Patent Publication Number: US-2016237518-A1

Title: High tensile strength steel wire

Description:
TECHNICAL FIELD 
     The present invention relates to a high tensile strength steel wire, to a process for manufacturing a high tensile strength steel wire and to the uses or applications of such a high tensile strength steel wire as spring wire or an element for producing a rope. 
     BACKGROUND ART 
     Springs are usually made from alloys of steel. The most common spring steels are music wire, oil tempered wire, chrome silicon, chrome vanadium, and 302 and 17-7 stainless. Spring wires made of chrome silicon or chrome vanadium are higher quality, higher strength versions of oil tempered wire. 
     Spring steel used in applications such as automotive valve springs is in general required to have a very high tensile and yield strength. Tensile strength is a material&#39;s ability to resist forces that attempt to pull apart or stretch it. Tensile strength is an important property for wires for spring applications. For instance, extension springs operating above their tensile strength will break. 
     In general, when producing small sized, high strength springs, drawn steel wire for high strength spring use is quenched and tempered to impart higher material strength in the drawn steel wire, and then is cold coiled to obtain a coil spring shape. For this reason, first drawn then heat treated steel wire for high strength spring use is required to have not only high strength, but also to have a high enough workability that it will not break at the cold coiling. 
     The springs, in particular which are used for automobile engines, clutches, etc. are being required to offer more advanced performance in order to deal with the trend toward lighter weights and higher performances of automobiles. For this reason, steel wires with higher strength and higher durability are desired for springs. A major trend for improving properties is to adjust the composition of steels for spring wires. An example is disclosed in US 2012/0291927 A1, wherein the contents of C, Si, Mn and Cr in the steel wire are proposed to be strictly controlled and in the meantime both Cr and Si in the steel wire are set at a suitable amount. It has been found, nevertheless, that increasing mechanical strength beyond certain limits causes such steels to have inadequate ductility, taking into account the pre-shaping and bending operations that have to be carried out with the spring wire. A lot of effort has been done on the improvement of steel wires to have higher tensile strength and simultaneously have acceptable ductility. 
     DISCLOSURE OF INVENTION 
     It is an object of the present invention to provide a high tensile strength steel wire with an acceptable ductility. 
     It is another object of the present invention to provide a high tensile strength steel wire suitable to be used as spring wires. 
     It is still another object of the present invention to provide suitable process to manufacture a high tensile strength steel wire with an acceptable ductility. 
     The present invention describes a steel wire having very high strength and ductility thanks to the oriented martensitic microstructure, and a method to produce such a steel wire in a continuous process. 
     According to a first aspect of the present invention, there is provided a high tensile strength steel wire with following steel composition:
         a carbon content ranging from 0.20 weight percent to 1.00 weight percent, e.g. from 0.3 weight percent to 0.85 weight percent, e.g. from 0.4 weight percent to 0.7 weight percent, e.g. from 0.5 weight percent to 0.6 weight percent,   a silicon content ranging from 0.05 weight percent to 2.0 weight percent, e.g. from 0.2 weight percent to 1.8 weight percent, e.g. from 1.2 weight percent to 1.6 weight percent,   a manganese content ranging from 0.40 weight percent to 1.0 weight percent, e.g. from 0.5 weight percent to 0.9 weight percent,   a chromium content ranging from 0.0 weight percent to 1.0 weight percent, e.g. from 0.5 weight percent to 0.8 weight percent,   a sulfur and phosphor content being individually limited to 0.05 weight percent, e.g. limited to 0.025 weight percent,   contents of nickel, vanadium, aluminum, copper or other micro-alloying elements all being individually limited to 0.5 weight percent, e.g. limited to 0.2 weight percent, e.g. limited to 0.08 weight percent,
 
the remainder being iron,
 
said steel wire having martensitic structure,
 
wherein at least 10 volume percent of martensite are oriented.
       

