Patent Publication Number: US-4483722-A

Title: Low alloy cold-worked martensitic steel

Description:
BACKGROUND OF THE INVENTION 
     Mechanical properties of high strength steels generally depend upon melting practices, alloying elements, and heat treatments to provide particular mechanical characteristics for the intended purpose of the steel. High strength steel characterized by high tensile strengths, yield strengths, and toughness generally require strengthening, toughening, and hardening elements to attain the desired properties. As a general rule, alloying elements in steel promote a general decrease in the rate of austenite transformation to other phases, such as pearlite, bainite, and martensite, depending upon the rate of cooling. Typical alloying ingredients to enhance mechanical properties of steel are chromium, manganese, molybdenum, nickel, and silicon. Chromium increases the resistance to corrosion and oxidation, while increasing hardenability and promoting strength at high temperatures. Manganese increases the hardenability, and is relatively inexpensive. Molybdenum raises the grain coarsening temperature of austenite, deepens hardening, counteracts temper brittleness, and raises hot and creep strengths of the steel. Nickel strengthens unquenched steels, while silicon strengthens low alloy steels. 
     It has been the objective of metallurgists to provide optimum mechanical properties in steels while employing relatively low percentages of alloying elements. An example of such efforts is represented by the disclosure of U.S. Pat. No. 3,379,582, the disclosure of which is incorporated herein by reference, wherein the patentee produces a low alloy, high strength steel having a martensitic microstructure. According to that patent, the patentee provides an iron base alloy having from 0.20% to 0.30% carbon, 0.80% to 1.2% manganese, 3.25% to 4.00% nickel, 1.25% to 2.00% chromium, 0.25% to 0.50% molybdenum, 0.20% to 0.50% silicon and residual amounts of other elements. The patentee heat treats the alloy by heating above the critical temperature to form austenite, and then preferably air-cools the steel to form a martensitic microstructure. The alloying ingredients permit slow cooling by decreasing the rate of austenite transformation so that the microstructure is substantially all martensitic. The steel is tempered at about 500° F. to raise the yield strength to 170,000 psi or higher and to slightly decrease the ultimate tensile strengths to about 215,000 psi. The hardness obtained in 5-inch and 8-inch sections was at least Rockwell C 38 (about 365 Brinell), which was measured at the bar center. Thus, the alloy produced by the patentee exhibits excellent tensile and yield strengths, while maintaining a relatively high hardness. Because of the ability of this alloy to air-cool to a martensitic structure, it was believed that the alloy could be formed into useful shapes by hot working techniques. It was assumed that cold working such a hard martensitic alloy would result in cracking, and that such working would deleteriously increase the hardness while reducing the ductility of the steel. As a general rule, cold working increases the tensile strength and hardness of the steel, while it reduces ductility, percentage elongation, and yield strength. 
    
    
     SUMMARY AND DETAILED DESCRIPTION OF THE INVENTION 
     This invention provides a technique for producing a low alloy, high strength, martensitic steel which is cold-worked in such a manner as to increase tensile strength and yield strength whie maintaining the as-tempered hardness of the steel. 
     According to this invention, a low alloy, high strength steel is prepared generally according to the teachings of U.S. Pat. No. 3,379,582, the subject matter of which is incorporated herein by reference. That is to say, a melt is prepared containing between about 0.20% to 0.30% carbon, 0.80% to 1.2% manganese, 3.25% to 4.00% nickel, 1.25% to 2.00% chromium, 0.25% to 0.50% molybdenum, 0.20% to 0.50% silicon, and the balance iron with residual amounts of other elements. A specific example of such an alloy was prepared having 0.20% carbon, 0.90% manganese, 3.45% nickel, 1.45% chromium, 0.30% molybdenum, and 0.28% silicon. The alloy was prepared by conventional vacuum degassing techniques in an electric arc furnace and was then cast into ingots. The ingots were hot forged to rods 1.5 inches (Table I), 2 inches (Table II), 3 inches (Table III), and 4 inches (Table IV) in diameter. After cooling, the rods were normalized at an austenitizing temperature above the A c1  temperature of the alloy. A typical normalizing temperature for the alloy set forth above is 1750° F. Substantially higher temperatures tend to cause grain coarsening with deleterious effects upon subsequent transformations. After through heating the rods, the rods were cooled in still air to a temperature below the M s  temperature of the steel to transform the steel to an essentially martensitic structure. Thereafter the rods were tempered by heating the rods to a temperature below their A 1  temperature. Alternately, the rods could be martempered by cooling the steel from its austenitizing temperature to a temperature below its A 1  by, for example, quenching the austenitized steel in molten salt maintained at the desired tempering temperature. After the tempering step, some of the rods, as noted below, were cold-worked by reducing their diameter between about 1/32 and 1/8 inch, and preferably between about 1/32 and 1/16 inch. After cold working, the bars may be straightened by employing a Medart straightener, which not only straightens the bar but adds a degree of polish. 
     The cold-worked rods exhibited the following properties as compared to samples which were hot-rolled and heat-treated. In the following Tables, all of the samples were subjected to the identical heat treatment as set forth above, but samples 1, 2, and 3 were cold drawn, while samples 11, 22 and 33 were cold drawn and straightened. 
     
