Patent Application
20230357909
This invention relates to an austenitic stainless steel which has high formability with the TRIP (Transformation Induced Plasticity) effect and high corrosion resistance and optimized pitting resistance equivalent (PRE).
The transformation induced plasticity (TRIP) effect refers to the transformation of metastable retained austenite to martensite during plastic deformation as a result of imposed stress or strain. This property allows stainless steels having the TRIP effect to have a high formability, while retaining excellent strength.
It is well known from patent application WO 2015/114222 a method for manufacturing a ferritic-austenitic stainless steel having good formability and high elongation, which steel contains in weight % less than 0.04 weight % carbon, 0.2-0.8 weight % silicon, less than 2.0 weight % manganese, 16.5-19.5 weight % chromium, 3.0-4.7 weight % nickel, 1.0-4.0 weight % molybdenum, less than 3.5 weight % tungsten, less than 1 weight % copper, 0.13-0.26 weight % nitrogen, the rest being iron and inevitable impurities occurring in stainless steels. Sulphur is limited to less than 0.010 weight % and preferably less than 0.005 weight %, the phosphorus content is less than 0.040 weight % and the sum of sulphur and phosphorus (S+P) is less than 0.04 weight %, and the total oxygen content is below 100 ppm. The aluminium content is maximized to less than 0.04 weight % and preferably the maximum is less than 0.03 weight %. Further, boron, calcium and cerium are optionally added in small quantities; the preferred contents for boron and calcium are less than 0.004 weight % and for cerium less than 0.1 weight %. Optionally cobalt can be added up to 1 weight % for partial replacement of nickel. Also one or more of the group containing niobium, titanium and vanadium can be optionally added in the stainless steel of the invention, the contents of niobium and titanium being limited up to 0.1 weight % and the vanadium content being limited up to 0.2 weight %.
According to the WO patent application 2015/114222 the pitting resistance equivalent (PRE) has been optimized to give good corrosion resistance, being in the range of 30-36. The critical pitting temperature (CPT) is in the range of 34-45° C. Heat treatment is performed so that the microstructure of the stainless steel contains 45-80% austenite in the heat treated condition, the remaining microstructure being ferrite. Further, the measured Md30 temperature of the stainless steel is adjusted between −30 and 90° C., preferably in the range 10-60° C. in order to utilize the TRIP effect for improving the formability of the stainless steel.
It is known from the WO patent application 2011/135170 a method for manufacturing a ferritic-austenitic stainless steel having good formability and high elongation, which steel contains in weight % less than 0.05% C, 002-0.7% Si, 2-5% Mn, 19-20.5% Cr, 0.8-1.35% Ni, less than 0.6% Mo, less than 1% Cu, 0.16-0.24% N, the balance being iron and inevitable impurities. The stainless steel of the WO patent application 2011/135170 is heat treated so that the microstructure of the stainless steel contains 45-75% austenite in the heat treated condition, the remaining microstructure being ferrite. Further, the measured Md30 temperature of the stainless steel is adjusted between 0 and 50° C. in order to utilize the TRIP effect for improving the formability of the stainless steel.
Furthermore, it is know from the WO patent application 2013/034804 a ferritic austenitic stainless steel utilizing the TRIP effect, which contains less than 0.04 weight % C, less than 0.7 weight % Si, less than 2.5 weight % Mn, 18.5-22.5 weight % Cr, 0.8-4.5 weight % Ni, 0.6-1.4 weight % Mo, less than 1 weight % Cu, 0.10-0.24 weight % N, the rest being iron and inevitable impurities occurring in stainless steels. Sulphur is limited to less than 0.010 weight % and preferably less than 0.005 weight %, the phosphorus content is less than 0.040 weight % and the sum of sulphur and phosphorus (S+P) is less than 0.04 weight %, and the total oxygen content is below 100 ppm. The stainless steel optionally contains one or more added elements in the following: the aluminium content is maximized to less than 0.04 weight % and preferably the maximum is less than 0.03 weight %. Further, boron, calcium and cerium are optionally added in small quantities; the preferred contents for boron and calcium are less than 0.003 weight % and for cerium less than 0.1 weight %. Optionally cobalt can be added up to 1 weight % for partial replacement of nickel, and tungsten can be added up to 0.5 weight % as partial replacement of molybdenum. Also one or more of the group containing niobium, titanium and vanadium can be optionally added in the stainless steel of the invention, the contents of niobium and titanium being limited up to 0.1 weight % and the vanadium content being limited up to 0.2 weight %.
