Abstract:
Steel strips and methods for producing steel strips are provided. In an illustrated embodiment, a method for producing steel strips includes continuously casting molten steel into a strip, said molten steel comprising a concentration of residuals of 2.0 wt % or less is selected with regard to the microstructure of the finished strip to provide a desired yield strength; and cooling the strip to transform the strip from austenite to ferrite in the temperature range of 850° C. to 400° C. Cast steel with improved yield strength properties is produced by such method.

Description:
[0001]    This application claims priority to Australian Provisional Patent Application No. PR0460, filed Oct. 2, 2000.  
         BACKGROUND AND SUMMARY OF THE INVENTION  
         [0002]    The present invention relates to a method of producing steel strip and the cast strip produced according to the method.  
           [0003]    In particular, the present invention relates to producing steel strip in a continuous strip caster.  
           [0004]    The term “strip” as used in the specification is to be understood to mean a product of 5 mm thickness or less.  
           [0005]    The applicants have carried out extensive research and development work in the field of casting steel strip in a continuous strip caster in the form of a twin roll caster.  
           [0006]    In general terms, casting steel strip continuously in a twin roll caster involves introducing molten steel between a pair of contra-rotated horizontal casting rolls which are internally water cooled so that metal shells solidify on the moving rolls surfaces and are brought together at the nip between them to produce a solidified strip delivered downwardly from the nip between the rolls, the term “nip” being used to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed. The casting of steel strip in twin roll casters of this kind is for example described in U.S. Pat. Nos. 5,184,668, 5,277,243 and 5,934,359.  
           [0007]    The concentration of residuals in the steel composition can have a significant effect on the finished microstructure, and in turn affect yield strength properties of cast strip. In particular, higher concentrations of residuals make it possible to use lower cooling rates to transform the strip from austenite to ferrite in a temperature range between 850° C. and 400° C. to produce microstructures in cast strip that provide high yield strengths. It is understood that the transformation temperature range is within the range between 850° C. and 400° C. and not that entire temperature range. The precise transformation temperature range will vary with the chemistry of the steel composition and processing characteristics.  
           [0008]    There is provided a method of producing steel strip which includes the steps of:  
           [0009]    (a) continuously casting molten steel into a strip including austenite grains, said molten steel comprising a concentration of residuals in the steel composition selected with regard to the microstructure of the strip that is required to provide required mechanical properties; and  
           [0010]    (b) cooling the cast strip to transform the austenite grains in the strip to ferrite in a temperature range between 850° C. and 400° C.  
           [0011]    The continuous caster may be a twin roll caster.  
           [0012]    The term “residuals” covers levels of elements, such as copper, tin, zinc, nickel, chromium, and molybdenum, that are included in relatively small amounts, and are usually as a consequence of standard steel making. By way of example, the elements may be present as a result of using scrap steel to produce steel.  
           [0013]    In one embodiment, the total amount of the residuals is 1.2 wt % or less. These residuals may be up to 2.0 wt % where harder steel strip is desired with yield strengths up to and in excess of 700 MPa. This weight percent is the total weight percent in the steel strip including the residuals from scrap steel and steel processing.  
           [0014]    In one embodiment, the cast strip produced in step (a) may have a thickness of no more than 2 mm.  
           [0015]    In one embodiment, the cast strip produced in step (a) may include austenite grains that are columnar.  
           [0016]    The steel may be low carbon steel. The term “low carbon steel” is understood to be mean steel of the following composition, in wt %:  
           [0017]    C: 0.02-0.08  
           [0018]    Si: 0.5 or less;  
           [0019]    Mn: 1.0 or less;  
           [0020]    residuals: 1.2 or less; and  
           [0021]    Fe: balance.  
           [0022]    The low carbon steel may be silicon/manganese killed and may have the following composition by weight:  
                                                       Carbon    0.02-0.08%           Manganese    0.30-0.80%           Silicon    0.10-0.40%           Sulphur   0.002-0.05%           Aluminum   less than 0.01%                      
 
           [0023]    The low carbon steel may be aluminum killed and may have the following composition by weight:  
                                                       Carbon    0.02-0.08%           Manganese   0.40% max           Silicon   0.05% max           Sulphur   0.002-0.05%           Aluminum   0.05% max                      
 
           [0024]    The aluminum killed steel may be calcium treated.  
           [0025]    The method may further include the step of in-line hot rolling the cast strip after step (a) and prior to step (b).  
           [0026]    Step (b) may include cooling the strip to transform the strip from the austenite to ferrite at a selected cooling rate of at least 0.01° C./sec, and usually at least 0.1° C./sec, to produce a microstructure that provides required yield strength properties of the cast strip, the microstructure being selected from a group that includes microstructures that are:  
           [0027]    (i) predominantly polygonal ferrite;  
           [0028]    (ii) a mixture of polygonal ferrite and low temperature transformation products; and  
           [0029]    (iii) predominantly low temperature transformation products.  
           [0030]    It is understood that most embodiments of the present invention will have microstructures of types (ii) and (iii).  
           [0031]    The term “low temperature transformation products” includes Widmanstatten ferrite, acicular ferrite, bainite, and martensite. 
       
