Abstract:
The present invention discloses a magnesium alloy sheet with low Gd content and high ductility and its hot rolling technology, which belongs to the field of metal material technology. The chemical components of the magnesium alloy sheet, based on the mass percent, take up respectively: 0.9˜2.1% as Zn, 0.2˜0.8% as rare earth element, namely Gd, 0˜0.9% as Mn, and the rest as Mg. The magnesium alloy sheet of the present invention is added with relatively lower rare earth element, Gd, which reduces the alloy costs; in addition, magnesium alloy has good rolling performance, which can realize continuous, multi-pass and large-deformation rolling, and also ensure the sheets rolled have non-basal texture and high room-temperature elongation which reaches 35˜50%, wherein the elongation, δ, in the rolling direction is no less than 35% and that in the horizontal direction no less than 45%.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of metal material technology, and more especially, to a magnesium alloy sheet with low Gd content, high ductility and the hot rolling technology thereof. 
       DESCRIPTION OF RELATED ART 
       [0002]    Boasting advantages such as small density, high specific strength, high damping, high thermal conductivity, good vibration reducing performance and convenience for recycling, magnesium alloy becomes more and more popular in the market. Currently, die-cast magnesium alloy has been largely applied to various industrial areas, like automobile and 3C electronic product shell. Subsequent surface treatment technologies of die-cast magnesium alloy are tedious, complex and easy to pollute environment, thus the industrial circle hopes to adopt stamping, press forging and other secondary processing methods which have higher productivity to form magnesium alloy sheets directly into automobiles and 3C electronic product shells. 
         [0003]    However, due to poor rolling performance, long rolling process and low rolling yield of AZ31 magnesium alloy used in the existing industry, as well as low room-temperature plasticity (15˜20% generally), high anisotropy and small strain hardening factor of sheets, the ability of secondary plastic processing and forming under room and low temperatures is insufficient and secondary forming becomes difficult, thus, secondary processing, usually, can be conducted only under high or medium temperature, which causes low productivity, high production and application costs in sheet application. Therefore, one of the key points in current research and development of magnesium alloy sheets is to develop low-lost magnesium alloy sheets with high ductility and adapting to room temperature forming as well as the efficient rolling technology, which is of significance to expand the application scale of magnesium alloy sheets. 
         [0004]    One of the most economical and efficient methods of preparing magnesium alloy sheets is hot rolling, by which it is realizable, not only to produce wide sheets, but also to adjust grain size, structure and texture distribution through repeated rolling and heat treatment, thus achieving various specifications of sheets with excellent mechanical properties. However, the existing commercial magnesium alloy sheets, such as AZ31, show strong basal texture and microstructure and property anisotropy after rolling process, which constitutes the main cause of disadvantages, such as high flow stress and poor plastic flow stability, during subsequent secondary plastic processing, as well as inability of conducting secondary plastic processing and forming under room and low temperatures; besides, due to strong tension-compression asymmetry of the strength and plasticity of magnesium alloy sheets caused by anisotropy, cracks may occur on the compression side thereof in the course of bending under room and low temperatures, thus making the formed parts scrapped. According to researches, those magnesium alloy sheets with weaker basal texture have higher strain hardening rate (index) under medium and low temperatures, thus ensuring the stability of plastic flow to achieve higher plasticity. Therefore, it is acceptable to enhance the forming performance of magnesium alloy by optimizing sheet texture, wherein the weaker the basal texture component of the texture is, the lower the sheet forming temperature will be and the better the forming performance will become too. 
