Steel Sheet Suitable for Enamelling and Method for Producing Such a Sheet

Abstract

The present invention is related to a rolled steel sheet suitable for enamelling, said sheet having a carbon profile, defined by a gradient in the C-level from a level C surface at at least one surface of the sheet, to a level C bulk in the bulk of the sheet, C bulk being higher than C surface , and with: C bulk higher than 0 and lower than or equal to 0.08 wt %, C surface between 0 and 0.015 wt %, Al between 0.012 wt % and 0.07 wt %, Mn between 0.12 wt % and 0.45 wt %, O lower than 0.01 wt % and optionally: Cu between 0.025 wt % and 0.1 wt %, S between 0.008 wt % and 0.04 wt %, Ca between 0.0005 wt % and 0.005 wt %, the balance being Fe and incidental impurities, and wherein the depth where the C-level reaches C bulk +C surface )/2, is higher than 75 μm. The invention is equally related to a method for producing said steel sheet.

1 . A rolled steel sheet suitable for enamelling, said sheet having a carbon profile, defined by a gradient in the C-level from a level C surface  at at least one surface of the sheet, to a level C bulk  in the bulk of the sheet, C bulk  being higher than C surface , and with:
 C bulk  higher than 0 and lower than or equal to 0.08 wt %,   C surface  between 0 and 0.015 wt %,   Al between 0.012 wt % and 0.07 wt %,   Mn between 0.12 wt % and 0.45 wt %,   O lower than 0.01 wt %   and optionally:
 Cu between 0.025 wt % and 0.1 wt %, 
 S between 0.008 wt % and 0.04 wt %, 
 Ca between 0.0005 wt % and 0.005 wt %, 
   the balance being Fe and incidental impurities, and wherein the depth where the C-level reaches (C bulk +C surface )/2, is higher than 75 μm.   
     
     
         2 . Steel sheet according to  claim 1 , having an r m  value between 1.8 and 2.1. 
     
     
         3 . Steel sheet according to  claim 1 , wherein C surface  is between 0.005 wt % and 0.015 wt %. 
     
     
         4 . Steel sheet according to  claim 1 , wherein C surface  is between 0 and 0.005 wt %. 
     
     
         5 . Steel sheet according to  claim 1 , wherein C bulk  is between 0.02 wt % and 0.08 wt %. 
     
     
         6 . Steel sheet according to  claim 5 , wherein C bulk  is between 0.025 wt % and 0.08 wt %. 
     
     
         7 . Steel sheet according to  claim 5 , wherein C bulk  is between 0.025 wt % and 0.06 wt %. 
     
     
         8 . Steel sheet according to  claim 1 , wherein the Al-level is between 0.02 wt % and 0.06 wt %. 
     
     
         9 . Steel sheet according to  claim 1 , wherein said depth is between 130 μm and 200 μm. 
     
     
         10 . Enamelled steel sheet consisting of a steel sheet according to  claim 1 , provided with an enamel layer. 
     
     
         11 . Steel product produced from a sheet according to  claim 1 . 
     
     
         12 . Enamelled steel product consisting of a product according to  claim 11 , provided with an enamel layer. 
     
     
         13 . A method for producing a rolled steel sheet for enamelling, comprising the steps of:
 subjecting a steel slab to hot rolling followed by coiling, and cold rolling, so as to obtain a cold-rolled steel sheet, said slab comprising the following initial composition:
 C between 0.02 wt % and 0.08 wt %, 
 Al between 0.012 wt % and 0.07 wt %, 
 Mn between 0.12 wt % and 0.45 wt %, 
 O lower than 0.01 wt % 
 and optionally:
 Cu between 0.025 wt % and 0.1 wt %, 
 S between 0.008 wt % and 0.04 wt %, 
 Ca between 0.0005 wt % and 0.005 wt %, 
 
 the balance being Fe and incidental impurities, 
 subjecting said cold-rolled sheet to continuous annealing step, wherein said sheet is exposed during a decarburizing time to a decarburizing atmosphere comprising water vapour and hydrogen gas, wherein the H 2  content is between 1 vol % and 95 vol %, the H 2 O content between 0.04 vol % and 33 vol %, the remainder being mainly nitrogen gas, the ratio pH 2 O/pH 2  being between 0.04 and 0.5. 
   
     
     
         14 . Method according to  claim 13 , wherein said continuous annealing takes place at an anneal temperature between 760° C. and 850° C., and during a decarburizing time between 45 s and 300 s. 
     
     
         15 . Method according to  claim 14 , wherein the anneal temperature is between 800° C. and 850° C. 
     
     
         16 . Method according to  claim 13 , wherein the initial C-level is between 0.025 wt % and 0.08 wt %. 
     
     
         17 . Method according to  claim 16 , wherein the initial C-level is between 0.025 wt % and 0.06 wt %. 
     
     
         18 . Method according to  claim 13 , wherein the initial Al-level is between 0.02 wt % and 0.06 wt %. 
     
     
         19 . Method according to  claim 13 , wherein the ratio pH 2 O/pH 2  is between 0.04 and 0.25. 
     
     
         20 . Method according to  claim 13 , further comprising an over-ageing step at a temperature between 350° C. and 450° C. during a timespan between 100 s and 500 s. 
     
     
         21 . Method according to  claim 20 , further comprising a skinpass step with a reduction of between 0.3% and 1.5%.