Patent Publication Number: US-2011061776-A1

Title: Process for manufacturing sheet of austenitic stainless steel having high mechanical properties and sheet thus obtained

Description:
The present invention relates to the manufacture of hot-rolled sheet made of austenitic stainless steel having high mechanical properties and especially a very advantageous combination of mechanical strength and uniform elongation. 
     For the manufacture of structural components in the automotive industry, it is common practice to use various grades of coated carbon steel sheets having more or less complex microstructures. The parts are produced from sheets having a thickness ranging from 1 to 3 mm. However, for some parts it would be desirable to have both a higher corrosion resistance combined with a high deformability so as to produce parts with a complex drawing operation. Moreover, it is known that austenitic stainless steels are widely used because of their excellent corrosion resistance and their mechanical properties, in particular their high ductility. For example, austenitic stainless steels denoted according to the EN 10088-1 standard by the reference 1.4318 are known in which the composition contains (in contents expressed by weight): C≦0.030%, Si≦1.00%, Mn≦2.00%, P≦0.045%, S≦0.015%, Cr: 16.50 to 18.50%, Ni: 6.00 to 8.00%, N: 0.10 to 0.20%. These steels have high mechanical properties owing to the formation of martensite during deformation at room temperature. Typical mechanical properties of these steels in the annealed state are the following: yield strength R p0.2  (conventional yield strength corresponding to a 0.2% strain): 300-400 MPa; uniform elongation: A≧45%, R m  (maximum strength)≧700 MPa; product P=R p0.2  (MPa)×uniform elongation=about 15750 MPa. %. It is possible to use these grades in the state work-hardened by cold rolling: C850, C1000-EN-10088-2 standard, these designations corresponding to a minimum strength of 850 and 1000 MPa respectively. The increase in yield strength conferred by this operation (R p0.2 ≧600 MPa) is manifested by a simultaneous reduction in elongation (A=30%). The product P then reaches about 18000 MPa. %. These properties are satisfactory for certain applications. However, they remain insufficient if high strength in service is desired, for example for an increase in lightening, and a high capability for prior forming operations. 
     An alternative method to work hardening by cold rolling is work hardening by hot rolling at a sufficiently low temperature. This method gives a better elongation-strength compromise, but has the major drawback of leading to local deformations during forming, resulting in vermicular defects. To avoid these vermicular defects on a standard 1.4318 steel not recrystallized after hot rolling, it is necessary to carry out an annealing operation after the hot rolling. 
     The object of the invention is therefore to provide hot-rolled sheets of austenitic stainless steel having mechanical properties superior or equivalent to those of grades of the 1.4318 type mentioned above, which are inexpensive to manufacture and are insensitive to the appearance of vermicular defects. 
     The object of the invention is also to provide hot-rolled sheets made of austenitic stainless steel having a product P greater than 21000 MPa. %, which may be combined with a yield strength R p0.2  of greater than 650 MPa, or else of a uniform elongation of greater than 45%. 
     For this purpose, the subject of the invention is a hot-rolled sheet made of austenitic stainless steel, the product P (R p0.2  (MPa)×uniform elongation (%) of which is greater than 21000 MPa. % and the chemical composition of which comprises, the contents being expressed by weight: 0.015%≦C≦0.030%, 0.5%≦Mn≦2%, Si≦2%, 16.5%≦Cr≦18%, 6%≦Ni≦7%, S≦0.015%, P≦0.045%, Al≦0.050%, 0.15%≦Nb≦0.31%, 0.12%≦N≦0.16%, the Nb and N contents being such that: 
     Nb/8+0.1%≦N≦Nb/8+0.12%, optionally: 0.0005%≦B≦0.0025%, Mo≦0.6%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting. 
     According to a preferred embodiment, the niobium and nitrogen contents of the steel, expressed by weight, are such that: 0.20%≦Nb≦0.31%, 0.12%≦N≦0.16%. 
     The subject of the invention is also a hot-rolled sheet made of austenitic stainless steel according to any one of the above compositions, the yield strength R p0.2  of which is greater than 650 MPa, characterized in that the mean austenitic grain size of the steel is less than 6 microns, in that the non-recrystallized surface fraction is between 30 and 70% and in that the niobium is completely in the form of precipitates. 
     The subject of the invention is also hot-rolled sheet made of austenitic stainless steel according to any one of the above features, the uniform elongation of which is greater than 45%, characterized in that the niobium is not completely precipitated. 
