 μm. 
     
     
         26 . The method according to  claim 1 , wherein the work rolls of the last stand having a roughness Ra between 0.6 μm and 4.0 μm. 
     
     
         27  . The method according to  claim 1 , wherein the work rolls of the last stand having a roughness Ra between 0.6 μm and 2.0 μm.