Patent Publication Number: US-2004050459-A1

Title: Steel and method for producing an intermediate product

Description:
[0001] Martensitic, corrosion-resistant tool steels are used for the production of tools which in practical application are exposed to corrosive media and which at the same time have to meet exacting requirements concerning their hardness.  
       [0002] Metal-cutting production methods constitute a significant component of industrial production technology and are a principal cost unit in the production of tools for plastics processing. The economical use of steels of the type mentioned in the introduction thus depends to a significant degree on their machinability and their corrosion-resistance, with the latter in turn being decisively influenced by the chromium content of the steels. In this context, the term “machinability” refers to the characteristics of a material to be machined under certain conditions.  
       [0003] Particular requirements concerning the corrosion resistance of tools made from steels of the type mentioned above arise in the plastics-processing industry segment. In this industry segment, contact with the cooling media and cleaning media, with the surrounding atmosphere and with the plastics themselves which are being processed, in many cases leads to exposure to corrosion of the respective tool.  
       [0004] A martensitic stainless steel of good workability is known from EP 0 721 513 B1. The known steel contains 10 to 14 mass-% chromium. In order to improve its machinability, said steel also comprises at least 0.15% sulphur and 1.0 to 3.5% copper. The addition of copper has an additional positive influence on the hardness of the alloy.  
       [0005] Apart from the steel described in the above-mentioned European patent specification, a multitude of chromium-alloyed corrosion-resistant steels are known whose chromium content is between 11.0 and 17.0 mass-%. These include for example the steels which in the steel-iron list are designated with the material numbers of 1.2080, 1.2082, 1.2083, 1.2085, 1.2201, 1.2314, 1.2316, 1.2319, 1.2361, 1.2376, 1.2378, 1.2379, 1.2380, 1.2436, and 1.2601. These steels are regularly alloyed with carbon, silicon and manganese. Optionally, they also comprise carbide formers such as molybdenum, vanadium or tungsten.  
       [0006] Processing of the known steels is carried out depending on the respective carbon content and carbide content. For example, the steels of the type at issue are used by the tool manufacturers in a heat-treated state at a hardness of between 285 and 325 HB. At this hardness, the material can still be machined. These steels are also processed in a soft-annealed state, with the hardness of the steels in this case being max. 250 HB. While such steels, which are less hard, can be processed better, they require heat treatment after processing in order to achieve the usually required installation hardness of 46 to 60 HRC. Subsequently, finishing is required.  
       [0007] With the extreme end hardness required by users, economical metal-cutting processing is no longer possible in the known steels. While this problem is solved by processing in the soft annealed state with subsequent heat treatment, such finishing heat treatment is however associated not only with the disadvantage of costs of this additional process step but also with the danger of crack formation and with distortion of the component occurring as a result of heating.  
       [0008] The known steels which are shown in the steel-iron list are associated with a further disadvantage in that their weldability is reduced due to the carbon content and the alloy composition. However, good weldability is absolutely essential in the field of plastics processing. Due to subsequent changes in the design and due to necessary repairs, there is often a need for welding work on the tool.  
       [0009] Determining a steel which meets the requirements of practical application, in particular the problems encountered in plastics processing, is further complicated by the fact that such a steel must not only be corrosion-resistant, easily machinable and easily weldable, but it must also be adequately ductile so as to be able to absorb the forces experienced in practical operation. Otherwise there is a danger that the considerable bending forces, torsional forces, compressive forces and tensile forces also cause cracks.  
       [0010] It has been shown that the known steels do not meet all these requirements at the same time. For example, the steels which due to their increased sulphur content are easy to machine exhibit insufficient ductility, while steels which are harder due to an increase in their carbon content are insufficiently corrosion-resistant.  
       [0011] It is thus the object of the invention to provide a steel which is suitable in particular for the production of tools for the plastics-processing industry, with said steel not only being very hard and corrosion-resistant but also sufficiently ductili, machinable and weldable, thus meeting the practical requirements. Furthermore, the invention provides a method for producing intermediate products from such a steel. In this context, the term “intermediate products” also includes long products, flat products or other objects which will subsequently be subjected to further processing.  
