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
  • C22C38/18—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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present invention relates to an austenitic stainless steel alloy of low nickel content.
• the invention also relates to an article manufactured from the steel alloy.
• Austenitic stainless steel is a common material for various applications since these types of steels exhibit good corrosion resistance, good mechanical properties as well as good workability.
• Standard austenitic stainless steels comprise at least 17 percent chromium, 8 percent nickel and the rest iron. Other alloying elements are also often included.
• the steels described above exhibit good hot workability and high deformation hardening. These are properties which are important for the manufacturing of articles of large dimensions, such as heavy sheets. However, the steels described above have proven unsuitable for certain articles which require cold working including large reduction ratios.
• WO0026428 describes a low nickel steel alloy in which the amount of alloy elements have been combined to achieve a formable steel which exhibit good resistance to corrosion and work hardening. Further, the steel contains expensive alloy elements. Another steel alloy is described in JP2008038191. In this steel alloy, the elements have been balanced for improving the surface conditions of the steel. However, the properties of the above mentioned steel alloys make them unsuitable for processes involving cold working including large reduction ratios.
• one object of the present invention is to provide a low nickel austenitic stainless steel alloy, which can be cold worked with large reduction ratios.
• the inventive austenitic stainless steel alloy is referred to as the steel alloy.
• the inventive steel alloy should have good mechanical properties, comparable to the known steel grade AISI 302, as well as good corrosion properties.
• the composition of the steel alloy should be carefully balanced with regard to the influence of each alloy element so that a cost effective steel alloy is achieved, which fulfils the demands on productivity and final properties.
• the steel alloy should exhibit good hot workability properties.
• the steel alloy should further be so ductile and stable against deformation hardening such that it can be cold worked at high productivity at high reduction ratios without cracking or becoming brittle.
• a further object of the present invention is to provide an article manufactured from the improved austenitic stainless steel alloy.
• an austenitic stainless steel alloy having the following composition in percent of weight (wt%):
• MD30 (551 -462 * ([%C]+ [%N]) -9.2 * [%Si] -8.1 * [%Mn] -13.7 * [%Cr] - 29 * ([%Ni]+ [%Cu]) -68 * [%Nb] -18.5 * [%Mo]) 0 C,
• the particular composition provides a cost effective low nickel austenitic stainless steel alloy with excellent mechanical properties, excellent workability properties and improved resistance to corrosion compared to other low nickel austenitic stainless steel alloys.
• the workability properties of the steel alloy are optimized with regard to cold forming and reduced nickel content.
• the steel alloy is especially suitable for manufacturing processes which involve large reduction ratios of the steel.
• Articles of small dimensions for example springs
• wires may readily be manufactured from the steel alloy by cold drawing.
• Other examples of articles include, but are not limited to, strips, tubes, pipes, bars and products manufactured by cold-heading and forging.
• An advantage of the inventive steel alloy is that that it allows for the manufacturing of an article by cold working in fewer production steps since the number of intermediate heat treatments can be reduced.
• Articles produced by the steel alloy have proven very cost effective since the amounts of the alloying elements are carefully optimized with regard to their effect on the properties of the steel alloy.
• the contents of the alloy elements in the steel alloy may preferably be adjusted such that the following condition is fulfilled:
• the contents of the alloy elements in the steel alloy may preferably be adjusted such that the following condition is fulfilled: Nieqv + 0.85 * Cr eqv ⁇ 31.00 whereby the risk of a too high deformation hardening of the untransformed austenitic phase can be avoided and the formation of unwished phases such as Cr 2 N and N 2 (gas) can be controlled, which guarantees that optimal mechanical properties are achieved in the steel alloy.
• the contents of the alloy elements in the steel alloy may preferably be balanced such that the following condition is fulfilled: Nieqv + 0.85 * Cr eqv ⁇ 30.00 whereby the risk of a too high deformation hardening of the untransformed austenitic phase can be avoided and the formation of unwished phases such as Cr 2 N and N 2 (gas) can be controlled, which guarantees that optimal mechanical properties, are achieved in the steel alloy.
• the steel alloy may advantageously be included in an article, for example a wire, a spring, a strip, a tube, a pipe, a bar, and products manufactured by cold-heading and forging.
• the steel alloy is optimal for use in the manufacture of an article, for example a wire, a spring, a strip, a tube, a pipe, a cold-headed article or a forged article or an article produced by cold pressing/ cold forming.
