Patent Publication Number: US-11377711-B2

Title: 780MPa cold-rolled duel-phase strip steel and method for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application represents the national stage entry of PCT International Application No. PCT/CN2013/076184 filed May 24, 2013, which claims priority of Chinese Patent Application No. 201310021998.9 filed Jan. 22, 2013, the disclosures of which are incorporated by reference here in their entirety for all purposes. 
     TECHNICAL FIELD 
     The present invention relates to a dual-phase steel and a method for manufacturing the same, particularly to an iron-based dual-phase steel and a method for manufacturing the same. 
     BACKGROUND ART 
     Due to the requirements concerning weight reduction and safety, an increasing amount of steel plate with smaller thickness and higher strength is needed in the automobile industry market. Dual-phase strip steel having a tensile strength of 780 MPa has a good prospect of application because it represents good properties of strength and formability. 780 MPa dual-phase strip steel is expected to be a substitute for 590 MPa cold-rolled dual-phase steel in the future market and become the most widely used dual-phase steel. Dual-phase steel is made by strengthening via phase transformation. In order to guarantee certain hardening capacity, an amount of carbon and alloy elements have to be added into steel to ensure that supercooled austenite would be converted into martensite during the cooling of the dual-phase steel. However, high contents of carbon and alloy elements are unfavorable for the weldability of steel plate. Moreover, alloy elements tend to segregate in the course of casting, resulting in banded structure in cold-rolled strip steel. Consequently, cold-rolled dual-phase steel differentiates significantly in different directions, leading to a series of problems in practical use. 
     Carbon equivalent of steel mainly depends on carbon content, alloy element content and impurity element content in the steel. Carbon equivalent may be characterized using a variety of formulae, and is usually represented by Pcm value for automobile steel: Pcm=C+Si/30+Mn/20+2P+4S. Generally, Pcm value may be used to characterize the embrittlement tendency of steel plate after welding and cooling. When Pcm is higher than 0.24, welding spot tends to crack at the interface. It is safe when Pcm is lower than 0.24. 
     Steel is an anisotropic material in nature. As a continuous process is used for the production of strip steel, an orientational distribution exists in the steel structure to varying extent. In other words, an elongated band-like distribution is exhibited along the rolling direction. Due to high alloy element content in high-strength steel, composition segregation occurs easily. Furthermore, it is difficult to eliminate the segregation of substitutional alloy elements. The structure of steel is deformed and elongated during hot rolling and cold rolling, and finally forms a banded structure. Generally, the banded structure contains high contents of alloy elements and carbon, such that hard and brittle martensite having a band-like distribution is formed in the dual-phase steel after quenching, which is considerably detrimental to the properties of the steel. Therefore, alleviation of the banded structure to obtain a homogeneously distributed structure is the key to acquire good properties for high-strength dual-phase strip steel. 
     A Chinese patent literature that has a publication number of CN102212745A and was published on Oct. 12, 2011 and titled “High-plasticity 780 MPa Cold-rolled Dual-phase Steel and Manufacturing Method Thereof” discloses a method for manufacturing a high-plasticity 780 MPa cold-rolled dual-phase steel which has the following chemical composition: 0.06-0.08% C, 1.0-1.3% Si, 2.1-2.3% Mn, 0.02-0.07% Al, S≤0.01%, N≤0.005%, P≤0.01%, and the balance amounts of Fe and other unavoidable impurities. The end rolling temperature for hot rolling is 890° C., the coiling temperature is 670° C., the cold rolling reduction amount is 50-70%, and a conventional gas jet cooling continuous annealing is used. 
     An American patent literature that has a publication number of US20040238082A1 and was published on Dec. 2, 2004 and titled “High-strength Cold-rolled Steel Plate and Method for Production Thereof” discloses a method for manufacturing high-strength steel having good hole-expanding property, wherein the steel has the following chemical composition: 0.04-0.1% C, 0.5-1.5% Si, 1.8-3% Mn, P≤0.020%, S≤0.01%, 0.01˜0.1% Al, N≤0.005%, and the balance amounts of Fe and other unavoidable impurities. The steel plate is hot rolled between Ar3-870° C., coiled at a temperature below 620° C., and annealed at 750-870° C. Rapid cooling begins at 550-750° C. at a rapid cooling speed≥100° C./s, and ends at a temperature below 300° C. Finally, cold-rolled high-strength steel having a tensile strength of higher than 780 MPa and a hole-expanding ratio of at least 60% is obtained. Relatively high contents of Mn and Si are employed in the composition design of this steel plate. 
     A Japanese patent literature that has a publication number of JP Publication 2007-138262 and was published on Jun. 7, 2007 and titled “High-strength Cold-rolled Steel Plate With Small Variation Of Mechanical Properties And Manufacturing Method Thereof” relates to a high-strength cold-rolled steel plate which has the following chemical composition: 0.06-0.15% C, 0.5-1.5% Si, 1.5-3.0% Mn, 0.5-1.5% Al, S≤0.01%, P≤0.05%, and the balance amounts of Fe and other unavoidable impurities. The manufacturing process comprises the following steps: holding at Ac1˜Ac3 for 10 s, cooling to 500-750° C. at a cooling speed of 20° C./s, and cooling to a temperature below 100° C. at a cooling speed of higher than 100° C./s. 780 MPa high-strength steel plate having a hole-expanding ratio≥60 may be obtained. 
     None of the above literatures describe control over the banded structure in the steel, nor do they propose relevant solutions to the improvement of the anisotropy. Thus, the above patents do not relate to improvement of anisotropic mechanical properties of dual-phase steel. 
     SUMMARY 