According to the WO patent application 2013/034804 the pitting resistance equivalent (PRE) has been optimized to give good corrosion resistance, being in the range of 27-29.5. The critical pitting temperature (CPT) is in the range of 20-33° C., preferably 23-31° C. The TRIP (Transformation Induced Plasticity) effect in the austenite phase is maintained in accordance with the measured Md30 temperature at the range of 0-90° C., preferably in the range of 10-70° C., in order to ensure the good formability. The proportion of the austenite phase in the microstructure of the stainless steel of the invention is in the heat treated condition 45-75 volume %, advantageously 55-65 volume %, the rest being ferrite, in order to create favourable conditions for the TRIP effect. The heat treatment can be carried out using different heat treatment methods, such as solution annealing, high-frequency induction annealing or local annealing, at the temperature range from 900 to 1200° C., preferably from 950 to 1150° C.
Japanese patent application 2014001422A, shows an austenitic stainless steel with composition in mass %, C: 0.02 to 0.30%, Cr: 10.0 to 25.0%, Ni: 3.5 to 10.0%, Si: 0.1 to 3.0%, Mn: 0.5% to 5.0%, N: 0.10 to 0.40%, Mo: 0 to 3.0%, Cu: 0 to 3.0%, Ti: 0 to 0.10%, Nb: 0 to 0.50%, V: 0 to 1.0%, with a calculated Md30 of 0-40 based on a formula very similar to Nohara. The chemical range is very wide but the examples given in the invention show predominantly low levels of Mo and N which give a low PRE and corrosion resistance. The invention shows that austenitic alloys with a balanced Ni and N content can have a low stability but this is done only with a low PRE and corrosion resistance due to low Mo and in reality the real Md30 is likely to be much higher than the claimed region.
The present invention describes a new austenitic stainless steel utilizing the TRIP effect described in the prior art with an addition of a high pitting resistance equivalent (PRE) and giving therefore superior corrosion resistance combined with a TRIP effect with improved formability. In comparison to other commercially available austenitic stainless steels such TRIP 301 and slightly more stable 304 the new invention has a PRE and corrosion resistance that is far superior. In comparison to 316, the TRIP effect and PRE is substantially higher. In comparison to 904L the PRE and corrosion resistance of the invention is up to a similar level, but with a far superior TRIP effect that is not otherwise observed in any other high alloyed austenitic stainless steel. In addition to all of the reference alloys mentioned, the new invention has a much leaner composition in particular considering the Ni-content. This gives the invention a unique combination of high corrosion, TRIP and a cost effective alternative. The essential features of the invention are enlisted in the appended claims.
According to the invention, the austenitic stainless steel contains less than 0.04 weight % C, 0.2-0.8 weight % Si, 0-2.0 weight % Mn, 16.0-19.0 weight % Cr, 4.0-6.5 weight % Ni, 1.0-4.0 weight % Mo, 0-4.0 weight % W, 0-2.0 weight % Cu, 0.20-0.30 weight % N, the rest being iron and inevitable impurities occurring in stainless steels. Sulphur is limited to less than 0.010 weight % and preferably less than 0.005 weight %, the phosphorus content is less than 0.040 weight % and the sum of sulphur and phosphorus (S+P) is less than 0.04 weight %, and the total oxygen content is below 100 ppm.
The austenitic stainless steel of the invention optionally contains one or more added elements in the following: the aluminium content is maximized to less than 0.04 weight % and preferably the maximum is less than 0.03 weight %.
Further, boron, calcium and cerium are optionally added in small quantities; the preferred contents for boron and calcium are less than 0.004 weight % and for cerium less than 0.1 weight %. Optionally cobalt can be added up to 1 weight % for partial replacement of nickel. Also one or more of the group containing niobium, titanium and vanadium can be optionally added in the austenitic stainless steel of the invention, the contents of niobium and titanium being limited up to 0.1 weight % and the vanadium content being limited up to 0.2 weight %.
