Patent Publication Number: US-2004040633-A1

Title: Method for the production of hot strip or sheet from a micro-alloyed steel

Description:
[0001] The invention relates to a method for producing hot strip or hot plate from microalloyed steel.  
       [0002] From the Patent Abstract of Japan concerning JP 61-281814 A, a method for producing high-strength hot strip is known. The hot strip comprises a steel which in weight % comprises max. 0.3% C, max. 0.5% Si, 0.3-2.0% Mn, ≦0.05% Al, 0.01-0.02% Nb, 0.01-0.2% V as well as, if need be, 0.01-0.2% Mo and 0.01-0.2% Ti, with the remainder being iron and unavoidable impurities. A raw material made from this steel is finish rolled at a final rolling temperature of at least 750° C. to form hot strip which immediately thereafter is cooled to a coiling temperature of max. 650° C. Subsequently, the hot strip is subjected to final heat treatment.  
       [0003] Practical experiments have shown that the strength of hot strip of a thickness of 8 mm or more produced according to the known method does not meet the requirements demanded, for example, in the construction of structural members used in motor vehicle chassis.  
       [0004] It is the object of the invention, starting from the state of the art explained above, to state an economic method for the production of hot strip which is of high strength even at larger thicknesses.  
       [0005] According to the invention, this object is met by a method for producing hot strip or hot plate with a minimum yield point of 700 N/mm 2  at a thickness of 8 mm; in which a microalloyed steel which, apart from microalloying elements, comprises (in weight %) 0.05-0.12% C; 0.2-0.5% Si; 1.5-2.2% Mn; =0.025% P; =0.01% S; with the remainder being iron and unavoidable impurities, is cast to form a raw material such as slabs, thin slabs or blooms; in which the raw material is heated to a temperature of 1300-1350° C.; in which the heated raw material is rough rolled at a degree of deformation of 36% to 43%; in which the rough rolled raw material is thermomechanically hot rolled at a final rolling temperature which exceeds the Ac 3  temperature so as to form a hot strip; in which the hot strip is cooled at a cooling rate of at least 15° C./s to a coiling temperature of at least 590° C. and at most 630° C. at which temperature the cooled hot strip is finally coiled.  
       [0006] Surprisingly, it has been shown that by keeping the contents of the individual alloying elements to those prescribed by the invention, and by targeted matching of the final rolling temperature, of the cooling rate and of the coiling temperature, hot strip can be produced which is of extremely high strength even at a thickness exceeding 8 mm. Thus, in a hot strip which is 16 mm thick and which is produced with a composition according to the invention, the yield point is still at least 700 N/mm 2 . For a thickness of 13 mm it is possible to achieve a yield point of at least 760 N/mm 2 . There is no need for any supplementary heat treatment following cooling in the coil.  
       [0007] The high strength of steel produced according to the invention is achieved by combined application of precipitation strengthening, fine grain strengthening and solid-solution strengthening. In this process, the selected content of the alloying elements C and Mn result in the desired solid-solution strengthening. By heating the raw material to a temperature between 1300° C. and 1350° C. before rough rolling, precipitation of any alloying elements Ti, V and Nb, which may be present, are completely brought to solution. The degree of deformation set during subsequent rough deformation result in a fine, evenly distributed and recrystallised austenite grain.  
       [0008] Its outstanding strength values render hot strip which has been produced according to the invention particularly suitable for producing structural members of motor vehicles, for example highly loaded longitudinal members of trucks.  
       [0009] Preferably, the steel comprises at least one of the microalloying elements V, Mo, Ti, Nb, in the following percentages (in weight %):0.08-0.12% V; 0.1-0.2% Mo; 0.08-0.11% Ti; and 0.05-0.06% Nb. The Al content should be between 0.02 and 0.05 weight %. The microalloying element niobium impedes austenite grain growth. In addition, it impedes recrystallisation of the austenite grains during hot rolling.  
       [0010] Recrystallisation during hot rolling is further avoided by the fact that, according to the invention, hot-rolling takes place at temperatures which exceed the Ac 3  temperature. Conversion of austenite/ferrite takes place during strip cooling behind the last stand of the hot-rolling finishing train. In this way, a recrystallised, fine-grained microstructure with a low perlite content is obtained, with the size of the ferrite grains being between 11 and 14 ASTM.  
       [0011] A cooling speed of at least 15° C./s is selected in order to sufficiently quickly cool the hot strip from the final hot-rolling temperature, which is preferably at least 840° C., to the coiling temperature. During this cooling in the coil, which starts at the coiling temperature, the precipitation hardening maximum is achieved by the microalloying elements Ti and V as well as Nb. This effect can be achieved particularly safely if the coiling temperature ranges somewhere between 600° C. and 620° C. In the method according to the invention, hot strip with a yield point of at least 700 N/mm 2  even at a thickness of 16 mm can be produced particularly reliably if the steel contains (in weight %):0.06-0.08% C; 0.2-0.3% Si; 1.95-2.1% Mn; ≦0.02% P and ≦0.005% S. If Ti is present, its content should not exceed 0.1 weight %.  
       [0012] According to another variant of the invention, hot strip whose yield point at a thickness of 13 mm is at least 760 N/mm 2 , can be produced safely in that the steel comprises (in weight %):0.10-0.12% C; 0.4-0.5% Si; 1.95-2.1% Mn; =0.02% P; ≦0.005% S. The steel preferably comprises one of the microalloying elements V, Mo, Ti, Nb in the following percentages (in weight %): 0.08-0.10% V; 0.1-0.2% Mo; 0.09-0.11% Ti; and 0.05-0.06% Nb.  
       [0013] As a result of the production process or in order to achieve particular characteristics, the steel used according to the invention can optionally comprise one or several of the elements N, Cu, Ni, Sn, B or As. The total content of these elements should, however, not exceed 0.1 weight %.  
       [0014] If slabs are used as raw material, then the holding time during heating of the slabs should be at least 135 minutes so as to ensure safe heat soaking of the raw material.  
       [0015] Below, the invention is explained by means of exemplary embodiments: 
     
