Patent Publication Number: US-2004050464-A1

Title: Method for producing a cold rolled strip that is cold formed with low degrees of deformation

Description:
[0001] The invention pertains to a method for manufacturing a cold strip that is cold-worked with low degrees of deformation and produced from a steel of conventional composition.  
       [0002] In known methods of this type, a hot strip is initially hot-rolled from an input stock, e.g., slabs, thin slabs or cast strips. A cold strip is then cold-rolled from the hot strip. After the cold-rolling, this cold strip is subjected to additional cold-working steps, for example, planishing, temper rolling, rerolling and forming steps. This cold-working process also includes the production steps that are combined under the term “flexible rolling” among experts. In such a “flexible rolling” process, the roll gap is variably adjusted during the rolling process such that the manufacture of sheet metals with a defined thickness profile in the direction of rolling that can be individually adapted to the respective loads is shortened.  
       [0003] Cold strips are usually subjected to a recrystallization annealing process before they are cold-worked in order to achieve the ductility required for the cold-working process.  
       [0004] Another recrystallization annealing process would be advantageous for also ensuring favorable conditions in subsequent processing steps after the cold-working process. However, such an additional annealing process is usually not carried out if only low degrees of deformation were attained during the course of the cold-working process, namely because one would otherwise have to expect the formation of coarse grain in the microstructure of the cold-worked strips. The formation of coarse grain leads to a reduced ductility that, in particular, complicates deep-drawing processes and results in a scarred surface structure of the deep-drawn parts (so-called “orange peel effect”).  
       [0005] Although the formation of coarse grain can be prevented in practical applications by avoiding a recrystallization annealing-process, one must accept a certain limitation in the ductility and consequently in the processing ability, in particular, the deformability during the deep-drawing of the cold strip or during comparable working steps.  
       [0006] Based on the previously discussed state of the art, the invention aims to disclose a method for manufacturing recrystallization-annealed, cold-worked cold strips consisting of mild steels of conventional composition without risking the formation of coarse grain.  
       [0007] This objective is attained with a method, in which an input stock, e.g., a slab, a thin slab or a cast strip, that is produced from a steel containing no more than 0.2 wt. % C, no more than 1.0 wt. % Si, no more than 1.0 wt. % Mn, no more than 0.2 wt. % P, no more than 0.2 wt. % S, no more than 0.2 wt. % Al, no more than 0.02 wt. % N, no more than 0.2 wt. % Ti, no more than 0.2 wt. % V, no more than 0.2 wt. % Nb and no more than 0.01 wt. % B, the remainder being iron and unavoidable impurities, is hot-rolled into a hot strip, wherein the hot strip is cold-rolled into a cold strip, wherein the cold strip is subjected to a crystal regeneration annealing process at an annealing temperature that is lower than the recrystallization temperature, wherein the cold strip that was subjected to the crystal regeneration annealing process is cold-worked with low degrees of deformation, and wherein the cold-worked cold strip is then subjected to a second recrystallization annealing process.  
       [0008] Since the cold strip is, after the cold-rolling process, annealed within a temperature range that only suffices for a crystal regeneration, a complete recrystallization of its microstructure is prevented in this processing step. It was surprisingly determined that a cold strip annealed at temperatures that lie below the recrystallization annealing temperature not only has a sufficient ductility for ensuing processing steps, but can also be subjected to a recrystallization annealing process, in which no formation of coarse grain occurs, after the cold-working is completed. Due to the defined limitation of the temperature during the annealing process carried out before the cold-working, the invention makes it possible to produce a recrystallization-annealed cold strip without the formation of a microstructure that is unfavourable for the usage properties of the cold strip.  
       [0009] The temperature during the-annealing process carried out before the cold-working is preferably adjusted in such a way that it lies in the upper range of the crystal regeneration. This makes it possible to achieve an optimized ductility without risking the formation of coarse grain during the final recrystallization annealing process. If the steel being processed consists of a mild steel, the annealing temperature during the crystal regeneration annealing process should correspondingly be no lower than 450° C. and no higher than 550° C. However, if the steel being processed consists of an IF-steel, the annealing temperature during the crystal regeneration annealing process should be no lower than 550° C. and no higher than 650° C. Regardless of the type of steel being processed, the annealing temperature should always be chosen such that the cold strip is recrystallized by no more than 90% after the crystal regeneration annealing process.  
       [0010] The annealing temperature during the recrystallization annealing process carried out after the cold-working is chosen such that a complete recrystallization is ensured. According to previous experience, the required annealing temperatures for the types of steel in question lie at no less than 650° C. and no more than 850° C.  
       [0011] The degrees of deformation achieved during the course of the cold-working usually amount to no more than 40%.  
       [0012] The method according to the invention is particularly advantageous if the cold-working is carried out in the form of a flexible rolling process. Sheet metals for load-oriented components can be produced with this cold-working process.  
       [0013] The method according to the invention makes it possible to manufacture cold-worked cold strips, the microstructure of which has a grain size of no more than 6 ASTM. 
     
