JP5845806B2 - Steel material with rust layer with excellent chloride ion barrier properties - Google Patents

Description

  The present invention relates to a steel material with a rust layer, which is a corrosion-resistant material suitable for use in a highly humid environment rich in chloride ions, which are corrosive factors, and particularly excellent in chloride ion barrier properties.

  In weathering steel and corrosion resistant steel used in a severe corrosive environment containing salt, corrosion resistance is imparted by applying protective rust and coating formed on the steel surface in addition to the anticorrosion method. Regarding protective rust, as described in Japanese Industrial Standards JIS G 3114 for weathering steel for bridges and Patent Documents 1 to 5, additive elements such as Ni, Cr, Cu, and W are concentrated. Covering with a rust layer can significantly reduce the corrosion rate and often withstand service for decades without painting.

  On the other hand, as a specific environment where the coating treatment is performed, there is a case where it is exposed to a severe environment of high salinity and high wettability for a long period of time such as a corrosion-resistant steel for shipbuilding. In this case, there is a problem that a highly protective rust layer is difficult to be formed, and excellent corrosion resistance that can withstand practical use cannot be obtained. Therefore, a steel material subjected to anticorrosion measures such as painting is used.

JP-A-11-172370 Japanese Patent Laid-Open No. 2-125839 JP-A-5-51668 JP 2007-254881 A JP 2007-46148 A

However, all of the weathering steels described in Patent Documents 1 to 5 require the addition of at least one alloy additive element of Ni, Cr, Cu, and W, and are resistant to corrosion in a high salinity environment. In order to obtain the function, a relatively large amount of alloy addition of these expensive alloy addition amounts of up to about 10% by mass is required. Recently, although weather resistant steel added with about 1 to 3% by mass has been developed, it can be said that problems such as cost reduction of raw materials for production and search for alternative elements for corrosion resistance remain. . In addition, among the alloy elements used, there is an element that is easily restricted by the environment and method of use, such as rather deteriorating the corrosion resistance in a high salinity environment, and an example is Cr.
Furthermore, when anti-corrosion measures such as painting are performed, there is a problem that the coating film deteriorates over time and regular repair work is required, and there is a problem that enormous costs are required for its maintenance. Originally desirable for painting.

  An object of the present invention is to provide a steel material with a rust layer that is excellent in chloride ion barrier properties and can be used without coating in a site exposed to a highly humid environment rich in salt.

  As a result of intensive investigations on a steel material having excellent corrosion resistance that can be used without coating in a portion exposed to a highly humid environment rich in salt, the present inventors have found the following.

(I) It was selected from Sb and Sn for the steel material side portion (rust underlayer) of the rust layer formed on the surface of the steel material exposed for 7 days in an environment of temperature 35 ° C to 50 ° C and relative humidity 95% RH. If one or two elements are contained and more preferably concentrated, a steel material having excellent corrosion resistance that can be used without coating in a portion exposed to a high humidity environment rich in salt is provided. it can.

(Ii) Further, the average crystal grain size of the steel material side portion (rust lower layer) of the rust layer is set to 50 nm or less, thereby promoting and strengthening the formation of a fine and dense protective rust layer that contributes to the improvement of corrosion resistance. Excellent corrosion resistance is obtained.

The present invention has been made based on such findings. Specifically, in terms of mass%, C: 0.01 to 0.20%, Si: 0.05 to 0.5%, Mn: 0.1 to 2.0%, P: 0.025 % Or less, S: 0.0001 to 0.02%, Al: 0.01 to 0.10%, and one or two elements selected from Sb: 0.34 to 0.8% and Sn: 0.50 to 0.8% in total It has a rust layer on the surface of a steel material containing 0.34 to 1.0%, the balance being Fe and inevitable impurities, the steel material side portion of the rust layer contains Sb and / or Sn, and the average crystal grain size is 50 nm It is the steel material with a rust layer excellent in the chloride ion barrier property characterized by being the following.

  According to the present invention, it is a corrosion-resistant material suitable for use in a highly humid environment rich in chloride ions that are corrosive factors, and it is possible to provide a steel material with a rust layer that is particularly excellent in chloride ion barrier properties. it can.

