JP2008505247A - Ferritic stainless steel - Google Patents

Abstract

  Chemical composition in wt%: C: ≦ 0.1, Si: ≦ 2, Mn: 0.1-2, S: 0.08-0.4, Cr: 16-25, Ni: ≦ 2, Mo: 1 to 5, Cu: 0.01 to 3.0, Ca: ≦ 0.006, Sn: ≦ 0.15, B: ≦ 0.02, X: 0.01 to 0.5 and / or REM : 0.01 to 1, balance: Fe and impurities, X is 2 × Te + 1 × Se + 1 × Bi. This steel has better machinability and corrosion resistance than the corresponding lead-containing steel.

Description

  The present invention relates to a ferritic stainless steel produced by a conventional method with improved machinability for applications requiring low cutting speed / small cross section. The present invention belongs to the group of 20Cr2Mo steel and is a lead-free material having high workability, machinability, and corrosion resistance.

  Conventionally, most of stainless steel for machining a small cross section at a low cutting speed is ferritic steel to which sulfur, lead and tellurium are added as machinability improving components. With the development of environmental bills, the use of lead as a steel alloy component is likely to be prohibited or restricted.

  In the field of ferritic stainless steel with improved machinability, 430F steel and 18Cr2Mo steel are common. The steel of the present invention is assumed to be used when a low cutting speed and a small cross section are required. Therefore, 20Cr2Mo, which is representative in this application field, is a comparison target. The 20Cr2Mo steel group is both excellent in machinability and corrosion resistance, but contains lead that is being reduced or completely eliminated in this field of application. The corrosion type of this type of material is pitting corrosion. There is a PRE value (Pitting Resistance Equivalent) as a simple measure of pitting corrosion resistance, and it is represented by PRE =% Cr + 3.3 ×% Mo + 16 ×% N.

  The steel of the present invention is a lead-free material, and is a material having high machinability and sufficient corrosion resistance as compared to a typical material in this technical field of the market.

  Ferritic stainless steel described in U.S. Pat. . This steel has a PRE value of 19 or more.

  Moreover, the ferritic stainless steel described in JP 2001-098352 A has sulfur added as a machinability improving component. This steel may also contain tellurium, lead, selenium and bismuth.

  Further, the ferritic stainless steel described in JP-A-10-130794 may contain sulfur, lead, selenium, tellurium and calcium as machinability improving components, and molybdenum and copper as corrosion resistance improving components. Good. This steel has a PRE value of 20 or more.

  An object of this invention is to provide the ferritic stainless steel which improved machinability.

  Another object of the present invention is to provide a steel that can meet the demands of existing and future environmental bills, and at the same time has better machinability and corrosion resistance than typical existing steels in the same technical field. .

In order to achieve the above object, according to the present invention, in wt%:
C: ≦ 0.1
Si: ≦ 2
Mn: 0.1-2
S: 0.08 to 0.4
Cr: 16-25
Ni: ≦ 2
Mo: 1-5
Cu: 0.01-3.0
X: 0.01 to 0.5 and / or REM: 0.01 to 1
Remainder: Fe and impurities, X is [2 × Te + 1 × Se + 1 × Bi]
Ferritic stainless steel is provided.

  In addition to impurities, Ca, Sn, and B may be contained as additive elements.

  The present invention is described in further detail below. All of the various compositions shown below are exemplary and do not limit the present invention.

The ferritic stainless steel of the present invention is wt%:
C: ≦ 0.1
Si: ≦ 2
Mn: 0.1-2
S: 0.08 to 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 to 0.5, and / or REM: 0.01 to 1
Remainder: Fe and impurities, X is [2 × Te + 1 × Se + 1 × Bi]
including.

  Sulfur (S) improves the machinability by forming sulfides such as MnS and CrS. These sulfides promote chip formation and crushing, thereby reducing machining costs and tool wear. However, when the sulfur content increases, a problem arises in hot workability and the corrosion resistance also decreases. The sulfur content should be 0.4 wt% or less, preferably in the range of 0.08 to 0.4 wt%, preferably 0.1 to 0.4 wt%, most preferably 0.15 to 0.35 wt%. Should.