     Preferably, at least 20 volume percent of martensite are oriented. More preferably, at least 30 volume percent of martensite are oriented. Most preferably, at least 40 volume percent of martensite are oriented. 
     It is known that martensitic steel is a polycrystalline material. When the grains of polycrystalline material are randomly oriented, the polycrystalline material is not oriented or non-textured. Under specific conditions, the grains of polycrystalline material can be preferably oriented, and in this case the polycrystalline material is called to be “oriented” or “textured”. Two types of orientations are often confronted, i.e. “crystallographic orientation” and “microstructural orientation”. Crystallographic orientation means grains are crystallographically oriented, such as with preferred alignment or orientation of certain crystallographic planes or crystallographic directions. Preferred crystallographic orientation is usually determined from an analysis of the orientation dependence of the diffraction peak intensities (such as by X-Ray Diffraction (XRD) analysis or Electron Backscatter Diffraction (EBSD)) that have been measured in different spatial directions within the coordinate system of the sample. On the other hand, if the grains of polycrystalline material have morphologically anisotropic shape, the grains can also have “microstructural orientation” by such as uniaxial compression during formation of the polycrystalline. “Microstructural orientation” implies that the anisotropic shaped grains are morphologically oriented in preferred directions or planes. This can be detected by image analysis such as scanning electron microscope (SEM). Moreover, crystallographic orientation is often linked with microstructural orientation since the shape anisotropy of grains is often related to their crystallography. 
     Martensite occurs as lath- or plate-shaped crystal grains. When viewed in cross section, the lenticular (lens-shaped) crystal grains are sometimes described as Acicular (needle-shaped). According to the present application, in the produced martensitic steel wire, at least 10 volume percent of martensite are oriented. The term “oriented” means that the lenticular grains are either crystallographically oriented or microstructurally oriented, or oriented both crystallographically and microstructurally. 
     The volume percentage of the crystallographical alignment or orientation can be obtained by means of X-Ray Diffraction (XRD) analysis or Electron Backscatter Diffraction (EBSD). The volume percentage of the microstructural alignment or orientation can be evaluated by image analysis. 
     Herein, the term “oriented” does not only mean that the crystallographic axis or the axis of lenticular grains are exactly oriented at the same direction as illustrated by a 1  and a 2  in  FIG. 1 , but also refer to the orientation within a tolerance. When the directions of certain axes of grains (or certain crystallographic directions) are deviated, as presented by angle α in  FIG. 1 , within 20°, preferably within 10°, more preferably within 5°, these grains are also considered as oriented. 
     The alignment or orientation at least refers to one dimensional preferred orientation, e.g. in the direction perpendicular to the plane of lenticular grains (direction as shown by a 1 , a 2 , e.g. [001], in  FIG. 1 ). For one dimensional orientation, the lenticular grains are randomly distributed in the directions on the lenticular plane (directions as shown by a 4 , a 5 , in  FIG. 1 ). 
     Preferably, the steel wire according to the present application has a yield strength Rp0.2 which is at least 80 percent of the tensile strength Rm. Rp0.2 is the yield strength at 0.2% permanent elongation. More preferably, the yield to tensile ratio, i.e. Rp0.2/Rm, is between 80 percent to 95 percent. Therefore, the steel wire after elastic deformation can be still deformed to certain extent before breaking. 
     A steel wire according to the present application preferably has a corrosion resistance coating. More preferably, the steel wire has a corrosion resistance coating selected from any one of zinc, nickel, silver and copper, or their alloys. In such a case, the wires have a prolonged life time even in a harsh corrosive environment. 
     The steel wire according to the present application may be in a cold-drawn state and have a round cross-section. The steel wire may have a tensile strength Rm of at least 2000 MPa for wire diameter above 5.0 mm, at least 2100 MPa for wire diameter above 3.0 mm and at least 2200 MPa for wire diameters above 0.5 mm. Preferably, the steel wire has a reduction in area after fracture of at least 45% and more preferably of at least 50%. 
     Herein, the ductility of steel wires is obtained by a tensile test. The ductility of the steel wire is indicated by the reduction in area after fracture. The “reduction in area” is the difference between original cross sectional area of a specimen and the area of its smallest cross section after testing. It is usually expressed as % decrease in original cross section. The smallest cross section is measured after fracture for steel wires. 
     Wire drawing is a metal working process used to reduce the cross-section of a wire by pulling the wire through a single, or series of, drawing die(s). It is known that wire drawing increases the tensile strength Rm of the steel wire and meanwhile decreases the ductility. However, in comparison with traditional cold-drawn steel wires, the invention steel wire with specific composition has a comparative ductility and an extremely high tensile strength. 
     According to a second aspect of the present invention, the steel wire may be used as spring wire or an element for producing a rope. 
     According to a third aspect the present invention, there is provided a process of manufacturing a high tensile strength steel wire, said steel wire having as steel composition:
         a carbon content ranging from 0.20 weight percent to 1.00 weight percent, e.g. from 0.3 weight percent to 0.85 weight percent, e.g. from 0.4 weight percent to 0.7 weight percent, e.g. from 0.5 weight percent to 0.6 weight percent,   a silicon content ranging from 0.05 weight percent to 2.0 weight percent, e.g. from 0.2 weight percent to 1.8 weight percent, e.g. from 1.2 weight percent to 1.6 weight percent,   a manganese content ranging from 0.40 weight percent to 1.0 weight percent, e.g. from 0.5 weight percent to 0.9 weight percent,   a chromium content ranging from 0.0 weight percent to 1.0 weight percent, e.g. from 0.5 weight percent to 0.8 weight percent,   a sulfur and phosphor content being individually limited to 0.05 weight percent, e.g. limited to 0.025 weight percent,   contents of nickel, vanadium, aluminum, copper or other micro-alloying elements all being individually limited to 0.5 weight percent, e.g. limited to 0.2 weight percent, e.g. limited to 0.08 weight percent,
 
