Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten

Definitions

• the present invention relates to a conventionally manufactured ferritic, stainless steel alloy having improved machinability to be used when low cutting speeds/weak dimensions are required.
• the invention belongs to the group of 20Cr2Mo-steel and has high workability, machinability and corrosion resistance in combination with the material being lead-free.
• the currently dominant stainless steel for machining in small diameters with low cutting speeds is a ferritic material alloyed with sulphur, lead and tellurium as machinability-promoting additives.
• lead may become prohibited or limited as alloying material in steel.
• the steel according to the invention is a lead-free material where the machi- nability properties are better and the corrosion properties are good, in com ⁇ parison with materials predominant on the market within the technical field.
• US 6 033 625 describes a ferritic, stainless steel alloy which may be alloyed with lead, tellurium, selenium, calcium and sulphur as machinability-improving additives as well as molybdenum, copper and nickel as corrosion resistance improving additives.
• the PRE-value is at least 19.
• JP 2001098352 A describes a ferritic, stainless steel alloy alloyed with sulphur as machinability-improving additive. This alloy may also contain addi ⁇ tives of tellurium, lead, selenium or bismuth.
• JP 10130794 A discloses a ferritic, stainless steel alloy, which may contain sulphur, lead, selenium, tellurium and calcium as machinability- improving additives and molybdenum and copper as corrosion resistance improving additives.
• the PRE-value is at least 20.
• the object of the present invention is to provide a ferritic stainless steel alloy having improved machinability.
• An additional object of the present invention is to provide a steel alloy that complies with the requirements according to existing and possibly coming environmental legislation, but in spite of this possess properties such as good machinability and good corrosion resistance, in comparison with the currently predominant steel alloy within the field in question.
• X 0,01-0,5 and/or REM 0,01-1 the balance Fe as well as impurities, where X is (2*Te + 1*Se + 1*Bi).
• the alloy may also contain additives of the elements Ca, Sn and B.
• Figure 1 shows the number of holes drilled with one and the same tungsten carbide drill for materials according to the invention in comparison with the reference material, 20Cr2Mo.
• Figure 2 shows the calculated fraction MnS in a composition containing 0,03 %C, 0,5 %Si, 1 ,5 %Mn, 21 %Cr, 0,5 %Ni, 2,5 %Mo, 1 %Cu and 0,05 %N, wherein the S content varies 0,10-0,35 %.
• Figure 3 shows the calculated fraction of M23C6 carbides in a composition containing 0,5 %Si, 1,5 %Mn, 0,35 %S, 21 %Cr, 0,5 %Ni, 2,5 %Mo, 1 %Cu and 0,05 % N, wherein the C content varies 0,01-0,1 %.
• Figure 4 shows the calculated fraction of nitrides in a composition containing 0,5 %Si, 1 ,5 %Mn, 0,35 %S, 21 %Cr, 0,5 %Ni, 2,5 %Mo, 1 %Cu and 0,03 % C, wherein the N content varies 0,04-0,05 %.
• Figure 5 shows the calculated sigma phase content in a composition consisting of 0,03 %C, 0,5 %Si, 1 ,5 %Mn, 0,35 %S, 0,5 %Ni, 1 %Cu, 2,5 % Mo and 0,05 %N, wherein the Cr-content is 20-25 %.
• Figure 6 shows the calculated sigma phase content in a composition consisting of 0,03 %C, 0,5 %Si, 1 ,5 %Mn, 0,35 %S, 0,5 %Ni, 1 %Cu, 21% Cr and 0,05 %N, wherein the Mo-content is 1,85-2,5 %.
• the present invention relates to a ferritic stainless steel alloy having the following composition, all contents in percent by weight:
• X 0,01-0,5 and/or REM 0,01-1 the balance Fe as well as impurities, where X is (2*Te + 1*Se + i*Bi).
• Sulphur improves the machinability by forming sulphides, e.g., MnS and CrS. These sulphides promote chip formation and chip breaking and thereby lowering machining costs and tool wear.
• high contents of sulphur may lead to problems in hot working and decrease the corrosion resistance.
• the contents of sulphur should not exceed 0,4 weight-%, the content should be between 0,08 and 0,4 weight-%, preferably in the interval of 0,1-0,4 weight- %, most preferably 0,15-0,35 weight-%.