     The subject of the invention is also a process for manufacturing a hot-rolled sheet made of austenitic stainless steel, the yield strength R p0.2  of which is greater than 650 MPa, in which: a semi-finished product made of steel having the composition according to any one of the above compositions is supplied; then said semi-finished product is reheated to a temperature of between 1250° C. and 1320° C.; and then said semi-finished product is rolled with an end-of-rolling temperature below 990° C. and a cumulative reduction ratio E on the last two finishing stands of greater than 30%. 
     According to one particular embodiment, a semi-finished product made of steel having the composition above, containing 0.20%≦Nb≦0.31%, 0.12%≦N≦0.16%, is supplied and then said semi-finished product is rolled with an end-of-rolling temperature below 970° C. 
     The subject of the invention is also a process for manufacturing a hot-rolled sheet made of austenitic stainless steel, the uniform elongation of which is greater than 45%, in which: a semi-finished product made of steel having the composition according to any one of the above compositions is supplied; then said semi-finished product is reheated to a temperature of between 1250° C. and 1320° C.; and then said semi-finished product is rolled with an end-of-rolling temperature above 1000° C. 
     The subject of the invention is also a process for manufacturing a hot-rolled sheet made of austenitic stainless steel, the product P (R p0.2  (MPa)×uniform elongation (%)) of which is greater than 21000 MPa. %, in which: a semi-finished product made of a steel having the composition according to any one of the above compositions is supplied; then said semi-finished product is reheated to a temperature of between 1250° C. and 1320° C.; and then said semi-finished product is hot-rolled. 
     The subject of the invention is also the use of a hot-roiled sheet made of stainless steel according to any one of the above features or manufactured by any one of the above processes, for the manufacture of structural components in the automotive field. 
     Other features and advantages of the invention will become apparent over the course of the description below given by way of example. 
     After many trials, the inventors have shown that the various requirements mentioned above are satisfied by observing the following conditions: 
     As regards the chemical composition of the steel, the carbon content must be equal to or less than 0.030% so as to avoid the risk of sensitivity to intergranular corrosion. For the purpose of obtaining a yield strength of greater than 650 MPa, the carbon content must be equal to or greater than 0.015%. 
     Manganese, like silicon, is an element known for its deoxidizing properties in its liquid state and for increasing the hot ductility, in particular by being combined with sulphur. Moreover, at ambient temperature, manganese promotes stability of the austenitic phase and reduces the stacking fault energy. It also increases the solubility of nitrogen. These favourable effects are obtained inexpensively when the manganese content is between 0.5 and 2%. 
     Like manganese, silicon is an element usually added for the purpose of deoxidizing the liquid steel. Silicon also increases the yield strength and the tensile strength, by solid-solution hardening or by its action on the content of ferrite δ. However, above 2%, the weldability and hot ductility are reduced. Chromium is an element well known for increasing the oxidation resistance and corrosion resistance in aqueous medium. This effect is obtained satisfactorily when its content is between 16.5% and 18%. 
     Nickel is an essential element for ensuring sufficient stability of the austenitic structure of the steel at ambient temperature. The optimum content must be determined in relation to other elements of the composition promoting alpha-phase formation, such as chromium, or those promoting gamma-phase formation, such as carbon and nitrogen. Its effect on the stability of the structure is sufficient when its content is equal to or greater than 6%. Above 7%, the production cost increases excessively because of the expense of this addition element. 
     Molybdenum enables the pitting resistance to be increased. Optionally, an addition of molybdenum in an amount ranging up to 0.6% may be carried out. Boron is used to improve the forgibility of the steel. Optionally, an addition of boron in an amount of between 0.0005 and 0.0025% may be carried out. An addition with a greater amount would critically reduce the burning temperature. 
     Sulphur is an element that particularly degrades the hot forgibility and the corrosion resistance—its content must be kept equal to or less than 0.015%. Phosphorus likewise degrades the hot ductility—its content must less than 0.045% in order to obtain satisfactory results. 
     Aluminium is a powerful agent for deoxidizing the liquid metal. In combination with the abovementioned silicon and manganese contents, an optimum effect is obtained when its content is equal to or less than 0.050%. 
     Niobium and nitrogen are important elements of the invention for the purpose of manufacturing austenitic stainless steels having high mechanical properties. 