                       TABLE I                                                     
______________________________________                                    
                       Yield                                              
                       Strength                                           
              Tensile  psi    Elong.                                      
                                    Red. of                               
      Size    Strength 0.2%   % in  Area  Hardness                        
Sample                                                                    
      Inches  psi      offset &#34;2&#34;   %     BHN                             
______________________________________                                    
A     1.495   161,500  149,500                                            
                              16.0  57.8  363                             
      1.500                                                               
 1    1.489   159,500  149,000                                            
                              15.0  60.1  341                             
      1.494                                                               
11    1.489   176,000  170,000                                            
                              12.0  54.1  341                             
      1.494                                                               
B     1.508   171,000  152,500                                            
                              16.0  55.7  363                             
      1.511                                                               
 2    1.461   177,500  172,500                                            
                              11.0  55.7  341                             
      1.462                                                               
22    1.461   180,000  163,000                                            
                              11.5  50.8  363                             
      1.462                                                               
C     1.502   171,000  153,000                                            
                              17.0  56.8  363                             
      1.504                                                               
 3    1.442   184,500  180,000                                            
                              11.0  52.5  341                             
33     1.4405 186,000  177,000                                            
                               11.00                                      
                                    53.4  363                             
       1.4410                                                             
______________________________________                                    
 A, B, C = Hot Rolled                                                     
 1, 2, 3 = Cold Drawn between 1/32 and 1/8 inch                           
 11, 22, 33 = Cold Drawn between 1/32 and 1/8 inch and straightened       
 
    
     
                       TABLE II                                                    
______________________________________                                    
                       Yield                                              
                       Strength                                           
              Tensile  psi    Elong.                                      
                                    Red. of                               
      Size    Strength 0.2%   % in  Area  Hardness                        
Sample                                                                    
      Inches  psi      offset &#34;2&#34;   %     BHN                             
______________________________________                                    
D     3.031   156,750  133,250                                            
                              16.0  50.8  341                             
      3.052                                                               
D     3.031   156,750  133,500                                            
                              16.0  54.4  341                             
      3.052                                                               
 1    2.987   165,000  162,500                                            
                              11.5  51.1  352                             
      2.989                                                               
 1    2.987   165,500  163,250                                            
                              12.5  51.1  352                             
      2.989                                                               
11    2.990   175,500  173,750                                            
                              12.0  51.4  363                             
      2.991                                                               
11    2.990   176,000  164,250                                            
                              12.0  50.0  363                             
      2.991                                                               
E     3.026   156,000  135,000                                            
                              15.5  50.8  341                             
      3.045                                                               
E     3.026   156,750  136,250                                            
                              16.0  53.3  341                             
      3.045                                                               
 2    2.986   162,000  159,000                                            
                              13.0  53.2  341                             
      2.988                                                               
 2    2.986   161,000  157,500                                            
                              12.0  54.7  341                             
      2.988                                                               
22    2.887   174,000  171,500                                            
                              11.0  48.6  363                             
      2.991                                                               
22    2.887   175,000  172,500                                            
                              11.5  49.5  363                             
      2.991                                                               
F     3.023   161,000  139,000                                            
                              16.0  52.2  352                             
      3.048                                                               
F     3.023   161,500  135,000                                            
                              15.0  50.0  352                             
      3.048                                                               
 3    2.987   170,000  166,250                                            
                              12.0  50.0  363                             
      2.989                                                               
 3    2.987   170,750  168,250                                            
                              11.0  48.1  363                             
      2.989                                                               
33    2.990   177,500  172,500                                            
                              11.5  48.4  363                             
      2.993                                                               
33    2.990   181,500  180,000                                            
                              10.5  46.9  363                             
      2.993                                                               
______________________________________                                    
 D, E, F = Hot Rolled                                                     
 1, 2, 3 = Cold Drawn between 1/32 and 1/8 inch                           
 11, 22, 33 = Cold Drawn between 1/32 and 1/8 inch and straightened       
 
    
     