    
    
     [0014] Embodiments of the invention are described in greater detail below.  
     [0015] The alloying contents of a conventional mild steel A and an IF-Steel B are listed in the following table.  
                                                               Steel                                       Type   C   Si   Mn   P   S   Al   N   Ti                  A   0.022   0.01   0.16   0.008   0.006   0.033   0.0031   —       B   0.002   0.01   0.08   0.007   0.003   0.021   0.0022   0.059                  
 
     [0016] The steel types A, B were cast into slabs that were then hot-rolled into hot strips.  
     [0017] Subsequently, the hot strips produced from steel type A were cold-rolled into cold strips A1, A2, and the hot strips produced from steel type B were cold-rolled into cold-strips B1, B2.  
     [0018] The first cold strips A1 and B1 were then respectively subjected to a crystal regeneration annealing process. During the crystal regeneration annealing process of the cold strip A1, the annealing temperature was adjusted to 500 ° C., whereas the annealing temperature for the cold strip B1 was adjusted to 600° C. The degree of recrystallization achieved during this annealing process amounted to less than 90%.  
     [0019] For comparison purposes, the second cold strips A2 and B2 were subjected to a complete recrystallization annealing process. The annealing temperature was respectively adjusted to 720° C.  
     [0020] After the respective crystal regeneration and recrystallization annealing processes, the cold strips A1, A2 and B1, B2 were cold-worked at different degrees of deformation between 0% and 50%. This cold-working comprised a “flexible rolling” process, in which differentiated degrees of reduction between approximately 5% and 50% were adjusted.  
     [0021] The cold-worked cold strips A1, A1 and B1, B2 were ultimately subjected to a recrystallization annealing process at an annealing temperature of 720° C.  
     [0022] In Diagram 1, the grain size Gs resulting after the final recrystallization annealing process for the cold-worked cold strips A1, A2 produced from steel type A are plotted in ASTM as a function of the degree of deformation ε that is indicated in %. The individual measuring values are symbolized by circles. One can ascertain that the grain size of the cold strip A2 that was subjected to a complete recrystallization annealing process before the cold-working is significantly higher at all degrees of deformation than those of the cold strip A1 that, according to the invention, was merely subjected to a crystal-regeneration annealing process before the cold-working. The deviation is particularly significant at very low degrees of deformation on the order of less than 15%.  
     [0023] Diagram 2 shows an illustration of the grain size of the cold-strips B1, B2 produced from steel type B referred to, the degree of cold-rolling, wherein this illustration corresponds to that shown in Diagram 1. Also in this case, the grain size of the cold strip B2 increases significantly as the degree of deformation decreases. However, the grain size of the cold strip B1 manufactured in accordance with the invention lies at a uniform, significantly lower level.