         [0005]    Texture weakening of magnesium alloy pertains to second phase, solid solution atoms and lattice constant change, etc., among which the solid solution atom is the key factor influencing the texture. Adding a small quantity of rare earth elements to magnesium alloy will randomize the orientation of dynamic recrystallized grains during deformation, thus forming non-basal texture. The texture adjustment by alloying trace quantities of rare earth has positive significance in developing magnesium alloy sheets with high plasticity. According to the phase equilibrium thermodynamics principle and phase diagram of magnesium alloy, assume that structure containing fine second phase particles to be obtained by adding rare earth elements, such as Y, Nd and Gd, and magnesium alloy sheets with weak basal texture to be formed by annealing after rolling, so as to reduce sheets&#39; anisotropy, high stress hardening index, tension-compression asymmetry, ensure plastic flow stability during secondary processing and enhance sheets&#39; plasticity and secondary forming performance. 
         [0006]    Therefore, preparing magnesium alloy sheets with low anisotropy, weak texture, high stress hardening index and high plasticity by designing and optimizing Mg—RE alloy components, adopting traditional technologies like hot rolling and heat treatment to refine grains, obtain even structure and adjust texture according to the rules of the influence of rare earth elements over magnesium alloy structure, texture and performance becomes one of the research and development emphases in magnesium alloy material field. 
         [0007]    In addition, roller heating technology is important for industrial continuous rolling of magnesium alloy sheets in the future, by which it is possible to ensure billet temperature during rolling, realize multi-pass continuous rolling, reduce annealing times and improve production efficiency. Research shows that roller has the least influence upon the final structure, texture and mechanical properties of the sheets of the present invention when under a heating temperature of 25-400° C., which can ensure the characteristics of the sheets of the present invention. 
         [0008]    Thus, this invention application hopes to prepare a magnesium alloy sheet with non-basal texture and high ductility under room temperature via common rolling and heat treatment technologies by utilizing the unique effect of rare earth elements in magnesium alloy. Al and Zn are main alloying elements in magnesium alloy, but due to Al&#39;s strong adhesion to rare earth, Al—RE phase will be easily formed, which reduces the content of solid solution rare earth atoms in the matrix, thus presenting unobvious adjustment effect; so, select Zn as the second alloying element in addition to rare earth. As a micro-alloying element commonly used in magnesium alloy, Mn can not only enhance alloy corrosion resistance, but also inhibit the growth of crystallized grains, besides, will not affect texture adjustment effect, for which an appropriate amount of Mn must be added to alloy. 
         [0009]    Through literature retrieval, it is discovered that two patents at present relate to relevant technologies of the present invention patent: Shanghai Jiao Tong University disclosed an authigenic quasicrystal phase-reinforced high-plastic wrought magnesium alloy (patent application number: 200610026842.X), with the component and weight percentages being: Zn of 3-7%, Gd of 0.5-3% and Zr of 0-0.5%, wherein the tensile strength under room temperature of the extruded alloy with the components is 260-320 Mpa; the tensile elongation is 20-26%. Xi&#39;an Jiaotong University reported an in-situ synthesized quasicrystal phase-reinforced high-strength magnesium alloy (publication number: CN1789458A), with the component and weight