• the inventors of the present invention have found that by carefully balancing the amounts of the alloy elements described below both with regard to the effects of each separate element and to the combined effect of several elements a steel alloy is achieved which has excellent ductility and workability properties as well as improved corrosion resistance compared to other low nickel austenitic stainless steel alloys. In particular it was found that optimal properties are achieved in the steel alloy when the amounts of the alloying elements are balanced according to relationships described below.
• Carbon (C) stabilizes the austenitic phase of the steel alloy at high and low temperatures. Carbon also promotes deformation hardening by increasing the hardness of the martensitic phase, which to some extent is desirable in the steel alloy. Carbon further increases the mechanical strength and the aging effect of the steel alloy. However, a high amount of carbon drastically reduces the ductility and the corrosion resistance of the steel alloy. The amount of carbon should therefore be limited to a range from 0.02 to 0.06 wt%.
• Silicon (Si) is necessary for removing oxygen from the steel melt during manufacturing of the steel alloy. Silicon increases the aging effect of the steel alloy. Silicon also promotes the formation of ferrite and in high amounts, silicon increases the tendency for precipitation of intermetallic phases.
• the amount of silicon in the steel alloy should therefore be limited to a maximum of 1.0 wt%. Preferably is the amount of silicon limited to a range from 0.2 to 0.6 wt%.
• Manganese (Mn) stabilizes the austenite phase and is therefore an important element as a replacement for nickel, in order to control the amount of ferrite phase formed in the steel alloy. However, at very high contents, manganese will change from being an austenite stabilizing element to become a ferrite stabilizing element.
• manganese Another positive effect of manganese is that it promotes the solubility of nitrogen in the solid phase, and by that also indirectly increases the stability of the austenitic microstructure. Manganese will however increase the deformation hardening of the steel alloy, which increases the deformation forces and lowers the ductility, causing an enlarged risk of formation of cracks in the steel alloy during cold working. Increased amounts of manganese also reduces the corrosion resistance of the steel alloy, especially the resistance against pitting corrosion.
• the amount of manganese in the steel alloy should therefore be limited to a range from 2.0 to 6.0 wt%, preferably is the amount of manganese limited to a range from 2.0 to 5.5 wt%, more preferably to a range from 2.0 to 5.0 wt%.
• Nickel (Ni) is an expensive alloying element giving a large contribution to the alloy cost of a standard austenitic stainless steel alloy. Nickel promotes the formation of austenite and thus inhibits the formation of ferrite and improves ductility and to some extent the corrosion resistance. Nickel also stabilizes the austenite phase in the steel alloy from transforming into martensite phase (deformation martensite) during cold working. However, to achieve a proper balance between the austenite, ferrite and martensite phases on one hand, and the total alloy element cost of the steel alloy on the other hand, the amount of nickel should be in the range from 2.0 to 4.5 wt%, preferably is the amount of nickel limited to a range from 2.5 to 4.0 wt%.
• Chromium (Cr) is an important element of the stainless steel alloy since it provides corrosion resistance by the formation of a chromium-oxide layer on the surface of the steel alloy. An increase in chromium content can therefore be used to compensate for changes in other elements, causing reduced corrosion properties, in order to accomplish an optimal corrosion resistance of the steel alloy. Chromium promotes the solubility of nitrogen in the solid phase which has a positive effect on the mechanical strength of the steel alloy. Chromium also reduces the amount of deformation martensite during cold working, and by that indirectly helps to maintain the austenitic structure, which improves the cold workability of the steel alloy.
• the amount of chromium in the steel alloy should therefore be in the range from 17 wt% to 19 wt%, preferably is the amount of chromium limited to a range from 17.5 to 19 wt%.
• Copper increases the ductility of the steel and stabilizes the austenite phase and thus inhibits the austenite-to-martensite transformation during deformation which is favourable for cold working of the steel. Copper will also reduce the deformation hardening of the untransformed austenite phase during cold working, caused by an increase in the stacking fault energy of the steel alloy. At high temperatures, a too high amount of copper sharply reduces the hot workability of the steel, due to an extended risk of exceeding the solubility limit for copper in the matrix and to the risk of forming brittle phases. Besides that, additions of copper will improve the strength of the steel alloy during tempering, due to an increased precipitation hardening.
• copper promotes the formation of chromium nitrides which may reduce the corrosion resistance and the ductility of the steel alloy.
• the amount of copper in the steel alloy should therefore be limited to a range from 2.0 wt% to 4.0 wt%.