FIG. 1 (a) is a photograph when element mapping analysis is performed using an electron probe microanalyzer (EPMA) on a cross section of a steel material with a rust layer (Sb: containing 0.34% by mass) according to the present invention. (B) is a chart when the X-ray intensities of Sb and Cl are measured at positions on the line indicated by the white broken line in the depth direction of FIG. FIG. 2 (a) is a photograph when element mapping analysis is performed using an electron probe microanalyzer (EPMA) on the cross section of a steel material with a rust layer (Sn: 0.35% by mass) according to the present invention. (B) is a chart when the X-ray intensities of Sn and Cl are measured at positions on the line indicated by the white broken line in the depth direction of FIG. 3 (a) and 3 (b) are electron diffraction photographs of a rust layer of a steel material with a rust layer (Sb: 0.34% by mass) according to the present invention, and FIG. 3 (a) shows a high Sb content in the rust layer. Part and FIG.3 (b) are low Sb content parts in a rust layer.

Next, embodiments of the present invention will be described in detail below.
The steel material with a rust layer according to the present invention is in mass%, C: 0.01 to 0.20%, Si: 0.05 to 0.5%, Mn: 0.1 to 2.0%, P: 0.025% or less, S: 0.0001 to 0.02%, Al: 0.01 Contains ~ 0.10%, and further contains one or two elements selected from Sb: 0.34 ~ 0.8% and Sn: 0.50 ~ 0.8% in total, 0.34 ~ 1.0%, the balance being Fe and inevitable The surface of the steel material made of impurities has a rust layer, the steel material side portion of the rust layer contains Sb and / or Sn, and the average crystal grain size is 50 nm or less.

(1) About the component composition of the steel material with a rust layer of this invention "%" which is a unit of the content of the component shown below shall mean "mass%" unless otherwise indicated.

・ C: 0.01 ～ 0.20%
C is an effective element for improving the strength of the steel material, and in order to ensure the desired strength, it is necessary to contain 0.01% or more, but if it exceeds 0.20%, the weldability and toughness deteriorate. For this reason, C content shall be 0.01 to 0.20%.

・ Si: 0.05-0.5%
Si is an element added to increase the strength of the deoxidizer and steel material during steelmaking, and is contained in an amount of 0.05% or more in order to ensure a predetermined strength. On the other hand, if the Si content exceeds 0.5%, the toughness and weldability deteriorate significantly, so the Si content is in the range of 0.05 to 0.5%.

・ Mn: 0.1-2.0%
Mn is an element that improves strength and toughness, and needs to be contained in an amount of 0.1% or more in order to ensure a predetermined strength. However, if the Mn content exceeds 2.0%, the toughness and weldability deteriorate. For this reason, Mn content is taken as 0.1 to 2.0% of range.

-P: 0.025% or less P is an element that improves corrosion resistance, but when the P content exceeds 0.025%, weldability deteriorates. Therefore, the P content is 0.025% or less.

・ S: 0.0001-0.02%
Since S is an element that deteriorates weldability and toughness, it must be 0.02% or less. On the other hand, even if the S content is excessively reduced to less than 0.0001%, the production cost is remarkably increased, which is not preferable. Therefore, the S content is 0.0001 to 0.02%.

・ Al: 0.01-0.10%
Al is an element necessary for deoxidation during steelmaking, but if the Al content is less than 0.01%, a sufficient deoxidation effect cannot be expected. On the other hand, if the Al content is more than 0.10%, the Al ³⁺ eluted by the corrosion of the base iron lowers the pH of the base iron surface and deteriorates the corrosion resistance. Therefore, the Al content is set to 0.01 to 0.10%.

・ Sb: 0.34 to 0.8% and Sn: 0.34 to 0.8% of one or two elements selected from 0.34 to 0.8% in total
Sb and Sn are important elements of the present invention, and are effective elements that contribute to the improvement of corrosion resistance. Sb and Sn have a function of refining magnetite (Fe ₃ O ₄ ), thereby suppressing corrosion of the steel sheet due to suppression of chloride ion intrusion. In order to exhibit this corrosion resistance effect, it is necessary to add 0.34% or more in total of one or two elements selected from Sb: 0.34 to 0.8% and Sn: 0.34 to 0.8%. In addition, although there is an effect which suppresses rust formation, so that there is much content of Sb and Sn, if it exceeds 0.8% independently and exceeds 1.0% in total, the problem of embrittlement that a crack will occur at the time of manufacture will arise. . Therefore, the content of Sb and Sn is 0.34 to 0.8% for both of Sb and Sn added alone, and 0.34 to 1.0% in total for the combined addition of both Sb and Sn. The range.

