Patent Publication Number: US-2019177814-A1

Title: Lean duplex stainless steel having excellent bending processability

Description:
TECHNICAL FIELD 
     The present disclosure relates to a lean duplex stainless steel, and more particularly, to a lean duplex stainless steel having excellent bending processability. 
     BACKGROUND ART 
     In general, austenitic stainless steels having excellent processability and corrosion resistance include iron (Fe), as a base metal, and chromium (Cr) and nickel (Ni), as main raw materials, and have been developed to a variety of steel types suitable for various applications by adding and other elements such as molybdenum (Mo) and copper (Cu) thereto. 
     Since 300 series stainless steels having excellent corrosion resistance and processability include high-priced raw materials such as Ni and Mo, 400 series stainless steels have been discussed as alternatives thereto. However, there is a problem that formability of 400 series stainless steels cannot reach that of 300 series stainless steels. Although corrosion resistance levels of 400 series stainless steels are applicable, depending on the environment in use, to thick plates which are less processed than hot/cold-rolled stainless steels which have gone through more processing stages, the 400 series stainless steels have many limitations in use as thick plates due to poor impact properties and deterioration of welds. 
     Meanwhile, duplex stainless steels in which an austenite phase and a ferrite phase are mixed have advantages of both stainless steels and ferritic stainless steels and various types of duplex stainless steels have been developed. 
     Since alloy elements of Ni, Mo, and the like are expensive, inexpensive stainless steels have continuously received increasing attentions. As a result, attempts have been made to develop lean alloys having low amounts of expensive alloy elements. This trend has also been confirmed in duplex stainless steels whose microstructure is formed of a ferrite phase and an austenite phase. 
     Patent Document 1 discloses an austenoferritic stainless steel having a low Ni content and a high N content to constitute a lean duplex stainless steel having a high elongation simultaneously with high strength by adjusting stability of an austenite phase. Lean duplex stainless steels have excellent corrosion resistance and high strength due to effects of grain size refinement caused by a dual phase structure, and thus the use thereof is gradually increasing. 
     Stainless ornamental tubes or structural tubes are available in various shapes depending on applications thereof and also require various properties (such as corrosion resistance and formability). Thus, 400 series, 200 series, and 300 series stainless steels have been used therefor according to shapes and material requirements and relatively inexpensive 400 series and 200 series stainless steels have mainly been used therefor. 
     Although lean duplex stainless steels are expected to be applied not only to outdoor decoration pipes due to excellent corrosion resistance but also to structural tubes due to high strength, the use of the lean duplex stainless steels has been limited due to relatively high prices thereof in comparison with 400 series and 200 series stainless steels and easy occurrence of cracks when the lean duplex stainless steels bent are bent. Therefore, in order to replace 400 series and 200 series stainless steel tubes, amounts of expensive alloy elements need to be reduced and bending processability need to be improved to manufacture a tube having a complicated cross-section such as a rectangular cross-section. 
     Patent Document 1: Korean Patent Laid-Open Publication No. 10-2009-0005252 (Published on Jan. 12, 2009) 
     DISCLOSURE 
     Technical Problem 
     The present disclosure is directed to providing a lean duplex stainless steel having a dual phase structure, in which an austenite phase and a ferrite phase co-exist, and having improved bending processability by minimizing amounts of high-priced alloy elements such as Ni, Cu, and Mo and optimizing a sum of amounts of Cr and Mn and a ratio of Cr to Mn among elements constituting duplex stainless steels. 
     Technical Solution 
     One aspect of the present disclosure provides a lean duplex stainless steel having excellent bending processability and including, in percent (%) by weight of the entire composition, 0.01 to 0.06% of carbon (C), 0.2 to 1.0% of silicon (Si), 3.5 to 6.5% of manganese (Mn), 18.5 to 22.5% of chromium (Cr), 0.05 to 0.25% of nitrogen (N), and the remainder of iron (Fe) and other inevitable impurities, wherein a sum of amounts of Cr and Mn is from 26.0 to 28.5% and a Cr/Mn ratio is from 3.4 to 4.1. 
     The lean duplex stainless steel may include 0.5% or less of nickel (Ni), 0.5% or less or copper (Cu), and 0.5% or less of molybdenum (Mo). 
     A volume fraction of a ferrite matrix structure in a microstructure may be from 50 to 75%. 
     An elongation of the stainless steel may be from 30 to 40%. 
     Advantageous Effects 
     According to the embodiments of the present disclosure, alloy elements such as Ni, Cu, and Mo are controlled as impurities among the elements constituting a duplex stainless steel and these alloy elements are minimized or excluded therefrom, and therefore resources may be saved and manufacturing costs for the duplex stainless steel may be reduced. 
     In addition, bending processability of stainless steels may be improved by optimizing a sum of amounts of Cr and Mn and a ratio of Cr to Mn. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a photograph of a processed surface of a lean duplex stainless steel according to an example of the present disclosure after 180° bending processing. 
         FIG. 2  is a photograph of a processed surface of a lean duplex stainless steel according to a comparative example of the present disclosure after 180° bending processing. 
     
