Patent Application
20170130305
This invention relates to a duplex ferritic 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.
The EP patent application 2172574 and the JP patent application 2009052115 discolose a ferritic austenitic stainless steel which contains in weight % 0,002-0,1% C, 0,05-2% Si, 0,05-5% Mn, 17-25% Cr, 0,01-0,15% N, optionally less than 5% Ni, optionally less than 5% Cu, optionally less than 5% Mo, optionally less than 0,5% Nb and optionally less than 0,5% Ti. The Md temperature has been calculated from the chemical composition in the austenite phase which volume fraction in the steel is 10-50% using the formula
M
d=551−462(C+N)−9,2 Si−8,1 Mn−13,7 Cr−29(Ni+Cu)−18,5 Mo.
The Md temperature is limited to the range −10° C.≦Md≦110° C. The pitting resistance equivalent (PRE), which is calculated using the formula
PRE=% Cr+3,3*(% Mo)+10*% N−% Mn,
is described to be over 18. In the EP patent application 2172574 and the JP patent application 2009052115 the Mo content is only optional, and for the calculation of the Md temperature is based on the chemical composition of the austenite phase being only 10-50 vol % of the whole microstructure.
The EP patent application 1715073 discoses an austenitic ferritic stainless steel containing in weight % less than 0,2% C, less than 4% Si, less than 12% Mn, 15-35% Cr, less than 3% Ni, 0,05-0,6% N, optionally less than 4% Cu, optionally less than 4% Mo, optionally less than 0,5% V and optionally less than 0,1% Al. The volume fraction of the austenite phase is in a range from 10 to 85%, and the amount of (C+N) in the austenite phase is in the range from 0,16 to 2 weight %. The EP patent application 1715073 also has molybdenum (Mo) as an optional element.
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, 0,2-0,7% 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 duplex 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 duplex 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 a partial replacement to nickel, and tungsten can be added up to 0,5 weight % as partial replacement to molybdenum. Also one or more of the group containing niobium, titanium and vanadium can be optionally added in the duplex 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 at the range of 27-29,5. 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 at the range of 10-70° C., in order to ensure the good formability. The proportion of the austenite phase in the microstructure of the duplex 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.
The object of the present invention is to improve the properties of the duplex stainless steels described in the prior art and to achieve a new duplex ferritic austenitic stainless steel utilizing the TRIP effect with high pitting resistance equivalent (PRE) and giving therefore superior corrosion resistance. The essential features of the invention are enlisted in the appended claims.
According to the invention, the duplex ferritic austenitic stainless steel contains less than 0,04 weight % C, 0,2-0,8 weight % Si, 0,3-2,0 weight % Mn, 14,0-19,0 weight % Cr, 2,0-5,0 weight % Ni, 4,0-7,0 weight % Mo, less than 4,5 weight % W, 0,1-1,5 weight % Cu, 0,14-0,23 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 duplex 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, cerium and magnesium are optionally added in small quantities; the preferred contents for boron and calcium are less than 0,004 weight %. for cerium less than 0,1 weight % and for magnesium less than 0,05 weight %. Optionally cobalt can be added up to 1 weight % for a partial replacement to nickel. Also one or more of the group containing niobium, titanium and vanadium can be optionally added in the duplex 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 invention it is noticed that increasing the molybdenum content to the range of 4,0-7,0 weight %, it is necessary to decrease the chromium content to the range of 14,0-19,0 weight %. Within this condition, the sum of molybdenum, chromium and optional tungsten contents in weight per cents calculating with the formula Cr+Mo+0,5W is in the range of 20-23,5 weight %, where the ratio Cr/(Mo+0,5 W) is in the range of 2-4,75.
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 35-42. The TRIP (Transformation Induced Plasticity) effect in the austenite phase is maintained in accordance with the measured Md30 temperature at the range of −30-+90° C., preferably at the range of 0-+60° C., in order to ensure the good formability. 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. The proportion of the austenite phase in the microstructure of the duplex stainless steel of the invention is in the heat treated condition 50-80 volume %, advantageously 55-70 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, local annealing, or any other type of heat treatment at the temperature range from 900 to 1200° C., preferably from 950 to 1150° C.
