Steel sheet having a two-layer crystallization structure and process for producing such a steel sheet

Abstract

A method for producing a steel sheet having a two-layer recrystallization structure includes: providing a cold-rolled steel sheet made of a steel with a carbon content of 10 to 1000 ppm, based on the weight, and a specified recrystallization temperature; applying a barrier layer which is at least partly impermeable to nitrogen onto a first face; and heating the steel sheet to a heating temperature. The heating is carried out at least temporarily in a nitrogenizing gas atmosphere at least until the recrystallization temperature is reached, whereby nitrogen from the gas atmosphere is diffused at least in a region near the surface on a second face of the steel sheet upon heating and is stored in the region, whereby the recrystallization temperature of the steel is raised in the region. The heating temperature is higher than or equal to the specified recrystallization temperature and lower than the raised recrystallization temperature.

1 - 24 . (canceled) 
     
     
         25 . A method of manufacturing a steel sheet for packaging, the method comprising:
 providing a cold rolled steel sheet of a steel having a carbon content by weight of 10 to 1000 ppm and a predetermined recrystallization temperature, the steel sheet having a first side and a second side;   applying a barrier layer, which is substantially impermeable to nitrogen, to the first side of the steel sheet; and   heating the steel sheet to a heating temperature,   wherein the heating takes place in a nitriding gas atmosphere until the recrystallization temperature is at least temporarily reached, whereby during heating of the steel sheet, nitrogen diffuses from the gas atmosphere at least into a near surface region on the second side of the steel sheet and is incorporated in the near surface region, as a result of which the recrystallization temperature of the steel in the near surface region is raised by a value of ΔT;   wherein the heating temperature is greater than or equal to the recrystallization temperature and less than the recrystallization temperature increased by the value of ΔT in the near surface region.   
     
     
         26 . The method of  claim 25 , wherein the steel of the cold-rolled steel sheet has the following composition by weight:
 C: more than 0.001% and less than 0.1%;   Mn: more than 0.01% and less than 0.6%;   P: less than 0.04%;   S: less than 0.04%;   Al: more than 50 ppm and less than 0.08%;   Si: less than 0.1%;   an average nitrogen content by weight after heating of the cold-rolled steel sheet in the nitriding gas atmosphere of at least 0.005%;   residual iron and unavoidable impurities.   
     
     
         27 . The method of  claim 25 , wherein the steel of the cold-rolled steel sheet has the following composition by weight:
 C: more than 0.001% and less than 0.1%;   Mn: more than 0.01% and less than 0.6%;   P: less than 0.04%;   S: less than 0.04%;   Al: more than 50 ppm and less than 0.08%;   Si: less than 0.1%;   Cu: less than 0.1%;   Cr: less than 0.1%;   Ni: less than 0.1%;   Ti: less than 0.1% and more than 0.02%;   Nb: less than 0.08% and more than 0.01%;   Mo: less than 0.08%;   Sn: less than 0.05%;   B: less than 0.01% and more than 0.0005%;   N: more than 0.001% and less than 0.016% before the heating and at least 0.005% after the heating of the cold-rolled steel sheet in the nitriding gas atmosphere;   residual iron and unavoidable impurities   
     
     
         28 . The method of  claim 25 , wherein the cold-rolled steel sheet is heated from room temperature to the heating temperature within a predetermined heating time and, after reaching the heating temperature, the cold-rolled steel sheet is kept at the heating temperature for a predetermined annealing time, wherein the heating time is in the range from 1.0 to 300 seconds and the annealing time is in the range from 1.0 seconds to 80 seconds. 
     
     
         29 . The method of  claim 25 , wherein the steel of the cold-rolled steel sheet has an initial nitrogen content and, during heating of the cold-rolled steel sheet, the average nitrogen content in the near surface region increases to a value averaged over the near surface region being between 50 and 1000 ppm above the initial nitrogen content of the steel, wherein a gradient of the nitrogen content is established with a decreasing nitrogen content from the near surface region on the second side of the steel sheet to the first side of the steel sheet. 
     
     
         30 . The method of  claim 29 , wherein during the heating at least temporarily a nitriding of the near surface region on the second side of the cold-rolled steel sheet takes place and during the annealing time at least partially a recrystallization annealing of the cold-rolled steel sheet takes place in a first region on the first side of the steel sheet outside the near surface region, whereas the near surface region on the second side of the steel sheet is not recrystallized. 
     
     
         31 . The method of  claim 25 , wherein the value by which the recrystallization temperature in the near surface region is increased by incorporation of nitrogen during heating of the steel sheet, is greater than 30° C. 
     
     
         32 . The method of  claim 25 , wherein the barrier layer is formed by applying a sol-gel layer on the surface of the first side of the steel sheet. 
     
     
         33 . A steel sheet for packaging having a first side, a second side and a thickness of less than 0.5 mm, and a carbon content by weight of 10 to 1000 ppm, wherein a barrier layer which is substantially impermeable to nitrogen is present on a surface of the first side, wherein the steel sheet has a two-layer crystallization structure with a first region on the first side and a second region on the second side, wherein the first region is at least substantially recrystallized and the second region is not or at least not completely recrystallized. 
     
     
         34 . The steel sheet of  claim 33 , wherein the steel sheet has the following composition by weight:
 C: more than 0.001% and less than 0.1%, preferably less than 0.06%;   Mn: more than 0.01% and less than 0.6%;   P: less than 0.04%;   S: less than 0.04% and preferably more than 0.001%;   Al: less than 0.08% and preferably more than 0.005%;   Si: less than 0.1%;   a nitrogen content averaged over the thickness of the steel sheet of at least 0.005%;   rest iron and unavoidable impurities.   
     
     
         35 . The steel sheet of  claim 33 , wherein the first region has a thickness in the range from 50 μm to 450 μm and the second region has a thickness in the range from 1 μm to 50 μm. 
     
     
         36 . The steel sheet of  claim 33 , wherein the steel sheet has a tensile strength of more than 500 MPa and an elongation at break of more than 5%. 
     
     
         37 . The steel sheet of  claim 33 , wherein a gradient of the nitrogen content is present at least in the second region with the nitrogen content decreasing from the second side to the first side of the steel sheet. 
     
     
         38 . The steel sheet of  claim 33 , wherein the second region has a higher hardness and a higher tensile strength than the first region. 
     
     
         39 . The steel sheet of  claim 33 , wherein the second region has a degree of recrystallization of less than 30% and the first region has a degree of recrystallization of more than 70%. 
     
     
         40 . The steel sheet of  claim 33 , wherein the barrier layer comprises a sol-gel layer containing at least one of SiO 2 , TiO 2  and ZrO 2 . 
     
     
         41 . The steel sheet of  claim 33 , wherein the barrier layer has a thickness of less than 1 μm or a coating weight of less than 10 mg/m 2 . 
     
     
         42 . The steel sheet of  claim 33 , wherein, after a forming of the steel sheet, the surface of the second region has a bending radius in the range of 8 mm to 14 mm, wherein the second region is lying on the outside of the bending radius and has a roughness of less than 1.0 μm. 
     
     
         43 . A container made of a steel sheet according to  claim 34 , wherein the container has at least one convexly deformed portion and the second region of the steel sheet is located on a convex outer side of the deformed portion.