the remainder being iron,
 
said steel wire having martensitic structure,
 
wherein at least 10 volume percent of martensite are oriented.
 
said process comprising the following steps in order:
 
a) austenitizing a steel wire rod or steel wire above Ac3 temperature during a period less than 120 seconds,
 
b) quenching said austenitized steel wire rod or steel wire below 100° C. during a period less than 60 seconds,
 
c) tempering said quenched steel wire rod or steel wire between 320° C. and 500° C. during a period ranging from 10 seconds to 600 seconds,
 
d) work hardening said quenched and tempered steel wire rod or steel wire.
       

     In the prior art, such as in the disclosure of U.S. Pat. No. 5,922,149 A, the steel wire/wire rod was first deformed or work hardened to final dimension and thereafter quenched and tempered, as schematically shown in  FIG. 2 . In contradiction, in the present invention, the steel wire is first quenched to form martensitic microstructure. Tempering is followed thereafter. The tempered martensitic steel wire is then deformed or work hardened, e.g. by drawing, into final dimension, as schematically shown in  FIG. 3 . 
     Present invention receives unexpected technical results and advantages. Usually in wire processing quenching and tempering is the final step, and martensite has always been claimed as detrimental for drawing. The tensile strength of the martensitic wire according to the present invention is very high and the combination of the level of tensile strength with the high level of ductility is uncommon. The surprising result obtained by drawing the tempered martensite may be due to the special alloying of the steel (microalloyed with Cr and Si) versus conventional eutectoid steels. The synergy effect of the composition and the process of the present application results in a martensitic steel wire having a preferred martensite orientation. The orientation of martensite in the cold-drawn steel wire is the result of applied compression force via drawing on the quenched and tempered martensitic steel wires. 
     The process may further comprise a step of e) aging said work hardened steel wire at a temperature between 100° C. and 250° C. 
     Preferably, in the process said work hardening occurs at a temperature below 400° C. More preferably, said work hardening is cold drawing. Cold drawing has an added effect of work hardening and strengthening the material, and thus further improves the material&#39;s mechanical properties. It also improves the surface finish and holds tighter tolerances allowing desirable qualities that cannot be obtained by hot deformation. Alternatively, said work hardening is a warm drawing occurring between 200° C. and 700° C., e.g. 200° C. to 400° C. For a similar reduction, the application of warm drawing significantly reduces the passes and simplifies the process. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS 
       The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which: 
         FIG. 1  schematically shows grain alignment or orientation in poly-crystallographical materials. 
         FIG. 2  illustrates a thermo-mechanical process for steel wires according to the prior art. 
         FIG. 3  illustrates the thermo-mechanical process for steel wires according to the present invention. 
         FIG. 4  illustrates a temperature versus time curve for a thermal process according to the present invention. 
         FIG. 5  compares the strain hardening curves of prior art patented steel wire with the invention steel wire according to the first embodiment of the present invention. 
         FIG. 6  compares the tensile strength as a function of section reduction of three passes drawing process with six passes drawing process. 
         FIG. 7 ( a )  shows the scanning electron microstructure (SEM) of longitudinal cross-section of the steel wire according to the present invention while  FIG. 7 ( b )  shows the scanning electron microstructure of longitudinal cross-section of a reference steel wire at a same magnification. 
         FIG. 8 ( a )  shows the scanning electron microstructure (SEM) of longitudinal cross-section of the steel wire according to the present invention at a lower magnification while  FIG. 8 ( b )  shows the scanning electron microstructure (SEM) of longitudinal cross-section of a reference steel wire at a same magnification. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     Embodiment 1 
       FIG. 4  illustrates a suitable temperature versus time curve applied to a steel wire or wire rod with a diameter of 5.29 mm and with following steel composition:
         % wt C=0.55   % wt Si=1.4   % wt Cr=0.6   % wt Mn=0.7
 