• Nitrogen (N) increases the resistance of the steel alloy towards pitting corrosion. Nitrogen also promotes the formation of austenite and depresses the transformation of austenite into deformation martensite during cold working. Nitrogen also increases the mechanical strength of the steel alloy after completed cold working, which can be further improved by a precipitation hardening, normally produced by a precipitation of small particles in the steel alloy during a subsequent tempering operation. However, higher amounts of nitrogen lead to increasing deformation hardening of the austenitic phase, which has a negative impact on the deformation force. Even higher amounts of nitrogen also increase the risk of exceeding the solubility limit for nitrogen in the solid phase, giving rise to gas phase (bubbles) in the steel. To achieve a correct balance between the effect of stabilization of the austenitic phase and the effect of precipitation hardening and deformation hardening, the content of nitrogen in the steel alloy should be limited to a range from 0.15 to 0.25 wt%.
• molybdenum is an expensive alloying element and it also has a strong stabilizing effect on the ferrite phase. Therefore, the amount of molybdenum in the steel alloy should be limited to a range from 0 to 1.0 wt%, preferably 0 to 0.5 wt%.
• Tungsten stabilizes the ferrite phase and has a high affinity to carbon.
• high contents of tungsten in combination with high contents of Cr and Mo increase the risk of forming brittle inter-metallic precipitations.
• Tungsten should therefore be limited to a range from 0 to 0.3 wt%, preferably 0 to 0.2 wt%, more preferably 0 to 0.1 wt%.
• Vanadium (V) stabilizes the ferrite phase and has a high affinity to carbon and nitrogen. Vanadium is a precipitation hardening element that will increase the strength of the steel after tempering. Vanadium should be limited to a range from 0 to 0.3 wt% in the steel alloy, preferably 0 to 0.2 wt%, more preferably 0 to 0.1 wt%.
• Titanium (Ti) stabilizes the delta ferrite phase and has a high affinity to nitrogen and carbon. Titanium can therefore be used to increase the solubility of nitrogen and carbon during meting or welding and to avoid the formation of bubbles of nitrogen gas during casting. However, an excessive amount of Ti in the material causes precipitation of carbides and nitrides during casting, which can disrupt the casting process. The formed carbon-nitrides can also act as defects causing a reduced corrosion resistance, toughness, ductility and fatigue strength. Titanium should be limited to a range from 0 to 0.5 wt%, preferably 0 to 0.2 wt%, more preferably 0 to 0.1 wt%.
• Aluminium is used as de-oxidation agent during melting and casting of the steel alloy. Aluminium also stabilizes the ferrite phase and promotes precipitation hardening. Aluminium should be limited to a range from 0 to 1.0 wt%, preferably 0 to 0.2 wt%, more preferably 0 to 0.1 wt%.
• Niobium (Nb) stabilizes the ferrite phase and has a high affinity to nitrogen and carbon. Niobium can therefore be used to increase the solubility of nitrogen and carbon during melting or welding. Niobium should be limited to a range from 0 to 0.5 wt%, preferably 0 to 0.2 wt%, more preferably 0 to 0.1 wt%.
• Co Co has properties that are intermediate between those of iron and nickel. Therefore, a minor replacement of these elements with Co, or the use of Co-containing raw materials will not result in any major change in properties of the steel alloy.
• Co can be used to replace some Ni as an austenite-stabilizing element and increases the resistance against high temperature corrosion.
• Cobalt is an expensive element so it should be limited to a range from 0 to 1.0 wt%, preferably 0 to 0.5 wt%.
• the steel alloy may also contain minor amounts of normally occurring contamination elements, for example sulphur and phosphorus. These elements should not exceed 0.05 wt% each.
• delta ferrite The balance between the alloy elements which promotes stabilization of the austenite and ferrite (delta ferrite) phases is important since the hot and cold workability of the steel alloy generally depends on the amount of delta ferrite in the steel alloy. If the amount of delta ferrite in the steel alloy is too high, the steel alloy may exhibit a tendency towards hot cracking during hot rolling and reduced mechanical properties such as strength and ductility during cold working. Additionally, delta ferrite can act as precipitation sites for chromium nitrides, carbides or inter-metallic phases. Delta ferrite will also drastically reduce the corrosion resistance of the steel alloy.
• the chromium equivalent is a value corresponding to the ferrite stability and its effect on the phases formed in the microstructure during solidification of the steel alloy.
• the chromium equivalent may be derived from the modified Schaeffler DeLong diagram and is defined as:
• the nickel equivalent is a value corresponding to the austenite stability and its effect on the phases formed in the microstructure during solidification of the steel alloy.
• the nickel equivalent may also be derived from the modified Schaeffler DeLong diagram and is defined as: [%Ni]+[%Co]+0,5 * [%Mn]+0.3 * [%Cu]+25 * [%N]+30 * [%C].