    
    
     BEST MODE 
     A lean duplex stainless steel having excellent bending processability according to an embodiment of the present disclosure includes, in percent (%) by weight of the entire composition, 0.01 to 0.06% of carbon (C), 0.2 to 1.0% of silicon (Si), 3.5 to 6.5% of manganese (Mn), 18.5 to 22.5% of chromium (Cr), 0.05 to 0.25% of nitrogen (N), and the remainder of iron (Fe) and other inevitable impurities, wherein a sum of amounts of Cr and Mn is from 26.0 to 28.5% and a Cr/Mn ratio is from 3.4 to 4.1. 
     [Modes of the Invention] 
     Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the concept of the present disclosure to those skilled in the art. The present disclosure is not limited to these embodiments, and may be embodied in another form. In the drawings, parts unrelated to the descriptions may be omitted for clear description of the disclosure, and sizes of components may be exaggerated for easy understanding. 
     A lean duplex stainless steel having excellent bending processability according to an embodiment of the present disclosure includes, in percent (%) by weight of the entire composition, 0.01 to 0.06% of carbon (C), 0.2 to 1.0% of silicon (Si), 3.5 to 6.5% of manganese (Mn), 18.5 to 22.5% of chromium (Cr), 0.05 to 0.25% of nitrogen (N), and the remainder of iron (Fe) and other inevitable impurities. 
     The amount of C is from 0.01 to 0.06%. 
     C, which is an austenite phase-forming element, may be used as an alternative to a high-priced element such as Ni and is an effective element for increasing strength of a steel by solid solubility enhancement. 
     An excess of C may cause carbon segregation and formation of coarse carbides at central regions of a steel, thereby adversely affecting subsequent operations of hot-rolling, annealing, cold-rolling, and cold-annealing processes and C may easily bind to a carbide-forming element such as Cr which is effective for corrosion resistance at a boundary between ferrite and austenite phases to lower the amount of Cr around grains, thereby reducing corrosion resistance. Thus, to maximize corrosion resistance, the amount of C may be 0.06% or less. Thus, the amount of C may be from 0.01 to 0.06%. 
     The amount of Si is from 0.2 to 1.0%. 
     S, added in a small amount for deoxidation effects, is a ferrite-forming element enriched in ferrite by annealing. 
     The Si content needs to be added in an amount of 0.2% or more to obtain a proper fraction of a ferrite phase. However, an excess of Si greater than 1.0% may rapidly increase hardness of the ferrite phase resulting in a decrease in elongation, lower fluidity of a slag during a steelmaking process, bind to oxygen to form inclusions thereby impairing corrosion resistance. Thus, the amount of Si may be in the range of from 0.2 to 1.0%. 
     Mn is added in an amount of from 3.5 to 6.5%. 
     Mn, as an element adjusting fluidity of molten metal, serving as a deoxidizer, and increasing solid solubility of nitrogen, is added to replace a high-priced Ni as an austenite-forming element. 
     When the amount of Mn is less than 3.5%, it is difficult to obtain a proper fraction of an austenite phase even when the amounts of the other austenite-forming elements such as N are adjusted in the case where Ni and Cu are controlled as impurities. When the amount of Mn is greater than 6.5%, it is difficult to obtain corrosion resistance and to control the phase fraction due to an excessive austenite phase. Thus, the amount of Mn may be limited to the range of from 3.5 to 6.5%. 
     The amount of Cr is from 18.5 to 22.5%. 
     Cr, as an element stabilizing the ferrite phase together with Si, plays a major role in obtaining the ferrite phase and is essentially added to obtain corrosion resistance. 