• Tellurium is an additive used in order to modify the shape of the sulphide inclusions. Tellurium combines with manganese and alters the morphology of the MnS-inclusions. High contents of tellurium may give rise to poor hot workability, especially if the relation between tellurium and manganese is low. Tellurium should not be added in contents above 0,2 weight-% and should be in the interval of 0,01 weight-% to 0,2 weight-%, preferably in the interval of 0,01 to 0,015 weight-%, most preferably in the interval of 0,01 to 0,1 weight-%. Also selenium (Se) and bismuth (Bi) may be added for the same purpose. In order to obtain the desired result, the content of 2*Te + Se + Bi has to be within the interval of 0,01-0,5 weight-%, preferably 0,02-0,4 weight-%, most preferably 0,02-0,2 weight-%.
• Manganese (Mn) combines with sulfur forming manganese sulphides that enhance the machinability of the steel.
• the amount of manganese in the steel affects the morphology of the sulphide inclusions.
• Manganese is an austenite stabiliser, which entails that the content of manganese has to be kept low.
• the content of manganese in stainless steel is usually limited by the fact that the corrosion resistance is negatively affected at increasing content of manganese.
• Manganese should not be added in contents above 2,0 weight-% and should be in the interval of 0,1 to 2,0 weight-%, preferably in the interval of 0,2 to1,5 weight-%, most preferably in the interval of 0,4 to1 ,5 weight-%.
• Chromium (Cr) is a very important alloying element concerning the corrosion resistance of the material. This is due to the capability of chromium of forming a passive layer of CT 2 O 3 on the surface of the steel. In order to obtain a ferritic structure, the content of chromium in the material should be above 16 weight-%. In order for the material to get good resistance to pitting, a content of chromium of at least 19 weight-% is required. Therefore, the content of chromium should be in the interval of 16 to 25 weight-%, preferably be in the interval of 18 to 22 weight-%, most preferably in the interval of 19 to 21 weight-%.
• Silicon (Si) has a ferrite-stabilizing effect. Silicon is a precipitation-hardening element. At too high a content of silicon, the hot working becomes poor. How ⁇ ever, a certain quantity of silicon is required in order to deoxidize the material. Silicon should not be added in contents exceeding 2 weight-%, preferably maximum 1 weight-%, most preferably maximum 0,5 weight-%. Calcium (Ca) affects the morphology of the oxide inclusions. At high Ca/O ratios the melting point of the oxides is high and therefore the deformation capability of the oxides during cutting is low. This may give an increase of the tool wear. Calcium should not be added in contents above 0,006 weight-% and should be in the interval of 0 to 0,002 weight-%, preferably in the interval of O to 0,001 weight-%.
• Molybdenum is a ferrite-stabilizing element that has a highly beneficial effect on the corrosion resistance in chloride environments.
• the content of molybdenum should be in the interval of 1 ,0 to 5,0 weight-%, preferably in the interval of 1 ,5 to 2,5 weight-%, most preferably in the interval of 1 ,85 to 2,5 weight-%.
• Copper (Cu) has a positive effect on the machinability in respect of service life of the tool during machining.
• the reason for this is that copper precipitations in the size of 1 nm are precipitated along the grain boundaries in the material. Negative effects with high contents of copper may be deteriorations of the hot workability as well as the chip breaking of the material.
• the content of copper has to be in the interval of 0,01 to 3,0, preferably in the interval of 0,5 to 2,0 weight-%, most preferably in the interval of 0,7 to 2,0 weight-%
• Carbon (C) has a strong tendency to combine with chromium, which means that chromium carbides are precipitated in the grain boundaries. Accordingly, the surrounding bulk is depleted of chromium. This entails that the material becomes sensitive to intercrystalline corrosion. Therefore, the content of car ⁇ bon has to be kept as low as possible, maximum 0,1 weight-%, preferably maximum 0,05 weight-%, most preferably maximum 0,03 weight-%.
• Boron (B) contributes to increase the hot workability. It is to be added in a small amount, too great an amount gives poor hot workability.
• the content of boron should be between 0 to 0,02 weight-%, preferably in the interval of 0,0005 to 0,01 weight-%, most preferably in the interval of 0,001 to 0,01 weight-%.
• Nitrogen (N) is an austenite-forming element. In ferritic materials the solubility of nitrogen in the matrix is low. Even though nitrogen has a strong positive influence on the PRE-value a too high nitrogen content can be detrimental to the corrosion resistance. If preciptation of chromium nitride is formed in the material, these can be working as initiation points for corrosion. Even the workability of the material can be negatively affected by a high nitrogen content. The content of nitrogen therefore has to be kept as low as possible. The content of nitrogen must not exceed 0,05 weight-%.
• REM Rotary Earth Metals
• REM is used as machinability-improving additives.
• REM is a generic name of many elements, for instance cerium, lanthanum, praseo ⁇ dymium and neodymium. They modify the shape and composition of the non- metal inclusions.