percentages being: Zn of 3-10%, Y of 0.5-3.5%, Ce of 1% and Nd of 0-1%, wherein after rapid solidification and reciprocating large plastic extrusion, the tensile strength under room temperature is no less than 500 MPa; the elongation is no less than 20%. In the two inventions above, the contents of Zn in alloys are both no less than 3%, or even reach 10%. As we know, the increase of Zn content may form second phase with low melting point in the matrix, which not only leads to hot cracking during casting, but also cause poor rolling performance of magnesium alloy, narrow rolling temperature interval and small single-pass rolling deformation amount (less than 20% generally), thus the product productivity and yield are low. Usually, it is required to take the processing method of three-dimensional compressive stress, such as extrusion, which is not suitable for producing wide sheets, so the two patents adopt “extrusion process” and “rapid solidification+extrusion process” respectively. Meanwhile, materials of the two inventions have good strength, but with the plasticity being around 20%, they cannot meet the performance requirement for room temperature forming of magnesium alloy sheets. Thus, products of the two patents are indeed not appropriate for being sheet products with high ductility. In the magnesium alloy of the present invention application, Zn takes up no more than 2.1%, and the alloy has good rolling performance, with which single-pass rolling reduction can reach 50% or even more, so it is acceptable to produce wide magnesium alloy sheets efficiently in a short process via ordinary rolling method. 
         [0010]    Chongqing University once reported an Mg—Zn—Mn—Ce alloy which can realize rapid extrusion, with the component and weight percentages being: Zn of 1.8-4%, Mn of 0.5-1.5% and Ce of 0.15-0.80%, wherein the tensile strength of extruded magnesium alloy under room temperature is 285 Mpa; the tensile elongation is 20%. The patent prepares materials through extrusion process with rare earth, Ce, whose solid solubility is only 0.01wt % in magnesium, and by forming second-phase refined grains at grain boundary, prohibiting deformed grains from growing up and enhancing recrystallization temperature, high-temperature rapid extrusion can be achieved. The room-temperature elongation of extruded materials is no more than 20%. The patent takes Ce whose solid solubility is quite small to generate second phase at grain boundary and refine grains, so as to enhance strength. Since Ce does not weaken the texture of magnesium alloy, the enhancement of magnesium alloy plasticity will be unobvious. For the Mg—Zn—RE sheet of the present invention application, Gd with the solid solubility being 23.49wt % in magnesium is adopted and the weakening effect of Gd solid solution atoms Gd upon the texture of magnesium alloy is utilized, so as to enhance alloy rolling performance and change the grain orientation of magnesium alloy sheets after rolling to obtain non-basal texture, thus improving room-temperature plasticity and forming performance of rolled sheets. 
         [0011]    In the early period, the applicant has already developed a high-ductility Mg—Zn—RE magnesium alloy and its sheet rolling technology (application number: 200910011111), with the component and weight percentages being: Zn of 0-5%, RE of 0.1-10% and Mg of the balance amount. Due to wide range and high content of Zn and Gd, high-content Zn may reduce alloy plasticity while high content and high-cost Gd not only increases alloy costs, but also reduces the rolling performance and mechanical properties of alloy, thus limiting the industrial scale application of alloy. Thus, the present invention application hopes to further optimize and concretize the above patent. By comparing the weakening effects of rare earth elements, like Y and Gd, over texture, Gd is found to work better than Y, so it is selected from RE as alloying element, and at the same time, it is hoped to minimize Gd content on the basis of ensuring Gd&#39;s weakening of rolled sheet texture and enhancement of the room-temperature forming performance, thus