According to the invention, the sum of chromium, molybdenum and optional tungsten with the formula Cr+Mo+0,5 W is critical to maintain the Md30 temperature in the desired range in order to ensure good formability.
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 duplex 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. Because nitrogen can be added to stainless steels in larger extent than carbon without adverse effects on corrosion resistance the nitrogen contents from 0,14 to 0,23% 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 stabilizes the ferrite phase in duplex stainless steels but 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 0,3-2,0%.
Chromium (Cr) is the main addition to make the steel resistant to corrosion. Being ferrite stabilizer chromium is also the main addition to create a proper phase balance between the austenite phase and the ferrite phase. In addition, and together with molybdenum, chromium strongly increases the resistance to martensite formation. In order to provide a high PRE whilst maintaining an optimal TRIP effect, the range of chromium is limited to 14,0%-19,0% thanks to the increase in the molybdenum content. Preferably the chromium content is 14,0-18,0%.
Nickel (Ni) is an essential alloying element for stabilizing the austenite phase and for good ductility and at least 2,0% 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. Further, because of nickel's high cost and price fluctuation nickel should be maximized in the stainless steel of the invention to 5,0%.
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. The copper additions can also increase the resistance to sigma phase. An intentional addition up to the range 0,1-1,5% can be made, but preferably the copper content is in the range 0,1-0,7%, more preferably in the range 0,1-0,5%.
Molybdenum (Mo) is a ferrite stabilizer that can be added to strongly increase the corrosion resistance and, therefore, molybdenum shall have a content at least 4,0% in order to achieve the high PRE. Further, molybdenum, like chromium, strongly increases the resistance to martensite formation and reduces the TRIP effect. Therefore, molybdenum is added to the stainless steel of the invention to counter balance the effect of chromium in terms of TRIP and PRE. For this purpose molybdenum should be maximised to 7.0%, preferably 6,5%.
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 less than 7,0%, preferably 4,0-6,6%, 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 3,5%, but preferably at least 0,5% when tungsten is used to replace molybdenum.
In order to have optimal conditions for the TRIP effect and the desired value for PRE according to the invention, the co-effect of the chromium, molybdenum and optional tungsten contents in weight % is in the range of 20<(Cr+Mo+0,5 W)<23,5 where the ratio Cr/(Mo+0,5 W) is in the range of 2-4,75.
Boron (B), calcium (Ca) and cerium (Ce) are added in small quantities in duplex 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%.
Magnesium (Mg) is a strong oxide and sulphide former. When added as a final steelmaking step it forms magnesium sulphide (MgS) and transforms a potential low melting sulphide eutectic phase to a more stable morphology with a higher melting temperature thus improving the hot ductility of the alloy. The magnesium content is limited to less than 0,05%.
Sulphur (S) in duplex 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 duplex 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 super duplex 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 duplex 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 duplex 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 where
Based on the effects of the elements the duplex ferritic austenitic stainless steel according to the invention is presented with the chemical compositions A to P as named in the table 1. The table 1 contains also the chemical composition for the reference duplex stainless steel of commonly known as 2205 (Q) and the reference duplex stainless steels of the WO patent application 2011/135170 named as R and the WO patent application 2013/034804 named as S, all the contents of the table 1 in weight %.
The alloys A-P were manufactured in a vacuum induction furnace in 1 kg laboratory scale to small slabs that were forged and cold rolled down to 1,5 mm thickness.
The referred alloys Q to S 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 chromium, nickel, molybdenum and tungsten in the duplex stainless steels of the invention are significantly different from the reference stainless steels Q, R and S
The properties, the values for the Md30 temperature and PRE 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 austenite phase in the table 2 was calculated using the Nohara expression (1) established for austenitic stainless steels
M
d30=551−462(C+N)−9,2 Si−8,1 Mn−13,7 Cr−29(Ni+Cu)−18,5 Mo−68 Nb (1)
when annealed at the temperature of 1050° C.