According to the stainless steel of the invention, the pitting resistance equivalent (PRE) has been optimized to give good corrosion resistance, being at the range of 27-35. The critical pitting temperature (CPT) is in the range of 30-50° C., The TRIP (Transformation Induced Plasticity) effect in the austenite phase is maintained in accordance with the calculated Md30 temperature at the range of −70° C. to +60° C. The Md30-temperature, which is a measure for the austenite stability to the TRIP effect, is defined as the temperature at which 0.3 true strain yields 50% transformation of the austenite to martensite. In addition, the stacking fault energy of the invented stainless steels is much lower than commercially available stainless steels and is between 10-16 mJ/m2 according to formula (3). The proportion of the ferrite phase in the microstructure of the austenitic stainless steel of the invention is in the heat treated condition <10 volume %. The heat treatment can be carried out using different heat treatment methods, such as solution annealing, high-frequency induction annealing, local annealing or any other type of heat treatment at the temperature range from 900 to 1200° C., preferably from 950 to 1150° C.
Effects of different elements in the microstructure are described in the following, the element contents being described in weight %:
Carbon (C) partitions to the austenite phase and has a strong effect on austenite stability. Carbon can be added up to 0.04% but higher levels have detrimental influence on corrosion resistance.
Nitrogen (N) is an important austenite stabilizer in stainless steels and like carbon it increases the stability against martensite. Nitrogen also increases strength, strain hardening and corrosion resistance. The general empirical expressions on the Md30 temperature indicate that nitrogen and carbon have the same strong influence on austenite stability, but this is lower than previously reported in other Md30 expressions. Because nitrogen can be added to stainless steels in larger extent than carbon without adverse effects on corrosion resistance the nitrogen contents from 0.20 up 0.30% are effective in present stainless steels.
Silicon (Si) is normally added to stainless steels for deoxidizing purposes in the melt shop and should not be below 0.2%. Silicon has a stronger stabilizing effect on austenite stability against martensite formation than shown in current expressions. For this reason silicon is maximized to 0.8%, preferably to 0.5%.
Manganese (Mn) is an important addition to stabilize the austenite phase and to increase the solubility of nitrogen in the stainless steel. Manganese can partly replace the expensive nickel and bring the stainless steel to the right phase balance. Too high level in the content will reduce the corrosion resistance. Manganese has a stronger effect on austenite stability against deformation martensite and, therefore, the manganese content must be carefully addressed. The range of manganese shall be 0-2.0%, preferably 0-1.5%.
Chromium (Cr) is the main addition to make the steel resistant to corrosion. Further, chromium strongly increases the resistance to martensite formation and, therefore, reduces the TRIP effect. Also being a strong ferrite stabilizer the level of Cr needs to be limited in the austenitic stainless steel. To bring about these functions the chromium level should be at least 16.0% and the maximum level 19.0%. Preferably the chromium content is 16.5-18.7%
Nickel (Ni) is an essential alloying element for stabilizing the austenite phase and for good ductility and at least 4.5% must be added to the stainless steel of the invention. Having a large influence on austenite stability against martensite formation nickel has to be present in a narrow range. Ni also has a significant influence on increasing the stacking fault energy of stainless steels. Further, because of nickel's high cost and price fluctuation nickel should be maximized in the stainless steel of the invention to 6.5%, preferably to 6.2%
Copper (Cu) is normally present as a residual of 0.1-0.5% in most stainless steels, when the raw materials to a great deal are in the form of stainless scrap containing this element. Copper is a weak stabilizer of the austenite phase but has a strong effect on the resistance to martensite formation and must be considered in evaluation of formability of the present stainless steels. An intentional addition up to 2.0% can be made, but preferably the copper content is up to 1.6%.
Molybdenum (Mo) be added to significantly increase the PRE and corrosion resistance and, therefore, molybdenum shall have a content at least 1.0%, preferably at least 1.5%. Further it has been found that, molybdenum, like chromium, strongly increases the resistance to martensite formation and reduces the TRIP effect significantly more than previous expected. Therefore, molybdenum cannot be added to more than 4.0.