  Tellurium (Te) is added to adjust the shape of the sulfide inclusions. Tellurium combines with manganese to change the morphology of MnS interstitial grains. When the tellurium content increases, the hot workability decreases when the relationship between tellurium and manganese is weak. The tellurium content should be 0.2 wt% or less, within the range of 0.01-0.2 wt%, preferably 0.01-0.015 wt%, most preferably 0.01-0.1. Should. Selenium and bismuth are also added for the same purpose as tellurium. To obtain the desired result, the content of 2 × Te + Se + Bi is in the range of 0.01 to 0.5 wt%, preferably 0.02 to 0.4 wt%, most preferably 0.02 to 0.2 wt%. must not.

  Manganese (Mn) combines sulfur to form manganese sulfide and enhances the machinability of the steel. The amount of manganese in steel affects the morphology of sulfide inclusions. Since manganese is an austenite stabilizing element, the manganese content must be reduced. Usually, the manganese content of stainless steel is limited because the corrosion resistance is adversely affected as the manganese content increases. The addition amount of manganese should be 2.0 wt% or less, within the range of 0.1 to 2.0 wt%, preferably 0.2 to 1.5 wt%, most preferably 0.4 to 1.5 wt%. Should be.

Chromium (Cr) is a very important element with respect to corrosion resistance. This is because Cr has the ability to form a passive layer of Cr ₂ O _{3 on} the steel surface. In order to secure a ferrite structure, the chromium content should be 16 wt% or more. In order to ensure good pitting corrosion resistance, the chromium content needs to be 19 wt% or more. Accordingly, the chromium content should be in the range of 16-25 wt%, desirably 18-22 wt%, most desirably 19-21 wt%.

  Silicon (Si) has a ferrite stabilizing effect. Silicon is a precipitation hardening element. When there is too much silicon content, hot workability will fall. However, Si needs a certain amount as a deoxidizer. The amount of silicon added should be 2 wt% or less, preferably 1 wt% or less, and most preferably 0.5 wt% or less.

  Calcium (Ca) affects the form of oxide inclusions. When the Ca / O ratio increases, the melting point of the oxide increases, so that the deformability of the oxide during cutting decreases. As a result, tool wear increases. The amount of calcium added should be 0.006 wt% or less, in the range of 0 to 0.002 wt%, desirably in the range of 0 to 0.001 wt%.

  Molybdenum (Mo) is a ferrite stabilizing element that is extremely effective for corrosion resistance in chloride environments. The molybdenum content should be in the range of 1.0 to 5.0 wt%, desirably 1.5 to 2.5 wt%, most desirably 1.85 to 2.5.

  Copper (Cu) has a good effect on machinability in terms of tool life during machining. The reason is that 1 nm-sized copper precipitates are deposited along the grain boundaries. An adverse effect when the copper content becomes large is deterioration of hot workability and chip crushability. The copper content should be in the range of 0.01 to 3.0 wt%, preferably 0.5 to 2.0 wt%, most preferably 0.7 to 2.0 wt%.

  Carbon (C) has a strong bonding tendency with chromium, chromium carbide precipitates at the grain boundary, and the surrounding matrix is deficient in chromium. As a result, intergranular corrosion is likely to occur. Therefore, the carbon content should be as low as possible, 0.1 wt% or less, desirably 0.05 wt% or less, and most desirably 0.03 wt% or less.

  Boron (B) contributes to improvement of hot workability. However, since hot workability deteriorates when added in a large amount, the amount added should be small. The boron addition amount is in the range of 0 to 0.02 wt%, desirably in the range of 0.0005 to 0.01 wt%, and most desirably in the range of 0.001 to 0.01 wt%.

  Nitrogen (N) is an austenite forming element. The amount of nitrogen in the ferrite material is low. Nitrogen has a strong positive effect on the PRE value, but if the nitrogen content is too high, the corrosion resistance deteriorates. When chromium nitride is deposited, it acts as a starting point for corrosion. Furthermore, if the nitrogen content is high, the hot workability is also adversely affected. Therefore, the nitrogen content is made as low as possible. The nitrogen content is 0.05 wt% or less.

  REM (rare earth metal) is used as a machinability improving additive. REM is a generic name for many elements, and includes, for example, cerium, lanthanum, praseodymium, neodymium, and the like. REM changes the shape of non-metallic inclusions. REM can be added as misch metal or as a single element. In practice, the amount of REM added is 1 wt% or less, preferably 0.1 wt% or less.

