WO2006004486A1

WO2006004486A1 - Ferritic stainless steel alloy

Info

Publication number: WO2006004486A1
Application number: PCT/SE2005/000914
Authority: WO
Inventors: Mattias SANDSTRÖM; Anna Heedman; Ylva Trogen
Original assignee: Sandvik Intellectual Property Ab
Priority date: 2004-06-30
Filing date: 2005-06-15
Publication date: 2006-01-12
Also published as: US20090053092A1; SE0401686L; SE0401686D0; SE528680C2; JP2008505247A; CN1977062A; KR20070026683A; EP1774051A1

Abstract

The application describes a lead-free ferritic stainless steel alloy having the following composition, all contents given in percent by weight: C < 0,1 Si ≤ 2 Mn 0,1-2 S 0,08-0,4 Cr 16-25 Ni ≤ 2 Mo 1-5 Cu 0,01-3,0 Ca ≤ 0,006 Sn ≤ 0,15 B ≤ 0,02 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 has good machinability and good corrosion resistance, in comparison with the corresponding alloys containing lead.

Description

FERRITIC STAINLESS STEEL ALLOY

Technical Field of the Invention

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.

Background of the Invention

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. However, the development within environmental legislation indicates that lead may become prohibited or limited as alloying material in steel.

On the market for stainless, ferritic steel alloys having improved machinability, 430F-steels and 18Cr2Mo-steels are common. The steel according to the invention is intended to be used when low cutting speeds/weak dimensions are required. Then, it is foremost to be compared with 20Cr2Mo-steel, which dominates within this field. These steels have a combination of good machi¬ nability properties and good corrosion resistance, but contain lead, which the market tries to reduce or exclude entirely. A type of corrosion of this type of materials is pitting. The resistance to pitting may simply be described by the PRE-value (Pitting Resistance Equivalent), PRE=%Cr + 3,3*%Mo + 16*%N.

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. In this alloy, the PRE-value is at least 19.

Also 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.

Furthermore, 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. In this alloy, the PRE-value is at least 20.

Summary of the Invention

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.

This object is met according to the present invention by a ferritic, stainless steel alloy containing (in by weight %):

C < 0,1

Si ≤ 2

Mn 0,1-2

S 0,08-0,4

Cr 16-25

Ni < 2

Mo 1-5

Cu 0,01-3,0

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).

In addition to impurities, the alloy may also contain additives of the elements Ca, Sn and B.

Description of the Drawings

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 %. Detailed Description of the Invention

The invention will now be described in more detail. All exemplified composi¬ tions are to be regarded as examples and hence not limiting for the invention according to the present application.

The present invention relates to a ferritic stainless steel alloy having the following composition, all contents in percent by weight:

C < 0,1

Si < 2

Mn 0,1-2

S 0,08-0,4

Cr 16-25

Ni < 2

Mo 1-5

Cu 0,01-3,0

Ca < 0,006

Sn < 0,15

B < 0,02

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 (S) 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. However, 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 (Te) 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 CT₂O₃ 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 (Mo) 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 (Rare Earth Metals) are 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. In practice, 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-%.

Description of the Test Procedure

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.

In order to evaluate the machinability properties, the 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). 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.

*) Other REM additives in addition to Ce have not een examined.

Drill test

In the drill test, the number of items that could be drilled with one and the same drill was examined. The operation procedure was first center drilling, then drilling with a solid tungsten carbide drill.

In the course of drilling, the drill was examined at regular intervals. Problems such as chipping of the cutting edge and built-up edge formation was noted together with comments about of the geometry of the chips. In those cases chipping of the cutting edge limited the service life to 440 items or less, a re- test was made. In figure 1 , a summary of the number of drilled holes for each tested composition is shown. In those cases where two tests were carried out, the result is calculated as a mean value.

In figure 1 , it is seen that of the examined test materials, there are four compositions having better drillability than the others. By considering the possible problems of built-up edge formation and chipping that have been noticed, these four materials can be ranked according to table 2.

Table 2. The best materials in the drill test

Turning test

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 operation procedure in the manufacture of the items was contour turning followed by parting. A coated cemented carbide insert was used.

After each 100 manufactured items, ten items were selected in order to, by measuring the diameter, be able to study how the dimension of the items varies with the total period of engagement. In total, 1100 items per material were manufactured.

The alteration of the maximum diameter with the period of engagement can be described by a trend line. By assuming that the trend line is linear and can be written in the form

maximum diameter = inclination * time of contact + C

C is the point where the line intersects the diameter axis. From this, the incli¬ nation of the trend line may be determined.

Theoretically, the dimension of the selected test items should increase as the edge of the tool is worn down and the inclination of the trend line should be positive. A summary of the results in the turning test is given in table 3.

Table 3. Inclination of the trend line of the maximum dimension of the selected sam¬ ples. Excluding the ten first selected samples.

Assuming that small negative values can be set equal to zero and that a low value of the inclination of the trend line means a slow tool wear, a ranking according to table 4 can be set up for the materials. Table 4. The best test materials in the turning test

Chip breaking

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.

From the results in table 5, it is seen that all test materials have equivalent or better chip-breaking properties than the reference material 20Cr2Mo. The very best chips were obtained when turning the tin-alloyed material 98314. Second best chips, but still short, fine chips were obtained from the materials having REM additive, 98315 and 98316, as well as two of the test melts including tellurium, 98310 and 98311. Two of the melts with tellurium ended up on approximately the same level as the reference material, and were worst in the chip-breaking test. Table 5. Chip-breaking test.

