Patent Publication Number: US-2006005899-A1

Title: Steel composition for use in making tillage tools

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
      Not Applicable.  
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
      Not Applicable.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a steel composition for use in making tillage tools that function to penetrate and slice through the earth during agricultural soil preparation operations.  
      2. Description of the Related Art  
      Agricultural tillage systems are used to penetrate, part, and in some cases, level soil in soil preparation operations. Example tillage systems can be found in U.S. Pat. Nos. 6,276,462 and 4,403,662 which are incorporated herein by reference. The sharp edged discs, blades, chisels, knives and the like that are used on various tillage tools for penetrating and parting the soil during planting, cultivating and tilling undergo severe wear as the soil impacts and abrades surfaces of the tillage tools.  
      Tillage tools operate in varying field conditions and therefore require a combination of wear resistance, strength and impact resistance (toughness). Typically, plain carbon steels or low alloy steels are used for tillage tools. It is well known that increasing the carbon content of a steel part results in an increase in wear resistance and strength but a decrease in impact resistance (toughness). It is also well known that increasing the hardness of a steel part results in an increase in wear resistance and strength but a decrease in impact resistance (toughness). For these reasons, a balance between hardness and carbon content must be established to give the best combination of wear resistance, strength and impact resistance (toughness) for tillage tools.  
      Traditionally, a high carbon steel, such as a steel having 0.85 weight percent carbon, is used in making tillage tools. The high carbon steel is heat treated to a high hardness in order to enhance the wear resistance and strength of the tillage tools. However, as stated above, increases in carbon content and hardness decrease impact resistance (toughness), an important property for tillage tools. In order to restore the impact resistance (toughness) to heat-treated high carbon parts, the hardness of the part must be reduced by tempering to a Rockwell hardness typically in the range of Rc 40-43, which reduces the wear resistance and some of the benefit of using high carbon materials.  
      What is needed therefore is an improved steel composition for use in making tillage tools wherein the steel composition is tailored to provide a superior combination of wear resistance and impact toughness for the tillage tools.  
     SUMMARY OF THE INVENTION  
      The foregoing needs are met by a steel composition according to the invention particularly suitable for use in making tillage tools. A steel composition chemistry has been developed to meet the hardness requirements, maintain the impact strength, and optimize the wear resistance of tillage tools. The invention involves the use of a steel with a chemical composition tailored to provide a superior combination of wear resistance and impact toughness for tillage tools. The specially tailored chemical composition takes advantage of the good toughness imparted through the use of a steel having a lower carbon content than steels commonly used in making tillage tools. Wear tests have shown that components formed from a steel according to the invention possess an unexpectedly high level of wear resistance for the level of carbon content. This is attained through a heat treating process which produces maximum or near maximum possible hardness for the carbon content of the steel used.  
      Because of the relationship between carbon content and wear resistance, lower carbon steels have not traditionally been used in ground engaging applications. We have discovered that wear resistance is affected by both carbon content and hardness, and in laboratory wear tests, we have shown a significantly higher level of wear resistance than previously expected for a steel having a lower carbon content than steels commonly used in making tillage tools. These tests have shown the wear resistance benefit of a higher carbon content can be offset, while maintaining sufficient impact resistance (toughness) by utilizing a steel with a carbon content in the range of 0.25-0.40 weight percent and Rockwell hardness typically in the range of Rc 48-52.  
      In one aspect, the invention is a steel composition comprising: from 0.25 to 0.40% of carbon; from 0.90 to 1.40% of manganese; from 0.0005 to 0.005% of boron; from 0.15 to 0.30% of silicon; not more than 0.30% of nickel; from 0.10 to 0.30% of chromium, the nickel and chromium together being not more than 0.40%; and at least 97% iron and incidental impurities, wherein all percentages are percentages by weight of the total composition. In a preferred form, the invention is a steel composition consisting essentially of from 0.30 to 0.35% of carbon; from 1.00 to 1.25% of manganese; from 0.0005 to 0.003% of boron; from 0.15 to 0.30% of silicon; not more than 0.30% of nickel; from 0.10 to 0.30% of chromium, the nickel and chromium together being not more than 0.40%; from 0.001 to 0.004% of calcium; not more than 0.035% of phosphorous; not more than 0.015% of sulfur; not more than 0.10% of copper; not more than 0.040% of molybdenum; the balance being iron and incidental impurities, wherein all percentages are percentages by weight of the total composition.  