       [0012] In relation to the material, this object is met by a steel, in particular for tools exposed to corrosion, of the following composition (in mass-%):  
       [0013] C: min. 0.02 and max. 0.12%;  
       [0014] Si: max. 1.5%;  
       [0015] Mn: more than 1.0-2.50%;  
       [0016] P: max. 0.035%;  
       [0017] S: min. 0.04% and less than 0.15%;  
       [0018] Cr: more than 8.0% and less than 12%;  
       [0019] Mo: more than 0.0% and max. 0.20%;  
       [0020] V: more than 0.0% and max. 0.25%;  
       [0021] Nb: more than 0.1% and max. 0.5%;  
       [0022] N: at least 0.02% and max. 0.12%;  
       [0023] Ni: max. 0.5%;  
       [0024] B: max. 0.005%;  
       [0025] Cu: max. 0.3%;  
       [0026] Al: max. 0.035%;  
       [0027] Sn: max. 0.035%;  
       [0028] As: max. 0.02%;  
       [0029] at least one of the elements Ca, Mg or Ce, wherein the sum of the contents of these elements is more than 0.0002% and max. 0.015%;  
       [0030] with the remainder being iron and unavoidable impurities.  
       [0031] The niobium-alloyed tool steel according to the invention features an optimised combination of machinability, hardness, corrosion resistance, weldability and ductility. It achieves a hardness that lies between 300 and 450 HB. Despite its relatively high sulphur content its ductility is good for steels of the generic type, with the hardness meeting the requirements of practical application.  
       [0032] In order to improve their machinability, steels according to the invention are sulphur-alloyed, with the sulphur content in each case being less than 0.15 mass-%. Preferably the steel comprises at least 0.04 mass-% which safely ensures good machinability. Even better machinability can be achieved, taking into account the other conditions required of the composition, if steel according to the invention comprises at least 0.07 mass-% of sulphur.  
       [0033] In spite of such a sulphur content, the steel according to the invention displays good ductility. This is achieved in that the steel together with the sulphur comprises at least one of the elements calcium, manganese or cerium at quantities wherein the sum is more than 0.0002 but at the most 0.015 mass-%. These elements make it possible for sulphides to spheroidise in the matrix of the steel, thus improving the ductility of said steel. This can for example safely be achieved if the steel according to the invention comprises 0.001-0.009 mass-% of calcium.  
       [0034] By using low carbon contents of max. 0.12 mass-% as well as low nitrogen contents of max. 0.12 mass-% and a niobium content of 0.11 to 0.5 mass-%, in the steel according to the invention, hard phases are formed which contribute to the achieved hardness of 300 to 450 HB. At the same time, the respective hard phases are precipitated in a particularly fine and even distribution, which has a positive influence on the toughness characteristics.  
       [0035] These advantageous characteristics of alloying are particularly noticeable with niobium, if the niobium content is set so that the hardness factor Hf in steel according to the invention meets the following condition: 
       0.047 &lt;H   f ≦0.095, 
       [0036] wherein the hardness factor H f  is calculated according to the equation 
         H   f =0.11−%Nb/7.14 
       [0037] with %Nb designating the respective Nb content of the steel. If the Niobium content is selected in this way, the existing carbon and nitrogen are largely set, by the element niobium, to form hard phases so that in steel according to the invention the chromium which is contained in the matrix at a content of less than 12% is fully available for forming corrosion-inhibiting passive layers. In this way, steel according to the invention has outstanding corrosion-resistance in spite of the relatively low chromium contents, and at the same time a high degree of hardness.  
       [0038] Furthermore, in steel according to the invention the contents of such elements which can lead to crack formation in the weld seam are reduced to a minimum. Optimum weldability of steel according to the invention can be ensured in that the weld factor Sf which is calculated according to the equation 
         S   f =%C+5 x %B+2 x %Cu+(%P+%S)/2+(%Mo+%Cr)/4+%Mn/10 
       [0039] in steel according to the invention meets the following condition: 
       S f &lt;3.99, 
       [0040] wherein %C, %B, %Cu, %P, %S, %Mo, %Cr, %Mn designate the respective contents of C, B, Cu, P, S, Mo, Cr and Mn of the steel.  
       [0041] The ductility of the known tool steels mentioned in the introduction is negatively affected by the carbon content and carbide content as well as by the extent of the sulphur content, and by the distribution and morphology of the sulphides. Steel according to the invention only comprises max. 0.12% carbon. In this way, its carbide content, too, is limited. Furthermore, because the content of grain-boundary effective elements is reduced to a minimum in a steel according to the invention, its ductility is increased when compared to other sulphur alloyed steels.  