  Tin (Sn) acts as an additive that improves the machinability during low-speed cutting. The tin content is 0.15 wt% or less, preferably 0.10 wt% or less.

<Explanation of test method>
The test material was melted in a high frequency furnace and produced by casting, heating and forging. After forging, the sample was fully polished, rolled and quenched. The sample was annealed, cooled with water, and then drawn with a conventional drawing device. Finally, the material was linearized and then polished for testing.

  In order to evaluate machinability, the material under test was tested for drilling, turning and chip breaking. Furthermore, in order to evaluate corrosion resistance, neutral salt spray test (NSS), copper chloride accelerated salt spray test (CASS), pitting corrosion test (CPT) Went. 20Cr2Mo steel was used as a comparative material for drilling and turning, and 20Cr2Mo steel was mainly used as a comparative material hereinafter.

  Table 1 shows chemical compositions and PRE values (both wt%) for the test materials and comparative materials. About REM containing material, it added with the misch metal.

<Drilling test>
In the drilling test, the number that could be processed with the same drill was examined. Regarding the processing procedure, the center hole was first drilled and then processed with an integrated tungsten carbide drill.

  During drilling, the drill was inspected at regular intervals. The chipping of the cutting edge and the formation of the constituent cutting edge were recorded, and the shape of the chip was also recorded. When the specification life was limited to 440 or less due to chipping of the cutting edge, a retest was performed. FIG. 1 summarizes the number of drilled holes for each tested steel composition. When the test was performed twice, the average value was shown.

  As shown in FIG. 1, the drillability of four types of steel compositions is superior to others. These four types of materials were ranked as shown in Table 2 from the viewpoints of forming the cutting edge and chipping the cutting edge.

<Turning test>
Samples used in the turning test can be contoured simultaneously on the same turning insert to test for testing in various machining directions (plunge cutting, longitudinal machining with constant or varying depth of cut) Designed as follows.

  The machining procedure was cut after the external turning. A coated cemented carbide insert was used.

  After producing 100 pieces, 10 pieces were taken out and the diameter was measured to examine the dimensional change during the whole processing period. A total of 1100 pieces were produced for one type of material.

  Changes in the maximum diameter during processing can be represented by trend lines. Assuming that the trend line is a straight line, it can be expressed by the following equation.

Maximum diameter = slope x time constant + C
C is the point where the straight line intersects the diameter axis. From this, the inclination of the trend line is obtained.

  Theoretically, as the tool blade wears down, the workpiece diameter increases and the slope of the trend line should be positive. Table 3 summarizes the results of the turning test.

  Assuming a small negative value of zero and a small slope of the trend line means slow tool wear, the ranks of the tested materials are as shown in Table 4.

<Chip crushability>
The chip crushability was tested in a longitudinal turning using a coated cemented carbide insert with a feed level of 2 and dimensions of 2 levels. For each combination of feed and turning diameter, the chips were collected and judged in 5 stages and shown in Table 5. The lowest judgment step, that is, “impossible” is a judgment for a long chip that has not been crushed, and the judgment stage becomes better as the chip becomes shorter.

  From the results of Table 5, it can be seen that all the test materials have a chip crushability equal to or higher than that of the comparative material 20Cr2Mo steel. The best chip friability was obtained by turning the Sn-added material 98314. The next best chips were shorter and finer chips, obtained with two materials 98315 and 98316 with REM addition and two materials 98310 and 98311 with tellurium addition. Two of the tellurium-added materials were at the same level as the comparative materials, with the lowest results in the chip friability test.

<Machinability test results>
According to the results of the three types of machinability tests performed above, it can be seen that various materials exhibit different machinability under different processing conditions. For example, the tellurium additive material has a large particle size and a large round sulfide, with the best results in the drilling test. However, in order to distinguish materials with generally good machinability, each ferrite material was weighted relatively. From experience, for multiple products in the intended field of application, drilling is the most important form of machining, followed by chip friability and finally turning. Therefore, the final judgment puts the most emphasis on drill workability, the next was chip crushability, and finally turning. This is shown in Table 6.

<Corrosion test>
The following corrosion test was conducted.