     Although corrosion resistance is improved by an increase in the amount of Cr, the amounts of the high-priced Ni and the other austenite-forming elements need to be increased to maintain phase fractions. Thus, the amount of Cr may be limited to the range of from 18.5 to 22.5%. 
     The amount of N is from 0.05 to 0.25%. 
     N, as an element contributing to stabilizing the austenite phase together with C and Ni, is enriched in the austenite phase by annealing. 
     As the amount of N is increased, both corrosion resistance and strength may additionally be improved. However, an excess of N may generate nitrogen pores during casting due to an excessive solid solubility of N, resulting in surface defects, and thus it is difficult to stably manufacture steels. Therefore, the amount of N may be limited to the range of from 0.05 to 0.25%. 
     For example, the lean duplex stainless steel according to an embodiment may include 0.5% or less of Ni, 0.5% or less of Cu, and 0.5% or less of Mo. 
     Ni, as an austenite-stabilizing element together with Mn, Cu, and N, plays a main role in improving stability of the austenite phase. 
     However, the balance of the fractions of the phases may be maintained by increasing the amounts of Mn and N, which are also austenite-forming elements when the amounts of the high-priced Ni is minimized in order to reduce manufacturing costs. The high-priced Ni may not be added to prevent an increase in manufacturing costs thereby. Therefore, the amount of Ni may be limited to 0.5% or less (including zero (0)) in consideration of the amount as an impurity. 
     Cu is an element inhibiting work hardening caused by formation of a strain-induced martensite phase and contributing to softening of an austenitic stainless steel. 
     However, Cu may not be added to prevent an increase in manufacturing costs of products by the high-priced Cu. Thus, the amount of Cu may be limited to 0.5% or less (including zero (0)) in consideration of the amount as an impurity. 
     Mo is a very effective element capable of improving corrosion resistance while stabilizing a ferrite together with Cr. 
     However, Mo may not be added to prevent an increase in manufacturing costs by the high-priced Mo. Thus, the amount of Mo may be limited to 0.5% or less (including zero (0)) in consideration of the amount as an impurity. 
     That is, since the alloy elements such as Ni, Cu, and Mo are controlled as impurities and minimized or excluded from the elements constituting the duplex stainless steel according to an embodiment of the present disclosure, resources may be saved and manufacturing costs for the duplex stainless steel may be minimized. Thus, when each of the amounts of Ni, Cu, and Mo exceeds 0.5%, manufacturing costs may be increased due to increases in the amounts of the high-priced metals of Ni, Cu, and Mo. 
     Furthermore, in the duplex stainless steel according to an embodiment of the present disclosure, a sum of amounts of Cr and Mn is from 26.0 to 28.5%, and a Cr/Mn ratio is from 3.4 to 4.1. 
     The present disclosure relates to a composition of a low-priced lean duplex stainless steel to replace 400 series and 200 series stainless ornamental tubes. While carrying out research into formability of low-priced lean duplex stainless steels including high-priced alloy elements of Ni, Cu, and Mo as impurities rather than target elements, the present inventors have found an unusual phenomenon in which a stainless steel having a lower elongation measured by a tensile strength test has better bending processability than a stainless steel having a higher elongation. 
     Based on this unusual phenomenon, it was found that excellent bending processability are obtained only when a sum of the amounts of Cr and Mn is in the range of from 26.0 to 28.5% by weight and the Cr/Mn ratio is in the range of from 3.4 to 4.1. 