reducing the costs of the alloy and sheet products of the original patent. The present invention defines the scope of the lowest effective content of Gd for non-basal texture, which greatly reduces alloy costs and can meet the requirement of civil products-used magnesium alloy for low cost, and meanwhile, re-designs and re-optimizes the chemical components of Mg—Zn—Gd (—Mn) alloy based on Mn&#39;s favourable role in enhancing magnesium alloy corrosion resistance and inhibiting grain growth and on the premise of not affecting Gd&#39;s ability of texture weakening, which is an effective improvement and optimization of the previous patent of the applicant. 
       SUMMARY OF THE INVENTION 
       [0012]    Aiming at poor rolling performance, strong sheet basal texture, poor room-temperature ductility, high anisotropy and low strain hardening index of current commercial magnesium alloy, such as AZ31 alloy, as well as the problem that high content of rare earth in some alloy leads to overly high alloy costs, the present invention provides a new magnesium alloy sheet with low Gd content, high ductility, good room-temperature ductility and forming performance as well as its hot rolling technology, whose principle is to fully utilize the weakening effect of trace quantities of Gd solid solution atom upon magnesium alloy texture during rolling to determine the lowest effective content of Gd for texture weakening and thus reduce alloy costs. The prepared magnesium alloy sheets have non-basal texture with the room-temperature elongation being 35˜50%, wherein the elongation in the rolling direction is no less than 35% and that in the horizontal direction no less than 45%. 
         [0013]    The technical solution of this invention is as follows: 
         [0014]    A magnesium alloy sheet with low Gd content and high ductility, wherein the magnesium alloy belongs to Mg—Zn—Gd series and calculated as per mass percent, with chemical components: 0.9˜2.1% as Zn, 0.2˜0.8% as rare earth, Gd, 0˜0.9% as Mn and the balance amount as Mg. 
         [0015]    The hot rolling technology of the above-mentioned magnesium alloy sheet with low Gd content and high ductility includes the following steps: 
         [0016]    1) Homogenization treatment of ingots: Maintain the temperature of the ingots of magnesium alloy with the chemical components for 0˜120 hours under 300˜525° C.; the ingots, round or square, are manufactured with metal mold and sand gravity casting or with semi-continuous casting method; 
         [0017]    2) Hot rolling of ingots: Rolling temperature: 250˜525° C. (roller preheating temperature: room temperature to 400° C.); rolling reduction of each pass: 35˜50%; return the ingots back to the furnace every time after rolling of 1˜5 passes, heat them up to the rolling temperature and keep the temperature for 10˜60 minutes before further rolling, with the total rolling reduction being 80˜95%; 
         [0018]    3) Annealing of rolled sheets: the rolled sheets are annealed for 0.5˜120 hours under 250˜500° C. 
         [0019]    The present invention has the following advantages: 
         [0020]    1. In the magnesium alloy of the present invention, the content of rare earth, Gd, is very low, only 0.2-0.8%, which reduces alloy costs on the basis of ensuring texture weakening and room-temperature plasticity, and excludes costly Zr, thus enabling enterprises to accept the alloy costs. 
         [0021]    2. In the present invention, the alloy has good rolling performance and the rolling deformation amount of each pass can reach 50% or above, which reduces the frequency and time of reheating during rolling, shortens the technological process and enhances the productivity, thus achieving high product yield and reducing the total costs of products; it is acceptable to adopt the existing rolling equipment and technology, featuring simple technology and easy control, for industrial continuous production. 