Tungsten (W) has similar properties as molybdenum and can sometimes replace molybdenum. However, tungsten and molybdenum promote sigma phase precipitation and the sum of the molybdenum and tungsten contents according to the formula (Mo+0.5 W) should be 0-4.0%, preferably 2.0-4.0%, where the promotion of sigma and chi phases are possible to handle in technically relevant processes. The most important influence of tungsten is the surprisingly positive impact on the TRIP effect which in turn could be related to the effect on the stacking fault energy of the alloy since the stacking fault energy controls the deformation response in terms of dislocation glide, twinning or martensite formation. For this purpose, tungsten should be limited up to 3.8%, but preferably at least 1.0% when tungsten is used to replace molybdenum. Boron (B), calcium (Ca) and cerium (Ce) are added in small quantities in austenitic steels to improve hot workability and not at too high contents as this can deteriorate other properties. The preferred contents for boron and calcium in the stainless steel of the invention are less than 0.004% and for cerium less than 0.1%.
Sulphur (S) in austenitic steels deteriorates hot workability and can form sulphide inclusions that influence pitting corrosion resistance negatively. The content of sulphur should therefore be limited to less than 0.010% and preferably less than 0.005%.
Phosphorus (P) deteriorates hot workability and can form phosphide particles or films that influence corrosion resistance negatively. The content of phosphorus should therefore be limited to less than 0.040%, and so that the sum of sulphur and phosphorus (S+P) contents is less than 0.04%.
Oxygen (O) together with other residual elements has an adverse effect on hot ductility. The presence of oxide inclusions may reduce corrosion resistance (pitting corrosion) depending on type of inclusion. High oxygen content also reduces impact toughness. In a similar manner as sulphur oxygen improves weld penetration by changing the surface energy of the weld pool. For the stainless steel of the invention the advisable maximum oxygen level is below 100 ppm. In a case of a metallic powder the maximum oxygen content can be up to 250 ppm.
Aluminium (Al) should be kept at a low level in the austenitic stainless steel of the invention with high nitrogen content as these two elements can combine and form aluminium nitrides that will deteriorate the impact toughness. The aluminium content is limited to less than 0.04% and preferably to less than 0.03%.
Cobalt (Co) has similar metallurgical behaviour as its sister element, nickel, and cobalt may be treated in much the same way in steel and alloy production. Cobalt inhibits grain growth at elevated temperatures and considerably improves the retention of hardness and hot strength. Cobalt increases the cavitation erosion resistance and the strain hardening. Cobalt reduces the risk of sigma phase formation in stainless steels. The cobalt content is limited up to 1.0%.
The “micro-alloying” elements titanium (Ti), vanadium (V) and niobium (Nb) belong to a group of additions so named because they significantly change the steels properties at low concentrations, often with beneficial effects in carbon steel but in the case of austenitic stainless steels they also contribute to undesired property changes, such as reduced impact properties, higher surface defects levels and reduced ductility during casting and hot rolling. Many of these effects depend on their strong affinity for carbon and in particular nitrogen in the case of modern austenitic stainless steels. In the present invention niobium and titanium should be limited to maximum level of 0.1% whereas vanadium is less detrimental and should be less than 0.2%.
The present invention is described in more details referring to the drawings, of which
In all
Based on the effects of the elements the austenitic stainless steel according to the invention is presented with the chemical compositions A to S as named in table 1. Table 1 contains also the typical chemical composition for the reference commercially available austenitic stainless steels named as T to X, all the contents of the table 1 in weight %.
The alloys A-S were manufactured in an induction furnace in 1 kg laboratory scale to small slabs.
The referred alloys T and X were produced in 100 ton production scale followed by hot rolling and cold rolling to coil form with varying final dimensions.
When comparing the values in the Table 1 the contents of, nickel, nitrogen and tungsten in the austenitic stainless steels of the invention are significantly different from the reference stainless steels T to X and R.
The properties, the values for the Md30 temperature, the critical pitting temperature (CPT), Pitting Resistant Equivalent PRE and Stacking Fault Energy (SFE) were determined for the chemical compositions of the table 1 and the results are presented in the following table 2.
The predicted Md30 temperature (Md30 Nohara) of the steels in the table 2 was calculated using the Nohara expression (1) established for austenitic stainless steels
Md30=551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)−18.5Mo−68Nb (1),
The calculated Md30 temperatures in the table 2 were achieved in accordance with a mathematical constraint of optimization.
The critical pitting temperature (CPT) is measured in a 1 M sodium chloride (NaCl) solution according to the ASTM G150 test, and below this critical pitting temperature (CPT) pitting is not possible and only passive behaviour is seen.