     That is, when the sum of the amounts of Cr and Mn is adjusted within the range of from 26.0 to 28.5% by weight and the Cr/Mn ratio is adjusted within the range of from 3.4 to 4.1, a low-priced stainless steel having excellent formability during bending processing may be provided. 
     For example, a volume fraction of a ferrite matrix structure in a microstructure of the lean duplex stainless steel according to an embodiment may be from 50 to 75%. When the volume fraction of the ferrite matrix structure is less than 50%, sufficient corrosion resistance may not be obtained. When the volume fraction of the ferrite matrix structure is greater than 75%, a volume fraction of the austenite matrix structure relatively decreases, and thus sufficient processability may not be obtained. 
     For example, an elongation of the lean duplex stainless steel according to an embodiment may be from 30 to 40%. 
     When the elongation is less than 30%, processability may deteriorate. When the elongation is greater than 40%, cracks may occur during bending processing. Since processibilty is generally expected to be improved as elongation increases, bending processability is also expected to be improved. However, when the amounts of the elements, the sum of amounts of Cr and Mn, and the Cr/Mn ratio satisfy criteria according to the present disclosure, there is a problem that cracks occur while a steel is bent when an elongation exceeds 40%. 
     The lean duplex stainless steel according to an embodiment is manufactured by hot-rolling a duplex stainless steel slab including, in percent (%) by weight of the entire composition, 0.01 to 0.06% of C, 0.2 to 1.0% of Si, 3.5 to 6.5% of Mn, 18.5 to 22.5% of Cr, 0.05 to 0.25% of N, 0.5 or less of Ni, 0.5 or less of Cu, 0.5 or less of Mo, and the remainder of Fe and other inevitable impurities, wherein a sum of the amounts of Cr and Mn is from 26.0 to 28.5%, and a Cr/Mn ratio is from 3.4 to 4.1, hot-annealing the hot-rolled steel sheet at a temperature of from 1,050 to 1,150° C., cold-rolling the hot-annealed steel sheet, cold-annealing the cold-rolled steel sheet at a temperature of from 1,050 to 1,150° C., and acid-pickling the resultant. 
     The lean duplex stainless steel slab having the above composition may be rolled to manufacture a thick plate using any method well known in the art, and the hot-rolled steel sheet may have a thickness of from 4 to 20 mm. For example, the hot-rolled steel sheet may be annealed at a temperature of from 1,050 to 1,150° C. for 30 seconds to 60 minutes. 
     Thereafter, the hot-rolled steel sheet may be cold-rolled according to any method well known in the art, and the cold-rolled steel sheet may have a thickness of from 0.1 to 5 mm. For example, the cold-rolled steel sheet may be annealed at a temperature of from 1,050 to 1,150° C. for 10 seconds to 60 minutes. 
     Hereinafter, one or more exemplary embodiments will be described in detail with reference to the following examples and comparative examples. 
     Steels of Examples and Comparative Examples 
     Steels respectively including elements as shown in Table 1 below were prepared according to the following examples and comparative examples. Each of the steels was cast into an ingot weighting 50 kg and having a thickness of 140 mm in a vacuum induction melting furnace. The cast ingot was aged in a heating furnace at a temperature of 1,250° C. for 3 hours, hot-rolled to a width of 200 mm and a thickness of 4 mm, and air-cooled. The air-cooled hot-rolled steel sheet was hot-annealed at a temperature of 1,100° C. for 1 minute and cold-rolled to a thickness of 0.5 mm after acid-pickling. The cold-rolled steel sheet was cold-annealed at a temperature of 1,100° C. for 30 seconds and acid-pickled to prepare a duplex stainless cold-rolled steel sheet sample. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 Cr + 
                   