◆ Neutral salt spray test (NSS)
◆ Copper chloride accelerated salt spray test (CASS)
◆ The critical pitting temperature (CPT) was determined electrochemically.

  Corrosion tests were performed on the three types of test materials with the best machinability and the comparative material 20Cr2Mo steel. NSS was performed according to SS-ISO9227. CASS was in accordance with SS-ISO 9227, but 16 hours instead of 96 hours and was performed at 25 ° C. instead of 50 ° C.

<NSS and CASS>
Three samples for each composition were defatted and weighed. Samples were visually inspected after the test was completed and the extent of corrosive organisms recorded. Samples were evaluated as follows.

A = No corrosion observed B = Some corrosion (less than 20% of the surface)
C = considerable corrosion (20-70% of the surface)
D = severe corrosion (over 70% of the surface)
The sample was pickled and cleaned and then weighed to calculate weight loss. Finally, the samples were scanned with a real microscope to check for corrosion pits. For each composition, an average value and a distribution were obtained for each of the three types of tests. Table 8 shows the results of the corrosion test.

<CPT>
The pitting corrosion resistance test was performed by immersing the entire sample in a solution containing chloride ions by a constant potential method. Table 7 shows the experimental conditions. The solution was purged with nitrogen gas and deaired. A voltage was applied to the sample to impart polarity, and the electrochemical reaction on the sample surface was controlled. While keeping other parameters constant, the temperature was increased from 20 ° C. in increments of 5 ° C. CPT value is defined as the temperature at which the current exceeded 10 μA / cm ² . If the sample temperature exceeded 95 ° C., this temperature was recorded and the test was terminated.

  The material with the highest CPT value has the best pitting corrosion resistance in a chloride ion-containing environment. The test was conducted 6 times for each composition, and the average value and distribution were obtained. Table 8 shows the results.

<Corrosion test results>
In the CPT test, the three types of test materials were better than the comparative materials. In other tests, the material with the highest corrosion resistance among the test materials was equivalent to the comparative material. Other test materials were somewhat inferior.

  From the above test results, it is clear that the steel of the present invention has good machinability and good corrosion resistance. In addition, the steel of the present invention is lead free.

  The steel of the present invention is desirably produced by a conventional method, but may be produced by a powder metallurgy method.

<Theoretical calculation>
In addition to the test materials, several theoretical calculations were performed using Thermo-Calc (version Q, data base CCTSS) to evaluate the presence of sulfides, carbides and nitrides. Since these calculations assume an equilibrium state, they are merely a measure of the actual phenomenon.

  In the calculation, S, C, N, Mo, and Cr were each changed independently (other components were constant). Te, Se, and Bi were not included in the calculation because the database used did not contain data for entering these elements. The same applies to Cu exceeding 1.0 wt%.

  FIG. 2 contains 0.03% C, 0.5% Si, 1.5% Mn, 21% Cr, 0.5% Ni, 2.5% Mo, 1% Cu, 0.05% N. The calculated value of MnS content is shown about a composition. The sulfur content was varied in the range of 0.10% to 0.35%. The MnS content increases as the S content increases.

FIG. 3 contains 0.5% Si, 1.5% Mn, 0.35% S, 21% Cr, 0.5% Ni, 2.5% Mo, 1% Cu, 0.05% N. The calculated value of M ₂₃ C ₆ content is shown for the composition (M is Cr alone or a combination of Cr and Mo). The carbon content was varied in the range of 0.01 to 0.1%.

FIG. 4 contains 0.5% Si, 1.5% Mn, 0.35% S, 21% Cr, 0.5% Ni, 2.5% Mo, 1% Cu, 0.03% C. It shows the calculated values of the Cr ₂ N content the composition. The N content was varied in the range of 0.04 to 0.05%.

  Furthermore, the risk of sigma phase formation when the Mo content and Cr content were changed was evaluated. FIG. 5 shows 0.03% C, 0.5% Si, 1.5% Mn, 0.35% S, 0.5% Ni, 2.5% Mo, 1% Cu, 0.05% N. 6 containing 0.025% C, 0.5% Si, 1.5% Mn, 0.35% S, 21% Cr, 0.005%, and about 20 to 25% of the composition. A calculation result is shown about the composition which contained 5% Ni, 1% Cu, 0.05% N, and made Mo content 1.85-2.5%, respectively. When the Cr content and the Mo content increase, the risk of sigma phase formation increases. However, since the generation of the sigma phase greatly depends on the reaction time during production, it is unclear whether or not the sigma phase actually exists. For the actual composition shown in Table 1, no sigma phase was observed. Therefore, the steel of the present invention can avoid the sigma phase as long as an appropriate manufacturing method is used.