  The steel material with a rust layer of the present invention has the above-described components as a basic composition, and the balance is composed of Fe and inevitable impurities. The inevitable impurities described here include Ni: 0.009% or less, Cr: 0.009% or less, Cu: 0.01% or less, N: 0.004% or less, and O: 0.004% or less. Here, Cr may be a factor causing deterioration of corrosion resistance in a high salinity environment, it is desirable to avoid addition from the viewpoint of ease of handling, as described above, preferably 0.009% or less .

(2) About the rust layer of the steel material with a rust layer of the present invention The steel material with a rust layer of the present invention not only has the above steel composition, but also the steel material side portion (rust lower layer) of the rust layer contains Sb and / or Sn. In addition, it is an essential invention specific matter that the average grain size of the steel material side portion of the rust layer is 50 nm or less. Below, the reason for limitation will be described.

  First, the inventors have a steel type A containing C: 0.144%, Si: 0.28%, Mn: 1.18%, P: 0.012%, S: 0.002%, Al: 0.022% and Sb: 0.34% by mass. Steel material (steel plate) I produced using C and 0.1% by mass, C: 0.144%, Si: 0.28%, Mn: 1.18%, P: 0.014%, S: 0.002%, Al: 0.022% and Sn: 0.35% The steel material (steel plate) II produced using the steel type B to be evaluated was evaluated for corrosion resistance, particularly chloride ion blocking ability, by a corrosion test simulating a highly humid environment rich in chloride ions as follows.

  The two steel types A and B are melted and rolled to produce steel materials I and II. Each steel material I and II is cut into test pieces of 3 mm × 15 mm × 15 mm and shot blasted on the surface of each test piece. Treated. Then, each test piece is immersed in artificial seawater (5% NaCl) solution, corrosion cycle conditions: [5% NaCl solution immersion (15 minutes, twice a week), 35 ° C., 95% relative humidity (RH), 2 [Time → 60 ° C., 30% RH, 4 hours → 50 ° C., 95% RH, 2 hours], a corrosion cycle test was conducted for 7 days, simulating a humid environment with high salinity.

  In order to determine the positional relationship between the presence position of Sb or Sn in the rust layer after the corrosion test and chloride ions that are corrosive factors, each test piece is sheared to obtain a cross section, and the cross section is observed. Electron probe microanalyzer (EPMA) analysis was carried out by embedding and fixing with resin (diameter: 25 mm) as possible. The rust cross section was polished to # 4000 using ethanol (no water was used).

  FIG. 1A shows the result of element mapping of a polished rust cross section of a steel sheet I manufactured using steel type A, and FIG. 1B shows a white broken line in the depth direction of FIG. FIG. 2A shows the result of elemental mapping of a polished rust cross section of a steel plate II produced using steel type B, and the chart when measuring the X-ray intensity of Sb and Cl for the position on the line, and FIG. 2B shows a chart when the X-ray intensities of Sn and Cl are measured at positions on the line indicated by the white broken line in the depth direction of FIG.

From the results shown in FIGS. 1 and 2, there is a portion where the X-ray intensity of Sb or Sn in the rust layer 3 is clearly higher than the X-ray intensity level of Sb or Sn in the base material part (steel plate) 1. From this, it can be seen that Sn or Sb contained in the steel sheet is also present in the rust layer. Furthermore, the results shown in FIG. 1 and FIG. 2 show that a large amount of chloride ions are present on the surface side of the rust layer rather than the high content portion (concentrated portion) of Sb or Sn existing on the steel material side (rust lower layer) of the rust layer. This indicates that the high content part (concentration part) of Sb and Sn existing on the steel material side (rust lower layer) of the rust layer is from the steel material surface to the steel material inside. It was confirmed that the movement (permeation) of chloride ions in the slag was suppressed.
As a result of performing scanning electron microscope (SEM) analysis, focused ion beam processing (FIB) -transmission electron microscope (TEM) analysis and scanning transmission electron microscope (STEM) analysis on the rust structure of this concentrated part, Can be considered.

In the experiment, a thinned rust with an area of about 10 μm ² was prepared by FIB method from the concentrated part position specified by EPMA, and TEM observation was performed. The position confirmation of the Sb and Sn-containing part was evaluated by energy dispersive X-ray analysis (EDS) to determine whether or not it contained. The beam diameter is about 500 nm. Further, from the analysis of the electron diffraction pattern generated by transmitting the electron beam, the type of rust crystal and the size of the crystallite can be estimated. The result of the steel plate I manufactured using the steel type A will be described.

3A and 3B show electron diffraction photographs of a high Sb-containing part and a non-Sb-containing part (low Sb-containing part) existing in the rust layer of the steel sheet I. FIG. From the results of FIGS. 3A and 3B, the high Sb-containing part and the non-Sb-containing part (low Sb-containing part) present in the rust layer are both magnetites (Fe ₃ O ₄ ) having an inverted spinel structure. there were. In addition, the high Sb-containing part has a ring shape as shown in FIG. 3 (a), and the non-Sb-containing part (low Sb-containing part) has a spot shape as shown in FIG. 3 (b). The rust layer of the Sb-containing part shows that the crystal grains are finer than that of the non-Sb-containing part (low Sb-containing part). Furthermore, as a result of directly observing the size of the magnetite crystal grains present in the concrete rust layer with a TEM bright field image, the crystal grain size of the non-Sb-containing part (low Sb-containing part) is much larger than 50 nm. Some of them are large and about 1 μm at the maximum, while those having a high Sb content are 50 nm at the maximum. Moreover, it was confirmed by SEM observation backscattered electron image and STEM observation high-angle annular dark field image (HAADF) that the high Sb content part is relatively dense compared to the non-Sb content part (low Sb content part). . Thereby, it is thought that the rust layer of a high Sb content part is comprised with the fine and dense magnetite.

  Furthermore, the steel plate II was analyzed in the same manner, and it was confirmed that the same result as in the case of Sb was obtained.

  From the above, the steel material with a rust layer of the present invention not only has the above steel composition, but the steel material side portion of the rust layer contains Sb and / or Sn, and the average crystal grain size of the steel material side portion of the rust layer Was 50 nm or less.

In addition, it has been confirmed that the valence of Sb and Sn in the rust layer is mainly +5 valence and +4 valence, respectively, and the Sb valence in the rust layer is +5 and / or Sn atoms. And salts ([SbO ₃ ] ⁻ , [SnO ₃ ] ²⁻ ) and / or oxides (Sb ₂ O ₅ , SnO ₂ ) formed from the valence +4. In other words, the presence of Sb and / or Sn inhibits the crystal growth of magnetite and refines the magnetite, which contributes to the formation of a protective rust layer that suppresses migration (permeation) of chloride ions into the steel. and further these oxygen acid ions ^{_{[SbO 3] -, [SnO}} 3] 2- and chloride ion Cl ^- by permeation suppressing corrosive factors due to the Coulomb repulsion of the chloride ion, a chloride ion blocking further enhance It is estimated that

  Regarding the Sb and Sn contents necessary for the formation of a rust layer with chloride ion blocking action by refining the above magnetite, energy dispersive X-ray analyzer (EDS) quantitative analysis by TEM in the fine magnetite region (spatial resolution 500 nm). As a result, it has been found that the ratio of Sb and / or Sn atoms to one iron atom in the rust layer is preferably at least 0.01 or more, and more preferably 0.03 or more. The larger the ratio, the greater the effect. However, when the ratio is too large, the effect of chloride ion blocking (ion selective permeability) by oxygen acid ions of Sb and Sn is saturated, so Sb and / or for one iron atom in the rust layer The ratio of Sn atoms is preferably less than 2.0.

The “rust layer” as used in the present invention mainly includes magnetite (Fe ₃ O ₄ ), and in addition, goethite (α-FeOOH), akaganeite (β-FeOOH) and lipidocrosite (γ-FeOOH). It refers to rust containing one or more of them and an inevitable phase.
Moreover, in this invention, it is preferable that the thickness of this rust layer shall be about 1-500 micrometers, More preferably, it is 10-100 micrometers. Furthermore, in this invention, the steel material side part (rust lower layer) of a rust layer points out the thickness area | region of about 1/3 with respect to the whole rust layer thickness from the steel material surface.

  The above description is merely an example of the embodiment of the present invention, and various modifications can be made within the scope of the claims.

  Steel types No. 1 to 13 having the composition shown in Table 1 were manufactured, and the above-described corrosion cycle test was simulated on a steel plate surface cut into 3 mm × 15 mm × 15 mm test pieces for 7 days to simulate a high salt moisture environment. A rust layer was added to prepare a sample. At this time, the weight of the steel material not including the rust layer (the weight of the steel material before the test (A)) was obtained by measuring the weight of the sample after derusting. The thickness of the entire rust layer was determined by a conventionally known method by observing the cross section of each sample with a scanning electron microscope. Further, the content of Sb and Sn in the steel material side portion of the rust layer was judged by EPMA observation in a region having a thickness of one third of the entire rust layer thickness from the steel material surface. In addition, the average grain size of the rust layer steel material side part can be obtained by directly measuring the grain size of the magnetite crystal grains existing in the region of one third of the thickness of the entire rust layer from the steel surface by a TEM bright field image. Observed and determined by a conventionally known method. In addition, the same corrosion test was further performed for 43 days (main test) on the sample in which the corrosion cycle test was performed for 7 days to form a rust layer, and the chloride ion blocking property was evaluated. Corrosion rate (mm / year) by this test was calculated from the difference between the weight of steel after derusting after this test and the weight (A) of steel before this test to determine the amount of corrosion by this test. Table 1 shows the results of evaluating the chloride ion barrier properties by comparing the corrosion rate (B) of steel type No. 9 containing no Sn or Sb as a reference and the corrosion rate (C) of each sample. In addition, the numerical value of the chloride ion blocking property in Table 1 shows the improvement rate of the chloride ion blocking property as a percentage based on the steel type No. 9 containing no Sn or Sb, and is 100 × (B− C) / B. The larger the value, the better the chloride ion blocking property.

From the results shown in Table 1, none of the inventive examples 1 to 8 aiming at optimizing the content of Sb and Sn in the steel and the average crystal grain size of the rust layer contain Sb and Sn in the steel. In addition, the average crystal grain size of the rust layer is improved by 31% or more in terms of chloride ion blocking ability as compared with Comparative Example 9 which is outside the proper range of the present invention. On the other hand, Comparative Examples 10 and 11 in which the content of Sb and Sn in the steel is less than the range of the present invention and the average crystal grain size of the rust layer is outside the range of the present invention are both chloride ion blocking properties. The improvement rate was 10% or less.
Further, Comparative Examples 12 and 13 in which Cr was further added to the steel material in which the Sb and Sn contents within the present invention were added to the steel of the present invention had deteriorated ion barrier properties as compared with Invention Examples 1 and 4.
When compared in Invention Examples 1 to 8, Invention Examples 7 and 8 containing the proper amounts of both Sb and Sn in the steel and Invention Example 6 containing the maximum amount of Sn had a 54% improvement in chloride ion barrier properties. Inventive Examples 4 and 5 containing the proper amount of Sn alone in steel and Inventive Example 3 containing the maximum amount of Sb in the steel are the largest and the improvement rate of the chloride ion blocking property is as large as 42% or more. Inventive Examples 1 and 2, which contain an appropriate amount of Sb alone in the steel, the improvement rate of the chloride ion blocking property is 31% or more, and particularly Sn in the steel has a remarkable effect of improving the chloride ion blocking property. Met. In the prior art, the chloride ion barrier property was not improved unless Sb and Sn were added together with W and Ni, but in the present invention, by optimizing the content of Sn and Sb and the average crystal grain size of the rust layer, The chloride ion barrier property can be improved without containing W or Ni.

  According to the present invention, it is possible to provide a corrosion-resistant material suitable for use in a highly humid environment rich in chloride ions, which are corrosive factors, and can provide a steel material with a rust layer that is particularly excellent in chloride ion barrier properties. became.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Steel plate surface and rust layer interface 3 Rust layer 4 High Sb content part 5 Resin 6 High Sn content part

Claims (1)

In mass%, C: 0.01-0.20%, Si: 0.05-0.5%, Mn: 0.1-2.0%, P: 0.025% or less, S: 0.0001-0.02%, Al: 0.01-0.10%, Contains one or two elements selected from Sb: 0.34 to 0.8% and Sn: 0.50 to 0.8% in total, with a balance of 0.34 to 1.0% on the surface of the steel material consisting of Fe and inevitable impurities. Have
A steel material with a rust layer excellent in chloride ion barrier property, wherein the steel material side portion of the rust layer contains Sb and / or Sn and has an average crystal grain size of 50 nm or less.