     The object of the invention is to provide a 780 MPa cold-rolled dual-phase strip steel and a method for manufacturing the same, wherein a dual-phase strip steel having a homogeneous microstructure, good phosphating property and small anisotropy of mechanical properties is expected to be obtained by a design featuring low carbon equivalent, so that the cold-rolled dual-phase strip steel may meet the bi-directional demands of automobile industry on smaller thickness and higher strength of steel. 
     In order to achieve the above object of the invention, the invention provides a 780 MPa cold-rolled dual-phase strip steel, wherein the strip steel has a microstructure of fine equiaxed ferrite matrix and martensite islands distributed homogeneously on the ferrite matrix, and comprises the following the chemical elements in mass percentages: 
     C 0.06-0.1%; 
     Si≤0.28%; 
     Mn 1.8-2.3%; 
     Cr 0.1-0.4%; 
     Mo not added when Cr≥0.3%; Mo=0.3%—Cr when Cr≤0.3%; 
     Al 0.015-0.05%; 
     at least one of Nb and Ti elements, wherein Nb+Ti is in the range of 0.02-0.05%; 
     the balance amounts of Fe and other unavoidable impurities. 
     The principle for designing the various chemical elements in the 780 MPa cold-rolled dual-phase strip steel of the invention is as follows: 
     C: C may increase the strength of martensite and influence the content of martensite. It has much influence on the strength, but increased carbon content is not good to weldability of strip steel. The strength will be insufficient if carbon content is less than 0.06%, whereas the weldability will be decreased if carbon content is higher than 0.1%. Therefore, carbon content of 0.06-0.1 wt % is selected in the technical solution of the invention. 
     Si: Si acts to strengthen solid solution in dual-phase steel. Si can enhance the activity of carbon element, facilitate segregation of C in the Mn rich zone, and increase the carbon content in the band-like zone. However, Si is undesirable for the phosphating property of strip steel. Hence, an upper limit for Si content has to be set. The technical solution of the invention requires Si≤0.28 wt %. 
     Mn: Mn may increase the hardenability of steel and enhance the strength of steel effectively. But Mn will deteriorate the weldability of strip steel. Mn segregates in steel, and tends to be rolled into Mn rich zone having band-like distribution in the course of hot rolling, so as to form a banded structure which is undesirable for the structure homogeneity of dual-phase steel. When Mn is less than 1.8%, the hardenability and strength of strip steel will be insufficient. When Mn is more than 2.3%, the banded structure in strip steel will be exasperated and the carbon equivalent will be increased. Therefore, the content of Mn is set to be 1.8-2.3 wt %. 
     Cr: Cr may increase the hardenability of strip steel. Meanwhile, addition of Cr may make up the function of Mn. When Cr is less than 0.1%, the effect is not obvious. But when Cr is more than 0.4%, unduly high strength and decreased plasticity will be resulted. Thus, the Cr content in the technical solution of the invention is controlled to be 0.1-0.4 wt %. 
     Mo: Mo may increase the hardenability of steel and enhance the strength of strip steel effectively. Furthermore, Mo can ameliorate the distribution of carbides. Both Mo and Cr can assist in the hardenability of strip steel. Therefore, in the present technical solution, the addition of Mo is related to Cr. When the Cr content is lower than 0.3 wt %, the addition amount of shall be (0.3—Cr). When the Cr content is higher than 0.3 wt %, no addition of Mo is needed. 
     Al: Al has the function of deoxygenation and grain refinement in steel. The technical solution of the invention requires Al in the range of 0.015-0.05 wt %. 
     Nb, Ti: Nb and Ti are strengthening elements for precipitation, and have the function of grain refinement. They may be added separately or in combination, but the total amount to be added shall be controlled to be 0.02-0.05 wt %. 
     Furthermore, the following chemical elements are defined for the 780 MPa cold-rolled dual-phase strip steel of the invention: C 0.07-0.09 wt %; Mn 1.9-2.2 wt %; Al 0.02-0.04 wt %. 
     In the aspect of composition design, relatively low carbon content, relatively low total addition amount of alloy elements, and a manner of adding a multiplicity of alloy elements in combination are employed for the 780 MPa cold-rolled dual-phase strip steel of the invention. For the present technical solution, the selection of relatively low carbon content may decrease the enrichment degree of C in steel and hamper the tendency of forming a banded structure. The selection of decreased content of the main alloy element Mn in dual-phase steel may effectively reduce the probability of the occurrence of a banded structure in strip steel and abate the undesirable impact on the phosphating property. Strict restriction on the addition of Si may reduce C atom segregation resulting from the change of C atom activity caused by Si. Addition of a certain amount of Cr, Mo and other alloy elements may compensate the decreased hardenability resulting from relatively low content of Mn. Such a composition design may efficiently control the carbon equivalent Pcm in steel to be lower than 0.24. As such, not only welding cruciform tensile fastener-like crack can be obtained, but also no less than 780 MPa of steel strength can be guaranteed. As the microstructure of the strip steel comprises fine equiaxed ferrite matrix and martensite islands distributed homogeneously on the ferrite matrix, the banded structure exhibited therein is minute. Therefore, the strip steel shows small anisotropy in its mechanical properties and has good cold bending property and hole expanding property. 
     Correspondingly, the invention also provides a method for manufacturing the 780 MPa cold-rolled dual-phase strip steel, comprising the following steps:
         1) Smelting;   2) Casting: A secondary water-cooling process is used wherein the water jet capacity is not less than 0.7 L water/kg steel blank;   3) Hot rolling: The end rolling temperature is controlled to be 820-900° C., followed by rapid cooling after rolling;   4) Coiling: The coiling temperature is controlled to be 450-650° C.;   5) Cold rolling;   6) Continuous annealing: holding at 800-860° C., cooling to 640-700° C. at a cooling speed of not less than 5° C./s, further cooling to 220-280° C. at a cooling speed of 40-100° C./s, and tempering at 220-280° C. for 100-300 s.       