                       TABLE III                                                   
______________________________________                                    
                       Yield                                              
                       Strength                                           
              Tensile  psi    Elong.                                      
                                    Red. of                               
      Size    Strength 0.2%   % in  Area  Hardness                        
Sample                                                                    
      Inches  psi      offset &#34;2&#34;   %     BHN                             
______________________________________                                    
G     2.014   156,750  138,000                                            
                              17.0  57.8  341                             
      2.020                                                               
G     2.014   155,750  135,500                                            
                              17.0  57.3  341                             
      2.020                                                               
 1    1.993   162,500  157,000                                            
                              17.0  57.8  341                             
      1.995                                                               
 1    1.993   163,500  156,000                                            
                              15.0  56.8  341                             
      1.995                                                               
11    1.993   165,770  160,000                                            
                              13.5  55.7  352                             
      1.996                                                               
11    1.993   165,000  152,000                                            
                              14.5  56.2  352                             
      1.996                                                               
H     2.014   164,000  147,500                                            
                              17.0  56.0  363                             
      2.026                                                               
H     2.014   165,000  145,500                                            
                              16.5  54.4  363                             
      2.026                                                               
 2    1.994   168,500  163,500                                            
                              15.0  56.0  352                             
      1.996                                                               
 2    1.994   168,500  162,500                                            
                              13.0  53.8  352                             
      1.996                                                               
22    1.992   174,000  170,000                                            
                              11.5  50.3  363                             
      1.996                                                               
22    1.992   174,000  165,000                                            
                              13.0  54.7  363                             
      1.996                                                               
J     2.012   165,000  146,250                                            
                              16.5  54.7  363                             
      2.021                                                               
J     2.012   164,500  146,000                                            
                              16.5  54.9  363                             
      2.021                                                               
 3    1.994   170,500  167,500                                            
                              12.0  50.6  341                             
      1.996                                                               
 3    1.994   169,000  168,500                                            
                              12.0  53.0  341                             
      1.996                                                               
33    1.993   172,500  166,750                                            
                              12.0  51.9  363                             
      1.994                                                               
33    1.993   172,500  166,750                                            
                              12.0  52.5  363                             
      1.994                                                               
______________________________________                                    
 G, H, J = Hot Rolled                                                     
 1, 2, 3 = Cold Drawn between 1/32 and 1/8 inch                           
 11, 22, 33 = Cold Drawn between 1/32 and 1/8 inch and straightened       
 
    
     
                       TABLE IV                                                    
______________________________________                                    
                       Yield                                              
                       Strength                                           
              Tensile  psi    Elong.                                      
                                    Red. of                               
      Size    Strength 0.2%   % in  Area  Hardness                        
Sample                                                                    
      Inches  psi      offset &#34;2&#34;   %     BHN                             
______________________________________                                    
K     4.022   159,250  135,000                                            
                              16.0  53.8  352                             
      4.032                                                               
K     4.022   158,750  137,500                                            
                              16.5  54.7  352                             
      4.032                                                               
 1    3.996   162,500  162,500                                            
                              13.0  53.3  341                             
      3.999                                                               
 1    3.996   167,500  165,000                                            
                              12.0  55.2  341                             
      3.999                                                               
11    3.998   165,000  165,000                                            
                              14.0  55.2  341                             
      4.000                                                               
11    3.998   162,500  161,000                                            
                              14.0  53.8  341                             
      4.000                                                               
L     4.030   159,500  139,000                                            
                              16.0  55.5  352                             
      4.042                                                               
L     4.030   160,000  139,500                                            
                              16.0  54.1  352                             
      4.042                                                               
 2    3.995   160,750  156,500                                            
                              15.0  55.5  352                             
      3.999                                                               
 2    3.995   161,500  157,500                                            
                              13.5  51.7  352                             
      3.999                                                               
22    3.994   163,750  163,750                                            
                              12.5  53.3  341                             
      3.997                                                               
22    3.994   163,750  163,500                                            
                              12.0  52.2  341                             
      3.997                                                               
M     4.023   156,500  134,500                                            
                              16.0  52.2  363                             
      4.033                                                               
M     4.023   157,500  133,250                                            
                              16.5  55.5  363                             
      4.033                                                               
 3    3.996   161,250  161,250                                            
                              14.0  54.4  331                             
      3.999                                                               
 3    3.996   159,500  159,500                                            
                              14.0  53.6  331                             
      3.999                                                               
33    3.994   158,250  155,000                                            
                              13.5  54.4  331                             
      3.996                                                               
33    3.994   158,000  157,500                                            
                              13.0  53.8  331                             
      3.996                                                               
______________________________________                                    
 K, L, M = Hot Rolled                                                     
 1, 2, 3 = Cold Drawn between 1/32 and 1/8 inch                           
 11, 22, 33 = Cold Drawn between 1/32 and 1/8 inch and straightened       
 
    
     As may be seen, hot rolled samples A, B, C, D, E, F, G, H, J, K, L, and M exhibit excellent tensile strengths, yield strengths, elongation, and hardness. With such steels, increases in tensile strengths and yield strengths are to be expected upon cold rolling or drawing. It would also be expected that hardness would increase along with tensile and yield strengths. However, as is evident from the foregoing Tables, the Brinell hardness in many cases remained the same after cold working with a reduction in diameter of between 1/32 and 1/8 inch, while, surprisingly, in some of the cases, the Brinell hardness actually dropped. 
     It is evident from the foregoing that a low-carbon, low-alloy martensitic steel has been provided which exhibits acceptably high tensile and yield strengths without an undue increase in hardness. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.