The actual measured Md30 temperatures (Md30 measured) of the table 2 were established by straining the tensile samples to 0,30 true strain at different temperatures and by measuring the fraction of the transformed martensite with Satmagan equipment. Satmagan is a magnetic balance in which the fraction of ferromagnetic phase is determined by placing a sample in a saturating magnetic field and by comparing the magnetic and gravitational forces induced by the sample.
The calculated Md30 temperatures (Md30 calc) in the table 2 were achieved in accordance with a mathematical constraint of optimization.
The pitting resistance equivalent (PRE) is calculated using the formula (2):
PRE=% Cr+3,3*(% Mo+0,5% W)+30% N−% Mn (2).
The sums of the element contents for C+N, Cr+Si, Cu+Mo+0,5 W, Mn+Ni and Cr+Mo+0,5 W 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. The sum formula Cr+Mo+0,5 W is critical to maintain the Md30 temperature in the optimal range in order to ensure the good formality.
When comparing the values in the Table 2 the PRE value having the range of 35 -42 is much higher than the PRE value in the referred duplex stainless steels R and S which means that the corrosion resistance of the alloys A-P is higher. The PRE is of the same level or slightly higher than the reference alloy Q.
The predicted Md30 temperatures using the Nohara expression (1) are essentially different from the measured Md30 temperatures for the alloys on the table 2. Further, from the table 2 it is noticed that the calculated Md30 temperatures agree well with the measured Md30 temperatures, and the mathematical constraint of optimization used for the calculation is thus very suitable for the duplex stainless steels of the invention.
The calculated Md30 temperatures for the alloys A-P are considerably higher than the reference alloy R.
The sums of the element contents for C+N, Si+Cr, Mn+Ni, Cu+Mo+0,5 W and Cr+Mo+0,5 W in weight % for the duplex 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 chemical composition window, which lies within the frame of the area a′, b′, c′, d′, e and f′ in
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 alloys of the present invention A-P as well as the reference materials Q, R and S above were further tested by determining the yield strengths Rp0.2 and Rp1.0 and the tensile strength Rm as well as the elongation values for A50, A5 and Ag in the longitudinal direction where Ag is the uniform elongation or elongation to plastic instability. The work hardening rate of the alloys are described by the n-values derived from the equation (3)
σ=Kεn (3)
where σ is the stress, K is the strength index, ε is the plastic strain and n is the strain hardening exponent.
Due to the TRIP effect of the alloys of the present invention the n-values are derived within the strain intervals ε=10-15% (n(10-15%)) and ε=15-20% (n(15-20%)), since it is not possible to fit the equation (3) to the whole strain interval.
The table 7 contains the results of the tests for the alloys A-P of the invention as well as the respective values for the reference duplex stainless steels Q, R and S.
The results in the table 7 show that the yield strength values Rp0.2 and Rp1.0 for the alloys A-P are lower than the respective values for the reference duplex stainless steels Q, R and S and the tensile strength value Rm is similar to the reference duplex stainless steels Q, R and S. The elongation values A50, A5 and Ag of the alloys A-P are higher than the reference alloy Q with a similar PRE. Because the alloys A-P according to the invention are manufactured in the laboratory scale and the reference duplex stainless steels Q, R and S are produced in the production scale, the strength values of the table 7 are not directly comparable with each other.
The n-values of the alloys A-P are all higher than the reference alloy Q indicating the importance of the TRIP effect for the work hardening rate. Compared to the reference alloys R and S the n(10-15%) values are somewhat higher while the n(15-20%) values are considerably higher indicating the optimized work hardening rate for the alloys A-P of the present invention utilizing the TRIP effect.
For the alloys of the present invention n value is greater than 0,2 at ε=10-15% and the elongation Ag is greater than 19, preferably greater than 25.
The duplex 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 |
|---|---|---|---|
| 20145575 | Jun 2014 | FI | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FI2015/050415 | 6/11/2015 | WO | 00 |