     Further, the above method for manufacturing the 780 MPa cold-rolled dual-phase strip steel also comprises step 7): temper rolling. 
     Further, the cold rolling reduction rate is 40-60% in the above step 5). 
     Still further, the temper rolling elongation is 0.1-0.4% in the above step 7). 
     In the aspect of manufacturing process, the use of a secondary water-cooling process in the continuous casting step to cool the steel blank rapidly and evenly with a large cooling water jet capacity at a rapid cooling speed may refine the structure of the continuously cast blank. As such, fine carbides are dispersively distributed on the ferrite matrix in the form of particles. Relatively low end rolling temperature is used in the hot rolling step, and relatively low coiling temperature is used in the coiling step similarly. This may refine grains, and decrease the distribution continuity of the banded structure. Relatively high annealing and holding temperatures are used in the continuous annealing step, which may restrain the formation of the banded structure in the steel. Rapid cooling after homogeneous heating is also favorable for lessening segregation of carbon and inhibiting formation of the banded structure. After the above process steps, the microstructure of the 780 MPa cold-rolled dual-phase strip steel described herein exhibits fine equiaxed ferrite matrix and martensite islands distributed homogeneously on the ferrite matrix. The mechanical properties thereof show small anisotropy, and the structure is homogeneous. 
     Compared with the prior art, the 780 MPa cold-rolled dual-phase strip steel described herein shows homogeneous distribution of martensite, a minute banded structure, a fine and dense phosphating film on the surface, good weldability, superior homogeneity of mechanical properties, excellent phosphating property, and small difference between the longitudinal and lateral properties. It is desirable for stamping of dual-phase steel, can satisfy the requirements of high-strength dual-phase steel in terms of strength and formability, and can be used widely in automobile manufacture and other fields. 
     According to the method for manufacturing the 780 MPa cold-rolled dual-phase strip steel described herein, high-strength cold-rolled dual-phase strip steel having a homogeneous microstructure, good cold bending and hole expanding properties, and small anisotropy in mechanical properties is obtained by a suitable composition design and modified manufacturing steps without adding any difficulty to the procedures. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the as-cast microstructure of the 780 MPa cold-rolled dual-phase strip steel according to Example 3. 
         FIG. 2  shows the microstructure of the 780 MPa cold-rolled dual-phase strip steel according to Example 3. 
     
