Abstract:
The invention relates to a tool steel composition comprising, expressed in weight percentage: 
     C 0.3%-0.4% 
     Cr 2.0%-4.0% 
     Mo 0.8%-3.0% 
     V 0.4%-1.0% 
     W 1.5%-3.0% 
     Co 1.0%-5.0% 
     Si 0 %-1.0% 
     Mn 0 %-1.0% 
     Ni 0 %-1.0% 
     the balance being mainly constituted by iron and inevitable impurities, and also to a method of preparing the composition.

Description:
This application is a 371 of PCT/FR99/00735 filed Mar. 30, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a steel of the “3% to 5% by weight chromium” family as used for making tools that withstand heat and that work under high levels of stress, such as dies for stamping and forging, dies for wire drawing, molds for static casting or for casting under pressure, using various alloys such as alloys of aluminum, copper, or titanium. 
     Such steels are alloyed with chromium, molybdenum, and vanadium, elements which give them the required hot strength properties. More precisely, they are subdivided into three families of compositions having properties that are similar, such that these three families are used in the same applications. These are compositions that comprise the following alloying elements, with percentages expressed by weight: 
     5% chromium, 1.3% molybdenum, 0.5% to 1.3% vanadium, approximately; or 
     3% chromium, 3% molybdenum, 0.5% vanadium, approximately; or else 
     5% chromium, 3% molybdenum, 0.8% vanadium, approximately. 
     Some of those steels are specified in the AISI nomenclature in the United States under the terms H11, H12, and H13, or in the DIN nomenclature in Germany under the names W1.2343, W1.2606, and W1.2344, and they are mentioned in French standard NF A 35-590. 
     In use, the surface of the tooling comes into contact with materials that are heated to high temperature, for example liquid aluminum at 600° C.-750° C. or steel that is to be forged and that has been preheated to 1200° C. 
     Consequently, the surface of the tooling is itself raised to high temperature. As a result, temperature conditions are established within the tooling between its working portion which is subjected to heating and the remainder of the part which is cooled either by natural conditions or by forced cooling. 
     Under severe conditions of use implementing high surface temperatures and high levels of mechanical stress, a tool is destroyed quickly by two processes: 
     the mechanical strength of material decreases smoothly with increasing temperature; and 
     the material loses its initial properties which were imparted thereto by preliminary heat treatment because of the metallurgical transformations that take place under the combined effects of stresses and temperature giving rise to its mechanical strength weakening and then collapsing. 
     Thus, rapid or even catastrophic deterioration is observed of such tooling employed under severe conditions because the working surface softens, creeps, deforms plastically, and is subject to thermal fatigue. 
     SUMMARY OF THE INVENTION 
     The tool steel of the present invention overcomes these deficiencies and includes by weight percent 0.3-0.4 C, 2.0-4.0 Cr, 0.8-3.0 Mo, 0.4-1.0 V, 1.5-3.0W, 1.0-5.0 Co, 0-1.0 Si, 0-1.0 Mn and 0-1.0 Ni with the balance being mainly iron and inevitable impurities. The present invention further includes a method of preparing a tool steel with the aforesaid composition including steps of heating the steel to a temperature of 1020° C. to 1100° C. followed by staged quenching at temperatures of 250° C. to 320° C. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In a first aspect, the present invention provides a steel composition that withstands said severe operating conditions well. 
     The composition of the invention comprises, in weight percentage: 
     C 0.3%-0.4% 
     Cr 2.0%-4.0% 
     Mo 0.8%-3.0% 
     V 0.4%-1.0% 
     W 1.5%-3.0% 
     Co 1.0%-5.0% 
     Si 0-1.0% 
     Mn 0-1.0% 
     Ni 0-1.0% 
     the balance being mainly constituted by iron and inevitable impurities. 
     Preferably, the composition lies within the following ranges: 
     0.33%-0.37% 
     Cr 2.58%-3.50% 
     Mo 1.20%-2.20% 
     V 0.6%-0.9% 
     W 1.8%-2.6% 
     Co 1.5%-3.0% 
     Si 0.2%-0.5% 
     Mn 0.2%-0.5% 
     Ni 0-0.3% 
     In more particularly preferred manned, the composition of the invention has concentrations of P, Sb, Sn, and As, expressed in weight percentages, which satisfy the following relationships: 
     P≦0.008% 
     Sb≦0.002% 
     Sn≦0.003% 
     As≦0.005% 
     while the value given by Bruscato&#39;s relationship: 
     