         [0022]    3. The sheet technically prepared for the present invention has non-basal texture, low anisotropy and high strain hardening rate with the room-temperature elongation being 35˜50%, which can realize room-temperature secondary forming of sheets, reduce costs of secondary plastic forming and enhance productivity, for which the sheet will be extensively applied to fields like electronic product shell and automobile. 
         [0023]    4. The alloy of the present invention not only applies to rolled sheets, but also can be popularized and applied to the production of profiles, tubes and pipes, free forgings and die forgings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1 , (a)-(b) are the microphotographs of the rolled sheet of magnesium alloy; wherein: (a) refers to the Mg-2.0Zn-0.2Gd-0.8Mn alloy of Example 1; (b) refers to the Mg-1.8Zn-0.4Gd alloy of Example 2; (c) refers to the Mg-3.1Zn-0.9Gd alloy of Comparative Example 2; (d) refers to the Mg-1.2Zn-4.9Gd alloy of Comparative Example 3. 
           [0025]      FIG. 2 , (a)-(d) are the structures of the rolled sheet of magnesium alloy; wherein: (a) refers to the Mg-2.0Zn-0.2Gd-0.8Mn alloy of Example 1; (b) refers to the Mg-1.8Zn-0.4Gd alloy of Example 2; (c) refers to the Mg-1.9Zn-0.6Gd alloy of Example 3; (d) refers to the Mg-0.9Zn-0.7Gd-0.6Mn alloy of Example 4; (e) refers to the Mg-1.8Zn-0.1Gd alloy of Comparative Example 1. 
           [0026]      FIG. 3 , (a)-(d) are the structures of the rolled sheet of magnesium alloy annealed under different temperatures; wherein, (a) shows the Mg-2.0Zn-0.2Gd-0.8Mn sheet annealed for 2 hours under 250° C. of Example 1; (b) shows the Mg-1.8Zn-0.4Gd sheet annealed for 3 hours under 325° C. of Example 2; (c) shows the Mg-1.9Zn-0.6Gd sheet annealed for 1 hour under 350° C. of Example 3; (d) shows the Mg-0.9Zn-0.7Gd-0.6Mn sheet annealed for 0.5 hours under 400° C. of Example 4; (e) shows the Mg-1.8Zn-0.1Gd alloy sheet annealed for 1 hour under 400° C. of Comparative Example 1. 
           [0027]      FIG. 4  shows the basal (0002) structures of the rolled sheet after annealing; wherein, (a) shows the Mg-2.0Zn-0.2Gd-0.8Mn sheet annealed for 2 hours under 250° C. of Example 1 (texture strength levels: 1.07, 1.23, 1.41, 1.62, 1.86, 2.14, 2.46 and 2.82); (b) shows the Mg-1.8Zn-0.4Gd sheet annealed for 3 hours under 325° C. of Example 2 (texture strength levels: 1.08, 1.26, 1.47, 1.71, 1.86, 2.14, 2.46 and 2.82); (c) shows the Mg-1.9Zn-0.6Gd sheet annealed for 1 hour under 350° C. of Example 3 (texture strength levels: 1.09, 1.28, 1.50, 1.77, 2.08, 2.45, 2.89 and 3.40); (d) shows the Mg-0.9Zn-0.7Gd-0.6Mn sheet annealed for 0.5 hours under 400° C. of Example 4 (texture strength levels: 1.08, 1.27, 1.48, 1.74, 2.03, 2.38, 2.79 and 3.27); (e) shows the Mg-1.8Zn-0.1Gd alloy sheet annealed for 1 hour under 400° C. of Comparative Example 1 (texture strength levels: 1.1, 2.0, 3.3, 5.0, 6.9, 8.5, 10.1 and 12.4). 
           [0028]      FIG. 5  shows the tensile stress-strain curves of the rolled sheet after annealing; wherein, (a) shows the Mg-2.0Zn-0.2Gd-0.8Mn sheet annealed for 2 hours under 250° C. of Example 1; (b) shows the Mg-1.8Zn-0.4Gd sheet annealed for 3 hours under 325° C. of Example 2; (c) shows the Mg-1.9Zn-0.6Gd sheet annealed for 1 hour under 305° C. of Example 3; (d) shows the Mg-0.9Zn-0.7Gd-0.6Mn sheet annealed for 0.5 hours under 400° C. of Example 4; (e) shows the Mg-1.8Zn-0.1Gd alloy sheet annealed for 1 hour under 400° C. of Comparative Example 1. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0029]    The present invention is further detailed in combination with the figures and the examples. It must be emphasized that, the following examples are for explaining the present invention, other than limiting the present invention. Table 1 shows the chemical composition of the Mg—Zn—Gd alloys of examples 1-4 of the present invention (the data herein are the results of chemical analysis and based on mass percent), wherein the formulas are only partial components within the protective scope. Table 2 shows the chemical composition of the Mg—Zn—Gd alloys of comparative examples 1-3 (based on mass percent). 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Chemical Composition of the Mg—Zn—Gd Alloys of Examples 1-4 
               