               
               
                   
                 Cr 
                 Mn 
                 Si 
                 C 
                 N 
                 Mn 
                 Cr/Mn 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Steel of Example 1 
                 20.9 
                 6.10 
                 0.42 
                 0.022 
                 0.178 
                 27.0 
                 3.43 
               
               
                 Steel of Example 2 
                 22.1 
                 6.02 
                 0.41 
                 0.015 
                 0.192 
                 28.1 
                 3.67 
               
               
                 Steel of Example 3 
                 21.2 
                 6.00 
                 0.39 
                 0.050 
                 0.195 
                 27.2 
                 3.53 
               
               
                 Steel of Example 4 
                 22.1 
                 5.99 
                 0.40 
                 0.050 
                 0.183 
                 28.1 
                 3.69 
               
               
                 Steel of Example 5 
                 21.1 
                 5.91 
                 0.40 
                 0.058 
                 0.215 
                 27.0 
                 3.57 
               
               
                 Steel of Compara- 
                 19.0 
                 6.00 
                 0.41 
                 0.019 
                 0.184 
                 
                   25.0 
                 
                 
                   3.17 
                 
               
               
                 tive Example 1 
               
               
                 Steel of Compara- 
                 19.7 
                 5.85 
                 0.42 
                 0.018 
                 0.182 
                 
                   25.6 
                 
                 
                   3.37 
                 
               
               
                 tive Example 2 
               
               
                 Steel of Compara- 
                 19.0 
                 6.07 
                 0.37 
                 0.049 
                 0.188 
                 
                   25.1 
                 
                 
                   3.13 
                 
               
               
                 tive Example 3 
               
               
                 Steel of Compara- 
                 19.9 
                 6.10 
                 0.39 
                 0.050 
                 0.189 
                 26.0 
                 
                   3.26 
                 
               
               
                 tive Example 4 
               
               
                 Steel of Compara- 
                 20.9 
                 5.01 
                 0.71 
                 
                   0.100 
                 
                 0.105 
                 
                   25.9 
                 
                 
                   4.17 
                 
               
               
                 tive Example 5 
               
               
                 Steel of Compara- 
                 21.2 
                 3.92 
                 0.38 
                 
                   0.098 
                 
                 0.103 
                 
                   25.1 
                 
                 
                   5.41 
                 
               
               
                 tive Example 6 
               
               
                   
               
            
           
         
       
     
     A ferrite fraction of a steel was measured for the steel having a thickness of 4 mm in a hot-annealed state by using a Ferritescope. The Ferritescope is a device for measuring the fraction of a ferrite phase using magnetic properties of a steel, and ferrite fractions measured by a “Ferritescope MP30” manufactured by Fisher are shown in Table 2 below. 
     A sample having a length of 180 mm and a width of 20 mm obtained from a cold-annealed steel sheet having a thickness of 1.5 mm in a direction perpendicular to the rolling was processed and subjected to a bending test. The bending test was performed by bending the sample 90° first and further bending the sample up to 180° with a force of 10 ton using a punch having a corner radius of 1.5 mm. Breakage of the sample was determined based on the results of the 180° bending test and the results are shown in Table 2 below. 
     Tensile strength test results of the steels according to the examples and comparative examples are shown in Table 2 below. A sample having a gage length of 50 mm and a width of 12.5 mm was obtained from a cold-annealed steel sheet having a thickness of 1.5 mm in a direction perpendicular to the rolling and subjected to a tensile strength test at a tensile speed of 20 mm/min at room temperature. Properties of the steels after performing the tensile strength test five times for each sample are shown in Table 2 below. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Ferrite 
                   
                   
                   
                   
               