    
    
     DETAILED DESCRIPTION 
     The technical solution of the invention will be further demonstrated with reference to the following specific examples and accompanying drawings. 
     The 780 MPa cold-rolled dual-phase strip steel described herein was made according to the following steps:
         1) Smelting: the proportions of the chemical elements were controlled as shown in Table 1;   2) Casting: A secondary water-cooling process was used wherein the water jet capacity was not less than 0.7 L water/kg steel blank;   3) Hot rolling: The end rolling temperature was controlled to be 820-900° C., followed by rapid cooling after rolling;   4) Coiling: The coiling temperature was controlled to be 450-650° C.;   5) Cold rolling: The cold rolling reduction rate was 40-60%;   6) Continuous annealing: holding at 800-860° C., cooling to 640-700° C. at a cooling speed of not less than 5° C./s, further cooling to 220-280° C. at a cooling speed of 40-100° C./s, and tempering at 220-280° C. for 100-300 s;   7) temper rolling: The temper rolling elongation was 0.1-0.4% (this step was not performed in Example 1).       

     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Chemical elements (wt %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 C 
                 Si 
                 Mn 
                 Cr 
                 Mo 
                 Al 
                 Nb 
                 Ti 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Ex. 1 
                 0.06 
                 0.2 
                 2.3 
                 0.4 
                 0 
                 0.015 
                 0.02 
                 0.03 
               
               
                 Ex. 2 
                 0.07 
                 0.28 
                 1.8 
                 0.3 
                 0 
                 0.05 
                 0.03 
                 0.01 
               
               
                 Ex. 3 
                 0.08 
                 0.25 
                 1.9 
                 0.25 
                 0.05 
                 0.02 
                 0.025 
                 0.025 
               
               
                 Ex. 4 
                 0.09 
                 0.1 
                 2.1 
                 0.2 
                 0.1 
                 0.03 
                 0.02 
                 0.02 
               
               
                 Ex. 5 
                 0.1 
                 0.03 
                 2.0 
                 0.1 
                 0.2 
                 0.04 
                 0.015 
                 0.015 
               
               
                 Ex. 6 
                 0.085 
                 0.15 
                 2.2 
                 0.22 
                 0.08 
                 0.035 
                 0.01 
                 0.01 
               
               
                   
               
            
           
         
       
     
     Table 2 shows the specific process parameters of the examples. Examples 2-1 and 2-2 indicate that they both used the component proportions of Example 2 shown in Table 1, and Examples 5-1 and 5-2 indicate that they both used the component proportions of Example 5 shown in Table 1. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Continuous annealing 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Casting 
                   
                   
                 Inlet 
                 Outlet 
                   
                   
               
               
                   
                 Secondary 
                 Hot rolling 
                   
                 temperature 
                 temperature 
                 Rapid 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 cooling 
                 End 
                   
                   
                 Slow 
                 for 
                 for 
                 cooling 
                   
                   
                 Temper 
               
               
                   
                 water 
                 rolling 
                 Coiling 
                 Holding 
                 cooling 
                 rapid 
                 rapid 
                 speed 
                 Temper 
                   
                 rolling 
               
               
                   
                 capacity 
                 temperature 
                 temperature 
                 temperature 
                 speed 
                 cooling 
                 cooling 
                 (° C./ 
                 temperature 
                 Temper time 
                 elongation 
               
               
                 No. 
                 (L/kg) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (° C./s) 
                 (° C.) 
                 (° C.) 
                 s) 
                 (° C.) 
                 (s) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Ex. 1 
                 0.8 
                 830 
                 450 
                 805 
                 11 
                 690 
                 250 
                 100 
                 250 
                 250 
                 / 
               