     Niobium retards recrystallization during hot rolling—for a given end-of-hot-rolling temperature, its addition results in a higher work-hardening factor being maintained (the hot rolling is said to be “work hardening”), thus increasing the tensile strength of this steel. It is generally used like Ti to combat the formation of chromium carbides (EN 1.4580 and EN 1.4550 Nb stabilized austenitic stainless steels). Finally, it may lead to the formation of an intermetallic phase giving an improvement in hot creep resistance. 
     Nitrogen is an element hardening in interstitial solid solution, which most particularly increases the yield strength in this regard. It is also known, in solid solution, as a powerful stabilizer for the austenitic phase and as a retarder for the precipitation of chromium carbides Cr 23 C 6 . The solubility of nitrogen during solidification goes through a maximum—too high a content results in the formation of volume defects in the metal. 
     The combined addition of niobium and nitrogen for the purpose of hardening is somewhat unusual in austenitic stainless steels. Within the context of the invention, it has been demonstrated that stainless steels having a composition close to that of the abovementioned 1.4318 steels advantageously benefit from a particular combined addition of niobium and nitrogen, optimized for the purpose of obtaining certain mechanical properties under precise conditions, that are mentioned below: 
     Firstly, it has been demonstrated that a nitrogen content ranging from 0.12 to 0.16%, together with a niobium content ranging from 0.15 to 0.31%, the niobium and nitrogen contents being such that: Nb/8+0.1%≦N≦Nb/8+0.12% (relationship 1), make it possible to manufacture a hot-rolled sheet having high mechanical properties intended to be drawn, without the need for annealing after rolling as in conventional 1.4318 steels, the drawn part not being subject to the formation of vermicular defects. 
     The precipitation of nitrides NbN, which occurs during the end of hot rolling, reduces the amount of nitrogen in solid solution. The above relationship (1) keeps as much nitrogen in solid solution, after complete precipitation of all the available niobium, as in the 1.4318 grade (N≧0.1%). 
     This therefore makes it possible to obtain the same metastability of the austenite at ambient temperature. The possibility of reducing the Ni content by increasing the N content is limited by the solubility limit of nitrogen in the steel during solidification. For the Cr, Mn and Ni contents of the steel according to the invention, the nitrogen content must be equal to or less than 0.16%. 
     A sufficient amount of niobium must be present so as to obtain a hardening effect and to retard the recrystallization. This amount must be adapted so as to obtain an NbN solvus above the end-of-rolling temperature in order to obtain precipitation at the end of hot rolling. 
     The niobium and nitrogen contents according to the invention enable substantial precipitation of NbN after hot rolling to be obtained. 
     A combined addition of 0.15 to 0.31% niobium (preferably 0.20 to 0.31% niobium) and 0.12 to 0.16% nitrogen, the niobium and nitrogen contents being such that: Nb/8+0.1%≦N≦Nb/8+0.12%, makes it possible to obtain an advantageous yield strength/elongation combination, the product P of which is greater than 21000 MPa. %. 
     Apart from iron, the remainder of the composition consists of inevitable impurities resulting from the smelting, such as for example Sn or Pb. 
     The manufacturing process according to the invention is implemented as follows: 
     A steel having a composition explained above is smelted. This smelting may be followed by the steel being cast into ingots or, in the most general case, cast continuously, for example in the form of slabs ranging from 150 to 250 mm in thickness. The casting may also be carried out in the form of thin slabs a few tens of millimetres in thickness between steel counter rotating rolls. These cast semi-finished products are firstly heated to a temperature between 1250 and 1320° C. The purpose of the 1250° C. temperature is to dissolve any niobium-based precipitates (nitrides and carbonitrides). However, the temperature must be below 1320° C. for fear of being too close to the solidus temperature which could be reached in certain segregated zones and of causing a local onset of a liquid state that would be deleterious to hot forming. In the case of direct casting of thin slabs between counter rotating rolls, the step of hot-rolling these semi-finished products starting at a temperature below 1250° C. may take place directly after casting so that an intermediate reheat step is unnecessary in this case. 
     The rolling is generally carried out on a continuous hot-rolling mill comprising in particular roughing stands and finishing stands. It has been demonstrated that a particularly high yield strength of R p0.2  is obtained by especially controlling the reduction ratio in the last two finishing stands: if the thickness of the sheet entering the penultimate finishing stand is denoted by e N-2  and the thickness of the sheet exiting the last finishing stand is denoted by e N , the cumulative reduction ratio over the last two finishing stands is defined by: 
     
       
         
           
             ɛ 
             = 
             
               
                 
                   
                     e 
                     
                       N 
                       - 
                       2 
                     
                   
                   - 
                   
                     e 
                     N 
                   
                 
                 
                   e 
                   
                     N 
                     - 
                     2 
                   
                 
               
               . 
             
           
         
       
     
     According to the invention, it has been demonstrated that when the end-of-rolling temperature is below 990° C. and when the cumulative reduction ratio ε is greater than 30%, the yield strength R p0.2  of the final product obtained is greater than 650 MPa, the niobium then being completely in the form of precipitates. 
     For a Nb content of between 0.20 and 0.31% and a nitrogen content between 0.12 and 0.16%, this 650 MPa minimum value is obtained when the end-of-rolling temperature is below 970° C. and ε is greater than 30%. 
     According to the invention, it has also been demonstrated that it is possible to obtain a hot-rolled sheet with a uniform elongation of greater than 45% when the end-of-rolling temperature is above 1000° C. In this case, the niobium is partially precipitated. 
     After hot rolling, a sheet is obtained that is not sensitive to the appearance of vermicular defects and does not require intermediate annealing. 
     As a non-limiting example, the following results will show the advantageous characteristics conferred by the invention. 
    
    
     EXAMPLE 
     Semi-finished products were produced by casting steel having the composition presented in the table below (in wt %): 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Composition of the steels (in wt %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Steel 
                 C 
                 Mn 
                 Si 
                 Cr 
                 Ni 
                 Mo 
                 S 
                 P 
                 Al 
                 Nb 
                 N 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 I1 
                 0.023 
                 1.100 
                 0.48 
                 17.45 
                 6.67 
                 0.25 
                 0.005 
                 0.020 
                 0.002 
                 0.152 
                 0.13 
               
               
                 (according 
               
               
                 to the 
               
               
                 invention) 
               
               
                 I2 
                 0.024 
                 1.19 
                 0.55 
                 17.36 
                 6.66 
                 0.25 
                 0.005 
                 0.020 
                 0.002 
                 0.302 
                 0.15 
               
               
                 (according 
               
               
                 to the 
               
               
                 invention) 
               
               
                 R 
                 0.026 
                 1.030 
                 0.6 
                 17.5 
                 6.6 
                 0.25 
                 0.0008 
                 0.026 
                 0.002 
                 
                   0.002 
                 
                 0.13 
               
               
                 (reference) 
               
               
                   
               
               
                 Underlined values: not according to the invention 
               
            
           
         
       
     
     The semi-finished steel products were reheated at 1280° C. for 30 minutes. A hot-rolling operation was then carried out by varying the end-of-rolling temperature between 900 and 1100° C. and the cumulative reduction ratio ε, so as to reach a final thickness of 3 mm. Steel sheets I1-1, I1-2, I1-3, etc. denote sheets obtained from the same semi-finished product I1 rolled under different conditions. The microstructure of the steel obtained was characterized by measuring in particular the surface fraction of recrystallized austenitic phase, the fraction of precipitated niobium relative to the total niobium and the average grain size. In the case of an incompletely recrystallized structure, the latter measurement was carried out on the recrystallized part of the structure. The tensile mechanical properties were also determined, in particular the yield strength R p0.2  and the uniform elongation. The possible presence of local deformation during the tensile trial was also recorded. It is known that the presence of such a local deformation is associated with the appearance of vermicular defects during forming operations. 
     The results are given in Table 2 below: 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Manufacturing conditions, microstructural characteristics and 
               
               
                 mechanical properties of hot-rolled sheets 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Average 
                 Non- 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 grain size 
                 recrystallized 
                 Niobium 
               
               
                 Trial 
                 EOR 
                   
                 less than 
                 fraction between 
                 completely 
                 R p0.2   
                 A 
                 R p0.2  × A 
                 Localized 
               
               
                 No. 
                 (° C.) 
                 ε &gt; 30% 
                 6 microns 
                 30 and 70% 
                 precipitated 
                 (MPa) 
                 (%) 
                 (MPa. %) 
                 deformation 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 I1-1 
                 905 
                 Yes 
                 Yes 
                 Yes 
                 Yes 
                 689 
                 40 
                 27628 
                 No 
               
               
                 I1-2 
                 935 
                 Yes 
                 Yes 
                 Yes 
                 Yes 
                 651 
                 40 
                 25520 
                 No 
               
               
                 I1-3 
                 1040 
                 Yes 
                 No 
                 No (&lt;30%) 
                 No 
                 432 
                 49 
                 21340 
                 No 
               