             
          
           
               
                   
                 No. 
                 Zn 
                 Gd 
                 Mn 
                 Mg 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 2.0 
                 0.2 
                 0.8 
                 Residual 
               
               
                   
                 2 
                 1.8 
                 0.4 
                 0 
                 Residual 
               
               
                   
                 3 
                 1.9 
                 0.6 
                 0 
                 Residual 
               
               
                   
                 4 
                 0.9 
                 0.7 
                 0.6 
                 Residual 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Chemical Composition of the Mg—Zn—Gd Alloys of Comparative 
               
               
                 Examples 1-3 
               
             
          
           
               
                   
                 No. 
                 Zn 
                 Gd 
                 Mn 
                 Mg 
               
               
                   
                   
               
               
                   
                 1 
                 1.8 
                 0.1 
                 0 
                 Residual 
               
               
                   
                 2 
                 3.1 
                 0.9 
                 0 
                 Residual 
               
               
                   
                 3 
                 1.2 
                 4.9 
                 0 
                 Residual 
               
               
                   
                   
               
             
          
         
       
     
       EXAMPLE 1 
       [0030]    1) A 150 mm×200 mm×200 mm ingot poured into by metal mold gravity casting and conventional magnesium alloy smelting, with the mass percentages of alloy components as below: Zn: 2.0%, Gd: 0.2%, Mn: 0.8% and Mg: balance amount (1 of Table 1); 
         [0031]    2) After maintaining the temperature of the ingot under 450° C. for 10 hours and conducting homogenization treatment, cut the ingot into 150 mm×100 mm×20 mm billet and mill the face, keep the temperature of the milled billet for 2 hours under 250° C. and roll it; the roller temperature is 300° C.; the rolling reduction of the first pass is 35%, afterwards, the rolling reduction of each pass is 35-45%; return the ingot back to the furnace every time after rolling of 1 pass and keep the temperature for 5˜10 minutes before further rolling, until the sheet has a thickness of 2 mm and a total rolling reduction of 85%, wherein, the edges and surface of the sheet have no cracks, see (a) of  FIG. 1 , besides, dynamic recrystallization occurs in the course of rolling and the sheet has fine grain size, see (a) of  FIG. 2 ; 
         [0032]    3) After 2 hours of annealing under 250° C., the rolled sheet achieves more even equiaxed grain structure, see (a) of  FIG. 3 . The annealed sheet has non-basal texture and presents a double peak texture deviating by □40□ laterally, as shown in (a) of  FIG. 4 , which contributes to enhancing sheet plasticity. 
         [0033]    4) See §3.6.2 of Chinese standard GB 6397-86 for the tensile mechanical performance samples of sheet samples required to be prepared, (a) of  FIG. 5  for the stress-strain curves in the rolling and horizontal directions under room temperature, and Table 3 for the mechanical properties of the sheet after annealing from heat treatment. Of the rolled sheet, in the rolling direction, the tensile strength is 246 Mpa, the yield strength is 165 Mpa and the elongation 39%; in the horizontal direction, the tensile strength is 229 Mpa, the yield strength is 101 Mpa and elongation 45%. 
       EXAMPLE 2 
       [0034]    1) A 150 mm×200 mm×200 mm ingot poured into by metal mold gravity casting and conventional Mg—Zn—Gd magnesium alloy smelting, with the mass percentages of alloy components as below: Zn: 1.8%, Gd: 0.4% and Mg: balance amount (2 of Table 1); 
         [0035]    2) After maintaining the temperature of the ingot under 420° C. for 10 hours and conducting homogenization treatment, cut the ingot into 150 mm×100 mm×20 mm billet and mill the face, keep the temperature of the milled billet for 2 hours under 400° C. and roll it; the roller temperature is the room temperature; the rolling reduction of the first pass is 35%, afterwards, the rolling reduction of each pass is 45%; return the ingot back to the furnace every time after rolling of 2 passes and keep the temperature for 5˜10 minutes before further rolling, until the sheet has a thickness of 3.2 mm and a total rolling reduction of 84%, wherein, the edges and surface of the sheet have no cracks, see (b) of  FIG. 1 , besides, dynamic recrystallization occurs in the course of rolling and the sheet has fine grain size, see (b) of  FIG. 2 ; 
         [0036]    3) After 3 hours of annealing under 320° C., the rolled sheet exhibits static recrystallization and achieves evener structure, see (b) of  FIG. 3 . The annealed sheet has non-basal texture and presents a double peak texture deviating by 40° laterally, as shown in (b) of  FIG. 4 , which contributes to enhancing sheet plasticity. 