the balance being iron and unavoidable impurities.
       

     The starting temperature of martensite transformation M s  of this steel is about 280° C. and the temperature M f , at which martensite formation ends is about 100° C. 
     The various steps of the process are as follows:
         a first austenitizing step ( 10 ) during which the steel wire stays in a furnace at about 950° C. during 120 seconds,   a second quenching step ( 12 ) for martensite transformation in oil at a temperature below 100° C. during at least 20 seconds;   a third tempering step ( 14 ) for increase the toughness at a temperature above 320° C. during less than 60 seconds; and   a fourth cooling step ( 16 ) at room temperature during 20 or more seconds.       

     Curve  18  is the temperature curve in the various equipment parts (furnace, bath . . . ) and curve  19  is the temperature of the steel wire. 
     The steel wire or wire rod after above thermal treatment mainly has martensitic microstructure. Since martensite is sensible to H-embrittlement, thermo-treated steel wire is cold drawn directly without pickling and oil can act as lubricant for the later drawing process. 
     The formed martensitic steel wire or wire rod is continued with a series of wire drawing process, e.g. of six passes. 
     The diameter, diameter reduction, section reduction, cumulative section reduction, tensile strength, tensile strength variation and reduction in area after each pass of the steel wire for this six passes process are summarized in table 1. Herein, the “diameter reduction” and “section reduction” are referred to the reduction after each pass of drawing. The “diameter reduction” implies the difference of the diameter of the steel wire before and after each pass and is expressed as % diameter decrease to its original diameter before passing the wire drawing dies. Similarly, the “section reduction” implies the difference of the cross-section areas of the steel wire before and after each pass and is expressed as % section decrease to its original section before passing the wire drawing dies. 
     As shown in table 1, the diameter reduction is about 5% for each pass. The tensile strength of the steel wire further increases by passing more passes. After being drawn in six passes, the steel wire has a diameter of 3.86 mm and a tensile strength of 2151 N/mm 2 . Over six passes, the yield strength Rp0.2 of the steel wire is at least 80 percent of the tensile strength Rm. In addition, the steel wire overall has sufficient ductility illustrated by the reduction in area being above 46.5% and the total elongation at fracture of the drawn wire being more than 2%. 
     A strain hardening curve of the cold drawn wire (Q&amp;T CrSi) according to the invention in comparison with a reference wire (R-SW) is shown in  FIG. 5 . The reference wire contains 0.8 wt % Carbon and is patented in lead. The reference wire has an initial diameter of 6.5 mm and a tensile strength of 1360 N/mm 2 . By replacing the patenting operation of the reference wire by quenching and tempering, fine tempered martensite can be obtained with tensile strength being at least 400 N/mm 2  higher than for a patented wire. The strain hardening curve of the cold drawn tempered martensitic wire (Q&amp;T CrSi) has a similar slope to that of a patented wire (R-SW). This means that both steel wires showed a comparable strength increase for a same or similar section reduction. For a same amount of section reduction, the invention wire will be at least 400 N/mm 2  stronger than a wire drawn after patenting. 
     This extremely high tensile strength of the invention wire may be attributed to the martensitic microstructure formation and in particular to the oriented certain percentage of martensitic grains, which are observed in image analysis, in the steel wires after deformation or work hardening. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Properties of a steel wire with an initial diameter of 5.29 mm 
               
               
                 drawn in six passes to a diameter of 3.86 mm. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Cumulative 
                   
                 Tensile 
                   
               
               