FIG. 1 shows a comparison of the number of holes drilled by the same tungsten carbide drill for each material of the present invention and the comparative material 20Cr2Mo. FIG. 2 shows 0.03% C, 0.5% Si, 1.5% Mn, 21% Cr, 0.5% Ni, 2.5% Mo, 1% Cu, 0.05% N, S The calculated value of a MnS fraction is shown about the composition which changed content by 0.10-0.35%. FIG. 3 shows 0.5% Si, 1.5% Mn, 0.35% S, 21% Cr, 0.5% Ni, 2.5% Mo, 1% Cu, 0.05% N, C the composition with varying content in 0.01% to 0.1% showing the calculated values of _M 23 _{C 6} parts per. FIG. 4 shows 0.5% Si, 1.5% Mn, 0.35% S, 21% Cr, 0.5% Ni, 2.5% Mo, 1% Cu, 0.03% C, N The calculated value of a MnS fraction is shown about the composition which changed content by 0.04-0.05%. FIG. 5 shows 0.03% C, 0.5% Si, 1.5% Mn, 0.35% S, 0.5% Ni, 1% Cu, 2.5% Mo, 0.05% N. The calculation value of (sigma) phase fraction is shown about the composition which changed Cr content by 20 to 25%. FIG. 6 shows 0.03% C, 0.5% Si, 1.5% Mn, 0.35% S, 0.5% Ni, 1% Cu, 21% Cr, 0.05% N, Mo The calculated value of σ phase fraction is shown for a composition whose content is changed from 1.85 to 2.5%.

Claims (9)

At chemical composition wt%:
C: ≦ 0.1
Si: ≦ 2
Mn: 0.1-2
S: 0.08 to 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 to 0.5 and / or REM: 0.01 to 1
Remainder: Fe and impurities, X is 2 × Te + 1 × Se + 1 × Bi
Ferritic stainless steel characterized by   2. The ferrite according to claim 1, wherein the Cr content is 18 to 22 wt%, preferably 19 to 21 wt%, and / or the Ni content is ≦ 1 wt%, preferably ≦ 0.5 wt%. Stainless steel.   3. The ferritic stainless steel according to claim 1 or 2, wherein the C content is ≦ 0.05 wt%, preferably ≦ 0.03 wt%.   The Mn content according to any one of claims 1 to 3, wherein the Mn content is 0.2 to 1.5 wt%, preferably 0.4 to 1.5 wt%, and / or the S content is 0.1 Ferritic stainless steel characterized in that it is ˜0.4 wt%, preferably 0.15 to 0.35 wt%.   5. The ferritic stainless steel according to claim 4, wherein the Ca content is ≦ 0.002 wt%, preferably ≦ 0.001 wt%.   6. The Si content according to claim 1, wherein the Si content is ≦ 1 wt%, preferably ≦ 0.5 wt%, and / or the Mo content is 1.5 to 2.5 wt%, preferably Ferritic stainless steel characterized by being 1.85 to 2.5 wt%.   7. The Cu content according to any one of claims 1 to 6, wherein the Cu content is 0.5 to 2 wt%, desirably 0.7 to 2 wt%, and / or the B content is 0.0005 to 0.01 wt%. %, Preferably 0.001 to 0.01 wt% ferritic stainless steel.   8. The content of X, i.e. 2 x Te + 1 x Se + 1 x Bi, in any one of claims 1 to 7 is 0.02 to 0.4 wt%, preferably 0.02 to 0.2 wt%, and / or Or the content of REM is 0.01-0.1, Ferritic stainless steel characterized by the above-mentioned. In any one of Claim 1-8, content of N, P, and O which is an impurity is wt%:
N: ≦ 0.05
P: ≦ 0.03
O: ≦ 0.05
Ferritic stainless steel characterized by

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