Machinability results

The results from the three machinability examinations made, show that differ¬ ent materials work differently well in different situations. For instance, the tellu¬ rium-alloyed materials having great grains and great rounded sulphides got the best results in the drill test. In order to, in spite of this, be able to separate some or a few materials having a generally good machinability, the ferritic materials have been weighted versus each other. From experience, drilling is the most critical operation for a plurality of products within the intended fields of application, and then comes chip breaking, and finally turning. As a conse¬ quence of this, the drilling has been given the greatest significance for the final marking, then the chip breaking, and finally the turning, see table 6.

Corrosion test

Corrosion tests have been carried out:

• in neutral salt spray (NSS)

• in copper chloride accelerated salt spray (CASS)

• electrochemically, where the critical pitting temperature (CPT) has been determined.

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 ⁰C instead of 50 ⁰C. ^

NSS and CASS

Three samples of each composition were degreased and weighed. After finalized testing, the samples were visually inspected and the extent of corro- sion products was noted. The samples were evaluated in the following way:

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 samples were pickled until they were clean, after which they were weighed so that the weight loss could be calculated. Finally, the samples were scanned for pits using a stereomicroscope. For each composition and test method, three tests were made, from which a mean value and a distribution could be obtained. Table 8 shows the result from the corrosion test.

CPT

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 μA/cm² is exeeded. If the sample passed up to 95 degrees, this temperature was registered, and the test was finished.

The test material having the highest CPT-value is the test material that is most resistant to pitting in an environment including chloride ions. For each composition, six tests were made, from which a mean value and a distribution could be obtained. The result is accounted for in table 8. Table 7. Experimental data in CPT-measurement.

*SCE (Standard Calomel Electrode) is a reference electrode. It displays a voltage of 242 mV positive vs. the SHE (Standard Hydrogen Electrode).

Corrosion test results

In the CPT-test, the three test materials did better than the reference material. In the other tests, the most corrosion resistant of the test materials were equally good as the reference material. The other test materials were somewhat worse.

Table 8. Results corrosion test

From the test results accounted for above, it is clearly seen that 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.

Theoretical calculations

In addition to the tested compositions, some theoretical calculations were conducted using Thermo-Calc (version Q, data base CCTSS) in order to evaluate the presence of sulphides, carbides and nitrides. It should be noted that these calculations assume equilibrium and should therefore only serve as guidance to what might be expected in reality.

In the calculations, the contents of S, C, N, Mo and Cr were varied while the contents of the other elements were held constant. Te, Se and Bi were not included in the calculations since the data base used does not contain data in order to include these elements. The same is valid for Cu contents above 1 ,0 wt-%.

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 M₂3C₆, (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,

21 %Cr, 0,5 %Ni, 2,5 %Mo, 1 %Cu and 0,05 % N. The C content was varied between 0,01 and 0,1 %.

In figure 4, the calculated fraction of Cr₂N 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 %.

Also, the risk of obtaining sigma phase in the alloys was evaluated when varying the Mo content and the Cr content, respectively. Figure 5 illustrates 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 %; and Figure 6 illustrates 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 %. It is clear that high contents of Cr and Mo increase the risk of sigma phase formation. However, it is not clear if sigma phase will be present in reality since this is highly dependent on the reaction times during manufacturing. No sigma phase was observed in the actual compositions given in Table 1. Therefore, it is concluded that with a proper manufacturing process, sigma phase can be avoided in the alloy according to the invention.

Claims

1. Ferritic stainless steel alloy characterized in that it has the following composition, all contents in percent by weight:

C <0,1

Si < 2

Mn 0,1-2

S 0,08-0,4

Cr 16-25

Ni <2

Mo 1-5

Cu 0,01-3,0

Ca < 0,006

Sn <0,15

B <0,02

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.

2. Ferritic stainless steel alloy according to claim 1, characterized in that the content of Cr is 18-22 weight-%, preferably 19-21 weight-%, and/or that the content of Ni is < 1 weight-%, preferably ≤ 0,5 weight-%.

3. Ferritic stainless steel alloy according to claim 1 or 2, characterized in that the content of C is < 0,05 weight-%, preferably < 0,03 weight-%.

4. Ferritic stainless steel alloy according to any one of the preceding claims, characterized in that the content of Mn is 0,2-1 ,5 weight-%, pref¬ erably 0,4-1 ,5 weight-%, and/or that the content of S is 0,1-0,4 weight%, preferably 0,15-0,35 weight-%.

5. Ferritic stainless steel alloy according to claim 4, characterized in that the content of Ca is < 0,002 weight-%, preferably < 0,001 weight-%.

6. Ferritic stainless steel alloy according to any one of the preceding claims, characterized in that the content of Si is < 1 weight-% , preferably < 0,5 weight-%, and/or that the content Mo is 1,5-2,5 weight-%, preferably 1,85-2,5 weight-%.

7. Ferritic stainless steel alloy according to any one of the preceding claims, characterized in that the content of Cu is 0,5-2 weight-%, prefera¬ bly 0,7-2 weight-%, and/or that the content of B is 0,0005-0,01, preferably 0,001-0,01 weight-%.

8. Ferritic stainless steel alloy according to any one of the preceding claims, characterized in that the content of X, i.e., 2*Te + 1*Se + 1*Bi, is 0,02-0,4 weight-%, preferably 0,02-0,2 weight-%, and/or that the content of REM is 0,01-0,1 weight-%.

9. Ferritic stainless steel alloy according to any one of the preceding claims, characterized in that the impurities N, P and O are present in the following contents, all in percent by weight:

N < 0,05 P ≤ 0,03

O < 0,05