       [0042] It has been found that the grain boundary effective elements in steels of the type at issue segregate at the grain boundaries during the solidification process as well as during hot forming and/or during heat treatment at certain temperatures. Such segregation leads to a reduction in cohesion and is thus often the reason for crack formation. As a result of the embrittlement factor KG f  in a steel according to the invention meeting the following conditions, the negative influence of the grain-boundary effective elements, and thus the associated danger of crack formation, can be minimised in a targeted way: 
       KG f &lt;1.07 
       [0043] wherein the embrittlement factor KG f  is calculated according to the equation 
         KG   f =2.97 x %Cu+3.2 x (%Sn+%As)+0.55 x %Al+5.42 x %P+0.98%N 
       [0044] with %Cu, %Sn, %As, %Al, %P and %N designating the respective contents of Cu, Sn, As, Al, P and N of the steel.  
       [0045] In relation to the method for producing an intermediate product for the production of components, in particular for the production of a tool exposed to corrosion, made from a steel of a composition according to the invention, the above-mentioned object is met in that at least the following production steps are carried out:  
       [0046] melting a steel according to the invention;  
       [0047] casting the steel to form a raw material such as ingots, slabs, continuous-cast bars, thin slabs or cast strip;  
       [0048] diffusion annealing of the raw material at a temperature between 1200 and 1280° C.; and  
       [0049] hot forming the annealed raw material to form the component.  
       [0050] Diffusion annealing of the raw material, which annealing is carried out in the temperature range selected according to the invention, results in a compensation of the segregation due to solidification, so that even distribution of the alloying elements contained is achieved. During the subsequent hot forming of the raw material to form the intermediate product, the microstructure and the material isotropy are influenced. By carrying out hot forming using a deformation degree φ of at least 1.5, an improved microstructure and a higher isotropy of the material can be achieved.  
       [0051] Within the context of the method according to the invention, hot forming can be carried out by way of forging, or, if larger dimensions are to be produced, by way of hot rolling. Hot forming preferably takes place at temperatures between 850° C. and 1100° C. In this temperature range, the material used according to the invention has a low yield stress and good ductility, which result in optimum deformability. Hot forming can thus be carried out quickly, economically and at a high output.  
       [0052] Following hot forming, the workpiece generated according to the invention is removed from the deformation heat and preferably held where it is exposed to air. When held so as to be exposed to air, the material slowly and completely makes the transition from the austenitic to the martensitic state. Such slow cooling not only sets the desired hardness of the material of up to 450 HB, but heat stress and transformation stress are largely avoided so that no distortions or stress cracks occur in the finished intermediate product.  
       [0053] If necessary by carrying out an additional heat treatment at temperatures between 850° C. and 1050° C. with subsequent controlled cooling with the use of a cooling medium such as air, oil, water or a polymer, which heat treatment is preferably followed by tempering at temperatures between 400° C. and 650° C., a hardness in the intermediate product can be produced which differs from the hardness which is present in products which after the deformation heat have been held exposed to air. In particular, lower hardness values down to a lower limit of 300 HB can also be achieved by this heat treatment. 
     
    
    
     [0054] Below, the invention is explained in more detail by means of some exemplary embodiments. The following are shown:  
     [0055] Diag. 1 cutter wear during the drilling test, with reference to the distance travelled by the drill;  
     [0056] and  
     [0057] Diag. 2 The impact bending work determined for various steels, with reference to the embrittlement factor KG f .  
     [0058] Table 1 is a comparison between the alloys of steels A, B, C according to the invention and four comparison steels D, E, F, G which do not form part of the invention. Table 2 additionally shows the Brinell hardness values of steels A to G, as well as the hardness factors (H f ), weld factors (S f ) and embrittlement factors (KG f ).  
     [0059] In order to test the machinability of steels A-G, drilling tests were carried out on components produced from these steels, with non-coated helical drills made from high-speed steel with material number 1.3343. For this purpose, 24 mm deep holes were drilled in the steels which had a hardness of 300 to 400 HB. In each instance, the cutting speed was 12 m/min and forward feed was 0.12 mm/revolution.  
     [0060] After total drill travel of 200, 1200 and 2400 mm, wear on the cutting edges of the helical drills was measured. It became evident that despite their greater hardness, steels A, B and C according to the invention generate less wear on the cutting edges of the drills (Diag. 1). Their machinability is thus clearly improved when compared to that of conventional steels D, E, F and G which do not form part of the invention.  
     [0061] In order to determine the ductility of tool steels, the impact bending test according to steel-iron test sheet 1314 was carried out. This test determines the impact bending work which is needed to shatter unnotched specimens, which provides a measure of the ductility of a material. The specimens used which measured 7×10×55 mm were taken from the direction of deformation of the surveyed steels A-G, which steels had a hardness of 300 to 400 HB.  