• the amount of delta ferrite stabilizing alloying elements according to equation 1 and the amount of austenite stabilizing alloying elements according to equation 2 should be balanced such that condition B2 is fulfilled.
• the amount of delta ferrite stabilizing alloying elements according to equation 1 and the amount of austenite stabilizing alloying elements according to equation 2 should be balanced such that condition B4 is fulfilled.
• the amount of delta ferrite stabilizing alloying elements according to equation 1 and the amount of austenite stabilizing alloying elements according to equation 2 should be balanced such that condition B5 is fulfilled.
• the combination of ferrite and austenite forming alloy elements in the steel alloy is excellent.
• the amount of delta ferrite in the austenite matrix is balanced as well as the stability of the austenite phase and the amount of deformation martensite.
• the steel alloy therefore exhibits excellent mechanical and workability properties and good corrosion resistance.
• the properties of the steel alloy may further be improved by optimizing the balance between ferrite and austenite forming alloy elements according to relationships B2, B4 and B5.
• Alloy compositions that do not fulfil relationship B1 generally have too high amount of austenite stabilizing elements in relation to the ferrite stabilizing elements, and in view of the low amounts of delta ferrite phase formed.
• a high austenite stability is mainly accomplished by an increase in the manganese or nitrogen contents, causing a high stability of the austenite phase, followed by an increased deformation hardening of this phase during working.
• Alloy compositions that fulfil relationship B2 exhibit increased ductility during working and improved corrosion resistance since the amount of ferrite stabilizing elements in relation to the austenite stabilizing elements is balanced such that an optimal amount of delta ferrite phase is achieved in the steel alloy.
• Alloy compositions that fulfil relationship B3 exhibit reduced deformation hardening and an increased ductility, mainly during cold working.
• the improvement of these properties is mainly due to that the amounts of both ferrite and austenite stabilizing elements are high enough to cause a stable austenite phase with low amounts of deformation martensite.
• the relationship between alloying elements which depress the formation of martensite in the steel alloy is important for strength and ductility of the steel alloy.
• Low ductility at room temperature depends to a certain extent on deformation hardening, which is caused by the transformation of austenite into martensite during cold working of the steel alloy. Martensite increases the strength and hardness of the steel. However, if too much martensite is formed in the steel, it may be difficult to work in cold conditions, due to increased deformation forces. Too much martensite also decreases the ductility and may cause cracks in the steel during cold working of the steel alloy.
• the stability of the austenite phase in the steel alloy during cold deforming may be determined by the MD30 value of the steel alloy.
• a decreased MD30 temperature corresponds to an increased austenite stability, which will lower the deformation hardening during cold working, due to a reduced formation of deformation martensite.
• the MD30 value of the inventive steel alloy is defined as:
• MD30 (551 -462 * ([%C]+ [%N]) -9.2 * [%Si] -8.1 * [%Mn] -13.7 * [%Cr] - 29 * ([%Ni]+ [%Cu]) -68 * [%Nb] -18.5*[%Mo]) 0 C. (3)
• Figure 1 shows a S-N curve at 90% security against failure of tempered springs coiled from wire 1.0 mm in diameter.
• S is the stress in MPa and N is the number of cycles.
• the mean stress is 450 MPa.
• Heats of steel alloys according to the invention named: A, B, C were prepared. As comparison were also heats of comparative steel alloys named D, E 1 F, G, H, I, J, K, L.
• the heats were prepared on laboratory scale by melting of component elements in a crucible placed in an induction furnace. The composition of each heat is shown in table 1 a and 1 b.
• Equations 1-3 were calculated for each heat of steel alloy, table 2 shows the results from the calculations. The results from table 2 were then compared with the conditions for each equation, B1-B6 and it was determined if the test heats fulfilled the conditions B1-B6. Table 3 shows the result of the comparison. A "YES” means that the condition is fulfilled, a "NO” means that the condition is not fulfilled.
• the melts were cast into small ingots and samples of steel alloy having dimensions of 4x4x3 mm 3 were prepared from each heat.
• Table 1a Composition in wt% of inventive steel alloys.
• Table 1 b Composition in wt% of comparative steel alloys.
• each heat was then determined by a series of tests, described below, performed on the sample taken from each heat.
• each sample was subjected to plastic deformation by pressing of the sample in a hydraulic press under increasing force until a thickness reduction corresponding to 60% plastic deformation was accomplished.
• the applied maximum force in kN was measured for each sample. The results are shown in table 4.