               
                 Ex. 
                 0.85 
                 850 
                 500 
                 800 
                 10 
                 700 
                 280 
                 80 
                 270 
                 150 
                 0.2 
               
               
                 2-1 
               
               
                 Ex. 
                 0.9 
                 860 
                 550 
                 820 
                 9 
                 670 
                 260 
                 60 
                 260 
                 200 
                 0.3 
               
               
                 2-2 
               
               
                 Ex. 3 
                 0.95 
                 890 
                 600 
                 840 
                 6 
                 680 
                 240 
                 50 
                 240 
                 100 
                 0.4 
               
               
                 Ex. 4 
                 1 
                 840 
                 650 
                 860 
                 7 
                 660 
                 230 
                 40 
                 230 
                 300 
                 0.3 
               
               
                 Ex. 
                 0.82 
                 880 
                 610 
                 850 
                 5 
                 640 
                 220 
                 45 
                 220 
                 250 
                 0.2 
               
               
                 5-1 
               
               
                 Ex. 
                 0.87 
                 870 
                 520 
                 800 
                 10 
                 645 
                 280 
                 50 
                 280 
                 180 
                 0.3 
               
               
                 5-2 
               
               
                 Ex. 6 
                 0.93 
                 900 
                 570 
                 835 
                 8 
                 650 
                 270 
                 70 
                 240 
                 120 
                 0.1 
               
               
                   
               
            
           
         
       
     
     Table 3 shows the properties of the cold-rolled dual-phase steel of the examples according to the present technical solution. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Lateral sampling tensile 
                 Longitudinal sampling 
                 Lateral 
                 Longitudinal 
                 Hole 
               
               
                   
                 properties 
                 tensile properties 
                 bending 
                 bending 
                 expanding 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 σs 
                 σb 
                 δ 
                 σs 
                 σb 
                 δ 
                 (180° cold 
                 (180° cold 
                 ratio 
               
               
                 No. 
                 (Mpa) 
                 (Mpa) 
                 (%) 
                 (Mpa) 
                 (Mpa) 
                 (%) 
                 bending) 
                 bending) 
                 (%) 
               
               
                   
               
               
                 Ex. 1 
                 415 
                 790 
                 22 
                 420 
                 785 
                 23 
                 1a 
                 2a 
                 35 
               
               
                 Ex. 2-1 
                 420 
                 810 
                 22 
                 415 
                 815 
                 22 
                 1a 
                 2a 
                 34 
               
               
                 Ex. 2-2 
                 435 
                 820 
                 20 
                 430 
                 810 
                 20 
                 1a 
                 2a 
                 40 
               
               
                 Ex. 3 
                 450 
                 840 
                 19 
                 430 
                 845 
                 20 
                 1a 
                 2a 
                 50 
               
               
                 Ex. 4 
                 460 
                 840 
                 19 
                 450 
                 830 
                 19 
                 1a 
                 2a 
                 45 
               
               
                 Ex. 5-1 
                 470 
                 860 
                 18 
                 450 
                 855 
                 19 
                 1a 
                 2a 
                 55 
               
               
                 Ex. 5-2 
                 455 
                 830 
                 21 
                 440 
                 810 
                 20 
                 1a 
                 2a 
                 36 
               
               
                 Ex. 6 
                 485 
                 855 
                 19 
                 470 
                 845 
                 19 
                 1a 
                 2a 
                 51 
               
               
                   
               
            
           
         
       
     
     As shown in Table 3, the 780 MPa cold-rolled dual-phase strip steel described herein has high strength, good elongation, small anisotropy in mechanical properties, and can replace the 590 MPa cold-rolled dual-phase steel for use in the field of automobile manufacture. 
       FIG. 1  shows the as-cast microstructure of Example 3, and  FIG. 2  shows the microstructure of this example. As shown in  FIG. 1 , the as-cast structure of the cold-rolled dual-phase steel comprises cementite distributed dispersively on the ferrite grains. As shown in  FIG. 2 , the microstructure of the cold-rolled dual-phase steel comprises fine equiaxed ferrite matrix and martensite islands distributed homogeneously on the ferrite matrix, and the banded structure is minute. 
     An ordinary skilled person in the art would recognize that the above examples are only intended to illustrate the invention without limiting the invention in any way, and all changes and modifications to the above examples will fall in the scope of the claims of the invention so long as they are within the scope of the substantive spirit of the invention.