     [0062] The test was carried out at room temperature. As is shown by the values contained in Diag. 2 (average values from 3 individual specimens tested), as the embrittlement factor KGf increases, there is a clear reduction in the measured impact bending work. With values which are clearly above 200 J, steels A, B and C according to the invention have the desired high ductility level, while the comparison steels D, E, F and G with an increase in the embrittlement factor only returned values between 50 and 150 J, i.e. their ductility was clearly inferior.  
     [0063] In order to check the corrosion-resistance of the steels shown in Table 1, immersion tests were carried out in a 0.5 % aqueous solution of sodium chloride. After exposure for 1 h, the specimens were dried by exposure to air for half an hour before being immersed again. After a total of nine such immersion and drying cycles, the appearance of the specimens, which had originally been finish-ground, was assessed.  
     [0064] After completion of the tests, practically no rust was evident on the surface of the specimens of steels A to C according to the invention. This suggests adequate corrosion resistance. In contrast, the comparison steels D, E and G showed heavy corrosion as a result of the test solution, so that most of the surface area was already corroded after the test cycles. Only comparison steel F was more resistant to corrosion, due to its high chromium content and due to the absence of sulphur. However, due to the lack of sulphur in the composition, this steel F had the least favourable machinability of all steels tested.  
     [0065] The examples explained show that steel according to the invention not only safely achieves the desired hardness of 300 HB to 450 HB, but also displays good machinability. In contrast, steels which do not form part of the invention, which steels do not meet the conditions of the hardness factor Hf which are to be taken into account according to the invention do not achieve this combination of characteristics.  
     [0066] The situation is similar in the context of the value which the steels according to the invention have to meet in connection with the weldability factor S f . Thus, the comparison steels, whose weld factor S f  is above the limiting value envisaged according to the invention, have a significantly more unfavourable weld behaviour than do steels according to the invention. This is particularly evident in the occurrence of weld cracks, the prevention of which requires expensive preheating and subsequent treatment in the steels which do not form part of the invention.  
     [0067] Finally, the examples document that as a result of the limitation according to the invention of the contents of grain-boundary effective elements such as Cu, Sn, As, Al, P and N in steels A, B, C the respective embrittlement factor KG f  is kept low, and consequently good ductility has been achieved for steels of the type at issue.  
                                                                                       TABLE 1                       Steel   C   N   Si   Mn   P   S   Cr   Mo   Ni   Cu   Al   Ti   Nb   V   B   Sn   As   Ca                                                                                                            Invention                                                                               A   0.04   0.034   0.42   1.32   0.019   0.113   10.53   0.21   0.14   0.18   0.005   &lt;0.001   0.151   0.05   0.0002   0.021   0.007   0.0034       B   0.07   0.051   0.78   1.45   0.017   0.133   10.36   0.09   0.45   0.07   0.002   &lt;0.001   0.324   0.11   0.0001   0.009   0.011   0.0091       C   0.06   0.067   0.28   1.24   0.024   0.127   11.89   0.11   0.24   0.14   0.014   &lt;0.001   0.391   0.05   0.0002   0.018   0.005   0.0054       Comparison       D   0.04   0.021   0.99   0.82   0.017   0.161   12.45   0.04   0.44   0.89   0.013   &lt;0.001   0.001   0.02   0.0011   0.037   0.022   &lt;0.0001       E   0.34   0.005   0.34   1.15   0.029   0.054   16.12   0.04   0.54   0.28   0.006   &lt;0.001   0.001   0.01   0.0005   0.022   0.008   0.0027       F   0.36   0.008   0.15   0.89   0.018   0.001   15.28   1.08   0.45   0.22   0.025   0.001   0.001   0.02   0.0001   0.038   0.027   &lt;0.0001       G   0.38   0.001   0.57   0.66   0.036   0.091   13.84   0.02   1.56   1.24   0.006   0.023   0.782   0.22   0.0050   0.036   0.018   0.0020                          
 
     [0068]                               TABLE 2                           Hardness                   Steel   [HB]   S f     KG f     K f                                                      Invention                       A   395   3.28   0.76   0.0889       B   380   3.04   0.42   0.0646       C   370   3.54   0.69   0.0552       Comparison       D   330   5.12   2.95   0.1099       E   300   5.10   1.09   0.1099       F   315   4.99   0.98   0.1099       G   325   6.48   4.06   0.0005