               
                   
                 fraction 
                 YS 
                 TS 
                 EL 
                 Cracks at 
               
               
                   
                 (%) 
                 (MPa) 
                 (MPa) 
                 (%) 
                 bending 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Steel of Example 1 
                 63 
                 423 
                 670 
                 33.7 
                 No cracks 
               
               
                 Steel of Example 2 
                 71 
                 398 
                 643 
                 31.4 
                 No cracks 
               
               
                 Steel of Example 3 
                 56 
                 456 
                 703 
                 34.8 
                 No cracks 
               
               
                 Steel of Example 4 
                 64 
                 423 
                 670 
                 32.5 
                 No cracks 
               
               
                 Steel of Example 5 
                 60 
                 406 
                 672 
                 33.3 
                 No cracks 
               
               
                 Steel of Comparative 
                 
                   45 
                 
                 473 
                 839 
                 33.4 
                 Cracks 
               
               
                 Example 1 
               
               
                 Steel of Comparative 
                 55 
                 434 
                 689 
                 
                   45.1 
                 
                 Cracks 
               
               
                 Example 2 
               
               
                 Steel of Comparative 
                 
                   36 
                 
                 476 
                 883 
                 33.8 
                 Cracks 
               
               
                 Example 3 
               
               
                 Steel of Comparative 
                 
                   45 
                 
                 476 
                 754 
                 
                   43.7 
                 
                 Cracks 
               
               
                 Example 4 
               
               
                 Steel of Comparative 
                 59 
                 415 
                 678 
                 32.7 
                 Cracks 
               
               
                 Example 5 
               
               
                 Steel of Comparative 
                 62 
                 401 
                 668 
                 31.7 
                 Cracks 
               
               
                 Example 6 
               
               
                   
               
            
           
         
       
     
     Referring to Tables 1 and 2, while cracks did not occur in the steels according to the examples according to the present disclosure, cracks occurred in all of the steels according to the comparative examples. 
       FIG. 1  is a photograph of a processed surface of a lean duplex stainless steel according to an example of the present disclosure after 180° bending processing.  FIG. 2  is a photograph of a processed surface of a lean duplex stainless steel according to a comparative example of the present disclosure after 180° bending processing. 
     Specifically,  FIG. 1  is a photograph of a processed surface of the steel according to Example 1 after 180° bending processing, and  FIG. 2  is a photograph of a processed surface of the steel according to Comparative Example 6 after 180° bending processing. 
     That is, referring to  FIG. 1 , cracks did not occur even after the bending processing, and thus it may be confirmed that the steel had excellent bending processability. Referring to  FIG. 2 , it may be confirmed that serious surface cracks occurred by the bending processing. 
     Referring to Table 2 above, it may be confirmed that bending processability deteriorated in the case where the parameters of the sum of the amounts of Cr and Mn and the Cr/Mn ratio according to the present disclosure are not satisfied, even when the stainless steel has a high elongation measured by the tensile strength test. 
     More particularly, although the steels of Comparative Examples 2 and 4 had elongations of 4.5% and 43.7%, respectively, which are higher than those of the other steels, cracks occurred after the 180° bending processing. Therefore, it may be confirmed that the control of the parameters relating to Cr and Mn, i.e., the sum of the amounts of Cr and Mn, and the Cr/Mn ratio, as described herein is important to obtain excellent bending processability. 
     Therefore, to obtain excellent bending processability of the duplex stainless steel, the sum of the amounts of Cr and Mn, as elements of the steel, needs to be controlled from 26.0 to 28.5% by weight and the ratio of Cr to Mn (Cr/Mn), in weight percent, needs to be controlled from 3.4 to 4.1. 
     While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure. 
     INDUSTRIAL AVAILABILITY 
     The lean duplex stainless steel having excellent bending processability according to the present disclosure may be applied to various fields such as outdoor decoration tubes or interior structural tubes due to excellent bending processability.