    
    
       [0032]    In order that the invention may be more fully explained, an example will be described with reference to the accompanying drawings, in which:  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIG. 1 illustrates a strip casting installation incorporating an in-line hot rolling mill and coiler;  
         [0034]    [0034]FIG. 2 illustrates details of the twin roll strip caster; and  
         [0035]    [0035]FIG. 3 illustrates the effect of residuals on yield strength of cast strip. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    The following description is in the context of continuous casting steel strip using a twin roll caster. The present invention is not limited to the use of twin roll casters and extends to other types of continuous strip casters.  
         [0037]    [0037]FIG. 1 illustrates successive parts of a production line whereby steel strip can be produced in accordance with the present invention. FIGS. 1 and 2 illustrate a twin roll caster denoted generally as  11  which produces a cast steel strip  12  that passes in a transit path  10  across a guide table  13  to a pinch roll stand  14  comprising pinch rolls  14 A. Immediately after exiting the pinch roll stand  14 , the strip passes into a hot rolling mill  16  comprising a pair of reduction rolls  16 A and backing rolls  16 B by in which it is hot rolled to reduce its thickness. The rolled strip passes onto a run-out table  17  on which it may be force cooled by water jets  18  and through a pinch roll stand  20  comprising a pair of pinch rolls  20 A, and thence to a coiler  19 .  
         [0038]    As shown in FIG. 2, twin roll caster  11  comprises a main machine frame  21  which supports a pair of parallel casting rolls  22  having a casting surfaces  22 A. Molten metal is supplied during a casting operation from a ladle (not shown) to a tundish  23 , through a refractory shroud  24  to a distributor  25  and thence through a metal delivery nozzle  26  into the nip  27  between the casting rolls  22 . Molten metal thus delivered to the nip  27  forms a pool  30  above the nip and this pool is confined at the ends of the rolls by a pair of side closure dams or plates  28  which are applied to the ends of the rolls by a pair of thrusters (not shown) comprising hydraulic cylinder units connected to the side plate holders. The upper surface of pool  30  (generally referred to as the “meniscus” level) may rise above the lower end of the delivery nozzle so that the lower end of the delivery nozzle is immersed within this pool.  
         [0039]    Casting rolls  22  are water cooled so that shells solidify on the moving roll surfaces and are brought together at the nip  27  between them to produce the solidified strip  12  which is delivered downwardly from the nip between the rolls.  
         [0040]    The twin roll caster may be of the kind which is illustrated and described in some detail in U.S. Pat. Nos. 5,184,668 and 5,277,243 or U.S. Pat. No. 5,488,988 and reference may be made to those patents for appropriate constructional details which form no part of the present invention.  
         [0041]    Typically, the strip passing from the twin roll caster will be of the order of 1400° C. and the temperature of the strip presented to the hot rolling mill may be about 900-1100° C. The strip may have a width in the range of 0.9 m to 2.0 m and a thickness in the range of 0.7 mm to 2.0 mm. The strip speed may be in the order of 1.0 m/sec.  
         [0042]    The cooling rate in transforming the strip from austenite to ferrite in a temperature range between 850° C. and 400° C. is selected to be at least 0.01° C./sec, preferably at least 0.1° C./sec, and may be in excess of 100° C./sec. With such cooling rates for low carbon steel it is possible to produce cast strip having microstructures including:  
         [0043]    (i) predominantly polygonal ferrite;  
         [0044]    (ii) a mixture of polygonal ferrite and low temperature transformation products, such as a acicular ferrite, Widmanstatten ferrite, and bainite; and  
         [0045]    (iii) predominantly low temperature transformation products.  
         [0046]    It is understood that most embodiments of the present invention will have microstructures of types (ii) and (iii).  
         [0047]    In the case of low carbon steels, such a range of microstructures can produce yield strengths in excess of 450 MPa.  
         [0048]    The concentration of residuals in the steel is selected having regard to the finished microstructure of the cast strip that is required to provide required mechanical properties for the strip.  
         [0049]    The present disclosure is based on experimental work that has found the presence of a high amount of residuals (0.2% Cr, 0.2% Ni, 0.2% Mo, 0.4% Cu, 0.2% Sn) has produced a strip with improved microstructure.  
         [0050]    The experimental findings include that the austenite microstructure of strip cast at 75 m/min was similar to the microstructure of strip without residuals. However, when the cast strip with residuals was subjected to a standard cooling rate of 10-15° C./sec, the resultant finished microstructure was very different from that of the cast strip without residuals cooled at the same rate.  
         [0051]    The observed microstructure of cooled cast strip with residuals was predominantly bainitic with only a thin band of grain boundary ferrite appearing along the prior austenite grain boundaries, indicating a severely suppressed ferrite transformation caused by the presence of residuals. The mechanical properties of the resultant product are very desirable, with typical values of 540 MPa yield strength, 650 MPa tensile strength and 15% total elongation. Such values could be achieved in the past by microalloying which added considerable cost to the production of the cast strip.  
         [0052]    The effect of residuals was to enhance the proportion of low temperature transformation products (particularly the bainites) by lowering austenite to ferrite transformation temperatures and slowing the kinetics of polygonal ferrite formation.  
         [0053]    One, but not the only one, of the important consequences of this finding is that an increase in the concentration of residuals effects a reduction in the cooling rate that is required to transform austenite to ferrite to form a required microstructure to provide high yield strengths.  
         [0054]    Although the invention has been illustrated and described in detail in the foregoing drawings and description with reference to several embodiments, it should be understood that the description is illustrative and not restrictive in character, and that the invention is not limited to the disclosed embodiments. Rather, the present invention covers all variations, modifications and equivalent structures that come within the scope and spirit of the invention. Additional features of the invention will become apparent to those skilled in the art upon consideration of the detailed description, which exemplifies the best mode of carrying out the invention as presently perceived. Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention.