STEEL MATERIAL HAVING EXCELLENT ALCOHOL-INDUCED PITTING CORROSION RESISTANCE AND ALCOHOL-INDUCED SCC RESISTANCE
A steel material excellent in alcohol-induced pitting corrosion resistance and alcohol-induced SCC resistance enables application in large structures without the need of alloy treatment or addition of inhibitors, by improving the pitting corrosion resistance and the SCC resistance of the steel material itself, by having a chemical composition containing, by mass % C: 0.03% to 0.3%, Si: 0.01% to 1.0%, Mn: 0.1% to 2.0%, P: 0.03% or less, S: 0.01% or less and Al: 0.1% or less, and one or both of Mo: 0.03% to 1.0% and W: 0.03% to 1.0%, and at least two of Sb: 0.005% to 0.5%, Sn: 0.01% to 0.3% and Nb: 0.005% to 0.1%, and the balance including Fe and incidental impurities.
This disclosure relates to a steel material having excellent alcohol-induced corrosion resistance in particular, alcohol-induced pitting corrosion resistance and alcohol-induced SCC resistance.
Particularly, the disclosure relates to steel material having excellent alcohol-induced pitting corrosion resistance and alcohol-induced SCC resistance which is preferably applicable in parts which directly contact bio-alcohol, examples thereof including steel material used in tanks that store bio-alcohols such as bio-ethanol, tanks inside vessels or tanks for automobiles for the purpose of transportation, and steel material used for pipeline transportation.
BACKGROUNDAmong bio-alcohol, for example, bio-ethanol is produced mainly by decomposing and purifying sugar content in corn, wheat, or the like. Recently, bio-ethanol is being used widely throughout the world, as an alternative fuel for petroleum (gasoline) or as fuel mixed with gasoline. The usage amount thereof is increasing every year.
However, despite the increase in the amount of bio-ethanol being used in processes such as storing and transporting bio-ethanol or mixing with gasoline, the high local corrosiveness of bio-ethanol, to be specific, the generation of pitting corrosion or SCC (Stress Corrosion Cracking) makes handling bio-ethanol difficult.
Some reasons that increase the corrosiveness of bio-ethanol are the fact that acetic acid or chloride ions exist as infinitesimal impurities in the production process thereof, or the fact that it absorbs water or takes in dissolved oxygen when being stored.
Because of this, bio-ethanol has a drawback in that it can be safely handled only in facilities provided with ethanol resistance measures, for example, facilities using as tanks, organic coating material, stainless steel or stainless clad steel which have excellent ethanol-induced SCC resistance. Further, for transportation of bio-ethanol, conventional pipelines or the like for transporting petroleum could not be used.
Due to the reasons described above, there is a problem in that facilities that handle bio-ethanol require a considerable cost.
To solve the above problem, for example, JP 2011-26669 A proposes as a measure to deal with biofuels a method of applying a zinc-nickel alloy containing 5% to 25% of Ni to the steel material for tanks for biofuels, and applying on the alloy a chemical conversion treatment containing no hexavalent chromium. JP 2011-26669 A describes that, by adopting that method, the corrosion resistance property of the steel material in gasoline containing ethanol would be satisfactory.
Further, as a measure to deal with fuel vapor of bio-ethanol and the like, JP 2011-231358 A proposes a steel sheet for manufacturing pipes having excellent corrosion resistance, obtained by applying a “Zn—Co—Mo alloy where the composition ratio of Co to Zn in the alloy layer is 0.2 to 4.0 at %” on the steel sheet surface.
Further, in “F. Gui, J. A. Beavers and N. Sridhar, Evaluation of ammonia hydroxide for mitigating stress corrosion cracking of carbon steel in fuel grade ethanol, NACE Corrosion Paper, No. 11138 (2011)”, investigation is made on the inhibitor effect of ammonium hydroxide of a steel material inside a simulated liquid of bio-ethanol against SCC (Stress Corrosion Cracking). Gui, et al. states that by adding ammonium hydroxide, crack extension is suppressed and SCC is mitigated.
It is considered that the zinc-nickel alloy disclosed in JP 2011-26669 A would be effective for improving corrosion resistance. However, such zinc-nickel alloy requires electroplating treatment. For this reason, although the alloy can be applied for small tanks such as fuel tanks for automobiles without any problem, for thick steel material used for large structures such as storage tanks with a capacity of 1000 kL or more, or line pipes, treatment costs become very large, and therefore cannot be applied. Further, when coating failure or the like occurs, pitting corrosion progresses more easily and causes SCC to occur more easily, in that part. Therefore, it cannot be said that sufficient pitting corrosion resistance and SCC resistance would be obtained.
The Zn—Co—Mo alloy disclosed in JP 2011-231358 A requires electroplating treatment as well, and due to the same reasons as JP 2011-26669 A, the alloy cannot be applied for thick steel material used for large structures. Further, again, due to the same reasons as JP 2011-26669 A, it cannot be said that sufficient pitting corrosion resistance and SCC resistance would be obtained.
Further, according to the disclosure of Gui, et al., corrosion phenomena such as SCC are certainly mitigated by adding inhibitors. However, it cannot be said that the effect thereof would be sufficient. The reason is that, while inhibitors exhibit their effect by adsorbing on the steel sheet surface, the adsorption behavior thereof is largely affected by the surrounding pH and the like. Therefore, if corrosion occurs locally, adsorption may not be sufficient. Further, there is a risk of pollution due to effluence of inhibitors to the environment, and it cannot be said that the method of Gui, et al. is a preferable measure to prevent corrosion.
As described above, an anti-corrosion method using an alloy is not suitable for large structures, and the effect thereof regarding pitting corrosion resistance is not sufficient. Further, as for inhibitors, the effect of reducing corrosion is unstable. In view of the above, for large structures, it is advantageous to improve corrosion resistance of the steel material itself inside bio-ethanol, from the viewpoint of costs as well.
It could therefore be helpful to provide a steel material with excellent alcohol-induced pitting corrosion resistance and alcohol-induced SCC resistance enabling application in large structures without the need of alloy treatment or addition of inhibitors, by improving corrosion resistance, in particular, pitting corrosion resistance and SCC resistance of the steel material itself.
SUMMARYWe discovered that addition of Mo and W is effective in inhibiting corrosion, in particular, pitting corrosion and SCC inside bio-ethanol, and also that by adding Sb, Sn, and Nb along with Mo and W, pitting corrosion and SCC of steel inside bio-ethanol are significantly inhibited.
We thus provide:
1. A steel material excellent in alcohol-induced pitting corrosion resistance and alcohol-induced SCC resistance containing, by mass % C: 0.03% to 0.3%, Si: 0.01% to 1.0%, Mn: 0.1% to 2.0%, P: 0.03% or less, S: 0.01% or less and Al: 0.1% or less, and one or both of Mo: 0.03% to 1.0% and W: 0.03% to 1.0%, and at least two of Sb: 0.005% to 0.5%, Sn: 0.01% to 0.3% and Nb: 0.005% to 0.1%, and the balance including Fe and incidental impurities.
2. The steel material according to aspect 1, wherein the total amount of Mo and W, and Sb, Sn and Nb satisfies, by mass % a range of 0.15% (Mo+W+Sb+Sn+Nb) 1.0%, and the total amount of Mo and W satisfies, by mass % 0.08% (Mo+W).
3. The steel material according to aspect 1 or 2, wherein the steel material further contains, by mass % Ca in a range that satisfies Ca/S 0.5 and 0.01% or less.
4. The steel material according to any one of aspects 1 to 3, wherein the steel material further contains, by mass % B: 0.0002% to 0.03%.
5. The steel material according to any one of aspects 1 to 4, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
By using our steel materials for storage tanks or transport tanks for bio-ethanol, or pipelines, use for a longer period compared to conventional steel material is possible. Further, it is possible to avoid accidents caused by bio-ethanol leakage resulting from pitting corrosion or SCC, and to provide various facilities relating to the above tanks and pipelines at low costs. Therefore, our steel materials are very useful in industrial terms.
DETAILED DESCRIPTIONDetails of our steel materials are described below.
First, reasons for specifying the chemical composition of a steel material to the aforementioned range will be explained. In addition, although the unit of content of each element included in the chemical composition of the steel material is “mass %,” it will be simply expressed by “%,” unless otherwise specified.
C: 0.03% to 0.3%C is a necessary element to provide strength of steel, and to provide our target strength (400 MPa or more), it is contained in an amount of at least 0.03%. On the other hand, if the content thereof exceeds 0.3%, weldability decreases and restrictions are placed at the time of welding. Therefore, the upper limit of the content thereof is 0.3%. The content thereof is preferably 0.03% to 0.2%.
Si: 0.01% to 1.0%Si is added for the purpose of deoxidation. However, if the content thereof is less than 0.01%, the deoxidation effect is limited. On the other hand, if the content thereof exceeds 1.0%, toughness and weldability deteriorate. Therefore, Si content is 0.01% to 1.0%. The content thereof is preferably 0.05% to 0.5%.
Mn: 0.1% to 2.0%Mn is added for the purpose of improving strength and toughness. However, if the content thereof is less than 0.1%, the effect thereof is not sufficient. On the other hand, if the content thereof exceeds 2.0%, weldability deteriorates. Therefore, Mn content is 0.1% to 2.0%. The content thereof is preferably 0.3% to 1.6%.
P: 0.03% or less
P is contained as an incidental impurity. However, since it deteriorates toughness and weldability, P content is 0.03% or less. The content thereof is preferably 0.025% or less. Further, since excessive dephosphorization causes an increase in costs, the lower limit of P content is preferably 0.0003%. Therefore, the content thereof is preferably 0.0003% to 0.03%.
S: 0.01% or less
S is also contained as an incidental impurity. However, if the content thereof increases, not only does toughness and weldability decrease, but inclusions such as MnS increase and serve as the origin of SCC to decrease SCC resistance. Therefore, it is desirable to minimize S content, although a content thereof of 0.01% or less would be acceptable. Further, since excessive desulfurization causes an increase in costs, the lower limit of S content is preferably 0.0001%. Therefore, the content thereof is preferably 0.0001% to 0.01%.
Al: 0.100% or less
Al is added as a deoxidizer. However, Al content exceeding 0.100% decreases the toughness of the weld metal part when the steel is subjected to welding. Therefore, the content thereof is 0.100% or less. Further, from the viewpoint of guaranteeing a deoxidation effect, the lower limit of the content thereof is preferably 0.005%. More preferably, the content thereof is 0.005% to 0.070%.
One or both of Mo: 0.03% to 1.0% and W: 0.03% to 1.0%
Mo: 0.03% to 1.0%Mo is an important pitting corrosion resistance/SCC resistance improving element for the steel material. Mo forms an oxysalt as a corrosion product, and when a crack which serves as the origin of stress corrosion cracking occurs, the corrosion product functions to immediately protect the crack tip, and inhibit development of the crack. Further, with Mo being incorporated into the oxide film of the steel material surface, the solubility resistance of the oxide film under acid environment caused by an acetic acid contained in bio-ethanol as an impurity improves, and while reducing non-uniform corrosion, Mo also provides an effect of inhibiting pitting corrosion. However, if the content thereof is less than 0.03%, improving effects on pitting corrosion resistance and SCC resistance are limited. On the other hand, if the content thereof exceeds 1.0% , it is disadvantageous in terms of costs. Therefore, Mo content is 0.03% to 1.0%. Further, to prevent costs from increasing, the content thereof is preferably 0.03% to 0.5%.
W: 0.03% to 1.0%W is an important pitting corrosion resistance/SCC resistance improving element for the steel material. W, as well as Mo, forms an oxysalt as a corrosion product, and when a crack which serves as the origin of stress corrosion cracking occurs, the corrosion product functions to immediately protect the crack tip, and inhibit development of the crack. Further, with W being incorporated into the oxide film of the steel material surface, the solubility resistance of the oxide film under acid environment caused by an acetic acid contained in bio-ethanol as an impurity improves, and while reducing non-uniform corrosion, W also provides an effect of inhibiting pitting corrosion. However, if the content thereof is less than 0.03%, improving effects on pitting corrosion resistance and SCC resistance are limited. On the other hand, if the content thereof exceeds 1.0%, it is disadvantageous in terms of costs. Therefore, W content is 0.03% to 1.0%. Further, to prevent costs from increasing, the content thereof is preferably 0.03% to 0.5%. At least two of Sb: 0.005% to 0.5%, Sn: 0.01% to 0.3% and Nb: 0.005% to 0.1% Sb: 0.005% to 0.5%
Sb is an effective element in improving pitting corrosion resistance and SCC resistance under acid environment caused by an acetic acid contained in bio-ethanol as an impurity. However, if the content thereof is less than 0.005%, it is ineffective. On the other hand, if the content thereof exceeds 0.5%, limitations are caused in terms of steel material manufacturing. Therefore, Sb content is 0.005% to 0.5%. The content thereof is preferably 0.01% to 0.3%. Sn: 0.01% to 0.3%
Sn, as well as Sb, improves pitting corrosion resistance and SCC resistance under acid environment. However, if the content thereof is less than 0.01%, the addition effect is limited. On the other hand, if the content thereof exceeds 0.3%, the effect not only reaches a plateau but limitations are caused in terms of steel material manufacturing. Therefore, Sn content is 0.01% to 0.3%. The content thereof is preferably 0.02% to 0.2%.
Nb: 0.005% to 0.1%Nb is also an effective element in improving pitting corrosion resistance and SCC resistance under acid environment caused by an acetic acid. However, if the content thereof is less than 0.005%, the effect is not expressed. On the other hand, if the content thereof exceeds 1.0%, mechanical properties of the weld decrease. Therefore, Nb content is 0.005% to 0.1%. The content thereof is preferably 0.005% to 0.05%.
Among the above components, Mo and W, and Sb, Sn and Nb are particularly important, and by containing these components in a total amount of 0.15% to 1.0%, and by containing Mo and W which are particularly important in a total amount of 0.08% or more, it is possible to further improve pitting corrosion resistance and SCC resistance.
The basic components are as described above. The following components may also be contained according to necessity.
Ca: Ca/S≧0.5 and 0.01% or less
Ca is added for the purpose of performing morphological control of precipitates of S (e.g. MnS) which are incidental impurities and preventing cracks such as SCC. Therefore, Ca is preferably added depending on S content, and with Ca/S (mass ratio) being 0.5 or more, Ca provides the effect of preventing cracks. Ca/S is more preferably 1.0 or more. However, if Ca is added excessively, coarse inclusions are formed to deteriorate toughness of the base material. Therefore, the upper limit of Ca content is preferably 0.01%.
B: 0.0002% to 0.03%B is an element that enhances strength of the steel material and can be contained according to necessity. To obtain such an effect, B is preferably contained in an amount of 0.0002% or more. However, if B is added in an amount exceeding 0.03%, toughness deteriorates. Therefore, B is preferably contained in a range of 0.0002% to 0.03%. More preferably, the content thereof is 0.0003% to 0.003%.
Zr: 0.005% to 0.1%, V: 0.005% to 0.1%, Ti: 0.005% to 0.1%To further improve mechanical properties of the steel material, one or more of Zr, V and Ti may be contained. All of these elements have a limited addition effect if the contents thereof are less than 0.005%. On the other hand, if the contents thereof exceed 0.1%, mechanical properties of the weld decrease. Therefore, contents of these elements are 0.005% to 0.1%. The contents of these elements are preferably 0.005% to 0.05%.
Further, this disclosure is not intended to exclude other components that are not described herein, without losing the advantages of the disclosure. For example, in addition to these components, a small amount of REM can be added as a deoxidizer.
In our steel material, components other than those described above are Fe and incidental impurities.
A preferred method of manufacturing our steel material will now be described below.
Molten steel with the above preferable chemical composition is obtained by steelmaking in known furnaces such as a converter, an electric furnace and the like, and made into steel raw material such as slabs and billets by known methods such as the continuous casting method or the ingot casting method. When obtaining molten steel by steelmaking, vacuum degassing refining or the like may be performed.
As methods of adjusting components of molten steel, known steel refining methods may be followed.
Then, when hot rolling the above steel raw material into a desirable dimension, the material is heated to a temperature of 1000° C. to 1350° C. A heating temperature below 1000° C. results in a large deformation resistance, which makes it difficult to perform hot rolling. On the other hand, a heating temperature exceeding 1350° C. may lead to generation of surface flaws, or an increase in scale loss and fuel consumption rate. The heating temperature is preferably 1050° C. to 1300° C. If the temperature of the steel raw material is already 1000° C. to 1350° C., the material may be subjected to hot rolling directly, without heating.
Further, in hot rolling, it is necessary to control finisher delivery temperature, and a temperature of 600° C. or higher and 850° C. or lower is preferable. With a finisher delivery temperature of lower than 600° C., the increase in deformation resistance causes an increase in rolling load and makes it difficult to perform rolling.
On the other hand, if the temperature exceeds 850° C., a desirable strength may not be obtained. As cooling to perform after hot finish rolling, air cooling or accelerated cooling with a cooling rate of 150° C./s or less is preferable. In performing accelerated cooling, the cooling stop temperature is preferably 300° C. to 750° C. After cooling, re-heating treatment may be performed.
ExamplesExamples of our steel materials will now be described below. It should be noted that our steel materials are not intended to be limited to the disclosed examples.
Molten steel with the chemical composition shown in Table 1 was obtained by steelmaking using a vacuum melting furnace or a converter, and subjected to continuous casting to obtain slabs. Then, the slabs were heated to 1230° C., and then subjected to hot rolling under a condition of finisher delivery temperature of 820° C. to obtain steel sheets with thickness of 13 mm.
These steel sheets were subjected to the following pitting corrosion test and stress corrosion cracking test.
(1) Pitting Corrosion Test Using a Simulated Liquid of Bio-EthanolA steel material was cut out into pieces of 10 mm×25 mm×3.5 mm t, subjected to wet polishing using emery polishing paper on both sides until reaching #2000, and then subjected to ultrasonic degreasing in acetone for 5 minutes, and then subjected to air drying to obtain corrosion test material. A solution obtained by adding water: 10 ml, methanol: 5 ml, acetic acid: 560 mg, NaCl: 132 mg to ethanol: 985 ml was used as a simulated liquid of bio-ethanol. The solution was put into a test tube and the test material immersed therein at room temperature. After immersing in the solution for 30 days, the test material was taken out and rust on the surface thereof rinsed using a sponge or the like. Then, corrosion products were removed in an acid with an inhibitor added thereto. The test material was washed using pure water, washed in ethanol, and then air dried. Then, the pitting corrosion depth of the surface of the test material was measured using a 3D laser microscope, and the maximum pitting corrosion depth was evaluated.
Test materials with maximum pitting corrosion depth of less than 70% with respect to base steel (comparative example 1) were evaluated as having excellent pitting corrosion resistance.
(2) Stress Corrosion Cracking Test by SSRT (Slow Strain Rate Technique) in Simulated Liquid of Bio-EthanolA steel material was processed into a round bar of 130 mm×6.35 mm φ. Then, both ends thereof were subjected to screw processing, and at the same time, the round bar was processed to have a diameter of 3.81 mm φ over the length of 12.7 mm from the center part toward both ends. The test material was subjected to ultrasonic degreasing in acetone for 5 minutes, and then attached to an SSRT tester. A solution obtained by adding water: 10 ml, methanol: 5 ml, acetic acid: 56 mg, NaCl: 52.8 mg to ethanol: 985 ml was used as a simulated liquid of bio-ethanol. Strains were applied at a strain rate of 2.54×10−5 mm/s in dry air atmosphere to the cells covering test materials under the condition of being filled with a simulated liquid of bio-ethanol and the condition without the liquid, respectively. Then, the ratio of total elongation until fracture occurs ([total elongation with solution/total elongation without solution]×100 (%)) was calculated, and SCC resistance was evaluated based on the following criteria.
-
- Excellent: 95% or more
- Good: 90% or more and less than 95%
- Fair: 85% or more and less than 90%
- Poor: less than 85%
The obtained results are shown in Table 2.
As is clear from Table 2, in all of our examples, pitting corrosion in the simulated liquid of bio-ethanol is inhibited, and SCC resistance is also significantly improved. In contrast, in all of the comparative examples where the chemical composition was out of the scope of our disclosure, pitting corrosion depth was not particularly inhibited, and SCC resistance was not significantly improved.
By comparing the results of our examples with those of the comparative examples, it is clear that our steel materials have improved effects.
Claims
1.-5. (canceled)
6. A steel material excellent in alcohol-induced pitting corrosion resistance and alcohol-induced SCC resistance containing, by mass % C: 0.03% to 0.3%, Si: 0.01% to 1.0%, Mn: 0.1% to 2.0%, P: 0.03% or less, S: 0.01% or less and Al: 0.1% or less, and one or both of Mo: 0.03% to 1.0% and W: 0.03% to 1.0%, and at least two of Sb: 0.005% to 0.5%, Sn: 0.01% to 0.3% and Nb: 0.005% to 0.1%, and the balance including Fe and incidental impurities.
7. The steel material according to claim 6, wherein the total amount of Mo and W, and Sb, Sn and Nb satisfies, by mass % a range of 0.15%≦(Mo+W+Sb+Sn+Nb)≦1.0%, and the total amount of Mo and W satisfies, by mass % 0.08%≦(Mo+W).
8. The steel material according to claim 6, wherein the steel material further contains, by mass % Ca in a range that satisfies Ca/S 0.5 and 0.01% or less.
9. The steel material according to claim 6, wherein the steel material further contains, by mass % B: 0.0002% to 0.03%.
10. The steel material according to claim 6, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
11. The steel material according to claim 7, wherein the steel material further contains, by mass % Ca in a range that satisfies Ca/S≧0.5 and 0.01% or less.
12. The steel material according to claim 7, wherein the steel material further contains, by mass % B: 0.0002% to 0.03%.
13. The steel material according to claim 8, wherein the steel material further contains, by mass % B: 0.0002% to 0.03%.
14. The steel material according to claim 11, wherein the steel material further contains, by mass % B: 0.0002% to 0.03%.
15. The steel material according to claim 7, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
16. The steel material according to claim 8, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
17. The steel material according to claim 9, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
18. The steel material according to claim 11, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
19. The steel material according to claim 12, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
20. The steel material according to claim 13, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
21. The steel material according to claim 14, wherein the steel material contains, by mass % one or more of Zr: 0.005% to 0.1%, V: 0.005% to 0.1% and Ti: 0.005% to 0.1%.
Type: Application
Filed: Dec 2, 2013
Publication Date: Oct 29, 2015
Inventors: Itaru Samusawa (Kurashiki), Kazuhiko Shiotani (Kurashiki), Tsutomu Komori (Kurashiki)
Application Number: 14/649,059