      In another aspect, the invention is a tillage tool comprising a steel composition comprising: from 0.25 to 0.40% of carbon; from 0.90 to 1.40% of manganese; from 0.0005 to 0.005% of boron; from 0.15 to 0.30% of silicon; not more than 0.30% of nickel; from 0.10 to 0.30% of chromium, the nickel and chromium together being not more than 0.40%; and at least 97% iron and incidental impurities, wherein all percentages are percentages by weight of the total composition. In a preferred form, the invention is a tillage tool comprising a steel composition consisting essentially of from 0.30 to 0.35% of carbon; from 1.00 to 1.25% of manganese; from 0.0005 to 0.003% of boron; from 0.15 to 0.30% of silicon; not more than 0.30% of nickel; from 0.10 to 0.30% of chromium, the nickel and chromium together being not more than 0.40%; from 0.001 to 0.004% of calcium; not more than 0.035% of phosphorous; not more than 0.015% of sulfur; not more than 0.10% of copper; not more than 0.040% of molybdenum; the balance being iron and incidental impurities, wherein all percentages are percentages by weight of the total composition.  
      It is therefore an advantage of the present invention to provide a steel composition tailored to provide a superior combination of wear resistance and impact toughness for tillage tools.  
      These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, drawing, and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a graph illustrating wear tests comparing a steel composition according to the invention and a conventional steel used in tillage tools. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A steel composition according to the invention has been developed to meet the hardness requirements, maintain the impact strength (toughness), and optimize the wear resistance of tillage tools. The invention involves the use of a steel with a chemical composition tailored to provide a superior combination of wear resistance and impact toughness for tillage tools. The specially tailored chemical composition takes advantage of the good toughness imparted through the use of a steel having a lower carbon content than steels commonly used in making tillage tools.  
      A steel composition according to the invention includes at least 97% iron and incidental impurities remaining from a standard steel making process. The composition also includes various alloying elements as described below.  
      A steel composition according to the invention includes from 0.25 weight percent to 0.40 weight percent of carbon based on the total weight of the steel composition. The lower limit of carbon must be selected such that the steel may be hardened to a sufficient hardness for the tillage tools. A lower limit of 0.25 weight % carbon is selected to achieve a minimum hardness as measured on the Rockwell “C” scale of 48. An upper limit of 0.40 weight % carbon is selected as carbon levels above 0.40 weight % may adversely affect impact toughness. Preferably, a steel composition according to the invention includes from 0.30 weight percent to 0.35 weight percent of carbon based on the total weight of the steel composition.  
      A steel composition according to the invention includes from 0.90 weight percent to 1.40 weight percent of manganese based on the total weight of the steel composition. A lower limit of 0.90 weight % manganese is selected at a level to enhance the hardenability of the steel. An upper limit of 1.40 weight % manganese is selected to insure a consistent response to heat treatment. Preferably, a steel composition according to the invention includes from 1.00 weight percent to 1.25 weight percent of manganese based on the total weight of the steel composition.  
      A steel composition according to the invention includes from 0.0005 weight percent to 0.005 weight percent of boron based on the total weight of the steel composition. A lower limit of 0.0005 weight % boron is selected at a level to enhance the hardenability of the steel. An upper limit of 0.005 weight % boron is selected as boron levels above 0.005 weight % do not further enhance hardenability. Preferably, a steel composition according to the invention includes from 0.0005 weight percent to 0.003 weight percent of boron based on the total weight of the steel composition.  
      A steel composition according to the invention includes not more than 0.30 weight percent of nickel based on the total weight of the steel composition. The lower limit of nickel is selected at a level to enhance the hardenability of the steel. An upper limit of 0.30 weight % nickel is selected at a level to enhance the hardenability of the steel without adding significant cost.  
      A steel composition according to the invention includes from 0.10 weight percent to 0.30 weight percent of chromium based on the total weight of the steel composition. A lower limit of 0.10 weight % chromium is selected at a level to enhance the hardenability of the steel. An upper limit of 0.30 weight % chromium is selected to insure a consistent response to heat treatment. The nickel and chromium together are selected as being not more than 0.40 weight % of the steel composition to insure a consistent response to heat treatment without a significant increase in material cost.  
      A steel composition according to the invention may include from 0.001 to 0.004 weight % of calcium based on the total weight of the steel composition. The lower and upper limits for calcium are selected for inclusion shape control wherein heat treatment produces 85% minimum globular sulfide inclusions to improve the impact resistance. Cerium may be used in place of calcium. If cerium is used, the cerium/sulfur ratio must be 3.0 minimum.  
      A steel composition according to the invention may include not more than 0.015 weight % of sulfur based on the total weight of the steel composition. The tight limitation of sulfur to 0.015% maximum adds to the impact resistance.  
      A steel composition according to the invention may include from 0.15 weight percent to 0.30 weight percent of silicon based on the total weight of the steel composition. Silicon is used in the steel making process for deoxidizing.  
      A steel composition according to the invention may further include: (i) not more than 0.035 weight % of phosphorous based on the total weight of the steel composition; (ii) not more than 0.10% of copper based on the total weight of the steel composition; and (iii) not more than 0.040% of molybdenum based on the total weight of the steel composition.  