• REM may be added either as a misch metal or as pure elements.
• REM-metals are added in contents of maximum 1 weight-%, preferably maximum 0,1 weight-%.
• Tin (Sn) acts as a machinability-improving additive at low cutting speeds.
• the content of tin should not exceed 0,15 weight-%, preferably maximum 0,10 weight-%.
• Test materials were manufactured by melting in a high-frequency furnace, casting, and subsequent heating and forging. After the forging, the blanks were fully ground, rolled and quenched. The blanks were annealed, water- cooled and then drawn in a conventional drawing machine. Finally, the material was straightened and ground in order to be tested.
• test materials were examined upon drilling, turning as well as chip breaking. Furthermore, the corrosion properties were evaluated by a neutral salt spray test (NSS), a copper chloride accelerated salt spray test (CASS), and by a pitting corrosion test (CPT).
• NSS neutral salt spray test
• CASS copper chloride accelerated salt spray test
• CPT pitting corrosion test
• Reference material for drilling and turning was a 20Cr2Mo-steel, hereinafter the reference material is denominated 20Cr2Mo.
• the chemical compositions and the PRE-value, in percent by weight, of the test materials, including the reference material, are found in table 1. REM, when included, have been added as misch metal.
• the item used for the turning test was designed so that the contour thereof could be formed with one and the same turning insert at the same time as a number of different machining directions were to be tested (plunge cutting as well as longitudinal turning with constant and varying, respectively, cutting depths).
• the alteration of the maximum diameter with the period of engagement can be described by a trend line.
• the trend line is linear and can be written in the form
• C is the point where the line intersects the diameter axis. From this, the incli ⁇ nation of the trend line may be determined.
• the chip-breaking test was carried out as a longitudinal turning operation, with a coated cemented carbide insert, using two different feedings and at two different dimensions. For each combination of feed and turned diameter, chips were collected, which then were assessed according to a marking scale divided into five degrees, see table 5. The lowest marking, not satisfactory, was given for long unbroken chips, after which the marking became better with decreasing chip length.
• CPT critical pitting temperature
• Corrosion tests have been carried out on the three test materials that gave the best machinability data and for the reference material 20Cr2Mo.
• NSS has been carried out according to SS-ISO 9227.
• CASS has been carried out according to SS-ISO 9227 with the nonconformity that the test has proceeded for 16 h instead of 96 h and 25 0 C instead of 50 0 C. ⁇
• A no appreciable corrosion
• B some corrosion ( ⁇ 20 % of the surface)
• C significant corrosion (20-70 % of the surface)
• D heavy corrosion (>70 % of the surface)
• the resistance to pitting was examined by using a constant potential, with the sample entirely immersed in a solution including chloride ions.
• the experimental data are disclosed in Table 7.
• the solution was de-aired by purging with nitrogen gas.
• the sample was polarized by connecting a voltage to the sample so as to control the electrochemical processes on the surface of the sample. Meanwhile the other variables were kept constant the tempera ⁇ ture was raised in 5-degree steps starting at 20 degrees.
• the CPT-value is defined as the temperature where a current of 10 ⁇ A/cm 2 is exeeded. If the sample passed up to 95 degrees, this temperature was registered, and the test was finished.
• test material having the highest CPT-value is the test material that is most resistant to pitting in an environment including chloride ions.
• Table 7 Experimental data in CPT-measurement.
• SCE Standard Calomel Electrode
• the alloy according to the present invention has good machinability and good corrosion resistance. Furthermore, the alloy is lead-free. ⁇
• the alloy according to the invention is preferably produced in a conventional way, it is however also possible to produce it in a powder-metallurgical way.
• the calculated content of MnS in a composition with 0,03 %C, 0,5 %Si, 1,5 %Mn, 21 %Cr, 0,5 %Ni, 2,5 %Mo, 1 %Cu and 0,05 %N is illustrated in Figure 2.
• the sulphur content varies between 0,10 % and 0,35 %. It is clear that the content of MnS increases with increasing S content.
• Figure 3 illustrates the calculated content of carbides of the form M 2 3C 6 , (M stands for chromium and possible also for a combination of chromium and molybdenum), in a composition containing 0,5 %Si, 1 ,5 %Mn, 0,35 %S,
• the calculated fraction of Cr 2 N is illustrated for a composition containing 0,5 %Si, 1 ,5 %Mn, 0,35 %S, 21 %Cr, 0,5 %Ni, 2,5 %Mo, 1 %Cu and 0,03 % C, and wherein the N content varies 0,04-0,05 %.
• Figure 5 illustrates a . -

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