         [0037]    4) See §3.6.2 of Chinese standard GB 6397-86 for the tensile mechanical performance samples of sheet samples required to be prepared, (b) of  FIG. 5  for the stress-strain curves in the rolling and horizontal directions under room temperature, and Table 3 for the mechanical properties. Of the rolled sheet, in the rolling direction, the tensile strength is 253 Mpa, the yield strength is 201 Mpa and the elongation 35%; in the horizontal direction, the tensile strength is 231 Mpa, the yield strength is 132 Mpa and elongation 50%. 
       EXAMPLE 3 
       [0038]    1) A 150 mm×200 mm×200 mm ingot poured into by metal mold gravity casting and conventional Mg—Zn—Gd magnesium alloy smelting, with the mass percentages of alloy components as below: Zn: 1.9%, Gd: 0.6% and Mg: balance amount (3 of Table 1); 
         [0039]    2) Cut the ingot into 150 mm×100 mm×20 mm billet and mill the face without homogenization heat treatment, keep the temperature of the billet directly under 380° C. for rolling; the roller temperature is 250° C.; the rolling reduction of the first pass is 35%, subsequently, the rolling reduction of each pass is 35-45%; return the ingot back to the furnace every time after rolling of 2 passes and maintain the temperature for 5˜10 minutes before further rolling, until the sheet has a thickness of 3 mm and a total rolling reduction of 85%, wherein, the edges and surface of the sheet have no cracks; in addition, the sheet appears deformation structure and not fully recrystallized, see (c) of  FIG. 2 ; 
         [0040]    3) The rolled sheet is annealed after 1 hour of heat insulation under 350° C., with obvious static recrystallization generated and even equiaxed grain structure obtained, see (c) of  FIG. 3 . The annealed sheet has non-basal texture and presents a double peak texture deviating by □40□ laterally, as shown in (c) of  FIG. 4 , which contributes to enhancing sheet plasticity; 
         [0041]    4) See §3.6.2 of Chinese standard GB 6397-86 for the tensile mechanical performance samples of sheet samples required to be prepared, (c) of  FIG. 5  for the stress-strain curves in the rolling and horizontal directions under room temperature, and Table 3 for the mechanical properties. Of the rolled sheet, in the rolling direction, the tensile strength is 248 Mpa, the yield strength is 172 Mpa and the elongation 37%; in the horizontal direction, the tensile strength is 237 Mpa, the yield strength is 165 Mpa and elongation 45%. 
       EXAMPLE 4 
       [0042]    1) A 150 mm×200 m×200 mm ingot poured into by metal mold gravity casting and conventional Mg—Zn—Gd magnesium alloy smelting, with the mass percentages of alloy components as below: Zn: 0.9%, Gd: 0.7%, Mn: 0.6 and Mg: balance amount (4 of Table 1); 
         [0043]    2) Cut the ingot into 150 mm×100 mm×20 mm billet and mill the face without homogenization heat treatment, keep the temperature of the billet directly under 320° C. for rolling; the roller temperature is the room temperature; the rolling reduction of the first pass is 35%, subsequently, the rolling reduction of each pass is 45%; return the ingot back to the furnace every time after rolling of 2 passes and maintain the temperature for 5˜10 minutes before further rolling, until the sheet has a thickness of 2 mm and a total rolling reduction of 85%, wherein, the edges and surface of the sheet have no cracks, and the sheet appears recrystallization structure, see (d) of  FIG. 2 ; 
         [0044]    3) The rolled sheet is annealed after 0.5 hours of heat insulation under 400° C., with evener equiaxed grain structure obtained, see (d) of  FIG. 3 . The annealed sheet has non-basal texture and presents a double peak texture deviating by □40□ laterally, as shown in (d) of  FIG. 4 , which contributes to enhancing sheet plasticity. 
         [0045]    4) See §3.6.2 of Chinese standard GB 6397-86 for the tensile mechanical performance samples of sheet samples required to be prepared, (d) of  FIG. 5  for the stress-strain curves in the rolling and horizontal directions under room temperature, and Table 3 for the mechanical properties. Of the rolled sheet, in the rolling direction, the tensile strength is 248 Mpa, the yield strength is 172 Mpa and the elongation 37%; in the horizontal direction, the tensile strength is 237 Mpa, the yield strength is 165 Mpa and elongation 45%. 
       COMPARATIVE EXAMPLE 1 
       [0046]    1) A 150 mm×200 mm×200 mm ingot poured into by metal mold gravity casting and conventional magnesium alloy smelting, with the mass percentages of alloy components as below: Zn: 1.8%, Gd: 0.1% and Mg: balance amount (1 of Table 2); 
         [0047]    2) After maintaining the temperature of the ingot under 450° C. for 10 hours and conducting homogenization treatment, cut the ingot into 150 mm×100 mm×20 mm billet and mill the face, keep the temperature of the milled billet for 2 hours under 250° C. and roll it; the roller temperature is 25° C.; the rolling reduction of the first pass is 30%, afterwards, the rolling reduction of each pass is 30-45%; return the ingot back to the furnace every time after rolling of 1 pass and keep the temperature for 5˜10 minutes before further rolling, until the sheet has a width of 2 mm and a total rolling reduction of 85%, wherein, the edges and surface of the sheet have no cracks; in addition, dynamic recrystallization occurs in the course of rolling and the sheet has bigger grain size than the alloys of other numbers, see (e) of  FIG. 2 ; 