      In a preferred embodiment, the invention is a steel composition consisting essentially of from 0.30 to 0.35% of carbon; from 1.00 to 1.25% of manganese; from 0.0005 to 0.003% of boron; from 0.15 to 0.30% of silicon; from 0.10 to 0.30% of nickel; from 0.10 to 0.30% of chromium, the nickel and chromium together being not more than 0.40%; from 0.001 to 0.004% of calcium; not more than 0.035% of phosphorous; not more than 0.015% of sulfur; not more than 0.10% of copper; not more than 0.040% of molybdenum; the balance being iron and incidental impurities, wherein all percentages are percentages by weight of the total composition. In another preferred embodiment, the invention is a tillage tool comprising this preferred steel composition. Non-limiting examples of tillage tools include harrow disks, cultivator sweeps, coulters, opener disks, chisel points, and chisel shovels. The steel composition is particularly useful in making harrow disk blades subjected to highly abrasive sandy soil or rocky soil conditions.  
      Because of the relationship between carbon content and wear resistance, lower carbon steels have not traditionally been used in ground engaging applications such as tillage tools. However, wear resistance is affected by both carbon content and hardness. A steel chemistry according to the invention meets the hardness requirements of 48-52 HRC, maintains the impact strength (toughness), and optimizes the wear resistance of tillage tools. The specially tailored chemical composition takes advantage of the good toughness imparted through the use of low carbon steel. A steel composition according to the invention may be formed into tillage tools within a specified hardness range using quenching and tempering techniques well known to those skilled in the art.  
     EXAMPLE  
      Laboratory wear tests were conducted using the ASTM Standard Test Method for Conducting Wet/Sand/Rubber Wheel Abrasion Tests (Designation: ASTM G 105-89). Normalized laboratory wear data were obtained from several tests of a lower carbon steel according to the invention heat treated to a higher hardness level than conventional tillage tools versus a higher carbon steel at a slightly lower hardness that is representative of conventional tillage tools. The tests were conducted using: (1) the following steels according to the invention having 0.30-0.32 weight percent carbon and heat treated to a hardness of Rockwell “C” 49-50 (Examples 1-3), and (2) conventional steels having 0.75-0.8 weight percent carbon and heat treated to a hardness of Rockwell “C” 41-43 (Examples 4-7).  
                                                  Invention   Conventional                                             Element   Ex. 1   Ex. 2   Ex. 3   Ex. 4   Ex. 5   Ex. 6   Ex. 7                                                     Carbon   0.30   0.32   0.31   0.76   0.8   0.75   0.76       Phosphorous   0.01   0.019   0.014   0.01   0.02   0.01   0.01       Sulfur   &lt;0.01   0.003   0.001   &lt;0.01   0.02   0.02   0.02       Manganese   1.18   1.11   1.1   0.82   1.46   0.92   1.38       Silicon   0.22   0.12   0.25   0.17   0.12   0.11   0.13       Chromium   0.21   0.21   0.19   0.03   0.52   0.03   0.12       Molybdenum   &lt;0.01   &lt;0.01   0.01   &lt;0.01   0.06   0.01   0.03       Nickel   0.03   0.03   0.04   0.01   0.11   0.01   0.08       Copper   0.02   0.02   0.08   0.02   0.37   0.05   0.34       Boron   0.002   0.0026   0.0029   0.0003   0   0   0                         Hardness   Rockwell “C” 49-50   Rockwell “C” 41-43                  
 
      The test data were plotted as normalized weight loss in milligrams versus steel hardness on the Rockwell “C” scale. The graph is shown in  FIG. 1 . The average weight loss for the conventional steel was 86.6 milligrams and the average weight loss for a steel according to the invention was 64.3 milligrams. A steel according to the invention shows an average decrease in weight loss of 26% during testing. The tests as depicted in the graph in  FIG. 1  show a significantly higher level of wear resistance than previously expected for a decrease in carbon content of 0.55% (i.e., from 0.85 weight percent carbon in the conventional steel to 0.30 weight percent in the steel according to the invention). The tests as depicted in the graph in  FIG. 1  also show a significantly higher level of wear resistance than previously expected for an increase in hardness of only 10 HRC from the conventional steel in use for tillage tools. The test results depicted in  FIG. 1  show that the wear resistance benefit of a higher carbon content can be offset, while maintaining sufficient impact resistance by utilizing a steel according to the invention with a nominal carbon content of 0.30% and nominal hardness of 50 HRC.  
      Thus, there has been provided an improved steel composition for use in making tillage tools wherein the steel composition is tailored to provide a superior combination of wear resistance and impact toughness for the tillage tools. The steel composition meets the hardness requirements, maintains the impact strength (toughness), and optimizes the wear resistance of tillage tools.  
      Although the present invention has been described with reference to certain embodiments, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein.