Steel sheet suitable for enamelling and method for producing such a sheet

Abstract

A rolled steel sheet suitable for enamelling, 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 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 % and O lower than 0.01 wt % and wherein the depth where the C-level reaches (C bulk +C surface )/2, is higher than 75 μm.

What is claimed is: 
     
         1 . A method for producing a rolled steel sheet for enameling to have 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 comprising:
 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 method 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. 
     
     
         2 . The method according to  claim 1 , 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. 
     
     
         3 . The method according to  claim 2 , wherein the anneal temperature is between 800° C. and 850° C. 
     
     
         4 . The method according to  claim 1 , wherein the initial C-level is between 0.025 wt % and 0.08 wt %. 
     
     
         5 . The method according to  claim 4 , wherein the initial C-level is between 0.025 wt % and 0.06 wt %. 
     
     
         6 . The method according to  claim 1 , wherein the initial Al-level is between 0.02 wt % and 0.06 wt %. 
     
     
         7 . The method according to  claim 1 , wherein the ratio pH 2 O/pH 2  is between 0.04 and 0.25. 
     
     
         8 . The method according to  claim 1 , further comprising an over-ageing step at a temperature between 350° C. and 450° C. during a timespan between 100 s and 500 s. 
     
     
         9 . The method according to  claim 8 , further comprising a skinpass step with a reduction of between 0.3% and 1.5%. 
     
     
         10 . The method of  claim 1 , further comprising producing a particular depth where the C-level reaches a midpoint C-level of (C bulk +C surface )/2 that is higher than 75 micron, the midpoint C-level being a fixed value greater than 0 that is a single point on the gradient.