         [0048]    3) After 1 hour of annealing under 400° C., the grains of the rolled sheet grow up obviously and are close to 50 μm, see (e) of  FIG. 3 . The annealed sheet has obvious basal texture, which is different from the non-basal texture of other alloy sheets. As shown in (e) of  FIG. 4 , this type of texture is adverse to sheet plasticity enhancement. 
         [0049]    4) See §3.6.2 of Chinese standard GB 6397-86 for the tensile mechanical performance samples of sheet samples required to be prepared, (e) of  FIG. 5  for the stress-strain curves in the rolling and horizontal directions under room temperature, and Table 3 for the mechanical properties of the sheet after annealing from heat treatment. Of the rolled sheet, in the rolling direction, the tensile strength is 248 Mpa, the yield strength is 220 Mpa and the elongation 25%; in the horizontal direction, the tensile strength is 251 Mpa, the yield strength is 218 Mpa and elongation 32%. 
       COMPARATIVE EXAMPLE 2 
       [0050]    1) A 150 mm×200 mm×200 mm ingot poured into by metal mold gravity casting and conventional magnesium alloy smelting, with the mass percentages of alloy components as below: Zn: 3.1%, Gd: 0.9% and Mg: balance amount (2 of Table 2); 
         [0051]    2) After maintaining the temperature of the ingot under 420° C. for 5 hours and conducting homogenization treatment, cut the ingot into 150 mm×100 mm×20 mm billet and mill the face, keep the temperature of the milled billet for 2 hours under 250° C. and roll it; the roller temperature is 25° C.; the rolling reduction of the first pass is 30%, afterwards, the rolling reduction of each pass is 30-45%; return the ingot back to the furnace every time after rolling of 1 pass and keep the temperature for 5˜10 minutes before further rolling, until the sheet has a thickness of 2 mm, wherein, the sheet has serious cracks on both sides, see (c) of  FIG. 1 , besides, the rolling performance is relatively poor and the sheet yield low. 
       COMPARATIVE EXAMPLE 3 
       [0052]    1) A 150 mm×200 mm×200 mm ingot poured into by metal mold gravity casting and conventional magnesium alloy smelting, with the mass percentages of alloy components as below: Zn: 1.2%, Gd: 4.9% and Mg: balance amount (3 of Table 2); 
         [0053]    2) After maintaining the temperature of the ingot under 470° C. for 8 hours and conducting homogenization treatment, cut the ingot into 150 mm×100 mm×20 mm billet and mill the face, keep the temperature of the milled billet for 2 hours under 250° C. and roll it; the roller temperature is 25° C.; the rolling reduction of the first pass is 30%, afterwards, the rolling reduction of each pass is 30-45%; return the ingot back to the furnace every time after rolling of 1 pass and keep the temperature for 5˜10 minutes before further rolling, wherein, the sheet is seriously cracked and there is no guarantee for rolling the sheet provided with greater deformation amount, see (d) of  FIG. 1 . 
         [0054]    See Table 3 for the mechanical properties of the sheet in the rolling and horizontal directions after Mg—Zn—Gd rolling and annealing in the above examples and comparative examples. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Yield strength 
                 Tensile strength 
                 Elongation 
               
               
                 Alloy 
                 Direction 
                 (MPa) 
                 (MPa) 
                 (%) 
               
               
                   
               
             
             
               
                 Example 1 
                 Rolling 
                 165 
                 246 
                 39 
               
               
                   
                 Horizontal 
                 101 
                 229 
                 45 
               
               
                 Example 2 
                 Rolling 
                 201 
                 253 
                 35 
               
               
                   
                 Horizontal 
                 132 
                 231 
                 50 
               
               
                 Example 3 
                 Rolling 
                 172 
                 248 
                 37 
               
               
                   
                 Horizontal 
                 165 
                 237 
                 45 
               
               
                 Example 4 
                 Rolling 
                 164 
                 253 
                 40 
               
               
                   
                 Horizontal 
                 120 
                 243 
                 49 
               
               
                 Comparative 
                 Rolling 
                 220 
                 248 
                 25 
               
               
                 Example 1 
                 Horizontal 
                 218 
                 251 
                 32