                   
                   
                 Diameter 
                 Section 
                 section 
                 Tensile 
                 Strength 
                 Reduction 
               
               
                   
                 Diameter 
                 reduction 
                 reduction 
                 reduction 
                 Strength 
                 Variation 
                 in area 
               
               
                 Pass 
                 (mm) 
                 (%) 
                 (%) 
                 (%) 
                 (N/mm 2 ) 
                 (N/mm 2 ) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 0 
                 5.29 
                 0 
                 0 
                 0.0 
                 1863 
                 0 
                 55.3 
               
               
                 1 
                 5.02 
                 5.2 
                 10.1 
                 10.1 
                 1935 
                 72 
                 50.1 
               
               
                 2 
                 4.72 
                 5.8 
                 11.3 
                 20.3 
                 1975 
                 112 
                 49.8 
               
               
                 3 
                 4.49 
                 5.0 
                 9.8 
                 28.1 
                 2024 
                 161 
                 50.7 
               
               
                 4 
                 4.27 
                 4.7 
                 9.3 
                 34.8 
                 2049 
                 186 
                 52.7 
               
               
                 5 
                 4.06 
                 5.1 
                 9.9 
                 41.2 
                 2115 
                 252 
                 46.5 
               
               
                 6 
                 3.86 
                 4.7 
                 9.2 
                 46.6 
                 2151 
                 288 
                 47.1 
               
               
                   
               
            
           
         
       
     
     Embodiment 2 
     In this embodiment, a similar thermal treatment of embodiment 1 was applied to a steel wire with a diameter of 3.75 mm and with the following steel composition:
         % wt C=0.55   % wt Si=1.4   % wt Cr=0.6   % wt Mn=0.7
 
the balance being iron and unavoidable impurities.
       

     The steel wire after thermal treatment mainly has martensitic microstructure. The steel wire further undergoes six passes drawing steps with a diameter reduction to 2.8 mm. The properties of the steel wire after each pass are shown in table 2. Although an extreme high tensile strength is obtained after six passes, the steel wire still has sufficient ductility as indicated by a reduction in area of 52.8%. Moreover, the ductility of the steel wire is ensured during the whole drawing process, which can be verified by the reductions in area of the steel wires after one to six passes all being above 52.8% as shown in table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Properties of a steel wire with an initial diameter of 3.75 mm 
               
               
                 drawn in six passes to a diameter of 2.8 mm. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Cumulative 
                   
                 Tensile 
                   
               
               
                   
                   
                 Diameter 
                 Section 
                 section 
                 Tensile 
                 Strength 
                 Reduction 
               
               
                   
                 Diameter 
                 reduction 
                 reduction 
                 reduction 
                 Strength 
                 Variation 
                 in area 
               
               
                 Pass 
                 (mm) 
                 (%) 
                 (%) 
                 (%) 
                 (N/mm 2 ) 
                 (N/mm 2 ) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 0 
                 3.75 
                 0 
                 0 
                 0 
                 1930 
                 0 
                 58.0 
               
               
                 1 
                 3.59 
                 4.3 
                 8.4 
                 8.4 
                 2010 
                 80 
                 57.2 
               
               
                 2 
                 3.35 
                 6.7 
                 12.9 
                 20.2 
                 2060 
                 130 
                 56.8 
               
               
                 3 
                 3.17 
                 5.4 
                 10.5 
                 28.5 
                 2065 
                 135 
                 57.2 
               
               
                 4 
                 3.02 
                 4.7 
                 9.2 
                 35.1 
                 2110 
                 180 
                 54.3 
               
               
                 5 
                 2.9 
                 4.0 
                 7.8 
                 40.2 
                 2180 
                 250 
                 53.1 
               
               
                 6 
                 2.8 
                 3.4 
                 6.8 
                 44.2 
                 2220 
                 290 
                 52.8 
               
               
                   
               
            
           
         
       
     