       
         B=(10P+5Sb+4Sn+As)×0.01 
       
     
     is not greater than 0.10%. 
     The set of alloying elements whose actions complement one another is balanced so as to provide sufficient quenchability as is required for obtaining uniform properties throughout the thickness of parts of large size. 
     Carbon is the basic hardening element, and its level is adjusted so as to obtain sufficient mechanical strength while ensuring that eutectic carbides do not form during solidification because carbon concentration is too high. Its concentration in the alloy of the invention lies in the range 0.3% to 0.4% by weight, and preferably in the range 0.33% to 0.37% by weight. 
     Chromium and molybdenum contribute to quenchability and to hardening after quenching and tempering by forming alloyed carbides during tempering heat treatment. The concentrations of these elements must not be excessive so as to avoid excessively encouraging the formation of chromium-molybdenum carbides to the detriment of vanadium and tungsten carbides. The concentration of chromium in the alloy of the invention is 2.0% to 4.0% by weight, preferably 2.50% to 3.50% by weight, while the concentration of molybdenum is 0.8% to 3.0% by weight, and preferably 1.20% to 2.20% by weight. 
     Vanadium contributes to hardening during tempering treatment by forming specific carbides, thereby making it possible to increase structural resistance to heating, and thus to raise the highest acceptable operating temperatures. An excess of this element is prejudicial to toughness because eutectic carbides are formed on solidification, and because of the segregating nature of this element. Its concentration in the alloy of the invention is 0.4% to 1.0% by weight, and preferably 0.6% to 0.9% by weight. 
     Similarly, tungsten complements the action of vanadium by mechanisms of the same type and thereby contribute to raising the temperatures which are compatible with use, and in the same manner, excess tungsten is prejudicial to toughness and to structural uniformity. Its concentration in the alloy of the invention is 1.5% to 3.0% by weight, and preferably 1.8% to 2.6% by weight. 
     It is the complementary and appropriately balanced effects of these four carbide-generating elements Cr, Mo, V, and W that impart new properties to the alloy of the invention. 
     Cobalt improves mechanical strength when hot. Its concentration in the alloy of the invention is 1.0% to 5.0% by weight, and preferably 1.5% to 3.0% by weight. 
     The concentrations of silicon and of manganese in the alloy of the invention are each 0% to 1.0% by weight, and preferably 0.20% to 0.50% by weight. The concentration of nickel in the alloy of the invention is 0% to 1.0% by weight, and preferably 0% to 0.30% by weight. 
     More generally, although there is no desire to be tied to any particular theory, it is believed that the obtention of good characteristics for such steels depends on balancing the elements of the alloy; it is the result of the individual properties of each of the elements, and also of the way they interact. 
     The effect of tungsten stems from the formation of carbides, with this element contributing to the composition thereof. It is in competition with chromium and molybdenum, given that a predominance of chromium carbides is harmful for stability in operation. 
     Nevertheless, the crystallographic nature of the carbides formed depending on the steel is still poorly known at present, and 
     the effects of these carbides on the properties and the structural stability are known only in broad outline. 
     The steel of the invention is made using the methods applicable to the usual materials referred to. 
     The invention also provides a method of preparing tool steel having the above-defined composition, and in which, in a particular implementation, an appropriate tempering treatment is performed prior to the heat treatment of use, so as to obtain a metallographic structure that presents carbides which are fine and well distributed. 
     In a particular implementation, quenching is performed by heating the part to a temperature lying in the range 1020° C. to 1100° C., and preferably in the range 1040° C. to 1070° C., and then cooling by stepped quenching to 250° C. to 320° C. by any appropriate means. 
     In a particular implementation, the desired properties are obtained after performing two tempering treatments after quenching, the first tempering treatment being performed in the temperature range 550° C. to 580° C., and the second in the range 580° C. to 680° C. with adjustment as a function of the desired hardness in use. 
     In another particular implementation of the method of the invention, starting from metal produced by a conventional steelmaking method, remelting is performed by means of a consumable electrode under a vacuum or by means of a consumable electrode under slag, thereby giving the material improved inclusion properties and improved chemical uniformity, which has the effect of increasing its toughness properties and consequently its strength in operation. 
    
    
     The invention is described below by means of the following examples. 
     EXAMPLES 
     A test cast of a steel A of the invention having the composition given in the table below was made in order to perform various tests: 
     C 0.354% 
     Cr 3.09% 
     Mo 1.36% 
     V 0.81% 
     W 2.26% 
     Co 2.00% 
     Si 0.31% 
     Mn 0.30% 
     Ni 0.08% 
     P 0.007% 
     the balance being constituted by iron and inevitable impurities. 
     The various reference materials used for testing were 5% chromium steels containing varying quantities of molybdenum and vanadium. 
     The symbols used below have the following meanings: 
     R m : maximum strength; 
     R p0.2 : conventional elastic limit at 0.2%; 
     HRC: Rockwell hardness. 
     Example 1—Hot Traction Tests 
     These tests were performed at various temperatures on steel A of the invention, and on three other conventional grades of 5% chromium steel containing molybdenum and vanadium. The results are given in Table 1 below. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Test 
                   
                   
                 Intended 
               
               
                   
                 temperature 
                 R m   
                 R p0.2   
                 hardness 
               
               
                 Material 
                 (° C.) 
                 (MPa) 
                 (MPa) 
                 (HRC) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A 
                 520 
                 1092 
                 916 
                 46 
               