• the Vickers hardness [HV1] of each sample was thereafter measured according to standard measurement procedure (SS112517). The results from the hardness measurement are shown in table 4.
• Samples from heats D, G, H and I exhibited too high hardness after deforming, ranging from 474 to 484 HV, to be suitable for cold working into fine dimensions, A high number of cracks, 87 and 41 , were observed in samples from heats G and I.
• Samples from heats E, F, J, K and L exhibited too high deformation force, 180 to 193 N, to be suitable for cold working with high reduction ratios.
• Samples from heats K and L exhibited in addition thereto relatively high hardness, 487 and 458 HV.
• a high number of cracks, 43 and 53 were also observed in samples from heats F and J.
• a heat of the inventive steel alloy named M was prepared.
• Two heats named N and O of a slightly different composition were prepared for comparison.
• P of steel alloy AISI 302 a standard spring steel alloy, prepared as well as one heat, named Q of steel alloy AISI 204Cu, a standard steel alloy of low nickel content.
• Ingots of heat M as well as ingots of heats N, O, P, and Q of the comparative steel alloys were heated to a temperature of 1200 0 C and formed by rolling into square bars of a final dimension of 150 x 150 mm 2 .
• the square bars were then heated to a temperature of 1250 0 C and rolled into wire of a diameter of 5.5 mm.
• the wire rod was annealed directly after rolling at 1050 0 C. All heats had good hot working properties.
• the hot rolled wires were finally cold drawn in several steps with intermediate annealing at 1050 0 C, into a final diameter of 1.4 mm, 1.0 mm. 0.60 mm and 0.66 mm. Wire was also cold rolled to a dimension of 2.75 x 0.40 mm 2 . Samples were taken from the cold drawn wires.
• the properties of the steel alloy of each heat were analyzed during cold working of the steel alloys and the results were documented. It was observed that the steel alloy of heat M had excellent workability, low deformation hardening and high ductility. All these properties were better or at the same level in comparison to heats P and Q of the standard AISI 302 or 204Cu grade steel. It was also observed that heat O had good workability but the deformation hardening was higher than AISI 302. Heat N became brittle already at low reductions and tension cracks were observed.
• the tensile strength was determined according to standard SSEM 10002-1 on samples from wire rod (5.50 mm) and cold drawn wire from heats M, N, O and P. All samples were drawn and annealed with the same production parameters. The amount of martensite in the samples having a diameter of 5.50 mm by a magnetic balance equipment. The amount of martensite was again measured in samples that were drawn to a diameter of 1.4 mm and the increase in martensite phase was calculated. Table 8 shows the results from the tensile test and the amount of deformation martensite in the samples.
• the tempering effect is important for many applications, especially for springs.
• a high tempering response will benefit many spring properties like spring force, relaxation and fatigue resistance.
• the tensile increase for samples from heat M is much higher than samples from heat P (AISI 302).
• a high tensile increase is important for many applications, especially for spring applications.
• the high tempering response of heat M depends mainly on the high copper and nitrogen content, which increases the precipitation hardening of the steel alloy.
• Relaxation is a very important parameter for spring applications. Relaxation is the spring force that the spring looses over time.
• the relaxation property was determined for heats M and P. Samples of 1.0 mm wire were taken from each heat. Each wire sample was coiled to a spring and tempered at 35O 0 C for 1 hour. Each spring was thereafter stretched to a length that corresponds to a stress of 800, 1000, 1200 and 1400 MPa, respectively. The loss of spring force in Newton (N) was measured over 24 hours at room temperature. The relaxation is the loss of spring force measured in percent. The results from the test are shown in table 10.
• the fatigue strength was determined on samples from heats M and P.
• the resistance against pitting corrosion was determined on the samples from heat M and from heat P (AISI 302) and heat Q (AISI 204Cu) by measuring the Critical Pitting Temperature (CPT) during electrochemical testing.
• CPT Critical Pitting Temperature
• a 5.5 mm wire rod sample was taken from each steel heat. Each sample was grinded and polished to reduce the influence of surface properties. The samples were immersed in a 0.1% NaCI solution at a constant potential of 30OmV. The temperature of the solution was increased by 5°C each 5 min until the point where corrosion on the samples could be registred. The result of the CPT testing is shown in table 11.
• Table 11 shows that Heat M exhibit adequate resistance to pitting corrosion in comparison to Heat P (AISI 302). The results from the corrosion tests further show that heat M exhibits higher resistance to corrosion than heat Q (AISI 204Cu).
• CPT Critical pitting temperature

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