    
    
     EXAMPLE 1  
     [0016] A steel melt comprising (in weight %)  
                                                      C:   0.075%           Si:   0.254%           Mn:   2.011%           P:   0.015%           S:   0.003%           Al:   0.02%           N:   0.007%           Cu:   0.017%           Cr:   0.039%           Ni:   0.021%           Sn:   0.004%           V:   0.098%           Mo:   0.121%           Ti:   0.080%           Nb:   0.060%           B:   0.0003%           As:   0.002%                      
 
     [0017] with the remainder being iron and unavoidable impurities, was cast to form slabs. The slabs were then preheated for 135 minutes at a temperature of 1350° C. and subsequently rough rolled to a thickness of 55 mm, at a degree of deformation of between 36% and 43%. In a finishing hotroll line, the rough rolled slabs were thermomechanically finish rolled to a thickness of 16 mm. During this hot rolling, temperature management and degrees of deformation were matched so that the finish-hot-rolled hot strip A comprises a microstructure which is optimal for the intended use. The final rolling temperature was 840° C.  
     [0018] Immediately after the hot strip A left the finishing roll line it was water cooled to a coiling temperature of 610° C. and coiled. During water cooling, cooling rates of at least 15° C./s were achieved.  
     [0019] Specimens A1-A8, which were obtained from hot strip A produced in this way, were subjected to tensile tests, the results of which are shown in Table 1:  
                                       TABLE 1                                   R eh     R m         A 5     A 80             [N/mm 2 ]   [N/mm 2 ]   R eh /R m     [%]   [%]                                                                    A1   725   830   87.3   19.5   19.6           A2   701   816   85.9   21.5   21.6           A3   712   824   86.4   20.2   20.2           A4   753   842   89.5   21.5   20.8           A5   763   841   90.7   19.9   19.1           A6   772   846   91.2   21.0   20.3           A7   761   840   90.6   20.2   19.4           A8   760   858   88.5   19.0   18.4                      
 
     [0020] Furthermore, specimens A9-A12 of the hot strip were subjected to notched-bar impact bend tests, the results of which are shown in Table 2:  
                                   TABLE 2                                           Test temp.   Kv           Specimen   Direction   [K]   [J]                          A9   Longitudinal   233   86.6           A10   Transverse   233   49.5           A11   Longitudinal   233   86.0           A12   Transverse   233   58.6                      
 
     EXAMPLE 2  
     [0021] A steel melt comprising (in weight %)  
                                                      C:   0.115%           Si:   0.449%           Mn:   1.988%           P:   0.019%           S:   0.003%           Al:   0.03%           N:   0.007%           Cu:   0.017%           Cr:   0.037%           Ni:   0.02%           Sn:   0.003%           V:   0.092%           Mo:   0.108%           Ti:   0.101%           Nb:   0.055%           B:   0.0003%           As:   0.003%                      
 
     [0022] with the remainder being iron and unavoidable impurities, was cast to form slabs. As in the example of hot strip A, the slabs were preheated for 135 minutes at a temperature of 1350° C. and subsequently rough rolled to a thickness of 55 mm and then hot rolled in a finishing roll line at a final rolling temperature of 840° C. The thickness of the finish-rolled hot strip was 13 mm.  
     [0023] As is the case with hot strip A, hot strip B, too, was water cooled to a coiling temperature of 610° C. and coiled as soon as it left the finishing roll line. Again, cooling rates of 15° C./s were achieved during water cooling.  
     [0024] Specimens B1-B8, which were obtained from hot strip B produced in this way, were subjected to tensile tests, the results of which are shown in Table 3:  
                                       TABLE 3                                   R eh     R m         A 5     A 80             [N/mm 2 ]   [N/mm 2 ]   R eh /R m     [%]   [%]                                                                    B1   767   899   85.3   15.5   15.4           B2   771   893   86.3   18.1   18.1           B3   783   916   85.5   15.8   15.8           B4   796   875   91.0   18.5   18.0           B5   784   872   89.9   19.0   18.4           B6   792   871   90.9   19.9   19.3           B7   794   890   89.1   19.9   19.2           B8   776   871   89.0   18.6   18.1                      
 
     [0025] The results of the notched-bar impact bend tests, to which specimens B9-B12 of the hot strip B were also subjected, are shown in Table 4:  
                                   TABLE 4                                           Test temp.   Kv           Specimen   Direction   [K]   [J]                          B9   Longitudinal   233   55.5           B10   Transverse   233   36.2           B11   Longitudinal   233   78.4           B12   Transverse   233   37.4                      
 
     [0026] The tests carried out clearly confirm the outstanding mechanical characteristics of both hot strip A and B produced according to the invention.