     Embodiment 3 
     Different from the samples of embodiment 2, in this example, after a similar thermal treatment, the martensitic steel wire with 3.75 mm diameter is drawn by three passes. 
     The diameter, diameter reduction, section reduction, cumulative section reduction, tensile strength, tensile strength variation and reduction in area after each pass of the steel wire drawn by this three passes process are summarized in table 3. 
     The average diameter reduction of each pass is about 9.5% for three passes process which is almost a double of that of six passes process as shown in embodiment 1 and 2. The tensile strength (Rm) of three passes drawn wire (SW3) as a function of section reduction (Δs) is plot in  FIG. 6  in comparison with the tensile strength of six passes drawn wires of embodiment 1 (SW1) and embodiment 2 (SW2). As shown in  FIG. 6 , the increase of tensile strength is almost proportional to the increase of section reduction for both the three and the six passes drawn steel wires. Compared with the wire undergone six passes process (SW1 and SW2), as shown in  FIG. 6  the slope of tensile strength trend curve of wire undergone three passes process (SW3) is slightly bigger, i.e. the tensile strength increases even higher for a similar section reduction. The wire undergone three passes shows an average strength increase of 8 N/mm 2  for 1% section reduction while the wire undergone six passes shows an average strength increase of 6 N/mm 2  for 1% section reduction. In addition, the steel wires drawn by three passes have even better ductility. The reductions in area of the steel wires after one to three passes are all above 53.6%. The drawn steel wire after three pass has excellent properties: tensile strength is 2300 N/mm 2  and reduction in area is 53.6%, which are exceeded the standard requirement for quenched and tempered spring wires. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Properties of a steel wire with an initial diameter of 3.75 mm 
               
               
                 drawn in three passes to a diameter of 2.8 mm. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Cumulative 
                   
                 Tensile 
                   
               
               
                   
                   
                 Diameter 
                 Section 
                 section 
                 Tensile 
                 Strength 
                 Reduction 
               
               
                   
                 Diameter 
                 reduction 
                 reduction 
                 reduction 
                 Strength 
                 Variation 
                 in area 
               
               
                 Pass 
                 (mm) 
                 (%) 
                 (%) 
                 (%) 
                 (N/mm 2 ) 
                 (N/mm 2 ) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 0 
                 3.75 
                 0 
                 0 
                 0 
                 1930 
                 0 
                 58.0 
               
               
                 1 
                 3.38 
                 9.9 
                 18.8 
                 18.8 
                 2080 
                 150 
                 57.2 
               
               
                 2 
                 3.07 
                 9.2 
                 17.5 
                 33.0 
                 2175 
                 245 
                 54.2 
               
               
                 3 
                 2.8 
                 8.8 
                 16.8 
                 44.2 
                 2300 
                 370 
                 53.6 
               
               
                   
               
            
           
         
       
     
     This very high tensile strength can be a consequence of oriented martensitic grains of the steel wires after drawing. The microstructure of the drawn steel wire according to the invention is investigated. Taken as a reference, is a steel wire treated by a traditional process, i.e. first drawn and then quenched and tempered as shown in  FIG. 2 . The composition, section reduction and the thermal treatment of the invention steel wire and reference steel wire are quite similar. 
     The microstructure of the longitudinal cross-section of the steel wire undergone three passes according to the present invention is shown in  FIG. 7 ( a )  while the microstructure of the longitudinal cross-section of the reference wire is shown in  FIG. 7( b ) . The longitudinal cross-section is a section in the longitudinal or lengthwise direction of the steel wire. As shown in  FIG. 7( b ) , the reference wire appears a homogeneous martensitic microstructure. The martensitic grains are randomly oriented over the whole area. In contrast, for the invention steel wire, it presents a martensitic microstructure and the martensitic grains are preferably oriented as shown in  FIG. 7( a ) . In this longitudinal cross-section view, the martensitic grains appear acicular (needle-shaped) and the long axis of acicular is aligned parallel to the drawing direction (a direction parallel to the scale bar in  FIG. 7 ). This indicates that the normal of the lenticular (lens-shaped) crystal grains is preferably oriented perpendicular to the drawing direction. 
       FIGS. 8 ( a ) and ( b )  are respectively a microstructure of the longitudinal cross-section of an invention steel wire and a reference wire at lower magnification. It confirms an oriented martensitic microstructure ( FIG. 8 ( a ) ) of the steel wire according to the present invention vs. a randomly distributed martensitic microstructure of the reference wire ( FIG. 8( b ) ). 
     By image analysis, the steel wire according to the present invention undergone one passes shows at least 10 volume percent of oriented martensite and the steel wire undergone three passes shows at least 20 volume percent of oriented martensite.