               
                 5Cr 1.3Mo 0.5V 
                   
                 1088 
                 851 
               
               
                 A 
                 550 
                 918 
                 753 
               
               
                 5Cr 1.3Mo 0.5V 
                   
                 916 
                 709 
                 42 
               
               
                 5Cr 3Mo 0.5V 
                   
                 842 
                 664 
               
               
                 5Cr 1.5Mo 1V 
                   
                 901 
                 702 
               
               
                 A 
                 560 
                 1028 
                 830 
                 46 
               
               
                 5Cr 1.3Mo 0.5v 
                   
                 979 
                 710 
               
               
                 A 
                 600 
                 955 
                 745 
                 46 
               
               
                 5Cr 1.3Mo 0.5V 
                   
                 796 
                 552 
               
               
                   
               
             
          
         
       
     
     Compared with the reference materials, it can be seen that hot strength as described by the traction test is improved, in particular for operating temperatures in excess of 550° C. 
     Example 2—Hot Traction Tests after being Maintained at Temperature 
     These tests were performed at a temperature of 550° C. after being maintained at 550° C. for 50 hours, and they were performed on steel A of the invention and also on the three other grades described above in Example 1. The results are shown in Table 2 below. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Test 
                   
                   
                 Intended 
               
               
                   
                 temperature 
                 ΔR m   
                 ΔR p0.2   
                 hardness 
               
               
                 Material 
                 (° C.) 
                 (MPa) 
                 (MPa) 
                 (HRC) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A 
                 550 
                 −15 
                 −13 
                 42 
               
               
                 5Cr 1.3Mo 0.5V 
                   
                 −50 
                 −40 
                 42 
               
               
                 5Cr 3Mo 0.5V 
                   
                 −18 
                 −41 
                 42 
               
               
                 5Cr 1.5Mo 1V 
                   
                 −101 
                 −104 
                 42 
               
               
                   
               
             
          
         
       
     
     In the same manner, it can be seen that the hot strength as described by the traction test is less reduced by prolonged maintenance at the operating temperature (for 50 hours) with the steel of the invention as compared with the reference steels. 
     Example 3—Rupture Testing under Stress 
     These tests were performed on steel A of the invention, and also on another grade of steel having 5% chromium, 1.2% molybdenum, and 0.5% vanadium, and the purpose of the test was to determine the stress required to cause the test pieces to rupture after 100 hours. The results are given in Table 3 below. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Test 
                   
                   
               
               
                   
                   
                 temperature 
                 Stress 
                 Treated for 
               
               
                   
                 Material 
                 (° C.) 
                 (MPa) 
                 (HRC) 
               
               
                   
                   
               
             
             
               
                   
                 A 
                 520 
                 695 
                 42 
               
               
                   
                   
                 560 
                 555 
               
               
                   
                   
                 600 
                 360 
               
               
                   
                 A 
                 520 
                 795 
                 46 
               
               
                   
                   
                 560 
                 610 
               
               
                   
                   
                 600 
                 400 
               
               
                   
                 5Cr 1.2Mo 0.5V 
                 520 
                 670 
                 46 
               
               
                   
                   
                 560 
                 420 
               
               
                   
                   
                 600 
                 195 
               
               
                   
                 5Cr 1.2Mo 0.5V 
                 520 
                 795 
                 50 
               
               
                   
                   
                 560 
                 425 
               
               
                   
                   
                 600 
                 188 
               
               
                   
                   
               
             
          
         
       
     
     In the same manner as before, it can be seen that the creep strength expressed as the stress which leads to rupture in 100 hours is greater for the steel of the invention. 
     Example 4—Deformation Tests under Stress 
     These tests were performed on steel A of the invention, and also on the same grade of steel as was used in Example 3, and the tests were intended to determine the stress required to obtain 1% deformation of the test pieces in 100 hours. The results are given in Table 4 below 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Test 
                   
                   
               
               
                   
                   
                 temperature 
                 Stress 
                 Treated for 
               
               
                   
                 Material 
                 (° C.) 
                 (MPa) 
                 (HRC) 
               
               
                   
                   
               
             
             
               
                   
                 A 
                 560 
                 500 
                 42 
               
               
                   
                 A 
                 560 
                 640 
                 46 
               
               
                   
                 5Cr 1.2Mo 0.5V 
                 560 
                 350 
                 46 
               
               
                   
                 5Cr 1.2Mo 0.5V 
                 560 
                 370 
                 50 
               
               
                   
                   
               
             
          
         
       
     
     In the same manner as before, it can be seen that the creep strength expressed as the stress which leads to 1% deformation in 100 hours is better for the steel of the invention. 
     Naturally, the embodiments of the tool steel composition of the invention that are described above are given purely by way of non-limiting indication, and numerous modifications can easily be provided by the person skilled in the art without thereby going beyond the ambit of the invention.