The pitting resistance equivalent (PRE) is calculated using the formula (2):
PRE=% Cr+3.3(% Mo+0.5% W)+16N (2)
The sums of the element contents for C+N, Cr+Si, Cu+Mo+0.5 W and Mn+Ni in weight % are also calculated for the alloys of the table 1 in the table 2. The sums C+N and Mn+Ni represent austenite stabilizers, while the sum Si+Cr represents ferrite stabilizers and the sum Cu+Mo+0.5 W elements having resistance to martensite formation.
When comparing the values in the Table 2 the PRE value of for alloys A-S having the range of 27-35 is higher than the PRE value in the referred stainless steels T-V which means that the corrosion resistance of the alloys A—S is higher. The critical pitting temperature CPT is in the range of 31-48° C., which is a high level of corrosion resistance and much higher than the CPT for the referred austenitic stainless steels T to V.
The predicted Md30 temperatures using the Nohara expression (1) agree well with the known Md30 for the reference alloys T-V, but are not considered to be accurate for the inventive alloys A to S. This is because the inventive alloys A-S have a chemical composition unique from the austenitic steels that the Nohara expression was originally derived. In particular, the effect of N on the austenite stability and Md30, although still high has been found to be considerably lower in these inventive steels. The Nohara expression was originally derived for austenitic steels with low N and therefore the total effect of nitrogen in those steels was little. In the current invention, to increase the austenitic content, given that the Ni is low, and to have a high corrosion resistant stainless steel, i.e. increase the PRE, the N content is much higher than in the alloys. According to Nohara this would make the inventive steels very stable. The majority of the steels A-S have −50 or lower when using the Md30 expression of Nohara. The evaluation of the microstructure of steels A-S in this invention, together with previous prior art knowledge of measured Md30 values of alloys with similar composition levels in stainless steels as in WO patent application 2015/114222, has shown that the Md30 of alloys A-S are higher than predicted by Nohara. As such, a new Md30 expression has been derived for the invented alloys. As can be seen in table 2 the calculated Md30 values for the reference allows agree very well with the predicted values using Nohara, which also agree with the known real measured values for these commercially available grades. The new expression therefore is a good measure of the Md30 even for low N-contained stainless steel grades. In
The stacking fault energies were calculated using the formula (3) of G. Meric de Bellefon, J. C. van Duysen, and K. Sridharan, “Composition-dependence of stacking fault energy in austenitic stainless steels through linear regression with random intercepts,” J. Nucl. Mater., vol. 492, for austenitic stainless steels.
SFE(mJ/m2)=2.2+1.9Ni−2.9Si+0.77Mo+0.5Mn+40C−0.016Cr−3.6N (3)
The stacking fault energies (SFE) of the austenitic stainless steels of the invention are lower than the commercially available austenitic stainless steel. The SFE of the reference alloys, T and X, that are relatively low alloyed with a low PRE (<27) and consequently a higher Md30, >0 is greater than 16 mJ/m2. In addition the SFE for the higher alloyed and high PRE (>35) with low Md30 (<−70) grade 904L is also greater than 16 mJ/m2. Alloys A to S of the invention are unique in the sense that all have SFE lower than 16 mJ/m2 whilst having a PRE between 27-35 and an Md30 between −70 to +60.
The sums of the element contents for C+N, Si+Cr, Mn+Ni and Cu+Mo+0.5 W in weight % for the austenitic stainless steel of the present invention were used in the mathematical constraint of optimization to establish the dependence in one hand between C+N and Mn+Ni, and in another hand between Si+Cr and Cu+Mo+0.5 W. In accordance with this mathematical constraint of optimization the sums of Cu+Mo+0.5 W and Si+Cr, respectively the sums Mn+Ni and C+N, form the x and y axis of a coordination in the
In accordance with
The effect of the limitations for C+N and Mn+Ni with the preferred ranges for the element contents of the invention is that the chemical composition window of
The austenitic ferritic austenitic stainless steel of the invention can be produced as ingots, slabs, blooms, billets and flat products such as plates, sheets, strips, coils, and long products such as bars, rods, wires, profiles and shapes, seamless and welded tubes and/or pipes. Further, additional products such as metallic powder, formed shapes and profiles can be produced.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20193794.3 | Sep 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/073959 | 8/31/2021 | WO |