               
                 I1-4 
                 1050 
                 Yes 
                 No 
                 No (&lt;30%) 
                 No 
                 467 
                 46 
                 21715 
                 No 
               
               
                 I2-1 
                 930 
                 Yes 
                 Yes 
                 Yes 
                 Yes 
                 677 
                 38 
                 25997 
                 No 
               
               
                 I2-2 
                 965 
                 Yes 
                 Yes 
                 Yes 
                 Yes 
                 681 
                 39 
                 26559 
                 No 
               
               
                 I2-3 
                 980 
                 No 
                 No 
                 Yes 
                 Yes 
                 631 
                 41 
                 26186 
                 No 
               
               
                 I2-4 
                 1000 
                 No 
                 Yes 
                 No (&lt;30%) 
                 No 
                 627 
                 46 
                 28277 
                 No 
               
               
                 I2-5 
                 1100 
                 Yes 
                 No 
                 No (&lt;30%) 
                 No 
                 547 
                 53 
                 29100 
                 No 
               
               
                 R-1 
                 900 
                 Yes 
                 — 
                 Yes 
                 — 
                 702 
                 29 
                 20428 
                 Yes 
               
               
                 R-2 
                 925 
                 Yes 
                 — 
                 Yes 
                 — 
                 638 
                 29 
                 18566 
                 Yes 
               
               
                 R-3 
                 950 
                 Yes 
                 — 
                 Yes 
                 — 
                 632 
                 30 
                 19150 
                 Yes 
               
               
                 R-4 
                 1020 
                 Yes 
                 — 
                 No (&lt;30%) 
                 — 
                 482 
                 31 
                 14749 
                 No 
               
               
                   
               
               
                 EOR: End-of-rolling temperature; 
               
               
                 R p0.2: : Conventional yield strength at 0.2% strain; 
               
               
                 A: Uniform elongation; 
               
               
                 ε: Cumulative reduction ratio of the last two rolling passes. 
               
            
           
         
       
     
     Thus, the above table shows that steels I1 and I2 according to the invention have a particularly advantageous product R p0.2 ×A of greater than 21000 MPa. %, whereas the reference R steel does not have such a product, irrespective of the rolling conditions. 
     This table also shows that, when the non-recrystallized fraction is between 30 and 70% and when the average grain size is less than 6 microns, the yield strength R p0.2  is greater than 650 MPa (trials I1-1, I1-2, I2-1, I2-2). Moreover, when the non-recrystallized fraction is greater than 70%, the elongation tends to be reduced. 
     These properties are obtained for steels having a niobium content of between 0.15 and 0.31%, and a nitrogen content of between 0.12 and 0.16%, the niobium and nitrogen contents being such that: Nb/8+0.1%≦N≦Nb/8+0.12%, the end-of-rolling temperature being below 990° C. and the cumulative reduction ratio ε being greater than 30%. 
     In the case of steels having a niobium content of between 0.20% and 0.31% and a nitrogen content of between 0.12 and 0.16%, the niobium and nitrogen contents being such that: Nb/8+0.1%≦N≦Nb/8+0.12%, these properties are obtained when the end-of-rolling temperature is below 970° C. and when the cumulative reduction ratio ε is greater than 30% (trials I2-1 and I2-2). 
     When the niobium is not completely precipitated (trials I1-3, I1-4, I2-4, I2-5), the uniform elongation is greater than 45%. For the steel compositions according to the invention, this result is obtained when the end-of-rolling temperature is above 1000° C. For comparison, the reference steel does not offer such properties. 
     Therefore, certain manufacturing conditions (end-of-rolling temperature and cumulative reduction ratio) will be more particularly chosen depending on whether it is desired to produce a steel sheet having a particularly high yield strength or instead one having a high elongation capability. 
     Moreover, the stress-strain curves of the steels according to the invention show no plateau indicating local deformation, whatever the hot-rolling conditions, in contrast with the reference steel that exhibits local deformation whenever it is partially recrystallized (trials R-1, R-2, R-3). This point is particularly advantageous for the forming operation, by ensuring that there are no vermicular defects. 
     Thus, because of their particularly high mechanical properties, and especially their very advantageous yield strength×uniform elongation product, the hot-rolled steel sheets according to the invention will be advantageously used for applications requiring good formability and high corrosion resistance. When they are used in the automotive industry, their advantages will be profitably enjoyed for the economic manufacture of structural components.